Alveolar progenitor cells develop in mouse mammary glands independent of pregnancy and lactation

Integration of Single-Cell Transcriptomics With a High Throughput Functional Screening Assay to Resolve Cell Type, Growth Kinetics, and Stemness Heterogeneity Within the Comma-1D Cell Line

Cell lines are one of the most frequently implemented model systems in life sciences research as they provide reproducible high throughput testing. Differentiation of cell cultures varies by line and, in some cases, can result in functional modifications within a population. Although research is increasingly dependent on these in vitro model systems, the heterogeneity within cell lines has not been thoroughly investigated. Here, we have leveraged high throughput single-cell assays to investigate the Comma-1D mouse cell line that is known to differentiate in culture. Using scRNASeq and custom single-cell phenotype assays, we resolve the clonal heterogeneity within the referenced cell line on the genomic and functional level. We performed a cohesive analysis of the transcriptome of 5,195 sequenced cells, of which 85.3% of the total reads successfully mapped to the mm10-3.0.0 reference genome. Across multiple gene expression analysis pipelines, both luminal and myoepithelial lineages were observed. Deep differential gene expression analysis revealed eight subclusters identified as luminal progenitor, luminal differentiated, myoepithelial differentiated, and fibroblast subpopulations-suggesting functional clustering within each lineage. Gene expression of published mammary stem cell (MaSC) markers Epcam, Cd49f, and Sca-1 was detected across the population, with 116 (2.23%) sequenced cells expressing all three markers. To gain insight into functional heterogeneity, cells with patterned MaSC marker expression were isolated and phenotypically investigated through a custom single-cell high throughput assay. The comparison of growth kinetics demonstrates functional heterogeneity within each cell cluster while also illustrating significant limitations in current cell isolation methods. We outlined the upstream use of our novel automated cell identification platform-to be used prior to single-cell culture-for reduced cell stress and improved rare cell identification and capture. Through compounding single-cell pipelines, we better reveal the heterogeneity within Comma-1D to identify subpopulations with specific functional characteristics.

Asymmetric cell division of mammary stem cells

Somatic stem cells are distinguished by their capacity to regenerate themselves and also to produce daughter cells that will differentiate. Self-renewal is achieved through the process of asymmetric cell division which helps to sustain tissue morphogenesis as well as maintain homeostasis. Asymmetric cell division results in the development of two daughter cells with different fates after a single mitosis. Only one daughter cell maintains "stemness" while the other differentiates and achieves a non-stem cell fate. Stem cells also have the capacity to undergo symmetric division of cells that results in the development of two daughter cells which are identical. Symmetric division results in the expansion of the stem cell population. Imbalances and deregulations in these processes can result in diseases such as cancer. Adult mammary stem cells (MaSCs) are a group of cells that play a critical role in the expansion of the mammary gland during puberty and any subsequent pregnancies. Furthermore, given the relatively long lifespans and their capability to undergo self-renewal, adult stem cells have been suggested as ideal candidates for transformation events that lead to the development of cancer. With the possibility that MaSCs can act as the source cells for distinct breast cancer types; understanding their regulation is an important field of research. In this review, we discuss asymmetric cell division in breast/mammary stem cells and implications on further research. We focus on the background history of asymmetric cell division, asymmetric cell division monitoring techniques, identified molecular mechanisms of asymmetric stem cell division, and the role asymmetric cell division may play in breast cancer.

ΔN63 suppresses the ability of pregnancy-identified mammary epithelial cells (PIMECs) to drive HER2-positive breast cancer

While pregnancy is known to reduce a woman's life-long risk of breast cancer, clinical data suggest that it can specifically promote HER2 (human EGF receptor 2)-positive breast cancer subtype (HER2+ BC). HER2+ BC, characterized by amplification of HER2, comprises about 20% of all sporadic breast cancers and is more aggressive than hormone receptor-positive breast cancer (the majority of cases). Consistently with human data, pregnancy strongly promotes HER2+ BC in genetic mouse models. One proposed mechanism of this is post-pregnancy accumulation of PIMECs (pregnancy-identified mammary epithelial cells), tumor-initiating cells for HER2+ BC in mice. We previously showed that p63, a homologue of the tumor suppressor p53, is required to maintain the post-pregnancy number of PIMECs and thereby promotes HER2+ BC. Here we set to test whether p63 also affects the intrinsic tumorigenic properties of PIMECs. To this end, we FACS-sorted YFP-labeled PIMECs from p63+/-;ErbB2 and control p63+/+;ErbB2 females and injected their equal amounts into immunodeficient recipients. To our surprise, p63+/- PIMECs showed increased, rather than decreased, tumorigenic capacity in vivo, i.e., significantly accelerated tumor onset and tumor growth, as well as increased self-renewal in mammosphere assays and proliferation in vitro and in vivo. The underlying mechanism of these phenotypes seems to be a specific reduction of the tumor suppressor TAp63 isoform in p63+/- luminal cells, including PIMECs, with concomitant aberrant upregulation of the oncogenic ΔNp63 isoform, as determined by qRT-PCR and scRNA-seq analyses. In addition, scRNA-seq revealed upregulation of several cancer-associated (Il-4/Il-13, Hsf1/HSP), oncogenic (TGFβ, NGF, FGF, MAPK) and self-renewal (Wnt, Notch) pathways in p63+/-;ErbB2 luminal cells and PIMECs per se. Altogether, these data reveal a complex role of p63 in PIMECs and pregnancy-associated HER2+ BC: maintaining the amount of PIMECs while suppressing their intrinsic tumorigenic capacity.

PC3-Secreted Microprotein Is Expressed in Glioblastoma Stem-Like Cells and Human Glioma Tissues

Glioblastoma multiforme (GBM) is the most prevalent malignant primary brain tumor with a high recurrence rate. Despite multimodal therapy including surgical resection, chemotherapy, and radiotherapy, the median survival time after the initial diagnosis of GBM is approximately 14 months. Since cancer stem cells (CSCs) are considered the leading cause of cancer recurrence, glioblastoma stem cell-targeted therapy is a promising strategy for the treatment of GBM. However, because CSC heterogeneity has been implicated in the difficulties of CSC-target therapy, more in-depth knowledge of CSC biology is still required to develop novel therapies. In this study, we established single cell-derived tumorspheres from human glioblastoma U87MG cells. One of these tumorspheres, P4E8 clone, showed CSC-like phenotypes, such as self-renewal capacity, expression of CSC markers, resistance to anti-cancer agents, and in vivo tumorigenicity. Therefore, we used P4E8 cells as a cell-based model of glioblastoma stem cells (GSCs). Gene expression analysis using microarray indicated that the most highly expressed genes in P4E8 cells compared to the parental U87MG were PC3-secreted microprotein (MSMP). Furthermore, MSMP was expressed in patient-derived GSCs and human glioma tissues at the protein level, implying that MSMP might contribute to glioma development and progression.

Breast Cancer During Pregnancy: A Marked Propensity to Triple-Negative Phenotype

Breast and cervical cancers comprise 50% of all cancers during pregnancy. In particular, gestational breast cancer is considered one of the most aggressive types of cancers, which is a rare but fatal disease. However, the incidence of this type of cancer is increasing over the years and its prevalence is expected to rise further as more women delay childbearing. Breast cancer occurring after pregnancy is generally triple negative with specific characterizations of a poorer prognosis and outcome. On the other hand, it has been pointed out that this cancer is associated with a specific group of genes which can be used as precise targets to manage this deadly disease. Indeed, combination therapies consisting of gene-based agents with other cancer therapeutics is presently under consideration. We herein review recent progress in understanding the development of breast cancer during pregnancy and their unique subtype of triple negative which is the hallmark of this type of breast cancer.

