Advanced concepts in estrogen receptor biology and breast cancer endocrine resistance: implicated role of growth factor signaling and estrogen receptor coregulators

Estrogen receptor (ER), mediating estrogen-signaling stimuli, is a dominant regulator and a key therapeutic target in breast cancer etiology and progression. Endocrine therapy, blocking the ER pathway, is one of the most important systemic therapies in breast cancer management, but de novo and acquired resistance is still a major clinical problem. New research highlights the role of both genomic and nongenomic ER activities and their intimate molecular crosstalk with growth factor receptor and other signaling kinase pathways in endocrine resistance. These signaling pathways, when overexpressed and/or hyperactivated, can modulate both activities of ER, resulting in endocrine resistance. Thus, these signal transduction receptors and signaling molecules may serve as both predictive markers and novel therapeutic targets to circumvent endocrine resistance. Compelling experimental and clinical evidence suggest that the epidermal growth factor/HER2/neu receptor (EGFR/HER2) pathway might play a distinct role in endocrine resistance, and especially in resistance to selective estrogen receptor modulators (SERMs) such as tamoxifen. Results from preclinical studies of treatment combinations with various endocrine therapy drugs together with several potent anti-EGFR/HER2 inhibitors are very promising, and clinical trials to see whether this new strategy is effective in patients are now ongoing.     

Antiestrogen resistance in ER positive breast cancer cells

Summary Acquisition of the antiestrogen resistance by breast cancer cellsin vivo may result from a variety of mechanisms. The main pathway appears to involve loss of estrogen receptor (ER) expression or selection for ER negative cells among heterogenous population of tumor cells. However, clinical data suggest that, in about 30% of the cases, antiestrogen resistance arises even in the presence of estrogen receptors. Postulated mechanisms leading to the latter phenotype include selection for variant receptor forms during treatment, development of novel metabolic pathways for the drug, loss of nuclear co-factors, or activation of signal transduction pathway that cross activate ER signals. We have used anin vitro experimental system utilizing LY-2 cell line, an ER positive and antiestrogen resistant MCF-7 cell variant, to study the mechanism of antiestrogen resistance in the presence of functional ER. Result from a complementation experiment suggests that LY-2 phenotype is a recessive trait. Cloning of the genetic defect in the LY-2 cells would provide further insight for the mechanism of antiestrogen resistance in ER positive breast cancer cells.     

Prognostic role of estrogen receptor determination in breast cancer

The authors report on 283 primary, non-metastatic, breast cancer cases consecutively referred after surgery and followed-up from a minimum of 10 months to a maximum of 3.5 years. All cases were studied according to the presence of estrogen receptors (ER). ER presence was correlated with age and menstrual status, with ER+ cases more frequent in older patients. No correlation was found between ER and nodal status. Prognosis was evaluated in terms of disease-free survival at 2 years (actuarial method). No correlation between ER and survival was evident for N- cases, whereas a better prognosis was recorded for ER+N+ patients compared to ER-N+, although the difference was not statistically significant. The observed results are compared with recent literature data and agree with other recent reports, which did not confirm the previously undiscussed statement regarding the prognostic role of ER determination. According to these studies and to the present study, the prognostic role of ER determination seems at least questionable and particularly the postoperative adjuvant treatment of ER-N- cases should be reconsidered.     

Let-7 family miRNAs regulate estrogen receptor alpha signaling in estrogen receptor positive breast cancer

In order to understand how microRNAs (miRNAs) regulate breast cancer tumorigenesis, a miRNA expression microarray screening was performed using RNA from formalin-fixed paraffin-embedded (FFPE) breast tissues, which included benign (n = 13), ductal carcinoma in situ (DCIS) (n = 16), and invasive ductal carcinoma (IDC) (n = 15). Twenty-five differentially expressed miRNAs (P < 0.01) were identified, of which let-7 family miRNAs were down-regulated in human breast cancer tissues at stages of DCIS and IDC compared to benign stage. We further found that there was an inverse correlation between ER-α expression and several members of let-7 family in the FFPE tissues. Next, we performed bioinformatics analysis and found that let-7 miRNA sequences match sequence in the 3′-UTR of estrogen receptor alpha (ER-α), suggesting ER-α may be a target of let-7, which was further confirmed by a number of experimental assays, including luciferase assay, protein expression, and mRNA expression. Overexpression of let-7 miRNAs in ER-positive breast cancer MCF7 cell line negatively affected ER-α activity. As expected, dampening of the ER-α signaling by let-7 miRNAs inhibited cell proliferation, and subsequently triggered the cell apoptotic process in MCF7 cells. In conclusion, our findings indicate a new regulatory mechanism of let-7 miRNAs in ER-α mediated cellular malignant growth of breast cancer.     

