Establishment of yeast reporter assay systems to detect ligands of thyroid hormone receptors alpha and beta.

Thyroid hormones are essential for proper development and differentiation in vertebrates. Recently, concern over the disruption of thyroid hormone homeostasis by industrial chemicals and environmental pollutants has been spreading. To evaluate these chemicals, several bioassays have been developed to detect thyroid hormone ligand activity. Nevertheless, a simple and useful assay is required for the assessment of an enormous number of environmental chemicals. We established yeast reporter assays by expression of full-length thyroid hormone receptor (TRalpha or TRbeta) cDNA and of the TR-dependent reporter gene in yeasts. By additional introduction of the general coactivator SRC-1 cDNA into the yeasts, a higher response to endogenous thyroid hormones, thyroxine (T4), and triiodothyronine (T3) was obtained. The EC50 values for T3 were 35 and 1.5nM for TRalpha and TRbeta assay yeasts, respectively. We tested four chemicals, tetrabromobisphenol A, tetramethylbisphenol A, 2-isopropylphenol, and o-t-butylphenol, which are suspected to have thyroid hormone-disrupting activity. All four chemicals showed agonistic activities in both assay yeasts; however, their activities were weak in comparison with endogenous TR ligands. Antagonist activities of 2-isopropylphenol and o-t-butylphenol were also found in the TRalpha yeast assay. Taken together, these assay yeasts will be powerful tools for assessing TR ligand activity of industrial chemicals and environmental pollutants.