Studies on oxysterols : Origins, properties and roles

Abstract: Oxysterols are derivatives of cholesterol present at low concentrations in a variety of physiological compartments. The major oxysterols in human circulation are formed from pools of cholesterol that may be distinguished by their miscibility with plasma cholesterol. This was demonstrated by the differential enrichment of the major plasma oxysterols with exogenously supplied deuterium labelled cholesterol and lead to the definition of three functionally distinct pools. 7alpha-Hydroxycholesterol was found to represent the rapidly miscible pool and 27-hydroxycholesterol a pool of intermediate miscibility. The final pool is essentially non-exchangeable and is represented by 24S-hydroxycholesterol. The origin of the cholestenoic acids present in the circulation was also investigated. 3betaHydroxy-5-cholestenoic acid and 3beta,7alpha-dihydroxy-5-cholestenoic acid were found to be formed from a lung specific sub-pool of cholesterol. 7alpha-Hydroxy-3-oxo-4- cholestenoic acid appears to be formed from the rapidly miscible pool, and its presence in the plasma is probably a result of secretion by the liver. The position of the oxygen function was found to be an important determinant of the overall function of an oxysterol, with the rate of intermembrane transfer related to the distance between the 3beta-hydroxyl group and the second oxygen function. Moreover, as clear differences were observed in the kinetics of circulating oxysterols, this molecular relationship also appears to be conserved at a physiological level. It was found that determination of the concentration of these oxysterols in plasma and CSF from patients with a variety of neurological disorders provides useful information. Specifically the ratios of these oxysterols were found to reflect the severity of the blood-brain barrier dysfunction. Cholesterol was found to be able to interfere with the binding of 24S-hydroxycholesterol to LXRalpha at cholesterol: oxysterol ratios similar to those found in vivo. The implication is that there are few compartments with oxysterol: cholesterol ratios suitable for activation of LXR. In summary the present results demonstrate that oxysterols are formed from functionally distinct pools of cholesterol, and that their in vivo properties are critically dependent on the position of the secondary oxygen function. These properties are important in relations to their use as diagnostic markers. It is evident from the present investigation that there are few compartments with oxysterol: cholesterol ratios suitable for activation of LXR, under in vivo conditions.