Institute of Interdisciplinary Research, Free University of Brussels, Campus Erasme, B-1070 Brussels, Belgium
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The literature on intracellular signal transduction presents a confusing picture: every regulatory factor appears to be regulated by all signal transduction cascades and to regulate all cell processes. This contrasts with the known exquisite specificity of action of extracellular signals in different cell types in vivo. The confusion of the in vitro literature is shown to arise from several causes: the inevitable artifacts inherent in reductionism, the arguments used to establish causal effect relationships, the use of less than adequate models (cell lines, transfections, acellular systems, etc.), and the implicit assumption that networks of regulations are universal whereas they are in fact cell and stage specific. Cell specificity results from the existence in any cell type of a unique set of proteins and their isoforms at each level of signal transduction cascades, from the space structure of their components, from their combinatorial logic at each level, from the presence of modulators of signal transduction proteins and of modulators of modulators, from the time structure of extracellular signals and of their transduction, and from quantitative differences of expression of similar sets of factors.
signal transduction; effect of hormone modulations; isoforms; combinatorial logic
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
CELL SIGNAL TRANSDUCTION ENCOMPASSES
all the biological and biochemical phenomena that lead from the
perception of a signal by a cell to the response of the cell. The
signal transduction machinery of a cell integrates all the signals it
recognizes and translates them in a coordinated behavior. A signal for
a cell is whatever is recognized as such by a receptor that itself
initiates a response to this signal. A receptor is the structure that
recognizes and reacts to the signal and interprets the specificity of
the signal. These are circular definitions. Our physiology, which integrates the requirements of a living organism (for metabolism, growth, reproduction) and its responses to the outside world, uses
thousands of signals. For each cell, these include hormones and
neurotransmitters, signals from neighboring cells, soluble such as
paracrine factors, or membrane bound such as ephrins, and signals from
the inert substratum such as fibronectin. Because each signal may be
recognized by different receptors (e.g., for norepinephrine or
serotonin), the number of receptors is a multiple of the number of
signals; hence the tremendous complexity and specificity of signals and
their receptors. One category of receptors, the seven transmembrane
receptors, are coded by ~700 genes. i.e., ~2% of the number of
genes in the human genome. This contrasts with the rather limited
repertoire of known signal transduction pathways that are modulated by
such receptors, with only a few ubiquitous intracellular signal
molecules (cAMP, cGMP, Ca2+, etc.), phosphorylation
cascades, and other pathways [nuclear factor (NF)-B, etc.].
The cross signalings (anthropomorphically called cross talks) between
the various cascades further simplify the picture in appearance:
everything seems to modulate everything (Fig.
1). See, for example, a recent title:
"Signaling networksdo all roads lead to the same genes?"
(261). Furthermore, the opposite cross signalings between
cascades, as reported, give a totally incoherent picture
(141). It is striking that excellent reviews on known signal transduction proteins or cascades mention so many demonstrated effects obtained in different cell models that the authors have great
difficulties in drawing a coherent picture or end up by concluding for
each cascade, or even enzyme: "X is regulated by all signal
transduction cascades and regulates almost all cellular processes, from
gene expression to cell death" or "these results suggest that the
pathway linking A to B involves the integration of numerous signal
transduction steps by a highly complex network" (6, 18, 19, 25,
26, 28, 42, 77, 92, 93, 121, 125, 179, 260, 271, 272, 275,
298-300, 359, 360). The very restricted phenotypes of
knockout mice models in which a supposedly essential protein is absent
do not fit with such statements. The impression left was recently
summarized: "elegant complexity coupled with hopeless confusion
better defines our current state of knowledge" (309). In
fact, such reviews give a comprehensive picture of all the interactions
that may exist in mammalian cells, i.e., of the toolkit available for
differentiation. On the other hand, attempts to give a unifying picture
lead to unwarranted generalizations and/or selective consideration of
the literature and scientific myths (e.g., "Ca2+ causes
cell growth"). Thus a first paradox, which has been spelled out for a
few cascades (226, 227, 230), opposes on the one hand the
multiplicity and specificity of signals, their receptors, and their
effects and on the other hand the nonspecificity or promiscuity of the
few signaling cascades (78, 167, 205).
|
Another paradox arises from the use of simplified models for the study
of normal physiology, namely, the behavior of the cell within the whole
organism, preferably human. Because of the limitations of clinical
investigation and ever-increasing restrictions of animal
experimentation, one has to rely on models that, from experimental animals to reconstituted systems, may lose in physiological relevance what they gain in simplicity and in definition (Table
1). In the 1960s hundreds of articles,
published in the best journals, on the direct action of thyroid hormone
on mitochondria or on various enzymes were undoubtedly true but
physiologically irrelevant. There is no doubt that work on cell lines
has tremendously enriched our knowledge of signal transduction, of the
actors involved, and of their interactions. However, the literature on
signal transduction shows that similar pathways may have different,
sometimes opposite, effects in different cells. This suggests that, if
we are interested in the human thyroid cell, it is this cell type that
we must study. In fact, the choice of a model dictates our concepts, of
which we become prisoners. If enough researchers use a model, they tend to forget about the caveats and to reject in their research or as
referees the ugly facts that may question their way of life. They
become a constituency of the model. Furthermore, it is easier to
publish clean data and mechanisms on cell lines than more disperse and
less clear-cut in vivo results. But then, as one mentor asked after a
seminar on some peculiar properties of a much-used cell line model,
"So what?" Of course, this should not detract from work on cell
lines because this allows us to identify new partners and interactions
and defines what is possible in signal transduction. On the other hand,
work on physiological models defines what is relevant in the physiology
of a given cell type at a given stage.
|
In this review we analyze the physiological relevance of signal transduction data and discuss the mechanisms that, despite the apparently generalized "textbook" schemes, account for the exquisite specificity of in vivo cell signaling. Furthermore, we explore the possible biological consequences of a loosening of this specificity. For graphic representation of signal transduction pathways, we use a recently proposed system (267).
![]() |
MANY RELATIONS DESCRIBED IN SIGNAL TRANSDUCTION PATHWAYS MAY NOT BE PHYSIOLOGICALLY RELEVANT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Attempts to synthesize any field of the signal transduction literature these days either simplify it in a personal, selective, and therefore distorted way or end up giving a very confusing picture. Part of the confusion may be clarified by considering the systems used to obtain the data and their possible artifacts (188, 309).
Loss of Biologically Important Information in Simplified Systems
Simplification of the systems used for the study of signal transduction is a double-edged sword. At each level from the in vivo study of humans to the precise molecular definition of proteins by X-ray crystallography, what we gain in precision, rigor, and definition we may lose in relevant biology (Table 1).There are numerous examples of apparent discrepancies between findings
in simplified systems, e.g., in vitro and the situation in
vivo. For instance, expression of the "cell
proliferation signal" epidermal growth factor (EGF) in transgenic
mice causes growth retardation (41). Lack of
phosphoinositol-3-kinase (PI3K)-, an essential element of
proliferation cascades, leads to colorectal carcinomas
(292). There are great differences, not commented on by
the authors, between the specificities of G proteins and their
-subunits for their controlling receptors in membrane preparations and in whole cells (122). Similarly, the
history of knockout mice is rich in genes whose protein product is
essential for a cell process in vitro but not in vivo.
The crucial fact that teratocarcinoma cells aggregated with normal morulas give rise to normal cells in adult animals (265, 148) or that bone marrow cells injected in heart regenerate myocardium (253) testifies to the importance of the proper tissue environment in the behavior of the cell. Similarly, the interrelations between cancer cells and their neighboring cells are fundamental to their biology (89). Normal ovarian stromal cells promote the growth of normal ovarian epithelial cells but inhibit the corresponding tumor cells in vivo (258). These observations led to the development of new transgenic models in which oncogenes can be activated by a spontaneous recombination event rather than systematically in all cells of a given type (161).
Autocrine and paracrine effects may differ in culture and in vivo because of the dilution of factors in vitro, the washout of factors in vivo, or the absence of a necessary factor in the medium. In COS7 cells, for instance, the stimulatory effect of insulin-like growth factor (IGF)-I on the mitogen-activated protein (MAP) kinase (MAPK) cascade is secondary to the autocrine release of EGF (287). The simple manipulation of cells in culture introduces new variables: change of medium or even minor mechanical stress causes ATP release and activation of the ubiquitous purinergic receptors (254, 255). Because many biological effects require the conjunction of several factors, simplified systems may lose properties, including the specificity of interactions. For instance, direct interactions of transcription factors with DNA in acellular systems are much more promiscuous than in vivo (43).
Yet, to define mechanisms, simplified systems and even pure molecular species are necessary. To fully understand the reaction of a pharmacophore with its target one must define the precise structures involved. Still, the biological relevance of the findings needs to be validated up to the level of the human organism. Animal models, transgenics or knockouts, general or local, permanent, permanently or transiently inducible, allow us to test precisely the role of a given gene in vivo. Defined human genetic diseases, when they exist, allow extension of the conclusions to humans.
Arguments In Favor of a Causal Relationship May Not Be Proofs
The best argument in favor of the hypothesis that a biological event is necessary for a signal transduction pathway is to show that its suppression inhibits the downstream events. Suppression can be achieved by pharmacological inhibitors, antibodies, dominant-negative or competing peptides or proteins, deletion of the protein by inhibition of its synthesis (e.g., antisense, RNA interference), or gene knockout. For inhibitors, the postulated suppression must take place in the system studied under the conditions used and it must be specific. Such controls are often missing. Many hormonal effects have been related to protein kinase A (PKA) because they are inhibited by the supposedly specific H89, which, in the same concentration range, inhibits MAP kinase-stimulated kinase (MSK1), a kinase downstream of MAPK (332).In addition, the fact that an event is necessary does not necessarily
imply that it is a required step in the causal relationship sequence
(Fig. 2A). Metabolites or an
O2 supply are necessary for the survival of many cells and
therefore for the operation of their signal transduction pathways, but
they are not part of these pathways!
|
Demonstration that experimental induction of what is supposed to be the primary event causes the downstream steps of the pathway is an argument but no more. Constitutively active forms of signal transduction proteins are often used for such purpose, although their specificity may also have been altered, as shown for EGF receptors (EGFRs) (210). Failure to induce the consequences may just indicate that parallel events are also necessary (Fig. 2B). Overexpression of the primary event will be ineffective or inhibitory if the effect is biphasic vs. concentration (182), i.e., in hormesis (36). This is the case for cAMP induction of proliferation in granulosa cells (282) (281) or for p53 and apoptosis (187) (Fig. 2C). Constant expression of the primary event will be ineffective or inhibitory in a sequential process in which each successive step requires the arrest of the previous one (e.g., in phagocytosis with extension of the membrane, engulfment, scission of the vesicle from the membrane, etc.; Refs. 12, 228). Induction per se indicates that the initial event may cause the downstream consequences, but the fact that this event actually takes place in the pathway remains to be proven. Preferably, as laid out by Robison et al. (280) in their rules for cAMP causal relationships, activation/inhibition of each of the actors of a cascade should be shown in the intact cell, direct activation/inhibition of each actor by its upstream modulator should be demonstrated, and the kinetics and concentration effect relationships should be compatible with the proposed scheme.
Validity of Reported Relations May Be Restricted to Artificial Systems
Many interrelations within and between elements of the signal transduction pathways are protein-protein or protein-DNA interactions. The methods used to define such interactions have provided a tremendous yield of new information. However, their results should be properly assessed and their physiological relevance validated. Protein-protein interactions are difficult to study at the low concentrations that prevail in normal cells. For instance, coimmunoprecipitations depend on the affinity and specificity of the antibodies used, on the dissociation rate of the interaction, and on the concentrations of the targets to be demonstrated in Western blots. Of course, the investigator can diversify the antibodies and modulate their concentrations, the washings, etc. To overcome such difficulties the overexpression of one or several proteins in transfected systems has become very general. Factors of overexpressions up to 100-fold are common. At these concentrations, weak, nonphysiological interactions and effects may well take place and the specificity of action of isoforms may disappear (see, for example, pRb and p107; Ref. 158, Fig. 3, A and B). For example, proteins that associate with the GABA receptors and cluster them in transfected cells may not do so in their native neurons (171, 172). Moreover, interactions that are normally impaired by compartmentation may occur in such systems. If an interaction is constrained by stoichiometric binding in a protein scaffold, even a doubling of the concentration of one of the locked proteins may be sufficient to allow spillover outside (Fig. 3C). On the other hand, overexpression of a scaffold protein will segregate its binding proteins in separate complexes and thus have an inhibitory effect (Refs. 197, 252; Fig. 3D). Expression constructs can interfere with nuclear receptors or transcription factors. From such artificial interactions authors infer effects on the corresponding endogenous proteins. In this case, the cell is no more than a glorified test tube.
|
Other confounding factors in transfection studies are the use of constitutively activated mutants whose persistent activity does not reproduce the temporally organized activation of the natural proteins or, conversely, the use of transient transfections that do not reproduce the sustained activity of oncogenes, or the use of dominant-negative mutants whose actions may be much more diverse than foreseen (309). Similarly, the whole field of gene regulation, which has relied on cotransfections of plasmids expressing transcription factors and promoters with reporter genes, is undergoing a reappraisal now that it is realized that genes in normal chromatin may behave differently, or at least in a more sophisticated way, than genes in the naked or poorly "chromatinized" plasmids. In Drosophila, for instance, there is little correlation between binding in vitro of transcription factors to DNA fragments and DNA binding in vivo (21).
In vitro acellular systems with proteins, purified or not, are also
used and thus have little relation to physiology. They may allow us to
estimate thermodynamic properties or to pinpoint possible interactions.
However, the concentrations used may be much higher than in the cell
and may generate many false positives. Moreover, the artificial system
used may lack components that would confer specificity. Interactions of
Hox transcription factors with naked DNA are much less specific than
they are in vivo (150, 151). G complexes lose their
target specificity in acellular systems (73).
Similar reservations hold for yeast double-hybrid systems. For instance, steroid receptors in double-hybrid systems respond to all ligands by activating the downstream promoters, whereas in vivo some ligands act as agonists and others as antagonists. To reproduce the in vivo situation, Yamamoto et al. (371) had to introduce also in yeast the needed coupling factors. Still, as a first step to define the repertoire of possible interactions, this method has allowed gigantic steps (150, 151).
All interactions proposed on the basis of transfection, double-hybrid, or acellular experiments should therefore be validated at the normal concentrations in the cells in which they are supposed to take place, which is easier said than done (247). In each case, of course, the investigator can overexpress to detect unfavorable interactions (for example, interactions that would require the nonexisting phosphorylation of the protein) or underexpress to indicate specificity. Conviction about the validity of interactions demonstrated in vitro arises from the convergence of independent arguments.
Possible Relations May Not Apply Because the Proteins Involved Are Not Expressed in the Same Cells in the Tissue or in the Same Compartment in the Cell
To interact, proteins must be expressed in the same cell. Evidence for this assumption may be invalid for several reasons. The detection by PCR of the corresponding mRNA in the cell may be due to minor "illegitimate transcription" as, with enough cycles of PCR reaction, every mRNA can be detected in every tissue. With such evidence, in the thyroid field, eye muscles would express thyroglobulin, thyroperoxidase, and thyrotropin (TSH) receptor and could be considered as pseudo-thyroid follicular cells! On the other hand, tissue distribution studies (for proteins or mRNA) do not indicate which cells contain the protein studied unless one relies on immunohistochemical or in situ hybridization evidence. Human thyroids contain many muscarinic receptors, but, in contrast to the dog thyroid, the receptors are not in their follicular cells.For an effect to take place (e.g., DNA synthesis after growth factor action), all the elements of the corresponding signal transduction cascade should be present in the cell. In fact, illicit expression of protooncogenes in cells in which they are not normally expressed is a major cause of cancer (274). The finding that EGFR activation by G protein-coupled receptors (GPCRs) and the consequent cell division may require the processing of cell-bound pro-EGF by a metalloprotease, itself activated by the cascade downstream of the GPCR (269), explains why such a mechanism operates only in some cases (58). In addition, quite a few different mechanisms have been proposed in different cell types to explain GPCR activation of the growth factor receptor and MAPK cascades, with no attempt to disprove other hypotheses (120, 125, 170, 214, 215, 231, 294, 354, 382). Most of these articles refer to "the mechanism of EGFR activation by GPCRs."
Similarly, proteins may be restricted to different cell
compartments or even to distinct macromolecular assemblies (scaffolds) that insulate them from others and thus prevent interaction, even though all are present in the same cell. This compartmentation generates functional modules, i.e., discrete entities whose function is
separable from that of other molecules (133). By
generating such a module with the elements of the MAPK cascade, the
yeast scaffold protein Ste 5 confers to nonspecific enzymes the
specificity of action of mating hormones (Fig.
4; Refs. 61,
95).
|
Demonstrated Interactions May Only Occur in Some Cell Types, in Some Species, or in Model Cells, Which Explains Why a Cascade May Have Different and Even Opposite Effects in Different Cells
Differentiation in about 200 different cell types implies a specific program of protein expression for each of them. A clear indication of specificity is given by the numerous examples of opposite results of the same cascade in different cells. The same Ras oncogene product blocks proliferation in human fibroblasts and induces it in human thyrocytes and immortalized fibroblasts (57, 112, 113, 250). It induces cyclin D in an intestinal cell line, while causing cyclin D1 phosphorylation and degradation in Rat1 fibroblasts (303). The same cAMP cascade inhibits cell proliferation in many cells (e.g., fibroblasts and other cells of mesodermal origin) but triggers it in some others (thyrocytes, somatotrophs, etc.); depending on the cell type, it activates, inhibits, or does not modulate MAPKs (285). In tadpoles the same hormone, triiodothyronine (T3), acting on the same T3 receptor, induces cell death in the tail and cell proliferation in the rest of the body (308, 328). NF-Activation of the same pathway in the same cell type in different species may also lead to opposite results (209). The TSH receptor in human thyroid cells activates both the cAMP and phospholipase C cascades and accordingly activates thyroid secretion by the former and thyroid hormone synthesis by the latter, whereas in dog thyrocytes it only activates the cAMP pathway, which activates both functions (348). The same end result of TSH is obtained by different pathways in dog and human. In vertebrates as in viruses, evolution often conserves the function but may achieve it by different mechanisms.
