title: Comparison of tamoxifen ligands on estrogen receptor interaction with estrogen response elements.
abstract: The estrogen receptor (ER) is a ligand-activated transcription factor that binds to specific DNA sequences, estrogen response elements (EREs). Estradiol-liganded ER (E2-ER) binds cooperatively to stereoaligned EREs that are surrounded by naturally-occurring AT-rich sequences with a stoichiometry of one E2-ER dimer per ERE. When ER is bound by 4-hydroxytamoxifen (4-OHT), the active metabolite of the widely used therapeutic antiestrogen tamoxifen (TAM), the receptor binds to EREs with high affinity. However, one molecule of 4-OHT ligand dissociates from the ER dimer apparently during the process of binding to DNA, yielding a stoichiometry of one [3H]4-OHT molecule per ERE. To determine whether DNA-binding induced ligand dissociation is a general property of type I antiestrogens that are not covalently attached to the ER, we examined the interaction of ER liganded by tamoxifen (TAM) with EREs. We demonstrate that TAM-ER binds EREs with lower affinity than E2-ER, 4-OHT-ER, or ER liganded by the covalent antiestrogen tamoxifen aziridine. Unlike E2-ER, both TAM and 4-OHT-ER bind EREs non-cooperatively. Like 4-OHT, TAM appears to dissociate from the liganded ER as the receptor binds EREs. Additionally, partial proteolysis of ERE-bound ER by trypsin revealed different cleavage patterns for E2 versus 4-OHT and TAM. These findings indicate that the behavior of the ER liganded by TAM is generally similar to that of the antiestrogen 4-OHT.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Louisville School of Medicine, KY 40292, USA. cmklin01@ulkyvm.louisville.edu
authors.forename: C M
authors.initials: CM
authors.lastname: Klinge
authors.forename: A L
authors.initials: AL
authors.lastname: Studinski-Jones
authors.forename: P C
authors.initials: PC
authors.lastname: Kulakosky
authors.forename: R A
authors.initials: RA
authors.lastname: Bambara
authors.forename: R
authors.initials: R
authors.lastname: Hilf
mesh_terms: D000818:Animals; D002417:Cattle; D004247:DNA / Q000235:genetics / Q000378:metabolism*; D004965:Estrogen Antagonists / Q000378:metabolism / Q000494:pharmacology*; D004967:Estrogens / Q000378:metabolism / Q000494:pharmacology; D006801:Humans; D008024:Ligands; D011869:Radioligand Assay; D011960:Receptors, Estrogen / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D013629:Tamoxifen / Q000378:metabolism / Q000494:pharmacology*
chemical_list: D004965:Estrogen Antagonists; D004967:Estrogens; D008024:Ligands; D011960:Receptors, Estrogen; D011994:Recombinant Proteins; D013629:Tamoxifen; D004247:DNA