Estrogen receptor mRNA splice variants in pre- and postmenopausal human endometrium and endometrial carcinoma

The evolving role of receptors as predictive biomarkers for metastatic breast cancer

INTRODUCTION

In breast cancer, estrogen receptor (ER) and human epidermal growth factor receptor 2 (HER2) are essential biomarkers to predict response to endocrine and anti-HER2 therapies, respectively. In metastatic breast cancer, the use of these receptors and targeted therapies present additional challenges: temporal heterogeneity, together with limited sampling methodologies, hinders receptor status assessment, and the constant evolution of the disease invariably leads to resistance to treatment. Areas covered: This review summarizes the genomic abnormalities in ER and HER2, such as mutations, amplifications, translocations, and alternative splicing, emerging as novel biomarkers that provide an insight into underlying mechanisms of resistance and hold potential predictive value to inform treatment selection. We also describe how liquid biopsies for sampling of circulating markers and ultrasensitive detection technologies have emerged which complement ongoing efforts for biomarker discovery and analysis. Expert commentary: While evidence suggests that genomic aberrations in ER and HER2 could contribute to meeting the pressing need for better predictive biomarkers, efforts need to be made to standardize assessment methods and better understand the resistance mechanisms these markers denote. Taking advantage of emerging technologies, research in upcoming years should include prospective trials incorporating these predictors into the study design to validate their potential clinical value.

Recurrent hormone-binding domain truncated ESR1 amplifications in primary endometrial cancers suggest their implication in hormone independent growth

The estrogen receptor alpha (ERα) is highly expressed in both endometrial and breast cancers, and represents the most prevalent therapeutic target in breast cancer. However, anti-estrogen therapy has not been shown to be effective in endometrial cancer. Recently it has been shown that hormone-binding domain alterations of ERα in breast cancer contribute to acquired resistance to anti-estrogen therapy. In analyses of genomic data from The Cancer Genome Atlas (TCGA), we observe that endometrial carcinomas manifest recurrent ESR1 gene amplifications that truncate the hormone-binding domain encoding region of ESR1 and are associated with reduced mRNA expression of exons encoding the hormone-binding domain. These findings support a role for hormone-binding alterations of ERα in primary endometrial cancer, with potentially important therapeutic implications.

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.

Oestrogen receptor splice variants in the pathogenesis of disease

The full-length oestrogen receptor (ER) exists in most vertebrates as two separately encoded isoforms. ER splice variants represent truncated or otherwise modified versions of the full-length alpha or beta isoforms of the parent receptor. ERalpha is found on chromosome 6q and encodes a 595 amino acid protein, while ERbeta is found on chromosome 14q and encodes a 530 amino acid protein. These receptors possess differing ligand affinities, are differentially expressed in a tissue-specific fashion and may act antagonistically. Their altered expression has been implicated in the pathophysiology of a diverse range of conditions from cancer progression in hormone-responsive tissues to neurodegenerative disease. Variously co-expressed with full-length ERs, ER splice variants may have a positive or negative influence on transcription either by modifying the effect of the parent receptor or through their own intrinsic activity. To date, the vast majority of studies have used generic primers or antibodies against the full-length receptors and would not distinguish ER-mediated effects associated with various splice variants. Thus the evidence base of the influence of ER splice variants in normal developmental physiology and in the pathogenesis of disease is weak and greater understanding of their role will undoubtedly lead to new therapeutic strategies for disease intervention and treatment. This review aims to compile the current evidence for the presence of ER splice variants in humans, their physiological roles and clinical sequelae.

