Proof of region-specific multipotent progenitors in human breast epithelia

Organoid Cultures for the Study of Mammary Biology and Breast Cancer: The Promise and Challenges

During the last decade, biomedical research has experienced a resurgence in the use of three-dimensional culture models for studies of normal and cancer biology. This resurgence has been driven by the development of models in which primary cells are grown in tissue-mimicking media and extracellular matrices to create organoid or organotypic cultures that more faithfully replicate the complex architecture and physiology of normal tissues and tumors. In addition, patient-derived tumor organoids preserve the three-dimensional organization and characteristics of the patient tumors ex vivo, becoming excellent preclinical models to supplement studies of tumor xenografts transplanted into immunocompromised mice. In this perspective, we provide an overview of how organoids are being used to investigate normal mammary biology and as preclinical models of breast cancer and discuss improvements that would enhance their utility and relevance to the field.

Unlocking Translational Potential: Conditionally Reprogrammed Cells in Advancing Breast Cancer Research

Preclinical in vitro models play an important role in studying cancer cell biology and facilitating translational research, especially in the identification of drug targets and drug discovery studies. This is particularly relevant in breast cancer, where the global burden of disease is quite high based on prevalence and a relatively high rate of lethality. Predictive tools to select patients who will be responsive to invasive or morbid therapies (radiotherapy, chemotherapy, immunotherapy, and/or surgery) are relatively lacking. To be clinically relevant, a model must accurately replicate the biology and cellular heterogeneity of the primary tumor. Addressing these requirements and overcoming the limitations of most existing cancer cell lines, which are typically derived from a single clone, we have recently developed conditional reprogramming (CR) technology. The CR technology refers to a co-culture system of primary human normal or tumor cells with irradiated murine fibroblasts in the presence of a Rho-associated kinase inhibitor to allow the primary cells to acquire stem cell properties and the ability to proliferate indefinitely in vitro without any exogenous gene or viral transfection. This innovative approach fulfills many of these needs and offers an alternative that surpasses the deficiencies associated with traditional cancer cell lines. These CR cells (CRCs) can be reprogrammed to maintain a highly proliferative state and reproduce the genomic and histological characteristics of the parental tissue. Therefore, CR technology may be a clinically relevant model to test and predict drug sensitivity, conduct gene profile analysis and xenograft research, and undertake personalized medicine. This review discusses studies that have applied CR technology to conduct breast cancer research.

An Early Neoplasia Index (ENI10), Based on Molecular Identity of CD10 Cells and Associated Stemness Biomarkers, is a Predictor of Patient Outcome in Many Cancers

An accurate estimate of patient survival at diagnosis is critical to plan efficient therapeutic options. A simple and multiapplication tool is needed to move forward the precision medicine era. Taking advantage of the broad and high CD10 expression in stem and cancers cells, we evaluated the molecular identity of aggressive cancer cells. We used epithelial primary cells and developed a breast cancer stem cell–based progressive model. The superiority of the early-transformed isolated molecular index was evaluated by large-scale analysis in solid cancers. BMP2-driven cell transformation increases CD10 expression which preserves stemness properties. Our model identified a unique set of 159 genes enriched in G2–M cell-cycle phases and spindle assembly complex. Using samples predisposed to transformation, we confirmed the value of an early neoplasia index associated to CD10 (ENI10) to discriminate premalignant status of a human tissue. Using a stratified Cox model, a large-scale analysis (>10,000 samples, The Cancer Genome Atlas Pan-Cancer) validated a strong risk gradient (HRs reaching HR = 5.15; 95% confidence interval: 4.00–6.64) for high ENI10 levels. Through different databases, Cox regression model analyses highlighted an association between ENI10 and poor progression-free intervals for more than 50% of cancer subtypes tested, and the potential of ENI10 to predict drug efficacy. The ENI10 index constitutes a robust tool to detect pretransformed tissues and identify high-risk patients at diagnosis. Owing to its biological link with refractory cancer stem cells, the ENI10 index constitutes a unique way of identifying effective treatments to improve clinical care.

SIGNIFICANCE

We identified a molecular signature called ENI10 which, owing to its biological link with stem cell properties, predicts patient outcome and drugs efficiency in breast and several other cancers. ENI10 should allow early and optimized clinical management of a broad number of cancers, regardless of the stage of tumor progression.

Imagine beyond: recent breakthroughs and next challenges in mammary gland biology and breast cancer research

On 8 December 2022 the organizing committee of the European Network for Breast Development and Cancer labs (ENBDC) held its fifth annual Think Tank meeting in Amsterdam, the Netherlands. Here, we embraced the opportunity to look back to identify the most prominent breakthroughs of the past ten years and to reflect on the main challenges that lie ahead for our field in the years to come. The outcomes of these discussions are presented in this position paper, in the hope that it will serve as a summary of the current state of affairs in mammary gland biology and breast cancer research for early career researchers and other newcomers in the field, and as inspiration for scientists and clinicians to move the field forward.

