Mammosphere Formation Efficiency Research Articles

7961 EMT Plasticity Induced By GRHL2 Overexpression Stimulates Dormancy And Enriches Stem Cell Characteristics In ER+ Breast Cancer Cells

Abstract Disclosure: C. Zheng: None. K.O. Allen: None. T. Liu: None. N.M. Solodin: None. K. Salem: None. A.M. Fowler: None. J. Vera: None. P.K. Tsourkas: None. S. McIlwain: None. M.S. Ozers, PhD: Owner/Co-Owner; Self; Co-Founder and CSO of Proteovista LLC. E.T. Alarid: None. Metastasis is governed by an epithelial to mesenchymal transition (EMT) - the ability for an epithelial cell to adopt a mesenchymal phenotype to promote migration and tumor progression. Though EMT can be articulated as a distinctive transition between its binary states, a cell’s capacity to become plastic, and thus, express a hybrid EMT state with both epithelial and mesenchymal markers has re-defined the literature’s understanding of metastasis. To improve our comprehension of EMT, the nuclear transcription factor responsible for driving epithelial cell fate Grainyhead-like protein 2 (GRHL2) became the protein of interest as clinical evidence suggests high GRHL2 expression is indicative of poorer prognosis in breast cancer (BC). A tetracycline-inducible GRHL2 overexpression model in the estrogen receptor (ER) positive BC MCF7 cell line was developed to understand GRHL2’s role in EMT. Surprisingly, overexpression of the EMT suppressor GRHL2 induces EMT plasticity in ER-positive BC with increased E-cadherin and vimentin expression assessed by immunocytochemistry and mRNA expression. This GRHL2 overexpression model was used to further explore the dynamic nature of EMT plasticity. Gene expression analyses was performed to assess representative genes associated with a partial EMT in purified populations of GRHL2 overexpressing cells. Gene set enrichment analysis (GSEA) determined statistically significant hallmark pathways associated with GRHL2 overexpression. Isolated RNA of GRHL2 overexpressing cells revealed increased mRNA expression of p27 and NR2F1 via RT-qPCR, consistent with a dormancy phenotype. Interestingly, mammosphere and flow cytometry further expanded on the dynamic states of EMT plasticity by emphasizing on the enrichment of stem cell characteristics with GRHL2 overexpression. Extensive GRHL2 overexpression provides cells self-renewal capacity, as examined by mammosphere formation efficiency. Flow cytometry for stem cell markers CD44 and ALDH1 expression demonstrates the increased representation of stem cell markers in EMT plastic populations. To evaluate how GRHL2 overexpression influences tumor progression in vivo, immunocompromised mice were injected with GRHL2 overexpressing cells to examine tumor growth and the potential of cells to create micro-metastases using IVIS. Tumor growth responses differed between parental and GRHL2 overexpressing xenografts, with increased GRHL2 expression decreasing tumor growth. mRNA expression of harvested tumors mirrored in vitro expression of p27 and NR2F1. Together, these data indicates that GRHL2 overexpression induces EMT plasticity, in turn stimulating dormancy and stem-cell acquisition in vitro and in vivo, allowing hybrid cells the ability to adjust to its environment. Presentation: 6/3/2024

Continuous exposure to doxorubicin induces stem cell-like characteristics and plasticity in MDA-MB-231 breast cancer cells identified with the SORE6 reporter

PurposeCancer stem cells (CSCs) account for recurrence and resistance to breast cancer drugs, rendering them a cause of mortality and therapeutic failure. In this study, we examined the effects of exposure to low concentrations of doxorubicin (Dox) on CSCs and non-CSCs from TNBC.MethodsThe effects of Dox were studied using the SORE6 reporter system. We examined the enrichment of the CSCs population, as well as the proliferation, and death of the reporter-positive fraction (GFP + cells) by flow cytometry. The resistant and stemness phenotypes were analyzed by viability and mammosphere formation assay, respectively. We identified differentially expressed and coregulated genes by RNA-seq analysis, and the correlation between gene expression and clinical outcome was evaluated by Kaplan-Mayer analysis using public databases.ResultsIn MDAMB231 and Hs578t cells, we identified enriched subsets in the CSCs population after continuous exposure to low concentrations of Dox. Cells from these enriched cultures showed resistance to toxic concentrations of Dox and increased efficiency of mammosphere formation. In purified GFP + or GFP- cells, Dox increased the mammosphere-forming efficiency, promoted phenotypic switches in non-CSCs populations to a CSC-like state, reduced proliferation, and induced differential gene expression. We identified several biological processes and molecular functions that partially explain the development of doxorubicin-resistant cells and cellular plasticity. Among the genes that were regulated by Dox exposure, the expression of ITGB1, SNAI1, NOTCH4, STAT5B, RAPGEF3, LAMA2, and GNAI1 was significantly associated with poor survival, the stemness phenotype, and chemoresistance.ConclusionThe generation of chemoresistant cells that have characteristics of CSCs, after exposure to low concentrations of Dox, involves the differential expression of genes that have a clinical impact.

Abstract PO2-24-06: Cancer-Associated Fibroblasts Promote Tumor Stemness in Aggressive Breast Cancers via NDRG1

