Apoptosis induction by activator protein 2α involves transcriptional repression of Bcl-2

Activator protein 2 alpha ( AP-2 alpha) induces cytotoxicity by inducing cell cycle arrest and apoptosis. In this study we investigated the mechanism of apoptosis induction by AP-2 alpha. We found that AP-2 alpha induced apoptosis efficiently in cells treated with benzyloxycarbonyl-IETD-fluoromethyl ketone or FADD-silenced cells but failed to do so in benzyloxycarbonyl-LEHD-fluoromethyl ketone-treated or apoptosis protease activation factor-1 ( Apaf1)-silenced cells, suggesting the central role of mitochondria in AP-2 alpha-induced apoptosis. In good correlation, cells overexpressing AP-2 alpha showed a reduction in mitochondrial membrane potential ( Delta Psi(m)), cytochrome c and Smac/DIABLO release into cytosol, and Bax translocation into mitochondria. We found that the pro-apoptotic protein Bax is important for AP-2 alpha-induced apoptosis as adenovirus AP2 failed to induce apoptosis in HCT116 Bax(-/-) cells. However, we found the IAP ( inhibitor of apoptosis) inhibitor Smac/DIABLO may have a limited role in AP-2 alpha-induced apoptosis as we found the IAP member Survivin down-regulated by AP-2 alpha. Although the total Bax level remains unaltered, we found a time-dependent increase in the activated form of Bax in adenovirus AP2-infected cells. In addition, we show that AP-2 alpha transcriptionally represses Bcl-2 by binding to its promoter both in vitro and in vivo and that this is essential for AP-2 alpha-induced apoptosis as ectopic expression of Bcl-2 efficiently inhibited apoptosis induced by AP-2 alpha. Furthermore, we show that chemotherapy-induced endogenous AP-2 alpha down-regulates Bcl-2 and induces apoptosis in an AP-2 alpha-dependent manner. Moreover, we demonstrate that inhibition of okadaic acid or staurosporine-sensitive pathways in AP-2 alpha overexpressing breast cancer cells resulted in AP-2 alpha-dependent apoptosis induction. These results suggest that AP-2 alpha induces apoptosis by down-regulating Bcl-2 and utilizing a bax/cytochrome c/Apaf1/caspase 9-dependent mitochondrial pathway.