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

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.

Cholesteryl esterase-treated LDL augments oxidized LDL-mediated cholesteryl ester deposition in mouse peritoneal macrophages

Arterial unesterified cholesterol, phospholipid particles have been isolated from atherosclerotic lesions and characterized. However, the role of these 'liposomes' in macrophage foam cell formation is unclear. Recently, LDL, after trypsin and cholesteryl esterase treatment (T/CE LDL), was shown to have physical properties similar to the unesterified cholesterol, phospholipid particles isolated from atherosclerotic lesions. Yet, when mouse peritoneal macrophages were incubated with these model particles in culture medium (DMEM and 5% LPDS), only an insignificant accumulation of cellular cholesteryl esters was observed. Previously, we demonstrated that complex formation between unesterified cholesterol, phosphatidylcholine liposomes and cupric sulfate-oxidized LDL dramatically enhances the ability of the liposomes to augment cellular cholesterol accretion (Greenspan P, Yu H, Mao F, Gutman RL. J Lipid Res 1997;38:101-109). When T/CE LDL, another cholesterol-rich phospholipid particle, was substituted for unesterified cholesterol phosphatidylcholine liposomes in our complex, mouse peritoneal macrophages accumulated a significant amount of both cellular unesterifed cholesterol (61 microg/mg cell protein) and cholesteryl esters (76 microg/mg cell protein) after 48 h of incubation. These results demonstrate again that the interaction of two cholesterol-bearing particles (T/CE LDL and oxidized LDL), which individually can not promote significant cholesterol accumulation in cells, will, when combined, produce macrophage foam cells.

High fat, high cholesterol diets alter low density lipoprotein size and binding affinity in monkeys

The purpose of this study was to examine the effects of various dietary fats on low density lipoprotein (LDL) binding in an in vitro system where receptor number is not regulated. Cynomolgus monkeys were fed diets containing 37% of energy from fat, with various degrees of saturation, and 0.4 mg/kcal cholesterol or low-fat (13% of energy), low cholesterol (0.03 mg/kcal) chow. Plasma LDL was isolated after 16 weeks. The fatty acid composition of LDL showed enrichment corresponding to the dietary fats consumed, and the high fat, high cholesterol diets produced marked hypercholesterolemia compared to chow feeding. Of those fed the high fat diets, monkeys fed the fish oil diet had the highest LDL cholesterol concentrations, 13.25 +/- 0.77 mmol/l, while those fed the safflower oil diet had the lowest, 7.51 +/- 3.31. LDL from chow fed monkeys had the lowest binding affinity; the Kd was 26.2 +/- 8.7 microg/ml, nearly twice that of the high fat diets (P = 0.003). No significant differences in binding were found between the different high fat diets, although there was a trend toward lower affinity in the diets enriched in polyunsaturated fat. LDL size was affected by diet with chow fed monkeys having the smallest average LDL, 259.3 +/- 1.7 A compared to the other groups (P = 0.03). Monkeys fed the fish oil diet tended to have smaller LDL, but this was not significantly different from the other high fat diets. Binding affinity was correlated with LDL size, r = 0.54, P < 0.01. LDL composition, as measured by apo B/cholesterol ratio, was altered by feeding a high fat, high cholesterol diet. The ratio was reduced in the LDL samples from monkeys fed the high fat diets compared to those fed chow, but this ratio was not significantly correlated with binding. Thus, it appears that increasing dietary fat and cholesterol intake increases LDL size and binding affinity, such that LDL metabolism may be altered independently from effects on receptor number; the type of dietary fat does not seem to influence this process when fat and cholesterol content is very high.

Iron-ascorbate-phospholipid mediated modification of low density lipoprotein

LDL can be oxidized by a variety of agents to form a modified lipoprotein which is capable of being avidly metabolized by macrophages. While previous in vitro studies have focused exclusively on the oxidation of LDL, other lipids found in the atheroma are also subject to oxidation and its lipoperoxide byproducts may contribute to the process of LDL modification. To examine the relationship between the oxidation of phospholipids and the subsequent modification of LDL, we incubated 250 microM phosphatidylcholine with 10 microM ferrous sulfate and 50 microM ascorbic acid in 10 mM Tris (pH 7.0). After 18 h at 37 degrees C, significant amounts of thiobarbituric acid reactive substances (TBARS) were formed. The inclusion of LDL (100 micrograms protein/ml) elevated the TBARS and increased the electrophoretic mobility of the lipoprotein. LDL treated with iron and ascorbate in the absence of phosphatidylcholine did not result in the modification of this lipoprotein. LDL that was incubated with phosphatidylcholine, iron and ascorbate was found to be metabolized by macrophages to a far greater extent than native LDL or LDL treated with phosphatidylcholine alone. Probucol (10 microM) inhibited the LDL modification process. These results demonstrate that while iron and ascorbate cannot oxidize LDL directly, the addition of phosphatidylcholine to these initiators of lipid peroxidation can mediate and lead to the modification of LDL.