Estrogen receptor-alpha splice variants in the human brain

In the present review we discuss recent findings showing that, in addition to the canonical estrogen receptor-alpha (ERalpha), the level of various ERalpha splice variants is changed in the human brain in aging and Alzheimer's disease (AD) at both the mRNA and protein level and that they should be considered for the understanding of estrogen effects on the brain and estrogen therapy pitfalls. Indeed, the expression pattern of certain splice forms is brain area-specific. Thus, the major isoform found in the mamillary body (MB) appeared to be del.7 (deletion of exon 7), while in the hippocampus del.4 (exon 4 omitted) was expressed at the highest level. Furthermore, while transcripts missing exons 7 and 2 declined with aging in the MB of patients >60 years old, no age-related alterations were determined for a number of splice variants in the hippocampus. A novel MB1 isoform with a 168-bp deletion within the transactivation function 1 of ERalpha turned out to accumulate in the histaminergic tuberomamillary nucleus of postmenopausal women. Finally, the level of alternatively spliced ERalpha may also change in AD in a brain area-specific manner and so affect the sensitivity to estrogen therapy.

Hormonal heterogeneity of endometrial cancer

Endometrial cancer is the most common malignant tumor of the female genital tract in the developed world. Increasing evidence suggests that the majority of cases can be divided into two different types ofendometrial cancer based on clinico-pathological and molecular characteristics. Type I is associated with an endocrine milieu of estrogen predominance. These tumors are ofendometroid histology and develop from endometrial hyperplasia. They have good prognosis and are sensitive to endocrine treatment. Type II endometrial cancers are not associated with a history of unopposed estrogens and develop from the atrophic endometrium of elderly women. Mainly, they are of serous papillary or clear cell morphology, have a poor prognosis and do not react to endocrine treatment. Both types of endometrial cancer probably differ markedly with regard to the molecular mechanisms of transformation. The transition from normal endometrium to a malignant tumor is thought to involve a stepwise accumulation of alterations in cellular mechanisms leading to dysfunctional cell growth. This chapter reviews the current knowledge of the molecular mechanisms commonly associated with development of type I and type II endometrial cancer.

Coexpression index of estrogen receptor alpha mRNA isoforms in simple, complex hyperplasia without atypia, complex atypical hyperplasia and adenocarcinoma

OBJECTIVE

Estrogen receptor isoforms are postulated to play an important role in modulating the estrogen response. To clarify the molecular mechanisms through which malignant changes are activated in endometrium, this study aims to examine the expression profiles of wild-type ER-alpha and their splice variants and to assess the number of coexisting mRNA isoforms of ER-alpha in normal endometrium as well as in endometrial hyperplasia and endometrial endometrioid adenocarcinoma.

METHODS

Human endometrium and specimens including endometrial hyperplasia and endometrial cancer were obtained during surgery. Endometrial data were classified into four groups: simple hyperplasia (n=24), complex hyperplasia (n=15), atypical hyperplasia (n=11), endometrial endometrioid adenocarcinoma (n=19) (grade 1, grade 2 morphological degree) and proliferative endometrium (n=24) as a control group. Total cellular RNA was extracted from endometrial tissues using Total RNA Prep Plus. A real-time quantitative RT-PCR assay was developed to quantify the wild-type ER-alpha and ER-alpha mRNA isoforms copy numbers. We have evaluated the variation in ERs mRNA level between normal endometrium and endometrial hyperplasia and adenocarcinoma. We also evaluated the "sharing indicator". It is a factor of mRNA ER-alpha holding shares in whole mRNA it assume quotient of ER-alpha slicing variant to all variants of mRNA ER-alpha.

RESULTS

It was found that the number of coexisting mRNA isoforms was significantly higher in adenocarcinoma endometrium than that evaluated for various degrees of hyperplasia endometrium and normal proliferative endometrium (p<0.05, the Kruskal-Wallis test).

CONCLUSION

The risk for progression of endometrial hyperplasia to atypical hyperplasia and eventually endometrioid adenocarcinoma may be accompanied by an increase in the number of alternative splicing variants of mRNA ER-alpha.

