Quantitation of human mammary epithelial stem cells with in vivo regenerative properties using a subrenal capsule xenotransplantation assay

A xenotransplantation mouse model to study physiology of the mammary gland from large mammals

Although highly conserved in structure and function, many (patho)physiological processes of the mammary gland vary drastically between mammals, with mechanisms regulating these differences not well understood. Large mammals display variable lactation strategies and mammary cancer incidence, however, research into these variations is often limited to in vitro analysis due to logistical limitations. Validating a model with functional mammary xenografts from cryopreserved tissue fragments would allow for in vivo comparative analysis of mammary glands from large and/or rare mammals and would improve our understanding of postnatal development, lactation, and premalignancy across mammals. To this end, we generated functional mammary xenografts using mammary tissue fragments containing mammary stroma and parenchyma isolated via an antibody-independent approach from healthy, nulliparous equine and canine donor tissues to study these species in vivo. Cryopreserved mammary tissue fragments were xenotransplanted into de-epithelialized fat pads of immunodeficient mice and resulting xenografts were structurally and functionally assessed. Preimplantation of mammary stromal fibroblasts was performed to promote ductal morphogenesis. Xenografts recapitulated mammary lobule architecture and contained donor-derived stromal components. Mammatropic hormone stimulation resulted in (i) upregulation of lactation-associated genes, (ii) altered proliferation index, and (iii) morphological changes, indicating functionality. Preimplantation of mammary stromal fibroblasts did not promote ductal morphogenesis. This model presents the opportunity to study novel mechanisms regulating unique lactation strategies and mammary cancer induction in vivo. Due to the universal applicability of this approach, this model serves as proof-of-concept for developing mammary xenografts for in vivo analysis of virtually any mammals, including large and rare mammals.

Differential DNA damage repair and PARP inhibitor vulnerability of the mammary epithelial lineages

It is widely assumed that all normal somatic cells can equally perform homologous recombination (HR) and non-homologous end joining in the DNA damage response (DDR). Here, we show that the DDR in normal mammary gland inherently depends on the epithelial cell lineage identity. Bioinformatics, post-irradiation DNA damage repair kinetics, and clonogenic assays demonstrated luminal lineage exhibiting a more pronounced DDR and HR repair compared to the basal lineage. Consequently, basal progenitors were far more sensitive to poly(ADP-ribose) polymerase inhibitors (PARPis) in both mouse and human mammary epithelium. Furthermore, PARPi sensitivity of murine and human breast cancer cell lines as well as patient-derived xenografts correlated with their molecular resemblance to the mammary progenitor lineages. Thus, mammary epithelial cells are intrinsically divergent in their DNA damage repair capacity and PARPi vulnerability, potentially influencing the clinical utility of this targeted therapy.

Mammary epithelial cells have lineage-rooted metabolic identities

Cancer metabolism adapts the metabolic network of its tissue of origin. However, breast cancer is not a disease of a single origin. Multiple epithelial populations serve as the culprit cell of origin for specific breast cancer subtypes, yet our knowledge of the metabolic network of normal mammary epithelial cells is limited. Using a multi-omic approach, here we identify the diverse metabolic programmes operating in normal mammary populations. The proteomes of basal, luminal progenitor and mature luminal cell populations revealed enrichment of glycolysis in basal cells and of oxidative phosphorylation in luminal progenitors. Single-cell transcriptomes corroborated lineage-specific metabolic identities and additional intra-lineage heterogeneity. Mitochondrial form and function differed across lineages, with clonogenicity correlating with mitochondrial activity. Targeting oxidative phosphorylation and glycolysis with inhibitors exposed lineage-rooted metabolic vulnerabilities of mammary progenitors. Bioinformatics indicated breast cancer subtypes retain metabolic features of their putative cell of origin. Thus, lineage-rooted metabolic identities of normal mammary cells may underlie breast cancer metabolic heterogeneity and targeting these vulnerabilities could advance breast cancer therapy.