Association of negatively-charged phospholipids with low-density lipoprotein (LDL) increases its uptake and the deposition of cholesteryl esters by macrophages

LDL, the major carrier of cholesterol in blood, is poorly metabolized by macrophages. In contrast, macrophages can recognize and endocytose anionic phospholipids such as phosphatidylserine, phosphatidylglycerol and cardiolipin. Since macrophages can take up large amounts of these phospholipids, experiments were performed to ascertain whether pre-incubation of native LDL with negatively-charged phospholipids would enhance the metabolism of LDL by macrophages. When 125I-LDL was incubated with cardiolipin liposomes for 18 h at 37 degrees C before addition to macrophages, an approx. 40-fold increase of LDL metabolism by these cells was observed. Similar results were found when LDL was pre-incubated with phosphatidylserine or phosphatidylglycerol; however, pre-incubation of LDL with phosphatidylcholine liposomes did not lead to an increase of LDL metabolism. The macrophage uptake of LDL pre-incubated with cardiolipin was reduced to approx. 40% of control values in the presence of dextran sulfate and fucoidin, inhibitors of anionic phospholipid uptake. Cytochalasin D, an inhibitor of phagocytosis, reduced the lysosomal degradation of LDL pre-incubated with cardiolipin to approx. 10% of control values. When the LDL-cardiolipin mixture was chromatographed on agarose gel, two peaks containing LDL were observed in the elution profile: the first peak appeared at the void volume and the second peak was detected just ahead of native LDL. The LDL in both peaks was much more extensively metabolized by macrophages than was native LDL; the LDL in the first peak was metabolized at a rate that was 8 times the second peak. The results demonstrate that negatively-charged phospholipids can form a complex with LDL which facilitates its phagocytosis by macrophages.

The influence of dietary fat source (sunflower oil or olive oil) on LDL composition and serum lipid levels in miniature swine (Sus scrofa)

A total of 24 miniature swine (Sus scrofa) were fed with two diets of 9% fat content, differing only in the quality of the fat source (sunflower oil and olive oil). Two groups of animals were fed for a 12-week period, and the other two groups were fed for a 50-week period. After the two experimental periods, the influence of the dietary fat on serum lipids and protein and fatty acid composition of isolated LDLs was studied. In the short term, the serum cholesterol level was slightly higher in the olive oil group but, with the time of adaptation to the diet, serum levels of TC, FC and PL increased significantly in the sunflower group. In the long term, LDL and HDL were also significantly higher in the sunflower group when compared to the monounsaturated diet. In the sunflower group, PROT/TC and PROT/LIP ratios decreased significantly with the experimental period, while in the olive oil group they increased, due to the decrease in EC and TG fractions. The LDL particle in the olive group contained fewer saturated fatty acids and more monounsaturated fatty acids, specially oleic acid, than the LDL in the sunflower group. The changes found in chemical and fatty acid compositions of LDL, according to the saturation degree of the predominant fat of the diet, could alter its cellular metabolism.

Oral contraceptive treatment decreases arterial low density lipoprotein degradation in female cynomolgus monkeys

The effect of oral contraceptive therapy on early events in atherogenesis was studied in female cynomolgus monkeys. After a 1-month dietary challenge, monkeys were randomized into three groups stratified by total plasma cholesterol and high density lipoprotein cholesterol concentrations. The monkeys were then fed a cholesterol-containing diet for 16 weeks. This relatively short period ensured that studies were done before any treatment-induced differences in arterial morphology occurred. Monkeys were treated with either diet alone (control group), with the addition of a monophasic oral contraceptive (equivalent to a human dose of 50 micrograms ethinyl estradiol and 500 micrograms norgestrel per day), or with a triphasic oral contraceptive (equivalent to a human dose of 30-40 micrograms ethinyl estradiol and 50-125 micrograms levonorgestrel per day). Twenty-four hours before necropsy, low density lipoproteins (LDLs) labeled with 131I and LDLs labeled with the residualizing label 125I-tyramine cellobiose were injected into the animals. The arterial LDL degradation rate, amount of undegraded LDLs, and total LDL accumulation were then determined. Although there were regional differences in LDL metabolism, both treatments decreased the rate of LDL degradation and LDL accumulation in the coronary arteries and other arterial sites. Treatment also resulted in significantly lower LDL molecular weights. Despite a trend toward a more atherogenic lipid profile (decreased high density lipoprotein cholesterol and increased total plasma/high density lipoprotein cholesterol ratio), oral contraceptive treatment may inhibit atherogenesis by decreasing arterial LDL degradation.

Increased receptor binding of low-density lipoprotein from individuals consuming a high-carbohydrate, low-saturated-fat diet

The substitution of saturated fat by complex carbohydrate, according to current dietary recommendations, results in a decrease of plasma and low-density lipoprotein (LDL) cholesterol levels. To determine whether this decrease might result from structural and thus functional changes in LDL particles, the binding internalization and degradation of 125I-LDL were measured using TR715-19 cells, a mutant CHO line into which has been transfected the human LDL receptor, and in which measurements of binding are highly reproducible. Eleven nondiabetic subjects (35 +/- 4 years, 27% +/- 3% body fat) were studied after they had 15% protein, and 560 mg cholesterol/d and the other containing 21% fat (6% saturated), 65% carbohydrate, 14% protein, and 524 mg cholesterol/d.LDL cholesterol levels decreased form 125 +/- 6 to 108 +/- 5 mg/dl (P < .01) on the high-carbohydrate diet. There was an increase in the binding affinity of LDL (Kd 6.6 +/- 2.6 v 7.3 +/- 2.7 micrograms/mL +/- SD; P < .02), and internalization (P < .10), and degradation (P < .05) were also higher. The data suggest that decreasing dietary saturated fat may cause alterations in LDL composition that result in increased receptor clearance; this may partially explain the LDL-decreasing effect of this dietary change.

