Differential effects of isolated lipoproteins from normal and hypercholesterolemic rhesus monkeys on cholesterol esterification and accumulation in arterial smooth muscle cells in culture

Intake of trans fatty acids and low-density lipoprotein size in a Costa Rican population

Intervention studies show that dietary composition altered low-density lipoprotein (LDL) particle size, but population studies are scarce, and the potential effects of trans fatty acids (FA) on LDL size are unknown. Trans FA intake has been associated with a more atherogenic lipid profile and increased coronary heart disease (CHD). We examined the association between dietary intake, including trans FA and LDL size, in 414 randomly selected subjects living in Puriscal, Costa Rica. Dietary intake was assessed by a validated food frequency questionnaire (FFQ). Women had larger LDL size (A) compared with men (263 v 261), and large LDL particles were correlated with increased intake (% energy) of protein (P =.005), animal fat (P =.041), trans FA (P <.0001), and decreased intake of carbohydrate (P =.052) in sex-, age-, and total energy intake-adjusted models. The correlation between trans FA intake and large LDL was significant in multivariate models that included dietary and nondietary factors; a 1% difference in trans FA was associated with a 2.44 A increase in LDL size (P =.004). In sum, it is possible that the effects of dietary factors, such as intake of trans FA on CHD are mediated through their effects on LDL size.

Effects of LDL enriched with different dietary fatty acids on cholesteryl ester accumulation and turnover in THP-1 macrophages

LDL enriched with either saturated, monounsaturated, n-6 polyunsaturated, or n-3 polyunsaturated fatty acids were used to study the effects of dietary fatty acids on macrophage cholesteryl ester (CE) accumulation, physical state, hydrolysis, and cholesterol efflux. Incubation of THP-1 macrophages with acetylated LDL (AcLDL) from each of the four diet groups resulted in both CE and triglyceride (TG) accumulation, in addition to alterations of cellular CE, TG, and phospholipid fatty acyl compositions reflective of the individual LDLs. Incubation with monounsaturated LDL resulted in significantly higher total and CE accumulation when compared with the other groups. After TG depletion, intracellular anisotropic lipid droplets were visible in all four groups, with 71% of the cells incubated with monounsaturated AcLDL containing anisotropic lipid droplets, compared with 30% of cells incubated with n-3 AcLDL. These physical state differences translated into higher rates of both CE hydrolysis and cholesterol efflux in the n-3 group. These data suggest that monounsaturated fatty acids may enhance atherosclerosis by increasing both cholesterol delivery to macrophage foam cells and the percentage of anisotropic lipid droplets, while n-3 PUFAs decrease atherosclerosis by creating more fluid cellular CE droplets that accelerate the rate of CE hydrolysis and the efflux of cholesterol from the cell.

The possible role of copper ions in atherogenesis: the Blue Janus

It has been proposed that the oxidative modification of low density lipoprotein (LDL) is a key event in human atherogenesis. Copper ions can catalyse the oxidative modification of LDL in vitro and there is some evidence that they may also participate in the oxidation of LDL within the arterial wall. However, copper ions also form an intrinsic constituent of superoxide dismutase and caeruloplasmin, enzymes that may be involved in preventing oxidative injury. Atherosclerotic lesions frequently contain considerable quantities of extracellular matrix molecules. These may contribute to the expansion of the arterial neointima, causing luminal narrowing. They may also play a beneficial role by stabilising the plaque. Copper is an essential component of lysyl oxidase, an enzyme involved in the biosynthesis of collagen, which is a major constituent of the extracellular matrix. The impact of alterations in body copper status on atherogenesis is therefore difficult to predict. Experimental and epidemiological data are conflicting and therefore do not provide a clear resolution of this issue. We have reviewed the biochemical and cellular effects of copper ions that may play a role in atherogenesis.

