Understanding mammary gland development through the imbalanced expression of growth regulators

Bringing androgens up a NOTCH in breast cancer

While it has been known for decades that androgen hormones influence normal breast development and breast carcinogenesis, the underlying mechanisms have only been recently elucidated. To date, most studies have focused on androgen action in breast cancer cell lines, yet these studies represent artificial systems that often do not faithfully replicate/recapitulate the cellular, molecular and hormonal environments of breast tumours in vivo. It is critical to have a better understanding of how androgens act in the normal mammary gland as well as in in vivo systems that maintain a relevant tumour microenvironment to gain insights into the role of androgens in the modulation of breast cancer development. This in turn will facilitate application of androgen-modulation therapy in breast cancer. This is particularly relevant as current clinical trials focus on inhibiting androgen action as breast cancer therapy but, depending on the steroid receptor profile of the tumour, certain individuals may be better served by selectively stimulating androgen action. Androgen receptor (AR) protein is primarily expressed by the hormone-sensing compartment of normal breast epithelium, commonly referred to as oestrogen receptor alpha (ERa (ESR1))-positive breast epithelial cells, which also express progesterone receptors (PRs) and prolactin receptors and exert powerful developmental influences on adjacent breast epithelial cells. Recent lineage-tracing studies, particularly those focussed on NOTCH signalling, and genetic analysis of cancer risk in the normal breast highlight how signalling via the hormone-sensing compartment can influence normal breast development and breast cancer susceptibility. This provides an impetus to focus on the relationship between androgens, AR and NOTCH signalling and the crosstalk between ERa and PR signalling in the hormone-sensing component of breast epithelium in order to unravel the mechanisms behind the ability of androgens to modulate breast cancer initiation and growth.

Ltbp1L is focally induced in embryonic mammary mesenchyme, demarcates the ductal luminal lineage and is upregulated during involution

Introduction

Latent TGFβ binding proteins (LTBPs) govern TGFβ presentation and activation and are important for elastogenesis. Although TGFβ is well-known as a tumor suppressor and metastasis promoter, and LTBP1 is elevated in two distinct breast cancer metastasis signatures, LTBPs have not been studied in the normal mammary gland.

Methods

To address this we have examined Ltbp1 promoter activity throughout mammary development using an Ltbp1L-LacZ reporter as well as expression of both Ltbp1L and 1S mRNA and protein by qRT-PCR, immunofluorescence and flow cytometry.

Results

Our data show that Ltbp1L is transcribed coincident with lumen formation, providing a rare marker distinguishing ductal from alveolar luminal lineages. Ltbp1L and Ltbp1S are silent during lactation but robustly induced during involution, peaking at the stage when the remodeling process becomes irreversible. Ltbp1L is also induced within the embryonic mammary mesenchyme and maintained within nipple smooth muscle cells and myofibroblasts. Ltbp1 protein exclusively ensheaths ducts and side branches.

Conclusions

These data show Ltbp1 is transcriptionally regulated in a dynamic manner that is likely to impose significant spatial restriction on TGFβ bioavailability during mammary development. We hypothesize that Ltbp1 functions in a mechanosensory capacity to establish and maintain ductal luminal cell fate, support and detect ductal distension, trigger irreversible involution, and facilitate nipple sphincter function.

Stress and breast cancer: from epidemiology to molecular biology

Stress exposure has been proposed to contribute to the etiology of breast cancer. However, the validity of this assertion and the possible mechanisms involved are not well established. Epidemiologic studies differ in their assessment of the relative contribution of stress to breast cancer risk, while physiological studies propose a clear connection but lack the knowledge of intracellular pathways involved. The present review aims to consolidate the findings from different fields of research (including epidemiology, physiology, and molecular biology) in order to present a comprehensive picture of what we know to date about the role of stress in breast cancer development.

