Abstract ES4-1: Adaptive Resistance to Hormonal Therapy

While recurrences are delayed by the use of endocrine therapy (ET), resistance can evolve with the development of metastatic breast cancer (MBC) in about one-fourth of ER-positive patients. MBC is incurable with a median 5-year survival rate of less than 25%. We know that ET resistance can result from an adaptive process in which escape survival mechanisms are used to evade treatment. One of the best-characterized ET resistance mechanisms is transcriptional activation of growth factor receptor (GFR) signaling, and subsequent estrogen-independent activation of ER via “cross-talk” with downstream signaling from GFRs. We will discuss distinct mechanisms of ligand-independent activation of ER discovered using transcriptional profiling of metastatic, ET-resistant tumors. It is now estimated that ~20% of therapy-resistant MBCs contain ESR1 mutations. Recent data has also shown that mutations in the MAPK/RAS pathway and transcription factor regulatory programs (for example, MYC) only account for an additional 22% of recurrent breast tumors. Thus, acquired mutations alone do not account for the majority of ET resistance mechanisms in MBC. ER-positive breast cancer cell lines display profound transcriptional variability at the single cell level associated with the acquisition of therapy resistance. Recently, bulk and single cell RNA sequencing was used on sequential samples from patients with ER-positive, ET-resistant patients and identified RNA expression patterns that represented different cellular phenotypes, including an increased epithelial mesenchymal transition state that could provide a selective advantage for selective phenotypic subclones to dominate. We will focus our discussion of transcriptional reprograming as a major mechanism of ET resistance.

Citation Format: S Fuqua. Adaptive Resistance to Hormonal Therapy [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr ES4-1.

Abstract P1-02-03: The Y537S ESR1 mutation carries unique metabolomics profiling in breast cancer

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 changed metabolomics and whether these metabolomics change could affect metastasis.

Experimental design and methods:

We generated ESR1 Y537S homozygous mutations using CRISPR Casp-9 technology. Globe metabolites screening was performed using 6550 Agilent QTOF instrument. Athymic mice were used in tumor xenograft studies. Affymetrix microarrays were performed to compare gene expression changes in Y537S mutant compared with parental cells. Enriched metabolite pathways and gene expression integrated analysis was analyzed by using online analysis tool http://www.metaboanalyst.ca.

Results:

We generated CRISPR ESR1 Y537S mutation homozygous knock-in clones in MCF-7 cells. In vivo experiments revealed that mutant cells are dominant drivers of metastasis. Transcriptome profiling revealed elevated expression of Hallmark pathways, including EMT and estrogen-regulated gene expression. We performed globe metabolites screening using MCF-7 Y537S and MCF-7 parental and identified 134 metabolites. Serum starvation media was used and estrogen was used as control for both cell lines. As we expected estrogen treatment induced metabolites changes in parental cells. However, metabolites in mutant cells were not changed significantly under estrogen treatment. Interestingly, metabolites in the mutant cells at baseline were remarkably upregulated (78 out of 134 identified total metabolites) indicating mutant cells in serum starvation condition had significantly different metabolomics compared with parental cells. Top upregulated pathways include protein biosynthesis, betaine metabolism and ammonia recycling. Integration of microarray gene expression and metabolites reviewed several metabolomics pathways significantly changed in mutant compared with parental cells including for example aminoacyl-tRNA biosynthesis, arginine and proline metabolism and alanine, aspartate and glutamate metabolism.

Conclusion: The Y537S ER mutation is a driver of distant metastasis in ER-positive breast cancer cells. Y537S ER mutant had globe changes of metabolites expression which was confirmed by integrated analysis combining microarray gene expression. The roles of these metabolites need to be studied to correlate with metastasis. Enzymes responsible for converting these metabolites changes could be served as potential therapeutic targets.

Citation Format: Gu G, Piyarathna B, Coarfa C, Ellis L, Ando' S, Fuqua S. The Y537S ESR1 mutation carries unique metabolomics profiling in breast cancer [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 P1-02-03.

Abstract P4-08-09: Targeting Thyroid Receptor b in Estrogen Receptor Negative Breast Cancer

Background: The treatment of estrogen receptor (ER)-negative breast cancer (BC) is a major clinical problem due to the lack of useful therapeutic targets. Nuclear receptors (NRs) are potential targets in these patients because they regulate global transcriptional events and many already have agonists/antagonists available.

