Triglyceride and cholesteryl ester hydrolysis

Cultured rabbit aortic smooth muscle cells were converted to foam cells by exposure to sonicated lipid droplets of defined composition using an inverted culture technique. Uptake of the lipid droplets by the cells was shown to be dependent on the time of exposure to the droplets and on the mass of droplets presented to the cells. A comparison of the hydrolysis of triolein and cholesteryl oleate by cells that had been exposed to isotropic lipid droplets containing equimolar amounts of the two lipids revealed that the rate of hydrolysis of triglyceride was 3 to 4 times faster than that for cholesteryl ester. The hydrolysis of cholesteryl oleate from cells loaded with the isotropic droplets was approximately 1.5 times as fast as that from cells loaded with anisotropic droplets containing only cholesteryl oleate. A comparison of the hydrolysis of cholesteryl ester in the presence and absence of Sandoz compound 58-035, an inhibitor of acyl CoA:cholesterol acyl transferase, by cells loaded with isotropic droplets showed that about 30% of the free cholesterol liberated by hydrolysis was reesterified regardless of the mass of free cholesterol. a We conclude that cultured smooth muscle cells have a greater capacity to hydrolyze triglyceride than cholesteryl ester. and that the rate of hydrolysis of cholesteryl ester appears to be related to the physical state of the droplet in which the cholesteryl ester is stored. In addition, it appears that the smooth muscle cells have a cholesteryl ester cycle that is inefficient in the reesterification of excess free cholesterol.Minor, L. K., G. H. Rothblat, and J. M. Glick. Triglyceride and cholesteryl ester hydrolysis in a cell culture model of smooth muscle foam cells. J Lipid Res. 1989. 30: 189-197. Supplementary key words lipid droplets ACAT The development of atherosclerotic plaque is a prolonged, multifactorial process wherein macrophages and smooth muscle cells accumulate large deposits of cellular lipid and thus acquire the phenotypic character of foam cells. The origin of the lipid in foam cells has been variously attributed to LDL (l), modified LDL such as malondialdehyde-LDL (2, 3), P-VLDL (4, 5), postprandial lipoproteins (6), extracellular lipid (7, 8), and macrophage lipid droplets (9), all of which are likely to play a contributing role. The intracellular lipid droplets in foam cells are thought to be in a dynamic state, wherein the lipid is constantly hydrolyzed and reesterified (10, 11). While the predominant lipid in plaque is cholesteryl ester (primarily cholesteryl oleate), a substantial amount of free cholesterol is also present. Although lipoproteins such as P-VLDL and postprandial lipoproteins might be expected to provide triglyceride as well as cholesteryl ester, relatively little triglyceride is found in plaque (12, 13). Recent work in our laboratory has focused on an hypothesis regarding the mechanism underlying the formation of smooth muscle foam cells. We proposed that smooth muscle cells in the vessel wall might acquire lipid inclusions that were originally synthesized by macrophages (9). To test this hypothesis, we developed a cell culture model to demonstrate that cultured smooth muscle cells can become foam cells by the uptake of lipid droplets isolated from cultured macrophages. We also demonstrated that the lipid in these inclusions is metabolically available to the smooth muscle cells. An intriguiing observation in these early experiments (9) was made using polarizing light microscopy which suggested selective clearance of triglyceride by the smooth muscle cells. In the present study, we have used sonicated lipid droplets as a model of the cellular inclusions to compare more quantitatively the relative metabolism of cholesteryl esters and triglyceride in smooth muscle cells. MATERIALS AND METHODS