Cellular expression of estrogen and progesterone receptors in mammary glands: regulation by hormones, development and aging

Transcriptomic Analysis of Pubertal and Adult Virgin Mouse Mammary Epithelial and Stromal Cell Populations

Conflicting data exist as to how mammary epithelial cell proliferation changes during the reproductive cycle. To study the effect of endogenous hormone fluctuations on gene expression in the mouse mammary gland, we performed bulk RNAseq analyses of epithelial and stromal cell populations that were isolated either during puberty or at different stages of the adult virgin estrous cycle. Our data confirm prior findings that proliferative changes do not occur in every mouse in every cycle. We also show that during the estrous cycle the main gene expression changes occur in adipocytes and fibroblasts. Finally, we present a comprehensive overview of the Wnt gene expression landscape in different mammary gland cell types in pubertal and adult mice. This work contributes to understanding the effects of physiological hormone fluctuations and locally produced signaling molecules on gene expression changes in the mammary gland during the reproductive cycle and should be a useful resource for future studies investigating gene expression patterns in different cell types across different developmental timepoints.

Dual role of pregnancy in breast cancer risk

Reproductive history is one of the strongest risk factors for breast cancer in women. Pregnancy can promote short-term breast cancer risk, but also reduce a woman's lifetime risk of breast cancer. Changes in hormone levels before and after pregnancy are one of the key factors in breast cancer risk. This article summarizes the changes in hormone levels before and after pregnancy, and the roles of hormones in mammary gland development and breast cancer progression. Other factors, such as changes in breast morphology and mammary gland differentiation, changes in the proportion of mammary stem cells (MaSCs), changes in the immune and inflammatory environment, and changes in lactation before and after pregnancy, also play key roles in the occurrence and development of breast cancer. This review discusses the dual effects and the potential mechanisms of pregnancy on breast cancer risk from the above aspects, which is helpful to understand the complexity of female breast cancer occurrence.

Changes in the mammary gland during aging and its links with breast diseases

The functional capacity of organisms declines in the process of aging. In the case of breast tissue, abnormal mammary gland development can lead to dysfunction in milk secretion, a primary function, as well as the onset of various diseases, such as breast cancer. In the process of aging, the terminal duct lobular units (TDLUs) within the breast undergo gradual degeneration, while the proportion of adipose tissue in the breast continues to increase and hormonal levels in the breast change accordingly. Here, we review changes in morphology, internal structure, and cellular composition that occur in the mammary gland during aging. We also explore the emerging mechanisms of breast aging and the relationship between changes during aging and breast-related diseases, as well as potential interventions for delaying mammary gland aging and preventing breast disease.

PAQR8 and PAQR9 expression is altered in the ventral tegmental area of aged rats infected with varicella zoster virus

Infection with varicella zoster virus (VZV) results in chicken pox and reactivation of VZV results in herpes zoster (HZ) or what is often referred to as shingles. Patients with HZ experience decreased motivation and increased emotional distress consistent with functions of the ventral tegmental area (VTA) of the brain. In addition, activity within the ventral tegmental area is altered in patients with HZ. HZ primarily affects individuals that are older and the VTA changes with age. To begin to determine if the VTA has a role in HZ symptoms, a screen of 10,000 genes within the VTA in young and old male rats was completed after injecting the whisker pad with VZV. The two genes that had maximal change were membrane progesterone receptors PAQR8 (mPRβ) and PAQR9 (mPRε). Neurons and non-neuronal cells expressed both PAQR8 and PAQR9. PAQR8 and PAQR9 protein expression was significantly reduced after VZV injection of young males. In old rats PAQR9 protein expression was significantly increased after VZV injection and PAQR9 protein expression was reduced in aged male rats versus young rats. Consistent with previous results, pain significantly increased after VZV injection of the whisker pad and aged animals showed significantly more pain than young animals. Our data suggests that PAQR8 and PAQR9 expression is altered by VZV injection and that these changes are affected by age.

Medicinal Mushroom Leucocalocybe mongolica Imai Extracts Improve Mammary Gland Differentiation in Lactating Rats via Regulating Protein Expression

Leucocalocybe mongolica is a known medicinal mushroom in China. It possesses many biological activities. This study investigated the effect of L. mongolica petroleum ether and water extracts (200, 500, and 1,000 mg/kg BW) on mammary gland differentiation during lactation. However, prolactin, growth hormone, progesterone, and estrogen levels were determined in serum by ELISA assay. Immunofluorescence, western blot, and real-time PCR were utilized to evaluate the expression levels of β-casein, α-Lactalbumin, prolactin receptor, progesterone receptor, and STAT-5a. The immunohistochemistry staining was used to detect the presence of steroid receptors. The results showed that petroleum ether and water extracts increased milk yield and milk content of calcium, total fat, total carbohydrate, and total protein. Prolactin and growth hormone levels were significantly upregulated in all treated groups compared with the control group. In contrast, progesterone and estrogen were downregulated. The high doses of petroleum ether and water extracts increased the expression levels of β-Cas, α-Lactalb, PRLR, PR, and STAT-5a. The observation of histological sections showed that the extracts induced higher mammary gland differentiation than the control group. This study is the first to use mushrooms as nutritional supplements to improve milk production and mammary gland differentiation during lactation.

Effects of Energy and Dietary Fiber on the Breast Development in Gilt

To study the effects of energy and dietary fiber on breast development in gilts and its possible mechanisms, 32 gilts (Landrace × Yorkshire) were randomly allocated into a 2 × 2 factorial design to receive a diet with low or high energy [LE: 33.37 MJ/d digestible energy (DE); HE: 41.87 MJ/d DE] and low or high fiber (LF: 0.3 kg/d dietary fiber, HF: 0.6 kg/d dietary fiber). The weight of breast tissue was recorded. The mammary glands were collected for further analyses. The high energy intake increased the relative weight of breast tissue (p < 0.05) and the content of breast fat (p < 0.05). At the same time, the oil red staining of breast slices also showed an increase in breast fat content in high-energy treatment. High energy intake increased the DNA concentration in breast tissues (p < 0.05). In addition, high energy intake increased the concentration of triglycerides, free fatty acids, and total cholesterol in the blood of gilts (p < 0.05), and the supplementation of high fiber tended to reduce free fatty acids, total cholesterol, and estradiol (p < 0.1). Proteomic analysis suggested that there were notable differences in the cytoskeleton, intracellular non–membrane-bounded organelle, apoptosis, receptor activity, and endopeptidase inhibitor activity in molecular function between the energy and fiber effects (p < 0.05). High fiber intake also decreased the mRNA expression of 5-HT7, Bax, and caspase-3 in the breast tissue of gilts (p < 0.05), which further confirmed the importance of fiber in regulating breast development in gilt. Our results indicate that increasing gilt energy intake improved breast weight and fat deposition and increased breast cell apoptosis. Increased fiber intake reduced breast fat deposition and breast cell apoptosis at high energy intake in gilts. These results provide a potential strategy for dietary intervention against high energy intake in gilts and even in humans.

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.

