Isolation of mouse mammary epithelial progenitor cells with basal characteristics from the Comma-Dbeta cell line

Designing Organoid Models to Monitor Cancer Progression, Plasticity and Resistance: The Right Set Up for the Right Question

The recent trend in 3D cell modeling has fostered the emergence of a wide range of models, addressing very distinct goals ranging from the fundamental exploration of cell–cell interactions to preclinical assays for personalized medicine. It is clear that no single model will recapitulate the complexity and dynamics of in vivo situations. The key is to define the critical points, achieve a specific goal and design a model where they can be validated. In this report, we focused on cancer progression. We describe our model which is designed to emulate breast carcinoma progression during the invasive phase. We chose to provide topological clues to the target cells by growing them on microsupports, favoring a polarized epithelial organization before they are embedded in a 3D matrix. We then watched for cell organization and differentiation for these models, adding stroma cells then immune cells to follow and quantify cell responses to drug treatment, including quantifying cell death and viability, as well as morphogenic and invasive properties. We used model cell lines including Comma Dβ, MCF7 and MCF10A mammary epithelial cells as well as primary breast cancer cells from patient-derived xenografts (PDX). We found that fibroblasts impacted cell response to Docetaxel and Palbociclib. We also found that NK92 immune cells could target breast cancer cells within the 3D configuration, providing quantitative monitoring of cell cytotoxicity. We also tested several sources for the extracellular matrix and selected a hyaluronan-based matrix as a promising alternative to mouse tumor basement membrane extracts for primary human cancer cells. Overall, we validated a new 3D model designed for breast cancer for preclinical use in personalized medicine.

Single-cell analysis reveals Comma-1D as a unique cell model for mammary gland development and breast cancer

The mammary epithelial tree contains two distinct populations, luminal and basal. The investigation of how this heterogeneity is developed and how it influences tumorigenesis has been hampered by the need to perform these studies using animal models. Comma-1D is an immortalized mouse mammary epithelial cell line that has unique morphogenetic properties. By performing single-cell RNA-seq studies we found that Comma-1D cultures consist of two main populations with luminal and basal features and a smaller population with mixed lineage and bipotent characteristics. We demonstrated that multiple transcription factors associated with the differentiation of the mammary epithelium in vivo also modulate this process in Comma-1D cultures. Additionally, we found that only cells with luminal features were able to acquire transformed characteristics after an oncogenic HER2 mutant was introduced in their genomes. Overall, our studies characterize at a single-cell level the heterogeneity of the Comma-1D cell line and illustrate how Comma-1D cells can be used as an experimental model to study both the differentiation and the transformation processes in vitro.

Inhibitor of Differentiation 4 (ID4) represses mammary myoepithelial differentiation via inhibition of HEB

Inhibitor of differentiation (ID) proteins dimerize with basic HLH (bHLH) transcription factors, repressing transcription of lineage-specification genes across diverse cellular lineages. ID4 is a key regulator of mammary stem cells; however, the mechanism by which it achieves this is unclear. Here, we show that ID4 has a cell autonomous role in preventing myoepithelial differentiation of basal cells in mammary organoids and in vivo. ID4 positively regulates proliferative genes and negatively regulates genes involved in myoepithelial function. Mass spectrometry reveals that ID4 interacts with the bHLH protein HEB, which binds to E-box motifs in regulatory elements of basal developmental genes involved in extracellular matrix and the contractile cytoskeleton. We conclude that high ID4 expression in mammary basal stem cells antagonizes HEB transcriptional activity, preventing myoepithelial differentiation and allowing for appropriate tissue morphogenesis. Downregulation of ID4 during pregnancy modulates gene regulated by HEB, promoting specialization of basal cells into myoepithelial cells.

Recent Advances in Experimental Models of Breast Cancer Exosome Secretion, Characterization and Function

Breast cancer (BC) is responsible for 15% of all the cancer deaths among women in the USA. The tumor microenvironment (TME) has the potential to act as a driver of breast cancer progression and metastasis. The TME is composed of stromal cells within an extracellular matrix and soluble cytokines, chemokines and extracellular vesicles and nanoparticles that actively influence cell behavior. Extracellular vesicles include exosomes, microvesicles and large oncosomes that orchestrate fundamental processes during tumor progression through direct interaction with target cells. Long before tumor cell spread to future metastatic sites, tumor-secreted exosomes enter the circulation and establish distant pre-metastatic niches, hospitable and permissive milieus for metastatic colonization. Emerging evidence suggests that breast cancer exosomes promote tumor progression and metastasis by inducing vascular leakiness, angiogenesis, invasion, immunomodulation and chemoresistance. Exosomes are found in almost all physiological fluids including plasma, urine, saliva, and breast milk, providing a valuable resource for the development of non-invasive cancer biomarkers. Here, we review work on the role of exosomes in breast cancer progression and metastasis, and describe the most recent advances in models of exosome secretion, isolation, characterization and functional analysis. We highlight the potential applications of plasma-derived exosomes as predictive biomarkers for breast cancer diagnosis, prognosis and therapy monitoring. We finally describe the therapeutic approaches of exosomes in breast cancer.

TGF-β regulates Sca-1 expression and plasticity of pre-neoplastic mammary epithelial stem cells

The epithelial-mesenchymal plasticity, in tight association with stemness, contributes to the mammary gland homeostasis, evolution of early neoplastic lesions and cancer dissemination. Focused on cell surfaceome, we used mouse models of pre-neoplastic mammary epithelial and cancer stem cells to reveal the connection between cell surface markers and distinct cell phenotypes. We mechanistically dissected the TGF-β family-driven regulation of Sca-1, one of the most commonly used adult stem cell markers. We further provided evidence that TGF-β disrupts the lineage commitment and promotes the accumulation of tumor-initiating cells in pre-neoplastic cells.

