Lipoprotein Disorders and Cardiovascular Disease

Biochemistry of Lipids

Life requires fats. The biochemistry of lipids and lipoproteins is complex. The clinically relevant lipoproteins ( Table 27.1 ), apolipoproteins ( Table 27.2 ), receptors and processing enzymes ( Table 27.3 ), and current (and potential) drug targets are shown. Biologic lipids usually refer to a broad grouping of naturally occurring molecules that include fatty acids, waxes, eicosanoids, monoglycerides, diglycerides, triglycerides, phospholipids, sphingolipids, sterols, terpenes, prenols, and fat-soluble vitamins (A, D, E, and K), in contrast to the other major groupings of biologic molecules, namely, nucleic acids, proteins, amino acids, and carbohydrates. The major biologic functions of lipids include critical contributions to biologic membranes, energy storage, and the backbones or modifiers of many signaling molecules. Certain lipids, especially fatty acids, readily undergo oxidation and can generate substances highly toxic to cells. Fatty acids can be metabolized in the mitochondrion by beta-oxidation, whereas the sterol nucleus resists enzymatic degradation. Elimination of cholesterol therefore requires excretion as bile acids or conversion into steroidal hormones.

Lipids generally do not dissolve in water. The lipid transport system has evolved in animals to carry hydrophobic molecules (fat) from sites of origin (the intestines and the liver) to sites of use (muscles and rapidly dividing tissues) through the aqueous (water) environment of plasma. Proteins highly conserved through evolution, termed apolipoproteins (apo), mediate this process. Most apolipoproteins derive from an ancestral gene and contain both hydrophilic and hydrophobic domains. This amphipathic structure enables these proteins to bridge the interface between the aqueous environment of plasma and the phospholipid constituents of lipoprotein. The major types of lipids that circulate in plasma include cholesterol and cholesteryl esters, glycerophospholipids, sphingolipids, and glycerolipids (triglycerides) ( Fig. 27.1 ). The LIPID maps (Lipid Metabolites and Pathways Strategy) consortium has provided standardized nomenclature for lipids, although this area is rapidly evolving with advanced lipidomics technologies.

The membranes of mammalian cells and their subcellular organelles require cholesterol . This lipid gives rise to steroid hormones and bile acids and contributes to the integrity of the epidermis. Many cell functions depend critically on membrane cholesterol, and cells regulate tightly their cholesterol content. Importantly, all mammalian cells have retained the ability to synthetize cholesterol de novo from acetyl coenzyme A (CoA). Most of the cholesterol in plasma circulates as cholesteryl esters in the core of lipoprotein particles. The enzyme lecithin-cholesterol acyltransferase (LCAT) forms cholesteryl esters in the blood compartment by transferring a fatty acyl chain from phosphatidylcholine to cholesterol.

Glycerolipids ( triglycerides ) consist of a three-carbon glycerol backbone covalently linked to three fatty acid chains (R _1–3 ). The fatty acid composition varies in terms of chain length and the presence of double bonds (degree of saturation). The highly hydrophobic triglyceride molecules circulate in the core of the lipoprotein. Hydrolysis of triglycerides by lipases generates the free fatty acids (FFAs) used for energy.

Glycerophospholipids , found in all cellular membranes, consist of a glycerol molecule linked to two fatty acids (designated R; see Fig. 27.1 ). Fatty acids differ in length and in the number of double bonds. The third carbon of the glycerol backbone carries a phosphate group linked to one of four molecules: choline (phosphatidylcholine, also called lecithin), ethanolamine (phosphatidylethanolamine), serine (phosphatidylserine), or inositol (phosphatidylinositol). More complex phospholipids include phosphatidylglycerol (cardiolipin is formed by the fusion of two phosphatidylglycerol molecules -antibodies against cardiolipin often occur in systemic lupus), and plasmalogens, an important constituent of eukaryotic membranes. Another phospholipid, sphingomyelin , has special functions in the plasma membrane in the formation of membrane microdomains such as rafts and caveolae. The structure of sphingomyelin resembles that of phosphatidylcholine. The backbone of sphingolipids uses the amino acid serine rather than glycerol. Phospholipids are polar molecules, more water soluble than triglycerides or cholesterol or its esters. Phospholipids participate in signal transduction pathways: hydrolysis by membrane-associated phospholipases generates second messengers, including diacylglycerols, lysophospholipids, phosphatidic acids, and FFAs such as arachidonate, that regulate many cell functions. The phosphorylation of phosphatidylinositol contributes critically to membrane and cell organelle signaling and transport.

