Phosphorylation of the c-Fos transrepression domain by mitogen-activated protein kinase and 90-kDa ribosomal S6 kinase

Nerve growth factor activates extracellular signal-regulated kinase and p38 mitogen-activated protein kinase pathways to stimulate CREB serine 133 phosphorylation

The mechanisms by which growth factor-induced signals are propagated to the nucleus, leading to the activation of the transcription factor CREB, have been characterized. Nerve growth factor (NGF) was found to activate multiple signaling pathways that mediate the phosphorylation of CREB at the critical regulatory site, serine 133 (Ser-133). NGF activates the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinases (MAPKs), which in turn activate the pp90 ribosomal S6 kinase (RSK) family of Ser/Thr kinases, all three members of which were found to catalyze CREB Ser-133 phosphorylation in vitro and in vivo. In addition to the ERK/RSK pathway, we found that NGF activated the p38 MAPK and its downstream effector, MAPK-activated protein kinase 2 (MAPKAP kinase 2), resulting in phosphorylation of CREB at Ser-133. Inhibition of either the ERK/RSK or the p38/MAPKAP kinase 2 pathway only partially blocked NGF-induced CREB Ser-133 phosphorylation, suggesting that either pathway alone is sufficient for coupling the NGF signal to CREB activation. However, inhibition of both the ERK/RSK and the p38/MAPKAP kinase 2 pathways completely abolished NGF-induced CREB Ser-133 phosphorylation. These findings indicate that NGF activates two distinct MAPK pathways, both of which contribute to the phosphorylation of the transcription factor CREB and the activation of immediate-early genes.

pp90rsk1 regulates estrogen receptor-mediated transcription through phosphorylation of Ser-167

The estrogen receptor alpha (ER), a member of the steroid receptor superfamily, contains an N-terminal hormone-independent transcriptional activation function (AF-1) and a C-terminal hormone-dependent transcriptional activation function (AF-2). Here, we used in-gel kinase assays to determine that pp90rsk1 activated by either epidermal growth factor (EGF) or phorbol myristate acetate specifically phosphorylates Ser-167 within AF-1. In vitro kinase assays demonstrated that pp90rsk1 phosphorylates the N terminus of the wild-type ER but not of a mutant ER in which Ser-167 was replaced by Ala. In vivo, EGF stimulated phosphorylation of Ser-167 as well as Ser-118. Ectopic expression of active pp90rsk1 increased the level of phosphorylation of Ser-167 compared to that of either a mutant pp90rsk1, which is catalytically inactive in the N-terminal kinase domain, or to that of vector control. The ER formed a stable complex with the mutant pp90rsk1 in vivo. Transfection of the mutant pp90rsk1 depressed ER-dependent transcription of both a wild-type ER and a mutant ER that had a defective AF-2 domain (ER TAF-1). Furthermore, replacing either Ser-118 or Ser-167 with Ala in ER TAF-1 showed similar decreases in transcription levels. A double mutant in which both Ser-118 and Ser-167 were replaced with Ala demonstrated a further decrease in transcription compared to either of the single mutations. Taken together, our results strongly suggest that pp90rsk1 phosphorylates Ser-167 of the human ER in vivo and that Ser-167 aids in regulating the transcriptional activity of AF-1 in the ER.

MAPKAPK5, a novel mitogen-activated protein kinase (MAPK)-activated protein kinase, is a substrate of the extracellular-regulated kinase (ERK) and p38 kinase

A novel protein kinase that has significant sequence homology to mitogen-activated protein kinase (MAPK)-activated protein kinase (MAPKAPK) was identified. This novel protein kinase has a nucleotide sequence that encodes a protein of 473 amino acids and shares 45%, 46%, and 44% amino acid sequence identities to MAPKAPK2, 3 and 4 respectively. Northern blot analysis revealed that it has a wide tissue distribution. This novel protein kinase designated MAPKAPK5 can be phosphorylated by extracellular-regulated kinase (ERK), and p38 kinase but not by c-jun N-terminal kinase (JNK) in vitro. Recombinant GST-MAPKAPK5 protein can phosphorylate a peptide derived from the regulatory light chain of myosin II. Phosphorylation of MAPKAPK5 by ERK and p38 kinase increased its activity by 9 and 15 fold respectively. Taken together, these data suggest that MAPKAPK5 is a novel in vitro substrate for ERK and p38 kinase.

Urea activates ribosomal S6 kinase (RSK) in a MEK-dependent fashion in renal mIMCD3 cells

Urea activates a characteristic subset of signaling pathways in a tissue-specific fashion, including transcription of immediate early genes through activation of the mitogen-activated protein kinase (MAPK), ERK (extracellular signal-regulated kinase), and activation of its transcription factor substrate, Elk-1. The ability of urea to activate the ERK effector and pivotal regulatory kinase, ribosomal S6 kinase (RSK), was investigated in mIMCD3 renal inner medullary collecting duct cells. Urea upregulated RSK activity in a time-dependent fashion in serum-deprived mIMCD3 cells; the effect was maximal at 5 min. Activation by hypertonic NaCl, in contrast, was negligible at 5 min and peaked at 15 min. Both stimuli induced the nuclear translocation of cytosolic RSK, as determined via immunofluorescence. Importantly, activation of RSK by both solutes was MAPK/ERK kinase (MEK) dependent, as determined by the ability of the specific MEK inhibitor, PD-98059, to abrogate the response. Taken together, these data indicate that urea activates the ERK effector, RSK, in cells of the renal medulla in an ERK-dependent fashion, further emphasizing the functional significance of urea signaling through ERK activation in renal medullary cells.

