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

Differential regulation of Tec1 by Fus3 and Kss1 confers signaling specificity in yeast development

Transcriptional regulation by mitogen-activated protein (MAP) kinase signaling cascades is a major control mechanism for eukaryotic development. In budding yeast, Fus3 and Kss1 are two MAP kinases that control two distinct developmental programs-mating and invasive growth. We investigated whether signal-specific activation of mating and invasive growth involves regulation of the transcription factor Tec1 by Fus3 and Kss1. We present evidence that, during mating, Fus3 phosphorylates Tec1 to downregulate this invasive growth-specific transcription factor and its target genes. This function of Fus3 is essential for correct execution of the mating program and is not shared by Kss1. We find that Kss1 controls the activity of Tec1 mainly during invasive growth by control of TEC1 gene expression. Our study suggests that signaling specificity can arise from differential regulation of a single transcription factor by two MAP kinases with shared functions in distinct developmental programs.

The unique C-terminal tail of the mitogen-activated protein kinase ERK5 regulates its activation and nuclear shuttling

ERK5 is unique among mitogen-activated protein kinases (MAPKs) in that it contains a large C-terminal tail. We addressed the question of how this tail could affect the signaling capacity of ERK5. Gradual deletion of the C-terminal domains resulted in a drastic increase of ERK5 kinase activity, which was dependent on the up-stream MAPK cascade, thus indicating a possible auto-inhibitory function of the tail. It is interesting that ERK5 was able to autophosphorylate its own tail. Moreover, ERK5, which was found to be expressed in virtually all kinds of cell lines, localized to nuclear as well as cytoplasmic compartments. The localization of ERK5 was determined by its C-terminal domains, which were also required for appropriate nucleocytoplasmic shuttling. Taken together, these results indicate that ERK5 signaling is directed by the presence of its unique C-terminal tail, which might be the key to understanding the key role of ERK5 in MAPK signaling.

The hinge-helix 1 region of peroxisome proliferator-activated receptor gamma1 (PPARgamma1) mediates interaction with extracellular signal-regulated kinase 5 and PPARgamma1 transcriptional activation: involvement in flow-induced PPARgamma activation in endothelial cells

Peroxisome proliferator-activated receptors (PPAR) are ligand-activated transcription factors that form a subfamily of the nuclear receptor gene family. Since both flow and PPARgamma have atheroprotective effects and extracellular signal-regulated kinase 5 (ERK5) kinase activity is significantly increased by flow, we investigated whether ERK5 kinase regulates PPARgamma activity. We found that activation of ERK5 induced PPARgamma1 activation in endothelial cells (ECs). However, we could not detect PPARgamma phosphorylation by incubation with activated ERK5 in vitro, in contrast to ERK1/2 and JNK, suggesting a role for ERK5 as a scaffold. Endogenous PPARgamma1 was coimmunoprecipitated with endogenous ERK5 in ECs. By mammalian two-hybrid analysis, we found that PPARgamma1 associated with ERK5a at the hinge-helix 1 region of PPARgamma1. Expressing a hinge-helix 1 region PPARgamma1 fragment disrupted the ERK5a-PPARgamma1 interaction, suggesting a critical role for hinge-helix 1 region of PPARgamma in the ERK5-PPARgamma interaction. Flow increased ERK5 and PPARgamma1 activation, and the hinge-helix 1 region of the PPARgamma1 fragment and dominant negative MEK5beta significantly reduced flow-induced PPARgamma activation. The dominant negative MEK5beta also prevented flow-mediated inhibition of tumor necrosis factor alpha-mediated NF-kappaB activation and adhesion molecule expression, including vascular cellular adhesion molecule 1 and E-selectin, indicating a physiological role for ERK5 and PPARgamma activation in flow-mediated antiinflammatory effects. We also found that ERK5 kinase activation was required, likely by inducing a conformational change in the NH(2)-terminal region of ERK5 that prevented association of ERK5 and PPARgamma1. Furthermore, association of ERK5a and PPARgamma1 disrupted the interaction of SMRT and PPARgamma1, thereby inducing PPARgamma activation. These data suggest that ERK5 mediates flow- and ligand-induced PPARgamma activation via the interaction of ERK5 with the hinge-helix 1 region of PPARgamma.

Tid1, the Human Homologue of a Drosophila Tumor Suppressor, Reduces the Malignant Activity of ErbB-2 in Carcinoma Cells

The ErbB-2/HER-2 receptor tyrosine kinase is overexpressed in a wide range of solid human tumors. The ErbB-2 gene product is a transmembrane glycoprotein belonging to the epidermal growth factor receptor family, and its cytoplasmic domain is responsible for sending the mitogenic signals into cells. We discovered that this domain of ErbB-2 interacts with Tid1 protein, the human counterpart of the Drosophila tumor suppressor Tid56, whose null mutation causes lethal tumorigenesis during the larval stage. Tid1 also is known as a cochaperone of heat shock protein 70 (HSP70) and binds to HSP70 through its conserved DnaJ domain. We found that increased expression of Tid1 in human mammary carcinomas overexpressing ErbB-2 suppresses the expression level of ErbB-2 and attenuates the resultant ErbB-2-dependent oncogenic extracellular signal-regulated kinase 1/2 and big mitogen-activated protein kinase 1 signaling pathways leading to programmed cell death (PCD). A functional DnaJ domain of Tid1 also is required for its inhibition of ErbB-2 expression and the consequent PCD of carcinoma cells resulting from increased Tid1 expression. Importantly, ErbB-2-dependent tumor progression in animals is inhibited by increased expression of Tid1 in tumor cells. Collectively, these results suggest that Tid1 modulates the uncontrolled proliferation of ErbB-2-overexpressing carcinoma cells by reducing ErbB-2 expression and as a result suppresses the ErbB-2-dependent cancerous signaling and tumor progression. Moreover, the cochaperonic and regulatory functions of Tid1 on HSP70 most likely play an essential role in this antitumor function of Tid1 in carcinoma cells.

Role of the BMK1/ERK5 signaling pathway: lessons from knockout mice

Mitogen-activated protein (MAP) kinase cascades play a central role in mediating extracellular stimuli-induced intracellular signaling during cell activation. The fourth and least studied mammalian MAP kinase pathway, big MAP kinase 1 (BMK1), also known as extracellular signal regulated kinase 5 (ERK5), is activated in response to growth factors and stress. Activation of this signaling pathway has been implicated not only in physiological functions such as cell survival, proliferation and differentiation but also in pathological processes such as carcinogenesis, cardiac hypertrophy and atherosclerosis. In recent years a series of gene-targeted mice lacking components within the BMK1 cascade have been generated, which have enabled us to investigate the role of the BMK1 pathway within different tissues. Analyses of these knockout mice have led to major discoveries in the role of BMK1 signaling in angiogenesis and in cardiac development. Moreover, studies using conditional BMK1 knockout mice, which circumvent the early embryonic lethality of BMK1 knockouts, have unveiled the importance of BMK1 in endothelial survival and maintenance of vascular integrity during adulthood. Here we summarize current understanding of the function of BMK1, as well as include new data generated from a series of tissue-specific BMK1 knockout mice in an attempt to dissect the role of the BMK1 pathway in various cell types in animals.

Regulation of hepatic stellate cell activation and growth by transcription factor myocyte enhancer factor 2

BACKGROUND & AIMS

Hepatic stellate cells (HSCs) undergo activation during the development of liver fibrosis. Transcriptional regulation plays a key role in this process. We studied the role of transcription factor myocyte enhancer factor 2 (MEF2) during HSC activation.

METHODS

Culture of HSCs isolated from rat liver on plastic dishes and HSC-T6 on a basement membrane-like matrix were used as models of HSC activation and deactivation, respectively. The expression and activity of MEF2 were correlated with HSC activation. The roles of MEF2 during HSC activation were assessed in vitro and in vivo by animal models of fibrosis.

RESULTS

Early induction of MEF2 messenger RNA and protein accompanied culture-induced HSC activation. This was associated with enhanced MEF2 DNA binding and transactivation activity. p38 mitogen-activated protein kinase but not extracellular signal-regulated kinase pathway was required for increased MEF2 activity during HSC activation. Increased MEF2 protein also correlated with fibrosis in vivo. Reversal of HSC activation was paralleled by a marked decrease in MEF2 protein and activity. Functionally, enhancing MEF2 significantly increased the expression of alpha-smooth muscle actin (alpha-SMA), activated collagen I promoter activity, and stimulated HSC proliferation. MEF2 interference RNA significantly inhibited expression of alpha-SMA, collagen alpha1(I), and proliferating cell nuclear antigen.

CONCLUSIONS

The studies provide the first evidence for the presence of MEF2 in the liver and show that MEF2 regulates multiple aspects of HSC activation. These studies show a novel role of MEF2 as a key nuclear mediator that may participate in the pathologic process of liver fibrogenesis in vivo.

