The comparative pathology of human and mouse mammary glands

Optical parameters of healthy and tumor breast tissues in mice

Knowledge of the optical parameters of tumors is important for choosing the correct laser treatment parameters. In this paper, optical properties and refraction indices of breast tissue in healthy mice and a 4T1 model mimicking human breast cancer have been measured. A significant decrease in both the scattering and refractive index of tumor tissue has been observed. The change in tissue morphology has induced the change in the slope of the scattering spectrum. Thus, the light penetration depth into tumor has increased by almost 1.5-2 times in the near infrared "optical windows." Raman spectra have shown lower lipid content and higher protein content in tumor. The difference in the optical parameters of the tissues under study makes it possible to reliably differentiate them. The results may be useful for modeling the distribution of laser radiation in healthy tissues and cancers for deriving optimal irradiation conditions in photodynamic therapy.

Rat Models of Hormone Receptor-Positive Breast Cancer

Hormone receptor-positive (HR+) breast cancer (BC) is the most common type of breast cancer among women worldwide, accounting for 70-80% of all invasive cases. Patients with HR+ BC are commonly treated with endocrine therapy, but intrinsic or acquired resistance is a frequent problem, making HR+ BC a focal point of intense research. Despite this, the malignancy still lacks adequate in vitro and in vivo models for the study of its initiation and progression as well as response and resistance to endocrine therapy. No mouse models that fully mimic the human disease are available, however rat mammary tumor models pose a promising alternative to overcome this limitation. Compared to mice, rats are more similar to humans in terms of mammary gland architecture, ductal origin of neoplastic lesions and hormone dependency status. Moreover, rats can develop spontaneous or induced mammary tumors that resemble human HR+ BC. To date, six different types of rat models of HR+ BC have been established. These include the spontaneous, carcinogen-induced, transplantation, hormone-induced, radiation-induced and genetically engineered rat mammary tumor models. Each model has distinct advantages, disadvantages and utility for studying HR+ BC. This review provides a comprehensive overview of all published models to date.

Nonlinear progression across the occult transition establishes cancer lethality

Cancer screening is based upon a linear model of growth and invasion. Yet, early dissemination during the lengthy pre-diagnostic phase suggests that nonlinearity in growth can also occur. Therefore, we quantitatively traced the invisible and visible phases of tumorigenesis in the mammary gland for more than two-thousand tumors. Dynamic mathematical models of the invisible phase revealed an occult checkpoint resulting in nonlinear progression of transformed field cells. We found that expansile fields have increased dwell time at the occult checkpoint resulting in a large reservoir of image detectable precursors prior to invasion. In contrast, slowly proliferating lesions disseminate early and then transition rapidly through an occult checkpoint in a process we term nascent lethality. Our data illustrate how nonlinear growth across an occult checkpoint can account for a paradoxical increase in early-stage cancer detection without a dramatic reduction in metastatic burden.

Highlights

Growth during the invisible phase of tumorigenesis is a nonlinear processField size and field growth rate are uncoupled from metastatic potentialOccult transition rates vary by genotypeNascent lethal lesions are currently undetectable.

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.

Stromal heterogeneity may explain increased incidence of metaplastic breast cancer in women of African descent

The biologic basis of genetic ancestry-dependent variability in disease incidence and outcome is just beginning to be explored. We recently reported enrichment of a population of ZEB1-expressing cells located adjacent to ductal epithelial cells in normal breasts of women of African ancestry compared to those of European ancestry. In this study, we demonstrate that these cells have properties of fibroadipogenic/mesenchymal stromal cells that express PROCR and PDGFRα and transdifferentiate into adipogenic and osteogenic lineages. PROCR + /ZEB1 + /PDGFRα+ (PZP) cells are enriched in normal breast tissues of women of African compared to European ancestry. PZP: epithelial cell communication results in luminal epithelial cells acquiring basal cell characteristics and IL-6-dependent increase in STAT3 phosphorylation. Furthermore, level of phospho-STAT3 is higher in normal and cancerous breast tissues of women of African ancestry. PZP cells transformed with HRasG12V ± SV40-T/t antigens generate metaplastic carcinoma suggesting that these cells are one of the cells-of-origin of metaplastic breast cancers.

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.

A Non-Invasive Deep Photoablation Technique to Inhibit DCIS Progression and Induce Antitumor Immunity

Ductal carcinoma in situ (DCIS) of the breast is often managed by lumpectomy and radiation or mastectomy, despite its indolent features. Effective non-invasive treatment strategies could reduce the morbidity of DCIS treatment. We have exploited the high heat shock protein 90 (HSP90) activity in premalignant and malignant breast disease to non-invasively detect and selectively ablate tumors using photodynamic therapy (PDT). PDT with the HSP90-targeting photosensitizer, HS201, can not only ablate invasive breast cancers (BCs) while sparing non-tumor tissue, but also induce antitumor immunity. We hypothesized that HS201-PDT would both non-invasively ablate DCIS and prevent progression to invasive BC. We tested in vitro selective uptake and photosensitivity of HS201 in DCIS cell lines compared to the non-selective parental verteporfin, and assessed in vivo antitumor efficacy in mammary fat pad and intraductal implantation models. Selective uptake of HS201 enabled treatment of intraductal lesions while minimizing toxicity to non-tumor tissue. The in vivo activity of HS201-PDT was also tested in female MMTV-neu mice prior to the development of spontaneous invasive BC. Mice aged 5 months were administered HS201, and their mammary glands were exposed to laser light. HS201-PDT delayed the emergence of invasive BC, significantly prolonged disease-free survival (DFS) (p = 0.0328) and tended to improve overall survival compared to the no-treatment control (p = 0.0872). Systemic administration of anti-PD-L1 was combined with HS201-PDT and was tested in a more aggressive spontaneous tumor model, HER2delta16 transgenic mice. A single PDT dose combined with anti-PD-L1 improved DFS compared to the no-treatment control, which was significantly improved with repetitive HS201-PDT given with anti-PD-L1 (p = 0.0319). In conclusion, a non-invasive, skin- and tissue-sparing PDT strategy in combination with anti-PD-L1 antibodies effectively prevented malignant progression of DCIS to invasive BC. This non-invasive treatment strategy of DCIS may be safe and effective, while providing an option to reduce the morbidity of current conventional treatment for patients with DCIS. Clinical testing of HS201 is currently underway.

