Novel effects of the acyl-coenzyme A:cholesterol acyltransferase inhibitor 58-035 on foam cell development in primary human monocyte-derived macrophages

We examined the effect of acyl-coenzyme A:cholesterol acyltransferase (ACAT) inhibitors on intracellular cholesterol stores in primary human monocyte-derived macrophages (HMMs) during foam cell formation. HMMs were exposed to acetylated low density lipoprotein (acLDL, 500 μg protein per mL) with or without 58-035 (1 to 10 μg/mL) or CI-976 (2 μg/mL) for 2 to 48 hours. Total cholesterol (TC) and esterified cholesterol (EC) mass was significantly lower while unesterified cholesterol (UC) increased slightly in cells incubated with acLDL plus ACAT inhibitors. Sterol mass was also measured in cells coincubated with acLDL (500 μg protein per mL) with or without 58-035 (2 μg/mL), high density lipoprotein (HDL, 400 μg protein per mL), or HDL+58-035 for 48 hours. TC and EC were 23% and 55% lower, respectively (P < 0.0004), while UC was 11% higher (P < 0.04) in cells incubated with acLDL plus 58-035. In contrast, coincubation with HDL alone did not significantly affect TC, EC, or UC mass compared with acLDL alone. The effect of 58-035 could not be explained by cytotoxicity, because adenine release, secreted lactate dehydrogenase, glucose utilization, and cell protein were similar in cells exposed to acLDL regardless of the presence of 58-035. We investigated several potential mechanisms for the decreased TC mass, including increased UC efflux and decreased acLDL binding and uptake. Efflux was measured in cells exposed to [1,2-₃H]cholesteryl oleate-labeled acLDL, unlabeled control acLDL, and native untreated acLDL (500 μg protein per mL) with or without 58-035 (5 μg/mL) for 24 or 48 hours. UC efflux increased in a time-dependent manner from cells exposed to acLDL plus 58-035 compared with cells exposed to acLDL alone (P < 0.04). High-affinity binding was measured in cells exposed to ¹²⁵I-acLDL (5 μg protein per mL) with or without excess unlabeled acLDL (100 or 500 μg protein per mL) for 4 hours at 4 °C. Specific acLDL binding, uptake, and total degradation were significantly lower when 58-035 was present during cholesterol enrichment compared with cells exposed to acLDL alone (P < 0.001). Unlike the effects of ACAT inhibitors on foam cell formation in rodent macrophages, these compounds lowered TC accumulation in HMMs during foam cell formation by limiting the uptake of acLDL and enhancing UC efflux. They may offer promise as drug therapies for atherosclerosis.