Assessment of thyroid hormone activity of halogenated bisphenol A using a yeast two-hybrid assay

The thyroid hormone agonist/antagonist activities of halogenated derivatives of bisphenol A (BPA) were assessed using a yeast two-hybrid assay incorporating the human thyroid hormone α (TRα), both with and without possible metabolic activation by rat liver S9 preparation. In the absence of the rat liver S9 preparation, 3,3′,5,5′-tetrabromobisphenol A (TBBPA), 3,3′,5,5′-tetrachlorobisphenol A (TCBPA), and 3,3′,5-trichlorobisphenol A (3,3′,5-triClBPA) exhibited agonist activity, whereas 3-chlorobisphenol A (3-ClBPA), 3,5-dichlorobisphenol A (3,5-diClBPA), 3,3′-dichlorobisphenol A (3,3′-diClBPA), and BPA did not. The activities of TBBPA and TCBPA increased markedly (7.6-fold and 3.1-fold, respectively) after their metabolic activation with the rat liver S9 preparation. TBBPA, TCBPA, and 3,3′,5-triClBPA inhibited the binding of triiodothyronine (T3) to TRα at 2 × 10⁻⁵ M without rat liver S9 treatment and 4 × 10⁻⁶ M with rat liver S9 treatment, demonstrating their T3 antagonist activity. These results revealed that metabolic activation by rat liver S9 significantly increased the agonist/antagonist potential of some halogenated BPAs.