Health benefits of high-density lipoproteins in preventing cardiovascular diseases

Journal of Clinical Lipidology (2012) 6, 524–533

Health benefits of high-density lipoproteins in preventing cardiovascular diseases Hicham Berrougui, PhD*, Claudia N. Momo, PhD, Abdelouahed Khalil, PhD Research Centre on Aging, Sherbrooke University Geriatric Institute, 1036 rue Belvedere south, Sherbrooke, QC, Canada J1H 4C4 (Drs. Berrougui, Momo, and Khalil); and Department of Biology, University Sultan Moulay Slimane, Beni–Mellal, Morocco (Dr. Berrougui) KEYWORDS: Atherogenesis; Cardiovascular diseases; Cholesterol efflux; HDL; Paraoxonase 1; Reverse cholesterol transport

Abstract: Plasma levels of high-density lipoprotein (HDL) are strongly and inversely correlated with atherosclerotic cardiovascular diseases. However, it is becoming clear that a functional HDL is a more desirable target than simply increasing HDL-cholesterol levels. The best known antiatherogenic function of HDL particles relates to their ability to promote reverse cholesterol transport from peripheral cells. However, HDL also possesses antioxidant, anti-inflammatory, and antithrombotic effects. This review focuses on the state of knowledge regarding assays of HDL heterogeneity and function and their relationship to cardiovascular diseases. Ó 2012 National Lipid Association. All rights reserved.

A half century of investigations, including epidemiologic, interventional, and molecular studies, have revealed that there is an inverse relationship between levels of plasma high-density lipoprotein (HDL) and cardiovascular disease (CVD). In addition, low HDL-cholesterol (C) was identified as the most frequent dyslipoproteinemia encountered in patients with premature coronary heart disease (CHD). As was observed in the Framingham cohort, the risk for atherosclerosis and coronary events is associated with reduced HDL, whereas elevated HDL is protective.1 It has been estimated that, for an increase of 1 mg/dL HDL, the risk of death or myocardial infarction is decreased by approximately 3%.2 The protective effects attributed to HDL are most often explained by the concept initially proposed by Glomset, reverse cholesterol transport (RCT).3 In RCT, HDL particles serves as a shuttle, removing excess cholesterol from * Corresponding author. E-mail address: [email protected] Submitted June 20, 2011. Accepted for publication April 5, 2012.

peripheral tissues and delivering it to the liver for final excretion with bile. Cholesterol efflux (CE) is the result of unspecific and passive as well as specific and active processes. According to the RCT hypothesis, CE occurs via three pathways. The first pathway is aqueous diffusion by which free cholesterol molecules spontaneously desorb from the plasma membrane, diffuse through the aqueous phase, and become adsorbed on acceptor particles by collision. The second pathway involves scavenger receptor class B type I (SR-BI)–mediated bidirectional free cholesterol exchanges that depend on the direction of the cholesterol gradient.4 The third pathway involves the ATP-binding cassette receptors ABCA1 and ABCG1, which mediate CE in a unidirectional manner to lipid-poor apolipoprotein (apo)A-I and mature HDL, respectively. In addition to its role in promoting CE, HDL also exhibits anti-inflammatory proprieties in endothelial cells and macrophages. HDL has been shown to inhibit the production of E-selectin, interleukin (IL)-8 and IL-1, expression of monocyte chemotactic protein-1, intracellular adhesion molecule-1 (ICAM-1), and vascular cell adhesion

1933-2874/$ - see front matter Ó 2012 National Lipid Association. All rights reserved. doi:10.1016/j.jacl.2012.04.004

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Health benefits of HDL in cardiovascular disease

molecule-1 (VCAM-1), factors that contribute to binding of leukocytes and early atheroma formation. HDL represents a carrier of several anti-oxidant enzymes, and it also acts as an effective scavenger of certain oxidants, thereby protecting other targets (ie, low-density lipoprotein [LDL]) from oxidative modification and to ensure an antithrombotic and antiplatelet effects as well as improving endothelial function.5 In this review, we highlight the potential mechanisms by which HDL may exert its beneficial effects in human health, particularly in the prevention of atherosclerosis, the underlying factor in coronary and vascular heart diseases.

Structure and heterogeneity of HDL HDLs are a class of heterogeneous lipoproteins containing approximately equal amounts of lipids (cholesterol and phospholipids) and proteins.6 The density of HDL particles ranges between 1.063 and 1.21 g/mL and are small in size (diameter 5–17 nm).5 HDL subclasses differ in their content of lipids, apolipoprotein (mainly, apoA-I and apoA-II), enzymes, and lipids transfer proteins, including PON1, PAF-AH or Lp-PLA2, lecithin cholesterol acetyltransferase (LCAT), cholesteryl ester transfer protein (CETP), and phospholipid transfer protein.7 HDL and their subclasses are currently separated by several methods all based on

different physical and chemical properties (density, size, electrophoretic mobility, apolipoprotein content, etc.), each of which are determined by the lipid and/or protein concentrations of different particles. Classification of HDL particles as a function of size and density on the basis of their ultracentrifugal rate principally revealed the presence of smaller, denser HDL3 (1.125–1.21 g/mL) and larger, more buoyant HDL2 (1.063–1.125 g/mL), whereas larger HDL designed HDL1 represent a relatively minor species in most individuals.8,9 HDL2 and HDL3 can be further divided on gradient gel electrophoresis (on the basis of the particle diameter) into five subclasses: HDL3c (7.6 nm), HDL3b (8 nm), HDL3a (8.4 nm), HDL2a (9.2 nm), and HDL2b (10.6nm).10 HDL2b is strongly correlated with HDL cholesterol and is inversely related to CHD risk, whereas increased HDL3b is associated with the atherogenic lipoprotein phenotype (increased triglycerides; small, dense LDL; and reduced HDL2b).8 HDLs can also be separated on the basis of size and charge, which leads to five major HDLs (pre–b-1-HDL, a-4 HDL, a-3 HDL a-2 HDL, and a-1 HDL), and there are large pre–b-migrating HDLs known as pre–b-2-HDL (Table 1). Pre–b-1HDLs are most the efficient and specifically interact with ABCA1 to promote CE, a-1 HDLs interact with SR-BI, and intermediate-size a-3 HDLs are most efficient in interacting with ABCG1.11,12

Heterogeneity of HDL particles

Property

Technical separation

HDL subpopulations

Differing in shape

Gel electrophoresis

Discoidal HDL Spherical HDL

Apoprotein composition

Charge and dimension

HDL 2

HDL3a HDL3b HDL3c

HDL3

Ultracentrifugation separatin

Immunoafinity chromatography separation

Two-dimensional gel electrophoresis

7,6

Pre - 2 Pre - 3

α1 α2 α3 α4

Pre - 1 Premigration

HDL, high-density lipoprotin; LpA, lipoprotein A.

