Selective estrogen receptor modulators (SERMs) mimic estrogen action in certain tissues while opposing it in others. The therapeutic effectiveness of SERMs such as tamoxifen and raloxifene in breast cancer depends on their antiestrogenic activity. In the uterus, however, tamoxifen is estrogenic. Here, we show that both tamoxifen and raloxifene induce the recruitment of corepressors to target gene promoters in mammary cells. In endometrial cells, tamoxifen, but not raloxifene, acts like estrogen by stimulating the recruitment of coactivators to a subset of genes. The estrogen-like activity of tamoxifen in the uterus requires a high level of steroid receptor coactivator 1 (SRC-1) expression. Thus cell type- and promoter-specific differences in coregulator recruitment determine the cellular response to SERMs.

https://science.sciencemag.org/content/sci/295/5564/2465.full.pdf

Molecular determinants for the tissue specificity of SERMs.

I. Introduction II. Nuclear Receptors: General Concepts A. Anatomy of nuclear receptors B. Mechanisms of action III. Orphan Nuclear Receptors A. Definition B. Nomenclature C. Structural and functional diversity IV. Novel Hormone Response Systems: RXR and Its Heterodimeric Partners A. RXR: rexinoids B. PPAR: multiple ligands, multiple functions C. PXR: pregnanes, xenobiotic compounds, and benzoate derivatives D. CAR (constitutive androstane receptor): androstanes and phenobarbital E. LXR: control of cholesterol metabolism by oxysterols F. FXR: bile acids receptor V. Orphans in Search of a Home A. HNF4: diabetes and possible regulation by acyl-coenzyme A (CoA) thioesters B. FTZ-F1: steroidogenesis and sexual development C. Rev-Erb: singular members of the superfamily D. ROR: neuron development and T cell selection E. TR2: the testis receptors F. TLX: forebrain development G. COUP-TF: neurogenesis, angiogenesis, and heart development H. ERR: placenta development and control of lipid metabolism I. NGFI-B: hyp...

/pdf/orphan-nuclear-receptors-from-gene-to-function-28jgs4505b.pdf

Orphan nuclear receptors: from gene to function.

Mixed antiestrogens, such as 4-hydroxytamoxifen (4HT), act as either partial agonists or antagonists of estrogen receptor (ER) function in a tissue-, cell-, and promoter-specific manner, suggesting that intracellular factors modulate their ability to regulate transcription. To determine whether coactivators and corepressors have the capacity to modulate the relative agonist/antagonist activity of 4HT, ER-dependent gene expression was measured in the absence or presence of expression vectors for SRC-1 (steroid receptor coactivator-1) or SMRT (silencing mediator of retinoic acid and thyroid hormone receptors). In Hep G2 cells in which 4HT is an agonist, exogenous SRC-1 enhanced estradiol (E2)- and 4HT-stimulated transcription in a dose-dependent manner, while SMRT overexpression strongly reduced basal and 4HT-stimulated gene expression with no effect on E2 activity. These observations were not cell- or promoter-specific inasmuch as similar results were obtained in HeLa cells under conditions in which 4HT is an antagonist. A protein-protein interaction assay indicated that the full-length ER binds to SMRT in vitro. To assess whether relative coactivator and corepressor expression within a given cell could modulate the balance of 4HT agonist/antagonist activity, SRC-1 and SMRT were coexpressed. SMRT overexpression blocked SRC-1 coactivation of 4HT-stimulated gene expression and preferentially inhibited 4HT agonist activity whether or not exogenous SRC-1 was present. The cumulative data in this model system indicate that the relative expression of coactivators and corepressors can modulate 4HT regulation of ER transcriptional activity and suggest they could contribute to the tissue-specific ability of mixed antiestrogens to activate or inhibit ER-mediated gene expression.

/pdf/coactivator-and-corepressor-regulation-of-the-agonist-4523jc821y.pdf

Coactivator and corepressor regulation of the agonist/antagonist activity of the mixed antiestrogen, 4-hydroxytamoxifen

Evaluation of Chemicals with Endocrine Modulating Activity in a Yeast-Based Steroid Hormone Receptor Gene Transcription Assay

The endocrine pancreas is organized into clusters of cells called islets of Langerhans comprising four well-defined cell types: alpha beta, delta and PP cells. While recent genetic studies indicate that islet development depends on the function of an integrated network of transcription factors, the specific roles of these factors in early cell-type specification and differentiation remain elusive. Nkx2.2 is a member of the mammalian NK2 homeobox transcription factor family that is expressed in the ventral CNS and the pancreas. Within the pancreas, we demonstrate that Nkx2.2 is expressed in alpha, beta and PP cells, but not in delta cells. In addition, we show that mice homozygous for a null mutation of Nkx2.2 develop severe hyperglycemia and die shortly after birth. Immunohistochemical analysis reveals that the mutant embryos lack insulin-producing beta cells and have fewer glucagon-producing alpha cells and PP cells. Remarkably, in the mutants there remains a large population of islet cells that do not produce any of the four endocrine hormones. These cells express some beta cell markers, such as islet amyloid polypeptide and Pdx1, but lack other definitive beta cell markers including glucose transporter 2 and Nkx6.1. We propose that Nkx2.2 is required for the final differentiation of pancreatic beta cells, and in its absence, beta cells are trapped in an incompletely differentiated state.

