Lipid and Lipoprotein Basics

Normal Lipid Physiology

Lipids are critical structural components of cells and circulating plasma lipoproteins and play important regulatory roles in the body. Cholesterol and triglycerides are the lipids of greatest clinical importance.

Cholesterol

Cholesterol is a sterol synthesized by all animal cells (Figure 2-1). About 25% of cholesterol is synthesized in the liver. The adrenal glands, reproductive organs, and intestines also have higher rates of cholesterol synthesis. Synthesis begins with one molecule each of acetyl CoA and acetoacetyl-CoA, which are metabolized through multiple steps to cholesterol (Figure 2-2). The rate-limiting step in cholesterol synthesis is the reduction to mevalonate by 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase, the target of statins. Downstream metabo­lites of mevalonate include several biologically active molecules. Cholesterol synthesis is regulated by intra­cellular cholesterol levels primarily via sterol regulator element-binding proteins (SREBP) 1 and 2.

Cholesterol is largely transported and stored as cholesteryl ester. Cholesteryl esters are taken up by macrophage foam cells, the initiating step in the development of atherosclerosis, with continued accumulation contributing to atherosclerotic plaque growth.

Cholesterol is the backbone for the biosynthesis of steroid hormones. Cholesterol is also required to maintain cell membrane structural integrity and fluidity (Figure 2-3). Human cell membranes are composed of a bilayer of phospholipids (which contain 2-fatty acid chains) interspersed with proteins and other structures. The hydroxyl group of the cholesterol molecule interacts with the polar head groups of membrane phospholipids and sphingolipids. The bulky steroid group and hydrocarbon chain are embedded in the membrane with the nonpolar fatty acids of other lipids. Cholesterol also plays a critical role in intracellular transport via endocytosis and cell signaling via lipid rafts that bring receptor proteins into close proximity with messenger molecules (Figure 2-3). Cholesterol is a chief component of the myelin sheath, providing insulation for more efficient nerve conduction. It is possible that some of the “pleiotropic” effects of statins are a downstream result of alterations in cholesterol levels affecting these other functions.

Adults synthesize about 100 mg of cholesterol per day. The average daily dietary intake of cholesterol is about 300 mg. Cholesterol is recycled in the body. Most cholesterol inside of cells is esterified. Non-esterified cholesterol is secreted in the bile and about 50% is reabsorbed back into the bloodstream in the small intestine.

Cholesterol is largely hydrophobic and therefore must be transported in the blood by lipoproteins. Lipoproteins are complex particles, with the water soluble proteins and polar lipid head facing outward and the water insoluble cholesteryl esters and fatty acid tails of the triglyceride molecules facing inward. The primary circulating lipoprotein transporting cholesterol to the body is low-density lipoprotein (LDL). High-density lipoproteins (HDL) transport cholesterol from the periphery to the liver.

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Triglycerides

Triglycerides are comprised of three fatty acids bound to glycerol (Figure 2-4). Triglycerides are synthesized by the liver or come from dietary fat intake. Triglycerides are the primary method of transporting fatty acids to and from the liver and adipose tissue.

There are three major classes of fatty acids of interest: saturated, monounsaturated and polyunsaturated (see Figure 2-4 for examples of each attached to glycerol backbone). Polyunsaturated fatty acids have two or more double bonds between the carbons in the chain. They are liquid at room temperature because the molecules do not stack together as easily due to the nonlinear conformation of the fatty acid carbon chains. Monounsaturated fatty acids have one double bond and saturated fatty acids have no double bonds. Hydrogenation is a method of transforming poly- or monounsaturated oils into saturated fatty acids, which are more likely to be solid at room temperature because they can stack together. The fatty composition of the diet contributes to the risk of atherosclerotic cardiovascular disease (ASCVD) (see Cardiovascular Disease Prevention).

The structure of circulating plasma lipoproteins are influenced by the bipolar nature of triglycerides (similar to the membrane bilayer assembly of phospholipids). The hydrophilic polar glycerol group of the triglyceride is exposed to the cellular milieu, while the hydrophobic nonpolar fatty acids are sequestered inside the bilayer. Triglycerides are major components of chylomicrons and very low-density lipoproteins (VLDL). Other lipoproteins, such as LDL and HDL, contain varying amounts of triglycerides.

