From the Departments of Pharmacology and ¶ Cell Biology and Neuroscience, The University of Texas Southwestern Medical Center, Dallas, Texas 75390-9041 and the § Department of Biological Research-Oncology, Schering-Plough Research Institute, Kenilworth, New Jersey 07033
Received for publication, October 25, 2000, and in revised form, December 1, 2000
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ABSTRACT |
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We have previously demonstrated an
involvement of MEK5 and ERK5 in RafBXB-stimulated focus
formation in NIH3T3 cells. We find here that MEK5 and ERK5 cooperate
with the RafBXB effectors MEK1/2 and ERK1/2 to induce foci. To
further understand MEK5-ERK5-dependent signaling, we
examined potential MEK5-ERK5 effectors that might influence
focus-forming activity. Consistent with results from our
focus-formation assays, constitutively active variants of MEK5 and MEK1
synergize to activate NF- The transmission of extracellular stimuli to the nucleus is a
complex process that often involves the coordinated activation of one
or more three-kinase pathways, known as
MAP1 kinase cascades (1, 2).
These cascades consist of a MEK kinase, or MEKK, that phosphorylates
and activates a MAP/ERK kinase, or MEK. The dual specificity MEKs
phosphorylate MAP kinases on tyrosine and threonine residues, which
substantially increases the activity of the MAP kinases. The active MAP
kinases subsequently phosphorylate transcription factors, downstream
kinases, and other substrates.
The currently known members of the MAP kinase family include ERKs 1, 2, 3, 5, and 7, and multiple c-Jun NH2-terminal kinase and p38
isoforms. ERK5, with its 816 amino acids, is close to twice the size of
many of the other MAP kinases (3, 4). The difference in size is because
of a stretch of ~400 amino acids C-terminal to the kinase domain that
do not display any sequence similarity to known proteins and have
no known function. On the other hand, the catalytic domain, located at
its N terminus, is ~50% identical to ERK2.
ERK5 is regulated by a wide variety of mitogens and cell stresses by
mechanisms that sometimes involve Ras or Src (5-12). ERK5
activity appears to be influenced by the MEK kinases MEKK3 and Tpl-2
(13, 14). Supporting one of these connections, overexpressed MEKK3
immunoprecipitated from EGF-stimulated 293 cells phosphorylates MEK5, the only known ERK5 kinase (13), in vitro.
Although its activity is not sufficient to activate ERK5, Raf-1 appears
to be involved in regulation of ERK5 by oncogenic Ras in 293 cells (7,
8).
Several activators of ERK5 have been identified; however, less is known
about its downstream effectors. In vitro ERK5 substrates include the MADS box transcription factors myocyte
enhancer factor 2A, C, and D (10, 13, 14) and the Ets class
transcription factor Sap1a (9). The ability of ERK5 to activate MEF2
isoforms appears to allow it to positively regulate intracellular
concentrations of c-Jun (14).
We have previously shown that increasing the activity of the ERK5
pathway by expression of MEK5DD, an activated mutant of MEK5, in NIH3T3
cells is not sufficient to stimulate the formation of foci (8). We
were, however, able to uncover a role for the ERK5 pathway in focus
formation when observed in a RafBXB-dependent signaling
background. Foci induced by RafBXB are increased in number by enhancing
the activity of the ERK5 pathway and are decreased in number by
disruption of MEK5 or ERK5 function (8). Thus, under some conditions,
MEK5 and ERK5 can impact growth and morphological transformation.
Transfection of RafBXB into cells alters the activity and localization
of a large repertoire of signaling molecules including ERK1/2 (15-17).
