Luminal Breast Cancer Models Research Articles

Aptamer-guided graphene oxide quantum dots for targeted suicide gene therapy in an organoid model of luminal breast cancer

Breast cancer is one of the most common cancers in women. One of the best therapeutic methods against breast cancer is gene therapy, while having an appropriate gene carrier is the biggest challenge of gene therapy. Hence, developing carriers with low cytotoxicity and high gene transfection efficiency, and preferentially with the selective function of gene delivery is a critical demand for this method. In the present study, we introduce a novel targeted carrier to deliver the inducible caspase-9 suicide gene (pLVSIN-iC9) into breast cancer cells. The carrier is composed of graphene oxide quantum dots decorated with polyethyleneimine, and S2.2; an aptamer with high affinity to MUC1 (GOQD-PEI/S2.2). Due to the overexpression of MUC1 in breast cancer cells, the designed GOQD-PEI/S2.2/pLVSIN-iC9 can selectively target cancer cells. Moreover, to better mimic solid tumor conditions, and to evaluate the selective effect of the GOQD-PEI/S2.2/pLVSIN-iC9, an organoid model derived from human dermal fibroblasts (HDF) and MCF-7 cells (coculture organoid) was generated and characterized. The results demonstrate that the coculture organoid model adapts the tissue structure of luminal breast cancer, as well. Therefore, the organoids were subjected to treatment with targeted gene therapy using GOQD-PEI/S2.2/pLVSIN-iC9. Our evidence supports the targeted killing effect of iC9 on the breast cancer cells of the organoids and suggests the good potential of the newly introduced carriers in targeted gene delivery.

Decorin, an exercise-induced secretory protein, is associated with improved prognosis in breast cancer patients but does not mediate anti-tumorigenic tissue crosstalk in mice

BackgroundRegular exercise can reduce incidence and progression of breast cancer, but the mechanisms for such effects are not fully understood. MethodsWe used a variety of rodent and human experimental model systems to determine whether exercise training can reduce tumor burden in breast cancer and to identify mechanism associated with any exercise training effects on tumor burden. ResultsWe show that voluntary wheel running slows tumor development in the mammary specific polyomavirus middle T antigen overexpression (MMTV-PyMT) mouse model of breast cancer but only when mice are not housed alone. We identify the proteoglycan decorin as a contraction-induced secretory factor that systemically increases in patients with breast cancer immediately following exercise. Moreover, high expression of decorin in tumors is associated with improved prognosis in patients, while treatment of breast cancer cells in vitro with decorin reduces cell proliferation. Notwithstanding, when we overexpressed decorin in murine muscle or injected recombinant decorin systemically into mouse models of breast cancer, elevated plasma decorin concentrations did not result in higher tumor decorin levels and tumor burden was not improved. ConclusionExercise training is anti-tumorigenic in a mouse model of luminal breast cancer, but the effect is abrogated by social isolation. The proteoglycan decorin is an exercise-induced secretory protein, and tumor decorin levels are positively associated with improved prognosis in patients. The hypothesis that elevated plasma decorin is a mechanism by which exercise training improves breast cancer progression in humans is not, however, supported by our pre-clinical data since elevated circulating decorin did not increase tumor decorin levels in these models.

Antiprogestins for breast cancer treatment: We are almost ready

The development of antiprogestins was initially a gynecological purpose. However, since mifepristone was developed, its application for breast cancer treatment was immediately proposed. Later, new compounds with lower antiglucocorticoid and antiandrogenic effects were developed to be applied to different pathologies, including breast cancer. We describe herein the studies performed in the breast cancer field with special focus on those reported in recent years, ranging from preclinical biological models to those carried out in patients. We highlight the potential use of antiprogestins in breast cancer prevention in women with BRCA1 mutations, and their use for breast cancer treatment, emphasizing the need to elucidate which patients will respond. In this sense, the PR isoform ratio has emerged as a possible tool to predict antiprogestin responsiveness. The effects of combined treatments of antiprogestins together with other drugs currently used in the clinic, such as tamoxifen, CDK4/CDK6 inhibitors or pembrolizumab in preclinical models is discussed since it is in this scenario that antiprogestins will be probably introduced. Finally, we explain how transcriptomic or proteomic studies, that were carried out in different luminal breast cancer models and in breast cancer samples that responded or were predicted to respond to the antiprogestin therapy, show a decrease in proliferative pathways. Deregulated pathways intrinsic of each model are discussed, as well as how these analyses may contribute to a better understanding of the mechanisms involved.

Clonal heterogeneity in ER+ breast cancer reveals the proteasome and PKC as potential therapeutic targets

Intratumoral heterogeneity impacts the success or failure of anti-cancer therapies. Here, we investigated the evolution and mechanistic heterogeneity in clonal populations of cell models for estrogen receptor positive breast cancer. To this end, we established barcoded models of luminal breast cancer and rendered them resistant to commonly applied first line endocrine therapies. By isolating single clones from the resistant cell pools and characterizing replicates of individual clones we observed inter- (between cell lines) and intra-tumor (between different clones from the same cell line) heterogeneity. Molecular characterization at RNA and phospho-proteomic levels revealed private clonal activation of the unfolded protein response and respective sensitivity to inhibition of the proteasome, and potentially shared sensitivities for repression of protein kinase C. Our in vitro findings are consistent with tumor-heterogeneity that is observed in breast cancer patients thus highlighting the need to uncover heterogeneity at an individual patient level and to adjust therapies accordingly.

