Overcoming endocrine resistance in breast cancer: are signal transduction inhibitors the answer?

Mechanisms and Therapeutic Strategies for Endocrine Resistance in Breast Cancer: A Comprehensive Review and Meta-Analysis

Background: As per recent scenarios, drug resistance is a significant challenge in treating breast cancer for several reasons, such as genetic mutations, altered signaling pathways, and tumor microenvironment. Endocrine resistance is one of the biggest significant barriers to treatment, particularly in hormone receptor-positive (HR+) breast cancers, which depends on estrogen or progesterone signaling for growth. While therapies such as tamoxifen, aromatase inhibitors, and selective estrogen receptor degraders (SERDs) have effectively targeted these pathways, many patients develop resistance, rendering them less effective over time, which is driving a need for innovative therapeutics to treat breast cancer and overcome drug resistance and better treatment outcomes. Recent studies suggest that combining the different therapies, including immunotherapy, targeted therapy, chemotherapy, etc., with endocrine therapy, may bypass the endocrine resistance. Methodology: We conducted a comprehensive systematic review and meta-analysis examining the molecular mechanisms of endocrine resistance and evaluating randomized clinical trial outcomes, overall survival and progression-free survival in endocrine-resistant breast cancer patients treated with endocrine therapy, targeted therapy, immunotherapy, or chemotherapy. Results: We have analyzed 35 randomized clinical trial studies for different therapies along with combination therapy, and our results demonstrated that supplementary or additional therapies in endocrine resistance breast cancer patients have better progression-free and overall survival. Conclusions: The current study has demonstrated that combination therapies may have good survival results and patient outcomes in endocrine resistance. Also, This review sheds light on current challenges in drug resistance and the future direction of cancer treatment through a comprehensive analysis of these emerging treatment approaches to improve patient outcomes.

Palbociclib: Randomized Studies and Real-world Evidence as the Basis for Therapeutic Planning in Metastatic Breast Cancer

Endocrine-based combination therapy with an inhibitor of the cyclin-dependent kinases 4 and 6 (CDK4/6 inhibitors) is currently the first-line therapy of choice for patients with hormone receptor-positive (HR+) and human epidermal growth factor receptor 2-negative (HER2-), locally advanced or metastatic breast cancer (mBC). The efficacy and safety of the treatment with palbociclib, the first CDK4/6 inhibitor approved for this indication, have been confirmed in large randomized controlled clinical trials (RCTs) with strictly defined patient cohorts. Since then, many relevant questions about CDK4/6 inhibition with palbociclib for mBC have been investigated in RCTs and real-world studies. Based on this evidence, palbociclib is widely used in clinical practice since many years because of its efficacy and good tolerability. The aim of this review is to summarize findings from RCTs and RWE considering clinically relevant aspects such as safety, tolerability, quality of life and efficacy with a focus on specific questions and patient characteristics. A critical discussion and review of the overall evidence for endocrine-based therapy with the CDK4/6 inhibitor palbociclib can contribute to support therapy decisions in daily clinical practice.

Metastatic ER+ Breast Cancer: Mechanisms of Resistance and Future Therapeutic Approaches

Endocrine therapy is the main treatment for hormone receptor-positive (HR+) breast cancer. However, advanced tumors develop resistance to endocrine therapy, rendering it ineffective as the disease progresses. There are several molecular mechanisms of primary and secondary endocrine resistance. Resistance can develop due to either alteration of the estrogen receptor pathway (e.g., ESR1 mutations) or upstream growth factors signaling pathways (e.g., PI3K/Akt/mTOR pathway). Despite progress in the development of molecularly targeted anticancer therapies, the emergence of resistance remains a major limitation and an area of unmet need. In this article, we review the mechanisms of acquired endocrine resistance in HR+ advanced breast cancer and discuss current and future investigational therapeutic approaches.

Promising Therapeutic Impact of a Selective Estrogen Receptor Downregulator, Fulvestrant, as Demonstrated In Vitro upon Low-Grade Serous Ovarian Carcinoma Cell Lines

Few studies have reported hormonal agent use in the treatment of low-grade serous ovarian carcinomas (LGSOCs), which are chemoresistant. Considering the need for novel effective therapies, we investigated the hormone receptor expression and hormonal inhibition efficacy in LGSOCs. Using immunohistochemistry, we assessed the estrogen receptor (ER) expression status in 33 cases of histologically confirmed serous ovarian tumors, including 10, 11, and 12 cases of LGSOCs, serous borderline tumors (SBTs), and serous cystadenomas (SCAs), respectively. The genetic background reported in our previous study was used in the current study. MPSC1 cells, which were established from LGSOCs, were used in cell proliferation assays. We observed a higher ER expression in LGSOCs and SBTs than in SCAs (70%, 81%, and 50%, respectively). Thus, LGSOCs and SBTs exhibit higher ER expression than SCAs. Moreover, the PIK3CA mutation positively correlated with ER expression in LGSOCs (p = 0.0113). MPSC1 cells showed low ER expression on Western blotting. MPSC1 cell proliferation was significantly inhibited by fulvestrant (a selective ER downregulator). The activation of ER and PI3K/AKT signaling pathways may play an important role in LGSOC carcinogenesis. ER downregulation with fulvestrant or combination therapy with PI3K inhibitors is a possible novel treatment for patients with LGSOCs.

Understanding and overcoming tumor heterogeneity in metastatic breast cancer treatment

Rational development of targeted therapies has revolutionized metastatic breast cancer outcomes, although resistance to treatment remains a major challenge. Advances in molecular profiling and imaging technologies have provided evidence for the impact of clonal diversity in cancer treatment resistance, through the outgrowth of resistant clones. In this Review, we focus on the genomic processes that drive tumoral heterogeneity and the mechanisms of resistance underlying metastatic breast cancer treatment and discuss implications for future treatment strategies.

