(-)-Gossypol, a natural BH-3 mimetic, chemosensitizes human prostate cancer in vitro and in vivo via p53-independent induction of PUMA and NOXA

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Chemoresistance markedly impairs the efficacy of cancer therapy and involves anti-apoptotic signal transduction pathways that prevent the treatment-induced cell death. Anti-apoptotic Bcl-2 family proteins such as Bcl-xL, Bcl-2 and Mcl-1 are overexpressed in prostate cancer and contribute to prostate tumor initiation, progression and resistance to chemotherapy. They are promising molecular targets for modulating therapeutic resistance of prostate cancer.
 (-)-Gossypol, a natural product from cottonseed, has recently been identified as a potent small molecule inhibitor of Bcl-2/Bcl-xL and is now in Phase II clinical trials. In the current study, we tested our hypothesis that (-)-gossypol may improve prostate cancer's response to chemotherapy by inhibiting the anti-apoptosis activity of Bcl-2/Bcl-xL and investigated its molecular mechanism of action. (-)-Gossypol inhibited tumor cell growth and induced apoptosis in cancer cells with high levels of Bcl-2/Bcl-xL proteins, but had minimal effect on normal cells. (-)-Gossypol induced apoptosis by disrupting the mitochondrial transmembrane potential and releasing AIF and cytochrome c into cytoplasm. Fluorescence resonance energy transfer (FRET) assay demonstrated that (-)-gossypol potently blocked the interactions of Bcl-xL with Bax or Bad in live cells, in a dose-dependent manner, indicating that (-)-gossypol induced apoptosis through inhibiting the heterodimerization of anti-apoptotic protein Bcl-xL with proapoptotic protein Bax/Bad. In Bcl-xL-transfected FL5.12 model cell lines, (-)-Gossypol overcomed the Bcl-xL protection of FL5.12 cells upon IL-3-withdrawal. When used in combination with chemotherapeutic agents, (-)-Gossypol increased apoptosis induced by docetaxel and cisplatin in human prostate cancer PC-3 cells with high Bcl-2/Bcl-xL. (-)-Gossypol synergistically enhanced the anti-tumor effect of docetaxel both in vitro and in vivo. (-)-Gossypol dose-dependently induced PUMA and Noxa protein expression which was augmented by combination with docetaxel, while increased Mcl-1 and ubiquitin protein levels, suggesting the antitumor activity of (-)-gossypol might also involve, at least in part, proteosome inhibition.
 The data support that (-)-gossypol exerts its anti-tumor activity not only through inhibition of the anti-apoptotic proteins such as Bcl-2 and Bcl-xL, also through the p53-independent induction of Noxa and PUMA. (-)-Gossypol significantly enhances the anti-tumor activity of chemotherapy in vitro and in vivo, and may represent a promising new regime for treatment of hormone-refractory human prostate cancer with Bcl-2/Bcl-xL overexpression.