Our focus assays with RafBXB and MEK5DD suggested that Raf activates
the ERK1/2 pathway to produce the synergy in focus formation observed
with MEK5 and ERK5 (8). In support of this idea, we now find that
MEK5DD and MEK1R4F, an active mutant of MEK1, cooperate to form foci of
growth and morphologically transformed NIH3T3 cells. In addition, we
explore potential mechanisms by which the MEK5-ERK5 pathway
might influence focus formation. We find that MEK5DD and MEK1R4F
synergize to activate an NF- Plasmids, Reagents, and Expression of Recombinant
Proteins--
3× SRE-Luc, pCMV5-NF- Luciferase Reporter Assays--
NIH3T3 cells were
maintained as described (18). Calcium phosphate precipitates were
prepared using standard protocols. For 3× SRE-Luc assays 60-mm dishes
were transfected with 1 µg of 3× SRE-Luc and 1 µg of empty vector,
pCMV5Myc-MEK5DD, pCMV5-MEK1R4F, pCMV5-RafBXB, pCMV5Myc-MEK5KM, or
pCMV5Myc-ERK5KM as indicated. Either 4 µg of pCMV5- Immunoprecipitations and Kinase Assays--
COS-7 cells were
transfected with 1 µg of pCEP4HA-RSK and 0.5 µg of pCMV5Myc-MEK5DD
and PCDNA3FLAG-ERK5 as indicated using FuGENE6 transfection reagent
according to the manufacturer's protocol (Roche Molecular
Biochemicals). The appropriate amount of empty vector was used
to maintain a final amount of 2 µg of DNA in each transfection.
18-24 h post-transfection, medium was replaced with Dulbecco's
modified Eagle's medium. After 24 h without serum lysates were
prepared as described previously (8). Immunoprecipitations were
performed as described (7) using anti-HA antibody. Kinase assays were
performed as described (8) using histone 7S as indicated.
Focus Assays--
Focus assays were performed as described
previously (21).
MEK5DD and MEK1R4F Synergize to Induce Growth and Morphological
Transformation--
A primary mediator of Raf-dependent
signaling is the MEK1/2-ERK1/2 MAP kinase cascade. However, Raf-1
coordinates the activities of signaling pathways that are independent
of ERK1/2 (22). Despite the fact that Raf-1 does not apparently
activate MEK5, our earlier results demonstrate that ERK5 and MEK5 are
required for formation of foci of growth and morphologically
transformed cells induced by the activated Raf variant RafBXB (8).
These results suggest that the ERK5 pathway is one of the other
Raf-1-dependent pathways.
To define further the other Raf-mediated events that allow MEK5 and
ERK5 to influence focus formation, we tested the cooperativity of ERK5
and ERK1/2 pathways to induce cellular transformation. NIH3T3 cells
were transfected with vector alone, MEK5DD, an active mutant of MEK1
(MEK1R4F), or MEK5DD and MEK1R4F together. MEK5DD alone, as expected,
did not induce foci. MEK1R4F clearly induced foci (Fig.
1), although much less efficiently than
RafBXB (not shown; see Ref. 8). The combination of MEK5DD and MEK1R4F
produced significantly more foci than MEK1R4F alone (Fig. 1). These
findings indicate that the ERK1/2 and ERK5 pathways can act
synergistically to induce foci of growth and morphologically
transformed cells.
MEK5DD and MEK1R4F Synergize to Activate NF-
NF- MEK5 and ERK5 Are Involved in Stimulation of NF-
Activation of the serum response element by RafBXB often correlates
with focus formation (30). We therefore tested whether the expression
of dominant negative MEK5 or ERK5 affects RafBXB activation of a
luciferase reporter gene fused to the SRE. Neither dominant negative
MEK5 nor ERK5 had an effect on the ability of RafBXB to activate the
SRE-dependent reporter under conditions in which they do
inhibit RafBXB activation of NF- MEK5DD Does Not Influence the Transcription of HbEGF--
In
NIH3T3 cells the Raf-MEK-ERK pathway stimulates NF- Activation of ERK5 Is Sufficient to Increase NF- Coexpression of MEK5DD and ERK5 Activates p90 RSK--
A number of
kinases, including RSKs 1-3, mitogen-activated protein
kinase-associated protein kinase 2, Msk, and Mnk, are regulated by mitogen-activated protein kinase phosphorylation (1, 33-35). However, with the exception of Mnk (7), a role for ERK5 in the
regulation of these kinases has not been explored. p90 RSK has been
implicated in the regulation of NF- Proliferation and differentiation require the function of a
diverse array of signaling pathways. Molecules, such as Raf, that can
induce these events must coordinate the actions of multiple downstream
effector pathways (15, 16). We have previously demonstrated a
requirement for MEK5 and ERK5, in addition to ERK1/2, in the induction
of foci by RafBXB (8). Here we show that MEK1RF4 and MEK5DD supply
complementary activities to promote bypass of contact inhibition of
growth. In most cases, focus formation correlates with formation of
tumors in nude mice. We have explored possible mechanisms through which
MEK5 and ERK5 might impact focus formation. We find that MEK5 and ERK5
are required for Raf stimulation of NF- Deregulated NF- p90 RSK activity is increased by oncogenes in cell culture systems (42,
43) and, like NF- Activating both ERK5 and ERK1/2, based on their known substrate
profiles, broadens the range of transcription factors and other
downstream effectors that may be targeted. A coordinated increase in
the activity of both kinase pathways may also result in a more
pronounced effect on individual signaling activities involved in focus
formation than can be generated by either pathway alone. For instance,
NF-B, and MEK5 and ERK5 are required for
activation of NF-
B by RafBXB. The MEK5-ERK5 pathway is also
sufficient to activate both NF-
B and p90 ribosomal S6 kinase.
