Interactions of synthetic estrogens with human estrogen receptors.

Synthetic estrogens have diverse chemical structures and may either positively or negatively affect the estrogenic signaling pathways through interactions with the estrogen receptors (ERs). Modeling studies suggest that 4-(1-adamantyl)phenol (AdP) and 4,4'-(1,3-adamantanediyl)diphenol (AdDP) can bind in the ligand binding site of ERalpha. We used fluorescence polarization (FP) to compare the binding affinities of AdP, AdDP and 2-(1-adamantyl)-4-methylphenol (AdMP) for human ERalpha and ERbeta with the binding affinities of the known ER ligands, diethylstilbestrol (DES) and 4hydroxytamoxifen (4OHT). Competition binding experiments show that AdDP has greater affinity for both ERs than does AdP, while AdMP does not bind the receptor proteins. The relative binding affinities of AdDP and AdP are weaker than the affinity of DES or 4OHT for both ERs with the exception of AdDP, which binds ERbeta with higher affinity than does 4OHT. We also found that AdDP and AdP cause differential conformational changes in ERalpha and ERbeta, which result in altered affinities of the ERs for fluorescein-labeled estrogen response elements (EREs) using a direct binding FP assay. The results show that ERbeta liganded with either AdDP or AdP has greater affinity for human pS2 ERE than the ERbeta-4OHT complex. The data suggest that synthetic molecules like adamantanes may function as biologically active ligands for human ERs. This demonstrates the importance of considering the potential of novel classes of synthetic compounds as selective ER modulators.

Interactions of dietary estrogens with human estrogen receptors and the effect on estrogen receptor-estrogen response element complex formation.

Epidemiologic and experimental studies support the hypothesis that dietary estrogens from plant sources (phytoestrogens) may play a role in the prevention of breast and prostate cancer. The molecular mechanisms for such chemopreventive effect are still unclear. We investigated the possibility that phytoestrogens may bind differentially to estrogen receptor proteins (ER[alpha] and ERss) and affect the interactions of the ligand-ER complexes with different estrogen response element (ERE) sequences. We used fluorescence polarization to measure the binding affinities of genistein, coumestrol, daidzein, glyceollin, and zearalenone for human ER[alpha] and ERss. Competition binding experiments revealed higher affinity of the phytoestrogens for ERss than for ER[alpha]. Genistein [median inhibitory concentration 12nM] is the most potent and has the same relative binding affinity for ERss as 17ss-estradiol. We also studied the effect of these phytoestrogens on the ability of ER[alpha] and ERss to associate with specific DNA sequences (EREs). The direct binding of human recombinant estrogen receptors to fluorescein-labeled EREs indicates that phytoestrogens can cause conformational changes in both human ERs, which results in altered affinities of the complexes for the ERE from the Xenopus vitellogenin A2 gene and an ERE from the human pS2 gene.