Bcl-2 phosphorylation by p38 MAPK: Identification of target sites and biologic consequences.

The antiapoptotic role of Bcl-2 can be regulated by its phos- phorylation in serine and threonine residues located in a non- structured loop that links BH3 and BH4 domains. p38 MAPK has been identified as one of the kinases able to mediate such phosphorylation, through direct interaction with Bcl-2 protein in the mitochondrial compartment. In this study, we identify, by using mass spectrometry techniques and specific anti-phos- phopeptide antibodies, Ser87 and Thr56 as the Bcl-2 residues phosphorylated by p38 MAPK and show that phosphorylation of these residues is always associated with a decrease in the antiapo- ptotic potential of Bcl-2 protein. Furthermore, we obtained evi- dence that p38 MAPK-induced Bcl-2 phosphorylation plays a key role in the early events following serum deprivation in embryonic fibroblasts. Both cytochrome c release and caspase activation trig- gered by p38 MAPK activation and Bcl-2 phosphorylation are absent in embryonic fibroblasts from p38- knock-out mice, whereas they occur within 12 h of serum with- drawal in p38--/- MEF; moreover, they can be prevented by p38 MAPK inhibitors and are not associated with the synthesis of the proapoptotic proteins Bax and Fas. Thus, Bcl-2 phosphorylation by activated p38 MAPK is a key event in the early induction of apopto- sis under conditions of cellular stress.