Effects of enrichment of fibroblasts with unesterified cholesterol on the efflux of cellular lipids to apolipoprotein A-I

Is reverse cholesterol transport regulated by active cholesterol?

This review considers the hypothesis that a small portion of plasma membrane cholesterol regulates reverse cholesterol transport in coordination with overall cellular homeostasis. It appears that almost all of the plasma membrane cholesterol is held in stoichiometric complexes with bilayer phospholipids. The minor fraction of cholesterol that exceeds the complexation capacity of the phospholipids is called active cholesterol. It has an elevated chemical activity and circulates among the organelles. It also moves down its chemical activity gradient to plasma HDL, facilitated by the activity of ABCA1, ABCG1, and SR-BI. ABCA1 initiates this process by perturbing the organization of the plasma membrane bilayer, thereby priming its phospholipids for translocation to apoA-I to form nascent HDL. The active excess sterol and that activated by ABCA1 itself follow the phospholipids to the nascent HDL. ABCG1 similarly rearranges the bilayer and sends additional active cholesterol to nascent HDL, while SR-BI simply facilitates the equilibration of the active sterol between plasma membranes and plasma proteins. Active cholesterol also flows downhill to cytoplasmic membranes where it serves both as a feedback signal to homeostatic ER proteins and as the substrate for the synthesis of mitochondrial 27-hydroxycholesterol (27HC). 27HC binds the LXR and promotes the expression of the aforementioned transport proteins. 27HC-LXR also activates ABCA1 by competitively displacing its inhibitor, unliganded LXR. § Considerable indirect evidence suggests that active cholesterol serves as both a substrate and a feedback signal for reverse cholesterol transport. Direct tests of this novel hypothesis are proposed.

Bergaptol from blossoms of Citrus aurantium L. var. amara Engl inhibits LPS-induced inflammatory responses and ox-LDL-induced lipid deposition

Aberrant activation of inflammation and excess accumulation of lipids play pivotal roles in atherosclerosis (AS) progression. Constituents from Citrus aurantium Linn variant amara Engl (CAVA) were effectively investigated for their various bioactivities, especially anti-inflammation. Bergaptol (BER) is particularly abundant in Citrus products. Accumulating studies have confirmed its predominant anti-cancer and antioxidant functions, whereas few studies focused on its antiatherogenic functions. In the current study, BER was isolated from CAVA for the first time. Macrophages were stimulated with lipopolysaccharides (LPSs) or oxidized low-density lipoproteins (ox-LDL) to mimic inflammatory responses and AS development. BER treatment significantly inhibited LPS-induced production of nitric oxide (NO), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α), and gene expression of inducible nitric oxide synthase (iNOS), IL-6, TNF-α, interleukin-1 beta (IL-1β) and cyclooxygenase-2 (COX-2). BER also potently blocked LPS-induced mitogen-activated protein kinase (MAPK) phosphorylation and nuclear factor-kappa B (NF-κB) activation, as evidenced by the inhibitory effects on c-Jun N-terminal kinase (JNK), P38, P65, IκBα and IκKα/β phosphorylation, and NF-κB nuclear translocation. Furthermore, BER treatment markedly mitigated ox-LDL-induced foam cell formation by inhibiting scavenger receptor class A type I (SRA1) and cluster of differentiation 36 (CD36)-dependent cholesterol uptake. In conclusion, BER might be a novel therapeutic agent for AS prevention through inhibiting inflammatory responses and cholesterol uptake.

Melittin-Induced Lipid Extraction Modulated by the Methylation Level of Phosphatidylcholine Headgroups

Protein- and peptide-induced lipid extraction from membranes is a critical process for many biological events, including reverse cholesterol transport and sperm capacitation. In this work, we examine whether such processes could display specificity for some lipid species. Melittin, the main component of dry bee venom, was used as a model amphipathic α-helical peptide. We specifically determined the modulation of melittin-induced lipid extraction from membranes by the change of the methylation level of phospholipid headgroups. Phosphatidylcholine (PC) bilayers were demethylated either by substitution with phosphatidylethanolamine (PE) or chemically by using mono- and dimethylated PE. It is shown that demethylation reduces the association of melittin with membranes, likely because of the resulting tighter chain packing of the phospholipids, which reduces the capacity of the membranes to accommodate inserted melittin. This reduced binding of the peptide is accompanied by an inhibition of the lipid extraction caused by melittin. We demonstrate that melittin selectively extracts PC from PC/PE membranes. This selectivity is proposed to be a consequence of a PE depletion in the surroundings of bound melittin to minimize disruption of the interphospholipid interactions. The resulting PC-enriched vicinity of melittin would be responsible for the observed formation of PC-enriched lipid/peptide particles resulting from the lipid efflux. These findings reveal that modulating the methylation level of phospholipid headgroups is a simple way to control the specificity of lipid extraction from membranes by peptides/proteins and thereby modulate the lipid composition of the membranes.

HDL particle size is a critical determinant of ABCA1-mediated macrophage cellular cholesterol export

RATIONALE

High-density lipoprotein (HDL) is a heterogeneous population of particles. Differences in the capacities of HDL subfractions to remove cellular cholesterol may explain variable correlations between HDL-cholesterol and cardiovascular risk and inform future targets for HDL-related therapies. The ATP binding cassette transporter A1 (ABCA1) facilitates cholesterol efflux to lipid-free apolipoprotein A-I, but the majority of apolipoprotein A-I in the circulation is transported in a lipidated state and ABCA1-dependent efflux to individual HDL subfractions has not been systematically studied.

OBJECTIVE

Our aims were to determine which HDL particle subfractions are most efficient in mediating cellular cholesterol efflux from foam cell macrophages and to identify the cellular cholesterol transporters involved in this process.

METHODS AND RESULTS

We used reconstituted HDL particles of defined size and composition, isolated subfractions of human plasma HDL, cell lines stably expressing ABCA1 or ABCG1, and both mouse and human macrophages in which ABCA1 or ABCG1 expression was deleted. We show that ABCA1 is the major mediator of macrophage cholesterol efflux to HDL, demonstrating most marked efficiency with small, dense HDL subfractions (HDL3b and HDL3c). ABCG1 has a lesser role in cholesterol efflux and a negligible role in efflux to HDL3b and HDL3c subfractions.

CONCLUSIONS

Small, dense HDL subfractions are the most efficient mediators of cholesterol efflux, and ABCA1 mediates cholesterol efflux to small dense HDL and to lipid-free apolipoprotein A-I. HDL-directed therapies should target increasing the concentrations or the cholesterol efflux capacity of small, dense HDL species in vivo.

