The Pea3 Ets transcription factor regulates differentiation of multipotent progenitor cells during mammary gland development

Cancer invasion and metastasis: Insights from murine pubertal mammary gland morphogenesis

Cancer invasion and metastasis accounts for the majority of cancer related mortality. A better understanding of the players that drive the aberrant invasion and migration of tumors cells will provide critical targets to inhibit metastasis. Postnatal pubertal mammary gland morphogenesis is characterized by highly proliferative, invasive, and migratory normal epithelial cells. Identifying the molecular regulators of pubertal gland development is a promising strategy since tumorigenesis and metastasis is postulated to be a consequence of aberrant reactivation of developmental stages. In this review, we summarize the pubertal morphogenesis regulators that are involved in cancer metastasis and revisit pubertal mammary gland transcriptome profiling to uncover both known and unknown metastasis genes. Our updated list of pubertal morphogenesis regulators shows that most are implicated in invasion and metastasis. This review highlights molecular linkages between development and metastasis and provides a guide for exploring novel metastatic drivers.

Critical role of the MCAM-ETV4 axis triggered by extracellular S100A8/A9 in breast cancer aggressiveness

Metastatic breast cancer is the leading cause of cancer-associated death in women. The progression of this fatal disease is associated with inflammatory responses that promote cancer cell growth and dissemination, eventually leading to a reduction of overall survival. However, the mechanism(s) of the inflammation-boosted cancer progression remains unclear. In this study, we found for the first time that an extracellular cytokine, S100A8/A9, accelerates breast cancer growth and metastasis upon binding to a cell surface receptor, melanoma cell adhesion molecule (MCAM). Our molecular analyses revealed an important role of ETS translocation variant 4 (ETV4), which is significantly activated in the region downstream of MCAM upon S100A8/A9 stimulation, in breast cancer progression in vitro as well as in vivo. The MCAM-mediated activation of ETV4 induced a mobile phenotype called epithelial-mesenchymal transition (EMT) in cells, since we found that ETV4 transcriptionally upregulates ZEB1, a strong EMT inducer, at a very high level. In contrast, downregulation of either MCAM or ETV4 repressed EMT, resulting in greatly weakened tumor growth and lung metastasis. Overall, our results revealed that ETV4 is a novel transcription factor regulated by the S100A8/A9-MCAM axis, which leads to EMT through ZEB1 and thereby to metastasis in breast cancer cells. Thus, therapeutic strategies based on our findings might improve patient outcomes.

Comprehensive profiling of transcriptional networks specific for lactogenic differentiation of HC11 mammary epithelial stem-like cells

The development of mammary gland as a lactogenic tissue is a highly coordinated multistep process. The epithelial cells of lactiferous tubules undergo profound changes during the developmental window of puberty, pregnancy, and lactation. Several hormones including estrogen, progesterone, glucocorticoids and prolactin act in concert, and orchestrate the development of mammary gland. Understanding the gene regulatory networks that coordinate proliferation and differentiation of HC11 Mammary Epithelial stem-like Cells (MEC) under the influence of lactogenic hormones is critical for elucidating the mechanism of lactogenesis in detail. In this study, we analyzed transcriptome profiles of undifferentiated MEC (normal) and compared them with Murine Embryonic Stem Cells (ESC) using next-generation mRNA sequencing. Further, we analyzed the transcriptome output during lactogenic differentiation of MEC following treatment with glucocorticoids (primed state) and both glucocorticoids and prolactin together (prolactin state). We established stage-specific gene regulatory networks in ESC and MEC (normal, priming and prolactin states). We validated the top up-and downregulated genes in each stage of differentiation of MEC by RT-PCR and found that they are comparable with that of RNA-seq data. HC11 MEC display decreased expression of Pou5f1 and Sox2, which is crucial for the differentiation of MEC, which otherwise ensure pluripotency to ESC. Cited4 is induced during priming and is involved in milk secretion. MEC upon exposure to both glucocorticoids and prolactin undergo terminal differentiation, which is associated with the expression of several genes, including Xbp1 and Cbp that are required for cell growth and differentiation. Our study also identified differential expression of transcription factors and epigenetic regulators in each stage of lactogenic differentiation. We also analyzed the transcriptome data for the pathways that are selectively activated during lactogenic differentiation. Further, we found that selective expression of chromatin modulators (Dnmt3l, Chd9) in response to glucocorticoids suggests a highly coordinated stage-specific lactogenic differentiation of MEC.

ETV4 transcription factor and MMP13 metalloprotease are interplaying actors of breast tumorigenesis

BACKGROUND

The ETS transcription factor ETV4 is involved in the main steps of organogenesis and is also a significant mediator of tumorigenesis and metastasis, such as in breast cancer. Indeed, ETV4 is overexpressed in breast tumors and is associated with distant metastasis and poor prognosis. However, the cellular and molecular events regulated by this factor are still misunderstood. In mammary epithelial cells, ETV4 controls the expression of many genes, MMP13 among them. The aim of this study was to understand the function of MMP13 during ETV4-driven tumorigenesis.

