Evidence for a previously unidentified upstream exon in the human oestrogen receptor gene

Positive Regulation of Estrogen Receptor Alpha in Breast Tumorigenesis

Estrogen receptor alpha (ERα, NR3A1) contributes through its expression in different tissues to a spectrum of physiological processes, including reproductive system development and physiology, bone mass maintenance, as well as cardiovascular and central nervous system functions. It is also one of the main drivers of tumorigenesis in breast and uterine cancer and can be targeted by several types of hormonal therapies. ERα is expressed in a subset of luminal cells corresponding to less than 10% of normal mammary epithelial cells and in over 70% of breast tumors (ER+ tumors), but the basis for its selective expression in normal or cancer tissues remains incompletely understood. The mapping of alternative promoters and regulatory elements has delineated the complex genomic structure of the ESR1 gene and shed light on the mechanistic basis for the tissue-specific regulation of ESR1 expression. However, much remains to be uncovered to better understand how ESR1 expression is regulated in breast cancer. This review recapitulates the current body of knowledge on the structure of the ESR1 gene and the complex mechanisms controlling its expression in breast tumors. In particular, we discuss the impact of genetic alterations, chromatin modifications, and enhanced expression of other luminal transcription regulators on ESR1 expression in tumor cells.

Intrinsic and Extrinsic Factors Governing the Transcriptional Regulation of ESR1

Transcriptional regulation of ESR1, the gene that encodes for estrogen receptor α (ER), is critical for regulating the downstream effects of the estrogen signaling pathway in breast cancer such as cell growth. ESR1 is a large and complex gene that is regulated by multiple regulatory elements, which has complicated our understanding of how ESR1 expression is controlled in the context of breast cancer. Early studies characterized the genomic structure of ESR1 with subsequent studies focused on identifying intrinsic (chromatin environment, transcription factors, signaling pathways) and extrinsic (tumor microenvironment, secreted factors) mechanisms that impact ESR1 gene expression. Currently, the introduction of genomic sequencing platforms and additional genome-wide technologies has provided additional insight on how chromatin structures may coordinate with these intrinsic and extrinsic mechanisms to regulate ESR1 expression. Understanding these interactions will allow us to have a clearer understanding of how ESR1 expression is regulated and eventually provide clues on how to influence its regulation with potential treatments. In this review, we highlight key studies concerning the genomic structure of ESR1, mechanisms that affect the dynamics of ESR1 expression, and considerations towards affecting ESR1 expression and hormone responsiveness in breast cancer.

The Role of ERα36 in Development and Tumor Malignancy

Estrogen nuclear receptors, represented by the canonical forms ERα66 and ERβ1, are the main mediators of the estrogen-dependent pathophysiology in mammals. However, numerous isoforms have been identified, stimulating unconventional estrogen response pathways leading to complex cellular and tissue responses. The estrogen receptor variant, ERα36, was cloned in 2005 and is mainly described in the literature to be involved in the progression of mammary tumors and in the acquired resistance to anti-estrogen drugs, such as tamoxifen. In this review, we will first specify the place that ERα36 currently occupies within the diversity of nuclear and membrane estrogen receptors. We will then report recent data on the impact of ERα36 expression and/or activity in normal breast and testicular cells, but also in different types of tumors including mammary tumors, highlighting why ERα36 can now be considered as a marker of malignancy. Finally, we will explain how studying the regulation of ERα36 expression could provide new clues to counteract resistance to cancer treatments in hormone-sensitive tumors.

Single CpG site methylation controls estrogen receptor gene transcription and correlates with hormone therapy resistance

Hormone therapy is the most effective treatment for patients with estrogen receptor α-positive breast cancers. However, although resistance occurs during treatment in some cases and often reflects changed estrogen receptor α status, the relationship between changes in estrogen receptor α expression and resistance to therapy are poorly understood. In this study, we identified a mechanism for altered estrogen receptor α expression during disease progression and acquired hormone therapy resistance in aromatase inhibitor-resistant breast cancer cell lines. Subsequently, we investigated promoter switching and DNA methylation status of the estrogen receptor α promoter, and found marked changes of methylation at a single CpG site (CpG4) in resistant cells. In addition, luciferase reporter assays showed reduced transcriptional activity from this methylated CpG site. This CpG region was also completely conserved among species, suggesting that it acts as a methylation-sensitive Ets-2 transcription factor binding site, as confirmed using chromatin immunoprecipitation assays. In estrogen receptor α-positive tumors, CpG4 methylation levels were inversely correlated with estrogen receptor α expression status, suggesting that single CpG site plays an important role in the regulation of estrogen receptor α transcription.

Molecular characterization of an estrogen receptor and estrogen-related receptor and their autoregulatory capabilities in two Mytilus species

Vertebrate-like sex steroid hormones have been widely detected in mollusks, and numerous experiments have shown the importance of steroids in gonad development. Nevertheless, their signaling pathways in invertebrates have not been uncovered yet. Steroid receptors are an ancient class of transcription factors with multiple roles in not only vertebrates but also invertebrates. Estrogen signaling is thought to have major roles in mollusk physiology, but the full repertoire of estrogen receptors is unknown. We presented the successful cloning of two novel forms of estrogen receptor-like genes. These receptors are present in two closely related species of Mytilus: Mytilus edulis and Mytilus galloprovincialis, commonly known and widely distributed sentinel species. Our phylogenetic analysis revealed that one of these receptors is an estrogen receptor (ER) and the other one is an estrogen-related receptor (ERR). Studies of expression analysis showed that both receptor mRNAs were localized in the oocytes and follicle cells in contact with developing oocytes in the ovary and Sertoli cells in the testis, and in the ciliated cells of the gill. In addition, we have evidence that one (ER) of these may have a capacity to autoregulate its own expression in the gonadal cells by estrogen (E2) and that this gene is responsive to estrogenic compounds.

Sexually dimorphic transcription of estrogen receptors in cod gonads throughout a reproductive cycle

The role of sex steroid regulation in gonadal maturation is a very complex process that is far from being fully understood. Hence, we have investigated seasonal changes in gonadal expression of estrogen receptors (ERs) in Atlantic cod (Gadus morhua L.), a batch spawner, throughout the annual reproductive cycle. Three nuclear ER partial cDNA sequences (esr1, esr2a, and esr2b) were cloned and all esr transcripts were detected mainly in liver and gonads of fish of both sexes. In situ hybridization of esrs along with germ cell (vasa) and gonadal somatic cell markers (gonadal soma-derived factor (gsdf), 3β-hydroxysteroid dehydrogenase (3βhsd), and anti-Müllerian hormone (amh) for testicular, or gsdf for ovarian somatic cells) showed that all three esrs were preferentially localized within interstitial fibroblasts composed of immature and mature Leydig cells in testis, whereas they were differentially expressed in both follicular cells and oocytes in ovary. Quantitative real-time PCR analysis revealed a sexually dimorphic expression pattern of the three esr paralogs in testis and ovary. A significant increase in esr2a expression was identified in testis and of esr2b in ovary, whereas esr1 transcripts were elevated in both testis and ovary in February and March before the spawning period. The localization and sexually dimorphic expression of esr genes in gonads indicate a direct function of estrogen via ERs in gonadal somatic cell growth and differentiation for Leydig cell in testis and follicular cells in ovary throughout the annual reproductive cycle in Atlantic cod.