Material and Methods: We used microarray analysis of 227 ER-negative tumors to identify NR targets, and performed hierarchical clustering using 41 NRs. Expressed receptors were scored using prediction analysis of microarrays (PAM) across clustered groups. Cell lines were matched to subtypes using previously described data (Neve et al. 2006). Candidate gene expression levels were confirmed by qRT-PCR using TaqMan probes. pGIPZ lentiviral vectors encoding shRNA were used to knockdownselected candidates. MTT and soft agar assays were used to measure chemosensitivity and growth following treatment with Docetaxel (Doc), Doxorubicin (Dox), or Cisplatin (Cis). Statistical analysis was performed using Red-R.

Results: The 41 NRs clustered tumors into 5 groups. For each group we selected genes representing the highest ranked discriminators, and examined their effects in cell lines matching each groups' gene signature. Thyroid hormone receptor b (THRβ) was selected from group V. The expression levels of this receptor were confirmed by qRT-PCR and Western blot analysis.

Knockdown of THRβ in ER-negative HCC2185 cells rendered cells more resistant to all chemotherapeutics by using MTT assay. Similar results were confirmed in ER-negative MDA-MB-453 and HCC202 cells. Knockdown of THRβ enhanced colony forming potential in anchorage-independent soft agar assays in MDA-MB-453 and HCC202 cells. Statistical analysis using clinical data from Sabatier et al. (BCRT 2011) showed that patients with low THRb have a worse clinical outcome. In order to translate these findings into the clinic, we treated cells with a specific THRβ agonists, GC-1 and KB-141. GC-1 inhibited cell growth in growth assays, and synergistic effects were observed when cells were treated with GC-1 and Docetaxel in combination. Re-expression of ERα protein was observed in ER-nagative cells lines after treatment with GC-1 and KB141, suggesting that modulation of THRβ may also extend hormonal therapy to this hormonally insensitive group of patients.

Conclusion: Clinical targeting of NRs in ER-negative BCs is a novel strategy since receptors can be specifically targeted with ligands. Our data suggest that chemotherapy response in ER-negative patients overexpressing THRβ could be enhanced with a THRβ agonist. Similarly, functional re-activation of ERα by activating THRβ might extend hormonal therapies to these patients as well.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P4-08-09.

Tamoxifen regulates cell fate through mitochondrial estrogen receptor beta in breast cancer

Tamoxifen has both cytostatic and cytotoxic properties for breast cancer. Tamoxifen engaged mitochondrial estrogen receptor beta (ERβ) as an antagonist in MCF-7 BK cells, increasing reactive oxygen species (ROS) concentrations from the mitochondria that were required for cytotoxicity. In part this derived from tamoxifen down-regulating manganese superoxide dismutase (MnSOD) activity through nitrosylating tyrosine 34, thereby increasing ROS. ROS activated protein kinase C delta and c-jun N-terminal kinases, resulting in the mitochondrial translocation of Bax and cytochrome C release. Interestingly, tamoxifen failed to cause high ROS levels or induce cell death in MCF7BK-TR cells due to stimulation of MnSOD activity through agonistic effects at mitochondrial ERβ. In several mouse xenograft models, lentiviral shRNA-induced knockdown of MnSOD caused tumors that grew in the presence of tamoxifen to undergo substantial apoptosis. Tumor MnSOD and mitochondrial ERβ are therefore targets for therapeutic intervention to reverse tamoxifen resistance and enhance a cell death response.

Phosphorylation of the Mutant K303R Estrogen Receptor α at Serine 305 Impacts Aromatase Inhibitor Sensitivity

