Estrogen receptor mRNA splice variants produced from the distal and proximal promoter transcripts

Demethylation of promoter C region of estrogen receptor alpha gene is correlated with its enhanced expression in estrogen-ablation resistant MCF-7 cells

Long-term estrogen deprivation (LTED) MCF-7 cells showing estrogen-independent growth, express estrogen receptor (ER) alpha at a much higher level than wild-type MCF-7 cells. Enhanced expression of ERalpha associated with partial localization of ERalpha to the plasma membranes in LTED cells is thought to be an important step for acquisition of estrogen-ablation resistance. In this study, we compared the regulation of ERalpha gene expression between wild type and LTED cells, examining the usage of the promoters A and C as well as their methylation status. We found that transcription from the promoter C was drastically enhanced in LTED cells, compared with that in wild-type cells. Furthermore, the promoter C region was highly unmethylated in LTED cells, but partially methylated in wild-type cells. Our findings imply that demethylation of promoter C region in the ERalpha gene is in part responsible for the enhanced expression of ERalpha gene in LTED cells.

A dose-response study on the estrogenic activity of benzophenone-2 on various endpoints in the serum, pituitary and uterus of female rats

The tetrahydroxylated biphenyl-ketone 2,2',4,4'-tetrahydroxybenzophenone (BP2), one of twelve benzophenone-derived UV-filters, is used in cosmetic products and in packaging materials to protect these products from light induced damage. Recently published studies showed that BP2 exerts estrogenic activity; thus, it is an endocrine active chemical. We present data from a pharmacodynamic dose-response experiment with five dosages of BP2 applied per gavage to adult ovariectomized (ovx) rats for 5 days. Estradiol-valerate (E2) served as a control compound. The uterotrophic assay, proposed by the OECD, was modified to have a broader view on endocrine activity outside the urogenital tract to prevent that undesirable actions in other organs regulated by estrogens are missed. The gene expression levels of marker genes of estrogenic action were measured by semi-quantitative RT-PCR. Metabolic parameters were assessed by determination of the serum concentrations of leptin, cholesterol, high- and low-density lipoproteins, and triglycerides in the serum. Administration of BP2 at dosages of 10-1,000 mg/kg bodyweight led to changes of these parameters comparable to the changes in the E2 group with 0.6 mg/kg bodyweight. For the observed estrogenic activities of BP2, the "no observed adverse effect levels" were determined. Additionally, the data were further analyzed using the benchmark approach. If BP2 is transcutaneously absorbed in the human, the obtained threshold values would suggest refraining from the further use of BP2 as UV-filter in cosmetic products although additional toxicological studies should be conducted to clarify possible adverse effects.

Multi-organic risk assessment of estrogenic properties of octyl-methoxycinnamate in vivo

Sun protection products contain a variety of UV-filters, among others, octyl-methoxycinnamate (OMC). Recently, an uterotrophic effect in immature rats has been reported, indicating that OMC might have estrogenic properties and thus is an endocrine active chemical (EAC). However, determination of an estrogenic activity in the uterus only is a restricted approach with the potential risk of missing undesirable actions in other organs regulated by estrogens. A pharmacodynamic experiment with 5 dosages of OMC in adult ovariectomized (ovx) rats was carried out to quantify the multi-organic estrogenic properties of OMC. As control compound, estradiol-valerate (E2) was included. Animals were treated per gavage for 5 days. The expression levels of markers of estrogenic action in several organs were measured by RT-PCR. Effects on metabolic parameters were assessed by determination of the serum concentrations of leptin, cholesterol, high and low density lipoproteins (HDL and LDL), glucose and triglycerides. Observed changes upon OMC treatment were analyzed using the NO(A)EL and the benchmark dose approach. From the obtained pharmacodynamic data of the most sensitive parameter (truncated estrogen receptor protein 1 gene expression in the pituitary) we obtained threshold values that are exceeded by the recommended use of OMC containing formulations for skin protection in humans, therefore we propose to reduce the use of OMC in cosmetic products. In addition to estrogenic actions of OMC, non-estrogenic effects have been found for this chemical supporting the need of a multi-organic risk assessment of putative EACs.