Effect of E5324, a novel inhibitor of acyl-CoA:cholesterol acyltransferase, on cholesteryl ester synthesis and accumulation in macrophages

The in vitro potencies of a novel inhibitor of acyl-CoA:cholesterol acyltransferase (ACAT), E5324 (n-butyl-N'-[2-[3-(5-ethyl-4-phenyl-1H-imidazol-1-yl)propoxy]-6- methylphenyl]urea), were studied. E5324 was found to be a potent ACAT inhibitor in microsomes from a various tissues and in cultured cell homogenate, with IC50 values in the range of 0.044 to 0.19 microM. The kinetic study on E5324 showed that the inhibition of rat intestine ACAT was competitive with respect to oleoyl CoA. E5324 inhibited [3H]olate incorporation into cholesteryl [3H]oleate in phorbol ester-treated THP-1 cell lines (IC50 = 0.44 microM). The rate of [3H]oleate incorporation into phospholipids and triglycerides was not affected by E5324. In an experiment with [3H]cholesterol as the substrate for ACAT, E5324 also inhibited [3H]cholesteryl ester synthesis (IC50 = 0.41 microM). Furthermore, E5324 prevented accumulation of both esterified and total cholesterol in acetyl low density lipoprotein-loaded THP-1 cells. These results indicate that E5324 is a potent and selective ACAT inhibitor and prevents cholesteryl ester accumulation in macrophages.

The effect of interferon on the metabolism of LDLs

Interferons have been shown to lower low density lipoprotein (LDL) cholesterol concentrations by 20-50%. To evaluate the effect of interferons on LDL metabolic behavior in individuals with normal and mildly elevated LDL cholesterol levels, autologous LDL labeled with 125I was administered to subjects at baseline and during interferon treatment. Interferon beta serine (IFN-beta serine) was administered intravenously at 4.5 x 10(6) units daily for at least 3 weeks before the start of kinetic study and continued for an additional 2 weeks. Results were analyzed by using a multicompartmental model that allows for two intravascular LDL compartments. In normal subjects, IFN-beta serine reduced LDL cholesterol and apolipoprotein (apo) B levels by 25% and 27%, respectively (p less than 0.05); LDL apo B synthesis was decreased by 59% (p less than 0.05). In hypercholesterolemic subjects, IFN-beta serine reduced LDL cholesterol levels by 38% (p less than 0.05); however, apo B concentrations and production rates were not significantly decreased. Clearance of LDL from the first intravascular apo B pool was markedly reduced in these subjects, resulting in a shift in the distribution of LDL apo B from the second to the first intravascular LDL apo B pool. We conclude that interferon's actions on LDL metabolism differ in normocholesterolemic and hypercholesterolemic subjects. In normal subjects, interferon decreased LDL cholesterol and apo B levels through a reduction in the LDL apo B production rate. However, in hypercholesterolemic subjects, interferon reduced LDL cholesterol by altering the distribution of apo B mass between LDL subspecies.

Long-term treatment of neonatal aortic smooth muscle cells with beta VLDL induces cholesterol accumulation

A model for smooth muscle derived foam cells was developed by treating smooth muscle cells isolated from the aortae of neonatal rabbits with beta VLDL for up to 1 month. Hyperlipidemic beta VLDL isolated from cholesterol fed rabbits induced proliferation of the cells that were maintained in lipid deficient serum. In addition, the lipoprotein fraction stimulated [14C]oleic acid incorporation into [14C]cholesteryl ester, even in cultures that had been chronically exposed to the lipoprotein. The accumulation of cholesterol was evaluated and small amounts of cholesteryl ester were demonstrated in cultures treated for 3 days with beta VLDL. However, continued exposure to the lipoprotein resulted in larger elevations in total cholesterol, approximately 65% of which was in the esterified form in cultures treated with 100 micrograms beta VLDL/ml for 24 days. When cholesterol levels were examined as a function of time, it was determined that both total cholesterol and cholesteryl ester levels increased. Approximately 2-3 weeks after lipoprotein was introduced to the culture, maximum levels were attained. Triglyceride levels were also measured and found to increase more than two-fold in cultures that had been incubated in the presence of beta VLDL for 24 days, when compared to cultures incubated in its absence. Examination of the cultures by electron microscopy revealed intracytoplasmic lipid droplets in beta VLDL treated cells. These results suggest that beta VLDL treatment of neonatal aortic smooth muscle cells provides an ideal model in which to study the lipid laden smooth muscle cells that characterize the atherosclerotic plaque.

