Puneet Varma

High density lipoprotein

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High-density lipoproteins (HDL) are one of the five major groups of lipoproteins. Lipoproteins are complex particles composed of multiple proteins which transport all fat molecules (lipids) around the body within the water outside cells. They are typically composed of 80-100 proteins per particle (organized by one, two or three ApoA; more as the particles enlarge picking up and carrying more fat molecules) and transporting up to hundreds of fat molecules per particle. Unlike the larger lipoprotein particles which deliver fat molecules to cells, HDL particles remove fat molecules from cells which need to export fat molecules. The fats carried include cholesterol, phospholipids, and triglycerides; amounts of each are quite variable.

Contents

Increasing concentrations of HDL particles are strongly associated with decreasing accumulation of atherosclerosis within the walls of arteries. This is important because atherosclerosis eventually results in sudden plaque ruptures, cardiovascular disease, stroke and other vascular diseases. HDL particles are sometimes referred to as "good cholesterol" because they can transport fat molecules out of artery walls, reduce macrophage accumulation, and thus help prevent or even regress atherosclerosis. However, studies have shown that HDL-lacking mice still have the ability to transport cholesterol to bile, suggesting that there are alternative mechanisms for cholesterol removal.

Testing

Because of the high cost of directly measuring HDL and LDL protein particles, blood tests are commonly performed for the surrogate value, HDL-C, i.e. the cholesterol associated with ApoA-1/HDL particles. In healthy individuals, about 30% of blood cholesterol, along with other fats, is carried by HDL. This is often contrasted with the amount of cholesterol estimated to be carried within low-density lipoprotein particles, LDL, and called LDL-C. HDL particles remove fats and cholesterol from cells, including within artery wall atheroma, and transport it back to the liver for excretion or re-utilization; thus the cholesterol carried within HDL particles (HDL-C) is sometimes called "good cholesterol" (despite being the same as cholesterol in LDL particles). Those with higher levels of HDL-C tend to have fewer problems with cardiovascular diseases, while those with low HDL-C cholesterol levels (especially less than 40 mg/dL or about 1 mmol/L) have increased rates for heart disease. Higher native HDL levels are correlated with better cardiovascular health; however, it does not appear that further increasing one's HDL improves cardiovascular outcomes.

The remainder of the serum cholesterol after subtracting the HDL is the non-HDL cholesterol. The concentration of these other components, which may cause atheroma, is known as the non-HDL-C. This is now preferred to LDL-C as a secondary marker as it has been shown to be a better predictor and it is more easily calculated.

Structure and function

HDL is the smallest of the lipoprotein particles. It is the densest because it contains the highest proportion of protein to lipids. Its most abundant apolipoproteins are apo A-I and apo A-II. A rare genetic variant, ApoA-1 Milano, has been documented to be far more effective in both protecting against and regressing arterial disease; atherosclerosis. The liver synthesizes these lipoproteins as complexes of apolipoproteins and phospholipid, which resemble cholesterol-free flattened spherical lipoprotein particles; the complexes are capable of picking up cholesterol, carried internally, from cells by interaction with the ATP-binding cassette transporter A1 (ABCA1). A plasma enzyme called lecithin-cholesterol acyltransferase (LCAT) converts the free cholesterol into cholesteryl ester (a more hydrophobic form of cholesterol), which is then sequestered into the core of the lipoprotein particle, eventually causing the newly synthesized HDL to assume a spherical shape. HDL particles increase in size as they circulate through the bloodstream and incorporate more cholesterol and phospholipid molecules from cells and other lipoproteins, for example by the interaction with the ABCG1 transporter and the phospholipid transport protein (PLTP).

HDL transports cholesterol mostly to the liver or steroidogenic organs such as adrenals, ovary, and testes by both direct and indirect pathways. HDL is removed by HDL receptors such as scavenger receptor BI (SR-BI), which mediate the selective uptake of cholesterol from HDL. In humans, probably the most relevant pathway is the indirect one, which is mediated by cholesteryl ester transfer protein (CETP). This protein exchanges triglycerides of VLDL against cholesteryl esters of HDL. As the result, VLDLs are processed to LDL, which are removed from the circulation by the LDL receptor pathway. The triglycerides are not stable in HDL, but are degraded by hepatic lipase so that, finally, small HDL particles are left, which restart the uptake of cholesterol from cells.

