Downregulation of c-FLIP and upregulation of DR-5 by cantharidin sensitizes TRAIL-mediated apoptosis in prostate cancer cells via autophagy flux

Tumor necrosis factor (TNF)-related apop-tosis-inducing ligand (TRAIL), a type II transmembrane protein, is a part of the TNF superfamily of cytokines. Cantharidin, a type of terpenoid, is extracted from the blister beetles (Mylabris genus) used in Traditional Chinese Medicine. Cantharidin elicits antibiotic, antiviral and antitumor effects, and can affect the immune response. The present study demonstrated that a cantharidin and TRAIL combination treatment regimen elicited a synergistic outcome in TRAIL-resistant DU145 cells. Notably, it was also identified that cantharidin treatment initiated the downregulation of cellular FLICE-like inhibitory protein (c-FLIP) and upregulation of death receptor 5 (DR-5), and sensitized cells to TRAIL-mediated apoptosis by initiating autophagy flux. In addition, cantharidin treatment increased lipid-modified microtubule-associated proteins 1A/1B light chain 3B expression and significantly attenuated sequestosome 1 expression. Attenuation of autophagy flux by a specific inhibitor such as chloroquine and genetic modification using ATG5 small interfering RNA abrogated the cantharidin-mediated TRAIL-induced apoptosis. Overall, the results of the present study revealed that cantharidin effectively sensitized cells to TRAIL-mediated apoptosis and its effects are likely to be mediated by autophagy, the downregulation of c-FLIP and the upregulation of DR-5. They also suggested that the combination of cantharidin and TRAIL may be a successful therapeutic strategy for TRAIL-resistant prostate cancer.