The Influence of Physical Training on the Immune System of Rats during N-methyl-N-nitrosourea-Induced Carcinogenesis

Aim: To assess the effect of physical training on the selected parameters of the immune system regarding CD3, CD4, CD8, CD11, CD161, CD45A cell counts in rats treated with N-methyl-N-nitrosourea (MNU). Material and Methods: Thirty-eight female Sprague-Dawley rats were injected intraperitoneally with MNU and were divided into three groups, i.e., sedentary control (SC), the group of moderate-intensity training (MIT) and the group of high-intensity training (HIT). Physical training was supervised immediately after MNU administration and was conducted 5 days per week for 12 weeks on a three-position treadmill. Results: A significant difference was found between SC and training groups in terms of the number of induced tumors per rat (1.57 vs. 0.4, p = 0.05) and in the following lymphocyte subpopulations: CD4+/CD8+ (p = 0.01), CD3−/CD11b+ (p = 0.02), CD3−/CD161+ (p = 0.002), CD3−/CD161− (p = 0.002), CD3+/CD45RA+ (p = 0.003) and CD3−/CD45RA+ (p = 0.005). In terms of the intensity of physical training, the highest efficacy was found for MIT and the following lymphocyte subpopulations: CD3−/CD11b+ (SC vs. MIT, p < 0.001), CD3−/CD161+ (SC vs. MIT, p = 0.002), CD3−/CD161− (SC vs. MIT, p = 0.002), CD3+/CD45RA+ (SC vs. MIT, p = 0.02) and CD3−/CD45RA+ (SC vs. MIT, p < 0.001, MIT vs. HIT, p = 0.02). Furthermore, negative correlations were found between the number of apoptotic cells and CD3−/CD11b (r = −0.76, p = 0.01) in SC and between the number of induced tumors and CD3+/CD8+ (r = −0.61, p = 0.02) and between their volume and CD+/CD8+ (r = −0.56, p = 0.03) in the group of rats undergoing training. Conclusions: Physical training, particularly MIT, affected immune cell function and an altered immune response can be considered a mechanism underlying the effect of exercise on breast cancer development.

Best practices to quantify the impact of reproductive toxicants on development, function, and diseases of the rodent mammary gland

Work from numerous fields of study suggests that exposures to hormonally active chemicals during sensitive windows of development can alter mammary gland development, function, and disease risk. Stronger links between many environmental pollutants and disruptions to breast health continue to be documented in human populations, and there remain concerns that the methods utilized to identify, characterize, and prioritize these chemicals for risk assessment and risk management purposes are insufficient. There are also concerns that effects on the mammary gland have been largely ignored by regulatory agencies. Here, we provide technical guidance that is intended to enhance collection and evaluation of the mammary gland in mice and rats. We review several features of studies that should be controlled to properly evaluate the mammary gland, and then describe methods to appropriately collect the mammary gland from rodents. Furthermore, we discuss methods for preparing whole mounted mammary glands and numerous approaches that are available for the analysis of these samples. Finally, we conclude with several examples where analysis of the mammary gland revealed effects of environmental toxicants at low doses. Our work argues that the rodent mammary gland should be considered in chemical safety, hazard and risk assessments. It also suggests that improved measures of mammary gland outcomes, such as those we present in this review, should be included in the standardized methods evaluated by regulatory agencies such as the test guidelines used for identifying reproductive and developmental toxicants.

Heterogeneity and function of macrophages in the breast during homeostasis and cancer

Macrophages are diverse immune cells populating all tissues and adopting a unique tissue-specific identity. Breast macrophages play an essential role in the development and function of the mammary gland over one's lifetime. In the recent years, with the development of fate-mapping, imaging and scRNA-seq technologies we grew a better understanding of the origin, heterogeneity and function of mammary macrophages in homeostasis but also during breast cancer development. Here, we aim to provide a comprehensive review of the latest improvements in studying the macrophage heterogeneity in healthy mammary tissues and breast cancer.

How Lineage Tracing Studies Can Unveil Tumor Heterogeneity in Breast Cancer

Lineage tracing studies have become a well-suited approach to reveal cellular hierarchies and tumor heterogeneity. Cellular heterogeneity, particularly in breast cancer, is still one of the main concerns regarding tumor progression and resistance to anti-cancer therapies. Here, we review the current knowledge about lineage tracing analyses that have contributed to an improved comprehension of the complexity of mammary tumors, highlighting how targeting different mammary epithelial cells and tracing their progeny can be useful to explore the intra- and inter-heterogeneity observed in breast cancer. In addition, we examine the strategies used to identify the cell of origin in different breast cancer subtypes and summarize how cellular plasticity plays an important role during tumorigenesis. Finally, we evaluate the clinical implications of lineage tracing studies and the challenges remaining to address tumor heterogeneity in breast cancer.

Newly characterized bovine mammary stromal region with epithelial properties supports representative epithelial outgrowth development from transplanted stem cells

Limited outgrowth development of bovine mammary epithelial stem cells transplanted into de-epithelialized mouse fat pads restricts advanced studies on this productive organ's development and renewal. We challenged the mouse-bovine incompatibility by implanting parenchymal adjacent or distant bovine stromal layers (close and far stroma, respectively) into the mouse fat pad to serve as an endogenous niche for transplanted stem cells. The close stroma better supported stem cell take rate and outgrowth development. The diameter of these open duct-like structures represented and occasionally exceeded that of the endogenous ducts and appeared 8.3-fold wider than the capsule-like structures developed in the mouse fat pad after similar cell transplantation. RNA-Seq revealed lower complement activity in this layer, associated with secretion of specific laminins and WNT proteins favoring epithelial outgrowth development. The close stroma appeared genetically more similar to the parenchyma than to the far stroma due to epithelial characteristics, mainly of fibroblasts, including expression of epithelial markers, milk protein genes, and functional mammary claudins. Gene markers and activators of the mesenchymal-to-epithelial transition were highly enriched in the epithelial gene cluster and may contribute to the acquired epithelial properties of this stromal layer.