Single-cell analysis of autophagy activity in normal and de novo transformed human mammary cells

Assessment of autophagy activity has historically been limited to investigations of fixed tissue or bulk cell populations. To address questions of heterogeneity and relate measurements to functional properties of viable cells isolated from primary tissue, we created a lentiviral (RFP-GFP-MAP1LC3B) vector that allows the autophagosome and autolysosome content of transduced cells to be monitored at the single-cell level. Use of this strategy to analyze purified subsets of normal human mammary cells showed that both the luminal progenitor-containing (LP) subset and the basal cells (BCs) display highly variable but overall similar autophagic flux activity despite differences suggested by measurements of the proteins responsible (i.e., LC3B, ATG7 and BECLIN1) in bulk lysates. Autophagosome content was also highly variable in the clonogenic cells within both the LPs and BCs, but the proliferative response of the BCs was more sensitive to autophagy inhibition. In addition, use of this vector showed cells with the lowest autophagosome content elicited the fastest tumor growth in 2 different models of human mammary tumorigenesis. These results illustrate the utility of this vector to define differences in the autophagy properties of individual cells in primary tissue and couple these with their responses to proliferative and oncogenic stimuli.

CXCL12γ induces human prostate and mammary gland development

BACKGROUND

Epithelial stem cells (ESCs) demonstrate a capacity to maintain normal tissues homeostasis and ESCs with a deregulated behavior can contribute to cancer development. The ability to reprogram normal tissue epithelial cells into prostate or mammary stem-like cells holds great promise to help understand cell of origin and lineage plasticity in prostate and breast cancers in addition to understanding normal gland development. We previously showed that an intracellular chemokine, CXCL12γ induced cancer stem cells and neuroendocrine characteristics in both prostate and breast adenocarcinoma cell lines. However, its role in normal prostate or mammary epithelial cell fate and development remains unknown. Therefore, we sought to elucidate the functional role of CXCL12γ in the regulation of ESCs and tissue development.

METHODS

Prostate epithelial cells (PNT2) or mammary epithelial cells (MCF10A) with overexpressed CXCL12γ was characterized by quantitative real-time polymerase chain reaction, Western blots, and immunofluorescence for lineage marker expression, and fluorescence activated cell sorting analyses and sphere formation assays to examine stem cell surface phenotype and function. Xenotransplantation animal models were used to evaluate gland or acini formation in vivo.

RESULTS

Overexpression of CXCL12γ promotes the reprogramming of cells with a differentiated luminal phenotype to a nonluminal phenotype in both prostate (PNT2) and mammary (MCF10A) epithelial cells. The CXCL12γ-mediated nonluminal type cells results in an increase of epithelial stem-like phenotype including the subpopulation of EPCAM^Lo /CD49f^Hi /CD24^Lo /CD44^Hi cells capable of sphere formation. Critically, overexpression of CXCL12γ promotes the generation of robust gland-like structures from both prostate and mammary epithelial cells in in vivo xenograft animal models.

CONCLUSIONS

CXCL12γ supports the reprogramming of epithelial cells into nonluminal cell-derived stem cells, which facilitates gland development.

Basal-like Breast Cancers: From Pathology to Biology and Back Again

Human breast cancers referred to as "basal-like" are of interest because they lack effective therapies and their biology is poorly understood. The term basal-like derives from studies demonstrating tumor gene expression profiles that include some transcripts characteristic of the basal cells of the normal adult human mammary gland and others associated with a subset of normal luminal cells. Elucidating the mechanisms responsible for the profiles of basal-like tumors is an active area of investigation. More refined molecular analysis of patients' samples and genetic strategies to produce breast cancers de novo from defined populations of normal mouse mammary cells have served as complementary approaches to identify relevant pathway alterations. However, both also have limitations. Here, we review some of the underlying reasons, including the unifying concept that some normal luminal cells have both luminal and basal features, as well as some emerging new avenues of investigation.

