Acyl-CoA synthetase 1 is required for oleate and linoleate mediated inhibition of cholesterol efflux through ATP-binding cassette transporter A1 in macrophages.

Diabetes and insulin resistance increase the risk of cardiovascular disease caused by atherosclerosis through mechanisms that are poorly understood. Lipid-loaded macrophages are key contributors to all stages of atherosclerosis. We have recently shown that diabetes associated with increased plasma lipids reduces cholesterol efflux and levels of the reverse cholesterol transporter ABCA1 (ATP-binding cassette transporter A1) in mouse macrophages, which likely contributes to macrophage lipid accumulation in diabetes. Furthermore, we and others have shown that unsaturated fatty acids reduce ABCA1-mediated cholesterol efflux, and that this effect is mediated by the acyl-CoA derivatives of the fatty acids. We therefore investigated whether acyl-CoA synthetase 1 (ACSL1), a key enzyme mediating acyl-CoA synthesis in macrophages, could directly influence ABCA1 levels and cholesterol efflux in these cells. Mouse macrophages deficient in ACSL1 exhibited reduced sensitivity to oleate- and linoleate-mediated ABCA1 degradation, which resulted in increased ABCA1 levels and increased apolipoprotein A-I-dependent cholesterol efflux in the presence of these fatty acids, as compared with wildtype mouse macrophages. Conversely, overexpression of ACSL1 resulted in reduced ABCA1 levels and reduced cholesterol efflux in the presence of unsaturated fatty acids. Thus, the reduced ABCA1 and cholesterol efflux in macrophages subjected to conditions of diabetes and elevated fatty load may, at least in part, be mediated by ACSL1. These observations raise the possibility that ABCA1 levels could be increased by inhibition of acyl-CoA synthetase activity in vivo. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Obesity and weight loss result in increased adipose tissue ABCG1 expression in db/db mice.

The prevalence of obesity has reached epidemic proportions and is associated with several co-morbid conditions including diabetes, dyslipidemia, cancer, atherosclerosis and gallstones. Obesity is associated with low systemic inflammation and an accumulation of adipose tissue macrophages (ATMs) that are thought to modulate insulin resistance. ATMs may also modulate adipocyte metabolism and take up lipids released during adipocyte lipolysis and cell death. We suggest that high levels of free cholesterol residing in adipocytes are released during these processes and contribute to ATM activation and accumulation during obesity and caloric restriction. Db/db mice were studied for extent of adipose tissue inflammation under feeding conditions of ad libitum (AL) and caloric restriction (CR). The major finding was a marked elevation in epididymal adipose ABCG1 mRNA levels with obesity and CR (6-fold and 16-fold, respectively) over that seen for lean wild-type mice. ABCG1 protein was also elevated for CR as compared to AL adipose tissue. ABCG1 is likely produced by cholesterol loaded ATMs since this gene is not highly expressed in adipocytes and ABCG1 expression is sterol mediated. Our data supports the concept that metabolic changes in adipocytes due to demand lipolysis and cell death lead to cholesterol loading of ATMs. Based on finding cholesterol-loaded peritoneal leukocytes with elevated levels of ABCG1 in CR as compared to AL mice, we suggest that pathways for cholesterol trafficking out of adipose tissue involve ATM egress as well as ABCG1 mediated cholesterol efflux. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Cholesterol accumulation regulates expression of macrophage proteins implicated in proteolysis and complement activation.

