Effect of hyperlipemic serum on cholesterol accumulation in monkey aortic medial cells

Pluronic block copolymers inhibit low density lipoprotein self-association

Little is known about exogenous inhibitors of low-density lipoprotein (LDL) aggregation. The search for nontoxic and bioavailable inhibitors of LDL aggregation is of interest, especially considering that the suppression of the aggregation of LDL might represent a therapeutic approach. We hypothesized that amphiphilic copolymers of propylene oxide and ethylene oxide, the so-called Pluronic block copolymers, can be used to influence the aggregation of LDL. In this work we used Pluronic® P85, L61 and F68. A comparative study of the effects of Pluronic block copolymers with various hydrophilic-lipophilic properties on the aggregation process of LDL showed that Pluronic copolymers with strong hydrophobic properties (P85 and L61) at concentrations close to or greater than the respective critical concentration of micelle formation inhibited the aggregation process of LDL; however, the "hydrophilic" Pluronic F68 had no effect on the aggregation of LDL at any concentration. Thus, the study demonstrated for the first time that Pluronic® block copolymers inhibit LDL self-association. The possibility of modulating the aggregation of LDL by various Pluronic copolymers can be regarded as a prerequisite in the creation of new types of anti-atherosclerotic drugs.

Plasma kinetics of free and esterified cholesterol in familial hypercholesterolemia: Effects of simvastatin

The objective of this study was to evaluate the kinetics of both free and esterified forms of cholesterol contained in a emulsion that binds to LDL receptors (LDE) in subjects with heterozygous familial hypercholesterolemia (FH), and the same subjects under the effects of high-dose simvastatin treatment, as compared with a control normolipidemic group (NL). Twenty-one FH patients (44.0 +/- 13.0 yr, 12 females, LDL cholesterol levels 6.93 +/- 1.60 mmol/L) and 22 normolipidemic patients (44.0 +/- 15.0, 10 females, LDL cholesterol levels 3.15 +/- 0.62 mmol/L) were injected intravenously with 14C-cholesteryl ester and 3H-cholesterol. FH patients were also evaluated after 2 mon of 40 or 80 mg/d simvastatin treatment, and plasma samples were collected over 24 h to determine the residence time (RT, in h) of both LDE labels, expressed as the median (25%; 75%). The RT of both 14C-cholesteryl ester and 3H-cholesterol were greater in FH than in NL [FH: 36.0 (20.5; 1191.0), NL: 17.0 (12.0-62.5), P = 0.015; and FH: 52.0 (30.0; 1515.0); NL 20.5 (14.0-30.0) P < 0.0001]. Treatment reduced LDL cholesterol by 36% (P < 0.0001), RT of 14C-cholesteryl ester by 49% (P = 0.0029 vs. baseline), and 3H-cholesterol RT by 44% (P = 0.019 vs. baseline). After treatment, the RT values of 14C-cholesteryl ester in the FH group approached the NL values (P = 0.58), but the RT of 3H-cholesterol was still greater than those for the NL group (P = 0.01). The removal of LDE cholesteryl esters and free cholesterol was delayed in FH patients. Treatment with a high dose of simvastatin normalized the removal of cholesterol esters but not the removal of free cholesterol.

Analysis of cholesterol and cholesteryl esters in human serum using capillary supercritical fluid chromatography

Capillary supercritical fluid chromatography (SFC) using carbon dioxide as a mobile phase was applied for the determination of free cholesterol and cholesteryl esters in human serum. Serum samples were extracted with methanol-chloroform (2:1, v/v), and the extracts were analyzed by pressure programmed capillary SFC-flame-ionization detection (FID) without thermal degradation and derivatization. The total cholesterol concentrations obtained from SFC analysis were compared with those from GC or enzymatic analysis. The capillary SFC-FID method having high resolution gave an acceptable average relative standard deviation of 2.6%, and a detection limit of 4-6 pg. The quantitative results were acceptable for the simultaneous analysis of cholesterol and its esters in biological fluids. The concentration profiles of each compound in various samples, normal Korean human serum, Western human serum, and from high-cholesterol patient plasma, have been compared with this method.

