Ursodeoxycholic acid modulates the ubiquitin-proteasome degradation pathway of p53

p53/Mdm-2 interaction is a prime target of ursodeoxycholic acid (UDCA) for regulating apoptosis in primary rat hepatocytes. Here, we further explored the role of UDCA in downregulating p53 by Mdm-2. UDCA reduced the stability of p53 by decreasing protein half-life. Although proteasomal activity was slightly increased with UDCA, the effect was also observed for other bile acids. More importantly, immunoprecipitation assays revealed that UDCA promoted p53 ubiquitination, therefore leading to increased p53 degradation. In this regard, proteasome inhibition after UDCA pre-treatment resulted in accumulation of ubiquitinated p53, which in turn was prevented in cells overexpressing a mutated form of p53 that does not undergo Mdm-2 ubiquitination. The involvement of Mdm-2 in UDCA-mediated response was further confirmed by siRNA-mediated gene silencing experiments. Finally, the protective effect of UDCA against p53-induced apoptosis was abolished after inhibition of proteasome activity and prevention of p53 ubiquitination by Mdm-2. These findings suggest that UDCA protects cells from p53-mediated apoptosis by promoting its degradation via the Mdm-2-ubiquitin-proteasome pathway.

Research highlights
► Our results demonstrate that exposure of cells to UDCA decreased the half-life of p53, while slightly increasing proteasome activity.
► More importantly, when we chemically inhibited proteasome activity, pre-treatment with UDCA resulted in increased accumulation of ubiquitinated p53, and absence of protection against p53-induced apoptosis.
► Similarly, UDCA protection was prevented in cells overexpressing a mutated form of p53 that lacks part of the Mdm-2 binding site.
► These findings suggest that UDCA inhibits the p53 apoptotic pathway, in part, by promoting UPP degradation in the complex network of p53 regulation.