Intracellular transport of cholesteryl esters from lysosomes to cytoplasm in macrophages.

Abstract

ABSTRACT. The mechanism through which nonmembranous lipid inclusion bodies consisting of cholesteryl esters accumulate in the cytoplasm was studied. Most lipid inclusion bodies in macrophages after 24 h incubation with anisotropic cholesteryl oleate liquid crystals were surrounded by a limiting membrane. The limiting membrane, however, could not be observed after further incubation for 48 h in the presence of esterastin, which is known to be an inhibitor of lipase and esterase. Under these conditions, the levels of hydrolysis and re-esterification of cholesteryl esters were less than 15% and 5% of the control ones, respectively.These results suggest that the inclusion bodies were transferred from lysosomes to the cytoplasm, with partial hydrolysis of cholesteryl esters, in addition to through the pathway via microsomes.A prominent feature of atherosclerosis is the marked accumulation of lipids, especially cholesteryl esters, in the arterial walls (15). These lipids are observed as small droplets showing intense anisotropy on polarized microscopy (5). They have been isolated (13,18), and their biochemical (13, 18) and morphological characteristics (1, 7, 13, 18) have been examined. They consist mainly of cholesteryl esters (about 95%) and have been called cholesteryl ester-rich lipid droplets (7). Some of the lipid-laden foam cells are thought to be derived from macrophages (11,17, 23). Histochemical and ultrastructural observations have showen that the lipid droplets, with or without a limiting membrane, mainly accumulate in the cytoplasm of foam cells in early atherosclerotic lesions (16). Studies involving the quick-freeze, etching technique (1) also showed that the lipids that accumulated in foam cells were present as membrane-bound droplets in lysosomes and as membrane-free droplets, with a concentric lamillar structure, surrounded by 10 nm filaments.When macrophages are incubated with acetyl-LDL (acetylated low density lipoprotein), the acetyl-LDL is internalized through endocytosis and then delivered to lysosomes (2, 6). Cholesteryl esters in the lipoprotein are hydrolyzed in lysosomes, and free cholesterol is liberated. According to the hypothesis of Brown et al. (2), the free cholesterol is re-esterified by a microsomal acyl-CoA: cholesterol Abbreviations used: acetyl-LDL, acetylated low density lipoprotein; PBS(-), Can-, Mg2+ free phosphate-buffered saline; ACAT, acyl-CoA: cholesterol acyltransferase. 435