Abstract 1315: DNMT and HDAC inhibitors resensitize resistant tumors to antiprogestin therapy in a mouse breast cancer model

Abstract

Abstract Mouse mammary carcinomas induced by medroxyprogesterone acetate (MPA) are maintained by syngeneic transplantation and tumor variants with different hormone responsiveness have been generated. Tumors which grow without the MPA supply were defined as hormone independent (HI). Most of these tumors regress after antiprogestin (mifepristone, MIF) administration, while others show constitutive resistance to hormone therapy. We have previously demonstrated that responsive tumors show higher levels of progesterone receptor A (PRA) than PRB, while the opposite occurs in resistant tumors, indicating that the PR isoform ratio might be a predictor of antiprogestin responsiveness. We have also demonstrated that in constitutive resistant tumors PRA is silenced by methylation of its promoter, and that treatment with 5azadC (demethylating agent) induces PRA expression and tumors partially recover antiprogestin responsiveness. The aim of this study was to investigate whether the co-treatment with DNA methyltransferase (DNMT) and histone deacetylase (HDAC) inhibitors would enhance even more the responsiveness to MIF. For this purpose we investigated the expression of HDAC1 in constitutive resistant tumors and whether treatment of these tumors with 5azadC and trichostatin A (TSA; HDAC inhibitor) restored MIF responsiveness better than 5azadC alone. The constitutive resistant tumor 59-HI showed a higher expression of HDAC1 as compared to the antiprogestin sensititive tumor 59-2-HI as determined by Western blot (p<0,001) and immunohistochemistry, suggesting that these tumors might be candidates to respond to an HDAC inhibitor. 59-HI tumors were implanted sc in BALB/c mice and when tumors were palpable they were treated with a) 5azadC (1 mg/kg/every other day; ip), b) MIF (12 mg/kg/daily; sc), c) 5azadC plus MIF or d) TSA (1 mg/kg/every other day; sc), e) TSA plus 5azadC, f) TSA, 5azadC and MIF, or g) vehicle. Interestingly the co-treatment of 5azadC, TSA and MIF induced a significant inhibitory effect that was even greater than the one induced by 5azadC plus MIF (p<0.001). No inhibitory effects were observed when each compound was used alone or with TSA and MIF. The expression of PR was evaluated in control, 5azadC- and TSA plus 5azadC- treated tumors. An increase in PRA expression was observed by western blot and immunohistochemistry. The increase of PRA was higher with TSA plus 5azadC than with 5azadC alone (p<0,001). In conclusion, we report for the first time that a) constitutive resistant mammary carcinomas show high levels of HDAC1 which might be responsible for histone acetylation within the methylated PRA promoter, b) the combined treatment of demethylating agents and HDAC inhibitors increases the efficiency in both, PR re-expression and antiprogestin sensitivity. These results support our hypothesis that tumors with high levels of PRA are those which respond to antiprogestin treatment. 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 1315. doi:10.1158/1538-7445.AM2011-1315