Extracellular signal-regulated kinases: ERKs in progress

Mitogen-activated protein kinases in schizophrenia

BACKGROUND

Mitogen-activated protein kinases (MAPKs) are important mediators of signal transduction from the cell surface to the nucleus and have been implicated in the integration of a variety of physiologic processes in most cells, including neurons. To investigate the possible involvement of MAPKs in schizophrenia, we compared the levels of the MAPK intermediates in postmortem brain tissue obtained from schizophrenic and control subjects. Our focus was on the cerebellar vermis because of evidence suggesting that schizophrenia is associated with abnormalities of structure, function, and signal transduction in this brain region.

METHODS

Cytosolic proteins were fractionated by gel electrophoresis and subjected to Western blot analysis using polyclonal MAPK antibody, which detects total extracellular signal-regulated kinases (ERKs) 1 and 2 levels, and monoclonal MAP kinase phosphatase (MKP) 2 antibody.

RESULTS

Schizophrenic subjects had increased levels of ERK2 [2763 +/- (SD) 203 vs. 2286 +/- 607 arbitrary units, U = 17, p < .05] in cerebellar vermis. The levels of a dual specificity tyrosine phosphatase, MKP2, were significantly decreased in cerebellar vermis (1716 +/- 465 versus 2372 +/- 429 arbitrary units, U = 12, p < .02) from schizophrenic patients. ERK1/MKP2 and ERK2/MKP2 ratios in cerebellar vermis, but not in other brain regions, were significantly different in schizophrenic subjects as compared to control subjects (U = 15, p < or = .027; U = 3, p < .001, respectively).

CONCLUSIONS

MAPK levels are elevated in the cerebellar vermis of schizophrenic subjects. This could result from a protein dephosphorylation defect in vivo and might be involved in the pathology of the disease.

A microchip-based enzyme assay for protein kinase A

A microchip-based enzyme assay for protein kinase A is described. The microchips were prepared by standard photolithographic techniques. The assay reagents were placed in wells on the microchips, and electroosmosis was used to transport aliquots of these reagents into the network of etched channels, where the enzymatic reaction takes place. Protein kinase A catalyzes the transfer of a phosphate group from ATP to the serine residue of the heptapeptide LeuArgArgAlaSerLeuGly (Kemptide). The outcome of the enzymatic reaction was assessed by performing an on-chip electrophoretic separation of the fluorescently labeled peptide substrate and product. All liquid-handling steps were performed by controlling the electroosmotically driven flow from reagent and buffer wells using electrical current. On-chip dilutions of the peptide substrate, ATP and H-89, a known protein kinase A inhibitor, were performed and the kinetic constants (K(m), K(i)) of these compounds were determined. This prototype assay demonstrates the usefulness of the microchips for performing enzymatic assays for which fluorogenic substrates cannot easily be designed.

LDL stimulates mitogen-activated protein kinase phosphatase-1 expression, independent of LDL receptors, in vascular smooth muscle cells

Low density lipoprotein (LDL) is a well-established risk factor for atherosclerosis, stimulating vascular smooth muscle cell (SMC) differentiation and proliferation, but the signal transduction pathways between LDL stimulation and cell proliferation are poorly understood. Because mitogen-activated protein kinases (MAPKs) play a crucial role in mediating cell growth, we studied the effect of LDL on the induction of MAPK phosphatase-1 (MKP-1) in human SMCs and found that LDL stimulated induction of MKP-1 mRNA and proteins in a time- and dose-dependent manner. Heparin, inhibiting LDL-receptor binding, did not influence LDL-stimulated MKP-1 mRNA expression, and human LDL also induced MKP-1 expression in rat SMCs and fibroblasts derived from LDL receptor-deficient mice, indicating an LDL receptor-independent process. Pretreatment of SMCs with pertussis toxin markedly inhibited LDL-induced MKP-1 expression. Depletion of protein kinase C (PKC) by phorbol 12-myristate 13 acetate or inhibition of PKC by calphostin C blocked MKP-1 induction, but the phospholipase C inhibitor U73122 had no effect. Pretreatment of SMCs with genistein or herbimycin A abrogated LDL-stimulated MKP-1 induction. The MAPK kinase inhibitor PD98059 abolished LDL-stimulated activation of extracellular signal-regulated protein kinases (ERKs) but not MKP-1 induction. Furthermore, constitutive expression of MKP-1 in vivo reduced LDL-induced expression of Elk-1-dependent reporter genes, and SMC lines overexpressing recombinant MKP-1 exhibited decreased ERK activities and retarded proliferation in response to LDL. Our findings demonstrate that LDL induces MKP-1 expression in SMCs via activation of PKC and tyrosine kinases, independent of LDL receptors and ERK-MAPKs, and that MKP-1 plays an important role in the regulation of LDL-initiated signal transductions leading to SMC proliferation.

Interaction of mitogen-activated protein kinases with the kinase interaction motif of the tyrosine phosphatase PTP-SL provides substrate specificity and retains ERK2 in the cytoplasm

ERK1 and ERK2 associate with the tyrosine phosphatase PTP-SL through a kinase interaction motif (KIM) located in the juxtamembrane region of PTP-SL. A glutathione S-transferase (GST)-PTP-SL fusion protein containing the KIM associated with ERK1 and ERK2 as well as with p38/HOG, but not with the related JNK1 kinase or with protein kinase A or C. Accordingly, ERK2 showed in vitro substrate specificity to phosphorylate GST-PTP-SL in comparison with GST-c-Jun. Furthermore, tyrosine dephosphorylation of ERK2 by the PTP-SLDeltaKIM mutant was impaired. The in vitro association of ERK1/2 with GST-PTP-SL was highly stable; however, low concentrations of nucleotides partially dissociated the ERK1/2.PTP-SL complex. Partial deletions of the KIM abrogated the association of PTP-SL with ERK1/2, indicating that KIM integrity is required for interaction. Amino acid substitution analysis revealed that Arg and Leu residues within the KIM are essential for the interaction and suggested a regulatory role for Ser(231). Finally, coexpression of PTP-SL and ERK2 in COS-7 cells resulted in the retention of ERK2 in the cytoplasm in a KIM-dependent manner. Our results demonstrate that the noncatalytic region of PTP-SL associates with mitogen-activated protein kinases with high affinity and specificity, providing a mechanism for substrate specificity, and suggest a role for PTP-SL in the regulation of mitogen-activated protein kinase translocation to the nucleus upon activation.

Very low density lipoprotein-mediated signal transduction and plasminogen activator inhibitor type 1 in cultured HepG2 cells

In normal subjects and in patients with cardiovascular disease, plasma triglycerides are positively correlated with plasminogen activator inhibitor type 1 (PAI-1) levels. Moreover, in vitro studies indicate that VLDLs induce PAI-1 synthesis in cultured cells, ie, endothelial and HepG2 cells. However, the signaling pathways involved in the effect of VLDL on PAI-1 synthesis have not yet been investigated. We report that VLDLs induce a signaling cascade that leads to an enhanced secretion of PAI-1 by HepG2 cells. In myo-[(3)H]inositol-labeled HepG2 cells, VLDL (100 microg/mL) caused a time-dependent increase in [(3)H]inositol phosphates, the temporal sequence being tris>bis>monophosphate. VLDL brought about a time-dependent stimulation of membrane-associated protein kinase C (PKC) activity and arachidonate release. Finally, VLDL stimulated mitogen-activated protein (MAP) kinase, and this effect was reduced by 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7), which suggests that PKC plays a pivotal role in MAP kinase phosphorylation. VLDL-induced PAI-1 secretion was completely prevented by U73122, a specific inhibitor of phosphatidylinositol-specific phospholipase C, by H7 or by PKC downregulation, and by mepacrine (all P<0.01 versus VLDL-treated cells). 3,4,5-Trimethoxybenzoic acid 8-(diethylamino)-octyl ester, which prevents Ca2+ release from intracellular stores, inhibited VLDL-induced PAI-1 secretion by 60% (P<0.05), and the MAP kinase/extracellular signal-regulated kinase kinase (MEK) inhibitor PD98059 completely suppressed both basal and VLDL-induced PAI-1 secretion. These data demonstrate that VLDL-induced PAI-1 biosynthesis results from a principal signaling pathway involving PKC-mediated MAP kinase activation.

Expression and phosphorylation of mitogen-activated protein kinases during spermatogenesis and epididymal sperm maturation in mice

The expression and phosphorylation/dephosphorylation of mitogen-activated protein (MAP) kinases during mouse spermatogenesis and epididymal sperm maturation have been investigated by immunoblotting and immunohistochemical staining with commercially available anti-ERK2 and anti-Active MAPK antibodies. Two forms of MAP kinases, p42ERK2 and p44ERK1, were expressed in a similar amount in spermatogenic cells at different stages. ERK1 and ERK2 were phosphorylated (activated) in early spermatogenic cells from primitive spermatogonia to zygotene primary spermatocytes, while only a small quantity of phosphorylated MAP kinases could be detected in pachytene primary spermatocytes and spermatids. MAP kinase activity in primative spermatogonia and preleptotene primary spermatocytes was the highest among spermatogenic cells. ERK1 and ERK2 were also present in epididymal spermatozoa, and their phosphorylation was increased while spermatozoa pass through epididymis and vas deferens for maturation. It would appear that MAP kinase activation may contribute to the mitotic proliferation of primative spermatogonia, an early phase of spermatogenic meiosis, and, later, sperm motility acquirement.

