Abstract
Thyroid hormones have critical roles in cardiac function and brain development, and the mechanism of action of thyroid hormones are mediated via the activation of thyroid hormone receptors (TRs). In vitro reporter gene assays are widely used to detect endocrine disruptors, and several human cell-based bioassays have been established in TR-responsive reporter gene assays. In the previous study, we developed yeast-based TRα and TRβ reporter gene assays for Homo sapiens and Xenopus tropicalis. In this study, we evaluated these four yeast-based reporter gene assay systems in terms of detection efficiencies of thyroid hormones and anti-thyroid hormone compounds. Additionally, the acceptance and pre-decision criteria were validated in two facilities. In 96-wells plates, yeasts were incubated with the various concentrations of test substances. The antagonistic activities of chemicals were examined by competition between the chemical and thyroid hormone simultaneously added at the following concentrations. To distinguish antagonist activity from the cytotoxicity, the CYC yeast strains established in W303a for each receptor that constitutively expressed the lacZ reporter were used for comparisons. In both of TRα and TRβ assays for Homo sapiens and Xenopus tropicalis, β-galactosidase activity was induced by thyroid hormones in dose-dependent manner. All anti-thyroid hormone compounds tested in this study inhibit thyroid hormone-induced in human TR assays, but some compounds had no antagonistic effects in the assays for Xenopus tropicalis. The results were consistent in both laboratories, suggesting these yeast-based TR assay systems to be robust and reproducible. We will continue to verify effectiveness of our TR assay systems by increasing the numbers of testing facilities and test substances.
