The N-terminal transactivation domain of rat estrogen receptor is localized in a hydrophobic domain of eighty amino acids

A yeast assay based on the gilthead sea bream (teleost fish) estrogen receptor beta for monitoring estrogen mimics

A yeast (Saccharomyces cerevisiae)-based assay was developed and tested with steroids and chemicals (mostly pesticides). The induction of beta-galactosidase activity was strictly dependent on the presence of seabream (Sparus aurata) betaa estrogen receptor (sbERbetaa) and substances known to have estrogenic activity. 17beta-Estradiol (E(2)) and diethylstilbestrol (DES), both agonists, were most active and the antagonist tamoxifen (TAM) was 14-fold less active than E(2). Among the chemicals tested bisphenol-A was most active, followed by pentachlorophenol and naphthalene. Ligand-binding assays with recombinant sbERbetaa and sbERalpha revealed that sbERbetaa binds E(2) with 6.5-fold higher affinity than sbERalpha, confirming the selection of a high sensitive receptor for the yeast assay. DES, ICI 182,780, estrone and TAM had higher relative binding affinity to E2 in sbERalpha than sbERbetaa, although there was no difference in IC50 for these steroids between the two receptors. These results reveal the usefulness of using the yeast-based receptor assay for detecting chemical interaction with steroid receptors from contaminated samples.

Surprises with antiprogestins: novel mechanisms of progesterone receptor action

When hormone antagonists have inappropriate agonist-like effects, the clinical consequences are grave. We describe novel molecular mechanisms by which antiprogestin-occupied progesterone receptors behave like agonists. These mechanisms include agonist-like transcriptional effects that do not require receptor binding to DNA at progesterone response elements, or that result from cross-talk between progesterone receptors and other signalling pathways. We discuss the complex structural organization of progesterone receptors, and demonstrate that the B receptor isoform has a unique third activation domain that may confer agonist-like properties in the presence of antiprogestins, whereas the A receptor isoform is a dominant-negative inhibitor. We argue that these novel mechanisms play a role in the apparent hormone resistance of breast cancers and the variable tissue-specific responses to antagonists.

Colocalization and ligand-dependent discrete distribution of the estrogen receptor (ER)alpha and ERbeta

To investigate the relationships between the loci expressing functions of estrogen receptor (ER)alpha and that of ERbeta, we analyzed the subnuclear distribution of ERalpha and ERbeta in response to ligand in single living cells using fusion proteins labeled with different spectral variants of green fluorescent protein. Upon activation with ligand treatment, fluorescent protein-tagged (FP)-ERbeta redistributed from a diffuse to discrete pattern within the nucleus, showing a similar time course as FP-ERalpha, and colocalized with FP-ERalpha in the same discrete cluster. Analysis using deletion mutants of ERalpha suggested that the ligand-dependent redistribution of ERalpha might occur through a large part of the receptor including at least the latter part of activation function (AF)-1, the DNA binding domain, nuclear matrix binding domain, and AF-2/ligand binding domain. In addition, a single AF-1 region within ERalpha homodimer, or a single DNA binding domain as well as AF-1 region within the ERalpha/ERbeta heterodimer, could be sufficient for the cluster formation. More than half of the discrete clusters of FP-ERalpha and FP-ERbeta were colocalized with hyperacetylated histone H4 and a component of the chromatin remodeling complex, Brg-1, indicating that ERs clusters might be involved in structural changes of chromatin.

Jun dimerization protein 2 functions as a progesterone receptor N-terminal domain coactivator

