Neutral lipid transfer and lipolysis convert high molecular weight LDL from cholesterol-fed nonhuman primates towards normal: a molecular analysis

Dietary and genetic probes of atherogenic dyslipidemia

A goal of dietary management of cardiovascular disease risk in patients with obesity and metabolic syndrome is improvement in the atherogenic dyslipidemia comprising elevated triglyceride, reduced high-density lipoprotein (HDL) cholesterol, and increased numbers of small, dense low-density lipoprotein (LDL) particles. Individuals with a genetically influenced trait characterized by a high proportion of small, dense LDL (phenotype B) respond to a low-fat, high-carbohydrate diet with greater reduction of LDL cholesterol, apoprotein B, and mid-sized LDL2 particles than unaffected subjects (phenotype A). In contrast, in phenotype A subjects there is a reciprocal shift from large LDL1 to small LDL3 such that a high proportion convert to phenotype B. There is evidence for heritable effects on these diet-induced subclass changes and for the involvement of specific genes. For example, a haplotype of the APOA5 gene associated with increased plasma triglyceride and small, dense LDL predicts greater diet-induced reduction of LDL2, a haplotype-specific effect that is strongly correlated with both increased VLDL precursors and LDL4 products. Understanding of such diet-genotype interactions may help to elucidate mechanisms that are responsible for phenotype B and for its differential dietary responsiveness. This information may also ultimately help in identifying those individuals who are most likely to achieve cardiovascular risk benefit from specific dietary interventions.

Associations of low density lipoprotein particle composition with atherogenicity

PURPOSE OF REVIEW

A growing body of data suggests that in addition to LDL-cholesterol concentrations, compositional properties of LDL, including size and fatty acid composition, are important in determining the relative degree of atherogenicity. This review examines current research in this field to evaluate which properties of LDL may most directly influence the risk of coronary heart disease.

RECENT FINDINGS

The presence of small dense LDL has been correlated with an increased risk of coronary heart disease, but this has not been shown to be fully independent of related factors such as elevated plasma triacylglycerol concentrations. An increased susceptibility of small dense LDL to in-vitro oxidation has also been demonstrated, but its importance to coronary heart disease risk has not been established. Other studies have found that the presence of enlarged LDL, modified (oleate enriched) fatty acyl composition of LDL, and higher numbers of LDL particles in plasma also are endpoints associated with an increased risk of coronary heart disease.

SUMMARY

LDL size may indicate a metabolic condition associated with increased CHD risk as opposed to the direct promotion of atherosclerosis by specific particle types of LDL. In most claims of detrimental effects of small dense LDL, neither LDL particle concentrations nor the fatty acid composition of the particles were established, both factors being important in contributing to the atherogenic potential of LDL. The predisposition to premature coronary heart disease cannot currently be objectively assigned to any one type of LDL particle.

CETP and lipid transfer inhibitor protein are uniquely affected by the negative charge density of the lipid and protein domains of LDL

Lipoprotein surface charge influences cholesteryl ester transfer protein (CETP) activity and its association with lipoproteins; however, the relationship between these events is not clear. Additionally, although CETP and its regulator, lipid transfer inhibitor protein (LTIP), bind to lipoproteins, it is not known how the charge density of lipoprotein protein and lipid domains influences these factors. Here, the electronegativity of the protein (by acetylation) and surface lipid (oleate addition) domains of LDL were modified. LDL-only lipid transfer assays measured changes in CETP and LTIP activities. CETP activity was stimulated by <10 microM oleate but completely suppressed by >20 microM. The same electronegative potential induced by acetylation mildly stimulated CETP. Modification-induced enhanced binding of CETP did not correlate with CETP activity. LTIP activity was completely blocked by approximately 10 microM oleate but only mildly suppressed by acetylation. LTIP binding to LDL was not decreased by oleate. Thus, the negative charge of LDL surface lipids, but not protein, is an important regulator of CETP and LTIP activity. Altered binding could not explain changes in CETP activity, suggesting that the extent of CETP binding is not normally rate limiting to its activity. Physiologic and pathophysiologic conditions that modify the negative charge of lipoprotein surface lipids will suppress LTIP activity first, followed by CETP.

