Analysis and physiologic significance of cholesterol epoxide in animal tissues

Identification and pharmacological characterization of cholesterol-5,6-epoxide hydrolase as a target for tamoxifen and AEBS ligands

The microsomal antiestrogen binding site (AEBS) is a high-affinity target for the antitumor drug tamoxifen and its cognate ligands that mediate breast cancer cell differentiation and apoptosis. The AEBS, a hetero-oligomeric complex composed of 3beta-hydroxysterol-Delta8-Delta7-isomerase (D8D7I) and 3beta-hydroxysterol-Delta7-reductase (DHCR7), binds different structural classes of ligands, including ring B oxysterols. These oxysterols are inhibitors of cholesterol-5,6-epoxide hydrolase (ChEH), a microsomal epoxide hydrolase that has yet to be molecularly identified. We hypothesized that the AEBS and ChEH might be related entities. We show that the substrates of ChEH, cholestan-5alpha,6alpha-epoxy-3beta-ol (alpha-CE) and cholestan-5beta,6beta-epoxy-3beta-ol (beta-CE), and its product, cholestane-3beta,5alpha,6beta-triol (CT), are competitive ligands of tamoxifen binding to the AEBS. Conversely, we show that each AEBS ligand is an inhibitor of ChEH activity, and that there is a positive correlation between these ligands' affinity for the AEBS and their potency to inhibit ChEH (r2=0.95; n=39; P<0.0001). The single expression of D8D7I or DHCR7 in COS-7 cells slightly increased ChEH activity (1.8- and 2.6-fold), whereas their coexpression fully reconstituted ChEH, suggesting that the formation of a dimer is required for ChEH activity. Similarly, the single knockdown of D8D7I or DHCR7 using siRNA partially inhibited ChEH in MCF-7 cells, whereas the knockdown of both D8D7I and DHCR7 abolished ChEH activity by 92%. Taken together, our findings strongly suggest that the AEBS carries out ChEH activity and establish that ChEH is a new target for drugs of clinical interest, polyunsaturated fatty acids and ring B oxysterols.

Protective role of glutathione and glutathione transferases in mutagenesis and carcinogenesis

Glutathione (GSH) alone detoxifies electrophiles with an effectiveness which depends on the rate of the reaction and the concentration of GSH. If electrophiles are substrates for GSH transferase isoenzymes, the effectiveness of detoxication is much enhanced due to the increased rate of reaction and it is also independent of GSH concentration to low levels of GSH depletion, since the Km for GSH is approximately 0.1 mM. In this paper detoxication of electrophilic metabolites of the hepatocarcinogen N-methyl-4-aminoazobenzene which are not substrates for GSH transferases and the carcinogenic electrophile derived from the hepatocarcinogen aflatoxin B1 which is a poor substrate is compared with detoxication of electrophiles which are good substrates and which although bacterial mutagens are not carcinogenic in organs containing the appropriate GSH transferases. GSH transferases detoxify not only electrophiles derived from xenobiotics, but also endogenous electrophiles which are usually the consequence of free radical damage in the presence of oxygen to lipids and DNA and include lipid and DNA hydroperoxides and alkenals arising from the decomposition of lipid hydroperoxides. Studies in the rat and other mammals show the GSH transferases to be dimers in which the subunits are members of a gene super-family. There are three, perhaps four multigene families namely, alpha containing subunits 1, 2, 8 and 10; mu containing subunits 3, 4, 6 and 9; pi containing subunit 7 and subunits 5 and 5* which are so far unassigned. Subunit 5* is apparently restricted to the nucleus and is noteworthy for its activity towards DNA hydroperoxides. Studies in the human are not as advanced as in the rat but so far reveal close similarities. The ability of GSH transferases to detoxify electrophiles is important in carcinogenesis at a number of points. They may inhibit initiation and tumour proportion, but they may be advantageous to the developing tumour cell, and may be acquired in increased amounts during malignant progression. In many tumour cells the development of lines resistant to anticancer drugs is associated with an increased expression of GSH transferases, particularly GSH transferase pi in human cells.