1,21

LpA-I; LpA-I:A-II

Particle diameter

Density and size

HDL2b HDL2a

1,063

Diameter (nm)

10,6

Density (g/ml)

Table 1

525

αmigration

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On the basis of the apolipoprotein composition (apo-AI and apo-AII), HDL particles can also be classified as LpAI, Lp-AII, and Lp-AI:AII. Lp-AI is initially secreted as lipid-poor apo-AI complex which interacts with ABCA1 transporters and facilitates CE, resulting in the formation of pre–b-HDL, which in turn can under LCAT action be converted to a-HDL–containing LP-AI (Table 1).13

overall hypothesis: the detoxification of lipid hydroperoxides by HDL is potentially antiatherogenic. Among plasma lipoproteins, PON1, unlike PAF-AH, is almost exclusively found in association with HDL. PON1 exhibits a wide range of hydrolytic activities, such as arylesterase, phosphodiesterase, and lactonase activities. PON1 is an HDL-associated calcium-dependent enzyme, able to hydrolyse oxidized fatty acids from phospholipids and to reduce the proinflammatory response. Human serum PON1 activity was shown to be inversely related to the risk of cardiovascular diseases, and low serum PON1 activities were observed in atherosclerosis, hypercholesterolemic, and diabetic patients. In animals, the role of PON1 in atherosclerosis development was demonstrated in studies in which the authors used mice lacking or overexpressing PON1.21 In previous studies, authors reported that PON1 inhibits transition metal ions and free radical generator-induced LDL and HDL oxidation. The mechanism by which PON1 exerts its antioxidant effect remains unclear. However, it has been reported that PON1 purified from human HDL catalyses the hydrolysis of the oxidized polyunsaturated fatty acids at the sn-2 position of phospholipids in oxidized LDL (ox-LDL).22 In a recent study,23 we demonstrated that human-purified or recombinant PON1 inhibits LDL or HDL oxidation by reducing the formation of conjugated diene and lysophosphatidylcholine. It was also reported that PON1 decreases the oxidative activity of macrophages and the extent to which macrophages infiltrate the artery wall.24 It may, therefore, be central to the antiinflammatory and oxidative properties of HDL.

HDL as antioxidant particles A growing body of evidence suggests that HDL exert part of its antiatherogenic effect by counteracting LDL oxidation. The antioxidant properties of HDL in vivo can be separated into direct and indirect actions. HDL can directly inhibit the oxidation of LDL by, for example, transferring oxidation products from LDL to HDL so that HDL serves as a ‘‘sink’’ for oxidized lipids. In addition, inhibition of oxidative events and oxidative stress in vivo may be achieved indirectly via other functions of HDL, such as mediating CE or through their anti-inflammatory functions.14 HDL inhibits LDL oxidation by the transition of metal ions but also prevents 12-lipooxygenase–mediated formation of lipid hydroperoxides. Several of HDL components contribute to this antioxidant effect, including some of its protein components: apoA-I, PON-1, PAF-AH, LCAT, and glutathione peroxidase (GPX).15 HDL obtained from the serum of apoA-I–overexpressing transgenic animals inhibits oxidation of LDL two or three times more effectively than HDL from wild-type animals. The same effects were observed when apoA-I was injected in mice; a reduction of the susceptibility of LDL to lipid peroxidation was observed.16 Infusion of apoA-I into humans renders their LDL resistant to oxidation by human artery wall cell co-cultures. Navab et al17 have also indicated that treating human coronary aortic artery cells with lipid-free-apoA-I, PON1, or HDL increases the resistance of LDL to oxidation. Garner et al18 demonstrated that apoA-I reduces peroxides of both phospholipids and cholesteryl esters and also remove hydroperoxyeicosatetraenoic acid and hydroperoxyoctadecadienoic acid, which are products of 12-lipooxygenase, from native LDL and are necessary for induction of the nonenzymatic oxidation of lipoprotein phospholipids. This process is accompanied by the formation of oxidized forms of apoA-I containing methionine sulfoxides. However, paradoxical data have been reported related to the impact of such ‘‘sulfoxidation of apoA-I’’ on its biological properties. In a previous study, Jonas et al19 concluded that these alterations are not sufficient to disrupt the overall properties of the apoA-I complexes with lipid or to impair their ability to activate LCAT. However, Shao et al20 have recently demonstrated that a single methionin-148 oxidation in apoA-I impair LCAT activation, a critical step in RCT. In contrast, Panzenbock et al9 have demonstrated that selective oxidation of methionine (86 and 112) residues to methionine sulfoxides (apoA-I132) enhances rather than diminishes known antiatherogenic activities of apoA-I, which is consistent with the

HDL and inflammation HDL protects against endothelial dysfunction and modulates the inflammatory signaling excreted by ox-LDL and cytokines, which takes part in their general antiatherogenic effects. The evidence of the anti-inflammatory effect of HDL has been reported in both in vivo and in vitro studies. Cockerill et al25 were the first to report that HDL inhibits inflammation in vitro. In their study authors have demonstrated that physiologically relevant concentrations of plasma HDL and discoidal rHDL (reconstituted HDL) inhibited VCAM-1, ICAM-1, and E-selectin expression in activated human umbilical vein endothelial cells. These effects can be explained by the fact that HDL inhibits sphingosine kinase, extracellular signal-regulated kinase, and nuclear factor kB signalling cascades. Moreover, the anti-inflammatory properties of HDL have also been linked with the inhibition of monocyte chemotactic protein-1 expression and transmigration of monocytes into cells that have been stimulated with ox-LDL. However, it have been reported that these anti-inflammatory effects of isolated HDL from human plasma varied widely, especially regarding VCAM-1 expression.26 One of the underlying reasons for this variation may be related to the fact that

