Low-density lipoprotein

Low-density lipoprotein (LDL) is one of the five major groups of lipoproteins, which, in order of molecular size, largest to smallest, are chylomicrons, very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), LDL, and high-density lipoprotein (HDL). Lipoprotein molecules enable the transportation of lipids (fats), such as cholesterol, phospholipids, and triglycerides, within the water around cells (extracellular fluid), including the bloodstream. Studies have shown that higher levels of type-B LDL particles (as opposed to type-A LDL particles) are associated with health problems, including cardiovascular disease.

Although the nickname is overly simplistic and thus misleading, LDL molecules are often informally called bad cholesterol because they can transport their content of many fat molecules into artery walls, attract macrophages, and thus drive atherosclerosis. In contrast, HDL molecules are frequently referred to as good cholesterol or healthy cholesterol, because they can remove fat molecules from macrophages in the wall of arteries.

Testing
Blood tests typically report LDL-C, the amount of cholesterol contained in LDL. In clinical context, mathematically calculated estimates of LDL-C are commonly used to estimate how much low density lipoproteins are driving progression of atherosclerosis.

Direct LDL measurements are also available and better reveal individual issues but are less often promoted or done due to slightly higher costs and being available from only a couple of laboratories in the United States. In 2008, the ADA and ACC recognized direct LDL particle measurement by NMR as superior for assessing individual risk of cardiovascular events.

Structure
Each native LDL particle enables emulsification, i.e surrounding/packaging all fatty acids being carried, enabling these fats to move around the body within the water outside cells. Each particle contains a single apolipoprotein B-100 molecule (Apo B-100, a protein that has 4536 amino acid residues and a mass of 514 kDa), along with 80 to 100 additional ancillary proteins. Each LDL has a highly hydrophobic core consisting of polyunsaturated fatty acid known as linoleate and hundreds to thousands (about 1500 commonly cited as an average) esterified and non-esterified cholesterol molecules. This core also carries varying numbers of triglycerides and other fats and is surrounded by a shell of phospholipids and unesterified cholesterol, as well as the single copy of Apo B-100. LDL particles are approximately 22 nm (0.00000087 in.) in diameter and have a mass of about 3 million daltons. Since LDL particles contain a variable and changing number of fatty acid molecules, there is a distribution of LDL particle mass and size. Determining the structure of LDL has been a tough task because of its heterogeneous structure. The structure of LDL at human body temperature in native condition, with a resolution of about 16 Angstroms using cryo-electron microscopy, has been recently described.

LDL subtype patterns
LDL particles vary in size and density, and studies have shown that a pattern that has more small dense LDL particles, called Pattern B, equates to a higher risk factor for coronary heart disease (CHD) than does a pattern with more of the larger and less-dense LDL particles (Pattern A). This is thought to be because the smaller particles are more easily able to penetrate the endothelium. Pattern I, for intermediate, indicates that most LDL particles are very close in size to the normal gaps in the endothelium (26 nm). According to one study, sizes 19.0–20.5 nm were designated as pattern B and LDL sizes 20.6–22 nm were designated as pattern A.

Some in the medical community have suggested the correspondence between Pattern B and CHD is stronger than the correspondence between the LDL number measured in the standard lipid profile test. Tests to measure these LDL subtype patterns have been more expensive and not widely available, so the common lipid profile test is used more often.

There has also been noted a correspondence between higher triglyceride levels and higher levels of smaller, denser LDL particles and alternately lower triglyceride levels and higher levels of the larger, less dense LDL.

With continued research, decreasing cost, greater availability and wider acceptance of other lipoprotein subclass analysis assay methods, including NMR spectroscopy, research studies have continued to show a stronger correlation between human clinically obvious cardiovascular events and quantitatively measured particle concentrations.

Transport into the cell
When a cell requires additional cholesterol (beyond its current internal HMGCoA production pathway), it synthesizes the necessary LDL receptors, and inserts them into the plasma membrane. The LDL receptors diffuse freely until they associate with clathrin-coated pits. LDL particles in the bloodstream bind to these extracellular LDL receptors. The clathrin-coated pits then form vesicles that are endocytosed into the cell.

After the clathrin coat is shed, the vesicles deliver the LDL and their receptors to early endosomes, onto late endosomes to lysosomes. Here the cholesterol esters in the LDL are hydrolysed. The LDL receptors are recycled back to the plasma membrane.