Role for EPS8 in squamous carcinogenesis

We have investigated the role of the signaling intermediate, EPS8, in tumor progression using a model system and in vivo. HN4 primary tumor cells express low levels of EPS8, similar to normal keratinocytes, and show minimal invasion in vitro in response to epidermal growth factor, whereas HN12 cells express high levels of EPS8 and are highly motile in vitro and tumorigenic in vivo. Additional independent tumor cell lines also showed elevated EPS8 expression compared with normal keratinocytes. Using retroviral transduction, we generated HN4 cell lines expressing EPS8 (HN4/EPS8) at levels equivalent to those present in HN12 cells. HN4/EPS8 cells showed increased proliferation and migration compared with controls, together with elevated expression and activity of matrix metalloprotease (MMP)-9, which was dependent on protein kinase B (AKT) activity. Introduction of plasmids that direct synthesis of EPS8 short hairpin RNA (shRNA) into HN12 cells resulted in decreased EPS8 expression in these cells, which correlated with a decrease in their capacity to migrate and invade in vitro. In addition, shRNA-mediated knockdown of EPS8 reduced expression and activity of MMP-9 produced by these cells and reduced MMP-9 promoter activity. EPS8 knockdown cells showed decreased tumorigenicity in vivo compared with controls and lower MMP-9 expression. Conversely, overexpression of EPS8 in HN4 cells was sufficient to induce growth of these non-tumorigenic cells in orthotopic transplantation assays. Furthermore, EPS8 expression in clinical samples of squamous cell carcinoma showed variable expression levels and broadly paralleled expression of MMP-9. The data support a role for EPS8 in squamous carcinogenesis.

ETV6-NTRK3 fusion oncogene initiates breast cancer from committed mammary progenitors via activation of AP1 complex

To better understand the cellular origin of breast cancer, we developed a mouse model that recapitulates expression of the ETV6-NTRK3 (EN) fusion oncoprotein, the product of the t(12;15)(p13;q25) translocation characteristic of human secretory breast carcinoma. Activation of EN expression in mammary tissues by Wap-Cre leads to fully penetrant, multifocal malignant breast cancer with short latency. We provide genetic evidence that, in nulliparous Wap-Cre;EN females, committed alveolar bipotent or CD61(+) luminal progenitors are targets of tumorigenesis. Furthermore, EN transforms these otherwise transient progenitors through activation of the AP1 complex. Given the increasing relevance of chromosomal translocations in epithelial cancers, such mice serve as a paradigm for the study of their genetic pathogenesis and cellular origins, and generation of preclinical models.

Ultrastructural Morphometry of Mammary Gland in Transgenic and Non-transgenic Rabbits

The mammary gland of transgenic animals has been used for the production of recombinant proteins of therapeutic and nutraceutical use. The objective of this study was to compare the ultrastructure of transgenic and non-transgenic rabbit mammary gland tissue. New Zealand White transgenic rabbits were obtained by breeding non-transgenic rabbits with transgenic founder rabbits containing a whey acidic protein-human factor VIII (WAP-hFVIII) transgene integrated into their genome. Samples of mammary gland tissue from lactating rabbit females were isolated by surgical procedures. These samples were examined by optical and electron microscopy and photographs were taken. Measurements of ultrastructural organelles were made from digital images of the mammary cells. No differences were found in the cellular structure of mammary tissue, but significant differences t((0.001)) in the relative volume of mitochondria and vacuoles between transgenic and non-transgenic mammary gland epithelium were observed.

C/EBPdelta is a crucial regulator of pro-apoptotic gene expression during mammary gland involution

The STAT3 transcription factor is an important initiator of mammary gland involution in the mouse. This work shows that the STAT3 target gene CCAAT/enhancer binding protein delta (C/EBPdelta) is a crucial mediator of pro-apoptotic gene expression events in mammary epithelial cells. In the absence of C/EBPdelta, involution is delayed, the pro-apoptotic genes encoding p53, BAK, IGFBP5 and SGP2/clusterin are not activated, while the anti-apoptotic genes coding for BFL1 and Cyclin D1 are not repressed. Consequently, p53 targets such as survivin, BRCA1, BRCA2 and BAX are not regulated appropriately and protease activation is delayed. Furthermore, expression of MMP3 and C/EBPdelta during the second phase of involution is perturbed in the absence of C/EBPdelta. In HC11 cells, C/EBPdelta alone is sufficient to induce IGFBP5 and SGP2. It also suppresses Cyclin D1 expression and cooperates with p53 to elicit apoptosis. This study places C/EBPdelta between STAT3 and several pro- and anti-apoptotic genes promoting the physiological cell death response in epithelial cells at the onset of mammary gland involution.