TET1 Isoforms Have Distinct Expression Pattern, Localization and Regulation in Breast Cancer.

TET1 regulates gene expression by demethylating their regulatory sequences through the conversion of 5-methylcytosine to 5-hyroxymethylcytosine. TET1 plays important roles in tissue homeostasis. In breast cancer, TET1 was shown to play controversial roles. Moreover, TET1 has at least two isoforms (long and short) that have distinct expression pattern and apparently different functions in tissue development and disease including breast cancer. We hypothesized that TET1 isoforms have different expression patterns, localization and regulation in different types of breast cancer. To prove our hypothesis, we studied the expression of TET1 isoforms in basal and luminal breast cancer cell lines, as well as in basal and luminal breast cancer animal models. We also studied the effect of different hormones on the expression of the two isoforms. Moreover, we assessed the distribution of the isoforms between the cytoplasm and nucleus. Finally, we overexpressed the full length in a breast cancer cell line and tested its effect on cancer cell behavior. In this study, we demonstrate that while Estrogen and GnRH downregulate the expression of long TET1, they lead to upregulation of short TET1 expression. In addition, we uncovered that luminal cells show higher expression level of the long isoform. We also show that while all TET1 isoforms are almost depleted in a basal breast cancer animal model, the expression of the short isoform is induced in luminal breast cancer model. The short form is expressed mainly in the cytoplasm while the long isoform is expressed mainly in the nucleus. Finally, we show that long TET1 overexpression suppresses cell oncogenic phenotypes. In conclusion, our data suggest that TET1 isoforms have distinct expression pattern, localization and regulation in breast cancer and that long TET1 suppresses oncogenic phenotypes, and that further studies are necessary to elucidate the functional roles of different TET1 isoforms in breast cancer.

Loss of ASAP1 in the MMTV-PyMT model of luminal breast cancer activates AKT, accelerates tumorigenesis, and promotes metastasis

ASAP1 is a multi-domain adaptor protein that regulates cytoskeletal dynamics, receptor recycling and intracellular vesicle trafficking. Its expression is associated with poor prognosis in a variety of cancers, and can promote cell migration, invasion and metastasis. Although amplification and expression of ASAP1 has been associated with poor survival in breast cancer, we found that in the autochthonous MMTV-PyMT model of luminal breast cancer, ablation of ASAP1 resulted in an earlier onset of tumor initiation and increased metastasis. This was due to tumor cell-intrinsic effects of ASAP1 deletion, as ASAP1 deficiency in tumor, but not in stromal cells was sufficient to replicate the enhanced tumorigenicity and metastasis observed in the ASAP1-null MMTV-PyMT mice. Loss of ASAP1 in MMTV-PyMT mice had no effect on proliferation, apoptosis, angiogenesis or immune cell infiltration, but enhanced mammary gland hyperplasia and tumor cell invasion, indicating that ASAP1 can accelerate tumor initiation and promote dissemination. Mechanistically, these effects were associated with a potent activation of AKT. Importantly, lower ASAP1 levels correlated with poor prognosis and enhanced AKT activation in human ER+/luminal breast tumors, validating our findings in the MMTV-PyMT mouse model for this subtype of breast cancer. Taken together, our findings reveal that ASAP1 can have distinct functions in different tumor types and demonstrate a tumor suppressive activity for ASAP1 in luminal breast cancer.

Centrosome Abnormalities and Polyploidy in Murine Mammary Carcinomas with Different Degrees of Hormone Responsiveness

Centrosome amplification leads to aberrant mitosis, giving rise to aneuploidy and it has been associated with poor prognosis in human cancers. This study aimed to evaluate the relationship between polyploidy, centrosome abnormalities, and response to endocrine treatment in progestin-induced mouse mammary carcinomas. We found cells with three or more centrosomes in the polyploid tumors. The endocrine unresponsive tumors showed a higher average number of centrosomes per cell than the responsive tumors. The results suggest an association between polyploidy and centrosome amplification with the resistance to endocrine therapy in this luminal breast cancer model.

