Characterization of two lipoproteins containing apolipoproteins B and E from lesion-free human aortic intima

Lesion-derived low density lipoprotein and oxidized low density lipoprotein share a lability for aggregation, leading to enhanced macrophage degradation

In this study we assessed whether low density lipoproteins (LDL) isolated from minced aortic atherosclerotic plaques obtained at autopsy (A-LDL) shared structural and functional properties with LDL oxidized by incubation with Cu2+ for 8-18 hours at 20 degrees C (Ox-LDL). Although both A-LDL and Ox-LDL represented monomeric particles about the size of LDL, both differed from LDL in that they showed an increase in electrophoretic mobility relative to LDL, an increase in cholesterol to protein ratio, and an increase in reactivity with a monoclonal antibody that recognizes epitopes on malondialdehyde (MDA)-modified proteins. In addition, both showed an increase in fluorescence at 360 nm excitation, 430 nm emission, an increase in fragmentation of apolipoprotein B with patterns that were quite similar, and an increase in recognition by the scavenger receptor on mouse peritoneal macrophages (MPMs) based on competition of 125I-A-LDL and 125I-Ox-LDL degradation by excess acetylated LDL. In addition, inhibition of degradation by MPMs of 125I-A-LDL and 125I-Ox-LDL by excess unlabeled Ox-LDL and A-LDL were similar. When MDA was added in increasing amounts to labeled LDL and A-LDL, less MDA was required to modify A-LDL than LDl to obtain ligands that were degraded by MPMs to the same degree. Finally, both A-LDL and Ox-LDL but not LDL underwent aggregation (increased metastability) when concentrated to levels exceeding 1 mg protein/ml and showed enhanced macrophage uptake via phagocytosis (inhibition by cytochalasin D). These results demonstrate that A-LDL and Ox-LDL share properties additional to those previously reported, suggesting that oxidation may be a major mode of modification of LDL accumulating in atherosclerotic lesions. This could lead to lipid loading of macrophages induced by phagocytosis of aggregated particles, in addition to unregulated uptake via the scavenger receptor of monomeric particles.

Estrogens inhibit copper and cell-mediated modification of low density lipoprotein

The effects of estrogens on LDL modification by copper ions, U 937 monocyte-like cells or endothelial cells was studied by determination of the lipid peroxidation product content and measurement of the relative electrophoretic mobility. The presence of estradiol, estriol and estrone inhibited LDL oxidation in a dose-dependent manner in the range of concentrations from 5 to 50 microM. In the case of oxidation by Cu2+, the decreasing order of efficiency was: estradiol, estriol, estrone. In monocyte-induced oxidation, the protective effect of estrogens was more marked, and the order of efficiency was the same, except that estrone was as active as estriol. Pretreatment of monocyte cells with estrogens also inhibited the subsequent modification of LDL by these cells, tested in the absence of the hormones. Testosterone had no effect in all the studied systems. Furthermore, the degradation by J774 macrophage like cells of LDL modified either by Cu2+ or monocytes was markedly reduced when modification has been performed in the presence of estrogens. Since oxidative modification of LDL is believed to be involved in the appearance of atherosclerotic plaques, this effect of estrogens might be related to their protective action against atherosclerosis.

Dietary eicosapentaenoic acid and docosahexaenoic acid from fish oil. Their incorporation into advanced human atherosclerotic plaques