Mutagenic characterization of cholesterol epoxides in Chinese hamster V79 cells

The uptake, metabolism and alkylating properties of the diastereomeric cholesterol epoxides were studied using Chinese hamster lung fibroblasts (V79 cells). Specific emphasis is given to the comparative cyto- and geno-toxic effects of cholesterol 5 beta,6 beta-epoxide (beta CE) and cholesterol 5 alpha,6 alpha-epoxide (alpha CE) and data are provided for the first time indicating that beta CE can induce more 6-thioguanine-resistant cells than alpha CE. Cholesterol 5 beta,6 beta-epoxide induced colonies of cells resistant to 6-thioguanine at 2-3-fold the frequencies observed with the alpha-isomer, but neither compound produced ouabain-resistant colonies. The cytotoxicity (LD50) of alpha CE was estimated to be 45-50 microM whereas beta CE displayed an LD50 of 25-29 microM. Inhibition of DNA synthesis (IC50) was observed over the same dose ranges as the LD50 for each epoxide isomer. The epoxides were assimilated by cells to an equal extent, however, beta CE was metabolized to cholestane 3 beta,5 alpha-6 beta-triol twice as rapidly as the alpha-isomer. Both epoxides reacted with 4-(4'-nitrobenzyl)-pyridine to a similar extent, and with identical nucleophilic selectivity at pH 7.4, but their alkylating activity was estimated on this basis to be two orders of magnitude less than methyl methanesulfonate. Binding experiments with the DNA or cultured V79 cells or with calf-thymus DNA indicated that interactions were noncovalent and DNA binding did not correlate with the potency of the epoxides to induce the 6-thioguanine-resistant phenotype. Our results could be interpreted as indicating that both cholesterol epoxide isomers are weak mutagens or that they might induce some epigenetic event repressing the hypoxanthine guanine-phosphoribosyltransferase gene. The similarity of the epoxides' alkylating activity and their DNA-binding properties are inconsistent with their different potencies in inducing the 6-thioguanine-resistant phenotype, suggesting that the mechanism leading to this phenotype is not necessarily the result of DNA alkylation.

The cytotoxic and mutagenic properties of cholesterol oxidation products

The oxidation of cholesterol proceeds as part of the lipid peroxidation process in membranes. Several oxidation products characteristic of a free-radical mechanism are formed and some can serve as indices of the nature and extent of cholesterol oxidation and of lipid peroxidation in general. Among the most typical oxidation products of lipid peroxide-dependent propagation reactions are the enantiomeric 5,6-epoxides and 7-ketocholestanol. Small amounts of these compounds may persist in tissues experiencing lipid peroxidation at a low but steady flux of free-radical reactions. Supporting evidence includes the routine detection of small quantities of cholesterol epoxides in tissues of normal animals, and the increase of these epoxides under conditions of oxidant stress or antioxidant deficiency. Conversion of cholesterol epoxides to cholestane triol is expected in cells possessing cholesterol epoxide hydrolase. All of these oxidation products possess remarkable cytotoxicity (at least part of which may be due to effects on the cell membrane) causing an increase in intracellular calcium. The cholesterol epoxides are also weakly mutagenic, although the mechanism for this mutagenicity remains to be clarified. In contrast, the other lipid epoxides normally encountered in tissues (chiefly fatty acid epoxides) are not mutagenic, and are much less toxic than the oxysterols described. The cytotoxicity of several oxysterols may be due to a number of mechanisms. That only the epoxides are mutagenic suggests that genotoxicity is a function of their electrophilic reactivity. This is not consistently apparent with the other compounds examined.

Anti-inflammatory properties of an oxidized sterol

A polar photoproduct of cholesterol oxidation, 7-ketocholesterol, was able to inhibit in a dose-dependent manner the mouse ear-swelling response to irritants such as croton oil or cantharidin. Its anti-inflammatory properties were much less than equivalent concentrations of hydrocortisone, but the oxidized sterol did not induce any systemic effects (as measured by thymolytic activity), as did topical hydrocortisone. It is concluded that 7-ketocholesterol has weak anti-inflammatory activity, and its mode of action may be different from that of glucocorticoids.

