Overcoming Resistance to Endocrine Therapy in Breast Cancer: New Approaches to a Nagging Problem

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.

Small molecule inhibition of TFF3 overcomes tamoxifen resistance and enhances taxane efficacy in ER+ mammary carcinoma

Even though tamoxifen has significantly improved the survival of estrogen receptor positive (ER+) mammary carcinoma (MC) patients, the development of drug resistance with consequent disease recurrence has limited its therapeutic efficacy. Trefoil factor-3 (TFF3) has been previously reported to mediate anti-estrogen resistance in ER+MC. Herein, the efficacy of a small molecule inhibitor of TFF3 (AMPC) in enhancing sensitivity and mitigating acquired resistance to tamoxifen in ER+MC cells was investigated. AMPC induced apoptosis of tamoxifen-sensitive and resistant ER+MC cells and significantly reduced cell survival in 2D and 3D culture in vitro. In addition, AMPC reduced cancer stem cell (CSC)-like behavior in ER+MC cells in a BCL2-dependent manner. Synergistic effects of AMPC and tamoxifen were demonstrated in ER+MC cells and AMPC was observed to improve tamoxifen efficacy in tamoxifen-sensitive cells and to re-sensitize cells to tamoxifen in tamoxifen-resistant ER+MC in vitro and in vivo. Additionally, tamoxifen-resistant ER+MC cells were concomitantly resistant to anthracycline, platinum and fluoropyrimidine drugs, but not to Taxanes. Taxane treatment of tamoxifen-sensitive and resistant ER+MC cells increased TFF3 expression indicating a combination vulnerability for tamoxifen-resistant ER+MC cells. Taxanes increased CSC-like behavior of tamoxifen-sensitive and resistant ER+MC cells which was reduced by AMPC treatment. Taxanes synergized with AMPC to promote apoptosis and reduce CSC-like behavior in vitro and in vivo. Hence, AMPC restored the sensitivity of tamoxifen and enhanced the efficacy of Taxanes in tamoxifen-resistant ER+MC. In conclusion, pharmacological inhibition of TFF3 may serve as an effective combinatorial therapeutic strategy for the treatment of tamoxifen-resistant ER+MC.

EGFR inhibition reverses epithelial‑mesenchymal transition, and decreases tamoxifen resistance via Snail and Twist downregulation in breast cancer cells

Tamoxifen resistance remains a major obstacle in the treatment of estrogen receptor (ER)‑positive breast cancer. In recent years, the crucial role of the epithelial‑mesenchymal transition (EMT) process in the development of drug resistance in breast cancer has been underlined. However, the central molecules inducing the EMT process during the development of tamoxifen resistance remain to be elucidated. In the present study, it was demonstrated that tamoxifen‑resistant breast cancer cells underwent EMT and exhibited an enhanced cell motility and invasive behavior. The inhibition of snail family transcriptional repressor 1 (Snail) and twist family BHLH transcription factor 1 (Twist) reversed the EMT phenotype and decreased the tamoxifen resistance, migration and invasion of tamoxifen‑resistant breast cancer cells. In addition, it was observed that the inhibition of epidermal growth factor receptor (EGFR) reversed the EMT phenotype in tamoxifen‑resistant MCF7 (MCF‑7/TR) cells via the downregulation of Snail and Twist. Notably, the EGFR inhibitor, gefitinib, decreased tamoxifen resistance, migration and invasion through the inhibition of Snail and Twist. On the whole, the results of the present study suggest that EGFR may be a promising therapeutic target for tamoxifen‑resistant breast cancer. Moreover, it was suggested that gefitinib may serve as a potent novel therapeutic strategy for breast cancer patients, who have developed tamoxifen resistance.

SETDB1 interactions with PELP1 contributes to breast cancer endocrine therapy resistance

Background

Methyltransferase SETDB1 is highly expressed in breast cancer (BC), however, the mechanisms by which SETDB1 promotes BC progression to endocrine therapy resistance remains elusive. In this study, we examined the mechanisms by which SETDB1 contribute to BC endocrine therapy resistance.