VL30 retrotransposition is associated with induced EMT, CSC generation and tumorigenesis in HC11 mouse mammary stem‑like epithelial cells

Retrotransposons copy their sequences via an RNA intermediate, followed by reverse transcription into cDNA and random insertion, into a new genomic locus. New retrotransposon copies may lead to cell transformation and/or tumorigenesis through insertional mutagenesis. Methylation is a major defense mechanism against retrotransposon RNA expression and retrotransposition in differentiated cells, whereas stem cells are relatively hypo-methylated. Epithelial-to-mesenchymal transition (EMT), which transforms normal epithelial cells into mesenchymal-like cells, also contributes to tumor progression and tumor metastasis. Cancer stem cells (CSCs), a fraction of undifferentiated tumor-initiating cancer cells, are reciprocally related to EMT. In the present study, the outcome of long terminal repeat (LTR)-Viral-Like 30 (VL30) retrotransposition was examined in mouse mammary stem-like/progenitor HC11 epithelial cells. The transfection of HC11 cells with a VL30 retrotransposon, engineered with an EGFP-based retrotransposition cassette, elicited a higher retrotransposition frequency in comparison to differentiated J3B1A and C127 mouse mammary cells. Fluorescence microscopy and PCR analysis confirmed the specificity of retrotransposition events. The differentiated retrotransposition-positive cells retained their epithelial morphology, while the respective HC11 cells acquired mesenchymal features associated with the loss of E-cadherin, the induction of N-cadherin, and fibronectin and vimentin protein expression, as well as an increased transforming growth factor (TGF)-β1, Slug, Snail-1 and Twist mRNA expression. In addition, they were characterized by cell proliferation in low serum, and the acquisition of CSC-like properties indicated by mammosphere formation under anchorage-independent conditions. Mammospheres exhibited an increased Nanog and Oct4 mRNA expression and a CD44⁺/CD24^−/low antigenic phenotype, as well as self-renewal and differentiation capacity, forming mammary acini-like structures. DNA sequencing analysis of retrotransposition-positive HC11 cells revealed retrotransposed VL30 copies integrated at the vicinity of EMT-, cancer type- and breast cancer-related genes. The inoculation of these cells into Balb/c mice produced cytokeratin-positive tumors containing pancytokeratin-positive cells, indicative of cell invasion features. On the whole, the findings of the present study demonstrate, for the first time, to the best of our knowledge, that stem-like epithelial HC11 cells are amenable to VL30 retrotransposition associated with the induction of EMT and CSC generation, leading to tumorigenesis.

A clonal stem cell line established from a mouse mammary placode with ability to generate functional mammary glands

The mammary gland develops from the placode at ectodermal invagination. The rudimentary parenchyma (mammary bud) develops mammary trees and alveolar structures, suggesting that the mammary bud consists of stem/progenitor cells. Here, we established a clonal stem cell line from a mammary bud of a p53 null female embryo at day 14.5. FP5-3-1 line was a homogeneous cell population with polygonal epithelial morphology and spontaneously became heterogeneous during passages. Recloning gave rise to four sublines; three sublines have basal epithelial property and one subline has luminal epithelial property. The former sublines generate functional mammary glands when injected into cleared fat pads and the latter subline does not. The cell lines also express many stemness-related genes. The clonal cell lines established in the present study are shown to be mammary stem cells and not tumorigenic. They provide useful models for normal and tumor biology of the mammary gland in vivo and in vitro.

Real-time imaging of integrin β4 dynamics using a reporter cell line generated by Crispr/Cas9 genome editing

The ability to monitor changes in the expression and localization of integrins is essential for understanding their contribution to development, tissue homeostasis and disease. Here, we pioneered the use of Crispr/Cas9 genome editing to tag an allele of the β4 subunit of the α6β4 integrin. A tdTomato tag was inserted with a linker at the C-terminus of integrin β4 in mouse mammary epithelial cells. Cells harboring this tagged allele were similar to wild-type cells with respect to integrin β4 surface expression, association with the α6 subunit, adhesion to laminin and consequent signaling. These integrin β4 reporter cells were transformed with YAP (also known as YAP1), which enabled us to obtain novel insight into integrin β4 dynamics in response to a migratory stimulus (scratch wound) by live-cell video microscopy. An increase in integrin β4 expression in cells proximal to the wound edge was evident, and a population of integrin β4-expressing cells that exhibited unusually rapid migration was identified. These findings could shed insight into integrin β4 dynamics during invasion and metastasis. Moreover, these integrin β4 reporter cells should facilitate studies on the contribution of this integrin to mammary gland biology and cancer.This article has an associated First Person interview with the first author of the paper.

Delivery of biologically active miR-34a in normal and cancer mammary epithelial cells by synthetic nanoparticles

Functional RNAs, such as microRNAs, are emerging as innovative tools in the treatment of aggressive and incurable cancers. In this study, we explore the potential of silica dioxide nanoparticles (SiO₂NPs) in the delivery of biologically active miRNAs. Focusing on the tumor-suppressor miR-34a, we evaluated miRNAs delivery by SiO₂NPs into the mammary gland, using in vitro as well as in vivo model systems. We showed that silica nanoparticles can efficiently deliver miR-34a into normal and cancer epithelial cells grown in culture without major signs of toxicity. Delivered miRNA retained the ability to silence artificial as well endogenous targets and can reduce the growth of mammospheres in 3D culture. Finally, miR-34a delivery through intra-tumor administration of SiO₂NPs leads to a reduced mammary tumor growth. In conclusion, our studies suggest that silica nanoparticles can mediate the delivery of miR-34a directly into mammary tumors while preserving its molecular and biological activity.

Microarray and pathway analysis of two COMMA-Dβ derived clones reveal important differences relevant to their developmental capacity in-vivo

Microarray technologies were used to analyze transcriptomes from Comma-Dβ and clonal derivatives, SP3 (Lobule-competent) and NSP2 (Lobule-incompetent), during different mouse mammary growth phases: in-vitro, in-vivo 5-weeks, and in-vivo 12-weeks. A differentially expressed gene (DEG) algorithm was used to enrich for genes associated with cellular proliferation, differentiation, cell cycle regulation, and carcinogenesis. A pairwise comparison analysis, of SP3 vs. NSP2 in-vitro, revealed a total of 45 DEGs significantly up-regulated in SP3. Of the 45 DEGs, only Ccnd1 (Cyclin D1), Id2 (Inhibitor of DNA binding 2) and Sox9 (SRY Box 9) were identified to be associated with cellular proliferation, regulation of G1/S mitotic cell cycle, mammary gland and alveolar development in SP3. During the regenerative growth phase, in-vivo 5-weeks, we identified a total of 545 DEGs. 308 DEGs, of the 545 DEGs, were significantly up-regulated and 237 DEGs were significantly down-regulated in SP3 vs. NSP2. In addition, we identified 9 DEGs significantly up-regulated, within SP3's cell cycle pathway and a persistent overexpression of Cyclin D1, Id2, and Sox9, consistent with our in-vitro study. During the maintenance phase, in-vivo 12-weeks, we identified 407 DEGs. Of these, 336 DEGs were up-regulated, and 71 were down-regulated in SP3 vs. NSP2. Our data shows 15 DEGs significantly up-regulated, simultaneously, affecting 8 signal transducing carcinogenic pathways. In conclusion, increased expression of Cyclin D1, Id2 and Sox9 appear to be important for lobular genesis in SP3. Also, in-vivo 12 week displays increase expression of genes and pathways, involved in tumorigenesis.