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VCAM-1 expression in human umbilical vein endothelial cells depends on the length and degree of unsaturation of HDL phospholipids acyl chains as reported by Baker et al.27 The anti-inflammatory properties of rHDL have also been investigated in vivo. Recently, Patel et al28 demonstrate that infusion of apo-AI in its free lipid form or as a constituent of rHDL markedly inhibits acute vascular inflammation in normo-cholesterolemic New Zealand white rabbits. This effect may be explained by the up-regulating action of apoA-I on the expression of arterial 3-hydroxysteroid-24 reductase (DHCR24, an enzyme that catalyzes the final step of cholesterol synthesis).28 Moreover, Tang et al29 reported that apoA-I can also exert an indirect anti-inflammatory effect through its interaction with ABCA1. The interaction of apoA-I with ABCA1-expressing macrophages suppressed the ability of lyso-polysaccharide to induce the inflammatory cytokines, IL-1, IL-6, and tumor necrosis factor-a, which was reversed by silencing signal transducers and activators of transcription-3 or ABCA1. In a recent work, Nobecourt et al30 have demonstrated that the reduced antiinflammatory properties of nonenzymatically glycated apoA-I were attributed to a reduced ability to inhibit nuclear factor-kB activation and reactive oxygen species. Genetic deletion of apoA-I in (apoA-I(2/2)) mice reduces total and HDL cholesterol but increased pro-inflammatory HDL and decreases PON1 activity.31 HDLs are considered proinflammatory in the presence of systemic inflammation such as atherosclerosis because of the damage to apoA-I, PON1, and HDL-associated enzymes caused by the oxidants produced in the inflammatory reaction.32 Although hepatic PON1 production can be modulated by inflammatory cytokines, a consistent relationship between serum PON1 and cytokine concentration has proven t o be hard to find in humans. Two theories have been proposed to explain the anti-inflammatory of PON-1. The first consists of a coupled antioxidant effect of PON-1 towards ox-LDL that increases proinflammatory products (as discussed previously); second consists of enhancing the mobilization of cholesterol from macrophages. However, the most convincing evidence of the anti-inflammatory actions of PON1 comes from in vivo studies. Overexpressing human PON1 in mouse models of atherosclerosis reduces systemic inflammation and ox-LDL concentration in both plasma and artery walls. By contrast, PON1 knockout mice have been shown to exhibit proinflammatory HDL.33 In a recent study, we attempted to understand how PON1 may exert its anti-inflammatory effects.23 Interestingly, the anti-inflammatory versus the proinflammatory effect of PON1 depends on whether PON1 is in presence of ox-LDL or ox-HDL. In the presence of ox-LDL, PON1 induced a significant increase of ICAM-1 expression in the (Eahy 926) endothelial cells. However, an anti-inflammatory effect (3-fold reduction of ICAM-1 expression) was observed in presence of ox-HDL and PON1, as compared with ox-HDL in the absence of PON1. The anti-inflammatory effect of PON1 on ox-HDL was increased in the presence of LCAT

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(2-fold reduction of ICAM-1 expression) when ox-HDL was incubated in the presence of PON1 and LCAT. Moreover, our study revealed that PON1 significantly decreased the expression of ICAM-1 induced by tumor necrosis factor-a or purified Lyso-PC. The use of reconstituted rHDL as well as LCAT and platelet-activating factor acetylhydrolase inhibitors suggested a cooperative effect between PON1 and LCAT in the hydrolysis and inactivation of oxidized phospholipids.

HDL and blood homeostasis Antithrombotic effects of HDL The direct antithrombotic effect of HDL was clearly demonstrated in the rat model of acute arterial thrombosis.34 In this study, infusion of apoA-IMilano into rats into whom a thrombus formation was induced prolonged the period required for the development of thrombus and diminished their body weight. In several studies researchers demonstrated an inverse relationship between HDL and apoA-I levels in plasma and the incidence of recurrent venous thromboembolism. In other studies, infusion of rHDL in healthy and type 2 diabetes mellitus subjects reduced ex vivo reactivity of platelets towards agonists such as collagen, adenosine diphosphate, and arachidonic acid.35 However, the predictive effects of HDL remain controversial because no evidence for increased thromboembolic risk in subjects with low levels of HDL, HDL2, HDL3, or apoA-I has been found among the 19,049 participants of the prospective Longitudinal Investigation Of Thromboembolism study.36 Although native and rHDL have direct effects on platelet function,37 albeit through ill-defined mechanisms, it is generally considered that the major antithrombotic effects of HDL occur indirectly via effects on the vasculature. For example, HDL reduces endothelial cell surface expression of adhesion molecules, modulates blood flow through alterations in nitric oxide production, and decreases tissue factor production.

Antiplatelet effects of HDL It has been demonstrated that HDL inhibits thrombin-, collagen-, adenosine diphosphate-, and adrenalin-induced platelet aggregation.38 Interaction of HDL with platelets may be mediated in several mechanisms. It has been suggested that this interaction depends on the platelets–nitric oxide production induced by apoE contained in HDL because inhibition of nitric oxide synthase diminished the ability of HDL to inhibit platelet activation. Platelets express the apoE receptor LRP8, as well as other HDLbinding proteins such as GPIIb/IIIa (aIIbb3 integrin) and CD36. This notion has been supported by the impairment of HDL-induced signaling in aIIbb3-deficient platelets. However, controversial results have been reported concerning the interaction of HDL and their subclasses with this binding protein.

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Although large HDL (HDL2/apoE-rich HDL) exerts a direct inhibitory effect on platelet activation, data regarding the effects of smaller HDL3 are discrepant.39 In fact, Pedreno et al40 have shown that neither the GPIIb–IIIa complex nor GPIIb or GPIIIa individually are the membrane binding proteins for HDL3 on intact resting platelets. First, specific ligands for platelet were unable to inhibit the binding of HDL3 to intact resting platelets. Second, the HDL3 binding characteristics, the activation of protein kinase C and the inhibition of thrombin-induced inositoltriphosphate (IP3) formation and calcium (Ca21) mobilization mediated by HDL3 particles were similar in platelets from control subjects and patients characterized by total and partial lack of GPIIb–IIIa and fibrinogen.40 SR-BI is also expressed on platelets and recent studies have suggested a role for SR-BI in platelet function; however, its role in haemostasis is unknown. It has been reported that HDL binding to platelets was reduced by SR-B1 ligands. Furthermore, both native HDL and other SR-B1 ligands failed to inhibit thrombin-induced activation platelets from SR-B1– deficient mice. However, it has been reported that even high concentration of native HDL do not have significant direct effects on agonists induced platelets activation, whereas ox-HDL are able to inhibit platelets aggregation.41

