Bcl-2 family proteins regulate mitochondrial reactive oxygen production and protect against oxidative stress

Bcl-2 family proteins protect against a variety of forms of cell death, including acute oxidative stress. Previous studies have shown that overexpression of the antiapoptotic protein Bcl-2 increases cellular redox capacity. Here we report that cell lines transfected with Bcl-2 paradoxically exhibit increased rates of mitochondrial H2O2 generation. Using isolated mitochondria, we determined that increased H2O2 release results from the oxidation of reduced nicotinamide adenine dinucleotide-linked substrates. Antiapoptotic Bcl-2 family proteins Bcl-xL and Mcl-1 also increase mitochondrial H2O2 release when overexpressed. Chronic exposure of cells to low levels of the mitochondrial uncoupler carbonyl cyanide 4-(triflouromethoxy)phenylhydrazone reduced the rate of H2O2 production by Bcl-xL overexpressing cells, resulting in a decreased ability to remove exogenous H2O2 and enhanced cell death under conditions of acute oxidative stress. Our results indicate that chronic and mild elevations in H2O2 release from Bcl-2, Bcl-xL, and Mcl-1 overexpressing mitochondria lead to enhanced cellular antioxidant defense and protection against death caused by acute oxidative stress.