OBJECTIVE: Cholesterol accumulation by macrophages plays a key role in atherogenesis. To begin to develop a global picture of this process, we used proteomics and transcriptomics to analyze foam cells generated with acetyl-low-density lipoprotein, a classic ligand for scavenger receptors. METHODS AND RESULTS: Tandem mass spectrometry and stringent statistical analysis revealed that foam cells differentially expressed 15 of 542 proteins (2.8%) detected in macrophage-conditioned medium. Apolipoprotein E was one of the most upregulated proteins, confirming that proteins involved in lipid metabolism are important targets for regulation by sterol accumulation. However, levels of proteins linked to complement activation and lysosomal proteolysis also changed markedly. Transcriptional analysis demonstrated that 698 of 19,700 genes (3.5%) were regulated in foam cells, including many genes important in sterol metabolism. We also found that cholesterol accumulation regulated genes implicated in complement activation but failed to affect genes linked to proteolysis and macrophage polarization. Changes in protein levels in macrophage-conditioned medium were largely independent of changes in mRNA levels. CONCLUSIONS: Loading sterol into macrophages regulates levels of complement proteins and lysosomal proteases-key players in the immune system and plaque rupture. Posttranscriptional mechanisms are likely important for controlling levels of most of the proteins detected in macrophage medium.

Serum amyloid A facilitates the binding of high-density lipoprotein from mice injected with lipopolysaccharide to vascular proteoglycans.

OBJECTIVE: Levels of serum amyloid A (SAA), an acute-phase protein carried on high-density lipoprotein (HDL), increase in inflammatory states and are associated with increased risk of cardiovascular disease. HDL colocalizes with vascular proteoglycans in atherosclerotic lesions. However, its major apolipoprotein, apolipoprotein A-I, has no proteoglycan-binding domains. Therefore, we investigated whether SAA, which has proteoglycan-binding domains, plays a role in HDL retention by proteoglycans. METHODS AND RESULTS: HDL from control mice and mice deficient in both SAA1.1 and SAA2.1 (SAA knockout mice) injected with bacterial lipopolysaccharide (LPS) was studied. SAA mRNA expression in the liver and plasma levels of SAA increased dramatically in C57BL/6 mice after LPS administration, although HDL cholesterol did not change. Fast protein liquid chromatography analysis showed most of the SAA to be in HDL. Mass spectrometric analysis indicated that HDL from LPS-injected control mice had high levels of SAA1.1/2.1 and reduced levels of apolipoprotein A-I. HDL from LPS-injected control mice demonstrated high-affinity binding to biglycan relative to normal mouse HDL. In contrast, HDL from LPS-injected SAA knockout mice showed very little binding to biglycan, consistent with SAA facilitating the binding of HDL to vascular proteoglycans. CONCLUSION: SAA enrichment of HDL under inflammatory conditions plays an important role in the binding of HDL to vascular proteoglycans.

Modifying apolipoprotein A-I by malondialdehyde, but not by an array of other reactive carbonyls, blocks cholesterol efflux by the ABCA1 pathway.

Dysfunctional high density lipoprotein (HDL) is implicated in the pathogenesis of cardiovascular disease, but the underlying pathways remain poorly understood. One potential mechanism involves covalent modification by reactive carbonyls of apolipoprotein A-I (apoA-I), the major HDL protein. We therefore determined whether carbonyls resulting from lipid peroxidation (malondialdehyde (MDA) and hydroxynonenal) or carbohydrate oxidation (glycolaldehyde, glyoxal, and methylglyoxal) covalently modify lipid-free apoA-I and inhibit its ability to promote cellular cholesterol efflux by the ABCA1 pathway. MDA markedly impaired the ABCA1 activity of apoA-I. In striking contrast, none of the other four carbonyls were effective. Liquid chromatography-electrospray ionization-tandem mass spectrometry of MDA-modified apoA-I revealed that Lys residues at specific sites had been modified. The chief adducts were MDA-Lys and a Lys-MDA-Lys cross-link. Lys residues in the C terminus of apoA-I were targeted for cross-linking in high yield, and this process may hinder the interaction of apoA-I with lipids and ABCA1, two key steps in reverse cholesterol transport. Moreover, levels of MDA-protein adducts were elevated in HDL isolated from human atherosclerotic lesions, suggesting that lipid peroxidation might render HDL dysfunctional in vivo. Taken together, our observations indicate that MDA damages apoA-I by a pathway that generates lysine adducts at specific sites on the protein. Such damage may facilitate the formation of macrophage foam cells by impairing cholesterol efflux by the ABCA1 pathway.

