Protein kinase C δ is essential for etoposide-induced apoptosis in salivary gland acinar cells

Abstract

We have previously shown that parotid C5 salivary acinar cells undergo apoptosis in response to etoposide treatment as indicated by alterations in cell morphology, caspase-3 activation, DNA fragmentation, sustained activation of c-Jun N-terminal kinase, and inactivation of extracellular regulated kinases 1 and 2. Here we report that apoptosis results in the caspase-dependent cleavage of protein kinase C-δ (PKCδ) to a 40-kDa fragment, the appearance of which correlates with a 9-fold increase in PKCδ activity. To understand the function of activated PKCδ in apoptosis, we have used the PKCδ-specific inhibitor, rottlerin. Pretreatment of parotid C5 cells with rottlerin prior to the addition of etoposide blocks the appearance of the apoptotic morphology, the sustained activation of c-Jun N-terminal kinase, and inactivation of extracellular regulated kinases 1 and 2. Inhibition of PKCδ also partially inhibits caspase-3 activation and DNA fragmentation. Immunoblot analysis shows that the PKCδ cleavage product does not accumulate in parotid C5 cells treated with rottlerin and etoposide together, suggesting that the catalytic activity of PKCδ may be required for cleavage. PKCδ and PKCβ1 activities also increase during etoposide-induced apoptosis. Inhibition of these two isoforms with Go6976 slightly suppresses the apoptotic morphology, caspase-3 activation, and DNA fragmentation, but has no effect on the sustained activation of c-Jun N-terminal kinase or inactivation of extracellular regulated kinase 1 and 2. These data demonstrate that activation of PKCδ is an integral and essential part of the apoptotic program in parotid C5 cells and that specific activated isoforms of PKC may have distinct functions in cell death.