The effect of an exopolysaccharide probiotic molecule from Bacillus subtilis on breast cancer cells

Introduction

Many well-known risk factors for breast cancer are associated with dysbiosis (an aberrant microbiome). However, how bacterial products modulate cancer are poorly understood. In this study, we investigated the effect of an exopolysaccharide (EPS) produced by the commensal bacterium Bacillus subtilis on breast cancer phenotypes. Although B. subtilis is commonly included in probiotic preparations and its EPS protects against inflammatory diseases, it was virtually unknown whether B. subtilis-derived EPS affects cancer.

Methods

This work investigated effects of EPS on phenotypes of breast cancer cells as a cancer model. The phenotypes included proliferation, mammosphere formation, cell migration, and tumor growth in two immune compromised mouse models. RNA sequencing was performed on RNA from four breast cancer cells treated with PBS or EPS. IKKβ or STAT1 signaling was assessed using pharmacologic or RNAi-mediated knock down approaches.

Results

Short-term treatment with EPS inhibited proliferation of certain breast cancer cells (T47D, MDA-MB-468, HCC1428, MDA-MB-453) while having little effect on others (MCF-7, MDA-MB-231, BT549, ZR-75-30). EPS induced G1/G0 cell cycle arrest of T47D cells while increasing apoptosis of MDA-MB-468 cells. EPS also enhanced aggressive phenotypes in T47D cells including cell migration and cancer stem cell survival. Long-term treatment with EPS (months) led to resistance in vitro and promoted tumor growth in immunocompromised mice. RNA-sequence analysis showed that EPS increased expression of pro-inflammatory pathways including STAT1 and NF-κB. IKKβ and/or STAT1 signaling was necessary for EPS to modulate phenotypes of EPS sensitive breast cancer cells.

Discussion

These results demonstrate a multifaceted role for an EPS molecule secreted by the probiotic bacterium B. subtilis on breast cancer cell phenotypes. These results warrant future studies in immune competent mice and different cancer models to fully understand potential benefits and/or side effects of long-term use of probiotics.

Abstract P3-11-01: Increasing DAXX Expression in ER+ Breast Cancer Cells to Overcome Endocrine Therapy Resistance

Background: New treatment paradigms are needed to overcome resistance to endocrine therapy (ET; tamoxifen or aromatase inhibitors, AI) in ER+ breast cancer (BC). ET resistance is due to survival of breast cancer stem cells (BCSCs) that contribute to relapse of ER+ BC. Notch signaling drives BCSCs. In order to identify Notch specific biomarkers for the purpose of patient selection for anti-Notch therapy, we conducted a pre-surgical biomarker window study combining ET plus MK-0752, a -secretase inhibitor (GSI). Death Associated Protein 6 (DAXX) was discovered to be a novel Notch1 target gene and necessary for GSI-mediated inhibition of BCSCs. Subsequently, we found that DAXX alone was sufficient to inhibit BCSCs. In this current study, we investigated the mechanism by which high DAXX expression inhibited growth of ET resistant ER+ BC cells in vitro and in vivo. Methods: Isogenic ER+ BC cell lines (parental MCF-7, ET resistant MCF-7/5C, parental T47D, and ET resistant T47D-ED) were used. Cells were cultured in estrogen-deprived medium for more than 1 year to mimic AI use. DAXX was depleted using siRNA or overexpressed using a pCMV-expression vector. Bulk cell proliferation was analyzed in response to estrogen depletion or increasing concentrations of 17-estradiol. BCSC survival was measured using the mammosphere-forming assay. Tumor onset and burden were measured by injecting DAXX-expressing or depleted mammospheres into mammary fat pads of female, athymic nude mice. Recurrence of an ER+ PDX tumor (BCM 5097) was measured after withdrawal of estrogen. RNA sequencing identified enriched genes and pathways that required DAXX. Based on these results, cell death was assessed using Annexin V/7-AAD flow cytometry, PARP-1 and Caspase 8 cleavage, phosphorylation of JNK, and expression of apoptotic protein regulators, BIM, BAX, Bcl-2, and Bcl-xL. JNK signaling was inhibited using SB600125. Results: Estradiol stimulated proliferation of parental, ER+ MCF-7 and T47D cells. In contrast, estradiol inhibited proliferation of isogenic ET resistant MCF-7/5C and T47D-ED cells in a concentration and ER-dependent manner. Estradiol induced the DAXX protein in both ET sensitive and resistant cells. DAXX was required for BCSC survival in ET sensitive cells. However, once ER+ cells acquired resistance to ET, DAXX was necessary and sufficient to inhibit both bulk cell proliferation and BCSC survival, suggesting that increasing DAXX might be a novel approach to overcome ET resistance. In mice, high DAXX expression significantly inhibited tumor onset and burden of ET resistant tumors compared to DAXX-depleted tumors. Low DAXX expression was significantly associated with recurrence of an ER+ PDX tumor (BCM 5097) after withdrawal of estrogen. RNA sequencing revealed that DAXX activated an anti-neoplastic gene signature, including transcription factors that regulate cell death genes including the Bcl2-family. DAXX was required for high BIM expression and low levels of Bcl-xL. DAXX was necessary and sufficient to induce apoptosis, PARP-1 cleavage, and phosphorylation of JNK in ET resistant cells. A selective JNK inhibitor, SB600125 rescued DAXX-mediated inhibition of ET resistant bulk cell proliferation and BCSC survival, suggesting that high DAXX expression activates JNK signaling to regulate apoptotic proteins to induce cell death of BCSCs-derived from ET resistant BC. Conclusions: Expressing high DAXX levels is a potent method to inhibit ET-resistant BC cell proliferation and BCSC survival. The mechanism by which DAXX inhibits ET-resistant BC is through activation of JNK signaling, regulation of pro-apoptotic genes, and induction of apoptosis. The translational impact of this research is to identify novel agents that can increase DAXX expression and test them pre-clinically and in clinical trials for patients with ET-resistant breast cancer.

Citation Format: Clodia Osipo, Kathy S. Albain, Daniel Peiffer, Debra Wyatt. Increasing DAXX Expression in ER+ Breast Cancer Cells to Overcome Endocrine Therapy Resistance [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P3-11-01.

Abstract P3-05-03: KMT2D as a novel therapeutic target for HER2+ breast cancers

Background: Breast cancer is the second leading cause of cancer-related deaths among women in the US. Major factors that contribute to breast cancer carcinogenesis include disruptions in both genetic and epigenetic processes. Histone modifications and DNA-methylation are common epigenetic changes that have been implicated in several breast cancers, including an aggressive subset known as human epidermal growth factor receptor 2 (HER2+) breast cancers. One prognostic marker for poor survival in women with HER2+ breast cancer is high RNA expression of the critical epigenetic regulator, Histone-lysine methyltransferase 2D (KMT2D). KMT2D belongs to a family of histone modifying proteins which plays an essential role in regulating developmental processes. Specifically, KMT2D is a histone methyltransferase that monomethylates histone 3 lysine 4 at enhancers throughout the genome and is required for transcriptional activation. Interestingly, in HER2+ cells (BT474), KMT2D mRNA was found to be significantly increased in stem-like tumor initiating cells (TICs) compared to bulk cells. BT474 cells that have acquired resistance to trastuzumab (BTR) from prolonged treatment express higher levels of KMT2D than those of the isogenic parental sensitive cells (BTS). Furthermore, siRNA-specific knockdown of KMT2D significantly decreased bulk cell proliferation in both BTS and BTR cells, and significantly inhibited TIC survival in BTS cells. Thus, we hypothesized that KMT2D is a critical epigenetic regulator that contributes to resistance by promoting bulk cell proliferation and TIC survival. Methods: RNA-sequencing was performed in BTS and BTR cells-expressing or depleted for KMT2D. Differentially regulated genes were identified and pathway analysis was performed. Chromatin immunoprecipitation (ChIP) PCR and sequencing were performed to identify novel KMT2D-bound regions in both BTS and BTR cells. SiRNA induced silencing was also utilized to study various gene functions in BT474 cells. Results: We identified and confirmed by RT-qPCR several differentially expressed genes that were positively and negatively regulated by KMT2D, suggesting they may be critical direct targets of KMT2D. One gene, ITGB6, encodes for integrin subunit beta 6 and was found to be decreased upon KMT2D depletion. ITGB6 regulates multiple processes which contribute to cancer progression and metastases, including cellular proliferation and epithelial-mesenchymal transition. Similar to KMT2D knockdown, ITGB6 knockdown resulted in decreased BTS and BTR bulk cell proliferation and a reduction in TICs. Interestingly, KMT2D enrichment at an ITGB6 enhancer region was lower in resistant cells compared to sensitive cells. Future studies will include knockdown of KMT2D with simultaneous rescue of wild type or a mutant of ITGB6 to determine if KMT2D-mediated increase in ITGB6 is required for HER2+ cell growth and/or TIC survival. Furthermore, our data also suggests an anti-apoptotic and cell-cell adhesion role for KMT2D, which may in part be due to its direct effects on ITGB6. Additional future studies will investigate the mechanism of how KMT2D regulates its target genes through ChIP-sequencing analysis as well as studies to evaluate KMT2D’s role in vivo. Conclusions: Our results suggest a critical role for KMT2D in HER2+ breast cancer progression, and may yield novel therapeutic targets and pathways for the treatment of HER2+ breast cancers.

Citation Format: Emily Ma, Andrei Zlobin, Debra Wyatt, Jeffrey Ng, Andrew Dingwall, Clodia Osipo. KMT2D as a novel therapeutic target for HER2+ breast cancers [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P3-05-03.

Abstract P1-08-22: The role of DAXX as a critical growth regulator of triple negative breast cancer

Background: Triple negative breast cancer (TNBC) is a heterogeneous disease that frequently presents at a metastatic stage or upon initial early stage diagnosis, and may recur quickly. Treatment options are limited outside of cytotoxic chemotherapy in the adjuvant or metastatic setting and there is an immediate need to identify and validate novel biomarkers that predict sensitivity to chemotherapy. Previously, it was shown that the pro-apoptotic protein DAXX is required for paclitaxel-mediated breast cancer cell death. Based on these data and our recent discoveries that DAXX is a potent inhibitor of breast cancer stem cells in ER+ breast cancer (Peiffer et al., Cancer Res. 2019; Peiffer et al., NPJ Breast Cancer. 2020), we next explored the role of DAXX in TNBC. We found that high DAXX expression in TNBC cells regulates cell proliferation and activities of PARP-1 and JNK; two pathways required for cell death. The goals of the current work were to investigate the mechanism by which DAXX regulates proliferation at the level of PARP-1 and/or JNK and test if DAXX expression predicts sensitivity to carboplatin, paclitaxel, doxorubicin, the PARP-1 inhibitor olaparib, or the JNK inhibitor SP600125 in TNBC cells. Methods: Proliferation and cell cycle analysis were performed in three TNBC cell lines (MDA-MB-231, BT549, and MDA-MB-468) under DAXX expression and depletion conditions using an siRNA approach. Sensitivity to carboplatin, paclitaxel, and doxorubicin was determined by measuring proliferation in a dose-dependent manner. PARP-1 activity was assessed by detecting global protein PARylation levels using Western blotting. JNK activation was analyzed by measuring phosphorylated JNK levels. A JNK inhibitor (SP600125) was used to determine if JNK signaling was responsible for restricting cell proliferation and if DAXX was required for the elevated JNK activity. Olaparib was used to assess the role of PARP-1 activation in DAXX-depleted cells. Results: The TNBC cell lines expressed high levels of DAXX. Depletion of DAXX increased cell proliferation by promoting cell cycle progression through the S-phase. The consequence of this increased cell proliferation was decreased sensitivity to carboplatin and paclitaxel, but not to doxorubicin. Mechanistically, at least one chemotherapeutic agent, carboplatin induced JNK activation and DAXX was required for the increased JNK activity. The JNK inhibitor (SP600125) partially reversed resistance to carboplatin in DAXX-depleted cells. Additionally, genetic knockdown of DAXX resulted in elevated PARP-1 activity as measured by global PARylation of proteins. PARP-1 blockade using olaparib also partially reversed resistance to carboplatin in DAXX-depleted cells. Conclusions: These results suggest that DAXX is a critical growth regulator and predictor of response to carboplatin and paclitaxel in triple negative breast cancer cells. DAXX is necessary to activate the JNK pathway in response to carboplatin and it is this carboplatin-mediated JNK signaling that inhibits cell proliferation. Furthermore, DAXX limits PARP-1 activity and is necessary for increased sensitivity to carboplatin or paclitaxel. Data herein support assessing DAXX expression in triple negative tumors to determine if high DAXX expression predicts increased sensitivity to carboplatin, paclitaxel, and if DAXX-low expressing TNBC cells require addition of a JNK or PARP-1 inhibitor.