Fish oils and eicosapentaenoic acid (EPA) n-3 fatty acids are essential polyunsaturated fatty acids (PUFAs) that are commonly found in plants and marine life. Beyond their triglyceride lowering effects, they have pleiotropic actions with beneficial cardiovascular effects that are key to resolving inflammation and regulating adaptive immunity. n-3 fatty acids derive their name from the first double, which is located on the n-3 position (between the third and fourth carbons from the terminal methyl [omega] end). n-3 metabolites serve as precursors of potent lipid mediators (eicosanoids), and have effects on other lipids (ceramides, lysophosphatidylcholines, diacylglycerols) and amino acids (leucine, glutamine). Moreover, n-3 derived eicosanoids form downstream potent oxylipins which play a critical role in regulating inflammation, vascular tone, and endothelial function. n-3 fatty acids may have favorable antiarrhythmic effects by stabilizing the plasma membrane, metabolic effects through insulin signaling and energy use, and at high doses may also block coagulation. These pleiotropic effects have given rise to the hypothesis that dietary n-3 treatment improves cardiovascular clinical outcomes. However, results of several large randomized clinical trials have been conflicting (see discussion under “ Fish Oils and Pure Eicosapentaenoic Acid”).

PUFAs are metabolized by three pathway enzymes: cyclooxygenase (COX), lipoxygenase (LOX), and CypP450. n-3 FA include EPA (20:5, i.e., 20 carbons in length with 5 double bonds) and docosahexaenoic acid (DHA, 22:6). Alpha-linolenic acid (ALA) is metabolized into EPA, and EPA is modified into DHA. EPA and DHA are oxidized by COX and LOX enzymes to generate eicosanoid subfamilies including prostaglandins, prostacyclins, resolvins, and thromboxanes. Certain n-3 derived eicosanoids have potent antiinflammatory and pro-resolving effects, independently, as well as by inhibiting proinflammatory n-6 PUFA-derived eicosanoids. EPA and DHA-derived eicosanoids compete for and inhibit arachidonic acid eicosanoid production and receptor stimulation (particularly in the COX pathway). EPA can also be metabolized into resolvin E1 which may be cardioprotective, or it can be elongated into the 22-carbon docosapentaenoic acid (DPA) which has resolvin-like antiinflammatory effects and can shift arachidonic acid metabolism from COX to LOX pathway. DHA is thought to be cardioprotective after it is incorporated in cell membranes via mechanisms that improve receptor and ion channel function. Genetic variants in fatty acid desaturases ( FADS ) – enzymes that modulate eicosanoid and other fatty acid levels – and in other genes encoding enzymes involved in eicosanoid metabolism (COX, LOX, and CypP450), also associate with CVD risk. Deciphering the specific roles of n3-derived lipid metabolites in inflammatory and immune signaling would provide insight into their role as therapeutic and nutritional agents.

Lipoproteins, Apolipoproteins, Receptors, and Processing Enzymes

Lipoproteins are complex macromolecular structures coated by a water compatible envelope of phospholipids, free cholesterol, and apolipoproteins covering a hydrophobic core of cholesteryl esters and triglycerides ( Fig. 27.2 ). Lipoproteins vary in size, density in the aqueous environment of plasma, and lipid and apolipoprotein content (see Table 27.1 , Fig. 27.1B , inset ). The classification of lipoproteins reflects their density in plasma (the density of plasma is 1.006 g/mL) as gauged by flotation in an ultracentrifuge. The TRLs, which consist of chylomicrons , chylomicron remnants , very low-density lipoprotein (VLDL) and intermediate density lipoproteins (IDL), have a density of less than 1.006 g/mL. The rest (bottom fraction) of the ultracentrifuged plasma consists of LDL , HDL , and Lp(a) .

