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Cholesterol Epoxide Is a Direct-Acting Mutagen (Lipid Oxidation/Mutagenesis/Carcinogenesis) A

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Cholesterol Epoxide Is a Direct-Acting Mutagen (Lipid Oxidation/Mutagenesis/Carcinogenesis) A Proc. Nati. Acad. Sci. USA Vol. 81, pp. 4198-4202, July 1984 Medical Sciences Cholesterol epoxide is a direct-acting mutagen (lipid oxidation/mutagenesis/carcinogenesis) A. SEVANIAN AND A. R. PETERSON Institute for Toxicology, Department of Pathology, and the Cancer Center, University of Southern California, Los Angeles, CA 90033 Communicated by Bruce N. Ames, March 15, 1984* ABSTRACT A 24-hr treatment of V79 Chinese hamster impair lipogenesis (21), as this compound has been reported lung fibroblasts with 12.4 ,uM cholesterol 5a,6a-epoxide in- to be cytotoxic (14, 22). duced 8-azaguanine-resistant mutants at frequencies 4.6- to In the present study, we have measured Chol epoxide up- 11.8-fold higher than the spontaneous mutation rate. We show take and apparent metabolism as well as its genotoxicity in that cholesterol epoxide, which is produced by in vivo choles- parallel experiments with V79 Chinese hamster cells. Using terol oxidation, is a weak direct-acting mutagen. Cholesterol established procedures (23, 24), we found that Chol epoxide epoxide was found to be accumulated by cells and transformed is mutagenic in Chinese hamster V79 cells, and we describe to cholestane-3.8,5x,6.8-triol, which was more toxic and a relationships between this mutagenicity and cytotoxicity, in- more potent inhibitor of DNA synthesis than the epoxide but, hibition of DNA synthesis, and transformation of Chol epox- at concentrations less than 17.8 ,.M, was not significantly mu- ide to cholestane triol. tagenic. Consideration of the rates of cholesterol epoxide con- version to cholestane triol shows that this conversion can result in abolition of the mutagenicity of the epoxide. Conditions un- MATERIALS AND METHODS der which conversion of the epoxide to the triol is low, as in the Chemicals, Radiochemicals, and Cell Culture Supplies. case of low epoxide hydrolase activity, favor mutagenicity Chol, Chol epoxide, and cholestane triol were purchased whereas rapid conversion to triol favors cytotoxicity. from Steraloids (Wilton, NH). Chol epoxide was found to be contaminated with =8% Chol and -2% cholestane triol as Cholesterol 5a,6a-epoxide, a recognized oxidation product determined by thin-layer and gas chromatographic analysis of cholesterol (Chol) (1-4), may also be a metabolic interme- (25). We therefore used purified Chol epoxide and choles- diate in bile acid biosynthesis (2, 5) and has been found to tane triol for all experiments. Both compounds were purified accumulate in such cases as hypercholesterolemia in humans by silicic acid column chromatography using benzene/ethyl (6). The enzymatic oxidation of Chol is coupled with hydra- acetate (3:2) as the eluting solvent. Alternatively, Chol epox- tion reactions that form cholestane-3f3,5a,6f3-triol (7). ide and radiolabeled Chol epoxide and cholestane triol were (Throughout this paper, the terms Chol epoxide and choles- prepared by reaction of [4-1 C]Chol (New England Nuclear), tane triol will refer to the Sa,6a and 3,1,5a,613 isomers, re- or unlabeled Chol (Sigma), with monoperphthalic acid as de- spectively, unless otherwise specified.) Since Chol epoxide scribed (25). The purity and characteristics of labeled and possesses an electrophilic oxirane group, it could be expect- unlabeled Chol epoxide were checked by gas chromatogra- ed to be genotoxic and carcinogenic (8, 9). The detection of phy/mass spectrometry using a Hewlett-Packard model Chol epoxide in UV-induced skin cancer (9) and cholestane 5992 GC/MS fitted with a 6 ft (1.86 m) 3% OV-101 packed triol in human colon cancer (10) suggests that the etiology of column and operated isothermally at 270°C. Further charac- these cancers may be associated with Chol epoxide, which is terization was accomplished by thin-layer radiochromatog- not xenobiotic but appears to originate in vivo or be assimi- raphy (26) and by HPLC using a Perkin-Elmer series 4 liquid lated as a food contaminant. Petrakis et al. (11) have recently chromatograph equipped with a 150 x 4.6 mm 3-,m Spheri- reported that human breast fluid can contain enormous lev- sorb silica column (Chromanetics, Jessup, MD). The eluting els (up to 780 ,M) of Chol epoxide. The origin of this Chol solvent for HPLC was 4.2% isopropanol in hexane at a flow epoxide is unknown although its levels are directly related to rate of 1.0 ml/min. Detection was accomplished by differen- breast fluid cholesterol content, which increases progres- tial refractometry. The 5a,6a- and 5/3,6,f3epoxides were elut- sively with advancing age. ed in 7.5 and 8.4 min, respectively (k = 2.75 and 3.20), which Studies have shown that Chol epoxide induces chromo- permits further purification of Chol 5a,6a- and 5,8,6,B3epox- somal damage, DNA