Nuclear estrogen receptor molecular heterogeneity in the mouse uterus

Estrogen Receptor Alpha Splice Variants, Post-Translational Modifications, and Their Physiological Functions

The importance of estrogenic signaling for a broad spectrum of biological processes, including reproduction, cancer development, energy metabolism, memory and learning, and so on, has been well documented. Among reported estrogen receptors, estrogen receptor alpha (ERα) has been known to be a major mediator of cellular estrogenic signaling. Accumulating evidence has shown that the regulations of ERα gene transcription, splicing, and expression across the tissues are highly complex. The ERα promoter region is composed of multiple leader exons and 5′-untranslated region (5′-UTR) exons. Differential splicing results in multiple ERα proteins with different molecular weights and functional domains. Furthermore, various post-translational modifications (PTMs) further impact ERα cellular localization, ligand affinity, and therefore functionality. These splicing isoforms and PTMs are differentially expressed in a tissue-specific manner, mediate certain aspects of ERα signaling, and may work even antagonistically against the full-length ERα. The fundamental understanding of the ERα splicing isoforms in normal physiology is limited and association studies of the splicing isoforms and the PTMs are scarce. This review aims to summarize the functional diversity of these ERα variants and the PTMs in normal physiological processes, particularly as studied in transgenic mouse models.

Hypomorphism of a Novel Long ERα Isoform Causes Severe Reproductive Dysfunctions in Female Mice

Estrogen receptor alpha (ERα)-mediated estrogen signaling plays a pivotal role in both reproductive and nonreproductive functions. Transcriptional regulation of the ERα gene is highly complex, with multiple transcript variants being differentially produced across the tissues. However, tissue-specific variation and physiological specificity of the ERα variants are not yet fully understood. In an attempt to generate a Cre-dependently restorable ERα-null mouse for functional genetic studies, we unexpectedly produced ERα hypomorphic mice with biased downregulation of a previously unappreciated long ERα isoform that is enriched in the female reproductive organs (uterus and ovaries) and the pituitary but minimally expressed in the brain. Female homozygous mutant mice were capable of pregnancy but displayed irregular estrus cycle and rarely kept newborn pups alive. No significant morphological and pathological changes in reproductive system or disruption of body weight homeostasis were seen in female homozygous mutant mice. Collectively, our results define a tissue-specific enriched long ERα isoform and its preferential role in female reproductive function rather than body weight homeostasis.

Estren behaves as a weak estrogen rather than a nongenomic selective activator in the mouse uterus

A proposed membrane-mediated mechanism of rapid nongenomic response to estrogen has been the intense focus of recent research. Estren, a synthetic steroid, is reported to act selectively through a rapid membrane-mediated pathway, rather than through the classical nuclear estrogen receptor (ER)-mediated pathway, to maintain bone density in ovariectomized mice without uterotropic effects. To evaluate the mechanism and physiological effects of estren, we studied responses in adult ovariectomized mice. In a 3-d uterine bioassay, we found that 300 microg estren significantly increased uterine weight; in comparison, a more maximal response was seen with 1 mug estradiol (E2). The estren response was partly ERalpha independent, because ERalpha knockout (alphaERKO) uteri also exhibited a more moderate weight increase. Estren induced epithelial cell proliferation in wild-type, but not alphaERKO, mice, indicating ERalpha dependence of the epithelial growth response. Examination of estren-regulated uterine genes by microarray indicated that early (2 h) changes in gene expression are similar to the early responses to E2. These gene responses are ERalpha dependent, because they are not seen in alphaERKO mice. Later estren-induced changes in gene expression (24 h) are blunted compared with those seen 24 h after E2. In contrast to early genes, these later estren responses are independent of ERalpha, because the alphaERKO shows a similar response to estren at 24 h. We found that E2 or estren treatments lead to depletion of ERalpha in the uterine cytosol fraction and accumulation in the nuclear fraction within 30-60 min, consistent with the ability of estren to regulate genes through a nuclear ERalpha rather than a nongenomic mechanism. Interestingly, estren, but not E2, induces accumulation of androgen receptor (AR) in the nuclear fraction of both wild-type and alphaERKO samples, suggesting that AR might be involved in the later ERalpha-independent genomic responses to estren. In conclusion, our studies suggest that estren is weakly estrogenic in the mouse uterus and might induce nuclear ERalpha- and AR-mediated responses. Given its activity in our uterine model, the use of estren as a bone-selective clinical compound needs to be reconsidered.