Citation Format: Clodia Osipo, Debra Wyatt, Michelle Fernandez, Daniel S Peiffer, Kathy S Albain. The role of DAXX as a critical growth regulator of triple negative breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-08-22.

Abstract P5-12-10: A commensal’s exopolysaccharide inhibits breast cancer proliferation in vitro but promotes tumor growth in vivo

Background: Recent decades have started to realize the importance of the microbiome in cancer initiation, progression and response to therapy. Recent findings suggest that both the local and distal microbiome may influence tumor growth. Although many studies focused on the gut microbiome, the breast itself also possesses a unique microbiome which differs between healthy and breast cancer patients, and between tumor subtypes. However, it is poorly understood whether these bacteria in the gut or the breast microenvironment may interact with breast cancer cells and if that interaction alters tumor growth. Bacteria produce metabolites and other signaling molecules to alter the host’s response. We have identified and purified an exopolysaccharide (EPS) produced by the commensal bacterium Bacillus subtilis, which was shown to act directly on the host’s innate immune cells to protect against various inflammatory diseases. This study aims to investigate if, and how the bacterial product (EPS) alters proliferation and tumor growth of breast cancer cells.Methods: Cell proliferation of bulk cells was conducted on PBS or EPS treated breast cancer cells (T47D, MDA-MB-468, MDA-MB-453, HCC1428, BT549, MDA-MB-231, ZR-75-30, and MCF-7) using trypan blue exclusion and the XTT assay. Flow cytometry was used to assess apoptosis with Annexin V, and cell cycle progression with propidium iodide. Cancer stem cell survival was assessed using the mammosphere forming assay. CRISPR/Cas9-mediated knockout of the Toll-like-receptor 4 (TLR4) was used to assess the requirement of TLR4 signaling on EPS-mediated phenotypes. RNA-sequencing was performed on both EPS-sensitive and resistant breast cancer cell lines after treatment with PBS or EPS for 20 hours. T47D xenografts studies were performed in vivo using EPS pretreatment and continual treatment via i.p. injection.Results: EPS inhibited proliferation of some breast cancer cell lines (T47D, MDA-MB-468, MDA-MB-453, HCC1428) in a concentration dependent manner while having little effect on others (BT549, MDA-MB-231, ZR-75-30, MCF7). EPS acted via distinct mechanisms depending on the cell line, increasing apoptosis in MDA-MB-468 cells while inducing G0/G1 cell cycle arrest in T47D cells. Although EPS directly inhibited bulk cell proliferation, the cancer stem cell population of T47D cells was increased by EPS. EPS is known to require TLR4 signaling to modulate immune cells. CRISPR/Cas9 knockout of TLR4 demonstrated that it was not required for EPS-mediated inhibition of T47D breast cancer cells, suggesting a novel mechanism of action in this cell line. RNA-sequence analysis showed that EPS altered interferon signaling and cell cycle progression pathways. Interferon signaling activates the STAT family of transcription factors, and western blots confirmed that 3h of EPS treatment induced STAT1 phosphorylation and p21 accumulation, indicating that EPS activated this pathway. Contrary to in vitro proliferation data, in vivo results showed that EPS-treated cells formed significantly larger tumors compared to PBS controls (273mg vs 59mg). The slope of tumor growth was also significantly faster in the EPS-treated group. Conclusions: These findings demonstrated the multifacet of EPS’s effect on breast cancer cells. EPS inhibits the proliferation of certain breast cancer cells in vitro, possibly via activation of STAT1 and p21 signaling, but also increases survival of cancer stem cells and promotes more aggressive tumor growth in vivo. This highlights the complexity of the influence of commensal bacteria on breast cancer growth, which can be highly context-dependent. Deeper understanding of this complex interplay between the host-microbe-tumor will be vital to the development of novel bacterial strategies in the future.

Citation Format: Mai Rachel Nguyen, Katherine L Knight, Clodia Osipo. A commensal’s exopolysaccharide inhibits breast cancer proliferation in vitro but promotes tumor growth in vivo [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-12-10.

Abstract PS18-03: Differential gene expression in luminal-type invasive lobular carcinoma and invasive ductal carcinoma by MammaPrint risk stratification

Background: Invasive lobular carcinoma (ILC) comprises 10-15% of breast tumors and is the second most common histological type after invasive ductal carcinoma (IDC). Patients with ILC are often diagnosed at an older age and more advanced stage than those with IDC. Late recurrences and worse long-term survival suggest the need for improved approaches to treatment optimization and exploration of molecular pathways unique to ILC. Although previous reports have described comprehensive transcriptomic profiling of ILC, these were limited by small sample sizes. Furthermore, differential gene expression between ILC and IDC within genomic risk groups and molecular subtypes has yet to be explored. Here we characterize differential gene expression between ILC and IDC in a large, age-matched patient subset categorized by 70-gene signature/MammaPrint (MP) risk and 80-gene signature/BluePrint (BP) subtype.

Methods: The prospective FLEX Registry (NCT03053193) includes stage I-III primary invasive breast cancer patients who receive MP/BP testing and consent to full transcriptome and clinical data collection. This sub-analysis included 450 ILC patients enrolled from 2017 to present. Compared with a random selection of IDC patients (n=450, mean age, 60 years), ILC patients were older (mean, 63 years, p<0.001). Thus, we selected an age-matched subset for differential gene expression analysis. There were few non-Luminal ILCs; thus, gene expression analyses were limited to BP Luminal tumors. A subset of 413 age-matched pairs (n=826) of ILC and IDC were used for analysis. Gene expression data were quantile normalized using R limma package, and differentially expressed genes (DEGs) were compared between groups. DEGs with an adjusted p<0.05 and log2 fold change > ± 1.0 were considered significant.

Results: ILC represented 13% of FLEX cases (n=450/3562), and were 81% lymph node-negative, 99% ER+, 94% HER2-negative, and 68% MP Low Risk (LR). By BP, ILC were 99% Luminal, 1% HER2, and <1% Basal type. BP Luminal ILC were predominantly grade 2 (63%), T1 (61%), node-negative (84%), and MP LR (69%). Menopausal status, nodal status, ethnicity, BMI distribution, and frequency of type 2 diabetes mellitus were similar between ILC and IDC. However, IDC were more likely to be MP HR (46% IDC vs. 31% ILC, p<0.001) and grade 3 (15% IDC vs. 4% ILC, p<0.001). ILC were more likely to be T3 (10% ILC vs. 1% IDC, p<0.001). We found 4 DEGs common to all comparisons: all Luminal ILC vs. IDC, MP LR ILC vs. IDC, and MP HR ILC vs. IDC. ILC had lower expression of CDH1 (E-cadherin) than IDC, regardless of MP risk. Including CDH1, 6 unique genes were differentially expressed in LR ILC compared with IDC, and 21 genes were differentially expressed in HR ILC compared with IDC. Genes with increased expression in HR ILC were related to immune cell migration/chemotaxis, hormone signaling, and growth factor signaling. HR ILCs were also enriched for TGFβ signaling and angiogenesis pathway genes.

Conclusions: Here we report differential clinical and molecular characteristics between ILC and IDC in a large, age-matched patient subset. Regardless of MP risk, expression of CDH1 was lower in ILC compared with IDC. Approximately one-third of ILCs were MP HR, and we report a greater number and diversity of DEGs between HR ILC and HR IDC compared with LR tumors, in particular genes related to TGFβ signaling. TGFβ pathway genes play a variety of roles in the tumor microenvironment, including induction of angiogenesis, fibroblast growth factor stimulation, and inhibition and/or exclusion of an immune response. These results suggest that therapeutic strategies targeting the TGFβ pathway may be future avenues of exploration in ILC, although further studies are warranted to characterize underlying molecular mechanisms.

Citation Format: Beth-Ann Lesnikoski, Jennifer A. Crozier, Gordan Srkalovic, Patricia Robinson, Clodia Osipo, Kaylan Banda, Heather M. Kling, Josien Haan, William Audeh, FLEX Investigators Group. Differential gene expression in luminal-type invasive lobular carcinoma and invasive ductal carcinoma by MammaPrint risk stratification [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS18-03.

Notch Signaling in Breast Cancer: A Role in Drug Resistance

Breast cancer is a heterogeneous disease that can be subdivided into unique molecular subtypes based on protein expression of the Estrogen Receptor, Progesterone Receptor, and/or the Human Epidermal Growth Factor Receptor 2. Therapeutic approaches are designed to inhibit these overexpressed receptors either by endocrine therapy, targeted therapies, or combinations with cytotoxic chemotherapy. However, a significant percentage of breast cancers are inherently resistant or acquire resistance to therapies, and mechanisms that promote resistance remain poorly understood. Notch signaling is an evolutionarily conserved signaling pathway that regulates cell fate, including survival and self-renewal of stem cells, proliferation, or differentiation. Deregulation of Notch signaling promotes resistance to targeted or cytotoxic therapies by enriching of a small population of resistant cells, referred to as breast cancer stem cells, within the bulk tumor; enhancing stem-like features during the process of de-differentiation of tumor cells; or promoting epithelial to mesenchymal transition. Preclinical studies have shown that targeting the Notch pathway can prevent or reverse resistance through reduction or elimination of breast cancer stem cells. However, Notch inhibitors have yet to be clinically approved for the treatment of breast cancer, mainly due to dose-limiting gastrointestinal toxicity. In this review, we discuss potential mechanisms of Notch-mediated resistance in breast cancer cells and breast cancer stem cells, and various methods of targeting Notch through γ-secretase inhibitors, Notch signaling biologics, or transcriptional inhibitors. We also discuss future plans for identification of novel Notch-targeted therapies, in order to reduce toxicity and improve outcomes for women with resistant breast cancer.

Abstract 1859: Elucidating the role of KMT2D as a novel therapeutic target for resistant HER2+ breast cancer

HER2 targeted agents have improved clinical outcomes for women with HER2+ breast cancer. However, drug resistance remains a critical challenge for curing HER2+ disease. Cancer stem cells (CSCs)-enriched during anti-HER2 therapy promote resistance and recurrence. Therefore, novel targets that drive survival of CSCs are needed. Recently, the histone-lysine N-methyltransferase 2D (KMT2D) has been shown to be a prognostic marker for poor recurrence free survival in women with HER2+ breast cancer. Preliminary data showed that KMT2D RNA was increased in HER2+ CSC-enriched mammospheres compared to bulk cells. Further, KMT2D protein is increased in resistant compared to sensitive cells. These data suggest that KMT2D is necessary for survival of CSCs and resistance to anti-HER2 therapy. To address this hypothesis, RNAi-mediated knockdown of KMT2D was performed in trastuzumab sensitive and resistant cells. KMT2D depletion in sensitive cells resulted in almost complete blockade of proliferation in 2-D culture and CSC survival in 3-D culture. In contrast, proliferation and CSC survival was only moderately inhibited by KMT2D depletion in resistant cells. ChIP studies confirmed that KMT2D is enriched on the MYC enhancer in sensitive cells but only moderately in resistant cells. Based on these data, KMT2D is necessary for growth of HER2+ breast cancer and is possibly inactivated during acquisition of resistance. To determine the mechanism, RNA sequencing was performed in both sensitive and resistance cells expressing or depleted for KMT2D. Several differentially expressed genes that were positively or negatively regulated by KMT2D were identified. Specifically, transcript levels of ITGB6 and CLIC3 were decreased upon KMT2D knockdown, while PGR and AR transcripts were increased. Initial results showed that ITGB6 depletion resulted in decreased bulk cell proliferation and survival of CSCs similar to KMT2D knockdown. These results suggest that ITGB6 could be a direct target gene of KMT2D and thus mediates the phenotypic effects of KMT2D. Overall, our results suggest that KMT2D is necessary for both proliferation and CSC survival in HER2+ breast cancer sensitive to anti-HER2 therapy and a combination approach of targeting both HER2 and KMT2D could prevent enrichment of CSCs and acquired resistance.

Citation Format: EMILY MA, Andrei Zlobin, Debra Wyatt, Jeffrey Ng, Andrew Dingwall, Clodia Osipo. Elucidating the role of KMT2D as a novel therapeutic target for resistant HER2+ breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1859.

