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

Role of MEKK2-MEK5 in the regulation of TNF-alpha gene expression and MEKK2-MKK7 in the activation of c-Jun N-terminal kinase in mast cells

Cross-linking of the high-affinity IgE receptor (FcepsilonRI) on mast cells with IgE and multivalent antigen triggers mitogen-activated protein (MAP) kinase activation and cytokine gene expression. We report here that MAP kinase kinase 4 (MKK4) gene disruption does not affect either MAP kinase activation or cytokine gene expression in response to cross-linking of FcepsilonRI in embryonic stem cell-derived mast cells. MKK7 is activated in response to cross-linking of FcepsilonRI, and this activation is inhibited by MAP/ERK kinase (MEK) kinase 2 (MEKK2) gene disruption. In addition, expression of kinase-inactive MKK7 in the murine mast cell line MC/9 inhibits c-Jun NH(2)-terminal kinase (JNK) activation in response to cross-linking of FcepsilonRI, whereas expression of kinase-inactive MKK4 does not affect JNK activation by this stimulus. However, FcepsilonRI-induced activation of the tumor necrosis factor-alpha (TNF-alpha) gene promoter is not affected by expression of kinase-inactive MKK7. We describe an alternative pathway by which MEKK2 activates MEK5 and big MAP kinase1/extracellular signal-regulated kinase 5 in addition to MKK7 and JNK, and interruption of this pathway inhibits TNF-alpha promoter activation. These findings suggest that JNK activation by antigen cross-linking is dependent on the MEKK2-MKK7 pathway, and cytokine production in mast cells is regulated in part by the signaling complex MEKK2-MEK5-ERK5.

Granulocyte colony-stimulating factor induces ERK5 activation, which is differentially regulated by protein-tyrosine kinases and protein kinase C. Regulation of cell proliferation and survival

Granulocyte colony-stimulating factor (G-CSF) plays a major role in the regulation of granulopoiesis. Treatment of cells with G-CSF has been shown to activate multiple signal transduction pathways. We show here that Erk5, a novel member of the MAPK family, and its specific upstream activator MEK5 were activated in response to incubation of cells with G-CSF. Different from other members of the MAPK family including Erk1/2, JNK, and p38, maximal activation of Erk5 by G-CSF required the C-terminal region of the G-CSF receptor. Genistein, a specific inhibitor of protein-tyrosine kinases, blocked G-CSF-induced Erk5 activation. In contrast, inhibition of protein kinase C activity increased G-CSF-mediated activation of Erk5 and MEK5, whereas stimulation of protein kinase C activity inhibited activation of the two kinases by G-CSF. The proliferation of BAF3 cells in response to G-CSF was inhibited by expression of a dominant-negative MEK5 but potentiated by expression of a constitutively active MEK5. Expression of the constitutively active MEK5 also increased the survival of BAF3 cells cultured in the absence of or in low concentrations of G-CSF. Together, these data implicate Erk5 as an important signaling component in the biological actions of G-CSF.

Molecular cloning of mouse ERK5/BMK1 splice variants and characterization of ERK5 functional domains

The mitogen-activated protein kinases (MAPKs) play important roles in regulation of cell growth and survival. Human MAPK 5 (ERK5) or Big MAP kinase 1 (BMK1) is a recently cloned member of the MAPK family. To identify ERK5-related kinases, we searched the GenBanktrade mark expressed sequence tag (EST) data base for mouse cDNAs with homology to human ERK5. A full-length mouse cDNA that was highly homologous to the human ERK5 was identified. Further analysis of ERK5 polymerase chain reaction products generated from mouse embryo cDNA yielded three mouse ERK5 cDNAs (mERK5a, mERK5b, and mERK5c). Sequence analysis showed that these cDNAs are alternative splice products of the mouse ERK5 gene. Interestingly, expressed mERK5b and mERK5c act as dominant negative inhibitors based on inhibition of mERK5a kinase activity and mERK5a-mediated MEF2C transactivation. However, the physiological significance of mERK5b and mERK5c is not fully understood. Further investigation using these mouse ERK5 splice variants and other constructed mutants identified functional roles of several regions of mERK5, which appear to be important for protein-protein interaction and intracellular localization. Specifically, we found that the long C-terminal tail, which contains a putative nuclear localization signal, is not required for activation and kinase activity but is responsible for the activation of nuclear transcription factor MEF2C due to nuclear targeting. In addition, the N-terminal domain spanning amino acids (aa) 1-77 is important for cytoplasmic targeting; the domain from aa 78 to 139 is required for association with the upstream kinase MEK5; and the domain from aa 140-406 is necessary for oligomerization. Taken together, these observations indicate that ERK5 is regulated by distinct mechanisms determined by its unique structure and presumably the presence of multiple splice variants.

Differential regulation of mitogen-activated protein kinases ERK1/2 and ERK5 by neurotrophins, neuronal activity, and cAMP in neurons

Activation of the extracellular signal-regulated kinase 1 (ERK1) and ERK2 by neurotrophins, neuronal activity, or cAMP has been strongly implicated in differentiation, survival, and adaptive responses of neurons during development and in the adult brain. Recently, a new member of the mitogen-activated protein (MAP) kinase family, ERK5, was discovered. Like ERK1 and ERK2, ERK5 is expressed in neurons, and ERK5 stimulation by epidermal growth factor is blocked by the MAP kinase/ERK kinase 1 (MEK1) inhibitors PD98059 and U0126. This suggests the interesting possibility that some of the functions attributed to ERK1/2 may be mediated by ERK5. However, the regulatory properties of ERK5 in primary cultured neurons have not been reported. Here we examined the regulation of ERK5 signaling in primary cultured cortical neurons. Our data demonstrate that, similar to ERK1/2, ERK5 is activated by neurotrophins including brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and NT-4. BDNF stimulation of ERK5 required the activity of MEK5. Surprisingly, ERK5 was not stimulated by cAMP or neuronal activity induced by glutamate or membrane depolarization. In contrast to ERK1/2, ERK5 strongly activated the transcriptional activity of myocyte enhancer factor 2C (MEF2C) in pheochromocytoma 12 (PC12) cells and was required for neurotrophin stimulation of MEF2C transcription in both PC12 cells and cortical neurons. Furthermore, ERK1/2, but not ERK5, induced transcription from Elk1 and the cAMP/ Ca(2+) response element in PC12 cells. Our data suggest that mechanisms for regulation of ERK5 and downstream transcriptional pathways regulated by ERK5 are distinct from those of ERK1/2 in neurons. Furthermore, ERK5 is the first MAP kinase identified whose activity is stimulated by neurotrophins but not by neuronal activity.

