Differential expression of estrogen receptor mRNA splice variants in the tamoxifen resistant human breast cancer cell line, MCF-7/TAMR-1 compared to the parental MCF-7 cell line

SMAD4 depletion contributes to endocrine resistance by integrating ER and ERBB signaling in HR + HER2- breast cancer

Endocrine resistance poses a significant clinical challenge for patients with hormone receptor-positive and human epithelial growth factor receptor 2-negative (HR + HER2-) breast cancer. Dysregulation of estrogen receptor (ER) and ERBB signaling pathways is implicated in resistance development; however, the integration of these pathways remains unclear. While SMAD4 is known to play diverse roles in tumorigenesis, its involvement in endocrine resistance is poorly understood. Here, we investigate the role of SMAD4 in acquired endocrine resistance in HR + HER2- breast cancer. Genome-wide CRISPR screening identifies SMAD4 as a regulator of 4-hydroxytamoxifen (OHT) sensitivity in T47D cells. Clinical data analysis reveals downregulated SMAD4 expression in breast cancer tissues, correlating with poor prognosis. Following endocrine therapy, SMAD4 expression is further suppressed. Functional studies demonstrate that SMAD4 depletion induces endocrine resistance in vitro and in vivo by enhancing ER and ERBB signaling. Concomitant inhibition of ER and ERBB signaling leads to aberrant autophagy activation. Simultaneous inhibition of ER, ERBB, and autophagy pathways synergistically impacts SMAD4-depleted cells. Our findings unveil a mechanism whereby endocrine therapy-induced SMAD4 downregulation drives acquired resistance by integrating ER and ERBB signaling and suggest a rational treatment strategy for endocrine-resistant HR + HER2- breast cancer patients.

Endocrine therapy resistance: what we know and future directions

Endocrine resistance is a major hurdle in the treatment of estrogen receptor (ER)-positive breast cancer. When abnormally regulated, molecular signals responsible for cellular proliferation, as well as ER itself, allow for cellular evasion of ER-dependent treatments. Therefore, pharmacological treatments that target these evasion mechanisms are beneficial for the treatment of endocrine-resistant breast cancers. This review summarizes currently understood molecular signals that contribute to endocrine resistance and their crosstalk that stem from mitogen-activated protein kinase (MAPK), phosphoinositol-3 kinase/protein kinase B (PI3K/AKT), mechanistic target of rapamycin (mTOR), cyclin-dependent kinases 4 and 6 (CDK4/6) and aberrant ER function. Recent clinical trials that target these molecular signals as a treatment strategy for endocrine-resistant breast cancer are also highlighted.

Inhibition of STAT3 enhances sensitivity to tamoxifen in tamoxifen-resistant breast cancer cells

BACKGROUND

The mechanisms of endocrine resistance are complex, and deregulation of several oncogenic signalling pathways has been proposed. We aimed to investigate the role of the EGFR and Src-mediated STAT3 signalling pathway in tamoxifen-resistant breast cancer cells.

METHODS

The ER-positive luminal breast cancer cell lines, MCF-7 and T47D, were used. We have established an MCF-7-derived tamoxifen-resistant cell line (TamR) by long-term culture of MCF-7 cells with 4-hydroxytamoxifen. Cell viability was determined using an MTT assay, and protein expression levels were determined using western blot. Cell cycle and annexin V staining were analysed using flow cytometry.

RESULTS

TamR cells showed decreased expression of estrogen receptor and increased expression of EGFR. TamR cells showed an acceleration of the G1 to S phase transition. The protein expression levels of phosphorylated Src, EGFR (Y845), and STAT3 was increased in TamR cells, while phosphorylated Akt was decreased. The expression of p-STAT3 was enhanced according to exposure time of tamoxifen in T47D cells, suggesting that activation of STAT3 can cause tamoxifen resistance in ER-positive breast cancer cells. Both dasatinib (Src inhibitor) and stattic (STAT3 inhibitor) inhibited cell proliferation and induced apoptosis in TamR cells. However, stattic showed a much stronger effect than dasatinib. Knockdown of STAT3 expression by siRNA had no effect on sensitivity to tamoxifen in MCF-7 cells, while that enhanced sensitivity to tamoxifen in TamR cells. There was not a significant synergistic effect of dasatinib and stattic on cell survival. TamR cells have low nuclear p21(Cip1) expression compared to MCF-7 cells and inhibition of STAT3 increased the expression of nuclear p21(Cip1) in TamR cells.

CONCLUSIONS

The EGFR and Src-mediated STAT3 signalling pathway is activated in TamR cells, and inhibition of STAT3 may be a potential target in tamoxifen-resistant breast cancer. An increase in nuclear p21(Cip1) may be a key step in STAT3 inhibitor-induced cell death in TamR cells.

Downregulation of 15-hydroxyprostaglandin dehydrogenase during acquired tamoxifen resistance and association with poor prognosis in ERα-positive breast cancer

Aim:

Tamoxifen (TAM) resistance remains a clinical issue in breast cancer. The authors previously reported that 15-hydroxyprostaglandin dehydrogenase (HPGD) was significantly downregulated in tamoxifen-resistant (TAMr) breast cancer cell lines. Here, the authors investigated the relationship between HPGD expression, TAM resistance and prediction of outcome in breast cancer.

Methods:

HPGD overexpression and silencing studies were performed in isogenic TAMr and parental human breast cancer cell lines to establish the impact of HPGD expression on TAM resistance. HPGD expression and clinical outcome relationships were explored using immunohistochemistry and in silico analysis.

Results:

Restoration of HPGD expression and activity sensitised TAMr MCF-7 cells to TAM and 17β-oestradiol, whilst HPGD silencing in parental MCF-7 cells reduced TAM sensitivity. TAMr cells released more prostaglandin E₂ (PGE₂) than controls, which was reduced in TAMr cells stably transfected with HPGD. Exogenous PGE₂ signalled through the EP4 receptor to reduce breast cancer cell sensitivity to TAM. Decreased HPGD expression was associated with decreased overall survival in ERα-positive breast cancer patients.

Conclusions:

HPGD downregulation in breast cancer is associated with reduced response to TAM therapy via PGE₂-EP4 signalling and decreases patient survival. The data offer a potential target to develop combination therapies that may overcome acquired tamoxifen resistance.