Apolipoproteins have four major roles: (1) assembly and secretion of the lipoprotein (apoA-I, B100, and B48), (2) structural integrity of the lipoprotein (apo B, E, A-I, and A-II), (3) coactivators or inhibitors of enzymes (apoA-I, A-V, C-I, C-II, and C-III), and (4) binding or docking to specific receptors and proteins for cellular uptake of the entire particle or selective uptake of a lipid component (apoA-I, B100, and E) (see Table 27.2 ). The role of several apolipoproteins (A-IV, A-V, D, H, J, L, and M) remains incompletely understood.

Many proteins regulate the synthesis, secretion, and metabolic fate of lipoproteins; their characterization has provided insight into molecular cellular physiology and targets for drug development (see Table 27.3 ). Discovery of the LDL receptor (LDL-R) represented a landmark in understanding cholesterol metabolism and receptor-mediated endocytosis. The LDL-R regulates the entry of cholesterol into cells, and tight control mechanisms alter its expression on the cell surface, depending on intracellular cholesterol. The LDL-R belongs to a superfamily of membrane receptors that include LDL-R, VLDL-R, LDL-R–mediated peptide type 1 (LRP1; apo E receptor), LRP1B, LRP4 (MGEF7), LRP5 and LRP6 (involved in the process of bone formation), LRP8 (apo E receptor-2), and LRP9. LRP1, which mediates the uptake of chylomicron remnants and VLDL, preferentially recognizes apo E. LRP1 also interacts with hepatic lipase. The complex interaction between hepatocytes and the various lipoproteins containing apo E involves cell surface proteoglycans that provide scaffolding for lipolytic enzymes (lipoprotein lipase [LPL] and hepatic lipase) involved in recognition of remnant lipoproteins. Macrophages express receptors that bind modified (especially oxidized) lipoproteins. These scavenger lipoprotein receptors mediate the uptake of oxidatively modified LDL into macrophages. In contrast to the exquisitely regulated LDL-R, high cellular cholesterol content does not suppress scavenger receptors, thereby enabling intimal macrophages to accumulate abundant cholesterol, become foam cells, and form fatty streaks. Sterol accumulation in the endoplasmic reticulum may lead to cell apoptosis via the unfolded protein response. Endothelial cells can also take up modified lipoproteins through specific receptors such as the oxidized LDL-R, LOX-1.

At least three physiologically relevant receptors interact with HDL particles: the scavenger receptor class B (SR-B1) and the adenosine triphosphate (ATP)-binding cassette transporters A1 (ABCA1) and G1 (ABCG1). SR-B1 is a receptor for HDL (also for LDL and VLDL, but with less affinity). SR-B1 mediates the selective uptake of HDL cholesteryl esters in steroidogenic tissues, hepatocytes, and endothelium. ABCA1 mediates cellular phospholipid (and possibly cholesterol) efflux and HDL formation. The ABCG1 transporter transfers cellular cholesterol to already formed HDL particles.

Lipoprotein Metabolism and Transport

The lipoprotein transport system has two major roles: efficient transport of triglycerides from the intestine and liver to sites of utilization (fat tissue or muscle), and transport of cholesterol to peripheral tissues for membrane synthesis and steroid hormone production or to the liver for bile acid synthesis.

Intestinal Pathway (Chylomicrons To Chylomicron Remnants)

Fat typically furnishes 20% to 40% of the daily calories. Triglycerides account for the major portion of ingested fats. For an individual consuming 2000 kcal/day, with 30% in the form of fat, this represents approximately 66 g of triglycerides and 250 mg (0.250 g) of cholesterol per day. The intestine has very efficient fat absorption mechanisms, probably evolved to maximize provision of the organism with nutrients under circumstances of limited or irregular availability of food.

