High-density lipoproteins and atherosclerosis

High-Density Lipoproteins and Atherosclerosis Daniel J. Rader,

MD

High-density lipoproteins (HDLs) are strongly related to risk of atherosclerotic cardiovascular disease. Low levels of HDL cholesterol are a major cardiovascular risk factor, and overexpression of the major HDL protein, apolipoprotein (apo) A-I, markedly inhibits progression and even induces regression of atherosclerosis in animal models. Clinical data regarding the effect of increasing HDL cholesterol on vascular events are limited. HDL remains an important potential target for therapeutic in-

tervention. A variety of gene products are involved in the regulation of HDL metabolism. Yet, the mechanisms by which HDL inhibits atherosclerosis are not yet fully understood. There remains much to be learned about HDL metabolism and its relation to atherosclerosis and other cardiovascular risk factors. 䊚2002 by Excerpta Medica, Inc. Am J Cardiol 2002;90(suppl):62i–70i

here is a strong inverse association between plasma high-density lipoprotein (HDL) cholesterol T levels and incidence of coronary artery disease (CAD)

HDL particles interact with peripheral cells and acquire cholesterol and phospholipid through a transport process facilitated by the cellular protein adenosine triphosphate (ATP)– binding cassette protein A1 (ABCA1; Figure 1). Unesterified cholesterol is esterified to cholesteryl ester within the HDL particle by the enzyme lecithin cholesterol acyltransferase (LCAT). HDL cholesteryl ester can be taken up selectively by the liver through the action of the scavenger receptor class BI (SR-BI; Figure 1). Cholesteryl ester can also be selectively transferred to apo B– containing lipoproteins in exchange for triglyceride through the action of cholesteryl ester transfer protein. Conversely, the phospholipid transfer protein mediates transfer of phospholipids from apo B– containing lipoproteins to HDL. Several members of the triglyceride lipase gene family influence the metabolism of HDL. Hydrolysis of triglycerides in triglyceride-rich lipoproteins by lipoprotein lipase results in transfer of lipids and apolipoproteins to HDL. Hepatic lipase hydrolyzes HDL triglyceride and phospholipids, generating smaller lipid-depleted HDL particles. Endothelial lipase hydrolyzes HDL phospholipids and promotes HDL catabolism.

that is independent of other known risk factors.1 Low levels of HDL cholesterol are frequently found in patients with CAD,2 whereas genetic syndromes of high HDL cholesterol are often associated with decreased CAD and longevity. This epidemiologic association led to the recommendation for the inclusion of HDL cholesterol in routine screening of all adults and as an independent risk factor in the assessment of cardiovascular risk.3 Recent attention has also focused on low HDL cholesterol as a potential target for therapeutic intervention. Low HDL cholesterol is frequently found in association with elevated levels of atherogenic lipoproteins, including very low-density lipoprotein (VLDL) and small, dense low-density lipoprotein (LDL), as well as with a metabolic syndrome that includes insulin resistance, glucose intolerance, and hypertension.4 Therefore, low HDL cholesterol is, in part, a marker for the presence of other cardiovascular risk factors. There remains much to be learned about HDL metabolism and its relation to atherosclerosis and other cardiovascular risk factors.

HIGH-DENSITY LIPOPROTEIN METABOLISM A schematic diagram depicting HDL metabolism is shown in Figure 1. HDL is composed of lipids (including phospholipid, cholesterol, and triglyceride) as well as apolipoproteins, the major one of which is apo A-I.5 Apo A-I is synthesized and secreted by both the intestine and the liver. Nascent apo A-I– containing From the University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, USA. Daniel J. Rader is an Established Investigator of the American Heart Association and a recipient of the Burroughs Wellcome Foundation Clinical Scientist Award in Translational Research and is supported by National Institutes of Health grants from the National Heart, Lung, and Blood Institute, the National Institute of Diabetes, Digestive, and Kidney Diseases, and the National Center for Research Resources. Address for reprints: Daniel J. Rader, MD, University of Pennsylvania School of Medicine, 654 BRB II/III, 421 Curie Boulevard, Philadelphia, Pennsylvania 19104. E-mail: [email protected].

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©2002 by Excerpta Medica, Inc. All rights reserved.

