Mechanism of action of angiotensin II in human isolated subcutaneous resistance arteries

1. Human isolated subcutaneous arteries were mounted in a myograph and isometric tension measured. In some experiments, intracellular calcium [Ca(2+)]i was also measured using fura-2. 2. Angiotensin II (100 pM - 1 microM) increased [Ca(2+)]i and tone in a concentration-dependent manner. The effects of angiotensin II (100 nM) were inhibited by an AT1-receptor antagonist, candesartan (100 pM). 3. Ryanodine (10 microM), had no effect on angiotensin II-induced responses, but removal of extracellular Ca(2+) abolished angiotensin II-induced rise in [Ca(2+)]i and tone. Inhibition of Ca(2+) entry by Ni(2+) (2 mM), also inhibited angiotensin II responses. The dihydropyridine, L-type calcium channel antagonist, amlodipine (10 microM), only partially attenuated angiotensin II responses. 4. Inhibition of protein kinase C (PKC) by chelerythrine (1 microM), or by overnight exposure to a phorbol ester (PDBu; 500 nM) had no effect on angiotensin II-induced contraction. 5. Genistein (10 microM), a tyrosine kinase inhibitor, inhibited angiotensin II-induced contraction, but did not inhibit the rise in [Ca(2+)]i, suggesting that at this concentration it affected the calcium sensitivity of the contractile apparatus. Genistein did not affect responses to norepinephrine (NE) or high potassium (KPSS). 6. A selective MEK inhibitor, PD98059 (30 microM), inhibited both the angiotensin II-induced contraction and rise in [Ca(2+)]i, but had no effect on responses to NE or KPSS. 7. AT1 activation causes Ca(2+) influx via L-type calcium channels and a dihydropyridine-insensitive route, but does not release Ca(2+) from intracellular sites. Activation of tyrosine kinase(s) and the ERK 1/2 pathway, but not classical or novel PKC, also play a role in angiotensin II-induced contraction in human subcutaneous resistance arteries.

Active erk regulates microtubule stability in H-ras-transformed cells

Increasing evidence suggests that activated erk regulates cell functions, at least in part, by mechanisms that do not require gene transcription. Here we show that the map kinase, erk, decorates microtubules (MTs) and mitotic spindles in both parental and mutant active ras-transfected 10T1/2 fibroblasts and MCF10A breast epithelial cells. Approximately 20% of total cellular erk decorated MTs in both cell lines. A greater proportion of activated erk was associated with MTs in the presence of mutant active H-ras than in parental cells. Activation of erk by the ras pathway coincided with a decrease in the stability of MT, as detected by a stability marker. The MKK1 inhibitor, PD98059 and transfection of a dominant negative MKK1 blocked ras-induced instability of MTs but did not modify the association of erk with MTs or affect MT stability of the parental cells. These results indicate that the subset of active erk kinase that associates with MTs contributes to their instability in the presence of a mutant active ras. The MT-associated subset of active erk likely contributes to the enhanced invasive and proliferative abilities of cells containing mutant active H-ras.

Role of the extracellular signal-regulated kinase (Erk) signal transduction cascade in alpha(2) adrenoceptor-mediated vasoconstriction in porcine palmar lateral vein

The mechanism of alpha(2) adrenoceptor-mediated vasoconstriction is unknown, but may involve activation of voltage-sensitive calcium channels, and/or a protein tyrosine kinase. Recently the extracellular signal-regulated kinase (Erk) cascade, often an event downstream of tyrosine kinase activation, has been shown to mediate vasoconstriction to a variety of agents. The aim of this present study was to determine the involvement of the Erk signal transduction cascade in alpha(2) adrenoceptor-mediated vasoconstriction, and to confirm the involvement of activation of voltage-sensitive calcium channels, and protein tyrosine kinase. Contractions to the alpha(2) adrenoceptor agonist UK14304 in the porcine palmar lateral vein in vitro were reduced 70 - 80% by the MEK inhibitors PD98059 (10 - 50 microM) and U0126 (10 - 50 microM), indicating the involvement of the Erk signal transduction cascade. Immunoblots also demonstrated an increase in the phosphorylated (activated) form of Erk in palmar lateral vein segments after contraction with UK14304, which was inhibited by PD98059 and U0126. The calcium channel blockers nifedipine and verapamil, or removal of extracellular calcium inhibited UK14304-induced contractions and phosphorylation of Erk, demonstrating the importance of an influx of extracellular calcium. UK14304-induced contractions were inhibited by PP2 (1 - 10 microM), a selective inhibitor of Src tyrosine kinases, but not by PP3, an inactive analogue. PP2 also prevented the phosphorylation of Erk by UK14304. These data demonstrate that alpha(2) adrenoceptor-mediated vasoconstriction in the porcine palmar lateral vein is dependent upon activation of the Erk signal transduction cascade, which is downstream of an influx of extracellular calcium, and activation of Src tyrosine kinases.