Parthenogenetic Activation of Porcine Oocytes After Nuclear Transfer

Mature porcine oocytes are arrested at metaphase II of meiosis. At fertilization, like all mammalian oocytes they exhibit a low frequency Ca(2+) oscillation lasting several hours. This oscillation is thought to be the signal that triggers resumption of meiosis and activates the developmental program of the oocyte. The signal transduction mechanism of the sperm-induced Ca(2+) signal is not known in detail, and attempts to generate the oscillation artificially have met with little success. Nevertheless, artificial activation of the oocyte is a crucial step during nuclear transfer. Methods are available to induce a transient elevation in the intracellular free Ca(2+) concentration to surpass the meiotic arrest and induce development of the constructed embryo. Further studies concentrating on the mechanism of Ca(2+) signaling during fertilization will help to improve the efficiency of the procedures used for parthenogenetic activation of the oocyte.

Vascular effects of non-esterified fatty acids: implications for the cardiovascular risk factor cluster

Insulin resistance emerges as a central component of the risk factor cluster and is a likely contributor to vascular disease independently of traditional risk factors such as hypertension and diabetes mellitus. However, the intermediary mechanisms by which atherosclerosis is accelerated among patients with the insulin resistance syndrome remain inadequately defined. Most of the attention has centered on hyperinsulinemia and defects of insulin-mediated glucose disposal. However, we observed that obese hypertensive patients have elevated plasma concentrations of non-esterified fatty acids (NEFAs), including oleic acid, which are highly resistant to suppression by insulin. Resistance to insulin's fatty acid lowering action correlate with blood pressure in obese subjects independently of defects in glucose disposal. This observation raises the possibility that NEFAs have biologically significant effects on the cardiovascular system. In fact, oleic acid impairs nitric oxide synthase activity and endothelium-dependent vasorelaxation in vitro. Moreover, raising NEFAs in normal human volunteers to levels observed in obese hypertensive patients impairs lower extremity endothelium-dependent vasodilation and augments local and systemic vascular alpha1-adrenoceptor reactivity in normal volunteers. Thus, raising NEFAs replicates in healthy subjects important functional vascular changes implicated in the hypertension and atherosclerosis observed in patients with the risk factor cluster. At a molecular level, experiments in cultured vascular smooth muscle cells demonstrate that oleic acid activates a mitogenic signaling cascade which includes protein kinase C, reactive oxygen species and extracellular signal-regulated kinases. Each of these signaling events has been implicated in the structural and functional vascular changes which accompany the risk factor cluster. Collectively, these observations raise the possibility that fatty acids contribute to functional and structural vascular changes among insulin-resistant individuals. A better understanding of the signaling mechanisms by which NEFAs exert their vascular effects may facilitate novel and more effective therapeutic approaches to managing the cardiovascular risk factor cluster.

Peptide inhibitors of the mitogen-activated protein kinase pathway: a structure -mimetic peptide corresponding to the conserved inter-DFG-APE region in the kinase domain

The signal transduction pathway mediated by mitogen-activated protein kinases is an attractive target for the design of pharmacologically effective inhibitors. Two specific cell-permeant small molecule inhibitors of this pathway have been reported. However, under certain circumstances, nonpermeable inhibitors, such as neutralizing antibodies and peptide inhibitors, are also useful. We present here a novel approach for such peptide inhibitor design. The procedure is based on the synthesis of a structure-mimetic peptide corresponding to a short peptide segment in the target molecule. The results obtained so far show that a peptide designed in such a way is an effective inhibitor of the pathway. The possible application of such peptides and antipeptide antibodies as probes for protein kinase regulation mechanisms is also evaluated.

Protein phosphorylation and signal transduction

It is now generally accepted that protein phosphorylation-dephosphorylation has a role in the regulation of essentially all cellular functions. Thus, it is of interest that this process is involved in signal transduction. Nonetheless, the extent to which protein phosphorylation participates in signaling is truly remarkable. Almost every known signaling pathway eventually impinges on a protein kinase, or in some instances, a protein phosphatase. The diversity of these enzymes is noteworthy, and it is of interest that many biotechnology companies are eyeing them as potentially important targets for drugs. Such drugs may have important therapeutic applications, and in any event, they certainly will be useful to investigators who study signal transduction. Indeed, this already has been proven to be true.

Both Gs and Gi proteins are critically involved in isoproterenol-induced cardiomyocyte hypertrophy

Activation of beta-adrenoreceptors induces cardiomyocyte hypertrophy. In the present study, we examined isoproterenol-evoked intracellular signal transduction pathways leading to activation of extracellular signal-regulated kinases (ERKs) and cardiomyocyte hypertrophy. Inhibitors for cAMP and protein kinase A (PKA) abolished isoproterenol-evoked ERK activation, suggesting that Gs protein is involved in the activation. Inhibition of Gi protein by pertussis toxin, however, also suppressed isoproterenol-induced ERK activation. Overexpression of the Gbetagamma subunit binding domain of the beta-adrenoreceptor kinase 1 and of COOH-terminal Src kinase, which inhibit functions of Gbetagamma and the Src family tyrosine kinases, respectively, also inhibited isoproterenol-induced ERK activation. Overexpression of dominant-negative mutants of Ras and Raf-1 kinase and of the beta-adrenoreceptor mutant that lacks phosphorylation sites by PKA abolished isoproterenol-stimulated ERK activation. The isoproterenol-induced increase in protein synthesis was also suppressed by inhibitors for PKA, Gi, tyrosine kinases, or Ras. These results suggest that isoproterenol induces ERK activation and cardiomyocyte hypertrophy through two different G proteins, Gs and Gi. cAMP-dependent PKA activation through Gs may phosphorylate the beta-adrenoreceptor, leading to coupling of the receptor from Gs to Gi. Activation of Gi activates ERKs through Gbetagamma, Src family tyrosine kinases, Ras, and Raf-1 kinase.

Gialpha but not gqalpha is linked to activation of p21(ras) in human airway smooth muscle cells

Airway smooth muscle hypertrophy contributes to the narrowing of asthmatic airways. Activation of the mitogen-activated protein kinases is an important event in mediating cell proliferation. Because the monomeric G protein p21(ras) is an important intermediate leading to activation of mitogen-activated protein kinases, we questioned which heterotrimeric G protein-coupled receptors were linked to the activation of p21(ras) in cultured human airway smooth muscle and which of the heterotrimeric G protein subunits (alpha or betagamma) transmitted the activation signal. Carbachol and endothelin-1 increased GTP-bound p21(ras) in a pertussis toxin-sensitive manner [ratio of [32P]GTP to ([32P]GTP + [32P]GDP): control, 30 +/- 1.7; 3 min of 1 microM carbachol, 39 +/- 1.1; 3 min of 1 microM endothelin-1, 40 +/- 1.2], whereas histamine, bradykinin, and KCl were without effect. Transfection of an inhibitor of the G protein betagamma-subunit [the carboxy terminus (Gly495-Leu689) of the beta-adrenoceptor kinase 1] failed to inhibit the carbachol-induced activation of p21(ras). These data suggest that Gi- but not Gq-coupled receptors activate p21(ras) in human airway smooth muscle cells, and this effect most likely involves the alpha-subunit.

Mechanisms of mitogen-activated protein kinase activation in experimental diabetes

Various growth factors and vasoactive substances are implicated in the pathogenesis of renal growth seen in early diabetes mellitus (DM). Mitogen-activated protein kinase (MAPK) is an important mediator of these extracellular stimuli. Protein kinase C (PKC), an enzyme known to be stimulated in DM, also activates MAPK. Thus, MAPK activity was examined in glomeruli from streptozotocin-induced DM rats. MAPK activity, measured as myelin basic protein kinase, was elevated by approximately 50% in DM versus controls (CON). Increased protein contents of p42mapk and p44mapk, as well as increased tyrosine phosphorylation and mobility shift of p42mapk, were also observed in DM. Tyrosine dephosphorylation of pp42mapk, on the other hand, assessed by incubating glomerular membrane with or without sodium orthovanadate (vanadate), was significantly diminished in DM. Protein expression of MAPK phosphatase-1 (MKP-1), a dual specificity phosphatase that inactivates MAPK, was approximately 60% of CON. Reduction in MKP-1 was reproduced in cultured mesangial cells grown under high glucose (30 mM; HG). The suppression of MKP-1 was PKC-dependent since incubation of HG cells with phorbol 12-myristate 13-acetate for 24 h abolished it. Furthermore, calcium ionophore A23187 reversed the suppression, suggesting that blunted Ca2+ signalling, characteristic of HG cells secondary to PKC stimulation, may be the cause. These results demonstrate that glomerular MAPK is activated in DM by multiple mechanisms i.e., increases in protein contents, increased phosphorylation, and decreased dephosphorylation of the enzyme due to suppression of MKP-1. These alterations may have an implication in the pathogenesis of diabetic nephropathy.