HIGH-DENSITY LIPOPROTEIN AND ATHEROSCLEROSIS IN ANIMALS Although it is still not entirely clear whether the association of HDL cholesterol levels with CAD is causal in nature, a wealth of evidence in animals indicates that HDL and apo A-I are directly antiatherogenic. Repeated intravenous injection of human HDL into cholesterol-fed rabbits results in regression of atherosclerotic lesions,6 and repeated intravenous injection of rabbit apo A-I into cholesterol-fed rabbits results in reduced progression of atherosclerotic lesions.7 Hepatic overexpression of human apo A-I in transgenic mice reduces progression of atherosclerosis in C57BL/6 mice fed a high fat, high cholesterol diet8 in apo E– deficient mice9,10 and in apo (a) transgenic mice.11 Somatic gene transfer and hepatic expression of human apo A-I induced significant regression of 0002-9149/02/$ – see front matter PII S0002-9149(02)02635-8

FIGURE 1. High-density lipoprotein (HDL) and reverse cholesterol transport. Peripheral cells, both macrophages and nonmacrophages, must efflux excess free cholesterol (FC) to acceptors in the extracellular environment. Lipid-poor apolipoprotein A-I (A-1) interacts with peripheral cells and acquires FC and phospholipid (PC) through a transport process facilitated by the cellular protein adenosine triphosphate– binding cassette protein A1 (ABCA1). Mature HDL can also acquire cholesterol from macrophages via the scavenger receptor class BI (SR-BI). Unesterified cholesterol in HDL is converted to cholesteryl ester (CE) within the HDL particle by the enzyme lecithin cholesterol acyltransferase (LCAT). HDL cholesterol can be taken up selectively by the liver through the action of SR-BI. The liver secretes free cholesterol directly into the bile or converts it into bile acids (BA), which are then secreted into the bile. Ultimately, biliary sterols are excreted in the feces. HDL CE can also be selectively transferred to apolipoprotein B– containing lipoproteins in exchange for triglyceride through the action of CE transfer protein (CETP) (not shown). Hepatic lipase (HL) and endothelial lipase (EL) hydrolyze HDL lipids, generating smaller HDL particles and promoting their catabolism.

preexisting atherosclerotic lesions in LDL receptor– deficient mice12 and reduced the progression of atherosclerosis in apo E– deficient mice.13 Transgenic overexpression of human apo A-I in the livers of cholesterol-fed rabbits reduced the development of aortic atherosclerosis.14 Expression of apo A-I in macrophages was also demonstrated to reduce atherosclerosis progression.15 Studies of the effect of apo A-I deficiency on the development of atherosclerosis in animals have been of relatively limited utility. Studies of the effect of apo A-I– deficient mice have reduced levels of HDL cholesterol, but when fed a chow diet or even when fed an atherogenic diet, they do not spontaneously develop atherosclerosis.16 Apo A-I– deficient mice crossed with human apo B transgenic mice failed to develop significant atherosclerosis on a chow diet but did develop moderately increased atherosclerosis when fed a western-type diet.17 Apo A-I⫺/⫺/apo B transgenic mice fed a cholate-containing atherogenic diet had a 39% increase in atherosclerosis relative to apo B transgenic mice.18

HIGH-DENSITY LIPOPROTEIN AND REVERSE CHOLESTEROL TRANSPORT

Role of ABCA1 in reverse cholesterol transport: Apo A-I is thought to protect against atherosclerosis at least in part by promoting efflux of excess cholesterol from macrophages in the arterial wall and returning that cholesterol to the liver for excretion into the bile,

a process known as reverse cholesterol transport (Figure 1).19 Lipid-poor apo A-I (sometimes referred to as pre␤ HDL) is an excellent acceptor of free cholesterol from cells, such as macrophages, by means of a transport process promoted by ABCA1 (Figure 1). Mutations in ABCA1 are the molecular cause of Tangier disease,20 a codominant condition of very low HDL cholesterol and apo A-I levels and marked cholesterol accumulation in macrophages. The ABCA1 knockout mouse has markedly reduced HDL cholesterol levels and evidence of macrophage lipid accumulation.21 However, when ABCA1 knockout mice were reconstituted with wild-type bone marrow (and thus macrophages), HDL cholesterol and apo A-I levels were only slightly increased. Conversely, transplantation of ABCA1 deficient marrow into wild-type mice did not affect HDL cholesterol and apo A-I levels.22 Therefore, although macrophages are the cell type most affected by ABCA1 deficiency with regard to cholesterol accumulation, macrophages themselves contribute little to the bulk lipidation of plasma apo A-I and therefore to plasma HDL cholesterol levels. It is believed that liver ABCA1 is the most important contributor to lipidation of lipid-poor apo A-I and therefore to plasma HDL cholesterol levels.23 Importantly, whereas total ABCA1 deficiency was not associated with increased atherosclerosis in LDL receptor– deficient or apo E– deficient mice,24 selective deficiency of ABCA1 in macrophages was found

