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

TAB1beta (transforming growth factor-beta-activated protein kinase 1-binding protein 1beta ), a novel splicing variant of TAB1 that interacts with p38alpha but not TAK1

The mitogen-activated protein kinases (MAPKs) play an important role in a variety of biological processes. Activation of MAPKs is mediated by phosphorylation on specific regulatory tyrosine and threonine sites. We have recently found that activation of p38alpha MAPK can be carried out not only by its upstream MAPK kinases (MKKs) but also by p38alpha autophosphorylation. p38alpha autoactivation requires an interaction of p38alpha with TAB1 (transforming growth factor-beta-activated protein kinase 1-binding protein 1). The autoactivation mechanism of p38alpha has been found to be important in cellular responses to a number of physiologically relevant stimuli. Here, we report the characterization of a splicing variant of TAB1, TAB1beta. TAB1 and TAB1beta share the first 10 exons. The 11th and 12th exons of TAB1 were spliced out in TAB1beta, and an extra exon, termed exon beta, downstream of exons 11 and 12 in the genome was used as the last exon in TAB1beta. The mRNA of TAB1beta was expressed in all cell lines examined. The TAB1beta mRNA encodes a protein with an identical sequence to TAB1 except the C-terminal 69 amino acids were replaced with an unrelated 27-amino acid sequence. Similar to TAB1, TAB1beta interacts with p38alpha but not other MAPKs and stimulates p38alpha autoactivation. Different from TAB1, TAB1beta does not bind or activate TAK1. Inhibition of TAB1beta expression with RNA interference in MDA231 breast cancer cells resulted in the reduction of basal activity of p38alpha and invasiveness of MDA231 cells, suggesting that TauAlphaBeta1beta is involved in regulating p38alpha activity in physiological conditions.

Protein kinases and their involvement in the cellular responses to genotoxic stress

Cells are constantly subjected to genotoxic stress, and much has been learned regarding their response to this type of stress during the past year. In general, the cellular genotoxic response can be thought to occur in three stages: (1) damage sensing; (2) activation of signal transduction pathways; (3) biological consequences and attenuation of the response. The biological consequences, in particular, include cell cycle arrest and cell death. Although our understanding of the molecular mechanisms underlying cellular genotoxic stress responses remains incomplete, many cellular components have been identified over the years, including a group of protein kinases that appears to play a major role. Various DNA-damaging agents can activate these protein kinases, triggering a protein phosphorylation cascade that leads to the activation of transcription factors, and altering gene expression. In this review, the involvement of protein kinases, particularly the mitogen-activated protein kinases (MAPKs), at different stages of the genotoxic response is discussed.

ATF1 phosphorylation by the ERK MAPK pathway is required for epidermal growth factor-induced c-jun expression

Epidermal growth factor induction of c-jun expression requires ATF1 and MEF2 sites in the c-jun promoter. We find that activation of the c-jun promoter through the ATF1 site requires phosphorylation of ATF1 at serine 63. A serine 63 to alanine mutation of ATF1 acts to block epidermal growth factor (EGF) induction of a transfected c-jun gene. ATF1 can be phosphorylated by mitogen- and stress-activated protein kinase 1 (MSK1), which is activated by EGF and ERK1/2. Kinase-dead MSK1 mutants blocked EGF induction of a transfected c-jun gene suggesting that MSK1 or a similar family member is required for induced c-jun expression. Use of the MEK1 inhibitor U0126 and dominant negative MEK1 further showed that MSK1 activation and c-jun induction require the ERK pathway. In contrast, a JNK inhibitor blocked EGF induction of c-jun expression but not ATF1 phosphorylation. These results show that the two MAPK pathways, ERK and JNK, are required for EGF-induced c-jun expression and that the ERK pathway acts through downstream phosphorylation of ATF1.

Gastrin stimulates cyclooxygenase-2 expression in intestinal epithelial cells through multiple signaling pathways. Evidence for involvement of ERK5 kinase and transactivation of the epidermal growth factor receptor

Gastrin is a hormone produced by G-cells in the normal gastric antrum. However, colorectal carcinoma cells may aberrantly produce gastrin and exhibit increased expression of cholecystokinin B (CCK-B)/gastrin receptors. Gastrin is trophic for the normal gastric oxyntic mucosa and exerts a growth-promoting action on gastrointestinal malignancy. Thus, gastrin may act as an autocrine/paracrine or endocrine factor in the initiation and progression of colorectal carcinoma. The molecular mechanisms involved have not been elucidated. Hypergastrinemia induced by Helicobacter pylori infection is associated with increased cyclooxygenase-2 (COX-2) expression in gastric and colorectal tissues, suggesting the possibility that gastrin up-regulates COX-2 expression in these tissues; this has not been confirmed. We report here that gastrin significantly increases the expression of COX-2 mRNA and protein, the activity of the COX-2 promoter, and the release of prostaglandin E(2) from a rat intestinal epithelial cell line transfected with the CCK-B receptor. These actions were dependent upon the activation of multiple MAPK signal pathways, including ERK5 kinase; transactivation of the epidermal growth factor receptor; and the increased expression and activities of transcription factors ELK-1, activating transcription factor-2, c-Fos, c-Jun, activator protein-1, and myocyte enhancer factor-2. Thus, our findings identify the signaling pathways coupling the CCK-B receptor with up-regulation of COX-2 expression. This effect may contribute to this hormone-dependent gastrointestinal carcinogenesis, especially in the colon.

Up-regulation of c-jun mRNA in cardiac myocytes requires the extracellular signal-regulated kinase cascade, but c-Jun N-terminal kinases are required for efficient up-regulation of c-Jun protein

Cardiac hypertrophy, an important adaptational response, is associated with up-regulation of the immediate early gene, c- jun, which encodes the c-Jun transcription factor. c-Jun may feed back to up-regulate its own transcription and, since the c-Jun N-terminal kinase (JNK) family of mitogen-activated protein kinases (MAPKs) phosphorylate c-Jun(Ser-63/73) to increase its transactivating activity, JNKs are thought to be the principal factors involved in c- jun up-regulation. Hypertrophy in primary cultures of cardiac myocytes is induced by endothelin-1, phenylephrine or PMA, probably through activation of one or more of the MAPK family. These three agonists increased c- jun mRNA with the rank order of potency of PMA approximately endothelin-1>phenylephrine. Up-regulation of c- jun mRNA by endothelin-1 was attenuated by inhibitors of protein kinase C (GF109203X) and the extracellular signal-regulated kinase (ERK) cascade (PD98059 or U0126), but not by inhibitors of the JNK (SP600125) or p38-MAPK (SB203580) cascades. Hyperosmotic shock (0.5 M sorbitol) powerfully activates JNKs, but did not increase c- jun mRNA. These data suggest that ERKs, rather than JNKs, are required for c- jun up-regulation. However, endothelin-1 and phenylephrine induced greater up-regulation of c-Jun protein than PMA and phosphorylation of c-Jun(Ser-63/73) correlated with the level of c-Jun protein. Up-regulation of c-Jun protein by endothelin-1 was attenuated by inhibitors of protein kinase C and the ERK cascade, probably correlating with a primary input of ERKs into transcription. In addition, SP600125 inhibited the phosphorylation of c-Jun(Ser-63/73), attenuated the increase in c-Jun protein induced by endothelin-1 and increased the rate of c-Jun degradation. Thus whereas ERKs are the principal MAPKs required for c- jun transcription, JNKs are necessary to stabilize c-Jun for efficient up-regulation of the protein.