Aromatase inhibitors (AIs) are challenging tamoxifen as the treatment of choice for both early and advanced breast cancer in postmenopausal women with estrogen receptor (ER) α-positive diseases. However, resistance frequently occurs. Previously, we identified a lysine to arginine transition at residue 303 (K303R) in ERα in premalignant breast lesions and invasive breast cancers, which confers estrogen hypersensitivity and resistance to the AI Anastrozole (Ana) when transfected in MCF-7 breast cancer cells. To identify genes whose expression was associated with the development of aromatase inhibitor resistance (AIR), we performed microarray analysis, and found a marked increase in the gene expression of different IGF family members in the mutant cells. Immunoblot analysis also revealed elevated constitutive phosphorylation of the IGF-1R/IRS-1/Akt pathway in K303R-expressing cells. Treatment with IGF-1R and Akt inhibitors drastically inhibited proliferation of the K303R-mutant expressing cells, while wild-type ERα-expressing clones showed little reduction of growth by these inhibitors, confirming the increased IGF signaling activation in the mutant cells.Post-translational modifications of ERα, such as phosphorylation, tightly regulate its function, and the K303R mutation resides at a major post-translational modification site, serine (S) residue 305. We found that the mutant was more efficiently phosphorylated than the WT receptor by Akt in vitro, and that the mutant cells exhibited enhanced S305 phosphorylation in vivo. Mutation of S305 to alanine (A) to destroy this phosphorylation site prevented in vitro and in vivo Akt-mediated phosphorylation.The ERα S305 residue is an important site that modifies response to tamoxifen; thus, we questioned whether the S305 site could also influence AI response. To address the role of S305 phosphorylation on AIR, we generated pools of stable transfectants expressing exogenous WT, K303R, or K303R/S305A mutant receptors, and then evaluated for the effects of Ana in soft agar assays. Expression of the K303R/S305A mutant resulted in a significant reduction in the basal non-stimulated growth and sensitivity to Ana compared to K303R ERα-expressing pools, thus mimicking the same response profile of wild-type ERα clones. A selective blocking peptide (S305 peptide, residues 298-308) able to antagonize this phosphorylation reversed AIR. Blockade of S305 phosphorylation also resulted in a specific inhibition of IGF-1R/IRS-1/Akt activation, but only in the mutant cells.Our data suggest that the K303/S305 residues of the ERα mutation may be a novel determinant of aromatase inhibitor response in breast cancer, and blockade of S305 ERα phosphorylation represents a new therapeutic strategy to overcome resistance to hormonal therapies in tumors with the ERα mutation.

Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 5053.

MTA2 Enhances Breast Cancer Metastasis by Regulating the Rho Signaling Pathway

Background: Metastasis is the ultimate cause of death for patients succumbing to breast cancer. The metastasis tumor associated (MTA) gene family has been linked with metastasis in breast cancer. Previously, we found that MTA2 overexpression enhanced the metastatic ability of the estrogen receptor-alpha (ERα)-negative cell line MDA-MB-231. This was accompanied by an increase in cell rounding, indicating a change in cytoskeletal organization. Here we explored the molecular pathways contributing to the MTA2-enhanced metastatic phenotype.Materials and Methods: MDA-MB-231 cells were engineered to overexpress MTA2. Changes in protein expression were determined by immunoblot assay. Differential gene expression was determined using Affymetrix microarrays and analyzed using dChip, BRB Array Tools, and R. Pathway analysis was carried out using the Database for Visualization, Annotation, and Integrated Discovery (DAVID). Validation was done by comparing to publicly available datasets. MTA2-overexpressing or control cells were transfected with a Rho GTP dissociation inhibitor alpha (Rho GDIα) or vector control expression constructs. Anchorage-independent growth was measured using soft-agar assays. Analysis of the impact of MTA2 and Rho GDIα combined expression was carried out using publically available datasets.Results: Analysis of enriched ontologies from significantly different genes between MTA2-overexpressing cells and clinical breast cancer samples of the basal subtype previously published by Richardson et al (1) indicated that the focal adhesion pathway was enhanced in MTA2-overexpressing samples. We found that a negative regulator of the focal adhesion pathway, Rho GDIα, was decreased in MTA2-overexpressing cells at the protein level. When a yellow fluorescent protein-tagged Rho GDIα expression vector was re-expressed in MTA2-overexpressing cells, there was a concomitant decrease in the expression of MTA2 and increased expression of endogenous Rho GDIα. When tested in soft agar assays, re-expression of Rho GDIα significantly reduced the colony-forming ability of MTA2-overexpressing cells. We examined the publically available dataset published by Wang et al. (2), and found that ERα-negative patients with high MTA2 expression and low Rho GDIa levels had earlier recurrence compared with all other patients (P=0.02).Conclusion: MTA2 overexpression was associated with an aggressive phenotype in MDA-MB-231 cells. These data indicate that MTA2 overexpression may be associated with increased signaling through the Rho pathway. We found that MTA2 and Rho GDIα form a regulatory loop which reinforces either MTA2 high or MTA2 low expression levels. Furthermore, we found that combined high MTA2 and low Rho GDIα expression had a negative impact on patient outcomes in the ERα-negative population. We are currently exploring the use of Rho pathway inhibitors for their potential to block this phenotype, and to explore the MTA2/Rho GDIα regulatory loop.1) Richardson AL, Wang ZC, De Nicolo A, Lu X et al. X chromosomal abnormalities in basal-like human breast cancer. Cancer Cell 2006 Feb;9(2):121-32.2) Wang Y, Klijn JG, Zhang Y, Sieuwerts AM et al. Gene-expression profiles to predict distant metastasis of lymph-node-negative primary breast cancer. Lancet 2005 Feb 19-25;365(9460):671-9.

Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 6143.

Effect of estrogen receptor-positive progenitor cells on a tamoxifen resistance phenotype in breast cancer cells

1042

Background: The antiestrogen tamoxifen and aromatase inhibitors are the most frequently prescribed hormonal agents for the treatment of estrogen receptor (ER) α-positive breast cancer. An important question is whether there is a group of hormone resistant, ERα-positive patients who may derive additional benefit from the addition of chemotherapy to endocrine therapy, or who may be candidates for “targeted” biologics. Dicer1 is an RNase III-containing enzyme that processes microRNA precursors into mature microRNA, which have been implicated in breast tumor invasion and metastasis. BCRP1 is a transmembrane transport protein known to efflux a number of chemotherapeutic agents, but also steroid hormones. In the present study, we investigated whether Dicer might affect response to tamoxifen in breast cancer cells, and generated estrogen receptor-positive MCF-7 human breast cancer cells stably overexpressing Dicer1, and they exhibited elevated BCRP1 protein. Methods: We utilized preclinical approaches to study the function of BCRP1 in Dicer-overexpressing breast cancer cells using in vitro growth assays in soft agar, mammosphere formation assays, and in vivo tumor initiation. Results: Microarray analyses of human breast tumors, suggested that Dicer overexpression was associated with tamoxifen resistance. Dicer-overexpressing MCF-7 cells express elevated levels of BCRP1, ALDH, and cErbB2/HER-2 evident by immunoblot analysis. The Dicer1-overexpressing cells formed soft agar colonies in the presence of tamoxifen, however Fumitremorgin C (FTC) or MBLI-97, both BCRP inhibitors, reversed resistance, and sensitized cells to tamoxifen therapy. Preclinical in vivo tumor xenograft experiments confirmed the tamoxifen-resistant phenotype. Mammosphere potential was enhanced in Dicer-overexpressing cells suggesting an enrichment of stem-like breast cancer cells. Conclusions: Our results suggest that Dicer-overexpressing breast cancer cells are a novel preclinical model for an estrogen receptor-positive breast cancer progenitor phenotype and tamoxifen resistance. Based on our data Dicer1 is a potential predictive biomarker in breast cancer, and predicts that clinical BCRP1 inhibition may facilitate tumor sensitization to hormonal therapy.

No significant financial relationships to disclose.

A phase II study of anastrazole and fulvestrant in combination with gefitinib in patients with newly diagnosed ER-positive breast cancer

1050

Background: Endocrine therapy for ER+ breast cancer is effective and relatively nontoxic, but is limited by de novo and acquired resistance. Preclinical studies suggest that complete ER blockade along with inhibition of co-expressed growth factor receptors enhances endocrine response and overcomes resistance. Methods: We conducted a phase II trial of combined anastrazole and fulvestrant with gefitinib, an EGFR tyrosine kinase inhibitor, as intial therapy in postmenopausal women with locoregional or metastatic ER+ breast cancer. A tumor core biopsy was done at baseline and after 3 weeks. Patients received treatment for 4 months. Surgery was then offered if the tumor was operable. Primary endpoint was clinical response as defined by RECIST criteria and secondary endpoints were safety/tolerability, complete pathologic response rate (pCR), and biomarker analysis. Planned sample size was 60 patients, but the study closed after 15 patients were enrolled because of poor accrual. Results: Median age at diagnosis was 67 years. Median clinical tumor size was 7 cm and 4 patients had metastases. 11 patients (73%) had ER+/PgR+ tumors and 4 (27%) had ER+/PgR- disease. 3 patients withdraw from the study before response was assessed. Of the12 remaining patients there were 2 complete responses (17%), 5 partial responses (42%), 5 with stable disease (42%), and 2 (17%) with progressive disease. Of the 7 patients who had surgery at 4 months, none had pCR. Most common adverse events were rash in 4 patients, Diarrhea in 4, joint symptoms in 3, and abnormal LFT’s in 3. All adverse events were reversible. Biomarkers, including arrays and Ki67, are pending and will be presented at the meeting. Conclusions: Complete ER blockade using anastrazole and fulvestrant with EGFR inhibition is well-tolerated and has activity in this cohort of patients with relatively advanced ER+ breast cancer. Despite its documented benefit and low toxicity, accrual to neoadjuvant endocrine therapy studies remains poor in the US, largely because of physician hesitance to refer patients for non-chemotherapy studies. More effort is needed to increase physician awareness about the value of endocrine therapy, and to increase accrual to trials that address the question of endocrine resistance.