The molecular basis of mammary gland development and epithelial differentiation

Our understanding of the molecular events underpinning the development of mammalian organ systems has been increasing rapidly in recent years. With the advent of new and improved next-generation sequencing methods, we are now able to dig deeper than ever before into the genomic and epigenomic events that play critical roles in determining the fates of stem and progenitor cells during the development of an embryo into an adult. In this review, we detail and discuss the genes and pathways that are involved in mammary gland development, from embryogenesis, through maturation into an adult gland, to the role of pregnancy signals in directing the terminal maturation of the mammary gland into a milk producing organ that can nurture the offspring. We also provide an overview of the latest research in the single-cell genomics of mammary gland development, which may help us to understand the lineage commitment of mammary stem cells (MaSCs) into luminal or basal epithelial cells that constitute the mammary gland. Finally, we summarize the use of 3D organoid cultures as a model system to study the molecular events during mammary gland development. Our increased investigation of the molecular requirements for normal mammary gland development will advance the discovery of targets to predict breast cancer risk and the development of new breast cancer therapies.

Exposure to Propylparaben During Pregnancy and Lactation Induces Long-Term Alterations to the Mammary Gland in Mice

The mammary gland is a hormone sensitive organ that is susceptible to endocrine-disrupting chemicals (EDCs) during the vulnerable periods of parous reorganization (ie, pregnancy, lactation, and involution). Pregnancy is believed to have long-term protective effects against breast cancer development; however, it is unknown if EDCs can alter this effect. We examined the long-term effects of propylparaben, a common preservative used in personal care products and foods, with estrogenic properties, on the parous mouse mammary gland. Pregnant BALB/c mice were treated with 0, 20, 100, or 10 000 µg/kg/day propylparaben throughout pregnancy and lactation. Unexposed nulliparous females were also evaluated. Five weeks post-involution, mammary glands were collected and assessed for changes in histomorphology, hormone receptor expression, immune cell number, and gene expression. For several parameters of mammary gland morphology, propylparaben reduced the effects of parity. Propylparaben also increased proliferation, but not stem cell number, and induced modest alterations to expression of ERα-mediated genes. Finally, propylparaben altered the effect of parity on the number of several immune cell types in the mammary gland. These results suggest that propylparaben, at levels relevant to human exposure, can interfere with the effects of parity on the mouse mammary gland and induce long-term alterations to mammary gland structure. Future studies should address if propylparaben exposures negate the protective effects of pregnancy on mammary cancer development.

An Integrative Single-cell Transcriptomic Atlas of the Post-natal Mouse Mammary Gland Allows Discovery of New Developmental Trajectories in the Luminal Compartment

The mammary gland is a highly dynamic organ which undergoes periods of expansion, differentiation and cell death in each reproductive cycle. Partly because of the dynamic nature of the gland, mammary epithelial cells (MECs) are extraordinarily heterogeneous. Single cell RNA-seq (scRNA-seq) analyses have contributed to understand the cellular and transcriptional heterogeneity of this complex tissue. Here, we integrate scRNA-seq data from three foundational reports that have explored the mammary gland cell populations throughout development at single-cell level using 10× Chromium Drop-Seq. We center our analysis on post-natal development of the mammary gland, from puberty to post-involution. The new integrated study corresponds to RNA sequences from 53,686 individual cells, which greatly outnumbers the three initial data sets. The large volume of information provides new insights, as a better resolution of the previously detected Procr⁺ stem-like cell subpopulation or the identification of a novel group of MECs expressing immune-like markers. Moreover, here we present new pseudo-temporal trajectories of MEC populations at two resolution levels, that is either considering all mammary cell subtypes or focusing specifically on the luminal lineages. Interestingly, the luminal-restricted analysis reveals distinct expression patterns of various genes that encode milk proteins, suggesting specific and non-redundant roles for each of them. In summary, our data show that the application of bioinformatic tools to integrate multiple scRNA-seq data-sets helps to describe and interpret the high level of plasticity involved in gene expression regulation throughout mammary gland post-natal development.

Progesterone and Breast Cancer

Synthetic progestogens (progestins) have been linked to increased breast cancer risk; however, the role of endogenous progesterone in breast physiology and carcinogenesis is less clearly defined. Mechanistic studies using cell culture, tissue culture, and preclinical models implicate progesterone in breast carcinogenesis. In contrast, limited epidemiologic data generally do not show an association of circulating progesterone levels with risk, and it is unclear whether this reflects methodologic limitations or a truly null relationship. Challenges related to defining the role of progesterone in breast physiology and neoplasia include: complex interactions with estrogens and other hormones (eg, androgens, prolactin, etc.), accounting for timing of blood collections for hormone measurements among cycling women, and limitations of assays to measure progesterone metabolites in blood and progesterone receptor isotypes (PRs) in tissues. Separating the individual effects of estrogens and progesterone is further complicated by the partial dependence of PR transcription on estrogen receptor (ER)α-mediated transcriptional events; indeed, interpreting the integrated interaction of the hormones may be more essential than isolating independent effects. Further, many of the actions of both estrogens and progesterone, particularly in "normal" breast tissues, are driven by paracrine mechanisms in which ligand binding to receptor-positive cells evokes secretion of factors that influence cell division of neighboring receptor-negative cells. Accordingly, blood and tissue levels may differ, and the latter are challenging to measure. Given conflicting data related to the potential role of progesterone in breast cancer etiology and interest in blocking progesterone action to prevent or treat breast cancer, we provide a review of the evidence that links progesterone to breast cancer risk and suggest future directions for filling current gaps in our knowledge.

Progesterone receptor expression in proliferating cancer cells of hormone-receptor-positive breast cancer

Breast cancer has been suggested to have two distinct driving mechanisms: the hormone receptor and the growth factor receptor pathways. We hypothesized that each driving system produces a different expression pattern of estrogen-regulated genes, such as progesterone receptor, in proliferating cells. Progesterone receptor and Ki67 expressions were assessed by dual-fluorescence immunohistochemistry in estrogen-receptor-positive breast cancer tissues. Two distinct proliferating cell populations were observed: progesterone-receptor-positive and progesterone-receptor-negative. In the training cohort, tissues with progesterone-receptor-positive proliferating cells were associated with lower grade and better disease-free survival (p = 0.0055 and 0.0026, respectively). These associations were confirmed in the validation cohort from the neoadjuvant endocrine trial JFMC34 (p = 0.033 and 0.0003, respectively). In the validation cohort, patients with progesterone-receptor-positive proliferating cells responded better to endocrine therapy and had a lower Oncotype DX Recurrence Score. In the multivariate analysis, progesterone receptor status of proliferating cells, but not progesterone receptor or Ki67 alone, was an independent predictor of disease-free survival in both cohorts (p = 0.0043 and 0.0026). In conclusion, the progesterone receptor status of proliferating cancer cells was associated with histological grade and Recurrence Score, and a potent prognostic factor in estrogen-receptor-positive breast cancers. Results suggest that different driving systems generate different expression patterns of progesterone receptor in proliferating cancer cells. Further studies are warranted to validate the findings.

Hormones induce the formation of luminal-derived basal cells in the mammary gland

In the mammary gland, it is widely believed that the luminal cells are unipotent after birth, contributing only to the luminal compartment in normal development. Here, by lineage tracing, we uncovered an unexpected potential of luminal cells that can give rise to basal cells during pregnancy. These luminal-derived basal cells (LdBCs) persisted through mammary regression and generated more progeny in successive rounds of pregnancies. LdBCs express basal markers as well as estrogen receptor α (ERα). In ovariectomized (OVX) mice, stimulation with estrogen and progesterone promoted the formation of LdBCs. In serial transplantation assays, LdBCs were able to reconstitute new mammary glands in a hormone-dependent manner. Transcriptome analysis and genetic experiments suggest that Wnt/β-catenin signaling is essential for the formation and maintenance of LdBCs. Our data uncover an unexpected bi-potency of luminal cells in a physiological context. The discovery of ERα⁺ basal cells, which can respond to hormones and are endowed with stem cell-like regenerative capacity in parous mammary gland, provides new insights into the association of hormones and breast cancer.

