Progesterone induces adult mammary stem cell expansion

Luminal breast cancer metastases and tumor arousal from dormancy are promoted by direct actions of estradiol and progesterone on the malignant cells

Introduction

Luminal, estrogen receptor-positive (ER⁺) breast cancers can metastasize but lie dormant for years before recurrences prove lethal. Understanding the roles of estrogen (E) or progestin (P) in development of luminal metastases or in arousal from dormancy is hindered by few preclinical models. We have developed such models.

Methods

Immunocompromised, ovariectomized (ovx’d) mice were intracardiac-injected with luminal or basal human breast cancer cells. Four lines were tested: luminal ER⁺PR⁺ cytokeratin 5-negative (CK5⁻) E3 and MCF-7 cells, basal ER⁻PR⁻CK5⁺ estrogen withdrawn-line 8 (EWD8) cells, and basal ER⁻PR⁻CK5⁻ MDA-MB-231 cells. Development of micrometastases or macrometastases was quantified in ovx’d mice and in mice supplemented with E or P or both. Metastatic deposits were analyzed by immunohistochemistry for luminal, basal, and proliferation markers.

Results

ER⁻PR⁻ cells generated macrometastases in multiple organs in the absence or presence of hormones. By contrast, ovx’d mice injected with ER⁺PR⁺ cells appeared to be metastases-free until they were supplemented with E or E+P. Furthermore, unlike parental ER⁺PR⁺CK5⁻ cells, luminal metastases were heterogeneous, containing a significant (6% to 30%) proportion of non-proliferative ER⁻PR⁻CK5⁺ cells that would be chemotherapy-resistant. Additionally, because these cells lack receptors, they would also be endocrine therapy-resistant. With regard to ovx’d control mice injected with ER⁺PR⁺ cells that appeared to be metastases-free, systematic pathologic analysis of organs showed that some harbor a reservoir of dormant micrometastases that are ER⁺ but PR⁻. Such cells may also be endocrine therapy- and chemotherapy-resistant. Their emergence as macrometastases can be triggered by E or E+P restoration.

Conclusions

We conclude that hormones promote development of multi-organ macrometastases in luminal disease. The metastases display a disturbing heterogeneity, containing newly emergent ER⁻PR⁻ subpopulations that would be resistant to endocrine therapy and chemotherapy. Similar cells are found in luminal metastases of patients. Furthermore, lack of hormones is not protective. While no overt metastases form in ovx’d mice, luminal tumor cells can seed distant organs, where they remain dormant as micrometastases and sheltered from therapies but arousable by hormone repletion. This has implications for breast cancer survivors or women with occult disease who are prescribed hormones for contraception or replacement purposes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-014-0489-4) contains supplementary material, which is available to authorized users.

Embryonic cells contribute directly to the quiescent stem cell population in the adult mouse mammary gland

INTRODUCTION

Studies have identified multi-potent stem cells in the adult mammary gland. More recent studies have suggested that the embryonic mammary gland may also contain stem/progenitor cells that contribute to initial ductal development. We were interested in determining whether embryonic cells might also directly contribute to long-lived stem cells that support homeostasis and development in the adult mammary gland.

METHODS

We used DNA-label retention to detect long label-retaining cells in the mammary gland. Mouse embryos were labeled with 5-ethynl-2'-deoxyuridine (EdU) between embryonic day 14.5 and embryonic day 18.5 and were subsequently sacrificed and examined for EdU retention at various intervals after birth. EdU retaining cells were co-stained for various lineage markers and identified after fluorescence activated cell sorting analysis of specific epithelial subsets. EdU-labeled mice were subjected to subsequent 5-bromo-2'-deoxyuridine administration to determine whether EdU-labeled cells could re-enter the cell cycle. Finally, EdU-labeled cells were grown under non-adherent conditions to assess their ability to form mammospheres.

RESULTS

We demonstrate embryonically-derived, long label-retaining cells (eLLRCs) in the adult mammary gland. eLLRCs stain for basal markers and are enriched within the mammary stem cell population identified by cell sorting. eLLRCs are restricted to the primary ducts near the nipple region. Interestingly, long label retaining cells (labeled during puberty) are found just in front of the eLLRCs, near where the ends of the ducts had been at the time of DNA labeling in early puberty. A subset of eLLRCs becomes mitotically active during periods of mammary growth and in response to ovarian hormones. Finally, we show that eLLRCs are contained within primary and secondary mammospheres.

CONCLUSIONS

Our findings suggest that a subset of proliferating embryonic cells subsequently becomes quiescent and contributes to the pool of long-lived mammary stem cells in the adult. eLLRCs can re-enter the cell cycle, produce both mammary lineages and self-renew. Thus, our studies have identified a putative stem/progenitor cell population of embryonic origin. Further study of these cells will contribute to an understanding of how quiescent stem cells are generated during development and how fetal exposures may alter future breast cancer risk in adults.

Wip1 phosphatase in breast cancer

Understanding the factors contributing to tumor initiation, progression and evolution is of paramount significance. Among them, wild-type p53-induced phosphatase 1 (Wip1) is emerging as an important oncogene by virtue of its negative control on several key tumor suppressor pathways. Originally discovered as a p53-regulated gene, Wip1 has been subsequently found amplified and more recently mutated in a significant fraction of human cancers including breast tumors. Recent development in the field further uncovered the utility of anti-Wip1-directed therapies in delaying tumor onset or in reducing the tumor burden. Furthermore, Wip1 could be an important factor that contributes to tumor heterogeneity, suggesting that its inhibition may decrease the rate of cancer evolution. These effects depend on several signaling pathways modulated by Wip1 phosphatase in a spatial and temporal manner. In this review we discuss the recent development in understanding how Wip1 contributes to tumorigenesis with its relevance to breast cancer.

