The Wip1 phosphatase and Mdm2: cracking the "Wip" on p53 stability

The p53 tumor suppressor is essential in maintaining genomic integrity in response to cellular stresses. In response to DNA damage, p53 is activated and stabilized largely through post-translational modifications, including phosphorylation by DNA damage responsive kinases such as ATM and ATR. Activated p53 transactivates a battery of genes that can mediate either cell cycle arrest or apoptosis. In those instances where p53 facilitates cell cycle arrest, a means to return the cell to a pre-stress state with low p53 levels is important. The E3 ubiquitin ligase Mdm2 is one p53 transcriptional target that accumulates after damage and promotes p53 ubiquitination and degradation. Thus, p53 and Mdm2 form a critical negative feedback regulatory loop that helps to maintain appropriate p53 levels in the presence or absence of stress. We propose here that Wip1 (Wildtype p53-Induced Phosphatase 1), also known as PPM1D, plays an important role in the p53-Mdm2 autoregulatory loop. We have recently shown that Wip1, also a p53 target gene, dephosphorylates Mdm2 at Ser395 (an ATM target site), resulting in stabilization of Mdm2, enhanced Mdm2-p53 binding, and enhanced ubiquitination of p53 by Mdm2. Thus, Wip1 facilitates Mdm2-mediated degradation of p53. The p53 inhibitory role of Wip1 implicates it as a potential oncogene and indeed Wip1 is amplified and overexpressed in a number of human cancers. Wip1 may inhibit p53 signaling by multiple mechanisms, but our data suggests that its largest effects are due to dephosphorylation of Mdm2.