Abstract B09: Targeting the anti-apoptotic protein Mcl-1 with the mTOR inhibitor RAD001 sensitizes luminal breast cancers to the Bcl-2/Bcl-xL inhibitor ABT-263

In an effort to enhance therapeutic tumor cell killing in luminal breast cancers, we investigated the inhibition of the anti-apoptotic Bcl-2 family proteins Bcl-2, Bcl-xL and Mcl-1. Anti-apoptotic Bcl-2 proteins are inhibitors of the intrinsic apoptotic pathway that sequester and inhibit pro-apoptotic Bcl-2 family members. Sustained overexpression of anti-apoptotic Bcl-2 family members frequently occurs in cancers and supports tumor initiation, tumor progression, therapeutic resistance and poor patient survival. While only 2/324 and 1/324 luminal breast cancers curated by The Cancer Genome Atlas (TCGA) had gene amplification of BCL2 and BCL2L1 (encoding Bcl-xL) respectively, 21/324 ( MCL1 amplification. Wap-Myc transgenic mouse mammary tumors, which are highly enriched for the luminal A transcription signature, displayed profoundly increased Mcl-1 levels as compared to normal mammary tissue, suggesting a role for Mcl-1 in luminal breast cancer biology. Using a panel of four MCL1 -amplified breast cancer cell lines, we found that two were marginally sensitive to treatment with ABT-263, a BH3-mimetic that targets Bcl-2 and Bcl-xL. Conversely, ectopic overexpression of Mcl-1 decreased sensitivity to ABT-263. ABT-263 induced minimal cell killing in MCF7 xenografts and had no effect on tumor growth. Similarly, ABT-263 did not affect tumor growth in luminal-like Wap-Myc mouse mammary tumors. Thus, luminal breast cancer cells expressing increased Mcl-1 are poorly responsive to Bcl-2/Bcl-xL inhibition. Upregulation of Mcl-1 expression and activity, an established ABT-263-resistance mechanism in leukemias and lymphomas, was observed in MCL1 -amplified luminal breast cancer cell lines treated with ABT-263 in culture and in vivo. ABT-263 also increased the anti-apoptotic activity of Mcl-1 towards the pro-apoptotic Bcl-2 family member BIM, suggesting that Mcl-1 upregulation compensates for inhibition of Bcl-2 and Bcl-xL by ABT-263. There are currently no Mcl-1 specific inhibitors. However, Mcl-1 undergoes cap-dependent translation, and thus inhibition of cap-dependent translation has been proposed as a method to inhibit the protein expression of Mcl-1. Importantly, the mTOR pathway is required for efficient cap-dependent translation in many cancer cells. We found that the rapalogue RAD001/everolimus, an mTOR inhibitor, decreased Mcl-1 protein levels in MCL1 -amplified luminal breast cancer cell lines. Furthermore, RAD001 blocked Mcl-1 protein upregulation in response to ABT-263, producing increased cell killing and decreased cell growth in all MCL1 -amplified luminal breast cancer cell lines. Treatment of luminal Wap-Myc mammary tumors with RAD001 decreased tumor volume by 40%, while treatment with ABT-263 had no effect on tumor volume. These data suggest that Mcl-1 targeting may be superior to Bcl-2/Bcl-xL targeting in luminal breast cancers, particularly in luminal breast cancers with MCL1 -amplification. Furthermore, we have identified the mTOR pathway as a potential therapeutic target for blocking Mcl-1 expression in luminal breast cancers. Citation Format: Michelle M. Williams, Linus Lee, Meghan M. Morrison, Andrew J. Williams, Violeta Sanchez, Donna Hicks, Rebecca Cook. Targeting the anti-apoptotic protein Mcl-1 with the mTOR inhibitor RAD001 sensitizes luminal breast cancers to the Bcl-2/Bcl-xL inhibitor ABT-263. [abstract]. In: Proceedings of the AACR Special Conference: Targeting the PI3K-mTOR Network in Cancer; Sep 14-17, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(7 Suppl):Abstract nr B09.