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to significantly increase atherosclerosis in apo E– deficient25 and LDL receptor– deficient mice.24 Mice overexpressing ABCA1 in liver and macrophages had reduced atherosclerosis when fed a high-fat cholate diet but increased atherosclerosis on the background of apo E deficiency.26 Because lipid-poor apo A-I stimulates ABCA1-mediated cholesterol efflux from macrophages in vitro, increasing the concentration of apo A-I in vivo might be expected to promote cholesterol efflux from macrophages and therefore macrophage-specific RCT. Role of LCAT in reverse cholesterol transport: Apo A-I activates LCAT, which transfers a fatty acid from phosphatidylcholine to free cholesterol, creating cholesteryl ester (Figure 1).27 Transgenic overexpression of LCAT in mice28,29 and rabbits30 results in substantial increases in HDL cholesterol levels. Overexpression of human LCAT in human apo A-I transgenic mice31 leads to even greater increases in the plasma concentrations of a cholesteryl ester– enriched, large HDL. Conversely, LCAT-deficient mice have markedly reduced levels of HDL cholesterol and apo AI.32,33 The relation of LCAT to atherosclerosis is complex. Overexpression of human LCAT in cholesterolfed rabbits has been associated with increased HDL cholesterol levels34 and markedly reduced atherosclerosis,35 but this effect requires the presence of functional LDL receptors.36 By contrast, transgenic overexpression of human LCAT in mice either results in increased atherosclerosis37 or affords no evidence of protection from it.38 When human LCAT was overexpressed in mice that were also transgenic for human cholesteryl ester transfer protein expression, atherosclerosis was significantly reduced,39 suggesting that the antiatherogenic effect of LCAT requires the presence of cholesteryl ester transfer protein. The impact of LCAT deficiency on atherosclerosis in mice is uncertain. In 1 report, aortic atherosclerosis was significantly reduced in 3 different mouse models with LCAT deficiency.33 However, in another model, LCAT deficiency was associated with increased atherosclerosis in LDL receptor knockout and apo E knockout mice.40 Therefore, the relation between LCAT deficiency and atherosclerosis remains uncertain. Role of SR-BI in reverse cholesterol transport: Apo A-I binds to SR-BI, initiating a process of selective uptake of cholesterol from HDL into the liver (Figure 1).41 Hepatic overexpression of SR-BI in mice through gene transfer42 or germ-line transgenesis43,44 results in markedly reduced HDL cholesterol levels. Transgenic mice overexpressing SR-BI in the liver have reduced atherosclerosis but also have markedly reduced plasma levels of atherogenic apo B– containing lipoproteins.45,46 Hepatic overexpression of SR-BI using gene transfer does not reduce plasma levels of apo B– containing lipoproteins and yet still significantly reduces atherosclerosis (despite reducing plasma HDL cholesterol levels).47 Mice with homozygous null mutations in the SR-BI gene48 or bearing an insertion in the promoter region of the SR-BI gene, which results in about 50% of normal hepatic SR-BI 64i THE AMERICAN JOURNAL OF CARDIOLOGY姞

expression,49 exhibit increased plasma HDL cholesterol levels. Genetic deficiency of SR-BI increased atherosclerosis in apo E– deficient mice despite higher HDL cholesterol levels50 and is associated with markedly premature death resulting from apparent myocardial infarctions in the mice.51 It is widely believed, although not proved, that SR-BI deficiency is associated with reduced reverse cholesterol transport. It is unknown whether deficiency in macrophage SR-BI, hepatic SR-BI, or both contribute to the increased atherosclerosis. Role of cholesterol ester transfer protein in reverse cholesterol transport: Transfer of cholesteryl ester from