Survival rates of breast cancer and predictive factors: a hospital-based study from western Amazon area in Brazil

Abstract Breast cancer survival in Latin America countries is below Central European countries. Hospital-based breast cancer survival studies in western Amazon, Brazil, are lacking. This article aims to estimate hospital-based breast cancer survival in Rio Branco, Acre, and predictor factors. Hospital-based cohort study of all women diagnosed with breast cancer (2007-2012) was proceeded. Information were obtained from medical reports, and follow-up was until 2013. One-, 2- and 5- years breast cancer specific-survival were estimated by Kaplan-Meier method. Crude and adjusted Harzards Ratios (HR) were estimated by proportional Cox regression model. One-, 2-, and 5-year overall breast cancer survival were 95.5%, 83.7%, and 87.3% respectively. Surgery combined to radiotherapy significantly affected 1-, 2-, and 5-year survival (99%, 94%, and 90.6%, respectively) as compared to other treatments (77%,57.1%, and 37.5%, respectively). Comparing to surgery combined to radiotherapy treatment, surgery alone increased the risk of death, independently of age and stage (HR = 7.23;95%CI:2.29-22.83). In Rio Branco, Acre, 5-year breast cancer survival is similar to more developed areas in Brazil. Surgery combined to radiotherapy was independently associated to a lower risk of death as compared to surgery alone and other treatment.

Attenuation of Mammary Gland Dysplasia and Feeding Difficulties in Tabby Mice by Fetal Therapy

Hypohidrotic ectodermal dysplasias (HED) are hereditary differentiation disorders of multiple ectodermal structures including the mammary gland. The X-linked form of HED (XLHED) is caused by a lack of the secreted signaling molecule ectodysplasin A1 (EDA1) which is encoded by the gene EDA and belongs to the tumor necrosis factor (TNF) superfamily. Although male patients (hemizygous) are usually more severely affected by XLHED, heterozygous female carriers of an EDA mutation may also suffer from a variety of symptoms, in particular from abnormal development of their breasts. In Tabby mice, a well-studied animal model of XLHED, EDA1 is absent. We investigated the effects of prenatal administration of Fc-EDA, a recombinant EDA1 replacement protein, on mammary gland development in female Tabby mice. Intra-amniotic delivery of Fc-EDA to fetal animals resulted later in improved breastfeeding and thus promoted the growth of their offspring. In detail, such treatment led to a normalization of the nipple shape (protrusion, tapering) that facilitated sucking. Mammary glands of treated female Tabby mice also showed internal changes, including enhanced branching morphogenesis and ductal elongation. Our findings indicate that EDA receptor stimulation during development has a stable impact on later stages of mammary gland differentiation, including lactation, but also show that intra-amniotic administration of an EDA1 replacement protein to fetal Tabby mice partially corrects the mammary gland phenotype in female adult animals.

Oxybenzone Alters Mammary Gland Morphology in Mice Exposed During Pregnancy and Lactation

Hormones and endocrine-disrupting chemicals are generally thought to have permanent “organizational” effects when exposures occur during development but not adulthood. Yet, an increasing number of studies have shown that pregnant females are disrupted by endocrine-disrupting chemical exposures, with some effects that are permanent. Here, we examined the long-term effects of exposure to oxybenzone, an estrogenic chemical found in sunscreen and personal care products, on the morphology of the mammary gland in mice exposed during pregnancy and lactation. Female mice were exposed to vehicle or 30, 212, or 3000 µg oxybenzone/kg/d, from pregnancy day 0 until weaning. A nulliparous group, receiving vehicle treatment, was also evaluated. Mammary glands were collected 5 weeks after involution for whole-mount, histological, immunohistochemical, and molecular analyses. Exposure to 3000 µg oxybenzone/kg/d induced permanent changes to ductal density that was significantly different from both the nulliparous and vehicle groups. The two highest doses of oxybenzone similarly induced an intermediate phenotype for expression of progesterone receptor. A monotonic, dose-dependent increase in cell proliferation was also observed in the oxybenzone-treated females, becoming statistically significant at the highest dose. Finally, oxybenzone exposure induced an intermediate phenotype for Esr1 expression in all oxybenzone-treated groups. These data suggest that oxybenzone, at doses relevant to human exposures, produces long-lasting alterations to mammary gland morphology and function. Further studies are needed to determine if exposure to this chemical during pregnancy and lactation will interfere with the known protection that pregnancy provides against breast cancer.

Lineage-Restricted Mammary Stem Cells Sustain the Development, Homeostasis, and Regeneration of the Estrogen Receptor Positive Lineage

The mammary gland (MG) is composed of different cell lineages, including the basal and the luminal cells (LCs) that are maintained by distinct stem cell (SC) populations. LCs can be subdivided into estrogen receptor (ER)⁺ and ER⁻ cells. LCs act as the cancer cell of origin in different types of mammary tumors. It remains unclear whether the heterogeneity found in luminal-derived mammary tumors arises from a pre-existing heterogeneity within LCs. To investigate LC heterogeneity, we used lineage tracing to assess whether the ER⁺ lineage is maintained by multipotent SCs or by lineage-restricted SCs. To this end, we generated doxycycline-inducible ER-rtTA mice that allowed us to perform genetic lineage tracing of ER⁺ LCs and study their fate and long-term maintenance. Our results show that ER⁺ cells are maintained by lineage-restricted SCs that exclusively contribute to the expansion of the ER⁺ lineage during puberty and their maintenance during adult life.

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ER⁺ stem cells mediate expansion and maintenance of the ER⁺ lineage

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ER⁺ stem cells expand and differentiate into ER⁺ cells following transplantation

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ER⁺ stem cells survive involution and repopulate the ER⁺ lineage

NF-κB at the Crossroads of Normal Mammary Gland Biology and the Pathogenesis and Prevention of BRCA1-Mutated Breast Cancer

Recent studies have shown that progesterone receptor (PR)-expressing cells respond to progesterone in part through the induction of the receptor activator of NF-κB ligand (RANKL), which acts in a paracrine manner to induce expansion of a RANK-expressing luminal progenitor cell population. The RANK⁺ population in human breast tissue from carriers of BRCA1 mutations (BRCA1^mut/+) as well as the luminal progenitor population in Brca1-deficient mouse mammary glands is abnormally amplified. Remarkably, mouse Brca1^+/- and human BRCA1^mut/+ progenitor cells are able to form colonies in vitro in the absence of progesterone, demonstrating a hormone-independent proliferative capacity. Our research has demonstrated that proliferation in BRCA1-deficient cells results in a DNA damage response (DDR) that activates a persistent NF-κB signal, which supplants progesterone/RANKL signaling for an extended time period. Thus, the transcriptional targets normally activated by RANKL that promote a proliferative response in luminal progenitors can contribute to the susceptibility of mammary epithelial cells to BRCA1-mutated breast cancers as a consequence of DDR-induced NF-κB. Together, these latest findings mark substantial progress in uncovering the mechanisms driving high rates of breast tumorigenesis in BRCA1 mutation carriers and ultimately reveal possibilities for nonsurgical prevention strategies. Cancer Prev Res; 11(2); 69-80. ©2017 AACR.

