Role of mitogen-activated protein kinase phosphatase during the cellular response to genotoxic stress. Inhibition of c-Jun N-terminal kinase activity and AP-1-dependent gene activation

Activation of c-Jun N-terminal kinase 1 and caspase 3 in the tamoxifen-induced apoptosis of rat glioma cells

PURPOSE

The mechanisms of the antitumor effects of tamoxifen upon gliomas are still unclear. In this study, we investigated the role of c-Jun N-terminal kinase-1 (JNK1) and caspase 3 in the tamoxifen-induced apoptosis of rat glioma cells.

METHODS

Glioma cells were treated with tamoxifen, followed by a cytotoxicity assay to study its effects on the cells, and then a flow-activated cell sorter (FACS) analysis was performed to analyze the cellular apoptosis of the glioma cells. The expression of JNK1 and phospho-specific JNK1 in glioma cells treated with tamoxifen was investigated by Western blot analysis. The activity of caspase 3 in glioma cells was analyzed by caspase activity assay.

RESULTS

Tamoxifen was demonstrated to exert cytotoxic effects upon and induced apoptosis of the glioma cells in a concentration- and time-dependent manner (P<0.05). Western blot analysis demonstrated that tamoxifen increased the expression of phospho-specific JNK1 in glioma cells, and an increasing concentration of tamoxifen induced an increasing expression of phospho-specific JNK1. Four-hour 50-microM tamoxifen treatment increased the expression of phospho-specific JNK1 to 3.2 times that of the control level in glioma cells. Tamoxifen also increased the activity of caspase 3 in glioma cells. Pretreatment of glioma cells with the antisense oligonucleotide (OGN) of JNK1 immediately prior to tamoxifen treatment suppressed the expression of phospho-specific JNK1 and the activity of caspase 3. The apoptosis fraction of glioma cells induced by 4-h treatment with 50 microM tamoxifen was decreased from 51% to 28% by pretreatment with the antisense OGN of JNK1 (P<0.003), and to 20% by pretreatment with caspase 3 inhibitor (DEVD-CHO) (P<0.0008).

CONCLUSIONS

The results suggest that the tamoxifen-induced apoptosis of rat glioma cells is related to the activation of the JNK1/caspase 3 signaling pathway; however, the confirmation of the occurrence of such activation in vivo needs further investigation.

Nerve growth factor-dependent survival of CESS B cell line is mediated by increased expression and decreased degradation of MAPK phosphatase 1

The sIgG(+) lymphoblastoid B cell line CESS spontaneously produces a high amount of nerve growth factor (NGF) and expresses both high affinity (p140(Trk-A)) and low affinity (p75(NTR)) NGF receptors. Autocrine production of NGF maintains the survival of CESS cells through the continuous deactivation of p38 MAPK, an enzyme able to induce Bcl-2 phosphorylation and subsequent cytochrome c release and caspase activation. In this paper, we show that NGF induces transcriptional activation and synthesis of MAPK phosphatase 1 (MKP-1), a dual specificity phosphatase that dephosphorylates p38 MAPK, thus preventing Bcl-2 phosphorylation. Furthermore, NGF increases MKP-1 protein stability by preventing its degradation through the proteasome pathway. Following NGF stimulation, MKP-1 protein mainly localizes on mitochondria, suggesting an interaction with p38 MAPK in this compartment. Incubation of CESS cells with MKP-1-specific antisense oligonucleotides induces cell death, which was not prevented by exogenous NGF. By contrast, overexpression of native MKP-1, but not of its catalytically impaired form, inhibits apoptosis induced by NGF neutralization in CESS cells. Thus, the molecular mechanisms underlying the survival function of NGF in CESS B cell line predominantly consist in maintaining elevated levels of MKP-1 protein, which controls p38 MAPK activation.

Suppression of inducible nitric oxide synthase and cyclooxygenase-2 in downregulating nuclear factor-kappa B pathway by Garcinol

Garcinol is a polyisoprenylated benzophenone derivative of Garcinia indica fruit rind and other species. Recent studies have demonstrated that garcinol exhibited antioxidative effects in vitro. In this study, we found that garcinol inhibited the expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) in lipopolysaccharide (LPS)-activated macrophages. Western blot analyzes and gel-shift assays revealed that garcinol strongly blocks the activation of eukaryotic transcription factor nuclear factor-kappa B (NF-kappa B)-induced by LPS. Moreover, transient transfection experiments showed that garcinol inhibited the NF-kappa B-dependent transcriptional activity. Based on these data, we demonstrated that inhibition of LPS-induced NF-kappa B activation occurred through suppressing the phosphorylation of I kappa B alpha and p38 mitogen-activated kinase (MAPK). Garcinol also lowers the LPS-induced increase of intracellular reactive oxygen species (ROS), which contributes to the activation of NF-kappa B. The NF-kappa B signaling pathway leads to inflammatory reaction and our results suggest that garcinol suppresses the expression of iNOS in this pathway.

Alterations in subcellular localization of p38 MAPK potentiates endothelin-stimulated COX-2 expression in glomerular mesangial cells

Endothelin-1 (ET-1) is a potent vasoconstrictor peptide with mitogenic actions linked to activation of tyrosine kinase signaling pathways. ET-1 induces cyclooxygenase-2 (COX-2), an enzyme that converts arachidonic acid to pro-inflammatory eicosanoids. Activation of each of the three major mitogen-activated protein kinase (MAPK) pathways, ERK1/2, JNK/SAPK, and p38 MAPK (p38), have been shown to enhance the expression of COX-2. Negative regulation of MAPK may occur via a family of dual specificity phosphatases referred to as mitogen-activated protein kinase phosphatases (MKP). The goal of this work was to test the hypothesis that wild type MKP-1 regulates the expression of ET-1-induced COX-2 expression by inhibiting the activation of p38 in cultured glomerular mesangial cells (GMC). An adenovirus expressing both wild type and a catalytically inactive mutant of MKP-1 (MKP-1/CS) were constructed to study ET-1-regulated MAPK signaling and COX-2 expression in cultured GMC. ET-1 stimulated the phosphorylation of ERK and p38 alpha MAPK and induced the expression of COX-2. Expression of COX-2 was partially blocked by U0126, a MEK inhibitor, and SB 203580, a p38 MAPK inhibitor. Adenoviral expression of MKP-1/CS augmented basal and ET-1-induced phosphorylation of p38 alpha MAPK with less pronounced effects on ERK1/2 phosphorylation. Ectopic expression of wild type MKP-1 blocked the phosphorylation of p38 alpha MAPK by ET-1 but increased the phosphorylation of p38 gamma MAPK. Co-precipitation studies demonstrated association of MKP-1 with p38 alpha MAPK and ERK1/2. Immunofluorescent image analysis demonstrated trapping of phospho-p38 MAPK in the cytoplasm by MKP-1/CS/green fluorescent protein. ET-1-stimulated expression of COX-2 was increased in MKP-1/CS versus LacZ or green fluorescent protein-infected control cells. These results indicate that MKP-1 demonstrates a relative selectivity for p38 alpha MAPK versus p38 gamma MAPK in GMC and is likely to indirectly regulate the expression of COX-2.

Evidence for antagonism of BMP-4 signals by MAP kinase during Xenopus axis determination and neural specification

We have previously shown that mitogen-activated protein (MAP) kinase activity is required for neural specification in Xenopus. In mammalian cells, the BMP-4 effector Smad1 is inhibited by phosphorylation at MAP kinase sites (Kretzschmar et al., 1997). To test the hypothesis that MAP kinase inhibits the BMP-4/Smad1 pathway during early Xenopus development, we have generated a Smad1 mutant lacking the MAP kinase phosphorylation sites (M4A-Smad1) and compared the effects of wild-type (WT)- and M4A-Smad1 on axial pattern and neural specification in Xenopus embryos. Although overexpression of either WT- or M4A-Smad1 produced ventralized embryos, at each mRNA concentration, M4A-Smad1 had a greater ventralizing effect than WT-Smad1. Interestingly, overexpression of either form of Smad1 in ventral blastomeres disrupted posterior pattern and morphogenesis; again, more severe defects were produced by expression of M4A-Smad1 than by equal amounts of WT-Smad1. Ectodermal expression of M4A-Smad1 disrupted expression of the anterior neural gene otx2 in vivo and inhibited neural specification in response to endogenous signals in mesoderm-ectoderm recombinates. In contrast, overexpression of WT-Smad1 at identical levels had little effect on either neural specification or otx2 expression. Comparisons of protein levels following overexpression of either WT- or M4A-Smad1 indicate that WT-Smad1 may be slightly more stable than M4A-Smad1; thus, differences in stability cannot account for the increased effectiveness of M4A-Smad1. Our results demonstrate that mutations disrupting the MAPK phosphorylation sites act collectively as a gain-of-function mutation in Smad1 and that inhibitory phosphorylation of Smad1 may be a significant mechanism for the regulation of BMP-4/Smad1 signals during Xenopus development.