Our results support the hypothesis that NF-
B and p90 ribosomal S6
kinase are involved in MEK5-ERK5-dependent focus formation
and may serve as integration points for ERK5 and ERK1/2 signaling.
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
B-sensitive reporter, MEK5 and ERK5 are
involved in RafBXB activation of NF-
B, and that the MEK5-ERK5
pathway is sufficient to activate NF-
B. We also identified p90
ribosomal S6 kinase (RSK) as a target of MEK5-ERK5 signaling.
MATERIALS AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
B-inducing kinase (NIK),
and 2× NF-
B-Luc (18); heparin-binding epidermal growth factor-like
growth factor (HbEGF)-Luc (19), pCMV5Myc-MEK5DD,
pCMV5Myc-MEK5KM, pCMV5-RafBXB, pCMV5Myc-ERK5KM, and
pCEP4HA-ERK5CatKM (8); and pCMV5-
-GAL, pCMV5-MEK1R4F (7),
PCDNA3FLAG-ERK5 (10), and pCEP4HA-RSK (20) were as described.
Expression of proteins was monitored using the following antibodies:
HA, 12CA5, BAbCO (Covance Research Products, Cumberland, VA),
and M2
FLAG, which were from Sigma.
-GAL or 2 µg
of PRL-TK was used to control for transfection efficiency. For
2× NF-
B-Luc assays 60-mm dishes were transfected with 1.5 µg of
2× NF-
B-Luc and 1 µg of empty vector, pCMV5Myc-MEK5DD,
pCMV5-MEK1R4F, pCMV5-RafBXB, pCMV5Myc-MEK5KM, pCMV5Myc-ERK5KM, or
pCEP4HA-ERK5CatKM or 0.3 µg of pCMV5-NIK as indicated. 2 µg of
either pCMV5-
-GAL or PRL-TK was used to control for transfection
efficiency. For HbEGF-Luc assays 60-mm plates were transfected with 1 µg of HbEGF-Luc, 2 µg of PRL-TK, and 1 µg of empty vector,
pCMV5Myc-MEK5DD, or pCMV5-MEK1R4F as indicated. 18-24 h
post-transfection medium was replaced with Dulbecco's modified
Eagle's medium plus 0.5% calf serum. After 24 h in low serum
lysates were prepared in 0.5 ml of luciferase lysis buffer (18).
Lysates were assayed for firefly luciferase and firefly Renilla
activity using a dual luciferase assay kit (Promega) and the Turner
Designs luminometer. Reporter gene induction was calculated by
normalizing luciferase activity to either Renilla or
-galactosidase activity (18).
RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Fig. 1.
MEK5DD enhances focus formation by
MEK1R4F. NIH3T3 cells plated in 5% calf serum were transfected
with 500 ng of the indicated construct. Plates were scored for the
appearance of foci of morphologically and growth-transformed cells 14 days post-transfection. The values represented were normalized to foci
observed in cells transfected with MEK1R4F. Shown is the average of two
independent experiments performed in duplicate. Error bars
show S.E. A range of 5-10 foci/plate was observed for
MEK1R4F-transfected cells and 50-100 foci/plate for cells transfected
with MEK5DD and MEK1R4F.
B but Not the Serum
Response Element--
SRE, integration points for ERK1/2, c-Jun
NH2-terminal kinase, and p38 signaling are present in many
serum-induced genes, notably that of the proto-oncogene c-Fos (23-27).