ERK5 MAPK regulates embryonic angiogenesis and acts as a hypoxia-sensitive repressor of vascular endothelial growth factor expression

During angiogenesis, endothelial cells undergo proliferation, reorganization, and stabilization to establish a mature vascular network. This process is critical for establishing a functional circulatory system during development and contributes to the pathological process of tumor growth. Here we report that embryos deficient for the ERK5 MAPK die between embryonic days 10.5 and 11.5 with angiogenic failure and cardiovascular defects. We show that ERK5 deficiency leads to an increased expression of the vascular endothelial growth factor (VEGF), dysregulation of which has been shown to impede angiogenic remodeling and vascular stabilization. Our data also reveal that ERK5 negatively regulates transcription from the vegf locus during hypoxic responses. Importantly, ERK5 is required at an earlier developmental stage than p38alpha, and p38alpha does not compensate for ERK5 deficiency. These results demonstrate that ERK5 plays a specific role in the regulation of early angiogenesis.

Role of p38 and p44/42 mitogen-activated protein kinases in microglia

Although microglial cells are thought to play a beneficial role in the regeneration and plasticity of the central nervous system (CNS), recent studies have indicated that at least some molecules released by microglia may be harmful in acute brain insults and neurodegenerative diseases. Therefore, the pathways mediating the synthesis and release of these neurotoxic compounds are of importance. p38 and p44/42 families of mitogen-activated protein kinases (MAPKs) in microglia respond strongly to various extracellular stimuli, such as ATP, thrombin, and beta-amyloid, a peptide thought to be responsible for the neuropathology in Alzheimer's disease. In this review we describe in vivo evidence implicating that p38 and p44/42 MAPKs may play a critical role in harmful microglial activation in acute brain injury, such as stroke, and in more chronic neurodegenerative diseases, such as Alzheimer's disease. We also clarify the extracellular signals responsible for activation of p38 and p44/42 MAPK in microglia and review the responses so far reported to be mediated by these kinases.

Adhesion-related kinase repression of gonadotropin-releasing hormone gene expression requires Rac activation of the extracellular signal-regulated kinase pathway

Recent studies suggest that adhesion-related kinase (Ark) plays a role in gonadotropin-releasing hormone (GnRH) neuronal physiology. Ark promotes migration of GnRH neurons via Rac GTPase and concomitantly suppresses GnRH gene expression via homeodomain and myocyte enhancer factor-2 (MEF2) transcription factors. Here, we investigated the signaling cascade required for Ark inhibition of the GnRH promoter in GT1-7 GnRH neuronal cells. Ark repression was blocked by the MEK/ERK pathway inhibitor, PD98059, and dominant negative MEK1 but was unaffected by dominant negative Ras. Inhibitors of the Rho family GTPases, Clostridium difficile toxin B (Rho/Rac/Cdc42 inhibitor) and Clostridium sordellii lethal toxin (Rac/Cdc42 inhibitor), blocked Ark inhibition of GnRH transcription. Moreover, dominant negative Rac blunted both Ark activation of ERK and repression of the GnRH promoter, demonstrating an essential role for Rac in coupling Ark to ERK activation. Like Ark, a constitutively active mutant of Rac suppressed GnRH transcription in an ERK-dependent manner. Finally, Ark-mediated repression was significantly attenuated by a dominant negative MEF2C, whereas repression induced by constitutively active Rac was unaffected, indicating that MEF2 proteins are not targets of the Ark --> Rac --> MEK --> ERK cascade. The data suggest that Ark suppresses GnRH gene expression via the coordinated activation of a Rac --> ERK signaling pathway and a distinct MEF2- dependent mechanism.

Variable apoptotic response of NSCLC cells to inhibition of the MEK/ERK pathway by small molecules or dominant negative mutants

To evaluate the role of the MEK/ERK pathway in NSCLC survival, we analyzed NSCLC cell lines that differed in tumor histology and status of p53, Rb, and K-ras. Constitutive ERK1/2 activity was demonstrated in 17 of 19 cell lines by maintenance of ERK1/2 phosphorylation with serum deprivation. Phosphorylation of ERK1/2 correlated with phosphorylation of MEK1/2 and p90RSK, but was inversely correlated with phosphorylation of c-Raf at S259. With serum deprivation, the MEK inhibitors, PD98059 and U0126, inhibited ERK1/2 activity but did not increase apoptosis. PD98059 and U0126 induced cell cycle arrest in G(0)/G(i) in cells with the highest levels of ERK1/2 activity, which correlated with induction of p27 but not p21. To confirm the cytostatic response to MEK inhibitors, we performed transient transfections with dominant negative forms of MEK or ERK. Surprisingly, dominant negative MEK and ERK mutants increased apoptosis without affecting cell cycle or p27 levels. When combined with paclitaxel, MEK inhibitors had no effect on apoptosis. In contrast, dominant negative ERK2 potentiated paclitaxel-induced apoptosis. Our studies show that constitutive ERK1/2 activity in NSCLC cells promotes cellular survival and chemotherapeutic resistance. Moreover, our data are the first to demonstrate divergent cellular responses to inhibition of the MEK/ERK pathway by small molecule inhibitors or dominant negative mutants.

Cell-cycle arrest by PD184352 requires inhibition of extracellular signal-regulated kinases (ERK) 1/2 but not ERK5/BMK1

Serum and growth factors activate both the canonical extracellular signal-regulated kinase (ERK) 1/2 pathway and the ERK5/big mitogen-activated protein kinase 1 (BMK) 1 pathway. Pharmacological inhibition of the ERK1/2 pathway using PD98059 and U0126 prevents cyclin D1 expression and inhibits cell proliferation, arguing that the ERK1/2 pathway is rate limiting for cell-cycle re-entry. However, both PD98059 and U0126 also inhibit the ERK5/BMK1 pathway, raising the possibility that the anti-proliferative effect of such drugs may be due to inhibition of ERK5 or both pathways. Here we characterize the effect of the novel mitogen-activated protein kinase/ERK kinase (MEK) inhibitor, PD184352, on the ERK1/2 and ERK5 pathways in the Chinese hamster fibroblast cell line CCl39. In quiescent cells, serum-stimulated ERK1 activity was completely inhibited by PD184352 with an IC50 below 1 microM, whereas ERK5 activation was unaffected even at 20 microM. Serum-stimulated DNA synthesis and cyclin D1 expression was inhibited by low doses of PD184352, which abolished ERK1 activity but had no effect on ERK5. Similarly, in cycling cells PD184352 caused a dose-dependent G1 arrest and inhibition of cyclin D1 expression at low doses, which inhibited ERK1 but were without effect on ERK5. These results indicate that the anti-proliferative effect of PD184352 is due to inhibition of the classical ERK1/2 pathway and does not require inhibition of the ERK5 pathway.

Overexpression of chemokine-like factor 2 promotes the proliferation and survival of C2C12 skeletal muscle cells

Chemokine-like factor 1 (CKLF1) is a novel cytokine first cloned from U937 cells. It contains different splicing forms and has chemotactic effects on a wide spectrum of cells both in vitro and in vivo; it can also stimulate the regeneration of skeletal muscle cells in vivo, but the mechanism remains unclear. To probe the myogenesis function of CKLF2, which is the largest isoform of CKLFs, C2C12 murine myoblasts were stably transfected with human CKLF2 eukaryotic expression vector. Compared with control vector transfected C2C12 cells, CKLF2 overexpression causes accelerated myoblast proliferation as determined by cell counting and [(3)H]TdR incorporation assays. In addition, CKLF2 overexpression also promotes cell differentiation, which was determined by higher expression levels of myogenin, creatine kinase, myosin and the accelerated myoblast fusion. Further analysis also indicates that CKLF2 could activate the transcription activity of the bHLH/MyoD and MEF2 families. Finally, DNA synthesis and myotube formation could also be promoted by growing C2C12 cells in conditioned media from CKLF2-transfected cells. These findings strongly suggest a role for human CKLF2 in regulation of skeletal muscle myogenesis.