The incorporation of fatty acids from dietary fish oil was measured in obstructive atherosclerotic plaques removed from 11 patients fed fish oil, rich in omega-3 fatty acids, for 6-120 days before a planned arterial endarterectomy. The fatty acids of plasma and atheroma were analyzed with special reference to docosahexaenoic acid (DHA, 22:6) and eicosapentaenoic acid (EPA, 20:5), the principal omega-3 fatty acids of fish oil. The omega-3 fatty acid content increased greatly in plasma from 0.9% of fatty acids to 14.8% in cholesteryl esters, from 3.8% to 22.1% in phospholipids, and from 1.3% to 21.9% in triglycerides. The omega-3 fatty acid content of the atherosclerotic plaques was also greater when compared with that of plaques removed from 18 non-fish oil-fed controls. The omega-3 fatty acid in cholesteryl esters of the plaques was 4.9% in the experimental group versus 1.4% in control plaque, in phospholipids it was 8.8% versus 1.8%, and in triglycerides it was 4.7% versus 0.7% (p less than 0.001 for each lipid class). The two major omega-3 fatty acids (DHA and EPA) behaved differently. Compared with their respective plasma levels, relatively more DHA than EPA was deposited into the plaques. Whereas the increase of omega-3 fatty acids in plasma reached a plateau 3 weeks after initiation of fish oil feeding, a linear increase in plaque omega-3 fatty acids continued with time. As a result of the changes in fatty acid composition, the lipid classes of both plasma and plaque had a higher unsaturation index in the fish oil-fed group.(ABSTRACT TRUNCATED AT 250 WORDS)

Macrophage degradation of LDL extracted from human aortic plaques: effect of isolation conditions

Several laboratories have recently reported on the structural and functional characteristics of an LDL fraction isolated from atherosclerotic lesions, designated A-LDL. Given the wide variety of tissue sources and isolation conditions that have been employed, we have addressed whether several procedures currently used affect the interaction of A-LDL with macrophages, and, if so, by what mechanisms. We isolated A-LDL from human aortic plaques by ultracentrifugation and gel filtration chromatography. Although some differences in the chromatographic elution profiles on gel filtration were apparent between homogenized and nonhomogenized extracts, A-LDL isolated from the same pool of plaque minces with or without homogenization showed no differences in macrophage degradation or inhibition of this degradation by excess acetyl-LDL. A-LDL isolated from plaques obtained at surgery or at autopsy less than 12 hr after death also showed no major differences in macrophage recognition, suggesting that post-mortem changes were probably not affecting cell recognition. However, A-LDL particles underwent aggregation when subjected to concentration, when stored for periods of 2 weeks or more, or when subjected to vortexing. The aggregated A-LDL was degraded more readily by macrophages than unaggregated A-LDL, and inhibition of degradation of aggregated A-LDL by excess acetyl-LDL was less than for unaggregated A-LDL. Collectively, these studies show that although post-mortem changes and tissue homogenization do not appreciably affect the interaction of A-LDL with macrophages in culture, other isolation and preparation conditions have dramatic effects which could explain some of the diversity of A-LDL metabolism reported in the literature.

Macrophages and oxidized low density lipoproteins in the pathogenesis of atherosclerosis

Oxidized low density lipoprotein (LDL) may play an important role in the pathogenesis of atherosclerosis. Recent evidence strongly suggests that oxidized LDL is present in atherosclerotic lesions in vivo: 1) LDL isolated from human and rabbit lesions (but not from normal intima) resembles oxidized LDL in its physical, chemical and immunological properties; 2) Oxidized LDL and/or oxidation specific lipid-protein adducts can be demonstrated in human and rabbit lesions by immunocytochemical techniques; 3) Human and rabbit serum contains autoantibodies against oxidized LDL and oxidation specific lipid-protein adducts; 4) atherosclerotic lesions contain IgG that recognizes oxidized LDL and 5) antioxidant therapy slows the development of atherosclerotic lesions in rabbits. Atherosclerosis in human and rabbit arteries may be linked to macrophage-induced oxidative modification of LDL mediated by 15-lipoxygenase which leads to an enhanced uptake of LDL in macrophages by way of the scavenger receptor(s). The identification of LDL oxidation as one of the key events in the early pathogenesis of atherosclerosis offers an interesting possibility to reduce atherosclerosis by antioxidants, enzyme inhibitors and other compounds that protect LDL against oxidative damage and/or reduce the subsequent harmful effects of oxidized LDL on various cellular functions.