METHODS

Different constructs of the MMP13 gene promoter were used to study the direct regulation of MMP13 by ETV4. Moreover, cell proliferation, migration, invasion, anchorage-independent growth, and in vivo tumorigenicity were assayed using models of mammary epithelial and cancer cells in which the expression of MMP13 and/or ETV4 is modulated. Importantly, the expression of MMP13 and ETV4 messenger RNA was characterized in 456 breast cancer samples.

RESULTS

Our results revealed that ETV4 promotes proliferation, migration, invasion, and anchorage-independent growth of the MMT mouse mammary tumorigenic cell line. By investigating molecular events downstream of ETV4, we found that MMP13, an extracellular metalloprotease, was an ETV4 target gene. By overexpressing or repressing MMP13, we showed that this metalloprotease contributes to proliferation, migration, and anchorage-independent clonogenicity. Furthermore, we demonstrated that MMP13 inhibition disturbs proliferation, migration, and invasion induced by ETV4 and participates to ETV4-induced tumor formation in immunodeficient mice. Finally, ETV4 and MMP13 co-overexpression is associated with poor prognosis in breast cancer.

CONCLUSION

MMP13 potentiates the effects of the ETV4 oncogene during breast cancer genesis and progression.

Serotonergic system antagonists target breast tumor initiating cells and synergize with chemotherapy to shrink human breast tumor xenografts

Breast tumors comprise an infrequent tumor cell population, termed breast tumor initiating cells (BTIC), which sustain tumor growth, seed metastases and resist cytotoxic therapies. Hence therapies are needed to target BTIC to provide more durable breast cancer remissions than are currently achieved. We previously reported that serotonergic system antagonists abrogated the activity of mouse BTIC resident in the mammary tumors of a HER2-overexpressing model of breast cancer. Here we report that antagonists of serotonin (5-hydroxytryptamine; 5-HT) biosynthesis and activity, including US Federal Food and Drug Administration (FDA)-approved antidepressants, targeted BTIC resident in numerous breast tumor cell lines regardless of their clinical or molecular subtype. Notably, inhibitors of tryptophan hydroxylase 1 (TPH1), required for 5-HT biosynthesis in select non-neuronal cells, the serotonin reuptake transporter (SERT) and several 5-HT receptors compromised BTIC activity as assessed by functional sphere-forming assays. Consistent with these findings, human breast tumor cells express TPH1, 5-HT and SERT independent of their molecular or clinical subtype. Exposure of breast tumor cells ex vivo to sertraline (Zoloft), a selective serotonin reuptake inhibitor (SSRI), reduced BTIC frequency as determined by transplanting drug-treated tumor cells into immune-compromised mice. Moreover, another SSRI (vilazodone; Viibryd) synergized with chemotherapy to shrink breast tumor xenografts in immune-compromised mice by inhibiting tumor cell proliferation and inducing their apoptosis. Collectively our data suggest that antidepressants in combination with cytotoxic anticancer therapies may be an appropriate treatment regimen for testing in clinical trials.

Serotonin transporter antagonists target tumor-initiating cells in a transgenic mouse model of breast cancer

Accumulating data suggests that the initiation and progression of human breast tumors is fueled by a rare subpopulation of tumor cells, termed breast tumor-initiating cells (BTIC), which resist radiotherapy and chemotherapy. Consequently, therapies that abrogate BTIC activity are needed to achieve durable cures for breast cancer patients. To identify such therapies we used a sensitive assay to complete a high-throughput screen of small molecules, including approved drugs, with BTIC-rich mouse mammary tumor cell populations. We found that inhibitors of the serotonin reuptake transporter (SERT) and serotonin receptors, which include approved drugs used to treat mood disorders, were potent inhibitors of mouse BTIC activity as determined by functional sphere-forming assays and the initiation of tumor formation by transplant of drug-exposed tumor cells into syngeneic mice. Moreover, sertraline (Zoloft), a selective serotonin reuptake inhibitor (SSRI), synergized with docetaxel (Taxotere) to shrink mouse breast tumors in vivo. Hence drugs targeting the serotonergic system might be repurposed to treat breast cancer patients to afford more durable breast cancer remissions.

Pubertal Mammary Gland Development: Elucidation of In Vivo Morphogenesis Using Murine Models

During the past 25 years, the combination of increasingly sophisticated gene targeting technology with transplantation techniques has allowed researchers to address a wide array of questions about postnatal mammary gland development. These in turn have significantly contributed to our knowledge of other branched epithelial structures. This review chapter highlights a selection of the mouse models exhibiting a pubertal mammary gland phenotype with a focus on how they have contributed to our overall understanding of in vivo mammary morphogenesis. We discuss mouse models that have enabled us to assign functions to particular genes and proteins and, more importantly, have determined when and where these factors are required for completion of ductal outgrowth and branch patterning. The reason for the success of the mouse mammary gland model is undoubtedly the suitability of the postnatal mammary gland to experimental manipulation. The gland itself is very amenable to investigation and the combination of genetic modification with accessibility to the tissue has allowed an impressive number of studies to inform biology. Excision of the rudimentary epithelial structure postnatally allows genetically modified tissue to be readily transplanted into wild type stroma or vice versa, and has thus defined the contribution of each compartment to particular phenotypes. Similarly, whole gland transplantation has been used to definitively discern local effects from indirect systemic effects of various growth factors and hormones. While appreciative of the power of these tools and techniques, we are also cognizant of some of their limitations, and we discuss some shortcomings and future strategies that can overcome them.