Matrix Composition Modulates Vitamin D3's Effects on 3D Collagen Fiber Organization by MCF10A Cells

Vitamin D3 (vitD3) has been implicated in various cellular functions affecting multiple tissue types. Epidemiological and laboratory studies suggest that vitD3 may be effective as a preventive or therapeutic option for breast cancer. However, randomized clinical trials have yet to confirm these suggestions. Breast neoplasias can arise from developmental alterations; based on this evidence, we seek to understand vitD3's role in normal breast development, particularly its role in epithelial morphogenetic processes such as ductal elongation, branching, and alveolar formation. These processes require extensive changes in the extracellular microenvironment, such as collagen fiber organization, and are largely influenced by hormones. Here, we build upon our past work to shed light on calcitriol's effects on collagen fiber organization by breast epithelial cells, and how such effects are modulated by extracellular matrix composition. We embedded MCF10A normal human breast epithelial cells in two different matrices-collagen type I and collagen type I + 10% Matrigel; treatment with calcitriol resulted in flatter epithelial structures. Next, using two-photon microscopy, we examined changes in collagen fiber organization and corresponding changes in epithelial structures. Applying a novel three-dimensional (3D) image analysis method, we show that increasing doses of calcitriol result in denser collagen fiber bundles in the localized area surrounding the epithelial structures, and that these bundles are aligned in a more parallel direction to epithelial structures when exposed to the highest vitD3 dose. Changed patterns in fiber organization may explain the flattening of epithelial structures; in turn, changes in biophysical forces in the matrix abutting these structures may be responsible for changes in the referred patterns. Addition of 10% Matrigel dampened the effects of calcitriol on both epithelial morphogenesis and fiber organization. Overall, we report novel functions of calcitriol in the breast epithelium and add to the growing body of evidence documenting how hormones affect biophysical processes.

Endocrine disrupting chemicals and the mammary gland

Development of the mammary gland requires coordination of hormone signaling pathways including those mediated by estrogen, progesterone, androgen and prolactin receptors. These hormones play important roles at several distinct stages of life including embryonic/fetal development, puberty, pregnancy, lactation, and old age. This also makes the gland sensitive to perturbations from environmental agents including endocrine disrupting chemicals (EDCs). Although there is evidence from human populations of associations between EDCs and disruptions to breast development and lactation, these studies are often complicated by the timing of exposure assessments and the latency to develop breast diseases (e.g., years to decades). Rodents have been instrumental in providing insights-not only to the basic biology and endocrinology of the mammary gland, but to the effects of EDCs on this tissue at different stages of development. Studies, mostly but not exclusively, of estrogenic EDCs have shown that the mammary gland is a sensitive tissue, that exposures during perinatal development can produce abnormal mammary structures (e.g., alveolar buds, typically seen in pregnant females) in adulthood; that exposures during pregnancy can alter milk production; and that EDC exposures can enhance the response of the mammary tissue to hormones and chemical carcinogens. Other studies of persistent organic pollutants have shown that EDC exposures during critical windows of development can delay development of the gland, with lifelong consequences for the individual. Collectively, this work continues to support the conclusion that EDCs can harm the mammary gland, with effects that depend on the period of exposure and the period of evaluation.

Three lactation-related hormones: Regulation of hypothalamus-pituitary axis and function on lactation

The endocrine system plays a central role in many aspects of lactation, including mammogenesis (mammary gland development), lactogenesis (onset of lactation), and galactopoiesis (maintenance of milk secretion). Many hormones of the endocrine system directly or indirectly regulate lactation process. The secretion of prolactin (PRL), one of the most important lactation-related hormones, is inhibited by hypothalamus-pituitary dopaminergic system and stimulated by hypothalamus-pituitary oxytocinergic system. This hormone is essential in all stages of lactation. The growth hormone (GH) regulates metabolism and the distribution of nutrients between tissues mammary glands, and stimulates the production of IGF-I from the liver which binds to IGF-IR of mammary epithelial cells (MECs) to indirectly promote lactation. The synthesis and secretion of estrogen (E) are affected by the hypothalamus-pituitary axis. The hormone regulates duct morphogenesis and MECs proliferation. It also modulates the synthesis and secretion of PRL and GH, which together regulate the lactation in female animals. In this article, we reviewed the three main lactation-related hormones (PRL, GH, and E), summarize their regulation by the hypothalamus-pituitary axis and how they influence lactation.

Preclinical Models to Study Obesity and Breast Cancer in Females: Considerations, Caveats, and Tools

Obesity increases the risk for breast cancer and is associated with poor outcomes for cancer patients. A variety of rodent models have been used to investigate these relationships; however, key differences in experimental approaches, as well as unique aspects of rodent physiology lead to variability in how these valuable models are implemented. We combine expertise in the development and implementation of preclinical models of obesity and breast cancer to disseminate effective practices for studies that integrate these fields. In this review, we share, based on our experience, key considerations for model selection, highlighting important technical nuances and tips for use of preclinical models in studies that integrate obesity with breast cancer risk and progression. We describe relevant mouse and rat paradigms, specifically highlighting differences in breast tumor subtypes, estrogen production, and strategies to manipulate hormone levels. We also outline options for diet composition and housing environments to promote obesity in female rodents. While we have applied our experience to understanding obesity-associated breast cancer, the experimental variables we incorporate have relevance to multiple fields that investigate women's health.

Dimethylbenz(a)anthracene-induced mammary tumorigenesis in mice

Every year, over 2 million women are diagnosed with breast cancer. Although considerable progress was made within the last years in cancer prevention, diagnosis and treatment, breast cancer is still responsible for over 600,000 of deaths per year. Over the years, numerous mouse models have been developed to understand breast cancer etiology and progression. Among those, mammary carcinomas induced by carcinogen, such as 7,12-dimethylbenz[a]anthracene (DMBA), has been widely used. Generally, 30-70% of mice exposed to 4-6 weekly doses of 1mg of DMBA during the peripubertal period (4-10 weeks of age) will develop mammary tumors within 150-200 days after the first exposure, that sometime metastasize to the lungs. As a result, DMBA-induced tumorigenesis is thought to be an accurate and relevant model to study breast cancer as it closely mimics this multistep process. This chapter presents the typical protocol used in mice to induce mammary gland tumors using DMBA. The influence of the number of doses and the total burden of DMBA given, as well as of the age and strain of the mice on mammary gland incident and on tumor onset are discussed. The current knowledge regarding mechanisms involved in DMBA-induced tumorigenesis is also presented.