Aeromonas hydrophila Cytotoxic Enterotoxin Activates Mitogen-activated Protein Kinases and Induces Apoptosis in Murine Macrophages and Human Intestinal Epithelial Cells

A cytotoxic enterotoxin (Act) of Aeromonas hydrophila possesses several biological activities, induces an inflammatory response in the host, and causes apoptosis of murine macrophages. In this study, we utilized five target cell types (a murine macrophage cell line (RAW 264.7), bone marrow-derived transformed macrophages, murine peritoneal macrophages, and two human intestinal epithelial cell lines (T84 and HT-29)) to investigate the effect of Act on mitogen-activated protein kinase (MAPK) pathways and mechanisms leading to apoptosis. As demonstrated by immunoprecipitation/kinase assays or Western blot analysis, Act activated stress-associated p38, c-Jun NH(2)-terminal kinase (JNK), and extracellular signal-regulated kinase 1/2 (ERK1/2) in these cells. Act also induced phosphorylation of upstream MAPK factors (MAPK kinase 3/6 (MKK3/6), MKK4, and MAP/ERK kinase 1 (MEK1)) and downstream effectors (MAPK-activated protein kinase-2, activating transcription factor-2, and c-Jun). Act evoked cell membrane blebbing, caspase 3-cleavage, and activation of caspases 8 and 9 in these cells. In macrophages that do not express functional tumor necrosis factor receptors, apoptosis and caspase activities were significantly decreased. Immunoblotting of host whole cell lysates revealed Act-induced up-regulation of apoptosis-related proteins, including the mitochondrial proteins cytochrome c and apoptosis-inducing factor. However, mitochondrial membrane depolarization was not detected in response to Act. Taken together, the data demonstrated for the first time Act-induced activation of MAPK signaling and classical caspase-associated apoptosis in macrophages and intestinal epithelial cells. Given the importance of MAPK pathways and apoptosis in inflammation-associated diseases, this study provided new insights into the mechanism of action of Act on host cells.

Prolonged extracellular signal-regulated kinase 1/2 activation during fibroblast growth factor 1- or heregulin beta1-induced antiestrogen-resistant growth of breast cancer cells is resistant to mitogen-activated protein/extracellular regulated kinase kinase inhibitors

Increased growth factor receptor signaling is implicated in antiestrogen-resistant breast tumors suggesting that abrogation of such signaling could restore or prolong sensitivity to antihormonal agents. Activation of the mitogen-activated protein/extracellular regulated kinase kinase (MEK)-extracellular regulated kinase (ERK)1/2 cascade is a common component of such pathways. We investigated the ability of the MEK activation inhibitor U0126 to block the increased growth of estrogen receptor-positive MCF-7 breast cancer cells caused by fibroblast growth factor 1 (FGF-1), heregulin beta1 (HRGbeta1), and epidermal growth factor (EGF) in the presence of the pure antiestrogen ICI 182780 (Faslodex; fulvestrant). We found that either FGF-1 or HRGbeta1 but not EGF substantially reduced the inhibitory effects of U0126 on growth and ERK1/2 activation, including the combined inhibitory effects of U0126 and ICI 182780. FGF-1 and HRGbeta1 also reduced the inhibition of ERK1/2 phosphorylation by the MEK inhibitors PD98059 and PD184161. Interestingly, a transiently transfected dominant-negative MEK1 completely abrogated activation of a coexpressed green fluorescent protein-ERK2 reporter by all three of the factors. Despite a short-lived activation of Ras and Raf-1 by all three of the growth factors, both FGF-1 and HRGbeta1, unlike EGF, induced a prolonged activation of MEK and ERK1/2 in these cells. Thus, activation of FGF-1- and HRGbeta1-specific signaling causes MEK-dependent prolonged activation of ERK1/2, which is incompletely susceptible to known MEK inhibitors. We also demonstrate that the cytosolic phospholipase A2 inhibitor arachidonyl trifluoro methyl ketone and the pan PKC inhibitor bisindolymaleimide abrogated U0126-resistant phosphorylation of ERK1/2 induced by HRGbeta1 but not by FGF-1. Phosphorylation of ERK5 by all three of the factors was also resistant to U0126 suggesting that its activation is not sufficient to overturn growth inhibition due to diminished ERK1/2 activation. Therefore, therapy combining antiestrogens and MEK inhibitors may be ineffective in some antiestrogen-resistant estrogen receptor-positive breast cancers.

CARRIE web service: automated transcriptional regulatory network inference and interactive analysis

We present an intuitive and interactive web service for CARRIE (Computational Ascertainment of Regulatory Relationships Inferred from Expression). CARRIE is a computational method that analyzes microarray and promoter sequence data to infer a transcriptional regulatory network from the response to a specific stimulus. This service displays an interactive graph of the inferred network and provides easy access to the evidence for the involvement of each gene in the network. We provide functionality to include network data in KEGG XML (KGML) format in this graph. Our service also provides Gene Ontology annotation to aid the user in forming hypotheses about the role of each gene in the cellular response. The CARRIE web service is freely available at http://zlab.bu.edu/CARRIE-web.

Multiple kinase cascades mediate prolactin signals to activating protein-1 in breast cancer cells

The importance of prolactin (PRL) in physiological proliferation and differentiation of the mammary gland, together with high levels of PRL receptors in breast tumors, the association of circulating PRL with incidence of breast cancer, and the recognition of locally produced PRL, point to the need for greater understanding of PRL actions in mammary disease. Although PRL has been shown to activate multiple kinase cascades in various target cells, relatively little is known of its signaling pathways in the mammary gland apart from the Janus kinase 2/ signal transducer and activator of transcription 5 pathway, particularly in tumor cells. Another potential effector is activating protein-1 (AP-1), a transcription complex that regulates processes essential for neoplastic progression, including proliferation, survival and invasion. We demonstrate that PRL activates AP-1 in MCF-7 cells, detectable at 4 h and sustained for at least 24 h. Although Janus kinase 2 and ERK1/2 are the primary mediators of PRL-induced signals, c-Src, phosphatidylinositol 3'-kinase, protein kinase C, and other MAPKs contribute to maximal activity. PRL activation of these pathways leads to increased c-Jun protein and phosphorylation, JunB protein, and phosphorylation of c-Fos, elevating the levels of AP-1 complexes able to bind DNA. These active AP-1 dimers may direct expression of multiple target genes, mediating some of PRL's actions in mammary disease.

Extracellular signal-regulated kinases regulate dendritic growth in rat sympathetic neurons

NGF activates several signaling cascades in sympathetic neurons. We examined how activation of one of these cascades, the ERK/MAP (extracellular signal-regulated kinase/mitogen-activated protein) kinase pathway, affects dendritic growth in these cells. Dendritic growth was induced by exposure to NGF and BMP-7 (bone morphogenetic protein-7). Exposure to NGF increased phosphorylation of ERK1/2. Unexpectedly, two MEK (MAP kinase kinase) inhibitors (PD 98059 and U 0126) enhanced dendritic growth, and a ligand, basic FGF, that activates the ERK pathway inhibited the growth of these processes. The enhancement of dendritic growth by PD 98059 was associated with an increase in the number of axo-dendritic synapses, and it appeared to represent a specific morphogenic effect because neither axonal growth nor cell survival was affected. In addition, increased dendritic growth was not observed after exposure to inhibitors of other signaling pathways, including the phosphatidylinositol-3-kinase inhibitor LY 294002. Dendritic growth was also increased in cells transfected with dominant-negative mutants of MEK1 and ERK2 but not with dominant-negative mutants of MEK5 and ERK5, suggesting that ERK1/2 is the primary mediator of this effect. Exposure to BMP-7 induces nuclear translocation of Smad1 (Sma- and Mad-related protein 1), and PD 98059 treatment potentiated nuclear accumulation of Smad-1 induced by BMP-7 in sympathetic neurons, suggesting a direct enhancement of BMP signaling in cells treated with an MEK inhibitor. These observations indicate that one of the signaling cascades activated by NGF can act in an antagonistic manner in sympathetic neurons and reduce the dendritic growth induced by other NGF-sensitive pathways.