DAXX-inducing phytoestrogens inhibit ER+ tumor initiating cells and delay tumor development

Recurrence of estrogen receptor (ER)-positive breast tumors despite curative-intent adjuvant therapy is thought to be due to enrichment of tumor initiating cells (TIC) during endocrine therapy (ET). Recently, it was identified that by antagonizing the ER, ET promotes rapid degradation of the death-associated factor 6 (DAXX) protein, which is necessary and sufficient to potently inhibit TICs. Thus, the goal of the current study was to identify a DAXX-inducing agent to inhibit TICs and prevent proliferation of the tumor. Phytoestrogens (naringenin, resveratrol, genistein, apigenin, and quercetin) were screened for DAXX protein expression, anti-TIC and anti-proliferative efficacy in vitro and in vivo. Specific DAXX-inducing phytoestrogens were tested to assess selectivity towards ERα and/or ERβ. Results showed that phytoestrogens tested induced DAXX protein expression and inhibited survival of TICs from ER+ MCF-7 and T47D cells. Only naringenin, resveratrol, and quercetin did not stimulate total cell proliferation. Naringenin, resveratrol, but not quercetin inhibited survival of TICs in vitro and in vivo in a DAXX-dependent manner. Naringenin-induced DAXX protein expression and inhibition of TICs seemed to be more selective towards ERβ while resveratrol was more selective through ERα. Naringenin or resveratrol inhibited the rate of tumor initiation and rate of tumor growth in a DAXX-dependent manner. These results suggest that a therapeutic approach using a phytoestrogen to induce DAXX protein expression could potently inhibit TICs within a tumor to delay or prevent tumor initiation. Therefore, a DAXX-promoting phytoestrogen should be explored for prevention of tumor progression in advanced disease and relapse in the adjuvant setting.

Molecular profiles and treatment recommendations for invasive lobular carcinoma in a real-world prospective breast cancer registry

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Background: Invasive lobular carcinoma (ILC), the second most common histological breast cancer type, comprises 10-15% of breast tumors. Although ILC is thought to have a low risk of relapse, patient survival data vary. ILC treatment guidelines mirror those for invasive ductal carcinoma (IDC); however, these were extrapolated from trials that included predominantly IDC. The FLEX Registry captures data from real-world breast cancer patients; the current sub-study evaluated molecular profiles and treatment recommendations in ILC. Methods: The FLEX Registry (NCT03053193) includes stage I-III primary invasive breast cancer patients who receive 70-gene signature (70-GS)/80-gene signature (80-GS) testing and consent to full transcriptome and clinical data collection. This sub-analysis includes 335 ILC patients and 2,179 IDC patients enrolled from 2017 to present. 70-GS stratified tumors by risk of distant metastasis [High (HR), Low (LR), Ultralow (UL) Risk], and 80-GS classified tumors by molecular subtype (Luminal, HER2, or Basal type). Genomic and clinical data were compared for ILC and IDC using chi-square or Fisher’s exact test. Results: ILC represented 13% of FLEX cases (n = 335/2752); 81% were lymph node-negative, 99% ER-positive, and 94% HER2-negative. Clinical risk assessment (MINDACT criteria) classified 61% of ILC and 53% of IDC as low risk (p = 0.03). 70-GS risk distribution in ILC (18% UL, 51% LR, 31% HR) differed from IDC (13% UL, 34% LR, 53% HR; p < 0.01). Discordance between 70-GS and clinical risk was greater in ILC (43%) than IDC (30%, p < 0.01). 80-GS results also differed between ILC (98% Luminal, 1.6% HER2, 0.3% Basal) and IDC (85% Luminal, 3% HER2, 11% Basal; p < 0.01). Inclusion of chemotherapy (CT) in treatment plans was associated with 70-GS risk (p < 0.01); treatment plans that disagreed with 70-GS results were associated with discordant clinical risk (p < 0.05). Treatment plans disagreed with 70-GS in 15% of ILC and 12% of IDC total cases (p = 0.08), and more frequently in HR cases (25% in ILC, 14% in IDC; p < 0.01). Conclusions: The FLEX Registry includes patients with ILC consistent with real-world breast cancer frequencies. ILC demonstrated genomic risk and subtype profiles distinct from IDC. Treatment plans largely agreed with 70-GS results; however, discordance was more frequent in HR ILC than IDC. Future studies will evaluate clinical outcomes; however, these results demonstrate the added value of molecular profiling in ILC and the utility of real-world registry data in evaluating uncommon breast tumor types. Clinical trial information: NCT03053193 .

DAXX Suppresses Tumor-Initiating Cells in Estrogen Receptor-Positive Breast Cancer Following Endocrine Therapy

Estrogen receptor (ER)-positive breast cancer recurrence is thought to be driven by tumor-initiating cells (TIC). TICs are enriched by endocrine therapy through NOTCH signaling. Side effects have limited clinical trial testing of NOTCH-targeted therapies. Death-associated factor 6 (DAXX) is a newly identified marker whose RNA expression inversely correlates with NOTCH in human ER⁺ breast tumor samples. In this study, knockdown and overexpression approaches were used to investigate the role of DAXX on stem/pluripotent gene expression, TIC survival in vitro, and TIC frequency in vivo, and the mechanism by which DAXX suppresses TICs in ER⁺ breast cancer. 17β-Estradiol (E₂)-mediated ER activation stabilized the DAXX protein, which was required for repressing stem/pluripotent genes (NOTCH4, SOX2, OCT4, NANOG, and ALDH1A1), and TICs in vitro and in vivo. Conversely, endocrine therapy promoted rapid protein depletion due to increased proteasome activity. DAXX was enriched at promoters of stem/pluripotent genes, which was lost with endocrine therapy. Ectopic expression of DAXX decreased stem/pluripotent gene transcripts to levels similar to E₂ treatment. DAXX-mediated repression of stem/pluripotent genes and suppression of TICs was dependent on DNMT1. DAXX or DNMT1 was necessary to inhibit methylation of CpGs within the SOX2 promoter and moderately within the gene body of NOTCH4, NOTCH activation, and TIC survival. E₂-mediated stabilization of DAXX was necessary and sufficient to repress stem/pluripotent genes by recruiting DNMT1 to methylate some promoters and suppress TICs. These findings suggest that a combination of endocrine therapy and DAXX-stabilizing agents may inhibit ER⁺ tumor recurrence. SIGNIFICANCE: Estradiol-mediated stabilization of DAXX is necessary and sufficient to repress genes associated with stemness, suggesting that the combination of endocrine therapy and DAXX-stabilizing agents may inhibit tumor recurrence in ER⁺ breast cancer.

Abstract 5845: Roles for MLL2/ KMT2D or MLL3/ KMT2C in HER+ breast cancer stem cells

A major clinical challenge for the treatment of breast cancer (BC) is drug resistance. Cancer stem cells (CSCs) may contribute to drug resistance despite therapy. The HER2-positive subtype of BC contains a gene amplification for the ERBB2 proto-oncogene and these tumors are treated with anti-HER2 therapy. Anti-HER2 therapy, such as trastuzumab or lapatinib, increases Notch signaling and is required for HER2+ breast tumor resistance and tumor recurrence. Notch promotes survival of BC CSCs cells. MLL2/KMT2D (Lysine Methyltransferase 2D) and MLL3/KMT2C (Lysine Methyltransferase 2C) are histone methyltransferases that directly control gene enhancer activity through methylation of histone3 lysine4 within enhancer nucleosomes. These complexes collaborate with transcription factors, such as Notch, to drive gene expression by helping to shape the epigenetic landscape of a cell. Recently, it was shown that lapatinib or a PI3K inhibitor increases MLL2 activity and this increase in MLL2 contributes to resistance. Both MLL2 and MLL3 were found to be required for CSCs differentiation, while MLL2 was required for upregulation of c-Myc in HER2+ cells. Recent breast cancer xeno-engraftment studies found that clonal selection and expansion of primary and metastatic breast tumors was associated with amplification of MLL3 in 80% of breast cancer xenografts, suggesting that increased MLL3 expression may contribute to the survival of CSCs in breast tumors. High expression of MLL2 and/or MLL3 predict poor outcome for women with HER2+ breast cancer from cBio portal and Kaplan-Meier Plotter datasets. We measured the expression levels of MLL2 and MLL3 transcripts in both bulk HER2+ breast cancer cells that are trastuzumab sensitive, cells that have acquired resistance to trastuzumab, and cancer stem-enriched mammospheres. Results from PCR analysis showed that MLL2 and MLL3 transcripts were increased in mammospheres compared to bulk cells. Mammosphere forming efficiency of trastuzumab sensitive cells was decreased when MLL2 or MLL3 was knocked down using RNAi. Mammosphere forming efficiency of anti-HER2 therapy resistant (BT474 TR) cells was significantly decreased upon MLL3 knockdown, while HCC1954 mammospheres were inhibited by either MLL2 or MLL3 knockdown. We next measured transcript levels of Notch genes and other stem, mesenchymal, and luminal genes to determine whether MLL2 or MLL3 is necessary for the proper expression of cell fate genes. We found that Notch3 transcripts were decreased in both BT474 and HCC1954 cells upon MLL2 knockdown. SNAIL and Notch1 transcripts were also decreased, suggesting that MLL2 is required for the expression of genes involved in regulating cancer stem cell fates. These results suggest that MLL2/KMT2D and its paralog MLL3/KMT2C are increased in CSCs-derived from HER2+ breast cancer and may regulate CSC genes such Notch1 and Notch3 to promote resistance to anti-HER2 based therapy.

Citation Format: Andrei Zlobin, Debra Wyatt, Mary Varsanik, Andrew Dingwall, Clodia Osipo.. Roles for MLL2/ KMT2D or MLL3/ KMT2C in HER+ breast cancer stem cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5845.

Abstract 1831: PKCδ as a novel target for HER2-positive, trastuzumab resistant breast cancer

Breast cancer is the most common cancer diagnosis and second leading cause of cancer related deaths among women in the United States. This poor prognosis is in part due to the high rates of resistance. In particular, resistance to anti-HER2-based therapy remains a clinical challenge for women with breast cancer suggesting that better treatment strategies are needed. Results demonstrate that resistance to trastuzumab correlates with lower PKC activity compared to trastuzumab sensitive breast cancer cells, suggesting that downregulation of PKC activity may contribute to trastuzumab resistance. The exact mechanism by which PKCs contribute to trastuzumab resistance is not well understood. Here, we hypothesized that activation of PKCs could prevent trastuzumab resistance and thus inhibit the survival and recurrence of HER2+ breast cancer. Our results show that in two distinct trastuzumab-resistant cell lines (HCC1954 and BT474), activation of PKC by the phorbol ester TPA inhibits both proliferation of bulk cells and survival of cancer stem cells (CSC) as assessed by mammosphere formation. Further, treatment using a pharmacological PKC inhibitor (GF109203X) almost completely rescued the anti-proliferative and anti-CSC survival effects of TPA in two distinct trastuzumab-resistant cell lines. Measurement of PKC isoforms at the transcript and protein levels showed that PKCδ was overexpressed in the resistant cell line compared to the sensitive cell line. PKCδ depletion using a siRNA prevented the anti-proliferative and anti-CSC survival effects of TPA, suggesting that PKCδ was the target of TPA activation in trastuzumab resistant cells. TPA-mediated activation of PKCδ was found to cause G2/M cell cycle arrest, almost complete inhibition of AKT phosphorylation at serine 473, and decreased expression of the CSC marker, ALDH1A1. These results suggest that phorbol ester-mediated activation of PKCδ could be a novel therapeutic strategy for HER2+ breast cancer that is resistant to trastuzumab-based therapy.

Citation Format: Debra L. Wyatt, Andrei Zlobin, Emily Ma, Mitchell F. Denning, Clodia Osipo. PKCδ as a novel target for HER2-positive, trastuzumab resistant breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1831.

Inhibition of HER2 Increases JAGGED1-dependent Breast Cancer Stem Cells: Role for Membrane JAGGED1

Purpose: HER2-positive breast cancer is driven by cells possessing stem-like properties of self-renewal and differentiation, referred to as cancer stem cells (CSC). CSCs are implicated in radiotherapy, chemotherapy resistance, and tumor recurrence. NOTCH promotes breast CSC survival and self-renewal, and overexpression of NOTCH1 and the NOTCH ligand JAGGED1 predict poor outcome. Resistance to anti-HER2 therapy in HER2⁺ breast cancer requires NOTCH1, and that combination of trastuzumab and a gamma secretase inhibitor (GSI) prevents tumor relapse in xenograft models.Experimental Design: The current study investigates mechanisms by which HER2 tyrosine kinase activity regulates NOTCH-dependent CSC survival and tumor initiation.Results: Lapatinib-mediated HER2 inhibition shifts the population of HER2⁺ breast cancer cells from low membrane JAGGED1 expression to higher levels, independent of sensitivity to anti-HER2 treatment within the bulk cell population. This increase in membrane JAGGED1 is associated with higher NOTCH receptor expression, activation, and enrichment of CSCs in vitro and in vivo Importantly, lapatinib treatment results in growth arrest and cell death of JAGGED1 low-expressing cells while the JAGGED1 high-expressing cells continue to cycle. High membrane JAGGED1 protein expression predicts poor overall cumulative survival in women with HER2⁺ breast cancer.Conclusions: These results indicate that higher membrane JAGGED1 expression may be used to either predict response to anti-HER2 therapy or for detection of NOTCH-sensitive CSCs posttherapy. Sequential blockade of HER2 followed by JAGGED1 or NOTCH could be more effective than simultaneous blockade to prevent drug resistance and tumor progression. Clin Cancer Res; 24(18); 4566-78. ©2018 AACR.

