Genomic profiling of murine mammary tumors identifies potential personalized drug targets for p53-deficient mammary cancers

Targeted therapies against basal-like breast tumors, which are typically 'triple-negative breast cancers (TNBCs)', remain an important unmet clinical need. Somatic TP53 mutations are the most common genetic event in basal-like breast tumors and TNBC. To identify additional drivers and possible drug targets of this subtype, a comparative study between human and murine tumors was performed by utilizing a murine Trp53-null mammary transplant tumor model. We show that two subsets of murine Trp53-null mammary transplant tumors resemble aspects of the human basal-like subtype. DNA-microarray, whole-genome and exome-based sequencing approaches were used to interrogate the secondary genetic aberrations of these tumors, which were then compared to human basal-like tumors to identify conserved somatic genetic features. DNA copy-number variation produced the largest number of conserved candidate personalized drug targets. These candidates were filtered using a DNA-RNA Pearson correlation cut-off and a requirement that the gene was deemed essential in at least 5% of human breast cancer cell lines from an RNA-mediated interference screen database. Five potential personalized drug target genes, which were spontaneously amplified loci in both murine and human basal-like tumors, were identified: Cul4a, Lamp1, Met, Pnpla6 and Tubgcp3 As a proof of concept, inhibition of Met using crizotinib caused Met-amplified murine tumors to initially undergo complete regression. This study identifies Met as a promising drug target in a subset of murine Trp53-null tumors, thus identifying a potential shared driver with a subset of human basal-like breast cancers. Our results also highlight the importance of comparative genomic studies for discovering personalized drug targets and for providing a preclinical model for further investigations of key tumor signaling pathways.

Abstract IA08: Developmental insights into breast cancer intratumoral heterogeneity

Breast cancer is no longer considered a single disease, but instead is made up of multiple subtypes with genetically and epigenetically heterogeneous tumors composed of numerous clones. Both the hierarchical cancer stem cell and clonal evolution models have been invoked to help explain this intratumoral heterogeneity. Several recent studies have helped define the functional interactions among the different cellular subpopulations necessary for the evolution of this complex ecosystem. These interactions involve paracrine interactions that include locally acting Wnt family members, reminiscent of the signaling pathways important for normal mammary gland development and stem cell self-renewal. For example, mammary gland stem cell self-renewal is regulated by paracrine signaling involving Wnt4 and RSPO1 secreted by differentiated mammary luminal cells under the influence of systemic steroid hormones. Furthermore, interactions between non-canonical and canonical Wnt pathways play an important role in mammary gland development and cell fate determination [1]. Using a genetically engineered p53 null triple negative breast cancer model, we described a novel feedback loop in which the breast cancer stem cells(CSC) function through the action of paracrine mediators secreted by the CSC progeny[2], similar to what is observed in the normal mammary gland. We demonstrated the existence of cross-talk between tumor cells via Wnt2, Cxcl12 and IL6 signaling, indicating the non-cell-autonomous properties and importance of cooperativity between tumor subpopulations in tumor initiation and progression. Ligands expressed in the mesenchymal progeny cells produced through the asymmetric division of CSCs interacted with receptors on the CSCs. Knockdown of the ligands in the mesenchymal cells and their respective receptors in the CSCs resulted in decreased mammosphere formation in co-cultures. Furthermore, knockdown of Wnt 2 in the mesenchymal cells resulted in increased tumor latency when limiting numbers of CSCs were transplanted into the cleared mammary fat pad. The functional interactions between CSCs and mesenchymal population of cells may provide an improved niche microenvironment, especially for those tumors initiated from extremely low numbers of the CSCs. These studies indicate that similar regulatory mechanisms may exist for the maintenance of both normal and tumor stem cells with respect to their symmetric and asymmetric division. A better understanding of these interactions, especially in the metastatic setting, will be important for the development of improved combinatorial therapies designed to prevent relapse and to ultimately decrease mortality.

References:

1. Roarty, K., Shore, A.N., Creighton, C.J., and Rosen, J.M. (2015) Ror2 regulates branching, differentiation, and actin-cytoskeletal dynamics with the mammary epithelium. J. Cell Biol. 208: 351-66. PMID:25624393. PMC4315251.

2. Zhang, M., Tsimelzon, A., Chang, C-H., Wolff, A., Hilsenbeck, S.G. and Rosen, J.M.,(2015) Intratumoral heterogeneity in a Trp53-null mouse model of human breast cancer, Cancer Discovery 5:520-33. PMID:25735774. PMC4420701

Citation Format: Jeffrey M. Rosen, Kevin Roarty, Mei Zhang. Developmental insights into breast cancer intratumoral heterogeneity. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr IA08.

Abstract A05: The tumor suppressor function of Plk2 in triple-negative breast cancer may be mediated through regulation of Plk1

Triple-negative breast cancers (TNBCs) are highly aggressive, associated with poor prognosis and lack targeted therapies. Current breast cancer therapies target the estrogen (ER), progesterone (PR) and human epidermal growth factor (HER2) receptors, which are absent in TNBCs. Developing new treatment strategies for TNBCs requires a better understanding of the signaling networks regulating TNBCs. Polo-like kinase 1 (Plk1) is a putative oncogene in TNBC. Plk1 is frequently overexpressed and promotes mitotic cell division, making it an attractive target for cancer therapy. Several inhibitors of Plk1 exist, one of which has accelerated to phase III clinical trials for acute myeloid leukemia. However, these drugs also inhibit Plk2, another polo-like kinase family member. The impact that the presence of Plk2 has on the effectiveness of Plk1 inhibitors as a cancer therapy is unknown.

We reported recently that a loss of Plk2 in the developing mammary gland results in increased proliferation, hyperbranching, misoriented mitotic spindle assembly and defects in polarity (Villegas et al Development 2015). Loss of Plk2 was accompanied by increased expression of Plk1. Genetic rescue experiments, knocking down Plk1 in Plk2 null mouse mammary epithelium, and bimolecular fluorescence complementation assays, using wildtype Plk2 and a kinase dead mutant (KD) of Plk2 as bait, revealed that Plk2 regulates these processes through its direct interaction with Plk1. Our preliminary data suggest that loss of Plk2 results in increased Plk1 protein but not RNA expression. We propose that Plk2 functions as a tumor suppressor by decreasing Plk1 stability in TNBCs. Loss of Plk2, therefore, may sensitize tumors to treatment with Plk1 inhibitors if these tumors display Plk1 oncogene addiction. We hypothesize that Plk2, through targeted degradation of Plk1, inhibits tumorigenesis in TNBC.

We observed that loss of Plk2 alone is not sufficient to generate mouse mammary tumors, however more lesions form after multiple pregnancies in Plk2 null glands than wildtype. To examine the tumor suppressor function of Plk2 through its regulation of Plk1 in TNBC, we are generating preclinical mouse mammary tumor models integrating the germline loss of Plk2 with p53 loss or c-Myc overexpression frequently observed in TNBC. Finally, to investigate the clinical relevance of Plk2 in TNBC, we will use available tissue microarrays of TNBC patient derived xenograft (PDX) mouse models to identify those that exhibit loss of Plk2. We will treat the identified PDX models with Plk1 inhibitors to confirm that Plk2 loss promotes Plk1 in human TNBC samples. With these studies, we expect to find that Plk2 is involved in the targeted degradation of Plk1 in TNBC, sensitizing this aggressive breast cancer subtype to treatment with Plk1 inhibitors. The results of these studies should help validate whether Plk2 is a new biomarker for determining which patients will benefit from Plk1 targeted TNBC treatment. Supported by Susan G. Komen Foundation grant SAC110031.

Citation Format: Deanna Acosta, Elizabeth Villegas, Elena Kabotyanski, Celina Montemayor, Sarah J. Kurley, Rocio Dominguez-Vidana, Chad A. Shaw, Thomas F. Westbrook, Jeffrey M. Rosen. The tumor suppressor function of Plk2 in triple-negative breast cancer may be mediated through regulation of Plk1. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr A05.

The mammary stem cell hierarchy: a looking glass into heterogeneous breast cancer landscapes

The mammary gland is a dynamic organ that undergoes extensive morphogenesis during the different stages of embryonic development, puberty, estrus, pregnancy, lactation and involution. Systemic and local cues underlie this constant tissue remodeling and act by eliciting an intricate pattern of responses in the mammary epithelial and stromal cells. Decades of studies utilizing methods such as transplantation and lineage-tracing have identified a complex hierarchy of mammary stem cells, progenitors and differentiated epithelial cells that fuel mammary epithelial development. Importantly, these studies have extended our understanding of the molecular crosstalk between cell types and the signaling pathways maintaining normal homeostasis that often are deregulated during tumorigenesis. While several questions remain, this research has many implications for breast cancer. Fundamental among these are the identification of the cells of origin for the multiple subtypes of breast cancer and the understanding of tumor heterogeneity. A deeper understanding of these critical questions will unveil novel breast cancer drug targets and treatment paradigms. In this review, we provide a current overview of normal mammary development and tumorigenesis from a stem cell perspective.

