BMK1/ERK5 regulates serum-induced early gene expression through transcription factor MEF2C

Differential regulation of p90 ribosomal S6 kinase and big mitogen-activated protein kinase 1 by ischemia/reperfusion and oxidative stress in perfused guinea pig hearts

Reactive oxygen species (ROS) activate members of the Src kinase and mitogen-activated protein kinase superfamily, including big mitogen-activated protein kinase 1 (BMK1) and extracellular signal-regulated kinases (ERK1/2). A potentially important downstream effector of ERK1/2 is p90 ribosomal S6 kinase (p90RSK), which plays an important role in cell growth through the activation of several transcription factors, as well as the Na(+)/H(+) exchanger. Previously, we showed that Src regulates BMK1 via a redox-sensitive signaling pathway. Because ROS are generated during ischemia and reperfusion after ischemia, we assessed the effects of these stimuli (H(2)O(2), ischemia, and reperfusion) in the activation of ERK1/2, p90RSK, Src, and BMK1 in perfused guinea pig hearts. H(2)O(2) (100 micromol/L) significantly activated all kinases. Ischemia alone stimulated p90RSK, Src, and BMK1 but not ERK1/2. These results suggest that p90RSK activation through ischemia occurs via a pathway other than ERK1/2. A role of Src in ischemia-mediated BMK1 activation was demonstrated through inhibition with the Src inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine. Reperfusion after ischemia stimulated both p90RSK and ERK1/2. In contrast, although ROS increase during reperfusion after ischemia, the activities of both BMK1 and its upstream regulator, Src, were markedly attenuated by reperfusion after ischemia. The activation of C-terminal Src kinase during ischemia but not during reperfusion suggests that the attenuation of Src and BMK1 activity by reperfusion was not regulated by C-terminal Src kinase activity. The antioxidant N-2-mercaptopropionylglycine completely inhibited ERK1/2 and p90RSK activation by reperfusion but only partially inhibited ischemia-induced Src and BMK1 activation. The present study is the first to show the coregulation of Src and BMK1 by reperfusion after ischemia, which we propose to occur via a novel, ROS-independent pathway.

Induction of endogenous genes following infection of human endothelial cells with an E1(-) E4(+) adenovirus gene transfer vector

Recombinant adenovirus (Ad) gene transfer vectors are effective at transferring exogenous genes to a variety of cells and tissue types both in vitro and in vivo. However, in the process of gene transfer, the Ad vectors induce the expression of target cell genes, some of which may modify the function of the target cell and/or alter the local milieu. To develop a broader understanding of Ad vector-mediated induction of endogenous gene expression, genes induced by first-generation E1(-) E4(+) Ad vectors in primary human umbilical vein endothelial cells were identified by cDNA subtraction cloning. The identified cDNAs included signaling molecules (lymphoid blast crisis [LBC], guanine nucleotide binding protein alpha type S [Galpha-S], and mitogen kinase [MEK5]), calcium-regulated/cytoskeletal proteins (calpactin p11 and p36 subunits, vinculin, and spinocerebellar ataxia [SCA1]), growth factors (insulin-like growth factor binding protein 4 and transforming growth factor beta2), glyceraldehyde-6-phosphate dehydrogenase, an expressed sequence tag, and a novel cDNA showing homology to a LIM domain sequence. Two- to sevenfold induction of the endogenous gene expression was observed at 24 h postinfection, and induction continued up to 72 h, although the timing of gene expression varied among the identified genes. In contrast to that observed in endothelial cells, the Ad vector-mediated induction of gene expression was not found following Ad vector infection of primary human dermal fibroblasts or human alveolar macrophages. Empty Ad capsids did not induce endogenous gene expression in endothelial cells. Interestingly, additional deletion of the E4 gene obviated the upregulation of genes in endothelial cells by the E1(-) E3(-) Ad vector, suggesting that genes carried by the E4 region play a central role in modifying target cell gene expression. These findings are consistent with the notion that efficient transfer of exogenous genes to endothelial cells by first-generation Ad vectors comes with the price that these vectors also induce the expression of a variety of cellular genes.

BWMK1, a novel MAP kinase induced by fungal infection and mechanical wounding in rice

The activation of the mitogen-activated protein (MAP) kinases by different environmental stresses has been previously observed in several dicot plant species. Here, we report the isolation of a novel MAP kinase in rice that is induced during infection by the blast fungus Magnaporthe grisea or upon mechanical wounding. The gene is designated as BWMK1 for blast- and wound-induced MAP kinase. The cDNA of BWMK1 was isolated from rice leaves challenged by the blast pathogen. Transcripts of the corresponding gene accumulated in rice leaves 4 h after blast inoculation and 30 min after mechanical wounding. This gene encodes a 506 amino acid protein that contains a new dual-phosphorylation activation motif TDY and about 150 unique amino acids on its C terminus. In-gel kinase activity and immunoprecipitation assays confirmed that BWMK1 is a functional MAP kinase. These results show that BWMK1 is a new member of the plant MAP kinase family and may mediate both defense and wound signaling in rice.