Abnormalities of protein kinases in neurodegenerative diseases

In neurodegenerative diseases such as ALS and AD there is evidence for abnormal regulation of protein kinases. In these diseases, altered activities and protein levels of several specific kinases suggest that abnormal phosphorylation is present and this aberrant phosphorylation may be involved in the pathogenesis of these diseases. The observation that regulation of the NMDA receptor ion channel is altered in tissue from ALS patients may arise from the abnormal phosphorylation state of the protein kinase regulating NMDA receptor function. Whether the abnormalities of these protein kinases is a primary event leading to altered receptor regulation or vice versa is still poorly understood. The seemingly multiple pathogenic mechanisms of ALS and AD create complexity in assessing a primary cause that may lead to cell death. The mechanisms causing cell death (apoptosis or necrosis) may be overlapping with integrated events among the components interacting and contributing to a final pathway for neuron death. Thus, evidence of impairment in protein kinase signalling in these diseases may be a primary cause, a secondary event, or a compensatory mechanism. To further study this issue, different model systems could be beneficial to obtain a better understanding of these diseases.

Enhanced Gi-protein-mediated mitogenesis following chronic ethanol exposure in a rat model of experimental hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is associated with increased expression and function of inhibitory guanine nucleotide regulatory proteins (Gi-proteins). This study addresses the effects of chronic ethanol exposure on the expression and function of adenylyl cyclase (AC)-linked G-proteins (Gs and Gi) and growth in experimental HCC. G-protein expression and function was determined by immunoblot in the hepatic tumorigenic H4IIE cell line and isolated cultured hepatocytes in the absence or presence of ethanol (5-100 mmol/L). Chronic exposure (24 hours) to ethanol dose-dependently increased Gialpha1/2 expression in the H4IIE cell line, but not in cultured hepatocytes. Gsalpha-protein expression remained unchanged in both H4IIE cells and cultured hepatocytes following ethanol treatment. In addition, ethanol directly activated a Gi-protein, because pertussis toxin (PTx)-catalyzed, adenosine diphosphate (ADP)-dependent ribosylation of Gialpha substrates decreased following ethanol treatment. The increased functional activity of Gialpha1/2-protein expression was confirmed by demonstrating that ethanol dose-dependently inhibited basal and stimulated AC activity in H4IIE cells, while not significantly altering basal AC activity in isolated cultured hepatocytes. Furthermore, while ethanol had no significant effect on basal mitogenesis in H4IIE cells or hepatocytes, increased mitogenesis caused by direct Gialpha-protein stimulation (mastoparan M7; 10-5,000 nmol/L) was further enhanced in the presence of ethanol, an effect that was completely blocked following Gi-protein inhibition (PTx; 100 ng/mL). In contrast, activation of Gi-proteins using M7 failed to alter cellular mitogenesis in isolated cultured hepatocytes, whether in the absence or presence of ethanol. Finally, analysis of mitogen-activated protein kinase (MAPK) activity demonstrated that chronic ethanol treatment further enhanced Gi-protein-stimulated MAPK activity in hepatic tumorigenic cells. In conclusion, these data demonstrate that ethanol enhances cellular mitogenesis in experimental HCC as a result of, at least in part, a Gi-MAPK-dependent pathway. Furthermore, this effect may be caused by ethanol's direct up-regulation of the expression and activity of Gi-proteins in HCC.

Extracellular signal-regulated kinase mediates renal regeneration in rats with myoglobinuric acute renal injury

In vitro data support that extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK), members of mitogen-activated protein (MAP) kinases, mediate the signal transduction pathways responsible for the cell proliferation. However, in vivo role of these MAP kinases is poorly understood. Intramuscular injection of 50% glycerol solution induces acute renal failure in rats. This injury is known as a model of rhabdomyolysis in human. To investigate the molecular mechanism of the signaling pathway in this injury, we examined the role of ERK and JNK. After the glycerol injection JNK was rapidly and transiently activated at about 4 h, while the activation of ERK was gradually increased and the levels were sustained at least to 24 h. Next, we examined the expression of cell-cycle related proteins after the glycerol injection using Western blot analysis. The levels of proliferating cell nuclear antigen (PCNA) protein as a marker for cell proliferation were induced at 2 h and significantly increased to 24 h after the injection. In addition, cyclins D1, D2, and D3 as markers for G1 phase also increased with similar time courses. To examine whether activation of ERK and/or JNK are involved in the renal regeneration after the glycerol injection, we examined the effect of genistein, which is an inhibitor of tyrosine kinase, on the activation of ERK and JNK. Administration of genistein to rats with this injury decreased the activation of ERK, but not JNK. The induction of PCNA and cyclin D1 was also prevented by this treatment. In this condition, renal function was further worsened as compared to control rats. These results provide the first evidence that ERK may be involved in the repair process of renal tubules damaged by this injury.

Differential regulation of transcription: repression by unactivated mitogen-activated protein kinase Kss1 requires the Dig1 and Dig2 proteins

Kss1, a yeast mitogen-activated protein kinase (MAPK), in its unphosphorylated (unactivated) state binds directly to and represses Ste12, a transcription factor necessary for expression of genes whose promoters contain filamentous response elements (FREs) and genes whose promoters contain pheromone response elements (PREs). Herein we show that two nuclear proteins, Dig1 and Dig2, are required cofactors in Kss1-imposed repression. Dig1 and Dig2 cooperate with Kss1 to repress Ste12 action at FREs and regulate invasive growth in a naturally invasive strain. Kss1-imposed Dig-dependent repression of Ste12 also occurs at PREs. However, maintenance of repression at PREs is more dependent on Dig1 and/or Dig2 and less dependent on Kss1 than repression at FREs. In addition, derepression at PREs is more dependent on MAPK-mediated phosphorylation than is derepression at FREs. Differential utilization of two types of MAPK-mediated regulation (binding-imposed repression and phosphorylation-dependent activation), in combination with distinct Ste12-containing complexes, contributes to the mechanisms by which separate extracellular stimuli that use the same MAPK cascade can elicit two different transcriptional responses.

PTP-SL and STEP protein tyrosine phosphatases regulate the activation of the extracellular signal-regulated kinases ERK1 and ERK2 by association through a kinase interaction motif

Protein kinases and phosphatases regulate the activity of extracellular signal-regulated kinases 1 and 2 (ERK1/2) by controlling the phosphorylation of specific residues. We report the physical and functional association of ERK1/2 with the PTP-SL and STEP protein tyrosine phosphatases (PTPs). Upon binding, the N-terminal domains of PTP-SL and STEP were phosphorylated by ERK1/2, whereas these PTPs dephosphorylated the regulatory phosphotyrosine residues of ERK1/2 and inactivated them. A sequence of 16 amino acids in PTP-SL was identified as being critical for ERK1/2 binding and termed kinase interaction motif (KIM) (residues 224-239); it was shown to be required for phosphorylation of PTP-SL by ERK1/2 at Thr253. Co-expression of ERK2 with catalytically active PTP-SL in COS-7 cells impaired the EGF-induced activation of ERK2, whereas a PTP-SL mutant, lacking PTP activity, increased the ERK2 response to EGF. This effect was dependent on the presence of the KIM on PTP-SL. Furthermore, ERK1/2 activity was downregulated in 3T3 cells stably expressing PTP-SL. Our findings demonstrate the existence of a conserved ERK1/2 interaction motif within the cytosolic non-catalytic domains of PTP-SL and STEP, which is required for the regulation of ERK1/2 activity and for phosphorylation of the PTPs by these kinases. Our findings suggest that PTP-SL and STEP act as physiological regulators of the ERK1/2 signaling pathway.

Induction of mitogen-activated protein kinase phosphatase-1 by arachidonic acid in vascular smooth muscle cells

Arachidonic acid (AA) and its metabolites play important roles in a variety of biological processes, such as signal transduction, contraction, chemotaxis, and cell proliferation and differentiation. It was demonstrated recently that AA can activate mitogen-activated protein kinases (MAPKs), which are crucial for transducing signals initiating cell growth and apoptosis. Here we studied the effect of AA on the induction of MAPK phosphatase-1 (MKP-1) in vascular smooth muscle cells (VSMCs) and found that AA stimulated induction of MKP-1 mRNA and proteins in VSMCs in a time- and dose-dependent manner. Specific inhibitors of cyclooxygenase-, lipoxygenase-, and cytochrome P450-dependent metabolism did not affect AA-induced MKP-1 expression, indicating that eicosanoid biosynthesis was not involved in this process. The glutathione precursor N-acetylcysteine, an antioxidant, abolished AA-stimulated MKP-1 gene expression, whereas inhibition of protein kinase C by calphostin C had no influence on MKP-1 induction. VSMC pretreatment with genistein, a tyrosine kinase inhibitor, completely blocked AA-stimulated MKP-1 induction. MAPK kinase inhibitor PD 98059 did abolish AA-stimulated activation of extracellular signal-regulated kinases but not MKP-1 induction. Furthermore, agonists that increase AA release stimulated MKP-1 induction and activation of MAPKs, including extracellular signal-regulated kinases and c-Jun NH2-terminal protein kinases or stress-activated protein kinases. Taken together, our findings demonstrate that AA induced MKP-1 expression in VSMCs via activation of tyrosine kinases involving AA-induced free radical generation, suggesting an important role for MKP-1 in the regulation of AA-initiated signal transduction in VSMCs.