Drosophila Fos mediates ERK and JNK signals via distinct phosphorylation sites

During Drosophila development Fos acts downstream from the JNK pathway. Here we show that it can also mediate ERK signaling in wing vein formation and photoreceptor differentiation. Drosophila JNK and ERK phosphorylate D-Fos with overlapping, but distinct, patterns. Analysis of flies expressing phosphorylation site point mutants of D-Fos revealed that the transcription factor responds differentially to JNK and ERK signals. Mutations in the phosphorylation sites for JNK interfere specifically with the biological effects of JNK activation, whereas mutations in ERK phosphorylation sites affect responses to the EGF receptor-Ras-ERK pathway. These results indicate that the distinction between ERK and JNK signals can be made at the level of D-Fos, and that different pathway-specific phosphorylated forms of the protein can elicit different responses.

Multiple distinct signal pathways, including an autocrine neurotrophic mechanism, contribute to the survival-promoting effect of depolarization on spiral ganglion neurons in vitro

We have shown previously that BDNF, neurotrophin-3 (NT-3), chlorphenylthio-cAMP (cpt-cAMP) (a permeant cAMP analog), and membrane depolarization promote spiral ganglion neuron (SGN) survival in vitro in an additive manner, depolarization having the greatest efficacy. Expression of both BDNF and of NT-3 is detectable in cultured SGNs after plating in either depolarizing or nondepolarizing medium. These neurotrophins promote survival by an autocrine mechanism; TrkB-IgG or TrkC-IgG, which block neurotrophin binding to, respectively, TrkB and TrkC, partially inhibit the trophic effect of depolarization. The mitogen-activated protein kinase kinase inhibitor PD98059 and the phosphatidylinositol-3-OH kinase inhibitor LY294002 both abolish trophic support by neurotrophins but only partially inhibit support by depolarization. Inhibition by these compounds is not additive with inhibition by Trk-IgGs. The cAMP antagonist Rp-adenosine-3',5'-cyclic-phosphorothioate (Rp-cAMPS) abolishes survival attributable to cpt-cAMP but has no effect on that attributable to neurotrophins, nor do inhibitors of neurotrophin-dependent survival affect survival attributable to cpt-cAMP. However, Rp-cAMPS does partially inhibit depolarization-dependent survival, an inhibition that is additive with that by Trk-IgGs, PD98059, or LY294002. Moreover, Rp-cAMPS prevents depolarization-dependent survival of PC12 cells maintained in subthreshold levels of NGF. Inhibition of Ca(2+)/calmodulin-dependent protein kinases (CaMKs) with KN-62 reduces SGN survival independently of Rp-cAMPS, Trk-IgGs, and LY294002 and additively with them. Combined inhibition of Trk, cAMP, and CaMK signaling prevents depolarization-dependent survival. Thus, survival of SGNs under depolarizing conditions involves additivity among a depolarization-independent autocrine pathway, a cAMP-dependent pathway, and a CaMK-dependent pathway.

Involvement of the ERK mitogen-activated protein kinase in cell resistance to complement-mediated lysis

Sublytic doses of complement desensitize cells and make them resistant to lytic complement doses. This process, named complement-induced protection, requires calcium ion influx, protein kinase C activation and protein synthesis. The involvement of the extracellular signal-regulated kinase, ERK, in cell desensitization by sublytic complement was examined in erythroleukaemia K562 cells and in COS-7 cells. As shown here, ERK is activated in K562 and COS-7 cells within 10 min of sublytic immune attack and then shows a decline and a second peak of activation at 20 min. C7- and C8-deficient human sera have a small effect on ERK activity. However, a significant increase in ERK activation is observed when C7 or C8, respectively, is added back to these sera. Complement-induced ERK activation was blocked in cells treated with GF109203X or Go6976, two selective PKC inhibitors, as well as by treatment with PD098059, an inhibitor of MEK1, the ERK kinase. PD098059 treatment also sensitized K562 cells to complement-mediated lysis and prevented complement-induced protection. COS-7 cells transfected with a dominant-negative MEK plasmid were incapable of undergoing the process of complement-induced protection. In conclusion, cell desensitization by sublytic doses of the complement membrane attack complex involves a signalling cascade that includes PKC-mediated ERK activation.

