ESR1 mutations and therapeutic resistance in metastatic breast cancer: progress and remaining challenges

Breast cancer accounts for 25% of the cancers in women worldwide. The most common subtype of breast cancer diagnosed is hormone receptor positive, which expresses the oestrogen receptor (ER). Targeting of the ER with endocrine therapy (ET) is the current standard of care for ER-positive (ER+) breast cancer, reducing the mortality by up to 40%. Resistance to ET, however, remains a major issue for ER + breast cancer, leading to recurrence and metastasis. One major driver of ET resistance is mutations in the ER gene (ESR1) leading to constitutive transcriptional activity and reduced ET sensitivity. These mutations are particularly detrimental in metastatic breast cancer (MBC) as they are present in as high as 36% of the patients. This review summarises the pre-clinical characterisation of ESR1 mutations and their association with clinical outcomes in MBC and primary disease. The clinically approved and investigational therapeutic options for ESR1 mutant breast cancer and the current clinical trials evaluating ESR1 mutations and ET resistance are also discussed. Finally, this review addresses pre-clinical models and multi-'omics' approaches for developing the next generation of therapeutics for ESR1 mutant and ET-resistant breast cancer.

Metastasis: complexity thwarts precision targeting

Metastasis is the spread of cancer cells to new areas of the body by way of the lymph system or bloodstream. Mechanism-based therapeutics have transformed its treatment. This issue of British Journal of Cancer will highlight recent advances in our understanding of metastasis, and how to block its spread.

Hormonal modulation of ESR1 mutant metastasis

Estrogen receptor alpha gene (ESR1) mutations occur frequently in ER-positive metastatic breast cancer, and confer clinical resistance to aromatase inhibitors. Expression of the ESR1 Y537S mutation induced an epithelial-mesenchymal transition (EMT) with cells exhibiting enhanced migration and invasion potential in vitro. When small subpopulations of Y537S ESR1 mutant cells were injected along with WT parental cells, tumor growth was enhanced with mutant cells becoming the predominant population in distant metastases. Y537S mutant primary xenograft tumors were resistant to the antiestrogen tamoxifen (Tam) as well as to estradiol (E2) withdrawal. Y537S ESR1 mutant primary tumors metastasized efficiently in the absence of E2; however, Tam treatment significantly inhibited metastasis to distant sites. We identified a nine-gene expression signature, which predicted clinical outcomes of ER-positive breast cancer patients, as well as breast cancer metastasis to the lung. Androgen receptor (AR) protein levels were increased in mutant models, and the AR agonist dihydrotestosterone significantly inhibited estrogen-regulated gene expression, EMT, and distant metastasis in vivo, suggesting that AR may play a role in distant metastatic progression of ESR1 mutant tumors.

Estrogen-induced transcription at individual alleles is independent of receptor level and active conformation but can be modulated by coactivators activity

Steroid hormones are pivotal modulators of pathophysiological processes in many organs, where they interact with nuclear receptors to regulate gene transcription. However, our understanding of hormone action at the single cell level remains incomplete. Here, we focused on estrogen stimulation of the well-characterized GREB1 and MYC target genes that revealed large differences in cell-by-cell responses, and, more interestingly, between alleles within the same cell, both over time and hormone concentration. We specifically analyzed the role of receptor level and activity state during allele-by-allele regulation and found that neither receptor level nor activation status are the determinant of maximal hormonal response, indicating that additional pathways are potentially in place to modulate cell- and allele-specific responses. Interestingly, we found that a small molecule inhibitor of the arginine methyltransferases CARM1 and PRMT6 was able to increase, in a gene specific manner, the number of active alleles/cell before and after hormonal stimulation, suggesting that mechanisms do indeed exist to modulate hormone receptor responses at the single cell and allele level.

ESR1 mutations in breast cancer

The acquisition of ligand-independent ESR1 mutations during aromatase inhibitor therapy in metastatic estrogen receptor (ER)-positive breast cancer is a common mechanism of hormonal therapy resistance. Preclinical and clinical studies have demonstrated that ESR1 mutations can preexist in primary tumors and can be enriched during metastasis. Furthermore, ESR1 mutations express a unique transcriptional profile that favors tumor progression, suggesting that selected ESR1 mutations may influence metastasis. Several groups have used sensitive detection methods using patient liquid biopsies to track ESR1 or truncal somatic mutations to predict treatment outcome and tumor progression, and some of these techniques may eventually be used to guide sequential treatment options in patients. Further development and standardization of mutation tracking in circulating tumor DNA is ongoing. Clinically, patients with ESR1 mutations derive clinical benefit when treated with fulvestrant and CDK4/6-targeted therapies, but the development of more potent selective ER degraders and/or new targeted biotherapies are needed to overcome the endocrine-resistant phenotype of ESR1 mutant-bearing tumors. In this review, we discuss the mechanisms of resistance and dissemination of ESR1 mutations as well as the detection methods for ESR1 mutation tracking, newly discovered potential therapeutic targets, and the clinical implications and treatment options for treating patients with ESR1 mutant-bearing tumors.

A combinatorial biomarker predicts pathologic complete response to neoadjuvant lapatinib and trastuzumab without chemotherapy in patients with HER2+ breast cancer

BACKGROUND

HER2-positive (+) breast cancers, defined by HER2 overexpression and/or amplification, are often addicted to HER2 to maintain their malignant phenotype. Yet, some HER2+ tumors do not benefit from anti-HER2 therapy. We hypothesize that HER2 amplification levels and PI3K pathway activation are key determinants of response to HER2-targeted treatments without chemotherapy.

PATIENTS AND METHODS

Baseline HER2+ tumors from patients treated with neoadjuvant lapatinib plus trastuzumab [with endocrine therapy for estrogen receptor (ER)+ tumors] in TBCRC006 (NCT00548184) were evaluated in a central laboratory for HER2 amplification by fluorescence in situ hybridization (FISH) (n = 56). HER2 copy number (CN) and FISH ratios, and PI3K pathway status, defined by PIK3CA mutations or PTEN levels by immunohistochemistry were available for 41 tumors. Results were correlated with pathologic complete response (pCR; no residual invasive tumor in breast).

RESULTS

Thirteen of the 56 patients (23%) achieved pCR. None of the 11 patients with HER2 ratio <4 and/or CN <10 achieved pCR, whereas 13/45 patients (29%) with HER2 ratio ≥4 and/or CN ≥10 attained pCR (P = 0.0513). Of the 18 patients with tumors expressing high PTEN or wild-type (WT) PIK3CA (intact PI3K pathway), 7 (39%) achieved pCR, compared with 1/23 (4%) with PI3K pathway alterations (P = 0.0133). Seven of the 16 patients (44%) with HER2 ratio ≥4 and intact PI3K pathway achieved pCR, whereas only 1/25 (4%) patients not meeting these criteria achieved pCR (P = 0.0031).

CONCLUSIONS

Our findings suggest that there is a clinical subtype in breast cancer with high HER2 amplification and intact PI3K pathway that is especially sensitive to HER2-targeted therapies without chemotherapy. A combination of HER2 FISH ratio and PI3K pathway status warrants validation to identify patients who may be treated with HER2-targeted therapy without chemotherapy.

Proteomic profiling identifies key coactivators utilized by mutant ERα proteins as potential new therapeutic targets

Approximately 75% of breast cancers are estrogen receptor alpha (ERα)-positive and are treatable with endocrine therapies, but often patients develop lethal resistant disease. Frequent mutations (10–40%) in the ligand-binding domain (LBD) codons in the gene encoding ERα (ESR1) have been identified, resulting in ligand-independent, constitutively active receptors. In addition, ESR1 chromosomal translocations can occur, resulting in fusion proteins that lack the LBD and are entirely unresponsive to all endocrine treatments. Thus, identifying coactivators that bind to these mutant ERα proteins may offer new therapeutic targets for endocrine-resistant cancer. To define coactivator candidate targets, a proteomics approach was performed profiling proteins recruited to the two most common ERα LBD mutants, Y537S and D538G, and an ESR1-YAP1 fusion protein. These mutants displayed enhanced coactivator interactions as compared to unliganded wild-type ERα. Inhibition of these coactivators decreased the ability of ESR1 mutants to activate transcription and promote breast cancer growth in vitro and in vivo. Thus, we have identified specific coactivators that may be useful as targets for endocrine-resistant breast cancers.

The Impact of ESR1 Mutations on the Treatment of Metastatic Breast Cancer

After nearly 20 years of research, it is now established that mutations within the estrogen receptor (ER) gene, ESR1, frequently occur in metastatic breast cancer and influence response to hormone therapy. Though early studies presented differing results, sensitive sequencing techniques now show that ESR1 mutations occur at a frequency between 20 and 40% depending on the assay method. Recent studies have focused on several "hot spot mutations," a cluster of mutations found in the hormone-binding domain of the ESR1 gene. Throughout the course of treatment, tumor evolution can occur, and ESR1 mutations emerge and become enriched in the metastatic setting. Sensitive techniques to continually monitor mutant burden in vivo are needed to effectively treat patients with mutant ESR1. The full impact of these mutations on tumor response to different therapies remains to be determined. However, recent studies indicate that mutant-bearing tumors may be less responsive to specific hormonal therapies, and suggest that aromatase inhibitor (AI) therapy may select for the emergence of ESR1 mutations. Additionally, different mutations may respond discretely to targeted therapies. The need for more preclinical mechanistic studies on ESR1 mutations and the development of better agents to target these mutations are urgently needed. In the future, sequential monitoring of ESR1 mutational status will likely direct personalized therapeutic regimens appropriate to each tumor's unique mutational landscape.