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

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

Mammalian mitogen-activated protein kinase signal transduction pathways activated by stress and inflammation

The molecular details of mammalian stress-activated signal transduction pathways have only begun to be dissected. This, despite the fact that the impact of these pathways on the pathology of chronic inflammation, heart disease, stroke, the debilitating effects of diabetes mellitus, and the side effects of cancer therapy, not to mention embryonic development, innate and acquired immunity, is profound. Cardiovascular disease and diabetes alone represent the most significant health care problems in the developed world. Thus it is not surprising that understanding these pathways has attracted wide interest, and in the past 10 years, dramatic progress has been made. Accordingly, it is now becoming possible to envisage the transition of these findings to the development of novel treatment strategies. This review focuses on the biochemical components and regulation of mammalian stress-regulated mitogen-activated protein kinase (MAPK) pathways. The nuclear factor-kappa B pathway, a second stress signaling paradigm, has been the subject of several excellent recent reviews (258, 260).

BMK1 mediates growth factor-induced cell proliferation through direct cellular activation of serum and glucocorticoid-inducible kinase

Activation of the mammalian mitogen-activated protein kinase known as BMK1 is required for growth factor-induced cell proliferation. To understand the mechanism by which BMK1 mediates this cellular response, this kinase was used as bait in a yeast two-hybrid-based library screening. Here, we report the identification of serum and glucocorticoid-inducible kinase (SGK) as a cellular protein that physically interacts with BMK1. During growth factor-induced cell stimulation, BMK1 activates SGK by phosphorylation at serine 78. This BMK1-mediated phosphorylation event is necessary for the activation of SGK and, more importantly, for cell proliferation induced by growth factors.

Differential MAPK pathways utilized for HGF- and EGF-dependent renal epithelial morphogenesis

Cells derived from the inner medullary collecting duct undergo in vitro branching tubulogenesis to both the c-met receptor ligand hepatocyte growth factor (HGF) as well as epidermal growth factor (EGF) receptor ligands. In contrast, many other cultured renal epithelial cells respond in this manner only to HGF, suggesting that these two receptors may use independent signaling pathways during morphogenesis. We have therefore compared the signaling pathways for mIMCD-3 cell morphogenesis in response to EGF and HGF. Inhibition of the p42/44 mitogen-activated protein kinase (MAPK) pathway with the mitogen-activated protein kinase kinase (MKK1) inhibitor PD98059 (50 microm) markedly inhibits HGF-induced cell migration with only partial inhibition of EGF-induced cell motility. Similarly, HGF-dependent, but not EGF-dependent, branching morphogenesis was more greatly inhibited by the MKK1 inhibitor. Examination of EGF-stimulated cells demonstrated that extracellular-regulated kinase 5 (ERK5) was activated in response to EGF but not HGF, and that activation of ERK5 was only 60% inhibited by 50 microm PD98059. In contrast, the MKK inhibitor U0126 markedly inhibited both ERK1/2 and ERK5 activation and completely prevented HGF- and EGF-dependent migration and branching process formation. Expression of dominant negative ERK5 (dnBMK1) likewise inhibited EGF-dependent branching process formation, but did not affect HGF-dependent branching process formation. Our results indicate that activation of the ERK1/ERK2 signaling pathway is critical for HGF-induced cell motility/morphogenesis in mIMCD-3 cells, whereas ERK5 appears to be required for EGF-dependent morphogenesis.

ERK5 and ERK2 cooperate to regulate NF-kappaB and cell transformation

We have previously demonstrated an involvement of MEK5 and ERK5 in RafBXB-stimulated focus formation in NIH3T3 cells. We find here that MEK5 and ERK5 cooperate with the RafBXB effectors MEK1/2 and ERK1/2 to induce foci. To further understand MEK5-ERK5-dependent signaling, we examined potential MEK5-ERK5 effectors that might influence focus-forming activity. Consistent with results from our focus-formation assays, constitutively active variants of MEK5 and MEK1 synergize to activate NF-kappaB, and MEK5 and ERK5 are required for activation of NF-kappaB by RafBXB. The MEK5-ERK5 pathway is also sufficient to activate both NF-kappaB and p90 ribosomal S6 kinase. Our results support the hypothesis that NF-kappaB and p90 ribosomal S6 kinase are involved in MEK5-ERK5-dependent focus formation and may serve as integration points for ERK5 and ERK1/2 signaling.

MEK7-dependent activation of p38 MAP kinase in keratinocytes

Previous studies suggest that a PKC/Ras/MEKK1 cascade regulates involucrin (hINV) gene expression in human epidermal keratinocytes. MEK7, which is expressed in epidermis, has been identified as a member of this cascade (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). However, the kinase that functions downstream of MEK7 has not been identified. Our present studies show that MEK7 expression in keratinocytes markedly activates p38alpha and modestly activates JNK. Activation of p38 MAPK by MEK7 is a novel finding, as previous reports have assigned MEK7 as a JNK regulator. We also demonstrate that this regulation is physiologically important, as the p38alpha- and JNK-dependent activities regulate hINV promoter activity and expression of the endogenous hINV gene.