On ingestion, lingual and pancreatic lipases hydrolyze triglycerides into FFAs and monoglycerides or diglycerides. Emulsification by bile salts leads to the formation of intestinal micelles. Micelles resemble lipoproteins insofar as they consist of phospholipids, free cholesterol, bile acids, diglycerides and monoglycerides, FFAs, and glycerol. The mechanism of micelle uptake by intestinal brush border cells still engenders debate. The Niemann-Pick C1-like 1 (NPC1L1) protein is part of an intestinal cholesterol transporter complex and the target for the selective cholesterol absorption inhibitor ezetimibe. After uptake into intestinal cells, fatty acids undergo re-esterification to form triglycerides and packaging into chylomicrons inside the intestinal cell and enter the portal circulation ( Fig. 27.3 , part 1 ). Chylomicrons contain apo B48, the amino-terminal component of apo B100. In the intestine, the apo B gene is modified during transcription into mRNA by substitution of a uracil for a cytosine via an apo B48–editing enzyme complex (ApoBec). This mechanism involves a cytosine deaminase and leads to a termination codon at residue 2153 and a truncated form of apo B. Only intestinal cells express ApoBec. Apo B48 does not bind to LDL-R. Intestinal cells absorb plant sterols (sitosterol, campesterol), sort these compounds into a separate cellular compartment, and re-secrete them into the intestinal lumen via the ABCG5/8 heterodimeric transporter. Mutations of the ABCG5/8 genes cause the rare disorder sitosterolemia.

Chylomicrons rapidly enter the plasma compartment after meals. In capillaries of adipose tissue or muscle cells in the peripheral circulation, chylomicrons encounter the enzyme LPL attached to heparan sulfate proteoglycans on the luminal surface of endothelial cells (see Fig. 27.3 , part 2 ). Apo C-II and apo A-V activate, and apo C-III inhibits LPL activity. LPL has broad specificity for triglycerides; it cleaves all fatty acyl residues attached to glycerol and in the process generates three molecules of FFA for each molecule of glycerol. Muscle cells rapidly take up fatty acids. Fatty acids provide the energy substrate for muscle contraction by the generation of ATP during beta-oxidation of fatty acyl residues in mitochondria. Adipose cells can store triglycerides made from fatty acids for energy utilization, a process that requires insulin. The triglyceride lipase hormone–sensitive lipase, that is activated by cyclic adenosine monophosphate (cAMP) in response to stress, releases stored fatty acids from adipose tissues. Fatty acids can also travel to the liver bound to fatty acid–binding proteins or albumin and undergo repackaging into VLDL. Peripheral resistance to insulin can thus increase the delivery of FFAs to the liver with a consequent increase in VLDL secretion and increased apo B particles in plasma, a characteristic of the “metabolic syndrome” and type 2 diabetes. The remnant particles, derived from chylomicrons following LPL action, contain apo E and enter the liver for degradation and reutilization of their core constituents (see Fig. 27.3 , part 3).

Hepatic Pathway (Very Low-Density Lipoprotein to Intermediate-Density Lipoprotein)

Food is not always available, and dietary fat content varies. The body requires ready availability of triglyceride to meet energy demands. Hepatic secretion of VLDL particles serves this function (see Fig. 27.3 , part 4 ). VLDLs are TRLs smaller than chylomicrons (see Table 27.1 and Fig. 27.2 ). They contain apo B100 as their main lipoprotein. As opposed to apo B48, apo B100 contains a domain recognized by LDL-R (the apo B/E receptor). VLDL particles follow the same catabolic pathway as chylomicrons through LPL (see Fig. 27.3 , part 2 ). During hydrolysis of TRLs by LPL, an exchange of proteins and lipids takes place: VLDL particles (and chylomicrons) acquire apo Cs and apo E, in part from HDL particles. VLDLs also exchange triglycerides for cholesteryl esters from HDL (mediated by cholesteryl ester transfer protein [CETP]) (see Fig. 27.3 , part 9). Such bidirectional transfer of constituents between lipoproteins serves several purposes: acquisition of specific apolipoproteins by lipoproteins that will dictate their metabolic fate, transfer of phospholipids onto nascent HDL particles mediated by phospholipid transfer protein (PLTP) (during loss of the core triglycerides, the phospholipid envelope becomes redundant and sheds apoA-I to form new HDL particles), and transfer of cholesterol from HDL to VLDL remnants so that it can be metabolized in the liver. This exchange constitutes a major part of the “reverse cholesterol transport pathway.”

