High-Density Lipoprotein Subclass 2 (HDL2-C)

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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:

  1. 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.
  2. 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.
  3. 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.
  4. 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.

Clinical Interpretation

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

Treatment Considerations

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


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