Effect of dietary fish oil on coronary artery and aortic atherosclerosis in African green monkeys

Studies were carried out for 2.5 to 3 years in adult male African green monkeys (grivet subspecies) fed diets containing 22% of calories as lard or fish oil with 40% of calories as fat and 0.75 mg cholesterol/Kcal to determine if isocaloric substitution of menhaden fish oil for lard affects coronary artery atherosclerosis. The average total plasma cholesterol concentrations during the experimental period were significantly lower for the fish-oil group (231 +/- 37 mg/dl) compared to the lard group (360 +/- 44 mg/dl), but this difference did not become apparent until after 5 months of experimental diet consumption. High density lipoprotein cholesterol concentrations were 30% lower (p less than 0.01) for the fish-oil group also (57 +/- 5 vs. 82 +/- 6 mg/dl). Plasma triglyceride concentrations were low for both groups, but after about 5 months of diet consumption, they were higher for the animals fed fish oil (25 +/- 2 mg/dl) compared to their lard-fed counterparts (15 +/- 1 mg/dl). Coronary artery intimal area (in this case a measure of early atherosclerotic lesion size) was low in all animals but was significantly less (p less than 0.03) for the fish oil vs. lard groups (0.01 +/- 0.002 vs. 0.03 +/- 0.009 mm2). More atherosclerosis was found in other arteries, and a trend was seen of less atherosclerosis in the thoracic aorta and common carotid arteries of the fish-oil group. The size of lesions in the abdominal aorta was similar between diet groups, but microscopic examination of arteries of the lard group revealed relatively more cholesterol monohydrate crystals compared to the arteries of the fish-oil group. Chemical analysis showed that there was less esterified cholesterol (1.46 +/- 0.71 vs. 3.43 +/- 0.74 mg/g, p = 0.04) and free cholesterol (3.7 +/- 2.15 vs. 7.05 +/- 1.68 mg/g, p = 0.08) in the abdominal aortas taken from the animals fed fish oil. There was a significant correlation between low density lipoprotein (LDL) cholesteryl ester (CE) fatty acid ratio (i.e., saturated + monounsaturated/polyunsaturated species) and the amount of esterified (r = 0.59) and free (r = 0.63) cholesterol in the abdominal aortas. Compared to the lard group, animals fed fish oil had significantly lower LDL CE melting temperatures (26 +/- 1 vs. 38 +/- 1 degree C) and significantly smaller LDL particles (2.68 +/- 0.10 vs. 3.25 +/- 0.38 g/mumol). Therefore, the potentially antiatherogenic effects of dietary fish oil include its ability to decrease the concentration, size, CE content, and CE melting temperature of plasma LDL.

Triggerlike stimulation of cholesterol accumulation and DNA and extracellular matrix synthesis induced by atherogenic serum or low density lipoprotein in cultured cells

A 72-hour incubation of cultured cells with blood sera or plasma of patients suffering from coronary heart disease (CHD) with angiographically assessed coronary atherosclerosis caused a threefold to fourfold elevation of intracellular cholesterol. An elevated cholesterol level in the cells precultured with patients' sera was retained several days after the removal of the examined serum from culture. The accumulation of intracellular cholesterol was accompanied by enhanced synthesis of DNA, total protein, collagen, sulfated glycosaminoglycans, and hyaluronic acid. Enhanced DNA and total protein synthesis was retained for at least 9 days after the serum had been removed from culture. The obtained results suggest that the sera of CHD patients possess an atherogenic potential that manifests itself at the arterial cell level in the stable stimulation of atherosclerotic cellular processes: proliferation, lipidosis, and fibrosis. The examined sera of healthy donors were devoid of such an atherogenic potential. The low density lipoprotein (LDL) fraction (density, 1.030-1.050 g/cm3) obtained from an atherogenic serum had the same atherogenic potential as a whole serum. Atherosclerotic alterations in cultured intimal cells caused by atherogenic LDL were retained for at least 3 days after the removal of the lipoprotein from culture. Preincubation of intimal cells with LDL obtained from healthy donors had no effect on the intracellular cholesterol level or the synthesis of DNA and extracellular matrix. One may assume that the atherogenic potential of CHD patients' sera is related to the presence of LDLs that are qualitatively different from the LDL of healthy subjects.