Mechanisms responsible for the compositional heterogeneity of nascent high density lipoprotein

Apolipoprotein (apo) A-I-containing nascent HDL particles produced by the ATP binding cassette transporter A1 have different sizes and compositions. To understand the molecular basis for this heterogeneity, the HDL particles produced by apoA-I-mediated solubilization of phospholipid (PL)/free (unesterified) cholesterol (FC) bilayer membranes in cell and cell-free systems are compared. Incubation of apoA-I with ATP binding cassette transporter A1-expressing baby hamster kidney cells leads to formation of two populations of FC-containing discoidal nascent HDL particles. The larger 11-nm diameter particles are highly FC-enriched (FC/PL = 1.2/1 mol/mol) relative to the smaller 8 nm particles and the cell plasma membrane (FC/PL = 0.4/1). ApoA-I-mediated spontaneous solubilization of either multilamellar or unilamellar vesicles made of a membrane-PL mixture and FC yields discoidal HDL particles with diameters in the range 9–17 nm and, as found with the cell system, the larger particles are relatively enriched in FC despite the fact that all particles are created by solubilization of a common FC/PL membrane domain. The size-dependent distribution of FC among HDL particles is due to varying amounts of PL being sequestered in a boundary layer by interaction with apoA-I at the disc edge. The presence of a relatively large boundary layer in smaller discoidal HDL promotes preferential distribution of phosphatidylserine to such particles. However, phosphatidylcholine and sphingomyelin which are the primary PL constituents of nascent HDL do not exhibit selective incorporation into HDL discs of different sizes. This understanding of the mechanisms responsible for the heterogeneity in lipid composition of nascent HDL particles may provide a basis for selecting subspecies with preferred cardio-protective properties.

Global proteomic profiling reveals altered proteomic signature in schizophrenia serum

Schizophrenia is one of the most severe psychiatric disorders affecting 1% of the world population. There is yet no empirical method to validate the diagnosis of the disease. The identification of an underlying molecular alteration could lead to an improved disease understanding and may yield an objective panel of biomarkers to aid in the diagnosis of this devastating disease. Presented is the largest reported liquid chromatography-mass spectrometry-based proteomic profiling study investigating serum samples taken from first-onset drug-naive patients compared with samples collected from healthy volunteers. The results of this large-scale study are presented along with enzyme-linked immunosorbent assay-based validation data.

High density lipoprotein structure-function and role in reverse cholesterol transport

High density lipoprotein (HDL) possesses important anti-atherogenic properties and this review addresses the molecular mechanisms underlying these functions. The structures and cholesterol transport abilities of HDL particles are determined by the properties of their exchangeable apolipoprotein (apo) components. ApoA-I and apoE, which are the best characterized in structural terms, contain a series of amphipathic alpha-helical repeats. The helices located in the amino-terminal two-thirds of the molecule adopt a helix bundle structure while the carboxy-terminal segment forms a separately folded, relatively disorganized, domain. The latter domain initiates lipid binding and this interaction induces changes in conformation; the alpha-helix content increases and the amino-terminal helix bundle can open subsequently. These conformational changes alter the abilities of apoA-I and apoE to function as ligands for their receptors. The apoA-I and apoE molecules possess detergent-like properties and they can solubilize vesicular phospholipid to create discoidal HDL particles with hydrodynamic diameters of ~10 nm. In the case of apoA-I, such a particle is stabilized by two protein molecules arranged in an anti-parallel, double-belt, conformation around the edge of the disc. The abilities of apoA-I and apoE to solubilize phospholipid and stabilize HDL particles enable these proteins to be partners with ABCA1 in mediating efflux of cellular phospholipid and cholesterol, and the biogenesis of HDL particles. ApoA-I-containing nascent HDL particles play a critical role in cholesterol transport in the circulation whereas apoE-containing HDL particles mediate cholesterol transport in the brain. The mechanisms by which HDL particles are remodeled by lipases and lipid transfer proteins, and interact with SR-BI to deliver cholesterol to cells, are reviewed.

A low‐saturated‐fat, low‐cholesterol diet decreases plasma CETP activity and pre β‐HDL formation but does not affect cellular cholesterol efflux to plasma from type 1 diabetic patients

The aim of this study was to evaluate the effect of a low-saturated-fat, low-cholesterol diet on plasma lipopoproteins, pre beta-high density lipoprotein (HDL) formation, lecithin:cholesterol acyltransferase (LCAT), cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP) activities, as well as on the ability of plasma to stimulate cellular cholesterol efflux. Twelve male type 1 diabetic patients with plasma cholesterol >5.0 mmol/L were studied while consuming their usual diet and after 6 weeks of a low-fat, low-cholesterol diet. Pre beta-HDL formation was measured using crossed immuno-electrophoresis. Plasma LCAT, CETP and PLTP activities were assayed by exogenous substrate methods. The ability of plasma to promote cellular cholesterol efflux out of Fu5AH rat hepatoma cells and out of human skin fibroblasts was also determined. Saturated fat intake was lowered (p = 0.001) due to replacement with carbohydrates, while mono- and polyunsaturated fat intake remained unchanged. Cholesterol intake decreased as well (p = 0.003). The changes in plasma total cholesterol, very low and low-density lipoprotein (VLDL+LDL) cholesterol, HDL cholesterol, HDL phospholipids, apolipoprotein (apo) A-I, plasma LCAT activity and PLTP activity were not significant. Plasma CETP activity (p = 0.008) and pre beta-HDL formation (p = 0.008) decreased. The ability of plasma to promote cholesterol efflux out of fibroblasts and Fu5AH cells remained unchanged. Reduction in dietary saturated fat and cholesterol intake does not adversely affect cellular cholesterol efflux to plasma from type 1 diabetic patients, despite a drop in pre beta-HDL formation.

Human Endothelial Cells of the Placental Barrier Efficiently Deliver Cholesterol to the Fetal Circulation via ABCA1 and ABCG1

Although maternal–fetal cholesterol transfer may serve to compensate for insufficient fetal cholesterol biosynthesis under pathological conditions, it may have detrimental consequences under conditions of maternal hypercholesterolemia leading to preatherosclerotic lesion development in fetal aortas. Maternal cholesterol may enter fetal circulation by traversing syncytiotrophoblast and endothelial layers of the placenta. We hypothesized that endothelial cells (ECs) of the fetoplacental vasculature display a high and tightly regulated capacity for cholesterol release. Using ECs isolated from human term placenta (HPECs), we investigated cholesterol release capacity and examined transporters involved in cholesterol efflux pathways controlled by liver-X-receptors (LXRs). HPECs demonstrated 2.5-fold higher cholesterol release to lipid-free apolipoprotein (apo)A-I than human umbilical vein ECs (HUVECs), whereas both cell types showed similar cholesterol efflux to high-density lipoproteins (HDLs). Interestingly, treatment of HPECs with LXR activators increased cholesterol efflux to both types of acceptors, whereas no such response could be observed for HUVECs. In line with enhanced cholesterol efflux, LXR activation in HPECs increased expression of ATP-binding cassette transporters ABCA1 and ABCG1, while not altering expression of ABCG4 and scavenger receptor class B type I (SR-BI). Inhibition of ABCA1 or silencing of ABCG1 decreased cholesterol efflux to apoA-I (−70%) and HDL 3 (−57%), respectively. Immunohistochemistry localized both transporters predominantly to the apical membranes of placental ECs in situ. Thus, ECs of human term placenta exhibit unique, efficient and LXR-regulated cholesterol efflux mechanisms. We propose a sequential pathway mediated by ABCA1 and ABCG1, respectively, by which HPECs participate in forming mature HDL in the fetal blood.

