Abstract

Human UDP-glucuronosyltransferase (UGT) 2B7 is the major isoform catalyzing the glucuronidation of a variety of endogenous compounds including bile acids. To determine the role of bile acids in the regulation of UGT2B7 expression, Caco-2 cells were incubated with the natural human farnesoid X receptor (hFXR) ligand, chenodeoxycholic acid, as well as the secondary bile acid, lithocholic acid, derived from chenodeoxycholic acid. Incubation of Caco-2 cells with lithocholic acid in the absence of exogenous hFXR resulted in a dose-dependent down-regulation of UGT2B7 mRNA levels, with an IC₅₀ of 13 μM. Similar down-regulation was also observed with chenodeoxycholic acid; however, much higher concentrations were required. Transient transfection of Caco-2 cells with hFXR suppressed UGT2B7 mRNA expression both in the absence and presence of ligand. UGT2B7 promoter transfection experiments and deletion/mutation analysis showed that lithocholic acid-activated hFXR decreased UGT2B7 promoter activity via a negative hFXR response element (NFRE) located between nucleotides –148 and –134. Cotransfection with hFXR and/or human retinoid X receptor further enhanced the repression. Electrophoretic mobility shift assays additionally confirmed the role of NFRE in UGT2B7 down-regulation by lithocholic acid. These findings suggest that lithocholic acid, an activator of nuclear hFXR, acts as a negative regulator of UGT2B7 expression, indicating that hFXR may play an essential role in lithocholic acid homeostasis through negative regulation of this UGT that is involved in lithocholic acid biotransformation. Therefore, it is postulated that lithocholic acid toxicity may be due to down-regulation of genes involved in its detoxification, including UGT2B7, leading to limited excretion of lithocholic acid from the body.