Lipid transfer inhibitor protein defines the participation of high density lipoprotein subfractions in lipid transfer reactions mediated by cholesterol ester transfer protein (CETP)

Cholesterol ester transfer protein (CETP) moves triglyceride (TG) and cholesteryl ester (CE) between lipoproteins. CETP has no apparent preference for high (HDL) or low (LDL) density lipoprotein as lipid donor to very low density lipoprotein (VLDL), and the preference for HDL observed in plasma is due to suppression of LDL transfers by lipid transfer inhibitor protein (LTIP). Given the heterogeneity of HDL, and a demonstrated ability of HDL subfractions to bind LTIP, we examined whether LTIP might also control CETP-facilitated lipid flux among HDL subfractions. CETP-mediated CE transfers from [3H]CE VLDL to various lipoproteins, combined on an equal phospholipid basis, ranged 2-fold and followed the order: HDL3 > LDL > HDL2. LTIP inhibited VLDL to HDL2 transfer at one-half the rate of VLDL to LDL. In contrast, VLDL to HDL3 transfer was stimulated, resulting in a CETP preference for HDL3 that was 3-fold greater than that for LDL or HDL2. Long-term mass transfer experiments confirmed these findings and further established that the previously observed stimulation of CETP activity on HDL by LTIP is due solely to its stimulation of transfer activity on HDL3. TG enrichment of HDL2, which occurs during the HDL cycle, inhibited CETP activity by approximately 2-fold and LTIP activity was blocked almost completely. This suggests that LTIP keeps lipid transfer activity on HDL2 low and constant regardless of its TG enrichment status. Overall, these results show that LTIP tailors CETP-mediated remodeling of HDL3 and HDL2 particles in subclass-specific ways, strongly implicating LTIP as a regulator of HDL metabolism.

The surface cholesteryl ester content of donor and acceptor particles regulates CETP: a liposome-based approach to assess the substrate properties of lipoproteins

Cholesteryl ester transfer protein (CETP) activity is regulated, in part, by lipoprotein composition. We previously demonstrated that CETP activity follows saturation kinetics as cholesteryl ester (CE) levels in the phospholipid surface of donor particles are increased. We propose here that the plateau of CETP activity occurs because the surface concentration of CE in the acceptor becomes rate limiting. This hypothesis was tested in CETP assays between synthetic liposomes whose CE content was varied independently. As donor CE increased, CETP activity followed saturable kinetics, but the slope of the first-order portion of the curve and the maximum achievable CE transfer rate were linearly related to the acceptor's surface CE concentration. These findings, plus studies with free cholesterol-modified LDL, strongly suggest that CE-rich donor liposomes can measure the CETP-accessible CE in acceptor lipoproteins. CETP activity from CE-rich liposomes to multiple control LDLs ranged 1.8-fold despite equivalent CETP binding capacity, suggesting that LDLs vary widely in their capacity to present CE to CETP. Thus, CETP activity depends on the surface availability of substrate lipids in the donor and acceptor. Donor liposomes with high CE content can be used to assess how subtle changes in composition alter the substrate potential of plasma lipoproteins.

Lipid transfer inhibitor protein defines the participation of lipoproteins in lipid transfer reactions: CETP has no preference for cholesteryl esters in HDL versus LDL

