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 secre te 27-oxygenated metabolites of cholesterol, This mechanism was compar ed with the classical high density lipoprotein (HDL)-dependent reverse cholesterol transport, Under standard conditions, macrophage preparat ions 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 relati ve amounts of 3 beta-hydroxy-5-cholestenoic acid produced by the macro phages were also highest, Macrophages derived from monocytes of patien ts with sterol 27-hydroxylase deficiency did not secrete 27-oxygenated products, demonstrating that sterol 27-hydroxylase is the critical en zyme for the conversion of cholesterol into the 27-oxygenated steroids , That sterol 27-hydroxylase is responsible not only for 27-hydroxylat ion 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-hydroxyl ase. Secretion of 27-oxygenated products by the cultured macrophages a s well as the ratio between the alcohol and the acid appeared to be de pendent upon total 27-hydroxylase activity, the availability of substr ate cholesterol, and the presence of an acceptor for 27-hydroxy-choles terol in the medium, With albumin as extracellular acceptor, the major secreted product was 3 beta-hydroxy-5-cholestenoic acid. Under such c onditions, 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-media ted efflux from the cells. The results support the contention that the sterol 27-hydroxylase-mediated elimination of cholesterol is more imp ortant in macrophages than in endothelial cell. This mechanism may be an alternative and/or a complement to the classical HDL-mediated rever se cholesterol transport in macrophages, in particular when the concen tration of HDL is low.