Cellular Plasticity and Heterotypic Interactions during Breast Morphogenesis and Cancer Initiation

Simple Summary

This review aims to discuss the structure, function and dynamics of the breast gland and how changes to the function of the breast’s cells can lead to different types of cancer.

Abstract

The human breast gland is a unique organ as most of its development occurs postnatally between menarche and menopause, a period ranging from 30 to 40 years. During this period, the monthly menstruation cycle drives the mammary gland through phases of cell proliferation, differentiation, and apoptosis, facilitated via a closely choreographed interaction between the epithelial cells and the surrounding stroma preparing the gland for pregnancy. If pregnancy occurs, maximal differentiation is reached to prepare for lactation. After lactation, the mammary gland involutes to a pre-pregnant state. These cycles of proliferation, differentiation, and involution necessitate the presence of epithelial stem cells that give rise to progenitor cells which differentiate further into the luminal and myoepithelial lineages that constitute the epithelial compartment and are responsible for the branching structure of the gland. Maintaining homeostasis and the stem cell niche depends strongly on signaling between the stem and progenitor cells and the surrounding stroma. Breast cancer is a slowly progressing disease whose initiation can take decades to progress into an invasive form. Accumulating evidence indicates that stem cells and/or progenitor cells at different stages, rather than terminally differentiated cells are the main cells of origin for most breast cancer subgroups. Stem cells and cancer cells share several similarities such as increased survival and cellular plasticity which is reflected in their ability to switch fate by receiving intrinsic and extrinsic signals. In this review, we discuss the concept of cellular plasticity in normal breast morphogenesis and cancer, and how the stromal environment plays a vital role in cancer initiation and progression.

Sci-Seq of Human Fetal Salivary Tissue Introduces Human Transcriptional Paradigms and a Novel Cell Population

Multiple pathologies and non-pathological factors can disrupt the function of the non-regenerative human salivary gland including cancer and cancer therapeutics, autoimmune diseases, infections, pharmaceutical side effects, and traumatic injury. Despite the wide range of pathologies, no therapeutic or regenerative approaches exist to address salivary gland loss, likely due to significant gaps in our understanding of salivary gland development. Moreover, identifying the tissue of origin when diagnosing salivary carcinomas requires an understanding of human fetal development. Using computational tools, we identify developmental branchpoints, a novel stem cell-like population, and key signaling pathways in the human developing salivary glands by analyzing our human fetal single-cell sequencing data. Trajectory and transcriptional analysis suggest that the earliest progenitors yield excretory duct and myoepithelial cells and a transitional population that will yield later ductal cell types. Importantly, this single-cell analysis revealed a previously undescribed population of stem cell-like cells that are derived from SD and expresses high levels of genes associated with stem cell-like function. We have observed these rare cells, not in a single niche location but dispersed within the developing duct at later developmental stages. Our studies introduce new human-specific developmental paradigms for the salivary gland and lay the groundwork for the development of translational human therapeutics.

Ductal keratin 15+ luminal progenitors in normal breast exhibit a basal-like breast cancer transcriptomic signature

Normal breast luminal epithelial progenitors have been implicated as cell of origin in basal-like breast cancer, but their anatomical localization remains understudied. Here, we combine collection under the microscope of organoids from reduction mammoplasties and single-cell mRNA sequencing (scRNA-seq) of FACS-sorted luminal epithelial cells with multicolor imaging to profile ducts and terminal duct lobular units (TDLUs) and compare them with breast cancer subtypes. Unsupervised clustering reveals eleven distinct clusters and a differentiation trajectory starting with keratin 15⁺ (K15⁺) progenitors enriched in ducts. Spatial mapping of luminal progenitors is confirmed at the protein level by staining with critical duct markers. Comparison of the gene expression profiles of normal luminal cells with those of breast cancer subtypes suggests a strong correlation between normal breast ductal progenitors and basal-like breast cancer. We propose that K15⁺ basal-like breast cancers originate in ductal progenitors, which emphasizes the importance of not only lineages but also cellular position within the ductal-lobular tree.

