Distinction of the binding modes for human nuclear receptor ERR  between bisphenol A and 4-hydroxytamoxifen

Bisphenol A (BPA) strongly binds to human estrogen-related receptor gamma (ERRgamma). BPA is an oestrogenic endocrine disruptor that influences various physiological functions at very low doses. BPA functions as an inverse-type antagonist of ERRgamma to retain its high basal constitutive activity by inhibiting the deactivating inverse agonist activity of 4-hydroxytamoxifen (4-OHT). We recently demonstrated that ERRgamma receptor residues Glu275 and Arg316 function as the intrinsic binding site of BPA's phenol-hydroxyl group. We also determined the chief importance of phenol-hydroxyl<-->Arg316 hydrogen bonding and the corroborative role of phenol-hydroxyl<-->Glu275 hydrogen bonding. However, there appeared to be a distinct difference between the receptor binding modes of BPA and 4-OHT. In the present study, using tritium-labelled or non-labelled BPA and 4-OHT, we evaluated in detail the receptor binding capabilities of wild-type ERRgamma and its mutants with amino acid alterations at positions 275 and 316. Both compounds exhibited a strong binding ability to wild-type ERRgamma due to the hydrogen bonding to Glu275 and Arg316. However, 4-OHT revealed significantly reduced occupancy for both wild-type and mutant receptors. The data obtained suggest that 4-OHT barely binds to ERRgamma due to the strong ability of Glu275 and Arg316 to recruit phenol compounds.