Developmental Insights into Breast Cancer Intratumoral Heterogeneity

Breast cancer is no longer considered a single disease, but instead is made up of multiple subtypes with genetically and most likely epigenetically heterogeneous tumors composed of numerous clones. Both the hierarchical cancer stem cell and clonal evolution models have been invoked to help explain this intratumoral heterogeneity. Several recent studies have helped define the functional interactions among the different cellular subpopulations necessary for the evolution of this complex ecosystem. These interactions involve paracrine interactions that include locally acting Wnt family members, reminiscent of the signaling pathways important for normal mammary gland development and stem cell self-renewal. In this review, we discuss the interactions among various cell populations in both normal and tumor tissues. A better understanding of these interactions, especially in the metastatic setting, will be important for the development of improved combinatorial therapies designed to prevent relapse and to ultimately decrease mortality.

Upregulation of EGFR signaling is correlated with tumor stroma remodeling and tumor recurrence in FGFR1-driven breast cancer

Introduction

Despite advances in early detection and adjuvant targeted therapies, breast cancer is still the second most common cause of cancer mortality among women. Tumor recurrence is one of the major contributors to breast cancer mortality. However, the mechanisms underlying this process are not completely understood. In this study, we investigated the mechanisms of tumor dormancy and recurrence in a preclinical mouse model of breast cancer.

Methods

To elucidate the mechanisms driving tumor recurrence, we employed a transplantable Wnt1/inducible fibroblast growth factor receptor (FGFR) 1 mouse mammary tumor model and utilized an FGFR specific inhibitor, BGJ398, to study the recurrence after treatment. Histological staining was performed to analyze the residual tumor cells and tumor stroma. Reverse phase protein array was performed to compare primary and recurrent tumors to investigate the molecular mechanisms leading to tumor recurrence.

Results

Treatment with BGJ398 resulted in rapid tumor regression, leaving a nonpalpable mass of dormant tumor cells organized into a luminal and basal epithelial layer similar to the normal mammary gland, but surrounded by dense stroma with markedly reduced levels of myeloid-derived tumor suppressor cells (MDSCs) and decreased tumor vasculature. Following cessation of treatment the tumors recurred over a period of 1 to 4 months. The recurrent tumors displayed dense stroma with increased collagen, tenascin-C expression, and MDSC infiltration. Activation of the epidermal growth factor receptor (EGFR) pathway was observed in recurrent tumors, and inhibition of EGFR with lapatinib in combination with BGJ398 resulted in a significant delay in tumor recurrence accompanied by reduced stroma, yet there was no difference observed in initial tumor regression between the groups treated with BGJ398 alone or in combination with lapatinib.

Conclusion

These studies have revealed a correlation between tumor recurrence and changes of stromal microenvironment accompanied by altered EGFR signaling.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-015-0649-1) contains supplementary material, which is available to authorized users.

PTEN is required to maintain luminal epithelial homeostasis and integrity in the adult mammary gland

In the mammary gland, PTEN loss in luminal and basal epithelial cells results in differentiation defects and enhanced proliferation, leading to the formation of tumors with basal epithelial characteristics. In breast cancer, PTEN loss is associated with a hormone receptor-negative, basal-like subtype that is thought to originate in a luminal epithelial cell. Here, we show that luminal-specific PTEN loss results in distinct effects on epithelial homeostasis and mammary tumor formation. Luminal PTEN loss increased proliferation of hormone receptor-negative cells, thereby decreasing the percentage of hormone receptor-positive cells. Moreover, luminal PTEN loss led to misoriented cell divisions and mislocalization of cells to the intraluminal space of mammary ducts. Despite their elevated levels of activated AKT, Pten-null intraluminal cells showed increased levels of apoptosis. One year after Pten deletion, the ducts had cleared and no palpable mammary tumors were detected. These data establish PTEN as a critical regulator of luminal epithelial homeostasis and integrity in the adult mammary gland, and further show that luminal PTEN loss alone is not sufficient to promote the progression of mammary tumorigenesis.

Mammary Stem Cells and Tumor-Initiating Cells Are More Resistant to Apoptosis and Exhibit Increased DNA Repair Activity in Response to DNA Damage

Adult stem cells and tumor-initiating cells (TICs) often employ different mechanisms of DNA damage response (DDR) as compared to other tissue cell types. However, little is known about how mammary stem cells (MaSCs) and mammary TICs respond to DNA damage. Using the mouse mammary gland and syngeneic p53-null tumors as models, we investigated the molecular and physiological consequences of DNA damage in wild-type MaSCs, p53-null MaSCs, and p53-null TICs. We showed that wild-type MaSCs and basal cells are more resistant to apoptosis and exhibit increased non-homologous end joining (NHEJ) activity. Loss of p53 in mammary epithelium affected both cell-cycle regulation and DNA repair efficiency. In p53-null tumors, we showed that TICs are more resistant to ionizing radiation (IR) due to decreased apoptosis, elevated NHEJ activity, and more-rapid DNA repair. These results have important implications for understanding DDR mechanisms involved in both tumorigenesis and therapy resistance.

•

MaSCs are more resistant to apoptosis and exhibit increased NHEJ activity

•

Loss of p53 in MECs impairs cell-cycle regulation and DNA repair efficiency

•

TICs exhibit decreased apoptosis, increased DNA repair efficiency, and NHEJ activity

•

TICs are highly proliferative and exhibit improper cell-cycle regulation after IR

STAT3 signaling is activated preferentially in tumor-initiating cells in claudin-low models of human breast cancer

In breast cancer, a subset of tumor-initiating cells (TIC) or "cancer stem cells" are thought to be responsible for tumor maintenance, treatment resistance, and disease recurrence. While current breast cancer stem cell markers (e.g., CD44(high) /CD24(low/neg) , ALDH positive) have allowed enrichment for such cells, they are not universally expressed and may actually identify distinct TIC subpopulations in the same tumor. Thus, additional markers of functional stem cells are needed. The STAT3 pathway is a critical regulator of the function of normal stem cells, and evidence is accumulating for its important role in breast cancer stem cells. However, due to the lack of a method for separating live cells based on their level of STAT3 activity, it remains unknown whether STAT3 functions in the cancer stem cells themselves, or in surrounding niche cells, or in both. To approach this question, we constructed a series of lentiviral fluorescent (enhanced green fluorescent protein, EGFP) reporters that enabled flow cytometric enrichment of cells differing in STAT3-mediated transcriptional activity, as well as in vivo/in situ localization of STAT3 responsive cells. Using in vivo claudin-low cell line xenograft models of human breast cancer, we found that STAT3 signaling reporter activity (EGFP(+) ) is associated with a subpopulation of cancer cells enriched for mammosphere-forming efficiency, as well as TIC function in limiting dilution transplantation assays compared to negative or unsorted populations. Our results support STAT3 signaling activity as another functional marker for human breast cancer stem cells thus making it an attractive therapeutic target for stem-cell-directed therapy in some breast cancer subtypes.

Expression of miR-200c in claudin-low breast cancer alters stem cell functionality, enhances chemosensitivity and reduces metastatic potential

Claudin-low tumors are a highly aggressive breast cancer subtype with no targeted treatments and a clinically documented resistance to chemotherapy. They are significantly enriched in cancer stem cells (CSCs), which makes claudin-low tumor models particularly attractive for studying CSC behavior and developing novel approaches to minimize CSC therapy resistance. One proposed mechanism by which CSCs arise is via an epithelial-mesenchymal transition (EMT), and reversal of this process may provide a potential therapeutic approach for increasing tumor chemosensitivity. Therefore, we investigated the role of known EMT regulators, miR-200 family of microRNAs in controlling the epithelial state, stem-like properties, and therapeutic response in an in vivo primary, syngeneic p53^null claudin-low tumor model that is normally deficient in miR-200 expression. Using an inducible lentiviral approach, we expressed the miR-200c cluster in this model and found that it changed the epithelial state, and consequently, impeded CSC behavior in these mesenchymal tumors. Moreover, these state changes were accompanied by a decrease in proliferation and an increase in the differentiation status. miR-200c expression also forced a significant reorganization of tumor architecture, affecting important cellular processes involved in cell-cell contact, cell adhesion, and motility. Accordingly, induced miR200c expression significantly enhanced the chemosensitivity and decreased the metastatic potential of this p53^null claudin-low tumor model. Collectively, our data suggest that miR-200c expression in claudin-low tumors offers a potential therapeutic application to disrupt the EMT program on multiple fronts in this mesenchymal tumor subtype, by altering tumor growth, chemosensitivity, and metastatic potential in vivo.