Microquantification of cholesterol and cholesteryl esters in rat peritoneal macrophages by reverse-phase high-performance liquid chromatography

A simple and rapid method for the microquantification of cholesterol and cholesteryl esters by reverse-phase high performance liquid chromatography has been established. Comparison of elution patterns of authentic cholesterol and cholesteryl esters revealed that a mu Bondasphere reverse-phase C8 (300-A) column was more suitable than a corresponding reverse-phase C4 or C18 column in terms of rapidity and sensitivity. Recovery of cholesterol and cholesteryl esters from a C8 column was greater than 98% when determined either by radioactive cholesterol and cholesteryl oleate or by cholesteryl heptadecanoate. The sensitivity of the quantification ranged from 5 ng to 50 micrograms for both cholesterol and cholesteryl esters. This method was applied to determination of cellular cholesterol and cholesteryl esters of rat peritoneal macrophages. Lipid extracts of these cells were found to contain 38.01 +/- 2.60 micrograms of cholesterol and 3.18 +/- 0.36 micrograms of cholesteryl esters per milligram of cell protein. When the cells were loaded with cholesteryl esters by incubation for 24 h with various concentrations of acetylated low-density lipoprotein, a cellular level of cholesteryl esters showed a dose-dependent increase and reached a maximal level of 106.60 +/- 3.05 micrograms/mg cell protein. Thus, the present method is useful for the microquantification of cholesterol and cholesteryl esters from lipid extracts of biological samples.

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.

Effect of normolipemic and hyperlipemic serum on biosynthetic response to cyclic stretching of aortic smooth muscle cells

Arterial smooth muscle cells synthesize matrix macromolecules in response to mechanical stimulation. Exposure to serum lipids also stimulates connective tissue fiber accumulation. To assess the effect of serum lipids on the biosynthetic response to tensile stress, we subjected rabbit aortic smooth muscle cells that were cultured on purified elastin membranes to cyclic stretching and relaxation 50 times per minute in the presence of serum-free medium (SFM), normolipemic serum (NLS), or hyperlipemic serum (HLS). Incorporation of 14C-proline into proline and into hydroxyproline was taken as a measure of protein and collagen synthesis. When cells were grown in plastic Petri dishes, exposure to NLS or HLS increased both protein and collagen production to the same extent compared to synthesis in SFM (1.7 times for NLS and 1.6 times for HLS; p less than 0.001 compared to SFM). For cells grown on stationary elastin membranes, NLS and HLS also increased protein and collagen synthesis compared to SFM. The effect of NLS was 1.35 times that of HLS for protein and 1.43 times greater for collagen (p less than 0.03). Cyclic stretching in SFM doubled synthesis for both protein (p less than 0.002) and collagen (p less than 0.002) compared to stationary controls, but had no effect on synthesis in NLS. In HLS, however, cyclic stretching elevated synthesis to the same level as was found in NLS (p less than 0.003). We conclude that the relative inhibition of synthesis on stationary membranes by HLS was not due to a toxic effect, since HLS increased synthesis both in Petri dishes and on elastin membranes, and the amplifying effect of cyclic stretching in HLS was similar to that seen in SFM.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The sympathetic nervous system and atherosclerosis

Morphometric and chemical changes in the arterial wall were studied after 12 months of diet-induced atherosclerosis in rhesus monkeys treated with either bilateral surgical thoracic sympathectomy or propranolol. There was a marked reduction in the progression of atherosclerosis in the carotid arteries and a moderate reduction in the disease found in the thoracic aorta of monkeys treated initially with a sympathectomy, in comparison to control monkeys fed an atherogenic diet alone. Propranolol at a dose of 40 mg/12 hrs also seemed to reduce the progression of atherosclerosis in the carotid arteries and thoracic aorta, although the differences were less dramatic. There were minimal differences in the extent of atherosclerosis in the abdominal aorta or femoral arteries of animals in either treatment group as compared with the control group. Similarly, the chemical composition of these same major vessels showed no significant differences. Therefore, in the face of severe atherogenic stimuli, chemical or surgical sympathectomy may be useful in controlling atherosclerosis in specific arterial beds.

