Endocytic pathway of acetylated low-density lipoprotein in rat peritoneal macrophages.

Abstract

It has been generally admitted that cholesteryl ester-loaded foam cells, one of the major cell components of atherosclerotic lesions, are derived from macrophages or smooth muscle cells and that low-density lipoprotein (LDL) is the major atherogenic lipoprotein (8,17) . The discovery of the LDL pathway by Goldstein and Brown not only contributes to our understanding of cholesterol metabolism but also suggests the importance of a receptor-mediated endocytic mechanism in biological systems. However, endocytosis of LDL via this pathway by most cultured cells does not lead to substantial intracellular cholesterol accumulation and this receptor is poorly expressed by macrophages or macrophage-derived cells. Moreover, atheromatous plaques comprised of typical foam cells develop in the patients genetally devoid of this receptor. A subsequent discovery that incubation of cultured macrophages with acetylated LDL (acetyl-LDL) resulted in massive intracellular accumulation of cholesteryl esters has shed light on this dilemma (9). The scavenger function of macrophages or macrophage-derived cells for chemically modified proteins has since been intensively studied from its potential link to atherosclerosis (4).These studies have suggested that the endocytosis of LDL preparations chemically modified by acetylation, acetoacetylation, malondialdehyde treatment or modified by in vitro incubation with endothelial cells is mediated by a scavenger receptor termed the acetyl-LDL receptor (7,9,10,22). This has been demonstrated in macrophages or macrophage-derived cells, such as mouse peritoneal macrophages, human monocytes or sinusoidal liver cells (2,7,9,11,14,22).In the present communication, morphological approaches have been attempted to reveal the endocytic pathway for acetyl-LDL and subsequent foam cell formation in rat peritoneal macrophages. The results are presented with discussions on the pathophysiological significance of the scavenger function in cholesterol metabolism of macrophages or macrophage-derived cells. RESULTS AND DISCUSSIONIncubation of rat peritoneal macrophages with 50 ug/ml of native unmodified human LDL for 48 hr at 37°C did not change their morphology (Fig. 1B). On the other hand, upon incubation with acetyl-LDL under the same conditions, many lipid vacuoles appeared in the cytoplasm, exhibiting typical foam cells (Fig. 1A). Further biochemical approaches using either 125I-LDL or 125I-acetyl-LDL also showed the contrasting behavior of these two radiolabeled ligands in the interaction with macrophages; 125I-LDL did not display significant binding to the cells nor 145