Intratumoral heterogeneity in a Trp53-null mouse model of human breast cancer

Intratumoral heterogeneity correlates with clinical outcome and reflects the cellular complexity and dynamics within a tumor. Such heterogeneity is thought to contribute to radio- and chemoresistance because many treatments may target only certain tumor cell subpopulations. A better understanding of the functional interactions between various subpopulations of cells, therefore, may help in the development of effective cancer treatments. We identified a unique subpopulation of tumor cells expressing mesenchymal-like markers in a Trp53-null mouse model of basal-like breast cancer using fluorescence-activated cell sorting and microarray analysis. Both in vitro and in vivo experiments revealed the existence of cross-talk between these "mesenchymal-like" cells and tumor-initiating cells. Knockdown of genes encoding ligands upregulated in the mesenchymal cells and their corresponding receptors in the tumor-initiating cells resulted in reduced tumorigenicity and increased tumor latency. These studies illustrate the non-cell-autonomous properties and importance of cooperativity between tumor subpopulations.

SIGNIFICANCE

Intratumoral heterogeneity has been considered one important factor in assessing a patient's initial response to treatment and selecting drug regimens to effectively increase tumor response rate. Elucidating the functional interactions between various subpopulations of tumor cells will help provide important new insights in understanding treatment response and tumor progression.

The osteogenic niche promotes early-stage bone colonization of disseminated breast cancer cells

Breast cancer bone micrometastases can remain asymptomatic for years before progressing into overt lesions. The biology of this process, including the microenvironment niche and supporting pathways, is unclear. We find that bone micrometastases predominantly reside in a niche that exhibits features of osteogenesis. Niche interactions are mediated by heterotypic adherens junctions (hAJs) involving cancer-derived E-cadherin and osteogenic N-cadherin, the disruption of which abolishes niche-conferred advantages. We elucidate that hAJ activates the mTOR pathway in cancer cells, which drives the progression from single cells to micrometastases. Human data set analyses support the roles of AJ and the mTOR pathway in bone colonization. Our study illuminates the initiation of bone colonization, and provides potential therapeutic targets to block progression toward osteolytic metastases.

Ror2 regulates branching, differentiation, and actin-cytoskeletal dynamics within the mammary epithelium

Intricate cross-talk between classical and alternative Wnt signaling pathways includes an essential role for Ror2 in mammary epithelial development and differentiation.

Wnt signaling encompasses β-catenin–dependent and –independent networks. How receptor context provides Wnt specificity in vivo to assimilate multiple concurrent Wnt inputs throughout development remains unclear. Here, we identified a refined expression pattern of Wnt/receptor combinations associated with the Wnt/β-catenin–independent pathway in mammary epithelial subpopulations. Moreover, we elucidated the function of the alternative Wnt receptor Ror2 in mammary development and provided evidence for coordination of this pathway with Wnt/β-catenin–dependent signaling in the mammary epithelium. Lentiviral short hairpin RNA (shRNA)-mediated depletion of Ror2 in vivo increased branching and altered the differentiation of the mammary epithelium. Microarray analyses identified distinct gene level alterations within the epithelial compartments in the absence of Ror2, with marked changes observed in genes associated with the actin cytoskeleton. Modeling of branching morphogenesis in vitro defined specific defects in cytoskeletal dynamics accompanied by Rho pathway alterations downstream of Ror2 loss. The current study presents a model of Wnt signaling coordination in vivo and assigns an important role for Ror2 in mammary development.

The oncogenic STP axis promotes triple-negative breast cancer via degradation of the REST tumor suppressor

Defining the molecular networks that drive breast cancer has led to therapeutic interventions and improved patient survival. However, the aggressive triple-negative breast cancer subtype (TNBC) remains recalcitrant to targeted therapies because its molecular etiology is poorly defined. In this study, we used a forward genetic screen to discover an oncogenic network driving human TNBC. SCYL1, TEX14, and PLK1 ("STP axis") cooperatively trigger degradation of the REST tumor suppressor protein, a frequent event in human TNBC. The STP axis induces REST degradation by phosphorylating a conserved REST phospho-degron and bridging REST interaction with the ubiquitin-ligase βTRCP. Inhibition of the STP axis leads to increased REST protein levels and impairs TNBC transformation, tumor progression, and metastasis. Expression of the STP axis correlates with low REST protein levels in human TNBCs and poor clinical outcome for TNBC patients. Our findings demonstrate that the STP-REST axis is a molecular driver of human TNBC.

Chk1 versus Cdc25: Chking One’s Levels of Cellular Proliferation

This review summarizes recent studies which have provided new insight into the mechanisms by which the DNA damage response kinase, Chk1 inhibits the dual specificity phosphatase, Cdc25, and thereby regulates cell cycle progression. Recently, Chk1 has been shown to not only regulate Cdc25A degradation but also its ability to interact with various Cdk complexes through phosphorylation of the carboxy-terminus of the phosphatase. Surprisingly, these effects appear to be specific for Chk1, but not Chk2, which may explain the recently reported in vivo haploinsufficiency phenotype observed in the mammary gland using a Chk1 conditional mouse model.

Wnt-responsive cancer stem cells are located close to distorted blood vessels and not in hypoxic regions in a p53-null mouse model of human breast cancer

Cancer stem cells (CSCs, or tumor-initiating cells) may be responsible for tumor formation in many types of cancer, including breast cancer. Using high-resolution imaging techniques, we analyzed the relationship between a Wnt-responsive, CSC-enriched population and the tumor vasculature using p53-null mouse mammary tumors transduced with a lentiviral Wnt signaling reporter. Consistent with their localization in the normal mammary gland, Wnt-responsive cells in tumors were enriched in the basal/myoepithelial population and generally located in close proximity to blood vessels. The Wnt-responsive CSCs did not colocalize with the hypoxia-inducible factor 1α-positive cells in these p53-null basal-like tumors. Average vessel diameter and vessel tortuosity were increased in p53-null mouse tumors, as well as in a human tumor xenograft as compared with the normal mammary gland. The combined strategy of monitoring the fluorescently labeled CSCs and vasculature using high-resolution imaging techniques provides a unique opportunity to study the CSC and its surrounding vasculature.

Regulation of mammary epithelial cell homeostasis by lncRNAs

The epithelial cells of the mammary gland develop primarily after birth and undergo surges of hormonally regulated proliferation, differentiation, and apoptosis during both puberty and pregnancy. Thus, the mammary gland is a useful model to study fundamental processes of development and adult tissue homeostasis, such as stem and progenitor cell regulation, cell fate commitment, and differentiation. Long noncoding RNAs (lncRNAs) are emerging as prominent regulators of these essential processes, as their extraordinary versatility allows them to modulate gene expression via diverse mechanisms at both transcriptional and post-transcriptional levels. Not surprisingly, lncRNAs are also aberrantly expressed in cancer and promote tumorigenesis by disrupting vital cellular functions, such as cell cycle, survival, and migration. In this review, we first broadly summarize the functions of lncRNAs in mammalian development and cancer. Then we focus on what is currently known about the role of lncRNAs in mammary gland development and breast cancer. This article is part of a Directed Issue entitled: The Non-coding RNA Revolution.

Plk2 regulates mitotic spindle orientation and mammary gland development

Disruptions in polarity and mitotic spindle orientation contribute to the progression and evolution of tumorigenesis. However, little is known about the molecular mechanisms regulating these processes in vivo. Here, we demonstrate that Polo-like kinase 2 (Plk2) regulates mitotic spindle orientation in the mammary gland and that this might account for its suggested role as a tumor suppressor. Plk2 is highly expressed in the mammary gland and is required for proper mammary gland development. Loss of Plk2 leads to increased mammary epithelial cell proliferation and ductal hyperbranching. Additionally, a novel role for Plk2 in regulating the orientation of the mitotic spindle and maintaining proper cell polarity in the ductal epithelium was discovered. In support of a tumor suppressor function for Plk2, loss of Plk2 increased the formation of lesions in multiparous glands. Collectively, these results demonstrate a novel role for Plk2 in regulating mammary gland development.

Paracrine signaling in mammary gland development: what can we learn about intratumoral heterogeneity?

Paracrine signaling mechanisms play a critical role in both normal mammary gland development and breast cancer. Dissection of these mechanisms using genetically engineered mouse models has provided significant insight into our understanding of the mechanisms that guide intratumoral heterogeneity. In the following perspective, we briefly review some of the emerging concepts in this field and emphasize why elucidation of these pathways will be important for future progress in devising new and improved combinatorial therapeutic approaches for breast and other solid cancers.