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

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

Role for retinoblastoma protein family members in UV-enhanced expression from the human cytomegalovirus immediate early promoters

The expression from a reporter construct driven by a cytomegalovirus (CMV) immediate early (IE) promoter is strongly inducible by UV in human fibroblasts. This response is induced at lower UV fluences in transcription-coupled repair (TCR)-deficient fibroblasts compared with normal fibroblasts and is absent in their simian virus 40-transformed counterparts. In this study we demonstrate that expression of human papilloma virus (HPV) E7 (but not of HPV E6) can attenuate UV-induced expression from the human CMV-IE-driven reporter construct in human fibroblasts. Furthermore, UV-induced expression from the reporter construct appears impaired in murine fibroblasts harboring inactivating mutations in the retinoblastoma (Rb) gene family members p107 and pRb but not in fibroblasts harboring such mutations in the p53 gene. Taken together, these data suggest that one or more members of the pRb family (but not p53) play an essential role in mediating UV-induced expression from the CMV-IE promoter. In this study we report normal UV-upregulation of reporter expression in xeroderma pigmentosum (XP) group E fibroblasts, consistent with normal TCR. Because XP-E cells deficient in the p48 subunit of the damaged DNA-binding protein are impaired in E2F-1-activated transcription, these results also suggest that the (pRb-regulated) transcription factor E2F-1 does not play an essential role in UV-enhanced expression from the CMV-IE promoter.

Antioxidant NAC and AMPA/KA receptor antagonist DNQX inhibited JNK3 activation following global ischemia in rat hippocampus

c-Jun N-terminal kinase-3 (JNK3), the only neural-specific isoform, may play an important role in excitotoxicity and neuronal injury. To analyze the variation of JNK3 activation, levels of phospho-JNK3 were measured at various time points of ischemia and selected time points of reperfusion, respectively. Our study illustrated that JNK3 was rapidly activated and translocated from cytosol to nucleus during ischemia. During reperfusion, two peaks of JNK3 activation occurred at 30 min and 3 days, respectively. To further define the mechanism of JNK3 activation, antioxidant N-acetylcysteine (NAC), alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate (KA) receptor antagonist 6,7-dinitro-quinoxaline-2,3(1H,4H)-dione (DNQX), N-methyl-D-aspartate (NMDA) receptor antagonist ketamine and L-type voltage-gated Ca(2+) channel (L-VGCC) antagonist nifedipine were given to the rats 20 min prior to ischemia. The results showed that NAC obviously inhibited JNK3 activation during the early reperfusion, whereas DNQX preferably attenuated JNK3 activation during the latter reperfusion. Ketamine and nifedipine had no significant effects on JNK3 activation during reperfusion. Consequently, reactive oxygen species (ROS) and AMPA/KA receptor were closely associated with JNK3 activation following global ischemia.

Osmotic regulation of insulin-induced mitogen-activated protein kinase phosphatase (MKP-1) expression in H4IIE rat hepatoma cells

A contribution of intracellular dehydration to insulin resistance has been established in human subjects and in different experimental systems. Here the effect of hyperosmolarity (405 mosmol/l) on insulin-induced mitogen-activated protein (MAP) kinase phosphatase (MKP)-1 expression was studied in H4IIE rat hepatoma cells. Insulin induces robust MKP-1 expression which correlates with a vanadate-sensitive decay of extracellular-signal-regulated kinase (Erk-1/Erk-2) activity. Hyperosmolarity delays MKP-1 accumulation by insulin and this corresponds to impaired MKP-1 synthesis, whereas MKP-1 degradation remains unaffected by hyperosmolarity. Rapamycin, which inhibits signalling downstream from the mammalian target of rapamycin (mTOR) and a peptide inhibiting protein kinase C (PKC) zeta/lambda abolish insulin-induced MKP-1 protein but not mRNA expression, suggesting the involvement of the p70 ribosomal S6 protein kinase (p70S6-kinase) and/or the eukaryotic initiation factor 4E-binding proteins (4E-BPs) as well as atypical PKCs in MKP-1 translation. Hyperosmolarity induces sustained suppression of p70S6-kinase and 4E-BP1 hyperphosphorylation by insulin, whereas insulin-induced tyrosine phosphorylation of the insulin receptor (IR) beta subunit and the IR substrates IRS1 and IRS2, recruitment of the phosphoinositide 3-kinase (PI 3-kinase) regulatory subunit p85 to the receptor substrates as well as PI 3-kinase activation, and Ser-473 phosphorylation of protein kinase B and Thr-410/403 phosphorylation of PKC zeta/lambda are largely unaffected under hyperosmotic conditions. The hyperosmotic impairment of both, MKP-1 expression and p70S6-kinase hyperphosphorylation by insulin is insensitive to K(2)CrO(4), calyculin A and vanadate, and inhibition of the Erk-1/Erk-2 and p38 pathways. The suppression of MKP-1 may further contribute to insulin resistance under dehydrating conditions by allowing unbalanced MAP kinase activation.

High glucose activates the p38 MAPK pathway in cultured human peritoneal mesothelial cells

BACKGROUND

Peritoneal fibrosis is a serious complication in long-term continuous ambulatory peritoneal dialysis (CAPD) patients, but the underlying mechanism is not well understood. Since high glucose activates the p38 mitogen-activated protein kinase (MAPK) pathway in various kinds of cells, and because mesothelial cells are always exposed to high glucose dialysate, we examined the activity and expression of p38 MAPK members in cultured human peritoneal mesothelial cells (HPMCs) under high glucose conditions.

METHODS

HPMCs were isolated from omentum and subcultured. After serum restriction, HPMCs were exposed to 5.6 mmol/L glucose (low glucose), 5.6 mmol/L glucose + 34.5 mmol/L mannitol (low glucose + mannitol), or 40 mmol/L glucose (high glucose) for 3 minutes to 48 hours with or without SB203580. Reverse transcription-polymerase chain reaction (RT-PCR) and Western blot were performed to determine mRNA and protein expression, respectively.

RESULTS

p38 MAPK and cyclic adenosine monophosphate (cAMP)-responsive element binding protein (CREB) activities and mRNA expressions were significantly increased in HPMCs exposed to high glucose compared to low glucose or low glucose + mannitol after 10 minutes and remain at higher levels to 48 hours (P < 0.05), but total p38 MAPK and CREB protein expressions did not differ. MAPK kinase 3/6 (MKK3/6) activity and mRNA expression were also higher in high glucose cells after 3 minutes (P < 0.05), and fibronectin mRNA expression was significantly increased in HPMCs exposed to high glucose after 2 hours (P < 0.05). In contrast, high glucose significantly inhibited MAPK phosphatase-1 (MKP-1) protein and mRNA expression after 10 minutes (P < 0.05). SB203580 (1 micromol/L) pretreatment for 1 hour significantly reduced high glucose-induced CREB activity and fibronectin mRNA expression by 89% and 75%, respectively (P < 0.05).

CONCLUSION

p38 MAPK activity was increased in HPMCs exposed to high glucose, in parallel with increased MKK3/6 activity and decreased MKP-1 expression, resulting in CREB activation. This activated p38 MAPK pathway may play a role in the pathogenesis of peritoneal fibrosis.

Discovery of CEP-1347/KT-7515, an inhibitor of the JNK/SAPK pathway for the treatment of neurodegenerative diseases

Apoptosis has been proposed as a mechanism of cell death in Alzheimer's, Huntington's and Parkinson's diseases and the occurrence of apoptosis in these disorders suggests a common mechanism. Events such as oxidative stress, calcium toxicity, mitochondria defects, excitatory toxicity, and deficiency of survival factors are all postulated to play varying roles in the pathogenesis of the diseases. However, the transcription factor c-jun may play a role in the pathology and cell death processes that occur in Alzheimer's disease. Parkinson's disease (PD) is also a progressive disorder involving the specific degeneration and death of dopamine neurons in the nigrostriatal pathway. In Parkinson's disease, dopaminergic neurons in the substantia nigra are hypothesized to undergo cell death by apoptotic processes. The commonality of biochemical events and pathways leading to cell death in these diseases continues to be an area under intense investigation. The current therapy for PD and AD remains targeting replacement of lost transmitter, but the ultimate objective in neurodegenerative therapy is the functional restoration and/or cessation of progression of neuronal loss. This chapter will describe a novel approach for the treatment of neurodegenerative diseases through the development of kinase inhibitors that block the active cell death process at an early transcriptional independent step in the stress activated kinase cascade. In particular, preclinical data will be presented on the c-Jun Amino Kinase pathway inhibitor, CEP-1347/KT-7515, with respect to it's properties that make it a desirable clinical candidate for treatment of various neurodegenerative diseases.

Restraint of proinflammatory cytokine biosynthesis by mitogen-activated protein kinase phosphatase-1 in lipopolysaccharide-stimulated macrophages

Exposure of macrophages to LPS elicits the production of proinflammatory cytokines, such as TNF-alpha, through complex signaling mechanisms. Mitogen-activated protein (MAP) kinases play a critical role in this process. In the present study, we have addressed the role of MAP kinase phosphatase-1 (MKP-1) in regulating proinflammatory cytokine production using RAW264.7 macrophages. Analysis of MAP kinase activity revealed a transient activation of c-Jun N-terminal kinase (JNK) and p38 after LPS stimulation. Interestingly, MKP-1 was induced concurrently with the inactivation of JNK and p38, whereas blocking MKP-1 induction by triptolide prevented this inactivation. Ectopic expression of MKP-1 accelerated JNK and p38 inactivation and substantially inhibited the production of TNF-alpha and IL-6. Induction of MKP-1 by LPS was found to be extracellular signal-regulated kinase dependent and involved enhanced gene expression and increased protein stability. Finally, MKP-1 expression was also induced by glucocorticoids as well as cholera toxin B subunit, an agent capable of preventing autoimmune diseases in animal models. These findings highlight MKP-1 as a critical negative regulator of the macrophage inflammatory response, underscoring its premise as a potential target for developing novel anti-inflammatory drugs.

