PKCδ Is Required for Mitochondrial-dependent Apoptosis in Salivary Epithelial Cells

We report here that the novel protein kinase C isoform, PKCδ, is required at or prior to the level of the mitochondria for apoptosis induced by a diverse group of cell toxins. We have used adenoviral expression of a kinase-dead (KD) mutant of PKCδ to explore the requirement for PKCδ in the mitochondrial-dependent apoptotic pathway. Expression of PKCδKD, but not PKCαKD, in salivary epithelial cells resulted in a dose-dependent inhibition of apoptosis induced by etoposide, UV-irradiation, brefeldin A, and paclitaxel. DNA fragmentation was blocked up to 71% in parotid C5 cells infected with the PKCδKD adenovirus, whereas caspase-3 activity was inhibited up to 65%. The activation of caspase-9-like proteases by all agents was also inhibited in parotid C5 cells expressing PKCδKD. The ability of PKCδKD to block the loss of mitochondrial membrane potential was similarly determined. Expression of PKCδKD blocked the decrease in mitochondrial membrane potential observed in cells treated with etoposide, UV, brefeldin A, or paclitaxel in a dose-dependent manner. In contrast to the protective function of PKCδKD, expression of PKCδWT resulted in a potent induction of apoptosis, which could be inhibited by co-infection with PKCδKD. These results suggest that PKCδ is a common intermediate in mitochondrial-dependent apoptosis in salivary epithelial cells.