Mechanisms of cholesterol accumulation in the arterial wall

Lipids and the endothelium: bidirectional interactions

The endothelium is often viewed solely as the barrier that prevents the penetration of circulating lipoproteins into the arterial wall. However, recent research has demonstrated that the endothelium has an important part in regulating circulating fatty acids and lipoproteins, and is in turn affected by these lipids/lipoproteins in ways that appear to have important repercussions for atherosclerosis. Thus, a number of potentially toxic lipids are produced during lipolysis of lipoproteins at the endothelial cell surface. Catabolism of triglyceride-rich lipoproteins creates free fatty acids that are readily taken up by endothelial cells, and, likely through the action of acyl-CoA synthetases, exacerbate inflammatory processes. In this article, we review how the endothelium participates in lipoprotein metabolism, how lipids alter endothelial functions, and how lipids are internalized, processed, and transported into the subendothelial space. Finally, we address the many endothelial changes that might promote atherogenesis, especially in the setting of diabetes.

Methyl-beta-cyclodextrin suppresses hyaluronan synthesis by down-regulation of hyaluronan synthase 2 through inhibition of Akt

Hyaluronan synthases (HAS1-3) are integral plasma membrane proteins that synthesize hyaluronan, a cell surface and extracellular matrix polysaccharide necessary for many biological processes. It has been shown that HAS is partly localized in cholesterol-rich lipid rafts of MCF-7 cells, and cholesterol depletion with methyl-beta-cyclodextrin (MbetaCD) suppresses hyaluronan secretion in smooth muscle cells. However, the mechanism by which cholesterol depletion inhibits hyaluronan production has remained unknown. We found that cholesterol depletion from MCF-7 cells by MbetaCD inhibits synthesis but does not decrease the molecular mass of hyaluronan, suggesting no major influence on HAS stability in the membrane. The inhibition of hyaluronan synthesis was not due to the availability of HAS substrates UDP-GlcUA and UDP-GlcNAc. Instead, MbetaCD specifically down-regulated the expression of HAS2 but not HAS1 or HAS3. Screening of signaling proteins after MbetaCD treatment revealed that phosphorylation of Akt and its downstream target p70S6 kinase, both members of phosphoinositide 3-kinase-Akt pathway, were inhibited. Inhibitors of this pathway suppressed hyaluronan synthesis and HAS2 expression in MCF-7 cells, suggesting that the reduced hyaluronan synthesis by MbetaCD is due to down-regulation of HAS2, mediated by the phosphoinositide 3-kinase-Akt-mTOR-p70S6K pathway.

Removal of cholesteryl esters from diet-induced atherosclerotic lesions in two long-term studies in rhesus monkeys

In two long-term studies of dietary-cholesterol-induced atherosclerosis lesions in rhesus monkeys, we determined the fatty acid composition of cholesteryl esters in the arterial intima-media preparations. In the first study (2-year study) monkeys were fed an atherogenic diet for 2 years; in the second study, (5-year study), monkeys were fed the atherogenic diet for about 5.4 years. Dietary cholesterol was removed from regressed animals and the regression periods in the 2-year study were 30 and 52 weeks and in the 5-year study were 20, 30, 52, 101, and 191 weeks. In both studies in thoracic and abdominal aortic segments, the percent removal of 18:2 cholesteryl ester (CE) was small, whereas percent removal of 18:1 CE and other CEs was much higher. We postulate that 18:2 CE is not hydrolyzed to the extent comparable to 18:1 and other CEs in lesions. Perhaps 18:2 CE is not a preferred substrate for arterial neutral cholesterol ester hydrolase.

Cholesterol crystal uptake and metabolism by P388D1 macrophages

Cholesterol monohydrate crystals are frequently detected in intermediate and advanced atherosclerotic lesions. Little is known regarding mobilization of this molecular form of cholesterol into metabolically active pools. To study a potential mechanism for mobilization of crystalline cholesterol, we examined its uptake by a mouse macrophage cell line (P388D1). Crystals were overlayered on a P388D1 cell monolayer maintained in a serum-free medium. Following incubation, the monolayer was washed, and the cells were harvested and analyzed for crystal internalization. By transmission electron microscopy, crystals were found intracellularly surrounded by a bilayer membrane. Analyses of the cellular cholesterol ester content by gas-liquid chromatography and esterification of [14C]cholesterol indicated the conversion of crystalline cholesterol to cholesterol esters. This pathway for solubilization of cholesterol crystals by macrophages could play an important role in the regression of atherosclerotic lesions.