Long-term effects of verapamil on aortic smooth muscle cells cultured in the presence of hypercholesterolemic serum

Smooth muscle cells derived from rabbit and bovine aorta were cultured for up to 5 weeks in the presence of d less than 1.019 g/ml fraction of hypercholesterolemic rabbit serum. When this fraction was added to serum containing culture medium, there was a significant increase in DNA, protein, and cholesteryl ester per dish. Addition of 50 microM verapamil markedly reduced the stimulatory effect of the d less than 1.019 g/ml fraction on both DNA and protein content per dish. The effect of verapamil on cholesteryl ester content was more complex: there was an increase within the first week, but later the net accumulation of cholesteryl ester per dish was lower than in untreated dishes. The recovery of less DNA in verapamil-treated dishes was not due to increased cell loss, as evidenced by retention of a residualizing marker, 3H-cholesteryl linoleyl ether. Moreover, verapamil did reduce incorporation of 3H-thymidine into DNA. In verapamil-treated dishes, there was flattening and a cobblestone appearance of the cells. A hypothesis is proposed to explain the inhibitory effect of verapamil on the development of atheroma formation in cholesterol-fed rabbits: Assuming that macrophages play an active role in cholesteryl ester removal from atheroma, verapamil, which reduces lysosomal cholesteryl ester hydrolysis in macrophages, would permit the lipid-laden macrophage to remove more cholesteryl ester per cell from the arterial wall. In addition, the presently reported results support the possibility that verapamil may impede the development of atheroma formation by reduction of smooth muscle cell proliferation.

The transfer of lipids from protein-free lipoprotein models to human fibroblasts in culture

Lipid microemulsions were prepared by sonication of mixtures of cholesteryl ester, triacylglycerol, phosphatidylcholine and cholesterol in aqueous dispersions and were purified by gel filtration. The resulting emulsion particles were characterized by differential scanning calorimetry, electron microscopy and analytical gel filtration and were shown to have the size and general organization of low-density lipoprotein. The lipid microemulsions were used as protein-free plasma lipoprotein models for studies of the receptor-independent transfer of lipids to human fibroblasts in culture. The transfer rate of [3H]cholesterol increased with the donor concentration and with the molar ratio between cholesterol and phosphatidylcholine in the donor particles. A maximal transfer value of 1 nmol per mg protein per h was obtained for cholesterol/phosphatidylcholine 1:1 particles. There was a profound temperature effect on the cholesterol transfer. The effect of altering the core lipid of the emulsion particles on the [3H]cholesterol transfer rate was small giving a somewhat higher rate with cholesteryl oleate and cholesteryl stearate than with cholesteryl linoleate. Addition of trioleoylglycerol to the cholesteryl ester core had no effect on the transfer rate. The transfer rate of palmitoyl[14C]oleoylphosphatidylcholine was found to be about 1/5 of that obtained for [3H]cholesterol. About 50% of the cell-associated [14C]cholesteryl oleate was found in the trypsin-releasable pool, while 25% was internalized by the cells at a rate of 0.06 nmol X mg-1 X h-1. Trioleoylglycerol was internalized at the same rate as the cholesteryl ester. Our data suggest that the lipoprotein lipid composition may play a role in the receptor-independent cellular uptake of cholesterol.

