Metabolism of an oxysterol, 7-ketocholesterol, by sterol 27-hydroxylase inHepG2 cells

7-Ketocholesterol (7K) is a quantitatively important oxysterol in both athe rosclerotic lesions and macrophage foam cells. We reported recently that ra diolabeled 7K delivered to rodents in a modified lipoprotein or chylomicron remnant-like emulsion, both cleared predominantly by the liver, was rapidl y excreted into the intestine as water-soluble products, presumably bile ac ids. Herein, we aimed to elucidate, the early or initial reactions in 7K me tabolism. The hypothesis was tested that sterol 27-hydroxylase, a mitochond rial cytochrome P450 and the first enzyme of the acidic bile acid pathway, is responsible for the initial metabolism of 7K by HepG2 cells, a human hep atoblastoma cell-line. The 27-hydroxylated product of 7K (27OH-7K) was show n to be the initial, lipid-soluble product of 7K metabolism. It was produce d in mitochondrial incubations and whole cells and was readily released int o the media from cells. intact cells generated metabolites of 7K that had u ndergone conversion from lipid-soluble precursors to water-soluble products rapidly and extensively. Their production was ablated with cyclosporin A, a sterol 27-hydroxylase inhibitor. Furthermore, we demonstrated the effecti veness of two novel selective inhibitors of this enzyme, GW273297X and G126 8267X. These inhibitors also ablated the production of water-soluble produc ts by cells; and the inhibitor of choice, GW273297X, decreased the producti on of 27OH-7K in mitochondrial preparations. This is the first study to dem onstrate that sterol 27-hydroxylase plays an important role in the metaboli sm of oxysterols such as 7K in liver cells.