repair (12), and oncogenic transforma- ides. In these experiments, further purification of the Chol tion (13, 14) of mammalian cells. Conversely, negative find- epoxide was not attempted and the substance was used at ings were obtained with Chol epoxide in the Ames Salmonel- the 95% purity level described below. A portion of the Chol la mutagenicity assay (15, 16), and Chol epoxide did not epoxide was converted to cholestane triol by reaction for 24 induce tumors nor promote N-methyl-N'-nitro-N-nitroso- hr at room temperature with 5% perchloric acid in tetrahy- guanidine carcinogenesis in rats and mice (15, 17, 18), al- drofuran/water/acetone, 4:1:0.5 (vol/vol/vol). Reactants and though careful determinations of Chol epoxide uptake and products were separated chromatographically after neutral- transformation were not made in several of these studies. ization of the reaction mixture with 0.1 M KOH (26). Furthermore, the proportions of the Sa,6a isomer and its di- The sources of N-methyl-N'-nitro-N-nitrosoguanidine and astereomer, Chol 5P,6p-epoxide, both of which may be [6-3H]thymidine and of the V79 Chinese hamster lung fibro- formed enzymatically or by peroxidation (1, 2, 4) were not blasts and the media and plasticware used in their culture indicated. Nonabsorption (19) or low levels of uptake and have been described (24). All reagents were of spectrophoto- metabolism of Chol epoxide (20) would be expected to affect metric grade. its genotoxicity, while conversion to cholestane triol could Abbreviations: Chol, cholesterol; zsGr, 8-azaguanine resistant. The publication costs of this article were defrayed in part by page charge *Communication of this paper was initiated by Charles Heidelberger payment. This article must therefore be hereby marked "advertisement" and, after his death (January 18, 1983), completed by Bruce N. in accordance with 18 U.S.C. §1734 solely to indicate this fact. Ames. 4198 Downloaded by guest on September 26, 2021 Medical Sciences: Sevanian and Peterson Proc. NatL. Acad. Sci. USA 81 (1984) 4199 Treatment of Cells for DNA Synthesis, Cytotoxicity, and purity of [14C]Chol epoxide was substantiated by HPLC. Mutagenesis Assays. For a single measurement of cytotoxic- The results of these analyses suggest that the biological ef- ity, 200 cells in each of four 60-mm dishes, or for a single fects measured are probably attributed to the a-isomer. measurement of DNA synthesis, 105 cells in each of two 100- The highest concentrations of Chol epoxide and Chol used mm dishes, were treated as follows. Cells were incubated in in our experiments were 62 ,bM and 25 ,tM, respectively. medium containing 0.5% acetone, which was used as the de- Concentrations greater than these produced precipitates and livery solvent and served as the negative control. N-methyl- therefore were not used. The highest concentration of cho- N'-nitro-N-nitrosoguanidine (positive control) at 6.8 /LM, lestane triol used, 24.8 .uM, caused cells to detach from the Chol, Chol epoxide, and cholestane triol were added to Dul- dishes; therefore, concentrations below 24.8 uM were used becco's medium containing 5% dialyzed fetal calf serum and to study its effects on V79 cells. 5% dialyzed calf serum at 370C. Cells were then incubated The time course of Chol epoxide (62 ,tM) incorporation for intervals of 2-24 hr. The 60-mm and 100-mm dishes con- over a 24-hr interval by cultures of V79 cells in which Chol tained 5 ml and 10 ml, respectively, of medium, which was epoxide produced significant mutagenesis is shown in Fig. aspirated after incubation. The monolayers were then 1A. It is assumed that any of the compound not washed from washed twice with complete medium and incubated with the monolayer was assimilated into a cellular compartment. fresh complete medium. A linear increase was found over the first 8 hr of incubation Assay of DNA Synthesis, Cytotoxicity, and Mutagenesis. and thereafter the rate of uptake decreased. Also shown in These assay procedures have been characterized (23, 24, the figure is the total amount of cholestane triol recovered 27). DNA synthesis was assayed by measuring [3H]thymi- from the cells and medium combined. By 24 hr the amount of dine incorporation into the cold-trichloroacetic acid-insolu- cholestane triol recovered from the medium represented ble fraction of cells that had been incubated in medium for 45 -35% of the total formed. Incubation of Chol epoxide in cul- min and then in medium containing [3H]thymidine (5.0 ture medium alone revealed very low levels of hydrolysis kCi/ml) for 30 min (23). Cytotoxicity was assayed by the (0.15 nmol/24 hr), and the cholestane triol produced nonen- plating-efficiency method (24). The frequencies of 8-aza- zymatically was subtracted to give the corrected enzymati- guanine-resistant (zsGr) mutants were determined using an cally dependent rate. After 24 hr of treatment, -5% of the expression time of 6 days with replating as described (24).
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