Preclinical study of a Kv11.1 potassium channel activator as antineoplastic approach for breast cancer

Potassium ion (K⁺) channels have been recently found to play a critical role in cancer biology. Despite that pharmacologic manipulation of ion channels is recognized as an important therapeutic approach, very little is known about the effects of targeting of K⁺ channels in cancer. In this study, we demonstrate that use of the Kv11.1 K⁺ channel activator NS1643 inhibits tumor growth in an in vivo model of breast cancer.

Tumors exposed to NS1643 had reduced levels of proliferation markers, high expression levels of senescence markers, increased production of ROS and DNA damage compared to tumors of untreated mice. Importantly, mice treated with NS1643 did not exhibit significant cardiac dysfunction. In conclusion, pharmacological stimulation of Kv11.1 activity produced arrested TNBC-derived tumor growth by generating DNA damage and senescence without significant side effects. We propose that use of Kv11.1 channels activators could be considered as a possible pharmacological strategy against breast tumors.

Abstract B38: Exploring targeting potassium channels in cancer: A novel strategy for therapeutic intervention

Failing of anticancer therapies is mostly due to our limited understanding of biochemical signaling in cancer cells. Therefore, identification of new targets and new strategies for anticancer drug discovery is greatly needed.

Potassium ion (K+) channels are transmembrane associated proteins that control several biological processes including cell proliferation and cell motility in both physiological and pathological conditions by allowing an outward flux of K+ ion to cross the surface membrane. Furthermore, several K+ ion channels have been found expressed in cancers of different histogenesis but not in tissues from which cancer has been generated. This suggests that changes in K+ ion gradients play a key role in cancer biology. In contrast to blockers, K+ channels activators are well tolerated in mammals and several FDA approved K+ channels activators are readily available on the market suggesting that these agents could be used as potential anticancer drug. Nevertheless, very little has been done to understand whether these proteins can be used as therapeutic targets against cancers.

We present data showing for the first time that pharmacologic stimulation of specific K+ channels with small molecules strongly reduced tumor growth and metastatic spread in different animal model of cancer biology including Drosophila and mice without discernible side effects.

For example, use of two chemically distinct Kv11.1 potassium channel activators showed growth arrest in an in vivo Drosophila tumor model and strongly inhibited tumor growth in SCID mice bearing the aggressive TNBC cell line MDA-MB-231.

In addition, the metastatic spread of this cancer cell line in NOD-scid-IL2Rγnull was significantly reduced in terms of organs affected and metastatic burden.

Interestingly, analysis of the biochemical signaling linking activation of specific K+ channels to tumor growth inhibition revealed that pharmacological stimulation of K+ channel activated a cellular senescent program that is characterized by increased expression of the tumor suppressors (e.g. p21, p16, HMGA2), formation of heterochromatin and permanent growth arrest of cancer cells independently of their molecular subtypes (including p53 negative cancer cells, breast, skin or ovarian cancer cells).

Investigating on the possible mechanism linking Kv11.1 stimulation to inhibition of metastasis revealed that Kv11.1 agonists strongly inhibit cancer cell motility by suppressing nuclear function of bβ-catenin in transcribing for Epithelial to Mesenchymal Transition (EMT) markers including vimentin, CD44, N-cadherin. Concurrently, K+ channel agonists increased cytoplasmic β-catenin protein level by repressing its degradation, increased levels of the adhesive molecule E-cadherin and promoted formation of the proteins complex bβ-catenin/E-cadherin at the surface membrane. Overall, our data strongly suggest that use of K+ channel agonists can inhibit tumor progression by activating senescence and inhibit metastasis by reversing mesenchymal into epithelial phenotype (MET).

We think that these are important discoveries because they offer an opportunity to identify in K+ channels proteins that can be therapeutically targeted to eliminate tumor growth and metastasis.

Citation Format: Eun-Kyoung Breuer, Clodia Osipo, Jeremiah Zartman, Claire Wells, Michael Nishimura, Walter Jones, Saverio Gentile. Exploring targeting potassium channels in cancer: A novel strategy for therapeutic intervention [abstract]. In: Proceedings of the AACR Precision Medicine Series: Opportunities and Challenges of Exploiting Synthetic Lethality in Cancer; Jan 4-7, 2017; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2017;16(10 Suppl):Abstract nr B38.

Abstract 4769: Targeting HER2 enriches Jagged1 high cancer stem cells in breast cancer

The human epidermal growth factor receptor 2 (HER2) positive subtype of breast cancer is characterized by gene amplification and/or protein overexpression of HER2. It is driven by a subpopulation of cells possessing stem cell properties of self-renewal and differentiation, known as Cancer Stem cells (CSCs). CSCs are implicated in tumor growth as well as radiotherapy and chemotherapy associated resistance. Notch receptors promote breast CSCs survival and self-renewal, and overexpression of a Notch ligand Jagged1 mRNA predicts poor prognosis in women with breast cancer. Our lab has published that Jagged1 or Notch1 is a critical target in trastuzumab/lapatinib (LAP) resistant HER2+ breast cancer. The study aimed to determine whether anti-HER2 therapy selects for Jagged-1/Notch-dependent CSCs that are responsible for tumor initiation. Surface expression of Notch1 and Jagged1 upon HER2 blockade using LAP was measured in HER2+ breast cancer cell lines (MDA-MB-453 and HCC1954) using flow cytometry. LAP treatment increased the Jagged1-positive subpopulation compared to vehicle. Cells were sorted based on Jagged1 cell surface expression and assessed for CSC-like properties (i.e. mammosphere forming efficiency, Aldefluor activity, expression of CD44high /CD24low, Notch target transcripts expression, and limiting dilution tumor initiating potential in athymic, nude mice). In addition, immunohistochemistry was performed on 145 HER2+ breast tumor microarray to detect cytoplasmic, membrane, or nuclear Jagged1 protein expression. Kaplan-Meier analysis was performed to determine overall survival. The results showed that upon HER2 inhibition, Jagged1 cell surface expression increased and Notch1 cell surface expression was unchanged. The Jagged1high subpopulation of cells showed elevated levels of Aldehyde dehydrogenase activity, Notch target gene transcripts, and mammosphere formation efficiency compared to vehicle treated cells. The MRK-003 γ-secretase inhibitor (GSI) prevented mammosphere formation in the Jagged1high cells indicating that Notch activation drives Jagged1high CSC survival. Also, we confirmed that Jagged1 expression is required for the enrichment of CSCs using a Jagged1 siRNA. Combined knockdown of Notch1 and Notch3 receptors was necessary to reduce LAP-enriched mammospheres suggesting that targeting HER2 enriches for a Jagged1-Notch1+Notch3 driven CSC phenotype. Importantly, higher membrane expression of Jagged1 protein in 145 HER2+ breast tumor specimens correlated with significantly lower overall cumulative survival. These results reveal that HER2 blockade in breast cancer cells enriches for a Jagged1high subpopulation that has higher CSC potential and is resistant to HER2 inhibitors. The implications of this work are that dual blockade of Jagged-1/Notch1/Notch3 and HER2 could be more effective than either therapy alone to eliminate both HER2 and Jagged-1-dependent cancer cells.

Citation Format: Deep S. Shah, Debra Wyatt, Andrew Baker, Andrew Green, Aleksandra Filipovic, Lucio Miele, Clodia Osipo. Targeting HER2 enriches Jagged1 high cancer stem cells in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4769. doi:10.1158/1538-7445.AM2017-4769

Abstract 1335: Deubiquitinase OTUD6B isoforms are important regulators of growth and proliferation

Deubiquitinases (DUBs) are increasingly linked to the regulation of fundamental processes in normal and cancer cells, including DNA replication and repair, programmed cell death, and oncogenes and tumor suppressors signaling. Here evidence is presented that the deubiquitinase OTUD6B regulates protein synthesis in non-small cell lung cancer (NSCLC) cells, operating downstream from mTORC1. OTUD6B associates with the protein synthesis initiation complex and modifies components of the 48S preinitiation complex. The two main OTUD6B splicing isoforms seem to regulate protein synthesis in opposing fashions: the long OTUD6B-1 isoform is inhibitory, while the short OTUD6B-2 isoform stimulates protein synthesis. These properties affect NSCLC cell proliferation, since OTUD6B-1 represses DNA synthesis while OTUD6B-2 promotes it. Mutational analysis and downstream mediators suggest that the two OTUD6B isoforms modify different cellular targets. OTUD6B-2 influences the expression of cyclin D1 by promoting its translation while regulating (directly or indirectly) c-Myc protein stability. This phenomenon appears to have clinical relevance as NSCLC cells and human tumor specimens have a reduced OTUD6B-1/OTUD6B-2 mRNA ratio compared to normal samples. The global OTUD6B expression level does not change significantly between non-neoplastic and malignant tissues, suggesting that modifications of splicing factors during the process of transformation are responsible for this isoform switch. Because protein synthesis inhibition is a viable treatment strategy for NSCLC, these data indicate that OTUD6B isoform 2, being specifically linked to NSCLC growth, represents an attractive, novel therapeutic target and potential biomarker for early diagnosis of malignant NSCLC.

Note: This abstract was not presented at the meeting.

Citation Format: Maurizio Bocchetta, Clodia Osipo, Anna Sobol. Deubiquitinase OTUD6B isoforms are important regulators of growth and proliferation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1335. doi:10.1158/1538-7445.AM2017-1335

Deubiquitinase OTUD6B Isoforms Are Important Regulators of Growth and Proliferation

Deubiquitinases (DUB) are increasingly linked to the regulation of fundamental processes in normal and cancer cells, including DNA replication and repair, programmed cell death, and oncogenes and tumor suppressor signaling. Here, evidence is presented that the deubiquitinase OTUD6B regulates protein synthesis in non-small cell lung cancer (NSCLC) cells, operating downstream from mTORC1. OTUD6B associates with the protein synthesis initiation complex and modifies components of the 48S preinitiation complex. The two main OTUD6B splicing isoforms seem to regulate protein synthesis in opposing fashions: the long OTUD6B-1 isoform is inhibitory, while the short OTUD6B-2 isoform stimulates protein synthesis. These properties affect NSCLC cell proliferation, because OTUD6B-1 represses DNA synthesis while OTUD6B-2 promotes it. Mutational analysis and downstream mediators suggest that the two OTUD6B isoforms modify different cellular targets. OTUD6B-2 influences the expression of cyclin D1 by promoting its translation while regulating (directly or indirectly) c-Myc protein stability. This phenomenon appears to have clinical relevance as NSCLC cells and human tumor specimens have a reduced OTUD6B-1/OTUD6B-2 mRNA ratio compared with normal samples. The global OTUD6B expression level does not change significantly between nonneoplastic and malignant tissues, suggesting that modifications of splicing factors during the process of transformation are responsible for this isoform switch.

IMPLICATIONS

Because protein synthesis inhibition is a viable treatment strategy for NSCLC, these data indicate that OTUD6B isoform 2, being specifically linked to NSCLC growth, represents an attractive, novel therapeutic target and potential biomarker for early diagnosis of malignant NSCLC. Mol Cancer Res; 15(2); 117-27. ©2016 AACR.

Abstract LB-121: DAXX is a novel Notch-1 gene target and biomarker of GSI-sensitivity in ER+ breast cancer

Breast cancer is a heterogeneous disease that is best treated based on the expressed biomarker profile. Currently, the major clinical challenges are drug resistance and metastatic spread. Cancer stem cells (CSCs) are believed to be responsible for drug resistance and disease progression despite therapy. We previously identified novel biomarkers that could potentially be used with novel therapeutic strategies that target CSCs. We demonstrated in a presurgical window biomarker study of 20 breast cancer patients with ER-positive disease that signaling through Notch receptors mediates expression of 18 genes. Twenty women with ER-positive tumors were treated with 14 days of ET (tamoxifen or letrozole), with the addition of the oral GSI MK-0752 on day 15 (3 days on, 4 days off, 3 days on). Definitive surgery was on day 25. Analysis of biomarkers using microarray and RT-PCR during ET combined with MK-0752 GSI revealed statistically significant modulation of 18 genes: pro-apoptotic DAXX and NOXA; a tumor suppressor LFNG; Notch signaling such as NOTCH1, NOTCH4, HEYL, HES1, and HEY2; as well as proliferation-associated transcripts MIK67, CCND1, CCNA2; stem cell markers RUNX1 and ALDH1; and novel genes such as RICTOR, RPTOR, MMP7, ADAM19, and PgR (Albain et al. Proc SABCS 2014). The goal of the current study was to identify whether Notch1 directly regulated the 18 identified genes. We measured binding of Notch1 to CBF-1 (CSL/RPBJκ) binding sites of the 18 genes using a Chromatin Immunoprecipitation assay (ChIP) to determine whether Notch1 directly regulates the 18 genes. We scanned and designed primers 5 kb upstream and downstream of transcription start sites of the 18 genes and demonstrated that Notch1 was recruited to CBF-1 sites of 16 out of 18 genes. Different distribution of Notch1 binding was observed across all of the genes. Classical Notch targets, HEY1 and HES1 were among the most responsive genes and used as positive controls. Notch1 was found to be highly enriched on CBF-1 regulatory elements for the DAXX and NOXA genes. Binding of Notch1 to DAXX promoter elements increased in response to estrogen deprivation and this increase was attenuated upon GSI treatment. The biological activity of most genes was measured using the mammosphere-forming assay as a surrogate for CSC survival. Estrogen deprivation increased mammosphere-forming efficiency of ER+ breast cancer cells more than 2 fold compared to estrogen treatment. GSI blocked mammosphere formation by 95%-98% in response to anti-estrogen treatment. DAXX expression, was found to be necessary for GSI-mediated blockade of mammosphere formation. ChIP and mammosphere forming data calculated by ANOVA were found to be statistically significant. These results demonstrated that Notch1 is a direct transcriptional regulator of 16 genes identified by the clinical trial and in particular, DAXX, a pro-apoptotic gene that could serve as a cancer stem cell biomarker for anti-Notch therapy in ER+ breast cancer. Supported by a grant from the Breast Cancer Research Foundation.