Triglycerides are stored in adipose tissue and their breakdown into fatty acids provides a major fuel source for cellular metabolism. Triglycerides are not taken up directly by cells but require lipolysis to liberate free fatty acids, which can be taken up directly by the cell. Glucagon signals hormone sensitive lipase to break down stored triglycerides into free fatty acids. The brain does not metabolize free fatty acids but does utilize the glycerol component by converting it into glucose via gluconeogenesis.

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Other Lipids

Other lipids, such as sphingosine, sphingomyelin and ceramides, also play important structural and signaling roles in the body. Sphingolipids are included in circulating lipoproteins and atherosclerotic plaque. However, the role of these lipids in the development of ASCVD is not well understood, and they are not affected by current drug therapies.

Lipoprotein Metabolism

Cholesterol and triglycerides are either synthesized by the liver or come from the diet. Lipoproteins transport cholesterol and triglycerides in the blood for use throughout the body. Chylomicrons transport dietary fatty acids as triglycerides for uptake by cells throughout the body, including the liver. In the liver, triglycerides are stored, used for energy, or used to construct VLDL. The liver assembles VLDL from triglycerides (either from the diet or synthesized in the liver), apolipoprotein B and cholesteryl esters. VLDL is then secreted into the blood.

Constituent triglycerides and cholesterol are removed by peripheral cells as VLDL circulates through the body, with short-lived intermediate-density lipoproteins (IDLs) as intermediaries, finally resulting in the longer-lasting LDL particle. LDL comprises the largest fraction of circulating lipoproteins, constituting about 60% to 70% of total cholesterol. The relative size, composition and atherogenicity of the circulating lipoproteins are shown in Figure 2-5.

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HDL

HDL is the smallest and densest of the circulating lipoproteins, with the highest proportion of protein to lipid content. HDL carries apo AI and AII rather than apoB100, which characterizes the atherogenic lipoproteins.

HDL is synthesized in the liver as a complex of apolipoproteins AI and AII and phospholipids in a discoid particle called nascent HDL or pre-ß HDL (Figure 2-6). As this particle circulates thought the body and interacts with peripheral cells, it acquires cholesterol and additional phospholipid by binding to adenosine triphosphate-binding cassette protein 1 (ABC1), ultimately resulting in mature HDL. This process is called reverse cholesterol transport. HDL carries about 30% of blood cholesterol.

Inside the HDL particle, unesterified cholesterol is esterified by lecithin-cholesterol acyl transferase (LCAT). Esterified cholesterol can then be taken up by the liver via scavenger receptor B1 (SR-B1) or in the plasma can be exchanged for triglycerides from apoB-containing lipoproteins via cholesteryl ester transfer protein (CETP). CETP inhibitors are under development that markedly raise HDL-C levels, with variable effects on LDL-C.

Phospholipids can be transferred from apoB-containing lipoproteins to HDL via phospholipid transfer protein (PLTP). HDL may also receive triglycerides hydrolyzed from VLDL and chylomicrons by lipoprotein lipase. Hepatic lipase hydrolyzes the triglycerides in HDL, forming smaller HDL particles.

HDL, HDL-C and Apo AI are all inversely associated with ASCVD risk in epidemiologic studies. An HDL-C <40 mg/dL in men and <50 mg/dL in women is associated with increased ASCVD risk. However, raising HDL-C with drug therapy has yet to be shown to reduce ASCVD risk (see Treatment Approaches). An uncommon apo AI mutation, Apo AI-milano, causes low HDL-C levels but is not associated with increased ASCVD risk. This is thought to be due to more efficient reverse cholesterol transport. Infusions of Apo AI-milano have been shown to reduce atherosclerosis in animals and humans, but development has not moved forward to date.

Apo AII has been associated with increased ASCVD risk, although the mechanisms are unclear.

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Chylomicrons

Chylomicrons are the key transporter of fatty acids from the intestine, and also transport a small amount of cholesterol. Dietary triglycerides are broken down by pancreatic lipase in the duodenum. The intestine absorbs free fatty acids, monoglycerides (one glycerol and one fatty acid), and some diglycerides. The enterocyte then reassembles the free fatty acids, monoglycerides and diglycerides into triglycerides which are then packaged with dietary cholesterol, apoB48, and apo A I and II, apo CI, CII, and CIII into chylomicrons. Chylomicrons are secreted by the enterocyte into the lymph system, which drains into the inferior vena cava, and ultimately into the blood.