Regional differences in arterial low density lipoprotein metabolism in surgically postmenopausal cynomolgus monkeys. Effects of estrogen and progesterone replacement therapy

To determine if arterial lipoprotein metabolism may be involved in mediating well-known anatomic regional differences in susceptibility to atherosclerosis, arterial low density lipoprotein (LDL) metabolism and extent of atherosclerosis were studied in 17 ovariectomized female cynomolgus monkeys. The animals were fed an atherogenic diet for 18 weeks, during which time one group received 17 beta-estradiol and cyclic progesterone treatment (n = 9) and the controls received no hormone replacement therapy (n = 8). As reported previously, hormone replacement markedly reduced the accumulation of LDL in coronary arteries without affecting plasma lipoprotein patterns. We report here that LDL metabolism differed among arterial sites. LDL accumulation, LDL degradation rate, and the concentration of undegraded LDL were greatest in the coronary arteries and carotid bifurcations compared with the aorta, iliac arteries, and cerebral arteries. Although hormone replacement decreased indexes of LDL metabolism, there was no effect on intimal thickness or indexes of endothelial injury, such as leukocyte adhesion and endothelial cell turnover rate. There were, however, regional differences in these morphological parameters. The intima was thickest in the aorta, and leukocyte adhesion and endothelial cell turnover rates were greatest in the carotid bifurcation and thoracic aorta. The decreased accumulation and metabolism of LDL caused by hormone replacement therapy was specific to the arterial system and did not occur in the liver or other peripheral tissues.

Role of apolipoprotein E on cholesteryl ester-enriched low density lipoprotein particles in coronary artery atherosclerosis of hypercholesterolemic nonhuman primates

Significant differences among individuals occur in the lipoprotein response to atherogenic diets in cynomolgus and African green monkeys. The range of concentrations of total plasma cholesterol (TPC) was 100-600 mg/dl and of apolipoprotein (apo) E (quantified by enzyme-linked immunosorbent assay) was 3-20 mg/dl in the animal groups of this study. The correlation between the concentrations of TPC and of apo E was r = 0.89 in these animals. To determine which lipoprotein classes contained the majority of apo E, agarose gel-filtration chromatography was used to subfractionate whole plasma. In hypercholesterolemic monkeys, the majority of the apo E and apo B-100 coeluted within the region of low density lipoprotein (LDL). In normocholesterolemic monkeys, the majority of apo E coeluted with apo A-I and high density lipoproteins. A strong positive correlation was seen between the concentrations of plasma apo E and LDL cholesterol (r = 0.9), but there was no significant correlation between high density lipoprotein apo E and either TPC or plasma apo E concentrations. A positive correlation of r = 0.8 was found between the LDL apo E to apo B-100 molar ratio and the average LDL particle size, suggesting an increase in the number of apo E molecules on the larger LDL particles. Within individual animals, LDL were heterogeneous and the LDL subfractions of lower density (1.02 less than d less than 1.03 g/ml) had the highest proportion of apo E, although apo E was present on LDL of all densities. A strong positive correlation between plasma apo E concentration and coronary artery atherosclerosis was identified, and in stepwise regression analysis, plasma apo B concentration and the apo E to apo B molar ratio of LDL together accounted for more than 90% of the variation in the atherosclerosis end point of coronary artery intimal area. These data strongly suggest that the enrichment of LDL with cholesteryl esters and apo E, which occurs in hypercholesterolemic primates, is an atherogenic feature of the plasma lipoproteins.

Lipoprotein receptor activity of peritoneal macrophages from insulin-deficient mice

Binding, uptake and degradation of 125I-labeled normal very low density lipoprotein (125I-n-VLDL) from normal swine plasma and 125I-labeled beta-migrating VLDL (125I-beta-VLDL) from hypercholesterolemic rabbit plasma by peritoneal macrophages of mice rendered insulin-deficient by streptozotocin (250 mg/kg) were studied. It was found that the amount of binding, uptake and degradation of 125I-n-VLDL by macrophages from the diabetic mice was 2-fold or 2.5-fold higher than by macrophages from normal mice, resulting from an increase in the binding capacity of VLDL receptors on the macrophages from the insulin-deficient rodents. In contrast, the binding, uptake and degradation of 125I-beta-VLDL by macrophages from diabetic mice were reduced to only about 45% of normal levels because of a decrease in the number and affinity of the receptors for beta-VLDL. These experimental results indicate that n-VLDL is more important than beta-VLDL in the pathogenesis of atherosclerosis in insulin-dependent diabetes.

