Comparison of tamoxifen ligands on estrogen receptor interaction with estrogen response elements

• Published in: Molecular and cellular endocrinology 1998-08, Vol.143 (1), p.79-90
• Main Authors: Klinge, Carolyn M., Studinski-Jones, April L., Kulakosky, Peter C., Bambara, Robert A., Hilf, Russell
• Format: Article
• Language: English
• Subjects: 4-Hydroxytamoxifen; Animals; Cattle; DNA - genetics; DNA - metabolism; Estrogen Antagonists - metabolism; Estrogen Antagonists - pharmacology; Estrogen receptor; Estrogen response element (calf uterus); Estrogens - metabolism; Estrogens - pharmacology; Humans; Ligands; Radioligand Assay; Receptors, Estrogen - antagonists & inhibitors; Receptors, Estrogen - metabolism; Recombinant Proteins - metabolism; Tamoxifen; Tamoxifen - metabolism; Tamoxifen - pharmacology; Tamoxifen aziridine
• ISSN: 0303-7207; 1872-8057
• DOI: 10.1016/S0303-7207(98)00130-0

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 with a stoichiometry of one E 2-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 E 2-ER, 4-OHT-ER, or ER liganded by the covalent antiestrogen tamoxifen aziridine. Unlike E 2-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 E 2 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.