High-density lipoprotein suppresses the type I interferon response, a family of potent antiviral immunoregulators, in macrophages challenged with lipopolysaccharide.

BACKGROUND: High-density lipoprotein (HDL) protects the artery wall by removing cholesterol from lipid-laden macrophages. However, recent evidence suggests that HDL might also inhibit atherogenesis by combating inflammation. METHODS AND RESULTS: To identify potential antiinflammatory mechanisms, we challenged macrophages with lipopolysaccharide, an inflammatory microbial ligand for Toll-like receptor 4. HDL inhibited the expression of 30 (277 of 911) of the genes normally induced by lipopolysaccharide, microarray analysis revealed. One of its major targets was the type I interferon response pathway, a family of potent viral immunoregulators controlled by Toll-like receptor 4 and the TRAM/TRIF signaling pathway. Unexpectedly, the ability of HDL to inhibit gene expression was independent of macrophage cholesterol stores. Immunofluorescent studies suggested that HDL promoted TRAM translocation to intracellular compartments, which impaired subsequent signaling by Toll-like receptor 4 and TRIF. To examine the potential in vivo relevance of the pathway, we used mice deficient in apolipoprotein A-I, the major protein of HDL. After infection with Salmonella typhimurium, a Gram-negative bacterium that expresses lipopolysaccharide, apolipoprotein A-I-deficient mice had 6-fold higher plasma levels of interferon-ß, a key regulator of the type I interferon response, than did wild-type mice. CONCLUSIONS: HDL inhibits a subset of lipopolysaccharide-stimulated macrophage genes that regulate the type I interferon response, and its action is independent of sterol metabolism. These findings raise the possibility that regulation of macrophage genes by HDL might link innate immunity and cardioprotection.

Oxidation of apolipoprotein A-I by myeloperoxidase impairs the initial interactions with ABCA1 required for signaling and cholesterol export.

A key cardioprotective effect of high-density lipoprotein involves the interaction of its major protein, apolipoprotein A-I (apoA-I) with ATP-binding cassette transporter A1 (ABCA1), a macrophage cholesterol exporter. ApoA-I is thought to remove cholesterol from macrophages by a cascade of events. First it binds directly to ABCA1, activating signaling pathways, and then it binds to and solubilizes lipid domains generated by ABCA1. HDL isolated from human atherosclerotic lesions and blood of subjects with established coronary artery disease contains elevated levels of 3-chlorotyrosine and 3-nitrotyrosine, two characteristic products of myeloperoxidase (MPO), a heme protein secreted by macrophages. Here we show that chlorination (but not nitration) of apoA-I by the MPO pathway impairs its ability to interact directly with ABCA1, to activate the Janus kinase 2 signaling pathway, and to promote efflux of cellular cholesterol. In contrast, oxidation of apoA-I has little effect on its ability to stabilize ABCA1 protein or to solubilize phospholipids. Our results indicate that chlorination of apoA-I by the MPO pathway selectively inhibits two critical early events in cholesterol efflux: (1) the binding of apoA-I to ABCA1 and (2) the activation of a key signaling pathway. Therefore, oxidation of apoA-I in the artery wall by MPO-generated chlorinating intermediates may contribute to atherogenesis by impairing cholesterol efflux from macrophages.

Diabetes reduces the cholesterol exporter ABCA1 in mouse macrophages and kidneys.