Aging-Associated Alterations in Mammary Epithelia and Stroma Revealed by Single-Cell RNA Sequencing

Aging is closely associated with increased susceptibility to breast cancer, yet there have been limited systematic studies of aging-induced alterations in the mammary gland. Here, we leverage high-throughput single-cell RNA sequencing to generate a detailed transcriptomic atlas of young and aged murine mammary tissues. By analyzing epithelial, stromal, and immune cells, we identify age-dependent alterations in cell proportions and gene expression, providing evidence that suggests alveolar maturation and physiological decline. The analysis also uncovers potential pro-tumorigenic mechanisms coupled to the age-associated loss of tumor suppressor function and change in microenvironment. In addition, we identify a rare, age-dependent luminal population co-expressing hormone-sensing and secretory-alveolar lineage markers, as well as two macrophage populations expressing distinct gene signatures, underscoring the complex heterogeneity of the mammary epithelia and stroma. Collectively, this rich single-cell atlas reveals the effects of aging on mammary physiology and can serve as a useful resource for understanding aging-associated cancer risk.

Using single-cell RNA-sequencing, Li et al. compare mammary epithelia and stroma in young and aged mice. Age-dependent changes at cell and gene levels provide evidence suggesting alveolar maturation, functional deterioration, and potential pro-tumorigenic and inflammatory alterations. Additionally, identification of heterogeneous luminal and macrophage subpopulations underscores the complexity of mammary lineages.

Pregnancy and Breast Cancer: Pathways to Understand Risk and Prevention

Several studies have made strong efforts to understand how age and parity modulate the risk of breast cancer. A holistic understanding of the dynamic regulation of the morphological, cellular, and molecular milieu of the mammary gland offers insights into the drivers of breast cancer development as well as into potential prophylactic interventions, the latter being a longstanding ambition of the research and clinical community aspiring to eradicate the disease. In this review we discuss mechanisms that react to pregnancy signals, and we delineate the nuances of pregnancy-associated dynamism that contribute towards either breast cancer development or prevention. Further definition of the molecular basis of parity and breast cancer risk may allow the elaboration of tools to predict and survey those who are at risk of breast cancer development.

Does the Mouse Mammary Gland Arise from Unipotent or Multipotent Mammary Stem/Progenitor Cells?

The presence of long-lived lineage restricted progenitor and multipotent progenitor cells in adult mouse mammary gland for cancer development is compelling. Mammary cancers are phenotypically diverse This might be explained by transformation of long-lived, lineage-limited progenitor subpopulations. Mammary multipotent epithelial stem cells and their environmental niches must be considered, since their niche(s), once empty might be occupied by lineage-limited progenitors that are proximal. The existence of premalignant mammary populationst that manifest characteristics of lineage limitation argues strongly for this proposition.

Expression of the mRNA stability regulator Tristetraprolin is required for lactation maintenance in the mouse mammary gland

Tristetraprolin (TTP), an mRNA-binding protein that negatively controls levels of inflammatory factors, is highly expressed in the lactating mouse mammary gland. To determine the biological relevance of this expression profile, we developed bi-transgenic mice in which this protein is specifically down-regulated in the secretory mammary epithelium in the secretory mammary epithelium during lactation. Our data show that TTP conditional KO mice produced underweight litters, possibly due to massive mammary cell death induced during lactation without the requirement of additional stimuli. This effect was linked to overexpression of inflammatory cytokines, activation of STAT3 and down-regulation of AKT phosphorylation. Importantly, blocking TNFα activity in the lactating conditional TTP KO mice inhibited cell death and similar effects were observed when this treatment was applied to wild-type animals during 48 h after weaning. Therefore, our results demonstrate that during lactation TTP wards off early involution by preventing the increase of local inflammatory factors. In addition, our data reveal the relevance of locally secreted TNFα for triggering programmed cell death after weaning.

Tissue specific microenvironments: a key tool for tissue engineering and regenerative medicine

The accumulated evidence points to the microenvironment as the primary mediator of cellular fate determination. Comprised of parenchymal cells, stromal cells, structural extracellular matrix proteins, and signaling molecules, the microenvironment is a complex and synergistic edifice that varies tissue to tissue. Furthermore, it has become increasingly clear that the microenvironment plays crucial roles in the establishment and progression of diseases such as cardiovascular disease, neurodegeneration, cancer, and ageing. Here we review the historical perspectives on the microenvironment, and how it has directed current explorations in tissue engineering. By thoroughly understanding the role of the microenvironment, we can begin to correctly manipulate it to prevent and cure diseases through regenerative medicine techniques.

Mammary extracellular matrix directs differentiation of testicular and embryonic stem cells to form functional mammary glands in vivo

Previously, we demonstrated the ability of the normal mammary microenvironment (niche) to direct non-mammary cells including testicular and embryonic stem cells (ESCs) to adopt a mammary epithelial cell (MEC) fate. These studies relied upon the interaction of transplanted normal MECs with non-mammary cells within the mammary fat-pads of recipient mice that had their endogenous epithelium removed. Here, we tested whether acellular mammary extracellular matrix (mECM) preparations are sufficient to direct differentiation of testicular-derived cells and ESCs to form functional mammary epithelial trees in vivo. We found that mECMs isolated from adult mice and rats were sufficient to redirect testicular derived cells to produce normal mammary epithelial trees within epithelial divested mouse mammary fat-pads. Conversely, ECMs isolated from omental fat and lung did not redirect testicular cells to a MEC fate, indicating the necessity of tissue specific components of the mECM. mECM preparations also completely inhibited teratoma formation from ESC inoculations. Further, a phenotypically normal ductal outgrowth resulted from a single inoculation of ESCs and mECM. To the best of our knowledge, this is the first demonstration of a tissue specific ECM driving differentiation of cells to form a functional tissue in vivo.

FGF2 and EGF Are Required for Self-Renewal and Organoid Formation of Canine Normal and Tumor Breast Stem Cells

Recent studies suggest that human tumors are generated from cancer cells with stem cell (SC) properties. Spontaneously occurring cancers in dogs contain a diversity of cells that like for human tumors suggest that certain canine tumors are also generated from cancer stem cells (CSCs). CSCs, like normal SCs, have the capacity for self-renewal as mammospheres in suspension cultures. To understand how cells with SC properties contribute to canine mammary gland tumor development and progression, comparative analysis between normal SCs and CSCs, obtained from the same individual, is essential. We have utilized the property of sphere formation to develop culture conditions for propagating stem/progenitor cells from canine normal and tumor tissue. We show that cells from dissociated mammospheres retain sphere reformation capacity for several serial passages and have the capacity to generate organoid structures ex situ. Utilizing various culture conditions for passaging SCs and CSCs, fibroblast growth factor 2 (FGF2) and epidermal growth factor (EGF) were found to positively or negatively regulate mammosphere regeneration, organoid formation, and multi-lineage differentiation potential. The response of FGF2 and EGF on SCs and CSCs was different, with increased FGF2 and EGF self-renewal promoted in SCs and repressed in CSCs. Our protocol for propagating SCs from normal and tumor canine breast tissue will provide new opportunities in comparative mammary gland stem cell analysis between species and anticancer treatment and therapies for dogs. J. Cell. Biochem. 118: 570-584, 2017. © 2016 Wiley Periodicals, Inc.