Effects May Not Occur at All Times in a Given Cell
Receptors and signal transduction proteins are differentially expressed during embryogenesis, growth, and even under different physiological conditions. Effects may therefore differ at different stages. In fact, because signals during embryogenesis act through a few cascades, embryogenic development could not occur if these cascades were not interpreted differently by the target cells at different stages (62, 102). The whole early tissue organization in Xenopus development depends on the well-defined, orderly time sequence of the action of four inducing factors on cells, which becomes different at each stage (314). The same Ras activation applied to Drosophila imaginal tissue at different stages leads to proliferation, apoptosis, or suppression of apoptosis (162). The evolution of cells in embryogenesis is now mimicked in vitro in embryonic stem (ES) cells (212). A maturating dendritic cell or T lymphocyte changes its panel of secreted cytokines and cytokine receptors, i.e., its signaling systems, within a few hours (27, 249).The same stimulus may achieve the same result by different mechanisms
at different times of the life of a cell, e.g., protein kinase C
(PKC)- is required for T cell receptor-induced NF-
B activation in
mature but not immature T lymphocytes (323).
Many effects depend on the cellular environment. Vasopressin, acting
through its V1a receptor, activates Gq in proliferating Swiss 3T3 cells but Gq and G13 in cells in the
G0/G1 phase of the cell cycle (1).
In hepatocytes in culture, norepinephrine stimulates DNA synthesis
through -receptors at low density and through
-receptors at high
density (166). Muscarinic receptors stimulate growth of
quiescent NIH 3T3 cells but inhibit it when the cells are growing
(245). The same increase in cytosolic Ca2+ may
push or retract the growth cone of a nerve, depending on preexisting
Ca2+ or cAMP level (378). Malignant mammary
epithelial cells may revert to a normal phenotype in a specific
intracellular matrix environment (23).
Cell Lines May Not Be Good Models of Their In Vivo Counterparts
Most articles on cell lines extrapolate their findings to the in vivo cell counterpart, e.g., extrapolating to the human thyroid cell what has been found in FRTL5 rat thyroid cell line. An ever greater part of the literature on cell signaling bears on cell lines as models in part because of the ease of working with them. In fact, cell lines are poor models of their in vivo counterparts. First, by definition, contrary to normal somatic cells, they are immortal, i.e., they reproduce indefinitely. The process by which they are obtained implies a selection over several generations of genetically altered cells having the desired properties, very different from those of the cells of origin. For instance, whereas tyrosine kinases of the Src family are necessary for the proliferating effect of platelet-derived growth factor (PDGF) in normal fibroblasts, they are not in NIH 3T3 cells or other cell lines (32). It is striking that similar rat thyroid cell lines selected by different criteria, the PCCl3 and the FRTL5 cell lines, exhibit quite different properties. For example, they require one (FRTL5) or two (PCCl3) oncogenes to become transformed. Finally, cell lines may evolve: the FRTL5 cells described at their origin required insulin and TSH to grow. Presently available samples require only one of the two or, in some laboratories, only insulin, TSH having just a complementary role. Some FRTL5 cells used in different laboratories are stimulated by EGF, some not; in some, serum enhances TSH mitogenic action, in some not. What is the interest for physiologists of the differences between two thyroid cell lines or between the same cell line having evolved in different laboratories? Another example of a molecular evolution of cell lines is the progressive accumulation and selection of cells with larger alleles due to unstable triplet repeats (117).Similarly, cancer cell lines are often studied as representative of in vivo cancer cells even though they have developed new characteristics: p53 is mutated in thyroid and esophageal carcinoma cell lines but not in the corresponding primary tumors (327, 368). p16INK4 is inactivated in thyroid tumor cell lines but not in thyroid tumors (37). PTEN mutations are detected in melanoma cell lines but little in melanomas (379-381). Even in cancer cells the characteristics of disseminated cells can be very different from those of the tumor that releases them (177). How relevant to metastasis are human cancer cell lines injected in animals (318)?
Intuitively, one would guess that the variations between similar cells (e.g., thyrocytes of different species) would be much greater at the initial steps of the cascades than at their core mechanisms: there are hundreds of receptors modulating cAMP levels but only six isoenzymes of cAMP-dependent kinase, and these have the same substrates. However, this is not always true, as variations may also occur at the core of signal transduction pathways: in dog thyrocytes cAMP activates cyclin D/cyclin-dependent kinase (CDK)4 complexes without inducing cyclin Ds, whereas in FRTL5 cells it does so by inducing cyclin D1 (65).
Conclusions on Physiological Relevance of Reported Effects
It is therefore dangerous to extrapolate to other systems the data obtained in one species. One sees articles in which data on human, dog, or pig thyroid cells in primary cultures or rat thyroid cell lines are combined in reasoning about the nonexistent paradigmatic "the thyroid cell." Although the existence of a given mechanism is often demonstrated only in one system, articles imply, implicitly or explicitly, that the mechanism described is general (e.g., "activation of the EGFR by seven transmembrane receptor," "cAMP activates MAPK"). The specificity of cell signaling in different cells, even for the same or similar extracellular signals, and even through the same initial receptor, is demonstrated a contrario in reviews that attempt to synthesize our present knowledge. In a recent article on seven transmembrane receptors and cell proliferation, no two of the systems described work in the same way, and even when one receptor is considered, its mitogenic cascade differs from one model to another (125). The buzzwords of such reviews are "complex, pivotal, subtle..." (260, 261). This explains the wrong impression of confusion emerging from such reviews.The map of all possible interactions and causal relations in signal transduction should therefore be considered as a map of possibilities, only few of which really take place at a given time in a given cell type. The exquisite cell- and stage specificity in signal transduction is fortunate for the pharmacologist (364) who aims at such a specificity for his drugs.
![]() |
HOW IS CELL SPECIFICITY OF ACTION OF SIGNAL TRANSDUCTION CASCADES ACHIEVED? |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Cell Responses Depend on the Pattern of Their Protein and Isoform Expression
The specificity of response to one cascade in different cell types depends on its differentiation, i.e., on its protein composition and, therefore, on the genes whose promoters are accessible. For example, in kinase cascades the response to the same cAMP and cAMP-dependent kinase depends on the population of phosphorylable proteins in a specific cell type, i.e., on its differentiation. Similarly, the response to a similar transcription factor in different cells depends on the nature of the gene promoters that are accessible.At each step of most cascades, several isoforms of proteins perform overlapping functions. They may be encoded by different genes or result from different mRNA splicing of the same gene or from postranslational processing. In mammalian cells there are at the present time 10 adenylate cyclases, more than 40 cyclic nucleotide phosphodiesterases (315), 70 A-kinase-anchoring proteins (AKAPs) (74), and 11 families of nonreceptor tyrosine kinases (279). Evolution multiplies the varieties of possible isoforms at each stage, from one kinase at each step of the MAPK STE pathway in yeast, Caenorhabditis elegans, and Drosophila to several kinases in mammalian cells (61). This explains why work on such simple model organisms is fundamental to demonstrate basic mechanisms and schemes, their actors, and their interactions and thus give the foundation of signal transduction. It also explains why direct extrapolation to mammalian cells is risky.
Because the isoforms have relatively similar properties, they are
often, when discovered, lumped together as though interchangeable. In
fact, more detailed investigations reveal different (sometimes opposite) and overlapping regulatory properties, e.g., the positive or
negative effect of phosphorylation by MAPK on phosphodiesterase 4D
isoforms (217) and the opposite and qualitatively
different effects of p53 isoforms (239, 240). They also
present cell type-specific expressions (30), intracellular
localizations (206) as determined by specific docking
domains (305), effectors (e.g., for receptors or kinases;
Ref. 126), and controls in expression at the
transcriptional, translational, and posttranslational level (70,
123). These differences may give the isoforms entirely different
physiological roles. They may also respond differently to direct and
cross signalings. Isoforms of glucose transporters are usually tissue
specific, with a conserved transmembrane catalyzing transport domain
and different cytoplasmic tails allowing specific regulations
(236-238, 333). The same Ca2+ signal will
enhance or decrease cAMP accumulation depending on the type of
adenylate cyclase present (147). Whereas the - and
-subunits of the G proteins transducing the action of seven
transmembrane receptors were long thought to be interchangeable, it
appears more and more that the response to a given receptor requires
the presence of a defined set of
-,
-, and
-subunits
(140, 160, 278). E2F2 and E2F4 have opposite roles
on cell differentiation (257). With only some of the
possible isoforms present in a given cell, of all the possible controls
only the few permitted by this selection will operate in this cell.
This has been well demonstrated, e.g., for G protein
-subunits in
the different cells of human fetal adrenal gland (30) or
for AU-rich element (ARE) binding proteins that regulate the stability
and translation of mRNA in embryos (142).
The nature of the expressed isoforms may itself depend on the physiological state of the cell: depolarization induces, through Ca2+ and Ca2+/calmodulin-dependent protein kinase, a different splicing of the pre-mRNA of the Slo channels and therefore the expression of different proteins with different allosteric properties (369).
The relatively low number of genes in the human genome seems to put a
lid over the number of possible isoforms of any protein. In fact, any
gene coding for an isoform of a signal transduction protein may also
code for several isoforms by mRNA alternative splicing (24,
119), e.g., each cyclic nucleotide phosphodiesterase gene for
3-10 alternatives (24). In these, the presence or
absence of one motif of protein-protein interaction in an alternative form of a protein may channel a pathway in a given direction or not
(24, 119) according to the protein recognition code
(321). The presence of one or another intraprotein
signaling module such as a protein phosphorylation motif may confer
positive or negative regulation by PKA (Ref. 66; Fig.
5) or by MAPK (340). A
splice variant of phospholipase C behaves as a negative regulator of phospholipase C (242). Truncation by alternative
splicing of fosB into
fosB leads to a different transcriptional
repertoire (290). Splice forms of an Eph receptor inverse
the adhesion/repulsion response caused by this receptor
(137). Variations in the upstream open reading frames of
mRNAs also greatly change the life of the mRNA and its translocation
efficiency (236-238).
|
Finally, the repertoire of possible effectors of each signal transduction protein, as presently known, will probably expand in the future. When an action of such a protein is discovered, it is often assumed to be the only one until we are shown otherwise. Thus we restrict the role of GPCRs to their effects on G proteins (129, 134) and the role of PTEN to its protein phosphatase activity (68) to discover later that other effects exist. The specificity of isoform expression thus goes a long way in explaining cell specificity of responses.
Operation of a Pathway May Depend on Spatial Structure of Its Constitutive Elements: Subcellular, Membrane Localizations, Multiprotein Complexes
Spatial structure in the cell refers to cell compartments and to supramolecular complexes. Nuclear or cytoplasm compartmentation prevents many possible interactions, sequestrating active molecules from each other. For example, sequestration of MDM2 in the nucleolus by p19ARF blocks its inhibition of p53. The regulation of the cell cycle and of transcription represents a ballet of nuclear to cytosol import-export dynamics (110, 232, 266). Similar dynamic controls of protein localization operate even in bacteria (154, 155, 304). Thus the existence or not of a translocation mechanism or of its regulation may greatly differentiate the effects of a signal transduction pathway in different cell types. Loss of spatial structure of signal transduction pathways is a cause of several diseases (243).The targeting or nontargeting of a protein at the membrane may change
the whole pattern of its interactions. Such targeting may involve
protein-lipid or protein-protein interactions (193). It
may require specified mRNA localization and protein production (313). Nonprenylated Ras or, in some cells,
nonmyristoylated cGMP-activated kinase does not activate its cascade
(136, 219). Insulin and WNT both inhibit glycogen synthase
kinase 3, but this leads exclusively to increased glycogen synthesis
for insulin and exclusively to increased availability of
-catenin
for WNT (71). Integrin necessarily stimulates at
defined contacts (298-300). Compartmentalization goes
further with the segregation of some membrane proteins within or
without lipid rafts and caveolae (107). GPCRs may have
only access to their compatible G proteins in "raft" subdomains of
the membrane (254). This could explain the discrepancy between TSH promiscuous effects on G proteins in isolated membranes and
its more restricted effects in intact cells (5).
The numerous proteins whose main function is to anchor signal
transduction proteins (e.g., the AKAPs for cAMP-dependent protein
kinase) to definite structures show the importance of such
localizations (86).
Similarly, scaffold proteins also have the role of assembling supramolecular complexes, bringing together signal transduction proteins in one permanent or transitory functional unit, module, or "signalosome" (35, 133, 260, 261). Such complex functional multiprotein assemblies were first described for metabolic enzymes, accounting for metabolic channeling (256). Their properties are more than the sum of the properties of their individual constituents (106). For instance, by associating tightly phospholipase C and its G protein, the INAD scaffold protein allows the former to activate the GTPase activity of the latter and thus to shorten the signaling of the photoreceptor (51, 95). For cAMP, inositol 1,4,5-trisphosphate (IP3), and even phosphatidylinositol-3,4,5-trisphosphate (PIP3), colocalization of the signal generation effector and remover allows highly localized effects in dendritic spines (169). The synapse or the neuromuscular junction is a permanent multimeric complex constituted sequentially (84, 329-331) and dependent on specific targeting (44-46, 132, 171, 172). In yeast the Ste5 scaffold protein channels the activation of the MAPK pathway by mating factor to mating-specific genes (Refs. 61, 108; Fig. 4). 14-3-3 Proteins have a similar role in vertebrates (241, 370). Activation of the T cell or B cell receptor requires the constitution of a large integrated multimeric complex in membrane rafts (181, 326, 336, 358). The stability of such complexes varies from very transient to quasi-permanent (97).
Specificity of Response of Different Cell Types May Result from a Combinatorial Logic
From specific combination of elements involved in parallel.
In this type of regulation, a few factors may combine to specify many
different instructions. A signal can be compared to a letter in a word:
it has no meaning per se, only the combination of several letters has a
meaning and the same letter used in a different word or combination may
have a different or opposite meaning. Such regulations have been
demonstrated, for example, for gene expression and for odor
discrimination by the olfactory system (34, 109, 220). In
the former, it is generally the combination of several complementary
DNA regulatory elements and their specific transcription factors that
confers specificity and strength to a promoter (204). Thus
even broadly overlapping sets of regulated transcription factors may
have very different end effects (Refs. 94,
371; Figs. 6 and
7). Assuming that in humans, as in other
species, transcription factors may represent 5% of the 100,000 expressed mRNAs, the number of possible combinations of three (with
repetitions) is 2 × 1010! The distribution of the 30 Ets transcription factors in different cell types can be considered as
a fingerprint of each type (223). The expression of
thyroid-specific genes depends on three transcription factors [thyroid
transcription factor (TTF)1, TTF2, Pax8], each of which is expressed
in the thyrocyte and in at least one other cell type, but all three are
only coexpressed in the thyrocyte (60). Similarly
different cascades with partially overlapping sets of induced genes may
also exhibit the same combinatorial logic (Ref. 94; Fig.
8).
|
|
|
|
From combination of unregulated parallel factors and of one
regulated factor in each cell type: the triggering reaction or
"switch."
Many different signals operate in their target cell by inducing one or
several rather ubiquitous transcription factors, i.e., early-immediate
genes. C-Fos and Egr1, which are induced in many cells in response to
all sorts of signals, are an obvious example (372).
Cell-specific response is given by the other cell-specific transcription factors that are also necessary for induction of a
specific gene in the cell. The nonspecific Egr1, in conjunction with
SF1, leads in gonadotroph cells to the subsequent induction of LH
gene (165, 338) and, in conjunction with WT1, induces Mullerian inhibiting substance in Sertoli cells (Ref. 310;
Fig. 10). The interleukin (IL)-6
promoter in monocytes requires cAMP response element-binding protein
(CREB), AP1, and cellular enhancer-binding protein (CEBP) but is
activated by the newly released NF-
B (344). Just as in an electric circuit the response to an electric switch corresponds to the system downstream (light, air conditioning, heating,
etc.), the transcription response to a signaling pathway and its
general triggering transcription factor depends on the existing
tissue-specific transcription factors and their regulatory elements.
This concept would account for synexpression, i.e., the expression with
a similar pattern of a set of genes in a cell type in response to a
stimulus (124, 246), and in some cases for cell
differentiation (124). It goes a long way in explaining the puzzling question of how a promiscuous signaling cascade can achieve unique effects in a given cell type. The phosphorylation of
histones H1 by cAMP-dependent kinases in different cell types presumably causes a general loosening of the chromatin structure, which
might facilitate later, more promoter-specific transcription effects,
fits with the same concept. This concept of a single switch whose
meaning is only determined by the specific existing elements of the
cell also applies to early steps of signal transduction cascades. The
effect of a general signal such as Ca2+ or cAMP in a cell
depends on its complement of existing protein substrates of calmodulin
or PKA (Fig. 11). Similarly, the
pattern of gene expression induced by EGF in a single cell line depends on the composition of the cell matrix (373).
|
|
From specific combination of sequential factors.
In the specificity of nuclear receptors action at least six different
factors are involved, each of which can switch the sign (+ or ) of
the response: the regulatory element in the promoter DNA, the nuclear
receptor or the transcription factor binding to the regulatory element,
the hormone binding to the receptor, the phosphorylation of the
receptor, the other transcription factors present on the promoters, the
coactivator or corepressor binding to the receptor, and their
modulators and adaptors (157, 192, 221, 283, 371).