Autophagy regulator BECN1 suppresses mammary tumorigenesis driven by WNT1 activation and following parity

Earlier studies reported allelic deletion of the essential autophagy regulator BECN1 in breast cancers implicating BECN1 loss, and likely defective autophagy, in tumorigenesis. Recent studies have questioned the tumor suppressive role of autophagy, as autophagy-related gene (Atg) defects generally suppress tumorigenesis in well-characterized mouse tumor models. We now report that, while it delays or does not alter mammary tumorigenesis driven by Palb2 loss or ERBB2 and PyMT overexpression, monoallelic Becn1 loss promotes mammary tumor development in 2 specific contexts, namely following parity and in association with wingless-type MMTV integration site family, member 1 (WNT1) activation. Our studies demonstrate that Becn1 heterozygosity, which results in immature mammary epithelial cell expansion and aberrant TNFRSF11A/TNR11/RANK (tumor necrosis factor receptor superfamily, member 11a, NFKB activator) signaling, promotes mammary tumorigenesis in multiparous FVB/N mice and in cooperation with the progenitor cell-transforming WNT1 oncogene. Similar to our Becn1(+/-);MMTV-Wnt1 mouse model, low BECN1 expression and an activated WNT pathway gene signature correlate with the triple-negative subtype, TNFRSF11A axis activation and poor prognosis in human breast cancers. Our results suggest that BECN1 may have nonautophagy-related roles in mammary development, provide insight in the seemingly paradoxical roles of BECN1 in tumorigenesis, and constitute the basis for further studies on the pathophysiology and treatment of clinically aggressive triple negative breast cancers (TNBCs).

Laurent J, Wronski A, Caballero S, Lyle SR, Naber SP, Kuperwasser C. Human breast progenitor cell numbers are regulated by WNT and TBX3

BACKGROUND

Although human breast development is mediated by hormonal and non-hormonal means, the mechanisms that regulate breast progenitor cell activity remain to be clarified. This limited understanding of breast progenitor cells has been due in part to the lack of appropriate model systems to detect and characterize their properties.

METHODS

To examine the effects of WNT signaling and TBX3 expression on progenitor activity in the breast, primary human mammary epithelial cells (MEC) were isolated from reduction mammoplasty tissues and transduced with lentivirus to overexpress WNT1 or TBX3 or reduce expression of their cognate receptors using shRNA. Changes in progenitor activity were quantified using characterized assays. We identified WNT family members expressed by cell populations within the epithelium and assessed alterations in expression of WNT family ligands by MECs in response to TBX3 overexpression and treatment with estrogen and progesterone.

RESULTS

Growth of MECs on collagen gels resulted in the formation of distinct luminal acinar and basal ductal colonies. Overexpression of TBX3 in MECs resulted in increased ductal colonies, while shTBX3 expression diminished both colony types. Increased WNT1 expression led to enhanced acinar colony formation, shLRP6 decreased both types of colonies. Estrogen stimulated the formation of acinar colonies in control MEC, but not shLRP6 MEC. Formation of ductal colonies was enhanced in response to progesterone. However, while shLRP6 decreased MEC responsiveness to progesterone, shTBX3 expression did not alter this response.

CONCLUSIONS

We identified two phenotypically distinguishable lineage-committed progenitor cells that contribute to different structural elements and are regulated via hormonal and non-hormonal mechanisms. WNT signaling regulates both types of progenitor activity. Progesterone favors the expansion of ductal progenitor cells, while estrogen stimulates the expansion of acinar progenitor cells. Paracrine WNT signaling is stimulated by estrogen and progesterone, while autocrine WNT signaling is induced by the embryonic T-box transcription factor TBX3.

Integrin αvβ3 drives slug activation and stemness in the pregnant and neoplastic mammary gland

Although integrin αvβ3 is linked to cancer progression, its role in epithelial development is unclear. Here, we show that αvβ3 plays a critical role in adult mammary stem cells (MaSCs) during pregnancy. Whereas αvβ3 is a luminal progenitor marker in the virgin gland, we noted increased αvβ3 expression in MaSCs at midpregnancy. Accordingly, mice lacking αvβ3 or expressing a signaling-deficient receptor showed defective mammary gland morphogenesis during pregnancy. This was associated with decreased MaSC expansion, clonogenicity, and expression of Slug, a master regulator of MaSCs. Surprisingly, αvβ3-deficient mice displayed normal development of the virgin gland with no effect on luminal progenitors. Transforming growth factor β2 (TGF-β2) induced αvβ3 expression, enhancing Slug nuclear accumulation and MaSC clonogenicity. In human breast cancer cells, αvβ3 was necessary and sufficient for Slug activation, tumorsphere formation, and tumor initiation. Thus, pregnancy-associated MaSCs require a TGF-β2/αvβ3/Slug pathway, which may contribute to breast cancer progression and stemness.

A role for β3-integrins in linking breast development and cancer

Pregnancy induces a rapid and controlled expansion of mammary stem cells. In this issue of Developmental Cell, Desgrosellier et al. (2014) show that β3-integrin is required downstream of hormonal signaling and TGFβ2 to regulate mammary stem cell number and alveolar development specifically during early pregnancy.

Mammary stem cells have myoepithelial cell properties

Contractile myoepithelial cells dominate the basal layer of the mammary epithelium and are considered to be differentiated cells. However, we observe that up to 54% of single basal cells can form colonies when seeded into adherent culture in the presence of agents that disrupt actin-myosin interactions, and on average, 65% of the single-cell-derived basal colonies can repopulate a mammary gland when transplanted in vivo. This indicates that a high proportion of basal myoepithelial cells can give rise to a mammary repopulating unit (MRU). We demonstrate that myoepithelial cells, flow-sorted using two independent myoepithelial-specific reporter strategies, have MRU capacity. Using an inducible lineage-tracing approach we follow the progeny of myoepithelial cells that express α-smooth muscle actin and show that they function as long-lived lineage-restricted stem cells in the virgin state and during pregnancy.

R-spondin1 is a novel hormone mediator for mammary stem cell self-renewal

Cai et al. find that Rspo1 cooperates with another hormonal mediator, Wnt4, to promote mammary stem cell (MaSC) self-renewal through Wnt/β-catenin signaling. Hormonal treatment that stimulates the expression of both Rspo1 and Wnt4 can completely substitute for exogenous Wnt proteins, potently expand MaSCs, and maintain their full development potential in transplantation. This study shows that hormones can induce a collaborative local niche environment for stem cells.

