17β-Estradiol Enhances the Flux of Cholesterol Through the Cholesteryl Ester Cycle in Human Macrophages

Abstract

Estrogens have been shown to have many positive effects on the function of arterial wall, and recent evidence suggest that 17β-estradiol has a direct action in reducing the accumulation of cholesteryl ester in macrophages. The mechanisms underlying the effects of 17β-estradiol on foam cell formation, however are poorly understood. The aim of this study is to investigate the role of 17β-estradiol in the regulation of the cholesteryl ester cycle and cholesterol efflux in human macrophages. In addition, the influence of 17β-estradiol on apolipoprotein E (apoE) and lipoprotein lipase (LDL) secretion by the cells was also tested. Human Monocyte Derived Macrophages (HMDM), matured in the presence or the absence of 17β-estradiol, were loaded with [3H]-cholesteryl ester-labeled-acetyl LDL (low density lipoprotein) and the efflux of radioactivity into the medium was measured. The effect of 17β-estradiol on cellular activities of acyl coenzyme A: cholesterol acyl transferase (ACAT), and both neutral and acid cholesteryl ester hydrolase (CEH) and the secretion of apoE and LDL into the medium, were also studied. The results indicate that 17β-estradiol induces an increase in the amount of labeled cholesterol released from the cells and, the data obtained from the measurements of ACAT and CEH activities showed that, in estrogen-treated HMDM, the cholesteryl ester cycle favors the hydrolysis of lipoprotein cholesterol by CEH in comparison with its acylation by ACAT. In particular, for the first time a strong enhancement of neutral and acid CEH in human macrophages by 17β-estradiol, was demonstrated. ApoE and LDL secretion increased during the maturation of monocytes to macrophages, and was not modified by 17β-estradiol. In contrast, loading the cells with cholesterol by incubation in the presence of acetylated or oxidized LDL produced an increase in the levels of apoE secreted by both estrogen-treated and control macrophages. The activity of LPL found in the cell medium, on the other hand, in lipid loaded cells tended to be increased only in estrogen treated macrophages, suggesting that the effects of estrogen on unloaded macrophages are different from those produced on lipid-loaded macrophages. On the whole, the present findings suggest that one of the mechanisms by which 17β-estradiol acts to reduce cholesterol accumulation in macrophages is by increasing reverse cholesterol transport through the enhancement of the cholesteryl ester cycle, so that the generation of intracellular unesterified cholesterol for excretion from the cells is favored.