Thrombin activates two stress-activated protein kinases, c-Jun N-terminal kinase and p38, in HepG2 cells

PAR-1 mediated apoptosis of breast cancer cells by V. cholerae hemagglutinin protease

Bacterial toxins have emerged as promising agents in cancer treatment strategy. Hemagglutinin (HAP) protease secreted by Vibrio cholerae induced apoptosis in breast cancer cells and regresses tumor growth in mice model. The success of novel cancer therapies depends on their selectivity for cancer cells with limited toxicity for normal tissues. Increased expression of Protease Activated Receptor-1 (PAR-1) has been reported in different malignant cells. In this study we report that HAP induced activation and over expression of PAR-1 in breast cancer cells (EAC). Immunoprecipitation studies have shown that HAP specifically binds with PAR-1. HAP mediated activation of PAR-1 caused nuclear translocation of p50-p65 and the phosphorylation of p38 which triggered the activation of NFκB and MAP kinase signaling pathways. These signaling pathways enhanced the cellular ROS level in malignant cells that induced the intrinsic pathway of cell apoptosis. PAR-1 mediated apoptosis by HAP of malignant breast cells without effecting normal healthy cells in the same environment makes it a good therapeutic agent for treatment of cancer.

Systematic Analysis of Quantitative Logic Model Ensembles Predicts Drug Combination Effects on Cell Signaling Networks

A major challenge in developing anticancer therapies is determining the efficacies of drugs and their combinations in physiologically relevant microenvironments. We describe here our application of "constrained fuzzy logic" (CFL) ensemble modeling of the intracellular signaling network for predicting inhibitor treatments that reduce the phospho-levels of key transcription factors downstream of growth factors and inflammatory cytokines representative of hepatocellular carcinoma (HCC) microenvironments. We observed that the CFL models successfully predicted the effects of several kinase inhibitor combinations. Furthermore, the ensemble predictions revealed ambiguous predictions that could be traced to a specific structural feature of these models, which we resolved with dedicated experiments, finding that IL-1α activates downstream signals through TAK1 and not MEKK1 in HepG2 cells. We conclude that CFL-Q2LM (Querying Quantitative Logic Models) is a promising approach for predicting effective anticancer drug combinations in cancer-relevant microenvironments.

Protease activated receptor-1 inhibits the Maspin tumor-suppressor gene to determine the melanoma metastatic phenotype

The thrombin receptor protease activated receptor-1 (PAR-1) is overexpressed in metastatic melanoma cell lines and tumor specimens. Previously, we demonstrated a significant reduction in tumor growth and experimental lung metastasis after PAR-1 silencing via systemic delivery of siRNA encapsulated into nanoliposomes. Gene expression profiling identified a 40-fold increase in expression of Maspin in PAR-1-silenced metastatic melanoma cell lines. Maspin promoter activity was significantly increased after PAR-1 silencing, suggesting that PAR1 negatively regulates Maspin at the transcriptional level. ChIP analyses revealed that PAR-1 decreases binding of Ets-1 and c-Jun transcription factors to the Maspin promoter, both known to activate Maspin transcription. PAR-1 silencing did not affect Ets-1 or c-Jun expression; rather it resulted in increased expression of the chromatin remodeling complex CBP/p300, as well as decreased activity of the CBP/p300 inhibitor p38, resulting in increased binding of Ets-1 and c-Jun to the Maspin promoter and higher Maspin expression. Functionally, Maspin expression reduced the invasive capability of melanoma cells after PAR-1 silencing, which was abrogated after rescuing with PAR-1. Furthermore, tumor growth and experimental lung metastasis was significantly decreased after expressing Maspin in a metastatic melanoma cell line. Moreover, silencing Maspin in PAR-1-silenced cells reverted the inhibition of tumor growth and experimental lung metastasis. Herein, we demonstrate a mechanism by which PAR-1 negatively regulates the expression of the Maspin tumor-suppressor gene in the acquisition of the metastatic melanoma phenotype, thus attributing an alternative function to PAR-1 other than coagulation.