Reduced alternative splicing of estrogen receptor alpha in the endometrium of women with endometriosis

Endometriosis is a condition which involves the presence of uterine stroma and glands outside of the uterine cavity and represents one of the most prevalent disorders of the female reproductive tract. The key symptom of endometriosis is pain, including dysmenorrhea, deep dyspareunia, and chronic pelvic pain. As such, endometriosis has significant economic consequences within the healthcare system and can influence the daily quality of life in affected patients. However, the pathophysiology of this disease and the mechanisms in which this condition generates pain are very unclear. This study, involving 30 women with endometriosis and 28 controls without endometriosis, aimed to investigate relative levels of estrogen receptor alpha (ERα) splice variants in the endometrium of women with and without endometriosis and investigate potential links to the severity of pain. Wild type (wt)-ERα was dominantly expressed in human endometrium while the expression of ERα-del.4, ERα-del.7, and ERα-del.3,4 was significantly reduced in endometriosis patients compared with healthy patients (p < 0.05). Furthermore, the relative ratios of wtERα:ERα-del.4, and wtERα:ERα-del.3,4 were associated with the severity of pain in endometriosis patients (p < 0.05). Consequently, analyzing differences in the relative levels of four types of ERα splice variant in the endometrium of patients with endometriosis may help in the development of endometriosis-targeted treatment and the development of appropriate therapies.

Identification of a Novel Human E-Cadherin Splice Variant and Assessment of Its Effects Upon EMT-Related Events

Epithelial Cadherin (E-cadherin) is involved in calcium-dependent cell-cell adhesion and signal transduction. The E-cadherin decrease/loss is a hallmark of Epithelial to Mesenchymal Transition (EMT), a key event in tumor progression. The underlying molecular mechanisms that trigger E-cadherin loss and consequent EMT have not been completely elucidated. This study reports the identification of a novel human E-cadherin variant mRNA produced by alternative splicing. A bioinformatics evaluation of the novel mRNA sequence and biochemical verifications suggest its regulation by Nonsense-Mediated mRNA Decay (NMD). The novel E-cadherin variant was detected in 29/42 (69%) human tumor cell lines, expressed at variable levels (E-cadherin variant expression relative to the wild type mRNA = 0.05-11.6%). Stable transfection of the novel E-cadherin variant in MCF-7 cells (MCF7Ecadvar) resulted in downregulation of wild type E-cadherin expression (transcript/protein) and EMT-related changes, among them acquisition of a fibroblastic-like cell phenotype, increased expression of Twist, Snail, Zeb1, and Slug transcriptional repressors and decreased expression of ESRP1 and ESRP2 RNA binding proteins. Moreover, loss of cytokeratins and gain of vimentin, N-cadherin and Dysadherin/FXYD5 proteins was observed. Dramatic changes in cell behavior were found in MCF7Ecadvar, as judged by the decreased cell-cell adhesion (Hanging-drop assay), increased cell motility (Wound Healing) and increased cell migration (Transwell) and invasion (Transwell w/Matrigel). Some changes were found in MCF-7 cells incubated with culture medium supplemented with conditioned medium from HEK-293 cells transfected with the E-cadherin variant mRNA. Further characterization of the novel E-cadherin variant will help understanding the molecular basis of tumor progression and improve cancer diagnosis. J. Cell. Physiol. 232: 1368-1386, 2017. © 2016 Wiley Periodicals, Inc.

Fibroblast Subtypes Regulate Responsiveness of Luminal Breast Cancer to Estrogen

Purpose: Antiendocrine therapy remains the most effective treatment for estrogen receptor-positive (ER⁺) breast cancer, but development of resistance is a major clinical complication. Effective targeting of mechanisms that control the loss of ER dependency in breast cancer remains elusive. We analyzed breast cancer-associated fibroblasts (CAF), the largest component of the tumor microenvironment, as a factor contributing to ER expression levels and antiendocrine resistance.Experimental Design: Tissues from patients with ER⁺ breast cancer were analyzed for the presence of CD146-positive (CD146^pos) and CD146-negative (CD146^neg) fibroblasts. ER-dependent proliferation and tamoxifen sensitivity were evaluated in ER⁺ tumor cells cocultured with CD146^pos or CD146^neg fibroblasts. RNA sequencing was used to develop a high-confidence gene signature that predicts for disease recurrence in tamoxifen-treated patients with ER⁺ breast cancer.Results: We demonstrate that ER⁺ breast cancers contain two CAF subtypes defined by CD146 expression. CD146^neg CAFs suppress ER expression in ER⁺ breast cancer cells, decrease tumor cell sensitivity to estrogen, and increase tumor cell resistance to tamoxifen therapy. Conversely, the presence of CD146^pos CAFs maintains ER expression in ER⁺ breast cancer cells and sustains estrogen-dependent proliferation and sensitivity to tamoxifen. Conditioned media from CD146^pos CAFs with tamoxifen-resistant breast cancer cells are sufficient to restore tamoxifen sensitivity. Gene expression profiles of patient breast tumors with predominantly CD146^neg CAFs correlate with inferior clinical response to tamoxifen and worse patient outcomes.Conclusions: Our data suggest that CAF composition contributes to treatment response and patient outcomes in ER⁺ breast cancer and should be considered a target for drug development. Clin Cancer Res; 23(7); 1710-21. ©2016 AACR.

ERα dimerization: a key factor for the weak estrogenic activity of an ERα modulator unable to compete with estradiol in binding assays

Estrothiazine (ESTZ) is a weak estrogen sharing structural similarities with coumestrol. ESTZ failed to compete with [³H]17β-estradiol ([³H]17β-E₂) for binding to the estrogen receptor α (ERα), questioning its ability to interact with the receptor. However, detection by atomic force spectroscopy (AFS) of an ESTZ-induced ERα dimerization has eliminated any remaining doubts. The effect of the compound on the proliferation of ERα-positive and negative breast cancer cells confirmed the requirement of the receptor. The efficiency of ESTZ in MCF-7 cells was weak without any potency to modify the proliferation profile of estradiol and coumestrol. Growth enhancement was associated with a proteasomal degradation of ERα without substantial recruitment of LxxLL coactivators. This may be related to an unusual delay between the acquisition by the receptor of an ERE-binding capacity and the subsequent estrogen-dependent transcription. A complementary ability to enhance TPA-induced AP-1 transcription was observed, even at concentrations insufficient to activate the ERα, suggesting a partly independent mechanism. ESTZ also rapidly and transiently activated ERK1/2 likely through membrane estrogenic pathways provoking a reorganization of the actin network. Finally, the systematic absence of biological responses with an ESTZ derivative unable to induce ERα dimerization stresses the importance of this step in the action of the compound, as reported for conventional estrogens. In view of the existence of many other ERα modulators (endocrine disruptors such as, for example, pesticides, environmental contaminants or phytoestrogens) with extremely weak or similar apparent lack of binding ability, our work may appear as pilot investigation for assessing their mechanism of action.