Dendritic cell survival and maturation are regulated by different signaling pathways

Although dendritic cell (DC) activation is a critical event for the induction of immune responses, the signaling pathways involved in this process have not been characterized. In this report, we show that DC activation induced by lipopolysaccharide (LPS) can be separated into two distinct processes: first, maturation, leading to upregulation of MHC and costimulatory molecules, and second, rescue from immediate apoptosis after withdrawal of growth factors (survival). Using a DC culture system that allowed us to propagate immature growth factor-dependent DCs, we have investigated the signaling pathways activated by LPS. We found that LPS induced nuclear translocation of the nuclear factor (NF)-kappaB transcription factor. Inhibition of NF-kappaB activation blocked maturation of DCs in terms of upregulation of major histocompatibility complex and costimulatory molecules. In addition, we found that LPS activated the extracellular signal-regulated kinase (ERK), and that specific inhibition of MEK1, the kinase which activates ERK, abrogated the ability of LPS to prevent apoptosis but did not inhibit DC maturation or NF-kappaB nuclear translocation. These results indicate that ERK and NF-kappaB regulate different aspects of LPS-induced DC activation: ERK regulates DC survival whereas NF-kappaB is responsible for DC maturation.

Reactive oxygen species are critical in the oleic acid-mediated mitogenic signaling pathway in vascular smooth muscle cells

Obese hypertensive patients with cardiovascular risk factor clustering have increased plasma nonesterified fatty acid levels and are at high risk for atherosclerotic events. Our previous studies demonstrated that oleic acid induces a mitogenic response in rat aortic smooth muscle cells (RASMCs) through protein kinase C (PKC)- and extracellular signal-regulated kinase (ERK)-dependent pathways. In the present study we investigated the possibility that the generation of reactive oxygen species (ROS) constitutes a critical component of the oleic acid-induced mitogenic signaling pathway in RASMCs. We studied the effect(s) of oleic acid on the generation of ROS using the oxidant-sensitive fluoroprobe 2',7'-dichlorofluorescin diacetate. Relative fluorescence intensity and fluorescent images were obtained with laser confocal scanning microscopy from 1 to 5 minutes, since preliminary studies demonstrated that the peak fluorescence intensity occurred within 5 minutes. Oleic acid (100 micromol/L) induced a time-dependent increase of cell fluorescence that was >8-fold of that seen in control cells at 5 minutes. This was blocked by catalase, which suggests that H2O2 was the principal ROS. The oleic acid-induced increases in H2O2 were blocked when PKC was inhibited with the use of bisindolylmaleimide and when PKC activity was downregulated by exposing RASMCs to phorbol 12-myristate 13-acetate for 24 hours. Stearic and elaidic acids, which are weak PKC activators, did not significantly increase H2O2 production. The increase of H2O2 in response to oleic acid was inhibited by the antioxidant N-acetylcysteine. N-Acetylcysteine also completely blocked ERK activation and the increase of thymidine incorporation in response to oleic acid. The data suggest that generation of H2O2 in RASMCs exposed to oleic acid is PKC dependent. Moreover, H2O2 production emerges as a critical intermediary event in the oleic acid-mediated mitogenic signaling pathway between the activation of PKC and ERK. These observations raise the possibility that the elevated plasma nonesterified fatty acids, including oleic acid, in obese hypertensive patients contribute to vascular growth and remodeling by a PKC-dependent mechanism to generate ROS that subsequently activate ERK.

Isolation and functional characterization of cDNA of serum amyloid A-activating factor that binds to the serum amyloid A promoter

Serum amyloid A (SAA), a plasma protein inducible in response to many inflammatory conditions, is associated with the pathogenesis of several diseases including reactive amyloidosis, rheumatoid arthritis, and atherosclerosis. We have previously reported an element of the SAA promoter, designated SAA-activating sequence (SAS), that is involved in the inflammation-induced SAA expression, and a nuclear factor, SAS-binding factor (SAF), that interacts with the SAS element has been identified previously (A. Ray and B. K. Ray, Mol. Cell. Biol. 16:1584-1594, 1996). To evaluate how SAF is involved in SAA promoter activation, we have investigated structural features and functional characteristics of this transcription factor. Our studies indicate that SAF belongs to a family of transcription factors characterized by the presence of multiple zinc finger motifs of the Cys2-His2 type at the carboxyl end. Of the three cloned SAF cDNAs (SAF-1, SAF-5, and SAF-8), SAF-1 isoform showed a high degree of homology to MAZ/ZF87/Pur-1 protein while SAF-5 and SAF-8 isoforms are unique and are related to SAF-1/MAZ/ZF87/Pur-1 at the zinc finger domains but different elsewhere. Although structurally distinct, all members are capable of activating SAS element-mediated expression and display virtually identical sequence specificities. However, varying levels of expression of members of this gene family were observed in different tissues. Functional activity of SAF is regulated by a posttranslational event as SAF DNA-binding and transactivation abilities are increased by a protein phosphatase inhibitor, okadaic acid, and inhibited by a protein kinase inhibitor, H7. Consistent with this observation, increased DNA binding of the cloned SAF and its hyperphosphorylation, in response to okadaic acid treatment of the transfected cells, were observed. Taken together, our results suggest that, in addition to tissue-specific expression, SAFs, a family of zinc finger transcription factors, undergo a modification by a posttranslational event that confers their SAA promoter-binding activity and transactivation potential.

Sperm-triggered calcium oscillations during meiosis in ascidian oocytes first pause, restart, then stop: correlations with cell cycle kinase activity

We have investigated the relationship between the sperm-triggered Ca2+ oscillations and the activities of two cell cycle kinases (MPF activity and MAP kinase activity) at fertilisation of Ascidiella aspersa oocytes. Maturation Promoting Factor (MPF) activity is elevated in the metaphase I (MI)-arrested unfertilised oocyte (as measured by phosphorylation of exogenous histone H1) and falls 5 minutes after fertilisation to remain at low levels for 5 minutes. The first polar body (pb1) is extruded when the MPF activity is low. The MPF activity is elevated again 15 minutes after fertilisation and finally becomes inactivated 25 minutes after fertilisation when the pb2 is extruded. MAP kinase activity increases from an initially elevated level to reach maximal activity 10 minutes after fertilisation and subsequently falls to reach low levels 25 minutes after fertilisation. Sperm trigger a series of Ca2+ oscillations that pause for 5 minutes while only the MPF activity is low and are present when both MPF and MAP kinase activity are elevated. We next attempted to determine whether the second phase of calcium oscillations is required to reactivate the MPF activity that precedes extrusion of the second polar body. To do this, we triggered a monotonic Ca2+ signal. This leads to the inactivation of MPF followed by MPF reactivation. The MPF activity then remains elevated for an extended period of time. During this period, the chromatin remains condensed and a metaphase II (MII) spindle forms. Fertilisation of these MII oocytes triggers extrusion of pb2 in 7 minutes. Interestingly, the second phase of Ca2+ oscillations is completely absent when MII oocytes are fertilised. Thus, in both MI and MII oocytes, the sperm-triggered Ca2+ oscillations follow the MPF activity. Finally we discuss our finding that the Ca2+ release system remains sensitive during the metaphase-like state (including the period when the Ca2+ oscillations pause).

Inhibition of MAPK pathway by a synthetic peptide corresponding to the activation segment of MAPK

Mitogen-activated protein kinase (MAPK) is activated by phosphorylation within its activation segment. Upon phosphorylation, the activation segment refolds to provide the active conformation of the enzyme. We reported previously that a phosphorylation-sensitive secondary structure could be formed in a 26-amino-acid long synthetic peptide corresponding to the activation segment of Xenopus MAPK, termed IDA (Inter-DFG-APE) MAPK peptide (Tokmakov, A. A., et al. 1997, Biochem. Biophys. Res. Commun. 236, 243-247). Here, we show that unphosphorylated IDA MAPK peptide can inhibit in vitro both MAPK and MAPK kinase activities with the inhibition constants of 82 and 18 microM, respectively. Phosphorylated forms of the peptide were of little effect. IDA MAPK peptide did not inhibit significantly the activity of some other protein kinases, including MAPK homologue p38 kinase, suggesting the specificity for MAPK and MAPK kinase. Microinjection of unphosphorylated IDA MAPK peptide into immature Xenopus oocytes significantly suppressed progesterone-induced oocyte maturation by inhibiting activation of both MAPK and maturation promoting factor. Similar inhibition of maturation was registered upon oocyte treatment with another specific inhibitor of MAPK pathway, PD098059. These results depict IDA MAPK peptide as a selective inhibitor of the MAPK pathway that can be used for the investigations of MAPK-mediated signaling.

Synergistic action of R-Ras and IGF-1 on Bcl-xL expression and caspase-3 inhibition in BaF3 cells: R-Ras and IGF-1 control distinct anti-apoptotic kinase pathways

R-Ras and insulin-like growth factor-1 (IGF-1) synergistically inhibit apoptosis of BaF3 cells upon interleukin-3 deprivation. To characterize the mechanism of this synergistic inhibition, we examined the effect of R-Ras and IGF-1 on several apoptosis-related proteins. Extracellular signal-regulated kinase (ERK) was activated by IGF-1, but not by R-Ras. In contrast, Akt was activated strongly by R-Ras, but weakly by IGF-1. It was also found that R-Ras and IGF-1 cooperatively induced Bcl-xL expression and inhibited caspase-3 activation.

