ABCG1 has a critical role in mediating cholesterol efflux to HDL and preventing cellular lipid accumulation

Here we demonstrate that the ABC transporter ABCG1 plays a critical role in lipid homeostasis by controlling both tissue lipid levels and the efflux of cellular cholesterol to HDL. Targeted disruption of Abcg1 in mice has no effect on plasma lipids but results in massive accumulation of both neutral lipids and phospholipids in hepatocytes and in macrophages within multiple tissues following administration of a high-fat and -cholesterol diet. In contrast, overexpression of human ABCG1 protects murine tissues from dietary fat-induced lipid accumulation. Finally, we show that cholesterol efflux to HDL specifically requires ABCG1, whereas efflux to apoA1 requires ABCA1. These studies identify Abcg1 as a key gene involved in both cholesterol efflux to HDL and in tissue lipid homeostasis.     

Deletion of the transmembrane transporter ABCG1 results in progressive pulmonary lipidosis

We show that mice lacking the ATP-binding cassette transmembrane transporter ABCG1 show progressive and age-dependent severe pulmonary lipidosis that recapitulates the phenotypes of different respiratory syndromes in both humans and mice. The lungs of chow-fed Abcg1(-/-) mice, >6-months old, exhibit extensive subpleural cellular accumulation, macrophage, and pneumocyte type 2 hypertrophy, massive lipid deposition in both macrophages and pneumocytes and increased levels of surfactant. No such abnormalities are observed at 3 months of age. However, gene expression profiling reveals significant changes in the levels of mRNAs encoding key genes involved in lipid metabolism in both 3- and 8-month-old Abcg1(-/-) mice. These data suggest that the lungs of young Abcg1(-/-) mice maintain normal lipid levels by repressing lipid biosynthetic pathways and that such compensation is inadequate as the mice mature. Studies with A-549 cells, a model for pneumocytes type 2, demonstrate that overexpression of ABCG1 specifically stimulates the efflux of cellular cholesterol by a process that is dependent upon phospholipid secretion. In addition, we demonstrate that Abcg1(-/-), but not wild-type macrophages, accumulate cholesterol ester droplets when incubated with surfactant. Together, these data provide a mechanism to explain the lipid accumulation in the lungs of Abcg1(-/-)mice. In summary, our results demonstrate that ABCG1 plays essential roles in pulmonary lipid homeostasis.     

ABCG1 influences the brain cholesterol biosynthetic pathway but does not affect amyloid precursor protein or apolipoprotein E metabolism in vivo

Cholesterol homeostasis is of emerging therapeutic importance for Alzheimer's disease (AD). Agonists of liver-X-receptors (LXRs) stimulate several genes that regulate cholesterol homeostasis, and synthetic LXR agonists decrease neuropathological and cognitive phenotypes in AD mouse models. The cholesterol transporter ABCG1 is LXR-responsive and highly expressed in brain. In vitro, conflicting reports exist as to whether ABCG1 promotes or impedes Abeta production. To clarify the in vivo roles of ABCG1 in Abeta metabolism and brain cholesterol homeostasis, we assessed neuropathological and cognitive outcome measures in PDAPP mice expressing excess transgenic ABCG1. A 6-fold increase in ABCG1 levels did not alter Abeta, amyloid, apolipoprotein E levels, cholesterol efflux, or cognitive performance in PDAPP mice. Furthermore, endogenous murine Abeta levels were unchanged in both ABCG1-overexpressing or ABCG1-deficient mice. These data argue against a direct role for ABCG1 in AD. However, excess ABCG1 is associated with decreased levels of sterol precursors and increased levels of SREBP-2 and HMG-CoA-reductase mRNA, whereas deficiency of ABCG1 leads to the opposite effects. Although functions for ABCG1 in cholesterol efflux and Abeta metabolism have been proposed based on results with cellular model systems, the in vivo role of this enigmatic transporter may be largely one of regulating the sterol biosynthetic pathway.     

