[Macrophage lipoprotein metabolism]

In this review, attention has been focused on specific receptors on macrophages where mechanisms allow macrophages to take up and digest cholesterol-containing lipoproteins. These receptor-mediated mechanisms differ from those in other cell types like fibroblasts and smooth muscle cells. There are at least two types of lipoprotein receptors on macrophages: "scavenger" receptors and beta-VLDL receptors. Modified low-density lipoproteins are recognized by non-regulated "scavenger" receptors. Poorly regulated receptor mechanism for beta-VLDL particles enriched in cholesterol esters are operative in humans with familial dysbetalipoproteinemia. Macrophage cholesterol esters are processed by cholesterol-ester cycle. Without an available cholesterol acceptor in extracellular fluid, free cholesterol is re-esterified which may result in an accumulation of cholesterol in the macrophages and hence, in the arterial wall. The scavenging of cholesterol by macrophages appears to be a protective mechanism against "foam cell" formation in atherosclerosis.