Peroxidasin Enhances Basal Phenotype and Inhibits Branching Morphogenesis in Breast Epithelial Progenitor Cell Line D492

The human breast is composed of terminal duct lobular units (TDLUs) that are surrounded by stroma. In the TDLUs, basement membrane separates the stroma from the epithelial compartment, which is divided into an inner layer of luminal epithelial cells and an outer layer of myoepithelial cells. Stem cells and progenitor cells also reside within the epithelium and drive a continuous cycle of gland remodelling that occurs throughout the reproductive period. D492 is an epithelial cell line originally isolated from the stem cell population of the breast and generates both luminal and myoepithelial cells in culture. When D492 cells are embedded into 3D reconstituted basement membrane matrix (3D-rBM) they form branching colonies mimicking the TDLUs of the breast, thereby providing a well-suited in vitro model for studies on branching morphogenesis and breast development. Peroxidasin (PXDN) is a heme-containing peroxidase that crosslinks collagen IV with the formation of sulfilimine bonds. Previous studies indicate that PXDN plays an integral role in basement membrane stabilisation by crosslinking collagen IV and as such contributes to epithelial integrity. Although PXDN has been linked to fibrosis and cancer in some organs there is limited information on its role in development, including in the breast. In this study, we demonstrate expression of PXDN in breast epithelium and stroma and apply the D492 cell line to investigate the role of PXDN in cell differentiation and branching morphogenesis in the human breast. Overexpression of PXDN induced basal phenotype in D492 cells, loss of plasticity and inhibition of epithelial-to-mesenchymal transition as is displayed by complete inhibition of branching morphogenesis in 3D culture. This is supported by results from RNA-sequencing which show significant enrichment in genes involved in epithelial differentiation along with significant negative enrichment of EMT factors. Taken together, we provide evidence for a novel role of PXDN in breast epithelial differentiation and mammary gland development.

Current Status of Breast Organoid Models

Breast cancer (BC) is the most frequently diagnosed malignancy among women globally. Although mouse models have been critical in advancing the knowledge of BC tumorigenesis and progression, human breast models comprising the breast tissue microenvironment are needed to help elucidate the underlying mechanisms of BC risk factors. As such, it is essential to identify an ex vivo human breast tissue mimetic model that can accurately pinpoint the effects of these factors in BC development. While two-dimensional models have been invaluable, they are not suitable for studying patient-specific tumor biology and drug response. Recent developments in three-dimensional (3D) models have led to the prominence of organized structures grown in a 3D environment called “organoids.” Breast organoids can accurately recapitulate the in vivo breast microenvironment and have been used to examine factors that affect signaling transduction, gene expression, and tissue remodeling. In this review, the applications, components, and protocols for development of breast organoids are discussed. We summarize studies that describe the utility of breast organoids, including in the study of normal mammary gland development and tumorigenesis. Finally, we provide an overview of protocols for development of breast organoids, and the advantages and disadvantages of different techniques in studies are described. The included studies have shown that breast organoids will continue to serve as a crucial platform for understanding of progression of BC tumors and the testing of novel therapeutics.

Desensitization of human breast progenitors by a transient exposure to pregnancy levels of estrogen

Full term pregnancy at an early age is the only factor known to consistently protect against breast cancer. Because hormone receptor positive progenitors in the human breast relay endocrine signaling, we here sought to determine whether an experimental mimicry of the third trimester surge of hormones would change their susceptibility to growth stimulation. Hormone receptor positive, reduction mammoplasty-derived human breast epithelial progenitors were exposed to a short-term, pregnancy-level of estradiol, and their subsequent response to estradiol stimulation was analyzed. Exposure to pregnancy-level of estradiol results in subsequent lower sensitivity to estrogen-induced proliferation. Expression array and immunoblotting reveal upregulation of S100A7 and down-regulation of p27, both associated with parity and epithelial differentiation. Notably, we find that the epithelial differentiation is accompanied by upregulation of E-cadherin and down-regulation of vimentin as well as by diminished migration and more mature luminal epithelial differentiation in a mouse transplantation model. Our findings are in support of a de-sensitization mechanism for pregnancy-induced prevention against breast cancer.

Specific cell differentiation in breast cancer: a basis for histological classification

Breast parenchyma progenitor cells show a high degree of phenotypic plasticity reflected in the wide range of morphology observed in benign and malignant breast tumours. Although there is evidence suggesting that all breast cancer (BC) arises from a common epithelial progenitor or stem cell located at the terminal duct lobular units (TDLUs), BC shows a broad spectrum of morphology with extensive variation in histological type and grade. This is related to the complexity of BC carcinogenesis including initial genetic changes in the cell of origin, subsequent genetic and epigenetic alterations and reprogramming that occur at various stages of BC development and the interplay with the surrounding microenvironment, factors which influence the process of differentiation. Differentiation in BC determines the morphology, which can be measured using histological grade and tumour type. Histological grade, which measures the similarity to the TDLUs, reflects the degree of differentiation whereas tumour type reflects the type of differentiation. Understanding BC phenotypic differentiation facilitates the accurate diagnosis and histological classification of BC with corresponding clinical implications in terms of disease behaviour, prognosis and management plans. In this review, we highlight the potential pathways that BC stem cells follow resulting in the development of different histological types of BC and how knowledge of these pathways impacts our ability to classify BC in diagnostic practice. We also discuss the role of cellular differentiation in producing metaplastic and neuroendocrine carcinomas of the breast and how the latter differ from their counterparts in other organs, with emphasis on clinical relevance.