Inhibition of lobuloalveolar development by FOXC1 overexpression in the mouse mammary gland

The forkhead box transcription factor FOXC1 plays a critical role in embryogenesis and the development of many organs. Its mutations and high expression are associated with many human diseases including breast cancer. Although FOXC1 knockout mouse studies showed that it is not required for mammary gland development during puberty, it is not clear whether its overexpression alters normal mammary development in vivo. To address this question, we generated transgenic mice with mammary-specific FOXC1 overexpression. We report that transgenic FOXC1 overexpression suppresses lobuloalveologenesis and lactation in mice. This phenotype is associated with higher percentages of estrogen receptor-, progesterone receptor-, or ki67-positive mammary epithelial cells in the transgenic mice at the lactation stage. We also show that expression of the Elf5 transcription factor, a master regulator of mammary alveologenesis and luminal cell differentiation, is markedly reduced in mammary epithelial cells of transgenic mice. Likewise, levels of activated Stat5, another inducer of alveolar expansion and a known mediator of the Elf5 effect, are also lowered in those cells. In contrast, the cytokeratin 8-positive mammary cell population with progenitor properties is elevated in the transgenic mice at the lactation stage, suggesting inhibition of mammary cell differentiation. These results may implicate FOXC1 as a new important regulator of mammary gland development.

Bisphenol S Alters the Lactating Mammary Gland and Nursing Behaviors in Mice Exposed During Pregnancy and Lactation

High doses of estrogenic pharmaceuticals were once prescribed to women to halt lactation. Yet, the effects of low-level xenoestrogens on lactation remain poorly studied. We investigated the effects of bisphenol S (BPS), an estrogen receptor (ER) agonist, on the lactating mammary gland; the arcuate nucleus, a region of the hypothalamus important for neuroendocrine control of lactational behaviors; and nursing behavior in CD-1 mice. Female mice were exposed to vehicle, 2 or 200 µg BPS/kg/d from pregnancy day 9 until lactational day (LD) 20, and tissues were collected on LD21. Tissues were also collected from a second group at LD2. BPS exposure significantly reduced the fraction of the mammary gland comprised of lobules, the milk-producing units, on LD21, but not LD2. BPS also altered expression of Esr1 and ERα in the mammary gland at LD21, consistent with early involution. In the arcuate nucleus, no changes were observed in expression of signal transducer and activator of transcription 5, a marker of prolactin signaling, or ERα, suggesting that BPS may act directly on the mammary gland. However, observations of nursing behavior collected during the lactational period revealed stage-specific effects on both pup and maternal nursing behaviors; BPS-treated dams spent significantly more time nursing later in the lactational period, and BPS-treated pups were less likely to initiate nursing. Pup growth and development were also stunted. These data indicate that low doses of BPS can alter lactational behaviors and the maternal mammary gland. Together, they support the hypothesis that pregnancy and lactation are sensitive to low-dose xenoestrogen exposures.

Long-term effects of repeated superovulation on the uterus and mammary gland in rhesus monkeys

PURPOSE

The aim of this study is to evaluate the effect of repeated controlled ovarian hyperstimulation (COH) on the structure and function of the uterus and mammary gland.

METHODS

Three adult female rhesus monkeys were superovulated up to four times, and three spontaneously ovulating monkeys were used as controls. After a 5-year period, the uterus and mammary gland tissue samples were collected for examination of their structure and function. Further, the expression of certain tumor markers was examined to assess the cancer risk for each organ.

RESULTS

Expression of Wnt7a (associated with the functional/developmental status of the uterus) was significantly decreased in the uterus of superovulated monkeys, and decreased expression of proliferation marker PCNA was found in uterine cells. Meanwhile, abnormal Golgi-derived secretory vesicles with an irregular shape were observed in the mammary glands of the superovulated monkeys, and decreased PCNA expression together with increased expression of caspase-3 (an apoptosis marker) was indicated in the mammary cells. The expression of tumor molecular markers of the uterus and mammary gland was not significantly different between the two groups.

CONCLUSIONS

Repeated COH affects the expression of the uterine development-related gene several years later, and uterine cells exhibited a low proliferation status. The ultrastructure of the mammary gland epithelial cells was abnormal, and the cells exhibited both low proliferation and high apoptosis status. Cancer risk for these organs was not observed. Given that primates are the closest relatives of humans, the results obtained from this study provide more intuitive information for optimization of clinical COH.

Steroid hormone secretion in inflammatory breast cancer cell lines

Inflammatory breast carcinoma (IBC) is a special type of breast cancer with a poor survival rate. Though several IBC cell lines have been established, recently a first IMC cell line was established. The aims of this study were: (1) to validate a highly sensitive, reliable, accurate and direct amplified enzyme immunoassay (EIA) to measure several cell-secreted steroid hormones: progesterone (P4), androstenedione (A4), testosterone (T), 17β-estradiol (E2) and estrone sulfate (SO4E1) in the culture medium. (2) To assess whether hormone production profile by IPC-366 cells validates the IMC model for human IBC. We validated a non-competitive amplified EIA for inflammatory breast cancer cell lines based on the results of accuracy, precision, sensitivity and parallelism. The low detection limits of the technique were: P4=13.2 pg/well, A4=2.3 pg/well, T=11.4 pg/well, E2=1.9 pg/well and SO4E1=4.5 pg/well. Intra- and inter-assay coefficient of variation percentages were <10%. The mean recovery rate of hormone added to the culture medium was >90%. In all hormones studied SUM149 have higher levels (1.4 times, but not significant) than IPC-366, and the correlation index between SUM149 and IPC-366 concentrations were >97%. We can coclude that cells of both cell lines, IPC-366 and SUM149, are capable to produce steroid hormone in culture media. The presented EIA methodology is very valuable for the detection of steroid production in culture media and could be used in hormone regulation studies and therapeutic agents in cell lines of inflammatory and non-inflammatory mammary carcinoma or other cancer cell lines in preclinical studies.

Differences in the Rate of in Situ Mammary Gland Development and Other Developmental Endpoints in Three Strains of Female Rat Commonly Used in Mammary Carcinogenesis Studies: Implications for Timing of Carcinogen Exposure

The potential of chemicals to alter susceptibility to mammary tumor formation is often assessed using a carcinogen-induced study design in various rat strains. The rate of mammary gland (MG) development must be considered so that the timing of carcinogen administration is impactful. In this study, in situ MG development was assessed in females of the Harlan Sprague-Dawley (Hsd:SD), Charles River Sprague-Dawley (Crl:SD), and Charles River Long-Evans (Crl:LE) rat strains at postnatal days 25, 33, and 45. Development was evaluated by physical assessment of growth parameters, developmental scoring, and quantitative morphometric analysis. Although body weight (BW) was consistently lower and day of vaginal opening (VO) occurred latest in female Hsd:SD rats, they exhibited accelerated pre- and peripubertal MG development compared to other strains. Glands of Crl:SD and Crl:LE rats exhibited significantly more terminal end buds (TEBs) and TEB/mm than Hsd:SD rats around the time of VO. These data suggest a considerable difference in the rate of MG development across commonly used strains, which is independent of BW and timing of VO. In mammary tumor induction studies employing these strains, administration of the carcinogen should be timed appropriately, based on strain, to specifically target the peak of TEB occurrence.

