Tyrosine Kinase Inhibitors Increase MCL1 Degradation and in Combination with BCLXL/BCL2 Inhibitors Drive Prostate Cancer Apoptosis

Purpose: Clinically available BH3 mimetic drugs targeting BCLXL and/or BCL2 (navitoclax and venetoclax, respectively) are effective in some hematologic malignancies, but have limited efficacy in solid tumors. This study aimed to identify combination therapies that exploit clinical BH3 mimetics for prostate cancer. Experimental Design: Prostate cancer cells or xenografts were treated with BH3 mimetics as single agents or in combination with other agents, and effects on MCL1 and apoptosis were assessed. MCL1 was also targeted directly using RNAi, CRISPR, or an MCL1-specific BH3 mimetic, S63845. Results: We initially found that MCL1 depletion or inhibition markedly sensitized prostate cancer cells to apoptosis mediated by navitoclax, but not venetoclax, in vitro and in vivo, indicating that they are primed to undergo apoptosis and protected by MCL1 and BCLXL. Small-molecule EGFR kinase inhibitors (erlotinib, lapatinib) also dramatically sensitized to navitoclax-mediated apoptosis, and this was associated with markedly increased proteasome-dependent degradation of MCL1. This increased MCL1 degradation appeared to be through a novel mechanism, as it was not dependent upon GSK3 beta-mediated phosphorylation and subsequent ubiquitylation by the ubiquitin ligases beta TRCP and FBW7, or through other previously identified MCL1 ubiquitin ligases or deubiquitinases. Inhibitors targeting additional kinases (cabozantinib and sorafenib) similarly caused GSK3 beta-independent MCL1 degradation, and in combination with navitoclax drove apoptosis in vitro and in vivo. Conclusions: These results show that prostate cancer cells are primed to undergo apoptosis and that cotargeting BCLXL and MCL1, directly or indirectly through agents that increase MCL1 degradation, can induce dramatic apoptotic responses. (C) 2018 AACR.