EFFECT OF PREGNANE X RECEPTOR (PXR) AND CONSTITUTIVE ANDROSTANE RECEPTOR (CAR) ON CYTOCHROME P3A ENZYMES IN AN EXPERIMENTAL MODEL OF CHOLESTASIS

Background and Aims: PXR and CAR are nuclear receptors playing an important role in the homeostasis of bilirubin and bile acids. In cholestasis, CAR and PXR up-regulate the alternative hepatic export routes and induce the synthesis of detoxification enzymes including the cytochrome P450–3A (CYP3A); however, the cellular mechanism(s) underlying these phenomena have not been completely clarified. The aim of this study was to correlate the expression of PXR, CAR and CYP3A with the degree of cholestasis. Methods: Cholestasis, obtained by bile duct ligation, was induced in 5Wistar rats; 10 sham-operated rats were used as controls. Severity of cholestasis was defined on the basis of histological examination of liver sections, and serum concentration of albumin, ALT, GGT, bilirubin. PXR, CAR, CYP3A protein expression was measured by Western blot at baseline, 4 and 8 weeks after ligation. Kinetic parameters of CYP3A activity were measured using 6b-testosteronehydroxylation. Results: PXR expression significantly increased in rats with severe cholestasis, whereas CAR level significantly decreased in the same rats. A significant 2- and 6-fold increase was observed in CYP3A expression at 4 and 8 weeks after ligation, respectively. A significant increase in Vmax of 6b-testosterone-hydroxylation was observed in severe cholestasis (p < 0.05), but not in mild cholestasis (994±263, 1370±296, 1661±502 pmol/mg/min at baseline, at 4 and 8 weeks respectively). Conclusions: PXR and CYP3A are up-regulated in this experimental model of cholestasis, whereas CAR is down-regulated. Since alterations of detoxification routes play a pivotal role in cholestatic liver injury, the regulation of hepatic enzymes by nuclear receptors may be targeted therapeutically.