Pygopus 2: tilting the Wnt-Notch balance in mammary epithelial lineage determination

The mammary gland requires the coordinated efforts of multiple epithelial cell lineages to build an organized bilayered ductal network. How epigenetic regulators direct decisions of cell-fate and lineage determination in cooperation with intrinsic transcriptional factors and extrinsic signaling factors remains a fundamental question in the field of developmental biology. Recent work sheds new light on the role of the histone methylation reader Pygopus 2 in coordinating the balance of self-renewal Wnt signals with luminal-specific Notch signals in mammary epithelial lineage determination.

Abstract P4-07-02: MicroRNA 204 mediated negative regulation of the IGF2R promotes breast cancer progression and is a potential mechanism driving breast cancer disparity

Breast cancer (BC) is a worldwide health issue as it represents the leading cause of cancer in women and the second leading cause of cancer-related mortality in women, with an increasing incidence. In the US, African American (AA) women have a significantly higher rate of mortality due to BC compared to Caucasian American (CA) women. It is becoming increasingly apparent that racial disparity in cancer exists due to molecular differences in tumor biology as well as, or in addition to, socioeconomic and standard of care issues. Sparse information exists regarding the molecular mechanisms that promote BC health disparity therefore, a greater understanding of the risk factors and biological links associated with BC, will significantly impact AA women. Our studies have identified a race specific mechanistic link between microRNA-204 (miR-204) and the Insulin-like Growth Factor-2 Receptor. MicroRNAs are small non-coding RNAs that function to inhibit gene expression through translational repression. Our published studies identified miR-204 as a novel oncomir and we recently identified IGF2R as a direct target. The IGF2R has been proposed to be a tumor suppressor gene in several cancers including breast cancer. We show that over-expression of miR-204 results in a decrease in IGF2R protein levels. We propose that the inhibition of IGF2R allows IGF-2 to bind the IGF1R leading to hyperactivation of this pathway which results in increased proliferation, migration and invasion, processes that are required for tumor progression. Indeed, we show that miR-204 expression results in the activation of the IGF1R/IRS-1/ERK pathway together with an increase in proliferation, migration and invasion. Published studies have shown that exogenous expression of activated IGF1R increases migration in non-transformed breast cells. We have shown that exogenous expression of miR-204 also increases migration in these cells. However, when miR-204 and IGF1R are both exogenously expressed no additional increase in migration was observed, suggesting that mIR-204 mediated increase in migration is through activation of the IGF1R pathway. Reduced IGF2R expression correlates with poor patient prognosis in BC patients and a recent study showed significantly higher levels of IGF2R in CA compared to AA tumor samples, suggesting that decreased IGF2R expression may contribute to BC disparity. We examined serum from BC patients and found elevated levels of miR-204 in AA compared to CA women. Circulating miRNAs can serve as accessible biomarkers for diagnosis & prognosis and may define a novel area of potential therapeutic intervention to reduce BC disparity. Our data also suggest that miR-204 mediated inhibition of IGF2R could be a potential biological mechanism driving BC disparity. A major hurdle to the identification of biological mechanisms conferring cancer health disparity is a lack of suitable experimental models with which to investigate race specific differences in tumor biology. We have developed a unique inducible miR-204 transgenic mouse model to define in vivo the oncogenic potential of miR-204 and the mechanism and functional consequences of IGF2R loss.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P4-07-02.

Transcriptomic classification of genetically engineered mouse models of breast cancer identifies human subtype counterparts

Background

Human breast cancer is a heterogeneous disease consisting of multiple molecular subtypes. Genetically engineered mouse models are a useful resource for studying mammary cancers in vivo under genetically controlled and immune competent conditions. Identifying murine models with conserved human tumor features will facilitate etiology determinations, highlight the effects of mutations on pathway activation, and should improve preclinical drug testing.

Results

Transcriptomic profiles of 27 murine models of mammary carcinoma and normal mammary tissue were determined using gene expression microarrays. Hierarchical clustering analysis identified 17 distinct murine subtypes. Cross-species analyses using three independent human breast cancer datasets identified eight murine classes that resemble specific human breast cancer subtypes. Multiple models were associated with human basal-like tumors including TgC3(1)-Tag, TgWAP-Myc and Trp53^-/-. Interestingly, the TgWAPCre-Etv6 model mimicked the HER2-enriched subtype, a group of human tumors without a murine counterpart in previous comparative studies. Gene signature analysis identified hundreds of commonly expressed pathway signatures between linked mouse and human subtypes, highlighting potentially common genetic drivers of tumorigenesis.

Conclusions

This study of murine models of breast carcinoma encompasses the largest comprehensive genomic dataset to date to identify human-to-mouse disease subtype counterparts. Our approach illustrates the value of comparisons between species to identify murine models that faithfully mimic the human condition and indicates that multiple genetically engineered mouse models are needed to represent the diversity of human breast cancers. The reported trans-species associations should guide model selection during preclinical study design to ensure appropriate representatives of human disease subtypes are used.

Abstract IA13: Wnt and Fgf signaling in mammary stem cells and breast cancer

Some of the earliest evidence implicating both Wnt and FGF signaling in mammary tumorigenesis came from the observation of tumors in mouse mammary tumor virus (MMTV)- infected mice. Tumors arising from MMTV proviral insertion often exhibited preferential activation of Wnt and FGF pathway members. Interestingly, tumors with MMTV-induced expression of these FGF pathway ligands also showed MMTV-induced activation of Wnt pathway members, and functional cooperativity of MMTV-driven Wnt 1 and Fgf-3 transgenes in mammary tumorigenesis was observed. High throughput MMTV insertional mutagenesis studies have indicated that activation of Wnt and FGF pathway components is the most common occurrence in resulting tumors, providing definitive genetic proof for the cooperativity between these two pathways. Unraveling the complexity of these pathways in the context of development and tumorigenesis remains an evolving challenge. While the Wnt/ß-catenin cascade plays a central role in a range of biological processes, it has emerged as a key regulator of stem cell dynamics in multiple tissues, including the mammary gland. Significant evidence now implicates the Wnt/ß-catenin pathway in uncontrolled self-renewal of cancer stem cells and their associated radiation resistance during treatment. In the mammary gland, localized Wnt signaling most likely plays a critical role in regulating oriented cell division and cell fate. As with many signaling pathways, this is not an “on and off switch”, but is instead influenced by the level and duration of the signaling event. There are also Wnt-mediated ß-catenin-independent pathways, known as noncanonical Wnt cascades. For instance, noncanonical Wnt signaling through the receptor Ror2 helps to specify the extent and location of canonical Wnt/ß-catenin signaling. With regard to FGF signaling, FGFR1 and FGFR2, the two primary FGFRs expressed in mouse mammary epithelium, also are critical for mammary stem cell maintenance. Cooperation of the FGF and Wnt pathways in mammary tumorigenesis is based in part on the activation of protein translational pathways that result in, but are not limited to, increased expression of Wnt/β-catenin target genes at the level of protein translation. Lastly, Wnt and Fgf ligands can be regulated by systemic steroid hormones and act in a paracrine manner to influence mammary stem cells, providing another layer to the signaling hierarchy. Supported by grant NIH-CA16303.

Citation Format: Jeffrey M. Rosen, Xue Bin, Kevin Roarty. Wnt and Fgf signaling in mammary stem cells and breast cancer. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr IA13.

Abstract A039: The role of long noncoding RNAs in epithelial to mesenchymal transition and cancer stem cells

The claudin-low subtype is generally triple (ER, PR, HER2) negative and there are currently no targeted agents directed at them. These tumors express low levels of tight and adherens junction genes including claudin 3 and E-cadherin, and high levels of markers associated with epithelial-mesenchymal transition (EMT) including Snail, Twist, and Zeb1/2. Claudin-low tumors are also enriched in signatures derived from human tumor-initiating cells and a sorted population enriched for human mammary stem cells. miRNAs are differentially expressed in claudin-low tumors including low expression of the miR-200 family - regulators of EMT and stemness. MiR-200 overexpression in claudin-low cell lines causes them to lose this classification and to adopt an expression profile of another subtype. While there has been considerable emphasis on miRNAs, our knowledge is still lacking about the role of long noncoding RNAs (lncRNAs) that comprise the majority of the mammalian transcriptome. Here, we have examined the expression profiles of >17,000 lncRNAs in a large set of breast tumors. Like mRNAs and miRNAs, lncRNAs differ dramatically in expression across subtypes and can be used for classification. LncRNAs that are differentially regulated in cell lines induced to undergo EMT are associated with claudin-low tumors and we have identified some of these lncRNAs as potential regulators of the EMT/CSC phenotype. We have begun to study the subcellular localization and potential function of a couple of these candidate lncRNAs using RNA FISH and siRNA knockdown respectively. These results suggest major roles for noncoding RNAs in claudin-low breast tumors and in the regulation of breast cancer stem cells.

