Identification of MAP kinase domains by redirecting stress signals into growth factor responses

Innate Immune Nod1/RIP2 Signaling Is Essential for Cardiac Hypertrophy but Requires Mitochondrial Antiviral Signaling Protein for Signal Transductions and Energy Balance

BACKGROUND

Cardiac hypertrophy is a key biological response to injurious stresses such as pressure overload and, when excessive, can lead to heart failure. Innate immune activation by danger signals, through intracellular pattern recognition receptors such as nucleotide-binding oligomerization domain 1 (Nod1) and its adaptor receptor-interacting protein 2 (RIP2), might play a major role in cardiac remodeling and progression to heart failure. We hypothesize that Nod1/RIP2 are major contributors to cardiac hypertrophy, but may not be sufficient to fully express the phenotype alone.

METHODS

To elucidate the contribution of Nod1/RIP2 signaling to cardiac hypertrophy, we randomized Nod1^-/-, RIP2^-/-, or wild-type mice to transverse aortic constriction or sham operations. Cardiac hypertrophy, fibrosis, and cardiac function were examined in these mice.

RESULTS

Nod1 and RIP2 proteins were upregulated in the heart after transverse aortic constriction, and this was paralleled by increased expression of mitochondrial proteins, including mitochondrial antiviral signaling protein (MAVS). Nod1^-/- and RIP2^-/- mice subjected to transverse aortic constriction exhibited better survival, improved cardiac function, and decreased cardiac hypertrophy. Downstream signal transduction pathways that regulate inflammation and fibrosis, including NF (nuclear factor) κB and MAPK (mitogen-activated protein kinase)-GATA4/p300, were reduced in both Nod1^-/- and RIP2^-/- mice after transverse aortic constriction compared with wild-type mice. Coimmunoprecipitation of extracted cardiac proteins and confocal immunofluorescence microscopy showed that Nod1/RIP2 interaction was robust and that this complex also included MAVS as an essential component. Suppression of MAVS expression attenuated the complex formation, NF κB signaling, and myocyte hypertrophy. Interrogation of mitochondrial function compared in the presence or ablation of MAVS revealed that MAVS serves to suppress mitochondrial energy output and mediate fission/fusion related dynamic changes. The latter is possibly linked to mitophagy during cardiomyocytes stress, which may provide an intriguing link between innate immune activation and mitochondrial energy balance under stress or injury conditions.

CONCLUSIONS

We have identified that innate immune Nod1/RIP2 signaling is a major contributor to cardiac remodeling after stress. This process is critically joined by and regulated through the mitochondrial danger signal adapter MAVS. This novel complex coordinates remodeling, inflammatory response, and mitochondrial energy metabolism in stressed cardiomyocytes. Thus, Nod1/RIP2/MAVS signaling complex may represent an attractive new therapeutic approach toward heart failure.

Hierarchical Organization Endows the Kinase Domain with Regulatory Plasticity

The functional diversity of kinases enables specificity in cellular signal transduction. Yet how more than 500 members of the human kinome specifically receive regulatory inputs and convey information to appropriate substrates-all while using the common signaling output of phosphorylation-remains enigmatic. Here, we perform statistical co-evolution analysis, mutational scanning, and quantitative live-cell assays to reveal a hierarchical organization of the kinase domain that facilitates the orthogonal evolution of regulatory inputs and substrate outputs while maintaining catalytic function. We find that three quasi-independent "sectors"-groups of evolutionarily coupled residues-represent functional units in the kinase domain that encode for catalytic activity, substrate specificity, and regulation. Sector positions impact both disease and pharmacology: the catalytic sector is significantly enriched for somatic cancer mutations, and residues in the regulatory sector interact with allosteric kinase inhibitors. We propose that this functional architecture endows the kinase domain with inherent regulatory plasticity.

High-Salt Enhances the Inflammatory Response by Retina Pigment Epithelium Cells following Lipopolysaccharide Stimulation

High-salt has been shown to play a role in the pathogenesis of autoimmune disease. In this study, we investigated the effect of high-salt on the production of inflammatory mediators by ARPE-19 cells and the possible mechanisms involved. ARPE-19 cells were cultured with LPS in DMEM to which extra NaCl had been added (20 mM and 40 mM). NaCl had no influence on the apoptosis and proliferation of ARPE-19. Addition of 40 mM NaCl significantly induced IL-6 and MCP-1 production but had no effect on IL-8 secretion. High mannitol, as an osmotic stress control, did not affect the secretion of inflammatory mediators by ARPE-19 cells indicating that the effect was not mediated by osmolarity. Coculture of ARPE-19 cells with NaCl resulted in significant increases in the phosphorylation of p38 MAPK, Akt, and NF-κB and an upregulation of the transcription factors NFAT5 and SGK1. High-salt significantly promotes IL-6 and MCP-1 production by ARPE-19 cells and is associated with activation of the p38 MAPK, Akt, and NF-κB pathway and NFAT-SGK1 pathways.

Role of H-Ras/ERK signaling in carbon nanotube-induced neoplastic-like transformation of human mesothelial cells

Rapid development and deployment of engineered nanomaterials such as carbon nanotubes (CNTs) in various commercial and biomedical applications have raised concerns about their potential adverse health effects, especially their long-term effects which have not been well addressed. We demonstrated here that prolonged exposure of human mesothelial cells to single-walled CNT (SWCNT) induced neoplastic-like transformation as indicated by anchorage-independent cell growth and increased cell invasiveness. Such transformation was associated with an up-regulation of H-Ras and activation of ERK1/2. Downregulation of H-Ras by siRNA or inactivation of ERK by chemical inhibitor effectively inhibited the aggressive phenotype of SWCNT-exposed cells. Integrin alpha V and cortactin, but not epithelial-mesenchymal transition (EMT) transcriptional regulators, were up-regulated in the SWCNT-exposed cells, suggesting their role in the aggressive phenotype. Cortactin expression was shown to be controlled by the H-Ras/ERK signaling. Thus, our results indicate a novel role of H-Ras/ERK signaling and cortactin in the aggressive transformation of human mesothelial cells by SWCNT.

Uridine 5'-triphosphate promotes in vitro Schwannoma cell migration through matrix metalloproteinase-2 activation

In response to peripheral nerve injury, Schwann cells adopt a migratory phenotype and modify the extracellular matrix to make it permissive for cell migration and axonal re-growth. Uridine 5′-triphosphate (UTP) and other nucleotides are released during nerve injury and activate purinergic receptors expressed on the Schwann cell surface, but little is known about the involvement of purine signalling in wound healing. We studied the effect of UTP on Schwannoma cell migration and wound closure and the intracellular signaling pathways involved. We found that UTP treatment induced Schwannoma cell migration through activation of P2Y₂ receptors and through the increase of extracellular matrix metalloproteinase-2 (MMP-2) activation and expression. Knockdown P2Y₂ receptor or MMP-2 expression greatly reduced wound closure and MMP-2 activation induced by UTP. MMP-2 activation evoked by injury or UTP was also mediated by phosphorylation of all 3 major mitogen-activated protein kinases (MAPKs): JNK, ERK1/2, and p38. Inhibition of these MAPK pathways decreased both MMP-2 activation and cell migration. Interestingly, MAPK phosphorylation evoked by UTP exhibited a biphasic pattern, with an early transient phosphorylation 5 min after treatment, and a late and sustained phosphorylation that appeared at 6 h and lasted up to 24 h. Inhibition of MMP-2 activity selectively blocked the late, but not the transient, phase of MAPK activation. These results suggest that MMP-2 activation and late MAPK phosphorylation are part of a positive feedback mechanism to maintain the migratory phenotype for wound healing. In conclusion, our findings show that treatment with UTP stimulates in vitro Schwannoma cell migration and wound repair through a MMP-2-dependent mechanism via P2Y₂ receptors and MAPK pathway activation.