Isolation and characterization of a new basal-like luminal progenitor in human breast tissue

Background

Adult stem cells and progenitors are responsible for breast tissue regeneration. Human breast epithelial progenitors are organized in a lineage hierarchy consisting of bipotent progenitors (BPs), myoepithelial- and luminal-restricted progenitors (LRPs) where the LRP differentiation into mature luminal cells requires estrogen receptor (ER) signaling. However, the experimental evidence exploring the relationship between the BPs and LRPs has remained elusive. In this study, we report the presence of a basal-like luminal progenitor (BLP) in human breast epithelial cells.

Methods

Breast reduction samples were used to obtain different subsets of human breast epithelial cell based on cell surface marker expression using flow cytometry. Loss of function and gain of function studies were employed to demonstrate the role of NOTCH3 (NR3)-FRIZZLED7 (FZD7) signaling in luminal cell fate commitment.

Results

Our results suggest that, NR3-FZD7 signaling axis was necessary for luminal cell fate commitment. Similar to LRPs, BLPs (NR3^highFZD7^highCD90⁺MUC1⁻ER⁻) differentiate to generate NR3^medFZD7^medCD90⁻MUC1⁺ER⁺ luminal cells. Unlike LRPs however, BLP’s proliferation and differentiation potentials depend on NR3 and regulated in part by FZD7 signaling. Lastly, we show that BLPs have a higher colony-forming potential than LRPs and that they are continuously generated from the NOTCH3⁻FZD7^low subset of the bipotent progenitors.

Conclusion

Our data indicate that BPs differentiate to generate basal-like luminal progenitors that in turn differentiate into LRPs. These results provide new insights into the hierarchical organization of human breast epithelial cell and how cooperation between the Notch and Wnt signaling pathways define a new progenitor cell type.

Electronic supplementary material

The online version of this article (10.1186/s13287-019-1361-3) contains supplementary material, which is available to authorized users.

Transcriptional regulation of normal human mammary cell heterogeneity and its perturbation in breast cancer

The mammary gland in adult women consists of biologically distinct cell types that differ in their surface phenotypes. Isolation and molecular characterization of these subpopulations of mammary cells have provided extensive insights into their different transcriptional programs and regulation. This information is now serving as a baseline for interpreting the heterogeneous features of human breast cancers. Examination of breast cancer mutational profiles further indicates that most have undergone a complex evolutionary process even before being detected. The consequent intra-tumoral as well as inter-tumoral heterogeneity of these cancers thus poses major challenges to deriving information from early and hence likely pervasive changes in potential therapeutic interest. Recently described reproducible and efficient methods for generating human breast cancers de novo in immunodeficient mice transplanted with genetically altered primary cells now offer a promising alternative to investigate initial stages of human breast cancer development. In this review, we summarize current knowledge about key transcriptional regulatory processes operative in these partially characterized subpopulations of normal human mammary cells and effects of disrupting these processes in experimentally produced human breast cancers.

Assays for functionally defined normal and malignant mammary stem cells

The discovery of rare, heterogeneous self-renewing stem cells with shared developmental and molecular features within epithelial components of mammary gland and breast cancers has provided a conceptual framework to understand cellular composition of these tissues and mechanisms that control their number. These normal mammary epithelial stem cells (MaSCs) and breast cancer stem cells (BCSCs) were identified and analyzed using transplant assays (namely mammary repopulating unit (MRU) assay, mammary tumor-initiating cell (TIC) assay), which reveal their latent ability to regenerate respective normal and malignant epithelial tissues with self-renewing units displaying hierarchical cellular differentiation over multiple generations in recipient mice. "Next-generation" methods using "barcoded" normal and malignant mammary cells, with the help of next-generation sequencing (NGS) technology, have revealed hidden complexity and heterogeneous growth potential of MaSCs and BCSCs. Several single markers or combinations of markers have been reported to prospectively enrich MaSCs and BCSCs. Such markers and the extent to which they enrich for MaSCs and BCSCs activity require a critical appraisal. Also, knowledge of the functional assays and their limitations and harmonious reporting of results is a prerequisite to improve our understanding of MaSCs and BCSCs. This chapter describes evolution of the concept of MaSCs and BCSCs, and specific methodologies to investigate them.