Apo CIII, a small but important 79 amino acid peptide has a high affinity for TRLs and attenuates the activity of LPL and the clearance of TRLs, thus contributing to elevated triglycerides. Apo CIII also resides within HDL that seems to act as a “reservoir” for this apolipoprotein. Recent work identified an intracellular role for apo CIII for the assembly and secretion of VLDL. Mendelian randomization experiments as well as epidemiologic studies have established that apo CIII can contribute causally to ASCVD. This recognition has spurred therapeutic efforts to decrease apo CIII (see below).

After hydrolysis of triglycerides removes some triglycerides from VLDL, these particles have relatively more cholesterol, shed several apolipoproteins (especially the C apolipoproteins), and acquire apo E. The VLDL remnant lipoprotein, called intermediate-density lipoprotein (IDL), undergoes liver uptake via its apo E moiety (see Fig. 27.3 , part 3 ) or further delipidation by hepatic lipase to form LDL particles (see Fig. 27.3 , part 6 ). At least four receptors take up TRLs, TRL remnants, and apo B–containing lipoproteins: VLDL-R, the remnant receptor (apo ER2), LDL-R (also called the apo B/E receptor), and LRP1. Most hepatic receptors share the ability to recognize apo E, an engagement that mediates the uptake of several classes of lipoproteins, including VLDL and IDL. The complex interaction between apo E and its ligand involves the “docking” of TRLs on heparan sulfate proteoglycans to present the ligand to its receptor.

Low-Density Lipoproteins

LDL particles contain predominantly cholesteryl esters packaged with apo B100. Normally, triglycerides constitute only 4% to 8% of the LDL mass (see Table 27.1 ). In conditions with elevated plasma triglyceride concentrations, LDL particles can acquire triglycerides and deplete their core cholesteryl esters. Such changes in core constituents influence LDL particle size: an increase in triglycerides and a relative decrease in cholesteryl esters yields smaller, denser LDL particles.

Humans are unusual among mammals because they generate LDL as a cholesterol-rich lipoprotein. Nonhuman primates fed a cholesterol-enriched diet also carry cholesterol in LDL. In other mammals, such as rodents or rabbits, HDL particles transport most of the cholesterol. Cells can either make cholesterol from acyl CoA through enzymatic reactions requiring at least 33 enzymatic steps or obtain it as cholesteryl esters from HDL or LDL particles. Cells internalize LDL via LDL-R ( Fig. 27.4 ). LDL particles contain one molecule of apo B. Although several highly lipophilic domains of apo B associate with phospholipids, a region surrounding residue 3500 binds with high affinity to LDL-R. LDL-R localizes in a region of the plasma membrane rich in the protein clathrin (see Fig. 27.4 ; Fig. 27.3 , part 7 ). Once bound to the receptor, clathrin polymerizes and forms an endosome that contains LDL bound to its receptor, a portion of the plasma membrane, and clathrin. This internalized particle then fuses with lysosomes whose hydrolytic enzymes (cholesteryl ester hydrolase, cathepsins) release free cholesterol and degrade apo B. LDL-R releases its ligand and can recycle to the plasma membrane. The chaperone proprotein convertase subtilisin/kexin type 9 (PCSK9), secreted by hepatocytes, undergoes auto-catalytic cleavage and binds to the LDL-R. Association with PCSK9 diverts the complex to the lysosomal degradative pathway, thus preventing the recycling of the LDL-R ( Fig. 27.4 ). Gain-of-function mutations in the PCSK9 gene causes autosomal dominant hypercholesterolemia, whereas loss-of-function mutations increase LDL-R and lower LDL-C substantially. ^,