MEF2 activation in differentiated primary human skeletal muscle cultures requires coordinated involvement of parallel pathways

The myocyte enhancer factor (MEF)2 transcription factor is important for development of differentiated skeletal muscle. We investigated the regulation of MEF2 DNA binding in differentiated primary human skeletal muscle cells and isolated rat skeletal muscle after exposure to various stimuli. MEF2 DNA binding activity in nonstimulated (basal) muscle cultures was almost undetectable. Exposure of cells for 20 min to 120 nM insulin, 0.1 and 1.0 mM hydrogen peroxide, osmotic stress (400 mM mannitol), or 1.0 mM 5-aminoimidazole-4-carboxamide-1-β- d-ribofuranoside (AICAR) led to a profound increase in MEF2 DNA binding. To study signaling pathways mediating MEF2 activity, we preincubated human skeletal muscle cell cultures or isolated rat epitrochlearis muscles with inhibitors of p38 mitogen-activated protein kinase (MAPK) (10 μM SB-203580), MEK1 (50 μM PD-98059), PKC (1 and 10 μM GF109203X), phosphatidylinositol (PI) 3-kinase (10 μM LY-294002), or AMP-activated protein kinase (AMPK; 20 μM compound C). All stimuli resulted primarily in activation of MEF2D DNA binding. Exposure of cells to osmotic or oxidative stress increased MEF2 DNA binding via pathways that were completely blocked by MAPK inhibitors and partially blocked by inhibitors of PKC, PI 3-kinase, and AMPK. In epitrochlearis muscle, MAPK inhibitors blocked contraction but not AICAR-mediated MEF2 DNA binding. Thus activation of MEF2 in skeletal muscle is regulated via parallel intracellular signaling pathways in response to insulin, cellular stress, or activation of AMPK.

Role of extracellular signal regulated kinase 5 in neuronal survival

The newest member of the mitogen activated protein (MAP) kinase family of proteins, extracellular signal regulated kinase 5 (ERK5; also known as big-mitogen activated kinase 1 or BMK1) is widely expressed in many tissues including the brain. Although growth factor activation of ERK5 in non-neuronal cells has been shown to contribute to cell proliferation, differentiation and transformation, until recently no information was available on the role of ERK5 in neuronal survival. Recent data suggests that ERK5 is activated by neurotrophic factors in primary neuronal cells and plays an important role in neurotrophin mediated neuronal survival. These data also suggest that the mechanism of ERK5-mediated survival involves transcriptional regulation.

Mitogen-activated protein kinases in apoptosis regulation

Cells are continuously exposed to a variety of environmental stresses and have to decide 'to be or not to be' depending on the types and strength of stress. Among the many signaling pathways that respond to stress, mitogen-activated protein kinase (MAPK) family members are crucial for the maintenance of cells. Three subfamilies of MAPKs have been identified: extracellular signal-regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs), and p38-MAPKs. It has been originally shown that ERKs are important for cell survival, whereas JNKs and p38-MAPKs were deemed stress responsive and thus involved in apoptosis. However, the regulation of apoptosis by MAPKs is more complex than initially thought and often controversial. In this review, we discuss MAPKs in apoptosis regulation with attention to mouse genetic models and critically point out the multiple roles of MAPKs.

Targeted deletion of BMK1/ERK5 in adult mice perturbs vascular integrity and leads to endothelial failure

Big mitogen-activated protein kinase 1 (BMK1), also known as ERK5, is a member of the MAPK family. Genetic ablation of BMK1 in mice leads to embryonic lethality, precluding the exploration of pathophysiological roles of BMK1 in adult mice. We generated a BMK1 conditional mutation in mice in which disruption of the BMK1 gene is under the control of the inducible Mx1-Cre transgene. Ablation of BMK1 in adult mice led to lethality within 2-4 weeks after the induction of Cre recombinase. Physiological analysis showed that the blood vessels became abnormally leaky after deletion of the BMK1 gene. Histological analysis revealed that, after BMK1 ablation, hemorrhages occurred in multiple organs in which endothelial cells lining the blood vessels became round, irregularly aligned, and, eventually, apoptotic. In vitro removal of BMK1 protein also led to the death of endothelial cells partially due to the deregulation of transcriptional factor MEF2C, which is a direct substrate of BMK1. Additionally, endothelial-specific BMK1-KO leads to cardiovascular defects identical to that of global BMK1-KO mutants, whereas, surprisingly, mice lacking BMK1 in cardiomyocytes developed to term without any apparent defects. Taken together, the data provide direct genetic evidence that the BMK1 pathway is critical for endothelial function and for maintaining blood vessel integrity.

BMK1 is activated in glomeruli of diabetic rats and in mesangial cells by high glucose conditions

BACKGROUND

High glucose causes renal cell injury through various signal transduction pathways, including mitogen-activated protein (MAP) kinases cascades. Big MAP kinase 1 (BMK1), also known as extracellular signal-regulated kinase 5 (ERK5), is a recently identified MAP kinase family member and was reported to be sensitive to osmotic and oxidative stress. However, the role of BMK1 in diabetic nephropathy has not been elucidated yet.

METHODS

We investigated whether BMK1 is activated in the glomeruli of Otsuka Long Evans Tokushima Fatty (OLETF) rats, a model of type 2 diabetes mellitus in comparison with the control Long Evans Tokushima Otsuka (LETO) rats. We also examined the effect of high glucose on BMK1 activity in cultured rat mesangial cells.

RESULTS

BMK1 and ERK1/2 but not p38 were activated in the glomeruli of OLETF rats, which showed diabetic nephropathy at 52 weeks of age. High glucose, in addition to a high concentration of raffinose, caused rapid and significant activation of BMK1 in rat mesangial cells. MAP kinase/ERK kinase (MEK) inhibitors, U0126 and PD98059, both inhibited BMK1 activation by high glucose in a concentration-dependent manner. Protein kinase C (PKC) inhibition by GF109203X and PKC down-regulation with long-time phorbol myristate acetate (PMA) treatment both inhibited BMK1 and Src kinase activation. Src kinase inhibitors, herbimycin A and PP2, also inhibited high glucose-induced BMK1 activation. PKC inhibitors, Src inhibitors and MEK inhibitors, all inhibited cell proliferation by high glucose. Finally, transfection of dominant-negative MEK5, which is an upstream regulator of BMK1, abolished the BMK1-mediated rat mesangial cell proliferation stimulated by high glucose.

CONCLUSION

In the present study, we demonstrated that high glucose activates BMK1 both in vivo and in vitro. It was suggested that high glucose induces PKC- and c-Src-dependent BMK1 activation. It could not be denied that BMK1 activation is induced through an osmotic stress-sensitive mechanism. BMK1-mediated mesangial cell growth may be involved in the pathogenesis of diabetic nephropathy.

MEK/MAPK as a signaling element in ATP control of endothelial myosin light chain

Phosphorylation of endothelial myosin light chains (MLC) is a key mechanism in control of endothelial contractile machinery. Extracellular ATP influences endothelial MLC phosphorylation by either activation of Ca(2+)-dependent MLC kinase or Ca(2+)-independent MLC phosphatase. Here, the role of the MEK/MAPK pathway in this signaling was investigated in porcine aortic endothelial cells. Phosphorylation of ERK2 and phosphorylation of MLC were analyzed in cultured aortic endothelial cells. ATP (10 microM) increased ERK2 phosphorylation from basal 17 +/- 3 to 53 +/- 4%, an effect suppressed in the presence of the MEK inhibitors PD-98059 (20 microM) or U0126 (10 microM). Phosphorylation of ERK2 was not dependent on the ATP-induced cytosolic Ca(2+) rise, because it was unaltered when this was suppressed by the Ca(2+) chelator BAPTA (10 microM) or xestospongin C (3 microM), an inhibitor of the inositol 1,4,5-trisphosphate-sensitive Ca(2+) release mechanism of the endoplasmic reticulum. Phosphorylation of ERK2 was neither induced by the adenosine analog 5'-(N-ethylcarboxamido)adenosine (1 microM) nor inhibited in the presence of the adenosine receptor antagonist 8-phenyltheophylline (10 microM). ATP increased MLC kinase activity, and this was blocked in presence of PD-98059. ATP also increased MLC phosphatase activity, which was not inhibited by PD-98059. The MEK/MAPK pathway is a Ca(2+)-independent part of ATP signaling toward MLC kinase but not of ATP signaling toward MLC phosphatase.

MEKK2 regulates the coordinate activation of ERK5 and JNK in response to FGF-2 in fibroblasts

Mitogen-activated protein kinases (MAPKs) are regulated by MAPK kinases (MKKs), which are in turn regulated by MKK kinases (MKKKs). While a single MKKK can regulate several different MAPK family members, and several MKKKs can often activate the same MAPK, emerging evidence indicates a unique role for individual MKKKs in acting as signaling nodes to coordinately activate different subsets of MAPKs in response to specific cellular stimuli. Thus, while there is much apparent overlap in MAPK regulation by different MKKKs, each MKKK serves a specific purpose in regulation of unique cellular functions. The purpose of this study was to define the specific role of MEKK2, an MKKK, in MAPK regulation and cell function. MEKK2 coordinately activates the ERK5 and JNK pathways. Targeted disruption of MEKK2 expression causes loss of ERK5 and JNK activation in response to FGF-2 in mouse embryonic fibroblasts (MEFs). FGF-2 receptor signaling requires MEKK2 for induction of mRNA for c-Jun, Fra-1, and Fra-2, components of the AP-1 transcription complex. In FGF-2-stimulated MEKK2-/- fibroblasts, c-Jun phosphorylation is inhibited, consistent with a loss of JNK activation. Thus, MEKK2 regulates AP-1 activity at two levels, by regulating both expression of AP-1 components and c-Jun N-terminal phosphorylation. One function of the AP-1 transcription complex is to regulate cytokine gene expression. Expression of IL-1alpha, IL-1beta, IL-6, and TNFalpha is inhibited in MEKK2-/- fibroblasts. Bacterial lipopolysaccharide (LPS) and TNFalpha neither activate ERK5 nor require MEKK2 for JNK activation, demonstrating specificity of MEKK2 in FGF-2 receptor signaling and control of cytokine gene expression.