Estrogen and progesterone replacement therapy reduces low density lipoprotein accumulation in the coronary arteries of surgically postmenopausal cynomolgus monkeys

The effect of estrogen and progesterone replacement therapy on the initiating events in atherogenesis was studied in surgically postmenopausal cynomolgus monkeys. Monkeys were ovariectomized and divided randomly into two groups, one receiving 17 beta-estradiol and cyclic progesterone treatment (n = 9) and ovariectomized controls receiving no hormone replacement therapy (n = 8). The monkeys were fed a moderately atherogenic diet for 18 wk to accelerate the early pathogenic processes but not to be of sufficient duration to produce grossly visible atherosclerotic lesions. Sex hormone replacement therapy decreased the accumulation of LDL and products of LDL degradation in the coronary arteries by greater than 70% while having no significant effect on plasma lipid, lipoprotein, or apoprotein concentrations. Arterial intimal lesions were small with no difference between groups. The reduction in arterial LDL metabolism occurred very early in the pathogenesis of atherosclerosis and was independent of indices of endothelial cell injury, such as enhanced endothelial cell turnover or leukocyte adhesion to the endothelium. Results of this study suggest that one mechanism by which sex hormone treatment inhibits the initiation of atherosclerosis is a direct effect at the level of the arterial wall by suppressing the uptake and/or degradation of LDL.

In vivo clearance of low-density lipoproteins and beta-very-low-density lipoproteins in normal and hypercholesterolemic White Carneau pigeons

Low-density lipoproteins (hLDL) and beta-migrating-very-low-density lipoproteins (beta-VLDL) were isolated from the plasma of cholesterol-fed White Carneau (WC) pigeons and low-density lipoproteins (nLDL) were isolated from the plasma of grain-fed WC pigeons. The lipoproteins were radiolabeled with 125I or 131I and injected into normocholesterolemic or hypercholesterolemic WC pigeons to determine their rate of clearance from the plasma. The fractional catabolic rate (FCR) of nLDL and hLDL in normocholesterolemic pigeons averaged 0.202 and 0.206 pools/h.respectively. beta-VLDL was cleared at a significantly slower rate of 0.155 pools/h. The FCR of the same lipoproteins injected into hypercholesterolemic pigeons was reduced by 17% for nLDL, 50% for hLDL and 57% for beta-VLDL, indicating that the effect of hypercholesterolemia on clearance in vivo was different for the three lipoproteins. The FCR of reductively methylated pigeon LDL (MeLDL), which gives a measure of receptor-independent clearance of LDL, was shown previously to be 0.037 pools/h. These studies suggest therefore that LDL and beta-VLDL are cleared from the plasma of normocholesterolemic and hypercholesterolemic pigeons at a rate substantially greater than that predicted for non-specific processes. Despite the reduction in the clearance rate of hLDL and beta-VLDL due to cholesterol feeding, the absolute amount of cholesterol that was cleared from the plasma by these lipoproteins was increased from approx. 200 mg/kg body weight per day in the normocholesterolemic pigeons to greater than 1000 mg/kg body weight per day in the hypercholesterolemic pigeons. This is due principally to the enrichment in cholesterol relative to protein of the lipoproteins isolated from cholesterol-fed pigeons and the failure of hypercholesterolemia to completely inhibit receptor-dependent clearance of LDL and beta-VLDL. The lower rate of clearance of beta-VLDL relative to LDL is in marked contrast to mammalian beta-VLDL, which is cleared much faster than LDL, but is consistent with the lack of apo E on pigeon lipoproteins. Apo E is the apoprotein that is thought to be responsible for the rapid clearance of beta-VLDL in normocholesterolemic mammals. The low rate of beta-VLDL clearance in pigeons also suggests that pigeons lack an apolipoprotein that function like mammalian apo E.

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.

Beta-VLDL metabolism by pigeon macrophages. Evidence for two binding sites with different potentials promoting cholesterol accumulation

Previous studies from our laboratory (J Lipid Res 1988;29:643-656) have shown that thioglycolate-elicited peritoneal macrophages from White Carneau and Show Racer pigeons, like mammalian macrophages, have on their surfaces specific receptors for acetylated low density lipoprotein (acLDL) and beta-migrating very low density lipoproteins (beta-VLDL). The binding kinetics of beta-VLDL were complex, however, suggesting more than one binding site. The purpose of the present study was to further characterize these beta-VLDL binding sites. Scatchard analysis of 125I-beta-VLDL binding curves indicated at least two classes of binding sites. The first binds pigeon beta-VLDL and LDL with high affinity (Kd approximately 7 micrograms/ml), is down-regulated by cholesterol loading, requires calcium, and is destroyed by the proteolytic enzyme, pronase. This pigeon beta-VLDL receptor is specific for pigeon beta-VLDL and LDL and does not recognize HDL, acLDL, methyl LDL, cynomolgus monkey LDL, or rabbit beta-VLDL. Like the mammalian macrophage beta-VLDL receptor, the "pigeon beta-VLDL receptor" has many of the characteristics of an LDL receptor. The second class of binding sites is relatively nonspecific, recognizing both pigeon and rabbit beta-VLDL, LDL, acLDL, methyl LDL, and HDL. Binding to this site is not altered by incubation of macrophages with pronase or by cholesterol loading. This binding site has low affinity for beta-VLDL (Kd approximately 100 micrograms/ml), but high capacity. We have called this the "lipoprotein binding site," a term used by others to describe similar lipoprotein binding characteristics on a variety of cells. Not only does binding to this site promote the internalization and degradation of lipoproteins, but it may also facilitate the independent uptake of cholesterol. This conclusion is based on the observation that more cholesterol accumulates in cells incubated with rabbit beta-VLDL, which binds only to the lipoprotein binding site, than can be accounted for by beta-VLDL uptake and degradation. Since the lipoprotein binding site recognizes a variety of normal, as well as abnormal, lipoproteins, it would not require the generation of abnormal lipoprotein products, as must occur with the scavenger receptor, to promote the accumulation of cholesteryl esters in macrophages of atherosclerotic lesions. This, coupled with the fact that the lipoprotein binding site is not down-regulated by cholesterol loading, suggests that it could provide an alternative mechanism to the scavenger receptor pathway for the formation of foam cells.

