Mechanism Of Tamoxifen Resistance Research Articles

Mechanisms of tamoxifen resistance: insight from long non-coding RNAs.

Breast cancer(BC) is the second most prevalent tumor in the world and one of the most lethal tumors in women. Patients with estrogen receptor-positive breast cancer can obtain significant advantages from endocrine therapies including tamoxifen, aromatase inhibitors, and others. However, the development of primary or acquired drug resistance ultimately leads to discontinuation of treatment with adverse consequences for breast cancer patients, and the underlying mechanisms have not been fully elucidated. Long non-coding RNAs (lncRNAs) play pivotal roles in orchestrating fundamental biochemical and cellular processes. They exert regulatory control over various processes including epigenetics, gene transcription, post-transcriptional modifications, and translation. Additionally, they influence key biological events like cell cycle progression, cell differentiation, and development. For the past few years, the relationship between lncRNAs and endocrine resistance has gained increasing attention, leading to a surge in related studies. LncRNAs mediate tamoxifen resistance in cancer by utilizing a variety of molecular mechanisms, including enhanced estrogen receptor (ER) signaling, inhibition of apoptosis, autophagy, exosome-mediated transfer, epigenetic alterations, epithelial-to-mesenchymal transition, and acting as competitive endogenous RNAs(ceRNAs). In this comprehensive review, we systematically summarize the critical role and intricate molecular mechanisms by which lncRNAs influence the development of tamoxifen resistance in breast cancer. Furthermore, we propose the potential clinical significance of lncRNAs as innovative therapeutic targets and prognostic biomarkers for breast cancer.

A Distinct Chromatin State Drives Therapeutic Resistance in Invasive Lobular Breast Cancer.

A unique FOXA1-ER axis in invasive lobular breast cancer promotes disease progression and tamoxifen resistance, highlighting a potential therapeutic avenue for clinical investigations dedicated to this disease. See related commentary by Blawski and Toska, p. 3668.

Multi-time scale transcriptomic analysis on the dynamic process of tamoxifen resistance development in breast cancer cell lines.

Approximately 30% of breast cancer patients develop endocrine resistance after tamoxifen therapy. There still lacks a comprehensive understanding on the mechanism of tamoxifen resistance. This study aims to explore the dynamic process of ER + breast cancer resistance to tamoxifen through the time course transcriptomic analysis. The transcriptome profiles of human breast cancer cell line MCF-7 treated with tamoxifen at different time scales were collected from LINCS, SRA and GEO databases. Differentially expressed genes (DEGs) were identified in the short-term tamoxifen treatment and tamoxifen-resistant cell lines. The time course analysis was used to explore the dynamic development of tamoxifen resistance using the transcriptome profiles of tamoxifen-cultured MCF-7 for 1-12weeks. After the short-term treatment of MCF-7 with tamoxifen for 6h or 24h, the expression level of gene PRSS23 was significantly reduced. However, its expression recovered in the resistant cell lines. The time course analysis identified 9 clusters of the DEGs based on the temporal trend of their expression levels. Gene PRSS23 belongs to cluster 2 in which the expression levels were significantly down-regulated in the first 4weeks but gradually recovered afterwards. Functional enrichment analysis of the DEGs in cluster 2 showed that they are significantly enriched in DNA replication, mismatch repair and cell cycle pathways. Their specific role in the resistance development needs to be further explored. The protein-protein interaction network analysis indicates that gene PRSS23 participates in the drug resistance by regulating multiple tamoxifen drug targets. The acquired drug resistance in ER + breast cancer is a complex and dynamic biological process. PRSS23 plays an important role in the development of resistance and is a potential target for overcoming resistance.

