Objective-ABCG1 has recently been identified as a facilitator of cellular cholesterol and phospholipid efflux to high-density lipoprotein (HDL). Its expression in macrophages is induced during cholesterol uptake in macrophages and by liver X receptor (LXR). The role of macrophage ABCG1 in atherosclerotic lesion development is, however, still unknown. Methods and Results-To assess the role of macrophage ABCG1 in atherosclerosis, we generated low-density lipoprotein (LDL) receptor knockout (LDLr Ϫ/Ϫ ) mice that are selectively deficient in macrophage ABCG1 by using bone marrow transfer (ABCG1 Ϫ/Ϫ 3 LDLr Ϫ/Ϫ ). Peritoneal macrophages isolated from donor ABCG1 Ϫ/Ϫ mice exhibited a 22% (Pϭ0.0007) decrease in cholesterol efflux to HDL. To induce atherosclerosis, transplanted mice were fed a high-cholesterol diet containing 0.25% cholesterol and 15% fat for 6 and 12 weeks. Serum lipid levels and lipoprotein profiles did not differ significantly between ABCG1 Ϫ/Ϫ 3 LDLr Ϫ/Ϫ mice and controls. In lungs of ABCG1 Ϫ/Ϫ 3 LDLr Ϫ/Ϫ mice a striking accumulation of lipids was observed in macrophages localized to the subpleural region. After 6 weeks of high-cholesterol diet feeding the atherosclerotic lesion size was 49Ϯ12ϫ10 3 m 2 for ABCG1 ϩ/ϩ 3 LDLr Ϫ/Ϫ mice versus 65Ϯ15ϫ10 3 m 2 for ABCG1 Ϫ/Ϫ 3 LDLr Ϫ/Ϫ mice and after 12 weeks of high-cholesterol diet feeding 124Ϯ17ϫ10 3 m 2 for ABCG1 ϩ/ϩ 3 LDLr Ϫ/Ϫ mice versus 168Ϯ17ϫ10 3 m 2 for ABCG1 Ϫ/Ϫ 3 LDLr Ϫ/Ϫ mice. Atherosclerotic lesion size depended on both time and the macrophage ABCG1 genotype (Pϭ0.038 by 2-way ANOVA, nՆ8), indicating a moderately 33% to 36% increase in lesion formation in the absence of macrophage ABCG1. Conclusions-Macrophage ABCG1 deficiency does lead to heavy lipid accumulation in macrophages of the lung, and also a moderately significant effect on atherosclerotic lesion development was observed.

The function of scavenger receptor class B, type I (SR-BI) in the liver as a high-density lipoprotein receptor that promotes the selective uptake of cholesteryl esters is well defined. Its role in macrophages, however, is primarily unknown, because it functions in the uptake of (modified) lipoproteins as well as the secretion of cholesterol to high-density lipoproteins. In this study, the biological role of SR-BI on bone marrow-derived cells, including macrophages, in lipid metabolism and atherosclerosis was assessed by selective disruption of SR-BI in bone marrow in two established models of atherosclerosis: low-density lipoprotein (LDL) receptor-deficient mice that develop extensive atherosclerosis on a Westerntype diet and wild-type mice that develop fatty streak lesions when fed a high-cholesterol diet containing 0.5% cholate. The presence of SR-BI in bone marrowderived cells in LDLr؊/؊ mice decreased lesion development after 9 and 12 weeks of Western-type diet feeding, indicating that macrophage SR-BI protects against lesion development.

Objective-The purpose of this study was to evaluate the effect of the combined deletion of ABCA1 and ABCG1 expression in macrophages on foam cell formation and atherosclerosis. Methods and Results-LDL receptor knockout (KO) mice were transplanted with bone marrow from ABCA1/ABCG1 double KO (dKO) mice. Plasma cholesterol levels after 6 weeks of Western-type diet (WTD) feeding were significantly lower in dKO transplanted mice than ABCA1 KO, ABCG1 KO, and control transplanted animals. Extreme foam cell formation was present in macrophages of various tissues and the peritoneal cavity of dKO transplanted animals. Furthermore, severe hypoplasia of the thymus and a significant decrease in CD4-positive T cells in blood was observed. Despite relatively low plasma cholesterol levels dKO transplanted animals developed lesion sizes of 156Ϯ19ϫ10 3 m 2 after only 6 weeks of WTD feeding. Lesions, however, were smaller than single ABCA1 KO transplanted animals (226Ϯ30ϫ10 3 m 2 ; PϽ0.05) and not significantly different from single ABCG1 KO (117Ϯ22ϫ10 3 m 2 ) and WT transplanted mice (112Ϯ15ϫ10 3 m 2 ). Conclusions-Macrophage ABCA1 and ABCG1 play a crucial role in the prevention of macrophage foam cell formation, whereas combined deletion only modestly influences atherosclerosis which is associated with an attenuated increase in WTD-induced plasma cholesterol and decreased proinflammatory CD4-positive T cell counts. Key Words: ABCA1 Ⅲ ABCG1 Ⅲ atherosclerosis Ⅲ macrophage Ⅲ transplantation Ⅲ cholesterol T he transport of excess cholesterol by HDL from macrophages in the periphery back to the liver for catabolism and excretion in bile and feces, called reverse cholesterol transport (RCT), plays an important protective role in the development of atherosclerosis. 1 Several ATP-binding cassette (ABC) transporters have been implicated in macrophage lipid metabolism, RCT, and atherosclerosis. 2,3 The fulltransporter ABCA1 is highly induced in lipid-laden macrophages where it facilitates cellular cholesterol and phospholipid efflux to lipid-poor apoproteins like apoAI or ApoE. 4 As cholesterol efflux from macrophages results in decreased cellular lipid accumulation, macrophage ABCA1 expression has been suggested to protect against atherosclerosis. Information on the critical role of macrophage ABCA1 in lesion formation and progression was provided by bone marrow transplantation studies, using mostly LDL receptor (LDLr) KO mice as recipients with ABCA1 KO mice or ABCA1 overexpressor mice as donors leading to the anticipated induction and inhibition of lesion formation, respectively. [5][6][7] More recently, in macrophages a second ABC-transporter, ABCG1, was identified as a transporter for cholesterol from cells to HDL. 8 -10 Because, similarly to ABCA1, ABCG1 is highly induced in lipid-laden macrophages and able to facilitate efflux of cholesterol from macrophages, 8,9 it was anticipated that ABCG1 would add to the protective function of ABCA1 in lesion formation. This hypothesis was strengthened by data from Kennedy et al who showed th...