position on agarose gels. These various HDL particles interact differently with key proteins in the pathway, including the cell surface receptors ABCA1, ABCG1, and SR-BI, which mediate cholesterol transport between HDL and the cell. Efflux to Pre-b-HDL particles is ABCA1 dependent, whereas mature HDLs (HDL3, HDL2), react with ABCG1 transporter and SR-BI receptor. After pre–b1-HDL cholesterol uptake from peripheral cells, pre-b1-HDLs are converted into pre–b2-HDL.45 The free cholesterol present in these particles is then converted into cholesteryl ester as the result of the action of LCAT, which is dependent on apo-AI activation. This process leads to the transition from discoidal HDL to large spherical a3-HDLs, which are then transformed into even larger a2-HDL and a1-HDL as they become enriched with esterified cholesterol.45 Interestingly, we have found that during this transformation, PON1 activity was decreased in a-HDL compared with b-HDL.46 In two other studies, investigators have reported that nearly all of the lipoprotein-associated PON1 activity is located in the HDL3 fraction whereas HDL2 retains only 1% to 5% of the total activity.47 The final uptake of HDL2 by the liver involves the selective SR-BI receptors. Several transfer proteins and enzymes are involved in cholesterol uptake by the liver. CETP mediates the exchange of cholesteryl ester for triglycerides, the transfer of phospholipids by phospholipid transfer protein, and the hydrolysis of TG (triglycerides) and phospholipids by HL (hepatic lipase). These enzymatic reactions result in a3HDL and lipid-poor apoA-I regeneration. The particles become rapidly enriched in lipids by cellular cholesterol and phospholipid to form pre-b1-HDL molecules.48

HDL and RCT The RCT pathway is defined by HDL removing cholesterol from peripheral tissues and transporting it back to the liver where it is processed. The RCT pathway is complex and the precise mechanisms, receptors, and functions have not been fully elucidated. It suggested that the RCT process in the major cardio-protective function of HDL.42 The relationship between RCT and atherosclerosis was first suggested by Ross and Glomset.43 These authors proposed that the development of atherosclerotic lesions was initiated when an imbalance occurred between the deposit and removal of arterial cholesterol after endothelial injury. According to the RCT mechanism, HDL serves as a shuttle that transports excess cholesterol from peripheral cells to the liver for elimination. Dating back to the 1970s, epidemiologic, clinical, and fundamental studies have suggested that HDL levels should be increased to increase the clearance of cholesterol from the arterial wall to prevent CVD. Because RCT clearly occurs in all peripheral cells, it has been measured and proposed as a general peripheral physiological process.44 However, the macrophage is the primary cell that is overloaded with cholesterol in atherosclerotic lesions. Consequently, it is more appropriate to conceptualize and measure RCT as a macrophage-specific phenomenon in the case of atherosclerosis. The first step in RCT involves small disc-shaped, lipidpoor associated apo-AI particles or pre–b-HDL that are synthesized in the liver and the small intestine or that are produced by hydrolysis of triglyceride-rich particles. Pre– b-HDLs, one of the initial acceptors of cellular cholesterol, are fractions migrating electrophoretically in the pre-b

Aqueous diffusion of cholesterol According to the aqueous diffusion model, cholesterol molecules spontaneously desorb from the plasma membrane, diffuse through the aqueous phase, and are then incorporated into HDL particles by simple collision.49 Phospholipid vesicles, phospholipid/albumin complexes, and triglyceride/ phospholipid emulsions efficiently remove cholesterol from cells via this mechanism, which does not require interactions with specific cellular receptors. Consequently, the transfer of cholesterol is passive and driven by the concentration of the cholesterol gradient. Passive aqueous diffusion of cholesterol between the cell plasma membrane and HDL involves a bidirectional exchange of cholesterol. The exchange of cholesterol mass occurs when the rates of efflux and influx are equal. A net transfer of cholesterol mass in either direction can occur by mass action effects when either the cell membrane or HDLs are relatively enriched in cholesterol. The rate of cellular CE by aqueous diffusion is highly dependent on the structure of the acceptor particle, the phospholipid acyl chain content, and the sphigomyelinto-phosphatdylicholine ratio.50 The size of the acceptor particle is also an important determinant of the rate of cholesterol exchange because it affects the diffusion-mediated

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collisions with cholesterol molecules present in the aqueous phase. Large particles are inefficient acceptors.51 The transfer of cholesterol by aqueous diffusion from phospholipid layers occurs during a period of several hours.

SR-BI–dependent free cholesterol movement The movement of free cholesterol via SR-BI is bidirectional and, similar to aqueous diffusion, the net transit of free cholesterol will depend on the direction of the cholesterol gradient.4SR-BI is a cell-surface glycoprotein composed of a 509 amino acid with a molecular mass of 82 kDa and a member of the CD36 family of proteins.52 The protein possesses an extracellular domain that is crucial for mediating the bidirectional fluxes of free cholesterol.53 SR-BI regulates the selective uptake of other lipoprotein lipids, including cholesteryl ester, phosopholipids, and triglycerides. SR-BI is interacts with a broad range of acceptors, including HDL, LDL, oxidized LDL, acetylated LDL, and small unilamellar vesicles.51 It is mainly found in caveolae and transports internalized lipids to nonendosomal and nonlysosomal compartments.54 However, no efflux of cholesterol to lipid-free apolipoprotein occurs via SR-BI, even if lipid-free apoA-I binds to SR-BI,55 a specific amino acid sequence in apoA-I is not required for the interaction, and there is more than one lipoprotein binding site on this receptor.56 However, the binding of HDL to SR-BI is influenced by the conformation of apoA-I so that larger HDL particles bind better than smaller particles. Recently, Guo et al57 demonstrated that C323 is required for SR-BI-mediated HDL binding/cholesteryl ester uptake and for regulating plasma cholesterol levels in vivo and that change in redox status might be a regulatory factor that modulates SR-BI-mediated cholesterol transport. SR-BI– mediated CE also depends on the presence and the nature of the phospholipids in the acceptor particles. In a number of studies researchers have shown that enriching HDL with phosphatidylcholine increases SR-BI– mediated CE, whereas treating HDL with phospholipase A2 decreases efflux. Moreover, enrichment of HDL with sphingomyelin results in a small increase of CE and a large decrease in SR-BI–mediated cholesterol influx. The decrease in cholesterol influx is not mainly accomplished by tethering the acceptor particle to the membrane but is rather largely attributable to changes in the organization of the lipids in the plasma membrane. Because part of the population of SR-BI tends to locate in caveolae, it is likely that the SR-BI–induced changes in plasma membrane organization involve caveolae and/or lipid rafts.58

ABC transporters and CE ABCA1- and ABCG1-dependent CE ABCA1 and ABCG1 are members of a large family of ATP binding cassette (ABC) family of membrane