Other independent or dependent factors regulating the outcome are the
various feedbacks between transcription factors (276), the
methylation of the promoter and enhancer, and the state of the
chromatin (histone acetylation and phosphorylation, high-mobility
group (HMG) protein binding, polycomb group protein binding, etc.), DNA
methylation and chromatin structure being generally linked (40,
47, 152, 277, 297, 339, 353). "Coordination of large
sets of genes could be accomplished by affecting the function of
specific components of the transcriptional machinery" itself
(139). Moreover, some genes like that of the human
fibroblast growth factor (FGF)-1 have four different promoters,
differently regulated, directing the expression of four alternatively
spliced transcript variants (48).
Specificity of Response of a Cell to Different Cascades Having Apparently Similar Consequences May Result From Specific Combination of Biochemical Effects of Each Cascade: Common "Awakening" Reaction and Specific Effects
Some cell responses are common to many if not all signaling pathways in a given cell type. At the posttranslational level CREB and cAMP-responsive element modulator (CREM) phosphorylation and activation are caused by the cAMP cascade, intracellular Ca2+, and growth factors acting through MAPK, stress, and p38 and MAPK-activated protein (MAPKAP) kinase, etc. (63). Glycogen synthase kinase 3
|
Specificity of Response to a Cascade May Depend on Timing of Stimulus
Timing may also explain specificity. A signal may be short or long, immediate or delayed, continuous or oscillatory. The multiple qualitative differences in effect that such modalities confer have been well studied in the case of the intracellular signal Ca2+ (18, 19, 75, 82, 83, 99, 202-204, 233). For example, the frequency of calcium transients dictates NF-
|
Cell Specificity of Response May Depend on Qualitative Difference of Protein Expression of Modulators
Modulators are proteins not involved in the signal transduction cascade themselves but which positively or negatively modulate the proteins of this cascade (Table 2). When a given protein has opposite actions, differential modulations of them will lead to one result or the opposite, e.g., cMyc, which induces cell proliferation or apoptosis (50). Examples of such inhibitory modulating proteins abound: starting from natural antagonists of extracellular signals, soluble receptors that compete with extracellular signals for membrane-bound receptors, kinase inhibitors, G protein inhibitors [regulator of G protein signaling (RGS) proteins; Ref. 312], and Bin1 or Groucho inhibitors of MYC or LLT1/TCF
|
Some modulators even change the receptivity of a protein from one
signal to another. Receptor activity-modifying protein (RAMP) proteins
switch the specificity of the cGRP receptor to one or the other
hormone, RAMP1 to CGRP, RAMP2 to adrenomodulin (196, 236-238, 302). For hormone nuclear receptors, besides the
controls mentioned above, there are modulators of coactivators [e.g.,
pCIP for CREB binding protein (CBP)] and competitors.
There are even modulators of modulators such as p34SEI-I,
which antagonizes the inhibition by p16INK4a of
cyclin-CDK complexes (322), and prothymosin-, which
sequestrates the repressor of estrogen activity (224).
Finally, signal transduction cascades may induce, depending on the cell
type, the synthesis and secretion of autocrine or paracrine factors or
of proteases generating such factors, i.e., extracellular modulators
from inactive precursors (183, 195). Just as the field of
nuclear receptors is now discovering all the modulators of coactivators
and corepressors that provide specificity of action in different cell
types, the field of signal transduction cascades is now identifying its
modulators. The role of multifaceted proteins should also be considered
at this level.
Specificity of Response to a Cascade Depends on Quantitative Differences of Expression or Activity
Many cascades activate two or more different branches that may have parallel, synergic, or opposite effects. When the effects are opposite the expected behavior of the system may be very complex (341-343) (Fig. 14). Moreover, quantitative differences of expression may lead to opposite results: Fas receptor activates the caspase cascade, leading to apoptosis, and the MAPK cascade which inhibits it (138). Depending on the strength of the two effects, TNF or Fas receptor cause apoptosis in some cells but not in others (9, 10). The complex SMAD2/SMAD4, activated by transforming growth factor (TGF)-
|
Conclusion on Cell Specificity
Thus the mechanisms endowing cell types with an exquisite specificity in signaling in response to similar stimuli come down to the level and time structure of the stimuli, to the unique qualitative and quantitative pattern of expression of isoforms and modulators, and to their spatial arrangement in compartments and supramolecular complexes (293). ![]() |
LOSS OF SPECIFICITY MAY BE A CHARACTERISTIC OF DEGENERATE SYSTEMS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
As discussed above, much work on signal transduction and "cross talk" is carried out on cell lines, mostly for reasons of convenience. Cross talk, originally designating interference in radiocommunication (247), may thus represent a loss of specificity in signaling. The question may then be raised whether this loss is not a characteristic of these cell lines and of the way they have been generated. This is suggested by many examples. Oncogenes would not have transformed NIH 3T3 cells and thus would not have been discovered if these cells had not been already half-transformed (130, 186). Whereas PDGF action on fibroblasts has a stringent requirement for Src-type kinases, this requirement is lost in NIH 3T3 cells or other cell lines (Ref. 32).
Our knowledge of thyroid cell lines such as the FRTL5 cell line supports such conclusions. Generated by continuous culture of rat thyroid primary cultures, the cells were selected by their property of multiplying only in the presence of serum, TSH, and insulin. In the first report the cells did not multiply in the presence of insulin or TSH alone. Now, as used after many generations, the cells multiply in the presence of either insulin or TSH. This is inconsistent with in vivo work showing that mice or human thyroid cells do not multiply in the absence of TSH or when its receptor is inactive and that they do not respond to TSH in the absence of IGF-I. Moreover, aging of the cell line leads to an increase in the unstimulated proliferation rate. At the biochemical level, whereas the pathways of insulin and TSH are clearly distinct in primary cultures (with the cAMP-PKA response specific to TSH and the Ras MAPK and PI3 kinase responses specific to insulin), the pattern blurs in the FRTL5 cells, in which TSH and cAMP have been reported also to activate the Ras-MAPK and the PI3K pathways. The specificity of the TSH-cAMP and IGF-I-PI3K pathways has completely disappeared (175). Similarly, Ras oncogene induces proliferation with no effect on differentiation in human thyroid cells in primary culture, but it enlarges its action to dedifferentiation in the rat thyroid cell lines (112, 113). Whereas the proliferation effect in human cells requires at least MAPK and PI3K activations and another factor, in the WRT cells either MAPK, PI3K, or Ral GDS is sufficient (112, 113). It is obvious that such cells in which the activation of one pathway is sufficient for mitogenesis will be more prone to uncontrolled proliferation. Similar discrepancies have been reported for pituitary cell lines.
Thus the pattern of signaling is blurred in cell lines. This may result from genetic or, more likely, from epigenetic changes, from quantitative dysregulations leading to differences in expression, or from qualitative changes due to mutations. When specificity of an enzyme is constrained by its localization on a scaffold, even a slight increase of expression may lead to escape, new cross signalings, and scrambling of a network. The development of the embryo illustrates how a relatively shallow gradient, i.e., concentration difference, can lead to qualitatively different differentiation. These arguments should be related to recent findings in Drosophila. In these flies, mutations of HSP90 induce phenotypic variations in nearly all tissues, suggesting that HSP90 may be a part of a molecular buffering system that keeps cryptic signaling cassette variants silent (247). Disruption of such a system would lead to blurring in signaling.
There are indications that, besides protooncogene activation and antioncogene inhibition, a similar blurring is taking place in cancer cells. Illegitimate transcription leads to the synthesis and eventual secretion of heterotypic hormones in these cells. Heterotypic receptors also appear. Cancer in multiple endocrine neoplasia is accompanied by a loss of substrate specificity by tyrosine protein kinase Ret (in Men 2B) (291, 379-381). A clear example of the role of blurring signal transduction specificity in cancer is given by the loss of specificity of androgen hormone receptors and their corresponding abnormal responses in prostate cancers (361, 349, 377).
The normal hypermutation process of lymphocytes, when wrongly targeted, is at the source of many lymphomas (259). Such a blurring probably confers a selective growth advantage, as no inactivation, negative control, or checkpoint can stop a cell from proliferating when cross signaling ensures the bypass of any block. As the null phenotype of a cell is to grow and multiply, blurring of the controls should favor growth (229, 316). Moreover, if a loss of signaling specificity is a disadvantage for the growth of a cell, it will be selected against and disappear. If it is an advantage, the cell will outgrow the others in what becomes a Darwinian process (130). The same reasoning applies to random methylation of promoters, which accounts for some cancers (20, 194), presumably by loss of tumor suppressor gene activity (234), to the role of the conjunction of mutations in several cascades, and to the role of aneuploidy (225) in cancer. The condition is described as "gene addiction," in which the full malignant phenotype depends on the continued interaction between pathways (357).
It would be interesting to explore whether aging per se induces such loosening of controls. Normal human mammary epithelial cells spontaneously escape senescence and acquire genomic changes akin to those of the earliest lesions of breast cancer (286). This would explain the multitude of benign tumors arising with age. Extensive cross signaling may therefore represent not only a consequence of evolution and selection, i.e., of the carcinogenetic process, but also a factor in the initiation or further proliferation of cancer cells or cell lines.
The blurring of specificity and extension of cross signaling in cell lines or cancer cells could be called signaling entropy. Its mechanisms have been little studied. However, one might speculate that they involve the loosening of all the controls described earlier in this article that ensure cell specificity of signaling, e.g., loss of space or time structure, broadening of the substrate repertoire of enzymes, and increased illegitimate transcription, that is, an increase of entropy at all levels of signal transduction pathways. This in turn could result from a loosening of the very strict specificity of transcription.
![]() |
UTILITY OF NETWORKS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Among the infinite possibilities of cross signalings between signal transduction pathways in their superimposed layers of controls, only a restricted number will operate in a given cell at a given time and the pattern of these controls will be very different from one cell type to another, reflecting the diversity of function and constraints of the cells (133). One may wonder about the interest of these control networks. Their value may lie in their robustness: intuitively, we guess that the more complex regulatory web is also the more robust (14, 52, 207, 352). This has been proved experimentally in ecology (268) and by the Internet. However, such statements should be qualified. Communication networks display a high degree of robustness, the ability of their nodes to communicate being unaffected even by unrealistically high failure rates. "However, error tolerance comes at a price in that these networks are extremely vulnerable to removal of a few key nodes" (3). This and functional redundancies (178, 185, 376) explain why null genotypes for so many genes have no or a very poor phenotype whereas a minority are lethal. In fact, in yeast, the proteins that have the largest potential repertoire of possible interactions, i.e., the so-called "nodes" of network, or more mundanely the common bottlenecks of regulation networks, are the only ones whose disruption is detrimental to the cell (156). Another role for cross signaling in signal transduction cascades is that, just as metabolic pathways, as they diverge and converge, they may be controlled at multiple steps for efficiency (52). Finally, complex networks may allow fine tuning in regulation, which when applied to many cells, may become very important at the level of the organism. Small advantages may be very important in long-term survival and thus in evolution. On the other hand, a converse explanation for some complex signalings may be that they have no role and are irrelevant, innocuous relics of evolutionary history. Simulating and possibly understanding such complicated networks will require new tools, sophisticated algorithms, perhaps similar to those used in the study of neural networks (99).
![]() |
GENERAL CONCLUSIONS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The significance of a signal on a given cell depends on the network in which it is inserted. The response of a cell to a signal depends on the timing and strength of the signal, on the cooperativity of the response, on the cell environment, on the cell population, on the subcellular localization of the signal transduction proteins, on the nature of the isoenzymes present at each level, on the positive and negative feedbacks and feedforward controls, and on the synergic and/and or either/or controls, etc., that is, on its program, i.e., the regulation network, namely, the nature, quantitative importance, and localization of its constituent proteins, the actors in the game. Proteomics, i.e., the definition of the protein population in a cell type in a given condition, could give this information (198, 199) and thus, with the use of a database of direct or indirect protein-protein interactions and controls, theoretically allow us to draw the awfully complex regulatory network of a given cell at a given time in its history (91). There are several caveats to be considered. First, the existence of a regulatory pathway does not necessarily indicate an important physiological role. We and others have delineated the mechanisms of action of neurotransmitters on the dog and rat thyroids without ever being able to demonstrate a role for these controls. Second, given the fact that the addition of one factor or even one motif in a protein may change the whole result of a network, the task may be enormous. Third, anybody who has tried his hand at quantitative simulation knows that by choosing the right parameters in even very simple models almost any possible behavior can be predicted. Fourth, the requisites of Pollard (as quoted in Ref. 39) for successful computational prediction are rather tough: molecular inventory, molecular structures, molecular partners, kinetic constants, genetic and pharmacological phenotypes. At each step enzymes may be location specific or not, inhibited and/or stimulated, controlling and/or controlled. In fact, the functioning of a network may only be predicted when it is completely known. As expressed in a recent review, "it ain't over til it's over" (309). Moreover, even simple cross signaling may elicit very complex behaviors (189, 341-343). Present simulations on general schemes based on data from different systems (163) may be useful didactic intellectual exercises but have little predictive value. The possibility of predicting the behaviors of a cell by knowing its protein composition will be tested by the very thorough work of the Alliance on a few cell types as pioneered by A. Gilman (111).
The research in signal transduction therefore follows, legitimately, a three-pronged approach, each prong of which supports the others but should not be confused with them. The study of simple models, from yeast to Caenorhabditis, or even Drosophila, defines basic mechanisms, actions, and their relations (273, 355). After all, the basic steps of apoptosis and of the EGF pathway were demonstrated first in C. elegans and Drosophila, respectively (98). The study of various types of mammalian cell lines defines the many possible variations that evolution and the multiplication at each level of isoforms and new modulators and cross signalings have provided, i.e., the available toolkit for differentiation. It is precisely in these variations and cross signalings that different cell types will mainly differ. The study of a given cell type in its physiological context defines which of the almost infinite possible combinations of controls applies to this cell type.
Thus regulatory schemes can be presented and interpreted in two ways. The first way is as maps of all the possible interactions like the conventional metabolic pathway maps. Reviews on signal transduction pathways and protein belong to this category. They should provide information about all possible interactions, the "toolkits" of cell biology, but refrain from specific functional interpretation. Second, regulatory schemes can be presented and interpreted as the regulating scheme applied to one cell type at a given time in its history. In this case, the cell type, species, history, environment, and experimental condition should be defined. The exquisite specificity of cell types is illustrated by the exquisite cell specificity of the phenotypes of knockout models for supposedly ubiquitously important genes. Reasoning about the behavior of a given cell type should only be applied to the second type of scheme. In such reasoning one should never lose what Barbara McClintock called "the feeling for the organism," i.e., in this case, how signal transduction in a cell fits in with physiology. One outstanding example of success of the study in depth of one cell type is the study of T lymphocyte signaling (181). Editors might recommend modesty in titles and conclusions of articles, restricting specifically the domain of application of conclusions to the systems really studied or at least specifying that the controls described are possible but not necessarily universal: "cAMP can activate the MAPK pathway" is tolerable; the "tabloid"-type title "cAMP activates the MAPK pathway" is not. Otherwise, as Bacon said, "words turn back and reflect their power upon the understanding" (as quoted in Ref. 164), i.e., vagueness of language leads to confused thinking.
In cases in which existing schemes do not explain the results, modulators of the proteins in a cascade and even modulators of modulators should be sought after. The research on nuclear hormone receptors has shown the way to the whole signal transduction field in this regard.
For physiological relevance the choice of the right model is paramount. For human physiologists, this is the human cell in vivo. Any departure from this material entails the risk of irrelevance. Who cares about the peculiar properties of the FRTL5 cells used in our laboratory? When we use model systems the validity of the scheme we outline should be assessed in "real cells," physiological or pathological. To get our story on the role of TSH through cAMP in the control of thyroid function and growth accepted, we needed to generate transgenic mice in which constitutive activation of this cascade in the thyroid led to goiter and hyperthyroidism and to demonstrate the same mechanism and consequences in human autonomous thyroid adenomas (175, 285, 347)! Similarly, the positive role of cyclin D1 on proliferation and its negative control by p27 have been definitely validated by double-knockout mice (335). This increasingly recognized need explains the expanding literature on gene knockouts and especially on cell-specific and inducible gene knockouts.
One example from our group illustrates the necessity of in vivo validation. An enzyme hydrolyzing the intracellular signal PIP3 had been cloned in our lab: SHIP2 (263). PIP3 has been involved in various cells in the stimulation of protein synthesis, proliferation, etc., in the prevention of apoptosis, and in the action of growth factors IGF-I and insulin. Our prediction was therefore that mice in which SHIP2 had been knocked out would present growth anomalies and tumors. In fact, the main effect of this knockout was not predicted: a greatly increased sensitivity to insulin with rapid death from hypoglycemia. The affected mice were born with normal weight (49). The phenotype is extraordinarily restricted, which suggests that the enzyme mostly deals with the PIP3 generated by the insulin receptor! It is interesting that, whereas the 3' phosphatase PTEN also inactivates PIP3 and in transfected cells inhibits insulin action (244), its half-knockout in mice does not, as the SHIP2 half-knockout, suffer from hypoglycemia. Thus for the physiologist the safest approaches to signal transduction may be at the two extremes of the experimental spectrum: on in vivo models (transgenics and gene knockout) and on the structure of the relevant proteins. With regard to the in vivo models, their use in the study of specific organs in laboratories specialized in these organs would provide a wealth of information for both these groups and the groups generating the models.
