Transcriptional activation of genes by 17 beta-estradiol through estrogen receptor-Sp1 interactions

Regulation of prothymosin alpha by estrogen receptor alpha: molecular mechanisms and relevance in estrogen-mediated breast cell growth

Prothymosin alpha (PTalpha) is a small highly acidic protein found in the nuclei of virtually all mammalian tissues. Its high conservation in mammals and wide tissue distribution suggest an essential biological role. While the exact mechanism of action of PTalpha remains elusive, the one constant has been its relationship with the proliferative state of the cell and its requirement for cellular growth and survival. Recently PTalpha was found to promote transcriptional activity by sequestering the anticoactivator, REA from the Estrogen Receptor (ER) complex. We now report that Estradiol (E2) upregulates PTalpha mRNA and protein expression. Further studies indicate that ERalpha regulates PTalpha gene transcriptional activity. We have also delimited the region of PTalpha gene promoter involved in ERalpha-mediated transcriptional regulation and identified a novel ERalpha-binding element. Increased intracellular PTalpha expression in the presence of estrogens is accompanied by increased nuclear/decreased cytoplasmic localization. Increased nuclear expression of PTalpha is correlated with increased proliferation as measured by expression of Ki67 nuclear antigen. Conversely, inhibition of nuclear PTalpha expression in breast cancer cells using antisense methodology resulted in the inhibition of E2-induced breast cancer cell proliferation. Overall these studies underscore the importance of PTalpha in estrogen-induced breast cell proliferation.

Conditional over-expression of estrogen receptor alpha in a transgenic mouse model

Attempts to delineate the mechanisms of estrogen action have promoted the creation of several estrogen receptor alpha (ERalpha) mouse models in the past decade. These traditional models are limited by the fact that the receptors are either absent or present throughout all stages of development. The purpose of this work was to develop a conditional transgenic model that would provide an in vivo method of controlling the spatial and temporal regulation of ERalpha expression. The tetracycline responsive system was utilized. Three lines of transgenic mice carrying a transgene composed of the coding sequence for murine ERalpha placed under the regulatory control of a tet operator promoter (tet-op) were generated. These three lines of tet-op-mERa mice were each mated to an established line of transgenic mice expressing a tetracycline-dependent transactivator protein (tTA) from the mouse mammary tumor virus-long terminal repeat (MMTV-LTR). Double transgenic MMTV-tTA/tet-op-mERalpha mice were produced. All three lines demonstrated dominant gain of ERalpha shown by RT-PCR, immunoprecipitation, and immunohistochemistry. Transgene-specific ERalpha was expressed in numerous tissues including the mammary gland, salivary gland, testis, seminal vesicle, and epididymis. Expression was silenced by administration of doxycycline in the drinking water. This model can be utilized to evaluate the consequences of ERalpha dominant gain in targeted tissues at specific times during development. In this study dominant gain of ERalpha was associated with a reduction in epididymal/vas deferens and seminal vesicle weights consistent with the proposed action of ERalpha on fluid transport in the male reproductive tract. Combining this model with other dominant gain and gene knockout mouse models will be useful for testing effects of ERalpha action in combination with specific gene products and to evaluate if developmental and stage-specific expression of ERalpha can rescue identified phenotypes in gene knockout mice.

1,25-dihydroxyvitamin D(3)-independent stimulatory effect of estrogen on the expression of ECaC1 in the kidney

Estrogen deficiency results in a negative Ca(2+) balance and bone loss in postmenopausal women. In addition to bone, the intestine and kidney are potential sites for estrogen action and are involved in Ca(2+) handling and regulation. The epithelial Ca(2+) channel ECaC1 (or TRPV5) is the entry channel involved in active Ca(2+) transport. Ca(2+) entry is followed by cytosolic diffusion, facilitated by calbindin-D(28K) and/or calbindin-D(9k), and active extrusion across the basolateral membrane by the Na(+)/Ca(2+)-exchanger (NCX1) and plasma membrane Ca(2+)-ATPase (PMCA1b). In this transcellular Ca(2+) transport, ECaC1 probably represents the final regulatory target for hormonal control. The aim of this study was to determine whether 17beta-estradiol (17beta-E(2)) is involved in Ca(2+) reabsorption via regulation of the expression of ECaC1. The ovariectomized rat model was used to investigate the regulation of ECaC1, at the mRNA and protein levels, by 17beta-E(2) replacement therapy. Using real-time quantitative PCR and immunohistochemical analyses, this study demonstrated that 17beta-E(2) treatment at pharmacologic doses increased renal mRNA levels of ECaC1, calbindin-D(28K), NCX1, and PMCA1b and increased the protein abundance of ECaC1. Furthermore, the involvement of 1,25-dihydroxyvitamin D(3) in the effects of 17beta-E(2) was examined in 25-hydroxyvitamin D(3)-1alpha-hydroxylase-knockout mice. Renal mRNA expression of calbindin-D(9K), calbindin-D(28K), NCX1, and PMCA1b was not significantly altered after 17beta-E(2) treatment. In contrast, ECaC1 mRNA and protein levels were both significantly upregulated. Moreover, 17beta-E(2) treatment partially restored serum Ca(2+) levels, from 1.63 +/- 0.06 to 2.03 +/- 0.12 mM. In conclusion, this study suggests that 17beta-E(2) is positively involved in renal Ca(2+) reabsorption via the upregulation of ECaC1, an effect independent of 1,25-dihydroxyvitamin D(3).

