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

Contractile C2C12 myotube model for studying exercise-inducible responses in skeletal muscle

Adequate exercise leads to a vast variety of physiological changes in skeletal muscle as well as other tissues/organs and is also responsible for maintaining healthy muscle displaying enhanced insulin-responsive glucose uptake via GLUT4 translocation. We generated highly developed contractile C(2)C(12) myotubes by manipulating intracellular Ca(2+) transients with electric pulse stimulation (EPS) that is endowed with properties similar to those of in vivo skeletal muscle in terms of 1) excitation-induced contractile activity as a result of de novo sarcomere formation, 2) activation of both the AMP kinase and stress-activated MAP kinase cascades, and 3) improved insulin responsiveness as assessed by GLUT4 recycling. Tbc1d1, a Rab-GAP implicated in exercise-induced GLUT4 translocation in skeletal muscle, also appeared to be phosphorylated on Ser(231) after EPS-induced contraction. In addition, a switch in myosin heavy-chain (MHC) expression from "fast type" to "slow type" was observed in the C(2)C(12) myotubes endowed with EPS-induced repetitive contractility. Taking advantage of these highly developed contractile C(2)C(12) myotubes, we identified myotube-derived factors responsive to EPS-evoked contraction, including the CXC chemokines CXCL1/KC and CXCL5/LIX, as well as IL-6, previously reported to be upregulated in contracting muscles in vivo. Importantly, animal treadmill experiments revealed that exercise significantly increased systemic levels of CXCL1/KC, perhaps derived from contracting muscle. Taken together, these results confirm that we have established a specialized muscle cell culture model allowing contraction-inducible cellular responses to be explored. Utilizing this model, we identified contraction-inducible myokines potentially linked to the metabolic alterations, immune responses, and angiogenesis induced by exercise.

ERK5 regulation in naïve T-cell activation and survival

ERK5 has been implicated in regulating the MEF2-dependent genes Klf2 and nur77 downstream of the TCR and the maintenance of expression of CD62L on peripheral T cells. Based on this data, knockout of ERK5 would be predicted to compromise T-cell development and the maintenance of T cells in the periphery. Using an ERK5 conditional knockout, driven by CD4-CRE or Vav-CRE transgenes resulting in the loss of ERK5 in T cells, we have found that ERK5 is not required for T-cell development. In addition, normal numbers of T cells were found in the spleens and lymph nodes of these mice. We also find that TCR stimulation is not a strong signal for ERK5 activation in primary murine T cells. ERK5 was found to contribute to the induction of Klf2 but not nur77 mRNA following TCR activation. Despite the reduction in Klf2 mRNA, no effect was seen in ERK5 knockouts on either the mRNA levels for the Klf2 target genes CD62L, CCR7 and S1P, or the cell surface expression of CD62L. These results suggest that while ERK5 does contribute to Klf2 regulation in T cells, it is not essential for the expression of CD62L or T-cell survival.

Erk5 controls Slug expression and keratinocyte activation during wound healing

Reepithelialization during cutaneous wound healing involves numerous signals that result in basal keratinocyte activation, spreading, and migration, all linked to a loosening of cell-cell adhesion structures. The transcription factor Slug is required for this process, and EGF treatment of human keratinocytes induced activating phosphorylation of Erk5 that coincides with slug transcription. Accordingly, ectopic activation of Erk5 led to increased Slug mRNA levels and faster wound healing, whereas keratinocyte migration was totally blocked by Erk5 pathway inhibition. Expression of a shRNA specific for Erk5 strongly diminished Erk5 levels in keratinocytes and significantly decreased their motility response to EGF, along with induction of Slug expression. These Erk5-deprived keratinocytes showed an altered, more compact morphology, along with disruption of desmosome organization. Accordingly, they displayed an altered ability to form cell aggregates. These results implicate a novel EGFR/Erk5/Slug pathway in the control of cytoskeleton organization and cell motility in keratinocytes treated with EGF.

ERK5/MAPK is activated by TGFbeta in hepatocytes and required for the GSK-3beta-mediated Snail protein stabilization

Extracellular signal-regulated protein kinase 5 (ERK5) is a mitogen-activated protein kinase, specifically activated by MEK5, and involved in the regulation of many cellular functions including proliferation, survival, differentiation and apoptosis. MEK5/ERK5 module is an important element of different signal transduction pathways. The aim of this study was to investigate whether ERK5 participates to the signalling of the multifunctional cytokine TGFbeta, known to play an important role in the regulation of hepatic growth. Here, we reported that ERK5 is phosphorylated and activated by TGFbeta in hepatocytes, with a rapid and sustained kinetic, through a Src-dependent pathway. Moreover, we demonstrated that ERK5 participates to the TGFbeta-induced Snail protein regulation being required for its stabilization. We also found that the functional inactivation of ERK5 impedes the TGFbeta-mediated glycogen synthase kinase-3beta inactivation suggesting this as mechanism responsible for ERK5-mediated Snail stabilization. Thus, results presented in this study uncovered for the first time a role for ERK5 in the TGFbeta-induced cellular responses.

TGF-beta activates ERK5 in human renal epithelial cells

The role of the MAP kinase, extracellular signal-regulated kinase 5 (ERK5) remains unknown, however it is involved in cell differentiation and survival as highlighted by the embryonic lethality of the ERK5 knockout. ERK5 can be activated by growth factors and other extracellular signals. TGF-beta, a powerful controller of epithelial cell phenotype, is known to activate the MAP kinase, ERK1/2 however its effect on ERK5 remains unknown. This study demonstrates, fort the first time, ERK5 activation by TGF-beta, observed in both transformed and primary adult human PTEC; activation required ALK-5 receptor activity. In addition this work demonstrates expression of myocyte enhancer factor-2 (MEF2C) by PTEC and that TGF-beta increased the association of MEK5 with phospho-ERK5 and MEF2C. ERK5 activation by either TGF-beta or epidermal growth factor (EGF) was also inhibited by the p38 MAP kinase inhibitor, SB-202190.

ERK5 promotes Src-induced podosome formation by limiting Rho activation

Increased Src activity, often associated with tumorigenesis, leads to the formation of invasive adhesions termed podosomes. Podosome formation requires the function of Rho family guanosine triphosphatases and reorganization of the actin cytoskeleton. In addition, Src induces changes in gene expression required for transformation, in part by activating mitogen-activated protein kinase (MAPK) signaling pathways. We sought to determine whether MAPK signaling regulates podosome formation. Unlike extracellular signal–regulated kinase 1/2 (ERK1/2), ERK5 is constitutively activated in Src-transformed fibroblasts. ERK5-deficient cells expressing v-Src exhibited increased RhoA activation and signaling, which lead to cellular retraction and an inability to form podosomes or induce invasion. Addition of the Rho-kinase inhibitor Y27632 to ERK5-deficient cells expressing v-Src led to cellular extension and restored podosome formation. In Src-transformed cells, ERK5 induced the expression of a Rho GTPase-activating protein (RhoGAP), RhoGAP7/DLC-1, via activation of the transcription factor myocyte enhancing factor 2C, and RhoGAP7 expression restored podosome formation in ERK5-deficient cells. We conclude that ERK5 promotes Src-induced podosome formation by inducing RhoGAP7 and thereby limiting Rho activation.

