Illiana Meurs

Genetic variant of the scavenger receptor BI in humans

BACKGROUND In mice, the scavenger receptor class B type I (SR-BI) is essential for the delivery of high-density lipoprotein (HDL) cholesterol to the liver and steroidogenic organs. Paradoxically, elevated HDL cholesterol levels are associated with increased atherosclerosis in SR-BI-knockout mice. It is unclear what role SR-BI plays in human metabolism. METHODS We sequenced the gene encoding SR-BI in persons with elevated HDL cholesterol levels and identified a family with a new missense mutation (P297S). The functional effects of the P297S mutation on HDL binding, cellular cholesterol uptake and efflux, atherosclerosis, platelet function, and adrenal function were studied. RESULTS Cholesterol uptake from HDL by primary murine hepatocytes that expressed mutant SR-BI was reduced to half of that of hepatocytes expressing wild-type SR-BI. Carriers of the P297S mutation had increased HDL cholesterol levels (70.4 mg per deciliter [1.8 mmol per liter], vs. 53.4 mg per deciliter [1.4 mmol per liter] in noncarriers; P<0.001) and a reduced capacity for efflux of cholesterol from macrophages, but the carotid artery intima-media thickness was similar in carriers and in family noncarriers. Platelets from carriers had increased unesterified cholesterol content and impaired function. In carriers, adrenal steroidogenesis was attenuated, as evidenced by decreased urinary excretion of sterol metabolites, a decreased response to corticotropin stimulation, and symptoms of diminished adrenal function. CONCLUSIONS We identified a family with a functional mutation in SR-BI. The mutation carriers had increased HDL cholesterol levels and a reduction in cholesterol efflux from macrophages but no significant increase in atherosclerosis. Reduced SR-BI function was associated with altered platelet function and decreased adrenal steroidogenesis. (Funded by the European Community and others.).

Macrophage ABCG1 Deletion Disrupts Lipid Homeostasis in Alveolar Macrophages and Moderately Influences Atherosclerotic Lesion Development in LDL Receptor-Deficient Mice

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−/−) mice that are selectively deficient in macrophage ABCG1 by using bone marrow transfer (ABCG1−/− → LDLr−/−). Peritoneal macrophages isolated from donor ABCG1−/− mice exhibited a 22% (P=0.0007) 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−/− → LDLr−/− mice and controls. In lungs of ABCG1−/− → LDLr−/− 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 49±12×103 &mgr;m2 for ABCG1+/+ → LDLr−/− mice versus 65±15×103 &mgr;m2 for ABCG1−/− → LDLr−/− mice and after 12 weeks of high-cholesterol diet feeding 124±17×103 &mgr;m2 for ABCG1+/+ → LDLr−/− mice versus 168±17×103 &mgr;m2 for ABCG1−/− → LDLr−/− mice. Atherosclerotic lesion size depended on both time and the macrophage ABCG1 genotype (P=0.038 by 2-way ANOVA, n≥8), 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.

Combined Deletion of Macrophage ABCA1 and ABCG1 Leads to Massive Lipid Accumulation in Tissue Macrophages and Distinct Atherosclerosis at Relatively Low Plasma Cholesterol Levels

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 156±19×103 &mgr;m2 after only 6 weeks of WTD feeding. Lesions, however, were smaller than single ABCA1 KO transplanted animals (226±30×103 &mgr;m2; P<0.05) and not significantly different from single ABCG1 KO (117±22×103 &mgr;m2) and WT transplanted mice (112±15×103 &mgr;m2). 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.

Regulation of cholesterol homeostasis in macrophages and consequences for atherosclerotic lesion development

Foam cell formation due to excessive accumulation of cholesterol by macrophages is a pathological hallmark of atherosclerosis. Macrophages cannot limit the uptake of cholesterol and therefore depend on cholesterol efflux pathways for preventing their transformation into foam cells. Several ABC‐transporters, including ABCA1 and ABCG1, facilitate the efflux of cholesterol from macrophages. These transporters, however, also affect membrane lipid asymmetry which may have important implications for cellular endocytotic pathways. We propose that in addition to the generally accepted role of these ABC‐transporters in the prevention of foam cell formation by induction of cholesterol efflux from macrophages, they also influence the macrophage endocytotic uptake.