Abstract PR7: Epigenetic deregulation in LSD1-deficient luminal mammary tumor cells leads to their increased metastasis via reprogramming of the immune microenvironment

Abstract It has become increasingly recognized that epigenetic deregulation makes significant contributions to metastasis. Mutations or deregulated expression of epigenetic regulators can alter expression of key cancer-related genes in tumor cells and may contribute to metastatic progression by affecting interaction of tumor cells with their immune microenvironment. LSD1 (KDM1A) is a histone demethylase that mainly serves as a transcriptional repressor by removing methyl groups from mono- and di-methylated lysine 4 on histone H3. In human breast cancer, LSD1 homozygous deletions and mutations are found in ~1-2% metastatic cases, suggesting that it may function as a metastasis suppressor. Consistent with this, in a mouse model of luminal breast cancer, MMTV-PyMT, we found that induced loss or inhibition of LSD1 led to a significant increase in lung metastasis. Characterization of the affected tumor cells showed a profound upregulation of many immune-related genes in LSD1-deficient cells, including genes for most MHC class I (MHC-I) (classical and non-classical) molecules, interferon and immune checkpoint pathways. FACS analysis revealed significant upregulation of the CD4+ T cell and monocytic myeloid-derived suppressor cell (MDSC) populations, and downregulation of CD8+ T-cell and NK-cell populations, in tumors with LSD1 deletion/inhibition. MHC-Is are ligands for inhibitory receptors on NK cells, and their interaction (e.g., interaction between a non-classical MHC-I, HLA-E [Qa-1 in mice] and CD94/NKG2A [on NK cells]) is a self-guarding mechanism to avoid killing of healthy “self” cells by NK cells. Upregulation of MHC-Is in Lsd1-null PyMT tumor cells may lead to suppression of NK cells. In fact, in a coculture system, NK cells exhibited reduced cytotoxicity toward Lsd1-null tumor cells. Under the in vivo setting, intraductal injection of PyMT tumor organoid cells to mammary glands of immunodeficient mice led to development of both primary tumors and lung metastasis; importantly, in NSG recipients (lack NK cells), both Lsd1-null and Lsd1-wild-type (wt) tumor cells formed lung metastatic lesions with similar sizes, whereas in SCID recipients (have NK cells), the sizes of metastatic lesions formed from Lsd1-wt tumor cells were significantly smaller than Lsd1-null cells. Collectively, these data suggest that epigenetic deregulation in LSD1-deficient luminal mammary tumor cells reprograms the tumor immune microenvironment, leading to reduced clearance of Lsd1-null PyMT tumor cells by NK cells, which may contribute to their increased metastasis. This study is not only important for understanding the immune mechanism underlying the metastasis-suppressive role of LSD1 in breast cancer, but also has important implications for immunotherapy: currently a strategy of combining checkpoint inhibitors with epigenetic drugs (e.g., LSD1 inhibitor) is being tested for improving immunotherapy; our study may help minimize the risk of (increased) metastasis potentially associated with this strategy. This abstract is also being presented as Poster B6. Citation Format: Dongxi Xiang, Zhe Li. Epigenetic deregulation in LSD1-deficient luminal mammary tumor cells leads to their increased metastasis via reprogramming of the immune microenvironment [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr PR7.

LSD1 suppresses invasion, migration and metastasis of luminal breast cancer cells via activation of GATA3 and repression of TRIM37 expression.

LSD1 (KDM1A) is a histone demethylase that plays both oncogenic and tumor suppressor roles in breast cancer. However, the exact contexts under which it plays these opposite functions remain largely elusive. By characterizing its role in luminal breast epithelial cells, here we show that inhibition of LSD1 by both genetic and pharmacological approaches increases their invasion and migration, whereas its inhibition by genetic approach, but not by pharmacological approach, impairs their proliferation/survival. Induced loss of LSD1 in luminal cells in a mouse model of luminal breast cancer, MMTV-PyMT, leads to a profound increase in lung metastasis. Mechanistically, LSD1 interacts with GATA3, a key luminal-specific transcription factor (TF), and their common target genes are highly related to breast cancer. LSD1 positively regulates GATA3 expression. It also represses expression of TRIM37, a breast epithelial oncogene encoding a histone H2A ubiquitin ligase, and ELF5, a key TF gene for luminal progenitors and alveolar luminal cells. LSD1-loss also leads to reduced expression of several cell-cell adhesion genes (e.g., CDH1, VCL, CTNNA1), possibly via TRIM37-upregulation and subsequently TRIM37-mediated repression. Collectively, our data suggest LSD1 largely plays a tumor suppressor role in luminal breast cancer and the oncogenic program associated with LSD1-inhibition may be suppressed via TRIM37-inhibition.

In-cell determination of Lactate Dehydrogenase Activity in a Luminal Breast Cancer Model ⁻ ex vivo Investigation of Excised Xenograft Tumor Slices Using dDNP Hyperpolarized [1-13C]pyruvate.

[1-13C]pyruvate, the most widely used compound in dissolution-dynamic nuclear polarization (dDNP) magnetic resonance (MR), enables the visualization of lactate dehydrogenase (LDH) activity. This activity had been demonstrated in a wide variety of cancer models, ranging from cultured cells, to xenograft models, to human tumors in situ. Here we quantified the LDH activity in precision cut tumor slices (PCTS) of breast cancer xenografts. The Michigan Cancer Foundation-7 (MCF7) cell-line was chosen as a model for the luminal breast cancer type which is hormone responsive and is highly prevalent. The LDH activity, which was manifested as [1-13C]lactate production in the tumor slices, ranged between 3.8 and 6.1 nmole/nmole adenosine tri-phosphate (ATP) in 1 min (average 4.6 ± 1.0) on three different experimental set-ups consisting of arrested vs. continuous perfusion and non-selective and selective RF pulsation schemes and combinations thereof. This rate was converted to an expected LDH activity in a mass ranging between 3.3 and 5.2 µmole/g in 1 min, using the ATP level of these tumors. This indicated the likely utility of this approach in clinical dDNP of the human breast and may be useful as guidance for treatment response assessment in a large number of tumor types and therapies ex vivo.

