BACKGROUND High-density lipoprotein (HDL) may provide cardiovascular protection by promoting reverse cholesterol transport from macrophages. We hypothesized that the capacity of HDL to accept cholesterol from macrophages would serve as a predictor of atherosclerotic burden. METHODS We measured cholesterol efflux capacity in 203 healthy volunteers who underwent assessment of carotid artery intima–media thickness, 442 patients with angiographically confirmed coronary artery disease, and 351 patients without such angiographically confirmed disease. We quantified efflux capacity by using a validated ex vivo system that involved incubation of macrophages with apolipoprotein B–depleted serum from the study participants. RESULTS The levels of HDL cholesterol and apolipoprotein A-I were significant determinants of cholesterol efflux capacity but accounted for less than 40% of the observed variation. An inverse relationship was noted between efflux capacity and carotid intima–media thickness both before and after adjustment for the HDL cholesterol level. Furthermore, efflux capacity was a strong inverse predictor of coronary disease status (adjusted odds ratio for coronary disease per 1-SD increase in efflux capacity, 0.70; 95% confidence interval [CI], 0.59 to 0.83; P<0.001). This relationship was attenuated, but remained significant, after additional adjustment for the HDL cholesterol level (odds ratio per 1-SD increase, 0.75; 95% CI, 0.63 to 0.90; P = 0.002) or apolipoprotein A-I level (odds ratio per 1-SD increase, 0.74; 95% CI, 0.61 to 0.89; P = 0.002). Additional studies showed enhanced efflux capacity in patients with the metabolic syndrome and low HDL cholesterol levels who were treated with pioglitazone, but not in patients with hypercholesterolemia who were treated with statins. CONCLUSIONS Cholesterol efflux capacity from macrophages, a metric of HDL function, has a strong inverse association with both carotid intima–media thickness and the likelihood of angiographic coronary artery disease, independently of the HDL cholesterol level. (Funded by the National Heart, Lung, and Blood Institute and others.)

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Scavenger receptor BI (SR-BIThe levels of plasma high density lipoproteins (HDL) 1 are inversely related to the incidence of atherosclerosis and coronary artery disease (1, 2). The protective effect of HDL is thought to involve the reverse transport of cholesterol from cells in the arterial wall to the liver for disposal (3, 4). The transfer of cholesterol from cells to HDL may result from aqueous diffusion (5, 6) and/or the interaction between a cell surface receptor and HDL (7). A number of HDL-binding proteins have been described (5, 7) but none has been shown to be an authentic HDL receptor mediating cholesterol efflux. Recently a member of the scavenger receptor family, scavenger receptor type B class I (SR-BI), was shown to bind HDL with high affinity and to mediate the selective cellular uptake of HDL cholesteryl ester (CE) (8). SR-BI is highly expressed in steroidogenic tissues and the liver (8 -11), and in vivo evidence suggests that SR-BI expression is under feedback regulation (10). While these results show that SR-BI is an HDL receptor that is likely to provide sterol for steroidogenesis, the exact role of SR-BI in the regulation of HDL metabolism and the maintenance of general cholesterol homeostasis is unknown.In the present study we used SR-BI-transfected cells to evaluate a possible role of SR-BI in HDL-mediated cellular cholesterol efflux. We also sought to establish a relationship between cholesterol efflux and the level of SR-BI expression in a variety of cell types. The results, together with our finding that SR-BI mRNA is expressed in the thickened intima of atheromatous aorta, suggest a potentially important role of SR-BI in the initial steps of cholesterol efflux in the arterial wall.