HDAC4, a human histone deacetylase related to yeast HDA1, is a transcriptional corepressor

Histone acetylation plays an important role in regulating chromatin structure and thus gene expression. Here we describe the functional characterization of HDAC4, a human histone deacetylase whose C-terminal part displays significant sequence similarity to the deacetylase domain of yeast HDA1. HDAC4 is expressed in various adult human tissues, and its gene is located at chromosome band 2q37. HDAC4 possesses histone deacetylase activity intrinsic to its C-terminal domain. When tethered to a promoter, HDAC4 represses transcription through two independent repression domains, with repression domain 1 consisting of the N-terminal 208 residues and repression domain 2 containing the deacetylase domain. Through a small region located at its N-terminal domain, HDAC4 interacts with the MADS-box transcription factor MEF2C. Furthermore, HDAC4 and MEF2C individually upregulate but together downmodulate c-jun promoter activity. These results suggest that HDAC4 interacts with transcription factors such as MEF2C to negatively regulate gene expression.

Contribution of the ERK5/MEK5 pathway to Ras/Raf signaling and growth control

The activity of the catalytic domain of the orphan MAP kinase ERK5 is increased by Ras but not Raf-1 in cells, which suggests that ERK5 might mediate Raf-independent signaling by Ras. We found that Raf-1 does contribute to Ras activation of ERK5 but in a manner that does not correlate with Raf-1 catalytic activity. A clue to the mechanism of action of Raf-1 on ERK5 comes from the observation that endogenous Raf-1 binds to endogenous ERK5, suggesting the involvement of regulatory protein-protein interactions. This interaction is specific because Raf-1 binds only to ERK5 and not ERK2 or SAPK. Finally, we demonstrate the ERK5/MEK5 pathway is required for Raf-dependent cellular transformation and that a constitutively active form of MEK5, MEK5DD, synergizes with Raf to transform NIH 3T3 cells. These observations suggest that ERK5 plays a large role in Raf-1-mediated signal transduction.

Shear stress stimulation of p130(cas) tyrosine phosphorylation requires calcium-dependent c-Src activation

Fluid shear stress (flow) modulates endothelial cell function via specific intracellular signaling events. Previously we showed that flow activated ERK1/2 in an integrin-dependent manner (Takahashi, M., and Berk, B. C. (1996) J. Clin. Invest. 98, 2623-2631). p130 Crk-associated substrate (Cas), a putative c-Src substrate, was originally identified as a highly phosphorylated protein that is localized to focal adhesions and acts as an adapter protein. Recent reports have shown that Cas is important in cardiovascular development and actin filament assembly. Flow (shear stress = 12 dynes/cm(2)) stimulated Cas tyrosine phosphorylation within 1 min in human umbilical vein endothelial cells. Phosphorylation peaked at 5 min (3.5 +/- 0.7-fold) and was sustained to 20 min. Tyrosine phosphorylation of Cas was functionally important because flow stimulated association of Cas with Crk in a time- and force-dependent manner. Flow-mediated activation of c-Src, phosphorylation of Cas, and association of Cas with Crk were all inhibited by calcium chelation and pretreatment with the Src family-specific tyrosine kinase inhibitor PP1. To determine the role of c-Src in flow-stimulated phosphorylation of Cas, we transduced cells with adenovirus encoding kinase-inactive Src. Expression of kinase-inactive Src prevented flow-induced Cas tyrosine phosphorylation but not ERK1/2 activation. Calcium-dependent activation of c-Src and tyrosine phosphorylation of Cas defines a new flow-stimulated signal pathway, different from ERK1/2 activation. This pathway may be involved in focal adhesion remodeling and actin filament assembly.

Activation of the protein kinase ERK5/BMK1 by receptor tyrosine kinases. Identification and characterization of a signaling pathway to the nucleus

ERK5 (also known as BMK1), a member of the mitogen-activated protein kinase (MAPK) superfamily, was known to be activated strongly by oxidant and osmotic stresses. Here we have found that ERK5 is strongly activated by epidermal growth factor and nerve growth factor, whose receptors are tyrosine kinases. The activation of ERK5 was inhibited by expression of dominant-negative Ras and induced by expression of active Ras in PC12 cells, indicating a requirement for Ras in ERK5 activation. The epidermal growth factor-induced activation of ERK5 was found to be inhibited by PD98059 and U0126 inhibitors, which were previously thought to act specifically on classical MAPK kinase (also known as MEK1) and readily reversed by CL100 and MKP-3 dual-specificity phosphatases for which classical MAPKs were previously shown to serve as preferred substrates. The reporter assays demonstrated that the serum-induced enhancement of transcription from serum response element was significantly inhibited by expression of a dominant-negative form of MEK5, which was a direct and specific activator for ERK5 and that transcription from serum response element mediated by the Ets-domain transcription factor Sap1a, but not by Elk1, was stimulated by coexpression of ERK5 and active MEK5. In addition, Sap1a was shown to be phosphorylated by ERK5 in vitro and by the activation of the ERK5 pathway in cells. Moreover, the serum-induced c-Fos expression was markedly inhibited by expression of dominant-negative MEK5. These results reveal a novel signaling pathway to the nucleus mediated by ERK5 that functions downstream of receptor tyrosine kinases to induce immediate early genes, in parallel with the classical MAPK cascade.

