BackgroundVarious 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.ObjectivesIn 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.MethodsWe 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 KD of 5.50 nM at a Bmax 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.ConclusionThese results indicate that the phenol derivatives are potent candidates for the endocrine disruptor that binds to ERR-γ.

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Many lines of evidence reveal that bisphenol A (BPA) functions at very low doses as an endocrine disruptor. The human estrogen-related receptor gamma (ERR gamma) behaves as a constitutive activator of transcription, although the endogenous ligand is unknown. We have recently demonstrated that BPA binds strongly to ERR gamma (K(D) = 5.5 nM), but not to the estrogen receptor (ER). BPA preserves the ERR gamma's basal constitutive activity, and protects the selective ER modulator 4-hydroxytamoxifen from its deactivation of ERR gamma. In order to shed light on a molecular mechanism, we carried out the X-ray analysis of crystal structure of the ERR gamma ligand-binding domain (LBD) complexed with BPA. BPA binds to the receptor cavity without changing any internal structures of the pocket of the ERR gamma-LBD apo form. The hydrogen bonds of two phenol-hydroxyl groups, one with both Glu275 and Arg316, the other with Asn346, anchor BPA in the pocket, and surrounding hydrophobic bonds, especially with Tyr326, complete BPA's strong binding. Maintaining the 'activation helix' (helix 12) in an active conformation would as a result preserve receptor constitutive activity. Our results present the first evidence that the nuclear receptor forms complexes with the endocrine disruptor, providing detailed molecular insight into the interaction features.

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