Signals from the niche play pivotal roles in regulating adult stem cell self-renewal. Previous studies indicated that the steroid hormones can expand mammary stem cells (MaSCs) in vivo. However, the facilitating local niche factors that directly contribute to the MaSC expansion remain unclear. Here we identify R-spondin1 (Rspo1) as a novel hormonal mediator in the mammary gland. Pregnancy and hormonal treatment up-regulate Rspo1 expression. Rspo1 cooperates with another hormonal mediator, Wnt4, to promote MaSC self-renewal through Wnt/β-catenin signaling. Knockdown of Rspo1 and Wnt4 simultaneously abolishes the stem cell reconstitution ability. In culture, hormonal treatment that stimulates the expression of both Rspo1 and Wnt4 can completely substitute for exogenous Wnt proteins, potently expand MaSCs, and maintain their full development potential in transplantation. Our data unveil the intriguing concept that hormones induce a collaborative local niche environment for stem cells.

Progesterone downregulation of miR-141 contributes to expansion of stem-like breast cancer cells through maintenance of progesterone receptor and Stat5a

Progesterone (P4) has emerged as an important hormone regulating mammary stem cell populations. In breast cancer, P4 and synthetic analogs increase the number of stem-like cells within luminal estrogen receptor (ER) and progesterone receptor (PR) positive breast cancers. These cells gain expression of de-differentiated cell markers CD44 and cytokeratin 5 (CK5), lose luminal markers ER and PR, and are more therapy resistant. We previously described that P4-downregulation of microRNA (miR)-29a contributes to the expansion of CD44^high and CK5⁺ cells. Here we investigated P4-downregulation of miR-141, a member of the miR-200 family of tumor suppressors, in facilitating an increase in stem-like breast cancer cells. miR-141 was the sole member of the miR-200 family P4-downregulated at the mature miRNA level in luminal breast cancer cell lines. Stable inhibition of miR-141 alone increased the CD44^high population, and potentiated P4-mediated increases in both CD44^high and CK5⁺ cells. Loss of miR-141 enhanced both mammosphere formation and tumor initiation. miR-141 directly targeted both PR and Stat5a, transcription factors important for mammary stem cell expansion. miR-141 depletion increased PR protein levels, even in cells lines where PR expression is estrogen-dependent. Stat5a suppression via siRNA or a small molecule inhibitor reduced the P4-dependent increase in CK5⁺ and CD44^high cells. These data support a mechanism by which P4-triggered loss of miR-141 facilitates breast cancer cell de-differentiation through deregulation of PR and Stat5a, two transcription factors important for controlling mammary cell fate.

RANKL expression in normal and malignant breast tissue responds to progesterone and is up-regulated during the luteal phase

The receptor activator of nuclear factor-κB ligand (RANKL) acts as a paracrine factor in progesterone-induced mammary epithelial proliferation and tumorigenesis. This evidence comes mainly from mouse models. Our aim was to examine whether RANKL expression in human normal and malignant breast is under the control of progesterone throughout the menstrual cycle. Breast epithelial samples were obtained by random fine needle aspiration (rFNA) of the contralateral unaffected breasts (CUB) of 18 breast cancer patients, with simultaneous serum hormone measurements. Genes correlated with serum progesterone levels were identified through Illumina microarray analysis. Validation was performed using qRT-PCR in rFNA samples from CUB of an additional 53 women and using immunohistochemistry in tissue microarrays of 61 breast cancer samples. Expression of RANKL, DIO2, and MYBPC1 was correlated with serum progesterone in CUB, and was significantly higher in luteal phase. RANKL and MYBPC1 mRNA expression were highly correlated between CUB and matched tumor samples. RANKL protein expression was also significantly increased in the luteal phase and highly correlated with serum progesterone levels in cancer samples, especially in hormone receptor positive tumors. The regulatory effects of progesterone on the expression of RANKL, DIO2, and MYBPC1 were confirmed in three-dimensional cultures of normal breast organoids. In normal breast and in breast cancer, RANKL mRNA and protein expression fluctuate with serum progesterone with highest levels in the luteal phase, suggesting that RANKL is a modulator of progesterone signaling in normal and malignant breast tissue and a potential biomarker of progesterone action and blockade.

The mammary cellular hierarchy and breast cancer

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

Mammary stem cells and the differentiation hierarchy: current status and perspectives

Based on transplantation and lineage tracing studies, a hierarchy of stem and progenitor cells has been shown to exist among the mammary epithelium. In this review, Visvader and Stingl integrate recent data on the mammary stem cell differentiation hierarchy and its control at the transcriptional and epigenetic levels. They also discuss the relevance of the evolving hierarchy to the identification of “cells of origin” of breast cancer.

The mammary epithelium is highly responsive to local and systemic signals, which orchestrate morphogenesis of the ductal tree during puberty and pregnancy. Based on transplantation and lineage tracing studies, a hierarchy of stem and progenitor cells has been shown to exist among the mammary epithelium. Lineage tracing has highlighted the existence of bipotent mammary stem cells (MaSCs) in situ as well as long-lived unipotent cells that drive morphogenesis and homeostasis of the ductal tree. Moreover, there is accumulating evidence for a heterogeneous MaSC compartment comprising fetal MaSCs, slow-cycling cells, and both long-term and short-term repopulating cells. In parallel, diverse luminal progenitor subtypes have been identified in mouse and human mammary tissue. Elucidation of the normal cellular hierarchy is an important step toward understanding the “cells of origin” and molecular perturbations that drive breast cancer.