The transcription factor MEF2C-null mouse exhibits complex vascular malformations and reduced cardiac expression of angiopoietin 1 and VEGF

The MEF2 family of transcription factors has been implicated in transcriptional regulation in a number of different cell types. Targeted deletion of the MEF2C gene, in particular, revealed its importance for early cardiogenesis (Q. Lin et al., 1997, Science 276, 1404-1407). We report here that this deletion also resulted in vascular anomalies characterized by extreme variability in lumen size and defects in remodeling. While primary vascular networks formed in the yolk sac of the mutants, they failed to remodel into more complex vascular structures. Likewise, although the primordia of the dorsal aortae formed normally, anomalies were observed in these vessels later in development. Dorsal and anterior to the heart, the aortae exhibited abnormally small lumens, as did the anterior cardinal veins and intersegmental arteries. In contrast, the dorsal aortae and intersegmental arteries caudal to the heart were grossly enlarged. Cranial vessels were also enlarged and less branched than normal. Endocardiogenesis in the mutant was abnormal with the endothelial cells exhibiting a number of aberrant phenotypes. These endocardial defects were accompanied by a notable reduction in angiopoietin 1 and VEGF mRNA production by the myocardium, indicating that MEF2C is required for myocardial expression of these important endothelial-directed cytokines and thus for correct endocardial morphogenesis.

Post-translational control of the MEF2A transcriptional regulatory protein

Myocyte enhancer factor 2 (MEF2) transcriptional regulatory proteins are key regulators of muscle-specific gene expression and also play a general role in the cellular response to growth factors, cytokines and environmental stressors. To identify signaling pathway components that might mediate these events, the potential role of MAP kinase and PKC signaling in the modulation of MEF2A phosphorylation and transcriptional activity were therefore studied. In transient transfection reporter assays, activated p38 MAP kinase potently increased MEF2A trans -activating potential, PKC[delta] and [epsiv] isotypes enhanced MEF2A transactivation to a lesser extent, while the ERK1/2 and JNK/SAPK pathways were without effect. A GAL4-based assay system showed that p38 MAP kinase and PKC[delta] target the MEF2A transactivation domain. We also observed an increase in p38 MAP kinase activity in congruence with the increase in MEF2A expression in differentiating primary muscle cells. COS cells overexpressing MEF2A alone or with one of the kinases were metabolically labeled with [32P]orthophosphate and MEF2A was immunoprecipitated using specific anti-MEF2A antibodies. MEF2A from cells co-transfected with activated p38 MAP kinase showed a decreased electrophoretic mobility due to phosphorylation. Subsequent phosphopeptide mapping and phosphoamino acid analysis indicated the appearance of several phoshopeptides due to p38 MAP kinase activation of MEF2A which were due to phosphorylation on serine and threonine residues. These studies position MEF2A as a nuclear target for the p38 MAP kinase signaling pathway.

Phosphorylation of MAP kinases by MAP/ERK involves multiple regions of MAP kinases

Mitogen-activated protein (MAP) kinases are activated with great specificity by MAP/ERK kinases (MEKs). The basis for the specific activation is not understood. In this study chimeras composed of two MAP kinases, extracellular signal-regulated protein kinase 2 and p38, were assayed in vitro for phosphorylation and activation by different MEK isoforms to probe the requirements for productive interaction of MAP kinases with MEKs. Experimental results and modeling support the conclusion that the specificity of MEK/MAP kinase phosphorylation results from multiple contacts, including surfaces in both the N- and C-terminal domains.

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

Mitogen-activated protein (MAP) kinase-mediated signalling to the nucleus is an important event in the conversion of extracellular signals into a cellular response. However, the existence of multiple MAP kinases which phosphorylate similar phosphoacceptor motifs poses a problem in maintaining substrate specificity and hence the correct biological response. Both the extracellular signal-regulated kinase (ERK) and c-Jun NH2-terminal kinase (JNK) subfamilies of MAP kinases use a second specificity determinant and require docking to their transcription factor substrates to achieve maximal substrate activation. In this study, we demonstrate that among the different MAP kinases, the MADS-box transcription factors MEF2A and MEF2C are preferentially phosphorylated and activated by the p38 subfamily members p38alpha and p38beta2. The efficiency of phosphorylation in vitro and transcriptional activation in vivo of MEF2A and MEF2C by these p38 subtypes requires the presence of a kinase docking domain (D-domain). Furthermore, the D-domain from MEF2A is sufficient to confer p38 responsiveness on different transcription factors, and reciprocal effects are observed upon the introduction of alternative D-domains into MEF2A. These results therefore contribute to our understanding of signalling to MEF2 transcription factors and demonstrate that the requirement for substrate binding by MAP kinases is an important facet of three different subclasses of MAP kinases (ERK, JNK, and p38).

A network of mitogen-activated protein kinases links G protein-coupled receptors to the c-jun promoter: a role for c-Jun NH2-terminal kinase, p38s, and extracellular signal-regulated kinase 5

The expression of the c-jun proto-oncogene is rapidly induced in response to mitogens acting on a large variety of cell surface receptors. The resulting functional activity of c-Jun proteins appears to be critical for cell proliferation. Recently, we have shown that a large family of G protein-coupled receptors (GPCRs), represented by the m1 muscarinic receptor, can initiate intracellular signaling cascades that result in the activation of mitogen-activated protein kinases (MAPK) and c-Jun NH2-terminal kinases (JNK) and that the activation of JNK but not of MAPK correlated with a remarkable increase in the expression of c-jun mRNA. Subsequently, however, we obtained evidence that GPCRs can potently stimulate the activity of the c-jun promoter through MEF2 transcription factors, which do not act downstream from JNK. In view of these observations, we set out to investigate further the nature of the signaling pathway linking GPCRs to the c-jun promoter. Utilizing NIH 3T3 cells, we found that GPCRs can activate the c-jun promoter in a JNK-independent manner. Additionally, we demonstrated that these GPCRs can elevate the activity of novel members of the MAPK family, including ERK5, p38alpha, p38gamma, and p38delta, and that the activation of certain kinases acting downstream from MEK5 (ERK5) and MKK6 (p38alpha and p38gamma) is necessary to fully activate the c-jun promoter. Moreover, in addition to JNK, ERK5, p38alpha, and p38gamma were found to stimulate the c-jun promoter by acting on distinct responsive elements. Taken together, these results suggest that the pathway linking GPCRs to the c-jun promoter involves the integration of numerous signals transduced by a highly complex network of MAPK, rather than resulting from the stimulation of a single linear protein kinase cascade. Furthermore, our findings suggest that each signaling pathway affects one or more regulatory elements on the c-jun promoter and that the transcriptional response most likely results from the temporal integration of each of these biochemical routes.

