Regulation of BAD phosphorylation at serine 112 by the Ras-mitogen-activated protein kinase pathway

The function of the pro-apoptotic molecule BAD is regulated by phosphorylat ion of two sites, serine-112 (Ser-112) and serine-136 (Ser-136), Phosphoryl ation at either site results in loss of the ability of BAD to heterodimeriz e with the survival proteins BCL-X-L or BCL-2, Phosphorylated BAD binds to 14-3-3 and is sequestered in the cytoplasm, It has been shown that phosphor ylation of BAD at Ser-136 is mediated by the serine/threonine protein kinas e Akt-1/PKB which is downstream of phosphatidylinositol 3-kinase (PI3K), Th e signaling process leading to phosphorylation of BAD at Ser-112 has not be en identified. In this study, we show that phosphorylation of the two serin e residues of BAD is differentially regulated. While Ser-136 phosphorylatio n is concordant with activation of Akt, Ser-112. phosphorylation does not c orrelate with Akt activation. Instead, we demonstrate that activated Ras an d Raf, which are upstream of mitogen-activated protein kinases (MAPK), stim ulate selective phosphorylation of BAD at Ser-112, Furthermore, phosphoryla tion of Ser-112, but not Ser-136 requires activation of the MAPK pathway as the MEK inhibitor, PD 98059, blocks EGF-, as web as activated Ras- or Raf- mediated phosphorylation of BAD at Ser-112, Therefore, the PI3K-Akt and Ras -MAPK pathways converge at BAD by mediating phosphorylation of distinct ser ine residues.