Citation Format: Jason I. Herschkowitz, Cristian Coarfa, Aleix Prat, Michael J. Toneff, Katherine A. Hoadley, Marcel E. Dinger, John S. Mattick, Sendurai A. Mani, Charles M. Perou, Jeffrey M. Rosen. The role of long noncoding RNAs in epithelial to mesenchymal transition and cancer stem cells. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr A039.

Three-dimensional vasculature reconstruction of tumour microenvironment via local clustering and classification

The vasculature inside breast cancers is one important component of the tumour microenvironment. The investigation of its spatial morphology, distribution and interactions with cancer cells, including cancer stem cells, is essential for elucidating mechanisms of tumour development and treatment response. Using confocal microscopy and fluorescent markers, we have acquired three-dimensional images of vasculature within mammary tumours and normal mammary gland of mouse models. However, it is difficult to segment and reconstruct complex vasculature accurately from the in vivo three-dimensional images owing to the existence of uneven intensity and regions with low signal-to-noise ratios (SNR). To overcome these challenges, we have developed a novel three-dimensional vasculature segmentation method based on local clustering and classification. First, images of vasculature are clustered into local regions, whose boundaries well delineate vasculature even in low SNR and uneven intensity regions. Then local regions belonging to vasculature are identified by applying a semi-supervised classification method based on three informative features of the local regions. Comparison of results using simulated and real vasculature images, from mouse mammary tumours and normal mammary gland, shows that the new method outperforms existing methods, and can be used for three-dimensional images with uneven background and low SNR to achieve accurate vasculature reconstruction.

Fibroblast growth factor receptor signaling is essential for normal mammary gland development and stem cell function

Fibroblast growth factor (FGF) signaling plays an important role in embryonic stem cells and adult tissue homeostasis, but the function of FGFs in mammary gland stem cells is less well defined. Both FGFR1 and FGFR2 are expressed in basal and luminal mammary epithelial cells (MECs), suggesting that together they might play a role in mammary gland development and stem cell dynamics. Previous studies have demonstrated that the deletion of FGFR2 resulted only in transient developmental defects in branching morphogenesis. Using a conditional deletion strategy, we investigated the consequences of FGFR1 deletion alone and then the simultaneous deletion of both FGFR1 and FGFR2 in the mammary epithelium. FGFR1 deletion using a keratin 14 promoter-driven Cre-recombinase resulted in an early, yet transient delay in development. However, no reduction in functional outgrowth potential was observed following limiting dilution transplantation analysis. In contrast, a significant reduction in outgrowth potential was observed upon the deletion of both FGFR1 and FGFR2 in MECs using adenovirus-Cre. Additionally, using a fluorescent reporter mouse model to monitor Cre-mediated recombination, we observed a competitive disadvantage following transplantation of both FGFR1/R2-null MECs, most prominently in the basal epithelial cells. This correlated with the complete loss of the mammary stem cell repopulating population in the FGFR1/R2-attenuated epithelium. FGFR1/R2-null MECs were partially rescued in chimeric outgrowths containing wild-type MECs, suggesting the potential importance of paracrine mechanisms involved in the maintenance of the basal epithelial stem cells. These studies document the requirement for functional FGFR signaling in mammary stem cells during development.

Predicting drug responsiveness in human cancers using genetically engineered mice

PURPOSE

To use genetically engineered mouse models (GEMM) and orthotopic syngeneic murine transplants (OST) to develop gene expression-based predictors of response to anticancer drugs in human tumors. These mouse models offer advantages including precise genetics and an intact microenvironment/immune system.

EXPERIMENTAL DESIGN

We examined the efficacy of 4 chemotherapeutic or targeted anticancer drugs, alone and in combination, using mouse models representing 3 distinct breast cancer subtypes: Basal-like (C3(1)-T-antigen GEMM), Luminal B (MMTV-Neu GEMM), and Claudin-low (T11/TP53-/- OST). We expression-profiled tumors to develop signatures that corresponded to treatment and response, and then tested their predictive potential using human patient data.

RESULTS

Although a single agent exhibited exceptional efficacy (i.e., lapatinib in the Neu-driven model), generally single-agent activity was modest, whereas some combination therapies were more active and life prolonging. Through analysis of RNA expression in this large set of chemotherapy-treated murine tumors, we identified a pair of gene expression signatures that predicted pathologic complete response to neoadjuvant anthracycline/taxane therapy in human patients with breast cancer.

CONCLUSIONS

These results show that murine-derived gene signatures can predict response even after accounting for common clinical variables and other predictive genomic signatures, suggesting that mice can be used to identify new biomarkers for human patients with cancer.

Separation by cell size enriches for mammary stem cell repopulation activity

Mammary gland reconstitution experiments, as well as lineage tracing experiments, have provided evidence for the existence of adult mammary stem cells (MaSCs). In addition, cell sorting techniques for specific cell surface markers (CD24(+)CD29(H)CD49f(H)Sca1(-)) have been used to prospectively isolate MaSC-enriched populations. Although these markers enrich for cell subpopulations that harbor MaSCs, they do not identify regenerative stem cells uniquely. Here, we report that MaSCs can be further defined by the property of cell size. Fluorescence-activated cell sorting was used to analyze sizing beads and further separate populations of cells with varying degrees of forward scatter (FSC). Cells with a low FSC that were approximately <10 μm in size lacked outgrowth potential and failed to reconstitute the mammary gland when transplanted into the cleared fat pads of syngeneic mice. In contrast, cells >10 μm in size with a higher FSC had increased outgrowth potential as compared with lineage-negative (LIN(-)) control cells. Limiting dilution transplantation assays indicated that the repopulating ability of LIN(-)CD24(+)CD29(H) cells that were >10 μm in size was significantly increased as compared with cells marked by CD24 and CD29 alone. These results suggest that MaSCs can be further isolated by sorting based on size/FSC. These findings have critical implications for understanding mammary gland stem cell biology, an important requisite step for understanding the etiology of breast cancer.

FOXC2 expression links epithelial-mesenchymal transition and stem cell properties in breast cancer

Resistance to chemotherapy and metastases are the major causes of breast cancer-related mortality. Moreover, cancer stem cells (CSC) play critical roles in cancer progression and treatment resistance. Previously, it was found that CSC-like cells can be generated by aberrant activation of epithelial-mesenchymal transition (EMT), thereby making anti-EMT strategies a novel therapeutic option for treatment of aggressive breast cancers. Here, we report that the transcription factor FOXC2 induced in response to multiple EMT signaling pathways as well as elevated in stem cell-enriched factions is a critical determinant of mesenchymal and stem cell properties, in cells induced to undergo EMT- and CSC-enriched breast cancer cell lines. More specifically, attenuation of FOXC2 expression using lentiviral short hairpin RNA led to inhibition of the mesenchymal phenotype and associated invasive and stem cell properties, which included reduced mammosphere-forming ability and tumor initiation. Whereas, overexpression of FOXC2 was sufficient to induce CSC properties and spontaneous metastasis in transformed human mammary epithelial cells. Furthermore, a FOXC2-induced gene expression signature was enriched in the claudin-low/basal B breast tumor subtype that contains EMT and CSC features. Having identified PDGFR-β to be regulated by FOXC2, we show that the U.S. Food and Drug Administration-approved PDGFR inhibitor, sunitinib, targets FOXC2-expressing tumor cells leading to reduced CSC and metastatic properties. Thus, FOXC2 or its associated gene expression program may provide an effective target for anti-EMT-based therapies for the treatment of claudin-low/basal B breast tumors or other EMT-/CSC-enriched tumors.

Pregnancy-induced noncoding RNA (PINC) associates with polycomb repressive complex 2 and regulates mammary epithelial differentiation

Pregnancy-induced noncoding RNA (PINC) and retinoblastoma-associated protein 46 (RbAp46) are upregulated in alveolar cells of the mammary gland during pregnancy and persist in alveolar cells that remain in the regressed lobules following involution. The cells that survive involution are thought to function as alveolar progenitor cells that rapidly differentiate into milk-producing cells in subsequent pregnancies, but it is unknown whether PINC and RbAp46 are involved in maintaining this progenitor population. Here, we show that, in the post-pubertal mouse mammary gland, mPINC is enriched in luminal and alveolar progenitors. mPINC levels increase throughout pregnancy and then decline in early lactation, when alveolar cells undergo terminal differentiation. Accordingly, mPINC expression is significantly decreased when HC11 mammary epithelial cells are induced to differentiate and produce milk proteins. This reduction in mPINC levels may be necessary for lactation, as overexpression of mPINC in HC11 cells blocks lactogenic differentiation, while knockdown of mPINC enhances differentiation. Finally, we demonstrate that mPINC interacts with RbAp46, as well as other members of the polycomb repressive complex 2 (PRC2), and identify potential targets of mPINC that are differentially expressed following modulation of mPINC expression levels. Taken together, our data suggest that mPINC inhibits terminal differentiation of alveolar cells during pregnancy to prevent abundant milk production and secretion until parturition. Additionally, a PRC2 complex that includes mPINC and RbAp46 may confer epigenetic modifications that maintain a population of mammary epithelial cells committed to the alveolar fate in the involuted gland.