17β-Estradiol alters oxidative damage and oxidative stress response protein expression in the mouse mammary gland

Although substantial evidence has demonstrated that parity and 17β-estradiol (E2) reduce mammary carcinogenesis, it is not clear how this protection is conferred. Thus, we examined the effects of parity and E2 treatment in the mammary glands of ovariectomized 15 week-old virgin mice, 15 week-old primiparous mice, and 9 month-old retired breeders. E2 treatment significantly increased lipid peroxidation, protein carbonylation, and protein nitrosylation in the virgin mice, but not in the age-matched primiparous mice or retired breeders. Mammary gland expression of the oxidative stress response protein Cu/Zn superoxide dismutase was consistently reduced in all of the E2-treated mice regardless of parity. Expression of the oxidative stress and DNA repair protein apurinic endonuclease (Ape1) was significantly increased only in the mammary glands of the E2-treated retired breeders. These findings suggest that E2 and parity help to reduce mammary oncogenesis by maintaining the structure and function of proteins, lipids, and DNA.

Cripto-1 ablation disrupts alveolar development in the mouse mammary gland through a progesterone receptor-mediated pathway

Cripto-1, a member of the epidermal growth factor-Cripto-1/FRL-1/Cryptic family, is critical for early embryonic development. Together with its ligand Nodal, Cripto-1 has been found to be associated with the undifferentiated status of mouse and human embryonic stem cells. Several studies have clearly shown that Cripto-1 is involved in regulating branching morphogenesis and epithelial-mesenchymal transition of the mammary gland both in vitro and in vivo and together with the cofactor GRP78 is critical for the maintenance of mammary stem cells ex vivo. Our previous studies showed that mammary-specific overexpression of human Cripto-1 exhibited dramatic morphological alterations in nulliparous mice mammary glands. The present study shows a novel mechanism for Cripto-1 regulation of mammary gland development through direct effects on progesterone receptor expression and pathways regulated by progesterone in the mammary gland. We demonstrate a strict temporal regulation of mouse Cripto-1 (mCripto-1) expression that occurs during mammary gland development and a stage-specific function of mCripto-1 signaling during mammary gland development. Our data suggest that Cripto-1, like the progesterone receptor, is not required for the initial ductal growth but is essential for subsequent side branching and alveologenesis during the initial stages of pregnancy. Dissection of the mechanism by which this occurs indicates that mCripto-1 activates receptor activator NF-κB/receptor activator NF-κB ligand, and NF-κB signaling pathways.

Hormone-sensing mammary epithelial progenitors: emerging identity and hormonal regulation

The hormone-sensing mammary epithelial cell (HS-MEC-expressing oestrogen receptor-alpha (ERα) and progesterone receptor (PGR)) is often represented as being terminally differentiated and lacking significant progenitor activity after puberty. Therefore while able to profoundly influence the proliferation and function of other MEC populations, HS-MECs are purported not to respond to sex hormone signals by engaging in significant cell proliferation during adulthood. This is a convenient and practical simplification that overshadows the sublime, and potentially critical, phenotypic plasticity found within the adult HS-MEC population. This concept is exemplified by the large proportion (~80 %) of human breast cancers expressing PGR and/or ERα, demonstrating that HS-MECs clearly proliferate in the context of breast cancer. Understanding how HS-MEC proliferation and differentiation is driven could be key to unraveling the mechanisms behind uncontrolled HS-MEC proliferation associated with ERα- and/or PGR-positive breast cancers. Herein we review evidence for the existence of a HS-MEC progenitor and the emerging plasticity of the HS-MEC population in general. This is followed by an analysis of hormones other than oestrogen and progesterone that are able to influence HS-MEC proliferation and differentiation: androgens, prolactin and transforming growth factor-beta1.

ERrrr…where are the progenitors? Hormone receptors and mammary cell heterogeneity

The mammary epithelium is a highly heterogenous and dynamic tissue that includes a range of cell types with varying levels of proliferative capacity and differentiation potential, from stem to committed progenitor and mature cells. Generation of mature cells through expansion and specification of immature precursors is driven by hormonal and local stimuli. Intriguingly, although circulating hormones can be directly sensed only by a subset of mammary cells, they also regulate the behaviour of cells lacking their cognate receptors through paracrine mechanisms. Thus, mapping the hormonal signalling network on to the emerging mammary cell hierarchy appears to be a difficult task. Nevertheless, a first step towards a better understanding is the characterization of the hormone receptor expression pattern across individual cell types in the mammary epithelium. Here we review the most relevant findings on the cellular distribution of hormone receptors in the mammary gland, taking into account differences between mice and humans, the methods employed to assess receptor expression as well as the variety of approaches used to resolve the mammary cell heterogeneity.

Form and function: how estrogen and progesterone regulate the mammary epithelial hierarchy

The mammary gland undergoes dramatic post-natal growth beginning at puberty, followed by full development occurring during pregnancy and lactation. Following lactation, the alveoli undergo apoptosis, and the mammary gland reverses back to resemble the nonparous gland. This process of growth and regression occurs for multiple pregnancies, suggesting the presence of a hierarchy of stem and progenitor cells that are able to regenerate specialized populations of mammary epithelial cells. Expansion of epithelial cell populations in the mammary gland is regulated by ovarian steroids, in particular estrogen acting through its receptor estrogen receptor alpha (ERα) and progesterone signaling through progesterone receptor (PR). A diverse number of stem and progenitor cells have been identified based on expression of cell surface markers and functional assays. Here we review the current understanding of how estrogen and progesterone act together and separately to regulate stem and progenitor cells within the human and mouse mammary tissues. Better understanding of the hierarchal organization of epithelial cell populations in the mammary gland and how the hormonal milieu affects its regulation may provide important insights into the origins of different subtypes of breast cancer.

Minireview: Tipping the balance: ligand-independent activation of steroid receptors

Steroid receptors are prototypical ligand-dependent transcription factors and a textbook example for allosteric regulation. According to this canonical model, binding of cognate steroid is an absolute requirement for transcriptional activation. Remarkably, the simple one ligand-one receptor model could not be farther from the truth. Steroid receptors, notably the sex steroid receptors, can receive multiple inputs. Activation of steroid receptors by other signals, working through their own signaling pathways, in the absence of the cognate steroids, represents the most extreme form of signaling cross talk. Compared with cognate steroids, ligand-independent activation pathways produce similar but not identical outputs. Here we review the phenomena and discuss what is known about the underlying molecular mechanisms and the biological significance. We hypothesize that steroid receptors may have evolved to be trigger happy. In addition to their cognate steroids, many posttranslational modifications and interactors, modulated by other signals, may be able to tip the balance.

Dynamic reciprocity between cells and their microenvironment in reproduction

Dynamic reciprocity (DR) refers to the ongoing, bidirectional interaction between cells and their microenvironment, specifically the extracellular matrix (ECM). The continuous remodeling of the ECM exerts mechanical force on cells and modifies biochemical mediators near the cell membrane, thereby initiating cell-signaling cascades that produce changes in gene expression and cell behavior. Cellular changes, in turn, affect the composition and organization of ECM components. These continuous interactions are the fundamental principle behind DR, and its critical role throughout development and adult tissue homeostasis has been extensively investigated. While DR in the mammary gland has been well described, we provide direct evidence that similar dynamic interactions occur in other areas of reproductive biology as well. In order to establish the importance of DR in the adaptive functioning of the female reproductive tract, we present our most current understanding of DR in reproductive tissues, exploring the mammary gland, ovary, and uterus. In addition to explaining normal physiological function, investigating DR may shed new light into pathologic processes that occur in these tissues and provide an exciting opportunity for novel therapeutic intervention.