Prenatal Mammary Gland Development in the Mouse: Research Models and Techniques for Its Study from Past to Present

Mammary gland development starts during prenatal life, when at designated positions along the ventrolateral boundary of the embryonic or fetal trunk, surface ectodermal cells coalesce to form primordia for mammary glands, instead of differentiating into epidermis. With the wealth of genetically engineered mice available as research models, our understanding of the prenatal phase of mammary development has recently greatly advanced. This understanding includes the recognition of molecular and mechanistic parallels between prenatal and postnatal mammary morphogenesis and even tumorigenesis, much of which can moreover be extrapolated to human. This makes the murine embryonic mammary gland a useful model for a myriad of questions pertaining to normal and pathological breast development. Hence, unless indicated otherwise, this review describes embryonic mammary gland development in mouse only, and lists mouse models that have been examined for defects in embryonic mammary development. Techniques that originated in the field of developmental biology, such as explant culture and tissue recombination, were adapted specifically to research on the embryonic mammary gland. Detailed protocols for these techniques have recently been published elsewhere. This review describes how the development and adaptation of these techniques moved the field forward from insights on (comparative) morphogenesis of the embryonic mammary gland to the understanding of tissue and molecular interactions and their regulation of morphogenesis and functional development of the embryonic mammary gland. It is here furthermore illustrated how generic molecular biology and biochemistry techniques can be combined with these older, developmental biology techniques, to address relevant research questions. As such, this review should provide a solid starting point for those wishing to familiarize themselves with this fascinating and important subdomain of mammary gland biology, and guide them in designing a relevant research strategy.

ETS1 is a genome-wide effector of RAS/ERK signaling in epithelial cells

The RAS/ERK pathway is commonly activated in carcinomas and promotes oncogenesis by altering transcriptional programs. However, the array of cis-regulatory elements and trans-acting factors that mediate these transcriptional changes is still unclear. Our genome-wide analysis determined that a sequence consisting of neighboring ETS and AP-1 transcription factor binding sites is enriched near cell migration genes activated by RAS/ERK signaling in epithelial cells. In vivo screening of candidate ETS proteins revealed that ETS1 is specifically required for migration of RAS/ERK activated cells. Furthermore, both migration and transcriptional activation through ETS/AP-1 required ERK phosphorylation of ETS1. Genome-wide mapping of multiple ETS proteins demonstrated that ETS1 binds specifically to enhancer ETS/AP-1 sequences. ETS1 occupancy, and its role in cell migration, was conserved in epithelial cells derived from multiple tissues, consistent with a chromatin organization common to epithelial cell lines. Genome-wide expression analysis showed that ETS1 was required for activation of RAS-regulated cell migration genes, but also identified a surprising role for ETS1 in the repression of genes such as DUSP4, DUSP6 and SPRY4 that provide negative feedback to the RAS/ERK pathway. Consistently, ETS1 was required for robust RAS/ERK pathway activation. Therefore, ETS1 has dual roles in mediating epithelial-specific RAS/ERK transcriptional functions.

Transcriptional activation of OCT4 by the ETS transcription factor PEA3 in NCCIT human embryonic carcinoma cells

We examined the molecular mechanism of OCT4 gene regulation by polyomavirus enhancer activator 3 (PEA3) in NCCIT cells. Endogenous PEA3 and OCT4 were significantly elevated in undifferentiated cells and reduced upon differentiation. PEA3 knockdown led to a reduction in OCT4 levels. OCT4 promoter activity was significantly up-regulated by dose-dependent PEA3 overexpression. Deletion and site-directed mutagenesis of the OCT4 promoter revealed a putative binding site within the conserved region 2 (CR2). PEA3 interacted with the binding element within CR2 in NCCIT cells. This study reveals the molecular details of the mechanism by which the oncogenic factor PEA3 regulates OCT4 gene expression as a transcriptional activator.

Integrating gene expression and epidemiological data for the discovery of genetic interactions associated with cancer risk

Dozens of common genetic variants associated with cancer risk have been identified through genome-wide association studies (GWASs). However, these variants only explain a modest fraction of the heritability of disease. The missing heritability has been attributed to several factors, among them the existence of genetic interactions (G × G). Systematic screens for G × G in model organisms have revealed their fundamental influence in complex phenotypes. In this scenario, G × G overlap significantly with other types of gene and/or protein relationships. Here, by integrating predicted G × G from GWAS data and complex- and context-defined gene coexpression profiles, we provide evidence for G × G associated with cancer risk. G × G predicted from a breast cancer GWAS dataset identified significant overlaps [relative enrichments (REs) of 8-36%, empirical P values < 0.05 to 10(-4)] with complex (non-linear) gene coexpression in breast tumors. The use of gene or protein data not specific for breast cancer did not reveal overlaps. According to the predicted G × G, experimental assays demonstrated functional interplay between lipoma-preferred partner and transforming growth factor-β signaling in the MCF10A non-tumorigenic mammary epithelial cell model. Next, integration of pancreatic tumor gene expression profiles with pancreatic cancer G × G predicted from a GWAS corroborated the observations made for breast cancer risk (REs of 25-59%). The method presented here can potentially support the identification of genetic interactions associated with cancer risk, providing novel mechanistic hypotheses for carcinogenesis.

