Cholesterol accumulation and content in regions with defined endothelial integrity in the normal rabbit aorta

Psychosocial stress causes endothelial injury in cynomolgus monkeys via beta1-adrenoceptor activation

Current evidence links psychosocial factors to exacerbation of diet-induced atherosclerosis in monkeys via activation of the sympathetic nervous system. However, it is uncertain whether these factors can potentiate initial lesion formation, and do so even in the absence of dietary provocation, and whether any such effects can be prevented by beta-adrenergic blockade. As endothelial injury has been considered an initiating event in atherogenesis, we studied the effect of psychosocial stress on endothelial integrity in 48 adult male cynomolgus monkeys (Macaca fascicularis). All animals were housed in 12 social groups of four monkeys each for 11 weeks. The monkeys in half of the groups were exposed to a socially unstable ('stressed') condition for 72 h and received saline (n = 8), a lipophilic beta1-blocker (metoprolol, 0.30 mg/kg per h; n = 8), or hydrophillic beta1-blocker (atenolol, 0.15 mg/kg per h; n = 8). The remaining six social groups were assigned to the socially stable (non-stressed) condition; for 72 h these animals all remained in their social groups and were similarly treated with saline (n = 8), metoprolol (n = 8), or atenolol (n = 8). The frequency of IgG-positive (injured) endothelial cells was estimated on en face (Häutchen) preparations from the thoracic aorta and coronary arteries. Psychosocial stress caused a significant increase in the number of injured endothelial cells in the circumostial areas of the descending thoracic aorta in the placebo group (0.3 vs. 0.8%, P < 0.02), an effect that had not been demonstrated previously. Moreover, beta-blockade significantly (P < 0.01) inhibited the stress effect, with no differences between the two beta-blocking agents. The number of injured endothelial cells in the non-branched portions of the aorta and coronary arteries were low and indistinguishable among groups; irregularities in the size and location of branching points in the coronary arteries precluded analysis of these sites. This study demonstrated that psychosocial stress induces endothelial injury, and that this effect is mediated via beta1-adrenoceptor activation.

Effects of injury on apoB kinetics and concentration in rabbit aorta

Endothelial injury or dysfunction and deposition of lipoproteins and cholesterol are key events during the development of atherosclerosis. We have studied the lipoprotein kinetics in arterial tissue in relation to endothelial injury and re-endothelialization. Endothelial injury was induced in rabbits by use of a balloon catheter. With a specific immunoradiometric assay, apoB levels in arterial tissue were measured at different time points for up to 10 weeks after injury. Forty-five minutes before being killed, the rabbits were injected with 125I-LDL, and influx of LDL was calculated from the accumulation of radioactivity in the arterial tissue. The concentration of apoB in the injured arterial tissue was four times higher than that in control arterial tissue (P < .0001). Within the lesion the concentration was as high in nonendothelialized as in re-endothelialized regions. The tissue pool of apoB was divided into a loosely bound fraction and a tightly bound fraction. The increase of apoB in the injured areas was primarily due to an increase in the tightly bound fraction. The influx of apoB was severalfold higher in nonendothelialized tissue than in re-endothelialized tissue or control areas (P < .005). When retention time was calculated, this was found to dramatically increase (by seven times) the tightly bound pool of apoB in the re-endothelialized areas. In addition to the large increase of a tightly bound apoB pool in injured areas, we found a prolonged retention time of apoB in the lesions, but only in the re-endothelialized areas.(ABSTRACT TRUNCATED AT 250 WORDS)

Cationic polypeptides modulate in vitro association of low density lipoprotein with arterial proteoglycans, fibroblasts, and arterial tissue

Polymers of lys (plys) and arg (parg) were found to be efficient inhibitors of the formation of complexes between low density lipoprotein (LDL) and human chondroitin-6-sulfate-rich proteoglycans. Displacement curves indicate that efficiency was dependent on molecular weight. Inclusion of alanine in the polymer up to a 1:1 molar ratio (plys,ala) has a moderate effect on displacing capacity. Poly-L-lys (plys) and poly-L-arg (parg) exhibited similar displacing ability. Inclusion of tryptophan in plys and parg diminished their effect, whereas inclusion of serine in plys,ser (3:1) improved it. Plys (18.3 kD) stimulates LDL binding to human fibroblasts. This may be due to the association of polylysine to LDL, leading to an increase in its positive charge. These more positively charged LDL may have an increased association with the negatively charged region of the apolipoprotein B/E receptor. Perfusion experiments on rabbit aortic segments were used to measure the influx of 125I-LDL into the intima and to study the effect of basic polypeptides. Plys in a 10:1 molar ratio decreased the LDL uptake by approximately 25% when added to the system together with the LDL or in experiments in which the tissue segments were pre-perfused with plys, and LDL was added after elimination of the plys. The results suggest that polycationic polypeptides, due to their strong affinity for sulfated proteoglycans, interfere with the interactions of LDL with components of the arterial extracellular matrix.