During pregnancy, epithelial cells of the mammary gland begin to undergo differentiation into functional alveolar cells that, during lactation, will produce and secrete milk proteins, thereby providing nourishment to offspring. Following lactation, the majority of alveolar cells die and the mammary gland remodels to a pre-pregnancy-like state in a process called involution. However, some alveolar cells survive involution, and these cells are thought to serve as alveolar progenitors that are able to rapidly proliferate and differentiate into milk-producing cells in subsequent pregnancies. Keeping alveolar cells from undergoing terminal differentiation during pregnancy and involution is vital for the preservation of an alveolar progenitor population. Here, we show that the long noncoding RNA, PINC, is downregulated in the mammary gland between late pregnancy and early lactation, when alveolar cells begin to terminally differentiate. This reduction of PINC levels may be necessary for lactation, as overexpression of PINC inhibits differentiation, while knockdown of PINC enhances differentiation of mammary epithelial cells. Finally, we find that PINC interacts with the chromatin-modifying complex PRC2, suggesting epigenetic regulation may be involved in maintaining alveolar progenitors in the pregnant and involuting mammary gland. These results emphasize the potential importance of lncRNA-PRC2 involvement in regulating cell fate during development.

Abstract 5334: Novel lentiviral barcoding strategy for lineage tracing of cancer stem cells

The cancer stem cell theory posits the existence of a subset of tumorigenic cells with self-renewal potential and the ability to wholly reconstitute original tumors by lineage restriction. The corollary follows that these tumor-initiating cells (TICs) may be defined in part by their progeny, non-TICs that can contribute to tumor bulk, but lose their self-renewal potential. Although the expression of surface markers have been employed to identify highly enriched populations of TICs, no lineage-tracing experiments have been performed to date to trace the fate of TICs. To characterize TIC growth kinetics in mammary tumor cells transplanted into syngeneic recipient mice containing intact immune system, we developed a lentiviral barcoding system as a means of genetic lineage tracing of cell subpopulations sorted by surface marker expression. Combining antibody-based flow sort analysis with lentiviral-borne 60mer genetic barcodes, we have examined both clonality and propagation kinetics of barcodes in the tumor outgrowth. These studies are based on the hypothesis that TICs represent a limited subpopulation in breast cancers that give rise to tumor heterogeneity. By uniquely barcoding cells sorted by candidate surface marker expression (e.g. CD29 and CD24), then re-pooling barcoded cell fractions for transplantation into recipient mice, the abundance of each barcode in the reconstituted tumor reflects the frequency of TIC progeny from individual sorted cell fractions. In theory, only barcodes integrated into TICs should propagate throughout the heterogeneous tumor cell population. Thus, lineage tracing of each barcode is possible when the transplant-derived tumors are again FACS-analyzed using the same antibodies of the original sort. Alternatively, if there is extensive plasticity in tumors, non-TIC may acquire self-renewal properties and give rise to a new population of TICs with altered genetic and epigenetic properties. Here we present preliminary results in the previously characterized mouse p53 null stochastic model of basal-like breast cancer. Using qPCR, we demonstrate that barcodes integrate stably and are present throughout the reconstituted tumor cell population. We further determined that CD29hi/CD24hi subfraction consistently out-compete other subpopulations while giving rise to self and progeny populations not of the CD29hi/CD24hi immuno-profile. Our results indicate that CD29hi/CD24hi subfraction retains the two cardinal features of self-renewal and ability to give rise to progeny through lineage restriction ascribed to TICs. Our novel lentiviral barcoding strategy not only directly tests the CSC hypothesis but also provides future framework for defining specific biomarkers and evaluating therapeutics directed towards cancer stem cells. This lineage tracing methodology should be applicable not only to genetically engineered mouse models but also to primary human breast cancer xenografts.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5334. doi:1538-7445.AM2012-5334

Noncoding RNAs involved in mammary gland development and tumorigenesis: there's a long way to go

The mammalian genome encodes thousands of noncoding RNAs. These noncoding transcripts are broadly categorized into short noncoding RNAs, such as microRNAs (miRNAs), and long noncoding RNAs (lncRNAs) of greater than 200 nt. While the role of miRNAs in development and cancer biology has been extensively studied, much less is known about the vast majority of noncoding transcripts represented by lncRNAs. LncRNAs are emerging as key regulators of developmental processes and as such, their frequent misregulation in tumorigenesis and disease in not unexpected. The role of lncRNAs in mammary gland development and breast cancer is just beginning to be elucidated. This review will discuss the role of lncRNAs in mammalian and mammary gland development. In addition, we will review the contributions of lncRNAs to the stepwise progression of tumorigenesis, highlighting the role of lncRNAs in breast cancer.

Activation of Wnt signaling by chemically induced dimerization of LRP5 disrupts cellular homeostasis

Wnt signaling is crucial for a variety of biological processes, including body axis formation, planar polarity, stem cell maintenance and cellular differentiation. Therefore, targeted manipulation of Wnt signaling in vivo would be extremely useful. By applying chemical inducer of dimerization (CID) technology, we were able to modify the Wnt co-receptor, low-density lipoprotein (LDL)-receptor-related protein 5 (LRP5), to generate the synthetic ligand inducible Wnt switch, iLRP5. We show that iLRP5 oligomerization results in its localization to disheveled-containing punctate structures and sequestration of scaffold protein Axin, leading to robust β-catenin-mediated signaling. Moreover, we identify a novel LRP5 cytoplasmic domain critical for its intracellular localization and casein kinase 1-dependent β-catenin signaling. Finally, by utilizing iLRP5 as a Wnt signaling switch, we generated the Ubiquitous Activator of β-catenin (Ubi-Cat) transgenic mouse line. The Ubi-Cat line allows for nearly ubiquitous expression of iLRP5 under control of the H-2K^b promoter. Activation of iLRP5 in isolated prostate basal epithelial stem cells resulted in expansion of p63⁺ cells and development of hyperplasia in reconstituted murine prostate grafts. Independently, iLRP5 induction in adult prostate stroma enhanced prostate tissue regeneration. Moreover, induction of iLRP5 in male Ubi-Cat mice resulted in prostate tumor progression over several months from prostate hyperplasia to adenocarcinoma. We also investigated iLRP5 activation in Ubi-Cat-derived mammary cells, observing that prolonged activation results in mammary tumor formation. Thus, in two distinct experimental mouse models, activation of iLRP5 results in disruption of tissue homeostasis, demonstrating the utility of iLRP5 as a novel research tool for determining the outcome of Wnt activation in a precise spatially and temporally determined fashion.

The chromatin landscape of the casein gene locus

For several decades, the regulation of casein gene expression by the lactogenic hormones, prolactin and glucocorticoids, has provided an excellent model system in which to study how steroid and peptide hormones regulate gene expression. Early studies of casein gene regulation defined conserved sequence elements in the 5' flanking region of these genes, including one of which was identified as a γ-interferon activation sequence (GAS). Although this site was thought to interact with a mammary gland-specific factor, purification and cloning of this factor by Bernd Groner and his colleagues revealed it was instead a new member of the signal transducers and activators of transcription family, Stat5, which was expressed in many tissues. The exquisite tissue-specific expression of the casein genes was subsequently shown to depend not on a single transcription factor but on composite response elements that interacted with a number of ubiquitous transcription factors in response to the combinatorial effects of peptide and steroid hormone signaling. More recent studies have defined cooperative effects of prolactin and glucocorticoids as well as antagonistic effects of progesterone on the chromatin structure of both the casein gene proximal promoter region as well as a distal enhancer. Local chromatin modifications as well as long-range interactions facilitated by DNA looping are required for the hormonal regulation of β-casein gene expression. The casein genes are part of a large gene cluster, and the chromatin landscape of the entire cluster is regulated in a tissue-specific and developmental manner. Finally, newly discovered large non coding RNAs, such as the pregnancy-induced non coding RNA (PINC) may play an important role in the epigenetic regulation of mammary gland differentiation.

Stem cell antigen-1 (sca-1) regulates mammary tumor development and cell migration

BACKGROUND

Stem cell antigen-1 (Sca-1 or Ly6A) is a glycosyl phostidylinositol (GPI)-anchored cell surface protein associated with both stem and progenitor activity, as well as tumor initiating-potential. However, at present the functional role for Sca-1 is poorly defined.