MEK5 and ERK5 are localized in the nuclei of resting as well as stimulated cells, while MEKK2 translocates from the cytosol to the nucleus upon stimulation

The ERK5 signaling cascade acts through sequential activation of MEKK2/3, MEK5 and ERK5 and transmits signals to a variety of stress and mitogenic related targets. In this study we examined the subcellular localization of the components of the ERK5 cascade and found that in resting, as well as in EGF-stimulated HeLa and Rat-1 cells, endogenous ERK5 is localized mainly in the nucleus. This location is different from the previously described location of exogenous ERK5, in the cytosol of resting cells, which is confirmed in this study. The reason for the different localization could be a saturation of anchoring moieties by the endogenous ERK5. Indeed, in situ detergent extraction analysis using Nonidet P-40, revealed that ERK5 is bound to detergent resistant moieties in the nucleus, while the exogenous protein fails to interact with those anchors. The upstream activator MEK5 is also localized in the nucleus both before and after EGF stimulation and is resistant to NP-40 extraction in resting cells. ERK5 remains bound to these nuclear moieties even after stimulation, while MEK5 is detached from the anchors but remains localized in the nucleus. Unlike ERK5 and MEK5, their upstream activator MEKK2 is localized mainly in the cytosol of resting cells, and translocates into the nucleus upon EGF stimulation, allowing transmission of signals to the nuclear MEK5. The nuclear localization of MEK5 and ERK5 is different from that of ERK1/2 and MEK1/2 in resting cells, indicating that each MAPK cascade uses distinct mechanisms to transmit extracellular signals to their nuclear targets.

Src-dependent ERK5 and Src/EGFR-dependent ERK1/2 activation is required for cell proliferation by asbestos

Crocidolite asbestos elicits oxidative stress and cell proliferation, but the signaling cascades linked to these outcomes are unclear. To determine the role of mitogen-activated protein kinases (MAPK) in asbestos-induced cell signaling, we evaluated the effects of crocidolite asbestos, EGF and H2O2, on MAPK activation in murine lung epithelial cells (C10 line). In contrast to rapid and transient activation of extracellular signal-regulated kinase 5 (ERK5) by EGF or H2O2, asbestos caused protracted oxidant-dependent ERK5 activation that was inhibited by an Src kinase inhibitor (PP2), but not by an inhibitor of epidermal growth factor receptor (EGFR) phosphorylation (AG1478). ERK1/2 activation by asbestos was inhibited by either PP2 or AG1478. To confirm the involvement of Src in ERK1/2 and ERK5 activation, a dominant-negative Src construct was used. These experiments showed that Src was essential for ERK1/2 and also ERK5 phosphorylation by asbestos. Time frame studies indicated immediate activation of Src by asbestos fibers, whereas EGFR phosphorylation occurred subsequently. Data suggest that asbestos causes activation of ERK5 through an EGFR-independent pathway, whereas ERK1/2 activation is dependent on Src through a mechanism involving phosphorylation of the EGFR. Furthermore, Src, ERK1/2 and ERK5 activation are essential for cell proliferation by asbestos. The use of a dominant-negative ERK5 construct caused selective downregulation of c-jun expression, whereas inhibition of Src by PP2 or MEK1 by PD98059 caused decreases in c-fos, fra-1 and c-jun expression in asbestos-exposed C10 cells. These observations may have broad relevance to cell proliferation by carcinogenic mineral fibers and oxidants.

Big Mitogen-Activated Protein Kinase (BMK1)/ERK5 Protects Endothelial Cells From Apoptosis

Blood flow that is steady and laminar is known to be atheroprotective. One likely mechanism is enhanced endothelial cell (EC) survival. Because the mitogen-activated protein kinases (MAPKs) are known regulators of cell survival, we investigated the role of Big MAPK-1 (BMK1 or ERK5), which is potently stimulated by fluid shear stress. To activate BMK1, we overexpressed constitutively active (CA)-MEK5 in bovine lung microvascular ECs (BLMECs). Cell apoptosis was induced by growth factor deprivation (0% serum for 24 hours). Analysis of cell viability with MTT assay showed that activation of BMK1 by CA-MEK5 significantly improved cell viability from 48% to 87% and decreased apoptotic cells from 49% to 10%. Growth factor deprivation induced caspase-3 activity 5.2-fold, which was inhibited (approximately 60%) by CA-MEK5 overexpression. In contrast, inhibiting BMK1 activity by overexpressing dominant-negative BMK1 (DN-BMK1) stimulated apoptosis in BLMECs. Steady laminar fluid shear stress inhibited BLMEC apoptosis, and this protective effect was also reduced significantly by overexpressing DN-BMK1. Analysis of antiapoptotic mechanisms showed that both shear stress and CA-MEK5 stimulated phosphorylation of Bad on Ser112 and Ser136, whereas DN-BMK1 inhibited phosphorylation. Phosphorylation of Bad induced by BMK1 activation was independent of Akt, PKA, or p90RSK kinase activity. These results suggest that BMK1 activation by steady laminar flow is atheroprotective by inhibiting EC apoptosis via phosphorylation of Bad.

In vivo heat-shock response in the brain: signalling pathway and transcription factor activation

We analysed the expression of the hsp70 gene, the phosphorylation status of different members of the mitogen-activated protein kinase (MAPK) family, the behaviour of the Akt-GSK3 pathway, as well as the DNA-binding activity of several transcription factors, potential targets of these kinases, in the brain of rats exposed to a fever-like increase in body temperature. Two different brain regions, the cerebellum and the hippocampus, were studied. Hyperthermia caused HSF activation and the induction of hsp70 mRNA and protein to a greater extent in the cerebellum than in the hippocampus. In the cerebellum, ERK1/2 and p38 MAPK phosphorylation were increased by hyperthermia and returned to basal levels during the recovery from heat stress, whereas JNK3 phosphorylation decreased and recovered to above control levels within 60 min of recovery. JNK1 phosphorylation was never modified. In the hippocampus, ERK phosphorylation did not increase but rather decreased, whereas the behaviour of p38 MAPK and JNK was similar to that observed in the cerebellum. Akt phosphorylation increased after hyperthermia and was accompanied by an increased phosphorylation of two substrates, GSK3 and FKHRL1, in both brain areas, with a major effect in the cerebellum. DNA-binding activities of AP-1, NF-kappaB, and MEF2 were activated by heat shock in the cerebellum, whereas only MEF2 was activated in the hippocampus. Our data indicate that a physiologically relevant increase in body temperature induces brain injury and survival response to it as demonstrated by induction of hsp70 gene expression and activation of specific signalling pathways. Reprogramming of gene expression, by the specific transcription factors activated, probably plays a central role in cell adaptation and survival to heat stress. The hippocampus shows less responsiveness to hyperthermia than the cerebellum.

Activation of ERK5 is mediated by N-methyl-d-aspartate receptor and L-type voltage-gated calcium channel via Src involving oxidative stress after cerebral ischemia in rat hippocampus

Activation (phosphorylation) and the possible mechanism of extracellular signal-regulated kinase 5 (ERK5) were evaluated after cerebral ischemia-reperfusion (I/R) in the hippocampus in a four-vessel occlusion model of Sprague-Dawley rats. Western blotting showed that ERK5 was strongly activated from 10 min to 1 day and peaked at 30 min of reperfusion after 15 min ischemia. Pretreatment with N-acetylcysteine, a free radical scavenger, effectively inhibited ERK5 activation in a dose-dependent manner. Consistently, ERK5 activation was significantly suppressed by genistein (protein-tyrosine kinase inhibitor), PP2 (specific inhibitor of Src family kinases), nifedipine (L-VGCC blocker) and dextromethorphan (NMDA receptor antagonist), but not 6,7-dinitroquinoxaline-2, 3(1H, 4H)-dione (AMPA receptor antagonist). These results suggested that ERK5 could be significantly activated by I/R, which might be mediated by NMDA receptor and L-VGCC through Src kinase pathway involving oxidative stress in rat hippocampus.

Knockout of ERK5 causes multiple defects in placental and embryonic development

Backgroud

ERK5 is a member of the mitogen activated protein kinase family activated by certain mitogenic or stressful stimuli in cells, but whose physiological role is largely unclear.