Abstract GS3-10: Partitioning of cancer therapeutics in nuclear condensates

Abstract The molecules of the cell are compartmentalized into membrane- and non-membrane-bound organelles. Many non-membrane-bound organelles are phase-separated biomolecular condensates with distinct physicochemical properties that can absorb and concentrate specific proteins and nucleic acids involved in discrete biochemical processes. We reasoned that selective condensate partitioning might also occur with small molecule drugs whose targets occur within condensates, and that the therapeutic index and efficacy of such compounds might therefore relate to their ability to partition into condensates. To test this idea, we focused our study on nuclear condensates reported in cell lines, demonstrated they occur in normal human and malignant breast cancer, and developed assays to test clinically active antineoplastic small molecule drugs relative to these condensates.To study the behavior of drugs within condensates, these were modeled in vitro with purified proteins and visualized by fluorescent confocal microscopy. We found that cisplatin, tamoxifen, JQ1, THZ1, and mitoxantrone are concentrated in specific protein condensates in vitro, and that this occurs through physicochemical properties independent of the drug target. For each drug, the small molecule partitioned into the same condensate in vitro in which its established target resides in vivo. A screen of a chemically diverse fluorescent probes and mutant-protein condensates demonstrated that pi-system interactions between aromatic moieties in the protein and small molecule govern concentration in condensates. These results show that clinically important drugs partition into specific protein condensates in vitro by virtue of defined chemical properties, thereby altering their local concentration.Alkylating agents are a class of commonly used antineoplastic compounds, of which cisplatin is a prominent example. In vitro droplet assays revealed that cisplatin is selectively concentrated in transcriptional condensates, and that this ability is required for efficient platination of target DNA. In cell studies revealed that cisplatin preferentially targets DNA contained within MED1 condensates, and disrupts the genetic regulatory elements that compose phase-separated transcriptional condensates. Live cell imaging demonstrated that transcriptional condensates are dissolved by cisplatin, whereas other condensates remain intact. Thus, we conclude that cisplatin preferentially modifies transcriptional condensate-associated DNA in cells, and that this causes selective condensate disruption. The mechanisms that produce drug resistance can provide clues to drug activity in the clinical setting. Investigating the behavior of tamoxifen within ER transcriptional condensates demonstrated that it disrupts these condensates in vitro and on oncogenes in cells; hormonal therapy resistant ESR1 mutations render these condensates resistant. MED1 overexpression, a poorly understood mechanism of tamoxifen resistance, increased the size of ER-MED1 condensates, thereby rending tamoxifen more dilute and ineffective when concentrated therein. This suggest that altering the size and nature of transcription condensates in breast cancer can mediate drug resistance in the clinical setting.Our results show that antineoplastic drugs partition selectively into condensates, that this can occur through physicochemical properties independent of their molecular targets, and that resistance to drugs may occur through condensate altering mechanisms. These results have implications for development of efficacious cancer therapeutics; effective target engagement will depend on factors such as drug partitioning in condensates. Assays of the type described here may thus help optimize condensate partitioning, target engagement, and the therapeutic index of drugs for cancer treatment. Citation Format: Isaac Klein, Ann Boija, Lena Afeyan, Susana Wilson Hawken, Mengyang Fan, Alessandra Dall'Agnese, Ozgur Oksuz, Jonathan Henninger, Krishna Shrinivas, Benjamin Sabari, Ido Sagi, Victoria Clark, Jesse Platt, Mrityunjoy Kar, Patrick McCall, Alicia Zamudio, John Mantiega, Eliot Coffey, Charles Li, Nancy Hannett, Yang Guo, Tim-Michael Decker, Tong Lee, Tinghu Zhang, Jing-Ke Weng, Dylan Taatjes, Arup Chakraborty, Phillip Sharp, Young Tae Chang, Anthony Hyman, Nathanael Gray, Richard Young. Partitioning of cancer therapeutics in nuclear condensates [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr GS3-10.

Tamoxifen resistance mechanisms in breast cancer treatment

Therapies targeting estrogen receptor (ER) are being widely used to treat ER+ breast cancer patients. Despite early detection and improved survival outcomes, tamoxifen resistance, either intrinsic or acquired- is a major obstacle in effective disease management. Current review will summarize different molecular mechanisms of tamoxifen resistance both intrinsic and acquired in breast cancer treatment. This review not only provides basis to understand the nature of tamoxifen drug resistance but also suggests the mechanisms for its control leading to improved therapeutic interventions.