Abstract P4-04-08: Therapeutic strategy for ERα mutation driven-endocrine resistance in ER-positive breast cancers

Background: Although estrogen receptor (ER)-positive breast cancers are treated with endocrine therapy, 25% of these patients are at risk of relapse and the development of acquired endocrine resistance. Recently mutations in the ER gene (ESR1) have been identified which induce resistance to endocrine therapy. The most frequent ESR1 mutation, Y537S, promotes ligand-independent ER activity. It is known that ER regulates the cell cycle in a ligand-dependent manner. In this study, we examined the effects of the Y537S ESR1 mutation on cell cycle signaling and therapeutic response to checkpoint inhibitor.

Material and Methods: MCF-7 cells expressing the Y537S ESR1 mutation were generated by CRISPR-Cas9 knock-in techniques. Cells were incubated in phenol red minus medium containing 5% charcoal-dextran treated serum for 5 days to remove exogenous hormones. Cell cycle and apoptosis were examined by Flow cytometry and Annexin-V assays. Proliferation was analyzed by BrdU incorporation. Cell senescence was determined using beta-galactosidase assays. Cell cycle checkpoint kinases were examined by western blot analysis. Cell growth was analyzed using soft agar or MTT assays.

Results: Levels of p53 and apoptosis pathway proteins were significantly elevated in Y537S ESR1 mutant cells using RNA expression and reverse-phase protein microarrays. The ATM/ATR and Chk1/Chk2 mediated checkpoint signaling, the upstream pathway of p53, was activated in ESR1 Y537S mutation, which was repressed with fulvestrant treatment. ESR1 Y537 mutant cells accumulated about 5 fold in S phase and 1.7 fold in G2/M phase compared to control cells in estrogen deprived (ED) condition. BrdU incorporation was also increased about 2.5 fold compared with parental cells in estrogen-free medium. In addition, ESR1 Y537 mutant cells expressed a DNA double-strand break marker, gamma-H2AX protein in ED condition. Apoptosis and senescence were observed in ESR1 Y537S mutant cells in regular medium, however, apoptosis was not shown in ED medium. Chk1 inhibitor, PF477736 sensitized MCF-7 expressing ESR1 Y537S mutation to endocrine treatments such as fulvestrant, tamoxifen and AZD9496.

Conclusion: Combination therapy with cell cycle checkpoint kinase inhibitor may lead better prognosis in ER mutation driven-endocrine resistance in postmenopausal breast cancers.

Citation Format: Kim J-A, Fuqua SAW. Therapeutic strategy for ERα mutation driven-endocrine resistance in ER-positive breast cancers [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P4-04-08.

Low PTEN levels and PIK3CA mutations predict resistance to neoadjuvant lapatinib and trastuzumab without chemotherapy in patients with HER2 over-expressing breast cancer

PURPOSE

Aberrant activation of the PI3K pathway has been implicated in resistance to HER2-targeted therapy, but results of clinical trials are confounded by the co-administration of chemotherapy. We investigated the effect of perturbations of this pathway in breast cancers from patients treated with neoadjuvant anti-HER2-targeted therapy without chemotherapy.

PATIENTS AND METHODS

Baseline tumor samples from patients with HER2-positive breast cancer enrolled in TBCRC006 (NCT00548184), a 12-week neoadjuvant clinical trial with lapatinib plus trastuzumab [plus endocrine therapy for estrogen receptor (ER)-positive tumors], were assessed for PTEN status by immunohistochemistry and PIK3CA mutations by sequencing. Results were correlated with pathologic complete response (pCR).

RESULTS

Of 64 evaluable patients, PTEN immunohistochemistry and PIK3CA mutation analysis were performed for 59 and 46 patients, respectively. PTEN status (dichotomized by H-score median) was correlated with pCR (32% in high PTEN vs. 9% in low PTEN, p = 0.04). PIK3CA mutations were identified in 14/46 tumors at baseline (30%) and did not correlate with ER or PTEN status. One patient whose tumor harbored a PIK3CA mutation achieved pCR (p = 0.14). When considered together (43 cases), 1/25 cases (4%) with a PIK3CA mutation and/or low PTEN expression levels had a pCR compared to 7/18 cases (39%) with wild-type PI3KCA and high PTEN expression levels (p = 0.006).

CONCLUSION

PI3K pathway activation is associated with resistance to lapatinib and trastuzumab in breast cancers, without chemotherapy. Further studies are warranted to investigate how to use these biomarkers to identify upfront patients who may respond to anti-HER2 alone, without chemotherapy.

Estrogen receptors in breast and bone: from virtue of remodeling to vileness of metastasis

Bone metastasis is a prominent cause of morbidity and mortality in cancer. High rates of bone colonization in breast cancer, especially in the subtype expressing estrogen receptors (ERs), suggest tissue-specific proclivities for metastatic tumor formation. The mechanisms behind this subtype-specific organ-tropism remains largely elusive. Interestingly, as the major driver of ER+ breast cancer, ERs also have important roles in bone development and homeostasis. Thus, any agents targeting ER will also inevitably affect the microenvironment, which involves the osteoblasts and osteoclasts. Yet, how such microenvironmental effects are integrated with direct therapeutic responses of cancer cells remain poorly understood. Recent findings on ER mutations, especially their enrichment in bone metastasis, raised even more provocative questions on the role of ER in cancer-bone interaction. In this review, we evaluate the importance of ERs in bone metastasis and discuss new avenues of investigation for bone metastasis treatment based on current knowledge.

Abstract S4-02: The Y537S ESR1 mutation is a dominant driver of distant ER-positive breast cancer metastasis

Background: Estrogen receptor (ESR1) mutations occur at a high frequency in metastatic breast tumors in patients treated with hormonal therapy in the metastatic setting. We do not know if these mutations are involved in metastasis.

Experimental design and methods: We generated ESR1 Y537S homozygous mutations using CRISPR Casp-9 technology. Treatment synergy was evaluated using Compusyn. Athymic mice were used in tumor xenograft studies. ChIP-Seq and transcriptome analyses were performed.

Results: We generated CRISPR ESR1 Y537S mutation homozygous knock-in clones and lentiviral stable pools in MCF-7 cells. Transcriptome profiling revealed elevated expression of Hallmark pathways, including EMT and estrogen-regulated gene expression. The EMT in mutant cells was associated with a switch from E-cadherin to vimentin, and increased expression of SNAIL and TWIST. Mutant cell growth was resistant to tamoxifen, but responsive to fulvestrant treatment. Synergistic treatment effects were observed with fulvestrant and everolimus or palbociclib. CRISPR Y537S mutant knock-in cells grown in the mammary fat-pad of athymic mice spontaneously metastasized to distant organs including the lung, intestine, and kidneys. In the presence of estrogen, there was no difference in the frequency of distant macrometastases between parental wild-type ER and CRISPR Y537S mutant ER mice. However, in the absence of estrogen, 80% of CRISPR Y537S mutant ER mice displayed overt distant macrometastases, but none were observed in parental wild-type ER mice (p=0.04). Interestingly, although CRISPR Y537S mutant ER tumors grown in the mammary fat-pad were unresponsive to tamoxifen treatment, tamoxifen significantly inhibited the growth of mutant tumors at the distant microenvironment (8-fold). Distant tumors retained ER expression and hormone sensitivity. Comparison of residual tamoxifen-treated metastatic tumors with tumors grown at the primary mammary fat-pad site using immunoblot analysis demonstrated significant reduction in estrogen-regulated gene expression, but no effect on the expression of biomarkers associated with EMT, suggesting a disconnect between EMT and distant metastasis in mutant cells. EMT genes were also identified as direct binding site targets in Y537S mutant cells compared with wide-type ER using ChIPSeq. We discovered that expression of the Y537S mutant was dominant, driving the growth of distant metastatic tumors when co-expressed with wild-type ER cells. A Y537S ER mutant-specific gene expression signature predicted poor disease-free survival of ER-positive patients using the METABRIC database, and lung-specific metastasis-free survival in a Memorial Sloan Kettering dataset.

Conclusion: The Y537S ER mutation is a driver of distant metastasis in ER-positive breast cancer cells. Although tamoxifen treatment was ineffective at reducing the growth of mutant cells grown at the primary site, it was effective at reducing distant metastasis. A Y537S ER mutant-specific gene expression signature predicted poor disease-free, and distant lung metastasis in ER-positive patients. Mutation status is a potential new predictive factor for hormone therapy of metastatic breast cancer patients on maintenance hormonal therapy.

Citation Format: Fuqua SAW, Gu G, Rechoum Y, Gelsomino L, Dustin DJ, Corona-Rodriguez A, Beyer AR, Pejerrey SM, Gao M, Tsimelzon A, Tian L, Zhang X, Nagi C, Ando' S. The Y537S ESR1 mutation is a dominant driver of distant ER-positive breast cancer metastasis [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr S4-02.

Tumour and cellular distribution of activated forms of PR in breast cancers: a novel immunohistochemical analysis of a large clinical cohort

Background

The progesterone receptor (PR) is expressed by ∼70% of early breast tumours and is implicated in the progression of breast cancer. In cancerous tissues PR may be activated in the absence of a ligand, or when ligand concentrations are very low, resulting in aberrantly activated PR (APR). The presence of APR may indicate that patients with breast cancer are more likely to respond to antiprogestins. The aims of this study were to describe and classify the histological subnuclear morphology of active and inactive PR in archival breast cancer samples.

Methods

Archived tumour specimens from 801 women with invasive breast cancer were collected. Tissue samples (n=789) were analysed for PR isoforms A and B (PRA and PRB), Ki67 and estrogen receptors (ERα) status, using immunohistochemistry. Medical records were used to determine human epidermal growth factor 2 (HER2) status, tumour stage and grade.

