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The major neutral lipids transported through the blood stream, triglycerides and cholesteryl esters, are insoluble in aqueous solutions and therefore must be protected from plasma by a coating of amphipathic (both hydrophobic and hydrophilic) molecules. Lipoproteins are macromolecular complexes carrying various lipids and proteins in plasma that provide such protection to triglycerides and cholesteryl esters. The hydrophobic triglyceride and cholesteryl ester molecules comprise the core of the lipoproteins and are enveloped by an amphipathic monolayer of phospholipids, free cholesterol, and proteins. The proteins, called apoproteins (or apolipoproteins), are critical regulators of lipid transport. Although the lipoproteins comprise a continuum of particles differing gradually in density and in lipid and apoprotein composition, there are accumulations of relatively distinct subclasses that can be isolated by various physical methods. Thus, several major classes of lipoproteins have been defined by their physical-chemical characteristics
: chylomicrons, very-low-density lipoprotein (VLDL) intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL). The physical-chemical characteristics of the major lipoprotein classes are presented in Table 1.
Table 1PHYSICAL-CHEMICAL CHARACTERISTICS OF THE MAJOR LIPOPROTEIN CLASSES
The major lipid in chylomicrons and VLDL is triglyceride. Triglycerides serve as energy substrates in the liver and peripheral tissues, particularly muscle. Excess energy is stored as triglyceride in adipose tissue. Lipoprotein triglyceride synthesis occurs in the small intestine and liver. Once in the plasma, the majority of triglyceride molecules in chylomicrons and VLDL are hydrolyzed by the actions of lipoprotein lipase (LPL) and hepatic lipase (HL). The fatty acids released by these reactions are taken up by liver, muscle, and adipose tissue. Triglycerides in VLDL and chylomicrons can also be exchanged for cholesteryl esters in LDL and HDL. This bimolecular exchange is mediated by cholesteryl ester transfer protein (CETP).
Cholesterol
Cholesterol, which serves as a component of cell membranes and as a precursor of adrenal and gonadal steroids and hepatic bile acids, is the major core lipid of LDL and HDL. VLDL and chylomicron remnants have the potential to carry significant quantities of cholesterol as well. Cholesterol is synthesized in most tissues; the rate-limiting enzyme is HMGCoA reductase, an enzyme that is regulated both transcriptionally and postranslationally. Transcriptional regulation is via sterol effects on a cytosolic transcription factor, sterol response element binding protein (SREBP).
In lipoproteins, cholesterol is carried mainly as cholesteryl ester in the core of the lipoprotein particle. A small proportion of lipoprotein cholesterol is carried as free cholesterol in the surface monolayer of the lipoprotein particle. Free cholesterol on the lipoprotein surface can be esterified in plasma by the enzyme lecithin cholesterol acyl transferase (LCAT). This allows cholesterol to move from the surface into the core of the particle.
Phospholipids
The vast majority of the surface of lipoproteins is made of phospholipids that form monolayers that act as interfaces with both the polar plasma components and the nonpolar lipids of the lipoprotein core. Lecithin (phosphatidylcholine) is the major phospholipid in plasma and is the source of linoleate for cholesteryl ester formation by the LCAT reaction. Severe phospholipid deficiency can result in reduced assembly and secretion of VLDL from the liver.
Apoproteins
The ten major apoproteins (apos) are listed in Table 2. Found on the surface of lipoproteins, apos provide structural stability and have critical roles in regulating lipoprotein metabolism. Some apos act as cofactors for plasma lipid-modifying enzymes.
Table 2CHARACTERISTICS OF THE MAJOR APOLIPOPROTEINS
Apolipoprotein
Molecular Weight
Lipoproteins
Metabolic Functions
apo A-I
28,016
HDL, chylomicrons
Structural component of HDL; LCAT activator
apo A-II
17,414
HDL, chylomicrons
Unknown
apo A-IV
46,465
HDL, chylomicrons
Unknown; possibly facilitates transfer of other apos between HDL and chylomicrons
apo B-48
264,000
Chylomicrons
Necessary for assembly and secretion of chylomicrons from the small intestine
apo B-100
540,000
VLDL, IDL, LDL
Necessary for assembly and secretion of VLDL from the liver; structural protein of VLDL, IDL, LDL; ligand for LDL receptor
apo C-I
6630
Chylomicrons, VLDL, IDL, HDL
May inhibit hepatic uptake of chylomicron and VLDL remnants
apo C-II
8900
Chylomicrons, VLDL, IDL, HDL
Activator of lipoprotein lipase
apo C-III
8800
Chylomicrons, VLDL, IDL, HDL
Inhibitor of lipoprotein lipase; may inhibit hepatic uptake of chylomicron and VLDL remnants
apo E
34,145
Chylomicrons, VLDL, IDL, HDL
Ligand for binding of several lipoproteins to the LDL receptor and possibly to the LDL receptor-related receptor
apo(a)
250,000–800,000
Lp(a)
Composed of LDL with apo B covalently linked to apo(a); function not defined but is an independent predictor of coronary artery disease
Apo B100 (hereafter referred to as either apo B100 or simply apo B) is a large hydrophobic protein of 4563 amino acids with a molecular weight of approximately 540 kD that is synthesized to significant degrees in the liver. It is the major apo of VLDL, IDL, and LDL, comprising approximately 30%, 60%, and 95% of the protein in these respective lipoproteins. Apo B contains both hydrophobic lipid-binding regions, which probably participate in the assembly of nascent lipoprotein complexes, as well as hydrophilic sequences, which interact with the polar aqueous environment of plasma. Apo B contains an LDL receptor binding domain located in the region between amino acids 3100 and 3600 that is involved in the uptake of plasma LDL and possibly some VLDL and IDL by tissues. Unlike other apoproteins, apo B (including apo B48) does not exchange between lipoproteins. Apo B is necessary for the initial assembly and secretion of VLDL by the liver, and individuals who do not synthesize normal apo B do not secrete VLDL. In the plasma via interaction with the LDL receptor, apo B has a crucial role in the catabolism of LDL and probably VLDL and IDL as well.
