PIG3 regulates the stability of p53 via Mdm2

Abstract

The tumour suppressor p53 induces apoptosis or cell-cycle arrest in response to genotoxic and other stresses. One of the most important regulators of p53 is mouse double minute 2 (Mdm2), a RING domain E3 ligase that ubiquitinates p53, leading to both proteasomal degradation and relocation of p53 from the nucleus to the cytoplasm1. The p53-inducible gene 3 (Pig3) is a transcriptional target of p53 and is thought to play a role in apoptosis, but its other functions are not clear now. In this study, we showed that knocking down Pig3 by siRNA accelerated p53 and Mdm2 degradation, and decreased the half-life of wild-type p53 in several of the human cell lines; overexpression of PIG3 inhibited ubiquitination of exogenous p53 in a dose-dependent manner, whereas knocking down Pig3 by siRNA enhanced ubiquitination of endogenous p53. Using a coimmuno-precipitation assay, the interaction between endogenous PIG3 and Mdm2 and p53 was detected, it indicated that PIG3 binded with Mdm2 in normal condition, and released after UV treatment, but PIG3 binded with p53 after UV treatment. And furthermore, PIG3 and Mdm2 competitve binded with p53 in vitro. To confirm the function of PIG3 in the p53 system, the effect of PIG3 on p53-mediated apoptosis was examined. FACS and Western Blotting results showed that downregulation of PIG3 strongly decreased UV-induced apoptosis in HCT116. These results suggest that as the transcriptional target of p53, PIG3 regulated p53 stability via it and Mdm2 competitive binding with p53, which decreaed the ubiquitinatin of p53; and PIG3 promote apoptosis in p53-dependent manner, it is a feedback loop.