Assessment of the effects of metabolism on the estrogenic activity of xenoestrogens: A two-stage approach coupling human liver microsomes and a yeastestrogenicity assay

Concern that the reproductive health of humans is being affected by exposur e to xenoestrogens has led to the development of various in vitro and in vi vo screening assays for the identification of suspected xenoestrogens. Howe ver, the estrogenic activity of a chemical determined in vitro may not nece ssarily predict its activity in vivo if the chemical is metabolized during the assay and/or in vivo. Therefore, to investigate the role of metabolism in modulating the estrogenic activity of suspected xenoestrogens, we have d evised a two-stage approach coupling incubations with either human or rat h epatic microsomes with a yeast estrogenicity (transcription) assay. We have assessed the activity of the proestrogenic pesticide 99.5% methoxychlor [1 ,1,1-trichloro-2,2-bis-(4-methoxyphenyl) ethane, MXC] (EC50 = 4.45 +/- 1.9 muM, n = 6) and a structural analog, methoxybisphenol A [2,2-bis-(4-methoxy phenyl) propane, MBPA], in the yeast estrogenicity assay and also establish ed that yeast (Saccharomyces cerevisiae), unlike human liver microsomes, ar e not able to demethylate MXC or MBPA to estrogenic metabolites. This indic ates that the proestrogen MXC has weak intrinsic estrogenic activity. Using 99.5% MXC and 17 beta -estradiol as paradigms, we have demonstrated how me tabolism can enhance or suppress, respectively, estrogenic activity. The ef fect of metabolism on the activities of the weak xenoestrogens 3,17 beta -b isdesoxyestradiol [1,3,5(10)-estratriene] and 6-hydroxytetralin (5,6,7,8-te trahydro-2-naphthol) was also assessed. This two-stage approach can disting uish the estrogenic activity of a suspect chemical from the activity due to its more, or less, active metabolites and will aid in the evaluation of no vel xenoestrogens and, more importantly, proestrogens.