Results

A total of 79% of tumours stained positive for either PRA or PRB, and of these 25% of PRA-positive and 23% of PRB-positive tumours had PR present in the activated form. APRA was associated with higher tumour grade (p=0.001). APRB was associated with a higher tumour grade (p=0.046) and a trend for a more advanced stage. Patients with PR-positive tumours treated with antiestrogens had better disease-free survival (DFS) than those with PR-negative tumours (p<0.0001). Cumulative progression rate and DFS were similar irrespective of APR status. Both APRA and APRB were independent of HER2, ERα and Ki67 expression.

Conclusions

APR had a binary mode of expression in the breast cancer specimens tested, allowing separation into two tumour subsets. APR is an independent target at the cellular and tumour level and may therefore be a suitable predictive marker for antiprogestins, such as onapristone. Using the described technique, a companion diagnostic is under development to identify APR in solid tumours.

Erratum to: Estrogen receptor beta binds Sp1 and recruits a corepressor complex to the estrogen receptor alpha gene promoter

Erratum to: Breast Cancer Res Treat (2012), 134:569–581, DOI 10.1007/s10549-012-2090-9. Uunfortunately, authors could not find the original film from which the figure was drawn. Therefore, as suggested by the Editor, they have repeated the relative experiment, and ask to publish this new figure as a correction. The authors apologize for any inconvenience that it may cause.

ESR1 mutations affect anti-proliferative responses to tamoxifen through enhanced cross-talk with IGF signaling

The purpose of this study was to address the role of ESR1 hormone-binding mutations in breast cancer. Soft agar anchorage-independent growth assay, Western blot, ERE reporter transactivation assay, proximity ligation assay (PLA), coimmunoprecipitation assay, silencing assay, digital droplet PCR (ddPCR), Kaplan-Meier analysis, and statistical analysis. It is now generally accepted that estrogen receptor (ESR1) mutations occur frequently in metastatic breast cancers; however, we do not yet know how to best treat these patients. We have modeled the three most frequent hormone-binding ESR1 (HBD-ESR1) mutations (Y537N, Y537S, and D538G) using stable lentiviral transduction in human breast cancer cell lines. Effects on growth were examined in response to hormonal and targeted agents, and mutation-specific changes were studied using microarray and Western blot analysis. We determined that the HBD-ESR1 mutations alter anti-proliferative effects to tamoxifen (Tam), due to cell-intrinsic changes in activation of the insulin-like growth factor receptor (IGF1R) signaling pathway and levels of PIK3R1/PIK3R3. The selective estrogen receptor degrader, fulvestrant, significantly reduced the anchorage-independent growth of ESR1 mutant-expressing cells, while combination treatments with the mTOR inhibitor everolimus, or an inhibitor blocking IGF1R, and the insulin receptor significantly enhanced anti-proliferative responses. Using digital drop (dd) PCR, we identified mutations at high frequencies ranging from 12 % for Y537N, 5 % for Y537S, and 2 % for D538G in archived primary breast tumors from women treated with adjuvant mono-tamoxifen therapy. The HBD-ESR1 mutations were not associated with recurrence-free or overall survival in response in this patient cohort and suggest that knowledge of other cell-intrinsic factors in combination with ESR1 mutation status will be needed determine anti-proliferative responses to Tam.

ESR1 Mutations in Breast Cancer: Proof-of-Concept Challenges Clinical Action

Wang and colleagues demonstrate that digital droplet PCR (ddPCR) identified ESR1 mutations in 7% of primary breast cancers. ESR1 mutations were also readily detected in metastatic tissues and circulating tumor DNA in the blood. These results suggest that ddPCR may be amendable for monitoring tumor burden, and to predict relapse. See related article by Wang et al., p. 1130.

Phosphatase PTP4A3 Promotes Triple-Negative Breast Cancer Growth and Predicts Poor Patient Survival

Triple-negative breast cancer (TNBC) has the worst prognosis of all breast cancers, and women diagnosed with TNBC currently lack targeted treatment options. To identify novel targets for TNBC, we evaluated phosphatase expression in breast tumors and characterized their contributions to in vitro and in vivo growth of TNBC. Using Affymetrix microarray analysis of 102 breast cancers, we identified 146 phosphatases that were significantly differentially expressed in TNBC compared with estrogen receptor (ER)-positive tumors. Of these, 19 phosphatases were upregulated (0.66-fold; FDR = 0.05) in TNBC compared with ER-positive breast cancers. We knocked down 17 overexpressed phosphatases in four triple-negative and four ER-positive breast cancer lines using specific siRNAs and found that depletion of six of these phosphatases significantly reduced growth and anchorage-independent growth of TNBC cells to a greater extent than ER-positive cell lines. Further analysis of the phosphatase PTP4A3 (also known as PRL-3) demonstrated its requirement for G1-S cell-cycle progression in all breast cancer cells, but PTP4A3 regulated apoptosis selectively in TNBC cells. In addition, PTP4A3 inhibition reduced the growth of TNBC tumors in vivo Moreover, in silico analysis revealed the PTP4A3 gene to be amplified in 29% of basal-like breast cancers, and high expression of PTP4A3 could serve as an independent prognostic indicator for worse overall survival. Collectively, these studies define the importance of phosphatase overexpression in TNBC and lay the foundation for the development of new targeted therapies directed against phosphatases or their respective signaling pathways for TNBC patients. Cancer Res; 76(7); 1942-53. ©2016 AACR.

Role of MTA2 in human cancer

Metastasis is the ultimate cause of death for most cancer patients. While many mechanisms have been delineated for regulation of growth and tumor initiation of the primary tumor, very little is known about the process of metastasis. Metastasis requires dynamic alteration of cellular processes in order for cells to disseminate from the primary tumor to distant sites. These alterations often involve dramatic changes in the regulation of cytoskeletal and cell-environment interactions. Furthermore, controlled refinement of these interactions requires feedback to regulatory networks in the nucleus. MTA2 is a member of the metastasis tumor-associated family of transcriptional regulators and is a central component of the nucleosome remodeling and histone deacetylation complex. MTA2 acts as a central hub for cytoskeletal organization and transcription and provides a link between nuclear and cytoskeletal organization. We will focus on MTA2 in this chapter, especially its role in breast cancer metastasis.

Comprehensive genomic analysis identifies novel subtypes and targets of triple-negative breast cancer

PURPOSE

Genomic profiling studies suggest that triple-negative breast cancer (TNBC) is a heterogeneous disease. In this study, we sought to define TNBC subtypes and identify subtype-specific markers and targets.

EXPERIMENTAL DESIGN

RNA and DNA profiling analyses were conducted on 198 TNBC tumors [estrogen receptor (ER) negativity defined as Allred scale value ≤ 2] with >50% cellularity (discovery set: n = 84; validation set: n = 114) collected at Baylor College of Medicine (Houston, TX). An external dataset of seven publically accessible TNBC studies was used to confirm results. DNA copy number, disease-free survival (DFS), and disease-specific survival (DSS) were analyzed independently using these datasets.

RESULTS

We identified and confirmed four distinct TNBC subtypes: (i) luminal androgen receptor (AR; LAR), (ii) mesenchymal (MES), (iii) basal-like immunosuppressed (BLIS), and (iv) basal-like immune-activated (BLIA). Of these, prognosis is worst for BLIS tumors and best for BLIA tumors for both DFS (log-rank test: P = 0.042 and 0.041, respectively) and DSS (log-rank test: P = 0.039 and 0.029, respectively). DNA copy number analysis produced two major groups (LAR and MES/BLIS/BLIA) and suggested that gene amplification drives gene expression in some cases [FGFR2 (BLIS)]. Putative subtype-specific targets were identified: (i) LAR: androgen receptor and the cell surface mucin MUC1, (ii) MES: growth factor receptors [platelet-derived growth factor (PDGF) receptor A; c-Kit], (iii) BLIS: an immunosuppressing molecule (VTCN1), and (iv) BLIA: Stat signal transduction molecules and cytokines.

CONCLUSION

There are four stable TNBC subtypes characterized by the expression of distinct molecular profiles that have distinct prognoses. These studies identify novel subtype-specific targets that can be targeted in the future for the effective treatment of TNBCs.

AR collaborates with ERα in aromatase inhibitor-resistant breast cancer

Androgen receptor (AR) is an attractive target in breast cancer because of its frequent expression in all the molecular subtypes, especially in estrogen receptor (ER)-positive luminal breast cancers. We have previously shown a role for AR overexpression in tamoxifen resistance. We engineered ER-positive MCF-7 cells to overexpress aromatase and AR (MCF-7 AR Arom cells) to explore the role of AR in aromatase inhibitor (AI) resistance. Androstendione (AD) was used as a substrate for aromatization to estrogen. The nonsteroidal AI anastrazole (Ana) inhibited AD-stimulated growth and ER transcriptional activity in MCF-7 Arom cells, but not in MCF-7 AR Arom cells. Enhanced activation of pIGF-1R and pAKT was found in AR-overexpressing cells, and their inhibitors restored sensitivity to Ana, suggesting that these pathways represent escape survival mechanisms. Sensitivity to Ana was restored with AR antagonists, or the antiestrogen fulvestrant. These results suggest that both AR and ERα must be blocked to restore sensitivity to hormonal therapies in AR-overexpressing ERα-positive breast cancers. AR contributed to ERα transcriptional activity in MCF-7 AR Arom cells, and AR and ERα co-localized in AD + Ana-treated cells, suggesting cooperation between the two receptors. AR-mediated resistance was associated with a failure to block ER transcriptional activity and enhanced up-regulation of AR and ER-responsive gene expression. Clinically, it may be necessary to block both AR and ERα in patients whose tumors express elevated levels of AR. In addition, inhibitors to the AKT/IGF-1R signaling pathways may provide alternative approaches to block escape pathways and restore hormone sensitivity in resistant breast tumors.