Glial cell line-derived neurotrophic factor (GDNF) induced migration of spermatogonial cells in vitro via MEK and NF-kB pathways

Glial cell line-derived neurotrophic factor (GDNF) regulates spermatogonial stem cell (SSC) maintenance. In the present study, we examined the levels and the cellular origin of GDNF in mouse testes during age-development, and the capacity of GDNF to induce migration of enriched GFR-α1 positive cells in vitro. The involvement of MAP kinase (MEK) and NF-kB signal pathways were examined. Our results show high levels of GDNF in testicular tissue of one-week-old mice which significantly decreased with age when examined by ELISA, real time PCR (qPCR) and immunofluorescence staining (IF) analysis. GDNF receptor (GFR-α1) expression was similar to GDNF when examined by qPCR analysis. Only Sertoli cell cultures (SCs) from one-week-old mice produced GDNF compared to SCs from older mice. However, peritubular cells from all the examined ages did not produce GDNF. The addition of recombinant GDNF (rGDNF) or supernatant from SCs from one-week-old mice to GFR-α1 positive cells induced their migration in vitro. This effect was significantly reduced by the addition of inhibitors to MEK (PD98059, U0126), NF-kB (PDTC) and IkB protease inhibitor (TPCK). Our results show for the first time the capacity of rGDNF and supernatant from SCs to induce migration of enriched GFR-α1 positive cells, and the possible involvement of MEK, NF-kB and IkB in this process. This study may suggest a novel role for GDNF in the regulation SSC niches and spermatogenesis.

Multimolecular analysis of stable immunological synapses reveals sustained recruitment and sequential assembly of signaling clusters

The formation of the immunological synapse between T cells and antigen-presenting cells (APC) begins within minutes of contact and can take hours for full T-cell activation. Although early phases of the synapse have been extensively studied for a select number of proteins, later phases have not yet been examined in detail. We studied the signaling network in stable synapses by measuring the simultaneous localization of 25 signaling and structural molecules over 2 h at the level of individual synapses using multi-epitope ligand cartography (MELC). Signaling proteins including phospho(p)ZAP70, pSLP76, pCD3ζ, and pLAT, along with proteins that influence synapse structure such as F-actin, tubulin, CD45, and ICAM-1, were localized in images of synapses and revealed the multidimensional construction of a mature synapse. The construction of the stable synapse included intense early TCR signaling, a phase of recruitment of structural proteins, and a sustained increase in signaling molecules and colocalization of TCR and pLAT signaling clusters in the center of the synapse. Consolidation of TCR and associated proteins resulted in formation of a small number of discrete synaptic microclusters. Development of synapses and cSMAC composition was greatly affected by the absence of Vav1, with an associated loss in PLCγ1 recruitment, pSLP76, and increased CXCR4. Together, these data demonstrate the use of multi-epitope ligand cartography to quantitatively analyze synapse formation and reveal successive recruitment of structural and signaling proteins and sustained phosphorylation at the mature synapse.

Modification of collagen by 3-deoxyglucosone alters wound healing through differential regulation of p38 MAP kinase

Background

Wound healing is a highly dynamic process that requires signaling from the extracellular matrix to the fibroblasts for migration and proliferation, and closure of the wound. This rate of wound closure is impaired in diabetes, which may be due to the increased levels of the precursor for advanced glycation end products, 3-deoxyglucosone (3DG). Previous studies suggest a differential role for p38 mitogen-activated kinase (MAPK) during wound healing; whereby, p38 MAPK acts as a growth kinase during normal wound healing, but acts as a stress kinase during diabetic wound repair. Therefore, we investigated the signaling cross-talk by which p38 MAPK mediates wound healing in fibroblasts cultured on native collagen and 3DG-collagen.

Methodology/Principal Findings

Using human dermal fibroblasts cultured on 3DG-collagen as a model of diabetic wounds, we demonstrated that p38 MAPK can promote either cell growth or cell death, and this was dependent on the activation of AKT and ERK1/2. Wound closure on native collagen was dependent on p38 MAPK phosphorylation of AKT and ERK1/2. Furthermore, proliferation and collagen production in fibroblasts cultured on native collagen was dependent on p38 MAPK regulation of AKT and ERK1/2. In contrast, 3DG-collagen decreased fibroblast migration, proliferation, and collagen expression through ERK1/2 and AKT downregulation via p38 MAPK.

Conclusions/Significance

Taken together, the present study shows that p38 MAPK is a key signaling molecule that plays a significantly opposite role during times of cellular growth and cellular stress, which may account for the differing rates of wound closure seen in diabetic populations.

C-kit binding properties of hesperidin (a major component of KMP6) as a potential anti-allergic agent

Accumulation of mast cells can be causally related to several allergic inflammations. Stem cell factor (SCF) as a mast cell chemotaxin induces mast cell migration. To clarify a new effect of Pyeongwee-San extract (KMP6, a drug for indigestion) for the treatment of allergy, we investigated the effects of KMP6 on SCF-induced migration of rat peritoneal mast cells (RPMCs). A molecular docking simulation showed that hesperidin, a major component of KMP6, controls the SCF and c-kit binding by interaction with the active site of the c-kit. KMP6 and hesperidin significantly inhibited SCF-induced migration of RPMCs (P<0.05). The ability of the SCF to enhance morphological alteration and F-actin formation was also abolished by treatment with KMP6 or hesperidin. KMP6 and hesperidin inhibited SCF-induced p38 MAPK activation. In addition, SCF-induced inflammatory cytokine production was significantly inhibited by treatment with KMP6 or hesperidin (P<0.05). Our results show for the first time that KMP6 potently regulates SCF-induced migration, p38 MAPK activation and inflammatory cytokines production through hindrance of SCF and c-kit binding in RPMCs. Such modulation may have functional consequences during KMP6 treatment, especially mast cell-mediated allergic inflammation disorders.

MK591, a leukotriene biosynthesis inhibitor, induces apoptosis in prostate cancer cells: synergistic action with LY294002, an inhibitor of phosphatidylinositol 3'-kinase

MK591 is a synthetic compound which specifically inhibits the activity of 5-Lox and is currently under development for the treatment of asthma. We observed that human prostate cancer cells treated with MK591 undergo apoptosis within hours of treatment. Apoptosis involves severe morphological alteration, externalization of phosphatidyl-serine, cleavage of PARP, and degradation of chromatin-DNA. MK591 also induced rapid activation of the stress kinase, c-Jun N-terminal kinase (JNK), which plays an important role in the apoptosis process. The phosphatidylinositol 3'-kinase-Akt/protein kinase B (PI3K/Akt) axis is a well-known pro-survival pathway which prevents apoptosis through defined anti-apoptotic mechanisms in a variety of cancer cells. Interestingly, we observed that MK591 triggers apoptosis in prostate cancer cells without inhibition of PI3K-Akt, or ERK. Moreover, it was observed that MK591 and LY294002 (an inhibitor of PI3K) exert synergistic effect in inducing apoptosis in prostate cancer cells. Altogether, these findings indicate that 5-Lox inhibition-induced apoptosis in prostate cancer cells occurs without inhibition of PI3K-Akt, or ERK, and suggest for the existence of an Akt- and ERK-independent survival mechanism(s) in these cancer cells maintained via signals generated by metabolites of 5-Lox.

Glial cell-derived neurotrophic factor increases migration of human chondrosarcoma cells via ERK and NF-kappaB pathways

Invasion of tumor cells is the primary cause of therapeutic failure in the treatment of malignant chondrosarcomas. Glial cell-derived neurotrophic factor (GDNF) plays a crucial role in migration and metastasis of human cancer cells. Integrins are the major adhesive molecules in mammalian cells. Here we found that GDNF directed the migration and increased cell surface expression of alphav and beta3 integrin in human chondrosarcoma cells. Pretreated of JJ012 cells with MAPK kinase (MEK) inhibitors PD98059 or U0126 inhibited the GDNF-mediated migration and integrin expression. Stimulation of cells with GDNF increased the phosphorylation of MEK and extracellular signal-regulating kinase (ERK). In addition, NF-kappaB inhibitor (PDTC) or IkappaB protease inhibitor (TPCK) also inhibited GDNF-mediated cells migration and integrin up-regulation. Stimulation of cells with GDNF induced IkappaB kinase (IKKalpha/beta) phosphorylation, IkappaB phosphorylation, p65 Ser(536) phosphorylation, and kappaB-luciferase activity. Furthermore, the GDNF-mediated increasing of kappaB-luciferase activity was inhibited by PD98059, U0126, PDTC and TPCK or MEK, ERK, IKKalpha, and IKKbeta mutants. Taken together, these results suggest that the GDNF acts through MEK/ERK, which in turn activates IKKalpha/beta and NF-kappaB, resulting in the activations of alphavbeta3 integrin and contributing the migration of human chondrosarcoma cells.

