The current model for reverse cholesterol transport proposes that HDL transports excess cholesterol derived primarily from peripheral cells to the liver for removal. However, recent studies in ABCA1 transgenic mice suggest that the liver itself may be a major source of HDL cholesterol (HDL-C). To directly investigate the hepatic contribution to plasma HDL-C levels, we generated an adenovirus (rABCA1-GFP-AdV) that targets expression of mouse ABCA1-GFP in vivo to the liver. Compared with mice injected with control AdV, infusion of rABCA1-GFP-AdV into C57Bl/6 mice resulted in increased expression of mouse ABCA1 mRNA and protein in the liver. ApoA-I-dependent cholesterol efflux was increased 2.6-fold in primary hepatocytes isolated 1 day after rABCA1-GFP-AdV infusion. Hepatic ABCA1 expression in C57Bl/6 mice (n ‫؍‬ 15) raised baseline levels of TC, PL, FC, HDL-C, apoE, and apoA-I by 150-300% ( P Ͻ 0.05 all). ABCA1 expression led to significant compensatory changes in expression of genes that increase hepatic cholesterol, including HMG-CoA reductase (3.5-fold), LDLr (2.1-fold), and LRP (5-fold) in the liver.These combined results demonstrate that ABCA1 plays a key role in hepatic cholesterol efflux, inducing pathways that modulate cholesterol homeostasis in the liver, and establish the liver as a major source of plasma HDL-C. The role of HDL in the removal of excess cholesterol from peripheral cells was postulated nearly 30 years ago and designated reverse cholesterol transport (RCT) (1, 2). In the current working model of RCT, excess cholesterol is removed from peripheral cells and esterified by lecithin cholesterol acyltransferase. The cholesteryl ester is then transported back to the liver for removal from the body, either directly by HDL or following transfer by cholesterol ester transfer protein to the apoB-containing lipoproteins. This model proposes that cholesterol and lipids used for the formation and maturation of HDL are derived from nonhepatic, peripheral cells as well as from the metabolism and remodeling of the triglyceride-rich, apoBcontaining lipoproteins. Previous reports indicate that the liver is an important source of apoA-I and thus contributes significantly to the plasma pool of nascent HDL (3, 4). However, although the liver is the most important modulator of cholesterol homeostasis in the body, it has not been implicated as a major in vivo source of cholesterol to lipidate HDL in the circulation .A major advance in our understanding of the first step in reverse cholesterol transport was the identification of the ABCA1 transporter as the genetic defect in patients with Tangier disease (5-10). ABCA1 is the major transporter that facilitates the efflux of cholesterol and phospholipids to poorly lipidated apoA-I to form nascent or pre ␤ HDL. In the absence of a functional ABCA1 transporter, patients with Tangier disease are unable to efflux cholesterol to apoA-I and accumulate cholesteryl esters in many tissues, including arterial macrophages. Attie et al. reported similar findings in the ...

Abstract-High-density lipoproteins (HDL) protect against cardiovascular disease. HDL removes and transports excess cholesterol from peripheral cells to the liver for removal from the body. HDL also protects low-density lipoproteins (LDL) from oxidation and inhibits expression of adhesion molecules in endothelial cells, preventing monocyte movement into the vessel wall. The ABCA1 transporter regulates intracellular cholesterol levels in the liver and in peripheral cells by effluxing excess cholesterol to lipid-poor apoA-I to form nascent HDL, which is converted to mature ␣-HDL by esterification of cholesterol to cholesteryl esters (CE) by lecithin cholesterol acyltransferase. The hepatic ABCA1 transporter and apoA-I are major determinants of levels of plasma ␣-HDL cholesterol as well as poorly lipidated apoA-I, which interact with ABCA1 transporters on peripheral cells in the process of reverse cholesterol transport. Cholesterol in HDL is transported directly back to the liver by HDL or after transfer of CE by the cholesteryl ester transfer protein (CETP) by the apoB lipoproteins. Current approaches to increasing HDL to determine the efficacy of HDL in reducing atherosclerosis involve acute HDL therapy with infusions of apoA-I or apoA-I mimetic peptides and chronic long-term therapy with selective agents to increase HDL, including CETP inhibitors.

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bacterial adhesion ͉ internalization ͉ phagocytes ͉ cytosolic invasion ͉ high-density lipoprotein receptor C LA-1 and its splicing variant CLA-2, orthologues of rodent scavenger receptor class B type I (SR-BI) and its splicing variant SR-BII, are known as high-density lipoprotein (HDL) receptors. These receptors mediate HDL binding, followed by selective uptake of cholesteryl ester in the liver and steroidogenic tissues (for review, see refs.