RANK rs1805034 T>C polymorphism is associated with susceptibility of esophageal cancer in a Chinese population

Esophageal cancer remains the sixth leading cause of cancer associated death and eighth most common cancer worldwide. Genetic factors, such as single nucleotide polymorphisms (SNPs), may contribute to the carcinogenesis of esophageal cancer. Here, we conducted a hospital based case-control study to evaluate the genetic susceptibility of functional SNPs on the development of esophageal cancer. A total of 629 esophageal squamous cell carcinoma (ESCC) cases and 686 controls were enrolled for this study. The OPG rs3102735 T>C, rs2073618 G>C, RANK rs1805034 T>C, RANKL rs9533156 T>C and rs2277438 A>G were determined by ligation detection reaction method. Our findings suggested that RANK rs1805034 T>C is associated with the susceptibility of ESCC, which is more evident in male and elder (≥63) patients. Our study provides the first evidence that functional polymorphisms RANK rs1805034 T>C may be an indicator for individual susceptibility to ESCC. However, further larger studies among different ethnic populations are warranted to verify our conclusion.

Mammary stem cells: expansion and animal productivity

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

Breast cancer stem cells and the immune system: promotion, evasion and therapy

Cancer stem cells are believed to be a subset of heterogeneous tumour cells responsible for tumour initiation, growth, local invasion, and metastasis. In breast cancer, numerous factors have been implicated in regulation of cancer stem cells, but there is still a paucity of information regarding precise molecular and cellular mechanisms guiding their pathobiology. Components of both the adaptive and the innate immune system have been shown to play a crucial role in supporting breast cancer growth and spread, and recently some immune mediators, both molecules and cells, have been reported to influence breast cancer stem cell biology. This review summarises a small, pioneering body of evidence for the potentially important function of the "immuniche" in maintaining and supporting breast cancer stem cells.

Duration of tamoxifen use and the risk of contralateral breast cancer in BRCA1 and BRCA2 mutation carriers

Women with a mutation in BRCA1 or BRCA2 face a lifetime risk of breast cancer of approximately 80 %. Tamoxifen treatment of the first cancer has been associated with a reduction in the risk of a subsequent contralateral cancer. We studied 1,504 women with a known BRCA1 or BRCA2 mutation, 411 women with bilateral breast cancer (cases) and 1,093 women with unilateral breast cancer (controls) in a matched case-control study. Control women were of similar age and had a similar age of diagnosis of first breast cancer as the cases. For each woman who used tamoxifen, the starting and stopping dates were abstracted and the duration of tamoxifen use was calculated. Three hundred and thirty-one women had used tamoxifen (22 %); of these 84 (25 %) had completed four or more years of tamoxifen, the remainder stopped prematurely or were current users. For women with up to 1 year of tamoxifen use, the odds ratio for contralateral breast cancer was 0.37 (95 % CI 0.20-0.69; p = 0.001) compared to women with no tamoxifen use. Among women with 1-4 years of tamoxifen use the odds ratio was 0.53 (95 % CI 0.32-0.87; p = 0.01). Among women with four or more years of tamoxifen use the odds ratio was 0.83 (95 % CI 0.44-1.55; p = 0.55). Short-term use of tamoxifen for chemoprevention in BRCA1 and BRCA2 mutation carriers may be as effective as a conventional 5-year course of treatment.

Cyclin D1 and C/EBPβ LAP1 operate in a common pathway to promote mammary epithelial cell differentiation

Both cyclin D1 and the transcription factor C/EBPβ are required for mammary epithelial cell differentiation; however, the pathway in which they operate is uncertain. Previous analyses of the patterns of gene expression in human tumors suggested a connection between cyclin D1 overexpression and C/EBPβ, but whether this represents a cancer-specific gain of function for cyclin D1 is unknown. C/EBPβ is an intronless gene encoding three protein isoforms--LAP1, LAP2, and LIP. Here, we provide evidence that cyclin D1 engages C/EBPβ in an isoform-specific manner. Cyclin D1 binds to LAP1, an event that activates the transcriptional function of LAP1 by relieving its autoinhibited state effected by intramolecular interactions. Reexpression of LAP1 but not LAP2 or LIP restores the ability of C/EBPβ-deficient mammary epithelial cells to differentiate and does so in a manner dependent on cyclin D1. And cyclin D1-mediated activation of LAP1 participates in mammary epithelial cell differentiation. Our findings indicate that cyclin D1 and C/EBPβ LAP1 operate in a common pathway to promote mammary epithelial cell differentiation.

Concise review: breast cancer stem cells: regulatory networks, stem cell niches, and disease relevance

Accumulating evidence has shown that cancer stem cells (CSCs), the cancer cells that have long-term proliferative potential and the ability to regenerate tumors with phenotypically heterogeneous cell types, are important mediators of tumor metastasis and cancer relapse. In breast cancer, these cells often possess attributes of cells that have undergone an epithelial-mesenchymal transition (EMT). Signaling networks mediated by microRNAs and EMT-inducing transcription factors connect the EMT program with the core stem cell regulatory machineries. These signaling networks are also regulated by extrinsic niche signals that induce and maintain CSCs, contributing to metastatic colonization and promoting the reactivation of dormant tumor cells. Targeting these CSC pathways is likely to improve the efficacy of conventional chemo- and radiotherapies.

Growth hormone is secreted by normal breast epithelium upon progesterone stimulation and increases proliferation of stem/progenitor cells

Using in vitro and in vivo experimental systems and in situ analysis, we show that growth hormone (GH) is secreted locally by normal human mammary epithelial cells upon progesterone stimulation. GH increases proliferation of a subset of cells that express growth hormone receptor (GHR) and have functional properties of stem and early progenitor cells. In 72% of ductal carcinoma in situ lesions, an expansion of the cell population that expresses GHR was observed, suggesting that GH signaling may contribute to breast cancer development.

The molecular landscape of the normal human breast – defining normal

A key approach in understanding how breast cancer can occur is to determine the regulatory pathways at play in the normal breast and to identify precisely the normal developmental mechanisms subverted during early breast cancer progression. Using normal human breast tissue samples, Pardo and colleagues have identified the gene targets and pathways displaying fluctuating expression as a consequence of the menstrual cycle. Detailed characterization of how the human breast functions in its normal state, and how this may be perturbed at its earliest point, will provide a critical step toward the prevention of breast cancer.