Dual role for miR-34a in the control of early progenitor proliferation and commitment in the mammary gland and in breast cancer

The role of the tumour-suppressor miR-34 family in breast physiology and in mammary stem cells (MaSCs) is largely unknown. Here, we revealed that miR-34 family, and miR-34a in particular, is implicated in mammary epithelium homoeostasis. Expression of miR-34a occurs upon luminal commitment and differentiation and serves to inhibit the expansion of the pool of MaSCs and early progenitor cells, likely in a p53-independent fashion. Mutant mice (miR34-KO) and loss-of-function approaches revealed two separate functions of miR-34a, controlling both proliferation and fate commitment in mammary progenitors by modulating several pathways involved in epithelial cell plasticity and luminal-to-basal conversion. In particular, miR-34a acts as endogenous inhibitor of the Wnt/beta-catenin signalling pathway, targeting up to nine upstream regulators at the same time, thus modulating the expansion of the MaSCs/early progenitor pool. These multiple roles of miR-34a are maintained in a model of human breast cancer, in which chronic expression of miR-34a in triple-negative mesenchymal-like cells (enriched in cancer stem cells—CSCs) could promote a luminal-like differentiation programme, restrict the CSC pool, and inhibit tumour propagation. Hence, activation of miR-34a-dependent programmes could provide a therapeutic opportunity for the subset of breast cancers, which are rich in CSCs and respond poorly to conventional therapies.

A comprehensive bioinformatics analysis on multiple Gene Expression Omnibus datasets of nonalcoholic fatty liver disease and nonalcoholic steatohepatitis

Fatty liver disease is one of the leading causes of chronic damage in western countries. Approximately 25% of adults in the United States have fatty livers in the absence of excessive alcohol consumption, a condition termed nonalcoholic fatty liver disease (NAFLD). Little is known about the prevalence and genetic background of NAFLD or the factors that determine its development. In this study, we used the Gene-Cloud of Biotechnology Information bioinformatics platform to carry out a comprehensive bioinformatics analysis identifying differentially expressed genes (DEGs), key biological processes and intersecting pathways. We imported 3 Gene Expression Omnibus datasets (GSE31803, GSE49541, and GSE63067). Then, we assessed the expression of the DEGs in clinical samples. We found that CD24 was the only gene co-expressed in all 3 datasets. "Glycolysis/gluconeogenesis", "p53 signaling pathway" and "glycine, serine and threonine metabolism" were 3 common pathways related to the fatty liver process. In NAFLD tissues, CD24, COL1A1, LUM, THBS2 and EPHA3 were upregulated, and PZP was downregulated. CD24 is a core gene among these DEGs and have not yet been studied of its impact on NAFLD. Co-expressed genes, common biological processes and intersecting pathways identified in the study might play an important role in NAFLD progression. Further studies are needed to elucidate the mechanism of these potential genes and pathways in NAFLD.

Blimp-1/PRDM1 is a critical regulator of Type III Interferon responses in mammary epithelial cells

The transcriptional repressor Blimp-1 originally cloned as a silencer of type I interferon (IFN)-β gene expression controls cell fate decisions in multiple tissue contexts. Conditional inactivation in the mammary gland was recently shown to disrupt epithelial cell architecture. Here we report that Blimp-1 regulates expression of viral defense, IFN signaling and MHC class I pathways, and directly targets the transcriptional activator Stat1. Blimp-1 functional loss in 3D cultures of mammary epithelial cells (MECs) results in accumulation of dsRNA and expression of type III IFN-λ. Cultures treated with IFN lambda similarly display defective lumen formation. These results demonstrate that type III IFN-λ profoundly influences the behavior of MECs and identify Blimp-1 as a critical regulator of IFN signaling cascades.

Suppression of protein tyrosine phosphatase N23 predisposes to breast tumorigenesis via activation of FYN kinase

Zhang et al. identified PTPN23 as a suppressor of cell motility and invasion in mammary epithelial and breast cancer cells. They validated the underlying mechanism of PTPN23 function in breast tumorigenesis as that of a key phosphatase that normally suppresses the activity of FYN in two different models.

Disruption of the balanced modulation of reversible tyrosine phosphorylation has been implicated in the etiology of various human cancers, including breast cancer. Protein Tyrosine Phosphatase N23 (PTPN23) resides in chromosomal region 3p21.3, which is hemizygously or homozygously lost in some breast cancer patients. In a loss-of-function PTPome screen, our laboratory identified PTPN23 as a suppressor of cell motility and invasion in mammary epithelial and breast cancer cells. Now, our TCGA (The Cancer Genome Atlas) database analyses illustrate a correlation between low PTPN23 expression and poor survival in breast cancers of various subtypes. Therefore, we investigated the tumor-suppressive function of PTPN23 in an orthotopic transplantation mouse model. Suppression of PTPN23 in Comma 1Dβ cells induced breast tumors within 56 wk. In PTPN23-depleted tumors, we detected hyperphosphorylation of the autophosphorylation site tyrosine in the SRC family kinase (SFK) FYN as well as Tyr142 in β-catenin. We validated the underlying mechanism of PTPN23 function in breast tumorigenesis as that of a key phosphatase that normally suppresses the activity of FYN in two different models. We demonstrated that tumor outgrowth from PTPN23-deficient BT474 cells was suppressed in a xenograft model in vivo upon treatment with AZD0530, an SFK inhibitor. Furthermore, double knockout of FYN and PTPN23 via CRISPR/CAS9 also attenuated tumor outgrowth from PTPN23 knockout Cal51 cells. Overall, this mechanistic analysis of the tumor-suppressive function of PTPN23 in breast cancer supports the identification of FYN as a therapeutic target for breast tumors with heterozygous or homozygous loss of PTPN23.

Slug/Pcad pathway controls epithelial cell dynamics in mammary gland and breast carcinoma

Mammary gland morphogenesis results from the coordination of proliferation, cohort migration, apoptosis and stem/progenitor cell dynamics. We showed earlier that the transcription repressor Slug is involved in these functions during mammary tubulogenesis. Slug is expressed by a subpopulation of basal epithelial cells, co-expressed with P-cadherin (Pcad). Slug-knockout mammary glands showed excessive branching, similarly to Pcad-knockout. Here, we found that Slug unexpectedly binds and activates Pcad promoter through E-boxes, inducing Pcad expression. We determined that Pcad can mediate several functions of Slug: Pcad promoted clonal mammosphere growth, basal epithelial differentiation, cell-cell dissociation and cell migration, rescuing Slug depletion. Pcad also promoted cell migration in isolated cells, in association with Src activation, focal adhesion reorganization and cell polarization. Pcad, similarly to Slug, was required for in vitro 3D tubulogenesis. Therefore, Pcad appears to be responsible for epithelial-mesenchymal transition-linked plasticity in mammary epithelial cells. In addition, we found that genes from the Slug/Pcad pathway components were co-expressed and specifically correlated in human breast carcinomas subtypes, carrying pathophysiological significance.