The individual roles of hepatic versus intestinal ABCG5 and ABCG8 in sterol transport have not yet been investigated. To determine the specific contribution of liver ABCG5/G8 to sterol transport and atherosclerosis, we generated transgenic mice that overexpress human ABCG5 and ABCG8 in the liver but not intestine (liver G5/G8-Tg) in three different genetic backgrounds: C57Bl/6, apoE-KO, and low density lipoprotein receptor (LDLr)-KO. Hepatic overexpression of ABCG5/G8 enhanced hepatobiliary secretion of cholesterol and plant sterols by 1.5-2-fold, increased the amount of intestinal cholesterol available for absorption and fecal excretion by up to 27%, and decreased the accumulation of plant sterols in plasma by ϳ25%. However, it did not alter fractional intestinal cholesterol absorption, fecal neutral sterol excretion, hepatic cholesterol concentrations, or hepatic cholesterol synthesis. Consequently, overexpression of ABCG5/G8 in only the liver had no effect on the plasma lipid profile, including cholesterol, HDL-C, and non-HDL-C, or on the development of proximal aortic atherosclerosis in C57Bl/6, apoE-KO, or LDLr-KO mice. Thus, liver ABCG5/G8 facilitate the secretion of liver sterols into bile and serve as an alternative mechanism, independent of intestinal ABCG5/G8, to protect against the accumulation of dietary plant sterols in plasma. However, in the absence of changes in fractional intestinal cholesterol absorption, increased secretion of sterols into bile induced by hepatic overexpression of ABCG5/G8 was not sufficient to alter hepatic cholesterol balance, enhance cholesterol removal from the body or to alter atherogenic risk in liver G5/G8-Tg mice. These findings demonstrate that overexpression of ABCG5/G8 in the liver profoundly alters hepatic but not intestinal sterol transport, identifying distinct roles for liver and intestinal ABCG5/G8 in modulating sterol metabolism.Sitosterolemia, also known as phytosterolemia (1-9) is a rare genetic disorder characterized by elevated plasma and tissue levels of plant, shellfish, and animal sterols and the development of tendon and tuberous xanthomas, hemolytic episodes, arthritis, and premature coronary artery disease. The ATPbinding cassette (ABC) 1 half-transporters ABCG5 and ABCG8 (ABCG5/G8) have been recently identified as the genes defective in sitosterolemia (10 -12). The increased plasma and tissue levels of animal and plant sterols found in sitosterolemic patients have been attributed to enhanced absorption and decreased biliary excretion (1-4, 6, 8, 13-16). Unlike cholesterol, sitosterol accumulation does not inhibit HMGCoA reductase activity in human monocyte-derived macrophages (17). The inadequate down-regulation of HMGCoA reductase activity by plant sterols may promote foam cell formation and explain, in part, the increased risk of atherosclerosis in sitosterolemia (17).Several lines of evidence implicate ABCG5/G8 in intestinal and biliary sterol transport. First, ABCG5/G8 are expressed in the liver and small intestine and, to a lesser degree, in th...

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 ؋ 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 CYP7␣1, CYP27␣1, 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.

Summary. Aims: The aim of this study was to investigate associations between coronary heart disease risk and polymorphisms in the coagulation factor (F)VII gene in participants of a large prospective study. Methods: One thousand nine hundred and ®fty-seven men were genotyped for four FVII polymorphisms, À670A3C, À402G3A, a 10 base pair insertion at À323 (0 > 10) in the promoter, and R353Q in the structural gene. Associations among genotypes and estimated haplotypes, plasma FVII levels, and coronary heart disease risk were evaluated, and the function of the promoter polymorphisms was assessed in reporter gene assays. Results: The À670A3C and À402G3A polymorphisms were in complete allelic association. The haplotype containing À670C and À402A (frequency 0.23) was associated with signi®cantly increased plasma FVII coagulant activity and increased risk of an initial coronary event, particularly acute myocardial infarction, which remained after correction for conventional risk factors. In contrast, the À323 insertion and Q353 alleles (frequency 0.11 and 0.10, respectively) were associated with decreased plasma FVII levels, but hazard ratios for coronary events in carriers of these alleles were not signi®cantly different from unity. In transiently transfected hepatoma cells, increased basal expression of the reporter gene was directed by a promoter fragment with rare haplotype À670C/À630G/À402A rather than by a promoter fragment with common haplotype À670A/À630A/À402G; À402A was not responsible for this effect. Conclusions: The promoter haplotype, À670C/À630G/ 402A, was associated with signi®cantly increased plasma FVII coagulant activity, risk of an initial coronary event, particularly acute myocardial infarction, and reporter gene expression.

ABCG1 promotes cholesterol efflux from cells, but ABCG1(-/-) bone marrow transplant into ApoE(-/-) and LDLr(-/-) mice reduces atherosclerosis. To further investigate the role of ABCG1 in atherosclerosis, ABCG1 transgenic mice were crossed with LDLr-KO mice and placed on a high-fat western diet. Increased expression of ABCG1 mRNA was detected in liver (1.8-fold) and macrophages (2.7-fold), and cholesterol efflux from macrophages to HDL was also increased (1.4-fold) in ABCG1xLDLr-KO vs. LDLr-KO mice. No major differences were observed in total plasma lipids. However, cholesterol in the IDL-LDL size range was increased by approximately 50% in ABCG1xLDLr-KO mice compared to LDLr-KO mice. Atherosclerosis increased by 39% (10.1+/-0.8 vs 6.1+/-0.9% lesion area, p=0.02), as measured by en face analysis, and by 53% (221+/-98 vs 104+/-58x10(3)microm(2), p =0.01), as measured by cross-sectional analysis in ABCG1xLDLr-KO mice. Plasma levels for MCP-1 (1.5-fold) and TNF-alpha (1.2-fold) were also increased in ABCG1xLDLr-KO mice. In summary, these findings suggest that enhanced expression of ABCG1 increases atherosclerosis in LDLr-KO mice, despite its role in promoting cholesterol efflux from cells.