Tamoxifen through GPER upregulates aromatase expression: a novel mechanism sustaining tamoxifen-resistant breast cancer cell growth

Tamoxifen resistance is a major clinical challenge in breast cancer treatment. Aromatase inhibitors are effective in women who progressed or recurred on tamoxifen, suggesting a role of local estrogen production by aromatase in driving tamoxifen-resistant phenotype. However, the link between aromatase activity and tamoxifen resistance has not yet been reported. We investigated whether long-term tamoxifen exposure may affect aromatase activity and/or expression, which may then sustain tamoxifen-resistant breast cancer cell growth. We employed MCF-7 breast cancer cells, tamoxifen-resistant MCF-7 cells (MCF-7 TR1 and TR2), SKBR-3 breast cancer cells, cancer-associated fibroblasts (CAFs1 and CAFs2). We used tritiated-water release assay, realtime-RT-PCR, and immunoblotting analysis for evaluating aromatase activity and expression; anchorage-independent assays for growth; reporter-gene, electrophoretic-mobility-shift, and chromatin-immunoprecipitation assays for promoter activity studies. We demonstrated an increased aromatase activity and expression, which supports proliferation in tamoxifen-resistant breast cancer cells. This is mediated by the G-protein-coupled receptor GPR30/GPER, since knocking-down GPER expression or treatment with a GPER antagonist reversed the enhanced aromatase levels induced by long-term tamoxifen exposure. The molecular mechanism was investigated in ER-negative, GPER/aromatase-positive SKBR3 cells, in which tamoxifen acts as a GPER agonist. Tamoxifen treatment increased aromatase promoter activity through an enhanced recruitment of c-fos/c-jun complex to AP-1 responsive elements located within the promoter region. As tamoxifen via GPER induced aromatase expression also in CAFs, this pathway may be involved in promoting aggressive behavior of breast tumors in response to tamoxifen treatment. Blocking estrogen production and/or GPER signaling activation may represent a valid option to overcome tamoxifen-resistance in breast cancers.

Estrogen receptor (ER) α mutations in breast cancer: hidden in plain sight

The idea that somatic ERα mutations could play an important role in the evolution of hormone-dependent breast cancers was proposed some years ago (Fuqua J Mammary Gland Biol Neoplasia 6(4):407-417, 2001; Dasgupta et al. Annu Rev Med 65:279-292, 2013), but has remained controversial until recently. A significant amount of new data has confirmed these initial observations and shown their significance, along with much additional work relevant to the treatment of breast cancer. Thus, it is the purpose of this review to summarize the research to date on the existence and clinical consequences of ERα mutations in primary and metastatic breast cancer.

Abstract PD1-2: PIK3CA mutations and/or low PTEN predict resistance to combined anti-HER2 therapy with lapatinib and trastuzumab and without chemotherapy in TBCRC006, a neoadjuvant trial of HER2-positive breast cancer patients

We have recently reported that in patients with HER2-positive breast cancer, neoadjuvant targeted therapy with lapatinib and trastuzumab to more completely block the HER receptor layer, combined with endocrine therapy (in ER-positive tumors) and without chemotherapy led to a substantial 27% pathologic complete response (pCR) rate in the breast. Activation of downstream signaling pathways may lead to resistance to therapies targeting the HER pathway receptors. Aberrant activation of the PI3K pathway via decreased levels of PTEN and/or the presence of activating PIK3CA mutations has been implicated in resistance to targeted anti-HER2 therapy, but results of clinical trials are all confounded by the co-administration of chemotherapy and are inconsistent. We sought to clarify the role of these variables in predicting pCR, a surrogate for long-term outcome, in patients treated with potent targeted therapy alone in a prospective Phase II neoadjuvant trial in patients with HER2-positive breast cancer.

Patients with large tumors (median 6 cm) were given 12 weeks of lapatinib plus trastuzumab followed by surgery (Rimawi et al. JCO, 2013). Serial tissue biopsies were obtained from study participants. For this study, we focused on baseline pre-treatment characteristics. PTEN protein levels were measured by IHC and scored using the H-score. PIK3CA mutations were identified on extracted DNA using multiplex PCR with targeted next generation sequencing (the Ion Torrent 50-gene cancer mutation panel).

Of 64 evaluable patients, tissue was available on 59 for PTEN IHC, and sufficient DNA was available on 33 for the mutation panel. PTEN median H-score was 100 (range 0-300). PTEN status when dichotomized by the median was correlated with pCR (32% in high PTEN vs. 9% in low PTEN, p = 0.04). Activating PIK3CA mutations were identified in 12 out of 33 tumors (36%; 3 mutations in the helical and 9 in the catalytic domain) and were independent of ER status. None of the patients whose tumors harbored a PIK3CA mutation achieved pCR (p = 0.06). There was no association between PTEN status and PIK3CA mutation suggesting they are independent variables (p = 0.44). When PIK3CA mutations were considered together with PTEN status, there were 31 cases with data on both. The overall pCR rate in this cohort was 16% (lower than pCR rate observed in the overall trial). However, 0/17 cases (0%) with a mutation and/or PTEN low expression (&lt;100 H score) had a pCR compared to 5/14 cases (36%) with PI3KCA wild type and high PTEN levels (p = 0.01).

We conclude that PI3K pathway activation downstream of HER2 as a result of either low PTEN or activating PIK3CA mutation results in resistance to the combination of lapatinib and trastuzumab. This is the first report on patient tissue samples from a neoadjuvant trial using the combination of lapatinib and trastuzumab without chemotherapy. If validated in a larger cohort, our findings suggest that patients with HER2 positive tumors and who also harbor aberrant downstream PI3K pathway activation may benefit from the addition of PI3K/Akt/mTOR inhibitors to potent HER2 blockade.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr PD1-2.

Abstract ES04-2: Understanding nuclear receptor in breast cancer

Breast cancer cells have receptors on the surface, in the cytoplasm and in the nucleus. Chemical messengers such as steroid hormones bind to receptors, and this causes changes in the transcriptional program of the cells. Breast cancer cells may express two important steroid receptors: estrogen receptor s alpha and beta (ERs a and b, respectively), and/or progesterone receptor (PR). ER-positive cancer cells depend on estrogen for their growth, so they can be treated with drugs to block estrogen effects (e.g. tamoxifen), or which block estrogen synthesis (Cyp19 [aromatase] inhibitors or Cyp19 inhibitors which block both androgen and estrogen synthesis. Cells which express more of these receptors generally respond better to hormonal therapies.

The ERs are from a large family of nuclear receptor transcription factors (TFs). When bound by hormone (they bind to DNA directly or they can interact with other transcription factors bound to DNA to regulate gene expression. ER and PR acts by recruiting a complex of coactivator or corepressor proteins that modulate ER functions. These coactivators are important regulatory molecules, and evidence suggests that some are oncogenes, capable of causing breast cancer and showing gene amplification and/or over-expression in a ER-psositiv breast cancers, thus affecting tumor growth and treatment.

Many mechanisms for resistance to endocrine therapies have been postulated and the loss of ER over time although not common can happen. The evolution of receptor mutations over time perhaps in tumor subpopulations has also been postulated, and recent evidence suggests that they occur more frequently than once thought in ER-positive metastatic gtumors. In addition, the PR is frequently lost or altered, which can result from its activation from upstream growth factor receptors. Coactivators that are overexpressed can also modulate ER function so that tumors fail to respond to blocking ER therapies. The basic biology, function, and altered mechanism of action of ER/PR which occur during tumor evolution will be discussed in this educational session.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr ES04-2.

RNA sequencing of cancer reveals novel splicing alterations

Breast cancer transcriptome acquires a myriad of regulation changes, and splicing is critical for the cell to “tailor-make” specific functional transcripts. We systematically revealed splicing signatures of the three most common types of breast tumors using RNA sequencing: TNBC, non-TNBC and HER2-positive breast cancer. We discovered subtype specific differentially spliced genes and splice isoforms not previously recognized in human transcriptome. Further, we showed that exon skip and intron retention are predominant splice events in breast cancer. In addition, we found that differential expression of primary transcripts and promoter switching are significantly deregulated in breast cancer compared to normal breast. We validated the presence of novel hybrid isoforms of critical molecules like CDK4, LARP1, ADD3, and PHLPP2. Our study provides the first comprehensive portrait of transcriptional and splicing signatures specific to breast cancer sub-types, as well as previously unknown transcripts that prompt the need for complete annotation of tissue and disease specific transcriptome.