HDL, ABC transporters, and cholesterol efflux: implications for the treatment of atherosclerosis

High-density lipoprotein (HDL) has been identified as a potential target in the treatment of atherosclerotic vascular disease. The failure of torcetrapib, an inhibitor of cholesteryl ester transfer protein (CETP) that markedly increased HDL levels in a clinical trial, has called into doubt the efficacy of HDL elevation. Recent analysis suggests that failure may have been caused by off-target toxicity and that HDL is functional and promotes regression of atherosclerosis. New studies highlight the central importance of the ATP-binding cassette (ABC) transporters ABCA1 and ABCG1 in reducing macrophage foam cell formation, inflammation, and atherosclerosis. A variety of approaches to increasing HDL may eventually be successful in treating atherosclerosis.

Mechanism of ATP-binding cassette transporter A1-mediated cellular lipid efflux to apolipoprotein A-I and formation of high density lipoprotein particles

The ATP-binding cassette transporter A1 (ABCA1) plays a critical role in the biogenesis of high density lipoprotein (HDL) particles and in mediating cellular cholesterol efflux. The mechanism by which ABCA1 achieves these effects is not established, despite extensive investigation. Here, we present a model that explains the essential features, especially the effects of ABCA1 activity in inducing apolipoprotein (apo) A-I binding to cells and the compositions of the discoidal HDL particles that are produced. The apo A-I/ABCA1 reaction scheme involves three steps. First, there is binding of a small regulatory pool of apo A-I to ABCA1, thereby enhancing net phospholipid translocation to the plasma membrane exofacial leaflet; this leads to unequal lateral packing densities in the two leaflets of the phospholipid bilayer. Second, the resultant membrane strain is relieved by bending and by creation of exovesiculated lipid domains. The formation of highly curved membrane surface promotes high affinity binding of apo A-I to these domains. Third, this pool of bound apo A-I spontaneously solubilizes the exovesiculated domain to create discoidal nascent HDL particles. These particles contain two, three, or four molecules of apo A-I and a complement of membrane phospholipid classes together with some cholesterol. A key feature of this mechanism is that membrane bending induced by ABCA1 lipid translocase activity creates the conditions required for nascent HDL assembly by apo A-I. Overall, this mechanism is consistent with the known properties of ABCA1 and apo A-I and reconciles many of the apparently discrepant findings in the literature.

ApoA-I induces a preferential efflux of monounsaturated phosphatidylcholine and medium chain sphingomyelin species from a cellular pool distinct from HDL(3) mediated phospholipid efflux

Electrospray ionization tandem mass spectrometry (ESI-MS/MS) was used for a detailed analysis of cellular phospholipid and cholesterol efflux in free cholesterol (FC) loaded human primary fibroblasts and human monocyte-derived macrophages (HMDM) loaded with enzymatically modified LDL (E-LDL). Although both cell models differed significantly in their cellular lipid composition, a higher apoA-I specific efflux was found for monounsaturated phosphatidylcholine (PC) species together with a decreased contribution of polyunsaturated PC species in both cell types. Moreover, medium chain sphingomyelin (SPM) species SPM 14:0 and SPM 16:1 were translocated preferentially to apoA-I in both cell types. In contrast to fibroblasts, HMDM displayed a considerable proportion of cholesteryl esters (CE) in basal and apoA-I specific efflux media, most likely due to secretion of CE associated to apoE. Analysis of HDL(3) mediated lipid efflux from HMDM using D(9)-choline and (13)C(3)-FC stable isotope labeling revealed significantly different D(9)-PC and D(9)-SPM species pattern for apoA-I and HDL(3) specific efflux media, which indicates a contribution of distinct cellular phospholipid pools to apoA-I and HDL(3) mediated efflux. Together with a partial loading of fibroblasts and HMDM with HDL(3)-derived CE species, these data add further evidence for retroendocytosis of HDL. In summary, analysis of apoA-I/ABCA1 and HDL(3) mediated lipid efflux by ESI-MS/MS demonstrated a preferential efflux of monounsaturated PC and medium chain SPM to apoA-I. Moreover, this is the first study, which provides evidence for distinct cellular phospholipid pools used for lipid transfer to apoA-I and HDL(3) from the analysis of phospholipid species pattern in HMDM.

Ceramide structural features required to stimulate ABCA1-mediated cholesterol efflux to apolipoprotein A-I

Ceramide is a component of the sphingomyelin cycle and a well-established lipid signaling molecule. We recently reported that ceramide specifically increased ABCA1-mediated cholesterol efflux to apolipoprotein A-I (apoA-I), a critical process that leads to the formation of cardioprotective HDL. In this report, we characterize the structural features of ceramide required for this effect. C2 dihydroceramide, which contains a fully saturated acyl chain and is commonly used as a negative control for ceramide apoptotic signaling, stimulated a 2- to 5-fold increase in ABCA1-mediated cholesterol efflux to apoA-I over a 0-60 muM concentration range without the cell toxicity apparent with native C2 ceramide. Compared with C2 ceramide, C6 and C8 ceramides with medium-length N-acyl chains showed a similar extent of efflux stimulation (a 2- to 5-fold increase) but at a higher onset concentration than the less hydrophobic C2 ceramide. In contrast, the reduced and methylated ceramide analogs, N,N-dimethyl sphingosine and N,N,N-trimethyl sphingosine, failed to stimulate cholesterol efflux. We found that changes in the native spatial orientation at either of two chiral carbon centers (or both) resulted in an approximately 50% decrease compared with native ceramide-stimulated cholesterol efflux. These data show that the overall ceramide shape and the amide bond are critical for the cholesterol efflux effect and suggest that ceramide acts through a protein-mediated pathway to affect ABCA1 activity.

Lipid efflux by the ATP-binding cassette transporters ABCA1 and ABCG1

Plasma levels of high-density lipoproteins (HDL) and apolipoprotein A-I (apoA-I) are inversely correlated with the risk of cardiovascular disease. One major atheroprotective mechanism of HDL and apoA-I is their role in reverse cholesterol transport, i.e., the transport of excess cholesterol from foam cells to the liver for secretion. The ATP-binding cassette transporters ABCA1 and ABCG1 play a pivotal role in this process by effluxing lipids from foam cells to apoA-I and HDL, respectively. In the liver, ABCA1 activity is one rate-limiting step in the formation of HDL. In macrophages, ABCA1 and ABCG1 prevent the excessive accumulation of lipids and thereby protect the arteries from developing atherosclerotic lesions. However, the mechanisms by which ABCA1 and ABCG1 mediate lipid removal are still unclear. Particularly, three questions remain controversial and are discussed in this review: (1) Do apoA-I and HDL directly interact with ABCA1 and ABCG1, respectively? (2) Does cholesterol efflux involve retroendocytosis of apoA-I or HDL? (3) Which lipids are directly transported by ABCA1 and ABCG1?