[Table: see text]

Production and characterization of an estrogen receptor beta subtype-specific mouse monoclonal antibody

An important step in differentiating the unique physiological roles of the alpha and beta forms of estrogen receptor is to determine the precise expression pattern of each of these receptors. We report the generation and characterization of a murine IgG1 monoclonal antibody (MAb), ER15.64A that is ERbeta subtype-specific and capable of recognizing full-length human ERbeta as well as all of its known protein isoforms. ER15.64A, raised against a ERbeta peptide (aa2-18)-keyhole limpet hemocyanine conjugate, reacted to the immunizing peptide and the full-length E. coli expressed ERbeta in ELISA and BIAcore assays. It also immunostained nuclei of Sf9 insect cells that were infected with an ERbeta-baculovirus. In Western analysis, ER15.64A recognized ERbeta1 and ERbeta2 proteins from a reticulocyte in vitro transcription/translation preparation. This antibody did not cross-react with recombinant ERalpha in ELISA, BIAcore, immunocytochemistry, or Western blot analysis. The specificity of ER15.64A should make this antibody a useful tool for monitoring expression of ERbeta and its isoforms at the protein level and should aid in distinguishing the pattern of ERbeta receptor expression from that of ERalpha.

Low cell motility induced by hsp27 overexpression decreases osteolytic bone metastases of human breast cancer cells in vivo

The mechanisms controlling the formation of osteolytic bone metastases in patients with breast cancer are still poorly understood. To explore the role of motility in the establishment of osteolytic bone metastases, we have used a model of bone metastasis in which MDA-MB-231 breast cancer cells exhibiting low (hsp27-transfectants) and high (control-transfectant) endogenous cell motility were compared. We found that MDA-MB-231 cells exhibiting low cell motility were less capable of establishing osteolytic lesions. The number and the area of the osteolytic lesions in mice inoculated with low motility cells were both significantly smaller. Histomorphometry of bone lesions also demonstrated less tumor area in mice bearing hsp27 transfectants although there was no difference in the osteoclast number per square millimeter of tumor-bone interface. These data suggest that cell motility may be an important mechanism in the metastatic cascade of breast cancer cells to the bone and that controlling cell motility may be a useful target to prevent the establishment of osteolytic bone metastases.

The role of the estrogen receptor in tumor progression

We will discuss two estrogen receptor (ER) variants that may play a role in breast cancer initiation or progression. One is a truncated receptor, which has been named the exon 5 ER deletion variant, and the other is called a hyper-sensitive variant of the ER that is sensitive to low levels of hormone. Both ER variants differ from the wild-type ER in their hormone binding domains (HBD). The exon 5 ER deletion variant lacks a large portion of the HBD, while the hyper-sensitive ER variant has a point mutation in a conserved region of the HBD. The exon 5 ER deletion variant is transcriptionally active in the absence of hormone, and is thus considered constitutively-active. In contrast, the hyper-sensitive ER variant is functionally active in response to subphysiological concentrations of estrogen. We speculate that both ER variants may form productive heterodimers with wild-type receptor to modify the normal function of ER in the breast.

Molecular genetic studies of early breast cancer evolution

In the past few years there has been an explosion in the number of patients diagnosed with hyperplastic breast disease and in situ breast cancer. Based on epidemiological data, these morphologically defined lesions may be categorized as those with little malignant potential (e.g. typical hyperplasia or proliferative disease without atypia [PDWA], those with significant malignant potential which may already be "initiated" (e.g. atypical ductal hyperplasia [ADH]), and early "transformed" lesions which are malignant but not yet invasive (e.g. ductal carcinoma in situ [DCIS]). They may represent sequential evolutionary stages in the ontogeny of invasive breast cancer, with each morphologically defined stage resulting from accumulating genetic changes culminating in a transformed clonal lineage capable of invasion and metastasis. Using loss-of-heterozygosity (LOH) analysis, we are studying the genetic changes associated with these lesions in archival tissue samples. 50% (6/12) of the proliferative lesions (PDWA and ADH) and 80% of the DCIS shared their LOH patterns with more advanced lesions from the same breast, strongly supporting a precursor/product relationship between these lesion and the cancers they accompany.