Activation of mitogen-activated protein kinases (ERK/JNK) and AP-1 transcription factor in rat carotid arteries after balloon injury

Smooth muscle cell proliferation is a key event in neointimal formation after balloon angioplasty. The molecular signals that mediate this process have yet to be identified. Mitogen-activated protein (MAP) kinases are thought to play a pivotal role in transmitting transmembrane signals required for cell proliferation in vitro. The present studies were designed to investigate whether the signal transduction pathways of MAP kinases were involved in the development of restenosis in the injured arteries. Rat carotid arteries were isolated at various time points after balloon injury, and activities of MAP kinases, including extracellular signal-regulated kinases (ERK), and stress activated protein kinases (SAPK)/c-Jun N-terminal protein kinases (JNK), were determined in protein extracts of the vasculature using protein kinase assay and Western blot analysis. After balloon angioplasty, ERK2 and JNK1 activities in the vessel wall increased rapidly, reached a high level in 5 minutes and maintained for 1 hour. A sustained increase in ERK2 kinase activity was observed over the next 7 days in the arterial wall and 14 days in neointima after injury. In contrast, opposite and uninjured arteries did not show significant changes in these kinase activities. Concomitantly, Western blot analysis confirmed that the ERK2 kinase in the injured vessels was indeed activated or phosphorylated, showing a slowly migrating species of a 42-kDa protein containing phosphorylated tyrosine. Kinase activation is followed by an increase in c-fos and c-jun gene expression and enhanced activator protein 1 (AP-1) DNA-binding activity. Thus, balloon injury rapidly activates the MAP kinases in rat carotid arteries. These kinase activations may be crucial in mediating smooth muscle cell proliferation in response to vascular angioplasty.

Analysis of the phosphorylation of human heat shock transcription factor-1 by MAP kinase family members

The activation of heat shock transcription factor-1 (HSF-1) after treatment of mammalian cells with stresses such as heat shock, heavy metals, or ethanol induces the synthesis of heat shock proteins. HSF-1 is phosphorylated at normal growth temperature and is hyperphosphorylated upon stress. We recently presented evidence that HSF-1 can be phosphorylated by the mitogen activated protein kinase, ERK1, and that such phosphorylation appears to negatively regulate the activity of HSF-1. In this report, we have tested the ability of ERK1 to phosphorylate various HSF-1 deletion mutants. Our results show that ERK1 phosphorylation is dependent on a region of HSF-1 extending from amino acids 280 to 308. This region contains three serine residues that are potential ERK1 phosphorylation sites. The region falls within a previously defined regulatory domain of HSF-1. The possibility of protein kinases other than ERK1 phosphorylating HSF-1 was also examined using in-gel kinase assays. The results show that HSF-1 can be phosphorylated in a ras-dependent manner by other members of the MAP kinase family such as JNK and p38 protein kinases and possibly others.

Bacterial expression and purification of biologically active mouse c-Fos proteins by selective codon optimization

A simple strategy using selective codon optimization was devised to express mouse c-Fos protein in high levels in E. coli. Ten arginine codons located in the basic region were optimized to achieve high levels of protein expression. The c-Fos protein was purified to near homogeneity and was demonstrated to be biologically active by assaying its several biological activities.

Mitogenic stimulation of resting T cells causes rapid phosphorylation of the transcription factor LSF and increased DNA-binding activity

The mammalian transcription factor LSF (CP2/LBP-1c) binds cellular promoters modulated by cell growth signals. We demonstrate here that LSF-DNA-binding activity is strikingly regulated by induction of cell growth in human peripheral T lymphocytes. Within 15 min of mitogenic stimulation of these cells, the level of LSF-DNA-binding activity increased by a factor of five. The level of LSF protein in the nucleus remained constant throughout this interval. However, a rapid decrease in the electrophoretic mobility of LSF, attributable to phosphorylation, correlated with the increase in DNA-binding activity. pp44 (ERK1) phosphorylated LSF in vitro on the same residue that was phosphorylated in vivo, specifically at amino acid position 291, as indicated by mutant analysis. As direct verification of the causal relationship between phosphorylation and DNA-binding activity, treatment in vitro of LSF with phosphatase both increased the electrophoretic mobility of the protein and decreased LSF-DNA-binding activity. This modulation of LSF-DNA-binding activity as T cells progress from a resting to a replicating state reveals that LSF activity is regulated during cell growth and suggests that LSF regulates growth-responsive promoters.

Mitogen-activated protein kinases activate the serine/threonine kinases Mnk1 and Mnk2

Mitogen-activated protein (MAP) kinases bind tightly to many of their physiologically relevant substrates. We have identified a new subfamily of murine serine/threonine kinases, whose members, MAP kinase-interacting kinase 1 (Mnk1) and Mnk2, bind tightly to the growth factor-regulated MAP kinases, Erk1 and Erk2. MNK1, but not Mnk2, also binds strongly to the stress-activated kinase, p38. MNK1 complexes more strongly with inactive than active Erk, implying that Mnk and Erk may dissociate after mitogen stimulation. Erk and p38 phosphorylate MNK1 and Mnk2, which stimulates their in vitro kinase activity toward a substrate, eukaryotic initiation factor-4E (eIF-4E). Initiation factor eIF-4E is a regulatory phosphoprotein whose phosphorylation is increased by insulin in an Erk-dependent manner. In vitro, MNK1 rapidly phosphorylates eIF-4E at the physiologically relevant site, Ser209. In cells, Mnk1 is post-translationally modified and enzymatically activated in response to treatment with either peptide growth factors, phorbol esters, anisomycin or UV. Mitogen- and stress-mediated MNK1 activation is blocked by inhibitors of MAP kinase kinase 1 (Mkk1) and p38, demonstrating that Mnk1 is downstream of multiple MAP kinases. MNK1 may define a convergence point between the growth factor-activated and one of the stress-activated protein kinase cascades and is a candidate to phosphorylate eIF-4E in cells.

Exercise stimulates the mitogen-activated protein kinase pathway in human skeletal muscle

Physical exercise can cause marked alterations in the structure and function of human skeletal muscle. However, little is known about the specific signaling molecules and pathways that enable exercise to modulate cellular processes in skeletal muscle. The mitogen-activated protein kinase (MAPK) cascade is a major signaling system by which cells transduce extracellular signals into intracellular responses. We tested the hypothesis that a single bout of exercise activates the MAPK signaling pathway. Needle biopsies of vastus lateralis muscle were taken from nine subjects at rest and after 60 min of cycle ergometer exercise. In all subjects, exercise increased MAPK phosphorylation, and the activity of its downstream substrate, the p90 ribosomal S6 kinase 2. Furthermore, exercise increased the activities of the upstream regulators of MAPK, MAP kinase kinase, and Raf-1. When two additional subjects were studied using a one-legged exercise protocol, MAPK phosphorylation and p90 ribosomal S6 kinase 2, MAP kinase kinase 1, and Raf-1 activities were increased only in the exercising leg. These studies demonstrate that exercise activates the MAPK cascade in human skeletal muscle and that this stimulation is primarily a local, tissue-specific phenomenon, rather than a systemic response to exercise. These findings suggest that the MAPK pathway may modulate cellular processes that occur in skeletal muscle in response to exercise.

