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J. Biol. Chem., Vol. 277, Issue 50, 48094-48098, December 13, 2002

This Article Abstract Full Text (PDF) All Versions of this Article: 277/50/48094    most recent M208535200v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Pearson, G. W. Articles by Cobb, M. H. Search for Related Content PubMed PubMed Citation Articles by Pearson, G. W. Articles by Cobb, M. H. Social Bookmarking                      What's this?

Cell Condition-dependent Regulation of ERK5 by cAMP^*

Gray W. Pearson and Melanie H. Cobb^§

From the Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9041

Received for publication, August 20, 2002, and in revised form, September 16, 2002

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

ERK5 activity is increased by agents known to activate receptor tyrosine kinases, G-protein coupled receptors, and stress response pathways. We now find a role for cAMP in the regulation of ERK5. ERK5 is activated by forskolin, isoproterenol, and epinephrine in NIH3T3 cells and C2C12 myoblasts. ERK1/2 are also activated by cAMP in NIH3T3 cells, but not in C2C12 myoblasts, demonstrating differential regulation of ERK5 and ERK1/2 by cAMP. We examined the effect of cell context on activation of ERK5 and discovered ERK5 activity is inhibited, rather than activated, by cAMP in confluent, serum-deprived NIH3T3 cells and C2C12 myoblasts. Our results suggest that regulation of MAP kinase pathways by cAMP is not only dictated by cell type, but also by cell context.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

The transmission of extracellular stimuli into intracellular responses often involves the activation MAP¹ kinase cascades (1, 2). These cascades contain three protein kinases acting in series, a MAP kinase kinase kinase (MAP3K or MEKK) which activates MAP/ERK kinases (MAP2Ks or MEKs), which phosphorylate MAP kinases. Upon activation, MAP kinases regulate cellular responses through the phosphorylation of other protein kinases, cytoplasmic- and membrane-bound proteins, and transcription factors. One of these MAP kinase cascades includes the MAP kinase ERK5 and its upstream activator MEK5. The currently known MAP3Ks that activate MEK5 are MEKK2 and MEKK3 (3, 4).

MEK5-ERK5 signaling is involved in the regulation of cellular proliferation. A constitutively active variant of MEK5, MEK5DD, enhances focus formation induced by activated alleles of Raf-1 and MEK1 in NIH3T3 cells (5). ERK5 activity is required for proliferation induced by EGF and granulocyte colony-stimulating factor (6, 7) and focus formation resulting from the expression of an active mutant of Raf (5). ERK5 most likely regulates proliferative responses through the activation of downstream effectors, such as MEF2A, MEF2C, and MEF2D (8), p90 ribosomal S6 kinase (p90RSK), nuclear factor-B (NF-B) (9), and serum- and glucocorticoid-inducible kinase (SGK) (10).

While cAMP mediates the trophic actions of hormones that control endocrine gland function, including the thyroid, adrenal cortex, and reproductive organs (11), cAMP inhibits the proliferation of fibroblasts. Changes in MAP kinase activity are often part of the repertoire of events required for cAMP to induce the desired cellular response. For example, ERK1/2 activity is increased by cAMP, and this activation is required for cAMP-stimulated neurite outgrowth in PC12 cells (12, 13). ERK1/2 activation is also thought be involved in cAMP-induced long term potentiation (LTP) (14, 15). The ability of cAMP to inhibit proliferation has often been attributed to a correlative cAMP-dependent protein kinase (PKA)-dependent inhibition of growth factor-stimulated activation of ERK1/2 (16-18). It has been proposed that the variety of effects of cAMP on MAP kinase activity are derived from differences characteristic to different cell types (12, 13, 16-21).