METHODOLOGY/PRINCIPAL FINDINGS

To investigate the role of Sca-1 in mammary tumorigenesis, we used a mammary cell line derived from a MMTV-Wnt1 mouse mammary tumor that expresses high levels of endogenous Sca-1. Using shRNA knockdown, we demonstrate that Sca-1 expression controls cell proliferation during early tumor progression in mice. Functional limiting dilution transplantations into recipient mice demonstrate that repression of Sca-1 increases the population of tumor propagating cells. In scratch monolayer assays, Sca-1 enhances cell migration. In addition, knockdown of Sca-1 was shown to affect cell adhesion to a number of different extracellular matrix components. Microarray analysis indicates that repression of Sca-1 leads to changes in expression of genes involved in proliferation, cell migration, immune response and cell organization.

CONCLUSIONS/SIGNIFICANCE

Sca-1 exerts marked effects on cellular activity and tumorgenicity both in vitro and in vivo. A better understanding of Sca-1 function may provide insight into the broader role of GPI-anchored cell surface proteins in cancer.

P190A RhoGAP is required for mammary gland development

P190A and p190B Rho GTPase activating proteins (GAPs) are essential genes that have distinct, but overlapping roles in the developing nervous system. Previous studies from our laboratory demonstrated that p190B is required for mammary gland morphogenesis, and we hypothesized that p190A might have a distinct role in the developing mammary gland. To test this hypothesis, we examined mammary gland development in p190A-deficient mice. P190A expression was detected by in situ hybridization in the developing E14.5day embryonic mammary bud and within the ducts, terminal end buds (TEBs), and surrounding stroma of the developing virgin mammary gland. In contrast to previous results with p190B, examination of p190A heterozygous mammary glands demonstrated that p190A deficiency disrupted TEB morphology, but did not significantly delay ductal outgrowth indicating haploinsufficiency for TEB development. To examine the effects of homozygous deletion of p190A, embryonic mammary buds were rescued by transplantation into the cleared fat pads of SCID/Beige mice. Complete loss of p190A function inhibited ductal outgrowth in comparison to wildtype transplants (51% vs. 94% fat pad filled). In addition, the transplantation take rate of p190A deficient whole gland transplants from E18.5 embryos was significantly reduced compared to wildtype transplants (31% vs. 90%, respectively). These results suggest that p190A function in both the epithelium and stroma is required for mammary gland development. Immunostaining for p63 demonstrated that the myoepithelial cell layer is disrupted in the p190A deficient glands, which may result from the defective cell adhesion between the cap and body cell layers detected in the TEBs. The number of estrogen- and progesterone receptor-positive cells, as well as the expression levels of these receptors was increased in p190A deficient outgrowths. These data suggest that p190A is required in both the epithelial and stromal compartments for ductal outgrowth and that it may play a role in mammary epithelial cell differentiation.

Identification and gene expression profiling of tumor-initiating cells isolated from human osteosarcoma cell lines in an orthotopic mouse model

In the cancer stem cell model a cell hierarchy has been suggested as an explanation for intratumoral heterogeneity and tumor formation is thought to be driven by this tumor cell subpopulation. The identification of cancer stem cells in osteosarcoma (OS) and the biological processes dysregulated in this cell subpopulation, also known as tumor-initiating cells (TICs), may provide new therapeutic targets. The goal of this study, therefore, was to identify and characterize the gene expression profiles of TICs isolated from human OS cell lines. We analyzed the self-renewal capacity of OS cell lines and primary OS tumors based upon their ability to form sphere-like structures (sarcospheres) under serum-starving conditions. TICs were identify from OS cell lines using the long-term label retention dye PKH26. OS TICs and the bulk of tumor cells were isolated and used to assess their ability to initiate tumors in NOD/SCID mice. Gene expression profiles of OS TICs were obtained from fresh orthotopic tumor samples. We observed that increased sarcosphere efficiency correlated with an enhanced tumorigenic potential in OS. PKH26Hi cells were shown to constitute OS TICs based upon their capacity to form more sarcospheres, as well as to generate both primary bone tumors and lung metastases efficiently in NOD/SCID mice. Genomic profiling of OS TICs revealed that both bone development and cell migration processes were dysregulated in this tumor cell subpopulation. PKH26 labeling represents a valuable tool to identify OS TICs and gene expression analysis of this tumor cell compartment may identify potential therapeutic targets.

Progesterone receptor directly inhibits β-casein gene transcription in mammary epithelial cells through promoting promoter and enhancer repressive chromatin modifications

Differentiated HC-11 cells ectopically expressing progesterone receptor (PR) were used to explore the molecular mechanisms by which progesterone suppresses β-casein gene transcription induced by prolactin (PRL) and glucocorticoids in the mammary gland. As detected by chromatin immunoprecipitation assays, treatment of cells with the progestin agonist R5020 induced a rapid recruitment (5 min) of PR to the proximal promoter (-235 bp) and distal enhancer (-6 kb upstream of transcription start site) of β-casein. PR remained bound for 4 h and was dissociated by 24 h after treatment. Despite efficient binding, the hormone agonist-occupied PR did not stimulate transcription of the β-casein gene. Recruitment of signal transducer and activator of transcription 5a, glucocorticoid receptor, and the CCAAT enhancer binding protein β to the enhancer and proximal promoter of β-casein induced by PRL and glucocorticoids was blocked by progestin cotreatment, whereas PR binding was induced under these conditions. PRL/glucocorticoid-induced histone acetylation and the recruitment of the coactivator p300 and RNA polymerase II required for gene activation were also inhibited by progestin. In addition, progestin prevented dissociation of the corepressors Yin and Yang 1 and histone deacetylase 3 from the promoter, and demethylation of lysine 9 of histone 3 induced by PRL and glucocorticoids. These studies are consistent with the conclusion that progesterone interferes with PRL/glucocorticoid induction of β-casein transcription by a physical interaction of PR with the promoter and enhancer that blocks assembly of a transcriptional activation complex and dissociation of corepressors and promotes repressive chromatin modifications. These studies define a novel mechanism of steroid receptor-mediated transcriptional repression of a physiologically important gene in mammary gland development and differentiation.

Prospective isolation and characterization of committed and multipotent progenitors from immortalized mouse mammary epithelial cells with morphogenic potential

Introduction

Utilizing single-cell cloning of the COMMA-D cell line engineered to express β-galactosidase (CDβ) cell line, which exhibits normal in vivo morphogenesis, distinct multipotent, ductal-limited, alveolar-limited and luminal-restricted progenitors, have been isolated and characterized.

Methods

A single-cell suspension of CDβ cells was stained using Hoechst dye 33342, followed by analysis and sorting. Cells that effluxed the dye appeared on the left side of a FACS analysis panel and were referred to as side population (SP) cells. Cells that retained the dye appeared on the right side and were referred to as non-SP (NSP) cells. Cells from both SP and NSP regions were sorted and analyzed for outgrowth potential. Additionally, individual clones were derived from single cells sorted from each region.

Results

There was no difference in the outgrowth potential of the SP vs. NSP cells when 5,000 cells per fat pad were transplanted. However, individual clones derived from single cells sorted from either SP or NSP regions had varying growth potential. A total of nine clones were identified, four of which possessed in vivo mammary outgrowth potential and five of which lacked in vivo outgrowth potential. Two of the clones formed mammary lobuloalveolar structures that contained both ducts and alveoli and were termed multipotent. Two of the clones generated either ductal-only or alveolar-only structures and were referred to as ductal-limited or alveolar-limited progenitor clones, respectively. The ability to expand the clones in vitro allowed for the characterization of their unique molecular phenotypes. Among the mammary-specific markers tested, high cytokeratin 5 (CK5) expression was the only marker that correlated with the clones' outgrowth potential. Among the clones that did not show any in vivo outgrowth potential when transplanted alone, one clone showed in vivo growth and incorporated into the mammary lumen when mixed with normal mammary epithelial cells. This clone also showed the highest in vitro expression of CK8 and Elf5and may represent a luminal-restricted progenitor clone. In addition, six "biclones," each made from an SP cell plus an NSP cell, were analyzed. Of these six, three exhibited lobuloalveolar growth.

Conclusions

Distinct immortalized mammary progenitors have been isolated and characterized. Importantly, the results of this study provide further evidence for the existence of distinct ductal and alveolar mammary progenitors.

