Endocrine hormones and local signals during the development of the mouse mammary gland

The CAR agonist TCPOBOP inhibits lipogenesis and promotes fibrosis in the mammary gland of adolescent female mice

Constitutive androstane receptor (CAR) is a nuclear receptor that not only regulates drug-metabolizing enzymes but also influences energy metabolism. TC, 1, 4-bis [2-(3, 5-dichloropyridyloxy)] benzene (TCPOBOP) has been shown to inhibit lipogenesis in the liver and adipose tissues. The mammary gland is mainly composed of fat pads and duct systems in adolescent female mice. Here, activation of CAR by TC reduces the mammary gland weight, blocks lipid accumulation by inhibiting lipogenesis and gluconeogenesis, and accelerates collagen formation and fibrosis in the mammary fat pad of adolescent female mice. This information provides a reference for CAR activation, which may affect mammary gland development in adolescent females.

Low dose bisphenol S or ethinyl estradiol exposures during the perinatal period alter female mouse mammary gland development

Throughout life, mammary tissue is strongly influenced by hormones. Scientists have hypothesized that synthetic chemicals with hormonal activities could disrupt mammary gland development and contribute to breast diseases and dysfunction. Bisphenol S (BPS) is an estrogenic compound used in many consumer products. In this study, CD-1 mice were exposed to BPS (2 or 200 μg/kg/day) during pregnancy and lactation. Mice exposed to 0.01 or 1 μg/kg/day ethinyl estradiol (EE2), a pharmaceutical estrogen, were also evaluated. Mammary glands from female offspring were collected prior to the onset of puberty, during puberty, and in early adulthood. Growth parameters, histopathology, cell proliferation and expression of hormone receptors were quantified. Our evaluations revealed age- and dose-specific effects of BPS that were different from the effects of EE2, and distinct from the effects of BPA that have been reported previously. These assessments suggest that individual xenoestrogens may have unique effects on this sensitive tissue.

Prenatal Exposure to Unconventional Oil and Gas Operation Chemical Mixtures Altered Mammary Gland Development in Adult Female Mice

Unconventional oil and gas (UOG) operations, which combine hydraulic fracturing (fracking) and directional drilling, involve the use of hundreds of chemicals, including many with endocrine-disrupting properties. Two previous studies examined mice exposed during early development to a 23-chemical mixture of UOG compounds (UOG-MIX) commonly used or produced in the process. Both male and female offspring exposed prenatally to one or more doses of UOG-MIX displayed alterations to endocrine organ function and serum hormone concentrations. We hypothesized that prenatal UOG-MIX exposure would similarly disrupt development of the mouse mammary gland. Female C57Bl/6 mice were exposed to ~3, ~30, ~ 300, or ~3000 μg/kg/d UOG-MIX from gestational day 11 to birth. Although no effects were observed on the mammary glands of these females before puberty, in early adulthood, females exposed to 300 or 3000 μg/kg/d UOG-MIX developed more dense mammary epithelial ducts; females exposed to 3 μg/kg/d UOG-MIX had an altered ratio of apoptosis to proliferation in the mammary epithelium. Furthermore, adult females from all UOG-MIX-treated groups developed intraductal hyperplasia that resembled terminal end buds (i.e., highly proliferative structures typically seen at puberty). These results suggest that the mammary gland is sensitive to mixtures of chemicals used in UOG production at exposure levels that are environmentally relevant. The effect of these findings on the long-term health of the mammary gland, including its lactational capacity and its risk of cancer, should be evaluated in future studies.

The Ovarian Cycle

The "ovarian cycle" is an exquisite and dynamic endocrine system that includes ovarian events, hypothalamic-pituitary interactions, uterine endometrial and myometrial changes during implantation and pregnancy, cervical alterations in structure, and breast development. The ovarian cycle and the steroid hormones produced by the ovary also impact epithelial cancer development in the ovary, uterus, cervix, and breast. This chapter provides a personal view of recent developments that occur in this complex endocrine environment.