Modularity of MAP kinases allows deformation of their signalling pathways

Eukaryotic protein kinase pathways have both grown in number and changed their network architecture during evolution. We wondered if there are pivotal proteins in these pathways that have been repeatedly responsible for forming new connections through evolution, thus changing the topology of the network; and if so, whether the underlying properties of these proteins could be exploited to re-engineer and rewire these pathways. We addressed these questions in the context of the mitogen-activated protein kinase (MAPK) pathways. MAPK proteins were found to have repeatedly acquired new specificities and interaction partners during evolution, suggesting that these proteins are pivotal in the kinase network. Using the MAPKs Fus3 and Hog1 of the Saccharomyces cerevisiae mating and hyper-osmolar pathways, respectively, we show that these pivotal proteins can be re-designed to achieve a wide variety of changes in the input-output properties of the MAPK network. Through an analysis of our experimental results and of the sequence and structure of these proteins, we show that rewiring of the network is possible due to the underlying modular design of the MAPKs. We discuss the implications of our findings on the radiation of MAPKs through evolution and on how these proteins achieve their specificity.

Selectivity of docking sites in MAPK kinases

Protein kinases often recognize their substrates and regulators through docking interactions that occur outside of the active site; these interactions can help us to understand kinase networks, and to target kinases with drugs. During mitogen-activated protein kinase (MAPK) signaling, the ability of MAPK kinases (MKKs, or MEKs) to recognize their cognate MAPKs is facilitated by a short docking motif (the D-site) in the MKK N terminus, which binds to a complementary region on the MAPK. MAPKs then recognize many of their targets using the same strategy, because many MAPK substrates also contain D-sites. The extent to which docking contributes to the specificity of MAPK transactions is incompletely understood. Here we characterize the selectivity of the interaction between MKK-derived D-sites and MAPKs by measuring the ability of D-site peptides to inhibit MAPK-mediated phosphorylation of D-site-containing substrates. We find that all MKK D-sites bind better to their cognate MAPKs than they do to non-cognate MAPKs. For instance, the MKK3 D-site peptide, which is a remarkably potent inhibitor of p38alpha (IC(50) < 10 nm), does not inhibit JNK1 or JNK2. Likewise, MAPKs generally bind as well or better to cognate D-sites than to non-cognate D-sites. For instance, JNK1 and JNK2 do not appreciably bind to any D-sites other than their cognate D-sites from MKK4 and MKK7. In general, cognate, within-pathway interactions are preferred about an order of magnitude over non-cognate interactions. However, the selectivity of MAPKs and their cognate MKK-derived D-sites for each other is limited in some cases; in particular, ERK2 is not very selective. We conclude that MAPK-docking sites in MAPK kinases bind selectively to their cognate MAPKs.

The N-terminal domain of ERK1 accounts for the functional differences with ERK2

The Extracellular Regulated Kinase 1 and 2 transduce a variety of extracellular stimuli regulating processes as diverse as proliferation, differentiation and synaptic plasticity. Once activated in the cytoplasm, ERK1 and ERK2 translocate into the nucleus and interact with nuclear substrates to induce specific programs of gene expression. ERK1/2 share 85% of aminoacid identity and all known functional domains and thence they have been considered functionally equivalent until recent studies found that the ablation of either ERK1 or ERK2 causes dramatically different phenotypes. To search a molecular justification of this dichotomy we investigated whether the different functions of ERK1 and 2 might depend on the properties of their cytoplasmic-nuclear trafficking. Since in the nucleus ERK1/2 is predominantly inactivated, the maintenance of a constant level of nuclear activity requires continuous shuttling of activated protein from the cytoplasm. For this reason, different nuclear-cytoplasmic trafficking of ERK1 and 2 would cause a differential signalling capability. We have characterised the trafficking of fluorescently tagged ERK1 and ERK2 by means of time-lapse imaging in living cells. Surprisingly, we found that ERK1 shuttles between the nucleus and cytoplasm at a much slower rate than ERK2. This difference is caused by a domain of ERK1 located at its N-terminus since the progressive deletion of these residues converted the shuttling features of ERK1 into those of ERK2. Conversely, the fusion of this ERK1 sequence at the N-terminus of ERK2 slowed down its shuttling to a similar value found for ERK1. Finally, computational, biochemical and cellular studies indicated that the reduced nuclear shuttling of ERK1 causes a strong reduction of its nuclear phosphorylation compared to ERK2, leading to a reduced capability of ERK1 to carry proliferative signals to the nucleus. This mechanism significantly contributes to the differential ability of ERK1 and 2 to generate an overall signalling output.

Cumulating researches on the relationship between P38 MAPK signaling pathway and gastric carcinoma

The cascade reaction of mitogen-activated protein kinases (MAPKs) is one of the vital intracellular signal transduction systems, participating in many physiological progressions, such as cell growth, proliferation, differentiation and apoptosis. P38 is a member of MAPKs, mediating many cell reactions induced by stress, inflammatory cytokines or bacterial products and playing a key role in the regulation of cell cycle. For different cell lines of gastric carcinoma, P38 has different functions. The same phenomenon can be seen when the cells are presented under different stimulus. P38 pathway may be one candidate target of cancer therapy.

Upregulation of vascular endothelial growth factor by heat-killed Listeria monocytogenes in macrophages

We investigated the effect of heat-killed Listeria monocytogenes (HKLM) on the expression of vascular endothelial growth factor (VEGF) in RAW264.7 macrophage-like cells. The expression of VEGF was induced in RAW264.7 cells treated with HKLM. Pretreatment of cells with cycloheximide, a protein synthesis inhibitor, inhibited the induction of VEGF mRNA by HKLM. Induction of VEGF by HKLM was partially inhibited by treatment of cells with SB203580, a p38 mitogen-activated protein kinase (MAPK) inhibitor, or a neutralizing antibody against tumor necrosis factor-alpha (TNF-alpha). In addition, HKLM induced phosphorylation of p38 MAPK. These results suggest that p38 MAPK and TNF-alpha are involved in the VEGF expression induced by HKLM in RAW264.7 cells. We confirmed that increased VEGF expression is immunohistochemically detected in splenic macrophages of mice infected with L. monocytogenes (L. monocytogenes). VEGF is thought to be involved in inflammatory reactions induced by L. monocytogenes infection.

Glial cell-derived neurotrophic factor (GDNF) promotes low-grade Hs683 glioma cell migration through JNK, ERK-1/2 and p38 MAPK signaling pathways

Invasion of tumor cells is the primary cause of therapeutic failure in the treatment of malignant gliomas. In an attempt to investigate the properties of the malignant progression of glioma cells, we examined the correlation between cell migration and glial cell-derived neurotrophic factor (GDNF) secretion of two glioma cell lines which differ in their invasive phenotypes. Here, we show that the high-grade C6 cells are more migrative and secrete more GDNF than the low-grade Hs683 cells. GDNF signaling is more highly activated in C6 cells than in Hs683 cells. Treatment of the Hs683 cells with GDNF significantly increased migration comparable to the C6 cells, revealing the autocrine and/or paracrine effect of GDNF on promotion of the glioma cell migration. We then examined the involvement of mitogen-activated protein kinases (MAPKs) including c-Jun N-terminal protein kinase (JNK), extracellular signal-regulated kinases (ERKs) and p38 MAPK in Hs683 cell migration induced by GDNF. A prominent activation of JNK, ERKs and p38 MAPK was observed in the GDNF-treated cells. Functional studies showed that the activation of these MAPKs was critical for Hs683 cell migration induced by GDNF. Our findings revealing molecular mechanisms for the promoting effect of GDNF on glioma cell migration may provide an insight into a better understanding to the malignant progression of human gliomas.

