Expression and modulation of the UGT1A family of phase II metabolism genes via liganded vitamin D receptor

Abstract

Phase two metabolic genes are primary targets for a range of nuclear receptors within the body. These include but are not limited to PXR, FXR, and LXR all of which create a complex network of crosstalk pathways between members to establish and maintain efficient sensing, resulting ultimately in the elimination of potentially harmful endogenous/exogenous toxin. The Vitamin D Receptor (VDR) is a member of the nuclear receptor super family and is activated in a ligand dependent manner. VDRs neoclassical potential has been well established in transactivation roles within phase one and three metabolic gene families such as Cytochrome P450 members, and ATP-binding Cassette transporters, however very little is understood of VDRs prospective role within phase two metabolism. Uridine 5’-diphospho-glucuronosyltransferase (UGT) are one of the major phase two metabolic gene families and are highly expressed within the liver, lower intestinal and kidney to name a few. The extra hepatic activity of UGT members’ may now have an influence on the body’s pharmacokinetic potential (Nakamura et al., 2008b). tract chief among which is UGT1A1. UGT1A1 is involved in the glucuronidation of endogenous and exogenous compounds including steroids, chemotherapeutics such as Irinotecan used to treat colon cancer and small cell lung cancer, and bile acids, generating more hydrophilic metabolites. In many cancerous disease states including liver and biliary, UGT1A expression is down regulated leading to impaired metabolic functioning (Strassburg et al., 1997). This is also relevant in genetic diseases such as Gilbert’s Syndrome and Crigler-Najjar Syndrome. VDRs role in regulating UGTs was assessed through qRT-PCR analysis, exposing LS180 (colon), and HepG2 (liver) respectively to a range of VDR agonists, using 8 known regulators of UGT as positive controls. Further investigation was undertaken to identify the VDRE within the promoter of UGT1A1 and assessing its functionality as a nuclear receptor motif through site directed mutagenesis. It was noted that the isolated VDRE also is the binding motif for PXR therefore the loss of this motif resulted in the loss of both VDR and PXR as a regulator of UGT1A1. To conclude we have shown through luciferase activity that VDR has the potential to positively regulate UGT1A1 to a comparable level of already known nuclear receptor regulator PXR. This could lead to the potential regulation of UGT1A1 through diet supplementation of vitamin D using both natural and synthetic sources, in a range of inflammatory and genetic disease states allowing for a potential therapeutic to be established.

Date of Award	Sept 2018
Original language	English
Supervisor	Paul Thompson (Supervisor) & Murtaza M Tambuwala (Supervisor)