Fish oil inhibits development of atherosclerosis in rhesus monkeys

The effect of feeding fish oil (Menhaden) on the progression of rhesus monkey atherosclerosis was determined by feeding diets containing 2% cholesterol and either 25% coconut oil (Group I), 25% fish oil/coconut oil (1:1) (Group II), or 25% fish oil/coconut oil (3:1) (Group III) for 12 months (n = 8/group). The average serum cholesterol levels were 875 mg/dl for Group I, 463 mg/dl for Group II, and 405 mg/dl for Group III. HDL cholesterol levels were 49 mg/dl for Group I, 29 mg/dl for Group II, and 20 mg/dl for Group III. An average of 79% of the aortic intima was involved with atherosclerosis in Group I, 48% in Group II, and 36% in Group III. The aortas of both fish-oil groups (II or III) contained significantly less cholesterol (total, free, and esterified), as well as less acid lipase, cholesteryl esterase, and ACAT activities when compared to the coconut-oil group (I) (p less than 0.05). Microscopically, the aortic and carotid artery lesions were smaller in cross-sectional area and in thickness, and contained less macrophages in the fish-oil groups (II and III) when compared to the coconut-oil group (I) (p less than 0.05). This protective effect was not consistently enhanced by increasing the proportion of fish oil to 3:1 (Group III) over 1:1 (Group II). The results indicate that fish oil-containing diets reduce serum cholesterol levels and inhibit atherosclerosis even in the face of lowered HDL cholesterol levels when compared to a pure coconut oil/cholesterol diet in rhesus monkeys. Therefore, fish-oil diets exert effective protective control of progression of atherosclerosis during severe atherogenic stimuli.

Macrophage interaction with very-low-density lipoproteins results in triacylglycerol-enriched smooth muscle cells

Macrophage-conditioned medium containing very-low-density lipoproteins (VLDL) and its effects on smooth muscle cell triacylglycerol metabolism was investigated. Macrophages exposed to VLDL from normolipemic rats accumulated high levels of intracellular triacylglycerol, while similarly treated smooth muscle cells showed only slight changes. Medium, initially composed of VLDL and albumin, contained substantial levels of free fatty acids after exposure to macrophages. In the presence of albumin, the loss of VLDL triacylglycerol from the medium and the appearance of medium free fatty acids was consistent with a precursor-product relationship. The extent of medium fatty acid accumulation was dependent on the length of time of incubation with macrophages and was proportional to the concentration of VLDL and albumin added to the culture dish. This macrophage-conditioned medium, when given to smooth muscle cells, promoted a 6-12-fold increase in smooth muscle cell triacylglycerol levels over that produced by fresh VLDL and albumin. Similar increases in cell triacylglycerol levels could be produced using fresh medium approximating the oleate concentration and the fatty acid to albumin molar ratios found in macrophage-conditioned medium. In macrophage-conditioned medium with VLDL but without albumin, little free fatty acid was found. Other factors produced by macrophages did not seem to affect the metabolism of VLDL by smooth muscle cells since, in the absence of albumin, media with VLDL caused comparable responses in smooth muscle cell triacylglycerol accumulation whether or not the medium was previously exposed to macrophages. Thus, the minor changes in triacylglycerol content in smooth muscle cells promoted by medium containing VLDL and albumin were substantially enhanced by a prior exposure of the medium to macrophages, primarily due to the free fatty acids present in the macrophage-conditioned medium.

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.

Role of acid lipase in cholesteryl ester accumulation during atherogenesis. Correlation of enzyme activity with acid lipase-containing macrophages in rabbit and human lesions

Purified acid lipase was previously shown to hydrolyze the artificial substrate, alpha-naphthyl palmitate, as well as triglycerides and cholesteryl esters and to form cholesteryl esters. To determine to what extent these activities are associated with acid lipase-containing cells in atherosclerotic plaques, we examined rabbit aortas at different stages of experimental lesion induction and human atherosclerotic arteries. Assays of cholesteryl ester formation, and alpha-naphthyl palmitate and cholesteryl ester hydrolysis were performed on homogenates of lesions and the hydrolysis of the artificial fatty acid ester was used as a histochemical marker to identify acid lipase positive foam cells in sections of the same lesions. The volume of lesions occupied by cells stained for acid lipase correlated strongly with the enzyme activities of the arterial homogenates. These results suggest that acid lipase-containing cells may mediate the accumulation of cholesteryl ester during atherogenesis. Since acid lipase activity marks macrophages, these methods may be useful for relating macrophage distribution and function to lesion progression, regression, and complication.