Stromal SNAI2 Is Required for ERBB2 Breast Cancer Progression

SNAI2 overexpression appears to be associated with poor prognosis in breast cancer, yet it remains unclear in which breast cancer subtypes this occurs. Here we show that excess SNAI2 is associated with a poor prognosis of luminal B HER2⁺/ERBB2⁺ breast cancers in which SNAI2 expression in the stroma but not the epithelium correlates with tumor proliferation. To determine how stromal SNAI2 might influence HER2⁺ tumor behavior, Snai2-deficient mice were crossed with a mouse line carrying the ErbB2/Neu protooncogene to generate HER2⁺/ERBB2⁺ breast cancer. Tumors generated in this model expressed SNAI2 in the stroma but not the epithelium, allowing for the role of stromal SNAI2 to be studied without interference from the epithelial compartment. The absence of SNAI2 in the stroma of HER2⁺/ERBB2⁺ tumors is associated with: (i) lower levels of cyclin D1 (CCND1) and reduced tumor epithelium proliferation; (ii) higher levels of AKT and a lower incidence of metastasis; (iii) lower levels of angiopoietin-2 (ANGPT2), and more necrosis. Together, these results indicate that the loss of SNAI2 in cancer-associated fibroblasts limits the production of some cytokines, which influences AKT/ERK tumor signaling and subsequent proliferative and metastatic capacity of ERBB2⁺ breast cancer cells. Accordingly, SNAI2 expression in the stroma enhanced the tumorigenicity of luminal B HER2⁺/ERBB2⁺ breast cancers. This work emphasizes the importance of stromal SNAI2 in breast cancer progression and patients' prognosis. SIGNIFICANCE: Stromal SNAI2 expression enhances the tumorigenicity of luminal B HER2⁺ breast cancers and can identify a subset of patients with poor prognosis, making SNAI2 a potential therapeutic target for this disease. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/23/5216/F1.large.jpg.

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.

Branching morphogenesis

Over the past 5 years, several studies have begun to uncover the links between the classical signal transduction pathways and the physical mechanisms that are used to sculpt branched tissues. These advances have been made, in part, thanks to innovations in live imaging and reporter animals. With modern research tools, our conceptual models of branching morphogenesis are rapidly evolving, and the differences in branching mechanisms between each organ are becoming increasingly apparent. Here, we highlight four branched epithelia that develop at different spatial scales, within different surrounding tissues and via divergent physical mechanisms. Each of these organs has evolved to employ unique branching strategies to achieve a specialized final architecture.

Chemopreventive Effects of Propolis in the MNU-Induced Rat Mammary Tumor Model

Currently, one of the central problems in cancer management is the relapse of disease following conventional treatments, yet few therapeutic agents targeting resistance and tolerance exist. Propolis is known as a healing agent since ancient times. Therefore, over time, its curative properties have kept the interest of scientists, thus leading permanently to investigations of its other possible undiscovered effects. In this context, current experiments were performed to establish the chemopreventive potential of propolis extract (PE) (1.05 mg/kg BW/day) in N-methyl-N-nitrosourea- (MNU-) induced rat mammary tumors. MNU-inoculated/PE-treated rats had tumors of different physical attributes compared with control rats MNU-inoculated. The number of developed tumors (mean 49% versus 100%), incidence (mean 49% versus 100%), multiplicity (1.8 versus 3.7 (p < 0.001)), tumor volume (mean 10 cm³ versus 16 cm³ (p < 0.001)), and weight of the tumor mass (mean 7.42 g versus 9.00 g (p < 0.05)) were noted. The numbers of grade I tumors recorded for MNU-inoculated rats were 24 (Group 1) and 7 (Group 2) for MNU-induced/PE-treated rats. In the serum of rats MNU-inoculated/PE-treated were found higher levels of antioxidative enzymes (SOD, CAT, and GPx) than in MNU-induced. Taken together, these data indicate that propolis could be a chemopreventive agent against MNU-induced mammary carcinogenesis.

How to Choose a Mouse Model of Breast Cancer, a Genomic Perspective

Human breast cancer is a heterogeneous disease with numerous subtypes that have been defined through immunohistological, histological, and gene expression patterns. The diversity of breast cancer has made the study of its various underlying causes complex. To facilitate the examination of particular facets of breast cancer, mouse models have been generated, ranging from carcinogen induced models to genetically engineered mice. While mouse models have been generated to mimic the initiating event, including p53 loss, BRCA loss, or overexpression of HER2 / Neu / erbB2, other genomic events are often not well characterized. However, these secondary genetic events are often critical to the mouse tumor evolution, subtype, and outcome, just as they are in human breast cancer. As such, these other genomic events are a critical component of what models are chosen to study specific subtypes of human breast cancer. Here we review the genomic analyses that have been completed for various genetically engineered mouse models, how they compare to human breast cancer, and detail how this information can be used in choosing a mouse model for analysis.