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Macrophage ATP-binding cassette transporter A1 (ABCA1), scavenger receptor class B type I (SR-BI), and ABCG1 have been shown to promote cholesterol efflux to extracellular acceptors in vitro and influence atherosclerosis in mice, but their roles in mediating reverse cholesterol transport (RCT) from macrophages in vivo are unknown. Using an assay of macrophage RCT in mice, we found that primary macrophages lacking ABCA1 had a significant reduction in macrophage RCT in vivo, demonstrating the importance of ABCA1 in promoting macrophage RCT, however substantial residual RCT exists in the absence of macrophage ABCA1. Using primary macrophages deficient in SR-BI expression, we found that macrophage SR-BI, which was shown to promote cholesterol efflux in vitro, does not contribute to macrophage RCT in vivo. To investigate whether macrophage ABCG1 is involved in macrophage RCT in vivo, we used ABCG1-overexpressing, -knockdown, and -knockout macrophages. We show that increased macrophage ABCG1 expression significantly promoted while knockdown or knockout of macrophage ABCG1 expression significantly reduced macrophage RCT in vivo. Finally, we show that there was a greater decrease in macrophage RCT from cells where both ABCA1 and ABCG1 expression were knocked down than from ABCG1-knockdown cells. These results demonstrate that ABCA1 and ABCG1, but not SR-BI, promote macrophage RCT in vivo and are additive in their effects.

Reverse cholesterol transport (RCT) is a term used to describe the efflux of excess cellular cholesterol from peripheral tissues and its return to the liver for excretion in the bile and ultimately the feces. It is believed to be a critical mechanism by which HDL exert a protective effect on the development of atherosclerosis. In this paradigm, cholesterol is effluxed from arterial macrophages to extracellular HDLbased acceptors through the action of transporters such as ABCA1 and ABCG1. After efflux to HDL, cholesterol may be esterified in the plasma by the enzyme lecithin:cholesterol acyltransferase and is ultimately transported from HDL to the liver, either directly via the scavenger receptor BI or after transfer to apolipoprotein B-containing lipoproteins by the cholesteryl ester transfer protein. Methods for assessing the integrated rate of macrophage RCT in animals have provided insights into the molecular regulation of the process and suggest that the dynamic rate of macrophage RCT is more strongly associated with atherosclerosis than the steady-state plasma concentration of HDL cholesterol. Promotion of macrophage RCT is a potential therapeutic approach to preventing or regressing atherosclerotic vascular disease, but robust measures of RCT in humans will be needed in order to confidently advance RCT-promoting therapies in clinical development.-

Abstract-The removal of excess free cholesterol from cells by HDL or its apolipoproteins is important for maintaining cellular cholesterol homeostasis. This process is most likely compromised in the atherosclerotic lesion because the development of atherosclerosis is associated with low HDL cholesterol. Multiple mechanisms for efflux of cell cholesterol exist. Efflux of free cholesterol via aqueous diffusion occurs with all cell types but is inefficient. Efflux of cholesterol is accelerated when scavenger receptor class-B type I (SR-BI) is present in the cell plasma membrane. Both diffusion-mediated and SR-BI-mediated efflux occur to phospholipid-containing acceptors (ie, HDL and lipidated apolipoproteins); in both cases, the flux of cholesterol is bidirectional, with the direction of net flux depending on the cholesterol gradient. The ATP-binding cassette transporter AI (ABCA1) mediates efflux of both cellular cholesterol and phospholipid. In contrast to SR-BI-mediated flux, efflux via ABCA1 is unidirectional, occurring to lipid-poor apolipoproteins. The relative importance of the SR-BI and ABCA1 efflux pathways in preventing the development of atherosclerotic plaque is not known but will depend on the expression levels of the two proteins and on the type of cholesterol acceptors available. Key Words: cholesterol efflux Ⅲ scavenger receptor class-BI Ⅲ ATP-binding cassette transporter AI Ⅲ reverse cholesterol transport H DL levels are inversely correlated with the incidence of coronary artery disease. 1-4 A long-standing hypothesis to explain this protective effect of HDL against atherosclerosis is the process of reverse cholesterol transport (RCT). 5 In RCT, HDL or its apolipoproteins mediate the removal of excess free cholesterol (FC) from peripheral cells and, after a series of reactions in plasma, the cholesterol is delivered via either LDL or HDL to the liver for excretion into the bile. The flux of FC between cells and extracellular acceptors is important at two points in the RCT pathway: (1) the removal of FC from peripheral cells and (2) the delivery of HDL FC to the liver. There are 3 known mechanisms of FC flux: (1) aqueous diffusion, (2) SR-BI-mediated FC flux, and (3) ABCA1-mediated efflux (Figure 1). The purpose of this review is to discuss each mechanism and the relative importance of each mechanism to RCT. Aqueous DiffusionCholesterol molecules are sufficiently water-soluble to be able to transfer from either model 6 or cell membranes 7 to an acceptor by the so-called aqueous diffusion mechanism. 8 This process involves desorption of cholesterol molecules from the donor lipid-water interface and diffusion of these molecules through the intervening aqueous phase until they collide with and are absorbed by an acceptor. At a constant donor particle concentration, there is a hyperbolic dependence of cholesterol transfer rate on the concentration of acceptor particles; the kinetics can be described in terms of the rate constants for At lower acceptor concentrations, the transfer rate is dependent on th...