Abstract Background: Inflammatory breast cancer (IBC) and triple negative breast cancer (TNBC) have the lowest 5-year survival rates among breast cancers: 40% for IBC and 77% for TNBC. Effective treatments for these aggressive tumors are urgently needed. To develop better treatments, it is crucial to understand the underlying mechanisms driving their aggressive biology. Our recent research has identified N-myc downstream regulated gene 1 (NDRG1) as a crucial driver of tumor progression and metastasis in aggressive breast cancers. We have also found that NDRG1 expression in tumors is correlated with poorer outcomes in IBC patients. Additionally, we have observed that cancer-associated fibroblasts (CAFs), derived from breast cancer biopsies, stimulate NDRG1 expression and phosphorylation as well as tumor stemness. Based on these findings, we hypothesize that NDRG1 is a key regulator of tumor stemness and progression, and secreted stromal factors regulate tumor stemness through NDRG1. Methods: To assess tumor stemness, we conducted in vitro experiments by quantifying the CD44+/CD24- subpopulation and performing mammosphere assays in NDRG1 control and depleted cells (SUM149, BCX010, MDA-IBC3). In vivo, we employed a limiting dilution transplantation assay, where NDRG1 control and knockdown SUM149 cells were transplanted into the mammary fat pad at different dilutions. We treated NDRG1 control and knockdown breast cancer cells with different patient-derived CAF-conditioned media (CAFs-CM, 50%, 24h) or control medium. The expression of NDRG1/pNDRG1 and CD44+/CD24- cells was evaluated by immunoblotting and flow cytometry. qPCR array was performed to evaluate tumor stemness markers. Results: NDRG1 depletion significantly reduced the CD44+/CD24- subpopulation (p< 0.001) and the efficiency of mammosphere formation (p=0.001). In vivo limiting dilution experiments demonstrated a substantial reduction in the frequency of breast cancer stem cells in NDRG1 knockdown cells (p= 1 x 10-12). Treatment of IBC cells with CAF-CM derived from breast cancer patients stimulated the cancer stem cell subpopulation [CD44+/CD24-: Control medium (43.1 ± 0.1) vs CAF-CM (55.1 ± 0.7); p=0.004] and increased NDRG1 and phospho-NDRG1 expression. However, the stimulation of NDRG1 knockdown cells with CAF-CM did not significantly alter the cancer stem cell population (0.2 ± 0.2 in Control medium versus 0.4 ± 0.3 in CAF-CM) and the expression of NDRG1/phospho-NDRG1. Additionally, qPCR array analysis revealed the upregulation of several cancer stem cell and self-renewal markers in CAF-CM stimulated cells compared to control-medium treated cells. Conclusions: Our data strongly support the role of NDRG1 as a critical regulator of tumor stemness. Moreover, we have discovered that conditioned media from patient-derived CAFs induces tumor stemness in breast cancer cells, and this effect is dependent on the expression NDRG1. Citation Format: Emilly Villodre, Jenny M Hogstrom, Xiaoding Hu, Nina Kozlova, Debu Tripathy, Taru Muranen, Bisrat Debeb. Cancer-Associated Fibroblasts Promote Tumor Stemness in Aggressive Breast Cancers via NDRG1 [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-24-06.

Estrogen-related receptor alpha (ERRα) promotes the migration, invasion and angiogenesis of breast cancer stem cell-like cells.

Breast cancer progression, metastasis and recurrence are largely driven by breast cancer stem cells (BCSCs), which constitute a subset of tumor cells exhibiting stem cell characteristics. In this study, we evaluated the role of estrogen-related receptor alpha (ERRα) in the migration, invasion and angiogenesis of BCSCs. The inhibition of ERRα using XCT790 or knockdown of ERRα using shRNA inhibited the mammosphere formation efficiency, as well as the migration and invasion of BCSCs derived from the mammospheres of MCF7 and MDA-MB-231 (MB231) cells. Conversely, the overexpression of ERRα significantly increased the migration and invasion of BCSCs derived from the mammosphere. In addition, the XCT790 treatment or shERRα significantly downregulated the epithelial-mesenchymal transition (EMT), as evidenced by the downregulation in the expression of vimentin, Snail, Slug and N-cadherin in the mammospheres of MCF7 and MB231 cells. The chorioallantoic membrane assay showed that the conditioned media from XCT790-treated and shERRα cells significantly inhibited blood vessel formation and vessel length. Furthermore, XCT790 treatment or shERRα also downregulated the expression of molecular markers of angiogenesis, such as VEGF-A and Ang-2 in the mammospheres. Conversely, the overexpression of ERRα in MCF7 cells significantly increased both EMT and angiogenesis. These findings suggest that ERRα inhibits the migration, invasion and angiogenesis of BCSCs, suggesting as a potential target for breast cancer therapy.

Orai1α and Orai1β support calcium entry and mammosphere formation in breast cancer stem cells

Orai1 is the pore-forming subunit of the Ca2+-release activated Ca2+ channels that mediate store-operated Ca2+ entry (SOCE) in excitable and non-excitable cells. Two Orai1 forms have been identified in mammalian cells, the full-length variant Orai1α, and the short form Orai1β, lacking the N-terminal 63 amino acids. Stem cells were isolated from non-tumoral breast epithelial cells of the MCF10A cell line, and the most representative ER+ , HER2 or triple negative breast cancer cell lines MCF7, SKBR3 and MDA-MB-231, respectively. Orai and TRPC family members expression was detected by RT-PCR and Western blotting. Changes in cytosolic Ca2+ concentration were analyzed by confocal microscopy using Fluo 4 and the spheroid-forming ability and self-renewal was estimated in culture plates coated with pHEMA using a cell imaging system. Here, we have characterized the expression of Orai family members and several TRPC channels at the transcript level in breast stem cells (BSC) derived from the non-tumoral breast epithelial cell line MCF10A and breast cancer stem cells (BCSC) derived from the well-known estrogen receptor positive (ER+), HER2 and triple negative cell lines MCF7, SKBR3 and MDA-MB-231, respectively. Furthermore, we have evaluated the mammosphere formation efficiency and self-renewal of the BSC and BCSC. Next, through a combination of Orai1 knockdown by iRNA and the use of MDA-MB-231 KO cells, missing the native Orai1, transfected with plasmids encoding for either Orai1α or Orai1β, we show that Orai1 is essential for mammosphere formation and self-renewal efficiency in BCSC derived from triple negative and HER2 subtypes cell cultures, while this channel has a negligible effect in BCSC derived from ER+ cells as well as in non-tumoral BSC. Both, Orai1α, and Orai1β support SOCE in MDA-MB-231-derived BCSC with similar efficiency, as well as COX activation and mammosphere formation. These findings provide evidence of the functional role of Orai1α and Orai1β in spheroid forming efficiency and self-renewal in breast cancer stem cells.

Estrogen-Related Receptor Alpha (ERRα) Promotes Cancer Stem Cell-Like Characteristics in Breast Cancer.

