Targeting of p38 mitogen-activated protein kinases to MEF2 transcription factors

Caspase 3 activity is required for skeletal muscle differentiation

The cellular alterations associated with skeletal muscle differentiation share a high degree of similarity with key phenotypic changes usually ascribed to apoptosis. For example, actin fiber disassembly/reorganization is a conserved feature of both apoptosis and differentiating myoblasts and the conserved muscle contractile protein, myosin light chain kinase, is required for the apoptotic feature of membrane blebbing. As such, these observations suggest that the induction of differentiation and apoptosis in the myogenic lineage may use overlapping cellular mechanisms. Here, we report that skeletal muscle differentiation depends on the activity of the key apoptotic protease, caspase 3. Peptide inhibition of caspase 3 activity or homologous deletion of caspase 3 leads to dramatic reduction in both myotube/myofiber formation and expression of muscle-specific proteins. Subsequently, we have identified Mammalian Sterile Twenty-like kinase as a crucial caspase 3 effector in this cellular process. Mammalian Sterile Twenty-like kinase is cleavage-activated by caspase 3, and restoration of this truncated kinase in caspase 3 null myoblasts restores the differentiation phenotype. Taken together, these results confirm a unique and unanticipated role for a caspase 3-mediated signal cascade in the promotion of myogenesis.

The ETS domain transcription factor Elk-1 contains a novel class of repression domain

The ETS domain transcription factor Elk-1 serves as an integration point for different mitogen-activated protein (MAP) kinase pathways. Phosphorylation of Elk-1 by MAP kinases triggers its activation. However, while the activation process is well understood, its downregulation-inactivation is less well characterized. The ETS DNA-binding domain plays a role in the downregulation of Elk-dependent promoter activity following mitogenic activation by recruiting the mSin3A-HDAC complex. Here we have identified a novel evolutionarily conserved repression domain in Elk-1, termed the R motif, which serves to reduce the basal transcriptional activity of Elk-1 and dampen its response to mitogenic signals. This domain is highly potent and portable and can repress transcription in trans. The R motif is related to the CRD1 repression domain in p300 and can functionally replace this domain and confer p21(waf1/cip1) inducibility on p300. However, the R motif acts in a context-dependent manner and is not p21(waf1/cip1) responsive in Elk-1. Thus, the Elk-1 R motif and the p300 CRD1 motif represent a new class of repression domains that are regulated in a context-dependent manner.

Modular structure of a docking surface on MAPK phosphatases

Mitogen-activated protein kinases (MAPKs) must be precisely inactivated to achieve proper functions in the cells. Ten members of dual specificity phosphatases specifically acting on MAPKs, termed MAPK phosphatases (MKPs), have been reported. Each member has its own substrate specificity that should be tightly regulated. However, the molecular mechanism underlying the regulation of the specificity is largely unknown. In the MAPK signaling pathways, docking interactions, which are different from transient enzyme-substrate interaction, are known to regulate the enzymatic specificity. Here we have identified and characterized a docking surface of MKPs. Our results show that a docking surface is composed of a tandem alignment of three subregions (modules): a cluster of positively charged amino acids, a cluster of hydrophobic amino acids, and a cluster of positively charged amino acids (positive-hydrophobic-positive). This modular structure well fits the docking groove on MAPKs that we have previously identified and may contribute to regulating the docking specificity of the MKP family. The position, number, and species of charged amino acids in each module including the central hydrophobic subregion are important factors in regulation of docking to specific MAPKs. This modular structure in the docking interaction may define a novel model of protein-protein interaction that would also regulate other systems.

Crystal structures of MAP kinase p38 complexed to the docking sites on its nuclear substrate MEF2A and activator MKK3b

The structures of the MAP kinase p38 in complex with docking site peptides containing a phi(A)-X-phi(B) motif, derived from substrate MEF2A and activating enzyme MKK3b, have been solved. The peptides bind to the same site in the C-terminal domain of the kinase, which is both outside the active site and distinct from the "CD" domain previously implicated in docking site interactions. Mutational analysis on the interaction of p38 with the docking sites supports the crystallographic models and has uncovered two novel residues on the docking groove that are critical for binding. The two peptides induce similar large conformational changes local to the peptide binding groove. The peptides also induce unexpected and different conformational changes in the active site, as well as structural disorder in the phosphorylation lip.

Activation of the MEF2 transcription factor in skeletal muscles from myotonic mice

Becker syndrome, a recessive nondystrophic myotonia caused by mutations in the chloride channel 1 gene (CLCN1), is characterized by delayed muscle relaxation after contraction. The ADR (arrested development of righting response) mouse is an animal model for Becker syndrome. Skeletal muscles from ADR myotonic animals show an increased number of oxidative fibers with a lack of glycolytic fibers as well as signs of muscle hypertrophy. Through breeding ADR myotonic mice with mice harboring a MEF2-dependent reporter gene, we found that the transcriptional activity of MEF2 was dramatically enhanced in myotonic muscles. Post-translational induction of MEF2 transcriptional activity correlated with the activation of p38 MAPK and did not affect MEF2 DNA-binding affinity. Expression of class II histone deacetylases (HDACs), which repress MEF2-dependent gene expression, was significantly reduced in skeletal muscles from myotonic mice. These findings suggest that the combined effects of class II HDAC deficiency and p38 MAPK activation lead to potent upregulation of MEF2 transcriptional activity, which contributes to the long-term changes in gene expression and fiber-type transformation observed in myotonic skeletal muscles. These findings provide new molecular targets for potential treatment of congenital myotonia.

Transient protein-protein interactions and a random-ordered kinetic mechanism for the phosphorylation of a transcription factor by extracellular-regulated protein kinase 2

No thorough mechanistic study of extracellular signal-regulated protein kinase 2 (ERK2) has appeared in the literature. A recombinant protein termed EtsDelta138, which comprises of residues 1-138 of the transcription factor Ets-1 is an excellent substrate of ERK2 (Waas W. F., and Dalby, K. N. (2001) Protein Exp. Purif. 23, 191-197). The kinetic mechanism of ERK2 was examined, with excess magnesium, by initial velocity measurements, both in the absence and presence of products at 27 degrees C, pH 7.5, and ionic strength 0.1 m (KCl). The velocity data are consistent with a steady-state random-ordered ternary complex mechanism, where both substrates have unhindered access to binding sites on the enzyme. The mechanism and magnitude of product inhibition by monophosphorylated EtsDelta138 is consistent with, but does not prove, the notion that ERK2 forms a discrete interaction with EtsDelta138 in the absence of active site interactions, and that this "docking complex" facilitates intramolecular phosphorylation of the substrate. The approximation of the steady-state data to a rapid equilibrium model strongly suggests that the formation of ERK2.Ets138 complexes are transient in nature with dissociation constants of greater magnitude than the catalytic constant, of k(cat) = 17 s(-1).