Development of the human breast

Mammalia are so named based on the presence of the mammary gland in the breast. The mammary gland is an epidermal appendage, derived from the apocrine glands. The human breast consists of the parenchyma and stroma, originating from ectodermal and mesodermal elements, respectively. Development of the human breast is distinctive for several reasons. The human breast houses the mammary gland that produces and delivers milk through development of an extensive tree-like network of branched ducts. It is also characterized by cellular plasticity, with extensive remodeling in adulthood, a factor that increases its susceptibility to carcinogenesis. Also, breast development occurs in distinct stages via complex epithelial-mesenchymal interactions, orchestrated by signaling pathways under the regulation of systemic hormones. Congenital and acquired disorders of the breast often have a basis in development, making its study essential to understanding breast pathology.

Aldehyde dehydrogenase and estrogen receptor define a hierarchy of cellular differentiation in the normal human mammary epithelium

INTRODUCTION

Although estrogen and progesterone play a key role in normal mammary development and in breast cancer, the potential for proliferation and lineage differentiation as well as origin of cells that express the estrogen receptor (ER) in normal breast epithelium are not known. Some evidence suggests that normal human mammary stem/progenitor cells are ER-, but the identity of these cells and the cellular hierarchy of breast epithelium are still subjects of controversy. It is likely that elucidation of these aspects will bring insight into the cellular origin of breast cancer subtypes.

METHODS

We used fluorescence-activated cell sorting of primary human mammary epithelial cells along with in vitro and in vivo functional assays to examine the hierarchic relation between cells with aldehyde dehydrogenase enzymatic activity (ALDH+ cells) and ER+ cells in the normal human breast epithelium. We assessed the proliferation and lineage differentiation potential of these cells in vitro and in vivo. A gene reporter assay was used to separate live ER+ and ER- mammary epithelial cells. With shRNA-mediated knockdown, we investigated the role of ALDH isoforms in the functionality of mammary epithelial progenitor cells.

RESULTS

We describe a cellular hierarchy in the normal human mammary gland in which ER-/ALDH+ cells with functional properties of stem/progenitor cells generate ER+ progenitor cells, which in turn give rise to cells of luminal lineage. We show that the ALDH1A1 isoform, through its function in the retinoic acid metabolism, affects the proliferation and/or early differentiation of stem/progenitor cells and is important for branching morphogenesis.

CONCLUSIONS

This study presents direct evidence that ER+ cells are generated by ER-/ALDH+ stem/progenitor cells. We also show that ER+ cells are able to generate cell progeny of luminal lineage in vitro and in vivo. Loss of ALDH1A1 function impairs this process, as well as branching morphogenesis and clonogenicity in suspension culture. This latter effect is reversed by treatment with retinoic acid.

The dynamics of murine mammary stem/progenitor cells

The stem/progenitor cells in the murine mammary gland are a highly dynamic population of cells that are responsible for ductal elongation in puberty, homeostasis maintenance in adult, and lobulo-alveolar genesis during pregnancy. In recent years understanding the epithelial cell hierarchy within the mammary gland is becoming particularly important as these different stem/progenitor cells were perceived to be the cells of origin for various subtypes of breast cancer. Although significant advances have been made in enrichment and isolation of stem/progenitor cells by combinations of antibodies against cell surface proteins together with flow cytometry, and in identification of stem/progenitor cells with multi-lineage differentiation and self-renewal using mammary fat pad reconstitution assay and in vivo genetic labeling technique, a clear understanding of how these different stem/progenitors are orchestrated in the mammary gland is still lacking. Here we discuss the different in vivo and in vitro methods currently available for stem/progenitor identification, their associated caveats, and a possible new hierarchy model to reconcile various putative stem/progenitor cell populations identified by different research groups.

Somatic loss of p53 leads to stem/progenitor cell amplification in both mammary epithelial compartments, basal and luminal

Mammary epithelium comprises a layer of luminal cells and a basal myoepithelial cell layer. Both mammary epithelial compartments, basal and luminal, contain stem and progenitor cells, but only basal cells are capable of gland regeneration upon transplantation. Aberrant expansion of stem/progenitor cell populations is considered to contribute to breast tumorigenesis. Germline deletions of p53 in humans and mice confer a predisposition to tumors, and stem cell frequency is abnormally high in the mammary epithelium of p53-deficient mice. However, it is unknown whether stem/progenitor cell amplification occurs in both, basal and luminal cell populations in p53-deficient mammary tissue. We used a conditional gene deletion approach to study the role of p53 in stem/progenitor cells residing in the mammary luminal and basal layers. Using two- and three-dimensional cell culture assays, we showed that p53 loss led to the expansion of clonogenic stem/progenitor cells in both mammary epithelial cell layers. Moreover, following p53 deletion, luminal and basal stem/progenitor cells acquired a capacity for unlimited propagation in mammosphere culture. Furthermore, limiting dilution and serial transplantation assays revealed amplification and enhanced self-renewal in the basal regenerating cell population of p53-deficient mammary epithelium. Our data suggest that the increase in stem/progenitor cell activity may be, at least, partially mediated by the Notch pathway. Taken together, these results strongly indicate that p53 restricts the propagation and self-renewal of stem/progenitor cells in both layers of the mammary epithelium providing further insight into the impact of p53 loss in breast cancerogenesis.

Calcineurin/Nfatc1 signaling links skin stem cell quiescence to hormonal signaling during pregnancy and lactation

In most tissues, the prevailing view is that stem cell (SC) niches are generated by signals from within the nearby tissue environment. Here, we define genetic changes altered in hair follicle (HF) SCs in mice treated with a potent SC activator, cyclosporine A (CSA), which inhibits the phosphatase calcineurin (CN) and the activity of the transcription factor nuclear factor of activated T cells c1 (Nfatc1). We show that CN/Nfatc1 regulates expression of prolactin receptor (Prlr) and that canonical activation of Prlr and its downstream signaling via Jak/Stat5 drives quiescence of HF SCs during pregnancy and lactation, when serum prolactin (Prl) levels are highly elevated. Using Prl injections and genetic/pharmacological loss-of-function experiments in mice, we show that Prl signaling stalls follicular SC activation through its activity in the skin epithelium. Our findings define a unique CN-Nfatc1-Prlr-Stat5 molecular circuitry that promotes persistent SC quiescence in the skin.