The progesterone receptor (PR) contains two transcription activation function (AF) domains, constitutive AF-1 in the N terminus and AF-2 in the C terminus. AF-2 activity is mediated by a hormone-dependent interaction with a family of steroid receptor coactivators (SRCs). SRC-1 can also stimulate AF-1 activity through a secondary domain that interacts simultaneously with the primary AF-2 interaction site. Other protein interactions and mechanisms that mediate AF-1 activity are not well defined. By interaction cloning, we identified an AP-1 family member, Jun dimerization protein 2 (JDP-2), as a novel PR-interacting protein. JDP-2 was first defined as a c-Jun interacting protein that functions as an AP-1 repressor. PR and JDP-2 interact directly in vitro through the DNA binding domain (DBD) of PR and the basic leucine zipper (bZIP) region of JDP-2. The two proteins also physically associate in mammalian cells, as detected by coimmunoprecipitation, and are recruited in vivo to a progesterone-inducible target gene promoter, as detected by a chromatin immunoprecipitation (ChIP) assay. In cell transfection assays, JDP-2 substantially increased hormone-dependent PR-mediated transactivation and worked primarily by stimulating AF-1 activity. JDP-2 is a substantially stronger coactivator of AF-1 than SRC-1 and stimulates AF-1 independent of SRC-1 pathways. The PR DBD is necessary but not sufficient for JDP-2 stimulation of PR activity; the DBD and AF-1 are required together. JDP-2 lacks an intrinsic activation domain and makes direct protein interactions with other coactivators, including CBP and p300 CBP-associated factor (pCAF), but not with SRCs. These results indicate that JDP-2 stimulates AF-1 activity by the novel mechanism of docking to the DBD and recruiting or stabilizing N-terminal PR interactions with other general coactivators. JDP-2 has preferential activity on PR among the nuclear receptors tested and is expressed in progesterone target cells and tissues, suggesting that it has a physiological role in PR function.

The role of coactivators and corepressors in the biology and mechanism of action of steroid hormone receptors

Steroid hormone receptors are members of a superfamily of ligand-dependent transcription factors. As such they have a DNA binding domain that recognizes specific target gene sequences along with separate transcriptional activation domains. What sets steroid hormone receptors (and other nuclear hormone receptors) apart from other families of sequence specific transcriptional activators is the presence of a ligand binding domain (LBD) that acts as a molecular switch to turn on transcriptional activity when a hormonal ligand induces a conformational change in the receptor. Upon binding hormone, steroid receptors recruit a novel coactivator protein complex with an essential role in receptor-mediated transcriptional activation. Coactivators function as adaptors in a signaling pathway that transmits transcriptional responses from the DNA bound receptor to the basal transcriptional machinery. Hormone agonists induce a conformational change in the carboxyl-terminal transcriptional activation domain, AF-2, that creates a new protein interaction site on the surface of the LBD that is recognized by LXXLL motifs in the p160 family of coactivators. In contrast, steroid antagonists such as the antiestrogen tamoxifen for the estrogen receptor induce an alternate conformation in AF-2 that occludes the coactivator binding site and recruits corepressors that can actively silence steroid responsive genes. Thus, the cellular availability of coactivators and corepressors is an important determinant in the biological response to both steroid hormone agonists and antagonists. This paper provides an update on the properties and mechanism of action of nuclear receptor coactivators, the nature of the coactivator-binding site, and the structural and mechanistic basis for ligand-dependent binding of coactivators to receptors.

The estrogen receptor enhances AP-1 activity by two distinct mechanisms with different requirements for receptor transactivation functions

Estrogen receptors (ERs alpha and beta) enhance transcription in response to estrogens by binding to estrogen response elements (EREs) within target genes and utilizing transactivation functions (AF-1 and AF-2) to recruit p160 coactivator proteins. The ERs also enhance transcription in response to estrogens and antiestrogens by modulating the activity of the AP-1 protein complex. Here, we examine the role of AF-1 and AF-2 in ER action at AP-1 sites. Estrogen responses at AP-1 sites require the integrity of the ERalpha AF-1 and AF-2 activation surfaces and the complementary surfaces on the p160 coactivator GRIP1 (glucocorticoid receptor interacting protein 1), the NID/AF-1 region, and NR boxes. Thus, estrogen-liganded ERalpha utilizes the same protein-protein contacts to transactivate at EREs and AP-1 sites. In contrast, antiestrogen responses are strongly inhibited by ERalpha AF-1 and weakly inhibited by AF-2. Indeed, ERalpha truncations that lack AF-1 enhance AP-1 activity in the presence of antiestrogens, but not estrogens. This phenotype resembles ERbeta, which naturally lacks constitutive AF-1 activity. We conclude that the ERs enhance AP-1 responsive transcription by distinct mechanisms with different requirements for ER transactivation functions. We suggest that estrogen-liganded ER enhances AP-1 activity via interactions with p160s and speculate that antiestrogen-liganded ER enhances AP-1 activity via interactions with corepressors.