Influx and cellular degradation of low density lipoproteins in rabbit aorta determined in an in vitro perfusion system

The accumulation of 125I-low density lipoprotein (LDL) into normal and atherosclerotic arterial tissue and cellular uptake in arterial cells were studied in an in vitro perfusion system for rabbit aorta. The accumulation of 125I-LDL in normal tissue could be fitted to an inverse exponential function with an initial influx rate of 1.39 nl/mg wet weight/hour and an equilibration volume of about 2% of the tissue volume. The influx rate into atherosclerotic plaques was about 10 times faster and the equilibration volume, 50 times higher. In atherosclerotic tissue there was a steep concentration gradient between the plaque and the underlying media. The accumulation of 125I-LDL in the media under plaque and in normal tissue adjacent to plaques was similar to that seen in normal tissue. For studies of cellular uptake of LDL a trace label, 125I-tyramine-cellobiose (TC), was used. Normal or atherosclerotic rabbit aorta was perfused in vitro with medium containing 125I-TC-LDL. After perfusion the tissue was digested and the cells were isolated by density gradient centrifugation. Two main cell fractions with characteristics of smooth muscle cells and foam cells, respectively, were obtained. A 70-fold higher uptake was seen in the foam cells. In conclusion, these studies suggest a higher influx rate into atherosclerotic plaques, as well as a high LDL concentration in the plaque, compared with normal tissue or underlying media. We suggest that most of the cellular uptake of LDL in the arterial wall is caused by the foam cells.

The longest-lived patient with homozygous familial hypercholesterolemia secondary to a defect in internalization of the LDL receptor

The authors report the longest-lived patient with homozygous familial hypercholesterolemia, the seventh case of a defect in internalization of low-density lipoprotein (LDL). The patient is a 57-year-old man, whose plasma total cholesterol (TC) and LDL-cholesterol (LDL-C) concentrations were 465-660 mg/100 ml and 461 mg/100 ml, respectively, while his plasma high-density lipoprotein-cholesterol (HDL-C) was 13.6-16.9 mg/100 ml. He was the product of a consanguineous marriage, and his parents, brothers, and a sister had mild hypercholesterolemia. His coronary angiogram revealed diffuse coronary artery narrowing. Receptor studies revealed that his fibroblasts bound as much LDL as normal cells, but could not internalize or degrade LDL.

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.

The relationship between carotid plaque composition and neurologic symptoms

Variations in plaque composition, particularly an increased lipid concentration, could make carotid plaques unstable and prone to embolization. To investigate this hypothesis, 35 carotid bifurcation plaques from 31 patients undergoing carotid endarterectomy (20 symptomatic, 11 asymptomatic) were prospectively analyzed. Plaque total lipid, cholesterol, collagen, and Ca2+ content were determined, and the plaque collagen was fractionated into pepsin-soluble collagen (PSC) (indicative of less crosslinked, more recently synthesized collagen) and pepsin-insoluble collagen. Preoperative serum lipid and plasma lipoprotein levels were also obtained in all patients. Results revealed that plaques removed from symptomatic patients contained significantly more total lipid and cholesterol (P less than 0.001) than those removed from asymptomatic patients. There was also an increased amount of pepsin-soluble collagen in symptomatic plaques, potentially indicating increased metabolic activity (P less than 0.05). In addition, mean plasma low-density lipoprotein cholesterol was higher in symptomatic patients than in asymptomatic patients (P less than 0.05). Since lipid-laden plaques can potentially be identified by B-mode ultrasound, it may be important to remove these plaques in asymptomatic patients or, alternatively, to attempt to lower the plaque cholesterol by lipid-lowering maneuvers.