Parity-induced mouse mammary epithelial cells are pluripotent, self-renewing and sensitive to TGF-β1 expression

A parity-induced mammary population, marked by beta-galactosidase expression conditionally activated through cre-lox recombinase originates in WAP-Cre/Rosa-lox-STOP-lox-LacZ (WAP-Cre/Rosa-LacZ) female mice during pregnancy, lactation and involution. During subsequent pregnancies, these parity-induced mammary epithelial cells (PI-MEC) proliferated to produce new secretory acini composed of secretory luminal cells and myoepithelium. In serial transplantation assays, PI-MEC were able to self-renew over several transplant generations and to contribute significantly to the resulting mammary outgrowths. In limiting dilution transplantation, they proliferated to produce both luminal and myoepithelial cells, comprised both lobule-limited and duct-limited epithelial outgrowths, and differentiated into all the cellular subtypes recognized in murine mammary epithelium. TGF-beta1 expression from the whey acidic protein promoter (WAP) in triply transgenic females did not prevent the appearance of PI-MEC after pregnancy despite the absence of full lactation or their ability to proliferate and produce progeny with diverse cellular fates in situ upon subsequent pregnancies. However, in transplants from triple transgenic parous females, the WAP-TGF-beta1-positive PI-MEC did not contribute to the newly recapitulated mammary outgrowths, suggesting that they were incapable of expansive cellular proliferation (self-renewal). This result is consistent with our earlier publication that WAP-TGF-beta1 expression in mammary epithelium induces premature stem cell senescence in mammary transplants and decreases mammary cancer risk in mouse mammary tumor virus (MMTV)-infected females even after multiple pregnancies.

The significance of human jagged 1 mutations detected in severe cases of extrahepatic biliary atresia

Mutations of human jagged 1 (JAG1) gene are responsible for Alagille Syndrome (AGS), whose 2 main symptoms are intrahepatic bile duct hypoplasia and pulmonary stenosis. We examined the JAG1 mutation in extrahepatic biliary atresia (EHBA), which is similar in phenotype to AGS, although a different pathogenesis is suggested. In 102 cases of EHBA, 9 missense mutations were detected, including 2 intrafamilial expressions in the propositus and an aunt of one family. These mutations were all missense and sporadic except for those of this particular family. The JAG1 gene mutations were generally found in severely ill patients subjected to liver transplantation at less than 5 years of age. None of the 9 cases of EHBA revealed any of the 5 major symptoms of AGS nor any identical pathological findings after 3 years of follow-up. Our cases were clearly separated from AGS by pathological findings and clinical features, and could be diagnosed as EHBA and not as atypical AGS. The increase of interleukin 8 (IL-8) production induced by tumor necrosis factor alpha (TNF-alpha) in Huh 7 cells was suppressed by the coexistence of JAG1 protein. We examined the different influences between wild-type cells and the 3 kinds of mutants detected in EHBA on Huh 7 cells and found that 2 of 3 mutants showed about half of the repressed activity compared with that of wild type. In conclusion, these results suggest that the JAG1 gene abnormality may be an aggravating factor in EHBA.

Regulation of apoptosis during mammary involution by the p53 tumor suppressor gene

Regulation and functions of the p53 tumor suppressor gene have been studied extensively with respect to its critical role in maintaining the stability of genomic DNA following genotoxic insults. However, p53 is also induced by physiologic stimuli resulting in cell cycle arrest and apoptosis. In other situations, the activity of p53 must be repressed to prevent inappropriate removal of cells. The mammary gland provides a valuable system in which to study the mechanisms by which the expression and biological responses to p53 can be regulated under a variety of physiological circumstances. The pro-apoptotic role of p53 in the secretory mammary epithelium may be especially relevant to lactation in livestock. We have utilized p53-deficient mice to establish the molecular targets of p53 in the mammary gland and biological consequences when it is absent. The p21/WAF1 gene (Cdkn1a) is a transcriptional target gene of the p53 protein that responds to elevated levels of p53 during milk stasis providing an endogenous reporter of p53 activity. Abrogation of p53 resulted in delayed involution of the mammary epithelium, demonstrating the physiological role of p53 in regulating involution. Though delayed, stromal proteases were induced in the mammary gland by 5 d postweaning, providing a p53-independent mechanism that resulted in removal of the residual secretory epithelium. These processes can be interrupted by treatment with hydrocortisone. These data establish p53 as a physiological regulator of involution that acts to rapidly initiate apoptosis in the secretory epithelium in response to stress signals, but also indicate the presence of compensatory pathways to effect involution. Additional mechanisms involving intracellular stress signaling pathways (e.g., Stat3) and stromal-mediated pathways have been identified and, together with p53 pathways, may be used to identify animals with greater persistency of lactation.