The ERK cascade: a prototype of MAPK signaling

Sequential activation of protein kinases within the mitogen-activated protein kinase (MAPK) cascades is a common mechanism of signal transduction in many cellular processes. Four such cascades have been elucidated thus far, and named according to their MAPK tier component as the ERK1/2, JNK, p38MAPK, and ERK5 cascades. These cascades cooperate in transmitting various extracellular signals, and thus control cellular processes such as proliferation, differentiation, development, stress response, and apoptosis. Here we describe the classic ERK1/2 cascade, and concentrate mainly on the properties of MEK1/2 and ERK1/2, including their mode of regulation and their role in various cellular processes and in oncogenesis. This cascade may serve as a prototype of the other MAPK cascades, and the study of this cascade is likely to contribute to the understanding of mitogenic and other processes in many cell lines and tissues.

TGF-beta1-induced thrombospondin-1 expression through the p38 MAPK pathway is abolished by fluvastatin in human coronary artery smooth muscle cells

Thrombospondin-1 (TSP-1) and transforming growth factor-beta1 (TGF-beta1) are both implicated in the pathogenesis of in-stent restenosis. This study evaluated the hypothesis that the HMG-CoA reductase inhibitor fluvastatin inhibits TGF-beta1 induced TSP-1 expression via inhibition of p38 mitogen activated protein kinase (MAPK) phosphorylation in human coronary artery smooth muscle cells (HCASMC) and may therefore have anti-restenosis potential. Fluvastatin significantly reduced TSP-1 mRNA and protein expression in HCASMC in a concentration-dependent manner with a significant reduction in expression observed after treatment with 0.25 microM fluvastatin. TGF-beta1 (5 ng/ml) induced phosphorylation of p38 MAPK and induced TSP-1 mRNA and protein expression in HCASMC. Fluvastatin abolished TGF-beta1-induced phosphorylation of p38 MAPK and TGF-beta1-induced TSP-1 expression. Blockade of the p38 MAPK pathway with the upstream inhibitor SB-203580 also abolished TGF-beta1-induced TSP-1 expression. We conclude that fluvastatin decreases expression of TSP-1 and abolishes the ability of TGF-beta1 to induce TSP-1 expression in HCASMC; this may be achieved by preventing signalling through the p38 MAPK pathway. Targeted delivery of fluvastatin may therefore be a useful therapeutic objective for prevention of the intimal hyperplasia associated with in-stent restenosis.

Tanshinone: an inhibitor of proliferation of vascular smooth muscle cells

Tanshinone (Tan) is one of the active components of Radix Salvia miltiorrhiza (Lamiaceae), which is clinically used to treat cardiovascular diseases in China. The aim of this study was to estimate the effect of Tan on the proliferation of cultured vascular smooth muscle cells (VSMCs) induced by fatal bovine serum (FBS). It was shown that various concentrations of Tan inhibited the VSMCs proliferation in a dose-dependent manner. Tan significantly blocked VSMCs cell cycle in G(0)/G(1) phase. The anti-proliferative effect of Tan was associated with the inhibition of the extracellular signal-regulated kinase1/2 (ERK1/2). On the other hand, the decrement of Tan on the cyclin D1 protein may be related to the high expression of p21(waf/cip1). The data suggest that the anti-proliferative effect of Tan on VSMCs proliferation was associated with ERK1/2 signaling pathway.

The cAMP signalling pathway activates CREB through PKA, p38 and MSK1 in NIH 3T3 cells

Cyclic adenosine 3',5'-monophosphate (cAMP) was originally shown to induce gene transcription through activation of cAMP-dependent protein kinase (PKA), and subsequent phosphorylation of the transcription factor cAMP response element-binding protein, CREB, at serine-133. However, elevated cAMP levels may activate multiple signalling pathways with protein kinases that can phosphorylate CREB at serine-133. We analysed the pathways involved in CREB phosphorylation and activation in NIH 3T3 cells exposed to the cAMP elevating agent forskolin. PKA represented the predominant pathway during the burst phase, while the mitogen-activated protein kinase p38 pathway became activated in a PKA-dependent fashion in forskolin treated cells. The phosphorylation kinetics of p38 was delayed compared to PKA activation. Activated p38 stimulated CREB-mediated transcription and potentiated the transcriptional strength of CREB provoked by forskolin. The p38-mediated activation of CREB was inhibited by dominant negative mutants of MSK-1 and by the PKA/MSK-1 inhibitor H89, but not by dominant negative mutants of MSK-2/RSK-B and MAPKAPK2. Our results suggest that forskolin-induced CREB phosphorylation and activation in NIH 3T3 cells is mediated directly by PKA and by a time-delayed PKA-dependent p38/MSK-1 pathway. This bifurcation and time-dependent regulation of the cAMP-responsive signalling pathways may enable the cell to endure and/or enforce a cellular response provoked by a cAMP-elevating stimulus.

Microglia release activators of neuronal proliferation mediated by activation of mitogen-activated protein kinase, phosphatidylinositol-3-kinase/Akt and delta-Notch signalling cascades

Microglia, the resident macrophage of the brain, can release substances that aid neuronal development, differentiation and survival. We have investigated the effects of non-activated microglia on the survival of cultured rat cerebellar granule neurones. Microglial-conditioned medium, collected from primary rat microglial cultures, was used to treat 7-day-in-vitro neurones, and neuronal viability and proliferation was assessed following a further 1 or 7 days in culture. Microglial-conditioned medium enhanced neuronal survival by up to 50% compared with untreated neurones and this effect was completely abated by pretreatment of the microglia with l-leucine methyl ester. The expression of the proliferation marker Ki-67 increased in neuronal cultures treated with microglial-conditioned medium suggesting enhanced proliferation of precursor neurones. Microglial-induced neuronal proliferation could be attenuated by specific inhibition of mitogen-activated protein kinase or phosphatidylinositol-3-kinase/Akt signalling pathways, and by selective fractionation and immunodepletion of the microglial-conditioned medium. Activation of the Notch pathway was enhanced as antibody against the Notch ligand, delta-1, prevented the microglial-induced neuronal proliferation. These results show that microglia release stable neurotrophic factors that can promote neuronal precursor cell proliferation.

The Death Effector Domain Protein PEA-15 Prevents Nuclear Entry of ERK2 by Inhibiting Required Interactions

ERK2 nuclear-cytoplasmic distribution is regulated in response to hormones and cellular state without the requirement for karyopherin-mediated nuclear import. One proposed mechanism for the movement of ERK2 into the nucleus is through a direct interaction between ERK2 and nucleoporins present in the nuclear pore complex. Previous reports have attributed regulation of ERK2 localization to proteins that activate or deactivate ERK2, such as the mitogen-activated protein (MAP) kinase kinase MEK1 and MAP kinase phosphatases. Recently, a small non-catalytic protein, PEA-15, has also been demonstrated to promote a cytoplasmic ERK2 localization. We found that the MAP kinase insert in ERK2 is required for its interaction with PEA-15. Consistent with its recognition of the MAP kinase insert, PEA-15 blocked activation of ERK2 by MEK1, which also requires the MAP kinase insert to interact productively with ERK2. To determine how PEA-15 influences the localization of ERK2, we used a permeabilized cell system to examine the effect of PEA-15 on the localization of ERK2 and mutants that have lost the ability to bind PEA-15. Wild type ERK2 was unable to enter the nucleus in the presence of an excess of PEA-15; however, ERK2 lacking the MAP kinase insert largely retained the ability to enter the nucleus. Binding assays demonstrated that PEA-15 interfered with the ability of ERK2 to bind to nucleoporins. These results suggest that PEA-15 sequesters ERK2 in the cytoplasm at least in part by interfering with its ability to interact with nucleoporins, presenting a potential paradigm for regulation of ERK2 localization.

