Niemann-Pick C1 (NPC1) is a key participant in cellular cholesterol trafficking. Loss of NPC1 function leads to defective suppression of SREBP-dependent gene expression and failure to appropriately activate liver X receptor-mediated (LXR-mediated) pathways, ultimately resulting in intracellular cholesterol accumulation. To determine whether NPC1 contributes to regulation of macrophage sterol homeostasis in vivo, we examined the effect of NPC1 deletion in BM-derived cells on atherosclerotic lesion development in the Ldlr -/-mouse model of atherosclerosis. High-fat diet-fed chimeric Npc1 -/-mice reconstituted with Ldlr -/-Npc1 -/-macrophages exhibited accelerated atherosclerosis despite lower serum cholesterol compared with mice reconstituted with wild-type macrophages. The discordance between the low serum lipoprotein levels and the presence of aortic atherosclerosis suggested that intrinsic alterations in macrophage sterol metabolism in the chimeric Npc1 -/-mice played a greater role in atherosclerotic lesion formation than did serum lipoprotein levels. Macrophages from chimeric Npc1 -/-mice showed decreased synthesis of 27-hydroxycholesterol (27-HC), an endogenous LXR ligand; decreased expression of LXR-regulated cholesterol transporters; and impaired cholesterol efflux. Lower 27-HC levels were associated with elevated cholesterol oxidation products in macrophages and plasma of chimeric Npc1 -/-mice and with increased oxidative stress. Our results demonstrate that NPC1 serves an atheroprotective role in mice through regulation of LXR-dependent cholesterol efflux and mitigation of cholesterol-induced oxidative stress in macrophages. IntroductionMacrophages regulate tissue lipid homeostasis through the uptake and catabolism of atherogenic plasma lipoproteins (1). Macrophages express scavenger receptor A and CD36, which bind and internalize modified lipoproteins, such as oxidized LDL. Upon internalization, lipoprotein-derived cholesterol is delivered to a late endosomal organelle, where cholesterol esters are hydrolyzed to free cholesterol. Delivery of free cholesterol from the endocytic pathway to the plasma membrane and ER requires the coordinated actions of the late endosomal Niemann-Pick C1 (NPC1) and lysosomal NPC2 proteins (2). Delivery of cholesterol to the ER rapidly stimulates esterification and accumulation of cholesterol ester in cytoplasmic droplets. Because the scavenger receptors, in contrast to the LDL receptor (LDLR), are not controlled by a sterol-regulated negative feedback loop, macrophages accumulate massive quantities of lipoprotein-derived lipids.