Phosphorylation of the transactivation domain of Pax6 by extracellular signal-regulated kinase and p38 mitogen-activated protein kinase

The transcription factor Pax6 is required for normal development of the central nervous system, the eyes, nose, and pancreas. Here we show that the transactivation domain (TAD) of zebrafish Pax6 is phosphorylated in vitro by the mitogen-activated protein kinases (MAPKs) extracellular-signal regulated kinase (ERK) and p38 kinase but not by Jun N-terminal kinase (JNK). Three of four putative proline-dependent kinase phosphorylation sites are phosphorylated in vitro. Of these sites, the serine 413 (Ser413) is evolutionary conserved from sea urchin to man. Ser413 is also phosphorylated in vivo upon activation of ERK or p38 kinase. Substitution of Ser413 with alanine strongly decreased the transactivation potential of the Pax6 TAD whereas substitution with glutamate increased the transactivation. Reporter gene assays with wild-type and mutant Pax6 revealed that transactivation by the full-length Pax6 protein from paired domain-binding sites was strongly enhanced (16-fold) following co-transfection with activated p38 kinase. This enhancement was largely dependent on the Ser413 site. ERK activation, however, produced a 3-fold increase in transactivation which was partly independent of the Ser413 site. These findings provide a starting point for further studies aimed at elucidating a post-translational regulation of Pax6 following activation of MAPK signaling pathways.

Transcriptional activity of MEF2 during mouse embryogenesis monitored with a MEF2-dependent transgene

The four members of the MEF2 family of MADS-box transcription factors, MEF2-A, MEF2-B, MEF2-C and MEF2-D, are expressed in overlapping patterns in developing muscle and neural cell lineages during embryogenesis. However, during late fetal development and postnatally, MEF2 transcripts are also expressed in a wide range of cell types. Because MEF2 expression is controlled by translational and post-translational mechanisms, it has been unclear whether the presence of MEF2 transcripts in the embryo reflects transcriptionally active MEF2 proteins. To define the temporospatial expression pattern of transcriptionally active MEF2 proteins during mouse embryogenesis, we generated transgenic mice harboring a lacZ reporter gene controlled by three tandem copies of the MEF2 site and flanking sequences from the desmin enhancer, which is active in cardiac, skeletal and smooth muscle cells. Expression of this MEF2-dependent transgene paralleled expression of MEF2 mRNAs in developing myogenic lineages and regions of the adult brain. However, it was not expressed in other cell types that express MEF2 transcripts. Tandem copies of the MEF2 site from the c-jun promoter directed expression in a similar pattern to the desmin MEF2 site, suggesting that transgene expression reflects the presence of transcriptionally active MEF2 proteins, rather than other factors specific for DNA sequences flanking the MEF2 site. These results demonstrate the presence of transcriptionally active MEF2 proteins in the early muscle and neural cell lineages during embryogenesis and argue against the existence of lineage-restricted MEF2 cofactors that discriminate between MEF2 sites with different immediate flanking sequences. The discordance between MEF2 mRNA expression and MEF2 transcriptional activity in nonmuscle cell types of embryos and adults also supports the notion that post-transcriptional mechanisms regulate the expression of MEF2 proteins.

pRb is required for MEF2-dependent gene expression as well as cell-cycle arrest during skeletal muscle differentiation

BACKGROUND

The onset of differentiation-specific gene expression in skeletal muscle is coupled to permanent withdrawal from the cell cycle. The retinoblastoma tumor-suppressor protein (pRb) is a critical regulator of this process, required for both cell-cycle arrest in G0 phase and high-level expression of late muscle-differentiation markers. Although the cell-cycle defects that are seen in pRb-deficient myocytes can be explained by the well-described function of pRb as a negative regulator of the transition from G1 to S phase, it remains unclear how pRb positively affects late muscle-gene expression.

RESULTS

Here, we show that the myogenic defect in Rb-/- cells corresponds to a deficiency in the activity of the transcription factor MEF2. Without pRb, MyoD induces the accumulation of nuclear-localized MEF2 that is competent to bind DNA yet transcriptionally inert. When pRb is present, MyoD stimulates the function of the MEF2C transcriptional activation domain and the activity of endogenous MEF2-type factors. Co-transfection of MyoD together with an activated form of MEF2C containing the Herpesvirus VP16 transcriptional activation domain partially bypasses the requirement for pRb and induces late muscle-gene expression in replicating cells. This ectopic myogenesis is nevertheless significantly augmented by co-expression of an E2F1-pRb chimeric protein that blocks the cell cycle.

CONCLUSION

These findings indicate that pRb promotes the expression of late-stage muscle-differentiation markers by both inhibiting cell-cycle progression and cooperating with MyoD to promote the transcriptional activation activity of MEF2.