PEA-15 binding to ERK1/2 MAPKs is required for its modulation of integrin activation

Activation of Raf-1 suppresses integrin activation, potentially through the activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2). However, bulk ERK1/2 activation does not correlate with suppression. PEA-15 reverses suppression of integrin activation and binds ERK1/2. Here we report that PEA-15 reversal of integrin suppression depends on its capacity to bind ERK1/2, indicating that ERK1/2 function is indeed required for suppression. Mutations in either the death effector domain or C-terminal tail of PEA-15 that block ERK1/2 binding abrogated the reversal of integrin suppression. Furthermore, we used ERK/p38 chimeras and site-directed mutagenesis to identify ERK1/2 residues required for binding PEA-15. Mutations of residues that precede the alphaG helix and within the mitogen-activated protein kinase insert blocked ERK2 binding to PEA-15, but not activation of ERK2. These ERK2 mutants blocked the ability of PEA-15 to reverse suppression of integrin activation. Thus, PEA-15 regulation of integrin activation depends on its binding to ERK1/2. To directly test the role of ERK1/2 localization in suppression, we enforced membrane association of ERK1 and 2 by joining a membrane-targeting CAAX box sequence to them. Both ERK1-CAAX and ERK2-CAAX were membrane-localized and suppressed integrin activation. In contrast to suppression by membrane-targeted Raf-CAAX, suppression by ERK1/2-CAAX was not reversed by PEA-15. Thus, ERK1/2 are the Raf effectors for suppression of integrin activation, and PEA-15 reverses suppression by binding ERK1/2.

Inhibition of the stem cell factor-induced migration of mast cells by dexamethasone

Mast cell accumulation can be causally related to several allergic inflammations. Previous work has demonstrated that glucocorticoids decreased tissue mast cell number, and stem cell factor (SCF)-induced migration of mast cells required p38 MAPK activation. In the present study we investigated the effects of dexamethasone on SCF-induced migration of rat peritoneal mast cells (RPMCs). SCF significantly induced the migration of RPMCs at 4 h. Dexamethasone dose-dependently inhibited SCF-induced migration of RPMCs (approximately 90.1% at 100 nM; P < 0.05). The MAPK p38 inhibitor SB203580 (20 microM) also inhibited the SCF-induced migration. The ability of SCF to enhance morphological alteration and filamentous actin formation was also abolished by treatment with dexamethasone. Dexamethasone inhibited SCF-induced p38 MAPK activation to near-basal levels and induced MAPK phosphatase-1 expression. In addition, SCF-induced inflammatory cytokine production was significantly inhibited by treatment with dexamethasone or SB203580 (P < 0.01). Our results show that dexamethasone potently regulates SCF-induced migration, p38 MAPK activation, and inflammatory cytokine production through the expression of MKP-1 protein in RPMCs. Such modulation may have functional consequences during dexamethasone treatment, especially mast cell-mediated allergic inflammation disorders.

A bipartite mechanism for ERK2 recognition by its cognate regulators and substrates

Mitogen-activated protein (MAP) kinases control gene expression in response to extracellular stimuli and exhibit exquisite specificity for their cognate regulators and substrates. We performed a structure-based mutational analysis of ERK2 to identify surface areas that are important for recognition of its interacting proteins. We show that binding and activation of MKP3 by ERK2 involve two distinct protein-protein interaction sites in ERK2. Thus, the common docking (CD) site composed of Glu-79, Tyr-126, Arg-133, Asp-160, Tyr-314, Asp-316, and Asp-319 are important for high affinity MKP3 binding but not essential for ERK2-induced MKP3 activation. MKP3 activation requires residues Tyr-111, Thr-116, Leu-119, Lys-149, Arg-189, Trp-190, Glu-218, Arg-223, Lys-229, and His-230 in the ERK2 substrate-binding region, located distal to the common docking site. Interestingly, many of the residues important for MKP3 recognition are also used for Elk1 binding and phosphorylation. In addition to the shared residues, there are also residues that are unique to each target recognition. There is evidence indicating that the CD site and the substrate-binding region defined here are also utilized for MEK1 recognition, and indeed, we demonstrate that the binding of MKP3, Elk1, and MEK1 to ERK2 is mutually exclusive. Taken together, our data suggest that the efficiency and fidelity of ERK2 signaling is achieved by a bipartite recognition process. In this model, one part of the ERK2-binding proteins (e.g. the kinase interaction motif sequence) docks to the CD site located on the back side of the ERK2 catalytic pocket for high affinity association, whereas the interaction of the substrate-binding region with another structural element (e.g. the FXFP motif in MKP3 and Elk1) may not only stabilize binding but also provide contacts crucial for modulating the activity and/or specificity of ERK2 target molecules.

Inhibition of arachidonate 5-lipoxygenase triggers prostate cancer cell death through rapid activation of c-Jun N-terminal kinase

Previously, we reported that inhibition of arachidonate 5-lipoxygenase triggers massive apoptosis in both androgen-sensitive (LNCaP) and androgen-refractory (PC3) human prostate cancer cells within hours of treatment [Proc. Natl. Acad. Sci. USA 95 (1998) 13182-13187]. Apoptosis was prevented by exogenous 5(S)-HETE, a product of 5-lipoxygenase, indicating a role of this eicosanoid as an essential survival/anti-apoptotic factor for prostate cancer cells. However, nothing was clearly known about details of the underlying molecular mechanisms or events mediating the induction of fulminating apoptosis in these cells. This report documents the fact that inhibition of arachidonate 5-lipoxygenase induces rapid activation of c-Jun N-terminal kinase (JNK) in human prostate cancer cells which is prevented by the 5-lipoxygenase metabolite, 5(S)-HETE. Activation of JNK is unaffected by the cell-permeable tetra-peptide inhibitors of caspase 8 or caspase 3 (IETD-FMK and DEVD-FMK), though these inhibitors effectively blocked apoptosis triggering, suggesting that activation of JNK is independent or upstream of caspase activation. Both 5-lipoxygenase inhibition-induced activation of JNK and induction of apoptosis are prevented by curcumin, an inhibitor of JNK-signaling pathway. Apoptosis is also blocked by SP600125, a specific inhibitor of JNK activity, indicating that JNK activity is required for the induction of apoptosis in these cells. These findings suggest that the metabolites of arachidonate 5-lipoxygenase promote survival of prostate cancer cells involving down-regulation of stress-activated protein kinase.

Interleukin-1 beta and tumor necrosis factor-alpha induce MUC5AC overexpression through a mechanism involving ERK/p38 mitogen-activated protein kinases-MSK1-CREB activation in human airway epithelial cells

Mucin hypersecretion is commonly observed in many inflammatory diseases of the respiratory tract. MUC5AC is generally recognized to be a major airway mucin because MUC5AC is highly expressed in the goblet cells of human airway epithelium. Moreover, it is regulated by various inflammatory cytokines. However, the mechanisms by which the interleukin (IL)-1beta and tumor necrosis factor (TNF)-alpha induce MUC5AC gene expression in normal nasal epithelial cells, and the signal molecules involved, especially in the downstream signaling of mitogen-activated protein (MAP) kinases, remain unclear. Here we show that pharmacologic or genetic inhibition of either ERK or p38 MAP kinase pathway abolished IL-1beta- and TNF-alpha-induced MUC5AC gene expression in normal human nasal epithelial cells. Our results also indicate that the activation of mitogen- and stress-activated protein kinase 1 (MSK1) and cAMP-response element-binding protein and cAMP-response element signaling cascades via ERK and p38 MAP kinases are crucial aspects of the intracellular mechanisms that mediate MUC5AC gene expression. Taken together, these studies give additional insights into the molecular mechanism of IL-1beta- and TNF-alpha-induced MUC5AC gene expression and enhance our understanding on mucin hypersecretion during inflammation.