Cancer stem cells drive tumor initiation, invasion, metastasis and recurrence. In the present study, we have evaluated the role of ERRα in the maintenance of breast cancer stem cells (BCSCs) using breast cancer cell lines. The inhibition of ERRα with the inverse agonist, XCT-790, or the knockdown of ERRα in breast cancer cells significantly reduced the mammosphere formation efficiency and mammosphere size along with a significant reduction in the CD44+/CD24- BCSCs. Treatment with XCT-790 significantly downregulated expression of the transcription factors involved in stem cell maintenance such as Oct4, Klf4, Sox2, Nanog and c-Myc in the mammosphere forming stem cells of MCF7 and MDA-MB-231. In addition, XCT-790 induced cell cycle arrest and apoptosis in the mammosphere-forming cells. The knockdown or inhibition of ERRα downregulated the expression of Notch1 and β-catenin, whereas the overexpression of ERRα in MCF7 cells upregulated the expression of these proteins. Moreover, the inhibition of ERRα synergistically enhanced the efficacy of paclitaxel in inhibiting the BCSCs. These results show that ERRα is crucial for the maintenance of BCSCs and suggest that ERRα could be a potential target for breast cancer treatment.

Kruppel-like factor 8 regulates triple negative breast cancer stem cell-like activity.

Breast tumor development is regulated by a sub-population of breast cancer cells, termed cancer stem-like cells (CSC), which are capable of self-renewing and differentiating, and are involved in promoting breast cancer invasion, metastasis, drug resistance and relapse. CSCs are highly adaptable, capable of reprogramming their own metabolism and signaling activity in response to stimuli within the tumor microenvironment. Recently, the nutrient sensor O-GlcNAc transferase (OGT) and O-GlcNAcylation was shown to be enriched in CSC populations, where it promotes the stemness and tumorigenesis of breast cancer cells in vitro and in vivo. This enrichment was associated with upregulation of the transcription factor Kruppel-like-factor 8 (KLF8) suggesting a potential role of KLF8 in regulating CSCs properties. Triple-negative breast cancer cells were genetically modified to generate KLF8 overexpressing or KLF8 knock-down cells. Cancer cells, control or with altered KLF8 expression were analyzed to assess mammosphere formation efficiency, CSCs frequency and expression of CSCs factors. Tumor growth in vivo of control or KLF8 knock-down cells was assessed by fat-pad injection of these cell in immunocompromised mice. Here, we show that KLF8 is required and sufficient for regulating CSC phenotypes and regulating transcription factors SOX2, NANOG, OCT4 and c-MYC. KLF8 levels are associated with chemoresistance in triple negative breast cancer patients and overexpression in breast cancer cells increased paclitaxel resistance. KLF8 and OGT co-regulate each other to form a feed-forward loop to promote CSCs phenotype and mammosphere formation of breast cancer cells. These results suggest a critical role of KLF8 and OGT in promoting CSCs and cancer progression, that may serve as potential targets for developing strategy to target CSCs specifically.

Abstract 2442: KLF8 and OGT/O-GlcNAcylation regulate stem-like cell properties in breast cancer

Abstract Breast cancer is the most common cancer and the second leading cause of cancer death among women overall. One of the main challenges in fighting breast cancer is the intratumor heterogeneity as they are composed of different subpopulations of cancer cells. This heterogeneity of breast cancer is promoted and maintained by a subpopulation of cancer stem-like cells (CSC). CSC share properties of adult stem cells, capable of self-renewing, differentiating & repopulating a tumor. In addition to CSC role in promoting tumor growth, CSC are involved in metastasis, drug resistance and relapse of breast cancer. Therefore, understanding the mechanisms that drive and maintain CSC will help identify novel therapeutic targets in treating breast cancer. Functional activities of CSC are heavily influenced by modulations in the tumor microenvironment, such as alteration in nutrient status. One key component connecting signaling pathway and cancer cell metabolism is the enzyme O-GlcNAc Transferase (OGT), which utilize UDP-GlcNAc to modify nuclear and cytoplasmic proteins in a process termed O-GlcNAcylation. OGT and O-GlcNAc are upregulated in many cancers. Recently, we showed a direct connection between OGT/O-GlcNAc and CSC in breast cancer, in which OGT/O-GlcNAc is required and sufficient to promote the stemness of breast cancer cells in vitro, as well as tumorigenesis in vivo. Global transcriptomic analysis of CSCs-enriched mammospheres with OGT overexpression revealed Kruppel-like-factor 8 (KLF8) as a downstream target of OGT in CSC. Here, we show that KLF8 expression in breast cancer cells is regulated by OGT at the mRNA and protein level. Knockdown of KLF8 reduced mammosphere forming efficiency (MFE), reduced CSC population, and tumor growth in vivo. In contrast, KLF8 overexpression increased MFE and CSC population. Importantly, KLF8 knockdown blocked the effect of elevated OGT and O-GlcNAcylation in promoting MFE, while KLF8 overexpression rescued OGT inhibition in regulating in breast cancer cells. Consistently, KLF8 knockdown in breast cancer cells significantly reduced expression of stem cells markers at both mRNA and protein level, while KLF8 overexpression increased expression of these CSC factors. Interestingly, KLF8 knockdown in breast cancer cell reduced OGT expression, while KLF8 overexpression increased OGT and O-GlcNAcylation, suggesting a possible feed-forward loop between OGT and KLF8 in promoting CSC. Supporting the idea that KLF8 and OGT are critical for breast cancer progression, high expression of KLF8 and OGT is correlated with poor breast cancer patient outcome. Furthermore, overexpression of KLF8 showed increase in resistance to paclitaxel in triple-negative breast cancer cells in vitro, suggesting a possible role of KLF8 in promoting chemotherapy resistance in breast cancer. Together, our results suggested that KLF8 and OGT may work in concert in promoting breast cancer stem-like cells population. Citation Format: Giang Le Minh, Emily Esquea, Tejsi Dhamelia, Neha Akella, Mauricio J. Reginato. KLF8 and OGT/O-GlcNAcylation regulate stem-like cell properties in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2442.