Nuclear receptor coactivators: regulators of steroid action in brain and behaviour

Steroid hormones act in specific regions of the brain to alter behaviour and physiology. Although it has been well established that the bioavailability of the steroid and the expression of its receptor is critical for understanding steroid action in the brain, the importance of nuclear receptor coactivators in the brain is becoming more apparent. The present review focuses on the function of the p160 family of coactivators, which includes steroid receptor coactivator-1 (SRC-1), SRC-2 and SRC-3, in steroid receptor action in the brain. The expression, regulation and function of these coactivators in steroid-dependent gene expression in both brain and behaviour are discussed.

NFκB signaling is important for growth of antiestrogen resistant breast cancer cells

Resistance to endocrine therapy is a major clinical challenge in current treatment of estrogen receptor-positive breast cancer. The molecular mechanisms underlying resistance are yet not fully clarified. In this study, we investigated whether NFκB signaling is causally involved in antiestrogen resistant cell growth and a potential target for re-sensitizing resistant cells to endocrine therapy. We used an MCF-7-derived cell model for antiestrogen resistant breast cancer to investigate dependence on NFκB signaling for antiestrogen resistant cell growth. We found that targeting NFκB preferentially inhibited resistant cell growth. Antiestrogen resistant cells expressed increased p50 and RelB, and displayed increased phosphorylation of p65 at Ser529 and Ser536. Moreover, transcriptional activity of NFκB after stimulation with tumor necrosis factor α was enhanced in antiestrogen resistant cell lines compared to the parental cell line. Inhibition of NFκB signaling sensitized tamoxifen resistant cells to the growth inhibitory effects of tamoxifen but was not sufficient to fully restore sensitivity of fulvestrant resistant cells to fulvestrant. In support of this, depletion of p65 with siRNA in tamoxifen resistant cells increased sensitivity to tamoxifen treatment. Our data provide evidence that NFκB signaling is enhanced in antiestrogen resistant breast cancer cells and plays an important role for antiestrogen resistant cell growth and for sensitivity to tamoxifen treatment in resistant cells. Our results imply that targeting NFκB might serve as a potential novel treatment strategy for breast cancer patients with resistance toward antiestrogen.

Membrane estrogen signaling enhances tumorigenesis and metastatic potential of breast cancer cells via estrogen receptor-α36 (ERα36)

Protein kinase C (PKC) signaling can be activated rapidly by 17β-estradiol (E(2)) via nontraditional signaling in ERα-positive MCF7 and ERα-negative HCC38 breast cancer cells and is associated with tumorigenicity. Additionally, E(2) has been shown to elicit anti-apoptotic effects in cancer cells counteracting pro-apoptotic effects of chemotherapeutics. Supporting evidence suggests the existence of a membrane-associated ER that differs from the traditional receptors, ERα and ERβ. Our aim was to identify the ER responsible for rapid PKC activation and to evaluate downstream effects, such as proliferation, apoptosis, and metastasis. RT-PCR, Western blot, and immunofluorescence were used to determine the presence of ER splice variants in multiple cell lines. E(2) effects on PKC activity were measured with and without ER-blocking antibodies. Cell proliferation was determined by [(3)H]thymidine incorporation, and cell viability was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, (MTT) whereas apoptosis was determined by DNA fragmentation and TUNEL. Quantitative RT-PCR and sandwich ELISA were used to determine the effects on metastatic factors. The role of membrane-dependent signaling in cancer cell invasiveness was examined using an in vitro assay. The results indicate the presence of an ERα splice variant, ERα36, in ERα-positive MCF7 and ERα-negative HCC38 breast cancer cells, which localized to plasma membranes and rapidly activated PKC in response to E(2), leading to deleterious effects such as enhancement of proliferation, protection against apoptosis, and enhancement of metastatic factors. These findings propose ERα36 as a novel target for the development of therapies that can prevent progression of breast cancer in the primary tumor as well as during metastasis.

Multiple Routes to Oestrogen Antagonism

Several lines of evidence attest to the existence of alternative ligand binding sites on the oestrogen receptor (ER), including non-competitive inhibition by trilostane or tamoxifen. It is possible that the inhibitory action of conventional oestrogen agonists at high concentrations may indicate that they too interact at alternative ER sites, albeit at low affinity. To test this possibility an oestrogen reporter assay was used to compare the activity of different oestrogens and antagonists in breast cancer and prostate cell lines. All four cell lines tested contained different amounts of oestrogen receptor α (ERα), ERβ, progesterone receptor and coregulator mRNA. Though differences were observed in response to stimulation and inhibition, these correlated only with the presence or absence of ERα, and not with the other components. Thus stimulation of the reporter by oestradiol and oestrone was biphasic in the breast cancer cells, while prostate cells were unable to respond. Only T47D cells were stimulated by oestriol or diethylstilboestrol, however reporter activity of all the cell lines was repressed by 10μM diethylstilboestrol. Reporter activity of MCF-7 cells was inhibited by tamoxifen, raloxifene and ICI 182,780, but stimulated by trilostane, yet all these antioestrogens inhibited agonist-stimulated activity. Trilostane also inhibited the agonism seen in cells co-treated with E2 and tamoxifen. It is clear that several of the compounds tested may have either agonist or antagonist effects under different conditions and at different concentrations, acting through ERα alone. Though biphasic dose response curves, or hormesis, have been attributed to various mechanisms, we here provide evidence that alternative ligand binding sites may contribute to this phenomenon.