Methods

We utilized therapy sensitive (MCF7 and ZR75), therapy resistant (MCF7-TamR, MCF7-FR, MCF7-PELP1cyto, MCF7-SETDB1) estrogen receptor alpha positive (ER⁺)BC models and conducted in vitro cell viability, colony formation, 3-dimensional cell growth assays to investigate the role of SETDB1 in endocrine resistance. RNA-seq of parental and SETDB1 knock down ER⁺ BC cells was used to identify unique pathways. SETDB1 interaction with PELP1 was identified by yeast-two hybrid screen and confirmed by immunoprecipitation and GST-pull down assays. Mechanistic studies were conducted using Western blotting, reporter gene assays, RT-qPCR, and in vitro methylation assays. Xenograft assays were used to establish the role of PELP1 in SETDB1 mediated BC progression.

Results

RNA-seq analyses showed that SETDB1 regulates expression of a subset of estrogen receptor (ER) and Akt target genes that contribute to endocrine therapy resistance. Importantly, using yeast-two hybrid screen, we identified ER coregulator PELP1 as a novel interacting protein of SETDB1. Biochemical analyses confirmed SETDB1 and PELP1 interactions in multiple BC cells. Mechanistic studies confirmed that PELP1 is necessary for SETDB1 mediated Akt methylation and phosphorylation. Further, SETDB1 overexpression promotes tamoxifen resistance in BC cells, and PELP1 knockdown abolished these effects. Using xenograft model, we provided genetic evidence that PELP1 is essential for SETDB1 mediated BC progression in vivo. Analyses of TCGA datasets revealed SETDB1 expression is positively correlated with PELP1 expression in ER⁺ BC patients.

Conclusions

This study suggests that the PELP1/SETDB1 axis play an important role in aberrant Akt activation and serves as a novel target for treating endocrine therapy resistance in breast cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-022-01520-4.

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.

Androgens enhance the ability of intratumoral macrophages to promote breast cancer progression

Androgens are produced locally in breast carcinoma tissues by androgen‑producing enzymes such as 5α‑reductase type 1 (5αRed1) and affect not only breast cancer cells but the tumor microenvironment as well. Tumor‑associated macrophages (TAMs) are primary components of the tumor microenvironment and contribute to tumor progression. Although previous studies suggest that androgen/androgen receptor (AR) signaling in macrophages has important roles in human diseases, androgen action on TAMs has remained largely unknown. We immunolocalized macrophage marker CD163 as well as AR and 5αRed1 in 116 breast carcinomas and correlated them with clinicopathological parameters and clinical outcomes. Moreover, we examined the roles of androgens on macrophages in breast cancer progression using cell lines 4T1 (mouse breast cancer) and RAW264.7 (macrophage) in a tumor‑bearing female BALB/c mouse model. Double immunohistochemistry revealed that AR was sporadically expressed in the macrophages in breast carcinoma tissues. Macrophage infiltration was significantly correlated with an aggressive phenotype of breast carcinomas and worse prognosis, especially in the 5αRed1‑positive group. In a sphere‑forming assay using 4T1 and RAW‑AR cells, which stably express AR, the sphere size was significantly increased due to androgens when 4T1 cells were cocultured with RAW‑AR cells. Furthermore, in vivo experiments revealed that tumor growth and Ki67, a cell proliferation marker, were increased when androgens were stably produced in breast cancer cells and AR was expressed in macrophages. In conclusion, AR is expressed in intratumoral macrophages and is associated with an aggressive phenotype of breast carcinomas, especially when breast cancer cells actively produce androgens. Thus, androgens may enhance the ability of macrophages to promote breast cancer progression.