Tamoxifen resistance mechanisms in breast cancer treatment

Therapies targeting estrogen receptor (ER) are being widely used to treat ER+ breast cancer patients. Despite early detection and improved survival outcomes, tamoxifen resistance, either intrinsic or acquired- is a major obstacle in effective disease management. Current review will summarize different molecular mechanisms of tamoxifen resistance both intrinsic and acquired in breast cancer treatment. This review not only provides basis to understand the nature of tamoxifen drug resistance but also suggests the mechanisms for its control leading to improved therapeutic interventions.

Abstract 5281: Development of a panel of breast cancer patient-derived xenograft models (PDX) with estrogen independence and/or acquired resistance to endocrine treatment

Abstract Eighty five percent of preclinical agents entering oncology clinical trials fail to demonstrate sufficient safety or efficacy to gain regulatory approval (1). This failure rate shows a weak understanding of the complexity of human cancer, the continued limitations of the predictive value of existing preclinical models and the scale at which cancer models are interrogated in the preclinical setting (2). Consequently, there is a need of new experimental models that better replicate the diversity of human tumor biology in a preclinical setting. It is now evidenced that PDXs models recapitulate human tumor biology and predict patient drug response (3) by directly comparing drug responses in patients and their corresponding xenografts. To extend such observations to a greater number of human cancers, we have generated in collaboration with EISAI an extensive collection of breast cancer PDXs. Similar to what is observed in the clinic, 60% of our breast PDX panel is classified as estrogen receptor (ER) positive. Tested compounds which interfere with the ER pathway as anti-estrogens (fulvestrant) or estrogen deprivation (without estrogen supplementation, or ovariectomy), are able to decrease the tumor growth in PDX-bearing mice. However, in clinic, endocrine treatment efficacy is limited by intrinsic and acquired resistance (4). The main mechanisms of resistance to these therapies are lack of expression of ER, deregulation of ER-associated transcription factors, coactivators, activation of receptor tyrosine kinase signaling, and aberrant expression of cell-cycle regulators (5). In order to reconcile this need for relevant preclinical models and to study acquired resistance we developed a panel of breast cancer PDX models either resistant to fulvestrant or able to grow in the complete absence of estrogen (ovariectomy or without estrogen supplementation). Each generated tumor was then analyzed by IHC for their ER/PR expression. Whole exome mutation and gene expression were also analyzed and compared to the parental tumor. 1: Arrowsmith, J. & Miller, P. Trial watch: phase II and phase III attrition rates 2011-2012. Nat. Rev. Drug Discov. 12, 569 (2013). 2: Paul, S.M. et al. How to improve R&D productivity: the pharmaceutical industry's grand challenge. Nat. Rev. Drug Discov. 9, 203-214 (2010). 3: Hidalgo, M. et al. Patient-derived xenograft models: an emerging platform for translational cancer research. Cancer Discov. 4, 998-1013 (2014). 4. Ring, A. & Dowsett, M. Mechanisms of tamoxifen resistance. Endocr Relat Cancer 11:643-58 (2004). 5. Hanny, A. & Abdel-Hafiz. Epigenetic mechanisms of tamoxifen resistance in luminal breast cancer. Diseases Jul 6;5(3) (2017). Citation Format: Marc Hillairet de Boisferon, Ismahene Benzaid, Elodie Marie Dit Chatel, Nicolas Hoffmann, Bruce Littlefield. Development of a panel of breast cancer patient-derived xenograft models (PDX) with estrogen independence and/or acquired resistance to endocrine treatment [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5281.

GLUT1 participates in tamoxifen resistance in breast cancer cells through autophagy regulation

Tamoxifen is an estrogen modulator widely used in the treatment of patients with ESR/ER-positive breast cancer; however, resistance limits its clinical application. Autophagy alterations have recently been suggested as a new mechanism for tamoxifen resistance. Glucose transporter 1 (GLUT1) has been reported to be associated with the development and metastasis of breast cancer, but the relationship among GLUT1, autophagy, and endocrine resistance remains unclear. Our present study found that GLUT1 expression and autophagy flux were upregulated in the tamoxifen-resistant breast cancer cell line MCF-7/TAMR-1 and that knockdown of GLUT1 promoted sensitization to tamoxifen. Moreover, knockdown of GLUT1 significantly decreased the enhancement of autophagy flux in tamoxifen-resistant cell lines. Furthermore, inhibiting autophagy in tamoxifen-resistant cells resulted in sensitization to tamoxifen. We conclude that GLUT1 contributes to tamoxifen resistance in breast cancer and that tamoxifen-resistant cells become resensitized to tamoxifen after GLUT1 silencing. These findings suggest GLUT1 as a new factor clinically associated with resistance to tamoxifen.