Real-world data of fulvestrant as first-line treatment of postmenopausal women with estrogen receptor-positive metastatic breast cancer

Goals of endocrine therapy for advanced breast cancer (ABC) include prolonging survival rates, maintaining the quality of life, and delaying the initiation of chemotherapy. We evaluated the effectiveness of fulvestrant as first-line in patients with estrogen receptor (ER)-positive ABC with relapse during or after adjuvant anti-estrogenic therapy in real-world settings. Retrospective, observational study involving 171 postmenopausal women with ER-positive ABC who received fulvestrant as first-line between January 2011 and May 2018 in Spanish hospitals. With a median follow-up of 31.4 months, the progression-free survival (PFS) with fulvestrant was 14.6 months. No differences were seen in the visceral metastatic (14.3 months) versus non-visceral (14.6 months) metastatic subgroup for PFS. Overall response rate and clinical benefit rate were 35.2% and 82.8%. Overall survival was 43.1 months. The duration of the clinical benefit was 19.2 months. Patients with ECOG performance status 0 at the start of treatment showed a significant greater clinical benefit rate and overall survival than with ECOG 1–2. Results in real-world settings are in concordance with randomized clinical trials. Fulvestrant continues to demonstrate clinical benefits in real-world settings and appears be well tolerated as first-line for the treatment of postmenopausal women with ER-positive ABC.

Monitoring of Hematologic, Cardiac, and Hepatic Function in Post-Menopausal Women with HR+/HER2- Metastatic Breast Cancer

INTRODUCTION

In the treatment of metastatic breast cancer (mBC), regular monitoring is key in helping physicians to make informed clinical decisions, managing treatment side effects, and maintaining patients' quality of life. Therefore, we investigated the monitoring frequency in post-menopausal women with HR+/HER2- mBC stratified by first-line regimen.

METHODS

Treatment monitoring was assessed using two complementary data sources: a medical chart review (chart review analysis) and a commercial claims database (claims analysis). Women with post-menopausal HR+/HER2- mBC who initiated first-line therapy for mBC were selected and classified under three cohorts, based on treatment received: cyclin-dependent kinase 4/6 (CDK4/6) inhibitor (i.e., palbociclib-the only CDK4/6 approved at the time of the study), endocrine therapy (ET), and chemotherapy. Frequency of monitoring [complete blood count (CBC), electrocardiogram (EKG), and liver function test (LFT)] and laboratory abnormalities detected during the first line of therapy were analyzed.

RESULTS

In the chart review analysis, 64 US oncologists abstracted medical information on 401 eligible patients, including 210 CDK4/6 users, 121 ET users, 51 chemotherapy users; 19 patients used other regimens. All patients had ≥ 1 CBC; between 8.3% (ET users) and 39.5% (CDK4/6 users) had ≥ 1 EKG; and over 98% of patients had ≥ 1 LFT across all three cohorts. Among monitored patients, 64.6% had a CBC abnormality, with anemia (39.9%), leukopenia (27.4%), and neutropenia (26.7%) being the most common. Abnormal EKG readings were detected in 8.4, 0.0%, and 7.7% of CDK4/6, ET, and chemotherapy users, respectively. LFT abnormalities were detected in 14.1-26.0% of CDK4/6 and chemotherapy users, respectively. Similar frequency of monitoring was observed in the claims analysis, with the exception of EKG monitoring, for which the proportion of patients tested was higher.

CONCLUSION

Post-menopausal women with HR+/HER2- mBC receiving first-line therapy with CDK4/6, ET, or chemotherapy were regularly monitored regardless of the first-line regimen received.

FUNDING

Novartis Pharmaceuticals Corporation.

Overcoming endocrine resistance in metastatic hormone receptor-positive breast cancer

Endocrine therapy has historically formed the basis of treatment of metastatic hormone receptor-positive breast cancer. The development of endocrine resistance has led to the development of newer endocrine drug combinations. Use of the CDK4/6 inhibitors has significantly improved progression-free survival in this group of patients. There are multiple studies of the use of P13K inhibitors and mTOR inhibitors for use as subsequent lines of therapy, particularly for endocrine resistance. The optimal sequencing of therapy should be based on medical comorbidities, prior adjuvant therapies, quality of life, side-effect profile, and disease-free interval.

Identification of miRNAs as biomarkers for acquired endocrine resistance in breast cancer

Therapies targeting estrogen receptor α (ERα) including tamoxifen, a selective estrogen receptor modulator (SERM) and aromatase inhibitors (AI), e.g., letrozole, have proven successful in reducing the death rate for breast cancer patients whose initial tumors express ERα. However, about 40% of patients develop acquired resistance to these endocrine treatments. There is a critical need to develop sensitive circulating biomarkers that accurately identify signaling pathways altered in breast cancer patients resistant to endocrine therapies. Serum miRNAs have the potential to serve as biomarkers of the progression of endocrine-resistant breast cancer due to their cancer-specific expression and stability. Exosomal transfer of miRNAs has been implicated in metastasis and endocrine-resistance. This review focuses on miRNAs in breast tumors and in serum, including exosomes, from breast cancer patients that are associated with resistance to tamoxifen since it is best-studied.

Estrogen, progesterone, and HER2/neu receptor discordance between primary and metastatic breast tumours-a review

Discordance in estrogen (ER), progesterone (PR), and HER2/neu status between primary breast tumours and metastatic disease is well recognized. In this review, we highlight how receptor discordance between primary tumours and paired metastasis can help elucidate the mechanism of metastasis but can also effect patient management and the design of future trials. Discordance rates and ranges were available from 47 studies (3384 matched primary and metastatic pairs) reporting ER, PR, and HER2/neu expression for both primary and metastatic sites. Median discordance rates for ER, PR, and HER2/neu were 14 % (range 0-67 %, IQR 9-25 %), 21 % (range 0-62 %, IQR 15-41 %), and 10 % (range 0-44 %, IQR 4-17 %), respectively. Loss of receptor expression was more common (9.17 %) than gain (4.51 %). Discordance rates varied amongst site of metastasis with ER discordance being highest in bone metastases suggesting that discordance is a true biological phenomenon. Discordance rates vary for both the biomarker and the metastatic site. Loss of expression is more common than gain. This can affect patient management as it can lead to a reduction in both the efficacy and availability of potential therapeutic agents. Future studies are recommended to explore both the mechanisms of discordance as well as its impact on patient outcome and management.