Insolubilization of low density lipoprotein induces cholesterol accumulation in cultured subendothelial cells of human aorta

Primary cultures of typical and modified smooth muscle cells isolated from the intima of human aorta were used to study the mechanism whereby low density lipoprotein (LDL) induces accumulation of intracellular cholesterol. Incubation of intimal cells with native LDL obtained from human plasma did not lead to deposition of total cholesterol. LDL added to the cultures simultaneously with hyaluronic acid, heparin, chondroitin sulfate, fibronectin, and mouse monoclonal antibody against LDL also failed to alter the cellular cholesterol. On the other hand, 24-h incubation of the cells with LDL in the presence of dextran sulfate, gelatin, particles of aortic elastin, particles of collagenase-resistant aortic matrix, goat polyclonal antibodies against LDL or latex beads caused a significant (1.5-7-fold) increase in total cholesterol. The compounds which stimulated cholesterol deposition are able to form precipitating complexes with LDL. On the contrary, the agents which failed to induce cholesterol accumulation were unable to insolubilize LDL. A direct correlation (r = 0.927) was found between the cholesterol content of the insoluble complex and the increment of cholesterol in the cultured cells. To find out whether LDL plays a specific role in the deposition of intracellular cholesterol, very low density lipoproteins and high density lipoproteins were used. These lipoproteins stimulated the accumulation of intracellular cholesterol in the presence of agents capable of forming insoluble associates with them. Our data suggest that insolubilization of lipoproteins is a key event in the LDL-mediated accumulation of intracellular cholesterol induced by various agents.

Low-density lipoproteins isolated from the blood of patients with coronary heart disease induce the accumulation of lipids in human aortic cells

Smooth muscle cells cultured from the intima of unaffected human aorta accumulate lipids during incubation with the blood serum of patients with coronary heart disease (CHD). Blood sera of most healthy subjects fail to induce the deposition of lipids in cultured cells. Very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins of two subclasses (HDL2 and HDL3) were isolated from the blood of healthy subjects and CHD patients. LDL from the blood of healthy individuals did not raise intracellular lipid levels within 24 hr of cultivation (the maximal concentration used, 1000 micrograms/ml). During the same incubation period, LDL obtained from the blood of CHD patients (200 to 1000 micrograms/ml) caused a 2- to 5-fold rise in cholesteryl esters as well as a 1.5- to 3-fold rise in free cholesterol and triglycerides, whereas intracellular phospholipid levels remained unchanged. There was a direct correlation (r = 0.95) between cholesterol accumulation in the cells incubated with whole sera of CHD patients and cholesterol level in the cells incubated with LDL isolated from these sera. In one of the three cases, the ability to raise the intracellular level of cholesteryl esters was demonstrated by VLDL (500 micrograms/ml) derived from CHD patients' blood. HDL2 and HDL3 did not affect lipid levels in smooth muscle cells cultured from unaffected intima. HDL3 from the blood of CHD patients and healthy subjects (50 to 250 micrograms/ml) reduced cholesteryl ester levels in cells cultured from atherosclerotic plaques 1.5- to 2-fold. HDL2 also decreased the content of cholesteryl esters in plaque cells, though less effectively than HDL3. The data obtained suggest that circulating LDL and, possibly, VLDL in the blood of CHD patients are capable of inducing the accumulation of fat in vascular wall cells.