Role of MAP kinases in the 1,25-dihydroxyvitamin D3-induced transactivation of the rat cytochrome P450C24 (CYP24) promoter. Specific functions for ERK1/ERK2 and ERK5

The current study investigated the action of 1,25-dihydroxyvitamin D(3) (1,25D) at the genomic and signal transduction levels to induce rat cytochrome P450C24 (CYP24) gene expression. A rat CYP24 promoter containing two vitamin D response elements and an Ets-1 binding site was used to characterize the mechanism of actions for the 1,25D secosteroid hormone. The Ets-1 binding site was determined to function cooperatively with the most proximal vitamin D response element in a hormone-dependent fashion. Evidence was obtained for distinct roles of ERK1/ERK2 and ERK5 in the 1,25D-inductive actions. Specifically, 1,25D stimulated the activities of ERK1/ERK2 and ERK5 in a Ras-dependent manner. Promoter induction was inhibited by mitogen-activated protein (MAP) kinase inhibitors (PD98059 and U0126) and a dominant-negative Ras mutant (Ras17N). Induction of CYP24 by 1,25D was also inhibited by overexpression of dominant-negative mutants of ERK1 and MEK5 (ERK1K71R and MEK5(A)). The p38 and JNK MAP kinases were not required for the action of 1,25D. 9-cis retinoid X receptor alpha (RXR alpha) interacted with ERK2 but not ERK5 in intact cells, whereas Ets-1 interacted preferentially with ERK5. Increased phosphorylation of RXR alpha and Ets-1 was detected in response to 1,25D. Activated ERK2 and ERK5 specifically phosphorylated RXR alpha and Ets-1, respectively. Mutagenesis of Ets-1 (T38A) reduced CYP24 promoter activity to levels observed with the dominant-negative MEK5(A) and inhibited ERK5-directed phosphorylation. Mutated RXR alpha (S260A) inhibited 1,25D-induced CYP24 promoter activity and abolished phosphorylation by activated ERK2. The 1,25D-inductive action through ERK5 involved Ets-1 phosphorylation at threonine 38, whereas hormone stimulation of ERK1/ERK2 required RXR alpha phosphorylation on serine 260. The ERK1/ERK2 and ERK5 modules provide a novel mechanism for linking the rapid signal transduction and slower transcription actions of 1,25D to induce CYP24 gene expression.

Phosphotyrosine-specific phosphatase PTP-SL regulates the ERK5 signaling pathway

The duration and the magnitude of mitogen-activated protein kinase (MAPK) activation specifies signal identity and thus allows the regulation of diverse cellular functions by the same kinase cascade. A tight and finely tuned regulation of MAPK activity is therefore critical for the definition of a specific cellular response. We investigated the role of tyrosine-specific phosphatases (PTPs) in the regulation of ERK5. Although unique in its structure, ERK5 is activated in analogy to other MAPKs by dual phosphorylation of threonine and tyrosine residues in its activation motif. In this study we concentrated on whether and how PTP-SL, a kinase-interacting motif-containing PTP, might be involved in the down-regulation of the ERK5 signal. We found that both proteins interact directly with each other in vitro and in intact cells, resulting in mutual modulation of their enzymatic activities. PTP-SL is a substrate of ERK5 and independent of phosphorylation binding to the kinase enhances its catalytic phosphatase activity. On the other hand, interaction with PTP-SL not only down-regulates endogenous ERK5 activity but also effectively impedes the translocation of ERK5 to the nucleus. These findings indicate a direct regulatory influence of PTP-SL on the ERK5 pathway and corresponding downstream responses of the cell.

Molecular interpretation of ERK signal duration by immediate early gene products

The duration of intracellular signalling is associated with distinct biological responses, but how cells interpret differences in signal duration are unknown. We show that the immediate early gene product c-Fos functions as a sensor for ERK1 (extracellular-signal-regulated kinase 1) and ERK2 signal duration. When ERK activation is transient, its activity declines before the c-Fos protein accumulates, and under these conditions c-Fos is unstable. However, when ERK signalling is sustained, c-Fos is phosphorylated by still-active ERK and RSK (90K-ribosomal S6 kinase). Carboxy-terminal phosphorylation stabilizes c-Fos and primes additional phosphorylation by exposing a docking site for ERK, termed the FXFP (DEF) domain. Mutating the DEF domain disrupts the c-Fos sensor and c-Fos-mediated signalling. Other immediate early gene products that control cell cycle progression, neuronal differentiation and circadium rhythms also contain putative DEF domains, indicating that multiple sensors exist for sustained ERK signalling. Together, our data identify a general mechanism by which cells can interpret differences in ERK activation kinetics.

Identification of mitogen-activated protein kinase kinase as a chemoresistant pathway in MCF-7 cells by using gene expression microarray

BACKGROUND

Components of the mitogen-activated protein kinase (MAPK) cascade have been implicated in apoptotic regulation. This study used gene expression profiling analysis to identify and implicate mitogen-activated protein kinase kinase (MEK5)-BMK1 (big mitogen-activated kinase-1)/extracellular signal related protein kinase (ERK5) pathway as a novel target involved in chemoresistance.

METHODS

Differential gene expression between apoptotically sensitive (APO+) and apoptotically resistant (APO-) MCF-7 cell variants was determined by using microarray and confirmed by reverse transcriptase- polymerase chain reaction (RT-PCR). An apoptotic/viability reporter gene assay was used to deter-mine the effects of the transfection of a dominant-negative mutant of BMK1 (BMK1/DN) in conjunction with apoptotic-inducing agents (etoposide, tumor necrosis factor-alpha [TNF], or TNF-related apoptosis-inducing ligand [TRAIL]), with or without phorbol ester (PMA).

RESULTS

Of the 1186 genes detected through microarray analysis, MEK5 was increased 22-fold in APO- cells. Overexpression of MEK5 was confirmed by using RT-PCR analysis. Expression of BMK1/DN alone resulted in a dose-dependent increase in cell death versus control (P <.05). In addition, BMK1/DN enhanced the sensitivity of MCF-7 cells to treatment-induced cell death (P <.05). The ability of PMA to partially suppress TRAIL- and TNF-induced cell death was inhibited by BMK1/DN. However, only TRAIL-induced activity suppression reached statistical significance (P <.05).

CONCLUSIONS

The overexpression of MEK5 in APO- MCF-7 breast carcinoma cells shows that this MAPK signaling protein represents a potent survival molecule. Molecular inhibition of MEK5 signaling may represent a mechanism for sensitizing cancer cells to chemotherapeutic regimens.

PP1/PP2A phosphatases inhibitors okadaic acid and calyculin A block ERK5 activation by growth factors and oxidative stress

Okadaic acid is an inhibitor of the protein Ser/Thr phosphatases PP1 and PP2A, which blocks the activation of extracellular signal-regulated protein kinase 5 (ERK5), a member of the MAP kinase family activated by growth factors and several types of stressors. The blocking of ERK5 activation by okadaic acid was observed in HeLa cells exposed to epidermal growth factor and H(2)O(2) as well as in PC12 cells stimulated by nerve growth factor and H(2)O(2). Calyculin A, another PP1 and PP2A inhibitor, behaved similarly although these compounds are not structurally related. This suggests that either PP1 or PP2A or both are necessary for ERK5 activation. Protein kinase C (PKC) acts as a negative regulator of the ERK5 activation pathway, however our data suggest that the effects of PKC and the phosphatase are unrelated.

