Oestrogen and growth factor cross-talk and endocrine insensitivity and acquired resistance in breast cancer

Low levels of sex hormone-binding globulin predict an increased breast cancer risk and its underlying molecular mechanisms

Sex hormone-binding globulin (SHBG) is a serum glycoprotein exhibiting the unique feature of binding sex steroids with high affinity and specificity. Over the past few decades, there have been significant breakthroughs in our understanding of the function and regulation of SHBG. The biological role of SHBG has expanded from being considered a simple sex hormone transporter to being associated with several complex physiological and pathological changes in a variety of target tissues. Many factors can affect the plasma SHBG levels, with fluctuations in circulating levels affecting the development of various diseases, such as increasing the risk of developing breast cancer. This article reviews the clinical significance of changes in circulating SHBG levels in the development of breast cancer and the possible influence of these levels on endocrine drug resistance in hormone receptor-positive breast cancer. Higher levels of plasma SHBG significantly reduce the risk of estrogen receptor-positive breast cancer, especially in postmenopausal women. Moreover, the molecular mechanisms by which SHBG affects breast cancer risk are also summarized in detail. Finally, transcriptomics and proteomics data revealed that SHBG expression in breast tissue can effectively distinguish breast cancer from normal tissue. Additionally, the association between SHBG expression levels and various classical tumor-related pathways was investigated.

ROS1 altered breast cancers - a distinctive molecular subtype of PR- metastatic breast cancers: Expanding the scope of targeted therapeutics

BACKGROUND

Breast cancer, one of the leading causes of cancer-related mortality in women worldwide, exhibits wide-ranging histo-morphologic, clinical and molecular diversity.

OBJECTIVE

This study compares the genetic alterations of breast tumors with the histo-morphological, hormone receptor status and metastatic "organotropism".

MATERIALS AND METHODS

Twenty-two cases of primary invasive breast carcinoma with local/distant metastasis were retrieved from the pathology archives. The status of estrogen and progesterone receptors by immunohistochemistry was recorded along with other pertinent case data. Next generation sequencing was performed on formalin-fixed paraffin embedded blocks of tumor.

RESULTS

The mean age of the study subjects was 57.9 ± 13.3 years. TP53 mutation was the most common gene alteration in this study and was seen in 40.9% cases. ROS1 gene was mutated in 44.4% PR negative breast cancers while being wild type in the twelve PR positive tumors. (p = 0.021).STRING interaction network constructed with ROS1 and PR revealed a significantly higher number of interactions in this network than expected (p-value 0.000973).

CONCLUSION

This study highlights the significantly higher incidence of ROS1 gene alterations in metastatic PR- breast cancers, with STRING network analysis revealing higher nodal interaction in the nodal network comprised of PR and ROS1 exclusive of ER.

A novel immune subtype classification of ER-positive, PR-negative and HER2-negative breast cancer based on the genomic and transcriptomic landscape

BACKGROUND

The diversity and plasticity behind ER+/PR-/HER2- breast cancer have not been widely explored. It is essential to identify heterogeneous microenvironment phenotypes and investigate specific genomic events driving the formation of these phenotypes.

METHODS

Based on the immune-related gene expression profiles of 411 ER+/PR-/HER2- breast cancers in the METABRIC cohort, we used consensus clustering to identify heterogeneous immune subtypes and assessed their reproducibility in an independent meta-cohort including 135 patients collected from GEO database. We further analyzed the differences of cellular and molecular characteristics, and potential immune escape mechanism among immune subtypes. In addition, we constructed a transcriptional trajectory to visualize the distribution of individual patient.

RESULTS

Our analysis identified and validated five reproducible immune subtypes with distinct cellular and molecular characteristics, potential immune escape mechanisms, genomic drivers, as well as clinical outcomes. An immune-cold subtype, with the least amount of lymphocyte infiltration, had a poorer prognosis. By contrast, an immune-hot subtype, which demonstrated the highest infiltration of CD8+ T cells, DCs and NK cells, and elevated IFN-γ response, had a comparatively favorable prognosis. Other subtypes showed more diverse gene expression and immune infiltration patterns with distinct clinical outcomes. Finally, our analysis revealed a complex immune landscape consisting of both discrete cluster and continuous spectrum.

CONCLUSION

Overall, this study revealed five heterogeneous immune subtypes among ER+/PR-/HER2- breast cancer, also provided important implications for clinical translations.

Loss of ZIP facilitates JAK2-STAT3 activation in tamoxifen-resistant breast cancer

Tamoxifen is beneficial in treating estrogen receptor–positive breast cancer, but resistance to this treatment eventually ensues. A method to identify mechanisms of tamoxifen resistance identified the histone deacetylase ZIP, leading to the finding that increased expression of the tyrosine kinase JAK2 is one important factor. As a result of this discovery, it may be possible to use an inhibitor of JAK2 to block the aberrant activation of STAT3 caused by ZIP deficiency to help overcome or prevent tamoxifen resistance.

Tamoxifen, a widely used modulator of the estrogen receptor (ER), targets ER-positive breast cancer preferentially. We used a powerful validation-based insertion mutagenesis method to find that expression of a dominant-negative, truncated form of the histone deacetylase ZIP led to resistance to tamoxifen. Consistently, increased expression of full-length ZIP gives the opposite phenotype, inhibiting the expression of genes whose products mediate resistance. An important example is JAK2. By binding to two specific sequences in the promoter, ZIP suppresses JAK2 expression. Increased expression and activation of JAK2 when ZIP is inhibited lead to increased STAT3 phosphorylation and increased resistance to tamoxifen, both in cell culture experiments and in a mouse xenograft model. Furthermore, data from human tumors are consistent with the conclusion that decreased expression of ZIP leads to resistance to tamoxifen in ER-positive breast cancer.

Development and characterisation of acquired radioresistant breast cancer cell lines

BACKGROUND

Radiotherapy plays an important role in the multimodal treatment of breast cancer. The response of a breast tumour to radiation depends not only on its innate radiosensitivity but also on tumour repopulation by cells that have developed radioresistance. Development of effective cancer treatments will require further molecular dissection of the processes that contribute to resistance.

METHODS

Radioresistant cell lines were established by exposing MDA-MB-231, MCF-7 and ZR-751 parental cells to increasing weekly doses of radiation. The development of radioresistance was evaluated through proliferation and colony formation assays. Phenotypic characterisation included migration and invasion assays and immunohistochemistry. Transcriptomic data were also generated for preliminary hypothesis generation involving pathway-focused analyses.

RESULTS

Proliferation and colony formation assays confirmed radioresistance. Radioresistant cells exhibited enhanced migration and invasion, with evidence of epithelial-to-mesenchymal-transition. Significantly, acquisition of radioresistance in MCF-7 and ZR-751 cell lines resulted in a loss of expression of both ERα and PgR and an increase in EGFR expression; based on transcriptomic data they changed subtype classification from their parental luminal A to HER2-overexpressing (MCF-7 RR) and normal-like (ZR-751 RR) subtypes, indicating the extent of phenotypic changes and cellular plasticity involved in this process. Radioresistant cell lines derived from ER+ cells also showed a shift from ER to EGFR signalling pathways with increased MAPK and PI3K activity.

CONCLUSIONS

This is the first study to date that extensively describes the development and characterisation of three novel radioresistant breast cancer cell lines through both genetic and phenotypic analysis. More changes were identified between parental cells and their radioresistant derivatives in the ER+ (MCF-7 and ZR-751) compared with the ER- cell line (MDA-MB-231) model; however, multiple and likely interrelated mechanisms were identified that may contribute to the development of acquired resistance to radiotherapy.

