Akt Regulation of Mdm2-p53 Signaling in Cellular Stress Responses and Tumorigenesis

Abstract

In cells undergoing stress, the p53 transcription factor is stabilized and activates the expression of numerous genes contributing to p53-mediated tumor suppression. One p53 target gene is Mdm2, which encodes an oncoprotein that binds and ubiquitinates p53 for proteasomal degradation, thus limiting the amplitude and duration of the p53-mediated stress response. Our lab recently discovered that Mdm2 phosphorylation by ATM and c-Abl regulates the DNA damage response and tumorigenesis in mice. AKT has also been found in transfection studies to phosphorylate Mdm2 at serine residues 166 and 186 (mouse S163 and S183) to alter p53 activity. However, the physiological significance of Mdm2 phosphorylation by Akt remains unknown. Therefore, I generated Mdm2S163A or Mdm2S183A mice expressing mutant Mdm2 incapable of being phosphorylated by Akt. In contrast with our previous studies, Akt phosphorylation of Mdm2 does not alter spontaneous tumorigenesis or the DNA damage response to ionizing radiation. However, Akt phosphorylation of Mdm2-S183 (but not -S163) upregulates nuclear localization of Mdm2, destabilizes p53, and reduces p53-mediated senescence in response to elevated levels of reactive oxygen species (ROS). To examine the effects of Mdm2-S183 phosphorylation on p53 tumor suppression, I utilized three different mouse models of ROS-induced cancer. Increased levels of p53 and senescence in Mdm2S183A mice yielded reduced tumorigenesis in an activated Ras model of lung cancer, a phorbal ester-induced skin cancer model, and a diethylnitrosamine-induced model of hepatocellular carcinoma. Since AKT is also important regulator of cell metabolism, I explored the impact of the Mdm2-S183 allele on metabolic functions. Mdm2 phosphorylation by Akt reduced glucose metabolism via glycolysis in vitro, and reduced insulin tolerance in mice, without altering glucose tolerance and glucose-stimulated insulin secretion. Collectively, these findings document a unique physiologic role for the AKT-Mdm2-p53 signaling axis in regulating cell growth and tumorigenesis.