The DNA damage effector Chk1 kinase regulates Cdc14B nucleolar shuttling during cell cycle progression

Chk1 is a critical effector of DNA damage checkpoints necessary for the maintenance of chromosome integrity during cell cycle progression. Here we report, that Chk1 co-localized with the nucleolar marker, fibrillarin in response to radiation-induced DNA damage in human cells. Interestingly, in vitro studies using GST pull down assays identified the dual-specificity serine/threonine nucleolar phosphatase Cdc14B as a Chk1 substrate. Furthermore, Chk1, but not a kinase-dead Chk1 control, was shown to phosphorylate Cdc14B using an in vitro kinase assay. Co-immunoprecipitation experiments using exogenous Cdc14B transfected into human cells confirmed the interaction of Cdc14B and Chk1 during cell cycle. In addition, reduction of Chk1 levels via siRNA or UCN-01 treatment demonstrated that Chk1 activation following DNA damage was required for Cdc14B export from the nucleolus. These studies have revealed a novel interplay between Chk1 kinase and Cdc14B phosphatase involving radiation-induced nucleolar shuttling to facilitate error-free cell cycle progression and prevent genomic instability.

Thermal enhancement with optically activated gold nanoshells sensitizes breast cancer stem cells to radiation therapy

Breast cancer metastasis and disease recurrence are hypothesized to result from residual cancer stem cells, also referred to as tumor-initiating cells, which evade initial treatment. Using both syngeneic mouse and human xenograft models of triple-negative breast cancer, we have demonstrated that a subpopulation enriched in cancer stem cells was more resistant to treatment with 6 gray of ionizing radiation than the bulk of the tumor cells, and accordingly their relative proportion increased 48 to 72 hours after ionizing radiation treatment. In contrast, we achieved a larger reduction in tumor size without a concomitant increase in the percentage of cancer stem cells by treating with local hyperthermia for 20 minutes at 42°C after ionizing radiation using intravenously administered, optically activated gold nanoshells. Forty-eight hours after treatment, cells derived from the tumors treated with ionizing radiation plus hyperthermia exhibited both a marked decrease in tumorigenicity and a more differentiated phenotype than mock- and ionizing radiation-treated tumors. Thus, we have confirmed that these cancer stem cells are responsible for accelerated repopulation in vivo and demonstrated that hyperthermia sensitizes this cell population to radiation treatment. These findings suggest that local hyperthermia delivered by gold nanoshells plus radiation can eliminate radioresistant breast cancer stem cells.

Altered differentiation and paracrine stimulation of mammary epithelial cell proliferation by conditionally activated Smoothened

The Hedgehog (Hh) signaling network is critical for patterning and organogenesis in mammals, and has been implicated in a variety of cancers. Smoothened (Smo), the gene encoding the principal signal transducer, is overexpressed frequently in breast cancer, and constitutive activation in MMTV-SmoM2 transgenic mice caused alterations in mammary gland morphology, increased proliferation, and changes in stem/progenitor cell number. Both in transgenic mice and in clinical specimens, proliferative cells did not usually express detectable Smo, suggesting the hypothesis that Smo functioned in a non-cell autonomous manner to stimulate proliferation. Here, we employed a genetically tagged mouse model carrying a Cre-recombinase-dependent conditional allele of constitutively active Smo (SmoM2) to test this hypothesis. MMTV-Cre- or adenoviral-Cre-mediated SmoM2 expression in the luminal epithelium, but not in the myoepithelium, was required for the hyper-proliferative phenotypes. High levels of proliferation were observed in cells adjacent or in close-proximity to Smo expressing cells demonstrating that SmoM2 expressing cells were stimulating proliferation via a paracrine or juxtacrine mechanism. In contrast, Smo expression altered luminal cell differentiation in a cell-autonomous manner. SmoM2 expressing cells, purified by fluorescence activated cell sorting (FACS) via the genetic fluorescent tag, expressed high levels of Ptch2, Gli1, Gli2, Jag2 and Dll-1, and lower levels of Notch4 and Hes6, in comparison to wildtype cells. These studies provide insight into the mechanism of Smo activation in the mammary gland and its possible roles in breast tumorigenesis. In addition, these results also have potential implications for the interpretation of proliferative phenotypes commonly observed in other organs as a consequence of hedgehog signaling activation.

Small players with big roles: microRNAs as targets to inhibit breast cancer progression

As modulators of gene expression, microRNAs (miRNAs) are essential for normal development. Not surprisingly, aberrant expression of miRNAs is associated with many diseases, including cancer. Studies of various breast cancer subtypes have demonstrated that, like gene expression profiles and pathological differences, miRNA profiles can distinguish various tumor subtypes. Over the last few years, roles for miRNAs during many stages of breast cancer progression have been established. This includes potential breast cancer associated polymorphisms in miRNA target sites or miRNAs themselves, miRNAs that can act as tumor suppressors or oncogenes, and miRNAs that can modulate metastatic spread. Recent studies have also suggested key roles for miRNAs in regulating cancer stem cells. Thus, miRNAs have now become important therapeutic targets. This can be achieved by replacing miRNA expression where it has been lost or decreased, or conversely by inhibiting miRNA expression where it has been amplified or overexpressed in cancers. Ultimately, miRNAs should provide both important prognostic biomarkers as well as new targetable molecules for the treatment of breast cancer.

P190B RhoGAP has pro-tumorigenic functions during MMTV-Neu mammary tumorigenesis and metastasis

Introduction

Rho GTPases are overexpressed and hyperactivated in human breast cancers. Deficiency of p190B RhoGAP, a major inhibitor of the Rho GTPases, inhibits mouse mammary tumor virus long terminal repeat (MMTV)-Neu/ErbB2 mammary tumor formation and progression in part through effects within the stromal environment, suggesting that p190B function is pro-tumorigenic. To further investigate the potential pro-tumorigenic actions of p190B, we examined the effects of exogenous p190B expression within the mammary epithelium on MMTV-Neu tumor formation and progression.

Methods

Tetracycline (tet)-regulatable p190B transgenic mice were bred to MMTV-Neu mice, and the effects of exogenous p190B expression on tumor latency, multiplicity, growth rates, angiogenesis, and metastasis were examined. The effects of exogenous p190B expression on cell-matrix adhesion and invasion were tested using non-transformed primary mammary epithelial cells (MECs). Rho GTPase activity, oxidative stress as an indicator of reactive oxygen species (ROS) production, and downstream signaling pathways were analyzed.

Results

Altered p190B expression resulted in a two-fold increase in tumor multiplicity and a three-fold increase in metastases compared to control mice indicating that exogenous p190B expression in the mammary epithelium promotes MMTV-Neu mammary tumor formation and progression. Interestingly, non-transformed primary MECs expressing exogenous p190B displayed increased adhesion to laminin and type IV collagen and formed invasive structures in a three-dimensional culture assay. Ras related C3 botulinum toxin 1 (Rac1)-GTP levels were elevated in p190B transgenic tumors whereas Ras homologous A (RhoA) and cell division cycle 42 (Cdc42)-GTP levels were not significantly altered. Rac1 activity affects production of ROS, which regulate transformation, metastasis, and oxidative stress. Protein carbonylation, which is indicative of oxidative stress, was elevated 1.75-fold in p190B transgenic tumors as compared to control tumors suggesting that exogenous p190B expression may affect Rac1-dependent ROS production.

Conclusions

These studies indicate that paradoxically, p190B RhoGAP, a major inhibitor of the Rho GTPases in vitro, has pro-tumorigenic functions that enhance MMTV-Neu induced mammary tumor formation and metastasis. Furthermore, exogenous p190B expression enhances cell adhesion and invasion, which may facilitate metastasis. Rac1 activity and oxidative stress are elevated in tumors expressing exogenous p190B suggesting that p190B may promote tumorigenesis through a Rac1/ROS dependent mechanism.

Genetic manipulation of individual somatic mammary cells in vivo reveals a master role of STAT5a in inducing alveolar fate commitment and lactogenesis even in the absence of ovarian hormones

Assessing the molecular control of development and cell fate in individual cells in the intact mammary epithelium has not been possible to date. By exploiting an intraductal retrovirus (RCAS)-mediated gene delivery method to introduce a marker gene, we found that ductal epithelial cells are turned over with a half time of approximately 1month in adult virgin mice. However, following RCAS-mediated introduction of a constitutively activated STAT5a (caSTAT5a), caSTAT5a-activated ductal epithelial cells expand and replace other cells in the epithelium, eventually forming a mammary gland resembling that in a late pregnant mouse, suggesting that STAT5a activation alone is sufficient to mediate pregnancy-induced mammary cell expansion, alveolar cell fate commitment, and lactogenesis. Furthermore, such caSTAT5a-induced alveolar differentiation does not require ovarian functions, although caSTAT5a-induced cell proliferation is partly reduced in ovariectomized mice. In conclusion, in this first report of studying the developmental role of a gene in a few cells in a normally developed virgin mammary ductal tree, STAT5a activation causes alveolar fate commitment and lactogenesis, and with the help of ovarian hormones, drives alveolar expansion.