A novel estrogen receptor-related protein gamma splice variant lacking a DNA binding domain exon modulates transcriptional activity of a moderate range of nuclear receptors

A novel estrogen receptor-related protein (ERR) gamma splice variant cDNA (ERRgamma3) was found in human full-length cDNA libraries. ERRgamma3 cDNA consists of 3362 base pairs and has an open reading frame of 1188bp. The predicted peptide sequence of ERRgamma3 differs from both ERRgamma1 and ERRgamma2 in missing 39 amino acid residues corresponding to the second zinc finger motif of the DNA binding domain (DBD). ERRgamma3 gene consists of 8 exons including three unique 5'-terminal exons and lacks the exon encoding the second zinc finger motif. The expression of ERRgamma3 was confined to adipocytes and prostate while that of ERRgamma2 was fairly widespread. The ERRgamma3 product was shown by transactivation assay to have no ability to activate ERE-controlled transcription. However, ERRgamma3 has an ability to modulate the transcriptional activity of other nuclear hormone receptors. ERRgamma3 augmented the ligand-dependent transcriptional activities of ER (estrogen receptor) alpha, ERbeta, and thyroid receptor (TR) alpha by 1.3-, 4-, and 2.1-fold whereas it inhibited fully the activity of glucocorticoid receptor (GR). However, ERRgamma3 had no effect on Vitamin D3 receptor, retinoic acid receptor alpha or peroxisome proliferator activated receptor alpha, beta, and gamma. These findings will help to elucidate the physiological role of the ERRgamma subfamily.

Arzoxifene: the development and clinical outcome of an ideal SERM

Hormone-sensitive tumours are among the most common cancers in women. Specific inhibition of the estrogen receptor by selective estrogen receptor downregulators or selective estrogen receptor modulators (SERMs) is effective for the treatment of breast and endometrial cancers and may be used for the prevention of breast cancer. Due to differential recruitment of co-activators and corepressors, SERMs are tissue specific and may have antiestrogenic effects in some tissues, with estrogen agonist activity in others. The ideal SERM would have antiestrogenic effects on the breast and endometrium, but pro-estrogenic effects on bone and lipids. The SERM, arzoxifene (LY-353381.HCl) meets all of these criteria. This review summarises the development, preclinical studies and the clinical outcome of arzoxifene and places it in context with other modalities in the treatment of hormone receptor-positive tumours.

Expression of estrogen receptor beta exon-deleted variant mRNAs in ovary and uterine endometrium

Various estrogen receptor beta exon-deleted variant (ER-beta EDV) mRNAs were expressed in human ovary and uterine endometrium. Estrogen receptor beta (ER-beta) completely or partially deleted exon n is expressed as ER-beta EnDV or En'DV, respectively. The mRNAs for ER-beta single exon-deleted variant (EDV), ER-beta E2DV, E4DV, E5DV and E6DV; for ER-beta double exon-deleted variants, ER-beta E1'+2DV, E4+5DV and E5+6DV; and for ER-beta triple exon-deleted variants, ER-beta E2'+3+4DV and E4+5+6DV were detected. In ER-beta E2DV, E4+5DV, E5DV and E6DV mRNAs, the new stop codon is made in the exon following the deleted exon(s), and the new proteins may lack the corresponding domains. In ER-beta E1'+2DV, E2'+3+4DV, E4DV, E4+5+6DV and E5+6DV mRNAs, the original stop codon is still present, and the new proteins may conserve the new short amino acid sequences surrounding the deleted exons. ER-beta E1'+2DV, E2DV, E2'+3+4DV, E4DV, E4+5DV and E4+5+6 are unlikely to work as a transcription factors. On the other hand, ER-beta E5DV, E6DV and E5+6DV lack only the ligand-binding domain, and might work as dominant positive or negative factors. Therefore, ER-beta E5DV, E6DV and E5+6DV, constitutively expressed in human ovary and uterine endometrium might, in part regulate estrogen responsiveness.

Identification of two estrogen regulated genes associated with growth regulation of human breast cancer

We have identified two estrogen regulated gene products in the E(2) growth inhibited human breast cancer xenograft, T61; one showing 100% homology to the human BAC clone RP11-112E16, the other 100% homology to the human CPR3/DNJ3 gene. Verification by Northern blot analyses showed an up-regulation of the BAC clone RP11-112E16 and the CPR3/DNJ3 mRNAs upon E(2) treatment. Treatment of T61 tumors with tamoxifen, leading to static tumor growth, also increased the expression of the BAC clone RP11-112E16 and the CPR3/DNJ3 mRNAs. A similar association between growth inhibition and BAC clone RP11-112E16 and CPR3/DNJ3 mRNA induction was observed in MCF-7 cells treated with ICI 182.780. In MCF-7 cells, treatment with E(2) resulted in growth stimulation concomitant with a decrease in the BAC clone RP11-112E16 and CPR3/DNJ3 mRNA expression. Treatment with a combination of E(2) and ICI 182.780 abolished the anti-estrogen induced increase in BAC clone RP11-112E16 and CPR3/DNJ3 mRNA expression, indicating that regulation of the gene products is mediated through the ER. The association between growth inhibition and BAC clone RP11-112E16 or CPR3/DNJ3 mRNA expression was supported by high expression of both gene products in brain tissue. Further investigations are ongoing to clarify the biological function of these two gene products.

Steroid receptors and metastatic potential in endometrial cancers

The relative overexpression of estrogen receptor (ER)-alpha exon 5 splicing variant, the disrupted synchronization of ER-beta and ER-alpha expressions, and the suppression of progesterone receptor (PR) form A expression as a transcriptional repressor might be related to metastatic potential of uterine endometrial cancers, leading to poor patient prognosis related to estrogen refractoriness.

Increased expression of cytochrome p450 1A1 and 1B1 genes in anti-estrogen-resistant human breast cancer cell lines

The expression of CYP1A1 and CYP1B1, encoding enzymes known to play a central role in oxidative metabolism of a wide range of compounds including steroids, was significantly increased in anti-estrogen-resistant human breast cancer cell lines. This was a purely regulatory phenomenon because no gene amplification had occurred. In anti-estrogen-sensitive MCF-7 cells, the steroidal anti-estrogen, ICI 182780, is able to induce the expression of the arylhydrocarbon receptor (AhR)-regulated gene product, CYP1A1, via an estrogen receptor (ER)- mediated process. This observation suggests cross-talk between the AhR and ER systems. Surprisingly, the increased constitutive expression in anti-estrogen-resistant cells of CYP1A1 and CYP1B1 mRNAs, encoding detoxification enzymes, had no effect on the activity of the ICI 182780 compound. The ICI 182780 regulation of estradiol-inducible genes was found to be identical in the resistant and sensitive breast cancer cell lines. In conclusion, anti-estrogen resistance is not due to conversion of ICI 182780 to less active compounds.