The Role of Integrin α6 (CD49f) in Stem Cells: More than a Conserved Biomarker

Stem cells have the capacity for self-renewal and differentiation into specialized cells that form and repopulated all tissues and organs, from conception to adult life. Depending on their capacity for differentiation, stem cells are classified as totipotent (ie, zygote), pluripotent (ie, embryonic stem cells), multipotent (ie, neuronal stem cells, hematopoietic stem cells, epithelial stem cells, etc.), and unipotent (ie, spermatogonial stem cells). Adult or tissue-specific stem cells reside in specific niches located in, or nearby, their organ or tissue of origin. There, they have microenvironmental support to remain quiescent, to proliferate as undifferentiated cells (self-renewal), and to differentiate into progenitors or terminally differentiated cells that migrate from the niche to perform specialized functions. The presence of proteins at the cell surface is often used to identify, classify, and isolate stem cells. Among the diverse groups of cell surface proteins used for these purposes, integrin α6, also known as CD49f, may be the only biomarker commonly found in more than 30 different populations of stem cells, including some cancer stem cells. This broad expression among stem cell populations indicates that integrin α6 may play an important and conserved role in stem cell biology, which is reaffirmed by recent demonstrations of its role maintaining self-renewal of pluripotent stem cells and breast and glioblastoma cancer stem cells. Therefore, this review intends to highlight and synthesize new findings on the importance of integrin α6 in stem cell biology.

Diversity of Cnidarian Muscles: Function, Anatomy, Development and Regeneration

The ability to perform muscle contractions is one of the most important and distinctive features of eumetazoans. As the sister group to bilaterians, cnidarians (sea anemones, corals, jellyfish, and hydroids) hold an informative phylogenetic position for understanding muscle evolution. Here, we review current knowledge on muscle function, diversity, development, regeneration and evolution in cnidarians. Cnidarian muscles are involved in various activities, such as feeding, escape, locomotion and defense, in close association with the nervous system. This variety is reflected in the large diversity of muscle organizations found in Cnidaria. Smooth epithelial muscle is thought to be the most common type, and is inferred to be the ancestral muscle type for Cnidaria, while striated muscle fibers and non-epithelial myocytes would have been convergently acquired within Cnidaria. Current knowledge of cnidarian muscle development and its regeneration is limited. While orthologs of myogenic regulatory factors such as MyoD have yet to be found in cnidarian genomes, striated muscle formation potentially involves well-conserved myogenic genes, such as twist and mef2. Although satellite cells have yet to be identified in cnidarians, muscle plasticity (e.g., de- and re-differentiation, fiber repolarization) in a regenerative context and its potential role during regeneration has started to be addressed in a few cnidarian systems. The development of novel tools to study those organisms has created new opportunities to investigate in depth the development and regeneration of cnidarian muscle cells and how they contribute to the regenerative process.

A Quiescent Bcl11b High Stem Cell Population Is Required for Maintenance of the Mammary Gland

Stem cells in many tissues sustain themselves by entering a quiescent state to avoid genomic insults and to prevent exhaustion caused by excessive proliferation. In the mammary gland, the identity and characteristics of quiescent epithelial stem cells are not clear. Here, we identify a quiescent mammary epithelial cell population expressing high levels of Bcl11b and located at the interface between luminal and basal cells. Bcl11b^high cells are enriched for cells that can regenerate mammary glands in secondary transplants. Loss of Bcl11b leads to a Cdkn2a-dependent exhaustion of ductal epithelium and loss of epithelial cell regenerative capacity. Gain- and loss-of-function studies show that Bcl11b induces cells to enter the G₀ phase of the cell cycle and become quiescent. Taken together, these results suggest that Bcl11b acts as a central intrinsic regulator of mammary epithelial stem cell quiescence and exhaustion and is necessary for long-term maintenance of the mammary gland.

ΔNp63α expression induces loss of cell adhesion in triple-negative breast cancer cells

Background

p63, a member of the p53 protein family, plays key roles in epithelial development and carcinogenesis. In breast cancer, p63 expression has been found predominantly in basal-A (epithelial-type) triple-negative breast carcinomas (TNBC). To investigate the functional role of p63 in basal-A TNBC, we created MDA-MB-468 cell lines with inducible expression of the two major N-terminal p63 isoforms, TAp63α and ∆Np63α.

Results

TAp63α did not have significant effect on gene expression profile and cell phenotype, whilst the main effect of ΔNp63α was reduction of cell adhesion. Gene expression profiling revealed genes involved in cell adhesion and migration whose expression relies on overexpression of ΔNp63α. Reduced cell adhesion also led to decreased cell proliferation in vitro and in vivo. Similar data were obtained in another basal-A cell line, BT-20, but not in BT-549 basal-B (mesenchymal-like) TNBC cells.

Conclusions

In basal-A TNBC cells, ∆Np63α has much stronger effects on gene expression than TAp63α. Although p63 is mentioned mostly in connection with breast cell differentiation and stem cell regulation, we showed that a major effect of p63 is regulation of cell adhesion, a process important in metastasis and invasion of tumour cells. That this effect is not seen in mesenchymal-type TNBC cells suggests lineage-dependent functions, mirroring the expression of ∆Np63α in primary human breast cancers.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2808-x) contains supplementary material, which is available to authorized users.

Immobilized WNT Proteins Act as a Stem Cell Niche for Tissue Engineering

The timing, location, and level of WNT signaling are highly regulated during embryonic development and for the maintenance of adult tissues. Consequently the ability to provide a defined and directed source of WNT proteins is crucial to fully understand its role in tissue development and to mimic its activity in vitro. Here we describe a one-step immobilization technique to covalently bind WNT3A proteins as a basal surface with easy storage and long-lasting activity. We show that this platform is able to maintain adult and embryonic stem cells while also being adaptable for 3D systems. Therefore, this platform could be used for recapitulating specific stem cell niches with the goal of improving tissue engineering.