Non-redundant function of the MEK5-ERK5 pathway in thymocyte apoptosis

The mitogen-activated protein kinases (MAPKs) ERK1/2, p38, and JNK are thought to determine survival-versus-death fate in developing thymocytes. However, this view was challenged by studies using 'MEK1-ERK1/2-specific' pharmacological inhibitors, which block both positive and negative selection. Recently, these inhibitors were also shown to affect MEK5, an upstream activator of ERK5, another class of MAPK with homology to ERK1/2. To define the contribution of the MEK5-ERK5 pathway in T-cell development, we retrovirally expressed dominant-negative or constitutively activated form of MEK5 to inhibit or activate the MEK5-ERK5 pathway. We demonstrate that MEK5 regulates apoptosis of developing thymocytes but has no function in positive selection. ERK5 activity correlates with the levels of Nur77 family members but not that of Bim, two effector pathways of thymocyte apoptosis. These results illustrate the critical involvement of the MEK5-ERK5 pathway in thymocyte development distinct from that of ERK1/2 and highlight the importance of the MAPK network in mediating differential effects pertaining to T-cell differentiation and apoptosis.

ERK5/BMK1 is indispensable for optimal colony-stimulating factor 1 (CSF-1)-induced proliferation in macrophages in a Src-dependent fashion

CSF-1, by binding to its high-affinity receptor CSF-1R, sustains the survival and proliferation of monocyte/macrophages, which are central cells of innate immunity and inflammation. The MAPK ERK5 (also known as big MAPK-1, BMK1, or MAPK7) is a 98-kDa molecule sharing high homology with ERK1/2. ERK5 is activated by oxidative stress or growth factor stimulation. This study was undertaken to characterize ERK5 involvement in macrophage signaling that is elicited by CSF-1. Exposure to the CSF-1 of primary human macrophages or murine macrophage cell lines, as well as murine fibroblasts expressing ectopic CSF-1R, resulted in a rapid and sustained increase of ERK5 phosphorylation on activation-specific residues. In the BAC1.2F5 macrophage cell line, ERK5 was also activated by another mitogen, GM-CSF, while macrophage activators such as LPS or IFN-gamma and a number of nonproliferative cytokines failed. Src family kinases were found to link the activation of CSF-1R to that of ERK5, whereas protein kinase C or the serine phosphatases PP1 and PP2A seem not to be involved in the process. Treatment of macrophages with ERK5-specific small interfering RNA markedly reduced CSF-1-induced DNA synthesis and total c-Jun phosphorylation and expression, while increasing the expression of the cyclin-dependent kinase inhibitor p27. Following CSF-1 treatment, the active form of ERK5 rapidly translocated from cytosol to nucleus. Taken together, the results reported in this study show that ERK5 is indispensable for optimal CSF-1-induced proliferation and indicate a novel target for its control.

Effects of MEK5/ERK5 association on small ubiquitin-related modification of ERK5: implications for diabetic ventricular dysfunction after myocardial infarction

Diabetes mellitus (DM) contributes to the exacerbation of left ventricle (LV) dysfunction after myocardial infarction (MI). Activation of ERK5, an atypical mitogen activated protein kinase with transcriptional activity, inhibits apoptosis and LV dysfunction after doxorubicin treatment. SUMOylation has been proposed as a negative regulator of various transcription factors. In the current study, we investigated the role of ERK5-SUMOylation in ERK5 transcriptional activity as well as on DM-mediated exacerbation of LV dysfunction and apoptosis after MI. ERK5 wild-type transcriptional activity was inhibited by Ubc9 (SUMO E2 conjugase) or PIAS1 (E3 ligase), but not in the ERK5-SUMOylation-site defective mutant (K6R/K22R). H2O2 and high glucose, 2 well-known mediators of diabetes, induced ERK5-SUMOylation, and the K6R/K22R mutant, dominant negative form of Ubc9, and siRNA-PIAS1 reversed H2O2-mediated reduction of ERK5 transcriptional activity in cardiomyocytes, indicating the presence of SUMOylation-dependent ERK5 transcriptional repression. Constitutively active form of MEK5alpha (CA-MEK5alpha) inhibited ERK5-SUMOylation independent of kinase activity, but dependent on MEK5-ERK5 association. To investigate the pathological role of ERK5-SUMOylation in DM mice after MI, we used cardiac specific CA-MEK5alpha transgenic mice (CA-MEK5alpha-Tg). MI was induced in streptozotocin (STZ)-injected (DM+MI group) or vehicle-injected mice (MI group) by ligating the left coronary artery. The ERK5-SUMOylation was increased in the DM+MI, but not in the MI group. ERK5-SUMOylation, the exacerbation of LV dysfunction, and the number of TUNEL-positive cells in DM+MI was significantly inhibited in CA-MEK5alpha-Tg mice. Of note, we could not detect any difference of cardiac function after MI in non-diabetic CA-MEK5alpha-Tg and non-transgenic littermate control mice. These results demonstrated that ERK5 transcriptional activity is subject to downregulation by diabetes-dependent SUMOylation, which resulted in a proapoptotic condition contributing to poor post-MI LV function.

Transcription factor Mef2c is required for B cell proliferation and survival after antigen receptor stimulation

Calcineurin is required for B cell receptor (BCR)-induced proliferation of mature B cells. Paradoxically, loss of NFAT transcription factors, themselves calcineurin targets, induces hyperactivity, which suggests that calcineurin targets other than NFAT are required for BCR-induced proliferation. Here we demonstrate a function for the calcineurin-regulated transcription factor Mef2c in B cells. BCR-induced calcium mobilization was intact after Mef2c deletion, but loss of Mef2c caused defects in B cell proliferation and survival after BCR stimulation in vitro and lower T cell-dependent antibody responses and germinal center formation in vivo. Mef2c activity was specific to BCR stimulation, as Toll-like receptor and CD40 signaling induced normal responses in Mef2c-deficient B cells. Mef2c-dependent targets included the genes encoding cyclin D2 and the prosurvival factor Bcl-x(L). Our results emphasize an unrecognized but critical function for Mef2c in BCR signaling.

Homogeneous time-resolved fluorescence resonance energy transfer assay for measurement of Phox/Bem1p (PB1) domain heterodimerization

Twenty human proteins encode Phox/Bem1p (PB1) domains, which are involved in forming protein heterodimers. MEKK2, MEKK3, and MEK5 are 3 serine-threonine protein kinases that have PB1 domains. MEKK2, MEKK3, and MEK5 are the MAP3Ks and the MAP2K in the ERK5 mitogen-activated protein kinase (MAPK) signaling module. ERK5 is a critical MAPK for both development of the vasculature and vascular homeostasis in the adult, but no other MAPK has been shown to be critical in vascular maintenance in the adult animal. MEKK2 and MEKK3 are the only MAP3Ks shown to physically interact with and activate the MEK5-ERK5 signaling module. Interaction of MEKK2 or MEKK3 with MEK5 is mediated by heterodimerization of the MEKK2 (or MEKK3) PB1 and MEK5 PB1 domains. The authors have developed a homogeneous, time-resolved fluorescence resonance energy transfer (TR-FRET) assay to monitor PB1-PB1 domain heterodimerization. The assay uses a europium-chelate conjugated GST-MEK5 PB1 domain chimera, biotinylated MEKK2 PB1 domain, and streptavidin-Cy5. Interaction of the MEKK2 and MEK5 PB1 domains gives a robust FRET signal (Z' factor = 0.93), which is completely abrogated by mutation of 2 acidic residues (64D65E-->AA) within the MEK5 PB1 domain that causes loss of stable PB1-PB1 domain interaction. This assay can be used to study the specificity of PB1-PB1 domain interactions and to screen for molecules that can regulate MEKK2/MEKK3-MEK5 interactions. Disruption of PB1 domain interactions represents a novel approach for selectively regulating the ERK5 signaling pathway independent of kinase active site-directed adenosine triphosphate competitive inhibitors.