Novel splicing events and post-transcriptional regulation of human estrogen receptor α E isoforms

Expression of the estrogen receptor α (ERα) gene is subject to complex regulation. To elucidate the mechanisms of this regulation, the genomic organization and the physiological role of the multiple 5'-untranslated regions (5'-UTRs) must be determined. Here, we investigated the expression and splicing patterns of the human ERα E isoforms. We identified two novel untranslated exons, N1 and N2, in the 5'-region of the human ERα gene and multiple E isoform mRNA variants generated by alternative usage of non-coding internal exons. Expression of the N1-containing variants was observed only in the human breast adenocarcinoma cell line, MCF7, while the N2-containing variants were expressed in the adult liver and MCF7 cells. We examined post-transcriptional regulation of the variant mRNAs using luciferase reporter assays and quantitative PCR. The insertion of untranslated internal exons into the 5'-UTRs of the E isoforms reduced their translation efficiency, but barely influenced mRNA turnover. Our results indicate that the genomic organization of the human ERα gene and the splicing profiles of the human ERα E isoforms are more complicated than previously reported. Furthermore, the 5'-UTRs of the E isoforms post-transcriptionally control human ERα expression mainly through translational repression.

Expression of a dominant negative estrogen receptor alpha variant in transgenic mice accelerates uterine cancer induced by the potent estrogen diethylstilbestrol

ERΔ3 transgenic mice expressing a dominant negative estrogen receptor α (ERα) variant lacking the second zinc finger in the DNA binding domain were developed to examine its potential to inhibit estrogen action in vivo. To investigate if ERΔ3 expression influences uterine carcinogenesis, ERΔ3 transgenic mice were exposed to diethylstilbestrol (DES) on post-natal days 1-5. Neonatal DES treatment induced uterine adenocarcinomas in 81% of 8-month-old ERΔ3 mice compared to 49% of wild-type females (p<0.016). ERΔ3 did not inhibit the expression of the estrogen-responsive progesterone receptor and lactoferrin genes in the presence of ERα or modify their expression in ERα knockout (αERKO) mice. Higher circulating 17β-estradiol levels and non-classical signaling by ERΔ3 may be related to the earlier incidence of uterine cancer. These findings indicate that expression of this ERα variant can influence determining events in uterine cancer development and its natural occurrence in the human uterus would unlikely be protective.

Complex organization of the 5'-untranslated region of the mouse estrogen receptor α gene: identification of numerous mRNA transcripts with distinct 5'-ends

The 5'-untranslated region (5'-UTR) of the estrogen receptor α (ERα) gene plays an important role in determining its tissue-specific expression. We examined the 5'-UTRs of the mouse ERα mRNA variants in depth using the Basic Local Alignment Search Tool (BLAST), rapid amplification of 5'-cDNA ends (5'-RACE) and RT-PCR. We demonstrated the presence of multiple variants containing unique 5'-UTRs. We mapped the cDNA sequences onto the mouse genome, and found that both alternative splicing from four different leader exons (A, C, F1, and H) to exon 1, and combinations of 12 internal exons (X1, X2, X3, X4, F2/X5, X6, X7, X8, X9, X10, X11, and B) generate multiple ERα transcripts. Mouse exon B, that has homologies with human exon B and rat exon 0T, was used as an internal exon, not as a leader exon. RT-PCR analysis revealed distinct expression patterns of the variants, suggesting that the alternative promoter usage and alternative splicing are regulated in a tissue-specific manner. Our results indicate that the genomic organization of the mouse ERα gene is complicated as previously shown in the rat ERα gene. In addition, both alternative promoter usage and alternative splicing contribute to the remarkable mRNA diversity of the mouse ERα gene.

[Molecular mechanisms of regulation of estrogen receptor a expression level in breast cancer]

Estrogen receptor alpha (ERalpha) plays an important role in breast cancer development and progression and thus becomes a useful molecular target for breast cancer therapy. ERalpha is differentially expressed in breast cancer patients. Moreover, ERalpha expression levels may change at different stages of breast cancer even for the same patient. ERalpha expression is closely associated with the effect of endocrine therapy and prognosis. The mechanisms underlying ERalpha expression are complicated, because ERalpha expression is regulated at different levels, including chromatin, transcriptional, post- transcriptional, translational, and post-translational levels. Many proteins modulate the transcription of ERalpha gene at the chromatin and transcriptional levels through direct or indirect interaction with the ERa promoter. Some microRNAs decrease ERalpha levels possibly by induction of the degradation of ERalpha mRNA and/or repression of the mRNA translation. At the post-translational level, many proteins regulate ERalpha protein levels through ubiquitin-proteosome pathway. This review focuses on molecular mechanisms of regulation of ERalpha expression at different levels.

Alternative promoter usage and alternative splicing of the rat estrogen receptor alpha gene generate numerous mRNA variants with distinct 5'-ends

The 5'-untranslated region (UTR) of the estrogen receptor alpha (ERalpha) gene plays an important role in determining tissue-specific expression. To elucidate the regulatory mechanisms of rat ERalpha gene expression, the genomic organization must be investigated. We therefore analyzed the structure of the rat ERalpha mRNA 5'-UTR using rapid amplification of 5'-cDNA ends (5'-RACE) and RT-PCR. The analysis showed the presence of multiple variants containing unique 5'-UTRs. We mapped the cDNA sequences on the rat genome, and newly identified one leader exon (exon 0U) and ten untranslated internal exons (exons I1-10). Both splicing from four different leader exons (exons 0S, 0N, 0U, and 0/B) onto exon 1 and alternative splicing in combination with eleven internal exons (exons I1-10, and 0T) produce multiple transcripts. RT-PCR analysis revealed that each variant had preferred expression sites, suggesting that promoter usage and splicing are regulated in tissue-specific manners. Moreover, we determined a splicing event to yield Deltaexon 1 variants (0S-2-3-4-5-6-7-8), which are translated into rat 46 kDa ERalpha proteins. Our results indicate that the rat ERalpha gene is more complex than previously thought in terms of genomic organization and that both alternative promoter usage and alternative splicing contribute to the remarkable diversity of ERalpha mRNAs.