Ratiometric Array of Conjugated Polymers-Fluorescent Protein Provides a Robust Mammalian Cell Sensor

Supramolecular complexes of a family of positively charged conjugated polymers (CPs) and green fluorescent protein (GFP) create a fluorescence resonance energy transfer (FRET)-based ratiometric biosensor array. Selective multivalent interactions of the CPs with mammalian cell surfaces caused differential change in FRET signals, providing a fingerprint signature for each cell type. The resulting fluorescence signatures allowed the identification of 16 different cell types and discrimination between healthy, cancerous, and metastatic cells, with the same genetic background. While the CP-GFP sensor array completely differentiated between the cell types, only partial classification was achieved for the CPs alone, validating the effectiveness of the ratiometric sensor. The utility of the biosensor was further demonstrated in the detection of blinded unknown samples, where 121 of 128 samples were correctly identified. Notably, this selectivity-based sensor stratified diverse cell types in minutes, using only 2000 cells, without requiring specific biomarkers or cell labeling.

A new role of SNAI2 in postlactational involution of the mammary gland links it to luminal breast cancer development

Breast cancer is a major cause of mortality in women. The transcription factor SNAI2 has been implicated in the pathogenesis of several types of cancer, including breast cancer of basal origin. Here we show that SNAI2 is also important in the development of breast cancer of luminal origin in MMTV-ErbB2 mice. SNAI2 deficiency leads to longer latency and fewer luminal tumors, both of these being characteristics of pretumoral origin. These effects were associated with reduced proliferation and a decreased ability to generate mammospheres in normal mammary glands. However, the capacity to metastasize was not modified. Under conditions of increased ERBB2 oncogenic activity after pregnancy plus SNAI2 deficiency, both pretumoral defects-latency and tumor load-were compensated. However, the incidence of lung metastases was dramatically reduced. Furthermore, SNAI2 was required for proper postlactational involution of the breast. At 3 days post lactational involution, the mammary glands of Snai2-deficient mice exhibited lower levels of pSTAT3 and higher levels of pAKT1, resulting in decreased apoptosis. Abundant noninvoluted ducts were still present at 30 days post lactation, with a greater number of residual ERBB2+ cells. These results suggest that this defect in involution leads to an increase in the number of susceptible target cells for transformation, to the recovery of the capacity to generate mammospheres and to an increase in the number of tumors. Our work demonstrates the participation of SNAI2 in the pathogenesis of luminal breast cancer, and reveals an unexpected connection between the processes of postlactational involution and breast tumorigenesis in Snai2-null mutant mice.

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.

A pooled shRNA screen for regulators of primary mammary stem and progenitor cells identifies roles for Asap1 and Prox1

Background

The molecular regulators that orchestrate stem cell renewal, proliferation and differentiation along the mammary epithelial hierarchy remain poorly understood. Here we have performed a large-scale pooled RNAi screen in primary mouse mammary stem cell (MaSC)-enriched basal cells using 1295 shRNAs against genes principally involved in transcriptional regulation.

Methods

MaSC-enriched basal cells transduced with lentivirus pools carrying shRNAs were maintained as non-adherent mammospheres, a system known to support stem and progenitor cells. Integrated shRNAs that altered culture kinetics were identified by next generation sequencing as relative frequency changes over time. RNA-seq-based expression profiling coupled with in vitro progenitor and in vivo transplantation assays was used to confirm a role for candidate genes in mammary stem and/or progenitor cells.

Results

Utilizing a mammosphere-based assay, the screen identified several candidate regulators. Although some genes had been previously implicated in mammary gland development, the vast majority of genes uncovered have no known function within the mammary gland. RNA-seq analysis of freshly purified primary mammary epithelial populations and short-term cultured mammospheres was used to confirm the expression of candidate regulators. Two genes, Asap1 and Prox1, respectively implicated in breast cancer metastasis and progenitor cell function in other systems, were selected for further analysis as their roles in the normal mammary gland were unknown. Both Prox1 and Asap1 were shown to act as negative regulators of progenitor activity in vitro, and Asap1 knock-down led to a marked increase in repopulating activity in vivo, implying a role in stem cell activity.

Conclusions

This study has revealed a number of novel genes that influence the activity or survival of mammary stem and/or progenitor cells. Amongst these, we demonstrate that Prox1 and Asap1 behave as negative regulators of mammary stem/progenitor function. Both of these genes have also been implicated in oncogenesis. Our findings provide proof of principle for the use of short-term cultured primary MaSC/basal cells in functional RNAi screens.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1187-z) contains supplementary material, which is available to authorized users.

Mouse mammary stem cells express prognostic markers for triple-negative breast cancer

INTRODUCTION

Triple-negative breast cancer (TNBC) is a heterogeneous group of tumours in which chemotherapy, the current mainstay of systemic treatment, is often initially beneficial but with a high risk of relapse and metastasis. There is currently no means of predicting which TNBC will relapse. We tested the hypothesis that the biological properties of normal stem cells are re-activated in tumour metastasis and that, therefore, the activation of normal mammary stem cell-associated gene sets in primary TNBC would be highly prognostic for relapse and metastasis.

METHODS

Mammary basal stem and myoepithelial cells were isolated by flow cytometry and tested in low-dose transplant assays. Gene expression microarrays were used to establish expression profiles of the stem and myoepithelial populations; these were compared to each other and to our previously established mammary epithelial gene expression profiles. Stem cell genes were classified by Gene Ontology (GO) analysis and the expression of a subset analysed in the stem cell population at single cell resolution. Activation of stem cell genes was interrogated across different breast cancer cohorts and within specific subtypes and tested for clinical prognostic power.

RESULTS

A set of 323 genes was identified that was expressed significantly more highly in the purified basal stem cells compared to all other cells of the mammary epithelium. A total of 109 out of 323 genes had been associated with stem cell features in at least one other study in addition to our own, providing further support for their involvement in the biology of this cell type. GO analysis demonstrated an enrichment of these genes for an association with cell migration, cytoskeletal regulation and tissue morphogenesis, consistent with a role in invasion and metastasis. Single cell resolution analysis showed that individual cells co-expressed both epithelial- and mesenchymal-associated genes/proteins. Most strikingly, we demonstrated that strong activity of this stem cell gene set in TNBCs identified those tumours most likely to rapidly progress to metastasis.

CONCLUSIONS

Our findings support the hypothesis that the biological properties of normal stem cells are drivers of metastasis and that these properties can be used to stratify patients with a highly heterogeneous disease such as TNBC.

