# Low-density lipoprotein: Wikis

Note: Many of our articles have direct quotes from sources you can cite, within the Wikipedia article! This article doesn't yet, but we're working on it! See more info or our list of citable articles.

# Encyclopedia

Low-density lipoprotein (LDL) is a type of lipoprotein that transports cholesterol and triglycerides from the liver to peripheral tissues. LDL is one of the five major groups of lipoproteins; these groups include chylomicrons, very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), low-density lipoprotein, and high-density lipoprotein (HDL), although some alternative organizational schemes have been proposed. Like all lipoproteins, LDL enables fats and cholesterol to move within the water-based solution of the blood stream. LDL also regulates cholesterol synthesis at these sites. It is used medically as part of a cholesterol blood test, and since high levels of LDL cholesterol can signal medical problems like cardiovascular disease, it is sometimes called bad cholesterol, (as opposed to HDL, which is frequently referred to as good cholesterol or healthy cholesterol).[1]

## Biochemistry

### Structure

Each native LDL particle contains a single apolipoprotein B-100 molecule (Apo B-100, a protein with 4536 amino acid residues), which circulates the fatty acids, keeping them soluble in the aqueous environment. In addition, LDL has a highly-hydrophobic core consisting of polyunsaturated fatty acid known as linoleate and about 1500 esterified cholesterol molecules. This core is surrounded by a shell of phospholipids and unesterified cholesterol, as well as a single copy of B-100 large protein (514 kD). LDL particles are approximately 22 nm in diameter and have a mass of about 3 million daltons, but since LDL particles contain a changing number of fatty acids, they actually have a mass and size distribution.[2]

### 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 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).

The correspondence between Pattern B and CHD has been suggested by some in the medical community to be 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 has been used more commonly.

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.[3][4]

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 event and quantitatively-measured particle concentrations.

### Transport into the cell

When a cell requires cholesterol, 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 blood stream 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

Because LDLs transport cholesterol to the arteries and can be retained there by arterial proteoglycans starting the formation of plaques, increased levels are associated with atherosclerosis, and thus heart attack, stroke, and peripheral vascular disease. For this reason, cholesterol inside LDL lipoproteins is often called bad cholesterol. This is a misnomer. The cholesterol transported on LDL is the same as cholesterol transported on other lipoprotein particles. The cholesterol itself is not bad; rather, it is how and where the cholesterol is being transported, and in what amounts over time, that causes adverse effects.

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.[5] The healthiest pattern, 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; 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.

LDL is formed as VLDL lipoproteins lose triglyceride through the action of lipoprotein lipase (LPL) and become smaller and denser, containing a higher proportion of cholesterol.

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

LDL poses a risk for cardiovascular disease when it invades the endothelium and becomes oxidized, since the oxidized form is more easily retained by the proteoglycans. A complex set of biochemical reactions regulates the oxidation of LDL, chiefly stimulated by presence of free radicals in the endothelium.

A 4 minute animation of the atherosclerosis process, entitled "Pathogenesis of Acute MI", commissioned by Paul M. Ridker, MD, MPH, FACC, FAHA, at the Harvard Medical School, can be viewed at pri-med.com [1]. While the animation contains a few technical errors, it correctly illustrates the principal issues.

### 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. [6]

### Lowering LDL

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 in the pathway.

#### Pharmaceutical

The use of statins (HMG-CoA reductase inhibitors) is effective against high levels of LDL cholesterol. Statins inhibit the enzyme HMG-CoA reductase in the liver, the rate-limiting step of cholesterol synthesis. To compensate for the decreased cholesterol availability, synthesis of LDL receptors is increased , resulting in an increased clearance of LDL from the blood.