Transcriptional induction of mitogen-activated protein kinase phosphatase 1 by retinoids. Selective roles of nuclear receptors and contribution to the antiapoptotic effect

All-trans-retinoic acid (t-RA) inhibits hydrogen peroxide (H(2)O(2))-induced apoptosis by inhibiting the c-Jun N-terminal kinase (JNK)-activator protein 1 (AP-1) pathway. In this report, we examined the involvement of mitogen-activated protein kinase phosphatase 1 (MKP-1) in suppression of JNK and the antiapoptotic effect of t-RA and the roles of nuclear receptors in the regulation of MKP-1 by t-RA. We found that not only t-RA, but also a selective agonist of retinoic acid receptor (RAR), a selective agonist of retinoid X receptor (RXR), and a pan-agonist of RAR and RXR all induced MKP-1 at the transcriptional level. Activation of RAR was required for all of these triggering effects, but activation of RXR was required only for the RXR agonist-induced MKP-1 expression. Among the three RAR subtypes, RARalpha and RARgamma, but not RARbeta, mediated the t-RA-induced MKP-1 expression. The antiapoptotic effect of t-RA on H(2)O(2)-induced apoptosis in several cell types was correlated with the inducibility of MKP-1 by t-RA. Inhibition of MKP-1 by vanadate enhanced JNK phosphorylation and attenuated the antiapoptotic effect of t-RA. Furthermore, overexpression of MKP-1 inhibited H(2)O(2)-induced JNK phosphorylation and apoptosis. To our knowledge, this is the first to demonstrate that 1) MKP-1 is inducible by retinoids at the transcriptional level, 2) RXR and individual RAR subtypes have different roles in this process, and 3) the induced MKP-1 plays a significant role in mediating both JNK inhibition and the antiapoptotic effect of t-RA in oxidative stress.

Evidence that inhibition of p44/42 mitogen-activated protein kinase signaling is a factor in proteasome inhibitor-mediated apoptosis

The proteasome is emerging as a target for cancer therapy because small molecule inhibitors of its catalytic activity induce apoptosis in both in vitro and in vivo models of human malignancies and are proving to have efficacy in early clinical trials. To further elucidate the mechanism of action of these inhibitors, their impact on signaling through the p44/42 mitogen-activated protein kinase (MAPK) pathway was studied. Proteasome inhibition with either carbobenzoxy-leucyl-leucyl-phenylalaninal or lactacystin led to a loss of dually phosphorylated, activated p44/42 MAPK in A1N4-myc human mammary and MDA-MB-231 breast carcinoma cells in a dose- and time-dependent fashion. This correlated with an induction of the dual specificity MAPK phosphatases (MKP)-1 and -2, and blockade of MKP induction using either actinomycin D or Ro-31-8220 significantly decreased loss of activated p44/42 MAPK. Inhibition of p44/42 MAPK signaling by use of the MAPK kinase inhibitors PD 98059 or U0126, or by use of a dominant negative MAPK construct, enhanced proteasome inhibitor-mediated apoptosis. Conversely, activation of MAPK by epidermal growth factor, or use of a mutant MAPK resistant to MKP-mediated dephosphorylation, inhibited apoptosis. These studies support a role for inactivation of signaling through the p44/42 MAPK pathway in proteasome inhibitor-mediated apoptosis.

An anti-GD2 monoclonal antibody enhances apoptotic effects of anti-cancer drugs against small cell lung cancer cells via JNK (c-Jun terminal kinase) activation

Small cell lung cancer (SCLC) cell lines specifically express ganglioside GD2, and anti-GD2 monoclonal antibodies (mAbs) caused suppression of cell growth and induced apoptosis of SCLC cells with single use. Here, enhancement of the cytotoxic effects of various anti-cancer drugs with an anti-GD2 mAb was demonstrated. The cytotoxicity of all six drugs examined was markedly enhanced, i.e. 2.4 - 7.8-fold increase of cell sensitivity in terms of IC(50). In particular, the combination of cisplatin (CDDP) with an anti-GD2 mAb resulted in prominent enhancement of cytotoxicity even in low - moderate GD2-expressing lines. The anti-GD2 mAb induced weak activation of c-Jun terminal kinase (JNK) in SCLC cells, and all anti-cancer drugs also induced its activation to various degrees. When CDDP and an anti-GD2 mAb were used together, significantly stronger JNK activation was observed corresponding to the cytotoxic effects, suggesting that synergistic phosphorylation of JNK with two reagents induced prominent apoptosis. The essential role of JNK in the induction of SCLC apoptosis with CDDP and anti-GD2 mAb was confirmed by experiments with a JNK inhibitor, curcumin. These results suggest that anti-GD2 mAbs would be very efficient in combination with anti-cancer drugs, both to achieve SCLC-specific cytotoxicity and to enhance its magnitude.

A mammalian expression system for rapid production and purification of active MAP kinase phosphatases

Expression of enzymatically active mammalian proteins in Escherichia coli can proven to be a challenging task due to poor solubility, improper folding, and lack of adequate posttranslational modification. Expression of mammalian proteins using baculovirus or yeast systems is time-consuming and may also be subject to inadequate modification. In order to overcome these technical difficulties, we have developed a mammalian expression system for the convenient subcloning of cDNA fragments, high-level expression, and one-step purification of enzymatically active proteins. The mammalian expression vector pEBG that expresses glutathione S-transferase fusion proteins was modified to create an SrfI restriction site in the multiple cloning site. The protein coding sequences of MAP kinase phosphatase-1 (MKP-1), MAP kinase phosphatase-2 (MKP-2), and the tumor suppressor PTEN were PCR-amplified using Pfu DNA polymerase and cloned into the SrfI site through SrfI digestion-coupled ligation. The resulting plasmids were transiently transfected into 293T cells using FuGENE 6 transfection reagent. Forty eight hours after transfection, cells were harvested and bioactive recombinant proteins were purified by glutathione-Sepharose beads. Protein yield, which ranged from 200 to 700 microg, was more than adequate for biochemical studies. The usefulness of this versatile system for studying protein function and its potential application for proteomics research are discussed.

The carboxyl-terminal domains of MKP-1 and MKP-2 have inhibitory effects on their phosphatase activity

Both the mitogen-activated protein kinase (MAPK) phosphatases MKP-1 and MKP-2 exert important feedback control of MAPK-mediated signaling events. The function of MKP-1 and MKP-2 is regulated via complex mechanisms, ranging from increased transcription of the MKP-1 and MKP-2 genes to post-translational catalytic activation of MKP-1 and MKP-2 proteins upon binding to their substrate MAPKs. In addition, MKP-1 stability increases upon ERK-dependent phosphorylation of two serine residues in its C-terminus. The C-terminal regions of MKP-1 and MKP-2, but not those of other MKPs, are homologous. To investigate the role of this domain, we have deleted the C-terminal tails from MKP-1 and MKP-2 and examined the effect of these deletions on their enzymatic activity. C-terminally truncated MKP-1 and MKP-2 exhibited, both in vivo and in vitro, substantially greater phosphatase activity towards their substrate MAPKs than did the full-length counterparts. However, C-terminal truncations did not significantly change either their substrate affinity, or their substrate-mediated catalytic activation. Basal phosphatase activity of the truncated proteins was also significantly higher than that of the wild-type counterparts. Collectively, these results suggest that the C-terminal domain may potentially play a role in the regulation of MKP-1 and MKP-2.

Oxidant-induced signaling: effects of peroxynitrite and singlet oxygen

Following the requirement for cells to cope with oxidative stress, there are cellular adaptation mechanisms at the level of gene expression. Much of what is known about oxidant-induced signaling in mammalian cells was found in experiments using hydrogen peroxide as an oxidant. However, since the biochemical reactivities of various oxidants significantly differ, 'oxidative stress' is not necessarily identical independent of the oxidant employed to bring it about. Here, the biological actions of peroxynitrite and singlet oxygen are presented, focusing on signaling effects. Peroxynitrite is generated in biological systems in the diffusion-controlled reaction of superoxide with nitrogen monoxide and is thus likely to be produced in the vicinity of activated macrophages. Singlet oxygen is generated by stimulated neutrophils in vivo and may further be generated photochemically, e.g. upon exposure of cells to ultraviolet A radiation. Exposure of cells to either of these oxidants elicits a cellular stress response, entailing the activation of signaling cascades that regulate proliferative and apoptotic responses, such as mitogen-activated protein kinase cascades or the phosphoinositide 3-kinase/Akt cascade. Two mechanisms for the oxidant-induced activation of a signaling cascade may be envisaged: (i) the indirect targeting of the cascade by interrupting negative regulation, and (ii) an activating oxidation of one of the constituting components of the cascade. Examples for both mechanisms in relation to peroxynitrite and singlet oxygen are discussed.