Cx43 deficiency confers EMT-mediated tamoxifen resistance to breast cancer via c-Src/PI3K/Akt pathway

Tamoxifen (TAM) resistance has indicated a significant challenge during endocrine therapy for hormone-sensitive breast cancer. Thus, it is significant to elucidate the molecular events endowing TAM resistance to endocrine therapy. In this study, we found that epithelial-mesenchymal transition (EMT) was an important event to confer TAM resistance, and attenuating EMT by elevating connexin (Cx) 43 expression could reverse TAM resistance. Specifically, Cx43 overexpression improved TAM sensitivity, while Cx43 depletion facilitated TAM insensitivity by modulating EMT in T47D TAM-resistant and -sensitive cells, and transplanted xenografts. Importantly, we found a novel reciprocal regulation between Cx43 and c-Src/PI3K/Akt pathway contributing to EMT and TAM resistance in breast cancer. Moreover, we identified that Cx43 deficiency was significantly correlated with poor relapse-free survival in patients undergoing TAM treatment. Therefore, Cx43 represents a prognostic marker and an attractive target for breast cancer treatments. Therapeutic strategies designed to increase or maintain Cx43 function may be beneficial to overcome TAM resistance.

The Emerging Role of Extracellular Vesicles in Endocrine Resistant Breast Cancer

Simple Summary

Two-thirds of breast cancer patients present an estrogen receptor–positive tumor at diagnosis, and the main treatment options for these patients are endocrine therapies such as aromatase inhibitors, selective modulators of estrogen receptor activity or selective estrogen receptor down-regulators. Although endocrine therapies have high efficacy in early-stage breast cancers, the failure of the therapeutic response to these hormonal treatments remains the major clinical challenge. Recently, extracellular vesicles (EVs) have emerged as a novel mechanism of drug resistance. Indeed, EVs isolated from tumor and stromal cells act as key messengers in intercellular communications able to propagate traits of resistance and/or educate the microenvironment to sustain a breast cancer resistant phenotype. Understanding the EV-mediated molecular mechanisms involved in hormonal resistance can provide the rationale for novel and effective treatment modalities and allow for the identification of potential biomarkers to monitor therapy response in ER-positive breast cancer patients.

Abstract

Breast cancer is the most common solid malignancy diagnosed in females worldwide, and approximately 70% of these tumors express estrogen receptor α (ERα), the main biomarker of endocrine therapy. Unfortunately, despite the use of long-term anti-hormone adjuvant treatment, which has significantly reduced patient mortality, resistance to the endocrine treatments often develops, leading to disease recurrence and limiting clinical benefits. Emerging evidence indicates that extracellular vesicles (EVs), nanosized particles that are released by all cell types and responsible for local and systemic intercellular communications, might represent a newly identified mechanism underlying endocrine resistance. Unraveling the role of EVs, released by transformed cells during the tumor evolution under endocrine therapy, is still an open question in the cancer research area and the molecular mechanisms involved should be better defined to discover alternative therapeutic approaches to overcome resistance. In this review, we will provide an overview of recent findings on the involvement of EVs in sustaining hormonal resistance in breast cancer and discuss opportunities for their potential use as biomarkers to monitor the therapeutic response and disease progression.

Stromal CCL5 Promotes Breast Cancer Progression by Interacting with CCR3 in Tumor Cells

Chemokines secreted from stromal cells have important roles for interactions with carcinoma cells and regulating tumor progression. C-C motif chemokine ligand (CCL) 5 is expressed in various types of stromal cells and associated with tumor progression, interacting with C-C chemokine receptor (CCR) 1, 3 and 5 expressed in tumor cells. However, the expression on CCL5 and its receptors have so far not been well-examined in human breast carcinoma tissues. We therefore immunolocalized CCL5, as well as CCR1, 3 and 5, in 111 human breast carcinoma tissues and correlated them with clinicopathological characteristics. Stromal CCL5 immunoreactivity was significantly correlated with the aggressive phenotype of breast carcinomas. Importantly, this tendency was observed especially in the CCR3-positive group. Furthermore, the risk of recurrence was significantly higher in the patients with breast carcinomas positive for CCL5 and CCR3 but negative for CCR1 and CCR5, as compared with other patients. In summary, the CCL5-CCR3 axis might contribute to a worse prognosis in breast cancer patients, and these findings will contribute to a better understanding of the significance of the CCL5/CCRs axis in breast carcinoma microenvironment.

