title: Exploration of dimensions of estrogen potency: parsing ligand binding and coactivator binding affinities.
abstract: The estrogen receptors, ERα and ERβ, are ligand-regulated transcription factors that control gene expression programs in target tissues. The molecular events underlying estrogen action involve minimally two steps, hormone binding to the ER ligand-binding domain followed by coactivator recruitment to the ER·ligand complex; this ligand·receptor·coactivator triple complex then alters gene expression. Conceptually, the potency of an estrogen in activating a cellular response should reflect the affinities that characterize both steps involved in the assembly of the active ligand·receptor·coactivator complex. Thus, to better understand the molecular basis of estrogen potency, we developed a completely in vitro system (using radiometric and time-resolved FRET assays) to quantify independently three parameters: (a) the affinity of ligand binding to ER, (b) the affinity of coactivator binding to the ER·ligand complex, and (c) the potency of ligand recruitment of coactivator. We used this system to characterize the binding and potency of 12 estrogens with both ERα and ERβ. Some ligands showed good correlations between ligand binding affinity, coactivator binding affinity, and coactivator recruitment potency with both ERs, whereas others showed correlations with only one ER subtype or displayed discordant coactivator recruitment potencies. When ligands with low receptor binding affinity but high coactivator recruitment potencies to ERβ were evaluated in cell-based assays, elevation of cellular coactivator levels significantly and selectively improved their potency. Collectively, our results indicate that some low affinity estrogens may elicit greater cellular responses in those target cells that express higher levels of specific coactivators capable of binding to their ER complexes with high affinity.
authors.affiliation: Department of Chemistry, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: M
authors.initials: M
authors.lastname: Jeyakumar
authors.forename: Kathryn E
authors.initials: KE
authors.lastname: Carlson
authors.forename: Jillian R
authors.initials: JR
authors.lastname: Gunther
authors.forename: John A
authors.initials: JA
authors.lastname: Katzenellenbogen
mesh_terms: D002460:Cell Line; D047628:Estrogen Receptor alpha / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D047629:Estrogen Receptor beta / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D004967:Estrogens / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D006801:Humans; D008024:Ligands; D056918:Nuclear Receptor Coactivators / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary
chemical_list: C506487:ESR1 protein, human; D047628:Estrogen Receptor alpha; D047629:Estrogen Receptor beta; D004967:Estrogens; D008024:Ligands; C455500:NCOA7 protein, human; D056918:Nuclear Receptor Coactivators