Modification of lipoprotein patterns and retardation of atherogenesis by a fish oil supplement to a hyperlipidemic diet for swine

We have studied the effect of addition of 30 ml cod liver oil (FO) daily to a highly atherogenic butter (BT) diet for swine on lesion development in the coronary arteries and aorta, plasma lipoprotein (LP) patterns, plasma levels of thiobarbituric acid-reactive substances (TBARS) and on tritiated thymidine-labeling indices ([3H]TdR LI) of smooth muscle cells (SMC) and monocyte/macrophages (M/M phi) in the atherosclerotic lesions. Seventeen male Yorkshire swine (11.1 +/- 0.4 kg) were divided into 3 groups: BT (n = 6), BT + FO (n = 6) and mash (n = 5). They were fed the respective diets for 4 months. Terminally, fasting plasma was obtained and cholesterol contents were determined in various fractions of lipoproteins separated by density gradient ultracentrifugation, Pevikon block electrophoresis and immunoelectrophoresis. Apoprotein (B, A-I, E and C) contents of the plasma and lipoprotein fractions were determined by polyacrylamide gel electrophoresis and densitometry of gels stained with Coomassie blue. Swine were injected intramuscularly with 0.5 mCi/kg of [3H]TdR 2 h before death. The aorta and coronary arteries were perfusion fixed in situ under anesthesia. Samples were obtained for microscopic morphometry, autoradiography and immunohistochemistry from distal abdominal aorta, thoracic aorta, and proximal coronary arteries; left main (LM), left anterior descending (LAD), left circumflex (LCX), right main (RM), and right coronary artery (RCA). On the BT diet without FO there was extensive atherosclerotic (AS) lesion development, which was drastically reduced by the addition of FO to the BT diet in all sites by from 71 to 94%. The overall plasma cholesterol (CH) levels were reduced only modestly by the FO (816 +/- 64 to 629 +/- 14 mg/dl) but the distribution of CH in the various lipoprotein classes was remarkably altered. The CH in the large lipoprotein molecules containing both B and E apoproteins was reduced from 488 +/- 84 to 204 +/- 17 mg/dl by the FO with an almost corresponding increase in the conventional LDL molecules containing apo B only (158 +/- 29 to 344 +/- 15 mg/dl). We offer the hypothesis that the large apo B,E containing molecules are much more atherogenic than the smaller apo B containing molecules. This hypothesis is supported by a highly significant correlation between extent of lesion development in all arterial sites and plasma levels of CH in apo B,E containing lipoproteins. Plasma TBARS were elevated by the BT + FO diet but seemed to have no significant effect on the lesions.(ABSTRACT TRUNCATED AT 400 WORDS)

Low density lipoprotein interaction with artery derived proteoglycan: the influence of LDL particle size and the relationship to atherosclerosis susceptibility

An in vitro binding system was used to determine whether increases in LDL particle size and altered LDL chemical composition accompanying increased plasma cholesterol concentrations result in greater association of LDL with artery proteoglycan (PG) and whether the binding is related to atherosclerosis. LDL isolated from hypercholesterolemic atherosclerosis-susceptible White Carneau and resistant Show Racer pigeons was complexed to purified White Carneau pigeon aorta-derived high molecular weight PG under conditions whereby PG monomers were saturated. Using LDL of molecular weight greater than 5.0 x 10(6) daltons from both pigeon breeds, an inverse correlation between LDL size and the number of LDL particles bound per micrograms PG was demonstrated (r = 0.87, P less than 0.01). This relationship was attributed to the increased size of the LDL particle rather than any modification in chemical composition known to occur when LDL size increases, suggesting the major effect was attributed to steric hindrance. White Carneau pigeons with high molecular weight LDL had more severe atherosclerosis and the PG-LDL complexes contained excess cholesterol but no relationship was seen between atherosclerosis and number of LDL complexed. In animals with LDL between 3.6 x 10(6) and 4.8 x 10(6) daltons, considerable variability in PG binding was apparent, but this also was not related to LDL chemical composition. In this group of pigeons, which were all White Carneau, the positive relationship of PG-LDL binding and aorta cholesterol concentration was significant (r = 0.67, P less than 0.05). These results suggest that factors other than chemical composition (perhaps surface charge or apoprotein conformation changes) influence PG-LDL binding and that the assessment of PG-LDL binding is useful in predicting atherosclerosis in animals that do not respond to hypercholesterolemia by increasing LDL size.