Lipoproteins and atherosclerosis

The plasma lipoproteins are the primary means of transport of cholesterol among tissues. In particular, the apo B-containing lipoproteins (VLDL, IDL and LDL) are important for the delivery of cholesterol from the liver to peripheral tissues, while HDL appear to mediate the reverse process of movement of cholesterol from tissues back to the liver. Both of these transport processes are necessary for efficient whole body cholesterol homeostasis, because the liver is the major site of both the production and excretion of cholesterol. However, deviations from a proper balance of transport of cholesterol, either increases in LDL levels or decreases in HDL cholesterol flux, may result in accumulation of cholesterol in extrahepatic tissues. Increased risk of atherosclerosis and CHD may be associated with elevation in the number of LDL particles, increase or decrease in LDL particle size, or changes in the composition of plasma LDL. These modifications of plasma LDL may be brought about following perturbation of one of several aspects of LDL metabolism. These include decreased LDL receptor activity, increased VLDL production and cholesterol enrichment of the liver-derived VLDL. The events in the arterial wall that make some LDL particles apparently atherogenic are not well understood. In the case of nonhuman primates, large-size LDL are associated with an increased risk of CHD. One characteristic of these LDL is that their core lipids are rich in saturated cholesteryl esters and their transition temperatures are frequently above body temperature. The liquid crystalline cholesteryl ester cores of such LDL may modulate the conformation of apo B on the surface and thereby affect the interaction of these LDL with cellular receptors or connective tissue matrix proteoglycans. It is likely, though, that changes in LDL particle number, LDL particle size and LDL particle composition may each contribute to progression of atherosclerosis. The presumed metabolic events that make HDL protective against atherosclerosis have been termed reverse cholesterol transport, and suggest that small HDL that are deficient in free cholesterol acquire this lipid from cell membranes. The HDL cholesterol is esterified by LCAT in the circulation, forming large HDL that can then deliver the cholesteryl ester to the liver by both direct and indirect means. In most circumstances, it is assumed that an increase in plasma HDL cholesterol concentration reflects an increase in the rate at which HDL is removing cholesterol from tissues and, consequently, a decrease in atherosclerosis.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of a low-energy diet associated with egg supplementation on plasma cholesterol and lipoprotein levels in normal subjects: results of a cross-over study

1. The influence of a low-energy diet when associated with high-cholesterol intake was investigated in seventeen normal men during an 8-week cross-over study. The subjects were given a daily supplement of two whole eggs and two egg yolks (approximately 1 g cholesterol) either with their usual diet for 4 weeks or with a low-energy diet for 4 weeks. Each subject took part randomly in both dietary periods. 2. During the first part of the study, no changes occurred in the plasma cholesterol of the subjects with egg supplementation of the usual diet. 3. In contrast, the low-energy diet and associated weight loss markedly decreased tolerance to high-cholesterol intake resulting in increased plasma cholesterol. The mean rise was 22.7% but with wide individual variations in the response. This was almost completely normalized when the subjects returned to their usual energy intake indicating the involvement of weight reduction in the increase observed. 4. Changes in low-density-lipoprotein (LDL) cholesterol were parallel to those of total plasma cholesterol with an increase following the low-energy diet and normalization after body-weight recovery. 5. The opposite effect was shown with the low-energy diet after previous adaptation to the consumption of four eggs per day. This dietary regimen resulted in a decrease in plasma cholesterol although it was not significant. Moreover, the lipoprotein profile was improved with a decrease in very-low-density-lipoprotein (VLDL) cholesterol and an increase in high-density-lipoprotein (HDL) cholesterol. 6. High-cholesterol intake induced significant changes in lipoprotein composition whatever the energy ration. LDL and HDL were enriched in cholesterol esters as early as the 1st month of egg supplementation of the diet. 7. Taken together, the results emphasize the possible adverse effect of slimming diets when associated with high-cholesterol intake. The existence of 'high-responders' to these dietary conditions calls for special attention to be paid to the cholesterol content of restricted diets.

Metabolism of atherogenic lipoproteins by smooth muscle cells of different phenotype in culture

Smooth muscle cells of the rabbit aorta, when grown in vitro, express three distinguishable forms of phenotype (contractile, reversible synthetic, and irreversible synthetic). We compared the interactions of these three smooth muscle phenotypes with rabbit very low density lipoprotein (VLDL), low density lipoprotein (LDL), and very low density lipoprotein from cholesterol-fed rabbits (beta-VLDL). beta-VLDL showed saturable. high-affinity binding characteristics with each phenotype predominantly through the B/E receptor. The irreversible synthetic cells displayed the greatest binding capacity and the contractile cells, the least. Binding and degradation of normal VLDL was less than that of beta-VLDL and higher than that of LDL. Only the irreversible synthetic cells showed substantial (about threefold) cholesteryl ester formation and cholesterol accumulation, and then only when incubated with beta-VLDL. Substantial stainable lipid, shown chemically to include triglyceride, cholesterol and cholesteryl ester, was also observed only when irreversible synthetic cells were exposed to beta-VLDL. The high capacity of irreversible synthetic-state, smooth muscle cells to bind and accumulate beta-VLDL in contrast to the relative immunity of contractile cells may be relevant to the genesis of atherosclerosis in the rabbit and possibly also in humans.