Identification of a possible MAP kinase cascade in Arabidopsis thaliana based on pairwise yeast two-hybrid analysis and functional complementation tests of yeast mutants

A possible MAP kinase (MAPK) cascade of Arabidopsis thaliana was identified on the basis of both yeast 2-hybrid analysis and complementation analysis of yeast mutants. Specific protein-protein interactions between ATMPK4 (a MAPK) and MEK1 (a MAPKK) and interactions between MEK1 and ATMEKK1 (a MAPKKK) were detected by using the 2-hybrid system. A growth defect of the yeast mpk1delta mutant was reversed by coexpression of ATMPK4 and MEK1. Coexpression of the N-terminal deletion form of ATMEKK1 increased the ability of MEK1 to suppress a growth defect of the yeast pbs2delta mutant. These results suggest that ATMPK4, MEK1, and ATMEKK1 may interact with each other and constitute a specific MAPK cascade in Arabidopsis. This is the first demonstration of a possible MAPK cascade in plants.

Regulation of mitogen-activated protein kinase phosphatase-1 in vascular smooth muscle cells

Mitogen-activated protein (MAP) kinase cascades are major signaling systems by which cells transduce extracellular cues into intracellular responses. In general, MAP kinases are activated by phosphorylation on tyrosine and threonine residues and inactivated by dephosphorylation. Therefore, MAP kinase phosphatase-1 (MKP-1), a dual-specificity protein tyrosine phosphatase that exhibits catalytic activity toward both regulatory sites on MAP kinases, is suggested to be responsible for the downregulation of extracellular signal-regulated kinase (ERK), stress-activated protein kinase (SAPK), and p38 MAP kinase. In the present study, we examined the role of these MAP kinases in the induction of MKP-1 in vascular smooth muscle cells (VSMCs). Extracellular stimuli such as platelet-derived growth factor (PDGF), 12-O-tetradecanoylphorbol 13-acetate (TPA), and angiotensin II, which activated ERK but not SAPK/p38 MAP kinase, induced a transient induction of MKP-1 mRNA and its intracellular protein. In addition, PD 098059, an antagonist of MEK (MAP kinase/ERK kinase), the upstream kinase of ERK, significantly reduced the PDGF-induced activation of ERK and potently inhibited the expression of MKP-1 after stimulation with PDGF, thereby demonstrating the induction of MKP-1 in response to activation of the ERK signaling cascade. Furthermore, anisomycin, a potent stimulus of SAPK and p38 MAP kinase, also induced MKP-1 mRNA expression. This effect of anisomycin was significantly inhibited in the presence of the p38 MAP kinase antagonist SB 203580. These data suggest the induction of MKP-1, not only after stimulation of the cell growth promoting ERK pathway but also in response to activation of stress-responsive MAP kinase signaling cascades. We suggest that this pattern of MKP-1 induction may be a negative feedback mechanism in the control of MAP kinase activity in VSMCs.

Mechanisms of vascular growth-promoting effects of neuropeptide Y: role of its inducible receptors

We have previously reported that neuropeptide Y (NPY), a sympathetic cotransmitter and vasoconstrictor, is mitogenic for vascular smooth muscle cells (VSMCs), and now report on the mechanisms mediating these effects. In rat aortic A10 cell line, NPY's potency was greater than that of norepinephrine, and efficacy similar to that of platelet-derived growth factor, but less than that of the full serum, in stimulating cell proliferation; this effect was optimal in cell 60-80% cell density. At lower cell density and serum content, NPY stimulated DNA fragmentation/apoptosis. In rat aortic primary VSMCs (RASMCs), mitogenic effect of NPY was bimodal with the first peak at 1 pM, a decline at 1 nM, and a second peak at 10-100 nM; peptide YY had similar but less efficacious effects. The first NPY's peak was mimicked by Y2 agonists, and blocked by Y2 antagonist (T4-[NPY(33-36]4), and the second mimicked by Y1 agonist and partially blocked by Y1 antagonist, BIBP3226, suggesting a multireceptor mode of action. In A10 and in RASMCs, the expression of NPY receptors, Y1, Y2 and Y5, using RT-PCR was undetectable in quiescent cells but detected after pre-treatment with NPY. The receptor induction was NPY dose-dependent and also affected by incubation time and presence of serum. The NPY mitogenic effects were attenuated by calcium channel blockers, particularly verapamil. In primary cultures of rat coronary endothelial cells (where NPY is also mitogenic), NPY stimulated mitogen-activated protein kinase (MAPK) activity. Thus, the growth-promoting effects of NPY in vascular cells occur at concentrations lower than vasoconstrictive, and appear to be mediated by inducible Y1, Y2, and Y5 receptors, calcium entry and possibly MAPK activation.

Regulation of mitogen-activated protein kinase phosphatase-1 induction by insulin in vascular smooth muscle cells. Evaluation of the role of the nitric oxide signaling pathway and potential defects in hypertension

In this study, we examined the regulation of mitogen-activated protein kinase phosphatase (MKP-1) expression by insulin in primary vascular smooth muscle cell cultures. Insulin caused a rapid time- and dose-dependent induction of MKP-1 mRNA and protein expression. Blockade of nitric-oxide synthase (NOS) with NG-monomethyl-L-arginine acetate, and cGMP with RpcGMP, completely inhibited MKP-1 expression. Insulin-mediated MKP-1 expression was preceded by inducible NOS (iNOS) induction and cGMP production. Blockade of phosphatidylinositol 3-kinase (PI3-kinase) signaling with wortmannin inhibited insulin-mediated iNOS protein induction, cGMP production, and MKP-1 expression. To evaluate potential interactions between NOS and the mitogen-activated protein kinase (MAPK) signaling pathways, we employed PD98059 and SB203580, two specific inhibitors of ERKs and p38 MAPK. These inhibitors abolished the effect of insulin on MKP-1 expression. Only PD98059 inhibited insulin-mediated iNOS protein induction. Vascular smooth muscle cells from spontaneous hypertensive rats exhibited a marked decrease in MKP-1 induction due to defects in insulin-induced iNOS expression because of reductions in PI3-kinase activity. Treatment with sodium nitroprusside and 8-bromo-cGMP restored MKP-1 mRNA expression to levels comparable with controls. We conclude that insulin-induced MKP-1 expression is mediated by PI3-kinase-initiated signals, leading to the induction of iNOS and elevated cGMP levels that stimulates MKP-1 expression.

Beta-adrenergic and fibroblast growth factor receptors induce neuronal process outgrowth through different mechanisms

The mechanisms that initiate and direct neuronal process formation remain poorly understood. We have recently described a neuronal progenitor cell line, AS583-8.E4.22 (AS583-8) which undergoes neurite formation in response to beta2-adrenergic and basic fibroblast growth factor (bFGF) receptor activation [Kwon, J.H. et al., (1996) Eur. J. Neurosci., 8, 2042-2055]. In the present study, a comparison of these responses revealed that isoproterenol (ISO), a beta-adrenergic receptor agonist, induces multiple, highly branched processes within 30 min while bFGF induces fewer, unbranched processes within 24 h. In contrast to the ISO response, bFGF induces mitogen-activated protein kinase activation and c-fos expression in the cell line and results in neurite outgrowth that is dependent on new mRNA and protein synthesis. Two-dimensional isoelectric focusing-sodium dodecyl sulphate-polyacrylamide gel electrophoresis of cytoskeletal preparations revealed different patterns following ISO vs. bFGF exposure suggesting selective changes in protein expression and/or post-translational modifications. Immunoblot analysis of these preparations for beta-tubulin, tyrosinated alpha-tubulin and acetylated alpha-tubulin also revealed different patterns following each type of treatment. Follow-up confocal microscopy revealed that following ISO, the distribution of tyrosinated tubulin extends to the distal ends of processes whereas acetylated alpha-tubulin is diminished within distal ends. This pattern has been reported to be associated with enhanced microtubule dynamics, a state in which process outgrowth is facilitated. In contrast, following bFGF treatment the distributions of tyrosinated and acetylated alpha-tubulin were identical, a state associated with a diminution of microtubule dynamics. These results, a different time course of neurite formation, dependency on new gene expression and differential expression and cellular distribution of major cytoskeleton proteins suggest that neurite outgrowth induced by ISO vs. bFGF is mediated by two distinct intracellular effector mechanisms in AS583-8 cells. In addition, studies, using the differential distribution of post-translational modified alpha-tubulins in neurites of primary neuronal cultures as marker for the two distinct processes of neurite formation suggest, that similar mechanisms are present in vivo. Therefore, the AS583-8 cell line provides a useful model to study these signalling mechanisms that couple neurotransmitter and growth factor receptor activation to the cytoskeletal changes that mediate neurite formation.

Endogenous FGF1-induced activation and synthesis of extracellular signal-regulated kinase 2 reduce cell apoptosis in retinal-pigmented epithelial cells

Retinal-pigmented epithelial (RPE) cell survival is critical to the maintenance of the function of the neural retinal and in the development of various retina degenerations. We investigated molecular mechanisms involved in this function by assessing apoptosis in RPE cells following serum deprivation. Apoptosis induced by serum withdrawal is lower in aged RPE cells because of higher endogenous acidic fibroblast growth factor (FGF1) synthesis and secretion. These experiments examined several aspects of FGF signaling and the contribution of endogenous FGF1 to activation of the extracellular signal-regulated kinase 2 (ERK2). In aged RPE cells, FGFR1 was rapidly activated, and its autophosphorylation followed the kinetics of endogenous FGF1 secretion, before the onset of apoptosis. ERK2 phosphorylation, activity, and de novo synthesis increased at the same time. In marked contrast, no de novo JNK1 synthesis was observed. MEK1 inhibition resulted in lower levels of ERK2 activation and synthesis and higher levels of apoptosis. Treatment with neutralizing anti-FGF1 or blocking anti-FGFR1 antibodies mimics these effects. Thus, this study strongly suggests that the survival-increasing effect of FGF1 in aged RPE cells is because of an autocrine/paracrine loop in which the ERK2 cascade plays a pivotal role.