In fact, there is more coherence in the physiology of regulation systems than in the mechanisms that achieve this physiology. TSH activates both the synthesis and the secretion of thyroid hormones in all the species studied, but in some it activates both the phospholipase C and the cAMP pathways, which stimulate synthesis and secretion, respectively, whereas in others the hormone only stimulates the cAMP pathway, which there enhances synthesis and secretion (348). Similarly, somatostatin uses different cascades and mechanisms to inhibit cell proliferation in different systems (96). The diversity of mitogenic pathways used by GPCRs is even more striking, making any attempt to generalize illusory. Thus physiologists, as embryologists, rightly consider what regulates their system of interest and what the effects of these regulations are and define the elements of the signal transduction pathways black box later. For the physiologist, as well as for the pharmaceutical industry, those mechanisms operating in their cell of interest are most important. Cell specificity in signaling, even in very basic mechanisms, is the key to therapeutic targeting (375).
![]() |
ACKNOWLEDGEMENTS |
---|
The authors thank R. Beauwens, G. Rousseau, and G. Vassart for critical reading of the manuscript.
![]() |
FOOTNOTES |
---|
The work of the group is supported by the "Service du Premier Ministre Affaires Scientifiques, Techniques et Culturelles SSTC" (PAI), the "Fonds National de la Recherche Scientifique," "Fonds de la Recherche Scientifique Médicale," "Fonds Cancérologique Fortis," "Opération Télévie," and "Fédération Belge contre le Cancer."
Address for reprint requests and other correspondence: J. E. Dumont, Inst. of Interdisciplinary Research (I.R.I.B.H.N.), School of Medicine, Univ. of Brussels, 808 route de Lennik, B-1070 Brussels, Belgium (E-mail: jedumont{at}ulb.ac.be).
10.1152/ajpcell.00581.2001
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Abel, A,
Wittau N,
Wieland T,
Schultz G,
and
Kalkbrenner F.
Cell cycle-dependent coupling of the vasopressin V1a receptor to different G proteins.
J Biol Chem
275:
32543-32551,
2000
2.
Ahlgren, U,
Pfaff SL,
Jessell TM,
Edlund T,
and
Edlund H.
Independent requirement for ISL1 in formation of pancreatic mesenchyme and islet cells.
Nature
385:
257-260,
1997[ISI][Medline].
3.
Albert, R,
Jeong H,
and
Barabasi AL.
Error and attack tolerance of complex networks.
Nature
406:
378-382,
2000[ISI][Medline].
4.
Alexander, WS,
Starr R,
Fenner JE,
Scott CL,
Handman E,
Sprigg NS,
Corbin JE,
Cornish AL,
Darwiche R,
Owczarek CM,
Kay TW,
Nicola NA,
Hertzog PJ,
Metcalf D,
and
Hilton DJ.
SOCS1 is a critical inhibitor of interferon gamma signaling and prevents the potentially fatal neonatal actions of this cytokine.
Cell
98:
597-608,
1999[ISI][Medline].
5.
Allgeier, A,
Laugwitz KL,
Van Sande J,
Schultz G,
and
Dumont JE.
Multiple G-protein coupling of the dog thyrotropin receptor.
Mol Cell Endocrinol
127:
81-90,
1997[ISI][Medline].
6.
Aplin, AE,
Howe AK,
and
Juliano RL.
Cell adhesion molecules, signal transduction and cell growth.
Curr Opin Cell Biol
11:
737-744,
1999[ISI][Medline].
7.
Attisano, L,
and
Wrana JL.
Smads as transcriptional co-modulators.
Curr Opin Cell Biol
12:
235-243,
2000[ISI][Medline].
8.
Bajenaru, ML,
Donahoe J,
Corral T,
Reilly KM,
Brophy S,
Pellicer A,
and
Gutmann DH.
Neurofibromatosis 1 (NF1) heterozygosity results in a cell-autonomous growth advantage for astrocytes.
Glia
33:
314-323,
2001[ISI][Medline].
9.
Baker, DA,
Mille-Baker B,
Wainwright SM,
Ish-Horowicz D,
and
Dibb NJ.
Mae mediates MAP kinase phosphorylation of Ets transcription factors in Drosophila.
Nature
411:
330-334,
2001[ISI][Medline].
10.
Baker, SJ,
and
Reddy EP.
Modulation of life and death by the TNF receptor superfamily.
Oncogene
17:
3261-3270,
1998[ISI][Medline].
11.
Baldari, CT,
Telford JL,
and
Acuto O.
EMBO Workshop Report: lymphocyte antigen receptor and coreceptor signaling Siena, Italy, November 6-10, 1999.
EMBO J
19:
4857-4865,
2000
12.
Banyard, J,
Anand-Apte B,
Symons M,
and
Zetter BR.
Motility and invasion are differentially modulated by Rho family GTPases.
Oncogene
19:
580-591,
2000[ISI][Medline].
13.
Bardelli, A,
Basile ML,
Audero E,
Giordano S,
Wennstrom S,
Menard S,
Comoglio PM,
and
Ponzetto C.
Concomitant activation of pathways downstream of Grb2 and PI 3-kinase is required for MET-mediated metastasis.
Oncogene
18:
1139-1146,
1999[ISI][Medline].
14.
Barkai, N,
and
Leibler S.
Robustness in simple biochemical networks.
Nature
387:
913-917,
1997[ISI][Medline].
15.
Beguin, P,
Nagashima K,
Gonoi T,
Shibasaki T,
Takahashi K,
Kashima Y,
Ozaki N,
Geering K,
Iwanaga T,
and
Seino S.
Regulation of Ca2+ channel expression at the cell surface by the small G-protein kir/Gem.
Nature
411:
701-706,
2001[ISI][Medline].
16.
Benzing, T,
Yaffe MB,
Arnould T,
Sellin L,
Schermer B,
Schilling B,
Schreiber R,
Kunzelmann K,
Leparc GG,
Kim E,
and
Walz G.
14-3-3 Interacts with regulator of G protein signaling proteins and modulates their activity.
J Biol Chem
275:
28167-28172,
2000
17.
Berger, SL.
An embarrassment of niches: the many covalent modifications of histones in transcriptional regulation.
Oncogene
20:
3007-3013,
2001[ISI][Medline].
18.
Berridge, MJ,
Bootman MD,
and
Lipp P.
Calciuma life and death signal.
Nature
395:
645-648,
1998[ISI][Medline].
19.
Berridge, MJ,
Lipp P,
and
Bootman MD.
The versatility and universality of calcium signalling.
Nat Rev Mol Cell Biol
1:
11-21,
2000[ISI][Medline].
20.
Bialy, H.
Aneuploidy and cancerthe vintage wine revisited.
Nat Biotechnol
19:
22-23,
2001[ISI][Medline].
21.
Biggin, MD.
To bind or not to bind.
Nat Genet
28:
303-304,
2001[ISI][Medline].
22.
Bilodeau, ML,
Boulineau T,
Hullinger RL,
and
Andrisani OM.
Cyclic AMP signaling functions as a bimodal switch in sympathoadrenal cell development in cultured primary neural crest cells.
Mol Cell Biol
20:
3004-3014,
2000
23.
Bissell, MJ,
Weaver VM,
Lelievre SA,
Wang F,
Petersen OW,
and
Schmeichel KL.
Tissue structure, nuclear organization, and gene expression in normal and malignant breast.
Cancer Res
59:
1757s-1763s,
1999[ISI].
24.
Black, DL.
Protein diversity from alternative splicing: a challenge for bioinformatics and post-genome biology.
Cell
103:
367-370,
2000[ISI][Medline].
25.
Bos, JL,
de Rooij J,
and
Reedquist KA.
Rap1 signalling: adhering to new models.
Nat Rev Mol Cell Biol
2:
369-377,
2001[ISI][Medline].
26.
Bos, JL,
and
Zwartkruis FJ.
Signal transduction. Rhapsody in G proteins.
Nature
400:
820-821,
1999[ISI][Medline].
27.
Bouloc, A.
Les cellules dendritiques cutanées humaines.
Médecine/Sciences
17:
465-474,
2001.
28.
Bowman, T,
Garcia R,
Turkson J,
and
Jove R.
STATs in oncogenesis.
Oncogene
19:
2474-2488,
2000[ISI][Medline].
29.
Boyes, J,
Byfield P,
Nakatani Y,
and
Ogryzko V.
Regulation of activity of the transcription factor GATA-1 by acetylation.
Nature
396:
594-598,
1998[ISI][Medline].
30.
Breault, L,
Chamoux E,
Lehoux JG,
and
Gallo-Payet N.
Localization of G protein alpha-subunits in the human fetal adrenal gland.
Endocrinology
141:
4334-4341,
2000
31.
Brickman, JM,
Adam M,
and
Ptashne M.
Interactions between an HMG-1 protein and members of the Rel family.
Proc Natl Acad Sci USA
96:
10679-10683,
1999
32.
Broome, MA,
and
Courtneidge SA.
No requirement for src family kinases for PDGF signaling in fibroblasts expressing SV40 large T antigen.
Oncogene
19:
2867-2869,
2000[ISI][Medline].
33.
Brown, P.
Cinderella goes to the ball.
Nature
410:
1018-1020,
2001[ISI][Medline].
34.
Buck, LB.
The molecular architecture of odor and pheromone sensing in mammals.
Cell
100:
611-618,
2000[ISI][Medline].
35.
Burack, WR,
and
Shaw AS.
Signal transduction: hanging on a scaffold.
Curr Opin Cell Biol
12:
211-216,
2000[ISI][Medline].
36.
Calabrese, EJ,
and
Baldwin LA.
Hormesis: U-shaped dose responses and their centrality in toxicology.
Trends Pharmacol Sci
22:
285-291,
2001[ISI][Medline].
37.
Calabro, V,
Strazzullo M,
La Mantia G,
Fedele M,
Paulin C,
Fusco A,
and
Lania L.
Status and expression of the p16INK4 gene in human thyroid tumors and thyroid-tumor cell lines.
Int J Cancer
67:
29-34,
1996[ISI][Medline].
38.
Cambier, JC.
Inhibitory receptors abound?
Proc Natl Acad Sci USA
94:
5993-5995,
1997
39.
Carson, JH,
Cowan A,
and
Loew LM.
Computational cell biologists snowed in at Cranwell.
Trends Cell Biol
11:
236-238,
2001[Medline].
40.
Cavalli, G,
and
Paro R.
Epigenetic inheritance of active chromatin after removal of the main transactivator.
Science
286:
955-958,
1999
41.
Chan, SY,
and
Wong RW.
Expression of epidermal growth factor in transgenic mice causes growth retardation.
J Biol Chem
275:
38693-38698,
2000
42.
Chang, L,
and
Karin M.
Mammalian MAP kinase signalling cascades.
Nature
410:
37-40,
2001[ISI][Medline].
43.
Chariot, A,
Gielen J,
Merville MP,
and
Bours V.
The homeodomain-containing proteins: an update on their interacting partners.
Biochem Pharmacol
58:
1851-1857,
1999[ISI][Medline].
44.
Chen, C,
Edelstein LC,
and
Gelinas C.
The Rel/NF-kappaB family directly activates expression of the apoptosis inhibitor Bcl-x(L).
Mol Cell Biol
20:
2687-2695,
2000
45.
Chen, H,
Lin RJ,
Xie W,
Wilpitz D,
and
Evans RM.
Regulation of hormone-induced histone hyperacetylation and gene activation via acetylation of an acetylase.
Cell
98:
675-686,
1999[ISI][Medline].
46.
Chen, L,
Chetkovich DM,
Petralia RS,
Sweeney NT,
Kawasaki Y,
Wenthold RJ,
Bredt DS,
and
Nicoll RA.
Stargazing regulates synaptic targeting of AMPA receptors by two distinct mechanisms.
Nature
408:
936-943,
2000[ISI][Medline].
47.
Cheung, WL,
Briggs SD,
and
Allis CD.
Acetylation and chromosomal functions.
Curr Opin Cell Biol
12:
326-333,
2000[ISI][Medline].
48.
Chotani, MA,
Touhalisky K,
and
Chiu IM.
The small GTPases Ras, Rac, and Cdc42 transcriptionally regulate expression of human fibroblast growth factor 1.
J Biol Chem
275:
30432-30438,
2000
49.
Clement, S,
Krause U,
Desmedt F,
Tanti JF,
Behrends J,
Pesesse X,
Sasaki T,
Penninger J,
Doherty M,
Malaisse W,
Dumont JE,
Le Marchand-Brustel Y,
Erneux C,
Hue L,
and
Schurmans S.
The lipid phosphatase SHIP2 controls insulin sensitivity.
Nature
409:
92-97,
2001[ISI][Medline].
50.
Conzen, SD,
Gottlob K,
Kandel ES,
Khanduri P,
Wagner AJ,
O'Leary M,
and
Hay N.
Induction of cell cycle progression and acceleration of apoptosis are two separable functions of c-Myc: transrepression correlates with acceleration of apoptosis.
Mol Cell Biol
20:
6008-6018,
2000
51.
Cook, B,
Bar-Yaacov M,
Cohen B,
Goldstein RE,
Paroush Z,
Selinger Z,
and
Minke B.
Phospholipase C and termination of G-protein-mediated signalling in vivo.
Nat Cell Biol
2:
296-301,
2000[ISI][Medline].
52.
Cornish-Bowden, A,
and
Cardenas ML.
From genome to cellular phenotypea role for metabolic flux analysis?
Nat Biotechnol
18:
267-268,
2000[ISI][Medline].
53.
Corti, C,
Leclerc L' H,
Quadroni M,
Schmid H,
Durussel I,
Cox J,
Dainese HP,
James P,
and
Carafoli E.
Tyrosine phosphorylation modulates the interaction of calmodulin with its target proteins.
Eur J Biochem
262:
790-802,
1999
54.
Cossu, G,
and
Borello U.
Wnt signaling and the activation of myogenesis in mammals.
EMBO J
18:
6867-6872,
1999
55.
Coulonval, K,
Vandeput F,
Stein RC,
Kozma SC,
Lamy F,
and
Dumont JE.
Phosphatidylinositol 3-kinase, protein kinase B and ribosomal S6 kinases in the stimulation of thyroid epithelial cell proliferation by cAMP and growth factors in the presence of insulin.
Biochem J
348:
351-358,
2000[ISI][Medline].
56.
Cowan, CW,
He W,
and
Wensel TG.
RGS proteins: lessons from the RGS9 subfamily.
Prog Nucleic Acid Res Mol Biol
65:
341-359,
2001[ISI][Medline].
57.
Crespo, P,
and
Leon J.
Ras proteins in the control of the cell cycle and cell differentiation.
Cell Mol Life Sci
57:
1613-1636,
2000[ISI][Medline].
58.
Crouch, MF,
Davy DA,
Willard FS,
and
Berven LA.
Activation of endogenous thrombin receptors causes clustering and sensitization of epidermal growth factor receptors of swiss 3T3 cells without transactivation.
J Cell Biol
152:
263-273,
2001
59.
Czyzyk, J,
Leitenberg D,
Taylor T,
and
Bottomly K.
Combinatorial effect of T-cell receptor ligation and CD45 isoform expression on the signaling contribution of the small GTPases Ras and Rap1.
Mol Cell Biol
20:
8740-8747,
2000
60.
Damante, G,
Tell G,
and
Di Lauro R.
A unique combination of transcription factors controls differentiation of thyroid cells.
Prog Nucleic Acid Res Mol Biol
66:
307-356,
2000[ISI].
61.
Dan, I,
Watanabe NM,
and
Kusumi A.
The Ste20 group kinases as regulators of MAP kinase cascades.
Trends Cell Biol
11:
220-230,
2001[ISI][Medline].
62.
Darlington, GJ.
Molecular mechanisms of liver development and differentiation.
Curr Opin Cell Biol
11:
678-682,
1999[ISI][Medline].
63.
De Cesare, D,
Fimia GM,
and
Sassone-Corsi P.
Signaling routes to CREM and CREB: plasticity in transcriptional activation.
Trends Biochem Sci
24:
281-285,
1999[ISI][Medline].
64.
De Hoog, CL,
Fan WT,
Goldstein MD,
Moran MF,
and
Koch CA.
Calmodulin-independent coordination of Ras and extracellular signal-regulated kinase activation by Ras-GRF2.
Mol Cell Biol
20:
2727-2733,
2000
65.
Depoortere, F,
Van Keymeulen A,
Lukas J,
Costagliola S,
Bartkova J,
Dumont JE,
Bartek J,
Roger P,
and
Dremier S.
A requirement for cyclin D3-cyclin-dependent kinase (cdk)-4 assembly in the cyclic adenosine monophosphate-dependent proliferation of thyrocytes.
J Cell Biol
140:
1427-1439,
1998
66.
Depre, C,
Rider MH,
and
Hue L.
Mechanisms of control of heart glycolysis.
Eur J Biochem
258:
277-290,
1998[Abstract].
67.
De Vries, L,
and
Farquhar MG.
RGS proteins: more than just GAPs for heterotrimeric G proteins.
Trends Cell Biol
9:
138-143,
1999[ISI][Medline].
68.
Di Cristofano, A,
and
Pandolfi PP.
The multiple roles of PTEN in tumor suppression.
Cell
100:
387-390,
2000[ISI][Medline].
69.
Diebold, BA,
and
Bokoch GM.
Molecular basis for Rac2 regulation of phagocyte NADPH oxidase.
Nat Immunol
2:
211-215,
2001[ISI][Medline].
70.
Di Liegro, CM,
Bellafiore M,
Izquierdo JM,
Rantanen A,
and
Cuezva JM.
3'-Untranslated regions of oxidative phosphorylation mRNAs function in vivo as enhancers of translation.
Biochem J
352:
109-115,
2000[ISI][Medline].
