Downregulation of MCL-1 and upregulation of PUMA using mTOR inhibitors enhance antitumor efficacy of BH3 mimetics in triple-negative breast cancer

Triple-negative breast cancer (TNBC) shows a higher malignant and poorer clinical outcome compared with other breast cancer subtypes. Albeit that chemotherapy is the first choice for TNBC treatment, rapid emergence of chemoresistance and variability of chemotherapeutic responses in TNBC patients call for novel therapeutic strategies. Here, we reported evidences highlighting that combination of BH3 mimetics and mTOR inhibitors could be a promising therapeutic strategy to improve TNBC treatment. Our results showed that combination of the BH3 mimetic ABT263 and typical mTOR inhibitors, BEZ235 or AZD8055, leads to efficient apoptosis in vitro. Tumor regression was significantly improved by combination therapy compared with either drug alone in the xenograft model. Further mechanistic investigations revealed that mTOR inhibitors induced the suppression of MCL-1; concomitantly, the expression level of PUMA was significantly upregulated in a FOXO3a-dependent manner. The specific changes of MCL-1 and PUMA facilitated the release of the apoptotic regulators, such as BIM, BAX, and BAK, to induce the activation of mitochondrial apoptotic pathway, thereby sensitizing the ABT263 activity in TNBC. Therefore, our findings provided evidences that mTOR inhibitors can enhance antitumor efficacy of BH3 mimetics via downregulating MCL-1 and upregulating PUMA in TNBC; it could be a promising therapeutic strategy to treat TNBC.