Characterization of nascent HDL particles and microparticles formed by ABCA1-mediated efflux of cellular lipids to apoA-I

The nascent HDL created by ABCA1-mediated efflux of cellular phospholipid (PL) and free (unesterified) cholesterol (FC) to apolipoprotein A-I (apoA-I) has not been defined. To address this issue, we characterized the lipid particles released when J774 mouse macrophages and human skin fibroblasts in which ABCA1 is activated are incubated with human apoA-I. In both cases, three types of nascent HDL containing two, three, or four molecules of apoA-I per particle are formed. With J774 cells, the predominant species have hydrodynamic diameters of approximately 9 and 12 nm. These discoidal HDL particles have different FC contents and PL compositions, and the presence of acidic PL causes them to exhibit alpha-electrophoretic mobility. These results are consistent with ABCA1 located in more than one membrane microenvironment being responsible for the production of the heterogeneous HDL. Activation of ABCA1 also leads to the release of apoA-I-free plasma membrane vesicles (microparticles). These larger, spherical particles released from J774 cells have the same PL composition as the 12 nm HDL and contain CD14 and ganglioside, consistent with their origin being plasma membrane raft domains. The various HDL particles and microparticles are created concurrently, and there is no precursor-product relationship between them. Importantly, a large fraction of the cellular FC effluxed from these cells by ABCA1 is located in microparticles. Collectively, these results show that the products of the apoA-I/ABCA1 interaction include discoidal HDL particles containing different numbers of apoA-I molecules. The cellular PLs and cholesterol incorporated into these nascent HDL particles originate from different cell membrane domains.

Nutrigenomics and nutrigenetics: the emerging faces of nutrition

The recognition that nutrients have the ability to interact and modulate molecular mechanisms underlying an organism's physiological functions has prompted a revolution in the field of nutrition. Performing population-scaled epidemiological studies in the absence of genetic knowledge may result in erroneous scientific conclusions and misinformed nutritional recommendations. To circumvent such issues and more comprehensively probe the relationship between genes and diet, the field of nutrition has begun to capitalize on both the technologies and supporting analytical software brought forth in the post-genomic era. The creation of nutrigenomics and nutrigenetics, two fields with distinct approaches to elucidate the interaction between diet and genes but with a common ultimate goal to optimize health through the personalization of diet, provide powerful approaches to unravel the complex relationship between nutritional molecules, genetic polymorphisms, and the biological system as a whole. Reluctance to embrace these new fields exists primarily due to the fear that producing overwhelming quantities of biological data within the confines of a single study will submerge the original query; however, the current review aims to position nutrigenomics and nutrigenetics as the emerging faces of nutrition that, when considered with more classical approaches, will provide the necessary stepping stones to achieve the ambitious goal of optimizing an individual's health via nutritional intervention.

Absorption and lipoprotein transport of sphingomyelin

Dietary sphingomyelin (SM) is hydrolyzed by intestinal alkaline sphingomyelinase and neutral ceramidase to sphingosine, which is absorbed and converted to palmitic acid and acylated into chylomicron triglycerides (TGs). SM digestion is slow and is affected by luminal factors such as bile salt, cholesterol, and other lipids. In the gut, SM and its metabolites may influence TG hydrolysis, cholesterol absorption, lipoprotein formation, and mucosal growth. SM accounts for approximately 20% of the phospholipids in human plasma lipoproteins, of which two-thirds are in LDL and VLDL. It is secreted in chylomicrons and VLDL and transferred into HDL via the ABCA1 transporter. Plasma SM increases after periods of large lipid loads, during suckling, and in type II hypercholesterolemia, cholesterol-fed animals, and apolipoprotein E-deficient mice. SM is thus an important amphiphilic component when plasma lipoprotein pools expand in response to large lipid loads or metabolic abnormalities. It inhibits lipoprotein lipase and LCAT as well as the interaction of lipoproteins with receptors and counteracts LDL oxidation. The turnover of plasma SM is greater than can be accounted for by the turnover of LDL and HDL particles. Some SM must be degraded via receptor-mediated catabolism of chylomicron and VLDL remnants and by scavenger receptor class B type I receptor-mediated transfer into cells.

ATP-binding cassette transporter A1: a cell cholesterol exporter that protects against cardiovascular disease

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.

Headgroup-specific exposure of phospholipids in ABCA1-expressing cells

ABCA1 has been established to be required for the efflux of cholesterol and phospholipids to apolipoproteins such as apoA-I. At present, it is unclear whether ABCA1-mediated lipid exposure is specific with regard to lipid headgroups and whether it requires calcium activation and the presence of a lipid acceptor. In the present work, we found exofacial exposure of endogenous phosphatidylserine in the absence of apoA-I to be enhanced in ABCA1-GFP expressing MDCKII and HeLa cells compared with control cells. By using C6-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) (NBD)-labeled phospholipid analogues, we observed elevated redistribution of phosphatidylserine and phosphatidylethanolamine but not of phosphatidylcholine analogues from the cytoplasmic to the exoplasmic leaflet of the plasma membrane of ABCA1-GFP expressing cells. Whereas glyburide affected neither the level of exofacial endogenous PS nor the outward movement of the amino phospholipid analogues, the latter was sensitive to intracellular Ca2+ in ABCA1-GFP expressing cells, further enhancing outward analogue redistribution with respect to control cells. Both receptor-mediated endocytosis and fluidphase endocytosis were reduced in MDCKII cells expressing ABCA1-GFP. Glyburide raised the level of receptor-mediated endocytosis in the ABCA1-GFP expressing cell to the level of control cells in the absence of glyburide. In control cells, however, fluid-phase endocytosis but not receptor-mediated endocytosis was significantly reduced upon glyburide treatment.

Apolipoprotein A-I-dependent cholesterol esterification in patients with rheumatoid arthritis

Growing evidence suggests that atherogenesis is associated with inflammation or defective removal of cholesterol excess from peripheral cells. Apolipoprotein A-I [ApoA-I] activates the enzyme Lecithin-Cholesterol Acyl-Transferase to esterify cell cholesterol for transport to liver. Haptoglobin [Hpt] was previously found able to bind ApoA-I, and suggested to reduce the enzyme activation. The aim of this study was to demonstrate that enhanced levels of Hpt, as present during inflammation, are associated with low enzyme activity and increased thickness of the arterial wall. Enzyme activity and Hpt concentration were analysed in patients with rheumatoid arthritis having the same plasma levels of antioxidants (ascorbate, urate, alpha-tocopherol, retinol) or oxidation markers (nitrotyrosine, lipoperoxide) of healthy subjects. Cholesterol esterification, determined as ratio of cholesteryl esters with cholesterol in high-density lipoproteins, was lower in patients than in controls, and negatively correlated with the intima-media wall thickness of the common carotid. The ratio of Hpt with ApoA-I was negatively correlated with the enzyme activity, while positively correlated with intima-media wall thickness. The results suggest that high Hpt levels might severely impair the enzyme activity, thus contributing to cholesterol accumulation in vascular cells, and lesion formation in the endothelium.

