Novel membrane-targeted ERK1 and ERK2 chimeras which act as dominant negative, isotype-specific mitogen-activated protein kinase inhibitors of Ras-Raf-mediated transcriptional activation of c-fos in NIH 3T3 cells

Protease-activated receptor 4: a critical participator in inflammatory response

Protease-activated receptors (PARs) are G protein-coupled receptors of which four members PAR1, PAR2, PAR3, and PAR4 have been identified, characterized by a typical mechanism of activation involving various related proteases. The amino-terminal sequence of PARs is cleaved by a broad array of proteases, leading to specific proteolytic cleavage which forms endogenous tethered ligands to induce agonist-biased PAR activation. The biological effect of PARs activated by coagulation proteases to regulate hemostasis and thrombosis plays an enormous role in the cardiovascular system, while PAR4 can also be activated by trypsin, cathepsin G, the activated factor X of the coagulation cascade, and trypsin IV. Irrespective of its role in thrombin-induced platelet aggregation, PAR4 activation is believed to be involved in inflammatory lesions, as show by investigations that have unmasked the effects of PAR4 on neutrophil recruitment, the regulation of edema, and plasma extravasation. This review summarizes the roles of PAR4 in coagulation and other extracellular protease pathways, which activate PAR4 to participate in normal regulation and disease.

Protease-activated receptor 4 activation increases the expression of calcitonin gene-related peptide mRNA and protein in dorsal root ganglion neurons

Accumulating evidence demonstrates that nociceptor activation evokes a rapid change in mRNA and protein levels of calcitonin gene-related peptide (CGRP) in dorsal root ganglion (DRG) neurons. Although the colocalization of CGRP and protease-activated receptor-4 (PAR4), a potent modulator of pain processing and inflammation, was detected in DRG neurons, the role of PAR4 activation in the expression of CGRP has not been investigated. In the present study, the expression of CGRP and activation (phosphorylation) of extracellular signal-regulated kinases 1 and 2 (ERK1/2) in rat DRG neurons were measured by immunofluorescence, real-time PCR, and Western blotting after AYPGKF-NH2 (selective PAR4-activating peptide; PAR4-AP) intraplantar injection or treatment of cultured DRG neurons. The expression of CGRP in cultured DRG neurons was also assessed after treatment with AYPGKF-NH2 with preaddition of PD98059 (an inhibitor for ERK1/2 pathway). Results showed that PAR4-AP intraplantar injection or treatment of cultured DRG neurons evoked significant increases in DRG cells displaying CGRP immunoreactivity and cytoplasmic and nuclear staining for phospho-ERK1/2 (p-ERK1/2). Percentages of total DRG neurons expressing both CGRP and PAR4 or p-ERK1/2 also increased significantly at 2 hr after PAR4-AP treatment. Real-time PCR and Western blotting showed that PAR4-AP treatment significantly increased expression of CGRP mRNA and protein levels in DRG neurons. The PAR4 activation-evoked CGRP expression both at mRNA and at protein levels was significantly inhibited after p-ERK1/2 was inhibited by PD98059. These results provide evidence that activation of PAR4 upregulates the expression of CGRP mRNA and protein levels in DRG neurons via the p-ERK1/2 signal pathway.

MiR-494 is regulated by ERK1/2 and modulates TRAIL-induced apoptosis in non-small-cell lung cancer through BIM down-regulation

MicroRNAs (miRNAs) have an important role in the development of chemosensitivity or chemoresistance in different types of cancer. Activation of the ERK1/2 pathway is a major determinant of diverse cellular processes and cancer development and is responsible for the transcription of several important miRNAs. Here we show a link between the ERK1/2 pathway and BIM expression through miR-494. We blocked ERK1/2 nuclear activity through the overexpression of an ERK1/2 natural interactor, the protein PED/PEA15, and we performed a microRNA expression profile. miR-494 was the most down-regulated microRNA after ERK1/2 inactivation. Moreover, we found that miR-494 induced Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) resistance in non-small-cell lung cancer (NSCLC) through the down-modulation of BIM. Elucidation of this undiscovered ERK1/2 pathway that regulates apoptosis and cell proliferation through miR-494 in NSCLC will greatly enhance our understanding of the mechanisms responsible for TRAIL resistance and will provide an additional arm for the development of anticancer therapies.

Mouse tissues that undergo neoplastic progression after K-Ras activation are distinguished by nuclear translocation of phospho-Erk1/2 and robust tumor suppressor responses

