Selective inhibition of protein kinase C isozymes by Fas ligation

Activation of protein kinase C (PKC) can protect cells from apoptosis induced by various agents, including Fas ligation. To elucidate a possible interaction between Fas-mediated apoptotic signals and activation-related protective signals, we investigated the impact of Fas ligation on PKC activity. We demonstrate that engagement of Fas on human lymphoid Jurkat cells triggered apoptosis, and Fas ligation resulted in partial blockade of cellular PKC activity. The phorbol 12-myristate 13-acetate-mediated translocation of PKC theta from the cytoplasm to the membrane was inhibited by treatment with anti-Fas antibody, whereas the translocation of PKC alpha or epsilon was not affected. In vitro kinase assay of PKC alpha or epsilon phosphotransferase activity demonstrated that Fas ligation inhibited the ability of PKC alpha to phosphorylate histone H1 as substrate but did not inhibit epsilon isozyme activity. This inhibition of PKC alpha activity mediated by Fas ligation was reversed by okadaic acid, a phosphatase inhibitor, suggesting the involvement of a member of the protein phosphatase 2A subfamily in this component of Fas signaling, Identical patterns of PKC isozyme inhibition were obtained using mouse thymoma cells over-expressing the fog gene (LF(+)), These results suggest that the selective inhibition of a potentially protective, PKC-mediated pathway by Fas activation may, to some extent, contribute to Fas-induced apoptotic signaling.