Heterogeneity of Mammary Stem Cells

Adult female mammals are endowed with the unique ability to produce milk for nourishing their newborn offspring. Milk is secreted on demand by the mammary gland, an organ which develops during puberty, further matures during pregnancy and lactation, but reverts to a resting state after weaning. The glandular tissue (re)generated through this series of structural and functional changes is finely sourced by resident stem cells under the control of systemic hormones and local stimuli.Over the past decades a plethora of studies have been carried out in order to identify and characterize mammary stem cells, primarily in mice and humans. Intriguingly, it is now emerging that multiple mammary stem cell pools (co)exist and are characterized by distinctive molecular markers and context-dependent functions.This chapter reviews the heterogeneity of the mammary stem cell compartment with emphasis on the key properties and molecular regulators of distinct stem cell populations in both the mouse and human glands.

A robust cell culture system for large scale feeder cell-free expansion of human breast epithelial progenitors

Background

Normal human breast epithelial cells are maintained by the proliferation and differentiation of different human breast epithelial progenitors (HBEPs). However, these progenitor subsets can only be obtained at low frequencies, limiting their further characterization. Recently, it was reported that HBEPs can be minimally expanded in Matrigel cocultures with stromal feeder cells. However, variability of generating healthy feeder cells significantly impacts the effective expansion of HBEPs.

Methods

Here, we report a robust feeder cell-free culture system for large-scale expansion of HBEPs in two-dimensional cultures.

Results

Using this cell culture system HBEPs can be exponentially expanded as bulk cultures. Moreover, purified HBEP subtypes can also be separately expanded using our cell culture system. The expanded HBEPs retain their undifferentiated phenotype and form distinct epithelial colonies in colony forming cell assays.

Conclusions

The availability of a culture system enabling the large-scale expansion of HBEPs facilitates their application to screening platforms and other cell-based assays.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0994-y) contains supplementary material, which is available to authorized users.

Mammary molecular portraits reveal lineage-specific features and progenitor cell vulnerabilities

Casey et al. integrate epigenomic, transcriptomic, and proteomic profiling of primary basal and luminal mammary cells to identify master epigenetic regulators of the mammary epithelium and uncover stem and progenitor cell vulnerabilities. They develop a pipeline to identify drugs that abrogate progenitor cell activity in normal and high-risk breast cancer patient samples in vitro and in vivo.

The mammary epithelium depends on specific lineages and their stem and progenitor function to accommodate hormone-triggered physiological demands in the adult female. Perturbations of these lineages underpin breast cancer risk, yet our understanding of normal mammary cell composition is incomplete. Here, we build a multimodal resource for the adult gland through comprehensive profiling of primary cell epigenomes, transcriptomes, and proteomes. We define systems-level relationships between chromatin–DNA–RNA–protein states, identify lineage-specific DNA methylation of transcription factor binding sites, and pinpoint proteins underlying progesterone responsiveness. Comparative proteomics of estrogen and progesterone receptor–positive and –negative cell populations, extensive target validation, and drug testing lead to discovery of stem and progenitor cell vulnerabilities. Top epigenetic drugs exert cytostatic effects; prevent adult mammary cell expansion, clonogenicity, and mammopoiesis; and deplete stem cell frequency. Select drugs also abrogate human breast progenitor cell activity in normal and high-risk patient samples. This integrative computational and functional study provides fundamental insight into mammary lineage and stem cell biology.