ERK and p38 inhibit the expression of 4E-BP1 repressor of translation through induction of Egr-1

4E-BP1 plays a major role in translation by inhibiting cap-dependent translation initiation. Several reports have investigated the regulation of 4E-BP1 phosphorylation, which varies along with cell differentiation and upon various stimulations, but very little is known about the regulation of its expression. In a first part, we show that the expression of 4E-BP1 protein and transcript decreases in hematopoietic cell lines cultivated in the presence of phorbol 12-myristate 13-acetate (PMA). This decrease depends on the activation of the ERK/mitogen-activated protein kinases. 4E-BP1 expression also decreases when the p38/mitogen-activated protein kinase pathway is activated by granulocyte/macrophage colony-stimulating factor but to a lesser extent than with PMA. In a second part, we examine how 4e-bp1 promoter activity is regulated. PMA and granulocyte/macrophage colony-stimulating factor induce Egr-1 expression through ERK and p38 activation, respectively. Using a dominant negative mutant of Egr, ZnEgr, we show that this transcription factor is responsible for the inhibition of 4e-bp1 promoter activity. In a third part we show that histidine decarboxylase, whose activity and expression are inversely correlated with 4E-BP1 expression, is a potential target for the translational machinery. These data (i) are the first evidence of a new role of ERK and p38 on the translational machinery and (ii) demonstrate that 4E-BP1 is a new target for Egr-1.

Molecular recognitions in the MAP kinase cascades

The mitogen-activated protein kinase (MAPK) cascades play a pivotal role in many aspects of cellular functions, and are evolutionarily conserved from yeast to mammals. In mammals, there are four subfamily members in the MAPKs. Each MAPK has its own activators, substrates and inactivators. In order to achieve normal cellular functions, the MAPK cascades should transduce signals with high efficiency and fidelity. However, the molecular basis for the mechanism underlying the specific reactions in the MAPK cascades has not been fully understood. The MAPKs form a globular structure without a distinct domain specific for protein-protein interactions. Recent studies revealed two mechanisms regulating the signalling, the docking interaction and the scaffolding. The docking interaction is achieved through the common docking domain (the CD domain) on MAPKs, and is different from a transient enzyme-substrate interaction through the active centre of the enzymes. Almost all the MAPK-interacting molecules have a conserved motif interacting with the CD domain. The scaffolding usually utilizes a third molecule to tether several components of the MAPK cascades. Both of them are thought to regulate the enzymatic specificity and efficiency.

COM crystals activate the p38 mitogen-activated protein kinase signal transduction pathway in renal epithelial cells

Interaction of calcium oxalate monohydrate (COM) crystals with renal cells has been shown to result in altered gene expression, DNA synthesis, and cell death. In the current study the role of a stress-specific p38 MAP kinase-signaling pathway in mediating these effects of COM crystals was investigated. Exposure of cells to COM crystals (20 microg/cm(2)) rapidly stimulated strong phosphorylation and activation of p38 mitogen-activated protein kinase (p38 MAP kinase) and re-initiation of DNA synthesis. Inhibition of COM crystal binding to the cells by heparin blocked the effects of COM crystals on p38 MAPK activation. We also show that specific inhibition of p38 MAPK by 4-(4-fluorophenyl)-2-(4-methylsulfonylphenyl)-5-(4-pyridyl) imidazole (SB203580) or by overexpression of a dominant negative mutant of p38 MAP kinase abolishes COM crystal-induced re-initiation of DNA synthesis. The inhibition is dose-dependent and correlates with in situ activity of native p38 MAP kinase, determined as mitogen-activated protein kinase-activated protein kinase-2 (MAPKAP kinase-2) activity in cell extracts. In summary, inhibiting activation of p38 MAPK pathway abrogated the DNA synthesis in response to COM crystals. These data are the first demonstrations of activation of the p38 MAPK signaling pathway by COM crystals and suggest that, in response to COM crystals, this pathway transduces critical signals governing the re-initiation of DNA synthesis in renal epithelial cells.

Rac-PAK signaling stimulates extracellular signal-regulated kinase (ERK) activation by regulating formation of MEK1-ERK complexes

Utilizing mutants of extracellular signal-regulated kinase 2 (ERK2) that are defective for intrinsic mitogen-activated protein kinase or ERK kinase (MEK) binding, we have identified a convergent signaling pathway that facilitates regulated MEK-ERK association and ERK activation. ERK2-delta19-25 mutants defective in MEK binding could be phosphorylated in response to mitogens; however, signaling from the Raf-MEK pathway alone was insufficient to stimulate their phosphorylation in COS-1 cells. Phosphorylation of ERK2-delta19-25 but not of wild-type ERK2 in response to Ras V12 was greatly inhibited by dominant-negative Rac. Activated forms of Rac and Cdc42 could enhance the association of wild-type ERK2 with MEK1 but not with MEK2 in serum-starved adherent cells. This effect was p21-activated kinase (PAK) dependent and required the putative PAK phosphorylation sites T292 and S298 of MEK1. In detached cells placed in suspension, ERK2 was complexed with MEK2 but not with MEK1. However, upon replating of cells onto a fibronectin matrix, there was a substantial induction of MEK1-ERK2 association and ERK activation, both of which could be inhibited by dominant-negative PAK1. These data show that Rac facilitates the assembly of a mitogen-activated protein kinase signaling complex required for ERK activation and that this facilitative signaling pathway is active during adhesion to the extracellular matrix. These findings reveal a novel mechanism by which adhesion and growth factor signals are integrated during ERK activation.

Cell adhesion differentially regulates the nucleocytoplasmic distribution of active MAP kinases

Cells decide whether to undergo processes, such as proliferation,differentiation and apoptosis, based upon the cues they receive from both circulating factors and integrin-mediated adhesion to the extracellular matrix. Integrins control the activation of the early signaling pathways. For example, growth factor activation of the ERK cascade is enhanced when cells are adherent. In addition, adhesion receptors oversee the cellular localization of critical signaling components. We have recently shown that ERK signaling to the nucleus is regulated by cell adhesion at the level of nucleocytoplasmic trafficking. Since the ERKs are only one class of MAP kinase, we extended these studies to include both JNK and p38 MAP kinases. We have rendered JNK and p38 activation in NIH 3T3 fibroblasts anchorage-independent either by treatment with anisomycin or by expression of upstream activators. Under conditions whereby JNK activation is anchorage-independent, we show that localization of JNK to the nucleus and JNK-mediated phosphorylation of c-Jun and Elk-1 is not altered by loss of adhesion. Likewise, the ability of activated p38 to accumulate in the nucleus was similar in suspended and adherent cells. Finally, we show that expression of a form of ERK, which is activated and resistant to nuclear export, reverses the adhesion-dependency of ERK phosphorylation of Elk-1. Thus, adhesion differentially regulates the nucleocytoplasmic distribution of MAP kinase members; ERK accumulation in the nucleus occurs more efficiently in adherent cells, whereas nuclear accumulation of active p38 and active JNK are unaffected by changes in adhesion.

Characterization of a murine gene encoding a developmentally regulated cytoplasmic dual-specificity mitogen-activated protein kinase phosphatase

Mitogen-activated protein kinases (MAPKs) play a vital role in cellular growth control, but far less is known about these signalling pathways in the context of embryonic development. Duration and magnitude of MAPK activation are crucial factors in cell fate decisions, and reflect a balance between the activities of upstream activators and specific MAPK phosphatases (MKPs). Here, we report the isolation and characterization of the murine Pyst3 gene, which encodes a cytosolic dual-specificity MKP. This enzyme selectively interacts with, and is catalytically activated by, the 'classical' extracellular signal-regulated kinases (ERKs) 1 and 2 and, to a lesser extent, the stress-activated MAPK p38alpha. These properties define the ability of this enzyme to dephosphorylate and inactivate ERK1/2 and p38alpha, but not JNK (c-Jun N-terminal kinase) in vivo. When expressed in mammalian cells, the Pyst3 protein is predominantly cytoplasmic. Furthermore, leptomycin B causes a complete redistribution of the protein to the nucleus, implicating a CRM (chromosomal region maintenance)1/exportin 1-dependent nuclear export signal in determining the subcellular localization of this enzyme. Finally, whole-mount in situ hybridization studies in mouse embryos reveal that the Pyst3 gene is expressed specifically in the placenta, developing liver and in migratory muscle cells. Our results suggest that this enzyme may have a critical role in regulating the activity of MAPK signalling during early development and organogenesis.