The G protein-coupled receptor GPR30 mediates the proliferative effects induced by 17beta-estradiol and hydroxytamoxifen in endometrial cancer cells

The growth of both normal and transformed epithelial cells of the female reproductive system is stimulated by estrogens, mainly through the activation of estrogen receptor alpha (ERalpha), which is a ligand-regulated transcription factor. The selective ER modulator tamoxifen (TAM) has been widely used as an ER antagonist in breast tumor; however, long-term treatment is associated with an increased risk of endometrial cancer. To provide new insights into the potential mechanisms involved in the agonistic activity exerted by TAM in the uterus, we evaluated the potential of 4-hydroxytamoxifen (OHT), the active metabolite of TAM, to transactivate wild-type ERalpha and its splice variant expressed in Ishikawa and HEC1A endometrial tumor cells, respectively. OHT was able to antagonize only the activation of ERalpha by 17beta-estradiol (E2) in Ishikawa cells, whereas it up-regulated c-fos expression in a rapid manner similar to E2 and independently of ERalpha in both cell lines. This stimulation occurred through the G protein-coupled receptor named GPR30 and required Src-related and epidermal growth factor receptor tyrosine kinase activities, along with the activation of both ERK1/2 and phosphatidylinositol 3-kinase/AKT pathways. Most importantly, OHT, like E2, stimulated the proliferation of Ishikawa as well as HEC1A cells. Transfecting a GPR30 antisense expression vector in both endometrial cancer cell lines, OHT was no longer able to induce growth effects, whereas the proliferative response to E2 was completely abrogated only in HEC1A cells. Furthermore, in the presence of the inhibitors of MAPK and phosphatidylinositol 3-kinase pathways, PD 98059 and wortmannin, respectively, E2 and OHT did not elicit growth stimulation. Our data demonstrate a new mode of action of E2 and OHT in endometrial cancer cells, contributing to a better understanding of the molecular mechanisms involved in their uterine agonistic activity.

Estrogen receptor-alpha splice variants in the medial mamillary nucleus of Alzheimer's disease patients: identification of a novel MB1 isoform

Previously we have reported an increased nuclear estrogen receptor-alpha (ERalpha) in the medial mamillary nucleus (MMN) in Alzheimer's disease (AD). In the present study, we addressed the presence of specific ERalpha mRNA splice variants in this brain area of five AD cases compared with five controls using the RT-PCR and quantitative RT-PCR approach. Indeed, the occurrence of isoforms with the deletion of exons 7 (del.7), 4 (del.4), or 2 (del.2) was determined in all patients. However, there were no significant differences in the relative transcription levels of each of the mentioned splice variants between AD and control cases, although the ratio of the del.7 isoform to the canonical ERalpha mRNA was higher in controls. Given that exons 7 and 4 encode the ligand-binding domain of the ERalpha, whereas exon 2 encodes the DNA-binding domain, abundant expression of these splice variants suggests that much of the available ERalpha in the MMN of AD and elderly control patients is nonfunctional because they will be unable to bind either the ligand (del.7 and del.4 variants) or the estrogen-responsive elements on appropriate DNA (del.2 variant). Yet, the wild-type ERalpha mRNA appeared to be 2- to 3-fold up-regulated in AD, confirming the rise in the nuclear immunocytochemical staining and pointing to the potential for a beneficial effect of estrogen replacement therapy on the MMN-associated cognitive functions in AD because it represents the availability of potentially functional ERalpha in the MMN. Noteworthy, the expression of the wild-type, del.7, and del.2 mRNAs declined with advanced age in both AD and control patients. Interestingly, we have identified in two AD and two control patients a novel ERalpha splice variant that we called MB1 (mamillary body, exon 1) with a 168-nucleotide deletion corresponding to a U2-type intron inside exon 1 encoding the major portion of the transactivation function 1 domain of the receptor.