Biologic basis of sequential and combination therapies for hormone-responsive breast cancer

Although pharmacologic therapies that reduce or block estrogen signaling are effective treatments of estrogen receptor (ER)-positive breast cancer, acquired resistance to individual drugs can develop. Furthermore, this approach is ineffective as initial therapy for a subgroup of receptor-positive patients. The mechanisms of drug resistance are not completely understood, but the presence of alternative signaling pathways for activating ER response appears to play a significant role. Cross-talk between signaling pathways can activate ERs when conventional ER pathways are blocked or inactivated. For example, signaling via epidermal growth factor or HER-2 receptors, mitogen-activated protein kinases, phosphatidylinositol 3' kinase/protein kinase B, and vascular endothelial growth factor receptor can lead to estrogen-independent stimulation of ERs and tumor growth. The discovery that alternative pathways are involved in estrogen signaling has prompted development of newer endocrine therapies, such as aromatase inhibitors and pure estrogen antagonists, with distinct mechanisms for interrupting signal transduction. The existence of multiple pathways may explain the effectiveness of follow-up therapy with a different class of endocrine agents after failure of prior endocrine treatment. Because they do not have the partial agonist activity of tamoxifen that is enhanced by the adaptive hypersensitivity process, these alternative endocrine agents may play an increasingly important role in the treatment of ER-positive breast cancer. Although optimal sequencing of these agents has not been determined and is continuing to evolve, current evidence allows rational recommendations to be made. The multiple pathways involved in activating ERs also provide a rationale for combining endocrine and non-endocrine therapies that block different signaling pathways, which may have synergistic and overlapping interactions.

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.

Estrogen receptor mutations in human disease

As early as the 1800s, the actions of estrogen have been implicated in the development and progression of breast cancer. The estrogen receptor (ER) was identified in the late 1950s and purified a few years later. However, it was not until the 1980s that the first ER was molecularly cloned, and in the mid 1990s, a second ER was cloned. These two related receptors are now called ERalpha and ERbeta, respectively. Since their discovery, much research has focused on identifying alterations within the coding sequence of these receptors in clinical samples. As a result, a large number of naturally occurring splice variants of both ERalpha and ERbeta have been identified in normal epithelium and diseased or cancerous tissues. In contrast, only a few point mutations have been identified in human patient samples from a variety of disease states, including breast cancer, endometrial cancer, and psychiatric diseases. To elucidate the mechanism of action for these variant isoforms or mutant receptors, experimental mutagenesis has been used to analyze the function of distinct amino acid residues in the ERs. This review will focus on ERalpha and ERbeta alterations in breast cancer.

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.

Novel splicing events of untranslated first exons in human estrogen receptor alpha (ER alpha) gene

In order to analyze the structures of the 5'-untranslated region of estrogen receptor alpha (ER alpha) mRNA in human uterine endometrium (Em), total RNA from Em was analyzed by 5'-rapid amplification of the cDNA ends method with antisense primer located on exon 1 of human ER alpha gene. Three isoforms of 5'-RACE clones were obtained: ER alpha mRNAs containing exon (A) (the upstream region of exon 1), exon C, and exons F-E2 (we adopted the nomenclature of 5'-untranslated exons of the Gannon group). The results imply that the major isoforms of ER alpha mRNA expressed in Em are these three isoforms. Moreover, reverse transcription-polymerase chain reaction (RT-PCR) analysis was carried out on Em, ovary (Ov) and liver (Li) mRNAs to detect the novel isoforms of ER alpha mRNA in these tissues, using sense primers located on exons (A), B, C, F, and E1, and antisense primer located on exon 1. As a result, in addition to the previously reported ER alpha mRNA isoforms containing exons (A), B, C, F-E2 and E1-E2 on exon 1, we identified two novel isoform mRNAs in which exons F and E1 were directly spliced onto exon 1. Differential distributions of these isoforms of ER alpha mRNAs in Em, Ov and Li were demonstrated by RT-PCR-Southern blot analysis. These results, together with the previous reports by others, indicate that there are at least ten isoforms of ER alpha mRNA containing different 5'-untranslated regions, exons (A), B, C, D, T1-T2, T1, F-E2, F, E1-E2 and E1, expressed in human, and that these are involved in tissue specific expression of the gene.

Cloning of the novel isoform of the estrogen receptor beta cDNA (ERbeta isoform M cDNA) from the human testicular cDNA library

Our recent report has revealed the existence of the progesterone receptor (PR) isoform S, which consists of the novel PR exon S and exons 4-8 of the PR gene in the human testicular cDNA library. More recently, we have cloned the human estrogen receptor alpha (ERalpha) isoform S cDNA from the library. The ERalpha isoform S cDNA also contains the novel ERalpha exon S and exons 4-8 of the ERalpha cDNA. Based on these findings, we assumed that the novel isoform of cDNA like the PR- and ERalpha isoforms might exist in the human ER beta (ERbeta). In order to investigate this possibility, we have screened the human testicular cDNA library using the exons 4-8 corresponding sequence of the human ERbeta cDNA. Consequently, we have cloned a novel isoform of the ERbeta cDNA that consists of a previously unidentified 5'-sequence and the exons 5-8 of the ERbeta gene. We termed this isoform cDNA the "ERbeta isoform M cDNA". The 5'-sequence of the ERbeta isoform M cDNA was confirmed to be derived from a novel exon (termed the "exon M") by analysis of the genomic DNA. Moreover, we have analyzed the molecular size of the ERbeta isoform M encoded by the ERbeta isoform M mRNA by transient expression of the ERbeta isoform M cDNA in the 293T cell. The approximately 28 kDa protein, which was recognized by the anti-rat ERbeta antibody against the carboxyl-terminal region, was synthesized in the cells. Thus, we concluded that the ATG in the exon M could be used as the translation initiation codon. This report revealed for the first time the existence of the ERbeta mRNA isoform that is not caused by the skipping of one or more exons, by the alternative usage of the multiple exon 8s, nor by the alternative utilization of the untranslated 5'-exons located on the upstream region of the exon 1.

Genomic studies of the spleen protein tyrosine kinase locus reveal a complex promoter structure and several genetic variants

Here we show that the gene of the cytoplasmic tyrosine kinase SYK spans a region of 90kb with 13 coding exons, an alternative exon 14 and at least two 5' untranslated regions exons 1a and 1b. 5' RACE (Rapid amplification of cDNA ends) of human Syk cDNAs demonstrated a complex promoter usage and splicing pattern. We identified three common single nucleotide polymorphisms in the exon la promoter region of the Syk gene as well as a variant Syk cDNA haplotype. This haplotype was characterized by a constellation of 5 silent mutations in the Syk cDNA: 1065(C-T), 1302(G-C), 1338(G-A), 1521(C-T) and 1545(T-C). A hypervariable CATATA(n) repeat polymorphism was also localized to the intron between exons 11 and 12. These novel insights into the genomic organization, promoter structure and genetic variability of Syk will serve as a foundation for detailed molecular epidemiological investigation of its potential role in human cancer biology.

