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 in that 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 (see later).⁷ 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. 45-4, path 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 resecrete 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 LPL, an enzyme attached to heparan sulfate proteoglycans, and present on the luminal surface of endothelial cells (Fig. 45-4, path 2). LPL activity is modulated by apo C-II and apo A-V (activators) and by apo C-III (an inhibitor). 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. Conversely, hormone-sensitive lipase is a triglyceride lipase that is activated by cyclic adenosine monophosphate (cAMP) in response to stress and releases fatty acids from adipose tissues. Fatty acids can also bind to fatty acid–binding proteins and albumin and travel to the liver, where they are repackaged in 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. As discussed later, this is one of the consequences of 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 (Fig. 45-4, path 3).

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

Food is not always available, and dietary fat content varies. The body must ensure that triglyceride is readily available to meet energy demands. Hepatic secretion of VLDL particles serves this function (Fig. 45-4, path 4). VLDLs are TRLs smaller than chylomicrons (see Table 45-1 and Fig. 45-3). 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 through LPL as chylomicrons do (see Fig. 45-4, path 2). During hydrolysis of TRLs by LPL, an exchange of proteins and lipids takes place: VLDL particles (and chylomicrons) acquire apo C’s and apo E, in part from HDL particles. VLDLs also exchange triglycerides for cholesteryl esters from HDL (mediated by cholesteryl ester transfer protein [CETP]) (Fig. 45-4, path 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 apo A-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.”

After hydrolysis of triglycerides partly depletes VLDL of triglycerides, VLDL 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), is taken up by the liver via its apo E moiety (see Fig. 45-4, path 3) or is further delipidated by hepatic lipase to form an LDL particle (Fig. 45-4, path 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.⁵ The interaction between apo E and its ligand is complex and involves the “docking” of TRLs on heparan sulfate proteoglycans before presentation of the ligand to its receptor.

Low-Density Lipoproteins

LDL particles contain predominantly cholesteryl esters packaged with the protein moiety apo B100. Normally, triglycerides constitute only 4% to 8% of the LDL mass (see Table 45-1). In the presence of elevated plasma triglyceride levels, LDL particles can become enriched in triglycerides and depleted in core cholesteryl esters. Variation in LDL particle size results from changes in core constituents, with an increase in triglycerides and a relative decrease in cholesteryl esters leading to smaller, denser LDL particles.

Humans are unusual among mammals in that they use LDL as a major cholesterol transporter. Nonhuman primates fed a cholesterol-enriched diet also carry cholesterol in LDL. In other mammals, such as rodents or rabbits, VLDL carries triglycerides, and HDL particles transport most of the cholesterol. Cells can either make cholesterol from acyl coenzyme A (CoA) through enzymatic reactions requiring at least 33 steps, or obtain it as cholesteryl esters from HDL and LDL particles. Cells internalize LDL via LDL-R (Fig. 45-5A). LDL particles contain one molecule of apo B. Although several domains of apo B are highly lipophilic and associated with phospholipids, a region surrounding residue 3500 binds with high affinity to LDL-R. LDL-R is localized in a region of the plasma membrane rich in the protein clathrin (Fig. 45-5A; also Fig. 45-4, path 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 will detach itself from its ligand and recycle to the plasma membrane.

Cells tightly regulate their cholesterol content by (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 apo A-I and HDL) via the ABCA1 and ABCG1 transporters, and (4) intracellular cholesterol esterification via the enzyme acetyl-CoA acetyltransferase (ACAT) (Fig. 45-5A-C). SREBP-2 coordinately regulates the first two pathways at the level of gene transcription. Cellular cholesterol binds to SCAP (SREPB cholesterol-activated protein), which is localized 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 (NH₂) fragment of SREBP. The SREBP NH₂ fragment migrates to the nucleus and increases the transcriptional activity of genes involved in cellular cholesterol and fatty acid homeostasis. Cleavage of SREBP depends on a proprotein convertase related to the subtilisin/kexin family of convertases. Another member of the convertase superfamily, proprotein convertase subtilisin/kexin 9 (PCSK9), regulates the internalization and cellular processing of LDL-R; gain-of-function mutations in this gene cause autosomal dominant hypercholesterolemia, whereas loss-of-function mutations increase LDL-R and lower LDL-C significantly.⁸ The ACAT pathway regulates the cholesterol content in membranes. Humans express two separate forms of ACAT. ACAT1 and ACAT2 are derived from different genes and mediate cholesterol esterification in cytoplasm and in the endoplasmic reticulum lumen for lipoprotein assembly and secretion.

Regulation of cholesterol efflux from cells depends in part on the ABCA1 pathway, controlled in turn by hydroxysterols (especially 24- and 27-OH cholesterol, which act as ligands for the liver-specific receptor [LXR] family of transcriptional regulatory factors). In conditions of cholesterol sufficiency, the cell can decrease its input of cholesterol by decreasing the de novo synthesis of cholesterol. The cell can also decrease the amount of cholesterol that enters the cell via the LDL-R, thereby increasing the amount stored as cholesteryl esters, and promote the removal of cholesterol by increasing its movement to the plasma membrane for efflux.