Original Research: Featured Article: Imatinib mesylate (Gleevec) inhibits Notch and c-Myc signaling: Five-day treatment permanently rescues mammary development

Wap-Int3 transgenic females expressing the Notch4 intracellular domain (designated Int3) from the whey acidic protein promoter exhibit two phenotypes in the mammary gland: blockage of lobuloalveolar development and lactation, and tumor development with 100% penetrance. Previously, we have shown that treatment of Wap-Int3 tumor bearing mice with Imatinib mesylate (Gleevec) is associated with complete regression of the tumor. In the present study, we show that treatment of Wap-Int3 mice during day 1 through day 6 of pregnancy with Gleevec leads to the restoration of their lobuloalveolar development and ability to lactate in subsequent pregnancies in absence of Gleevec treatment. In addition, these mice do not develop mammary tumors. We investigated the mechanism for Gleevec regulation of Notch signaling and found that Gleevec treatment results in a loss of Int3 protein but not of Int3 mRNA in HC11 mouse mammary epithelial cells expressing Int3. The addition of MG-132, a proteasome inhibitor, shows increased ubiquitination of Int3 in the presence of Gleevec. Thus, Gleevec affects the stability of Int3 by promoting the degradation of Int3 via E3 ubiquitin ligases targeting it for the proteasome degradation. Gleevec is a tyrosine kinase inhibitor that acts on c-Kit and PDGFR. Therefore, we investigated the downstream substrate kinase GSK3β to ascertain the possible role that this kinase might play in the stability of Int3. Data show that Gleevec degradation of Int3 is GSK3β dependent. We have expanded our study of the effects Gleevec has on tumorigenesis of other oncogenes. We have found that anchorage-independent growth of HC11-c-Myc cells as well as tumor growth in nude mice is inhibited by Gleevec treatment. As with Int3, Gleevec treatment appears to destabilize the c-Myc protein but not mRNA. These results indicate that Gleevec could be a potential therapeutic drug for patients bearing Notch4 and/or c-Myc positive breast carcinomas.

Consumption of endophyte-infected fescue seed during the dry period does not decrease milk production in the following lactation

Ergot alkaloids in endophyte-infected grasses inhibit prolactin (PRL) secretion and may reduce milk production of cows consuming these grasses. We investigated the effects of consuming endophyte-infected fescue seed during late lactation and the dry period on mammary growth, differentiation, and milk production. Twenty-four multiparous Holstein cows were randomly assigned to 3 treatment groups. Starting at 90±4 d prepartum, cows were fed endophyte-free fescue seed (control; CON), endophyte-free fescue seed plus 3×/wk subcutaneous injections of bromocriptine (0.1mg/kg of body weight, positive control; BROMO), or endophyte-infected fescue seed (INF) as 10% of the diet on an as fed basis. Although milk yield of groups did not differ before treatment, at dry off (-60 d prepartum) INF and BROMO cows produced less milk than CON. Throughout the treatment period, basal concentrations of PRL and the prepartum increase in plasma PRL were reduced in INF and BROMO cows compared with CON cows. Three weeks after the end of treatment, circulating concentrations of PRL were equivalent across groups. In the subsequent lactation milk yield was not decreased; in fact, BROMO cows exhibited a 9% increase in milk yield relative to CON. Evaluation of mammary tissue during the dry period and the subsequent lactation, by quantitative histology and immunohistochemical analysis of proliferation markers and putative mammary stem or progenitor cell markers, indicated that feeding endophyte-infected fescue seed did not significantly affect mammary growth and development. Feeding endophyte-infected grasses during the dry period may permit effective utilization of feed resources without compromising milk production in the next lactation.

High-Throughput Single-Cell Derived Sphere Formation for Cancer Stem-Like Cell Identification and Analysis

Considerable evidence suggests that many malignancies are driven by a cellular compartment that displays stem cell properties. Cancer stem-like cells (CSCs) can be identified by expression of cell surface markers or enzymatic activity, but these methods are limited by phenotypic heterogeneity and plasticity of CSCs. An alternative phenotypic methodology based on in-vitro sphere formation has been developed, but it is typically labor-intensive and low-throughput. In this work, we present a 1,024-microchamber microfluidic platform for single-cell derived sphere formation. Utilizing a hydrodynamic capturing scheme, more than 70% of the microchambers capture only one cell, allowing for monitoring of sphere formation from heterogeneous cancer cell populations for identification of CSCs. Single-cell derived spheres can be retrieved and dissociated for single-cell analysis using a custom 96-gene panel to probe heterogeneity within the clonal CSC spheres. This microfluidic platform provides reliable and high-throughput sphere formation for CSC identification and downstream clonal analysis.

A Long-Lived Luminal Subpopulation Enriched with Alveolar Progenitors Serves as Cellular Origin of Heterogeneous Mammary Tumors

It has been shown that the mammary luminal lineage could be maintained by luminal stem cells or long-lived progenitors, but their identity and role in breast cancer remain largely elusive. By lineage analysis using Wap-Cre mice, we found that, in nulliparous females, mammary epithelial cells (MECs) genetically marked by Wap-Cre represented a subpopulation of CD61+ luminal progenitors independent of ovarian hormones for their maintenance. Using a pulse-chase lineage-tracing approach based on Wap-Cre adenovirus (Ad-Wap-Cre), we found that Ad-Wap-Cre-marked nulliparous MECs were enriched with CD61+ alveolar progenitors (APs) that gave rise to CD61- alveolar luminal cells during pregnancy/lactation and could maintain themselves long term. When transformed by different oncogenes, they could serve as cells of origin of heterogeneous mammary tumors. Thus, our study revealed a type of long-lived AP within the luminal lineage that may serve as the cellular origin of multiple breast cancer subtypes.

Form and function: how estrogen and progesterone regulate the mammary epithelial hierarchy

The mammary gland undergoes dramatic post-natal growth beginning at puberty, followed by full development occurring during pregnancy and lactation. Following lactation, the alveoli undergo apoptosis, and the mammary gland reverses back to resemble the nonparous gland. This process of growth and regression occurs for multiple pregnancies, suggesting the presence of a hierarchy of stem and progenitor cells that are able to regenerate specialized populations of mammary epithelial cells. Expansion of epithelial cell populations in the mammary gland is regulated by ovarian steroids, in particular estrogen acting through its receptor estrogen receptor alpha (ERα) and progesterone signaling through progesterone receptor (PR). A diverse number of stem and progenitor cells have been identified based on expression of cell surface markers and functional assays. Here we review the current understanding of how estrogen and progesterone act together and separately to regulate stem and progenitor cells within the human and mouse mammary tissues. Better understanding of the hierarchal organization of epithelial cell populations in the mammary gland and how the hormonal milieu affects its regulation may provide important insights into the origins of different subtypes of breast cancer.