HDL to apo B– containing lipoproteins in exchange for triglycerides by means of cholesterol ester transfer protein is 1 potential pathway of reverse cholesterol transport.52 It may be a major route by which HDL cholesterol is returned to the liver in humans.53 Genetic homozygous cholesterol ester transfer protein deficiency, which is found primarily in Japan, is associated with markedly increased HDL cholesterol levels.54 However, the relation between cholesteryl ester transfer protein deficiency and atherosclerosis is still in question. One report suggested that homozygous cholesteryl ester transfer protein deficiency was protective against CAD,55 although another suggested it may be associated with increased atherosclerotic disease.56 Heterozygous cholesteryl ester transfer protein deficiency, which is associated with modestly increased HDL cholesterol levels, was shown in 1 large epidemiologic study to be associated with an increased risk of CAD for those individuals with HDL cholesterol levels in the range of 40 to 60 mg/dL.57 Some reports suggested that expression of cholesteryl ester transfer protein in mice resulted in increased atherosclerosis.58,59 On the other hand, another study showed that expression of cholesteryl ester transfer protein reduced atherosclerosis in mice with hypertriglyceridemia,60 suggesting that the effect of cholesteryl ester transfer protein expression on atherosclerosis may depend on other aspects of lipoprotein metabolism. Studies of cholesteryl ester transfer protein inhibition in rabbits (which have high levels of cholesteryl ester transfer protein) have demonstrated a significant reduction in atherosclerosis, both using a small molecule inhibitor61 as well as an anti-cholesteryl ester transfer protein immunization strategy.62 Measurement of reverse cholesterol transport in animals and humans: There have been various efforts to

quantify reverse cholesterol transport or components of the process in animals. Notably, none of these methods have involved measuring reverse cholesterol transport specifically from the macrophage, the most important cholesterol-accumulating cell in atherosclerosis, and most have been in rodents that lack cholesteryl ester transfer protein. Bakkeren et al63 injected 3 H-cholesteryl ester–labeled acetylated LDL into rats and showed that it initially labeled liver endothelial cells, then subsequently became incorporated into HDL and was eventually converted in part to bile acids and excreted into the bile.63 Stein et al64 injected 3 H-cholesterol–labeled cationized LDL into skeletal

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muscle in mice and monitored loss of radioactivity from this depot over time. Using this approach, they found no evidence of increased rate of loss of 3Hcholesterol in human apo A-I transgenic mice.64 Another approach is based on the concept that in steady state, the rate at which peripheral tissue acquires cholesterol through de novo synthesis and uptake of lipoproteins must equal the rate of loss of cholesterol through efflux.65,66 Therefore, this method provides a way to estimate the rate of the first step in reverse cholesterol transport cholesterol efflux from total peripheral tissues. The method involves the administration of 3H-water for endogenous labeling of cholesterol to measure peripheral (extrahepatic) cholesterol synthesis rates and injection of labeled LDL to estimate the rate of delivery of cholesterol to peripheral tissues. Using this approach, Osono et al65 reported that under conditions of a 2-fold difference in plasma apo A-I concentrations, there was no difference in the rate of “net centripetal cholesterol flux” from the peripheral tissue, and Jolly et al66 reported that apo A-I knockout mice were not different from wild-type mice with regard to peripheral cholesterol efflux. Another approach is based on the concept that an intervention that affects the rate of transport of cholesterol from the periphery to the liver will result in a change in excretion of fecal sterols (both neutral sterols and bile acids). This method was first reported in a human study in which 4 patients with heterozygous familial hypercholesterolemia were injected with a bolus of apo A-I/phosphatidylcholine (PC) liposomes, and fecal sterol excretion was monitored 9 days before and 9 days after the injection; the fecal excretion of neutral sterols and bile acids were increased by 39% and 30%, respectively.67 Alam et al68 applied both of these methods in the most comprehensive study to date of manipulation of different steps in the reverse cholesterol transport pathway on measures of reverse cholesterol transport. Most of the interventions they used did not increase reverse cholesterol transport as measured by either or both of these methods. The only intervention that increased net centripetal cholesterol flux was injection of apo A-I/PC complexes. Finally, fecal sterol excretion and biliary cholesterol and bile acid secretion rates in the steady-state condition were used to estimate rates of reverse cholesterol transport from the periphery in ABCA1 knockout mice. Groen et al69 concluded, surprisingly, that there were no differences in ABCA1 knockout mice compared with wild-type mice. Therefore, despite continued enthusiasm for the concept of reverse cholesterol transport as a major mechanism by which HDL and apo A-I protect against atherosclerosis, no direct proof of this concept yet exists. Furthermore, the studies cited above have created substantial doubt as to whether HDL and apo A-I actually promote the rate of reverse cholesterol transport.70 However, the methods used have estimated rates of reverse cholesterol transport from entire peripheral tissue and not from macrophages specifically. Furthermore, the studies that have been done to date have generally involved overexpression or suppres-