Loss of a negative feedback loop involving pea3 and cyclin d2 is required for pea3-induced migration in transformed mammary epithelial cells

The Ets family transcription factor Pea3 (ETV4) is involved in tumorigenesis especially during the metastatic process. Pea3 is known to induce migration and invasion in mammary epithelial cell model systems. However, the molecular pathways regulated by Pea3 are still misunderstood. In the current study, using in vivo and in vitro assays, Pea3 increased the morphogenetic and tumorigenic capacity of mammary epithelial cells by modulating their cell morphology, proliferation, and migration potential. In addition, Pea3 overexpression favored an epithelial-mesenchymal transition (EMT) triggered by TGF-β1. During investigation for molecular events downstream of Pea3, Cyclin D2 (CCND2) was identified as a new Pea3 target gene involved in the control of cellular proliferation and migration, a finding that highlights a new negative regulatory loop between Pea3 and Cyclin D2. Furthermore, Cyclin D2 expression was lost during TGF-β1-induced EMT and Pea3-induced tumorigenesis. Finally, restored Cyclin D2 expression in Pea3-dependent mammary tumorigenic cells decreased cell migration in an opposite manner to Pea3. As such, these data demonstrate that loss of the negative feedback loop between Cyclin D2 and Pea3 contributes to Pea3-induced tumorigenesis.

IMPLICATIONS

This study reveals molecular insight into how the Ets family transcription factor Pea3 favors EMT and contributes to tumorigenesis via a negative regulatory loop with Cyclin D2, a new Pea3 target gene.

Expanding Sca-1(+) mammary stem cell in the presence of oestrogen and growth hormone

BACKGROUND

Sca-1 is controversial as a mammary stem cell marker in the literature, which may be due to the different isolation protocols and culture media used in different laboratories. The object of our study is to establish the Medium to promote the proliferation of mammary stem cell and explore the possibility of Sca-1 as mammary stem cell marker.

METHODS

We used BM medium supplemented with different concentration of 17Β-oestradiol and GH to find out MaECM medium which promoted the proliferation of mouse mammary epithelial cells and inhibited the growth of fibroblasts. Flow cytometry was used to isolate Sca-1(+) and Sca-1(-) cell populations from cultured mammary epithelial cells. Mammary fat pad transplantation and Mammosphere- forming assay were done to confirm the stem cell potential of Sca-1(+) cells. Differentiating culture was used to detect the differentiation potential of Sca-1(+) cells. Real-time PCR was carried out to analyse the expression of mammary stem cell-related genes in Sca-1(+) cells.

RESULTS

We first selected the medium suitable for mammary stem cell growth. Stem cell medium BM was used to culture mammary organoids, which generated many fibroblasts. We established MaECM medium supplemented with oestrogen and growth hormone (GH), in which oestrogen promoted mammary epithelial cell proliferation and inhibited fibroblast growth, and GH obviously enhanced the effect of oestrogen on mammary epithelial cell proliferation. Flow cytometry showed that 50% of cells were Sca-1(+) under the culture of MaECM medium. We confirmed that Sca-1(+) cells regenerated mammary outgrowths when transplanted in vivo, formed mammospheres in vitro and differentiated into luminal epithelial cells with milk-secreting function and myoepithelial cells under Matrigel culture. Furthermore, gene expression analysis by Real-time PCR revealed that Sca-1(+) cells expressed markedly higher levels of mammary stem cell-related genes in comparison to Sca-1(-) cells.

CONCLUSION

Our research demonstrates that Sca-1(+) mammary stem cells can be more easily isolated when cultured in the presence of oestrogen and GH.

Single unpurified breast tumor-initiating cells from multiple mouse models efficiently elicit tumors in immune-competent hosts

The tumor-initiating cell (TIC) frequency of bulk tumor cell populations is one of the criteria used to distinguish malignancies that follow the cancer stem cell model from those that do not. However, tumor-initiating cell frequencies may be influenced by experimental conditions and the extent to which tumors have progressed, parameters that are not always addressed in studies of these cells. We employed limiting dilution cell transplantation of minimally manipulated tumor cells from mammary tumors of several transgenic mouse models to determine their tumor-initiating cell frequency. We determined whether the tumors that formed following tumor cell transplantation phenocopied the primary tumors from which they were isolated and whether they could be serially transplanted. Finally we investigated whether propagating primary tumor cells in different tissue culture conditions affected their resident tumor-initiating cell frequency. We found that tumor-initiating cells comprised between 15% and 50% of the bulk tumor cell population in multiple independent mammary tumors from three different transgenic mouse models of breast cancer. Culture of primary mammary tumor cells in chemically-defined, serum-free medium as non-adherent tumorspheres preserved TIC frequency to levels similar to that of the primary tumors from which they were established. By contrast, propagating the primary tumor cells in serum-containing medium as adherent populations resulted in a several thousand-fold reduction in their tumor-initiating cell fraction. Our findings suggest that experimental conditions, including the sensitivity of the transplantation assay, can dramatically affect estimates of tumor initiating cell frequency. Moreover, conditional on cell culture conditions, the tumor-initiating cell fraction of bulk mouse mammary tumor cell preparations can either be maintained at high or low frequency in vitro thus permitting comparative studies of tumorigenic and non-tumorigenic cancer cells.