Comparison of stem/progenitor cell number and transcriptomic profile in the mammary tissue of dairy and beef breed heifers

Bovine mammary stem cells (MaSC) are a source of ductal and lobulo-alveolar tissue during the development of the mammary gland and its remodeling in repeating lactation cycles. We hypothesize that the number of MaSC, their molecular properties, and interactions with their niche may be essential in order to determine the mammogenic potential in heifers. To verify this hypothesis, we compared the number of MaSC and the transcriptomic profile in the mammary tissue of 20-month-old, non-pregnant dairy (Holstein-Friesian, HF) and beef (Limousin, LM) heifers. For the identification and quantification of putative stem/progenitor cells in mammary tissue sections, scanning cytometry was used with a combination of MaSC molecular markers: stem cell antigen-1 (Sca-1) and fibronectin type III domain containing 3B (FNDC3B) protein. Cytometric analysis revealed a significantly higher number of Sca-1^posFNDC3B^pos cells in HF (2.94 ± 0.35 %) than in LM (1.72 ± 0.20 %) heifers. In HF heifers, a higher expression of intramammary hormones, growth factors, cytokines, chemokines, and transcription regulators was observed. The model of mammary microenvironment favorable for MaSC was associated with the regulation of genes involved in MaSC maintenance, self-renewal, proliferation, migration, differentiation, mammary tissue remodeling, angiogenesis, regulation of adipocyte differentiation, lipid metabolism, and steroid and insulin signaling. In conclusion, the mammogenic potential in postpubertal dairy heifers is facilitated by a higher number of MaSC and up-regulation of mammary auto- and paracrine factors representing the MaSC niche.

CPEB1 promotes differentiation and suppresses EMT in mammary epithelial cells

Downregulation of CPEB1, a sequence-specific RNA-binding protein, in a mouse mammary epithelial cell line (CID-9) causes epithelial-to-mesenchymal transition (EMT), based on several criteria. First, CPEB1 knockdown decreases protein levels of E-cadherin and β-catenin but increases those of vimentin and Twist1. Second, the motility of CPEB1-depleted cells is increased. Third, CID-9 cells normally form growth-arrested, polarized and three-dimensional acini upon culture in extracellular matrix, but CPEB1-deficient CID-9 cells form nonpolarized proliferating colonies lacking a central cavity. CPEB1 downregulates Twist1 expression by binding to its mRNA, shortening its poly(A) tract and repressing its translation. CID-9 cultures contain both myoepithelial and luminal epithelial cells. CPEB1 increases during CID-9 cell differentiation, is predominantly expressed in myoepithelial cells, and its knockdown prevents expression of the myoepithelial marker p63. CPEB1 is present in proliferating subpopulations of pure luminal epithelial cells (SCp2) and myoepithelial cells (SCg6), but its depletion increases Twist1 only in SCg6 cells and fails to downregulate E-cadherin in SCp2 cells. We propose that myoepithelial cells prevent EMT by influencing the polarity and proliferation of luminal epithelial cells in a mechanism that requires translational silencing of myoepithelial Twist1 by CPEB1.

Oncogenic transformation of mammary epithelial cells by transforming growth factor beta independent of mammary stem cell regulation

Background

Transforming growth factor beta (TGFβ) is transiently increased in the mammary gland during involution and by radiation. While TGFβ normally has a tumour suppressor role, prolonged exposure to TGFβ can induce an oncogenic epithelial to mesenchymal transition (EMT) program in permissive cells and initiate the generation of cancer stem cells. Our objective is to mimic the transient exposure to TGFβ during involution to determine the persistent effects on premalignant mammary epithelium.

Method

CDβGeo cells, a transplantable mouse mammary epithelial cell line, were treated in vitro for 14 days with TGFβ (5 ng/ml). The cells were passaged for an additional 14 days in media without TGFβ and then assessed for markers of EMT and transformation.

Results

The 14-day exposure to TGFβ induced EMT and transdifferentiation in vitro that persists after withdrawal of TGFβ. TGFβ-treated cells are highly tumorigenic in vivo, producing invasive solid de-differentiated tumours (100%; latency 6.7 weeks) compared to control (43%; latency 32.7 weeks). Although the TGFβ-treated cells have initiated a persistent EMT program, the stem cell population was unchanged relative to the controls. The gene expression profiles of TGFβ-treated cells demonstrate de-differentiation with decreases in the expression of genes that define luminal, basal and stem cells. Additionally, the gene expression profiles demonstrate increases in markers of EMT, growth factor signalling, TGFβ2 and changes in extra cellular matrix.

Conclusion

This model demonstrates full oncogenic EMT without an increase in stem cells, serving to separate EMT markers from stem cell markers.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Molecular hierarchy of mammary differentiation yields refined markers of mammary stem cells

The partial purification of mouse mammary gland stem cells (MaSCs) using combinatorial cell surface markers (Lin(-)CD24(+)CD29(h)CD49f(h)) has improved our understanding of their role in normal development and breast tumorigenesis. Despite the significant improvement in MaSC enrichment, there is presently no methodology that adequately isolates pure MaSCs. Seeking new markers of MaSCs, we characterized the stem-like properties and expression signature of label-retaining cells from the mammary gland of mice expressing a controllable H2b-GFP transgene. In this system, the transgene expression can be repressed in a doxycycline-dependent fashion, allowing isolation of slowly dividing cells with retained nuclear GFP signal. Here, we show that H2b-GFP(h) cells reside within the predicted MaSC compartment and display greater mammary reconstitution unit frequency compared with H2b-GFP(neg) MaSCs. According to their transcriptome profile, H2b-GFP(h) MaSCs are enriched for pathways thought to play important roles in adult stem cells. We found Cd1d, a glycoprotein expressed on the surface of antigen-presenting cells, to be highly expressed by H2b-GFP(h) MaSCs, and isolation of Cd1d(+) MaSCs further improved the mammary reconstitution unit enrichment frequency to nearly a single-cell level. Additionally, we functionally characterized a set of MaSC-enriched genes, discovering factors controlling MaSC survival. Collectively, our data provide tools for isolating a more precisely defined population of MaSCs and point to potentially critical factors for MaSC maintenance.

Slug controls stem/progenitor cell growth dynamics during mammary gland morphogenesis

Background

Morphogenesis results from the coordination of distinct cell signaling pathways controlling migration, differentiation, apoptosis, and proliferation, along stem/progenitor cell dynamics. To decipher this puzzle, we focused on epithelial-mesenchymal transition (EMT) “master genes”. EMT has emerged as a unifying concept, involving cell-cell adhesion, migration and apoptotic pathways. EMT also appears to mingle with stemness. However, very little is known on the physiological role and relevance of EMT master-genes. We addressed this question during mammary morphogenesis. Recently, a link between Slug/Snai2 and stemness has been described in mammary epithelial cells, but EMT master genes actual localization, role and targets during mammary gland morphogenesis are not known and we focused on this basic question.

