Departments of Medicine and Physiology, Gastrointestinal Division, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() |
---|
Signal transduction in epithelial cells adds another level of complexity to the signaling that occurs in symmetrical cells, in the form of the need to coordinate and keep separate signals at the apical and basolateral membranes. Regulation by protein kinases of ileal NaCl absorption and its component brush-border Na+/H+ exchanger are used as an example of how signaling in epithelial cells must deal with spatial localization of signals, protein-protein interactions, signaling molecules, and the involvement of the transport protein being regulated in collecting and focusing the signals generated at the receptor and beyond.
intestinal epithelial cells; epidermal growth factor; carbachol; NHE3; signalosomes
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() |
---|
WE RELATE SEVERAL RECENT ADVANCES in the understanding of signal transduction to the specific challenges of signaling in epithelial cells. The advances reviewed here are related to 1) spatial localization of signaling proteins, 2) protein-protein interactions, 3) involvement of signalosomes (23) or signaling complexes, and 4) cross talk between G protein-linked and tyrosine kinase signaling pathways. Although the same ligands, receptors, and signaling proteins appear to be involved in signaling in epithelial and nonepithelial cells, an added complexity in epithelia is the separation of the epithelial cell plasma membrane into two domains, the brush-border and basolateral membranes. For those conditions in which the receptors, effectors, and targets are not located in the same membrane domain, there are the added complexities of 1) maintaining the domains separate, 2) ways for the domains to communicate, and 3) ways for the regulated process to focus the various components of the signals generated from the receptors.
![]() |
LOCATION OF TARGET: REGULATION OF NEUTRAL NACL ABSORPTION |
---|
The process used for illustration is the neutral NaCl-linked absorptive
process in the mammalian ileum and ascending colon (7). This process is
made up of a brush-border
Na+/H+
exchanger linked to an anion exchanger (functional
Cl/HCO
3
exchanger). The latter was previously thought to be a member of the AE
gene family and now may be the gene product of the DRA
(downregulated in adenoma) gene (8a). The brush-border
Na+/H+
exchangers are members of the mammalian
Na+/H+
exchanger gene family, of which at least six isoforms have now been
identified (6, 18, 19). Two of these, NHE2 and NHE3, are present in the
brush border of ileum and ascending colon of all species studied,
including the human, dog, rabbit, rat, mouse, and chicken (9). NHE3
appears to be the major functional brush-border isoform in mammalian
intestine. However, although only partially defined, the role of NHE2
seems to vary among species. For instance, in rabbit ileum, 50% of
basal
Na+/H+
exchange is due to NHE2 and 50% to NHE3 (24), whereas in the dog all
basal
Na+/H+
exchange is due to NHE3. In contrast, in the chicken, NHE2 makes up
85% of basal colonic
Na+/H+
exchange and all of the increase induced by elevating aldosterone (4).
Neutral NaCl absorption is important because it is
1) the major mechanism for ileal
Na+ absorption during the period
between meals, 2) accounts for the entire increase in canine ileal
Na+ absorption occuring in the
postprandial state, and 3) is the single transport process that becomes abnormally inhibited in most
diarrheal diseases. Short-term regulation of NaCl absorption by growth
factors and protein kinases, the topic of this review, appears to be
exerted by an effect on the
Na+/H+
exchanger component of neutral NaCl absorption. However, there has been
no detailed study of direct effects on the
Cl
/HCO
3
exchanger. Both neutral NaCl absorption and brush-border
Na+/H+
exchange are unusual for transport processes in that they are not
turned off under basal conditions. Rather, both neutral NaCl absorption
and brush-border
Na+/H+
exchange are set in a partially activated state. This allows rapid
stimulation and inhibition similar to that occurring in the
postprandial state (7). Although this was initially thought to be due
to the presence of a myriad of neurohumoral-paracrine mediators in the
normal intestine with different mixes of these mediators released in
