Bcl-2 Does Not Inhibit Bax Insertion During Intrinsic Apoptosis

Abstract

Mitochondrial outer membrane (MOM) permeabilization and cytochrome c release from mitochondria into the cytosol are considered to be the commitment steps of the intrinsic apoptotic pathway. Cytochrome c release is regulated by the Bcl-2 family proteins, which contains both pro-apoptotic (e.g. BAX) and anti-apoptotic (e.g. Bcl-2) members. It is now well established that after a death signal, cytosolic BAX is translocated to the mitochondrial outer membrane, inserted then in the double leaflet and activated through a conformational change. Activated BAX oligomerizes and might be associated to other mitochondrial proteins, leading to the formation of the Mitochondrial Apoptosis-induced Channel (MAC) into the MOM. This channel allows the release of cytochrome c into the cytosol. Bcl-2 inhibits MAC formation and therefore, cytochrome c release. However, the molecular mechanisms through which Bcl-2 affects earlier steps of BAX-mediated apoptosis are not fully understood.In this study we investigated the effects of Bcl-2 over-expression on BAX-mediated apoptosis. We were able to confirm that Bcl-2 over-expression inhibits BAX translocation to the MOM and activation/oligomerization, as previously reported. Bax translocation is generally considered as the primary target of Bcl-2. Surprisingly, Bcl-2 over-expression did not alter the insertion status of BAX into the MOM. These data point out the further step, BAX activation/oligomerization, as the primary target of Bcl-2. Since Bcl-2 does not inhibit BAX insertion, we hypothesize that an event occurring after cytochrome c release triggers somehow a positive feedback on Bax expression and translocation. Thus, the inhibition of cytochrome c release by Bcl-2 could explain the further blocking on BAX translocation.Supported by NYU Research Challenge Funds to LD.