OR26-1 Phosphorylated Progesterone Receptor Isoforms Mediate Opposing Stem-Like and Proliferative Breast Cancer Cell Fates

Progesterone receptors (PRs) are key modifiers of ER target gene selection and drivers of luminal breast cancer progression. In addition to Her2 and estrogen receptor (ER), total PRs rather than isoform-specific PR expression levels are measured in breast tumors as indicators of functional ER. Herein, we identified phenotypic differences between PR-A and PR-B in luminal breast cancer cell models with focus on tumorsphere biology. Our findings indicate that PR-A is a dominant driver of cancer stem and stem-like cell (CSC) expansion in T47D breast cancer models, whereas PR-B is a potent driver of anchorage-independent proliferation. PR-A+ tumorspheres were enriched for ALDH activity, CD44+/CD24-, and CD49f+/CD24- (i.e. basal-like) cell populations relative to PR-B+ tumorspheres. Progestin promoted heightened expression of known CSC-associated target genes (e.g. FOXO1, p21, KLF4, WNT4, PTGES, NOTCH2) in PR-A+ but not PR-B+ cells cultured in non-adherent tumorsphere (3D) conditions. In contrast to previous reports, we report robust phosphorylation of PR-A relative to PR-B Ser294 (i.e. Ser130 in PR-A) and show that this residue is required for PR-A-induced expression of CSC-associated genes and CSC behavior in vitro. Chromatin immunoprecipitation assays demonstrated that PR-A phosphorylation was required for receptor recruitment to PRE-containing promoter regions of CSC-associated genes (WNT4, KLF4, NOTCH2). Remarkably, cells expressing S294A-PR-A exhibited severely impaired CSC phenotypes but heightened anchorage-independent cell proliferation. The PR target gene and co-activator, FOXO1, promoted PR Ser294 phosphorylation and tumorsphere formation. The FOXO1 inhibitor (AS1842856) alone or in combination with onapristone (PR antagonist) blunted phospho-PR (Ser294) and tumorsphere formation in PR-A+ and PR-B+ T47D cells and unmodified ER+/PR+ MCF-7 and BT474 breast cancer models. Our data reveal unique isoform-specific functions of phospho-PRs as modulators of distinct and opposing pathways (i.e. CSC expansion versus proliferation) in luminal breast cancer models. A clear understanding of PR isoform-specific actions, phosphorylation events, and the role of co-factors such as FOXO1 may lead to novel biomarkers of advanced tumor behavior and reveal new approaches to pharmacologically target CSCs in luminal breast cancers.

Pitavastatin Selectively Kills PTEN Knock Out Cells and Cancer Organoids in Mouse Model via the Mevalonate Pathway

PTEN is one of the most frequently mutated tumor suppressors in human cancer. Loss of PTEN leads to hyperactivation of the PI3K–AKT signaling pathway, cell migration, and cell proliferation. Aiming to identify specific drugs that could target cancer cells, we screened 2650 FDA approved compounds and discovered that certain statins selectively kill Dictyostelium pten‐cells. This result carries over to mammalian cells: MCF10A cells lacking PTEN are more sensitive to statins than MCF10A wild type (WT) cells. By testing the differential sensitivity between MCF10A WT and pten−/− cells with multiple statins, we found that pitavastatin showed the best performance. Mevalonic acid and geranylgeranyl pyrophosphate (GGPP), but not farnesyl phyrophosphate or squalene, rescued pitavastatin treated MCF10A pten−/− cells. Pitavastatin induced an increase in RhoA expression suggesting that the requirement for geranylgeranylation of this GTPase may mediate the sensitivity of MCF10A pten−/− cells. The different sensitivity between MCF10A WT and pten−/− cells also carried over to three‐dimensional culture model (3D). Moreover, we found pitavastatin significantly decreased the growth and Twist1‐induced cell dissemination in 3D culture of murine mammary gland organoids. Finally, we tested the effects of pitavastatin on growth and survival of WT(FVB) and tumor organoids (MMTV‐PyMT (luminal breast cancer model), C31Tag (basal breast cancer model)) in 3D culture. Both MMTV‐PyMT and C31Tag were killed by 100 nM pitavastatin, while WT organoids survived. These findings implicate pitavastatin as a potential therapeutic candidate for tumors.Support or Funding InformationThis work was supported by NIH Grant R35 GM118177.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Abstract P5-05-06: Progesterone receptor (PR) isoform-specific expansion of breast cancer stem-like cells occurs through a phospho-PR/FOXO1 driven signaling axis