Novel Molecular Mediators in the Pathway Connecting G-protein-coupled Receptors to MAP Kinase Cascades

The family of receptors coupled to heterotrimeric GTP-binding proteins (G proteins) constitutes the largest group of integral membrane proteins involved in signal transduction. These receptors participate in many important biological functions, ranging from photoreception to neurotransmission and exocytosis, as well as in processes such as embryogenesis, angiogenesis, tissue regeneration and normal and aberrant cell growth. Initial studies addressing the functioning of these receptors had focused primarily on second messenger-generating systems. Here, the authors survey the current knowledge on how this family of receptors transduces signals to the nucleus through an intricate network of nucleotide exchange factors, small GTPases, and cytoplasmic kinases which, in turn, control gene expression by phosphorylating nuclear regulatory molecules.

Construction and sequence analysis of subtraction complementary DNA libraries from human preimplantation embryos

PURPOSE

Because stage-specific genetic expression in human preimplantation development is not sufficiently studied, we have undertaken the construction of a subtraction complementary DNA (cDNA) library enriched for transcripts specific for human blastocysts.

METHODS

For this purpose individual pools of cDNAs synthesized from four hatched blastocysts and three cleaving 8- to 10-cell embryos were exposed to suppression subtractive hybridization to minimize the presence of transcripts of housekeeping genes and other genes of maternal origin known to be expressed earlier in preimplantation development. Random clones of this library were sequenced and analyzed using the BLAST algorithm.

RESULTS

The resulting subtraction library had a complexity of 3 x 10(5) and an average size of inserts of about 0.8 kb. Sequencing of random library clones revealed the following human genes: CD9 antigen, fatty acid binding protein, ferritin heavy chain, amyloid precursor, MAP kinase messenger RNAs, DNA clone 127H14, messenger RNA for diacylglycerol kinase, a sequence homologous to C1 inhibitor, messenger RNA for the KIAA0145 gene, and others.

CONCLUSIONS

The presence of these genes in human preimplantation development suggests expression specific to the blastocyst stage.

p38 mitogen-activated protein kinase pathway promotes skeletal muscle differentiation. Participation of the Mef2c transcription factor

Differentiation of muscle cells is regulated by extracellular growth factors that transmit largely unknown signals into the cells. Some of these growth factors induce mitogen-activated protein kinase (MAPK) cascades within muscle cells. In this work we show that the kinase activity of p38 MAPK is induced early during terminal differentiation of L8 cells. Addition of a specific p38 inhibitor SB 203580 to myoblasts blocked their fusion to multinucleated myotubes and prevented the expression of MyoD and MEF2 family members and myosin light chain 2. The expression of MKK6, a direct activator of p38, or of p38 itself enhanced the activity of MyoD in converting 10T1/2 fibroblasts to muscle, whereas treatment with SB 203580 inhibited MyoD. Several lines of evidence suggesting that the involvement of p38 in MyoD activity is mediated via its co-activator MEF2C, a known substrate of p38, are presented. In these experiments we show that MEF2C protein and MEF2-binding sites are necessary for the p38 MAPK pathway to regulate the transcription of muscle creatine kinase reporter gene. Our results indicate that the p38 MAPK pathway promotes skeletal muscle differentiation at least in part via activation of MEF2C.

Extracellular signal-regulated kinase 7 (ERK7), a novel ERK with a C-terminal domain that regulates its activity, its cellular localization, and cell growth

Mitogen-activated protein (MAP) kinases play distinct roles in a variety of cellular signaling pathways and are regulated through multiple mechanisms. In this study, a novel 61-kDa member of the MAP kinase family, termed extracellular signal-regulated kinase 7 (ERK7), has been cloned and characterized. Although it has the signature TEY activation motif of ERK1 and ERK2, ERK7 is not activated by extracellular stimuli that typically activate ERK1 and ERK2 or by common activators of c-Jun N-terminal kinase (JNK) and p38 kinase. Instead, ERK7 has appreciable constitutive activity in serum-starved cells that is dependent on the presence of its C-terminal domain. Interestingly, the C-terminal tail, not the kinase domain, of ERK7 regulates its nuclear localization and inhibition of growth. Taken together, these results elucidate a novel type of MAP kinase whereby interactions via its C-terminal tail, rather than extracellular signal-mediated activation cascades, regulate its activity, localization, and function.

Transcriptional control of muscle development by myocyte enhancer factor-2 (MEF2) proteins

Metazoans contain multiple types of muscle cells that share several common properties, including contractility, excitability, and expression of overlapping sets of muscle structural genes that mediate these functions. Recent biochemical and genetic studies have demonstrated that members of the myocyte enhancer factor-2 (MEF2) family of MADS (MCM1, agamous, deficiens, serum response factor)-box transcription factors play multiple roles in muscle cells to control myogenesis and morphogenesis. Like other MADS-box proteins, MEF2 proteins act combinatorially through protein-protein interactions with other transcription factors to control specific sets of target genes. Genetic studies in Drosophila have also begun to reveal the upstream elements of myogenic regulatory hierarchies that control MEF2 expression during development of skeletal, cardiac, and visceral muscle lineages. Paradoxically, MEF2 factors also regulate cell proliferation by functioning as endpoints for a variety of growth factor-regulated intracellular signaling pathways that are antagonistic to muscle differentiation. We discuss the diverse functions of this family of transcription factors, the ways in which they are regulated, and their mechanisms of action.