sion of single steps in the reverse cholesterol transport pathway, but it may require simultaneous alteration of ⬎1 step in order to influence the rate of reverse cholesterol transport. Therefore, more work is needed to explore the relation between HDL, reverse cholesterol transport, and atherosclerosis.

OTHER POTENTIAL ANTIATHEROGENIC EFFECTS OF HIGH-DENSITY LIPOPROTEIN AND APOLIPOPROTEIN A-I HDL has a variety of other properties that have been demonstrated in vitro that could, in theory, contribute to its antiatherogenic effects. HDL is widely considered to have antioxidant properties.71 HDL was shown to protect LDL from copper-mediated oxidation ex vivo.72,73 LDL from mice genetically susceptible to fatty streak lesion formation was highly susceptible to oxidation by artery wall cells and was rendered resistant to oxidation after incubation with apo A-I in vitro.74 Treatment of human endothelial cells with HDL or apo A-I rendered the cells unable to oxidize LDL.75 A single intravenous injection of a large dose of human HDL3 to rabbits with induced hypercholesterolemia led to a decrease in conjugated dienes and trienes by 20% to 30%.76 Injection of apo A-I into mice and humans rendered their LDL resistant to oxidation.74 HDL is also believed to have direct anti-inflammatory properties. Pretreatment of human umbilical vein endothelial cells with HDL reduced the cytokine-induced upregulation of adhesion molecules, such as intercellular adhesive molecule–(ICAM-1), vascular cell adhesion molecule–(VCAM-1), and E-selectin.77– 82 HDL inhibited tumor necrosis factor-␬–stimulated sphingosine kinase activity in endothelial cells, resulting in a decrease in sphingosine-1 phosphate production and necrosis factor-␬B signaling cascades.83 In vivo, reconstituted HDL containing human apo A-I reduced VCAM-1 expression after carotid injury in apo E–deficient mice.84 In a porcine model of vascular inflammation, plasma HDL cholesterol levels were elevated by bolus injection of reconstituted discoidal HDL, which inhibited basal and interleukin-1␣–induced E-selectin expression by porcine microvascular endothelial cells in vivo. However, not all studies have shown these effects. In human coronary artery endothelial cells preincubated with increasing amounts of total HDL and then activated with tumor necrosis factor–␣, flow cytometric analysis failed to detect any downregulation of VCAM-1 or E-selectin expression by HDL.85 In an apo A-I transgenic mouse model there were no differences in endothelial VCAM-1 expression in apo A-I transgenic compared with wild-type mice.86 HDL has been shown to have a variety of other properties. HDL may promote the release and bioactivity of prostacyclin, a vasoactive prostaglandin synthesized by vascular endothelial and smooth muscle cells, through several possible mechanisms, including increasing cyclooxygenase-2 expression87 and prostacyclin stabilization.88 HDL has been shown to result in endothelial nitric oxide synthase (eNOS) activation

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and nitric oxide production.89 HDL also has antiplatelet and anticoagulant effects.90,91 The in vivo significance of these properties has generally not been established.

