Endothelial cells cholesterol oxides

Enhanced production of vasoconstrictor factors via eicosanoid and/or free radical-related mechanisms has been observed in several cardiovascular disease states. In addition to the well-established role of free radicals in promoting the oxidation of low density lipoprotein cholesterol (LDL-C), changes in free radical status may modify endogenous eicosanoid profiles and/or produce nonenzymatic lipid peroxidation products of the arachidonic acid (AA) cascade such as lipid hydroperoxides and isoprostanes, which have been shown to possess potent vasoactive properties (3). Furthermore, an excess of free radicals may interact with the vascular endothelial cell nitric oxide (NO) to produce highly reactive peroxynitrite radicals, resulting in tissue damage and vasoconstriction (4—6) (Fig. 2). 

Blake, 1989 Winyard et al., 1989). We suggest that within the inflamed rheumatoid joint (or the artery wall in atherogenesis), the production of ROM and proteases by endothelial cells and/or macrophages may cause the release of copper ions from Cp (see Section 2.2.3.2). It has been reported that Cp is cleaved faster in serum from patients with inflammatory diseases when compared to normal serum (Laurell, 1985). The oxidative modification of LDL by Cp-derived copper ions may explain the observation that increased serum cholesterol values are associated with accelerated atherosclerotic progression in men with high serum copper concentrations (Salonen et al., 1991). 

Recent data indicate that SR-BI is a nonspecific receptor for many lipophilic molecules (Lorenzi et al., 2008 Reboul et al., 2007b). Apart from HDLs, rodent SR-BI also binds to LDL, VLDL, acetylated LDL, oxidized LDL, and maleylated bovine serum albumin. SR-BII has a similar ligand specificity and function to that of SR-BI (Webb et al., 1998). However, it has been shown that vitamin E (which like carotenoids is carried in the bloodstream mainly by LDL and HDL) is transported more efficiently into the endothelial cells from HDLs than from LDLs (Balazs et al., 2004 Kaempf-Rotzoll et al., 2003 Mardones and Rigotti, 2004). This is in striking contrast to cholesterol, which is taken up much more efficiently from LDLs than HDLs by the RPE to the retina (Tserentsoodol et al., 2006b). It remains to be shown which lipoproteins are the main carriers for carotenoids transported from blood into the RPE. 

As mentioned earlier, oxidation of LDL is initiated by free radical attack at the diallylic positions of unsaturated fatty acids. For example, copper- or endothelial cell-initiated LDL oxidation resulted in a large formation of monohydroxy derivatives of linoleic and arachi-donic acids at the early stage of the reaction [175], During the reaction, the amount of these products is diminished, and monohydroxy derivatives of oleic acid appeared. Thus, monohydroxy derivatives of unsaturated acids are the major products of the oxidation of human LDL. Breuer et al. [176] measured cholesterol oxidation products (oxysterols) formed during copper- or soybean lipoxygenase-initiated LDL oxidation. They identified chlolcst-5-cnc-3(3, 4a-diol, cholest-5-ene-3(3, 4(3-diol, and cholestane-3 3, 5a, 6a-triol, which are present in human atherosclerotic plaques. 

Contrary to LDL, high-density lipoproteins (HDL) prevent atherosclerosis, and therefore, their plasma levels inversely correlate with the risk of developing coronary artery disease. HDL antiatherogenic activity is apparently due to the removal of cholesterol from peripheral tissues and its transport to the liver for excretion. In addition, HDL acts as antioxidants, inhibiting copper- or endothelial cell-induced LDL oxidation [180], It was found that HDL lipids are oxidized easier than LDL lipids by peroxyl radicals [181]. HDL also protects LDL by the reduction of cholesteryl ester hydroperoxides to corresponding hydroperoxides. During this process, HDL specific methionine residues in apolipoproteins AI and All are oxidized [182]. 

Figure 22.6 How various factors increase the risk of atherosclerosis, thrombosis and myocardial infarction. The diagram provides suggestions as to how various factors increase the risk of development of the trio of cardiovascular problems. The factors include an excessive intake of total fat, which increases activity of clotting factors, especially factor VIII an excessive intake of saturated or trans fatty acids that change the structure of the plasma membrane of cells, such as endothelial cells, which increases the risk of platelet aggregation or susceptibility of the membrane to injury excessive intake of salt - which increases blood pressure, as does smoking and low physical activity a high intake of fat or cholesterol or a low intake of antioxidants, vitamin 6 2 and folic acid, which can lead either to direct chemical damage (e.g. oxidation) to the structure of LDL or an increase in the serum level of LDL, which also increases the risk of chemical damage to LDL. A low intake of folate and vitamin B12 also decreases metabolism of homocysteine, so that the plasma concentration increases, which can damage the endothelial membrane due to formation of thiolactone.

