Downregulation of Bid is associated with PKC epsilon-mediated TRAIL resistance

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising anticancer agent as it selectively kills tumor cells but spares normal cells. Resistance to TRAIL by tumor cells limits its therapeutic use. We have previously shown that protein kinase C-epsilon (PKC epsilon) acts as an antiapoptotic protein in MCF-7 breast cancer cells. In the present study, we have investigated the mechanism(s) by which PKCe contributes to TRAIL resistance. Overexpression of PKCe inhibited caspase-8 and -9 activation, release of mitochondrial cytochrome c and cell death induced by TRAIL, but did not interfere with the recruitment of caspase-8 to the death-inducing signaling complex. Knockdown/inhibition of PKCe resulted in enhanced sensitivity to TRAIL. The level of Bcl-2 was increased and Bid was decreased by PKCe at both the protein and mRNA level but PKCe had no effect on Bax. Knockdown of Bcl-2 by siRNA reversed TRAIL resistance in PKC epsilon-overexpressing cells, whereas depletion of Bid contributed to TRAIL resistance in MCF-7 cells. A decrease in Bid content was also associated with inhibition of TRAIL-induced caspase-8 activation. Furthermore, PKC epsilon depletion or overexpression of DN-PKC epsilon was associated with a decrease in Bcl-2 protein level. Thus, our results suggest that PKC epsilon acts upstream of mitochondria and mediates TRAIL resistance via both Bcl-2 and Bid in MCF-7 cells.