Results

To help determine the function of ERK5 we have used gene targeting to inactivate this gene in mice. Here we report that ERK5 knockout mice die at approximately E10.5. In situ hybridisation for ERK5, and its upstream activator MKK5, showed strong expression in the head and trunk of the embryo at this stage of development. Between E9.5 and E10.5, multiple developmental problems are seen in the ERK5-/- embryos, including an increase in apoptosis in the cephalic mesenchyme tissue, abnormalities in the hind gut, as well as problems in vascular remodelling, cardiac development and placental defects.

Conclusion

Erk5 is essential for early embryonic development, and is required for normal development of the vascular system and cell survival.

WNK1 activates ERK5 by an MEKK2/3-dependent mechanism

WNK1 belongs to a unique protein kinase family that lacks the catalytic lysine in its normal position. Mutations in human WNK1 and WNK4 have been implicated in causing a familial form of hypertension. Here we report that overexpression of WNK1 led to increased activity of cotransfected ERK5 in HEK293 cells. ERK5 activation was blocked by the MEK5 inhibitor U0126 and expression of a dominant negative MEK5 mutant. Expression of dominant negative mutants of MEKK2 and MEKK3 also blocked activation of ERK5 by WNK1. Moreover, both MEKK2 and MEKK3 coimmunoprecipitated with endogenous WNK1 from cell lysates. WNK1 phosphorylated both MEKK2 and -3 in vitro, and MEKK3 was activated by WNK1 in 293 cells. Finally, ERK5 activation by epidermal growth factor was attenuated by suppression of WNK1 expression using small interfering RNA. Taken together, these results place WNK1 in the ERK5 MAP kinase pathway upstream of MEKK2/3.

14-3-3beta binds to big mitogen-activated protein kinase 1 (BMK1/ERK5) and regulates BMK1 function

Big mitogen-activated kinase 1 (BMK1/ERK5) is a member of the MAPK family activated by growth factors that mediates cell growth and survival. Previous data show that BMK1 can be activated by steady laminar flow and is atheroprotective by preventing endothelial cells from undergoing apoptosis. The primary structure of BMK1 is distinct from other MAPK members by virtue of a unique long C-tail, suggesting specific mechanisms of regulation. To characterize regulatory mechanisms for BMK1 function, we identified binding proteins by yeast two-hybrid analysis. Among these proteins, the scaffolding protein 14-3-3 was identified. BMK1 bound to 14-3-3beta in vitro and in vivo as demonstrated by glutathione S-transferase (GST)-14-3-3beta fusion protein pull-down assays and coimmunoprecipitation. Phosphorylation of BMK1 was most likely required for this interaction. GST-14-3-3beta pull-down assays using truncated constructs of BMK1 and site-directed BMK1 mutants demonstrated that the interaction requires serine 486 within the C terminus of BMK1. BMK1 bound to 14-3-3beta basally, and the interaction was greatly abrogated when BMK1 was activated. The interaction of 14-3-3beta and BMK1 inhibited kinase activities stimulated by constitutively active (CA)-MEK5 and epidermal growth factor. Mutation of serine 486 (BMK1-S486A) prevented the interaction with 14-3-3beta and enhanced BMK1 activity upon epidermal growth factor stimulation. These data demonstrate an inhibitory function for 14-3-3beta binding to BMK1 and show that serine 486 phosphorylation represents a novel regulatory mechanism for BMK1.

Thrombin protease-activated receptor-1 signals through Gq- and G13-initiated MAPK cascades regulating c-Jun expression to induce cell transformation

Although the ability of G protein-coupled receptors to stimulate normal and aberrant cell growth has been intensely investigated, the precise nature of the molecular mechanisms underlying their transforming potential are still not fully understood. In this study, we have taken advantage of the potent mitogenic effect of thrombin and the focus-forming activity of one of its receptors, protease-activated receptor-1, to dissect how this receptor coupled to Galphai, Galphaq/11, and Galpha12/13 transduces signals from the membrane to the nucleus to initiate transcriptional events involved in cell transformation. Using endogenous and transfected thrombin receptors in NIH 3T3 cells, ectopic expression of muscarinic receptors coupled to Galphaq and Galphai, and chimeric G protein alpha subunits and murine fibroblasts deficient in Galphaq/11, and Galpha12/13, we show here that, although coupling to Galphai is sufficient to induce ERK activation, the ability to couple to Galphaq and/or Galpha13 is necessary to induce c-jun expression and cell transformation. Furthermore, we show that Galphaq and Galpha13 can initiate the activation of MAPK cascades, including JNK, p38, and ERK5, which in turn regulate the activity of transcription factors controlling expression from the c-jun promoter. We also present evidence that c-Jun and the kinases regulating its expression are integral components of the transforming pathway initiated by protease-activated receptor-1.

Transcriptional regulation by the MAP kinase signaling cascades

A major function of the mitogen-activated protein kinase (MAPK) pathways is to control eukaryotic gene expression programmes in response to extracellular signals. MAPKs directly control gene expression by phosphorylating transcription factors. However, it is becoming clear that transcriptional regulation in response to MAPK signaling is more complex. MAPKs can also target coactivators and corepressors and affect nucleosomal structure by inducing histone modifications. Furthermore, multiple inputs into individual promoters can be elicited by MAPKs by targeting different components of the same coregulatory complex or by triggering different events on the same transcription factor. "Postgenomic approaches" are beginning to impact on our understanding of these gene regulatory networks. In this review, we summarise the current knowledge of MAPK-mediated gene regulation, and focus on how complexities in signaling outcomes are achieved and how this relates to physiological processes.

Differential role of MEK5alpha and MEK5beta in BMK1/ERK5 activation

Big mitogen-activated protein kinase 1/extracellular-regulated kinase 5 (BMK1/ERK5) is regulated sequentially by a series of upstream MAP kinase kinases (MEKs) in a signaling cascade. MEKs activate their downstream MAPK by phosphorylation of threonine and tyrosine in the T- X-Y motif. MEK5 is the upstream BMK1 kinase and exists as naturally occurring splice variants, MEK5alpha and MEK5beta. The full-length MEK5 (MEK5alpha) is 89 amino acids longer than MEK5beta at the N terminus, but the precise functional difference between the two splice variants is not known. Dual phosphorylation site mutation of MEK5alpha (Ser-311 --> Asp and Thr- 315 --> Asp; MEK5alpha(S311D/T315D)) activated BMK1, but the corresponding dual phosphorylation sites mutant of MEK5beta could not induce BMK1 kinase activation or nuclear translocation. Furthermore, MEK5beta inhibited epidermal growth factor-induced BMK1 activation and MEK5alpha(S311D/T315D)-induced MEF2 transcriptional activity. Both MEK5alpha and MEK5beta individually co-immunoprecipitated with BMK1, but the presence of MEK5beta prevented association of MEK5alpha with BMK1 suggesting a mechanistic basis for the dominant-negative behavior of MEK5beta on BMK1 activation. The ratio of MEK5alpha to MEK5beta expression was higher in cancer cell lines, and overexpression of MEK5beta-inhibited serum-induced DNA synthesis. These data suggest that alternative splicing of MEK5alpha and MEK5beta may play a critical role in BMK1 activation and subsequent cell proliferation.

The oncogene Nup98-HOXA9 induces gene transcription in myeloid cells

The nucleoporin Nup98 gene is frequently rearranged in acute myelogenous leukemia (AML). In most cases this results in fusion of the N terminus of Nup98 to the DNA binding domain of a homeodomain transcription factor. The prototype of these fusions, Nup98-HOXA9, is associated with human AML and induces AML in mouse models. To understand the mechanisms by which Nup98-HOXA9 causes AML, we expressed it in myeloid cells and identified its target genes using high density oligonucleotide microarrays. The analysis was performed in triplicate and was confirmed by quantitative real time PCR. Of the 102 Nup98-HOXA9 target genes identified, 92 were up-regulated, and only 10 were down-regulated, suggesting a transcriptional activation function. A similar analysis of wild-type HOXA9 revealed 13 target genes, 12 of which were up-regulated, and 1 was down-regulated. In contrast, wild-type Nup98 had no effect on gene expression, demonstrating that the HOXA9 DNA binding domain is required for gene regulation. Co-transfection experiments using a luciferase reporter linked to the promoter of one of the Nup98-HOXA9 target genes confirmed up-regulation at the transcriptional level by Nup98-HOXA9 but not by either HOXA9 or Nup98. These data indicate that Nup98-HOXA9 is an aberrant transcription factor whose activity depends on the HOXA9 DNA binding domain but has a stronger and wider transcriptional effect than HOXA9. Several of the genes regulated by Nup98-HOXA9 are associated with increased cell proliferation and survival as well as drug metabolism, providing insights into the pathogenesis and epidemiology of Nup98-HOXA9-induced AML.