Mammosphere formation assay optimization in the characterization of cancer stem cells of the primary breast tumor

Background/Purpose: Breast cancer is the most frequently diagnosed cancer and the leading cause of death from cancer among females worldwide. Breast cancer tumors that feature breast cancer stem cells (BCSCs) are known to cause drug resistance and metastasis. Culturing BCSCs from primary tumors as mammospheres is both difficult and costly. Therefore, the ability to form BCSC mammospheres in-vitro has become essential in assessing their characterization. Flow cytometric analysis of surface markers and measurement of aldehyde dehydrogenase (ALDH) activity are among other methods used to evaluate cancer cells' stem cell activity. 
 Methods:The research material consisted of BCSCs isolated from the tumor tissues collected from two patients with invasive ductal carcinoma breast tumors. Subsequently, several stem cell surface markers, i.e., cluster of differentiation 44 (CD44), CD24, and CD133, were analyzed using flow cytometry during the third passaging of the cells. ALDH assay is performed with negative control verapamil incubated cells. Two mammosphere forming methods, i.e., low attachment and agar-coated wells together with medium seeded in three different cell concentrations, were compared. 
 Results: CD44+, CD24- and CD133+antibody expressions showed that these cells could be tumor-initiating CSCs. ALDH assay results also indicated that these cells possessed stem cell features. In addition, the results of the mammosphere assay revealed that agar-coated wells at a concentration of 7000 cells/cm2 had more prominent floating features and viable spheres.
 Conclusion: The findings of this study supported the hypothesis that agar-coated culture dishes in mammosphere culture would increase the mammosphere formation efficiency (MFE) value and revealed the importance of the number of cells in elucidating the nature of BCSCs.

Abstract P6-10-08: Evaluation of potent novel second-generation MELK-selective inhibitor in aggressive breast cancers

Abstract Background Inflammatory breast cancer (IBC) and triple-negative breast cancer (TNBC) are aggressive breast cancer subtypes with poor clinical outcomes due to the lack of well-validated and actionable targets and the onset of chemo-resistant metastasis. TNBC and IBC account for 15-20% and 2-4% of all breast cancer diagnoses but result in 30% and 8%–10% of breast cancer-related deaths, respectively. MELK (maternal embryonic leucine zipper kinase) is a potential therapeutic target in both TNBC and triple-negative IBC (TN-IBC). We have shown that 1) MELK expression is higher in basal tumors (mostly TNBC) and in IBC than in non-IBC, 2) high MELK expression is an independent prognostic factor for poor overall survival (P=0.0002) and poor distant metastasis-free survival (P=0.008) in breast cancer, and 3) MELK knockout leads to significantly lower metastatic burden and prolonged survival in a xenograft model of TNBC (unpublished data). MELK inhibitor (MELKi) OTS167 is in clinical trials but it lacks specificity and cross-reacts with other essential kinases. It is critical to discover novel, selective inhibitors with good bioavailability that target MELK and minimize adverse effects. We have developed a second-generation small-molecule MELK inhibitor 30e that is a highly potent and slow-binding ATP-competitive inhibitor of MELK. 30e exhibits potent cellular inhibition of MELK (IC50< 10 nM), as assessed using a live cell NanoBRET assay, and is highly selective as assessed by live-cell KiNativ profiling experiments in MDA-MB-231 TNBC cells. Methods We screened a panel of TNBC and IBC cell lines for MELK expression by western blot and qPCR analysis. We assessed the effects of MELK inhibition with the 30e on migration and invasion. The role of MELK in regulating cancer stem cell phenotypes was assessed using the mammosphere assay and flow cytometry (CD44+/CD24-/low surface marker). MELK’s protumorigenic role was determined in vitro using a soft agar assay, and the anti-tumor activity of 30e was assessed in vivo using a TN-IBC orthotopic xenograft mouse model. Results We found that TNBC had high mRNA levels of MELK (62%) compared to non-TNBC cell lines (25%), which correlated with MELK protein levels. 30e inhibited cell viability in TNBC and IBC cells in a dose-dependent manner and with IC50s ranging from 0.45 to 1.76 μM (P≤0.05). However, no significant effect was observed when normal MCF-10A breast cells were treated with 30e (IC50>20 μM). 30e also significantly reduced colony formation ability. Further, we observed that 30e reduced mammosphere formation efficiency and the CD44+/CD24-/low subpopulation in both TNBC and IBC cells in a dose-dependent manner (P≤0.05), suggesting the potential of 30e to inhibit the stem cell population in vitro. The soft agar assay also showed a significant decrease in colony formation in the TNBC and IBC cells after treatment with 30e, an indirect indicator of in vivo tumorigenicity. To assess the efficacy of 30e treatment on tumor growth, we evaluated this inhibitor as a single agent in a SUM-149 (TN-IBC) orthotopic mouse model. The mice were treated with intraperitoneal injections daily with 3 doses (2.5, 5, and 10 mg/kg) of 30e. Our data show that 30e suppressed tumor growth in a dose-dependent manner (P< 0.007). Conclusions and Future Directions Our data demonstrate that a novel, potent and specific MELK inhibitor inhibits tumor growth by suppressing the cancer stem cell phenotype. MELK is a promising target for aggressive cancers such as TNBC and IBC. We are evaluating how MELK inhibition modulates the tumor microenvironment, which is a critical component of breast cancer response to treatment. We performed a cytokine antibody array and are validating the top targets. Further, we will evaluate 30e in combination with standard-of-care treatment for toxicity and inhibition of metastasis. Citation Format: Mohd Mughees, Moises Tacam, Alex Tan, Hector Gonzalez, Emilly S. Villodre, Bisrat Debeb, Debu Tripathy, Geoffrey Bartholomeusz, Juhyeon Lee, Kevin Dalby, Chandra Bartholomeusz. Evaluation of potent novel second-generation MELK-selective inhibitor in aggressive breast cancers [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 P6-10-08.