We tested the hypothesis that ERK5, like other MAP kinases, is activated by cAMP in some cell types and inhibited by cAMP in others. We found that increasing cAMP activates ERK5 in NIH3T3 cells and C2C12 myoblasts. Comparatively, ERK1/2 are only activated in NIH3T3 cells, which suggests differential mechanisms of cAMP-dependent activation of ERK5 and ERK1/2. Consistent with differential regulation of ERK5 and ERK1/2 by cAMP, ERK5 is inhibited, whereas ERK1/2 are activated by cAMP in PC12 cells. Furthermore, we have identified growth conditions in NIH3T3 cells and C2C12 myoblasts that make ERK5 susceptible to inhibition by cAMP. Our findings indicate that the ability of cAMP to increase or decrease ERK5 activity is dependent not only on cell type, but also on cell context.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

Cell Culture-- NIH3T3 cells were maintained as described previously (5). C2C12 myoblasts were cultured in Dulbecco's modified Eagle's medium containing 10% fetal bovine serum and 1% L-glutamine. Differentiation of C2C12 myoblasts into myotubes was induced by changing to medium consisting of Dulbecco's modified Eagle's medium supplemented with 2% horse serum.

Preparation of Cell Lysates-- After removal of medium, cells were washed once with cold phosphate-buffered saline. Cells were then scraped into lysis buffer containing a final concentration of 50 mM Hepes, 150 mM NaCl, 1.5 mM MgCl₂, 1 mM EGTA, 10% glycerol, 0.2 mM NaVO₄, 100 mM NaF, 50 mM -glycerophosphate, and 0.1% Nonidet P-40. Lysates were subjected to centrifugation for 15 min at 14,000 × g. Supernatants were used as described below.

Immune Complex Kinase Assays-- Lysates were precleared with 20 µl of nonspecific antibody and 40 µl of protein-G-Sepharose 1:1 slurry for one h or overnight. Samples were sedimented for 2 min at 14,000 × g, and supernatants were placed in clean tubes. Anti-ERK5 antibody (1 µl, Sigma) was added to these supernatants at 4 °C for 1 h, after which 40 ml of protein-G-Sepharose beads were added for an additional h. Beads were then pelleted and washed four times with wash buffer (1 M NaCl, 0.25 M Tris-HCl, pH 7.4, 0.1% Nonidet P-40, 0.1% deoxycholate) and two times with 10 mM Hepes, pH 7.4. Kinase reactions were performed for 30 min at 30 °C in 10 mM MgCl₂, 50 µM ATP, [-³²P]ATP, and 10 mM Hepes, pH 7.4, with GST-MEF2C-(204-321) as substrate. Reactions were terminated by the addition of 4× sample buffer. Reactions were then analyzed by SDS-PAGE and visualized with autoradiography. Where shown, fold activation was determined by measuring ³²P incorporation into substrate.

Western Blots-- Membranes were blocked for 1 h or overnight in blocking buffer, 1% milk, 1% bovine serum albumin in Tris-buffered saline plus 1.25% Tween 20 (TBST). Primary antibody incubation was performed for 1 h or overnight with a 1:1000 dilution of either anti-phospho-ERK1/2 (BIOSOURCE), or anti-ERK5 (Sigma) diluted in blocking buffer. Blots were washed three times with TBST. Secondary antibody incubation was for 15 min with goat anti-rabbit secondary antibody diluted 1:1500 in blocking buffer. Blots were again washed three times and visualized using enhanced chemiluminescence (ECL).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

Activation of ERK5 and ERK1/2 by Cyclic Nucleotides in NIH3T3 Cells-- Increases in cAMP concentrations can activate, inhibit, or have no effect on the activity of ERK1/2 (2, 12, 13, 16-21). We chose to investigate how cAMP influences ERK5 in NIH3T3 cells. In these cells ERK5 has an integral involvement in NF-B regulation under conditions that result in growth and morphological transformation (9). Forskolin (10 µM), a direct activator of adenylyl cyclase, or the general phosphodiesterase inhibitor isobutylmethylxanthine (IBMX, 50 µM) were sufficient to increase both ERK5 and ERK1/2 activity (Fig. 1A). Because treating cells with IBMX may raise cGMP as well as cAMP concentrations, we explored the possibility that both may regulate ERK5 and ERK1/2 activity. NIH3T3 cells were treated with the phosphodiesterase-resistant analogs, 8-bromo-cAMP and 8-p-chlorophenylthio (CPT)-cGMP; both caused a modest activation of ERK5 and ERK1/2 (Fig. 1B), suggesting that IBMX activates ERK5 and ERK1/2 by increasing the concentration of cAMP and cGMP. The increase in ERK5 activity was detected by increased phosphorylation of the substrate MEF2C and also by the presence of a slower migrating form of ERK5, which is due to autophosphorylation of ERK5 that occurs only if ERK5 is activated. In the experiment shown a small amount of this autophosphorylated form is detected in the control sample as well.