Cholesteryl ester transfer protein (CETP) catalyzes the net transfer of cholesteryl ester (CE) between lipoproteins in exchange for triglyceride (heteroexchange). It is generally held that CETP primarily associates with HDL and preferentially transfers lipids from this lipoprotein fraction. This is illustrated in normal plasma where HDL is the primary donor of the CE transferred to VLDL by CETP. However, in plasma deficient in lipid transfer inhibitor protein (LTIP) activity, HDL and LDL are equivalent donors of CE to VLDL (Arterioscler Thromb Vasc Biol. 1997;17:1716-1724). Thus, we have hypothesized that the preferential transfer of CE from HDL in normal plasma is a consequence of LTIP activity and not caused by a preferential CETP-HDL interaction. We have tested this hypothesis in lipid mass transfer assays with partially purified CETP and LTIP, and isolated lipoproteins. With a physiological mixture of lipoproteins, the preference ratio (PR, ratio of CE mass transferred from a lipoprotein to VLDL versus its CE content) for HDL and LDL in the presence of CETP alone was approximately 1 (ie, no preference). Fourfold variations in the LDL/HDL ratio or in the levels of HDL in the assay did not result in significant preferential transfer from any lipoprotein. On addition of LTIP, the PR for HDL was increased up to 2-fold and that for LDL decreased in a concentration-dependent manner. Under all conditions where LDL and HDL levels were varied, LTIP consistently resulted in a PR >1 for CE transfer from HDL. Short-term experiments with radiolabeled lipoproteins and either partially purified or homogenous CETP confirmed these observations and further demonstrated that CETP has a strong predilection to mediate homoexchange (bidirectional transfer of the same lipid) rather than heteroexchange (CE for TG); LTIP had no effect on the selection of CE or TG by CETP or its mechanism of action. We conclude, in contrast to current opinion, that CETP has no preference for CE in HDL versus LDL, suggesting that the previously reported stable binding of CETP to HDL does not result in selective transfer from this lipoprotein. These data suggest that LTIP is responsible for the preferential transfer of CE from HDL that occurs in plasma. CETP and LTIP cooperatively determine the extent of CETP-mediated remodeling of individual lipoprotein fractions.

In vitro production of beta-very low density lipoproteins and small, dense low density lipoproteins in mildly hypertriglyceridemic plasma: role of activities of lecithin:cholester acyltransferase, cholesterylester transfer proteins and lipoprotein lipase

As a model for the formation of beta-very low density lipoproteins (VLDL) and small, dense LDL by the intraplasma metabolic activities in vivo, lipoproteins in fresh plasma were interacted in vitro with endogenous lecithin:cholesterol acyltransferase (LCAT) and cholesterylester transfer proteins (CETP) and subsequently with purified lipoprotein lipase (LpL). The LCAT and CETP reactions in a mildly hypertriglyceridemic (HTG) plasma at 37 degrees C for 18 h resulted in (1) esterification of about 45% plasma unesterified cholesterol (UC), (2) a marked increase in cholesterylester (CE) (+129%) and a decrease in triglyceride (TG) (-45%) in VLDL, and (3) a marked increase of TG (+ 341%) with a small net decrease of CE (-3.6%) in LDL, causing a significant alteration in the TG/CE of VLDL (from 8.0 to 1.9) and of LDL (from 0.20 to 0.93). The LDL in LCAT and CETP-reacted plasma is larger and more buoyant than that in control plasma. In vitro lipolysis of control and LCAT and CETP-reacted plasma by LpL, which hydrolyzed >90% of VLDL-TG and about 50-60% of LDL-TG, converted most of VLDL in control plasma (>85%) but less than half (40%) of VLDL in LCAT and CETP-reacted plasma into the IDL-LDL density fraction and transformed the large, buoyant LDL in the LCAT and CETP-reacted plasma into particles smaller and denser than those in the control plasma. The remnants that accumulated in the VLDL density region of the postlipolysis LCAT and CETP-reacted plasma contained apo B-100 and E but little or no detectable apo Cs and consisted of particles having pre-beta and beta-electrophoretic mobilities. The inhibition of LCAT during incubation of plasma, which lessened the extent of alteration in VLDL and LDL core lipids, increased the extent of lipolytic removal of VLDL from the VLDL density region but lowered the extent of alteration in the size and density of LDL. The LCAT, CETP and/or LpL-mediated alterations in the density of LDL in normolipidemic fasting plasma were less pronounced than that in mildly HTG plasma, but they became highly pronounced upon increase of its TG-rich lipoprotein level by the addition of preisolated VLDL or by the induction of postprandial lipemia. Although the effect of LCAT, CETP and LpL reactions in non-circulating plasma in vitro may be different from that in vivo, the above data suggests that the plasma TG-rich lipoprotein level and the extent of intraplasma LCAT, CETP, LpL and likely hepatic lipase (HL) reactions in vivo may play a role in determining the LDL phenotype.