The evolving role of receptors as predictive biomarkers for metastatic breast cancer

INTRODUCTION

In breast cancer, estrogen receptor (ER) and human epidermal growth factor receptor 2 (HER2) are essential biomarkers to predict response to endocrine and anti-HER2 therapies, respectively. In metastatic breast cancer, the use of these receptors and targeted therapies present additional challenges: temporal heterogeneity, together with limited sampling methodologies, hinders receptor status assessment, and the constant evolution of the disease invariably leads to resistance to treatment. Areas covered: This review summarizes the genomic abnormalities in ER and HER2, such as mutations, amplifications, translocations, and alternative splicing, emerging as novel biomarkers that provide an insight into underlying mechanisms of resistance and hold potential predictive value to inform treatment selection. We also describe how liquid biopsies for sampling of circulating markers and ultrasensitive detection technologies have emerged which complement ongoing efforts for biomarker discovery and analysis. Expert commentary: While evidence suggests that genomic aberrations in ER and HER2 could contribute to meeting the pressing need for better predictive biomarkers, efforts need to be made to standardize assessment methods and better understand the resistance mechanisms these markers denote. Taking advantage of emerging technologies, research in upcoming years should include prospective trials incorporating these predictors into the study design to validate their potential clinical value.

ZIP restores estrogen receptor expression and response to Tamoxifen in estrogen receptor negative tumors

As a component of NURD histone deacetylase complex, ZIP serves as a tumor suppressor gene in the development of breast tumors. However, whether it takes part in chemotherapy resistance remains poorly defined. In the present study, we reported that ZIP enhanced the response to SERM chemotherapy in ER-negative cells. Overexpression of ZIP suppressed EGFR expression level and restored ERalpha protein level in cells resistant to Tamoxifen. In vivo data confirmed those in vitro findings.

Endocrine resistance in advanced breast cancer: current evidence and future directions

SUMMARY Hormone receptor-positive breast cancer constitutes a heterogeneous group representing the majority (60–75%) of patients with the disease. Hormonal approaches interfering with receptor signaling represents an established treatment strategy with demonstrated efficacy and favorable toxicity profile. However, due to either primary or acquired resistance, metastatic hormone receptor-positive breast cancer remains incurable. Mechanisms of resistance are complex and still poorly understood. Estrogen receptor signaling interactions with critical growth factor pathways and various downstream kinases have been implicated. Available evidence recommends a sequential single-agent approach to advanced hormone receptor-positive disease as a preferred therapeutic alternative. Recent information suggests that rational mTOR inhibition modulates hormonal resistance. Further advances in this field will require analysis of biopsies from metastatic sites at the time of progression.

Targeting the insulin growth factor receptor 1

The IGF axis is a tightly controlled endocrine system that regulates cell growth and development, known to have an important function in cancer biology. IGF1 and IGF2 can promote cancer growth in a GH-independent manner both through paracrine and autocrine secretion and can also confer resistance to chemotherapy and radiation. Many alterations of this system have been found in neoplasias, including increased expression of ligands and receptors, loss of heterozygosity of the IGF2 locus and increased IGF1R gene copy number. The IGF1 network is an attractive candidate for targeted therapy, including receptor blockade with monoclonal antibodies and small molecule inhibitors of receptor downstream signaling. This article reviews the role of the IGF axis in the initiation and progression of cancer, and describes the recent advances in IGF inhibition as a therapeutic tool.

The predictive value of epidermal growth factor receptor expression for sensitivity to vinorelbine in breast cancer

Breast cancer patients with positive epidermal growth factor receptor (EGFR) expression have significantly worse post-relapse prognosis than patients with negative EGFR expression. Vinorelbine (NVB) is usually reserved as a salvage therapy after anthracyclines and taxanes in patients with breast cancer. To see whether EGFR expression has a predictive value in NVB-mediated effect on human breast cancer cells, we examined 50 primary breast cancer samples. Of these, 42% were found to be NVB sensitive by ATP-tumour chemosensitivity assay. Sensitivity was correlated with EGFR expression level (p = 0.001). To dynamically examine EGFR's effect on NVB sensitivity in breast cancer cells, we used the real-time cell electronic sensing system with EGFR-positive and EGFR-negative breast cancer cell lines, MCF-7 and MDA-MB-435s, respectively. MCF-7 is NVB sensitive, while MDA-MB-435 is NVB resistant. NVB-induced cytotoxicity to MCF-7 can be partly reversed with inhibitory anti-EGFR antibody. NVB up-regulated EGFR expression in MCF-7 cells, which affects ERK1/2 phosphorylation. This cellular response mechanism may cause greater input to non-lethally damaged cells. These data suggest that EGFR expression can be used as a prognostic factor for breast cancer sensitivity to NVB, which could help identify appropriate treatments for breast cancer.

Adaptive changes results in activation of alternate signaling pathways and resistance to aromatase inhibitor resistance

Hormone therapy is an effective approach for the treatment of breast cancer. The antiestrogen tamoxifen has had a major impact on the disease. Recently, aromatase inhibitors which reduce estrogen synthesis have proved to be more effective and these agents are now used as first line therapy for postmenopausal breast cancer. Nevertheless, resistance to treatment eventually may occur. We have investigated mechanisms involved in resistance to AIs and devised strategies to overcome the resistance. Using a xenograft model, we have identified adaptive changes that results in activation of alternate signaling pathways in tumors resistant to aromatase inhibitors. Expression of ERα and aromatase was decreased in the tumors after long term treatment with AIs. In contrast increased expression was observed of tyrosine kinase receptors such as HER-2 and IGFR as well as of downstream signaling proteins, including MAPK. We have demonstrated functional activation of the MAPKinase pathway and shown dependency on growth factor receptor signaling in letrozole resistant cells. Furthermore, our studies indicate that HER-2 is a negative regulator of ER. Thus, when HER-2 was blocked with antibody (herceptin, trastuzumab) ER expression was increased rendering the cells and tumors responsive to aromatase inhibitors and resulting in tumor regression.

GP88 (PC-Cell Derived Growth Factor, progranulin) stimulates proliferation and confers letrozole resistance to aromatase overexpressing breast cancer cells

Background

Aromatase inhibitors (AI) that inhibit breast cancer cell growth by blocking estrogen synthesis have become the treatment of choice for post-menopausal women with estrogen receptor positive (ER⁺) breast cancer. However, some patients display de novo or acquired resistance to AI. Interactions between estrogen and growth factor signaling pathways have been identified in estrogen-responsive cells as one possible reason for acquisition of resistance. Our laboratory has characterized an autocrine growth factor overexpressed in invasive ductal carcinoma named PC-Cell Derived Growth Factor (GP88), also known as progranulin. In the present study, we investigated the role GP88 on the acquisition of resistance to letrozole in ER⁺ breast cancer cells

Methods

We used two aromatase overexpressing human breast cancer cell lines MCF-7-CA cells and AC1 cells and their letrozole resistant counterparts as study models. Effect of stimulating or inhibiting GP88 expression on proliferation, anchorage-independent growth, survival and letrozole responsiveness was examined.

Results

GP88 induced cell proliferation and conferred letrozole resistance in a time- and dose-dependent fashion. Conversely, naturally letrozole resistant breast cancer cells displayed a 10-fold increase in GP88 expression when compared to letrozole sensitive cells. GP88 overexpression, or exogenous addition blocked the inhibitory effect of letrozole on proliferation, and stimulated survival and soft agar colony formation. In letrozole resistant cells, silencing GP88 by siRNA inhibited cell proliferation and restored their sensitivity to letrozole.

Conclusion

Our findings provide information on the role of an alternate growth and survival factor on the acquisition of aromatase inhibitor resistance in ER⁺ breast cancer.