Mitogen-activated protein kinase: conservation of a three-kinase module from yeast to human

Mitogen-activated protein kinases (MAPK) are serine-threonine protein kinases that are activated by diverse stimuli ranging from cytokines, growth factors, neurotransmitters, hormones, cellular stress, and cell adherence. Mitogen-activated protein kinases are expressed in all eukaryotic cells. The basic assembly of MAPK pathways is a three-component module conserved from yeast to humans. The MAPK module includes three kinases that establish a sequential activation pathway comprising a MAPK kinase kinase (MKKK), MAPK kinase (MKK), and MAPK. Currently, there have been 14 MKKK, 7 MKK, and 12 MAPK identified in mammalian cells. The mammalian MAPK can be subdivided into five families: MAPKerk1/2, MAPKp38, MAPKjnk, MAPKerk3/4, and MAPKerk5. Each MAPK family has distinct biological functions. In Saccharomyces cerevisiae, there are five MAPK pathways involved in mating, cell wall remodelling, nutrient deprivation, and responses to stress stimuli such as osmolarity changes. Component members of the yeast pathways have conserved counterparts in mammalian cells. The number of different MKKK in MAPK modules allows for the diversity of inputs capable of activating MAPK pathways. In this review, we define all known MAPK module kinases from yeast to humans, what is known about their regulation, defined MAPK substrates, and the function of MAPK in cell physiology.

Regulation of the MEF2 family of transcription factors by p38

Members of the MEF2 family of transcription factors bind as homo- and heterodimers to the MEF2 site found in the promoter regions of numerous muscle-specific, growth- or stress-induced genes. We showed previously that the transactivation activity of MEF2C is stimulated by p38 mitogen-activated protein (MAP) kinase. In this study, we examined the potential role of the p38 MAP kinase pathway in regulating the other MEF2 family members. We found that MEF2A, but not MEF2B or MEF2D, is a substrate for p38. Among the four p38 group members, p38 is the most potent kinase for MEF2A. Threonines 312 and 319 within the transcription activation domain of MEF2A are the regulatory sites phosphorylated by p38. Phosphorylation of MEF2A in a MEF2A-MEF2D heterodimer enhances MEF2-dependent gene expression. These results demonstrate that the MAP kinase signaling pathway can discriminate between different MEF2 isoforms and can regulate MEF2-dependent genes through posttranslational activation of preexisting MEF2 protein.

Fluid shear stress stimulates big mitogen-activated protein kinase 1 (BMK1) activity in endothelial cells. Dependence on tyrosine kinases and intracellular calcium

Mitogen-activated protein (MAP) kinases including ERK1/2 and JNK play an important role in shear stress-mediated gene expression in endothelial cells (EC). A new MAP kinase termed big MAP kinase 1 (BMK1/ERK5) has been shown to phosphorylate and activate the transcription factor MEF2C, which is highly expressed in EC. To determine the effects of shear stress on BMK1, bovine aortic EC were exposed to steady laminar flow (shear stress = 12 dynes/cm2). Flow activated BMK1 within 10 min with peak activation at 60 min (7.1 +/- 0.6-fold) in a force-dependent manner. Flow was the most powerful activator of BMK1, significantly greater than H2O2 or sorbitol. An important role for non-Src tyrosine kinases in flow-mediated BMK1 activation was demonstrated by inhibition with herbimycin A, but not with the Src inhibitor PP1 or overexpression of kinase-inactive c-Src. BMK1 activation was calcium-dependent as shown by inhibition with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid/acetoxymethyl ester or thapsigargin. As shown by specific inhibitors or activators, flow-mediated BMK1 activation was not regulated by the following: intracellular redox state; intracellular NO; protein kinase A, C, or G; calcium/calmodulin-dependent kinase; phosphatidylinositol 3-kinase; or arachidonic acid metabolism. In summary, flow potently stimulates BMK1 in EC by a mechanism dependent on a tyrosine kinase(s) and calcium mobilization, but not on c-Src, redox state, or NO production.

Myocyte enhancer factor 2C and Nkx2-5 up-regulate each other's expression and initiate cardiomyogenesis in P19 cells

The Nkx2-5 homeodomain protein plays a key role in cardiomyogenesis. Ectopic expression in frog and zebrafish embryos results in an enlarged myocardium; however, expression of Nkx2-5 in fibroblasts was not able to trigger the development of beating cardiac muscle. In order to examine the ability of Nkx2-5 to modulate endogenous cardiac specific gene expression in cells undergoing early stages of differentiation, P19 cell lines overexpressing Nkx2-5 were differentiated in the absence of Me2SO. Nkx2-5 expression induced cardiomyogenesis in these cultures aggregated without Me2SO. During differentiation into cardiac muscle, Nkx2-5 expression resulted in the activation of myocyte enhancer factor 2C (MEF2C), but not MEF2A, -B, or -D. In order to compare the abilities of Nkx2-5 and MEF2C to induce cellular differentiation, P19 cells overexpressing MEF2C were aggregated in the absence of Me2SO. Similar to Nkx2-5, MEF2C expression initiated cardiomyogenesis, resulting in the up-regulation of Brachyury T, bone morphogenetic protein-4, Nkx2-5, GATA-4, cardiac alpha-actin, and myosin heavy chain expression. These findings indicate the presence of a positive regulatory network between Nkx2-5 and MEF2C and show that both factors can direct early stages of cell differentiation into a cardiomyogenic pathway.

