Mcl-1 Inhibition Modulates ERK-Mediated Resistance in Multiple Myeloma

Novel multiple myeloma (MM) treatments have significantly improved over the previous several decades, primarily on account of targeting bone marrow microenvironment (BMM) pathways. However, drug resistance and patient relapse remain major clinical problems. The role of BMM in the upregulation of anti-apoptotic protein Mcl-1 is well documented. The Mcl-1 protein plays a critical role in the progression and acquired drug resistance in MM. The regulation of Mcl-1, a protein characterized by a short half-life, from transcription to degradation is crucial for understanding its role in cell survival. The GSK3β and Erk play important role in the stability of Mcl-1. Also, overexpression of phospho Erk is associated with the acquired resistance. In this study, we investigated Mcl-1 regulation, focusing on transcriptional and post-translational modifications and their impact on protein stability in Mcl-1 inhibitor ( KS18) treated cells. The small molecule inhibitor KS18 induces Mcl-1Ser159/Thr163 phosphorylation and ubiquitination resulting in a sharp decline in Mcl-1 protein levels. Furthermore, we assessed the effects of the KS18 in a combination with ERK inhibitors on cell viability and found that blocking the Mcl-1 stabilization mechanism improves the effectiveness and potency of KS18. Furthermore, we compared KS18 to different classes of chemotherapeutic agents, such as GSK3β/α inhibitor (LY209031), ERK inhibitor (SEH77272), MEK inhibitor (PD18435), and Akt inhibitor (AZD5363). Interestingly, we found KS18 more potent than other agents. Combined, our results propose a strong rationale for novel combination therapies using selective KS18 and ERK inhibitors, which have the potential to markedly improve the outcome of MM treatment. This may also address one of the major clinical problems, drug resistance, and enhance the use of existing drugs