Abstract 363: Intracellular redox milieu regulates Mcl-1 and decreases overall mitochondrial priming in hematopoietic cancers

Abstract

Abstract Background: The dichotomy of redox signaling in cancer cell fate decisions has emerged as a topic of importance over the years. In particular, our group has highlighted the role of superoxide (O2 .-)-mediated signaling in the acquisition of drug resistance and cancer survival. Mechanistically, this involves post-translational modification of proteins involved in oncogenic signaling, such as c-Myc and NF-κB, as well as apoptosis resistance like Bcl-2 at the serine 70 phosphorylation site. Here we present evidence that O2 .--mediated signaling stabilizes anti-apoptotic protein Mcl-1, a commonly amplified gene in human cancers and is implicated in resistance to venetoclax, the Bcl-2 specific BH3 mimetic. Methods: Intracellular O2 .- levels was increased either pharmacologically or by siRNA mediated knockdown of SOD1 in MOLM14, RPMI8226, OCI-AML3 hematopoietic cell lines. Cell viability and apoptosis were assessed by MTT and Annexin V/DAPI staining. Protein expression was determined by Western blot analysis. BH3 profiling of patient-derived CLL cells was used to assess mitochondrial priming. Results: Elevated O2 .- levels increased Mcl-1 stability through Thr163 phosphorylation of Mcl-1, which in turn reduced ubiquitination-dependent degradation. Furthemore, venetoclax resistant cells possessed higher O2 .- levels and increase in intracellular O2 .- levels promoted resistance to Mcl-1 inhibitor, S63845 through increased binding between Mcl-1 and pro-apoptotic proteins NOXA and Bak. Contrarily, scavenging of O2 .- re-sensitized venetoclax-resistant leukemia cells to venetoclax. Importantly, BH3 profiling revealed that increase in intracellular O2 .- levels decreased overall mitochondrial priming in primary CLL cells by promoting sequestration of pro-apoptotic proteins that specifically target Mcl-1. Conclusion: Oncogenic activity of O2 .- is mediated by increased Mcl-1 stability through Thr163 phosphorylation, which resulted in a reduction in overall mitochondrial priming and increased binding of Mcl-1 to NOXA and Bak. Scavenging O2 .- reduces Mcl-1 levels and sensitizes resistant cells to venetoclax-induced apoptosis thereby demonstrating that manipulation of the redox microenvironment could be a potential vulnerability to be exploited in venetoclax resistant cancers Citation Format: Jolin Xiao Lai, Stephen Jun Chong, Matthew Davids, Shazib Pervaiz. Intracellular redox milieu regulates Mcl-1 and decreases overall mitochondrial priming in hematopoietic cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 363.