A novel 14-kilodalton protein interacts with the mitogen-activated protein kinase scaffold mp1 on a late endosomal/lysosomal compartment

We have identified a novel, highly conserved protein of 14 kD copurifying with late endosomes/lysosomes on density gradients. The protein, now termed p14, is peripherally associated with the cytoplasmic face of late endosomes/lysosomes in a variety of different cell types. In a two-hybrid screen with p14 as a bait, we identified the mitogen-activated protein kinase (MAPK) scaffolding protein MAPK/extracellular signal-regulated kinase (ERK) kinase (MEK) partner 1 (MP1) as an interacting protein. We confirmed the specificity of this interaction in vitro by glutathione S-transferase pull-down assays and by coimmunoprecipitation, cosedimentation on glycerol gradients, and colocalization. Moreover, expression of a plasma membrane-targeted p14 causes mislocalization of coexpressed MP1. In addition, we could reconstitute protein complexes containing the p14-MP1 complex associated with ERK and MEK in vitro.The interaction between p14 and MP1 suggests a MAPK scaffolding activity localized to the cytoplasmic surface of late endosomes/lysosomes, thereby combining catalytic scaffolding and subcellular compartmentalization as means to modulate MAPK signaling within a cell.

Blockade of NGF-induced neurite outgrowth by a dominant-negative inhibitor of the egr family of transcription regulatory factors

Although it is well established that members of the Egr family of transcription regulatory factors are induced in many neuronal plasticity paradigms, it is still unclear what role, if any, they play in this process. Because NGF stimulation of pheochromocytoma 12 cells elicits a robust induction of Egr family members, we have investigated their role in mediating long-term effects elicited by NGF in these cells by using the Egr zinc finger DNA-binding domain as a selective antagonist of Egr family-mediated transcription. We report that expression of this Egr inhibitor construct suppresses neurite outgrowth elicited by NGF but not by dibutyryl cAMP. To check that this Egr inhibitor construct does not act by blocking the MEK/ERK pathway, which is known to mediate NGF-induced neurite outgrowth, we confirmed that the Egr inhibitor construct does not block NGF activation of Elk1-mediated transcription, a response that is dependent on this pathway. Conversely, inhibition of MEK does not impair Egr family-mediated transcription. Thus, we conclude (1) that induction of Egr family members and activation of the MEK/ERK pathway by NGF are mediated by separate signaling pathways and (2) that both are required to trigger neurite outgrowth induced by NGF.

Reversal of the Ras-induced transformed phenotype by HR12, a novel ras farnesylation inhibitor, is mediated by the Mek/Erk pathway

We have used the selective farnesylation inhibitor HR12 [cysteine-N(methyl)valine-N(cyclohexyl) glycine-methionine-O-methyl-ester] to study the role of oncogenic Ras in cytoskeletal reorganization in Ha-ras(V12)-transformed Rat1 cells (Rat1/ras). Application of HR12 resulted in complete restoration of the cytoskeleton and associated cell adhesions disrupted by oncogenic Ras. This included an increase in the number and size of focal adhesions, accompanied by massive stress fiber formation and enhanced tyrosine phosphorylation. Furthermore, HR12 induced assembly of adherens junctions and dramatically elevated the level of the junctional components, cadherin and beta-catenin. HR12 was unable to restore the nontransformed phenotype in cells expressing farnesylation-independent, myristylated Ras. Examination of the main Ras-regulated signaling pathways revealed that HR12 induced a dose- and time-dependent decline in Erk1&2 activation (t(1/2) approximately 6 h), which correlated with the accumulation of nonfarnesylated oncogenic-Ras. Inhibition of the Mek/Erk pathway in Rat1/ras cells, using the Mek inhibitor, PD98059, resulted in complete cytoskeletal recovery, indistinguishable from that induced by HR12. Moreover, a constitutively active Mek mimicked the effect of ras transformation in Rat1 cells, and prevented HR12-induced cytoskeletal effects in Rat1/ras cells. No such effects were observed after treatment of Rat1/ras cells with the phosphatidylinositol 3-kinase inhibitor LY294002. These findings establish the Mek/Erk pathway as the dominant pathway involved in conferring the cytoskeletal and junctional manifestations of the Ras-induced transformed phenotype.