Identification of the activated form of the progesterone receptor (PR) in breast cancer (BC)

593

Background: Upon ligand binding, PRs dimerize and form a discrete focal subnuclear distribution pattern (FDP), which are associated with transcriptional activation of PR (APR). FDP and are observed in BC independently of menopausal status. The feasibility of using an IHC technique to characterize the PR functional status has previously been reported in BC and presence/absence of APR is hypothesized to predict anti-progestin activity. This study describes an immunohistochemistry (IHC) method, by which a biomarker could be developed to investigate this hypothesis. Methods: 303 paraffin embedded/formalin fixed archival BC samples were processed with PR-A or PR- B isotype specific antibodies. Nuclear morphology was analyzed with standard microscopy at x1000, interpretation of the IHC slides was done by an experienced pathologist. Standard PR, estrogen receptor (ER), and Ki67 testing were done. Tumor grading/stage was obtained from the patients’ records. Results: Histology was ductal 85%, lobular 13%, other 2%. Consistent with prior research observations, tumors had two PR nuclear morphologies: 1. Diffuse pattern (D) where the PR was distributed evenly in a fine granular pattern or, 2. Aggregate pattern (A) where the PR is distributed in discrete clumps or aggregates. This defined 3 tumor phenotypes: A cells only (A), D cells only (D), and a heterogenous mix of A+D cells (AD). The APRpos group is defined as the A and A+ D phenotypes. Tumors were PR positive in 76% for PRA and 80% for PRB. Tumors were APRpos for PRA in 23% and independent of PR intensity score, ER, Ki67, and HER2, but associated with higher PR % positivity and higher tumor grade. Tumors were APRpos for PRB in 22% and associated with lower PR intensity and higher tumor grade, independent of ERpos %, PRpos %, Ki67 and HER2. Conclusions: PR positive BC tumors can be grouped in two categories based on PR nuclear morphology: 1. a group with diffuse and homogenous nuclear staining, 2. a group with heterogeneous area of cells having a nuclear pattern consistent with a functional or activated PR (APR). The described IHC technique to identify APR has the potential to be developed as companion diagnostic as a potential predictor of anti-progestin efficacy in patients with BC.

Collaboration of AR and ERα in conferring resistance to an aromatase inhibitor

579

Background: We have previously shown a role for AR overexpression in tamoxifen resistance in ERα-positive MCF-7 breast cancer cells; here we hypothesized that AR overexpression might similarly be involved in resistance to the aromatase inhibitor anastrazole (Anas). Methods: MCF-7 cells were transfected to express the aromatase gene (MCF-7 Arom), or the aromatase and AR (MCF-7 AR Arom cells). Western blot analysis was used to evaluate protein levels, MTT and soft agar assays to evaluate proliferation, luciferase reporter assays to evaluate transcriptional activities and confocal microscopy was used for localization. Results: Anas inhibited androstendione (AD)-stimulated growth in MCF-7 Arom cells but not in MCF-7 AR Arom cells. Similarly, Anas did not inhibit ERα transcriptional activity in MCF-7 AR Arom cells. Enhanced activation of pIGF-1R, pIRS-1, pAKT, and pMAPK were also observed in AR Arom cells, suggesting constitutive activation of nongenomic signaling in these cells. Consistent with activation of these potential treatment “escape” mechanisms, inhibitors of AKT and IGF-1R restored sensitivity to Anas. Sensitivity to Anas was also restored using the AR antagonist MDV3100, however use of Abiraterone acetate as a single agent most effectively blocked proliferation of AR-overexpressing cells. These results suggest that both AR and ERα must be blocked to restore sensitivity to hormonal therapies in AR overexpressing ERα-positive breast cancers. Unexpectedly, AR contributed to ERα transcriptional activity in MCF-7 AR Arom cells, as shown by inhibition with the AR antagonist bicalutamide. AR and ERα co-localized both in the cytoplasm and in the nucleus of AD+Anas-treated cells, suggesting potential activation of both non-genomic and nuclear-mediated effects when AR is overexpressed in ERα-positive cells. We confirmed these findings in breast cancer cells with acquired resistance to tamoxifen. Conclusions: These results show the necessity to block both AR and ER in patients whose tumors express elevated levels of AR. In addition, inhibitors to the AKT/IGF-1R signaling pathways or direct inhibition of androgen/estrogen synthesis provide alternative approaches to restore hormone sensitivity in resistant breast tumors.

Independent characterization by duel staining of progesterone receptor (PR) and estrogen receptor (ER) in breast cancer (BC)

596

Background: The oncology literature indicates that ER and PR are linked and in BC the presence of PR usually indicates functional ER. BCs express both ER and PR to varying degree, but little has been published on expression of ER + PR in individual cancer cells. The goal of this study is to 1. Determine the expression ER and PR at the cellular level, 2. Determine if ER and PR are expressed in the same BC cells. If ER and PR are separate, this may indicate that antiestrogens and antiprogestins may target different cells. Methods: Archived 1° BC specimens were processed using standard IHC techniques for ER/PR testing. The procedure consisted of sequential double staining on the same microtone section, where the PR was visualized through brown staining using HRP/DAB, and the ER was visualized through red staining using AP/Permanent Red. The initial testing on 13 tumor samples utilized a duel anti-PR-A/B antibody (Ab) and an ER Ab. The next 63 tumor samples utilized; 1. anti-ER and anti-PR-A Abs, 2. anti-ER and anti-PR-B Abs. A pathologist experienced with IHC, interpreted and enumerated the cells staining positive for ER only, for PR only and cells staining positive for both ER & PR. The number of cells expected to be stained for both ER & PR by chance can be calculated as the rate of total ER by the rate of total PR and compared with the observed rate (paired rank test). ER/PR positivity is defined as ≥ 5% cells positive. Results: In the first series, 11/13 tumors were ERpos, 13/13 were PRpos, 7/13 tumors had both ER & PR expressed in 5-20% (median 5%) of the same tumor cells. In the 2nd series of 63 tumors; 1. 50/53 were ERpos, 52/53 were PRApos and 44/53 had both ER & PRA expressed in <5-20% (median 5%) of the same tumor cells, 2. 44/52 were ERpos, 52/52 were PRBpos and 42/52 tumors had both ER & PRB expressed in <5-20% (median 10%) of the same tumor cells. Areas of ER or PR only predominance were frequent. A paired rank test indicates that the observed rate (median 9%) of ER/PR duel staining is less than predicted (median 18%, p <0.000). Conclusions: ER & PR were expressed in the majority of tumors examined with a minority the tumor cells expressing both ER & PR. These data support evaluating antiprogestins as a different therapeutic target from ER.

In vitro mechanism for downregulation of ER-α expression by epigallocatechin gallate in ER+/PR+ human breast cancer cells

SCOPE

Exposure of the breast to estrogens and other sex hormones is an important cancer risk factor and estrogen receptor downregulators are attracting significant clinical interest. Epigallocatechin gallate (EGCG), a polyphenolic compound found in green tea, has gained considerable attention for its antitumor properties. Here we aimed to investigate the molecular mechanisms through which EGCG regulates ER-α expression in ER+ PR+ breast cancer cells.

MATERIAL AND METHODS

Western blotting analysis, real-time PCR, and transient transfections of deletion fragments of the ER-α gene promoter show that EGCG downregulates ER-α protein, mRNA, and gene promoter activity with a concomitant reduction of ER-α genomic and nongenomic signal. These events occur through p38(MAPK) /CK2 activation, causing the release from Hsp90 of progesterone receptor B (PR-B) and its consequent nuclear translocation as evidenced by immunofluorescence studies. EMSA, and ChIP assay reveal that, upon EGCG treatment, PR-B is recruited at the half-PRE site on ER-α promoter. This is concomitant with the formation of a corepressor complex containing NCoR and HDAC1 while RNA polymerase II is displaced. The events are crucially mediated by PR-B isoform, since they are abrogated with PR-B siRNA.

CONCLUSION

Our data provide evidence for a mechanism by which EGCG downregulates ER-α and explains the inhibitory action of EGCG on the proliferation of ER+ PR+ cancer cells tested. We suggest that the EGCG/PR-B signaling should be further exploited for clinical approach.

Leptin increases HER2 protein levels through a STAT3-mediated up-regulation of Hsp90 in breast cancer cells

Obesity condition confers risks to breast cancer development and progression, and several reports indicate that the adipokine leptin, whose synthesis and plasma levels increase with obesity, might play an important role in modulating breast cancer cell phenotype. Functional crosstalk occurring between leptin and different signaling molecules contribute to breast carcinogenesis. In this study, we show, in different human breast cancer cell lines, that leptin enhanced the expression of a chaperone protein Hsp90 resulting in increased HER2 protein levels. Silencing of Hsp90 gene expression by RNA interference abrogated leptin-mediated HER2 up-regulation. Leptin effects were dependent on JAK2/STAT3 activation, since inhibition of this signaling cascade by AG490 or ectopic expression of a STAT3 dominant negative abrogated leptin-induced HER2 and Hsp90 expressions. Functional experiments showed that leptin treatment significantly up-regulated human Hsp90 promoter activity. This occurred through an enhanced STAT3 transcription factor binding to its specific responsive element located in the Hsp90 promoter region as revealed by electrophoretic mobility shift assay and chromatin immunoprecipitation assay. Analysis of HER2, Akt and MAPK phosphorylation levels revealed that leptin treatment amplified the responsiveness of breast cancer cells to growth factor stimulation. Furthermore, we found that long-term leptin exposure reduced sensitivity of breast cancer cells to the antiestrogen tamoxifen. In the same experimental conditions, the combined treatment of tamoxifen with the Hsp90 inhibitor 17-AAG completely abrogated leptin-induced anchorage-independent breast cancer cell growth. In conclusion, our results highlight, for the first time, the ability of the adipocyte-secreted factor leptin to modulate Hsp90/HER2 expressions in breast cancer cells providing novel insights into the molecular mechanism linking obesity to breast cancer growth and progression.