the basal and postprandial state, it now appears that this partial
stimulation also occurs when NHE3 is expressed in the PS120 fibroblast
and the human colon cancer cell line Caco-2. For instance, in ileal
brush border, Caco-2 cells, and PS120 cells,
1)
Ca2+-calmodulin and
Ca2+-calmodulin kinase II inhibit
NaCl absorption and/or NHE3 on the apical membrane under basal
conditions, 2) tyrosine kinases
stimulate NaCl absorption and NHE3, and
3) serine/threonine phosphatases 1 and 2A (okadaic acid sensitive) inhibit NaCl absorption and NHE3 (3,
16). Whether these effects are exerted directly on NHE3 or on NHE3
regulatory proteins is not known. Relevant to the kinase regulation of
NHE3 expressed in PS120 cells and on the apical surface of Caco-2 cells
and rabbit ileum, protein kinase C (PKC) inhibits, whereas fibroblast
growth factor (FGF) or epidermal growth factor (EGF) stimulates, NHE3
(7, 15). This regulation involves changes in the maximal velocity
(Vmax) of NHE3
and, at least for EGF, FGF, and PKC, does not appear to involve changes
in the
K'(H+i)
(6, 15, 16). In contrast, the housekeeping isoform NHE1 is regulated by
kinases through changes in
K'(H+)i
(15). Changes in
Vmax could be due
to changes either in the amount of NHE3 on the plasma membrane, in the
turnover number of each NHE3 on the plasma membrane, or both. On the
basis of cell surface biotinylation studies of NHE3 expressed in PS120 fibroblasts and confocal microscopy studies of NHE3 in Caco-2 cells,
most regulation appears to be due to changes in both the amount of
plasma membrane NHE3 and in the NHE3 turnover number (10).
![]() |
REGULATORY MECHANISMS |
---|
In this review, two examples are used in which activation of basolateral membrane receptors, one G protein-coupled receptor and one tyrosine kinase receptor, alters apical membrane NaCl absorption and brush-border Na+/H+ exchange. 1) The cholinergic agonist carbachol binds to M3 muscarinic cholinergic receptors to inhibit NaCl absorption. 2) EGF binds to the EGF receptor (EGFR) in the basolateral membrane to stimulate NaCl absorption and increase brush-border Na+/H+ exchange. These examples were chosen to illustrate that separate signal transduction steps occur in the basolateral and apical membranes and therefore both membranes must be studied to define the signaling events. We refer to this as the asymmetrical aspect of signal transduction. Furthermore, these models were chosen because one stimulates (EGF) and one inhibits (carbachol) ileal NaCl absorption and brush-border Na+/H+ exchange, yet both involve similar classes of intermediate and cross talk of tyrosine kinase and G protein-coupled signaling.
![]() |
LOCALIZATION OF THE SIGNAL |
---|
Carbachol/Ca2+ signal in ileum (Fig. 1). As in most epithelial cells, carbachol binds to muscarinic cholinergic receptors, which are localized to the basolateral membrane. Although only partially defined, these appear to be M3 receptors on the basis of pharmacological and molecular classification. When activated, these receptors lead to a biphasic increase in intracellular free Ca2+ ([Ca2+]i). In spite of the lack of apical membrane carbachol receptors, the elevation of [Ca2+]i occurs first at the apical membrane, within 1 s of addition of ligand, and sweeps as waves from the apical to the basolateral surface. These waves occur with a near-maximal agonist concentration, but at lower concentrations the [Ca2+]i elevation remains restricted to the apical pole. This phenomenon is best characterized in pancreatic acinar cells and hepatocytes but has also been shown to occur in lacrimal gland epithelial cells, Madin-Darby canine kidney (MDCK) cells, and in a human colon cancer cell line, HT-29 (11). It appears that [Ca2+]i is elevated close to the apical membrane, where it acts to regulate NaCl absorption and brush-border Na+/H+ exchange; the additional increase in [Ca2+]i near the basolateral membrane probably regulates K+ channels. Ca2+ is known to be a poorly diffusible messenger (1). Baker showed in the 1980s that injection of boluses of Ca2+ into squid axon diffused little, and Allbritton et al. (1) demonstrated that the free Ca2+ diffusion length was less than 0.1 µm. This distance is too small to transmit a signal from the basolateral membrane to the apical membrane, given that intestinal cells are 20-35 µm in length.