Atherosclerosis regression is an important clinical goal. In previous studies of regression in mice, the rapid loss of plaque foam cells was explained by emigration to lymph nodes, a process reminiscent of dendritic cells. In the present study, plaque-containing arterial segments from apoE ؊/؊ mice were transplanted into WT recipient normolipidemic mice or apoE ؊/؊ mice. Three days after transplant, in the WT regression environment, plaque size decreased by Ϸ40%, and foam cell content by Ϸ75%. In contrast, both parameters increased in apoE ؊/؊ recipients. Foam cells were isolated by laser capture microdissection. In WT recipients, there were 3-to 6-fold increases in foam cells of mRNA for liver X receptor ␣ and cholesterol efflux factors ABCA1 and SR-BI. Although liver X receptor ␣ was induced, there was no detectable expression of its putative activator, peroxisome proliferator-activated receptor ␥. Expression levels of VCAM or MCP-1 were reduced to 25% of levels in pretransplant or apoE ؊/؊ recipient samples, but there was induction at the mRNA and protein levels of chemokine receptor CCR7, an essential factor for dendritic cell migration. Remarkably, when CCR7 function was abrogated in vivo by treatment of WT recipients with antibodies to CCR7 ligands CCL19 and CCL21, lesion size and foam cell content were substantially preserved. In summary, in foam cells during atherosclerosis regression, there is induction of CCR7 and a requirement for its function. Taken with the other gene expression data, these results in vivo point to complex relationships among the immune system, nuclear hormone receptors, and inflammation during regression.cholesterol efflux ͉ dendritic cell ͉ macrophage ͉ monocyte ͉ nuclear hormone receptor M ouse models of atherosclerosis regression are relatively few.However, similarities between atherosclerosis in humans and in mice deficient in apolipoprotein E (apoE Ϫ/Ϫ ) or the LDL receptor (e.g., see ref. 1) suggest that molecular mechanisms underlying regression in mouse models could be relevant to the reduction of the large plaque burden in the middle-aged and older human population.We have previously described a mouse model of regression. First, plaques were allowed to develop in apoE Ϫ/Ϫ mice, then a segment of either thoracic aortic (2) or aortic arch (3, 4) was transplanted into the abdominal aorta of a WT recipient, quickly changing the plasma lipid environment from hyper to normolipidemia. As a control, an aortic segment was transplanted into an apoE Ϫ/Ϫ mouse. By 1 month, in the regression environment (WT recipient), essentially all of the foam cells disappeared from plaques (4), with many no longer visible after 3 days (5). In contrast, in the progression environment (apoE Ϫ/Ϫ recipient), plaque size and foam cell content increased over time.In a recent study, we showed that the rapid depletion of foam cells in the regression environment was correlated with a substantial number of these cells emigrating to lymph nodes (5). Interestingly, the emigrating cells expressed markers of den...