Involvement of the extracellular signal-regulated kinase cascade for cocaine-rewarding properties

A central feature of drugs of abuse is to induce gene expression in discrete brain structures that are critically involved in behavioral responses related to addictive processes. Although extracellular signal-regulated kinase (ERK) has been implicated in several neurobiological processes, including neuronal plasticity, its role in drug addiction remains poorly understood. This study was designed to analyze the activation of ERK by cocaine, its involvement in cocaine-induced early and long-term behavioral effects, as well as in gene expression. We show, by immunocytochemistry, that acute cocaine administration activates ERK throughout the striatum, rapidly but transiently. This activation was blocked when SCH 23390 [a specific dopamine (DA)-D1 antagonist] but not raclopride (a DA-D2 antagonist) was injected before cocaine. Glutamate receptors of NMDA subtypes also participated in ERK activation, as shown after injection of the NMDA receptor antagonist MK 801. The systemic injection of SL327, a selective inhibitor of the ERK kinase MEK, before cocaine, abolished the cocaine-induced ERK activation and decreased cocaine-induced hyperlocomotion, indicating a role of this pathway in events underlying early behavioral responses. Moreover, the rewarding effects of cocaine were abolished by SL327 in the place-conditioning paradigm. Because SL327 antagonized cocaine-induced c-fos expression and Elk-1 hyperphosphorylation, we suggest that the ERK intracellular signaling cascade is also involved in the prime burst of gene expression underlying long-term behavioral changes induced by cocaine. Altogether, these results reveal a new mechanism to explain behavioral responses of cocaine related to its addictive properties.

Activation of ERK/MAP kinase in the amygdala is required for memory consolidation of pavlovian fear conditioning

Although much has been learned about the neurobiological mechanisms underlying Pavlovian fear conditioning at the systems and cellular levels, relatively little is known about the molecular mechanisms underlying fear memory consolidation. The present experiments evaluated the role of the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) signaling cascade in the amygdala during Pavlovian fear conditioning. We first show that ERK/MAPK is transiently activated-phosphorylated in the amygdala, specifically the lateral nucleus (LA), at 60 min, but not 15, 30, or 180 min, after conditioning, and that this activation is attributable to paired presentations of tone and shock rather than to nonassociative auditory stimulation, foot shock sensitization, or unpaired tone-shock presentations. We next show that infusions of U0126, an inhibitor of ERK/MAPK activation, aimed at the LA, dose-dependently impair long-term memory of Pavlovian fear conditioning but leaves short-term memory intact. Finally, we show that bath application of U0126 impairs long-term potentiation in the LA in vitro. Collectively, these results demonstrate that ERK/MAPK activation is necessary for both memory consolidation of Pavlovian fear conditioning and synaptic plasticity in the amygdala.

Multiple Ras-dependent phosphorylation pathways regulate Myc protein stability

Our recent work has shown that activation of the Ras/Raf/ERK pathway extends the half-life of the Myc protein and thus enhances the accumulation of Myc activity. We have extended these observations by investigating two N-terminal phosphorylation sites in Myc, Thr 58 and Ser 62, which are known to be regulated by mitogen stimulation. We now show that the phosphorylation of these two residues is critical for determining the stability of Myc. Phosphorylation of Ser 62 is required for Ras-induced stabilization of Myc, likely mediated through the action of ERK. Conversely, phosphorylation of Thr 58, likely mediated by GSK-3 but dependent on the prior phosphorylation of Ser 62, is associated with degradation of Myc. Further analysis demonstrates that the Ras-dependent PI-3K pathway is also critical for controlling Myc protein accumulation, likely through the control of GSK-3 activity. These observations thus define a synergistic role for multiple Ras-mediated phosphorylation pathways in the control of Myc protein accumulation during the initial stage of cell proliferation.

The importance of ERK activity in the regulation of cyclin D1 levels and DNA synthesis in human cultured airway smooth muscle

The relationship between persistent ERK (extracellular signal-regulated kinase) activity, cyclin D1 protein and mRNA levels and cell cycle progression in human cultured airway smooth muscle was examined in response to stimulation by ET-1 (endothelin-1), thrombin and bFGF (basic fibroblast growth factor). Thrombin (0.3 and 3 u ml(-1)) and bFGF (0.3 and 3 nM) increased ERK activity for more than 2 h and increased cell number, whereas ET-1 (100 nM) transiently stimulated ERK activity and was non-mitogenic. The MEK1 (mitogen-activated ERK kinase) inhibitor, PD 98059 (30 microM), inhibited both ERK phosphorylation and activity, and either prevented (thrombin 0.3 and 3 u ml(-1), bFGF 300 pM) or attenuated (bFGF 3 nM) DNA synthesis. Thrombin and bFGF increased both cyclin D1 mRNA and protein levels. PD 98059 decreased cyclin D1 protein levels stimulated by the lower but not higher thrombin concentrations. Moreover, increases in cyclin D1 mRNA levels were unaffected by PD 98059 pretreatment, irrespective of the mitogen or its concentration, suggesting that inhibition of cyclin D1 protein levels occurred by a post-transcriptional mechanism. These findings indicate that the control of cyclin D1 protein levels may occur independently of the MEK1/ERK signalling pathways. The inhibition of S phase entry by PD 98059 at higher thrombin concentrations appears to result from effects on pathways downstream or parallel to those regulating cyclin D1 protein levels. These findings suggest heterogeneity in the signalling of DNA synthesis in human cultured airway smooth muscle.

