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 3beta-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 3beta-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 the further oxidation of this steroid into 3beta-hydroxy-5-cholestenoic acid was shown with use of tritium-labeled 27-hydroxycholesterol 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-hydroxycholesterol in the medium. With albumin as extracellular acceptor, the major secreted product was 3beta-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 [4-14C]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 cells. 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.
Babiker
A
A
Department of Medical Laboratory Sciences and Technology, The Karolinska Institute, SE-141 86, Huddinge, Sweden.
Andersson
O
O
Lund
E
E
Xiu
R J
RJ
Deeb
S
S
Reshef
A
A
Leitersdorf
E
E
Diczfalusy
U
U
Björkhem
I
I
D000418:Albumins / Q000494:pharmacology; D000818:Animals; D001054:Apolipoproteins A / Q000494:pharmacology; D001692:Biological Transport; D002417:Cattle; D002478:Cells, Cultured; D053493:Cholestanetriol 26-Monooxygenase; D002784:Cholesterol / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004730:Endothelium, Vascular / Q000378:metabolism*; D006801:Humans; D008075:Lipoproteins, HDL / Q000378:metabolism*; D016676:Macrophages, Alveolar / Q000378:metabolism*; D013250:Steroid Hydroxylases / Q000378:metabolism*; D014316:Tritium; D019294:Xanthomatosis, Cerebrotendinous / Q000378:metabolism / Q000473:pathology
D000418:Albumins; D001054:Apolipoproteins A; D008075:Lipoproteins, HDL; D014316:Tritium; D003577:Cytochrome P-450 Enzyme System; D002784:Cholesterol; D013250:Steroid Hydroxylases; C506831:CYP27A1 protein, human; D053493:Cholestanetriol 26-Monooxygenase