Effects of binary mixtures of estrogen and antiestrogens on Japanese medaka (Oryzias latipes)

It is supposed that the estrogenic effects of estrogenic chemicals will be balanced by the presence of other endocrine-disrupting chemicals (EDCs) with different properties. However, little convincing research into this issue has been carried out to date. In the present study, the combined effects of EDCs with estrogenic and antiestrogenic activities were assessed using a 21-day reproduction assay with the Japanese medaka (Oryzias latipes) as an in vivo model. Natural estrogen (17β-estradiol, E2) and the pharmaceutical substances letrozole and tamoxifen, which are both model antiestrogens, were tested as reference compounds individually and in combination. Exposure to E2 (200 ng/L) alone resulted in significant biological changes in paired fish, including impaired reproductive capacity and plasma vitellogenin induction. Upon coexposure with tamoxifen or letrozole (10, 50 and 250 μg/L), the effects of estrogen on some biomarkers (e.g., plasma vitellogenin concentration) could be neutralized to some extent, but the impairments in reproductive performance were hardly ameliorated or even became more severe. Since reproductive performance is a more holistic parameter with population-level relevance, the possibility that the effects of estrogenic chemicals can be cancelled out by the presence of antiestrogens in aquatic organisms should be questioned. Our observations also illustrate the importance of considering multiple endpoints at different biological levels of organisms for accurate and adequate assessment of the integrated effects of EDCs, especially in environmentally relevant exposure situations.