Inhibition of Sterol Synthesis in Cultured Mouse Cells by ?a-Hydroxycholesterol, 7@-Hydroxycholesterol, and 7-Ketocholesterol*

Rates of sterol synthesis from acetate and the levels of 3-hydroxy-3-methylglutaryl-CoA reductase activity (HMGCoA reductase, EC 1.1.1.3.4) in liver cell cultures and in L cells were strongly inhibited by highly purified preparations of 7a-hydroxycholesterol, 7/3-hydroxycholesterol, and ‘I-ketocholesterol. These steroids specifically diminished the activity of 3-hydroxy-3-methylglutaryl-CoA reductase without altering rates of acetate metabolism to CO2 or fatty acids, or rates of protein and RNA synthesis. The inhibitory activities of these steroids were associated with specmc structural features which included the 7-ketone or 7-hydroxyl functions. The following steroids did not inhibit sterol synthesis significantly under the test conditions employed: purified cholesterol, 7-dehydrocholesterol, 24,25dihydrolanosterol, pregnenolone, cholestane, cholic acid, and chenodeoxycholic acid. 3-Keto-A4-cholesten-7ar-ol inhibited sterol synthesis slightly but did not depress 3-hydroxy3-methylglutaryl-CoA reductase activity. Other steroid preparations (cholestanol, cholestan-3-one, and cholest-4-en-d-one) appeared to be weakly inhibitory when they were added to liver cell cultures at relatively high concentrations. Sterol synthesis in L cells was more sensitive to inhibitory steroids than was that in liver cells, although primary liver cell cultures took up more [4-r4C]cholesterol from the medium than did L cells or long term liver cell cultures. Measurements of the inhibitory activities of fractions isolated from impure cholesterol and from a mixture of preputial gland sterols by thin layer chromatography suggest that there are other inhibitors of sterol synthesis not yet identified.