The Bcl-2 family protein Mcl-1 is often degraded in cancer cells subjected to effective therapeutic treatment, and defective Mcl-1 degradation has been associated with intrinsic and acquired drug resistance. However, a causal relationship between Mcl-1 degradation and anticancer drug responses has not been directly established, especially in solid tumor cells where Mcl-1 inhibition alone is insufficient to trigger cell death. In this study, we present evidence that Mcl-1 participates directly in determining effective therapeutic responses in colon cancer cells. In this setting, Mcl-1 degradation was induced by a variety of multi-kinase inhibitor drugs, where it relied upon GSK3β phosphorylation and FBW7-dependent ubiquitination. Specific blockade by genetic knock-in (KI) abolished apoptotic responses and conferred resistance to kinase inhibitors. Mcl-1-KI also suppressed the anti-angiogenic and anti-hypoxic effects of kinase inhibitors in the tumor microenvironment. Interestingly, these same inhibitors also induced the BH3-only Bcl-2 family protein PUMA, which is required for apoptosis. Degradation-resistant Mcl-1 bound and sequestered PUMA from other pro-survival proteins to maintain cell survival, which was abolished by small-molecule Mcl-1 inhibitors. Our findings establish a pivotal role for Mcl-1 degradation in the response of colon cancer cells to targeted therapeutics, and they provide a useful rational platform to develop Mcl-1-targeting agents that can overcome drug resistance.
- Received November 28, 2016.
- Revision received January 17, 2017.
- Accepted January 31, 2017.
- Copyright ©2017, American Association for Cancer Research.