Identification of novel point mutations in ERK2 that selectively disrupt binding to MEK1

Extracellular signal-regulated kinases 1 and 2 (ERK1 and ERK2) are essential components of pathways through which signals received at membrane receptors are converted into specific changes in protein function and gene expression. As with other members of the mitogen-activated protein (MAP) kinase family, ERK1 and ERK2 are activated by phosphorylations catalyzed by dual-specificity protein kinases known as MAP/ERK kinases (MEKs). MEKs exhibit stringent specificity for individual MAP kinases. Indeed, MEK1 and MEK2 are the only known activators of ERK1 and ERK2. ERK2 small middle dotMEK1/2 complexes can be detected in vitro and in vivo. The biochemical nature of such complexes and their role in MAP kinase signaling are under investigation. This report describes the use of a yeast two-hybrid screen to identify point mutations in ERK2 that impair its interaction with MEK1/2, yet do not alter its interactions with other proteins. ERK2 residues identified in this screen are on the surface of the C-terminal domain of the kinase, either within or immediately preceding alpha-helix G, or within the MAP kinase insert. Some mutations identified in this manner impaired the two-hybrid interaction of ERK2 with both MEK1 and MEK2, whereas others had a predominant effect on the interaction with either MEK1 or MEK2. Mutant ERK2 proteins displayed reduced activation in HEK293 cells following epidermal growth factor treatment, consistent with their impaired interaction with MEK1/2. However, ERK2 proteins containing MEK-specific mutations retained kinase activity, and were similar to wild type ERK2 in their activation following overexpression of constitutively active MEK1. Unlike wild type ERK2, proteins containing MEK-specific point mutations were constitutively localized in the nucleus, even in the presence of overexpressed MEK1. These data suggest an essential role for the MAP kinase insert and residues within or just preceding alpha-helix G in the interaction of ERK2 with MEK1/2.

Oxalate selectively activates p38 mitogen-activated protein kinase and c-Jun N-terminal kinase signal transduction pathways in renal epithelial cells

Oxalate, a metabolic end product, is an important factor in the pathogenesis of renal stone disease. Oxalate exposure to renal epithelial cells results in re-initiation of the DNA synthesis, altered gene expression, and apoptosis, but the signaling pathways involved in these diverse effects have not been evaluated. The effects of oxalate on mitogen- and stress-activated protein kinase signaling pathways were studied in LLC-PK1 cells. Exposure to oxalate (1 mM) rapidly stimulated robust phosphorylation and activation of p38 MAPK. Oxalate exposure also induced modest activation of JNK, as monitored by phosphorylation of c-Jun. In contrast, oxalate exposure had no effect on phosphorylation and enzyme activity of p42/44 MAPK. We also show that specific inhibition of p38 MAPK by 4(4-(fluorophenyl)-2-(4-methylsulfonylphenyl)-5-(4-pyridyl)imidazole (SB203580) or by overexpression of a kinase-dead dominant negative mutant of p38 MAPK abolishes oxalate induced re-initiation of DNA synthesis in LLC-PK1 cells. The inhibition is dose-dependent and correlates with in situ activity of native p38 MAP kinase, determined as MAPK-activated protein kinase-2 activity in cell extracts. Thus, this study not only provides the first demonstration of selective activation of p38 MAPK and JNK signaling pathways by oxalate but also suggests that p38 MAPK activity is essential for the effects of oxalate on re-initiation of DNA synthesis.

Different domains of the mitogen-activated protein kinases ERK3 and ERK2 direct subcellular localization and upstream specificity in vivo

Extracellular signal-regulated kinase 3 (ERK3) is a member of the mitogen-activated protein (MAP) kinase family. ERK3 is most similar in its kinase catalytic domain to ERK2, yet it displays many unique properties. Among these, unlike ERK2, which translocates to the nucleus following activation, ERK3 is constitutively localized to the nucleus, despite the lack of a defined nuclear localization sequence. We created two chimeras between ERK2 and the catalytic domain of ERK3 (ERK3DeltaC), and some mutants of these chimeras, to examine the basis for the different behaviors of these two MAP kinase family members. We find the following: 1) the N-terminal folding domain of ERK3 functions in phosphoryl transfer reactions with the C-terminal folding domain of ERK2; 2) the C-terminal halves of ERK2 and ERK3DeltaC are primarily responsible for their subcellular localization in resting cells; and 3) the N-terminal folding domain of ERK2 is required for its activation in cells, its interaction with MEK1, and its accumulation in the nucleus.

Inhibition of MEK1,2/ERK mitogenic pathway by estrogen with antiproliferative properties in rat aortic smooth muscle cells

Proliferation and migration of vascular smooth muscle cells (VSMCs) are believed to contribute significantly to intimal thickening in atheroscleosis, restenosis, and venous bypass graft disease. Estrogen inhibits proliferation and migration of VSMCs. However, antiproliferative mechanisms of estrogen were not well elucidated yet. In this study, we investigated the antiproliferative effect of estrogen to determine whether the transduction signals and protooncogenes were affected in rat aortic smooth muscle cells (RASMCs). Estrogen inhibited the proliferative response stimulated by 5% fetal bovine serum (FBS) dose-dependently in RASMCs (IC50: 40 nM). In 0.5% serum-treated RASMCs, estrogen dramatically inhibited the activity of extracellular signal-regulated kinases (ERK) followed by inhibition of MEK1,2 activity in dose-dependent manner without affecting the other mitogen-activating protein kinases (MAPKs), c-jun N-terminal kinases (JNK) and p38. Induction of Elk-1 mRNA was significantly reduced dose-dependently up to 100 nM of estrogen. These results indicate that the antiproliferative effects of estrogen in RASMCs involved ERK inhibition followed by the inactivation of MEK1,2 and downregulation of Elk-1 expression.

Involvement of the activation loop of ERK in the detachment from cytosolic anchoring

The Extracellular signal-regulated kinases (ERKs) are translocated into the nucleus in response to mitogenic stimulation. The mechanism of translocation and the residues in ERKs that govern this process are not clear as yet. Here we studied the involvement of residues in the activation loop of ERK2 in determining its subcellular localization. Substitution of residues in the activation loop to alanines indicated that residues 173-181 do not play a significant role in the phosphorylation and activation of ERK2. However, residues 176-181 are responsible for the detachment of ERK2 from MEK1 upon mitogenic stimulation. This dissociation can be mimicked by substitution of residues 176-178 to alanines and is prevented by deletion of these residues or by substitution of residues 179-181 to alanines. On the other hand, residues 176-181, as well as residues essential for ERK2 dimerization, do not play a role in the shuttle of ERK2 through nuclear pores. Thus, phosphorylation-induced conformational rearrangement of residues in the activation loop of ERK2 plays a major role in the control of subcellular localization of this protein.

Phosphorylation of MafA is essential for its transcriptional and biological properties

We previously described the identification of quail MafA, a novel transcription factor of the Maf bZIP (basic region leucine zipper) family, expressed in the differentiating neuroretina (NR). In the present study, we provide the first evidence that MafA is phosphorylated and that its biological properties strongly rely upon phosphorylation of serines 14 and 65, two residues located in the transcriptional activating domain within a consensus for phosphorylation by mitogen-activated protein kinases and which are conserved among Maf proteins. These residues are phosphorylated by ERK2 but not by p38, JNK, and ERK5 in vitro. However, the contribution of the MEK/ERK pathway to MafA phosphorylation in vivo appears to be moderate, implicating another kinase. The integrity of serine 14 and serine 65 residues is required for transcriptional activity, since their mutation into alanine severely impairs MafA capacity to activate transcription. Furthermore, we show that the MafA S14A/S65A mutant displays reduced capacity to induce expression of QR1, an NR-specific target of Maf proteins. Likewise, the integrity of serines 14 and 65 is essential for the MafA ability to stimulate expression of crystallin genes in NR cells and to induce NR-to-lens transdifferentiation. Thus, the MafA capacity to induce differentiation programs is dependent on its phosphorylation.

