COMPARISON OF TAMOXIFEN LIGANDS ON ESTROGEN-RECEPTOR INTERACTION WITHESTROGEN RESPONSE ELEMENTS

The estrogen receptor (ER) is a ligand-activated transcription factor that binds to specific DNA sequences, estrogen response elements (EREs ). Estradiol-liganded ER (E-2-ER) binds cooperatively to stereoaligned EREs that are surrounded by naturally-occurring AT-rich sequences wit h a stoichiometry of one E-2-ER dimer per ERE. When ER is bound by 4-h ydroxytamoxifen (4-OHT), the active metabolite of the widely used ther apeutic 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 [H-3]4-OHT molecule per ERE. To determine whet her DNA-binding induced ligand dissociation is a general property of t ype I antiestrogens that are not covalently attached to the ER, we exa mined the interaction of ER liganded by tamoxifen (TAM) with EREs. We demonstrate that TAM-ER binds EREs with lower affinity than E-2-ER, 4- OHT-ER, or ER liganded by the covalent antiestrogen tamoxifen aziridin e. Unlike E-2-ER, both TAM and 4-OHT-ER bind EREs non-cooperatively. L ike 4-OHT, TAM appears to dissociate from the liganded ER as the recep tor binds EREs. Additionally, partial proteolysis of ERE-bound ER by t rypsin revealed different cleavage patterns for E-2 versus 4-OHT and T AM. These findings indicate that the behavior of the ER liganded by TA M is generally similar to that of the antiestrogen 4-OHT. (C) 1998 Els evier Science Ireland Ltd. All rights reserved.