Nuclear receptor signaling in the control of cholesterol homeostasis: have the orphans found a home?

Cholesterol is essential for all mammalian cells. Cellular cholesterol requirements are met through de novo synthesis and uptake of plasma lipoproteins, homeostatic responses that are transcriptionally regulated by the sterol regulatory element-binding proteins (SREBPs). To prevent cytotoxicity attributable to accumulation of excess cholesterol, liver X receptors (LXRs) and the farnesoid X receptor (FXR), together with other members of the nuclear receptor superfamily, promote the storage, transport, and catabolism of sterols and their metabolites. Members of this metabolic nuclear receptor family include receptors for oxysterols (LXRs), bile acids (CAR, FXR, and PXR), and fatty acids (PPARs). Through coordinated regulation of transcriptional programs, these nuclear receptors regulate key aspects of cellular and whole-body sterol homeostasis, including cholesterol absorption, lipoprotein synthesis and remodeling, lipoprotein uptake by peripheral tissues, reverse cholesterol transport, and bile acid synthesis and absorption. This review focuses on the nuclear receptors that are central to the lipid metabolic signaling cascades, communication between lipid metabolites and their receptors, and the role of nuclear receptors in orchestrating the complex transcriptional programs that govern cholesterol and bile acid metabolism.

ACAT2 deficiency limits cholesterol absorption in the cholesterol-fed mouse: impact on hepatic cholesterol homeostasis

Acyl CoA:cholesterol acyltransferase (ACAT) 2 is the major cholesterol-esterifying enzyme in mouse enterocytes and hepatocytes. Male ACAT2(+/+) and ACAT2(-/ -) mice were fed chow containing added cholesterol (0%-0.500% w/w) for 24 days. Over this range, fractional cholesterol absorption in the ACAT2(+/+) mice fell from 41.4% +/- 6.6% to 21.0% +/- 5.2%, and in their ACAT2(-/-) counterparts it fell from 35.1% +/- 4.5% to 7.9% +/- 0.8%. The mass of dietary cholesterol absorbed (mg/d per 100 g body weight) increased from 1.2 +/- 0.2 to 14.7 +/- 4.4 in the ACAT2(+/+) mice and from 1.0 +/- 0.2 to 5.5 +/- 0.6 in those without ACAT2. In the ACAT2(+/+) mice, hepatic cholesterol concentrations increased as a function of intake despite compensatory changes in cholesterol and bile acid synthesis and in the expression of adenosine triphosphate-binding cassette transporter G5 (ABCG5) and ABC transporter G8 (ABCG8). In contrast, in ACAT2(-/-) mice in which the amount of cholesterol absorbed at the highest intake was only 37% of that in the ACAT2(+/+) mice, suppression of synthesis was a sufficient adaptive response; there was no change in bile acid synthesis, ABCG5/G8 expression, or hepatic cholesterol concentration. The expression of adenosine triphosphate-binding cassette transporter A1 (ABCA1) in the jejunum was markedly elevated in the ACAT2(-/-) mice, irrespective of dietary cholesterol level. In conclusion, although ACAT2 deficiency limits cholesterol absorption, the extent to which it impacts hepatic cholesterol homeostasis depends on cholesterol intake. Loss of ACAT2 activity may result in unesterified cholesterol being absorbed via an ABCA1-mediated basolateral efflux pathway.

Influence of ApoA-I structure on the ABCA1-mediated efflux of cellular lipids

The influence of apolipoprotein (apo) A-I structure on ABCA1-mediated efflux of cellular unesterified (free) cholesterol (FC) and phospholipid (PL) is not well understood. To address this issue, we used a series of apoA-I mutants to examine the contributions of various domains in the molecule to ABCA1-mediated FC and PL efflux from mouse J774 macrophages and human skin fibroblasts. Irrespective of the cell type, deletion or disruption of the C-terminal lipid-binding domain of apoA-I drastically reduced the FC and PL efflux ( approximately 90%), indicating that the C-terminal amphipathic alpha-helix is required for high affinity microsolubilization of FC and PL. Deletion in the N-terminal region of apoA-I also reduced the lipid efflux ( approximately 30%) and increased the K(m) about 2-fold compared with wild type apoA-I, whereas deletion of the central domain (Delta123-166) had no effect on either K(m) or V(max). These results indicate that ABCA1-mediated lipid efflux is relatively insensitive to the organization of the apoA-I N-terminal helix-bundle domain. Alterations in apoA-I structure caused parallel changes in its ability to bind to a PL bilayer and to induce efflux of FC and PL. Overall, these results are consistent with a two-step model for ABCA1-mediated lipid efflux. In the first step, apoA-I binds to ABCA1 and hydrophobic alpha-helices in the C-terminal domain of apoA-I insert into the region of the perturbed PL bilayer created by the PL transport activity of ABCA1, thereby allowing the second step of lipidation of apoA-I and formation of nascent high density lipoprotein particles to occur.

Contributions of domain structure and lipid interaction to the functionality of exchangeable human apolipoproteins

Exchangeable apolipoproteins function in lipid transport as structural components of lipoprotein particles, cofactors for enzymes and ligands for cell-surface receptors. Recent findings with apoA-I and apoE suggest that the tertiary structures of these two members of the human exchangeable apolipoprotein gene family are related. Characteristically, these proteins contain a series of proline-punctuated, 11- or 22-amino acid, amphipathic alpha-helical repeats that can adopt a helix bundle conformation in the lipid-free state. The amino- and carboxyl-terminal regions form separate domains with the latter being primarily responsible for lipid binding. Interaction with lipid induces changes in the conformation of the amino-terminal domain leading to alterations in function; for example, opening of the amino-terminal four-helix bundle in apolipoprotein E upon lipid binding is associated with enhanced receptor-binding activity. The concept of a two-domain structure for the larger exchangeable apolipoproteins is providing new molecular insights into how these apolipoproteins interact with lipids and other proteins, such as receptors. The ways in which structural changes induced by lipid interaction modulate the functionality of these apolipoproteins are reviewed.

ABCA1 mediates concurrent cholesterol and phospholipid efflux to apolipoprotein A-I

Prior studies provide data supporting the notion that ATP binding cassette transporter A1 (ABCA1) promotes lipid efflux to extracellular acceptors in a two-step process: first, ABCA1 mediates phospholipid efflux to an apolipoprotein, and second, this apolipoprotein-phospholipid complex accepts free cholesterol in an ABCA1-independent manner. In the current study using RAW264.7 cells, ABCA1-mediated free cholesterol and phospholipid efflux to apolipoprotein A-I (apoA-I) were tightly coupled to each other both temporally and after treatment with ABCA1 inhibitors. The time course and temperature dependence of ABCA1-mediated lipid efflux to apoA-I support a role for endocytosis in this process. Cyclodextrin treatment of RAW264.7 cells partially inhibited 8Br-cAMP-induced efflux of free cholesterol and phospholipid to apoA-I. ABCA1-expressing cells are more sensitive to cell damage by high-dose cyclodextrin and vanadate, leading to increased lactate dehydrogenase leakage and phospholipid release even in the absence of the acceptor apoA-I. Finally, we could not reproduce a two-step effect on lipid efflux using conditioned medium from ABCA1-expressing cells pretreated with cyclodextrin.