Chemosensitization role of fulvestrant in combination with chemotherapy in postmenopausal hormone receptor positive and human epidermal growth factor negative metastatic breast cancer

In metastatic breast cancer (MBC), hormone receptor positive (HR+), human epidermal growth factor negative (HER2-) subtype accounts for the majority. With various new modalities available to prolong life span in this group of patients, the effect is distant from optimum. Prevalent strategy of treating postmenopausal HR+ HER2- MBC is application of chemotherapy (CT) after progression of disease on endocrine therapy (ET) of several lines. Generally, ET targets HR+ ingredients and CT works better with HR- tumor cells. HR+ MBC, though hormone-sensitive, has HR- portion which reacts poorly to ET. Thus, sequential use of ET and CT neglects its insensitive part and gives rise to drug resistance, while alleviation of tumor burden is the top priority in metastatic setting. Chemohormonal therapy (i.e. concomitant use of ET and chemotherapy) complements for the shortcoming of current therapy strategy targeting both HR+ and HR- ingredients theoretically. Fulvestrant, a pure estrogen receptor antagonist and down-regulator, could be a promising agent using concurrently with CT based on chemosensitizing character shown in preclinical and pilot clinical studies. It is hypothesized in this article that chemohormonal therapy with concurrent fulvestrant and CT would be a promising strategy in postmenopausal HR+ HER2- MBC patients. Proof of this hypothesis would help control evolvement of tumor burden and acquirement of drug resistance over a short period of time.

Real-world effectiveness of everolimus-based therapy versus fulvestrant monotherapy in HR(+)/HER2(-) metastatic breast cancer

AIMS

Assessing real-world effectiveness of everolimus-based therapy (EVE) versus fulvestrant monotherapy (FUL) among postmenopausal women with hormone receptor-positive (HR(+))/HER2(-) metastatic breast cancer (mBC) after progression on nonsteroidal aromatase inhibitor (NSAI).

DATA & METHODS

Medical charts of community-based patients who received EVE or FUL for mBC after NSAI were examined. Progression-free survival (PFS), time on treatment and time to chemotherapy were compared using Kaplan-Meier curves and Cox proportional hazards models adjusting for line of therapy and patient characteristics.

RESULTS & CONCLUSION

192 patients received EVE and 156 FUL. After adjusting for patient characteristics, EVE was associated with significantly longer PFS than FUL (hazard ratio: 0.71; p = 0.045). EVE was associated with better PFS than FUL among NSAI-refractory postmenopausal HR(+)/HER2(-) mBC patients.

Endocrine therapy in post-menopausal women with metastatic breast cancer: From literature and guidelines to clinical practice

Current international guidelines recommend endocrine therapy as the initial treatment of choice in hormone receptor positive advanced breast cancer. Endocrine therapy has been a mainstay of hormone responsive breast cancer treatment for more than a century. To date it is based on different approaches,such as blocking the estrogen receptor through selective receptor estrogen modulators, depleting extragonadal peripheral estrogen synthesis by aromatase inhibitors or inducing estrogen receptor degradation using selective down-regulators. Despite estrogen and/or progesterone receptor positive status, up to a quarter of patients could be either primarily resistant to hormone therapies or will develop hormone resistance during the course of their disease. Different mechanisms, either intrinsic or acquired, could be implicated in endocrine resistance. In the present work available endocrine therapies and their appropriate sequences have been reviewed, and the most promising strategies to overcome endocrine resistance have been highlighted.

Amphiregulin Is a Critical Downstream Effector of Estrogen Signaling in ERα-Positive Breast Cancer

Estrogen stimulation promotes epithelial cell proliferation in estrogen receptor (ERα)-positive breast cancer. Many ERα target genes have been enumerated, but the identities of the key effectors mediating the estrogen signal remain obscure. During mouse mammary gland development, the estrogen growth factor receptor (EGFR) ligand amphiregulin acts as an important stage-specific effector of estrogen signaling. In this study, we investigated the role of amphiregulin in breast cancer cell proliferation using human tissue samples and tumor xenografts in mice. Amphiregulin was enriched in ERα-positive human breast tumor cells and required for estrogen-dependent growth of MCF7 tumor xenografts. Furthermore, amphiregulin levels were suppressed in patients treated with endocrine therapy. Suppression of EGF receptor signaling appeared necessary for the therapeutic response in this setting. Our findings implicate amphiregulin as a critical mediator of the estrogen response in ERα-positive breast cancer, emphasizing the importance of EGF receptor signaling in breast tumor pathogenesis and therapeutic response.

Nodes-and-connections RNAi knockdown screening: identification of a signaling molecule network involved in fulvestrant action and breast cancer prognosis

Although RNA interference (RNAi) knockdown screening of cancer cell cultures is an effective approach to predict drug targets or therapeutic/prognostic biomarkers, interactions among identified targets often remain obscure. Here, we introduce the nodes-and-connections RNAi knockdown screening that generates a map of target interactions through systematic iterations of in silico prediction of targets and their experimental validation. An initial RNAi knockdown screening of MCF-7 human breast cancer cells targeting 6560 proteins identified four signaling molecules required for their fulvestrant-induced apoptosis. Signaling molecules physically or functionally interacting with these four primary node targets were computationally predicted and experimentally validated, resulting in identification of four second-generation nodes. Three rounds of further iterations of the prediction–validation cycle generated third, fourth and fifth generation of nodes, completing a 19-node interaction map that contained three predicted nodes but without experimental validation because of technical limitations. The interaction map involved all three members of the death-associated protein kinases (DAPKs) as well as their upstream and downstream signaling molecules (calmodulins and myosin light chain kinases), suggesting that DAPKs play critical roles in the cytocidal action of fulvestrant. The in silico Kaplan–Meier analysis of previously reported human breast cancer cohorts demonstrated significant prognostic predictive power for five of the experimentally validated nodes and for three of the prediction-only nodes. Immunohistochemical studies on the expression of 10 nodal proteins in human breast cancer tissues not only supported their prognostic prediction power but also provided statistically significant evidence of their synchronized expression, implying functional interactions among these nodal proteins. Thus, the Nodes-and-Connections approach to RNAi knockdown screening yields biologically meaningful outcomes by taking advantage of the existing knowledge of the physical and functional interactions between the predicted target genes. The resulting interaction maps provide useful information on signaling pathways cooperatively involved in clinically important features of the malignant cells, such as drug resistance.