Mutation of K-Ras is a frequent oncogenic event in human cancers, particularly cancers of lungs, pancreas, and colon. It remains unclear why some tissues are more susceptible to Ras-induced transformation than others. Here, we globally activated a mutant oncogenic K-Ras allele (K-Ras(G12D)) in mice and examined the tissue-specific effects of this activation on cancer pathobiology, Ras signaling, tumor suppressor, DNA damage, and inflammatory responses. Within 5 to 6 weeks of oncogenic Ras activation, mice develop oral and gastric papillomas, lung adenomas, and hematopoietic hyperproliferation and turn moribund. The oral, gastric, and lung premalignant lesions display activated extracellular signal-regulated kinases (Erk)1/2 and NF-κB signaling as well as activated tumor suppressor and DNA damage responses. Other organs such as pancreas, liver, and small intestine do not exhibit neoplastic progression within 6 weeks following K-Ras(G12D) activation and do not show a potent tumor suppressor response. Even though robust Erk1/2 signaling is activated in all the tissues examined, the pErk1/2 distribution remains largely cytoplasmic in K-Ras(G12D)-refractory tissues (pancreas, liver, and intestines) as opposed to a predominantly nuclear localization in K-Ras(G12D)-induced neoplasms of lung, oral, and gastric mucosa. The downstream targets of Ras signaling, pElk-1 and c-Myc, are elevated in K-Ras(G12D)-induced neoplastic lesions but not in K-Ras(G12D)-refractory tissues. We propose that oncogenic K-Ras-refractory tissues delay oncogenic progression by spatially limiting the efficacy of Ras/Raf/Erk1/2 signaling, whereas K-Ras-responsive tissues exhibit activated Ras/Raf/Erk1/2 signaling, rapidly form premalignant tumors, and activate potent antitumor responses that effectively prevent further malignant progression.

Prenylation-deficient G protein gamma subunits disrupt GPCR signaling in the zebrafish

Prenylation of G protein gamma (gamma) subunits is necessary for the membrane localization of heterotrimeric G proteins and for functional heterotrimeric G protein coupled receptor (GPCR) signaling. To evaluate GPCR signaling pathways during development, we injected zebrafish embryos with mRNAs encoding Ggamma subunits mutated so that they can no longer be prenylated. Low-level expression of these prenylation-deficient Ggamma subunits driven either ubiquitously or specifically in the primordial germ cells (PGCs) disrupts GPCR signaling and manifests as a PGC migration defect. This disruption results in a reduction of calcium accumulation in the protrusions of migrating PGCs and a failure of PGCs to directionally migrate. When co-expressed with a prenylation-deficient Ggamma, 8 of the 17 wildtype Ggamma isoforms individually confer the ability to restore calcium accumulation and directional migration. These results suggest that while the Ggamma subunits possess the ability to interact with G Beta (beta) proteins, only a subset of wildtype Ggamma proteins are stable within PGCs and can interact with key signaling components necessary for PGC migration. This in vivo study highlights the functional redundancy of these signaling components and demonstrates that prenylation-deficient Ggamma subunits are an effective tool to investigate the roles of GPCR signaling events during vertebrate development.

The regulation of extracellular signal-regulated kinase (ERK) in mammalian cells

The mitogen-activated protein (MAP) kinase extracellular-signal-regulated kinases (ERKs) are activated by diverse mechanisms. These include ligation of receptor tyrosine kinases such as epidermal growth factor (EGF) and cell adhesion receptors such as the integrins. In general, ligand binding of these receptors leads to GTP loading and activation of the small GTPase Ras, which recruits Raf to the membrane where it is activated. Raf subsequently phosphorylates the dual specificity MAP/ERK kinase (MEK1/2) which in turn phosphorylates and thereby activates ERK. ERK is a promiscuous kinase and can phosphorylate more than 100 different substrates. Therefore activation of ERK can affect a broad array of cellular functions including proliferation, survival, apoptosis, motility, transcription, metabolism and differentiation. ERK activity is controlled by many distinct mechanisms. Scaffold proteins control when and where ERK is activated while anchoring proteins can restrain ERK localization to specific subcellular compartments. Meanwhile, phosphatases dephosphorylate and inactivate ERK thereby shutting off the pathway. Finally, several feedback mechanisms have been identified downstream of ERK activation. Here we will focus on the diverse mechanisms of ERK regulation in mammalian cells.

Interaction between ERK and GSK3beta mediates basic fibroblast growth factor-induced apoptosis in SK-N-MC neuroblastoma cells

The Ewing's sarcoma family of tumors (ESFT) includes Ewing's sarcoma (ES), Askin's tumor of the chest wall, and peripheral primitive neuroectodermal tumor. Basic fibroblast growth factor (FGF2) suppresses the growth of ESFT cells and causes their apoptosis. The underlying mechanism is unclear. Using a human peripheral primitive neuroectodermal tumor cell line, SK-N-MC, we demonstrated FGF2 stimulated phosphorylation of ERK1 and ERK2 (pERK1/2) and GSK3beta (pGSK3beta(Tyr-216)), all of which were primarily retained in the cytoplasm. FGF2 promoted the association between ERK and pGSK3beta(Tyr-216). Inhibitors for GSK3beta (TDZD and LiCl) and ERK (PD98059) protected cells from FGF2-induced apoptosis. On the other hand, inhibitors of GSK3beta, but not PD98059 decreased ERK/pGSK3beta(Tyr-216) association and caused a nuclear translocation of pERK1/2. Similarly, expression of a kinase-deficient (K85R) GSK3beta or GSK3beta-small interfering RNA inhibited FGF2-regulated ERK/pGSK3beta(Tyr-216) association and translocated pERK to the nucleus. Both K85R GSK3beta and small interfering RNA offered protection against FGF2-induced cell death. In contrast, overexpression of wild-type GSK3beta sensitized cells to FGF2 cytotoxicity. Hydrogen peroxide and ethanol enhanced FGF2-stimulated pGSK3beta(Tyr-216), ERK/pGSK3beta(Tyr-216) association, and cytoplasmic retention of pERK1/2. As a result, they potentiated FGF2-induced cell death. Taken together, our results suggested that FGF2-induced accumulation of pERK1/2 in the cytoplasm is toxic for SK-N-MC cells. The formation of an ERK.GSK3beta complex retained pERK1/2 in the cytoplasm. In contrast, disruption of the ERK.GSK3beta complex resulted in nuclear translocation of pERK1/2 and offered protection.