Expression of estrogen receptor alpha exon-deleted mRNA variants in the human and non-human primate frontal cortex

Although estrogen receptor alpha (ERalpha) mRNA has been detected in the primate frontal cortex, the types of ERalpha transcripts expressed, including exon-deleted variants (Delta), have not been determined in the monkey or human frontal cortex. Because the types of ERalpha mRNA expressed in brain could define neuronal responses to estrogens, we examined the transcript pool of ERalpha mRNAs expressed in normal adult and developing human and macaque frontal cortex. We reverse transcribed total RNA from the postmortem frontal cortex of 29 normal adult humans, 12 rhesus macaques, and 19 people ranging from infants to adults and employed two rounds of nested polymerase chain reaction (PCR) to generate ERalpha products spanning the coding domain. In a third nested PCR, we used primers specific for novel sequences of exon-exon junctions created when whole exons are missing. By sequencing PCR products, we detected 60 instances of 12 distinct DeltaERalpha mRNAs in adult humans and 94 instances of 13 distinct DeltaERalpha mRNAs in monkeys in differing patterns from one individual to another. In adult humans, 83% of individuals expressed at least 1 DeltaERalpha mRNA variant, and 100% of the monkeys expressed at least 1 DeltaERalpha mRNA variant. The single Delta2, Delta5, and Delta7 variants were frequently expressed in both human and monkey frontal cortex, Delta3 variants were rare in both species, and Delta6 variants were more frequently expressed in monkeys. In both species, we detected double, triple and quadruple Deltas, but these were less common than single Deltas. The pattern of human variant expression did not appear to change dramatically as a function of age. These findings imply the potential to produce different ERalpha proteins in frontal cortex, possibly with altered structure and function which may have physiological relevance for gene transcription by virtue of altered functional interactions with each other, other steroid hormone receptors, and genomic DNA.

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.

Estrogen receptor-alpha messenger RNA variants that lack exon 5 or exon 7 are coexpressed with wild-type form in human endometrium during all phases of the menstrual cycle

OBJECTIVE

We have assessed the expression levels of messenger RNA for estrogen receptor-alpha and splice variants lacking exon 5 or exon 7 that presumably exert dominant positive (splice variants lacking exon 5) and negative (splice variants lacking exon 7) effects, respectively, on estrogen responses in the human endometrium.

STUDY DESIGN

This was a prospective study that was conducted at an academic community-based hospital. The patients, aged 18 to 40 years, underwent hysterectomy for benign gynecologic causes. Eighty-one endometrial specimens (46 proliferative, 35 secretory) were analyzed with the use of reverse transcription-polymerase chain reaction assay for the messenger RNA levels of estrogen receptor-alpha, and splice variants lacking exon 5 and exon 7.

RESULTS

Wild-type estrogen receptor-alpha and splice variants splice variants lacking exon 5 and lacking exon 7 messenger RNAs were detected in all endometrial specimens throughout the menstrual cycle. In addition, a double-splice estrogen receptor-alpha messenger RNA variant (splice variants lacking exon 5 and exon 7) was detected at constant low levels of expression. Semiquantitative analysis showed higher levels of estrogen receptor-alpha messenger RNA in the early and mid proliferative endometrial phases than in late proliferative and secretory endometrium ( P <.05). The splice variant lacking exon 7 messenger RNA expression level was about 10-fold higher than the splice variant lacking exon 5 messenger RNA relative to wild-type estrogen receptor-alpha messenger RNA ( P <.001). The expression of splice variants lacking exon 5 compared with wild-type estrogen receptor-alpha messenger RNA is relatively constant throughout endometrial development. In contrast, an examination of the ratio of the levels of splice variants lacking exon 7 to wild-type estrogen receptor-alpha messenger RNA indicated a small, but significantly higher, splice variant lacking exon 7 level in the mid secretory phase (postovulatory days 5-8) than the mid proliferative and early secretory phases ( P <.05).

CONCLUSION

We found no evidence of differential coexpression of the positive dominant estrogen receptor variant, splice variants lacking exon 5, with wild-type estrogen receptor-alpha. We did find that the dominant negative splice variant lacking exon 7 was slightly increased relative to wild-type estrogen receptor-alpha in the postovulatory phase. Future investigation is required to suggest the biologic significance of the observed increased relative expression of the splice variants lacking exon 7 in secretory endometrium and to determine the function of splice variants lacking exon 5 and splice variants lacking exon 7.