71.
Ding, VW,
Chen RH,
and
McCormick F.
Differential regulation of glycogen synthase kinase 3beta by insulin and Wnt signaling.
J Biol Chem
275:
32475-32481,
2000
72.
Dinulescu, DM,
and
Cone RD.
Agouti and agouti-related protein: analogies and contrasts.
J Biol Chem
275:
6695-6698,
2000
73.
Diverse-Pierluissi, M,
McIntire WE,
Myung CS,
Lindorfer MA,
Garrison JC,
Goy MF,
and
Dunlap K.
Selective coupling of G protein beta gamma complexes to inhibition of Ca2+ channels.
J Biol Chem
275:
28380-28385,
2000
74.
Dodge, K,
and
Scott JD.
AKAP79 and the evolution of the AKAP model.
FEBS Lett
476:
58-61,
2000[ISI][Medline].
75.
Dolmetsch, RE,
Lewis RS,
Goodnow CC,
and
Healy JI.
Differential activation of transcription factors induced by Ca2+ response amplitude and duration.
Nature
386:
855-858,
1997[ISI][Medline].
76.
Donnadieu, E,
Jouvin MH,
and
Kinet JP.
A second amplifier function for the allergy-associated Fc(epsilon)RI-beta subunit.
Immunity
12:
515-523,
2000[ISI][Medline].
77.
Dotto, GP.
p21(WAF1/Cip1): more than a break to the cell cycle?
Biochim Biophys Acta
1471:
M43-M56,
2000[ISI][Medline].
78.
Downward, J.
The ins and outs of signalling.
Nature
411:
759-762,
2001[ISI][Medline].
79.
Dremier, S,
Vandeput F,
Zwartkruis FJT,
Bos JL,
Dumont JE,
and
Maenhaut C.
Activation of the small G protein Rap1 in dog thyroid cells by both cAMP-dependent and -independent pathways.
Biochem Biophys Res Commun
267:
7-11,
2000[ISI][Medline].
80.
Dufner, A,
and
Thomas G.
Ribosomal S6 kinase signaling and the control of translation.
Exp Cell Res
253:
100-109,
1999[ISI][Medline].
81.
Dumont, JE,
Lamy F,
Roger PP,
and
Maenhaut C.
Physiological and pathological regulation of thyroid cell proliferation and differentiation by thyrotropin and other factors.
Physiol Rev
72:
667-697,
1992
82.
Dupont, G,
Swillens S,
Clair C,
Tordjmann T,
and
Combettes L.
Hierarchical organization of calcium signals in hepatocytes: from experiments to models.
Biochim Biophys Acta
1498:
134-152,
2000[ISI][Medline].
83.
Durham, PL,
and
Russo AF.
Differential regulation of mitogen-activated protein kinase-responsive genes by the duration of a calcium signal.
Mol Endocrinol
14:
1570-1582,
2000
84.
Dustin, ML,
and
Shaw AS.
Costimulation: building an immunological synapse.
Science
283:
649-650,
1999
85.
Eastman, Q,
and
Grosschedl R.
Regulation of LEF-1/TCF transcription factors by Wnt and other signals.
Curr Opin Cell Biol
11:
233-240,
1999[ISI][Medline].
86.
Edwards, AS,
and
Scott JD.
A-kinase anchoring proteins: protein kinase A and beyond.
Curr Opin Cell Biol
12:
217-221,
2000[ISI][Medline].
87.
Ehrhardt, GR,
Korherr C,
Wieler JS,
Knaus M,
and
Schrader JW.
A novel potential effector of M-Ras and p21 Ras negatively regulates p21 Ras-mediated gene induction and cell growth.
Oncogene
20:
188-197,
2001[ISI][Medline].
88.
Ekholm, SV,
and
Reed SI.
Regulation of G(1) cyclin-dependent kinases in the mammalian cell cycle.
Curr Opin Cell Biol
12:
676-684,
2000[ISI][Medline].
89.
Elenbaas, B,
and
Weinberg RA.
Heterotypic signaling between epithelial tumor cells and fibroblasts in carcinoma formation.
Exp Cell Res
264:
169-184,
2001[ISI][Medline].
90.
Elion, EA.
Routing MAP kinase cascades.
Science
281:
1625-1626,
1998
91.
Endy, D,
and
Brent R.
Modelling cellular behaviour.
Nature
409, Suppl:
391-395,
2001[ISI][Medline].
92.
Erickson, JW,
and
Cerione RA.
Multiple roles for Cdc42 in cell regulation.
Curr Opin Cell Biol
13:
153-157,
2001[ISI][Medline].
93.
Exton, JH.
Regulation of phospholipase D.
Biochim Biophys Acta
1439:
121-133,
1999[ISI][Medline].
94.
Fambrough, D,
McClure K,
Kazlauskas A,
and
Lander ES.
Diverse signaling pathways activated by growth factor receptors induce broadly overlapping, rather than independent, sets of genes.
Cell
97:
727-741,
1999[ISI][Medline].
95.
Fanning, AS,
and
Anderson JM.
Protein modules as organizers of membrane structure.
Curr Opin Cell Biol
11:
432-439,
1999[ISI][Medline].
96.
Ferjoux, G,
Bousquet C,
Cordelier P,
Benali N,
Lopez F,
Rochaix P,
Buscail L,
and
Susini C.
Signal transduction of somatostatin receptors negatively controlling cell proliferation.
J Physiol (Paris)
94:
205-210,
2000[ISI][Medline].
97.
Ferreira, ST,
and
De Felice FG.
PABMB Lecture. Protein dynamics, folding and misfolding: from basic physical chemistry to human conformational diseases.
FEBS Lett
498:
129-134,
2001[ISI][Medline].
98.
Fiorini, M,
Alimandi M,
Fiorentino L,
Sala G,
and
Segatto O.
Negative regulation of receptor tyrosine kinase signals.
FEBS Lett
490:
132-141,
2001[ISI][Medline].
99.
Fisher, MJ,
Malcolm G,
and
Paton RC.
Spatio-logical processes in intracellular signalling.
Biosystems
55:
83-92,
2000[ISI][Medline].
100.
Flier, JS,
Harris M,
and
Hollenberg AN.
Leptin, nutrition, and the thyroid: the why, the wherefore, and the wiring.
J Clin Invest
105:
859-861,
2000
101.
Franklin, DS,
Godfrey VL,
O'Brien DA,
Deng C,
and
Xiong Y.
Functional collaboration between different cyclin-dependent kinase inhibitors suppresses tumor growth with distinct tissue specificity.
Mol Cell Biol
20:
6147-6158,
2000
102.
Fraser, SE,
and
Harland RM.
The molecular metamorphosis of experimental embryology.
Cell
100:
41-55,
2000[ISI][Medline].
103.
Freeman, M.
Feedback control of intercellular signalling in development.
Nature
408:
313-319,
2000[ISI][Medline].
104.
Frey, N,
McKinsey TA,
and
Olson EN.
Decoding calcium signals involved in cardiac growth and function.
Nat Med
6:
1221-1227,
2000[ISI][Medline].
105.
Fry, CJ,
and
Farnham PJ.
Context-dependent transcriptional regulation.
J Biol Chem
274:
29583-29586,
1999
106.
Gaertner, FH.
Unique catalytic properties of enzyme clusters.
Trends Biochem Sci
2:
63-65,
1978.
107.
Galbiati, F,
Razani B,
and
Lisanti MP.
Emerging themes in lipid rafts and caveolae.
Cell
106:
403-411,
2001[ISI][Medline].
108.
Garrington, TP,
and
Johnson GL.
Organization and regulation of mitogen-activated protein kinase signaling pathways.
Curr Opin Cell Biol
11:
211-218,
1999[ISI][Medline].
109.
Ghazi, A,
and
VijayRaghavan KV.
Developmental biology. Control by combinatorial codes.
Nature
408:
419-420,
2000[ISI][Medline].
110.
Gill, RM,
and
Hamel PA.
Subcellular compartmentalization of E2F family members is required for maintenance of the postmitotic state in terminally differentiated muscle.
J Cell Biol
148:
1187-1201,
2000
111.
Gilman, AG.
Wrong signals about alliance's scope and aim.
Nature
408:
133,
2000.
112.
Gire, V,
Marshall C,
and
Wynford-Thomas D.
PI-3-kinase is an essential anti-apoptotic effector in the proliferative response of primary human epithelial cells to mutant RAS.
Oncogene
19:
2269-2276,
2000[ISI][Medline].
113.
Gire, V,
and
Wynford-Thomas D.
RAS oncogene activation induces proliferation in normal human thyroid epithelial cells without loss of differentiation.
Oncogene
19:
737-744,
2000[ISI][Medline].
114.
Goi, T,
Rusanescu G,
Urano T,
and
Feig LA.
Ral-specific guanine nucleotide exchange factor activity opposes other Ras effectors in PC12 cells by inhibiting neurite outgrowth.
Mol Cell Biol
19:
1731-1741,
1999
115.
Goi, T,
Shipitsin M,
Lu Z,
Foster DA,
Klinz SG,
and
Feig LA.
An EGF receptor/Ral-GTPase signaling cascade regulates c-Src activity and substrate specificity.
EMBO J
19:
623-630,
2000
116.
Goldstein, LS.
Transduction. When worlds collidetrafficking in JNK.
Science
291:
2102-2103,
2001
117.
Gomes-Pereira, M,
Fortune MT,
and
Monckton DG.
Mouse tissue culture models of unstable triplet repeats: in vitro selection for larger alleles, mutational expansion bias and tissue specificity, but no association with cell division rates.
Hum Mol Genet
10:
845-854,
2001
118.
Gottifredi, V,
and
Prives C.
P53 and PML: new partners in tumor suppression.
Trends Cell Biol
11:
184-187,
2001[ISI][Medline].
119.
Graveley, BR.
Alternative splicing: increasing diversity in the proteomic world.
Trends Genet
17:
100-107,
2001[ISI][Medline].
120.
Grosse, R,
Roelle S,
Herrlich A,
Hohn J,
and
Gudermann T.
Epidermal growth factor receptor tyrosine kinase mediates Ras activation by gonadotropin-releasing hormone.
J Biol Chem
275:
12251-12260,
2000
121.
Gudermann, T,
Grosse R,
and
Schultz G.
Contribution of receptor/G protein signaling to cell growth and transformation.
Naunyn Schmiedebergs Arch Pharmacol
361:
345-362,
2000[ISI][Medline].
122.
Gudermann, T,
Kalkbrenner F,
Dippel E,
Laugwitz KL,
and
Schultz G.
Specificity and complexity of receptor-G-protein interaction.
Adv Second Messenger Phosphoprotein Res
31:
253-262,
1997[ISI][Medline].
123.
Guhaniyogi, J,
and
Brewer G.
Regulation of mRNA stability in mammalian cells.
Gene
265:
11-23,
2001[ISI][Medline].
124.
Guss, KA,
Nelson CE,
Hudson A,
Kraus ME,
and
Carroll SB.
Control of a genetic regulatory network by a selector gene.
Science
292:
1164-1167,
2001
125.
Gutkind, JS.
Cell growth control by G protein-coupled receptors: from signal transduction to signal integration.
Oncogene
17:
1331-1342,
1998[ISI][Medline].
126.
Hagemann, C,
and
Rapp UR.
Isotype-specific functions of Raf kinases.
Exp Cell Res
253:
34-46,
1999[ISI][Medline].
127.
Hagmann, M.
New insights into cystic fibrosis ion channel.
Science
286:
388-389,
1999
128.
Halfar, K,
Rommel C,
Stocker H,
and
Hafen E.
Ras controls growth, survival and differentiation in the Drosophila eye by different thresholds of MAP kinase activity.
Development
128:
1687-1696,
2001
129.
Hall, RA,
Premont RT,
and
Lefkowitz RJ.
Heptahelical receptor signaling: beyond the G protein paradigm.
J Cell Biol
145:
927-932,
1999
130.
Hanahan, D,
and
Weinberg RA.
The hallmarks of cancer.
Cell
100:
57-70,
2000[ISI][Medline].
131.
Harland, RM.
Developmental biology. A twist on embryonic signalling.
Nature
410:
423-424,
2001[ISI][Medline].
132.
Harlow, ML,
Ress D,
Stoschek A,
Marshall RM,
and
McMahan UJ.
The architecture of active zone material at the frog's neuromuscular junction.
Nature
409:
479-484,
2001[ISI][Medline].
133.
Hartwell, LH,
Hopfield JJ,
Leibler S,
and
Murray AW.
From molecular to modular cell biology.
Nature
402:
C47-C52,
1999[ISI][Medline].
134.
Heuss, C,
and
Gerber U.
G-protein-independent signaling by G-protein-coupled receptors.
Trends Neurosci
23:
469-475,
2000[ISI][Medline].
135.
Hilger-Eversheim, K,
Moser M,
Schorle H,
and
Buettner R.
Regulatory roles of AP-2 transcription factors in vertebrate development, apoptosis and cell-cycle control.
Gene
260:
1-12,
2000[ISI][Medline].
136.
Hoenderop, JG,
Vaandrager AB,
Dijkink L,
Smolenski A,
Gambaryan S,
Lohmann SM,
de Jonge HR,
Willems PH,
and
Bindels RJ.
Atrial natriuretic peptide-stimulated Ca2+ reabsorption in rabbit kidney requires membrane-targeted, cGMP-dependent protein kinase type II.
Proc Natl Acad Sci USA
96:
6084-6089,
1999
137.
Holmberg, J,
Clarke DL,
and
Frisen J.
Regulation of repulsion versus adhesion by different splice forms of an Eph receptor.
Nature
408:
203-206,
2000[ISI][Medline].
138.
Holmstrom, TH,
Tran SE,
Johnson VL,
Ahn NG,
Chow SC,
and
Eriksson JE.
Inhibition of mitogen-activated kinase signaling sensitizes HeLa cells to Fas receptor-mediated apoptosis.
Mol Cell Biol
19:
5991-6002,
1999
139.
Holstege, FC,
Jennings EG,
Wyrick JJ,
Lee TI,
Hengartner CJ,
Green MR,
Golub TR,
Lander ES,
and
Young RA.
Dissecting the regulatory circuitry of a eukaryotic genome.
Cell
95:
717-728,
1998[ISI][Medline].
140.
Hou, Y,
Azpiazu I,
Smrcka A,
and
Gautam N.
Selective role of G protein gamma subunits in receptor interaction.
J Biol Chem
275:
38961-38964,
2000
141.
Houslay, MD,
and
Kolch W.
Cell-type specific integration of cross-talk between extracellular signal-regulated kinase and cAMP signaling.
Mol Pharmacol
58:
659-668,
2000
142.
Houzet, L,
Morello D,
Defrance P,
Mercier P,
Huez G,
and
Kruys V.
Regulated control by granulocyte-macrophage colony- stimulating factor AU-rich element during mouse embryogenesis.
Blood
98:
1281-1288,
2001
143.
Hu, Q,
Deshpande S,
Irani K,
and
Ziegelstein RC.
[Ca2+]i oscillation frequency regulates agonist-stimulated NF-kappaB transcriptional activity.
J Biol Chem
274:
33995-33998,
1999
144.
Hu, X,
and
Lazar MA.
The CoRNR motif controls the recruitment of corepressors by nuclear hormone receptors.
Nature
402:
93-96,
1999[ISI][Medline].
145.
Hughes, P,
and
Dragunow M.
Induction of immediate-early genes and the control of neurotransmitter-regulated gene expression within the nervous system.
Pharmacol Rev
47:
133-178,
1995[ISI][Medline].
146.
Hupp, TR,
Lane DP,
and
Ball KL.
Strategies for manipulating the p53 pathway in the treatment of human cancer.
Biochem J
352:
1-17,
2000[ISI][Medline].
147.
Hurley, JH.
Structure, mechanism, and regulation of mammalian adenylyl cyclase.
J Biol Chem
274:
7599-7602,
1999
148.
Illmensee, K,
and
Mintz B.
Totipotency and normal differentiation of single teratocarcinoma cells cloned by injection into blastocysts.
Proc Natl Acad Sci USA
73:
549-553,
1976[Abstract].
149.
Isotani, S,
Hara K,
Tokunaga C,
Inoue H,
Avruch J,
and
Yonezawa K.
Immunopurified mammalian target of rapamycin phosphorylates and activates p70 S6 kinase alpha in vitro.
J Biol Chem
274:
34493-34498,
1999
150.
Ito, T,
Tashiro K,
Muta S,
Ozawa R,
Chiba T,
Nishizawa M,
Yamamoto K,
Kuhara S,
and
Sakaki Y.
Toward a protein-protein interaction map of the budding yeast: a comprehensive system to examine two-hybrid interactions in all possible combinations between the yeast proteins.
Proc Natl Acad Sci USA
97:
1143-1147,
2000
151.
Ito, Y,
Chen G,
Imanishi Y,
Morooka T,
Nishida E,
Okabayashi Y,
and
Kasuga M.
Differential control of cellular gene expression by diffusible and non-diffusible egf.
J Biochem (Tokyo)
129:
733-737,
2001[Abstract].
152.
Jacobs, JJ,
and
van Lohuizen M.
Cellular memory of transcriptional states by Polycomb-group proteins.
Semin Cell Dev Biol
10:
227-235,
1999[ISI][Medline].
153.
Janssens, V,
and
Goris J.
Protein phosphatase 2A: a highly regulated family of serine/threonine phosphatases implicated in cell growth and signalling.
Biochem J
353:
417-439,
2001[ISI][Medline].
154.
Jensen, CJ,
Buch MB,
Krag TO,
Hemmings BA,
Gammeltoft S,
and
Frodin M.
90-kDa ribosomal S6 kinase is phosphorylated and activated by 3-phosphoinositide-dependent protein kinase-1.