Loss of ABCG1 results in chronic pulmonary inflammation

ABCG1, a member of the ATP-binding cassette transporter superfamily, is highly expressed in multiple cells of the lung. Loss of ABCG1 results in severe pulmonary lipidosis in mice, with massive deposition of cholesterol in both alveolar macrophages and type 2 cells and the accumulation of excessive surfactant phospholipids. These observations are consistent with ABCG1 controlling cellular sterol metabolism. Herein, we report on the progressive and chronic inflammatory process that accompanies the lipidosis in the lungs of Abcg1-/- mice. Compared with wild-type animals, the lungs of aged chow-fed mice deficient in ABCG1 show distinctive signs of inflammation that include macrophage accumulation, lymphocytic infiltration, hemorrhage, eosinophilic crystals, and elevated levels of numerous cytokines and cytokine receptors. Analysis of bronchoalveolar lavages obtained from Abcg1-/- mice revealed elevated numbers of foamy macrophages and leukocytes and the presence of multiple markers of inflammation including crystals of chitinase-3-like proteins. These data suggest that cholesterol and/or cholesterol metabolites that accumulate in Abcg1-/- lungs can trigger inflammatory signaling pathways. Consistent with this hypothesis, the expression of a number of cytokines was found to be significantly increased following increased cholesterol delivery to either primary peritoneal macrophages or Raw264.7 cells. Finally, cholesterol loading of primary mouse macrophages induced cytokine mRNAs to higher levels in Abcg1-/-, as compared with wild-type cells. These results demonstrate that ABCG1 plays critical roles in pulmonary homeostasis, balancing both lipid/cholesterol metabolism and inflammatory responses.     

Murine 12/15-Lipoxygenase Regulates ATP-binding Cassette Transporter G1 Protein Degradation through p38- and JNK2-dependent Pathways

12/15-Lipoxygenase (12/15LO) plays a role in the pathogenesis of atherosclerosis and diabetes and has been implicated in low density lipoprotein oxidation. Murine macrophages express high levels of 12/15LO and are key cells involved in the accumulation and efflux of oxidized low density lipoprotein in the arterial wall. During this process, macrophages up-regulate scavenger receptors that regulate lipid uptake, and ATP-binding cassette (ABC) transporters, that regulate lipid efflux. We have previously demonstrated that 12/15LO enhances the turnover and serine phosphorylation of ABCG1. In the current study, we further elucidate the mechanisms by which 12/15LO regulates ABCG1. Proteasomal inhibitors blocked the down-regulation of ABCG1 expression and resulted in accumulation of phosphorylated ABCG1. Macrophages that lack 12/15LO have enhanced transporter expression, reduced ABCG1 phosphorylation, and increased cholesterol efflux. Conversely, macrophages that overexpress 12/15LO have reduced ABCG1 expression, increased transporter phosphorylation, and reduced cholesterol efflux. 12/15LO plays a key role in activating the MAPK pathway. Inhibition of the p38 or JNK pathways with pharmacological inhibitors or dominant negative constructs blocked 12S-hydroxyeicosatetranoic acid-mediated degradation of ABCG1. Moreover, we isolated macrophages from JNK1-, JNK2-, and MKK3-deficient mice to analyze the involvement of specific MAPK pathways. JNK2- and MKK3-, but not JNK1-deficient macrophages were resistant to the down-regulation of ABCG1 protein, reduction in efflux, and increase in serine phosphorylation by 12S-hydroxyeicosatetranoic acid. These findings provide evidence that 12/15LO regulates ABCG1 expression and function through p38- and JNK2-dependent mechanisms, and that targeting these pathways may provide novel approaches for regulating cholesterol homeostasis.     

α-Lipoic acid ameliorates foam cell formation via liver X receptor α-dependent upregulation of ATP-binding cassette transporters A1 and G1