Slowing down glioblastoma progression in mice by running or the anti-malarial drug dihydroartemisinin? Induction of oxidative stress in murine glioblastoma therapy

Influencing cancer metabolism by lifestyle changes is an attractive strategy as - if effective - exercise-induced problems may be less severe than those induced by classical anti-cancer therapies. Pursuing this idea, clinical trials evaluated the benefit of e.g. different diets such as the ketogenic diet, intermittent caloric restriction and physical exercise (PE) in the primary and secondary prevention of different cancer types. PE proved to be beneficial in the context of breast and colon cancer.Glioblastoma has a dismal prognosis, with an average overall survival of about one year despite maximal safe resection, concomitant radiochemotherapy with temozolomide followed by adjuvant temozolomide therapy. Here, we focused on the influence of PE as an isolated and adjuvant treatment in murine GB therapy.PE did not reduce toxic side effects of chemotherapy in mice administered in a dose escalating scheme as shown before for starvation. Although regular treadmill training on its own had no obvious beneficial effects, its combination with temozolomide was beneficial in the treatment of glioblastoma-bearing mice. As PE might partly act through the induction of reactive oxygen species, dihydroartemisinin - an approved anti-malarial drug which induces oxidative stress in glioma cells - was further evaluated in vitro and in vivo. Dihydroartemisinin showed anti-glioma activity by promoting autophagy, reduced the clonogenic survival and proliferation capacity of glioma cells, and prolonged the survival of tumor bearing mice. Using the reactive oxygen species scavenger n-acetyl-cysteine these effects were in part reversible, suggesting that dihydroartemisinin partly acts through the generation of reactive oxygen species.

Purification of Distinct Subsets of Epithelial Cells from Normal Human Breast Tissue

The mammary epithelium is composed of a variety of specialized cell types that function in a coordinated fashion to produce and eject milk through multiple cycles of pregnancy. The ability to identify and purify these subsets of cells in order to interrogate their growth and differentiation capacities, as well as to characterize the molecular mechanisms that regulate their behavior, is essential in identifying the processes associated with breast cancer initiation and progression. This methods chapter outlines the step-by-step methods for dissociating human breast reduction specimens to a single cell suspension of viable cells. As well, strategies for purifying four distinct subsets of epithelial cells by using fluorescence-activated cell sorting and protocols for interrogating the growth and differentiation properties of these purified cells at clonal densities in adherent culture are also described.

Human glandular organoid formation in murine engineering chambers after collagenase digestion and flow cytometry isolation of normal human breast tissue single cells

Women with high mammographic density (MD) are at increased risk of breast cancer (BC) after adjustment for age and body mass index. We have developed a murine biochamber model in which both high MD (HMD) and low MD (LMD) tissue can be propagated. Here, we tested whether cells isolated by collagenase digestion and fluorescence-activated cell sorting (FACS) from normal breast can be reconstituted in our biochamber model, which would allow cell-specific manipulations to be tested. Fresh breast tissue was collected from women (n = 7) undergoing prophylactic mastectomy. The tissue underwent collagenase digestion overnight and, in some cases, additional FACS enrichment to obtain mature epithelial, luminal progenitor, mammary stem, and stromal cells. Cells were then transferred bilaterally into biochambers in SCID mice (n = 5-7) and incubated for 6 weeks, before harvesting for histological analyses, and immunohistochemical staining for cytokeratins (CK), vimentin, Ki-67, murine macrophages, and Cleaved Caspase-3. Biochambers inoculated with single cells after collagenase digestion or with flow cytometry contained glandular structures of human origin (human vimentin-positive), which expressed CK-14 and pan-CK, and were proliferating (Ki-67-positive). Glandular structures from the digested tissues were smaller than those in chambers seeded with finely chopped intact mammary tissue. Mouse macrophage infiltration was higher in the chambers arising from digested tissues. Pooled single cells and FACS fractionated cells were viable in the murine biochambers and formed proliferating glandular organoids of human origin. This is among the first report to demonstrate the success of formed human glandular organoids from isolated primary mammary cells in the murine biochamber model.