The naturally occurring variant of estrogen receptor (ER) ERDeltaE7 suppresses estrogen-dependent transcriptional activation by both wild-type ERalpha and ERbeta

We have isolated and functionally characterized the exon 7-skipped variant (ERDeltaE7) of estrogen receptor (ER)alpha, which has emerged as the predominant variant expressed in multiple normal and tumoral tissues. However, to date no function has been established for this variant in mammalian cells. ERDeltaE7 exhibits a negligible ability to bind ligands, insensitivity to allosteric modulation by estrogen and antiestrogens, and loss of estrogen-dependent interaction with p160 coactivators such as SRC-1 and AIB1. ERDeltaE7 is able to form heterodimers with both ERalpha and ERbeta in a ligand-independent manner. Transient expression experiments in HeLa cells show that increasing amounts of ERDeltaE7 result in a progressive inhibition of the estrogen-dependent transcriptional activation by both wild-type ERalpha and ERbeta on estrogen response element-driven promoters. The inhibitory effect of ERDeltaE7 is due to the inhibition of binding of wild-type receptors to their responsive elements. Surprisingly, the activation function (AF)-1-dependent transactivation triggered by epithelial growth factor and phorbol-12-myristate-13-acetate is also abolished in ERDeltaE7 despite AF1 integrity, suggesting a cross-talk between AF1 and AF2 regions of the receptor. These results indicate that the naturally occurring variant ERDeltaE7 is a dominant negative receptor that, when expressed at high levels relative to wild-type ERs, might have profound effects on several estrogen-dependent functions.

Estrogen receptor alpha mRNA variant lacking exon 5 is co-expressed with the wild-type in endometrial adenocarcinoma

BACKGROUND

Endometrial adenocarcinoma is a typical estrogen-dependent neoplasia. The molecular mechanisms underlying carcinogenesis in the endometrium are still largely unknown. Recently, estrogen receptor (ER) mRNA splicing variants have been investigated in several normal and neoplastic human tissues. It has been suggested that the variant receptors compete with the wild-type receptors and thereby modulate the effects of estrogens and related steroids.

OBJECTIVE

To investigate the possible expression of the ER alpha mRNA variant-type lacking exon 5 (ERDelta5) in endometrial adenocarcinoma and peritumoral tissues, non-neoplastic endometrium of healthy women served as control.

STUDY DESIGN

The study included 16 patients divided in two groups. The first group (n=6) was submitted to hysteroscopy and endometrial biopsy for metrorrhage, showing normal proliferative (n=2) or secretory (n=4) endometrium, the second group (n=10) included patients submitted to hysterectomy for endometrial adenocarcinoma (stages Ib-IIIb). In this latter group, specimens from peritumoral tissues were also analyzed (n=3). Characterization of the variant and wild-type alpha estrogen receptor transcripts was performed by RT-PCR with primers located in exons 4 and 6, followed by southern hybridization with probes directed to a specific 29 nucleotide sequence of exon 6, internal to the amplified fragments.

RESULTS

The ER alpha mRNA variant was co-expressed with the wild-type ER in five or six samples of non-neoplastic endometrium and in 10/10 cases of adenocarcinoma, with a more intense hybridization signal corresponding to the wild-type 439bp band compared to the variant-type 300bp band. Specimens from peritumoral tissue also expressed the variant ERDelta5 along with wild-type ER.

CONCLUSION

The presence of alpha mRNA variant lacking exon 5 in both normal and endometrial adenocarcinoma do not support a major role of variant estrogens receptors in the biology of endometrial cancer.