Several adaptations have been identified in ileal cells that focus the cholinergic elevation of [Ca2+]i at the apical membrane. The cholinergic receptor-linked increase in [Ca2+]i involves a phosphatidylinositol (PI)-specific phospholipase C (PLC). There is an unusual asymmetrical aspect of carbachol signaling through PLC. In most cells there appears to be direct coupling of the M3 receptor and PI-specific PLC-EGF/tyrosine kinase receptor (Fig. 1).
In the adult intestine the EGFR is localized to the basolateral
membrane (22). Caco-2 cells grown as polarized cells have only a small
fraction of EGFR on the apical membrane. In Caco-2 cells, the normal
ligand for the EGFR is unknown; some Caco-2 cells make amphiregulin and
others make transforming growth factor-. It has not yet been
established whether EGF is physiologically involved in short-term
regulation of NaCl absorption. However, EGF at doses present in serum
acutely stimulates NaCl absorption. This effect occurs quickly,
reaching a maximum in 1-15 min, and is associated with an increase
in NHE3 Vmax. EGF
stimulation of NaCl absorption and brush-border
Na+/H+
exchange can be demonstrated in intact ileal tissue, brush-border vesicles made from ileal villus cells exposed in vitro to EGF, and NHE3
expressed on the apical surface of Caco-2 cells. This short-term EGF
stimulation of brush-border
Na+/H+
exchange occurs specifically, with no accompanying increase in Na+-dependent
D-glucose absorption over ~30
min in ileum or Caco-2 cells. However, longer exposure to EGF is
associated with an increase in
D-glucose uptake and in the size
of the ileal brush border. The EGF effects on brush-border NHE3
asymmetrically involve at least two signaling molecules at the brush
border, PI 3-kinase and c-Src. The EGF stimulation of brush-border
Na+/H+
exchange can be prevented by apical membrane exposure of ileum or
Caco-2 cells to the PI 3-kinase inhibitor wortmannin (13). In addition,
brush-border PI 3-kinase activity and amount and tyrosine
phosphorylation of the p85 subunit increase within 1 min of exposure to
EGF. These changes parallel the EGF stimulation of brush-border
Na+/H+
exchange activity, and wortmannin added to the apical membrane prevents
the EGF stimulation of
Na+/H+
exchange as well as the changes in the brush-border PI 3-kinase. In
ileal cells, although there is basolateral PI 3-kinase activity under
basal conditions, no change occurs with EGF exposure (13). Apical
membrane tyrosine kinases are involved in the increase in PI 3-kinase
at the apical membrane. Addition of the tyrosine kinase inhibitors
tyrphostin-23 and genistein to the apical membrane of ileal cells and
Caco-2 cells prevented the EGF-induced increase in PI 3-kinase activity
in Caco-2 cells and the increase in activity of PI 3-kinase and
tyrosine phosphorylation in ileal brush border. This effect occurred
when genistein was added to ileal brush-border vesicles that had been
exposed in vitro to EGF and had increased brush-border
Na+/H+
exchange. However, addition of wortmannin to these vesicles did not
reverse the EGF stimulation of brush-border
Na+/H+
exchange, suggesting that the PI 3-kinase effect had already occurred,
for instance, in delivering the NHE3 to the apical membrane.
![]() |
CROSSOVER BETWEEN SIGNALING BY G PROTEIN-COUPLED RECEPTORS AND TYROSINE KINASE RECEPTORS |
---|
Although initially thought to be entirely separate signaling pathways, crossover of tyrosine kinase and G protein-related signaling is now known to be a common event in signaling pathways. This adds a great deal of specificity to signaling based on integration of multiple components in an individual signaling pathway. Such crossover is recognized to occur in epithelial cells (2).
In the EGF signaling described above, there is stimulation of both
serine/threonine kinases and tyrosine kinases in the brush border. In
the brush border, c-Src is activated by EGF binding to the basolateral
membrane, but there is also stimulation of the serine/threonine kinases
Akt and PKC- at the brush border (M. Donowitz, C. Shih, and S. Khurana, unpublished observations). Thus crossover of EGFR
to serine/threonine kinases is not strictly a switch to
serine/threonine signaling at the apical membrane.