Small molecule antagonists of the Wnt/β-catenin signaling pathway target breast tumor-initiating cells in a Her2/Neu mouse model of breast cancer

Background

Recent evidence suggests that human breast cancer is sustained by a minor subpopulation of breast tumor-initiating cells (BTIC), which confer resistance to anticancer therapies and consequently must be eradicated to achieve durable breast cancer cure.

Methods/Findings

To identify signaling pathways that might be targeted to eliminate BTIC, while sparing their normal stem and progenitor cell counterparts, we performed global gene expression profiling of BTIC- and mammary epithelial stem/progenitor cell- enriched cultures derived from mouse mammary tumors and mammary glands, respectively. Such analyses suggested a role for the Wnt/Beta-catenin signaling pathway in maintaining the viability and or sustaining the self-renewal of BTICs in vitro. To determine whether the Wnt/Beta-catenin pathway played a role in BTIC processes we employed a chemical genomics approach. We found that pharmacological inhibitors of Wnt/β-catenin signaling inhibited sphere- and colony-formation by primary breast tumor cells and primary mammary epithelial cells, as well as by tumorsphere- and mammosphere-derived cells. Serial assays of self-renewal in vitro revealed that the Wnt/Beta-catenin signaling inhibitor PKF118–310 irreversibly affected BTIC, whereas it functioned reversibly to suspend the self-renewal of mammary epithelial stem/progenitor cells. Incubation of primary tumor cells in vitro with PKF118–310 eliminated their capacity to subsequently seed tumor growth after transplant into syngeneic mice. Administration of PKF118–310 to tumor-bearing mice halted tumor growth in vivo. Moreover, viable tumor cells harvested from PKF118–310 treated mice were unable to seed the growth of secondary tumors after transplant.

Conclusions

These studies demonstrate that inhibitors of Wnt/β-catenin signaling eradicated BTIC in vitro and in vivo and provide a compelling rationale for developing such antagonists for breast cancer therapy.

Small molecule antagonists of the Wnt/β-catenin signaling pathway target breast tumor-initiating cells in a Her2/Neu mouse model of breast cancer

BACKGROUND

Recent evidence suggests that human breast cancer is sustained by a minor subpopulation of breast tumor-initiating cells (BTIC), which confer resistance to anticancer therapies and consequently must be eradicated to achieve durable breast cancer cure.

METHODS/FINDINGS

To identify signaling pathways that might be targeted to eliminate BTIC, while sparing their normal stem and progenitor cell counterparts, we performed global gene expression profiling of BTIC- and mammary epithelial stem/progenitor cell- enriched cultures derived from mouse mammary tumors and mammary glands, respectively. Such analyses suggested a role for the Wnt/Beta-catenin signaling pathway in maintaining the viability and or sustaining the self-renewal of BTICs in vitro. To determine whether the Wnt/Beta-catenin pathway played a role in BTIC processes we employed a chemical genomics approach. We found that pharmacological inhibitors of Wnt/β-catenin signaling inhibited sphere- and colony-formation by primary breast tumor cells and primary mammary epithelial cells, as well as by tumorsphere- and mammosphere-derived cells. Serial assays of self-renewal in vitro revealed that the Wnt/Beta-catenin signaling inhibitor PKF118-310 irreversibly affected BTIC, whereas it functioned reversibly to suspend the self-renewal of mammary epithelial stem/progenitor cells. Incubation of primary tumor cells in vitro with PKF118-310 eliminated their capacity to subsequently seed tumor growth after transplant into syngeneic mice. Administration of PKF118-310 to tumor-bearing mice halted tumor growth in vivo. Moreover, viable tumor cells harvested from PKF118-310 treated mice were unable to seed the growth of secondary tumors after transplant.

CONCLUSIONS

These studies demonstrate that inhibitors of Wnt/β-catenin signaling eradicated BTIC in vitro and in vivo and provide a compelling rationale for developing such antagonists for breast cancer therapy.

