Expression and functional properties of transforming growth factor alpha and epidermal growth factor during mouse mammary gland ductal morphogenesis

ERBB Receptors and Their Ligands in the Developing Mammary Glands of Different Species: Fifteen Characters in Search of an Author

The ERBB tyrosine kinase receptors and their ligands belong to a complex family that has diverse biological effects and expression profiles in the developing mammary glands, where its members play an essential role in translating hormone signals into local effects. While our understanding of these processes stems mostly from mouse models, there is the potential for differences in how this family functions in the mammary glands of other species, particularly in light of their unique histomorphological features. Herein we review the postnatal distribution and function of ERBB receptors and their ligands in the mammary glands of rodents and humans, as well as for livestock and companion animals. Our analysis highlights the diverse biology for this family and its members across species, the regulation of their expression, and how their roles and functions might be modulated by varying stromal composition and hormone interactions. Given that ERBB receptors and their ligands have the potential to influence processes ranging from normal mammary development to diseased states such as cancer and/or mastitis, both in human and veterinary medicine, a more complete understanding of their biological functions should help to direct future research and the identification of new therapeutic targets.

Characterization of Organoid Cultures to Study the Effects of Pregnancy Hormones on the Epigenome and Transcriptional Output of Mammary Epithelial Cells

The use of mouse derived mammary organoids can provide a unique strategy to study mammary gland development across a normal life cycle, as well as offering insights into how malignancies form and progress. Substantial cellular and epigenomic changes are triggered in response to pregnancy hormones, a reaction that engages molecular and cellular changes that transform the mammary epithelial cells into "milk producing machines". Such epigenomic alterations remain stable in post-involution mammary epithelial cells and control the reactivation of gene transcription in response to re-exposure to pregnancy hormones. Thus, a system that tightly controls exposure to pregnancy hormones, epigenomic alterations, and activation of transcription will allow for a better understanding of such molecular switches. Here, we describe the characterization of ex vivo cultures to mimic the response of mammary organoid cultures to pregnancy hormones and to understand gene regulation and epigenomic reprogramming on consecutive hormone exposure. Our findings suggest that this system yields similar epigenetic modifications to those reported in vivo, thus representing a suitable model to closely track epigenomic rearrangement and define unknown players of pregnancy-induced development.

Pregnancy and Breast Cancer

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

Efficacy of recombinant bovine epidermal growth factor in the treatment of experimental subclinical Staphylococcus aureus mastitis in a ewe model

Staphylococcus aureus is the most common contagious mastitis pathogen of dairy cattle. Antimicrobial treatment of infected cattle results in variable cure rates. Epidermal growth factor (EGF) plays an important role in the modulation of host innate immune responses and the regulation of mammary epithelial regeneration, indicating that EGF may be useful as a treatment for mastitis. A pilot study was conducted to evaluate the efficacy of recombinant bovine EGF (rbEGF) for the treatment of S aureus intramammary infection (IMI) using an ovine model. Each ewe was experimentally infected with S aureus in both udder halves. One udder half of each ewe received one of two treatments: EGF (n=13) or pirlimycin (n=13). The contralateral udder half of each ewe received sterile saline as a control. The bacteriological cure rate following rbEGF was significantly lower (15 per cent) than that attained with pirlimycin hydrochloride (61 per cent) and did not differ from that following treatment with sterile saline. Cure rates following treatment with rbEGF were not significantly different to those following sterile saline. Given that EGF is associated with modulation of host immunity and wound healing, future studies into EGF should not focus on whether EGF increases cure rates of S aureus IMI.