Efficacy of fulvestrant 500 mg in Chinese postmenopausal women with advanced/recurrent breast cancer and factors associated with prolonged time-to-treatment failure: A retrospective case series

This study was to investigate the efficacy and safety of fulvestrant 500 mg for the treatment of hormone receptor positive advanced postmenopausal women, including ovarian ablation and investigated factors associated with prolonged time-to-treatment failure.Data from 60 women with metastatic breast cancer who were treated at Zhejiang Cancer Hospital. Patients received 500 mg (n = 60) between December 2011 and November 2012 were followed until November 2017. Main outcomes were clinical responses to fulvestrant, including best response, progressive disease, partial response, and stable disease lasting 12 months or more. Time to progression and time to progression-free-survival were also analyzed.Among the included 60 patients (mean age 47.18 years), 51 (85.0%) had received prior adjuvant therapy. During follow-up after fulvestrant treatment, the median PFS for the best response was derived as 7.0 months (inter-quartile = 4, 13.8 months). The observed median progression-free-survival time for best response was represented longer when fulvestrant was first-line treatment than when patients received prior endocrine and/or chemotherapy. Univariate analysis revealed that receiving either endocrine therapy only or endocrine therapy plus chemotherapy prior to fulvestrant treatment may be associated with median progression-free survival time to best response (P = .002, .026, .007, respectively).Fulvestrant treatment is safe and well-tolerated in women with hormone-sensitive advanced breast cancer, and first-line fulvestrant therapy increases progression-free-survival time, especially in patients without prior adjuvant treatment.

Enhancer reprogramming driven by high-order assemblies of transcription factors promotes phenotypic plasticity and breast cancer endocrine resistance

Acquired therapy resistance is a major problem for anticancer treatment, yet the underlying molecular mechanisms remain unclear. Using an established breast cancer cellular model, we show that endocrine resistance is associated with enhanced phenotypic plasticity, indicated by a general downregulation of luminal/epithelial differentiation markers and upregulation of basal/mesenchymal invasive markers. Consistently, similar gene expression changes are found in clinical breast tumours and patient-derived xenograft samples that are resistant to endocrine therapies. Mechanistically, the differential interactions between oestrogen receptor α and other oncogenic transcription factors, exemplified by GATA3 and AP1, drive global enhancer gain/loss reprogramming, profoundly altering breast cancer transcriptional programs. Our functional studies in multiple culture and xenograft models reveal a coordinated role of GATA3 and AP1 in re-organizing enhancer landscapes and regulating cancer phenotypes. Collectively, our study suggests that differential high-order assemblies of transcription factors on enhancers trigger genome-wide enhancer reprogramming, resulting in transcriptional transitions that promote tumour phenotypic plasticity and therapy resistance.

Rac1 activation in human breast carcinoma as a prognostic factor associated with therapeutic resistance

BACKGROUND

RAS-related C3 botulinus toxin substrate 1 (Rac1) is a molecular switch fluctuating between GDP-bound inactive form (Rac1-GDP) and GTP-bound active form (Rac1-GTP) and involved in diverse function in both normal and malignant cells such as breast carcinoma cells. Although several studies have demonstrated immunolocalization of Rac1 protein in human breast carcinoma tissues, activation status of Rac1 still remains to be elucidated.

METHODS

We immunolocalized active form of Rac1 (Rac1-GTP) as well as total Rac1 using antibody specific for them in 115 invasive breast carcinoma tissues and correlated with clinicopathological parameters and clinical outcomes.

RESULTS

Rac1-GTP was frequently immunolocalized in the cytoplasm or cell membrane of breast carcinoma cells and it was positively correlated with Ki-67 labeling index and total Rac1 while negatively correlated with progesterone receptor. On the other hand, immunohistochemical Rac1-GTP status was significantly correlated with increased risk of recurrence and breast cancer-specific mortality of breast cancer patients and multivariate analyses did demonstrate Rac1-GTP as an independent worse prognostic factor for both disease-free and breast cancer-specific survival. In addition, Rac1-GTP was still correlated with worse prognosis in the patients who had received adjuvant chemotherapy or endocrine therapy.