Rodent Models Assessing Mammary Tumor Prevention by Soy or Soy Isoflavones

While epidemiological studies performed in Asian countries generally show that high levels of dietary soy are associated with reduced breast cancer risk, studies in Western countries have typically failed to show this correlation. In an attempt to model the preventative actions of soy on mammary tumor development, rodent models have been employed. Thirty-four studies were identified that evaluated the impact of soy products or purified soy isoflavones on mammary tumor initiation (studies evaluating established mammary tumors or mammary tumor cell lines were not included) and these studies were separated into mammary tumors induced by chemical carcinogens or transgenic expression of oncogenes based on the timing of soy administration. Regardless of when soy-based diets or purified isoflavones were administered, no consistent protective effects were observed in either carcinogen-induced or oncogene-induced mammary tumors. While some studies demonstrated that soy or purified isoflavones could reduce mammary tumor incidence, other studies showed either no effect or tumor promoting effects of soy products or isoflavones. Most importantly, only five studies found a decrease in mammary tumor incidence and six studies observed a decrease in tumor multiplicity, two relevant measures of the tumor preventative effects of soy or isoflavones. The variable outcomes of the studies examined were not completely surprising given that few studies employed the same experimental design. Future studies should be carefully designed to more accurately emulate soy consumption observed in Asian cultures including lifetime exposure to less refined soy products and potentially the incorporation of multigenerational feeding studies.

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.

PDGFRα+ stromal adipocyte progenitors transition into epithelial cells during lobulo-alveologenesis in the murine mammary gland

The mammary gland experiences substantial remodeling and regeneration during development and reproductive life, facilitated by stem cells and progenitors that act in concert with physiological stimuli. While studies have focused on deciphering regenerative cells within the parenchymal epithelium, cell lineages in the stroma that may directly contribute to epithelial biology is unknown. Here we identify, in mouse, the transition of a PDGFRα⁺ mesenchymal cell population into mammary epithelial progenitors. In addition to being adipocyte progenitors, PDGFRα⁺ cells make a de novo contribution to luminal and basal epithelia during mammary morphogenesis. In the adult, this mesenchymal lineage primarily generates luminal progenitors within lobuloalveoli during sex hormone exposure or pregnancy. We identify cell migration as a key molecular event that is activated in mesenchymal progenitors in response to epithelium-derived chemoattractant. These findings demonstrate a stromal reservoir of epithelial progenitors and provide insight into cell origins and plasticity during mammary tissue growth.

Endogenously Synthesized n-3 Polyunsaturated Fatty Acids in Pregnant fat-1 Mice Decreases Mammary Cancer Risk of Female Offspring by Regulating Expression of Long Noncoding RNAs

SCOPE

The present study investigates the precise mechanism by which maternal n-3 PUFAs decrease mammary cancer risk of offspring in terms of epigenetics.

METHODS AND RESULTS

Transgenic fat-1 and wild-type C57BL/6J littermates are fed an n-6 PUFAs diet during pregnancy. Wild-genotype offspring of fat-1 mothers (fat-1 group) are compared with wild-genotype offspring of C57BL/6J mothers (control group) in breast cancer risk. Fat-1 group shows a significantly lower tumor incidence and smaller tumor volume compared with control group. n-3 PUFAs in fat-1 mothers change the expression of long noncoding RNA (lncRNA, 53 upregulated and 45 downregulated) in mammary glands of offspring. The lncRNA changes are associated with the changes of mRNA in multiple oncogenic signaling pathways, especially NF-κB, Jak-STAT, and MAPK pathways. Expression of key protein in these pathways, namely p65, p60, STAT3, Jak1, and p38, are significantly inhibited in fat-1 group. In line with these results, reduced proliferation and increased apoptosis are also observed in mammary epithelial of fat-1 group than control group.

CONCLUSION

The anticancer effect of maternal n-3 PUFAs is related to the regulation of lncRNA expression, which can further regulate the susceptibility of offspring to breast cancer.

Heterogeneity of Mammary Stem Cells

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

Modeling Human Ductal Carcinoma In Situ in the Mouse

Breast cancer development is a multi-step process in which genetic and molecular heterogeneity occurs at multiple stages. Ductal carcinoma arises from pre-invasive lesions such as atypical ductal hyperplasia (ADH) and ductal carcinoma in situ (DCIS), which progress to invasive and metastatic cancer. The feasibility of obtaining tissue samples from all stages of progression from the same patient is low, and thus molecular studies dissecting the mechanisms that mediate the transition from pre-invasive DCIS to invasive carcinoma have been hampered. In the past 25 years, numerous mouse models have been developed that partly recapitulate the histological and biological properties of early stage lesions. In this review, we discuss in vivo model systems of breast cancer progression from syngeneic mouse models to human xenografts, with particular focus on how accurately these models mimic human disease.

Detection of lithium in breast milk and in situ elemental analysis of the mammary gland

Breast feeding provides considerable benefits to the infant and mother. However, a lithium-based psychiatric medication may cause side effects in the child. Using laser induced breakdown spectroscopy (LIBS), trace lithium levels were observed in the breast milk of lactating rats administered with lithium treatment postpartum. Subsequently, the mammary glands of female rats were analyzed using LIBS, energy dispersive X-ray fluorescence spectroscopy, and inductively coupled plasma mass spectrometry. Key biological elements iron, magnesium, cobalt, calcium, phosphorus, sodium, iodine, potassium, sulfur, chlorine and zinc were observed. Lithium at 1.06 µg/g was measured in the mammary glands of treated subjects, but was below the limit of detection in controls. Lithium also increased iodine content in the glands. Lithium is present in the breast milk and mammary glands of lithium treated female subjects and this is the likely route of entry to breast-fed infants.

High Expression of CD200 and CD200R1 Distinguishes Stem and Progenitor Cell Populations within Mammary Repopulating Units

Aiming to unravel the top of the mammary epithelial cell hierarchy, a subset of the CD49f^highCD24^med mammary repopulating units (MRUs) was identified by flow cytometry, expressing high levels of CD200 and its receptor CD200R1. These MRU^{CD200/CD200R1} repopulated a larger area of de-epithelized mammary fat pads than the rest of the MRUs, termed MRU^{not CD200/CD200R1}. MRU^{CD200/CD200R1} maintained a much lower number of divergently defined, highly expressed genes and pathways that support better cell growth, development, differentiation, and progenitor activity than their MRU^{not CD200/CD200R1} counterparts. A defined profile of hierarchically associated genes supporting a single-lineage hypothesis was confirmed by in vitro mammosphere analysis that assembled 114 genes with decreased expression from MRU^{CD200/CD200R1} via MRU^{not CD200/CD200R1} toward CD200⁺CD200R1^- and CD200R1⁺CD200^- cells. About 40% of these genes were shared by a previously published database of upregulated genes in mammary/breast stem cells and may represent the core genes involved in mammary stemness.