Roles for miRNAs in endocrine resistance in breast cancer

Therapies targeting estrogen receptor alpha (ERα), including selective ER modulators such as tamoxifen, selective ER downregulators such as fulvestrant (ICI 182 780), and aromatase inhibitors such as letrozole, are successfully used in treating breast cancer patients whose initial tumor expresses ERα. Unfortunately, the effectiveness of endocrine therapies is limited by acquired resistance. The role of microRNAs (miRNAs) in the progression of endocrine-resistant breast cancer is of keen interest in developing biomarkers and therapies to counter metastatic disease. This review focuses on miRNAs implicated as disruptors of antiestrogen therapies, their bona fide gene targets and associated pathways promoting endocrine resistance.

Expression of the Androgen Receptor, pAkt, and pPTEN in Breast Cancer and Their Potential in Prognostication

BACKGROUND

Importance of androgen receptor (AR) as an independent prognostic marker in Pakistani women with breast cancer (BCa) remains unexplored. Our aim was to identify the expression and potential prognostic value of AR, its upstream regulator (pAkt) and target gene (pPTEN) in invasive BCa.

METHODS

This study used a cohort of 200 Pakistani women with invasive BCa diagnosed during 2002-2011. Expression of AR, pAkt and pPTEN was determined on formalin fixed paraffin embedded tissue sections by immunohistochemistry. The association of AR, pAkt and pPTEN with clinicopathological parameters was determined. Survival analyses were undertaken on patients with ≥5years of follow-up (n=82).

RESULTS

Expression of AR, pAkt and pPTEN was observed in 47.5%, 81.3% and 50.6% of patients, respectively. AR-expressing tumors were low or intermediate in grade (P<.001) and expressed ER (P=.002) and PR (P=.001). Patients with AR+ tumors had significantly higher OS (Mean OS=10.2±0.465years) compared to patients with AR- tumors (Mean OS=5.8±0.348years) (P=.047). Furthermore, AR-positivity was associated with improved OS in patients receiving endocrine therapy (P=.020). Patients with AR+ /pAkt+ /pPTEN- tumors, had increased OS (Mean OS=7.1±0.535years) compared to patients with AR-/pAkt+/pPTEN- tumors (Mean OS=5.1±0.738years).

CONCLUSION

AR-expressing tumors are frequently characterized by low or intermediate grade tumors, expressing ER and PR. In addition, expression of AR, pAkt and pPTEN, could be considered in prognostication of patients with invasive BCa.

Integrity of the LXXLL motif in Stat6 is required for the inhibition of breast cancer cell growth and enhancement of differentiation in the context of progesterone

Background

Progesterone is essential for the proliferation and differentiation of mammary gland epithelium. Studies of breast cancer cells have demonstrated a biphasic progesterone response consisting of an initial proliferative burst followed by sustained growth arrest. However, the transcriptional factors acting with the progesterone receptor (PR) to mediate the effects of progesterone on mammary cell growth and differentiation remain to be determined. Recently, it was demonstrated that signal transducer and activator of transcription 6 (Stat6) is a cell growth suppressor. Similar to progesterone-bound PR, Stat6 acts by inducing the expression of the G1 cyclin-dependent kinase inhibitors p21 and p27. The possible interaction between Stat6 and progesterone pathways in mammary cells was therefore investigated in the present study.

Methods

ChIP and luciferase were assayed to determine whether Stat6 induces p21 and p27 expression by recruitment at the proximal Sp1-binding sites of the gene promoters. Immunoprecipitation and Western blotting were performed to investigate the interaction between Stat6 and PR-B. The cellular DNA content and cell cycle distribution in breast cancer cells were analyzed by FACS.

Results

We found that Stat6 interacts with progesterone-activated PR in T47D cells. Stat6 synergizes with progesterone-bound PR to transactivate the p21 and p27 gene promoters at the proximal Sp1-binding sites. Moreover, Stat6 overexpression and knockdown, respectively, increased or prevented the induction of p21 and p27 gene expression by progesterone. Stat6 knockdown also abolished the inhibitory effects of progesterone on pRB phosphorylation, G1/S cell cycle progression, and cell proliferation. In addition, knockdown of Stat6 expression prevented the induction of breast cell differentiation markers, previously identified as progesterone target genes. Finally, Stat6 gene expression levels increased following progesterone treatment, indicating a positive auto-regulatory loop between PR and Stat6.

Conclusions

Taken together, these data identify Stat6 as a coactivator of PR mediating the growth-inhibitory and differentiation effects of progesterone on breast cancer cells.

Endocrine therapy for postmenopausal women with hormone receptor-positive her2-negative advanced breast cancer after progression or recurrence on nonsteroidal aromatase inhibitor therapy: a Canadian consensus statement

Approximately 22,700 Canadian women were expected to be diagnosed with breast cancer in 2012. Despite improvements in screening and adjuvant treatment options, a substantial number of postmenopausal women with hormone receptor positive (hr+) breast cancer will continue to develop metastatic disease during or after adjuvant endocrine therapy. Guidance on the selection of endocrine therapy for patients with hr+ disease that is negative for the human epidermal growth factor receptor 2 (her2-) and that has relapsed or progressed on earlier nonsteroidal aromatase inhibitor (nsai) therapy is of increasing clinical importance. Exemestane, fulvestrant, and tamoxifen are approved therapeutic options in this context. Four phase iii trials involving 2876 patients-efect, sofea, confirm, and bolero-2-have assessed the efficacy of various treatment options in this clinical setting. Data from those trials suggest that standard-dose fulvestrant (250 mg monthly) and exemestane are of comparable efficacy, that doubling the dose of fulvestrant from 250 mg to 500 mg monthly results in a 15% reduction in the risk of progression, and that adding everolimus to exemestane (compared with exemestane alone) results in a 57% reduction in the risk of progression, albeit with increased toxicity. Multiple treatment options are now available to women with hr+ her2- advanced breast cancer recurring or progressing on earlier nsai therapy, although current clinical trial data suggest more robust clinical efficacy with everolimus plus exemestane. Consideration should be given to the patient's age, functional status, and comorbidities during selection of an endocrine therapy, and use of a proactive everolimus safety management strategy is encouraged.