CONCLUSION

These findings suggested Rac1 activation played pivotal roles in the progression and therapeutic resistance of breast cancers and Rac1 might be an important therapeutic target for improvement of the therapy for breast cancer patients.

The Ongoing Search for Biomarkers of CDK4/6 Inhibitor Responsiveness in Breast Cancer

CDK4 inhibitors (CDK4/6i), such as palbociclib, ribociclib, and abemaciclib, are approved in combination with hormonal therapy as a front-line treatment for metastatic HR+, HER2- breast cancer. Their targets, CDK4 and CDK6, are cell-cycle regulatory proteins governing the G1-S phase transition across many tissue types. A key challenge remains to uncover biomarkers to identify those patients that may benefit from this class of drugs. Although CDK4/6i addition to estrogen modulation therapy essentially doubles the median progression-free survival, overall survival is not significantly increased. However, in reality only a subset of treated patients respond. Many patients exhibit primary resistance to CDK4/6 inhibition and do not derive any benefit from these agents, often switching to chemotherapy within 6 months. Some patients initially benefit from treatment, but later develop secondary resistance. This highlights the need for complementary or companion diagnostics to pinpoint patients who would respond. In addition, because CDK4 is a bona fide target in other tumor types where CDK4/6i therapy is currently in clinical trials, the lack of target identification may obscure benefit to a subset of patients there as well. This review summarizes the current status of CDK4/6i biomarker test development, both in clinical trials and at the bench, with particular attention paid to those which have a strong biological basis as well as supportive clinical data.

Estrogen Receptor α (ERα)-targeting Compounds and Derivatives: Recent Advances in Structural Modification and Bioactivity

Breast cancer is the most common cancer suffered by female, and the second highest cause of cancer-related death among women worldwide. At present, hormone therapy is still the main treatment route and can be divided into three main categories: selective estrogen receptor modulators (SERMs), selective estrogen receptor downregulators (SERDs), and aromatase inhibitors (AIs). However, breast cancer is difficult to cure even after several rounds of anti-estrogen therapy and most drugs have serious side-effects. Here, we review the literature published over the past five years regarding the isolation and synthesis of analogs and their derivatives.

Redrawing the Lines: The Next Generation of Treatment in Metastatic Breast Cancer

Although not considered curative in nature, new therapeutic advances in metastatic breast cancer (MBC) have substantially improved patient outcomes. This article discusses the state-of-the-art and emerging therapeutic options for management of MBC. BC systemic therapy targets multiple key pathways, including estrogen receptor signaling, HER2 signaling, and phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) signaling. Other therapeutic strategies include targeting DNA repair, inhibiting immune checkpoints, and developing antibody-drug conjugates. Although surgery historically was reserved for palliation of symptomatic, large, or ulcerating masses, some data suggest a possibly expanding role for more aggressive locoregional therapy in combination with systemic therapy. As technology develops, biomarker-specific, line-agnostic, and receptor-agnostic treatment strategies will redraw the current lines of MBC care. However, tumor heterogeneity remains a challenge. To effectively reshape our approach to MBC, careful consideration of the patient perspective, the costs and value of novel treatments, and accessibility (especially in developing countries) is paramount.

DLG5 suppresses breast cancer stem cell-like characteristics to restore tamoxifen sensitivity by inhibiting TAZ expression

Tamoxifen (TAM) is a primary drug for treatment of estrogen receptor positive breast cancer. However, TAM resistance remains a serious threat to breast cancer patients and may be attributed to increased stemness of breast cancer. Here, we show that discs large homolog 5 (DLG5) expression is down‐regulated in TAM‐resistant breast cancer and cells. DLG5 silencing decreased the sensitivity to TAM and increased the frequency and stemness of CD44⁺/CD24⁻ breast cancer stem cells (BCSCs) and TAZ, a transducer of the Hippo pathway, expression in MCF7 cells while DLG5 overexpression had opposite effects. TAZ silencing restored the sensitivity to TAM and reduced the frequency and stemness in TAM‐resistant breast cancer cells. Taken together, our data indicate that down‐regulated DLG5 expression increases the stemness of breast cancer cells by enhancing TAZ expression, contributing to TAM resistance in breast cancer.