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

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

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

The challenges of modeling hormone receptor-positive breast cancer in mice

Estrogen receptor-positive (ER+) tumors account for 70-80% of all breast cancer (BC) cases and are characterized by estrogen dependency for their growth. Endocrine therapies using estrogen receptor antagonists or aromatase inhibitors represent a key component of the standard of care for these tumors. The occurrence of de novo or acquired resistance to estrogen withdrawal represents an important clinical problem, impacting on patient survival. In addition, despite an initially favorable outcome, a part of ER+ BC patients present with disease recurrence locally or at distant sites years or even decades after apparent remission. In vivo models that closely mimic human disease are urgently needed to study the biology of these tumors, investigate the molecular mechanisms underlying endocrine resistance and identify patients at risk of recurrence. Despite the similarities in the overall hormonal regulation of mammary gland development between mice and humans, the majority of the mammary carcinomas occurring in genetically engineered mouse models (GEMMs) are ER negative and most xenograft models are based on few ER+ cancer cell lines. We recently showed that the microenvironment is critical for ER+ cancer cells and discuss in this review the potential of intraductal xenograft model for basic and preclinical research.

Chronic expression of wild-type Ret receptor in the mammary gland induces luminal tumors that are sensitive to Ret inhibition

The receptor tyrosine kinase Ret, a key gain-of-function mutated oncoprotein in thyroid carcinomas, has recently been implicated in other cancer types. While Ret copy number gains and mutations have been reported at low frequencies in breast tumors, we and others have reported that Ret is overexpressed in about 40% of human tumors and this correlates with poor patient prognosis. Ret activation regulates numerous intracellular pathways related to proliferation and inflammation, but it is not known whether abnormal Ret expression is sufficient to induce mammary carcinomas. Using a novel doxycycline-inducible transgenic mouse model with the MMTV promoter controlling Ret expression, we show that overexpression of wild-type Ret in the mammary epithelium produces mammary tumors, displaying a morphology that recapitulates characteristics of human luminal breast tumors. Ret-evoked tumors are estrogen receptor positive and negative for progesterone receptor. Moreover, tumors rapidly regress after doxycycline withdrawal, indicating that Ret is the driving oncoprotein. Using next-generation sequencing, we examined the levels of transcripts in these tumors, confirming a luminal signature. Ret-evoked tumors have been passaged in mice and used to test novel therapeutic approaches. Importantly, we have determined that tumors are resistant to endocrine therapy, but respond successfully to treatment with a Ret kinase inhibitor. Our data provide the first compelling evidence for an oncogenic role of non-mutated Ret in the mammary gland and are an incentive for clinical development of Ret as a cancer biomarker and therapeutic target.

Aging Mouse Models Reveal Complex Tumor-Microenvironment Interactions in Cancer Progression

Mouse models and genetically engineered mouse models (GEMM) are essential experimental tools for the understanding molecular mechanisms within complex biological systems. GEMM are especially useful for inferencing phenocopy information to genetic human diseases such as breast cancer. Human breast cancer modeling in mice most commonly employs mammary epithelial-specific promoters to investigate gene function(s) and, in particular, putative oncogenes. Models are specifically useful in the mammary epithelial cell in the context of the complete mammary gland environment. Gene targeted knockout mice including conditional targeting to specific mammary cells can reveal developmental defects in mammary organogenesis and demonstrate the importance of putative tumor suppressor genes. Some of these models demonstrate a non-traditional type of tumor suppression which involves interplay between the tumor susceptible cell and its host/environment. These GEMM help to reveal the processes of cancer progression beyond those intrinsic to cancer cells. Furthermore, the, analysis of mouse models requires appropriate consideration of mouse strain, background, and environmental factors. In this review, we compare aging-related factors in mouse models for breast cancer. We introduce databases of GEMM attributes and colony functional variations.

Myosin 1e promotes breast cancer malignancy by enhancing tumor cell proliferation and stimulating tumor cell de-differentiation

Despite advancing therapies, thousands of women die every year of breast cancer. Myosins, actin-dependent molecular motors, are likely to contribute to tumor formation and metastasis via their effects on cell adhesion and migration and may provide promising new targets for cancer therapies. Using the MMTV-PyMT murine model of breast cancer, we identified Myosin 1e (MYO1E) as a novel tumor promoter. Tumor latency in mice lacking MYO1E was significantly increased, and tumors formed in the absence of MYO1E displayed unusual papillary morphology, with well-differentiated layers of epithelial cells covering fibrovascular cores, rather than solid sheets of tumor cells typically observed in this cancer model. These tumors were reminiscent of papillary breast cancer in humans that is typically non-invasive and often cured by tumor excision. MYO1E-null tumors exhibited decreased expression of the markers of cell proliferation, which was recapitulated in primary tumor cells derived from MYO1E-null mice. In agreement with our findings, meta-analysis of patient survival data indicated that MYO1E expression level was associated with reduced recurrence-free survival in basal-like breast cancer. Overall, our data suggests that MYO1E contributes to breast tumor malignancy and regulates the differentiation and proliferation state of breast tumor cells.

Luminal STAT5 mediates H2AX promoter activity in distinct population of basal mammary epithelial cells

Deregulated STAT5 activity in the mammary gland causes parity-dependent tumorigenesis. Epithelial cell cultures transfected with constitutively active STAT5 express higher levels of the histone H2AX than their non-transfected counterparts. Higher H2AX expression may be involved in tumorigenesis. Here, we aimed to link high STAT5 activity to H2AX–GFP expression by looking for distinct types of mammary cells that express these proteins. In vitro and in transgenic mice, only 0.2 and 0.02%, respectively, of the cells expressed the H2AX–GFP hybrid gene. Its expression correlated with that of the endogenous H2AX gene, suggesting that detectable H2AX–GFP expression marks high levels of H2AX transcript. Methylation of the H2AX promoter characterized non-GFP-expressing H2AX–GFP cells and was inversely correlated with promoter activity. Administration of 5-azacytidine increased H2AX promoter activity in an activated STAT5-dependent manner. In transgenic mice, H2AX–GFP expression peaked at pregnancy. The number of H2AX–GFP-expressing cells and GFP expression decreased in a Stat5a-null background and increased in mice expressing the hyperactivated STAT5. Importantly, H2AX–GFP activity was allocated to basal mammary cells lacking stem-cell properties, whereas STAT5 hyperactivity was detected in the adjacent luminal cells. Knockdown of RANKL by siRNA suggested its involvement in signaling between the two layers. These results suggest paracrine activation of H2AX via promoter demethylation in specific populations of basal mammary cells that is induced by a signal from neighboring luminal cells with hyper STAT5 activity. This pathway provides an alternative route for the luminally confined STAT5 to affect basal mammary cell activity.

