High density lipoproteins (HDLs) are found in human plasma in the density range 1.063-1.21 g/mL and consist by weight of approximately 50% protein, 25% phospholipid, 20% cholesterol, and 5% triglyceride.4,19 The cholesterol component of HDL has been inversely associated with the incidence of coronary heart disease.20-23 Total HDL levels in human plasma are approximately 250 mg/dL, and HDL has classically been divided into two density classes: HDL2 (d 1.063-1.125 g/mL), and HDL3 (d 1.125-1.21 g/mL).24
In reverse cholesterol transport, the liver first produces HDL as a globular protein that binds phospholipids and free cholesterol, assuming a flat, discoid shape, which is still cholesterol poor. The newly synthesized lipid-poor apolipoprotein A-I interacts with ABCA1, removing excess cellular cholesterol and forming pre-β–HDL. As pre-β – HDL circulates through the blood stream interacting with the enzyme lecithin cholesterol acyltransferase (LCAT) which is responsible for the esterification of cholesterol particles in pre-β–HDL-C, converting it to mature α-HDL (otherwise designated as HDL2 and HDL3). HDL3 is relatively cholesterol-poor. As cholesterol is accumulated from the circulation, it becomes engorged with cholesterol and gets transformed to a spherical HDL2 particle.
HDL2 is the largest and most cardio-protective of all the HDL subclasses. The fate of the lipids in HDL2, which are primarily cholesterol esters, involve multiple pathways:
- Cholesteryl esters (CE) in HDL can be trafficked directly to steroidogenic tissues (adrenal glands, gonads) or adipocytes. Alternatively, the HDL can deliver its CE to the liver where it can be delipidated or endocytosed or to the small intestine where delipidation can also occur. The receptor that delipidates mature HDLs is the scavenger receptor B1 (SR-B1) and hepatic receptors capable of endocytosing HDLs are the holoparticle receptor (beta chain apoA-I synthase) or the LDL receptor which binds to apoE on HDLs. When HDLs traffic CE to the liver or intestine the process is called direct RCT.
- CE in HDL can also be transferred by means of CETP to other HDL species or to VLDL or LDL in exchange for triglycerides (TG). Through this pathway, HDL-collected cholesterol from peripheral cells or the arterial wall is eventually transported to the liver via LDL and this process is termed indirect RCT.35 Total RCT is the sum of indirect and direct RCT. Because this is an ongoing, dynamic process in which HDLs are constantly being lipidated and delipidated, and exchanging core lipids, a serum HDL-C has no relationship to the process. Indeed, the last phase of RCT is delipidation of large HDLs.
- TG-rich HDL particles may interact with the hepatic lipase enzyme, or hepatic endothelial cell lipase (HEL), which catabolizes some of the core lipids in HDL2, thereby converting the larger HDL2 to smaller HDL3, which are then returned to the circulation for relipidation.36 HDLs lipidate and delipidate multiple times over their six-day half-life.
- HDL may have additional functions, unrelated to RCT, that play a role in its potential anti-atherogenic effect; these are most likely related to its proteome. HDL carries many proteins that perform a multitude of functions. Some HDL-associated proteins may decrease LDL oxidation and potentially have other antioxidant functions. HDLs also appear to stimulate the synthesis of leukocyte adhesion molecules in endothelial cells by supplying them with arachidonic acid.37 These unique functions are discussed in more detail in the later section on HDL2.
The cardioprotective effects of HDL are due in large part to reverse cholesterol transport. Several clinical studies support the association between CHD risk and HDL subclass levels. Cheung and colleagues demonstrated that coronary artery disease was more closely associated with HDL particle size than with actual HDL levels.2 Ballantyne et al. reported that in myocardial infarction survivors HDL2 but not HDL3 was decreased compared with control subjects.3
As the cholesterol content in HDL particles increases, these particles increase in size, forming larger, lipid- enriched, heterogeneous HDL particles. Human HDL’s separate into two major subfractions, which have been designated HDL2 (less dense) and HDL3 (more dense).16,17 The HDL subclasses differ in both structure and antiatherogenic properties. Because HDL2 particles reabsorb cholesterol more efficiently than other HDL subclasses, they are most closely associated with reverse cholesterol transport. HDL2 is also associated with paraoxanase, an antioxidant that inhibits the oxidation of LDL particles.1
The oxidative modification of low-density lipoprotein (LDL) is a key event in the initiation and acceleration of atherosclerosis.5,6 As an antiatherogenic mediator, HDL, aside from playing an important role in the reverse cholesterol transport, protects LDL against oxidation.6,8 The antioxidant property of HDL has been attributed in part to the HDL-bound enzyme paraoxonase-1 (PON1).9-11
Oxidatively modified LDL is a potent ligand for scavenger receptors on macrophages and contributes to the generation of macrophage-derived foam cells, the hallmark of early atherosclerotic fatty streak lesions.14,15 Studies have demonstrated that HDL2 suppresses LDL oxidation by interrupting the continuous chain of lipid peroxide (LOOH) oxidation and prevents the formation of the secondary oxidation products, such as malondialdehyde and 4-hydroynonenal, thereby suppressing the oxidation of apolipoprotein B of LDL particles.