Validation of an in vitro model of erbB2(+) cancer cell redirection

Overexpression of the oncoprotein erbB2/HER2 is present in 20-30% of breast cancer patients and inversely correlates with patient survival. Reports have demonstrated the deterministic power of the mammary microenvironment where the normal mammary microenvironment redirects cells of non-mammary origin or tumor-derived cells to adopt a mammary phenotype in an in vivo model. This phenomenon is termed tumor cell redirection. Tumor-derived cells that overexpress the erbB2 oncoprotein lose their tumor-forming capacity in this model. In this model, phosphorylation of erbB2 is attenuated thus reducing the tumor cell's tumor-forming potential. In this report, we describe our results using an in vitro model based on the in vivo model mentioned previously. Tumor-derived cells are mixed in predetermined ratios with normal mammary epithelial cells prior to seeding in vitro. In this in vitro model, the tumor-derived cells are redirected as determined by attenuated phosphorylation of the receptor and reduced sphere and colony formation. These results match those observed in the in vivo model. This in vitro model will allow expanded experimental options in the future to determine additional aspects of tumor cell redirection that can be translated to other types of cancer.

Effect of glycogen synthase kinase-3 inactivation on mouse mammary gland development and oncogenesis

Many components of the Wnt/β-catenin signaling pathway have critical functions in mammary gland development and tumor formation, yet the contribution of glycogen synthase kinase-3 (GSK-3α and GSK-3β) to mammopoiesis and oncogenesis is unclear. Here, we report that WAP-Cre-mediated deletion of GSK-3 in the mammary epithelium results in activation of Wnt/β-catenin signaling and induces mammary intraepithelial neoplasia that progresses to squamous transdifferentiation and development of adenosquamous carcinomas at 6 months. To uncover possible β-catenin-independent activities of GSK-3, we generated mammary-specific knockouts of GSK-3 and β-catenin. Squamous transdifferentiation of the mammary epithelium was largely attenuated, however, mammary epithelial cells lost the ability to form mammospheres suggesting perturbation of stem cell properties unrelated to loss of β-catenin alone. At 10 months, adenocarcinomas that developed in glands lacking GSK-3 and β-catenin displayed elevated levels of γ-catenin/plakoglobin as well as activation of the Hedgehog and Notch pathways. Collectively, these results establish the two isoforms of GSK-3 as essential integrators of multiple developmental signals that act to maintain normal mammary gland function and suppress tumorigenesis.

The mammary cellular hierarchy and breast cancer

Advances in the study of hematopoietic cell maturation have paved the way to a deeper understanding the stem and progenitor cellular hierarchy in the mammary gland. The mammary epithelium, unlike the hematopoietic cellular hierarchy, sits in a complex niche where communication between epithelial cells and signals from the systemic hormonal milieu, as well as from extra-cellular matrix, influence cell fate decisions and contribute to tissue homeostasis. We review the discovery, definition and regulation of the mammary cellular hierarchy and we describe the development of the concepts that have guided our investigations. We outline recent advances in in vivo lineage tracing that is now challenging many of our assumptions regarding the behavior of mammary stem cells, and we show how understanding these cellular lineages has altered our view of breast cancer.

Mammary stem cells: expansion and animal productivity

Identification and characterization of mammary stem cells and progenitor cells from dairy animals is important in the understanding of mammogenesis, tissue turnover, lactation persistency and regenerative therapy. It has been realized by many investigators that altered lactation, long dry periods (non-milking period between two consecutive lactation cycles), abrupt cessation of lactation (common in water buffaloes) and disease conditions like mastitis, greatly reduce milk yield thus render huge financial losses within the dairy sector. Cellular manipulation of specialized cell types within the mammary gland, called mammary stem cells (MaSCs)/progenitor cells, might provide potential solutions to these problems and may improve milk production. In addition, MaSCs/progenitor cells could be used in regenerative therapy against tissue damage caused by mastitis. This review discusses methods of MaSC/progenitor cell manipulation and their mechanisms in bovine and caprine animals. Author believes that intervention of MaSCs/progenitor cells could lessen the huge financial losses to the dairy industry globally.

p63 is a prosurvival factor in the adult mammary gland during post-lactational involution, affecting PI-MECs and ErbB2 tumorigenesis

In embryogenesis, p63 is essential to develop mammary glands. In the adult mammary gland, p63 is highly expressed in the basal cell layer that comprises myoepithelial and interspersed stem/progenitor cells, and has limited expression in luminal epithelial cells. In adult skin, p63 has a crucial role in the maintenance of epithelial stem cells. However, it is unclear whether p63 also has an equivalent role as a stem/progenitor cell factor in adult mammary epithelium. We show that p63 is essential in vivo for the survival and maintenance of parity-identified mammary epithelial cells (PI-MECs), a pregnancy-induced heterogeneous population that survives post-lactational involution and contain multipotent progenitors that give rise to alveoli and ducts in subsequent pregnancies. p63+/- glands are normal in virgin, pregnant and lactating states. Importantly, however, during the apoptotic phase of post-lactational involution p63+/- glands show a threefold increase in epithelial cell death, concomitant with increased activation of the oncostatin M/Stat3 and p53 pro-apoptotic pathways, which are responsible for this phase. Thus, p63 is a physiologic antagonist of these pathways specifically in this regressive stage. After the restructuring phase when involution is complete, mammary glands of p63+/- mice again exhibit normal epithelial architecture by conventional histology. However, using Rosa(LSL-LacZ);WAP-Cre transgenics (LSL-LacZ, lox-stop-lox β-galactosidase), a genetic in vivo labeling system for PI-MECs, we find that p63+/- glands have a 30% reduction in the number of PI-MEC progenitors and their derivatives. Importantly, PI-MECs are also cellular targets of pregnancy-promoted ErbB2 tumorigenesis. Consistent with their PI-MEC pool reduction, one-time pregnant p63+/- ErbB2 mice are partially protected from breast tumorigenesis, exhibiting extended tumor-free and overall survival, and reduced tumor multiplicity compared with their p63+/+ ErbB2 littermates. Conversely, in virgin ErbB2 mice p63 heterozygosity provides no survival advantage. In sum, our data establish that p63 is an important survival factor for pregnancy-identified PI-MEC progenitors in breast tissue in vivo.

Mammary gland development

The mammary gland develops through several distinct stages. The first transpires in the embryo as the ectoderm forms a mammary line that resolves into placodes. Regulated by epithelial–mesenchymal interactions, the placodes descend into the underlying mesenchyme and produce the rudimentary ductal structure of the gland present at birth. Subsequent stages of development—pubertal growth, pregnancy, lactation, and involution—occur postnatally under the regulation of hormones. Puberty initiates branching morphogenesis, which requires growth hormone (GH) and estrogen, as well as insulin-like growth factor 1 (IGF1), to create a ductal tree that fills the fat pad. Upon pregnancy, the combined actions of progesterone and prolactin generate alveoli, which secrete milk during lactation. Lack of demand for milk at weaning initiates the process of involution whereby the gland is remodeled back to its prepregnancy state. These processes require numerous signaling pathways that have distinct regulatory functions at different stages of gland development. Signaling pathways also regulate a specialized subpopulation of mammary stem cells that fuel the dramatic changes in the gland occurring with each pregnancy. Our knowledge of mammary gland development and mammary stem cell biology has significantly contributed to our understanding of breast cancer and has advanced the discovery of therapies to treat this disease.