Carbachol signaling involves both serine/threonine phosphorylation and tyrosine kinase activation in the apical membrane, as occurs with EGFR signaling. Carbachol binding to basolateral membrane receptors is associated with tyrosine kinase-dependent activation of PI-PLC and PLD in the brush border and activation of PKC as well. Which isoform of PKC is involved and whether its activation is dependent on tyrosine kinase have not been established, but the latter would be likely, since PKC activation is dependent on phospholipase activation (Fig. 1). PKC activation in the brush border is required for the inhibition of NHE3 by carbachol, and this effect occurs at the level of the brush border.
|
![]() |
PROTEIN-PROTEIN INTERACTIONS |
---|
A major advance in understanding signal transduction is the recognition
of the common involvement of physical attachment of proteins that
occurs as a part of signaling, which to a significant extent occurs by
means of interaction of specific domains (20). These include SH2, SH3,
WW, PDZ, PTB, 143
3, and PH domains. Interacting
proteins form collections of signaling molecules called signalosomes
(23), which accumulate at both tyrosine kinase receptors and at
seven-membrane-spanning domain/G protein-coupled receptors as part of
signaling initiation. Various approaches have proven sensitive enough
to at least suggest which molecules interact. These include yeast
two-hybrid screening and other interactive cloning approaches,
coprecipitations, and overlay techniques. In epithelial cells, these
approaches have demonstrated that signaling complexes also exist at the
brush border.
The presence of protein-protein interactions in signaling has provided some insight into a confusing aspect of studies of cloned Na+/H+ exchangers in model cell systems. Many aspects of NHE3 regulation in ileal brush border were similar in PS120 cells and Caco-2 cells. One exception is cAMP inhibition of ileal Na+/H+ exchange, because in PS120 cells and Caco-2 cells NHE3 is not regulated by cAMP. In another type of fibroblast, AP1 cells, cAMP does inhibit NHE3. At least part of this cell-specific response of NHE3 to cAMP appears to be due to two related proteins, E3KARP (NHE3 kinase A regulatory protein) and NHERF (Na+/H+ exchanger regulatory factor). These related proteins each have two PDZ domains, which are highly homologous to each other and are also highly conserved between E3KARP and NHERF. When transfected into PS120 cells, E3KARP and NHERF reconstitute cAMP inhibition of NHE3, establishing a role for these proteins in cAMP regulation (26). NHERF is a phosphoprotein under basal conditions, whereas E3KARP is not phosphorylated under basal conditions (G. Lamprecht and C. Yun, unpublished data), suggesting that their phosphorylation is not required for cAMP inhibition of NHE3. The human analog of NHERF, EBP50, binds ezrin, a protein that links the brush border to the actin cytoskeletal core, suggesting that a potential role of these proteins is to link transport proteins to cytoskeleton (21). This may serve to allow formation of a signaling complex that involves the transport protein being regulated. The role of these molecules in signaling has just begun to be understood, and our suggestion that they have a role in the formation of signalosomes is a hypothesis. In recognition of the fact that these proteins are involved in regulation of far more than Na+/H+ exchanges, we refer to them as the Regulatory Factor gene family.
The role of protein-protein interactions in signaling in general and in different locations in epithelial cells has just begun to be understood. For instance, there is very little information regarding the signaling complexes that involve the junctional complex, although these signaling complexes are known to be rich in tyrosine kinases.
![]() |
INVOLVEMENT OF THE TRANSPORT PROTEIN BEING REGULATED IN SIGNALING COMPLEXES |
---|
Signals initiated at plasma membrane receptors are focused at the
receptors by protein-protein interactions that are initiated by parts
of the receptor (20). Examples include the tyrosine kinase receptors,
in which a phosphorylated tyrosine in the cytoplasmic domain of the
receptor plus the three to five 3' amino acids
selectively bind SH2 domains, and perhaps a comparable sequence of
events in G protein-coupled receptors, in which phosphoserines of the receptor selectively bind 143
3 proteins. What serves
to focus the signal at the apical membrane in an epithelial cell? The
evidence we have generated suggests the presence of apical membrane
signaling complexes, but it seems very likely that, as occurs in
association with receptors, the signals are focused. We hypothesize
that one way that the signals are accumulated is that the proteins
being regulated focus the signals and signaling complexes. We use NHE3 as an example of a transport protein that is regulated in the brush
border and appears to focus the signal on itself. The COOH-terminal ~300 amino acids of NHE3 are cytoplasmic and are required for NHE3
regulation by growth factors, protein kinases, and changes in
osmolarity (6, 16). The COOH terminus of NHE3 is organized into a
series of domains, with part of the COOH terminus required for
stimulation and part for inhibition (Fig.