3pK, a novel mitogen-activated protein (MAP) kinase-activated protein kinase, is targeted by three MAP kinase pathways

Recently we have identified a mitogen-activated protein kinase (MAPK)-activated protein kinase, named 3pK (G. Sithanandam, F. Latif, U. Smola, R. A. Bernal, F.-M. Duh, H. Li, I. Kuzmin, V. Wixler, L. Geil, S. Shresta, P. A. Lloyd, S. Bader, Y. Sekido, K. D. Tartof, V. I. Kashuba, E. R. Zabarovsky, M. Dean, G. Klein, B. Zbar, M. I. Lerman, J. D. Minna, U. R. Rapp, and A. Allikmets, Mol. Cell. Biol. 16:868-876, 1996). In vitro characterization of the kinase revealed that 3pK is activated by ERK. It was further shown that 3pK is phosphorylated in vivo after stimulation of cells with serum. However, the in vivo relevance of this observation in terms of involvement of the Raf/MEK/ERK cascade has not been established. Here we show that 3pK is activated in vivo by the growth inducers serum and tetradecanoyl phorbol acetate in promyelocytic HL60 cells and transiently transfected embryonic kidney 293 cells. Activation of 3pK was Raf dependent and was mediated by the Raf/MEK/ERK kinase cascade. 3pK was also shown to be activated after stress stimulation of cells. In vitro studies with recombinant proteins demonstrate that in addition to ERK, members of other subgroups of the MAPK family, namely, p38RK and Jun-N-terminal kinases/stress-activated protein kinases, were also able to phosphorylate and activate 3pK. Cotransfection experiments as well as the use of a specific inhibitor of p38RK showed that these in vitro upstream activators also function in vivo, identifying 3pK as the first kinase to be activated through all three MAPK cascades. Thus, 3pK is a novel convergence point of different MAPK pathways and could function as an integrative element of signaling in both mitogen and stress responses.

Transient expression of the mitogen-activated protein kinase phosphatase MKP-1 (3CH134/ERP1) in the rat brain after limbic epilepsy

The immediate early gene-encoded enzyme, MAP kinase phosphatase 1 (MKP-1), is thought to be a key element in controlling cellular signalling pathways activated by MAP kinases. Since MAP kinase have been demonstrated to participate in neuronal stimulus-transcription coupling following seizure activity, the present study investigated the induction of MKP-1 in the rat brain after limbic epilepsy. MKP-1 expression was studied with a polyclonal antiserum by Western blots, immunocytochemistry and immuno-electron microscopy at different time periods between 1 and 24 h after kainic acid-induced limbic seizures. MKP-1 induction was identified in dentate granule cells of the hippocampus but not in pyramidal neurons, furthermore in neurons of the outer layers of the neocortex, as well as in neurons of the lateral nucleus of the bed of the stria terminalis. Immuno-electron microscopy demonstrated that MKP-1 was localized in the neuronal nucleus, where the substrate of MKP-1, activated MAP kinases, are also found. In view of the restricted areas of MKP-1 expression and the widespread areas of altered MAP kinases activity it can be concluded that in the majority of CNS populations other mechanisms than MKP-1 induction are responsible for the shut-off of MAP kinases following seizure activity. MKP-1 may contribute in the specific subpopulations where it is induced to the post-translational control of inducible transcription factors of the fos, jun and myc family.

Coupling of the RAS-MAPK pathway to gene activation by RSK2, a growth factor-regulated CREB kinase

A signaling pathway has been elucidated whereby growth factors activate the transcription factor cyclic adenosine monophosphate response element-binding protein (CREB), a critical regulator of immediate early gene transcription. Growth factor-stimulated CREB phosphorylation at serine-133 is mediated by the RAS-mitogen-activated protein kinase (MAPK) pathway. MAPK activates CREB kinase, which in turn phosphorylates and activates CREB. Purification, sequencing, and biochemical characterization of CREB kinase revealed that it is identical to a member of the pp90(RSK) family, RSK2. RSK2 was shown to mediate growth factor induction of CREB serine-133 phosphorylation both in vitro and in vivo. These findings identify a cellular function for RSK2 and define a mechanism whereby growth factor signals mediated by RAS and MAPK are transmitted to the nucleus to activate gene expression.

YMXM motifs and signaling by an insulin receptor substrate 1 molecule without tyrosine phosphorylation sites

Tyrosine phosphorylation of insulin receptor substrate 1 (IRS-1) by the activated receptors for insulin, IGF-1, and various cytokines creates binding sites for signaling proteins with Src homology 2 domains (SH2 proteins). Determining the role of specific SH2 proteins during insulin signaling has been difficult because IRS-1 possesses as many as 18 potential tyrosine phosphorylation sites, several of which contain redundant motifs. Using 32D cells, which contain no endogenous IRS proteins, we compared the signaling ability of an IRS-1 molecule in which 18 potential tyrosine phosphorylation sites were replaced by phenylalanine (IRS-1(F18)) with two derivative molecules which retained three YMXM motifs (IRS-1(3YMXM)) or the two COOH-terminal SHP2-Fyn binding sites (IRS-1(YCT)). During insulin stimulation, IRS-1(F18) failed to undergo tyrosine phosphorylation or mediate activation of the phosphotidylinositol (PI) 3'-kinase or p70(s6k); IRS-1(YCT) was tyrosine phosphorylated but also failed to mediate these signaling events. Neither IRS-1(3YMXM) nor IRS-1(YCT) mediated activation of mitogen-activated protein kinases. IRS-1(F18) and IRS-1(YCT) partially mediated similar levels of insulin-stimulated mitogenesis at high insulin concentrations, however, suggesting that IRS-1 contains phosphotyrosine-independent elements which effect mitogenic signals, and that the sites in IRS-l(YCT) do not augment this signal. IRS-1(3YMXM) mediated the maximal mitogenic response to insulin, although the response to insulin was more sensitive with wild-type IRS-1. By contrast, the association of IRS-1(3YMXM) with PI 3'-kinase was more sensitive to insulin than the association with IRS-1. Thus, the binding of SH2 proteins (such as PI 3'-kinase) by YMXM motifs in IRS-1 is an important element in the mitogenic response, but other elements are essential for full mitogenic sensitivity.