Disruption of 3D MCF-12A breast cell cultures by estrogens--an in vitro model for ER-mediated changes indicative of hormonal carcinogenesis

Introduction

Estrogens regulate the proliferation of normal and neoplastic breast epithelium. Although the intracellular mechanisms of estrogens in the breast are largely understood, little is known about how they induce changes in the structure of the mammary epithelium, which are characteristic of breast cancer. In vitro three dimensional (3D) cultures of immortalised breast epithelial cells recapitulate features of the breast epithelium in vivo, including formation of growth arrested acini with hollow lumen and basement membrane. This model can also reproduce features of malignant transformation and breast cancer, such as increased cellular proliferation and filling of the lumen. However, a system where a connection between estrogen receptor (ER) activation and disruption of acini formation can be studied to elucidate the role of estrogens is still missing.

Methods/Principal Findings

We describe an in vitro 3D model for breast glandular structure development, using breast epithelial MCF-12A cells cultured in a reconstituted basement membrane matrix. These cells are estrogen receptor (ER)α, ERβ and G-protein coupled estrogen receptor 1 (GPER) competent, allowing the investigation of the effects of estrogens on mammary gland formation and disruption. Under normal conditions, MCF-12A cells formed organised acini, with deposition of basement membrane and hollow lumen. However, treatment with 17β-estradiol, and the exogenous estrogens bisphenol A and propylparaben resulted in deformed acini and filling of the acinar lumen. When these chemicals were combined with ER and GPER inhibitors (ICI 182,780 and G-15, respectively), the deformed acini recovered normal features, such as a spheroid shape, proliferative arrest and luminal clearing, suggesting a role for the ER and GPER in the estrogenic disruption of acinar formation.

Conclusion

This new model offers the opportunity to better understand the role of the ER and GPER in the morphogenesis of breast glandular structure as well as the events implicated in breast cancer initiation and progression.

Phase II trial of exemestane in combination with fulvestrant in postmenopausal women with advanced, hormone-responsive breast cancer

INTRODUCTION

Exemestane, the irreversible steroidal aromatase inhibitor, and fulvestrant, the pure estrogen antagonist, are active as single drugs in postmenopausal women with advanced hormone-responsive breast cancer. We designed a phase II study with the purpose of determining whether combining these 2 drugs with different and potentially complementary mechanisms of action will improve the clinical benefit.

PATIENTS AND METHODS

Forty postmenopausal women with hormone-responsive advanced breast cancer received intramuscular injection of fulvestrant 250 mg every 28 days in combination with daily exemestane 25 mg until disease progression. We examined the influence of fulvestrant on exemestane pharmacokinetics and the effect of exemestane and fulvestrant on serum IGF-1 (insulin-like growth factor 1) and IGFBP-3 (IGF-binding protein 3) levels.

RESULTS

The observed proportion of patients free of progressive disease at 6 months after the initiation of treatment with exemestane and fulvestrant was 50%, a rate similar to that achieved with single-agent exemestane or fulvestrant in the first- or second-line setting. Pharmacokinetics parameters showed that coadministration of fulvestrant did not result in clinically relevant changes in exemestane plasma concentrations. A comparison of IGF-1 and IGFBP-3 levels demonstrated the increase of 35% and 12%, respectively, in mean levels from baseline to day 120.

CONCLUSIONS

The combination of exemestane and fulvestrant did not improve clinical benefit. The observed lack of improved efficacy was not related to altered drug exposure.

p38γ mitogen-activated protein kinase (MAPK) confers breast cancer hormone sensitivity by switching estrogen receptor (ER) signaling from classical to nonclassical pathway via stimulating ER phosphorylation and c-Jun transcription

Estrogen receptor (ER) α promotes breast cancer growth by regulating gene expression through classical estrogen response element (ERE) binding and nonclassical (interaction with c-Jun at AP-1 sites) pathways. ER is the target for anti-estrogens such as tamoxifen (TAM). However, the potential for classical versus nonclassical ER signaling to influence hormone sensitivity is not known. Moreover, anti-estrogens frequently activate several signaling cascades besides the target ER, and the implications of these "off-target" signaling events have not been explored. Here, we report that p38γ MAPK is selectively activated by treatment with TAM. This results in both phosphorylation of ER at Ser-118 and stimulation of c-Jun transcription, thus switching ER signaling from the classical to the nonclassical pathway leading to increased hormone sensitivity. Unexpectedly, phosphorylation at Ser-118 is required for ER to bind both p38γ and c-Jun, thereby promoting ER relocation from ERE to AP-1 promoter sites. Thus, ER/Ser-118 phosphorylation serves as a central mechanism by which p38γ regulates signaling transduction of ER with its inhibitor TAM.