Targeting RANKL in metastasis

Acting through its cognate receptor, receptor activator of nuclear factor-κB (RANK), RANK ligand (RANKL) is an essential mediator of osteoclast function and survival. Preclinical data have now firmly established that blockade of tumor-induced osteoclastogenesis by RANKL inhibition will not only protect against bone destruction but will also inhibit the progression of established bone metastases and delay the formation of de novo bone metastases in cancer models. In patients with bone metastases, skeletal complications are driven by increased osteoclastic activity and may result in pathological fractures, spinal cord compression and the need for radiotherapy to the bone or orthopedic surgery (collectively known as skeletal-related events (SREs)). Denosumab, a fully human monoclonal antibody against RANKL, has been demonstrated to prevent or delay SREs in patients with solid tumors that have metastasized to bone. In addition to its central role in tumor-induced osteolysis, bone destruction and skeletal tumor progression, there is emerging evidence for direct pro-metastatic effects of RANKL, independent of osteoclasts. For example, RANKL also stimulates metastasis via activity on RANK-expressing cancer cells, resulting in increased invasion and migration. Pharmacological inhibition of RANKL may also reduce bone and lung metastasis through blockade of the direct action of RANKL on metastatic cells. This review describes these distinct but potentially overlapping mechanisms by which RANKL may promote metastases.

Huntingtin regulates mammary stem cell division and differentiation

Little is known about the mechanisms of mitotic spindle orientation during mammary gland morphogenesis. Here, we report the presence of huntingtin, the protein mutated in Huntington’s disease, in mouse mammary basal and luminal cells throughout mammogenesis. Keratin 5-driven depletion of huntingtin results in a decreased pool and specification of basal and luminal progenitors, and altered mammary morphogenesis. Analysis of mitosis in huntingtin-depleted basal progenitors reveals mitotic spindle misorientation. In mammary cell culture, huntingtin regulates spindle orientation in a dynein-dependent manner. Huntingtin is targeted to spindle poles through its interaction with dynein and promotes the accumulation of NUMA and LGN. Huntingtin is also essential for the cortical localization of dynein, dynactin, NUMA, and LGN by regulating their kinesin 1-dependent trafficking along astral microtubules. We thus suggest that huntingtin is a component of the pathway regulating the orientation of mammary stem cell division, with potential implications for their self-renewal and differentiation properties.

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HTT regulates MaSC self-renewal and cell fate specification

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HTT is essential for mammary epithelial morphogenesis in vivo

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HTT regulates spindle orientation in a dynein-dependent manner

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HTT mediates the cortical localization of dynein/dynactin/LGN/NUMA through kinesin 1

RANKL/RANK - from bone physiology to breast cancer

RANK and its ligand RANKL are key molecules in bone metabolism and are critically involved in pathologic bone disorders. Deregulation of the RANK/RANKL system is for example a main reason for the development of postmenopausal osteoporosis, which affects millions of women worldwide. Another essential function of RANK and RANKL is the development of a functional lactating mammary gland during pregnancy. Sex hormones, in particular progesterone, induce RANKL expression resulting in proliferation of mammary epithelial cells. Moreover, RANK and RANKL have been shown to regulate mammary epithelial stem cells. RANK and RANKL were also identified as critical mechanism in the development of hormone-induced breast cancer and metastatic spread to bone. In this review, we will focus on the various RANK/RANKL functions ranging from bone physiology, immune regulation, and initiation of breast cancer.

Fully interlocking: a story of teamwork among breast epithelial cells

In this issue of Developmental Cell, Forster et al. (2014) show that the basal myoepithelial cell layer directs the final maturation of the adjacent luminal cell sheet during pregnancy. Do all mammary epithelial cells both give and take instructions from others to create the milk production machinery?

PKA signaling drives mammary tumorigenesis through Src

Protein kinase A (PKA) hyperactivation causes hereditary endocrine neoplasias; however, its role in sporadic epithelial cancers is unknown. Here, we show that heightened PKA activity in the mammary epithelium generates tumors. Mammary-restricted biallelic ablation of Prkar1a, which encodes for the critical type-I PKA regulatory subunit, induced spontaneous breast tumors characterized by enhanced type-II PKA activity. Downstream of this, Src phosphorylation occurs at residues serine-17 and tyrosine-416 and mammary cell transformation is driven through a mechanism involving Src signaling. The phenotypic consequences of these alterations consisted of increased cell proliferation and, accordingly, expansion of both luminal and basal epithelial cell populations. In human breast cancer, low PRKAR1A/high SRC expression defines basal-like and HER2 breast tumors associated with poor clinical outcome. Together, the results of this study define a novel molecular mechanism altered in breast carcinogenesis and highlight the potential strategy of inhibiting SRC signaling in treating this cancer subtype in humans.

Next-generation transcriptome sequencing of the premenopausal breast epithelium using specimens from a normal human breast tissue bank

Introduction

Our efforts to prevent and treat breast cancer are significantly impeded by a lack of knowledge of the biology and developmental genetics of the normal mammary gland. In order to provide the specimens that will facilitate such an understanding, The Susan G. Komen for the Cure Tissue Bank at the IU Simon Cancer Center (KTB) was established. The KTB is, to our knowledge, the only biorepository in the world prospectively established to collect normal, healthy breast tissue from volunteer donors. As a first initiative toward a molecular understanding of the biology and developmental genetics of the normal mammary gland, the effect of the menstrual cycle and hormonal contraceptives on DNA expression in the normal breast epithelium was examined.

Methods

Using normal breast tissue from 20 premenopausal donors to KTB, the changes in the mRNA of the normal breast epithelium as a function of phase of the menstrual cycle and hormonal contraception were assayed using next-generation whole transcriptome sequencing (RNA-Seq).