MiR-200 affects tamoxifen resistance in breast cancer cells through regulation of MYB

Resistance to tamoxifen is a major clinical challenge. Research in recent years has identified epigenetic changes as mediated by dysregulated miRNAs that can possibly play a role in resistance to tamoxifen in breast cancer patients expressing estrogen receptor (ER). We report here elevated levels of EMT markers (vimentin and ZEB1/2) and reduced levels of EMT-regulating miR-200 (miR-200b and miR-200c) in ER-positive breast cancer cells, MCF-7, that were resistant to tamoxifen, in contrast with the naïve parental MCF-7 cells that were sensitive to tamoxifen. Further, we established regulation of c-MYB by miR-200 in our experimental model. C-MYB was up-regulated in tamoxifen resistant cells and its silencing significantly decreased resistance to tamoxifen and the EMT markers. Forced over-expression of miR-200b/c reduced c-MYB whereas reduced expression of miR-200b/c resulted in increased c-MYB We further confirmed the results in other ER-positive breast cancer cells T47D cells where forced over-expression of c-MYB resulted in induction of EMT and significantly increased resistance to tamoxifen. Thus, we identify a novel mechanism of tamoxifen resistance in breast tumor microenvironment that involves miR-200-MYB signaling.

Autoantibodies Specific to ERα are Involved in Tamoxifen Resistance in Hormone Receptor Positive Breast Cancer.

Tamoxifen resistance is a major hurdle in the treatment of estrogen receptor (ER)-positive breast cancer. The mechanisms of tamoxifen resistance are not fully understood although several underlying molecular events have been suggested. Recently, we identified autoantibodies reacting with membrane-associated ERα (anti-ERα Abs) in sera of breast cancer patients, able to promote tumor growth. Here, we investigated whether anti-ERα Abs purified from sera of ER-positive breast cancer patients could contribute to tamoxifen resistance. Anti-ERα Abs inhibited tamoxifen-mediated effects on cell cycle and proliferation in MCF-7 cells. Moreover, anti-ERα Abs hampered the tamoxifen-mediated reduction of tumor growth in SCID mice xenografted with breast tumor. Notably, simvastatin-mediated disaggregation of lipid rafts, where membrane-associated ERα is embedded, restored tamoxifen sensitivity, preventing anti-ERα Abs effects. In conclusion, detection of serum anti-ERα Abs may help predict tamoxifen resistance and concur to appropriately inform therapeutic decisions concerning hormone therapy in ER-positive breast cancer patients.

Oestrogen receptor-regulated glutathione S-transferase mu 3 expression attenuates hydrogen peroxide-induced cytotoxicity, which confers tamoxifen resistance on breast cancer cells.

Glutathione S-transferase mu 3 (GSTM3) is an enzyme involving in the detoxification of electrophilic compounds by conjugation with glutathione. Higher GSTM3 mRNA levels were reported in patients with ERα-positive breast cancer who received only tamoxifen therapy after surgery. Thus, this study aimed to clarify the oncogenic characteristics of GSTM3 in breast cancer and the mechanism of tamoxifen resistance. GSTM3 expression in human breast tumour tissues (n = 227) was analysed by RT-PCR and quantitative PCR. Western blot, promoter activity assays, and chromatin immunoprecipitation (ChIP) assays were used to investigate the mechanism of GSTM3 gene regulation. Hydrogen peroxide (H2O2)-induced cytotoxicity in breast cancer cells was detected by MTT assays and flow cytometry. The oncogenic characteristics of GSTM3 in MCF-7 cells were examined by siRNA knockdown in soft agar assays and a xenograft animal model. GSTM3 mRNA was highly expressed in ER- and HER2-positive breast cancers. Moreover, patients who received adjuvant Herceptin had increased GSTM3 mRNA levels in tumour tissue. Oestrogen-activated GSTM3 gene expression through ERα-mediated recruitment of SP1, EP300, and AP-1 complexes. GSTM3-silenced MCF-7 cells were more sensitive to H2O2, with significantly inhibited proliferation and colony formation abilities. Tamoxifen-resistant (Tam-R) cells lacking GSTM3 showed enhanced sensitivity to H2O2, but this result was contrary to that obtained after short-term tamoxifen exposure. The animal model suggested that GSTM3 silencing might suppress the tumourigenic ability of MCF-7 cells and increase tumour cell apoptosis. ROS production is one mechanism by which cancer drugs kill tumour cells, and according to our evidence, GSTM3 may play an important role in preventing breast cancer treatment-induced cellular cytotoxicity.