Predicting response: identifying molecular determinants of endocrine response and resistance in breast cancer

Evaluation of: Fernandez-Cuesta L, Anaganti S, Hainaut P, Olivier M. p53 status influences response to tamoxifen but not to fulvestrant in breast cancer cell lines. Int. J. Cancer 128, 1813-1821 (2011). Blocking estrogen activity or production through the use of anti-estrogens such as tamoxifen and aromatase inhibitors, respectively, has had a significant impact on improving the survival of breast cancer patients. However, innate or acquired resistance to these endocrine therapies remains a major clinical problem and a challenge to the successful treatment of this disease. A recent article explored the role of the tumor-suppressor gene TP53 in the response of breast cancer cell lines to tamoxifen and the pure anti-estrogen, fulvestrant. Mutations in TP53, which occur frequently in breast cancer like many other types of neoplasia, are already known to negatively influence prognosis, but here their role in the response to anti-estrogen treatment was evaluated. This study found that cells harboring p53 mutations were more resistant to the cytotoxic effects of 4-hydroxytamoxifen than their p53 wild-type counterparts. Furthermore, mutant p53 cells were actually stimulated by low concentrations of 4-hydroxytamoxifen, with evidence that this may be mediated through enhanced growth factor signaling. By contrast, p53 status did not affect the response to fulvestrant. This article further delineates the role of p53 as a determinant of the endocrine response.

Loss of Rho GDIα and resistance to tamoxifen via effects on estrogen receptor α

BACKGROUND

Estrogen receptor (ER) α is a successful therapeutic target in breast cancer, but patients eventually develop resistance to antiestrogens such as tamoxifen.

METHODS

To identify genes whose expression was associated with the development of tamoxifen resistance and metastasis, we used microarrays to compare gene expression in four primary tumors from tamoxifen-treated patients whose breast cancers did not recur vs five metastatic tumors from patients whose cancers progressed during adjuvant tamoxifen treatment. Because Rho guanine dissociation inhibitor (GDI) α was underexpressed in the tamoxifen-resistant group, we stably transfected ERα-positive MCF-7 breast cancer cells with a plasmid encoding a short hairpin (sh) RNA to silence Rho GDIα expression. We used immunoblots and transcription assays to examine the role of Rho GDIα in ER-related signaling and growth of cells in vitro and as xenografts in treated nude mice (n = 8-9 per group) to examine the effects of Rho GDIα blockade on hormone responsiveness and metastatic behavior. The time to tumor tripling as the time in weeks from randomization to a threefold increase in total tumor volume over baseline was examined in treated mice. The associations of Rho GDIα and MTA2 levels with tamoxifen resistance were examined in microarray data from patients. All statistical tests were two-sided.

RESULTS

Rho GDIα was expressed at lower levels in ERα-positive tumors that recurred during tamoxifen treatment than in ERα-positive tamoxifen-sensitive primary tumors. MCF-7 breast cancer cells in which Rho GDIα expression had been silenced were tamoxifen-resistant, had increased Rho GTPase and p21-activated kinase 1 activity, increased phosphorylation of ERα at serine 305, and enhanced tamoxifen-induced ERα transcriptional activity compared with control cells. MCF-7 cells in which Rho GDIα expression was silenced metastasized with high frequency when grown as tumor xenografts. When mice were treated with estrogen or estrogen withdrawal, tripling times for xenografts from cells with Rho GDIα silencing were similar to those from vector-containing control cells; however, tripling times were statistically significantly faster than control when mice were treated with tamoxifen (median tripling time for tumors with Rho GDIα small interfering RNA = 2.34 weeks; for control tumors = not reached, hazard ratio = 4.13, 95% confidence interval = 1.07 to 15.96, P = .040 [adjusted for multiple comparisons, P = .119]). Levels of the metastasis-associated protein MTA2 were also increased upon Rho GDIα silencing, and combined Rho GDIα and MTA2 levels were associated with recurrence in 250 tamoxifen-treated patients.

CONCLUSION

Loss of Rho GDIα enhances metastasis and resistance to tamoxifen via effects on both ERα and MTA2 in models of ERα-positive breast cancer and in tumors of tamoxifen-treated patients.

Single nucleotide polymorphisms of CYP19A1 predict clinical outcomes and adverse events associated with letrozole in patients with metastatic breast cancer

PURPOSE

The CYP19A1 gene encodes the aromatase enzyme involved in the peripheral conversion of androgen to estrogen. We evaluated the efficacy of the aromatase inhibitor letrozole in patients with metastatic breast cancer (MBC) as related to DNA polymorphisms of CYP19A1.

PATIENTS AND METHODS

One hundred and nine patients with hormone receptor-positive MBC were treated with letrozole alone or in combination with a GnRH agonist. DNA was isolated from peripheral blood and genotyped for 46 single nucleotide polymorphisms (SNPs) of CYP19A1.

RESULTS

Among 46 SNPs examined, rs700518, rs10459592, and rs4775936 were significantly associated with higher clinical benefit rate (CBR, CR + PR + SD ≥ 6 months) (OR = 2.61 [95% CI; 1.13-6.03], P = 0.025; OR = 2.45 [95% CI; 1.06-5.65], P = 0.036; OR = 2.60 [95% CI; 1.12-6.02], P = 0.026, respectively). Median time to progression (TTP) was improved without statistical significance in patients having an over-dominant form of rs700518. In haplotype analysis, the specific haplotypes M_1_3 and M_2_1 showed a strong association with CBR (OR = 3.37 [95% CI 1.43-7.90], P = 0.005; OR = 5.33 [95% CI 1.63-17.45], P = 0.006, respectively). There was a statistically significant difference in TTP in patients with haplotype M_1_3 (5.61 months [95% CI 0.00-11.45] vs. 11.08 months [95% CI 6.75-15.42], P = 0.040) and M_2_1 (7.31 months [95% CI 4.63-9.99] vs. 12.95 months [95% CI 9.27-16.63], P = 0.038). Haplotypes M_3_5 (OR = 11.25 [95% CI 1.17-108.28], P = 0.01) and M_5_3 (OR = 4.12, [95% CI 1.09-15.61], P = 0.03) were associated with side effects of arthralgia and hot flash, respectively.

CONCLUSION

The genetic variations of CYP19A1 were significantly associated with clinical efficacy, suggesting potential predictive markers for letrozole treatment in patients with metastatic breast cancer.