Insulin-like growth factor-I receptor and estrogen receptor crosstalk mediates hormone-induced neurite outgrowth in PC12 cells

Estradiol (E(2)) and insulin-like growth factor-I (IGF-I) can act independently or in concert to promote neurite outgrowth in vivo and in cultured neurons. This study examined the role of crosstalk between estrogen receptor (ER)alpha and the IGF-I receptor as a critical mediator of hormone- and growth factor-dependent neurite outgrowth in a homogenous cell system. We used control PC12 cells and PC12 cells stably transfected with ER alpha, both of which express IGF-I receptor. Cells were treated for 1 week with vehicle, 1 nM E(2) or 100 ng/ml IGF-I alone or with E(2) or IGF-I in the presence of either the IGF-I receptor antagonist JB1 or the ER antagonist ICI 182,780. IGF-I significantly increased neurite outgrowth, as measured by the percentage of process-bearing cells, and absolute neurite length per cell in both control and ER alpha-transfected PC12 cells. In contrast, E(2) increased process formation and extension only in PC12 cells that were stably transfected with ER alpha. ICI 182,780 and JB1 blocked the IGF-I-induced increases in neurite length in both cell types. The efficacy of ICI 182,780 in control PC12 cells may have been due to the upregulation of ER alpha in these cells by the 7-day treatment with IGF-I. The ER and IGF-I receptor antagonists similarly blocked the E(2)-induced increase in neurite lengths in ER alpha-transfected cells. Immunofluorescent analysis of the cellular distribution of an axonal marker, phospho-neurofilament, verified that the processes extended by PC12 cells were neurites. These data suggest that receptor crosstalk between IGF-I receptors and ER alpha has an important role in neurite formation and extension even in a single-cell system.

Upstream regions of the estrogen receptor alpha proximal promoter transcript regulate ER protein expression through a translational mechanism

Estrogen receptor alpha (ER) mRNA is primarily transcribed from two promoters, the two transcripts share identical sequence encoding the same ER protein but differ in upstream regions. The 5' region of the two transcripts contain upstream open reading frames (uORFs) encoding potential peptides of 20 and 18 amino acids. The peptides have five C-terminal residues in common. These studies were undertaken to determine if the uORFs and encoded peptides differentially affected expression of ER. Expression of green fluorescent protein (GFP) reporter constructs containing upstream proximal promoter transcript sequences with the first 18 codons of ER fused to GFP was tested in HeLa cells. The cells expressed reduced levels of GFP as compared to the pEGFP-N1 parent vector; the effect was dependent on the presence of an intact proximal ER transcript uORF. Similar regulation by the uORF was seen in transfected MCF-7, MDA MB-231 and Ishikawa cells. Only protein expression was affected by eliminating the uORF; RNA levels were unchanged. This indicates the mechanism is translational rather than being an effect of the introduced point mutations on either mRNA stability or transcription. Eliminating the uORF did not significantly increase expression from similar distal promoter transcript ER-GFP constructs. However, study of in-frame fusions of GFP to the entire proximal and distal uORFs and to their translational start motifs showed that the translational start region of the distal uORF was inherently better at initiating translation than the AUG environment of the proximal promoter transcript uORF. The data indicate there are regulatory properties suppressing expression from the ER translation start which are specific to the unique regions of the ER proximal promoter transcript and these are likely associated with the proximal transcript uORF peptide product.

Phenotypic changes in MCF-7 cells during prolonged exposure to tamoxifen

MCF-7 breast tumor cells form multicellular nodules (foci) over a confluent monolayer in an estradiol (E2)-dependent, antiestrogen-sensitive reaction. A cell line cloned from MCF-7 that displays these phenotypes was probed to determine the effects of long term exposure to tamoxifen on the growth of foci, estrogen receptor alpha (ERalpha) status, and gene responsiveness to E2. In one of two experiments, a heterogeneous cell population emerged (TMX2) that over-expressed estrogen receptor alpha wild type mRNA (ERalpha mRNA) (approximately 20-fold) missing exon 3 (ERDelta3 mRNA) and its corresponding protein (ERDelta3P). On a per mRNA to protein basis, ERDelta3P and wild-type ERalpha were equivalently expressed. Return of the TMX2 population to medium without tamoxifen eventually selected for a population that expressed predominately wild-type ERalpha, whereas TMX2 clones over expressing ERDelta3 mRNA and ERDelta3P retained this phenotype in tamoxifen-free media. In both experiments, expression of all ERalpha mRNAs and proteins declined to barely detectable levels during 6-12 months exposure, concomitant with a progressive increase in the ability of the cells to form foci independently of E2 or tamoxifen. Selection for these various populations suggests that tamoxifen can induce and/or support certain cellular changes that lead to altered ERalpha expression, E2-independent cell growth and resistance to antiestrogens.