Medical relevance - Atherosclerosis
Because LDL particles can also transport cholesterol into the artery wall, retained there by arterial proteoglycans and attract macrophages that engulf the LDL particles and start the formation of plaques, increased levels are associated with atherosclerosis. Over time vulnerable plaques rupture, activate blood clotting and produce arterial stenosis, which if severe enough results in heart attack, stroke, and peripheral vascular disease symptoms and major debilitating events.

Increasing evidence has revealed that the concentration and size of the LDL particles more powerfully relates to the degree of atherosclerosis progression than the concentration of cholesterol contained within all the LDL particles. The healthiest pattern A, though relatively rare, is to have small numbers of large LDL particles and no small particles. Having small LDL particles, though common, is an unhealthy pattern B; high concentrations of small LDL particles (even though potentially carrying the same total cholesterol content as a low concentration of large particles) correlates with much faster growth of atheroma, progression of atherosclerosis and earlier and more severe cardiovascular disease events and death. This video, examining autopsy specimens from an actual heart attack resulting in sudden death, shows the seqence. These videos, and, illustrate the sequence of events and why, though the underlying process develops over decades, the symptoms are often of sudden onset.

LDL particles are formed as VLDL lipoproteins lose triglyceride through the action of lipoprotein lipase (LPL) and they become smaller and denser (i.e. fewer fat molecules with same protein transport shell), containing a higher proportion of cholesterol esters.

A hereditary form of high LDL is familial hypercholesterolemia (FH). Increased LDL is termed hyperlipoproteinemia type II (after the dated Fredrickson classification).

LDL particles pose a risk for cardiovascular disease when they invade the endothelium and become oxidized, since the oxidized forms are more easily retained by the proteoglycans. A complex set of biochemical reactions regulates the oxidation of LDL particles, chiefly stimulated by presence of necrotic cell debries and free radicals in the endothelium.

Role in the innate immune system
LDL lipoproteins interfere with the quorum sensing system that upregulates genes required for invasive Staphylococcus aureus infection. The mechanism of antagonism entails binding Apolipoprotein B, to a S. aureus autoinducer pheromone, preventing signaling through its receptor. Mice deficient in apolipoprotein B are more susceptible to invasive bacterial infection.

Lowering LDL-C
The mevalonate pathway serves as the basis for the biosynthesis of many molecules, including cholesterol. The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA reductase) is an essential component and performs the first of 37 steps within the cholesterol production pathway, and present in every animal cell.

Keep in mind that LCL-C is not a measurement of actual LDL particles; LDL-C is only an estimate (not measured from the individual's blood sample) of how much cholesterol is being transported by all LDL particles; either a smaller concentration of large particles or a high concentration of small particles. Also keep in mind that LDL particles carry many fat molecules (typically 3,000 to 6,000 fat molecules per LDL particle); cholesterol, triglycerides, phospholipids and others. Thus even if the hundreds to thousands of cholesterol molecules within an average LDL particle were measured, this does not reflect the other fat molecules or even the number of LDL particles.

Pharmaceutical

 * Statins reduce high levels of LDL particles by inhibiting the enzyme HMG-CoA reductase in cells, the rate-limiting step of cholesterol synthesis. To compensate for the decreased cholesterol availability, synthesis of hepatic LDL receptors is increased, resulting in an increased clearance of LDL particles from the blood.


 * Ezetimibe reduces intestinal absorption of cholesterol, thus can reduce LDL particle concentrations when combined with statins.


 * PCSK9 inhibitors, in phase 3 clinical trials, by several companies, appear to be far more effective for LDL reduction than the statins, even statins at high dose.


 * Niacin (B3), lowers LDL by selectively inhibiting hepatic diacyglycerol acyltransferase 2, reducing triglyceride synthesis and VLDL secretion through a receptor HM74 and HM74A or GPR109A.


 * Several CETP inhibitors have been researched to improve HDL concentrations, but so far, despite dramatically increasing HDL-C, have not had a consistent track record in reducing atherosclerosis disease events. Some have increased mortality rates compared with placebo.


 * Clofibrate is effective at lowering cholesterol levels, but has been associated with significantly increased cancer and stroke mortality, despite lowered cholesterol levels. Other, more recently developed and tested fibrates, e.g. fenofibric acid have had a better track record and are primarily promoted for lowering VLDL particles (triglycerides), not LDL particles, yet can help some in combination with other strategies.