Comparison of mammosphere formation from stem-like cells of normal breast, malignant primary breast tumors, and MCF-7 cell line

BackgroundMammosphere formation assay has become a versatile tool to quantify the activity of putative breast cancer stem cells in non-adherent in vitro cultures. However, optimizing the suspension culture system is crucial to establish mammosphere cultures from primary breast tumors.MethodsThis study aimed at determining the self-renewal and sphere-forming potential of breast cancer stem-like cells derived from human primary invasive ductal carcinoma and normal breast tissue samples, and MCF-7 breast cancer cell line using an optimal suspension culture system. Mammosphere-forming efficiency of the mammospheres generated from the tissue samples and cell line were compared. We evaluated the expression of CD44+/CD24−/low and CD49f+/EpCAM−/low phenotypes in the stem-like cells by flow cytometry. CK-18, CK-19, α-SMA, and EpCAM marker expression was assessed using immunohistochemical staining.ResultsBreast epithelial cells isolated from the three samples formed two-dimensional spheroids in suspension cultures. Interestingly, mammospheres formed from patient-derived primary breast tumors were enriched in breast cancer stem-like cells with the phenotype CD44+/CD24−/low and exhibited a relatively more number of large spheres when compared to the normal breast stem cells. MCF-7-derived SCs were more aggressive and resulted in the formation of a significantly higher number of spheroids. The expression of CK-18/CK-19 and α-SMA/EpCAM proteins was confirmed in breast cancer tissues.ConclusionsThus, the use of primary tumor specimens and breast cancer cell lines as suitable models for elucidating the breast cancer stem cell activity was validated using mammosphere culture system.

KLF8 and OGT/O‐GlcNAcylation regulate breast cancer stem‐like cells

One of the main challenges in treating breast cancer is the intra‐tumor heterogeneity, which, partly, is maintained and promoted by a sub‐population of breast cancer cells called breast cancer stem‐like cells (BCSCs). BCSCs shares traits of mammary stem cells, capable of self‐renewing and differentiating, while promotes invasion, metastasis, drug resistance and relapse. BCSCs are highly adaptive, capable of reprogramming metabolism and signaling activity in response to tumor microenvironment. One key component of cell nutrient sensing mechanism, linking alteration in metabolism to cell signaling, is O‐GlcNAc transferase (OGT). OGT is responsible to adding O‐GlcNAc moieties to target nuclear, cytoplasmic and mitochondrial proteins from UDP‐GlcNAc – final product of the hexosamine biosynthetic pathway. This modification is termed O‐GlcNAcylation and both OGT and O‐GlcNAcylation are upregulated in many cancers. Recently, we showed that OGT/O‐GlcNAcylation is enriched in BCSCs, promoting the stemness and tumorigenesis of breast cancer cells in vitro and in vivo. RNA‐seq analysis of BCSC‐enriched mammospheres with OGT overexpression revealed Kruppel‐like‐factor 8 (KLF8) as a downstream target of OGT in promoting BCSCs. Here, we show that KLF8 expression in breast cancer cells is regulated by OGT at the mRNA and protein level. Knockdown of KLF8 reduced mammosphere forming efficiency (MFE) and NANOG expression and ALDH activity in breast cancer cells, while overexpressing KLF8 increased MFE, NANOG+ and ALDH+BCSC population. Knockdown of KLF8 also reduced breast tumor growth in vivo. Importantly, KLF8 knockdown abolished the effect of elevated OGT and O‐GlcNAcylation in breast cancer cell MFE and BCSC population, while KLF8 overexpression rescued OGT inhibition in MFE and BCSC population. suggesting that KLF8 is required in promoting BCSCs. Consistently, KLF8 knockdown in breast cancer cells significantly reduced expression of stem cells markers at both mRNA and protein level. Interestingly, KLF8 knockdown in breast cancer cell reduced OGT expression, while KLF8 overexpression increased OGT and O‐GlcNAcylation, suggesting a possible feed‐forward loop between OGT and KLF8 in promoting BCSCs. Supporting the idea that KLF8 and OGT are critical for breast cancer progression, high expression of KLF8 and OGT is correlated with poor breast cancer patient outcome. Furthermore, overexpression of KLF8 showed increase in resistance to paclitaxel in triple‐negative breast cancer cells in vitro, suggesting a possible role of KLF8 in promoting chemotherapy resistance in breast cancer. Together, our results suggested that KLF8 and OGT may work in concert in promoting breast cancer stem‐like cells population.

BCL6 and the Notch pathway: a signaling axis leading to a novel druggable biotarget in triple negative breast cancer.

The transcriptional repressor B-cell lymphoma 6 (BCL6) is dysregulated in several neoplasms, but its role in triple negative breast cancer (TNBC), a highly aggressive subtype which lacks effective treatment, is unclear. The presence of intratumoral cancer stem cells (CSCs) is a main cause of tumor relapse. The Notch signaling pathway is crucial for regulating CSC self-renewal and promoting breast cancer (BC) development and resistance to anticancer therapies. Here, we investigated signaling cascades of BCL6 in the CSC compartment of TNBCs, and the mechanisms that govern its activity, mainly through Notch signaling. Gene expression, somatic copy number alterations and clinical data from the Cancer Genome Atlas and METABRIC were accessed through theXena and cbioportal browsers. Public transcriptome profiles from TNBC datasets were retrieved from theGene Expression Omnibus. Mammosphere formation efficiency was calculated after BCL6 knockdown via transient siRNAtransfection, stable silencing or pharmacological inhibition. The effects exhibited via BCL6 inhibition in putative TNBC stem-like cells were evaluated by immunofluorescence and qRT-PCR analyses. Chromatin immunoprecipitation experiments were performed to validate a putative BCL6 responsive element located in the first intron of the Numb gene and to define the circuit of corepressors engaged by BCL6 following its inhibition. Immunoprecipitation assays were carried out to investigate a novel interaction at the basis of BCL6 control of CSC activity in TNBC. In silico analyses of benchmarked public datasets revealed a significant enrichment of BCL6 in cancer stemness related pathways, particularly of Notch signaling in TNBC. In vitro stable inhibition of BCL6 significantly reduced tumor cell growth and, accordingly, we found thatthe mammosphere formation efficiency of BCL6 silencedcells was significantly impaired by pharmacological inhibition of Notch signaling. BCL6 was found to be expressed at significantly higher levels in TNBC mammospheres than in their adherent counterparts, and loss of BCL6 function significantly decreased mammosphere formation with preferential targeting of CD44-positive versus ALDH-positive stem-like cells. Functional interplay between BCL6 and the chromatin remodeling factor EZH2 triggered the BCL6/Notch stemness signaling axis via inhibition of Numb transcription. Our results may be instrumental for the prospective design of combination treatment strategies that selectively target novel TNBC-associated biomarker(s) whose activity is implicated in the regulation of cancer stemness (such as BCL6) and molecules in developmentally conserved signaling pathways (such as Notch) to achieve long-lasting tumor control and improve patient outcomes.