Chronic ethanol feeding potentiates Fas Jo2-induced hepatotoxicity: role of CYP2E1 and TNF-alpha and activation of JNK and P38 MAP kinase

We have previously shown that treatment of mice with pyrazole or acute ethanol potentiated Fas agonistic Jo2 antibody-induced liver injury by a mechanism involving induction of CYP2E1 and elevated oxidative stress. The current study evaluated whether chronic alcohol feeding potentiates Fas-induced liver injury and whether CYP2E1 plays a role in any enhanced hepatotoxicity. Wild-type and CYP2E1 knockout mice were fed ethanol or isocaloric dextrose for 4 weeks followed by a single treatment with either saline or Jo2. Mice were killed 8 h after the Jo2 challenge. There were three- to five fold increases in transaminases and more extensive eosinophilic necrosis, hemorrhage, and infiltration of inflammatory cells in the central zone of the hepatic lobule in the ethanol-fed mice treated with Jo2 compared to the dextrose/Jo2- or ethanol/saline-treated mice. Liver injury was blunted in ethanol-fed CYP2E1 knockout mice treated with Jo2. The chronic ethanol feeding produced steatosis, elevation of CYP2E1, and oxidative stress in wild-type but not CYP2E1 knockout mice. These changes in wild-type mice fed ethanol were similar after saline or Jo2 treatment. The Jo2 treatment produced activation of JNK and P38 MAP kinase, increased activity of caspase-8 and -3, and lowered hepatic GSH levels in both the dextrose- and the alcohol-fed mice. JNK was activated at early times after Jo2 treatment in the ethanol-fed mice. Serum TNF-alpha levels were strikingly elevated in the wild-type ethanol/Jo2 group, which showed liver injury, compared to all the other groups, which did not show liver injury. Inhibition of JNK or P38 MAPK partially, but not completely, prevented the elevated liver injury in the wild-type ethanol/Jo2 mice. These results show that chronic ethanol feeding enhances Fas-induced liver injury by a mechanism associated with induction of CYP2E1, elevated serum TNF-alpha levels, and activation of MAPK.

Role of the coagulation system in acetaminophen-induced hepatotoxicity in mice

Acetaminophen (N-acetyl-p-aminophenol [APAP]) is one of the leading causes of acute liver failure, and APAP hepatotoxicity is associated with coagulopathy in humans. We tested the hypothesis that activation of the coagulation system and downstream protease-activated receptor (PAR)-1 signaling contribute to APAP-induced liver injury. Fasted C57BL/J6 mice were treated with either saline or APAP (400 mg/kg intraperitoneally) and were euthanized 0.5-24 hours later. Hepatotoxicity and coagulation system activation occurred by 2 hours after administration of APAP. Treatment with APAP also caused a rapid and transient increase in liver procoagulant activity. In addition, significant deposition of fibrin was observed in the liver by 2 hours, and the concentration of plasminogen activator inhibitor-1 in plasma increased between 2 and 6 hours. Pretreatment with heparin attenuated the APAP-induced activation of the coagulation system and hepatocellular injury and diminished hepatic fibrin deposition at 6 hours. Loss of hepatocellular glutathione was similar in APAP-treated mice pretreated with saline or heparin, suggesting that heparin did not diminish bioactivation of APAP. In mice deficient in tissue factor, the principal cellular activator of coagulation, APAP-induced liver injury, activation of coagulation, and hepatic fibrin deposition were reduced at 6 hours. Formation of the tissue factor-factor VIIa complex leads to the generation of thrombin that can activate cells through cleavage of PAR-1. Mice lacking PAR-1 developed less injury and hepatic fibrin deposits at 6 hours in response to APAP than control mice.

CONCLUSION

Activation of the coagulation system and PAR-1 signaling contribute significantly to APAP-induced liver injury.

PAR1-mediated NFkappaB activation promotes survival of prostate cancer cells through a Bcl-xL-dependent mechanism

We have previously reported that protease-activated receptor 1 (PAR1 or thrombin receptor) is over-expressed in metastatic prostate cancer cell lines compared to prostate epithelial cells. In this study, we examined 1,074 prostate biopsies by tissue microarray analysis and demonstrated that PAR1 expression is significantly increased in prostate cancer compared to normal prostate epithelial cells and benign prostatic hyperplasia. We hypothesized that PAR1 activation contributed to prostate cancer cell progression. We demonstrated that stimulation of PAR1 by thrombin or thrombin receptor activating peptide (TRAP6), in androgen-independent DU145 and PC-3 cells resulted in increased DNA binding activity of the NFkappaB p65 subunit. IL-6 and IL-8 levels were also elevated in conditioned media by at least two-fold within 4-6 h of PAR1 activation. This induction of cytokine production was abrogated by pretreatment of cells with the NFkappaB inhibitor caffeic acid phorbol ester. The p38 and ERK1/2 MAPK signaling cascades were also activated by PAR1 stimulation, whereas the SAPK/JNK pathway was unaffected. Inhibition of p38 and ERK1/2 by SB-203589 and PD-098059, respectively, did not abrogate NFkappaB activity, suggesting an independent induction of NFkappaB by PAR1 stimulation. Furthermore, TUNEL assay showed that activation of PAR1 attenuated docetaxel induced apoptosis through the upregulation of the Bcl-2 family protein Bcl-xL. Akt activation was not observed, suggesting that drug resistance induced by PAR1 was independent of PI3K signaling pathway. Because thrombin and PAR1 are over-expressed in prostate cancer patients, targeting the inhibition of their interaction may attenuate NFkappaB signaling transduction resulting in decreased drug resistance and subsequent survival of prostate cancer cells.