Effect of hypoxia on cholesterol accumulation in cultured rabbit aortic smooth muscle cells

We studied the effect of hypoxia on cholesterol accumulation in cultured rabbit aortic smooth muscle cells, which were incubated in a medium with normolipemic rabbit serum (NRS) or hyperlipemic rabbit serum (HRS). The cells were incubated in a humidified atmosphere of either 20% O2, 75% N2 and 5% CO2 (control cells) or 2% O2, 93% N2 and 5% CO2 (hypoxic cells). In a medium containing 20% NRS, the free cholesterol level of hypoxic cells was only a little higher than that of control cells, and there was no significant difference in esterified cholesterol content. On the other hand, in a medium containing 20% HRS, the free cholesterol level was slightly higher and the esterified cholesterol level was markedly higher in hypoxic cells compared with control cells. These results show that hypoxia promotes the accumulation of cholesterol, especially as ester, in smooth muscle cells cultured with hyperlipemic serum. These in vitro experiments indicate that hypoxia in the arterial wall associated with hyperlipidemia may play an important role in atherogenesis, although the precise mechanism remains unclear.

Enhanced cholesterol delivery to cells in culture by low density lipoproteins from hypercholesterolemic monkeys

Previous studies have shown that the large molecular weight low density lipoprotein (LDL) of abnormal composition isolated from hypercholesterolemic nonhuman primates stimulates greater cholesterol accumulation in cells in culture than does the same amount of normal LDL. The purpose of the present study was to determine if a correlation existed over a range of LDL molecular weights with cholesterol accumulation in cells in culture, if the differential in cholesterol accumulation was the result of increased delivery of cholesterol to the cells and to evaluate the extent to which this differential was dependent on a functional LDL receptor pathway. Monkey and human skin fibroblasts were incubated for 24 hours with LDL isolated from individual normal or hypercholesterolemic cynomolgus monkeys having LDL molecular weights ranging from 2.58-6.39 x 10(6), and the cellular free and esterified cholesterol contents were determined. There was no correlation of LDL molecular weight with accumulation of cellular free or esterified cholesterol with LDL from normal animals having molecular weight of 2.58 to 3.08 X 10(6) or from hypercholesterolemic animals with LDL molecular weights greater than 4.5 x 10(6). There was a positive and significant correlation of LDL molecular weight with the accumulation of cellular free and esterified cholesterol with LDL molecular weights of 3.0 x 4.5 x 10(6). These differences were present when the LDL were added at equivalent protein or cholesterol concentrations and cannot be entirely explained by the increased amounts of cholesterol in LDL particles of larger molecular weight. The enhanced cellular cholesterol accumulation with hypercholesterolemic LDL seems to be the result of increased delivery of LDL cholesterol to the cells as shown by the increased rate of suppression of cellular sterol synthesis and LDL receptor activity, the increased stimulation of cholesterol esterification, and the increased accumulation of cellular 3H-cholesterol from LDL labeled with 3H-cholesteryl oleate. This difference in cellular cholesterol accumulation requires that the LDL must be both bound and internalized by a functional LDL receptor pathway, since cells that lack LDL receptors or are unable to internalize their LDL receptors do not show increased accumulation of cholesterol when incubated with hypercholesterolemic LDL.

Structural and functional changes of rhesus serum low density lipoproteins during cycles of diet-induced hypercholesterolemia

Over the course of a 2-year study, two male rhesus monkeys underwent episodes of diet-induced hypercholesterolemia (from a diet supplemented with 25% coconut oil and 2% cholesterol) followed by regression phases in which the animals received a low fat Purina chow diet. During the induction of hypercholesterolemia, serum cholesterol, apo B, saturation of low density lipoprotein (LDL) cholesteryl ester fatty acyl chains, and the ability of the serum to stimulate cholesterol esterification by smooth muscle cells rose immediately and in parallel, whereas there was a lag period before the serum became mitogenic to smooth muscle cells. Concurrently, there were important changes in the density, size, chemistry, and concentration of the LDL species in the rhesus serum; induced LDL shifted from the LDL-II to the LDL-I density region with increasing cholesterol concentration. Both structural and functional changes were reversed upon return to a normal Purina chow diet, although at different rates. Serum cholesterol, apo B, and the rate of cholesterol esterification in smooth muscle cells promoted by the serum declined in parallel while the mitogenicity of the serum to smooth muscle cells and the degree of saturation of LDL cholesteryl ester fatty acids took longer to return to normal values. In fact, there was an immediate and dramatic rise in saturation upon reversal before the LDL cholesteryl ester fatty acyl chains returned to their normal composition. The Lp(a) particles did not increase in either concentration or size in response to the test diet, although the change in their lipid composition was similar to those of the other LDL species. The studies indicate that dietary manipulations affect the physicochemical properties of the LDL particles, and that the resultant structural alterations are accompanied by changed in vitro cellular response, suggestive of a greater atherogenicity.