Cells regulate their cholesterol content tightly through highly conserved cellular pathways, including: (1) synthesis of cholesterol in the smooth endoplasmic reticulum (via the rate-limiting step hydroxymethylglutaryl-CoA [HMG-CoA] reductase), (2) receptor-mediated endocytosis of LDL (two mechanisms under the control of steroid-responsive element binding protein-2 [SREBP-2]), (3) efflux of cholesterol from the plasma membrane to cholesterol acceptor particles (predominantly apoA-I and HDL) via the ABCA1and ABCG1 transporters, and (4) intracellular cholesterol esterification via the enzyme acetyl-CoA acetyltransferase (ACAT). SREBP-2 coordinately regulates the first two pathways at the level of gene transcription. Cellular cholesterol binds to SCAP (SREPB cholesterol-activated protein), which localizes on the endoplasmic reticulum. Cholesterol inhibits the interaction of SCAP with SREPB. In the absence of cholesterol, SCAP mediates the cleavage of SREBP at two sites by specific proteases with the release of an amino-terminal fragment of SREBP. This SREBP fragment migrates to the nucleus and increases the transcriptional activity of genes involved in cellular cholesterol and fatty acid homeostasis. The ACAT pathway regulates the cholesterol content in membranes. Humans express two separate forms of ACAT. ACAT1 and ACAT2 derive from different genes and mediate cholesterol esterification in cytoplasm and in the endoplasmic reticulum lumen for lipoprotein assembly and secretion.

Definitions

Time and new knowledge have stimulated changes in the classification of lipoprotein disorders. The original classification of lipoprotein disorders by Fredrickson, Lees, and Levy (1967) which depended on analysis of lipoprotein patterns by ultracentrifugation or electrophoresis has fallen into disuse (see prior editions for details). Most clinicians now classify lipoprotein disorders by which specific lipoprotein lipid is elevated and, when sufficiently characterized, by the genetic defect, e.g., familial hypercholesterolemia (FH). For example, a young patient with eruptive xanthomas and a plasma triglyceride level of 22 mmol/L (2000 mg/dL) probably has familial hyperchylomicronemia as a result of LPL deficiency or other monogenic defects. A 38-year-old woman with a strong family history of ASCVD, tendinous xanthomas, and an untreated LDL-C of 240 mg/dL (6.4 mmol/L) likely has heterozygous familial hypercholesterolemia (HeFH). An obese, hypertensive middle-aged man with a cholesterol level of 6.4 mmol/L (245 mg/dL), a triglyceride level of 3.1 mmol/L (274 mg/dL), an HDL-C level of 0.8 mmol/L (31 mg/dL), and a calculated LDL-C level of 4.2 mmol/L (162 mg/dL) probably has metabolic syndrome, and this should trigger the clinician to seek other components of this cluster, including poor lifestyle, hypertension, and hyperglycemia. Conversely, an obese middle-aged man with a plasma triglyceride level of 7 mmol/L (620 mg/dL) probably has mutations in several genes associated with plasma triglyceride levels.

The clinical usefulness of apolipoprotein levels has stirred debate (see Chapter 25 ). Taken as a single measurement, the apo B level provides information on the number of potentially atherogenic particles and can be used as a goal of lipid-lowering therapy. Similarly, LDL particle size correlates highly with plasma HDL-C and triglyceride levels, and most studies do not show it to be an independent cardiovascular risk factor in particular after adjusting for apo B or LDL particle concentration. Small, dense LDL particles tend to track with features of metabolic syndrome, which usually involves dyslipoproteinemia with elevated plasma triglyceride and reduced HDL-C levels. While there is some debate on the value of apo B or non-HDL-C as a better predictive biomarker of ASCVD, the Emerging Risk Factors Collaboration studies and the UK Biobank (comprising 346,686 participants) have shown that measurement of non–HDL-C is equivalent to measurement of apo B in determination of cardiovascular risk. The measurement of apoB remains a useful tool for cardiovascular risk prediction, especially in the primary prevention setting in the presence of features of the metabolic syndrome. Similarly, HDL-C tracks as well with CVD risk as apoA-I does.

