Metabolism of 27-, 25- and 24-hydroxycholesterol in rat glial cells and neurons

Neurotoxicity of cholesterol oxides on cultured cerebellar granule cells

Cultured rat cerebellar granule cells were used to determine the potential neurotoxicity of cholesterol oxides. The cholesterol oxides tested included: 7-beta-OH-, 7-keto-, 19-OH-, 22(R)-OH-, 22(S)-OH- and 25-OH- cholesterol. Among them, 7-beta-OH- and 7-keto-cholesterol were the most efficacious in causing neuronal death such that 20 microg/ml (50 microM) of these agents killed more than 80% of cells in 2 days. 7-beta-OH-cholesterol at this concentration killed 50% of cells in approximately 7 h. A number of pharmacological agents were tested for their abilities to prevent neuronal death induced by cholesterol oxides. Among them, aurintricarboxylic acid, vitamin E and methyl-beta-cyclodextrin were able to prevent cholesterol oxide-induced neurotoxicity in a dose-dependent manner. These results suggest that, in addition to causing pathological changes in cells directly involved in atherosclerosis, cholesterol oxides may induce toxicity in neurons of the central nervous system.

Biochemical characterization of the 7alpha-hydroxylase activities towards 27-hydroxycholesterol and dehydroepiandrosterone in pig liver microsomes

Microsomal cytochrome P-450 catalyzing the 7alpha-hydroxylation of 27-hydroxycholesterol and dehydroepiandrosterone was partially purified from pig liver. This enzyme fraction also catalyzed 7alpha-hydroxylation of 25-hydroxycholesterol and pregnenolone but did not 7alpha-hydroxylate cholesterol or testosterone. Studies with extrahepatic tissues have suggested the possibility of one common enzyme responsible for the 7alpha-hydroxylation of 27-hydroxycholesterol and dehydroepiandrosterone. A series of experiments was performed to study if there are one or several enzymes 7alpha-hydroxylating these steroids in the liver. The activities towards the two substrates copurified but the ratio between 27-hydroxycholesterol and dehydroepiandrosterone 7alpha-hydroxylation varied considerably in different purification steps and between different preparations. The enzyme inhibitors disulfiram, N-bromosuccinimide, ketoconazole, metyrapone and alpha-naphthoflavone affected the activities in a similar way. Dehydroepiandrosterone inhibited 27-hydroxycholesterol 7alpha-hydroxylation whereas 27-hydroxycholesterol had almost no inhibitory effect on dehydroepiandrosterone 7alpha-hydroxylation. Experiments to examine the nature of inhibition by dehydroepiandrosterone indicated that the two steroids did not compete for the same active site. The results of this study do not rule out the possibility of one single enzyme catalyzing 7alpha-hydroxylation of the two steroids. However, taken together the data suggest that hepatic microsomal 7alpha-hydroxylation of 27-hydroxycholesterol and dehydroepiandrosterone involves at least two, probably closely related, enzymes. (c) 1998 Elsevier Science B. V.

7alpha-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 7alpha-hydroxylated with subsequent oxidation to 7alpha-hydroxy-3-oxo-delta4 steroids. A minor fraction of 27-hydroxycholesterol was also metabolised to 3beta-hydroxy-5-cholestenoic, 3beta,7alpha-dihydroxy-5-cholestenoic and 7alpha-hydroxy-3-oxo-4-cholestenoic acids. The 7alpha-hydroxylase was found to be localised to the thymic epithelial cells and the reaction was stimulated by interleukin-1beta and inhibited by metyrapone and RU486. In contrast to 25-hydroxycholesterol and 27-hydroxycholesterol, the 7alpha-hydroxylated metabolites, 7alpha,25-dihydroxycholesterol, 7alpha,25-dihydroxy-4-cholesten-3-one and 7alpha,27-dihydroxy-4-cholesten-3-one did not induce thymocyte apoptosis. The results suggest that 7alpha-hydroxylation may be of regulatory importance, possibly by protecting the developing thymocytes against toxic effects by oxysterols.