Molecular interactions between apoE and ABCA1: impact on apoE lipidation

Apolipoprotein E (apoE)/ABCA1 interactions were investigated in human intact fibroblasts induced with 22(R)-hydroxycholesterol and 9-cis-retinoic acid (stimulated cells). Here, we show that purified human plasma apoE3 forms a complex with ABCA1 in normal fibroblasts. Lipid-free apoE3 inhibited the binding of (125)I-apoA-I to ABCA1 more efficiently than reconstituted HDL particles (IC(50) = 2.5 +/- 0.4 microg/ml vs. 12.3 +/- 1.3 microg/ml). ApoE isoforms showed similar binding for ABCA1 and exhibited identical kinetics in their abilities to induce ABCA1-dependent cholesterol efflux. Mutation of ABCA1 associated with Tangier disease (C1477R) abolished both apoE3 binding and apoE3-mediated cholesterol efflux. Analysis of apoE3-containing particles generated during the incubation of lipid-free apoE3 with stimulated normal cells showed nascent apoE3/cholesterol/phospholipid complexes that exhibited prebeta-electrophoretic mobility with a particle size ranging from 9 to 15 nm, whereas lipid-free apoE3 incubated with ABCA1 mutant (C1477R) cells was unable to form such particles. These results demonstrate that 1). apoE association with lipids reduced its ability to interact with ABCA1; 2). apoE isoforms did not affect apoE binding to ABCA1; 3). apoE-mediated ABCA1-dependent cholesterol efflux was not affected by apoE isoforms in fibroblasts; and 4). the lipid translocase activity of ABCA1 generates apoE-containing high density-sized lipoprotein particles. Thus, ABCA1 is essential for the biogenesis of high density-sized lipoprotein containing only apoE particles in vivo.

ABCA1 and amphipathic apolipoproteins form high-affinity molecular complexes required for cholesterol efflux

Apolipoproteins, such as apolipoprotein A-I (apoA-I), can stimulate cholesterol efflux from cells expressing the ATP binding cassette transporter A1 (ABCA1). The nature of the molecular interaction between these cholesterol acceptors and ABCA1 is controversial, and models suggesting a direct protein-protein interaction or indirect association have been proposed. To explore this issue, we performed competition binding and chemical cross-linking assays using six amphipathic plasma proteins and an 18 amino acid amphipathic helical peptide. All seven proteins stimulated lipid efflux and inhibited the cross-linking of apoA-I to ABCA1. Cross-linking of apoA-I to ABCA1 was saturable and occurred at high affinity (Kd of 7.0 +/- 1.9 nM), as was cross-linking of apoA-II. After binding to ABCA1, apoA-I rapidly dissociated (half-life of 25 min) from the complex and was released back into the medium. A mutant form of ABCA1 (W590S) that avidly binds apoA-I but fails to promote cholesterol efflux released apoA-I with similar kinetics but without transfer of cholesterol to apoA-I. Thus, a high-affinity, saturable, protein-protein interaction occurs between ABCA1 and all of its amphipathic protein ligands. Dissociation of the complex leads to the cellular release of cholesterol and the apolipoprotein. However, dissociation is not dependent on cholesterol transfer, which is a clearly separable event, distinguishable by ABCA1 mutants.

Apolipoprotein A-1 interaction with plasma membrane lipid rafts controls cholesterol export from macrophages

Cholesterol efflux to apolipoprotein A-1 (apoA-1) from cholesterol-loaded macrophages is an important anti-atherosclerotic mechanism in reverse cholesterol transport. We recently provided kinetic evidence for two distinct pathways for cholesterol efflux to apoA-1 [Gaus et al. (2001) Biochemistry 40, 9363]. Cholesterol efflux from two membrane pools occurs sequentially with different kinetics; a small pool rapidly effluxed over the first hour, followed by progressive release from a major, slow efflux pool over several hours. In the present study, we propose that the rapid and slow cholesterol efflux pools represent cholesterol derived from lipid raft and nonraft domains of the plasma membrane, respectively. We provide direct evidence that apoA-1 binds to both lipid raft and nonraft domains of the macrophage plasma membrane. Conditions that selectively deplete plasma membrane lipid raft cholesterol, such as incorporation of 7-ketocholesterol or rapid exposure to cyclodextrins, block apoA-1 binding to these domains but also inhibit cholesterol efflux from the major, slow pool. We propose that cholesterol exported to apoA-1 from this major slow efflux pool derives from nonraft regions of the plasma membrane but that the interaction of apoA-1 with lipid rafts is necessary to stimulate this efflux.

Potential involvement of dissociated apoA-I in the ABCA1-dependent cellular lipid release by HDL

Helical apolipoproteins of high density lipoprotein (HDL) remove phospholipid and cholesterol from cells and generate HDL particles being mediated by ATP binding cassette transporter A1 (ABCA1). In murine macrophage cell line RAW264 cells, cAMP induced expression of ABCA1, release of cellular phospholipid and cholesterol by apolipoprotein A-I (apoA-I), and reversible binding of apoA-I to the cell. The apoA-I-dependent lipid release was directly proportional to the cAMP-induced binding of apoA-I, and was inhibited 70% by a monoclonal antibody selective to lipid-free apoA-I, 725-1E2. In contrast, apparent cellular cholesterol release to HDL was substantial even without ABCA1 induction, and it was increased only by 27% after the cAMP treatment. The antibody inhibited this increment by 70%. Lipid-free apoA-II liberated apoA-I from HDL by displacement and thereby markedly expanded the cAMP-induced part of the cholesterol release to the HDL-containing medium, and the antibody inhibited this part also by 70%. Binding experiments of the double-labeled reconstituted HDL showed that cAMP induced reversible binding of apoA-I but not the association of cholesteryl ester with the cells. The effect of the antibody on the cellular cholesterol release to the reconstituted HDL was similar to that of the HDL-mediated release. The data implicated that the ABCA1-dependent cholesterol release to HDL is mediated by apoA-I dissociated from HDL.

Effects of apolipoprotein A-I on ATP-binding cassette transporter A1-mediated efflux of macrophage phospholipid and cholesterol: formation of nascent high density lipoprotein particles

The mechanism of formation of high density lipoprotein (HDL) particles by the action of ATP-binding cassette transporter A1 (ABCA1) is not defined completely. To address this issue, we monitored efflux to apoA-I of phosphatidylcholine (PC), sphingomyelin (SM), and unesterified (free) cholesterol (FC) from J774 macrophages, in which ABCA1 is up-regulated, and investigated the nature of the particles formed. The various apoA-I/lipid particles appearing in the extracellular medium were separated by gel filtration chromatography. The presence of apoA-I in the extracellular medium led to the simultaneous formation of more than one type of poorly lipidated apoA-I-containing particle: there were 9- and 12-nm diameter particles containing approximately 3:1 and 1:1 phospholipid/FC (mol/mol), respectively, which were present together with 6-nm monomeric apoA-I. Removal of the C-terminal alpha-helix (residues 223-243) of apoA-I reduced phospholipid and FC efflux and prevented formation of the 9- and 12-nm HDL particles; the apoA-I variant formed larger particles that eluted in the void volume. FC loading of the J774 cells also led to the formation of larger apoA-I-containing particles that were highly enriched in FC. Besides creating HDL particles, ABCA1 mediated release of larger (20-450-nm diameter) FC-rich particles that were not involved in HDL formation and that are probably membrane vesicles. These particles contained 1:1 PC/SM in contrast to the HDL particles, which contained 2:1 PC/SM. This is consistent with lipid raft and non-raft plasma membrane domains being involved primarily in ABCA1-mediated vesicle release and nascent HDL formation, respectively.