 * Some Tocotrienols, especially delta- and gamma-tocotrienols, are being promoted as statin alternative non-prescription agents to treat high cholesterol, having been shown in vitro to have an effect. In particular, gamma-tocotrienol appears to be another HMG-CoA reductase inhibitor, and can reduce cholesterol production. As with statins, this decrease in intra-hepatic (liver) LDL levels may induce hepatic LDL receptor up-regulation, also decreasing plasma LDL levels. As always, a key issue is how benefits and complications of such agents compare with statins&mdash;molecular tools that have been analyzed in large numbers of human research and clinical trials since the mid-1970s.


 * Phytosterols are widely recognized as having a proven LDL cholesterol lowering efficacy. Current supplemental guidelines recommend doses of phytosterols in the 1.6-3.0 grams per day range (Health Canada, EFSA, ATP III,FDA) with a recent meta-analysis demonstrating an 8.8% reduction in LDL-cholesterol at a mean dose of 2.15 gram per day. However, plant sterols and stanols, if absorbed (intestinal cells generally block), greatly accelerate progression of atherosclerosis more than cholesterol delivered into the arterial walls by lipoprotein particles.


 * Insulin induces HMG-CoA reductase activity, whereas glucagon diminishes HMG-CoA reductase activity. While glucagon production is stimulated by dietary protein ingestion, insulin production is stimulated by dietary carbohydrate.  The rise of insulin is, in general, determined by the digestion of carbohydrates into glucose and subsequent increase in serum glucose levels. In non-diabetics, glucagon levels are very low when insulin levels are high; however, those who have become diabetic no longer suppress glucagon output after eating.


 * A ketogenic diet may have similar response to taking niacin (lowered LDL and increased HDL) through beta-hydroxybutyrate, a ketone body, coupling the niacin receptor (HM74A).


 * Lowering the blood lipid concentration of triglycerides helps lower the concentration of small LDL particles, because fat rich VLDL particles convert in the bloodstream into Small LDL particles.

Dietary

 * The most effective approach has been minimizing fat stores located inside the abdominal cavity (visceral body fat) in addition to minimizing total body fat. Visceral fat, which is more metabolically active than subcutaneous fat, has been found to produce many enzymatic signals, e.g. resistin, which increase insulin resistance and circulating VLDL particle concentrations, thus both increasing LDL particle concentrations and accelerating the development of Diabetes Mellitus.

Importance of antioxidants
Because LDL particles appear harmless until they are within the blood vessel walls and oxidized by free radicals, it is postulated that ingesting antioxidants and minimizing free radical exposure may reduce LDL's contribution to atherosclerosis, though results are not conclusive. Studies have reported the benefits of green tea in helping to reduce LDL; some studies have focused on the antioxidant qualities of the unfermented green tea leaves, others looked at green-tea compounds called catechins which are thought to decrease cholesterol absorption in the gut.

Estimation of LDL particles via cholesterol content
Chemical measures of lipid concentration have long been the most-used clinical measurement, not because they have the best correlation with individual outcome, but because these lab methods are less expensive and more widely available.

The lipid profile does not measure LDL particles directly but instead estimates them using the Friedewald equation by subtracting the amount of cholesterol associated with other particles, such as HDL and VLDL, assuming a prolonged fasting state, etc.:
 * $$L \approx C - H - kT$$
 * where H is HDL cholesterol, L is LDL cholesterol, C is total cholesterol, T are triglycerides, and k is 0.20 if the quantities are measured in mg/dl and 0.45 if in mmol/l.

There are limitations to this method, most notably that samples must be obtained after a 12 to 14 h fast and that LDL-C cannot be calculated if plasma triglyceride is >4.52 mmol/L (400 mg/dL). Even at triglyceride levels 2.5 to 4.5 mmol/L, this formula is considered inaccurate. If both total cholesterol and triglyceride levels are elevated then a modified formula, with quantities in mg/dl, may be used
 * $$L = C - H - 0.16T$$

This formula provides an approximation with fair accuracy for most people, assuming the blood was drawn after fasting for about 14 hours or longer.

