Regulation of Prostate Cancer Cell Survival by Protein Kinase Cε Involves Bad Phosphorylation and Modulation of the TNFα/JNK Pathway

Protein kinase C epsilon (PKC epsilon), a diacyglycerol- and phorbol ester-responsive serine-threonine kinase, has been implicated in mitogenic and survival control, and it is markedly overexpressed in human tumors, including in prostate cancer. Although prostate cancer cells undergo apoptosis in response to phorbol ester stimulation via PKC delta-mediated release of death factors, the involvement of PKC epsilon in this response is not known. PKC epsilon depletion by RNAi or expression of a dominant negative kinase-dead PKC epsilon mutant potentiated the apoptotic response of PMA and sensitized LNCaP cells to the death receptor ligand TNF alpha. On the other hand, overexpression of PKC epsilon by adenoviral means protected LNCaP cells against apoptotic stimuli. Interestingly, PKC epsilon RNAi depletion significantly enhanced the release of TNF alpha in response to PMA and greatly potentiated JNK activation by this cytokine. Further mechanistic analysis revealed that PMA fails to promote phosphorylation of Bad in Ser(112) in PKC epsilon-depleted LNCaP cells, whereas PKC epsilon overexpression greatly enhanced Bad phosphorylation. This effect was independent of Akt, ERK, or p90Rsk, well established kinases for Ser(112) in Bad. Moreover, expression of a S112A-Bad mutant potentiated PMA-induced apoptosis. Finally, we found that upon activation PKC epsilon accumulated in mitochondrial fractions in LNCaP cells and that Bad was a substrate of PKC epsilon in vitro. Our results established that PKC epsilon modulates survival in prostate cancer cells via multiple pathways.