Search

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).

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-β, 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.

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.

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.

SummaryCholesteryl ester accumulation by macrophages is a critical early event in atherogenesis. To test the hypothesis that sterol loading promotes foam cell formation and vascular disease by perturbing a network of interacting proteins, we used a global approach to identify proteins that are differentially expressed when macrophages are loaded with cholesterol in vivo. Our analysis revealed a sterol-responsive network that is highly enriched in proteins with known physical interactions, established roles in vesicular transport, and demonstrated atherosclerotic phenotypes in mice. Pharmacologic intervention with a statin or rosiglitazone and use of mice deficient in LDL receptor or apolipoprotein E implicated the network in atherosclerosis. Biochemical fractionation revealed that most of the sterol-responsive proteins resided in microvesicles, providing a physical basis for the network's functional and biochemical properties. These observations identify a highly integrated network of proteins whose expression is influenced by environmental, genetic, and pharmacological factors implicated in atherogenesis.

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.

ABCA1 exports cholesterol and phospholipids from cells 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. Here we used a mutational analysis approach to evaluate the relationship between these events. We prepared seven naturally occurring mutants and one artificial missense mutant of ABCA1 with varying degrees of impaired function, expressed them to similar levels as wild-type ABCA1 on the cell surface of BHK cells, and measured ABCA1-dependent lipid export, apolipoprotein A-I (apoA-I) binding, and signaling activities. Linear regression analyses showed that cholesterol and phospholipid efflux and cellular apoA-I binding correlated significantly with the ability of ABCA1 to form cell surface lipid domains. Lipid export and cellular apoA-I binding activities and formation of lipid domains also correlated with the amount of apoA-I that could be cross-linked to ABCA1. Moreover, each of these lipid export and apoA-I binding activities correlated with apoA-I-induced Janus kinase 2 (JAK2) activation. Thus, these missense mutations in ABCA1 impair lipid export, apoA-I binding, and apoA-I-stimulated JAK2 activities to similar extents, indicating that these processes are highly interactive components of a pathway that functions to export lipids from cells.

Phospholipid lipid transfer protein (PLTP) mimics high-density lipoprotein apolipoproteins in removing cholesterol and phospholipids from cells through the ATP-binding cassette transporter A1 (ABCA1). Because amphipathic α-helices are the structural determinants for ABCA1 interactions, we examined the ability of synthetic peptides corresponding to helices in PLTP to remove cellular cholesterol by the ABCA1 pathway. Of the seven helices tested, only one containing PLTP residues 144–163 (p144), located at the tip of the N-terminal barrel, promoted ABCA1-dependent cholesterol efflux and stabilized ABCA1 protein. Mutating methionine 159 (Met-159) in this helix in PLTP to aspartate (M159D) or glutamate (M159E) nearly abolished the ability of PLTP to remove cellular cholesterol and dramatically reduced PLTP binding to phospholipid vesicles and its phospholipid transfer activity. These mutations impaired PLTP binding to ABCA1-generated lipid domains and PLTP-mediated stabilization of ABCA1 but increased PLTP binding to ABCA1. PLTP interactions with ABCA1 also mimicked apolipoproteins in activating Janus kinase 2; however, the M159D/E mutants were also able to activate this kinase. Structural analyses showed that the M159D/E mutations had only minor effects on PLTP conformation. These findings indicate that PLTP helix 144–163 is critical for removing lipid domains formed by ABCA1, stabilizing ABCA1 protein, interacting with phospholipids, and promoting phospholipid transfer. Direct interactions with ABCA1 and activation of signaling pathways likely involve other structural determinants of PLTP.

High-density lipoprotein (HDL) inhibits atherosclerosis by removing cholesterol from artery wall macrophages. Additionally, HDL is anti-inflammatory in animal studies, suggesting that this property might also be important for its cardioprotective effects. Recent studies in subjects with established cardiovascular disease (CVD) demonstrate that myeloperoxidase targets HDL for oxidation and blocks the lipoprotein's ability to remove excess cholesterol from cells, raising the possibility that the enzyme provides a specific mechanism for generating dysfunctional HDL in humans. Shotgun proteomic analysis of HDL identified multiple complement regulatory proteins, protease inhibitors, and acute-phase response proteins, supporting a central role for HDL in inflammation. Mass spectrometry and biochemical analyses demonstrated that HDL(3) from CVD subjects was selectively enriched in apolipoprotein E, suggesting that it carries a unique cargo of proteins in humans with clinically significant CVD. Thus, oxidative modifications to HDL and changes in its protein composition might be useful biomarkers-and perhaps mediators-of CVD.

Cholesterol plays an important role during brain development, since it is involved in glial cell proliferation, neuronal survival and differentiation, and synaptogenesis. Astrocytes produce large amounts of brain cholesterol and produce and release lipoproteins containing apoE that can extract cholesterol from CNS cells for elimination. We hypothesized that some of the deleterious effects of ethanol in the developing brain may be due to the disruption of cholesterol homeostasis in astrocytes. This study investigates the effect of ethanol on cholesterol efflux mediated by ATP-binding cassette (ABC) cholesterol transporters. In fetal rat astrocytes in culture, ethanol caused a concentration-dependent increase in cholesterol efflux and increased the levels of ABCA1 starting at 25 mm. Similar effects of ethanol on cholesterol efflux and ABCA1 were also observed in fetal human astrocytes. In addition, ABCA1 levels were increased in the brains of 7-day-old pups treated for 3 days with 2, 4, or 6 g/kg ethanol. Ethanol also increased apoE release from fetal rat astrocytes, and conditioned medium prepared from ethanol-treated astrocytes extracted more cholesterol than conditioned medium from untreated cells. In addition, ethanol increased the levels of another cholesterol transporter, ABCG1. Ethanol did not affect cholesterol synthesis and reduced the levels of intracellular cholesterol in rat astrocytes. Retinoic acid, which induces teratogenic effects similarly to ethanol, also caused up-regulation of ABCA1 and ABCG1.