Progesterone receptor mRNA variant containing novel exon insertions between exon 4 and exon 5 in human uterine endometrium

The presence of human progesterone receptor (PR) mRNA variants has been demonstrated in uterine endometrium and breast tissues as well as in cancer cells of these tissues. While exon deletions by the alternative splicing in these variants have been reported, there are few reports available on the PR mRNA variants with exon insertion. In the present study, we attempted to detect a PR mRNA variant containing the exon insertions in normal uterine endometrium. Endometrial tissues were subjected to reverse transcription-polymerase chain reaction (RT-PCR) with PCR primers which were located in exons 3 and 8. Analysis of the RT-PCR products revealed the presence of a novel PR mRNA variant which contained a 232 bp inserted nucleotide sequence between exons 4 and 5. We termed this transcript the "i45 PR mRNA variant". Genomic analysis indicated that the inserted sequence was derived from two novel independent exons of 123 bp and 109 bp, termed "exon i45a" and "exon i45b", respectively, which are located between exons 4 and 5 of the human PR gene. The i45 PR mRNA variant was further detected in uterine endometrial cancer tissues as well as in the normal uterine endometrium. These results demonstrate the presence of a novel PR mRNA variant with exon insertions in the human tissue for the first time. The i45 PR variant protein, possibly transcribed from this i45 PR mRNA variant, may play physiological and/or pathological roles in the human uterine endometrium.

Genetics of osteoporosis: role of steroid hormone receptor gene polymorphisms

Osteoporosis is a common skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue with a consequent increase in bone fragility and susceptibility to fracture. In the past years, twin and family study have shown that this disease recognizes a strong genetic component and that genetic factors play an important role in regulating bone mineral density (BMD). While in few isolate conditions osteoporosis can be inherited in a simple Mendelian pattern, due to single gene mutations, in the majority of cases has to be considered a multifactorial polygenic disease in which genetic determinants are modulated by hormonal, environmental and nutritional factors. Given the important role that steroid hormones play in bone cell development and in the maintenance of normal bone architecture, polymorphisms at receptor of the steroid/thyroid hormone receptor superfamily, such as estrogen receptor alpha (ERalpha) and Vitamin D receptor (VDR) have been thoroughly investigated in the last years and appeared to represent important candidate genes. The individual contribution of these genetic polymorphisms to the pathogenesis of osteoporosis remains to be universally confirmed and an important aim in future work will be to define their functional molecular consequences and how these polymorphisms interact with each other and with the environment to cause the osteoporotic phenotype. A further promising application of genetic studies in osteoporosis comes from their pharmacogenomic implications, with the possibility to give a better guidance for therapeutic agents commonly used to treat this invalidating disorder or to identify target molecules for new therapeutic agents.

The multiple untranslated first exons system of the human estrogen receptor beta (ER beta) gene

In order to investigate the regulatory mechanism of the expression of the human estrogen receptor beta (ER beta) gene, we have analyzed the structure of the 5'-untranslated region of the ER beta mRNA in the normal uterine endometrium and liver using the 5'-rapid amplification of the cDNA ends method. The sequence analysis revealed the presence of the two isoforms of the ER beta mRNA containing the distinct 5'-untranslated regions. The genomic analysis revealed that the two isoforms of the message originated from the two distinct untranslated first exons, termed the exon 0K and exon 0N, which were spliced to the exon 1. We termed the two isoforms of the message the ER beta mRNA (0K-1) and ER beta mRNA (0N-1). We further analyzed the distribution of the ER beta mRNA (0K-1) and ER beta mRNA (0N-1) in the ejaculated spermatozoa, liver, uterine endometrium and myometrium, and peripheral leukocytes using the reverse transcription-polymerase chain reaction. The distributions of the two mRNA isoforms were different from each other. From these results, it is indicated for the first time that the expression of the human estrogen receptor beta (ER beta) gene is regulated, at least in part, by the multiple untranslated first exons and promoters system.

Estrogen receptors in the human forebrain and the relation to neuropsychiatric disorders

The steroid hormone estrogen influences brain function and neuropsychiatric disorders, but neuroanatomical information about the estrogen receptors (ERs) are rather limited. The main focus of this article is to provide an overview of the current status of the ER distribution and possible function in the human brain. The ERs are ligand activated transcription factors that belong to the steroid hormone receptors, included in the nuclear receptor superfamily. To date, there are two known ER subtypes, alpha and beta. In the human forebrain, both estrogen receptor subtypes are predominantly expressed in limbic-related areas, although they show distinct distribution patterns. The ERalpha mRNA expression appears to dominate in the hypothalamus and amygdala, indicating that the alpha-subtype might modulate neuronal cell populations involved in autonomic and reproductive neuroendocrine functions as well as emotional interpretation and processing. In contrast, the hippocampal formation, entorhinal cortex, and thalamus appear to be ERbeta dominant areas, suggesting a putative role for ERbeta in cognition, non-emotional memory and motor functions. Clinical observations of estrogenic effects together with the information available today regarding ER expression in the primate brain provide important clues as to the functional aspects of the two ER subtypes. However, further characterization of the different phenotypes of the ER expressing cells in the human brain is needed as well as the delineation of the genes which are regulated by the ERs and how this transcriptional control correlates with human behavior and mental status.

Genomic structure of the promoters of the human estrogen receptor-alpha gene demonstrate changes in chromatin structure induced by AP2gamma

Expression of human estrogen receptor-alpha (ERalpha) involves the activity from several promoters that give rise to alternate untranslated 5' exons. However, the genomic locations of the alternate 5' exons have not been reported previously. We have developed a contig map of the human ERalpha gene that includes all of the known alternate 5' exons. By using S1 nuclease and 5'- rapid amplification of cDNA ends, the cap sites for the alternate ERalpha transcripts E and H were identified. DNase I-hypersensitive sites specific to ERalpha-positive cells were associated with each of the cap sites. A DNase I-hypersensitive site, HS1, was localized to binding sites for AP2 in the untranslated region of exon 1 and was invariably present in the chromatin structure of ERalpha-positive cells. Overexpression of AP2gamma in human mammary epithelial cells generated the HS1-hypersensitive site. The ERalpha promoter was induced by AP2gamma in mammary epithelial cells, and trans-activation was dependent upon the region of the promoter containing the HS1 site. These results demonstrate that AP2gamma trans-activates the ERalpha gene in hormone-responsive tumors by inducing changes in the chromatin structure of the ERalpha promoter. These data are further evidence for a critical role for AP2 in the oncogenesis of hormone-responsive breast cancers.