Cholesterol accumulation in aortic smooth muscle cells exposed to low density lipoproteins. Contribution of free cholesterol transfer

Incubation of cultured arterial smooth muscle cells with large concentrations of low density lipoproteins (LDL) resulted in a net increase in cell cholesterol and cholesteryl ester mass that was dependent on LDL concentration and time of incubation. Use of an inhibitor of acyl-CoA:cholesterol acyl-transferase (ACAT) reduced the accumulation of cholesteryl ester mass by 40% (range 25% to 50%), suggesting that a significant proportion of the cholesteryl ester mass that accumulated from LDL did so without being hydrolyzed and re-esterified. Quiescent arterial smooth muscle cells exposed for 48 hours to 0.5 mg/ml of 125I-LDL accumulated 115 nmol total sterol/mg cell protein. However, these cells took up and degraded only 21 micrograms of 125I-LDL protein, which contains 64 nmol total cholesterol. Hence, only about 60% of the increase in cell-associated cholesterol mass was accounted for by LDL particle uptake and degradation. Further, when cells were incubated with 3H cholesteryl linoleyl ether-labeled LDL, the net increase of total cell cholesterol was 81 nmol/mg cell protein. However, only 49 nmol of total cholesterol was taken up by LDL particle uptake, as calculated from the uptake of the 3H cholesteryl linoleyl ether tracer. It thus appears that about 40% of the accumulated cholesterol mass was derived independent of LDL particle uptake, suggesting the possibility of transfer of free cholesterol from the surface of LDL to the cell surface. The occurrence of cholesterol surface transfer was independently verified by the measurement of the uptake and cellular distribution of LDL-derived free 3H-cholesterol. A substantial fraction of the accumulated cell cholesterol mass (approximately 40%) was derived from surface transfer of LDL free cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of fatty acid and triacylglycerol metabolism of macrophages and smooth muscle cells

The response of macrophages and smooth muscle cells to culture in free fatty acid has been compared. Because oleate and linoleate promoted triacylglycerol enrichment of smooth muscle cells, whereas palmitate had little effect, oleate was used for these studies. The kinetics of the accumulation of triacylglycerol produced by oleate was comparable between smooth muscle cells and macrophages. When grown in increasing concentrations of oleic acid at various fatty acid to albumin molar ratios, the extent of triacylglycerol accumulation in both cell types was dependent on the concentration of oleate, the concentration of albumin, and the oleate to albumin molar ratio. However, macrophages contained 2.6-fold more triacylglycerol than smooth muscle cells in the presence of oleate at 0.36 mM or greater and at levels of albumin higher than 0.15 mM. The cellular triacylglycerol content of macrophages was linearly related to the oleate to albumin molar ratio at both a constant albumin concentration and a constant oleate concentration, whereas the accumulation of triacylglycerol in smooth muscle cells showed a curvilinear relationship. When cells were preloaded with triacylglycerol, smooth muscle cells showed a greater loss of lipid when exposed to albumin than macrophages did. Over a two-hr time period, macrophages incorporated twice as much labeled fatty acid as smooth muscle cells. Thus, while smooth muscle cells and macrophages showed similar responses to exogenous fatty acid and albumin, there were also significant quantitative distinctions.

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)

Mechanisms and consequences of cellular cholesterol exchange and transfer

It is apparent from consideration of the reactions involved in cellular cholesterol homeostasis that passive transfer of unesterified cholesterol molecules plays a role in cholesterol transport in vivo. Studies in model systems have established that free cholesterol molecules can transfer between membranes by diffusion through the intervening aqueous layer. Desorption of free cholesterol molecules from the donor lipid-water interface is rate-limiting for the overall transfer process and the rate of this step is influenced by interactions of free cholesterol molecules with neighboring phospholipid molecules. The influence of phospholipid unsaturation and sphingomyelin content on the rate of free cholesterol exchange are known in pure phospholipid bilayers and similar effects probably occur in cell membranes. The rate of free cholesterol clearance from cells is determined by the structure of the plasma membrane. It follows that the physical state of free cholesterol in the plasma membrane is important for the kinetics of cholesterol clearance and cell cholesterol homeostasis, as well as the structure of the plasma membrane. Bidirectional flux of free cholesterol between cells and lipoproteins occurs and rate constants characteristic of influx and efflux can be measured. The direction of any net transfer of free cholesterol is determined by the relative free cholesterol/phospholipid molar ratios of the donor and acceptor particles. Cholesterol diffuses down its gradient of chemical potential generally partitioning to the phospholipid-rich particle. Such a surface transfer process can lead to delivery of cholesterol to cells. This mechanism operates independently of any lipoprotein internalization by receptor-mediated endocytosis. The influence of enzymes such as lecithin-cholesterol acyltransferase and hepatic lipase on the direction of net transfer of free cholesterol between lipoproteins and cells can be understood in terms of their effects on the pool sizes and the rate constants for influx and efflux. Excess accumulation of free cholesterol in cells stimulates the rate of cholesteryl ester formation and induces deposition of cholesteryl ester inclusions in the cytoplasm similar to the situation in the 'foam' cells of atherosclerotic plaque. Clearance of cellular cholesteryl ester requires initial hydrolysis to free cholesterol followed by efflux of this free cholesterol. The rate of clearance of cholesteryl ester from cytoplasmic droplets is influenced by the physical state of the cholesteryl ester; liquid-crystalline cholesteryl ester is removed more slowly than cholesteryl ester in a liquid state.(ABSTRACT TRUNCATED AT 400 WORDS)

Relationships between lymphocyte cholesterol homeostasis and LDL-cholesterol

The homeostasis of cholesterol was studied in lymphocytes freshly isolated from the blood and cultured with or without low-density lipoprotein (LDL). The content of cholesterol decreased in the lymphocytes cultured without LDL, whereas LDL substituted for cellular cholesterol losses, in spite of almost suppressed LDL-receptor and lymphocyte cholesterol synthesis. Free cholesterol was taken up from LDL mainly via cholesterol exchange and, in contrast to esterified cellular cholesterol, rapidly excreted into the medium. In vitro stimulation of lymphocyte cholesterol synthesis was correlated to the ratio of esterified to free LDL-cholesterol in the blood from which the lymphocytes had been isolated. This result probably reflects the different rates of influx and efflux of esterified or free cholesterol between plasma lipoproteins and lymphocytes. These effects should be taken into account if LDL-cholesterol is determined in plasma for the evaluation of an individual's atherosclerotic risk.

