Gossypol induces Bax/Bak-independent activation of apoptosis and cytochrome c release via a conformational change in Bcl-2

Cells without Bak and Bax are largely resistant to apoptosis (1; 2), despite the presence of other key components of the apoptotic machinery. We screened 7,800 natural compounds and found several that could specifically induce caspase activation and the release of cytochrome c (cyto c) in the bak(-/-)/bax(-/-) cells. One of these was gossypol, a polyphenolic compound naturally found in cottonseed that has been used in antifertility trials. We found that gossypol, but not other Bcl-2-interacting molecules, induced cyto c release and loss of mitochondrial membrane potential (Delta Psi m) independently of mPTP and Bak/Bax activation. Furthermore, we found that gossypol induced an allosteric change in Bcl-2 in both bak(-/-)/bax(-/-) cells and Bcl-2 overexpressing cells. This change in Bcl-2 conformation led to the release of cyto c in the presence of Bcl-2 and Bcl-xL in reconstituted proteoliposomes. We also observed that gossypol substantially reduced the growth of tumor xenografts from Bcl- 2 overexpressing cells in nude mice. We conclude that gossypol converts the antiapoptotic molecule Bcl-2 into a proapoptotic molecule that can mediate the release of cyto c and induce apoptosis