The arterial barrier to lipoprotein influx in the hypercholesterolemic rabbit. 2. Long-term studies in deendothelialized and reendothelialized aortas

These studies consider whether a mild aortic injury that does not increase cholesteryl ester influx during the first few days promotes atheromatosis in the hypercholesterolemic rabbit. The cholesteryl ester influx in uninjured, deendothelialized, and reendothelialized aorta was also measured in order to account for the different cholesteryl ester contents in these areas. By 32-33 days after localized aortic injury which was made after 5-7 days of cholesterol feeding, uninjured (control) areas of the thoracic aortas had accumulated 48 micrograms/cm2 each of esterified and nonesterified cholesterol due to continued cholesterol feeding. However, the previously injured deendothelialized and reendothelialized areas of the aortas had accumulated 6 and 10 times as much esterified cholesterol, and 2 and 5 times as much nonesterified cholesterol, respectively, as the adjacent uninjured areas. Esterified cholesterol influx was low during the second day after injury but increased with time so that 30-31 days later the esterified cholesterol influx in deendothelialized and reendothelialized aorta was respectively 44 and 7 times as great as the 0.1 microgram/h/cm2 in the adjacent uninjured aorta. However, expressed per mg aortic cholesteryl ester, cholesteryl ester influx in reendothelialized aorta was no greater than in noninjured aorta. These studies suggest that an injury that does not initially increase cholesteryl ester influx can promote atheromatosis and that the higher rate of cholesteryl ester influx that develops with time in reendothelialized areas is closely related to its increased cholesteryl ester content.

Transendothelial transport of low density lipoprotein in association with cell mitosis in rat aorta

Atherosclerosis is characterized by focal areas of lipid accumulation and intimal smooth muscle cell proliferation in large arteries. In vivo studies on rat aorta with Evans blue-albumin conjugate (EBA) have shown that there are preferential sites of increased permeability with an increased uptake of the conjugate. It has been shown that these blue areas are associated with a high endothelial cell turnover rate and an enhanced permeability to lipids. In a previous study, we demonstrated that 99% of endothelial cells in the mitotic (M) phase as identified by hematoxylin staining of the dividing nuclei exhibited EBA leakage and that these dividing cells accounted for 30% of all leakage sites. In the present study, experiments were performed on the thoracic aortas of 10 adult male Sprague-Dawley rats to determine the statistical frequency of isolated leaks to Lucifer yellow-low density lipoprotein conjugate (LY-LDL) at the level of individual cells and to assess the relationship of such leaks to the cell turnover processes. Leakage of LY-LDL around individual endothelial cells was visualized by fluorescence microscopy, and cells in mitosis on the same specimens were identified by hematoxylin staining. Although endothelial cell mitosis is infrequent (0.034%), 80% of dividing cells in the M phase were associated with LY-LDL leakage. These dividing cells accounted for 45% of all leakage spots. These findings lend support to our recent hypothesis that transiently open junctions surrounding the endothelial cells undergoing cell turnover provide pathways through which LDL enters the subendothelial space, resulting in lipid accumulation.

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.

Prostacyclin formation around human abdominal aortic ostia

Earlier studies have demonstrated an age dependent effect in the extent of sudanophilic and atherosclerotic lesions around aortic ostia in men. As prostacyclin may be a key determinant in the development of atherosclerosis we examined its formation around abdominal aortic ostia (coeliac, superior mesenteric and inferior mesenteric arteries) in-vitro. Prostacyclin formation was higher distal to the orifice as compared to proximal areas, but without reaching the level of significance. Prostacyclin formation was always higher in females than in males and in nonsmokers than in smokers, showing the greatest difference distal to the ostium, but again without significance. From the limited number of vessels examined, no trend for an age dependency can be noted. The relation between prostacyclin generation and morphological data is discussed.