Adult Murine Pancreatic Progenitors Require Epidermal Growth Factor and Nicotinamide for Self-Renewal and Differentiation in a Serum- and Conditioned Medium-Free Culture

Adult pancreatic stem and progenitor cells may serve as an alternative source of insulin-secreting endocrine cells in cell replacement therapy for type 1 diabetes, but much remained unknown about these cells. We previously identified adult murine pancreatic progenitor-like cells that displayed in vitro self-renewal and tri-lineage differentiation activities in a three-dimensional colony/organoid assay containing 1% methylcellulose and 5% Matrigel. However, the presence of other undefined culture components, such as serum and conditioned medium, has prevented a complete understanding of the signals required for progenitor cell growth. Here, we have established a serum-free, conditioned medium-free colony assay with the inclusion of seven defined factors: epidermal growth factor (EGF), R-Spondin 1 (RSPO1), Noggin, nicotinamide, exendin-4, activin B, and vascular endothelial growth factor (VEGF)-A. The requirements for colony growth were characterized and we found that EGF and nicotinamide were necessary and sufficient for the colony growth and long-term self-renewal of these progenitors. However, the seven factor (7F) culture medium better induced colony size and self-renewal in long-term culture than EGF plus nicotinamide alone. Individual 3-week-old colonies grown in the 7F culture medium expressed ductal, acinar, and endocrine lineage markers, suggesting that tri-lineage differentiation of the tri-potent progenitors was occurring without genetic manipulation. A delayed inhibition of Notch signaling using small molecules in 2-week-old cultures enhanced endocrine gene expression in 3-week-old colonies. This better-defined colony assay system will enable our and other laboratories for in-depth mechanistic studies on the biology of these progenitor cells.

An "elite hacker": breast tumors exploit the normal microenvironment program to instruct their progression and biological diversity

The year 2011 marked the 40 year anniversary of Richard Nixon signing the National Cancer Act, thus declaring the beginning of the "War on Cancer" in the United States. Whereas we have made tremendous progress toward understanding the genetics of tumors in the past four decades, and in developing enabling technology to dissect the molecular underpinnings of cancer at unprecedented resolution, it is only recently that the important role of the stromal microenvironment has been studied in detail. Cancer is a tissue-specific disease, and it is becoming clear that much of what we know about breast cancer progression parallels the biology of the normal breast differentiation, of which there is still much to learn. In particular, the normal breast and breast tumors share molecular, cellular, systemic and microenvironmental influences necessary for their progression. It is therefore enticing to consider a tumor to be a "rogue hacker"--one who exploits the weaknesses of a normal program for personal benefit. Understanding normal mammary gland biology and its "security vulnerabilities" may thus leave us better equipped to target breast cancer. In this review, we will provide a brief overview of the heterotypic cellular and molecular interactions within the microenvironment of the developing mammary gland that are necessary for functional differentiation, provide evidence suggesting that similar biology--albeit imbalanced and exaggerated--is observed in breast cancer progression particularly during the transition from carcinoma in situ to invasive disease. Lastly we will present evidence suggesting that the multigene signatures currently used to model cancer heterogeneity and clinical outcome largely reflect signaling from a heterogeneous microenvironment-a recurring theme that could potentially be exploited therapeutically.

Molecular regulators of pubertal mammary gland development

The pubertal mammary gland is an ideal model for experimental morphogenesis. The primary glandular branching morphogenesis occurs at this time, integrating epithelial cell proliferation, differentiation, and apoptosis. Between birth and puberty, the mammary gland exists in a relatively quiescent state. At the onset of puberty, rapid expansion of a pre-existing rudimentary mammary epithelium generates an extensive ductal network by a process of branch initiation, elongation, and invasion of the mammary mesenchyme. It is this branching morphogenesis that characterizes pubertal mammary gland growth. Tissue-specific molecular networks interpret signals from local cytokines/growth factors in both the epithelial and stromal microenvironments. This is largely orchestrated by secreted ovarian and pituitary hormones. Here, we review the major molecular regulators of pubertal mammary gland development.

