Mutational analysis of cysteine residues within the hormone-binding domain of the human estrogen receptor identifies mutants that are defective in both DNA-binding and subcellular distribution

Characterization of the promoter region of the human somatostatin receptor subtype 2 gene and localization of sequences required for estrogen-responsiveness

Somatostatin receptor (sst) subtype 2 mRNA is potently regulated by 17beta-estradiol (E2) in a time and dose-dependent manner in the estrogen receptor (+) human breast cancer cell line T47D. To explore the mechanism involved in E2 regulated expression of sst2, we isolated and characterized a genomic clone containing over 5.3 kilobase pairs (kb) of the 5' flanking region of the human sst2 gene. The 5'-flanking region lacks both TATA and CCAAT boxes. Primer extension and RNAase protection analysis revealed the existence of two transcriptional start sites, located within an initiator-like sequence, 85 and 82 bp upstream of the translational initiation methionine. The 5.3-kb 5'-flanking region was an active promoter in T47D cells but was inactive in MDA MB 435s cells, a human breast cancer cell line that does not express sst2. Deletion analysis identified both positive and negative regulatory elements within the 5.3-kb fragment. Furthermore, transcriptional regulation by E2 was mediated by a distal 1.5-kb fragment located 3.8 kb from the transcriptional start sites. Determining the precise ERE will provide important insights into the complex regulation of sst2 expression in normal and neoplastic tissue.

Stability of the ligand-estrogen receptor interaction depends on estrogen response element flanking sequences and cellular factors

To determine whether accessory proteins mediate the ligand- and DNA sequence-dependent specificity of estrogen receptor (ER) interaction with DNA, the binding of partly purified vs highly purified bovine ER to various estrogen response elements (EREs) was measured in the presence of different ER ligands. Partly purified estradiol-liganded ER (E2-ER) binds cooperatively to stereoaligned tandem EREs flanked by naturally occurring AT-rich sequences, with a stoichiometry of one E2-ER dimer per ERE. In contrast, highly purified E2-ER binds with a 10-fold lower affinity and non-cooperatively to EREs flanked by the AT-rich region. Moreover, the binding stoichiometry of highly purified E2-ER was 0.5 E2-ER dimer, or one monomer per ERE, independent of the ERE flanking sequence. Interestingly, the binding of ER liganded with the antiestrogen 4-hydroxytamoxifen (4-OHT-ER) was non-cooperative with an apparent stoichiometry of 0.5 4-OHT-ER dimer per ERE, regardless of ER purity or ERE flanking sequence. We recently showed that when 4-OHT-ER binds DNA, one molecule of 4-OHT dissociates from the dimeric 4-OHT-ER-ERE complex, accounting for the reduced apparent binding stoichiometry. In contrast, ER covalently bound by tamoxifen aziridine (TAz) gave an ERE binding stoichiometry of one TAz-ER dimer per ERE, and TAz-ER binds cooperatively to multiple AT-rich EREs, regardless of the purity of the receptor. We have obtained evidence that purification of ER removes an accessory protein(s) that interacts with ER in a sequence- and/or DNA conformational-dependent manner, resulting in stabilization of E2, but not 4-OHT, in the ligand binding domain when the receptor binds to DNA. We postulate that retention of ligand by ER maintains the receptor in a conformation necessary to achieve high-affinity, cooperative ERE binding.

Molecular cloning and characterization of Japanese eel estrogen receptor cDNA

A cDNA encoding the Japanese eel, Anguilla japonica, estrogen receptor (ER) was isolated and sequenced. This cDNA contains a complete open reading frame encoding 573 amino acid residues, and the molecular weight of this protein is calculated to be 63417. The amino acid sequence of the eel ER shows high homology of the DNA binding (80%) and ligand binding (55%) domains with those of other species. However, the other domains show greatly reduced homology (10-20%). When the cDNA was ligated to the pSVL vector and transfected into COS7 cells, a protein was produced that had high affinity for estradiol-17 beta (E2) and specifically bound estrogens. Northern analysis showed that three ER mRNAs with lengths of 5.6, 3.8 and 1.2 kb were expressed in eel liver. Their expression was E2 inducible, with the 5.6 kb mRNA being strongly dependent on E2 stimulation.