Eicosapentaenoic acid and docosahexaenoic acid modulate MAP kinase enzyme activity in human T-cells

In order to investigate the implication of docosahexacnoic acid (DHA) and eicosapentaenoic acid (EPA) in T signalling, we assessed their effects on the activation of two mitogen activated protein (MAP) kinases, i.e. extracellularly-regulated kinases 1 and 2 (ERK1/ERK2) in Jurkat T-cells. The n-3 polyunsaturated fatty acids (PUFAs) alone failed to induce MAP kinase (MAPK) enzyme activity. To elucidate whether DHA and EPA act via protein kinase C (PKC) dependent and independent pathways, we employed their respective activators, i.e. phorbol 12-myristate 13-acetate (PMA) and antiCD3 antibodies. We observed that U0126, an inhibitor of MAPK kinase-ERK kinase 1/2 (MEK1/2), abolished the actions of these two agents on MAPK activation, suggesting that they act upstream of MEK1/2. Further EPA and DHA diminished both the PMA- and antiCD3 antibodies-induced enzyme activity of ERK1/ERK2 in Jurkat T-cells. Interestingly, okadaic acid (OA), a phosphatase inhibitor seems to act downstream of MEK1/2 as U0126 failed to inhibit the OA-induced MAPK activation. It is noteworthy that EPA and DHA not only failed to curtail the OA-induced MAPK activity but also these n-3 PUFAs at 20 microM potentiated the action of OA. Therefore, EPA and DHA seem to modulate MAPK activation upstream and downstream of MEK1/2. On the hand, arachidonic acid, an n-6 PUFA potentiated the MAPK enzyme activity. In conclusion, our study shows that EPA and DHA may regulate T-cells functions by modulating MAPK enzyme activity.

Regulation of stress-activated protein kinase signaling pathways by protein phosphatases

Stress-activated protein kinase (SAPK) signaling plays essential roles in eliciting adequate cellular responses to stresses and proinflammatory cytokines. SAPK pathways are composed of three successive protein kinase reactions. The phosphorylation of SAPK signaling components on Ser/Thr or Thr/Tyr residues suggests the involvement of various protein phosphatases in the negative regulation of these systems. Accumulating evidence indicates that three families of protein phosphatases, namely the Ser/Thr phosphatases, the Tyr phosphatases and the dual specificity Ser/Thr/Tyr phosphatases regulate these pathways, each mediating a distinct function. Differences in substrate specificities and regulatory mechanisms for these phosphatases form the molecular basis for the complex regulation of SAPK signaling. Here we describe the properties of the protein phosphatases responsible for the regulation of SAPK signaling pathways.

ATM-dependent activation of the gene encoding MAP kinase phosphatase 5 by radiomimetic DNA damage

Cellular responses to DNA damage are mediated by an extensive network of signaling pathways. The ATM protein kinase is a master regulator of the response to double-strand breaks (DSBs), the most cytotoxic DNA lesion caused by ionizing radiation. ATM is the protein missing or inactive in patients with the pleiotropic genetic disorder ataxia-telangiectasia (A-T). A major response to DNA damage is altered expression of numerous genes. While studying gene expression in control and A-T cells following treatment with the radiomimetic chemical neocarzinostatin (NCS), we identified an expressed sequence tag that represented a gene that was induced by DSBs in an ATM-dependent manner. The corresponding cDNA encoded a dual specificity phosphatase of the MAP kinase phosphatase family, MKP-5. MKP-5 dephosphorylates and inactivates the stress-activated MAP kinases JNK and p38. The phosphorylation-dephosphorylation cycle of JNK and p38 by NCS was attenuated in A-T cells. Thus, ATM modulates this cycle in response to DSBs. These results further highlight ATM as a link between the DNA damage response and major signaling pathways involved in proliferative and apoptotic processes.

Evidence against high glucose as a mediator of ERK1/2 or p38 MAPK phosphorylation in rat skeletal muscle

Hyperglycemia leads to multiple changes in insulin signaling in skeletal muscle from people with type 2 diabetes. We hypothesized that mitogen-activated protein kinase (MAPK) signaling cascades may be directly activated by an acute exposure to high extracellular glucose concentrations. We determined whether an elevation in the extracellular glucose concentration would induce signal transduction in skeletal muscle via MAPK cascades. Epitrochlearis muscles were incubated in the presence of 5 or 25 mM glucose. Exposure of muscle to either hyperosmosis (600 mM mannitol) or insulin (6 nM) led to a marked increase in extracellular signal-regulated protein kinase (ERK)1/2 phosphorylation. Hyperosmosis elicited a 5.2-fold increase in p38 phosphorylation (P < 0.05), whereas insulin was without effect. ERK1/2 phosphorylation was not increased by high glucose exposure. After a 20-min exposure to 25 mM glucose, a tendency toward repressed (23%) p38 phosphorylation was observed (P = 0.06). No effect of high glucose was noted on signal transduction to signal transducer and activator of transcription 3 and Akt. In conclusion, short-term exposure of skeletal muscle to high levels of glucose does not appear to alter ERK1/2 or p38 MAPK phosphorylation.

Transcriptional induction of MKP-1 in response to stress is associated with histone H3 phosphorylation-acetylation

Mitogen-activated protein (MAP) kinase phosphatase 1 (MKP-1) has been shown to play a critical role in mediating the feedback control of MAP kinase cascades in a variety of cellular processes, including proliferation and stress responsiveness. Although MKP-1 expression is induced by a broad array of extracellular stimuli, the mechanisms mediating its induction remain poorly understood. Here we show that MKP-1 mRNA was potently induced by arsenite and ultraviolet light and modestly increased by heat shock and hydrogen peroxide. Interestingly, arsenite also dramatically induces phosphorylation-acetylation of histone H3 at a global level which precedes the induction of MKP-1 mRNA. The transcriptional induction of MKP-1, histone H3 modification, and elevation in MKP-1 mRNA in response to arsenite are all partially prevented by the p38 MAP kinase inhibitor SB203580, suggesting that the p38 pathway is involved in these processes. Finally, analysis of the DNA brought down by chromatin immunoprecipitation (ChIP) reveals that arsenite induces phosphorylation-acetylation of histone H3 associated with the MKP-1 gene and enhances binding of RNA polymerase II to MKP-1 chromatin. ChIP assays following exposure to other stress agents reveal various degrees of histone H3 modification at the MKP-1 chromatin. The differential contribution of p38 and ERK MAP kinases in mediating MKP-1 induction by different stress agents further illustrates the complexity and versatility of stress-induced MKP-1 expression. Our results strongly suggest that chromatin remodeling after stress contributes to the transcriptional induction of MKP-1.

Decreased mitogen activated protein kinase activities in congenital diaphragmatic hernia-associated pulmonary hypoplasia

BACKGROUND/PURPOSE

The mechanisms that cause pulmonary hypoplasia associated with congenital diaphragmatic hernia (CDH) currently are unknown. The authors proposed that the reduced size and immaturity of these lungs may be associated with differences in the levels of mitogen activated protein (MAP) kinase phosphorylation (extracellular signal regulated protein kinases, ERK-1 and -2).

METHODS

ERK-1 activities were measured using immune-complex kinase assays on fetal whole-lung lysates obtained from both nitrofen and olive oil-treated (control) pregnant rats. In addition, ERK-1 and ERK-2 functional activities were estimated by semiquantitative Western blot analysis, using an antibody specific for the diphosphorylated (dp-ERK, activated) forms of the enzymes.

RESULTS

ERK-1 activities, measured using immune-complex kinase assays, were reduced in CDH lungs compared with olive oil-treated controls (P <.02). In addition, dp-ERK-1 and dp-ERK-2 levels were found to be reduced in CDH lungs compared with controls (dp-ERK-1, P =.003; dp-ERK-2, P =.04), whereas ERK-1 and ERK-2 protein levels were unchanged.

CONCLUSIONS

The lower values of ERK-1 activity and reduced amounts of dp-ERK-1 and dp-ERK-2 in lung tissue from CDH animals, suggests that ERK-1 and ERK-2 activities are reduced in pulmonary hypoplasia associated with CDH. The observed reduction in ERK-1 and ERK-2 activities implicates attenuated cell signaling upstream of the ERK-1 and -2 enzymes.

Discordance between the binding affinity of mitogen-activated protein kinase subfamily members for MAP kinase phosphatase-2 and their ability to activate the phosphatase catalytically

MKP-2 is a member of the mitogen-activated protein (MAP) kinase phosphatase family which has been suggested to play an important role in the feedback control of MAP kinase-mediated gene expression. Although MKP-2 preferentially inactivates extracellular signal-regulated kinase (ERK) and c-Jun NH(2)-terminal kinase (JNK) MAP kinase subfamilies, the mechanisms underlying its own regulation remain unclear. In this report, we have examined the MKP-2 interaction with and catalytic activation by distinct MAP kinase subfamilies. We found that the catalytic activity of MKP-2 was enhanced dramatically by ERK and JNK but was affected only minimally by p38. By contrast, p38 and ERK bound MKP-2 with comparably strong affinities, whereas JNK and MKP-2 interacted very weakly. Through site-directed mutagenesis, we defined the ERK/p38-binding site as a cluster of arginine residues in the NH(2)-terminal domain of MKP-2. Mutation of the basic motif abrogated its interaction with both ERK and p38 and severely compromised the catalytic activation of MKP-2 by these kinases. Unexpectedly, such mutations had little effect on JNK-triggered catalytic activation. Both in vitro and in vivo, wild type MKP-2 effectively inactivated ERK2 whereas MKP-2 mutants incapable of binding to ERK/p38 did not. Finally, in addition to its role as a docking site for ERK and p38, the MKP-2 basic motif plays a role in regulating its nuclear localization. Our studies provided a mechanistic explanation for the substrate preference of MKP-2 and suggest that catalytic activation of MKP-2 upon binding to its substrates is crucial for its function.