Erk5 null mice display multiple extraembryonic vascular and embryonic cardiovascular defects

Erk5 is a mitogen-activated protein kinase, the biological role of which is largely undefined. Therefore, we deleted the erk5 gene in mice to assess its function in vivo. Inactivation of the erk5 gene resulted in defective blood-vessel and cardiac development leading to embryonic lethality around embryonic days 9.5-10.5. Cardiac development was retarded largely, and the heart failed to undergo normal looping. Endothelial cells that line the developing myocardium of erk5-/- embryos displayed a disorganized, rounded morphology. Vasculogenesis occurred, but extraembryonic and embryonic blood vessels were disorganized and failed to mature. Furthermore, the investment of embryonic blood vessels with smooth muscle cells was attenuated. Together, these data define an essential role for Erk5 in cardiovascular development. Moreover, the inability of Erk5-deficient mice to form a complex vasculature suggests that Erk5 may play an important role in controlling angiogenesis.

Invited review: intracellular signaling in contracting skeletal muscle

Physical exercise is a significant stimulus for the regulation of multiple metabolic and transcriptional processes in skeletal muscle. For example, exercise increases skeletal muscle glucose uptake, and, after exercise, there are increases in the rates of both glucose uptake and glycogen synthesis. A single bout of exercise can also induce transient changes in skeletal muscle gene transcription and can alter rates of protein metabolism, both of which may be mechanisms for chronic adaptations to repeated bouts of exercise. A central issue in exercise biology is to elucidate the underlying molecular signaling mechanisms that regulate these important metabolic and transcriptional events in skeletal muscle. In this review, we summarize research from the past several years that has demonstrated that physical exercise can regulate multiple intracellular signaling cascades in skeletal muscle. It is now well established that physical exercise or muscle contractile activity can activate three of the mitogen-activated protein kinase signaling pathways, including the extracellular signal-regulated kinase 1 and 2, the c-Jun NH(2)-terminal kinase, and the p38. Exercise can also robustly increase activity of the AMP-activated protein kinase, as well as several additional molecules, including glycogen synthase kinase 3, Akt, and the p70 S6 kinase. A fundamental goal of signaling research is to determine the biological consequences of exercise-induced signaling through these molecules, and this review also provides an update of progress in this area.

BMK1 (ERK5) regulates squamous differentiation marker SPRR1B transcription in Clara-like H441 cells

Various toxicants and carcinogens upregulate the expression of small proline-rich protein 1B (SPRR1B), a squamous differentiation marker, in bronchial epithelial cells both in vivo and in vitro. We have recently shown that phorbol 13-myristate 12-acetate (PMA)-stimulated SPRR1B transcription in Clara-like H441 cells is mainly mediated by activator protein-1 (AP-1) and c-Jun N-terminal kinase-1 (JNK1). Though mitogen-activated protein kinase (MAPK) kinase (MEK)-1/2 pathway inhibitors strongly suppressed both basal and PMA-inducible SPRR1B transcription, overexpression of dominant negative (dn) forms of extracellular signal-regulated kinase (ERK)-1 and/or -2 did not have any significant effect indicating the involvement of another ERK-like MAPK in this pathway. Here, we report for the first time the involvement of ERK5 in PMA-inducible SPRR1B transcription in H441 cells. PMA significantly induced ERK5 activation in H441 cells. Overexpression of dn-ERK5 strongly suppressed both basal and PMA-inducible SPRR1B transcription, whereas wild-type ERK5 upregulated it. Consistent with this, a mutant form of MEK-5, an upstream activator of ERK5, strongly suppressed PMA-inducible promoter activity. However, coexpression of c-Jun restored promoter activation suppressed by dn-ERK5. Thus, in addition to JNK1, the activation of MEK5-ERK5 MAPK pathway probably plays a pivotal role in transcriptional regulation of AP-1-mediated SPRR1B expression in the distal bronchiolar region.

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.

Mitogen-activated protein kinases and activator protein 1 are required for proliferation and cardiomyocyte differentiation of P19 embryonal carcinoma cells

Mitogen-activated protein kinases (MAPKs) have been implicated as regulators of differentiation. The biological effect of MAPK signaling in the nucleus is achieved by signal-responsive transcription factors. Here we have investigated MAPK signaling and activation of AP-1 transcription factors in P19 embryonal carcinoma cells undergoing cardiomyocyte differentiation. We show that aggregation and Me(2)SO treatment, which trigger the differentiation response, result in sustained activation of JNK1, p38, and ERK1/2 MAPKs and acquisition of AP-1 DNA binding activity. The induced AP-1 activity consists of c-Jun, JunD, and Fra-2 proteins and is accompanied with the increased expression of these proteins. JNK is involved in c-Jun phosphorylation, whereas ERK and p38 activities are essential for maximal c-Jun and Fra-2 expression, and AP-1 DNA binding activity. While the inhibition of ERK can partially prevent the formation of beating cardiomyocytes, the activity of p38 is absolutely required for the differentiation. Expression of dominant negative c-Jun(bZIP) in P19 cells can also inhibit the differentiation response. Surprisingly, however, expression of dominant negative SEK or JNK causes an inhibition of P19 cell proliferation. Taken together, the results show that ERK, JNK, p38, and AP-1 are activated in a coordinated and sustained manner, and contribute to proliferation and cardiomyocyte differentiation of P19 cells.

AP-1 as a regulator of cell life and death

The transcription factor AP-1 (activator protein-1) is involved in cellular proliferation, transformation and death. Using mice and cells lacking AP-1 components, the target-genes and molecular mechanisms mediating these processes were recently identified. Interestingly, the growth-promoting activity of c-Jun is mediated by repression of tumour suppressors, as well as upregulation of positive cell cycle regulators. Mostly, c-Jun is a positive regulator of cell proliferation, whereas JunB has the converse effect. The intricate relationships between the different Jun proteins, their activities and the mechanisms that mediate them will be discussed.

Vascular smooth muscle cell proliferation requires both p38 and BMK1 MAP kinases

Vascular smooth muscle cell (VSMC) proliferation is a key event in the progression of atherosclerosis. Induction of both c-fos (through the transcription factor Elk-1) and c-jun, both immediate early genes, is important for the stimulation of VSMC proliferation and migration. It was earlier found that p38 mitogen-activated protein (MAP) kinase upregulates c-jun gene transcription through phosphorylation of two myocyte enhancer factor 2 (MEF2) family transcription factors, MEF2A and MEF2C, while big MAP kinase 1 (BMK1) may upregulate c-jun gene transcription through MEF2A, MEF2C, and also MEF2D. Here, we report that inhibition of BMK1 by a dominant negative form of MEK5 or pharmacologic inhibition of p38 by SB 203580 additively suppress serum-induced VSMC proliferation. This additive effect of p38 and BMK1 inhibition implies that these two kinases coordinately regulate MEF2 transcription factors. The exclusive activation of MEF2D by BMK1 appears required for this cooperative upregulation of c-jun in VSMC, and coactivation of p38 and BMK1 also has additive effects on the activation of a reporter gene linked to the c-jun promoter in our experimental system. Thus, coordinate activity of both the p38 and BMK1 pathways appears necessary for optimal transcription of c-jun and, pari pasu, VSMC proliferation. These results may have implications for the future design of pharmacologic agents for inhibition of VSMC growth.

