Functional Studies Of A Novel P53 Negative Regulatory Molecules Pact

The p53 tumor suppressor is a transcriptional factor that can induce either growth arrest or apoptosis and is frequently mutated or deleted in more than 50% of human cancers. However, the steady-state levels and transcriptional activity of p53 increase dramatically in cells that sustain various types of stress. In addition, many tumors retaining wide-type p53 often have defects in activating or responding to p53. Tight regulation of p53 is essential for maintaining normal cell growth and this occurs primarily through post-translation modifications of p53. PACT (also known as P2P-R, RBBP6) is a RING finger-containing protein that can bind p53 in vitro and in vivo. Here, we demonstrated that PACT can inhibit the accumulation and promote the degradation of p53. Endogenous PACT can also interact with Hdm2 and promotes Hdm2-mediated ubiquitination of p53 due to enhancement of the assembly of the p53-Hdm2 complex. Depletion of endogenous PACT by RNAi results in p53 accumulation due to a reduction of p53 ubiquitination in vivo and induces apoptosis. Furthermore, disruption of PACT in mice leads to early embryonic lethality before 7.5 dpc accompanied by widespread apoptosis. More interestingly, introduction of a p53 null mutation into PACT^-/- embryos partially rescues the lethality phenotype and prolonges survival until E10.5. Taken together, these findings identify PACT as a potential cofactor for Hdm2 in the negative regulation of p53 and suggest a possible role for PACT in development and tumorigenesis.