Differential transcriptional regulation by the alpha- and gamma-catalytic subunit isoforms of cAMP-dependent protein kinase

The C gamma and C alpha isoforms of the cAMP-dependent protein kinase (PKA) share 83% identity including all critical catalytic and substrate-binding residues defined to date. Compared to C alpha, C gamma has a different substrate specificity and a selective pseudosubstrate specificity, exhibiting inhibition by regulatory subunits, but not by the protein kinase inhibitor. In these studies, C gamma-mediated gene transcription regulation was compared with that of C alpha in four cell lines using transient transfection/dual luciferase assays. As compared to C gamma, C alpha more efficiently activated a cAMP-response element (CRE)-regulated fragment of the human alpha-glycoprotein hormone promoter which was coupled to a firefly luciferase reporter gene (pGH alpha-fluc). This occurred in Cos7, Y1, and Kin8 adrenal cells by 23-, 6.5-, and 1.4-fold, respectively. In contrast, C gamma, but not C alpha, activated the Sp1RE-regulated herpes simplex virus thymidine kinase promoter which was coupled to a Renilla luciferase reporter (pTK-rluc). In Sp1-deficient Sf9 cells, pGH alpha-fluc expression was maintained for both isoforms, but cotransfection with an Sp1 expression plasmid was necessary and sufficient for activation of pTK-rluc expression by C gamma. In all cell lines, cotransfection with a PDK1 expression plasmid enhanced the transcriptional activation of both C alpha and C gamma (1.5- to 3-fold), while a catalytically inactive PDK1 mutant (PDK.KD) did not. These results suggest that both C alpha and C gamma can activate CRE-responsive genes; however, C alpha does so with better efficiency than C gamma. In contrast to C alpha, C gamma activates transcription of genes containing pTK-like Sp1RE sites. Activation of different C subunit isoforms can provide a means to diversify cAMP-mediated transcription, possibly affecting cell phenotype.

Opposing action of estrogen receptors alpha and beta on cyclin D1 gene expression

Induction of cyclin D1 gene transcription by estrogen receptor alpha (ERalpha) plays an important role in estrogen-mediated proliferation. There is no classical estrogen response element in the cyclin D1 promoter, and induction by ERalpha has been mapped to an alternative response element, a cyclic AMP-response element at -57, with possible participation of an activating protein-1 site at -954. The action of ERbeta at the cyclin D1 promoter is unknown, although evidence suggests that ERbeta may inhibit the proliferative action of ERalpha. We examined the response of cyclin D1 promoter constructs by luciferase assay and the response of the endogenous protein by Western blot in HeLa cells transiently expressing ERalpha, ERalphaK206A (a derivative that is superactive at alternative response elements), or ERbeta. In each case, ER activation at the cyclin D1 promoter is mediated by both the cyclic AMP-response element and the activating protein-1 site, which play partly redundant roles. The activation by ERbeta occurs only with antiestrogens. Estrogens, which activate cyclin D1 gene expression with ERalpha, inhibit expression with ERbeta. Strikingly, the presence of ERbeta completely inhibits cyclin D1 gene activation by estrogen and ERalpha or even by estrogen and the superactive ERalphaK206A. The observation of the opposing action and dominance of ERbeta over ERalpha in activation of cyclin D1 gene expression has implications for the postulated role of ERbeta as a modulator of the proliferative effects of estrogen.