Identification and characterization of estrogen receptor variants in prostate cancer cell lines

A sensitive semi-nested reverse transcriptase-polymerase chain reaction (RT-PCR) amplification was performed to evaluate estrogen receptor-alpha (ER-alpha) mRNA expression in prostate cancer cell lines. We demonstrated the presence of wild-type ER-alpha (wt ER-alpha) and five ER-alpha variants, designated ER-alphaA, B, C, D, and E. Unlike ER-alphaA and D, ER-alphaB, C, and E were not previously reported in normal or cancerous mammalian cells. DNA sequencing analysis of these ER-alpha variants revealed the genetic changes to be either in-frame or out-of-frame deletions. The expression of each ER-alpha variant differs significantly depending on the androgen responsiveness, tumorigenic and metastatic potentials of each prostate cancer cell line. The potential functional significance of ER-alpha variants was assessed in yeast two-hybrid and ERE promoter-reporter mammalian transcription assay systems. The results of these studies indicated that none of the ER-alpha variants can form homo- or heterodimers either with wt ER-alpha or among themselves in vivo, and that these ER-alpha variants have no demonstrable transcriptional or dominant-negative activity, as assessed in vitro.

Analysis of estrogen receptor alpha and beta in endometrial carcinomas: correlation with ER beta and clinicopathologic findings in 45 cases

Estrogens play important roles in the pathogenesis of the great majority of endometrial endometrioid adenocarcinoma. Recently, a novel estrogen receptor (ER), ER beta, has been characterized, but little is known about the status of ER beta in endometrial carcinoma. We therefore examined expression of both ER alpha and ER beta in 45 cases of endometrioid endometrial adenocarcinoma using mRNA in situ hybridization, reverse transcription and polymerase chain reaction (RT-PCR), and immunohistochemistry. We also correlated the findings with various clinicopathologic parameters in these cases to examine their possible biologic significance. Accumulation of mRNA hybridization signals for both ER alpha and ER beta was detected predominantly in the cytoplasm of carcinoma cells, and to a lesser extent in some stromal cells. ER beta mRNA was detected in 16/45 cases (35.6%), and ER alpha mRNA hybridization signals were detected in 36/45 cases (80.0%). Among the 16 ER beta positive cases, 15 cases also had ER alpha mRNA hybridization signals. In the cases that expressed both ER alpha and ER beta, ER alpha mRNA hybridization signals were more widely distributed than ER beta mRNA. In 21 cases, carcinoma cells had ER alpha mRNA hybridization signals but not ER beta mRNA. There was a statistically significant positive correlation between the results of mRNA in situ hybridization and semiquantitative RT-PCR or immunohistochemistry for both ER alpha and ER beta. There were no significant correlations between ER beta mRNA expression and PR labeling index, Ki67 LI, age, or histologic grade. The results from our study indicate that ER beta is coexpressed with ER alpha, and that the estrogenic effects occur predominantly through ER alpha in endometrial carcinomas.

High incidence of alternatively spliced forms of deoxycytidine kinase in patients with resistant acute myeloid leukemia

Deficiency of functional deoxycytidine kinase (dCK) is a common characteristic for in vitro resistance to cytarabine (AraC). To investigate whether dCK is also a target for induction of AraC resistance in patients with acute myeloid leukemia (AML), we determined dCK messenger RNA (mRNA) expression in (purified) leukemic blasts and phytohemagglutinin-stimulated T cells (PHA T cells) from patients with chemotherapy-sensitive and chemotherapy-resistant AML. In control samples from healthy donors (PHA T cells and bone marrow), only wild-type dCK complementary DNA (cDNA) was amplified. Also, in (purified) leukemic blasts from patients with sensitive AML, only wild-type dCK cDNAs were observed. These cDNAs coded for active dCK proteins in vitro. However, in 7 of 12 (purified) leukemic blast samples from patients with resistant AML, additional polymerase chain reaction fragments with a deletion of exon 5, exons 3 to 4, exons 3 to 6, or exons 2 to 6 were detected in coexpression with wild-type dCK. Deletion of exons 3 to 6 was also identified in 6 of 12 PHA T cells generated from the patients with resistant AML. The deleted dCK mRNAs were formed by alternative splicing and did code for inactive dCK proteins in vitro. These findings suggest that the presence of inactive, alternatively spliced dCK mRNA transcripts in resistant AML blasts may contribute to the process of AraC resistance in patients with AML.

High incidence of alternatively spliced forms of deoxycytidine kinase in patients with resistant acute myeloid leukemia

Deficiency of functional deoxycytidine kinase (dCK) is a common characteristic for in vitro resistance to cytarabine (AraC). To investigate whether dCK is also a target for induction of AraC resistance in patients with acute myeloid leukemia (AML), we determined dCK messenger RNA (mRNA) expression in (purified) leukemic blasts and phytohemagglutinin-stimulated T cells (PHA T cells) from patients with chemotherapy-sensitive and chemotherapy-resistant AML. In control samples from healthy donors (PHA T cells and bone marrow), only wild-type dCK complementary DNA (cDNA) was amplified. Also, in (purified) leukemic blasts from patients with sensitive AML, only wild-type dCK cDNAs were observed. These cDNAs coded for active dCK proteins in vitro. However, in 7 of 12 (purified) leukemic blast samples from patients with resistant AML, additional polymerase chain reaction fragments with a deletion of exon 5, exons 3 to 4, exons 3 to 6, or exons 2 to 6 were detected in coexpression with wild-type dCK. Deletion of exons 3 to 6 was also identified in 6 of 12 PHA T cells generated from the patients with resistant AML. The deleted dCK mRNAs were formed by alternative splicing and did code for inactive dCK proteins in vitro. These findings suggest that the presence of inactive, alternatively spliced dCK mRNA transcripts in resistant AML blasts may contribute to the process of AraC resistance in patients with AML.

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

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

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.

Influence of estrogen receptor variants on the determination of ER status in human breast cancer

Determination of estrogen receptor alpha (ER) status in breast cancer is an important predictive factor for clinical response to endocrine therapy. We have recently shown that discrepancies in ER status determined by immunohistochemical assay (ER-IHA) can occur between amino-terminal (1D5) and carboxyl-terminal (AER-311) targeted ER antibodies and that those tumors which demonstrate discordance are associated with increased expression of truncated ER variant mRNAs. In this study, we have explored this observation to examine if ER variant expression can exert a direct effect on ER-IHA or whether this association is attributable to the characteristics of the antibodies. ER negative cos-1 cells were transfected with expression vectors containing wild type ER (wt-ER) and/or a frequently expressed truncated variant, ER-clone-4 variant. We found that ER-IHA performed with the same N- and C-terminal targeting ER antibodies on cos-1 cells expressing wt-ER alone demonstrated no difference in signals by western blot (P > 0.1). However, co-expression of wt-ER and the truncated ER-clone-4 variant, resulted in discordant IHA results with relatively higher ER-IHA H-scores from N-terminal antibodies (P < 0.03). Furthermore, re-examination of a subset of breast tumors previously studied by ER-IHA showed persistent concordance in 4/5 cases and persistent differences in 3/5 cases with a different pair of ER antibodies. We conclude that the presence of truncated ER variant proteins can interfere with the interpretation of ER status determined by IHA and that this may account for some of the inconsistencies between ER status and response to endocrine therapy.