Accumulation of cholesterol in arterial macrophages may contribute to diabetes-accelerated atherosclerotic cardiovascular disease. The ATP-binding cassette transporter ABCA1 is a cardioprotective membrane protein that mediates cholesterol export from macrophages. Factors elevated in diabetes, such as reactive carbonyls and free fatty acids, destabilize ABCA1 protein in cultured macrophages, raising the possibility that impaired ABCA1 plays an atherogenic role in diabetes. We therefore examined the modulation of ABCA1 in two mouse models of diabetes. We isolated peritoneal macrophages, livers, kidneys, and brains from type 1 non-obese diabetic (NOD) mice and mice made diabetic by viral-induced autoimmune destruction of pancreatic beta-cells, and we measured ABCA1 protein and mRNA levels and cholesterol contents. ABCA1 protein levels and cholesterol export activity were reduced by 40-44% (P<0.01) in peritoneal macrophages and protein levels by 48% (P<0.001) in kidneys in diabetic NOD mice compared with nondiabetic animals, even though ABCA1 mRNA levels were not significantly different. A similar selective reduction in ABCA1 protein was found in peritoneal macrophages (33%, P<0.05) and kidneys (35%, P<0.05) from the viral-induced diabetic mice. In liver and brain, however, diabetes had no effect or slightly increased ABCA1 protein and mRNA levels. The reduced ABCA1 in macrophages and kidneys was associated with increased cholesterol content. Impaired ABCA1-mediated cholesterol export could therefore contribute to the increased atherosclerosis and nephropathy associated with diabetes.

The macrophage cholesterol exporter ABCA1 functions as an anti-inflammatory receptor.

ATP-binding cassette transporter A1 (ABCA1) is a cell membrane protein that exports excess cholesterol from cells to apolipoprotein (apo) A-I, the major protein in high density lipoproteins. Genetic studies have shown that ABCA1 protects against cardiovascular disease. The interaction of apoA-I with ABCA1 promotes cholesterol removal and activates signaling molecules, such as Janus kinase 2 (JAK2), that optimize the lipid export activity of ABCA1. Here we show that the ABCA1-mediated activation of JAK2 also activates STAT3, which is independent of the lipid transport function of ABCA1. ABCA1 contains two candidate STAT3 docking sites that are required for the apoA-I/ABCA1/JAK2 activation of STAT3. The interaction of apoA-I with ABCA1-expressing macrophages suppressed the ability of lysopolysaccaride to induce the inflammatory cytokines interleukin-1beta, interleukin-6, and tumor necrosis factor-alpha, which was reversed by silencing STAT3 or ABCA1. Thus, the apoA-I/ABCA1 pathway in macrophages functions as an anti-inflammatory receptor through activation of JAK2/STAT3. These findings implicate ABCA1 as a direct molecular link between the cardioprotective effects of cholesterol export from arterial macrophages and suppressed inflammation.

The cell cholesterol exporter ABCA1 as a protector from cardiovascular disease and diabetes.

ATP-binding cassette transporter A1 (ABCA1) is an integral cell membrane protein that exports cholesterol from cells and suppresses macrophage inflammation. ABCA1 exports cholesterol by a multistep pathway that involves forming cell-surface lipid domains, solubilizing these lipids by apolipoproteins, binding of apolipoproteins to ABCA1, and activating signaling processes. Thus, ABCA1 behaves both as a lipid exporter and a signaling receptor. ABCA1 transcription is highly induced by sterols, and its expression and activity are regulated post-transcriptionally by diverse processes. ABCA1 mutations can reduce plasma HDL levels, accelerate cardiovascular disease, and increase the risk for type 2 diabetes. Genetic manipulations of ABCA1 expression in mice also affect plasma HDL levels, inflammation, atherogenesis, and pancreatic beta cell function. Metabolites elevated in individuals with the metabolic syndrome and diabetes destabilize ABCA1 protein and decrease cholesterol export from macrophages, raising the possibility that an impaired ABCA1 pathway contributes to the enhanced atherogenesis associated with common inflammatory and metabolic disorders. The ABCA1 pathway has therefore become a promising new therapeutic target for treating cardiovascular disease and diabetes.

Shotgun proteomics implicates protease inhibition and complement activation in the antiinflammatory properties of HDL.