SNAI2 upregulation is associated with an aggressive phenotype in fulvestrant-resistant breast cancer cells and is an indicator of poor response to endocrine therapy in estrogen receptor-positive metastatic breast cancer

Background

Endocrine resistance in estrogen receptor-positive (ER+) breast cancer is a major clinical problem and is associated with accelerated cancer cell growth, increased motility and acquisition of mesenchymal characteristics. However, the specific molecules and pathways involved in these altered features remain to be detailed, and may be promising therapeutic targets to overcome endocrine resistance.

Methods

In the present study, we evaluated altered expression of epithelial-mesenchymal transition (EMT) regulators in ER+ breast cancer cell models of tamoxifen or fulvestrant resistance, by gene expression profiling. We investigated the specific role of increased SNAI2 expression in fulvestrant-resistant cells by gene knockdown and treatment with a SNAIL-p53 binding inhibitor, and evaluated the effect on cell growth, migration and expression of EMT markers. Furthermore, we evaluated SNAI2 expression by immunohistochemical analysis in metastatic samples from two cohorts of patients with breast cancer treated with endocrine therapy in the advanced setting.

Results

SNAI2 was found to be significantly upregulated in all endocrine-resistant cells compared to parental cell lines, while no changes were observed in the expression of other EMT-associated transcription factors. SNAI2 knockdown with specific small interfering RNA (siRNA) converted the mesenchymal-like fulvestrant-resistant cells into an epithelial-like phenotype and reduced cell motility. Furthermore, inhibition of SNAI2 with specific siRNA or a SNAIL-p53 binding inhibitor reduced growth of cells resistant to fulvestrant treatment. Clinical evaluation of SNAI2 expression in two independent cohorts of patients with ER+ metastatic breast cancer treated with endocrine therapy in the advanced setting (N = 86 and N = 67) showed that high SNAI2 expression in the metastasis correlated significantly with shorter progression-free survival on endocrine treatment (p = 0.0003 and p = 0.004).

Conclusions

Our results suggest that SNAI2 is a key regulator of the aggressive phenotype observed in endocrine-resistant breast cancer cells, an independent prognostic biomarker in ER+ advanced breast cancer treated with endocrine therapy, and may be a promising therapeutic target in combination with endocrine therapies in ER+ metastatic breast cancer exhibiting high SNAI2 levels.

Electronic supplementary material

The online version of this article (10.1186/s13058-018-0988-9) contains supplementary material, which is available to authorized users.

Exosome-Mediated Transfer of Cancer Cell Resistance to Antiestrogen Drugs

Exosomes are small vesicles which are produced by the cells and released into the surrounding space. They can transfer biomolecules into recipient cells. The main goal of the work was to study the exosome involvement in the cell transfer of hormonal resistance. The experiments were performed on in vitro cultured estrogen-dependent MCF-7 breast cancer cells and MCF-7 sublines resistant to SERM tamoxifen and/or biguanide metformin, which exerts its anti-proliferative effect, at least in a part, via the suppression of estrogen machinery. The exosomes were purified by differential ultracentrifugation, cell response to tamoxifen was determined by MTT test, and the level and activity of signaling proteins were determined by Western blot and reporter analysis. We found that the treatment of the parent MCF-7 cells with exosomes from the resistant cells within 14 days lead to the partial resistance of the MCF-7 cells to antiestrogen drugs. The primary resistant cells and the cells with the exosome-induced resistance were characterized with these common features: decrease in ERα activity and parallel activation of Akt and AP-1, NF-κB, and SNAIL1 transcriptional factors. In general, we evaluate the established results as the evidence of the possible exosome involvement in the transferring of the hormone/metformin resistance in breast cancer cells.

