The novel isoform of the estrogen receptor-alpha cDNA (ERalpha isoform S cDNA) in the human testis

Estrogen Receptor α Isoforms Generated by Alternative Use of Cryptic Exons

Estrogen receptor α (ERα) regulates several physiological functions. In pathophysiological conditions, ERα is involved in the development and progression of estrogen-sensitive tumors. The ERα gene contains multiple 5'-untranslated exons and eight conventional coding exons and presents multiple isoforms generated by alternative promoter usage and alternative splicing. This gene also possesses non-conventional exons in the 3'- and intronic regions, and alternative use of cryptic exons contributes to further diversity of ERα mRNAs and proteins. Recently, the genomic organization of ERα genes and the splicing profiles of their transcripts were comparatively analyzed in humans, mice, and rats, and multiple ERα isoforms with distinct structures and functions were identified. These transcripts contain cryptic sequences that encode insertion-containing or truncated ERα proteins. In particular, alternative cryptic exons with in-frame stop codons yield transcripts encoding C-terminally-truncated ERα proteins. The C-terminally-truncated ERα isoforms lack part or all of the ligand-binding domain but possess an isoform-specific sequence. Some of these isoforms exhibit constitutive transactivation and resistance to estrogen receptor antagonists. Although numerous studies have reported conflicting results regarding their functions, the critical determinant for their gain-of-function has been identified structurally. Here we review recent progress in ERα variant research concerning the genomic organization of ERα genes, splicing profiles of ERα transcripts, and transactivation properties of ERα isoforms.

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

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

Identification of a novel C-terminally truncated estrogen receptor α variant (ERαi34) with constitutive transactivation and estrogen receptor antagonist resistance

Constitutively active estrogen receptor α (ERα) variants with C-terminal truncation are candidate molecules for gain of both endocrine- and chemotherapy-resistance in estrogen-sensitive tumors. Our previous reports using artificially truncated ERα constructs demonstrated that ERα variants encoded in 1-2-3-cryptic exon- and 1-2-3-4-cryptic exon-types of transcripts have potentials to display constitutive transactivation of an estrogen response element reporter. However, naturally occurring 1-2-3-cryptic exon-type ERα variants have not been cloned yet. Therefore, the present study was designed to identify naturally occurring ERα variants encoded in 1-2-3-cryptic exon-type ERα transcripts. We cloned a novel C-terminally truncated ERα variant (ERαi34) encoded in a 1-2-3-i34 transcript from MCF-7 cells and characterized its constitutive and ER antagonist-resistant transactivation in transfected cells. Stable transfection of the variant into MCF-7 cells augmented basal cell proliferation insensitive to fulvestrant. Collectively, we validated the structure-based mechanisms underlying constitutive and ER antagonist-resistant transactivation by C-terminally truncated ERα variants.

Sex steroid hormones as neuroprotective elements in ischemia models

Among sex steroid hormones, progesterone and estradiol have a wide diversity of physiological activities that target the nervous system. Not only are they carried by the blood stream, but also they are locally synthesized in the brain and for this reason, estradiol and progesterone are considered 'neurosteroids'. The physiological actions of both hormones range from brain development and neurotransmission to aging, illustrating the importance of a deep understanding of their mechanisms of action. In this review, we summarize key roles that estradiol and progesterone play in the brain. As numerous reports have confirmed a substantial neuroprotective role for estradiol in models of neurodegenerative disease, we focus this review on traumatic brain injury and stroke models. We describe updated data from receptor and signaling events triggered by both hormones, with an emphasis on the mechanisms that have been reported as 'rapid' or 'cytoplasmic actions'. Data showing the therapeutic effects of the hormones, used alone or in combination, are also summarized, with a focus on rodent models of middle cerebral artery occlusion (MCAO). Finally, we draw attention to evidence that neuroprotection by both hormones might be due to a combination of 'cytoplasmic' and 'nuclear' signaling.