Citation Format: Andrei Zlobin, Debra Wyatt, Jeffrey C. Bloodworth, Susan Hilsenbeck, Suzanne Fuqua, Lucio Miele, Kathy S. Albain, Clodia Osipo. DAXX is a novel Notch-1 gene target and biomarker of GSI-sensitivity in ER+ breast cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-121.

ATG9A loss confers resistance to trastuzumab via c-Cbl mediated Her2 degradation

Acquired or de novo resistance to trastuzumab remains a barrier to patient survival and mechanisms underlying this still remain unclear. Using stable isotope labelling by amino acids in cell culture (SILAC)-based quantitative proteomics to compare proteome profiles between trastuzumab sensitive/resistant cells, we identified autophagy related protein 9A (ATG9A) as a down-regulated protein in trastuzumab resistant cells (BT474-TR). Interestingly, ATG9A ectopic expression markedly decreased the proliferative ability of BT474-TR cells but not that of the parental line (BT474). This was accompanied by a reduction of Her2 protein levels and AKT phosphorylation (S473), as well as a decrease in Her2 stability, which was also observed in JIMT1 and MDA-453, naturally trastuzumab-resistant cells. In addition, ATG9A indirectly promoted c-Cbl recruitment to Her2 on T1112, a known c-Cbl docking site, leading to increased K63 Her2 polyubiquitination. Whereas silencing c-Cbl abrogated ATG9A repressive effects on Her2 and downstream PI3K/AKT signaling, its depletion restored BT474-TR proliferative rate. Taken together, our findings show for this first time that ATG9A loss in trastuzumab resistant cells allowed Her2 to escape from lysosomal targeted degradation through K63 poly-ubiquitination via c-Cbl. This study identifies ATG9A as a potentially druggable target to overcome resistance to anti-Her2 blockade.

Abstract P3-06-15: Notch3 as a predictor of GSI sensitivity in distinct subtypes of triple negative breast cancer

Background: Breast cancer is the second leading cause of cancer-associated death in women. Triple negative breast cancer (TNBC) accounts for 10-15% of breast cancer, lacks expression of ER, PR, and HER2 gene amplification. Due to lack of targeted therapy, TNBC has the worst prognosis. TNBC is a heterogeneous disease with six distinct subtypes. Current treatment includes combination of surgery, radiation therapy and chemotherapy. Notch signaling is increased in TNBC and predicts for worst overall outcome. The goal of the study is to identify novel Notch-biomarkers that predict sensitivity of subtypes of TNBC to Notch inhibition in both bulk and cancer stem cells.

Methods: Two subtypes of TNBC, Mesenchymal stem-like MDA-MB-231 and basal like MDA-MB-468 cells were used. Notch signaling was evaluated using both RNA-sequencing and quantitative real time PCR (q-RTPCR). Cancer stem cell markers were evaluated using Aldefluor assay, CD44 high/ CD24 low expression, and mammosphere forming assay. To study the effect of chemotherapy and Notch, MDA-MB-231 and MDA-MB-468 cells were treated with carboplatin or a pan Notch inhibitor- gamma secretase inhibitor (GSI) or combination of both and cell viability was measured using the trypan blue exclusion test.

Results: The RNA-seq and RT-PCR results showed that Notch3 is differentially expressed in the two cell lines. Notch3 was knocked down using siRNA and its effect on cell viability was assessed after GSI or carboplatin or combination treatment. MDA-MB-468 cells had higher Notch receptors and activity, most notably Notch3. MDA-MB-468 cells had high levels of Aldefluor whereas MDA-MB-231 cells showed higher levels of CD44 High/CD24 Low expression. MDA-MB-468 cells had higher mammosphere forming efficiency (MFE) compared to MDA-MB-231. Notch inhibition decreased MFE of MDA-MB-468 whereas it increased MFE of MDA-MB-231 cells. MDA-MB-468 cells have lower IC50 for carboplatin or GSI compared to MDA-MB-231 cells, as measured by cell viability assay. However, combination treatment lowered IC50 for GSI in MDA-MB-231 cells as compared to MDA-MB-468 cells. Since Notch3 levels were strikingly different between the two cell lines, we hypothesized that Notch3 levels are necessary for GSI sensitivity. Carboplatin treatment increased Notch3 in MDA-MB-231 cells and the increase in Notch3 could be the reason for lower IC50 of GSI during combination treatment as increased Notch3 would provide the substrate for gamma-secretase. In MDA-MB-468 cells, Notch3 knockdown significantly reduced cell viability and was similar to GSI, suggesting that GSI acts through Notch3. Furthermore, Notch3 knockdown and carboplatin treatment reduced viability comparable to carboplatin and GSI treatment, reinforcing that the GSI acts through Notch3 in TNBC. Using the Kaplan-Meier Plotter, high Notch3 predicted poor recurrence free survival post chemotherapy in patients with ER-, HER2-, basal breast cancer.

Conclusions: GSI acts through Notch3 in two TNBC subtypes and combination of chemotherapy with Notch inhibition results in a better outcome as compared to either drug alone. Future experiments would elucidate the role of Notch3 inhibition in targeting cancer stem cells post chemotherapy treatment in different subtypes of TNBC.

Citation Format: Shah D, Osipo C. Notch3 as a predictor of GSI sensitivity in distinct subtypes of triple negative breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P3-06-15.

PKCα Attenuates Jagged-1-Mediated Notch Signaling in ErbB-2-Positive Breast Cancer to Reverse Trastuzumab Resistance

PURPOSE

Breast cancer is the second leading cause of cancer mortality among women worldwide. The major problem with current treatments is tumor resistance, recurrence, and disease progression. ErbB-2-positive breast tumors are aggressive and frequently become resistant to trastuzumab or lapatinib. We showed previously that Notch-1 is required for trastuzumab resistance in ErbB-2-positive breast cancer.

EXPERIMENTAL DESIGN

Here, we sought to elucidate mechanisms by which ErbB-2 attenuates Notch signaling and how this is reversed by trastuzumab or lapatinib.

RESULTS

The current study elucidates a novel Notch inhibitory mechanism by which PKCα downstream of ErbB-2 (i) restricts the availability of Jagged-1 at the cell surface to transactivate Notch, (ii) restricts the critical interaction between Jagged-1 and Mindbomb-1, an E3 ligase that is required for Jagged-1 ubiquitinylation and subsequent Notch activation, (iii) reverses trastuzumab resistance in vivo, and (iv) predicts better outcome in women with ErbB-2-positive breast cancer.

CONCLUSIONS

The clinical impact of these studies is PKCα is potentially a good prognostic marker for low Notch activity and increased trastuzumab sensitivity in ErbB-2-positive breast cancer. Moreover, women with ErbB-2-positive breast tumors expressing high Notch activation and low PKCα expression could be the best candidates for anti-Notch therapy.

Human Epidermal Growth Factor Receptor 2 (HER2) Impedes MLK3 Kinase Activity to Support Breast Cancer Cell Survival

Human epidermal growth factor receptor 2 (HER2) is amplified in ∼ 15-20% of human breast cancer and is important for tumor etiology and therapeutic options of breast cancer. Up-regulation of HER2 oncogene initiates cascades of events cumulating to the stimulation of transforming PI3K/AKT signaling, which also plays a dominant role in supporting cell survival and efficacy of HER2-directed therapies. Although investigating the underlying mechanisms by which HER2 promotes cell survival, we noticed a profound reduction in the kinase activity of a pro-apoptotic mixed lineage kinase 3 (MLK3) in HER2-positive (HER2+) but not in HER2-negative (HER2-) breast cancer tissues, whereas both HER2+ and HER2- tumors expressed a comparable level of MLK3 protein. Furthermore, the kinase activity of MLK3 was inversely correlated with HER2+ tumor grades. Moreover, HER2-directed drugs such as trastuzumab and lapatinib as well as depletion of HER2 or HER3 stimulated MLK3 kinase activity in HER2+ breast cancer cell lines. In addition, the noted inhibitory effect of HER2 on MLK3 kinase activity was mediated via its phosphorylation on Ser(674) by AKT and that pharmacological inhibitors of PI3K/AKT prevented trastuzumab- and lapatinib-induced stimulation of MLK3 activity. Consistent with the pro-apoptotic function of MLK3, stable knockdown of MLK3 in the HER2+ cell line blunted the pro-apoptotic effects of trastuzumab and lapatinib. These findings suggest that HER2 activation inhibits the pro-apoptotic function of MLK3, which plays a mechanistic role in mediating anti-tumor activities of HER2-directed therapies. In brief, MLK3 represents a newly recognized integral component of HER2 biology in HER2+ breast tumors.

Osteopontin mediates an MZF1-TGF-β1-dependent transformation of mesenchymal stem cells into cancer-associated fibroblasts in breast cancer

Interactions between tumor cells and cancer-associated fibroblasts (CAFs) in the tumor microenvironment (TMEN) significantly influence cancer growth and metastasis. Transforming growth factor-β (TGF-β) is known to be a critical mediator of the CAF phenotype, and osteopontin (OPN) expression in tumors is associated with more aggressive phenotypes and poor patient outcomes. The potential link between these two pathways has not been previously addressed. Utilizing in vitro studies using human mesenchymal stem cells (MSCs) and MDA-MB231 (OPN+) and MCF7 (OPN−) human breast cancer cell lines, we demonstrate that OPN induces integrin-dependent MSC expression of TGF-β1 to mediate adoption of the CAF phenotype. This OPN-TGF-β1 pathway requires the transcription factor, myeloid zinc finger 1 (MZF1). In vivo studies with xenotransplant models in NOD-scid mice showed that OPN expression increases cancer growth and metastasis by mediating MSC-to-CAF transformation in a process that is MZF1- and TGF-β1-dependent. We conclude that tumor-derived OPN engenders MSC-to-CAF transformation in the microenvironment to promote tumor growth and metastasis via the OPN-MZF1-TGF-β1 pathway.

Notch-EGFR/HER2 Bidirectional Crosstalk in Breast Cancer

The Notch pathway is a well-established mediator of cell-cell communication that plays a critical role in stem cell survival, self-renewal, cell fate decisions, tumorigenesis, invasion, metastasis, and drug resistance in a variety of cancers. An interesting form of crosstalk exists between the Notch receptor and the Epidermal Growth Factor Receptor Tyrosine Kinase family, which consists of HER-1, -2, -3, and -4. Overexpression of HER and/or Notch occurs in several human cancers including brain, lung, breast, ovary, and skin making them potent oncogenes capable of advancing malignant disease. Continued assessment of interplay between these two critical signaling networks uncovers new insight into mechanisms used by HER-driven cancer cells to exploit Notch as a compensatory pathway. The compensatory Notch pathway maintains HER-induced downstream signals transmitted to pathways such as Mitogen Activated Protein Kinase and Phosphatidylinositol 3-Kinase (PI3K), thereby allowing cancer cells to survive molecular targeted therapies, undergo epithelial to mesenchymal transitioning, and increase cellular invasion. Uncovering the critical crosstalk between the HER and Notch pathways can lead to improved screening for the expression of these oncogenes enabling patients to optimize their personal treatment options and predict potential treatment resistance. This review will focus on the current state of crosstalk between the HER and Notch receptors and the effectiveness of current therapies targeting HER-driven cancers.

Abstract 1823: Novel regulation of Jagged1 by ErbB2 in breast cancer: implications for anti-ErbB2 therapy

We have demonstrated that Notch1 is required for trastuzumab resistance in ErbB2 positive breast cancer. This indicates that ErbB2 suppresses Notch1 in breast cancer and therapeutic intervention targeting ErbB2 might have an unintended consequence which is aberrant up regulation of Notch1 which is a breast oncogene.