Abstract P5-06-09: Rivaroxaban targets the procoagulant tumour microenvironment in vitro and thereby inhibits breast cancer progression

Abstract Introduction: Breast cancer patients have a four-fold increased risk of developing a venous thromboembolism and those that do have a significantly increased risk of mortality despite adjusting for cancer stage. Tissue Factor (TF) is expressed by breast cancer-associated fibroblasts as well as breast cancer epithelial cells, and at significantly higher levels than by normal breast fibroblasts. TF signals via PAR-1 and PAR-2 to induce proliferation, invasion, angiogenesis and metastasis. Rivaroxaban is a licensed oral anticoagulant that inhibits the TF-Factor VIIa-Factor Xa complex and could therefore be repurposed to target the procoagulant tumour microenvironment in breast cancer. We hypothesise that a procoagulant microenvironment will induce breast cancer progression in vitro and that these promoting effects will be inhibited by anticoagulants, including Rivaroxaban. Methods: Lentivirally transduced TF over-expressing fibroblasts (TFF) and their control (CF) or conditioned media (TFFCM and CFCM), were cultured with oestrogen receptor positive (MCF-7), triple negative (MDA-MB-231) and HER2 positive (BT474) breast cancer cells, in the presence or absence of Rivaroxaban or anti-TF antibody 10H10. Proliferation (sulforhodamine-B/EdU assay), migration (scratch/transwell assay) and stem cell activity (mammosphere forming efficiency (MFE) assay) were assessed. The underlying mechanism was analysed with western blotting and quantitative PCR. The TFFCM and CFCM were analysed with cytokine arrays, enzyme-linked immunosorbent assays (ELISA) and mass spectrometry whilst the TFF and CF were analysed using RNA sequencing. Results: 3D co-culture of MCF-7s with TFF as compared to CF promoted cancer cell migration (p=0.04) and stem cell activity (MFE: p<0.0001), with these effects abrogated by Rivaroxaban (migration: p= 0.0341, MFE: p=0.0003) and 10H10 (migration: p=0.01, MFE: p=0.0028). TFFCM promoted proliferation, migration and stem cell activity in MCF-7 cells (p<0.05) compared to CFCM, with these effects abrogated by Rivaroxaban (migration, MFE: p<0.05) and 10H10 (proliferation, migration, MFE: p<0.05). The cancer-promoting effects of TFFCM in MCF-7 cells was associated with a decrease in CXCL8 (p<0.0001) and an increase in VEGFA (p<0.05) when compared to CFCM; changes which were reversed by Rivaroxaban (VEGFA: p<0.03) and 10H10 (CXCL8, VEGFA: p<0.05). TFFCM also promoted stem cell activity in both MDA-MB-231 (p<0.006) and BT474 (p<0.002) cell lines compared to CFCM. This increased MFE in BT474 was abrogated by Rivaroxaban (p=0.02) and 10H10 (p<0.005) and associated with an increase in VEGFA (p<0.001) and CXCL8 (p<0.0001). Cytokine array and ELISA analysis of TFFCM versus CFCM indicated increased levels of IL-10 (p<0.006), GRO-α (p<0.0001) and IL1-α (p<0.02). GRO-α and IL1-α production was inhibited by fibroblast treatment with Rivaroxaban (GRO-α, p<0.0001 and IL1-α, p<0.001) and 10H10 (p<0.001). TFFCM versus CFCM had elevated levels of plasminogen activator inhibitor 1, integrin linked kinase, thrombospondin 4 and α -fetoprotein on mass spectrometry. RNA sequencing analysis of the TFF as compared to the CF revealed upregulation of AEBP1 (adipocyte enhancer binding protein 1) (p<0.01), NKX2-5 (homeobox protein NKX-2.5) (p<0.0001), TRIM58 (tripartite motif containing 58) (p<0.0001) and KRT81 (keratin 81) (p<0.0001), all of which were decreased by fibroblast treatment with Rivaroxaban (NKX2-5: p<0.01, KRT81: p<0.0001). Conclusion: A procoagulant microenvironment promotes proliferation, migration and stem cell activity in breast cancer in vitro, with these effects abrogated by Rivaroxaban. Targeting the procoagulant tumour microenvironment is a promising future cancer treatment. Citation Format: Emma L Blower, John Castle, Angelica Santiago-Gomez, Robert Clarke, Cliona C Kirwan. Rivaroxaban targets the procoagulant tumour microenvironment in vitro and thereby inhibits breast cancer progression [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 P5-06-09.

Antiviral Drug 2-thio-6-azauridine Sensitizes Paclitaxel-Resistant Triple Negative Breast Cancer Cells by Targeting Mammosphere Formation and ABC Transporters

Background: 2-thio-6-azauridine (TAU) is a nucleoside analog and potential antiviral drug. The antiproliferative activity of TAU has been evaluated in limited cancer cell lines. The present study is aimed to evaluate the effect of TAU on drug sensitization mechanism in paclitaxel (PTX) resistant triple-negative breast cancer (TNBC) cells.Methods: The cell death mechanism was determined using MTT, BrdU incorporation, apoptosis, and DNA damage Western blot and RT-PCR assays. A specific ELISA method was used to determine the caspase-3 activity and expression levels of MRP1, MDR1, BCRP, and MRP8. Western blot analysis was used to assess the expression of CD151, MRP1, MDR1, and BCRP in CD151 overexpressing PTX-resistant TNBC cells.Results: The combination of TAU and PTX (10:20nM) synergistically inhibited the 50% viability of 12-fold PTX-resistant TNBC cells. Mechanistically, the combination inhibited the proliferation by arresting the cell cycle at the G2M phase and induced apoptosis by altering cell integrity and nuclear morphology as well as damaging DNA. The combination sensitized the PTX-resistant TNBC cells by increasing BAX and decreasing Bcl-2 expression, activating caspase-3, and reducing the expression of ABC transporters MRP1 and MDR1. The combination reduced the expression of MRP1 and MDR1 in CD151 overexpressing PTX-resistant TNBC cells, indicating the role of CD151in TAU mediated sensitization of PTX-resistant TNBC cells. The combination also reduced the mammosphere formation efficiency of PTX-resistant TNBC cells. Conclusion: Overall, the present study illustrated the promising ability of TAU in sensitizing drug-resistant TNBC cells to PTX.