Characterization of mammary epithelial stem/progenitor cells and their changes with aging in common marmosets

Age is the number one risk factor for breast cancer, yet the underlying mechanisms are unexplored. Age-associated mammary stem cell (MaSC) dysfunction is thought to play an important role in breast cancer carcinogenesis. Non-human primates with their close phylogenetic relationship to humans provide a powerful model system to study the effects of aging on human MaSC. In particular, the common marmoset monkey (Callithrix jacchus) with a relatively short life span is an ideal model for aging research. In the present study, we characterized for the first time the mammary epithelial stem/progenitor cells in the common marmoset. The MaSC-enriched cells formed four major types of morphologically distinct colonies when cultured on plates pre-seeded with irradiated NIH3T3 fibroblasts, and were also capable of forming mammospheres in suspension culture and subsequent formation of 3D organoids in Matrigel culture. Most importantly, these 3D organoids were found to contain stem/progenitor cells that can undergo self-renewal and multi-lineage differentiation both in vitro and in vivo. We also observed a significant decrease of luminal-restricted progenitors with age. Our findings demonstrate that common marmoset mammary stem/progenitor cells can be isolated and quantified with established in vitro and in vivo assays used for mouse and human studies.

Barcoding reveals complex clonal dynamics of de novo transformed human mammary cells

Most human breast cancers have diversified genomically and biologically by the time they become clinically evident. Early events involved in their genesis and the cellular context in which these events occur have thus been difficult to characterize. Here we present the first formal evidence of the shared and independent ability of basal cells and luminal progenitors, isolated from normal human mammary tissue and transduced with a single oncogene (KRAS(G12D)), to produce serially transplantable, polyclonal, invasive ductal carcinomas within 8 weeks of being introduced either subrenally or subcutaneously into immunodeficient mice. DNA barcoding of the initial cells revealed a dramatic change in the numbers and sizes of clones generated from them within 2 weeks, and the first appearance of many 'new' clones in tumours passaged into secondary recipients. Both primary and secondary tumours were phenotypically heterogeneous and primary tumours were categorized transcriptionally as 'normal-like'. This system challenges previous concepts that carcinogenesis in normal human epithelia is necessarily a slow process requiring the acquisition of multiple driver mutations. It also presents the first description of initial events that accompany the genesis and evolution of malignant human mammary cell populations, thereby contributing new understanding of the rapidity with which heterogeneity in their properties can develop.

Estrogen regulates luminal progenitor cell differentiation through H19 gene expression

Although the role of estrogen signaling in breast cancer development has been extensively studied, the mechanisms that regulate the indispensable role of estrogen in normal mammary gland development have not been well studied. Because of the unavailability of culture system to maintain estrogen-receptor-positive (ERα(+)) cells in vitro, the molecular mechanisms that regulate estrogen/ERα signaling in the normal human breast are unknown. In the present study, we examined the effects of estrogen signaling on ERα(+) human luminal progenitors using a modified matrigel assay and found that estrogen signaling increased the expansion potential of these progenitors. Furthermore, we found that blocking ERα attenuated luminal progenitor expansion and decreased the luminal colony-forming potential of these progenitors. Additionally, blocking ERα decreased H19 expression in the luminal progenitors and led to the development of smaller luminal colonies. We further showed that knocking down the H19 gene in the luminal progenitors significantly decreased the colony-forming potential of the luminal progenitors, and this phenotype could not be rescued by the addition of estrogen. Lastly, we explored the clinical relevance of the estrogen-H19 signaling axis in breast tumors and found that ERα(+) tumors exhibited a higher expression of H19 as compared with ERα(-) tumors and that H19 expression showed a positive correlation with ERα expression in those tumors. Taken together, the present results indicate that the estrogen-ERα-H19 signaling axis plays a role in regulating the proliferation and differentiation potentials of the normal luminal progenitors and that this signaling network may also be important in the development of ER(+) breast cancer tumors.

Combined Single-Cell Functional and Gene Expression Analysis Resolves Heterogeneity within Stem Cell Populations

Heterogeneity within the self-renewal durability of adult hematopoietic stem cells (HSCs) challenges our understanding of the molecular framework underlying HSC function. Gene expression studies have been hampered by the presence of multiple HSC subtypes and contaminating non-HSCs in bulk HSC populations. To gain deeper insight into the gene expression program of murine HSCs, we combined single-cell functional assays with flow cytometric index sorting and single-cell gene expression assays. Through bioinformatic integration of these datasets, we designed an unbiased sorting strategy that separates non-HSCs away from HSCs, and single-cell transplantation experiments using the enriched population were combined with RNA-seq data to identify key molecules that associate with long-term durable self-renewal, producing a single-cell molecular dataset that is linked to functional stem cell activity. Finally, we demonstrated the broader applicability of this approach for linking key molecules with defined cellular functions in another stem cell system.