Estrogen receptor beta binds Sp1 and recruits a corepressor complex to the estrogen receptor alpha gene promoter

Human estrogen receptors alpha and beta are crucially involved in the regulation of mammary growth and development. Normal breast tissues display a relative higher expression of ER beta than ER alpha, which drastically changes during breast tumorogenesis. Thus, it is reasonable to suggest that a dysregulation of the two estrogen receptor subtypes may induce breast cancer development. However, the molecular mechanisms underlying the potential opposing roles played by the two estrogen receptors on tumor cell growth remain to be elucidated. In the present study, we have demonstrated that ER beta overexpression in breast cancer cells decreases cell proliferation and down-regulates ER alpha mRNA and protein content, along with a concomitant repression of estrogen-regulated genes. Transient transfection experiments, using a vector containing the human ER alpha promoter region, showed that elevated levels of ER beta down-regulated basal ER alpha promoter activity. Furthermore, site-directed mutagenesis and deletion analysis revealed that the proximal GC-rich motifs at -223 and -214 are critical for the ER beta-induced ER alpha down-regulation in breast cancer cells. This occurred through ER beta-Sp1 protein-protein interactions within the ER alpha promoter region and the recruitment of a corepressor complex containing the nuclear receptor corepressor NCoR, accompanied by hypoacetylation of histone H4 and displacement of RNA-polymerase II. Silencing of NCoR gene expression by RNA interference reversed the down-regulatory effects of ER beta on ER alpha gene expression and cell proliferation. Our results provide evidence for a novel mechanism by which overexpression of ER beta through NCoR is able to down regulate ER alpha gene expression, thus blocking ER alpha's driving role on breast cancer cell growth.

Transcriptomic landscape of breast cancers through mRNA sequencing

Breast cancer is a heterogeneous disease with a poorly defined genetic landscape, which poses a major challenge in diagnosis and treatment. By massively parallel mRNA sequencing, we obtained 1.2 billion reads from 17 individual human tissues belonging to TNBC, Non-TNBC, and HER2-positive breast cancers and defined their comprehensive digital transcriptome for the first time. Surprisingly, we identified a high number of novel and unannotated transcripts, revealing the global breast cancer transcriptomic adaptations. Comparative transcriptomic analyses elucidated differentially expressed transcripts between the three breast cancer groups, identifying several new modulators of breast cancer. Our study also identified common transcriptional regulatory elements, such as highly abundant primary transcripts, including osteonectin, RACK1, calnexin, calreticulin, FTL, and B2M, and "genomic hotspots" enriched in primary transcripts between the three groups. Thus, our study opens previously unexplored niches that could enable a better understanding of the disease and the development of potential intervention strategies.

Leptin mediates tumor-stromal interactions that promote the invasive growth of breast cancer cells

Obesity confers risks to cancer development and progression but the mechanisms underlying these risks remain unclear. In this study, we identify a role for the obesity cytokine leptin, which has been implicated previously in breast cancer development, as a determinant for the tumor-promoting activity of cancer-associated fibroblasts (CAF) in both wild-type (WT) and K303R mutant estrogen receptor-α (ERα)-expressing breast cancer cells. Human CAFs stimulated a greater increase in the proliferation and migration of breast cancer cells expressing the K303R-ERα hyperactive receptor than WT-ERα-expressing cells. A concomitant increase was seen in leptin receptor isoform expression and activation of the leptin signaling pathway in cells expressing K303R-ERα compared with WT-ERα, correlating with leptin effects on cell growth, motility, and invasiveness in mutant cells. Epidermal growth factor and other factors secreted by K303R-ERα cells stimulated CAF proliferation, migration, and subsequent leptin secretion. Moreover, K303R-ERα expression generated a leptin hypersensitive phenotype in vivo. Together, our results reveal a bidirectional cross-talk between breast cancer cells and "educated" CAFs that drives tumor progression via leptin signaling. In elucidating a mechanism that connects obesity and cancer, these findings reinforce the concept that blocking cancer-stromal cell communication may represent an effective strategy for targeted therapy of breast cancer.

P1-12-04: EBP50 – A Novel Biomarker for Resistance to Endocrine and HER2−Targeted Therapies in Breast Cancer

Background: Therapeutic strategies directed at inhibiting the action of ERa using antiestrogens, such as tamoxifen (Tam), or inhibiting HER2 signalling using a monoclonal antibody-Trastuzumab (Tras) are the standard therapies offered to women with ERa or ERa/HER2−positive cancer. However, not all patients respond to these targeted therapies, and a large number of patients who do respond will eventually develop disease progression or recurrence while on therapy. EBP50 functions as a molecular scaffold to coordinate a diverse range of regulatory processes and a number of studies suggest a role for EBP50 in cancer progression.

Material and Methods: Microarray profiling was used to identify genes associated with response to tamoxifen in primary as compared to metastatic breast tumor recurrences. Stable transfection of EBP50 shRNA was performed in ZR75B and BT-474 cells. Cell response to Tam and/or Tras was evaluated using soft agar and MTT growth assays. The molecular signaling pathways affected by EBP50 knockdown were explored using immunoblot, and immunoprecipitation. Cellular localization was determined using confocal microscopy. Mammosphere potential was used to evaluate the stem cell renewal capacity in BT-474 cells.

Results: The mean levels of EBP50 were reduced in the tamoxifen-resistant, metastatic breast tumors compared to tamoxifen-sensitive tumors. Knockdown of EBP50 levels decreased Tam sensitivity in ZR75B cells. Thus reduction in EBP50 levels analygous to that discovered in resistant patients, conferred resistance to the growth inhibitory effects of Tam. Cells with knockdown of EBP50 (shEBP50 cells) showed higher levels of phosphorylated HER2, EGFR and HER3, as well as their downstream signalling pathways. PI3K and AKT inhibitors were able to restore Tam sensitivity in shEBP50 cells. After long term treatment of ZR75-B cells with Tam resulted in decreased endogenous levels of EBP50, suggesting that only cells evolving with lower EBP50 levels were able to survive Tam treatment. Similarly, knockdown EBP50 in ERa/HER2 positive BT-474 cells significantly increased phosphorylated HER2 and these cells were resistant not only to Tam, but also to Tras treatment. BT-474 cells acquired Tras resistance concomittant with decreased EBP50 levels in a dose-dependent manner respective to parental cells. Interestingly, shEBP50 cells demonstrated an enhanced capacity to form mammospheres compared to vector control cells. We demonstrate that EBP50 is able to bind HER2 using immunoprecipitation, suggesting that EBP50 interacts directly with HER2. Confocal microscope analysis demonstrated the colocalization of these two proteins. Inhibitors to c-Src, PI3K, AKT and EGFR were used in combination with Tras; shEBP50 cells were sensitive to both PI3K and AKT inhibitors, and EGFR inhibitors were able to restore Tras sensitivity.

Discussion: Our data suggest that EBP50 is a novel negative regulator of HER2 signaling, and its loss conferred resistance to both Tam and Tras. EBP50 loss might function to stabilize HER2, and enhance dimerization with EGFR and HER3. We hypothesize that EBP50 levels might be a new predictive biomarker for targeted therapy; patients with low EBP50 levels might best be treated with a combination of therapies including PI3K/AKT inhibitors.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P1-12-04.

PD01-07: AR Overexpression and Aromatase Inhibitor Resistance in Breast Cancer

Background: Aromatase inhibitors (AIs) have emerged as the therapy of choice for the treatment of estrogen receptor alpha (ERα)-positive breast cancer. However, many patients develop resistance to AI treatment. Although the involvement of the ERα in AI resistance is well established, the role of the androgen receptor (AR) is not known. It has been estimated that about 60%-70% of ERα-positive breast cancer co-express the AR, and that AR agonists can either inhibit or stimulate breast cancer cell proliferation. Thus it is important to determine if there are biomarkers predicting AR's effects in breast tumors. We have previously shown a role for AR-overexpression in tamoxifen resistance in ERα-positive MCF-7 breast cancer cells; here we hypothesized that AR overexpression might similarly be involved in resistance to the AI anastrazole (Anas).

Materials and Methods: Stable transfection of MCF-7 cells was performed to generate cell lines that express the aromatase gene (MCF-7 BK Arom) and then co-transfected with an AR expression vector (MCF-7 AR Arom). Aromatase and AR expression levels were evaluated by western blot analysis, and the enzyme activity was evaluated using aromatase activity assays. Proliferation was tested using anchorage independent soft agar assays and MTT in the presence of the androgen substrate androstenedione (AD), or AD plus Anas. ERα and AR transcriptional activities were tested with ERE-luciferase reporter assays. Localization of ERα and AR within the cells was visualized using immunofluorescence microscopy.

Results: ERα-positive MCF-7 cells were stably transfected with either aromatase, or aromatase plus AR. MCF-7 aromatase clones overexpressing AR were resistant to the growth inhibitory effects of Anas when stimulated with the androgen AD. Resistance was not mediated through changes in aromatase expression or activity. The growth of several of the AR Arom-overexpressing cells was stimulated with treatment of Anas alone, suggesting that Anas was acting as an agonist. As expected, AD treatment stimulated ERα transcriptional activity, but Anas was unable to block AD-stimulated activity in AR Arom-overexpressing cells using ERE-Luciferase reporter assay. Anas was able to enhance AR and ERα colocalization in AR-overxpressing cells. Resistance was not associated with activation of known mechanisms of resistance, such as HER2, IGF-1R, or MAPK. However AR-overexpressing cells had higher constitutive phosphorylation of Akt. Accordingly, resistance to Anas was blocked using an Akt1/2 inhibitor.

Conclusion: Using a model of ERα-positive breast cancer cells expressing exogenous aromatase and AR, we have demonstrated that AR overexpression confers resistance to the AI Anas. These results suggest that in patients recurring on hormonal therapy whose tumors express elevated levels of AR, targeted therapy to Akt might restore hormone sensitivity.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr PD01-07.

Dicer-mediated upregulation of BCRP confers tamoxifen resistance in human breast cancer cells

PURPOSE

Tamoxifen (Tam) is the most prescribed hormonal agent for treatment of estrogen receptor α (ERα)-positive breast cancer patients. Using microarray analysis, we observed that metastatic breast tumors resistant to Tam therapy had elevated levels of Dicer.

