Cholesterol-esterifying enzymes of foam cells isolated from atherosclerotic rabbit intima

Mouse models of atherosclerosis: a historical perspective and recent advances

Atherosclerosis represents a significant cause of morbidity and mortality in both the developed and developing countries. Animal models of atherosclerosis have served as valuable tools for providing insights on its aetiology, pathophysiology and complications. They can be used for invasive interrogation of physiological function and provide a platform for testing the efficacy and safety of different pharmacological therapies. Compared to studies using human subjects, animal models have the advantages of being easier to manage, with controllable diet and environmental risk factors. Moreover, pathophysiological changes can be induced either genetically or pharmacologically to study the harmful effects of these interventions. There is no single ideal animal model, as different systems are suitable for different research objectives. A good understanding of the similarities and differences to humans enables effective extrapolation of data for translational application. In this article, we will examine the different mouse models for the study and elucidation of the pathophysiological mechanisms underlying atherosclerosis. We also review recent advances in the field, such as the role of oxidative stress in promoting endoplasmic reticulum stress, mitochondrial dysfunction and mitochondrial DNA damage, which can result in vascular inflammation and atherosclerosis. Finally, novel therapeutic approaches to reduce vascular damage caused by chronic inflammation using microRNA and nano-medicine technology, are discussed.

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.

The effect of bezofibrate and clofibrate on cholesterol accumulation, esterification and removal in cultured 3T3 fibroblasts

3T3 mouse fibroblasts were used to determine the effect of bezafibrate and clofibrate on the cellular metabolism of cholesterol. In cells incubated in normal medium these agents decreased the incorporation of 3H-labelled oleic acid relative to 14C-labelled linoleic acid into the cholesterol ester fraction. When the 3T3 fibroblasts were incubated with cationised low density lipoprotein (LDL) the amount of esterified cholesterol which accumulated in the cells was greatly increased. This accumulation of cholesterol ester was reduced by bezafibrate and clofibrate. These agents decreased the incorporation of both 3H-labelled oleic acid and 14C-labelled linoleic acid into the cholesterol ester fraction of the cells, with a preferential effect on oleic acid as indicated by a reduction in the 3H/14C ratio. When cells which had been preincubated with cationised LDL were reincubated in normal medium, the removal of esterified cholesterol from the cells was increased by both bezafibrate and clofibrate. The mechanism of the effects of these agents on the metabolism of cellular cholesterol is discussed.

Inhibition of cholesterol esterification in rabbit aorta by prostaglandin E2

Cholesterol esterification in the arterial wall was investigated with cell-free preparations of intima-media from control rabbits and rabbits rendered atherosclerotic by feeding a diet containing 1% cholesterol. In the presence of 2 mM ATP and 0.1 mM CoA, the major activity for esterification of [4-14C] cholesterol added in vitro was found in the 12,000 g and 105,000 g pellets. In control animals, the activity in the latter pellet was twice that in the former. After cholesterol-feeding for 6 months, the activity increased 5-fold in the 105,000 g pellet and 2-fold in the 12,000 g pellet of the atherosclerotic intima-media. Prostaglandin E2 (PGE2) in concentrations between 2 and 12 X 10(-7) M exhibited a dose-dependent inhibition of the esterifying activity in both particulate preparations. The inhibition was 97% at PGE2 concentrations greater than 1.2 X 10(-6) M in preparations from control animals. Inhibition by PGE2 in preparations from atherosclerotic rabbits was also observed. These results suggest a possible regulatory role of PGE2 in cholesterol esterification in the arterial wall.

Synthesis and removal of different cholesterol esters by aortic smooth muscle cells in culture

The incorporation of 3H-labelled oleic acid and of 14C-labelled linoleic acid into phospholipid, triglyceride and cholesterol ester in smooth muscle cells grown in incubation medium supplemented with either 5% normal or 5% hyperlipemic serum has been studied. Both fatty acids were incorporated into cholesterol esters to a greater extent when cells grown in incubation medium containing hyperlipemic serum. Oleic acid was incorporated into cholesterol esters in preference to linoleic acid. The addition of hyperlipemic serum to the incubation medium did not increase the incorporation fo either 3H-labelled oleic acid or of 14C-labelled linoleic acid into phospholipid or triglyceride. The removal of labelled lipid fractions has also been followed for four days in cells pulse labelled for 24 hours with 3H-labelled oleic acid and 14C-labelled linoleic acid. Both 3H- and 14C-labelled cholesterol esters were removed more rapidly when the smooth muscle cells were grown in medium containing normal serum than in medium containing hyperlipemic serum. The removal of both phospholipid and triglyceride was similar in normal and hyperlipemic serum. Comparison of the 3H/14C ratio indicated that the cholesterol oleate and cholesterol linoleate were removed at similar rates.