SIRT1 and Estrogen Signaling Cooperation for Breast Cancer Onset and Progression

Breast cancer remains a significant female mortality cause. It constitutes a multifactorial disease for which research on environmental factors offers little help in predicting onset or progression. The pursuit for its foundations by analyzing hormonal changes as a motive for disease development, indicates that increased exposure to estrogens associates with increased risk. A prevalent number of breast cancer cases show dependence on the increased activity of the classic nuclear estrogen receptor (ER) for cell proliferation and survival. SIRT1 is a Type III histone deacetylase which is receiving increasing attention due to its ability to perform activities over relevant non-histone proteins and transcription factors. Interestingly, concomitant SIRT1 overexpression is commonly found in ER-positive breast cancer cases. Both proteins had been shown to directly interact, in a process related to altered intracellular signaling and aberrant transcription, then promoting tumor progression. Moreover, SIRT1 activities had been also linked to estrogenic effects through interaction with the G-protein coupled membrane bound estrogen receptor (GPER). This work aims to summarize present knowledge on the interplay between SIRT1 and ER/GPER for breast cancer onset and progression. Lastly, evidences on the ability of SIRT1 to interact with TGFß signaling, a concurrent pathway significantly involved in breast cancer progression, are reported. The potential of this research field for the development of innovative strategies in the assessment of orphan breast cancer subtypes, such as triple negative breast cancer (TNBC), is discussed.

EZH2 inhibition sensitizes tamoxifen‑resistant breast cancer cells through cell cycle regulation

Enhancer of zeste homologue 2 (EZH2), a catalytic subunit of polycomb repressive complex 2, is overexpressed in a number of different tumors including breast cancer, and serves important roles in cell cycle regulation, proliferation, apoptosis, tumorigenesis and drug resistance. However, it remains unclear whether EZH2 contributes to tamoxifen resistance in breast cancer. In the present study, the role of EZH2 in tamoxifen resistance in MCF‑7 cells was investigated. EZH2 was overexpressed in MCF‑7 tamoxifen‑resistant (MCF‑7 TamR) cells. EZH2 overexpression decreased the sensitivity of MCF‑7 cells to tamoxifen, and EZH2 knockdown improved the sensitivity of MCF‑7 TamR cells to tamoxifen. Furthermore, EZH2 knockdown induced cell cycle arrest in MCF‑7 TamR cells, accompanied by a decrease in cyclin D1 expression and an increase in p16 expression. EZH2 knockdown reduced p16 gene methylation in MCF‑7 TamR cells. These findings suggested that EZH2 overexpression may contribute to tamoxifen resistance in breast cancer, and EZH2 inhibition may reverse tamoxifen resistance in breast cancer by regulating the cell cycle via the demethylation of the p16 gene. Thus, EZH2 inhibitors may be effective for treating tamoxifen resistance in breast cancer.

Sex Differences and Bone Metastases of Breast, Lung, and Prostate Cancers: Do Bone Homing Cancers Favor Feminized Bone Marrow?

Sex-associated differences in bone metastasis formation from breast, lung, and prostate cancer exist in clinical studies, but have not been systematically reviewed. Differences in the bone marrow niche can be attributed to sexual dimorphism, to genetic variations that affect sex hormone levels, or to the direct effects of sex hormones, natural or exogenously delivered. This review describes the present understanding of sex-associated and sex hormone level differences in the marrow niche and in formation of bone metastasis during the transition of these three cancers from treatable disease to an often untreatable, lethal metastatic one. Our purpose is to provide insight into some underlying molecular mechanisms for hormonal influence in bone metastasis formation, and to the potential influence of sexual dimorphism, genetic differences affecting sex assignment, and sex hormone level differences on the bone niche and its favorability for metastasis formation. We reviewed publications in PubMed and EMBASE, including full length manuscripts, case reports, and clinical studies of relevance to our topic. We focused on bone metastasis formation in breast, lung, and prostate cancer because all three commonly present with bone metastases. Several clear observations emerged. For breast cancer bone metastasis formation, estrogen receptor (ER) signaling pathways indicate a role for ER beta (ERβ). Estrogen influences the bone microenvironment, creating and conditioning a favorable niche for colonization and breast cancer progression. For lung cancer, studies support the hypothesis that females have a more favorable bone microenvironment for metastasis formation. For prostate cancer, a decrease in the relative androgen to estrogen balance or a “feminization” of bone marrow favors bone metastasis formation, with a potentially important role for ERβ that may be similar to that in breast cancer. Long-term estrogen administration or androgen blockade in males may feminize the bone marrow niche to one more favorable for bone metastases in prostate cancer. Administration of androgens in females, especially combined with mastectomy, may reduce risk of developing bone metastatic breast cancer. We conclude that it should be considered that females, those with female-leaning genetic variations, or hormonal states that feminize the bone marrow, may offer favorable sites for bone metastases.

