Low-density lipoprotein
Low-density lipoprotein is one of the five major groups of lipoprotein that transport all fat molecules around the body in extracellular water. These groups, from least dense to most dense, are chylomicrons, very low-density lipoprotein, intermediate-density lipoprotein, low-density lipoprotein and high-density lipoprotein. LDL delivers fat molecules to cells.
Lipoproteins transfer lipids around the body in the extracellular fluid, making fats available to body cells for receptor-mediated endocytosis. Lipoproteins are complex particles composed of multiple proteins, typically 80–100 proteins per particle. A single LDL particle is about 22–27.5 nanometers in diameter, typically transporting 3,000 to 6,000 fat molecules per particle and varying in size according to the number and mix of fat molecules contained within. The lipids carried include all fat molecules with cholesterol, phospholipids, and triglycerides dominant; amounts of each vary considerably.
Elevated LDL is an established causal factor for the development of atherosclerotic cardiovascular disease. A normal non-atherogenic LDL-C level is 20–40 mg/dl. Guidelines recommend maintaining LDL-C under 2.6 mmol/L and under 1.8 mmol/L for those at high risk.
Biochemistry
Structure
Each native LDL particle enables emulsification, i.e. surrounding the 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, 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 of esterified and unesterified 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 to 27.5 nm 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 difficult for biochemists because of its heterogeneous structure. However, the structure of LDL at human body temperature in native condition, with a resolution of about 16 Angstroms using cryogenic electron microscopy, has been described in 2011.Physiology
LDL particles are formed when triglycerides are removed from VLDL by the lipoprotein lipase enzyme, and they become smaller and denser, containing a higher proportion of cholesterol esters.Transport into the cell
When a cell requires more cholesterol than its HMG-CoA pathway can produce, it synthesizes the necessary LDL receptors as well as PCSK9, a proprotein convertase that marks the LDL receptor for degradation. LDL receptors are inserted into the plasma membrane and diffuse freely until they associate with clathrin-coated pits. When LDL receptors bind LDL particles in the bloodstream, the clathrin-coated pits are endocytosed into the cell.Vesicles containing LDL receptors bound to LDL are delivered to the endosomes. In the presence of low pH, such as that found in the endosome, LDL receptors undergo a conformation change, releasing LDL. LDL is then shipped to the lysosomes, where cholesterol esters in the LDL are hydrolysed. LDL receptors are typically returned to the plasma membrane, where they repeat this cycle. If LDL receptors bind to PCSK9, however, transport of LDL receptors is redirected to the lysosome, where they are degraded.
Innate immune system
LDL interferes 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.Size patterns
LDL can be grouped based on its size: large low-density LDL particles are described as pattern A, and small high-density LDL particles are pattern B. Pattern B has been associated by some with a higher risk for coronary artery disease. This is thought to be because the smaller particles are more easily able to penetrate the endothelium of arterial walls. Pattern I, or intermediate, indicates that most LDL particles are very close in size to the normal gaps in the endothelium. 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 evidence suggests the correlation between pattern B and coronary artery disease 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 standard 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 shown a stronger correlation between clinically evident human cardiovascular events and quantitatively measured particle concentrations.
Oxidized
Oxidized LDL is a general term for LDL particles with oxidatively modified structural components. As a result, from free radical attack, both lipid and protein parts of LDL can be oxidized in the vascular wall. Besides the oxidative reactions in the vascular wall, oxidized lipids in LDL can also be derived from oxidized dietary lipids. Oxidized LDL is known to associate with the development of atherosclerosis, and it is therefore widely studied as a potential risk factor of cardiovascular diseases. Atherogenicity of oxidized LDL has been explained by lack of recognition of oxidation-modified LDL structures by the LDL receptors, preventing the normal metabolism of LDL particles and leading eventually to the development of atherosclerotic plaques. Of the lipid material contained in LDL, various lipid oxidation products are known as the ultimate atherogenic species. Acting as a transporter of these injurious molecules is another mechanism by which LDL can increase the risk of atherosclerosis.The LOX-1 scavenge receptor does take up oxLDL, but the liver does not naturally express it. It is instead expressed by endothelial cells, platelets, macrophages, smooth muscle cells, and cardiomyocytes as an innate immune scavenge receptor. When activated, pro-inflammatory signals are generated in the cell, and damaging compounds are released as well. As a result, these cells are most sensitive to the effects of oxLDL. SR-BI and CD36, two class B scavenge receptors, also take up oxLDL into the macrophage.
Despite lower recognition efficacy by the LDL receptor, the liver does remove oxLDLs from the circulation. This is achieved by Kupffer cells and liver sinusoidal endothelial cells. In LSECs, stabilin-1 and stabilin-2 mediate most of the uptake. Uptake of oxLDLs causes visible disruption to the structure of the LSEC in rats. Doing the same also damages human LSEC cultures.
Acetyl
Acetyl LDL is a construct generated in vitro. When scientists produced such a modified version of LDL, they found that a class of scavenge receptors, now called SR-A, can recognize them and take them up. Because scavenge receptors work much faster than the downregulated native LDL receptor of a macrophage, oxLDL and acLDL can both fill up a macrophage quickly, turning it into a foam cell.Testing
commonly report LDL-C: the amount of cholesterol that is estimated to be contained with LDL particles, on average, using a formula, the Friedewald equation. In a clinical context, mathematically calculated estimates of LDL-C are commonly used to estimate how much low-density lipoproteins drive the progression of atherosclerosis. The problem with this approach is that LDL-C values are commonly discordant with both direct measurements of LDL particles and actual rates of atherosclerosis progression.Direct LDL measurements are also available and better reveal individual issues but are less often promoted or done due to slightly higher costs and are available from only a couple of laboratories in the United States. In 2008, the American Diabetes Association and American College of Cardiology recognized direct LDL particle measurement by NMR as superior for assessing individual risk of cardiovascular events.
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 outcomes but because these lab methods are less expensive and more widely available.The lipid profile does not measure LDL particles. It only 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.:
where H is HDL cholesterol, L is LDL cholesterol, C is total cholesterol, T is triglycerides, and k is 0.20 if the quantities are measured in mg/dL and 0.45 in mmol/L.
L is often reported as CLDL-C. This is the most common way of estimating the amount of cholesterol carried by low-density lipoprotein.
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. Even at triglyceride levels of 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
This formula provides an approximation with fair accuracy for most people, assuming the blood was drawn after fasting for about 14 hours or longer, but does not reveal the actual LDL particle concentration because the percentage of fat molecules within the LDL particles, which are cholesterol, varies as much as 8:1 variation. There are several formulas published addressing the inaccuracy in LDL-C estimation. The inaccuracy is based on the assumption that VLDL-C is always one-fifth of the triglyceride concentration. Other formulae address this issue by using an adjustable factor or using a regression equation. There are few studies which have compared the LDL-C values derived from this formula and values obtained by direct enzymatic method. Direct enzymatic methods are found to be accurate and must be the test of choice in clinical situations. In resource-poor settings, the option to use the formula has to be considered.
However, the concentration of LDL particles, and to a lesser extent, their size, has a stronger and consistent correlation with individual clinical outcomes 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 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 is low, and cardiovascular events are also low.