G-protein-coupled estrogen receptor GPR30 and tamoxifen resistance in breast cancer

Recently, we have shown that the new G-protein-coupled estrogen receptor GPR30 plays an important role in the development of tamoxifen resistance in vitro. This study was undertaken to evaluate the correlation between GPR30 and tamoxifen resistance in breast cancer patients. GPR30 protein expression was evaluated by immunohistochemical analysis in 323 patients with primary operable breast cancer. The association between GPR30 expression and tamoxifen resistance was confirmed in a second cohort of 103 patients treated only with tamoxifen. Additionally, we evaluated GPR30 expression in 33 primary tumors and in recurrent tumors from the same patients. GPR30 expression was detected in 56.7% of the breast cancer specimens investigated and it correlated with overexpression of HER-2 (P = 0.021), EGFR (P = 0.024) and lymph node status (P = 0.047). In a first cohort, survival analysis showed that GPR30 was negatively correlated with relapse-free survival (RFS) only in patients treated with tamoxifen (tamoxifen with or without chemotherapy). GPR30 expression was associated with shorter RFS (HR = 1.768; 95% CI, 1.156–2.703; P = 0.009). In a subset of patients treated only with tamoxifen, multivariate analysis revealed that GPR30 expression is an independent unfavorable factor for RFS (HR = 4.440; 95% CI, 1.408–13.997; P = 0.011). In contrast, GPR30 tended to be a favorable factor regarding RFS in patients who did not receive tamoxifen. In 33 paired biopsies obtained before and after adjuvant therapy, GPR30 expression significantly increased only under tamoxifen treatment (P = 0.001). GPR30 expression in breast cancer independently predicts a poor RFS in patients treated with tamoxifen.     

Role of estrogen receptor alpha in modulating IGF-I receptor signaling and function in breast cancer

The insulin-like growth factor I (IGF-I) receptor (IGF-IR) is a multifunctional transmembrane tyrosine kinase that has been implicated in neoplastic transformation. The tumorigenic potential of IGF-IR relies on its strong anti-apoptotic and mitogenic activity. The growth and survival signals of IGF-IR are mediated through multiple intracellular pathways, many of which emanate from insulin receptor substrate 1 (IRS-1). In hormone-dependent breast cancer cells, IGF-IR and IRS-1 are often co-expressed with the estrogen receptor alpha (ERalpha), and IGF-I and ER systems are engaged in a powerful functional cross-talk. Most notably, activation of ERalpha upregulates the expression of IRS-1, IGF-IR, and IGF-1, which results in amplification of IGF-I responses. Reciprocally, stimulation of IGF-IR increases the phosphorylation and activity of ERalpha. In contrast, in ERalpha-negative breast cancer cells and tumors, the levels of IGF-IR and IRS-1 are often decreased and IGF-I is non-mitogenic. Our data suggest that defective IGF-IR signaling in ERalpha-negative cells is related, at least in part, to improper activation of the IRS-1/PI-3K/Akt/GSK-3 pathway and lack of Rb1 phosphorylation. These defects are partially reversed by re-expression of ERalpha. Interestingly, some non-mitogenic IGF-I responses, such as migration and invasion are retained in the absence of ERalpha, suggesting that IGF-IR function in breast cancer cells might depend on the ERalpha status. The understanding of how ERalpha may dictate IGF-I responses will help in devising rational anti-IGF-IR strategies for breast cancer treatment.     

Cryptotanshinone inhibits breast cancer cell growth by suppressing estrogen receptor signaling

Estrogen receptor (ER) is a major therapeutic target for the treatment of breast cancer, because of the crucial role of estrogen signaling deregulation in the development and progression of breast cancer. In this study, we report the identification of a novel ERα binding compound, cryptotanshinone (CPT), by screening the CADD database. We also show that CPT effectively inhibits estrogen-induced ER transactivation and gene expression of ER target genes. Furthermore, we showed that CPT suppressed breast cancer cell growth mainly in an ERα dependent manner. Finally, we confirmed the potential therapeutic efficiency of CPT using xenograft experiments in vivo. Taken together, our results describe a novel mechanism for the anticancer activity of CPT and provide supporting evidence for its use as a potential therapeutic agent to treat patients with ERα positive breast cancer.     