2). NHE1 and NHE3 are regulated partially
by changes in their phosphorylation, but although they are
phosphoproteins in the basal state, some kinase/growth factor
regulation is not associated with changes in NHE phosphorylation (6,
18, 25). It appears that some of the regulatory subdomains in the NHE
COOH termini act as hooks or scaffolds to interact with associated
regulatory proteins. Some of these associated regulatory proteins help
change the NHE phosphorylation, whereas others appear to regulate the
NHEs in different ways, including the formation of signaling complexes.
An example includes the regulatory factors E3KARP and NHERF, in their
reconstitution of cAMP regulation of NHE3. E3KARP binds to NHE3 close
to the domain necessary for cAMP inhibition (Fig. 2; C. Yun and G. Lamprecht, unpublished observations). NHE3 phosphorylation is increased
by cAMP on amino acid Ser605 (14). NHE3 is not
phosphorylated by the regulatory factors, since they are not protein
kinases. As a hypothesis, we suggest that the regulatory factors either
change the conformation of the COOH terminus of NHE3 or change the
location of NHE3, perhaps clustering the molecule in the plasma
membrane, to make it accessible to protein kinase A. One likely protein
involved is the cytoskeletal linking protein ezrin, which has recently
been shown to bind NHERF and is known to bind protein kinase A (21). We
suggest the involvement of a complex, possibly formed between ezrin
with the attached protein kinase A (20), E3KARP or NHERF, and NHE3
(Fig. 2). A second regulatory protein that binds to the COOH terminus
of NHE3 is calmodulin (3). Calmodulin appears to be involved in the regulation of NHE1, NHE2, and NHE3 (5, 6, 8, 14). Calmodulin binds to
the COOH-terminal 85 amino acids of NHE3, and inhibitors of calmodulin
(W13) and calmodulin kinase II (KN-62) stimulate NHE3. The effects of
W13 and KN-62 require the COOH-terminal 76 amino acids of NHE3 (5, 16).
How calmodulin regulates NHE3 and whether a signaling complex forms
around the calmodulin is not known. However, the fact that two
signaling molecules have been shown to bind to the COOH terminus of
NHE3 (Fig. 2) suggests that signaling complexes may form around
specific domains of NHE3 and that NHE3 might focus regulation by acting
as a scaffold for signaling proteins.
|
![]() |
ACKNOWLEDGEMENTS |
---|
This study was supported in part by National Institutes of Health Grants RO1-DK-26523, PO1-DK-44484, RO1-DK-51116, American Digestive Health Foundation/American Gastroenterological Association/Hoechst Marion Rousel Research Scholar Award, and the Meyerhoff Digestive Diseases Center.
![]() |
FOOTNOTES |
---|
* Third in a series of invited articles on G Protein-Coupled Receptors in Gastrointestinal Physiology.
Of the many studies quoted, only selected references were listed due to space limitations.
Address for reprint requests: M. Donowitz, Johns Hopkins Univ. School of Medicine, 925 Ross Research Bldg., 720 Rutland Ave., Baltimore, MD 21205-2195.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() |
---|
1.
Allbritton, N. L.,
T. Meyer,
and
L. Stryer.
Regulation of the messenger action of calcium ion and inositol 1,4,5-trisphosphate.
Science
258:
1812-1815,
1992[Medline].
2.
Barrett, K. E.
Integrated regulation of intestinal epithelial transport: intercellular and intracellular pathways (Bowditch lecture).
Am. J. Physiol.
272 (Cell Physiol. 41):
C1069-C1076,
1997
3.
Cohen, M. E.,
L. Reinlib,
A. J. Watson,
F. Gorelick,
K. Rys-Sikora,
M. Tse,
R. P. Rood,
A. J. Czernik,
G. W. Sharp,
and
M. Donowitz.
Rabbit ileal villus cell brush border Na+/H+ exchange is regulated by Ca2+/calmodulin-dependent protein kinase II, a brush border membrane protein.