J Biol Chem
274:
27168-27176,
1999
155.
Jensen, RB,
and
Shapiro L.
Proteins on the move: dynamic protein localization in prokaryotes.
Trends Cell Biol
10:
483-488,
2000[ISI][Medline].
156.
Jeong, H,
Mason SP,
Barabasi AL,
and
Oltvai ZN.
Lethality and centrality in protein networks.
Nature
411:
41-42,
2001[ISI][Medline].
157.
Jepsen, K,
Hermanson O,
Onami TM,
Gleiberman AS,
Lunyak V,
McEvilly RJ,
Kurokawa R,
Kumar V,
Liu F,
Seto E,
Hedrick SM,
Mandel G,
Glass CK,
Rose DW,
and
Rosenfeld MG.
Combinatorial roles of the nuclear receptor corepressor in transcription and development.
Cell
102:
753-763,
2000[ISI][Medline].
158.
Jiang, H,
Karnezis AN,
Tao M,
Guida PM,
and
Zhu L.
pRB and p107 have distinct effects when expressed in pRB-deficient tumor cells at physiologically relevant levels.
Oncogene
19:
3878-3887,
2000[ISI][Medline].
159.
Joazeiro, CA,
Wing SS,
Huang H,
Leverson JD,
Hunter T,
and
Liu YC.
The tyrosine kinase negative regulator c-Cbl as a RING-type, E2-dependent ubiquitin-protein ligase.
Science
286:
309-312,
1999
160.
Johansen, PW,
Lund HW,
and
Gordeladze JO.
Specific combinations of G-protein subunits discriminate hormonal signalling in rat pituitary (GH3) cells in culture.
Cell Signal
13:
251-256,
2001[ISI][Medline].
161.
Johnson, L,
Mercer K,
Greenbaum D,
Bronson RT,
Crowley D,
Tuveson DA,
and
Jacks T.
Somatic activation of the K-ras oncogene causes early onset lung cancer in mice.
Nature
410:
1111-1116,
2001[ISI][Medline].
162.
Joneson, T,
and
Bar-Sagi D.
Suppression of Ras-induced apoptosis by the Rac GTPase.
Mol Cell Biol
19:
5892-5901,
1999
163.
Jordan, JD,
Landau EM,
and
Iyengar R.
Signaling networks: the origins of cellular multitasking.
Cell
103:
193-200,
2000[ISI][Medline].
164.
Judson, HF.
Talking about the genome.
Nature
409:
769,
2001[ISI][Medline].
165.
Kaiser, UB,
Halvorson LM,
and
Chen MT.
Sp1, steroidogenic factor 1 (SF-1), and early growth response protein 1 (egr-1) binding sites form a tripartite gonadotropin-releasing hormone response element in the rat luteinizing hormone-beta gene promoter: an integral role for SF-1.
Mol Endocrinol
14:
1235-1245,
2000
166.
Kajiyama, Y,
and
Ui M.
Differential mitogenic actions of alpha 1- and beta-adrenergic agonists on rat hepatocytes.
Cell Signal
10:
241-251,
1998[ISI][Medline].
167.
Kalo, MS,
and
Pasquale EB.
Signal transfer by Eph receptors.
Cell Tissue Res
298:
1-9,
1999[ISI][Medline].
168.
Karandikar, M,
and
Cobb MH.
Scaffolding and protein interactions in MAP kinase modules.
Cell Calcium
26:
219-226,
1999[ISI][Medline].
169.
Katz, PS,
and
Clemens S.
Biochemical networks in nervous systems: expanding neuronal information capacity beyond voltage signals.
Trends Neurosci
24:
18-25,
2001[ISI][Medline].
170.
Keely, SJ,
Calandrella SO,
and
Barrett KE.
Carbachol-stimulated transactivation of epidermal growth factor receptor and mitogen-activated protein kinase in T84 cells is mediated by intracellular Ca2+, PYK-2, and p60src.
J Biol Chem
275:
12619-12625,
2000
171.
Kennedy, MB.
Signal-processing machines at the postsynaptic density.
Science
290:
750-754,
2000
172.
Kennedy, MB.
Sticking together.
Proc Natl Acad Sci USA
97:
11135-11136,
2000
173.
Kikuchi, A.
Roles of Axin in the Wnt signalling pathway.
Cell Signal
11:
777-788,
1999[ISI][Medline].
174.
Kim, AS,
Kakalis LT,
Abdul-Manan N,
Liu GA,
and
Rosen MK.
Autoinhibition and activation mechanisms of the Wiskott-Aldrich syndrome protein.
Nature
404:
151-158,
2000[ISI][Medline].
175.
Kimura, T,
Van Keymeulen A,
Golstein J,
Fusco A,
Dumont JE,
and
Roger PP.
Regulation of thyroid cell proliferation by tsh and other factors: a critical evaluation of in vitro models.
Endocr Rev
22:
631-656,
2001
176.
Kjoller, L,
and
Hall A.
Signaling to Rho GTPases.
Exp Cell Res
253:
166-179,
1999[ISI][Medline].
177.
Klein, CA.
The biology and analysis of single disseminated tumour cells.
Trends Cell Biol
10:
489-493,
2000[ISI][Medline].
178.
Klinghoffer, RA,
Sachsenmaier C,
Cooper JA,
and
Soriano P.
Src family kinases are required for integrin but not PDGFR signal transduction.
EMBO J
18:
2459-2471,
1999
179.
Kolch, W.
Meaningful relationships: the regulation of the Ras/Raf/MEK/ERK pathway by protein interactions.
Biochem J
351:
289-305,
2000[ISI][Medline].
180.
Kouzarides, T.
Acetylation: a regulatory modification to rival phosphorylation?
EMBO J
19:
1176-1179,
2000
181.
Krawczyk, C,
and
Penninger JM.
Molecular controls of antigen receptor clustering and autoimmunity.
Trends Cell Biol
11:
212-220,
2001[ISI][Medline].
182.
Krsmanovic, LZ,
Mores N,
Navarro CE,
Tomic M,
and
Catt KJ.
Regulation of Ca2+-sensitive adenylyl cyclase in gonadotropin-releasing hormone neurons.
Mol Endocrinol
15:
429-440,
2001
183.
Kveiborg, M,
Flyvbjerg A,
Eriksen EF,
and
Kassem M.
Transforming growth factor-beta1 stimulates the production of insulin-like growth factor-I and insulin-like growth factor-binding protein-3 in human bone marrow stromal osteoblast progenitors.
J Endocrinol
169:
549-561,
2001
184.
Lahuna, O,
Rastegar M,
Maiter D,
Thissen JP,
Lemaigre FP,
and
Rousseau GG.
Involvement of STAT5 (signal transducer and activator of transcription 5) and HNF-4 (hepatocyte nuclear factor 4) in the transcriptional control of the hnf6 gene by growth hormone.
Mol Endocrinol
14:
285-294,
2000
185.
Lam, EW,
Glassford J,
Banerji L,
Thomas NS,
Sicinski P,
and
Klaus GG.
Cyclin D3 compensates for loss of cyclin D2 in mouse B-lymphocytes activated via the antigen receptor and CD40.
J Biol Chem
275:
3479-3484,
2000
186.
Land, H,
Parada LF,
and
Weinberg RA.
Tumorigenic conversion of primary embryo fibroblasts requires at least two cooperating oncogenes.
Nature
304:
596-602,
1983[ISI][Medline].
187.
Lassus, P,
Bertrand C,
Zugasti O,
Chambon JP,
Soussi T,
Mathieu-Mahul D,
and
Hibner U.
Anti-apoptotic activity of p53 maps to the COOH-terminal domain and is retained in a highly oncogenic natural mutant.
Oncogene
18:
4699-4709,
1999[ISI][Medline].
188.
Lawlor, MA,
and
Alessi DR.
PKB/Akt: a key mediator of cell proliferation, survival and insulin responses?
J Cell Sci
114:
2903-2910,
2001
189.
Leclercq, J,
and
Dumont JE.
Boolean analysis of cell regulation networks.
J Theor Biol
104:
507-534,
1983[ISI][Medline].
190.
LeCouter, J,
Kowalski J,
Foster J,
Hass P,
Zhang Z,
Dillard-Telm L,
Frantz G,
Rangell L,
DeGuzman L,
Keller GA,
Peale F,
Gurney A,
Hillan KJ,
and
Ferrara N.
Identification of an angiogenic mitogen selective for endocrine gland endothelium.
Nature
412:
877-884,
2001[ISI][Medline].
191.
Lee, YK,
Dell H,
Dowhan DH,
Hadzopoulou-Cladaras M,
and
Moore DD.
The orphan nuclear receptor SHP inhibits hepatocyte nuclear factor 4 and retinoid X receptor transactivation: two mechanisms for repression.
Mol Cell Biol
20:
187-195,
2000
192.
Lefstin, JA,
and
Yamamoto KR.
Allosteric effects of DNA on transcriptional regulators.
Nature
392:
885-888,
1998[ISI][Medline].
193.
Lemmon, MA,
Falasca M,
Ferguson KM,
and
Schlessinger J.
Regulatory recruitment of signalling molecules to the cell membrane by pleckstrin-homology domains.
Trends Cell Biol
7:
237-242,
1997[ISI].
194.
Leonhardt, H,
and
Cardoso MC.
DNA methylation, nuclear structure, gene expression and cancer.
J Cell Biochem Suppl
35:
78-83,
2000.
195.
Le Roith, D,
Bondy C,
Yakar S,
Liu JL,
and
Butler A.
The somatomedin hypothesis: 2001.
Endocr Rev
22:
53-74,
2001
196.
Leuthauser, K,
Gujer R,
Aldecoa A,
McKinney RA,
Muff R,
Fischer JA,
and
Born W.
Receptor-activity-modifying protein 1 forms heterodimers with two G-protein-coupled receptors to define ligand recognition.
Biochem J
351:
347-351,
2000[ISI][Medline].
197.
Levchenko, A,
Bruck J,
and
Sternberg PW.
Scaffold proteins may biphasically affect the levels of mitogen-activated protein kinase signaling and reduce its threshold properties.
Proc Natl Acad Sci USA
97:
5818-5823,
2000
198.
Lewis, KA,
Gray PC,
Blount AL,
MacConell LA,
Wiater E,
Bilezikjian LM,
and
Vale W.
Betaglycan binds inhibin and can mediate functional antagonism of activin signalling.
Nature
404:
411-414,
2000[ISI][Medline].
199.
Lewis, TS,
Hunt JB,
Aveline LD,
Jonscher KR,
Louie DF,
Yeh JM,
Nahreini TS,
Resing KA,
and
Ahn NG.
Identification of novel MAP kinase pathway signaling targets by functional proteomics and mass spectrometry.
Mol Cell
6:
1343-1354,
2000[ISI][Medline].
200.
Leyns, L,
Bouwmeester T,
Kim SH,
Piccolo S,
and
De Robertis EM.
Frzb-1 is a secreted antagonist of Wnt signaling expressed in the Spemann organizer.
Cell
88:
747-756,
1997[ISI][Medline].
201.
Lezcano, N,
Mrzljak L,
Eubanks S,
Levenson R,
Goldman-Rakic P,
and
Bergson C.
Dual signaling regulated by calcyon, a D1 dopamine receptor interacting protein.
Science
287:
1660-1664,
2000
202.
Li, M,
Wang X,
Meintzer MK,
Laessig T,
Birnbaum MJ,
and
Heidenreich KA.
Cyclic AMP promotes neuronal survival by phosphorylation of glycogen synthase kinase 3beta.
Mol Cell Biol
20:
9356-9363,
2000
203.
Li, W,
Llopis J,
Whitney M,
Zlokarnik G,
and
Tsien RY.
Cell-permeant caged InsP3 ester shows that Ca2+ spike frequency can optimize gene expression.
Nature
392:
936-941,
1998[ISI][Medline].
204.
Li, X,
Eastman EM,
Schwartz RJ,
and
Draghia-Akli R.
Synthetic muscle promoters: activities exceeding naturally occurring regulatory sequences.
Nat Biotechnol
17:
241-245,
1999[ISI][Medline].
205.
Lichtstein, D,
and
Rodbard D.
A second look at the second messenger hypothesis.
Life Sci
40:
2041-2051,
1987[ISI][Medline].
206.
Liscovitch, M,
Czarny M,
Fiucci G,
and
Tang X.
Phospholipase D: molecular and cell biology of a novel gene family.
Biochem J
345:
401-415,
2000[ISI][Medline].
207.
Little, JW,
Shepley DP,
and
Wert DW.
Robustness of a gene regulatory circuit.
EMBO J
18:
4299-4307,
1999
208.
Lo, RS,
Wotton D,
and
Massague J.
Epidermal growth factor signaling via Ras controls the Smad transcriptional co-repressor TGIF.
EMBO J
20:
128-136,
2001
209.
Logsdon, CD.
The influence of the cellular context on receptor function: a necessary consideration for physiologic interpretations of receptor expression studies.
Life Sci
64:
369-374,
1999[ISI][Medline].
210.
Lorimer, IA,
and
Lavictoire SJ.
Activation of extracellular-regulated kinases by normal and mutant EGF receptors.
Biochim Biophys Acta
1538:
1-9,
2001[ISI][Medline].
211.
Lotem, J,
Kama R,
and
Sachs L.
Suppression or induction of apoptosis by opposing pathways downstream from calcium-activated calcineurin.
Proc Natl Acad Sci USA
96:
12016-12020,
1999
212.
Lumelsky, N,
Blondel O,
Laeng P,
Velasco I,
Ravin R,
and
McKay R.
Differentiation of embryonic stem cells to insulin-secreting structures similar to pancreatic islets.
Science
292:
1389-1394,
2001
213.
Luo, X,
Zeng W,
Xu X,
Popov S,
Davignon I,
Wilkie TM,
Mumby SM,
and
Muallem S.
Alternate coupling of receptors to Gs and Gi in pancreatic and submandibular gland cells.
J Biol Chem
274:
17684-17690,
1999
214.
Luttrell, LM,
Daaka Y,
and
Lefkowitz RJ.
Regulation of tyrosine kinase cascades by G-protein-coupled receptors.
Curr Opin Cell Biol
11:
177-183,
1999[ISI][Medline].
215.
Luttrell, LM,
Ferguson SS,
Daaka Y,
Miller WE,
Maudsley S,
Della RG,
Lin F,
Kawakatsu H,
Owada K,
Luttrell DK,
Caron MG,
and
Lefkowitz RJ.
Beta-arrestin-dependent formation of beta2 adrenergic receptor-Src protein kinase complexes.
Science
283:
655-661,
1999
216.
Machide, M,
Kamitori K,
and
Kohsaka S.
Hepatocyte growth factor-induced differential activation of phospholipase cgamma 1 and phosphatidylinositol 3-kinase is regulated by tyrosine phosphatase SHP-1 in astrocytes.
J Biol Chem
275:
31392-31398,
2000
217.
MacKenzie, SJ,
Baillie GS,
McPhee I,
Bolger GB,
and
Houslay MD.
ERK2 mitogen-activated protein kinase binding, phosphorylation, and regulation of the PDE4D cAMP-specific phosphodiesterases. The involvement of COOH-terminal docking sites and NH2-terminal UCR regions.
J Biol Chem
275:
16609-16617,
2000
218.
Maestes, DC,
Potter RM,
and
Prossnitz ER.
Differential phosphorylation paradigms dictate desensitization and internalization of the N-formyl peptide receptor.
J Biol Chem
274:
29791-29795,
1999
219.
Magee, T,
and
Marshall C.
New insights into the interaction of Ras with the plasma membrane.
Cell
98:
9-12,
1999[ISI][Medline].
220.
Malnic, B,
Hirono J,
Sato T,
and
Buck LB.
Combinatorial receptor codes for odors.
Cell
96:
713-723,
1999[ISI][Medline].
221.
Maniatis, T,
Falvo JV,
Kim TH,
Kim TK,
Lin CH,
Parekh BS,
and
Wathelet MG.
Structure and function of the interferon-beta enhanceosome.
Cold Spring Harb Symp Quant Biol
63:
609-620,
1998[ISI][Medline].
222.
Marmorstein, R.
Protein modules that manipulate histone tails for chromatin regulation.
Nat Rev Mol Cell Biol
2:
422-432,
2001[ISI][Medline].
223.
Maroulakou, IG,
and
Bowe DB.
Expression and function of Ets transcription factors in mammalian development: a regulatory network.
Oncogene
19:
6432-6442,
2000[ISI][Medline].
224.
Martini, PG,
Delage-Mourroux R,
Kraichely DM,
and
Katzenellenbogen BS.
Prothymosin alpha selectively enhances estrogen receptor transcriptional activity by interacting with a repressor of estrogen receptor activity.
Mol Cell Biol
20:
6224-6232,
2000
225.
Marx, J.
Cell biology. Do centrosome abnormalities lead to cancer?
Science
292:
426-429,
2001
226.
Massague, J,
Blain SW,
and
Lo RS.
TGFbeta signaling in growth control, cancer, and heritable disorders.
Cell
103:
295-309,
2000[ISI][Medline].
227.
Massague, J,
and
Wotton D.
Transcriptional control by the TGF-beta/Smad signaling system.
EMBO J
19:
1745-1754,
2000
228.
May, RC,
and
Machesky LM.
Phagocytosis and the actin cytoskeleton.
J Cell Sci
114:
1061-1077,
2001
229.
Mazia D. The Cell, edited by B. Mirsky. New York:
Academic, 1961, vol. III, p. 80.
230.
McCormick, F.
Signalling networks that cause cancer.
Trends Cell Biol
9:
M53-M56,
1999[ISI][Medline].
231.