High-throughput surface marker screen on primary human breast tissues reveals further cellular heterogeneity

BACKGROUND

Normal human breast tissues are a heterogeneous mix of epithelial and stromal subtypes in different cell states. Delineating the spectrum of cellular heterogeneity will provide new insights into normal cellular properties within the breast tissue that might become dysregulated in the initial stages of cancer. Investigation of surface marker expression provides a valuable approach to resolve complex cell populations. However, the majority of cell surface maker expression of primary breast cells have not been investigated.

METHODS

To determine the differences in expression of a range of uninvestigated cell surface markers between the normal breast cell subpopulations, primary human breast cells were analysed using high-throughput flow cytometry for the expression of 242 cell surface proteins in conjunction with EpCAM/CD49f staining.

RESULTS

We identified 35 surface marker proteins expressed on normal breast epithelial and/or stromal subpopulations that were previously unreported. We also show multiple markers were equally expressed in all cell populations (e.g. CD9, CD59, CD164) while other surface markers were confirmed to be enriched in different cell lineages: CD24, CD227 and CD340 in the luminal compartment, CD10 and CD90 in the basal population, and CD34 and CD140b on stromal cells.

CONCLUSIONS

Our dataset of CD marker expression in the normal breast provides better definition for breast cellular heterogeneity.

Cytokeratin 5 determines maturation of the mammary myoepithelium

At invasion, transformed mammary epithelial cells expand into the stroma through a disrupted myoepithelial (ME) cell layer and basement membrane (BM). The intact ME cell layer has thus been suggested to act as a barrier against invasion. Here, we investigate the mechanisms behind the disruption of ME cell layer. We show that the expression of basal/ME proteins CK5, CK14, and α-SMA altered along increasing grade of malignancy, and their loss affected the maintenance of organotypic 3D mammary architecture. Furthermore, our data suggests that loss of CK5 prior to invasive stage causes decreased levels of Zinc finger protein SNAI2 (SLUG), a key regulator of the mammary epithelial cell lineage determination. Consequently, a differentiation bias toward luminal epithelial cell type was detected with loss of mature, α-SMA-expressing ME cells and reduced deposition of basement membrane protein laminin-5. Therefore, our data discloses the central role of CK5 in mammary epithelial differentiation and maintenance of normal ME layer.

Volume-constrained microcontainers enable myoepithelial functional differentiation in highly parallel mammary organoid culture

A long-standing constraint on organoid culture is the need to add exogenous substances to provide hydrogel matrix, which limits the study of fully human or fully native organoids. This paper introduces an approach to culture reconstituted mammary organoids without the impediment of exogenous matrix. We enclose organoids in nanoliter-scale, topologically enclosed, fluid compartments surrounded by agar. Organoids cultured in these “microcontainers” appear to secrete enough extracellular matrix to yield a self-sufficient microenvironment without exogenous supplements. In microcontainers, mammary organoids exhibit contractility and a high-level, physiological, myoepithelial (MEP) behavior that has not been previously reported in reconstituted organoids. The presence of contractility suggests that microcontainers elicit MEP functional differentiation, an important milestone. Microcontainers yield thousands of substantially identical and individually trackable organoids within a single culture vessel, enabling longitudinal studies and statistically powerful experiments, such as the evaluation of small effect sizes. Microcontainers open new doors for researchers who rely on organoid models.

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Microcontainers are volume-constrained microwells with hydrogel lids

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Microcontainers enable statistically robust experimental design with organoids

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Organoids produce their own extracellular matrix within microcontainers

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Myoepithelial cells in mammary organoids achieve fully functional differentiation

Breast cancers, mammary stem cells, and cancer stem cells, characteristics, and hypotheses

The cellular heterogeneity of breast cancers still represents a major therapeutic challenge. The latest genomic studies have classified breast cancers in distinct clusters to inform the therapeutic approaches and predict clinical outcomes. The mammary epithelium is composed of luminal and basal cells, and this seemingly hierarchical organization is dependent on various stem cells and progenitors populating the mammary gland. Some cancer cells are conceptually similar to the stem cells as they can self-renew and generate bulk populations of nontumorigenic cells. Two models have been proposed to explain the cell of origin of breast cancer and involve either the reprogramming of differentiated mammary cells or the dysregulation of mammary stem cells or progenitors. Both hypotheses are not exclusive and imply the accumulation of independent mutational events. Cancer stem cells have been isolated from breast tumors and implicated in the development, metastasis, and recurrence of breast cancers. Recent advances in single-cell sequencing help deciphering the clonal evolution within each breast tumor. Still, few clinical trials have been focused on these specific cancer cell populations.