Atherogenic hyperlipoproteinemia. The cellular and molecular biology of plasma lipoproteins altered by dietary fat and cholesterol

Diets high in saturated fat and cholesterol cause alterations in the plasma lipoproteins, and these alterations cause certain of the lipoproteins to deliver cholesterol to the cells of the arterial wall. Other changes in lipoprotein are induced as an attempt to compensate for the delivery of cholesterol to cells. Atherosclerosis results when influx of cholesterol into the arterial wall exceeds egress of cholesterol from the tissues. The interactions of the various plasma lipoproteins are described in order to generate a reasonable hypothesis characterizing atherogenic hyperlipoproteinemia.

Cholesterol transfer from normal and atherogenic low density lipoproteins to Mycoplasma membranes

The purpose of this study was to determine whether the free cholesterol of hypercholesterolemic low density lipoprotein from cholesterol-fed nonhuman primates has a greater potential for surface transfer to cell membranes than does the free cholesterol of normal low density lipoprotein. The low density lipoproteins were isolated from normal and hypercholesterolemic rhesus and cynomolgus monkeys, incubated with membranes from Acholeplasma laidlawii, a mycoplasma species devoid of cholesterol in its membranes, and the mass transfer of free cholesterol determined by measuring membrane cholesterol content. Since these membranes neither synthesize nor esterify cholesterol, nor degrade the protein or cholesterol ester moieties of low density lipoprotein, they are an ideal model with which to study differences in the cholesterol transfer potential of low density lipoprotein independent of the uptake of the intact low density lipoprotein particle. When added at an equivalent particle concentration, there was greater enrichment of membranes with free cholesterol from hypercholesterolemic low density lipoprotein. Hypercholesterolemic low density lipoprotein, however, contains more cholesterol per particle than normal low density lipoprotein; yet calculations on the basis of equivalent free cholesterol content showed no difference in either the rate or extent of free cholesterol transfer from normal or hypercholesterolemic low density lipoprotein. This was true for the transfer of at least 90% of the free cholesterol from both lipoproteins. These studies indicate that, even though there are marked differences in the cholesterol composition of normal and hypercholesterolemic low density lipoproteins, this does not result in a greater chemical potential for surface transfer of free cholesterol. Consequently, if a difference in the surface transfer of free cholesterol is responsible for the enhanced ability of hypercholesterolemic low density lipoprotein to promote cellular cholesterol accumulation and, perhaps, also atherosclerosis, it must be the result of differences in the interaction to the hypercholesterolemic low density lipoprotein with the more complicated mammalian cell membranes, rather than differences in the chemical potential for cholesterol transfer.

The mass uptake of cholesterol ester from low density lipoproteins by cultured smooth muscle and adventitial cells of human aortas

Cultured human smooth muscle and adventitial cells were incubated with human serum and low density lipoprotein (LDL) to study the uptake and accumulation of cholesterol ester from exogenous LDL. The cellular total cholesterol varied with the amount of LDL cholesterol in the medium. The cholesterol ester content increased 4-fold after 2 hours of incubation. A 6-fold rise occurred by 24 hours and continued to 72 hours. The cholesterol ester of the adventitial cells was markedly depleted by incubation with abetalipoproteinemic serum or with a lipid-depleted plasma fraction. By the use of 14C-labeled LDL free cholesterol in the incubation medium, we calculated that some 70-80% of the total accumulated cholesterol ester after 24 hours of incubation was derived from LDL cholesterol ester, and only 20-30% was synthesized by the cells. These studies demonstrated conclusively that human cells greatly increase their cholesterol ester mass after incubation with LDL.