High-density lipoproteins (HDLs) protect against cardiovascular disease by removing cholesterol from arterial macrophages. One important pathway involves HDL apolipoproteins, which remove cellular cholesterol and phospholipids by an active process mediated by a cell membrane protein termed ATP-binding cassette transporter (ABC)A1. The interaction of apolipoproteins with ABCA1-expressing cells elicits several biological responses that coordinate removal of cellular lipids, including solubilizing membrane lipid domains, stabilizing ABCA1 protein and transducing signals. The apolipoprotein structural determinant responsible for all these activities is the amphipathic α-helix. Human and animal studies have shown that a reduced apolipoprotein supply and impaired macrophage ABCA1 activity are atherogenic. Metabolic factors associated with diabetes, the metabolic syndrome and inflammation can damage apolipoproteins and ABCA1, raising the possibility that impaired cholesterol removal from arterial cells contribut...

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.

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.

Evidence indicates that high density lipoprotein (HDL) is cardioprotective and that several mechanisms are involved. One important pathway is a membrane-associated ATP-binding cassette transporter, ABCA1, that clears cholesterol from macrophage foam cells. Anti-inflammatory and antioxidant properties also might contribute to HDL's ability to inhibit atherosclerosis.Myeloperoxidase targets HDL for oxidation, raising the possibility that the enzyme provides a specific mechanism for generating dysfunctional HDL in humans. Myeloperoxidase-dependent oxidation of apolipoprotein A-I, the major protein in HDL, blocks HDL's ability to remove excess cholesterol from cells by the ABCA1 pathway. Analysis of mutated forms of apoA-I and oxidized apoA-I treated with methionine sulfoxide reductase implicate oxidation of specific tyrosine and methionine residues in impairing the ABCA1 transport activity of apoA-I. The crystal structure of lipid-free apoA-I suggests that such oxidative damage might disrupt negatively charged regions on the protein's surface or alter its remodeling, resulting in conformations that fail to interact with ABCA1.Oxidation of HDL by myeloperoxidase may represent a specific molecular mechanism for converting the cardioprotective lipoprotein into a dysfunctional form, raising the possibility that the enzyme represents a potential therapeutic target for preventing vascular disease in humans. Moreover, oxidized HDL might prove useful as a blood marker for clinically significant cardiovascular disease in humans.

High density lipoprotein (HDL) isolated from human atherosclerotic lesions and the blood of patients with established coronary artery disease contains elevated levels of 3-chlorotyrosine. Myeloperoxidase (MPO) is the only known source of 3-chlorotyrosine in vivo, indicating that MPO oxidizes HDL in humans. We previously reported that Tyr-192 is the major site that is chlorinated in apolipoprotein A-I (apoA-I), the chief protein in HDL, and that chlorinated apoA-I loses its ability to promote cholesterol efflux from cells by the ATP-binding cassette transporter A1 (ABCA1) pathway. However, the pathways that promote the chlorination of specific Tyr residues in apoA-I are controversial, and the mechanism for MPO-mediated loss of ABCA1-dependent cholesterol efflux of apoA-I is unclear. Using site-directed mutagenesis, we now demonstrate that lysine residues direct tyrosine chlorination in apoA-I. Importantly, methionine residues inhibit chlorination, indicating that they can act as local, protein-bound antioxidants. Moreover, we observed near normal cholesterol efflux activity when Tyr-192 of apoA-I was mutated to Phe and the oxidized protein was incubated with methionine sulfoxide reductase. Thus, a combination of Tyr-192 chlorination and methionine oxidation is necessary for depriving apoA-I of its ABCA1-dependent cholesterol transport activity. Our observations suggest that biologically significant oxidative damage of apoA-I involves modification of a limited number of specific amino acids, raising the feasibility of producing oxidation-resistant forms of apoA-I that have enhanced anti-atherogenic activity in vivo.

Blood high-density lipoprotein (HDL) levels are inversely related to risk for cardiovascular disease, implying that factors associated with HDL metabolism are atheroprotective. One of these factors is ATP-binding cassette transporter A1 (ABCA1), a cell membrane protein that mediates the transport of cholesterol, phospholipids, and other metabolites from cells to lipid-depleted HDL apolipoproteins. ABCA1 transcription is highly induced by sterols, a major substrate for cellular export, and its expression and activity are regulated posttranscriptionally by diverse processes. Liver ABCA1 initiates formation of HDL particles, and macrophage ABCA1 protects arteries from developing atherosclerotic lesions. ABCA1 mutations can cause a severe HDL deficiency syndrome characterized by cholesterol deposition in tissue macrophages and prevalent atherosclerosis. Genetic manipulations of ABCA1 expression in mice also affect plasma HDL levels and atherogenesis. Metabolites elevated in individuals with the metabolic syndrome and diabetes destabilize ABCA1 protein and decrease cholesterol export from macrophages. Moreover, oxidative modifications of HDL found in patients with cardiovascular disease reduce the ability of apolipoproteins to remove cellular cholesterol by the ABCA1 pathway. These observations raise 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 an important new therapeutic target for treating cardiovascular disease.

Abnormal high density lipoprotein (HDL) metabolism among patients with diabetes and insulin resistance may contribute to their increased risk of atherosclerosis. ATP-binding cassette transporter ABCA1 mediates the transport of cholesterol and phospholipids from cells to HDL apolipoproteins and thus modulates HDL levels and atherogenesis. Unsaturated fatty acids, which are elevated in diabetes, impair the ABCA1 pathway in cultured cells by destabilizing ABCA1 protein. Here we examined the cellular pathway that mediates the ABCA1 destabilizing effects of fatty acids. The long-chain acyl-CoA synthetase inhibitor triacsin C completely reversed fatty acid-induced ABCA1 destabilization, indicating that fatty acids need to be activated to their CoA derivatives to enhance ABCA1 degradation. Unsaturated but not saturated fatty acids stimulated phospholipase D (PLD) activity, the PLD inhibitor 1-butanol prevented the unsaturated fatty acid-induced reduction in ABCA1 levels, and the PLD2 activator mastoparan markedly reduced ABCA1 protein levels, implicating a role for PLD2 in the ABCA1 destabilizing effects of fatty acids. Unsaturated fatty acids and mastoparan increased phosphorylation of ABCA1 serines. PLD2 small interfering RNA abolished the ability of unsaturated fatty acids to inhibit lipid transport activity, to reduce protein levels, and to increase serine phosphorylation of ABCA1. The diacylglycerol analog oleoylacetylglycerol also reduced ABCA1 protein levels and increased its serine phosphorylation, suggesting that PLD2-generated diacylglycerols promote the destabilizing phosphorylation of ABCA1. These data provide evidence that intracellular unsaturated acyl-CoA derivatives destabilize ABCA1 by activating a PLD2 signaling pathway.

