Liver X receptors inhibit human monocyte-derived macrophage foam cell formation by inhibiting fluid-phase pinocytosis of LDL

Abstract

Liver X receptors (LXRs) are ligand-activated transcription factors involved in the control of lipid metabolism and inflammation. Several studies have recently shown that LXRs promote reverse cholesterol transport and inhibit atherosclerosis. Our study investigated whether LXRs affect macrophage uptake of LDL by human monocyte-derived macrophages. We have previously shown that human monocytes differentiated into macrophages with macrophage-colony-stimulating factor (M-CSF) constitutively take up large amounts of native LDL by receptor-independent, fluid-phase pinocytosis. In the research reported here, human monocytes were differentiated to macrophages in the presence of M-CSF with or without the LXR agonists T0901317 or 22(R)-hydroxycholesterol. Then, macrophages were incubated with native (125)I-LDL to determine LDL uptake. T0901317 and 22(R)-hydroxycholesterol inhibited (125)I-LDL uptake by 68 +/- 1% and 69 +/- 2%, respectively, and decreased pinocytotic vacuoles in the macrophages. (125)I-BSA uptake, a measure of fluid-phase pinocytosis, and (125)I-LDL uptake were the same, and T0901317 treatment inhibited uptake of both to the same degree. T0901317 did not affect receptor-mediated uptake of acetylated LDL, showing that the LXR effect is specific for fluid-phase pinocytosis of lipoproteins. Our results show that LXRs downregulate macrophage pinocytosis of LDL. The findings reveal an additional new mechanism by which LXR agonists may inhibit macrophage cholesterol accumulation and atherosclerosis, namely, by inhibiting macrophage uptake of LDL.