The extracellular signal-regulated kinase pathway phosphorylates AML1, an acute myeloid leukemia gene product, and potentially regulates its transactivation ability

AML1 (also called PEBP2alphaB, CBFA2, or CBFalpha2) is one of the most frequently disrupted genes in chromosome abnormalities seen in human leukemias. It has been reported that AML1 plays several pivotal roles in myeloid hematopoietic differentiation and other biological phenomena, probably through the transcriptional regulation of various relevant genes. Here, we investigated the mechanism of regulation of AML1 functions through signal transduction pathways. The results showed that AML1 is phosphorylated in vivo on two serine residues within the proline-, serine-, and threonine-rich region, with dependence on the activation of extracellular signal-regulated kinase (ERK) and with interleukin-3 stimulation in a hematopoietic cell line. These in vivo phosphorylation sites of AML1 were phosphorylated directly in vitro by ERK. Although differences between wild-type AML1 and phosphorylation site mutants in DNA-binding affinity were not observed, we have shown that ERK-dependent phosphorylation potentiates the transactivation ability of AML1. Furthermore the phosphorylation site mutations reduced the transforming capacity of AML1 in fibroblast cells. These data indicate that AML1 functions are potentially regulated by ERK, which is activated by cytokine and growth factor stimuli. This study provides some important clues for clarifying unidentified facets of the regulatory mechanism of AML1 function.

Activation of p90rsk during meiotic maturation and first mitosis in mouse oocytes and eggs: MAP kinase-independent and -dependent activation

Mitogen-activated protein kinases (MAPK) become activated during the meiotic maturation of oocytes from many species; however, their molecular targets remain unknown. This led us to characterize the activation of the ribosomal subunit S6 kinase of Mr 82 X 10(3) - 92 X 10(3) (p90rsk; a major substrate of MAPK in somatic cells) in maturing mouse oocytes and during the first cell cycle of the mouse embryo. We assessed the phosphorylation state of p90rsk by examining the electrophoretic mobility shifts on immunoblots and measured the kinase activity of immunoprecipitated p90rsk on a S6-derived peptide. Germinal vesicle stage (GV) oocytes contained a doublet of Mr 82 × 10(3) and 84 × 10(3) with a low S6 peptide kinase activity (12% of the maximum level found in metaphase II oocytes). A band of Mr 86 × 10(3) was first observed 30 minutes after GV breakdown (GVBD) and became prominent within 2 to 3 hours. MAPK was not phosphorylated 1 hour after GVBD, when the p90rsk-specific S6 kinase activity reached 37 % of the M II level. 2 hours after GVBD, MAPK became phosphorylated and p90rsk kinase activity reached 86% of the maximum level. The p90rsk band of Mr 88 × 10(3), present in mature M II oocytes when S6 peptide kinase activity is maximum, appeared when MAPK phosphorylation was nearly complete (2.5 hours after GVBD). In activated eggs, the dephosphorylation of p90rsk to Mr 86 X 10(3) starts about 1 hour after the onset of pronuclei formation and continues very slowly until the beginning of mitosis, when the doublet of Mr 82 X 10(3) and 84 X 10(3) reappears. A role for a M-phase activated kinase (like p34cdc2) in p90rsk activation was suggested by the reappearance of the Mr 86 X 10(3) band during first mitosis and in 1-cell embryos arrested in M phase by nocodazole. The requirement of MAPK for the full activation of p90rsk during meiosis was demonstrated by the absence of the fully active Mr 88 X 10(3) band in maturing c-mos −/− oocytes, where MAPK is not activated. The inhibition of kinase activity in activated eggs by 6-DMAP after second polar body extrusion provided evidence that both MAPK- and p90rsk-specific phosphatases are activated at approximately the same time prior to pronuclei formation.

Regulation of transcription by MAP kinase cascades

Kinases belonging to the mitogen-activated protein kinase (MAPK) family are used throughout evolution to control the cellular responses to external signals such as growth factors, nutrient status, stress or inductive signals. Many important substrates for MAPKs are transcription factors, and both the genetic and the biochemical links between MAPKs and transcription factors are becoming increasingly well understood.

Evidence for two catalytically active kinase domains in pp90rsk

Mitogen-activated protein kinase and one of its targets, pp90rsk (ribosomal S6 kinase [RSK]), represent two serine/threonine kinases in the Ras-activated signalling cascade that are capable of directly regulating gene expression. pp90rsk has been shown to have two highly conserved and distinct catalytic domains. However, whether both domains are active and which domain is responsible for its various identified phosphotransferase activities have not been determined. Here we demonstrate that the N-terminal domain is responsible for its phosphotransferase activity towards a variety of substrates which contain an RXXS motif at the site of in vitro phosphorylation, including serum response factor, c-Fos, Nur77, and the 40S ribosomal protein S6. We also provide evidence that the C-terminal domain is catalytically active and can be further activated by mitogen-activated protein kinase phosphorylation.

Genetic interactions of pokkuri with seven in absentia, tramtrack and downstream components of the sevenless pathway in R7 photoreceptor induction in Drosophila melanogaster

The sevenless (sev) cascade plays an inductive role in formation of the R7 photoreceptor, whilst the pokkuri (pok) and tramtrack (ttk) gene products are known to repress R7 induction in developing ommatidia of Drosophila melanogaster. To elucidate how these positive and negative signalling mechanisms co-operate in the normal fate determination of R7, genetic interactions of mutations in the pok locus with ttk and downstream elements of sev including Gap1, raf1, rolled (r1) and seven in absentia (sina) were examined. The eye phenotype of a weak hypomorph, pok ¹⁵, was enhanced dominantly by Gap1-^mip, a recessive mutation in a gene encoding a down-regulator of Ras1, producing multiple R7 in ommatidia. Ras1 has been reported to activate r1-encoded mitrogen-activated protein (MAP) kinase via Raf1 that is associated physically with Rasl. Ommatidia of raf1 ^c110 and rl ²/rl^EMS64 typically lacked R7 and a few outer photoreceptors. The pok ¹ mutation suppressed dominantly the rafl ^c110 rl²/rl^EMS64 eye phenotypes, allowing single R7 cells to develop in ommatidia. The rafl ^c110 mutation improved adult viability of pok ¹ homozygotes. An in vitro experiment demonstrated that MAP kinase phosphorylates Pok protein. Ttk is a transcriptional repressor which binds to the regulatory sequence upstream of the fushi-tarazu (ftz), even skipped (eve) and engrailed (en) coding region. A reduced activity in ttk resulted in enhancement of the pok phenotype. ttk mutations produced extra R7 cells even in sina homozygotes whilst the pok mutation did not. This result indicates that Ttk represses R7 induction downstream of the sites where Pok and Sina function.