The novel 5'-untranslated first exon, exon 0H, of the rat estrogen receptor beta gene

The multiple untranslated first exons and promoters system has been reported to be involved in the tissue-specific expression of the estrogen receptor alpha (ERalpha) in humans and rats. However, a few reports are available concerning tissue-specific regulation of the expression of the estrogen receptor beta (ERbeta) gene. To investigate the mechanism regulating the expression of the rat ERbeta gene, we analyzed the structure of the 5'-untranslated region (UTR) of the rat testicular ERbeta mRNA using 5'-rapid amplification of the cDNA ends (5'-RACE) method. Sequence analysis revealed the presence of two isoforms of the ERbeta mRNA containing distinct 5'-UTRs. Although the 5'-UTR of one isoform of the messages was identical to the 5'-UTR of the previously reported ERbeta cDNA, the other isoform had a novel sequence in its 5'-UTR. Genomic analysis revealed that the 5'-UTRs of these two mRNA isoforms originated from two distinct untranslated first exons, the previously identified exon termed "exon 0N," and the novel exon we termed "exon 0H," both of which were spliced onto exon 1. We termed these isoforms of the messages containing the exon 0N and exon 0H, the ERbeta mRNA (0N-1) and ERbeta mRNA (0H-1), respectively. Furthermore, the distributions of these mRNA isoforms in several rat tissues were analyzed using the reverse transcription-polymerase chain reaction (RT-PCR) method. The distributions of the two mRNA isoforms differed; the ERbeta mRNA (0N-1) was widely distributed in the tissues examined, while expression of the ERbeta mRNA (0H-1) was restricted to a few tissues such as the anterior pituitary, amygdala, and some peripheral tissues. In conclusion, our findings indicate that the tissue-specific expression of the rat ERbeta gene is regulated, at least in part, by the multiple untranslated first exons system which consists of exon 0N and exon 0H.

Estradiol stabilizes estrogen receptor messenger ribonucleic acid in sheep endometrium via discrete sequence elements in its 3'-untranslated region

The preovulatory surge of estrogen up-regulates estrogen receptor-alpha (ER) gene expression in the uterus during the estrous/menstrual cycles of female mammals. Previously, we demonstrated that the 5-fold increase in ER mRNA levels in endometrium of ovariectomized ewes treated with a physiological dose of estradiol (E2) is entirely due to an increase in ER mRNA stability. Our current work confirms that the E2 effect is specific to ER mRNA. The sequence of ER mRNA, cloned from sheep endometrium, shows a high degree of conservation with those of other species, even in the 5'- and the very long 3'-untranslated regions. In a cell-free assay, ER mRNA demonstrates greater stability with endometrial extracts from E2-treated ewes compared with those from untreated ovariectomized ewes. The E2-enhanced stability of ER mRNA was ablated by prior treatment of the extracts with proteinase K, 70 C heat, and oxidizing and alkylating reagents, indicating that a protein is responsible for stabilization of the message. The 3'-untranslated region of ER mRNA contains discrete sequences required for E2-enhanced stability, four of which were identified by extensive deletion mutant analyses. Transfer of two of the four minimal E2-modulated stability sequences conferred E2-enhanced stability to a heterologous RNA. These minimal E2-modulated stability sequences contain a common 10-base, uridine-rich sequence that is predicted to reside in a loop structure. Throughout our studies, estrogen stabilization of ER mRNA in sheep endometrium resembled that of vitellogenin mRNA in frog liver, indicating conservation of this ancient mechanism for enhancing gene expression in response to estrogen.

Gender-dependent expression of alpha and beta estrogen receptors in human nontumor and tumor lung tissue