Elevated LDL triglyceride concentrations in subjects heterozygous for the hepatic lipase S267F variant

Although naturally occurring loss-of-function mutations in human hepatic lipase (HL) have been described, the biochemical phenotype of heterozygous HL deficiency remains ill defined. This may be due to the relatively small numbers of heterozygous adult carriers of HL mutations in index kindreds. We have identified several new heterozygotes for the catalytically inactive, nonsecreted HL variant S267F in the kindred that was originally ascertained because of hypertriglyceridemia due to the mutant, secreted, circulating apolipoprotein (apo) CII variant apo CII-T. Pairwise comparisons with family controls showed that only the plasma low density lipoprotein triglycerides (LDL TGs) were higher in 11 simple heterozygotes for HL S267F (P=0.002). In contrast, both plasma total TGs and LDL TGs were significantly higher in 12 simple heterozygotes for apo CII-T than in family-matched control subjects (P=0.005 and 0.009, respectively). These findings suggest that the TG content of LDL is increased by heterozygosity for 2 different mutations that affect different proteins involved in lipolysis. However, the mechanisms underlying this compositional change in LDL appear to be different for the 2 mutations, because the total TGs are also elevated in subjects heterozygous for apo CII-T but not in subjects heterozygous for HL S267F.

Compared with dietary monounsaturated and saturated fat, polyunsaturated fat protects African green monkeys from coronary artery atherosclerosis

Atherogenic diets enriched in saturated, n-6 polyunsaturated, and monounsaturated fatty acids were fed to African green monkeys for 5 years to define effects on plasma lipoproteins and coronary artery atherosclerosis. The monkeys fed polyunsaturated and monounsaturated fat had similar plasma concentrations of LDL cholesterol, and these values were significantly lower than for LDL in the animals fed saturated fat. Plasma HDL cholesterol concentrations were comparable in animals fed saturated and monounsaturated fat and were significantly higher than in animals fed polyunsaturated fat. Thus, the monounsaturated fat group had the lowest LDL/HDL ratio. LDL particle size was largest in the saturated and monounsaturated fat groups, significantly larger than in the polyunsaturated fat group. LDL particle enrichment with cholesteryl oleate was the greatest in the animals fed monounsaturated fat, next greatest in the saturated fat-fed animals, and was least in the polyunsaturated fat-fed animals. Coronary artery atherosclerosis as measured by intimal area was less in the polyunsaturated fat compared with the saturated fat groups, was less in the animals fed polyunsaturated fat compared with the monounsaturated fat-fed animals, but did not differ between the monounsaturated and saturated fat groups. Cholesteryl ester, particularly cholesteryl oleate, accumulation in the coronary arteries was also similar between groups fed monounsaturated and saturated fat but was minimal in the animals fed polyunsaturated fat. In sum, the monkeys fed monounsaturated fat developed equivalent amounts of coronary artery atherosclerosis as those fed saturated fat, but monkeys fed polyunsaturated fat developed less. The beneficial effects of the lower LDL and higher HDL in the animals fed monounsaturated fat apparently were offset by the atherogenic shifts in LDL particle composition. Dietary polyunsaturated fat appears to result in the least amount of coronary artery atherosclerosis because it prevents cholesteryl oleate accumulation in LDL and the coronary arteries in these primates.