HIGH-DENSITY LIPOPROTEIN HETEROGENEITY There are 2 broad issues related to heterogeneity of HDL: apolipoprotein composition and size/density. The major issue related to apolipoprotein composition is the content of apo A-I and apo A-II, segregating HDL particles into 2 broad categories: those that do not contain apo A-II and therefore contain apo A-I as the major apolipoprotein (these are often referred to as LpA-I), and those that contain both apo A-I and apo A-II (these are often referred to as LpA-I:A-II). In general, LpA-I:A-II particles represent approximately two thirds of the HDL particles, and LpA-I particles represent approximately one third. There has been considerable interest in the concept that these 2 types of HDL particles differ fundamentally in their metabolism and especially in their ability to inhibit atherosclerosis. Apo A-I, but not apo A-II, is produced in the intestine, so the intestine produces only the LpA-I particle.92 The liver makes both apo A-I and apo A-II, so it makes both LpA-I and LpA-I:A-II particles. Relatively little is known about the specific biosynthetic pathways of these particles. Studies of the metabolism of these 2 types of particles using radiolabeled particles in humans showed that the LpA-I particle turns over faster than the LpA-I:A-II particle.93 Some data suggest that the LpA-I:A-II particle is a less optimal substrate for hepatic lipase than the LpA-I particle.94,95 There has been substantial interest in the concept that these particles may differ in their ability to promote cholesterol efflux from cells. In an adipocyte cell line, LpA-I was more effective in promoting cholesterol efflux than LpA-I:A-II.96 Furthermore, overexpression of human apo A-II in mice offset the protective effect of human apo A-I overexpression.97 However, since the discovery of the ABCA1-mediated cholesterol efflux pathway, there are clear data that apo A-II itself can promote efflux,98 and therefore it is unclear whether these particles are different in their efflux-promoting capacity. Finally, although some human studies suggest that LpA-I might be a better predictor of protection from risk,99 other studies do not support that conclusion.100 In fact, in general, concentrations of both particles, like HDL cholesterol itself, seem to be inverse predictors of cardiovascular risk. HDL is clearly very heterogeneous with regard to its density and the very closely related issue of size. Like all lipoproteins, HDL consists of a hydrophobic lipid core of cholesteryl ester and some triglyceride, as well as phospholipids and apolipoproteins on the surface of the particle. Factors that affect the lipid composition of the HDL particle determine both the size and the density. Some of the fundamental separations of HDL were initially based on density and included 66i THE AMERICAN JOURNAL OF CARDIOLOGY姞

separation of HDL into HDL2 (the larger, more buoyant particle) and HDL3 (the smaller and denser particle). Less well characterized by ultracentrifugation but representing the apo A-I found at a density heavier than HDL3 is a smaller particle that is referred to as lipid-poor apo A-I, pre␤-HDL, or nascent HDL. The current paradigm is that nascent HDL (or lipid-poor apo A-I or pre␤-HDL) acts as an acceptor of excess cholesterol from cells by means of ABCA1mediated transport, whereupon the cholesterol is esterified by LCAT, forming cholesteryl ester and creating HDL3. Further acquisition of cholesterol results in an increase in particle size and a reduction in particle density, resulting in formation of HDL2. HDL also interacts with apo B– containing lipoproteins involving lipid transfer by means of cholesteryl ester transfer protein and phospholipid transfer protein, and this also contributes to the heterogeneity of the HDL particle. Finally, HDL undergoes a remodeling process involving delipidation in which larger particles are converted to smaller particles, which also has an important effect on the distribution of cholesterol among different-sized HDL particles. Patients with coronary disease generally have smaller HDL particles,101 leading to the theory that larger HDL particles may be associated with greater protection from CAD. However, the data regarding the predictive ability of large (ie, HDL2) versus small (ie, HDL3) HDL particles for CAD risk are conflicting.102 Remodeling of HDL is a critical process that regulates HDL size/density heterogeneity.103 Hepatic lipase is a factor that plays a role in the conversion of the larger HDL2 particles to the smaller HDL3 particles through hydrolysis of HDL triglyceride and possibly phospholipids.104 Overexpression of hepatic lipase in rabbits105 and mice106,107 results in decreased levels of HDL cholesterol and smaller HDL particles. Female hepatic lipase– deficient mice have very modestly elevated HDL cholesterol levels.108 Overexpression of hepatic lipase in mice reduced atherosclerosis106; conversely, hepatic lipase deficiency has been shown to reduce atherosclerosis in apo E– deficient mice.109 In humans, high plasma hepatic lipase activity is associated with reduced HDL cholesterol levels and smaller HDL particles.110 Both increased111 and decreased112 postheparin plasma hepatic lipase activity levels have been reported in humans with CAD compared with controls. Genetic deficiency in hepatic lipase is associated with modestly elevated HDL cholesterol levels and larger HDL particles113,114 but paradoxically may be associated with increased risk for atherosclerotic vascular disease,114 possibly because of higher levels of atherogenic lipoproteins. Lower levels of hepatic lipase activity and increased levels of HDL2 are found in association with a common single nucleotide polymorphism in the hepatic lipase promoter in many115–117 but not all118 studies. The regression of coronary atherosclerosis resulting from intensive lipid-lowering therapy has been shown to be associated with reduction in hepatic lipase activity.119 Endothelial lipase is a member of the same triglyceride lipase gene family as lipoprotein lipase and