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

At present, there is an extensive body of literature documenting the participation of estrogen receptors (ER) and progesterone receptors (PR) in mammary gene expression. Yet, the precise roles of these receptors in regulating mammary development, carcinogenesis and the growth of a subset of tumors still remain unclear. Mammary glands are composed of various cell types with different developmental potentials. Further, ultimately, that it is their mutual interactions which dictate the behavior of mammary epithelial cells. Therefore, to resolve the roles of ER and PR in normal mammary growth, differentiation and carcinogenesis, analyses for the expression of these receptors at the level of individual cell types is of paramount importance. Accordingly, in the present studies using immunolocalization techniques, we document the ontogeny and cellular distribution of ER and PR during mammary development and in response to ovarian hormones and aging. In addition, we discuss the potential biological significances of the expression patterns of ER and PR during various physiological states. We believe that the observations reported here should provide a conceptual framework(s) for elucidating the roles of ER and PR in normal and neoplastic mammary tissues.

Apoptosis in normal and neoplastic mammary gland development

Apoptosis plays important roles in mammary development from early embryonic formation of the mammary gland to the regression that follows cessation of cycling. The most dramatic occurrence of apoptosis is found during mammary involution. Most of the secretory epithelium in the lactating breast undergoes apoptosis as the mammary gland regresses and is reorganized for another cycle of lactation. We used the morphology, biochemical changes, and gene expression detected in apoptotic mammary epithelium during involution as a model for studying cell death during other stages of mammary development and for approaching the failure of apoptosis found in mammary hyperplasia. Morphological studies and gene expression have suggested that apoptosis during involution is comprised of two phases: an early limited apoptosis in response to hormone ablation and later protease promoted widespread apoptosis in response to altered cell-matrix interactions and loss of anchorage. We examined protein expression during involution for changes associated with loss of hormone stimulation and altered cell-matrix interactions. One of the proteins whose expression is able to inhibit apoptosis, and is altered during mammary epithelial cell was the serine-threonine protein kinase, Akt 1. Akt 1 activation is common to hormone, growth factor, and anchorage-mediated survival of epithelial cells. We found regulated expression of activated Akt 1 in the mammary gland during involution. Akt 1 activation peaked in pregnancy and lactation, and decreased significantly during apoptosis in mammary involution. Mechanisms of Akt 1 action include modulation of the ratio bcl-2 family members implicated in control of apoptosis. Bcl-2 family proteins were also expressed in pattern consistent with Akt 1 regulation. These observations led us to examine expression of activated Akt 1 and bcl-2 family proteins in premalignant hyperplasias. Akt 1 activation was increased; expression of anti-apoptotic proteins bcl-2 and bcl-x was strongly increased while pro-apoptotic bax was greatly diminished in three different lines of transplantable premalignant mammary hyperplasia. This data suggest that activation of Akt 1 by hormone- or anchorage-mediated pathways regulates survival of mammary epithelium and can contribute to initiation of neoplasia. These data suggest that perturbation of normal cell turnover can contribute to initiation of neoplasia.

Transforming growth factor alpha- and c-myc-induced mammary carcinogenesis in transgenic mice

The growth factor transforming growth factor alpha (TGFalpha) and the nuclear transcription factor c-myc often are overexpressed by human breast cancer cells. To produce models of breast disease with these etiologies, mice were generated that carried TGF-alpha- or c-myc-encoding transgenes. Transgene targeting employed the whey acidic protein (WAP) gene promoter, which is expressed in pregnant and lactating mammary epithelial cells. Non-virgin WAP-TGFalpha transgenic mice displayed accelerated mammary development during pregnancy, delayed post-parturient mammary involution, a progressive increase in the number of hyperplastic alveolar nodules (HANs), and development of mammary carcinoma with a mean latency of 9 months. Non-virgin WAP-c-myc transgenic mice displayed accelerated mammary gland development during pregnancy and development of mammary carcinomas with a latency of 8 months. Bitransgenic mice carrying both WAP-TGFalpha and WAP-c-myc displayed a dramatic acceleration of tumor development. These models identify the overexpression of TGFalpha or c-myc as etiological factors in the development of mammary neoplasia and demonstrate the increased severity of disease when both molecular alterations are present in the same cell.

Interplay of Notch and FGF signaling restricts cell fate and MAPK activation in the Drosophila trachea

The patterned branching in the Drosophila tracheal system is triggered by the FGF-like ligand Branchless that activates a receptor tyrosine kinase Breathless and the MAP kinase pathway. A single fusion cell at the tip of each fusion branch expresses the zinc-finger gene escargot, leads branch migration in a stereotypical pattern and contacts with another fusion cell to mediate fusion of the branches. A high level of MAP kinase activation is also limited to the tip of the branches. Restriction of such cell specialization events to the tip is essential for tracheal tubulogenesis. Here we show that Notch signaling plays crucial roles in the singling out process of the fusion cell. We found that Notch is activated in tracheal cells by Branchless signaling through stimulation of Δ expression at the tip of tracheal branches and that activated Notch represses the fate of the fusion cell. In addition, Notch is required to restrict activation of MAP kinase to the tip of the branches, in part through the negative regulation of Branchless expression. Notch-mediated lateral inhibition in sending and receiving cells is thus essential to restrict the inductive influence of Branchless on the tracheal tubulogenesis.