Mammary Organoids and 3D Cell Cultures: Old Dogs with New Tricks

3D cell culture methods have been an integral part of and an essential tool for mammary gland and breast cancer research for half a century. In fact, mammary gland researchers, who discovered and deciphered the instructive role of extracellular matrix (ECM) in mammary epithelial cell functional differentiation and morphogenesis, were the pioneers of the 3D cell culture techniques, including organoid cultures. The last decade has brought a tremendous increase in the 3D cell culture techniques, including modifications and innovations of the existing techniques, novel biomaterials and matrices, new technological approaches, and increase in 3D culture complexity, accompanied by several redefinitions of the terms "3D cell culture" and "organoid". In this review, we provide an overview of the 3D cell culture and organoid techniques used in mammary gland biology and breast cancer research. We discuss their advantages, shortcomings and current challenges, highlight the recent progress in reconstructing the complex mammary gland microenvironment in vitro and ex vivo, and identify the missing 3D cell cultures, urgently needed to aid our understanding of mammary gland development, function, physiology, and disease, including breast cancer.

Heterogeneity of CEACAM5 in breast cancer

CEACAM5 is overexpressed in many primary breast carcinomas. However, the exact role of CEACAM5 in breast cancer tumorigenesis remains unresolved. Here, we examined a repository of 110 cryopreserved primary breast carcinomas by immunohistochemistry to assess the distribution of CEACAM5 in tumor subtypes. The majority of estrogen receptor-positive and HER2-overexpressing tumors were CEACAM5-positive, whereas most of Triple-negative tumors were negative. Assessing sample sets of paired primary breast cancers and corresponding lymph node lesions from a total of 59 patients revealed a high correlation between primary tumor and lymph node with regard to CEACAM5-status. However, a notable subset of sample sets demonstrated intratumoral heterogeneity in the primary tumor, the metastatic lesion or both, suggesting that both CEACAM5-positive and –negative cells can play a role in tumor dissemination. When examining the consequence of expression of CEACAM5 in breast cancer cell lines in culture assays we found that CEACAM5-expressing cells were less invasive. In survival analysis, using cohort studies of breast cancer, expression of CEACAM5 predicted different clinical outcomes depending on molecular subtypes. Altogether, our analysis suggests that CEACAM5 plays a context-dependent role in breast cancer that warrants further investigation.

AXL Is a Driver of Stemness in Normal Mammary Gland and Breast Cancer

The receptor tyrosine kinase AXL is associated with epithelial plasticity in several solid tumors including breast cancer and AXL-targeting agents are currently in clinical trials. We hypothesized that AXL is a driver of stemness traits in cancer by co-option of a regulatory function normally reserved for stem cells. AXL-expressing cells in human mammary epithelial ducts co-expressed markers associated with multipotency, and AXL inhibition abolished colony formation and self-maintenance activities while promoting terminal differentiation in vitro. Axl-null mice did not exhibit a strong developmental phenotype, but enrichment of Axl ⁺ cells was required for mouse mammary gland reconstitution upon transplantation, and Axl-null mice had reduced incidence of Wnt1-driven mammary tumors. An AXL-dependent gene signature is a feature of transcriptomes in basal breast cancers and reduced patient survival irrespective of subtype. Our interpretation is that AXL regulates access to epithelial plasticity programs in MaSCs and, when co-opted, maintains acquired stemness in breast cancer cells.

Fibroblasts direct differentiation of human breast epithelial progenitors

BACKGROUND

Breast cancer arises within specific regions in the human breast referred to as the terminal duct lobular units (TDLUs). These are relatively dynamic structures characterized by sex hormone driven cyclic epithelial turnover. TDLUs consist of unique parenchymal entities embedded within a fibroblast-rich lobular stroma. Here, we established and characterized a new human breast lobular fibroblast cell line against its interlobular counterpart with a view to assessing the role of region-specific stromal cues in the control of TDLU dynamics.

METHODS

Primary lobular and interlobular fibroblasts were transduced to express human telomerase reverse transcriptase (hTERT). Differentiation of the established cell lines along lobular and interlobular pathways was determined by immunocytochemical staining and genome-wide RNA sequencing. Their functional properties were further characterized by analysis of mesenchymal stem cell (MSC) differentiation repertoire in culture and in vivo. The cells' physiological relevance for parenchymal differentiation was examined in heterotypic co-culture with fluorescence-activated cell sorting (FACS)-purified normal breast primary luminal or myoepithelial progenitors. The co-cultures were immunostained for quantitative assessment of epithelial branching morphogenesis, polarization, growth, and luminal epithelial maturation. In extension, myoepithelial progenitors were tested for luminal differentiation capacity in culture and in mouse xenografts. To unravel the significance of transforming growth factor-beta (TGF-β)-mediated crosstalk in TDLU-like morphogenesis and differentiation, fibroblasts were incubated with the TGF-β signaling inhibitor, SB431542, prior to heterotypic co-culture with luminal cells.

