151 - Rac1-induced ROS is implicated in the sustained activation of anti-apoptotic protein Bcl-2 in cancer cells

Abstract

The anti-apoptotic protein Bcl-2 is strongly associated with the development of therapy resistance in a host of hematopoietic and other cancers. To that end, the post-translational modification of Bcl-2, in particular its mono- or multi-site phosphorylation, have been reported to impact its apoptosis inhibitory activity. While multi-site phosphorylation compromises its death inhibitory activity, our recent work has highlighted the critical involvement of serine 70 phosphorylation (S70pBcl-2) in stabilizing its anti-apoptotic activity. S70pBcl-2 is a function of either an amplification of the activities of various kinases involved in this post-translational modification or a loss of function of the phosphatase regulating this modification. The pleiotropic phosphatase, PP2A, is specifically involved in the dephosphorylation of Bcl-2 at S70. In our search to identify redox dependent pathways impacting the sustained activation of Bcl-2 via S70P, we provided evidence that redox modification of the B56δ sub-unit of PP2A resulted in the sustained S70P of Bcl-2, thereby promoting its death inhibitory activity in cancer cells. Interestingly, here we present substantial evidence for the existence of a similar redox-dependent regulation of Bcl-2 phosphorylation in cells expressing GTP-bound active Rac1. We show that active Rac1 promotes sustained S70pBcl2 levels in cancer cells via physical interaction with Bcl-2, thereby permitting Rac1-induced production of superoxide (O2–) in close proximity to inactivate PP2A holoenzyme assembly and specific activity. Corroborating this, pharmacological inhibition of the interaction with BH3 mimetics such as HA14-1 or ABT199 as well as scavengers of O2– such as Tiron reduced S70pBcl2. Importantly, the interaction between Rac1 and Bcl-2 to promote S70pBcl2 as well as the oxidative inhibition of PP2A are re-capitulated in primary cells derived from lymphoma patients. Collectively, our findings demonstrate a novel redox mechanism of active Rac1 in sustaining S70pBcl2 in cancer cells.