Oxidized sterols inhibit the formation of podophyllin-induced metaphase figures in mouse vaginal epithelia

The antimitotic activity of oxidized derivatives of cholesterol was investigated using an assay developed by Van Scott and Bonder. In this assay, a drug that has antimitotic activity and is not a metaphase-blocking agent will inhibit the formation of podophyllin-induced metaphase figures, as counted on histologic specimens. Mouse vaginal epithelia were classified as being estrogen or progesterone predominant on the basis of histologic criteria. Podophyllin-injected mice in the estrogenic phase of the estrus cycle demonstrated high metaphase-figure counts, with an average of 284.86 +/- 132.01. In this group, all intravaginally administered compounds, inhibited the formation of metaphase figures, including a propylene-glycol ethanol vehicle (60% suppression); thus, it is concluded that animals in this phase are not a suitable model for assaying antimitotic activity. Mice in the progesterone-predominant phase of the estrus cycle had lower counts of podophyllin-induced metaphase figures, i.e., 142.13 +/- 39.29. In this group, 25-OH-cholesterol was the most effective inhibitor (59% suppression), followed by 7-ketocholesterol (48% suppression) and methotrexate (40% suppression). Cholesterol (5% suppression) and vehicle (20% suppression) did not have any significant effects. Progesterone-predominant epithelium was only susceptible to methotrexate and oxidized sterols. This suggests that oxidized sterols may have antimitotic activity.

Cholesterol epoxide is a direct-acting mutagen

A 24-hr treatment of V79 Chinese hamster lung fibroblasts with 12.4 microM cholesterol 5 alpha, 6 alpha-epoxide induced 8-azaguanine-resistant mutants at frequencies 4.6- to 11.8-fold higher than the spontaneous mutation rate. We show that cholesterol epoxide, which is produced by in vivo cholesterol oxidation, is a weak direct-acting mutagen. Cholesterol epoxide was found to be accumulated by cells and transformed to cholestane-3 beta, 5 alpha, 6 beta-triol, which was more toxic and a more potent inhibitor of DNA synthesis than the epoxide but, at concentrations less than 17.8 microM, was not significantly mutagenic. Consideration of the rates of cholesterol epoxide conversion to cholestane triol shows that this conversion can result in abolition of the mutagenicity of the epoxide. Conditions under which conversion of the epoxide to the triol is low, as in the case of low epoxide hydrolase activity, favor mutagenicity whereas rapid conversion to triol favors cytotoxicity.

5 alpha,6 alpha-Epoxy-cholestan-3 beta-ol (cholesterol alpha-oxide): A specific substrate for rat liver glutathione transferase B

A semi-micro assay was developed for the conjugation of 5 alpha,6 alpha-epoxy-cholestan-3 beta-ol (cholesterol alpha-oxide) with glutathione. The soluble supernatant of rat liver homogenate catalysed the reaction at a rate of 0.2-0.5 pmol . min-1 . mg protein-1 with 4 microM cholesterol alpha-oxide, while the reaction in the presence of GSH alone was barely detectable. Enzymic activity in the soluble supernatant was due equally to the two forms of glutathione transferase B (approximately 100 pmol . min-1 . mg protein-1), glutathione transferases AA, A, C and E being unreactive. The activity of purified glutathione transferase B was about 5-times that expected from the activity of the soluble supernatant. Complex enzyme kinetics were obtained suggestive of substrate inhibition.

Effects of vitamin E deficiency and nitrogen dioxide exposure on lung lipid peroxidation: use of lipid epoxides and malonaldehyde as measures of peroxidation

The effect of vitamin E deficiency in male Sprague-Dawley rats upon lipid peroxidation in lung tissue was examined by measuring malonaldehyde and lipid epoxide production. In addition to controls, some animals were also exposed to 3 +/- 0.1 ppm NO2 continuously for 7 d in order to study the effects of oxidant stress on lung lipid peroxidation and vitamin E content. The observed changes in malonaldehyde and epoxide content could serve as good indices of lipid peroxidation, particularly under conditions of vitamin E deprivation. The responses measured indicated an inverse relation in the lung between tissue vitamin E content and quantity of lipid peroxidation products. Measurement of lipid epoxides served as a reliable indicator of lung tissue lipid peroxidation. Finally, NO2 inhalation appeared to elicit a response characterized by increased assimilation of vitamin E into lung tissue.