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Selective inhibition of protein kinase C isozymes by Fas ligation
1Cancer Research Center and Departments of Medicine, Biochemistry, Pediatrics, Microbiology, Pathology and Laboratory Medicine, Boston University School of Medicine, Boston, Massachusetts 02118, USA.
Abstract:
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 PKCtheta from the cytoplasm to the membrane was inhibited by treatment with anti-Fas antibody, whereas the translocation of PKCalpha or epsilon was not affected. In vitro kinase assay of PKCalpha or epsilon phosphotransferase activity demonstrated that Fas ligation inhibited the ability of PKCalpha to phosphorylate histone H1 as substrate but did not inhibit epsilon isozyme activity. This inhibition of PKCalpha 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 overexpressing the fas 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.
Insights
Fas ligation triggers apoptosis by partially blocking protein kinase C (PKC) activity. This selective inhibition of PKCalpha, a protective pathway, may contribute to Fas-induced cell death signaling.
Area of Science:
- Cellular Biology
- Immunology
- Molecular Signaling
Background:
- Protein kinase C (PKC) activation confers cellular protection against apoptosis.
- Fas ligation is a known inducer of apoptosis in various cell types.
Purpose of the Study:
- To investigate the interaction between Fas-mediated apoptotic signals and PKC-mediated protective signals.
- To determine the impact of Fas ligation on PKC activity and its specific isozymes.
Main Methods:
- Utilized human lymphoid Jurkat cells and mouse thymoma cells overexpressing Fas.
- Applied anti-Fas antibody treatment and phorbol ester to study PKC translocation.
- Conducted in vitro kinase assays to assess PKCalpha and PKCepsilon activity.
- Employed okadaic acid, a phosphatase inhibitor, to investigate signaling pathways.
Main Results:
- Fas ligation induced apoptosis and partially inhibited overall PKC activity.
- Fas engagement selectively inhibited the translocation and kinase activity of PKCalpha, but not PKCepsilon or PKCtheta.
- Inhibition of PKCalpha activity was reversed by okadaic acid, suggesting protein phosphatase 2A involvement.
- Similar inhibition patterns were observed in Fas-overexpressing mouse cells.
Conclusions:
- Fas activation selectively inhibits PKCalpha activity, a potentially protective signaling pathway.
- This selective inhibition may contribute to the induction of apoptosis mediated by Fas signaling.
- Highlights a crosstalk between apoptotic and protective signaling pathways involving PKC isozymes.