Identification of regulatory oxysterols, 24(S),25-epoxycholesterol and 25-hydroxycholesterol, in cultured fibroblasts.

Biosynthetically tritiated sterols from Chinese hamster lung (Dede) cells were fractionated by high performance liquid chromatography, and fractions were assayed for their ability to repress 3-hydroxy-3-methylglutaryl-CoA reductase in L cell cultures. Most of the activity found was associated with two oxysterols, 24(S),25-epoxycholesterol and 25-hydroxycholesterol. The identities of the two sterols were established by co-chromatography with authentic samples and by isotopic dilution and recrystallization. Only low levels of repressor activity were found in other fractions of the sterol extract. The endogenous concentrations of 24(S),25-epoxycholesterol (7.2 fg/cell) and 25-hydroxycholesterol (1.5 fg/cell) appear to be within the ranges required for the regulation of HMG-CoA reductase.