Activation of BMK1 via tyrosine 1062 in RET by GDNF and MEN2A mutation

Big mitogen-activated protein kinase 1 (BMK1) is a new member of mitogen-activated protein kinase (MAPK) family. In the present study, we investigated whether glial cell line-derived neurotrophic factor (GDNF) can induce activation of BMK1 through RET tyrosine kinase. Its activation reached a maximal level at 30 min and continued at least for 120 min after GDNF stimulation. In addition, we detected BMK1 activation in NIH3T3 cells expressing RET with a multiple endocrine neoplasia (MEN) 2A mutation. The level of BMK1 activation markedly decreased by replacement of tyrosine 1062 with phenylalanine (designated Y1062F) in RET, indicating the importance of downstream signaling via tyrosine 1062. However, although both RAS/MAPK and phosphatidylinositol 3-kinase (PI3-K)/AKT signaling pathways are activated via tyrosine 1062, BMK1 activation by GDNF was not significantly impaired by treatment with an MEK1 inhibitor, PD98059, or two distinct PI3-K inhibitors, LY294002 and wortmannin, suggesting that the RAS and PI3-K signaling pathways are not crucial for BMK1 activation by GDNF. Moreover, luciferase reporter assays revealed that RET-MEN2A mutant proteins can activate the MEF2C transcription factor that is known to be a cellular target for BMK1, and that its activation is impaired by the Y1062F mutation or by expression of a dominant negative form of MEK5.

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

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

A novel mitogen-activated protein kinase is responsive to Raf and mediates growth factor specificity

The proto-oncogene Raf is a major regulator of growth and differentiation. Previous studies from a number of laboratories indicate that Raf activates a signaling pathway that is independent of the classic MEK1,2-ERK1,2 cascade. However, no other signaling cascade downstream of Raf has been identified. We describe a new member of the mitogen-activated protein kinase family, p97, an ERK5-related kinase that is activated and Raf associated when cells are stimulated by Raf. Furthermore, p97 is selectively responsive to different growth factors, providing a mechanism for specificity in cellular signaling. Thus, p97 is activated by the neurogenic factor fibroblast growth factor (FGF) but not the mitogenic factor epidermal growth factor (EGF) in neuronal cells. Conversely, the related kinase ERK5 is activated by EGF but not FGF. p97 phosphorylates transcription factors such as Elk-1 and Ets-2 but not MEF2C at transactivating sites, whereas ERK5 phosphorylates MEF2C but not Elk-1 or Ets-2. Finally, p97 is expressed in a number of cell types including primary neural and NIH 3T3 cells. Taken together, these results identify a new signaling pathway that is distinct from the classic Raf-MEK1,2-ERK1,2 kinase cascade and can be selectively stimulated by growth factors that produce discrete biological outcomes.

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

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

Inhibitory role for dual specificity phosphatase VHR in T cell antigen receptor and CD28-induced Erk and Jnk activation

The 21-kDa dual specific protein phosphatase VH1-related (VHR) is one of the smallest known phosphatases, and its function has remained obscure. We report that this enzyme is expressed in lymphoid cells and is not induced by T cell antigen receptor like other dual specificity phosphatases. Introduction of exogenous VHR into Jurkat T cells caused a marked decrease in the transcriptional activation of a nuclear factor of activated T cells and an activator protein-1-driven reporter gene in response to ligation of T cell antigen receptors. The inhibition was dose-dependent and was similar at different doses of anti-receptor antibody. Catalytically inactive VHR mutants caused an increase in gene activation, suggesting a role for endogenous VHR in this response. In contrast, the activation of a nuclear factor kappaB-driven reporter was not affected. The inhibitory effects of VHR were also seen at the level of the mitogen-activated kinases Erk1, Erk2, Jnk1, Jnk2, and on reporter genes that directly depend on these kinases, namely Elk, c-Jun, and activator protein-1. In contrast, p38 kinase activation was not affected by VHR, and p38-assisted gene activation was less sensitive. Our results suggest that VHR is a negative regulator of the Erk and Jnk pathways in T cells and, therefore, may play a role in aspects of T lymphocyte physiology that depend on these kinases.

Smad proteins function as co-modulators for MEF2 transcriptional regulatory proteins

An emerging theme in transforming growth factor-ss (TGF-ss) signalling is the association of the Smad proteins with diverse groups of transcriptional regulatory proteins. Several Smad cofactors have been identified to date but the diversity of TGF-ss effects on gene transcription suggests that interactions with other co-regulators must occur. In these studies we addressed the possible interaction of Smad proteins with the myocyte enhancer-binding factor 2 (MEF2) transcriptional regulators. Our studies indicate that Smad2 and 4 (Smad2/4) complexes cooperate with MEF2 regulatory proteins in a GAL4-based one-hybrid reporter gene assay. We have also observed in vivo interactions between Smad2 and MEF2A using co-immunoprecipitation assays. This interaction is confirmed by glutathione S:-transferase pull-down analysis. Immunofluorescence studies in C2C12 myotubes show that Smad2 and MEF2A co-localise in the nucleus of multinuclear myotubes during differentiation. Interestingly, phospho-acceptor site mutations of MEF2 that render it unresponsive to p38 MAP kinase signalling abrogate the cooperativity with the Smads suggesting that p38 MAP Kinase-catalysed phosphorylation of MEF2 is a prerequisite for the Smad-MEF2 interaction. Thus, the association between Smad2 and MEF2A may subserve a physical link between TGF-ss signalling and a diverse array of genes controlled by the MEF2 cis element.

Inhibition of p38 mitogen-activated protein kinase augments lipopolysaccharide-induced cell proliferation in CD14-expressing Chinese hamster ovary cells

CD14-expressing Chinese hamster ovary (CD14-CHO) cells, established by transfection of human CD14 DNA, acquired high responsiveness to lipopolysaccharide (LPS) through membrane-bound CD14 expression. LPS induced DNA synthesis and activated a series of mitogen-activated protein (MAP) kinases, extracellular signal-regulated kinase 1/2 (Erk1/2), p38, and c-Jun N-terminal kinase/stress-activated protein kinase, in CD14-CHO cells but not in mock-transfected CHO cells. Anti-CD14 antibody completely abrogated both LPS-induced DNA synthesis and LPS-induced phosphorylation of those MAP kinases, suggesting a critical role of membrane-bound CD14 in LPS signaling. A p38 MAP kinase inhibitor, SB203580, markedly augmented LPS-induced DNA synthesis in CD14-CHO cells, whereas an Erk1/2 inhibitor, PD98059, had no affect. On the other hand, SB203580 exhibited no effect on epidermal growth factor-induced DNA synthesis in CD14-CHO cells, although PD98059 inhibited it significantly. The activation and inactivation of p38 MAP kinase with dominant negative and dominant positive mutants also suggested the participation of p38 MAP kinase in LPS-induced DNA synthesis. It was therefore suggested that the activation of p38 MAP kinase can negatively regulate LPS-induced cell proliferation in CD14-CHO cells.