The leucine-rich repeat protein SUR-8 enhances MAP kinase activation and forms a complex with Ras and Raf

Caenorhabditis elegans sur-8 encodes a positive regulator of Ras signaling. We investigated the mechanism by which the human Sur-8 homolog can positively regulate Ras-MAP kinase signaling in mammalian cells. Sur-8 expression enhances Ras- or EGF-induced Raf and ERK activation but has no effect on ERK activation induced by active Raf or MEK. Furthermore, Sur-8 expression does not increase AKT or JNK activation. Sur-8 interacts with Ras and Raf and is able to form a ternary complex with the two proteins. Thus, Sur-8 may function as a scaffold that enhances Ras-MAP kinase signal transduction by facilitating the interaction between Ras and Raf.

Estrogen-induced activation of the mitogen-activated protein kinase cascade in the cerebral cortex of estrogen receptor-alpha knock-out mice

We have shown previously in the developing cerebral cortex that estrogen elicits the rapid and sustained activation of multiple signaling proteins within the mitogen-activated protein (MAP) kinase cascade, including B-Raf and extracellular signal-regulated kinase (ERK). Using estrogen receptor (ER)-alpha gene-disrupted (ERKO) mice, we addressed the role of ER-alpha in mediating this action of estrogen in the brain. 17beta-Estradiol increased B-Raf activity and MEK (MAP kinase/ERK kinase)-dependent ERK phosphorylation in cerebral cortical explants derived from both ERKO and their wild-type littermates. The ERK response was stronger in ERKO-derived cultures but, unlike that of wild-type cultures, was not blocked by the estrogen receptor antagonist ICI 182,780. Surprisingly, both the ER-alpha selective ligand 16alpha-iodo-17beta-estradiol and the ER-beta selective ligand genistein failed to elicit ERK phosphorylation, suggesting that a different mechanism or receptor may mediate estrogen-induced ERK phosphorylation in the cerebral cortex. Interestingly, the transcriptionally inactive stereoisomer 17alpha-estradiol did elicit a strong induction of ERK phosphorylation, which, together with the inability of the ER-alpha- and ER-beta-selective ligands to elicit ERK phosphorylation, and of ICI 182,780 to block the actions of estradiol in ERKO cultures, supports the hypothesis that a novel, estradiol-sensitive and ICI-insensitive estrogen receptor may mediate 17beta-estradiol-induced activation of ERK in the brain.

ErbB2 is necessary for induction of carcinoma cell invasion by ErbB family receptor tyrosine kinases

The epidermal growth factor (EGF) family of tyrosine kinase receptors (ErbB1, -2, -3, and -4) and their ligands are involved in cell differentiation, proliferation, migration, and carcinogenesis. However, it has proven difficult to link a given ErbB receptor to a specific biological process since most cells express multiple ErbB members that heterodimerize, leading to receptor cross-activation. In this study, we utilize carcinoma cells depleted of ErbB2, but not other ErbB receptor members, to specifically examine the role of ErbB2 in carcinoma cell migration and invasion. Cells stimulated with EGF-related peptides show increased invasion of the extracellular matrix, whereas cells devoid of functional ErbB2 receptors do not. ErbB2 facilitates cell invasion through extracellular regulated kinase (ERK) activation and coupling of the adaptor proteins, p130CAS and c-CrkII, which regulate the actin-myosin cytoskeleton of migratory cells. Overexpression of ErbB2 in cells devoid of other ErbB receptor members is sufficient to promote ERK activation and CAS/Crk coupling, leading to cell migration. Thus, ErbB2 serves as a critical component that couples ErbB receptor tyrosine kinases to the migration/invasion machinery of carcinoma cells.

