UBIQUITINATION AND SUMOYLATION OF PREGNANE X RECEPTOR

Abstract

Abstract Pregnane X receptor (PXR, NR1I2) is a ligand-activated nuclear receptor (NR) superfamily member expressed at high levels in the liver and intestine of mammals. PXR can be activated by a broad range of structurally diverse xenobiotics and endobiotics. As a key regulator of xenobiotic metabolism and clearance, activated PXR up-regulates the expression of genes encoding phase I (oxidation) and phase II (conjugation) metabolizing enzymes and phase III transporters to increase the metabolism and clearance of drugs and xenobiotics from the body, thus protecting the body from potential toxic insults. Besides xenobiotic metabolism and clearance, activation of PXR also involves in the regulation of many other important biochemical pathways, like inflammation and bile acid homeostasis. While ligand-binding is the primary mechanism for NRs activation, recent research indicates that post-translational modifications of NRs also help to determine their activities under different physiological conditions and represent new modes of regulation for NRs. Studies on post-translational modifications of PXR have just begun to emerge, how post-translational modifications regulate PXR activity is not well-understood. This dissertation focuses on ubiquitination and SUMOylation of PXR. These post-translational modifications of PXR were characterized and their effects on PXR activities were studied in both primary cultures of hepatocytes and immortalized cell lines. Data presented here indicate that PXR is a target of the ubiquitin proteasome system, and inhibition of proteasome activity decreases the transactivation of PXR. The E3s and SENPs (Sentrin-specific Protease) that regulate PXR SUMOylation and de-SUMOylation are identified. Utilizing the newly identified SENPs, SUMOylation is further confirmed to be indispensable for PXR to repress inflammatory response. Furthermore, the crosstalk between ubiquitination and SUMOylation at the level of PXR is explored. Our data indicate that SUMOylation increases the presence of ubiquitinated PXR, and many other substrates of ubiquitin. Taken together, this dissertation contributes to the understanding of post-translational modifications of PXR and their regulatory effects on drug metabolism and inflammation, which is expected to produce new opportunities for the development of novel and safe therapeutic strategies.