Estrogen receptor variants ERdelta5 and ERdelta7 down-regulate wild-type estrogen receptor activity

The estrogen receptor (ER) plays a key role in mediating the effect of estrogens. It is the primary target for endocrine therapy for many diseases, including breast cancer. The ER contains six domains that are associated with distinct functions; the presence of all six domains is required for ligand-dependent receptor activity. ER variants, reported in breast tumors and other neoplasms, usually lack one or more domains or a part of a domain. Such deletions can have dramatic effects on ER activity, cellular response to hormone, and response to hormonal therapy. We used simple and rapid yeast systems to understand more clearly how ER variants alter the response of wild-type ER (wtER) to estrogen and antiestrogens. We co-expressed ER variant, ERdelta5 or ERdelta7, with wtER in yeast containing an ERE-LacZ reporter. We found that ERdelta5 and ERdelta7 decreased the response of wtER to 1 nM 17beta-estradiol by 41-43 and 24-34%, respectively. Alone, ERdelta5 displayed weak hormone-independent transcriptional activity that was not affected by tamoxifen or ICI 182,780. ERdelta7, in contrast, showed no constitutive activity and no response to ligands. To further understand whether ERdelta5 and ERdelta7 affect wtER activity by forming a variant:wtER heterodimer, we used the yeast two-hybrid system. The protein-protein interaction results showed that ERdelta5 and ERdelta7 could form neither homodimers with themselves nor heterodimers with wtER. This finding suggests that the influence of ERdelta5 and ERdelta7 on wtER is not mediated by suppressing wtER through heterodimerization.

Expression of Niemann-Pick type C transcript in rodent cerebellum in vivo and in vitro

This study assesses the developmental expression of the Niemann-Pick type C mRNA in vivo and in vitro in rat cerebellum. NPC is an autosomal recessive neurovisceral lipid storage disease associated with an alteration in cholesterol trafficking. In the mouse model of NPC and in the early onset form of human NPC, Purkinje neurons are among the first neurological targets, suffering stunted growth during postnatal development and dying, leading to ataxia. Recently, the genes responsible for human (NPC1) and mouse (Npc1) NPC disease have been cloned. Based on a highly homologous domain, we designed primers to look for levels of Npc1 mRNA with a semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) approach using cyclophilin as an internal standard. Total RNA was isolated from various postnatal developmental stages of the rat cerebellum as template for the analyses. Npc1 transcripts were observed at postnatal day 0 and at later stages of development, both in vivo and in vitro from primary cerebellar cultures. To identify the location of Npc1 inside the cerebellum, we performed immunostaining with an anti-Npc1 antibody in primary rat cerebellar cultures identifying reactive Purkinje neurons by double-labeling with the Purkinje specific marker calbindin and sub-populations of glial cells. In summary, Npc1 is expressed in rat cerebellum in vivo and in vitro and is expressed during early postnatal development as well as in the adult cerebellum. Since Npc1 is expressed at similar levels throughout development, the vulnerability of Purkinje neurons to this disease is likely to involve disruption of an interaction with other developmentally-regulated proteins.

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.

Differential expression of three classes of voltage-gated Ca(2+) channels during maturation of the rat cerebellum in vitro

Voltage-gated Ca(2+) channels provide a mode of Ca(2+) influx that is essential for intracellular signaling in many cells. Semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) was used to assess the relative amounts of mRNAs encoding three classes of Ca(2+) channels (alpha1A, alpha1B and alpha1E) during development, in cultures established from prenatal rat cerebellar cortex. Ca(2+) channel transcript levels were standardized to a constitutive marker (cyclophilin). For all three classes of Ca(2+) channels, transcript levels were highest at early stages (4-10 days in vitro) and declined with age. This developmental pattern was differentially regulated by a depolarizing agent, tetraethylammonium chloride (TEA, 1 mM). Chronic depolarization yielded a significant elevation in transcript levels for alpha1B (N-type) and alpha1E (R-type) Ca(2+) channels during neuronal maturation (10-21 days in vitro), but dramatically suppressed transcript levels for the alpha1A (P-type) Ca(2+) channel at all stages of development. The effects of TEA on alpha1A, alpha1B and alpha1E transcript levels were mimicked by increasing external K(+) (from 5 to 10 mM). The regulatory effects of depolarization on transcript levels were dependent on extracellular Ca(2+) for alpha1E but not for alpha1A. For alpha1B, transcript levels depended on extracellular Ca(2+) only for increased K(+) as the depolarizing stimulus, but not for TEA. These results suggest that levels of Ca(2+) channel transcripts in rat cerebellum are developmentally regulated in vitro and can be influenced differentially by transmembrane signaling via chronic depolarization and Ca(2+) entry. Dynamic regulation of Ca(2+) channel expression may be relevant to the different functional roles of Ca(2+) channels and their regional localization within neurons.

Differential regulation of specific genes in MCF-7 and the ICI 182780-resistant cell line MCF-7/182R-6

To elucidate the mechanisms involved in anti-oestrogen resistance, two human breast cancer cell lines MCF-7 and the ICI 182780-resistant cell line, MCF-7/182R-6, have been compared with regard to oestrogen receptor (ER) expression, ER function, ER regulation, growth requirements and differentially expressed gene products. MCF-7/182R-6 cells express a reduced level of ER protein. The ER protein is functional with respect to binding of oestradiol and the anti-oestrogens tamoxifen, 4-hydroxy-tamoxifen and ICI 182780, whereas expression and oestrogen induction of the progesterone receptor is lost in MCF-7/182R-6 cells. The ER protein and the ER mRNA are regulated similarly in the two cell lines when subjected to treatment with oestradiol or ICI 182780. Oestradiol down-regulates ER mRNA and ER protein expression. ICI 182780 has no initial effect on ER mRNA expression whereas the ER protein level decreases rapidly in cells treated with ICI 182780, indicating a severely decreased stability of the ER protein when bound to ICI 182780. In vitro growth experiments revealed that the ICI 182780-resistant cell line had evolved to an oestradiol-independent phenotype, able to grow with close to maximal growth rate both in the absence of oestradiol and in the presence of ICI 182780. Comparison of gene expression between the two cell lines revealed relatively few differences, indicating that a limited number of changes is involved in the development of anti-oestrogen resistance. Identification of the differentially expressed gene products are currently in progress.