McDonald, PH,
Chow CW,
Miller WE,
Laporte SA,
Field ME,
Lin FT,
Davis RJ,
and
Lefkowitz RJ.
Beta-arrestin 2: a receptor-regulated MAPK scaffold for the activation of JNK3.
Science
290:
1574-1577,
2000
232.
McKinsey, TA,
Zhang CL,
Lu J,
and
Olson EN.
Signal-dependent nuclear export of a histone deacetylase regulates muscle differentiation.
Nature
408:
106-111,
2000[ISI][Medline].
233.
Missiaen, L,
Robberecht W,
van den Bosch L,
Callewaert G,
Parys JB,
Wuytack F,
Raeymaekers L,
Nilius B,
Eggermont J,
and
De Smedt H.
Abnormal intracellular Ca2+ homeostasis and disease.
Cell Calcium
28:
1-21,
2000[ISI][Medline].
234.
Momparler, RL,
and
Bovenzi V.
DNA methylation and cancer.
J Cell Physiol
183:
145-154,
2000[ISI][Medline].
235.
Morgan, JI,
and
Curran T.
Stimulus-transcription coupling in neurons: role of cellular immediate-early genes.
Trends Neurosci
12:
459-462,
1989[ISI][Medline].
236.
Morris, AJ,
and
Malbon CC.
Physiological regulation of G protein-linked signaling.
Physiol Rev
79:
1373-1430,
1999
237.
Morris, B.
The insulin-sensitive glucose transporter.
Rev Cytol
137A:
259-297,
1992.
238.
Morris, DR,
and
Geballe AP.
Upstream open reading frames as regulators of mRNA translation.
Mol Cell Biol
20:
8635-8642,
2000
239.
Morrison, DK.
KSR: a MAPK scaffold of the Ras pathway?
J Cell Sci
114:
1609-1612,
2001
240.
Morrison, RS,
and
Kinoshita Y.
Development. p73Guilt by association?
Science
289:
257-258,
2000
241.
Muslin, AJ,
and
Xing H.
14-3-3 Proteins: regulation of subcellular localization by molecular interference.
Cell Signal
12:
703-709,
2000[ISI][Medline].
242.
Nagano, K,
Fukami K,
Minagawa T,
Watanabe Y,
Ozaki C,
and
Takenawa T.
A novel phospholipase C delta4 (PLCdelta4) splice variant as a negative regulator of PLC.
J Biol Chem
274:
2872-2879,
1999
243.
Nakagawa, T,
and
Sheng M.
Neurobiology. A stargazer foretells the way to the synapse.
Science
290:
2270-2271,
2000
244.
Nakashima, N,
Sharma PM,
Imamura T,
Bookstein R,
and
Olefsky JM.
The tumor suppressor PTEN negatively regulates insulin signaling in 3T3-L1 adipocytes.
J Biol Chem
275:
12889-12895,
2000
245.
Nicke, B,
Detjen K,
and
Logsdon CD.
Muscarinic cholinergic receptors activate both inhibitory and stimulatory growth mechanisms in NIH3T3 cells.
J Biol Chem
274:
21701-21706,
1999
246.
Niehrs, C,
and
Pollet N.
Synexpression groups in eukaryotes.
Nature
402:
483-487,
1999[ISI][Medline].
247.
Noselli, S,
and
Perrimon N.
Signal transduction. Are there close encounters between signaling pathways?
Science
290:
68-69,
2000
248.
Nusse, R.
Developmental biology. Making head or tail of Dickkopf.
Nature
411:
255-256,
2001[Medline].
249.
O'Garra, A,
and
Arai N.
The molecular basis of T helper 1 and T helper 2 cell differentiation.
Trends Cell Biol
10:
542-550,
2000[ISI][Medline].
250.
Ohtani, N,
Zebedee Z,
Huot TJ,
Stinson JA,
Sugimoto M,
Ohashi Y,
Sharrocks AD,
Peters G,
and
Hara E.
Opposing effects of Ets and Id proteins on p16INK4a expression during cellular senescence.
Nature
409:
1067-1070,
2001[ISI][Medline].
251.
Olayioye, MA,
Neve RM,
Lane HA,
and
Hynes NE.
The ErbB signaling network: receptor heterodimerization in development and cancer.
EMBO J
19:
3159-3167,
2000
252.
O'Neill, GM,
Fashena SJ,
and
Golemis EA.
Integrin signalling: a new Cas(t) of characters enters the stage.
Trends Cell Biol
10:
111-119,
2000[ISI][Medline].
253.
Orlic, D,
Kajstura J,
Chimenti S,
Jakoniuk I,
Anderson SM,
Li B,
Pickel J,
McKay R,
Nadal-Ginard B,
Bodine DM,
Leri A,
and
Anversa P.
Bone marrow cells regenerate infarcted myocardium.
Nature
410:
701-705,
2001[ISI][Medline].
254.
Ostrom, RS,
Gregorian C,
and
Insel PA.
Cellular release of and response to ATP as key determinants of the set-point of signal transduction pathways.
J Biol Chem
275:
11735-11739,
2000
255.
Ostrom, RS,
Post SR,
and
Insel PA.
Stoichiometry and compartmentation in G protein-coupled receptor signaling: implications for therapeutic interventions involving G(s).
J Pharmacol Exp Ther
294:
407-412,
2000
256.
Ovadi, J,
and
Srere PA.
Macromolecular compartmentation and channeling.
Int Rev Cytol
192:
255-280,
2000[ISI][Medline].
257.
Paramio, JM,
Segrelles C,
Casanova ML,
and
Jorcano JL.
Opposite functions for E2F1 and E2F4 in human epidermal keratinocyte differentiation.
J Biol Chem
275:
41219-41226,
2000
258.
Parrott, JA,
Nilsson E,
Mosher R,
Magrane G,
Albertson D,
Pinkel D,
Gray JW,
and
Skinner MK.
Stromal-epithelial interactions in the progression of ovarian cancer: influence and source of tumor stromal cells.
Mol Cell Endocrinol
175:
29-39,
2001[ISI][Medline].
259.
Pasqualucci, L,
Neumeister P,
Goossens T,
Nanjangud G,
Chaganti RS,
Kuppers R,
and
Dalla-Favera R.
Hypermutation of multiple proto-oncogenes in B-cell diffuse large-cell lymphomas.
Nature
412:
341-346,
2001[ISI][Medline].
260.
Pawson, T,
and
Nash P.
Protein-protein interactions define specificity in signal transduction.
Genes Dev
14:
1027-1047,
2000
261.
Pawson, T,
and
Saxton TM.
Signaling networksdo all roads lead to the same genes?
Cell
97:
675-678,
1999[ISI][Medline].
262.
Peifer, M,
and
Polakis P.
Wnt signaling in oncogenesis and embryogenesisa look outside the nucleus.
Science
287:
1606-1609,
2000
263.
Pesesse, X,
Deleu S,
De Smedt F,
Drayer L,
and
Erneux C.
Identification of a second SH2-domain-containing protein closely related to the phosphatidylinositol polyphosphate 5-phosphatase SHIP.
Biochem Biophys Res Commun
239:
697-700,
1997[ISI][Medline].
264.
Petrone, A,
and
Sap J.
Emerging issues in receptor protein tyrosine phosphatase function: lifting fog or simply shifting?
J Cell Sci
113:
2345-2354,
2000
265.
Pierce, GB,
and
Wallace C.
Differentiation of malignant to benign cells.
Cancer Res
31:
127-134,
1971[ISI][Medline].
266.
Pines, J.
Four-dimensional control of the cell cycle.
Nat Cell Biol
1:
E73-E79,
1999[ISI][Medline].
267.
Pirson, I,
Fortemaison N,
Jacobs C,
Dremier S,
Dumont JE,
and
Maenhaut C.
The visual display of regulatory information and networks.
Trends Cell Biol
10:
404-408,
2000[ISI][Medline].
268.
Polis, GA.
Stability is woven by complex webs.
Nature
395:
744-745,
1998[ISI].
269.
Prenzel, N,
Zwick E,
Daub H,
Leserer M,
Abraham R,
Wallasch C,
and
Ullrich A.
EGF receptor transactivation by G-protein-coupled receptors requires metalloproteinase cleavage of proHB-EGF.
Nature
402:
884-888,
1999[ISI][Medline].
270.
Proost, P,
Struyf S,
Couvreur M,
Lenaerts JP,
Conings R,
Menten P,
Verhaert P,
Wuyts A,
and
Van Damme J.
Posttranslational modifications affect the activity of the human monocyte chemotactic proteins MCP-1 and MCP-2: identification of MCP-2(6-76) as a natural chemokine inhibitor.
J Immunol
160:
4034-4041,
1998
271.
Qiu, W,
Zhuang S,
von Lintig FC,
Boss GR,
and
Pilz RB.
Cell type-specific regulation of B-Raf kinase by cAMP and 14-3-3 proteins.
J Biol Chem
275:
31921-31929,
2000
272.
Qiu, Y,
and
Kung HJ.
Signaling network of the Btk family kinases.
Oncogene
19:
5651-5661,
2000[ISI][Medline].
273.
Raabe, T.
The sevenless signaling pathway: variations of a common theme.
Biochim Biophys Acta
1496:
151-163,
2000[ISI][Medline].
274.
Rabbitts, TH.
Chromosomal translocations in human cancer.
Nature
372:
143-149,
1994[ISI][Medline].
275.
Rane, SG,
and
Reddy EP.
Janus kinases: components of multiple signaling pathways.
Oncogene
19:
5662-5679,
2000[ISI][Medline].
276.
Rastegar, M,
Lemaigre FP,
and
Rousseau GG.
Control of gene expression by growth hormone in liver: key role of a network of transcription factors.
Mol Cell Endocrinol
164:
1-4,
2000[ISI][Medline].
277.
Razin, A.
CpG methylation, chromatin structure and gene silencinga three-way connection.
EMBO J
17:
4905-4908,
1998
278.
Richardson, M,
and
Robishaw JD.
The alpha2A-adrenergic receptor discriminates between Gi heterotrimers of different betagamma subunit composition in Sf9 insect cell membranes.
J Biol Chem
274:
13525-13533,
1999
279.
Robinson, DR,
Wu YM,
and
Lin SF.
The protein tyrosine kinase family of the human genome.
Oncogene
19:
5548-5557,
2000[ISI][Medline].
280.
Robison, GA,
Butcher RW,
and
Sutherland EW.
Cyclic AMP.
Annu Rev Biochem
37:
149-174,
1968[ISI][Medline].
281.
Robker, RL,
and
Richards JS.
Hormonal control of the cell cycle in ovarian cells: proliferation versus differentiation.
Biol Reprod
59:
476-482,
1998
282.
Robker, RL,
and
Richards JS.
Hormone-induced proliferation and differentiation of granulosa cells: a coordinated balance of the cell cycle regulators cyclin D2 and p27Kip1.
Mol Endocrinol
12:
924-940,
1998
283.
Robyr, D,
Wolffe AP,
and
Wahli W.
Nuclear hormone receptor coregulators in action: diversity for shared tasks.
Mol Endocrinol
14:
329-347,
2000
284.
Roesler, WJ.
What is a cAMP response unit?
Mol Cell Endocrinol
162:
1-7,
2000[ISI][Medline].
285.
Roger, PP,
Reuse S,
Maenhaut C,
and
Dumont JE.
Multiple facets of the modulation of growth by cAMP.
Vitam Horm
51:
59-191,
1995[ISI][Medline].
286.
Romanov, SR,
Kozakiewicz BK,
Holst CR,
Stampfer MR,
Haupt LM,
and
Tisty TD.
Normal human mammary epithelial cells spontaneously escape senescence and acquire genomic changes.
Nature
409:
633-637,
2001[ISI][Medline].
287.
Roudabush, FL,
Pierce KL,
Maudsley S,
Khan KD,
and
Luttrell LM.
Transactivation of the EGF receptor mediates IGF-1-stimulated shc phosphorylation and ERK1/2 activation in COS-7 cells.
J Biol Chem
275:
22583-22589,
2000
288.
Rutherford, SL,
and
Lindquist S.
Hsp90 as a capacitor for morphological evolution.
Nature
396:
336-342,
1998[ISI][Medline].
289.
Ryan, KM,
Ernst MK,
Rice NR,
and
Vousden KH.
Role of NF-kappaB in p53-mediated programmed cell death.
Nature
404:
892-897,
2000[ISI][Medline].
290.
Sabatakos, G,
Sims NA,
Chen J,
Aoki K,
Kelz MB,
Amling M,
Bouali Y,
Mukhopadhyay K,
Ford K,
Nestler EJ,
and
Baron R.
Overexpression of DeltaFosB transcription factor(s) increases bone formation and inhibits adipogenesis.
Nat Med
6:
985-990,
2000[ISI][Medline].
291.
Salvatore, D,
Melillo RM,
Monaco C,
Visconti R,
Fenzi G,
Vecchio G,
Fusco A,
and
Santoro M.
Increased in vivo phosphorylation of ret tyrosine 1062 is a potential pathogenetic mechanism of multiple endocrine neoplasia type 2B.
Cancer Res
61:
1426-1431,
2001
292.
Sasaki, T,
Irie-Sasaki J,
Horie Y,
Bachmaier K,
Fata JE,
Li M,
Suzuki A,
Bouchard D,
Ho A,
Redston M,
Gallinger S,
Khokha R,
Mak TW,
Hawkins PT,
Stephens L,
Scherer SW,
Tsao M,
and
Penninger JM.
Colorectal carcinomas in mice lacking the catalytic subunit of PI(3)Kgamma.
Nature
406:
897-902,
2000[ISI][Medline].
293.
Schlessinger, J.
Cell signaling by receptor tyrosine kinases.
Cell
103:
211-225,
2000[ISI][Medline].
294.
Schmidt, M,
Frings M,
Mono ML,
Guo Y,
Weernink PA,
Evellin S,
Han L,
and
Jakobs KH.
G protein-coupled receptor-induced sensitization of phospholipase C stimulation by receptor tyrosine kinases.
J Biol Chem
275:
32603-32610,
2000
295.
Schnurr, M,
Then F,
Galambos P,
Scholz C,
Siegmund B,
Endres S,
and
Eigler A.
Extracellular ATP and TNF-alpha synergize in the activation and maturation of human dendritic cells.
J Immunol
165:
4704-4709,
2000
296.
Schomerus, C,
Korf HW,
Laedtke E,
Weller JL,
and
Klein DC.
Selective adrenergic/cyclic AMP-dependent switch-off of proteasomal proteolysis alone switches on neural signal transduction: an example from the pineal gland.
J Neurochem
75:
2123-2132,
2000[ISI][Medline].
297.
Schubeler, D,
Lorincz MC,
Cimbora DM,
Telling A,
Feng YQ,
Bouhassira EE,
and
Groudine M.
Genomic targeting of methylated DNA: influence of methylation on transcription, replication, chromatin structure, and histone acetylation.
Mol Cell Biol
20:
9103-9112,
2000
298.
Schwartz, J.
Intercellular communication in the anterior pituitary.
Endocr Rev
21:
488-513,
2000
299.
Schwartz, MA,
and
Assoian RK.
Integrins and cell proliferation: regulation of cyclin-dependent kinases via cytoplasmic signaling pathways.
J Cell Sci
114:
2553-2560,
2001[ISI][Medline].
300.
Schwartz, MA,
and
Baron V.
Interactions between mitogenic stimuli, or, a thousand and one connections.
Curr Opin Cell Biol
11:
197-202,
1999[ISI][Medline].
301.
Sellers, LA,
Alderton F,
Carruthers AM,
Schindler M,
and
Humphrey PP.
Receptor isoforms mediate opposing proliferative effects through gbetagamma-activated p38 or Akt pathways.
Mol Cell Biol
20:
5974-5985,
2000
302.
Sexton, PM,
Albiston A,
Morfis M,
and
Tilakaratne N.
Receptor activity modifying proteins.
Cell Signal
13:
73-83,
2001[ISI][Medline].
303.
Shao, J,
Sheng H,
DuBois RN,
and
Beauchamp RD.
Oncogenic Ras-mediated cell growth arrest and apoptosis are associated with increased ubiquitin-dependent cyclin D1 degradation.
J Biol Chem
275:
22916-22924,
2000
304.
Shapiro, L,
and
Losick R.
Dynamic spatial regulation in the bacterial cell.
Cell
100:
89-98,
2000[ISI][Medline].
305.
Sharrocks, AD,
Yang SH,
and
Galanis A.
Docking domains and substrate-specificity determination for MAP kinases.
Trends Biochem Sci
25:
448-453,
2000[ISI][Medline].
306.
Shaywitz, AJ,
and
Greenberg ME.
CREB: a stimulus-induced transcription factor activated by a diverse array of extracellular signals.
Annu Rev Biochem
68:
821-861,
1999[ISI][Medline].
307.
Sheikh, MS,
Huang Y,
Fernandez-Salas EA,
El-Deiry WS,
Friess H,
Amundson S,
Yin J,
Meltzer SJ,
Holbrook NJ,
and
Fornace AJJ
The antiapoptotic decoy receptor TRID/TRAIL-R3 is a p53-regulated DNA damage-inducible gene that is overexpressed in primary tumors of the gastrointestinal tract.
Oncogene
18:
4153-4159,
1999[ISI][Medline].
308.
Shi, YB,
and
Ishizuya-Oka A.
Thyroid hormone regulation of apoptotic tissue remodeling: implications from molecular analysis of amphibian metamorphosis.
Prog Nucleic Acid Res Mol Biol
65:
53-100,
2000[ISI].
309.
Shields, JM,
Pruitt K,
McFall A,
Shaub A,
and
Der CJ.
Understanding Ras: "it ain't over 'til it's over".
