Modification of low density lipoprotein by endothelial cells involves lipid peroxidation and degradation of low density lipoprotein phospholipids.

Abstract

Low density lipoprotein (LDL) incubated with cultured endothelial cells from rabbit aorta or human umbilical vein is altered in several ways (EC-modified): (i) It is degraded by macrophages much faster than LDL similarly incubated in the absence of cells or incubated with fibroblasts.(ii) Its electrophoretic mobility is increased. (iii) Its density is increased. We report here that antioxidants completely prevent these changes. We also report that these changes do not take place if transition metals in the medium are chelated with EDTA. During EC-modification as much as 40% of the LDL phosphatidylcholine is degraded to lysophosphatidylcholine by a phospholipase A2-like activity. When incubation conditions in the absence of cells were selected to favor oxidation-for example, by extending the time of incubation of LDL at low concentrations, or by increasing the Cu2+ concentration-LDL underwent changes very similar to those occurring in the presence of cells, including degradation of phosphatidylcholine. Hence, some phospholipase activity appears to be associated with the isolated LDL used in these studies. The results suggest a complex process in which endothelial cells modify LDL by mechanisms involving generation of free radicals and action of phospholipase (s).The lipid-laden foam cells in atherosclerotic lesions are derived largely or in part from monocyte/macrophages (1, 2). These cells have only low levels of the classical low density lipoprotein (LDL) receptor and take up native LDL in vitro at relatively low rates, insufficient to cause lipid accumulation to the extent found in vivo (3). It has been suggested that this paradox may be explained if the LDL particle is in some way altered in vivo to a form taken up more readily than native LDL. Certain chemically modified forms of LDL, including acetylated LDL, are indeed taken up much more rapidly than native LDL by macrophages, and this uptake involves a different receptor, designated the acetyl-LDL receptor (3-5). Incubation of LDL with cultured endothelial cells generates a modified form (or forms) of LDL (endothelial cell-modified LDL; EC-modified LDL) that is taken up 3-to 10-fold more rapidly by macrophages and at least in part by way of the acetyl-LDL receptor (6-8). The modification in biological properties is accompanied by a marked increase in electrophoretic mobility and hydrated density but the mechanisms involved are still poorly understood.LDL is highly sensitive to metal-catalyzed oxidation (9-11) and oxidized LDL has been shown to be toxic to some cultured cells (12,13). Endothelial cells in culture have been shown to be capable of oxidizing LDL (14). Since the modification of LDL by endothelial cells involves long incubation under aerobic conditions, we examined the possible role of oxidative changes in the process. In the present paper, we report that generation of EC-modified LDL is associated with lipid peroxidation and with extensive hydrolysis of LDL phosphatidylcholine (PtdCho) to lysophosphatidylcholine (lyso-PtdCho).MATERIALS...

Mechanism of the antioxidant to pro-oxidant switch in the behavior of dehydroascorbate during LDL oxidation by copper(II) ions

“…Abbreviations: BCDS, bathocuproine disulphonic acid; DMPO,5, EPR, electron paramagnetic resonance; 13-HPODE, 13(S)-hydroperoxyoctadeca-9Z,11E-dienoic acid 2…”

“…A number of large clinical trials have shown no decrease in cardiovascular disease after antioxidant supplementation, however, and the importance of oxidised LDL in atherosclerosis is therefore an unresolved issue [2]. Several studies have demonstrated that LDL can be oxidised in vitro by cells associated with atherosclerotic lesions [3][4][5][6][7]. The mechanisms of LDL oxidation in vivo are controversial [1].…”

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Subject area: Systems biochemistryAbbreviations: BCDS, bathocuproine disulphonic acid; DMPO,5, EPR, electron paramagnetic resonance; 13-HPODE, 13(S)-hydroperoxyoctadeca-9Z,11E-dienoic acid 2

ABSTRACTOxidised low density lipoprotein (LDL) may be involved in the pathogenesis of atherosclerosis.We have therefore investigated the mechanisms underlying the antioxidant/pro-oxidant behavior of dehydroascorbate, the oxidation product of ascorbic acid, toward LDL incubated with Cu 2+ ions. By monitoring lipid peroxidation through the formation of conjugated dienes and lipid hydroperoxides, we show that the pro-oxidant activity of dehydroascorbate is critically dependent on the presence of lipid hydroperoxides, which accumulate during the early stages of oxidation.

Using electron paramagnetic resonance spectroscopy, we show that dehydroascorbate amplifies the generation of alkoxyl radicals during the interaction of copper ions with the model alkyl hydroperoxide, tert-butylhydroperoxide.

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Transition metal ions within human atherosclerotic lesions can catalyse the oxidation of low density lipoprotein by macrophages

“…Most workers find that LDL oxidation by cells requires the presence of transition metal ions, usually iron [7,9], but copper ions will also catalyse the oxidation [9,10]. A sufficiently high concentration of iron [7,9] or copper [10,11] can catalyse the oxidation of LDL in the absence of cells.…”

Characterization of a receptor for oxidized low-density lipoproteins on rat Kupffer cells: similarity to macrosialin

“…During the oxidation process, a number of chemical changes take place within the lipoprotein particle, including an increase in density, a decrease in content of esterified cholesterol, fragmentation of apolipoprotein B and an increase in negative charge, leading to increased electrophoretic mobility [1][2][3]. As a consequence, oxLDL are no longer recognized by the native LDL receptor, but instead interaction with the so-called scavenger receptor is induced.…”