Aberrant expression of extracellular signal-regulated kinase 5 in human prostate cancer

Abnormal intracellular signaling contributes to carcinogenesis and may represent novel therapeutic targets. mitogen/extracellular signal-regulated kinase kinase-5 (MEK5) overexpression is associated with aggressive prostate cancer. In this study, we examined the role of extracellular signal-regulated kinase (ERK5, an MAPK and specific substrate for MEK5) in prostate cancer. ERK5 immunoreactivity was significantly upregulated in high-grade prostate cancer when compared to benign prostatic hyperplasia (P<0.0001). Increased ERK5 cytoplasmic signals correlated closely with Gleason sum score (P<0.0001), bony metastases (P=0.0044) and locally advanced disease at diagnosis (P=0.0023), with a weak association with shorter disease-specific survival (P=0.036). A subgroup of patients showed strong nuclear ERK5 localization, which correlated with poor disease-specific survival and, on multivariant analysis, was an independent prognostic factor (P<0.0001). Analysis of ERK5 expression in matched tumor pairs (before and after hormone relapse, n=26) revealed ERK5 nuclear expression was significantly associated with hormone-insensitive disease (P=0.0078). Similarly, ERK5 protein expression was increased in an androgen-independent LNCaP subline. We obtained the following in vitro and in vivo evidence to support the above expression data: (1) cotransfection of ERK5wt and MEK5D constructs in PC3 cells results in predominant ERK5 nuclear localization, similar to that observed in aggressive clinical disease; (2) ERK5-overexpressing PC3 cells have enhanced proliferative, migrative and invasive capabilities in vitro (P<0.0001), and were dramatically more efficient in forming tumors, with a shorter mean time for tumors to reach a critical volume of 1000 mm(3), in vivo (P<0.0001); (3) the MEK1 inhibitor, PD184352, blocking ERK1/2 activation at low dose, did not suppress proliferation but did significantly decrease proliferation at a higher dose required to inhibit ERK5 activation. Taken together, our results establish the potential importance of ERK5 in aggressive prostate cancer.

Integration of protein kinases mTOR and extracellular signal-regulated kinase 5 in regulating nucleocytoplasmic localization of NFATc4

The target of rapamycin (TOR) signaling regulates the nucleocytoplasmic shuttling of transcription factors in yeast. Whether the mammalian counterpart of TOR (mTOR) also regulates nucleocytoplasmic shuttling is not known. Using a phospho-specific monoclonal antibody, we demonstrate that mTOR phosphorylates Ser(168,170) of endogenous NFATc4, which are conserved gate-keeping Ser residues that control NFAT subcellular distribution. The mTOR acts as a basal kinase during the resting state to maintain NFATc4 in the cytosol. Inactivation and nuclear export of NFATc4 are mediated by rephosphorylation of Ser(168,170), which can be a nuclear event. Kinetic analyses demonstrate that rephosphorylation of Ser(168,170) of endogenous NFATc4 is mediated by mTOR and, surprisingly, by extracellular signal-regulated kinase 5 (ERK5) mitogen-activated protein kinase as well. Ablation of ERK5 in the Erk5(-/-) cells ascertains defects in NFATc4 rephosphorylation and nucleocytoplasmic shuttling. In addition, phosphorylation of NFATc4 by ERK5 primes subsequent phosphorylation mediated by CK1alpha. These results demonstrate that distinct protein kinases are integrated to phosphorylate the gate-keeping residues Ser(168,170) of NFATc4, to regulate subcellular distribution. These data also expand the repertoire of physiological substrates of mTOR and ERK5.

The MADS transcription factor Mef2c is a pivotal modulator of myeloid cell fate

Mef2c is a MADS (MCM1-agamous–deficient serum response factor) transcription factor best known for its role in muscle and cardiovascular development. A causal role of up-regulated MEF2C expression in myelomonocytic acute myeloid leukemia (AML) has recently been demonstrated. Due to the pronounced monocytic component observed in Mef2c-induced AML, this study was designed to assess the importance of Mef2c in normal myeloid differentiation. Analysis of bone marrow (BM) cells manipulated to constitutively express Mef2c demonstrated increased monopoiesis at the expense of granulopoiesis, whereas BM isolated from Mef2cΔ/− mice showed reduced levels of monocytic differentiation in response to cytokines. Mechanistic studies showed that loss of Mef2c expression correlated with reduced levels of transcripts encoding c-Jun, but not PU.1, C/EBPα, or JunB transcription factors. Inhibiting Jun expression by short-interfering RNA impaired Mef2c-mediated inhibition of granulocyte development. Moreover, retroviral expression of c-Jun in BM cells promoted monocytic differentiation. The ability of Mef2c to modulate cell-fate decisions between monocyte and granulocyte differentiation, coupled with its functional sensitivity to extracellular stimuli, demonstrate an important role in immunity—and, consistent with findings of other myeloid transcription factors, a target of oncogenic lesions in AML.

Vascular endothelial growth factor induces MEF2C and MEF2-dependent activity in endothelial cells

PURPOSE

Vascular endothelial growth factor is a key regulator of physiological and pathologic angiogenesis. Although much is known about the major upstream signaling pathways of VEGF in endothelial cells, less is known about key transcription factors involved in VEGF action. The transcription factor myocyte enhancer factor (MEF)-2C is thought to play an important role in vasculogenesis and angiogenesis during vascular development. The purpose of this study was to investigate the regulation of MEF2C expression and MEF2-dependent activity in endothelial cells by VEGF.

METHODS

Expression of MEF2C in human retinal endothelial cells and human umbilical vein endothelial cells was assayed by real-time PCR and Western blot. VEGF regulation of MEF2-dependent transcription was studied using an MEF2-luciferase reporter construct containing three copies of a high-affinity MEF2 binding site. The effect of MEF2 on endothelial cell migration was evaluated using a dominant-negative MEF2C mutant.

RESULTS

VEGF induced MEF2C expression in a dose- and time-dependent fashion. This induction was completely abrogated by the inhibition of protein kinase C and was partially blocked by the inhibition of PKC-beta and PKC-delta. In addition to regulating MEF2C expression, VEGF stimulated transcription from an MEF2-dependent promoter. VEGF stimulation of transcription was significantly reduced by the inhibition of calcineurin, CaMKII, p38 MAPK, and PKC, but not by the inhibition of ERK1/2 or BMK1/ERK5. Transfection of a dominant-negative mutant of MEF2C significantly inhibited VEGF-stimulated endothelial cell migration.

CONCLUSIONS

These results implicate VEGF as a key regulator of MEF2C and suggest that MEF2 may be an important mediator of VEGF in endothelial cells.

Myocyte enhancer factor 2A is transcriptionally autoregulated

MEF2 (myocyte enhancer factor 2) proteins are a small family of transcription factors that play pivotal roles in striated muscle differentiation, development, and metabolism, in neuron survival and synaptic formation, and in lymphocyte selection and activation. Products of the four mammalian MEF2 genes, MEF2A, MEF2B, MEF2C, and MEF2D, are expressed with overlapping but distinct temporospatial patterns. Toward analysis of MEF2A functions and the determinants of its regulated expression, we have mapped and begun studies of the transcriptional control regions of this gene. Heterogeneous 5'-untranslated regions of MEF2A mRNAs result from use of alternative promoters and splicing patterns. The two closely approximated TATA-less promoters are approximately 65 kb upstream of the exon containing the sole initiation codon. Ribonuclease protection and primer extension assays show that each promoter is active in various adult tissues. A canonical MEF2 site overlies the major promoter 1 transcription start site. This element specifically binds MEF2 factors, including endogenous nuclear MEF2A according to chromatin immunoprecipitation studies, and is critical to MEF2A transcription in myocytes. The site exerts reciprocal control of the alternative promoters, silencing promoter 1 and activating promoter 2 under some conditions. Erk5 and p38 MAPK signaling stimulate MEF2A expression by activating both promoters from the MEF2 element. MEF2A transcription is therefore subject to positive or negative regulation by its protein products, depending on signaling activities that influence MEF2 factor trans-activity. The sole MEF2 gene of the cephalochordate amphioxus has a similar regulatory region structure, suggesting that this mode of autoregulatory control is conserved among higher metazoan MEF2 genes.