Microphthalmia transcription factor is a target of the p38 MAPK pathway in response to receptor activator of NF-kappa B ligand signaling

Receptor activator of NF-kappaB ligand (RANKL) activates signaling pathways that regulate osteoclast differentiation, function, and survival. The microphthalmia transcription factor (MITF) is required for terminal differentiation of osteoclasts. To determine whether MITF could be a target of RANKL signaling, a phosphospecific MITF antibody directed against conserved residue Ser(307), a potential mitogen-activated protein kinase (MAPK) site, was produced. Using this antibody, we could demonstrate that MITF was rapidly and persistently phosphorylated upon stimulation of primary osteoclasts with RANKL and that phosphorylation of Ser(307) correlated with expression of the target gene tartrate-resistant acid phosphatase. MITF phosphorylation at Ser(307) also correlated with persistent activation of p38 MAPK, and p38 MAPK could utilize MITF Ser(307) as a substrate in vitro. The phosphorylation of MITF and activation of target gene expression in osteoclasts were blocked by p38 MAPK inhibitor SB203580. In transient transfections, a constitutively active Rac1 or MKK6 gene could collaborate with MITF to activate the tartrate-resistant acid phosphatase gene promoter dependent on Ser(307). Dominant negative p38 alpha and beta could inhibit the collaboration between upstream signaling components and MITF in the transient assays. These results indicate that MITF is a target for the RANKL signaling pathway in osteoclasts and that phosphorylation of MITF leads to an increase in osteoclast-specific gene expression.

Specificity determinants in MAPK signaling to transcription factors

One critical component in determining the specificity, fidelity, and efficiency of MAPK substrate phosphorylation is the presence of distinct docking domains in the substrate proteins. These docking domains are found in a range of substrates, including the transcription factors myocyte enhancer factor-2A and SAP-1. However, the sequences of these motifs differ, as does their targeting preferences by MAPKs, with SAP-1 being targeted by both the ERK and p38 isoforms, whereas myocyte enhancer factor-2A is targeted only by certain members of the p38 subfamily. Here, we have investigated the specificity determinants within these motifs and generated a model for how specificity is generated. We demonstrate that residues throughout the docking domains play important roles in the recognition process. However, residues located at different positions are important for discriminating between ERK and p38 MAPKs. Furthermore, the docking domains can be further subdivided into submotifs, which are differentially required for phosphorylation by ERK or p38 MAPKs. We have used loss- and gain-of-function mutagenesis to identify residues that discriminate between ERK and p38 MAPKs, residues that act to promote suboptimal interactions, and regions that are differentially required depending on the kinase involved. A model is proposed to explain how specificity is generated within these short docking domains.

MEF2-mediated recruitment of class II HDAC at the EBV immediate early gene BZLF1 links latency and chromatin remodeling

In B lymphocytes induced to proliferate in vitro by the Epstein-Barr virus (EBV), extra-chromosomal viral episomes packaged in chromatin persist in the nucleus, and there is no productive cycle. A switch from this latency to the productive cycle is observed after induced expression of the EBV BZLF1 gene product, the transcription factor EB1. We present evidence that, during latency, proteins of the myocyte enhancer binding factor 2 (MEF2) family are bound to the BZLF1 promoter and recruit class II histone deacetylases. Furthermore, we propose that latency is determined primarily by a specific and local recruitment of class II histone deacetylase (HDAC) by MEF2D to the BZLF1 gene promoter. The switch from latency to the productive cycle could be due in part to post-translational modification of MEF2 proteins and changes in the local acetylation state of the chromatin.

Docking interactions in the mitogen-activated protein kinase cascades

Regulation of cellular functions and responses utilizes a number of the signal transduction pathways. Each pathway should transduce signals with high efficiency and fidelity to avoid unnecessary crosstalks. The mitogen-activated protein kinase (MAPK) cascades regulate a wide variety of cellular functions, including cell proliferation, differentiation, and stress responses. MAPK is activated by MAPK kinase; phosphorylates various targets, including transcription factors and MAPK-activated protein kinases; and is inactivated by several phosphatases. Recent studies have provided a cue to understand the molecular mechanism underlying the signal transduction through the MAPK cascades. In the MAPK cascades, docking interactions, which are achieved through a site outside the catalytic domain of MAPKs, regulate the efficiency and specificity of the enzymatic reactions. The docking interaction is different from a transient enzyme-substrate interaction through the active center. It has been shown that activators, substrates, and inactivators of MAPKs utilize a common site on MAPKs in the docking interaction. Then, the docking interaction may regulate not only the efficiency and specificity of the cascades, but also the ordered and integrated signaling.

Deletion of calcineurin and myocyte enhancer factor 2 (MEF2) binding domain of Cabin1 results in enhanced cytokine gene expression in T cells

Cabin1 binds calcineurin and myocyte enhancer factor 2 (MEF2) through its COOH-terminal region. In cell lines, these interactions were shown to inhibit calcineurin activity after T cell receptor (TCR) signaling and transcriptional activation of Nur77 by MEF2. The role of these interactions under physiological conditions was investigated using a mutant mouse strain that expresses a truncated Cabin1 lacking the COOH-terminal calcineurin and MEF2 binding domains. T and B cell development and thymocyte apoptosis were normal in mutant mice. In response to anti-CD3 stimulation, however, mutant T cells expressed significantly higher levels of interleukin (IL)-2, IL-4, IL-9, IL-13, and interferon gamma than wild-type T cells. The enhanced cytokine gene expression was not associated with change in nuclear factor of activated T cells (NF-AT)c or NF-ATp nuclear translocation but was preceded by the induction of a phosphorylated form of MEF2D in mutant T cells. Consistent with the enhanced cytokine expression, mutant mice had elevated levels of serum immunoglobulin (Ig)G1, IgG2b, and IgE and produced more IgG1 in response to a T cell-dependent antigen. These findings suggest that the calcineurin and MEF2 binding domain of Cabin1 is dispensable for thymocyte development and apoptosis, but is required for proper regulation of T cell cytokine expression probably through modulation of MEF2 activity.

Activation of MEF2 by muscle activity is mediated through a calcineurin-dependent pathway

Gene expression in skeletal muscles of adult vertebrates is altered profoundly by changing patterns of contractile work. Here we observed that the functional activity of MEF2 transcription factors is stimulated by sustained periods of endurance exercise or motor nerve pacing, as assessed by expression in trans genic mice of a MEF2-dependent reporter gene (desMEF2-lacZ). This response is accompanied by transformation of specialized myofiber subtypes, and is blocked either by cyclosporin A, a specific chemical inhibitor of calcineurin, or by forced expression of the endogenous calcineurin inhibitory protein, myocyte-enriched calcineurin interacting protein 1. Calcineurin removes phosphate groups from MEF2, and augments the potency of the transcriptional activation domain of MEF2 fused to a heterologous DNA binding domain. Across a broad range, the enzymatic activity of calcineurin correlates directly with expression of endogenous genes that are transcriptionally activated by muscle contractions. These results delineate a molecular pathway in which calcineurin and MEF2 participate in the adaptive mechanisms by which skeletal myofibers acquire specialized contractile and metabolic properties as a function of changing patterns of muscle contraction.