MAP kinase pathways in the yeast Saccharomyces cerevisiae

A cascade of three protein kinases known as a mitogen-activated protein kinase (MAPK) cascade is commonly found as part of the signaling pathways in eukaryotic cells. Almost two decades of genetic and biochemical experimentation plus the recently completed DNA sequence of the Saccharomyces cerevisiae genome have revealed just five functionally distinct MAPK cascades in this yeast. Sexual conjugation, cell growth, and adaptation to stress, for example, all require MAPK-mediated cellular responses. A primary function of these cascades appears to be the regulation of gene expression in response to extracellular signals or as part of specific developmental processes. In addition, the MAPK cascades often appear to regulate the cell cycle and vice versa. Despite the success of the gene hunter era in revealing these pathways, there are still many significant gaps in our knowledge of the molecular mechanisms for activation of these cascades and how the cascades regulate cell function. For example, comparison of different yeast signaling pathways reveals a surprising variety of different types of upstream signaling proteins that function to activate a MAPK cascade, yet how the upstream proteins actually activate the cascade remains unclear. We also know that the yeast MAPK pathways regulate each other and interact with other signaling pathways to produce a coordinated pattern of gene expression, but the molecular mechanisms of this cross talk are poorly understood. This review is therefore an attempt to present the current knowledge of MAPK pathways in yeast and some directions for future research in this area.

MAP kinases and vascular smooth muscle function

The mitogen-activated protein (MAP) kinase family members are ubiquitously expressed protein kinases activated in response to a variety of extracellular stimuli and shown to be involved in cell growth, transformation, differentiation and apoptosis. MAP kinases have been implicated in both growth and apoptosis of vascular smooth muscle cells (VSMC) which suggests that they play important roles in cardiovascular diseases such as essential hypertension, atherosclerosis, and restenosis followed angioplasty. The MAP kinases are themselves components of specific kinase cascades characterized by activation by specific stimuli, families of related serine and threonine kinases and downstream substrates that include other kinases, transcription factors, membrane receptors and other cell mediators. Cross-talk among the different MAP kinases results in direct modulation of signal transduction. In addition, increased expression and activation of MAP kinase phosphatases plays an important role in MAP kinase inactivation. Our laboratory has used angiotensin II (AngII), a potent activator of all MAP kinases in VSMC, to study mechanisms by which MAP kinases are regulated by vasoactive peptides. In this review, we describe the mechanisms by which AngII activates MAP kinases, and potential roles for MAP kinases in AngII-dependent effects on VSMC function.

Requirement of the MADS-box transcription factor MEF2C for vascular development

The embryonic vasculature develops from endothelial cells that form a primitive vascular plexus which recruits smooth muscle cells to form the arterial and venous systems. The MADS-box transcription factor MEF2C is expressed in developing endothelial cells and smooth muscle cells (SMCs), as well as in surrounding mesenchyme, during embryogenesis. Targeted deletion of the mouse MEF2C gene resulted in severe vascular abnormalities and lethality in homozygous mutants by embryonic day 9.5. Endothelial cells were present and were able to differentiate, but failed to organize normally into a vascular plexus, and smooth muscle cells did not differentiate in MEF2C mutant embryos. These vascular defects resemble those in mice lacking the vascular-specific endothelial cell growth factor VEGF or its receptor Flt-1, both of which are expressed in MEF2C mutant embryos. These results reveal multiple roles for MEF2C in vascular development and suggest that MEF2-dependent target genes mediate endothelial cell organization and SMC differentiation.

Bmk1/Erk5 is required for cell proliferation induced by epidermal growth factor

Epidermal growth factor (EGF) induces cell proliferation in a variety of cell types by binding to a prototype transmembrane tyrosine kinase receptor. Ligation of this receptor by EGF activates Erk1 and Erk2, members of the mitogen-activated protein (MAP) kinase family, through a Ras-dependent signal transduction pathway. Despite our detailed understanding of these events, the exact mechanism by which EGF causes cells to proliferate is unclear. Big MAP kinase (Bmk1), also known as Erk5, is a member of the MAP kinase family that is activated in cells in response to oxidative stress, hyperosmolarity and treatment with serum. Here we show that EGF is a potent activator of Bmk1. In contrast to Erk1/2, EGF-mediated activation of Bmk1 occurs independently of Ras and requires the MAP-kinase kinase Mek5. Expression of a dominant-negative form of Bmk1 blocks EGF-induced cell proliferation and prevents cells from entering the S phase of the cell cycle. These results demonstrate that Bmk1 is part of a distinct MAP-kinase signalling pathway that is required for EGF-induced cell proliferation and progression through the cell cycle.

Interaction of myocyte enhancer factor 2 (MEF2) with a mitogen-activated protein kinase, ERK5/BMK1

Myocyte enhancer factor 2 (MEF2) has been implicated in the complex hierarchical regulation of muscle-specific gene expression and differentiation. While the MyoD family members are able to initiate the skeletal muscle differentiation program, whether MEF2 is sufficient in directing skeletal muscle differentiation is still controversial. Furthermore, how MEF2 transactivates its target genes is not fully understood. It has been suggested that the interactions of MEF2 with other factors modify its transcriptional activity. Therefore, the identification of MEF2-interacting factors may be important in understanding the mechanism by which MEF2 activates its target genes. In this study, a mitogen-activated protein kinase (MAP kinase), ERK5/BMK1 was found to interact with MEF2 in a yeast two hybrid screen. The interaction was confirmed by a glutathione S -transferase-pull down assay and a co-immunoprecipitation study indicating that endogenous ERK5 and MEF2 interact with each other in vivo . The interacting domain of MEF2 was mapped to the N-terminus which contains the highly conserved MADS and MEF2 domains. Functionally, ERK5/BMK1 was able to phosphorylate MEF2 in vitro . Furthermore, when cotransfected with ERK5/BMK1, the transactivation capacity of MEF2 was enhanced. These results suggest that the functions of MEF2 could be regulated through ERK5/BMK1.