Clofibrate is effective at lowering cholesterol levels, but has been associated with significantly increased cancer and stroke mortality, despite lowered cholesterol levels.[7]

Niacin (B3), lowers LDL by selectively inhibiting hepatic diacyglycerol acyltransferase 2, reducing triglyceride synthesis and VLDL secretion through a receptor HM74[8] and HM74A or GPR109A.[9]

Tocotrienols, especially δ- and γ-tocotrienols, have been shown to be effective nutritional agents to treat high cholesterol in vitro in recent research programs. In particular, γ-tocotrienol appears to act on a specific enzyme called 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase) and suppresses the production of this enzyme, which results in less cholesterol being manufactured by liver cells.[10] This decrease in hepatic (liver) LDL levels causes hepatic LDL receptor up-regulation, further decreasing plasma LDL levels as it is taken in by the liver.

#### Dietary

Insulin induces HMG-CoA reductase activity, whereas glucagon downregulates it.[11] 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. Glucagon levels are very low when insulin levels are high.

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).[9]

Lowering the blood lipid concentration of triglycerides helps lower the amount of LDL, because VLDL gets converted in the bloodstream into LDL.

Fructose, a component of sucrose as well as high-fructose corn syrup, upregulates hepatic VLDL synthesis.[12]

### Importance of antioxidants

Because LDL appears to be harmless until oxidized by free radicals,[13] it is postulated that ingesting antioxidants and minimizing free radical exposure may reduce LDL's contribution to atherosclerosis, though results are not conclusive.[14]

## Measurement of LDL

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. However, there is increasing evidence and recognition of the value of more sophisticated measurements. To be specific, LDL particle number (concentration), and to a lesser extent size, have shown much tighter correlation with atherosclerotic progression and cardiovascular events than is obtained using chemical measures of total LDL concentration contained within the particles. LDL cholesterol concentration can be low, yet LDL particle number high and cardiovascular events rates are high. Also, LDL cholesterol concentration can be relatively high, yet LDL particle number low and cardiovascular events are also low. If LDL particle concentration is tracked against event rates, many other statistical correlates of cardiovascular events, such as diabetes mellitus, obesity, and smoking, lose much of their additional predictive power.

The lipid profile does not measure LDL level directly but instead estimates it using the Friedewald equation[4][15] using levels of other cholesterol such as HDL:

$H \approx C - L - kT$
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 LDL-C levels 2.5 to 4.5 mmol/L, this formula is considered to be inaccurate.[16] If both total cholesterol and triglyceride levels are elevated then a modified formula, with quantities are in mg/dl, may be used

L = CH − 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 far tighter correlation with clinical outcome than the content of cholesterol with the LDL particles, even if the LDL-C estimation is about correct.)

### 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 2003, these guidelines were:[17][18][19]

Level mg/dL Level mmol/L Interpretation
<100 <2.6 Optimal LDL cholesterol, corresponding to reduced, but not zero, risk for heart disease
100 to 129 2.6 to 3.3 Near optimal LDL level
130 to 159 3.3 to 4.1 Borderline high LDL level
160 to 189 4.1 to 4.9 High LDL level
>200 >4.9 Very high LDL level, corresponding to highest increased risk of heart disease

These guidelines were based on a goal of presumably decreasing death rates from cardiovascular disease to less than 2% to 3% per year or less than 20% to 30% every 10 years. Note that 100 is not considered optimal; less than 100 is optimal, though it is unspecified how much less.

Over time, with more clinical research, these recommended levels keep being reduced because LDL reduction, including to abnormally low levels, has been the most effective strategy for reducing cardiovascular death rates in large double blind, randomized clinical trials;[20] 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. 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 LDLipoprotein concentrations, probably not the better 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.