Activation status of Wnt/ß-catenin signaling in normal and neoplastic breast tissues: relationship to HER2/neu expression in human and mouse

Wnt/ß-catenin signaling is strongly implicated in neoplasia, but the role of this pathway in human breast cancer has been controversial. Here, we examined Wnt/ß-catenin pathway activation as a function of breast cancer progression, and tested for a relationship with HER2/neu expression, using a human tissue microarray comprising benign breast tissues, ductal carcinoma in situ (DCIS), and invasive carcinomas. Cores were scored for membranous ß-catenin, a key functional component of adherens junctions, and for nucleocytoplasmic ß-catenin, a hallmark of Wnt/ß-catenin pathway activation. Only 82% of benign samples exhibited membrane-associated ß-catenin, indicating a finite frequency of false-negative staining. The frequency of membrane positivity was similar in DCIS samples, but was significantly reduced in carcinomas (45%, P<0.001), consistent with loss of adherens junctions during acquisition of invasiveness. Negative membrane status in cancers correlated with higher grade (P = 0.04) and estrogen receptor-negative status (P = 0.03), both indices of poor prognosis. Unexpectedly, a substantial frequency of nucleocytoplasmic ß-catenin was observed in benign breast tissues (36%), similar to that in carcinomas (35%). Positive-staining basal nuclei observed in benign breast may identify putative stem cells. An increased frequency of nucleocytoplasmic ß-catenin was observed in DCIS tumors (56%), suggesting that pathway activation may be an early event in human breast neoplasia. A correlation was observed between HER2/neu expression and nucleocytoplasmic ß-catenin in node-positive carcinomas (P = 0.02). Furthermore, cytoplasmic ß-catenin was detected in HER2/neu-induced mouse mammary tumors. The Axin2^NLSlacZ mouse strain, a previously validated reporter of mammary Wnt/ß-catenin signaling, was utilized to define in vivo transcriptional consequences of HER2/neu-induced ß-catenin accumulation. Discrete hyperplastic foci observed in mammary glands from bigenic MMTV/neu, Axin2^NLSlacZ mice, highlighted by robust ß-catenin/TCF signaling, likely represent the earliest stage of mammary intraepithelial neoplasia in MMTV/neu mice. Our study thus provides provocative evidence for Wnt/ß-catenin signaling as an early, HER2/neu-inducible event in breast neoplasia.

Synthetic transactivation screening reveals ETV4 as broad coactivator of hypoxia-inducible factor signaling

The human prolyl-4-hydroxylase domain (PHD) proteins 1–3 are known as cellular oxygen sensors, acting via the degradation of hypoxia-inducible factor (HIF) α-subunits. PHD2 and PHD3 genes are inducible by HIFs themselves, suggesting a negative feedback loop that involves PHD abundance. To identify novel regulators of the PHD2 gene, an expression array of 704 transcription factors was screened by a method that allows distinguishing between HIF-dependent and HIF-independent promoter regulation. Among others, the E-twenty six transcription factor ETS translocation variant 4 (ETV4) was found to contribute to PHD2 gene expression particularly under hypoxic conditions. Mechanistically, complex formation between ETV4 and HIF-1/2α was observed by mammalian two-hybrid and fluorescence resonance energy transfer analysis. HIF-1α domain mapping, CITED2 overexpression and factor inhibiting HIF depletion experiments provided evidence for cooperation between HIF-1α and p300/CBP in ETV4 binding. Chromatin immunoprecipitation confirmed ETV4 and HIF-1α corecruitment to the PHD2 promoter. Of 608 hypoxically induced transcripts found by genome-wide expression profiling, 7.7% required ETV4 for efficient hypoxic induction, suggesting a broad role of ETV4 in hypoxic gene regulation. Endogenous ETV4 highly correlated with PHD2, HIF-1/2α and several established markers of tissue hypoxia in 282 human breast cancer tissue samples, corroborating a functional interplay between the ETV4 and HIF pathways.

Tamoxifen downregulates ets oncogene family members ETV4 and ETV5 in benign breast tissue: implications for durable risk reduction

Five years of tamoxifen reduces breast cancer risk by nearly 50% but is associated with significant side effects and toxicities. A better understanding of the direct and indirect effects of tamoxifen in benign breast tissue could elucidate new mechanisms of breast carcinogenesis, suggest novel chemoprevention targets, and provide relevant early response biomarkers for phase II prevention trials. Seventy-three women at increased risk for breast cancer were randomized to tamoxifen (20 mg daily) or placebo for 3 months. Blood and breast tissue samples were collected at baseline and posttreatment. Sixty-nine women completed all study activities (37 tamoxifen and 32 placebo). The selected biomarkers focused on estradiol and IGFs in the blood; DNA methylation and cytology in random periareolar fine-needle aspirates; and tissue morphometry, proliferation, apoptosis, and gene expression (microarray and reverse transcriptase PCR) in the tissue core samples. Tamoxifen downregulated Ets oncogene transcription factor family members ETV4 and ETV5 and reduced breast epithelial cell proliferation independent of CYP2D6 genotypes or effects on estradiol, ESR1, or IGFs. Reduction in proliferation was correlated with downregulation of ETV4 and DNAJC12. Tamoxifen reduced the expression of ETV4- and ETV5-regulated genes implicated in epithelial-stromal interaction and tissue remodeling. Three months of tamoxifen did not affect breast tissue composition, cytologic atypia, preneoplasia, or apoptosis. A plausible mechanism for the chemopreventive effects of tamoxifen is restriction of lobular expansion into stroma through downregulation of ETV4 and ETV5. The human equivalent of murine multipotential progenitor cap cells of terminal end buds may be the primary target.

