In vivo formation of 25-hydroxycholesterol from endogenous cholesterol after a single meal, dietary cholesterol challenge.

The role of oxysterols as regulatory molecules in the suppression of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity was investigated in the intact rat in response to an acute dietary cholesterol challenge. When rats were fed highly purified cholesterol as a single meal at a level of 5% of the diet, maximal inhibition of enzyme activity (66%) occurred 120 min after the completion of the meal. Furthermore, when nonsaponifiable liver extracts were chromatographically resolved and analyzed by high performance liquid chromatography (HPLC) and capillary gas chromatography-mass spectrometry (GC-MS), 25-hydroxycholesterol was identified in the livers of rats 120 min after the completion of the single cholesterol meal. Significantly, only barely detectable amounts of 25-hydroxycholesterol were observed in the livers from control rats fed a sterol-free diet. The biosynthetic origin of 25-hydroxycholesterol was investigated with the use of deuterated water. Rats were fed deuterium oxide (33%) ad libitum for 3 days and then killed 120 min after the completion of a single cholesterol meal. As before, 25-hydroxycholesterol was detected in the livers from cholesterol-fed rats, but not to a significant extent in livers from control-fed rats receiving a sterol-free diet. Isotope ratio mass spectrometry revealed that the fractional incorporation of deuterium into 25-hydroxycholesterol (21%) was less than that observed for cholesterol (24%) isolated from the same livers, indicating that 25-hydroxycholesterol was produced endogenously from exogenous cholesterol and not from autoxidation of cholesterol. In a separate experiment it was also shown that [3H]mevalonate was incorporated into 25-hydroxycholesterol after a single meal cholesterol challenge, but was barely detected in the livers of control rats. The evidence obtained in the present article supports the hypothesis that 25-hydroxycholesterol is endogenously produced from cholesterol at early time intervals after an acute dietary cholesterol challenge. In addition, rat liver HMG-CoA reductase was inhibited by the administration of a single intragastric dose (1 microgram/kg) of an aqueous solution of 25-hydroxycholesterol. Thus, the results provide strong support for the conclusion that 25-hydroxycholesterol plays a significant role in the in vivo regulation of rat liver cholesterol biosynthesis after an acute dietary cholesterol challenge.