Proteinase-activated receptors: transducers of proteinase-mediated signaling in inflammation and immune response

Serine proteinases such as thrombin, mast cell tryptase, trypsin, or cathepsin G, for example, are highly active mediators with diverse biological activities. So far, proteinases have been considered to act primarily as degradative enzymes in the extracellular space. However, their biological actions in tissues and cells suggest important roles as a part of the body's hormonal communication system during inflammation and immune response. These effects can be attributed to the activation of a new subfamily of G protein-coupled receptors, termed proteinase-activated receptors (PARs). Four members of the PAR family have been cloned so far. Thus, certain proteinases act as signaling molecules that specifically regulate cells by activating PARs. After stimulation, PARs couple to various G proteins and activate signal transduction pathways resulting in the rapid transcription of genes that are involved in inflammation. For example, PARs are widely expressed by cells involved in immune responses and inflammation, regulate endothelial-leukocyte interactions, and modulate the secretion of inflammatory mediators or neuropeptides. Together, the PAR family necessitates a paradigm shift in thinking about hormone action, to include proteinases as key modulators of biological function. Novel compounds that can modulate PAR function may be potent candidates for the treatment of inflammatory or immune diseases.

jun-N-terminal kinase regulates thrombin-induced PAI-1 gene expression in proximal tubular epithelial cells

BACKGROUND

Interstitial activation of the coagulation cascade is a common finding in acute and chronic tubulointerstitial damage. We previously demonstrated that thrombin may induce proximal tubular epithelial cells (PTEC) proliferation and regulate, through plasminogen activator inhibitor (PAI)-1 and urokinase-type plasminogen activator (u-PA), their profibrotic activity. The signaling pathways leading to these effects are still unknown. The PAI-1 promoter contains several activator protein-1 (AP-1) consensus sequences. AP-1 activation is induced by different agonists through jun-N-terminal kinase (JNK). Thus, we investigated the role of the JNK-AP-1 axis on thrombin-induced PAI-1 and u-PA expression in immortalized PTEC and its modulation by PKC and src, two key signaling enzymes.

METHODS

JNK and src activation was investigated by Western blotting, PAR-1 cellular surface expression by flow cytometry, PAI-1 and u-PA gene expression by Northern blotting, AP-1 activation by transient transfection, and DNA synthesis by (3)H-thymidine uptake.

RESULTS

Thrombin and PMA induced a time-dependent increase of JNK phosphorylation in immortalized PTEC that was inhibited by PKC down-regulation. Both thrombin and PMA caused AP-1 activation, significantly reduced by src inhibition. Phorbol 12-myristate 13-acetate (PMA), indeed, induced an increase in src phosphorylation. Both PMA- and thrombin-stimulated PAI-1 gene expression was abolished by JNK, protein kinase C (PKC), and src inhibition, and this effect was regulated at the trascriptional level. PKC, but not src or JNK inhibition, abolished thrombin-elicited u-PA expression. Finally, JNK inhibition did not influence thrombin-induced proliferation.

CONCLUSION

Our data suggest that thrombin activates the JNK-AP-1 axis in a PKC- and src-dependent manner in PTEC. This axis, necessary for thrombin-stimulated PAI-1 expression, but not for its fibrinolytic and regenerative effect, may represent a therapeutic target in acute and chronic tubulointerstitial damage.