Regulation of tamoxifen resistance in estrogen receptor positive MCF-7 breast cancer cells by microRNA-10b targeting histone deacetylase 4

Objective To investigate the effect of microRNA-10b (miR-10b) targeting histone deacetylase 4 (HDAC4) on the resistance of estrogen receptor positive (ER+ ) MCF-7 breast cancer cells to tamoxifen. Methods ER+ and tamoxifen resistant MCF-7 cell line MCF-7-TR was established, and miR-10b expression level in MCF-7 cells and MCF-7-TR cells was detected by reverse transcription real-time quantitative polymerase chain reaction (Real-time PCR). Methyl thiazol tetrazolium (MTT) and trypan blue were used to detect the proliferation and activity of MCF-7 cells and MCF-7-TR cells transfected with pre-miR-10b, miR-10b inhibitor and siHDAC4 at different tamoxifen concentrations (0, 5, 10, 15, 20, 30 μmol/L), respectively. The expression of HDAC4 protein was detected by Western blotting. Results Taken expression level and invasive ability of miR-10b in MCF-7 breast cancer cells as the standard, the miR-10b expression level and invasion ability of MCF-7-TR cell breast cancer cells were 7.2±1.1 and 5.3±1.3 respectively. With the increase of the concentration of tamoxifen, decreased the rate of cell proliferation in MCF-7 cells, which is most obvious, followed by MCF-7-TR+ miR-10b inhibitor + siHDAC4 and MCF-7+ pre-miR-10b+ HDAC4 cells, and the proliferation of MCF-7-TR+ siHDAC4 cells and MCF-7-TR cells decreased the rate of the lowest. The migration ability of MCF-7 cells and MCF-7-TR+ miR-10b cells of the inhibitor decreased most, MCF-7+ pre-miR-10b+ and tamoxifen (10 μmol/L) MCF-7-TR+ cells and tamoxifen (10 μmol/L) had no significant difference in the ability of cell migration (t=1.116, P=0.272), but was significantly higher than that of MCF-7 cells and MCF-7-TR+ miR-10b cells (t=0.124, 10.769, 14.569, 11.577, P=0.000). The expression level of HDAC4 protein in MCF-7+ pre-miR-10b cells was lower than that in MCF-7 cells, MCF-7-TR cells, HDAC4 protein expression level, MCF-7-TR+ miR-10b inhibitor cells, MCF-7+ siHDAC4 cells HDAC4 protein expression level was lower than MCF-7 cells. Conclusion The miR-10b-HDAC4 signaling pathway may be a molecular mechanism of tamoxifen resistance in breast cancer. Key words: Breast cancer; Estrogen receptor positive; Tamoxifen resistance; MicroRNA-10b; Histone deacetylase 4

Reactivation of cAMP Pathway by PDE4D Inhibition Represents a Novel Druggable Axis for Overcoming Tamoxifen Resistance in ER-positive Breast Cancer.