The SRE is regulated by both serum response factors and ternary complex
factors, including Elk-1 and Sap1a. ERK5 might influence SRE activity,
because it has been shown to phosphorylate and activate Sap1a (9). It is possible then that a synergistic enhancement in the transcription of
SRE-sensitive genes could occur when MEK5DD and MEK1R4F are coexpressed, which might contribute to the cooperation seen in our
focus assays. We tested this possibility by transfecting NIH3T3 cells
with an SRE-driven reporter and empty vector, MEK5DD, MEK1R4F, or
MEK5DD and MEK1R4F together. MEK5DD neither significantly influenced the SRE reporter by itself, nor did it enhance MEK1R4F activation of
the reporter (Fig. 2A). It
seems unlikely then that the MEK5-ERK5 pathway influences morphological
transformation through genes containing serum response elements.
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Fig. 2.
MEK5DD and MEK1R4F synergize to activate
NF- B but not the serum response element.
A, NIH3T3 cells were transfected with the SRE reporter,
along with plasmid encoding no insert, MEK5DD, MEK1R4F, or MEK5DD and
MEK1R4F as indicated. Shown is the average of at least three
independent experiments performed in duplicate. Relative activity
refers to the increase in luciferase activity compared with cells
transfected with control vector and reporter. Transfection efficiency
was monitored using a Renilla firefly luciferase reporter driven by the
thymidine kinase promoter. Error bars show S.E.
B, same as described in A, except the activity of
the NF-
B reporter was determined.
B activity is influenced by MAP kinases and is required for focus
formation induced by oncogenic Ras and RafBXB (28, 29). Using a
luciferase reporter linked to a multimerized NF-
B-sensitive element,
we tested the ability of MEK5DD and MEK1R4F to synergize in activating
NF-
B family transcription factors. NIH3T3 cells were cotransfected
with the NF-
B reporter and vector alone, MEK5DD, MEK1R4F, or MEK5DD
plus MEK1R4F. MEK5DD was unable to activate the
NF-
B-dependent reporter on its own; however, it did
enhance NF-
B reporter activity stimulated by MEK1R4F (Fig.
2B). These results are consistent with the idea that MEK5DD
may influence MEK1R4F-stimulated focus formation by enhancing
activation of NF-
B by MEK1R4F.
B by
RafBXB--
We have previously found that the MEK5-ERK5 module
influences RafBXB-stimulated transformation of NIH3T3 cells. Although
neither MEK5 nor ERK5 is activated directly by RafBXB, focus assays
indicate that MEK5 and ERK5 participate in a subset of
RafBXB-stimulated molecular events (8). Our current data indicate that
the MEK5-ERK5 pathway influences NF-
B activity and, therefore, may
also be involved in activation of NF-
B by RafBXB. To test this
possibility we assayed the effects of MEK5KM, ERK5KM, and ERK5CatKM (a
truncated variant containing amino acids 1-451 of ERK5) on activation
of the NF-
B reporter by RafBXB. MEK5KM and ERK5KM inhibited RafBXB activation of the NF-
B reporter (Fig.
3A). Interestingly, deletion of amino acids 452-816 renders ERK5 unable to influence NF-
B activity (Fig. 3A). Consistent with this result, ERK5CatKM
does not inhibit RafBXB-induced focus formation (not shown). MEK5KM and
ERK5KM do not affect the ability of the NIK (Fig. 3B) or
MEKK1 (not shown) to activate the NF-
B reporter, which suggests that the ability of MEK5-ERK5 to influence NF-
B activation is
pathway-specific.
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Fig. 3.
MEK5 and ERK5 are required for maximal
RafBXB-stimulated NF- B activity.
A, NIH3T3 cells were transfected with cDNAs encoding the
NF-
B reporter, along with empty vector or RafBXB. Some cells were
transfected with RafBXB, together with the kinase-defective mutants
MEK5KM, ERK5KM, or ERK5CatKM. Shown is the average of at least three
independent experiments performed in duplicate. Relative activity
refers to the average luciferase activity as a fraction of that in
cells expressing RafBXB.