Hypoxia and hypoxia/reoxygenation activate Raf-1, mitogen-activated protein kinase kinase, mitogen-activated protein kinases, and S6 kinase in cultured rat cardiac myocytes

In response to hypoxia and reoxygenation, mammalian cells are known to express a variety of genes to adapt to these external stresses or lead to further cell damage. We investigated the intracellular signaling cascades in cultured rat cardiac myocytes subjected to hypoxia followed by reoxygenation (hypoxia/reoxygenation). Here, we show that both hypoxia and hypoxia/reoxygenation caused rapid activation of the mitogen-activated protein kinase kinase kinase (MAPKKK), activity of Raf-1. This was followed by the sequential activation of mitogen-activated protein kinase kinase (MAPKK), mitogen-activated protein (MAP) kinases, and S6 kinase (p90rsk). Furthermore, hypoxia caused hyperphosphorylation of Raf-1. The maximal hyperphosphorylation of Raf-1 appeared to be accompanied by a significant decrease in MAPKKK activity. These results strongly suggest the following: (1) Intracellular signals initiated by both hypoxia and hypoxia/reoxygenation converge on Raf-1 and activate its MAPKKK activity. Then, Raf1 activates downstream serine/threonine kinases including MAPKK, MAP kinases and p90rsk. (2) Raf-1 is not only located upstream from MAPKK and MAP kinases but also may be phosphorylated by MAP kinases directly or indirectly, and at least Raf-1 kinase activity may be downregulated by this feedback mechanism.

Mitogenic signals and transforming potential of Nyk, a newly identified neural cell adhesion molecule-related receptor tyrosine kinase

Nyk/Mer is a recently identified receptor tyrosine kinase with neural cell adhesion molecule-like structure (two immunoglobulin G-like domains and two fibronectin III-like domains) in its extracellular region and belongs to the Ufo/Axl family of receptors. The ligand for Nyk/Mer is presently unknown, as are the signal transduction pathways mediated by this receptor. We constructed and expressed a chimeric receptor (Fms-Nyk) composed of the extracellular domain of the human colony-stimulating factor 1 receptor (Fms) and the transmembrane and cytoplasmic domains of human Nyk/Mer in NIH 3T3 fibroblasts in order to investigate the mitogenic signaling and biochemical properties of Nyk/Mer. Colony-stimulating factor 1 stimulation of the Fms-Nyk chimeric receptor in transfected NIH 3T3 fibroblasts leads to a transformed phenotype and generates a proliferative response in the absence of other growth factors. We show that phospholipase C gamma, phosphatidylinositol 3-kinase/p70 S6 kinase, Shc, Grb2, Raf-1, and mitogen-activated protein kinase are downstream components of the Nyk/Mer signal transduction pathways. In addition, Nyk/Mer weakly activates p90rsk, while stress-activated protein kinase, Ras GTPase-activating protein (GAP), and GAP-associated p62 and p190 proteins are not activated or tyrosine phosphorylated by Nyk/Mer. An analysis comparing the Nyk/Mer signal cascade with that of the epidermal growth factor receptor indicates substrate preferences by these two receptors. Our results provide a detailed description of the Nyk/Mer signaling pathways. Given the structural similarity between the Ufo/Axl family receptors, some of the information may also be applied to other members of this receptor tyrosine kinase family.

Mitogen and stress response pathways: MAP kinase cascades and phosphatase regulation in mammals and yeast

Evolutionarily conserved from yeast to man, mitogen-activated protein kinase (MAPK) pathways respond to a variety of disparate signals which induce differentiation, proliferation, or changes in intracellular enzyme regulation. Recent advances have identified two new mammalian MAPK relatives, JNK1 and p38, and the pathways which are responsible for their activation.

Unexpected transcriptional signals in normal and mitotically defective cells mediated through cytokine and growth factor receptors

Polypeptide growth factors and cytokines mediate their biochemical functions through their responsive receptors. Known cytokine receptors do not possess intrinsic kinase domains whereas several polypeptide growth factor receptors do. Nevertheless, both classes of ligands are capable of activating sets of overlapping genes. In human epidermoid carcinoma cells, for example, both cytokines and epidermal growth factor (EGF) promote a common transcriptional activation signal through the tyrosine phosphorylation of stat91 (signal transducer and activator of transcription) proteins. The stat family of cytoplasmic proteins also appear to have dual functions. Tyrosine phosphorylated 'stats' are employed for signal transduction and, second, for activation of transcription of several genes. The transcription factor-SIE-DNA binding patterns are now known to be different for EGF and interferon-gamma IFN-gamma-treated cells. Nevertheless, in the active DNA-bound complex, the stat91 polypeptide is a component found in either EGF or INF-gamma-treated extracts. Other stat family members of transcription factors may also be present in the complexes. In this case, tyrosine phosphorylated stat91 polypeptides may form into homodimeric or heterodimeric assemblies with other stat-related transcription factors. We describe a novel stat-related factor, p93, that is found in EGF-treated A431 cell extracts but appears to be absent in bovine fibroblast growth factor (bFGF), IFN-gamma, tumor necrosis factor-alpha (TNF-alpha), and untreated cells. p93 appears to be antigenically related to stat91. p185c-neu+, EGFr+ (M1), and p185c-neu- kinase inactive, EGFr+ (NEN757) expressing cells undergo different mitotic responses to EGF. M1 can respond to EGF mitotically while NEN757 cannot. Both cell lines respond to 10 ng/ml of EGF and also to IFN-gamma in transducing transcriptional activation signals to the nucleus, despite the distinct growth response to EGF. Our work has analyzed the stat pathway in these types of cells and found similar patterns of usage despite the distinct EGF-responsive features. Cytoplasmic nonreceptor tyrosine kinases Jak1 and Jak2 may be involved in the activation of stat91 and other transcription factors in EGF and IFN-gamma signaling pathways. Collectively, these studies suggest that the major EGF-stimulated mitotic growth pathways may not be absolutely linked to the stat91 signaling pathways and that such transcription complexes are more complex than previously reported.