MEK5, a new target of the atypical protein kinase C isoforms in mitogenic signaling

The MEK5-extracellular signal-regulated kinase (ERK5) tandem is a novel mitogen-activated protein kinase cassette critically involved in mitogenic activation by the epidermal growth factor (EGF). The atypical protein kinase C isoforms (aPKCs) have been shown to be required for cell growth and proliferation and have been reported to interact with the adapter protein p62 through a short stretch of acidic amino acids termed the aPKC interaction domain. This region is also present in MEK5, suggesting that it may be an aPKC-binding partner. Here we demonstrate that the aPKCs interact in an EGF-inducible manner with MEK5 and that this interaction is required and sufficient for the activation of MEK5 in response to EGF. Consistent with the role of the aPKCs in the MEK5-ERK5 pathway, we show that zetaPKC and lambda/iotaPKC activate the Jun promoter through the MEF2C element, a well-established target of ERK5. From all these results, we conclude that MEK5 is a critical target of the aPKCs during mitogenic signaling.

Differential regulation of mitogen-activated protein kinases ERK1/2 and ERK5 by neurotrophins, neuronal activity, and cAMP in neurons

Activation of the extracellular signal-regulated kinase 1 (ERK1) and ERK2 by neurotrophins, neuronal activity, or cAMP has been strongly implicated in differentiation, survival, and adaptive responses of neurons during development and in the adult brain. Recently, a new member of the mitogen-activated protein (MAP) kinase family, ERK5, was discovered. Like ERK1 and ERK2, ERK5 is expressed in neurons, and ERK5 stimulation by epidermal growth factor is blocked by the MAP kinase/ERK kinase 1 (MEK1) inhibitors PD98059 and U0126. This suggests the interesting possibility that some of the functions attributed to ERK1/2 may be mediated by ERK5. However, the regulatory properties of ERK5 in primary cultured neurons have not been reported. Here we examined the regulation of ERK5 signaling in primary cultured cortical neurons. Our data demonstrate that, similar to ERK1/2, ERK5 is activated by neurotrophins including brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and NT-4. BDNF stimulation of ERK5 required the activity of MEK5. Surprisingly, ERK5 was not stimulated by cAMP or neuronal activity induced by glutamate or membrane depolarization. In contrast to ERK1/2, ERK5 strongly activated the transcriptional activity of myocyte enhancer factor 2C (MEF2C) in pheochromocytoma 12 (PC12) cells and was required for neurotrophin stimulation of MEF2C transcription in both PC12 cells and cortical neurons. Furthermore, ERK1/2, but not ERK5, induced transcription from Elk1 and the cAMP/ Ca(2+) response element in PC12 cells. Our data suggest that mechanisms for regulation of ERK5 and downstream transcriptional pathways regulated by ERK5 are distinct from those of ERK1/2 in neurons. Furthermore, ERK5 is the first MAP kinase identified whose activity is stimulated by neurotrophins but not by neuronal activity.

Mouse B1 cell line responds to lipopolysaccharide via membrane-bound CD14

The role of membrane-bound CD14 in the response of mouse B1 cell lines to lipopolysaccharide (LPS) was studied. The surface profile of mouse TH2.52 B cells was positive for CD5, IgM, B220, CD11b and F4/80, suggesting that TH2.52 cells carried the typical phenotype of B1 cells. Furthermore, TH2.52 B1 cells were found to express membrane-bound CD14, which plays a critical role in LPS recognition. TH2.52 B1 cells responded to a very low concentration of LPS and exhibited: (i) augmentation of DNA synthesis; (ii) activation of nuclear factor (NF)-kappaB; and (iii) phosphorylation of extracellular signal regulated kinase 1/2 (Erk1/2). They were markedly inhibited by anti-CD14 antibody. Therefore, the expression of membrane-bound CD14 was suggested to provide high sensitivity to LPS for TH2.52 B1 cells.

Activation of the myocyte enhancer factor-2 transcription factor by calcium/calmodulin-dependent protein kinase-stimulated binding of 14-3-3 to histone deacetylase 5

Skeletal muscle differentiation is controlled by interactions between myocyte enhancer factor-2 (MEF2) and myogenic basic helix-loop-helix transcription factors. Association of MEF2 with histone deacetylases (HDAC) -4 and -5 results in repression of MEF2 target genes and inhibition of myogenesis. Calcium/calmodulin-dependent protein kinase (CaMK) signaling promotes myogenesis by disrupting MEF2-HDAC complexes and stimulating HDAC nuclear export. To further define the mechanisms that confer CaMK responsiveness to HDAC4 and -5, we performed yeast two-hybrid screens to identify HDAC-interacting factors. These screens revealed interactions between HDAC4 and members of the 14-3-3 family of proteins, which function as signal-dependent intracellular chaperones. HDAC4 binds constitutively to 14-3-3 in yeast and mammalian cells, whereas HDAC5 binding to 14-3-3 is largely dependent on CaMK signaling. CaMK phosphorylates serines -259 and -498 in HDAC5, which subsequently serve as docking sites for 14-3-3. Our studies suggest that 14-3-3 binding to HDAC5 is required for CaMK-dependent disruption of MEF2-HDAC complexes and nuclear export of HDAC5, and implicate 14-3-3 as a signal-dependent regulator of muscle cell differentiation.

