PKCδ is required for mitochondrial-dependent apoptosis in salivary epithelial cells

We report here that the novel protein kinase C isoform, PKC delta, 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 PKC delta to explore the requirement for PKC delta in the mitochondrial-dependent apoptotic pathway. Expression of PKC delta KD, but not PKC delta KD, 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 PKC delta KD 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 PKC delta KD. The ability of PKC delta KD to block the loss of mitochondrial membrane potential was similarly determined. Expression of PKC delta KD 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 PKC delta KD, expression of PKC delta WT resulted in a potent induction of apoptosis, which could be inhibited by coinfection with PKC delta KD. These results suggest that PKC delta is a common intermediate in mitochondrial-dependent apoptosis in salivary epithelial cells.