Role of MAPK Phosphatase-1 in Sustained Activation of JNK during Ethanol-induced Apoptosis in Hepatocyte-like VL-17A Cells

Ethanol metabolism plays a central role in activating the mitogen-activated protein kinase (MAPK) cascade leading to inflammation and apoptosis. Sustained activation of c-Jun N-terminal kinase (JNK), one of the MAPKs, has been shown to induce apoptosis in hepatocytes. MAPK phosphatase-1 (MKP-1) has been shown to dephosphorylate MAPKs in several cells. The aim of the study is to evaluate the role of MKP-1 in sustained JNK activation as a mechanism to explain ethanol-induced hepatocyte apoptosis. VL-17A cells (HepG2 cells overexpressing alcohol dehydrogenase and cytochrome P450-2E1) were exposed to ethanol for different time periods. Western blots were performed for MKP-1, phospho-JNK, phosphotyrosine, and protein kinase Cδ (PKCδ). Electrophoretic mobility shift assays for AP-1 were performed. Apoptosis was measured by caspase-3 activity assay, TUNEL, and 4′,6-diamidino-2-phenylindole staining. Reactive oxygen species were neutralized by overexpressing both superoxide dismutase-3 and catalase genes using lentiviral vectors in VL-17A cells. Ethanol incubation markedly decreased the MKP-1 protein levels to 15% of control levels and was associated with sustained phosphorylation of p46 JNK and p54 JNK, as well as increased apoptosis. VL-17A cells overexpressing superoxide dismutase-3 and catalase, treatment with a tyrosine kinase inhibitor, or incubation of the cells with PKCδ small interference RNAs significantly inhibited the ethanol-induced MKP-1 degradation and apoptosis. Ethanol-induced oxidative stress enhanced the tyrosine phosphorylation of PKCδ, which in turn caused the proteasomal degradation of MKP-1, leading to sustained JNK activation and increased apoptosis in VL-17A cells.