Elimination of cholesterol in macrophages and endothelial cells by the sterol 27-hydroxylase mechanism - Comparison with high density lipoprotein-mediated reverse cholesterol transport

Cultured macrophages and endothelial cells have been reported to secrete 27-oxygenated metabolites of cholesterol, This mechanism was compared with the classical high density lipoprotein (HDL)-dependent reverse cholesterol transport, Under standard conditions, macrophage preparations had considerably higher capacity to secrete 27-hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid than had endothelial cells and fibroblasts, Western blotting showed that lung macrophages contained the most sterol 27-hydroxylase protein of the cells tested, The relative amounts of 3 beta-hydroxy-5-cholestenoic acid produced by the macrophages were also highest, Macrophages derived from monocytes of patients with sterol 27-hydroxylase deficiency did not secrete 27-oxygenated products, demonstrating that sterol 27-hydroxylase is the critical enzyme for the conversion of cholesterol into the 27-oxygenated steroids, That sterol 27-hydroxylase is responsible not only for 27-hydroxylation of cholesterol but also for time further oxidation of this steroid into 3 beta-hydroxy-5-cholestenoic acid was shown with use of tritium-labeled 27-hydroxy-cholesterol and an inhibitor of sterol 27-hydroxylase. Secretion of 27-oxygenated products by the cultured macrophages as well as the ratio between the alcohol and the acid appeared to be dependent upon total 27-hydroxylase activity, the availability of substrate cholesterol, and the presence of an acceptor for 27-hydroxy-cholesterol in the medium, With albumin as extracellular acceptor, the major secreted product was 3 beta-hydroxy-5-cholestenoic acid. Under such conditions, secretion of labeled 27-oxygenated products was higher than that of labeled cholesterol from lung alveolar macrophages preloaded with [4-C-14]cholesterol. With HDL as acceptor, 27-hydroxycholesterol was the major secreted product, and the total secretion of labeled 27-oxygenated products was only about 10% of that of labeled cholesterol, Thus, 27-hydroxycholesterol and cholesterol may compete for HDL-mediated efflux from the cells. The results support the contention that the sterol 27-hydroxylase-mediated elimination of cholesterol is more important in macrophages than in endothelial cell. This mechanism may be an alternative and/or a complement to the classical HDL-mediated reverse cholesterol transport in macrophages, in particular when the concentration of HDL is low.