PIG3 Regulates p53 Stability by Suppressing Its MDM2-Mediated Ubiquitination

Under normal, non-stressed conditions, intracellular p53 is continually ubiquitinated by MDM2 and targeted for degradation. However, in response to severe genotoxic stress, p53 protein levels are markedly increased and apoptotic cell death is triggered. Inhibiting the ubiquitination of p53 under conditions where DNA damage has occurred is therefore crucial for preventing the development of cancer, because if cells with severely damaged genomes are not removed from the population, uncontrolled growth can result. However, questions remain about the cellular mechanisms underlying the regulation of p53 stability. In this study, we show that p53-inducible gene 3 (PIG3), which is a transcriptional target of p53, regulates p53 stability. Overexpression of PIG3 stabilized both endogenous and transfected wild-type p53, whereas a knockdown of PIG3 lead to a reduction in both endogenous and UV-induced p53 levels in p53-proficient human cancer cells. Using both in vivo and in vitro ubiquitination assays, we found that PIG3 suppressed both ubiquitination- and MDM2-dependent proteasomal degradation of p53. Notably, we demonstrate that PIG3 interacts directly with MDM2 and promoted MDM2 ubiquitination. Moreover, elimination of endogenous PIG3 in p53-proficient HCT116 cells decreased p53 phosphorylation in response to UV irradiation. These results suggest an important role for PIG3 in regulating intracellular p53 levels through the inhibition of p53 ubiquitination.

The oncogenic effects of p53-inducible gene 3 (PIG3) in colon cancer cells

The p53-inducible gene 3 (PIG3), initially identified as a gene downstream of p53, plays an important role in the apoptotic process triggered by p53-mediated reactive oxygen species (ROS) production. Recently, several studies have suggested that PIG3 may play a role in various types of cancer. However, the functional significance of PIG3 in cancer remains unclear. Here, we found that PIG3 was highly expressed in human colon cancer cell lines compared to normal colonderived fibroblasts. Therefore, we attempted to elucidate the functional role of PIG3 in colon cancer. PIG3 overexpression increases the colony formation, migration and invasion ability of HCT116 colon cancer cells. Conversely, these tumorigenic abilities were significantly decreased in in vitro studies with PIG3 knockdown HCT116 cells. PIG3 knockdown also attenuated the growth of mouse xenograft tumors. These results demonstrate that PIG3 is associated with the tumorigenic potential of cancer cells, both in vitro and in vivo, and could play a key oncogenic role in colon cancer.

Dose-dependent pattern of inducible mRNA expression of PIG3 gene in normal human lymphoblastoid cells by 60Co γ-rays / 中华放射医学与防护杂志

Objective To investigate the dose-response pattern on the inducible expression of PIG3 mRNA in normal human lymphoblastoid AHHI cells by 60Co γ-rays,and its possibility for developing novel radiation biodosimeter.Methods Laser confocal fluorescent microscopy was used to detect the γ-H2AX foci,a biomarker of DNA double-strand break.After irradiation with 0,1,2,4,6,8 and 10 Gy of 60Coγ- rays,AHH-1 cells were harvested at 4,10 and 24 h post-irradiation,and subjected to total RNA extraction and detection of PIG3 mRNA expression by quantitative real-time PCR.Results PIG3 protein foci were formed in the nuclei at 30 min after irradiation,and a part of these PIG3 foci were colocalized with γH2AX foci.After irradiation,PIG3 mRNA level was enhanced with the increased time of postirradiation,and remained an increased level at least till 24 h.The radiation-inducible expression of PIG3 mRNA was demonstrated in a dose-dependent manner.The dose-dependent range at 4 h post-irradiation was 0 - 6 Gy,and at 10 h and 24 h was 0 - 10 Gy.Conclusions PIG3 involves in the cellular response to DNA double-strand break.The dose-dependent inducible response of PIG3 mRNA expression might provide a valuable candidate for developing a novel radiation biodosimeter.