Combining Src inhibitors and aromatase inhibitors: a novel strategy for overcoming endocrine resistance and bone loss

Aromatase inhibitors have largely replaced tamoxifen as the first-line treatment for postmenopausal women with metastatic, hormone receptor-positive (HR+) breast cancer. However, many patients develop clinical resistance with prolonged treatment, and oestrogen deprivation following aromatase inhibition can result in loss of bone mineral density. Furthermore, most patients with metastatic breast cancer develop bone metastases, and the resulting adverse skeletal-related events are a significant cause of patient morbidity. Src, a non-receptor tyrosine kinase, is a component of signalling pathways that regulate breast cancer cell proliferation, invasion and metastasis as well as osteoclast-mediated bone turnover. Preclinical evidence also suggests a role for Src in acquired endocrine resistance. As such, Src inhibition represents a logical strategy for the treatment of metastatic breast cancer. In vitro, combination therapy with Src inhibitors and endocrine agents, including aromatase inhibitors, has been shown to inhibit the proliferation and metastasis of both endocrine-responsive and endocrine-resistant breast cancer cell lines more effectively than either of the therapy alone. Src inhibition has also been shown to suppress osteoclast formation and activity. Combination therapy with aromatase inhibitors and Src inhibitors therefore represents a novel approach through which the development of both acquired resistance and bone pathology could be delayed. Data from clinical trials utilising such combinations will reveal if this strategy has the potential to improve patient outcomes.

Designer monotransregulators provide a basis for a transcriptional therapy for de novo endocrine-resistant breast cancer

The main circulating estrogen hormone 17beta-estradiol (E2) contributes to the initiation and progression of breast cancer. Estrogen receptors (ERs), as transcription factors, mediate the effects of E2. Ablation of the circulating E2 and/or prevention of ER functions constitute approaches for ER-positive breast cancer treatments. These modalities are, however, ineffective in de novo endocrine-resistant breast neoplasms that do not express ERs. The interaction of E2-ERs with specific DNA sequences, estrogen responsive elements (EREs), of genes constitutes one genomic pathway necessary for cellular alterations. We herein tested the prediction that specific regulation of ERE-driven genes by an engineered monomeric and constitutively active transcription factor, monotransregulator, provides a basis for the treatment of ER-negative breast cancer. Using adenovirus infected ER-negative MDA-MB-231 cells derived from a breast adenocarcinoma, we found that the monotransregulator, but not the ERE-binding defective counterpart, repressed cellular proliferation and motility, and induced apoptosis through expression of genes that required ERE interactions. Similarly, the monotransregulator suppressed the growth of ER-negative BT-549 cells derived from a breast-ductal carcinoma. Moreover, the ERE-binding monotransregulator repressed xenograft tumor growth in a nude mice model. Thus, specific regulation of genes bearing EREs could offer a therapeutic approach for de novo endocrine-resistant breast cancers.

Androgen receptor overexpression induces tamoxifen resistance in human breast cancer cells

Although the androgen receptor (AR) is a known clinical target in prostate cancer, little is known about its possible role in breast cancer. We have investigated the role of AR expression in human breast cancer in response to treatment with the antiestrogen tamoxifen. Resistance to tamoxifen is a major problem in treating women with breast cancer. By gene expression profiling, we found elevated AR and reduced estrogen receptor (ER) alpha mRNA in tamoxifen-resistant tumors. Exogenous overexpression of AR rendered ERalpha-positive MCF-7 breast cancer cells resistant to the growth-inhibitory effects of tamoxifen in anchorage-independent growth assays and in xenograft studies in athymic nude mice. AR-overexpressing cells remained sensitive to growth stimulation with dihydrotestosterone. Treatment with the AR antagonist Casodex (bicalutamide) reversed this resistance, demonstrating the involvement of AR signaling in tamoxifen resistance. In AR-overexpressing cells, tamoxifen induced transcriptional activation by ERalpha that could be blocked by Casodex, suggesting that AR overexpression enhances tamoxifen's agonistic properties. Our data suggest a role for AR overexpression as a novel mechanism of hormone resistance, so that AR may offer a new clinical therapeutic target in human breast cancers.

Aromatase inhibitors and breast cancer

We have developed a breast cancer intratumoral aromatase model to simulate the postmenopausal breast cancer patient in order to compare the antitumor efficacy of aromatase inhibitors (AIs) and antiestrogens (AEs). The AI letrozole sustained growth inhibition longer than the AE tamoxifen. Nevertheless, eventually tumors began to grow despite continued treatment. Estrogen receptor-alpha (ER-alpha) levels decreased below control levels concomitant with increased phosphorylation of ER-alpha and unaltered progesterone receptor (PgR) levels. Expression of Her-2, p-Shc, Grb-2, p-Raf, p-Mekl/2, and p-MAPK was increased in the letrozole-resistant tumors. When cells isolated from letrozole-resistant tumors (LTLTCa cells) were treated with inhibitors of the Her-2 signaling pathway, such as trastuzumab (herceptin), ER-alpha was restored. Furthermore, sensitivity of LTLTCa cells to AIs and AEs was regained. These findings suggest cross-talk between ER and Her-2 signaling. To prevent activation of the Her-2 pathway and resistance to AIs, mice were treated with a combination of an AI anastrozole and the ER downregulator fulvestrant. This resulted in no increase in Her-2 and p-MAPK levels, and tumor growth was significantly inhibited. Thus, blocking both ER and Her-2 signaling delayed development of resistance to AIs. This hypothesis was supported by the finding that growth of letrozole-resistant tumors was reduced when xenografts were treated with trastuzumab combined with letrozole. In addition, resistance to letrozole could be reversed by discontinuing letrozole. Our findings indicate that after letrozole treatment is stopped, the antitumor effect of letrozole can be restored when the AI treatment is resumed.

IGF and insulin receptor signaling in breast cancer

Major molecular abnormalities in breast cancer include the deregulation of several components of the IGF system. It is well recognized that the epithelial breast cancer cells commonly overexpress the IGF-I receptor while IGF-II is expressed by the tumor stroma. In view to the fact that the IGF-IR has mitogenic, pro-invasive and anti-apoptotic effects and mediates resistance to a variety of anti-cancer therapies, breast cancer is expected to be a candidate to therapeutic approaches aimed to inhibit the IGF-IR. However, there is increasing awareness that IGF system in cancer undergoes signal diversification by various mechanisms. One of these mechanisms is the aberrant expression of insulin receptor (IR) isoform A (IR-A), which is a high affinity receptor for both insulin and IGF-II, in breast cancer cells. Moreover, overexpression of both IGF-IR and IR-A in breast cancer cells, leads to overexpression of hybrid IR/IGF-IR receptors (HRs) as well. Upon binding to IGF-II, both IR-A and HRs may activate unique signaling patterns, which predominantly mediate proliferative effects. A better understanding of IGF system signal diversification in breast cancer has important implications for cancer prevention measures, which should include control of insulin resistance and associated hyperinsulinemia. Moreover, in addition to the IGF-IR, both IR-A and HRs should be also considered as molecular targets for anti-cancer therapies.

Combination of anastrozole with fulvestrant in the intratumoral aromatase xenograft model

Although the aromatase inhibitor anastrozole has been shown to be very effective in the treatment of hormone-dependent postmenopausal breast cancer, some patients with advanced disease will develop resistance to treatment. To investigate therapeutic strategies to overcome resistance to anastrozole treatment, we have used an intratumoral aromatase model that simulates postmenopausal breast cancer patients with estrogen-dependent tumors. Growth of the tumors in the mice was inhibited by both anastrozole and fulvestrant compared with the control tumors. Nevertheless, tumors had doubled in size at 5 weeks of treatment. We therefore investigated whether switching the original treatments to anastrozole or fulvestrant alone or the combination of anastrozole plus fulvestrant would reduce tumor growth. The results showed that the best strategy to reverse the insensitivity to anastrozole or fulvestrant is to combine the two agents. Additionally, the tumors treated with anastrozole plus fulvestrant from the beginning had only just doubled their size after 14 weeks of treatment, whereas the anastrozole and fulvestrant treatments alone resulted in 9- and 12-fold increases in tumor size, respectively, in the same time period. Anastrozole plus fulvestrant from the beginning or in sequence was associated with down-regulation of signaling proteins involved in the development of hormonal resistance such as insulin-like growth factor type I receptor beta, mitogen-activated protein kinase (MAPK), p-MAPK, AKT, mammalian target of rapamycin (mTOR), p-mTOR, and estrogen receptor alpha compared with tumors treated with anastrozole or fulvestrant alone. These results suggest that blocking the estrogen receptor and aromatase may delay or reverse the development of resistance to aromatase inhibitors in advanced breast cancer patients.