Cardiovascular heart diseases (CHD) are considered as the clinical expression of advanced atherosclerosis. One of the initial steps in atherogenesis is the oxidative modification of LDL and the uptake of the modified lipoprotein particles by macrophages, which in turn become lipid laden cholesterol-rich cells, so-called foam cells [159]. An accumulation of foam cells in the arterial wall is the first visible sign of atherosclerosis and is termed fatty streak, the precursor to the development of the occlusive plaque [160]. It is well known that oxidation of LDL can be initiated in vitro by incubating isolated LDL particles with cells (macrophages, lymphocytes, smooth muscle cells, or endothelial cells), metal ions (copper or iron), enzymes, oxygen radicals, or UV-light. However less is known about the mechanisms by which... 

Parenchymal liver cells Kupffer cells Liver endothelial cells Leucocytes Galactose, polymeric IgA, cholesterol ester-VLDL, LDL Mannose-fucose, galactose (particles), (oxidized) LDL Mannose, acetylated LDL Chemotactic peptide, complement C3b... 

Sevanian, A., Hodis, H.N., Hwang, J., McLeod, L.L., Peterson, H. 1995. Characterization of endothelial cell injury by cholesterol oxidation products found in oxidized LDL. J. Lipid Res. 36, 1971-1986. 

LDL is oxidatively modified when incubated in vitro with three major cellular constituents of the vascular wall endothelial cells [35], vascular smooth muscle cells [35] and macrophages [35-37], The uptake of oxidised LDL occurs via the scavenger-receptor pathway, and expression of scavenger receptors has been demonstrated on macrophages, endothelial cells [38], fibroblasts [39] and smooth muscle cells [39]. Unlike the LDL receptor, expression of the scavenger receptor is not down-regulated by an increase in intracellular cholesterol [40]. Therefore, uptake of Ox-LDL contributes to the accumulation of cholesteryl esters in foam cells of atherosclerotic lesions [40]. Now, the question is Does oxidation of LDL-lipids influence the development of atherosclerosis ... 

As the knowledge of the pathogenesis of atherosclerosis rapidly increases, it appears that an active vascular endothelium, smooth muscle cells, and blood-borne cells such as monocytes and macrophages all play active roles in the atherosclerotic disease process. Risk factors, such as elevated plasma levels of certain lipids, prooxidants, and cytokines, may contribute to the chronic activation/stimulation as well as to the damage of the endothelium and other vascular tissues (160). There is evidence that supports the hypothesis that it is not only pure cholesterol and saturated fats but rather oxidation products of cholesterol and unsaturated fats (and possibly certain pure unsaturated fats) that are atherogenic, possibly by causing endothelial cell injury/dysfiinction. Lipid-mediated endothelial cell dysfunction may lead to adhesion of monocytes, increased permeability of the endothelium to macromolecules, i.e., a decrease in endothelial barrier function, and disturbances in growth control of the vessel wall. 

Relaxation of blood vessels appears to be at least partially under the control of endothelial cells and their secreted products, especially endothelium-derived relaxation factor (EDRF). Oxidized LDL directly inhibits the endothelial cell-associated vessel relaxation. The generation of increased reactive oxygen species in association with elevated levels of blood cholesterol has also been reported. One of these reactive oxygen species, superoxide (O2), may interact with vasoactive EDRF (nitric oxide) locally in the artery wall, preventing endothelial cell-dependent vasodilation. In addition, a product of the reaction of nitric oxide and superoxide, the reactive peroxynitrite, may act to stimulate lipoprotein oxidation, which, as noted above, is regarded as an early step in atherosclerotic plaque generation. 

A recent study has provided insight into the association of ambient air pollution with increased cardiovascular morbidity and mortality. In this study, when human microvascular endothelial cells were exposed to a combination of ultrafine diesel exhaust particles and oxidized lipid components, a synergistic effect on the expression profiles of several gene modules that correspond to pathways relevant to inflammatory pathways such as atherosclerosis was observedJ57l The implications of this study include a greatly increased risk of heart disease in those with high cholesterol who breathe polluted air. 

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