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hepatic lipase. It was cloned independently by 2 different groups.120,121 Endothelial lipase has triglyceride lipase activity122 but relative to lipoprotein lipase and hepatic lipase has substantially greater phospholipase activity, putting it at the other end of the lipolytic spectrum from lipoprotein lipase.123 Plasma concentrations of HDL cholesterol and apo A-I were markedly reduced by overexpression of endothelial lipase in mice.120 Therefore, like hepatic lipase, endothelial lipase may play a role in converting larger HDL to smaller HDL particles.

MANAGEMENT OF PATIENTS WITH LOW HIGH-DENSITY LIPOPROTEIN CHOLESTEROL All adults should be screened with a full fasting lipid profile including triglycerides, total cholesterol, HDL cholesterol, and LDL cholesterol levels, according to the most recent National Cholesterol Education Program (NCEP) Adult Treatment Panel (ATP) III guidelines.124 These guidelines raise the threshold for low HDL cholesterol from 35 to 40 mg/dL.124 According to the guidelines, low HDL cholesterol (HDL cholesterol ⬍40 mg/dL in men and ⬍50 mg/dL in women) is 1 of the 5 criteria for diagnosis of the metabolic syndrome.124 The first step in the management of patients with low HDL cholesterol is counseling patients on therapeutic lifestyle changes. Regular aerobic exercise can help increase HDL cholesterol levels, but its effects are modest unless accompanied by weight loss.125 Drug therapy targeted toward patients with low HDL cholesterol should be considered in selected patients. Statins increase HDL cholesterol levels only slightly, but significantly reduce risk in patients with low HDL cholesterol.126 Fibric acid derivatives, or fibrates, work through activation of peroxisome proliferatoractivated receptor (PPAR)–␣.127 They decrease triglyceride levels, increase HDL cholesterol levels modestly, and significantly reduce cardiovascular risk in patients with low HDL cholesterol.128 Thiazolidinediones, activators of ␥ PPAR␥ and insulin sensitizers used for the treatment of type 2 diabetes, have HDL cholesterol–increasing effects.129,130 Nicotinic acid, or niacin, is the most effective HDL-increasing drug available. There are few clinical outcome studies, but the Coronary Drug Project showed a significant reduction in total mortality after 15 years of followup.131 Immediate-release niacin causes flushing, but an extended-release form of niacin is better tolerated.132 For patients on a statin with persistently low HDL cholesterol, the addition of a fibrate127 or niacin133,134 can help increase HDL cholesterol levels. The combination of a statin and a fibrate can be associated with myopathy, whereas a statin and niacin combination is sometimes associated with abnormal liver function tests, both of which must be carefully monitored in patients on combination therapy. New therapies based on targeting HDL metabolism and reverse cholesterol transport include inhibition of cholesteryl ester transfer protein, new PPAR agonists, upregulation of ABCA1 through nuclear receptor ago-

nists, and intravenous infusions of phospholipid liposomes.

SUMMARY HDL metabolism is complex, and HDL exists in a variety of different types of particles that undoubtedly have different functions. Much more research is needed to better understand the relation between HDL metabolism and subfractions, reverse cholesterol transport, and atherosclerosis, as well as the clinical role of HDL subfractionation in cardiovascular risk prediction and the potential roles of new therapeutic approaches to HDL metabolism and reverse cholesterol transport. 1. Gordon DJ, Rifkind BM. High-density lipoproteins—the clinical implications of recent studies. N Engl J Med 1989;321:1311–1316. 2. Genest J, Bard JM, Fruchart JC, Ordovas JM, Schaefer EJ. Familial hypoal-