HDL lowers the risk for atherosclerotic cardiovascular disease by promoting cholesterol efflux from macrophage foam cells. However, other antiatherosclerotic properties of HDL are poorly understood. To test the hypothesis that the lipoprotein carries proteins that might have novel cardioprotective activities, we used shotgun proteomics to investigate the composition of HDL isolated from healthy subjects and subjects with coronary artery disease (CAD). Unexpectedly, our analytical strategy identified multiple complement-regulatory proteins and a diverse array of distinct serpins with serine-type endopeptidase inhibitor activity. Many acute-phase response proteins were also detected, supporting the proposal that HDL is of central importance in inflammation. Mass spectrometry and biochemical analyses demonstrated that HDL3 from subjects with CAD was selectively enriched in apoE, raising the possibility that HDL carries a unique cargo of proteins in humans with clinically significant cardiovascular disease. Collectively, our observations suggest that HDL plays previously unsuspected roles in regulating the complement system and protecting tissue from proteolysis and that the protein cargo of HDL contributes to its antiinflammatory and antiatherogenic properties.

Myeloperoxidase: an oxidative pathway for generating dysfunctional high-density lipoprotein.

Accumulation of low-density lipoprotein (LDL)-derived cholesterol by artery wall macrophages triggers atherosclerosis, the leading cause of cardiovascular disease. Conversely, high-density lipoprotein (HDL) retards atherosclerosis by promoting cholesterol efflux from macrophages by the membrane-associated ATP-binding cassette transporter A1 (ABCA1) pathway. HDL has been proposed to lose its cardioprotective effects in subjects with atherosclerosis, but the underlying mechanisms are poorly understood. One potential pathway involves oxidative damage by myeloperoxidase (MPO), a heme enzyme secreted by human artery wall macrophages. We used mass spectrometry to demonstrate that HDL isolated from patients with established cardiovascular disease contains elevated levels of 3-chlorotyrosine and 3-nitrotyrosine, two characteristic products of MPO. When apolipoprotein A-I (apoA-I), the major HDL protein, was oxidized by MPO, its ability to promote cellular cholesterol efflux by ABCA1 was impaired. Moreover, oxidized apoA-I was unable to activate lecithin:cholesterol acyltransferase (LCAT), which rapidly converts free cholesterol to cholesteryl ester, a critical step in HDL maturation. Biochemical studies implicated tyrosine chlorination and methionine oxygenation in the loss of ABCA1 and LCAT activity by oxidized apoA-I. Oxidation of specific residues in apoA-I inhibited two key steps in cholesterol efflux from macrophages, raising the possibility that MPO initiates a pathway for generating dysfunctional HDL in humans.

ATP-Binding cassette cholesterol transporters and cardiovascular disease.

A hallmark of atherosclerotic cardiovascular disease (CVD) is the accumulation of cholesterol in arterial macrophages. Factors that modulate circulating and tissue cholesterol levels have major impacts on initiation, progression, and regression of CVD. Four members of the ATP-binding cassette (ABC) transporter family play important roles in this modulation. ABCA1 and ABCG1 export excess cellular cholesterol into the HDL pathway and reduce cholesterol accumulation in macrophages. ABCG5 and ABCG8 form heterodimers that limit absorption of dietary sterols in the intestine and promote cholesterol elimination from the body through hepatobiliary secretion. All 4 transporters are induced by the same sterol-sensing nuclear receptor system. ABCA1 expression and activity are also highly regulated posttranscriptionally by diverse processes. ABCA1 mutations can cause a severe HDL-deficiency syndrome characterized by cholesterol deposition in tissue macrophages and prevalent atherosclerosis. ABCG5 or ABCG8 mutations can cause sitosterolemia, in which patients accumulate cholesterol and plant sterols in the circulation and develop premature CVD. Disrupting Abca1 or Abcg1 in mice promotes accumulation of excess cholesterol in macrophages, and manipulating mouse macrophage ABCA1 expression affects atherogenesis. Overexpressing ABCG5 and ABCG8 in mice attenuates diet-induced atherosclerosis in association with reduced circulating and liver cholesterol. Metabolites elevated in individuals with the metabolic syndrome and diabetes destabilize ABCA1 protein and inhibit transcription of all 4 transporters. Thus, impaired ABC cholesterol transporters might contribute to the enhanced atherogenesis associated with common inflammatory and metabolic disorders. Their beneficial effects on cholesterol homeostasis have made these transporters important new therapeutic targets for preventing and reversing CVD.