Regulation of MCF-7 breast cancer cell growth by beta-estradiol sulfation

Estrogen stimulation is an important factor in human breast cancer cell growth and development. Metabolism of beta-estradiol (E2), the major endogenous human estrogen, is important in regulating both the level and activity of the hormone in breast tissues. Conjugation of E2 with a sulfonate moiety is an inactivation process since the sulfate ester formed by this reaction can not bind and activate the estrogen receptor. In human tissues including the breast, estrogen sulfotransferase (EST, SULT1E1) is responsible for high affinity E2 sulfation activity. EST is expressed in human mammary epithelial (HME) cells but not in most cultured breast cancer cell lines, including estrogen responsive MCF-7 cells. Stable expression of EST in MCF-7 cells at levels similar to those detected in HME cells significantly inhibits cell growth at physiologically relevant E2 concentrations. The mechanism of cell growth inhibition involves the abrogation of responses observed in growth factor expression in MCF-7 cells following E2 stimulation. MCF-7 cells expressing EST activity did not show a decrease in estrogen receptor-alpha levels, nor a characteristic increase in progesterone receptor or decrease in transforming growth factor-beta expression upon exposure to 100 pM or 1 nM E2. The lack of response in these MCF-7 cells is apparently due to the rapid sulfation and inactivation of free E2 by EST. These results suggest that loss of EST expression in the transformation of normal breast tissues to breast cancer may be an important factor in increasing the growth responsiveness of preneoplastic or tumor cells to estrogen stimulation.

Vitamin D receptor-mediated gene regulation mechanisms and current concepts of vitamin D analog selectivity

Lipophilic hormones of steroidal origin such as the sex hormones and 1,25-dihydroxy vitamin D(3) (1,25[OH](2)D(3)) function by regulating patterns of gene expression in cells. The mediators of such actions are nuclear receptors that recognize these ligands with high affinity and selectivity and function through several mechanisms as gene specific transcription factors. As a result of the mechanistic complexity of nuclear receptor action, recent studies have revealed that both synthetic analogs as well as novel mimetics of a receptor's natural hormonal ligand are capable of modulating functional responses in both cell- and gene-selective manners. These findings have given rise to the term selective receptor modulators, typified by such synthetic estrogen receptor ligands as tamoxifen and raloxifene. A number of vitamin D analogs have been prepared that appear to exhibit tissue-selective activity--most notable through their inability to induce levels of hypercalcemia typical of the activity of the natural hormone 1,25(OH)(2)D(3). Because this debilitating yet normal feature of the natural ligand limits its usefulness in a variety of clinical indications, including its application to prevent bone disease caused by secondary hyperparathyroidism, this feature of many of the new analogs is especially welcome. This article discusses what constitutes a selective receptor modulator and whether the current vitamin D analogs represent such entities.

Regulation of DNA replication fork genes by 17beta-estradiol

The steroid hormone estrogen can stimulate mitogenesis in hormone-responsive breast cancer epithelial cells. This action is attributed to the transcriptional activity of the ER, a ligand-dependent transcription factor. However, the exact molecular mechanism underlying estrogen-induced proliferation has yet to be completely elucidated. Using custom cDNA microarrays containing many genes implicated in cell cycle progression and DNA replication, we examined the gene expression of a hormone-responsive breast cancer cell line (MCF-7) treated with a mitogenic dose of estrogen in the absence of confounding growth factors found in serum. Gene expression changes were monitored 1, 4, 12, 24, 36, and 48 h after estrogen stimulation so that RNA levels at critical times throughout cell cycle progression could be monitored. Significant changes include the altered transcript levels of genes implicated in transcription, cellular signaling, and cell cycle checkpoints. At time points during which increased numbers of cells were progressing through S phase, a majority of the genes associated with the DNA replication fork were also found to be induced. The coexpression of DNA replication fork genes by estrogen without the support of serum growth factors indicates an important estrogen regulatory component of the molecular mechanism driving estrogen-induced mitogenesis.

Diversity in the mechanisms of gene regulation by estrogen receptors

The sequencing of the human genome has opened the way for using bioinformatics to identify sets of genes controlled by specific regulatory signals. Here, we review the unexpected diversity of DNA response elements mediating transcriptional regulation by estrogen receptors (ERs), which control the broad physiological effects of estrogens. Consensus palindromic estrogen response elements are found in only a few known estrogen target genes, whereas most responsive genes contain only low-affinity half palindromes, which may also control regulation by other nuclear receptors. ERs can also regulate gene expression in the absence of direct interaction with DNA, via protein-protein interactions with other transcription factors or by modulating the activity of upstream signaling components, thereby significantly expanding the repertoire of estrogen-responsive genes. These diverse mechanisms of action must be taken into account in screening for potential estrogen-responsive sequences in the genome or in regulatory regions of target genes identified by expression profiling.