The detection of MAPK signaling

Mitogen-activated protein kinase (MAPK) cascades are central pathways that participate in the intracellular transmission of extracellular signals. Each of the MAPK signaling cascades seems to consist of three to five tiers of protein kinases that sequentially activate each other by phosphorylation. Since the majority of MAPK cascade components are kinases, the methods used to detect their activation involve determining phosphorylation state and protein kinase activities. The primary method describes the use of immunoblotting with specific anti-phospho antibody to detect activation of MAPK components. Alternative methods described are immunoprecipitation of desired protein kinases followed by phosphorylation of specific substrates and the use of an in-gel kinase assay. These methods have proven useful in the study of the MAPK signaling cascades.

Betagamma subunits of G(i/o) suppress EGF-induced ERK5 phosphorylation, whereas ERK1/2 phosphorylation is enhanced

Extracellular signal-regulated kinases (ERKs) play important physiological roles in proliferation, differentiation and gene expression. ERK5 is twice the size of ERK1/2, the amino-terminal half contains the kinase domain that shares the homology with ERK1/2 and TEY activation motif, whereas the carboxy-terminal half is unique. In this study, we examined the cross-talk mechanism between G-protein-coupled receptors (GPCRs) and receptor tyrosine kinases, focusing on ERK1/2 and 5. The pretreatment of rat pheochromocytoma cells (PC12) with pertussis toxin (PTX) specifically enhanced epidermal growth factor (EGF)-induced ERK5 phosphorylation. In addition, lysophosphatidic acid (LPA) attenuated the EGF-induced ERK5 phosphorylation in LPA(1) receptor- and G(i/o)-dependent manners. On the other hand, LPA alone activated ERK1/2 via Gbetagamma subunits and Ras and potentiated EGF-induced ERK1/2 phosphorylation at late time points. These results suggest G(i/o) negatively regulates ERK5, while it positively regulates ERK1/2. LPA did not affect cAMP levels after EGF treatment, and the reagents promoting cAMP production such as forskolin and cholera toxin also attenuated the EGF-induced ERK5 phosphorylation, indicating that the inhibitory effect of LPA on ERK5 inhibition via G(i/o) is not due to inhibition of adenylyl cyclase by Galpha(i/o). However, the inhibitory effect of LPA on ERK5 was abolished in PC12 cells stably overexpressing C-terminus of GPCR kinase2 (GRK2), and overexpression of Gbeta(1) and gamma(2) subunits also suppressed ERK5 phosphorylation by EGF. In response to LPA, Gbetagamma subunits interacted with EGF receptor in a time-dependent manner. These results strongly suggest that LPA negatively regulates the EGF-induced ERK5 phosphorylation through Gbetagamma subunits.

Protein kinase A represses skeletal myogenesis by targeting myocyte enhancer factor 2D

Activation of protein kinase A (PKA) by elevation of the intracellular cyclic AMP (cAMP) level inhibits skeletal myogenesis. Previously, an indirect modulation of the myogenic regulatory factors (MRFs) was implicated as the mechanism. Because myocyte enhancer factor 2 (MEF2) proteins are key regulators of myogenesis and obligatory partners for the MRFs, here we assessed whether these proteins could be involved in PKA-mediated myogenic repression. Initially, in silico analysis revealed several consensus PKA phosphoacceptor sites on MEF2, and subsequent analysis by in vitro kinase assays indicated that PKA directly and efficiently phosphorylates MEF2D. Using mass spectrometric determination of phosphorylated residues, we document that MEF2D serine 121 and serine 190 are targeted by PKA. Transcriptional reporter gene assays to assess MEF2D function revealed that PKA potently represses the transactivation properties of MEF2D. Furthermore, engineered mutation of MEF2D PKA phosphoacceptor sites (serines 121 and 190 to alanine) rendered a PKA-resistant MEF2D protein, which efficiently rescues myogenesis from PKA-mediated repression. Concomitantly, increased intracellular cAMP-mediated PKA activation also resulted in an enhanced nuclear accumulation of histone deacetylase 4 (HDAC4) and a subsequent increase in the MEF2D-HDAC4 repressor complex. Collectively, these data identify MEF2D as a primary target of PKA signaling in myoblasts that leads to inhibition of the skeletal muscle differentiation program.

Gene Ontology-driven transcriptional analysis of CD34+ cell-initiated megakaryocytic cultures identifies new transcriptional regulators of megakaryopoiesis

Differentiation of hematopoietic stem and progenitor cells is an intricate process controlled in large part at the level of transcription. While some key megakaryocytic transcription factors have been identified, the complete network of megakaryocytic transcriptional control is poorly understood. Using global gene expression microarray analysis, Gene Ontology-based functional annotations, and a novel interlineage comparison with parallel, isogenic granulocytic cultures as a negative control, we closely examined the mRNA level of transcriptional regulators in megakaryocytes derived from human mobilized peripheral blood CD34(+) hematopoietic cells. This approach identified 199 differentially expressed transcription factors or transcriptional regulators. We identified and detailed the transcriptional kinetics of most known megakaryocytic transcription factors including GATA1, FLI1, and MAFG. Furthermore, many genes with transcription factor activity or transcription factor binding activity were identified in megakaryocytes that had not previously been associated with that lineage, including BTEB1, NR4A2, FOXO1A, MEF2C, HDAC5, VDR, and several genes associated with the tumor suppressor p53 (HIPK2, FHL2, and TADA3L). Protein expression and nuclear localization were confirmed in megakaryocytic cells for four of the novel candidate megakaryocytic transcription factors: FHL2, MXD1, E2F3, and RFX5. In light of the hypothesis that transcription factors expressed in a particular differentiation program are important contributors to such a program, these data substantially expand our understanding of transcriptional regulation in megakaryocytic differentiation of stem and progenitor cells.

HGF mediates cell proliferation of human mesothelioma cells through a PI3K/MEK5/Fra-1 pathway

The ligand hepatocyte growth factor/scatter factor (HGF) and its receptor tyrosine kinase, c-Met, are highly expressed in most human malignant mesotheliomas (MMs) and may contribute to their increased growth and viability. Based upon our observation that RNA silencing of fos-related antigen 1 (Fra-1) inhibited c-met expression in rat mesotheliomas (1), we hypothesized that Fra-1 was a key player in HGF-induced proliferation in human MMs. In three of seven human MM lines evaluated, HGF increased Fra-1 levels and phosphorylation of both extracellular signal-regulated kinase 5 (ERK5) and AKT that were inhibited by the phosphatidylinositol 3-kinase (PI3K) inhibitor, LY290042. HGF-dependent phosphorylation and Fra-1 expression were decreased after knockdown of Fra-1, whereas overexpression of Fra-1 blocked the expression of mitogen/extracellular signal-regulated kinase kinases (MEK)5 at the mRNA and protein levels. Stable MM cell lines using a dnMEK5 showed that basal Fra-1 levels were increased in comparison to empty vector control lines. HGF also caused increased MM cell viability and proliferating cell nuclear antigen (PCNA) expression that were abolished by knockdown of MEK5 or Fra-1. Data suggest that HGF-induced effects in some MM cells are mediated via activation of a novel PI3K/ERK5/Fra-1 feedback pathway that might explain tumor-specific effects of c-Met inhibitors on MM and other tumors.