ATP-Binding Cassette Transporters A1 and G1, HDL Metabolism, Cholesterol Efflux, and Inflammation: Important Targets for the Treatment of Atherosclerosis

Atherosclerosis has been characterized as a chronic inflammatory response to cholesterol deposition in arteries. Plasma high density lipoprotein (HDL) levels bear a strong independent inverse relationship with atherosclerotic cardiovascular disease. One central antiatherogenic role of HDL is believed to be its ability to remove excessive peripheral cholesterol back to the liver for subsequent catabolism and excretion, a physiologic process termed reverse cholesterol transport (RCT). Cholesterol efflux from macrophage foam cells, the initial step of RCT is the most relevant step with respect to atherosclerosis. The ATP-binding cassette (ABC) transporters ABCA1 and ABCG1 play crucial roles in the efflux of cellular cholesterol to HDL and its apolipoproteins. Moreover, ABCA1 and ABCG1 affect cellular inflammatory cytokine secretion by modulating cholesterol content in the plasma membrane and within intracellular compartments. In humans, ABCA1 mutations can cause a severe HDL-deficiency syndrome characterized by cholesterol deposition in tissue macrophages and prevalent atherosclerosis. Disrupting Abca1 or Abcg1 in mice promotes accumulation of excessive cholesterol in macrophages, and physiological manipulation of ABCA1 expression affects atherogenesis. Here we review recent advances in the role of ABCA1 and ABCG1 in HDL metabolism, macrophage cholesterol efflux, inflammation, and atherogenesis. Next, we summarize the structure, expression, and regulation of ABCA1 and ABCG1. Finally, we give an update on the progress and pitfalls of therapeutic approaches that target ABCA1 and ABCG1 to stimulate the flux of lipids through the RCT pathway.

Total Body ABCG1 Expression Protects Against Early Atherosclerotic Lesion Development in Mice

Objective—ABCG1 has recently been identified as a facilitator of cholesterol and phospholipid efflux from macrophages to HDL. In bone marrow transplantation studies, we and others have now shown that the absence of macrophage ABCG1 may differentially influence atherosclerotic lesions dependent on the experimental setting and/or the stage of atherosclerotic lesion development. To further define the role of ABCG1 in atherogenesis, we investigated in the current study the effect of total body deficiency of ABCG1 on atherosclerotic lesion development. Methods and Results—ABCG1−/− mice and wild-type littermates were fed an atherogenic diet for 12 weeks to induce atherosclerotic lesion formation. Both before and after the start of the atherogenic diet, serum lipid levels and lipoprotein profiles did not differ significantly between the two groups. In addition no significant difference in serum apoE levels was found after diet feeding. In wild-type mice the atherogenic diet induced the formation of macrophage-rich early lesions (size: 24±7×103 &mgr;m2 [n=6]). Feeding ABCG1−/− mice the atherogenic diet led to a significant 1.9-fold stimulation of atherosclerotic lesion size (46±6x103 &mgr;m2 [n=7]; Student t test P=0.034 and Mann–Whitney test P=0.050) compared with controls, suggesting a clear antiatherogenic role for ABCG1. At the same time, excessive lipid accumulation was observed in macrophage-rich areas of the lungs and spleens of ABCG1−/− mice as compared with wild-type mice. Conclusions—Total body ABCG1 expression protects against early atherosclerotic lesion development.

Absence of HDL cholesteryl ester uptake in mice via SR-BI impairs an adequate adrenal glucocorticoid-mediated stress response to fasting.

Receptor-mediated cholesterol uptake has been suggested to play a role in maintaining the adrenal intracellular free cholesterol pool and the ability to produce hormones. Therefore, in the current study, we evaluated the importance of scavenger receptor class B type I (SR-BI)-mediated cholesteryl ester uptake from HDL for adrenal glucocorticoid hormone synthesis in vivo. No difference was observed in the plasma level of corticosterone between SR-BI-deficient and wild-type mice under ad libitum feeding conditions. Overnight fasting (∼16 h) stimulated the plasma level of corticosterone by 2-fold in wild-type mice. In contrast, no effect of fasting on plasma corticosterone levels was observed in SR-BI-deficient mice, leading to a 44% lower plasma corticosterone level compared with their wild-type littermate controls. In parallel, an almost complete depletion of lipid stores in the adrenal cortex of fasted SR-BI-deficient mice was observed. Plasma adrenocorticotropic hormone levels were increased by 5-fold in fasted SR-BI-deficient mice. SR-BI deficiency induced marked changes in the hepatic expression of the glucocorticoid-responsive genes cholesterol 7α-hydroxylase, HMG-CoA synthase, apolipoprotein A-IV, corticosteroid binding globulin, interleukin-6, and tumor necrosis factor-α, which coincided with a 42% decreased plasma glucose level under fasting conditions. In conclusion, we show that the absence of adrenal HDL cholesteryl ester uptake in SR-BI-deficient mice impairs the adrenal glucocorticoid-mediated stress response to fasting as a result of adrenal glucocorticoid insufficiency and attenuated liver glucocorticoid receptor signaling, leading to hypoglycemia under fasting conditions.