Cell surface restriction of EGFR by a tenascin cytotactin-encoded EGF-like repeat is preferential for motility-related signaling

The 14th EGFL-repeat (Ten14) of human tenascin cytotactin activates the epidermal growth factor receptor (EGFR) with micromolar affinity; however, unlike EGF, Ten14-mediated activation of EGFR does not lead to receptor internalization. As the divergent signaling pathways downstream of EGFR have been shown to be triggered from plasma membrane and cytosolic locales, we investigated whether Ten14-mediated surface restriction of EGFR resulted in altered biochemical and cellular responses as compared to EGF. Molecules associated with migratory cascades were activated to a relatively greater extent in response to Ten14, with very weak activation of proliferation-associated cascades. Activation of phospholipase C gamma (PLCgamma) and m-calpain, associated with lamellipod protrusion and tail retraction, respectively, were noted at even at sub-saturating doses of Ten14. However, activation of ERK/MAPK, p90RSK, and Elk1, factors affecting proliferation, remained low even at high Ten14 concentrations. Similar activation profiles were observed for EGF-treated cells at 4 degrees C, a maneuver that limits receptor internalization. We demonstrate a concurrent effect of such altered signaling on biophysical responses-sustained migration was observed at levels of Ten14 that activated PLCgamma, but did not stimulate proliferation significantly. Here, we present a novel class of EGFR ligands that can potentially signal as a part of the extracellular matrix, triggering specific intracellular signaling cascades leading to a directed cellular response from an otherwise pleiotropic receptor. This work extends the signaling paradigm of EGFL repeat being presented in a restricted fashion as part of the extracellular matrix.

The RhoGAP RGA-2 and LET-502/ROCK achieve a balance of actomyosin-dependent forces inC. elegansepidermis to control morphogenesis

Embryonic morphogenesis involves the coordinate behaviour of multiple cells and requires the accurate balance of forces acting within different cells through the application of appropriate brakes and throttles. In C. elegans, embryonic elongation is driven by Rho-binding kinase (ROCK) and actomyosin contraction in the epidermis. We identify an evolutionary conserved, actin microfilament-associated RhoGAP (RGA-2) that behaves as a negative regulator of LET-502/ROCK. The small GTPase RHO-1 is the preferred target of RGA-2 in vitro, and acts between RGA-2 and LET-502 in vivo. Two observations show that RGA-2 acts in dorsal and ventral epidermal cells to moderate actomyosin tension during the first half of elongation. First,time-lapse microscopy shows that loss of RGA-2 induces localised circumferentially oriented pulling on junctional complexes in dorsal and ventral epidermal cells. Second, specific expression of RGA-2 in dorsal/ventral, but not lateral, cells rescues the embryonic lethality of rga-2 mutants. We propose that actomyosin-generated tension must be moderated in two out of the three sets of epidermal cells surrounding the C. elegans embryo to achieve morphogenesis.

Activation of MAPK pathways links LMNA mutations to cardiomyopathy in Emery-Dreifuss muscular dystrophy

Mutations in LMNA, which encodes nuclear Lamins A and C cause diseases affecting various organs, including the heart. We have determined the effects of an Lmna H222P mutation on signaling pathways involved in the development of cardiomyopathy in a knockin mouse model of autosomal dominant Emery-Dreifuss muscular dystrophy. Analysis of genome-wide expression profiles in hearts using Affymetrix GeneChips showed statistically significant differences in expression of genes in the MAPK pathways at the incipience of the development of clinical disease. Using real-time PCR, we showed that activation of MAPK pathways preceded clinical signs or detectable molecular markers of cardiomyopathy. In heart tissue and isolated cardiomyocytes, there was activation of MAPK cascades and downstream targets, implicated previously in the pathogenesis of cardiomyopathy. Expression of H222P Lamin A in cultured cells activated MAPKs and downstream target genes. Activation of MAPK signaling by mutant A-type lamins could be a cornerstone in the development of heart disease in autosomal dominant Emery-Dreifuss muscular dystrophy.