Control of muscle development by dueling HATs and HDACs

Skeletal muscle cells have provided an especially auspicious system in which to dissect the roles of chromatin structure in the control of cell growth, differentiation, and development. The MyoD and MEF2 families of transcription factors act cooperatively to regulate the expression of skeletal muscle-specific genes. Recent studies have shown that these two classes of transcription factors associate with histone acetyltransferases and histone deacetylases to control the activation and repression, respectively, of the muscle differentiation program. Signaling systems that regulate the growth and differentiation of muscle cells act, at least in part, by regulating the intracellular localization and associations of these chromatin remodeling enzymes with myogenic transcription factors. We describe the molecules and mechanisms involved in chromatin remodeling during skeletal muscle development.

Keratinocyte growth factor activates p38 MAPK to induce stress fibre formation in human prostate DU145 cells

FGF7/Keratinocyte growth factor (KGF) regulates the differentiation and development of the prostate epithelium, while over-expression of FGF8 and FGF1 are implicated in carcinogenesis of the prostate. We tested the hypothesis that different members of the FGF family function through different signalling molecules. In prostate DU145 cells, both FGF1 and FGF2 activated ERK1/2 potently and p38 moderately. KGF was however most efficient in inducing p38 activities but had no effect on ERK1/2 function. JNK and STAT activities were not induced by FGFs in prostate cells. In vitro expression of the transcription factors Elk-1 and MEF2A (substrates for ERK1/2 and p38, respectively) for functional quantification, confirmed the pattern of FGF-induced MAPK activations in COS-7 cells. Furthermore, KGF was more efficient than FGF1 and FGF2 in inducing actin stress fibres, and the specific p38 inhibitor SB202190 completely abolished this in a dose-dependent manner. The MEK1/2 inhibitor, U0126, had no effect on FGF-induced stress fibre formation. This study demonstrates the selective activation of MAPK family members by FGFs resulting in activation of transcription factors and stress fibre formation. As multiple FGFs are over-expressed in human prostate cancer, characterization of the distinct signalling pathway by FGFs may reveal new specific targets for therapy.

Differential localization of HDAC4 orchestrates muscle differentiation

The class II histone deacetylases HDAC4 and HDAC5 interact specifically with the myogenic MEF2 transcription factor and repress its activity. Here we show that HDAC4 is cytoplasmic during myoblast differentiation, but relocates to the nucleus once fusion has occurred. Inappropriate nuclear entry of HDAC4 following overexpression suppresses the myogenic programme as well as MEF2-dependent transcription. Activation of the Ca(2+)/calmodulin signalling pathway via constitutively active CaMKIV prevents nuclear entry of HDAC4 and HDAC4-mediated inhibition of differentiation. Consistent with a role of phosphorylation in HDAC4 cytoplasmic localisation, HDAC4 binds to 14-3-3 proteins in a phosphorylation-dependent manner. Together these data establish a role for HDAC4 in muscle differentiation. Recently, HDAC5 has also been implicated in muscle differentiation. However, despite the functional similarities of HDAC4 and HDAC5, their intracellular localisations are opposed, suggesting a distinct role for these enzymes during muscle differentiation.

Intestinal epithelial cell differentiation involves activation of p38 mitogen-activated protein kinase that regulates the homeobox transcription factor CDX2

The intracellular signaling pathways responsible for cell cycle arrest and differentiation along the crypt-villus axis of the human small intestine remain largely unknown. p38 mitogen-activated protein kinases (MAPKs) have recently emerged as key modulators of various vertebrate cell differentiation processes. In order to elucidate further the mechanism(s) responsible for the loss of proliferative potential once committed intestinal cells begin to differentiate, the role and regulation of p38 MAPK with regard to differentiation were analyzed in both intact epithelium as well as in well established intestinal cell models recapitulating the crypt-villus axis in vitro. Results show that phosphorylated and active forms of p38 were detected primarily in the nuclei of differentiated villus cells. Inhibition of p38 MAPK signaling by 2-20 microm SB203580 did not affect E2F-dependent transcriptional activity in subconfluent Caco-2/15 or HIEC cells. p38 MAPK activity dramatically increased as soon as Caco-2/15 cells reached confluence, whereas addition of SB203580 during differentiation of Caco-2/15 cells strongly attenuated sucrase-isomaltase gene and protein expression as well as protein expression of villin and alkaline phosphatase. The binding of CDX2 to the sucrase-isomaltase promoter and its transcriptional activity were significantly reduced by SB203580. Pull-down glutathione S-transferase and immunoprecipitation experiments demonstrated a direct interaction of CDX3 with p38. Finally, p38-dependent phosphorylation of CDX3 was observed in differentiating Caco-2/15 cells. Taken together, our results indicate that p38 MAPK may be involved in the regulation of CDX2/3 function and intestinal cell differentiation.

The p38 MAPK pathway is required for cell growth inhibition of human breast cancer cells in response to activin

Activin, a member of the TGFbeta family inhibits cell growth in various target tissues. Activin interacts with a complex of two receptors that upon activation phosphorylate specific intracellular mediators, the Smad proteins. The activated Smads interact with diverse DNA binding proteins and co-activators of transcription in a cell-specific manner, thus leading to various activin biological effects. In this study, we investigated the role and mechanism of action of activin in the human breast cancer T47D cells. We found that activin treatment of T47D cells leads to a dramatic decrease in cell growth. Thus activin appears as a potent cell growth inhibitor of these breast cancer cells. We show that activin induces the Smad pathway in these cells but also activates the p38-mitogen-activated protein kinase pathway, further leading to phosphorylation of the transcription factor ATF2. Finally, specific inhibitors of the p38 kinase (SB202190, SB203580, and PD169316) but not an inactive analogue (SB202474) or the MEK-1 inhibitor PD98059 completely abolish the activin-mediated cell growth inhibition of T47D cells. Together, these results define a new role for activin in human breast cancer T47D cells and highlight a new pathway utilized by this growth factor in the mediation of its biological effects in cell growth arrest.