Metabolism of low density lipoproteins by pigeon skin fibroblasts and aortic smooth muscle cells. Comparison of cells from atherosclerosis-susceptible and atherosclerosis-resistant pigeons

Aortic smooth muscle cells from atherosclerosis-susceptible White Carneau (WC) pigeons lack a functional low density lipoprotein (LDL) receptor pathway. The purpose of the present study was to determine if atherosclerosis-resistant Show Racer pigeons (SR) shared this lack of an LDL receptor pathway and if LDL from normal and hypercholesterolemic pigeons were metabolized similarly. The amount of LDL bound, internalized, and degraded by skin fibroblasts, embryo fibroblasts, and aortic smooth muscle cells from WC and SR pigeons were similar and averaged from 2% to 25% of that seen with monkey smooth muscle cells incubated with the same LDL. LDL uptake by pigeon cells was due largely to nonspecific processes, while specific uptake predominated in monkey cells. A similar lack of specific uptake was obtained with LDL from normal and hypercholesterolemic pigeons. Sterol synthesis and HMG-CoA reductase activity were 10- to 35-fold higher in pigeon cells than in monkey cells incubated in serum-containing medium. LDL had little effect on cholesterol esterification and cholesteryl ester accumulation in pigeon cells. These results indicate that despite major changes in the size and composition of LDL from hypercholesterolemic pigeons, this LDL, like normal pigeon and monkey LDL, was not metabolized by specific uptake processes by pigeon cells. Cells from both WC and SR pigeons lack a functional LDL receptor pathway.

Effect of egg yolk feeding on the concentration and composition of serum lipoproteins in man

The effect of egg yolk consumption on the composition of LDL and on the concentration of HDL subclasses was studied in healthy subjects. Six volunteers consumed a diet low in cholesterol for 10 days and then daily added 6 egg yolks to their diet for another 10 days; the experiment was repeated 1 year later with the same subjects. Egg yolk consumption caused the cholesterol intake to increase by 1600 mg/day, and the fat intake by 7 energy % at the expense of carbohydrates; this increase was due almost exclusively to monounsaturated fatty acids. Upon egg yolk feeding the mean level of serum total cholesterol rose by 13%; the bulk of this rise was due to LDL cholesterol, which increased by 21%. VLDL and IDL cholesterol decreased by 19 and 11%, and serum total triglycerides by 17%. Marked relative increases of 35 and 36% were seen in the cholesterol level of the HDL subfractions with densities of 1.055-1.075 g/ml (HDL1) and 1.075-1.100 g/ml (HDL2), respectively. The HDL2/LDL cholesterol ratio increased by 16%. No change in cholesterol in HDL3 (d greater than 1.100 g/ml) was observed. The increase in cholesterol in HDL isolated by density gradient ultracentrifugation significantly exceeded the increase in cholesterol in heparin-Mn2+ soluble HDL. This suggests the formation of apo E-containing HDL, i.e. HDLc, which has HDL density but is not soluble in heparin-Mn2+. The composition of the LDL particles was significantly altered; the core became enriched in esterified cholesterol at the expense of triglycerides, and the ratio of core components to surface components increased by 7%.

New concepts about the pathogenesis of atherosclerosis in diabetes mellitus

New concepts about the pathogenesis of atherosclerosis in diabetes mellitus are presented. Emphasis is given to alterations of endothelial function, as indicated by von Willebrand factor activity, prostacyclin release, and fibrinolytic activity in diabetes mellitus. Previous work on platelet aggregation and arachidonic acid metabolism is updated and recent findings are emphasized. The atherogenic mix of elevated low-density lipoprotein cholesterol and low high-density lipoprotein cholesterol levels in uncontrolled diabetes mellitus is noted. The lipid hypothesis is extended by consideration of very low-density lipoprotein and intermediate-density lipoprotein metabolism in diabetes. Lipoprotein-cell interactions that may contribute to atherosclerosis are reviewed and suggestions are made for future research in order to clarify the pathogenesis of atherosclerosis in diabetes mellitus.