Mechanism of vascular smooth muscle relaxation by estrogen in depolarized rat and mouse aorta. Role of nuclear estrogen receptor and Ca2+ uptake

17 beta-Estradiol induces vasodilation in vitro and in vivo, which has been suggested to contribute to the cardiovascular protection by this ovarian steroid hormone. However, the exact mechanism of vasorelaxation by estrogens remains to be elucidated. In this study, we analyzed the potential role of genomic mechanisms involving the nuclear estrogen receptor and inhibition of entry of extracellular Ca2+ in 17 beta-estradiol-induced vasorelaxation in depolarized aortic rings, isolated from male and female rats and male mice. In both male and female rat aortic rings without endothelium and in intact male mouse aortic rings treated with NG-nitro-L-arginine, 17 beta-estradiol caused dose-dependent (0.3 to 30 mumol/L) relaxation of contraction evoked by high-K+ depolarization (30 and 45 mmol/L KCl, respectively). The estrogen receptor antagonist ICI 164384 had no effect on 17 beta-estradiol-induced relaxations. 125I-17 beta-estradiol binding studies showed the presence of high-affinity cytosolic-nuclear estrogen receptors in control male mouse aortas. Comparable relaxations of aortic rings isolated from control and estrogen receptor-deficient transgenic mice provided direct evidence that the nuclear estrogen receptor is not involved in this response. 17 beta-Estradiol-induced relaxation of rat aortic rings could not be prevented by cycloheximide or actinomycin D, suggesting that the response was not mediated by de novo protein synthesis or gene transcription. In rat aortic rings, 17 beta-estradiol inhibited the increase of 45Ca uptake by 30 mmol/L KCl at concentrations (10 and 30 mumol/L) that caused vasorelaxation in the same tissue, suggesting that inhibition of Ca2+ entry contributes to the response. 17 alpha-Estradiol was less effective, and estrone was devoid of vasorelaxing activity. Vasorelaxation by estrogens in female and male rat aortas was similar, indicating no gender difference in vascular responses under these conditions.

Two populations of the estrogen receptor separated and characterized using aqueous two-phase partitioning

Two populations of the rat uterine estrogen receptor (ER) were separated and characterized using aqueous two-phase partitioning. Countercurrent distribution of rat uterine cytosolic ER allowed rapid and efficient separation of two populations, one population partitioned preferentially into the top phase (T, K(obs) = 3-6) and the other into the bottom phase (B, K(obs) = 0.01-0.03). The majority of unoccupied cytosolic ER is in the T population. Upon estrogen binding and/or heating to 30 degrees C in vitro the T population is converted to the B population. The transition from T to B does not exclusively involve loss of heat shock protein 90 and does not alter the ligand binding ability of the steroid binding domain. Using the human ER steroid binding domain overproduced in Escherichia coli and the steroid binding domain generated by partial trypsinization of the rat uterine ER, we demonstrate that the characteristic distinguishing T and B populations is not localized to this domain alone but may be associated with the amino terminal half of the ER (the A/B and DNA binding domains). The T to B transition of the ER also occurs in human MCF-7 breast cancer cells upon treatment with estrogen at 37 degrees C.

Vascular estrogen receptors and endothelium-derived nitric oxide production in the mouse aorta. Gender difference and effect of estrogen receptor gene disruption

The present study was designed to test the hypothesis that estrogen receptors (ER) in the blood vessel wall play a role in the modulation of the release of endothelium-derived nitric oxide (EDNO). Both basal and stimulated release of EDNO were determined in aortic rings isolated from female and male wild-type and male homozygous estrogen receptor knock-out (ERKO) mice. 125I-17beta-estradiol binding in aortic tissue showed significantly more high affinity cytosolic- nuclear-binding sites in male compared with female wildtype mice. Estrogen receptor transcripts were present in the aorta of male wild-type mice, but they were absent in male ERKO animals. Basal release of EDNO (determined by endothelium-dependent contraction caused by NG-nitro-arginine) was significantly higher in aorta of wild-type male mice compared with wild-type female mice, and significantly lower in the aorta of male ERKO compared with male wild-type mice. Acetylcholine-induced endothelium-dependent relaxation was similar in all groups studied. No difference was observed in the activity of calcium-dependent nitric oxide synthase in homogenates of lungs and brain taken from male wild-type and ERKO mice. These studies show a significant association between the number of estrogen receptors and basal release of EDNO in the aorta of mice, and suggest that decreased vascular estrogen receptor number may represent a novel risk factor for cardiovascular diseases.

