Regulation of the mitochondrial checkpoint in p53-mediated apoptosis confers resistance to cell death.

Abstract

The p53 tumor suppressor protein inhibits tumor formation, in part by inducing apoptosis, which is inhibited by anti-apoptotic Bcl-2 family members Bcl-2 and adenovirus E1B 19K. We have identified p53-apoptotic signaling events which are targeted for inhibition by E1B 19K. Apoptotic signaling by p53 induced a Bid-independent conformational change in Bax, a Bax-Bak interaction, release of cytochrome c and Smac/DIABLO from mitochondria, caspase-9 and -3 activation, cleavage of known caspase substrates, and apoptosis. When p53-dependent apoptosis was blocked by E1B 19K expression, E1B 19K bound Bak, and the Bax-Bak interaction was inhibited. Cytochrome c and Smac/DIABLO release from mitochondria was also inhibited in E1B 19K expressing cells and cells remained viable. After a prolonged p53 death stimulus, the inhibition of the mitochondrial death checkpoint by E1B 19K failed, and cytochrome c and Smac/DIABLO were released from mitochondria, and became degraded. Despite this eventual failure to inhibit the mitochondrial checkpoint, caspase-9 and -3 were not activated, and cells remained viable even upon treatment with an exogenous death stimulus. Thus, p53 induces apoptosis in part through Bax and Bak, and even an incomplete inhibition of this mitochondrial checkpoint may be sufficient to confer resistance to cell death.