Physicochemical transfer of [3H]cholesterol from plasma lipoproteins to cultured human fibroblasts

The transfer of free cholesterol from [3H]cholesterol-labelled plasma lipoproteins to cultured human lung fibroblasts was studied in a serum-free medium. The uptake of [3H]cholesterol depended upon time of incubation, concentration of lipoprotein in the medium, and temperature. Modified (reduced and methylated) low-density lipoprotein (LDL), which did not enter the cells by the receptor pathway, gave a somewhat lower transfer rate than unmodified LDL, but if the transfer values for native LDL were corrected for the receptor-mediated uptake of cholesterol the difference was eliminated. The initial rates of transfer of [3H]cholesterol from LDL and high-density lipoprotein (HDL) were of the same order of magnitude (0.67 +/- 0.05 and 0.75 +/- 0.06 nmol of cholesterol/h per mg of cell protein, respectively) while that from very-low-density lipoprotein (VLDL) was much lower (0.23 +/- 0.02 nmol of cholesterol/h per mg) (means +/- S.D., n = 5). The activation energy for transfer of cholesterol from reduced, methylated LDL to fibroblasts was determined to be 57.5 kJ/mol. If albumin was added to the incubation medium the transfer of [3H]cholesterol was enhanced, while that of [14C]dipalmitoyl phosphatidylcholine was decreased compared with the protein-free system. The results demonstrate that, in spite of its low water solubility, free cholesterol can move from lipoproteins to cellular membranes, probably by aqueous diffusion. We propose that physicochemical transfer of free cholesterol may be a significant mechanism for net uptake of the sterol into the artery during atherogenesis.

Studies of hypercholesterolemia in the nonhuman primate. I. Changes that lead to fatty streak formation

Morphologic studies resulting from events that occur during the development of the lesions of atherosclerosis were studied in chronic, diet-induced hypercholesterolemia in a series of nonhuman primates. Within 12 days of hypercholesterolemia in Macaca nemestrina, monocytes became adherent to the surface of the endothelium. These monocytes appeared to migrate subendothelially, accumulate lipid, and become lipid-laden macrophages (foam cells). Within a month, a "serofibrinous insudate" formed together with variable numbers of subendothelial lipid-laden macrophages. By the second month, foam cells increased in number, often in multilayers, to form a fatty streak. Concomitantly, the luminal surface of the arteries became increasingly irregular due to the subendothelial accumulation of foam cells. Numerous monocytes continued to attach to the endothelial surface over the fatty streaks, and many of them appeared to enter the intima and participate in the growth of the fatty streaks. Lipid-laden smooth muscle cells appeared in small numbers and formed two to four layers between the macrophages and the internal elastic lamella at 2 to 3 months. During the third month of hypercholesterolemia, endothelial cell continuity over the lipid-laden macrophages became interrupted, exposing the underlying foam cells to circulating blood. Foam cells were then readily observed in whole blood smears, suggesting that many of the lipid-laden macrophages leave the intima and enter the circulation. After 4 months, significant endothelial denudation was found in the iliac artery and many exposed macrophages were covered by adherent platelets in the form of a mural thrombus. Thus, the early components of atherosclerosis induced by chronic hypercholesterolemia centered around the monocyte-macrophage and its interaction with endothelium in the induction of the fatty streak. Subsequent changes that lead to macrophage-smooth muscle interactions, platelet-macrophage interactions, and platelet-endothelial interactions appeared to set the stage for the development of more advanced proliferative lesions.

LDL-induced cytotoxicity and its inhibition by HDL in human vascular smooth muscle and endothelial cells in culture

Human aortic medial smooth muscle cells (SMC) and umbilical vein endothelial cells (EC) in culture were exposed to various concentrations of plasma low density (LDL) and high density (HDL) lipoproteins prepared from normolipemic donors in order to assess their effects on cell growth. So that the effects of each lipoprotein could be evaluated separately and in combination, lipoproteins were added to culture medium containing lipoprotein deficient serum (LPDS, d greater than 1.25 g/ml at a protein concentration of 4.5 mg/ml of medium). The addition of LDL at cholesterol concentrations of 160 microgram/ml of culture medium, resulted in significant reductions in both the number of SMC and EC cells per dish within 3 days of exposure (P less than 0.001, SMC; P less than 0.01, EC), when compared with LPDS controls and the starting cell numbers. This cytotoxic phenomenon was dose-related, and only at LDL cholesterol concentrations equal to or below 50 microgram/ml were no marked changes observed. In contrast, HDL at all concentrations tested produced no such deleterious effects. Autoradiographic assessment of DNA synthesis confirmed these findings. After 48 h of continuous exposure to tritiated thymidine, labeling indexes reached much lower plateaus in the LDL-treated groups.