Results

In total, 255 genes representing 1.4% of all genes were deemed to have statistically significant differential expression between the two phases of the menstrual cycle. The overwhelming majority (221; 87%) of the genes have higher expression during the luteal phase. These data provide important insights into the processes occurring during each phase of the menstrual cycle. There was only a single gene significantly differentially expressed when comparing the epithelium of women using hormonal contraception to those in the luteal phase.

Conclusions

We have taken advantage of a unique research resource, the KTB, to complete the first-ever next-generation transcriptome sequencing of the epithelial compartment of 20 normal human breast specimens. This work has produced a comprehensive catalog of the differences in the expression of protein-coding genes as a function of the phase of the menstrual cycle. These data constitute the beginning of a reference data set of the normal mammary gland, which can be consulted for comparison with data developed from malignant specimens, or to mine the effects of the hormonal flux that occurs during the menstrual cycle.

Bovine mammary stem cells: new perspective for dairy science

Mammary stem cells provide opportunities for the cyclic remodelling of the bovine mammary gland. Therefore, understanding the character and regulation of mammary stem cells is important for increasing animal health and productivity. The exciting possibility that stem cell expansion can influence milk production is currently being investigated by several researchers. In fact, appropriate regulation of mammary stem cells could hopefully benefit milk yield, persistency of lactation, dry period management and tissue repair. Accordingly, we and others have attempted to characterize and regulate the function of bovine mammary stem cells. However, research on mammary stem cells requires tissue biopsies, which represents a limitation for the management of animal welfare. Interestingly, different studies recently reported the identification of putative mammary stem cells in human breast milk. The possible identification of primitive cell types within cow's milk may provide a non-invasive source of relevant mammary cells for a wide range of applications. In this review, we have summarized the main achievements in this field for dairy cow science and described the interesting perspectives open to manipulate milk persistency during lactation and to cope with oxidative stress during the transition period by regulating mammary stem cells.

Exposure to lard-based high-fat diet during fetal and lactation periods modifies breast cancer susceptibility in adulthood in rats

The present study investigated whether early life exposure to high levels of animal fat increases breast cancer risk in adulthood in rats. Dams consumed a lard-based high-fat (HF) diet (60% fat-derived energy) or an AIN93G control diet (16% fat-derived energy) during gestation or gestation and lactation. Their 7-week-old female offspring were exposed to 7,12-dimethyl-benzo[a]anthracene to induce mammary tumors. Pregnant dams consuming an HF diet had higher circulating leptin levels than pregnant control dams. However, compared to the control offspring, significantly lower susceptibility to mammary cancer development was observed in the offspring of dams fed an HF diet during pregnancy (lower tumor incidence, multiplicity and weight), or pregnancy and lactation (lower tumor multiplicity only). Mammary epithelial elongation, cell proliferation (Ki67) and expression of NFκB p65 were significantly lower and p21 expression and global H3K9me3 levels were higher in the mammary glands of rats exposed to an HF lard diet in utero. They also tended to have lower Rank/Rankl ratios (P=.09) and serum progesterone levels (P=.07) than control offspring. In the mammary glands of offspring of dams consuming an HF diet during both pregnancy and lactation, the number of terminal end buds, epithelial elongation and the BCL-2/BAX ratio were significantly lower and serum leptin levels were higher than in the controls. Our data confirm that the breast cancer risk of offspring can be programmed by maternal dietary intake. However, contrary to our expectation, exposure to high levels of lard during early life decreased later susceptibility to breast cancer.

Mammographic density-a review on the current understanding of its association with breast cancer

There has been considerable recent interest in the genetic, biological and epidemiological basis of mammographic density (MD), and the search for causative links between MD and breast cancer (BC) risk. This report will critically review the current literature on MD and summarize the current evidence for its association with BC. Keywords 'mammographic dens*', 'dense mammary tissue' or 'percent dens*' were used to search the existing literature in English on PubMed and Medline. All reports were critically analyzed. The data were assigned to one of the following aspects of MD: general association with BC, its relationship with the breast hormonal milieu, the cellular basis of MD, the generic variations of MD, and its significance in the clinical setting. MD adjusted for age, and BMI is associated with increased risk of BC diagnosis, advanced tumour stage at diagnosis and increased risk of both local recurrence and second primary cancers. The MD measures that predict BC risk have high heritability, and to date several genetic markers associated with BC risk have been found to also be associated with these MD risk predictors. Change in MD could be a predictor of the extent of chemoprevention with tamoxifen. Although the biological and genetic pathways that determine and perhaps modulate MD remain largely unresolved, significant inroads are being made into the understanding of MD, which may lead to benefits in clinical screening, assessment and treatment strategies. This review provides a timely update on the current understanding of MD's association with BC risk.

Polymorphisms in the RANK/RANKL genes and their effect on bone specific prognosis in breast cancer patients

The receptor activator of NF-κB (RANK) pathway is involved in bone health as well as breast cancer (BC) pathogenesis and progression. Whereas the therapeutic implication of this pathway is established for the treatment of osteoporosis and bone metastases, the application in adjuvant BC is currently investigated. As genetic variants in this pathway have been described to influence bone health, aim of this study was the prognostic relevance of genetic variants in RANK and RANKL. Single nucleotide polymorphisms in RANK(L) (rs1054016/rs1805034/rs35211496) were genotyped and analyzed with regard to bone metastasis-free survival (BMFS), disease-free survival, and overall survival for a retrospective cohort of 1251 patients. Cox proportional hazard models were built to examine the prognostic influence in addition to commonly established prognostic factors. The SNP rs1054016 seems to influence BMFS. Patients with two minor alleles had a more favorable prognosis than patients with at least one common allele (HR 0.37 (95% CI: 0.17, 0.84)), whereas other outcome parameters remained unaffected. rs1805034 and rs35211496 had no prognostic relevance. The effect of rs1054016(RANKL) adds to the evidence that the RANK pathway plays a role in BC pathogenesis and progression with respect to BMFS, emphasizing the connection between BC and bone health.