A comparison of transcriptional activation by ER alpha and ER beta

We have compared the ability of ER alpha and ER beta to stimulate transcription from a number of reporter genes in different cell lines and demonstrate that the activity of AF1 in ER beta is negligible compared with that of ER alpha on ERE based reporters. The activity of AF2 in ER alpha and ER beta is similar and this is likely to reflect their similar ability to bind coactivators. As a consequence, when transcription from a gene depends on both AF1 and AF2 the activity of ER alpha greatly exceeds that of ER beta but when AF1 is not required ER alpha and ER beta have similar transcriptional activities.

Two transcription activation functions in the amino terminus of the mouse estrogen receptor that are affected by the carboxy terminus

To determine the characteristics of the N-terminal transactivation domain (AF-1) of the mouse estrogen receptor (ER), we constructed a number of deletion mutants. Wild-type and mutant receptors were expressed in yeast cells and assayed for their ability to transactivate an estrogen-responsive reporter plasmid (ERE-CYCl-LacZ) that contained a single estrogen response element of the vitellogenin A2 gene promoter. Deletion of the N-terminal 121 amino acids from the mouse ER resulted in a 50% reduction in transactivation activity compared with the full-length wild-type ER. Deletion of the first 150 amino acids resulted in loss of 90% transactivation activity. An ER deletion mutant lacking residues 121-154 retained full transcriptional activity, suggesting that this region plays a significant transacting role only when the first portion is deleted. A point mutation was introduced in the C-terminal region at Met-521 in order to study the possible interaction between the C-terminal ligand-binding domain and the N-terminal AF-1 region. This mutant ER, M521G, exhibited 150% of the transcriptional activity of the wild-type ER. An M521G mutant lacking the N-terminal 121 amino acids retained full transactivation activity, whereas, M521G lacking 150 amino acids resulted in only 10% of wild-type activity. These results suggest that residues 121-154 might interact with the C terminus to affect transcription. In summary, multiple N-terminal regions in the ER were identified that function in transactivation. Furthermore, a point mutation in the C-terminal portion of the ER may change the conformation of the ER ligand-binding domain, producing a more stable receptor/ligand complex that increases transcriptional activity. These data suggest that the N- and C-terminal portions of the ER interact in a cooperative manner to activate transcription from target genes.

An N-terminal deletion mutant of estrogen receptor exhibits increased synergism with upstream activators and enhanced binding to the estrogen response element

To study the role of the N-terminal region of the estrogen receptor (ER) in transcription activation and in DNA binding, we constructed a mutant of the Xenopus laevis ER which lacks amino acids 1-159 (XER160/586). In transient transfections, XER160/586 exhibited < 10% of the activity of wild-type XER on a synthetic promoter containing two estrogen response elements (EREs). To examine transcriptional synergism by XER and by XER160/586, we determined the activity of promoters containing EREs and binding sites for either the vitellogenin activator, NF1, or AP1 upstream activator protein. For the three promoters transcription by XER was 2.8-fold greater than expected for additive activities, and transcription by XER160/586 was 6.2-fold greater. These data demonstrate that an upstream activator protein bound near the promoter can partially compensate for the loss of the internal N-terminal (AF1) transactivation domain in XER160/586. Using a promoter interference assay to study the intracellular interaction between ER and the estrogen response element, we found that XER160/586 exhibited a significant increase in affinity for the ERE. Its low basal activity and enhanced affinity for the ERE make XER160/586 an effective dominant negative mutant. When co-expressed with wild-type XER at 1:1 and 5:1 ratios, XER160/586 suppressed the activity of wild-type XER by 57% and > 80%, respectively.