Rifaximin, a pregnane X receptor (PXR) activator regulates apoptosis in a murine model of breast cancer

The present study was proposed to investigate the effect of rifaximin (RFX) on methyl nitrosourea (MNU) induced mammary gland carcinoma in albino wistar rats.

Characterization of primary human mammary epithelial cells isolated and propagated by conditional reprogrammed cell culture

Purpose

Conditional reprogramming methods allow for the inexhaustible in vitro proliferation of primary epithelial cells from human tissue specimens. This methodology has the potential to enhance the utility of primary cell culture as a model for mammary gland research. However, few studies have systematically characterized this method in generating in vitro normal human mammary epithelial cell models.

Results

We show that cells derived from fresh normal breast tissues can be propagated and exhibit heterogeneous morphologic features. The cultures are composed of CK18, desmoglein 3, and CK19-positive luminal cells and vimentin, p63, and CK14-positive myoepithelial cells, suggesting the maintenance of in vivo heterogeneity. In addition, the cultures contain subpopulations with different CD49f and EpCAM expression profiles. When grown in 3D conditions, cells self-organize into distinct structures that express either luminal or basal cell markers. Among these structures, CK8-positive cells enclosing a lumen are capable of differentiation into milk-producing cells in the presence of lactogenic stimulus. Furthermore, our short-term cultures retain the expression of ERα, as well as its ability to respond to estrogen stimulation.

Materials and Methods

We have investigated conditionally reprogrammed normal epithelial cells in terms of cell type heterogeneity, cellular marker expression, and structural arrangement in two-dimensional (2D) and three-dimensional (3D) systems.

Conclusions

The conditional reprogramming methodology allows generation of a heterogeneous culture from normal human mammary tissue in vitro. We believe that this cell culture model will provide a valuable tool to study mammary cell function and malignant transformation.

Pregnancy and Breast Cancer

Breast cancer is the most commonly diagnosed type of cancer among women worldwide. The majority of breast cancers are sporadic and the etiology is not well understood. Several factors have been attributed to altering the risk of breast cancer. A full-term pregnancy is a crucial factor in altering the risk. Early full-term pregnancy has been shown to reduce the lifetime risk of breast cancer, while a later first full-term pregnancy increases breast cancer risk. Epidemiological and experimental data demonstrate that spontaneous or induced abortions do not significantly alter the risk of breast cancer. In this study, we briefly discuss the different types and stages of breast cancer, various risk factors, and potential mechanisms involved in early full-term pregnancy-induced protection against breast cancer. Understanding how early full-term pregnancy induces protection against breast cancer will help design innovative preventive and therapeutic strategies. This understanding can also help in the development of molecular biomarkers that can be of tremendous help in predicting the risk of breast cancer in the general population.

Pathobiology of the 129:Stat1 -/- mouse model of human age-related ER-positive breast cancer with an immune infiltrate-excluded phenotype

Background

Stat1 gene-targeted knockout mice (129S6/SvEvTac-Stat1^tm1Rds) develop estrogen receptor-positive (ER⁺), luminal-type mammary carcinomas at an advanced age. There is evidence for both host environment as well as tumor cell-intrinsic mechanisms to initiate tumorigenesis in this model. In this report, we summarize details of the systemic and mammary pathology at preneoplastic and tumor-bearing time points. In addition, we investigate tumor progression in the 129:Stat1^−/− host compared with wild-type 129/SvEv, and we describe the immune cell reaction to the tumors.

Methods

Mice housed and treated according to National Institutes of Health guidelines and Institutional Animal Care and Use Committee-approved methods were evaluated by histopathology, and their tissues were subjected to immunohistochemistry with computer-assisted quantitative image analysis. Tumor cell culture and conditioned media from cell culture were used to perform macrophage (RAW264.7) cell migration assays, including the 129:Stat1^−/−-derived SSM2 cells as well as control Met1 and NDL tumor cells and EpH4 normal cells.

Results

Tumorigenesis in 129:Stat1^−/− originates from a population of FoxA1⁺ large oval pale cells that initially appear and accumulate along the mammary ducts in segments or regions of the gland prior to giving rise to mammary intraepithelial neoplasias. Progression to invasive carcinoma is accompanied by a marked local stromal and immune cell response composed predominantly of T cells and macrophages. In conditioned media experiments, cells derived from 129:Stat1^−/− tumors secrete both chemoattractant and chemoinhibitory factors, with greater attraction in the extracellular vesicular fraction and inhibition in the soluble fraction. The result appears to be recruitment of the immune reaction to the periphery of the tumor, with exclusion of immune cell infiltration into the tumor.

Conclusions

129:Stat1^−/− is a unique model for studying the critical origins and risk reduction strategies in age-related ER⁺ breast cancer. In addition, it can be used in preclinical trials of hormonal and targeted therapies as well as immunotherapies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-017-0892-8) contains supplementary material, which is available to authorized users.

Engineered Hydrogels in Cancer Therapy and Diagnosis

Over the last decade, numerous investigations have attempted to clarify the intricacies of tumor development to propose effective approaches for cancer treatment. Thanks to the unique properties of hydrogels, researchers have made significant progress in tumor model reconstruction, tumor diagnosis, and associated therapies. Notably, hydrogel-based systems can be adjusted to respond to cancer-specific hallmarks and/or external stimuli. These well-known drug reservoirs can be used as smart carriers for multiple cargos, including both naked and nanoparticle-encapsulated chemotherapeutics, genes, and radioisotopes. Recent works have attempted to specialize hydrogels for cancer research; we comprehensively review this topic for the first time, synthesizing past results and defining paths for future work.