Luminal progenitors restrict their lineage potential during mammary gland development

The hierarchical relationships between stem cells and progenitors that guide mammary gland morphogenesis are still poorly defined. While multipotent basal stem cells have been found within the myoepithelial compartment, the in vivo lineage potential of luminal progenitors is unclear. Here we used the expression of the Notch1 receptor, previously implicated in mammary gland development and tumorigenesis, to elucidate the hierarchical organization of mammary stem/progenitor cells by lineage tracing. We found that Notch1 expression identifies multipotent stem cells in the embryonic mammary bud, which progressively restrict their lineage potential during mammary ductal morphogenesis to exclusively generate an ERα^neg luminal lineage postnatally. Importantly, our results show that Notch1-labelled cells represent the alveolar progenitors that expand during pregnancy and survive multiple successive involutions. This study reveals that postnatal luminal epithelial cells derive from distinct self-sustained lineages that may represent the cells of origin of different breast cancer subtypes.

Stem cells in the embryonic mammary gland that express the Notch1 receptor are initially multipotent and highly regenerative, but they progressively restrict their lineage potential to the lumen of the mammary duct, where they may give rise to breast cancer.

Tissue-specific stem cells are believed to be multipotent, thus able to generate all cell types of their tissue of origin. In the mammary gland epithelium, however, the existence of multipotent versus unipotent adult stem cells is currently under debate. In this study, we have identified and characterized a population of mammary luminal progenitors that express the Notch1 receptor. Using lineage tracing experiments, we found that these cells are self-sustained unipotent adult progenitors with high self-renewal capacity. Although they lack estrogen and progesterone hormone receptors, these cells are highly responsive to hormones. Importantly, Notch1-expressing cells are multipotent during embryonic mammary development, when they can give rise to all mammary cell types, while they become lineage-restricted postnatally. The cells characterized in this study also present extensive plasticity, as they can repopulate the entire mammary gland in transplantation experiments. Our study reveals that the Notch1 receptor is a specific marker for the identification of luminal progenitors that lack expression of hormone receptors and that can be critical for breast cancer initiation.

Anatomical localization of progenitor cells in human breast tissue reveals enrichment of uncommitted cells within immature lobules

INTRODUCTION

Lineage tracing studies in mice have revealed the localization and existence of lineage-restricted mammary epithelial progenitor cells that functionally contribute to expansive growth during puberty and differentiation during pregnancy. However, extensive anatomical differences between mouse and human mammary tissues preclude the direct translation of rodent findings to the human breast. Therefore, here we characterize the mammary progenitor cell hierarchy and identify the anatomic location of progenitor cells within human breast tissues.

METHODS

Mammary epithelial cells (MECs) were isolated from disease-free reduction mammoplasty tissues and assayed for stem/progenitor activity in vitro and in vivo. MECs were sorted and evaluated for growth on collagen and expression of lineages markers. Breast lobules were microdissected and individually characterized based on lineage markers and steroid receptor expression to identify the anatomic location of progenitor cells. Spanning-tree progression analysis of density-normalized events (SPADE) was used to identify the cellular hierarchy of MECs within lobules from high-dimensional cytometry data.

RESULTS

Integrating multiple assays for progenitor activity, we identified the presence of luminal alveolar and basal ductal progenitors. Further, we show that Type I lobules of the human breast were the least mature, demonstrating an unrestricted pattern of expression of luminal and basal lineage markers. Consistent with this, SPADE analysis revealed that immature lobules were enriched for basal progenitor cells, while mature lobules consisted of increased hierarchal complexity of cells within the luminal lineages.