Preclinical modeling of endocrine response and resistance: focus on aromatase inhibitors

The authors developed a breast cancer intratumoral aromatase model system to compare the antitumor efficacy of several aromatase inhibitors (AIs) and antiestrogens (AEs). Although the AI letrozole caused sustained growth inhibition, tumors eventually began to grow, even when treatment was maintained. For the current study, the mechanisms of resistance to letrozole during the course of treatment were investigated. Estrogen receptor alpha (ER-alpha) levels decreased below control levels in letrozole-resistant tumors. The decrease was simultaneous to an increase in phosphorylation of ER-alpha and an unaltered expression of progesterone receptor (PgR). Expression levels of HER-2, activated (phosphorylated) SHC-adaptor protein (p-Shc), growth factor receptor-bound protein 2 (Grb-2), p-Raf, phosphorylated mitogen-activated protein kinase kinase 1/2 (p-Mekl/2), and phosphorylated mitogen-activated protein kinase (p-MAPK) were increased. When cells isolated from letrozole-resistant tumors (LTLTCa cells) were treated with inhibitors of the HER-2 signaling pathway, ER-alpha expression and estradiol-stimulated transactivation was restored. The HER-2 blocker trastuzumab also restored the sensitivity of LTLTCa cells to AIs and AEs. These findings suggested that there is crosstalk between ER and HER-2 signaling. To prevent activation of the HER-2 pathway and resistance to AIs, mice were treated with a combination of AIs and the ER down-regulator fulvestrant. There was no increase in HER-2 or p-MAPK expression, and tumor growth was inhibited significantly. When trastuzumab was added to unresponsive tumors under letrozole treatment, it significantly inhibited tumors growth compared with switching to trastuzumab alone. However, the trastuzumab plus letrozole combination was more effective than letrozole alone only in refractory breast tumors. These results suggested that blocking both ER and HER-2 signaling may delay the development of resistance to AIs in patients with recurrent breast cancer.

Transgenic models to study actions of prolactin in mammary neoplasia

Transgenic models to explore the role of prolactin and its interactions with other factors in mammary oncogenesis have begun to reveal the dynamic contributions of prolactin to the development and progression of this disease. Targeting prolactin to mammary epithelial cells mimics the local production of this hormone that is prominent in women, and permits studies in the absence of effects on the ovarian steroid milieu. These models have demonstrated that local production of prolactin is sufficient to induce mammary tumors after a long latency. Prolactin also can potentiate actions of other oncogenic stimuli, decreasing tumor latency and increasing incidence in several models. Augmented proliferation, without alteration of apoptosis, is a consistent feature. Pathways in addition to the well-characterized Jak2-Stat5 pathway, including ERK1/2 and Akt1/2, are implicated in these actions. These studies have also revealed a complex relationship with estrogen; while prolactin increases ERalpha expression, it does not require estrogenic ligand for lesion development, and indeed, in combination with the EGFR ligand, TGFalpha, prolactin can contribute to estrogen insensitivity. These studies highlight the utility of these models to decipher the interplay between prolactin and other oncogenic factors in breast cancer, with implications for preventative and therapeutic strategies.

Endogenous sex hormones, prolactin and mammographic density in postmenopausal Norwegian women

The associations between endogenous sex hormone levels and breast cancer risk in postmenopausal women are well established. Mammographic density is a strong risk factor for breast cancer, and possibly an intermediate marker. However, the results from studies on the associations between endogenous sex hormones and mammographic density are conflicting. The authors examined the associations between circulating levels of sex hormones, sex hormone binding globulin (SHBG) and prolactin and mammographic densities among postmenopausal women not currently using postmenopausal hormone therapy (HT). The authors also examined if insulin-like growth factor-I (IGF-I) levels influenced the association between estrogen and mammographic density. Altogether, 722 postmenopausal participants in the Norwegian governmental mammographic screening program had endogenous hormone concentrations measured. Mammograms were classified according to percent and absolute mammographic density using a previously validated computer-assisted method. After adjustment for age, number of children, age at menopause, body mass index and HT use, both plasma concentrations of SHBG (p-trend = 0.003) and estrone (p-trend = 0.07) were positively associated with percent mammographic density. When the analyses were stratified according to median IGF-I concentration, the weak association between estrone and mammographic density was strengthened among women with IGF-I levels below median, while the association disappeared among women with over median IGF-I levels (p for interaction = 0.02). In summary, the authors found a positive association between plasma SHBG levels and mammographic densities among 722 postmenopausal Norwegian women not currently using HT. Further, the authors found a positive but weak association between plasma estrone concentration and mammographic density, which appeared to be modified by IGF-I levels.

Growth factor signalling in endocrine and anti-growth factor resistant breast cancer

Growth factors provide powerful mitogenic and survival signals to breast cancer cells and it is therefore not surprising that they are able to subvert inhibitory responses to anti-hormonal drugs. In this review we discuss several mechanisms by which this may be achieved and expand our observations to encompass recently emerging anti-growth factor treatments. The information presented is underpinned by inhibitor studies that show the targeting of such mechanisms in advance of anti-hormone or anti-growth factor resistance development is able to substantially delay this event, thus pointing the way forward to intelligent combination therapies relevant to the future management of breast cancer.

BEX2 is overexpressed in a subset of primary breast cancers and mediates nerve growth factor/nuclear factor-kappaB inhibition of apoptosis in breast cancer cell lines

We have identified a novel subtype of estrogen receptor (ER)-positive breast cancers with improved outcome after tamoxifen treatment and characterized by overexpression of the gene BEX2. BEX2 and its homologue BEX1 have highly correlated expression and are part of a cluster enriched for ER response and apoptosis genes. BEX2 expression is induced after estradiol (E2) treatment with a peak at 3 h, suggesting BEX2 is an estrogen-regulated gene. BEX2 belongs to a family of genes, including BEX1, NGFRAP1 (alias BEX3), BEXL1 (alias BEX4), and NGFRAP1L1 (alias BEX5). Both BEX1 and NGFRAP1 interact with p75NTR and modulate nerve growth factor (NGF) signaling through nuclear factor-kappaB (NF-kappaB) to regulate cell cycle, apoptosis, and differentiation in neural tissues. In breast cancer cells, NGF inhibits C2-induced apoptosis through binding of p75NTR and NF-kappaB activation. Here, we show that BEX2 expression is necessary and sufficient for the NGF-mediated inhibition (through NF-kappaB activation) of C2-induced apoptosis. We also show that BEX2 modulates apoptosis of breast cancer cells in response to E2 (50 nmol/L) and tamoxifen (5 and 10 micromol/L). Furthermore, BEX2 overexpression enhances the antiproliferative effect of tamoxifen at pharmacologic dose (1 micromol/L). These data suggest that a NGF/BEX2/NF-kappaB pathway is involved in regulating apoptosis in breast cancer cells and in modulating response to tamoxifen in primary tumors.

Mechanisms of acquired resistance to endocrine therapy in hormone-dependent breast cancer cells

Acquired resistance is a major problem limiting the clinical benefit of endocrine therapy. To investigate the mechanisms involved, two in vitro models were developed from MCF-7 cells. Long-term culture of MCF-7 cells in estrogen deprived medium (LTED) mimics aromatase inhibition in patients. Continued exposure of MCF-7 to tamoxifen represents a model of acquired resistance to antiestrogens (TAM-R). Long-term estrogen deprivation results in sustained activation of the ERK MAP kinase and the PI3 kinase/mTOR pathways. Using a novel Ras inhibitor, farnesylthiosalicylic acid (FTS), to achieve dual inhibition of the pathways, we found that the mTOR pathway plays the primary role in mediation of proliferation of LTED cells. In contrast to the LTED model, there is no sustained activation of ERK MAPK but enhanced responsiveness to rapid stimulation induced by E(2) and TAM in TAM-R cells. An increased amount of ERalpha formed complexes with EGFR and c-Src in TAM-R cells, which apparently resulted from extra-nuclear redistribution of ERalpha. Blockade of c-Src activity drove ERalpha back to the nucleus and reduced ERalpha-EGFR interaction. Prolonged blockade of c-Src activity restored sensitivity of TAM-R cells to tamoxifen. Our results suggest that different mechanisms are involved in acquired endocrine resistance and the necessity for individualized treatment of recurrent diseases.