Methodology/Principal Findings

Using a Slug–lacZ transgenic model and immunolocalization, we located Slug in a distinct subpopulation covering about 10–20% basal cap and duct cells, mostly cycling cells, coexpressed with basal markers P-cadherin, CK5 and CD49f. During puberty, Slug-deficient mammary epithelium exhibited a delayed development after transplantation, contained less cycling cells, and overexpressed CK8/18, ER, GATA3 and BMI1 genes, linked to luminal lineage. Other EMT master genes were overexpressed, suggesting compensation mechanisms. Gain/loss-of-function in vitro experiments confirmed Slug control of mammary epithelial cell luminal differentiation and proliferation. In addition, they showed that Slug enhances specifically clonal mammosphere emergence and growth, cell motility, and represses apoptosis. Strikingly, Slug-deprived mammary epithelial cells lost their potential to generate secondary clonal mammospheres.

Conclusions/Significance

We conclude that Slug pathway controls the growth dynamics of a subpopulation of cycling progenitor basal cells during mammary morphogenesis. Overall, our data better define a key mechanism coordinating cell lineage dynamics and morphogenesis, and provide physiological relevance to broadening EMT pathways.

Differentiation of cancer cell type and phenotype using quantum dot-gold nanoparticle sensor arrays

We demonstrate rapid and efficient sensing of mammalian cell types and states using nanoparticle-based sensor arrays. These arrays are comprised of cationic quantum dots (QDs) and gold nanoparticles (AuNPs) that interact with cell surfaces to generate distinguishable fluorescence responses based on cell surface signatures. The use of QDs as the recognition elements as well as the signal transducers presents the potential for direct visualization of selective cell surface interactions. Notably, this sensor is unbiased, precluding the requirement of pre-knowledge of cell state biomarkers and thus providing a general approach for phenotypic profiling of cell states, with additional potential for imaging applications.

Integrins in mammary development

Integrins are ubiquitously expressed major cell surface receptors for extracellular matrix. Integrin interaction with their extracellular ligands triggers activation of the intracellular signaling pathways that control cell shape, motility, proliferation, survival, cell-type-specific gene expression. In this review, we summarize recent studies analyzing contribution of integrins to the control of the mammary morphogenesis and differentiation, function and maintenance of mammary stem and progenitor cells and resume the data from mouse models revealing the contribution of the integrin-mediated signaling to mammary tumorigenesis.

The splicing factor SRSF1 regulates apoptosis and proliferation to promote mammary epithelial cell transformation

The splicing-factor oncoprotein SRSF1 (also known as SF2/ASF) is upregulated in breast cancers. We investigated SRSF1’s ability to transform human and mouse mammary epithelial cells in vivo and in vitro. SRSF1-overexpressing COMMA-1D cells formed tumors, following orthotopic transplantation to reconstitute the mammary gland. In 3-D culture, SRSF1-overexpressing MCF-10A cells formed larger acini than control cells, reflecting increased proliferation and delayed apoptosis during acinar morphogenesis. These effects required the first RNA-recognition motif and nuclear functions of SRSF1. SRSF1 overexpression promoted alternative splicing of BIM and BIN1 isoforms that lack pro-apoptotic functions and contribute to the phenotype. Finally, SRSF1 cooperated specifically with MYC to transform mammary epithelial cells, in part by potentiating eIF4E activation, and these cooperating oncogenes are significantly co-expressed in human breast tumors. Thus, SRSF1 can promote breast cancer, and SRSF1 itself or its downstream effectors may be valuable targets for therapeutics development.

Stem cell antigen-1 deficiency enhances the chemopreventive effect of peroxisome proliferator-activated receptorγ activation

Stem cell antigen-1 (Sca-1, Ly6A) is a glycerophosphatidylinositol (GPI)-anchored protein that was identified as a murine marker of bone marrow stem cells. Although Sca-1 is widely used to enrich for stem and progenitor cells in various tissues, little is known about its function and associated signaling pathways in normal and malignant cells. Here, we report that the absence of Sca-1 in the mammary gland resulted in higher levels of PPARγ and PTEN, and a reduction of pSer84PPARγ, pERK1/2, and PPARδ. This phenotype correlated with markedly increased sensitivity of Sca-1 null mice to PPARγ agonist GW7845 and insensitivity to PPARδ agonist GW501516. Reduction of Sca-1 expression in mammary tumor cells by RNA interference resulted in a phenotype similar to the Sca-1 deficient mammary gland, as evidenced by increased PPARγ expression and transcriptional activity, resulting in part from a lesser susceptibility to proteasomal degradation. These data implicate Sca-1 as a negative regulator of the tumor suppressor effects of PPARγ.

Radiation acts on the microenvironment to affect breast carcinogenesis by distinct mechanisms that decrease cancer latency and affect tumor type

Tissue microenvironment is an important determinant of carcinogenesis. We demonstrate that ionizing radiation, a known carcinogen, affects cancer frequency and characteristics by acting on the microenvironment. Using a mammary chimera model in which an irradiated host is transplanted with oncogenic Trp53 null epithelium, we show accelerated development of aggressive tumors whose molecular signatures were distinct from tumors arising in nonirradiated hosts. Molecular and genetic approaches show that TGFβ mediated tumor acceleration. Tumor molecular signatures implicated TGFβ, and genetically reducing TGFβ abrogated the effect on latency. Surprisingly, tumors from irradiated hosts were predominantly estrogen receptor negative. This effect was TGFβ independent and linked to mammary stem cell activity. Thus, the irradiated microenvironment affects latency and clinically relevant features of cancer through distinct and unexpected mechanisms.

Molecular regulators of pubertal mammary gland development

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

Stem cell antigen-1 enhances tumorigenicity by disruption of growth differentiation factor-10 (GDF10)-dependent TGF-beta signaling

Stem cell antigen (Sca)-1/Ly6A, a glycerophosphatidylinositol-linked surface protein, was found to be associated with murine stem cell- and progenitor cell-enriched populations, and also has been linked to the capacity of tumor-initiating cells. Despite these interesting associations, this protein's functional role in these processes remains largely unknown. To identify the mechanism underlying the protein's possible role in mammary tumorigenesis, Sca-1 expression was examined in Sca-1(+/EGFP) mice during carcinogenesis. Mammary tumor cells derived from these mice readily engrafted in syngeneic mice, and tumor growth was markedly inhibited on down-regulation of Sca-1 expression. The latter effect was associated with significantly elevated expression of the TGF-β ligand growth differentiation factor-10 (GDF10), which was found to selectively activate TGF-β receptor (TβRI/II)-dependent Smad3 phosphorylation. Overexpression of GDF10 attenuated tumor formation; conversely, silencing of GDF10 expression reversed these effects. Sca-1 attenuated GDF10-dependent TGF-β signaling by disrupting the heterodimerization of TβRI and TβRII receptors. These findings suggest a new functional role for Sca-1 in maintaining tumorigenicity, in part by acting as a potent suppressor of TGF-β signaling.