Myocyte enhancer factor-related B-MEF2 is developmentally expressed in B cells and regulates the immunoglobulin J chain promoter

Immunoglobulin J chain gene expression is induced by the delivery of a lymphokine signal to antigen-activated B cells in a primary immune response. A major interleukin 2 (IL-2)-responsive region that contains two adjacent control elements (JA and JB) exists within the J chain promoter. Transcription factor PU.1 positively regulates J chain gene expression by binding to one of the control elements (JB) in the J chain promoter. In the present study we have determined that a myocyte enhancer factor 2 (MEF2)-related nuclear factor, named B-MEF2, positively regulates the J chain gene promoter activity via the second control element (JA). An in vitro translated MEF2 family member, MEF2C, was found to bind the JA site with identical properties as endogenously expressed B-MEF2 in B cell lines. Moreover, in vivo experiments showed that a dominant negative mutant of MEF2C blocked B-MEF2 regulation of the J chain promoter. Consistent with its role as positive regulator of J chain gene expression, B-MEF2 levels were enhanced in highly differentiated B cells. In addition, induction of an IL-2-responsive presecretor cell line BCL1 with IL-2 or IL-5 (which up-regulates J chain gene expression) resulted in an increased expression of B-MEF2. We conclude that a MEF2-related transcriptional factor, B-MEF2, acts as a stage-specific positive regulator of J chain gene expression in the B cell lineage.

Stress-activated protein kinases are negatively regulated by cell density

Stimulation by UV irradiation, TNFalpha, as well as PDGF or EGF activates the JNK/SAPK signalling pathway in mouse fibroblasts. This results in the phosphorylation of the N-terminal domain of c-Jun, increasing its transactivation potency. Using an antibody that specifically recognizes c-Jun phosphorylated at Ser63, we show that culture confluency drastically inhibited c-Jun N-terminal phosphorylation due to the inhibition of the JNK/SAPK pathway. Transfection experiments demonstrate that the inhibition occurs at the same level as, or upstream of, the small G-proteins cdc42 and Rac1. In contrast, the classical MAPK pathway was insensitive to confluency. The inhibition of JNK/SAPK activation depended on the integrity of the actin microfilament network. These results were confirmed and extended in monolayer wounding experiments. After PDGF, EGF or UV stimulation, c-Jun was predominantly phosphorylated in cells bordering the wound, which are the cells that move to occupy the wounded area. Thus, modulation of the stress-dependent signal cascade by confluency will restrict c-Jun N-terminal phosphorylation in response to mitogenic or chemotactic agents to cells that border a wounded area.

Mitogen-activated protein kinases and transcriptional responses in renal injury and repair

Mammalian cells respond to external stimuli by activation of a variety of signal transduction pathways which culminate in stereotypical responses important in renal disease, such as proliferation, growth arrest, hypertrophy, differentiation, or apoptosis. A set of intracellular signalling events occurs ultimately leading to the transcription of genes whose encoded proteins mediate the response. In vertebrates many of the stimuli which result in these important cellular responses initiate intracellular signalling events which converge on a set of cellular kinase cascades which are collectively called the mitogen-activated protein (MAP) kinase cascades. There are three families of MAP kinases that have been identified in mammalian cells. These kinase pathways as well as other cellular signalling pathways are critically important for the regulation of transcriptional events. In this review, we will discuss recently published information on how MAP kinases and transcription factors regulated by these kinases may be implicated in renal injury and repair.

p38 Mitogen-activated protein kinase is a critical component of the redox-sensitive signaling pathways activated by angiotensin II. Role in vascular smooth muscle cell hypertrophy

Angiotensin II induces an oxidant stress-dependent hypertrophy in cultured vascular smooth muscle cells. To investigate the growth-related molecular targets of H2O2, we examined the redox sensitivity of agonist-stimulated activation of the mitogen-activated protein kinase (MAPK) family. We show here that angiotensin II elicits a rapid increase in intracellular H2O2 and a rapid and robust phosphorylation of both p42/44MAPK (16-fold) and p38MAPK (15-fold). However, exogenous H2O2 activates only p38MAPK (14-fold), and diphenylene iodonium, an NADH/NADPH oxidase inhibitor, attenuates angiotensin II-stimulated phosphorylation of p38MAPK, but not p42/44MAPK. Furthermore, in cells stably transfected with human catalase, angiotensin II-induced intracellular H2O2 generation is almost completely blocked, resulting in inhibition of phosphorylation of p38MAPK, but not p42/44MAPK, and a subsequent partial decrease in angiotensin II-induced hypertrophy. Specific inhibition of either the p38MAPK pathway with SB203580 (4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H- imidaz ole) or the p42/44MAPK pathway with PD98059 (2-(2'-amino-3'-methoxyphenyl)oxanaphthalen-4-one) also partially, but significantly, attenuates angiotensin II-induced hypertrophy; however, simultaneous blockade of both pathways has an additive inhibitory effect, indicating that the hypertrophic response to angiotensin II requires parallel, independent activation of both MAPK pathways. These results provide the first evidence that p38MAPK is a critical component of the oxidant stress (H2O2)-sensitive signaling pathways activated by angiotensin II in vascular smooth muscle cells and indicate that it plays a crucial role in vascular hypertrophy.