Apo B48 is transcribed from the gene for apo B100. In the small intestine, the mRNA is edited at base 6666 so that a codon for glycine becomes a stop codon. Hence, the translation of apo B100 mRNA stops at this codon in the human intestine. Found on chylomicrons, apo B48 is necessary for intestinal assembly and secretion of these lipoproteins.
Apo B48 seems to have no role other than to direct the initial assembly and secretion of chylomicrons; it has no binding site for the LDL receptor.
Apo C-I, apo C-II, and apo C-III, although often discussed together in reviews of lipoprotein physiology, actually have unique metabolic roles. The liver is the site of synthesis of all three of the C apoproteins. Apo C-I is a minor component of chylomicrons, VLDL, IDL, and HDL. Recent studies have better defined the functions of apo C-I, which include a role in the binding of chylomicrons and VLDL to the LDL receptor and to the LDL receptor–related protein (LRP). These finding are consistent with the ability of apo C-I to block uptake of lipoproteins by the liver. Transgenic mice overexpressing apo C-I have moderate elevations in both plasma triglyceride and cholesterol levels and accumulate both apo B48- and apo B100-containing lipoproteins.
Apo C-II is found on chylomicrons, VLDL, IDL, and HDL to varying degrees and is a necessary activator of LPL. Severe hypertriglyceridemia with chylomicronemia is present in individuals lacking apo C-II.
Unexpectedly, transgenic mice overexpressing apo C-II have hypertriglyceridemia, apparently because excess apo C-II (and apo C-III) interferes with the binding of triglyceride-rich lipoproteins to the surface of endothelial cells. This binding is necessary for LPL-associated triglyceride hydrolysis.
Apo C-III is a major component of VLDL and accounts for about 40% of the protein. It is also present on chylomicrons, IDL, and HDL. In vitro and in vivo studies indicate that apo C-III inhibits both LPL action and hepatic uptake of chylomicron and VLDL remnants.
Apolipoprotein B metabolism in subjects with deficiency of apolipoprotein C-III and A-I: Evidence that apolipoprotein C-III inhibits lipoprotein lipase in vivo.
; apo C-III knockout mice have similar alterations in triglyceride catabolism. Transgenic mice expressing the human gene for apo C-III have marked hypertriglyceridemia.
Both apo C-III and the ratio of apo C-III to apo C-II are increased in individuals with hypertriglyceridemia, raising the possibility that some individuals may have an underlying dysregulation in apo C-III production. This possibility is supported by the associations observed between polymorphisms in and around the apo A-I/C-III/A-IV gene cluster and both isolated hypertriglyceridemia and combined hyperlipidemia.
Apo E has a critical role in the removal of remnant lipoproteins from plasma by interacting with several receptors in the liver, including the LDL receptor and LRP. Recently, a VLDL receptor has been identified that recognizes apo E–containing lipoproteins. Its physiologic role has not been determined, and it is not present in the liver. Apo E has three major alleles that differ in DNA sequence and in amino acid composition; the proteins can be identified as isoforms on isoelectric focusing gels. The normal variant, apo E-3, is associated with normal VLDL and LDL metabolism. Apo E-2 cannot bind to the LDL receptor and its presence, particularly in the homozygous state, can be associated with the accumulation of cholesteryl ester–enriched VLDL. Apo E-4 binds normally to the LDL receptor but is associated with higher levels of LDL cholesterol.
The major protein present on HDL is apo A-I, comprising about 70% to 80% of the protein mass.
It is synthesized in both the liver and small intestines. It is secreted from the intestine on nascent chylomicrons but quickly transfers to HDL during lipolysis of the chylomicron triglyceride. Nascent HDL-like particles enter with plasma containing apo A-I. As an activator of the enzyme LCAT, which esterifies free cholesterol in the smaller more dense HDL subclass HDL3,
apo A-I has a critical role in reverse cholesterol transport. Apo AI may also have a critical role in maintaining the integrity of HDL particles in the plasma and may be important as a ligand for the recently described scavenger receptor B1, which may have a role in the selective delivery of cholesteryl ester from HDL to cells.
accounting for 10% to 15% of HDL protein. Apo A-II has been reported to activate HL in vitro, although in vivo data from transgenic mice suggest that apo A-II may inhibit HL action on HDL. The exact role of apo A-II in lipoprotein physiology remains to be determined.
Apo A-IV is a minor component of HDL and chylomicrons; most apo A-IV circulates lipoprotein-free.
No definite function has been attributed to this apoprotein. Apo A-IV may have a role in the activation of LCAT or may be necessary for full activation of LPL by HDL apo C-II.
Apo (a) is a protein that is present on a subpopulation of LDL particles.