α-Lipoic acid (α-LA), a key cofactor in cellular energy metabolism, has protective activities in atherosclerosis, yet the detailed mechanisms are not fully understood. In this study, we examined whether α-LA affects foam cell formation and its underlying molecular mechanisms in murine macrophages. Treatment with α-LA markedly attenuated oxidized low-density lipoprotein (oxLDL)-mediated cholesterol accumulation in macrophages, which was due to increased cholesterol efflux. Additionally, α-LA treatment dose-dependently increased protein levels of ATP-binding cassette transporter A1 (ABCA1) and ABCG1 but had no effect on the protein expression of SR-A, CD36, or SR-BI involved in cholesterol homeostasis. Furthermore, α-LA increased the mRNA expression of ABCA1 and ABCG1. The upregulation of ABCA1 and ABCG1 by α-LA depended on liver X receptor α (LXRα), as evidenced by an increase in the nuclear levels of LXRα and LXRE-mediated luciferase activity and its prevention of the expression of ABCA1 and ABCG1 after inhibition of LXRα activity by the pharmacological inhibitor geranylgeranyl pyrophosphate (GGPP) or knockdown of LXRα expression with small interfering RNA (siRNA). Consistently, α-LA-mediated suppression of oxLDL-induced lipid accumulation was abolished by GGPP or LXRα siRNA treatment. In conclusion, LXRα-dependent upregulation of ABCA1 and ABCG1 may mediate the beneficial effect of α-LA on foam cell formation.     

Restoration of PPARγ reverses lipid accumulation in alveolar macrophages of GM-CSF knockout mice

Pulmonary alveolar proteinosis (PAP) is a lung disease characterized by a deficiency of functional granulocyte macrophage colony-stimulating factor (GM-CSF) resulting in surfactant accumulation and lipid-engorged alveolar macrophages. GM-CSF is a positive regulator of PPARγ that is constitutively expressed in healthy alveolar macrophages. We previously reported decreased PPARγ and ATP-binding cassette transporter G1 (ABCG1) levels in alveolar macrophages from PAP patients and GM-CSF knockout (KO) mice, suggesting PPARγ and ABCG1 involvement in surfactant catabolism. Because ABCG1 represents a PPARγ target, we hypothesized that PPARγ restoration would increase ABCG1 and reduce macrophage lipid accumulation. Upregulation of PPARγ was achieved using a lentivirus expression system in vivo. GM-CSF KO mice received intratracheal instillation of lentivirus (lenti)-PPARγ or control lenti-eGFP. Ten days postinstillation, 79% of harvested alveolar macrophages expressed eGFP, demonstrating transduction. Alveolar macrophages showed increased PPARγ and ABCG1 expression after lenti-PPARγ instillation, whereas PPARγ and ABCG1 levels remained unchanged in lenti-eGFP controls. Alveolar macrophages from lenti-PPARγ-treated mice also exhibited reduced intracellular phospholipids and increased cholesterol efflux to HDL, an ABCG1-mediated pathway. In vivo instillation of lenti-PPARγ results in: 1) upregulating ABCG1 and PPARγ expression of GM-CSF KO alveolar macrophages, 2) reducing intracellular lipid accumulation, and 3) increasing cholesterol efflux activity.     

转运蛋白ABCG1/4 和ABCA1对脑胆固醇的调节作用

脑是富含胆固醇的器官,机体大约有25%的胆固醇集中在脑组织中.ATP结合盒超家族转运蛋白对脑组织中胆固醇的膜外转运和动态平衡起着重要的调节作用.研究发现,ATP结合盒超家族转运蛋白亚体ABCG1、ABCG4和ABCA1在成体脑组织中存在不同程度的表达,一种或多种亚体的缺失可以导致神经退行性病变.然而,ATP结合盒超家族转运蛋白亚体对脑发育过程中脑胆固醇动态变化的调节缺乏相关性的报道.在本研究中,从低胆固醇饮食喂养的C57BL/6J小鼠中获取出生后不同发育时期的脑组织,对ABCG1、ABCG4和ABCA1的mRNA与蛋白质表达水平进行测定,并对脑组织和血清中ATP结合盒超家族转运蛋白的表达水平与胆固醇水平的相关性进行研究.同时,使用ABCG1、ABCG4单一基因敲除鼠和ABCG1、ABCG4双基因敲除鼠,研究ATP结合盒超家族转运蛋白对与胆固醇合成的相关基因表达的影响以及对脑组织胆固醇代谢的调节作用.结果发现,ABCG1、ABCG4和ABCA1在机体多个器官中均有表达,但ABCG1和ABCG4在小鼠脑组织中表达量最高.在脑组织发育过程中,ABCG1和ABCG4mRNA水平呈现明显的表达时效性,小鼠于出生后42天达到峰值,而ABCA1 mRNA的表达水平无明显变化.血清和脑组织中中酯化型胆固醇水平呈双高峰分布,也于出生后42天达到最高.基因敲除鼠模型显示,单一敲除ABCG1或者ABCG4基因对脑组织胆固醇水平无明显影响,而ABCG1和ABCG4基因的同时缺失导致脑胆固醇水平显著升高,并明显降低胆固醇合成相关基因的表达水平.本研究表明,在脑发育成熟过程中,ATP结合盒超家族转运蛋白亚体ABCG1和ABCG4,而非ABCA1,以调节脑胆固醇的膜外转运;ABCG1和ABCG4互补调控脑胆固醇的动态平衡.     