PEA-15 inhibits tumor cell invasion by binding to extracellular signal-regulated kinase 1/2

Phosphoprotein enriched in astrocytes of 15 kDa (PEA-15) binds to extracellular signal-regulated kinase 1 and 2 (ERK1/2) mitogen-activated protein (MAP) kinases to alter ERK1/2 cellular localization and target preferences and binds to adaptors in the extrinsic cell death pathway to block apoptosis. Here, we report that PEA-15 protein expression is inversely correlated with the invasive behavior of breast cancer in an immunohistochemical analysis of a breast cancer progression tissue microarray. Short hairpin RNA-mediated inhibition of PEA-15 expression increased the invasion of PEA-15-expressing tumor cells in vitro, suggesting a causative role for PEA-15 in the inhibition of invasion. This causative role was confirmed by the finding that the enforced expression of PEA-15 in invasive tumor cells reduced invasion. The effect of PEA-15 on tumor invasion is mediated by its interaction with ERK1/2 as shown by the following: (a) PEA-15 mutants that fail to bind ERK1/2 did not inhibit invasion; (b) overexpression of ERK1 or activated MAP/ERK kinase (MEK) reversed the inhibitory effect of PEA-15; (c) when an inhibitor of ERK1/2 activation reduced invasion, PEA-15 expression did not significantly reduce invasion further. Furthermore, we find that the effect of PEA-15 on invasion seems to relate to the nuclear localization of activated ERK1/2. PEA-15 inhibits invasion by keeping ERK out of the nucleus, as a PEA-15 mutant that cannot prevent ERK nuclear localization was not able to inhibit invasion. In addition, membrane-localized ERK1, which sequesters endogenous ERK1 to prevent its nuclear localization, also inhibited invasion. These results reveal that PEA-15 regulates cancer cell invasion via its ability to bind ERK1/2 and indicate that nuclear entry of ERK1/2 is important in tumor behavior.

The MEK/ERK cascade: from signaling specificity to diverse functions

The ERK signaling cascade is a central MAPK pathway that plays a role in the regulation of various cellular processes such as proliferation, differentiation, development, learning, survival and, under some conditions, also apoptosis. The ability of this cascade to regulate so many distinct, and even opposing, cellular processes, raises the question of signaling specificity determination by this cascade. Here we describe mechanisms that cooperate to direct MEK-ERK signals to their appropriate downstream destinations. These include duration and strength of the signals, interaction with specific scaffolds, changes in subcellular localization, crosstalk with other signaling pathways, and presence of multiple components with distinct functions in each tier of the cascade. Since many of the mechanisms do not function properly in cancer cells, understanding them may shed light not only on the regulation of normal cell proliferation, but also on mechanisms of oncogenic transformation.

Chronic fluoxetine administration inhibits extracellular signal-regulated kinase 1/2 phosphorylation in rat brain

Accumulating evidence indicates that antidepressants alter intracellular signalling mechanisms resulting in long-term synaptic alterations which probably account for the delay in clinical action of these drugs. Therefore, we investigated the effects of chronic fluoxetine administration on extracellular signal-regulated kinase (ERK) 1 and 2, a group of MAPKs that mediate signal transduction from the cell surface downstream to the nucleus. Our data demonstrate that 3-week fluoxetine treatment resulted in long-lasting reduction of phospho-ERK 1 and 2. Such an effect depends on the length of the treatment given that no changes were observed after a single drug injection or after 2 weeks of treatment and it is region specific, being observed in hippocampus and frontal cortex but not in striatum. Finally, phospho-ERK 1 and 2 were differently modulated within nucleus and cytosol in hippocampus but similarly reduced in the same compartments of the frontal cortex, highlighting the specific subcellular compartmentalization of fluoxetine. Conversely, imipramine did not reduce the hippocampal phosphorylation of both ERK subtypes whereas it selectively increased ERK 1 phosphorylation in the cytosolic compartment of frontal cortex suggesting a drug-specific effect on this intracellular target. These results point to modulation of phosphorylation, rather than altered expression, as the main target in the action of fluoxetine on this pathway. The reduction of ERK 1/2 function herein reported may be associated with the therapeutic effects of fluoxetine in the treatment of depression.

Ectodomain shedding of membrane-anchored heparin-binding EGF like growth factor and subcellular localization of the C-terminal fragment in the cell cycle

Heparin-binding EGF-like growth factor (HB-EGF) is initially synthesized as a type I transmembrane protein (proHB-EGF). The proHB-EGF is shed by specific metalloproteases, releasing the N-terminal fragment into the extracellular space as a soluble growth factor (HB-EGF) and the C-terminal fragment (HB-EGF-C) into the intracellular space, where it prevents transcriptional repression by the promyelocytic leukemia zinc finger protein (PLZF). The goal of the present study was to characterize regulation of proHB-EGF shedding and study its temporal variations in HB-EGF-C localization throughout the cell cycle. Quantitative combination analyses of cell surface proHB-EGF and HB-EGF in conditioned medium showed that proHB-EGF shedding occurred during the G(1) cell cycle phase. Laser scanning cytometry (LSC) revealed that HB-EGF-C was internalized into the cytoplasm during the late G1 phase and accumulated in the nucleus beginning in the S phase. Subsequent nuclear export of PLZF occurred during the late S phase. Further, HB-EGF-C was localized around the centrosome following breakdown of the nuclear envelope and was localized to the interzonal space with chromosome segregation in the late M phase. Temporal variations in HB-EGF localization throughout the cell cycle were also characterized by time-lapse imaging of cells expressing YFP-tagged proHB-EGF, and these results were consistent with those obtained in cytometry studies. These results indicate that proHB-EGF shedding and subsequent HB-EGF-C signaling are related with progression of the cell cycle and may provide a clue to understand the unique biological significance of non-receptor-mediated signaling of proHB-EGF in cell growth.