Activated raf kinase inhibits muscle cell differentiation through a MEF2-dependent mechanism

Muscle cell development is dependent on the activity of cell type-specific basic-helix-loop-helix transcription factors, MyoD, Myf-5, myogenin, and MRF4 which collaborate with myocyte enhancer factor 2 proteins to activate muscle-specific gene expression. Growth factors and activated Ras prevent differentiation of myoblasts in culture but the downstream signalling pathways are not well understood. Here, we demonstrate that active Raf kinase (Raf-BxB) completely inhibits myogenic conversion of 10T1/2 cells mediated by Myf-5 and differentiation of L6 myoblasts as indicated by the absence of myotubes, lack of myogenin expression, and markedly reduced expression of myosin heavy chain. However, activated Raf inhibits transcriptional activation by Myf-5 only partially suggesting that other potential targets of Ras/Raf signalling may be involved. Significantly, we observed that elevated Raf kinase activity in L6 muscle cells suppresses the accumulation of MEF2 protein in nuclei, while MEF2 transcription appears unaffected. Moreover, forced expression of MEF2A in 10T1/2 cells rescues MyoD dependent myogenic conversion in the presence of constitutively active Raf kinase and partially restores transactivation of a myogenin promoter-dependent reporter gene in L6 muscle cells containing activated Raf kinase. From these observations we conclude that persistent activation of Raf signalling affects nuclear MEF2 functions which may explain why myogenin expression and myoblast differentiation are inhibited.

Phosphoinositide 3-kinase induces the transcriptional activity of MEF2 proteins during muscle differentiation

The activity of phosphoinositide 3-kinase (PI3-K) is essential for the differentiation of skeletal muscle cells by largely unknown mechanisms. Here we show that inhibition of PI3-K activity by the pharmacological agent LY294002 affects early processes of myoblast differentiation including the transcriptional activation of myogenin. Previous studies indicated that transcription of myogenin was dependent on MyoD and MEF2 proteins. We find that expression of a dominant negative form of PI3-K or growth in the presence of LY294002 inhibits cellular activity of MEF2 but not of MyoD. Evidence reveals that whereas MEF2 transcriptional activity is inhibited, its DNA binding activity remains unaffected. Recent studies demonstrated that phosphorylation by p38 mitogen-activated protein kinase (MAPK) induced transcriptional activity of MEF2 proteins. We show that the phosphorylation of MEF2 occurring during muscle differentiation is prevented if the activity of PI3-K is inhibited. However, our results also indicate that p38 MAPK is not affected by PI3-K in muscle cells. Nevertheless, p38 MAPK can substitute for PI3-K in the induction of MEF2 and muscle transcription. Together, these findings indicate that PI3-K affects skeletal muscle differentiation by inducing phosphorylation and transcriptional activity of MEF2 proteins in a parallel but distinct route from p38 MAPK.

Signal-dependent nuclear export of a histone deacetylase regulates muscle differentiation

Members of the myocyte enhancer factor-2 (MEF2) family of transcription factors associate with myogenic basic helix-loop-helix transcription factors such as MyoD to activate skeletal myogenesis. MEF2 proteins also interact with the class II histone deacetylases HDAC4 and HDAC5, resulting in repression of MEF2-dependent genes. Execution of the muscle differentiation program requires release of MEF2 from repression by HDACs, which are expressed constitutively in myoblasts and myotubes. Here we show that HDAC5 shuttles from the nucleus to the cytoplasm when myoblasts are triggered to differentiate. Calcium/calmodulin-dependent protein kinase (CaMK) signalling, which stimulates myogenesis and prevents formation of MEF2-HDAC complexes, also induces nuclear export of HDAC4 and HDAC5 by phosphorylation of these transcriptional repressors. An HDAC5 mutant lacking two CaMK phosphorylation sites is resistant to CaMK-mediated nuclear export and acts as a dominant inhibitor of skeletal myogenesis, whereas a cytoplasmic HDAC5 mutant is unable to block efficiently the muscle differentiation program. Our results highlight a mechanism for transcriptional regulation through signal- and differentiation-dependent nuclear export of a chromatin-remodelling enzyme, and suggest that nucleo-cytoplasmic trafficking of HDACs is involved in the control of cellular differentiation.

ERK5 is a novel type of mitogen-activated protein kinase containing a transcriptional activation domain

Previous studies have shown that upregulation of the orphan steroid receptor Nur77 is required for the apoptosis of immature T cells in response to antigen receptor signals. Transcriptional upregulation of Nur77 in response to antigen receptor signaling involves two binding sites for the MEF2 family of transcription factors located in the Nur77 promoter. Calcium signals greatly increase the activity of MEF2D in T cells via a posttranslational mechanism. The mitogen-activated protein (MAP) kinase ERK5 was isolated in a yeast two-hybrid screen using the MADS-MEF2 domain of MEF2D as bait. ERK5 resembles the other MAP kinase family members in its N-terminal half, but it also contains a 400-amino-acid C-terminal domain of previously uncharacterized function. We report here that the C-terminal region of ERK5 contains a MEF2-interacting domain and, surprisingly, also a potent transcriptional activation domain. These domains are both required for coactivation of MEF2D by ERK5. The MEF2-ERK5 interaction was found to be activation dependent in vivo and inhibitable in vitro by the calcium-sensitive MEF2 repressor Cabin 1. The transcriptional activation domain of ERK5 is required for maximal MEF2 activity in response to calcium flux in T cells, and it can activate the endogenous Nur77 gene when constitutively recruited to the Nur77 promoter via MEF2 sites. These studies provide insights into a mechanism whereby MEF2 activity can respond to calcium signaling and suggest a novel, unexpected mechanism of MAP kinase function.

Control of the eukaryotic cell cycle by MAP kinase signaling pathways

In an often rapidly changing environment, cells must adapt by monitoring and reacting quickly to extracellular stimuli detected by membrane-bound receptors and proteins. Reversible phosphorylation of intracellular regulatory proteins has emerged as a crucial mechanism effecting the transmission and modulation of such signals and is determined by the relative activities of protein kinases and phosphatases within the cell. These are often arranged into complex signaling networks that may function independently or be subject to cross-regulation. Recently, genetic and biochemical analyses have identified the universally conserved mitogen-activated protein (MAP) kinase cascade as one of the most ubiquitous signal transduction systems. This pathway is activated after a variety of cellular stimuli and regulates numerous physiological processes, particularly the cell division cycle. Progression through the cell cycle is critically dependent on the presence of environmental growth factors and stress stimuli, and failure to correctly integrate such signals into the cell cycle machinery can lead to the accumulation of genetic damage and genomic instability characteristic of cancer cells. Here we focus on the MAP kinase cascade and discuss the molecular mechanisms by which these extensively studied signaling pathways influence cell growth and proliferation.

