24(S),25-Epoxycholesterol. Evidence consistent with a role in the regulation of hepatic cholesterogenesis

Previously we showed that 24(S),25-epoxycholesterol is formed from acetate, via squalene 2,3(S),22(S),23-dioxide and 24(S),25-oxidolanosterol, during the normal course of cholesterol biosynthesis in S10 rat liver homogenate (Nelson, J. A., Steckbeck, S. R., and Spencer, T. A. (1981) J. Biol. Chem. 256, 1067-1068; Nelson, J. A., Steckbeck, S. R., and Spencer, T. A. (1981) J. Am. Chem. Soc. 103, 6974-6975). Herein we demonstrate that the nonsaponifiable extract from human liver tissue contains 24(S),25-epoxycholesterol in an amount approximately 10(-3) relative to cholesterol. We show that 24(S),25-epoxycholesterol, like many other oxygenated sterols, represses hydroxymethylglutaryl-CoA reductase activity in cultured cells and binds to the cytosolic oxysterol-binding protein. Furthermore, we show that this epoxide is not rapidly metabolized in cultured cells. These results suggest that 24(S),25-epoxycholesterol may participate in the regulation of hepatic cholesterol metabolism in vivo.