Repurposing of mTOR Complex Inhibitors Attenuates MCL-1 and Sensitizes to PARP Inhibition.

MCL-1, a member of the antiapoptotic BCL-2 family, is a prosurvival protein with an essential DNA repair function. This study aims to test whether inhibition of protein synthesis by mTOR complex (mTORC) inhibitors depletes MCL-1, suppresses homologous recombination (HR) repair, and sensitizes cancer cells to PARP inhibitors. Treatment with everolimus decreases MCL-1 in colorectal carcinomas and small cell lung cancer (SCLC) cells but not glioblastoma multiforme (GBM) cells with a PTEN mutational background. However, AZD2014, a dual mTORC inhibitor, depletes MCL-1 in GBMs. Further, we show that everolimus decreases 4EBP1 phosphorylation only in colorectal carcinoma, whereas AZD2014 decreases 4EBP1 phosphorylation in both colorectal carcinoma and GBM cells. Combination therapy using everolimus or AZD2014 with olaparib inhibits the growth of clone A and U87-MG xenografts in in vivo and decreases clonogenic survival in in vitro compared with monotherapy. Reintroduction of MCL-1 rescues the survival of cancer cells in response to combination of everolimus or AZD2014 with olaparib. Treatment of cells with mTORC inhibitors and olaparib increases γ-H2AX and 53BP1 foci, decreases BRCA1, RPA, and Rad51 foci, impairs phosphorylation of ATR/Chk1 kinases, and induces necroptosis. In summary, mTORC inhibitors deplete MCL-1 to suppress HR repair and increase sensitivity to olaparib both in in vitro and in xenografts. IMPLICATIONS: Targeting the DNA repair activity of MCL-1 in in vivo for cancer therapy has not been tested. This study demonstrates that depleting MCL-1 sensitizes cancer cells to PARP inhibitors besides eliciting necroptosis, which could stimulate antitumor immunity to improve the therapeutic intervention of cancers.