Akt enhances Mdm2-mediated ubiquitination and degradation of p53

p53 plays a key role in DNA damage-induced apoptosis. Recent studies have reported that the phosphatidylinositol 3-OH-kinase-A-kt pathway inhibits p53-mediated transcription and apoptosis, although the underlying mechanisms have yet to be determined. Mdm2, a ubiquitin ligase for p53, plays a central role in regulation of the stability of p53 and serves as a good substrate for Akt. In this study, we find that expression of Akt reduces the protein levels of p53, at least in part by enhancing the degradation of p53. Both Akt expression and serum treatment induced phosphorylation of Mdm2 at Ser(186). Akt-mediated phosphorylation of Mdm2 at Ser(186) had little effect on the subcellular localization of Mdm2. However, both Akt expression and serum treatment increased Mdm2 ubiquitination of p53. The serum-induced increase in p53 ubiquitination was blocked by LY294002, a phosphatidylinositol 3-OH-kinase inhibitor. Moreover, when Ser(186) was replaced by Ala, Mdm2 became resistant to Akt enhancement of p53 ubiquitination and degradation. Collectively, these results suggest that Akt enhances the ubiquitination-promoting function of Mdm2 by phosphorylation of Ser(186), which results in reduction of p53 protein. This study may shed light on the mechanisms by which Akt promotes survival, proliferation, and tumorigenesis.