The human brain has distinct regional expression patterns of estrogen receptor alpha mRNA isoforms derived from alternative promoters

The human estrogen receptor (ER) alpha gene is transcribed from multiple promoters, generating mRNA isoforms with unique 5' ends in the untranslated region. In the present study, alternative promoters were shown to regulate the ERalpha gene expression in different neuronal populations of the human brain. By using in situ hybridization histochemistry, the A and B promoters, but not the C promoter, in the ERalpha gene were found to be active in the human forebrain. The mRNA isoform transcribed from the A promoter was expressed in low levels in most of the brain areas where ERalpha mRNA was present. In contrast, the B promoter mRNA isoform was more restricted, localized predominantly in high-expressing ERalpha mRNA regions. The gross anatomical distribution of the different mRNA isoforms analyzed with RT-PCR generally supported the results obtained by the in situ hybridization. Estrogen is known to modulate many different brain functions, such as neuroendocrine events associated with reproduction, mood, and cognition, likely to be mediated by different neuronal populations. Thus, the current findings of alternative ERalpha promoter expression in distinct neuronal populations suggest that multiple promoter usage is a possible mechanism to achieve differentiated regulation of the ERalpha expression, dependent on the cell phenotype and consequently the functions mediated by the specific neuron.

Tissue-specific expression of multiple mRNA variants of the mouse estrogen receptor alpha gene

The isolation of five new mouse estrogen receptor alpha mRNA variants (mERalpha A, B, F1, F2 and H) is described. All of these transcripts are generated by alternative splicing and all encode the 66 kDa ERalpha protein that the previously identified mRNA C variant generates. However, these transcripts differ in their 5' untranslated regions. RT-PCR and S1 nuclease protection assays revealed a tissue- and sex-specific expression pattern of all variants. The C and F mRNA variants are the predominantly expressed mERalpha variants in mouse. The expression of mERalpha H mRNA is restricted to liver, although female mice produce around a five fold higher level of this transcript than males. Our results show that the mERalpha gene is a complex genomic unit in mice that exhibits alternative splicing which is regulated in a tissue-specific manner.

Assessment of hypermethylated DNA in two promoter regions of the estrogen receptor alpha gene in human endometrial diseases

OBJECTIVE

DNA methylation in the promoter regions of many genes is associated with the regulation of gene expression. We examined the frequency of DNA hypermethylation at two nucleotide positions, the proximal promoter region (PPR) in exon 1 and the distal promoter region (DPR) in exon 1', of the estrogen receptor alpha (ERalpha) gene in 111 cases of various human endometrial diseases.

METHODS

The degree of hypermethylation of PPR and DPR was examined by semi-quantitative competitive polymerase chain reaction assay using restriction enzymes (HpaII, NotI, and SacII).

RESULTS

Endometrial tissues in the proliferative phase obtained from patients with leiomyomas and/or adenomyosis and no significant hormonal abnormalities did not show hypermethylation at the HpaII cleavage position of DPR. In 6 of 16 (37.5%) cases of simple endometrial hyperplasia, the PPR was hypermethylated, whereas in cases of atypical endometrial hyperplasia and endometrioid adenocarcinoma the frequencies were extremely low. Hypermethylation of these promoter regions did not correlate with lack of ERalpha protein in the 46 cases that were analyzed by enzyme immunoassay.

CONCLUSIONS

ERalpha gene transcription from the distal promoter, rather than from the proximal promoter, is predominant in the proliferative phase of the normal menstrual cycle, because the PPR, but not the DPR, is frequently hypermethylated under those conditions. Conversely, ERalpha gene transcription from the proximal promoter is predominant in precancerous lesions such as atypical hyperplasia and endometrial cancer. However, hypermethylation at the promoter sites we examined was not related to the loss of ERalpha protein in endometrial disease.

Selective promoter usage of the human estrogen receptor-alpha gene and its regulation by estrogen

Three promoters have been identified for the human estrogen receptor-alpha gene. The positions of promoters A and B are known whereas that of the recently identified promoter C is not. Cloning and hybridization experiments demonstrated that promoter C is located more than 21 kb upstream of promoter A. The use of the three promoters was examined in estrogen receptor-positive breast cancer cell lines, cell lines derived from other malignancies, and some normal tissues by RT-PCR and transient transfection. All estrogen-responsive breast cancer cell lines used all three promoters, apart from ZR-75 cells, which did not use promoter B in one of two sublines examined. Cell lines derived from other malignancies and other normal tissues that express lower levels of estrogen receptor-alpha showed more selective promoter usage. This suggests that the level of expression of estrogen receptor-alpha is determined by the number of promoters used, rather than the selective use of specific promoters. We also show that promoter C is used more widely than suggested by others. Analysis of a series of estrogen receptor-positive primary breast tumors showed that all three promoters were used in all the tumors. All three promoters were modulated by estrogen in estrogen-responsive breast cancer cell lines: all three promoters were down-regulated by estrogen in MCF-7 cells in which estrogens reduce receptor expression whereas all promoters used were upregulated in T47D, ZR-75, and EFM-19 cells in which estrogens increase receptor expression. This suggests that it is the repertoire of transcription factors present within a cell rather than the selective use of a specific promoter that determines whether estrogen receptor-alpha expression is increased or decreased by estrogen.

Single nucleotide polymorphisms (SNPs) in the estrogen receptor gene and breast cancer susceptibility

In order to evaluate the role of inherited variation in the estrogen receptor (ESR1) gene in human breast cancer, we determined intronic sequences flanking each ESRI exon; identified multiple SNPs and length polymorphisms in the ESR1 coding sequence, splice junctions and regulatory regions; and genotyped families at high risk of breast cancer and population-based breast cancer patients and controls. Of 10 polymorphic sites in ESR1, four are synonymous SNPs, two are nonsynonymous SNPs and four are length polymorphisms; five are novel. No ESR1 polymorphisms were associated with breast cancer, either in the high-risk families or the case-control study. We therefore conclude that inherited genetic variation is not a mechanism by which the estrogen receptor is commonly involved in breast cancer development.

Hypermethylation of the Estrogen Receptor Alpha Gene Is Not Related to Lack of Receptor Protein in Human Breast Cancer

BACKGROUND: Expression and transcriptional activity of genes are regulated byseveral factors, including DNA methylation. We examined the frequency of DNA hypermethylation at two nucleotide positions, the proximal promoter region (PPR) in exon 1 and the distal promoter region (DPR) in exon 1' of the estrogen-receptor alpha (ER alpha) gene in human breast cancer, and the correlation between ER and progesterone receptor (PgR) status. METHODS: The frequency of hypermethylation of PPR and DPR in 124 breast cancerswas examined by the semiquantitative competitive polymerase chain reaction (COM-PCR) assay with restriction enzymes. ER and PgR proteins were analyzed by enzyme immunoassay (EIA; fmol/mg) to determine whether DNA hypermethylation influences the status of either protein. RESULTS: There were no significant differences in ER protein status between DNAmethylated and unmethylated groups for either PPR (71.2 +/- 190.1 versus 60.7 +/- 88.4) or DPR (70.0 +/- 183.6 versus 60.9 +/- 89.3). There was a significant differencein PgR protein status between these two groups for PPR (46.8 +/- 67.1 versus 169.1 +/- 394.9, P< 0.01). When one positional methylation was regarded as the criterion for hypermethylation, the frequency of hypermethylation in the ER(+)PgR(-)phenotype was significantly higher than in the ER(+)PgR(+), ER(-)PgR( +) and ER(-)PgR(-) phenotypes (72.7% versus 31.3%, 40.0% and 28.6%, P< 0.01). CONCLUSION: Hypermethylation of the 5'-upstream side of the ER alpha gene did notcorrelate with lack of ER, but did correlate with lack of PgR, and particularlywith the expression of the ER(+)PgR(-) phenotype. We conclude that DNA hypermethylation of PPR and DPR in the Er alpha gene reflects the expression of the ER target gene rather than the ER gene itself and may account for anti-estrogen resistance in ER-positive breast cancer.