Advanced glycation end product precursors impair ABCA1-dependent cholesterol removal from cells.

Abnormal HDL metabolism may contribute to the increased atherosclerosis associated with diabetes. The ATP-binding cassette transporter A1 (ABCA1) is an atheroprotective cell protein that mediates cholesterol transport from cells to apolipoprotein (apo) A-I, the major protein in HDL. Because formation of advanced glycation end products (AGEs) is associated with diabetic vascular complications, we examined the effects of carbonyls implicated in AGE formation on the ABCA1 pathway in cultured fibroblasts and macrophages. Treating cells with glycolaldehyde (GA) and glyoxal (GO) strongly inhibited ABCA1-dependent transport of cholesterol from cells to apoA-I, while methylglyoxal had little effect. This occurred under conditions where other lipoprotein receptors or lipid metabolic pathways were little affected, indicating that ABCA1 was uniquely sensitive to these carbonyls. GA and GO destabilized ABCA1 and nearly abolished its binding of apoA-I, indicating that these carbonyls directly modified ABCA1. Immunohistology of coronary arteries from hyperlipidemic swine revealed that inducing diabetes with streptozotocin increased atherosclerotic lesion area and dramatically reduced the fraction of macrophages that expressed detectable ABCA1. These results raise the possibility that reactive carbonyl-mediated damage to ABCA1 promotes accumulation of cholesterol in arterial macrophages and thus contribute to the increased cardiovascular disease associated with diabetes, insulin resistance, and other inflammatory conditions.

Pathways for oxidation of high-density lipoprotein in human cardiovascular disease.

The cardioprotective effect of high-density lipoprotein (HDL) is thought to involve, in part, the membrane-associated ATP-binding cassette transporter ABCA1, which clears cholesterol from lipid-laden macrophages in the artery wall. If HDL is unable to interact with this transporter because of oxidative damage, cholesterol clearance is impaired. Important insights into the mechanisms that oxidize proteins in the human artery wall have come from the mass spectrometric (MS) detection of oxidized amino acids that result from specific reaction pathways. Recent MS studies indicate that HDL isolated from patients with cardiovascular disease contains elevated levels of 3-chlorotyrosine and 3-nitrotyrosine, which are two characteristic products of myeloperoxidase (MPO), a heme enzyme secreted by macrophages. MPO-dependent chlorination of apolipoprotein A-I (apoA-I), the major HDL protein, impairs its ability to remove excess cellular cholesterol by the ABCA1 pathway. Tandem MS analysis of apoA-I has demonstrated that this loss of activity is associated with methionine oxidation and chlorination of a single tyrosine residue. Analysis of mutated forms of apoA-I has implicated lysine residues in the regiospecific chlorination of tyrosine. It is further suggested that the tyrosine chlorination and methionine oxidation of apoA-I impairs ABCA1 transport activity. The crystal structure of lipid-free apoA-I suggests a potential mechanism for rendering this protein dysfunctional. Collectively, these observations indicate that MPO oxidatively damages HDL in humans and suggest that oxidation of specific amino acid residues in apoA-I may contribute to atherogenesis by impairing cholesterol efflux from macrophages.

Molecular basis of cholesterol homeostasis: lessons from Tangier disease and ABCA1.

High-density lipoproteins (HDLs) play a role in transporting cholesterol from peripheral tissues to the liver for elimination from the body. Two hallmarks of cardiovascular disease are the presence of sterol-laden macrophages in the artery wall and reduced plasma HDL levels. A cell-membrane protein called ABCA1 mediates the secretion of excess cholesterol from cells into the HDL metabolic pathway. Mutations in ABCA1 cause Tangier disease, a severe HDL deficiency syndrome characterized by accumulation of cholesterol in tissue macrophages and prevalent atherosclerosis. Because of its ability to deplete macrophages of cholesterol and to raise plasma HDL levels, ABCA1 has become a promising therapeutic target for preventing cardiovascular disease.

Acrolein modifies apolipoprotein A-I in the human artery wall.