雌激素受体与乳腺癌内分泌治疗耐药的相关性研究

目的探讨雌激素受体β(ERβ)与乳腺癌内分泌治疗耐药的相关性。方法选取2010年1月至2015年6月间广东省惠州市中心人民医院收治的行乳腺癌改良根治术且接受他莫昔芬(TAM)内分泌治疗的100例早中期绝经后乳腺癌患者。检测乳腺癌患者ERβ表达情况,计算患者无肿瘤生存时间,比较不同ERβ表达患者间临床因素和预后的差异,分析ERβ表达差异与TAM内分泌治疗耐药的关系。结果 ERβ表达与原癌基因人类表皮生长因子受体2(HER-2)的表达相关,差异有统计学意义(P<0.05),与患者年龄、肿瘤大小、淋巴结转移、化疗及放疗无关,差异无统计学意义(P>0.05)。ERβ阳性表达患者无肿瘤生存率明显低于ERβ阴性表达患者,差异有统计学意义(P<0.05)。Cox多因素分析显示,ERβ阳性表达患者中,淋巴结转移是乳腺癌内分泌治疗预后不良的独立危险因素,差异有统计学意义(P<0.05)。结论 ERβ阳性表达在乳腺癌患者内分泌治疗耐药中具有重要作用,可能是导致患者预后不佳的危险因素之一。     

Selenium disrupts estrogen signaling by altering estrogen receptor expression and ligand binding in human breast cancer cells

Cancer prevention studies suggest that selenium is effective in reducing the incidence of cancers including prostate, colon, and lung cancers. Previous reports showed that selenium inhibits premalignant human breast MCF-10AT1 and MCF10AT3B cell growth in vitro and reduces mammary tumor incidence after exposure to carcinogens in tumor models. Because estrogen is critical to the development and differentiation of estrogen target tissues, including the breast, the present study was designed to examine the effect of selenium on estrogen receptor (ER) expression and activation using methylseleninic acid (MSA), an active form of selenium in vitro. Selenium decreased the levels of expression of ERalpha mRNA and protein and reduced the binding of labeled estradiol to estrogen receptor in MCF-7 cells. Selenium inhibited the trans-activating activity of estrogen receptor in MCF-7 cells expressing functional estrogen receptor using a luciferase reporter construct linked to estrogen responsive element. Selenium decreased the binding of estrogen receptor to the estrogen responsive element site using an electrophoretic mobility gel shift assay. Selenium suppressed estrogen induction of the endogenous target gene c-myc. In contrast to the effect on ERalpha in MCF-7 cells, selenium increased ERbeta mRNA expression in MDA-MB231 human breast cancer cells. Thus, differential regulation of ERalpha and ERbeta in breast cancer cells may represent a novel mechanism of selenium action and provide a rationale for selenium breast cancer prevention trial.     

乳腺癌组织ER与Hedgehog信号通路分子相关性

目的 探讨乳腺浸润性导管癌中雌激素受体(estrogen receptor,ER)的表达与Hedgehog信号通路关键分子Gli1和Ptch之间的关系及临床意义.方法 收集2012-01-01-2013-01-30滨州医学院附属医院甲乳外科手术切除60例ER阳性和60例ER阴性乳腺浸润性导管癌标本,采用RT-PCR和蛋白质印迹法检测Hedgehog信号通路关键分子Gli1和Ptch mRNA和蛋白的表达情况,分析其相关性及临床意义.结果 RT-PCR检测结果显示,ER阳性组Gli1 mRNA表达量为1.05±0.27,ER阴性组中为2.70±0.26,差异有统计学意义,t=20.751,P＜0.01;ER阳性组Ptch mRNA表达量为1.04±0.29,ER阴性组中为2.34±0.35,差异有统计学意义,t=18.413,P＜0.01;蛋白质印迹法检测结果显示,ER阳性组中Gli1蛋白表达量为0.43±0.007,ER阴性组中为0.71±0.011,差异有统计学意义,t＝11.086,P＜0.01;ER阳性组Ptch蛋白表达量为0.36±0.008,在ER阴性组中为0.58±0.012,差异有统计学意义,t=16.437,P＜0.01.Gli1和Ptch与ER阴性患者的年龄和肿瘤大小无关,P值均＞0.05;Gli1(x2＝16.33,P＜0.001)和Ptch(x2＝24.33,P＜0.001)与TNM分期存在相关性;Gli1(x2=6.50,P＜0.05)和Ptch(x2 =5.09,P＜0.05)与腋窝淋巴结转移存在相关性.结论 在ER阴性乳腺浸润性导管癌中,Hedgehog信号通路呈明显活化状态,可能与TNM分期与腋窝淋巴结的转移有关,阻断Hedgehog信号通路可能成为治疗ER阴性乳腺癌的新靶点.     