Abstract Luminal breast cancers account for ˜75% of cases and are positive for progesterone receptor (PR) and estrogen receptor (ER) expression. PR is a classical ER-target gene and is used as a biomarker of ER activity; however, a growing body of evidence supports the role of PR as an important modifier of ER actions and a key driver of luminal breast cancer progression. Progesterone signaling is mediated by two PR isoforms: full-length PR-B and truncated PR-A, which lacks the N-terminal 164 amino acids. Little is known about PR isoform-specific actions in PR-expressing breast tumors, given that total PR expression is measured clinically. Herein, we sought to identify phenotypic differences in luminal breast cancer cells (T47D) overexpressing PR-A (T47D-YA) or PR-B (T47D-YB). We demonstrate that PR-B expression is required for anchorage-independent colony formation, while PR-A expressing cells fail to proliferate in soft agar. PR-B driven proliferation has been mapped to PR phosphorylation events, which include MAPK or CDK consensus sites such as Ser294. We demonstrate that in contrast to previous reports, PR-A is well phosphorylated at Ser294 in response to progestins (e.g. R5020) using our custom phospho-PR (Ser294) polyclonal antibody. Interestingly, Ser294 phosphorylation of PR-A occurs more rapidly and robustly following hormone treatment compared to PR-B expressing cells. Our findings indicate that PR-A is a dominant driver of stem-like expansion in breast cancer cells. PR-A tumorspheres exhibited enriched ALDH+ and CD44+/CD24- populations compared to PR-B and promoted heightened gene expression of stem-like genes (e.g. FOXO1). We demonstrate that the PR target gene and co-activator FOXO1 promotes both PR-A and PR-B phosphorylation at Ser294 and augments tumorsphere formation. Direct inhibition of FOXO1 levels abrogates phospho-PR (Ser294) levels and tumorsphere formation in PR expressing cells. Finally, we show that Ser294 is required for PR-A induced expression of stem-like genes (e.g. FOXO1) and stem-like behavior as measured by ALDH+/CD44+ tumorspheres. Taken together, our data reveal unique functions of PR isoforms as modulators of distinct and opposing pathways (i.e. proliferation versus stem-like expansion) in luminal breast cancer models. A clear understanding of PR isoform-specific actions, phosphorylation events, and the role of co-factors such as FOXO1 may lead to novel biomarkers of advanced tumor behavior and reveal new approaches to pharmacologically target PR isoforms in luminal breast cancers. Citation Format: Truong TH, Dwyer AR, Diep CH, Lange CA. Progesterone receptor (PR) isoform-specific expansion of breast cancer stem-like cells occurs through a phospho-PR/FOXO1 driven signaling axis [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P5-05-06.

Phosphorylated Progesterone Receptor Isoforms Mediate Opposing Stem Cell and Proliferative Breast Cancer Cell Fates.

Progesterone receptors (PRs) are key modifiers of estrogen receptor (ER) target genes and drivers of luminal breast cancer progression. Total PR expression, rather than isoform-specific PR expression, is measured in breast tumors as an indicator of functional ER. We identified phenotypic differences between PR-A and PR-B in luminal breast cancer models with a focus on tumorsphere biology. Our findings indicated that PR-A is a dominant driver of cancer stem cell (CSC) expansion in T47D models, and PR-B is a potent driver of anchorage-independent proliferation. PR-A+ tumorspheres were enriched for aldehyde dehydrogenase (ALDH) activity, CD44+/CD24-, and CD49f+/CD24- cell populations relative to PR-B+ tumorspheres. Progestin promoted heightened expression of known CSC-associated target genes in PR-A+ but not PR-B+ cells cultured as tumorspheres. We report robust phosphorylation of PR-A relative to PR-B Ser294 and found that this residue is required for PR-A-induced expression of CSC-associated genes and CSC behavior. Cells expressing PR-A S294A exhibited impaired CSC phenotypes but heightened anchorage-independent cell proliferation. The PR target gene and coactivator, FOXO1, promoted PR phosphorylation and tumorsphere formation. The FOXO1 inhibitor (AS1842856) alone or combined with onapristone (PR antagonist), blunted phosphorylated PR, and tumorsphere formation in PR-A+ and PR-B+ T47D, MCF7, and BT474 models. Our data revealed unique isoform-specific functions of phosphorylated PRs as modulators of distinct and opposing pathways relevant to mechanisms of late recurrence. A clear understanding of PR isoforms, phosphorylation events, and the role of cofactors could lead to novel biomarkers of advanced tumor behavior and reveal new approaches to pharmacologically target CSCs in luminal breast cancer.