ETV7 regulates breast cancer stem-like cell features by repressing IFN-response genes

Cancer stem cells (CSCs) represent a population of cells within the tumor able to drive tumorigenesis and known to be highly resistant to conventional chemotherapy and radiotherapy. In this work, we show a new role for ETV7, a transcriptional repressor member of the ETS family, in promoting breast cancer stem-like cells plasticity and resistance to chemo- and radiotherapy in breast cancer (BC) cells. We observed that MCF7 and T47D BC-derived cells stably over-expressing ETV7 showed reduced sensitivity to the chemotherapeutic drug 5-fluorouracil and to radiotherapy, accompanied by an adaptive proliferative behavior observed in different culture conditions. We further noticed that alteration of ETV7 expression could significantly affect the population of breast CSCs, measured by CD44+/CD24low cell population and mammosphere formation efficiency. By transcriptome profiling, we identified a signature of Interferon-responsive genes significantly repressed in cells over-expressing ETV7, which could be responsible for the increase in the breast CSCs population, as this could be partially reverted by the treatment with IFN-β. Lastly, we show that the expression of the IFN-responsive genes repressed by ETV7 could have prognostic value in breast cancer, as low expression of these genes was associated with a worse prognosis. Therefore, we propose a novel role for ETV7 in breast cancer stem cells’ plasticity and associated resistance to conventional chemotherapy and radiotherapy, which involves the repression of a group of IFN-responsive genes, potentially reversible upon IFN-β treatment. We, therefore, suggest that an in-depth investigation of this mechanism could lead to novel breast CSCs targeted therapies and to the improvement of combinatorial regimens, possibly involving the therapeutic use of IFN-β, with the aim of avoiding resistance development and relapse in breast cancer.

Abstract 3084: KLF8 is a required downstream effector of OGT/O-GlcNAcylation in regulating breast cancer stem-like cells

Abstract One of the main challenges in treating breast cancer is the intra-tumor heterogeneity, which, partly, is maintained and promoted by a sub-population of breast cancer cells called breast cancer stem-like cells (BCSCs). BCSCs shares traits of mammary stem cells, capable of self-renewing and differentiating, while promotes invasion, metastasis, drug resistance and relapse. BCSCs are highly adaptive, capable of reprogramming metabolism and signaling activity in response to tumor microenvironment. One key component of cell nutrient sensing mechanism, linking alteration in metabolism to cell signaling, is O-GlcNAc transferase (OGT). OGT is responsible to adding O-GlcNAc moieties to target nuclear, cytoplasmic and mitochondrial proteins from UDP-GlcNAc - final product of the hexosamine biosynthetic pathway. This modification is termed O-GlcNAcylation and both OGT and O-GlcNAcylation are upregulated in many cancers. Recently, we showed that OGT/O-GlcNAcylation is enriched in BCSCs, promoting the stemness and tumorigenesis of breast cancer cells in vitro and in vivo. RNA-seq analysis of BCSC-enriched mammospheres with OGT overexpression revealed Kruppel-like-factor 8 (KLF8) as a downstream target of OGT in promoting BCSCs. Here, we showed that KLF8 expression in breast cancer cells is regulated by OGT at protein and mRNA level. Knockdown of KLF8 reduced mammosphere forming efficiency (MFE) and NANOG expression in breast cancer cells, while overexpressing KLF8 increased MFE, NANOG+ and CD44H/CD24L BCSC population. Importantly, KLF8 knockdown abolished the effect of elevated OGT and O-GlcNAcylation in breast cancer cell MFE and BCSC population, while KLF8 overexpression rescued OGT inhibition in MFE and BCSC population. suggesting that KLF8 is required in promoting BCSCs. Interestingly, KLF8 knockdown in breast cancer cell reduced OGT expression, while KLF8 overexpression increased OGT and O-GlcNAcylation, suggesting a possible feed-forward loop between OGT and KLF8 in promoting BCSCs. Supporting the idea that KLF8 and OGT are critical for breast cancer progression, breast cancer patient survival was lower with high KLF8 or OGT levels. Furthermore, overexpression of KLF8 showed increase in resistance to paclitaxel in triple-negative breast cancer cells in vitro, suggesting a possible role of KLF8 in promoting chemotherapy resistance in breast cancer[LM1] . Together, our results suggested that KLF8 is a downstream target of OGT/O-GlcNAcylation in promoting breast cancer stem-like cells population. Citation Format: Giang Le Minh, Neha M. Akella, Tejsi Dhameliya, Mauricio J. Reginato. KLF8 is a required downstream effector of OGT/O-GlcNAcylation in regulating breast cancer stem-like cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3084.