Phosphorylation of tyrosine 537 on the human estrogen receptor is required for binding to an estrogen response element

We report here that the phosphorylation of tyrosine 537 on the human estrogen receptor (hER) controls the receptor's dimerization and DNA binding ability. The DNA-binding form of both the hER from human MCF-7 mammary carcinoma cells and the hER overexpressed in Sf9 insect cells was isolated using estrogen response element (ERE) affinity chromatography. Western blot analyses demonstrated that the DNA-binding form of the hER from MCF-7 or Sf9 cells was (i) phosphorylated at tyrosine 537, (ii) localized in the nucleus of estradiol-treated MCF-7 cells with an apparent molecular mass of 67 kDa, and (iii) hyperphosphorylated at serine residue(s). The non-DNA-binding form of the hER was (i) devoid of phosphorylation at tyrosine 537, (ii) cytosolic with an apparent molecular mass of 66 kDa, and (iii) hypophosphorylated at serine residue(s). The dephosphorylation of the purified hER at phosphotyrosine 537 with a tyrosine phosphatase eliminated binding to an ERE in a gel mobility shift assay. The binding of the tyrosine-dephosphorylated hER to an ERE was restored by the rephosphorylation of tyrosine 537 with Src family tyrosine kinases, p60c-src or p56lck. Mutation of tyrosine 537 to phenylalanine confirmed that the phosphorylation of tyrosine 537 is necessary for the hER to bind an ERE. An anti-hER antibody restored the binding of the tyrosine-dephosphorylated hER to an ERE, indicating that the bivalent anti-hER antibody brought together the two inactive hER monomers. A far-Western blot confirmed that phosphotyrosine 537 is required for hER homodimerization. These experiments establish that dimerization of the hER and DNA binding are regulated by phosphorylation at tyrosine 537. This is the first demonstration of the regulation of dimerization of a steroid hormone receptor by phosphorylation. These results are significant since p60c-src is overexpressed in estrogen-dependent breast cancers and may act to enhance the activity of the hER.

Cross talk between peptide growth factor and estrogen receptor signaling systems

Epidermal growth factor reproduces many of the effects of estrogen on the murine female reproductive tract and may partially mediate estrogen-induced growth and differentiation. The mechanism by which the actions of estrogens and epidermal growth factor (EGF) converge is unknown. The studies described herein were performed to investigate the possibility that some of the actions of EGF may be mediated through the estrogen receptor. A specific estrogen receptor (ER) antagonist inhibited estrogenlike effects of EGF in the mouse uterus, specifically induction of DNA synthesis and phosphatidylinositol turnover. In addition, EGF elicited enhanced nuclear localization of uterine ER and formation of a unique nuclear form of ER that is present after estrogen treatment. These in vivo observations indicated that EGF may elicit some of its actions by activation of nuclear ER. Thus, the effect of peptide growth factors on activation of a consensus estrogen response element was assessed in Ishikawa human endometrial adenocarcinoma cells, which contain negligible ER levels, and in BG-1 human ovarian adenocarcinoma cells, which contain abundant ER. EGF and TGF alpha induced transcriptional activation of a consensus estrogen response element (ERE) in an ER-dependent manner in both cell types. In addition, insulinlike growth factor I (IGF-I) was as potent as 17 beta-estradiol in BG-1 cells. Synergism between growth factors and estrogen was observed in both cell types, although synergism was not observed between the different classes of growth factors [i.e., transforming growth factor alpha (TGF alpha) and IGF-I] in BG-1 cells. The most potent activator of ERE-dependent transcription was a protein kinase C activator (TPA), which acted synergistically with 17 beta-estradiol. A protein kinase C inhibitor abolished the effect of TPA but not that of 17 beta-estradiol, IGF-I, or TGF alpha. A protein kinase A activator elicited ER-dependent activation of transcription and did not synergize with estrogen or growth factors. In conclusion, some physiologic actions of peptide growth factors are dependent on ER. Indeed, growth factors are capable of eliciting ER-dependent activation of an ERE. Both the protein kinase A and protein kinase C pathways can elicit ER-dependent transcriptional activation; however, it is unlikely that these pathways mediate the effects of peptide growth factors on the ER in BG-1 cells.