Xenograft models for aromatase inhibitor studies

As several aromatase inhibitors are now available for treating breast cancer, we developed a model system to compare their antitumor efficacy and to explore strategies for their optimal use. Tumors are grown in ovariectomized, immunodeficient mice from MCF-7 human breast cancer cells transfected with the aromatase gene (MCF-7Ca) and can therefore synthesize as well as respond to estrogen. Results from this model have been predictive of clinical outcome. Thus, inhibiting estrogen action and estrogen synthesis by treating mice with the aromatase inhibitor letrozole and the antiestrogen tamoxifen in combination did not result in synergy. Moreover, when tamoxifen treatment was no longer effective, tumor growth was significantly reduced in response to sequential letrozole treatment. However, our findings indicate that letrozole alone was better than all other treatments. Although letrozole resulted in long sustained growth inhibition, tumors eventually grew despite continued treatment. Mechanisms of resistance to letrozole were investigated during the course of treatment. ER was initially upregulated in responding tumors, but subsequently decreased below control levels in tumors no longer responsive to letrozole. Her-2 as well as adapter proteins (p-Shc and Grb-2) and signaling proteins in the MAPK cascade (p-Raf, p-Mekl/2, and p-MAPK), were all increased in letrozole resistant tumors. In LTLT cells, isolated from the letrozole resistant tumors and treated with inhibitors of the MAPKinase pathway, MAPK activity was decreased and ER expression restored to control levels. Inhibitors of EGFR/Her-2 also restored the sensitivity of LTLT cells to letrozole. These results suggest that crosstalk occurs between ER and tyrosine kinase receptor signaling. Therefore, to investigate whether down-regulating ER would prevent activation of MAPK and resistance to letrozole, xenografts were treated with letrozole and faslodex in combination. Her-2 and MAPK were not increased and tumor growth was inhibited throughout 29 weeks of treatment. These results suggest that blocking both ER and growth factor mediated transcription may delay development of resistance to letrozole and maintain its growth inhibition of breast cancer.

Dietary Flaxseed Interaction With Tamoxifen Induced Tumor Regression in Athymic Mice With MCF-7 Xenografts by Downregulating the Expression of Estrogen Related Gene Products and Signal Transduction Pathways

Our previous short-term study has shown that 10% flaxseed (FS) inhibits the growth of human estrogen dependent estrogen receptor positive breast tumors (MCF-7) xenografts in ovariectomized (OVX) athymic mice and enhances the tumor inhibitory effect of tamoxifen (TAM). This study determined the long-term effect of 5% and 10% FS, with or without TAM, on the growth of MCF-7 xenografts in athymic mice and the potential mechanisms of actions. OVX mice with established MCF-7 tumors were treated with basal diet (control), 5% FS (5FS), 10% FS (10FS), and TAM (5 mg/pellet, 60-day release), alone or in combination, for 16 wk without estrogen supplementation. Tumor growth was monitored weekly. At sacrifice, the tumors were analyzed by immunohistochemistry for cell proliferation, apoptosis, and expression of estrogen-related genes and signal transduction pathways. Both 5FS and 10FS regressed the pretreatment tumor size by over 90% similar to control. TAM initially regressed the tumors but then induced a regrowth; thus, only a final 6% reduction from pretreatment tumor size was achieved, which was attenuated by combining TAM with 10FS but not with 5FS. TAM combined with 10FS regressed tumors to 55% of pretreatment tumor size due to decreased cell proliferation and increased apoptosis. The expressions of cyclin D1, estrogen receptor alpha, human epidermal growth factor receptor 2, and insulin-like growth factor I receptor in the TAM group were significantly reduced when TAM was combined with 5FS or 10FS. In conclusion, after long-term treatment, FS did not stimulate tumor growth and combined with TAM, regressed tumor size in part due to downregulation of the expression of estrogen-related gene products and signal transduction pathways.

Long-term treatment with tamoxifen facilitates translocation of estrogen receptor alpha out of the nucleus and enhances its interaction with EGFR in MCF-7 breast cancer cells

The therapeutic benefit of tamoxifen in patients with hormone-dependent breast cancer is limited by acquired resistance to this drug. To investigate the biological alterations responsible for tamoxifen resistance, an in vitro model was established. After 6-month continuous exposure to tamoxifen (10(-7) mol/L), growth of MCF-7 breast cancer cells was no longer inhibited by this antiestrogen. Although there was no significant increase in the basal levels of activated mitogen-activated protein kinase (MAPK), tamoxifen-resistant (TAM-R) cells exhibited enhanced sensitivity to epidermal growth factor (EGF) and estradiol stimulated activation of MAPK. Tamoxifen elicited rapid phosphorylation of MAPK, in contrast to its antagonistic activity in control cells. Blockade of the EGF receptor (EGFR)/MAPK pathway caused more dramatic inhibition of growth of TAM-R cells than the control cells. An increased amount of estrogen receptor alpha (ERalpha) was coimmunoprecipitated with EGFR from TAM-R cells although the total levels of these receptors were not increased. Notably, ERalpha seemed to redistribute to extranuclear sites in TAM-R cells. Increased ERalpha immunoreactivity in the cytoplasm and plasma membrane of TAM-R cells was shown by fluorescent microscopy and by Western analysis of isolated cellular fractions. In TAM-R cells, an increased amount of c-Src was coprecipitated with EGFR or ERalpha. Blockade of c-Src activity resulted in redistribution of ERalpha back to the nucleus and in reduction of its interaction with EGFR. Prolonged blockade of c-Src activity restored sensitivity of TAM-R cells to tamoxifen. Our results suggest that enhanced nongenomic function of ERalpha via cooperation with the EGFR pathway is one of the mechanisms responsible for acquired tamoxifen resistance.

Effects of novel retinoic acid metabolism blocking agent (VN/14-1) on letrozole-insensitive breast cancer cells

Aromatase inhibitors are proving to be more effective than tamoxifen for postmenopausal estrogen receptor (ER)-positive breast cancer. However, the inevitable development of resistance to treatment is a concern. We investigated the effects of novel retinoic acid metabolism blocking agent, VN/14-1, in overcoming letrozole resistance in long-term letrozole cultured (LTLC) cells. Compared with MCF-7 cells stably transfected with aromatase (MCF-7Ca), LTLC cells were no longer sensitive to growth inhibition by aromatase inhibitors. The HER-2/phosphorylated mitogen-activated protein kinase (pMAPK) growth factor signaling pathways were activated, and ERalpha and coactivator amplified in breast cancer 1 (AIB1) were up-regulated approximately 3-fold in LTLC cells. VN/14-1 inhibited aromatase activity and growth values of in MCF-7Ca cells with IC(50) of 8.5 and 10.5 nmol/L, respectively. In human placental microsomes, aromatase activity was inhibited with IC(50) of 8.0 pmol/L. The IC(50) in LTLC cells was 0.83 nmol/L, similar to letrozole (IC(50), 0.3 nmol/L) in MCF-7Ca cells. LTLC cells were 10-fold more sensitive to growth inhibition by VN/14-1 than MCF-7Ca cells. VN/14-1 treatment effectively down-regulated ERalpha, AIB1, pMAPK, HER-2, cyclin D1, cyclin-dependent kinase 4 (CDK4), and Bcl2 and up-regulated cytokeratins 8/18, Bad, and Bax. Tumor growth of LTLC cells in ovariectomized nude mice was independent of estrogens but was inhibited by VN/14-1 (20 mg/kg/d; P < 0.002). Decreases in ERalpha, cyclin D1, CDK4, and pMAPK and up-regulation of cytokeratins, Bad, and Bax with VN/14-1 in tumor samples may be responsible for the efficacy of this compound in inhibiting LTLC cell growth in vitro and in vivo.

