Vacuolar H + ‐ATPase inhibitors overcome Bcl‐xL‐mediated chemoresistance through restoration of a caspase‐independent apoptotic pathway

The anti‐apoptotic oncoproteins Bcl‐2 and Bcl‐xL play crucial roles in tumorigenesis and chemoresistance, and are thus therapeutic cancer targets. We searched for small molecules that disturbed the anti‐apoptotic function of Bcl‐2 or Bcl‐xL, and found vacuolar H + ‐ATPase (V‐ATPase) inhibitors, such as bafilomycin A1 (BMA), that showed such activity. Bcl‐xL‐overexpressing Ms‐1 cells displayed resistance to anticancer drugs, but underwent apoptosis following treatment with a combination of V‐ATPase inhibitors at doses similar to those that caused inhibitory activities of V‐ATPase. We investigated the apoptosis mechanism induced by cotreatment of Bcl‐xL‐overexpressing Ms‐1 cells with BMA as a V‐ATPase inhibitor and taxol (TXL) as an anticancer drug. With BMA, TXL triggered mitochondrial membrane potential loss and cytochrome c release, whereas downstream caspase activation was not observed. In contrast, pronounced nuclear translocation of mitochondrial apoptosis‐inducing factor and endonuclease G, known as effectors of caspase‐independent apoptosis, was observed with BMA and TXL cotreatment. Moreover, depletion of apoptosis‐inducing factor and endonuclease G using each siRNA significantly rescued cells from BMA‐ and TXL‐induced apoptosis. Hence, the apoptosis‐inducing factor‐ and endonuclease G‐dependent pathway was critical for apoptosis induction by BMA and TXL cotreatment. Our data suggest that V‐ATPase inhibitors could not only suppress anti‐apoptotic Bcl‐2 nor Bcl‐xL but could also facilitate the caspase‐independent apoptotic pathway. V‐ATPase inhibition will be a promising therapeutic approach for Bcl‐2‐ or Bcl‐xL‐overexpressing malignancies. ( Cancer Sci 2009)