View larger version (29K): [in this window] [in a new window]   Fig. 1.   Cyclic nucleotides activate ERK5 and ERK1/2 in NIH3T3 cells. A, NIH3T3 cells at 80% confluence were cultured in 0.5% calf serum for 18 h prior to stimulation for 5 min with 10 µM forskolin or 50 µM IBMX. Top panel, ERK5 immune complex kinase assay using a fragment of MEF2C as substrate. Bottom panel, ERK1/2 activity monitored with an antibody that recognizes the dually phosphorylated forms of ERK1/2. B, NIH3T3 cells cultured as described in A were stimulated with 50 µM 8-bromo-cAMP or 25 µM 8-pCPT-cGMP. ERK5 activation was detected by immunoblotting of a slower migrating autophosphorylated ERK5 band in cell lysates. The fold increase in ERK5 activity determined in immune complex kinase assays as described in A was plotted and is the average of two independent experiments. Error bars show the range. Results in A are representative of five and in B of two independent experiments.

Physiological activation of adenylyl cyclase often results from hormonal stimulation of -adrenergic receptors (22). To determine whether ERK5 is activated by physiologically induced changes in cAMP concentrations, we treated NIH3T3 cells with the -adrenergic receptor agonists isoproterenol and epinephrine (10 µM). ERK5 and ERK1/2 activity were increased by both these hormones within 5 min of stimulation (Fig. 2A); 10 nM isoproterenol was found to be sufficient to activate the kinases (data not shown). To confirm that activation of ERK5 occurs as a result of -adrenergic receptor activation, we utilized the competitive -adrenergic receptor antagonist propranolol. Incubating cells with 10 µM propranolol for 30 min prior to isoproterenol treatment completely abrogates activation of ERK5 and ERK1/2, proving the increase in activity is the result of -adrenergic receptor stimulation (Fig. 2B).

View larger version (33K): [in this window] [in a new window]   Fig. 2.   Activation of ERK5 and ERK1/2 in NIH3T3 cells through -adrenergic receptors. A, NIH3T3 cells were treated with 10 µM isoproterenol or 10 µM epinephrine as indicated. Top panel, ERK5 immune complex kinase assays. Bottom panel, ERK1/2 activity determined as described in the legend to Fig. 1. B, NIH3T3 cells were stimulated for 5 min with 10 or 100 nM isoproterenol. Cells from the indicated samples were treated with 10 µM propranolol or vehicle for 30 min prior to isoproterenol stimulation. The activities of ERK5 (top panel) and ERK1/2 (bottom panel) were determined as described in the legend to A. Results in A and B are representative of at least three independent experiments.

Activation of ERK5, but Not ERK1/2, by cAMP in C2C12 Myoblasts-- It has been reported that in corticol neurons and PC12 cells, ERK1/2 are activated by forskolin whereas ERK5 is not (23). Those findings and our results in NIH3T3 cells suggest that the ability of cAMP to activate the two pathways is uncoupled at some upstream regulatory point. To support this observation we decided to search for the converse situation, one in which ERK5 is activated by cAMP and ERK1/2 are not. We compared the regulation of ERK5 and ERK1/2 in C2C12 myoblasts, because it has been previously demonstrated that ERK1/2 activity is unchanged by agents that raise cAMP levels in skeletal muscle (24). C2C12 myoblasts were treated with forskolin, isoproterenol, epinephrine, or EGF and the activity of the MAP kinases compared. We found that ERK1/2 and ERK5 were both activated by EGF; however, only ERK5 activity was elevated in response to an increase in cAMP (Fig. 3A). Activation of ERK5 by forskolin and isoproterenol was noted at 5 and 15 min (Fig. 3B). In addition to suggesting differential regulation of the ERK5 and ERK1/2 pathways by cAMP, these results show activation of ERK5 by cAMP is not exclusive to NIH3T3 cells.

