Regulation of neutral cholesterol esterase activity

The effect of phospholipids on cholesteryl ester hydrolysis by neutral cholesterol esterase in alveolar macrophages was studied. Among the phospholipids used as emulsifiers, those with a negative charge, such as phosphatidylserine, phosphatidic acid, phosphatidylinositol, and cardiolipin, gave a higher level of hydrolysis by neutral cholesterol esterase than other less negatively charged phospholipids, such as phosphatidylcholine, lysophosphatidylcholine, phosphatidylethanolamine, and sphingomyelin. Phospholipase D treatment of liposomes emulsified with phosphatidylcholine produced phosphatidic acid and enhanced cholesteryl ester hydrolysis. Phospholipase AS treatment produced lysophosphatidylcholine and decreased the hydrolysis. The hydrolysis of cholesteryl ester in lipid droplets obtained from cholesterol-laden macrophages elicited by thioglycollate in the rat peritoneal cavity was low compared to artificial liposomes emulsified with phosphatidylcholine. The reason for this was speculated to be that lipid droplets were low in total phospholipids and poor in phospholipids with strong negative charges but rich in phosphatidylethanolamine and sphingomyelin. M These results suggest that the polar heads of phospholipids may play an important role in cholesteryl ester hydrolysis by neutral cho1esterolesterase.-Ishii, I., R. Onozaki, E. Takahashi, S. Takahashi, N. Fujio, T. Harada, N. Morisaki, K. Shirai, Y. Saito, and S. Hirose. Regulation of neutral cholesterol esterase activity by phospholipids containing negative charges in substrate 1iposome.J Lipid Res. 1995. 3 6 2303-2310. Supplementary key words phospholipase A2 phospholipase D cholesteryl esters In atherosclerotic lesions, foam cells with large amounts of lipid droplets are found. The major component of lipid droplets is cholesteryl ester, but these droplets also contain other lipids (1, 2). Hata, Hower, and h u l l (1) reported that human atherosclerotic lesions contain many types of neutral lipids and phospholipids, and that the composition of lipids changed depending on the development of atherosclerosis from fatty streaks to fibrous plaques. Foam cells in early atherosclerotic lesions are believed to be derived mainly from macrophages. Macrophage cholesterol metabolism takes place as follows (3, 4). Macrophages take up denatured low density lipoproteins (LDL) (5-7) and &very low density lipoproteins (PVLDL) (8, 9) through specific receptors (10-13). These lipoproteins are rapidly delivered to lysosomes where their cholesteryl ester is hydrolyzed by acid cholesterol esterase (4, 14-16). A small portion of free cholesterol released is excreted from the cells (17) and the rest is re-esterified by acyl-CoAcholesterol acyltransferase (ACAT) in microsomes (18, 19). Then, the reesterified cholesterol is stored in lipid droplets together with other lipids (1). Furthermore, cholesteryl ester stored in lipid droplets is also hydrolyzed by neutral cholesterol esterase (20-25). Macrophages are converted into foam cells in cultured systems. Inflammatory macrophages, phorbol ester-treated monocyte-like cells (24, 26), thioglycollate (23, 27), and lipopolysaccharide-stimulated macrophages (28) take up BVLDL and accumulate cholesteryl ester intracellularly as lipid droplets. However, the reason why cholesteryl ester in lipid droplets is hardly hydrolyzed by neutral cholesterol esterase is unclear. On the other hand, alveolar macrophages have been shown to catabolize exogenous cholesterol derived from PVLDL, i.e., these cells take up PVLDL, release cholesterol as a result of the high activity of neutral cholesterol Abbreviations: VLDL, very low density lipoprotein; PC, phosphatidylcholine; PA, phosphatidic acid. 'To whom correspondence should be addressed at: Department of Biochemistry, Faculty of Pharmaceutical Sciences, Chiba University. 1-33, Yayoicho, Inage-ku, Chiba, 263, Japan. Journal of Lipid Research Volume 36,1995 2303 at P E N N S T A T E U N IV E R S IT Y , on F ebuary 1, 2013 w w w .j.org D ow nladed fom esterase, and, as a result, accumulate less cholesteryl ester (23). However, the mechanism of the intracellular regulation of this enzyme activity is also unclear. Lipids other than cholesteryl ester in lipid droplets may be important for the regulation of this enzyme activity. In this study, we investigated the role of the lipid composiElectron microscopy Lipid droplets and artificial liposomes were negatively stained and analyzed by electron microscopy (sRL, To-