Regulation of ABCA1 expression and cholesterol efflux during adipose differentiation of 3T3-L1 cells

Adipose cells specialized in energy storage, contain large intracellular triglyceride-rich lipid droplets, are enriched with free cholesterol, and express sterol-regulated transcription factors such as liver X receptor (LXR). The recent identification of the LXR-dependent ATP binding cassette transporter A1 (ABCA1) pathway for cholesterol release from peripheral cells has led us to address the question of the expression and function of ABCA1 in adipocytes. In 3T3-L1 adipose cells, we observed a strong induction of ABCA1 mRNA during adipose differentiation, but only limited variations in ABCA1 protein. Lipid efflux onto apolipoprotein A-I (apoA-I), which depends on ABCA1, was comparable in adipocytes and preadipocytes, demonstrating a differential regulation of ABCA1 mRNA and cholesterol efflux. We also found that total cell cholesterol remained stable during differentiation of 3T3-L1 cells, but membrane cholesterol was lower in adipocytes than in preadipocytes, suggesting redistribution of cholesterol to the lipid droplet. Finally, we show that under standard lipolytic stimulation, 3T3-L1 adipocytes do not release cholesterol onto apoA-I, a process that required long exposures to lipolytic agents (24 h). In conclusion, despite large induction of ABCA1 mRNA during differentiation, cholesterol efflux through the ABCA1 pathway remains limited in adipocytes and requires prolonged lipolysis. This is consistent with the view of the adipocyte behaving as a cholesterol sink, with plasma cholesterol-buffering properties.

Abnormal phospholipid composition impairs HDL biogenesis and maturation in mice lacking Abca1

Recent studies have demonstrated that the ATP-binding cassette transporter A1 (ABCA1) facilitates the efflux of phospholipids and cholesterol to apoprotein acceptors, leading to the synthesis of HDL. The purpose of this study was to determine the changes in the lipoprotein fractions in Abca1-deficient mice and study the mechanisms responsible for the low levels of HDL when ABCA1 is absent. Plasma phospholipid concentration was decreased by more than 75%, mostly due to a reduction of phosphatidylcholine (PC) in HDL. Abca1(-/-) HDL represents less than 2% of wild-type levels and is smaller and enriched in phospholipids (11.2-fold more than HDL from controls). Compared to wild-type littermates, Abca1(-/-) HDL had a 4-fold increase in PC, whereas lysophosphatidylcholine (LPC) (125-fold), sphingomyelin (SPH) (49-fold), and phosphatidylethanolamine (PE) (18-fold) showed even higher increases. As a consequence, the ratios of LPC/PC, SPH/PC, PE/PC, and phosphatidylinositol + phosphatidylserine (PI+PS)/PC were all much higher in HDL from Abca1(-/-), compared to wild-type HDL. Plasma phospholipid transfer protein (PLTP) and lecithin cholesterol acyltransferase (LCAT) activities were decreased by more than 80%, suggesting that the maturation of HDL is affected. To test this hypothesis, plasma from Abca1(-/-) mice was incubated with CHO cells that are known to express high levels of ABCA1 with the intent of restoring the flux of phospholipid and cholesterol onto apoAI. Compared to native plasma, no change in maturation of HDL was observed. In contrast, a 220% increase in the formation of mature HDL was observed when ABCA1 function and LCAT activities were restored. Taken together, these observations suggest that ABCA1 is necessary for the adequate lipidation of apoAI, which enables the interaction with LCAT and subsequent maturation.

ABCA1 redistributes membrane cholesterol independent of apolipoprotein interactions

ATP binding cassette transporter A1 (ABCA1) mediates the transport of phospholipids and cholesterol from cells to lipid-poor HDL apolipoproteins. Cholesterol loading of cells induces ABCA1, implicating cholesterol as its major physiologic substrate. It is believed, however, that ABCA1 is primarily a phospholipid transporter and that cholesterol efflux occurs by diffusion to ABCA1-generated phospholipid-rich apolipoproteins. Here we show that overexpression of ABCA1 in baby hamster kidney cells in the absence of apolipoproteins redistributed membrane cholesterol to cell-surface domains accessible to treatment with the enzyme cholesterol oxidase. The cholesterol removed by apolipoprotein A-I (apoA-I), but not by HDL phospholipids, was derived exclusively from these domains. ABCA1 overexpression also increased cholesterol esterification, which was prevented by addition of apoA-I, suggesting that some of the cell-surface cholesterol not removed by apolipoproteins is transported to the intracellular esterifying enzyme acyl-CoA:cholesterol acyltransferase. ABCA1 expression was essential for cholesterol efflux even when apolipoproteins had already acquired phospholipids during prior exposure to ABCA1-expressing cells. These studies show that ABCA1 redistributes cholesterol to cell-surface domains, where it becomes accessible for removal by apolipoproteins, consistent with a direct role of ABCA1 in cholesterol transport.

Fast flip-flop of cholesterol and fatty acids in membranes: implications for membrane transport proteins

PURPOSE OF REVIEW

The rates by which unesterified fatty acids and cholesterol move through and desorb from membranes have been difficult to measure, in part because of the simple structures of these lipids but also because methods have generally not clearly distinguished the two steps of membrane transport. Lack of definitive knowledge has given rise to speculation about the mechanism(s) of membrane 'transport' proteins for fatty acids and cholesterol.

RECENT FINDINGS

New biophysical and biochemical approaches have provided evidence that fatty acids and cholesterol exhibit rapid diffusion (flip-flop), as fast as milliseconds, across both protein-free phospholipid bilayers and cell membranes. In contrast, desorption of the cholesterol molecule from a membrane surface (hours) is much slower than that of common dietary fatty acids (milliseconds to seconds).

SUMMARY

Knowledge of these properties provides a framework for understanding transport and metabolism of cholesterol and fatty acids and how their putative membrane and intracellular transporters might function.

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.