Mitochondrial and peroxisomal beta-oxidations in pig coronary smooth muscle cells

1. The total subcellular membranes of pig coronary media were fractionated using a sucrose density gradient. 2. A fraction with high succinate dehydrogenase activity and a mean density of 1.165 was separated from a fraction with high catalase activity and a mean density of 1.145. 3. Acyl CoA beta-oxidation activity measured in the absence of BSA was present in both fractions with 47% of the total activity in the succinate dehydrogenase fraction and 47% in the catalase fraction. 4. In the succinate dehydrogenase fraction bovine serum albumin stimulated the acyl CoA beta-oxidation (maximal stimulation, 3.2 times at a concentration of 15 mg%) while in the catalase fraction it had no effect. 5. It is concluded that, in pig coronary media, the beta-oxidation system has two components, i.e. mitochondrial and peroxisomal beta-oxidation.

Chemical composition and physical state of lipid deposits in atherosclerosis

The composition, morphology, and physical properties of lipids in atherosclerotic lesions from human aortas were studied in order to elucidate the factors for the accumulation of cholesterol and its esters in the vessel wall. Lesions were classified histologically into 3 groups: fatty streak, fibrous plaque, and advanced plaque. The relative lipid composition of the lesions was plotted on the phase diagram of the 3 major lipids: cholesterol, cholesteryl ester, and phospholipid. Early fatty streaks had compositions within the 2-phase zone with a cholesterol-phospholipid liquid crystalline phase and a cholesteryl ester oily phase. Advanced fatty streaks and fibro-fatty plaques fell within the 3-phase zone with excess free cholesterol. Advanced plaques also had an average lipid composition within the 3-phase zone, but with a larger excess of free cholesterol. From the lipid-chemical point of view there is a continuous progression from early fatty streaks through advanced fatty streaks and fibro-fatty plaques to advanced plaques. In fatty streaks the cholesteryl esters accumulate in the form of isotropic and anisotropic droplets. The latter are in the smectic liquid crystalline state with the molecules arranged in layers and have surfaces that are spherical and smooth. Fibrous and advanced plaques showed beside droplets also amorphous lipids and cholesterol monohydrate crystals. Some of the amorphous lipids were solid up to about 45 degrees C and exhibited a smectic phase at cooling, indicating cholesteryl esters as the major component. The transition temperatures of high-melting cholesteryl esters, e.g. palmitate, are depressed by low-melting ones. Most of the triglycerides are present in the cholesteryl ester droplets and abolish the cholesteric liquid crystalline phase.

Cardiovascular features of homozygous familial hypercholesterolemia: analysis of 16 patients

Familial hypercholesterolemia (FH) is characterized by an autosomal codominant inheritance, an abnormality in low-density lipoprotein (LDL) receptor function, elevated plasma cholesterol levels and premature atherosclerosis. Sixteen patients with homozygous FH were studied to correlate the extent of their atherosclerotic disease with their lipid levels and receptor function. The age range at initial presentation was 3 to 38 years (mean 12), and at the last examination, 6 to 43 years (mean 20). The mean pretreatment total plasma cholesterol concentration for all patients was 729 +/- 58 mg/dl (+/- standard error of the mean), and the mean LDL cholesterol level was 672 +/- 58 mg/dl (normal 60 to 176). High-density lipoprotein cholesterol was 28 +/- 3 mg/dl (normal 30 to 74). In the 7 patients with FH who had symptoms of myocardial ischemia (Group I), the mean pretreatment LDL cholesterol value (817 +/- 62 mg/dl) was higher than that of the 9 asymptomatic patients (Group II) (560 +/- 74 mg/dl). In Group I, 5 of 7 patients had left or right coronary ostial narrowing and 3 had significant left ventricular outflow obstruction. Most coronary arterial narrowing occurred in the right coronary and left anterior descending arteries and the least amount in the left circumflex coronary artery. A femoral bruit was the physical finding that correlated best with the Group I population; brother:sister pairs revealed a milder clinical course for the female. Seven of the 16 patients have survived into their third decade without symptoms. Comparison of these persons with those in whom angina developed reveals a marked heterogeneity in their clinical course, which appears to be associated with receptor negative/defective status.

