Interaction of oxidized low density lipoprotein with macrophages in atherosclerosis, and the antiatherogenicity of antioxidants.

Macrophage cholesterol accumulation and foam cell formation, the hallmark of early atherosclerosis, is the result of enhanced cellular uptake of plasma low density lipoprotein (LDL). Native LDL, has to undergo oxidative modifications in order to be taken up at an enhanced rate by macrophages, leading to foam cell formation. Macrophage uptake of oxidized LDL involves its binding to scavanger receptors (including cellular proteoglycans) and this is followed by an impaired cellular cholesterol metabolism. Cells of the arterial wall including macrophages can oxidize LDL in a process that involves activation of cellular oxygenases, such as NADPH oxidase and 15-lipoxygenase. This process, however, also depends on the macrophage antioxidant environment, where glutathione peroxidase and reduced glutathione play an important protective role against cell-mediated oxidation of LDL. Macrophage phospholipids peroxidation under oxidative stress can also contribute to macrophage-mediated oxidation of LDL. Evidence for the occurrence of oxidized LDL in vivo is as follows: 1) In the atherosclerotic lesion [in humans, as well as in the transgenic, apolipoprotein E-deficient mice], LDL is oxidized (and as a result, it is also aggregated), in comparison to plasma LDL which is normally not oxidized. 2) Plasma LDL from patients at high risk for atherosclerosis (such as hypercholesterolaemic, hypertensive, diabetic and renal failure patients), as well as from the apolipoprotein E-deficient mice, demonstrates increased susceptibility to oxidation in comparison to normal LDL. In some groups of these patients LDL is minimally oxidized already in plasma. 3) Supplementation of nutritional antioxidants, which are rich in polyphenols (red wine, licorice, olive oil), or of selenium to humans or to the apolipoprotein E-deficient mice, as well as therapy with beta-hydroxy-beta-methyl-glutaryl-CoA reductase inhibitors (so-called "statins") in hyperocholesterolaemic patients, were shown to reduce the susceptibility of LDL to oxidation. This effect could be associated with a reduction in the size of the atherosclerotic lesion and may thus contribute to attenuation of the atherosclerotic process.