Cholesterol net transport, esterification, and transfer in human hyperlipidemic plasma

Cholesterol esterification, cholesteryl ester transfer between lipoproteins, and cholesterol transport between lipoproteins and cultured cells have been measured in the plasma of 22 patients with primary hyperlipidemia and 10 normolipidemic subjects. In hyperbetalipoproteinemia, increase in plasma low density lipoprotein levels was associated with a reduction of cholesteryl ester transfer rates, and with a reversal of the normal direction of sterol transport between fibroblasts and their plasma culture medium. Instead of net transport from cells to medium there was a net uptake of sterol from plasma by the cells, despite a level of plasma lecithin/cholesterol acyltransferase activity that was within the normal range. In dysbetalipoproteinemia, esterification rates were increased above normal levels, but cholesteryl ester transfer was reduced and the direction of sterol transport between the cells and plasma medium was reversed, as in the hyperbetalipoproteinemic group. In hypertriglyceridemia, those subjects with cardiovascular disease showed a metabolic pattern similar to the hyperbetalipoproteinemic group. The subjects in this group without symptoms of cardiovascular disease showed a normal direction of sterol transport, normal or raised rates of cholesteryl ester transfer between lipoproteins, and an increased rate of sterol esterification in plasma that decreased towards normal levels as plasma triglyceride levels decreased. Despite their quite distinct metabolic patterns there was no consistent difference between the two hypertriglyceridemic groups in triglyceride or cholesterol levels, very low density lipoprotein composition, or electrophoretic or isoelectric focussing patterns. All hypertriglyceridemic subjects with documented cardiovascular disease showed reversed cell-plasma sterol transport and all subjects without such disease showed a normal direction of cell-plasma sterol transport. The results of this study indicate major and reproducible abnormalities in plasma cholesterol metabolism in several groups of subjects with genetically distinct hyperlipidemias, who are at risk for atherosclerotic vascular disease. The possible predictive value of sterol metabolic measurements in the analysis of cardiovascular disease is discussed.

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.

Uptake of [3H]cholesterol from low density lipoprotein by cultured human fibroblasts

The uptake of [3H]cholesterol from low density lipoprotein (LDL) was studied in LDL receptor-positive and receptor-negative human fibroblasts. In both cell lines the uptake depended upon temperature, time of incubation and the concentration of LDL in the medium. Although the incorporation of 125I-labeled LDL was minimal after 2 h of incubation in the receptor-negative (homozygous familial hypercholesterolemia, FH) cells, the uptake of [3H]cholesterol was only slightly less than that of the receptor-positive (WI-38) cells. With longer periods of incubation, a larger difference in labeled cholesterol incorporation was observed; this appeared to be due to a continued accumulation of the steroid in the WI-38 cells. After 8 and 24 h of incubation, some of the [3H]cholesterol was present as the ester in the WI-38 cells, but not the FH cells. Modified (reduced and methylated) LDL did not enter WI-38 cells by the receptor-mediated pathway during 2 h of incubation, as indicated by 125I uptake. [3H]Cholesterol uptake, however, was not significantly different from modified and unmodified LDL. While experiments indicated that significant amounts of cholesterol moved rapidly from LDL to cultured cells with a dependence on time and LDL concentration, no increase in total cell cholesterol was detected in either cell line. FH cells contained less total cholesterol and had a higher 3H specific activity than the WI-38 cells. These data suggest that there may be important mechanisms in addition to the LDL pathway for the movement of lipids into cells.

Effects of dietary cholesterol and fatty acids on plasma lipoproteins

The effects of dietary cholesterol and fatty acids on low density and high density lipoproteins (LDL and HDL) were studied in 20 young men. After 2-3 wk of evaluations on ad lib. diets, basal diets, which consisted of 15% protein, 45% carbohydrates, 40% fat, and 300 mg/day of cholesterol, were given for 4-5 wk (Basal). The ratio of dietary polyunsaturated to saturated fatty acids (P/S) for different groups of subjects were 0.25, 0.4, 0.8, or 2.5. 750 and 1,500 mg/d of cholesterol were added to the basal diets as 3 and 6 eggs, respectively. Total cholesterol and LDL cholesterol were lower in all subjects on the basal diets than on the ad lib. diets. Addition of 750 mg cholesterol to the diet with P/S = 0.25-0.4 raised LDL cholesterol by 16 +/- 14 mg/dl to 115% of basal diet values (n = 11, P less than 0.01); 1,500 mg increased LDL cholesterol by 25 +/- 19 mg/dl to 125% (n = 9, P less than 0.01). On the diet with P/S = 0.8, 750 mg produced insignificant increases in LDL cholesterol, but 1,500 mg produced increases of 17 +/- 22 mg/dl to 115% of basal (n = 6, P less than 0.02). On the P/S = 2.5 diet, neither 750 nor 1,500 mg produced significant changes. Thus, both the cholesterol contents and P/S ratios of diets were important in determining LDL levels. The lipid and apoprotein compositions, flotation rates, molecular weights, and binding by cellular receptors of LDL were virtually unchanged by the addition of cholesterol to the diets high in saturated fat. These diets, therefore, caused an increase in the number of LDL particles of virtually unchanged physical and biological properties. On the diet with low P/S ratio, HDL2 rose, whereas this effect was absent on diets with high P/S ratios. The response of LDL to dietary manipulations is consonant with epidemiologic data relating diets high in cholesterol and saturated fat to atherogenesis. The response of HDL2, however, is opposite to that of its putative role as a negative risk factor. Further work is needed to clarify this interesting paradox.

Cholesterol transport between cells and body fluids. Role of plasma lipoproteins and the plasma cholesterol esterification system

The manner in which cells retain their sterol content is reviewed. Although most of what is known at the molecular level has been defived from studies in continuous cell culture, the findings appear to be broadly applicable to conditions in vivo. The main impetus to this research has come from the potential direct relevance of cell sterol balance to lipid disorders.

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.