Hydrogen peroxide stimulates c-Src-mediated big mitogen-activated protein kinase 1 (BMK1) and the MEF2C signaling pathway in PC12 cells: potential role in cell survival following oxidative insults

Reactive oxygen species, generated by reduction-oxidation (redox) reactions, have been recognized as one of the major mediators of ischemia and reperfusion injury in the brain. Reactive oxygen species-induced cerebral events are attributable, in part, to the change in intracellular signaling molecules including mitogen-activated protein (MAP) kinases. Big MAP kinase 1 (BMK1), also known as ERK5, is a newly identified member of the MAP kinase family and has been reported to be sensitive to oxidative stress. In the present study, we examined the effect of H(2)O(2) on BMK1 activity in PC12 cells, and we investigated the pathophysiological implication of BMK1. Findings showed that BMK1 was rapidly and significantly activated by H(2)O(2) in a concentration-dependent manner in PC12 cells. BMK1 activation by H(2)O(2) was inhibited by both PD98059 and U0126, which were reported to inhibit MEK5 as well as MEK1/2. c-Src was suggested to be involved in BMK1 activation from the experiments with herbimycin A and PP2, specific inhibitors of Src family kinases. Transfection of kinase-inactive Src also inhibited H(2)O(2)-induced BMK1 activation. In addition, H(2)O(2) treatment of cells induced an enhancement of DNA binding activity of MEF2C, a downstream transcription factor of BMK1 in PC12 cells. Finally, pretreatment of cells with PD98059 and U0126 resulted in an increase in cell death including apoptosis by H(2)O(2) in ERK1/2 down-regulated cells as well as in intact PC12 cells. These findings suggest that c-Src mediated BMK1 activation by H(2)O(2) may counteract ischemic cellular damage probably through the activation of MEF2C transcription factor.

Activation of big mitogen-activated protein kinase-1 regulates smooth muscle cell replication

This study examined the activation of big mitogen-activated protein (MAP) kinase-1 (BMK1) in rat carotid smooth muscle cells (SMCs). Platelet-derived growth factor, fibroblast growth factor-2, sorbitol, and serum all increased the activation of BMK1 in rat carotid SMCs, whereas angiotensin II, phorbol esters, and tumor necrosis factor-alpha had only slight effects. With the exception of tumor necrosis factor-alpha, all these factors phosphorylated extracellular signal-regulated kinase (ERK)1/2. The MAPK kinase inhibitor (MEKI), U0126 (1 micromol/L), blocked ERK1/2 phosphorylation and at higher doses (5 micromol/L) blocked BMK1 phosphorylation. This inhibitor also blocked SMC DNA synthesis in a dose-dependent manner. When SMCs were transfected with an adenoviral construct expressing dominant mutant BMK1 and stimulated with fibroblast growth factor-2, a significantly smaller increase in cyclin D1 and cyclin A expression and in retinoblastoma factor phosphorylation was detected compared with the increase in cells transfected with an adenoviral construct expressing green fluorescent protein (GFP). SMC DNA synthesis was significantly blocked in the cells transfected with the dominant mutant BMK1. These data support the suggestion that BMK1 is important and necessary for mitogen-induced SMC proliferation.

Location, location, location: a spatial view of neurotrophin signal transduction

Neurotrophins were originally identified as target-derived factors that regulate the survival and differentiation of innervating neurons. However, neurotrophins can also be released by presynaptic cells to stimulate postsynaptic neurons. Recent studies indicate that differences exist between the signaling pathways activated by neurotrophin stimulation of nerve terminals (retrograde signaling) and neurotrophin stimulation of cell bodies. Retrograde signaling relies on the formation of signaling endosomes, vesicles containing activated Trk receptors and their ligands. Signaling endosomes travel from the nerve terminals to remote cell bodies, where they selectively activate a novel MAP kinase, Erk5, as well as PI3 kinase, and thereby stimulate neuronal survival. The differences in the signaling pathways activated by neurotrophins, which depends on the location of stimulation, provide a mechanism by which neurons can interpret the 'where' as well as the 'what' of growth factor stimulation.

Synaptic activity-induced conversion of intronic to exonic sequence in Homer 1 immediate early gene expression

Three Homer genes regulate the activity of metabotropic glutamate receptors mGluR1a and mGluR5 and their coupling to releasable intracellular Ca2+ pools and ion channels. Only the Homer 1 gene evolved bimodal expression of constitutive (Homer 1b and c) and immediate early gene (IEG) products (Homer 1a and Ania 3). The IEG forms compete functionally with the constitutive Homer proteins. The complex expression of the Homer 1 gene, unique for IEGs, focused our attention on the gene organization. In contrast to most IEGs, which have genes that are <5 kb, the Homer 1 gene was found to span approximately 100 kb. The constitutive Homer 1b/c forms are encoded by exons 1-10, whereas the IEG forms are encoded by exons 1-5 and parts of intron 5. RNase protection demonstrated a >10-fold activity-dependent increase in mRNA levels exclusively for the IEG forms. Moreover, fluorescent in situ hybridization documented that new primary Homer 1 transcripts are induced in neuronal nuclei within a few minutes after seizure, typical of IEGs, and that Homer 1b-specific exons are excluded from the activity-induced transcripts. Thus, at the resting state of the neurons, the entire gene is constitutively transcribed at low levels to yield Homer 1b/c transcripts. Neuronal activity sharply increases the rate of transcription initiation, with most transcripts now ending within the central intron. These coordinate transcriptional events rapidly convert a constitutive gene to an IEG and regulate the expression of functionally different Homer 1 proteins.

MAPK-regulated transcription: a continuously variable gene switch?

Switching mechanisms that control genes could be viewed either as stable binary switches, in which genes exist in 'on' or 'off' states; or as quantitative rheostat-like switches, in which the rate of transcription is continuously variable and coupled directly to the strength of intracellular signalling events. Here, we discuss the biological need for quantitative gene regulation and, using mitogen-activated protein kinase (MAPK)-controlled transcription as a model, assess the evidence for its existence and postulate mechanisms by which it might occur.

MEF2: a calcium-dependent regulator of cell division, differentiation and death

The decision of a cell to divide, differentiate or die is dependent on the coupling of cytoplasmic signals to the activation and repression of specific sets of genes in the nucleus. Many of the signal transduction pathways that control these cellular decisions are activated by elevation of intracellular calcium. Recent studies have revealed a central role for the myocyte enhancer factor-2 (MEF2) family of transcription factors in linking calcium-dependent signaling pathways to the genes responsible for cell division, differentiation and death. This article describes the post-translational mechanisms that confer calcium-sensitivity to MEF2 and its downstream target genes, and considers how this transcription factor can control diverse and mutually exclusive cellular decisions.

Synaptic activity-induced conversion of intronic to exonic sequence in Homer 1 immediate early gene expression

Three Homer genes regulate the activity of metabotropic glutamate receptors mGluR1a and mGluR5 and their coupling to releasable intracellular Ca2+ pools and ion channels. Only the Homer 1 gene evolved bimodal expression of constitutive (Homer 1b and c) and immediate early gene (IEG) products (Homer 1a and Ania 3). The IEG forms compete functionally with the constitutive Homer proteins. The complex expression of the Homer 1 gene, unique for IEGs, focused our attention on the gene organization. In contrast to most IEGs, which have genes that are <5 kb, the Homer 1 gene was found to span approximately 100 kb. The constitutive Homer 1b/c forms are encoded by exons 1-10, whereas the IEG forms are encoded by exons 1-5 and parts of intron 5. RNase protection demonstrated a >10-fold activity-dependent increase in mRNA levels exclusively for the IEG forms. Moreover, fluorescent in situ hybridization documented that new primary Homer 1 transcripts are induced in neuronal nuclei within a few minutes after seizure, typical of IEGs, and that Homer 1b-specific exons are excluded from the activity-induced transcripts. Thus, at the resting state of the neurons, the entire gene is constitutively transcribed at low levels to yield Homer 1b/c transcripts. Neuronal activity sharply increases the rate of transcription initiation, with most transcripts now ending within the central intron. These coordinate transcriptional events rapidly convert a constitutive gene to an IEG and regulate the expression of functionally different Homer 1 proteins.