However, the mechanism of action by which ErbB2 restricts Notch1 activation is unknown. In this current study, we investigated the role of cis- and trans-activation of Notch signaling by Notch ligands which are developmentally conversed to tightly regulate Notch activation. To address this hypothesis, we performed co-culture studies using fibroblasts expressing no Notch ligands or over-expressing human Jagged1 or Deltalike1 and ErbB2 positive breast cancer cells. We performed flow cytometry to isolate breast cancer cells after co-culture and extracted RNA to measure expression of Notch gene targets as a measure of Notch activity. The results showed that trastuzumab, Lapatinib, or ErbB2 knockdown increased overall Notch activition. Similarly, Co-culture with Jagged1-expressing fibroblasts increased overall Notch activation. However, Knocked down of Jagged1 in the breast cancer cells had little effect on ligand-induced Notch activation relieving the possibility of cis-inhibition. In contrast, Jagged1 knocked down abrogated trastuzumab-induced Notch activation in the breast cancer cells. These results suggest that ErbB2 might restrict Notch activation by preventing Jagged1-mediated trans activation of Notch and not by promoting cis-inhibition. Confocal immunofluorescence showed that Jagged1 is localized with Notch1 when ErbB2 is hyperactive but is trafficked to the cell surface in response to trastuzumab. K44ADynamin abrogated Jagged1 expression on the cell surface as measured by IF and surface biotinylation studies. Furthermore, K44ADynamin expression abrogated trastuzumab-induced Notch1 activation. Importantly, we measured growth consequences of Jagged1-mediated Notch activation in response to trastuzumab and found that Jagged1 is necessary for survival of ErbB2 positive breast cancer cells and trastuzumab resistance as measured by cell cycle analysis and Annexin V staining. These results taken together indicate that ErbB2 restricts Notch by limiting Jagged1-mediated trans-activation.

Citation Format: Clodia Osipo, Kinnari Pandya, Debra Wyatt. Novel regulation of Jagged1 by ErbB2 in breast cancer: implications for anti-ErbB2 therapy. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1823. doi:10.1158/1538-7445.AM2014-1823

Abstract 1829: Regulation of Mixed Lineage Kinase-3 activity by Her2 and its implication in death or survival

Human Epidermal Growth Factor Receptor 2 (Her2) is a receptor tyrosine kinase that regulates cell growth and differentiation signaling pathways. The protein Her2 is significantly overexpressed in 20-30% of breast cancer. The available clinical data proves that Her2 amplification in breast cancer is associated with worst overall survival. Therefore, Her2 has become a very important candidate for drug targeting and currently Her2 positive patients are treated with humanized monoclonal antibody, Trastuzumab (Herceptin) and a small molecule, Lapatinib that blocks Her2 tyrosine kinase activity. It is reported that Her2 amplification leads to activation of multiple survival pathways, including PI3K-Akt. However, it is not clearly known how Trastuzumab promotes cell death and what pro-apoptotic pathways are activated downstream of Her2 blockage? It is also reported that patients on Trastuzumab develop subsequent resistance to the therapy.

Previously we published that in breast cancer tumors, a pro-apoptotic kinase, Mixed Lineage Kinase-3 (MLK-3) was inhibited, specifically in estrogen receptor (ER) positive tumors. The kinase activity of MLK-3 was down regulated by estrogen and this prevented MLK-3-mediated cell death. Since Her2 serves as a drug target in breast cancer, therefore, we asked whether Her2 plays any role in regulating MLK-3 kinase activation and whether MLK-3 plays any role in Trastuzumab resistance.

Our results showed that the endogenous MLK-3 kinase activity was up-regulated in SkBr3 cells upon treatment with Trastuzumab and Lapatinib. Whether the activation of MLK-3 upon Her2 blockage leads to cell death or survival is still not known and experiments are underway to investigate these aspects. We also observed that Her3 was the dimerization partner of Her2 in Trastuzumab- or Lapatinib- mediated MLK-3 activation. Not knowing any role of MLK-3 in Trastuzumab resistance, we used Trastuzumab sensitive and resistant BT474 cell model to investigate the role of MLK-3 in Trastuzumab resistance. Our data clearly showed that knockdown of MLK-3 caused spontaneous cell death in resistant cells. The Trastuzumab treatment further amplified cell death in MLK-3 knockdown resistant cells. The Akt1 activation in these cells was also diminished upon MLK-3 knockdown.

Taken together, our data demonstrate that like ER, the Her2 also down regulates MLK-3 kinase activity and perhaps promote cell survival. Furthermore, paradoxically, the MLK-3 knockdown promoted cell death in Trastuzumab resistance BT474 cells, suggesting that MLK-3 could also serve as survival factor in resistant cells. Whether MLK-3 serves as a pro-apoptotic kinase or survival kinase in Trastuzumab mediated pathway needs to be extensively examined in near future.

Citation Format: Subhasis Das, Gautam Sondarva, Navin Viswakarma, Rakesh Sathish Nair, Clodia Osipo, Basabi Rana, Ajay Rana. Regulation of Mixed Lineage Kinase-3 activity by Her2 and its implication in death or survival. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1829. doi:10.1158/1538-7445.AM2014-1829

Abstract P6-05-25: Ratio of notch receptors is critical for response to notch inhibition by a gamma-secretase inhibitor in triple negative breast cancer cells

Background: Women with triple-negative breast cancer have the worst prognosis, and have few targeted therapy options. Notch receptor genes are potent breast oncogenes, overexpressed in triple-negative breast cancer, and critical for survival of breast cancer stem cells. However, the contribution of each Notch receptor and the significance of the ratio of receptors to growth and sensitivity to Notch inhibitors have not yet been fully explored. We hypothesized that each Notch receptor has distinct roles in the etiology of triple negative breast cancer and designed a study to explore these roles in a panel of three triple negative cell lines and primary, human triple negative breast cancer tissue.

Methods: Human cell lines BT-549, MDA-MB-231, and MDA-MB-468 were used to measure endogenous mRNA transcript levels of Notch ligands, receptors, and gene targets important for canonical Notch signaling (i.e. Hes-1, Hes-5, Hey-1, and Deltex-1), apoptosis (Noxa), inflammation (MMP-9 and IL-8), and cancer stem cells (ALDH1) by means of real time PCR under conditions where all four receptors were inhibited by a pan-inhibitor, gamma-secretase inhibitor or each receptor was individually knocked down using RNA interference. Furthermore, both anchorage-dependent and –independent growth were measured by counting cells and methylcellulose colony forming assay, respectively. In addition, the RNA from formalin-fixed, paraffin-embedded specimen from women-diagnosed with TNBC who had undergone breast surgery was also analyzed for comparable gene expression for similar targets following Laser capture micro-dissection.

Results: The results showed that only MDA-MB-468 cells were sensitive to growth inhibition by a GSI while MDA-MB-231 and BT549 were resistant. Transcripts of Notch-4, Deltex-1, Hes-1 and Hes-5, as well as IL-8 and MMP-9 were significantly increased in MDA-MB-468 as compared to BT549 or MDA-MB-231 cells that were resistant. Interestingly, the cancer stem marker, ALDH1 was significantly increased only in MDA-MB-468 while the apoptotic marker, Noxa was decreased compared to resistant cells: BT549 and MDA-MB-231. The PCR and growth results from the Notch-1 knocked down were similar to GSI inhibition. However, knocked down of Notch-2 resulted in resistance to GSI. Notch-3 knocked down showed an increase in Notch-1, Notch-2, and Notch-4 transcripts implying that Notch-3 might suppress the other Notch receptors. Notch-4 knocked down alone had little effect on growth of any of the three cell lines. The results from human specimens showed an inverse relationship between Notch-1 and Notch-2, and Notch-1 and Notch-4. For example, Notch-1, Jagged-1, and Hes-5 genes were up-regulated in triple negative tumor tissue (n = 20), while the Notch-4 gene was down-regulated as compared to normal control specimens from reduction mammoplasty (n = 4).

Conclusions: The results from this study indicate that Notch-1 is probably the growth driver in certain triple negative cancer types.. Furthermore, the ratios of Notch receptors and their activity states could predict sensitivity to Notch inhibition by a GSI or other inhibitors. Lastly, these results suggest that the best predictor genes for Notch activation are: Notch-1, Hes-1, and ALDH1.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P6-05-25.

Crosstalk between PKCα and Notch-4 in endocrine-resistant breast cancer cells

The Notch pathway is functionally important in breast cancer. Notch-1 has been reported to maintain an estrogen-independent phenotype in estrogen receptor α (ERα)+ breast cancer cells. Notch-4 expression correlates with Ki67. Notch-4 also plays a key role in breast cancer stem-like cells. Estrogen-independent breast cancer cell lines have higher Notch activity than estrogen-dependent lines. Protein kinase Cα (PKCα) overexpression is common in endocrine-resistant breast cancers and promotes tamoxifen (TAM)-resistant growth in breast cancer cell lines. We tested whether PKCα overexpression affects Notch activity and whether Notch signaling contributes to endocrine resistance in PKCα-overexpressing breast cancer cells.Analysis of published microarray data from ERα+ breast carcinomas shows that PKCα expression correlates strongly with Notch-4. Real-time reverse transcription PCR and immunohistochemistry on archival specimens confirmed this finding. In a PKCα-overexpressing, TAM-resistant T47D model, PKCα selectively increases Notch-4, but not Notch-1, expression in vitro and in vivo. This effect is mediated by activator protein-1 (AP-1) occupancy of the Notch-4 promoter. Notch-4 knockdown inhibits estrogen-independent growth of PKCα-overexpressing T47D cells, whereas Notch-4IC expression stimulates it. Gene expression profiling shows that multiple genes and pathways associated with endocrine resistance are induced in Notch-4IC- and PKCα-expressing T47D cells. In PKCα-overexpressing T47D xenografts, an orally active γ-secretase inhibitor at clinically relevant doses significantly decreased estrogen-independent tumor growth, alone and in combination with TAM. In conclusion, PKCα overexpression induces Notch-4 through AP-1. Notch-4 promotes estrogen-independent, TAM-resistant growth and activates multiple pathways connected with endocrine resistance and chemoresistance. Notch inhibitors should be clinically evaluated in PKCα- and Notch-4-overexpressing, endocrine-resistant breast cancers.

Gamma secretase inhibitors of Notch signaling

The numerous processes involved in the etiology of breast cancer such as cell survival, metabolism, proliferation, differentiation, and angiogenesis are currently being elucidated. However, underlying mechanisms that drive breast cancer progression and drug resistance are still poorly understood. As we discuss here in detail, the Notch signaling pathway is an important regulatory component of normal breast development, cell fate of normal breast stem cells, and proliferation and survival of breast cancer initiating cells. Notch exerts a wide range of critical effects through a canonical pathway where it is expressed as a type I membrane precursor heterodimer followed by at least two subsequent cleavages induced by ligand engagement to ultimately release an intracellular form to function as a transcriptional activator. Notch and its ligands are overexpressed in breast cancer, and one method of effectively blocking Notch activity is preventing its cleavage at the cell surface with γ-secretase inhibitors. In the context of Notch signaling, the application of clinically relevant anti-Notch drugs in treatment regimens may contribute to novel therapeutic interventions and promote more effective clinical response in women with breast cancer.

Abstract 861: Analysis of predictive genes in triple negative breast cancer in response to a gamma-secretase inhibitor

Defined by the lack of estrogen and progesterone receptors and amplification of human epidermal growth factor receptor-2 (HER-2), triple negative breast cancer (TNBC) is a devastating disease with no approved, targeted therapy to date. Characterized by aggressive behavior which includes high rates of metastasis and very poor prognosis, TNBC is presently treated with DNA-damaging agents such as cisplatin or carboplatin. The overall survival outcomes for women with TNBC is dismal and consequently TNBC research requires identification of biomarkers that specifically mediate signaling pathways that lead to disease progression and metastasis. Notch receptors which are necessary for self-renewal and survival of breast cancer stem cells are excellent candidate biomarkers to investigate in various subtypes of TNBC. Based on the critical need to identify predictive biomarkers, we hypothesized that inhibition of the Notch pathway via γ-secretase inhibition (GSI) will provide an effective blockade of TNBC growth in specific subtypes where predictive Notch gene expression profiles are significantly up or downregulated. A panel of three TNBC cell lines were used in this study: MDA-MB-231 (mesenchymal stem-like), MDA-MB-468 (basal-like 1), and BT549 (luminal). Using real-time PCR, Notch pathway-associated genes were measured which include Notch-1 and Notch-4 receptors, Notch target genes (i.e. Deltex-1, Hes-1, Hes-5, and Hey-1), and genes such as IL-8 and MMP-9 which are associated with aggressive tumor behavior. Both anchorage-dependent and anchorage-independent growth assays were measured in response to MRK-003 GSI, carboplatin, and the combination. We have identified for the first time a TNBC line (MDA-MB-468) that expressed the highest level of Hes-1, a Notch gene target, and is the most sensitive to single agent MRK-003 GSI treatment. We also identified that this cell line with the highest sensitivity to MRK-003 GSI upregulates Notch-4 and IL-8 in response to the GSI to a greater extent than either of the two other TNBC lines. Furthermore, the addition of carboplatin at its IC50 concentration significantly increased the sensitivity of MRK-003 GSI in otherwise insensitive TNBC lines. These results suggest for the first time that: 1) Hes-1 could potentially be a valuable predictive biomarker of Notch activity in TBNC; 2) induction of Notch-4 and/or IL-8 could predict response to MRK-003 GSI; and 3) carboplatin might provide a means to increase sensitivity to MRK-003 GSI in resistant TNBC.

