Protein phosphatase 2A enhances the proapoptotic function of Bax through dephosphorylation

Bax is a major proapoptotic member of the Bcl2 family that is required for apoptotic cell death. We have recently discovered that Bax phosphorylation at serine 184 induced by nicotine through activation of protein kinase AKT abolishes its proapoptotic function in human lung cancer cells. Here we found that either treatment of cells with the protein phosphatase 2A (PP2A) inhibitor okadaic acid or specific disruption of PP2A activity by expression of SV40 small tumor antigen enhanced Bax phosphorylation, whereas C(2)-ceramide, a potent PP2A activator, reduced nicotine-induced Bax phosphorylation, suggesting that PP2A may function as a physiological Bax phosphatase. PP2A co-localized and interacted with Bax. Purified, active PP2A directly dephosphorylated Bax in vitro. Overexpression of the PP2A catalytic subunit (PP2A/C) suppressed nicotine-stimulated Bax phosphorylation in association with increased apoptotic cell death. By contrast, depletion of PP2A/C by RNA interference enhanced Bax phosphorylation and prolonged cell survival. Mechanistically C(2)-ceramide-induced Bax dephosphorylation caused a conformational change by exposure of the 6A7 epitope (amino acids 13-19) that is normally hidden at its N terminus that promoted the insertion of Bax into mitochondrial membranes and formation of Bax oligomers leading to cytochrome c release and apoptosis. In addition, PP2A directly disrupted the Bcl2/Bax association to liberate Bax from the heterodimer complex. Thus, PP2A may function as a physiological Bax regulatory phosphatase that not only dephosphorylates Bax but also activates its proapoptotic function.