Metabolism by cells in culture of low-density lipoproteins of abnormal composition from non-human primates with diet-induced hypercholesterolemia

Diet-induced hypercholesterolemia in non-human primates results in the production of a low-density lipoprotein (LDL) of abnormal size and composition. This LDL from hypercholesterolemic monkeys has been shown to be more atherogenic than the same amount of LDL from normocholesterolemic animals. Previous studies have demonstrated that hypercholesterolemic LDL is approximately twice as effective as normal LDL in stimulating cholesterol accumulation and esterification in arterial smooth muscle cells in culture. The purpose of the present study was determine whether this effect was secondary to differences in metabolism of the normal and hypercholesterolemic LDL. for this, the metabolism of 125I-labeled normal and hypercholesterolemic LDL from rhesus and cynomolgus monkeys was compared in several lines of skin fibroblasts and smooth muscle cells. Both normal and hypercholesterolemic LDL bound with high affinity to the same cell surface receptor. However, the affinity for binding of hypercholesterolemic LDL was about twice that of normal LDL (apparent dissociation constant for binding, Kd, was 2.63 micrograms protein/ml and 4.35 micrograms protein/ml, respectively). Conversely, only about 50% as many particles of hypercholesterolemic were able to bind to the receptor, compared with normal LDL. Those cells with the greatest capacity to metabolize LD generally accumulated the most cholesterol with either hypercholesterolemic or normal LDL. In all cell lines, nearly twice as much cholesterol accumulated in cells incubated with hypercholesterolemic LDL compared with normal LDL, and this differential could not be explained by differences in metabolism of the two lipoproteins, suggesting that some cholesterol entered the cells independent of the uptake of the intact LDL molecule. LDL receptors appear necessary for this to occur, since no difference in cholesterol accumulation was observed in cells genetically deficient in LDL receptors.

Malondialdehyde alteration of low density lipoproteins leads to cholesteryl ester accumulation in human monocyte-macrophages

Glutaraldehyde treatment of (125)I-labeled low density lipoprotein ((125)I-native-LDL) produced a modified LDL ((125)I-glut-LDL) with a molecular weight of 10 x 10(6) or more. Malondialdehyde treatment of (125)I-native-LDL produced a product ((125)I-MDA-LDL) with a molecular weight not appreciably different from that of the original lipoprotein. However, the electrophoretic mobility of MDA-LDL indicated a more negative charge than native-LDL. (125)I-MDA-LDL was degraded by two processes: a high-affinity saturable process with maximal velocity at 10-15 mug of protein per ml and a slower, nonsaturable process. The degradation of (125)I-MDA-LDL was readily inhibited by increasing concentrations of nonradioactive MDA-LDL but was not inhibited by acetylated LDL or native-LDL even at concentrations as high as 1600 mug of protein per ml. After exposure of native-LDL to blood platelet aggregation and release in vitro, 1.73 +/- 0.19 nmol of malondialdehyde per mg of LDL protein was bound to the platelet-modified-LDL. No detectable malondialdehyde was recovered from native-LDL that had been treated identically except that the platelets were omitted from the reaction mixture. After incubation with glut-LDL, MDA-LDL, or platelet-modified-LDL for 3 days, human monocyte-macrophages showed a dramatic increase in cholesteryl ester content whereas the cholesteryl ester content of cells incubated with the same concentration of native-LDL did not. Based on these experiments we propose that modification of native-LDL may be a prerequisite to the accumulation of cholesteryl esters within the cells of the atherosclerotic reaction. We further hypothesize that one modification of LDL in vivo may result from malondialdehyde which is released from blood platelets or is produced by lipid peroxidation at the site of arterial injury.

Atherogenesis: a postprandial phenomenon

The hypothesis that plasma chylomicrons in persons who ingest a cholesterol-rich diet are atherogenic is evaluated. Evidence is presented that in humans, and experimental animals, chylomicron remnants as well as low-density lipoproteins are taken up by arterial cells. In persons who do not have familial hyperlipoproteinemia, atherogenesis may occur during the postprandial period. Research directions that may contribute to the evaluation of chylomicron remnants as a risk factor for atherogenesis are discussed. Lipoprotein studies after administration of a test meal containing fat and cholesterol are urgently needed.