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.

Very low density lipoproteins promote triglyceride accumulation in macrophages

Incubation of mouse peritoneal macrophages with very low density lipoproteins (VLDL) from normal rats or rhesus monkeys markedly increased the levels of intracellular triglycerides by 10- to 56-fold and was accompanied by the production of oil red O positive vacuoles. The stimulation of triglyceride accumulation in macrophages was time- and concentration-dependent and was specific for VLDL. Three possible mechanisms for the VLDL-stimulated triglyceride accumulation in macrophages were explored: receptor-mediated uptake, action of lipoprotein lipase, and phagocytosis. Macrophage uptake and degradation of 125I-monkey and rat VLDL demonstrated saturable and nonsaturable components. Uptake of 125I-VLDL could be inhibited by unlabeled normal VLDL, although hyperlipemic VLDL was more effective. HDL did not compete to a significant extent. Heparin released lipoprotein lipase-like activity from peritoneal macrophages. Addition of heparin with VLDL resulted in a greater, more rapid elevation in intracellular triglycerides, which was partially inhibited by albumin. Free fatty acid and Intralipid also produced triglyceride accumulation in macrophages. The data showed that all three of the mechanisms examined could contribute to the metabolism of VLDL by macrophages and cause the production of triglyceride-rich cells with a "foamy" appearance, although the evidence suggested that the action of lipoprotein lipase was probably the most important in this process.

Arterial lesions and blood lipids in rhesus monkeys fed human diets

The results of three consecutive experiments are reported in which atherosclerotic lesions of adult male rhesus monkeys produced by a ration relatively rich in calories, cholesterol, and saturated fats and resembling the "average American" table-prepared fare were compared with the atherosclerotic lesions in similar monkeys fed a low-fat, low-cholesterol, and low-calorie, so-called "prudent" table-prepared ration. Each experiment lasted 2 years. The frequency and severity of gross and microscopic aortic atheromatous disease and microscopic coronary artery disease were compared. In addition, the qualitative features of the lesions, several types of analyses of blood lipids, and the reactions of aortic cells to in vivo deposited interstitial aortic deposits of lipoprotein fractions prepared from these animals are described. In general, the animals fed the average American ration had serum cholesterols that were consistently higher (383 +/- 35 mg%) than animals fed the prudent ration (199 +/- 13 mg%). The contrasts in lesion involvement varied from about 6:1 in severity for both aortic and coronary lesions to approximately 3:1 or 4:1 in frequency. The aortic gross surface area involvement at autopsy was 46% for monkeys fed the average American ration compared to 7% for the prudent diet group. In lesions of animals fed the average American ration much of the lipid was extracellular and both cell proliferation and fiber protein deposition were prominent. The small lesions in the animals fed the prudent ration were much more likely to be composed mostly of "foam cell" intimal thickenings. The lesions seen in the animals fed the average American ration resemble those seen in autopsies of many young Americans whose lesions have been studied recently and who demonstrate raised lesions in their coronary arteries and aortas in the third decade.

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.

High intralysosomal levels of lipoprotein cholesterol influence the endocytic activity and acid hydrolase content of CH-23 fibroblasts

Lipoprotein extracts isolated from hyperlipemic rabbit serum were used to study the effects of abnormal lipid levels on the functions of the lysosomal system in Chinese hamster CH-23 fibroblasts. The ability of cells enriched with lipid to endocystose [3H]inulin was largely unimpaired and utilising density gradient fractionation procedures the fusion between incoming inulin and low density lysosomes ladened with esterified cholesterol could be demonstrated. Studies in which cells were exposed to short 'bursts' of [3H]inulin indicated, however, that the rate of fusion between inulin and secondary lysosomes was reduced. The incorporation of lipid material also produced a rapid though transient increase in several acid hydrolase activities. The stimulus for increased enzyme activity does not appear to be the deposition of ingested lipid within the lysosomes but rather at some stage prior to this, possibly the formation of endocytic vesicles. The current findings suggest that the intralysosomal incorporation of lipid material which occurs in several pathological conditions has marked effects on lysosomal function.