Sensitivity to pertuzumab (2C4) in ovarian cancer models: cross-talk with estrogen receptor signaling

Pertuzumab (Omnitarg, rhuMab 2C4) is a humanized monoclonal antibody, which inhibits HER2 dimerization. Because it has shown some clinical activity in ovarian cancer, this study sought to identify predictors of response to this agent in a model of ovarian cancer. A panel of 13 ovarian cancer cell lines was treated with heregulin beta1 (HRGbeta1) or transforming growth factor-alpha, and cell proliferation was assessed. Both agents increased cell number in the majority of cell lines studied, the response to both being similar (r = 0.83; P = 0.0004, Pearson test). HRGbeta1 stimulation could be partially reversed by pertuzumab in 6 of 13 cell lines, with complete reversal in PE04 and PE06 cells. Addition of pertuzumab to transforming growth factor-alpha-stimulated cells produced growth inhibition in 3 of 13 cell lines (PE01, PE04, and PE06). The magnitude of HRGbeta1-driven growth stimulation correlated significantly with an increase in extracellular signal-regulated kinase 2 (P = 0.037) but not Akt (P = 0.99) phosphorylation. Such HRGbeta1-driven phosphorylation of extracellular signal-regulated kinase 1/2 and Akt could be reduced with pertuzumab, accompanied by changes in cell cycle distribution. In cell lines responsive to pertuzumab, HRGbeta1-enhanced phosphorylation of HER2 (Tyr(877)) was reduced. Estrogen-stimulated changes in growth, cell cycle distribution, and signaling were reversed by pertuzumab, indicating cross-talk between HER2 and estrogen signaling. These data indicate that there is a subset of ovarian cancer cell lines sensitive to pertuzumab and suggest possible predictors of response to identify patients who could benefit from this therapy. Furthermore, we have identified an interaction between HER2 and estrogen signaling in this disease.

Estrogen receptors in pancreatic tumors

OBJECTIVES

Although a potential role for estrogen receptors (ER) in pancreatic tumors has been debated for many years, the importance of the receptors in these neoplasms remains unknown. Even the expression of the 2 ER isoforms, ER-alpha and ER-beta, in histological subtypes of pancreatic neoplasms is controversial. The aim of the present study was to systematically review the available literature about ER expression in pancreatic tumors and to discuss the potential importance of estrogen signaling in them.

METHODS

We performed a comprehensive literature search and analyzed the results regarding ER expression in pancreatic tumors, with special emphasis on the specificity of the antibodies used for immunohistochemistry.

RESULTS

Many articles have been published investigating the expression of ERs in pancreatic tumors, but the results are inconsistent. Moreover, most studies used antibodies that detected only ER-alpha, not ER-beta. Thus, the expression pattern of ER-beta in pancreatic neoplasm remains especially unclear.

CONCLUSIONS

The lack of detailed studies evaluating the expression of both ER-alpha and ER-beta receptors using isoform-specific antibodies likely contributes to the inconsistency of published results concerning ER expression in pancreatic tumors. Available published evidence suggests that a thorough reexamination of the potential role of ERs in pancreatic neoplasms is warranted.

Insulin-like growth factor-I, insulin-like growth factor binding protein-3 and the risk of fibrocystic breast conditions among Chinese women

We investigated whether circulating insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding protein-3 (IGFBP-3) levels are associated with the risk of fibrocystic breast conditions (FBC), in a case-control study nested within a randomized trial of breast self-examination conducted in Shanghai, China. Participants were enrolled during 1989-1991 and were followed over 10 years for the development of breast diseases. Controls (n = 897) were frequency-matched by age to cases (n = 451), who were diagnosed with FBC between 1995 and 2000. Circulating IGF-I and IGFBP-3 levels and their molar ratio were positively associated with risk of FBC. The odds ratios (ORs) and 95% confidence intervals (CI) for the upper fourth of the distribution compared to the lowest fourth for IGF-I, IGFBP3 and their molar ratio were 3.02 (2.02-4.52), 1.92 (1.37-2.71) and 2.26 (1.52-3.36), respectively. The strength of the association between IGF-I levels and FBC was attenuated after adjustment for IGFBP-3 and that for IGFBP-3 was largely eliminated after adjustment for IGF-I. Increasing levels of IGF-I were particularly associated with increasing risk of FBC with proliferative elements (ORs and 95% CIs for the 2nd, 3rd and upper fourth of the distribution of IGF-I: 3.13 (1.50-6.53), 4.57 (2.22-9.39) and 6.30 (3.08-12.89), compared with the lowest fourth. Our results suggest that elevated levels of IGF-I may contribute to the development of FBC.

Estrogen receptors in membrane lipid rafts and signal transduction in breast cancer

Regulation of breast cancer growth by estrogen is mediated by estrogen receptors (ER) in nuclear and extranuclear compartments. We assessed the structure and functions of extranuclear ER that initiate downstream signaling to the nucleus. ER, including full-length 66-kDa ER and a 46-kDa ER splice variant, are enriched in lipid rafts from MCF-7 cells with (MCF-7/HER-2) or without (MCF-7/PAR) HER-2 gene overexpression and co-localize with HER-1 and HER-2 growth factor receptors, as well as with lipid raft marker flotillin-2. In contrast, ER-negative MCF-7 cells do not express nuclear or lipid raft ER. ER knockdown with siRNA also elicits a marked loss of ER in MCF-7 cell rafts. In MCF-7/PAR cells, estrogen enhances ER association with membrane rafts and induces rapid phosphorylation of nuclear receptor coactivator AIB1, actions not detected in ER-negative cells. Thus, nuclear and lipid raft ER derive from the same transcript, and extranuclear ER co-localizes with HER receptors in membrane signaling domains that modulate downstream nuclear events leading to cell growth.

Serum HER-2/neu conversion to positive at the time of disease progression in patients with breast carcinoma on hormone therapy

BACKGROUND

Prolonged exposure of breast carcinoma cells in vitro to tamoxifen results in tamoxifen resistance. Tamoxifen-resistant cells express increased HER-2/neu mRNA and protein. The objective of this study was to determine whether patients with metastatic or locally advanced breast carcinoma who have negative serum HER-2/neu status at the initiation of first-line hormone therapy with letrozole or tamoxifen convert to positive serum HER-2/neu status at the time of disease progression and to determine whether serum HER-2/neu conversion to positive status is associated with response to therapy and overall survival.

METHODS

Serum samples were obtained at baseline and at the time of disease progression from 240 patients who initially had negative serum HER-2/neu status (< 15 ng/mL). A manual microtiter, enzyme-linked immunosorbent assay that was specific for the extracellular domain of the HER-2/neu (c-erbB-2) oncoprotein product was used to quantitate serum levels.

RESULTS

Among 240 patients, 61 patients (26%) converted from serum HER-2/neu negative to positive (> 15 ng/mL) at the time of disease progression. Thirty-two of 129 patients (25%) who were treated with tamoxifen and 29 of 111 patients (26%) who were treated with letrozole became converted to positive serum HER-2/neu status at the time of disease progression. The response rate and the time to disease progression on first-line hormone therapy were not affected by serum HER-2/neu conversion. The survival of patients who converted to positive serum HER-2/neu status was significantly shorter compared with the survival of patients who remained negative for serum HER-2/neu. A multivariate analysis revealed that conversion to positive serum HER-2/neu status was an independent prognostic variable for survival.