Progesterone receptor variants found in breast cells repress transcription by wild-type receptors

Progesterone, through its nuclear receptors (PR), regulates the development and growth of breast cancers. PR also serve as markers of hormone dependence and prognosis in patients with this disease, and functional PR are required to mediate the antiproliferative effects of progestin therapies. We find that normal and malignant breast cells and tissues can express anomalous forms of PR transcripts. We have isolated four variant PR mRNAs that contain precise deletions of exons encoding sections of the DNA- and hormone-binding domains. The transcripts lack exon 2 (PRdelta2), exon 4 (PRdelta4), exon 6 (PRdelta6), or exons 5 and 6 (PRdelta5,6). On immunoblots, PRdelta4, delta6. and delta5, 6 cloned into the background of the PR A-isoform comigrate with similar proteins present in breast tumor extracts; delta6 and delta5, 6 are dominant-negative transcriptional inhibitors of wild-type A- and B-receptors. We propose that expression of variant PR can compromise the accuracy of receptor measurements as markers of hormone-dependent cancers, and can modify the responses of tumors to progestin therapies.

Down-regulation of laminin-binding integrins by 1 alpha,25-dihydroxyvitamin D3 in human melanoma cells in vitro

In the present investigation the effect of 1 alpha,25(OH)2D3 on the expression of the integrin laminin receptor on the melanoma cell line SK-MEL-28 has been examined. The SK-MEL-28 cells were shown to contain high-affinity receptors for 1 alpha,25(OH)2D3 and cell proliferation was found to be inhibited in a dose-dependent manner in response to the hormone. Using monoclonal antibodies against the alpha 6-sub-unit of the integrin laminin receptor, immunocytochemistry demonstrated that exposure of cells to 1 alpha,25(OH)2D3 for 5 days caused a reduced staining intensity. This observation was further confirmed by dot blot analysis, where a dose-dependent decline of alpha 6 expression was obtained after treatment of the cells with 1 alpha,25(OH)2D3 for 6 days. FACS-analysis was performed in order to quantify this decline, and it was found that the level of alpha 6-subunits on the cell surface was reduced by more than 40%. Additional investigations including Northern blot analyses of poly(A)+RNA extracts also showed a dose-dependent reduction of alpha 6 mRNA. Interestingly, the decrease of alpha 6 expression on the surface of SK-MEL-28 melanoma cells was accompanied by a reduced ability of the cells to adhere to an artificial basement membrane. In conclusion, the present investigation shows that besides having an antiproliferative effect on the SK-MEL-28 melanoma cells, 1 alpha,25(OH)2D3 is also able to inhibit the surface expression of the alpha 6-subunit of the integrin laminin receptor. Moreover, the results strongly indicate that 1 alpha,25(OH)2D3 exerts its regulatory effect on the alpha 6-subunit at the transcriptional level rather than at the protein level.

Regulation of Ca2+-dependent K+ channel expression in rat cerebellum during postnatal development

Potassium channels govern duration and frequency of excitable membrane events and may regulate signals that are important in neuronal development. This study assesses the developmental expression of the large conductance Ca2+-dependent K+ channel in vivo and in vitro in rat cerebellum. In vivo, transcript levels for the Ca2+-dependent K+ channel (KCa) were shown by Northern analysis to increase during development, whereas transcript levels for the voltage-gated K+ channel Kv3.1, a delayed rectifier (KD), remained relatively constant. A comparable pattern was demonstrated by expression in Xenopus oocytes of poly(A)-enriched RNA isolated from postnatal rat cerebella. In cerebellar cultures, increased external K+ provided a simple manipulation of cell excitability that influenced KCa transcript levels during development. With low external K+ (5.3 mM), the levels of KCa channel transcript (assessed by semiquantitative PCR) remained constant throughout development. However, in culture medium that supported significant dendritic outgrowth (10 mM extracellular K+), an upregulation of KCa transcript level was observed similar to that seen in vivo. Tetraethylammonium (TEA; 1 mM) similarly enhanced KCa expression, suggesting that depolarizing stimuli increased KCa expression. The stimulatory effects of increased K+ or TEA on KCa expression required extracellular Ca2+ and were abolished in low external calcium (0.1 mM, buffered with EGTA), although morphological development and survival were not impaired. The regulation of KCa channel expression by depolarization and Ca2+ entry provides evidence of a logical feedback mechanism governing Ca2+ signals that may be significant in cerebellar development.

Estrogen receptor variants and mutations

There is a large and increasing body of experimental and clinical data supporting the existence or variant estrogen receptor (ER) proteins in both normal and neoplastic estrogen target tissues including human breast. Therefore, future examination of ER signal transduction and/or measurement of ER protein must take into account variant ER expression. The functions of variant ER proteins, either physiological or pathological, remain unclear, although a role(s) for some ER variants in breast tumorigenesis and breast cancer progression would be consistent with the accumulated data. Possible tissue specific expression leads to the speculation that ER variants may have a role in tissue specific estrogen action. The following review focuses on the current knowledge available in the scientific literature with respect to the type and characteristics of estrogen receptor variants and mutations that have been identified to occur naturally in tissues and cell lines.

