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

HNRNP G and HTRA2-BETA1 regulate estrogen receptor alpha expression with potential impact on endometrial cancer

Background

Estrogen receptor alpha (ERa/ESR1) expression is regulated by alternative splicing. Its most frequently detectable exon7 skipping isoform (ERaD7) is a dominant negative variant. Elevated expression of ERaD7 was already detected in endometrial cancer (EC), while its potential prognostic significance has not been characterized so far. Exon7 contains potential binding sites for the two functional splicing regulatory opponents, HNRNPG and HTRA2-BETA1 known to trigger opposite effects on EC outcome.

This study served to elucidate the influence of HNRNPG and HTRA2-BETA1 on ERa exon7 splicing regulation and the impact of ERaD7 concentration on type 1 EC outcome.

Methods

Functional in vitro experiments for HNRNPG and HTRA2-BETA1 in regard to the regulatory impact on endogenous and exogenous ERaD7 splicing were performed. Additionally, real-time PCR determined mRNA levels of ERaD7, HNRNPG and HTRA2-BETA1 in 116 type 1 EC patients.

Results

HNRNPG and HTRA2-BETA1 were found to be specific regulators of ERa exon7 splicing. While HTRA2-BETA1 promoted exon7 inclusion, HNRNPG antagonized this effect by inducing exon7 skipping (p = 0.004). ERaD7 was detected in 71 out of 116 type 1 EC specimens. Statistical analyses revealed an inverse correlation between ERaD7 mRNA levels and tumor grading (p = 0.029), FIGO stage (p = 0.033) as well as lymph node metastases (p = 0.032), respectively. Furthermore, higher ERaD7 expression could be correlated to an improved disease-specific survival (p = 0.034).

Conclusions

Our study demonstrates antagonistic regulatory effects of HNRNPG and HTRA2-BETA1 on ERa exon7 splicing with potential impact on type 1 EC clinical outcome due to the consecutively variable expression levels of the ERa isoform D7.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1088-1) contains supplementary material, which is available to authorized users.

Regulation of transcriptional activation function of rat estrogen receptor α (ERα) by novel C-terminal splice inserts

Estrogen receptor α (ERα) mediates estrogen diverse actions on tissues. ERα gene has eight constitutively expressing exons and is known to have multiple isoforms generated by alternative initiation of transcription and splicing events including exon skipping. We have discovered two novel exons inserted between exon 5 and 6 of rat ERα that can add independently or in tandem 18 and 14 amino acids to the estrogen binding/activator function 2 domain of the receptor. Their transcript expression is three to six fold higher in heart compared to brain, aorta, liver, ovary and uterus. In heart, the new variants increased ~2 fold with animal growth from prenatal to adulthood, and had a minor increment in aged animals (28 months). Inclusion of these exons yields a receptor with practically no binding capacity for estrogen and reduced dimerization. The new variants show nuclear localization but are less efficient in binding to estrogen responsive elements (EREs) and failed to transcriptionally activate promoters containing EREs (mSlo, KCNE2). Thus, the new variants can regulate the wild-type receptor function and may contribute to the regulatory action of estrogen, especially in the maturing heart where they are more abundant.

Estrogen receptor α and β in the normal immune system and in lymphoid malignancies

Estrogens regulate various normal and pathophysiological processes including cancers. Cellular signaling by estrogens is mediated by estrogen receptor α (ERα) and β (ERβ), respectively. Binding of agonists to the ERs affects gene transcription. The main endogenous estrogen, 17β-estradiol (E2), binds to both ERα and ERβ with similar affinity. However, the ligand-binding pocket of ERα and ERβ are slightly different which has allowed the development of selective ER ligands. Importantly, while estrogens via ERα stimulate proliferation, signaling via ERβ inhibits proliferation and promotes apoptosis. In both normal and cancer cells the ERs are co-expressed with ER splice variants which may modify the transcriptional activity of the wild-type receptors. Estrogens have prominent effects on immune functions and both ERα and ERβ are expressed in immune cells and lymphoid malignancies. With regard to lymphoid malignancies, most show estrogen influence as several epidemiological studies of lymphoid cancers demonstrate gender differences in incidence and prognosis with males being more affected. In line with these findings, recent results generated by us have shown that ERβ selective agonists inhibit growth and induce apoptosis in human and murine lymphomas in vivo in xenograft experiments. This suggests that ERβ selective agonists in the future may be useful in the treatment of lymphomas.

Expression levels of hnRNP G and hTra2-beta1 correlate with opposite outcomes in endometrial cancer biology

HnRNP G is a member of heterogeneous nuclear ribonucleoprotein (hnRNP) family with potent tumor suppressive activities. Human transformer-2-beta1 (hTra2-beta1) belongs to the arginine-serine rich like proteins and is found over-expressed in various human cancers. It was recently shown that hnRNP G and hTra2-beta1 exert antagonistic effects on alternative splicing. In our study we explored the impact of these two factors in tumor biology of endometrial cancer (EC). EC tissues (n = 139) were tested for hnRNP G and hTra2-beta1 expression on mRNA level by real time PCR and on protein level by immunohistochemistry. HTra2-beta1 mRNA level was found being induced in advanced International Federation of Gynecology and Obstetrics (FIGO) stages (p = 0.016). HnRNP G protein nuclear expression was found more prominent in patients without distant organ metastases (p = 0.033) and in FIGO Stages I/II group (p < 0.001). HTra2-beta1 protein nuclear levels were elevated in poorly differentiated (p = 0.044) and lymph node metastases (p = 0.003) cancers. Kaplan-Meier survival curves revealed that elevated hnRNP G mRNA (p = 0.029) and protein (p = 0.022) levels were associated with a favorable patient outcome. Multivariate Cox-regression analyses identified nuclear hnRNP G level [hazard ratio (HR) 0.468, p = 0.026) as well as hTra2-beta1 level (hazard ratio 5.760, p = 0.004) as independent prognostic factors for EC progression-free survival. Our results indicate that the antagonistic functional effects of hnRNP G and hTra2-beta1 on alternative splicing correlate directly to their opposite clinical effects on EC patient outcome.

Elevated oestrogen receptor splice variant ERαΔ5 expression in tumour-adjacent hormone-responsive tissue

Susceptibility to prostate or endometrial cancer is linked with obesity, a state of oestrogen excess. Oestrogen receptor (ER) splice variants may be responsible for the tissue-level of ER activity. Such micro-environmental regulation may modulate cancer initiation and/or progression mechanisms. Real-time reverse transcriptase (RT) polymerase chain reaction (PCR) was used to quantitatively assess the levels of four ER splice variants (ERαΔ3, ERαΔ5, ERβ2 and ERβ5), plus the full-length parent isoforms ERα and ERβ1, in high-risk [tumour-adjacent prostate (n = 10) or endometrial cancer (n = 9)] vs. low-risk [benign prostate (n = 12) or endometrium (n = 9)], as well as a comparison of UK (n = 12) vs. Indian (n = 15) benign prostate. All three tissue groups expressed the ER splice variants at similar levels, apart from ERαΔ5. This splice variant was markedly raised in all of the tumour-adjacent prostate samples compared to benign tissues. Immunofluorescence analysis for ERβ2 in prostate tissue demonstrated that such splice variants are present in comparable, if not greater, amounts as the parent full-length isoform. This small pilot study demonstrates the ubiquitous nature of ER splice variants in these tissue sites and suggests that ERαΔ5 may be involved in progression of prostate adenocarcinoma.

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.