Function of the exon 7 deletion variant estrogen receptor alpha protein in an estradiol-resistant, tamoxifen-sensitive human endometrial adenocarcinoma grown in nude mice

BACKGROUND

In addition to hormone and DNA binding, interactions, including competition with other proteins, appear to be a critical component of transcriptional regulation by the estrogen receptor alpha (ER(alpha)). In vitro studies suggest that exon deletion (Delta exon) variant forms of ER(alpha) may also play an important role in determining the progression from hormone dependence to hormone independence in receptor positive tumors.

METHODS

We investigated the presence of ERalpha mRNA and protein variants and their possible role in a moderately differentiated human endometrial adenocarcinoma grown in nude mice. In addition to wild-type (wt), RT-PCR assay of the tumor revealed the presence of two mRNA variants, a low concentration of Delta5 and a high concentration of Delta7 ER(alpha). We detected wt, Delta7, and Delta5,7 mRNA by sequencing the transcripts after stable transfection of three HeLa cells with either splice variant. The linked in vitro translation/transcription assay of the transfected cells and the Western blot analysis of the original tumor generated both wt (66 kDa) and Delta7 (52 kDa), Delta5,7 (46 kDa) ER(alpha) proteins.

RESULTS

Tumor growth was characterized as estradiol and progesterone resistant but tamoxifen sensitive, i.e., neither estradiol nor progesterone treatment altered the growth rate, whereas tamoxifen treatment significantly increased the tumor volume doubling time. Estradiol treatment decreased the wt and increased the Delta7 variant ER(alpha) protein expression significantly in a dose-dependent manner. Tamoxifen treatment, however, increased the expression of both proteins whereas progesterone had no effect. Estradiol treatment did not influence expression of the Delta5,7 variant protein, which increased significantly in the tamoxifen-treated tumors. Gel mobility shift assays revealed that both wt and Delta7 ER(alpha) proteins bind to the consensus DNA sequence, whereas the Delta5,7 variant protein did not.

CONCLUSIONS

We conclude that estradiol, tamoxifen, and progesterone regulate wt and variant ER(alpha) mRNA and protein expression separately and differently and that this hormonal regulation probably occurs, via different mechanisms, at the transcriptional or posttranscriptional level. The Delta7 variant ER(alpha) may play a crucial role in the determination of hormone sensitivity and thus in the outcome of hormone treatment of human endometrial adenocarcinomas.

Immunohistochemical analysis of gynecologic tumors

Ancillary techniques such as immunohistochemistry (IHC) enable the surgical pathologist to extract additional information from fixed, deparaffinized tissue specimens and to provide data critical to optimal clinical management of the patient. In this review of applications of IHC to the analysis of gynecologic malignancies, the usefulness of immunohistochemical analysis of neoplasms of the cervix, endometrium, and ovary is summarized. In the uterine cervix, dysplasia is associated with qualitative and quantitative alterations in the expression of the Ki-67 antigen expression, as well as an ability to detect human papillomavirus. Endometrial endometrioid adenocarcinomas display a highly characteristic immunophenotype, with coexpression of cytokeratin and vimentin and demonstration of foci of high molecular weight cytokeratin expression; in addition, IHC analysis of estrogen and progesterone receptor and p53 expression can provide important prognostic information about this tumor. Stromal tumors of the endometrium may display a partial smooth muscle immunophenotype, but novel markers such as CD10 provide new tools for the identification of these tumors. The immunophenotypes of the normal ovarian surface epithelium (OSE) and corresponding tumors display significant overlap with, but important distinctions from, mesothelium, and important new markers such as the Wilms tumor gene product can prove useful in the identification of carcinomas of the OSE. Important prognostic markers for carcinomas of the OSE include the HER-2/neu gene product and p53, alterations of which can both be assessed by IHC techniques. Finally, the recent availability of markers of ovarian stroma, including Melan-A and inhibin-alpha, has provided a means for the positive identification of ovarian stromal tumors, which can manifest protean histological appearances.

