Activation of PKC is sufficient to induce an apoptotic program in salivary gland acinar cells

14-3-3 proteins mediate an essential anti-apoptotic signal

The 14-3-3 proteins are a family of highly conserved eukaryotic regulatory molecules that play important roles in many biological processes including cell cycle control and regulation of cell death. They are able to carry out these effects through binding and modulating the activity of a host of signaling proteins. The ability of 14-3-3 to inhibit Bad and other proapoptotic proteins argues that 14-3-3 can support cell survival. To examine this issue in a global sense, a specific inhibitor of 14-3-3/ligand interactions, difopein, was used. Difopein expression led to induction of apoptosis. Studies using various components of survival and death signaling pathways were consistent with a vital role for 14-3-3/ligand interactions in signal transduction from upstream pro-survival kinases to the core apoptotic machinery. Because these kinases often become activated during oncogenesis, the effect of difopein on cell death induced by antineoplastic drugs was examined. It was found that difopein enhances the ability of cisplatin to kill cells. These data support the model that 14-3-3, through binding to Bad and other ligands, is critical for cell survival signaling. Inhibition of 14-3-3 may represent a useful therapeutic target for treatment of cancer and other diseases involving inappropriate cell survival.

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

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