MCAM/CD146 promotes tamoxifen resistance in breast cancer cells through induction of epithelial-mesenchymal transition, decreased ERα expression and AKT activation

Tamoxifen resistance presents a prominent clinical challenge in endocrine therapy for hormone sensitive breast cancer. However, the underlying mechanisms that contribute to tamoxifen resistance are not fully understood. In this study, we established a tamoxifen resistant MCF-7 cell line (MCF-7-Tam-R) by continuously incubating MCF-7 cells with 4-OH-tamoxifen. We found that melanoma cell adhesion molecule (MCAM/CD146), a unique epithelial-to-mesenchymal transition (EMT) inducer, was significantly up-regulated at both mRNA and protein levels in MCF-7-Tam-R cells compared to parental MCF-7 cells. Mechanistic research demonstrated that MCAM promotes tamoxifen resistance by transcriptionally suppressing ERα expression and activating the AKT pathway, followed by induction of EMT. Elevated MCAM expression was inversely correlated with recurrence-free and distant metastasis-free survival in a cohort of 4142 patients with breast cancer derived from a public database, particularly in the subgroup only treated with tamoxifen. These results demonstrate a novel function of MCAM in conferring tamoxifen resistance in breast cancer. Targeting MCAM might be a promising therapeutic strategy to overcome tamoxifen resistance in breast cancer patients.

Molecular Portrait of the Normal Human Breast Tissue and Its Influence on Breast Carcinogenesis

Normal human breast tissue consists of epithelial and nonepithelial cells with different molecular profiles and differentiation grades. This molecular heterogeneity is known to yield abnormal clones that may contribute to the development of breast carcinomas. Stem cells that are found in developing and mature breast tissue are either positive or negative for cytokeratin 19 depending on their subtype. These cells are able to generate carcinogenesis along with mature cells. However, scientific data remains controversial regarding the monoclonal or polyclonal origin of breast carcinomas. The majority of breast carcinomas originate from epithelial cells that normally express BRCA1. The consecutive loss of the BRCA1 gene leads to various abnormalities in epithelial cells. Normal breast epithelial cells also express hypoxia inducible factor (HIF) 1α and HIF-2α that are associated with a high metastatic rate and a poor prognosis for malignant lesions. The nuclear expression of estrogen receptor (ER) and progesterone receptor (PR) in normal human breast tissue is maintained in malignant tissue as well. Several controversies regarding the ability of ER and PR status to predict breast cancer outcome remain. Both ER and PR act as modulators of cell activity in normal human breast tissue. Ki-67 positivity is strongly correlated with tumor grade although its specific role in applied therapy requires further studies. Human epidermal growth factor receptor 2 (HER2) oncoprotein is less expressed in normal human breast specimens but is highly expressed in certain malignant lesions of the breast. Unlike HER2, epidermal growth factor receptor expression is similar in both normal and malignant tissues. Molecular heterogeneity is not only found in breast carcinomas but also in normal breast tissue. Therefore, the molecular mapping of normal human breast tissue might represent a key research area to fully elucidate the mechanisms of breast carcinogenesis.