KRIBB11 accelerates Mcl-1 degradation through an HSF1-independent, Mule-dependent pathway in A549 non-small cell lung cancer cells

The Bc1-2 family protein, Mc1-1 is known to have anti-apoptotic functions, and depletion of Mcl-1 by cellular stresses favors the apoptotic process. Moreover, Mcl-1 levels are frequently increased in various cancer cells, including non-small cell lung cancer (NSCLC), and is implicated in resistance to conventional chemotherapy and in cancer metastasis. In this study, we demonstrated that KRIBBI1 accelerates the proteasomal degradation of Mcl-1 in the NSCLC cell line, A549. While KRIBBI1 is an inhibitor of HSF1, we found that KRIBB11 induced Mcl-1 degradation in an HSF1-independent manner. Furthermore, this process was triggered via increase ubiquitination by the E3 ligase, Mule, rather than via de-ubiquitination by USP9X. Additionally, we found that Mcl-1 levels were only transiently reduced by KRIBBI1: Mcl-1 levels were gradually restored as KRIBB11 activity diminished. However, we found that this effect was blocked in BIS (Bc1-2 interacting cell death suppressor, also called BAG3)-depleted cells, and that BIS prevents Mc1-1 from undergoing HSP70-driven proteasomal degradation, through an interaction with HSP70. Taken together, our results suggest that targeting Mcl-1 with KRIBBI1 treatment, while simultaneously downregulating BIS, could be a therapeutic strategy in NSCLC. (C) 2017 Elsevier Inc. All rights reserved.