Activation of the EGF receptor signaling pathway in airway epithelial cells exposed to Utah Valley PM

Exposure to ambient particulate matter (PM) in the Utah Valley has previously been associated with a variety of adverse health effects. To investigate intracellular signaling mechanisms for pulmonary responses to Utah Valley PM inhalation, human primary airway epithelial cells were exposed to aqueous extracts of PM collected from the year before (Y1), during (Y2), and after (Y3) the closure of a local steel mill located in the Utah Valley in this study. Transfection with kinase-deficient extracellular signal-regulated kinase (ERK) 1 constructs partially blocked Utah Valley PM-induced interleukin (IL)-8 promoter reporter activity. The mitogen-activated protein kinase/ERK kinase (MEK) activity inhibitor PD-98059 significantly abolished IL-8 released in response to Utah Valley PM, as did the epidermal growth factor (EGF) receptor kinase inhibitor AG-1478. Western blotting showed that Utah Valley PM induced phosphorylation of EGF receptor tyrosine, MEK1/2, and ERK1/2, which could be ablated with AG-1478 or PD-98059. For all findings, the potency of Utah Valley PM collected during Y2 was found to be lower relative to that of Y1 and Y3. These data demonstrate that Utah Valley PM can induce IL-8 expression partially through the activation of the EGF receptor signaling.

p38 MAPK and MAPK kinase 3/6 mRNA and activities are increased in early diabetic glomeruli

BACKGROUND

The p38 mitogen-activated protein kinase (MAPK) pathway is activated by several stress factors, potentially leading to cellular apoptosis and growth. Little is known about the pattern of glomerular p38 MAPK pathway activation during the course of diabetic nephropathy (DN). We examined the activity and expression of the p38 MAPK pathway members, p38 MAPK, MKK3/6, cAMP-responsive element binding protein (CREB), and MAPK phosphatase-1 (MKP-1), in experimental DN in rats over the course of four months.

METHODS

Control (C; N = 16) and diabetic (DM; N = 16) rats were studied. Four rats from each group were sacrificed monthly, and competitive reverse transcription-polymerase chain reaction and Western blot were performed with microdissected and sieved glomeruli, respectively.

RESULTS

Glomerular p38 MAPK mRNA expression was significantly higher in DM than C (P < 0.01) throughout the four-month period. Western blot revealed an average 3.1-fold increase in p38 MAPK protein throughout the study period (P < 0.05). However, p38 MAPK activity was significantly increased only in one- and two-month diabetic glomeruli. Glomerular MKK3/6 and CREB mRNA as well as activity were significantly increased only in one- and two-month DM compared with C. MKP-1 mRNA showed a similar pattern.

CONCLUSIONS

Glomerular p38 MAPK activity was increased in early DN. Parallel to this, we also showed, to our knowledge for the first time, that there were increased MKK3/6 and CREB activities and mRNA expression. This activated p38 MAPK pathway in diabetic glomeruli may, in part, play a role in the pathogenesis of early hypertrophy and extracellular matrix accumulation.

MKP-1 as a target for pharmacological manipulations in PC12 cell survival

Dual specificity mitogen activated protein kinase phosphatase-1 (MKP-1) inactivates extracellular signal-regulated kinase (ERK), p38 and/or c-jun N-terminal protein kinase (JNK) by dephosphorylation via a negative feed-back loop. The aim of the present study was to assess the role of expression of MKP-1 and phosphorylation status of mitogen-activated protein kinases (MAPKs) in promoting cell survival in PC12 cells. We used FK506 and three different monoperoxovanadium complexes (mpVs) as pharmacological tools for manipulation of MKP-1 expression. Peroxovanadium compounds, known to be insulinomimetic agents and protein tyrosine phosphatase inhibitors, are cytotoxic to the cells, they activate JNK and down-regulate MPK-1. On the other hand, FK 506 has transient effect on ERK activation. However, when the agents are used in combination, ERK phosphorylation is prolonged and intensified, MKP-1 expression is increased, and cell survival is enhanced. The concomitant alterations observed in intensities and duration of phospho-ERKs and phospho-JNKs signals suggest that monoperoxovanadium complexes in combination with FK 506 enhance survival of PC12 cells by an induction of MKP-1 expression.

Distinct binding determinants for ERK2/p38alpha and JNK map kinases mediate catalytic activation and substrate selectivity of map kinase phosphatase-1

Mitogen-activated protein (MAP) kinase phosphatase 1 (MKP-1/CL100) is an inducible nuclear dual specificity protein phosphatase that can dephosphorylate and inactivate both mitogen- and stress-activated protein kinases in vitro and in vivo. However, the molecular mechanism responsible for the substrate selectivity of MKP-1 is unknown. In addition, it has been suggested that the signal transducers and activators of transcription 1 (STAT1) transcription factor is a physiological non-MAP kinase substrate for MKP-1. We have used the yeast two-hybrid assay to demonstrate that MKP-1 is able to interact selectively with the extracellular signal-regulated kinase 1/2 (ERK1/2), p38alpha, and c-Jun NH(2)-terminal kinase (JNK) MAP kinase isoforms. Furthermore, this binding is accompanied by catalytic activation of recombinant MKP-1 protein in vitro, and these end points show an absolute correlation with MKP-1 substrate selectivity in vivo. In contrast, MKP-1 does not interact with STAT1. Recombinant STAT1 does not cause catalytic activation of MKP-1; nor does MKP-1 block tyrosine phosphorylation of STAT1 in vivo. Both binding and catalytic activation of MKP-1 are abrogated by mutation of a conserved docking site in ERK2, p38alpha, and JNK1 MAP kinases. Within MKP-1, MAP kinase binding is mediated by the amino-terminal noncatalytic domain of the protein. However, mutation of a conserved cluster of positively charged residues within this domain abolishes the binding and activation of MKP-1 by ERK2 and p38alpha but not JNK1, indicating that there are distinct binding determinants for these MAP kinase isoforms. We conclude that the substrate selectivity of MKP-1 is determined by specific protein-protein interactions coupled with catalytic activation of the phosphatase and that these interactions are restricted to members of the MAP kinase family of enzymes.

Constitutive ERK1/2 activation in esophagogastric rib bone marrow micrometastatic cells is MEK-independent

In this study, we examined the mitogen-activated protein kinase (MAPK) cascade in micrometastatic cell lines generated from rib bone marrow (RBM) of patients undergoing resection of esophagogastric malignancies. The molecular mechanism(s) involved in esophagogastric MAPK activation have not previously been investigated. Constitutive activation of both ERK1 and -2 isoforms was evident in each of the five RBM cell lines. Elk-1, a transcription factor activated by the ERK1/2 pathway was also found to be constitutively activated. Cell lines generated from metastases of involved lymph nodes (OC2) and ascites (OC1) of patients with esophageal cancer do not display, however, hyperphosphorylation of ERK1/2. Constitutive RBM ERK1/2 activation is protein kinase C and phosphatidylinositol 3-kinase dependent. Surprisingly, constitutive ERK1/2 activation is MEK-independent. Pharmacological inhibition of MEK with two specific inhibitors, PD 98059 and U0126, were both ineffective in blocking ERK activation. Similarly, the use of a dominant negative MEK mutant was without effect. Interestingly, experiments overexpressing two different dominant negative Pak1 mutants significantly reduced RBM ERK1/2 activation, albeit not to the same extent for all cell lines. We also examined the role of three different phosphatases, PAC1, MKP-1, and -2. While RBM ERK1/2 activation was found to be PAC1- and MKP-2-independent, surprisingly, MKP-1 was down-regulated in all five RBM cell lines. In conclusion, we provide evidence for the first time for a MEK-independent constitutive ERK1/2 activation pathway in esophagogastric RBM cell lines. These findings have important implications for drug treatment strategies which currently target MEK in other forms of cancer.

Stage-specific differential activation of mitogen-activated protein kinases in hypertrophied and failing rat hearts

Mitogen-activated protein kinases (MAPKs) are involved in the early development of cardiac hypertrophy, but their roles in chronic left ventricular hypertrophy (LVH) are unclear. We studied the angiotensin (Ang) II-induced cardiac MAPK activation of the hypertensive Dahl salt-sensitive (DS) rats in the subacute developing LVH stage, the chronic compensated LVH stage, and the congestive heart failure (CHF) stage. In the isolated, coronary-perfused heart preparation, Ang II infusion (1x10(-6)mol/l) activated extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK) and p38-MAPK in the LV myocardium. No substantial differences were observed in the Ang II-induced ERK activation between the normotensive control DS rats and the hypertensive DS rats in either stage. In contrast, the Ang II-induced activation of JNK and p38-MAPK was augmented in the subacute LVH stage of the hypertensive DS rats, but then progressively attenuated in the chronic LVH and CHF stages. Chronic treatment with an angiotensin converting enzyme inhibitor, temocapril (20 mg/kg/day), ameliorated the responsiveness of the JNK/p38-MAPK activation, suggesting that the decreased JNK/p38-MAPK activation is a consequence of negative feedback regulation for the activated cardiac renin-angiotensin system in chronic LVH and CHF. Thus, the Ang II-induced activation of multiple cardiac MAPK pathways are differentially regulated, depending on the stages of chronic hypertrophic process. The JNK and p38-MAPK activation may be involved in the early development of adaptive LVH. However, the responsiveness of the cardiac JNK/p38-MAPK pathways progressively decreased in chronic LVH and CHF under the chronic activation of tissue renin-angiotensin system.