Localization of apolipoprotein E in normal and atherosclerotic human aorta

To elucidate the role of apolipoprotein E (apo E) in atherogenesis, we have investigated the localization of apo E in normal and atherosclerotic aortas as well as in other tissues of 32 post-mortem individuals. Using double immunofluorescence it has been found that normal intima of individuals older than 20 years and some adolescents contained immunoreactive material that reacted with poly- and monoclonal antibodies to apo E. A staining pattern of apo E differed from that of apolipoprotein B, the latter being seen in normal intima of each child older than 7 years. Apo E was present extracellularly in lipid streaks and atheromatous plaques, where its staining was particularly intensive around the necrotic zone of plaques. Some macrophages in the plaques of 4 aortas exhibited apo E-positive staining, while aortic endothelial and smooth muscle cells never contained apo E. Apo E-positive staining was not found in the majority of vessel cells, it was always, however, observed in other types of cells including hepatocytes. Kupffer cells, spleen macrophages and cerebral astrocytes. Our findings indicate that only some macrophages in human aorta may be responsible for the production of apo E that can participate in reverse cholesterol transport. At the same time, apo E accumulation in the aortic wall may promote the development of atherosclerosis.

Arterial metabolism of lipoproteins in relation to atherogenesis

In this short review we have concentrated on the ways in which modification of LDL structure may account for foam cell formation. We have presented in vivo evidence as well as in vitro evidence supporting the proposition that modification of native LDL is a prerequisite for foam cell formation and atherogenesis. Actually, oxidized LDL can contribute to atherogenesis in other ways as well. Oxidized LDL is chemotactic for circulating monocytes, yet inhibits the motility of the tissue macrophage as shown by Quinn et al. Also, oxidized LDL is cytotoxic as discussed above and this could play a crucial role in the transition from the fatty streak lesion to the clinically more consequential fibrous plaque and complicated lesion. If further research supports the importance of LDL modification in atherogenesis, a whole new array of possibilities opens itself to us for intervention. Anything that interferes with the relevant modifications of the LDL structure would presumably be additive to interventions lowering the plasma concentration of LDL. At the moment, the only such intervention that appears to be feasible is prevention of LDL oxidation. Possibly we may find ways to interfere with immune mechanisms that are involved in some patients; conceivably we might be able to interfere with the aggregation of LDL with itself or with other complexes in the artery wall that appear also to favor initiation of the atherogenic process.

Morphological detection and quantification of lipoprotein(a) deposition in atheromatous lesions of human aorta and coronary arteries

Lipoprotein(a), as an atherogenic particle, represents an independent risk factor for coronary heart disease. In the present study the morphological distribution of apoprotein (a) and apoprotein B within the arterial wall is described. Apoprotein B, a constituent of very low-density lipoprotein, low-density lipoprotein and lipoprotein(a) has previously been demonstrated in atheromatous lesions. Lipoprotein(a) possesses an additional protein, designated apoprotein (a). Autopsy material (n = 74) from the left coronary artery and from the thoracic aorta has been examined by means of immunohistochemistry and both apoprotein (a) and apoprotein B were detected, primarily associated with the extracellular matrix and accumulating in lesions in the arterial wall. The staining pattern for both antigens was almost always found to be congruent, suggesting that the detection of (a)-antigen has to be attributed at least in part to the presence of lipoprotein(a). It is concluded that both low-density lipoprotein and lipoprotein(a) have an important role in the pathogenesis of atherosclerosis.

Noninvasive imaging of 99mtechnetium-labeled low density lipoprotein uptake by tendon xanthomas in hypercholesterolemic patients

Technetium-labeled low density lipoproteins (Tc-LDL) appear to be useful for describing LDL biodistribution in normal and dyslipidemic subjects. We injected 99mTc-LDL into subjects with large tendon xanthomas secondary to homozygous familial hypercholesterolemia or sitosterolemia. Rapid (4 hours) accumulation of Tc-99m activity in xanthomas was observed, and this accumulation increased over a 24-hour period. No comparable accumulations of Tc-99m activity were noted in normal subjects or in a subject with heterozygous familial hypercholesterolemia who had very small tendon xanthomas. These findings support previous biopsy data indicating active uptake of LDL by macrophages within xanthoma and suggest that 99mTc-LDL imaging of xanthomas may be useful in studies of the effects of diet and drugs on the accumulation of lipoproteins by atherosclerotic plaques.

