Bcl2 phosphorylation: a tie between cell survival, growth, and ROS

The proto-oncoprotein Bcl2 is a powerful antagonist of the mitochondrial pathway of apoptosis initiated by a variety of extra- and intracellular stresses. By localizing to the mitochondria, Bcl2 acts as a molecular barrier against the permeabilizing action on mitochondria membranes of proapoptotic Bcl-2 family members, such as Bax, Bak, and Bid, which results in the release of caspase-activating death messengers in the cytosol precipitating apoptosis. Bcl2's properties also extend to its ability to function as an antioxidant, exerting a particular buffering effect on mitochondrial reactive oxygen species (ROS) production, and to delay cell-cycle progression, particularly during entry into S phase from quiescence. Due to its multifaceted role in cell fate, it is not surprising that Bcl2 is subject to tight posttranslational control. Phosphorylation of Bcl2, in the flexible loop domain, has been identified as a major regulatory mechanism, modulating the Bcl2 cytoprotective function in response to cellular stresses as well as growth and survival factors. Previous work from Deng et al showed that Bcl2 phosphorylation by the growth-factor–stimulated extracellular signal–regulated kinase 1/2 and protein kinase C α kinases, either at the unique Ser70 residue or at multiple Thr69, Ser70, and Ser89 sites, positively regulates Bcl2 antiapoptotic function.¹  However, other investigators have shown that c-Jun NH2-terminal kinase–mediated multiple-site Bcl2 phosphorylation hinders Bcl2 survival function in paclitaxel-induced apoptosis.²  While it seems likely that the type of stimulus, the regulatory pathways involved, and the degree and duration of phosphorylation at specific Bcl2 residues produce different outcomes, many questions remain to be answered. Unclear, for instance, is whether the antiapoptotic, antioxidant, and cell-cycle inhibitory Bcl2 actions are functionally related and how phosphorylation influences them. In this issue of Blood, Deng and colleagues (page 3179) add a step forward in our understanding of this complex issue, providing evidence for a possible unifying role of Bcl2 phosphorylation in the regulation of cell survival, cell cycle, and the cellular redox state. To test the relationship between Bcl2 mono-(Ser70) or multiple-(Thr69, Ser70, Ser89) site phosphorylation, cell-cycle progression, and apoptosis, Deng and colleagues produced survival-deficient nonphosphorylatable and gain-of-function phosphomimetic Bcl2 mutants and stably expressed them in interleukin-3 (IL-3)–dependent myeloid cells. In so doing, these investigators showed that in cells expressing the phosphomimetic mutants of Bcl2, which mimic the constitutive phosphorylation of the protein, but not in the nonphosphorylatable Bcl2 mutant–expressing cells, the G1/S transition is stalled while resistance to diverse apoptotic stimuli is increased. Remarkably, these cells showed decreased levels of intracellular ROS and an increased expression of the Cdk2 inhibitor p27, implicating a link among the phosphorylation status of Bcl2, inhibition of cell growth, and the cellular redox state. This link was further strengthened by demonstrating that limited amounts of intracellular ROS, which are required as a growth-promoting signal for the G1/S transition, lead to decreased p27 expression and that both signals are suppressed by the potent antioxidant function of the phosphomimetic Bcl2 mutants, thereby retarding cell growth. The efficient antioxidant activity of the phosphorylated Bcl2 could serve as a key mechanism to slow cell growth and increase cell survival in rapidly dividing normal cells. This would provide the cells more time to monitor and possibly repair cellular damage provoked by increased ROS levels, generated by the higher mitochondrial respiration, at the critical point before the decision of whether to proceed in the cell cycle or die is taken. Looking at the other side of the coin, this finding also suggests that abnormal up-regulation of the signalling pathways leading to Bcl2 phosphorylation in cancer cells may render them less vulnerable to anticancer treatments by increasing cellular resistance to oxidative stress–induced apoptosis.FIG1