Gβγ signaling and Ca2+ mobilization co-operate synergistically in a Sos and Rac-dependent manner in the activation of JNK by Gq-coupled receptors

The mechanism by which G(q)-coupled receptors stimulate the c-Jun N-terminal kinase (JNK) activity has not been fully delineated. Here, we showed that stimulation of endogenous G(q)-coupled receptors in human hepatocarcinoma HepG2 cells resulted in an Src family kinase- and Ca(2+)-dependent JNK activation. Cos-7 cells transfected with HA-tagged JNK and various G(q)-coupled receptors also exhibited similar characteristics and provided further evidence for the involvement of Gbetagamma, an upstream intermediate for Src family kinases. The Ca(2+) and Gbetagamma signals operate in a high degree of independence. Transient expression of Gbetagamma subunits and elevation of cytoplasmic Ca(2+) level by thapsigargin activated JNK in a synergistic fashion. JNK activities triggered by G(q)-coupled receptors, Gbetagamma and thapsigargin were all suppressed by dominant negative (DN) mutants of Son of sevenless (Sos) and Rac. We propose that the co-operative effect between Gbetagamma-mediated signaling and the increased intracellular Ca(2+) level represents a robust mechanism for the stimulation of JNK by G(q)-coupled receptors.

The serine protease plasmin triggers expression of MCP-1 and CD40 in human primary monocytes via activation of p38 MAPK and janus kinase (JAK)/STAT signaling pathways

The mechanism of proinflammatory activation of human monocytes by plasmin is unknown. Here we demonstrate that in human primary monocytes, plasmin stimulates mitogen-activated protein kinase (MAPK) signaling via phosphorylation of MAPK kinase 3/6 (MKK3/6) and p38 MAPK that triggers subsequent DNA binding of transcription factor activator protein-1 (AP-1). The AP-1 complex contained phosphorylated c-Jun and ATF2, and its DNA binding activity was blocked by the p38 MAPK inhibitor SB203580. In addition, plasmin elicits Janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling, as detected by phosphorylation of JAK1 tyrosine kinase and STAT1 and STAT3 proteins. Plasmin-induced DNA binding of STAT1 and STAT3 was blocked by SB203580 and AG490, inhibitors of p38 MAPK and JAK, respectively, but not by U0126, an inhibitor of MKK1/2. DNA binding of NF-kappaB remained unaffected by any of these inhibitors. The plasmin-induced signaling led to expression of monocyte chemoattractant protein-1 (MCP-1) and CD40, which required activation of both p38 MAPK and JAK/STAT signaling pathways. Additionally, signaling through both p38 MAPK and JAK is involved in the plasmin-mediated monocyte migration, whereas the formylmethionylleucylphenylalanine-induced chemotaxis remained unaffected. Taken together, our data demonstrate a novel function of the serine protease plasmin in a proinflammatory signaling network.

Thrombin regulates insulin-like growth factor-1 receptor transcription in vascular smooth muscle: characterization of the signaling pathway

We have previously demonstrated that thrombin upregulation of insulin-like growth factor-1 receptor (IGF-1R) is essential for thrombin-induced mitogenic signaling. To characterize the mechanisms involved, we studied transcription of the IGF-1R gene in rat aortic smooth muscle cells. Thrombin markedly increased IGF-1R mRNA levels, peaking at 3 hours (112+/-7% above control). This effect was mimicked by the hexapeptide SFFLRN (that functions as a tethered ligand) and was blocked by the thrombin inhibitor hirudin. Nuclear run-on assays indicated that thrombin stimulated IGF-1R gene transcription by 2.1-fold, and this was confirmed with the use of actinomycin D. Thrombin-mediated upregulation of IGF-1R mRNA and protein levels was protein kinase C independent but was completely inhibited by the protein tyrosine kinase inhibitor genistein and by the antioxidants N-acetyl-L-cysteine and pyrrolidinedithiocarbamate, suggesting the involvement of reactive oxygen species. The thrombin-induced increase in IGF-1R mRNA was inhibitable by diphenyleneiodonium chloride but not by other inhibitors of cellular oxidase systems, suggesting that NAD(P)H oxidase was necessary for the increase. Furthermore, inhibitors of the epidermal growth factor receptor kinase, Janus kinase-2 kinase, and Src kinase did not block the effect. Thus, thrombin transcriptionally regulates the IGF-1R gene via a redox-sensitive protein tyrosine kinase-dependent pathway that does not require protein kinase C activation. In view of our prior data indicating that IGF-1R density is a critical determinant of vascular smooth muscle cell growth, our findings have particular relevance to understanding mechanisms whereby growth factors such as thrombin regulate vascular proliferation in vivo.