The effect of ABCG1 deficiency on atherosclerotic lesion development in LDL receptor knockout mice depends on the stage of atherogenesis

OBJECTIVE As ABCG1 plays a role in cholesterol efflux, macrophage ABCG1 expression has been suggested to protect against atherosclerosis. However, we and others observed varying effects of ABCG1 deficiency on atherosclerotic lesion size. The objective of this study was to define the effect of ABCG1 deficiency during atherosclerotic lesion progression in LDL receptor knockout (LDLr(-/-)) mice. METHODS AND RESULTS ABCG1(-/-)/LDLr(-/-) and ABCG1(+/+)/LDLr(-/-) littermates were fed a Western-type diet for 10 and 12 weeks in order to study the effect of ABCG1 deficiency in the exponential phase of atherosclerotic lesion formation. At 10 weeks of diet feeding, a significant 1.5-fold increase in early atherosclerotic lesion size (130±12×10(3) μm(2)) was observed in ABCG1(-/-)/LDLr(-/-) mice compared to ABCG1(+/+)/LDLr(-/-) mice (88±11×10(3) μm(2); p<0.05). Interestingly, in more advanced lesions, induced by 12 weeks of WTD feeding, ABCG1(-/-)/LDLr(-/-) mice showed a significant 1.7-fold decrease in atherosclerotic lesion size (160±20×10(3) μm(2) vs 273±19×10(3) μm(2) in control mice; p<0.01), indicating that in the ABCG1(-/-)/LDLr(-/-) mice progression of lesion formation is retarded as compared to ABCG1(+/+)/LDLr(-/-) mice. In addition, correlation analysis performed on 7 independent published studies and the current study confirmed that ABCG1 is atheroprotective in early lesions, while the development of advanced lesions is stimulated. CONCLUSIONS It appears that the effect of ABCG1 deficiency on lesion development in LDLr(-/-) mice depends on the stage of atherogenesis, whereby the absence of ABCG1 leads to increased lesions at sizes<167×10(3) μm(2) while in more advanced stages of atherosclerosis enhanced apoptosis and/or compensatory mechanisms lead to retarded lesion progression.

Deletion of the High-Density Lipoprotein Receptor Scavenger Receptor BI in Mice Modulates Thrombosis Susceptibility and Indirectly Affects Platelet Function by Elevation of Plasma Free Cholesterol

Objective—Scavenger receptor BI (SR-BI) is a cell surface receptor that promotes the selective uptake of cholesteryl esters from high-density lipoprotein (HDL) by the liver. In mice, SR-BI deficiency results in increased plasma HDL cholesterol levels and enhanced susceptibility to atherosclerosis. The aim of this study was to investigate the role of SR-BI deficiency on platelet function. Methods and Results—SR-BI-deficient mice were thrombocytopenic, and their platelets were abnormally large, probably because of an increased cholesterol content. The FeCl3 acute injury model to study arterial thrombosis susceptibility showed that SR-BI wild-type mice developed total arterial occlusion after 24±2 minutes. In SR-BI-deficient mice, however, the time to occlusion was reduced to 13±1 minutes (P=0.02). Correspondingly, in SR-BI-deficient mice, platelets circulated in an activated state and showed increased adherence to immobilized fibrinogen. In contrast, platelet-specific disruption of SR-BI by bone marrow transplantation in wild-type mice did not alter plasma cholesterol levels or affect platelet count, size, cholesterol content, or reactivity, suggesting that changes in plasma cholesterol levels were responsible for the altered responsiveness of platelets in SR-BI-deficient mice. Conclusion—The function of SR-BI in HDL cholesterol homeostasis and prevention of atherosclerosis is indirectly also essential for maintaining normal platelet function and prevention of thrombosis.

Restoration of High-Density Lipoprotein Levels by Cholesteryl Ester Transfer Protein Expression in Scavenger Receptor Class B Type I (SR-BI) Knockout Mice Does Not Normalize Pathologies Associated With SR-BI Deficiency

Objective—Disruption of scavenger receptor class B type I (SR-BI) in mice impairs high-density lipoprotein (HDL)–cholesterol (HDL-C) delivery to the liver and induces susceptibility to atherosclerosis. In this study, it was investigated whether introduction of cholesteryl ester transfer protein (CETP) can normalize HDL-C transport to the liver and reduce atherosclerosis in SR-BI knockout (KO) mice. Methods and Results—Expression of human CETP in SR-BIKO mice resulted in decreased plasma HDL-C levels, both on chow diet (1.8-fold, P<0.001) and on challenge with Western-type diet (1.6-fold, P<0.01). Furthermore, the presence of CETP partially normalized the abnormally large HDL particles observed in SR-BIKO mice. Unexpectedly, expression of CETP in SR-BIKO mice did not reduce atherosclerotic lesion development, probably because of consequences of SR-BI deficiency, including the persistence of higher VLDL-cholesterol (VLDL-C) levels, unchanged elevated free cholesterol/total cholesterol ratio, and the increased oxidative status of the animals. In addition, CETP expression did not normalize other characteristics of SR-BI deficiency, including female infertility, reticulocytosis, thrombocytopenia, and impaired platelet aggregation. Conclusion—CETP restores HDL-C levels in SR-BIKO mice, but it does not change the susceptibility to atherosclerosis and other typical characteristics that are associated with SR-BI disruption. This may indicate that the pathophysiology of SR-BI deficiency is not a direct consequence of changes in the HDL pool.