Exon deletions and variants of human estrogen receptor mRNA in endometrial hyperplasia and adenocarcinoma

Estrogen receptors (estrogen receptor alpha, ER) belong to a family of ligand-modulated transcription factors that play an important role in the progression of such tumors as breast and endometrial cancers. Functional domains, a set of mutations and variants produced by internal deletions of ER mRNA, have mainly been identified in breast cancer. Experimental results suggest that the presence of variants may result in different proteins which differ in activity and modulate the ER signaling pathway differently. We analyzed samples from 21 cases of endometrial hyperplasia and from 29 cases of endometrial cancer for the presence of internal exons and exon deletion variants of ER mRNA. ER and progesterone receptor (PgR) proteins were measured using Western blot technique in all endometrial cancer samples. We found that absence of the wild-type exon PCR product of ER mRNA in a sample increased in parallel with malignant potential in both sample types, whereas the number of exon deletion variants detected in the same sample decreased in cases of malignancy. The precise deletions of the respective exons suggest that they are probably the result of splicing errors. A relatively high number of variants in hyperplasia samples may indicate the important role of ER mRNA variants in the physiologic regulation of transcription in estrogen-sensitive genes. Eleven of 29 adenocarcinomas expressed a 62-kDa ER protein, truncated at the amino terminal, whereas all but one sample expressed a short 52 kDa variant ER protein. Our results suggest that differing ER proteins are generally present in human endometrial adenocarcinomas and that they may influence the estradiol signaling pathways.

The truncated estrogen receptor alpha variant lacking exon 5 is not involved in progesterone receptor expression in meningiomas

Human meningioma tissues are mostly estrogen receptor (ER) negative and progesterone receptor (PR) positive in ligand binding and enzyme immuno assays. To explain this apparently ER independent PR expression, we investigated the existence of a 'hidden' ER variant, which would be capable of activating transcription of the PR gene. Total RNA of seven meningiomas, two breast cancer tissues and of MCF7 cells was analyzed by RT-PCR using primers situated in exon 4 and exon 6. Differential hybridization of the PCR transcripts with probes in exon 4 and 5 respectively, revealed a wild type ER (wtER) fragment and an exon 5 deleted ER variant (ERDelta5). PCR products of two meningiomas were cloned for sequence analysis. The result confirmed the existence of a wtER and ERDelta5.RT-PCR followed by Southern analysis was performed on mRNA of 23 meningiomas to determine the amount of ERDelta5 relative to wtER, which was compared to the PR content of the tissues. In contrast to our initial hypothesis and literature data on breast cancer, there was no relationship between the ERDelta5/wtER ratio and PR protein concentration. It is therefore concluded that ERDelta5 mRNA does not play the dominant role in PR synthesis in meningioma tissue.

Estrogen receptor-alpha mRNA variants in murine and human tissues

A side-by-side comparison of several normal mouse and human tissues was undertaken in order to determine if exon-deleted variant ER-alpha mRNAs are expressed in the mouse. The data showed that the complex pattern of ER-alpha alternative splicing that is detected in multiple human tissues was not apparent in murine tissues. Only low levels of an exon-4 deleted ER-alpha transcript were detected in murine tissues, although multiple relatively abundant exon-deleted ER-alpha transcripts were detected in human tissues. The data support a species-specific difference in the expression of ER-alpha variant mRNAs between mouse and human.

Expression of estrogen receptor alpha mRNA and protein variants in human endometrial carcinoma