PB1 domains of MEKK2 and MEKK3 interact with the MEK5 PB1 domain for activation of the ERK5 pathway

MEKK2 and MEKK3 are MAPK kinase kinases that activate the ERK5 pathway by phosphorylating and activating the MAPK kinase, MEK5. Activated MEK5 then phosphorylates and activates ERK5. PB1 domains were first defined in the p67phox and Bem1p proteins and have been shown to mediate protein-protein heterodimerization. A PB1 domain is encoded within the N-terminal portion of MEKK2, MEKK3, and MEK5. Herein, we analyze the functional role of MEKK2, MEKK3, and MEK5 PB1 domains in the ERK5 activation pathway. The PB1 domains of MEKK2 and MEKK3 bind the PB1 domain of MEK5 but do not significantly homo- or heterodimerize with one another in vitro. Furthermore, co-immunoprecipitation of MEKK2 and MEK5 from cell lysates shows that they form a complex in vivo. Deletion or mutation of the MEKK2 PB1 domain abolishes MEKK2-MEK5 complexes, demonstrating that the PB1 domain interaction is required for MEKK2-MEK5 interactions. Expression in cells of the MEKK2 or MEKK3 PB1 domain inhibits ERK5 activation, whereas expression of a mutant MEKK2 unable to bind the MEK5 PB1 domain or expression of the p67phox PB1 domain has no effect on ERK5 activation. These findings demonstrate that the PB1 domain mediates the association of MEKK2 and MEKK3 with MEK5 and that the respective PB1 domains of these kinases are critical for regulation of the ERK5 pathway. The free PB1 domain of MEKK2 or MEKK3 functions effectively to inhibit the ERK5 pathway but not the p38 or JNK pathways, demonstrating the specific and unique requirement of the MEKK2 and MEKK3 PB1 domain in regulating ERK5 activation.

hMusTRD1alpha1 represses MEF2 activation of the troponin I slow enhancer

The novel transcription factor hMusTRD1alpha1 (human muscle TFII-I repeat domain-containing protein 1alpha1; previously named MusTRD1; O'Mahoney, J. V., Guven, K. L., Lin, J., Joya, J. E., Robinson, C. S., Wade, R. P., and Hardeman, E. C. (1998) Mol. Cell. Biol. 18, 6641-6652) was identified in a yeast one-hybrid screen as a protein that binds within an upstream enhancer-containing region of the skeletal muscle-specific gene, TNNI1 (human troponin I slow; hTnIslow). It has been proposed that hMusTRD1alpha1 may play an important role in fiber-specific muscle gene expression by virtue of its ability to bind to an Inr-like element (nucleotides -977 to -960) within the hTnIslow upstream enhancer-containing region that is necessary for slow fiber-specific expression. In this study we demonstrate that both MEF2C, a known regulator of slow fiber-specific genes, and hMusTRD1alpha1 regulate hTnIslow through the Inr-like element. Co-transfection assays in C2C12 cells and Cos-7 cells demonstrate that hMusTRD1alpha1 represses hTnIslow transcription and prevents MEF2C-mediated activation of hTnIslow transcription. Gel shift analysis shows that hMusTRD1alpha1 can abrogate MEF2C binding to its cognate site in the hTnIslow enhancer. Glutathione S-transferase pull-down assays demonstrate that hMusTRD1alpha1 can interact with both MEF2C and the nuclear receptor co-repressor. The data support the role of hMusTRD1alpha1 as a repressor of slow fiber-specific transcription through mechanisms involving direct interactions with MEF2C and the nuclear receptor co-repressor.

Interaction codes within the family of mammalian Phox and Bem1p domain-containing proteins

The Phox and Bem1p (PB1) domain constitutes a recently recognized protein-protein interaction domain found in the atypical protein kinase C (aPKC) isoenzymes, lambda/iota- and zeta PKC; members of mitogen-activated protein kinase (MAPK) modules like MEK5, MEKK2, and MEKK3; and in several scaffold proteins involved in cellular signaling. Among the last group, p62 and Par6 (partitioning-defective 6) are involved in coupling the aPKCs to signaling pathways involved in cell survival, growth control, and cell polarity. By mutation analyses and molecular modeling, we have identified critical residues at the interaction surfaces of the PB1 domains of aPKCs and p62. A basic charge cluster interacts with an acidic loop and helix both in p62 oligomerization and in the aPKC-p62 interaction. Subsequently, we determined the abilities of mammalian PB1 domain proteins to form heteromeric and homomeric complexes mediated by this domain. We report several novel interactions within this family. An interaction between the cell polarity scaffold protein Par6 and MEK5 was found. Furthermore, p62 interacts both with MEK5 and NBR1 in addition to the aPKCs. Evidence for involvement of p62 in MEK5-ERK5 signaling is presented.

MAPK pathways in radiation responses

Within the last 15 years, multiple new signal transduction pathways within cells have been discovered. Many of these pathways belong to what is now termed 'the mitogen-activated protein kinase (MAPK) superfamily.' These pathways have been linked to the growth factor-mediated regulation of diverse cellular events such as proliferation, senescence, differentiation and apoptosis. Based on currently available data, exposure of cells to ionizing radiation and a variety of other toxic stresses induces simultaneous compensatory activation of multiple MAPK pathways. These signals play critical roles in controlling cell survival and repopulation effects following irradiation, in a cell-type-dependent manner. Some of the signaling pathways activated following radiation exposure are those normally activated by mitogens, such as the 'classical' MAPK (also known as the ERK) pathway. Other MAPK pathways activated by radiation include those downstream of death receptors and procaspases, and DNA-damage signals, including the JNK and P38 MAPK pathways. The expression and release of autocrine growth factor ligands, such as (transforming growth factor alpha) and TNF-alpha, following irradiation can also enhance the responses of MAPK pathways in cells and, consequently, of bystander cells. Thus, the ability of radiation to activate MAPK signaling pathways may depend on the expression of multiple growth factor receptors, autocrine factors and Ras mutation. Enhanced basal signaling by proto-oncogenes such as K-/H-/N-RAS may provide a radioprotective and growth-promoting signal. In many cell types, this may be via the PI3K pathway; in others, this may occur through nuclear factor-kappa B or multiple MAPK pathways. This review will describe the enzymes within the known MAPK signaling pathways and discuss their activation and roles in cellular radiation responses.

Activation of cyclin D1 expression by the ERK5 cascade

Transcriptional activation of the cyclin D1 gene is a key step in cell proliferation. Accordingly, cyclin D1 overexpression is frequently an early step in neoplastic transformation, particularly in mammary epithelium. Numerous studies have linked elevated cyclin D1 promoter activity to a sustained activation of the ERK1/2 cascade. Here we show that the ERK5 cascade, a distinct mitogen-induced MAPK pathway, can also drive cyclin D1 expression. In CCL39 cells, serum induces a strong, prolonged peak of ERK1/2 and ERK5 phosphorylation, and subsequently elevates cyclin D1 mRNA and protein levels. Overexpression of constitutively active MEK5 and wt ERK5 induces a cyclin D1 reporter gene (D1 -973-luciferase) at least as well as constitutively active MEK1. Activation is blocked by kinase-dead mutants of ERK5 and ERK2, respectively. Mutation of the CRE at -50 in the cyclin D1 promoter decreases activation by the ERK5 but not the ERK1/2 cascade. Importantly, expression of kinase-dead ERK5 diminishes endogenous cyclin D1 protein induction by serum in CCL39 cells and the breast cancer cell lines MCF-7 and HS579. These data identify the cyclin D1 gene as a novel target of the ERK5 cascade, an observation with important implications in cancers involving cyclin D1 deregulation.

Activation of gp130 transduces hypertrophic signal through interaction of scaffolding/docking protein Gab1 with tyrosine phosphatase SHP2 in cardiomyocytes

Grb2-associated binder-1 (Gab1) is a scaffolding/docking protein and contains a Pleckstrin homology domain and potential binding sites for Src homology (SH) 2 and SH3 domains. Gab1 is tyrosine phosphorylated and associates with protein tyrosine phosphatase SHP2 and p85 phosphatidylinositol 3-kinase on stimulation with various cytokines and growth factors, including interleukin-6. We previously demonstrated that interleukin-6-related cytokine, leukemia inhibitory factor (LIF), induced cardiac hypertrophy through gp130. In this study, we report the role of Gab1 in gp130-mediated cardiac hypertrophy. Stimulation with LIF induced tyrosine phosphorylation of Gab1, and phosphorylated Gab1 interacted with SHP2 and p85 in cultured cardiomyocytes. We constructed three kinds of adenovirus vectors, those carrying wild-type Gab1 (AdGab1WT), mutated Gab1 lacking SHP2 binding site (AdGab1F627/659), and beta-galactosidase (Adbeta-gal). Compared with cardiomyocytes infected with Adbeta-gal, longitudinal elongation of cardiomyocytes induced by LIF was enhanced in cardiomyocytes infected with AdGab1WT but inhibited in cardiomyocytes infected with AdGab1F627/659. Upregulation of BNP mRNA expression by LIF was evoked in cardiomyocytes infected with Adbeta-gal and AdGab1WT but not in cardiomyocytes infected with AdGab1F627/659. In contrast, Gab1 repressed skeletal alpha-actin mRNA expression through interaction with SHP2. Furthermore, activation of extracellular signal-regulated kinase 5 (ERK5) was enhanced in cardiomyocytes infected with AdGab1WT compared with cardiomyocytes infected with Adbeta-gal but repressed in cardiomyocytes infected with AdGab1F627/659. Coinfection of AdGab1WT with adenovirus vector carrying dominant-negative ERK5 abrogated longitudinal elongation of cardiomyocytes induced by LIF. Taken together, these findings indicate that Gab1-SHP2 interaction plays a crucial role in gp130-dependent longitudinal elongation of cardiomyoctes through activation of ERK5.

