PKC activation contributes to caspase-mediated eIF2α phosphorylation and cell death

Back ground: Stress-induced phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF2 alpha), involved in translation, promotes cell suicide or survival. Since multiple signaling pathways are implicated in cell death, the present study has analyzed the importance of PKC activation in the stress-induced eIF2 alpha phosphorylation, caspase activation and cell death in the ovarian cells of Spodoptera frugiperda (Sf9) and in their extracts. Methods: Cell death is analyzed by flow cytometry. Caspase activation is measured by Ac-DEVD-AFC hydrolysis and also by the cleavage of purified recombinant PERK, an endoplasmic reticulum-resident eIF2 alpha kinase. Status of eIF2 alpha phosphorylation and cytochrome c levels are analyzed by western blots. Results: PMA, an activator of PKC, does not promote cell death or affect eIF2 alpha phosphorylation. However. PMA enhances late stages of UV-irradiation or cycloheximide-induced caspase activation, eIF2 alpha phosphorylation and apoptosis in Sf9 cells. PMA also enhances cytochrome c-induced caspase activation and eIF2 alpha phosphorylation in cell extracts. These changes are mitigated more efficiently by caspase inhibitor, z-VAD-fmk, than by calphostin, an inhibitor of PKC. In contrast, tunicamycin-induced eIF2 alpha phosphorylation that does not lead to caspase activation or cell death is unaffected by PMA, z-VAD-fmk or by calphostin. Conclusions: While caspase activation is a cause and consequence of eIF2 alpha phosphorylation, PKC activation that follows caspase activation further enhances caspase activation, eIF2 alpha phosphorylation, and cell death in Sf9 cells. General significance: Caspases can activate multiple signaling pathways to enhance cell death. (C) 2010 Elsevier B.V. All rights reserved.