Trends Cell Biol
10:
147-154,
2000[ISI][Medline].
310.
Shimamura, R,
Fraizer GC,
Trapman J,
Lau Y,
and
Saunders GF.
The Wilms' tumor gene WT1 can regulate genes involved in sex determination and differentiation: SRY, Mullerian-inhibiting substance, and the androgen receptor.
Clin Cancer Res
3:
2571-2580,
1997[Abstract].
311.
Shymko, RM,
Dumont E,
De Meyts P,
and
Dumont JE.
Timing-dependence of insulin-receptor mitogenic versus metabolic signalling: a plausible model based on coincidence of hormone and effector binding.
Biochem J
339:
675-683,
1999[ISI][Medline].
312.
Siderovski, DP,
Strockbine B,
and
Behe CI.
Whither goest the RGS proteins?
Crit Rev Biochem Mol Biol
34:
215-251,
1999
313.
Simmonds, AJ,
dosSantos G,
Livne-Bar I,
and
Krause HM.
Apical localization of wingless transcripts is required for wingless signaling.
Cell
105:
197-207,
2001[ISI][Medline].
314.
Slack, JM.
Inducing factors in Xenopus early embryos.
Curr Biol
4:
116-126,
1994[ISI][Medline].
315.
Soderling, SH,
and
Beavo JA.
Regulation of cAMP and cGMP signaling: new phosphodiesterases and new functions.
Curr Opin Cell Biol
12:
174-179,
2000[ISI][Medline].
316.
Sonneschein, C,
and
Soto AM.
The Society of Cells. New York: Springer, 1999.
317.
Spitzer, NC,
Lautermilch NJ,
Smith RD,
and
Gomez TM.
Coding of neuronal differentiation by calcium transients.
Bioessays
22:
811-817,
2000[ISI][Medline].
318.
Steinwaerder, DS,
Carlson CA,
Otto DL,
Li ZY,
Ni S,
and
Lieber A.
Tumor-specific gene expression in hepatic metastases by a replication-activated adenovirus vector.
Nat Med
7:
240-243,
2001[ISI][Medline].
319.
Stolz, DB,
and
Jacobson BS.
Macro- and microvascular endothelial cells in vitro: maintenance of biochemical heterogeneity despite loss of ultrastructural characteristics.
In Vitro Cell Dev Biol
27A:
169-182,
1991[ISI].
320.
Strahl, BD,
and
Allis CD.
The language of covalent histone modifications.
Nature
403:
41-45,
2000[ISI][Medline].
321.
Sudol, M.
From Src homology domains to other signaling modules: proposal of the "protein recognition code".
Oncogene
17:
1469-1474,
1998[ISI][Medline].
322.
Sugimoto, M,
Nakamura T,
Ohtani N,
Hampson L,
Hampson IN,
Shimamoto A,
Furuichi Y,
Okumura K,
Niwa S,
Taya Y,
and
Hara E.
Regulation of CDK4 activity by a novel CDK4-binding protein, p34(SEI-1).
Genes Dev
13:
3027-3033,
1999
323.
Sun, Z,
Arendt CW,
Ellmeier W,
Schaeffer EM,
Sunshine MJ,
Gandhi L,
Annes J,
Petrzilka D,
Kupfer A,
Schwartzberg PL,
and
Littman DR.
PKC-theta is required for TCR-induced NF-kappaB activation in mature but not immature T lymphocytes.
Nature
404:
402-407,
2000[ISI][Medline].
324.
Sweeney, C,
and
Carraway KL.
Ligand discrimination by ErbB receptors: differential signaling through differential phosphorylation site usage.
Oncogene
19:
5568-5573,
2000[ISI][Medline].
325.
Talarmin, H,
Rescan C,
Cariou S,
Glaise D,
Zanninelli G,
Bilodeau M,
Loyer P,
Guguen-Guillouzo C,
and
Baffet G.
The mitogen-activated protein kinase kinase/extracellular signal-regulated kinase cascade activation is a key signalling pathway involved in the regulation of G(1) phase progression in proliferating hepatocytes.
Mol Cell Biol
19:
6003-6011,
1999
326.
Tamir, I,
and
Cambier JC.
Antigen receptor signaling: integration of protein tyrosine kinase functions.
Oncogene
17:
1353-1364,
1998[ISI][Medline].
327.
Tanaka, H,
Shibagaki I,
Shimada Y,
Wagata T,
Imamura M,
and
Ishizaki K.
Characterization of p53 gene mutations in esophageal squamous cell carcinoma cell lines: increased frequency and different spectrum of mutations from primary tumors.
Int J Cancer
65:
372-376,
1996[ISI][Medline].
328.
Tata, JR.
Amphibian metamorphosis as a model for studying the developmental actions of thyroid hormone.
Biochimie
81:
359-366,
1999[ISI][Medline].
329.
Thomas, MJ,
and
Seto E.
Unlocking the mechanisms of transcription factor YY1: are chromatin modifying enzymes the key?
Gene
236:
197-208,
1999[ISI][Medline].
330.
Thomas, R.
Laws for the dynamics of regulatory networks.
Int J Dev Biol
42:
479-485,
1998[ISI][Medline].
331.
Thomas, U,
Ebitsch S,
Gorczyca M,
Koh YH,
Hough CD,
Woods D,
Gundelfinger ED,
and
Budnik V.
Synaptic targeting and localization of discs-large is a stepwise process controlled by different domains of the protein.
Curr Biol
10:
1108-1117,
2000[ISI][Medline].
332.
Thomson, S,
Clayton AL,
Hazzalin CA,
Rose S,
Barratt MJ,
and
Mahadevan LC.
The nucleosomal response associated with immediate-early gene induction is mediated via alternative MAP kinase cascades: MSK1 as a potential histone H3/HMG-14 kinase.
EMBO J
18:
4779-4793,
1999
333.
Thorens, B,
Charron MJ,
and
Lodish HF.
Molecular physiology of glucose transporters.
Diabetes Care
13:
209-218,
1990[Abstract].
334.
Thron, CD.
Bistable biochemical switching and the control of the events of the cell cycle.
Oncogene
15:
317-325,
1997[ISI][Medline].
335.
Tong, W,
and
Pollard JW.
Genetic evidence for the interactions of cyclin D1 and p27(Kip1) in mice.
Mol Cell Biol
21:
1319-1328,
2001
336.
Torgersen, KM,
Vaage JT,
Rolstad B,
and
Tasken K.
A soluble LAT deletion mutant inhibits T-cell activation: reduced recruitment of signalling molecules to glycolipid-enriched microdomains.
Cell Signal
13:
213-220,
2001[ISI][Medline].
337.
Traverse, S,
Gomez N,
Paterson H,
Marshall C,
and
Cohen P.
Sustained activation of the mitogen-activated protein (MAP) kinase cascade may be required for differentiation of PC12 cells. Comparison of the effects of nerve growth factor and epidermal growth factor.
Biochem J
288:
351-355,
1992[ISI][Medline].
338.
Tremblay, JJ,
and
Drouin J.
Egr-1 is a downstream effector of GnRH and synergizes by direct interaction with Ptx1 and SF-1 to enhance luteinizing hormone beta gene transcription.
Mol Cell Biol
19:
2567-2576,
1999
339.
Tyler, JK,
and
Kadonaga JT.
The "dark side" of chromatin remodeling: repressive effects on transcription.
Cell
99:
443-446,
1999[ISI][Medline].
340.
Ueyama, T,
Zhu C,
Valenzuela YM,
Suzow JG,
and
Stewart AF.
Identification of the functional domain in the transcription factor RTEF-1 that mediates alpha 1-adrenergic signaling in hypertrophied cardiac myocytes.
J Biol Chem
275:
17476-17480,
2000
341.
Van Cauter, E,
Dhaene M,
and
Dumont JE.
Intracellular mediators in the mechanism of action of hormones or neurotransmitters: reciprocal controls.
Biosystems
9:
23-33,
1977[ISI][Medline].
342.
Van Cauter, E,
and
Dumont JE.
Cross inhibition models for the transmission of hormonal signals.
J Theor Biol
73:
657-677,
1978[ISI][Medline].
343.
Van Cauter, E,
Hardman JG,
and
Dumont JE.
Implications of cross inhibitory interactions of potential mediators of hormone and neurotransmitter action.
Proc Natl Acad Sci USA
73:
2982-2986,
1976[Abstract].
344.
Vanden Berghe, W,
De Bosscher K,
Boone E,
Plaisance S,
and
Haegeman G.
The nuclear factor-kappaB engages CBP/p300 and histone acetyltransferase activity for transcriptional activation of the interleukin-6 gene promoter.
J Biol Chem
274:
32091-32098,
1999
345.
Van Der Hoeven, PC,
Van Der Wal JC,
Ruurs P,
Van Dijk MC,
and
Van Blitterswijk J.
14-3-3 Isotypes facilitate coupling of protein kinase C-zeta to Raf-1: negative regulation by 14-3-3 phosphorylation.
Biochem J
345:
297-306,
2000[ISI][Medline].
346.
Van Goor, F,
Zivadinovic D,
Martinez-Fuentes AJ,
and
Stojilkovic SS.
Dependence of pituitary hormone secretion on the pattern of spontaneous voltage-gated calcium influx. Cell type-specific action potential secretion coupling.
J Biol Chem
276:
33840-33846,
2001
347.
Van Sande, J,
Parma J,
Tonacchera M,
Swillens S,
Dumont JE,
and
Vassart G.
Genetic basis of endocrine disease. Somatic and germline mutations of the TSH receptor gene in thyroid diseases.
J Clin Endocrinol Metab
80:
2577-2585,
1995[ISI][Medline].
348.
Vassart, G,
and
Dumont JE.
The thyrotropin receptor and the regulation of thyrocyte function and growth.
Endocr Rev
13:
596-611,
1992[ISI][Medline].
349.
Veldscholte, J,
Berrevoets CA,
Ris-Stalpers C,
Kuiper GG,
Jenster G,
Trapman J,
Brinkmann AO,
and
Mulder E.
The androgen receptor in LNCaP cells contains a mutation in the ligand binding domain which affects steroid binding characteristics and response to antiandrogens.
J Steroid Biochem Mol Biol
41:
665-669,
1992[ISI][Medline].
350.
Vinos, J,
and
Freeman M.
Evidence that Argos is an antagonistic ligand of the EGF receptor.
Oncogene
19:
3560-3562,
2000[ISI][Medline].
351.
Vogelstein, B,
Lane D,
and
Levine AJ.
Surfing the p53 network.
Nature
408:
307-310,
2000[ISI][Medline].
352.
Wagner, A.
Robustness against mutations in genetic networks of yeast.
Nat Genet
24:
355-361,
2000[ISI][Medline].
353.
Wallberg, AE,
Wright A,
and
Gustafsson JA.
Chromatin-remodeling complexes involved in gene activation by the glucocorticoid receptor.
Vitam Horm
60:
75-122,
2000[ISI][Medline].
354.
Wang, H,
Doronin S,
and
Malbon CC.
Insulin activation of mitogen-activated protein kinases Erk1,2 is amplified via beta-adrenergic receptor expression and requires the integrity of the Tyr350 of the receptor.
J Biol Chem
275:
36086-36093,
2000
355.
Wassarman, DA,
Therrien M,
and
Rubin GM.
The Ras signaling pathway in Drosophila.
Curr Opin Genet Dev
5:
44-50,
1995[Medline].
356.
Webley, K,
Bond JA,
Jones CJ,
Blaydes JP,
Craig A,
Hupp T,
and
Wynford-Thomas D.
Posttranslational modifications of p53 in replicative senescence overlapping but distinct from those induced by DNA damage.
Mol Cell Biol
20:
2803-2808,
2000
357.
Weinstein, IB,
Begemann M,
Ping-Zhou A,
Han EKH,
Sgambato A,
Doki Y,
Arber N,
Ciapparone M,
and
Yamamoto H.
Disorders in cell circuitry associated with multistage carcinogenesis: exploitable targets for cancer prevention and therapy.
Clin Cancer Res
3:
2696-2702,
1997[Abstract].
358.
Werlen, G,
Hausmann B,
and
Palmer E.
A motif in the alphabeta T-cell receptor controls positive selection by modulating ERK activity.
Nature
406:
422-426,
2000[ISI][Medline].
359.
Whitmarsh, AJ,
Cavanagh J,
Tournier C,
Yasuda J,
and
Davis RJ.
A mammalian scaffold complex that selectively mediates MAP kinase activation.
Science
281:
1671-1674,
1998
360.
Whitmarsh, AJ,
and
Davis RJ.
A central control for cell growth.
Nature
403:
255-256,
2000[ISI][Medline].
361.
Wilding, G,
Chen M,
and
Gelmann EP.
Aberrant response in vitro of hormone-responsive prostate cancer cells to antiandrogens.
Prostate
14:
103-115,
1989[ISI][Medline].
362.
Wilkinson, MG,
and
Millar JB.
Control of the eukaryotic cell cycle by MAP kinase signaling pathways.
FASEB J
14:
2147-2157,
2000
363.
Wilusz, CJ,
Wormington M,
and
Peltz SW.
The cap-to-tail guide to mRNA turnover.
Nat Rev Mol Cell Biol
2:
237-246,
2001[ISI][Medline].
364.
Wisden, W,
and
Stephens DN.
Towards better benzodiazepines.
Nature
401:
751-752,
1999[ISI][Medline].
365.
Wolffe, AP,
and
Hansen JC.
Nuclear visions: functional flexibility from structural instability.
Cell
104:
631-634,
2001[ISI][Medline].
366.
Wotton, D,
Lo RS,
Lee S,
and
Massague J.
A Smad transcriptional corepressor.
Cell
97:
29-39,
1999[ISI][Medline].
367.
Wrana, JL.
Regulation of Smad activity.
Cell
100:
189-192,
2000[ISI][Medline].
368.
Wright, PA,
Lemoine NR,
Goretzki PE,
Wyllie FS,
Bond J,
Hughes C,
Roher HD,
Williams ED,
and
Wynford-Thomas D.
Mutation of the p53 gene in a differentiated human thyroid carcinoma cell line, but not in primary thyroid tumours.
Oncogene
6:
1693-1697,
1991[ISI][Medline].
369.
Xie, J,
and
Black DL.
A CaMK IV responsive RNA element mediates depolarization-induced alternative splicing of ion channels.
Nature
410:
936-939,
2001[ISI][Medline].
370.
Yaffe, MB,
and
Cantley LC.
Signal transduction. Grabbing phosphoproteins.
Nature
402:
30-31,
1999[ISI][Medline].
371.
Yamamoto, KR,
Darimont BD,
Wagner RL,
and
Iniguez-Lluhi JA.
Building transcriptional regulatory complexes: signals and surfaces.
Cold Spring Harb Symp Quant Biol
63:
587-598,
1998[ISI][Medline].
372.
Yan, SF,
Fujita T,
Lu J,
Okada K,
Shan ZY,
Mackman N,
Pinsky DJ,
and
Stern DM.
Egr-1, a master switch coordinating upregulation of divergent gene families underlying ischemic stress.
Nat Med
6:
1355-1361,
2000[ISI][Medline].
373.
Yarwood, SJ,
and
Woodgett JR.
Extracellular matrix composition determines the transcriptional response to epidermal growth factor receptor activation.
Proc Natl Acad Sci USA
98:
4472-4477,
2001
374.
Yeung, K,
Seitz T,
Li S,
Janosch P,
McFerran B,
Kaiser C,
Fee F,
Katsanakis KD,
Rose DW,
Mischak H,
Sedivy JM,
and
Kolch W.
Suppression of Raf-1 kinase activity and MAP kinase signalling by RKIP.
Nature
401:
173-177,
1999[ISI][Medline].
375.
Yu, Q,
Geng Y,
and
Sicinski P.
Specific protection against breast cancers by cyclin D1 ablation.
Nature
411:
1017-1021,
2001[ISI][Medline].
376.
Zhang, P.
The cell cycle and development: redundant roles of cell cycle regulators.
Curr Opin Cell Biol
11:
655-662,
1999[ISI][Medline].
377.
Zhao, XY,
Malloy PJ,
Krishnan AV,
Swami S,
Navone NM,
Peehl DM,
and
Feldman D.
Glucocorticoids can promote androgen-independent growth of prostate cancer cells through a mutated androgen receptor.
Nat Med
6:
703-706,
2000[ISI][Medline].
378.
Zheng, JQ.
Turning of nerve growth cones induced by localized increases in intracellular calcium ions.
Nature
403:
89-93,
2000[ISI][Medline].
379.
Zhou, H,
Lin A,
Gu Z,
Chen S,
Park NH,
and
Chiu R.
12-O-tetradecanoylphorbol-13-acetate (TPA)-induced c-Jun N-terminal kinase (JNK) phosphatase renders immortalized or transformed epithelial cells refractory to TPA-inducible JNK activity.
J Biol Chem
275:
22868-22875,
2000
380.
Zhou, S,
Carraway KL,
Eck MJ,
Harrison SC,
Feldman RA,
Mohammadi M,
Schlessinger J,
Hubbard SR,
Smith DP,
and
Eng C.
Catalytic specificity of protein-tyrosine kinases is critical for selective signalling.
Nature
373:
536-539,
1995[ISI][Medline].
381.
Zhou, XP,
Gimm O,
Hampel H,
Niemann T,
Walker MJ,
and
Eng C.
Epigenetic PTEN silencing in malignant melanomas without PTEN mutation.
Am J Pathol
157:
1123-1128,
2000
382.
Zwick, E,
Hackel PO,
Prenzel N,
and
Ullrich A.
The EGF receptor as central transducer of heterologous signalling systems.
Trends Pharmacol Sci
20:
408-412,
1999[ISI][Medline].