High-Density Lipoprotein and Reverse Cholesterol Transport

Epidemiologic studies have consistently shown an inverse relationship between plasma levels of HDL-C and the presence of CAD. HDL promotes reverse cholesterol transport and can prevent lipoprotein oxidation, exerts anti-inflammatory actions in vitro, and promotes cell proliferation and survival.⁹ In vitro, HDL has potent effects on vascular endothelial cells. It promotes the production of nitric oxide (NO) through several mechanisms and prevents the expression of vascular endothelial cell inflammatory mediators via modulation of nuclear factor kappa B (NF-κB). Mendelian randomization analyses have cast doubt on HDL’s causality as a protective cardiovascular risk factor. Mutations of the genes for ABCA1 (causing HDL deficiency) do not impart additional cardiovascular risk, and conversely, genetic polymorphisms of genes that increase HDL-C are not associated with protection from cardiovascular events.¹⁰

HDL has a complex and incompletely understood metabolism. The complexity arises because HDL particles acquire their components from several sources and these components undergo metabolism at different sites. In addition, steady-state levels of HDL in plasma may reflect the dynamic state of HDL-mediated cholesterol trafficking, in contrast to the situation with LDL. The intestine and liver synthesize apo A-I, the main protein of HDL. Approximately 80% of HDL originates from the liver and 20% from the intestine (Fig 45-4, path 5). Lipid-free apo A-I acquires phospholipids from cell membranes and from redundant phospholipids shed during the hydrolysis of TRLs. Lipid-free apo A-I binds to ABCA1 and promotes its phosphorylation via cAMP, which increases the net efflux of phospholipids and cholesterol onto apo A-I to form a nascent HDL particle (see Fig. 45-4, path 5). This particle contains apo A-I, phospholipids, and some free cholesterol (Fig. 45-5C).¹¹ These nascent HDL particles will mediate further cellular efflux of cholesterol. Currently, standard laboratory tests do not measure these HDL precursors because they contain little or no cholesterol. On reaching a cell membrane, the nascent HDL particles capture membrane-associated cholesterol and promote the efflux of free cholesterol onto other HDL particles (Fig. 45-4, path 0). Conceptually, the formation of HDL particles appears to involve two steps. The first step involves binding of HDL apo A-I to ABCA1 and generation of a specific membrane microdomain that allows the subsequent lipidation of apo A-I.¹² Efflux of cellular cholesterol from peripheral cells, such as macrophages, does not contribute importantly to overall HDL-C mass but may have an important effect on export of cholesterol from atheromas. Macrophages can transfer cholesterol to apo A-I and apo E, to nascent discoid or ellipsoid HDL particles via the ABCA1 transporter, or to more mature spherical HDL particles via the ABCG1 transporter (Fig 45-5D). The ABCG1 transporter does not promote cellular cholesterol efflux to lipid-free or lipid-poor apo A-I, but rather to mature HDL particles. HDL-mediated cellular cholesterol can be measured in plasma and is altered in many disease states, including diabetes and CAD.¹³ The plasma enzyme LCAT, an enzyme activated by apo A-I, then esterifies the free cholesterol (Fig. 45-4, path 8, and see Fig. 45-5C). LCAT transfers an acyl chain (a fatty acid) from the R₂ position of a phospholipid to the 3′-OH residue of cholesterol, which results in the formation of a cholesteryl ester (see Fig. 45-1). In a process called selective uptake of cholesterol, HDL also provides cholesterol to steroid hormone–producing tissues and the liver through the scavenger receptor SR-B1 (see Fig. 45-5B).

Because of their hydrophobicity, cholesteryl esters move to the core of the lipoprotein, and the HDL particle now assumes a spherical configuration (a particle denoted HDL₃). With further cholesterol esterification, the HDL particle increases in size to become the more buoyant HDL₂. The cholesterol within HDL particles can be exchanged with TRLs via CETP, which mediates an equimolar exchange of cholesterol from HDL to TRL and movement of triglyceride from TRL onto HDL (see Fig. 45-4, path 9). Inhibition of CETP increases HDL-C in blood and has undergone exploration as a therapeutic target for prevention of CVD. PLTP mediates the transfer of phospholipids between TRL and HDL particles. Triglyceride-enriched HDLs are denoted HDL_2b. Hepatic lipase can hydrolyze triglycerides and endothelial lipase can hydrolyze phospholipids within these particles and thereby convert them back to HDL₃ particles.

Reverse cholesterol transport involves the uptake of cellular cholesterol from extrahepatic sources, such as lipid-laden macrophages, and its esterification by LCAT, transport by large HDL particles, and exchange for one triglyceride molecule by CETP. Hepatic receptors can now take up the cholesterol molecule originally on an HDL particle and residing in a TRL or LDL particle after this exchange. HDL particles therefore act as shuttles between tissue cholesterol, TRL, and the liver.

Reverse cholesterol transport by HDL constitutes a small but potentially important portion of the plasma HDL mass. Indeed, selective inactivation of macrophage ABCA1 does not change HDL-C levels in mice but increases atherosclerosis. The catabolism of HDL particles has engendered debate among lipoprotein researchers. The protein component of HDL particles is exchangeable with lipoproteins of other classes. The kidneys appear to be a route of elimination of apo A-I and other HDL apolipoproteins. The lipid components of HDL particles also follow a different metabolic route (Fig. 45-5B-D).