The effects of vasectomy and exercise on cholesterol levels in rat aortas

The effects of vasectomy and exercise upon esterified, unesterified and total cholesterol in rat aortic tissue was determined. A factorial design with two activity levels (aerobic exercise, sedentary) and three treatments (vasectomy, sham-vasectomy, control) was employed. Vasectomy significantly increased the amount of esterified and total cholesterol in aortic tissue. The vasectomy sedentary group had significantly higher unesterified cholesterol levels than the three exercise groups. Total cholesterol levels were significantly higher for the sedentary groups than the exercise groups. There was no significant difference between exercise and sedentary groups in esterified cholesterol.

Selective resolution of plaques and treatment of atherosclerosis by biophysical alteration of "cellular" and "intracellular" properties

Presenting a totally new approach to the effective treatment of Atherosclerosis through the alteration of biophysical properties both "intracellularly" and "extracellularly." The experimental data is preliminary but the results obtained indicate that by allowing the atherosclerotic lesions to take up the magnetically excitable submicron particles and then applying an external alternating electromagnetic field, the atherosclerotic lesions may be selectively resolved without damaging normal blood vessels. This concept suggests many areas of investigation since there are a variety of means of biophysically altering the atherosclerotic plaques, and many substances may be utilized to enhance the process. This new technology and this initial experimentation introduces a "new era" in the effective treatment of Atherosclerosis.

Atherogenesis: a postprandial phenomenon

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

[Etiology and pathogenesis of arteriosclerosis]

Cardiovascular diseases are the leading cause of death in Western countries, with an enormous increase in death rate and involvement of younger age groups during the last decades. This applies especially to coronary heart disease and is mainly caused by first-degree risk factors: hypertension, hyperlipoproteinemia, cigarette smoking, gout, obesity, polycythemia, lack of physical activity, and stress. These risk factors are discussed with special reference to overnutrition and increased cholesterol levels. Recent resuults of research concerning lipids and their relation to atherosclerosis are reviewed.

Relative atherogenicity of different plasma lipoproteins

Originally, data from the Framingham study appeared to show that serum lipoprotein fractions were no better in predicting coronary heart disease than the total serum cholesterol. More recently, the concentration of high density lipoprotein has been shown to exhibit a strong negative correlation with coronary heart disease, especially in the older age groups in which total serum cholesterol shows little or no correlation with clinical events. Biochemical mechanisms that might clarify the role of high density lipoprotein in atherogenesis are forthcoming. Another aspect of the lipoprotein-atherogenesis question pertains to the possible role of chylomicrons. Animal experiments show that cholesterol feeding is frequently accompanied by an increase in beta-migrating, very low density lipoproteins which appear to be chylomicron remnants. The atherogenic effect of serum low- and very low-density lipoproteins, including chylomicron remnants, appears to depend more on their contribution to the total cholesterol concentration than to their origin in liver versus intestines.

The fatty acid composition of triglycerides in arteries, depot fat and serum of amputated diabetics

The triglyceride fatty acid pattern (TFAP) in arterial wall, adipose tissue and serum in vivo has been estimated in amputated diabetic subjects by gas-liquid chromatography simultaneously. Besides, in some probands the fatty acid pattern of cholesterol esters of vessel walls has been obtained. In arterial wall the percental content of lauric, myristic, myristoleic, palmitoleic, stearic, oleic and linolenic acid was high and that of palmitic and linoleic acid was low, when compared to serum, whereas eicosatrienoic, arachidonic and eicosapentaenoic acid were of similar magnitude. In comparison to adipose tissue myristic, palmitic and nervonic acid were decreased and eicosatrienoic, arachidonic and eicosapentaenoic acid were increased in arterial wall. In view of the results the speculation is obvious that certain fatty acids have distinct metabolic positions in several tissues, such as depot fatty acids and precursors of prostaglandins, for example.

