A new bioluminescent cellular assay to measure the transcriptional effects of chemicals that modulate the alpha-1 thyroid hormone receptor

Interactions of environmental pollutants with the thyroid endocrine axis have received much attention especially because thyroid hormones (THs) play a major role in mammalian brain development. In order to screen for compounds that act on the triiodothyronine (T3) signaling pathway, we developed a new reporter gene assay expressing luciferase under the control of the TH receptor (TR). PC12 cells expressing the α1-isoform of TR of avian origin were stably transfected with a luciferase gene controlled by the SV40 promoter, and enhanced by a four-spaced direct repeat (DR4) thyroid response element (TRE). The resulting PC-DR-LUC cells were used to optimize a T3 assay in multiwell microplates. This assay was highly sensitive (30 pM T3) and reproducible, and responded as expected to TH analogues. Several halogenated phenolic (3,3′,5,5′-tetrabromobisphenol A, 3,3′,5,5′-tetrachlorobisphenol A, 4-hydroxy-2′,3,4′,5,6′-pentachlorobiphenyl) and phenol (pentachlorophenol, 2,4,6-triiodophenol) compounds suspected of being thyroid-disrupting environmental chemicals induced partial agonistic and/or complex competitive/uncompetitive antagonistic responses in PC-DR-LUC cells at micromolar concentrations. A cell viability test indicated that these effects were not related to cytotoxicity of the chemicals. These results suggest that the PC-DR-LUC assay could be a valuable tool for the large-scale screening for thyroid receptor agonists and antagonists in vitro, and for detecting thyroid disruptors in the environment.