The detection of MAPK signaling

Mitogen-activated protein kinase (MAPK) cascades are central pathways that participate in the intracellular transmission of extracellular signals. Each of the MAPK signaling cascades seems to consist of three to five tiers of protein kinases that sequentially activate each other by phosphorylation. Since the majority of MAPK cascade components are kinases, the methods used to detect their activation involve determining phosphorylation state and protein kinase activities. The Basic Protocol describes the use of immunoblotting with specific anti-phospho antibody to detect activation of MAPK components. Alternative methods described are immunoprecipitation of desired protein kinases followed by phosphorylation of specific substrates and the use of an in-gel kinase assay. These methods have proven useful in the study of the MAPK signaling cascades.

ERK5 differentially regulates PDGF-induced proliferation and migration of hepatic stellate cells

BACKGROUND/AIMS

Hepatic stellate cells (HSC) are liver-specific pericytes implicated in liver tissue repair. Activation of signaling pathways in HSC modulates hepatic fibrogenesis, but no information is available on the possible role of ERK5, a member of the mitogen-activated protein kinase family, in this process. In this study, we investigated the role of ERK5 in the biologic responses triggered by platelet-derived growth factor (PDGF) in HSC.

METHODS

Human HSC were cultured on plastic and studied in their myofibroblast-like phenotype.

RESULTS

PDGF-BB rapidly induced ERK5 activation and translocation to the nucleus. EGF and PDGF-DD were also found to activate ERK5. Interfering with Src activation blocked PDGF-BB-dependent ERK5 phosphorylation. To establish the biological significance of ERK5 activation, HSC were transfected with non-targeting siRNA or siRNA targeting ERK5. ERK5 silencing inhibited PDGF-BB-induced cell proliferation, and expression and activation of c-Jun. In contrast, depletion of ERK5 was associated with significantly increased cell migration, both in the presence or absence of PDGF-BB. This effect was associated with a redistribution of focal contacts, and with decreased phosphorylation of FAK, paxillin, and PAK.

CONCLUSIONS

ERK5 modulates PDGF-dependent biologic activities in human HSC, generating positive signals for cell proliferation downregulating the ability of the cells to migrate.

MEF2A sequence variants in Turkish population

Myocyte enhancer factor 2 (MEF2) is present in skeletal, cardiac, and smooth muscles and in neurons. MEF2A gene encodes a transcription factor which was on 15q26. The objective was to study the MEF2A gene in patients with premature MI. The control group consisted of 87 subjects who were older than 45 years with no history of cardiovascular disease or MI and no family history of CAD. The premature MI group consisted of 69 patients with documented MI younger than 45 years. No abnormal bands with single strand conformation polymorphism were detected after screening exon 1 through exon 8. This is the first study that detected 145408: T>C polymorphism in intron 10. In both study groups, the rare polymorphism P279L in exon 7, T>C polymorphism in intron 10, and 21-bp deletion in exon 11 of the gene were not found. The data supported the previous studies indicating no association between MEF2A gene and premature MI.

Aberrant extracellular signal-regulated kinase (ERK) 5 signaling in hippocampus of suicide subjects

Extracellular signal-regulated kinase 5 (ERK5), the newest member of the mitogen-activated protein (MAP) kinase family, is regulated differently than the other MAP kinases. Emerging evidence suggest the role of ERK5 signaling in promoting cell proliferation, differentiation, neuronal survival, and neuroprotection. The present study investigates whether suicide brain is associated with alterations in components of the ERK5 signaling cascade. In the prefrontal cortex (PFC) and hippocampus of suicide subjects (n=28) and nonpsychiatric control subjects (n=21), we examined the catalytic activities and protein levels of ERK5 and upstream MAP kinase kinase MEK5 in various subcellular fractions; mRNA levels of ERK5 in total RNA; and DNA-binding activity of myocyte enhancer factor (MEF)2C, a substrate of ERK5. In the hippocampus of suicide subjects, we observed that catalytic activity of ERK5 was decreased in cytosolic and nuclear fractions, whereas catalytic activity of MEK5 was decreased in the total fraction. Further, decreased mRNA and protein levels of ERK5, but no change in protein level of MEK5 were noted. A decrease in MEF2C-DNA-binding activity in the nuclear fraction was also observed. No significant alterations were noted in the PFC of suicide subjects. The observed changes were not related to a specific psychiatric diagnosis. Our findings of reduced activation and/or expression of ERK5 and MEK5, and reduced MEF2C-DNA-binding activity demonstrate abnormalities in ERK5 signaling in hippocampus of suicide subjects and suggest possible involvement of this aberrant signaling in pathogenic mechanisms of suicide.

MEF2: a central regulator of diverse developmental programs

The myocyte enhancer factor 2 (MEF2) transcription factor acts as a lynchpin in the transcriptional circuits that control cell differentiation and organogenesis. The spectrum of genes activated by MEF2 in different cell types depends on extracellular signaling and on co-factor interactions that modulate MEF2 activity. Recent studies have revealed MEF2 to form an intimate partnership with class IIa histone deacetylases, which together function as a point of convergence of multiple epigenetic regulatory mechanisms. We review the myriad roles of MEF2 in development and the mechanisms through which it couples developmental, physiological and pathological signals with programs of cell-specific transcription.

Activation of a C-terminal transcriptional activation domain of ERK5 by autophosphorylation

ERK5 plays a crucial role in many biological processes by regulating transcription. ERK5 has a large C-terminal-half that contains a transcriptional activation domain. However, it has remained unclear how its transcriptional activation activity is regulated. Here, we show that the activated kinase activity of ERK5 is required for the C-terminal-half to enhance the AP-1 activity, and that the activated ERK5 undergoes autophosphorylation on its most C-terminal region. Changing these phosphorylatable threonine and serine residues to unphosphorylatable alanines significantly reduces the transcriptional activation activity of ERK5. Moreover, phosphomimetic mutants of the C-terminal-half of ERK5 without an N-terminal kinase domain are shown to be able to enhance the AP-1 activity in fibroblastic cells. These results reveal the role of the stimulus-induced ERK5 autophosphorylation in regulation of gene expression.

MEKK3 is required for endothelium function but is not essential for tumor growth and angiogenesis

Mitogen-activated protein kinase kinase kinase 3 (MEKK3) plays an essential role in embryonic angiogenesis, but its role in tumor growth and angiogenesis is unknown. In this study, we further investigated the role of MEKK3 in embryonic angiogenesis, tumor angiogenesis, and angiogenic factor production. We found that endothelial cells from Mekk3-deficient embryos showed defects in cell proliferation, apoptosis, and interactions with myocardium in the heart. We also found that MEKK3 is required for angiopoietin-1 (Ang1)-induced p38 and ERK5 activation. To study the role of MEKK3 in tumor growth and angiogenesis, we established both wild-type and Mekk3-deficient tumor-like embryonic stem cell lines and transplanted them subcutaneously into nude mice to assess their ability to grow and induce tumor angiogenesis. Mekk3-deficient tumors developed and grew similarly as control Mekk3 wild-type tumors and were also capable of inducing tumor angiogenesis. In addition, we found no differences in the production of VEGF in Mekk3-deficient tumors or embryos. Taken together, our results suggest that MEKK3 plays a critical role in Ang1/Tie2 signaling to control endothelial cell proliferation and survival and is required for endothelial cells to interact with the myocardium during early embryonic development. However, MEKK3 is not essential for tumor growth and angiogenesis.