Podoplanin regulates mammary stem cell function and tumorigenesis by potentiating Wnt/β-catenin signaling

Stem cells (SCs) drive mammary development, giving rise postnatally to an epithelial bilayer composed of luminal and basal myoepithelial cells. Dysregulation of SCs is thought to be at the origin of certain breast cancers; however, the molecular identity of SCs and the factors regulating their function remain poorly defined. We identified the transmembrane protein podoplanin (Pdpn) as a specific marker of the basal compartment, including multipotent SCs, and found Pdpn localized at the basal-luminal interface. Embryonic deletion of Pdpn targeted to basal cells diminished basal and luminal SC activity and affected the expression of several Wnt/β-catenin signaling components in basal cells. Moreover, Pdpn loss attenuated mammary tumor formation in a mouse model of β-catenin-induced breast cancer, limiting tumor-initiating cell expansion and promoting molecular features associated with mesenchymal-to-epithelial cell transition. In line with the loss-of-function data, we demonstrated that mechanistically Pdpn enhances Wnt/β-catenin signaling in mammary basal cells. Overall, this study uncovers a role for Pdpn in mammary SC function and, importantly, identifies Pdpn as a new regulator of Wnt/β-catenin signaling, a key pathway in mammary development and tumorigenesis.

Anatomical, Physiological, and Functional Diversity of Adipose Tissue

Adipose tissue depots can exist in close association with other organs, where they assume diverse, often non-traditional functions. In stem cell-rich skin, bone marrow, and mammary glands, adipocytes signal to and modulate organ regeneration and remodeling. Skin adipocytes and their progenitors signal to hair follicles, promoting epithelial stem cell quiescence and activation, respectively. Hair follicles signal back to adipocyte progenitors, inducing their expansion and regeneration, as in skin scars. In mammary glands and heart, adipocytes supply lipids to neighboring cells for nutritional and metabolic functions, respectively. Adipose depots adjacent to skeletal structures function to absorb mechanical shock. Adipose tissue near the surface of skin and intestine senses and responds to bacterial invasion, contributing to the body's innate immune barrier. As the recognition of diverse adipose depot functions increases, novel therapeutic approaches centered on tissue-specific adipocytes are likely to emerge for a range of cancers and regenerative, infectious, and autoimmune disorders.

Histochemical properties of bovine and ovine mammary glands during fetal development

In order to obtain more information on the development of bovine and ovine fetal mammary glands, a series of mammary glands from fetuses of different ages were analyzed. A total of 16 bovine fetuses with curved crown rump lengths ranging from 12 cm (80 days) to 75 cm (240 days) and 15 ovine fetuses ranging from 55 days to 131 days were examined. We used hematoxylin and eosin stain and Oil-Red-O stain to analyze the developmental and morphogenetic processes of mammary glands. In addition, we used immunohistochemical staining to determine the pattern of expression of cytokeratin 18 (CK18) during luminal epithelial differentiation, α-smooth-muscle actin (α-SMA) for myoepithelial differentiation, Ki-67 for cell proliferation, and estrogen receptor α (ERα). Our analyzes showed: (a) The primary mammary duct begin to proliferate in a lengthwise within the teat at 90 days in bovine fetuses and 63 days in ovine fetus; (b) luminal epithelial cells and myoepithelial cells appeared from 90 days in bovine fetuses and 63 days in ovine fetus; (c) proliferation of epithelial cells appeared to coincide with the development of the primary and secondary ducts; and (d) ERα was not found in the fetal mammary gland, but adipocytes showed the presence of ERα. Overall, these results indicate that the sequence of events in the prenatal development of the mammary gland of sheep is similar to that of cattle.