Gamma-secretase inhibitors target tumor-initiating cells in a mouse model of ERBB2 breast cancer

Human breast tumors comprise a minor sub-population of tumor-initiating cells (TICs), commonly termed cancer stem cells. TICs are thought to sustain tumor growth and to confer resistance to current anticancer therapies. Hence, targeting TIC may be essential to achieving durable cancer cures. To identify molecular targets in breast TIC, we employed a transgenic mouse model of ERBB2 breast cancer; tumors arising in this model comprise a very high frequency of TIC, which is maintained in tumor cell populations propagated in vitro as non-adherent tumorspheres. The Notch pathway is dysregulated in human breast tumors and overexpression of constitutively active Notch proteins induces mammary tumors in mice. The Notch pathway has also been implicated in stem cell processes including those of mammary epithelial stem cells. Hence, we investigated the potential that the Notch pathway is required for TIC activity. We found that an antagonist of Notch signaling, a gamma (γ)-secretase inhibitor termed MRK-003, inhibited the survival of tumorsphere-derived cells in vitro and eliminated TIC as assessed by cell transplantation into syngeneic mice. Whereas MRK-003 also inhibited the self-renewal and/or proliferation of mammosphere-resident cells, this effect of the inhibitor was reversible thus suggesting that it did not compromise the survival of these cells. MRK-003 administration to tumor-bearing mice eliminated tumor-resident TIC and resulted in rapid and durable tumor regression. MRK-003 inhibited the proliferation of tumor cells, and induced their apoptosis and differentiation. These findings suggest that MRK-003 targets breast TIC and illustrate that eradicating these cells in breast tumors ensures long-term, recurrence-free survival.

Dynamic modification of the ETS transcription factor PEA3 by sumoylation and p300-mediated acetylation

Transcription factor activity is often controlled through the dynamic use of post-translational modifications. Two such modifications are acetylation and sumoylation, which both occur on lysine residues, providing the opportunity for cross-talk at the molecular level. Here, we focussed on the ETS-domain transcription factor PEA3 and studied the potential interplay between these two modifications. We demonstrate that PEA3 is acetylated in a p300-dependent manner. ERK MAPK pathway signalling potentiates acetylation of PEA3, and enhances its trans-activation capacity. However, the major acetylation and sumoylation events take place on the same sites in PEA3 making simultaneous modification impossible. Indeed, manipulation of either the sumoylation or acetylation pathways causes reciprocal changes in PEA3 acetylation and sumoylation respectively. However, despite the mutually exclusive nature of these modifications, both contribute to the trans-activation capacity of PEA3, implying that a dynamic series of modification events occurs during the activation process.

Molecular regulators of pubertal mammary gland development

The pubertal mammary gland is an ideal model for experimental morphogenesis. The primary glandular branching morphogenesis occurs at this time, integrating epithelial cell proliferation, differentiation, and apoptosis. Between birth and puberty, the mammary gland exists in a relatively quiescent state. At the onset of puberty, rapid expansion of a pre-existing rudimentary mammary epithelium generates an extensive ductal network by a process of branch initiation, elongation, and invasion of the mammary mesenchyme. It is this branching morphogenesis that characterizes pubertal mammary gland growth. Tissue-specific molecular networks interpret signals from local cytokines/growth factors in both the epithelial and stromal microenvironments. This is largely orchestrated by secreted ovarian and pituitary hormones. Here, we review the major molecular regulators of pubertal mammary gland development.

The ERK MAP kinase-PEA3/ETV4-MMP-1 axis is operative in oesophageal adenocarcinoma. Mol Cancer. 2010;9:313. [PMID: 21143918 PMCID: PMC3009708 DOI: 10.1186/1476-4598-9-313]

BACKGROUND

Many members of the ETS-domain transcription factor family are important drivers of tumourigenesis. In this context, their activation by Ras-ERK pathway signaling is particularly relevant to the tumourigenic properties of many ETS-domain transcription factors. The PEA3 subfamily of ETS-domain transcription factors have been implicated in tumour metastasis in several different cancers.

RESULTS

Here, we have studied the expression of the PEA3 subfamily members PEA3/ETV4 and ER81/ETV1 in oesophageal adenocarcinomas and determined their role in oesophageal adenocarcinoma cell function. PEA3 plays an important role in controlling both the proliferation and invasive properties of OE33 oesophageal adenocarcinoma cells. A key target gene is MMP-1. The ERK MAP kinase pathway activates PEA3 subfamily members and also plays a role in these PEA3 controlled events, establishing the ERK-PEA3-MMP-1 axis as important in OE33 cells. PEA3 subfamily members are upregulated in human adenocarcinomas and expression correlates with MMP-1 expression and late stage metastatic disease. Enhanced ERK signaling is also more prevalent in late stage oesophageal adenocarcinomas.

CONCLUSIONS

This study shows that the ERK-PEA3-MMP-1 axis is upregulated in oesophageal adenocarcinoma cells and is a potentially important driver of the metastatic progression of oesophageal adenocarcinomas.