However, the concentration of LDL particles, and to a lesser extent their size, has a slightly stronger correlation with individual clinical outcome than the amount of cholesterol within LDL particles, even if the LDL-C estimation is approximately correct. There is increasing evidence and recognition of the value of more targeted and accurate measurements of LDL particles. Specifically, LDL particle number (concentration), and to a lesser extent size, have shown slightly stronger correlations with atherosclerotic progression and cardiovascular events than obtained using chemical measures of the amount of cholesterol carried by the LDL particles. It is possible that the LDL cholesterol concentration can be low, yet LDL particle number high and cardiovascular events rates are high. Correspondingly, it is possible that LDL cholesterol concentration can be relatively high, yet LDL particle number low and cardiovascular events are also low. If LDL particle concentration is used to predict cardiovascular events, many other correlates of these clinical outcomes, such as diabetes mellitus, obesity and smoking, lose most of their predictive accuracy.

Normal ranges
In the USA, the American Heart Association, NIH, and NCEP provide a set of guidelines for fasting LDL-Cholesterol levels, estimated or measured, and risk for heart disease. As of about 2005, these guidelines were:

Over time, with more clinical research, these recommended levels keep being reduced because LDL reduction, including to abnormally low levels, was the most effective strategy for reducing cardiovascular death rates in one large double blind, randomized clinical trial of men with hypercholesterolemia; far more effective than coronary angioplasty/stenting or bypass surgery

For instance, for people with known atherosclerosis diseases, the 2004 updated American Heart Association, NIH and NCEP recommendations are for LDL levels to be lowered to less than 70 mg/dL, unspecified how much lower. This low level of less than 70 mg/dL was recommended for primary prevention of 'very-high risk patients' and in secondary prevention as a 'reasonable further reduction'. Lack of evidence for such a recommendation is discussed in an article in the Annals of internal medicine. It should also be noted that statin drugs involved in such clinical trials have numerous physiological effects beyond simply the reduction of LDL levels.

It has been estimated from the results of multiple human pharmacologic LDL lowering trials that LDL should be lowered to about 50 to reduce cardiovascular event rates to near zero. For reference, from longitudinal population studies following progression of atherosclerosis-related behaviors from early childhood into adulthood, it has been discovered that the usual LDL in childhood, before the development of fatty streaks, is about 35 mg/dL. However, all the above values refer to chemical measures of lipid/cholesterol concentration within LDL, not measured Low Density Lipoprotein concentrations, the accurate approach.

The feasibility of these figures has been questioned by sceptics, claiming that many members of the AHA and NIH are heavily associated with pharmaceutical companies giving them bias towards lowering cholesterol levels and such guidelines giving rise to increased use of cholesterol lowering medicine such as statins.

A study was conducted measuring the effects of guideline changes on LDL cholesterol reporting and control for diabetes visits in the US from 1995 to 2004. It was found that although LDL cholesterol reporting and control for diabetes and coronary heart disease visits improved continuously between 1995 and 2004, neither the 1998 ADA guidelines nor the 2001 ATP III guidelines increased LDL cholesterol control for diabetes relative to coronary heart disease.

Moreover, there are publications regarding the risks of low-LDL cholesterol too.

Direct measurement of LDL particle concentrations
There are several competing methods for measurement of lipoprotein particle size although the evidence in favor of their superiority to existing methods is weak, even by the statements of proponents. Direct LDL particle measurement by NMR was mentioned by the ADA and ACC, in a 28 March 2008 joint consensus statement, as having advantages for predicting individual risk of atherosclerosis disease events, but the statement noted that the test is not widely available and is more expensive (about $98.00 US without insurance coverage) than existing tests. Furthermore the authors also said it is "...unclear whether LDL particle size measurements add value to measurement of LDL particle concentration." Since the later 1990s, because of the development of NMR measurements, it has been possible to clinically measure lipoprotein particles at lower cost [under $100 US (including shipping) versus the previous costs of >$400 to >$5,000] and high accuracy. There are also other (less expensive) homogeneous assays for LDL, however most only estimate LDL.

Using NMR, as pioneered by researcher Jim Otvos and the North Carolina State University academic research spinoff company LipoScience, the total LDL particle concentrations, in nmol/L plasma, are typically subdivided by percentiles referenced to the 5,382 men and women, not on any lipid medications, who are participating in the MESA trial.

Optimal ranges
The LDL particle concentrations are typically categorized by percentiles, <20%, 20–50%, 50th–80th%, 80th–95% and >95% groups of 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 the <20% group, with increased rates for the higher groups. Multiple other measures, including particle sizes, small LDL particle concentrations, total and large HDL particle concentrations, along with estimations of insulin resistance pattern and standard cholesterol lipid measurements (for comparison of the plasma data with the estimation methods discussed above) are also routinely provided.