Erk5 is activated and acts as a survival factor in mitosis

Erk5 is a recently discovered MAPK claimed to be responsible for some of the roles attributed to Erk1/2; here we report that it is activated in mitosis in comparison to G1/S. When Erk5 is inactivated pharmacologically or largely ablated by RNAi, cell survival in mitosis is diminished. We have previously shown Bim, a BH3-only protein of the Bcl-2 family, to be phosphorylated in mitosis, in a MEK-dependent manner (M. Grãos, A. D. Almeida, S. Chatterjee, Biochem. J. 388 (2005) 185). Inactivation of Erk5 in mitosis causes dephosphorylation of Bim. Bim is in the mitochondria in mitosis and when dephosphorylated interacts with Bax, inducing caspase activation. We also show that in mitosis Bim co-immunoprecipitates with Erk5 and Erk5 phosphorylates GST-Bim in in vitro kinase reaction. Taken together, our results identify a new target of the still largely mysterious Erk5 and suggest that Erk5 in mitosis may be a decisive step for the survival of proliferating cells.

Hyperosmotic stress signaling to the nucleus disrupts the Ran gradient and the production of RanGTP

The RanGTP gradient depends on nucleocytoplasmic shuttling of Ran and its nucleotide exchange in the nucleus. Here we show that hyperosmotic stress signaling induced by sorbitol disrupts the Ran protein gradient and reduces the production of RanGTP. Ran gradient disruption is rapid and is followed by early (10-20 min) and late (30-60 min) phases of recovery. Results from SB203580 and siRNA experiments suggest the stress kinase p38 is important for Ran gradient recovery. NTF2 and Mog1, which are transport factors that regulate the nuclear localization of Ran, showed kinetics of delocalization and recovery similar to Ran. Microinjection of a nuclear localization signal reporter protein revealed that sorbitol stress decreases the rate of nuclear import. Sorbitol stress also slowed RCC1 mobility in the nucleus, which is predicted to reduce RCC1 dissociation from chromatin and RanGTP production. This was tested using a FRET biosensor that registers nuclear RanGTP levels, which were reduced in response to sorbitol stress. Although sorbitol alters nucleotide levels, we show that inverting the GTP/GDP ratio in cells is not sufficient to disrupt the Ran gradient. Thus, the Ran system is a target of hyperosmotic stress signaling, and cells use protein localization-based mechanisms as part of a rapid stress response.

The nerve growth factor and its receptors in airway inflammatory diseases

The nerve growth factor (NGF) belongs to the neurotrophin family and induces its effects through activation of 2 distinct receptor types: the tropomyosin-related kinase A (TrkA) receptor, carrying an intrinsic tyrosine kinase activity in its intracellular domain, and the receptor p75 for neurotrophins (p75NTR), belonging to the death receptor family. Through activation of its TrkA receptor, NGF activates signalling pathways, including phospholipase Cgamma (PLCgamma), phosphatidyl-inositol 3-kinase (PI3K), the small G protein Ras, and mitogen-activated protein kinases (MAPK). Through its p75NTR receptor, NGF activates proapoptotic signalling pathways including the MAPK c-Jun N-terminal kinase (JNK), ceramides, and the small G protein Rac, but also activates pathways promoting cell survival through the transcription factor nuclear factor-kappaB (NF-kappaB). NGF was first described by Rita Levi-Montalcini and collaborators as an important factor involved in nerve differentiation and survival. Another role for NGF has since been established in inflammation, in particular of the airways, with increased NGF levels in chronic inflammatory diseases. In this review, we will first describe NGF structure and synthesis and NGF receptors and their signalling pathways. We will then provide information about NGF in the airways, describing its expression and regulation, as well as pointing out its potential role in inflammation, hyperresponsiveness, and remodelling process observed in airway inflammatory diseases, in particular in asthma.

MAP kinases and the control of nuclear events

The mitogen-activated protein kinases (MAPKs) are a family of serine/threonine kinases that play an essential role in signal transduction by modulating gene transcription in the nucleus in response to changes in the cellular environment. They include the extracellular signal-regulated protein kinases (ERK1 and ERK2); c-Jun N-terminal kinases (JNK1, JNK2, JNK3); p38s (p38alpha, p38beta, p38gamma, p38delta) and ERK5. The molecular events in which MAPKs function can be separated in discrete and yet interrelated steps: activation of the MAPK by their upstream kinases, changes in the subcellular localization of MAPKs, and recognition, binding and phosphorylation of MAPK downstream targets. The resulting pattern of gene expression will ultimately depend on the integration of the combinatorial signals provided by the temporal activation of each group of MAPKs. This review will focus on how the specificity of signal transmission by MAPKs is achieved by scaffolding molecules and by the presence of structural motifs in MAPKs that are dynamically regulated by phosphorylation and protein-protein interactions. We discuss also how MAPKs recognize and phosphorylate their target nuclear proteins, including transcription factors, co-activators and repressors and chromatin-remodeling molecules, thereby affecting an intricate balance of nuclear regulatory molecules that ultimately control gene expression in response to environmental cues.

Role of mitogen-activated protein kinase kinase kinases in signal integration

Mitogen-activated protein kinases (MAPKs) are members of a dynamic protein kinase network through which diverse stimuli regulate the spatio-temporal activities of complex biological systems. MAPKs regulate critical cellular functions required for homeostasis such as the expression of cytokines and proteases, cell cycle progression, cell adherence, motility and metabolism. MAPKs therefore influence cell proliferation, differentiation, survival, apoptosis and development. In vertebrates, five MAPK families are regulated by MAPK kinase kinase-MAPK kinase-MAPK (MKKK-MKK-MAPK) phosphorelay systems. There are at least 20 MKKKs that selectively phosphorylate and activate different combinations of the seven MKKs, resulting in a specific activation profile of members within the five MAPK families. MKKKs are differentially activated by upstream stimuli including cytokines, antigens, toxins and stress insults providing a mechanism to integrate the activation of different MAPKs with the cellular response to each stimulus. Thus, MKKKs can be considered as 'signaling hubs' that regulate the specificity of MAPK activation. In this review, we describe how the MKKK 'hub' function regulates the specificity of MAPK activation, highlighting MKKKs as targets for therapeutic intervention in cancer and other diseases.

The tyrosine kinase Abl is required for Src-transforming activity in mouse fibroblasts and human breast cancer cells

The cytoplasmic tyrosine kinase Src has been implicated in signal transduction induced by growth factors and integrins. Src also shows oncogenic activity when deregulated. Accumulating evidence indicates that the tyrosine kinase Abl is an important substrate for Src signalling in normal cells. Here we show that Abl is also required for Src-induced transformation of mouse fibroblasts. Abl does not mediate tyrosine phosphorylation of Stat3 and Shc, two important regulators of Src oncogenic activity. In contrast, Abl controls the activation of the small GTPase Rac for oncogenic signalling and active Rac partly rescued Src transformation in cells with inactive Abl. Moreover, Abl mediates Src-induced extracellular regulated kinase 5 (ERK5) activation to drive cell transformation. Finally, we find that Abl/Rac and Abl/ERK5 pathways also operate in human MCF7 and BT549 breast cancer cells, where neoplastic transformation depends on Src-like activities. Therefore, Abl is an important regulator of Src oncogenic activity both in mouse fibroblasts and in human cancer cells. Targeting these Abl-dependent signalling cascades may be of therapeutic value in breast cancers where Src-like function is important.