Purpose: Tamoxifen remains an important hormonal therapy for ER-positive breast cancer; however, development of resistance is a major obstacle in clinics. Here, we aimed to identify novel mechanisms of tamoxifen resistance and provide actionable drug targets overcoming resistance.Experimental Design: Whole-transcriptome sequencing, downstream pathway analysis, and drug repositioning approaches were used to identify novel modulators [here: phosphodiesterase 4D (PDE4D)] of tamoxifen resistance. Clinical data involving tamoxifen-treated patients with ER-positive breast cancer were used to assess the impact of PDE4D in tamoxifen resistance. Tamoxifen sensitization role of PDE4D was tested in vitro and in vivo Cytobiology, biochemistry, and functional genomics tools were used to elucidate the mechanisms of PDE4D-mediated tamoxifen resistance.Results: PDE4D, which hydrolyzes cyclic AMP (cAMP), was significantly overexpressed in both MCF-7 and T47D tamoxifen-resistant (TamR) cells. Higher PDE4D expression predicted worse survival in tamoxifen-treated patients with breast cancer (n = 469, P = 0.0036 for DMFS; n = 561, P = 0.0229 for RFS) and remained an independent prognostic factor for RFS in multivariate analysis (n = 132, P = 0.049). Inhibition of PDE4D by either siRNAs or pharmacologic inhibitors (dipyridamole and Gebr-7b) restored tamoxifen sensitivity. Sensitization to tamoxifen is achieved via cAMP-mediated induction of unfolded protein response/ER stress pathway leading to activation of p38/JNK signaling and apoptosis. Remarkably, acetylsalicylic acid (aspirin) was predicted to be a tamoxifen sensitizer using a drug repositioning approach and was shown to reverse resistance by targeting PDE4D/cAMP/ER stress axis. Finally, combining PDE4D inhibitors and tamoxifen suppressed tumor growth better than individual groups in vivoConclusions: PDE4D plays a pivotal role in acquired tamoxifen resistance via blocking cAMP/ER stress/p38-JNK signaling and apoptosis. Clin Cancer Res; 24(8); 1987-2001. ©2018 AACR.

Salinomycin overcomes acquired tamoxifen resistance through AIB1 and inhibits cancer cell invasion in endocrine resistant breast cancer.

Salinomycin is a monocarboxylic polyether ionophore isolated from Streptomyces albus. It has been widely used as an antibiotic in veterinary medicine in poultry. A recent study demonstrated that salinomycin selectively inhibits human breast cancer stem cells; one possible mechanism of tamoxifen resistance. Our results show that salinomycin is effective in inhibiting MCF-7/LCC2 and MCF-7/LCC9 cell lines which are well-established endocrine resistant cells and has a synergistic effect in combination with tamoxifen using MTT proliferation assay. The inhibitory effect of salinomycin on the reduction of critical ER co-activator; amplified breast 1 (AIB1) mRNA and protein expression is overcoming tamoxifen resistance. Moreover, salinomycin significantly inhibits cell invasion in Matrigel invasion assay. The effect was mediated at least in part by the decrease of matrix metalopeptidase 9 (MMP-9) which is one critical enzyme facilitated in the cell invasion process. In conclusion, salinomycin should be developed as a novel agent used alone or in combination for endocrine-resistant breast cancer.

Aspirin regulation of c-myc and cyclinD1 proteins to overcome tamoxifen resistance in estrogen receptor-positive breast cancer cells.

Tamoxifen is still the most commonly used endocrine therapy drug for estrogen receptor (ER)-positive breast cancer patients and has an excellent outcome, but tamoxifen resistance remains a great impediment to successful treatment. Recent studies have prompted an anti-tumor effect of aspirin. Here, we demonstrated that aspirin not only inhibits the growth of ER-positive breast cancer cell line MCF-7, especially when combined with tamoxifen, but also has a potential function to overcome tamoxifen resistance in MCF-7/TAM. Aspirin combined with tamoxifen can down regulate cyclinD1 and block cell cycle in G0/G1 phase. Besides, tamoxifen alone represses c-myc, progesterone receptor (PR) and cyclinD1 in MCF-7 cell line but not in MCF-7/TAM, while aspirin combined with tamoxifen can inhibit the expression of these proteins in the resistant cell line. When knocking down c-myc in MCF-7/TAM, cells become more sensitive to tamoxifen, cell cycle is blocked as well, indicating that aspirin can regulate c-myc and cyclinD1 proteins to overcome tamoxifen resistance. Our study discovered a novel role of aspirin based on its anti-tumor effect, and put forward some kinds of possible mechanisms of tamoxifen resistance in ER-positive breast cancer cells, providing a new strategy for the treatment of ER-positive breast carcinoma.