Breast cancer tissue has been shown to contain alternatively spliced estrogen receptor alpha (ER-alpha) mRNA variants, which have altered biological activities compared to the full-length ER-alpha. The development of endometrial cancer, as well as drug resistance in breast cancer patients undergoing tamoxifen therapy, may represent altered ER-alpha function secondary to specific exon deletions. While the literature is replete with ER mRNA variant data, little information is available regarding the presence and function of endometrial ER variant proteins. We evaluated the presence of human ER-alpha mRNA and protein variants in six premenopausal, six postmenopausal, and six endometrial carcinoma samples. Reverse transcription-polymerase chain reaction, DNA hybridization, and sequencing techniques identified exon 4, exon 5, and exon 7 mRNA splice variants in all patients as well as MCF-7 and Ishikawa cell lines. Presence of translated proteins for full-length ER-alpha, as well as splice variants, was investigated by Western blot analysis using antibodies directed against the N-terminus, hinge region, and C-terminus portions of the ER. These experiments confirmed the presence of immunopositive protein bands of approximately 64-66 kDa in all patients corresponding to wild-type ER-alpha. A protein band migrating at 41 kDa, consistent with an exon 5 splice variant, was only seen in endometrial adenocarcinoma samples. Premenopausal and postmenopausal endometrial samples did not contain detectable amounts of ER splice variant protein. Human ER-alpha mRNA variants are present in all human endometrial samples, but detectable levels of variant proteins are only observed in patients with endometrial adenocarcinoma.

Hormonal treatment of endometrial cancer

This article summarizes the endocrine background of women with endometrial cancer at both peripheral and tissue levels, and the current status of clinical trials of hormonal, cytotoxic, and combination regimens. Because significant advances in systemic therapy are required to improve the prognosis of endometrial cancer, recommendations for future clinical investigations will be based on these recent biologic observations.

Characterization of membrane estrogen binding proteins from rabbit uterus

Estrogens exert fast non-genomic actions in their target tissues which may involve the participation of receptors located at the cell membrane. Studies were performed to identify and characterize membrane-associated 17beta-estradiol binding proteins in rabbit uterus. Specific and saturable [3H]17beta-estradiol binding sites of high affinity (Kd = 0.36 nM) were detected in uterine microsomes at higher concentration than in cytosol (370 +/- 98 vs. 270 +/- 87 fmol/mg protein, respectively). Various other steroid hormones, the stereoisomer 17alpha-estradiol and the antiestrogen tamoxifen were significantly less effective than 17beta-estradiol to compete with the radioactive ligand for binding to the membranes. The microsome binding sites were trypsin-sensitive and could be extracted to a great extent (80-90%) with 0.4/0.6 M KCl. Assays of the marker enzyme glucose-6-P dehydrogenase excluded membrane contamination with cytosolic soluble components. Immunoblot analysis of particulate and soluble fractions using monoclonal antibodies against the transactivation, heat shock protein recognition, and steroid binding domains of the nuclear estrogen receptor (ER; 67 kDa), revealed lower concentrations of the ER in membranes and the presence of five additional immunoreactive proteins of 57, 50, 32, 28, and 11 kDa which were absent in cytosol. Moreover, the antibody against the steroid binding domain was as effective as an inhibitor for cytosolic and membrane specific radioligand binding. Extraction of microsomes with the nondenaturing detergent CHAPS allowed a 2-fold enrichment of ER-like binding proteins as shown by antibody labeling and [3H]17beta-estradiol binding analysis. The results of this work are consistent with the existence of novel 17beta-estradiol membrane binding proteins structurally related to the intracellular ER. Future studies should investigate whether any of these proteins are involved in the primary events (e.g. receptor function) mediating nongenomic estrogen effects.

Estrogen receptor variants

Estrogen receptor (ER)3 gene expression in breast epithelium is an intricately regulated event. The human ER gene is transcribed from at least three different promoters which are expressed in a cell- and tissue-specific manner, and result in mRNA isoforms with unique 5'-untranslated exons. The ER is overexpressed in about two thirds of breast tumors, and even in early premalignant breast lesions compared with adjacent normal breast epithelium. Furthermore, normal breast epithelium as well as breast cancer tissue contains alternatively spliced ER mRNA variants where single or multiple exons are skipped. It is still unclear if any or all of the ER mRNA splicing variants are translated in vivo, and if a change in the balance of ER variants could effect tumor development and progression to hormone-independent growth. Although infrequent in primary breast cancer, single amino acid changes within the ER in metastatic disease which might influence cell proliferation may also contribute to neoplastic progression of the mammary epithelium.