Yeast RLM1 encodes a serum response factor-like protein that may function downstream of the Mpk1 (Slt2) mitogen-activated protein kinase pathway

The MPK1 (SLT2) gene of Saccharomyces cerevisiae encodes a mitogen-activated protein kinase that is regulated by a kinase cascade whose known elements are Pkc1 (a homolog of protein kinase C), Bck1 (Slk1) (a homolog of MEK kinase), and the functionally redundant Mpk1 activators Mkk1 and Mkk2 (homologs of MEK). An activated mutation of MKK1, MKK1P386, inhibits growth when overexpressed. This growth-inhibitory effect was suppressed by the mpk1 delta mutation, suggesting that hyperactivation of the Mpk1 pathway is toxic to cells. To search for genes that interact with the Mpk1 pathway, we isolated both chromosomal mutations and dosage suppressor genes that ameliorate the growth-inhibitory effect of overexpressed Mkk1P386. One of the genes identified by the analysis of chromosomal mutations is RLM1 (resistance to lethality of MKK1P386 overexpression), which encodes a protein homologous to a conserved domain of the MADS (Mcm1, Agamous, Deficiens, and serum response factor) box family of transcription factors. Although rlm1 delta cells grow normally at any temperature, they display a caffeine-sensitive phenotype similar to that observed in mutants defective in BCK1, MKK1/MKK2, or MPK1. A gene fusion that provides Rlm1 with a transcriptional activation domain of Gal4 suppresses bck1 delta and mpk1 delta. A screening for dosage suppressors yielded the MSG5 genes, which encode a dual-specificity protein phosphatase. Our results suggest that Rlm1 functions as a transcription factor downstream of Mpk1 that is subject to activation by the Mpk1 mitogen-activated protein kinase pathway.

Activation of extracellular signal-regulated protein kinase (ERK/MAP kinase) following CD28 cross-linking: activation in cells lacking p56lck

T lymphocytes require two signals for activation. Recognition of antigen/MHC complexes by the T cell receptor delivers the first signal, while a second signal, delivered by the cell surface receptors CD80 and/or CD86 binding to the T cell surface molecule CD28, has been shown to be effective for the initiation of effective T cell responses. While some of the cytoplasmic effector molecules involved in T cell receptor signaling is known, little is known regarding those involved in the co-stimulation of T cells by CD28. Using the T cell leukemic cell line Jurkat as a model for T cell activation, we demonstrate that cross-linking CD28 using monoclonal antibodies causes tyrosine phosphorylation and activation of MAP kinase/ERK. This activation was rapid, peaking at approximately 5 minutes post CD28 cross-linking, and transient. Activation of MAP kinase/ERK occurred 3 fold less efficiently in a Jurkat line lacking functional p56lck (JCAM.1), and was almost undetectable in a line lacking CD45 (J45.01). These results suggest that CD28 cross-linking can activate intracellular signaling pathways via several different tyrosine kinases. Thus CD28 signaling can activate src family kinases lck and fyn, as well as the Tec family kinase emt/itk. Activation of any one or a combination of these tyrosine kinases may be sufficient for the activation of MAPK following CD28 cross-linking. Activation of MAPK has been shown to cause activation of AP-1 and other transcription factors via serine and/or threonine phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Divergent functional roles for p90rsk kinase domains

A unique and highly conserved structural feature of approximately 90-kDa ribosomal S6 kinase (p90rsk or RSK) is the presence of two non-identical kinase domains. To explore the mechanism of RSK activation, a cloned human RSK cDNA (RSK3) was used to generate and characterize several site-directed RSK mutants; K91A (N-Lys, NH2-terminal ATP-binding mutant), K444A (C-Lys, COOH-terminal ATP-binding mutant), N/C-Lys (double ATP-binding mutant) T570A (C-Thr, mutant of the putative MAPK phosphorylation site in subdomain VIII of the C-domain), S218A (N-Ser, mutant of the corresponding NH2-terminal residue). Epitope-tagged RSKs were expressed in transfected COS cells followed by immunoprecipitation with or without prior in vivo epidermal growth factor stimulation. Kinase activity (S6 peptide) of N/C-Lys and N-Lys was ablated (and partially impaired with N-Ser). In contrast, both C-Lys and C-Thr retained high levels of kinase activity and were capable of responding to stimulation. C-Lys also retained partial kinase activity toward other substrates (c-Fos, S40 ribosomes, protein phosphatase 1 G-subunit, histones, and Leu-Arg-Arg-Ala-Ser-Leu-Gly (Kemptide)) whereas N-Lys did not. The isolated NH2-and COOH-terminal domains were also expressed; the C-domain was inactive, whereas the N-domain retained partial activity. Relative to wild-type, both N-Lys and C-Lys (as well as N-Ser and C-Thr) underwent partial in vitro autophosphorylation that was further stimulated by EGF protein tyrosine phosphatase. We conclude that 1) the NH2-terminal RSK kinase domain mediates substrate phosphorylation; 2) both domains contribute to autophosphorylation; 3) the putative MAPK phosphorylation site is not required for growth factor-stimulated autophosphorylation or kinase activation.

RSK3 encodes a novel pp90rsk isoform with a unique N-terminal sequence: growth factor-stimulated kinase function and nuclear translocation

A novel pp90rsk Ser/Thr kinase (referred to as RSK3) was cloned from a human cDNA library. The RSK3 cDNA encodes a predicted 733-amino-acid protein with a unique N-terminal region containing a putative nuclear localization signal. RSK3 mRNA was widely expressed (but was predominant in lung and skeletal muscle). By using fluorescence in situ hybridization, the human RSK3 gene was localized to band q27 of chromosome 6. Hemagglutinin epitope-tagged RSK3 was expressed in transiently transfected COS cells. Growth factors, serum, and phorbol ester stimulated autophosphorylation of recombinant RSK3 and its kinase activity toward several protein substrates known to be phosphorylated by RSKs. However, the relative substrate specificity of RSK3 differed from that reported for other isoforms. RSK3 also phosphorylated potential nuclear target proteins including c-Fos and histones. Furthermore, although RSK3 was inactivated by protein phosphatase 2A in vitro, the enzyme was not activated by ERK2/mitogen-activated protein (MAP) kinase. In contrast, the kinase activity of another epitope-tagged RSK isoform (RSK-1) was significantly increased by in vitro incubation with ERK2/MAP kinase. Finally, we used affinity-purified RSK3 antibodies to demonstrate by immunofluorescence that endogenous RSK3 undergoes serum-stimulated nuclear translocation in cultured HeLa cells. These results provide evidence that RSK3 is a third distinct isoform of pp90rsk which translocates to the cell nucleus, phosphorylates potential nuclear targets, and may have a unique upstream activator. RSK3 may therefore subserve a discrete physiologic role(s) that differs from those of the other two known mammalian RSK isoforms.