Mammary branch initiation and extension are inhibited by separate pathways downstream of TGFβ in culture

During the branching morphogenesis process that builds epithelial trees, signaling from stimulatory and inhibitory growth factors is integrated to control branch initiation and extension into the surrounding stroma. Here, we examined the relative roles played by these stimulatory and inhibitory signals in the patterning of branch initiation and extension of model mammary epithelial tubules in culture. We found that although several growth factors could stimulate branching, they did not determine the sites at which new branches formed or the lengths to which branches extended. Instead, branch initiation and extension were defined by two separate signals downstream of the inhibitory morphogen, transforming growth factor (TGF)-β. Branch initiation was controlled by signaling through p38 mitogen-activated protein kinase, whereas branch extension was controlled by Smad-mediated induction of a second diffusible inhibitor, Wnt5a. These data suggest that mammary epithelial branching is patterned predominately by repulsive signaling, and that TGFβ activates multiple inhibitory pathways to refine the architecture of the tree.

The metastasis-promoting protein S100A4 regulates mammary branching morphogenesis

High levels of the S100 calcium binding protein S100A4 also called fibroblast specific protein 1 (FSP1) have been established as an inducer of metastasis and indicator of poor prognosis in breast cancer. The mechanism by which S100A4 leads to increased cancer aggressiveness has yet to be established; moreover, the function of this protein in normal mammary gland biology has not been investigated. To address the role of S100A4 in normal mammary gland, its spatial and temporal expression patterns and possible function in branching morphogenesis were investigated. We show that the protein is expressed mainly in cells of the stromal compartment of adult humans, and during active ductal development, in pregnancy and in involution of mouse mammary gland. In 3D culture models, topical addition of S100A4 induced a significant increase in the TGFα mediated branching phenotype and a concomitant increase in expression of a previously identified branching morphogen, metalloproteinase-3 (MMP-3). These events were found to be dependent on MEK activation. Downregulation of S100A4 using shRNA significantly reduced TGFα induced branching and altered E-cadherin localization. These findings provide evidence that S100A4 is developmentally regulated and that it plays a functional role in mammary gland development, in concert with TGFα by activating MMP-3, and increasing invasion into the fat pad during branching. We suggest that S100A4-mediated effects during branching morphogenesis provide a plausible mechanism for how it may function in breast cancer progression.

The role of estrogen receptors, erbB receptors, vascular endothelial growth factor and its receptors, and vascular endothelial growth inhibitor in the development of the rat mammary gland

We identified the localization and distribution of cell-specific epidermal growth factor receptors (EGFRs: erbB-1, erbB-2, erbB-3, erbB-4), vascular endothelial growth factor (VEGF), VEGF receptors [VEGFRs: VEGF-R1 (flt-1), VEGF-R2 (flk-1/KDR), VEGF-R3 (flt-4)], vascular endothelial growth inhibitor (VEGI), and estrogen receptor (ER), and determined whether or not these growth factors in rat mammary glands are functional. Thirty-five adult female Spraque-Dawley rats were randomly divided into five groups, each of which were at the 7th, 14th, and 21st day of pregnancy; 7th day post-delivery; and 7th day after weaning. It was determined that erbB, VEGF and its receptors, VEGI, and ER stained at different intensities. Intense staining was observed, in particular, in erbB receptors during pregnancy and involution, and also in VEGF and its receptors during lactation, while ER stained during the last periods of pregnancy and lactation. In conclusion, the expression of erbB, VEGF and its receptors, and ER were determined at varying intensities at different sites of the mammary gland during pregnancy, lactation, and involution periods.

The ADAM17-amphiregulin-EGFR axis in mammary development and cancer

In order to fulfill its function of producing and delivering sufficient milk to newborn mammalian offspring, the mammary gland first has to form an extensive ductal network. As in all phases of mammary development, hormonal cues elicit local intra- and inter-cellular signaling cascades that regulate ductal growth and differentiation. Among other things, ductal development requires the epidermal growth factor receptor (EGFR), its ligand amphiregulin (AREG), and the transmembrane metalloproteinase ADAM17, which can cleave and release AREG from the cell surface so that it may interact with its receptor. Tissue recombination and transplantation studies demonstrate that EGFR phosphorylation and ductal development proceed only when ADAM17 and AREG are expressed on mammary epithelial cells and EGFR is present on stromal cells, and that local administration of soluble AREG can rescue the development of ADAM17-deficient transplants. Thus proper mammary morphogenesis requires the ADAM17-mediated release of AREG from ductal epithelial cells, the subsequent activation of EGFR on stromal cells, and EGFR-dependent stromal responses that in return elicit a new set of epithelial responses, all culminating in the formation of a fully functional ductal tree. This, however, raises new issues concerning what may act upstream, downstream or in parallel with the ADAM17-AREG-EGFR axis, how it may become hijacked or corrupted during the onset and evolution of cancer, and how such ill effects may be confronted.

