LXRα regulates human CETP expression in vitro and in transgenic mice

Liver X receptors (LXRs), LXRα and LXRβ, are members of the nuclear receptor superfamily and regulate the expression of genes involved in the regulation of cholesterol and fatty acid metabolism. Human plasma, unlike mouse plasma, contains cholesteryl ester transfer protein (CETP), which plays an important role in reverse cholesterol transport (RCT). LXRs induce CETP transcription via a direct repeat 4 element in the CETP promoter. However, the specific roles of the individual LXR subtypes in CETP expression and their consequences on plasma lipoprotein metabolism are still unclear. Here we showed that synthetic LXR agonist enhanced plasma CETP activity and resulted in non-high density lipoprotein (non-HDL) increase and HDL decrease in cynomolgus monkeys and human CETP transgenic mice. To address the relative importance of the two LXR subtypes, we investigated the effect of the suppression of both LXR subtypes on CETP expression in HepG2 cells. CETP expression induced by the LXR agonist was significantly reduced by LXRα knock-down, but not by LXRβ. Consistent with these data, CETP promoter activity was enhanced by LXRα activation, whereas LXRβ activation had only a minor effect. Furthermore, we investigated the effect of genetic deficiency of both LXR subtypes in human CETP transgenic mice. LXRα deficiency abolished the augmentation of plasma CETP activity and hepatic CETP expression induced by the synthetic LXR agonist, consequently increasing HDL and decreasing non-HDL, whereas LXRβ deficiency did not affect CETP activation. These findings indicate that LXRα has an essential role in the regulation of CETP expression and maintaining RCT.