Carbonyl stress is implicated in accelerated vascular disease, but little is known about the factors that control the reactions of carbonyls with proteins. Acrolein is a reactive carbonyl generated by the oxidation of lipids and amino acids. It also forms during cigarette smoking. We therefore investigated the possibility that acrolein might react with apolipoprotein A-I (apoA-I), the major protein of high-density lipoprotein (HDL), which plays a critical role in mobilizing cholesterol from artery wall macrophages. Tandem mass spectrometric analysis demonstrated that lysine residues were the only amino acids in apoA-I that were modified by acrolein. Immunohistochemical studies with a monoclonal antibody revealed that acrolein adducts colocalized with apoA-I in human atherosclerotic lesions. Moreover, the ability of apoA-I to remove cholesterol from cultured cells was impaired after exposure to acrolein, suggesting that the carbonyl might interfere with apoA-I's normal function of promoting cholesterol efflux from artery wall cells. Our observations suggest that acrolein may interfere with normal HDL cholesterol transport by modifying apoA-I. This structural damage might play a critical role in atherogenesis by impairing cholesterol removal from artery wall cells.

HDL apolipoproteins and ABCA1: partners in the removal of excess cellular cholesterol.

It is widely believed that HDL protects against atherosclerosis by removing excess cholesterol from arterial cells. Lipid-poor HDL apolipoproteins promote efflux of cholesterol, phospholipids, and other lipophilic molecules from cells by an active process mediated by a cell-membrane transporter called the ATP binding cassette transporter A-1 (ABCA1). ABCA1 either directly or indirectly translocates phospholipids and cholesterol to the cell surface, where they appear to form lipid domains that interact with amphipathic alpha-helixes in apolipoproteins. This interaction solubilizes these lipids and generates nascent HDL particles that dissociate from the cell. Binding of apolipoproteins to ABCA1 may also enhance the activity of this lipid-transport pathway. Thus, the apolipoprotein/ABCA1 pathway efficiently clears cells of excess cholesterol that would otherwise accumulate as intracellular lipid droplets. ABCA1 expression is highly induced by cholesterol loading of cells and is also modulated by sterol-independent mechanisms at both the transcriptional and posttranslational level. Studies of human disease and animal models have shown that both an increased availability of apolipoproteins and an enhanced macrophage ABCA1 activity are atheroprotective. These findings implicate the apolipoprotein/ABCA1 pathway as an important therapeutic target for treating cardiovascular disease.

The cholesterol mobilizing transporter ABCA1 as a new therapeutic target for cardiovascular disease.

Atherosclerotic cardiovascular disease remains the leading cause of morbidity and mortality in Western societies. A hallmark of the developing atherosclerotic lesion is the appearance of cholesterol-laden macrophages in the artery wall. A cell membrane transporter called ABCA1 mediates the removal of excess cholesterol from macrophages into the lipoprotein pathway. This makes ABCA1 a promising new therapeutic target for reducing cholesterol deposits in tissues, eliminating excess cholesterol from the body, and preventing cardiovascular disease.

Direct effects of long-chain non-esterified fatty acids on vascular cells and their relevance to macrovascular complications of diabetes.

Diabetes leads to a marked increase in cardiovascular disease caused by atherosclerosis. Cardiovascular complications of diabetes are associated with lipid abnormalities, mainly manifested as elevated levels of triglycerides. Hydrolysis of triglycerides by lipases in the arterial wall is believed to cause increased levels of non-esterified fatty acids (NEFAs) in lesions of atherosclerosis. Recent research has shown that long-chain NEFAs have a multitude of direct effects on cell types involved in atherogenesis. Thus, some of the most common long-chained fatty acids present in triglycerides, oleic acid and linoleic acid, have been shown to induce adhesion molecule expression, cytokine expression and apoptosis in endothelial cells, to increase cholesterol uptake and reduce cholesterol efflux in macrophages, and to increase arterial smooth muscle cell proliferation and migration. Certain NEFAs liberated from triglycerides may therefore play an important role in accelerating atherosclerosis caused by diabetes by directly affecting the key cell types involved in atherogenesis.