Studies on a novel oxidative mechanism for elimination of extrahepatic cellular cholesterol

Recently, a new oxidative mechanism for elimination of cholesterol from extrahepatic cells by sterol 27-hydroxylase was described. This mechanism involves conversion of cholesterol into 27-hydroxycholesterol and cholestenoic acid. These products can be excreted from the cells, transported in the circulation to the liver where they can be taken up and excreted as bile acids in bile. The aim of the present study was to characterize and evaluate the importance of this mechanism in humans. Human cultured alveolar macrophages were found to have the ability to eliminate about 40% of their intracellular cholesterol as 27-oxygenated metabolites during 24 hours of culture. Inhibition of the enzyme in these cells increased the accumulation of cholesterol significantly. When alveolar macrophages were pre-loaded with [4-14C]cholesterol, the amount of secreted radio-labeled 27-oxygenated products were found to be about 10% of that of secreted radiolabeled cholesterol. The amount and the pattern of the 27-oxygenated products were found to be dependent on the amount of the enzyme in the cell, the availability of the substrate cholesterol and the type of acceptor outside the cell. In the presence of HDL, 27-hydroxycholesterol was the major secreted product in the medium whereas albumin was found to stimulate the secretion of cholestenoic acid. The total uptake of the 27-oxygenated products by the liver was estimated to be about 20 mg per day. Assuming total conversion into bile acids, this mechanism is then responsible for up to 4% of total bile acid formation. Alveolar macrophages as well as monocyte-derived macrophages were found to have higher capacity to secrete 27-oxygenated products than other types of cells when cultured under the same conditions. By measuring the production of cholestenoic acid by the lung in vivo and by comparing the level of this compound in circulation before and after pulmectomy, the lung was found to be the major source of 27-oxygenated products in human circulation. It is responsible for production of more than 70% of cholestenoic acid in the circulation under normal conditions. In the circulation, 27-hydroxycholesterol was found mainly in the esterified form. It is carried mainly in the HDL (50%) and LDL (40%) particles. On the other hand, almost all of the cholestenoic acid was found in the free form and transported exclusively in the lipoprotein-free fractions. Patients with advanced atherosclerosis tend to have higher and more heterogeneous levels of 27- oxygenated products than healthy volunteers. It is concluded that the sterol 27-hydroxylase is of quantitative significance for elimination of extrahepatic cholesterol. This mechanism may be an alternative and/or complement to the classical HDL-mediated reverse cholesterol transport, in particular when the concentration of HDL is low. It is suggested that the sterol 27-hydroxylase is antiatherogenic. Further studies are required, however, to evaluate a possible role of 27-hydroxylase for lung function as well as the importance of circulating levels of 27-oxygenated products as prognostic factor in patients with advanced atherosclerosis