In vivo reprogramming of non-mammary cells to an epithelial cell fate is independent of amphiregulin signaling

Amphiregulin (AREG)^-/- mice demonstrate impaired mammary development and form only rudimentary ductal epithelial trees; however, AREG^-/- glands are still capable of undergoing alveologenesis and lactogenesis during pregnancy. Transplantation of AREG^-/- mammary epithelial cells into cleared mouse mammary fat pads results in a diminished capacity for epithelial growth (∼15%) as compared to that of wild-type mammary epithelial cells. To determine whether estrogen receptor α (ERα, also known as ESR1) and/or AREG signaling were necessary for non-mammary cell redirection, we inoculated either ERα^-/- or AREG^-/- mammary cells with non-mammary progenitor cells (WAP-Cre/Rosa26LacZ+ male testicular cells or GFP-positive embryonic neuronal stem cells). ERα^-/- cells possessed a limited ability to grow or reprogram non-mammary cells in transplanted mammary fat pads. AREG^-/- mammary cells were capable of redirecting both types of non-mammary cell populations to mammary phenotypes in regenerating mammary outgrowths. Transplantation of fragments from AREG-reprogrammed chimeric outgrowths resulted in secondary outgrowths in six out of ten fat pads, demonstrating the self-renewing capacity of the redirected non-mammary cells to contribute new progeny to chimeric outgrowths. Nestin was detected at the leading edges of developing alveoli, suggesting that its expression may be essential for lobular expansion.

Molecular Signaling of Progesterone, Growth Hormone, Wnt, and HER in Mammary Glands of Dogs, Rodents, and Humans: New Treatment Target Identification

Mammary tumors are the most common form of neoplasia in the bitch. Female dogs are protected when they are spayed before the first estrus cycle, but this effect readily disappears and is already absent when dogs are spayed after the second heat. As the ovaries are removed during spaying, ovarian steroids are assumed to play an essential role in tumor development. The sensitivity toward tumor development is already present during early life, which may be caused by early mutations in stem cells during the first estrus cycles. Later on in life, tumors arise that are mostly steroid-receptor positive, although a small subset of tumors overexpressing human epidermal growth factor 2 (HER2) and some lacking estrogen receptor, progesterone receptor (PR), and HER2 (triple negative) are present, as is the situation in humans. Progesterone (P₄), acting through PR, is the major steroid involved in outgrowth of mammary tissue. PRs are expressed in two forms, the progesterone receptor A (PRA) and progesterone receptor B (PRB) isoforms derived from splice variants from a single gene. The dog and the whole family of canids have only a functional PRA isoform, whereas the PRB isoform, if expressed at all, is devoid of intrinsic biological activity. In human breast cancer, overexpression of the PRA isoform is related to more aggressive carcinomas making the dog a unique model to study PRA-related mammary cancer. Administration of P₄ to adult dogs results in local mammary expression of growth hormone (GH) and wing less-type mouse mammary tumor virus integration site family 4 (Wnt4). Both proteins play a role in activation of mammary stem cells. In this review, we summarize what is known on P₄, GH, and Wnt signaling in canine mammary cancer, how the family of HER receptors could interact with this signaling, and what this means for comparative and translational oncological aspects of human breast cancer development.

A Novel Effect of β-Adrenergic Receptor on Mammary Branching Morphogenesis and its Possible Implications in Breast Cancer