Roles of extracellular signal-regulated kinase 1/2 and p38 mitogen-activated protein kinase in the signal transduction of basic fibroblast growth factor in endothelial cells during angiogenesis

We examined the role of mitogen-activated protein (MAP) kinases in the signal transduction of basic fibroblast growth factor (bFGF)-mediated effects in endothelial cells (ECs). When MSS31 murine endothelial cells were stimulated with bFGF, three MAP kinase homologs, extracellular signal-regulated kinase (ERK) 1/2, c-Jun N-terminal kinase (JNK) 1, and p38 MAP kinase were activated. The inhibition of the ERK1/2 pathway with PD98059, a specific inhibitor of MEK1, or of the p38 MAP kinase pathway with SB203580, a specific inhibitor of p38 MAP kinase, abrogated bFGF-mediated tube formation by MSS31 cells in type I collagen gel. Tube formation in type I collagen gel requires proliferation and migration of these cells, and degradation of the extracellular matrix by these cells. Both PD98059 and SB203580 inhibited bFGF-stimulated DNA synthesis as well as migration of MSS31 cells. Cell migration requires cytoskeleton reorganization and cell adhesion. bFGF induced actin reorganization and vinculin assembly in the focal adhesion plaque, both of which were inhibited by SB203580 but not by PD98059. bFGF induced the expression of the transcription factor ETS-1 in MSS31 cells. ETS-1 is responsible for the expression of proteases as well as integrin beta 3 subunit in ECs, and converts ECs to invasive phenotype. PD98059 inhibited this induction of ETS-1, whereas SB203580 did not. These results indicate that ERK1/2 and p38 MAP kinase are requisite for the signal transduction of bFGF in ECs. The roles of these two MAP kinase homologs are not identical, but these kinases work in a coordinated fashion.

Changes in the balance of phosphoinositide 3-kinase/protein kinase B (Akt) and the mitogen-activated protein kinases (ERK/p38MAPK) determine a phenotype of visceral and vascular smooth muscle cells

The molecular mechanisms behind phenotypic modulation of smooth muscle cells (SMCs) remain unclear. In our recent paper, we reported the establishment of novel culture system of gizzard SMCs (Hayashi, K., H. Saga, Y. Chimori, K. Kimura, Y. Yamanaka, and K. Sobue. 1998. J. Biol. Chem. 273: 28860-28867), in which insulin-like growth factor-I (IGF-I) was the most potent for maintaining the differentiated SMC phenotype, and IGF-I triggered the phosphoinositide 3-kinase (PI3-K) and protein kinase B (PKB(Akt)) pathway. Here, we investigated the signaling pathways involved in de-differentiation of gizzard SMCs induced by PDGF-BB, bFGF, and EGF. In contrast to the IGF-I-triggered pathway, PDGF-BB, bFGF, and EGF coordinately activated ERK and p38MAPK pathways. Further, the forced expression of active forms of MEK1 and MKK6, which are the upstream kinases of ERK and p38MAPK, respectively, induced de-differentiation even when SMCs were stimulated with IGF-I. Among three growth factors, PDGF-BB only triggered the PI3-K/PKB(Akt) pathway in addition to the ERK and p38MAPK pathways. When the ERK and p38MAPK pathways were simultaneously blocked by their specific inhibitors or an active form of either PI3-K or PKB(Akt) was transfected, PDGF-BB in turn initiated to maintain the differentiated SMC phenotype. We applied these findings to vascular SMCs, and demonstrated the possibility that the same signaling pathways might be involved in regulating the vascular SMC phenotype. These results suggest that changes in the balance between the PI3-K/PKB(Akt) pathway and the ERK and p38MAPK pathways would determine phenotypes of visceral and vascular SMCs. We further reported that SMCs cotransfected with active forms of MEK1 and MKK6 secreted a nondialyzable, heat-labile protein factor(s) which induced de-differentiation of surrounding normal SMCs.

Induction of cyclo-oxygenase-2 expression by methyl arachidonyl fluorophosphonate in murine J774 macrophages: roles of protein kinase C, ERKs and p38 MAPK

1. Methyl arachidonyl fluorophosphonate (MAFP), an inhibitor of phospholipase A2 (PLA2), has been widely used to assess the roles of PLA2 in various cell functions. Here, we report on a novel action of this compound at concentrations similar to those used for PLA2 inhibition. 2. The murine macrophage J774 released a large amount of prostaglandin E2 (PGE2) by MAFP (1-30 microM), which was abolished by indomethacin and NS-398 but not by valeryl salicylate, and results from increased cyclo-oxygenase-2 (COX-2) protein levels and gene expression. 3. This PGE2 release was blocked by inhibitors of tyrosine kinase (genistein), protein kinase C (PKC) (Ro 31-8220, Go 6976 or LY 379196), mitogen-activated protein kinase kinase (MEK) (PD 098059) or p38 mitogen-activated protein kinase (MAPK) (SB 203580). 4. Consistent with these results, MAFP caused membrane translocation of PKCbetaI and betaII isoforms and activated extracellular signal-regulated kinase (ERK) and p38 MAPK. 5. In accordance with these effects of MAFP, PKC activator phorbol 12-myristate 13-acetate (PMA) increased PGE2 release and caused activation of PKCbeta, ERKs and p38 MAPK. 6. This is the first report that the PLA2 inhibitor, MAFP, can induce COX-2 gene expression and PGE2 synthesis via the PKC-, ERK- and p38 MAPK-dependent pathways. Thus, the use of MAFP as a PLA2 inhibitor should be treated with caution.

