Taxanes Sensitize Prostate Cancer Cells to TRAIL-Induced Apoptotic Synergy Via Endoplasmic Reticulum Stress

Docetaxel (DTX) and cabazitaxel (CBZ) are guideline-chemotherapy treatments for metastatic castration-resistant prostate cancer (mCRPC), which comprises the majority of prostate cancer (PCa) deaths. TNF-related apoptosis inducing ligand (TRAIL) is an anti-cancer agent that is selectively cytotoxic to cancer cells; however, many human cancers are resistant to TRAIL.In this study, we sensitized androgen-independent and TRAIL-resistant PCa cells to TRAIL-mediated apoptosis via taxane therapy and examined the mechanism of sensitization. DU145 and PC3 cells displayed no significant reduction in cell viability when treated with soluble TRAIL, DTX, or CBZ alone indicating that both cell lines are resistant to TRAIL and taxanes individually. Taxane and TRAIL combination synergistically amplified apoptosis strongly suggesting that taxanes sensitize prostate cancer cells to TRAIL. A Jun N-terminal kinases (JNK) inhibitor inhibited apoptosis in treated cells and significantly reduced death receptor expression indicating JNK activation by ER stress sensitizes PCa cells to TRAIL-induced apoptosis by upregulating DR4/DR5 expression. In addition, suppression of C/EBP homologous protein (CHOP) reduced TRAIL sensitization in both cell lines indicating that ER stress-related apoptosis is mediated, in part by CHOP. Cytochrome c knockdown showed a significant decrease in sensitivity in PC3 cells, but not in Bax-deficient DU145 cells. A computational model was used to simulate apoptosis for cells treated with taxane and TRAIL therapy as demonstrated in in vitro experiments. Pretreatment with taxanes sensitized cells to apoptosis induced by TRAIL-mediated apoptosis, demonstrating that combining TRAIL with ER stress inducers is a promising therapy to reverse TRAIL resistance to treat mCRPC.