Changes in the coronary vascular system following prolonged exposure to stress

Hearts of stressed rats showed marked changes in the coronary vasculature. It was suggested that such morphological changes could be explained on the basis of an increased coronary vascular permeability. Endogenous inflammatory substances could induce swelling of the endothelial cells and cause separation of the neighbouring cells from one another, thus allowing the passage of lipid molecules through the endothelial lining. In order to gain supportive evidence for the above hypotheses an electron microscope study was undertaken. The presence of junctional gaps in the endothelial lining of the coronary vascular system was observed following prolonged stress, as well as platelet aggregation. The use of lipid staining of frozen sections indicated the presence of large lipid deposits in the arteriole walls, corresponding to vacuoles seen previously. It would appear therefore, that prolonged exposure to stress may result in pathological changes in the myocardium associated with changes in the vascular endothelial permeability, and platelet aggregation. Pathological changes induced in this way, however, should be inhibitd by high glucocorticoid levels and should not be manifested until adaptation of the steroid response to stress has occurred. Measurement of plasma glucocorticoid levels over the period of prolonged stress shows a good correlation between the adaptation of the steroid response and the onset of a progressive degeneration of the coronary vascular system.

The heart in the Hurler syndrome: gross, histologic and ultrastructural observations in five necropsy cases

Clinical and morphologic features of the cardiovascular system are described in five necropsy patients with the Hurler syndrome. In all five patients the coronary arteries, four cardiac valves, mural endocardium of all four chambers, myocardial walls and aorta were affected in a characteristic manner. All of these sites contained large clear cells known as Hurler cells (readily visible by light microscopy). In addition, granular cells were observed in semi-thin (1 mu) sections and by electron microscopy in the coronary arteries, atrioventricular (A-V) valves and in myocardial interstitium. These latter cells appear to produce collagen in an abnormal way and are probably responsible for the heavy deposits of collagen in the cardiovascular system of patients with the Hurler syndrome. In the cardiac muscle cells, in smooth muscle cells of the coronary arteries and in fibroblasts, wherever located, deposits of acid mucopolysaccharides and glycolipids usually were also observed. The acid mucopolysaccharide deposits were observed easily with light microscopy except in the cardiac muscle cells where they were seen only with electron microscopy. The glycolipid depositis, observed only on examination of 1 mu thick sections or with electron microscopy, have not previously been observed in coronary arteries or in myocardial cells. The infiltration into the heart by these cells and deposits in all five patients resulted in severe narrowing of the extramural coronary arteries, considerable thickening of the cardiac valves (the left-sided more than the right-sided valves), generalized thickening of mural endocardium and "stiffening" of the myocardial walls. Thus, the cardiovascular lesions in the Hurler syndrome are specific and life-threatening.

Mechanism of inhibition of cholesterol uptake by the arterial wall

Experiments have been described dealing with lipid synthesis and cholesterol uptake in perfused human and pig coronary arteries, rabbit aortas, and with the inhibitory effect of 7-ketocholesterol on cholesterol uptake in these preparations and in rabbits in vivo. Human and pig coronary arteries failed to synthesize cholesterol in vitro. 7-ketocholesterol inhibited cholesterol uptake in human coronary arteries and aortas of pigs and rabbits in vitro and by rabbit aortas in vivo. The inhibitory effect in vivo could only be shown after repeated i.v. injections of 7-ketocholesterol after solubilizing the steroid with bile sale (Na-glycocholate). Although 7-ketocholesterol was absorbed from the G.I. tract, gastric feeding of the bile salt steroid complex was ineffective, probably because of inadequate blood levels of 7-ketocholesterol achieved. The metabolic fate of 7-ketocholesterol and the nature of its effect on cholesterol are discussed. It is not likely that inhibition of HMG-CoA reductase is responsible for the inhibition of cholesterol uptake. The possibility was discussed that both cholesterol and 7-ketocholesterol actively compete for identical and specific binding sites or that an increase in 7-ketocholesterol in plasma leads to an increase in intracellular concentrations of this steroid thus inhibiting cholesterol transfer across the cell membrane. However definite conclusions on the nature of inhibition must await further experimentation.