Erk5 participates in neuregulin signal transduction and is constitutively active in breast cancer cells overexpressing ErbB2

The four receptor tyrosine kinases of the ErbB family play essential roles in several physiological processes and have also been implicated in tumor generation and/or progression. Activation of ErbB1/EGFR is mainly triggered by epidermal growth factor (EGF) and other related ligands, while activation of ErbB2, ErbB3, and ErbB4 receptors occurs by binding to another set of EGF-like ligands termed neuregulins (NRGs). Here we show that the Erk5 mitogen-activated protein kinase (MAPK) pathway participates in NRG signal transduction. In MCF7 cells, NRG activated Erk5 in a time- and dose-dependent fashion. The action of NRG on Erk5 was dependent on the kinase activity of ErbB receptors but was independent of Ras. Expression in MCF7 cells of a dominant negative form of Erk5 resulted in a significant decrease in NRG-induced proliferation of MCF7 cells. Analysis of Erk5 in several human tumor cell lines indicated that a constitutively active form of this kinase was present in the BT474 and SKBR3 cell lines, which also expressed activated forms of ErbB2, ErbB3, and ErbB4. Treatments aimed at decreasing the activity of these receptors caused Erk5 inactivation, indicating that the active form of Erk5 present in BT474 and SKBR3 cells was due to a persistent positive stimulus originating at the ErbB receptors. In BT474 cells expression of the dominant negative form of Erk5 resulted in reduced proliferation, indicating that in these cells Erk5 was also involved in the control of proliferation. Taken together, these results suggest that Erk5 may play a role in the regulation of cell proliferation by NRG receptors and indicate that constitutively active NRG receptors may induce proliferative responses in cancer cells through this MAPK pathway.

Functional regulatory regions of human transcription factor MEF2C

Myocyte enhancer-binding factor 2C (MEF2C), a transcription factor expressed at high levels in muscle and brain, is implicated in the terminal differentiation and post-mitotic survival of neurons. In this study MEF2C deletion mutants and naturally-occurring isoforms were transfected into COS and P19 cells with two different reporter genes, to test the relative transcriptional activities of the MEF2C constructs. Deletion of parts of the carboxy terminus, in particular amino acids 387-473, enhanced transcriptional activation. A region rich in serine, threonine, proline, and tyrosine from amino acids 312-367 was sufficient to activate transcription at low levels when coupled to amino acids 1-86, which contain the DNA-binding (MADS/MEF) domain of MEF2C, but also depended on amino acids 87-311 for full effect. A construct with amino acids 312-350 missing showed significantly less transcriptional activation than proteins containing this sequence. MEF2C constructs were uniformly localized to the cell nucleus by immunostaining with an antibody to the constant N-terminal region of MEF2C. Western blot and gel shift studies of extracts from transfected cells and from in vitro transcription/translation suggest that variation in the amount of protein expressed or in DNA-binding properties does not account for observed differences in transcriptional activation. This structural information may be useful for elucidating the mechanisms of MEF2C in interacting with other factors to regulate target genes.

The nuclear localization domain of the MEF2 family of transcription factors shows member-specific features and mediates the nuclear import of histone deacetylase 4

Targeting of myocyte enhancer binding factor 2 (MEF2) proteins to the nucleus depends on a C-terminal bipartite nuclear localization signal (NLS). By expression of green fluorescent protein (GFP)/MEF2 fusion proteins in transfected myoblasts, we show that MEF2C contains an additional 13 amino acids domain, located immediately upstream of the NLS, which contributes to its nuclear retention. We also show that the NLS present in MEF2 proteins is required for efficient nuclear localization of histone deacetylase 4 (HDAC4). In muscle cells, transfected HDAC4 is largely cytoplasmic or, to a lesser extent, pancellular. Co-transfection of either MEF2A or MEF2C causes HDAC4 to accumulate in the nucleus in association with MEF2. This effect strongly depends on MEF2 NLS; it also requires the specific interaction of HDAC4 with MEF2, since the isolated NLS is not sufficient for targeting HDAC4 to the nucleus and other nuclear proteins, such as NF-Y, cannot substitute MEF2. Therefore, we demonstrate that HDAC4, different from HDAC5, is mainly a cytoplasmic resident protein, requiring a trans-acting NLS for nuclear localization. The physiological implications of MEF2 carrying its own inhibitor to the nucleus are discussed.

Src family kinase and adenosine differentially regulate multiple MAP kinases in ischemic myocardium: modulation of MAP kinases activation by ischemic preconditioning

Recent studies suggest that ischemia activates Src and members of the mitogen-activated protein (MAP) kinase superfamily and their downstream effectors, including big MAP kinase 1 (BMK1) and p90 ribosomal S6 kinase (p90RSK). It has also been reported that adenosine is released during ischemia and involved in triggering the protective mechanism of ischemic preconditioning. To assess the roles of Src and adenosine in ischemia-induced MAP kinases activation, we utilized the Src inhibitor PP2 (4-Amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine) and the adenosine receptor antagonist 8-(p-sulfophenyl) theophylline (SPT) in perfused guinea pig hearts. PP2 (1 microm) inhibited ischemia-induced Src, BMK1 and JNK activation but not JAK2 and p38 activation. SPT inhibited ischemia-mediated p38 and JNK activation. These results demonstrate that Src family kinase and adenosine regulate MAP kinases by parallel pathways. Preconditioning significantly improved both recovery of developed pressure and dp/dt in isolated guinea pig hearts. Since the protective effect of preconditioning was blocked by PP2 (1 microm) and SPT (50 microm), we next investigated the regulation of Src, MAP kinases and p90RSK during preconditioning. The activity and time course of ERK1/2 was not changed, but p90RSK activation by reperfusion was completely inhibited by preconditioning. In contrast, the activation by ischemia of Src, BMK1, p38 and JNK was significantly faster in preconditioned hearts. Maximal BMK1 activation by ischemia was also significantly enhanced by preconditioning. These data suggest important roles for Src family kinases and adenosine in mediating preconditioning, and suggest specific roles for individual MAP kinases in preconditioning.

Signaling to the nucleus by an L-type calcium channel-calmodulin complex through the MAP kinase pathway

Increases in the intracellular concentration of calcium ([Ca2+]i) activate various signaling pathways that lead to the expression of genes that are essential for dendritic development, neuronal survival, and synaptic plasticity. The mode of Ca2+ entry into a neuron plays a key role in determining which signaling pathways are activated and thus specifies the cellular response to Ca2+. Ca2+ influx through L-type voltage-activated channels (LTCs) is particularly effective at activating transcription factors such as CREB and MEF-2. We developed a functional knock-in technique to investigate the features of LTCs that specifically couple them to the signaling pathways that regulate gene expression. We found that an isoleucine-glutamine ("IQ") motif in the carboxyl terminus of the LTC that binds Ca2+-calmodulin (CaM) is critical for conveying the Ca2+ signal to the nucleus. Ca2+-CaM binding to the LTC was necessary for activation of the Ras/mitogen-activated protein kinase (MAPK) pathway, which conveys local Ca2+ signals from the mouth of the LTC to the nucleus. CaM functions as a local Ca2+ sensor at the mouth of the LTC that activates the MAPK pathway and leads to the stimulation of genes that are essential for neuronal survival and plasticity.

Neurotrophins use the Erk5 pathway to mediate a retrograde survival response

Growth factors synthesized and released by target tissues promote survival and differentiation of innervating neurons. This retrograde signal begins when growth factors bind receptors at nerve terminals. Activated receptors are then endocytosed and transported through the axon to the cell body. Here we show that the mitogen-activated protein kinase (MAPK) signaling pathways used by neurotrophins during retrograde signaling differ from those used following direct stimulation of the cell soma. During retrograde signaling, endocytosed neurotrophin receptors (Trks) activate the extracellular signal-related protein kinase 5 (Erk5) pathway, leading to nuclear translocation of Erk5, phosphorylation of CREB, and enhanced neuronal survival. In contrast, Erk1/2, which mediates nuclear responses following direct cell body stimulation, does not transmit a retrograde signal. Thus, the Erk5 pathway has a unique function in retrograde signaling. Differential activation of distinct MAPK pathways may enable an individual growth factor to relay information that specifies the location and the nature of stimulation.

