Reduction in cytochrome P-450 enzyme expression is associated with repression of CAR (constitutive androstane receptor) and PXR (pregnane X receptor) in mouse liver during the acute phase response

Drug-activated nuclear receptors CAR and PXR

The metabolism and elimination of drugs is mainly mediated by cytochrome P450 (CYP) enzymes, aided by conjugative enzymes and transport proteins. An integral aspect of this elimination process is the induction of drug metabolism through activation of gene expression of metabolic and transport proteins. There is compelling evidence that induction is regulated by drug-activated nuclear receptors constitutive androstane receptor (CAR) and pregnane X receptor (PXR). This review outlines the basic properties of CAR and PXR, their ligands and target genes, and the mechanisms of the induction process. The implications of nuclear receptor-mediated induction for drug research are also discussed.

Sick euthyroid syndrome is associated with decreased TR expression and DNA binding in mouse liver

Infection is associated with low serum thyroid hormones and thyrotropin levels. Here we demonstrate that infection also reduces thyroid hormone receptor (TR) expression. In gel shift experiments, retinoid X receptor (RXR)/TR DNA binding was reduced in mouse liver by 60 and 77%, respectively, 4 and 16 h after lipopolysaccharide (LPS) administration. Surprisingly, LPS did not decrease either TR-alpha or TR-beta protein levels at 4 h, but by 16 h TR-alpha(1), TR-alpha(2), and TR-beta levels were reduced by 55, 87, and 41%, respectively. We previously reported that LPS rapidly decreases RXR protein levels in liver. Therefore, we added RXR-beta to hepatic nuclear extracts prepared 4 h after LPS treatment, which restored RXR/TR DNA binding to a level comparable to that of controls. A similar experiment conducted on extracts prepared 16 h after LPS administration did not restore RXR/TR DNA binding. We propose that decreased RXR expression is limiting for RXR/TR DNA binding at 4 h, whereas the reduction in both TR and RXR levels results in further decreased binding at 16 h.