Inhibition of PKCα induces of PKCδ-dependent apoptotic program in salivary epithelial cells

Abstract

We have used expression of a kinase dead mutant of PKCα (PKCαKD) to explore the role of this isoform in salivary epithelial cell apoptosis. Expression of PKCαKD by adenovirus-mediated transduction results in a dose-dependent induction of apoptosis in salivary epithelial cells as measured by the accumulation of sub-G1 DNA, activation of caspase-3, and cleavage of PKCδ and PKCζ, known caspase substrates. Induction of apoptosis is accompanied by nine-fold activation of c-Jun-N-terminal kinase, and an approximately two to three-fold increase in activated mitogen-activated protein kinase (MAPK) as well as total MAPK protein. Previous studies from our laboratory have shown that PKCδ activity is essential for the apoptotic response of salivary epithelial cells to a variety of cell toxins. To explore the contribution of PKCδ to PKCαKD-induced apoptosis, salivary epithelial cells were cotransduced with PKCαKD and PKCδKD expression vectors. Inhibition of endogenous PKCδ blocked the ability of PKCαKD to induce apoptosis as indicated by cell morphology, DNA fragmentation, and caspase-3 activation, indicating that PKCδ activity is required for the apoptotic program induced under conditions where PKCα is inhibited. These findings indicate that PKCα functions as a survival factor in salivary epithelial cells, while PKCδ functions to regulate entry into the apoptotic pathway.