Selective inhibition of protein kinase C isozymes by Fas ligation

Activation of protein kinase C (PKC) can protect cells from apoptosis induc ed by various agents, including Fas ligation. To elucidate a possible inter action between Fas-mediated apoptotic signals and activation-related protec tive signals, we investigated the impact of Fas ligation on PKC activity. W e demonstrate that engagement of Fas on human lymphoid Jurkat cells trigger ed apoptosis, and Fas ligation resulted in partial blockade of cellular PKC activity. The phorbol 12-myristate 13-acetate-mediated translocation of PK C theta from the cytoplasm to the membrane was inhibited by treatment with anti-Fas antibody, whereas the translocation of PKC alpha or epsilon was no t affected. In vitro kinase assay of PKC alpha or epsilon phosphotransferas e activity demonstrated that Fas ligation inhibited the ability of PKC alph a to phosphorylate histone H1 as substrate but did not inhibit epsilon isoz yme activity. This inhibition of PKC alpha activity mediated by Fas ligatio n was reversed by okadaic acid, a phosphatase inhibitor, suggesting the inv olvement of a member of the protein phosphatase 2A subfamily in this compon ent of Fas signaling, Identical patterns of PKC isozyme inhibition were obt ained 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.