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transporters that share common structural motifs for the active transport of a variety of substrates.59 Unlike aqueous diffusion and SR-BI–mediated cholesterol flux, the transport of free cholesterol by ABCA1 and ABCG1 is unidirectional, and net efflux always occurs via this mechanism. ABCA1, a 220-KDa protein, is a member of the ATP binding cassette is expressed in many organs; however, placenta, liver, lungs, and adrenal glands express the greatest levels. The ABCA1 protein is a full-size ABC transporter containing two transmembrane domains of six alpha-helices and two intracellular nucleotide-binding domains. ABCA1 expression is increased by loading cells with cholesterol because the consequent increase in oxysterol level activates the nuclear x receptor. The transcription of ABCA1 is also induced by ligands for the retinoid x receptor and, in the case of murine macrophages, also by cyclic AMP.60 ApoA-I/ABCA1 interaction leads to the formation of nascent HDLs that are primarily discoidal particles containing 2, 3, or 4 apoA-I molecules61 and to the production of some monomeric apoA-I molecules that are associated with 3-4 phospholipid molecules; lipid poor apoA-I/(pre-b1-HDL). This pre–b1-HDL is a product that in turn serves as a substrate in that it can react further and be converted into larger discoidal particles.56,62 Furthermore, the cholesterol within nascent discoidal HDL is esterified by LCAT in a process that converts the pre–b-migrating disc into amigrating, spherical HDLs, which are further remodelled by CETP that transfers cholesteryl esters from HDL to other lipoproteins.61 Understanding the role of ABCA1 in cellular CE came with the discovery that cholesterol-enriched fibroblasts and macrophages from patients with Tangier disease who possess mutations in the ABCA1 gene cannot transfer phospholipids and free cholesterol to lipid-free apolipoproteins, but they can do so to mature HDLs.63 ABCA1 activity affects membrane morphology, likely by flipping phospholipids to the outer leaflet of the plasma membrane.64 However, an active ATPase is essential for apoA-I binding to the cell surface, suggesting that apoA-I binds to a region of the membrane disrupted by ABCA1. ApoA-I interacts directly and binds with high affinity to ABCA1 domains.65 In addition, ABCA1 is degraded rapidly after transcription (half-life of 1–2 hours) and its cellular level is sensitive to the presence of an apolipoprotein such apoA-I because it binds to ABCA1 and stabilizes it by modulating its phosphorylation, thereby protecting the transporter from calpain-mediated proteolysis.66 CE via ABCA1 is also associated with phosphatidylcholine- and cholesterol-enriched domains. Denis et al67 have shown that the physical interaction of apoA-I with ABCA1 does not depend on membrane phosphatidylcholine or sphingomyelin and that the association of apoA-I with lipids reduced its ability to interact with ABCA1. ApoA-I binding to the extracellular domain of ABCA1 results in the active export of cellular cholesterol and phospholipids to lipid-poor apolipoproteins in a variety of

530 cells. Although the molecular basis for the interaction between ABCA1 and apoA-I is not yet well elucidated, Denis et al67 have proposed a direct association model in which ABCA1 acts as a receptor for apoA-I resulting in a stimulation of the CE activity of ABCA1. Gaus et al68 provide evidence that the apoA-I–lipid raft interaction is an essential event in apoA-I–stimulated CE because disorder of lipid raft domain with short incubation with (ie, cyclodextrin) results in an impairment of apoA-I– mediated CE. The kinetic model suggested by Gaus et al is that efflux to apoA-I is a two-step process. In the first step, some of the plasma membrane cholesterol contributes to a fast initial efflux (possibly from lipid rafts) and leads to a second pathway that mobilizes intracellular cholesterol mobilization.68,69 Denis et al67 have reported that the initial rapid and transient binding of apoA-I to the homotetrameric ABCA1 complex allows the insertion of apoA-I into the adjacent phosphatidylcholine-containing, high-capacity binding site, which is most likely created by the phospholipid translocase activity of ABCA1. The interaction of apoA-I with the high-capacity binding site of ABCA1 allows the extraction of phospholipids and free cholesterol and the subsequent dissociation of the lipidated products,67 resulting in the formation of nascent HDL particles. In turn, these nascent HDL (pre–b1-HDL), under LCAT activity, transform to spherical HDLs that are able to interact with ABCG1 and SR-BI receptors, leading to the second pathway with high mobilization of intracellular cholesterol. Recent studies, including ours, have demonstrated that PON1 enhances HDL-mediated CE.70–72 Moreover, a new finding has been recently reported by our team with respect to the mechanism of action by which PON1 mediates the CE process.71 Experiments performed with human purified PON1 show that it was highly effective in stimulating CE via the ABCA1 pathway, whereas no effect was observed with respect to SR-BI receptors. PON1 induces ABCA1protein and gene up-regulation, especially during the first hours of PON1–ABCA1 interaction.71 This effect probably occurs by PON1-hydrolyzing oxidized phospholipids to form Lyso-PC, which has been reported to induce ABCA1 up-regulation.72,73 Interestingly, PON1-mediated CE occurs in two major steps: (1) fast (first hours) and small (low % of CE) CE, which is lipid raft dependent as we demonstrated by the impairment of PON1-mediated CE upon lipid raft disruption and (2) slow (16 hours) and consistent (high % of CE) efflux. During the second step, ABCA1 was down-regulated, which was probably related to the involvement of ABCG1 and SR-BI receptors.71 These findings suggest that PON1 may act via an apoA-I like mechanism to mediate CE (Fig. 1). The role of ABCG1 in lipid efflux was confirmed in 2004 when Wang et al74 and Kennedy et al75 showed that ABCG1-transfected cells had increased CE to HDL. Recent studies have shown that ABCG1 promoted CE from cells to mature HDL particles (HDL2 and HDL3) but not to lipid free or lipid-poor apoA-I.75 Unlike ABCA1-dependent lipid

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Figure 1 Proposed mechanism of the CE pathway induced by apoA-I and PON1 interactions with ABCA1. apoA-I and PON1 interact with ABCA1, which leads to the formation of lipid-poor apoA-I and pre-b-HDL, which in turn react with ABCA1 to form larger discoidal HDL particles. Continuous action of LCAT contributes to the maturation of these particles to form spherical and cholesteryl ester-rich HDL. PON1 hydrolyses oxidized-PL and form Lyso-PC, which stimulates CE via the PPARg–LXRa–ABCA1dependent pathway.