Phosphorylation of tristetraprolin, a potential zinc finger transcription factor, by mitogen stimulation in intact cells and by mitogen-activated protein kinase in vitro

Tristetraprolin (TTP) is a potential transcription factor that contains three PPPPG repeats and two putative CCCH zinc fingers. TTP is encoded by the early response gene Zfp-36, which is highly expressed in response to growth factors and in several hematopoietic cell lines. In the present studies, we investigated the possibility that TTP is phosphorylated in intact cells. In NIH/3T3 cells that were made to overexpress TTP constitutively, we found that the protein was phosphorylated on serine residues, and that this phosphorylation was rapidly (within 10 min) stimulated by several mitogens. In cell-free assays, recombinant mouse TTP was a substrate for the mitogen-activated protein (MAP) kinase. By a combination of protease digestion experiments and site-directed mutagenesis strategies, we found that serine 220 was phosphorylated by p42 MAP kinase in vitro. Expression of mutant TTP in fibroblasts confirmed that serine 220 was one of the major, mitogen-stimulated phosphorylation sites on the protein in intact cells. These results suggest that TTP may be phosphorylated by MAP kinases in vivo and that this phosphorylation may regulate its function.

Activation of mitogen-activated protein kinase in porcine carotid arteries

The thin-filament protein h-caldesmon (the high molecular weight isoform of caldesmon) is phosphorylated in resting and contracted porcine carotid arteries. Phosphorylation of h-caldesmon in intact tissue occurs at sites that are covalently modified by mitogen-activated protein kinase (MAPK) in vitro. In this study, we have evaluated MAPK activation in arteries in response to mechanical load and pharmacological stimulation. MAPK was extracted from resting and stimulated porcine carotid arteries and then partially purified by anion-exchange fast-performance liquid chromatography. MAPK activity was separated into two peaks corresponding to the tyrosine-phosphorylated 42- and 44-kD isoforms of MAPK (p42MAPK and p44MAPK, respectively). Of the total MAPK activity, 42% was associated with p42MAPK, and 58% was associated with p44MAPK, this percentage was not altered by stimulation of the muscles with either KCl (110 mmol/L) or phorbol 12,13-dibutyrate (PDBu, 1 mumol/L). Both p42MAPK and p44MAPK, purified from porcine carotid arteries, phosphorylated h-caldesmon at the same sites and to levels approaching or > 1 mol phosphate per mole protein. In unloaded muscle strips, MAPK activity was 39 pmol.min-1.mg protein-1 when assayed with the peptide substrate APRTPG-GRR. MAPK activity increased in response to incremental mechanical loading to a maximum of 99 pmol.min-1.mg protein-1 at 16 x 10(3) N/m2. MAPK activity could be further increased in loaded muscles by pharmacological stimulation. With KCl stimulation, MAPK activities rose to a peak of 205 pmol.min-1.mg protein-1 at 10 minutes and then declined to basal values at 30 and 60 minutes.(ABSTRACT TRUNCATED AT 250 WORDS)

Does mitogen-activated-protein kinase have a role in insulin action? The cases for and against

The discovery of the mitogen-activated protein (MAP) kinase family of protein kinases has sparked off an intensive effort to elucidate their role in the regulation of many cellular processes. These protein kinases were originally identified based on their rapid activation by insulin. In this review we concentrate on examining the evidence for and against a role for the MAP kinases Erk-1 and Erk-2 in mediating the effects of insulin. While there is good evidence in favour of a direct role for MAP kinase in the growth-promoting effects of insulin and the regulation of Glut-1 and c-fos expression, and AP-1 transcriptional complex activity, this is by no means conclusive. MAP kinase may also play a role in the control of mRNA translation by insulin. On the other hand, the evidence suggests that MAP kinase is not sufficient for the acute regulation of glucose transport (Glut-4 translocation), glycogen synthesis, acetyl-CoA carboxylase or pyruvate dehydrogenase activity. The findings suggest that insulin may utilise at least three distinct signalling pathways which do not involve MAP kinase.

Cloning and characterization of p97MAPK, a novel human homolog of rat ERK-3

Mitogen-activated protein kinases, or extracellular signal-regulated kinases (ERKs), are serine/threonine protein kinases that are activated in response to a wide variety of extracellular stimuli and are encoded by a multigene family. Little is known about the function of the ERK-3 subfamily. To explore the molecular diversity of the ERK-3 subfamily, we isolated a novel human cDNA, designated Hu-ERK-3, from a fetal skeletal muscle library. Analysis of the complete 3,920-bp nucleotide sequence revealed that this clone encodes a predicted protein of 721 amino acids. In vitro transcription-translation generates a 97-kDa protein referred to as p97MAPK. Of all of the sequences compared, p97MAPK is the most homologous to rat ERK-3. Interestingly, although p97MAPK is highly (98%) homologous to ERK-3 at the amino acid level within the N-terminal two-thirds of the coding region, it diverges at the carboxyl terminus as a result of a unique extension of 178 amino acids. Although expression of p97MAPK was detected in all of the tissues tested by Northern (RNA) analysis, the most abundant expression was seen in skeletal muscle. An antibody raised against the unique C terminus recognized a 97-kDa protein in human cells. By using this antibody in an immune complex protein kinase assay, we have shown that treatment of human fibroblasts with serum or phorbol esters activates a myelin basic protein and histone H1 kinase activity in immunoprecipitates. p97MAPK appears to be the human homolog of rat ERK-3, and a member of this family is an active protein kinase.