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.

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.

High density lipoprotein (HDL) is the major carrier of lipid hydroperoxides in plasma, but it is not yet established whether HDL proteins are damaged by reactive nitrogen species in the circulation or artery wall. One pathway that generates such species involves myeloperoxidase (MPO), a major constituent of artery wall macrophages. Another pathway involves peroxynitrite, a potent oxidant generated in the reaction of nitric oxide with superoxide. Both MPO and peroxynitrite produce 3-nitrotyrosine in vitro. To investigate the involvement of reactive nitrogen species in atherogenesis, we quantified 3-nitrotyrosine levels in HDL in vivo. The mean level of 3-nitrotyrosine in HDL isolated from human aortic atherosclerotic intima was 6-fold higher (619 +/- 178 micromol/mol Tyr) than that in circulating HDL (104 +/- 11 micromol/mol Tyr; p0.01). Immunohistochemical studies demonstrated striking colocalization of MPO with epitopes reactive with an antibody to 3-nitrotyrosine. However, there was no significant correlation between the levels of 3-chlorotyrosine, a specific product of MPO, and those of 3-nitrotyrosine in lesion HDL. We also detected 3-nitrotyrosine in circulating HDL, and linear regression analysis demonstrated a strong correlation between the levels of 3-chlorotyrosine and levels of 3-nitrotyrosine. These observations suggest that MPO promotes the formation of 3-chlorotyrosine and 3-nitrotyrosine in circulating HDL but that other pathways also produce 3-nitrotyrosine in atherosclerotic tissue. Levels of HDL isolated from plasma of patients with established coronary artery disease contained twice as much 3-nitrotyrosine as HDL from plasma of healthy subjects, suggesting that nitrated HDL might be a marker for clinically significant vascular disease. The detection of 3-nitrotyrosine in HDL raises the possibility that reactive nitrogen species derived from nitric oxide might promote atherogenesis. Thus, nitrated HDL might represent a previously unsuspected link between nitrosative stress, atherosclerosis, and inflammation.

ATP-binding cassette transporter A1 (ABCA1) mediates transport of cellular cholesterol and phospholipids to high density lipoprotein (HDL) apolipoproteins, such as apoA-I. ABCA1 mutations can cause a severe HDL deficiency and atherosclerosis. Here we show that the protein-tyrosine kinase (TK) Janus kinase 2 (JAK2) modulates the apolipoprotein interactions with ABCA1 required for removing cellular lipids. The protein kinase A (PKA) inhibitor H89, the TK inhibitor genistein, and the JAK2 inhibitor AG490 suppressed apoA-I-mediated cholesterol and phospholipid efflux from ABCA1-expressing cells without altering the membrane ABCA1 content. Whereas PKA inhibition had no effect on apoA-I binding to cells or to ABCA1, TK and JAK2 inhibition greatly reduced these activities. Conversely, PKA but not JAK2 inhibition significantly reduced the intrinsic cholesterol translocase activity of ABCA1. Mutant cells lacking JAK2 had a severely impaired apoA-I-mediated cholesterol and phospholipid efflux and apoA-I binding despite normal ABCA1 protein levels and near normal cholesterol translocase activity. Thus, although PKA modulates ABCA1 lipid transport activity, JAK2 appears to selectively modulate apolipoprotein interactions with ABCA1. TK-mediated phosphorylation of ABCA1 was undetectable, implicating the involvement of another JAK2-targeted protein. Acute incubation of ABCA1-expressing cells with apoA-I had no effect on ABCA1 phosphorylation but stimulated JAK2 autophosphorylation. These results suggest that the interaction of apolipoproteins with ABCA1-expressing cells activates JAK2, which in turn activates a process that enhances apolipoprotein interactions with ABCA1 and lipid removal from cells.

Phospholipid lipid transfer protein (PLTP) is ubiquitously expressed in animal tissues and plays multiple roles in lipoprotein metabolism, but the function of peripheral PLTP is still poorly understood. Here we show that one of its possible functions is to transport cholesterol and phospholipids from cells to lipoprotein particles by a process involving PLTP interactions with cellular ATP-binding cassette transporter A1 (ABCA1). When ABCA1 was induced in murine macrophages or ABCA1-transfected baby hamster kidney cells, PLTP gained the ability to promote cholesterol and phospholipid efflux from cells. Although PLTP alone had lipid efflux activity, its maximum activity was observed in the presence of high density lipoprotein particles. Pulsechase studies showed that the interaction of PLTP with ABCA1-expressing cells played a role in promoting lipid efflux. Overexpression of ABCA1 dramatically increased binding of both PLTP and apoA-I to common sites on the cell surface. Both PLTP and apoA-I were covalently cross-linked to ABCA1, each protein blocked cross-linking of the other, and both PLTP and apoA-I stabilized ABCA1 protein. These results are consistent with PLTP and apoA-I binding to ABCA1 at the same or closely related sites. Thus, PLTP mimics apolipoproteins in removing cellular lipids by the ABCA1 pathway, except that PLTP acts more as an intermediary in the transfer of cellular lipids to lipoprotein particles.