Understanding the mechanisms that govern normal mammary gland development is crucial to the comprehension of breast cancer etiology. β-adrenergic receptors (β-AR) are targets of endogenous catecholamines such as epinephrine that have gained importance in the context of cancer biology. Differences in β₂-AR expression levels may be responsible for the effects of epinephrine on tumor vs non-tumorigenic breast cell lines, the latter expressing higher levels of β₂-AR. To study regulation of the breast cell phenotype by β₂-AR, we over-expressed β₂-AR in MCF-7 breast cancer cells and knocked-down the receptor in non-tumorigenic MCF-10A breast cells. In MCF-10A cells having knocked-down β₂-AR, epinephrine increased cell proliferation and migration, similar to the response by tumor cells. In contrast, in MCF-7 cells overexpressing the β₂-AR, epinephrine decreased cell proliferation and migration and increased adhesion, mimicking the response of the non-tumorigenic MCF-10A cells, thus underscoring that β₂-AR expression level is a key player in cell behavior. β-adrenergic stimulation with isoproterenol induced differentiation of breast cells growing in 3-dimension cell culture, and also the branching of murine mammary epithelium in vivo. Branching induced by isoproterenol was abolished in fulvestrant or tamoxifen-treated mice, demonstrating that the effect of β-adrenergic stimulation on branching is dependent on the estrogen receptor (ER). An ER-independent effect of isoproterenol on lumen architecture was nonetheless found. Isoproterenol significantly increased the expression of ERα, Ephrine-B1 and fibroblast growth factors in the mammary glands of mice, and in MCF-10A cells. In a poorly differentiated murine ductal carcinoma, isoproterenol also decreased tumor growth and induced tumor differentiation. This study highlights that catecholamines, through β-AR activation, seem to be involved in mammary gland development, inducing mature duct formation. Additionally, this differentiating effect could be resourceful in a breast tumor context.

Accurate Control of 17β-Estradiol Long-Term Release Increases Reliability and Reproducibility of Preclinical Animal Studies

Estrogens are the subject of intensive researches aiming to elucidate their mechanism of action on the various tissues they target and especially on mammary gland and breast cancer. The use of ready-to-use slow releasing devices to administer steroids, especially estrogens, to small experimental animals remains the method of choice in terms of animal well-being and of safety for both the researcher and the animal. In this study, we evaluated and compared, in vitro and in vivo, the release kinetic of estradiol (E2) over sixty days from two different slow-releasing systems: the matrix pellet (MP) and the reservoir implant (RI). We compared the impact of these systems in three E2-sensitive mouse models : mammary gland development, human MCF7 adenocarcinoma xenograft and mouse melanoma progression. The real amount of E2 that is released from both types of devices could differ from manufacturer specifications due to inadequate release for MP and initial burst effect for RI. Compared to MP, the interindividual variability was reduced with RI thanks to a superior control of the E2 release. Depending on the dose-dependent sensitivity of the physiological or pathological readout studied, this could lead to an improvement of the statistical power of in vivo experiments and thus to a reduction of the required animal number. Altogether, our data draw attention on the importance to adequately select the slow-releasing device that is the most appropriated to a specific experiment to better fulfill the 3Rs rule (Replacement, Reduction, Refinement) related to animal welfare and protection.

Mammary cell-activating factor regulates the hormone-independent transcription of the early lactation protein (ELP) gene in a marsupial

The regulation of the tammar wallaby (Macropus eugenii) early lactation protein (ELP) gene is complex. ELP is responsive to the lactogenic hormones; insulin (I), hydrocortisone (HC) and prolactin (PRL) in mammary gland explants but could not be induced with lactogenic hormones in tammar primary mammary gland cells, nor in KIM-2 conditionally immortalised murine mammary epithelial cells. Similarly, ELP promoter constructs transiently-transfected into human embryonic kidney (HEK293T) cells constitutively expressing the prolactin receptor (PRLR) and Signal Transducer and Activator of Transcription (STAT)5A were unresponsive to prolactin, unlike the rat and mouse β-casein (CSN2) promoter constructs. Identification of the minimal promoter required for the hormone-independent transcription of tammar ELP in HEK293Ts and comparative analysis of the proximal promoters of marsupial ELP and the orthologous eutherian colostrum trypsin inhibitor (CTI) gene suggests that mammary cell-activating factor (MAF), an E26 transformation-specific (ETS) factor, may bind to an AGGAAG motif and activate tammar ELP.