Role of BMK1 in regulation of growth factor-induced cellular responses

Big mitogen-activated protein kinase (MAPK) 1 (BMK1), also known as ERK5, is a recently identified member of the mammalian MAPK family. Cellular stimulation of BMK1 is induced in response to growth factors, oxidative stress, and hyperosmolar conditions. Specific members of the myocyte enhancer factor 2 family of transcription factors that regulate growth factor-induced early gene expression have been identified as direct downstream targets of BMK1 activity. Recent studies have shown that growth factors of the epidermal growth factor family mediate the sequential activation of a kinase cascade consisting of MAPK kinase kinase 3, MAPK kinase 5, and BMK1. Most importantly, the activation of this signal transduction pathway has been shown to be required for growth factor-mediated cell proliferation and cell-cycle progression. Collectively, these studies establish BMK1 as an important regulator of growth factor-induced cellular responses.

MAPK signaling and the kidney

Following an overview of the biochemistry of mitogen-activated protein kinase (MAPK) pathways, the relevance of these signaling events to specific models of renal cell function and pathophysiology, both in vitro and in vivo, will be emphasized. In in vitro model systems, events activating the principal MAPK families [extracellular signal-regulated and c-Jun NH(2)-terminal kinase and p38] have been best characterized in mesangial and tubular epithelial cell culture systems and include peptide mitogens, cytokines, lipid mediators, and physical stressors. Several in vivo models of proliferative or toxic renal injury are also associated with aberrant MAPK regulation. It is anticipated that elucidation of downstream effector signaling mechanisms and a clearer understanding of the immediate and remote upstream activating pathways, when applied to these highly clinically relevant model systems, will ultimately provide much greater insight into the basis for specificity now seemingly absent from these signaling events.

Extracellular signals and scores of phosphatases: all roads lead to MAP kinase

MAP kinases function as key signal integration points for a vast number of external stimuli that affect the life and death of cells and are therefore subject to rigorous regulation. Here we review the numerous protein phosphatases that directly counteract MAP kinase activation. To simplify the complexity, we attempt to integrate the information into a 'sequential phosphatase model' of MAP kinase regulation.

Signaling from G protein-coupled receptors to ERK5/Big MAPK 1 involves Galpha q and Galpha 12/13 families of heterotrimeric G proteins. Evidence for the existence of a novel Ras AND Rho-independent pathway

The regulation of gene expression by cell surface receptors often involves the stimulation of signaling pathways including one or more members of the MAPK superfamily of serine-threonine kinases. Upon their activation in the cytosol, MAPKs can translocate to the nucleus and affect the activity of a variety of transcription factors. Recently, it has been observed that a novel member of the MAPK superfamily, ERK5, can be potently activated by transforming G protein-coupled receptors (GPCRs) and that ERK5 participates in the regulation of c-jun expression through the activation of MEF2 transcription factors. How cell surface receptors, including GPCRs, stimulate ERK5 is still poorly understood. In this study, we have used transiently transfected COS-7 cells to begin delineating the biochemical route linking GPCRs to ERK5. We show that receptors that can couple to the G(q) and G(12/13) families of heterotrimeric G proteins, m1 and thrombin receptors, respectively, but not those coupled to G(i), such as m2 receptors, are able to regulate the activity of ERK5. To investigate which heterotrimeric G proteins signal to ERK5, we used a chimeric system by which Galpha(q)- and Galpha(13)-mediated signaling pathways can be conditionally activated upon ligand stimulation. Using this system, as well as the expression of activated forms of G protein subunits, we show that the Galpha(q) and Galpha(12/13) families of heterotrimeric G proteins, but not the Galpha(i), Galpha(s), and betagamma subunits, are able to regulate ERK5. Furthermore, we provide evidence that the stimulation of ERK5 by GPCRs involves a novel signaling pathway, which is distinct from those regulated by Ras and Rho GTPases.

Regulation of mitogen-activated protein kinase signaling networks by G protein-coupled receptors

The family of receptors that transmit signals through the activation of heterotrimeric GTP-binding proteins (G proteins) constitutes the largest group of cell surface proteins involved in signal transduction. These receptors participate in a broad range of important biological functions and are implicated in a number of disease states. More than half of all drugs currently available influence G protein-coupled receptors (GPCRs). These receptors affect the generation of small molecules that act as intracellular mediators or second messengers, and can regulate a highly interconnected network of biochemical routes controlling the activity of several members of the mitogen-activated protein kinase (MAPK) superfamily. They include extracellular signal-regulated kinase 1 (ERK1) and ERK2 (or p44(MAPK) and p42(MAPK)), c-Jun NH(2)-terminal kinases (JNKs), ERK5 (or BMK), and p38 MAPKs, including p38alpha (or CSBP-1), p38beta, p38gamma (or SAPK3 or ERK6), and p38delta?(or SAPK4). This review will focus on the molecular mechanisms by which GPCRs signal to the nucleus through this intricate network of second messenger-generating systems and MAPK signaling pathways, thereby affecting the expression of genes whose products influence many biological processes, including normal and aberrant cell growth.

Involvement of the MKK6-p38gamma cascade in gamma-radiation-induced cell cycle arrest

The p38 group of kinases belongs to the mitogen-activated protein (MAP) kinase superfamily with structural and functional characteristics distinguishable from those of the ERK, JNK (SAPK), and BMK (ERK5) kinases. Although there is a high degree of similarity among members of the p38 group in terms of structure and activation, each member appears to have a unique function. Here we show that activation of p38gamma (also known as ERK6 or SAPK3), but not the other p38 isoforms, is required for gamma-irradiation-induced G(2) arrest. Activation of the MKK6-p38gamma cascade is sufficient to induce G(2) arrest in cells, and expression of dominant negative alleles of MKK6 or p38gamma allows cells to escape the DNA damage-induce G(2) delay. Activation of p38gamma is dependent on ATM and leads to activation of Cds1 (also known as Chk2). These data suggest a model in which activation of ATM by gamma irradiation leads to the activation of MKK6, p38gamma, and Cds1 and that activation of both MKK6 and p38gamma is essential for the proper regulation of the G(2) checkpoint in mammalian cells.