HDL subspecies Lp(A-I) and Lp(A-I,A-II) have different anti-atherogenic potentials. To determine the role of lipoprotein lipase (LPL) and hepatic lipase (HL) in regulating these particles, we measured these enzyme activities in 28 healthy subjects with well-controlled Type 1 diabetes, and studied their relationship with Lp(A-I) and Lp(A-I,A-II). LPL was positively correlated with the apolipoprotein A-I (apoA-I), cholesterol, and phospholipid mass in total Lp(A-I), and with the apoA-I in large Lp(A-I) (r ≥ 0.58, P ≥ 0.001). HL was negatively correlated with all the above Lp(A-I) parameters plus Lp(A-I) triglyceride (r ≥ −0.53, P ≤ 0.003). No correlation was detected between LPL and Lp(A-I,A-II). However, HL was inversely correlated with total Lp(A-I,A-II) phospholipid, and with large Lp(A-I,A-II) (r ≥ 0.50, P ≤ 0.006). Similar studies were performed with phospholipid transfer protein (PLTP). Only total Lp(A-I) triglyceride in women (not men) (r = 0.71, P = 0.009) was significantly correlated with PLTP activity. These observations indicate that LPL and HL play major roles in determining the level and composition of plasma Lp(A-I), particularly large Lp(A-I), but not with Lp(A-I,A-II) level. Furthermore, select correlations of LPL and/or HL with the apoA-I, cholesterol, and triglyceride of Lp(A-I) but not Lp(A-I,A-II) imply that the apoA-I and lipid of Lp(A-I) and Lp(A-I,A-II) are not fully equilibrated.

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 α-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.

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 named ATP-binding cassette transporter A1 (ABCA1) mediates secretion of excess cholesterol from cells into the HDL metabolic pathway. The discovery of ABCA1 in 1999 triggered a deluge of studies conducted to characterize the properties of this important transporter. The present review summarizes the more recent of those studies and evaluates their implications for the role of ABCA1 in cholesterol transport, HDL metabolism, and atherogenesis.Cell culture experiments have shown that ABCA1 transports cholesterol, phospholipids, and other lipophilic molecules across the plasma membrane, where they are picked up by apolipoproteins containing little or no lipids, but the mechanisms involved are still unclear. It is now apparent that factors in addition to sterols modulate ABCA1 expression by diverse transcriptional and post-transcriptional processes. Studies in humans and mice with ABCA1 mutations revealed that the relative activity of ABCA1 determines plasma HDL levels and influences susceptibility to cardiovascular disease. Mouse models are beginning to provide insights into the function of ABCA1 in vivo but are also raising new questions regarding the contribution of ABCA1 to total cholesterol flux.Recent studies underscore the critical role of ABCA1 in clearing excess cholesterol from macrophages and generating HDL particles, implicating ABCA1 as an attractive new therapeutic target for treating cardiovascular disease.

Abnormal high density lipoprotein metabolism may contribute to the increased atherosclerosis associated with diabetes and insulin resistance. The ATP-binding cassette transporter ABCA1 mediates cholesterol transport from tissue macrophages to apoA-I, the major high density lipoprotein protein component. Because fatty acids are elevated in diabetes, we examined the effects of fatty acids on ABCA1 activity in cultured macrophages. Results showed that unsaturated fatty acids markedly inhibited ABCA1-mediated cholesterol and phospholipid efflux from macrophages when ABCA1 was induced by a cAMP analog. This was accompanied by a reduction in the membrane content of ABCA1 and a decrease in apoA-I binding to whole cells and to ABCA1. In contrast, saturated fatty acids had no effect on these processes. Fatty acids did not alter ABCA1 mRNA abundance or incorporation of methionine into ABCA1, indicating that decreased ABCA1 transcription, enhanced mRNA decay, or impaired translation efficiency did not account for these inhibitory effects. Unsaturated fatty acids, however, increased ABCA1 turnover when protein synthesis was blocked by cycloheximide. We conclude that unsaturated fatty acids reduce the macrophage ABCA1 content by enhancing its degradation rate. These findings raise the possibility that an increased supply of unsaturated fatty acids in the artery wall promotes atherogenesis by impairing the ABCA1 cholesterol secretory pathway in macrophages.

Alpha-tocopherol (alpha-TOH) is associated with plasma lipoproteins and accumulates in cell membranes throughout the body, suggesting that lipoproteins play a role in transporting alpha-TOH between tissues. Here we show that secretion of alpha-TOH from cultured cells is mediated in part by ABCA1, an ATP-binding cassette protein that transports cellular cholesterol and phospholipids to lipid-poor high density lipoprotein (HDL) apolipoproteins such as apoA-I. Treatment of human fibroblasts and murine RAW264 macrophages with cholesterol and/or 8-bromo-cyclic AMP, which induces ABCA1 expression, enhanced apoA-I-mediated alpha-TOH efflux. ApoA-I lacked the ability to remove alpha-TOH from Tangier disease fibroblasts that have a nonfunctional ABCA1. BHK cells that lack an active ABCA1 pathway markedly increased secretion of alpha-TOH to apoA-I when forced to express ABCA1. ABCA1 also mediated a fraction of the alpha-TOH efflux promoted by lipid-containing HDL particles, indicating that HDL promotes alpha-TOH efflux by both ABCA1-dependent and -independent processes. Exposing apoA-I to ABCA1-expressing cells did not enhance its ability to remove alpha-TOH from cells lacking ABCA1, consistent with this transporter participating directly in the translocation of alpha-TOH to apolipoproteins. These studies provide evidence that ABCA1 mediates secretion of cellular alpha-TOH into the HDL metabolic pathway, a process that may facilitate vitamin transport between tissues and influence lipid oxidation.

It is widely believed that HDL functions to transport cholesterol from peripheral cells to the liver by reverse cholesterol transport, a pathway that may protect against atherosclerosis by clearing excess cholesterol from arterial cells. A cellular ATP-binding cassette transporter (ABC) called ABCA1 mediates the first step of reverse cholesterol transport: the transfer of cellular cholesterol and phospholipids to lipid-poor apolipoproteins. Mutations in ABCA1 cause Tangier disease (TD), a severe HDL deficiency syndrome characterized by accumulation of cholesterol in tissue macrophages and prevalent atherosclerosis. Studies of TD heterozygotes revealed that ABCA1 activity is a major determinant of plasma HDL levels and susceptibility to CVD. Drugs that induce ABCA1 in mice increase clearance of cholesterol from tissues and inhibit intestinal absorption of dietary cholesterol. Multiple factors related to lipid metabolism and other processes modulate expression and tissue distribution of ABCA1. Therefore, as the primary gatekeeper for eliminating tissue cholesterol, ABCA1 has a major impact on cellular and whole body cholesterol metabolism and is likely to play an important role in protecting against cardiovascular disease. —Oram, J. F., and R. M. Lawn. ABCA1: the gatekeeper for eliminating excess tissue cholesterol.