phalipoproteinemia in premature coronary artery disease. Arterioscler Thromb Vasc Biol 1993;13:1728 –1737. 3. Wilson PWF, D’Agostino RB, Levy D, Belanger AM, Silbershatz H, Kannel WB. Prediction of coronary heart disease using risk factor categories. Circulation 1998;97:1837–1847. 4. Reaven GM, Chen YDI, Jeppesen J, Maheux P, Krauss RM. Insulin resistance and hyperinsulinemia in individuals with small, dense, low density lipoprotein particles. J Clin Invest 1993;92:141–146. 5. Rader DJ, Ikewaki K. Unraveling high density lipoprotein-apolipoprotein metabolism in human mutants and animal models. Curr Opin Lipidol 1996;7: 117–123. 6. Badimon JJ, Badimon L, Fuster V. Regression of atherosclerotic lesions by high density lipoprotein plasma fraction in the cholesterol-fed rabbit. J Clin Invest 1990;85:1234 –1243. 7. Miyazaki A, Sakuma S, Morikawa W, Takiue T, Miake F, Terano T, Sakai M, Hakamata H, Sakamoto Y, Naito M, et al. Intravenous injection of rabbit apolipoprotein A-I inhibits the progression of atherosclerosis in cholesterol-fed rabbits. Arterioscler Thromb Vasc Biol 1995;15:1882–1888. 8. Rubin E, Krauss R, Spangler E, Verstuyft J, Clift S. Inhibition of early atherogenesis in transgenic mice by human apolipoprotein AI. Nature 1991;353: 265–267. 9. Plump A, Scott C, Breslow J. Human apolipoprotein A-I gene expression increases high density lipoprotein and suppresses atherosclerosis in the apolipoprotein E– deficient mouse. Proc Natl Acad Sci U S A 1994;91:9607–9611. 10. Paszty C, Maeda N, Verstuyft J, Rubin EM. Apolipoprotein AI transgene corrects apolipoprotein E deficiency–induced atherosclerosis in mice. J Clin Invest 1994;94:899 –903. 11. Liu AC, Lawn RM, Verstuyft JG, Rubin EM. Human apolipoprotein A-I prevents atherosclerosis associated with apolipoprotein[a] in transgenic mice. J Lipid Res 1994;35:2263–2267. 12. Tangirala RK, Tsukamoto K, Chun SH, Usher D, Pure E, Rader DJ. Regression of atherosclerosis induced by liver-directed gene transfer of apolipoprotein A-I in mice. Circulation 1999;100:1816 –1822. 13. Benoit P, Emmanuel F, Caillaud JM, Bassinet L, Castro G, Gallix P, Fruchart JC, Branellec D, Denefle P, Duverger N. Somatic gene transfer of human apoA-I inhibits atherosclerosis progression in mouse models. Circulation 1999;99:105– 110. 14. Duverger N, Kruth H, Emmanuel F, Caillaud J, Viglietta C, Castro G, Tailleux A, Fievet C, Fruchart J, Houdebine LM, Denefle P. Inhibition of atherosclerosis development in cholesterol-fed human apolipoprotein A-I-transgenic rabbits. Circulation 1996;94:713–717. 15. Ishiguro H, Yoshida H, Major AS, Zhu T, Babaev VR, Linton MF, Fazio S. Retrovirus-mediated expression of apolipoprotein A-I in the macrophage protects against atherosclerosis in vivo. J Biol Chem 2001;276:36742–36748. 16. Li H, Reddick R, Maeda N. Lack of apoA-I is not associated with increased susceptibility to atherosclerosis in mice. Arterioscler Thromb Vasc Biol 1993;13: 1814 –1821. 17. Voyiaziakis E, Goldberg IJ, Plump AS, Rubin EM, Breslow JL, Huang LS. ApoA-I deficiency causes both hypertriglyceridemia and increased atherosclerosis in human apoB transgenic mice. J Lipid Res 1998;39:313–321. 18. Hughes SD, Verstuyft J, Rubin EM. HDL deficiency in genetically engineered mice requires elevated LDL to accelerate atherogenesis. Arterioscler Thromb Vasc Biol 1997;17:1725–1729. 19. Barter PJ, Rye K. Molecular mechanisms of reverse cholesterol transport. Curr Opin Lipidol 1996;7:82–87. 20. Hobbs HH, Rader DJ. ABC1: connecting yellow tonsils, neuropathy, and very low HDL. J Clin Invest 1999;104:1015–1017. 21. McNeish J, Aiello RJ, Guyot D, Turi T, Gabel C, Aldinger C, Hoppe KL,

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