Tamoxifen resistance and metastasis of human breast cancer cells were mediated by the membrane-associated estrogen receptor ER-α36 signaling in vitro.

The drug resistance and tumor metastasis have been the main obstacles for the longer-term therapeutic effects of tamoxifen (TAM) on estrogen receptor-positive (ER+) breast cancer, but the mechanisms underlying the TAM resistance are still unclear. Here, we demonstrated that the membrane-associated estrogen receptor ER-α36 signaling, but not the G protein-coupled estrogen receptor 1 (GPER1) signaling, might be involved in the TAM resistance and metastasis of breast cancer cells. In this study, a model of ER+ breast cancer cell MCF-7 that involves the up-regulated expression of ER-α36 and unchanged expression of ER-α66 and GPER1 was established via the removal of insulin from the cell culture medium. The mechanism of TAM resistance in the ER+ breast cancer cell line MCF-7 was investigated, and the results showed that the stimulating effect of insulin on susceptibility of MCF-7 to TAM was mediated by ER-α36 and that the expression level of ER-α36 in TAM-resistant MCF-7 cells was also significantly increased. Both TAM and estradiol (E2) could promote the migration of triple negative (ER-α66-/PR-/HER2-) and ER-α36+/GPER1+ breast cancer cells MDA-MB-231. The migration of MDA-MB-231 cells was inhibited by the down-regulated intracellular expression of ER-α36 by transient transfection of specific small interfering RNA, whereas no effect of GPER1 down-regulation was observed. Meanwhile, the effect of TAM on the migration of ER-α36-down-regulated MDA-MB-231 cells was also reduced. Furthermore, it was found that TAM enhanced the distribution of integrin β1 on the cell surface but did not affect the expression of integrin β1 in MDA-MB-231 cells. Collectively, these data suggested that ER-α36 signaling might play critical roles in acquired and de novo TAM resistance and metastasis of breast cancer, and ER-α36 might present a potential biomarker of TAM resistance in the clinical diagnosis and treatment of ER+ breast cancer.

Tamoxifen Resistance: Emerging Molecular Targets.

17β-Estradiol (E2) plays a pivotal role in the development and progression of breast cancer. As a result, blockade of the E2 signal through either tamoxifen (TAM) or aromatase inhibitors is an important therapeutic strategy to treat or prevent estrogen receptor (ER) positive breast cancer. However, resistance to TAM is the major obstacle in endocrine therapy. This resistance occurs either de novo or is acquired after an initial beneficial response. The underlying mechanisms for TAM resistance are probably multifactorial and remain largely unknown. Considering that breast cancer is a very heterogeneous disease and patients respond differently to treatment, the molecular analysis of TAM’s biological activity could provide the necessary framework to understand the complex effects of this drug in target cells. Moreover, this could explain, at least in part, the development of resistance and indicate an optimal therapeutic option. This review highlights the implications of TAM in breast cancer as well as the role of receptors/signal pathways recently suggested to be involved in the development of TAM resistance. G protein—coupled estrogen receptor, Androgen Receptor and Hedgehog signaling pathways are emerging as novel therapeutic targets and prognostic indicators for breast cancer, based on their ability to mediate estrogenic signaling in ERα-positive or -negative breast cancer.

Mitogen-activated protein kinase phosphatase 1 is involved in tamoxifen resistance in MCF7cells.