Human C-terminally truncated ERα variants resulting from the use of alternative exons in the ligand-binding domain

The nuclear receptor genes contain alternative internal and terminal exons, with alternative exon incorporation yielding mRNA variants that encode various receptor types, including some with C-terminal truncation that exhibit constitutive activation or dominant-negative transcriptional transactivation. However, C-terminally truncated estrogen receptor α (ERα) variants with alternative sequences have rarely been reported in humans. Therefore, we assessed human ERα genomic organization and alternative splicing profiles, and identified both alternative exons and C-terminally truncated ERα variants. These naturally occurring C-terminally truncated ERα proteins were localized in the nuclei of transfected cells. In addition, ERαi45c and ERαΔ5 variants exhibited constitutive transactivation of an estrogen responsive element-driven promoter in transfected cells. We manufactured expression vectors encoding artificially truncated ERα constructs and evaluated their transactivation abilities to establish mechanisms determining the constitutive activity and dominant-negative properties of truncated variants. Lack of the region encoded in exon 8 eliminated basal and ligand-induced transcriptional transactivation. The C-terminally truncated ERα variants/constructs containing the helices 5 in their ligand-binding domains did not exhibit constitutive transactivation. Furthermore, we demonstrated that truncation from C-termini to helices 5 in the variant ligand-binding domains was required for constitutive activation and found that the remnant regions of the ligand-binding domains and variant-specific sequences influenced transcriptional transactivation efficiency. In conclusion, we elucidated the structural and functional features of novel C-terminally truncated ERα variants and revealed the mechanisms underlying constitutive transactivation by C-terminally truncated nuclear receptor variants.

Identification of androgen receptor variants in testis from humans and other vertebrates

The androgen receptor (AR) is a ligand-activated transcription factor member of the nuclear receptor superfamily. The existence of alternatively spliced variants is well recognised for several members of this superfamily, most of them having functional importance. For example, several testicular oestrogen receptor variants have been suggested to play a role in the regulation of spermatogenesis. However, information on AR variants is mostly related to cancer and androgen insensitivity syndrome (AIS) cases. The objective of this study was to investigate the expression of AR variants in the testis from humans and other vertebrates. Four AR variants [ARΔ2(Stop) , ARΔ2(23Stop) , ARΔ3 and ARΔ4(120)] were identified in human testis. ARΔ2(Stop) and ARΔ3, with exon 2 or 3 deleted, respectively, were also expressed in human liver, lung, kidney and heart. In addition, ARΔ2(Stop) was expressed in rat and gilthead seabream testis, while an ARΔ3 was detected in African clawed frog testis. This is the first report revealing the existence of AR variants in the testis of evolutionarily distant vertebrate species and in nonpathological tissues. These data suggest the functional importance of these novel AR forms and demonstrate a complexity in AR signalling that is not exclusive of pathological conditions.

Identification of C-terminally and N-terminally truncated estrogen receptor α variants in the mouse

We re-examined mouse ERα mRNA variants using rapid amplification of cDNA ends (RACE) and RT-PCR. Our analysis showed the presence of several mRNA variants containing unique 5'- or 3'-nucleotide sequences. We mapped the cDNA sequences on the mouse genome, and identified four novel 3'-terminal and 5'-leader exons in the intronic region between exons 4 and 5. RT-PCR analysis revealed that the expression patterns of the C-terminally truncated ERα products (CTERPs) were similar to that of Wild-type ERα and that the N-terminally truncated ERα products (NTERPs) appeared to have different expression profiles. Moreover, we constructed expression vectors and analyzed the subcellular localization and the transcriptional activation abilities of the variant proteins in transfected HEK293 cells using immunocytochemistry and luciferase reporter assay. The CTERP variants localized in the nuclei and constitutively activated estrogen response element (ERE)-driven promoters, while the NTERP variant was located in the extra-nuclear regions and had no ability to activate the ERE promoters in the presence or absence of 10 nM estradiol. Our results indicate that the mouse ERα gene is more complex than previously thought in terms of genomic organization and that alternative splicing and alternative usage of intronic promoters contribute to the remarkable diversity of ERα mRNAs and proteins.

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.

The progesterone receptor in human term amniochorion and placenta is isoform C

The mechanism that initiates human parturition has been proposed to be functional progesterone withdrawal whereby the 116-kDa B isoform of the progesterone receptor (PR-B) switches in favor of the 94-kDa A isoform (PR-A) in reproductive tissues. Recently other PR isoforms, PR-S, PR-C, and PR-M generated from the same gene have been identified and partially characterized. Using immunohistochemical, Western blotting, and RT-PCR techniques, evidence is provided that the major PR isoform present in human term fetal membranes (amnion and chorion) and syncytiotrophoblast of the placenta is neither of the classical nuclear PR-B or PR-A isoforms but is the N terminally truncated 60-kDa PR-C isoform. Evidence is also provided that the PR-C isoform resides in the cytoplasm of the expressing cell types. Data are also presented to show that PR-B, PR-A, and PR-S isoforms are essentially absent from the amnion and chorion, whereas PR isoforms A, B, C, and S are all present in the decidua, with PR-A being the major isoform. The syncytiotrophoblast of the placenta contains the cytoplasmic PR-C isoform but not PR-A, PR-B, or PR-S. The major PR isoform in the amnion, chorion, and placenta is PR-C, suggesting that the cytoplasmic PR-C isoform has a specific role in extraembryonic tissues and may be involved in the regulation of human parturition.