Integrin-uPAR signaling leads to FRA-1 phosphorylation and enhanced breast cancer invasion

BackgroundThe Fos-related antigen 1 (FRA-1) transcription factor promotes tumor cell growth, invasion and metastasis. Phosphorylation of FRA-1 increases protein stability and function. We identify a novel signaling axis that leads to increased phosphorylation of FRA-1, increased extracellular matrix (ECM)-induced breast cancer cell invasion and is prognostic of poor outcome in patients with breast cancer.MethodsWhile characterizing five breast cancer cell lines derived from primary human breast tumors, we identified BRC-31 as a novel basal-like cell model that expresses elevated FRA-1 levels. We interrogated the functional contribution of FRA-1 and an upstream signaling axis in breast cancer cell invasion. We extended this analysis to determine the prognostic significance of this signaling axis in samples derived from patients with breast cancer.ResultsBRC-31 cells display elevated focal adhesion kinase (FAK), SRC and extracellular signal-regulated (ERK2) phosphorylation relative to luminal breast cancer models. Inhibition of this signaling axis, with pharmacological inhibitors, reduces the phosphorylation and stabilization of FRA-1. Elevated integrin αVβ3 and uPAR expression in these cells suggested that integrin receptors might activate this FAK-SRC-ERK2 signaling. Transient knockdown of urokinase/plasminogen activator urokinase receptor (uPAR) in basal-like breast cancer cells grown on vitronectin reduces FRA-1 phosphorylation and stabilization; and uPAR and FRA-1 are required for vitronectin-induced cell invasion. In clinical samples, a molecular component signature consisting of vitronectin-uPAR-uPA-FRA-1 predicts poor overall survival in patients with breast cancer and correlates with an FRA-1 transcriptional signature.ConclusionsWe have identified a novel signaling axis that leads to phosphorylation and enhanced activity of FRA-1, a transcription factor that is emerging as an important modulator of breast cancer progression and metastasis.

Abstract 1634: CDK4/6 inhibition directly enhances an anti-tumor immune response in breast cancer

Abstract The cyclin D:cyclin-dependent kinases 4 and 6 (CDK4/6) axis is one of the most frequently dysregulated pathways in human cancers, and CDK4/6 inhibitors have shown significant activity against a number of solid tumors, including breast cancer. Analogous to clinical experience, we found that the CDK4/6 inhibitor, abemaciclib, caused significant tumor regression in the MMTV-rtTA/tetO-HER2 mouse model of luminal breast cancer. However, as CDK4/6 inhibitors are known to block tumor cell proliferation, but not directly induce tumor cell apoptosis, as we confirmed in our study, the mechanisms by which CDK4/6 inhibition caused tumor regression were not clear. Notably, abemaciclib therapy increased total CD3+ T cells in these tumors, while decreasing the immunosuppressive CD4+ regulatory T cell population. Further investigation revealed that CDK4/6 inhibition directly suppresses regulatory T cells by inhibiting their proliferation without impacting natural regulatory T cell formation in the thymus, the differentiation of inducible regulatory T cells, or regulatory T cell apoptosis. We are currently exploring potential effects of CDK4/6 inhibition on regulatory T cell function and lineage stability. Analysis of CD8+ and CD4+ T cells in abemaciclib-treated tumors revealed a marked reduction in the expression of the inhibitory immune checkpoint receptors PD-1, Tim-3, CTLA-4, and LAG3. Specifically, the fraction of CD8+ T cells expressing known markers of T cell exhaustion (PD-1/Tim-3 double-positivity or high expression of PD-1) was decreased in abemaciclib-treated tumors. Importantly, antibody-mediated depletion of CD8+ T cells established that response of MMTV-rtTA/tetO-HER2 tumors to abemaciclib is dependent on CD8+ T cells. Finally, given the observed immunomodulatory effects of abemaciclib, we sought to determine if abemaciclib treatment would sensitize MMTV-rtTA/tetO-HER2 tumors to immune checkpoint blockade. Pretreatment with abemaciclib followed by combination abemaciclib and α-PDL1 significantly enhanced tumor regression compared to abemaciclib or α-PDL1 alone. Our studies reveal a novel mechanism by which CDK4/6 inhibitors directly elicit an anti-tumor immune response. These results provide strong rationale for further investigations into combining CDK4/6 inhibitors with immune checkpoint blockade in breast cancer. Citation Format: Molly J. DeCristo*, Shom Goel*, April C. Watt, Haley BrinJones, Jaclyn Sceneay, Ben Li, Jessalyn M. Ubellacker, Shaozhen Xie, Susanne Ramm, Hye-Jung Kim, Sandra S. McAllister, Jean J. Zhao. CDK4/6 inhibition directly enhances an anti-tumor immune response 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 1634. doi:10.1158/1538-7445.AM2017-1634

Abstract 135: Targeted therapy of hormone receptor-positive breast cancer harboring PIK3CA and AKT1 genetic aberrations