Cell proliferation in the atherosclerotic plaques of cholesterol-fed rabbits. Part 3. Histological and radioautographic observations on glucocorticoids-treated rabbits

Tritiated thymidine radioautography was employed to study the effect of cortisol and other glucocorticoids on cellular proliferation in the aorta and pulmonary artery of rabbits with cholesterol atherosclerosis. Labelled cell counts showed that glucocorticoids, even after one day and at a relatively low dose, decrease sharply the deoxyribonucleic acid synthesis in the intimal plaques. The hormonal influence on [3H]thymidine uptake seems to be a dose-dependent process. The relative potency of these steroids in inhibiting DNA synthesis in the plaques parallels closely their anti-inflammatory effectiveness. Conversely mineralocorticoids, including aldosterone and deoxycorticosterone, increase the rate of DNA synthesis in the plaques. It is concluded that the antiatherogenic effect of glucocorticoids on cholesterol-fed rabbits may be due, at least partly, to the inhibitory effect of these steroids on the DNA synthesis of the cellular components of the intimal plaques.

The synthesis of lipids from [1-14C]acetate by human venous endothelium in tissue culture

Endothelial cells from human umbilical cords were harvested, using a trypsin technique, grown in tissue culture and lipid synthesis studied using [1-14C]acetate as a precursor. Radiosubstrate was incorporated into fatty acids, mono-, di- and triglycerides, cholesterol esters and phospholipids. Radioactivity was also present in the culture medium in the mono-and diglyceride fractions and in the phospholipids running with the solvent front.

Fatty acyl CoA synthetase activity in normal and atherosclerotic rabbit aortic tissue

The activity of fatty acyl CoA synthetase and fatty acyl CoA:cholesterol acyltransferase was determined in microsomal fractions from normal and atherosclerotic rabbit aortic tissue. No change in fatty acyl CoA synthetase activity was observed as a result of cholesterol feeding in contrast to the several-fold increase in the activity of fatty acyl CoA:cholesterol acyltransferase seen in atherosclerotic tissue. Inhibition of both enzymes was observed when clofibrate, or the tetrahydronapthyl analog of this drug were added in vitro. The inhibitory effects were most pronounced on the fatty acyl CoA:cholesterol acyltransferase.

Cholesteryl ester synthesis in normal and atherosclerotic aortas of rabbits and rhesus monkeys

The formation of cholesteryl ester in aortic tissue was studied using subcellular fractions from normal and atherosclerotic rabbit and rhesus monkey aortas. The properties of two enzyme systems capable of esterifying 1- 14 C-oleic acid into cholesteryl ester in vitro were investigated, and increased activity was demonstrated for both systems as a result of cholesterol feeding. Microsomal preparations were used to study the ATP, CoA-dependent esterification which involves two enzymes, fatty acyl CoA synthetase and fatty acyl CoA: cholesterol acyltransferase. The properties of both enzymes were investigated, and an increase of about fourfold in activity of the acryltransfrease was demonstrated in aortic microsomes as a result of cholesterol feeding for 3-6 months. Esterification of oleic acid into cholesteryl ester by aortic high-speed supernatant fractions at an acidic pH was also observed; the enzyme system involved did not require cofactors, and its activity greatly increased as a result of cholesterol feeding. Similar increases in the activity of both esterifying enzyme systems were found when normal and atherosclerotic rhesus monkey aortic fractions were compared. p -Chlorophenoxyisobutyrate (CPIB) and 2-methyl-2-[ p -(1, 2, 3, 4-tetrahydro-l-naphthyl)-phenoxy]propionic acid (TPIA) produced inhibition of both cholesterol-esterifying enzyme systems. TPIA was a more effective inhibitor than CPIB on both enzyme systems by at least an order of magnitude. These studies suggest that increased intracellular synthesis of cholesteryl ester by aortic tissue may contribute to its accumulation in atherosclerosis.