Mice lacking the homeodomain transcription factor Nkx2.2 have diabetes due to arrested differentiation of pancreatic beta cells

The estrogen receptor (ER) is a transcription factor involved in steroid hormone signal transduction in higher eukaryotes The receptor also functions as a ligand-dependent transcriptional activator when introduced into Saccharomyces cerevisiae (baker's yeast), which suggests that at least some of the components of the signal transduction pathway are conserved between yeast and mammalian cells, and, moreover, allows the possibility of using this simple eukaryotic organism to dissect receptor function However, whether the ER actually activates transcription in a mechanistically similar fashion in yeast and mammalian cells is unclear, since it has been reported that the transactivation function within the hormone binding domain (TAF-2) does not function in yeast In this report, we have characterized the activity of the transactivation functions of the ER in yeast Our results indicate that both TAF-2 and the N-terminal transactivation region (TAF-1) are functional in yeast and contribute synergistically t

/pdf/ligand-dependent-and-independent-function-of-the-3qubok5gof.pdf

Ligand-dependent and -independent function of the transactivation regions of the human estrogen receptor in yeast.

The estrogen receptor (ER) is a transcription factor involved in steroid hormone signal transduction in higher eukaryotes. The receptor also functions as a ligand-dependent transcriptional activator when introduced into Saccharomyces cerevisiae (baker's yeast), which suggests that at least some of the components of the signal transduction pathway are conserved between yeast and mammalian cells, and, moreover, allows the possibility of using this simple eukaryotic organism to dissect receptor function. However, whether the ER actually activates transcription in a mechanistically similar fashion in yeast and mammalian cells is unclear, since it has been reported that the transactivation function within the hormone binding domain (TAF-2) does not function in yeast. In this report, we have characterized the activity of the transactivation functions of the ER in yeast. Our results indicate that both TAF-2 and the N-terminal transactivation region (TAF-1) are functional in yeast and contribute synergistically to the receptor's total activity. These results are consistent with those obtained in mammalian cells. Furthermore, we show that in yeast the antagonistic effects of the antiestrogen nafoxidine arise from a modulation of the synergistic interactions of TAF-1 and TAF-2, and not simply from an inactivation of TAF-2 by antihormone. Finally, we characterize the effect of ER deletion mutants on chromatin structure in yeast. Our data lend support to the view that the formation of competent transcriptional initiation complexes requires a precise disruption of chromatin structure.

and -Independent Function of the Transactivation Regions of the Human Estrogen Receptor in Yeast

Band-shifting and DNase I-footprinting assays have been used to study the trans-acting factor(s) binding to an important promoter element (-53 to -46 relative to the transcription start) of the rat insulin II gene. A binding activity which footprints a region between -60 and -40 was found in both HIT, a hamster insulinoma cell line, and HeLa cells. A mutation within this region which drastically decreases promoter activity in vivo also greatly reduces binding activity in vitro. This binding activity was purified from HeLa cells and identified by competition and renaturation analyses as being the same as the COUP (chicken ovalbumin upstream promoter) transcription factor, a DNA-binding protein required for efficient transcription of the ovalbumin gene in vitro. Interestingly, the binding sequences of the COUP transcription factor in the ovalbumin and the insulin promoters have only limited similarities.

The COUP transcription factor binds to an upstream promoter element of the rat insulin II gene.

The 5'-flanking region of the rat insulin II gene (-448 to +50) is sufficient for tissue-specific expression. To further determine the tissue-specific cis-acting element(s), important sequences defined by linker-scanning mutagenesis were placed upstream of a heterologous promoter and transfected into insulin-producing and -nonproducing cells. Rat insulin promoter element 3 (RIPE3), which spans from -125 to -86, was shown to confer beta-cell-specific expression in either orientation. However, two subregions of RIPE3, RIPE3a and RIPE3b (defined by linker-scanning mutations), displayed only marginal activities. These results suggest that the two subregions cooperate to confer tissue specificity, presumably via their cognate binding factors.

Young-Ping Hwung

Papers

Ligand-dependent and -independent function of the transactivation regions of the human estrogen receptor in yeast.

and -Independent Function of the Transactivation Regions of the Human Estrogen Receptor in Yeast

The COUP transcription factor binds to an upstream promoter element of the rat insulin II gene.

Cooperativity of sequence elements mediates tissue specificity of the rat insulin II gene.

Transactivation functions facilitate the disruption of chromatin structure by estrogen receptor derivatives in vivo