Method to Detect Functional Estrogen Receptor Expression using Estrogen Receptor Probing Compound

A novel method to detect functional estrogen receptor expression with estrogen receptor probing compound (ERPC) has been established. ERPC is composed of estradiol-BSA-biotin. ERPC can be used to detect functional estrogen receptor expression in both Western blot and immunohistochemistry. ERPC based techniques are non-radioactive, sensitive, relatively inexpensive, and can be used with all species. The use of both ERPC and ERalpha- or ERbeta-specific antibodies provides complementary information in characterizing estrogen receptors' expression and functional binding to ligand.

Differential distribution of splice variants of estrogen receptor beta in human testicular cells suggests specific functions in spermatogenesis

A growing number of estrogen receptor beta (ER beta) splice variants are reported. Several of these have been discovered in testis, but with few exceptions little is known about their cellular localization. The aim of this study was to identify and elucidate the mRNA expression pattern of the different ER beta splice variants in human testicular cells. Northern analysis was performed on whole testis and fractions enriched in germ cells from untreated men and from estrogen-treated men undergoing sex change surgery. Probes were constructed in order to systematically screen for and identify various ER beta splice variants. Several ER beta bands were observed in the human testis, in which splice variants constituted the major part of total ER beta transcripts. Interestingly, only two ER beta wild-type transcripts were detected. These seem to be virtually absent from the haploid germ cells and are probably mainly located in somatic cells and/or primary spermatocytes. Several novel ER beta deletion variants were found in high levels in the haploid germ cell fractions and were nearly absent in testicular cells from the estrogen-treated men. The cell-dependent distribution raises the question whether splice variants may have specific functions in spermatogenesis, and whether the differential splicing of ER beta is regulated in a cell-specific manner.

Isoform/variant mRNAs for sex steroid hormone receptors in humans

The open reading frames of human sex steroid hormone receptors (hSSHRs) are composed of eight exons. In addition, the presence of various exons - including 5'-untranslated exons, alternative coding exons and novel 'intronic' exons - has been demonstrated in the genes encoding hSSHRs. The isoform/variant hSSHR mRNAs generated from thes e exons can be tentatively classified into seven types. In type 1, different mRNAs are generated with the use of alternative transcription start sites. In type 2, one or more exons are skipped. In type 3, one or more exons are duplicated. In type 4, distinct mRNAs containing different 5'-untranslated exon(s) are synthesized. In type 5, distinct mRNAs possessing different coding exon(s) are generated. In type 6, mRNA is synthesized by intronic exons and coding exons 4/5-8. In type 7, mRNA with insertion of intronic exon(s) is generated. Here, we review the isoform/variant hSSHR mRNAs and the structure of the genes encoding them.

Novel isoforms of the mRNA for human female sex steroid hormone receptors

In our recent reports, the novel isoform cDNAs of the ER alpha (ER alpha isoform S cDNA), ER beta (ER beta isoform M cDNA) and PR (PR isoform S and PR isoform T cDNAs) have been identified. These isoform cDNAs contained the previously unidentified 5'-sequences on exons 4-8 (ER alpha isoform S cDNA), exons 5-8 (ER beta isoform M cDNA) or exons 4-8 (PR isoform S and PR isoform T cDNAs). The genomic DNA analysis revealed that the 5'-sequences were derived from the novel independent exons, the ER alpha exon S, ER beta exon M, PR exon S and PR exon T, respectively. Furthermore, the existence of the novel variant mRNA, termed the i45 PR mRNA variant, with the insertion of the previously unidentified exons, termed the exons i45a and i45b, has been demonstrated by the reverse transcription-polymerase chain reaction on the RNA of the human uterine endometrium. From these results, we have concluded that the genes for the human female sex steroid hormone receptors contain the novel intronic exons, that the novel isoform mRNAs are transcribed using the intronic exon and exons 4-8 (or exons 5-8) of the gene, and that the novel variant mRNA is generated by the insertion of the intronic exons in the PR. In the present communication, our recent data along with others on the novel isoform/variant mRNAs for the human female sex steroid hormone receptors will be summarized.

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.