Regulation of the G2-M cell cycle progression by the ERK5-NFkappaB signaling pathway

Elucidation of mechanisms regulating cell cycle progression is of fundamental importance for cell and cancer biology. Although several genes and signaling pathways are implicated in G1–S regulation, less is known regarding the mechanisms controlling cell cycle progression through G2 and M phases. We report that extracellular signal–regulated kinase 5 (ERK5), a member of the mitogen-activated protein kinases, is activated at G2–M and required for timely mitotic entry. Stimulation of ERK5 activated nuclear factor κB (NFκB) through ribosomal S6 kinase 2 (RSK2)-mediated phosphorylation and degradation of IκB. Furthermore, selective inhibition of NFκB at G2–M phases substantially delayed mitotic entry and inhibited transcription of G2–M–specific genes, including cyclin B1, cyclin B2, Plk-1, and cdc25B. Moreover, inhibition of NFκB at G2–M diminished mitosis induced by constitutive activation of ERK5, providing a direct link between ERK5, NFκB, and regulation of G2–M progression. We conclude that a novel ERK5–NFκB signaling pathway plays a key role in regulation of the G2–M progression.

Activation of p38/MEF2C pathway by all-trans retinoic acid in cardiac myoblasts

Myocyte enhancer factor 2C (MEF2C) is a transcription factor particularly expressed in cardiac muscle. While the effects of all-trans retinoic acid (atRA) on embryonic heart are well described, the mechanism of atRA action on MEF2C activity in cardiomyocytes is less known. The aim of the present study was to investigate whether and how atRA regulates MEF2C activity in H9c2 rat ventricular cells. Here, our results, obtained from Western blot and protein kinase assays, showed that the phosphorylation of p38 mitogen-activated protein kinase (MAPK) and MEF2C was induced by atRA in H9c2 myocardial cells. And the result from luciferase assays showed that the transactivation activity of MEF2C was upregulated by p38. Furthermore, using confocal microscopy and immunoprecipitation, we found that atRA hastened p38 translocation into nuclei to interact with MEF2C, and SB202190 inhibited nuclear translocation of p38. These results suggest that atRA may mediate p38/MEF2C signaling pathway during heart development.

Mitogenic growth signalling, DNA replication licensing, and survival are linked in prostate cancer

Activation of mitogen/extracellular-signal-regulated kinase kinase 5/extracellular signal-regulated kinase-5 (MEK5/ERK5) growth signalling is coupled to increased cell proliferation in prostate cancer (PCa). Dysregulation of the DNA replication licensing pathway, a critical step in growth control downstream of transduction signalling pathways, is associated with development of PCa. In this study we have investigated linkages between the MEK5/ERK5 pathway and DNA replication licensing during prostate carcinogenesis. The effects of increased MEK5/ERK5 signalling on the expression of replication licensing factors Mcm2 and geminin and the proliferation marker Ki67 were studied in an ecdysone-inducible system expressing a constitutively activated mutant of MEK5 in EcR293 cells and in stable ERK5 over-expressing PC3 clones. In parallel, expression of these biomarkers in PCa biopsy specimens (n=58) was studied and compared to clinicopathological parameters. In both in vitro systems induction of MEK5 expression resulted in increased levels of phosphorylated ERK5 and Mcm2, geminin and Ki67 proteins. In PCa specimens average Mcm2 expression was greater than Ki67 and geminin expression (median labelling index (LI) 36.7, 18.1, and 3.4% respectively), consistent with their differential expression according to growth status (P<0.0001). Mcm2, geminin and Ki67 expression were significantly associated with Gleason grade (P=0.0002, P=0.0003, P=0.004); however there was no link with T or M stage. There was a significant relationship between increasing ERK5 expression and increasing Mcm2 (P=0.003) and Ki67 (P=0.009) expression, with non-significant trends seen with increasing MEK5 expression. There were significant associations between Gleason grade and the number of cells traversing G1 phase (Ki67_LI-geminin_LI; (P=0.001)), with high ERK5 levels associated with both an increase in replication licensed but non-cycling cells (Mcm2_LI-Ki67_LI; (P=0.01)) and accelerated cell cycle progression (geminin_LI/Ki67_LI; (P= 0.005)), all indicative of a shift towards increasing proliferative potential. While Mcm2 and Ki67 were both prognostic factors on univariate analysis, only Mcm2 remained an independent prognostic marker on multivariate analysis. Taken together, our data show that induction of MEK5/ERK5 signalling is linked to activation of the DNA replication licensing pathway in PCa, and that the strong prognostic value of MCM proteins may result from their function as relay stations coupling growth regulatory pathways to genome duplication.

Brain-derived neurotrophic factor stimulates the transcriptional and neuroprotective activity of myocyte-enhancer factor 2C through an ERK1/2-RSK2 signaling cascade

Neurotrophin activation of myocyte-enhancer factor (MEF) 2C is one of the strongest pro-survival signaling pathways in developing neurons. To date, neurotrophin stimulation of MEF2C has been largely attributed to its direct phosphorylation by extracellular signal-regulated kinase (ERK) 5. Because MEF2C is not directly phosphorylated by ERK1/2 in vitro, it is generally assumed that the ERK1/2 signaling cascade does not regulate MEF2C. Surprisingly, we discovered that ERK1/2 are required for both the transcriptional and neuroprotective activity of MEF2C in cortical neurons stimulated by brain-derived neurotrophic factor. ERK1/2 stimulation of MEF2C is mediated by p90 ribosomal S6 kinase 2 (RSK2), a Ser/Thr protein kinase downstream of ERK1/2. RSK2 strongly phosphorylates purified recombinant MEF2C protein in vitro. Furthermore, RSK2 can directly phosphorylate MEF2C on S192, a consensus RSK2-phosphorylation site located in the transactivation domain of MEF2C. Substitution of S192 with a non-phosphorylatable alanine diminishes both the transcriptional and neuroprotective activity of MEF2C to an extent similar to mutation on S387, an established activating phosphorylation site. Together, our data identifies ERK1/2-RSK2 signaling as a novel mechanism by which neurotrophins activate MEF2C and promote neuronal survival.

Hsp90 chaperone control over transcriptional regulation by the yeast Slt2(Mpk1)p and human ERK5 mitogen-activated protein kinases (MAPKs)

Cell integrity MAPK (mitogen-activated protein kinase) function can be provided in yeast cells by either the native Slt2(Mpk1)p of yeast or by a heterologously expressed human ERK5 (extracellular-signal-regulated kinase 5). Both of these MAPKs need the Hsp90 (heat-shock protein 90) chaperone for their activation, so that when Hsp90 function is compromised their activities are low. This, in turn, affects the capacity of these MAPKs to control the transcription factors that regulate cell integrity genes.

Neuroprotective role of ERK1/2 and ERK5 in a dopaminergic cell line under basal conditions and in response to oxidative stress

Loss of motor function in Parkinson's disease is due in part to degeneration of dopamine (DA) neurons. Pharmacological evidence suggests that the mitogen-activated protein kinase signaling pathways involving extracellular signal-regulated kinases (ERKs) play important roles in neuroprotection of DA neurons. However, the relative roles of the several ERK isoforms in the viability of DA neurons have not yet been determined. In the present study, we investigated the contributions of ERK5, as well as ERK1/2, to MN9D cell survival under basal conditions and in response to 6-hydroxydopamine (6-OHDA). We observed that U0126, an inhibitor of ERK activation, decreased basal survival of these cells. To differentiate between ERK1/2 and ERK5, cells were transfected with a dominant negative form of either ERK5 or MEK1, the upstream activator of ERK1/2. Transfection of MN9D cells with either dominant negative construct mimicked U0126, reducing cell survival. Moreover, transfection of the cells in such a way as to increase ERK5 or ERK1/2 activity inhibited 6-OHDA-induced cell death, although this effect was significant only in the case of ERK1/2 activation. These studies suggest that activations of ERK5 and ERK1/2 both promote basal DA cell survival and that ERK1/2 also protects DA cells from oxidative stress. These are the first studies to demonstrate a role for ERK5 in DA neuronal survival and to investigate the relative roles of ERK1/2 and ERK5 in basal DA survival and neuroprotection from oxidative stress.