Evidence for a lipoprotein carrier in human plasma catalyzing sterol efflux from cultured fibroblasts and its relationship to lecithin:cholesterol acyltransferase

Immunoaffinity chromatography has been used to study the determinants of sterol efflux and net transport from cultured fibroblasts to human plasma medium. Sterol efflux was highly (approximately 80%) dependent upon a minor lipoprotein fraction containing apolipoprotein A-I unassociated with other apolipoproteins. The remaining activity was associated with the lipoprotein-free fraction of plasma and could be replaced by apoprotein-free albumin. Efflux was independent of lecithin:cholesterol acyltransferase (EC 2.3.1.43) activity. Net transport (i.e., the excess of efflux over influx) was completely inhibited by inhibition of lecithin:cholesterol acyltransferase or its removal by affinity chromatography on immobilized antibodies to apolipoprotein A-I or D (components of the transfer complex in human plasma). In uninhibited plasma, efflux and net transport rates had similar kinetics, suggesting that these were linked functions and that net transport was initiated by a carrier-dependent efflux step that, in the absence of lecithin:cholesterol acyltransferase activity, was associated with an equivalent influx of free sterol to the cells and that, in the presence of lecithin:cholesterol acyltransferase, was associated with esterification and transfer protein activity. The cholesterol carrier lipoprotein function (approximately 5% of plasma apolipoprotein A-I) appears to be the first step of lecithin:cholesterol acyltransferase-linked sterol transport from cells.

Subcellular changes associated with culture of Chinese hamster fibroblasts in hyperlipemic medium

Chinese hamster fibroblasts grown in hyperlipemic rabbit serum, enriched 20-fold in total cholesterol, show accumulation of cholesterol ester to 5-6 times the normal level, whereas free cholesterol levels show minimal changes. Changes with serum concentration and time show the uptake to be saturable. Concomitant with accumulation of cholesterol ester, the median density of lysosomes as observed on analytical sub-cellular fractionation decreased markedly, indicating a lysosomal localisation of accumulated sterol. These changes were freely and rapidly reversible on return to normal medium. The implications of these findings and the potential use of the system as a model for investigation of the subcellular consequences of lipid uptake is discussed.

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.

Increase of serum high density lipoprotein with progression and regression of aortic lipid deposition in rats

The progression and regression of aortic lipid deposition was studied in male ExHC rats. Rats in the progression group were fed an atherogenic diet containing 2% cholesterol, 0.4% sodium cholate and 10% olive oil for periods up to 32 weeks. Rats in the regression group were first fed the atherogenic diet for 16 weeks, and then maintained on a basal low cholesterol diet. Half the rats were killed at 4 weeks and the other half at 16 weeks after cessation of the atherogenic diet. Rat aortas of the progression group contained progressively more lipid pari passu with the duration of cholesterol feeding, but connective-tissue proliferation was absent. Deposited lipid in the aorta of cholesterol-fed rats disappeared very slowly after the rats were returned to the basal diet. serum HDL decreased in the hypercholesterolemic ExHC rats fed the atherogenic diet for 4 weeks. By contrast, serum HDL and apo A-I increased in both hypercholesterolemic ExHC rats fed the atherogenic diet for 32 weeks and slightly hypercholesterolemic ExHC rats fed the basal diet for 16 weeks in the regression period.