The MEF2A and MEF2D isoforms are differentially regulated in muscle and adipose tissue during states of insulin deficiency

Previously we have demonstrated that striated muscle GLUT4 gene expression decreased following streptozotocin-induced diabetes due to a loss of MEF2A transcription factor expression without any significant effect on the MEF2D isoform (Mora, S. and J. E. Pessin (2000) J Biol Chem, 275:16323-16328). In contrast to both cardiac and skeletal muscle, adipose tissue displays a selective decrease in MEF2D expression in diabetes without any significant alteration in MEF2A protein content. Adipose tissue also expresses very low levels of the MEF2 transcription factors and nuclear extracts from white adipose tissue exhibit poor in vitro binding to the MEF2 element. However, addition of in vitro synthesized MEF2A to adipose nuclear extracts results in the formation of the expected MEF2/DNA complex. More importantly, binding to the MEF2 element was also compromised in the diabetic condition. Furthermore, in vivo overexpression of MEF2A selectively in adipose tissue did not affect GLUT4 or MEF2D expression and was not sufficient to prevent GLUT4 down-regulation that occurred in insulin-deficient states.

Monocyte Chemoattractant Protein-1 Production by Intestinal Epithelial CellsIn Vitro:A Role for p38 in Epithelial Chemokine Expression

The intestinal epithelial cell (IEC) represents the first cellular barrier to infection. Consistent with this sentinel role, IEC are known to produce a variety of chemokines in response to bacterial infection or proinflammatory cytokines. These chemokines act as potent leukocyte activators and chemoattractants in vivo. In this report, we begin to characterize the regulation of expression of the chemokine monocyte chemoattractant protein-1 (MCP-1) in the rat small intestinal IEC-18 line. Following stimulation with either interleukin-1beta (IL-1beta) or lipopolysaccharide (LPS), IEC-18 cells produced MCP-1, with IL-1 proving a more effective stimulus than LPS at both the mRNA and protein levels. Expression of MCP-1 due to either stimulus was inhibited by tyrosine kinase inhibitors, prompting us to investigate potential phosphotyrosine-dependent targets responsible for MCP-1 expression. We detected activation of p38, a member of the mitogen-activated protein kinase family, following either IL-1 or LPS treatment. Specific inhibition of this kinase using the compound SB203580 caused a destabilization of MCP-1 mRNA. These data point to a role for p38 in the regulation of MCP-1 mRNA expression by the IEC.

Mitogen-activated protein (MAP) kinase pathways: regulation and physiological functions

Mitogen-activated protein (MAP) kinases comprise a family of ubiquitous proline-directed, protein-serine/threonine kinases, which participate in signal transduction pathways that control intracellular events including acute responses to hormones and major developmental changes in organisms. MAP kinases lie in protein kinase cascades. This review discusses the regulation and functions of mammalian MAP kinases. Nonenzymatic mechanisms that impact MAP kinase functions and findings from gene disruption studies are highlighted. Particular emphasis is on ERK1/2.

Control of cardiac-specific transcription by p300 through myocyte enhancer factor-2D

The transcriptional integrator p300 regulates gene expression by interaction with sequence-specific DNA-binding proteins and local remodeling of chromatin. p300 is required for cardiac-specific gene transcription, but the molecular basis of this requirement is unknown. Here we report that the MADS (MCM-1, agamous, deficiens, serum response factor) box transcription factor myocyte enhancer factor-2D (MEF-2D) acts as the principal conduit for cardiac transcriptional activation by p300. p300 activation of the native 2130-base pair human skeletal alpha-actin promoter required a single hybrid MEF-2/GATA-4 DNA motif centered at -1256 base pairs. Maximal expression of the promoter in cultured myocytes and in vivo correlated with binding of both MEF-2 and p300, but not GATA-4, to this AT-rich motif. p300 and MEF-2 were coprecipitated from cardiac nuclear extracts by an oligomer containing this element. p300 was found exclusively in a complex with MEF-2D at this and related sites in other cardiac-restricted promoters. MEF-2D, but not other MEFs, significantly potentiated cardiac-specific transcription by p300. No physical or functional interaction was observed between p300 and other factors implicated in skeletal actin transcription, including GATA-4, TEF-1, or SRF. These results show that, in the intact cell, p300 interactions with its protein targets are highly selective and that MEF-2D is the preferred channel for p300-mediated transcriptional control in the heart.

Regulation of gene expression by the small GTPase Rho through the ERK6 (p38 gamma) MAP kinase pathway

Small GTP-binding proteins of the Rho-family, Rho, Rac, and Cdc42, have been traditionally linked to the regulation of the cellular actin-based cytoskeleton. Rac and Cdc42 can also control the activity of JNK, thus acting in a molecular pathway transmitting extracellular signals to the nucleus. Interestingly, Rho can also regulate gene expression, albeit by a not fully understood mechanism. Here, we found that activated RhoA can stimulate c-jun expression and the activity of the c-jun promoter. As the complexity of the signaling pathways controlling the expression of c-jun has begun to be unraveled, this finding provided a unique opportunity to elucidate the biochemical routes whereby RhoA regulates nuclear events. We found that RhoA can initiate a linear kinase cascade leading to the activation of ERK6 (p38 gamma), a recently identified member of the p38 family of MAPKs. Furthermore, we present evidence that RhoA, PKN, MKK3/MKK6, and ERK6 (p38 gamma) are components of a novel signal transduction pathway involved in the regulation of gene expression and cellular transformation.

MEKK2 associates with the adapter protein Lad/RIBP and regulates the MEK5-BMK1/ERK5 pathway

MEKK2 and MEKK3 are two closely related mitogen-activated protein kinase (MAPK) kinase kinases. The kinase domains of MEKK2 and MEKK3 are nearly identical, although their N-terminal regulatory domains are significantly divergent. By yeast two-hybrid library screening, we have identified MEK5, the MAPK kinase in the big mitogen-activated protein kinase 1 (BMK1)/ERK5 pathway, as a binding partner for MEKK2. MEKK2 expression stimulates BMK1/ERK5 activity, the downstream substrate for MEK5. Compared with MEKK3, MEKK2 activated BMK1/ERK5 to a greater extent, which might correlate with a higher affinity MEKK2-MEK5 interaction. A dominant negative form of MEK5 blocked the activation of BMK1/ERK5 by MEKK2, whereas activation of c-Jun N-terminal kinase (JNK) was unaffected, showing that MEK5 is a specific downstream effector of MEKK2 in the BMK1/ERK5 pathway. Activation of BMK1/ERK5 by epidermal growth factor and H2O2 in Cos7 and HEK293 cells was completely blocked by a kinase-inactive MEKK3 (MEKK3kin(-)), whereas MEKK2kin(-) had no effect. However, in D10 T cells, expression of MEKK2kin(-) but not MEKK3kin(-) inhibited BMK1/ERK5 activity. Two-hybrid screening also identified Lck-associated adapter/Rlk- and Itk-binding protein (Lad/RIBP), a T cell adapter protein, as a binding partner for MEKK2. MEKK2 and Lad/RIBP colocalize at the T cell contact site with antigen-loaded presenting cells, demonstrating cotranslocation of MEKK2 and Lad/RIBP during T cell activation. MEKK3 neither binds Lad/RIBP nor is recruited to the T cell contact with antigen presenting cell. MEKK2 and MEKK3 are differentially associated with signaling from specific upstream receptor systems, whereas both activate the MEK5-BMK1/ERK5 pathway.