View larger version (41K): [in this window] [in a new window]   Fig. 3.   cAMP activates ERK5, but not ERK1/2, in C2C12 myoblasts. A, C2C12 myoblasts were grown to 80% confluence, serum-deprived for 18 h prior, and then treated for 5 min with 1 µM concentration of the indicated agonist. The activities of ERK5 (top panel) and ERK1/2 (bottom panel) were determined as described in the legend to Fig. 1. The fold increase in ERK5 activity (middle panel) determined in immune complex kinase assays is the average of three independent experiments. Error bars show S.E. of the mean. B, C2C12 myoblasts were treated for 5 or 15 min with 10 µM forskolin or 1 µM isoproterenol as indicated. ERK5 activity was monitored as described in the legend to Fig. 1. Results in A and B are representative of at least four independent experiments.

Our results demonstrate that cAMP may simultaneously activate one of these MAPK pathways and inhibit the other. This difference, observed in multiple cell types, strongly supports the idea that the regulatory actions of cAMP on the two kinases are mediated by different mechanisms. ERK5 has been reported to be activated by Ras, Src, or PKC  (5, 25-27). Perhaps cAMP negatively impacts the intermediate targets of these ERK5 activating pathways. Activation of ERK5, like p38 MAP kinase, may require PKA (28-31).

Regulation of ERK5 Activity by cAMP Is Dependent on Cell Conditions-- It has been suggested that cell type determines how cAMP affects MAP kinase activity, but this conclusion is not consistent with our results. Although it has been reported that cAMP negatively regulates ERK1/2 activity in NIH3T3 cells (18, 21), we find that ERK1/2 are stimulated by cAMP in NIH3T3 cells. A simple explanation is that this discrepancy is due to subtle variations that can exist from cell line to cell line. A more intriguing possibility is suggested upon closer inspection of the experimental conditions, however. Since the organization and regulation of signaling pathways may be influenced by the confluence of cells and/or duration of serum withdrawal (32-35), it is possible that the effect of cyclic nucleotides on MAP kinase activity may not only depend on cell type, but also the context of cell growth. Negative regulation of MAP kinase activity in NIH3T3 cells or other cell types may occur in cells if they have been grown to confluence and serum-deprived for at least 24 h (16-18). In our experiments, we measured ERK5 and ERK1/2 activity in NIH3T3 cells grown to 80% confluence and cultured in 0.5% calf serum for 18 h prior to hormone treatment. Thus, we first examined how the presence of serum and reduction in cell confluence would affect activation of ERK5 and ERK1/2 by cAMP. In proliferating NIH3T3 cells, ERK5 and ERK1/2 are activated by cAMP in a time-dependent manner (Fig. 4). ERK5 activation was detected by its reduced electrophoretic mobility at 5 and 15 min. We next grew cells to confluence and deprived them of serum for 24 h. These cells were then treated for times ranging from 5 to 60 min with forskolin and IBMX. After the indicated treatment time, the cells were either lysed or treated for an additional 15 min with FGF and then lysed. Under this growth condition, we were unable to detect activation of ERK5 by forskolin or IBMX (compare Fig. 5A to 4). In addition, we found that treatment with forskolin and IBMX prior to FGF prevented activation of ERK5 by the growth factor (Fig. 5A). A modest reduction in ERK1/2 activation was also observed. Thus, manipulating the cell growth conditions altered the activities of both ERK5 and ERK1/2 in response to cAMP.

View larger version (24K): [in this window] [in a new window]   Fig. 4.   Forskolin and IBMX activate ERK5 and ERK1/2 in proliferating NIH3T3 cells. 50% confluent NIH3T3 cells growing in 10% calf serum were treated with 10 µM forskolin and 50 µM IBMX for 5, 15, 30, or 60 min. Top panel, an increase in slower migrating, autophosphorylated ERK5 is seen at 5 min and peaks at 15 min. ERK1/2 activity was monitored as described in the legend to Fig. 1. Results are representative of at least three independent experiments.