The cholesterol delivered to the liver is excreted into the bile and, hence, intestine either directly or indirectly after conversion into bile acids. Delivery of HDL cholesterol to adrenals, ovaries, and testes is important for the synthesis of steroid hormones.

Several steps in the metabolism of HDL can participate in the transport of cholesterol from lipid-laden macrophages of atherosclerotic arteries, termed foam cells, to the liver for secretion into the bile. This pathway has been termed reverse cholesterol transport and is considered as the classical protective function of HDL toward atherosclerosis.

However, HDL carries many lipid and protein species, several of which have very low concentrations but are biologically very active. For example, HDL and its protein and lipid constituents help to inhibit oxidation, inflammation, activation of the endothelium, coagulation, and platelet aggregation. All these properties may contribute to the ability of HDL to protect from atherosclerosis, and it is not yet known which are the most important. In addition, a small subfraction of HDL lends protection against the protozoan parasite Trypanosoma brucei brucei. This HDL subfraction, termed trypanosome lytic factor (TLF), contains specialized proteins that, while very active, are unique to the TLF molecule.

In the stress response, serum amyloid A, which is one of the acute-phase proteins and an apolipoprotein, is under the stimulation of cytokines (interleukin 1, interleukin 6), and cortisol produced in the adrenal cortex and carried to the damaged tissue incorporated into HDL particles. At the inflammation site, it attracts and activates leukocytes. In chronic inflammations, its deposition in the tissues manifests itself as amyloidosis.

It has been postulated that the concentration of large HDL particles more accurately reflects protective action, as opposed to the concentration of total HDL particles. This ratio of large HDL to total HDL particles varies widely and is measured only by more sophisticated lipoprotein assays using either electrophoresis (the original method developed in the 1970s) or newer NMR spectroscopy methods (See also nuclear magnetic resonance and spectroscopy), developed in the 1990s.

Subfractions

Five subfractions of HDL have been identified. From largest (and most effective in cholesterol removal) to smallest (and least effective), the types are 2a, 2b, 3a, 3b, and 3c.

Major lipids in the human body

The lipids are a heterogeneous group of compounds which are relatively insoluble in water and soluble in non-polar solvents. Triglycerides (TGs), cholesterol, and phospholipids are the major lipids in the body. They are transported as complexes of lipid and proteins known as lipoproteins.

TGs (triglycerides): TGs are formed by combining glycerol with three molecules of fatty acid. TGs, as major components of VLDL and chylomicrons, play an important role in metabolism. When the body requires fatty acids as an energy source, the hormone glucagon signals the breakdown of the TGs by lipase to release free fatty acids (FFA). TGs are water-insoluble, non-polar neutral fats. These are not the structural components of biological membranes.TGs synthesis and storage mostly occurs in liver and adipose tissue. FFA and glycerol must be activated prior to the synthesis of TGs into Acyl-CoA and glycerol-3-phosphate respectively.

Cholesterol: The name cholesterol originates from the Greek chole- (bile) and stereos (solid), and the chemical suffix-ol for an alcohol. It is an essential structural component of cell membrane, where it is required to establish proper membrane permeability and fluidity. In addition, cholesterol is an important component for the manufacture of bile acids, steroid hormones, and vitamin D. Although cholesterol is an important and necessary molecule, a high level of serum cholesterol is an indicator for diseases such as heart disease. About 20-25% of total daily cholesterol production occurs in the liver.

Phospholipids: Phospholipids are TGs that are covalently bonded to a phosphate group by an ester linkage. Phospholipids perform important functions including regulating membrane permeability and in maintaining electron transport chain in mitochondria. They participate in the reverse cholesterol transport and thus help in the removal of cholesterol from the body. They are involved in signal transmission across membranes and they act as detergents and help in solubilization of cholesterol.