It is linked to apo B by a disulfide bond, and apo particles with this combination are designated Lp(a). The gene for apo (a) varies in the number of repeated sequences for one portion of the protein, and there are therefore a wide range of size isoforms of apo (a) in the plasma, with molecular masses ranging from approximately 250,000 to 800,000 daltons. The plasma concentration of Lp (a) [and therefore the proportions of LDL particles that contain Lp (a)] varies widely in the population and is genetically determined to a high degree. In several populations the presence of high levels of Lp (a) has been shown to be a strong independent risk factor for the development of coronary heart disease. The function of Lp (a) or apo (a) is unknown; it is possible that Lp (a) does not interact with the LDL receptor as well as LDL. Apo (a) has a high degree of homology to plasminogen, a key component of the fibrinolytic pathway, and might therefore interfere with fibrinolysis.
PLASMA LIPID ENZYMES
Lipoprotein Lipase
Lipoprotein lipase converts lipoprotein triglyceride into free fatty acids and mono- and diglycerides, thus allowing uptake of fatty acids by peripheral tissues. LPL is synthesized in adipose tissue and muscle. Its secretion from those sites is regulated at several levels, including transcriptional and post-transcriptional stages. Once secreted, LPL is transported by ill-defined mechanisms to the surface of capillary endothelial cells where, bound to heparan sulfate proteoglycans, it interacts with triglyceride-rich lipoproteins.
The binding of LPL to the endothelial surface is incompletely understood but seems to involve heparan sulfate proteoglycans and perhaps additional nonproteoglycan molecules. Indeed, recent work indicates that an amino terminal piece of apo B, possibly synthesized in the endothelial cell, serves as a binding site for LPL. The critical role of LPL in lipoprotein metabolism is clearly demonstrated in patients and in knockout mice lacking LPL.
Severe hypertriglyceridemia, reduced high density lipoprotein, and neonatal death in lipoprotein lipase knockout mice: Mild hypertriglyceridemia with impaired very low density lipoprotein clearance in heterozygotes.
Recent studies have identified a number of relatively common polymorphisms and mutations in LPL that seem to predispose individuals to hypertriglyceridemia.
It is an enzyme with homology to LPL that is synthesized in the liver. It is thought to interact with several lipoprotein classes after it is secreted from the liver and to bind to the luminal surface of endothelial cells in hepatic sinusoids. Its physiologic actions are to remove triglycerides and phospholipids from chylomicrons and VLDL remnants and to possibly augment chylomicron uptake by the liver. It may also have a role in VLDL to LDL conversion and LDL subfraction patterns. In addition, HL removes phospholipids from LDL and HDL and may be the major enzyme responsible for interconversion of larger to smaller HDL subspecies. Individuals lacking HL accumulate VLDL remnants but have normal or elevated levels of HDL despite having hypertriglyceridemia. Transgenic animals overexpressing HL have reduced triglyceride levels.
Cholesteryl Ester Transfer Protein
Cholesteryl ester transfer protein is synthesized in the liver and is an important plasma protein that mediates the exchange of triglycerides in VLDL, chylomicrons, and remnants for cholesteryl esters in HDL and LDL.
The rate of transfer is regulated in vivo both by the amount of this protein and by the substrate lipoproteins on which it functions. Individuals with hypertriglyceridemia have both low HDL cholesterol and small dense LDL. Both of these alterations are caused in large part by CETP-mediated core lipid transfers. Whether CETP-mediated transfer of HDL cholesteryl esters to apo B–containing lipoproteins is anti- or proatherogenic is unclear.
Lecithin-Cholesterol Acyltransferase
This enzyme converts HDL cholesterol into cholesteryl ester. The newly formed cholesteryl esters can move from the surface of HDL to the core, allowing the particles to adsorb more free cholesterol onto their surface. Based on recent studies in transgenic animals, this may have a crucial role in regulating plasma HDL cholesterol concentrations.
Lecithin: Cholesterol acyltransferase overexpression generates hyperalpha-lipoproteinemia and a nonatherogenic lipoprotein pattern in transgenic rabbits.
Lecithin-cholesterol acyltransferase (LCAT) genetic deficiency syndromes are associated with very low levels of HDL.
TRANSPORT OF EXOGENOUS (DIETARY) LIPIDS
Chylomicrons are lipoproteins that transport dietary fats, cholesterol, and fat-soluble nutrients. The chylomicron surface is composed of phospholipid, apo B48, and apo A-I, A-II, and A-IV. The core is about 90% triglyceride by weight, and the density of this lipoprotein is therefore less than that of plasma. The assembly and secretion of apo B48–containing lipoproteins, based mainly on studies of primary cultures of intestinal cells and of a cultured intestinal carcinoma cell line, CaCO2 cells, seem to parallel those processes in the liver for apo B100–containing lipoproteins. The availability of core lipids, triglyceride, and cholesteryl ester drives the secretion of chylomicrons (Fig. 1).
Figure 1Transport of chlyomicron and chylomicron remnants. The key steps in the transport of dietary nutrients take place when the chylomicron interacts with lipoprotein lipase in adipose and muscle tissue and when the chylomicron remnants are removed from the circulation via receptor-mediated pathways in the liver.
