Abstract 5085: Radiosensitization of castration-resistant prostate cancer via modulating autophagic cell death

Abstract

Abstract Radioresistance markedly impair the efficacy of cancer therapy. Anti-apoptotic Bcl-2 family proteins such as Bcl-xL, Bcl-2 and Mcl-1 are overexpressed in castration-resistant prostate cancer (CRPC) and contribute to prostate tumor initiation, progression and resistance to radiotherapy. A natural BH3-mimetic, small molecule inhibitor of Bcl-2, (−)-gossypol, shows promise in ongoing Phase II clinical trials for human prostate cancer. We have recently shown that (−)-gossypol preferentially induces autophagy in CRPC cells that have high levels of Bcl-2 and are resistant to apoptosis, both in vitro and in vivo, but not in androgen-dependent cells with low Bcl-2 and sensitive to apoptosis. Our results demonstrate for the first time that (−)-gossypol can interrupt the interaction between Beclin1 and Bcl-2/Bcl-xL at the endoplasmic reticulum, thus releasing the BH3-only pro-autophagic protein Beclin1, which in turn triggers the autophagic cascade. (−)-Gossypol-induced autophagy is Beclin1- and Atg5-dependent, together with Bcl-2 downregulation and Beclin1 upregulation. (−)-Gossypol increased autophagic cell death induced by X-ray radiation in the CRPC cells. Orally administered (−)-gossypol achieved a much greater efficacy with long-term tumor regression when used in combination with ionizing radiation. We have also explored a series of gossypol derivatives and identified two novel BH3-mimetic lead compounds that are more potent than the parental compound in inducing autophagic cell death in CRPC. Taken together, (−)-gossypol significantly enhances the anti-tumor activity of radiotherapy in vitro and in vivo, and represents a promising new regime for treatment of castration-resistant human prostate cancer with overexpression of Bcl-2. Our data provide new insights into the mode of cell death induced by Bcl-2 inhibitors, which would facilitate the rational design of clinical trials by selecting patients who are most likely to benefit from the Bcl-2-targeted molecular therapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5085. doi:10.1158/1538-7445.AM2011-5085