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

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

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

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

Solution structure of the MEF2A-DNA complex: structural basis for the modulation of DNA bending and specificity by MADS-box transcription factors

The solution structure of the 33 kDa complex between the dimeric DNA-binding core domain of the transcription factor MEF2A (residues 1-85) and a 20mer DNA oligonucleotide comprising the consensus sequence CTA(A/T)(4)TAG has been solved by NMR. The protein comprises two domains: a MADS-box (residues 1-58) and a MEF2S domain (residues 59-73). Recognition and specificity are achieved by interactions between the MADS-box and both the major and minor grooves of the DNA. A number of critical differences in protein-DNA contacts observed in the MEF2A-DNA complex and the DNA complexes of the related MADS-box transcription factors SRF and MCM1 provide a molecular explanation for modulation of sequence specificity and extent of DNA bending ( approximately 15 versus approximately 70 degrees ). The structure of the MEF2S domain is entirely different from that of the equivalent SAM domain in SRF and MCM1, accounting for the absence of cross-reactivity with other proteins that interact with these transcription factors.

Inhibition of p38 MAP kinase as a therapeutic strategy

Since the discovery of p38 MAP kinase in 1994, our understanding of its biology has progressed dramatically. The key advances include (1) identification of p38 MAP kinase homologs and protein kinases that act upstream and downstream from p38 MAP kinase, (2) identification of interesting and potentially important substrates, (3) elucidation of the role of p38 MAP kinase in cellular processes and (4) the establishment of the mechanism by which the pyridinylimidazole p38 MAP kinase inhibitors inhibit enzyme activity. It is now known that there are four members of the p38 MAP kinase family. They differ in their tissue distribution, regulation of kinase activation and subsequent phosphorylation of downstream substrates. They also differ in terms of their sensitivities toward the p38 MAP kinase inhibitors. The best-studied isoform is p38 alpha, whose activation has been observed in many hematopoietic and non-hematopoietic cell types upon treatment with appropriate stimuli. The pyridinylimidazole compounds, exemplified by SB 203580, were originally prepared as inflammatory cytokine synthesis inhibitors that subsequently were found to be selective inhibitors of p38 MAP kinase. SB 203580 inhibits the catalytic activity of p38 MAP kinase by competitive binding in the ATP pocket. X-ray crystallographic studies of the target enzyme complexed with inhibitor reinforce the observations made from site-directed mutagenesis studies, thereby providing a molecular basis for understanding the kinase selectivity of these inhibitors. The p38 MAP kinase inhibitors are efficacious in several disease models, including inflammation, arthritis and other joint diseases, septic shock, and myocardial injury. In all cases, p38 activation in key cell types correlated with disease initiation and progression. Treatment with p38 MAP kinase inhibitors attenuated both p38 activation and disease severity. Structurally diverse p38 MAP kinase inhibitors have been tested extensively in preclinical studies.

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

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

Role of basal calcium in the EGF activation of MAP kinases

The role of intracellular Ca2+ pools in the regulation of growth factor signal transduction pathways and mitogenesis is not well understood. We have examined the roles of basal and transiently mobilized Ca2+ in the regulation of MAP kinases by EGF. To assess the influence of Ca2+ transients we utilized Plcg1-/- and Plcg1+/+ mouse embryonic fibroblasts, while BAPTA/AM was employed to chelate total intracellular Ca2+ in the same cell lines. The MAP kinases erk-1, erk-2 and erk-5 exhibited similar patterns of activation in wild-type and Plcg1-/- cells treated with EGF. However, pretreatment with BAPTA/AM significantly increased and prolonged erk-1 and erk-2 activation in both cell types. In contrast, BAPTA/AM prevented the EGF activation of erk-5 in wild-type and Plcg1-/- cells. These data indicate that basal Ca2+, but not growth factor provoked Ca2+ transients, has a significant influence on the activation of these MAP kinases. AG1478, a specific EGF receptor kinase inhibitor, abolished the prolonged erk-1 and erk-2 activation produced by EGF in cells pretreated with BAPTA/AM. This indicates that the prolonged activation of erk-1 and erk-2 produced in the presence of BAPTA/AM requires continuous signaling from the EGF receptor kinase.

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

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

Multiple mitogen-activated protein kinase signaling pathways connect the cot oncoprotein to the c-jun promoter and to cellular transformation

The serine/threonine kinase Cot is a member of the mitogen-activated protein kinase (MAPK) kinase kinase family implicated in cellular transformation. Enhanced expression of this protein has been shown to activate both the MAPK and the c-Jun N-terminal kinase (JNK) pathways and to stimulate the nuclear factor of activated T cells and NF-kappaB-dependent transcription. However, the nature of the normal functions of the Cot protein and the molecular mechanisms responsible for its oncogenic potential are still largely unknown. Here, we show that overexpression of the cot proto-oncogene is sufficient to stimulate the expression of c-jun and that, in turn, the activity of c-Jun is required for Cot-induced transformation. These observations prompted us to explore the molecular events by which Cot regulates c-jun expression. We found that Cot potently stimulates the activity of the c-jun promoter utilizing JNK-dependent and -independent pathways, the latter involving two novel members of the MAPK family, p38gamma (ERK6) and ERK5. Molecularly, this activity was found to be dependent on the ability of Cot to activate, in vivo, members of each class of the MAPK kinase superfamily, including MEK, SEK, MKK6, and MEK5. Furthermore, the use of dominant interfering molecules revealed that Cot requires JNK, p38s, and ERK5 to stimulate the c-jun promoter fully and to induce neoplastic transformation. These findings indicate that Cot represents the first example of a serine/threonine kinase acting simultaneously on all known MAPK cascades. Moreover, these observations strongly suggest that the transforming ability of Cot results from the coordinated activation of these pathways, which ultimately converge on the regulation of the expression and activity of the product of the c-jun proto-oncogene.