Attenuation of EGF signaling in senescent cells by caveolin

One of the characteristics of senescent cells is unresponsiveness to external stimuli like EGF. Although they have a normal level of receptors and downstream signaling molecules, EGF cannot induce the activation of Erk kinases and DNA synthesis in senescent cells as much as in young cells. Caveolin proteins directly interact with signaling molecules including EGF receptor and suppress the activation of EGFR upon EGF stimulation. We found that Erk activation after EGF stimulation in senescent human diploid fibroblasts was down-regulated. Those senescent cells showed an increased level of three isoforms of caveolin proteins. This change seems to lie in transcriptional control in senescent cells. We also demonstrated up-regulated caveolin proteins were co-localized with EGFR proteins in detergent-insoluble fractions. From these results, we suggest that the up-regulated expression of caveolin might explain the unresponsiveness of senescent fibroblasts to EGF stimulation.

Differential regulation of JNK activation and MKP-1 expression by peroxovanadium complexes

Bisperoxovanadium complexes have been identified as insulinomimetic agents and protein tyrosine phosphatase inhibitors. The aim of the present study was to examine the effects of the most potent bisperoxovanadium complex, potassium bisperoxo (1,10-phenanthroline) oxovanadate (V) [bpV(phen)], on expression and activation of c-jun N-terminal protein kinases (JNK) and on expression of mitogen-activated protein kinase phosphatase-1 (MKP-1) in different cell lines. We compared the effects of bpV(phen) with the effects of tumor necrosis factor-alpha (TNF-alpha), a known regulator of JNK phosphorylation and inducer of MKP-1. Treatment with bpV(phen) causes significant and sustained down-regulation of MKP-1 expression both in PC12 and HeLa cells. In contrast, TNF-alpha induces MKP-1 expression in PC12 cells and does not alter MKP-1 expression in HeLa cells. Both bpV(phen) and TNF-alpha induce MKP-1 expression in OVCAR-3 cell line but with different dynamics: TNF-alpha causes transient and bpV(phen) sustained induction of MKP-1 expression. Temporal pattern of level of MKP-1 expression correlates with the regulation of JNK phosphorylation by bpV(phen) and TNF-alpha in PC12 cells. However, no detectable phospho-JNK signal is observed in either OVCAR-3 or HeLa cells treated with bpV(phen). In contrast, TNF-alpha causes strong and sustained JNK phosphorylation in OVCAR-3 cell line, and strong but transient JNK activation in HeLa cells. BpV(phen) and TNF-alpha does not alter JNK expression in any of the cell lines studied. We demonstrate that the effect of two stressors, bpV(phen) and TNF-alpha, on MKP-1 expression and JNK phosphorylation are strikingly different, depending on the cell type. These results suggest the possible role of MKP-1 in regulation of JNK phosphorylation in both PC12 and OVCAR-3 cell lines treated with bpV(phen).

Portal hypertensive gastric mucosa has reduced activation of MAP kinase (ERK2) in response to alcohol injury: a key to impaired healing?

Portal hypertensive (PHT) gastric mucosa has increased susceptibility to injury and impaired mucosal healing. Because our previous study showed that ulcer-induced activation of mitogen-activated protein (MAP) kinase (ERK) plays a pivotal role in gastric mucosal healing, we investigated whether ERK activation is altered in PHT gastric mucosa following alcohol injury. We studied ERK2 phosphorylation and activity and expression of MAP kinase phosphatase-1 (MKP-1) in gastric mucosa of PHT and sham-operated (SO) normal rats both at baseline and following alcohol injury. In SO gastric mucosa, ERK2 phosphorylation and activity were significantly increased time-dependently following alcohol injury: by 221% and 137%, respectively at 24 h vs. baseline. In contrast, in PHT gastric mucosa following alcohol injury, neither ERK2 phosphorylation nor activity was increased versus baseline. In PHT gastric mucosa, MKP-1 mRNA and protein expression were increased at baseline versus SO rats and were increased further following alcohol injury with values higher by 20%-40% at each study time versus SO rats. Because ERK2 is crucial for mucosal healing, reduced ERK2 activation resulting from the overexpression of MKP-1 might be the basis for the impaired mucosal healing in PHT gastric mucosa.

Activation of mitogen-activated protein kinase phosphatase 2 by gonadotropin-releasing hormone

The aim of these studies was to identify the signaling mechanism(s) that contribute to GnRH-induced expression of MAPK phosphatase (MKP)-2, a dual specificity phosphatase that selectively inactivates MAPKs. GnRH receptor activation induced MKP-2 expression in both clonal (alphaT3-1) and primary gonadotropes. Activation of PKC isozymes was sufficient and required for MKP-2 induction. Inhibition of the extracellular signal-regulated kinase (ERK) or c-Jun N-terminal kinase (JNK) but not the p38 MAPK cascade was sufficient to block GnRH-induced MKP-2 expression. Induction of MKP-2 by GnRH was dependent on elevation in intracellular Ca(2+). Inhibition of Ca(2+) influx through L-type voltage-gated calcium channels blocked GnRH-induced MKP-2 expression. Depletion of intracellular Ca(2+) stores with thapsigargin blocked MKP-2 activation by GnRH independent of ERK and JNK activity. These results support the conclusion that MKP-2 induction by GnRH occurs via MAPK-dependent and -independent pathways. One mechanism requires GnRH-induced ERK and JNK activation, while a second MAPK-independent pathway requires a thapsigargin-sensitive calcium signal.

Insulin regulates MAP kinase phosphatase-1 induction in Hirc B cells via activation of both extracellular signal-regulated kinase (ERK) and c-Jun-N-terminal kinase (JNK)

Previously, we have reported that insulin induces the expression of the dual-specificity tyrosine phosphatase Mitogen-activated protein (MAP) kinase phosphatase-1 (MKP-1) and that this may represent a negative feedback mechanism to regulate insulin-stimulated MAP kinase activity. In this work, the mechanism of regulation of MKP-1 expression by insulin was examined, particularly the role of the MAP kinase superfamily. Inhibition of the ERK pathway attenuated insulin-stimulated MKP-1 mRNA expression. Expression of dominant negative molecules of the JNK pathway also abolished insulin-stimulated MKP-1 expression. However, inhibition of p38MAPK activity by SB202190 had no effect on insulin-stimulated MKP-1 induction. Simultaneous inhibition of the ERK and JNK pathways abolished the ability of insulin to stimulate MKP-1 expression, however, this combined inhibition was neither additive nor synergistic, suggesting these pathways converge to act on a common final effector. In conclusion, induction of MKP-1 mRNA expression in Hirc B cells by insulin requires activation of both the ERK and JNK pathways, but not p38MAPK.

CL100/MKP-1 modulates JNK activation and apoptosis in response to cisplatin

Treatment of cells with cisplatin induces a sustained activation of the stress activated protein kinase SAPK/JNK and the mitogen-activated protein kinase p38. Activation of JNK by cisplatin is necessary for the induction of apoptosis. Expression of the MAPK phosphatases CL100/MKP-1 and hVH-5 selectively prevents JNK/SAPK activation by cisplatin in a dose dependent fashion and results in protection against cisplatin-induced apoptosis. In contrast, expression of the ERK-specific phosphatase Pyst1 inhibits JNK/SAPK activity only when expressed at very high levels and does not confer protection against cisplatin. Furthermore, expression of a catalytically inactive mutant of CL100 in 293 cells decreases the IC50 for cisplatin and increases the toxicity of transplatin. This effect seems to be mediated by an increase in JNK activity since p38 activity is unaffected. These results suggest that dual-specificity MAPK phosphatases may be candidate drug targets in order to optimize cisplatin based therapeutic protocols.

Insulin activates p38 mitogen-activated protein (MAP) kinase via a MAP kinase kinase (MKK) 3/MKK 6 pathway in vascular smooth muscle cells

BACKGROUND

Induction of stress-activated mitogen-activated protein (MAP) kinases such as p38 could be important for the development of cardiovascular diseases since p38 MAP kinase activation stimulates apoptosis, cell growth, prostanoid formation and other cellular dysfunctions when induced by oxidants, hyperosmolarity, or pro-inflammatory cytokines. On the other hand, insulin resistance is one of the most important factors promoting atherogenesis, including cardiovascular diseases, but it is not clear how these different factors transmit their signals intracellularly at the cytosolic and nuclear levels. In this study, we investigated the effect of insulin on p38 mitogen-activated protein (MAP) kinase activation in cultured rat vascular smooth muscle cells (VSMC).

MATERIALS AND METHODS

VSMC were obtained from the aortas of male Wistar rats by the media explant technique. After being stimulated by insulin with SB-203580, PD-98059, or GF109203X, the cells were solubilized and the expressions of MAP kinases, MAP kinase kinases and p70 S6 kinase were examined by immunoblot analysis. The amount of DNA synthesis was measured by [3H]thymidine incorporation.

RESULTS

Insulin activated p38 MAP kinase phosphorylation in a dose-dependent manner, and the phosphorylation was specifically inhibited by SB-203580, a p38 MAP kinase-specific inhibitor, but not by PD-98059, a specific inhibitor of upstream kinase (MEK), of extracellular signal-regulated kinase (ERK), or GF209203X, a protein kinase C-specific inhibitor. Insulin also activated MAP kinase kinase (MKK) 3/MKK 6 phosphorylation, the upstream kinase of p38 MAP kinase, but neither stress-activated protein kinase (SAPK)/ERK kinase (SEK1/MKK4) nor SAPK/c-jun NH2-terminal protein kinase. Surprisingly, phosphorylation of p70 S6 kinase and an increase of DNA synthesis by insulin were suppressed dose dependently by SB-203580.