Cellular Cholesterol Transport Proteins in Diabetic Nephropathy

Background

Lipid accumulation has been shown to accelerate renal injury, and the intracellular accumulation of lipids may be caused by alterations in synthesis as well as lipid uptake and efflux. We have investigated the role of cellular cholesterol transport proteins including adenosine triphosphate binding cassette transporter A1 (ABCA1), G1 (ABCG1) and scavenger receptor class B type I (SR-BI) in diabetic nephropathy.

Methods

Protein expression and the ability to mediate cholesterol efflux of ABCA1, ABCG1 and SR-BI was determined in human renal mesangial cells and proximal tubular epithelial cells cultured under normal or high glucose conditions. Renal expression of these cholesterol transporters was examined in a murine model of streptozotocin-induced type 1 diabetes.

Results

ABCA1, ABCG1 and SR-BI were expressed in both human renal mesangial cells and proximal tubular epithelial cells, and mediated cholesterol efflux to apolipoprotein AI and HDL. In vitro, hyperglycemia reduced the expression and the ability to mediate cholesterol efflux of all three cholesterol transporters (p<0.05). In vivo studies showed that intra-renal accumulation of lipids was increased in diabetic mice, particularly in mice with nephropathy. This was associated with a significant reduction in the expression of ABCA1, ABCG1 and SR-BI in the kidneys. These changes were already seen in diabetic mice without nephropathy and preceded the development of nephropathy. Diabetic mice with nephropathy had the lowest level of these cholesterol transporters.

Conclusion

Inducing diabetes with streptozotocin significantly reduced renal expression of ABCA1, ABCG1 and SR-BI. Defects in cholesterol export pathway in renal cells could therefore promote cholesterol accumulation and might contribute to the development of diabetic nephropathy.     

Differential expression and function of ABCG1 and ABCG4 during development and aging

ABCG1 and ABCG4 are highly homologous members of the ATP binding cassette (ABC) transporter family that regulate cellular cholesterol homeostasis. In adult mice, ABCG1 is known to be expressed in numerous cell types and tissues, whereas ABCG4 expression is limited to the central nervous system (CNS). Here, we show significant differences in expression of these two transporters during development. Examination of β-galactosidase-stained tissue sections from Abcg1^−/−LacZ and Abcg4^−/−LacZ knockin mice shows that ABCG4 is highly but transiently expressed both in hematopoietic cells and in enterocytes during development. In contrast, ABCG1 is expressed in macrophages and in endothelial cells of both embryonic and adult liver. We also show that ABCG1 and ABCG4 are both expressed as early as E12.5 in the embryonic eye and developing CNS. Loss of both ABCG1 and ABCG4 results in accumulation in the retina and/or brain of oxysterols, in altered expression of liver X receptor and sterol-regulatory element binding protein-2 target genes, and in a stress response gene. Finally, behavioral tests show that Abcg4^−/− mice have a general deficit in associative fear memory. Together, these data indicate that loss of ABCG1 and/or ABCG4 from the CNS results in changes in metabolic pathways and in behavior.     