Proc. Natl. Acad. Sci. USA
87:
8990-8994,
1990[Abstract].
4.
Donowitz, M., C. De La Horra, M. L. Calonge, I. S. Wood, J. Dyer, S. M. Gribble, F. Sanchez, C. M. Tse, S. P. Shirazi-Beechey, and A. A. Ilundain. In birds NHE2 is the major brush border Na/H exchanger
in the colon and is increased by a low NaCl diet.
Am. J. Physiol. In press.
5.
Donowitz, M.,
R. Kambadur,
M. Zizak,
S. Nath,
S. Akhter,
and
M. Tse.
NHE3 is a calmodulin binding protein: calmodulin inhibits NHE3 by binding to the C-terminal 76 amino acids of NHE3 (Abstract).
Gastroenterology
112:
A360,
1997.
6.
Donowitz, M.,
S. A. Levine,
C. H. Yun,
S. R. Brant,
S. Nath,
J. Yip,
S. Hoogerwerf,
J. Pouyssegur,
and
C. M. Tse.
Molecular studies of members of the mammalian Na+/H+ exchanger gene family.
In: Molecular Biology of the Membrane Transport Disorders, edited by S. G. Schultz,
T. E. Andreoli,
A. M. Brown,
D. M. Fambrough,
J. F. Hoffman,
and M. J. Welsh. New York: Plenum, 1996, p. 259-275.
7.
Donowitz, M.,
and
M. J. Welsh.
Regulation of mammalian small intestinal electrolyte secretion.
In: Physiology of the Gastrointestinal Tract (3rd ed.), edited by L. R. Johnson. New York: Raven, 1987, p. 1351-1388.
8.
Dransfield, D. T.,
A. J. Bradford,
J. Smith,
M. Martin,
C. Roy,
P. H. Mangent,
and
J. R. Goldenring.
Ezrin is a cyclic AMP-dependent protein kinase anchoring protein.
EMBO J.
16:
35-43,
1997
8a.
Hoglund, P.,
S. Haila,
J. Socha,
L. Tomaszewski,
U. Saarialho-Kere,
M. L. Karjalainen-Lindsberg,
K. Airola,
C. Holmberg,
A. de la Chapelle,
and
J. Kere.
Mutations of the down-regulated in adenoma (DRA) gene cause congenital chloride diarrhoea.
Nat. Genet.
14:
316-319,
1996[Medline].
9.
Hoogerwerf, W. A.,
S. C. Tsao,
O. Devuyst,
S. A. Levine,
C. H. Yun,
J. W. Yip,
M. E. Cohen,
P. D. Wilson,
A. J. Lazenby,
C. M. Tse,
and
M. Donowitz.
NHE2 and NHE3 are human and rabbit intestinal brush-border proteins.
Am. J. Physiol.
270 (Gastrointest. Liver Physiol. 33):
G29-G41,
1996
10.
Janecki, A. J., M. H. Montrose, P. Zimniak,
A. Zweibaum, C. M. Tse, S. Khurana, and M. Donowitz.
Subcellular redistribution is involved in acute regulation of the
brush border
Na+/H+
exchanger NHE3 in human colon adenocarcinoma cell line Caco-2: protein
kinase C-mediated inhibition of the exchanger. J. Biol. Chem. In press.
11.
Kasai, H.,
and
O. H. Peterson.
Spatial dynamics of second messengers: IP3 and cAMP as long-range and associative messengers.
Trends Neurosci.
17:
95-101,
1994[Medline].
12.
Khurana, S.,
S. Kreydiyyeh,
A. Aronzon,
W. A. Hoogerwerf,
S. G. Rhee,
M. Donowitz,
and
M. E. Cohen.
Asymmetric signal transduction in polarized ileal Na(+)-absorbing cells: carbachol activates brush-border but not basolateral-membrane PIP2-PLC and translocates PLC-gamma1 only to the brush border.
Biochem. J.
313:
509-518,
1996[Medline].
13.
Khurana, S.,
S. K. Nath,
S. A. Levine,
J. M. Bowser,
C. M. Tse,
M. E. Cohen,
and
M. Donowitz.
Brush border phosphatidylinositol 3-kinase mediates epidermal growth factor stimulation of intestinal NaCl absorption and Na+/H+ exchange.