After entry into plasma from the lymph, chylomicrons acquire apo C-I, apo C-II, apo C-III, and apo E from the surface of HDL. Transfer of free and esterified cholesterol and of phospholipids from HDL also occurs at this time. Apo E may become a surface component somewhat later than apo C-II and apo C-III. After gaining apo C-II, the activator of LPL, the chylomicron interacts with the enzyme, and core triglyceride hydrolysis is initiated. Absence of either LPL or apo C-II results in lack of chylomicron triglyceride lipolysis. Apo C-III may modulate this step by its ability to inhibit the interaction of lipoproteins with the endothelial cell surface, LPL, or both. Thus, genetic variations in LPL and apo C-III (and possibly apo C-II) could affect initial metabolism of nascent chylomicrons.
LPL-mediated triglyceride hydrolysis is accompanied by a reduction in the core volume and surface area of the chylomicrons and by transfer of phospholipid, free cholesterol, apo C-II, and apo C-III back to HDL. The remaining remnant particles, which are cholesteryl ester–enriched (both from dietary sources and from HDL-derived cholesteryl ester) and apo E–enriched, can interact with receptors on hepatocytes and be removed rapidly from the circulation.
The pathway for uptake of chylomicron remnants by the liver involves LDL receptors that can recognize apo E, specific apo E receptors such as LRP, and cell surface proteoglycans that can bind apo E. LPL bound to remnants and HL on the surface of hepatic cells may also have roles by acting as bridges between remnants and either LRP or proteoglycans. Individuals lacking apo E or having only the LDL receptor binding defective isoform of apo E, apo E2, may accumulate chylomicron remnants in plasma.
Patients with no LDL receptors (homozygous familial hypercholesterolemia) have minimal, if any, hyperchylomicronemia after eating fat. This is most likely an indication of chylomicron remnant binding to hepatocyte proteoglycans.
TRANSPORT OF ENDOGENOUS LIPIDS
Assembly and Secretion of Apo B-Containing Lipoproteins
The liver assembles and secretes apo B–containing lipoproteins, mainly VLDL. They are assembled in the endoplasmic reticulum, with maturation occurring in the Golgi apparatus of hepatocytes prior to secretion. Although genetic variations in the gene for apo B have been associated with altered plasma levels of apo B in population studies, in vitro studies indicate that the major regulation of apo B secretion is at the post-translational stage.
Apo B mRNA levels do not change in many situations where apo B secretion from cultured liver cells is altered by changes in hormone or substrate availability.
Post-translational regulation of the secretion of apo B–containing lipoproteins seems to be the result of the complex structural information present within the extremely long apo B peptide chain. Thus, although secretory proteins are typically synthesized in association with the cytosolic surface of the rough endoplasmic reticulum (RER) and then directed to the lumen of the RER by rapid translocation through the membrane, this does not seem to be the case for apo B. This polypeptide becomes associated with the endoplasmic reticulum membrane either cotranslationally or very early in the post-translational period. This results in exposure of nascent apo B to the cytosol where it interacts with heat shock protein 70 and becomes available for degradation by the ubiquitin-proteasome pathway.
In vitro studies indicate that between 50% and 80% of newly synthesized apo B may be degraded in hepatocytes.
The major role of apo B is the transport of lipids out of the liver. It is not surprising, therefore, that the availability of the major lipoprotein lipids, triglycerides, cholesteryl esters, and phospholipids determines whether nascent apo B is degraded or secreted. Studies from several laboratories indicate that triglyceride availability is the major factor in the post-translational regulation of apo B secretion.
Hepatocyte free cholesterol and cholesteryl ester content, however, are also important in regulating apo B secretion. Thus, in vivo treatment with acyl CoA: cholesterol acyltransferase (ACAT) inhibitors inhibited VLDL secretion from miniature pigs and African green monkeys. Additionally, studies in humans,
Lovastatin therapy reduces low density lipoprotein apoB levels in subjects with combined hyperlipidemia by reducing the production of apoB-containing lipoproteins: Implications for the pathophysiology of apoB production.
rabbits, and miniature swine with inhibitors of HMGCoA reductase have demonstrated reduced apo B secretion.
The isolation and characterization of microsomal triglyceride transfer protein (MTP) significantly advanced our knowledge of the regulation of the association of apo B48 and apo B with lipids, leading to chylomicron and VLDL assembly. MTP is a heterodimeric protein, consisting of MTP large subunit (98 kDa) and protein disulfide isomerase (PDI), that exists in the lumen of the endoplasmic reticulum. MTP is necessary for the assembly and secretion of apo B-containing lipoproteins from both the intestine and the liver. The human disorder abetalipoproteinemia results from mutations in the gene for the MTP large subunit.
Apo B and MTP physically interact early during the translocation of apo B across the endoplasmic reticulum membrane, and inhibition of MTP blocks apo B secretion from cultured liver cells. Because MTP large subunit has a long half-life, short-term studies in cultured cells have not produced changes in protein levels even when mRNA levels have changed. Long-term high-fat diets in hamsters, however, caused increased gene expression and protein in both liver and intestine.
The size of the VLDL secreted is also determined by the availability of triglyceride. Large triglyceride-rich VLDL are secreted when excess triglycerides are synthesized, such as in obesity, in persons consuming a diet high in simple carbohydrates, in untreated diabetes mellitus,
or with alcohol consumption. VLDL apo B secretion also increases during continuous feeding, whereas the appearance of LDL apo B is concomitantly reduced. This shift in the density distribution of secreted apo B lipoproteins is consistent with increased incorporation of core triglycerides associated with feeding. Secretion of larger, more triglyceride-rich VLDL is also associated with estrogen treatment, although secretion of VLDL apo B is also increased. Large, triglyceride-rich VLDL seem to be characteristic of the type of apo B lipoprotein that is assembled and secreted by individuals with familial hypertriglyceridemia.