RESULTS

hTERT immortalized fibroblast cell lines retained critical phenotypic traits in culture and linked to primary fibroblasts. Cell culture assays and transplantation to mice showed that the origin of fibroblasts determines TDLU-like and ductal-like differentiation of epithelial progenitors. Whereas lobular fibroblasts supported a high level of branching morphogenesis by luminal cells, interlobular fibroblasts supported ductal-like myoepithelial characteristics. TDLU-like morphogenesis, at least in part, relied on intact TGF-β signaling.

CONCLUSIONS

The significance of the most prominent cell type in normal breast stroma, the fibroblast, in directing epithelial differentiation is largely unknown. Through establishment of lobular and interlobular fibroblast cell lines, we here demonstrate that epithelial progenitors are submitted to stromal cues for site-specific differentiation. Our findings lend credence to considering stromal subtleties of crucial importance in the development of normal breast and, in turn, breast cancer.

The Expression Pattern of Epidermal Differentiation Marker Keratin 10 in the Normal Human Breast and Breast Cancer Cells

Cells of the human breast gland express an array of keratins, of which some are used for characterizing both normal and neoplastic breast tissue. However, the expression pattern of certain keratins has yet to be detailed. Here, the expression of a differentiation marker of epidermal epithelium, keratin 10 (K10), was investigated in the human breast gland. While in normal breast tissue generally less than 1% of luminal epithelial cells expressed K10, in women >30 years of age glandular structures with K10-positive (K10^pos) cells were found at higher frequency than in younger women. K10^pos cells belong to a mature luminal compartment as they were negative for cKIT, positive for Ks20.8, and mostly non-cycling. In breast cancer, around 16% of primary breast carcinomas tested were positive for K10 by immunohistochemistry. Interestingly, K10^pos tumor cells generally exhibit features of differentiation similar to their normal counterparts. Although this suggests that K10 is a marker of tumor differentiation, data based on gene expression analysis imply that high levels of K10 dictate a worse outcome for breast cancer patients. These findings can form the basis of future studies that should unravel which role K10 may play as a marker of breast cancer.

Expression of ncRNAs on the DLK1-DIO3 Locus Is Associated With Basal and Mesenchymal Phenotype in Breast Epithelial Progenitor Cells

Epithelial-to-mesenchymal transition (EMT) and its reversed process mesenchymal-to-epithelial transition (MET) play a critical role in epithelial plasticity during development and cancer progression. Among important regulators of these cellular processes are non-coding RNAs (ncRNAs). The imprinted DLK1-DIO3 locus, containing numerous maternally expressed ncRNAs including the lncRNA maternally expressed gene 3 (MEG3) and a cluster of over 50 miRNAs, has been shown to be a modulator of stemness in embryonic stem cells and in cancer progression, potentially through the tumor suppressor role of MEG3. In this study we analyzed the expression pattern and functional role of ncRNAs from the DLK1-DIO3 locus in epithelial plasticity of the breast. We studied their expression in various cell types of breast tissue and revisit the role of the locus in EMT/MET using a breast epithelial progenitor cell line (D492) and its isogenic mesenchymal derivative (D492M). Marked upregulation of ncRNAs from the DLK1-DIO3 locus was seen after EMT induction in two cell line models of EMT. In addition, the expression of MEG3 and the maternally expressed ncRNAs was higher in stromal cells compared to epithelial cell types in primary breast tissue. We also show that expression of MEG3 is concomitant with the expression of the ncRNAs from the DLK1-DIO3 locus and its expression is therefore likely indicative of activation of all ncRNAs at the locus. MEG3 expression is correlated with stromal markers in normal tissue and breast cancer tissue and negatively correlated with the survival of breast cancer patients in two different cohorts. Overexpression of MEG3 using CRISPR activation in a breast epithelial cell line induced partial EMT and enriched for a basal-like phenotype. Conversely, knock down of MEG3 using CRISPR inhibition in a mesenchymal cell line reduced the mesenchymal and basal-like phenotype of the cell line. In summary our study shows that maternally expressed ncRNAs are markers of EMT and suggests that MEG3 is a novel regulator of EMT/MET in breast tissue. Nevertheless, further studies are needed to fully dissect the molecular pathways influenced by non-coding RNAs at the DLK1-DIO3 locus in breast tissue.