D1 Dopamine Receptor Mediates Dopamine-induced Cytotoxicity via the ERK Signal Cascade

Postsynaptic striatal neurodegeneration occurs through unknown mechanisms, but it is linked to high extracellular levels of synaptic dopamine. Dopamine-mediated cytotoxicity of striatal neurons occurs through two distinct pathways: autoxidation and the D1 dopamine receptor-linked signaling pathway. Here we investigated the mitogen-activated protein kinase (MAPK) signaling pathways activated upon the acute stimulation of D1 dopamine receptors. In SK-N-MC neuroblastoma cells, endogenously expressing D1 dopamine receptors, dopamine caused activation of phosphorylated (p-)ERK1/2 and of the stress-signaling kinases, p-JNK and p-p38 MAPK, in a time- and dose-dependent manner. Selective stimulation of D1 receptors with the agonist SKF R-38393 caused p-ERK1/2, but not p-JNK or p-p38 MAPK activation, in a manner sensitive to the receptor-selective antagonist SCH 23390, protein kinase A inhibition (KT5720), and MEK1/2 inhibition (U0126 or PD98059). Activation of ERK by D1 dopamine receptors resulted in oxidative stress and cytotoxicity. In cells transfected with a catalytically defective mutant of MEK1, the upstream ERK-specific kinase, both dopamine- and SKF R-38393-mediated cytotoxicity was markedly attenuated, confirming the participation of the ERK signaling pathway. Cell fractionation studies showed that only a small amount of p-ERK1/2 was translocated to the nucleus, with the majority retained in the cytoplasm. From coimmunoprecipitation studies, p-ERK was found to form stable heterotrimeric complexes with the D1 dopamine receptor and beta-arrestin2. In cells transfected with the dominant negative mutant of beta-arrestin2, the formation of such complexes was substantially inhibited. These data provide novel mechanistic insights into the role of ERK in the cytotoxicity mediated upon activation of the D1 dopamine receptor.

Subcellular Localization Determines the Protective Effects of Activated ERK2 against Distinct Apoptogenic Stimuli in Myeloid Leukemia Cells

ERKs, mitogen-activated protein kinases, are well characterized as key mediators in the conveyance of signals that promote cell survival in cells of hemopoietic origin, a key factor in the upbringing of leukemogenesis. It is also well known that ERKs phosphorylate a wide array of substrates distributed throughout distinct cellular locations such as the nucleus, cytoplasm, and cell periphery, but the relative contribution of these compartmentalized signal components to the overall survival signal generated by activation of ERKs has yet to be established. To this end, we have utilized constitutively activated forms of ERK2, whose expression is restricted to the nucleus or to the cytoplasm, to investigate the consequences of compartmentalized activation of ERK in the survival of chronic myelogenous leukemia cells subjected to distinct apoptogenic stimuli. We show that cytoplasmic ERK2 activity protected against apoptosis caused by prolonged serum starvation, whereas ERK2 activation restricted to the nucleus antagonized apoptosis induced by the Bcr-Abl inhibitor STI571. On the other hand, neither cytoplasmic nor nuclear ERK2 activities were effective in counteracting apoptosis induced by UV light. These results demonstrate that the protective effects of ERK2 against defined apoptogenic stimuli are strictly dependent on the cellular localization where ERK activation takes place. Furthermore, we present evidence suggesting that the complex I kappa B-NF kappa B participates on ERK2-mediated survival mechanisms, in a fashion dependent on the cellular location where ERK2 is active and on the causative apoptogenic stimulus.

Regulation of Ras-MAPK pathway mitogenic activity by restricting nuclear entry of activated MAPK in endoderm differentiation of embryonic carcinoma and stem cells

In response to retinoic acid, embryonic stem and carcinoma cells undergo differentiation to embryonic primitive endoderm cells, accompanied by a reduction in cell proliferation. Differentiation does not reduce the activation of cellular MAPK/Erk, but does uncouple mitogen-activated protein kinase (MAPK) activation from phosphorylation/activation of Elk-1 and results in inhibition of c-Fos expression, whereas phosphorylation of the cytoplasmic substrate p90RSK remains unaltered. Cell fractionation and confocal immunofluorescence microscopy demonstrated that activated MAPK is restricted to the cytoplasmic compartment after differentiation. An intact actin and microtubule cytoskeleton appears to be required for the restriction of MAPK nuclear entry induced by retinoic acid treatment because the cytoskeletal disrupting agents nocodazole, colchicine, and cytochalasin D are able to revert the suppression of c-Fos expression. Thus, suppression of cell proliferation after retinoic acid–induced endoderm differentiation of embryonic stem and carcinoma cells is achieved by restricting nuclear entry of activated MAPK, and an intact cytoskeleton is required for the restraint.

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.