ERK5 activation of MEF2-mediated gene expression plays a critical role in BDNF-promoted survival of developing but not mature cortical neurons

Extracellular signal-regulated kinase 5 (ERK5) is a member of the mitogen-activated protein kinase family whose biological function in the CNS has not been defined. In contrast to ERK1 and ERK2, which are activated by neurotrophins (NTs), cAMP, and neuronal activity in cortical neurons, ERK5 is activated only by NTs. Here, we report that ERK5 expression is high in the brain during early embryonic development but declines as the brain matures to almost undetectable levels by postnatal day (P) 49. Interestingly, expression of a dominant-negative ERK5 blocked brain-derived neurotrophic factor protection against trophic withdrawal in primary cortical neurons cultured from embryonic day (E) 17 but not P0. Furthermore, expression of a dominant-negative ERK5 induced apoptosis in E17 but not P0 cortical neurons maintained in the presence of serum. We also present evidence that ERK5 protection of E17 cortical neurons may be mediated through myocyte enhancer factor 2-induced gene expression. These data suggest that ERK5 activation of myocyte enhancer factor 2-induced gene expression may play an important and novel role in the development of the CNS by mediating NT-promoted survival of embryonic neurons.

A myocyte enhancer factor 2D (MEF2D) kinase activated during neuronal apoptosis is a novel target inhibited by lithium

Depolarization promotes the survival of cerebellar granule neurons via activation of the transcription factor myocyte enhancer factor 2D (MEF2D). Removal of depolarization induces hyperphosphorylation of MEF2D on serine/threonine residues, resulting in its decreased DNA binding and susceptibility to caspases. The subsequent loss of MEF2-dependent gene transcription contributes to the apoptosis of granule neurons. The kinase(s) that phosphorylates MEF2D during apoptosis is currently unknown. The serine/threonine kinase, glycogen synthase kinase-3 beta (GSK-3 beta), plays a pro-apoptotic role in granule neurons. To investigate a potential role for GSK-3 beta in MEF2D phosphorylation, we examined the effects of lithium, a non-competitive inhibitor of GSK-3 beta, on MEF2D activity in cultured cerebellar granule neurons. Lithium inhibited caspase-3 activation and chromatin condensation in granule neurons induced to undergo apoptosis by removal of depolarizing potassium and serum. Concurrently, lithium suppressed the hyperphosphorylation and caspase-mediated degradation of MEF2D. Moreover, lithium sustained MEF2 DNA binding and transcriptional activity in the absence of depolarization. Lithium also attenuated MEF2D hyperphosphorylation and apoptosis induced by calcineurin inhibition under depolarizing conditions, a GSK-3 beta-independent model of neuronal death. In contrast to lithium, MEF2D hyperphosphorylation was not inhibited by forskolin, insulin-like growth factor-I, or valproate, three mechanistically distinct inhibitors of GSK-3 beta. These results demonstrate that the kinase that phosphorylates and inhibits the pro-survival function of MEF2D in cerebellar granule neurons is a novel lithium target distinct from GSK-3 beta.

An analysis of the phosphorylation and activation of extracellular-signal-regulated protein kinase 5 (ERK5) by mitogen-activated protein kinase kinase 5 (MKK5) in vitro

MKK5 expressed as a glutathione S-transferase fusion protein in human embryonic kidney 293 cells activated full-length extracellular-signal-regulated protein kinase (ERK)5 (ERK5wt) as well as the isolated catalytic domain (ERK5cat) in vitro. Activation was accompanied by the phosphorylation of Thr(219) and Tyr(221), the former residue being phosphorylated preferentially. ERK5cat phosphorylated at Thr(219), but not Tyr(221), possessed 10% of the activity of the doubly phosphorylated protein towards myelin basic protein, whereas ERK5cat phosphorylated at Tyr(221) alone was much less active. Activated ERK5 phosphorylated itself at a number of residues, including Thr(28), Ser(421), Ser(433), Ser(496), Ser(731) and Thr(733). ERK5 phosphorylated at Thr(219), but not Tyr(221), phosphorylated itself at a similar rate to ERK5 phosphorylated at both Thr(219) and Tyr(221). Activated ERK5 also phosphorylated mitogen-activated protein kinase kinase 5 (MKK5) extensively at Ser(129), Ser(137), Ser(142) and Ser(149), which are located within the region in MKK5 that is thought to interact with ERK5.

Roles of cardiac transcription factors in cardiac hypertrophy

Different cell types, equipped with unique structure and function, synthesize different sets of proteins on the basis of different patterns of gene expression, even though their genomes are identical. Cardiac transcription factors have been reported to control a cardiac gene program and thus to play a crucial role in transcriptional regulation during embryogenesis. Recently, postnatal roles of cardiac transcription factors have been extensively investigated. Consistent with the direct transactivation of numerous cardiac genes reactivated in response to hypertrophic stimulation, cardiac transcription factors are profoundly involved in the generation of cardiac hypertrophy or in cardioprotection from cytotoxic stress in the adult heart. In this review, the regulation of a cardiac gene program by cardiac transcription factors is summarized, with an emphasis on their potential role in the generation of cardiac hypertrophy.

Regulation of epidermal growth factor-induced connexin 43 gap junction communication by big mitogen-activated protein kinase1/ERK5 but not ERK1/2 kinase activation

The gap junction protein, Cx43, plays a pivotal role in coupling cells electrically and metabolically, and the putative phosphorylation sites that modulate its function are reflected as changes in gap junction communication. Growth factor stimulation has been correlated with a decrease in gap junction communication and a parallel activation of ERK1/2; the inhibition of epidermal growth factor (EGF)-induced Cx43 gap junction uncoupling was observed by using the MEK1/2 inhibitor, PD98059. Because 1) BMK1/ERK5, another MAPK family member also activated by growth factors, possesses a phosphorylation motif similar to ERK1/2, and 2) it has been reported that PD98059 can inhibit not only MEK1/2-ERK1/2 but also MEK5-BMK1 activation, we investigated whether BMK1 can regulate EGF-induced Cx43 gap junction uncoupling and phosphorylation, comparing this to the role of ERK1/2 on Cx43 function and phosphorylation induced by EGF. Selective activation or inactivation of ERK1/2 by using a constitutively active form or a dominant negative form of MEK1 did not regulate Cx43 gap junction coupling. In contrast, we found that BMK1, selectively activated by constitutively active MEK5alpha, induced gap junction uncoupling, and the inhibition of BMK1 activation by transfection of dominant negative BMK1 prevented EGF-induced gap junction uncoupling. Activated BMK1 selectively phosphorylates Cx43 on Ser-255 in vitro and in vivo, but not on S279/S282, which are reported as the consensus phosphorylation sites for MAPK. Furthermore, by co-immunoprecipitation, we found that BMK1 directly associates with Cx43 in vivo. These data indicate that BMK1 is more important than ERK1/2 in EGF-mediated Cx43 gap junction uncoupling by association and Cx43 Ser- 255 phosphorylation.

Phosphorylation motifs regulating the stability and function of myocyte enhancer factor 2A

The phosphorylation status of the myocyte enhancer factor 2 (MEF2) transcriptional regulator is a critical determinant of its tissue-specific functions. However, due to the complexity of its phosphorylation pattern in vivo, a systematic inventory of MEF2A phosphorylation sites in mammalian cells has been difficult to obtain. We employed modern affinity purification techniques, combined with mass spectrometry, to identify several novel MEF2 phosphoacceptor sites. These include an evolutionarily conserved KSP motif, which we show is important in regulating the stability and function of MEF2A. Also, an indirect pathway in which a protein kinase casein kinase 2 phosphoacceptor site is phosphorylated by activation of p38 MAPK signaling was documented. Together, these findings identify several novel aspects of MEF2 regulation that may prove important in the control of gene expression in neuronal and muscle cells.