View larger version (44K): [in this window] [in a new window]   Fig. 5.   Inhibition of growth factor-stimulated ERK5 activity by cAMP in confluent, serum-deprived cells. A, NIH3T3 cells were grown to confluence and cultured in 0.5% calf serum for 24 h prior to stimulation. Cells were treated for 5, 15, 30, or 60 min with 10 µM forskolin and 50 µM IBMX and lysed, or stimulated for an additional 15 min with 25 ng/ml FGF prior to lysis. ERK5 activity was determined by mobility shift and ERK1/2 by phospho-ERK1/2 blot as described in the legend to Fig. 1. B, C2C12 myoblasts were grown to confluence and serum-deprived for 24 h prior to stimulation. Samples in the first four lanes were from cells treated as indicated and harvested. Samples in the last lane were from cells treated for 15 min with 10 µM forskolin and 50 µM IBMX prior to a 15-min stimulation with 1 ng/ml EGF. ERK5 and ERK1/2 activities were monitored as described in the legend to Fig. 1. Results in A and B are representative of at least six independent experiments.

To determine the generality of this finding, we next asked whether ERK5 is both positively and negatively regulated by cAMP in C2C12 cells. Consistent with the results in NIH3T3 cells, treatment with forskolin and IBMX inhibited activation of ERK5 by EGF in confluent, serum-deprived C2C12 myoblasts, although activation of ERK1/2 was unaffected (Fig. 5B).

We and others have observed that ERK5 is active during the differentiation of C2C12 myoblasts into myotubes (36). ERK2 activity is also elevated during the end stages of differentiation (37, 38). ERK1 activity is increased to a much smaller extent. Myotubes were induced using differentiating conditions for 4 days. Treatment of these myotubes with forskolin and IBMX reduced existing ERK2 activity, but had no effect on the activity of ERK5 (Fig. 6). Both ERK5 and ERK2 were inhibited by the MEK inhibitor U0126. Our results in NIH3T3 cells and C2C12 myoblasts show that the observed effect of cAMP on MAP kinase activity is dependent on cellular context.

View larger version (23K): [in this window] [in a new window]   Fig. 6.   cAMP inhibits ERK1/2 activity, but not ERK5 activity, in differentiated C2C12 myotubes. C2C12 myotubes were treated for 5, 15, 30, or 60 min with 10 µM forskolin and 50 µM IBMX. Where indicated U0126 was added for 60 min. ERK5 (top panel) and ERK1/2 (middle panel) activities were monitored as described in the legend to Fig. 1. Bottom panel, total ERK1/2 was determined by Western blot with a polyclonal antibody that recognizes both proteins. Results are representative of at least three independent experiments.

In NIH3T3 cells, the concentration of cAMP is inversely related to the amount of serum present in the growth medium and may directly correlate with cell confluency (39). We suggest two possible explanations based on this finding. One possibility is that activation of ERK5 by cAMP occurs when intracellular cAMP concentration is at the lowest just prior to stimulation, and inhibition of ERK5 activation occurs when cAMP is at its highest concentration. As cells grow to confluence and cAMP levels increase, the ERK5 activation pathway may become refractory to further stimulation. Perhaps a co-existing inhibitory pathway may not be similarly desensitized and thus still capable of negatively regulating ERK5. Alternatively, the context-dependent increase in cAMP may induce sufficient cAMP-dependent inhibition of ERK5 to raise the threshold for ERK5 activation beyond that generated with forskolin or isoproterenol stimulation. Finally, changes in confluence and serum concentration may induce changes in the properties of cAMP signaling pathways as a result of changes in protein expression. Expression of RI, one of two types of PKA regulatory subunits, is dominant in proliferating cells and tumors and may allow PKA to positively contribute to cell growth (40, 41). RII is more highly expressed in non-proliferating tissue and growth-arrested cells (42). Relative expression of RI and RII may dictate whether cAMP activates or inhibits ERK5 activity. Regardless of the mechanism, the observation that effects of cAMP on ERK5 are dependent on cell context requires investigators to examine the data closely before concluding that cyclic nucleotides are strictly stimulatory or inhibitory toward MAP kinases in a particular cell type.

Inhibition of ERK5, but Not ERK1/2, Activity by Forskolin in PC12 Cells-- It has been reported that ERK5 is not activated by forskolin in corticol neurons or PC12 cells (23). The possibility that ERK5 may instead be inhibited by cAMP was not previously explored. ERK1/2 are activated by cAMP in cells of neuronal origin, so this would also be the first demonstration of opposing regulation of ERK5 and ERK1/2. We found that serum-induced ERK5 activity is reduced by forskolin, whereas ERK1/2 are potently activated (Fig. 7A). We then tested the ability of forskolin to inhibit ERK5 activation by the neurotrophin NGF. Once again forskolin inhibited activation of ERK5, but did not reduce the activation of ERK1/2 (Fig. 7B). These results show that ERK5 and ERK1/2 are not only differentially regulated by cAMP, but that cAMP may have opposite effects on these MAP kinase pathways.