Targeting tyrosine-kinases and estrogen receptor abrogates resistance to endocrine therapy in breast cancer

Despite numerous therapies that effectively inhibit estrogen signaling in breast cancer, a significant proportion of patients with estrogen receptor (ER)-positive malignancy will succumb to their disease. Herein we demonstrate that long-term estrogen deprivation (LTED) therapy among ER-positive breast cancer cells results in the adaptive increase in ER expression and subsequent activation of multiple tyrosine kinases. Combination therapy with the ER down-regulator fulvestrant and dasatinib, a broad kinase inhibitor, exhibits synergistic activity against LTED cells, by reduction of cell proliferation, cell survival, cell invasion and mammary acinar formation. Screening kinase phosphorylation using protein arrays and functional proteomic analysis demonstrates that the combination of fulvestrant and dasatinib inhibits multiple tyrosine kinases and cancer-related pathways that are constitutively activated in LTED cells. Because LTED cells display increased insulin receptor (InsR)/insulin-like growth factor 1 receptor (IGF-1R) signaling, we added an ant-IGF-1 antibody to the combination with fulvestrant and dasatinib in an effort to further increase the inhibition. However, adding MK0646 only modestly increased the inhibition of cell growth in monolayer culture, but neither suppressed acinar formation nor inhibited cell migration in vitro and invasion in vivo. Therefore, combinations of fulvestrant and dasatinib, but not MK0646, may benefit patients with tyrosine-kinase-activated, endocrine therapy-resistant breast cancer.     

The role of estrogen receptor alpha in mediating chemoresistance in breast cancer cells

Introduction

Previous studies suggested that estrogen receptor alpha (ERα) plays an important role in the chemoresistance of breast cancers. However, large random trials failed to demonstrate any benefit of the concurrent estrogen antagonist tamoxifen on the chemotherapy efficacy. Thus, in the present study, the importance of the role of ERα in the chemoresistance of breast cancer cells was investigated.

Methods

The ERα-transfected Bcap37 cells and natural ERα-positive T47D breast cancer cells were treated using chemotherapeutic agents with or without 17-beta estradiol (E2) pretreatment. Their viabilities were assessed using 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide assays. The dead cell rates were determined using propidium iodide dye exclusion tests, and the expression levels of Bcl-2 and Bax were detected through Western blot analysis. The effects of E2 on the growth of breast cancer cells were also determined via cell growth curve and cell cycle analysis.

Results

ERα activation by E2 increased the sensitivity of natural ERα-positive T47D breast cancer cells to chemotherapeutic agents. However, the increase in ERα expression in ERα-negative Bcap37 breast cancer cells also significantly increased their resistance. These phenomena cannot be explained by asserting that ERα mediated the chemoresistance of breast cancer cells by regulating the expression of Bcl-2 and Bax. Our findings show that ERα activation upregulated the expression of Bcl-2 in natural ERα-positive T47D breast cancer cells, whereas ERα activation by E2 downregulated and upregulated the Bcl-2 and Bax expression levels, respectively, in ERα-transfected Bcap37 cells. This phenomenon was due to the influence of ERα on the growth of breast cancer cells. Specifically, ERα activation enhanced the growth of natural ERα-positive breast cancer cells and thus increased their sensitivity to chemotherapeutic agents. However, ERα activation also inhibited the growth of ERα-transfected Bcap37 cells and increased the resistance of cancer cells to chemotherapeutic agents. Chemoresistance of ERα-transfected Bcap37 cells was only due to the specific growth inhibition by E2, which is not applicable to common ERα-positive breast cancer cells.

Conclusions

Although ERα was associated with chemoresistance of breast cancers, ERα itself did not mediate this resistance process.     

The origins of estrogen receptor alpha-positive and estrogen receptor alpha-negative human breast cancer

Current hormonal therapies have benefited millions of patients with breast cancer. Their success, however, is often temporary and limited to a subset of patients whose tumors express estrogen receptor alpha (ER). The therapies are entirely ineffective in ER-negative disease. Recent studies suggest that there are many biological pathways and alterations involved in determining whether ER is expressed and how it is regulated during breast cancer evolution. Improving hormonal therapies, in addition to perfecting current strategies, will also target these newly discovered pathways and alterations, and others yet to be found. The present commentary will briefly highlight a few important observations and unanswered questions regarding ER status and growth regulation during breast cancer evolution, which hopefully will help to stimulate new thinking and progress in this important area of medial research.