CONCLUSIONS

These results reveal underlying differences in the human breast epithelial hierarchy and suggest that with increasing glandular maturity, the epithelial hierarchy also becomes more complex.

TGFβ induces the formation of tumour-initiating cells in claudinlow breast cancer

The role of transforming growth factor-beta (TGFβ) in the progression of different molecular subtypes of breast cancer has not been clarified. Here we show that TGFβ increases breast tumour-initiating cell (BTIC) numbers but only in claudin(low) breast cancer cell lines by orchestrating a specific gene signature enriched in stem cell processes that predicts worse clinical outcome in breast cancer patients. NEDD9, a member of the Cas family of integrin scaffold proteins, is necessary to mediate these TGFβ-specific effects through a positive feedback loop that integrates TGFβ/Smad and Rho-actin-SRF-dependent signals. In normal human mammary epithelium, TGFβ induces progenitor activity only in the basal/stem cell compartment, where claudin(low) cancers are presumed to arise. These data show opposing responses to TGFβ in both breast malignant cell subtypes and normal mammary epithelial cell subpopulations and suggest therapeutic strategies for a subset of human breast cancers.

Phenotypic and functional characterisation of the luminal cell hierarchy of the mammary gland

Introduction

The organisation of the mammary epithelial hierarchy is poorly understood. Our hypothesis is that the luminal cell compartment is more complex than initially described, and that an understanding of the developmental relationships within this lineage will help in understanding the cellular context in which breast tumours occur.

Methods

We used fluorescence-activated cell sorting along with in vitro and in vivo functional assays to examine the growth and differentiation properties of distinct subsets of human and mouse mammary epithelial cells. We also examined how loss of steroid hormones influenced these populations in vivo. Gene expression profiles were also obtained for all the purified cell populations and correlated to those obtained from breast tumours.

Results

The luminal cell compartment of the mouse mammary gland can be resolved into nonclonogenic oestrogen receptor-positive (ER⁺) luminal cells, ER⁺ luminal progenitors and oestrogen receptor-negative (ER^-) luminal progenitors. The ER⁺ luminal progenitors are unique in regard to cell survival, as they are relatively insensitive to loss of oestrogen and progesterone when compared with the other types of mammary epithelial cells. Analysis of normal human breast tissue reveals a similar hierarchical organisation composed of nonclonogenic luminal cells, and relatively differentiated (EpCAM⁺CD49f⁺ALDH^-) and undifferentiated (EpCAM⁺CD49f⁺ALDH⁺) luminal progenitors. In addition, approximately one-quarter of human breast samples examined contained an additional population that had a distinct luminal progenitor phenotype, characterised by low expression of ERBB3 and low proliferative potential. Parent-progeny relationship experiments demonstrated that all luminal progenitor populations in both species are highly plastic and, at low frequencies, can generate progeny representing all mammary cell types. The ER^- luminal progenitors in the mouse and the ALDH⁺ luminal progenitors in the human appear to be analogous populations since they both have gene signatures that are associated with alveolar differentiation and resemble those obtained from basal-like breast tumours.

Conclusion

The luminal cell compartment in the mammary epithelium is more heterogeneous than initially perceived since progenitors of varying levels of luminal cell differentiation and proliferative capacities can be identified. An understanding of these cells will be essential for understanding the origins and the cellular context of human breast tumours.

The biology of human breast epithelial progenitors

Current evidence suggests that similar to other tissues in the human body mammary epithelia cells are being maintained by the unique properties of stem cells, undifferentiated as well as lineage-restricted progenitors. Because of their longevity, proliferation and differentiation potentials these primitive breast epithelial cells are likely targets of transforming mutations that can cause them to act as cancer initiating cells. In this context, understanding the molecular mechanisms that regulate the normal functions of the human breast epithelial stem cells and progenitors and how alterations to these same mechanisms can confer a cancer stem cell phenotype on these rare cell populations is crucial to the development of new and more effective therapies again breast cancer. This review article will examine the current state of knowledge about the isolation and characterization of human breast epithelial progenitors and their relevance to breast cancer research.