Cloning and characterization of p97MAPK, a novel human homolog of rat ERK-3

Mitogen-activated protein kinases, or extracellular signal-regulated kinases (ERKs), are serine/threonine protein kinases that are activated in response to a wide variety of extracellular stimuli and are encoded by a multigene family. Little is known about the function of the ERK-3 subfamily. To explore the molecular diversity of the ERK-3 subfamily, we isolated a novel human cDNA, designated Hu-ERK-3, from a fetal skeletal muscle library. Analysis of the complete 3,920-bp nucleotide sequence revealed that this clone encodes a predicted protein of 721 amino acids. In vitro transcription-translation generates a 97-kDa protein referred to as p97MAPK. Of all of the sequences compared, p97MAPK is the most homologous to rat ERK-3. Interestingly, although p97MAPK is highly (98%) homologous to ERK-3 at the amino acid level within the N-terminal two-thirds of the coding region, it diverges at the carboxyl terminus as a result of a unique extension of 178 amino acids. Although expression of p97MAPK was detected in all of the tissues tested by Northern (RNA) analysis, the most abundant expression was seen in skeletal muscle. An antibody raised against the unique C terminus recognized a 97-kDa protein in human cells. By using this antibody in an immune complex protein kinase assay, we have shown that treatment of human fibroblasts with serum or phorbol esters activates a myelin basic protein and histone H1 kinase activity in immunoprecipitates. p97MAPK appears to be the human homolog of rat ERK-3, and a member of this family is an active protein kinase.

Transient activation of RAF-1, MEK, and ERK2 coincides kinetically with ternary complex factor phosphorylation and immediate-early gene promoter activity in vivo

We have investigated the early in vivo signaling events triggered by serum that lead to activation of the c-fos proto-oncogene in HeLa cells. Both RAF-1 and MEK kinase activities are fully induced within 3 min of serum treatment and quickly decrease thereafter, slightly preceding the activation and inactivation of p42MAPK/ERK2. ERK2 activity correlates tightly with a transient phosphatase-sensitive modification of ternary complex factor (TCF), manifested by the slower electrophoretic mobility of TCF-containing protein-DNA complexes. These induced complexes in turn correlate with the activity of the c-fos, egr-1, and junB promoters. Phorbol ester treatment induces the same events but with slower and prolonged kinetics. Inhibition of serine/threonine phosphatase activities by okadaic acid treatment reverses the repression of the c-fos promoter either after induction or without induction. This corresponds to the presence of the induced complexes and of ERK2 activity, as well as to the activation of a number of other kinases. Inhibition of tyrosine phosphatase activities by sodium vanadate treatment delays but does not block ERK2 inactivation, TCF dephosphorylation, and c-fos repression. The tight linkage in vivo between the activity of MAP kinase, TCF phosphorylation, and immediate-early gene promoter activity is consistent with the notion that a stable ternary complex over the serum response element is a direct target for the MAP kinase signaling cascade. Furthermore, serine/threonine phosphatases are implicated in regulating the kinase cascade, as well as the state of TCF modification and c-fos promoter activity, in vivo.

c-Fos transcriptional activity stimulated by H-Ras-activated protein kinase distinct from JNK and ERK

Ras proteins exert their mitogenic and oncogenic effects through activation of downstream protein kinases. An important question is how Ras-generated signals reach the nucleus to activate downstream target genes. AP-1, a heterodimeric complex of Jun and Fos proteins, which activates mitogen-inducible genes, is a major nuclear target of Ras. Ras can stimulate AP-1 activity by inducing c-fos transcription, a process which is probably mediated by the ERK1 and -2 mitogen-activated protein (MAP) kinases, which phosphorylate the transcription factor Elk-1/TCF. Besides inducing transcription from fos and jun genes, mitogens and Ras proteins enhance AP-1 activity through phosphorylation of c-Jun. Phosphorylation of the c-Jun activation domain leads to c-jun induction through an autoregulatory loop. Ras- and ultra-violet-responsive protein kinases that phosphorylate c-Jun on serine residues at positions 63 and 73 and stimulate its transcriptional activity have been identified. These proline-directed kinases, termed JNKs, are novel MAP kinases. It is not clear, however, whether c-Jun is the only recipient and JNK the only transducer of the Ras signal to AP-1 proteins. A short sequence surrounding the major JNK phosphorylation site of c-Jun is conserved in c-Fos and is part of its activation domain, suggesting that c-Fos may be similarly regulated. Here we show that Ras does indeed augment the transcriptional activity of c-Fos through phosphorylation at Thr 232, the homologue of Ser 73 of c-Jun. However, this is mediated by a novel Ras- and mitogen-responsive proline-directed protein kinase that is different from JNKs and ERKs. Therefore, at least three types of proline-directed kinases transmit Ras- and mitogen-generated signals to the transcriptional machinery.

Transient activation of RAF-1, MEK, and ERK2 coincides kinetically with ternary complex factor phosphorylation and immediate-early gene promoter activity in vivo

We have investigated the early in vivo signaling events triggered by serum that lead to activation of the c-fos proto-oncogene in HeLa cells. Both RAF-1 and MEK kinase activities are fully induced within 3 min of serum treatment and quickly decrease thereafter, slightly preceding the activation and inactivation of p42MAPK/ERK2. ERK2 activity correlates tightly with a transient phosphatase-sensitive modification of ternary complex factor (TCF), manifested by the slower electrophoretic mobility of TCF-containing protein-DNA complexes. These induced complexes in turn correlate with the activity of the c-fos, egr-1, and junB promoters. Phorbol ester treatment induces the same events but with slower and prolonged kinetics. Inhibition of serine/threonine phosphatase activities by okadaic acid treatment reverses the repression of the c-fos promoter either after induction or without induction. This corresponds to the presence of the induced complexes and of ERK2 activity, as well as to the activation of a number of other kinases. Inhibition of tyrosine phosphatase activities by sodium vanadate treatment delays but does not block ERK2 inactivation, TCF dephosphorylation, and c-fos repression. The tight linkage in vivo between the activity of MAP kinase, TCF phosphorylation, and immediate-early gene promoter activity is consistent with the notion that a stable ternary complex over the serum response element is a direct target for the MAP kinase signaling cascade. Furthermore, serine/threonine phosphatases are implicated in regulating the kinase cascade, as well as the state of TCF modification and c-fos promoter activity, in vivo.

Functional divergence of the MAP kinase pathway. ERK1 and ERK2 activate specific transcription factors

Growth factor-receptor interactions at the cell surface eventually leading to the transcriptional activation of immediate early genes is mediated by the mitogen-activated protein kinase (MAP kinase/MAPK) cascade. Here we show that overexpression of extracellular signal-regulated kinase 1 (ERK1) cDNA, encoding p44mapk, results in the activation of Elk-1, the serum response factor accessory protein. We also show that overexpression of ERK2, encoding p42mapk, activates Myc, but not Elk-1. Therefore, the MAP kinase cascade diverges with at least one specific target for each MAP kinase isoform and provides a novel mechanism for differential regulation of this signaling pathway.