Assessment of the proliferative capacity of the flavanones 8-prenylnaringenin, 6-(1.1-dimethylallyl)naringenin and naringenin in MCF-7 cells and the rat mammary gland

8-Prenylnaringenin (8-PN) and naringenin (Nar) are phytoestrogens found in food items and nutritional supplements, while 6-(1.1-dimethylallyl)naringenin (6-DMAN) is a component of an African plant. Besides their assumed beneficial effects they may promote mammary and endometrial cancer. We therefore assessed their proliferative and estrogenic potential on the mammary gland in vitro and in vivo. In competitive estrogen receptor (ER) ligand binding assays 8-PN displayed a high relative binding affinity for both ERs with a preference for ERα and had the strongest mitotic effect on MCF-7 cells among the test substances. In a three day exposure in young adult ovariectomized female rats 15 mg/kg 8-PN had the highest capacity to increase the number of terminal end buds (TEB) in the mammary gland and stimulated expression of proliferation markers in epithelial ductal cells, followed by 6-DMAN and Nar, but overall their capacity to stimulate proliferation was weak in comparison to 17β-Estradiol (E2).

Hormonal control of stem cell systems

Many organs respond to physiological challenges by changing tissue size or composition. Such changes may originate from tissue-specific stem cells and their supportive environment (niche). The endocrine system is a major effector and conveyor of physiological changes and as such could alter stem cell behavior in various ways. In this review, we examine how hormones affect stem cell biology in four different organs: the ovary, intestine, hematopoietic system, and mammary gland. Hormones control every stage of stem cell life, including establishment, expansion, maintenance, and differentiation. The effects can be cell autonomous or non-cell autonomous through the niche. Moreover, a single hormone can affect different stem cells in different ways or affect the same stem cell differently at various developmental times. The vast complexity and diversity of stem cell responses to hormonal cues allow hormones to coordinate the body's reaction to physiological challenges.

Does mammographic density reflect the expression of breast cancer markers?

Mammographic density reflects variation in breast tissue composition as detected on mammogram. It is associated with a number of well-known breast cancer risk factors and itself is considered one of the strongest risk factors for breast cancer. If the expression of several proteins and genes within the breast tissue influences mammographic density in the same way as it influences breast cancer risk, then mammographic density might serve as an intermediate biomarker in future epidemiological studies on breast cancer. This has the potential to provide a quick means for predicting the effect of changes in the breast microenvironment on breast cancer risk without having to wait for an eventual development of breast cancer. In this review, the expression of several proteins and genes (growth factors, enzymes, proteoglycans and pro-inflammatory markers) within the breast tissue is shown to be associated with mammographic density. These proteins and genes are suspected to play a role in breast carcinogenesis. More studies assessing differential expression of proteins and genes in mammary epithelium and stroma and their association with mammographic density among premenopausal and postmenopausal women are required. Identification of proteins and genes influencing mammographic density may provide further insight on the molecular causes of breast cancer.

Progesterone action in endometrial cancer, endometriosis, uterine fibroids, and breast cancer

Progesterone receptor (PR) mediates the actions of the ovarian steroid progesterone, which together with estradiol regulates gonadotropin secretion, prepares the endometrium for implantation, maintains pregnancy, and differentiates breast tissue. Separation of estrogen and progesterone actions in hormone-responsive tissues remains a challenge. Pathologies of the uterus and breast, including endometrial cancer, endometriosis, uterine fibroids, and breast cancer, are highly associated with estrogen, considered to be the mitogenic factor. Emerging evidence supports distinct roles of progesterone and its influence on the pathogenesis of these diseases. Progesterone antagonizes estrogen-driven growth in the endometrium, and insufficient progesterone action strikingly increases the risk of endometrial cancer. In endometriosis, eutopic and ectopic tissues do not respond sufficiently to progesterone and are considered to be progesterone-resistant, which contributes to proliferation and survival. In uterine fibroids, progesterone promotes growth by increasing proliferation, cellular hypertrophy, and deposition of extracellular matrix. In normal mammary tissue and breast cancer, progesterone is pro-proliferative and carcinogenic. A key difference between these tissues that could explain the diverse effects of progesterone is the paracrine interactions of PR-expressing stroma and epithelium. Normal endometrium is a mucosa containing large quantities of distinct stromal cells with abundant PR, which influences epithelial cell proliferation and differentiation and protects against carcinogenic transformation. In contrast, the primary target cells of progesterone in the breast and fibroids are the mammary epithelial cells and the leiomyoma cells, which lack specifically organized stromal components with significant PR expression. This review provides a unifying perspective for the diverse effects of progesterone across human tissues and diseases.

The hyperplastic phenotype in PR-A and PR-B transgenic mice: lessons on the role of estrogen and progesterone receptors in the mouse mammary gland and breast cancer

Progesterone receptor (PR) belongs to the superfamily of steroid receptors and mediates the action of progesterone in its target tissues. In the mammary gland, in particular, PR expression is restricted to the luminal epithelial cell compartment. The generation of estrogen receptor-α (ER) and PR knockout mice allowed the specific characterization of the roles of each of these in mammary gland development: ER is critical for ductal morphogenesis, whereas PR has a key role in lobuloalveolar differentiation. To further study the role PR isoforms have in mammary gland biology, transgenic mice overexpressing either the "A" (PR-A) or the "B" (PR-B) isoforms of PR were generated. Overexpression of the A isoform of PR led to increased side branching, multilayered ducts, loss of basement membrane integrity, and alterations in matrix metalloproteinase activation in the mammary gland. Moreover, levels of TGFβ1 and p21 were diminished and those of cyclin D1 increased. Interestingly, the phenotype was counteracted by antiestrogens, suggesting that ER is essential for the manifestation of the hyperplasias. Mice overexpressing the B isoform of PR had limited ductal growth but retained the ability to differentiate during pregnancy. Levels of latent and active TGFβ1 were increased compared to PR-A transgenics. The phenotypes of these transgenic mice are further discussed in the context of the impact of progesterone on mammary stem cells and breast cancer. We conclude that an adequate balance between the A and B isoforms of PR is critical for tissue homeostasis. Future work to further understand the biology of PR in breast biology will hopefully lead to new and effective preventive and therapeutic alternatives for patients.