Expression of PIK3CA mutant E545K in the mammary gland induces heterogeneous tumors but is less potent than mutant H1047R

The phosphoinositide 3-kinase (PI3K) signaling cascade is a key mediator of cellular growth, survival and metabolism and is frequently subverted in human cancer. The gene encoding for the alpha catalytic subunit of PI3K (PIK3CA) is mutated and/or amplified in ∼30% of breast cancers. Mutations in either the kinase domain (H1047R) or the helical domain (E545K) are most common and result in a constitutively active enzyme with oncogenic capacity. PIK3CA^H1047R was previously demonstrated to induce tumors in transgenic mouse models; however, it was not known whether overexpression of PIK3CA^E545K is sufficient to induce mammary tumors and whether tumor initiation by these two types of mutants differs. Here, we demonstrate that expression of PIK3CA^E545K in the mouse mammary gland induces heterogenous mammary carcinomas but with a longer latency than PIK3CA^H1047R-expressing mice. Our results suggest that the helical domain mutant PIK3CA^E545K is a less potent inducer of mammary tumors due to less efficient activation of downstream Akt signaling.

Mammospheres from murine mammary stem cell-enriched basal cells: clonal characteristics and repopulating potential

Identification of murine mammary stem cells (MaSCs) has been attempted with various in vitro and in vivo assays. While, the in vivo repopulation assay remains as the most definitive assay for MaSC detection, it is expensive, time-consuming, and technically challenging. The in vitro mammosphere assay was considered unreliable because of major concerns about its clonal origin. In the current study, co-culture experiments with mammary cells from fluorescent protein transgenic mice and time-lapse video microscopy revealed that >90% mammospheres formed from sorted basal epithelial-enriched cells were of clonal origin in terms of stem cell. These basal-cell derived mammospheres were further distinguished morphologically in a 3-dimensional extracellular matrix culture and functionally in the in vivo repopulation assay. Transplant of single mammospheres or the resultant 3-dimensional solid structures into gland-free mammary fat pads yielded a 70% success rate of multilineage mammary gland reconstitution. Thus, this in vitro sphere formation and differentiation assay is a reliable alternative to the in vivo repopulation assay for the study of MaSCs.

Comparative aspects of mammary gland development and homeostasis

Mammary glands are crucial to the reproductive strategy of mammals, and the milk of domesticated ruminants serves as an important source of nutrients for the human population. The majority of mammary gland development occurs postnatally, and the mammary gland undergoes cyclical periods of growth, differentiation, lactation, and regression that are coordinated to provide nutrients for offspring or are driven by strategies to manage reproduction and milk production of domesticated species. Growth and maintenance of the mammary epithelium depends on the function of mammary stem cells and progenitor cells. In this review, we provide an overview of postnatal mammary gland development, cyclical phases of mammary gland regression (regression during lactation and between successive lactations), and mammary stem cells and progenitor cells. Where possible, these processes are related to animal production and compared across species, particularly bovine, porcine, murine, and human.

Mammary gland stem cells: more puzzles than explanations

Mammary gland stem cells (MaSC) have not been identified in spite of extensive research spanning over several decades. This has been primarily due to the complexity of mammary gland structure and its development, cell heterogeneity in the mammary gland and the insufficient knowledge about MaSC markers. At present, Lin (-) CD29 (i) CD49f (i) CD24 (+/mod) Sca- 1 (-) cells of the mammary gland have been reported to be enriched with MaSCs. We suggest that the inclusion of stem cell markers like Oct4, Sox2, Nanog and the mammary gland differentiation marker BRCA-1 may further narrow down the search for MaSCs. In addition, we have discussed some of the other unresolved puzzles on the mammary gland stem cells, such as their similarities and/or differences with mammary cancer stem cells, use of milk as source of mammary stem cells and the possibility of in vitro differentiation of embryonic stem (ES) cells into functional mammary gland structures in this review. Nevertheless, it is the lack of identity for a MaSC that is curtailing the advances in some of the above and other related areas.

Bovine mammary stem cells: cell biology meets production agriculture

Mammary stem cells (MaSC) provide for net growth, renewal and turnover of mammary epithelial cells, and are therefore potential targets for strategies to increase production efficiency. Appropriate regulation of MaSC can potentially benefit milk yield, persistency, dry period management and tissue repair. Accordingly, we and others have attempted to characterize and alter the function of bovine MaSC. In this review, we provide an overview of current knowledge of MaSC gained from studies using mouse and human model systems and present research on bovine MaSC within that context. Recent data indicate that MaSC retain labeled DNA for extended periods because of their selective segregation of template DNA strands during mitosis. Relying on this long-term retention of bromodeoxyuridine-labeled DNA, we identified putative bovine MaSC. These label-retaining epithelial cells (LREC) are in low abundance within mammary epithelium (<1%). They are predominantly estrogen receptor (ER)-negative and localized in a basal or suprabasal layer of the epithelium throughout the gland. Thus, the response of MaSC to estrogen, the major mitogen in mammary gland, is likely mediated by paracrine factors released by cells that are ER-positive. This is consistent with considerable evidence for cross-talk within and between epithelial cells and surrounding stromal cells. Excision of classes of cells by laser microdissection and subsequent microarray analysis will hopefully provide markers for MaSC and insights into their regulation. Preliminary analyses of gene expression in laser-microdissected LREC and non-LREC are consistent with the concept that LREC represent populations of stem cells and progenitor cells that differ with regard to their properties and location within the epithelial layer. We have attempted to modulate the MaSC number by infusing a solution of xanthosine through the teat canal and into the ductal network of the mammary glands of prepubertal heifers. This treatment increased the number of putative stem cells, as evidenced by an increase in the percentage of LREC and increased telomerase activity within the tissue. The exciting possibility that stem cell expansion can influence milk production is currently under investigation.

Reprogramming non-mammary and cancer cells in the developing mouse mammary gland

The capacity of any portion of the murine mammary gland to produce a complete functional mammary outgrowth upon transplantation to an epithelium-divested fat pad is unaffected by the age or reproductive history of the donor. Likewise, through serial transplantations, no loss of potency is detected when compared to similar transplantations of the youngest mammary tissue tested. This demonstrates that stem cell activity is maintained intact throughout the lifetime of the animal despite aging and the repeated expansion and depletion of the mammary epithelium through multiple rounds of pregnancy, lactation and involution. These facts support the contention that mammary stem cells reside in protected tissue locales (niches), where their reproductive potency remains essentially unchanged through life. Disruption of the tissue, to produce dispersed cells results in the desecration of the protection afforded by the "niche" and leads to a reduced capacity of dispersed epithelial cells (in terms of the number transplanted) to recapitulate complete functional mammary structures. Our studies demonstrate that during the reformation of mammary stem cell niches by dispersed epithelial cells in the context of the intact epithelium-free mammary stroma, non-mammary cells, including mouse and human cancer cells, may be sequestered and reprogrammed to perform mammary epithelial cell functions including those ascribed to mammary stem/progenitor cells.