Characterization of organoid cultured human breast cancer

Organoid cultures are increasingly used to model human cancers experimentally with a view to tailoring personalized medicine and predicting drug responses. Breast cancer is no exception, but in particular, primary breast cancer poses some inherent difficulties due to the frequent presence of residual non-malignant cells in the biopsies. We originally developed an assay for the distinction between malignant and non-malignant structures in primary breast cancer organoid cultures (Petersen et al., Proc Natl Acad Sci (USA) 89(19):9064–8, 1992). Here, we apply this assay to assess the frequency of normal-like organoids in primary breast carcinoma cultures and the cellular composition as a consequence of passaging. We find that in consecutively collected samples of primary human breast cancers, residual non-malignant tissues were observed histologically in five out of ten biopsies. Based on relevant morphogenesis and correct polarization as recorded by expression in luminal epithelial cells of mucin 1 (Muc1), occludin, and keratin 19 (K19) and expression in basal cells of integrin β4, p63, and K14, non-malignant organoids were present in all primary human breast cancer-derived cultures. Furthermore, passaging in a contemporary culture medium was in favor of the selective expansion of basal-like cells. We conclude that organoid cultures of human breast cancers are most representative of the tissue origin in primary culture.

A CD146 FACS Protocol Enriches for Luminal Keratin 14/19 Double Positive Human Breast Progenitors

Human breast cancer is believed to arise in luminal progenitors within the normal breast. A subset of these are double positive (DP) for basal and luminal keratins and localizes to a putative stem cell zone within ducts. We here present a new protocol based on a combination of CD146 with CD117 and CD326 which provides an up to thirty fold enrichment of the DP cells. We show by expression profiling, colony formation, and morphogenesis that CD146^high/CD117^high/CD326^high DP cells belong to a luminal progenitor compartment. While these DP cells are located quite uniformly in ducts, with age a variant type of DP (vDP) cells, which is mainly CD146-negative, accumulates in lobules. Intriguingly, in specimens with BRCA1 mutations known to predispose for cancer, higher frequencies of lobular vDP cells are observed. We propose that vDP cells are strong candidates for tracing the cellular origin of breast cancer.

Stem Cells and the Differentiation Hierarchy in Mammary Gland Development

The mammary gland is a highly dynamic organ that undergoes profound changes within its epithelium during puberty and the reproductive cycle. These changes are fueled by dedicated stem and progenitor cells. Both short- and long-lived lineage-restricted progenitors have been identified in adult tissue as well as a small pool of multipotent mammary stem cells (MaSCs), reflecting intrinsic complexity within the epithelial hierarchy. While unipotent progenitor cells predominantly execute day-to-day homeostasis and postnatal morphogenesis during puberty and pregnancy, multipotent MaSCs have been implicated in coordinating alveologenesis and long-term ductal maintenance. Nonetheless, the multipotency of stem cells in the adult remains controversial. The advent of large-scale single-cell molecular profiling has revealed striking changes in the gene expression landscape through ontogeny and the presence of transient intermediate populations. An increasing number of lineage cell-fate determination factors and potential niche regulators have now been mapped along the hierarchy, with many implicated in breast carcinogenesis. The emerging diversity among stem and progenitor populations of the mammary epithelium is likely to underpin the heterogeneity that characterizes breast cancer.

Cancer stem cells in breast and prostate: Fact or fiction?

Since the introduction of the cancer stem cell (CSC) hypothesis, accumulating evidence shows that most cancers present stem-like niches. However, therapies aimed at targeting this niche have not been as successful as expected. New evidence regarding CSCs hierarchy, similarities with normal tissue stem cells and cell plasticity might be key in understanding their role in cancer biology and how to efficiently eliminate them. In this Chapter, we discuss what is known in breast and prostate CSCs from their initial discoveries to the current therapeutic efforts in the field. Future challenges towards better CSC identification and isolation strategies will be key to shed light into how CSCs could accurately be targeted in combination to traditional therapies to ultimately prolong patient survival.

Application of the D492 Cell Lines to Explore Breast Morphogenesis, EMT and Cancer Progression in 3D Culture

The human female breast gland is composed of branching epithelial ducts that extend from the nipple towards the terminal duct lobular units (TDLUs), which are the functional, milk-producing units of the gland and the site of origin of most breast cancers. The epithelium of ducts and TDLUs is composed of an inner layer of polarized luminal epithelial cells and an outer layer of contractile myoepithelial cells, separated from the vascular-rich stroma by a basement membrane. The luminal- and myoepithelial cells share an origin and in recent years, there has been increasing understanding of how these cell types interact and how they contribute to breast cancer. Accumulating evidence links stem/or progenitor cells in the mammary/breast gland to breast cancer. In that regard, much knowledge has been gained from studies in mice due to specific strains that have allowed for gene knock out/in studies and lineage tracing of cellular fates. However, there is a large histologic difference between the human female breast gland and the mouse mammary gland that necessitates that research needs to be done on human material where primary cultures are important due to their close relation to the tissue of origin. However, due to difficulties of long-term cultures and lack of access to material, human cell lines are of great importance to bridge the gap between studies on mouse mammary gland and human primary breast cells. In this review, we describe D492, a breast epithelial progenitor cell line that can generate both luminal- and myoepithelial cells in culture, and in 3D culture it forms branching ducts similar to TDLUs. We have applied D492 and its daughter cell lines to explore cellular and molecular mechanisms of branching morphogenesis and cellular plasticity including EMT and MET. In addition to discussing the application of D492 in studying normal morphogenesis, we will also discuss how this cell line has been used to study breast cancer progression.