Re-evaluation of the structure of low density lipoproteins

Low density lipoprotein (LDL) is an established atherogenic factor. Much effort has therefore been devoted to elucidation of its structure, yielding the generally accepted model according to which the neutral lipids (cholesterol ester and triglycerides) form a lipid core emulsified by phospholipids, cholesterol and the amphipathic Apolipoprotein B. Yet, the detailed structure of LDL is not clear. The present work was carried out with the aim of re-evaluating the LDL structure using the minimal number of assumptions: in view of the previously noted surface deficit (lack of sufficient PL and cholesterol to cover the surface of the lipid core) we have assumed that polar head groups are not covered by apo B. Other than that, we have 'allowed' Apo B to penetrate into the PL monolayers and the lipidic core and to pertrude into the solution (be elevated above the PL head group level). We have also 'allowed' neutral lipid penetration into the monolayer and variation of the thickness of the phospholipid monolayers within reasonable boundaries. Based on the established values of relevant constants (molecular weights and volumes, densities and surface areas) we have computed the radius of the particle, the penetration of Apo B into lipidic milieus and the fraction of the surface area covered by Apo B as functions of the LDL composition, the monolayer thickness and the 'elevation' of Apo B above this monolayer. These computations show that at least 40% of the LDL surface must be covered by protein and that the protein penetrates, on the average, only about a half of the PL monolayer. Thus it is not very likely to penetrate into the lipid core. These general features are preserved in the smaller LDL particles of hypertriglyceridemic patients. Assuming that no PL head group is covered by Apo B, the previously described immobilization of 20% of the phospholipids is likely to result from the interaction of Apo B with neighboring PL. According to our computations this can be regarded consistent with the previously proposed arrangement of the apo B as a '3-4 domain structure' or a long string configuration but inconsistent with 'one domain' or 'twenty domain' structures.

Low density lipoprotein heterogeneity in spontaneously hypercholesterolemic pigs

We previously described a strain of spontaneously hypercholesterolemic pigs carrying an apo-B allele termed Lpb5. Lpb5 pigs are heterogeneous with respect to the severity of their hypercholesterolemia. We have termed Lpb5 pigs with severe hypercholesterolemia Lpb 5.1 pigs, and those with moderate hypercholesterolemia Lpb 5.2, Lpb 5.1 animals have a dramatic increase in buoyant LDL relative to dense LDL, with a buoyant-to-dense LDL ratio of 2.2. In contrast, Lpb 5.2 and control pigs have buoyant-to-dense LDL ratios of 0.7 and 0.5 respectively. This ratio appears to be a stable characteristic of the Lpb 5.1 phenotype because sexually mature boars have a dramatic decrease in total plasma cholesterol concentration with no decrease in their ratio of buoyant-to-dense LDL. We have previously demonstrated a fourteen-fold overproduction of buoyant LDL in the Lpb 5.1 pigs, with very little conversion of dense LDL to buoyant LDL. In the current work, very low density lipoprotein (VLDL) turnover experiments were conducted to determine whether VLDL conversion to buoyant LDL was increased in the Lpb 5.1 pigs. VLDL conversion to buoyant LDL could not account for the increased production of buoyant LDL in Lpb 5.1 pigs. Thus, we cannot account for the increased production of buoyant LDL in the Lpb 5.1 pigs from any measurable plasma lipoprotein source. We have therefore termed this production of buoyant LDL in the Lpb 5.1 pigs direct buoyant LDL production.

Interaction of lipid transfer protein with plasma lipoproteins and cell membranes

The hydrophobic lipid components of lipoproteins, cholesteryl ester and triglyceride, are transferred between all lipoproteins by a specific plasma glycoprotein, termed lipid transfer protein (LTP). LTP facilitates lipid transfer by an exchange process in which cholesteryl ester and triglyceride compete for transfer. Thus, LTP promotes remodeling of the lipoprotein structure, and plays an important role in the intravascular metabolism of these particles and in the lipoprotein-dependent pathways of cholesterol clearance from cells. The properties of LTP, its mechanisms of action, its roles in lipoprotein metabolism, and its modes of regulation are reviewed along with recent data that suggest a possible role for this protein in directly modifying cellular lipid composition.