Induction of Oxidative Stress and Glutathione Depletion by ABT-737, a Novel Small Molecule Inhibitor of Bcl-2, Contributes to Cytotoxicity in Leukemia Cells.

The Bcl-2 family of proteins regulate the process of cell death. Bcl-2 and Bcl-xL are potent anti-apoptotic members that are overexpressed in a number of malignancies, providing a means to evade cell death, which has been described as a characteristic hallmark of cancer. Several lines of evidence suggest that Bcl-2 and Bcl-xL possess anti-oxidant properties. Overexpression of Bcl-2 is protective against menadione and hydrogen peroxide induced cell death and causes an increase in intracellular levels of glutathione (GSH), the most abundant antioxidant defense. In the present study, we utilized a novel small molecule to test effects of Bcl-2 inhibition on intracellular redox status. ABT-737 is a first generation inhibitor of Bcl-2, Bcl-xL and Bcl-w and and acts as a mimic of the BH3-only antagonists of these family members. Exposure of acute lymphocytic leukemia (ALL) cells to ABT-737 caused a dose dependent increase in intracellular levels reactive oxygen species (ROS), namely superoxide and hydrogen peroxide. This dose dependent increase in intracellular oxidants was significantly less pronounced in cells treated with the less active enantiomer of ABT-737. A greater than 50% decrease in intracellular GSH levels was seen with similar doses of ABT-737 and the combination of buthionine sulfoximine, an agent that depletes GSH levels further, with ABT-737, caused synergistic cell death. These data were verified using cells transfected with a tetracycline repressable Bcl-2 expression plasmid. Taken together, our data identifies a novel sequelae for Bcl-2 inhibition: the induction of oxidative stress. Combination therapies utilizing ABT-737 or its analogs may be devised based on the observed effects on the redox environment.