Transgenic models to study actions of prolactin in mammary neoplasia

Transgenic models to explore the role of prolactin and its interactions with other factors in mammary oncogenesis have begun to reveal the dynamic contributions of prolactin to the development and progression of this disease. Targeting prolactin to mammary epithelial cells mimics the local production of this hormone that is prominent in women, and permits studies in the absence of effects on the ovarian steroid milieu. These models have demonstrated that local production of prolactin is sufficient to induce mammary tumors after a long latency. Prolactin also can potentiate actions of other oncogenic stimuli, decreasing tumor latency and increasing incidence in several models. Augmented proliferation, without alteration of apoptosis, is a consistent feature. Pathways in addition to the well-characterized Jak2-Stat5 pathway, including ERK1/2 and Akt1/2, are implicated in these actions. These studies have also revealed a complex relationship with estrogen; while prolactin increases ERalpha expression, it does not require estrogenic ligand for lesion development, and indeed, in combination with the EGFR ligand, TGFalpha, prolactin can contribute to estrogen insensitivity. These studies highlight the utility of these models to decipher the interplay between prolactin and other oncogenic factors in breast cancer, with implications for preventative and therapeutic strategies.

Roles of transforming growth factor-alpha in mammary development and disease

Transforming growth factor-alpha (TGFalpha) is a member of the epidermal growth factor (EGF) family. Expression of TGFalpha is highly regulated in response to exogenous cellular signals including cytokines and other growth factors. The growth factor has been found to be indispensable for proper development of many tissues and organs. TGFalpha has also been implicated in numerous disease states including forms of breast cancer. This minireview summarizes the basic biology of TGFalpha and its actions during normal and pathogenic development of the mammary epithelium.

Amphiregulin is an essential mediator of estrogen receptor alpha function in mammary gland development

Most mammary gland development occurs after birth under the control of systemic hormones. Estrogens induce mammary epithelial cell proliferation during puberty via epithelial estrogen receptor alpha (ERalpha) by a paracrine mechanism. Epidermal growth factor receptor (EGFR) signaling has long been implicated downstream of ERalpha signaling, and several EGFR ligands have been described as estrogen-target genes in tumor cell lines. Here, we show that amphiregulin is the unique EGF family member to be transcriptionally induced by estrogen in the mammary glands of puberal mice at a time of exponential expansion of the ductal system. In fact, we find that estrogens induce amphiregulin through the ERalpha and require amphiregulin to induce proliferation of the mammary epithelium. Like ERalpha, amphiregulin is required in the epithelium of puberal mice for epithelial proliferation, terminal end buds formation, and ductal elongation. Subsequent stages, such as side-branching and alveologenesis, are not affected. When amphiregulin(-/-) mammary epithelial cells are in close vicinity to wild-type cells, they proliferate and contribute to all cell compartments of the ductal outgrowth. Thus, amphiregulin is an important paracrine mediator of estrogen function specifically required for puberty-induced ductal elongation, but not for any earlier or later developmental stages.

