Oxysterols in biological systems: sources, metabolism and pathophysiological relevance.

Oxysterols are the 27-carbon products of cholesterol oxidation by both enzymic and non-enzymic mechanisms. Their roles in cholesterol homeostasis, as well as in diseases in which oxidative damage and lipid peroxidation are implicated (e.g. atherosclerosis), have been investigated extensively. However, there are a number of important considerations regarding the physiological/pathophysiological functions and activities of the different oxysterols. First, in both normal and diseased tissues, the levels of oxysterols are very low when compared to the native sterol. Also, when assessing studies that have measured the levels of oxysterols in biological samples, there must be careful consideration as to the method of sample isolation, storage and sampling. This is because of the potential generation or loss of oxysterols during these procedures. Additionally, the relevance of in vitro studies which examine the effects of oxysterols upon cell function should be judged as to cellular oxysterol content (both in terms of the levels of oxysterol and the degree of esterification) resulting from the oxysterol treatment. We present evidence that the means by which oxysterol is delivered in vitro determines whether the oxysterol content reflects what has been found in vivo. Studies identifying the specific cellular targets of oxysterol indicate that several oxysterols may be regulators of cellular lipid metabolism via control of gene transcription.