Hydrophobic as Well as Charged Residues in Both MEK1 and ERK2 Are Important for Their Proper Docking

Docking between MEK1 and ERK2 is required for their stable interaction and efficient signal transmission. The MEK1 N terminus contains the ERK docking or D domain that consists of conserved hydrophobic and basic residues. We mutated the hydrophobic and basic residues individually and found that loss of either type reduced MEK1 phosphorylation of ERK2 in vitro and its ability to bind to ERK2 in vivo. Moreover, ERK2 was localized in both the cytoplasm and the nucleus when co-expressed with MEK1 that had mutations in either the hydrophobic or the basic residues. We then identified two conserved hydrophobic residues on ERK2 that play roles in docking with MEK1. Mutating these residues to alanine reduced the interaction of ERK2 with MEK1 in cells. These mutations also reduced the phosphorylation of MEK1 by ERK2 but had little effect on phosphorylation of MBP by ERK2. Finally, we generated docking site mutants in ERK2-MEK1 fusion proteins. Although the mutation of the MEK1 D domain significantly reduced ERK2-MEK1 activity, mutations of the putatively complementary acidic residues and hydrophobic residues on ERK2 did not change its activity. However, both types of mutations decreased the phosphorylation of Elk-1 caused by ERK2-MEK1 fusion proteins. These findings suggest complex interactions of MEK1 D domains with ERK2 that influence its activation and its effects on substrates.

UV-A-induced AP-1 activation requires the Raf/ERK pathway in human NCTC 2544 keratinocytes

UV-A irradiation causes a dose-dependent activation of ERK in human NCTC 2544 keratinocytes. The specific inhibition of either ERK activity or Raf kinase activity impedes the activation of AP-1 DNA binding induced by UV-A. In addition, UV-A raises AP-1 promoter transcriptional activity, which is downregulated in NCTC 2544 cells expressing an inactive mutant of Raf-1. We found that singlet oxygen might be one of the mediators in both UV-A-induced AP-1 DNA binding and transcriptional activity. These results strongly suggest that UV-A-induced AP-1 activity requires the Raf-ERK pathway and imply a singlet oxygen effector.

EGR-1, a UV-inducible gene in p53(-/-) mouse cells

Changes in gene expression were examined in p53(-/-) and p53(+/+) mouse cells after ultraviolet (UV) irradiation. Differential display was used to identify differentially expressed gene(s) in UV-treated p53(-/-) and p53(+/+) cells. One of the differentially expressed genes was EGR-1 (early growth response gene-1), which was shown to be induced only in p53(-/-) cells. The induction of this gene by UV was detected as early as 0.5 h, peaked at 2 h, and returned to normal levels by 4 h. De novo protein synthesis was not required for UV-induced EGR-1 expression in p53(-/-) cells. Pretreatment of p53(-/-) cells with suramin, an inhibitor of growth factor receptors, completely suppressed UV-induced EGR-1 expression, suggesting that the induction may be mediated via the growth factor receptors. The presence of wild-type p53 suppressed the induction of EGR-1 after UV treatment. Overexpression of EGR-1 promoted the UV-induced transformation in p53(+/+) cells, but not in p53(-/-) cells. These data suggested that EGR-1 may be an important player in the UV responses of mammalian cells and may influence UV-induced transformation.

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

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

A conserved docking site in MEKs mediates high-affinity binding to MAP kinases and cooperates with a scaffold protein to enhance signal transmission

The recognition of mitogen-activated protein kinases (MAPKs) by their upstream activators, MAPK/ERK kinases (MEKs), is crucial for the effective and accurate transmission of many signals. We demonstrated previously that the yeast MAPKs Kss1 and Fus3 bind with high affinity to the N terminus of the MEK Ste7, and proposed that a conserved motif in Ste7, the MAPK-docking site, mediates this interaction. Here we show that the corresponding sequences in human MEK1 and MEK2 are necessary and sufficient for the direct binding of the MAPKs ERK1 and ERK2. Mutations in MEK1, MEK2, or Ste7 that altered conserved residues in the docking site diminished binding of the cognate MAPKs. Furthermore, short peptides corresponding to the docking sites in these MEKs inhibited MEK1-mediated phosphorylation of ERK2 in vitro. In yeast cells, docking-defective alleles of Ste7 were modestly compromised in their ability to transmit the mating pheromone signal. This deficiency was dramatically enhanced when the ability of the Ste5 scaffold protein to associate with components of the MAPK cascade was also compromised. Thus, both the MEK-MAPK docking interaction and binding to the Ste5 scaffold make mutually reinforcing contributions to the efficiency of signaling by this MAPK cascade in vivo.

Elevated superoxide production by active H-ras enhances human lung WI-38VA-13 cell proliferation, migration and resistance to TNF-alpha

Accumulating evidence has suggested that cellular production of superoxide acts as an intracellular messenger to regulate gene expression and modulate cellular activities. In this report, we set out to investigate the role of active H-ras-mediated superoxide production on tumor cell malignancy in a SV-40 transformed human lung WI-38 VA-13 cell line. Stable transfection and expression of constitutively active mutant V12-H-ras (V12-H-ras) dramatically increased intracellular production of superoxide. The expression of V12-H-ras significantly enhanced cell proliferation, migration and resistance to TNF-alpha treatment compared to that of parental and vector control cells, while expression of wild type H-ras (WT-H-ras) only had modest effects. Upon scavenging by superoxide dismutase and other molecules that decrease the intracellular level of active H-ras mediated superoxide production, cell proliferation, migration and resistance to TNF-alpha were significantly reduced. Furthermore, we demonstrated that the activation of membrane NADPH oxidase activity by expression of active H-ras contributed to the intracellular superoxide production. The causal relationship between membrane superoxide production and increased cell proliferation, migration, and resistance to TNF-alpha by the expression of active H-ras, has provided direct evidence to demonstrate that superoxide acts as an intracellular messenger to cascade ras oncogenic signal relay and to modulate tumor malignant activity.

Mitogen-activated protein [MAP] kinase pathways in plants: versatile signaling tools

Mitogen-activated protein kinases (MAPKs) are important signaling tools in all eukaryotes, and function in mediating an enormous variety of external signals to appropriate cellular responses. MAPK pathways have been studied extensively in yeast and mammalian cells, and a large body of knowledge on their functioning has accumulated, which is summarized briefly. Plant MAPK pathways have attracted increasing interest, resulting in the isolation of a large number of different components of MAPK cascades. Studies on the functions of these components have revealed that MAPKs play important roles in the response to a broad variety of stresses, as well as in the signaling of most plant hormones and in developmental processes. Finally, the involvement of various plant phosphatases in the inactivation of MAPKs is discussed.

Identification of a docking groove on ERK and p38 MAP kinases that regulates the specificity of docking interactions

MAP kinases (MAPKs) form a complex with MAPK kinases (MAPKKs), MAPK-specific phosphatases (MKPs) and various targets including MAPKAPKs. These docking interactions contribute to regulation of the specificity and efficiency of the enzymatic reactions. We have previously identified a docking site on MAPKs, termed the CD (common docking) domain, which is utilized commonly for docking interactions with MAPKKs, MKPs and MAPKAPKs. However, the CD domain alone does not determine the docking specificity. Here we have identified a novel site on p38 and ERK2 MAPKs that regulates the docking specificity towards MAPKAPKs. Remarkably, exchange of two amino acids in this site of ERK2 for corresponding residues of p38 converted the docking specificity for MAPKAPK-3/3pk, which is a dominant target of p38, from the ERK2 type to the p38 type, and vice versa. Furthermore, our detailed analyses with a number of MAPKAPKs and MKPs suggest that a groove in the steric structure of MAPKs, which comprises the CD domain and the site identified here, serves as a common docking region for various MAPK-interacting molecules.