High density lipoprotein (HDL) phospholipids and apolipoproteins remove cellular lipids by two distinct mechanisms, but their relative contribution to reverse choles- terol transport is unknown. Whereas phospholipid-mediated cholesterol efflux from cultured cells reflects the activity of the HDL receptor SR-BI, apolipoprotein-mediated lipid re- moval is regulated in response to changes in cellular choles- terol content (positive) and cell proliferation rates (negative). Here we show that immortalization of human skin fibroblast lines with the papillomavirus E6/E7 oncogenes increased their proliferation rates and selectively reduced the activity of the apolipoprotein-mediated lipid removal pathway. This re- duction was accompanied by a decrease in cellular cAMP levels and was reversed by treatment with a cAMP analog. The stimulatory effect of cAMP was independent of changes in cellular phenotype or activities of cholesteryl ester cycle en- zymes. The severely impaired apolipoprotein-mediated lipid removal pathway in Tangier disease fibroblasts, which persisted after immortalization, was not improved by treat- ment with a cAMP analog, implying that the cellular defect in Tangier disease is upstream from this cAMP-dependent signaling pathway. These results indicate that papilloma- virus-induced immortalization of fibroblasts selectively re- duces the activity of the apolipoprotein-mediated lipid removal pathway by a cAMP-dependent process, perhaps to prevent loss of cellular lipids needed for continual mem- brane synthesis. —Oram, J. F., A. J. Mendez, J. Lymp, T. J. Kavanagh, and C. L. Halbert. Reduction in apolipoprotein- mediated removal of cellular lipids by immortalization of human fibroblasts and its reversion by cAMP: lack of effect with Tangier disease cells. J. Lipid Res. 1999. 40: 1769-1781.

Abstract Protein kinase C (PKC) seems to play an important role in many of HDL effects on cells, including removal of excess cholesterol. HDL removes cholesterol by at least two mechanisms. One mechanism involves desorption/diffusion of cholesterol from the plasma membrane onto the acceptor particle, whereas the second is mediated by apolipoproteins and may involve intracellular translocation of cholesterol to the plasma membrane for subsequent efflux. In this report, we examined the possibility that mitogen-activated protein (MAP) kinase is one of the downstream events from HDL activation of PKC. Using a gel kinase assay with myelin basic protein incorporated into the gel, HDL (50 μg protein/mL) stimulated multiple kinases of 42, 50, 52, 58, and 60 kDa. The 42-kDa protein kinase, corresponding to the unresolved MAP kinases ERK1 and ERK2 based on immunoblotting, was activated over 2-fold by HDL. HDL activated all identified kinases in a concentration- and time-dependent manner, which became maximal within 5 to 10 minutes and remained activated for at least 60 minutes. HDL activation of MAP kinase seems to be partially mediated by PKC, because down-regulation of PKC and known PKC inhibitors inhibited the HDL effect by 40 to 50%. Free apolipoproteins A-I (10 μg/mL) and A-II (10 μg/mL) had no significant effect on MAP kinase activation. Moreover, modifying HDL with trypsin or tetranitromethane, which abolishes apolipoprotein-mediated cholesterol efflux, had no effect on HDL activation of MAP kinase. These results suggest that HDL activates MAP kinase via multiple signal transduction pathways that are likely involved in an HDL effect unrelated to apolipoprotein-mediated cholesterol translocation and efflux.

It is widely believed that high density lipoprotein (HDL) protects against cardiovascular disease by removing excess cholesterol from cells of the artery wall. Recent cell culture studies have provided evidence that a major pathway for removing cholesterol and phospholipids from cells is mediated by the direct interactions of HDL apolipoproteins (apo) with plasma membrane domains. These interactions efficiently clear cells of excess sterol by targeting for removal pools of cholesterol that feed into the cholesteryl ester cycle. The precursors for this pathway in vivo are likely to be lipid-free or lipid-poor apolipoproteins generated either by dissociation from the surface of HDL particles or by de novo synthesis. Fibroblasts from subjects with a severe HDL deficiency syndrome called Tangier disease have a cellular defect that prevents apolipoproteins from removing both cholesterol and phospholipids from cells. This defect is associated with a near absence of plasma HDL, markedly below normal low density lipoprotein (LDL) levels, and the appearance of macrophage foam cells in tissues. Thus, an inability of nascent apoA-I to acquire cellular lipids results in a rapid clearance of apoA-I from the plasma, decreased production and increased clearance of LDL, and sterol deposition in tissue macrophages. Although the molecular properties of this pathway are still poorly understood, these studies imply that the apolipoprotein-mediated pathway for removal of cellular lipids is a major source of plasma cholesterol and phospholipids and plays an important role in clearing excess cholesterol from macrophages in vivo.

The strong inverse relationship between HDL level and risk for coronary artery disease has been attributed to different mechanisms. More than 30 years ago, Glomset1 proposed that HDL mediates the transfer of cholesterol from peripheral tissues to the liver, where the sterol and its oxygenated products are excreted into the bile. Subsequent studies demonstrated that HDL accepts cholesterol from macrophage foam cells, the cellular hallmark of the atherosclerotic lesion.2,3⇓ In this scenario, HDL accepts cholesterol from macrophage foam cells in the artery wall and transports it back to the liver for excretion.4,5⇓ Strong evidence for this process, termed reverse cholesterol transport, was provided by the recent identification of the molecular defect in Tangier disease as mutations in ABCA1, a membrane-associated ATP-binding cassette transporter.5–7⇓⇓ Patients with Tangier disease have abnormally low levels of HDL cholesterol and suffer from premature coronary artery disease.6 Importantly, macrophages with genetically engineered overexpression of ABCA1 fail to accumulate cholesteryl ester in vivo; in vitro, their ability to donate cholesterol to HDL is markedly increased.8 Studies of fibroblasts cultured from patients suffering from Tangier disease have shown that the mechanism involves the transfer of membrane-associated cholesterol from cells to poorly lipidated apolipoprotein A-I (apo A-I), the major protein component of HDL.9 See page 1583 A number of other properties of HDL could also contribute to its cardioprotective effects. Many lines of evidence support the hypothesis that oxidation converts LDL, the major carrier of blood cholesterol, into an atherogenic form.10,11⇓ Unmodified HDL protects LDL from oxidative modification by metal ion-dependent12 and -independent pathways.13 However, HDL that has been oxidized by …