CONCLUSION

These results have established that insulin activates the p38 MAP kinase cascade via an MKK 3/6 pathway in rat VSMC, independently of a MEK-ERK cascade, and partly regulates cell growth.

Adhesion to fibronectin enhances MKP-1 activation in human endothelial cells

Integrin-mediated substrate adhesion of endothelial cells leads to intracellular signaling, including the activation of ERK 1/2 (extracellular regulated kinases 1 and 2), members of the mitogen-activated protein kinase (MAPK) family. MKP-1 is a dual-specificity protein phosphatase that may play an important role in regulating MAPK activity through dephosphorylation of threonine and tyrosine. Adhesion of human umbilical vein endothelial cells to fibronectin increased MKP-1 protein and mRNA levels, which reached a maximum at 60 min, while MAPK activity was maximal at 30 min. The MEK inhibitor PD98059 blocked activation of MAPK as well as the induction of MKP-1 during adhesion. The transcription inhibitor actinomycin D blocked MKP-1 induction and produced prolonged MAPK activation during adhesion. In contrast, endothelial adhesion to poly-L-lysine did not alter MAPK activity or MKP-1 levels. These findings demonstrate that integrin-mediated adhesion of endothelial cells to fibronectin results in transcriptional activation of MKP-1 through a MAPK-dependent mechanism. Regulation of MKP-1 by MAPK likely represents an important negative-feedback mechanism.

Mechanical stress-initiated signal transductions in vascular smooth muscle cells

Mechanical force is an important modulator of cellular morphology and function in a variety of tissues, and is particularly important in cardiovascular systems. Vascular smooth muscle cell (VSMC) hypertrophy and proliferation contribute to the development of atherosclerosis, hypertension, and restenosis, where mechanical forces are largely disturbed. How VSMCs sense and transduce the extracellular mechanical signals into the cell nucleus resulting in quantitative and qualitative changes in gene expression is an interesting and important research field. Recently, it has been demonstrated that mechanical stress rapidly induced phosphorylation of platelet-derived growth factor (PDGF) receptor, activation of integrin receptor, stretch-activated cation channels, and G proteins, which might serve as mechanosensors. Once mechanical force is sensed, protein kinase C and mitogen-activated protein kinases (MAPKs) were activated, leading to increased c-fos and c-jun gene expression and enhanced transcription factor AP-1 DNA-binding activity. Interestingly, physical forces also rapidly resulted in expression of MAPK phosphatase-1 (MKP-1), which inactivates MAPKs. Thus, mechanical stresses can directly stretch the cell membrane and alter receptor or G protein conformation, thereby initiating signalling pathways, usually used by growth factors. These findings have significantly enhanced our knowledge of the pathogenesis of arteriosclerosis and provided promising information for therapeutic interventions for vascular diseases.

Involvement of hepatocyte epidermal growth factor receptor mediated activation of mitogen-activated protein kinase signaling pathways in response to growth inhibition by a novel K vitamin

Compound 5 (Cpd 5), a synthetic K vitamin analogue, or 2-(2-mercaptoethanol)-3-methyl-1,4-naphthoquinone, is a potent inhibitor of epidermal growth factor (EGF)-induced rat hepatocyte DNA synthesis and induces EGF receptor (EGFR) tyrosine phosphorylation. To understand the cellular responses to Cpd 5, its effects on the EGF signal transduction pathway were examined and compared to those of the stimulant, EGF. Cpd 5 induced a cellular response program that included the induction of EGFR tyrosine phosphorylation and the activation of the mitogen-activated protein kinase (MAPK) cascade. EGFR tyrosine phosphorylation was induced by Cpd 5 in a time- and dose-dependent manner. Coimmunoprecipitation studies demonstrated that both EGF and Cpd 5 induced tyrosine phosphorylation of EGFR was associated with increased amounts of adapter proteins Shc and Grb2, and the Ras GTP-GDP exchange protein Sos, indicating the formation of functional EGFR complexes. Although EGFR phosphorylation was induced both by the stimulant EGF and the inhibitor Cpd 5, the timing and intensity of activation by EGF and Cpd 5 were different. EGF activated EGFR transiently, whereas Cpd 5 induced an intense and sustained activation. Cpd 5-altered cells had a decreased ability to dephosphorylate tyrosine phosphorylated EGFR, providing evidence for an inhibition of tyrosine phosphatase activity. Both EGF and Cpd 5 caused an induction of phospho-extracellular response kinase (ERK), which was also more sustained with Cpd 5. Moreover, whereas Cpd 5 induced a striking translocation of phosphorylated ERK from cytosol to the nucleus, no significant nuclear translocation occurred after stimulation with EGF. The data suggest that this novel compound causes growth inhibition through antagonism of EGFR phosphatases and consequent induction of EGFR and ERK phosphorylation. This is supported by experiments with PD 153035 and PD 098059, antagonists of phosphorylation of EGFR and MAP kinase kinase (MEK), respectively, which both antagonized Cpd 5-induced phosphorylation and the inhibition of DNA synthesis. These results imply a mechanism of cell growth inhibition associated with intense and prolonged protein tyrosine phosphorylation. Protein tyrosine phosphatases may thus be a novel target for drugs designed to inhibit cell growth.

JNK and p38 stresskinases--degenerative effectors of signal-transduction-cascades in the nervous system

The c-Jun N-terminal kinases (JNKs, also called stress activated protein kinases. SAPKs) and p38 kinases constitute together with extracellular signal-regulated kinases (ERKs) the family of MAP kinases. Whereas the functions of JNKs under physiological conditions are largely unknown, there is raising evidence that JNKs are potent effectors of apoptosis or degeneration of neurons in vitro and in the brain. The activation of the inducible transcription factor c-Jun by N-terminal phosphorylation is a central event in JNK-mediated degenerative processes that depend on de novo protein synthesis. At the post-translational level, cytoplasmic degenerative actions of JNKs might comprise inhibition of Bcl-2 and steroid hormone-receptor signaling or hyperphosphorylation of tau; and at transcriptional level, JNKs might trigger the induction of the apoptotic effectors p53 and Fas-Ligand by phosphorylation of c-Jun. The role of p38 is the nervous system is poorly understood, but its activation is also considered as part of the neuronal stress response. This review informs about the genetic processing, the regulation of activity and the biochemical actions of JNK and p38 isoforms in general. In the second part, we summarize the findings on expression and activation of JNKs and p38 under neurodegenerative condition. A particular focus is also put on the putative function of JNK under physiological conditions and for neuroprotection.

mkp-1 encoding mitogen-activated protein kinase phosphatase 1, a verotoxin 1 responsive gene, detected by differential display reverse transcription-PCR in Caco-2 cells

The major cytotoxic effect of the verotoxins (VTs) produced by strains of VT-producing Escherichia coli is the inhibition of host-cell protein synthesis, but VTs are also suspected to play a role in apoptotic cell signaling and cytokine release. Four differentially expressed genes, including mkp-1 (encoding mitogen-activated protein kinase phospatase 1), were detected by differential display reverse transcription-PCR (DD RT-PCR) stimulated by VT1 in Caco-2 cells. Northern blot analysis showed the induction of mkp-1 mRNA 6 h after VT1 stimulation. Neither mutant VT1 (mutVT1), harboring two mutations in the A subunit (E167Q-R170L), nor cycloheximide induced mkp-1 mRNA, but mkp-1 mRNA was detected with both wild-type VT1 (wtVT1) and anisomycin, a 28S rRNA inhibitor. Therefore, we concluded that the A subunit of VT1 was essential for mkp-1 induction. Increased amounts of phosphorylated c-Jun protein were also found with wtVT1 and anisomycin. Although the precise mechanism of induction of MKP-1 is unknown, we hypothesized that 28S rRNA not only was a sensor for ribotoxic stress, but also was involved in the signal cascade of MKP-1. This is the first report of detection by DD RT-PCR of cellular genes induced by bacterial toxins.

Glucagon-induced expression of the MAP kinase phosphatase MKP-1 in rat hepatocytes

BACKGROUND & AIMS

Glucagon exerts pleiotropic effects on liver function, but the underlying signal transduction is incompletely understood. We investigated the effect of glucagon on the mitogen-activated protein (MAP) kinase phosphatase MKP-1 expression.

METHODS

The effect of glucagon on MKP-1 expression was studied in cultured rat hepatocytes.

RESULTS

Glucagon (10-100 nmol/L) and 8-CPT-cAMP (10 or 50 micromol/L) stimulated in rat hepatocytes the expression of MKP-1 messenger RNA and protein, which became maximal within 30 minutes and declined to nearly basal levels after 60 minutes. MKP-1 induction by glucagon was sensitive to inhibition of adenylate cyclase and protein kinase A. The protein kinases G and C, Ca(2+), MAP kinases, reactive oxygen intermediates, and cellular dehydration were not involved in the glucagon-induced signaling to MKP-1. MKP-1 expression correlated with glucagon-induced antagonization of MAP kinase phosphorylation by epidermal growth factor in hepatocytes.

CONCLUSIONS

The MKP-1 response to glucagon produces an additional level of interaction with MAP kinase-dependent processes, which may contribute to the regulation of liver function by glucagon or other cAMP-elevating agents.