Genetic Lipoprotein Disorders

Understanding of the genetics of lipoprotein metabolism has expanded rapidly (see Chapter 7 ). Classification of genetic lipoprotein disorders usually requires a biochemical phenotype in addition to a clinical phenotype. With the exception of FH, monogenic disorders tend to be very rare and constitute “orphan” diseases. Disorders considered heritable on careful family study may be difficult to characterize unambiguously because of age, sex, penetrance, and gene-gene and environmental interactions. Most common lipoprotein disorders encountered clinically result from the interaction of increasing age, lack of physical exercise, weight gain, and a suboptimal diet with individual genetic makeup. Genetic lipoprotein disorders can either raise or lower levels of LDL, Lp(a), remnant lipoproteins, TRLs (chylomicrons and VLDL), or HDL ( Table 27.4 ).

Triglyceride-Rich Lipoproteins (see also Chapter 25 )

TRLs are circulating apo B-carrying particles that carry predominantly hydrophobic triglycerides but also contain cholesterol ester, and include chylomicrons, VLDL, IDL, and remnant particles. In the fasting state, triglycerides are carried predominantly by VLDL particles and their remnants, while in the fed state triglycerides are additionally carried by chylomicrons and their remnants. Plasma triglycerides may arise from a higher TRL particle number (reflected by increased apo B, since each TRL carries one apoB moiety on it) and/or due to greater particle size. Postprandial elevations in triglycerides and TRLs, often due to underlying abnormalities in TRL metabolism, may not be captured in the fasting state, hence guidelines have shifted to measuring nonfasting or fasting lipids for general risk screening or assessment. Population studies have shown that nonfasting triglycerides are equally or even more strongly associated with cardiovascular endpoints than fasting levels.

Nonfasting triglycerides are higher than fasting levels (by approximately 0.3 mmol/L or 27 mg/dL), and guidelines consider nonfasting triglycerides ≥2 mmol/L (175 mg/dL) as abnormal, while for fasting triglycerides the corresponding level is ≥1.7 mmol/L (150 mg/dL). The 2018 American Heart Association/American College of Cardiology (AHA/ACC) cholesterol guideline and the 2019 AHA/ACC prevention guideline consider fasting or nonfasting triglycerides greater than 175 mg/dL as a risk enhancing factor that could prompt consideration for initiating or intensifying statin therapy. There are some differences in the guideline cutpoints for severe hypertriglyceridemia, defined as fasting triglycerides greater than 10 mmol/L (885 mg/dL) in European guidelines or ≥500 mg/dL (5.7 mmol/L) in US guidelines. Because the triglyceride to cholesterol ratio in TRLs progressively increases as the hypertriglyceridemia becomes more severe, the Friedewald equation (which assumes a fixed triglyceride to cholesterol ratio of triglycerides/5 [mg/dL] or triglycerides/2.2 [mmol/L]) to calculate LDL-C is inaccurate when triglycerides are greater than 4.5 mmol/L (400 mg/dL), as it underestimates the true LDL-C at high triglyceride levels. Instead, it is preferable to use non-HDL-C or apo B instead of calculated LDL-C for LDL-related treatment decisions in patients with hypertriglyceridemia, as direct LDL-C assays may also be inaccurate. ^,