-Galactosidase activity derived from a
-galactosidase expression vector or Renilla firefly luciferase
activity was used to determine transfection efficiency. B,
NIH3T3 cells were transfected with the NF-
B reporter, along with
control vector or NIK. Some cells were also transfected with NIK and
MEK5KM or ERK5KM. Shown is the average of at least three independent
experiments performed in duplicate. Relative activity refers to the
average luciferase activity as a fraction of that in cells expressing
NIK. Transfection efficiency was determined as described for
A. C, NIH3T3 cells were transfected with the SRE
reporter, along with control vector or RafBXB. Some cells were
cotransfected with RafBXB and MEK5KM or ERK5KM. Shown is the average of
at least three independent experiments performed in duplicate. Relative
activity is as in A. Transfection efficiency was determined
as described for A.
B (Fig. 3C). The failure
of the MEK5-ERK5 kinase-dead proteins to interfere with SRE activation
is a measure of their failure to block RafBXB-dependent activation of ERK1/2, as shown using a Raf mutant that cannot activate
ERK1/2 (22). Based on these results we conclude that dominant negative
MEK5 and ERK5 are able to inhibit a subset of RafBXB-stimulated
transcriptional responses, such as NF-
B activation; the inhibition
of NF-
B activity may be one of the factors that results in a
decreased number of foci.
B through the
production of autocrine factors, possibly HbEGF, which act on the EGF
receptor (31). MEK1R4F likely utilizes a similar pathway based on
epistasis. Consistent with this possibility, increases in ERK1/2
activity correlate with the production of HbEGF (32), a potential
convergence point for MEK5DD and MEK1R4F signaling. We used a
luciferase reporter gene linked to the mouse HbEGF promoter to monitor
the effects of MEK5DD and MEK1R4F on HbEGF transcription. We found that
MEK5DD did not enhance MEK1R4Fdependent activation of the HbEGF
reporter (Fig. 4). Thus, the effect of MEK5DD on NF-
B activity is most likely not manifested at the level
of HbEGF production.
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Fig. 4.
MEK5DD and MEK1R4F do not synergize to
activate the transcription of HbEGF. NIH3T3 cells were transfected
with cDNAs encoding the HbEGF reporter, along with empty vector,
MEK5DD, MEK1R4F, or MEK5DD and MEK1R4F as indicated. Shown is the
average of at least three independent experiments performed in
duplicate. Transfection efficiency and relative activity were
determined as described for Fig. 2.
B
Activity--
We and others have found that coexpression of
ERK5 with MEK5DD enhances the activation of a luciferase reporter
driven by GAL-MEF2C beyond that induced by MEK5DD alone (10). Thus, we tested the possibility that coexpression of ERK5 with MEK5DD may increase the activity of the NF-
B reporter. Coexpression of MEK5DD and ERK5 did stimulate a modest activation of the NF-
B luciferase reporter (Fig. 5). These findings are
consistent with the conclusion that activation of ERK5 is sufficient to
stimulate NF-
B.
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Fig. 5.
Coexpression of MEK5DD and ERK5 activates
NF- B. NIH3T3 cells were transfected with
cDNAs encoding the NF-
B reporter, along with control vector,
MEK5DD, ERK5, or MEK5DD and ERK5 together. Shown is the average of
three independent experiments performed in duplicate. Transfection
efficiency and relative activity were determined as described for Fig.
2.
B (36, 37), in addition to its
ability to phosphorylate effectors, such as c-Fos, (38) which might
impact morphological transformation. To test the possibility that p90
RSK may be a target of MEK5-ERK5 signaling, Cos-7 cells were
transfected with cDNAs encoding HA-RSK with MEK5DD alone, ERK5
alone, or MEK5DD plus ERK5. We found that coexpression of MEK5DD and
ERK5 resulted in an approximately 4-fold increase in RSK activity when
compared with the activity of RSK cotransfected with empty vector,
MEK5DD (Fig. 6), or ERK5 (not shown).
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Fig. 6.
Coexpression of MEK5DD and ERK5 activates p90
RSK. COS-7 cells were transfected with HA-RSK along with control
vector, MEK5DD, or MEK5DD and ERK5 as indicated. Immune-complex kinase
assays were performed on immunoprecipitated HA-RSK using histone 7S
(H7S) as substrate to determine HA-RSK activity. Top
panel, a representative autoradiogram from one of three
independent experiments performed in duplicate. Middle two
panels, immunoblots of lysates to determine HA-RSK and FLAG-ERK5
expression from a representative experiment. Bottom panel,
average HA-RSK activity from three experiments performed in duplicate.