It is known that the presence of anti-oxidative vitamins, such as ubiquinol, and anti-oxidative enzymes, paraoxinase,18-20 platelet-activating factor acetylhydrolase,21,22 and glutathione peroxidase,23 in HDL, suppresses the formation of lipid peroxides. It is possible that these substances or some other unknown substance interrupt lipid oxidation by breaking the chain oxidation reaction after LOOH formation, such that HDL2 can retard the further oxidation of partially oxidized LDL in a concentration-dependent manner.
Navab et al. reported that in the presence of systemic inflammation, antioxidant enzymes can be inactivated and HDL can accumulate oxidized lipid and proteins that make it pro-inflammatory.24 After native HDL2 inhibits further oxidative modification of partially oxidized LDL, native HDL2 becomes oxidized HDL2 and there is a possibility that oxidized HDL2 has a role in providing cholesterol to the liver and lymphocytes via LDL receptors.25,26
HDL2 particle concentration may be increased by exercise, fish oil, or alcohol consumption in moderation. Niacin, fibric acids, and combination therapy (statin + niacin) have been demonstrated to increase large HDL particle concentration.
In a pooled analysis of four trials of statins, individuals with a ≥ 7.5% increase in serum HDL-C levels showed statistically significant regression of coronary atherosclerosis, independent of the serum LDL-C level.26 In a post- hoc analysis in the “Treating to New Targets” study27, the serum HDL-C levels were inversely related to the risk of cardiovascular events during statin treatment, even among patients with serum LDL-C levels < 70 mg/dL. Thus, changes in the serum HDL-C level appear to be a predictor of atherosclerotic cardiovascular risk, independent of the serum LDL-C level.
Aerobic exercise has been reported to elevate the serum HDL-C level by 5-10%, with the increase related to the frequency and intensity of exercise.28 The mechanism by which exercise increases the serum HDL-C level is not yet fully understood, but the effect has been attributed to an elevation in the levels of lipoprotein lipase.29 In general, the effect of statins in raising the serum HDL-C level is known to be modest (5-15%).30 All of the above- described research observations indicate that reasonable elevations of the serum HDL-C level may be obtained by the addition of aggressive lifestyle interventions to statin therapy. Despite the dramatic reductions in the cardiovascular risk with LDL-C lowering therapy, the residual cardiovascular risk remains significant.30
Therefore, intensive lifestyle interventions to raise the serum HDL-C level may serve as an additional strategy for addressing the residual cardiovascular risk in CAD patients with already elevated serum HDL-C and lowered serum LDL-C levels achieved with statin therapy. Evidently, the addition of intensive lifestyle modification to statin therapy for suppressing coronary plaque development not only has a synergistic effect in improving lipid metabolism, but also other antiarteriosclerotic effects.31
- Graner M, James RW, Kahri J. Association of Paraoxonase-1 Activity and Concentration With Angiographic Severity and Extent of Coronary Artery Disease. J Am Coll Cardiol 2006;47:2429–35.
- Cheung MC, Brown BG, Wolf AC, et al. Altered particle size distribution of apolipoprotein A-I-containing lipoproteins in subjects with coronary artery disease. J Lipid Res 1991;32:383-394.
- Ballantyne FC, Clark RS, Simpson HS, et al. High density and low-density lipoprotein subfractions in survivors of myocardial infarction and in control subjects. Metabolism 1982;31(5):433-437.
- Berglund L, Oliver EH, Fontanez N et al. HDL-subpopulation patterns in response to reductions in dietary total and saturated fat intakes in healthy subjects. Am J Clin Nutr 1999;70:992-1000.
- Steinberg D, Parthasarathy S, Carew T, et al. Beyond cholesterol. Modifications of low-density lipoprotein that increases atherogenicity. N Engl J Med 1998;320:933–7.
- Lusis AL. Atherosclerosis. Nature 2000;407:233– 41. 5.
- Mackness MI, Abbott CA, Arrol S, et al. The role of high density lipoprotein and lipid soluble antioxidant vitamins in inhibiting low-density lipoprotein oxidation. Biochem J 1993;294:829–34.
- Parthasarathy S, Barnett J, Fong LG. High-density lipoprotein inhibits the oxidative modification of low-density lipoprotein. Biochim Biophys Acta 1990;1044:275–83.
- Mackness MI, Arrol S, Abbott CA, et al. Protection of low-density lipoprotein against oxidative modification by high-density lipoprotein associated paraoxonase. Atherosclerosis 1993;104:129 –35.
- Watson AD, Berliner JA, Hama SY, et al. Protective effect of highdensity lipoprotein associated paraoxonase: inhibition of the biological activity of minimally oxidized low-density lipoprotein. J Clin Invest 1995;96:2882–91.
- Aviram M, Rosenblat M, Bisgaier CL, et al. Paraoxonase inhibits high density lipoprotein (HDL) oxidation and preserves its functions: a possible peroxidative role for paraoxonase. J Clin Invest 1998;101:1581–90.
- Miller GJ, Miller NE. Plasma high-density lipoprotein concentration and the development of ischaemic heart disease. Lancet 1975;1:16–9.