Protein kinase C signaling and oxidative stress

Oxidative stress is involved in the pathogenesis of various degenerative diseases including cancer. It is now recognized that low levels of oxidants can modify cell-signaling proteins and that these modifications have functional consequences. Identifying the target proteins for redox modification is key to understanding how oxidants mediate pathological processes such as tumor promotion. These proteins are also likely to be important targets for chemopreventive antioxidants, which are known to block signaling induced by oxidants and to induce their own actions. Various antioxidant preventive agents also inhibit PKC-dependent cellular responses. Therefore, PKC is a logical candidate for redox modification by oxidants and antioxidants that may in part determine their cancer-promoting and anticancer activities, respectively. PKCs contain unique structural features that are susceptible to oxidative modification. The N-terminal regulatory domain contains zinc-binding, cysteine-rich motifs that are readily oxidized by peroxide. When oxidized, the autoinhibitory function of the regulatory domain is compromised and, consequently, cellular PKC activity is stimulated. The C-terminal catalytic domain contains several reactive cysteines that are targets for various chemopreventive antioxidants such as selenocompounds, polyphenolic agents such as curcumin, and vitamin E analogues. Modification of these cysteines decreases cellular PKC activity. Thus the two domains of PKC respond differently to two different type of agents: oxidants selectively react with the regulatory domain, stimulate cellular PKC, and signal for tumor promotion and cell growth. In contrast, antioxidant chemopreventive agents react with the catalytic domain, inhibit cellular PKC activity, and thus interfere with the action of tumor promoters.

Age-related decline in Ras/ERK mitogen-activated protein kinase cascade is linked to a reduced association between Shc and EGF receptor

Numerous studies have demonstrated that the proliferative capacity of cells declines with age. Using rat primary hepatocytes as a model system, we recently demonstrated that this age-related decline in the proliferative response to mitogenic stimulation is associated with decreased activities of both extracellular signal-regulated kinase (ERK) and p70 S6 kinase (p70(S6k)). To unravel the molecular basis for age-related defects in the ERK pathway, we have now characterized the upstream signaling events that occur after epidermal growth factor (EGF) stimulation in young and aged hepatocytes. As previously noted for ERK, the activities of both MEK (the kinase immediately upstream of ERK) and Ras following EGF stimulation were significantly lower in aged hepatocytes. An examination of the EGF receptor (EGFR) revealed a similar amount of EGFR in the two age groups. Likewise, EGFR and Shc, an adaptor protein that plays a crucial role in linking EGFR to Ras activation, underwent tyrosine phosphorylation to a similar degree in both young and aged hepatocytes. However, in aged cells Shc was unable to form stable complexes with EGFR after EGF stimulation. Our results suggest that a decrease in the association between Shc and EGFR in aged cells underlies the age-related declines in the ERK signaling cascade and in proliferative capacity.

Biomechanical stress-induced apoptosis in vein grafts involves p38 mitogen-activated protein kinases

The present study was designed to investigate whether apoptosis occurs in early-stage vein grafts and to determine the mechanisms by which mechanical stress contributes to apoptosis in vascular smooth muscle cells (SMCs). Apoptosis in vessel walls of mouse vein grafts was confirmed by morphological changes and by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL). TUNEL(+) cells in vein grafts 1, 4, and 8 wk postoperatively was 13%, 29%, and 21%, respectively, and apoptosis occurred mainly in veins grafted to arteries, remaining unchanged in vein-to-vein grafts. When mouse, rat, and human arterial SMCs were cultured on a flexible membrane and subjected to cyclic strain stress, apoptosis was observed in a time- and strength-dependent manner. All three types of SMCs showed apoptotic death as confirmed by TUNEL, propidium iodide, and annexin V staining. To further study the signal pathways leading to apoptosis, activities of p38, a subfamily of mitogen-activated protein kinases (MAPKs), were determined. Mechanical stress resulted in p38 MAPK activation, reaching high levels within 8 min. SB 202190, a specific inhibitor for p38 MAPKs, prevented SMC apoptosis in response to mechanical stress. SMC lines stably transfected with a dominant negative rac, an upstream signal transducer, or overexpressing MAPK phosphatase-1, a negative regulator for MAPKs, completely inhibited mechanical stress stimulated p38 activation and abolished mechanical stress-induced apoptosis. Thus, we provide solid evidence that one of the earliest events in venous bypass grafts is apoptosis, in which mechanical stress-induced p38-MAPK activation is responsible for transducing signals leading to apoptosis.-Mayr, M., Li, C., Zou, Y., Huemer, U., Hu, Y., Xu, Q. Biomechanical stress-induced apoptosis in vein grafts involves p38 mitogen-activated protein kinases.

Activation of p38 mitogen-activated protein kinase and c-Jun NH(2)-terminal kinase by double-stranded RNA and encephalomyocarditis virus: involvement of RNase L, protein kinase R, and alternative pathways

Double-stranded RNA (dsRNA) accumulates in virus-infected mammalian cells and signals the activation of host defense pathways of the interferon system. We describe here a novel form of dsRNA-triggered signaling that leads to the stimulation of the p38 mitogen-activated protein kinase (p38 MAPK) and the c-Jun NH(2)-terminal kinase (JNK) and of their respective activators MKK3/6 and SEK1/MKK4. The dsRNA-dependent signaling to p38 MAPK was largely intact in cells lacking both RNase L and the dsRNA-activated protein kinase (PKR), i. e., the two best-characterized mediators of dsRNA-triggered antiviral responses. In contrast, activation of both MKK4 and JNK by dsRNA was greatly reduced in cells lacking RNase L (or lacking both RNase L and PKR) but was restored in these cells when introduction of dsRNA was followed by inhibition of ongoing protein synthesis or transcription. These results are consistent with the notion that the role of RNase L and PKR in the activation of MKK4 and JNK is the elimination, via inhibition of protein synthesis, of a labile negative regulator(s) of the signaling to JNK acting upstream of SEK1/MKK4. In the course of these studies, we identified a long-sought site of RNase L-mediated cleavage in the 28S rRNA, which could cause inhibition of translation, thus allowing the activation of JNK by dsRNA. We propose that p38 MAPK is a general participant in dsRNA-triggered cellular responses, whereas the activation of JNK might be restricted to cells with reduced rates of protein synthesis. Our studies demonstrate the existence of alternative (RNase L- and PKR-independent) dsRNA-triggered signaling pathways that lead to the stimulation of stress-activated MAPKs. Activation of p38 MAPK (but not of JNK) was demonstrated in mouse fibroblasts in response to infection with encephalomyocarditis virus (ECMV), a picornavirus that replicates through a dsRNA intermediate. Fibroblasts infected with EMCV (or treated with dsRNA) produced interleukin-6, an inflammatory and pyrogenic cytokine, in a p38 MAPK-dependent fashion. These findings suggest that stress-activated MAPKs participate in mediating inflammatory and febrile responses to viral infections.

Role of Gadd45 in apoptosis

gadd45 is a p53-regulated growth arrest and DNA-damage-inducible gene that is also regulated in a p53-independent manner. Whether Gadd45 plays a direct role in apoptosis remains unclear. Microinjection of the exogenous gadd45 expression vector into human fibroblasts has been shown to cause G2 arrest but not apoptosis. Recent studies suggest that Gadd45 may mediate genotoxic stress or Brca1-induced apoptosis via activation of c-Jun N-terminal kinase (JNK) and/or p38 mitogen-activated protein kinase (MAPK). Analyses of gadd45-deficient mice and cells have revealed that Gadd45 appears to exhibit pleiotropic effects, including cell cycle arrest at G2/M, DNA damage repair, and control of genomic stability, but is not required for radiation-induced apoptosis. Furthermore, stress-induced activation of JNK and p38 MAPK is not altered in gadd45-deficient embryonic fibroblasts, suggesting that the lack of Gadd45 may not affect the JNK and p38 MAPK activity. Thus, although the evidence from gadd45-null cells suggests that Gadd45 probably does not play a direct role in genotoxic stress-induced apoptosis, more in-depth studies are needed to firmly establish this contention.

Hyperexpression and activation of extracellular signal-regulated kinases (ERK1/2) in atherosclerotic lesions of cholesterol-fed rabbits

A hallmark of hyperlipidemia-induced atherosclerosis is altered gene expression that initiates cell proliferation and (de)differentiation in the intima of the arterial wall. The molecular signaling that mediates this process in vivo has yet to be identified. Extracellular signal-regulated kinases (ERKs) are thought to play a pivotal role in transmitting transmembrane signals required for cell proliferation in vitro. The present studies were designed to investigate the activity, abundance, and localization of ERK1/2 in atherosclerotic lesions of cholesterol-fed rabbits. Immunofluorescence analysis revealed abundant and heterogeneous distribution of ERK1/2, mainly localized in the cap and basal regions of atheromas. A population of ERK-enriched cells was identified as alpha-actin-positive smooth muscle cells (SMCs). ERK1 and 2 were heavily phosphorylated on tyrosyl residues and coexpressed with proliferating cell nuclear antigen in atherosclerotic lesions. ERK1/2 protein levels in protein extracts from atherosclerotic lesions were 2- to 3-fold higher than the vessels of chow-fed rabbits, and their activities were elevated 3- to 5-fold over those of the normal vessel. SMCs derived from atherosclerotic lesions had increased migratory/proliferative ability and higher ERK activity in response to LDL stimulation compared with cells from the normal vessel. Inhibition of ERK activation by PD98059, a specific inhibitor of mitogen-activated protein kinase kinases (MEK1/2), abrogated LDL-induced SMC proliferation in vitro. Taken together, our findings support the proposition that persistent activation and hyperexpression of ERK1/2 may be a critical element to initiate and perpetuate cell proliferation during the development of atherosclerosis.