## Footnotes

1. ^ LDL and HDL Cholesterol: What's Bad and What's Good?
2. ^ Segrest JP, Jones MK, De Loof H, Dashti N (September 2001). "Structure of apolipoprotein B-100 in low density lipoproteins". Journal of Lipid Research 42 (9): 1346–67. PMID 11518754.
3. ^ Superko HR, Nejedly M, Garrett B (2002). "Small LDL and its clinical importance as a new CAD risk factor: a female case study". Progress in Cardiovascular Nursing 17 (4): 167–73. doi:10.1111/j.0889-7204.2002.01453.x. PMID 12417832.
4. ^ a b Warnick GR, Knopp RH, Fitzpatrick V, Branson L (January 1990). "Estimating low-density lipoprotein cholesterol by the Friedewald equation is adequate for classifying patients on the basis of nationally recommended cutpoints". Clinical Chemistry 36 (1): 15–9. PMID 2297909.
5. ^ Not All Calories Are Created Equal, Author Says. Talk of the Nation discussion of the book Good Calories, Bad Calories, by Gary Taubes. National Public Radio, 2 Nov 2007.
6. ^ Peterson MM, Mack JL, Hall PR, et al. (December 2008). "Apolipoprotein B Is an innate barrier against invasive Staphylococcus aureus infection". Cell Host & Microbe 4 (6): 555–66. doi:10.1016/j.chom.2008.10.001. PMID 19064256.
7. ^ "WHO cooperative trial on primary prevention of ischaemic heart disease with clofibrate to lower serum cholesterol: final mortality follow-up. Report of the Committee of Principal Investigators". Lancet 2 (8403): 600–4. September 1984. PMID 6147641.
8. ^ Meyers CD, Kamanna VS, Kashyap ML (December 2004). "Niacin therapy in atherosclerosis". Current Opinion in Lipidology 15 (6): 659–65. doi:10.1097/00041433-200412000-00006. PMID 15529025.
9. ^ a b Soudijn W, van Wijngaarden I, Ijzerman AP (May 2007). "Nicotinic acid receptor subtypes and their ligands". Medicinal Research Reviews 27 (3): 417–33. doi:10.1002/med.20102. PMID 17238156.
10. ^ Song, B.L.; DeBose-Boyd, R.A. (2006). "Insig-Dependent Ubiquitination and Degradation of 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase Stimulated by Delta- and Gamma-Tocotrienols". J. Biol. Chem. 281 (35): 25054–25601. doi:10.1074/jbc.M605575200.
11. ^ Regulation of Cholesterol Synthesis
12. ^ Fructose, insulin resistance, and metabolic dyslipidemia
13. ^ Inhibition of in vitro human LDL oxidation by phenolic antioxidants from grapes and wines. Teissedre, P.L. : Frankel, E.N. : Waterhouse, A.L. : Peleg, H. : German, J.B. J-sci-food-agric. Sussex : John Wiley : & : Sons Limited. Jan 1996. v. 70 (1) p. 55-61.
14. ^ Esterbauer H, Puhl H, Dieber-Rotheneder M, Waeg G, Rabl H (1991). "Effect of antioxidants on oxidative modification of LDL". Annals of Medicine 23 (5): 573–81. doi:10.3109/07853899109150520. PMID 1756027.
15. ^ Friedewald WT, Levy RI, Fredrickson DS (June 1972). "Estimation of the concentration of low-density lipoprotein cholesterol in plasma, without use of the preparative ultracentrifuge". Clinical Chemistry 18 (6): 499–502. PMID 4337382.
16. ^ Sniderman AD, Blank D, Zakarian R, Bergeron J, Frohlich J (October 2003). "Triglycerides and small dense LDL: the twin Achilles heels of the Friedewald formula". Clinical Biochemistry 36 (7): 499–504. doi:10.1016/S0009-9120(03)00117-6. PMID 14563441.
17. ^ "Cholesterol Levels". American Heart Association. Retrieved 2009-11-14.
18. ^ "What Do My Cholesterol Levels Mean?" (PDF). American Heart Association. September 2007. Retrieved 2009-11-14.
19. ^ "Third Report of the National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III) Executive Summary". National Heart, Lung, and Blood Institute (NHLBI). National Institutes of Health. May 2001.
20. ^ Shepherd J, Cobbe SM, Ford I, et al. (November 1995). "Prevention of coronary heart disease with pravastatin in men with hypercholesterolemia. West of Scotland Coronary Prevention Study Group". The New England Journal of Medicine 333 (20): 1301–7. doi:10.1056/NEJM199511163332001. PMID 7566020.