Citation Format: Roma Olsauskas-Kuprys, Clodia Osipo. Analysis of predictive genes in triple negative breast cancer in response to a gamma-secretase inhibitor. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 861. doi:10.1158/1538-7445.AM2013-861

The functional role of Notch signaling in triple-negative breast cancer

The term "triple-negative breast cancer" (TNBC) is a heterogeneous subtype of breast cancer. Unfortunately, due to the lack of expression of hormone receptors and human epidermal growth factor receptor-2, therefore the lack of US Food and Drug Administration-approved targeted therapies, TNBC has the worst prognosis of all subtypes of breast cancer. Notch signaling has emerged as a pro-oncogene in several human malignancies and has particularly been associated with the triple-negative subtype of breast cancer. This chapter explores the role of Notch signaling in triple negative and other subtypes of breast cancer, the relationship of Notch with other breast cancer biomarkers, prognostic indicators associated with Notch, and potential therapeutic strategies targeting Notch inhibition. Hopefully, better understanding of this signaling pathway in the future will lead to optimal molecular therapeutic treatments for TNBC patients, improving their quality of life and outcome.

Abstract P4-08-06: Notch-dependent Regulation of Novel Genes Associated with Trastuzumab Resistance

Background: We have shown previously that Notch-1 is critical for the development and maintenance of trasuzumab or lapatinib resistance in HER2 positive breast cancer cell lines. Here we sought to identify the critical genes that are regulated by Notch to promote anti-HER2 targeted resistance.

Methods: We measured expression of 84 known breast cancer-associated transcripts using a real-time PCR array in both trastuzumab sensitive and resistant BT474 cell lines. A 4 fold increase or decrease in transcripts levels were measured to be statisitically significant. To confirm the change in levels, we designed primers to each gene of interest and repeated the PCR at least three independent times. Furthermore, we specifically asked whether Notch-1 or it's transcriptional mediator, CBF-1, directly regulated any of the identified genes in HER2 positive breast cancer cells using a genetic knocked down approach. An ANOVA for multiple comparisons was used to compute statistical significance.

Results: We identified more than 4 fold increase in APC, ABCG2, ABCB1, Gata-3, Bcl-2, Id2, and HRG-2 transcripts in trastuzumab resistant versus sensitive cells. Conversley, Gli1 was downregulated in resistant versus sensitive cells. Interestingly, siRNA directed against Notch-1 or CBF-1 decreased ABCG2 and Gata-3, but conversely increased ABCB1 and HRG-2 in resistant cells.

Conclusions: Our findings demonstrate APC, ABCG2, ABCB1, Gata-3, Bcl-2, Id2, and HRG-2 transcripts are signficantly increased in trastuzumab resistant breast cancer cells compared to sensitive cells. More importantly, Notch-1 or it's direct transcriptional activator CBF-1 may contribute to traztuzumab resistance by directly regulating critical genes that contribute to multidrug resistance: ABCB1 and ABCG2 and/or genes associated with stem cell survival: Gata-3 and HRG-2.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P4-08-06.

Abstract P2-05-15: Assessment of Notch Signaling Pathway Components as Biomarkers for Triple Negative Breast Cancer: Comparison of Triple Negative Breast Cancer Cell Lines and Human Breast Cancer Samples

Background: Treatment options for patients presenting with TNBC are limited, primarily because there is no approved targeted therapy available. Canonical breast cancer targets such as estrogen receptor and HER2/neu proteins are absent in TNBC. Furthermore, there is an urgent need to uncover biomarkers in this disease in light of its costly treatment as compared to non-TNBC treatment and, more importantly, its poor prognosis as evidenced by aggressive tumor metastasis and high patient mortality. While Notch-1 target gene expression has been reported to be linked to various types of malignancies, we propose that since Notch signaling pathway is involved in cancer cell proliferation and survival it could be a likely oncogenic driver and possible target in certain TNBC patients.

Methods: Human TNBC cell lines BT-549, MDA-MB-231, and MDA-MB-468 were used to measure endogenous relative mRNA transcript levels of Notch genes and gene targets by means of real time PCR. In addition, the RNA from formalin-fixed, paraffin-embedded specimen from women-diagnosed with TNBC who had undergone breast surgery was also analyzed for comparable gene expression for similar targets following Laser capture micro-dissection.

Results: The in vitro results show that mRNA transcripts of Notch-4, Deltex-1, Hes-1 and -Hes5, as well as upstream Notch regulator PEA3 targets IL-8 and MMP-9 were increased in MDA-MB-468 and BT-549 cells as compared to MDA-MB-231 cells (n = 3–5). In the clinical sample specimens, Notch-1, Jagged-1, and Hes-5 genes were up-regulated, while the Notch-4 gene was down-regulated in TNBC patients as compared to normal control specimens from reduction mammoplasty (n = 4).

Conclusions: Our data present the heterogeneity of TNBC disease. They also indicate that the Notch signaling pathway is up-regulated in certain TNBC cell lines and human TNBC breast cancer tissue. Lastly, these results suggest that some components of Notch signaling may be viable biomarkers to better predict Notch activity for future therapeutic interventions using Notch inhibitors.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P2-05-15.

Evaluation of Biomarkers in Multifocal/Multicentric Invasive Breast Carcinomas

Background. The purpose of this study was to evaluate whether breast carcinoma biomarkers vary among separate tumor foci of multifocal/multicentric (MF/MC) breast carcinomas and whether this variation correlates with morphological features and tumor grade. Design. We reviewed the biomarker profiles of MF/MC invasive breast carcinomas diagnosed between January 2001 and June 2010 at our institution. Estrogen receptor (ER), progesterone receptor (PR), and human epidermal receptor protein (HER2) results were classified as positive or negative. Results. Out of the 51 patients included in the study, 6 cases had 2 tumor foci with different morphologies, 7 cases had 2 foci with similar morphology but different grades, and 38 cases had 2 tumor foci with similar morphologies and grades. Out of the 38 patients who had MF/MC tumors with the same morphology and grade, only 1 patient had a difference in ER and PR status between foci. Out of the 7 patients who had morphologically similar tumors with different grades, 4 had similar results in both tumor foci, 3 had different results for ER and PR, and another had differing results for HER2 between the foci. All 6 patients who had MF/MC foci with different morphologies exhibited similar ER, PR, and HER2 results between the foci. Conclusion. Regardless of the similarity in tumor morphology or grade, a small number of cases included foci that exhibited different tumor marker expression, which might affect the treatment strategy. Therefore, our results suggest that the evaluation of tumor markers in different foci should be considered in MF/MC tumors for accurate treatment strategies.

Abstract 939: ErbB-2 inhibits Notch-4 expression and activity in breast cancer independent of ER function

Amplification, over-expression, and/or hyperactivity of ErbB-2 occur in 50% of breast tumors which include both luminal A/B and the HER2+ subtypes. Metastatic breast tumors that overexpress ErbB-2 are generally resistant to anti-hormonal or anti-HER2-targeted therapy. We have published that either 17beta-estradiol or overexpression of ErbB-2 suppresses Notch activity and this is reversed by inhibitors of estrogen receptor (ER) or ErbB-2. Furthermore, we demonstrated that when breast tumors are treated with a Notch inhibitor (gamma-secretase inhibitor or GSI) in combination with tamoxifen (ER+ cells) or trastuzumab (ErbB-2+ cells), ER+ breast tumor xenografts regress or ErbB-2+ xenografts do not recur. Based on these published findings, we asked in the current study by what mechanism does ErbB-2 inhibit Notch signaling. We used both genetic and pharmacologic approaches to address the hypothesis that ErbB-2 hyperactivity was sufficient to suppress Notch independent of ER function. Our results showed that Notch-4 transcription is specifically and significantly decreased when ErbB-2 or Heregulin-beta1 is stably-overexpressed in MCF-7 cells. Furthermore, the data demonstrated that while 17beta-estradiol inhibits Notch-4 mRNA and protein expression, which was reversed by fulvestrant, the inhibition on Notch-4 by ErbB-2 or by Heregulin-beta1 overexpression was independent of ER activity. In addition, we confirmed that ER status was an independent factor by specifically targeting ErbB-2 using lapatinib in ER negative SKBr3 breast cancer cells and the results showed that Notch-4 mRNA and protein increased to more than 100 fold compared to vehicle treatment. We have recently published that c-Fos is a potential transcriptional repressor of Notch-4 in breast cancer cells and in agreement, we identified that c-Fos levels were significantly elevated in breast cancer cells stably-overexpressing either ErbB-2 or Heregulin-beta1. The biological significance of Notch-4 overexpression was investigated in two relevant resistant models in vitro and in vivo. Resistance to lapatinib was prevented by co-treatment of a GSI plus lapatinib. Resistance to anti-hormonal therapy using long-term estrogen deprived MCF-7 cells was reversed by a GSI treatment. MCF-7/HER2 breast tumor xenografts regressed to undetectable levels with the combination of a GSI plus trastuzumab under conditoions where estrogen was deprived. In conclusion, results from the current study suggest that ErbB-2 overexpression or hyperactivity via Heregulin-beta1 is a potent inhibitor of Notch-4 expression possibly by increasing c-Fos expression. Furthermore, the regulation of Notch-4 expression is independent of ER function. More importantly, anti-ER or anti-EbB-2 targeted therapy de-repress Notch-4 expression and thus sensitize breast cancer cells to a Notch inhibitor such as a GSI to prevent or reverse resistance.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 939. doi:1538-7445.AM2012-939

The Effect of Cold Ischemia Time and/or Formalin Fixation on Estrogen Receptor, Progesterone Receptor, and Human Epidermal Growth Factor Receptor-2 Results in Breast Carcinoma

Aims. To compare the results of estrogen and progesterone receptors (ER, PR), and human epidermal growth factor receptor-2 (HER2) expression status on biopsy and excision specimens and to evaluate the effect of cold ischemia time and/or formalin fixation on these biomarkers. Methods. Breast carcinomas that were diagnosed between 2007 and 2009 by core needle biopsy, and subsequently excised in our institution, were included in the study. Data regarding the tumor morphology, grade, and ER, PR, and HER2 status were retrospectively collected from the pathology reports. Results. Five out of 149 (3.4%) cases with ER-positive receptor status in the biopsy specimen became ER-negative in the subsequent excision specimen. Nine out of 126 (7.1%) cases with PR-positive receptor status in the biopsy specimen became PR-negative in the excision specimen. Receptor status change was predominantly seen in tumors with low ER and PR receptor expression. HER2 results were consistent between biopsy and excision specimens in all cases tested. Conclusions. Cold ischemia time and/or formalin fixation affect mainly ER and PR testing with low Allred scores and support the implementation of the ASCO/CAP guidelines. HER2 results, however, were not affected in our limited number of patients.

Targeting both Notch and ErbB-2 signalling pathways is required for prevention of ErbB-2-positive breast tumour recurrence

BACKGROUND

We reported that Notch-1, a potent breast oncogene, is activated in response to trastuzumab and contributes to trastuzumab resistance in vitro. We sought to determine the preclinical benefit of combining a Notch inhibitor (γ-secretase inhibitor (GSI)) and trastuzumab in both trastuzumab-sensitive and trastuzumab-resistant, ErbB-2-positive, BT474 breast tumours in vivo. We also studied if the combination therapy of lapatinib plus GSI can induce tumour regression of ErbB-2-positive breast cancer.

METHODS

We generated orthotopic breast tumour xenografts from trastuzumab- or lapatinib-sensitive and trastuzumab-resistant BT474 cells. We investigated the antitumour activities of two distinct GSIs, LY 411 575 and MRK-003, in vivo.

RESULTS

Our findings showed that combining trastuzumab plus a GSI completely prevented (MRK-003 GSI) or significantly reduced (LY 411 575 GSI) breast tumour recurrence post-trastuzumab treatment in sensitive tumours. Moreover, combining lapatinib plus MRK-003 GSI showed significant reduction of tumour growth. Furthermore, a GSI partially reversed trastuzumab resistance in resistant tumours.