ATP binding cassette G1-dependent cholesterol efflux during inflammation

ATP binding cassette transporter G1 (ABCG1) mediates the transport of cellular cholesterol to HDL, and it plays a key role in maintaining macrophage cholesterol homeostasis. During inflammation, HDL undergoes substantial remodeling, acquiring lipid changes and serum amyloid A (SAA) as a major apolipoprotein. In the current study, we investigated whether remodeling of HDL that occurs during acute inflammation impacts ABCG1-dependent efflux. Our data indicate that lipid free SAA acts similarly to apolipoprotein A-I (apoA-I) in mediating sequential efflux from ABCA1 and ABCG1. Compared with normal mouse HDL, acute phase (AP) mouse HDL containing SAA exhibited a modest but significant 17% increase in ABCG1-dependent efflux. Interestingly, AP HDL isolated from mice lacking SAA (SAAKO mice) was even more effective in promoting ABCG1 efflux. Hydrolysis with Group IIA secretory phospholipase A(2) (sPLA(2)-IIA) significantly reduced the ability of AP HDL from SAAKO mice to serve as a substrate for ABCG1-mediated cholesterol transfer, indicating that phospholipid (PL) enrichment, and not the presence of SAA, is responsible for alterations in efflux. AP human HDL, which is not PL-enriched, was somewhat less effective in mediating ABCG1-dependent efflux compared with normal human HDL. Our data indicate that inflammatory remodeling of HDL impacts ABCG1-dependent efflux independent of SAA.     

Differential effects of scavenger receptor BI deficiency on lipid metabolism in cells of the arterial wall and in the liver

Scavenger receptor class B, type I (SRBI) is a key regulator of high density lipoprotein (HDL) metabolism. It facilitates the efflux of cholesterol from cells in peripheral tissues to HDL and mediates the selective uptake of cholesteryl esters from HDL in the liver. We investigated the effects of SRBI deficiency in the arterial wall and in the liver using SRBI-deficient mice and wild-type littermates fed a Western-type diet. The SRBI-deficient mice showed massive accumulation of cholesterol-rich HDL in the circulation, reflecting impaired delivery to the liver. Strikingly, SRBI deficiency did not alter hepatic cholesterol (ester) content nor did it affect the expression of key regulators of hepatic cholesterol homeostasis, including HMG-CoA reductase, the low density lipoprotein receptor, and cholesterol 7alpha-hydroxylase. However, a approximately 40% reduction in biliary cholesterol content was observed, and the expression of ABCG8 and ABCG5, ATP half-transporters implicated in the transport of sterols from the liver to the bile, was attenuated by 70 and 35%, respectively. In contrast to the situation in the liver, SRBI deficiency did result in lipid deposition in the aorta and atherosclerosis. Vascular mRNA analysis showed increased expression of inflammatory markers as well as of genes involved in cellular cholesterol homeostasis. Our data show that, although hepatic cholesterol homeostasis is maintained upon feeding a Western-type diet, SRBI deficiency is associated with de-regulation of cholesterol homeostasis in the arterial wall that results in an increased susceptibility to atherosclerosis.     

ABCA1 and ABCG1 or ABCG4 act sequentially to remove cellular cholesterol and generate cholesterol-rich HDL

Recent developments in lipid metabolism have shown the importance of ATP binding cassette transporters (ABCs) in controlling cellular and total body lipid homeostasis. ABCA1 mediates the transport of cholesterol and phospholipids from cells to lipid-poor apolipoprotein A-I (apoA-I), whereas ABCG1 and ABCG4 mediate the transport of cholesterol from cells to lipidated lipoproteins. ABCA1, ABCG1, and ABCG4 are all expressed in cholesterol-loaded macrophages, and macrophages from ABCA1 and ABCG1 knockout mice accumulate cholesteryl esters. Here, we show that the lipidated particles generated by incubating cells overexpressing ABCA1 with apoA-I are efficient acceptors for cholesterol released from cells overexpressing either ABCG1 or ABCG4. The cholesterol released to the particles was derived from a cholesterol oxidase-accessible plasma membrane pool in both ABCG1 and ABCG4 cells, which is the same pool of cholesterol shown previously to be removed by high density lipoproteins. ABCA1 cells incubated with apoA-I generated two major populations of cholesterol- and phospholipid-rich lipoprotein particles that were converted by ABCG1 or ABCG4 cells to one major particle population that was highly enriched in cholesterol. These results suggest that ABCG1 and ABCG4 act in concert with ABCA1 to maximize the removal of excess cholesterol from cells and to generate cholesterol-rich lipoprotein particles.     

