RNF125 is a Ubiquitin-Protein Ligase that Promotes p53 Degradation

Background/Aims: Although early studies show that Mdm2 is the primary E3 ubiquitin ligase for the p53 tumor suppressor, an increasing amount of data suggests that p53 ubiquitination and degradation are more complex than once thought. Here, we investigated the role of RNF125, a non-Mdm2 ubiquitin-protein ligase, in the regulation of p53. Methods and Results: RNF125 physically interacted with p53 in exogenous/endogenous co-immunoprecipitation (IP) and GST-pull down assay, and a C72/75A mutation of RNF125 did not interfere with this interaction. Expression of RNF125 decreased the level of p53 in a dose-dependent manner, whereas knockdown of RNF125 by RNA interference increased the level of p53. As shown by Western blotting and ubiquitin assay, RNF125 ubiquitinated p53 and targeted it for proteasome degradation. Furthermore, RNF125 repressed p53 functions including p53-dependent transactivation and growth inhibition. Conclusion: Our data suggest that RNF125 negatively regulates p53 function through physical interaction and ubiquitin-mediated proteasome degradation

PSMA7 Directly Interacts with NOD1 and Regulates its Function

Background/Aims: Recent reports showed that proteasome subunit alpha type-7 (PSMA7) was overexpressed in colorectal cancer. To investigate the mechanism of PSMA7 in promotion of colorectal cancer, we screened for its interaction partners. Methods and Results: This study found that PSMA7 associated with nucleotide-binding oligomerization domain-containing protein 1 (NOD1) by yeast two-hybrid screening, co-immunoprecipitation (IP), and GST-pull down assay. As shown by Western blotting and ubiquitin assay, PSMA7 downregulated the expression of NOD1 in a proteasome-dependent manner. Overexpression of PSMA7 in HCT116 cells resulted in an inhibition of NOD1-mediated apoptosis and NF-κB activation, whereas knockdown of PSMA7 by RNA interference enhanced NOD1 activity. Conclusion: Our data suggest that PSMA7 is a negative regulator of the NOD1, and may promote tumor growth by its inhibitory role on NOD1