Tamoxifen resistance is a major clinical problem for ER-positive breast cancer, but the underlying mechanism is not completely elucidated. In the present study, we reported that mitogen-activated protein kinase (MAPK) phosphatase1 (MKP-1), a member of the family of MKPs, is involved in tamoxifen resistance. We found that MKP1 expression increased in tamoxifen resistant MCF7cells. To explore the possible role of MKP1 in tamoxifen resistance, siRNA targeting MKP1 was transfected into tamoxifen resistant MCF7cells. To our surprise, knockdown of MKP-1 promoted cell death induced by tamoxifen. On the other hand, the MKP1 overexpressed MCF7 cell clone was established and MKP1 overexpression effectively attenuated MCF7 cell death induced by tamoxifen. In addition, we revealed that MKP1 inhibited tamoxifen‑mediated JNK activation in tamoxifen resistant MCF7 and MCF7cells, and by this mechanism MKP1 was able to inhibit tamoxifen-induced cell death. We also showed that combined appliaction of MKP1 inhibitor triptolide and tamoxifen can effectively increase tamoxifen sensitivity in tamoxifen resistant MCF7cells. Collectively, our results indicated that MKP-1 can attenuate tamoxifen-induced cell death through inhibiting the JNK signal pathway, which represents a novel mechanism of tamoxifen resistance in MCF7cells.

IL6 induces TAM resistance via kinase-specific phosphorylation of ERα in OVCA cells.

About 40-60% of ovarian cancer (OVCA) cases express ERα, but only a small proportion of patients respond clinically to anti-estrogen treatment with estrogen receptor (ER) antagonist tamoxifen (TAM). The mechanism of TAM resistance in the course of OVCA progression remains unclear. However, IL6 plays a critical role in the development and progression of OVCA. Our recent results indicated that IL6 secreted by OVCA cells may promote the resistance of these cells to TAM via ER isoforms and steroid hormone receptor coactivator-1. Here we demonstrate that both exogenous (a relatively short period of treatment with recombinant IL6) and endogenous IL6 (generated as a result of transfection with a plasmid encoding sense IL6) increases expression of pERα-Ser118 and pERα-Ser167 in non-IL6-expressing A2780 cells, while deleting endogenous IL6 expression in IL6-overexpressing CAOV-3 cells (by transfection with a plasmid encoding antisense IL6) reduces expression of pERα-Ser118 and pERα-Ser167, indicating that IL6-induced TAM resistance may also be associated with increased expression of pERα-Ser118 and pERα-Ser167 in OVCA cells. Results of further investigation indicate that IL6 phosphorylates ERα at Ser118 and Ser167 by triggering activation of MEK/ERK and phosphotidylinositol 3 kinase/Akt signaling, respectively, to activate the ER pathway and thereby induce OVCA cells resistance to TAM. These results indicate that IL6 secreted by OVCA cells may also contribute to the refractoriness of these cells to TAM via the crosstalk between ER and IL6-mediated intracellular signal transduction cascades. Overexpression of IL6 not only plays an important role in OVCA progression but also promotes TAM resistance. Our results indicate that TAM-IL6-targeted adjunctive therapy may lead to a more effective intervention than TAM alone.

The role of captopril and losartan in prevention and regression of tamoxifen-induced resistance of breast cancer cell line MCF-7: An in vitro study

Innate and acquired tamoxifen (TAM) resistance in estrogen receptor positive (ER+) breast cancer is an important problem in adjuvant endocrine therapy. The underlying mechanisms of TAM resistance is yet unknown. In the present study, we evaluated the role of renin–angiotensin system (RAS) in the acquisition of TAM resistance in human breast cancer cell line MCF-7, and the potential role of captopril and captopril+losartan combination in the prevention and reversion of the TAM resistant phenotype. MCF-7 cells were continuously exposed to 1μmol/L TAM to develop TAM resistant cells (TAM-R). MTT cell viability assay was used to determine the growth response of MCF-7 and TAM-R cells, and quantitative real-time polymerase chain reaction (qRT-PCR) was used to assess angiotensin I converting enzyme (ACE), angiotensin II receptor type-1 and type-2 (AGTR1 and AGTR2) mRNA expressions. Preventive and therapeutic effects of RAS blockers – captopril and losartan – were examined on MCF-7 and TAM-R cells. Based on qRT-PCR, TAM-R cells compared to MCF-7 cells, had a mean±SD fold increase of 319.1±204.1 (P=0.002) in production of ACE mRNA level, 2211.8±777.9 (P=0.002) in AGTR1 mRNA level, and 265.9±143.9 (P=0.037) in production of AGTR2 mRNA level. The combination of either captopril or captopril+losartan with TAM led to the prevention and even reversion of TAM resistant phenotype.