EXPERIMENTAL DESIGN

We overexpressed Dicer in ERα-positive MCF-7 human breast cancer cells and observed a concomitant increase in expression of the breast cancer resistance protein (BCRP). We thus hypothesized that Tam resistance associated with Dicer overexpression in ERα-positive breast cancer cells may involve BCRP. We analyzed BCRP function in Dicer-overexpressing cells using growth in soft agar and mammosphere formation and evaluated intracellular Tam efflux.

RESULTS

In the presence of Tam, Dicer-overexpressing cells formed resistant colonies in soft agar, and treatment with BCRP inhibitors restored Tam sensitivity. Tumor xenograft studies confirmed that Dicer-overexpressing cells were resistant to Tam in vivo. Tumors and distant metastases could be initiated with as few as five mammosphere cells from both vector and Dicer-overexpressing cells, indicating that the mammosphere assay selected for cells with enhanced tumor-initiating and metastatic capacity. Dicer-overexpressing cells with elevated levels of BCRP effluxed Tam more efficiently than control cells, and BCRP inhibitors were able to inhibit efflux.

CONCLUSION

Dicer-overexpressing breast cancer cells enriched for cells with enhanced BCRP function. We hypothesize that it is this population which may be involved in the emergence of Tam-resistant growth. BCRP may be a novel clinical target to restore Tam sensitivity.

Resveratrol, through NF-Y/p53/Sin3/HDAC1 complex phosphorylation, inhibits estrogen receptor alpha gene expression via p38MAPK/CK2 signaling in human breast cancer cells

Agents to counteract acquired resistance to hormonal therapy for breast cancer would substantially enhance the long-term benefits of hormonal therapy. In the present study, we demonstrate how resveratrol (Res) inhibits human breast cancer cell proliferation, including MCF-7 tamoxifen-resistant cells (IC(50) values for viability were in the 30-45 μM range). We show that Res, through p38(MAPK) phosphorylation, causes induction of p53, which recruits at the estrogen receptor α (ERα) proximal promoter, leading to an inhibition of ERα expression in terms of mRNA and protein content. These events appear specifically p53 dependent, since they are drastically abrogated with p53-targeting siRNA. Coimmunoprecipitation assay showed specific interaction between p53, the Sin3A corepressor, and histone deacetylase 1 (HDAC1), which was phosphorylated. The enhancement of the tripartite complex p53/Sin3A/HDAC1, together with NF-Y on Res treatment, was confirmed by chromatin immunoprecipitation analyses, with a concomitant release of Sp1 and RNA polymerase II, thereby inhibiting the cell transcriptional machinery. The persistence of such effects in MCF-7 tamoxifen-resistant cells at a higher extent than parental MCF-7 cells addresses how Res may be considered a useful pharmacological tool to be exploited in the adjuvant settings for treatment of breast cancer developing hormonal resistance.

Farnesoid X receptor inhibits tamoxifen-resistant MCF-7 breast cancer cell growth through downregulation of HER2 expression

Tamoxifen (Tam) treatment is a first-line endocrine therapy for estrogen receptor-α-positive breast cancer patients. Unfortunately, resistance frequently occurs and is often related with overexpression of the membrane tyrosine kinase receptor HER2. This is the rationale behind combined treatments with endocrine therapy and novel inhibitors that reduce HER2 expression and signaling and thus inhibit Tam-resistant breast cancer cell growth. In this study, we show that activation of farnesoid X receptor (FXR), by the primary bile acid chenodeoxycholic acid (CDCA) or the synthetic agonist GW4064, inhibited growth of Tam-resistant breast cancer cells (termed MCF-7 TR1), which was used as an in vitro model of acquired Tam resistance. Our results demonstrate that CDCA treatment significantly reduced both anchorage-dependent and anchorage-independent epidermal growth factor (EGF)-induced growth in MCF-7 TR1 cells. Furthermore, results from western blot analysis and real-time reverse transcription-PCR revealed that CDCA treatment reduced HER2 expression and inhibited EGF-mediated HER2 and p42/44 mitogen-activated protein kinase (MAPK) phosphorylation in these Tam-resistant breast cancer cells. Transient transfection experiments, using a vector containing the human HER2 promoter region, showed that CDCA treatment downregulated basal HER2 promoter activity. This occurred through an inhibition of nuclear factor-κB transcription factor binding to its specific responsive element located in the HER2 promoter region as revealed by mutagenesis studies, electrophoretic mobility shift assay and chromatin immunoprecipitation analysis. Collectively, these data suggest that FXR ligand-dependent activity, blocking HER2/MAPK signaling, may overcome anti-estrogen resistance in human breast cancer cells and could represent a new therapeutic tool to treat breast cancer patients that develop resistance.

Abstract 940: AR overexpression confers resistance to an aromatase inhibitor in ERα-positive breast cancer cells

Background: Aromatase inhibitors (AIs) have emerged as the therapy of choice for the treatment of estrogen receptor alpha (ERα)-positive breast cancer. However, many patients develop resistance to AI treatment. Although the involvement of the ERα in AI resistance is well established, the role of the androgen receptor (AR) is not known. It has been estimated that about 60%-70% of ERα-positive breast cancer co-express the AR, and we have previously shown a role for AR-overexpression in tamoxifen resistance in ERα-positive MCF-7 breast cancer cells. Thus we hypothesized that AR overexpression might similarly be involved in resistance to the AI anastrazole (Anas).

Materials and Methods: Stable transfection of MCF-7 cells was performed to generate cell lines that express the aromatase gene (MCF-7 BK Arom) and then co-transfected with an AR expression vector (MCF-7 AR Arom). Aromatase and AR expression levels were evaluated by western blot analysis. Proliferation was tested using anchorage independent soft agar assays and MTT in the presence of the androgen substrate androstenedione (AD), or AD plus Anas. ERα and AR transcriptional activities were tested with ERE-luciferase reporter assays.

Results: Several clones expressing aromatase alone or aromatase plus AR were generated. MCF-7 aromatase clones overexpressing AR were resistant to the growth inhibitory effects of Anas when stimulated with the androgen AD. As expected AD treatment stimulated ERα transcriptional activity, but Anas was unable to block AD-stimulated activity in AR Arom-overexpressing cells. In addition, the growth of several of the AR Arom-overexpressing cells was stimulated with treatment of Anas alone. Resistance was not associated with activation of known mechanisms of resistance, such as HER2, or Akt activation. Inhibitors of various signaling and receptor growth pathways are currently being tested for their effects on blocking Anas resistance.

Conclusion: Using a model of ERα-positive breast cancer cells expressing endogenous aromatase and AR, we have demonstrated that AR overexpression confers resistance to the AI Anas.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 940. doi:10.1158/1538-7445.AM2011-940

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.

Abstract PD05-12: Combination of Notch Inhibitor MK-0752 and Endocrine Therapy for Early Stage ERα + Breast Cancer in a Presurgical Window Pilot Study

Background: Breast tumor initiating cells (TIC) use Notch receptors/ligands with other pathways for self renewal, resulting in tumor proliferation and progression. We showed that Notch inhibition with gamma secretase inhibitors (GSI) potentiates the effects of tamoxifen (tam) in xenografts (Rizzo et al. Cancer Res 2008). It is unknown whether GSIs plus endocrine therapy result in modulation of Notch and other proliferation markers in human breast cancer. Our objective was to add short exposure of the GSI MK-0752 to ongoing tam or letrozole (letr) during the presurgical window to determine 1) feasibility, 2) safety/tolerance, and 3) impact on biomarkers. We report the initial cohort of this pilot study (ClinTrials. gov NCT00756717).

Methods: Patients (pts) with early stage ERα + breast cancer were treated with 25 days of tam or letr. On day 15 MK-0752 was added to endocrine therapy (350 mg orally 3 days on, 4 days off, 3 days on), with definitive surgery day 25. Formalin fixed, paraffin embedded biopsies were obtained at baseline, day 14 and final surgery, with histologic confirmation of tumor content &gt;50% and RNA extraction by standard methods. Q-PCR was done for Notch1, Notch3, Notch4, Deltex, Jagged1, c-myc, HEY1, HEY2, HES1, PS2, C-Myc, Cyclin A2, NOXA (pro-apoptotic protein), Ki67, Dicer-1, RPL13 (internal control). Ct averages for 3 replicates were used and mRNA levels were calculated by the 2ΔΔCt method. Baseline gene expression levels were used as comparators for days 14 and 25 levels in each pt. The first cohort of 10 pts was analyzed to determine if enough signals were present to justify expanding the cohort at this dose to 20 pts and possibly test a second cohort on an alternate MK-0752 dose/schedule. Results: The initial cohort of 10 pts completed all therapy (4 tam, 6 letr), all biopsies and definitive surgery on schedule. One other pt withdrew prior to starting MK-0752 due to hypertension. Toxicity was minimal: grade 1 periorbital edema/cough, nausea, and axillary paresthesias in 1 pt each; grade 1 facial rash, 2 pts; and grade 2 fatigue, 1 pt. There was no diarrhea or surgical complications. Significant changes occurred in molecular marker levels after MK-0752 plus tam/letr (day 25) vs. end of tam/letr alone (day 14) as follows: Ki67 mRNA decreased in 9/10 pts; Notch4 decreased, 10/10; NOXA increased, 6/10; and Notch1 decreased, 6/10. Other markers showed inter-individual variations and will be presented, along with results of the global gene expression profiling (in progress). Conclusions: The addition of a short exposure of the GSI MK-0752 to ongoing endocrine therapy was feasible, safe, and well tolerated in pts with ERα + early breast cancer prior to definitive surgery. It results in anti-proliferative and pro-apoptotic effects at the molecular level. Notch4, which plays a key role in breast TIC, was the most consistent molecular marker of response in this setting. This suggests a potential anti-TIC effect of this combination and a role in overcoming endocrine resistance. Accrual to the expanded cohort is underway. If findings are confirmed, the second study with alternate MK-0752 dose/schedule may commence. Funding: Swim Across America, Inc. (clinical trial costs); Merck (drug supply, profiling)

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr PD05-12.