Life stage differences in mammary gland gene expression profile in non-human primates

Breast cancer (BC) is the most common malignancy of women in the developed world. To better understand its pathogenesis, knowledge of normal breast development is crucial, as BC is the result of disregulation of physiologic processes. The aim of this study was to investigate the impact of reproductive life stages on the transcriptional profile of the mammary gland in a primate model. Comparative transcriptomic analyses were carried out using breast tissues from 28 female cynomolgus macaques (Macaca fascicularis) at the following life stages: prepubertal (n = 5), adolescent (n = 4), adult luteal (n = 5), pregnant (n = 6), lactating (n = 3), and postmenopausal (n = 5). Mammary gland RNA was hybridized to Affymetrix GeneChip(®) Rhesus Macaque Genome Arrays. Differential gene expression was analyzed using ANOVA and cluster analysis. Hierarchical cluster analysis revealed distinct separation of life stage groups. More than 2,225 differentially expressed mRNAs were identified. Gene families or pathways that changed across life stages included those related to estrogen and androgen (ESR1, PGR, TFF1, GREB1, AR, 17HSDB2, 17HSDB7, STS, HSD11B1, AKR1C4), prolactin (PRLR, ELF5, STAT5, CSN1S1), insulin-like growth factor signaling (IGF1, IGFBP1, IGFBP5), extracellular matrix (POSTN, TGFB1, COL5A2, COL12A1, FOXC1, LAMC1, PDGFRA, TGFB2), and differentiation (CD24, CD29, CD44, CD61, ALDH1, BRCA1, FOXA1, POSTN, DICER1, LIG4, KLF4, NOTCH2, RIF1, BMPR1A, TGFB2). Pregnancy and lactation displayed distinct patterns of gene expression. ESR1 and IGF1 were significantly higher in the adolescent compared to the adult animals, whereas differentiation pathways were overrepresented in adult animals and pregnancy-associated life stages. Few individual genes were distinctly different in postmenopausal animals. Our data demonstrate characteristic patterns of gene expression during breast development. Several of the pathways activated during pubertal development have been implicated in cancer development and metastasis, supporting the idea that other developmental markers may have application as biomarkers for BC.

Decreased IGF type 1 receptor signaling in mammary epithelium during pregnancy leads to reduced proliferation, alveolar differentiation, and expression of insulin receptor substrate (IRS)-1 and IRS-2

The IGFs and the IGF type 1 receptor (IGF-1R) are essential mediators of normal mammary gland development in mice. IGF-I and the IGF-1R have demonstrated functions in formation and proliferation of terminal end buds and in ductal outgrowth and branching during puberty. To study the functions of IGF-1R during pregnancy and lactation, we established transgenic mouse lines expressing a human dominant-negative kinase dead IGF-1R (dnhIGF-1R) under the control of the whey acidic protein promoter. We provide evidence that the IGF-1R pathway is necessary for normal epithelial proliferation and alveolar formation during pregnancy. Furthermore, we demonstrate that the whey acidic protein-dnhIGF-1R transgene causes a delay in alveolar differentiation including lipid droplet formation, lumen expansion, and β-casein protein expression. Analysis of IGF-1R signaling pathways showed a decrease in P-IGF-1R and P-Akt resulting from expression of the dnhIGF-1R. We further demonstrate that disruption of the IGF-1R decreases mammary epithelial cell expression of the signaling intermediates insulin receptor substrate (IRS)-1 and IRS-2. No alterations were observed in downstream signaling targets of prolactin and progesterone, suggesting that activation of the IGF-1R may directly regulate expression of IRS-1/2 during alveolar development and differentiation. These data show that IGF-1R signaling is necessary for normal alveolar proliferation and differentiation, in part, through induction of signaling intermediates that mediate alveolar development.

The mouse mammary microenvironment redirects mesoderm-derived bone marrow cells to a mammary epithelial progenitor cell fate

Mammary stem cells reside in protected tissue locales (niches), where their reproductive potency remains essentially unchanged through life. Disruption of the tissue leads to a reduced capacity of dispersed epithelial cells to recapitulate complete functional mammary structures. Previous studies demonstrate that during the reformation of mammary stem cell niches by dispersed epithelial cells in the mammary stroma, nonmammary cells of ectodermal germ origin may be sequestered and reprogrammed to perform mammary epithelial cell (MEC) functions, including those ascribed to mammary stem/progenitor cells. To test whether tissue cells from organs derived from different germ layers could respond to mammary epithelial-specific signals, we utilized fluorescence-activated cell sorting-purified Lin(-) and Lin(-)/cKit+adult male bone marrow cells to mix with MECs. Our evidence shows that the signals provided by the mammary microenvironment are capable of redirecting mesoderm-derived adult progenitor cells to produce functional MEC progeny.

Functional characterization of stem cell activity in the mouse mammary gland

Any portion of the mouse mammary gland is capable of recapitulating a clonally derived complete and functional mammary tree upon transplantation into an epithelial divested mammary fat-pad of a recipient host. As such, it is an ideal model tissue for the study somatic stem cell function. This review will outline what is known regarding the function of stem/progenitor cells in the mouse mammary gland, including how progenitor populations can be functionally defined, the evidence for and potential role of selective DNA strand segregation, and the role of the niche in maintaining and controlling stem cell function.

Luminal expression of PIK3CA mutant H1047R in the mammary gland induces heterogeneous tumors

The phosphoinositide 3-kinase (PI3K) signaling cascade, a key mediator of cellular survival, growth, and metabolism, is frequently altered in human cancer. Activating mutations in PIK3CA, which encodes the α-catalytic subunit of PI3K, occur in approximately 30% of breast cancers. These mutations result in constitutive activity of the enzyme and are oncogenic, but it is not known whether they are sufficient to induce mammary carcinomas in mice. In the present study, we show that the expression of mutant PIK3CA H1047R in the luminal mammary epithelium evokes heterogeneous tumors that express luminal and basal markers and are positive for the estrogen receptor. Our results suggest that the PIK3CA H1047R oncogene targets a multipotent progenitor cell and, furthermore, show that this model recapitulates features of human breast tumors with PIK3CA H1047R.

Murine mammary epithelial stem cells: discovery, function, and current status

An entire mammary epithelial outgrowth, capable of full secretory differentiation, may comprise the progeny of a single cellular antecedent, i.e., may be generated from a single mammary epithelial stem cell. Early studies showed that any portion of an intact murine mammary gland containing epithelium could recapitulate an entire mammary epithelial tree on transplantation into an epithelium-free mammary fat pad. More recent studies have shown that a hierarchy of mammary stem/progenitor cells exists among the mammary epithelium and that their behavior and maintenance is dependent on signals generated both locally and systemically. In this review, we have attempted to develop the scientific saga surrounding the discovery and characterization of the murine mammary stem/progenitor cell hierarchy and to suggest further approaches that will enhance our knowledge and understanding of these cells and their role in both normal development and neoplasia.

Role of epithelial stem/progenitor cells in mammary cancer

Both mouse and human mammary glands contain stem/progenitor functional hierarchies that are maintained through the entire life span of the animal. Cells with such functional capacities are potential candidates for tumorigenesis as they are long lived, multipotent, and self-renewing. Using the mouse as a model, this review will discuss what is known about the mammary stem/progenitor hierarchy, the evidence that particular progenitor functions are susceptible to tumorigenic stimuli, how these findings in mice are relevant to the disease in humans, and the role of the local microenvironment in controlling tumorigenesis.