The epigenetic landscape of mammary gland development and functional differentiation

Most of the development and functional differentiation in the mammary gland occur after birth. Epigenetics is defined as the stable alterations in gene expression potential that arise during development and proliferation. Epigenetic changes are mediated at the biochemical level by the chromatin conformation initiated by DNA methylation, histone variants, post-translational modifications of histones, non-histone chromatin proteins, and non-coding RNAs. Epigenetics plays a key role in development. However, very little is known about its role in the developing mammary gland or how it might integrate the many signalling pathways involved in mammary gland development and function that have been discovered during the past few decades. An inverse relationship between marks of closed (DNA methylation) or open chromatin (DnaseI hypersensitivity, certain histone modifications) and milk protein gene expression has been documented. Recent studies have shown that during development and functional differentiation, both global and local chromatin changes occur. Locally, chromatin at distal regulatory elements and promoters of milk protein genes gains a more open conformation. Furthermore, changes occur both in looping between regulatory elements and attachment to nuclear matrix. These changes are induced by developmental signals and environmental conditions. Additionally, distinct epigenetic patterns have been identified in mammary gland stem and progenitor cell sub-populations. Together, these findings suggest that epigenetics plays a role in mammary development and function. With the new tools for epigenomics developed in recent years, we now can begin to establish a framework for the role of epigenetics in mammary gland development and disease.

Pea3 transcription factors and wnt1-induced mouse mammary neoplasia

The role of the PEA3 subfamily of Ets transcription factors in breast neoplasia is controversial. Although overexpression of PEA3 (E1AF/ETV4), and of the related factors ERM (ETV5) and ER81 (ETV1), have been observed in human and mouse breast tumors, PEA3 factors have also been ascribed a tumor suppressor function. Here, we utilized the MMTV/Wnt1 mouse strain to further interrogate the role of PEA3 transcription factors in mammary tumorigenesis based on our previous observation that Pea3 is highly expressed in MMTV/Wnt1 mammary tumors. Pea3 expression in mouse mammary tissues was visualized using a Pea3^NLSlacZ reporter strain. In normal mammary glands, Pea3 expression is predominantly confined to myoepithelial cells. Wnt1 transgene expression induced marked amplification of this cell compartment in nontumorous mammary glands, accompanied by an apparent increase in Pea3 expression. The pattern of Pea3 expression in MMTV/Wnt1 mammary glands recapitulated the cellular profile of activated β-catenin/TCF signaling, which was visualized using both β-catenin immunohistochemistry and the β-catenin/TCF-responsive reporter Axin2^NLSlacZ. To test the requirement for PEA3 factors in Wnt1-induced tumorigenesis, we employed a mammary-targeted dominant negative PEA3 transgene, ΔNPEA3En. Expression of ΔNPEA3En delayed early-onset tumor formation in MMTV/Wnt1 virgin females (P = 0.03), suggesting a requirement for PEA3 factor function for Wnt1-driven tumor formation. Consistent with this observation, expression of the ΔNPEA3En transgene was profoundly reduced in mammary tumors compared to nontumorous mammary glands from bigenic MMTV/Wnt1, MMTV/ΔNPEA3En mice (P = 0.01). Our data provide the first description of Wnt1-mediated expansion of the Pea3-expressing myoepithelial compartment in nontumorous mammary glands. Consistent with this observation, mammary myoepithelium was selectively responsive to Wnt1. Together these data suggest the MMTV/Wnt1 strain as a potential model of basal breast cancer. Furthermore, this study provides evidence for a protumorigenic role of PEA3 factors in breast neoplasia, and supports targeting the PEA3 transcription factor family in breast cancer.

Gene expression in murine mammary epithelial stem cell-like cells shows similarities to human breast cancer gene expression

Introduction

Mammary stem cells are bipotential and suggested to be the origin of breast cancer development, but are elusive and vaguely characterized. Breast tumors can be divided into subgroups, each one requiring specific treatment. To determine a possible association between mammary stem cells and breast cancer, a detailed characterization of the transcriptome in mammary stem cells is essential.

Methods

We have used a murine mammary epithelial stem-like cell line (HC11) and made a thorough investigation of global gene-expression changes during stepwise differentiation using dual-color comparative microarray technique. Subsequently, we have performed a cross-species comparison to reveal conserved gene expression between stem cells and subtype-specific and prognosis gene signatures, and correlated gene expression to in vivo mammary gland development.

Results

Our analysis of mammary stem-like and stepwise cell differentiation, and an in-depth description of our findings in a breast cancer perspective provide a unique map of the transcriptomic changes and a number of novel mammary stem cell markers. We correlate the alterations to in vivo mammary gland differentiation, and describe novel changes in nuclear receptor gene expression. Interestingly, our comparisons show that specific subtypes of breast cancers with poor prognosis and metastasizing capabilities show resemblance to stem-like gene expression.

Conclusions

The transcriptional characterization of these mammary stem-like cells and their differentiation-induced gene expression patterns is here made widely accessible and provides a basis for research on mammary stem-like cells. Our comparisons suggest that some tumors are more stem-like than others, with a corresponding worse prognosis. This information would, if established, be important for treatment decisions. We also suggest several marker candidates valuable to investigate further.