The C/EBPbeta transcription factor regulates epithelial cell proliferation and differentiation in the mammary gland

Studies of C/EBPbeta-deficient mice have demonstrated a pivotal role for this transcription factor in hematopoiesis, adipogenesis, and ovarian function. Here we show that C/EBPbeta is also essential for normal development and function of the mammary gland. Ductal morphogenesis in virgin C/EBPbeta-deficient mice was disrupted, with ducts displaying reduced growth and branching. To distinguish whether the effect of C/EBPbeta deficiency on mammary epithelium is indirect or cell autonomous, we performed ovarian and mammary gland transplants. Transplants of wild-type ovaries into mutant females partially restored ductal morphogenesis during puberty but failed to support mammopoiesis during pregnancy. At term, mutant mice harboring wild-type ovaries exhibited reduced alveolar proliferation and impaired epithelial cell differentiation, including a complete absence of milk protein expression. Mammary gland transplant experiments demonstrated that development of C/EBPbeta-deficient epithelium was defective within a wild-type stroma and host background. Cell proliferation during pregnancy was reduced and differentiation, as measured by the activity of milk protein genes, was inhibited. However, wild-type epithelium developed in a C/EBPbeta-deficient stroma. Thus, C/EBPbeta plays an essential, cell autonomous role in the proliferation and differentiation of mammary secretory epithelial cells and is required for the activation of milk protein genes.

Impaired mammary gland development and function in mice lacking LAR receptor-like tyrosine phosphatase activity

The LAR receptor-like protein tyrosine phosphatase is composed of two intracellular tyrosine phosphatase domains and a cell adhesion molecule-like extracellular region containing three immunoglubulin-like domains in combination with eight fibronectin type-III-like repeats. This architecture suggests that LAR may function in cellular signalling by the regulation of tyrosine phosphorylation through cell-cell or cell-matrix interactions. We used gene targeting in mouse embryonic stem cells to generate mice lacking sequences encoding both LAR phosphatase domains. Northern blot analysis of various tissues revealed the presence of a truncated LAR mRNA lacking the cytoplasmic tyrosine phosphatase domains and indicated that this LAR mutation is not accompanied by obvious changes in the expression levels of one of the LAR-like receptor tyrosine phosphatases PTPdelta or PTPsigma. LAR-/- mice develop and grow normally and display no appreciable histological tissue abnormalities. However, upon breeding we observed an abnormal neonatal death rate for pups from LAR-/- females. Mammary glands of LAR-/- females were incapable of delivering milk due to an impaired terminal differentiation of alveoli at late pregnancy. As a result, the glands failed to switch to a lactational state and showed a rapid involution postpartum. In wild-type mice, LAR expression is regulated during pregnancy reaching maximum levels around Day 16 of gestation. Taken together, these findings suggest an important role for LAR-mediated signalling in mammary gland development and function.

Inhibins and activins regulate mammary epithelial cell differentiation through mesenchymal-epithelial interactions

Inhibins and activins are members of the transforming growth factor beta (TGFbeta) family. Female mice in which both alleles encoding the inhibin betaB subunit have been deleted are unable to nurse their pups. We have now identified a cause of lactation failure in these mice. Ductal elongation and alveolar morphogenesis are retarded. During puberty and pregnancy, ductal outgrowth and alveolar development are limited and morphologically abnormal endbuds persist in the glands of postpartum females. The alveolar lumina fail to expand at parturition due to the absence of secreted milk. Transplantation experiments have been performed to determine whether the absence of systemic- or mammary-derived betaB subunits are the cause for the incomplete and aberrant development. While transplanted intact glands from wild-type mice grew normally in betaB-deficient hosts, betaB-deficient glands remained underdeveloped in wild-type hosts. However, betaB-deficient epithelium developed normally when transplanted into the fat pad of wild-type hosts. This demonstrates that ductal elongation and epithelial cell differentiation during puberty and pregnancy require activin/inhibin signalling from the stroma. The results further show that distinct, though related, activins and inhibins perform unique functions and are not able to compensate for the absence of activin B and AB and inhibin B in the process of mammogenesis. The betaB-deficient mice provide the first genetic evidence for stromal signalling in the adult mammary gland in vivo.