Notch and Wnt Dysregulation and Its Relevance for Breast Cancer and Tumor Initiation

Breast cancer is the second leading cause of cancer deaths among women in the world. Treatment has been improved and, in combination with early detection, this has resulted in reduced mortality rates. Further improvement in therapy development is however warranted. This will be particularly important for certain sub-classes of breast cancer, such as triple-negative breast cancer, where currently no specific therapies are available. An important therapy development focus emerges from the notion that dysregulation of two major signaling pathways, Notch and Wnt signaling, are major drivers for breast cancer development. In this review, we discuss recent insights into the Notch and Wnt signaling pathways and into how they act synergistically both in normal development and cancer. We also discuss how dysregulation of the two pathways contributes to breast cancer and strategies to develop novel breast cancer therapies starting from a Notch and Wnt dysregulation perspective.

Expression of Luminal Progenitor Marker CD117 in the Human Breast Gland

CD117 is a putative marker of luminal progenitor cells in the human breast. However, so far mapping the expression pattern of CD117 within the normal gland has not been reported. Here, we examined the anatomical distribution of CD117-expressing cells in lobular and ductal structures by immunohistochemistry. The presence of CD117-positive luminal cells could be divided into three distinct patterns: (1) contiguous, with coherent positive cells and rare negative cells interspaced; (2) patched, with a roughly equal frequency of positive and negative cells distributed focally; or (3) scattered, with few or no positive cells in the structure. Generally, a patched or scattered expression pattern was more frequent in lobules compared with ducts. Furthermore, an age-correlated increase in heterogeneity was observed. When comparing women below and above 21 years of age this heterogeneity was evident for both lobules and ducts. Although CD117-expression was generally segregated from luminal-lineage transcription factor GATA3-positive cells, some did co-express both markers. Finally, co-staining with Ki-67 revealed that a prominent part of cycling cells belonged to the CD117-positive population. Together these data demonstrate the presence of a CD117-expressing progenitor compartment with the capacity to replenish the luminal lineage of the breast gland.

Normal Breast-Derived Epithelial Cells with Luminal and Intrinsic Subtype-Enriched Gene Expression Document Interindividual Differences in Their Differentiation Cascade

Cell-type origin is one of the factors that determine molecular features of tumors, but resources to validate this concept are scarce because of technical difficulties in propagating major cell types of adult organs. Previous attempts to generate such resources to study breast cancer have yielded predominantly basal-type cell lines. We have created a panel of immortalized cell lines from core breast biopsies of ancestry-mapped healthy women that form ductal structures similar to normal breast in 3D cultures and expressed markers of major cell types, including the luminal-differentiated cell-enriched ERα-FOXA1-GATA3 transcription factor network. We have also created cell lines from PROCR (CD201)⁺/EpCAM^- cells that are likely the "normal" counterpart of the claudin-low subtype of breast cancers. RNA-seq and PAM50-intrinsic subtype clustering identified these cell lines as the "normal" counterparts of luminal A, basal, and normal-like subtypes and validated via immunostaining with basal-enriched KRT14 and luminal-enriched KRT19. We further characterized these cell lines by flow cytometry for distribution patterns of stem/basal, luminal-progenitor, mature/differentiated, multipotent PROCR⁺ cells, and organogenesis-enriched epithelial/mesenchymal hybrid cells using CD44/CD24, CD49f/EpCAM, CD271/EpCAM, CD201/EpCAM, and ALDEFLUOR assays and E-cadherin/vimentin double staining. These cell lines showed interindividual heterogeneity in stemness/differentiation capabilities and baseline activity of signaling molecules such as NF-κB, AKT2, pERK, and BRD4. These resources can be used to test the emerging concept that genetic variations in regulatory regions contribute to widespread differences in gene expression in "normal" conditions among the general population and can delineate the impact of cell-type origin on tumor progression.Significance: In addition to providing a valuable resource for the breast cancer research community to investigate cell-type origin of different subtypes of breast cancer, this study highlights interindividual differences in normal breast, emphasizing the need to use "normal" cells from multiple sources as controls to decipher the effects of cancer-specific genomic aberrations. Cancer Res; 78(17); 5107-23. ©2018 AACR.