Integrated regulation for very low density lipoprotein triglyceride and apolipoprotein-B kinetics in man: Normolipidemic subjects, familial hypertriglyceridemia and familial combined hyperlipidemia.
In contrast, when triglyceride but not cholesterol availability is limited, small VLDL and possibly IDL and LDL-like particles are secreted. For example, weight loss is associated with a shift in the pattern of apo B lipoprotein secretion in hypertriglyceridemic subjects. VLDL apo B secretion is decreased in association with reduced VLDL triglyceride secretion, whereas apo B secretion directly into LDL is increased. A similar shift in the density of secreted apo B lipoproteins was observed in studies when clofibrate was used to reduce plasma triglyceride secretion in patients with familial combined hyperlipidemia (FCHL). When apo B secretion is increased in the absence of concomitantly increased triglyceride synthesis, apo B is packaged with less core lipids, leading to the secretion of smaller, more dense apo B–containing lipoproteins.
In addition to impacting on lipoprotein size and density, the availability of lipids can determine the rate of particle secretion. This is probably best illustrated by the well-demonstrated link between rates of fatty acid flux and the secretion of apo B–containing lipoproteins. Numerous physiologic or pathophysiologic states in which fatty acid flux is increased are associated with increased rates of VLDL apo B secretion; obesity and diabetes mellitus are two well-defined examples. The link between insulin resistance and increased levels of plasma VLDL
Integrated regulation for very low density lipoprotein triglyceride and apolipoprotein-B kinetics in man: Normolipidemic subjects, familial hypertriglyceridemia and familial combined hyperlipidemia.
Metabolic basis of hyperapobetalipoproteinemia: Turnover of apolipoprotein B in low density lipoprotein and its precursors and subfractions compared with normal and familial hypercholesterolemia.
Insulin has important but complex roles in the regulation of VLDL secretion. In particular, the proposal that increased insulin availability (in the form of hyperinsulinemia) can actually increase VLDL synthesis and secretion has been controversial. Strong correlations exist between levels of plasma insulin, measurements of insulin action, levels of plasma VLDL, and rates of VLDL secretion into plasma in normal and diabetic subjects; however, a causal relationship between insulin secretion and VLDL production has been difficult to prove. Higher rates of free fatty acid flux to the liver are commonly associated with insulin resistance–hyperinsulinemia, and adequate insulinization in the presence of increased free fatty acids may be the basis for increased production of VLDL.
Interrelationship among insulin's antilipolytic and glucoregulatory effects and plasma triglycerides in nondiabetic and diabetic patients with endogenous hypertriglyceridemia.
Recent studies have shown that acute hyperinsulinemia can reduce VLDL triglyceride and apo B secretion from the liver. Although the results indicate that the ability of insulin to lower free fatty acid flux to the liver is a major factor in reducing apo B secretion, a significant proportion of the effects observed may be direct consequences of insulin action.
Insulin-resistant obese subjects did not show insulin-induced inhibition of VLDL secretion.
Very-Low-Density-Lipoprotein Catabolism
Catabolism of VLDL is characterized by several key branch points, and the outcome depends on which path individual particles take along the route. During part of this process, apo B seems to act as a bystander; at other points in the catabolic process, however, apo B may have a key role. The other apoproteins of VLDL, apo C-I, apo C-II, apo C-III, and apo E, all have significant roles in VLDL catabolism via their interactions with enzymes, cell surface proteoglycans, and specific receptors. Even the lipid composition of a particle has a role in determining its ultimate fate.
Interaction with LPL on endothelial cell surfaces initiates VLDL catabolism (Fig. 2). Apo E, which binds avidly to heparan sulfate proteoglycans, facilitates efficient VLDL catabolism by endothelial cell–bound LPL. As lipolysis proceeds, VLDL become smaller and more dense and are converted to IDL. Loss of VLDL triglyceride results in the transfer of surface cholesterol, phospholipids, and apoproteins to HDL. Apo E may be the last apoprotein to leave VLDL and IDL. At some point in the lipolytic process, the lipoprotein is released from the cell surface, apparently as a result of local increases in fatty concentrations that inhibit the binding of LPL to the proteoglycans and the interaction of LPL with lipoprotein. In addition, high local concentrations of free fatty acids prevent the interaction of apo C-II with LPL.
Figure 2Transport of VLDL, IDL, and LDL. The availability of core lipids, TG, and cholesterol in the liver stimulates the assembly and secretion of VLDL, which then interacts with LPL in adipose and muscle tissue. The VLDL remnant (IDL) can then be removed via receptor-mediated pathways in the liver or be converted to LDL. LDL is removed primarily by the liver and peripheral tissues via the LDL-receptor pathway.
The C apoproteins have important roles in regulating the enzymatic actions of LPL. Apo C-II is contained on the surface of lipoproteins and is the obligate cofactor for LPL action. Apo C-III decreases apo C-II stimulation of LPL and has been postulated to inhibit LPL in vivo. Indeed, subjects lacking apo C-III seem to have accelerated VLDL catabolism and very low levels of VLDL triglyceride,
Apolipoprotein B metabolism in subjects with deficiency of apolipoprotein C-III and A-I: Evidence that apolipoprotein C-III inhibits lipoprotein lipase in vivo.