CCAAT/enhancer binding protein beta regulates stem cell activity and specifies luminal cell fate in the mammary gland

The bZIP transcription factor C/EBP beta is important for mammary gland development and its expression is deregulated in human breast cancer. To determine whether C/EBP beta regulates mammary stem cells (MaSCs), we employed two different knockout strategies. Using both a germline and a conditional knockout strategy, we demonstrate that mammosphere formation was significantly decreased in C/EBP beta-deficient mammary epithelial cells (MECs). Functional limiting dilution transplantation assays indicated that the repopulating ability of C/EBP beta-deleted MECs was severely impaired. Serial transplantation experiments demonstrated that C/EBP beta deletion resulted in decreased outgrowth potential and premature MaSC senescence. In accord, fluorescence-activated cell sorting analysis demonstrated that C/EBP beta-null MECs contained fewer MaSCs, the loss of luminal progenitors and an increase in differentiated luminal cells as compared with wild-type. Gene profiling of C/EBP beta-null stem cells revealed an alteration in cell fate specification, exemplified by the expression of basal markers in the luminal compartment. Thus, C/EBP beta is a critical regulator of both MaSC repopulation activity and luminal cell lineage commitment. These findings have critical implications for understanding both stem cell biology and the etiology of different breast cancer subtypes.

Cyclin D1 kinase activity is required for the self-renewal of mammary stem and progenitor cells that are targets of MMTV-ErbB2 tumorigenesis

Transplantation studies have demonstrated the existence of mammary progenitor cells with the ability to self-renew and regenerate a functional mammary gland. Although these progenitors are the likely targets for oncogenic transformation, correlating progenitor populations with certain oncogenic stimuli has been difficult. Cyclin D1 is required for lobuloalveolar development during pregnancy and lactation as well as MMTV-ErbB2- but not MMTV-Wnt1-mediated tumorigenesis. Using a kinase-deficient cyclin D1 mouse, we identified two functional mammary progenitor cell populations, one of which is the target of MMTV-ErbB2. Moreover, cyclin D1 activity is required for the self-renewal and differentiation of mammary progenitors because its abrogation leads to a failure to maintain the mammary epithelial regenerative potential and also results in defects in luminal lineage differentiation.

Amphiregulin mediates self-renewal in an immortal mammary epithelial cell line with stem cell characteristics

Amphiregulin (AREG), a ligand for epidermal growth factor receptor, is required for mammary gland ductal morphogenesis and mediates estrogen actions in vivo, emerging as an essential growth factor during mammary gland growth and differentiation. The COMMA-D beta-geo (CDbetageo) mouse mammary cell line displays characteristics of normal mammary progenitor cells including the ability to regenerate a mammary gland when transplanted into the cleared fat pad of a juvenile mouse, nuclear label retention, and the capacity to form anchorage-independent mammospheres. We demonstrate that AREG is essential for formation of floating mammospheres by CDbetageo cells and that the mitogen activated protein kinase signaling pathway is involved in AREG-mediated mammosphere formation. Addition of exogenous AREG promotes mammosphere formation in cells where AREG expression is knocked down by siRNA and mammosphere formation by AREG(-/-) mammary epithelial cells. AREG knockdown inhibits mammosphere formation by duct-limited mammary progenitor cells but not lobule-limited mammary progenitor cells. These data demonstrate AREG mediates the function of a subset of mammary progenitor cells in vitro.

c-Myc affects mRNA translation, cell proliferation and progenitor cell function in the mammary gland

Background

The oncoprotein c-Myc has been intensely studied in breast cancer and mouse mammary tumor models, but relatively little is known about the normal physiological role of c-Myc in the mammary gland. Here we investigated functions of c-Myc during mouse mammary gland development using a conditional knockout approach.

Results

Generation of c-myc^fl/fl mice carrying the mammary gland-specific WAPiCre transgene resulted in c-Myc loss in alveolar epithelial cells starting in mid-pregnancy. Three major phenotypes were observed in glands of mutant mice. First, c-Myc-deficient alveolar cells had a slower proliferative response at the start of pregnancy, causing a delay but not a block of alveolar development. Second, while milk composition was comparable between wild type and mutant animals, milk production was reduced in mutant glands, leading to slower pup weight-gain. Electron microscopy and polysome fractionation revealed a general decrease in translational efficiency. Furthermore, analysis of mRNA distribution along the polysome gradient demonstrated that this effect was specific for mRNAs whose protein products are involved in milk synthesis. Moreover, quantitative reverse transcription-polymerase chain reaction analysis revealed decreased levels of ribosomal RNAs and ribosomal protein-encoding mRNAs in mutant glands. Third, using the mammary transplantation technique to functionally identify alveolar progenitor cells, we observed that the mutant epithelium has a reduced ability to repopulate the gland when transplanted into NOD/SCID recipients.

Conclusion

We have demonstrated that c-Myc plays multiple roles in the mouse mammary gland during pregnancy and lactation. c-Myc loss delayed, but did not block proliferation and differentiation in pregnancy. During lactation, lower levels of ribosomal RNAs and proteins were present and translation was generally decreased in mutant glands. Finally, the transplantation studies suggest a role for c-Myc in progenitor cell proliferation and/or survival.

See related minireview by Evan et al:

Wnt signaling can substitute for estrogen to induce division of ERalpha-positive cells in a mouse mammary tumor model

The interaction of estrogen with the estrogen receptor (ER, principally ERalpha) induces growth of human breast tumor cells. In contrast, ERalpha-positive cells have been described as non-dividing cells in normal breast (though estrogen stimulation of ERalpha cells directs the division of neighboring cells). However, there is a small sub-population of cells in normal mammary tissue that are ERalpha-positive, that can divide, and therefore share this property with human breast tumor cells. In order to investigate their pattern of growth regulation, we measured the fraction of dividing ERalpha(+) cells during normal growth and compared that to glands stimulated by oncogenic Wnt effectors. First, we found there was no difference between the rate of division of ERalpha(+) cells and ERalpha(-) cells, whether the population was responding to estrogen or Wnt mitogens. The proportion of dividing ERalpha(+) mammary epithelial cells was increased (10x) in response to pregnancy, and similar increases were observed in response to ectopic Wnt signaling. We propose that Wnt signaling can substitute for estrogen to drive total population growth (that includes ERalpha(+) cells). Although the E-ERalpha-derived mitogenic response is situated in a minority of the luminal cells, and the Wnt-LRP5/6-derived mitogenic response is situated in a minority of basal cells, overall, the growth response of the mammary epithelial population is remarkably similar.

Reprogramming cell fates in the mammary microenvironment

The capacity of any portion of the murine mammary gland to produce a complete functional mammary outgrowth upon transplantation to an epithelium-divested fat pad is unaffected by the age or reproductive history of the donor. Likewise, through serial transplantations, no loss of potency is detected when compared to similar transplantations of the youngest mammary tissue tested. This demonstrates that stem cell activity is maintained intact throughout the lifetime of the animal despite aging and the repeated expansion and depletion of the mammary epithelium through multiple rounds of pregnancy, lactation and involution. These facts support the contention that mammary stem cells reside in protected tissue locales (niches), where their reproductive potency remains essentially unchanged through life. Disruption of the tissue, to produce dispersed cells results in the desecration of the protection afforded by the "niche" and leads to a reduced capacity of dispersed epithelial cells (in terms of the number transplanted) to recapitulate complete functional mammary structures. Our studies demonstrate that during the reformation of mammary stem cell niches by dispersed epithelial cells in the context of the intact epithelium-free mammary stroma, non-mammary cells may be sequestered and reprogrammed to perform mammary epithelial cell functions including those ascribed to mammary stem/progenitor cells.

