Hydrogen peroxide-mediated cytosolic acidification is a signal for mitochondrial translocation of Bax during drug-induced apoptosis of tumor cells

Absence of the proapoptotic protein Bax renders tumor cells resistant to drug-induced apoptosis. We have shown that hydrogen peroxide (H2O2)-mediated cytosolic acidification is an effector mechanism during drug-induced apoptosis of tumor cells. Here, we report that Bax is critical in determining the sensitivity of tumor cells to H2O2-induced apoptosis. More importantly, exposure of colorectal carcinoma (HCT116) and leukemia cells (HL60 and CEM) to H2O2 or its intracellular production during drug-induced apoptosis is a signal for mitochondrial translocation of Bax. Furthermore, we provide evidence that drug-induced H2O2-mediated Bax translocation in tumor cells is caspase independent but involves cytosolic acidification. Inhibiting cytosolic acidification prevents Bax translocation, and contrarily enforced acidification of the intracellular milieu results in mitochondrial recruitment of Bax, even in the absence of a trigger. These findings provide a novel mechanism for mitochondrial translocation of Bax and directly implicate H2O2-mediated cytosolic acidification in the recruitment of the mitochondrial pathway during drug-induced apoptosis of tumor cells.