Transgenic expression of CYP7A1 in LDL receptor-deficient mice blocks diet-induced hypercholesterolemia

Constitutive expression of a cholesterol-7alpha-hydroxylase (CYP7A1) transgene in LDL receptor-deficient mice blocked the ability of a cholesterol-enriched diet to increase plasma levels of apolipoprotein B-containing lipoproteins. LDL receptor-deficient mice expressing the CYP7A1 transgene exhibited complete resistance to diet-induced hypercholesterolemia and to the accumulation of cholesterol in the liver. Hepatic mRNA expression of liver X receptor-inducible ABCG5 and ABCG8 was decreased in CYP7A1 transgenic, LDL receptor-deficient mice fed a cholesterol-enriched diet. Thus, increased biliary cholesterol excretion could not account for the maintenance of cholesterol homeostasis. CYP7A1 transgenic, LDL receptor-deficient mice fed the cholesterol-enriched diet exhibited decreased jejunal Niemann-Pick C1-Like 1 protein (NPC1L1) mRNA expression, an important mediator of intestinal cholesterol absorption. A taurocholate-enriched diet also decreased NPC1L1 mRNA expression in a farnesoid X receptor-independent manner. Reduced expression of NPC1L1 mRNA was associated with decreased cholesterol absorption ( approximately 20%; P < 0.05) exhibited by CYP7A1 transgenic LDL receptor-deficient mice fed the cholesterol-enriched diet. The combined data show that enhanced expression of CYP7A1 is an effective means to prevent the accumulation of cholesterol in the liver and of atherogenic apolipoprotein B-containing lipoproteins in plasma.     

ABCG1 regulates pulmonary surfactant metabolism in mice and men

Idiopathic pulmonary alveolar proteinosis (PAP) is a rare lung disease characterized by accumulation of surfactant. Surfactant synthesis and secretion are restricted to epithelial type 2 (T2) pneumocytes (also called T2 cells). Clearance of surfactant is dependent upon T2 cells and macrophages. ABCG1 is highly expressed in both T2 cells and macrophages. ABCG1-deficient mice accumulate surfactant, lamellar body-loaded T2 cells, lipid-loaded macrophages, B-1 lymphocytes, and immunoglobulins, clearly demonstrating that ABCG1 has a critical role in pulmonary homeostasis. We identify a variant in the ABCG1 promoter in patients with PAP that results in impaired activation of ABCG1 by the liver X receptor α, suggesting that ABCG1 basal expression and/or induction in response to sterol/lipid loading is essential for normal lung function. We generated mice lacking ABCG1 specifically in either T2 cells or macrophages to determine the relative contribution of these cell types on surfactant lipid homeostasis. These results establish a critical role for T2 cell ABCG1 in controlling surfactant and overall lipid homeostasis in the lung and in the pathogenesis of human lung disease.     

PLK1 promotes cholesterol efflux and alleviates atherosclerosis by up-regulating ABCA1 and ABCG1 expression via the AMPK/PPARγ/LXRα pathway

Polo-like kinase 1 (PLK1) is a serine/threonine kinase involving lipid metabolism and cardiovascular disease. However, its role in atherogenesis has yet to be determined. The aim of this study was to observe the impact of PLK1 on macrophage lipid accumulation and atherosclerosis development and to explore the underlying mechanisms. We found a significant reduction of PLK1 expression in lipid-loaded macrophages and atherosclerosis model mice. Lentivirus-mediated overexpression of PLK1 promoted cholesterol efflux and inhibited lipid accumulation in THP-1 macrophage-derived foam cells. Mechanistic analysis revealed that PLK1 stimulated the phosphorylation of AMP-activated protein kinase (AMPK), leading to activation of the peroxisome proliferator-activated receptor γ (PPARγ)/liver X receptor α (LXRα) pathway and up-regulation of ATP binding cassette transporter A1 (ABCA1) and ABCG1 expression. Injection of lentiviral vector expressing PLK1 increased reverse cholesterol transport, improved plasma lipid profiles and decreased atherosclerotic lesion area in apoE-deficient mice fed a Western diet. PLK1 overexpression also facilitated AMPK and HSL phosphorylation and enhanced the expression of PPARγ, LXRα, ABCA1, ABCG1 and LPL in the aorta. In summary, these data suggest that PLK1 inhibits macrophage lipid accumulation and mitigates atherosclerosis by promoting ABCA1- and ABCG1-dependent cholesterol efflux via the AMPK/PPARγ/LXRα pathway.     

