T-Screen as a tool to identify thyroid hormone receptor active compounds

The T-Screen represents an in vitro bioassay based on thyroid hormone dependent cell proliferation of a rat pituitary tumour cell line (GH3) in serum-free medium. It can be used to study interference of compounds with thyroid hormone at the cellular level, thus bridging the gap between limitations of assays using either isolated molecules (enzymes, transport proteins) or complex in vivo experiments with all the complex feedback mechanisms present. Compounds are tested both in the absence and presence of thyroid hormone (EC50 concentration of T3) to test for both agonistic and antagonistic potency. Thyroid hormones (3,3'-5-triiodothyronine: T3 and 3,3',5,5'-tetraiodothyroxine: T4) and compounds resembling the structure of thyroid hormones (3,3'-5-triiodothyroacetic acid: Triac; 3,3',5,5'-tetraiodothyroacetic acid: Tetrac) induced cell growth, with the rank order Triac > T3 > Tetrac > T4 (relative potency = 1.35 > 1 > 0.29 > 0.07), which is identical to published affinities of these compounds for nuclear thyroid hormone receptors. Exposure to 5,5'-diphenylhydantoin (DPH) in the presence of 0.25 nM T3 resulted in up to 60% decreased cell growth at 200 µM DPH. No effect of DPH on basal metabolic activity of GH3 cells was observed at this concentration. Fentinchloride (IC50 = 21 nM) decreased cell growth induced by 0.25 nM T3, whereas parallel exposure to these concentrations in the absence of T3 did not alter basal metabolic activities of GH3 cells. Apolar sediment extracts from the Dommel (34%) and Terneuzen (14%) decreased cell growth in the presence of 0.25 nM T3, whereas the extract from Hoogeveen increased cell growth (26%) and the extract from North Sea Channel had no effect. The T-Screen proved to be a fast and functional assay for assessing thyroid hormone receptor active potencies of pure chemicals or environmental mixtures.