View larger version (39K): [in this window] [in a new window]   Fig. 7.   cAMP inhibits ERK5 activity in PC12 cells. ERK5 and ERK1/2 activity were monitored as described in the legend to Fig. 1. A, PC12 cells cultured in 10% horse serum, 5% fetal bovine serum in RPMI were stimulated for the indicated times with 10 µM forskolin. Top panel, decreased ERK5 activity detected by the disappearance of the ERK5 band with reduced electrophoretic mobility. Bottom panel, ERK1/2 activity. B, PC12 cells were cultured in 1% horse serum, 0.5% fetal bovine serum in RPMI for 18 h before stimulation. PC12 cells were treated with Me2SO or 10 µM forskolin for the indicated time prior to a 15-min stimulation with 80 ng/ml NGF. Top panel, ERK5 activation detected by electrophoretic mobility. Bottom panel, ERK1/2 activity. Results in A and B are representative of at least three independent experiments.

Consequences of cAMP Effects on ERK5-- An important consequence of the fact that cAMP can have opposite effects on ERK1/2 and ERK5 is that it may aid in revealing the cellular processes that are dependent on ERK5. Distinguishing actions of ERK5 from those of ERK1/2 has been hampered by the fact that many agents activate both ERK1/2 and ERK5 under similar circumstances, and also because the pharmacological agents, PD98059 and U0126, which have been employed extensively to identify ERK1/2-dependent events, also block ERK5 activation. ERK5 may be involved in cAMP-dependent pathways in ways that were not initially be expected. For example, ERK5 has a potential input to regulation of cAMP response element-binding protein (CREB) through its ability to activate p90 Rsk, a role shared with ERK1/2 (9, 43). cAMP regulates transcription in part through phosphorylation of CREB on serine 133 by PKA (44). p90 Rsk also phosphorylates this site on CREB (45, 46), providing an alternative mechanism by which cAMP may control activation of CREB through ERK5 and Rsk. In several systems cAMP is known to utilize MAP kinases to control cellular events, for example, in the regulation of contractility in the heart (47), transcription of mitochondrial uncoupling protein 1 in adipocytes (29), stability of cycloxygenase-2 mRNA (48), and biosynthesis of VEGF in osteoblasts (49). We have demonstrated previously that ERK5 cooperates with ERK1/2 to regulate activation of NF-B and focus formation (9). It is possible that ERK5 cooperates with other MAP kinases under the circumstances described.

	ACKNOWLEDGEMENTS

We thank Ted Chrisman for cGMP reagents, Mike White and members of the Cobb laboratory for comments about the manuscript, and Dionne Ware for administrative assistance.

	FOOTNOTES

^* This work was supported by National Institutes of Health Grant DK34128 and Welch Foundation Grant I1243.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.

Supported by an NIGMS, National Institutes of Health, Predoctoral Training Grant in the pharmacological sciences. This work was done in partial fulfillment of the requirements for the Ph.D. degree.

^§ To whom correspondence should be addressed: Dept. of Pharmacology, 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, September 23, 2002, DOI 10.1074/jbc.M208535200

	ABBREVIATIONS

The abbreviations used are: MAP, mitogen-activated protein; ERK, extracellular signal-related kinase; EGF, epidermal growth factor; PKA, c-AMP-dependent protein kinase; MEK, mitogen-activated protein kinase/extracellular signal-related kinase kinase; MEKK, MEK kinase; IBMX, isobutylmethylxanthine; FGF, fibroblast growth factor; CREB, cAMP response element-binding protein.

	REFERENCES

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This Article Abstract Full Text (PDF) All Versions of this Article: 277/50/48094    most recent M208535200v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Pearson, G. W. Articles by Cobb, M. H. Search for Related Content PubMed PubMed Citation Articles by Pearson, G. W. Articles by Cobb, M. H. Social Bookmarking                      What's this?