Role of oxidatively modified LDL in atherosclerosis

Oxidative modification of LDL is accompanied by a number of compositional and structural changes, including increased electrophoretic mobility, increased density, fragmentation of apolipoprotein B, hydrolysis of phosphatidylcholine, derivatization of lysine amino groups, and generation of fluorescent adducts due to covalent binding of lipid oxidation products to apo B. In addition, oxidation of LDL has been shown to result in numerous changes in its biologic properties that could have pathogenetic importance, including accelerated uptake in macrophages, cytotoxicity, and chemotactic activity for monocytes. The present article summarizes very recent developments related to the mechanism of oxidation of LDL by cells, receptor-mediated uptake of oxidized LDL in macrophages, the mechanism of phosphatidylcholine hydrolysis during LDL oxidation, and other biologic actions of oxidized LDL including cytotoxicity, altered eicosanoid metabolism, and effects on the secretion of growth factors and chemotactic factors. In addition, this review will examine the evidence for the presence of oxidized LDL in vivo and the evidence that oxidized LDL plays a pathogenetic role in atherosclerosis.

Separation and quantitation of free cholesterol and cholesteryl esters in a macrophage cell line by high-performance liquid chromatography

A method for the direct high-performance liquid chromatographic (HPLC) determination of free cholesterol and the individual cholesteryl esters in cell culture experiments is described. The murine macrophage-like J774 cell line was loaded with cholesterol by incubation with low-density lipoproteins. After extraction of the cellular lipids with hexane-isopropanol (3:2, v/v), the cholesteryl esters were identified and quantified by isocratic HPLC. Unesterified cholesterol and its esters were eluted with acetonitrile-isopropanol (50:50, v/v) on a Zorbax ODS column within 25 min and detected at 210 nm. Cholesteryl heptadecanoate was used as an internal standard. The detection response is linear in the analytical range of interest; the overall coefficients of variation are less than 8% and the detection limit is between 50 and 150 ng. The results demonstrate that HPLC is suitable for the determination of cellular cholesteryl ester profiles and could usefully contribute to the understanding of the mechanism of foam cell formation during the development of atherosclerosis. This method can also be applied to all experimental systems involving the study of cholesteryl esters.

Lipoprotein composition changes induced by fenofibrate in dysbetalipoproteinemia type III

Fenofibrate (300 mg daily) was given to 9 subjects (7 men, 2 women) with dysbetalipoproteinemia type III. The treatment brought about important plasma level reductions in cholesterol (-35%), triglycerides (-56%), VLDL-cholesterol (-63%) and VLDL-triglycerides (-59%). The VLDL-C/TG ratio, which was 0.40 before treatment, was 0.30 after 4 weeks of fenofibrate, still suggestive of type III. LDL-C, when measured by conventional methods, was unchanged but isolation of the IDL (1.006-1.019 g/ml) fraction from the 1.006 g/ml infranatant revealed that true LDL-C levels actually increased in 6 individuals while IDL-C decreased considerably. The total HDL-C increase was mostly due to a 33% HDL3-C change. Apolipoprotein levels were considerably modified, notably apo B, C-III and E which were decreased, as well as the lipoprotein particles containing combinations of these apolipoproteins, namely LpE:B and LpC-III:B. Apo A-I was slightly modified as LpA-I: A-II particle levels increased and LpA-I decreased. There were marked compositional modifications of apo B-containing lipoproteins which corresponded to changes of the whole lipoprotein profile. Some abnormal classes of lipoproteins (e.g., beta-VLDL, dense LDL), characteristic of this disease, tended to disappear and were in some cases replaced by material of different size and density.