The DNA damage-induced decrease of Bcl-2 is secondary to the activation of apoptotic effector caspases

Apoptosis induced by DNA-damaging agents or radiation mainly proceeds through death receptor-independent caspase activation. The release of mitochondrial apoptogenic proteins, such as cytochrome c, into the cytoplasm leading to Apaf1-dependent activation of caspase-9 is a key event in this pathway. The permeability of the mitochondrial outer membrane is regulated by the various pro- and antiapoptotic Bcl-2 family proteins, and it is thought that DNA damage triggers apoptosis through the downregulation of antiapoptotic Bcl-2. Using murine embryonic fibroblasts (MEF) deficient and proficient in Apaf1, we show that DNA-damaging agents and radiation lead to a decline in Bcl-2 protein only in wt MEF, but not in apaf1(-/-) MEF, which are defective in the activation of effector caspases and apoptosis. In contrast, the induction of proapoptotic Noxa, the activation of Bax, the cytoplasmic release of cytochrome c, as well as a drop of the mitochondrial transmembrane potential Deltapsim are equally observed in wt and apaf1(-/-) MEF following DNA damage. Moreover, the loss of Bcl-2 protein occurring in wt MEF can be prevented by caspase inhibition. Hence, the activation of proapoptotic Bcl-2 family proteins rather than the downregulation of antiapoptotic Bcl-2 mediates the primary signal in the DNA damage-induced release of mitochondrial apoptogenic proteins in MEF.