The stimulatory effect of LXRα is blocked by SHP despite the presence of a LXRα binding site in the rabbit CYP7A1 promoter

Abstract

The transcription of the cholesterol 7α-hydroxylase gene (CYP7A1) is greatly decreased in cholesterol-fed rabbits. To determine whether the molecular structure of the promoter is responsible for this downregulation, we cloned the rabbit CYP7A1 promoter, identified the binding sites for α-fetoprotein transcription factor (FTF) and liver X receptor (LXRα), and studied the effects of FTF, LXRα, and SHP on its transcription. Adding LXRα/retinoid X receptor together with their ligands (L/R) to the promoter/reporter construct transfected into HepG2 cells greatly increased its activity. FTF did not increase promoter activity, nor did it enhance the stimulatory effect of L/R. Mutating the FTF binding site abolished the promoter baseline activity. Increasing amounts of SHP abolished the effect of L/R, and FTF enhanced the ability of SHP to decrease promoter activity below baseline levels. Thus, downregulation of CYP7A1 in cholesterol-fed rabbits is attributable secondarily to the activation of farnesoid X receptor, which increases SHP expression to override the positive effects of LXRα. Although FTF is a competent factor for maintaining baseline activity, it does not further enhance and may suppress CYP7A1 transcription.