Chylomicrons can be captured directly by cells, with the triglycerides used as a source of energy, or are stored in some cells. Lipoprotein lipase releases the fatty acids from triglycerides. Once most of the triglycerides are removed from the chylomicron, the remnant is cleared by the liver B/E receptor. Chylomicron clearance is variable, with slower clearance resulting in postprandial hypertriglyceridemia. Obesity, hypertriglyceridemia in the fasting state and diabetes slow chylomicron clearance.

Chylomicrons are not atherogenic but the chylomicron remnants may have some atherogenicity.

VLDL and IDLs

The majority of cholesterol in the body is syn­thesized in the liver. VLDL transports cholesterol synthesized in the liver to the peripheral tissues. In the liver, microsomal transport protein (MTP) forms VLDL from triglycerides and cholesteryl esters complexed with apoB, apo E, apo CII, apo CIII and phospholipids. An MTP inhibitor, lomitapide, is approved for lowering blood cholesterol only in patients with homozygous familial hypercholesterolemia because of increased hepatotoxicity, due at least in part to hepatic triglyceride accumulation.

Once VLDL is secreted into the circulation, triglycerides are removed via lipoprotein lipase to release fatty acids for storage or energy production. VLDL can also exchange triglycerides and phospholipids with HDL via CETP. As triglycerides continue to be removed by lipoprotein lipase and CETP, VLDL transitions to IDLs. When the cholesterol content exceeds the triglyceride content, IDLs become LDL. VLDL and IDLs can also bind hepatic VLDL receptors to apo E.

Insulin resistance, diabetes and obesity can result in increased production and decreased clearance of VLDL, manifested as moderate to severe hypertriglyc­eridemia.

VLDL and IDLs are considered atherogenic.

LDL

LDL is the final step in the pathway from VLDL, and the primary transporter of cholesterol in the body. LDL receptors on the liver bind the apoB-100 in the LDL particles. This complex is endocytosed into the cell, where the LDL is broken down while preserving the LDL receptor. The LDL receptor then returns to the cell surface, a recycling process that can occur up to 150 times. Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a key regulator of the LDL receptor. PCSK9 irreversibly binds to the LDL receptor-LDL complex, which results in the degradation of the LDL receptor along with LDL (Figure 23-1). PCSK9 inhibitors have been approved and result in dramatic LDL-C reductions (see Treatment Approaches). Intracellular cholesterol levels regulate the expression of both the LDL-receptor and PCSK9 via SREBP-1.

LDL plays a fundamental role in atherogenesis. Several drugs that lower LDL-C through an LDL receptor–mediated mechanism (statins and ezetimibe) have been shown to reduce ASCVD events.

LDL, enriched in triglycerides by CETP or other pathways, is a substrate for hepatic lipase, which hydrolyzes the triglycerides to produce smaller, denser LDL (small dense LDL). Small, dense LDL was thought to be more atherogenic than larger LDL particles. However, carefully done analyses have found small dense LDL to be a better marker of metabolic abnormalities that increase triglycerides, such as insulin resistance and diabetes.

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Lp(a)

Lipoprotein (a) (Lp(a)) consists of an LDL-like particle that is connected by apoB to apo (a) (Figure 2-5). Lp(a) is largely genetically determined at the apo (a) gene. The synthesis and regulation of lipoprotein (a) is not well understood. Lp(a) is assembled in the liver. Apo(a) varies in size due to a variable number of kringle IV protein repeats in the apo(a) gene, known as apo(a) isoforms. Apo(a) size is inversely related to Lp(a) concentration. This is thought to be a result of the longer time it takes to assemble an apo(a) isoform with a large number of kringle repeats.

The metabolism and function of Lp(a) is not well understood. There is homology between apo(a) and plasminogen activator, a thrombotic factor. Lp(a) also transports proinflammatory oxidized phospholipids that recruit inflammatory cells, which has direct inflammatory effects.

The distribution of Lp(a) levels in the population is highly skewed. Epidemiologic data have found an association between Lp(a) levels and ASCVD risk. Lp(a) levels of >50 mg/dL or >30 mg/dL have been identified as cut-points for increased risk. Data suggest Lp(a) level does add incremental information to traditional risk factors, and so might be considered as part of the clinician-patient discussion when deciding whether to initiate drug therapy based on risk.

Lifestyle modifications and most drugs do not influence Lp(a) levels. Niacin and PCSK9 inhibitors reduce Lp(a) levels by about 25% to 35%, but it is not clear whether reducing Lp(a) levels prevents ASCVD events.

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