Interaction of low density lipoproteins from normal and hyperlipemic Rhesus monkeys with arterial smooth muscle cells in culture

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.

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.

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

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

Metabolism of 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.

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.

The hypolipidemic activity of benzenetricarboxylic acids in rodents

A series of benzenetricarboxylic acids was shown to be potent hypolipidemic agents in rodents. Terephthalic acid proved to be one of the more potent agents, lowering serum cholesterol 42% and serum triglyceride 33 % at 20 mg/kg/day for 16 days. The ability to lower serum lipids by this agent appeared to be due to multiple modes of action: (1) terephthalic acid suppressed the activities both in vivo and in vitro of a number of regulatory enzymes involved in cholesterol, fatty acid, and triglyceride syntheses; (2) the drug inhibited cholesterol absorption from the GI tract by 43 %; and (3) the drug accelerated lipid excretion in the feces leading to a reduction of cholesterol in the tissue. Terephthalic acid was effective in lowering lipids in normal and hyperlipidemic animals and possessed a safe therapeutic index.

Prevention of the hyperlipidemic serum or LDL-induced cellular cholesterol ester accumulation by 22-hydroxycholesterol and its analogue

Incubation of monkey arterial smooth muscle cells with hyperlipidemic serum or low-density lipoproteins (LDL) produces intracellular cholesterol ester accumulation. Increased esterification of free cholesterol within the cell may account for this effect. To examine such a possibility, oxygenated sterols were used to block cholesterol esterification. The increased esterification of free cholesterol by smooth muscle cells and human skin fibroblasts, through their exposure to hyperlipidemic lipoproteins, was inhibited by 22-hydroxycholesterol and its analogue, SC-32561 (22-hydroxy-25-fluorocholesterol). The hyperlipidemic LDL-stimulated elevation in the cholesterol ester content of the smooth muscle cells was also prevented by these sterols. This reduction in cellular cholesterol ester did not coincide with an increase in free cholesterol. 22-Hydroxycholesterol also blocked the stimulation of the esterification of cholesterol due to 25-hydroxycholesterol. In the absence of lipoproteins, 22-hydroxycholesterol and SC-32561 had a minor effect on the incorporation of [14C]oleate into cholesterol esters, and efficiently reduced sterol synthesis in fibroblasts and smooth muscle cells. 22-Hydroxycholesterol and SC-32561 had the additional effect of lowering the number of cell-surface LDL receptors to a greater extent than did hyperlipidemic LDL. The presence of 22-hydroxycholesterol did not alter the interaction of normal LDL with the receptor. Oxygenated sterols recovered in the cell represented 4-21% of the total sterol content. The level of intracellular oxygenated sterols was significantly reduced by the presence of lipoproteins in the culture media. Due to the multiple effects of the oxysterols, they were not effective as tools in determining the contribution of acyl-coenzyme A:cholesterol acyltransferase enzyme activity to the intracellular pool of cholesterol esters. These results indicated that 22-hydroxycholesterol and SC-32561 effectively blocked the hyperlipidemic LDL-stimulated increase in smooth muscle cell cholesterol ester content by lowering cholesterol esterification, reducing cholesterol synthesis and down-regulating LDL cell-surface receptors.

Antihyperlipidemic activity of phthalimide analogues in rodents

Phthalimide analogues have been shown to effectively lower serum cholesterol and triglyceride levels in rats and mice. The mode of action of these agents was not to suppress the appetite of animals, but rather to reduce the activities of key enzymes in the early synthesis of liver cholesterol and fatty acids in the triglyceride pathway. Phthalimide analogues were effective in accelerating biliary excretion of cholesterol and blocking its absorption from the gut. After 16 days dosing, it was evident that higher levels of lipids were being excreted than in control mice. The major serum lipoprotein fractions were reduced in cholesterol, triglyceride, and neutral lipid content, but not phospholipid content in rats after 14 days of administration.

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.