CONCLUSIONS

Conversion to positive serum HER-2/neu status occurred in approximately 25% of patients who received first-line hormone therapy. Conversion to serum HER-2/neu-positive status occurred with equal frequency in antiestrogen and aromatase-inhibitor therapy. The current results showed that serum conversion to HER-2/neu-positive status was an independent risk factor for decreased survival in patients with breast carcinoma.

Estrogen receptor-positive, progesterone receptor-negative breast cancer: association with growth factor receptor expression and tamoxifen resistance

BACKGROUND

Clinical data indicate that estrogen receptor-positive/progesterone receptor-negative (ER+/PR-) breast cancers are less sensitive to tamoxifen than are ER+/PR+ tumors. It has also been reported that tamoxifen may be less effective in tumors that overexpress either HER-2 or HER-1 (epidermal growth factor receptor) and that signaling through these receptors reduces PR expression in experimental models. We hypothesized that ER+/PR- breast tumors are more likely than ER+/PR+ breast tumors to have an aggressive phenotype, to express HER-1 and overexpress HER-2, and are less likely to benefit from tamoxifen adjuvant therapy.

METHODS

Clinical and biological features of 31 415 patients with ER+/PR+ tumors were compared with those of 13,404 patients with ER+/PR- tumors. Association between disease-free survival (DFS) and HER-1 and HER-2 status was analyzed in a subset of 11,399 patients receiving adjuvant tamoxifen therapy. Hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated using Cox regression or Kaplan-Meier analyses, and all statistical tests were two-sided.

RESULTS

ER+/PR- tumors were more frequent in older patients, were larger in size, had a higher S-phase fraction, and were more likely to be aneuploid than ER+/PR+ tumors. Furthermore, three times as many ER+/PR- tumors as ER+/PR+ tumors expressed HER-1 (25% versus 8%; P < .001) and 50% more overexpressed HER-2 (21% versus 14%; P < .001). Among all tamoxifen-treated women, recurrence was higher among women with HER-1-expressing tumors than with HER-1-negative tumors (HR = 1.9, 95% CI = 1.0 to 3.5; P = .05); a stronger association between worse DFS and HER-2 overexpression was observed (HR = 2.3, 95% CI = 1.2 to 4.3; P = .006). However, results varied by PR status. Among tamoxifen-treated women with ER+/PR+ tumors, HER-1 or HER-2 status was not associated with worse DFS. Among women with ER+/PR- tumors, however, both HER-1 expression (HR = 2.4, 95% CI = 1.0 to 5.4; P = .036) and HER-2 overexpression (HR = 2.6, 95% CI = 1.1 to 6.0; P = .022) were associated with a higher likelihood of recurrence.

CONCLUSIONS

ER+/PR- tumors express higher levels of HER-1 and HER-2 and display more aggressive features than ER+/PR+ tumors. As in laboratory models, lack of PR expression in ER+ tumors may be a surrogate marker of aberrant growth factor signaling that could contribute to the tamoxifen resistance observed in these tumors.

Roles for neuregulins in human cancer

The human epidermal growth factor (EGF) receptor (HER) family of receptor tyrosine kinases has frequently been implicated in cancer. Apart from overexpression or mutation of these receptors, also the aberrant autocrine or paracrine activation of HERs by EGF-like ligands may be important in cancer progression. Neuregulins constitute a family of EGF-like ligands that bind to HER3 or HER4, preferably forming heterodimers with the orphan receptor HER2. Mesenchymal neuregulin typically serves as a pro-survival and pro-differentiation signal for adjacent epithelia. Disruption of the balance between proliferation and differentiation, because of autocrine production by the epithelial cells, increased sensitivity to paracrine signals or disruption of the spatial organization, may lead to constitutive receptor activation, in the absence of receptor overexpression. Consequently, the analysis of ligand expression and/or activated receptors in tumor samples may broaden the group of patients that can benefit from targeted therapies.

Activation of mitogen-activated protein kinase in xenografts and cells during prolonged treatment with aromatase inhibitor letrozole

Ovariectomized mice bearing tumor xenografts grown from aromatase-transfected estrogen receptor (ER)-positive human breast cancer cells (MCF-7Ca) were injected s.c. with 10 microg/d letrozole for up to 56 weeks. Western blot analysis of the tumors revealed that ERs (ERalpha) were increased at 4 weeks but decreased at weeks 28 and 56. Expression of erbB-2 and p-Shc increased throughout treatment, whereas growth factor receptor binding protein 2 (Grb2) increased only in tumors proliferating on letrozole (weeks 28 and 56). In cells isolated from tumors after 56 weeks and maintained as a cell line (LTLT-Ca) in 1 micromol/L letrozole, ERalpha was also decreased whereas erbB-2, adapter proteins (p-Shc and Grb2), and the signaling proteins in the mitogen-activated protein kinase (MAPK) cascade were increased compared with MCF-7Ca cells. Growth was inhibited in LTLT-Ca cells but not in MCF-7Ca cells treated with MAPK kinase 1/2 inhibitors U0126, and PD98059 (IC(50) approximately 25 micromol/L). PD98059 (5 micromol/L) also reduced MAPK activity and increased ERalpha to the levels in MCF-7Ca cells. Epidermal growth factor receptor kinase inhibitor, gefitinib (ZD1839) inhibited growth of LTLT-Ca cells (IC(50) approximately 10 micromol/L) and restored their sensitivity to tamoxifen and anastrozole. In xenografts, combined treatment with ER down-regulator fulvestrant and letrozole, prevented increases in erbB-2 and activation of MAPK and was highly effective in inhibiting tumor growth throughout 29 weeks of treatment. These results indicate that blocking both ER- and growth factor-mediated transcription resulted in the most effective inhibition of growth of ER-positive breast cancer cells.

Hormone Receptor Status Does Not Affect the Clinical Benefit of Trastuzumab Therapy for Patients with Metastatic Breast Cancer

BACKGROUND

Hormone receptor (HR) and HER2 signaling pathways are involved in the regulation of breast cancer proliferation. The impact of HR status on the clinical outcome of patients with HER2-overexpressing disease treated with the monoclonal antibody trastuzumab is unknown.

PATIENTS AND METHODS

To evaluate this, we conducted a retrospective analysis of 805 patients with metastatic breast cancer enrolled in 3 clinical trials comparing trastuzumab in combination with chemotherapy versus chemotherapy alone or trastuzumab monotherapy as first-, second-, or third-line treatment. Patients whose tumor samples overexpressed HER2 by fluorescence in situ hybridization (FISH) were stratified based on HR status, and clinical outcomes were compared.

RESULTS

Tumor samples from 596 of 805 patients were HER2overexpressing by FISH; 45% of these were HR-positive and 43% were HR-negative (HR status was unknown in 12%). Overall response rate (ORR) and time to progression (TTP) were significantly higher in patients treated with chemotherapy plus trastuzumab than in those treated with chemotherapy alone, irrespective of HR status. Median survival was longer for patients with HR-positive tumors receiving combination therapy compared with those with HR-negative tumors. In the trastuzumab monotherapy trials, ORR and TTP were similar for patients with HR-positive and HR-negative tumors. Median survival was longer for patients with HR-positive tumors compared with those with HR-negative tumors.

CONCLUSION

Hormone receptor status did not affect the clinical benefit of trastuzumab given as a single agent or combined with chemotherapy. The addition of trastuzumab to chemotherapy provides an improved clinical benefit compared with chemotherapy alone, regardless of HR status.