Atherogenic diets and neutral-lipid organization in plasma low density lipoproteins

The plasma low density lipoproteins (LDL) of rhesus monkeys fed 3 atherogenic diets exhibited thermal transitions at temperatures much higher (37--43 degrees C) than those observed in control animals or in normal humans (20--33 degrees C). The same differences were noted in the neutral lipids (cholesteryl esters and triglycerides) which were isolated from the respective lipoproteins. In particular, the difference in thermal properties between the normal and abnormal LDLs was attributable to subtle differences in their cholesteryl ester compositions (mainly an increase in the saturated and monounsaturated fatty acid moieties), with altered triglyceride contents playing only a minor role. Thus, at body temperature, the hyperlipidemia that follows the administration of atherogenic diets is associated with a high degree of order of the neutral lipids in the core of the LDL particle. This, in turn, may be related to the atherogenicity of the abnormal lipoprotein species.

Accumulation and loss of cholesterol esters in monkey arterial smooth muscle cells exposed to normal and hyperlipemic serum lipoproteins

The effects of high low and very low density lipoprotein fractions from normal or hyperlipemic rhesus monkey serum on the accumulation or removal of cholesterol esters from rhesus monkey smooth muscle cells in tissue culture were determined. Serum or serum lipoproteins were labeled with [14C] free cholesterol and adjusted to the same free cholesterol level in the incubation medium. Of the two normal lipoproteins examined, the LDL fraction caused more esterification than the HDL. Cells incubated in hyperlipemic serum showed a 2-fold stimulation in esterification as compared to cells in normal serum. This was contributed by hyperlipemic VLDL and LDL and led to a concomitant increase in cellular cholesterol ester content. Both hyperlipemic LDL and HDL stimulated esterification when compared to their normal counterparts. Cholesterol ester removal was examined by incubating the serum or lipoprotein fractions with cells enriched in cholesterol ester through a prior exposure to hyperlipemic serum. The cells incubated in normal or hyperlipemic HDL or lipoprotein-deficient serum had the lowest cholesterol ester content. Thus, the lipoprotein fractions which caused the lowest levels of cholesterol esterification were also the most efficient in the removal of cellular cholesterol esters.

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

Whole serum obtained from hypercholesterolemic rhesus monkeys was found to stimulate cholesterol esterification and cholesteryl ester accumulation in rhesus monkey arterial smooth muscle cells in culture to a significantly greater extent than normocholesterolemic serum. This was true even when the cholesterol concentration of the culture medium was equalized. Isolation and characterzation of the low density lipoproteins (LDL) from rhesus monkeys indicated that the LDL from hypercholesterolemic animals was 33% larger than LDL from normocholesterolemic animals due principally to an increase in the amount of cholesteryl ester per molecule. As a result, LDL from hypercholesterolemic animals transported over 50% more cholesterol per molecule than did normal LDL. The LDL of altered composition from hypercholesterolemic animals, when added to smooth muscle cells in culture, was nearly twice as effective in stimulating cholesterol esterification and cholesteryl ester accumulation than was LDL of normal composition. Results suggest that at least part of the exaggerated ability of whole hypercholesterolemic serum to stimulate the esterification and accumulation of cholesterol in cells in culture is due to the presence of LDL of altered composition.

Phospholipid metabolism of monkey smooth muscle cells grown in hyperlipemic serum

The phospholipid content and synthesis of monkey smooth muscle cells grown in tissue culture with normal or hyperlipemic monkey serum were examined. The pattern of incorporation of radioactively labeled inorganic phosphate into the phospholipids of these cells was measured using a 4 h pulse of 32P. Phosphatidylcholine and phosphatidylinositol were the predominant phospholipids labeled. Although phosphatidylcholine constituted 45% of the cellular phospholipid, phosphatidylinositol had the highest specific activity. Exposure of the smooth muscle cells to hyperlipemic monkey serum did not alter the phospholipid content, composition or synthesis of these cells. The total phospholipid content of the smooth muscle cells was independent of the concentration of lipid in the media. The distribution of 32P into the phospholipids of monkey alveolar macrophages, L-cell mouse fibroblasts, and segments of the intima-media from monkey aortas is reported.