X protein of hepatitis B virus inhibits Fas-mediated apoptosis and is associated with up-regulation of the SAPK/JNK pathway

The X protein from a chronic strain of hepatitis B virus (HBx) was determined to inhibit Fas-mediated apoptosis and promote cell survival. Fas-mediated apoptosis is the major cause of hepatocyte damage during liver disease. Experiments demonstrated that cell death caused by anti-Fas antibodies was blocked by the expression of HBx in human primary hepatocytes and mouse embryo fibroblasts. This effect was also observed in mouse erythroleukemia cells that lacked p53, indicating that protection against Fas-mediated apoptosis was independent of p53. Components of the signal transduction pathways involved in this protection were studied. The SAPK/JNK pathway has previously been suggested to be a survival pathway for some cells undergoing Fas-mediated apoptosis, and kinase assays showed that SAPK activity was highly up-regulated in cells expressing the HBx protein. Normal mouse fibroblasts expressing HBx were protected from death, whereas identical fibroblasts lacking the SEK1 component from the SAPK pathway succumbed to Fas-mediated apoptosis, whether HBx was present or not. Assays showed that caspase 3 and 8 activities and the release of cytochrome c from mitochondria were inhibited, in the presence of HBx, following stimulation with anti-Fas antibodies. Coprecipitation and confocal immunofluorescence microscopy experiments demonstrated that HBx localizes with a cytoplasmic complex containing MEKK1, SEK1, SAPK, and 14-3-3 proteins. Finally, mutational analysis of HBx demonstrated that a potential binding region for 14-3-3 proteins was essential for induction of SAPK/JNK activity and protection from Fas-mediated apoptosis.

Lithium regulates PKC-mediated intracellular cross-talk and gene expression in the CNS in vivo

It has become increasingly appreciated that the long-term treatment of complex neuropsychiatric disorders like bipolar disorder (BD) involves the strategic regulation of signaling pathways and gene expression in critical neuronal circuits. Accumulating evidence from our laboratories and others has identified the family of protein kinase C (PKC) isozymes as a shared target in the brain for the long-term action of both lithium and valproate (VPA) in the treatment of BD. In rats chronically treated with lithium at therapeutic levels, there is a reduction in the levels of frontal cortical and hippocampal membrane-associated PKC alpha and PKC epsilon. Using in vivO microdialysis, we have investigated the effects of chronic lithium on the intracellular cross-talk between PKC and the cyclic AMP (cAMP) generating system in vivo. We have found that activation of PKC produces an increase in dialysate cAMP levels in both prefrontal cortex and hippocampus, effects which are attenuated by chronic lithium administration. Lithium also regulates the activity of another major signaling pathway the c-Jun N-terminal kinase pathway--in a PKC-dependent manner. Both Li and VPA, at therapeutically relevant concentrations, increase the DNA binding of activator protein 1 (AP-1) family of transcription factors in cultured cells in vitro, and in rat brain ex vivo. Furthermore, both agents increase the expression of an AP-1 driven reporter gene, as well as the expression of several endogenous genes known to be regulated by AP-1. Together, these results suggest that the PKC signaling pathway and PKC-mediated gene expression may be important mediators of lithium's long-term therapeutic effects in a disorder as complex as BD.

The calcium-sensing receptor stimulates JNK in MDCK cells

The calcium-sensing receptor (CaR) stimulates ERK1 in rat fibroblasts, but its effect on other MAP kinases is not known. We used a model of renal distal tubule, the MDCK cell, to determine the effects of CaR stimulation on Jun kinase (JNK) activity. Stimulation of the CaR with 5 mM Ca(2+) resulted in a time-dependent increase in JNK activity. Activation of JNK occurred preferentially with stimulation on the basal surface relative to the apical surface. Basal administration of the CaR agonist gadolinium (30 microm) also stimulated JNK activity. Pertussis toxin blocked the ability of both CaR agonists to stimulate JNK, indicating that the effect was mediated through G(ialpha) class G proteins. Finally, we used confocal microscopy to determine that the CaR was located predominantly on the basal surface. These studies demonstrate for the first time that the CaR stimulates JNK activity.