export, which requires apolipoprotein binding, ABCG1dependent CE did not appear to require a direct interaction with lipoproteins.76 ABCG1, similarly to SR-BI, increases the free cholesterol content of the plasma membrane, as shown by the up-regulation in cholesterol oxidase susceptibility. ABCA1 and ABCG1 can act synergistically to remove cholesterol from cells. ABCA1 converts lipid-poor apoA-I to partially lipidated ‘‘nascent’’ lipoproteins that are effective acceptors for cholesterol exported by ABCG1. These findings raise the interesting possibility that ABCA1 and ABCG1 coordinate the removal of excess cholesterol from macrophages using a diverse array of lipid acceptor particles (Fig. 1).76 CE is also dependent on the phospholipid composition of HDL. Enriching HDL with phosphatidylcholine increases CE, whereas enriching HDL with sphingomyelin decreases cholesterol uptake by macrophages.77 However, Fournier et al78 have reported that CE decreased in the presence of phosphatidylethanolamine-rich HDL and increased in the presence of sphingomyelin-rich HDL. The phospholipid fatty acyl composition of lipoproteins has subtle but measurable effects on the fluidity of the lipoprotein phospholipidic layer. The structural and compositional integrity of HDL is essential for their antiatherogenic properties. In fact, oxidation may interfere with the biological role of HDL by impairing their antioxidant properties and their ability to promote CE.79

HDL heterogeneity and function Epidemiologic evidence indicates that HDL particles serve an antiatherogenic function and that high levels HDL-C are associated with a decreased risk of CVD. It is becoming increasingly apparent that these antiatherogenic effects of HDL are not only dependent on its concentration in circulating blood but also on its biological quality, which

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may be defined as the functionality of HDL.79 A topic of considerable interest is whether specific subfractions of HDL confer greater ability to predict cardiovascular risk than HDL-C itself. However, the data concerning the predictive ability of HDL subclasses for CHD risk are not conclusive. The analysis of data from the Framingham Study revealed that 44% of CVD events occurred in subjects with HDL levels in the normal range.80 Some large prospective population-based studies have suggested that apoA-I concentrations may be more predictive of future CHD events that HDL-C concentrations. Therefore, additional studies are needed to elucidate the impact of the functionality of HDL on their anti-atherogenic properties and determine factors that may affect this function. In summary, it’s known from the evidence that antiatherogenic properties of HDL (as discussed in this review) are lipid and protein dependent and are greatest in small, dense HDL. Moreover, the functional deficiency of HDL is intimately associated with changes in HDL more than in its plasma concentration.

Acknowledgment This work was supported by a grant from the Canadian Institutes of Health Research.

References 1. Gordon T, Castelli WP, Hjortland MC, Kannel WB, Dawber TR. High density lipoprotein as a protective factor against coronary heart disease. The Framingham Study. Am J Med. 1977;62:707–714. 2. Rubins HB, Robins SJ, Collins D, et al. Gemfibrozil for the secondary prevention of coronary heart disease in men with low levels of highdensity lipoprotein cholesterol. Veterans Affairs High-Density Lipoprotein Cholesterol Intervention Trial Study Group. N Engl J Med. 1999;341:410–418. 3. Glomset JA. The plasma lecithins:cholesterol acyltransferase reaction. J Lipid Res. 1968;9:155–167. 4. de La Llera-Moya M, Connelly MA, Drazul D, et al. Scavenger receptor class B type I affects cholesterol homeostasis by magnifying cholesterol flux between cells and HDL. J Lipid Res. 2001;42: 1969–1978. 5. Assmann G, Gotto AM Jr. HDL cholesterol and protective factors in atherosclerosis. Circulation. 2004;109:III8–III14. 6. Gordon DJ, Rifkind BM. High-density lipoprotein—the clinical implications of recent studies. N Engl J Med. 1989;321:1311–1316. 7. Link JJ, Rohatgi A, de Lemos JA. HDL cholesterol: physiology, pathophysiology, and management. Curr Probl Cardiol. 2007;32:268–314. 8. Navab M, Anantharamaiah GM, Fogelman AM. An apolipoprotein A-I mimetic works best in the presence of apolipoprotein A-I. Circ Res. 2005;97:1085–1086. 9. Panzenbock U, Kritharides L, Raftery M, Rye KA, Stocker R. Oxidation of methionine residues to methionine sulfoxides does not decrease potential antiatherogenic properties of apolipoprotein A-I. J Biol Chem. 2000;275:19536–19544. 10. Tang C, Vaughan AM, Anantharamaiah GM, Oram JF. Janus kinase 2 modulates the lipid-removing but not protein-stabilizing interactions of amphipathic helices with ABCA1. J Lipid Res. 2006;47:107–114. 11. Ji A, Wroblewski JM, Cai L, de Beer MC, Webb NR, van der Westhuyzen DR. Nascent HDL formation in hepatocytes and role of ABCA1, ABCG1 and SR-BI. J Lipid Res. 2012;53:446–455.