Synthetic amphipathic helical peptides promote lipid efflux from cells by an ABCA1-dependent and an ABCA1-independent pathway

In order to examine the necessary structural features for a protein to promote lipid efflux by the ABCA1 transporter, synthetic peptides were tested on ABCA1-transfected cells (ABCA1 cells) and on control cells. L-37pA, an l amino acid peptide that contains two class-A amphipathic helices linked by proline, showed a 4-fold increase in cholesterol and phospholipid efflux from ABCA1 cells compared to control cells. The same peptide synthesized with a mixture of l and d amino acids was less effective than L-37pA in solubilizing dimyristoyl phosphatidyl choline vesicles and in effluxing lipids. In contrast, the 37pA peptide synthesized with all d amino acids (D-37pA) was as effective as L-37pA. Unlike apoA-I, L-37pA and D-37pA were also capable, although at a reduced rate, of causing lipid efflux independent of ABCA1 from control cells, Tangier disease cells, and paraformaldehyde fixed ABCA1 cells. The ability of peptides to bind to cells correlated with their lipid affinity. In summary, the amphipathic helix was found to be a key structural motif for peptide-mediated lipid efflux from ABCA1, but there was no stereoselective requirement. In addition, unlike apoA-I, synthetic peptides can also efflux lipid by a passive, energy-independent pathway that does not involve ABCA1 but does depend upon their lipid affinity.

Stearoyl-CoA desaturase inhibits ATP-binding cassette transporter A1-mediated cholesterol efflux and modulates membrane domain structure

Liver X receptor/retinoid X receptor (LXR/RXR) transcription factors have been found to induce a number of genes involved in the regulation of cellular cholesterol efflux, including the ATP-binding cassette transporter A1 (ABCA1), which mediates the active efflux of cellular cholesterol and phospholipids to extracellular acceptors, such as apolipoprotein A-I (apoA-I). In a screen for macrophage LXR/RXR target genes, we identified stearoyl-CoA desaturases 1 and 2 (Scd1 and Scd2), and subsequently tested the hypothesis that SCD activity might modulate cellular cholesterol efflux. In HEK 293 cells co-transfection of ABCA1 with either SCD1 or SCD2 inhibited ABCA1-mediated cholesterol efflux but not phospholipid efflux. In Chinese hamster ovary (CHO) cells with moderate stable overexpression of SCD1, cholesterol efflux to apoA-I was inhibited by 73%, whereas phospholipid efflux and ABCA1 protein levels were unchanged. In contrast, cholesterol efflux to HDL(2), which is not dependent on ABCA1, was increased 2-fold in CHO-SCD1 cells. The effect of SCD on cholesterol efflux to apoA-I was independent of acyl-CoA:cholesterol acyltransferase (ACAT) activity. SCD activity led to an increased content of plasma membrane monounsaturated fatty acids (18:1) at the expense of saturated fatty acids (18:0). As shown by confocal microscopy, SCD overexpression led to a decrease of Triton X-100-resistant domains in the plasma membrane, indicating a decrease in membrane-ordered regions. The data suggest that SCD changes membrane organization and depletes a specific pool of membrane cholesterol supporting ABCA1-mediated efflux, whereas increasing availability of cholesterol for passive efflux by HDL(2). ABCA1-mediated cholesterol and phospholipid efflux may be uncoupled in pathological states associated with high SCD activity, as in hyperinsulinemic obese mice, or in animals treated with LXR activators.

ABCA1-mediated cholesterol efflux is defective in free cholesterol-loaded macrophages. Mechanism involves enhanced ABCA1 degradation in a process requiring full NPC1 activity

In advanced atherosclerosis, macrophage foam cells progressively accumulate large amounts of unesterified or "free" cholesterol (FC), a process that is thought to contribute to foam cell death and lesional necrosis. The cellular consequences of early FC accumulation, including those that lead to further FC accumulation, are poorly understood. In this context, we show that cholesterol and phospholipid efflux mediated by ABCA1, which is initially induced in the cholesterol-loaded macrophage, was inhibited by approximately 80% in pre-toxic FC-loaded macrophages. Cholesterol efflux to HDL(2), which is mediated by a non-ABCA1 pathway, was inhibited by only approximately 20% in FC-loaded macrophages. FC loading led to decreased levels of ABCA1 protein via increased degradation of ABCA1, and not by decreased transcription or translation of AbcA1 mRNA. The decrease in ABCA1 protein occurred relatively early and was not prevented by caspase inhibitors, indicating that it was not a consequence of FC-induced apoptosis. However, inhibition of proteasomal function by lactacystin largely prevented the degradation of ABCA1. Importantly, the FC-induced decrease in ABCA1 function and protein was almost entirely prevented in macrophages that had partial deficiency of npc1 or were exposed to nanomolar concentrations of U18666A, both of which lead to defective cholesterol trafficking to the endoplasmic reticulum, but leave trafficking to the plasma membrane largely intact. Thus, a relatively early event during FC loading of macrophages is increased degradation of ABCA1, which appears to require trafficking of cholesterol to a peripheral cellular site, but not bulk trafficking of excess cholesterol to the plasma membrane. These findings provide new insight into the post-translational regulation of ABCA1 and the pathobiology of the FC-loaded macrophage.

Aluminum accumulation and membrane fluidity alteration in synaptosomes isolated from rat brain cortex following aluminum ingestion: effect of cholesterol

In the present work, we studied the effect of cholesterol/phospholipid (CH/PL) molar ratio on aluminum accumulation and aluminum-induced alteration of membrane fluidity in rat brain cortex synaptosomes. We observed that sub-acute (daily supply of 1.00 g of AlCl(3) during 10 days) and chronic (daily supply of 0.03 g of AlCl(3) during 4 months) exposure to dietary aluminum leads to a synaptosomal aluminum enrichment of 45 and 59%, respectively. During chronic exposure to AlCl(3), the enhancement of aluminum content was prevented by administration of colestipol (0.31 g/day), which decreased the synaptosomal membrane CH/PL molar ratio (nmol/nmol) from 1.2 to 0.4. Fluorescence anisotropy analysis, using 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-(4-(trimethylamino)phenyl)-6-phenylhexa-1,3,5-triene (TMA-DPH), showed that after treatment with colestipol a decrease in membrane order occurs at the level of hydrophilic lipid-water surface and deeper hydrophobic region of the synaptosomal membrane. When the rats were exposed to aluminum, it was observed a significant enhancement of membrane fluidity, which was more pronounced at the level of the membrane hydrophilic regions. Meanwhile, when chronic exposure to dietary AlCl(3) was accompanied by treatment with colestipol, the aluminum-induced decrease in membrane order was negligible when compared to TMA-DPH and DPH anisotropy values measured upon colestipol treatment. In contrast, in vitro incubation of synaptosomes (isolated from control rats) with AlCl(3) induced a concentration-dependent rigidification of this more hydrophilic membrane region. The opposite action of aluminum on synaptosomal membrane fluidity, during in vivo and in vitro experiments, appears to be explained by alteration of synaptosomal CH/PL molar ratio, since a significant reduction (approximately 80%) of this parameter occurs during in vivo exposure to aluminum. In conclusion, during in vivo exposure to aluminum, fluidification of hydrophilic regions and reduction of CH/PL molar ratio of presynaptic membranes accompany the accumulation of this cation, which appear to restrict aluminum retention in brain cortex nerve terminals.