Detection and analysis of mammary gland stem cells

Emerging evidence from a variety of tissue types, including the mammary gland, suggests that normal stem and progenitor cells are the likely targets for malignant transformation, and that these transformed cells can function as cancer stem cells that drive tumour growth. In order to develop therapies that target these cancer stem cells, it is essential to determine the molecular mechanisms that regulate the growth and differentiation of these cells and their normal counterparts. To this end, a number of quantitative robust clonal assays have been developed that can detect the presence of human and mouse mammary stem and progenitor cells. These assays, when used in conjunction with cell-sorting strategies, have permitted the prospective isolation and characterization of a variety of cell types, including stem cells. Evidence to date indicates that these stem cells exhibit properties of basal mammary cells, possess extensive self-renewal properties, and are capable of generating a large number of phenotypically-distinct progenitor cells, many of which display characteristics of luminal cells. This review article will focus on the assays used to detect mammary stem and progenitor cells, some of the properties of these cells and their progeny and how they relate to the cancer stem cells that drive breast tumour growth.

Stem cells and the mammary microenvironment

An entire mammary epithelial outgrowth, capable of full secretory differentiation, may comprise the progeny of a single cellular antecedent. This conclusion is based upon the maintenance of retroviral insertion sites within the somatic DNA of successive transplant generations derived from a single mammary fragment. In addition, dissociation of these clonal dominant glands and implantation of dispersed cells at limiting dilution demonstrated that both duct-limited and lobule-limited outgrowths were developed as well as complete, fully differentiated glands. Thus, transplantation has revealed three distinct mammary epithelial progenitors in the mouse. Recently, using cre-lox conditional activation of reporter genes, the lobule-limited progenitor was lineally marked by lacZ expression. In situ, these cells were shown to regenerate secretory lobules upon successive pregnancies. In transplant studies, they demonstrated the capacity for self- renewal and contributed to the new generation of all of the epithelial cell types among mammary secretory lobules. Using this conditional activation model, cells isolated from other tissues of the WAP-Cre/Rosa26/lacZReporter mice, co-mingled with normal wild type mammary epithelial cells and transplanted into epithelium-divested mammary fat pads, were shown to be amenable to redirection of their cell fate by interaction with the mammary microenvironment in vivo. This suggests the ascendancy of the microenvironment over the intrinsic nature of somatic stem cells.

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

The Pea3 Ets transcription factor is overexpressed in breast tumors suggesting that it plays a role in mammary oncogenesis. However, the normal biological function of Pea3 in the mammary gland is not known. Here we report that Pea3 was expressed in the epithelium of the mouse mammary anlagen commensurate with their genesis, and at later times in the nipple and mammary ducts of female embryos. In adult mice Pea3 transcripts peaked at the onset of puberty and early pregnancy, times of active epithelial cell proliferation and differentiation. Pea3 was expressed in all progenitor cap cells and rare body cells of terminal end buds, and in the myoepithelial cells of ducts and alveoli. Analyses of the mammary glands of Pea3-null mice during puberty revealed an increased number of terminal end buds and an increased fraction of proliferating progenitor cells within these structures compared to their wild type littermates. Tissue transplant experiments demonstrated that these phenotypes were intrinsic to the Pea3-null mammary epithelium. During pregnancy, mammary glands isolated from Pea3-null females had impaired alveolar development as revealed by a decreased fraction of alveolar structures. We performed in vitro colony forming assays of mammary epithelial cells and discovered that loss of Pea3 altered the distribution of specific multipotent progenitor cells. Double-immunofluorescence confirmed that multipotential progenitors co-expressing markers of the myoepithelial and luminal epithelial lineage were amplified in the mammary glands of Pea3-null mice by comparison to their wild type counterparts. We propose that Pea3 functions in multipotential progenitors to regulate their lineage-specific differentiation potential.

The mammary microenvironment alters the differentiation repertoire of neural stem cells

A fundamental issue in stem cell biology is whether adult somatic stem cells are capable of accessing alternate tissue sites and continue functioning as stem cells in the new microenvironment. To address this issue relative to neurogenic stem cells in the mouse mammary gland microenvironment, we mixed wild-type mammary epithelial cells (MECs) with bona fide neural stem cells (NSCs) isolated from WAP-Cre/Rosa26R mice and inoculated them into cleared fat pads of immunocompromised females. Hosts were bred 6-8 weeks later and examined postinvolution. This allowed for mammary tissue growth, transient activation of the WAP-Cre gene, recombination, and constitutive expression of LacZ. The NSCs and their progeny contributed to mammary epithelial growth during ductal morphogenesis, and the Rosa26-LacZ reporter gene was activated by WAP-Cre expression during pregnancy. Some NSC-derived LacZ(+) cells expressed mammary-specific functions, including milk protein synthesis, whereas others adopted myoepithelial cell fates. Thus, NSCs and their progeny enter mammary epithelium-specific niches and adopt the function of similarly endowed mammary cells. This result supports the conclusion that tissue-specific signals emanating from the stroma and from the differentiated somatic cells of the mouse mammary gland can redirect the NSCs to produce cellular progeny committed to MEC fates.

Premalignant and malignant mammary lesions induced by MMTV and chemical carcinogens

This article reviews the types of mammary premalignant and malignant lesions induced in the mouse mammary gland after exposure to MMTV and chemical carcinogens. There are different morphological types of hyperplastic lesions in the mouse mammary gland. These lesions are designated as alveolar hyperplasia (HAN), ductal hyperplasia (DH), and cystic lesions; both non-keratinized and keratinized. The HAN and DH have been demonstrated to be precursors to invasive lesions. The HAN as a group tend to be ovarian-hormone independent for growth and transformation, mammary fat pad site-dependent for growth, possess unlimited replication potential (i.e., immortal), are at increased risk for progression to cancer, and are genetically stable. Serial transplantation demonstrates that any given stage of hyperplasia can progress to the next stage, sometimes rapidly and sometimes slowly. Multiple growth factor pathways are deregulated early in the development of HAN depending on etiology of the HAN. One general conclusion that emerges from such studies of HAN induced by different etiologies is that the early stages of premalignancy are a result of defects in cell cycle regulation with subsequent alterations playing a role in the acquisition of invasive phenotype. The predominant lesion induced by chemical carcinogens is the ductal hyperplasia (DH). Although DH show many of the essential biological alterations seen in HAN, they also exhibit a higher frequency of hormone-dependence and genetic instability, thus the DH appearing in chemical carcinogen treated mice and in transgenic mice mimic the histological, biological and genetic properties seen in human premalignant lesions more faithfully than do the HAN. The mammary tumors that arise from both general classes of premalignant lesions are very heterogeneous and exhibit different potentials for metastasis. The cell and molecular biology of metastasis represents an understudied area, in part because of the absence of suitable models to study the metastatic process. Newer transgenic mouse models provide a renewed opportunity to engage in the study of the mechanisms and processes underlying mammary metastasis.

Minireview: hormones and mammary cell fate--what will I become when I grow up?

Systemic hormones are key regulators of postnatal mammary gland development and play an important role in the etiology and treatment of breast cancer. Mammary ductal morphogenesis is controlled by circulating hormones, and these same hormones are also critical mediators of mammary stem cell fate decisions. Recent studies have helped further our understanding of the origin, specification, and fate of mammary stem cells during postnatal development. Here we review recent studies on the involvement of hormone receptors and several transcription factors in mammary stem/progenitor cell differentiation and lineage commitment.