531

12. Sankaranarayanan S, Oram JF, Asztalos BF, et al. Effects of acceptor composition and mechanism of ABCG1-mediated cellular free cholesterol efflux. J Lipid Res. 2009;50:275–284. 13. Kawano M, Miida T, Fielding CJ, Fielding PE. Quantitation of pre beta-HDL-dependent and nonspecific components of the total efflux of cellular cholesterol and phospholipid. Biochemistry. 1993;32: 5025–5028. 14. Ansell BJ, Fonarow GC, Fogelman AM. The paradox of dysfunctional high-density lipoprotein. Curr Opin Lipidol. 2007;18:427–434. 15. Jaouad L, Milochevitch C, Khalil A. PON1 paraoxonase activity is reduced during HDL oxidation and is an indicator of HDL antioxidant capacity. Free Radic Res. 2003;37:77–83. 16. Hayek T, Oiknine J, Dankner G, et al. HDL apolipoprotein A-I attenuates oxidative modification of low density lipoprotein: studies in transgenic mice. Eur J Clin Chem Clin Biochem. 1995;33: 721–725. 17. Navab M, Hama SY, Anantharamaiah GM, et al. Normal high density lipoprotein inhibits three steps in the formation of mildly oxidized low density lipoprotein: steps 2 and 3. J Lipid Res. 2000;41: 1495–1508. 18. Garner B, Witting PK, Waldeck AR, Christison JK, Raftery M, Stocker R. Oxidation of high density lipoproteins. I. Formation of methionine sulfoxide in apolipoproteins AI and AII is an early event that accompanies lipid peroxidation and can be enhanced by alpha-tocopherol. J Biol Chem. 1998;273:6080–6087. 19. Jonas A, von Eckardstein A, Churgay L, et al. Structural and functional properties of natural and chemical variants of apolipoprotein A-I. Biochim Biophys Acta. 1993;1166:202–210. 20. Shao B, Cavigiolio G, Brot N, et al. Methionine oxidation impairs reverse cholesterol transport by apolipoprotein A-I. Proc Natl Acad Sci U S A. 2008;105:12224–12229. 21. Tward A, Xia YR, Wang XP, et al. Decreased atherosclerotic lesion formation in human serum paraoxonase transgenic mice. Circulation. 2002;106:484–490. 22. Watson AD, Berliner JA, Hama SY, et al. Protective effect of high density lipoprotein associated paraoxonase. Inhibition of the biological activity of minimally oxidized low density lipoprotein. J Clin Invest. 1995;96:2882–2891. 23. Loued S, Isabelle M, Berrougui H, Khalil A. Anti-inflammatory effect of paraoxoanse 1. Atheroscler Suppl. 2009;10:1186. 24. Mertens A, Verhamme P, Bielicki JK, et al. Increased low-density lipoprotein oxidation and impaired high-density lipoprotein antioxidant defense are associated with increased macrophage homing and atherosclerosis in dyslipidemic obese mice: LCAT gene transfer decreases atherosclerosis. Circulation. 2003;107:1640–1646. 25. Cockerill GW, Rye KA, Gamble JR, et al. High-density lipoproteins inhibit cytokine-induced expression of endothelial cell adhesion molecules. Arterioscler Thromb Vasc Biol. 1995;15:1987–1994. 26. Tabet F, Rye KA. High-density lipoproteins, inflammation and oxidative stress. Clin Sci (Lond). 2009;116:87–98. 27. Baker PW, Rye KA, Gamble JR, et al. Phospholipid composition of reconstituted high density lipoproteins influences their ability to inhibit endothelial cell adhesion molecule expression. J Lipid Res. 2000;41:1261–1267. 28. Patel S, Di Bartolo BA, Nakhla S, et al. Anti-inflammatory effects of apolipoprotein A-I in the rabbit. Atherosclerosis. 2010;212:392–397. 29. Tang C, Liu Y, Kessler PS, et al. The macrophage cholesterol exporter ABCA1 functions as an anti-inflammatory receptor. J Biol Chem. 2009;284:32336–32343. 30. Nobecourt E, Tabet F, Lambert G, et al. Nonenzymatic glycation impairs the antiinflammatory properties of apolipoprotein A-I. Arterioscler Thromb Vasc Biol. 2010;30:766–772. 31. Wang W, Xu H, Shi Y, et al. Genetic deletion of apolipoprotein A-I increases airway hyperresponsiveness, inflammation, and collagen deposition in the lung. J Lipid Res. 2010;51:2560–2570. 32. Navab M, Anantharamaiah GM, Reddy ST, et al. Mechanisms of disease: proatherogenic HDL—an evolving field. Nat Clin Pract Endocrinol Metab. 2006;2:504–511.

532

Journal of Clinical Lipidology, Vol 6, No 6, December 2012

33. Shih DM, Gu L, Xia YR, et al. Mice lacking serum paraoxonase are susceptible to organophosphate toxicity and atherosclerosis. Nature. 1998;394:284–287. 34. Li D, Weng S, Yang B, et al. Inhibition of arterial thrombus formation by ApoA1 Milano. Arterioscler Thromb Vasc Biol. 1999;19:378–383. 35. Calkin AC, Drew BG, Ono A, et al. Reconstituted high-density lipoprotein attenuates platelet function in individuals with type 2 diabetes mellitus by promoting cholesterol efflux. Circulation. 2009;120:2095–2104. 36. Chamberlain AM, Folsom AR, Heckbert SR, et al. High-density lipoprotein cholesterol and venous thromboembolism in the Longitudinal Investigation of Thromboembolism Etiology (LITE). Blood. 2008; 112:2675–2680. 37. Nofer JR, Walter M, Kehrel B, et al. HDL3-mediated inhibition of thrombin-induced platelet aggregation and fibrinogen binding occurs via decreased production of phosphoinositide-derived second messengers 1,2-diacylglycerol and inositol 1,4,5-tris-phosphate. Arterioscler Thromb Vasc Biol. 1998;18:861–869. 38. Nofer JR, Kehrel B, Fobker M, et al. HDL and arteriosclerosis: beyond reverse cholesterol transport. Atherosclerosis. 2002;161:1–16. 39. Valiyaveettil M, Kar N, Ashraf MZ, et al. Oxidized high-density lipoprotein inhibits platelet activation and aggregation via scavenger receptor BI. Blood. 2008;111:1962–1971. 40. Pedreno J, de Castellarnau C, Masana L. Platelet HDL(3) binding sites are not related to integrin alpha(IIb)beta(3) (GPIIb-IIIa). Atherosclerosis. 2001;154:23–29. 41. Valiyaveettil M, Podrez EA. Platelet hyperreactivity, scavenger receptors and atherothrombosis. J Thromb Haemost. 2009;7(Suppl 1):218–221. 42. Murphy AJ, Chin-Dusting JP, Sviridov D, et al. The anti inflammatory effects of high density lipoproteins. Curr Med Chem. 2009;16:667–675. 43. Ross R, Glomset JA. Atherosclerosis and the arterial smooth muscle cell: proliferation of smooth muscle is a key event in the genesis of the lesions of atherosclerosis. Science. 1973;180:1332–1339. 44. Cuchel M, Rader DJ. Macrophage reverse cholesterol transport: key to the regression of atherosclerosis? Circulation. 2006;113:2548–2555. 45. Brufau G, Groen AK, Kuipers F. Reverse cholesterol transport revisited: contribution of biliary versus intestinal cholesterol excretion. Arterioscler Thromb Vasc Biol. 2011;31:1726–1733. 46. Berrougui H, Khalil A. Age-associated decrease of high-density lipoprotein-mediated reverse cholesterol transport activity. Rejuvenation Res. 2009;12:117–126. 47. Bergmeier C, Siekmeier R, Gross W. Distribution spectrum of paraoxonase activity in HDL fractions. Clin Chem. 2004;50:2309–2315. 48. Tanigawa H, Billheimer JT, Tohyama J, et al. Expression of cholesteryl ester transfer protein in mice promotes macrophage reverse cholesterol transport. Circulation. 2007;116:1267–1273. 49. Adorni MP, Zimetti F, Billheimer JT, et al. The roles of different pathways in the release of cholesterol from macrophages. J Lipid Res. 2007;48:2453–2462. 50. Rothblat GH, de la Llera-Moya M, Atger V, et al. Cell cholesterol efflux: integration of old and new observations provides new insights. J Lipid Res. 1999;40:781–796. 51. Yancey PG, Bortnick AE, Kellner-Weibel G, et al. Importance of different pathways of cellular cholesterol efflux. Arterioscler Thromb Vasc Biol. 2003;23:712–719. 52. Van Eck M, Pennings M, Hoekstra M, et al. Scavenger receptor BI and ATP-binding cassette transporter A1 in reverse cholesterol transport and atherosclerosis. Curr Opin Lipidol. 2005;16:307–315. 53. Connelly MA, de la Llera-Moya M, Monzo P, et al. Analysis of chimeric receptors shows that multiple distinct functional activities of scavenger receptor, class B, type I (SR-BI), are localized to the extracellular receptor domain. Biochemistry. 2001;40:5249–5259. 54. Graf GA, Connell PM, van der Westhuyzen DR, et al. The class B, type I scavenger receptor promotes the selective uptake of high density lipoprotein cholesterol ethers into caveolae. J Biol Chem. 1999;274: 12043–12048. 55. de la Llera-Moya M, Rothblat GH, Connelly MA, et al. Scavenger receptor BI (SR-BI) mediates free cholesterol flux independently of HDL tethering to the cell surface. J Lipid Res. 1999;40:575–580.