Colostrogenesis: candidate genes for IgG1 transcytosis mechanisms in primary bovine mammary epithelial cells

Bovine colostrogenesis is distinguished by the specific transfer of IgG1 from the blood to mammary secretions. The process has been shown to be initiated by hormones and occurs during the last weeks of pregnancy when steroid concentrations of estradiol (E2 ) and progesterone (P4 ) are highly elevated. Rodent intestinal uptake of immunoglobulin G is mediated by a receptor termed Fc fragment of IgG, Receptor, Transporter, alpha (FcGRT) and supported by light chain Beta-2-Microglobulin (β2M). We hypothesized that steroid hormone treatments (E2 and P4 ) of bovine mammary epithelial cells in vitro would induce up-regulation of IgG1 transcytosis candidate gene mRNA expression suggesting involvement in IgG1 transcytosis. Two different primary bovine mammary epithelial cell cultures were cultured on plastic and rat tail collagen and treated with hormonal combinations (steroids/lactogenic hormones). Evaluated mRNA components were bLactoferrin (bLf: a control), bFcGRT, β2M, and various small GTPases; the latter components are reported to direct endosomal movements in eukaryotic cells. All tested transcytosis components showed strong expression of mRNA in the cells. Expression of bFcGRT, bRab25 and bRhoB were significantly up-regulated (p < 0.05) by steroid hormones. bRab25 and bRhoB showed increased expression by steroid treatments, but also with lactogenic hormones. Analysis for the oestrogen receptor (ER) mRNA was mostly negative, but 25% of the cultures tested exhibited weak expression, while the progesterone receptor (PR) mRNA was always detected. bRab25 and bRhoB and likely bFcGRT are potential candidate genes for IgG1 transcytosis in bovine mammary cells.

Polymorphisms of Phase I and Phase II Enzymes and Breast Cancer Risk

Breast cancer is a complex disease which is provoked by a multitude of exogenous and endogenous factors including genetic variations. Recent genome-wide association studies identified a set of more than 18 novel low penetrant susceptibility loci, however, a limitation of this powerful approach is the hampered analysis of polymorphisms in DNA sequences with a high degree of similarity to other genes or pseudo genes. Since this common feature affects the majority of the highly polymorphic genes encoding phase I and II enzymes the retrieval of specific genotype data requires adapted amplification methods. With regard to breast cancer these genes are of certain interest due to their involvement in the metabolism of carcinogens like exogenous genotoxic compounds or steroid hormones. The present review summarizes the observed effects of functional genetic variants of phase I and II enzymes in well designed case control studies to shed light on their contribution to breast cancer risk.

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

Introduction

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

Methods

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

Results

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

Conclusion

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

The pharmacogenomics of sex hormone metabolism: breast cancer risk in menopausal hormone therapy

With women in western countries spending nearly one-third of their lifetime beyond menopause and a substantial number of these women facing severe menopausal symptoms, the goal of sex hormone pharmacogenomics is to promote the safe use of hormone replacement therapy (HRT). This could be achieved by providing molecular predictors for the upfront stratification of women in need of relief from menopausal symptoms into those with a likely benefit from HRT and those with a contraindication due to an HRT-associated breast cancer risk or other adverse effects. An increasing knowledge base of sex hormone metabolism and its variability, HRT outcomes and breast cancer susceptibility, as well as emerging examples of pharmacogenomic predictors, underscore the potential relevance of genetic variations for HRT outcome. The genes responsible for the metabolism, signaling and action of sex hormones are at the heart of this research; however, pharmacogenomic investigation of their therapeutic effects due to the enormous complexity of the biological pathways involved is still in its infancy. This article discusses the current knowledge, challenges and potential future directions towards the goal of genotype-guided safer HRT use.

The estrogen-responsive Agr2 gene regulates mammary epithelial proliferation and facilitates lobuloalveolar development

Agr2 is a putative protein disulfide isomerase (PDI) initially identified as an estrogen-responsive gene in breast cancer cell lines. While Agr2 expression in breast cancer is positively correlated with estrogen receptor (ER) expression, it is upregulated in both hormone dependent and independent carcinomas. Several in vitro and xenograft studies have implicated Agr2 in different oncogenic features of breast cancer; however, the physiological role of Agr2 in normal mammary gland development remains to be defined. Agr2 expression is developmentally regulated in the mammary gland, with maximum expression during late pregnancy and lactation. Using a mammary gland specific knockout mouse model, we show that Agr2 facilitates normal lobuloalveolar development by regulating mammary epithelial cell proliferation; we found no effects on apoptosis in Agr2(-/-) mammary epithelial cells. Consequently, mammary glands of Agr2(-/-) females exhibit reduced expression of milk proteins, and by two weeks post-partum their pups are smaller in size. Utilizing a conditional mouse model, we show that Agr2 constitutive expression drives precocious lobuloalveolar development and increased milk protein expression in the virgin mammary gland. In vitro studies using knock down and overexpression strategies in estrogen receptor positive and negative mammary epithelial cell lines demonstrate a role for Agr2 in estradiol-induced cell proliferation. In conclusion, the estrogen-responsive Agr2, a candidate breast cancer oncogene, regulates epithelial cell proliferation and lobuloalveolar development in the mammary gland. The pro-proliferative effects of Agr2 may explain its actions in early tumorigenesis.

Interplay between progesterone and prolactin in mammary development and implications for breast cancer

Progesterone and prolactin remodel mammary morphology during pregnancy by acting on the mammary epithelial cell hierarchy. The roles of each hormone in mammary development have been well studied, but evidence of signalling cross-talk between progesterone and prolactin is still emerging. Factors such as receptor activator of NFkB ligand (RANKL) may integrate signals from both hormones to orchestrate their joint actions on the epithelial cell hierarchy. Common targets of progesterone and prolactin signalling are also likely to integrate their pro-proliferative actions in breast cancer. Therefore, a thorough understanding of the interplay between progesterone and prolactin in mammary development may reveal therapeutic targets for breast cancer. This review summarises our understanding of Pg and PRL action in mammary gland development before focusing on molecular mechanisms of signalling cross-talk and the implications for breast cancer.

The biology of progesterone receptor in the normal mammary gland and in breast cancer

This paper reviews work on progesterone and the progesterone receptor (PR) in the mouse mammary gland that has been used extensively as an experimental model. Studies have led to the concept that progesterone controls proliferation and morphogenesis of the luminal epithelium in a tightly orchestrated manner at distinct stages of development by paracrine signaling pathways, including receptor activator of nuclear factor κB ligand (RANKL) as a major paracrine factor. Progesterone also drives expansion of stem cells by paracrine signals to generate progenitors required for alveologenesis. During mid-to-late pregnancy, progesterone has another role to suppress secretory activation until parturition mediated in part by crosstalk between PR and prolactin/Stat5 signaling to inhibit induction of milk protein gene expression, and by inhibiting tight junction closure. In models of hormone-dependent mouse mammary tumors, the progesterone/PR signaling axis enhances pre-neoplastic progression by a switch from a paracrine to an autocrine mode of proliferation and dysregulation of the RANKL signaling pathway. Limited experiments with normal human breast show that progesterone/PR signaling also stimulates epithelial cell proliferation by a paracrine mechanism; however, the signaling pathways and whether RANKL is a major mediator remains unknown. Work with human breast cancer cell lines, patient tumor samples and clinical studies indicates that progesterone is a risk factor for breast cancer and that alteration in progesterone/PR signaling pathways contributes to early stage human breast cancer progression. However, loss of PR expression in primary tumors is associated with a less differentiated more invasive phenotype and worse prognosis, suggesting that PR may limit later stages of tumor progression.