Additionally, as VLDL triglyceride concentration increases, the ratio of apo C-III to apo C-II increases, suggesting that hypertriglyceridemic VLDL may be less than an optimal substrate for LPL. Of interest, a transgenic mouse overexpressing human apo C-II also develops hypertriglyceridemia; experiments suggest that excess apo C-II, as well as excess apo C-III, can inhibit the interaction of VLDL particles with cell surface proteoglycans.
The term VLDL remnant has been used along with IDL to describe the product of LPL-mediated VLDL triglyceride catabolism. This particle is either removed from plasma as a remnant akin to the chylomicron remnant or converted to LDL. Although a method is not yet available to identify clearly and isolate VLDL that are destined for one or the other fate, evidence is emerging that provides insight into the mechanisms involved in both processes. Studies with radiolabeled Sf400-100 VLDL suggest that large, triglyceride-rich VLDL are mainly removed from plasma in the VLDL and IDL density range and are not efficiently converted to LDL.
The majority of Sf60-20 VLDL are, in contrast, converted to LDL. This suggests that the larger, more buoyant VLDL are predestined for removal as remnants, whereas the smaller denser VLDL are targeted for conversion to LDL. Whether conversion to LDL is predetermined by physical-chemical characteristics of secreted VLDL or is governed by the probability associated with residence time in plasma remains to be determined. Regulation of conversion is, however, supported by several human studies. When diets high in simple carbohydrates are used to increase hepatic secretion of triglyceride-rich VLDL, the proportion of apo B that is converted to LDL is reduced. When weight reduction or fibrate treatment is used to reduce hepatic triglyceride secretion, percent conversion of VLDL apo B to LDL is increased.
Two metabolic schemes have been proposed to explain why the larger, more triglyceride-enriched VLDL are removed directly from plasma, thereby preventing their conversion to LDL. The first is an apoprotein-based scheme focusing mainly on apo E. This apoprotein, acting as a ligand for the LDL receptor and possibly for one or more other receptors or surface-binding molecules, seems to have a critical role in the direct removal of some VLDL particles from the circulation and in the conversion of other VLDL particles to IDL and LDL.
The presence of increased numbers of apo E molecules on larger VLDL particles may be the basis for the observation that larger VLDL are not efficiently converted to LDL. Studies of the kinetics of apo E–enriched and apo E–poor VLDL in humans have concluded that these two subpopulations of apo E follow parallel but mostly separate metabolic fates. Apo E–enriched VLDL are cleared as a VLDL particle to a much greater extent than are apo E–poor VLDL. Studies by several groups have indicated that apo E is the major ligand directing cell uptake of partially lipolyzed triglyceride-rich apo B lipoproteins. Despite the presence of other receptor and nonreceptor pathways, recognition of one or more apo E molecules by the LDL receptor seems to be critical as suggested by studies of apo B metabolism in patients with homozygous familial hypercholesterolemia.
Identification of the LRP receptor ignited new interest in this area. Apo E–enriched lipoproteins bind to LRP as does LPL. Indeed, it has been postulated that LPL may not only produce the remnant but also direct it to its final site of tissue uptake. HL has also been demonstrated to bind to LRP and may also modify remnant uptake by the liver; however, after intensive investigation and ingenious use of transgenic and knockout mouse models, it seems that the LDL receptor is the major pathway for apo E–mediated remnant uptake by the liver, with LRP playing a “backup” role.
The importance of apo E in apo B remnant removal has been demonstrated in human and animal studies. Remnant particles are increased in the circulation of patients with the E2/E2 isoform. Rare patients with a genetic lack of apo E have been shown to have a large number of circulating cholesterol-rich remnant lipoproteins. A similar lipoprotein phenotype has been found in apo E knockout mice.
perhaps because they displace apo E from the cell surface. Apo C-I, apo C-II, and apo C-III have all been shown to inhibit the uptake of VLDL by the liver. This function of the apo C proteins offers another basis (in addition to defective interaction with LPL at the endothelial cell surface) for the hypertriglyceridemia seen in transgenic mice overexpressing apo C-II,
Either while circulating in plasma or during the period when VLDL triglycerides are being hydrolyzed, VLDL particles become enriched in cholesteryl ester as they transfer a triglyceride to HDL or LDL. This process generates a VLDL particle that has more cholesteryl ester than is present in LDL, and this excess cholesteryl ester, which cannot be hydrolyzed by LPL, results in a relatively buoyant remnant after LPL-mediated triglyceride hydrolysis is complete. Thus, apo B lipoproteins that enter the circulation with increased triglyceride content are not converted efficiently to LDL after removal of their core triglycerides by LPL because in the process they have become enriched with core cholesteryl ester. If apo E is attracted to cholesteryl ester–enriched lipoproteins, the apoprotein and core lipid schemes can merge, and the apo E–enriched remnant lipoproteins can be removed directly from plasma.
The catabolism of VLDL remnant is affected by HL as well. Studies suggest that HL has a role in removing triglycerides from partially catabolized VLDL or IDL and LDL. These data are supported by studies in a few subjects lacking HL who have increased VLDL remnant and IDL levels in plasma and by animal studies in which HL was either inhibited by passive immunization or overexpressed. Although LPL alone will hydrolyze VLDL triglyceride, the resulting denser lipoprotein is slightly larger and more phospholipid-rich than circulating LDL. Hence, it is thought that final conversion of VLDL to LDL requires HL. As noted previously, HL may also have a role in LRP-mediated removal of remnant lipoproteins.