Identification of a docking groove on ERK and p38 MAP kinases that regulates the specificity of docking interactions

MAP kinases (MAPKs) form a complex with MAPK kinases (MAPKKs), MAPK-specific phosphatases (MKPs) and various targets including MAPKAPKs. These docking interactions contribute to regulation of the specificity and efficiency of the enzymatic reactions. We have previously identified a docking site on MAPKs, termed the CD (common docking) domain, which is utilized commonly for docking interactions with MAPKKs, MKPs and MAPKAPKs. However, the CD domain alone does not determine the docking specificity. Here we have identified a novel site on p38 and ERK2 MAPKs that regulates the docking specificity towards MAPKAPKs. Remarkably, exchange of two amino acids in this site of ERK2 for corresponding residues of p38 converted the docking specificity for MAPKAPK-3/3pk, which is a dominant target of p38, from the ERK2 type to the p38 type, and vice versa. Furthermore, our detailed analyses with a number of MAPKAPKs and MKPs suggest that a groove in the steric structure of MAPKs, which comprises the CD domain and the site identified here, serves as a common docking region for various MAPK-interacting molecules.

Selective targeting of MAPKs to the ETS domain transcription factor SAP-1

MAPK pathways play important roles in regulating the key cellular processes of proliferation, differentiation, and apoptosis. There are multiple MAPK pathways, which are subject to different regulatory cues. It is important that these pathways maintain specificity in signaling to elicit the activation of a specific program of gene expression. MAPK-docking domains in several transcription factors have been shown to play important roles in determining the specificity and efficiency of their phosphorylation by MAPKs. Here we investigate the mechanisms by which MAPKs are targeted to the ETS domain transcription factor SAP-1. We demonstrate that SAP-1 contains two different domains that are required for its efficient phosphorylation in vitro and activation in vivo by ERK2 and a subset of p38 MAPKs. The D-domain is closely related to other MAPK-docking domains, but exhibits a novel specificity and serves to promote selective targeting of ERK2, p38alpha, and p38beta(2) to SAP-1. A second important region, the FXF motif, also plays an important role in directing MAPKs to phosphorylate SAP-1. The FXF motif promotes targeting by ERK2 and, to a lesser extent, p38alpha, but not p38beta(2). Our data therefore demonstrate that a modular system of motifs is responsible for directing specific MAPK subtypes to SAP-1, but also point to important distinctions in the mechanism of action of the D-domain and FXF motif.

Id helix-loop-helix proteins antagonize pax transcription factor activity by inhibiting DNA binding

The Id subfamily of helix-loop-helix (HLH) proteins plays a fundamental role in the regulation of cellular proliferation and differentiation. The major mechanism by which Id proteins are thought to inhibit differentiation is through interaction with other HLH proteins and inhibition of their DNA-binding activity. However, Id proteins have also been shown to interact with other proteins involved in regulating cellular proliferation and differentiation, suggesting a more widespread regulatory function. In this study we demonstrate functional interactions between Id proteins and members of the Pax-2/-5/-8 subfamily of paired-domain transcription factors. Members of the Pax transcription factor family have key functions in regulating several developmental processes exemplified by B lymphopoiesis, in which Pax-5 plays an essential role. Id proteins bind to Pax proteins in vitro and in vivo. Binding occurs through the paired DNA-binding domain of the Pax proteins and results in the disruption of DNA-bound complexes containing Pax-2, Pax-5, and Pax-8. In vivo, Id proteins modulate the transcriptional activity mediated by Pax-5 complexes on the B-cell-specific mb-1 promoter. Our results therefore demonstrate a novel facet of Id function in regulating cellular differentiation by functionally antagonizing the action of members of the Pax transcription factor family.

Thyroid-stimulating hormone and cyclic AMP activate p38 mitogen-activated protein kinase cascade. Involvement of protein kinase A, rac1, and reactive oxygen species

p38 mitogen-activated protein kinases (p38-MAPKs) are activated by cytokines, cellular stresses, growth factors, and hormones. We show here that p38-MAPKs are activated upon stimulation by thyroid-stimulating hormone (TSH) or cAMP. TSH caused the phosphorylation of p38-MAPK in Chinese hamster ovary cells stably transfected with the human TSH receptor but not in wild-type Chinese hamster ovary cells. The effect of TSH was fully mimicked by the adenylyl cyclase activator, forskolin, and by a permeant analog of cAMP. The effect of forskolin was reproduced in FRTL5 rat thyroid cells. TSH also stimulated the phosphorylation of MAPK kinase 3 or 6, over the same time scale as that of p38-MAPKs. TSH and forskolin stimulated the activity of the alpha-isoform of p38-MAPK assayed by phosphorylation of the transcription factor ATF2. The activity of MAPK-activated protein kinase-2 was stimulated by TSH and forskolin. This stimulation was abolished by SB203580, a specific inhibitor of p38-MAPKs. The protein kinase A inhibitor H89 inhibited the stimulation of phosphorylation of p38-MAPKs by forskolin, whereas inhibitors of protein kinase C, p70(S6k), and phosphatidylinositol 3-kinase were ineffective. Expression of the dominant negative form of Rac1, but not that of Ras, blocked forskolin-induced p38-MAPK activation. Diphenylene iodonium, a potent inhibitor of NADPH oxidase(s), and ascorbic acid, an effective free radical scavenger, suppressed TSH- or forskolin-stimulated p38-MAPK phosphorylation, indicating that the generation of reactive oxygen species plays a key role in signaling from cAMP to p38-MAPKs. Inhibition of the p38-MAPK pathway with SB203580 partially but significantly, attenuates cAMP- and TSH-induced expression of the sodium iodide symporter in FRTL-5 cells. These results point to a new signaling pathway for the G(s)-coupled TSH receptor, involving cAMP, protein kinase A, Rac1, and reactive oxygen species and resulting in the activation of a signaling kinase cascade that includes MAPK kinase 3 or 6, p38-MAPK, and MAPK-activated protein kinase-2.