CONCLUSION

Our data suggest that a combined inhibition of Notch and ErbB-2 signalling pathways could decrease recurrence rates for ErbB-2-positive breast tumours and may be beneficial in the treatment of recurrent trastuzumab-resistant disease.

Abstract 2553: Inhibition of γ-secretase in combination with Lapatinib induces tumor regression in vivo

Amplification, over-expression, and/or hyperactivity of ErbB-2 occur in 15-30% of breast tumors which include both luminal B and the HER2+ subtypes. Metastatic breast tumors that overexpress ErbB-2 are difficult to treat and generally resistant to trastuzumab. Lapatinib, a dual EGFR/ErbB-2 tyrosine kinase inhibitor, is approved for the treatment of ErbB-2-positive breast cancer that has advanced during or after trastuzumab treatment. However, resistance to lapatinib occurs in cell culture models in vitro. We have recently published that overexpression of ErbB-2 suppresses Notch-1 activity and this is reversed by inhibitors of ErbB-2, trastuzumab or a tyrosine kinase inhibitor. Furthermore, we demonstrated that trastuzumab resistance is reversed when Notch-1 is downregulated by siRNA or when cells are treated with a gamma-secretase inhibitor (MRK-003, GSI). Additionally, in an independent study we showed that GSI also prevented resistance to lapatinib. Since lapatinib is a very potent inhibitor of ErbB-2 tyrosine kinase activity in vitro, we asked whether a combination therapy based on lapatinib and GSI can prevent the resistant phenotype by using a pre-clinical in vivo model. BT474 breast tumor xenografts were generated in athymic mice and randomized to vehicle, GSI, lapatinib, or lapatinib plus a GSI. Tumor size was measured up to 13 weeks with treatments. The xenograft studies using BT474 lapatinib sensitive cells demonstrated that lapatinib treatment alone inhibited tumor growth by 40%. GSI alone had no effect on growth of tumors compared to vehicle control. However, GSI plus lapatinib significantly reduced tumor growth where regression was observed. Tumor histology of lapatinib plus GSI-treated tumors displayed a significant decrease in Ki67, a proliferation marker, and an increase in apoptotic cells as measured by TUNEL. Furthermore, both ERK1/2 and AKT1 phosphorylated proteins were undetectable by western blot in tumors treated with lapatinib plus GSI. These results suggest that both ErbB-2 and Notch activities must be inhibited to induce tumor regression of ErbB-2 positive breast cancer.

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 2553. doi:10.1158/1538-7445.AM2011-2553

Abstract 2284: Investigation of notch-1, ErbB-2, and ERα crosstalk in breast cancer

Notch-1 is a breast oncogene that is overexpressed with its ligand Jagged-1 in human breast cancers-associated with the poorest overall survival. Notch signaling can be inhibited by a gamma-secretase inhibitor (GSI). We have recently determined that the estrogen receptor-alpha or ErbB-2 inhibits Notch-1 activation in breast cancer cells. In the current study, we sought to uncover the complex crosstalk between ER, ErbB-2, and Notch-1 and ask whether triple combination treatment can disrupt this complex cross-talk model to inhibit growth of ER positive breast cancer cells (MCF-7) and possibly prevent anti-hormone resistance. The results demonstrated that following 1 week of estrogen deprivation, MCF-7 cells develop resistance and regrow. Canonical Notch gene targets: Hes-1, Hey-1, Hes-5, and Deltex-1 are significantly increased at this 1 week time point. Similarly, ErbB-2 mRNA increased significantly after 72 hours of estrogen deprivation. Upon treatment with fulvestrant, a pure ant-estrogen, ER transcriptional activity as measured by PS2 mRNA is inhibited after one week. Interestingly, the combination of fulvestrant and lapatinib, an anti-ErbB-2 tyrosine kinase inhibitor, synergistically increased both Deltex-1 and ErbB-2 mRNAs which were reduced upon GSI treatment. Western blot analysis showed an increase in ER and tyrosine-phosphorylated ErbB-2 and total ErbB-2 proteins. This increase in protein expression was further increased upon dual treatment with fulvestrant and lapatinib. Conversely, treatment with a Notch inhibitor, GSI caused a decrease in both total and tyrosine-phosphorylated ErbB-2. Cell cycle analysis showed that fulvestrant treatment significantly growth arrested cells in G1 while decreasing cells in S-phase. However, lapatnib or GSI or the combination treatments did not further enhance the growth inhibitory effects of fulvestrant. Similar results were observed using Annexin-V analysis by flow cytometry as a measure of early apoptosis. These results suggest that while inhibiting ER and ErbB-2 simultaneously increases Notch activation in a synergistic manner, this dramatic increase in Notch activity in the short term does not sensitize ER positive breast cancer cells to a Notch inhibitor to undergo cell death.

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 2284. doi:10.1158/1538-7445.AM2011-2284

Targeting Notch to target cancer stem cells

The cellular heterogeneity of neoplasms has been at the center of considerable interest since the "cancer stem cell hypothesis", originally formulated for hematologic malignancies, was extended to solid tumors. The origins of cancer "stem" cells (CSC) or tumor-initiating cells (TIC; henceforth referred to as CSCs) and the methods to identify them are hotly debated topics. Nevertheless, the existence of subpopulations of tumor cells with stem-like characteristics has significant therapeutic implications. The stem-like phenotype includes indefinite self-replication, pluripotency, and, importantly, resistance to chemotherapeutics. Thus, it is plausible that CSCs, regardless of their origin, may escape standard therapies and cause disease recurrences and/or metastasis after apparently complete remissions. Consequently, the idea of selectively targeting CSCs with novel therapeutics is gaining considerable interest. The Notch pathway is one of the most intensively studied putative therapeutic targets in CSC, and several investigational Notch inhibitors are being developed. However, successful targeting of Notch signaling in CSC will require a thorough understanding of Notch regulation and the context-dependent interactions between Notch and other therapeutically relevant pathways. Understanding these interactions will increase our ability to design rational combination regimens that are more likely to prove safe and effective. Additionally, to determine which patients are most likely to benefit from treatment with Notch-targeting therapeutics, reliable biomarkers to measure pathway activity in CSC from specific tumors will have to be identified and validated. This article summarizes the most recent developments in the field of Notch-targeted cancer therapeutics, with emphasis on CSC.

Abstract 614: ErbB-2 Suppresses Notch-1 Signaling by Regulating Jagged-1 Localization in ErbB-2 positive breast cancer

ErbB-2 is amplified and overexpressed in 20-30% of breast tumors. Today, ErbB-2 positive breast cancers are treated with trastuzumab plus chemotherapy. In spite of dramatic improvements in survival for women with ErbB-2+ breast tumors, resistance remains a major problem in metastatic disease. We have recently published that ErbB-2 suppresses Notch-1 activity and this is reversed by trastuzumab or a dual EGFR/ErbB-2 tyrosine kinase inhibitor similar to lapatinb. Furthermore, we demonstrated that trastuzumab resistance is reversed with a Notch-1 siRNA or a gamma-secretase inhibitor (GSI). Coincidentally, co-overexpression of Notch-1 and its ligand, Jagged-1, in breast cancer have been associated with the poorest overall survival. We know that activated Notch-1 plays an important role in ErbB-2 positive breast cancer and trastuzumab resistance; however, the precise mechanism by which Notch-1 is activated is not yet known. Thus, we hypothesized that overexpression of ErbB-2 inhibited expression and/or cell surface localization of Jagged-1 to suppress Notch-1 activity. This would be reversed in response to anti-ErbB-2 treatment. The results demonstrated that trastuzumab or lapatinib treatment of SKBr3 cells increased protein expression of Jagged-1 by Flow Cytometry. Notch-1 and Jagged-1 co-localized with early endosome antigen-1 (EEA-1) to submembraneous endosomes in ErbB-2 positive breast cancer cells as measured by confocal immunofluorescence. However upon trastuzumab treatment, Notch-1 and Jagged-1 exited EEA-1 positive vesicles and Notch-1 accumulated in the nucleus while Jagged-1 localized to the cell membrane. Cell surface biotinylation assays showed that Jagged-1 was enriched on the cell surface in response to trastuzumab treatment and this was mimicked by expression of K44A Dynamin. Cell surface accumulation of Jagged-1 was associated with an increase in Notch-1 transcriptional activity as measured by an increase in Deltex1 mRNA by real-time PCR. Conversely, knockdown of Jagged-1 by siRNA in SKbr3 and BT474 cells prevented the trastuzumab-induced increase in Deltex1 mRNA. Co-culture studies confirmed that culturing SKBr3 cells with fibroblast-expressing Jagged-1 or DLL4 increased Notch activity as measured by a CBF-1/Notch reporter assay. Taken together, these results suggest that ErbB-2 suppresses Notch-1 activity by retention of Jagged-1 or possibly other Notch ligands within early endosomes, which may be reversed by trastuzumab or lapatinib treatment.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 614.

Abstract 3918: PEA3 is a novel transcriptional activator of Notch-1 and Notch-4 in breast cancer: Opportunity for a new combination strategy

Women diagnosed with triple-negative breast cancer have the worst overall prognosis and frequently present with metastatic tumors. To date, there are no targeted therapies available to combat this aggressive form of breast cancer due to the lack of expression of well-known targets such as ERα, PR, or HER2/neu. Therefore, there is an immediate need to identify novel targets that are responsible for the proliferation, survival, and invasive phenotype. Notch-1 and Notch-4, both potent breast oncogenes, are overexpressed in triple-negative breast cancers-associated with the poorest overall survival. PEA3 (polyomavirus enhancer activator 3), a member of the Ets family of transcription factors, is overexpressed in triple-negative breast cancer and also correlates with aggressive behavior and poor overall survival. Here, we provide new evidence for transcriptional regulation of Notch in triple negative breast cancer and other subtypes of breast cancer. With focus on triple-negative breast cancer cells (MDA-MB-231), our results show that PEA3 is a transcriptional activator of both Notch-1 and Notch-4 as determined by using a PEA3 siRNA and measuring both transcripts and proteins. Chromatin immunoprecipitation confirmed enrichment of PEA3 within the promoter regions of both Notch-1 and Notch-4. Notch-1 recruitment appears to be AP-1 independent, whereas PEA3 recruitment on the Notch-4 promoter is dependent upon c-JUN. Furthermore, results showed that either c-Jun or Fra-1 were required for Notch-4 transcription while c-FOS was a repressor. Importantly, the combined inhibition of Notch signaling (via a gamma-secretase inhibitor, GSI) and PEA3 (via siRNA) arrested the cells in G1, decreased cell viability, reduced tumorgenicity in vitro, and increased apoptosis. Taken together, our results indicate that PEA3 is a novel transcriptional activator of both Notch-1 and Notch-4. PEA3-mediated Notch-1 transcription is AP-1 independent while Notch-4 transcription requires both PEA3 and AP-1 most probably composed of the c-Jun: Fra-1 complex. Moreover, both PEA3 and Notch signaling are essential for proliferation and survival of MDA-MB-231 breast cancer cells. Thus, dual targeting of both PEA3 and Notch pathways might provide a new therapeutic strategy for triple-negative breast cancer as well as additional therapeutic targeting in other breast cancer subtypes.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3918.

Notch versus the proteasome: what is the target of gamma-secretase inhibitor-I?

gamma-Secretase inhibitors are new anti-cancer agents targeting Notch signaling. Their specificity for Notch is as yet unclear. Han and colleagues investigated the effects of Z-LeuLeuNleu-CHO on growth of breast cancer cells. The results demonstrated a reduction in cell viability primarily via proteasome inhibition independent of Notch activity. Currently, gamma-secretase inhibitors in clinical trials are structurally distinct from Z-LeuLeuNleu-CHO. Their effects on the proteasome are yet to be determined. However, findings from Han and colleagues pose two critical questions: Is the level of proteasomal activity in breast tumors the driving force for growth? What does the Notch pathway contribute to this growth?

Trastuzumab resistance: role for Notch signaling

Epidermal growth factor receptor-2 (ErbB-2/HER2) is a potent breast oncogene that has been shown to be amplified in 20% of breast cancers. Overexpression of ErbB-2 predicts for aggressive tumor behavior, resistance to some cytotoxic and antihormonal therapies, and poor overall survival. Trastuzumab, the humanized, monoclonal antibody directed against ErbB-2 has shown tremendous efficacy and improved overall survival for women when combined with a taxane-based chemotherapy. However, resistance to trastuzumab remains a major concern, most notably in women with metastatic breast cancer. Numerous mechanisms that include overexpression of alternate receptor tyrosine kinases and/or loss of critical tumor suppressors have been proposed in the last several years to elucidate trastuzumab resistance. Here we review the many possible mechanisms of action that could contribute to resistance, and novel therapies to prevent or reverse the resistant phenotype. Moreover, we provide a critical role for Notch signaling cross-talk with overlapping or new signaling networks in trastuzumab-resistant breast.