Importance of a novel oxidative mechanism for elimination of intracellular cholesterol in humans

We have recently demonstrated that cultured human alveolar macrophages efficiently convert cholesterol into excretable 27-oxygenated products. We show here that increasing the intracellular concentration of cholesterol by a factor of 10 leads to about a twofold increase in the excretion of 27- oxygenated products from cultured macrophages. Inhibition of the sterol 27- hydroxylase caused a significant intracellular accumulation of cholesterol. A direct comparison was made between flux of cholesterol and 27-oxygenated products from macrophages preloaded with [4-¹⁴C]cholesterol. Under the specific conditions employed with fetal calf serum in the culture medium, the flux of 27-oxygenated products was about 10% of that of cholesterol. Since the sterol 27-hydroxylase, which converts cholesterol to 27-oxygenated products, is present in many cell types, we suggest that 27-oxygenation is a general mechanism for removal of intracellular cholesterol. To evaluate this hypothesis, we measured the net uptake by the human liver of circulating 27- oxygenated products, which was found to be about 20 mg/24 h. This uptake corresponds to ≃4% of the bile acid production, assuming quantitative conversion into bile acids. It is concluded that the 27-hydroxylase pathway is of significance for elimination of extrahepatic cholesterol.