Estrogen-induced activation of mitogen-activated protein kinase in cerebral cortical explants: convergence of estrogen and neurotrophin signaling pathways

We have shown that estrogen elicits a selective enhancement of the growth and differentiation of axons and dendrites (neurites) in the developing CNS. We subsequently demonstrated widespread colocalization of estrogen and neurotrophin receptors (trk) within developing forebrain neurons and reciprocal transcriptional regulation of these receptors by their ligands. Using organotypic explants of the cerebral cortex, we tested the hypothesis that estrogen/neurotrophin receptor coexpression also may result in convergence or cross-coupling of their signaling pathways. Estradiol elicited rapid (within 5-15 min) tyrosine phosphorylation/activation of the mitogen-activated protein (MAP) kinases, ERK1 and ERK2, that persisted for at least 2 hr. This extracellular signal-regulated protein kinase (ERK) activation was inhibited successfully by the MEK1 inhibitor PD98059, but not by the estrogen receptor (ER) antagonist ICI 182,780, and did not appear to result from estradiol-induced activation of trk. Furthermore, we also found that estradiol elicited an increase in B-Raf kinase activity. The latter and subsequent downstream events leading to ERK activation may be a consequence of our documentation of a multimeric complex consisting of, at least, the ER, hsp90, and B-Raf. These novel findings provide an alternative mechanism for some of the estrogen actions in the developing CNS and could explain not only some of the very rapid effects of estrogen but also the ability of estrogen and neurotrophins to regulate the same broad array of cytoskeletal and growth-associated genes involved in neurite growth and differentiation.

Multiple docking sites on substrate proteins form a modular system that mediates recognition by ERK MAP kinase

MAP kinases phosphorylate specific groups of substrate proteins. Here we show that the amino acid sequence FXFP is an evolutionarily conserved docking site that mediates ERK MAP kinase binding to substrates in multiple protein families. FXFP and the D box, a different docking site, form a modular recognition system, as they can function independently or in combination. FXFP is specific for ERK, whereas the D box mediates binding to ERK and JNK MAP kinase, suggesting that the partially overlapping substrate specificities of ERK and JNK result from recognition of shared and unique docking sites. These findings enabled us to predict new ERK substrates and design peptide inhibitors of ERK that functioned in vitro and in vivo.

Requirement of receptor internalization for opioid stimulation of mitogen-activated protein kinase: biochemical and immunofluorescence confocal microscopic evidence

Previously, we implicated the opioid receptor (OR), Gbetagamma subunits, and Ras in the opioid activation of extracellular signal-regulated protein kinase (ERK), a member of the mitogen-activated protein (MAP) kinase family involved in mitogenic signaling. We now report that OR endocytosis also plays a role in the opioid stimulation of ERK activity. COS-7 and HEK-293 cells were cotransfected with the cDNA of delta-, mu;-, or kappa-OR, dynamin wild-type (DWT), or the dominant suppressor mutant dynamin K44A, which blocks receptor endocytosis. The activation of ERK by opioid agonists in the presence of DWT was detected. In contrast, parallel ectopic coexpression of the K44A mutant with OR, followed by agonist treatment, resulted in a time-dependent attenuation of ERK activation. Immunofluorescence confocal microscopy of delta-OR and DWT-cotransfected COS-7 cells revealed that agonist exposure for 10 min resulted in an ablation of cell surface delta-OR immunoreactivity (IR) and an intensification of cytoplasmic (presumably endosomal) staining as seen in the absence of overexpressed DWT. After 1 hr of delta-agonist exposure the cells displayed substantial internalization of delta-OR IR. If the cells were cotransfected with delta-OR and dynamin mutant K44A, OR IR was retained on the cell surface even after 1 hr of delta-agonist treatment. Parallel immunofluorescence confocal microscopy, using an anti-ERK antibody, showed that agonist-induced time-dependent ERK IR trafficking into perinuclear and nuclear loci was impaired in the internalization-defective cells. Thus, both biochemical and immunofluorescence confocal microscopic evidence supports the hypothesis that the opioid activation of ERK requires receptor internalization in transfected mammalian cells.