Estrogen receptor (ER) expression in human lung has been understudied, particularly in light of its potential biological importance in the female lung cancer epidemic. Reverse transcription-polymerase chain reaction was used to probe mRNA expression of wild-type ERalpha and ERbeta and their splice variants in human bronchogenic tumor and adjacent nontumor specimens. In tumor tissue from 13 women and 13 men, ERalpha was expressed in 85% of women versus 15% in men [P=0.001]. ERbeta was expressed equally in tumors from women versus men [92% vs. 69%, P=ns]. Both ERalpha and beta forms were expressed simultaneously in the lung tumors of 77% of women versus 15% of men [P=0.005]. Among adjacent nontumor lung specimens, 31% of the women expressed ERalpha mRNA versus 0% of men [P=0.101], and 39% of women expressed ERbeta mRNA versus 31% of men [P=ns]; only one woman and no men expressed both ERalpha and beta in nontumor tissue. Females expressed ERalpha [P=0.017], ERbeta [P=0.013], and ERalpha+beta [P=0.002] more frequently in tumor versus nontumor tissue, whereas in males expression of ERalpha, beta and both alpha+beta was not clearly different for tumor versus nontumor tissue. In specimens expressing ERalpha mRNA, the transcript lacking exon 7 (delta7) was the major splice variant with varying contributions from the transcripts delta4, delta3+4, delta5 and others unidentified. Alternative splicing of ERbeta mRNA was observed, but not to as great an extent as for ERalpha mRNA. ERalpha promoter usage in tumors varied among individuals. When the ER receptors were co-expressed in tumors, ERalpha was quantitatively more abundant in the majority of cases than ERbeta. Within this small group of 26 patients, no correlation was found between age, smoking history, plasma nicotine, cotinine, estradiol concentrations or histopathologic type with tumor or nontumor estrogen receptor status of any type. However, several positive correlations imply that: (1) ERalpha expression occurs more often in the lungs of women than men; (2) ERbeta is expressed with approximately equal frequency in the lungs of both genders; and (3) tumors display a higher frequency of both receptor types than nontumors in women. We hypothesize that these putative gender-dependent differences in ERalpha and ERbeta expression could contribute unique phenotypic characteristics to lung cancer development or progression in women.

Regulation of cytochrome P450 (CYP) genes by nuclear receptors

Members of the nuclear-receptor superfamily mediate crucial physiological functions by regulating the synthesis of their target genes. Nuclear receptors are usually activated by ligand binding. Cytochrome P450 (CYP) isoforms often catalyse both formation and degradation of these ligands. CYPs also metabolize many exogenous compounds, some of which may act as activators of nuclear receptors and disruptors of endocrine and cellular homoeostasis. This review summarizes recent findings that indicate that major classes of CYP genes are selectively regulated by certain ligand-activated nuclear receptors, thus creating tightly controlled networks.

The anti-proliferative effects of 1alpha,25(OH)2D3 on breast and prostate cancer cells are associated with induction of BRCA1 gene expression

The anti-proliferative action of the seco-steroid hormone 1alpha, 25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] extends to some, but not all breast and prostate cancer cell lines. By elucidating the molecular mechanisms mediating the sensitivity of these cells, we can identify critical target genes regulated directly or indirectly by 1alpha,25(OH)2D3 and pathways potentially disrupted during transformation. In this study, we demonstrated the induction of expression of BRCA1 mRNA and protein as well as transcriptional activation from the BRCA1-promoter by 1alpha,25(OH)2D3 in the sensitive breast cancer cell line MCF-7. This was not observed in the 1alpha,25(OH)2D3-resistant breast cancer cell line MDA-MB-436. The induction of BRCA1 mRNA was blocked by cyclohexamide. This indicated that transcriptional activation was mediated indirectly by the vitamin D receptor (VDR). Inhibition of VDR protein levels by stable transformation of the anti-sense VDR in MCF-7 reduced the sensitivity of MCF-7 to 1alpha,25(OH)2D3 by 50-fold. In addition, the induction of BRCA1 protein and transcriptional activation of a BRCA1 promoter-luciferase reporter construct was abrogated in the stable transformant with the greatest reduction of VDR levels. Examination of other breast and prostate cancer cell lines revealed that sensitivity to the anti-proliferative effects of 1alpha, 25(OH)2D3 was strongly associated with an ability to modulate BRCA1 protein. Furthermore, the expression of the estrogen receptor in these cell lines strongly correlated with their sensitivity to 1alpha,25(OH)2D3 and their ability to modulate BRCA1 expression. Taken together, our data support a model whereby the anti-proliferative effects of 1alpha,25(OH)2D3 are mediated, in part, by the induction of BRCA1 gene expression via transcriptional activation by factors induced by the VDR and that this pathway is disrupted during the development of prostate and breast cancers.

Expression of an estrogen receptor alpha variant protein in cell lines and tumors