Correlation between low levels of estrogen receptors and estrogen responsiveness in two rat osteoblast-like cell lines

With the knowledge that estrogen replacement therapy can circumvent postmenopausal osteoporosis and with the discovery of estrogen receptors (ER) in cultures of normal osteoblast-like cells, extensive investigations have been directed toward understanding the role of the ER in normal bone homeostasis. ROS 17/2.8 and UMR-106-01, two established osteoblast-like cell lines derived from rat osteosarcomas, have been shown to have estrogen-regulated biologic responses. Only the ROS 17/2.8 cell line has been reported to contain ER. In this study, high-affinity, saturable binding sites characteristic of the ER were detected in UMR-106-01 cells by binding assays with the high-affinity ligand, [125I]17 beta-estradiol. An initial immunoconcentration step before western blot analysis also allowed detection of the full-length ER protein. In addition, northern blot analysis indicated that the entire ER transcript was expressed and that the half-life of the ER message was increased following cycloheximide treatment. Message levels were also regulated by removal of serum and treatment with estradiol. An estrogen-regulated reporter vector, ERET81CAT, was transfected into the UMR-106-01 cells to determine whether the detected level of ER was transcriptionally functional. Using this assay, estrogen responsiveness was evident; however, the response was inconsistent. Multiple factors, such as serum, estradiol, and cell density, influence the ER levels in these cells and probably cause fluctuations in the abundance of receptors available to induce the CAT response. When the cells are responsive, the ICI 164,384 antagonist could block the estrogen-induced activation of CAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Coupling of dual signaling pathways: epidermal growth factor action involves the estrogen receptor

Epidermal growth factor (EGF) reproduces many of the effects of estrogen on the murine female reproductive tract and may partially mediate estrogen-induced growth and differentiation. This study was performed to investigate the mechanism by which EGF elicits estrogen-like actions in the whole animal. EGF was administered to adult ovariectomized mice by slow release pellets implanted under the kidney capsule. The induction of uterine DNA synthesis and phosphatidylinositol lipid turnover by EGF or administration of diethylstilbestrol (5 micrograms/kg), a potent estrogen, was attenuated by the estrogen receptor antagonist ICI 164,384. Furthermore, EGF mimicked the effects of estrogen on enhanced nuclear localization of the estrogen receptor and the formation of a unique form of the estrogen receptor found exclusively in the nucleus. These results suggest that EGF may induce effects similar to those of estrogen in the mouse uterus by an interaction between the EGF signaling pathway and the classical estrogen receptor. The demonstration of cross-talk between polypeptide growth factors and steroid hormone receptors may be of importance to our understanding of the regulation of normal growth and differentiation as well as the mechanisms of transmission of extracellular mitogen signals to the nucleus.

Characterization of murine cell lines from diethylstilbestrol-induced uterine endometrial adenocarcinomas

Neonatal treatment with estrogens is associated with development of uterine adenocarcinomas in CD-1 mice. Treatment with the synthetic estrogen diethylstilbestrol (DES) on Days 1 to 5 after birth results in 90% incidence of these hormone-dependent lesions in 18-mo.-old mice. Three cell lines were established from these DES-associated tumors. Each of these cell lines exhibited morphologic and ultrastructural characteristics of transformed epithelial cells, including an increased nuclear:cytoplasmic ratio, enlarged and irregular nuclei with multiple nucleoli and areas of chromatin condensation, positive staining for cytokeratin, desmosomes, and microvilli. After subcutaneous injection into nude mice, all three cell lines formed solid tumors within 4 wk. Although the primary uterine tumors and tumor transplants in nude mice had been shown to be estrogen-dependent and estrogen-receptor positive, neither the monolayer growth nor the tumorigenicity of any of the three cell lines in this study was enhanced by or dependent on estrogen. Estrogen receptor levels were low in early and intermediate passage cells. Allele-specific oligonucleotide hybridization analysis of PCR-amplified cell line DNA revealed no point mutations in the 12th, 13th, or 61st codons of the K-ras or H-ras protooncogenes. Southern analysis revealed no changes in genomic organization of the putative tumor suppressor gene DCC, but demonstrated a three- to four-fold amplification of the c-myc gene in one cell line. Expression of c-myc RNA was concomitantly increased in the same cell line. These three transformed cell lines represent the end point in the process of hormone-associated tumorigenesis and as such should prove useful in investigating the molecular changes and the mechanisms involved in hormonal carcinogenesis.