Myocyte enhancer factor 2A and 2D undergo phosphorylation and caspase-mediated degradation during apoptosis of rat cerebellar granule neurons

Myocyte enhancer factor 2 (MEF2) proteins are important regulators of gene expression during the development of skeletal, cardiac, and smooth muscle. MEF2 proteins are also present in brain and recently have been implicated in neuronal survival and differentiation. In this study we examined the cellular mechanisms regulating the activity of MEF2s during apoptosis of cultured cerebellar granule neurons, an established in vitro model for studying depolarization-dependent neuronal survival. All four MEF2 isoforms (A, B, C, and D) were detected by immunoblot analysis in cerebellar granule neurons. Endogenous MEF2A and MEF2D, but not MEF2B or MEF2C, were phosphorylated with the induction of apoptosis. The putative sites that were phosphorylated during apoptosis are functionally distinct from those previously reported to enhance MEF2 transcription. The increased phosphorylation of MEF2A and MEF2D was followed by decreased DNA binding, reduced transcriptional activity, and caspase-dependent cleavage to fragments containing N-terminal DNA binding domains and C-terminal transactivation domains. Expression of the highly homologous N terminus of MEF2A (1-131 amino acids) antagonized the transcriptional activity and prosurvival effects of a constitutively active mutant of MEF2D (MEF2D-VP16). We conclude that MEF2A and MEF2D are prosurvival factors with high transcriptional activity in postmitotic cerebellar granule neurons. When these neurons are induced to undergo apoptosis by lowering extracellular potassium, MEF2A and MEF2D are phosphorylated, followed by decreased DNA binding and cleavage by a caspase-sensitive pathway to N-terminal fragments lacking the transactivation domains. The degradation of MEF2D and MEF2A and the generation of MEF2 fragments that have the potential to act as dominant-inactive transcription factors lead to apoptotic cell death.

Extracellular signal regulated kinase 5 (ERK5) is required for the differentiation of muscle cells

Extracellular signal regulated kinase 5 (ERK5) is a novel member of the mitogen-activated protein kinase (MAPK) family with a poorly defined physiological function. Since ERK5 and its upstream activator MEK5 are abundant in skeletal muscle we examined a function of the cascade during muscle differentiation. We show that ERK5 is activated upon induction of differentiation in mouse myoblasts and that selective activation of the pathway results in promoter activation of differentiation-specific genes. Moreover, myogenic differentiation is completely blocked when ERK5 expression is inhibited by antisense RNA. Thus, we conclude that the MEK5/ERK5 MAP kinase cascade is critical for early steps of muscle cell differentiation.

Src and multiple MAP kinase activation in cardiac hypertrophy and congestive heart failure under chronic pressure-overload: comparison with acute mechanical stretch

Activation of members of the mitogen-activated protein (MAP) kinase family and their downstream effectors has been proposed to play a key role in the pathogenesis of cell survival, ischaemic preconditioning, cardiac hypertrophy and heart failure. This study investigated the responses of Src kinase and multiple MAP kinases during the transition from compensated pressure-overload hypertrophy to decompensated congestive heart failure. Extracellular signal-regulated protein kinase (ERK) 1/2, p38, and Src were activated by chronic pressure-overload and their activity was sustained for 8 weeks after aortic banding. In contrast, while p90 ribosomal S6 kinase (90RSK) and big MAP kinase 1 (BMK1) were activated in compensated hypertrophy, their activities were significantly decreased in hearts with heart failure. No changes were found in C-Jun NH2 terminal kinase (JNK) activity after aortic banding. These data suggest that differential activation of MAP kinase family members may contribute to the transition from compensated to decompensated hypertrophy. We also examined acute effects of mechanical stretch on the activation of these kinases in normal and hypertrophied hearts. In the isolated coronary-perfused heart, a balloon in the left ventricle was inflated to achieve minimum end-diastolic pressure of 25 mmHg for 10-20 min. In normal guinea pig hearts, stretch activated ERK1/2, p90RSK, p38, Src, and BMK1 but not JNK. However in hypertrophied hearts, further activation of these kinases was not observed by acute mechanical stretch. Mechanical stretch-induced activation of ERK1/2 and p38 kinase in normal hearts was attenuated significantly by a protein kinase C inhibitor, chelerythrine. We demonstrate that ERK1/2, p90RSK, p38, Src, and BMK1 are activated by chronic pressure-overload and by acute mechanical stretch. These data suggest that Src, BMK1 and p90RSK play a role as novel signal transduction pathways leading to cardiac hypertrophy. In addition, the differential inhibition of p90RSK and BMK1 in hearts with congestive heart failure suggests the specific role of these two kinases to maintain cardiac function under chronic pressure-overload.

Myocyte enhancer factor 2A and 2D undergo phosphorylation and caspase-mediated degradation during apoptosis of rat cerebellar granule neurons

Myocyte enhancer factor 2 (MEF2) proteins are important regulators of gene expression during the development of skeletal, cardiac, and smooth muscle. MEF2 proteins are also present in brain and recently have been implicated in neuronal survival and differentiation. In this study we examined the cellular mechanisms regulating the activity of MEF2s during apoptosis of cultured cerebellar granule neurons, an established in vitro model for studying depolarization-dependent neuronal survival. All four MEF2 isoforms (A, B, C, and D) were detected by immunoblot analysis in cerebellar granule neurons. Endogenous MEF2A and MEF2D, but not MEF2B or MEF2C, were phosphorylated with the induction of apoptosis. The putative sites that were phosphorylated during apoptosis are functionally distinct from those previously reported to enhance MEF2 transcription. The increased phosphorylation of MEF2A and MEF2D was followed by decreased DNA binding, reduced transcriptional activity, and caspase-dependent cleavage to fragments containing N-terminal DNA binding domains and C-terminal transactivation domains. Expression of the highly homologous N terminus of MEF2A (1-131 amino acids) antagonized the transcriptional activity and prosurvival effects of a constitutively active mutant of MEF2D (MEF2D-VP16). We conclude that MEF2A and MEF2D are prosurvival factors with high transcriptional activity in postmitotic cerebellar granule neurons. When these neurons are induced to undergo apoptosis by lowering extracellular potassium, MEF2A and MEF2D are phosphorylated, followed by decreased DNA binding and cleavage by a caspase-sensitive pathway to N-terminal fragments lacking the transactivation domains. The degradation of MEF2D and MEF2A and the generation of MEF2 fragments that have the potential to act as dominant-inactive transcription factors lead to apoptotic cell death.

Ten ERK-related proteins in three distinct classes associate with AP-1 proteins and/or AP-1 DNA

We have identified seven ERK-related proteins ("ERPs"), including ERK2, that are stably associated in vivo with AP-1 dimers composed of diverse Jun and Fos family proteins. These complexes have kinase activity. We designate them as "class I ERPs." We originally hypothesized that these ERPs associate with DNA along with AP-1 proteins. We devised a DNA affinity chromatography-based analytical assay for DNA binding, the "nucleotide affinity preincubation specificity test recognition" (NAPSTER) assay. In this assay, class I ERPs do not associate with AP-1 DNA. However, several new "class II" ERPs do associate with DNA. p41 and p44 are ERK1/2-related ERPs that lack kinase activity and associate along with AP-1 proteins with AP-1 DNA. Class I ERPs and their associated kinase activity thus appear to bind AP-1 dimers when they are not bound to DNA and then disengage and are replaced by class II ERPs to form higher order complexes when AP-1 dimers bind DNA. p97 is a class III ERP, related to ERK3, that associates with AP-1 DNA without AP-1 proteins. With the exception of ERK2, none of the 10 ERPs appear to be known mitogen-activated protein kinase superfamily members.