In vivo models in breast cancer research: progress, challenges and future directions

Research using animal model systems has been instrumental in delivering improved therapies for breast cancer, as well as in generating new insights into the mechanisms that underpin development of the disease. A large number of different models are now available, reflecting different types and stages of the disease; choosing which one to use depends on the specific research question(s) to be investigated. Based on presentations and discussions from leading experts who attended a recent workshop focused on in vivo models of breast cancer, this article provides a perspective on the many varied uses of these models in breast cancer research, their strengths, associated challenges and future directions. Among the questions discussed were: how well do models represent the different stages of human disease; how can we model the involvement of the human immune system and microenvironment in breast cancer; what are the appropriate models of metastatic disease; can we use models to carry out preclinical drug trials and identify pathways responsible for drug resistance; and what are the limitations of patient-derived xenograft models? We briefly outline the areas where the existing breast cancer models require improvement in light of the increased understanding of the disease process, reflecting the drive towards more personalised therapies and identification of mechanisms of drug resistance.

Prenatal Mammary Gland Development in the Mouse: Research Models and Techniques for Its Study from Past to Present

Mammary gland development starts during prenatal life, when at designated positions along the ventrolateral boundary of the embryonic or fetal trunk, surface ectodermal cells coalesce to form primordia for mammary glands, instead of differentiating into epidermis. With the wealth of genetically engineered mice available as research models, our understanding of the prenatal phase of mammary development has recently greatly advanced. This understanding includes the recognition of molecular and mechanistic parallels between prenatal and postnatal mammary morphogenesis and even tumorigenesis, much of which can moreover be extrapolated to human. This makes the murine embryonic mammary gland a useful model for a myriad of questions pertaining to normal and pathological breast development. Hence, unless indicated otherwise, this review describes embryonic mammary gland development in mouse only, and lists mouse models that have been examined for defects in embryonic mammary development. Techniques that originated in the field of developmental biology, such as explant culture and tissue recombination, were adapted specifically to research on the embryonic mammary gland. Detailed protocols for these techniques have recently been published elsewhere. This review describes how the development and adaptation of these techniques moved the field forward from insights on (comparative) morphogenesis of the embryonic mammary gland to the understanding of tissue and molecular interactions and their regulation of morphogenesis and functional development of the embryonic mammary gland. It is here furthermore illustrated how generic molecular biology and biochemistry techniques can be combined with these older, developmental biology techniques, to address relevant research questions. As such, this review should provide a solid starting point for those wishing to familiarize themselves with this fascinating and important subdomain of mammary gland biology, and guide them in designing a relevant research strategy.

Evidence of two distinct functionally specialized fibroblast lineages in breast stroma

BACKGROUND

The terminal duct lobular unit (TDLU) is the most dynamic structure in the human breast and the putative site of origin of human breast cancer. Although stromal cells contribute to a specialized microenvironment in many organs, this component remains largely understudied in the human breast. We here demonstrate the impact on epithelium of two lineages of breast stromal fibroblasts, one of which accumulates in the TDLU while the other resides outside the TDLU in the interlobular stroma.

METHODS

The two lineages are prospectively isolated by fluorescence activated cell sorting (FACS) based on different expression levels of CD105 and CD26. The characteristics of the two fibroblast lineages are assessed by immunocytochemical staining and gene expression analysis. The differentiation capacity of the two fibroblast populations is determined by exposure to specific differentiating conditions followed by analysis of adipogenic and osteogenic differentiation. To test whether the two fibroblast lineages are functionally imprinted by their site of origin, single cell sorted CD271^low/MUC1^high normal breast luminal epithelial cells are plated on fibroblast feeders for the observation of morphological development. Epithelial structure formation and polarization is shown by immunofluorescence and digitalized quantification of immunoperoxidase-stained cultures.

RESULTS

Lobular fibroblasts are CD105^high/CD26^low while interlobular fibroblasts are CD105^low/CD26^high. Once isolated the two lineages remain phenotypically stable and functionally distinct in culture. Lobular fibroblasts have properties in common with bone marrow derived mesenchymal stem cells and they specifically convey growth and branching morphogenesis of epithelial progenitors.

CONCLUSIONS

Two distinct functionally specialized fibroblast lineages exist in the normal human breast, of which the lobular fibroblasts have properties in common with mesenchymal stem cells and support epithelial growth and morphogenesis. We propose that lobular fibroblasts constitute a specialized microenvironment for human breast luminal epithelial progenitors, i.e. the putative precursors of breast cancer.

Mammary Gland Evaluation in Juvenile Toxicity Studies: Temporal Developmental Patterns in the Male and Female Harlan Sprague-Dawley Rat

There are currently no reports describing mammary gland development in the Harlan Sprague-Dawley (HSD) rat, the current strain of choice for National Toxicology Program (NTP) testing. Our goals were to empower the NTP, contract labs, and other researchers in understanding and interpreting chemical effects in this rat strain. To delineate similarities/differences between the female and male mammary gland, data were compiled starting on embryonic day 15.5 through postnatal day 70. Mammary gland whole mounts, histology sections, and immunohistochemically stained tissues for estrogen, progesterone, and androgen receptors were evaluated in both sexes; qualitative and quantitative differences are highlighted using a comprehensive visual timeline. Research on endocrine disrupting chemicals in animal models has highlighted chemically induced mammary gland anomalies that may potentially impact human health. In order to investigate these effects within the HSD strain, 2,3,7,8-tetrachlorodibenzo-p-dioxin, diethylstilbestrol, or vehicle control was gavage dosed on gestation day 15 and 18 to demonstrate delayed, accelerated, and control mammary gland growth in offspring, respectively. We provide illustrations of normal and chemically altered mammary gland development in HSD male and female rats to help inform researchers unfamiliar with the tissue and may facilitate enhanced evaluation of both male and female mammary glands in juvenile toxicity studies.

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

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