Modulation of tau phosphorylation and intracellular localization by cellular stress

Tau is a microtubule-associated protein that is functionally modulated by phosphorylation and hyperphosphorylated in several neurodegenerative diseases. Because phosphorylation regulates both normal and pathological tau functioning, it is of great interest to identify the signalling pathways and enzymes capable of modulating tau phosphorylation in vivo. The present study examined changes in tau phosphorylation and localization in response to osmotic stress, which activates the stress-activated protein kinases (SAPKs), a family of proline-directed protein kinases shown to phosphorylate tau in vitro and hypothesized to phosphorylate tau in Alzheimer's disease. Immunoblot analysis with phosphorylation-dependent antibodies revealed that osmotic stress increased tau phosphorylation at the non-Ser/Thr-Pro sites Ser-262/356, within the microtubule-binding domain, as well as Ser/Thr-Pro sites outside of tau's microtubule-binding domain. Although all SAPKs examined were activated by osmotic stress, none of the endogenous SAPKs mediated the increase in tau phosphorylation. However, when transfected into SH-SY5Y cells, SAPK3, but not the other SAPKs examined, phosphorylated tau in situ in response to activation by osmotic stress. Osmotic-stress-induced tau phosphorylation correlated with a decrease in the amount of tau associated with the cytoskeleton and an increase in the amount of soluble tau. This stress-induced alteration in tau localization was only partially due to phosphorylation at Ser-262/356 by a staurosporine-sensitive, non-proline-directed, protein kinase. Taken together, these results suggest that osmotic stress activates at least two tau-directed protein kinases, one proline-directed and one non-proline-directed, that SAPK3 can phosphorylate tau on Ser/Thr-Pro residues in situ, and that Ser-262/356 phosphorylation only partially regulates tau localization in the cell.

Osmotic stress induces rapid activation of a salicylic acid-induced protein kinase and a homolog of protein kinase ASK1 in tobacco cells

In tobacco cells, osmotic stress induced the rapid activation of two protein kinases that phosphorylate myelin basic protein. Immunological studies demonstrated that the 48-kD kinase is the salicylic acid-induced protein kinase (SIPK), a member of the mitogen-activated protein kinase family. SIPK was activated 5 to 10 min after the cells were exposed to osmotic stresses, and its activity persisted for approximately 30 min. In contrast, the 42-kD kinase was activated within 1 min after osmotic stress, and its activity was maintained for approximately 2 hr. Moreover, in addition to myelin basic protein, the 42-kD kinase phosphorylated casein and two transcription factors, c-Jun and ATF-2. This latter enzyme was inactivated by a serine/threonine-specific phosphatase but, unlike SIPK, was not affected by a tyrosine-specific phosphatase. After the 42-kD kinase was purified to apparent homogeneity, tryptic peptide analysis indicated that it is a homolog of Arabidopsis serine/threonine kinase1 (ASK1).

Osmotic stress induces rapid activation of a salicylic acid-induced protein kinase and a homolog of protein kinase ASK1 in tobacco cells

In tobacco cells, osmotic stress induced the rapid activation of two protein kinases that phosphorylate myelin basic protein. Immunological studies demonstrated that the 48-kD kinase is the salicylic acid-induced protein kinase (SIPK), a member of the mitogen-activated protein kinase family. SIPK was activated 5 to 10 min after the cells were exposed to osmotic stresses, and its activity persisted for approximately 30 min. In contrast, the 42-kD kinase was activated within 1 min after osmotic stress, and its activity was maintained for approximately 2 hr. Moreover, in addition to myelin basic protein, the 42-kD kinase phosphorylated casein and two transcription factors, c-Jun and ATF-2. This latter enzyme was inactivated by a serine/threonine-specific phosphatase but, unlike SIPK, was not affected by a tyrosine-specific phosphatase. After the 42-kD kinase was purified to apparent homogeneity, tryptic peptide analysis indicated that it is a homolog of Arabidopsis serine/threonine kinase1 (ASK1).

The inhibitory action of butyrate on lipopolysaccharide-induced nitric oxide production in RAW 264.7 murine macrophage cells

The effect of butyrate, a natural bacterial product of colonic bacterial flora, on lipopolysaccharide (LPS)-induced nitric oxide (NO) production in RAW 264.7 murine macrophage cells was studied. Butyrate significantly reduced NO production in LPS-stimulated RAW cells. The inhibition was abolished by the removal of butyrate. Butyrate also inhibited the expression of inducible type NO synthase (iNOS) in LPS-stimulated RAW cells. Furthermore, butyrate prevented the activation of nuclear factor (NF)-kappaB through the stabilization of IkappaB-alpha and IkappaB-beta. Butyrate did not affect the phosphorylation of mitogen-activated protein (MAP) kinases by LPS. It was, therefore, suggested that butyrate down-regulated LPS-induced NO production in RAW cells through preventing the expression of iNOS, and that it was due to the inhibitory action of butyrate on the activation of NF-kappaB.

The p38 signal transduction pathway: activation and function

The p38 signalling transduction pathway, a Mitogen-activated protein (MAP) kinase pathway, plays an essential role in regulating many cellular processes including inflammation, cell differentiation, cell growth and death. Activation of p38 often through extracellular stimuli such as bacterial pathogens and cytokines, mediates signal transduction into the nucleus to turn on the responsive genes. p38 also transduces signals to other cellular components to execute different cellular responses. In this review, we summarize the characteristics of the major components of the p38 signalling transduction pathway and highlight the targets of this pathway and the physiological function of the p38 activation.

MEKK3 directly regulates MEK5 activity as part of the big mitogen-activated protein kinase 1 (BMK1) signaling pathway

Big mitogen-activated protein (MAP) kinase (BMK1), also known as ERK5, is a member of the MAP kinase family whose cellular activity is elevated in response to growth factors, oxidative stress, and hyperosmolar conditions. Previous studies have identified MEK5 as a cellular kinase directly regulating BMK1 activity; however, signaling molecules that directly regulate MEK5 activity have not yet been defined. Through utilization of a yeast two-hybrid screen, we have identified MEKK3 as a molecule that physically interacts with MEK5. This interaction appears to take place in mammalian cells as evidenced by the fact that cellular MEK5 and MEKK3 co-immunoprecipitate. In addition, we show that a dominant active form of MEKK3 stimulates BMK1 activity through MEK5. Moreover, we demonstrate that MEKK3 activity is required for growth factor mediated cellular activation of endogenous BMK1. Taken together, these results identify MEKK3 as a kinase that regulates the activity of MEK5 and BMK1 during growth factor-induced cellular stimulation.