Extracellular Signal-Regulated Kinase-2, But Not c-Jun NH2-Terminal Kinase, Activation Correlates with Surface IgM-Mediated Apoptosis in the WEHI 231 B Cell Line

Both extracellular signal-regulated kinase (ERK) and c-Jun NH2-terminal kinase (JNK) have been implicated in mediating the signaling events that precede apoptosis. We studied the activation of these kinases during apoptosis of WEHI 231 B cells. Surface IgM ligation induces apoptosis of WEHI 231 cells. This effect is augmented by simultaneous engagement of CD95 and is inhibited by costimulation with either CD40 or IL-4R. We determined that surface IgM ligation activates ERK2 to a much greater level than JNK, and that IgM-mediated ERK2 activation is enhanced by costimulation with anti-CD95. Costimulation with either IL-4 or anti-CD40 interferes with anti-IgM-stimulated ERK2 activation. Transient expression of mitogen-activated protein kinase phosphatase-1 (MKP-1) inhibits both ERK2 activation and cell death following stimulation with anti-IgM and the combination of anti-IgM plus anti-CD95. CD40 engagement alone activates JNK, but IL-4 stimulation does not. N-acetyl-l-cysteine pretreatment, which blocks CD40-mediated JNK activation, does not affect the ability of CD40 to inhibit anti-IgM-mediated ERK2 activation and apoptosis. Together, these data suggest that JNK activation is not required for CD40 inhibition of surface IgM-induced cell death and that ERK2 plays an active role in mediating anti-IgM-induced apoptosis of WEHI 231 B cells.

Vascular smooth muscle cell growth and insulin regulation of mitogen-activated protein kinase in hypertension

Hyperinsulinemia (HI) and insulin resistance (IR) are frequently associated with hypertension and atherosclerosis. However, the exact roles of HI and IR in the development of hypertension are unclear. Mitogen-activated protein kinases (MAPK) are well-characterized intracellular mediators of cell proliferation. In this study, we examined the contribution of MAPK pathway in insulin-stimulated mitogenesis using primary vascular smooth muscle cells (VSMCs) isolated from aortas of normotensive Wistar-Kyoto rats (WKY) and spontaneous hypertensive rats (SHR). VSMCs were grown to confluence in culture, serum starved, and examined for DNA synthesis (using [3H]thymidine (TDR), immunoprecipitated MAPK activity, and MAPK phosphatase (MKP-1) induction). Basal rate of TDR incorporation into DNA was twofold higher in SHR compared with WKY (P < 0.005). Insulin caused a dose-dependent increase in TDR incorporation (150% over basal levels with 100 nM in 12 h). Stimulation was sustained for 24 h with a decline toward basal in 36 h. Pretreatment with insulin-like growth factor I (IGF-I) receptor antibody did not abolish mitogenesis mediated by 10-100 nM insulin, suggesting that insulin effect is mediated via its own receptors. Insulin had a small mitogenic effect in WKY (33% over basal). Insulin-stimulated mitogenesis was accompanied by a dose-dependent increase in MAPK activity in SHR, with a peak activation (>2-fold over basal) between 5 and 10 min with 100 nM insulin. Insulin had very small effects on MAPK activity in WKY. In contrast, serum-stimulated MAPK activation was comparable in WKY and SHR. Pretreatment with MEK inhibitor, PD-98059, completely blocked insulin's effect on MAPK activation and mitogenesis. Inhibition of phosphatidylinositol 3-kinase with wortmannin also prevented insulin's effects on MAPK activation and mitogenesis. In WKY, insulin and IGF-I treatment resulted in a rapid induction of MKP-1, the dual-specificity MAPK phosphatase. In contrast, VSMCs from SHR were resistant to insulin with respect to MPK-1 expression. We conclude that insulin is mitogenic in SHR, and the effect appears to be mediated by sustained MAPK activation due to impaired insulin-mediated MKP-1 mRNA expression, which may act as an inhibitory feedback loop in attenuating MAPK signaling.

Activation of extracellular signal-regulated kinase 2 by a novel Abl-binding protein, ST5

The human ST5 gene encodes three proteins with predicted molecular masses of 126, 82, and 70 kDa. These widely expressed proteins share a C-terminal region that bears significant sequence homology to a group of GDP/GTP exchange proteins for the Rab3 family of small GTP binding proteins. The N-terminal region of the largest ST5 protein, p126, contains two proline-rich sequences, PR1 and PR2, with consensus motifs similar to Src homology 3 (SH3) binding regions and to mitogen-activated protein kinase (MAPK) phosphorylation sites. Based on these properties, we sought to investigate the activity of ST5 proteins in signal transduction pathways. In vitro, p126 displayed preferential binding to c-Abl SH3, as compared with other SH3 domains. This interaction was mediated by the PR2 sequence. In vivo, expression of p126, but not p82 or p70, activated MAPK/ERK2 in response to EGF in COS-7 cells. Expression of c-Abl with p126 greatly enhanced this activity. Deletion of PR1 blocked the ability of p126 to activate ERK2. Deletion of PR2 did not affect the basal activity, but blocked the stimulatory effect of c-Abl. Whereas p82 expression had no effect on ERK2 activation by p126, p70 completely abrogated this activity. These observations suggest that ST5 can function as a signaling protein and can provide a link between c-Abl and ERK2.

IL-16 Activates the SAPK Signaling Pathway in CD4+ Macrophages

IL-16 has been reported as a modulator of T cell activation and was shown to function as chemoattractant factor. The chemotactic activity of IL-16 depends on the expression of CD4 on the surface of target cells, but the intracellular signaling pathways are only now being deciphered. This report describes IL-16 as an additional activator of the stress-activated protein kinase (SAPK) pathway in CD4+ macrophages. Treatment of these cells with recombinant expressed IL-16 leads to the phosphorylation of SEK-1, resulting in activation of the SAPKs p46 and p54. IL-16 stimulation also leads to the phosphorylation of c-Jun and p38 MAPK (mitogen-activated protein kinase), without inducing MAPK-family members ERK-1 and ERK-2. Interestingly, the IL-16-mediated activation of SAPKs and p38 MAPK in macrophages alone induces no detectable apoptotic cell death. These observations suggest specific regulatory functions of IL-16 distinct from the proinflammatory cytokines TNF-α and IL-1β.

Endothelin and isoproterenol counter-regulate cAMP and mitogen-activated protein kinases

Mitogen-activated protein kinases (MAPK) and cAMP are important components of the intracellular signaling pathways. We studied the effects of endothelin-1 (ET-1) and isoproterenol (ISO) on the intracellular cAMP level in human pericardial smooth-muscle cells and investigated how these two ligands regulate the activity of MAPK (p42/p44 MAPK). ET-1 or ET-3 alone did not exhibit any effect on the cAMP level in these cells. In contrast, ISO at 10 microM caused a 12-fold increase in the accumulation of cAMP (370 +/- 70 pmol/ml vs. 31 +/- 5 pmol/ml). Addition of ET-1 attenuated ISO-stimulated cAMP accumulation by 50% in a dose-dependent manner, with an IC50 of 0.12 nM. ET-3 was 100-fold less potent (IC50 = 15 nM). The attenuating effect of ET-1 was completely blocked by 1 microM FR139317, suggesting that the effect is primarily mediated by the ETA receptor. In serum-deprived cells, the basal MAPK activity was low (0.07 +/- 0.01 nmoles Pi/mg/min). Addition of 10 nM ET-1 stimulated MAPK 15-fold within 5 min at 37 degrees C (1.08 +/- 0.02 nmoles Pi/mg/min). ISO alone (10 microM) had no significant effect on MAPK. However, ISO markedly attenuated ET-1-stimulated MAPK activity; a approximately 50% decrease in MAPK activity was observed in the presence of 10 microM ISO. Similar results were obtained when forskolin was tested. The effects of ISO and forskolin on attenuating ET-1-stimulated MAPK activity could be reversed by treating cells with H89, an inhibitor of protein kinase A. These results show that ET-1 partially attenuated the accumulation of cAMP induced by ISO, and that ISO attenuated the MAPK activity induced by ET-1, possibly via activation of protein kinase A. This study suggests that counter-regulation among various ligands and cross-talk among different signaling pathways may be required to modulate biologic functions in a living cell.