Hypertriglyceridemia correlates with risk of cardiovascular disease, and postprandial increase of TRL particles is an important factor in atherogenesis. However, it is less clear if triglycerides directly contribute to atherogenesis, or whether triglycerides are a marker of another atherogenic moiety of TRLs, such as the cholesterol (remnant/VLDL cholesterol) or apolipoproteins (e.g., apo B, apo CIII) that are also carried by these particles. Like LDL particles, all TRL particles carry one apo B per particle. While some studies found independent associations for triglycerides with cardiovascular disease risk, a meta-analysis of 68 prospective studies (>300,000 individuals) found that the association of triglycerides with cardiovascular risk was lost after adjusting for non-HDL-C or apo B. Genetic studies suggest a causal association between plasma triglycerides and cardiovascular risk, but many genetic variants are pleotropic and often also associate with differences in VLDL/remnant cholesterol, LDL-C, apo B, Lp(a), or HDL-C, making it challenging to identify the causal atherogenic moiety of TRLs. In a Mendelian randomization analysis of genetic scores composed of triglyceride-lowering variants in the LPL gene and LDL-C lowering variants in the LDLR gene, both sets of variants associated with similar risk of coronary disease per unit difference in apo B, suggesting that genetic variants that affect apoB are similarly atherogenic, regardless of differences in LDL-C or triglycerides. In a meta-analysis of randomized trials ( N = 374,358 from 25 statin trials and 24 nonstatin trials), triglyceride lowering associated with lower cardiovascular risk (approximately 15% lower risk per 1 mmol/L reduction in triglycerides), which was somewhat lower than for LDL-C (approximately 20% lower risk per 1 mmol/L reduction in LDL-C) and attenuated when the Reduction of Cardiovascular Events with Icosapent Ethyl-Intervention Trial (REDUCE-IT) was excluded.

Clearance of TRL particles requires LPL, hepatic lipase, apoE and other catalytically active apolipoproteins, and functional LDL receptors and LRP1. Enzymatic hydrolysis of TRLs by LPL and acquisition of cholesteryl esters from HDL by CETP results in the formation of cholesterol-enriched remnant particles that are depleted of part of their triglyceride content (see Fig. 27.3 , parts 2, 3, and 9 ). However, defective TRL clearance alone is often not sufficient (except in rare monogenic cases), and overproduction of TRL is usually necessary for hypertriglyceridemia.

Severe hypertriglyceridemia can result from poorly controlled diabetes or from genetic disorders (polygenic or monogenic) of the processing enzymes or apolipoproteins, often in the context of secondary factors. Severe hypertriglyceridemia is often due to polygenic and nongenetic determinants, often exacerbated in the presence of secondary nongenetic factors, and less commonly due to monogenic conditions. ^, In patients with insulin resistance, metabolic syndrome, or diabetes, elevation of plasma triglyceride levels is usually mild to moderate and occurs most often in the presence of visceral (abdominal) obesity and a diet rich in calories, carbohydrates, and saturated fats. It is now recognized that the genetic basis for severe hypertriglyceridemia is multifactorial due to the combination of poor lifestyle on a background susceptibility of multiple genetic defects each of which raises triglycerides by only a fraction of a mmol/L, the cumulative sum of which produces a clinical phenotype (e.g., familial hypertriglyceridemia, familial combined hyperlipidemia, or type III dyslipidemia). Thus, hypertriglyceridemia is best viewed as a complex disorder arising from the interaction of genetic and nongenetic factors. Primary causes of hypertriglyceridemia ( Table 27.5 ) include the rare monogenic syndromes (homozygous or compound heterozygote mutations) in several canonical genes related to LPL, and the much more common polygenic or multifactorial cases, which could include contributions from rare heterozygote variants in these canonical genes and/or other common variants associated with elevated triglycerides. ^, Genetic screening of siblings of patients with monogenic disorders is suggested.

High-Density Lipoproteins

Reduced plasma levels of HDL-C consistently correlate with the development or presence of ASCVD. Most cases of reduced HDL-C accompany elevated plasma triglycerides and often keep company with other features of metabolic syndrome. Genetic disorders of HDL can result from decreased production or abnormal maturation and increased catabolism. Genetic lipoprotein disorders leading to moderate to severe elevations in plasma triglycerides cause a reduction in HDL-C levels. Monogenic hyperchylomicronemia, polygenic hypertriglyceridemia, and combined hyperlipoproteinemia all associate with reduced HDL-C levels. Plasma triglyceride and HDL-C levels vary inversely. Several mechanisms contribute to this association: (1) decreased lipolysis of TRLs decreases the availability of substrate (phospholipids) for HDL maturation, (2) HDL enriched with triglyceride has an increased catabolic rate and hence reduced plasma concentration, and (3) the augmented pool of TRLs saps cholesterol from the HDL compartment by CETP-mediated exchange.