Examining the mechanism of Erk nuclear translocation using green fluorescent protein

In neuronal cells, the mitogen-activated protein kinase (MAP kinase) cascade is an important mediator of neurotrophin signaling from cell surface receptors to the nucleus, resulting in changes in gene expression. Nuclear localization of Erk is thought to be required for these effects. To examine the mechanism and regulation of Erk nuclear translocation, we have created a green fluorescent protein (GFP)-labeled Erk2 construct, which provides a sensitive means to follow the movement of Erk from the cytoplasm to the nucleus following receptor-mediated MAP kinase activation. Using this system in PC12 cells, we have examined a number of mechanisms that have been implicated in regulating the translocation of Erk. In PC12 cells, NGF and EGF induce a rapid translocation of GFP-Erk that requires Ras and Mek. We have found that prolonged phosphorylation of Erk is not required for the rapid and early influx of Erk into the nucleus following growth factor stimulation. Furthermore, following influx, GFP-Erk rapidly returned to the cytoplasm regardless of its phosphorylation state. The release of Erk from its cytoplasmic activator, Mek, followed by the dimerization of Erk, was sufficient to stimulate nuclear uptake, whereas Erk kinase activity was dispensable. PKA activity has been reported to be required for Erk translocation in PC12 cells. However, PKA activity was also not necessary for the early translocation of Erk into the nucleus by NGF or Ras, but it was able to induce a small influx of Erk that could be measured with GFP-Erk2.

ERK signaling pathway is involved in p15INK4b/p16INK4a expression and HepG2 growth inhibition triggered by TPA and Saikosaponin a

The signal pathway mediating induction of p15(INK4b) and p16(INK4a) during HepG2 growth inhibition triggered by the phorbol ester tumor promoter TPA (12-O-tetradecanoylphorbol 13-acetate) and the Chinese herb Saikosaponin a was investigated. Western blot of three activated forms of mitogen-activated protein kinase (MAPK) (p-ERK, p-JNK and p-p38) demonstrated that phosphorylation of ERK is dramatically induced (11.6-fold ) by TPA during 15 min to 1 h and significantly induced (2.5-fold) by Saikosaponin alpha at 30 min, whereas phosphorylation of JNK was induced only by TPA during 30 min to 1 h. Phosphorylation of p38 was not induced by either drug. During this period, phosphorylation of one of the downstream transcriptional factors of MAPK cascade, ATF2, was 3.2- and 2.0-fold induced by TPA and Saikosaponin a, respectively, whereas that of another transcriptional factor, c-jun, was induced by TPA only. On the other hand, expressions of proto-oncogene c-jun, junB and c-fos were induced by TPA and Saikosaponin a during 30 min to 6 h of treatment. Pretreatment of 20 microg/ml PD98059, an inhibitor of MEK which is the upstream kinase of ERK, prevents the TPA- and Saikosaponin a-triggered HepG2 growth inhibition by 50 and 30%, respectively, accompanied by a 50 - 85% decrease of the p15(INK4b)/p16(INK4a) RNAs and proteins induced by both drugs. Inductions of c-fos RNA by both drugs and c-jun phosphorylation by TPA were also significantly reduced by PD98059 pretreatment. In addition, AP-1 DNA-binding assay using nonisotopic capillary electrophoresis and laser-induced fluorescence (CE/LIF) demonstrated that the AP-1-related DNA-binding activity was significantly induced by TPA and Saikosaponin a, which can be reduced by PD98059 pretreatment. These results suggested that activation of ERK together with its downstream transcriptional machinery mediated p15(INK4b) and p16(INK4a) expression that led to HepG2 growth inhibition.

Protein kinase C isoforms involved in the transcriptional activation of cyclin D1 by transforming Ha-Ras

Transcriptional activation of the cyclin D1 by oncogenic Ras appears to be mediated by several pathways leading to the activation of multiple transcription factors which interact with distinct elements of the cyclin D1 promoter. The present investigations revealed that cyclin D1 induction by transforming Ha-Ras is MEK- and Rac-dependent and requires the PKC isotypes epsilon, lambda, and zeta, but not cPKC-alpha. This conclusion is based on observations indicating that cyclin D1 induction by transforming Ha-Ras was depressed in a dose-dependent manner by PD98059, a selective inhibitor of the mitogen-activated kinase kinase MEK-1, demonstrating that Ha-Ras employs extracellular signal-regulated kinases (ERKs) for signal transmission to the cyclin D1 promoter. Evidence is presented that PKC isotypes epsilon and zeta, but not lambda are required for the Ras-mediated activation of ERKs. Expression of kinase-defective, dominant negative (DN) mutants of nPKC-epsilon or aPKC-zeta inhibit ERK activation by constitutively active Raf-1. Phosphorylation within the TEY motif and subsequent activation of ERKs by constitutively active MEK-1 was significantly inhibited by DN aPKC-zeta, indicating that aPKC-zeta functions downstream of MEK-1 in the pathway leading to cyclin D1 induction. In contrast, TEY phosphorylation induced by constitutively active MEK-1 was not effected by nPKC-epsilon, suggesting another position for this kinase within the cascade investigated. Transformation by oncogenic Ras requires activation of several Ras effector pathways which may be PKC-dependent and converge on the cyclin D1 promoter. Therefore, we investigated a role for PKC isotypes in the Ras-Rac-mediated transcriptional regulation of cyclin D1. We have been able to reveal that cyclin D1 induction by oncogenic Ha-Ras is Rac-dependent and requires the PKC isotypes epsilon, lambda, and zeta, but not cPKC-alpha. Evidence is presented that aPKC-lambda acts upstream of Rac, between Ras and Rac, whereas the PKC isotypes epsilon and zeta act downstream of Rac and are required for the activation of ERKs.