T lymphocyte activation signals for interleukin-2 production involve activation of MKK6-p38 and MKK7-SAPK/JNK signaling pathways sensitive to cyclosporin A

p38/CSBP, a subgroup member of mitogen-activated protein kinase (MAPK) superfamily molecules, is known to be activated by proinflammatory cytokines and environmental stresses. We report here that p38 is specifically activated by signals that lead to interleukin-2 (IL-2) production in T lymphocytes. A p38 activator MKK6 was also markedly activated by the same stimulation. Pretreatment of cells with SB203580, a specific inhibitor of p38, as well as expression of a dominant-negative mutant of MKK6, suppressed the transcriptional activation of the IL-2 promoter. We also demonstrated that MKK7, a recently described MAPK kinase family member, plays a major role in the activation of stress-activated protein kinase (SAPK)/c-Jun NH2-terminal kinase (JNK) in T lymphocytes. Moreover, a dominant-negative mutant of MKK7 abrogated the transcriptional activation of the distal nuclear factor of activated T cells response element in the IL-2 promoter. Cyclosporin A, a potent immunosuppressant, inhibited activation of both p38 and SAPK/JNK pathways but not the MAPK/extracellular signal-regulated kinase (ERK) pathway. Our results indicate that both MKK6 to p38 and MKK7 to SAPK/JNK signaling pathways are activated in a cyclosporin A-sensitive manner and contribute to IL-2 gene expression in T lymphocytes.

Oleic acid and angiotensin II induce a synergistic mitogenic response in vascular smooth muscle cells

Oleic acid and angiotensin II (Ang II) are elevated and may interact to accelerate vascular disease in obese hypertensive patients. We studied the effects of oleic acid and Ang II on growth responses of rat aortic smooth muscle cells (VSMCs). Oleic acid (50 micromol/L) raised thymidine incorporation by 50% at 24 hours and cell number by 55% at 6 days (P<.05). Ang II (10(-11) to 10(-6) mol/L) did not significantly increase thymidine incorporation or VSMC number. Combining Ang II and 50 micromol/L oleic acid doubled thymidine incorporation and VSMC number. Losartan, an angiotensin type 1 (AT1) receptor antagonist, blocked the synergistic interaction between Ang II and oleic acid, whereas the AT2 receptor antagonist PD 123319 did not. Protein kinase C inhibition and downregulation, as well as inhibition of extracellular signal-regulated kinase (ERK) activation by PD 98059, eliminated the rise of thymidine incorporation in response to oleic acid and the synergistic interaction with Ang II. However, the response to 10% fetal bovine serum was unaffected. An antisense oligodeoxynucleotide to ERK-1 and ERK-2 reduced ERK protein expression and activation by 83% and 75%, respectively. Antisense prevented the rise of thymidine incorporation in response to oleic acid and the synergy with Ang II. Antisense reduced but did not prevent increased thymidine incorporation in response to serum. The data indicate that oleic acid and Ang II exert a synergistic mitogenic effect in VSMCs and suggest an important role for the AT1 receptor, PKC, and ERK in this synergy. The observations raise the possibility that a synergistic mitogenic interaction between oleic acid and Ang II accelerates vascular remodeling in obese hypertensive patients.

FGF-, BMP- and Shh-mediated signalling pathways in the regulation of cranial suture morphogenesis and calvarial bone development

The development of calvarial bones is tightly co-ordinated with the growth of the brain and needs harmonious interactions between different tissues within the calvarial sutures. Premature fusion of cranial sutures, known as craniosynostosis, presumably involves disturbance of these interactions. Mutations in the homeobox gene Msx2 as well as the FGF receptors cause human craniosynostosis syndromes. Our histological analysis of mouse calvarial development demonstrated morphological differences in the sagittal suture between embryonic and postnatal stages. In vitro culture of mouse calvaria showed that embryonic, but not postnatal, dura mater regulated suture patency. We next analysed by in situ hybridisation the expression of several genes, which are known to act in conserved signalling pathways, in the sagittal suture during embryonic (E15-E18) and postnatal stages (P1-P6). Msx1 and Msx2 were expressed in the sutural mesenchyme and the dura mater. FGFR2(BEK), as well as Bmp2 and Bmp4, were intensely expressed in the osteogenic fronts and Bmp4 also in the mesenchyme of the sagittal suture and in the dura mater. Fgf9 was expressed throughout the calvarial mesenchyme, the dura mater, the developing bones and the overlying skin, but Fgf4 was not detected in these tissues. Interestingly, Shh and Ptc started to be expressed in patched pattern along the osteogenic fronts at the end of embryonic development and, at this time, the expression of Bmp4 and sequentially those of Msx2 and Bmp2 were reduced, and they also acquired patched expression patterns. The expression of Msx2 in the dura mater disappeared after birth. &lt;P&gt; FGF and BMP signalling pathways were further examined in vitro, in E15 mouse calvarial explants. Interestingly, beads soaked in FGF4 accelerated sutural closure when placed on the osteogenic fronts, but had no such effect when placed on the mid-sutural mesenchyme. BMP4 beads caused an increase in tissue volume both when placed on the osteogenic fronts and on the mid-sutural area, but did not effect suture closure. BMP4 induced the expression of both Msx1 and Msx2 genes in sutural tissue, while FGF4 induced only Msx1. We suggest that the local application of FGF on the osteogenic fronts accelerating suture closure in vitro, mimics the pathogenesis of human craniosynostosis syndromes in which mutations in the FGF receptor genes apparently cause constitutive activation of the receptors. Taken together, our data suggest that conserved signalling pathways regulate tissue interactions during suture morphogenesis and intramembranous bone formation of the calvaria and that morphogenesis of mouse sagittal suture is controlled by different molecular mechanisms during the embryonic and postnatal stages. Signals from the dura mater may regulate the maintenance of sutural patency prenatally, whereas signals in the osteogenic fronts dominate after birth.

Integrin alphavbeta3 requirement for sustained mitogen-activated protein kinase activity during angiogenesis

Angiogenesis depends on growth factors and vascular cell adhesion events. Integrins and growth factors are capable of activating the ras/MAP kinase pathway in vitro, yet how these signals influence endothelial cells during angiogenesis is unknown. Upon initiation of angiogenesis with basic fibroblast growth factor (bFGF) on the chick chorioallantoic membrane (CAM), endothelial cell mitogen-activated protein (MAP) kinase (ERK) activity was detected as early as 5 min yet was sustained for at least 20 h. The initial wave of ERK activity (5-120 min) was refractory to integrin antagonists, whereas the sustained activity (4-20 h) depended on integrin alphavbeta3, but not beta1 integrins. Inhibition of MAP kinase kinase (MEK) during this sustained alphavbeta3-dependent ERK signal blocked the formation of new blood vessels while not influencing preexisting blood vessels on the CAM. Inhibition of MEK also blocked growth factor induced migration but not adhesion of endothelial cells in vitro. Therefore, angiogenesis depends on sustained ERK activity regulated by the ligation state of both a growth factor receptor and integrin alphavbeta3.

Cell type-specific angiotensin II-evoked signal transduction pathways: critical roles of Gbetagamma subunit, Src family, and Ras in cardiac fibroblasts

Angiotensin II (Ang II) induces hypertrophy of cardiac myocytes and hyperplasia of cardiac fibroblasts. To determine the molecular mechanism by which Ang II displayed different effects on cardiac myocytes and fibroblasts, we examined signal transduction pathways leading to activation of extracellular signal-regulated kinases (ERKs). Ang II-induced ERK activation was abolished by pretreatment with pertussis toxin and by overexpression of the Gbetagamma subunit-binding domain of the beta-adrenergic receptor kinase 1 in cardiac fibroblasts but not in cardiac myocytes. Inhibition of protein kinase C strongly inhibited activation of ERKs by Ang II in cardiac myocytes, whereas inhibitors of tyrosine kinases but not of protein kinase C abolished Ang II-induced ERK activation in cardiac fibroblasts. Overexpression of C-terminal Src kinase (Csk), which inactivates Src family tyrosine kinases, suppressed the activation of transfected ERK in cardiac fibroblasts. Ang II rapidly induced phosphorylation of Shc and association of Shc with Grb2. Cotransfection of the dominant-negative mutant of Ras or Raf-1 kinase abolished Ang II-induced ERK activation in cardiac fibroblasts. Overexpression of Csk or the dominant-negative mutant of Ras had no effects on Ang II-induced ERK activation in cardiac myocytes. These findings suggest that Ang II-evoked signal transduction pathways differ among cell types. In cardiac fibroblasts, Ang II activates ERKs through a pathway including the Gbetagamma subunit of Gi protein, tyrosine kinases including Src family tyrosine kinases, Shc, Grb2, Ras, and Raf-1 kinase, whereas Gq and protein kinase C are important in cardiac myocytes.

Trapidil inhibits platelet-derived growth factor-stimulated mitogen-activated protein kinase cascade

Trapidil, an antiplatelet drug, has been shown to reduce restenosis after angioplasty. It exerts its action, at least in part, by inhibiting vascular smooth muscle cell proliferation, antagonizing platelet-derived growth factor (PDGF). We examined its site of action on PDGF cellular signaling. Exposure of cultured rat vascular smooth muscle cells to increasing concentrations of trapidil for 18 hours resulted in a dose-dependent reduction in PDGF-BB-stimulated [3H] thymidine incorporation. Trapidil (400 microg/mL) increased PDGF beta-receptor protein by 28+/-8%, whereas PDGF-induced tyrosine phosphorylation of PDGF beta-receptor remained unchanged. PDGF-induced tyrosine phosphorylation of phospholipase Cgamma, the p85 regulatory subunit of phosphatidyl-inositol 3 kinase, Ras GTPase-activating protein, and an adaptor molecule Shc were also not altered. On the other hand, trapidil inhibited PDGF-stimulated mitogen-activated protein kinase (MAP kinase) activity by 35+/-7% at 10 minutes and by 32+/-10% at 6 hours. Activation of Raf-1, an upstream activator of MAP kinase, by PDGF was also attenuated by trapidil. Moreover, protein content of MAP kinase phosphatase-1, which inactivates MAP kinase, was elevated in trapidil-treated cells. These actions of trapidil may be mediated by cAMP. Thus, there was a 1.9-fold increase in cellular cAMP generation in trapidil-treated cells. The present results demonstrate that trapidil antagonizes PDGF-induced mitogenesis and MAP kinase activation in vascular smooth muscle cells, probably through cAMP.

