Macrophages Enhances Down-regulation of the Low Density Lipoprotein Receptor and 3-Hydroxy-3-methylglutaryl-Coenzyme A Reductase and Prevents Low Density Lipoprotein-induced Cholesterol Accumulation*

Cholesteryl ester accumulation in arterial wall macrophages (foam cells) is a prominent feature of atherosclerotic lesions. We have previously shown that 5774 macrophages accumulate large amounts of cholesteryl ester when incubated with unmodified low density lipoprotein (LDL) and that this is related to sluggish down-regulation of the 5774 LDL receptor and 3hydroxy-3-methylglutaryl-coenzyme A reductase. To further explore intracellular cholesterol metabolism and regulatory events in 5774 macrophages, we studied the effect of inhibitors of acyl-CoAcholesterol acyl transferase (ACAT) on the cells’ ability to accumulate cholesterol and to down-regulate receptor and reductase. Treatment of 5774 cells with LDL in the presence of ACAT inhibitor 58-035 (Sandoz) prevented both cholesteryl ester and total cholesterol accumulation. Furthermore, 58-035 markedly enhanced down-regulation of the 5774 LDL receptor and 3-hydroxy-3methylglutaryl-CoA reductase in the presence of LDL. In dose-response studies, down-regulation of the receptor by 58-035 paralleled its inhibition of ACAT activity. Compound 58-035 also increased the down-regulation of the 5774 LDL receptor in the presence of 25hydroxycholesterol and acetyl-LDL but not in the presence of cholesteryl hemisuccinate, which is not an ACAT substrate. The ability of 58-035 to enhance LDL receptor down-regulation was negated when cells were simultaneously incubated with recombinant high density lipoproteins discs, which promote cellular cholesterol efflux. In contrast to the findings with 5774 macrophages, down-regulation of the human fibroblast LDL receptor was not enhanced by 58-035. These data suggest that in 5774 macrophages, but not in fibroblasts, ACAT competes for a regulatory pool of intracellular cholesterol, contributing to diminished receptor and reductase down-regulation, LDL-cholesterol accumulation, and foam cell formation.