Low-Density Lipoprotein Production and Catabolism
LDL is generated mainly by the plasma catabolism of VLDL and IDL; however, numerous kinetic studies in animals and humans
Lovastatin therapy reduces low density lipoprotein apoB levels in subjects with combined hyperlipidemia by reducing the production of apoB-containing lipoproteins: Implications for the pathophysiology of apoB production.
have demonstrated production of LDL that seems to be independent of VLDL or IDL. Key issues that remain unanswered include the physical and compositional characteristics of these directly secreted LDL particles compared with those generated from VLDL and IDL and their fate relative to particles produced from the cascade.
Regardless of whether LDL are formed via the catabolism of more triglyceride-rich apo B precursors or by direct secretion from the liver, their core lipid composition can be significantly affected by the triglyceride-rich apo B lipoproteins via the action of CETP.
Excess numbers of triglyceride-rich lipoproteins can drive the exchange of LDL cholesteryl ester for chylomicron or VLDL triglyceride. The triglyceride-enriched LDL can then interact with LPL, or more likely HL, generating smaller more dense LDL that are relatively lipid-depleted. These smaller LDL do not interact as well with the LDL receptor and may also be more likely to undergo oxidative modification. Thus, small dense LDL might be more atherogenic.
The LDL receptor is a glycoprotein with a molecular weight of approximately 160,000 kD that is present on the cell surfaces of nearly all tissues in the body. After LDL apo B-100 or lipoproteins containing apo E have interacted with the LDL receptor, they are internalized and delivered to lysosomes. Within the lysosome, apoproteins are degraded to amino acids, and cholesteryl ester is converted to free cholesterol via the action of an acid lipase. The free cholesterol is then delivered to the cytoplasm where it interacts (in as yet incompletely defined ways) with the endoplasmic reticulum membrane protein SREBP. The interaction results in cleavage of SREBP, and the amino terminal portion enters the nucleus where it inhibits transcription of the genes for LDL receptors, HMG synthase, and HMGCoA reductase.
These regulatory pathways allow the cells to maintain cholesterol homeostasis. Lack of LDL receptors results in unregulated cellular cholesterol metabolism and markedly increased levels of plasma LDL cholesterol. The homozygous form of familial hypercholesterolemia is the paradigm for this abnormal state.
In normal individuals, approximately 60% to 80% of LDL can be cleared from plasma by LDL receptor–mediated pathways, with the remainder cleared via nonreceptor pathways. The latter may include fluid phase endocytosis, some of which may be facilitated by binding of lipoproteins to cell surface proteoglycans. It is also likely that some portion of what is defined as nonreceptor-mediated LDL clearance may represent clearance via other specific receptors, such as LRP and scavenger receptors (including those that recognize oxidatively modified LDL). Thus, increases in nonreceptor-mediated uptake could occur in situations in which LDL oxidation is increased.
HDL Metabolism
Numerous epidemiologic data indicate that higher levels of HDL cholesterol are associated with a reduced risk for atherosclerotic cardiovascular disease.
HDL comprise an extremely complex lipoprotein class. Subclasses of HDL varying in size, density, and lipid composition have been isolated by a variety of physical-chemical techniques. HDL with varying apoprotein composition have also been characterized. Apo A-I only and apo A-I/apo A-II particles may differ in metabolic characteristics and may have different epidemiologic impact, although these are controversial issues.
The scheme for HDL metabolism that is presented herein is simplified and focuses on the transport of cholesterol through HDL, the so-called reverse cholesterol transport system (Fig. 3).
Figure 3Transport of HDL. HDL are produced mainly in the plasma by fusion of apolipoproteins with phospholipids to form disks, such as pre-β HDL. The nascent HDL can adsorb free cholesterol from cell membranes. After the cholesterol is esterified by LCAT and moves from the surface to the core of the particle, the HDL can deliver the cholesterol to apo B-containing lipoproteins via CETP-mediated exchange for TG. Alternatively, HDL can deliver cholesterol to the liver (and other organs) via either selective uptake of core cholesteryl ester or whole particle uptake.
Recent evidence indicates that although the small intestine can assemble and secrete spherical HDL, the majority of mature HDL are formed by the apparent coalescence of individual phospholipid-apoprotein disks containing apo A-I, apo A-II, and possibly apo E. Thus, both the intestine and the liver secrete individual apoproteins with a small amount of phospholipid. These small, cholesterol-poor HDL disks with both pre-beta and alpha mobility on agarose electrophoresis can adsorb free cholesterol from cell membranes and become nascent HDL3 particles. This is thought to be the first step in reverse cholesterol transport.
Formation of cholesteryl ester increases the capacity of the HDL3 to accept more free cholesterol, because the core of the spherical particles can accumulate many more molecules of cholesterol ester than could be accommodated on the surface.
Continued adsorption of free cholesterol followed by LCAT-driven esterification enables HDL3 to enlarge such that they can accommodate apo C-II and apo C-III as well as more phospholipid on their surfaces. These latter components are derived from chylomicrons and VLDL (and to a lesser degree IDL) as these lipoproteins interact with LPL and lose their triglyceride. Apo E can also transfer to HDL at this time. The result of these processes is the formation of HDL2. The fate of HDL2 is very complicated as well but can be divided into two major categories: (1) transfer of the cholesterol components of HDL2 to other lipoproteins and to cells and (2) metabolism (with removal from plasma) of the HDL2 apoproteins.