Activation of the p42 mitogen-activated protein kinase pathway inhibits Cdc2 activation and entry into M-phase in cycling Xenopus egg extracts

We have added constitutively active MAP kinase/ERK kinase (MEK), an activator of the mitogen-activated protein kinase (MAPK) signaling pathway, to cycling Xenopus egg extracts at various times during the cell cycle. p42MAPK activation during entry into M-phase arrested the cell cycle in metaphase, as has been shown previously. Unexpectedly, p42MAPK activation during interphase inhibited entry into M-phase. In these interphase-arrested extracts, H1 kinase activity remained low, Cdc2 was tyrosine phosphorylated, and nuclei continued to enlarge. The interphase arrest was overcome by recombinant cyclin B. In other experiments, p42MAPK activation by MEK or by Mos inhibited Cdc2 activation by cyclin B. PD098059, a specific inhibitor of MEK, blocked the effects of MEK(QP) and Mos. Mos-induced activation of p42MAPK did not inhibit DNA replication. These results indicate that, in addition to the established role of p42MAPK activation in M-phase arrest, the inappropriate activation of p42MAPK during interphase prevents normal entry into M-phase.

Experience-dependent modifications in MAP2 phosphorylation in rat olfactory bulb

Microtubule-associated protein 2 (MAP2) is a neuron-specific cytoskeletal protein, enriched in dendrites and cell bodies, that helps determine dendritic shape. MAP2 regulates microtubule stability in a phosphorylation-dependent manner. The present study used immunocytochemistry with phosphoepitope-specific and phosphorylation state-independent antibodies to examine experience-dependent changes in MAP2 expression during postnatal development of the olfactory bulb. Our results demonstrate that immunoreactivity reflecting total MAP2 expression reaches a maximal level by postnatal day 20 (P20). The degree of staining for phosphoindependent forms of MAP2 is relatively unaffected by blocking odorant passage to one half the nasal epithelium via unilateral naris closure, a manipulation that attenuates physiological activity in the bulb. However, olfactory restriction from P1 dramatically reduces immunoreactivity for antibody AP18, which recognizes MAP2 only when phosphorylated on Ser136. Quantification of staining in the granule cell layer indicates that the greatest difference (64%) between control and experimental bulbs occurs after occlusion from P1 to P30 compared with animals deprived from P1 to P10 or P1 to P20. The shift in MAP2 phosphorylation occurs even when deprivation is delayed until P30, after the sensitive period for experience-dependent changes in bulb volume. Thus, the degree of the phosphorylation shift depends on the duration but not the time of onset of naris closure. Because staining for phosphorylation-independent forms of MAP2 is unchanged by naris closure, the total amount of the protein per unit area is probably not significantly altered. However, the large reductions of AP18-immunoreactivity in the bulb after olfactory restriction suggest that there is an activity-dependent stimulation of MAP2 phosphorylation.

Repression of mitogen-activated protein kinases ERK1/ERK2 activity by a protein tyrosine phosphatase in rat fibroblasts transformed by upstream oncoproteins

The observation that mitogen-activated protein (MAP) kinases ERK1 and ERK2 are constitutively activated in a number of oncogene-transformed cell lines has led to the hypothesis that prolonged activation of these enzymes is required for the transformation process. To investigate this question, we have examined the regulation of the ERK pathway in Rat1 fibroblasts transformed with activated c-Raf-1 (Raf22W), v-Ha-Ras, and v-Src. Expression of these oncoproteins had no effect on the enzymatic activity of ERK1 and ERK2 in either serum-starved or exponentially growing cells. Moreover, the stimulatory effect of serum on ERK1/ERK2 activity was substantially reduced or abrogated in these cells; this impairment was associated with a strong attenuation of c-fos gene induction. In contrast, expression of Raf22w, v-Ha-Ras, or v-Src resulted in the constitutive activation of the upstream kinases MEK1 and MEK2. Treatment of the cells with vanadate completely restored the activation of ERK1/ERK2 in oncogene-transformed cells, suggesting the involvement of a vanadate-sensitive tyrosine phosphatase. Northern blot analysis of VH1-like dual-specificity MAP kinase phosphatases did not reveal any significant difference in the mRNA expression pattern of these genes between parental and transformed Rat1 cells. Phosphoamino acid analysis indicated that ERK1 is phosphorylated on threonine, but not on tyrosine, in oncogene-transformed cells and that vanadate treatment restores tyrosine phosphorylation. We conclude from these results that ERK1/ERK2 activity is repressed by a single-specificity tyrosine phosphatase in oncogene-transformed rat fibroblasts.

Stimulation of MAP kinase and S6 kinase by vanadium and selenium in rat adipocytes

To explore the mechanism underlying the insulin-mimetic actions of vanadium and selenium we examined their effects on the mitogen activated protein/myelin basic protein kinases (MAPK) and ribosomal S6 protein kinases, which are among the best characterized of the kinases that comprise the phosphorylation cascade in insulin signal transduction. We observed a transient activation of MAPK and S6 kinases by insulin in rat adipocytes, while both sodium selenate and vanadyl sulphate produced prolonged activation of the kinases. Vanadyl sulphate stimulated the activity of MAPK and S6 kinase by as much as 6 fold and 15 fold, respectively. Pretreatment of the cells with genistein did not affect the activation of MAPK by insulin, but partially blocked the effects of sodium selenate and vanadyl sulphate. Genistein did not change the activation of S6 kinase by insulin, but blocked the activation in vanadyl sulphate- and sodium selenate-treated-cells, suggesting that a genistein sensitive tyrosine kinase may be involved in the activation by these two compounds. Rapamycin, a specific inhibitor of the p70s6k isoform of S6 kinase, partially reduced the activation of S6 kinase activity by sodium selenate, indicating a role for this kinase in the overall activity of the S6 kinase in sodium selenate-treated cells. A similar trend was noted in vanadyl sulphate-treated cells. Thus, this study supports the involvement of MAPK and S6 kinases in the insulin-mimetic actions of vanadium and selenium.

Experience-dependent modifications in MAP2 phosphorylation in rat olfactory bulb

Microtubule-associated protein 2 (MAP2) is a neuron-specific cytoskeletal protein, enriched in dendrites and cell bodies, that helps determine dendritic shape. MAP2 regulates microtubule stability in a phosphorylation-dependent manner. The present study used immunocytochemistry with phosphoepitope-specific and phosphorylation state-independent antibodies to examine experience-dependent changes in MAP2 expression during postnatal development of the olfactory bulb. Our results demonstrate that immunoreactivity reflecting total MAP2 expression reaches a maximal level by postnatal day 20 (P20). The degree of staining for phosphoindependent forms of MAP2 is relatively unaffected by blocking odorant passage to one half the nasal epithelium via unilateral naris closure, a manipulation that attenuates physiological activity in the bulb. However, olfactory restriction from P1 dramatically reduces immunoreactivity for antibody AP18, which recognizes MAP2 only when phosphorylated on Ser136. Quantification of staining in the granule cell layer indicates that the greatest difference (64%) between control and experimental bulbs occurs after occlusion from P1 to P30 compared with animals deprived from P1 to P10 or P1 to P20. The shift in MAP2 phosphorylation occurs even when deprivation is delayed until P30, after the sensitive period for experience-dependent changes in bulb volume. Thus, the degree of the phosphorylation shift depends on the duration but not the time of onset of naris closure. Because staining for phosphorylation-independent forms of MAP2 is unchanged by naris closure, the total amount of the protein per unit area is probably not significantly altered. However, the large reductions of AP18-immunoreactivity in the bulb after olfactory restriction suggest that there is an activity-dependent stimulation of MAP2 phosphorylation.

Redox regulation of genes that protect against carcinogens

Most carcinogens require activation to electrophilic metabolites or species that generate reactive oxygen in order to initiate the tumorigenic process. These reactive intermediates can, in turn, be detoxified by endogenous enzyme systems that and in the protection of cells from either toxic or mutagenic product formation. The levels of many of these enzymes are elevated by numerous compounds found in the diet, or by antioxidants. Recent evidence describes the mechanism for this induction of carcinogen detoxication enzymes to be regulated at the transcriptional level. Nuclear transcription factors bound to sites common among these carcinogen detoxication genes are activated by as yet unknown signal transduction pathways. The activity of these nuclear transcription factors are modulated by pro- and antioxidant reagents, suggesting that a redox-sensitive component governs the induction of enzymes involved in carcinogen metabolism. In this review, evidence for the redox regulation of the genes encoding carcinogen detoxication enzymes is presented. Evidence is also presented suggesting the participation of nuclear factor kappa B (NF-kappa B), mitogen-activated protein (MAP) kinase, and basic leucine zipper (bZIP) proteins and their activation pathways in this induction.