Site-directed estrogen receptor antibodies stabilize 4-hydroxytamoxifen ligand, but not estradiol, and indicate ligand-specific differences in the recognition of estrogen response element DNA in vitro

Conformational differences between type I antiestrogen-liganded estrogen receptor and estradiol (E2)-liganded estrogen receptor (ER) are thought to be responsible for differentiating agonist versus antagonist ER activity at individual genes. To examine the impact of ER ligand on estrogen-response element (ERE) binding kinetics and receptor conformation, we quantitated the effect of site-directed, ER-specific antibodies raised against synthetic peptides corresponding to the DNA-binding domain of human ER on ER-ERE binding in vitro. Although 4-hydroxytamoxifen-liganded-ER (4-OHT-ER) and E2-ER bind a consensus ERE with equal high affinity, the stoichiometry of 4-OHT-ER-ERE binding at saturation is approximately 50% lower than that of E2-ER binding to all ERE sequences tested. In contrast, the ERE binding stoichiometry of tamoxifen aziridine-liganded ER (TAz-ER) is identical to that of E2-ER: one receptor dimer bound per ERE. The difference in binding stoichiometry is caused by dissociation of one molecule of 4-OHT from the ER as the dimeric receptor binds DNA. Addition of low concentrations of ER-specific polyclonal antibodies AT3A or AT3B prevented 4-OHT ligand dissociation, yielding an increase in specific 4-OHT-ER-ERE binding to a level equal to that of E2-ER- or TAz-ER-ERE binding. However, higher amounts of AT3A or AT3B inhibited specific ERE binding of both 4-OHT- and E2-ER. We conclude that differences in ER conformation when liganded with 4-OHT versus E2 are revealed by these antibodies and that such differences in receptor conformation may influence subsequent interaction of the receptor with other proteins necessary for transactivation.

Dissociation of 4-hydroxytamoxifen, but not estradiol or tamoxifen aziridine, from the estrogen receptor as the receptor binds estrogen response element DNA

Estradiol-liganded estrogen receptor (E2-ER) binds EREs with a stoichiometry of one E2-ER dimer per estrogen response element (ERE). In contrast, although 4-hydroxytamoxifen (4-OHT)-liganded ER (4-OHT-ER) binds EREs with high affinity, its saturation ERE binding capacity is consistently half that of E2-ER, giving an apparent stoichiometry of one 4-OHT-ER monomer per ERE. Here we show that one molecule of 4-OHT ligand dissociates from the ER dimer apparently during the process of binding to DNA. Under equilibrium conditions, the type I antiestrogen tamoxifen aziridine (TAz), covalently attached to ER (TAz-ER), binds a single ERE with high affinity (Kd = 0.27 nM), comparable to that of E2-ER and 4-OHT-ER. In contrast to 4-OHT-ER, the ERE binding stoichiometry of TAz-ER was identical to that of E2-ER: one dimeric receptor per ERE. By measuring [3H]ligand that was initially bound to ER, a significant loss of [3H]4-OHT from ER was detected after ERE binding, whereas all [3H]E2 or [3H]TAz remained ER-bound. These results confirm that one molecule of 4-OHT ligand dissociates from the ER dimer as a consequence of ERE binding. Binding of 4-OHT and TAz are likely to induce a conformation in ER dimers that alters their capacity for gene activation. Upon ER binding to DNA, this conformation reveals itself by allowing 4-OHT dissociation, and predictably would allow TAz dissociation were it not bound covalently.

Binding properties of coumestrol to expressed human estrogen receptor

We have studied the binding of coumestrol, an inherently fluorescent analog of 17 beta-estradiol, to human estrogen receptor (hER) in extracts of transfected cultured cells. The binding of coumestrol to the hER as well as its dissociation from the receptor can be directly determined by the change in fluorescence intensity of the probe. In agreement with previous studies using calf uterine extracts, we find that coumestrol binds to the receptor with a ten-fold lower affinity than 17 beta-estradiol. However, the rate of dissociation appears to be close to that of the native ligand. Coumestrol can accept energy from Trp residues in the excited state and, using a C530W hER mutant, we have confirmed that residue 530 is close to the ligand-binding pocket. We also find that the presence of saturating amounts of the specific DNA binding sites (ERE) did not significantly alter the binding affinity of coumestrol to either wild type hER or the C530W mutant.