Clearance of excess cholesterol from cells by HDL is facilitated by the interaction of HDL apolipoproteins with cell-surface binding sites or receptors, a process that may be important in preventing atherosclerosis. In this study, synthetic peptides containing 18-mer amphipathic helices of the class found in HDL apolipoproteins (class A) were tested for their abilities to remove cholesterol and phospholipid from cultured sterol-laden fibroblasts and macrophages and to interact with cell-surface HDL binding sites. Lipid-free peptides containing two identical tandem repeats of class A amphipathic helices promoted cholesterol and phospholipid efflux from cells and depleted cellular cholesterol accessible for esterification by acyl CoA/cholesterol acyltransferase, similar to what was observed for purified apolipoprotein A-I. Peptide-mediated removal of plasma membrane cholesterol and depletion of acyl CoA/cholesterol acyltransferase-accessible cholesterol appeared to occur by separate mechanisms, as the latter process was less dependent on extracellular phospholipid. The dimeric amphipathic helical peptides also competed for high-affinity HDL binding sites on cholesterol-loaded fibroblasts and displayed saturable high-affinity binding to the cell surface. In contrast, peptides with a single helix had little or no ability to remove cellular cholesterol and phospholipid, or to interact with HDL binding sites, suggesting that cooperativity between two or more helical repeats is required for these activities. Thus, synthetic peptides comprising dimers of a structural motif common to exchangeable apolipoproteins can mimic apolipoprotein A-I in both binding to putative cell-surface receptors and clearing cholesterol from cells.

While high-density lipoprotein (HDL) is known to protect against a wide range of inflammatory stimuli, its anti-inflammatory mechanisms are not well understood. Furthermore, HDL's protective effects against saturated dietary fats have not been previously described. In this study, we used endothelial cells to demonstrate that while palmitic acid activates NF-κB signaling, apolipoprotein A–I, (apoA-I), the major protein component of HDL, attenuates palmitate-induced NF-κB activation. Further, vascular NF-κB signaling (IL-6, MCP-1, TNF-α) and macrophage markers (CD68, CD11c) induced by 24 weeks of a diabetogenic diet containing cholesterol (DDC) is reduced in human apoA-I overexpressing transgenic C57BL/6 mice compared to age-matched WT controls. Moreover, WT mice on DDC compared to a chow diet display increased gene expression of lipid raft markers such as Caveolin-1 and Flotillin-1, and inflammatory Toll-like receptors (TLRs) (TLR2, TLR4) in the vasculature. However apoA-I transgenic mice on DDC show markedly reduced expression of these genes. Finally, we show that in endothelial cells TLR4 is recruited into lipid rafts in response to palmitate, and that apoA-I prevents palmitate-induced TLR4 trafficking into lipid rafts, thereby blocking NF-κB activation. Thus, apoA-I overexpression might be a useful therapeutic tool against vascular inflammation.

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).

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.

Previous studies conducted within the framework of the Lipid Research Clinics Program showed a strong inverse correlation between high-density lipoprotein cholesterol (HDL-C) level and coronary heart disease (CHD) risk in American male populations, whereas in Russian populations such a correlation was less pronounced. It was assumed that HDL was less protective of CHD in Russian than in American males. This study compared the functional activity and lipid composition of HDL3 isolated from the blood plasma of men with low, normal, and high HDL-C levels from Moscow (Russia) and Seattle (United States) populations. Results obtained showed that American HDL3, irrespective of the plasma HDL-C level, had higher activity in stimulating both [3H]cholesterol and cholesterol mass efflux from cholesterol-loaded fibroblasts and in suppressing cellular cholesterol esterification when compared with Russian HDL3. American HDL3 remained more active than Russian HDL3, even when apolipoprotein E-containing particles were removed from HDL3 by heparin-Sepharose affinity chromatography. Russian and American 125I-HDL3 had similar binding to high-affinity cell-surface sites, but Russian HDL3 had a higher nonspecific binding component compared with American HDL3. This study demonstrates for the first time potential functional differences between HDL particles isolated from Russian and American populations. The lower activity of Russian HDL3 in promoting cellular cholesterol efflux may partly explain the higher CHD risk in the Russian population compared with the American one.

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 β-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.

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.

The interaction of high density lipoproteins (HDL) with the HDL receptor stimulates the translocation of cholesterol from intracellular pools to the plasma membrane where the cholesterol becomes available for removal by appropriate acceptors. The role of signal transduction through protein kinase C in HDL receptor-dependent cholesterol translocation and efflux was examined using cholesterol-loaded cultured human skin fibroblasts. Treatment of cells with HDL3 activated protein kinase C, demonstrated by a transient increase in membrane associated kinase activity. Kinase activation appeared to be dependent on binding of HDL3 to the HDL receptor, since tetranitromethane-modified HDL3, which does not bind to the receptor, was without effect. Translocation of intracellular sterol to the plasma membrane was stimulated by treatment of cells with the protein kinase C activators, dioctanoylglycerol and phorbol myristic acetate, and the calcium ionophore A23187. Conversely, treatment of cells with sphingosine, a protein kinase C inhibitor, reduced HDL3-mediated translocation and efflux of intracellular sterols. However, sphingosine had no effect on efflux of labeled cholesterol derived from the plasma membrane. Down-regulation of cellular protein kinase C activity by long term incubation with phorbol esters also inhibited HDL3-mediated efflux of intracellular sterols and abolished the ability of sphingosine to further inhibit HDL3-mediated efflux. These studies support the conclusion that HDL receptor-mediated translocation and efflux of intracellular cholesterol occurs through activation of protein kinase C.

