Liver X receptor and retinoic X receptor mediated ABCA1 regulation and cholesterol efflux in macrophage cells-messenger RNA measured by branched DNA technology

ABCA1 is an ATP binding cassette transporter that plays an essential role in cholesterol and phospholipid efflux and HDL biogenesis. ABCA1 expression in macrophage cells is subject to regulation by cAMP, cholesterol loading, and ligands of the nuclear receptors liver X receptor (LXR) and retinoid X receptor (RXR). We report here the development of a rapid and high volume branched DNA (bDNA) method to measure ABCA1 mRNA. By using the bDNA method, we show that both LXR and RXR ligands effectively regulate ABCA1 expression in three macrophage cell types: mouse RAW264.7 cell line, mouse peritoneal macrophage cells, and human macrophage THP-1 cells and their regulation is additive. Furthermore, by using a radiolabeled cholesterol efflux assay, we show that both LXR and RXR ligands are sufficient to mediate cholesterol efflux in macrophage cells and their efficacy correlates with ABCA1 regulation. These studies strengthen further the notion that LXR and RXR mediate ABCA1 expression and cholesterol efflux in macrophage cells as a permissive heterodimer and development of small molecule ligands of these nuclear receptors may represent a promising approach to modulating cholesterol efflux and plasma HDL cholesterol level in humans.