Effects of ABCG1 knockout on proteomic composition of HDL in mice on a chow diet and a high-fat diet

ATP-binding cassette transporter G1 (ABCG1) is a cellular transmembrane protein that transports oxysterol efflux from cells to high-density lipoprotein (HDL) particles in the plasma. Previous studies have demonstrated that an ABCG1 deficiency exerts an antiatherosclerotic function through the effects of oxysterol accumulation in cells to enhance apoptosis and regulate inflammatory processes. However, whether the deficiency of ABCG1 and the corresponding changes in the efflux of oxysterols could take a series of impacts on the proteomic composition of HDL remains unclear. Here, plasma HDL of ABCG1(−/−) mice and their wild-type controls on a normal chow diet (NCD) or a high-fat diet (HFD) were isolated by ultracentrifugation. The proportion of 7-ketocholesterol and the proteomic composition of samples were comparatively analyzed by LC-MS/MS. In NCD-fed mice, lipid metabolism-related protein (arachidonate 12-lipoxygenase) and antioxidative protein (pantetheinase) exhibited increased accumulation, and inflammatory response protein (alpha-1-antitrypsin) was decreased in accumulation in ABCG1(−/−) mice HDL. In HFD-fed mice, fewer proteins were detected than that of NCD-fed mice. The ABCG1(−/−) mice HDL exhibited increased accumulation of lipid metabolism-related proteins (e.g., carboxylesterase 1C, apolipoprotein (apo)C-4) and decreased accumulation of alpha-1-antitrypsin, as well as significantly reduced proportion of 7-ketocholesterol. Additionally, positive correlations were found between 7-ketocholesterol and some essential proteins on HDL, such as alpha-1-antitrypsin, apoA-4, apoB-100, and serum amyloid A (SAA). These results suggest a detrimental impact of oxysterols on HDL composition, which might affect the antiatherosclerotic properties of HDL.     

ABCA1 overexpression in the liver of LDLr-KO mice leads to accumulation of pro-atherogenic lipoproteins and enhanced atherosclerosis

The identification of ABCA1 as a key transporter responsible for cellular lipid efflux has led to considerable interest in defining its role in cholesterol metabolism and atherosclerosis. In this study, the effect of overexpressing ABCA1 in the liver of LDLr-KO mice was investigated. Compared with LDLr-KO mice, ABCA1-Tg x LDLr-KO (ABCA1-Tg) mice had significantly increased plasma cholesterol levels, mostly because of a 2.8-fold increase in cholesterol associated with a large pool of apoB-lipoproteins. ApoB synthesis was unchanged but the catabolism of (125)I-apoB-VLDL and -LDL were significantly delayed, accounting for the 1.35-fold increase in plasma apoB levels in ABCA1-Tg mice. We also found rapid in vivo transfer of free cholesterol from HDL to apoB-lipoproteins in ABCA1-Tg mice, associated with a significant 2.7-fold increase in the LCAT-derived cholesteryl linoleate content found primarily in apoB-lipoproteins. ABCA1-Tg mice had 1.4-fold increased hepatic cholesterol concentrations, leading to a compensatory 71% decrease in de novo hepatic cholesterol synthesis, as well as enhanced biliary cholesterol, and bile acid secretion. CAV-1, CYP2b10, and ABCG1 were significantly induced in ABCA1-overexpressing livers; however, no differences were observed in the hepatic expression of CYP7alpha1, CYP27alpha1, or ABCG5/G8 between ABCA1-Tg and control mice. As expected from the pro-atherogenic plasma lipid profile, aortic atherosclerosis was increased 10-fold in ABCA1-Tg mice. In summary, hepatic overexpression of ABCA1 in LDLr-KO mice leads to: 1) expansion of the pro-atherogenic apoB-lipoprotein cholesterol pool size via enhanced transfer of HDL-cholesterol to apoB-lipoproteins and delayed catabolism of cholesterol-enriched apoB-lipoproteins; 2) increased cholesterol concentration in the liver, resulting in up-regulated hepatobiliary sterol secretion; and 3) significantly enhanced aortic atherosclerotic lesions.