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.

Tumor-initiating function of nucleostemin-enriched mammary tumor cells

Nucleostemin (NS) is highly expressed in normal stem cells and tumors and is upregulated by estradiol in MCF7 breast cancer cells. To investigate the role of NS in mammary tumorigenesis, we established first that NS is expressed at higher levels in the basal cell type than in the luminal cell type in mouse mammary tumors and human breast cancer cells. NS expression was also increased during progression of mammary tumors in MMTV-Wnt1 and MMTV-PyMT transgenic mice and by the tumor sphere culture. To determine the function of NS-enriched tumor cells, we generated a bacterial artificial chromosome transgenic mouse line expressing green fluorescent protein (GFP) from the NS promoter and bred it to MMTV-Wnt1 mice, so that NS-expressing cells can be prospectively isolated based on their GFP levels. Notably, NS-enriched mammary tumor cells exhibited stronger in vitro and in vivo tumorigenic activities and expressed higher levels of K5, CD133, Oct4, telomerase reverse transcriptase, and C-X-C chemokine ligand 12 compared with NS-deficient mammary tumor cells. Furthermore, knockdown of NS dramatically reduced the sphere-forming activity of MDA-MB-231 and MCF7 human breast cancer cells. Our findings establish the tumor-initiating and molecular features of NS-enriched mammary tumor cells, suggesting that NS may offer a valuable therapeutic target.

Overexpression of ΔNp63α induces a stem cell phenotype in MCF7 breast carcinoma cell line through the Notch pathway

To elucidate a role of ΔNp63α in breast cancer, the expression levels of p63, estrogen receptor, progesterone receptor, p53, CK5, cerBb-2, and Notch1 were assayed in 50 clinical breast cancer specimens using immunochemistry. P63 was highly expressed in a subset of breast cancer with basal-like features. We then transfected MCF7 cells with ΔNp63α plasmid, and assayed its cancer stem cell-like features after transfection. Overexpression of ΔNp63α in MCF7 cells increased the percentage of CD24(-) CD44(+) subpopulation from 2.2±0.2% to 25.1±1.5% (P<0.05) and led to increased cancer cell proliferation, clonogenicity, anchorage-independent growth, and the incidence of xenograft grown in vivo. In addition, ΔNp63α overexpressing cancer cells were more drug resistant. Further studies suggested ΔNp63α-induced activation of the Notch pathway may play a role in these effects. Chromatin immunoprecipitation confirmed that ΔNp63α could directly bind to Notch1. In clinical breast cancer specimens, the expression level of p63 was also found to positively correlate with the expression level of Notch1. Our results suggest that ΔNp63α might serve as a tumor initiating transcription factor in breast cancer.

s-SHIP promoter expression marks activated stem cells in developing mouse mammary tissue

Mammary stem cells (MaSCs) play critical roles in normal development and perhaps tumorigenesis of the mammary gland. Using combined cell markers, adult MaSCs have been enriched in a basal cell population, but the exact identity of MaSCs remains unknown. We used the s-SHIP promoter to tag presumptive stem cells with GFP in the embryos of a transgenic mouse model. Here we show, in postnatal mammary gland development, that GFP(+) cap cells in puberty and basal alveolar bud cells in pregnancy each exhibit self-renewal and regenerative capabilities for all mammary epithelial cells of a new functional mammary gland upon transplantation. Single GFP(+) cells can regenerate the mammary epithelial network. GFP(+) mammary epithelial cells are p63(+), CD24(mod), CD49f(high), and CD29(high); are actively proliferating; and express s-SHIP mRNA. Overall, our results identify the activated MaSC population in vivo at the forefront of rapidly developing terminal end buds (puberty) and alveolar buds (pregnancy) in the mammary gland. In addition, GFP(+) basal cells are expanded in MMTV-Wnt1 breast tumors but not in ErbB2 tumors. These results enable MaSC in situ identification and isolation via a consistent single parameter using a new mouse model with applications for further analyses of normal and potential cancer stem cells.

Loss of CD24 expression promotes ductal branching in the murine mammary gland

CD24 is expressed on mammary stem cells and is used as a marker for their isolation, yet its function in the mammary gland still needs to be examined. Here we show that CD24 is expressed throughout the luminal epithelial cell layer, but only weakly in myoepithelial cells. During lactation, CD24 expression was suppressed within alveoli, but upregulated post-lactation, returning to a pre-pregnant spatial distribution. CD24-deficient mice exhibited an accelerated mammary gland ductal extension during puberty and an enhanced branching morphogenesis, resulting in increased furcation in the ductal structure. CD24-/- mammary epithelial cells were able to completely repopulate cleared mammary fat pads and to give rise to fully functional mammary glands. Together, these data suggest that while CD24 is expressed in mammary epithelium compartments thought to contain stem cells, CD24 is not a major regulator of mammary stem/progenitor cell function, but rather plays a role in governing branching morphogenesis.

A putative role for microRNA-205 in mammary epithelial cell progenitors

In an effort to understand the potential role of microRNAs (miRNAs) in mammary-gland stem or progenitor cells, miRNA microarrays were performed on subpopulations of the mouse mammary epithelial cell (MEC) line COMMA-DbetaGeo. This cell line contains a heterogeneous subpopulation of progenitors characterized by the expression of stem cell antigen 1 (Sca-1; encoded by Ly6a). Microarray analysis indicated that the Sca-1 subpopulations have distinct miRNA expression profiles. Functional studies were performed on miR-205, which was highly expressed in the Sca-1-positive (Sca-1(+)) cells. When miR-205 was overexpressed in vitro, the COMMA-DbetaGeo cells underwent several significant morphological and molecular changes. miR-205 overexpression led to an expansion of the progenitor-cell population, decreased cell size and increased cellular proliferation. In addition, the colony-forming potential of the two Sca-1 subpopulations was increased. Target prediction for miR-205 indicated that it might regulate the expression of the tumor-suppressor protein PTEN. Overexpression studies using reporter constructs confirmed that PTEN expression is regulated by miR-205. In addition to PTEN, several other putative and previously validated miR-205 targets were identified by microarray analysis, including the previously reported miR-205 targets ZEB1 and ZEB2. Additionally, in normal mouse MECs, high expression of miR-205 was observed in stem-cell-enriched cell populations isolated by FACS using established cell-surface markers.