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Dependence on Exogenous Metabolic Activation for Induction of Unscheduled DNA Synthesis in Syrian Hamster Embryo Cells by Diethylstilbestrol and Related Compounds

National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709 [G. H. D., A. W., J. A. M., J. C. B.]; Nippon Dental University Tokyo, 1-9-10 Fujiml, Chiyoda-ku, Tokyo 102, Japan [T. T., H. M.]; and Institute for Pharmacology and Toxicology, University of Wuerzburg, Versbachier Strasse 9, D-8700 Wuerzburg, Federal Republic of Germany [D. S.]

Diethylstilbestrol (DES) induces morphological and neoplastic transformation of Syrian hamster embryo cells in vitro in the absence of any measurable induction of gene mutations, which is consistent with the lack of genotoxicity of DES in a number of other assays. However, a few reports of a genotoxic activity of DES in certain systems have been published. In order to understand these differences, we have investigated whether DES induces unscheduled DNA synthesis (UDS) in Syrian hamster embryo cells under the conditions which result in cell transformation and have examined the role of an exogenous metabolic activation system on DES-induced UDS. DES, over a concentration range of 1 to 10 µg/ml, failed to induce any detectable UDS in the cells, while other known transforming agents, including UV irradiation (6 to 24 J/sq m), benzo(a)pyrene (0.1 to 1.0 µg/ml), and aflatoxin B₁ (10 to 100 µg/ml), induced significant levels of UDS. In contrast, UDS was induced in a dose-dependent manner by DES (1 to 10 µg/ml) after addition of an Aroclor-induced rat liver postmitochondrial supermatant fraction and other cofactors for exogenous metabolic activation. In order to probe the basis for this alteration in UDS induction, the ability of structural analogues and metabolites of DES to induce UDS was examined. In the absence of exogenous activation, the only oxidative metabolite of DES detected in the presence of the cells was cis,cis-dienestriol, which did not induce UDS by itself. In the presence of exogenous activation, cis,cis-dienestrol and its trans,trans-isomer induced UDS but not to a greater extent than DES. With the addition of the exogenous metabolizing system, increased metabolism of DES to cis,cis-dienestrol and additional polar derivatives of DES or dienestrol, possibly hydroxylated derivatives, were observed. With exogenous metabolic activation, tetrafluoro-DES and hexestrol, which differ in their ability to be peroxidatively metabolized to quinone and phenoxyradical intermediates, both induced UDS, although tetrafluorodiethylstilbestrol at 10 µg/ml stimulated a higher level of UDS. None of the DES-related compounds examined was active in the UDS assay without exogenous metabolic activation, but all of the compounds can potentially form phenoxyradical intermediates by a peroxidase-mediated reaction. The compounds which can be further oxidized to a quinone were most active in inducing UDS. These results are consistent with the hypothesis that this peroxidase-mediated pathway is important in the induction of UDS, although secondary metabolites may also be involved. Further studies on the nature of the reactive intermediates of DES which induce DNA damage are needed. However, this study demonstrates an important parameter in the genotoxicity of DES and possibly provides as explanation for some of the conflicting results regarding the DNA-damaging activity of DES. Further studies are needed to determine the biological relevance of the genotoxic activity of DES under these conditions.

¹ To whom requests for reprints should be addressed.

Received 5/24/83. Accepted 10/ 7/83.