Direct evidence revealing structural elements essential for the high binding ability of bisphenol a to human estrogen-related receptor-γ

Background: Various lines of evidence have shown that bisphenol A [BPA; HO-C₆H₄-C(CH₃)₂- C₆H₄-OH] acts as an endocrine disruptor when present in very low doses. We have recently demonstrated that BPA binds strongly to human estrogen-related receptor-γ (ERR-γ) in a binding assay using [³H]4-hydroxytamoxifen ([³H]4-OHT). We also demonstrated that BPA inhibits the deactivation activity of 4-OHT. Objectives: In the present study, we intended to obtain direct evidence that BPA interacts with ERR-γ as a strong binder, and also to clarify the structural requirements of BPA for its binding to ERR-γ. Methods: We examined [³H]BPA in the saturation binding assay using the ligand binding domain of ERR-γ and analyzed the result using Scatchard plot analysis. A number of BPA derivatives were tested in the competitive binding assay using [³H]BPA as a tracer and in the luciferase reporter gene assay. Results: [³H]BPA showed a K_D of 5.50 nM at a B_max of 14.4 nmol/mg. When we examined BPA derivatives to evaluate the structural essentials required for the binding of BPA to ERR-γ, we found that only one of the two phenol-hydroxyl groups was essential for the full binding. The maximal activity was attained when one of the methyl groups was removed. All of the potent BPA derivatives retained a high constitutive basal activity of ERR-γ in the luciferase reporter gene assay and exhibited a distinct inhibitory activity against 4-OHT. Conclusion: These results indicate that the phenol derivatives are potent candidates for the endocrine disruptor that binds to ERR-γ.