- Gordon DJ, Rifkind BM. High-density lipoprotein—the clinical implications or recent studies. N Engl J Med 1989;321:1311–6.
- Steinberg D, Parthasarathy S, Carew TE, Khoo JC, Witztum JL. Beyond cholesterol. Modifications of low-density lipoprotein that increase its atherogenicity. N Engl J Med 1989;320:915-924.
- Schwartz CJ, Valente AJ, Sprague EA, et al. The pathogenesis of atherosclerosis: an overview. Clin Cardiol 1991;14:11-16.
- Eisdenberg S. High density lipoprotein metabolism. J Lipid Res 1984;25:1017-1058.
- Sakuma N, Lin C, Matsumoto Y, Ikeuchi R, et al. Changes of HDL subfraction concentration and particle size by intralipid in vivo. Atherosclerosis 1988;74:91-98.
- Mackness MI, Arrol S, Abbott C, Durrington PN. Protection of low-density lipoprotein against oxidative modification by high-density lipoprotein associated paraxonase. Atherosclerosis 1993;104:129-135.
- Durrington PN, Mackness B, Mackness MI. Paroxonase and atherosclerosis. Atheroscler Thromb Vasc Biol 2001; 21:473-480.
- Kuremoto K, Watanabe Y, Ohmura H, et al. R/R genotype of human paraoxonase (PON1) is more protective against lipoprotein oxidation and coronary artery disease in Japanese subjects. J Atheroscler Thromb 2003;10:85-92.
- Stafforni DM, Zimmerman GA, McIntyre TM, Prescott SM The plasma PAF acetylhydrolase prevents oxidative modification of low-density lipoprotein. J Lipid Mediat Cell Signal 1994;10:53-56.
- Gardner AA, Reichert EC, Topham MK, Stafforni DM. Identification of a domain that mediates association of platelet-activating factor acetylhydrolase with high density lipoprotein. J Biol Chem 2008;283:17099-17106.\
- Chen N, Liu Y, Greiner CD, Holtzman JL. Physiologic concentrations of homocysteine inhibit the human plasma GSH peroxidase that reduces organic hydroperoxides. J Lab Clin Med 2000;136:58-65.
- Navab M, Anantharamaiah GM, Reddy ST, et al. Mechanisms of disease: pro-atherogenic HDL-an evolving field. Nat Clin Pract Endocrin Metab 2006;2:504-511.
- Sakuma N, Saeki T, Ito T, et al. HDL2 can inhibit further oxidative modification of partially oxidized LDL. J Athero Thromb 2010;17:229-34.
- Nicholls SJ, Tuzcu EM, Sipahi I, et al. Statins, high-density lipoprotein cholesterol, and regression of coronary atherosclerosis. JAMA 2007;297:499–508.
- Barter P, Gotto AM, LaRosa JC, et al. For the Treating to New Targets Investigators. HDL cholesterol, very low levels of LDL cholesterol, and cardiovascular events. N Engl J Med 2007;357:1301-10.
- King AC, Haskell WL, Young DR, et al. Longterm effects of varying intensities and formats of physical activity on participation rates, fitness, and lipoproteins in men and women aged 50 to 65 years. Circulation 1995;91:2596–2604.
- Thompson PD. What do muscles have to do with lipoproteins? Circulation 1990;81:1428–30.
- Baigent C, Keech A, Kearney PM, et al. Efficacy and safety of cholesterol-lowering treatment: Prospective meta-analysis of data from 90,056 participants in 14 randomised trials of statins. Lancet 2005;366:1267-78.
- Roberts CK, Chen AK, Barnard RJ. Effect of a short-term diet and exercise intervention in youth on atherosclerotic risk factors. Atherosclerosis 2007;19:98-106.
- Okada K, Maeda N, Tatsukawa M, et al. The influence of lifestyle modification on carotid artery intima-media thickness in a suburban Japanese population. Atherosclerosis 2004;173:329-37.
- Okazaki S, Yokoyama T, Miyauchi K, et al. Early statin treatment in patients with acute coronary syndrome: Demonstration of the beneficial effect on atherosclerotic lesions by serial volumetric intravascular ultrasound analysis during half a year after coronary event: The ESTABLISH Study. Circulation 2004; 110:1061-8.
- Tani S, Nagao K, Anazawa T, et al. Association of circulating leukocyte count with coronary atherosclerosis regression after pravastatin treatment. Atherosclerosis 2008;198:360–5.
- Inazu A, Brown ML, Hesler CD, et al. Increased high density lipoprotein levels caused by common cholesteryl-ester transfer protein gene mutation. N Engl J Med 1990; 323:1234.
- Navab M, Hama SY, Hough GP, et al. High density lipoprotein associated enzymes: Their role in vascular biology. Curr Opin Lipidol 1998;9: 449-456.
- Pomerantz KB, Fleisher LN, Tall AR, Cannon PJ. Enrichment of endothelial cell arachidonate by lipid transfer from high density lipoproteins: Relationship to prostaglandin I2 synthesis. Lipid Res 1985;26:1267.