Lipoprotein: These consist of a central core of a hydrophobic lipid (including TGs and cholesteryl esters) encased in a hydrophilic coat of polar phospholipid, free cholesterol and apolipoprotein. There are six main classes of lipoprotein, differing in the relative proportion of the core lipids and in the type of apoprotein.

  • Chylomicrons
  • VLDL-C particiles
  • IDL-C particles
  • LDL-C particles
  • HDL-C particle
  • Lipoprotein (a) [LP(a)]
  • Epimodility

    Men tend to have noticeably lower HDL levels, with smaller size and lower cholesterol content, than women. Men also have an increased incidence of atherosclerotic heart disease. Alcohol consumption tends to raise HDL levels, and moderate alcohol consumption is associated with lower cardiovascular and all-cause mortality. Recent studies confirm the fact that HDL has a buffering role in balancing the effects of the hypercoagulable state in type 2 diabetics and decreases the high risk of cardiovascular complications in these patients. Also, the results obtained in this study revealed that there was a significant negative correlation between HDL and activated partial thromboplastin time (APTT).

    Epidemiological studies have shown that high concentrations of HDL (over 60 mg/dL) have protective value against cardiovascular diseases such as ischemic stroke and myocardial infarction. Low concentrations of HDL (below 40 mg/dL for men, below 50 mg/dL for women) increase the risk for atherosclerotic diseases.

    Data from the landmark Framingham Heart Study showed that, for a given level of LDL, the risk of heart disease increases 10-fold as the HDL varies from high to low. On the converse, however, for a fixed level of HDL, the risk increases 3-fold as LDL varies from low to high.

    Even people with very low LDL levels are exposed to increased risk if their HDL levels are not high enough.

    Estimating HDL via associated cholesterol

    Clinical laboratories formerly measured HDL cholesterol by separating other lipoprotein fractions using either ultracentrifugation or chemical precipitation with divalent ions such as Mg2+, then coupling the products of a cholesterol oxidase reaction to an indicator reaction. The reference method still uses a combination of these techniques. Most laboratories now use automated homogeneous analytical methods in which lipoproteins containing apo B are blocked using antibodies to apo B, then a colorimetric enzyme reaction measures cholesterol in the non-blocked HDL particles. HPLC can also be used. Subfractions (HDL-2C, HDL-3C) can be measured, but clinical significance of these subfractions has not been determined. The measurement of apo-A reactive capacity can be used to measure HDL cholesterol but is thought to be less accurate.

    Recommended ranges

    The American Heart Association, NIH and NCEP provides a set of guidelines for fasting HDL levels and risk for heart disease.

    High LDL with low HDL level is an additional risk factor for cardiovascular disease.

    Measuring HDL concentration and sizes

    As technology has reduced costs and clinical trials have continued to demonstrate the importance of HDL, methods for directly measuring HDL concentrations and size (which indicates function) at lower costs have become more widely available and increasingly regarded as important for assessing individual risk for progressive arterial disease and treatment methods.

    Electrophoresis measurements

    Since the HDL particles have a net negative charge and vary by density & size, ultracentrifugation combined with electrophoresis have been utilized since before 1950 to enumerate the concentration of HDL particles and sort them by size with a specific volume of blood plasma. Larger HDL particles are carrying more cholesterol.

    NMR measurements

    Concentration and sizes of lipoprotein particles can be estimated using nuclear magnetic resonance fingerprinting.

    Optimal total and large HDL concentrations

    The HDL particle concentrations are typically categorized by event rate percentiles based on the people participating and being tracked in the MESA trial, a medical research study sponsored by the United States National Heart, Lung, and Blood Institute.

    The lowest incidence of atherosclerotic events over time occurs within those with both the highest concentrations of total HDL particles (the top quarter, >75%) and the highest concentrations of large HDL particles. Multiple additional measures, including LDL particle concentrations, small LDL particle concentrations, VLDL concentrations, estimations of insulin resistance and standard cholesterol lipid measurements (for comparison of the plasma data with the estimation methods discussed above) are routinely provided in clinical testing.

    Memory

    Fasting serum lipids have been associated with short term verbal memory. In a large sample of middle aged adults, low HDL cholesterol was associated with poor memory and decreasing levels over a five-year follow-up period were associated with decline in memory.