ERK2- and p90(Rsk2)-dependent pathways regulate the CCAAT/enhancer-binding protein-beta interaction with serum response factor

The serum response element (SRE) of the c-fos promoter is a convergence point for mitogenic signaling pathways. Several transcription factors regulate SRE, including serum response factor (SRF), ternary complex factors, and CCAAT/enhancer-binding protein-beta (C/EBPbeta). C/EBPbeta can interact with both SRF and the ternary complex factor family member Elk-1, but only in response to activated Ras. Transactivation of the SRE by C/EBPbeta is also greatly stimulated by Ras. The Ras effectors that signal to C/EBPbeta are unknown. In this report, we demonstrate that a consensus MAPK site in C/EBPbeta is necessary for Ras stimulation of both C/EBPbeta-SRF interaction and transactivation of the SRE by C/EBPbeta. To dissect signaling pathways activated downstream of Ras, different Ras effector constructs were analyzed. We show that activated forms of Raf and phosphatidylinositol 3-kinase stimulate C/EBPbeta-SRF interaction. We also show a novel selectivity for the MAPK family member ERK2, where dominant-negative ERK2, but not dominant-negative ERK1, blocks Ras stimulation of C/EBPbeta-SRF interaction. In addition, recombinant C/EBPbeta protein is phosphorylated by ERK2, but not by ERK1, in vitro. Finally, we demonstrate a requirement for p90(Rsk2) in regulation of C/EBPbeta-SRF interaction. These data show that multiple Ras effectors are required to regulate C/EBPbeta and SRF association.

Complex formation and cooperation of protein kinase C theta and Akt1/protein kinase B alpha in the NF-kappa B transactivation cascade in Jurkat T cells

Protein kinase C theta (PKC theta) is known to induce NF-kappa B, an essential transcriptional element in T cell receptor/CD28-mediated interleukin-2 production but also T cell survival. Here we provide evidence that PKC theta is physically and functionally coupled to Akt1 in this signaling pathway. First, T cell receptor/CD3 ligation was sufficient to induce activation as well as plasma membrane recruitment of PKC theta. Second, PKC theta selectively cooperated with Akt1, known to act downstream of CD28 co-receptor signaling, in activating a NF-kappa B reporter in T cells. Third, Akt1 function was shown to be required for PKC theta-mediated NF-kappa B transactivation. Fourth, PKC theta co-immunoprecipitated with Akt1; however, neither Akt1 nor PKC theta served as a prominent substrate for each other in vitro as well as in intact T cells. Finally, plasma membrane targeting of PKC theta and Akt1 exerted synergistic transactivation of the I-kappa B kinase beta/inhibitor of NF-kappa B/NF-kappa B signaling cascade independent of T cell activation. Taken together, these findings suggest a direct cross-talk between PKC theta and Akt1 in Jurkat T cells.

PEA-15 mediates cytoplasmic sequestration of ERK MAP kinase

The ERK 1/2 MAP kinase pathway controls cell growth and survival and modulates integrin function. Here, we report that PEA-15, a protein variably expressed in multiple cell types, blocks ERK-dependent transcription and proliferation by binding ERKs and preventing their localization in the nucleus. PEA-15 contains a nuclear export sequence required for its capacity to anchor ERK in the cytoplasm. Genetic deletion of PEA-15 results in increased ERK nuclear localization with consequent increased cFos transcription and cell proliferation. Thus, PEA-15 can redirect the biological outcome of MAP kinase signaling by regulating the subcellular localization of ERK MAP kinase.

Membrane proximal ERK signaling is required for M-calpain activation downstream of epidermal growth factor receptor signaling

Localization of signaling is critical in directing cellular outcomes, especially in pleiotropic signaling pathways. The extracellular signal-regulated kinase (ERK)/microtubule-associated protein kinase, which promotes cell migration, proliferation, and differentiation is found in the nucleus and throughout the cytoplasm. Recently, it has been shown that nuclear translocation of ERK is required for transcriptional changes and cell proliferation. However, the cellular consequences, of cytoplasmic signaling have not been defined. We explored whether cytoplasmic, specifically membrane-proximal, ERK signaling is involved in growth factor-induced cell motility. We previously have demonstrated that increased M-calpain activity downstream of epidermal growth factor receptor (EGFR)-mediated ERK activation is necessary for epidermal growth factor (EGF)-induced motility. Calpain isoforms also have been found in nuclear, cytosolic, and plasma membrane-associated compartments in a variety of cell types. We now employ cell engineering approaches to control localization of the upstream EGFR and ERK activities to examine the spatial effect of upstream signal locale on downstream calpain activity. With differential ligand-induced internalization and trafficking-restricted receptor variants, we find that calpain activity is triggered only by plasma membrane-restricted activated EGFR, not by internalized (although still active) EGFR. Cells transfected with membrane-targeted ERK1 and ERK2, which sequester endogenous ERKs, exhibited normal EGF-induced calpain activity. Transfection of an inactive ERK phosphatase (MKP-3/Pyst1) that sequesters ERK in the cytoplasm prevented calpain activation as well as de-adhesion. These data strongly suggest that EGF-induced calpain activity can be enhanced near sites of membrane-proximal EGFR-mediated ERK signaling, providing insights about how calpain activity might be regulated and targeted to enhance its effects on adhesion-related substrates.