Coordinate control of muscle cell survival by distinct insulin-like growth factor activated signaling pathways

Peptide growth factors control diverse cellular functions by regulating distinct signal transduction pathways. In cultured myoblasts, insulin-like growth factors (IGFs) stimulate differentiation and promote hypertrophy. IGFs also maintain muscle cell viability. We previously described C2 skeletal muscle lines lacking expression of IGF-II. These cells did not differentiate, but underwent progressive apoptotic death when incubated in differentiation medium. Viability could be sustained and differentiation enabled by IGF analogues that activated the IGF-I receptor; survival was dependent on stimulation of phosphatidylinositol 3-kinase (PI3-kinase). We now find that IGF action promotes myoblast survival through two distinguishable PI3-kinase-regulated pathways that culminate in expression of the cyclin-dependent kinase inhibitor, p21. Incubation with IGF-I or transfection with active PI3-kinase led to rapid induction of MyoD and p21, and forced expression of either protein maintained viability in the absence of growth factors. Ectopic expression of MyoD induced p21, and inhibition of p21 blocked MyoD-mediated survival, thus defining one PI3-kinase-dependent pathway as leading first to MyoD, and then to p21 and survival. Unexpectedly, loss of MyoD expression did not impede IGF-mediated survival, revealing a second pathway involving activation by PI3-kinase of Akt, and subsequent induction of p21. Since inhibition of p21 caused death even in the presence of IGF-I, these results establish a central role for p21 as a survival factor for muscle cells. Our observations also define a MyoD-independent pathway for regulating p21 in muscle, and demonstrate that distinct mechanisms help ensure appropriate expression of this key protein during differentiation.

Regulation of muscle regulatory factors by DNA-binding, interacting proteins, and post-transcriptional modifications

Skeletal muscle differentiation is influenced by multiple pathways, which regulate the activity of myogenic regulatory factors (MRFs)-the myogenic basic helix-loop-helix proteins and the MEF2-family members-in positive or negative ways. Here we will review and discuss the network of signals that regulate MRF function during myocyte proliferation, differentiation, and post-mitotic growth. Elucidating the mechanisms governing muscle-specific transcription will provide important insight in better understanding the embryonic development of muscle at the molecular level and will have important implications in setting out strategies aimed at muscle regeneration. Since the activity of MRFs are compromised in tumors of myogenic derivation-the rhabdomyosarcomas-the studies summarized in this review can provide a useful tool to uncover the molecular basis underlying the formation of these tumors.

The ternary complex factor Net contains two distinct elements that mediate different responses to MAP kinase signalling cascades

The ternary complex factors (TCFs), Elk-1, Sap-1a and Net, are key integrators of the transcriptional response to different signalling pathways. Classically, three MAP kinase pathways, involving ERK, JNK, and p38, transduce various extracellular stimuli to the nucleus. Net is a repressor that is converted into an activator by Ras/ERK signalling. Net is also exported from the nucleus in response to stress stimuli transduced through the JNK pathway, leading to relief from repression. Here we show that ERK and p38 bind to the D box and that binding is required for phosphorylation of the adjacent C-terminally located C-domain. The D box as well as the phosphorylation sites in the C-domain (the DC element) are required for transcription activation by Ras. On the other hand, JNK binds to the J box in the middle of the protein, and binding is required for phosphorylation of the adjacent EXport motif. Both the binding and phosphorylation sites (the JEX element) are important for Net export. In conclusion, specific targeting of Net by MAP kinase pathways involves two different docking sites and phosphorylation of two different domains. These two elements, DC and JEX, mediate two distinct functional responses.

Regulation of dual-specificity phosphatases M3/6 and hVH5 by phorbol esters. Analysis of a delta-like domain

Treatment of leukemic cells with phorbol 12-myristate 13-acetate (PMA) induces a short-lived phosphorylation and activation of stress-activated protein kinase (SAPK) and cellular differentiation. To investigate whether the rapid deactivation of SAPK results from dephosphorylation by dual-specificity phosphatases (DSPs), we studied regulation of the DSP hVH5 and its murine orthologue M3/6 in K562 human leukemia cells. PMA treatment rapidly induced hVH5 transcripts in these cells, and induced expression of M3/6 completely inhibited PMA-stimulated phosphorylation of SAPK, suggesting a feedback loop to control SAPK activity. Using both stable cell lines and transient transfection we demonstrate that activation of SAPK rapidly stimulated phosphorylation of M3/6. This phosphorylation did not regulate the half-life of total cellular M3/6. hVH5 and M3/6 shares with all sequenced mammalian DSPs an amino acid motif, XILPXLXL, located approximately 80 amino acids from the active site. The hVH5-M3/6 sequence, RILPHLYL, shares significant homology with the SAPK binding site of the c-Jun protein, called the delta domain. This motif was found to be important for DSP function, because deletion of RILPHLYL inhibits SAPK-mediated phosphorylation of M3/6, and deletion of this sequence or mutation of the LYL portion blocks the ability of this phosphatase to dephosphorylate SAPK.

Essential role for p38alpha mitogen-activated protein kinase in placental angiogenesis

The p38 family of mitogen-activated protein kinases (MAPKs) mediates signaling in response to environmental stresses and inflammatory cytokines, but the requirements for the p38 MAPK pathway in normal mammalian development have not been elucidated. Here, we show that targeted disruption of the p38alpha MAPK gene results in homozygous embryonic lethality because of severe defects in placental development. Although chorioallantoic placentation is initiated appropriately in p38alpha null homozygotes, placental defects are manifest at 10.5 days postcoitum as nearly complete loss of the labyrinth layer and significant reduction of the spongiotrophoblast. In particular, p38alpha mutant placentas display lack of vascularization of the labyrinth layer as well as increased rates of apoptosis, consistent with a defect in placental angiogenesis. Furthermore, p38alpha mutants display abnormal angiogenesis in the embryo proper as well as in the visceral yolk sac. Thus, our results indicate a requirement for p38alpha MAPK in diploid trophoblast development and placental vascularization and suggest a more general role for p38 MAPK signaling in embryonic angiogenesis.

Docking domains and substrate-specificity determination for MAP kinases

Signalling specificity in eukaryotic cells is maintained by several mechanisms. One mechanism by which mitogen-activated protein (MAP) kinases ensure their specificity of action is by interacting with their substrates through docking domains. These docking domains recruit the kinases to the correct substrates and enhance their fidelity and efficiency of action. Additional specificity determinants in the substrates serve to enhance the specificity of substrate phosphorylation by MAP kinases further.

Regulation of skeletal myogenesis by association of the MEF2 transcription factor with class II histone deacetylases

Skeletal muscle differentiation is controlled by associations between myogenic basic-helix-loop-helix and MEF2 transcription factors. We show that chromatin associated with muscle genes regulated by these transcription factors becomes acetylated during myogenesis and that class II histone deacetylases (HDACs), which interact with MEF2, specifically suppress myoblast differentiation. These HDACs do not interact directly with MyoD, yet they suppress its myogenic activity through association with MEF2. Elevating the level of MyoD can override the repression imposed by HDACs on muscle genes. HDAC-mediated repression of myogenesis also can be overcome by CaM kinase and insulin-like growth factor (IGF) signaling. These findings reveal central roles for HDACs in chromatin remodeling during myogenesis and as intranuclear targets for signaling pathways controlled by IGF and CaM kinase.