Previous studies have shown that nonenzymatic glycosylation of high-density lipoprotein (HDL) inhibits high-affinity binding to cultured cells and the candidate HDL-receptor protein. Because binding of HDL to its receptor is required for HDL-receptor–mediated cholesterol efflux from cells, we hypothesized that glycosylated HDL3 would have reduced ability to remove cholesterol from cells. HDL3 was glycosylated in vitro to achieve up to 40–50% reductions in free-lysine residues. Glycosylated HDL3 had a slightly greater ability than control HDL3 to sequester cholesterol directly from the plasma membrane, as predicted by changes in lipid composition. This process is independent of HDL-receptor binding and should not be influenced by reduced binding of HDL3. In contrast, efflux of intracellular cholesterol from cells, which is HDL-receptor dependent, was reduced 25–40%. The ability of glycosylated HDL3 to diminish cholesterol esterification was significantly reduced, indicating reduced net cholesterol efflux. Steady-state efflux of LDL-derived cholesterol was also markedly reduced. These findings suggest that nonenzymatically glycosylated HDL is functionally abnormal and might contribute to the accelerated development of atherosclerosis in patients with diabetes mellitus.

Among the known mechanisms of reverse cholesterol transport (RCT), ATP binding cassette transporter G1 (ABCG1)-mediated free cholesterol (FC) transport is the most recent and least studied. Here, we have characterized the efficiencies of different acceptors using baby hamster kidney (BHK) cells transfected with human ABCG1 cDNA, which is inducible upon treatment with mifepristone. When normalized on particle number and particle surface area, the acceptor efficiency for FC efflux was as follows: small unilamellar vesicles (SUV)LDLreconstituted HDLHDL(2) = HDL(3). Based on phospholipid content, the order was reversed. ABCG1 also mediated phospholipid efflux to human serum and HDL(3). ABCG1-mediated FC efflux correlated significantly with a number of HDL subfractions and components in serum collected from 25 normolipidemic individuals: apolipoprotein A-II (apoA-II) (r(2) = 0.7), apolipoprotein A-I (apoA-I) (r(2) = 0.5), HDL-C (r(2) = 0.4), HDL-PL (r(2) = 0.4), alpha-2 HDL (r(2) = 0.4), and prebeta HDL (r(2) = 0.2). ABCG1 did not enhance influx of FC or cholesteryl oleyl ether (COE) when cells were incubated with radiolabeled HDL(3). ABCG1 expression did not increase the association of HDL(3) with cells. Compared with control cells, ABCG1 expression significantly increased the FC pool available for efflux and the rate constant for efflux. In conclusion, composition and particle size determine the acceptor efficiency for ABCG1-mediated efflux. ABCG1 increases cell membrane FC pools and changes its rate of desorption into the aqueous phase without enhancing the association with the acceptor.

Population studies have shown a strong inverse relationship between plasma HDL levels and risk for cardiovascular disease, implying that factors associated with HDL metabolism are cardioprotective. One of these factors is ATP-binding cassette transporter A1 (ABCA1), an integral membrane protein that transports cholesterol and phospholipids to lipid-depleted HDL apolipoproteins, such as apolipoprotein A (apoA)-I (1). Studies of human HDL deficiencies and atherosclerosis-susceptible mouse models have shown that ABCA1 protects against atherogenesis and may prevent disease progression (2–4). These observations have made ABCA1 a promising new therapeutic target for treating cardiovascular disease. Thus, it is critical to understand how ABCA1 functions and is regulated. Despite this strong interest and intensive study over the last 8 years, we still do not have a clear picture of how and where ABCA1 mediates the transfer of cholesterol and phospholipids to apoA-I. Based on structural similarities with other better-characterized ABC transporters, it is assumed by many that ABCA1 pumps lipids to the cell surface where they become accessible for removal by apoA-I. There is evidence, however, that ABCA1bound apoA-I is endocytosed by macrophages and targeted to late endosomal or lysosomal compartments, where it picks up lipids and exports them from cells by an exocytosis process. The concept of this retroendocytosis pathway was introduced by Takahashi and Smith (5) before the discovery of ABCA1. They showed that apoA-I is internalized and resecreted by macrophages in association with cholesterol efflux from cells. Since then, several groups have shown that apoA-I and ABCA1 colocalize in endosomal compartments and that ABCA1 rapidly recycles between intracellular compartments and the plasma membrane (6–8). Cycling of proteins through cells, however, does not imply functional significance. It is well established that cultured cells, particularly macrophages, internalize and resecrete exogenous molecules without any apparent biological effect. In addition, cells rapidly recycle membrane proteins, including those bound to the surface. The possibility that an apoA-I retroendocytosis pathway actually functions to export lipids is supported largely by circumstantial evidence. Cells from subjects with lossof-function mutations in ABCA1 were shown to have defective late endocytotic trafficking and accumulation of cholesterol and phospholipids in late endocytotic vesicles (7), consistent with a role of ABCA1 in removing lipids from these compartments. Trapping ABCA1 in the plasma membrane by cyclosporine treatment was reported to reduce apoA-I-mediated cholesterol efflux (9). Similarly, deletion of a PEST sequence in ABCA1 inhibited its trafficking to late endosomes/lysosomes, and decreased apoA-I-mediated efflux of cholesterol derived from endocytosed and degraded lipoproteins (8). Taken together, these studies support the idea that ABCA1-bound apoA-I traffics to late endosomal vesicles and/or lysosomes, where it forms nascent lipoprotein particles that are subsequently secreted from the cell. There is strong evidence, however, that the ABCA1dependent transfer of cellular lipids to apoA-I occurs at the plasma membrane. Electron microscopy studies have shown that apoA-I selectively interacts with exovesiculated membrane protrusions on the surface of ABCA1expressing cells (10, 11). Inducing ABCA1 in the absence of apolipoproteins increases the amount of cholesterol that appears in plasma membrane domains accessible to the added enzyme cholesterol oxidase, and subsequent addition of apoA-I removes cholesterol exclusively from these domains (12). These findings indicate that ABCA1 can form lipid domains on the cell surface that are accessible for removal by apolipoproteins even before it has contact with apolipoproteins. It has also been shown that partitioning of ABCA1 to the plasma membrane can increase cholesterol efflux under some conditions (8, 13). As reported in the current issue of the Journal of Lipid Research, Faulkner et al. (14) used a three-pronged approach to test the functional relevance of an apoA-I/ ABCA1 retroendocytosis pathway in cultured RAW macrophages. They reasoned that if retroendocytosis plays an important role in apoA-I lipidation, apoA-I should be specifically internalized by cells in response to ABCA1 expression, the mass of apoA-I cycled through the cell should account for most of the lipid efflux, and the degree of apoA-I internalization should correlate with degree of cholesterol efflux. They generated fully functional fluores