J. Biol. Chem.
271:
9919-9927,
1996
14.
Kurashima, K.,
F. H. Yu,
A. G. Cabado,
E. Z. Szabo,
S. Grinstein,
and
J. Orlowski.
Identification of sites required for down-regulation of Na+/H+ exchanger NHE3 activity by cAMP-dependent protein kinase.
J. Biol. Chem.
272:
28672-28679,
1997
15.
Levine, S. A.,
M. H. Montrose,
C. M. Tse,
and
M. Donowitz.
Kinetics and regulation of three cloned mammalian Na+/H+ exchangers stably expressed in a fibroblast cell line.
J. Biol. Chem.
268:
25527-25535,
1993
16.
Levine, S. A.,
S. K. Nath,
C. H. Yun,
J. W. Yip,
M. Montrose,
M. Donowitz,
and
C. M. Tse.
Separate C-terminal domains of the epithelial specific brush border Na+/H+ exchanger isoform NHE3 are involved in stimulation and inhibition by protein kinases/growth factors.
J. Biol. Chem.
270:
13716-13725,
1995
17.
Nathanson, M. H.,
M. B. Fallon,
P. J. Padfield,
and
A. R. Morento.
Localization of the type 3 inositol 1,4,5-trisphosphate receptor in the Ca2+ wave trigger zone of pancreatic acinar cells.
J. Biol. Chem.
269:
4693-4696,
1994
18.
Noel, J.,
and
J. Pouyssegur.
Hormonal regulation, pharmacology, and membrane sorting of vertebrate Na+/H+ exchanger isoforms.
Am. J. Physiol.
268 (Cell Physiol. 37):
C283-C296,
1995
19.
Orlowski, J.,
and
S. Grinstein.
Na+/H+ exchangers of mammalian cells.
J. Biol. Chem.
272:
22373-22376,
1997
20.
Pawson, R.,
and
J. D. Scott.
Signaling through scaffold, anchoring and adaptor proteins.
Science
278:
2075-2080,
1977
21.
Reczek, D.,
M. Berryman,
and
A. Bretscher.
Identification of EBP50: a PDZ-containing phosphoprotein that associates with members of the Ezrin-Radixin-Moesin family.
J. Cell Biol.
139:
169-179,
1997
22.
Scheving, L. A.,
R. A. Shiurba,
T. D. Nguyen,
and
G. M. Gray.
Epidermal growth factor receptor of the intestinal enterocyte.
J. Biol. Chem.
264:
1735-1741,
1989
23.
Stancato, L. F.,
A. M. Silverstein,
J. K. Owens-Grillo,
Y. H. Chow,
R. Jove,
and
W. B. Pratt.
The hsp90-binding antibiotic geldanamycin decreases raf levels and epidermal growth factor signaling without disrupting formation of signaling complexes or reducing the specific enzymatic activity of raf kinase.
J. Biol. Chem.
272:
4013-4020,
1997
24.
Wormmeester, L.,
F. Sanchez de Medina,
F. Kokke,
C. M. Tse,
S. Khurana,
J. Bowser,
M. E. Cohen,
and
M. Donowitz.
Quantitative contribution of NHE2 and NHE3 to rabbit ileal brush-border Na+/H+ exchange.
Am. J. Physiol.
274 (Cell Physiol. 43):
C1261-C1272,
1998
25.
Yip, J. W.,
W. H. Ko,
G. Viberti,
R. L. Huganir,
M. Donowitz,
and
C. M. Tse.
Regulation of the epithelial brush border Na/H exchanger isoform 3 stably expressed in fibroblasts by fibroblast growth factor and phorbol esters is not through changes in phosphorylation of the exchange.
J. Biol. Chem.
272:
18473-18480,
1997
26.
Yun, C. H.,
S. Oh,
M. Zizak,
D. Steplock,
S. Tsao,
C. M. Tse,
E. J. Weinman,
and
M. Donowitz.
cAMP-mediated inhibition of the epithelial brush border Na+/H+ exchanger, NHE3, requires an associated regulatory protein.
Proc. Natl. Acad. Sci. USA
94:
3010-3015,
1997