Mcl-1 Phosphorylation without Degradation Mediates Sensitivity to HDAC Inhibitors by Liberating BH3-Only Proteins

Mcl-1, a prosurvival Bcl-2 family protein, is frequently overexpressed in cancer cells and plays a critical role in therapeutic resistance. It is well known that anticancer agents induce phosphorylation of Mcl-1, which promotes its binding to E3 ubiquitin ligases and subsequent proteasomal degradation and apoptosis. However, other functions of Mcl-1 phosphorylation in cancer cell death have not been well characterized. In this study, we show in colon cancer cells that histone deacetylase inhibitors (HDACi) induce GSK3β-dependent Mcl-1 phosphorylation, but not degradation or downregulation. The in vitro and in vivo anticancer effects of HDACi were dependent on Mcl-1 phosphorylation and were blocked by genetic knock-in of a Mcl-1 phosphorylation site mutant. Phosphorylation-dead Mcl-1 maintained cell survival by binding and sequestering BH3-only Bcl-2 family proteins PUMA, Bim, and Noxa, which were upregulated and necessary for apoptosis induction by HDACi. Resistance to HDACi mediated by phosphorylation-dead Mcl-1 was reversed by small-molecule Mcl-1 inhibitors that liberated BH3-only proteins. These results demonstrate a critical role of Mcl-1 phosphorylation in mediating HDACi sensitivity through a novel and degradation-independent mechanism. These results provide new mechanistic insights on how Mcl-1 maintains cancer cell survival and suggest that Mcl-1-targeting agents are broadly useful for overcoming therapeutic resistance in cancer cells.Significance: These findings present a novel degradation-independent function of Mcl-1 phosphorylation in anticancer therapy that could be useful for developing new Mcl-1-targeting agents to overcome therapeutic resistance. Cancer Res; 78(16); 4704-15. ©2018 AACR.