Antiapoptotic role of the p38 mitogen-activated protein kinase-myocyte enhancer factor 2 transcription factor pathway during neuronal differentiation

Myocyte enhancer factor 2 (MEF2) is in the MADS (MCM1agamous-deficiens-serum response factor) family of transcription factors. Although MEF2 is known as a myogenic factor, the expression pattern of the MEF2 family of genes (MEF2A-D) in developing brain also suggests a role in neurogenesis. Here we show that transfection with MEF2C, the predominant form in mammalian cerebral cortex, induces a mixed neuronal/myogenic phenotype in undifferentiated P19 precursor cells. During retinoic acid-induced neurogenesis of these cells, a dominant negative form of MEF2 enhances apoptosis but does not affect cell division. The mitogen-activated protein kinase p38alpha activates MEF2C. Dominant negative p38alpha also enhances apoptotic death of differentiating neurons, but these cells can be rescued from apoptosis by coexpression of constitutively active MEF2C. These findings suggest that the p38alpha/MEF2 pathway prevents cell death during neuronal differentiation.

Big mitogen-activated kinase regulates multiple members of the MEF2 protein family

Big mitogen-activated protein (MAP) kinase (BMK1), a member of the mammalian MAP kinase family, is activated by growth factors. The activation of BMK1 is required for growth factor-induced cell proliferation and cell cycle progression. We have previously shown that BMK1 regulates c-jun gene expression through direct phosphorylation and activation of transcription factor MEF2C. MEF2C belongs to the myocyte enhancer factor 2 (MEF2) protein family, a four-membered family of transcription factors denoted MEF2A, -2B, -2C, and -2D. Here, we demonstrate that, in addition to MEF2C, BMK1 phosphorylates and activates MEF2A and MEF2D but not MEF2B. The blocking of BMK1 signaling inhibits the epidermal growth factor-dependent activation of these three MEF2 transcription factors. The sites phosphorylated by activated BMK1 were mapped to Ser-355, Thr-312, and Thr-319 of MEF2A and Ser-179 of MEF2D both in vitro and in vivo. Site-directed mutagenesis reveals that the phosphorylation of these sites in MEF2A and MEF2D are necessary for the induction of MEF2A and 2D transactivating activity by either BMK1 or by epidermal growth factor. Taken together, these data demonstrate that, upon growth factor induction, BMK1 directly phosphorylates and activates three members of the MEF2 family of transcription factors thereby inducing MEF2-dependent gene expression.

p38 and extracellular signal-regulated kinases regulate the myogenic program at multiple steps

The extracellular signals which regulate the myogenic program are transduced to the nucleus by mitogen-activated protein kinases (MAPKs). We have investigated the role of two MAPKs, p38 and extracellular signal-regulated kinase (ERK), whose activities undergo significant changes during muscle differentiation. p38 is rapidly activated in myocytes induced to differentiate. This activation differs from those triggered by stress and cytokines, because it is not linked to Jun-N-terminal kinase stimulation and is maintained during the whole process of myotube formation. Moreover, p38 activation is independent of a parallel promyogenic pathway stimulated by insulin-like growth factor 1. Inhibition of p38 prevents the differentiation program in myogenic cell lines and human primary myocytes. Conversely, deliberate activation of endogenous p38 stimulates muscle differentiation even in the presence of antimyogenic cues. Much evidence indicates that p38 is an activator of MyoD: (i) p38 kinase activity is required for the expression of MyoD-responsive genes, (ii) enforced induction of p38 stimulates the transcriptional activity of a Gal4-MyoD fusion protein and allows efficient activation of chromatin-integrated reporters by MyoD, and (iii) MyoD-dependent myogenic conversion is reduced in mouse embryonic fibroblasts derived from p38alpha(-/-) embryos. Activation of p38 also enhances the transcriptional activities of myocyte enhancer binding factor 2A (MEF2A) and MEF2C by direct phosphorylation. With MEF2C, selective phosphorylation of one residue (Thr293) is a tissue-specific activating signal in differentiating myocytes. Finally, ERK shows a biphasic activation profile, with peaks of activity in undifferentiated myoblasts and postmitotic myotubes. Importantly, activation of ERK is inhibitory toward myogenic transcription in myoblasts but contributes to the activation of myogenic transcription and regulates postmitotic responses (i.e., hypertrophic growth) in myotubes.

The MEF2A isoform is required for striated muscle-specific expression of the insulin-responsive GLUT4 glucose transporter

Previously, we have demonstrated that an MEF2 consensus sequence located between -473/-464 in the human GLUT4 gene was essential for both tissue-specific and hormonal/metabolic regulation of GLUT4 expression (Thai, M. V., Guruswamy, S., Cao, K. T., Pessin, J. E., and Olson, A. L. (1998) J. Biol. Chem. 273, 14285-14292). To identify the specific MEF2 isoform(s) responsible for GLUT4 expression, we studied the pattern of expression of the MEF2 isoforms in insulin-sensitive tissues. Both heart and skeletal muscle were found to express the MEF2A, MEF2C, and MEF2D isoforms but not MEF2B. However, only the MEF2A protein was selectively down-regulated in insulin-deficient diabetes. Co-immunoprecipitation with isoform-specific antibodies revealed that, in the basal state, essentially all of the MEF2A protein was presented as a MEF2A-MEF2D heterodimer without any detectable MEF2A-MEF2A homodimers or MEF2A-MEF2C and MEF2C-MEF2D heterodimers. Electrophoretic mobility shift assays revealed that nuclear extracts from diabetic animals had reduced binding to the MEF2 binding site compared with extracts from control or insulin-treated animals. Furthermore, immunodepletion of the MEF2A-MEF2D complex from control extracts abolished binding to the MEF2 element. However, addition of MEF2A to diabetic nuclear extracts fully restored binding activity to the MEF2 element. These data strongly suggest that the MEF2A-MEF2D heterodimer is selectively decreased in insulin-deficient diabetes and is responsible for hormonally regulated expression of the GLUT4 gene.

mHDA1/HDAC5 histone deacetylase interacts with and represses MEF2A transcriptional activity

Recently we identified a new family of histone deacetylases in higher eukaryotes related to yeast HDA1 and showed their differentiation-dependent expression. Data presented here indicate that HDAC5 (previously named mHDA1), one member of this family, might be a potent regulator of cell differentiation by interacting specifically with determinant transcription factors. We found that HDAC5 was able to interact in vivo and in vitro with MEF2A, a MADS box transcription factor, and to strongly inhibit its transcriptional activity. Surprisingly, this repression was independent of HDAC5 deacetylase domain. The N-terminal non-deacetylase domain of HDAC5 was able to ensure an efficient repression of MEF2A-dependent transcription. We then mapped protein domains involved in the HDAC5-MEF2A interaction and showed that MADS box/MEF2-domain region of MEF2A interacts specifically with a limited region in the N-terminal part of HDAC5 which also possesses a distinct repressor domain. These data show that two independent class II histone deacetylases HDAC4 and HDAC5 are able to interact with members of the MEF2 transcription factor family and regulate their transcriptional activity, thus suggesting a critical role for these deacetylases in the control of cell proliferation/differentiation.