    Increasing HDL levels

    While higher HDL levels are correlated with cardiovascular health, no medication used to increase HDL has been proven to improve health. In other words, while high HDL levels might correlate with better cardiovascular health, specifically increasing one's HDL might not increase cardiovascular health. The remaining possibilities are that either good cardiovascular health causes high HDL levels, there is some third factor which causes both, or this is a coincidence with no causal link.

    Diet and exercise

    Certain changes in diet and exercise may have a positive impact on raising HDL levels:

  • Decreased intake of simple carbohydrates.
  • Aerobic exercise
  • Weight loss
  • Magnesium supplements raise HDL-C.
  • Addition of soluble fiber to diet
  • Consumption of omega-3 fatty acids such as fish oil or flax oil
  • Consumption of pistachio nuts.
  • Increased intake of cis-unsaturated fats
  • Consumption of medium-chain triglycerides (MCTs) such as caproic acid, caprylic acid, capric acid, and lauric acid.
  • Removal of trans fatty acids from the diet
  • Most saturated fats increase HDL cholesterol to varying degrees but also raise total and LDL cholesterol. A high-fat, adequate-protein, low-carbohydrate ketogenic diet may have similar response to taking niacin (vitamin B3) as described below (lowered LDL and increased HDL) through beta-hydroxybutyrate coupling the Niacin receptor 1.

    Recreational drugs

    HDL levels can be increased by smoking cessation, or mild to moderate alcohol intake.

    Cannabis in unadjusted analyses, past and current cannabis use were associated with lower levels of fasting insulin, glucose, HOMA-IR, BMI, and hemoglobin A1c but either current or past cannabis use was not associated with higher HDL-C levels. A study performed in 4635 patients demonstrated no effect on the HDL-C levels (P=0.78) [the mean (standard error) HDL-C values in control subjects (never used), past users and current users were 53.4 (0.4), 53.9 (0.6) and 53.9 (0.7) mg/dL, respectively].

    Pharmaceutical drugs and niacin

    Pharmacological therapy to increase the level of HDL cholesterol includes use of fibrates and niacin. Fibrates have not been proven to have an effect on overall deaths from all causes, despite their effects on lipids.

    Niacin (vitamin B3) increases HDL by selectively inhibiting hepatic diacylglycerol acyltransferase 2, reducing triglyceride synthesis and VLDL secretion through a receptor HM74 otherwise known as niacin receptor 2 and HM74A / GPR109A, niacin receptor 1.

    Pharmacologic (1- to 3-gram/day) niacin doses increase HDL levels by 10–30%, making it the most powerful agent to increase HDL-cholesterol. A randomized clinical trial demonstrated that treatment with niacin can significantly reduce atherosclerosis progression and cardiovascular events. However, niacin products sold as "no-flush", i.e. not having side-effects such as "niacin flush", do not contain free nicotinic acid and are therefore ineffective at raising HDL, while products sold as "sustained-release" may contain free nicotinic acid, but "some brands are hepatotoxic"; therefore the recommended form of niacin for raising HDL is the cheapest, immediate-release preparation. Both fibrates and niacin increase artery toxic homocysteine, an effect that can be counteracted by also consuming a multivitamin with relatively high amounts of the B-vitamins, however multiple European trials of the most popular B-vitamin cocktails, trial showing 30% average reduction in homocysteine, while not showing problems have also not shown any benefit in reducing cardiovascular event rates. A 2011 niacin study was halted early because patients adding niacin to their statin treatment showed no increase in heart health, but did experience an increase in the risk of stroke.

    In contrast, while the use of statins is effective against high levels of LDL cholesterol, most have little or no effect in raising HDL cholesterol. However, several statins – rosuvastatin and pitavastatin – have been demonstrated to significantly raise HDL levels. As statins are associated with side effects like myopathy which causes sore muscles, patients who experience these side effects may need to be given a lower dose of statin to control cholesterol.

    Lovaza has been shown to increase HDL-C. However, the best evidence to date suggests it has no benefit for primary or secondary prevention of cardiovascular disease.

    References

    High-density lipoprotein Wikipedia


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