The role of growth factor receptor pathways in human breast cancer cells adapted to long-term estrogen deprivation

To study the long-term effects of estrogen deprivation on breast cancer, MCF-7Ca human estrogen receptor-positive breast cancer cells stably transfected with human aromatase gene were cultured in the steroid-depleted medium for 6 to 8 months until they had acquired the ability to grow. Proliferation of these cells (UMB-1Ca) was accompanied by increased expression of human epidermal growth factor receptor 2, increased activation of AKT through phosphorylation at Ser473 and Thr308, and increased invasion compared with parental MCF-7Ca cells. Estrogen receptor expression was also increased 5-fold. Although growth was inhibited by the antiestrogen fulvestrant, the IC50 was 100-fold higher than for parental MCF-7Ca cells. Aromatase inhibitor letrozole also inhibited growth at 10,000-fold higher concentration than required for MCF-7Ca cells, whereas anastrozole, exemestane, formestane, and tamoxifen were ineffective at 100 nmol/L. Growth of UMB-1Ca cells was inhibited by phosphatidylinositol 3-kinase inhibitor wortmannin (IC50 approximately 25 nmol/L) and epidermal growth factor receptor kinase inhibitor gefitinib (ZD 1839; IC50 approximately 10 micromol/L) whereas parental MCF-7Ca cells were insensitive to these agents. Concomitant treatment of UMB-1Ca cells with the signal transduction inhibitors and anastrozole and tamoxifen restored their growth inhibitory effects. These studies show that estrogen deprivation results in up-regulation of growth factor signaling pathways, which leads to a more aggressive and hormone refractory phenotype. Cross-talk between ER and growth factor signaling was evident as inhibition of these pathways could restore estrogen responsiveness to these cells.

Upregulation of PKC-delta contributes to antiestrogen resistance in mammary tumor cells

Acquired resistance to tamoxifen (Tam) in breast cancer patients is a serious therapeutic problem. We have previously reported that protein kinase C-delta (PKC-delta) plays a major role in estrogen (E2)-mediated cell proliferation. To determine if PKC-delta is one of the major alternate signaling pathways that supports cell growth in the presence of Tam, we determined the levels of PKC isoforms in four different models of antiestrogen-resistant cells. Three out of four antiestrogen resistance cell lines (Tam/MCF-7, ICI/MCF-7 and HER-2/MCF-7) expressed significantly high levels of both total and activated PKC-delta levels compared to sensitive cells. Estrogen receptor (ER) alpha content and function are maintained in all the antiestrogen-resistant cell lines. Overexpressing active PKC-delta in Tam-sensitive MCF-7 cells (PKC-delta/MCF-7) led to Tam resistance both in vitro and in vivo. Inhibition of PKC-delta by rottlerin (a relatively specific inhibitor of PKC-delta) or siRNA significantly inhibited estrogen- and Tam-induced growth in antiestrogen-resistant cells. PKC-delta levels are significantly higher in Tam-resistant tumors compared to Tam-sensitive tumors in xenograft model (P<0.05). Taken together, these data suggest that PKC-delta plays a major role in antiestrogen resistance in breast tumor cells and thus provides a new target for treatment.

Characterization of a human breast cancer cell line, MCF-7/RU58R-1, resistant to the pure antiestrogen RU 58,668

Breast cancer is the most common cancer disease in women in the western world. Tamoxifen has been the standard first line endocrine therapy for patients with estrogen receptor (ER) positive tumors. Unfortunately, almost all patients with advanced disease develop tamoxifen resistance. This has lead to a search for new potent antiestrogens. One of the new compounds under development is the pure antiestrogen RU 58,668. To study the mechanisms behind acquired resistance to RU 58,668, the RU 58,668-resistant cell line MCF-7/RU58(R)-1 (RU58(R)-1) was developed. The RU58(R)-1 cell line was responsive to tamoxifen, but cross-resistant to ICI 182,780 and the estrogen-sensitivity was reduced compared to the parental MCF-7 cell line. The protein levels of ERalpha, IGF-I Receptor (IGF-IR) and Bcl-2 were severely reduced, when RU58(R)-1 cells were cultured with RU 58,668 and the expression of progesterone receptor (PR) was lost. The ERalpha level increased upon withdrawal of RU 58,668 and the ERalpha protein was destabilized by RU 58,668 in both cell lines. Regulation of most of the investigated estrogen-sensitive mRNAs was found to be normal in the resistant cells. The protein levels of IGF-IR, Bcl-2 and the IGF Binding Protein 2 (IGFBP2) reverted towards MCF-7 levels upon RU 58,668 withdrawal, but the resistant phenotype was maintained. Thus, it appears as acquired resistance to RU 58,668 is not a result of loss of the ERalpha expression or function and we suggest that in the presence of RU 58,668, the RU58(R)-1 cell line probably uses other mitogenic pathways than the ERalpha pathway for growth and survival.

Expression of insulin-like growth factor 1 receptor in primary breast cancer: immunohistochemical analysis

Insulin-like growth factor-1 receptor (IGF-1R) has been implicated in regulation in tumor growth. The results of previous studies performed by radioimmunoassay are conflicting, and the prognostic significance of IGF-1R expression in primary breast cancer is still controversial. IGF-1R expression was evaluated in formalin-fixed, paraffin-embedded tissue of 210 primary breast cancer patients by using anti-IGF-1R antibody. The clinicopathologic variables and 5-year disease-free survival were studied, and their correlations between IGF-1R expressions were investigated. IGF-1R overexpression was observed in 43.8% of tumors. IGF-1R overexpression had no correlation with prognosis or with other clinicopathologic parameters, such as age, tumor size, nodal status, histologic grade, hormone receptor status, and human epidermal growth factor 2 status. Though its prognostic value in breast cancer is limited, immunohistochemical evaluation of IGF-1R by using this monoclonal antibody may be useful in translational research using archived material.

Growth factor signalling networks in breast cancer and resistance to endocrine agents: new therapeutic strategies

Recent evidence demonstrates that growth factor networks are highly interactive with the estrogen receptor (ER) in the control of breast cancer growth and development. As such, tumor responses to anti-hormones are likely to be a composite of the ER and growth factor inhibitory activity of these agents, with alterations/aberrations in growth factor signalling providing a mechanism for the development of anti-hormone resistance. In this light, the current article focuses on illustrating the relationship between growth factor signalling and anti-hormone failure in our in-house tumor models of breast cancer and describes how we are now beginning to successfully target their actions to improve the effects of anti-hormonal drugs and to block aggressive disease progression.

Estrogen/EGF receptor interactions in breast cancer: rationale for new therapeutic combination strategies

In the therapy of estrogen receptor (ER) positive human mammary carcinomas, the treatment with the antiestrogen tamoxifen has been well established. However, the development of hormone resistance is an important factor in breast cancer progression against endocrine therapy. The presence of the receptor for EGF (EGFR) correlates with lack of response towards antiestrogen therapy. The EGFR is not only involved in tumor cell growth, survival signaling, cell migration, metastasis formation and angiogenesis, but also seems to confer reduced responses of tumor cells towards anti-hormones. Concomitant inhibition of both, the receptors for estrogen and EGF may be necessary to improve breast cancer therapy.

The biology of antihormone failure in breast cancer

Many estrogen receptor-positive breast cancer patients initially respond to treatment with antihormonal agents such as tamoxifen, but remissions are often followed by acquisition of resistance and ultimately disease relapse. The development of a rationale for the effective treatment of tamoxifen-resistant breast cancer requires an understanding of the complex signal transduction mechanisms that contribute towards loss of antiestrogen response. Interactions between estrogen and growth factor signaling pathways have been identified in estrogen-responsive cells that are thought to reinforce their individual cellular effects on growth and gene responses. Increasing evidence indicates that abnormalities occurring in growth factor signaling pathways, notably the epidermal growth factor receptor (EGFR) signaling pathway, could dramatically influence steroid hormone action and may be critical to antihormonal-resistant breast cancer cell growth. Thus, inhibitory agents targeting growth factor receptors, or their intracellular pathway components, may prove clinically beneficial in antihormone refractory disease. One example, gefitinib ('Iressa', ZD1839), an EGFR-tyrosine kinase inhibitor, is an interesting therapeutic option that may provide benefit in the treatment of antihormonal-resistant breast cancer. Rapid progress with pharmacological and molecular therapeutic agents is now being made. Therapies that target growth factor signaling pathways may prevent development of resistance.