Pregnane X Receptor SUMOylation and De-SUMOylation

Abstract

The pregnane x receptor (PXR, NR1I2) is a member of the nuclear receptor (NR) superfamily of ligand-activated transcription factors. PXR is activated by numerous lipophilic compounds, a variety of drugs and drug metabolites in clinical use. It regulates xenobiotic-inducible cytochrome P450 (CYP) expression in the liver and intestine which are major organs for xenobiotic biotransformation. While PXR has been identified as the positive regulator of many drug metabolizing enzymes (DMEs) and membrane transporter proteins, the effect of certain PXR activators is to repress the inflammatory response. Although it has been known for 40 years now that the PXR activator rifampicin inhibits immunological responses in liver cells, the mechanisms remain poorly understood. Previous results indicate that modification of PXR by small ubiquitin-related modifier (SUMO) likely contributes to this phenomenon. We hypothesize that PXR is SUMOylated to transrepress the inflammatory response genes in liver. Here, this thesis examines PXR SUMOylation/de-SUMOylation reaction mechanisms using in vitro and cell-based methods. Bacterial expression and purification systems were used in conjunction with in vitro SUMOylation reactions to further analyze PXR SUMOylation by both SUMO-1 and SUMO-3. The data reveal that the E3 SUMO-protein ligase protein inhibitor of activated STAT protein y (PIASy) potentiated SUMOylation of the PXR ligand binding domain (PXR-LBD) in vitro. Cell-based methods were used to characterize the de-SUMOylation of PXR using expression vectors encoding six different sentrin protease (SENP) enzymes including SENP1, SENP2, SENP3, SENP5, SENP6, and SENP7. Both SENP1 and SENP2 effectively de-conjugated both SUMO-1 and SUMO-3 from PXR, whereas SENP6 inhibited the formation of SUMO-3 chains on PXR. Collectively, the data suggest that (1) PXR can serve as a substrate for either SUMO-1 or SUMO-3 in vitro and in cell-based assays, (2) both SENP1 and SENP2 are able to remove SUMO moieties in a cellular environment, and (3) that SENP6 removes or prevents the formation of SUMO-chains on PXR. Taken together, the work presented in this thesis contributes to understanding the interface between PXR activators, the SUMOylation pathway and PXR activity. Future efforts should seek to determine the extent to which the biochemical mechanisms described here function in human liver and intestine in patients undergoing therapy with PXR activators. These studies are deemed critical for safe pharmacological strategies for addressing adverse drug reactions and provide a new paradigm for exploring novel approaches to repress inflammatory signaling in liver and intestine.