EGFR Signaling Regulates Maspin/SerpinB5 Phosphorylation and Nuclear Localization in Mammary Epithelial Cells

Maspin (SerpinB5) is a non-inhibitory serpin (serine protease inhibitor) with very diverse biological activities including regulation of cell adhesion, migration, death, control of gene expression and oxidative stress response. Initially described as a tumor and metastasis suppressor, clinical data brought controversies to the field, as some studies reported no correlation between SerpinB5 expression and prognosis value. These data underscore the importance of understanding SerpinB5 function in a normal physiological context and the molecular mechanism involved. Several SerpinB5 phosphoforms have been detected in different cell lines, but the signaling pathways involved and the biological significance of this post-translational modification in vivo remains to be explored. In this study we investigated SerpinB5 expression, subcellular localization and phosphorylation in different stages of the mouse mammary gland development and the signaling pathway involved. Here we show that SerpinB5 is first detected in late pregnancy, reaches its highest levels in lactation and remains at constant levels during post-lactational regression (involution). Using high resolution isoelectric focusing followed but immunoblot, we found at least 8 different phosphoforms of SerpinB5 during lactation, which decreases steadily at the onset of involution. In order to investigate the signaling pathway involved in SerpinB5 phosphorylation, we took advantage of the non-transformed MCF-10A model system, as we have previously observed SerpinB5 phosphorylation in these cells. We detected basal levels of SerpinB5 phosphorylation in serum- and growth factor-starved cells, which is due to amphiregulin autocrine activity on MCF-10A cells. EGF and TGF alpha, two other EGFR ligands, promote important SerpinB5 phosphorylation. Interestingly, EGF treatment is followed by SerpinB5 nuclear accumulation. Altogether, these data indicate that SerpinB5 expression and phosphorylation are developmentally regulated. In vitro analyses indicate that SerpinB5 phosphorylation is regulated by EGFR ligands, but EGF appears to be the only able to induce SerpinB5 nuclear localization.

Mechanisms of oestrogen receptor (ER) gene regulation in breast cancer

Most breast cancers are driven by a transcription factor called oestrogen receptor (ER). Understanding the mechanisms of ER activity in breast cancer has been a major research interest and recent genomic advances have revealed extraordinary insights into how ER mediates gene transcription and what occurs during endocrine resistance. This review discusses our current understanding on ER activity, with an emphasis on several evolving, but important areas of ER biology.

Progesterone and Overlooked Endocrine Pathways in Breast Cancer Pathogenesis

Worldwide, breast cancer incidence has been increasing for decades. Exposure to reproductive hormones, as occurs with recurrent menstrual cycles, affects breast cancer risk, and can promote disease progression. Exogenous hormones and endocrine disruptors have also been implicated in increasing breast cancer incidence. Numerous in vitro studies with hormone-receptor-positive cell lines have provided insights into the complexities of hormone receptor signaling at the molecular level; in vivo additional layers of complexity add on to this. The combined use of mouse genetics and tissue recombination techniques has made it possible to disentangle hormone action in vivo and revealed that estrogens, progesterone, and prolactin orchestrate distinct developmental stages of mammary gland development. The 2 ovarian steroids that fluctuate during menstrual cycles act on a subset of mammary epithelial cells, the hormone-receptor-positive sensor cells, which translate and amplify the incoming systemic signals into local, paracrine stimuli. Progesterone has emerged as a major regulator of cell proliferation and stem cell activation in the adult mammary gland. Two progesterone receptor targets, receptor activator of NfκB ligand and Wnt4, serve as downstream paracrine mediators of progesterone receptor-induced cell proliferation and stem cell activation, respectively. Some of the findings in the mouse have been validated in human ex vivo models and by next-generation whole-transcriptome sequencing on healthy donors staged for their menstrual cycles. The implications of these insights into the basic control mechanisms of mammary gland development for breast carcinogenesis and the possible role of endocrine disruptors, in particular bisphenol A in this context, will be discussed below.

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.