56. Lund-Katz S, Phillips MC. High density lipoprotein structure-function and role in reverse cholesterol transport. Subcell Biochem. 2010;51:183–227. 57. Guo L, Chen M, Song Z, et al. C323 of SR-BI is required for SR-BImediated HDL binding and cholesteryl ester uptake. J Lipid Res. 2011;52:2272–2278. 58. Ahmed SN, Brown DA, London E. On the origin of sphingolipid/cholesterol-rich detergent-insoluble cell membranes: physiological concentrations of cholesterol and sphingolipid induce formation of a detergent-insoluble, liquid-ordered lipid phase in model membranes. Biochemistry. 1997;36:10944–10953. 59. Dean M, Hamon Y, Chimini G. The human ATP-binding cassette (ABC) transporter superfamily. J Lipid Res. 2001;42:1007–1017. 60. Oram JF, Heinecke JW. ATP-binding cassette transporter A1: a cell cholesterol exporter that protects against cardiovascular disease. Physiol Rev. 2005;85:1343–1372. 61. Krimbou L, Marcil M, Genest J. New insights into the biogenesis of human high-density lipoproteins. Curr Opin Lipidol. 2006;17: 258–267. 62. Navab M, Reddy ST, Van Lenten BJ, et al. HDL and cardiovascular disease: atherogenic and atheroprotective mechanisms. Nat Rev Cardiol. 2011;8:222–232. 63. Voloshyna I, Reiss AB. The ABC transporters in lipid flux and atherosclerosis. Prog Lipid Res. 2011;50:213–224. 64. Burgess JW, Kiss RS, Zheng H, et al. Trypsin-sensitive and lipidcontaining sites of the macrophage extracellular matrix bind apolipoprotein A-I and participate in ABCA1-dependent cholesterol efflux. J Biol Chem. 2002;277:31318–31326. 65. Mulya A, Lee JY, Gebre AK, et al. Minimal lipidation of pre-beta HDL by ABCA1 results in reduced ability to interact with ABCA1. Arterioscler Thromb Vasc Biol. 2007;27:1828–1836. 66. Yokoyama S. Assembly of high-density lipoprotein. Arterioscler Thromb Vasc Biol. 2006;26:20–27. 67. Denis M, Haidar B, Marcil M, et al. Molecular and cellular physiology of apolipoprotein A-I lipidation by the ATP-binding cassette transporter A1 (ABCA1). J Biol Chem. 2004;279:7384–7394. 68. Gaus K, Kritharides L, Schmitz G, et al. Apolipoprotein A-1 interaction with plasma membrane lipid rafts controls cholesterol export from macrophages. FASEB J. 2004;18:574–576. 69. Gaus K, Gooding JJ, Dean RT, et al. A kinetic model to evaluate cholesterol efflux from THP-1 macrophages to apolipoprotein A-1. Biochemistry. 2001;40:9363–9373. 70. Rosenblat M, Gaidukov L, Khersonsky O, et al. The catalytic histidine dyad of high density lipoprotein-associated serum paraoxonase1 (PON1) is essential for PON1-mediated inhibition of low density lipoprotein oxidation and stimulation of macrophage cholesterol efflux. J Biol Chem. 2006;281:7657–7665. 71. Berrougui H, Loued S, Khalil A. Purified human paraoxonase-1 interacts with plasma membrane lipid rafts and mediates cholesterol efflux from macrophages. Free Radical Biol Medicine. 2012;52:1372–1381. 72. Rosenblat M, Vaya J, Shih D, et al. Paraoxonase 1 (PON1) enhances HDL-mediated macrophage cholesterol efflux via the ABCA1 transporter in association with increased HDL binding to the cells: a possible role for lysophosphatidylcholine. Atherosclerosis. 2005;179: 69–77. 73. Hou M, Xia M, Zhu H, et al. Lysophosphatidylcholine promotes cholesterol efflux from mouse macrophage foam cells via PPARgammaLXRalpha-ABCA1–dependent pathway associated with apoE. Cell Biochem Funct. 2007;25:33–44. 74. Wang N, Lan D, Chen W, et al. ATP-binding cassette transporters G1 and G4 mediate cellular cholesterol efflux to high-density lipoproteins. Proc Natl Acad Sci U S A. 2004;101:9774–9779. 75. Kennedy MA, Barrera GC, Nakamura K, et al. ABCG1 has a critical role in mediating cholesterol efflux to HDL and preventing cellular lipid accumulation. Cell Metab. 2005;1:121–131. 76. Oram JF, Vaughan AM. ATP-Binding cassette cholesterol transporters and cardiovascular disease. Circ Res. 2006;99:1031–1043. 77. Yancey PG, de la Llera-Moya M, Swarnakar S, et al. High density lipoprotein phospholipid composition is a major determinant of the

Berrougui et al

Health benefits of HDL in cardiovascular disease

bi-directional flux and net movement of cellular free cholesterol mediated by scavenger receptor BI. J Biol Chem. 2000;275: 36596–36604. 78. Fournier N, Paul JL, Atger V, et al. HDL phospholipid content and composition as a major factor determining cholesterol efflux capacity from Fu5AH cells to human serum. Arterioscler Thromb Vasc Biol. 1997;17:2685–2691.

533

79. Movva R, Rader DJ. Laboratory assessment of HDL heterogeneity and function. Clin Chem. 2008;54:788–800. 80. Ansell BJ, Navab M, Hama S, et al. Inflammatory/antiinflammatory properties of high-density lipoprotein distinguish patients from control subjects better than high-density lipoprotein cholesterol levels and are favorably affected by simvastatin treatment. Circulation. 2003;108: 2751–2756.