Regulation of estrogen receptor alpha gene mediated by promoter B responsible for its enhanced expressionin human breast cancer

We have previously reported that transcription from a distal promoter (promoter B) of the estrogen receptor alpha (ERalpha) gene is responsible for the increased expression of ERalpha in human breast carcinomas. This paper first characterized the promoter B region in terms of transient transfection experiments with luciferase using MCF-7 cells. Gradual deletions from the 5'-end of promoter B resulted in a decrease in promoter activity corresponding to the deleted lengths; a deletion of 39 bp in a non-coding exon 1a, drastically diminished the activity, indicating existence of an important cis -element. Furthermore, electrophoretic mobility shift assay and subsequent mutational analysis indicated that this element containing nucleotide sequence CTGGAAAG forms a specific DNA-protein complex. This element was capable of transactivating a heterogeneous SV40 promoter in MCF-7 cells, confirming that the element is a transcriptional enhancer; the trans -acting factor binding to the element was named ERBF-1 (estrogen receptor promoter B associated factor-1). The ERBF-1 was exclusively expressed in those cells expressing ERalpha mRNA transcribed from promoter B. Our findings indicate that ERBF-1 plays an important role in the expression of the ERalpha gene transcribed from promoter B, which is selectively utilized in breast cancer.

Multiple promoters direct the tissue-specific expression of novel N-terminal variant human vitamin D receptor gene transcripts

The effects of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] are mediated by the vitamin D receptor (VDR), a member of the nuclear receptor superfamily of transcriptional regulators. We have identified upstream exons of the human (h) VDR gene that are incorporated into variant transcripts, two of which encode N-terminal variant receptor proteins. Expression of the hVDR gene, which spans more than 60 kb and consists of at least 14 exons, is directed by two distinct promoters. A tissue-specific distal promoter generates unique transcripts in tissues involved in calcium regulation by 1, 25-(OH)2D3 and can direct the expression of a luciferase reporter gene in a cell line-specific manner. These major N-terminal differences in hVDR transcripts, potentially resulting in structural differences in the expressed receptor, may contribute to cellular responsiveness to 1,25-(OH)2D3 through tissue differences in the regulation of VDR expression.

The multiple untranslated first exons and promoters system of the oestrogen receptor gene in the brain and peripheral tissues of the rat and monkey and the developing rat cerebral cortex

Recent studies on the human oestrogen receptor (ER) gene have revealed the complex system with the multiple untranslated first exons and promoters in the ER gene expression. Little information is however available on the system in the ER gene of the rat or nonhuman primate. The rat genomic library was first screened by the rat ER cDNA (0-1) probe. One of the four positive clones (lambda rEgE1) was subcloned and sequenced. The nucleotide sequence was found to contain the exon 0, the intron 0, and the exon 1 with its 3'-ends. The novel untranslated first exons, the exon ON and the exon OS, were further identified. These results indicated the presence of at least four subtypes of the rat ER mRNAs; the messages transcribed from promoter P-0 (ER mRNA (0-1)), putative promoter P-1 (ER mRNA (1-1)), promoter P-ON (ER mRNA (ON-1)) and promoter P-OS (ER mRNA (OS-1)). The P-O- or P-1 driven message (0-1) or (1-1) appeared to be expressed most strongly in major oestrogen central- (anterior pituitary, AP, hypothalamus-preoptic area, HPOA, and amygdala, AMG) and peripheral targets (uterus and ovary). The message (ON-1) was strongly expressed in the liver and kidney, but not in the HPOA, AMG, cerebral cortex, CC, and cerebellum, Ce. The OS-1 message was expressed variably but generally in the tissues examined except for the CC and Ce. Thus, the region- and tissue specific expression of the rat ER gene is likely to be regulated by the multiple untranslated exons and promoters system. Furthermore, when the ER mRNA subtypes were examined in the rat neonatal CC where the ER protein level rose transiently, considered as a model for the development of the ER or progestin receptor A and B isoforms, the expression of the ER mRNAs seemed to be differential postnatally, implicating some stage dependent usage of the promoters in the development. In the monkey, we identified the untranslated first exon OS, the homologue of the rat exon OS. Interestingly, the exon C was found to consist of two different exons, the exon OK and the exon OG. By the alternative usage of the promoters and the alternative splicing, at least six ER mRNA subtypes, that is, ER mRNAs (0-1), (1-1), (OS-1), (OS-OG-1), (OK-1) and (OK-OG-1) were identified in the monkey tissues. These messages were also differentially distributed in the monkey brain and other tissues. It was noteworthy that the P-OK driven messages were expressed almost exclusively in the monkey liver. These results have suggested that the systems of the multiple untranslated first exons and promoters and the alternative splicing are involved in the regulation of the region- and tissue specific expression of the ER gene in the brain and peripheral tissues of the rat and monkey. Stage-related usage of the promoters was also suggested in the ER gene expression in the CC of the postnatal rat in development.

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

Relative proportions of the estrogen receptor (ER) alternatively spliced mRNA variants from the proximal (A) and distal (B) promoter pre-mRNA transcripts were measured in normal human uterus, an endometrial tumor, and in T47D, MCF-7, and BT-20 breast tumor cell lines. A single tube RNA-PCR method was developed to determine the proportions of the individual transcripts and a nested, competitive RNA-PCR method to determine the proportions of the alternatively spliced variants. Except for the BT-20 cells, the patterns of splice variants produced from each transcript were very similar. In BT-20 cells no splice variants were detected for the minor (< or = 1%) A promoter transcript, although the B promoter transcript was alternatively spliced similarly to the other samples, with the exon 7 variant as the major mRNA form. These results indicate that the mRNA spliced variant patterns in most tissues and tumors will be essentially unaffected by any changes in the A and B promoter ER mRNA transcript ratios that may occur. At least one exception does exist, however, and only more comprehensive studies can determine whether the BT-20 cells are unique or part of a larger subgroup.

