Enhanced production of 24S-hydroxycholesterol is not sufficient to drive liver X receptor target genes in vivo

Background: Oxysterols such as 24S-hydroxycholesterol (OHC) and 27-OHC are intermediates of Cholesterol excretion pathways. In addition, they are putative endogenous agonists of the liver X receptor (LXR) class of nuclear hormone receptors and are thought to be important mediators of cholesterol-dependent gene regulation. 24S-OHC is one of the most efficient endogenous LXR agonists known and is present in the brain and in the circulation at relatively high levels.

Objectives: To explore the regulatory importance of 24S-OHC in vivo.

Design: We developed a transgenic mouse model in which human Cholesterol 24-hydroxylase, the Enzyme responsible for the formation of 24S-OHC, was expressed under the control of a promoter derived from the β-actin gene.

Results: Both male and female transgenic mice had elevated levels of cerebral, plasma, biliary and faecal 24S-OHC. According to the faecal excretion results, production of 24S-OHC was increased four- to sevenfold. Gene expression profiling revealed that the elevated production of 24S-OHC did not result in the anticipated activation of LXR target genes in the brain or liver.

Conclusion: In spite of the fact that 24S-OHC is a highly effective agonist of LXRs in vitro, it is not a critical activator of target genes to this nuclear receptor in vivo, either in the brain or in the liver.