Nonenzymatic glycosylation of plasma proteins may contribute to the excess risk of developing atherosclerosis in patients with diabetes mellitus. Because high-density lipoprotein (HDL) is believed to protect against atherosclerosis and is glycosylated at increased levels in diabetic individuals, the effects of nonenzymatic glycosylation of HDL3 on binding of HDL3 to cultured fibroblasts and to the candidate HDL-receptor protein were examined. HDL3 was glycosylated in vitro with glucose alone or in combination with sodium cyanoborohydride. With this catalyst, up to 40–50% of the lysine residues could be glycosylated, resulting in a progressive drop to nearly 60% in high-affinity binding to cultured fibr oblasts at 4°C. Binding to the 110,000-Mr candidate HDL-receptor protein was reduced by almost 75%. At levels of HDL glycosylation equivalent to the 3–5% observed in diabetes, high-affinity binding to fibroblasts at 4°C was diminished by up to 15–20%. Binding kinetic studies paradoxically suggested that glycosylated HDL3 binds with higher affinity to a reduced number of binding sites. The findings in this study suggest that nonenzymatically glycosylated HDL may be functionally abnormal and might contribute to the development of atherosclerosis in patients with diabetes mellitus.

Abnormal HDL metabolism among patients with diabetes and insulin resistance may contribute to their increased risk of atherosclerosis. ABCA1 mediates the transport of cholesterol and phospholipids from cells to HDL apolipoproteins and thus modulates HDL levels and atherogenesis. Unsaturated fatty acids, which are increased in diabetes, impair the ABCA1 pathway in cultured cells by destabilizing ABCA1 protein. We previously reported that unsaturated fatty acids destabilize ABCA1 in murine macrophages and ABCA1-transfected baby hamster kidney cells by increasing its serine phosphorylation through a phospholipase D (PLD) pathway. Here, we examined the cellular pathway downstream of PLD that mediates the ABCA1-destabilizing effects of unsaturated fatty acids. The protein kinase C delta (PKCdelta)-specific inhibitor rottlerin and PKCdelta small interfering RNA completely abolished the ability of unsaturated fatty acids to inhibit lipid transport activity, to reduce protein levels, and to increase serine phosphorylation of ABCA1, implicating a role for PKCdelta in the ABCA1-destabilizing effects of fatty acids. These data indicate that unsaturated fatty acids destabilize ABCA1 by activating a PKCdelta pathway that phosphorylates ABCA1 serines.

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.

ABCA1 mediates the transport of cellular cholesterol and phospholipids to HDL apolipoproteins. Apolipoprotein A-I (apoA-I) interactions with ABCA1-expressing cells elicit several responses, including removing cellular lipids, stabilizing ABCA1 protein, and activating Janus kinase 2 (JAK2). Here, we used synthetic apolipoprotein-mimetic peptides to characterize the relationship between these responses. Peptides containing one amphipathic helix of L- or D-amino acids (2F, D-2F, or 4F) and a peptide containing two helices (37pA) all promoted ABCA1-dependent cholesterol efflux, competed for apoA-I binding to ABCA1-expressing cells, blocked covalent cross-linking of apoA-I to ABCA1, and inhibited ABCA1 degradation. 37pA was cross-linked to ABCA1, confirming the direct binding of amphipathic helices to ABCA1. 2F, 4F, 37pA, and D-37pA all stimulated JAK2 autophosphorylation. Inhibition of JAK2 greatly reduced peptide-mediated cholesterol efflux, peptide binding to ABCA1-expressing cells, and peptide cross-linking to ABCA1, indicating that these processes require an active JAK2. In contrast, apoA-I and peptides stabilized ABCA1 protein even in the absence of an active JAK2, implying that this process is independent of JAK2 and lipid efflux-promoting binding of amphipathic helices to ABCA1. These findings show that amphipathic helices coordinate the activity of ABCA1 by several distinct mechanisms that are likely to involve different cell surface binding sites.

Acrolein is a highly reactive alpha,beta-unsaturated aldehyde, but the factors that control its reactions with nucleophilic groups on proteins remain poorly understood. Lipid peroxidation and threonine oxidation by myeloperoxidase are potential sources of acrolein during inflammation. Because both pathways are implicated in atherogenesis and high density lipoprotein (HDL) is anti-atherogenic, we investigated the possibility that acrolein might target the major protein of HDL, apolipoprotein A-I (apoA-I), for modification. Tandem mass spectrometric analysis demonstrated that lysine 226, located near the center of helix 10 in apoA-I, was the major site modified by acrolein. Importantly, this region plays a critical role in the cellular interactions and ability of apoA-I to transport lipid. Indeed, we found that conversion of Lys-226 to N(epsilon)-(3-methylpyridinium)lysine by acrolein associated quantitatively with decreased cholesterol efflux from cells via the ATP-binding cassette transporter A1 pathway. In the crystal structure of truncated apoA-I, Glu-234 lies adjacent to Lys-226, suggesting that negatively charged residues might direct the modification of specific lysine residues in proteins. Finally, immunohistochemical studies with a monoclonal antibody revealed co-localization of apoA-I with acrolein adducts in human atherosclerotic lesions. Our observations suggest that acrolein might interfere with normal reverse cholesterol transport by HDL by modifying specific sites in apoA-I. Thus, acrolein might contribute to atherogenesis by impairing cholesterol removal from the artery wall.