Abstract Introduction: Molecular alterations in the PI3K/Akt/mTOR pathway is implicated in the pathogenesis of ER positive breast cancer with high frequency and therefore is the major focus of drug development. Activating mutations of this signaling pathway occur in more than 70% of breast tumors. The genetic alterations affecting major components of PI3K/Akt/mTOR include genes encoding the PI3K catalytic subunits p110α (PIK3CA) and the PI3K effector AKT1. Despite the efficacy of the available small molecule inhibitors for targeting different components of PI3K pathway, the contribution of PIK3CA and AKT1 genetic alterations to targeted therapy is not well understood. We speculated that ER positive breast tumors with PIK3CA and AKT1 alterations are more sensitive to PI3K and Akt inhibitors due to differential downstream pathway effectors. Methods: We used a panel of isogenic MCF7 cell lines with oncogenic mutations of PIK3CA and AKT1 established through somatic cell gene targeting. A series of pharmacological compounds currently approved for breast cancer therapy or in clinical trials in targeting PI3K/Akt/mTOR pathway were evaluated using viability, clonogenic and cell cycle analysis assays. We performed functional proteomic profiling using reverse phase proteomic analysis (RPPA) to investigate differential protein expression in response to PI3K pathway inhibition in PIK3CA and AKT1 mutants MCF7 cell lines. In vivo experiment was done to evaluate the efficacy of the inhibitors in suppressing tumor growth. Results: We demonstrated that PIK3CA (E545K) and AKT1 (E17K) sensitized cells to the inhibitory effects of BYL719 (p110α catalytic subunit inhibitor) and AZD5363 (pan-AKT kinase inhibitor) on survival, cell cycle progression and colony formation ability. The presence of PIK3CA and AKT1 mutations conferred growth advantage and exhibited increased proliferation in vitro and in vivo. These alterations dramatically increased Akt phosphorylation and induced activation of PI3K downstream effectors. RPPA analysis revealed several proteins differentially expressed in PIK3CA and AKT1 mutant cells compared with wild type (p-value < 0.05). Conclusion: PIK3CA and AKT1 mutations showed distinct effects on sensitivity to targeted pathway inhibitors in an isogenic luminal breast cancer model. The presence of these genetic alterations in estrogen receptor (ER) positive breast cancer provided proliferative advantage and enhanced their sensitivity to targeted PI3K inhibition. These results will contribute to identify single agent and combination therapies targeting PI3K pathway with maximal activity at tolerated dose and selection for individuals most likely to be responsive based on their genomic alterations. Citation Format: Maryam Shariati, Kurt W. Evans, Stephen M. Scott, Huiqin Chen, Funda Meric-Bernstam. Targeted therapy of hormone receptor-positive breast cancer harboring PIK3CA and AKT1 genetic aberrations [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 135. doi:10.1158/1538-7445.AM2017-135

Genetic causes of resistance to entinostat in luminal breast cancer model systems.

1065 Background: Based on promising phase II data, the histone deacetylase inhibitor Entinostat (ENT) is in phase III trials for patients with metastatic ER-positive breast cancer. Predictors of sensitivity and resistance, however, remain unknown. Methods: Luminal cell lines SKBR3 (ER-/HER2+), BT474 (ER+/HER2+) and MCF7 (ER+/HER2-) were treated with or without ENT at their IC50 doses and their gene expression profiles determined. In addition, a total of 27 MMTV-Neu mouse tumors (luminal) were untreated (N = 8), or treated with ENT at 12 mg/kg for 3 weeks (N = 5), 6 weeks (N = 6), or until progression after complete response (N = 8). We investigated their gene expression profiles by microarray and copy number (CN) by arrayCGH, and utilized the Dawnrank analysis, a network-based bioinformatics tool that integrates DNA and RNA data to identify driver genes, to find predictors of resistance to ENT. Results: Supervised analysis of gene expression data coming from the 3 treated cell lines showed significant upregulation of multiple MYC gene signatures. Therefore, we constitutively overexpressed MYC using lentiviral MYC shRNA in SKBR3 and MCF7, and MYC overexpression made cell lines more resistant to ENT. In MMTV-Neu mice, both MYC gene mRNA and gene signatures were downregulated while cells responded to ENT, and became upregulated when the tumors progressed. aCGH CN analysis revealed that a large portion of mouse Chromosome 4 had DNA CN loss and low gene expression in tumors that progressed while on ENT. Within this region, JUN was computationally identified to be a top driver gene associated with resistance. JUN was next knocked down using lentiviral JUN shRNA in BT474 and T47D, and JUN knock-down repeatedly made cell lines more resistant to ENT. MYC gene-expression was also upregulated in JUN-knockdown BT474 and T47D. Finally, JUN CN loss was found in 22% (132/588) of luminal tumors in The Genome Cancer Atlas breast cancer, and all the MYC signature scores were significantly higher in JUN-deleted TCGA samples. Conclusions: ENT was an effective drug for all of our luminal models, both in vitro and in vivo. Using these models, we selected for resistant variants and identified MYC signature expression, and JUN CN deletion as being associated with resistance.