Abstract 1078: Hypoxic microenvironment promotes resistance to targeted therapies in HER2-overexpressing breast cancer involving epithelial-to-mesenchymal transition features

Abstract Trastuzumab and trastuzumab emtansine (T-DM1) targeted therapies are the main choice for the treatment of HER2-overexpressing breast cancer patients. However, de novo or acquired resistance is still the major obstacle in clinical practice. It has been shown that several pathways, including HER2, can lead to HIF-α stabilization in breast cancer cells. Thus, the purpose of our study was to analyse the effect of hypoxia in acquired resistance to anti-HER2 therapies. We used HER2-overexpressing BT-474 and non-overexpressing MCF-7 human breast cancer cell lines. As an acute hypoxia model, we added CoCl2 (100 µM) to cell culture medium. The hypoxic status of the cells was evaluated by a Western blot analysis, showing a peak of HIF-1α expression after 6 hours of CoCl2 exposure. This result correlated with VEGF induction, as measured by RT-qPCR (p<0.05). Interestingly, under hypoxia, BT-474 cells treated with increasing concentrations of trastuzumab and T-DM1 for 72 hours presented a significantly higher viability compared with non-hypoxic cells (p<0.05). However, although the drugs decreased membrane HER2 protein expression (p<0.01), HER2 levels did not change under hypoxic conditions, as shown by flow cytometry and Western blot assays. Moreover, drug effects on MCF-7 cells did not change under hypoxic conditions. To further analyse this cytoprotective effect of hypoxia, proliferation-associated markers such as PCNA expression and AKT activation were evaluated by Western blot, while apoptosis frequency was measured by flow cytometry in BT-474 cells. PCNA and pAKT expression remained unaltered, while T-DM1-mediated apoptosis was significantly reduced under hypoxia (p<0.05). Then, using different open gene expression datasets, we identified a group of four genes (COL12A1, DKK1, CEACAM5 and ANTXR1) differentially expressed in T-DM1 resistant HER2-overexpressing breast cancer BT-474 and KPL-4 cell lines (p<0.05), which were coexpressed with hypoxia-related genes (HIF1A, EPAS1 and VEGFA) in HER2-overexpressing breast cancer tumours and cell lines from TCGA (p<0.01). Further in silico analyses revealed a direct association between those genes and the fibronectin gene (FN1, p<0.05). Remarkably, they were involved in TGF-β and angiogenesis pathways, as shown by a functional enrichment analysis (p<0.01). In addition, we found that BT-474 cells under hypoxia presented a higher mammosphere formation efficiency than non-hypoxic cells (p<0.05), showing a tendency towards a cancer stem cell phenotype. It is known that hypoxia represents a common feature of metastatic tumours and has a role in regulating stem cell behaviour. According to this, our results highlight a link between anti-HER2 therapy resistance and epithelial-to-mesenchymal transition characteristics, where hypoxia plays a key role which deserves further study. Citation Format: Virginia Judith Wolos, Ivana Jaqueline Tapia, Marianela Abrigo, Ezequiel Lacunza, Elisa Dora Bal de Kier Joffé, Gabriel León Fiszman. Hypoxic microenvironment promotes resistance to targeted therapies in HER2-overexpressing breast cancer involving epithelial-to-mesenchymal transition features [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1078.

Anti-Cancer Effects of an Optimised Combination of Ginsenoside Rg3 Epimers on Triple Negative Breast Cancer Models.

Key problems of chemotherapies, as the mainstay of treatment for triple-negative breast cancer (TNBC), are toxicity and development of tumour resistance. Using response surface methodology, we previously optimised the combination of epimers of ginsenoside Rg3 (Rg3) for anti-angiogenic action. Here, we show that the optimised combination of 50 µM SRg3 and 25 µM RRg3 (C3), derived from an RSM model of migration of TNBC cell line MDA-MB-231, inhibited migration of MDA-MB-231 and HCC1143, in 2D and 3D migration assays (p < 0.0001). C3 inhibited mammosphere formation efficiency in both cell lines and decreased the CD44+ stem cell marker in the mammospheres. Molecular docking predicted that Rg3 epimers had a better binding score with IGF-1R than with EGFR, HER-2 or PDGFR, and predicted an mTOR inhibitory function of Rg3. C3 affected the signalling of AKT in MDA-MB-231 and HCC1143 mammospheres. In a mouse model of metastatic TNBC, an equivalent dose of C3 (23 mg/kg SRg3 + 11 mg/kg RRg3) or an escalated dose of 46 mg/kg SRg3 + 23 mg/kg RRg3 was administered to NSG mice bearing MDA-MB-231-Luc cells. Calliper and IVIS spectrum measurement of the primary and secondary tumour showed that the treatment shrunk the primary tumour and decreased the load of metastasis in mice. In conclusion, this combination of Rg3 epimers showed promising results as a potential treatment option for TNBC patients.

The C-X-C Motif Chemokine Ligand 1 Sustains Breast Cancer Stem Cell Self-Renewal and Promotes Tumor Progression and Immune Escape Programs.

Breast cancer (BC) mortality is mainly due to metastatic disease, which is primarily driven by cancer stem cells (CSC). The chemokine C-X-C motif ligand-1 (CXCL1) is involved in BC metastasis, but the question of whether it regulates breast cancer stem cell (BCSC) behavior is yet to be explored. Here, we demonstrate that BCSCs express CXCR2 and produce CXCL1, which stimulates their proliferation and self-renewal, and that CXCL1 blockade inhibits both BCSC proliferation and mammosphere formation efficiency. CXCL1 amplifies its own production and remarkably induces both tumor-promoting and immunosuppressive factors, including SPP1/OPN, ACKR3/CXCR7, TLR4, TNFSF10/TRAIL and CCL18 and, to a lesser extent, immunostimulatory cytokines, including IL15, while it downregulates CCL2, CCL28, and CXCR4. CXCL1 downregulates TWIST2 and SNAI2, while it boosts TWIST1 expression in association with the loss of E-Cadherin, ultimately promoting BCSC epithelial-mesenchymal transition. Bioinformatic analyses of transcriptional data obtained from BC samples of 1,084 patients, reveals that CXCL1 expressing BCs mostly belong to the Triple-Negative (TN) subtype, and that BC expression of CXCL1 strongly correlates with that of pro-angiogenic and cancer promoting genes, such as CXCL2-3-5-6, FGFBP1, BCL11A, PI3, B3GNT5, BBOX1, and PTX3, suggesting that the CXCL1 signaling cascade is part of a broader tumor-promoting signaling network. Our findings reveal that CXCL1 functions as an autocrine growth factor for BCSCs and elicits primarily tumor progression and immune escape programs. Targeting the CXCL1/CXCR2 axis could restrain the BCSC compartment and improve the treatment of aggressive BC.