Estrogen controls the survival of BRCA1-deficient cells via a PI3K-NRF2-regulated pathway

Mutations in the tumor suppressor BRCA1 predispose women to breast and ovarian cancers. The mechanism underlying the tissue-specific nature of BRCA1's tumor suppression is obscure. We previously showed that the antioxidant pathway regulated by the transcription factor NRF2 is defective in BRCA1-deficient cells. Reactivation of NRF2 through silencing of its negative regulator KEAP1 permitted the survival of BRCA1-null cells. Here we show that estrogen (E2) increases the expression of NRF2-dependent antioxidant genes in various E2-responsive cell types. Like NRF2 accumulation triggered by oxidative stress, E2-induced NRF2 accumulation depends on phosphatidylinositol 3-kinase-AKT activation. Pretreatment of mammary epithelial cells (MECs) with the phosphatidylinositol 3-kinase inhibitor BKM120 abolishes the capacity of E2 to increase NRF2 protein and transcriptional activity. In vivo the survival defect of BRCA1-deficient MECs is rescued by the rise in E2 levels associated with pregnancy. Furthermore, exogenous E2 administration stimulates the growth of BRCA1-deficient mammary tumors in the fat pads of male mice. Our work elucidates the basis of the tissue specificity of BRCA1-related tumor predisposition, and explains why oophorectomy significantly reduces breast cancer risk and recurrence in women carrying BRCA1 mutations.

Molecular determinants of context-dependent progesterone receptor action in breast cancer

The ovarian steroid hormone, progesterone, and its nuclear receptor, the progesterone receptor, are implicated in the progression of breast cancer. Clinical trial data on the effects of hormone replacement therapy underscore the importance of understanding how progestins influence breast cancer growth. The progesterone receptor regulation of distinct target genes is mediated by complex interactions between the progesterone receptor and other regulatory factors that determine the context-dependent transcriptional action of the progesterone receptor. These interactions often lead to post-translational modifications to the progesterone receptor that can dramatically alter receptor function, both in the normal mammary gland and in breast cancer. This review highlights the molecular components that regulate progesterone receptor transcriptional action and describes how a better understanding of the complex interactions between the progesterone receptor and other regulatory factors may be critical to enhancing the clinical efficacy of anti-progestins for use in the treatment of breast cancer.

Pubertal bisphenol A exposure alters murine mammary stem cell function leading to early neoplasia in regenerated glands

Perinatal exposure to bisphenol A (BPA) has been shown to cause aberrant mammary gland morphogenesis and mammary neoplastic transformation. Yet, the underlying mechanism is poorly understood. We tested the hypothesis that mammary glands exposed to BPA during a susceptible window may lead to its susceptibility to tumorigenesis through a stem cell-mediated mechanism. We exposed 21-day-old Balb/c mice to BPA by gavage (25 μg/kg/d) during puberty for 3 weeks, and a subset of animals were further challenged with one oral dose (30 mg/kg) of 7,12-dimethylbenz(a)anthracene (DMBA) at 2 months of age. Primary mammary cells were isolated at 6 weeks, and 2 and 4 months of age for murine mammary stem cell (MaSC) quantification and function analysis. Pubertal exposure to the low-dose BPA increased lateral branches and hyperplasia in adult mammary glands and caused an acute increase of MaSC in 6-week-old glands and a delayed increase of luminal progenitors in 4-month-old adult gland. Most importantly, pubertal BPA exposure altered the function of MaSC from different age groups, causing early neoplastic lesions in their regenerated glands similar to those induced by DMBA exposure, which indicates that MaSCs are susceptible to BPA-induced transformation. Deep sequencing analysis on MaSC-enriched mammospheres identified a set of aberrantly expressed genes associated with early neoplastic lesions in patients with human breast cancer. Thus, our study for the first time shows that pubertal BPA exposure altered MaSC gene expression and function such that they induced early neoplastic transformation.

Oestrogen increases haematopoietic stem-cell self-renewal in females and during pregnancy

Sexually dimorphic mammalian tissues, including sexual organs and the brain, contain stem cells that are directly or indirectly regulated by sex hormones^1-6. An important question is whether stem cells also exhibit sex differences in physiological function and hormonal regulation in tissues that do not exhibit sex-specific morphological differences. The terminal differentiation and function of some haematopoietic cells are regulated by sex hormones^7-10 but haematopoietic stem cell (HSC) function is thought to be similar in both sexes. Here we show that mouse HSCs exhibit sex differences in cell cycle regulation by estrogen. HSCs in females divide significantly more frequently than in males. This difference depended on the ovaries but not the testes. Administration of estradiol, a hormone produced mainly in the ovaries, increased HSC cell division in males and females. Estrogen levels increased during pregnancy, increasing HSC division, HSC frequency, cellularity, and erythropoiesis in the spleen. HSCs expressed high levels of estrogen receptor α (ERα). Conditional deletion of ERα from HSCs reduced HSC division in female, but not male, mice and attenuated the increases in HSC division, HSC frequency, and erythropoiesis during pregnancy. Estrogen/ERα signaling promotes HSC self-renewal, expanding splenic HSCs and erythropoiesis during pregnancy.

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

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

How pregnancy at early age protects against breast cancer

Pregnancy at an early age has a strong protective effect against breast cancer in humans and rodents. Postulated mechanisms underlying this phenomenon include alterations in the relative dynamics of hormone and growth factor-initiated cell fate-determining signaling pathways within the hierarchically organized mammary gland epithelium. Recent studies in epithelial cell subpopulations isolated from mouse and human mammary glands have shown that early pregnancy decreases the proportion of hormone receptor-positive cells and causes pronounced changes in gene expression as well as decreased proliferation in stem/progenitor cells. The changes include downregulation of Wnt and transforming growth factor β (TGFβ) signaling. These new findings highlight the importance of cell-cell interactions within the mammary gland epithelium in modulating cancer risk and provide potential targets for breast cancer prevention strategies.