(-)-Epigallocatechin-3-gallate down-regulates EGFR, MMP-2, MMP-9 and EMMPRIN and inhibits the invasion of MCF-7 tamoxifen-resistant cells

The activation of the EGFR (epidermal growth factor receptor) signalling pathway is one of the key mechanisms underlying the development of resistance to tamoxifen in breast cancer patients. As EGCG [(-)-epigallocatechin-3-gallate], the most active catechin present in green tea, has been shown to down-regulate EGFR, we studied the effects of 10-100 μg/ml EGCG treatment on growth and invasion in a breast carcinoma cell line resistant to tamoxifen [MCF-7Tam (MCF-7 breast carcinoma cell line resistant to tamoxifen) cells] and parental MCF-7. A dose-dependent down-regulation of EGFR mRNA expression and protein level occurred after 50 μg/ml EGCG treatment of MCF-7Tam cells. EGFR molecules on the plasma membrane surface of MCF-7Tam cells significantly decreased. EGFR phosphorylation (Tyr-992, Tyr-1045 and Tyr-1068) was higher in MCF-7Tam than in MCF-7 and it was reduced by EGCG treatment. ERK (extracellular regulated kinase) and phospho-ERK p42/44 were also down-regulated by EGCG treatment and in vitro cell growth and invasion decreased. MMP-2 (matrix metalloproteinase-2) and MMP-9, which are implicated in cell invasion and metastasis, and EMMPRIN (extracellular matrix metalloproteinase inducer), a glycoprotein able to activate MMPs, were significantly reduced after 50 μg/ml EGCG treatment. In keeping with this, TIMP-1 (tissue inhibitor of metalloproteinases-1) and TIMP-2, which down-regulate MMPs, increased after EGCG treatment. Altogether, the present data demonstrated that EGCG could attenuate the tamoxifen-resistant phenotype of MCF-7Tam cells. EGCG could stop MCF-7Tam cell growth and in vitro invasion through down-regulation of EGFR and other molecules implicated in aggressive biological behaviour. The present data support the hypothesis that EGCG is an interesting molecule to be investigated in tamoxifen-resistant breast carcinoma.

Differential expression pattern of estrogen receptors, aromatase, and sulfotransferase in breast cancer tissue and corresponding lymph node metastases

Patients with hormone receptor positive breast cancer who are treated with endocrine therapy generally have a good prognosis. However, resistance to hormonal therapy and progression occurs, and the reasons for this are manifold. It has been proposed that the local estrogenic environment has a role in the process of local invasion and progression. We have determined the expression pattern of estrogen receptor α, estrogen receptor β, and the epithelial and stromal expression of the estrogen-metabolizing enzymes aromatase and sulfotransferase by immunohistochemistry in tissue arrays, containing 50 paraffin-embedded sets of tissues obtained from breast cancer and from corresponding metastatic axillary lymph nodes of the same patients. We have found statistically significant higher estrogen receptors α and β expression in primary tumors than in corresponding lymph node metastases (p = 0.0004 and p = 0.003, respectively). Aromatase was also expressed more frequently in epithelial as well as in stromal cells of the malignant tumor when compared to according lymph node metastases (p = 0.08 and p = 0.12, respectively). While in lymph node metastases only estrogen receptor α and stromal aromatase expression were correlated (p = 0.01), significant associations were seen between the estrogen receptor β and sromal aromatase, and epithelial sulfotransferase (p = 0.0006 and p = 0.03, respectively) in the primary tumor. We hypothesize that the decreased expression of local estrogens by aromatase, in combination with a decreased expression of estrogen receptors α and β in lymphatic metastases, renders these metastases hormone insensitive and could contribute to the poor response to endocrine therapy that is often seen in nodal-positive tumors.

Matrix metalloproteinase-1 expression can be upregulated through mitogen-activated protein kinase pathway under the influence of human epidermal growth factor receptor 2 synergized with estrogen receptor

In our previous work, Ets-1 upregulates human epidermal growth factor receptor 2 (HER2) induced matrix metalloproteinase 1 (MMP-1) expression. Based on the above knowledge and result, we hypothesized that estrogen receptor (ER) and its signaling pathway may affect MMP-1 expression under the influence of HER2. In addition, we investigated how the HER2 pathway cross-talk with the ER signaling pathway in genomic and nongenomic action of ER using reverse transcription-PCR, Western blot analysis, and ELISA assay. The results showed that ER-alpha expression increased MMP-1 expression under the presence of HER2. These upregulatory effects were mediated mainly by mitogen-activated protein kinase pathway and were reversed by downregulation of HER2 and/or ER. Activator protein DNA binding activity was involved in the MMP-1 expression. In summary, our results showed that ER can upregulate MMP-1 expression under the influence of HER2 in MCF-7 cells. In addition, this upregulatory effect was found to be mediated by mitogen-activated protein kinase pathway. MMP-1 might be an assigned target in interaction between ER and HER2.

The ErbB3 binding protein EBP1 regulates ErbB2 protein levels and tamoxifen sensitivity in breast cancer cells

The ErbB2/3 heterodimer plays a critical role in breast cancer progression and in the development of endocrine resistance. EBP1, an ErbB3 binding protein, inhibits HRG-stimulated breast cancer growth, decreases ErbB2 protein levels and contributes to tamoxifen sensitivity. We report here that ectopic expression of EBP1 in Estrogen Receptor (ER) positive breast cancers that express ErbB2 at both high and low levels decreased ErbB2 protein levels. ErbB2 protein expression was also increased in mammary glands of Ebp1 knock out mice. To define the mechanism of ErbB2 down regulation, we examined the effects of EBP1 on ErbB2 mRNA levels, transcription of the ErbB2 gene and ErbB2 protein stability. We found that ectopic expression of EBP1 decreased steady state levels of endogenous ErbB2 mRNA in all cell lines tested. EBP1 overexpression decreased the activity of an ErbB2 promoter reporter in cells which overxpress ErbB2. However, reporter activity was unchanged or increased in cells which express low endogenous levels of ErbB2. We also found that ectopic expression of EBP1 accelerated ErbB2 protein degradation and enhanced ErbB2 ubiquitination in cells which express both low and high levels of ErbB2. Treatment with proteasome inhibitors prevented this decrease in ErbB2 protein levels. Ablation of EBP1 expression led to tamoxifen resistance that was abrogated by inhibition of ErbB2 activity. These results suggest that EBP1 inhibits expression of ErbB2 protein levels by multiple mechanisms and that EBP1's effects on tamoxifen sensitivity are mediated in part by its ability to modulate ErbB2 levels.

