3D Models of MBP, a Biologically Active Metabolite of Bisphenol A, in Human Estrogen Receptor α and Estrogen Receptor β

Bisphenol A [BPA] is a widely dispersed environmental chemical that is of much concern because the BPA monomer is a weak transcriptional activator of human estrogen receptor alpha [ER alpha] and ER beta in cell culture. A BPA metabolite, 4-methyl-2,4-bis(4-hydroxyphenyl) pent-1-ene [MBP], has transcriptional activity at nM concentrations, which is 1000-fold lower than the concentration for estrogenic activity of BPA, suggesting that MBP may be an environmental estrogen. To investigate the structural basis for the activity of MBP at nM concentrations and the lower activity of BPA for human ER alpha and ER beta, we constructed 3D models of human ER alpha and ER beta with MBP and BPA for comparison with estradiol in these ERs. These 3D models suggest that MBP, but not BPA, has key contacts with amino acids in human ERa and ERb that are important in binding of estradiol by these receptors. Metabolism of BPA to MBP increases the spacing between two phenolic rings, resulting in contacts between MBP and ER alpha and ER beta that mimic those of estradiol with these ERs. Mutagenesis of residues on these ERs that contact the phenolic hydroxyls will provide a test for our 3D models. Other environmental chemicals containing two appropriately spaced phenolic rings and an aliphatic spacer instead of an estrogenic B and C ring also may bind to ERa or ERb and interfere with normal estrogen physiology. This analysis also may be useful in designing novel chemicals for regulating the actions of human ER alpha and ER beta.