Estrogen receptor mutations and changes in downstream gene expression and signaling

Estrogens play a crucial role in regulating the growth and differentiation of breast cancers, with approximately two thirds of all breast tumors expressing the estrogen receptor alpha (ERalpha). Therefore, therapeutic strategies directed at inhibiting the action of ERalpha by using anti-estrogens such as tamoxifen, or reducing estrogens levels by using aromatase inhibitors, such as letrozole, anastrozole, or exemestane, are the standard treatments offered to women with ERalpha-positive cancer. However, not all patients respond to endocrine therapies (termed de novo resistance), and a large number of patients who do respond will eventually develop disease progression or recurrence while on therapy (acquired resistance). Recently, variant forms of the receptor have been identified owing to alternative splicing or gene mutation. This article reviews these variant receptors and their clinical relevance in resistance to endocrine therapy, by addressing their molecular cross-talk with growth factor receptors and signaling components. Understanding the complexity of receptor-mediated signaling has promise for new combined therapeutic options that focus on more efficient blockade of receptor cross-talk.

Identification of a novel estrogen receptor-alpha variant and its upstream splicing regulator

Alternative splicing of precursor mRNA is a fundamental mechanism to generate multiple proteins from a single gene. Although constitutive and alternative mRNA splicing is temporally and spatially regulated, deregulation of mRNA splicing could cause development, progression, and metastasis of tumors. Through yeast two-hybrid screening of a human breast cDNA library using estrogen receptor-alpha (ERalpha) as bait, we identified a novel nuclear receptor box containing full-length protein, nuclear protein E3-3 (NPE3-3). Our results revealed that NPE3-3 associates with not only ERalpha but also with splicing factors, serine/arginine-rich protein (SRp)-30c, SRp40, and splicing factor SC-35, suggesting that NPE3-3 is likely to be involved in regulation of mRNA splicing. Accordingly, transient expression of NPE3-3 in cells resulted in expected splicing of the CD44 control minigene. We also discovered that NPE3-3-overexpressing clones produced a novel, previously unrecognized, alternatively spliced variant of ERalpha (termed ERalphaV), which had a molecular size of 37 kDa composed of only exons 1, 2, 7, and 8. ERalphaV was expressed and sequestered in the cytoplasm in MCF-7 cells stably overexpressing NPE3-3, suggesting its involvement in nongenomic hormone signaling. NPE3-3 clones exhibited up-regulation of ERK1/2 signaling, cyclin D1, and cathepsin D and enhanced tumor cell proliferation, migration, and tumorigenicity. Moreover, direct expression of the ERalphaV in breast cancer cells stimulated ERK1/2 up-regulation and cyclin D1 expression. We found that ERalphaV physically interacted with MAPK kinase (MEK)-1/2, and thus, an ERalphaV and MEK1/2 complex could lead to the activation of the ERK1/2 pathway. Interestingly, NPE3-3 was up-regulated in human breast tumors. These findings revealed a role for NPE3-3 in alternative splicing and suggest that ERalpha is a physiological target of NPE3-3, leading to a constitutive nongenomic signaling pathway in breast cancer cells.

G protein-coupled receptor 30 expression is up-regulated by EGF and TGF alpha in estrogen receptor alpha-positive cancer cells

In the present study, we evaluated the regulation of G protein-coupled receptor (GPR)30 expression in estrogen receptor (ER)-positive endometrial, ovarian, and estrogen-sensitive, as well as tamoxifen-resistant breast cancer cells. We demonstrate that epidermal growth factor (EGF) and TGF alpha transactivate the GPR30 promoter and accordingly up-regulate GPR30 mRNA and protein levels only in endometrial and tamoxifen-resistant breast cancer cells. These effects exerted by EGF and TGF alpha were dependent on EGF receptor (EGFR) expression and activation and involved phosphorylation of the Tyr(1045) and Tyr(1173) EGFR sites. Using gene-silencing experiments and specific pharmacological inhibitors, we have ascertained that EGF and TGF alpha induce GPR30 expression through the EGFR/ERK transduction pathway, and the recruitment of c-fos to the activator protein-1 site located within GPR30 promoter sequence. Interestingly, we show that functional cross talk of GPR30 with both activated EGFR and ER alpha relies on a physical interaction among these receptors, further extending the potential of estrogen to trigger a complex stimulatory signaling network in hormone-sensitive tumors. Given that EGFR/HER2 overexpression is associated with tamoxifen resistance, our data may suggest that ligand-activated EGFR could contribute to the failure of tamoxifen therapy also by up-regulating GPR30, which in turn could facilitates the action of estrogen. In addition, important for resistance is the ability of tamoxifen to bind to and activate GPR30, the expression of which is up-regulated by EGFR activation. Our results emphasize the need for new endocrine agents able to block widespread actions of estrogen without exerting any stimulatory activity on transduction pathways shared by the steroid and growth factor-signaling networks.

Combined low doses of PPARgamma and RXR ligands trigger an intrinsic apoptotic pathway in human breast cancer cells

Ligand activation of peroxisome proliferator-activated receptor (PPAR)gamma and retinoid X receptor (RXR) induces antitumor effects in cancer. We evaluated the ability of combined treatment with nanomolar levels of the PPARgamma ligand rosiglitazone (BRL) and the RXR ligand 9-cis-retinoic acid (9RA) to promote antiproliferative effects in breast cancer cells. BRL and 9RA in combination strongly inhibit of cell viability in MCF-7, MCF-7TR1, SKBR-3, and T-47D breast cancer cells, whereas MCF-10 normal breast epithelial cells are unaffected. In MCF-7 cells, combined treatment with BRL and 9RA up-regulated mRNA and protein levels of both the tumor suppressor p53 and its effector p21(WAF1/Cip1). Functional experiments indicate that the nuclear factor-kappaB site in the p53 promoter is required for the transcriptional response to BRL plus 9RA. We observed that the intrinsic apoptotic pathway in MCF-7 cells displays an ordinated sequence of events, including disruption of mitochondrial membrane potential, release of cytochrome c, strong caspase 9 activation, and, finally, DNA fragmentation. An expression vector for p53 antisense abrogated the biological effect of both ligands, which implicates involvement of p53 in PPARgamma/RXR-dependent activity in all of the human breast malignant cell lines tested. Taken together, our results suggest that multidrug regimens including a combination of PPARgamma and RXR ligands may provide a therapeutic advantage in breast cancer treatment.

Expression of the K303R estrogen receptor-alpha breast cancer mutation induces resistance to an aromatase inhibitor via addiction to the PI3K/Akt kinase pathway

Aromatase inhibitors (AI) are rapidly becoming the first choice for hormonal treatment of estrogen receptor-alpha (ERalpha)-positive breast cancer in postmenopausal women. However, de novo and acquired resistance frequently occurs. We have previously identified a lysine to arginine transition at residue 303 (K303R) in ERalpha in premalignant breast lesions and invasive breast cancers, which confers estrogen hypersensitivity and resistance to tamoxifen treatment. Thus, we questioned whether resistance to AIs could arise in breast cancer cells expressing the ERalpha mutation. As preclinical models to directly test this possibility, we generated K303R-overexpressing MCF-7 cells stably transfected with an aromatase expression vector. Cells were stimulated with the aromatase substrate, androstenedione, with or without the AI anastrozole (Ana). We found that Ana decreased androstenedione-stimulated growth of wild-type cells, whereas K303R-expressing cells were resistant to the inhibitory effect of Ana on growth. We propose that a mechanism of resistance involves an increased binding between the mutant receptor and the p85alpha regulatory subunit of phosphatidylinositol-3-OH kinase (PI3K), leading to increased PI3K activity and activation of protein kinase B/Akt survival pathways. Inhibition of the selective "addiction" to the PI3K/Akt pathway reversed AI resistance associated with expression of the mutant receptor. Our findings suggest that the K303R ERalpha mutation might be a new predictive marker of response to AIs in mutation-positive breast tumors, and that targeting the PI3K/Akt pathway may be a useful strategy for treating patients with tumors resistant to hormone therapy.

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.

Growth factor-induced resistance to tamoxifen is associated with a mutation of estrogen receptor alpha and its phosphorylation at serine 305

Estrogens play a crucial role in breast tumor growth, which is the rationale for the use of antiestrogens, such as tamoxifen, in women with estrogen receptor (ER)-alpha-positive breast cancer. However, hormone resistance is a major clinical problem. Altered growth factor signaling to the ERalpha pathway has been shown to be associated with the development of clinical resistance. We previously have identified a mutation that replaces arginine for lysine at residue 303 (K303R) of ERalpha, which confers hypersensitive growth in low levels of estrogen. To determine if the K303R mutation could participate in the evolution of hormone resistance, we generated MCF-7 breast cancer cells stably transfected with either wild-type (WT) or K303R ERalpha. We found that the mutation confers decreased sensitivity to tamoxifen in the presence of the growth factor heregulin, using anchorage-independent growth assays. K303R ERalpha-expressing cells were hypersensitive to growth factor signals. Our data suggest that phosphorylation of serine 305 within the hinge domain of ERalpha might play a key role in increasing ligand-independent activity of the mutant receptor. We hypothesize that the mutation adapts the receptor for enhanced bidirectional cross-talk with the HER2 growth factor receptor pathway, which then impacts on responsiveness to tamoxifen.