RSK activity is the -fold increase in HA-RSK activity when coexpressed
with the indicated cDNAs compared with the activity of HA-RSK
expressed alone.
DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
B, and MEK5DD synergizes with
MEK1R4F to activate NF-
B. We also find that when coexpressed, MEK5
and ERK5 are sufficient for activation of NF-
B and p90 RSK.
B activity, and abnormalities in NF-
B-encoding
genes or their expression, have been identified in a number of
different human tumors (39). Consistent with these clinical findings,
disrupting NF-
B activity is sufficient to reduce the number of foci
induced by oncogenic Ras and RafBXB in NIH3T3 cells (28, 29). NF-
B
may influence tumor formation and focus formation through promoting
cell survival and cell cycle entry (40, 41). ERK1/2 has been suggested
to stimulate NF-
B through the production of HbEGF (31). Increasing
ERK5 activity, however, has no effect on HbEGF transcription,
suggesting that ERK5 may modulate NF-
B through a novel mechanism.
The observation of synergy between MEK1R4F and MEK5DD in activating
NF-
B further suggests that ERK5 influences NF-
B by a distinct
mechanism from that used by ERK1/2.
B, may play a multifunctional role in the formation
of foci. p90 RSK is involved in the stabilization of the proto-oncogene
c-Fos and can inhibit apoptosis through the phosphorylation of
Bad or transcription of Bcl-2 and may stimulate chromatin remodeling in
a histone 3-dependent manner (42). In addition, p90 RSK
phosphorylates serine 32 of I
B, a required site of phosphorylation
for proteosome-mediated degradation of I
B, in response to phorbol
ester treatment and overexpression of v-Src (36, 37).
B and p90 RSK may be integration points for two distinct
activating signals. The MEK5DD and MEK1R4F activation signals could
converge at a regulatory site upstream of NF-
B, or alternatively, on
the NF-
B homo-and heterodimers themselves. Previous reports
suggest that p90 RSK can integrate the direct phosphorylation of
distinct sites by ERK1/2 and phosphoinositide-dependent protein
kinase to achieve a high activity state (44). The MEK5-ERK5 and
MEK1/2-ERK1/2 pathways may increase p90 RSK activity through a similar
integration mechanism. Further study will be necessary to discern
whether MEK5DD influences MEK1R4F-dependent cellular effects by enhancing the activity of factors regulated by MEK1R4F, through activation of additional factors, or a combination of both.
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ACKNOWLEDGEMENTS |
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We thank Jeff Cheng for contributing to an initial observation, Dale O. Henry for technical assistance, and Dionne Ware for administrative assistance. The HbEGF-Luc reporter construct was a gift from Dr. Gerhard Raab.
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FOOTNOTES |
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* This work was supported in part by Grants DK34128 (to M. H. C.) and CA71443 (to M. A. W.) from the National Institutes of Health and by Grants I1243 and I1414 from the Welch Foundation.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
In partial fulfillment of the requirements for the Ph.D.
Supported by Pharmacological Sciences Training Grant G1907062-25 at the
University of Texas Southwestern Medical Center.
To whom correspondence should be addressed: Dept of
Pharmacology, The University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-9041. Tel.: 214-648-3627; Fax:
214-648-3811; E-mail: mcobb@mednet.swmed.edu.
Published, JBC Papers in Press, December 15, 2000, DOI 10.1074/jbc.M009764200
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ABBREVIATIONS |
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The abbreviations used are:
MAP, mitogen-activated protein;
ERK, extracellular signal-regulated kinase;
MEK, mitogen-activated protein kinase/ERK kinase;
MEKK, MEK kinase;
EGF, epidermal growth factor;
RSK, ribosomal S6 kinase;
SRE, serum
response elements;
Luc, luciferase;
CMV, cytomegalovirus;
NIK, NF-B-inducing kinase;
HbEGF, heparin-binding EGF-like growth factor;
HA, hemagglutinin;
GAL, galactosidase.
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REFERENCES |
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