Differential screening and suppression subtractive hybridization identified genes differentially expressed in an estrogen receptor-positive breast carcinoma cell line

Differences in gene expression are likely to explain the phenotypic differences between hormone-responsive and hormone-unresponsive breast cancer. We have identified differentially expressed cDNAs in the estrogen receptor (ER)-positive MCF7 breast carcinoma cell line compared with the ER-negative MDA-MB-231 breast carcinoma cell line. Differential screening isolated four differentially expressed genes: cytokeratin 8, cytokeratin 18, Hsp27 and GPCR -Br. To identify differentially expressed genes of lower abundance, suppression subtractive hybridization was utilized and 29 differentially expressed clones were isolated. Sequence analysis revealed that 11 clones were from previously described genes: HEK8, neuropeptide Y receptor Y1, p21 WAF-1, p55 PIK, cytokeratin 18 (cloned twice), fructose-1,6-biphosphatase, cytokeratin 8, TGFbeta1 binding protein, elongation factor 1alpha2 and pS2. The remaining 18 clones did not match sequences in the GenBank/EMBL database, indicating that they may be novel genes. Expression of pS2, neuropeptide Y receptor Y1 and three novel clones was induced by estradiol, indicating estrogen-responsiveness. The expression pattern of one novel gene, DEME -6, correlated with expression of ER and ERF -1/ AP -2gamma in a panel of breast carcinoma cell lines. A 2.6 kb cDNA of DEME -6 was sequenced and contains an open reading frame of 574 amino acids that demonstrates 62.4% similarity with a gene from Caenorhabditis elegans chromosome III. Expression of DEME -6 was also detected in primary breast carcinomas but not in normal breast tissue, as determined by RT-PCR. These findings support the hypothesis that a set of genes coordinately regulated with ER , but not necessarily estradiol-responsive, are characteristic of the hormone-responsive breast cancer phenotype.

Transcriptional regulation of the steroid receptor genes

Steroid hormones, via their binding to specific receptors, are involved in the development, differentiation, and physiological response of cells to diverse stimuli. Activation by hormonal ligands induces conformational change in the receptor, enabling interaction with the target genes. The steroid receptor superfamily includes androgen, glucocorticoid, mineralocorticoid, progesterone, estrogen, thyroid, vitamin D, retinoic acid, and orphan receptors. This review will focus on the classic steroid receptors, i.e., the androgen, glucocorticoid, progesterone, and estrogen receptors, with emphasis on their transcriptional regulation. Readers are directed to several authoritative reviews for further details of steroid receptors (1-11).

The estrogen receptor gene: promoter organization and expression

The estrogen receptor (ER) is a ligand-activated transcription factor and a member of a large family of nuclear hormone receptors. As a mediator of estrogen hormone action, the ER is involved in many important physiological processes. ER gene expression has been demonstrated to be restricted to certain tissues and under complex hormonal control. However, the molecular mechanisms involved have remained largely unknown. Due to this lack of knowledge an investigation was undertaken to characterize the promoter organization of ER gene and investigate its expression. Approximately 3 kb of the 5' flanking region of the human ER (hER) gene was isolated and sequenced. By performing RT-PCR and RACE experiments it was shown that the hER gene is transcribed from three different promoters. Transcription of the hER gene from these promoters yields three different mRNA isoforms with unique 5' untranslated regions (5'UTRs), but identical coding regions. The expression pattern of the hER mRNA isoforms was investigated by RT-PCR. Both the A- and B-mRNA isoforms were found to be expressed in breast and uterus, whereas expression of the C-transcript was predominantly detected in liver. In bone cells only expression of the B-mRNA could be detected. The steady-state levels of the A- and B-transcripts in normal breast and uterus were quantified and compared with the hER mRNA levels in established cancer cell lines derived from the same tissues. This demonstrated approximately equal levels of the two transcripts in normal tissues whereas the A-mRNA was the most abundant isoform in the cancer cell lines investigated. Approximately 4.5 kb of the 5' flanking region of the rat ER (rER) gene were sequenced. Sequence analysis and PCR experiments suggested that the promoter organization of the rat and human ER genes is only partially conserved which might indicate species-specific differences in the regulation of ER expression. In conclusion, this work suggests tissue-specific alternative promoter usage as a mechanism in the regulation of human and rat ER gene expression.

Specific modulation of estrogen receptor mRNA isoforms in rat pituitary throughout the estrous cycle and in response to steroid hormones

We have identified several estrogen receptor (ER) mRNA isoforms in rat pituitary and characterized their regulation by gonadal steroids. The ER mRNAs correspond to splice variants in which either exon 4, exons 3 and 4, or exons 5 and 6 are deleted. A previously isolated pituitary-specific truncated mRNA, TERP-1, containing a unique 5'-end and exons 5 through 8 of the full-length ER, was also studied. The exon deletion variants were expressed in males and females, in pituitary, uterus, testes, heart, hypothalamus, and liver. An antibody to the ER C-terminus bound to full-length (64 kDa) and smaller (50 55 kDa and 40-45 kDa) ER proteins in uterus and pituitary and a pituitary-specific ER of 20-24 kDa corresponding to TERP-1. Estrogen (E) treatment in vivo stimulated full-length ER 2-3-fold, and TERP-1 7-10-fold, but had no effect on any exon deletion variant. Progesterone treatment, alone or with E, had no consistent effect on any ER mRNA form. TERP-1 mRNA was also dramatically and specifically modulated during the estrous cycle, increasing approximately 500-fold between the morning of diestrous and the afternoon of proestrus. Thus, ER mRNA variants exist in estrogen-responsive tissues; the pituitary contains at least one tissue-specific ER which is regulated by steroids and which may contribute to changes in regulated biological activity.

Human estrogen receptor (ER) gene promoter-P1: estradiol-independent activity and estradiol inducibility in ER+ and ER- cells

Estrogen receptor (ER) is expressed at a low level in normal tissues such as breast and uterus but at a high level in breast and endometrial carcinomas. A proximal element (ERF-1) located between positions +133 and +204 relative to the promoter P1 major initiation site has been recently identified in ER+ cells and contributes to the differential promoter activity between ER+ and ER- cells. In this study, MCF7 and HeLa cells were transfected with chloramphenicol acetyltransferase constructs containing ER gene promoter P1 sequences. We show here that the sequences lying between nucleotides +13 to +212 are also essential for transcription at the ER gene promoter P1 in ER- cells, which do not express ERF-1. Interestingly, on gel shift experiments, a complex specific to ER- cells forms in the region spanning nucleotides +123 to +210. We also show that promoter P1 is responsive to estradiol in cells expressing endogenous (MCF7) or exogenous ER. We further demonstrate, using mutational analysis and gel retardation assays, that the three half-estrogen response elements located between nucleotides -420 and -892 are responsible for the estradiol inducibility of promoter P1. Because estradiol has a mitogenic effect on both breast and endometrial epithelial cells, our data would give an insight into the role of estrogens in the occurrence of breast and endometrial carcinomas.

