7 alpha-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-hydroxy-cholesterol, 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 7 alpha-hydroxylated with subsequent oxidation to 7 alpha-hydroxy-3-oxo-Delta(4) steroids. A minor fraction of 27-hydroxycholesterol was also metabolised to 3 beta-hydroxy-5-cholestenoic, 3 beta,7 alpha-dihydroxy-5-cholestenoic and 7 alpha-hydroxy-3-oxo-4-cholestenoic acids, The 7 alpha-hydroxylase was found to be localised to the thymic epithelial cells and the reaction was stimulated by interleukin-1 beta and inhibited by metyrapone and RU486. In contrast to 25-hydroxy-cholesterol and 27-hydroxycholesterol, the 7 alpha-hydroxylated metabolites, 7 alpha,25-dihydroxycholesterol, 7 alpha,25-dihydroxy-4-cholesten-3-one and 7 alpha,27-dihydroxy-4-cholesten-3-one did not induce thymocyte apoptosis. The results suggest that 7 alpha-hydroxylation may be of regulatory importance, possibly by protecting the developing thymocytes against toxic effects by oxysterols.