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.

MEK6 regulates human involucrin gene expression via a p38alpha - and p38delta -dependent mechanism

A signaling cascade that includes protein kinase C (PKC), Ras, and MEKK1 regulates involucrin (hINV) gene expression in epidermal keratinocytes (Efimova, T., LaCelle, P., Welter, J. F., and Eckert, R. L. (1998) J. Biol. Chem. 273, 24387-24395 and Efimova, T., and Eckert, R. L. (2000) J. Biol. Chem. 275, 1601-1607). Because signal transfer downstream of MEKK1 may involve several MAPK kinases (MEKs), it is important to evaluate the regulatory role of each MEK isoform. In the present study we evaluate the role of MEK6 in transmitting this signal. Constitutively active MEK6 (caMEK6) increases hINV promoter activity and increases endogenous hINV levels. The caMEK6-dependent increase in gene expression is inhibited by the p38 MAPK inhibitor, SB203580, and is associated with a marked increase in p38alpha MAPK activity; JNK and ERK kinases are not activated. In addition, hINV gene expression is inhibited by dominant-negative p38alpha and increased when caMEK6 and p38alpha are co-expressed. caMEK6 also activates p38delta, but p38delta inhibits the caMEK6-dependent activation. These results suggest that MEK6 increases hINV gene expression by regulating the balance between activation of p38alpha, which increases gene expression, and p38delta, which decreases gene expression.

ERK7 is an autoactivated member of the MAPK family

Extracellular signal-regulated kinase 7 (ERK7) shares significant sequence homology with other members of the ERK family of signal transduction proteins, including the signature TEY activation motif. However, ERK7 has several distinguishing characteristics. Unlike other ERKs, ERK7 has been shown to have significant constitutive activity in serum-starved cells, which is not increased further by extracellular stimuli that typically activate other members of the mitogen-activated protein kinase (MAPK) family. On the other hand, ERK7's activation state and kinase activity appear to be regulated by its ability to utilize ATP and the presence of its extended C-terminal region. In this study, we investigated the mechanism of ERK7 activation. The results suggest that 1) MAPK kinase (MEK) inhibitors do not suppress ERK7 kinase activity; 2) intramolecular autophosphorylation is sufficient for activation of ERK7 in the absence of an upstream MEK; and 3) multiple regions of the C-terminal domain of ERK7 regulate its kinase activity. Taken together, these results indicate that autophosphorylation is sufficient for ERK7 activation and that the C-terminal domain regulates its kinase activity through multiple interactions.

Activated MEK5 induces serial assembly of sarcomeres and eccentric cardiac hypertrophy

Mitogen-activated protein kinase (MAPK) pathways couple intrinsic and extrinsic signals to hypertrophic growth of cardiomyocytes. The MAPK kinase MEK5 activates the MAPK ERK5. To investigate the potential involvement of MEK5-ERK5 in cardiac hypertrophy, we expressed constitutively active and dominant-negative forms of MEK5 in cardiomyocytes in vitro. MEK5 induced a form of hypertrophy in which cardiomyocytes acquired an elongated morphology and sarcomeres were assembled in a serial manner. The cytokine leukemia inhibitory factor (LIF), which stimulates MEK5 activity, evoked a similar response. Moreover, a dominant-negative MEK5 mutant specifically blocked LIF-induced elongation of cardiomyocytes and reduced expression of fetal cardiac genes without blocking other aspects of LIF-induced hypertrophy. Consistent with the ability of MEK5 to induce serial assembly of sarcomeres in vitro, cardiac-specific expression of activated MEK5 in transgenic mice resulted in eccentric cardiac hypertrophy that progressed to dilated cardiomyopathy and sudden death. These findings reveal a specific role for MEK5-ERK5 in the induction of eccentric cardiac hypertrophy and in transduction of cytokine signals that regulate serial sarcomere assembly.

SHP-2 complex formation with the SHP-2 substrate-1 during C2C12 myogenesis

Myogenesis is a highly ordered process that involves the expression of muscle-specific genes, cell-cell recognition and multinucleated myotube formation. Although protein tyrosine kinases have figured prominently in myogenesis, the involvement of tyrosine phosphatases in this process is unknown. SHP-2 is an SH2 domain-containing tyrosine phosphatase, which positively regulates growth and differentiation. We show that in C2C12 myoblasts, SHP-2 becomes upregulated early on during myogenesis and associates with a 120 kDa tyrosyl-phosphorylated complex. We have identified that the 120 kDa complex consists of the SHP-2 substrate-1 (SHPS-1) and the Grb2-associated binder-1 (Gab-1). SHPS-1, but not Gab-1, undergoes tyrosyl phosphorylation and association with SHP-2 during myogenesis, the kinetics of which correlate with the expression of MyoD. Either constitutive expression or inducible activation of MyoD in 10T½ fibroblasts promotes SHPS-1 tyrosyl phosphorylation and its association with SHP-2. It has been shown that p38 mitogen-activated protein kinase (MAPK) activity is required for the expression/activation of MyoD and MyoD-responsive genes. Inhibition of p38 MAPK by SB203580 in differentiating C2C12 myoblasts blocks MyoD expression, SHPS-1 tyrosyl phosphorylation and the association of SHPS-1 with SHP-2. These data suggest that SHPS-1/SHP-2 complex formation is an integral signaling component of skeletal muscle differentiation.

Growth inhibitory effect of green tea extract and (-)-epigallocatechin in Ehrlich ascites tumor cells involves a cellular thiol-dependent activation of mitogenic-activated protein kinases

The effect of green tea extract (GTE) in Ehrlich ascites tumor cells (EATC) was studied with respect to changes in the intracellular kinase system involving mitogen-activated protein kinases (MAPKs) and cellular thiol. We have previously shown a reduction in viability of EATC and tyrosine phosphorylation of 42 and 45 kDa proteins by GTE and its polyphenolic component, Epigallocatechin (EGC) (D.O. Kennedy, S. Nishimura, T. Hasuma, Y. Yoshihisa, S. Otani, I. Matsui-Yuasa, Involvement of protein tyrosine phosphorylation in the effect of green tea polyphenols on Ehrlich ascites tumor cells in vitro, Chem. Biol. Interact. 110 (1998) 159-172). Furthermore, GTE and EGC significantly decreased both cellular non-protein and protein sulfhydryl levels in EATC, but replenishing thiol stores with N-acetylcysteine (NAC) caused a recovery in cell viability, and therefore SH groups were identified as a novel target of green tea cytotoxicity (D.O. Kennedy, M. Matsumoto, A. Kojima, I. Matsui-Yuasa, Cellular thiol status and cell death in the effect of green tea polyphenols in Ehrlich ascites tumor cells, Chem. Biol. Interact. 122 (1999) 59-71). In this study, we have observed the stimulation of three forms of MAPK, namely ERK1/2, JNK/SAPK and p38, by EGC, which were dose and time-dependent. These MAPK stimulations were found to be cellular thiol status-dependent events as NAC reversed these stimulations. Furthermore, inhibition of the p38 MAPK pathway using the p38 inhibitor SB203580 caused a marked dose-dependent reduction in the decrease in cell viability caused by EGC treatment. Inhibiting the Erk1/2 MAPK pathway using the MEK inhibitor PD098059 caused a slight change in the decrease in cell viability by EGC. These may suggest that the cytotoxicity associated with EGC was more associated with the other MAPKs than with ERK1/2. This may be the first study of its kind providing a novel evidence of a role for different forms of MAPKs in the antitumor effect of green tea polyphenols, especially EGC, in Ehrlich ascites tumor cells.