7α-hydroxylation and 3-dehydrogenation abolish the ability of 25- hydroxycholesterol and 27-hydroxycholesterol to induce apoptosis in thymocytes

Oxygenated derivatives of sterols (oxysterols), including 25- hydroxycholesterol and 27-hydroxycholesterol have immunosuppressive effects. Oxysterols can directly induce apoptosis in immature thymocytes, cells which are inherently sensitive to induction of programmed cell death. For that reason, the metabolism of 25-hydroxycholesterol and 27-hydroxycholesterol in mouse thymus has been studied. When incubated with thymic tissue, both oxysterols were found to be 7α-hydroxylated with subsequent oxidation to 7α-hydroxy-3-oxo-Δ steroids. A minor fraction of 27-hydroxycholesterol was also metabolised to 3β-hydroxy-5-cholestenoic, 3β,7α-dihydroxy-5- cholestenoic and 7α-hydroxy-3-oxo-4-cholestenoic acids. The 7α-hydroxylase was found to be localised to the thymic epithelial cells and the reaction was stimulated by interleukin-β and inhibited by metyrapone and RU486. In contrast to 25-hydroxycholesterol and 27-hydroxycholesterol, the 7α- hydroxylated metabolites, 7a,25-dihydroxycholesterol, 7α,25-dihydroxy-4- cholesten-3-one and 7α,27-dihydroxy-4-cholesten-3-one did not induce thymocyte apoptosis. The results suggest that 7α-hydroxylation may be of regulatory importance, possibly by protecting the developing thymocytes against toxic effects by oxysterols.