Proteomic and transcriptomic profiling reveals a link between the PI3K pathway and lower estrogen-receptor (ER) levels and activity in ER+ breast cancer

INTRODUCTION

Accumulating evidence suggests that both levels and activity of the estrogen receptor (ER) and the progesterone receptor (PR) are dramatically influenced by growth-factor receptor (GFR) signaling pathways, and that this crosstalk is a major determinant of both breast cancer progression and response to therapy. The phosphatidylinositol 3-kinase (PI3K) pathway, a key mediator of GFR signaling, is one of the most altered pathways in breast cancer. We thus examined whether deregulated PI3K signaling in luminal ER+ breast tumors is associated with ER level and activity and intrinsic molecular subtype.

METHODS

We defined two independent molecular signatures of the PI3K pathway: a proteomic (reverse-phase proteomic array) PI3K signature, based on protein measurement for PI3K signaling intermediates, and a PI3K transcriptional (mRNA) signature based on the set of genes either induced or repressed by PI3K inhibitors. By using these signatures, we scored each ER+ breast tumor represented in multiple independent expression-profiling datasets (four mRNA, n = 915; one protein, n = 429) for activation of the PI3K pathway. Effects of PI3K inhibitor BEZ-235 on ER expression and activity levels and cell growth were tested by quantitative real-time PCR and cell proliferation assays.

RESULTS

Within ER+ tumors, ER levels were negatively correlated with the PI3K activation scores, both at the proteomic and transcriptional levels, in all datasets examined. PI3K signature scores were also higher in ER+ tumors and cell lines of the more aggressive luminal B molecular subtype versus those of the less aggressive luminal A subtype. Notably, BEZ-235 treatment in four different ER+ cell lines increased expression of ER and ER target genes including PR, and treatment with IGF-I (which signals via PI3K) decreased expression of ER and target genes, thus further establishing an inverse functional relation between ER and PI3K. BEZ-235 had an additional effect on tamoxifen in inhibiting the growth of a number of ER+ cell lines.

CONCLUSIONS

Our data suggest that luminal B tumors have hyperactive GFR/PI3K signaling associated with lower ER levels, which has been correlated with resistance to endocrine therapy. Targeting PI3K in these tumors might reverse loss of ER expression and signaling and restore hormonal sensitivity.

Role of GPR30 in the mechanisms of tamoxifen resistance in breast cancer MCF-7 cells

Tamoxifen is the most frequently used anti-hormonal drug for treatment of women with hormone-dependent breast cancer. The aim of this study is to investigate the mechanism of tamoxifen resistance and the impact of the new estrogen G-protein coupled receptor (GPR30). MCF-7 cells were continuously exposed to tamoxifen for 6 months to induce resistance to the inhibitory effect of tamoxifen. These tamoxifen-resistant cells (TAM-R) exhibited enhanced sensitivity to 17-ss-estradiol and GPR30 agonist, G1, when compared to the parental cells. In TAM-R cells, tamoxifen was able to stimulate the cell growth and MAPK phosphorylation. These effects were abolished by EGFR inhibitor AG1478, GPR30 anti-sense oligonucleotide, and the selective c-Src inhibitor PP2. Only EGFR basal expression was slightly elevated in the TAM-R cells, whereas GPR30 expression and the basal phosphorylation of Akt and MAPK remained unchanged when compared to the parental cells. Interestingly, estrogen treatment significantly increased GPR30 translocation to the cell surface, which was stronger in TAM-R cells. Continuous treatment of MCF-7 cells with GPR30 agonist G1 mimics the long-term treatment with tamoxifen and increases drastically its agonistic activity. This data suggests the important role of GPR30/EGFR receptor signaling in the development of tamoxifen resistance. The inhibition of this pathway is a valid option to improve anti-hormone response in breast cancer.

Prediction of hormone sensitivity for breast cancers

The classic action that leads to transcriptional activation of estrogen response genes mediated through estrogen receptors (ER) and the estrogen complex plays a pivotal role in the development of ER-positive breast cancers. In addition to this pathway, non-classic action and non-genomic action, both estrogen-dependent and estrogen-independent genomic actions have also been found to contribute to ER-positive tumor growth. Although the details of these mechanisms are not well known, participation of the growth factor signaling pathway is likely to be the most significant factor for acquisition of resistance to hormonal therapy. This resistance is mediated not only directly through cell growth promotion by growth factor signaling, but also through enhancement of alternative ER signaling pathways in addition to classic action. The reason why tamoxifen-insensitive ER-positive breast cancers respond to aromatase inhibitors may be explained, at least in part, by the different estrogen-related signaling pathways in which aromatase inhibitors may block estrogen signaling. In this paper we discuss the molecular mechanisms for resistance to hormonal therapy based on an understanding of estrogen signaling pathways.

CRK7 modifies the MAPK pathway and influences the response to endocrine therapy

Endocrine therapies, which inhibit estrogen receptor (ER)alpha signaling, are the most common and effective treatment for ERalpha-positive breast cancer. However, the use of these agents is limited by the frequent development of resistance. The cyclin-dependent kinase family member CRK7 (aka CRKRS) was identified from an RNA interference screen for modifiers of tamoxifen sensitivity. Here, we demonstrate that silencing of CRK7 not only causes resistance to tamoxifen but also leads to resistance to additional endocrine therapies including ICI 182780 and estrogen deprivation, a model of aromatase inhibition. We show that CRK7 silencing activates the mitogen-activated protein kinase (MAPK)-signaling pathway, which causes a loss of ER dependence, resulting in endocrine therapy resistance. This study identifies a novel role for CRK7 in MAPK regulation and resistance to estrogen signaling inhibitors.