Identification of two estrogen receptor transcripts with novel 5' exons isolated from a MCF7 cDNA library

Two novel transcripts of human estrogen receptor (ER) have been identified that differ in the 5' untranslated sequence. It has previously been determined that an alternate ER transcript is generated from transcription initiated upstream of the main ER cap site (P1), and utilizes a splice acceptor site at +163. Here we report the isolation of 21 ER clones from a MCF7 cDNA library. Eleven of these clones correspond to transcripts that initiate at the P1 cap site, whereas the remaining 10 clones are derived from two previously unidentified ER transcripts (designated E and H) that both utilize the +163 splice acceptor site. A panel of breast and endometrial carcinoma cell lines were screened by reverse transcriptase-polymerase chain reaction (RT-PCR) for expression of the E and H transcripts. It was found that all ER-positive cell lines expressed both of the novel transcripts. In addition, 10 primary human breast cancers were analyzed, of which six expressed the E transcript and five abundantly expressed the H transcript. These data indicate that expression of ER in human breast cancers can be dependent upon an alternate promoter at least 20 kb upstream of the primary cap site for ER.

Auto-regulation of the estrogen receptor promoter

The presence or absence of estrogen receptor (ER) plays a key role in the diagnosis and treatment of breast tumors. It is known that patients with breast tumors classified as ER-positive have a better prognosis. Observations such as this have led us to explore the question of what makes some breast tumors overexpress ER whereas others express either very low levels or none at all. To begin a study of ER regulation, we first chose to examine a 200 bp region of the ER promoter located immediately upstream from the transcribed sequence of the human ER gene. We found that this region of the ER promoter contained basal activity when transiently transfected into ER-negative HeLa cells. ER promoter activity was further increased by co-transfection of a wild-type ER expression vector, and this increased activity was hormone-dependent. Several ER deletion mutant constructs were also able to increase the activity of the ER promoter fragment, but none could support equivalent activity as was seen with the full-length ER. Therefore, we conclude that the ER can contribute to its own expression, and we hypothesize that this auto-regulation may contribute to its overexpression in some breast tumors.

A transcriptional enhancer required for the differential expression of the human estrogen receptor in breast cancers

Breast cancers lacking estrogen receptor (ER) expression have an adverse prognosis and fail to respond to endocrine therapy. We have identified a transcriptional enhancer in the human ER gene which is differentially active in ER-positive (ER+) and ER-negative (ER-) human breast cancer cell lines. Enhancer function was mapped to a 35-bp element located from -3778 to -3744 upstream of the major human ER mRNA start site, which we have termed ER-EH0 (for estrogen receptor enhancer). Gel retardation assays with ER+ and ER- cell lines identified multiple DNA-protein complexes which specifically form on this enhancer. One of these complexes could be supershifted by anti-Jun or anti-Fos antibodies, identifying it as an AP-1-containing complex. Methylation interference assays suggest binding of factors to both the AP-1 site and adjacent base pairs. Enhancer activity requires both the AP-1 site and these adjacent sequences. Mutations introduced into ER-EH0 and the recently described proximal promoter element ERF-1 in the context of the full-length promoter confirm ER-EH0 as the dominant cis-acting element involved in differential ER expression.

Identification of sequence alterations in the upstream regulatory region of the estrogen receptor gene in an ER-negative breast cancer cell line

Given the important role of the estrogen receptor (ER) in the development and physiology of the breast, it is essential to delineate the mechanisms responsible for its failed expression in some breast tumors. We have cloned and sequenced a portion of the ER upstream regulatory region from the ER-positive MCF-7 and the ER-negative MDA-MB-231 breast cancer cell lines to determine if sequence alterations in this region account for the ER-negative phenotype of some tumors. From this, we identified a number of variations between the sequences, two of which were determined to be associated with a 50% decrease in CAT activity.

Multiple splicing variants of the estrogen receptor are present in individual human breast tumors

Transcript variants of the estrogen receptor (ER) were investigated in 109 primary breast tumors using reverse transcription-polymerase chain reaction (RT-PCR) and primers allowing analysis of each internal exon. A high incidence of different ER variants was observed, each individual tumor often manifesting multiple variants coexisting with corresponding wildtype (wt) ER. These variants, by sequence analysis confirmed to represent exon splicing deletions, included ER deltaE2 (found in 41% of the tumors examined), ER deltaE3 (74%), ER deltaE4 (72%), ER deltaE5 (66%), and ER deltaE7 (88%). No evidence of transcripts lacking exon 6 was found, although a ER deltaE5,7 variant manifesting simultaneous deletion of exons 5 and 7 was observed. The presence of specific ER variants was not significantly correlated to the status of ER and progesterone receptor (PgR) protein expression, as assessed by routine analysis, although a trend towards a higher incidence of ER deltaE3 and increased expression of ER deltaE7 in ER+/PgR- tumors was observed, suggesting a dominant inhibitory effect on normal ER function to be involved. Moreover, ER deltaE4 was more common in ER+ tumors, possibly due to a cytoplasmic sequestring of this variant lacking a nuclear localization sequence. The presence of ER variants was not associated to clinicopathological variables, and equally frequent in tumors from patients having recurred or remained recurrence-free during adjuvant tamoxifen therapy. In conclusion, although ER splicing variants are abundant in breast cancer, the present study provides no evidence for a direct role of these ER variants in tumor development and tamoxifen resistance. It remains possible, however, that minor cell clones within the tumor, undetected by analysis of tumor homogenates, displaying extreme difference in content of ER variants, could be selected for during therapy or metastasis.

The untranslated first exon 'exon 0S' of the rat estrogen receptor (ER) gene

Recently, we have isolated the untranslated first exon 'exon ON' of the rat estrogen receptor (ER) gene from the liver by the use of the 5'-rapid amplification of cDNA ends (5'-RACE) method. To investigate the existence of other untranslated first exon(s), we further analyzed the 5'-untranslated region (UTR) of ER mRNA in the rat liver in this study. Total RNA from the livers of 8-week-old male Wistar rats was subjected to 5'-RACE with the antisense primers located in exon 1 of the rat ER gene. The inserts of four clones (clones 3, 4, 7 and 8) were sequenced. The nucleotide sequences of the clones revealed the existence of a previously unidentified untranslated first exon (we termed it 'exon OS') which was spliced onto exon I of the rat ER mRNA. The distribution of ER mRNA containing 'exon OS' (ER mRNA (OS-1)) in several brain regions and various peripheral tissues of 8-week-old male and female Wistar rats was further analyzed by the use of the reverse transcription-polymerase chain reaction. ER mRNA (OS-1) was found to be widely distributed in the rat brain and peripheral tissues. The distribution of the message was different from that of ER mRNA containing exon 0 (the first reported 5'-UTR form of rat ER mRNA) or of ER mRNA with exon ON which was reported in our recent report. These results indicate that (1) 'exon OS' is a novel untranslated first exon of the rat ER gene, (2) rat ER mRNAs possess at least three forms of 5'-UTRs which are exon 0, exon ON, and exon OS, (3) the tissue specific expression of ER is regulated, at least in part, by the usage of differential promoters in the rat.