MEF2 responds to multiple calcium-regulated signals in the control of skeletal muscle fiber type

Different patterns of motor nerve activity drive distinctive programs of gene transcription in skeletal muscles, thereby establishing a high degree of metabolic and physiological specialization among myofiber subtypes. Recently, we proposed that the influence of motor nerve activity on skeletal muscle fiber type is transduced to the relevant genes by calcineurin, which controls the functional activity of NFAT (nuclear family of activated T cell) proteins. Here we demonstrate that calcineurin-dependent gene regulation in skeletal myocytes is mediated also by MEF2 transcription factors, and is integrated with additional calcium-regulated signaling inputs, specifically calmodulin-dependent protein kinase activity. In skeletal muscles of transgenic mice, both NFAT and MEF2 binding sites are necessary for properly regulated function of a slow fiber-specific enhancer, and either forced expression of activated calcineurin or motor nerve stimulation up-regulates a MEF2-dependent reporter gene. These results provide new insights into the molecular mechanisms by which specialized characteristics of skeletal myofiber subtypes are established and maintained.

Signal-dependent activation of the MEF2 transcription factor by dissociation from histone deacetylases

Myocyte enhancer factor-2 (MEF2) transcription factors control muscle-specific and growth factor-inducible genes. We show that hypertrophic growth of cardiomyocytes in response to phenylephrine and serum is accompanied by activation of MEF2 through a posttranslational mechanism mediated by calcium, calmodulin-dependent protein kinase (CaMK), and mitogen-activated protein kinase (MAPK) signaling. CaMK stimulates MEF2 activity by dissociating class II histone deacetylases (HDACs) from the DNA-binding domain. MAPKs, which activate MEF2 by phosphorylation of the transcription activation domain, maximally stimulate MEF2 activity only when repression by HDACs is relieved by CaMK signaling to the DNA-binding domain. These findings identify MEF2 as an endpoint for hypertrophic stimuli in cardiomyocytes and demonstrate that MEF2 mediates synergistic transcriptional responses to the CaMK and MAPK signaling pathways by signal-dependent dissociation from HDACs.

MAP kinase pathways activated by stress: the p38 MAPK pathway

A stress-activated serine/threonine protein kinase, p38 mitogen-activated protein kinase (p38 MAPK), belongs to the MAP kinase superfamily. Diverse extracellular stimuli, including ultraviolet light, irradiation, heat shock, high osmotic stress, proinflammatory cytokines and certain mitogens, trigger a stress-regulated protein kinase cascade culminating in activation of p38 MAPK through phosphorylation on a TGY motif within the kinase activation loop. p38 MAPK appears to play a major role in apoptosis, cytokine production, transcriptional regulation, and cytoskeletal reorganization, and has been causally implicated in sepsis, ischemic heart disease, arthritis, human immunodeficiency virus infection, and Alzheimer's disease. The availability of specific inhibitors helps to clarify the role that p38 MAPK plays in these processes, and may ultimately offer therapeutic benefit for certain critically ill patients.

Crystal structure of MEF2A core bound to DNA at 1.5 A resolution

Members of the myocyte enhancer factor-2 (MEF2) family of transcription factors bind to and activate transcription through A+T-rich DNA sequences found primarily, but not exclusively, in the promoters of muscle-specific genes. Their importance has been established for myogenic development and in activation of the immediate-early gene, c-jun, and recently further functional roles in the immune system have emerged. The MEF2 factors belong to the MADS-box superfamily, sharing homology in a 58 amino acid domain that mediates DNA binding and dimerization. The structures of two MADS-box proteins, SRF and MCM1, bound to their cognate DNA have been previously reported and shown to share extensive similarity in their mode of DNA binding. We have solved the structure of MEF2A 2-78 bound to its DNA consensus sequence at 1.5 A resolution. It reveals how the absence of amino acids N-terminal to the MADS-box contributes to the DNA binding properties of MEF2 proteins and shows that the MEF domain C-terminal to the MADS-box adopts a conformation considerably different from the same region in SRF and MCM1.

Molecular determinants that mediate selective activation of p38 MAP kinase isoforms

The p38 mitogen-activated protein kinase (MAPK) group is represented by four isoforms in mammals (p38alpha, p38beta2, p38gamma and p38delta). These p38 MAPK isoforms appear to mediate distinct functions in vivo due, in part, to differences in substrate phosphorylation by individual p38 MAPKs and also to selective activation by MAPK kinases (MAPKKs). Here we report the identification of two factors that contribute to the specificity of p38 MAPK activation. One mechanism of specificity is the selective formation of functional complexes between MAPKK and different p38 MAPKs. The formation of these complexes requires the presence of a MAPK docking site in the N-terminus of the MAPKK. The second mechanism that confers signaling specificity is the selective recognition of the activation loop (T-loop) of p38 MAPK isoforms. Together, these processes provide a mechanism that enables the selective activation of p38 MAPK in response to activated MAPKK.

Induction of terminal differentiation by constitutive activation of p38 MAP kinase in human rhabdomyosarcoma cells

MyoD inhibits cell proliferation and promotes muscle differentiation. A paradoxical feature of rhabdomyosarcoma (RMS), a tumor arising from muscle precursors, is the block of the differentiation program and the deregulated proliferation despite MyoD expression. A deficiency in RMS of a factor required for MyoD activity has been implicated by previous studies. We report here that p38 MAP kinase (MAPK) activation, which is essential for muscle differentiation, is deficient in RMS cells. Enforced induction of p38 MAPK by an activated MAPK kinase 6 (MKK6EE) restored MyoD function and enhanced MEF2 activity in RMS deficient for p38 MAPK activation, leading to growth arrest and terminal differentiation. Stress and cytokines could activate the p38 MAPK in RMS cells, however, these stimuli did not promote differentiation, possibly because they activated p38 MAPK only transiently and they also activated JNK, which could antagonize differentiation. Thus, the selective and sustained p38 MAPK activation, which is distinct from the stress-activated response, is required for differentiation and can be disrupted in human tumors.