Evolution of immune functions of the mammary gland and protection of the infant

Abstract The evolution of immunological agents in milk is intertwined with the general aspects of the evolution of the mammary gland. In that respect, mammalian precursors emerged from basal amniotes some 300 million years ago. In contrast to the predominant dinosaurs, proto-mammals possessed a glandular skin. A secondary palate in the roof of the mouth that directed airflow from the nostrils to the oropharynx and thus allowed mammals to ingest and breathe simultaneously first appeared in cynodonts 230 million years ago. This set the stage for mammalian newborns to nurse from the future mammary gland. Interplays between environmental and genetic changes shaped mammalian evolution including the mammary gland from dermal glands some 160 millions of years ago. It is likely that secretions from early mammary glands provided nutrients and immunological agents for the infant. Natural selection culminated in milks uniquely suited to nourish and protect infants of each species. In human milk, antimicrobial, anti-inflammatory, and immunoregulatory agents and living leukocytes are qualitatively or quantitatively different from those in other mammalian milks. Those in human milk compensate for developmental delays in the immunological system of the recipient infant. Consequently, the immune system in human milk provided by evolution is much of the basis for encouraging breastfeeding for human infants.

Canine tumors: a spontaneous animal model of human carcinogenesis

The enormous biologic complexity of human cancer has stimulated the development of more appropriate experimental models that could resemble in a natural and spontaneous manner the physiopathologic aspects of cancer biology. Companion animals have many desired characteristics that fill the gap between in vitro and in vivo studies, and these characteristics have proven to be important in understanding many complex molecular aspects of human cancer. Spontaneous tumors in dogs share a wide variety of epidemiologic, biologic, and clinical features with human cancer, which makes this animal model both attractive and underused in oncology research. In this review, we summarize the importance of naturally occurring canine tumors as valuable tools for studying numerous aspects of human cancer as well as the potential use of this animal model for the development of new cancer treatments. We address specifically the use of canine mammary tumors as an increasingly powerful model to study human breast cancer.

Cell hierarchy and lineage commitment in the bovine mammary gland

The bovine mammary gland is a favorable organ for studying mammary cell hierarchy due to its robust milk-production capabilities that reflect the adaptation of its cell populations to extensive expansion and differentiation. It also shares basic characteristics with the human breast, and identification of its cell composition may broaden our understanding of the diversity in cell hierarchy among mammals. Here, Lin⁻ epithelial cells were sorted according to expression of CD24 and CD49f into four populations: CD24^medCD49f^pos (putative stem cells, puStm), CD24^negCD49f^pos (Basal), CD24^highCD49f^neg (putative progenitors, puPgt) and CD24^medCD49f^neg (luminal, Lum). These populations maintained differential gene expression of lineage markers and markers of stem cells and luminal progenitors. Of note was the high expression of Stat5a in the puPgt cells, and of Notch1, Delta1, Jagged1 and Hey1 in the puStm and Basal populations. Cultured puStm and Basal cells formed lineage-restricted basal or luminal clones and after re-sorting, colonies that preserved a duct-like alignment of epithelial layers. In contrast, puPgt and Lum cells generated only luminal clones and unorganized colonies. Under non-adherent culture conditions, the puPgt and puStm populations generated significantly more floating colonies. The increase in cell number during culture provides a measure of propagation potential, which was highest for the puStm cells. Taken together, these analyses position puStm cells at the top of the cell hierarchy and denote the presence of both bi-potent and luminally restricted progenitors. In addition, a population of differentiated luminal cells was marked. Finally, combining ALDH activity with cell-surface marker analyses defined a small subpopulation that is potentially stem cell- enriched.

Differential effects of exogenous androgen and an androgen receptor antagonist in the peri- and postpubertal murine mammary gland

There is emerging evidence that androgens inhibit proliferation of normal and malignant breast epithelial cells, but the actions of androgens in normal mammary gland morphogenesis are not well understood. In this study, we investigated whether development of the murine mammary gland could be altered by stimulating or suppressing androgen receptor (AR) signaling in vivo. Intact virgin female mice aged 5 wk (midpuberty) or 12 wk (postpuberty) were implanted with slow-release pellets containing either placebo, 5α-dihydrotestosterone (1.5 mg) or the AR antagonist flutamide (60 mg). Treatment with 5α-dihydrotestosterone from midpuberty to 12 wk of age-retarded ductal extension by 40% (P = 0.007), but treatment from 12-21 wk had no significant effect on gland morphology. In contrast, inhibition of AR signaling with flutamide from midpuberty had no effect on the mammary gland, but flutamide treatment from 12-21 wk increased ductal branching (P = 0.004) and proliferation (P = 0.03) of breast epithelial cells. The increased proliferation in flutamide-treated mice was not correlated with serum estradiol levels or estrogen receptor-α (ERα) expression. In control mice, the frequency and intensity of AR immunostaining in mammary epithelial cells was significantly increased in the 12- to 21-wk treatment group compared with the 5- to 12-wk group (P < 0.001). In contrast, no change in ERα occurred, resulting in a marked increase in the AR to ERα ratio from 0.56 (±0.12) to 1.47 (±0.10). Our findings indicate that androgen signaling influences development and structure of the adult mammary gland and that homeostasis between estrogen and androgen signaling in mature glands is critical to constrain the proliferative effects of estradiol.

The pivotal role of insulin-like growth factor I in normal mammary development

Mammary development begins in puberty in response to an estrogen (E(2)) surge. E(2) does not act alone. It relies on pituitary growth hormone (GH) to induce insulin-like growth factor I (IGF-I) production in the mammary stromal compartment. In turn, IGF-I permits E(2) (and progesterone) action. During puberty, E(2) and IGF-I synergize for ductal morphogenesis. During pregnancy, progesterone joins IGF-I and E(2) to stimulate secretory differentiation necessary to produce milk. Prolactin stimulates milk production, while transforming growth factor-β inhibits proliferation. The orchestrated action of hormones, growth factors, and receptors necessary for mammary development and function are also critical in breast cancer.

Pre- and postpubertal irradiation induces mammary cancers with distinct expression of hormone receptors, ErbB ligands, and developmental genes in rats

Childhood exposure to carcinogens renders a higher risk of breast cancer. The molecular mechanisms underlying cancer development after such exposure are not, however, well understood. Here we examined how the mechanism of cancer development relates to the age at exposure to ionizing radiation (IR) or the carcinogen 1-methyl-1-nitrosourea (MNU). Pre- and postpubertal (3- and 7-wk-old, respectively) female Sprague-Dawley rats were whole-body γ-irradiated (2 Gy), injected intraperitoneally with MNU (20 mg/kg) or left untreated and were autopsied at 50 wk of age. Mammary carcinomas were examined for estrogen receptor (ER) α, progesterone receptor (PR) and ErbB ligand expression and for expression microarrays. Early histological changes of the ovaries were also evaluated. The incidence of mammary cancer was higher after postpubertal, rather than prepubertal, IR exposure; the inverse was true for MNU. Most cancers were positive for both ERα and PR except for the prepubertal IR group. Cancers of the prepubertal IR group expressed a different set of ErbB ligands from those of the other groups and did not overexpress Areg, which encodes an estrogen-regulated ErbB ligand, or other developmentally related genes including those for hormonally regulated mammary gland development. Prepubertal IR exposure resulted in ovarian dysfunction as revealed by a reduced follicular pool. Evidence thus suggests that mammary carcinogenesis induced by prepubertal IR exposure is independent of ovarian hormones but requires certain ErbB ligands; induction by postpubertal exposure depends on ovarian hormones and different ErbB ligands. In contrast, the mechanism of MNU-induced carcinogenesis was less influenced by the age at exposure.