Calcium channel blocker verapamil accelerates gambogic acid-induced cytotoxicity via enhancing proteasome inhibition and ROS generation.

Verapamil (Ver), an inhibitor of the multidrug resistance gene product, has been proved to be a promising combination partner with other anti-cancer agents including proteasome inhibitor bortezomib. Gambogic acid (GA) has been approved for Phase II clinical trials in cancer therapy in China. We have most recently reported that GA is a potent proteasome inhibitor, with anticancer efficiency comparable to bortezomib but much less toxicity. In the current study we investigated whether Ver can enhance the cytotoxicity of GA. We report that (i) the combination of Ver and GA results in synergistic cytotoxic effect and cell death induction in HepG2 and K562 cancer cell lines; (ii) a combinational treatment with Ver and GA induces caspase activation, endoplasmic reticulum (ER) stress and reactive oxygen species (ROS) production; (iii) caspase inhibitor z-VAD blocks GA+Ver-induced apoptosis but not proteasome inhibition; (iv) cysteine-containing compound N-acetylcysteine (NAC) prevents GA+Ver-induced poly(ADP-ribose) polymerase cleavage and proteasome inhibition. These results demonstrate that Ver accelerates GA-induced cytotoxicity via enhancing proteasome inhibition and ROS production. These findings indicate that the natural product GA is a valuable candidate that can be used in combination with Ver, thus representing a compelling anticancer strategy.