Superoxide-mediated phosphorylation and stabilization of Mcl-1 by AKT underlie venetoclax resistance in hematologic malignancies.

Resistance to the Bcl-2-specific inhibitor, Venetoclax (VEN), poses a therapeutic challenge in the management of chronic lymphocytic leukemia and acute myeloid leukemia. Although VEN resistance has been linked to Mcl-1 upregulation, thereby switching survival dependence from Bcl-2 to Mcl-1, the mechanism underlying increased Mcl-1 expression remains elusive. Given that changes in cellular redox state affect cancer cell fate, we investigated the crosstalk between intracellular redox milieu and Mcl-1 upregulation in VEN-resistant cells. Results show that increased Mcl-1 protein levels in VEN-resistant hematologic malignant cells are associated with elevated intracellular superoxide (O2.-) levels. Validating that, augmenting intracellular O2.- in VEN-sensitive cells increases Mcl-1 phosphorylation at threonine-163 (T163pMcl-1) and protein stability via reduced Mcl-1 ubiquitination and degradation. Furthermore, redox-activated AKT/PKB is implicated in O2.--induced T163pMcl-1, as reducing intracellular O2.- or inhibiting AKT significantly decreases T163pMcl-1 and Mcl-1 accumulation, which amplifies mitochondrial apoptotic priming and restores VEN sensitivity. Importantly, combination therapy with AKT inhibitor, capivasertib, and VEN reduced VEN-resistant cells systemically and prolonged survival in a murine model. Collectively, a novel redox-dependent mechanism of Mcl-1 stability is demonstrated for the acquisition of VEN resistance, which has therapeutic implications for employing redox modulating strategies and AKT inhibitors against VEN-resistant hematologic malignancies.