Targeting proteins to membranes, using signal sequences for lipid modifications

Changing an existing lipid or appending a lipid to a cytosolic protein has emerged as an important technique for targeting proteins to membranes and for constitutively activating the membrane-bound protein. The potential for more precise or regulated interactions of lipidated proteins in membrane subdomains suggests that this method for membrane targeting will be of increasing usefulness.

Recent advances in our understanding of insulin action and insulin resistance

Insulin signaling at the target tissue results in a large array of biological outcomes. These events are essential for normal growth and development and for normal homeostasis of glucose, fat, and protein metabolism. Elucidating the intracellular events after activation of the IR has been the primary focus of a large number of investigators for decades, and for excellent reasons. Understanding the signaling pathways involved in insulin action could lead to a better understanding of the pathophysiology of insulin resistance associated with obesity and type 2 diabetes, and identifying key molecules and processes could lead to newer and more effective therapeutic agents for treating these common disorders.

This review summarizes our previous understanding of how insulin acts and outlines some recent developments in our understanding of insulin action and insulin resistance at the cellular level, beginning with a discussion on the discovery of evolutionarily conserved molecules of the insulin signaling pathways. This article will also provide a summary of a few in vitro and cellular models of insulin resistance and a description of some new paradigms in the cellular mechanisms of insulin action.

This review will not attempt to be all-inclusive; for a more comprehensive understanding, readers are referred to more complete reviews on insulin action (1–5).

Biochemical and biological functions of the N-terminal, noncatalytic domain of extracellular signal-regulated kinase 2

Extracellular signal-regulated kinase 1 (ERK1) and ERK2 are important components in signal transduction pathways involved in many cellular processes, including cell differentiation and proliferation. These proteins consist of a central kinase domain flanked by short N- and C-terminal noncatalytic domains. While the regulation of ERK2 by sequences within the kinase domain has been extensively studied, little is known about the small regions outside of the kinase domain. We performed mutational analysis on the N-terminal, noncatalytic domain of ERK2 in an attempt to determine its role in ERK2 function and regulation. Deleting or mutating amino acids 19 to 25 (ERK2-Delta19-25) created an ERK2 molecule that could be phosphorylated in response to growth factor and serum stimulation in a MEK (mitogen-activated protein kinase kinase or ERK kinase)-dependent manner but had little kinase activity and was unable to bind to MEK in vivo. Since MEK acts as a cytoplasmic anchor for the ERKs, the lack of a MEK interaction resulted in the aberrant nuclear localization of ERK2-Delta19-25 mutants in serum-starved cells. Assaying these mutants for their ability to affect ERK signaling revealed that ERK2-Delta19-25 mutants acted in a dominant-negative manner to inhibit transcriptional signaling through endogenous ERKs to an Elk1-responsive promoter in transfected COS-1 cells. However, ERK2-Delta19-25 had no effect on the phosphorylation of RSK2, an ERK2 cytoplasmic substrate, whereas a nonactivatable ERK (T183A) that retained these sequences could inhibit RSK2 phosphorylation. These results suggest that the N-terminal domain of ERK2 profoundly affects ERK2 localization, MEK binding, kinase activity, and signaling and identify a novel dominant-negative mutant of ERK2 that can dissociate at least some transcriptional responses from cytoplasmic responses.

SSeCKS, a major protein kinase C substrate with tumor suppressor activity, regulates G(1)-->S progression by controlling the expression and cellular compartmentalization of cyclin D

SSeCKS, first isolated as a G(1)-->S inhibitor that is downregulated in src- and ras-transformed cells, is a major cytoskeleton-associated PKC substrate with tumor suppressor and kinase-scaffolding activities. Previous attempts at constitutive expression resulted in cell variants with truncated ectopic SSeCKS products. Here, we show that tetracycline-regulated SSeCKS expression in NIH 3T3 cells induces G(1) arrest marked by extracellular signal-regulated kinase 2-dependent decreases in cyclin D1 expression and pRb phosphorylation. Unexpectedly, the forced reexpression of cyclin D1 failed to rescue SSeCKS-induced G(1) arrest. Confocal microscopy analysis revealed cytoplasmic colocalization of cyclin D1 with SSeCKS. Because the SSeCKS gene encodes two potential cyclin-binding motifs (CY) flanking major in vivo protein kinase C (PKC) phosphorylation sites (Ser(507/515)), we addressed whether SSeCKS encodes a phosphorylation-dependent cyclin scaffolding function. Bacterially expressed SSeCKS-CY bound cyclins D1 and E, whereas K-->S mutations within either CY motif ablated binding. Activation of PKC in vivo caused a rapid translocation of cyclin D1 to the nucleus. Cell permeable, penetratin-linked peptides encoding wild-type SSeCKS-CY, but not K-->S or phospho-Ser(507/515) variants, released cyclin D1 from its cytoplasmic sequestration and induced higher saturation density in cyclin D1-overexpressor cells or rat embryo fibroblasts. Our data suggest that SSeCKS controls G(1)-->S progression by regulating the expression and localization of cyclin D1. These data suggest that downregulation of SSeCKS in tumor cells removes gating checkpoints for saturation density, an effect that may promote contact independence.