Evaluation of agonist and antagonist radioligands for somatostatin receptor imaging of breast cancer using positron emission tomography

BackgroundThe somatostatin receptor subtype 2 (sstr2) is expressed on a majority of luminal breast cancers, however SPECT and scintigraphy imaging with agonistic sstr2 probes has been sub-optimal. High affinity antagonists can access more binding sites on the cell surface, resulting in higher tumor uptake and improved sensitivity. We compared the tumor uptake and biodistribution of the antagonist 68Ga-NODAGA-JR11 with two agonists 68Ga-DOTA-Tyr3-octreotide (68Ga-DOTATOC) and 68Ga-DOTA-Tyr3-octreotate (68Ga-DOTATATE), in the human, sstr2-positive, luminal breast cancer model: ZR-75-1.ResultsPeptides were assayed for binding affinity using a filtration-based competitive assay to sstr2. natGa-DOTATOC and natGa-DOTATATE had excellent affinity (inhibition constant Ki: 0.9 ± 0.1 nM and 1.4 ± 0.3 nM respectively) compared to natGa-NODAGA-JR11 (25.9 ± 0.2 nM). The number of binding sites on ZR-75-1 cells was determined in vitro by saturation assays. Agonist 67/natGa-DOTATOC bound to 6.64 ± 0.39 × 104 sites/cells, which was 1.5-fold higher than 67/natGa-NODAGA-JR11 and 2.3-fold higher than 67/natGa-DOTATATE. All three 68Ga-labeled peptides were obtained in good decay-corrected radiochemical yield (61-68%) and were purified by high performance liquid chromatography to ensure high specific activity (137 – 281 MBq/nmol at the end of synthesis). NOD scid gamma mice bearing ZR-75-1 tumors were injected intravenously with the labeled peptides and used for PET/CT imaging and biodistribution at 1 h post-injection. We found that 68Ga-DOTATOC had the highest tumor uptake (18.4 ± 2.9%ID/g), followed by 68Ga-DOTATATE (15.2 ± 2.2%ID/g) and 68Ga-NODAGA-JR11 (12.2 ± 0.8%ID/g). Tumor-to-blood and tumor-to-muscle ratios were also higher for the agonists (>40 and >150 respectively), compared to the antagonist (15.6 ± 2.2 and 45.2 ± 11.6 respectively).ConclusionsThe antagonist 68Ga-NODAGA-JR11 had the lowest tumor uptake and contrast compared to agonists 68Ga-DOTATOC and 68Ga-DOTATATE in ZR-75-1 xenografts.The main contributing factor to this result could be the use of an endogenously expressing cell line, which may differ from previously published transfected models in the number of low-affinity, antagonist-specific binding sites. The relative merit of agonists versus antagonists for sstr2 breast cancer imaging warrants further investigation, first in preclinical models with other sstr2-positive breast cancer xenografts, and ultimately in luminal breast cancer patients.

Posttranslationally modified progesterone receptors direct ligand-specific expression of breast cancer stem cell-associated gene programs

BackgroundEstrogen and progesterone are potent breast mitogens. In addition to steroid hormones, multiple signaling pathways input to estrogen receptor (ER) and progesterone receptor (PR) actions via posttranslational events. Protein kinases commonly activated in breast cancers phosphorylate steroid hormone receptors (SRs) and profoundly impact their activities.MethodsTo better understand the role of modified PRs in breast cancer, we measured total and phospho-Ser294 PRs in 209 human breast tumors represented on 2754 individual tissue spots within a tissue microarray and assayed the regulation of this site in human tumor explants cultured ex vivo. To complement this analysis, we assayed PR target gene regulation in T47D luminal breast cancer models following treatment with progestin (promegestone; R5020) and antiprogestins (mifepristone, onapristone, or aglepristone) in conditions under which the receptor is regulated by Lys388 SUMOylation (K388 intact) or is SUMO-deficient (via K388R mutation to mimic persistent Ser294 phosphorylation). Selected phospho-PR-driven target genes were validated by qRT-PCR and following RUNX2 shRNA knockdown in breast cancer cell lines. Primary and secondary mammosphere assays were performed to implicate phospho-Ser294 PRs, epidermal growth factor signaling, and RUNX2 in breast cancer stem cell biology.ResultsPhospho-Ser294 PR species were abundant in a majority (54%) of luminal breast tumors, and PR promoter selectivity was exquisitely sensitive to posttranslational modifications. Phospho-PR expression and target gene programs were significantly associated with invasive lobular carcinoma (ILC). Consistent with our finding that activated phospho-PRs undergo rapid ligand-dependent turnover, unique phospho-PR gene signatures were most prevalent in breast tumors clinically designated as PR-low to PR-null (luminal B) and included gene sets associated with cancer stem cell biology (HER2, PAX2, AHR, AR, RUNX). Validation studies demonstrated a requirement for RUNX2 in the regulation of selected phospho-PR target genes (SLC37A2). In vitro mammosphere formation assays support a role for phospho-Ser294-PRs via growth factor (EGF) signaling as well as RUNX2 as potent drivers of breast cancer stem cell fate.ConclusionsWe conclude that PR Ser294 phosphorylation is a common event in breast cancer progression that is required to maintain breast cancer stem cell fate, in part via cooperation with growth factor-initiated signaling pathways and key phospho-PR target genes including SLC37A2 and RUNX2. Clinical measurement of phosphorylated PRs should be considered a useful marker of breast tumor stem cell potential. Alternatively, unique phospho-PR target gene sets may provide useful tools with which to identify patients likely to respond to selective PR modulators that block PR Ser294 phosphorylation as part of rational combination (i.e., with antiestrogens) endocrine therapies designed to durably block breast cancer recurrence.