Re-employment of developmental transcription factors in adult heart disease

A finite number of transcription factors constitute a combinatorial code that orchestrates cardiac development and the specification and differentiation of myocytes. Many, if not all of these same transcription factors are re-employed in the adult heart in response to disease stimuli that promote hypertrophic enlargement and/or dilated cardiomyopathy, as part of the so-called "fetal gene program". This review will discuss the transcription factors that regulate the hypertrophic growth response of the adult heart, with a special emphasis on those regulators that participate in cardiac development.

Modification of glucocorticoid sensitivity by MAP kinase signaling pathways in glucocorticoid-induced T-cell apoptosis

OBJECTIVE

Glucocorticoid is widely used for the treatment of diseases such as hematological malignancies. Glucocorticoid sensitivity is different from person to person and the mechanism of the regulation of glucocorticoid sensitivity is not well known. Glucocorticoid resistance is a major clinical problem.

METHODS AND RESULTS

Here, using glucocorticoid-induced T-cell apoptosis, a model system for the analysis of the mechanism of glucocorticoid action, we clarified that mitogen-activated protein kinases (MAPKs) modify glucocorticoid sensitivity, namely that the activation of extracellular signal-regulated protein kinase (ERK) and p38 MAP kinase reduce and enhance glucocorticoid sensitivity, respectively.

CONCLUSION

These findings might provide new tools for overcoming glucocorticoid-resistance.

Brain-derived Neurotrophic Factor Activates ERK5 in Cortical Neurons via a Rap1-MEKK2 Signaling Cascade

The extracellular signal-regulated kinase 5 (ERK5) is activated in neurons of the central nervous system by neurotrophins including brain-derived neurotrophic factor (BDNF). Although MEK5 is known to mediate BDNF stimulation of ERK5 in central nervous system neurons, other upstream signaling components have not been identified. Here, we report that BDNF induces a sustained activation of ERK5 in rat cortical neurons and activates Rap1, a small GTPase, as well as MEKK2, a MEK5 kinase. Our data indicate that activation of Rap1 or MEKK2 is sufficient to stimulate ERK5, whereas inhibition of either Rap1 or MEKK2 attenuates BDNF activation of ERK5. Furthermore, BDNF stimulation of MEKK2 is regulated by Rap1. Our evidence also indicates that Ras and MEKK3, a MEK5 kinase in non-neuronal cells, do not play a significant role in BDNF activation of ERK5. This study identifies Rap1 and MEKK2 as critical upstream signaling molecules mediating BDNF stimulation of ERK5 in central nervous system neurons.

MAPK signalling: ERK5 versus ERK1/2

Extracellular-signal-regulated kinase 5 (ERK5) is a member of the mitogen-activated protein kinase (MAPK) family and, similar to ERK1/2, has the Thr-Glu-Tyr (TEY) activation motif. Both ERK5 and ERK1/2 are activated by growth factors and have an important role in the regulation of cell proliferation and cell differentiation. Moreover, both the ERK5 and the ERK1/2 pathways are sensitive to PD98059 and U0126, which are two well-known inhibitors of the ERK pathway. Despite these similarities, recent studies have revealed distinctive features of the ERK5 pathway: ERK5 has a key role in cardiovascular development and neural differentiation; ERK5 nuclear translocation is controlled by its own nuclear localizing and nuclear export activities; and the carboxy-terminal half of ERK5, which follows its kinase catalytic domain, has a unique function.

Expressed in the yeast Saccharomyces cerevisiae, human ERK5 is a client of the Hsp90 chaperone that complements loss of the Slt2p (Mpk1p) cell integrity stress-activated protein kinase

ERK5 is a mitogen-activated protein (MAP) kinase regulated in human cells by diverse mitogens and stresses but also suspected of mediating the effects of a number of oncogenes. Its expression in the slt2Delta Saccharomyces cerevisiae mutant rescued several of the phenotypes caused by the lack of Slt2p (Mpk1p) cell integrity MAP kinase. ERK5 is able to provide this cell integrity MAP kinase function in yeast, as it is activated by the cell integrity signaling cascade that normally activates Slt2p and, in its active form, able to stimulate at least one key Slt2p target (Rlm1p, the major transcriptional regulator of cell wall genes). In vitro ERK5 kinase activity was abolished by Hsp90 inhibition. ERK5 activity in vivo was also lost in a strain that expresses a mutant Hsp90 chaperone. Therefore, human ERK5 expressed in yeast is an Hsp90 client, despite the widely held belief that the protein kinases of the MAP kinase class are non-Hsp90-dependent activities. Two-hybrid and protein binding studies revealed that strong association of Hsp90 with ERK5 requires the dual phosphorylation of the TEY motif in the MAP kinase activation loop. These phosphorylations, at positions adjacent to the Hsp90-binding surface recently identified for a number of protein kinases, may cause a localized rearrangement of this MAP kinase region that leads to creation of the Hsp90-binding surface. Complementation of the slt2Delta yeast defect by ERK5 expression establishes a new tool with which to screen for novel agonists and antagonists of ERK5 signaling as well as for isolating mutant forms of ERK5.

ERK5 activation inhibits inflammatory responses via peroxisome proliferator-activated receptor delta (PPARdelta) stimulation

Peroxisome proliferator-activated receptors (PPAR) decrease the production of cytokine and inducible nitric-oxide synthase (iNOS) expression, which are associated with aging-related inflammation and insulin resistance. Recently, the involvement of the induction of heme oxygenase-1 (HO-1) in regulating inflammation has been suggested, but the exact mechanisms for reducing inflammation by HO-1 remains unclear. We found that overexpression of HO-1 and [Ru(CO)(3)Cl(2)](2), a carbon monoxide (CO)-releasing compound, increased not only ERK5 kinase activity, but also its transcriptional activity measured by luciferase assay with the transfection of the Gal4-ERK5 reporter gene. This transcriptional activity is required for coactivation of PPARdelta by ERK5 in C2C12 cells. [Ru(CO)(3)Cl(2)](2) activated PPARdelta transcriptional activity via the MEK5/ERK5 signaling pathway. The inhibition of NF-kappaB activity by ERK5 activation was reversed by a dominant negative form of PPARdelta suggesting that ERK5/PPARdelta activation is required for the anti-inflammatory effects of CO and HO-1. Based on these data, we propose a new mechanism by which CO and HO-1 mediate anti-inflammatory effects via activating ERK5/PPARdelta, and ERK5 mediates CO and HO-1-induced PPARdelta activation via its interaction with PPARdelta.

Epidermal growth factor protects prostate cancer cells from apoptosis by inducing BAD phosphorylation via redundant signaling pathways

Protection from apoptosis by receptor tyrosine kinases, resistant to the inhibition of phosphatidylinositol 3 '-kinase/Akt and Ras/MEK pathways, has been reported in several cell types, including fibroblasts and epithelial prostate cancer cells; however, mechanisms of this effect were not clear. Here we report that in prostate cancer cells, epidermal growth factor activates two antiapoptotic signaling pathways that impinge on the proapoptotic protein BAD. One signaling cascade operates via the Ras/MEK module and induces BAD phosphorylation on Ser112. Another pathway predominantly relies on Rac/PAK1 signaling that leads to BAD phosphorylation on Ser136. Each of these two pathways is sufficient to protect cells from apoptosis, and therefore both have to be inhibited simultaneously to block epidermal growth factor-dependent survival. Redundancy of antiapoptotic signaling pathways should be considered when therapies targeting antiapoptotic mechanisms are designed.