Human estrogen receptor alpha (ER) mRNA is a mixture of wild type and alternatively spliced variants. Many studies have examined the potential of ER mRNA profiles to serve as diagnostic/prognostic cancer biomarkers, but only a few have attempted to correlate ER mRNA profiles with protein expression. Representative ER mRNA pools were reproduced from the cDNAs of MCF-7 cells, a human breast tumor and human uterus and translated in a protease-free environment by reticulocyte lysates to determine relative translation efficiencies between the various ER mRNA transcripts and to facilitate identification of translated proteins. Cell line and tumor extracts were then examined for expression of the ER variant proteins identified in reticulocyte lysate translations. Each of the ER mRNA pools were translated by reticulocyte lysates into two ER proteins with molecular weights of approximately 60 and 52 kD. Western immunoblotting with various C- and N-terminal-directed, anti-ER antibodies and comparison with expressed ER protein standards established that the 52 kD protein (ERDelta7P) was translated from the predominant splice variant mRNA in each pool, which is missing exon 7. The 60 kD protein contained wild type ER sequence minus 61 C-terminal amino acids lost due to an intentional run off truncation. ERDelta7P expression was subsequently demonstrated in MCF-7 cells by Western immunoblotting with the site-directed antibodies. A protein corresponding to ERDelta7P was also detected in other ER positive breast tumor cell lines, and extracts of ER positive breast and uterine tumors. This widespread expression of ERDelta7P in vivo suggests that it may have some biological function. ERDelta7P may also affect immunohistochemical evaluation of ER positivity in tumors depending upon the level of its expression and the antibody used.

Regulation of cytochrome P450 (CYP) genes by nuclear receptors

Members of the nuclear-receptor superfamily mediate crucial physiological functions by regulating the synthesis of their target genes. Nuclear receptors are usually activated by ligand binding. Cytochrome P450 (CYP) isoforms often catalyse both formation and degradation of these ligands. CYPs also metabolize many exogenous compounds, some of which may act as activators of nuclear receptors and disruptors of endocrine and cellular homoeostasis. This review summarizes recent findings that indicate that major classes of CYP genes are selectively regulated by certain ligand-activated nuclear receptors, thus creating tightly controlled networks.

Estrogen receptor null mice: what have we learned and where will they lead us?

All scientific investigations begin with distinct objectives: first is the hypothesis upon which studies are undertaken to disprove, and second is the overall aim of obtaining further information, from which future and more precise hypotheses may be drawn. Studies focusing on the generation and use of gene-targeted animal models also apply these goals and may be loosely categorized into sequential phases that become apparent as the use of the model progresses. Initial studies of knockout models often focus on the plausibility of the model based on prior knowledge and whether the generation of an animal lacking the particular gene will prove lethal or not. Upon the successful generation of a knockout, confirmatory studies are undertaken to corroborate previously established hypotheses of the function of the disrupted gene product. As these studies continue, observations of unpredicted phenotypes or, more likely, the lack of a phenotype that was expected based on models put forth from past investigations are noted. Often the surprising phenotype is due to the loss of a gene product that is downstream from the functions of the disrupted gene, whereas the lack of an expected phenotype may be due to compensatory roles filled by alternate mechanisms. As the descriptive studies of the knockout continue, use of the model is often shifted to the role as a unique research reagent, to be used in studies that 1) were not previously possible in a wild-type model; 2) aimed at finding related proteins or pathways whose existence or functions were previously masked; or 3) the subsequent effects of the gene disruption on related physiological and biochemical systems. The alpha ERKO mice continue to satisfy the confirmatory role of a knockout quite well. As summarized in Table 4, the phenotypes observed in the alpha ERKO due to estrogen insensitivity have definitively illustrated several roles that were previously believed to be dependent on functional ER alpha, including 1) the proliferative and differentiative actions critical to the function of the adult female reproductive tract and mammary gland; 2) as an obligatory component in growth factor signaling in the uterus and mammary gland; 3) as the principal steroid involved in negative regulation of gonadotropin gene transcription and LH levels in the hypothalamic-pituitary axis; 4) as a positive regulator of PR expression in several tissues; 5) in the positive regulation of PRL synthesis and secretion from the pituitary; 6) as a promotional factor in oncogene-induced mammary neoplasia; and 7) as a crucial component in the differentiation and activation of several behaviors in both the female and male. The list of unpredictable phenotypes in the alpha ERKO must begin with the observation that generation of an animal lacking a functional ER alpha gene was successful and produced animals of both sexes that exhibit a life span comparable to wild-type. The successful generation of beta ERKO mice suggests that this receptor is also not essential to survival and was most likely not a compensatory factor in the survival of the alpha ERKO. In support of this is our recent successful generation of double knockout, or alpha beta ERKO mice of both sexes. The precise defects in certain components of male reproduction, including the production of abnormal sperm and the loss of intromission and ejaculatory responses that were observed in the alpha ERKO, were quite surprising. In turn, certain estrogen pathways in the alpha ERKO female appear intact or unaffected, such as the ability of the uterus to successfully exhibit a progesterone-induced decidualization response, and the possible maintenance of an LH surge system in the hypothalamus. [ABSTRACT TRUNCATED]