The role of estrogen receptor mutations in tamoxifen-stimulated breast cancer

During the past 20 years, the hormonal therapy of choice for the treatment of breast cancer has been the antiestrogen, tamoxifen. The use of tamoxifen has been proved to produce a favorable response and survival advantage in patients whose tumors are classified as estrogen receptor-positive (ER+)/progesterone receptor-positive (PR+). Additionally, tamoxifen is the only drug known to reduce the incidence of contralateral disease. This drug produces relatively few harmful side effects, while exhibiting several beneficial effects such as maintaining bone density and reducing the incidence of myocardial infarction in the postmenopausal woman. However, tumors eventually acquire a tamoxifen-resistant or tamoxifen-stimulated phenotype, resulting in disease recurrence. Several mechanisms have been proposed to account for tamoxifen-resistant breast cancer, in the hope of developing a more effective first-line or perhaps second-line treatment strategy. One popular theory is the occurrence of a mutation in the estrogen receptor, the drug target. A plethora of studies have reported the detection of estrogen receptor mRNA splice variants, and it has been suggested that the accumulation of these variant mRNAs are responsible for the development of tamoxifen-resistant breast cancer. In this review, several questions will be posed to address the suitability of both laboratory and clinical evidence to support this hypothesis. Although there is adequate data generated in the laboratory, there is, as yet, no compelling evidence to suggest that mutation of the estrogen receptor is the molecular mechanism producing tamoxifen-stimulated growth in human breast and endometrial cancer.

Oestrogen receptor variants and mutations in human breast cancer

Several oestrogen receptor variant and mutated mRNA species have been identified in human breast samples and cell lines. Over-expression and altered expression of some of these mRNAs have been correlated with breast tumourigenesis and progression. The following review focuses on the current knowledge available in the scientific literature with respect to the type and characteristics of oestrogen receptor variants and mutations that have been identified as occurring naturally in human breast tissues and cell lines.

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

Breast cancer tissue has been shown to contain alternatively spliced estrogen receptor (ER) mRNA variants which may result in alternate ER proteins. These ER variants lack specific functional domains and may alter breast cancer cells responses to both estrogen and antiestrogens. Specifically, ER variants might play a role in Tamoxifen resistance in breast cancer patients, as well as the development of endometrial carcinoma, an estrogen-dependent tumor, in patients taking this medication. We investigated the presence of ER variants in normal human endometrium and endometrial carcinoma. Ribonuclease protection assays (RPA) demonstrated ER mRNA variants in the DNA and hormone-binding domains. The reverse transcription-polymerase chain reaction (RT-PCR) assay was used to examine the ER complementary DNA (cDNA) from 25 patients, and generated two major products in both the exon 2 to 5 and 4 to 8 regions. Southern blot analysis of PCR products revealed exon 4 and 7 deletions in all 25 endometria without any qualitative differences in variant expression among premenopausal, postmenopausal, and adenocarcinoma samples. Cloning and sequencing of cDNA variants definitively identified exact deletions of either exon 4 or exon 7. These results demonstrate significant levels of ER mRNA splice variants as well as full-length ER mRNA in normal and neoplastic endometria.

Quantitation of estrogen receptor mRNA and its alternatively spliced mRNAs in breast tumor cells and tissues

Estrogen receptor (ER) mRNA exists as wild-type (full-length) and alternatively spliced variants in cell lines, normal tissues, and tumors. Most of the alternatively spliced variants discovered so far are missing one or more complete exons. RNase protection and RNA-PCR assays used previously to determine the relative concentration of a particular ER spliced-variant mRNA to wild-type mRNA have produced equivocal results because the probes/primers targeted only small regions within the nucleotide sequence. Variant ER mRNAs missing an exon outside the probe/primer region will react as if they were wild-type and any alternatively spliced variants containing a deletion at the probe/primer annealing site(s) will not be detected. A highly sensitive, competitive RNA-PCR assay has been developed that is quantitative with respect to the relative composition of wild-type ER and its alternatively spliced-mRNA forms, and semiquantitative with respect to their concentrations in cells and tissues. Separation and quantitation of the products are rapidly and accurately achieved by, respectively, capillary electrophoresis and laser-induced fluorescence. Wild-type ER mRNA concentration can be measured independently of all the reported exon deletion forms in a single PCR assay. Specific exon deletion forms can be measured by ER cDNA amplification with overlapping primer sets. Results obtained with RNAs isolated from two MCF-7 cell lines, a T-47D cell line, and five breast tumor tissues are presented.

Expression of estrogen receptor wild type and exon 5 splicing variant mRNAs in normal and endometriotic endometria during the menstrual cycle

Expression of estrogen receptor exon 5 splicing variant (ER-E5SV) mRNA contributes to dominant positive properties and metastatic potential of gynecological cancers. Therefore, in endometriotic endometria, which conserve implantation and tumor-like spreading potentials, the expression of ER-E5SV and estrogen receptor wild type (ER-WT) mRNA was analyzed, to investigate the biological implications. Estrogen receptor wild type mRNA in normal endometria was downregulated after ovulation, but not in endometriotic endometria. Therefore the wild type cascade in endometriotic endometria might be partly disorganized. The level of ER-E5SV mRNA in endometriotic endometria was lower than that in normal endometria, regardless of the stage in the menstrual cycle. However, there was no significant difference in the ratio of ER-E5SV to ER-WT mRNA in both normal and endometriotic endometria. On the other hand, the level of ER-E5SV mRNA in both normal and endometriotic endometria did not vary during the menstrual cycle. The dominant positive behavior of ER-E5SV might be masked by the functional cascade of ER-WT in normal endometria, but not in endometriotic endometria. This status might result in an incomplete response to endogenous steroids, and contribute to implantation and spreading potentials of endometriosis.

Expression of estrogen receptor exon 5 splicing variant (ER E5SV) mRNA in gynaecological cancers

Estrogen receptor exon 5 splicing variant (ER E5SV) mRNA has been found in tumours and the corresponding normal tissues, being transcriptionally active without ligand binding. Therefore, the expression of ER E5SV mRNA in gynaecological cancers was studied. The presence of ER E5SV mRNA was demonstrated in the normal ovary, uterine endometrium and cervix and their corresponding cancers. The ratio of ER E5SV/ER WT mRNA expression increased in some cases of metastatic tumour, but did not decrease in any case. Relative overexpression of ER E5SV mRNA might contribute to dominant positive properties and metastatic potential. Therefore, detection of ER E5SV mRNA abundance might be a useful indicator of metastatic potency in gynaecological cancers.

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.

Mechanisms of tamoxifen resistance in the treatment of advanced breast cancer

Multiple mechanisms may render breast cancer cells resistant to treatment with the antiestrogen tamoxifen. This review describes changes in the estrogen receptor (ER) signaling pathway which may lead to tamoxifen resistance: change in uptake or metabolism of tamoxifen, loss of expression of ER, decreased expression of ER, expression of mutant or variant forms of ER, intact ER but loss of cofactors, ligand-independent ER activation, modification of the estrogen response element, altered post-receptor events. Non-ER related alterations are also mentioned.