Cdk5-mediated inhibition of the protective effects of transcription factor MEF2 in neurotoxicity-induced apoptosis

Neurotoxic insults deregulate Cdk5 activity, which leads to neuronal apoptosis and may contribute to neurodegeneration. The biological activity of Cdk5 has been ascribed to its phosphorylation of cytoplasmic substrates. However, its roles in the nucleus remain unknown. Here we investigate the mechanism by which Cdk5 promotes neuronal apoptosis. We have identified the prosurvival transcription factor MEF2 as a direct nuclear target of Cdk5. Cdk5 phosphorylates MEF2 at a distinct serine in its transactivation domain to inhibit MEF2 activity. Neurotoxicity enhances nuclear Cdk5 activity, leading to Cdk5-dependent phosphorylation and inhibition of MEF2 function in neurons. MEF2 mutants resistant to Cdk5 phosphorylation restore MEF2 activity and protect primary neurons from Cdk5 and neurotoxin-induced apoptosis. Our studies reveal a nuclear pathway by which neurotoxin/Cdk5 induces neuronal apoptosis through inhibiting prosurvival nuclear machinery.

MEK kinase 2 and the adaptor protein Lad regulate extracellular signal-regulated kinase 5 activation by epidermal growth factor via Src

Lad is an SH2 domain-containing adaptor protein that binds MEK kinase 2 (MEKK2), a mitogen-activated protein kinase (MAPK) kinase kinase for the extracellular signal-regulated kinase 5 (ERK5) and JNK pathways. Lad and MEKK2 are in a complex in resting cells. Antisense knockdown of Lad expression and targeted gene disruption of MEKK2 expression results in loss of epidermal growth factor (EGF) and stress stimuli-induced activation of ERK5. Activation of MEKK2 and the ERK5 pathway by EGF and stress stimuli is dependent on Src kinase activity. The Lad-binding motif is encoded within amino acids 228 to 282 in the N terminus of MEKK2, and expression of this motif blocks Lad-MEKK2 interaction, resulting in inhibition of Src-dependent activation of MEKK2 and ERK5. JNK activation by EGF is similarly inhibited by loss of Lad or MEKK2 expression and by blocking the interaction of MEKK2 and Lad. Our studies demonstrate that Src kinase activity is required for ERK5 activation in response to EGF, MEKK2 expression is required for ERK5 activation by Src, Lad and MEKK2 association is required for Src activation of ERK5, and EGF and Src stimulation of ERK5-regulated MEF2-dependent promoter activity requires a functional Lad-MEKK2 signaling complex.

MEK5 overexpression is associated with metastatic prostate cancer, and stimulates proliferation, MMP-9 expression and invasion

The novel mitogen/extracellular-signal-regulated kinase kinase 5/extracellular signal-regulated kinase-5 (MEK5/ERK5) pathway has been implicated in the regulation of cellular proliferation. MEK5 expression has been detected in prostate cancer cells, although the significance of the MEK5/ERK5 pathway in human prostate cancer has not been tested. We examined MEK5 expression in 127 cases of prostate cancer and 20 cases of benign prostatic hypertrophy (BPH) by immunohistochemistry and compared the results to clinical parameters. We demonstrated that MEK5 expression is increased in prostate cancer as compared to benign prostatic tissue. Strong MEK5 expression correlates with the presence of bony metastases and less favourable disease-specific survival. Furthermore, among the patients with high Gleason score of 8-10, MEK5 overexpression has an additional prognostic value in survival. MEK5 transfection experiments confirm its ability to induce proliferation (P < 0.0001), motility (P = 0.0001) and invasion in prostate cancer cells (P = 0.0001). MEK5 expression drastically increased MMP-9, but not MMP-2 mRNA expression. Luciferase report assays suggest that the -670/MMP-9 promoter is upregulated by MEK5 and electromobility shift assay further suggests the involvement of activator protein-I (AP-1), but not the NF-kappa B, binding site in the MMP-9 promoter. Using an AP-1 luciferase construct, activation of MEK5 was confirmed to enhance AP-1 activities up to twofold. Taken together, our results establish MEK5 as a key signalling molecule associated with prostate carcinogenesis. As the MEK5/ERK5 interaction is highly specific, it represents a potential target of therapy.

Regulation of c-Fos and Fra-1 by the MEK5-ERK5 pathway

BACKGROUND

ERK5 is the newest subfamily member of the mitogen-activated protein kinase (MAPK) family, and is activated by various extracellular signals including growth factors. MEK5 is a specific activator of ERK5. c-Fos and Fra-1, well-known immediate early gene products, are members of the AP-1 family. We previously reported that activation of the MEK5-ERK5 pathway is able to induce expression of c-Fos.

RESULTS

We have found that activation of the MEK5-ERK5 pathway causes the phosphorylation and stabilization of c-Fos and Fra-1. Phosphorylation of c-Fos appears to be mediated by ERK5 and a kinase(s) lying downstream of ERK5, and the MEK5-ERK5 pathway-dependent phosphorylation sites on c-Fos are different from the ERK1/2 pathway-dependent ones. Interestingly, activation of the MEK5-ERK5 pathway, but not that of the ERK1/2 pathway, is found to markedly increase the transactivation activity of c-Fos. Furthermore, our results show that the C-terminal half of ERK5 is necessary for the maximal activation of the transactivation activity of c-Fos and Fra-1.

CONCLUSION

These results reveal a role of the MEK5-ERK5 pathway in modulating the function of the Fos family proteins which is different from the role of the ERK1/2 pathway.

Epidermal growth factor induces c-fos and c-jun mRNA via Raf-1/MEK1/ERK-dependent and -independent pathways in bovine luteal cells

Epidermal growth factor (EGF) modulates the actions of gonadotropins in the corpus luteum. The membrane-associated EGF receptors undergo rapid tyrosine phosphorylation and internalization upon ligand binding in ovarian cells, including luteal cells. However, little is known about the post-receptor signaling events induced by EGF that lead to the transcriptional regulation of EGF-responsive genes in the ovary. The present study was designed to examine in bovine luteal cells (1) activation of the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) signaling cascade (Raf/MEK/ERK) by EGF; (2) mRNA expression of AP-1 transcription factors, i.e. c-fos and c-jun, in response to EGF; and (3) the role of ERK in EGF-induced expression of c-fos and c-jun mRNA. Raf-1 and B-Raf, but not A-Raf, were activated by EGF (10 ng/ml) and the pharmacological protein kinase C (PKC) activator phorbol myristate acetate (PMA, 20 nM). Activation of Raf resulted in the phosphorylation and activation of MAPK kinase (MEK1) which subsequently activated ERKs. Treatment with EGF-induced the phosphorylation of both ERK2 and ERK1 in a time and concentration dependent manner. Additionally, activated ERK was found in the nucleus of the cells following treatment with EGF (10 ng/ml) and PMA (PMA, 20 nM) for 5 min. Depletion of PKC by chronic PMA treatment (2.5 microM, 24 h) only partially inhibited the stimulatory effects of EGF on Raf-1, ERK2 and ERK1. These data demonstrate that PKC-dependent and independent-mechanisms are involved in EGF activation of the Raf/MEK/ERK signaling cascade in bovine luteal cells. EGF rapidly and transiently stimulated the expression of c-fos and c-jun mRNA in bovine luteal cells. Maximal induction of c-fos and c-jun mRNA by EGF occurred within 30 min of treatment with 10 ng/ml EGF. Treatment with the MEK1 inhibitor PD098059 (50 microM) abolished EGF-induced ERK activation. However, blocking EGF-induced ERK activation by pretreatment with PD098059 only partially attenuated EGF-induced c-fos and c-jun mRNA expression. Thus, additional pathways are implicated in the regulation of c-fos and c-jun mRNA expression by EGF in bovine luteal cells.

ADP-ribosylation factor 4 small GTPase mediates epidermal growth factor receptor-dependent phospholipase D2 activation

The epidermal growth factor receptor (EGFR) plays a critical role in the development, proliferation, and differentiation of cells of epithelial and mesenchymal origin. These EGFR-dependent cellular processes are mediated by a repertoire of intracellular signaling pathways triggered by the activation of the EGFR cytoplasmic domain, which originates from ligand binding of its extracellular domain. To understand the molecular mechanisms by which the intracellular domain of EGFR transmits mitogenic messages to the downstream signaling pathways, we used the cytoplasmic region of EGFR as bait in yeast two-hybrid screening. We found that ADP-ribosylation factor 4 (ARF4) interacts with the intracellular part of EGFR and mediates the EGF-dependent cellular activation of phospholipase D2 (PLD2) but does not mediate the activation of PLD1. In addition, ARF4-mediated PLD2 activation leads to dramatic activation of the transcription factor activator protein 1 (AP-1), and, importantly, ARF4 activity is required for EGF-induced activation of cellular AP-1. Our findings indicate that ARF4 is a critical molecule that directly regulates cellular PLD2 activity and that this ARF4-mediated PLD2 activation stimulates AP-1-dependent transcription in the EGF-induced cellular response.