Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a highly promising candidate for the treatment of cancer because it elicits cell death in the majority of tumor cells while sparing most normal cells. Some cancers, however, display resistance to TRAIL, suggesting that treatment with TRAIL alone may be insufficient for cancer therapy. In the present study, we explored whether the apoptotic responsiveness of PC-3 prostate cancer cells to TRAIL could be enhanced by targeting the novel protein kinase C (PKC) isoform eta. Transfection of PC-3 cells with second-generation chimeric antisense oligonucleotides against PKCeta caused a time- and dose-dependent knockdown of PKCeta, as revealed by real-time RT-PCR and Western blot analyses. Knockdown of PKCeta resulted in a marked amplification of TRAIL's cytotoxic activity. Cell killing could be substantially prevented by the pan-caspase inhibitor z-VAD-fmk. In addition, PKCeta knockdown and administration of TRAIL significantly synergized in activation of caspase-3 and internucleosomal DNA fragmentation. Knockdown of PKCeta augmented TRAIL-induced dissipation of the mitochondrial transmembrane potential and release of cytochrome c from mitochondria into the cytosol, indicating that PKCeta acts upstream of mitochondria. We conclude that PKCeta represents a considerable resistance factor with respect to TRAIL and a promising target to exploit the therapeutic potential of TRAIL.