CETP mediates the transfer of cholesteryl ester from HDL to triglyceride-rich lipoproteins (chylomicrons and VLDL in the fed and fasted states, respectively).
As chylomicron remnants, VLDL remnants or IDL, and finally LDL are taken up by the liver via LDL or other receptor-mediated pathways, the CETP-transferred cholesteryl esters can be cleared as well. Selective uptake of HDL cholesteryl ester by tissues via interactions of HDL with cell membranes that do not involve uptake of the whole HDL particle has also been described in several tissues. Recently, the scavenger receptor B1 (SRB1) has been identified, which seems to be involved in selective cholesteryl ester delivery to tissues by HDL.
The relative importance of each of these pathways in reverse cholesterol transport is unknown but is under intensive study.
The mechanism for removal of apo A-I from plasma has not been fully determined, although some catabolism may occur when HDL containing apo E are removed from plasma in toto, particularly by the liver.
The kidney may be a significant site of apo A-I degradation. Because apo A-I and apo A-II are not always on the same particles, it is logical that these two proteins have different rates of clearance from plasma, and that their plasma levels are differentially regulated. The sites of apo A-II, apo A-IV, apo C-I, apo C-II, and apo C-III degradation are unknown.
Both the number of cholesterol molecules (particularly, esterified cholesterol) in each HDL particle and the number of HDL particles determine the plasma concentration of HDL cholesterol. Several points of regulation for each of these parameters can be hypothesized. First, the cholesteryl ester content of HDL could be reduced by an inability of HDL3 to accept cell membrane cholesterol or because of reduced LCAT activity. This might limit the initiation of reverse transport. Increased CETP-catalyzed cholesteryl ester transfer from HDL2 to triglyceride-rich lipoproteins, whether the result of increased CETP mass in plasma or acceptor lipoproteins (chylomicrons or VLDL), could also result in cholesteryl ester–depleted HDL particles. The rate of selective cholesteryl ester delivery to cells could have a role in regulation of the HDL cholesteryl ester content. Direct uptake of HDL particles via apo E binding to hepatic receptors could also regulate HDL particle number. The precise role of each of these pathways in the overall regulation of HDL levels is unknown. Several studies have indicated that increased apo A-I fractional catabolism is commonly observed in subjects with reduced HDL cholesterol levels.
Increased plasma and renal clearance of an exchangeable pool of apolipoprotein A-I in subjects with low levels of high density lipoprotein cholesterol.
The fact that accelerated removal of HDL apo A-I is common in subjects with high plasma triglyceride and low HDL cholesterol levels suggests that CETP-mediated increases in cholesteryl ester transfer from HDL to triglyceride-rich lipoproteins may reduce plasma HDL cholesterol concentrations by depleting HDL particles of their cholesteryl ester and secondarily reducing the number of HDL particles in the circulation.
Subjects with higher levels of LPL activity in postheparin plasma have higher plasma concentrations of HDL cholesterol and apo A-I.
This relationship may be derived from the low levels of chylomicrons and VLDL (acceptors for CETP-catalyzed transfer of cholesteryl ester from HDL) present in the plasma of individuals with high LPL activity or from the increased generation of surface remnants during lipolysis that are available for HDL formation. Subjects with high LPL activity might have relatively longer plasma residence times of surface apoproteins such as apo C-II and apo C-III (associated mainly with HDL2). The role of apo C-III in the regulation of HDL metabolism is suggested by work demonstrating an inverse relationship between the fractional catabolic rate of apo A-I and the apo C-III content of HDL.
SUMMARY
Lipoproteins are spherical macromolecular complexes in which hydrophobic molecules of triglyceride and cholesteryl ester are enveloped within a monolayer of amphipathic molecules of phospholipids, free cholesterol, and apoproteins. The major lipoprotein classes include intestinally derived chylomicrons that transport dietary fats and cholesterol, hepatic-derived VLDL, IDL, and LDL that can be atherogenic, and hepatic- and intestinally derived HDL that are anti-atherogenic. Apoprotein B is necessary for the secretion of chylomicrons (apo B48) and VLDL, IDL, and LDL (apo B100). Post-translational regulation of the assembly of apo B–containing lipoproteins by core lipid availability seems to be the major mechanism for variations in secretion. Plasma levels of VLDL triglycerides are determined mainly by rates of secretion and LPL lipolytic activity; plasma levels of LDL cholesterol are determined mainly by the secretion of apo B100 into plasma, the efficacy with which VLDL are converted to LDL and by LDL receptor–mediated clearance. Regulation of HDL cholesterol levels is complex and is affected by rates of synthesis of its apoproteins, rates of esterification of free cholesterol to cholesteryl ester by LCAT, levels of triglyceride-rich lipoproteins and CETP-mediated transfer of cholesteryl esters from HDL, and clearance from plasma of HDL lipids and apoproteins. Normal lipoprotein transport is associated with low levels of triglycerides and LDL cholesterol and high levels of HDL cholesterol. When lipoprotein transport is abnormal, lipoproteins levels can change in ways that predispose individuals to atherosclerosis.
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Address reprint requests to Henry N. Ginsberg, MD, Division of Preventive Medicine and Nutrition, Department of Medicine, Columbia University College of Physicians and Surgeons, 630 West 168th Street, New York, NY 10032
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