Lithium activates the c-Jun NH2-terminal kinases in vitro and in the CNS in vivo

The therapeutic efficacy of lithium in the treatment of mood disorders is delayed and only observed after chronic administration, a temporal profile that suggests alterations at the genomic level. Lithium has been demonstrated to modulate AP-1 DNA binding activity as well as the expression of genes regulated by AP-1, but the mechanisms underlying these effects have not been fully elucidated. In the present study, we found that the lithium-induced increases in AP-1 DNA binding activity were accompanied by increases in p-cJun and cJun levels in SH-SY5Y cells. Lithium also increased cJun-mediated reporter gene expression in a dose-dependent manner, with significant effects observed at therapeutically relevant concentrations. Lithium's effects on cJun-mediated reporter gene expression in SH-SY5Y cells were more pronounced in the absence of myo-inositol and were blocked by protein kinase C (PKC) inhibitors and by cotransfection with a PKCalpha dominant-negative mutant. Chronic in vivo lithium administration increased AP-1 DNA binding activity in frontal cortex and hippocampus and also increased the levels of the phosphorylated, active forms of c-Jun NH2-terminal kinases (JNKs) in both brain regions. These results demonstrate that lithium activates the JNK signaling pathway in rat brain during chronic in vivo administration and in human cells of neuronal origin in vitro; in view of the role of JNKs in regulating various aspects of neuronal function and their well-documented role in regulating gene expression, these effects may play a major role in lithium's long-term therapeutic effects.

Angiotensin II stimulates platelet-derived growth factor-B chain expression in newborn rat vascular smooth muscle cells and neointimal cells through Ras, extracellular signal-regulated protein kinase, and c-Jun N-terminal protein kinase mechanisms

Platelet-derived growth factors (PDGFs) have been implicated in the pathogenesis of vascular proliferative disorders. Vascular smooth muscle cells (VSMCs) are one of the cell types that produce PDGF-B chain in proliferative lesions, although the mechanism of regulation of PDGF-B chain production in these cells is not well understood. In the present study, we demonstrate that angiotensin II (Ang II), which is also implicated in vascular stenosis after angioplasty and atherosclerosis, markedly stimulates PDGF-B chain mRNA expression in cultured newborn rat medial VSMCs and neointimal VSMCs via an AT(1), but not in adult rat VSMCs. In newborn rat VSMCs, Ang II activates extracellular signal-regulated protein kinase (ERK), c-Jun N-terminal protein kinase (JNK), and p38 mitogen-activated protein kinase. The mitogen-activated protein/ERK (MEK) inhibitor PD98059, but not the p38 inhibitor SB203580, abrogates Ang II-induced PDGF-B mRNA expression. Transient transfection analysis using a PDGF-B promoter-luciferase gene reporter construct reveals that Ang II induces transcriptional activation of PDGF-B chain gene, which is abolished by the expression of a dominant negative form of either ERK or JNK, but not of p38. The expression of a dominant negative form of Ras abolishes the stimulatory effects of Ang II on ERK activity and PDGF-B mRNA expression. In adult rat VSMCs, Ang II activates ERK and JNK, but weakly induces Egr-1, a transcription factor implicated in PDGF-B chain gene expression, compared with newborn VSMCs. These data indicate that Ang II activates PDGF-B chain gene expression in VSMCs through mechanisms involving Ras-ERK and JNK.

Endogenous delta-opioid and ORL1 receptors couple to phosphorylation and activation of p38 MAPK in NG108-15 cells and this is regulated by protein kinase A and protein kinase C

The p38 mitogen-activated protein kinase (MAPK) cascade transduces multiple extracellular signals from cell surface to nucleus and is employed in cellular responses to cellular stresses and apoptotic regulation. The involvement of the p38 MAPK cascade in opioid- and opioid receptor-like receptor-1 (ORL1) receptor-mediated signal transduction was examined in NG108-15 neuroblastoma x glioma hybrid cells. Stimulation of endogenous delta-opioid receptor (DOR) or ORL1 resulted in activation of p38 MAPK. It also induced the activation of extracellular signal-regulated kinases (ERKs), another member of the MAPK family, with slower kinetics. Activation of p38 MAPK was abolished by selective antagonists of DOR or ORL1, pretreatment with pertussis toxin, or SB203580, a specific inhibitor of p38 MAPK. Inhibition of p38 MAPK had no significant effect on opioid-induced ERK activation, indicating that p38 MAPK activity was not required for ERK activation, though its stimulation preceded ERK activation. Inhibition of protein kinase A (PKA) strongly diminished p38 activation mediated by DOR or ORL1 but had no significant effect on ERK activation, and protein kinase C (PKC) inhibitors potentiated stimulation of p38 while inhibiting activation of ERKs. Taken together, our results provide the first evidence for coupling of DOR and ORL1 to the p38 MAPK cascade and clearly demonstrate that receptor-mediated activation of p38 MAPK both involves PKA and is negatively regulated by PKC.