Extracellular signal-regulated kinase (ERK) controls immediate early gene induction on corticostriatal stimulation

Activity-dependent changes in neuronal structure and synaptic remodeling depend critically on gene regulation. In an attempt to understand how glutamate receptor stimulation at the membrane leads to gene regulation in the nucleus, we traced intracellular signaling pathways targeting DNA regulatory elements of immediate early genes (IEGs). For this purpose we used an in vivo electrical stimulation of the glutamatergic corticostriatal pathway. We show that a transient activation of extracellular signal-regulated kinase (ERK) proteins (detected by immunocytochemistry with an anti-active antibody) is spatially coincident with the onset of IEG induction [c-fos, zif 268, and map kinase phosphatase-1 (MKP-1) detected by in situ hybridization] in the striatum, bilaterally. Both Elk-1 and CREB transcription factors (targeting SRE and CRE DNA regulatory elements, respectively) were hyperphosphorylated in register with ERK activation and IEG mRNA induction. However, their hyperphosphorylation occurred in different subcellular compartments: the cytoplasm and the nucleus for Elk-1 and the nucleus for CREB. The role of the ERK signaling cascade in gene regulation was confirmed after intrastriatal and unilateral injection of the specific ERK inhibitor PD 98059, which completely abolished c-fos, zif 268, and MKP-1 mRNA induction in the injected side. Of interest, both Elk-1 and CREB hyperphosphorylation also was impaired after PD 98059 injection. Thus two different ERK modules, one depending on the cytoplasmic activation of Elk-1 and the other one depending on the nuclear activation of CREB, control IEG transcriptional regulation in our model. Our findings provide significant insights into intracellular mechanisms underlying synaptic plasticity in the striatum.

Preconditioning with bright light evokes a protective response against light damage in the rat retina

Constant exposure to bright light induces photoreceptor degeneration and at the same time upregulates the expression of several neurotrophic factors in the retina. At issue is whether the induced neurotrophic factors protect photoreceptors. We used a preconditioning paradigm to show that animals preconditioned with bright light became resistant to subsequent light damage. The preconditioning consisted of a 12-48 hr preexposure, followed by a 48 hr "rest phase" of normal cyclic lighting. The greatest protection was achieved by a 12 hr preexposure. Preconditioning induces a prolonged increase in two endogenous neurotrophic factors: basic fibroblast growth factor (bFGF) and ciliary neurotrophic factor (CNTF). It also stimulates the phosphorylation of extracellular signal-regulated protein kinases (Erks) in both photoreceptors and Müller cells. These findings indicate that exposure to bright light initiates two opposing processes: a fast degenerative process that kills photoreceptors and a relatively slower process that leads to the protection of photoreceptors. The extent of light damage, therefore, depends on the interaction of the two processes. These results also suggest a role of endogenous bFGF and CNTF in photoreceptor protection and the importance of Erk activation in photoreceptor survival.

In vivo expression and regulation of Elk-1, a target of the extracellular-regulated kinase signaling pathway, in the adult rat brain

The transcription factor Elk-1, a nuclear target of extracellular-regulated kinases (ERKs), plays a pivotal role in immediate early gene induction by external stimuli. Notably, the degree of phosphorylation of Elk-1 is tightly correlated with the level of activation of transcription of c-fos by proliferative signals. No data yet indicate the role of Elk-1 in the adult brain in vivo. To address this question, we have analyzed in the present work (1) Elk-1 mRNA and protein expression in the adult rat brain, and (2) the regulation of Elk-1 (i.e., its phosphorylation state) in an in vivo model of immediate early gene (IEG) induction: an electrical stimulation of the cerebral cortex leading to c-fos and zif268 mRNA induction in the striatum. Using in situ hybridization, we show that Elk-1 mRNA is expressed in various brain structures of adult rat, and that this expression is exclusively neuronal. We demonstrate by immunocytochemistry using various specific Elk-1 antisera that the protein is not only nuclear (as shown previously in transiently transfected cell lines) but is also present in soma, dendrites, and axon terminals. On electrical stimulation of the glutamatergic corticostriatal pathway, we show a strict spatiotemporal correspondence among ERK activation, Elk-1 phosphorylation, and IEG mRNA induction. Furthermore, both activated proteins, analyzed by immunocytochemistry, are found in cytosolic and nuclear comparments of neuronal cells in the activated area. Our data suggest that the ERK signaling pathway plays an important role in regulating genes controlled by serum response element sites via phosphorylation of Elk-1 in vivo.