Uptake of labelled free and esterified cholesterol from plasma by the aortic intima--media tissue measured in vivo in three animal species

Hyperlipemic stilboestrol-treated cockerels, cholesterol-fed rabbits and minipigs, as well as normolipemic cockerels and rabbits were injected intravenously with homologous plasma of corresponding lipid concentration labelled in vivo with radioactive cholesterol. The ratios between labelled free cholesterol and labelled esterified cholesterol in the intima--media from the thoracic aorta of these 5 groups of animals were respectively 1-, 2-, 8-, 2- and 20-fold greater than the corresponding average tracer ratio in plasma during the uptake period of 4--6 h. The intima--media tissue in the coronary arteries studied in one minipig contained 2--5 times more labelled cholesterol per mg wet weight than corresponding aortic tissue. This arterial uptake of labelled cholesterol in the minipigs was measured concomitantly with the uptake of phosphatidylcholine and plasma protein labelled in vivo. The uptake for these various tracers in the minipig suggested entry of labelled free and esterified cholesterol into the arterial wall, mainly as part of the plasma lipoproteins, with subsequent hydrolysis in the arterial wall of some of the cholesterol ester. In the stilboestrol-treated cockerels hydrolysis of cholesterol ester seems to be absent. The relatively higher uptake in the minipig of the labelled plasma protein (albumin) than of the lipoprotein (as traced by its lipids) suggests a molecular weight-dependent arterial entry of these plasma macromolecules.

Vessel injury and atherosclerosis

Repeated endothelial injury causes lipid-rich lesion in animals on a normal diet. In severely thrombocytopenic animals these lesions do not form or are markedly inhibited. The occurrence of lipid in some experimental designs is related to continued or repeated deposition of thrombus. Lipid deposition occurs in areas where endothelium is repeatedly removed and regrows. Repeated deposition of thrombus may bring about changes in the metabolism of the neo-intima which favor lipid deposition.

Scanning electron microscopy in the evaluation of endothelial integrity of the fatty lesion in atherosclerosis

The luminal surface of fatty lesions of atherosclerosis was viewed by scanning electron microscopy (SEM). Endothelial cells were outlined by staining intercellular junctions with silver and the aortas were fixed in situ at physiological pressure. When aortas were dehydrated by passage through organic solvents followed by critical point drying from liquid CO2, there was considerable disruption of the luminal surface and it was not possible to correctly interpret the morphological integrity of the endothelium. In contrast, simple air-drying of aortas, without solvent dehydration after fixation, allowed the integrity of the cell layer overlying the lesion to be evaluated. The success of this technique was attributed to the retention of arterial lipids during dehydration of the tissue.

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.

Transfer of cholesterol in vitro between normal arterial smooth muscle tissue and serum lipoproteins of normo-lipidemic rabbits

During incubation of normal arterial tissue with serum lipoproteins, net transfer of cholesterol was observed in the direction from the lipoproteins into the arterial tissue. Such transfer was only observed during incubations with single, isolated lipoprotein fractions. It was independent of the type of lipoprotein, VLDL, LDL or HDL, which was incubated with the arterial tissue. On the other hand, no net transfer of cholesterol was observed during incubations of arterial tissue with a combination of serum lipoproteins equivalent to that in native serum. Studies on the transfer of radioactive cholesterol suggested that cholesterol elimination from arterial tissue in vitro was not affected by the composition of the incubation medium. Therefore, it is suggested that cholesterol accumulation more easily is influenced by the serum lipoprotein composition, and that serum lipoprotein dysbalance may promote cholesterol accumulation in the tissue. This effect may be present even when the dysbalance involves a decrease of specific serum lipoprotein fractions.

Mechanisms of cholesterol accumulation in the arterial wall

The rate of cholesterol accumulation is a function of three separate processes: the transfer of lipid or lipoprotein from blood plasma to the artery, the binding and sequestering of lipids in the arterial wall and the solubilization and removal of lipid from the artery. These processes have been studied with lipids or lipoproteins labeled with radioisotopes by autoradiographic and quantitative chemical procedures. More recently immunochemical procedures have been applied to this problem. Studies have been performed with intact animals, isolated organs and cell cultures. In addition, homogenates have been used successfully to study intraarterial transformations of lipids, (for example, cholesterol esterification). Although epidemiologic and clinical studies, as well as animal experiments, have provided evidence that the concentration of plasma low or very low density lipoproteins parallels the rate of atherogenesis, the nature of the causal chain linking plasma lipoproteins to atherosclerosis is as yet unclear. A possible link between plasma lipoproteins, arterial liproprotein lipase and atherogenesis has been postulated.