Acrylamide: review of toxicity data and dose-response analyses for cancer and noncancer effects

Acrylamide (ACR) is used in the manufacture of polyacrylamides and has recently been shown to form when foods, typically containing certain nutrients, are cooked at normal cooking temperatures (e.g., frying, grilling or baking). The toxicity of ACR has been extensively investigated. The major findings of these studies indicate that ACR is neurotoxic in animals and humans, and it has been shown to be a reproductive toxicant in animal models and a rodent carcinogen. Several reviews of ACR toxicity have been conducted and ACR has been categorized as to its potential to be a human carcinogen in these reviews. Allowable levels based on the toxicity data concurrently available had been developed by the U.S. EPA. New data have been published since the U.S. EPA review in 1991. The purpose of this investigation was to review the toxicity data, identify any new relevant data, and select those data to be used in dose-response modeling. Proposed revised cancer and noncancer toxicity values were estimated using the newest U.S. EPA guidelines for cancer risk assessment and noncancer hazard assessment. Assessment of noncancer endpoints using benchmark models resulted in a reference dose (RfD) of 0.83 microg/kg/day based on reproductive effects, and 1.2 microg/kg/day based on neurotoxicity. Thyroid tumors in male and female rats were the only endpoint relevant to human health and were selected to estimate the point of departure (POD) using the multistage model. Because the mode of action of acrylamide in thyroid tumor formation is not known with certainty, both linear and nonlinear low-dose extrapolations were conducted under the assumption that glycidamide or ACR, respectively, were the active agent. Under the U.S. EPA guidelines (2005), when a chemical produces rodent tumors by a nonlinear or threshold mode of action, an RfD is calculated using the most relevant POD and application of uncertainty factors. The RfD was estimated to be 1.5 microg/kg/day based on the use of the area under the curve (AUC) for ACR hemoglobin adducts under the assumption that the parent, ACR, is the proximate carcinogen in rodents by a nonlinear mode of action. When the mode of action in assumed to be linear in the low-dose region, a risk-specific dose corresponding to a specified level of risk (e.g., 1 x 10-5) is estimated, and, in the case of ACR, was 9.5 x 10-2 microg ACR/kg/day based on the use of the AUC for glycidamide adduct data. However, it should be noted that although this review was intended to be comprehensive, it is not exhaustive, as new data are being published continuously.

Effect of c-neu/ ErbB2 Expression Levels on Estrogen Receptor α–Dependent Proliferation in Mammary Epithelial Cells: Implications for Breast Cancer Biology

Mammary development and tumorigenesis are profoundly influenced by signaling pathways under the control of c-erbB2/c-neu and estrogen receptor alpha (ERalpha). Signaling through ERalpha is essential for ductal growth during puberty. In mice overexpressing wild-type c-neu in mammary epithelial cells, Tg (c-neu), ductal growth is impaired. An impeded signaling through ERalpha is also observed in a subset of human mammary tumors that overexpress erbB2. However, ductal growth is also impaired in the absence of c-neu in mouse mammary epithelial cells. To resolve this apparent paradox, we examined the relationship between c-neu expression and estrogen/ERalpha-dependent cell proliferation in pubertal Tg (c-neu). We report that proliferation in both terminal end buds and ducts is associated with ERalpha-positive cells, including those that coexpress c-neu, and is abolished in the absence of circulating estradiol. Tg (c-neu) contains hyperplastic mammary ducts with high proliferative index and coexpression of both ERalpha and c-neu in the dividing cells. These findings suggest that c-neu promotes ERalpha-dependent proliferation, and that this is responsible for the presence of hyperplastic ducts. Some of the hyperplastic ducts have acinar structures, indicative of morphologic differentiation. These ducts have low proliferative index and accompanied by a vast decrease in proliferation of ERalpha-positive cells, including those that express c-neu. As such, c-neu has dual but opposing effects on ERalpha-dependent proliferation in mammary epithelial cells. Therefore, depending on the physiologic setting, ductal morphogenesis will be compromised both in the absence and overexpression of c-neu, thus explaining the paradox.

Spontaneous and irradiation-induced tumor susceptibility in BRCA2 germline mutant mice and cooperative effects with a p53 germline mutation

Mutations in both p53 and BRCA2 are commonly seen together in human tumors suggesting that the loss of both genes enhances tumor development. To elucidate this interaction in an animal model, mice lacking the carboxy terminal domain of Brca2 were crossed with p53 heterozygous mice. Females from this intercross were then irradiated with an acute dose of 5 Gy ionizing radiation at 5 weeks of age and compared to nonirradiated controls. We found decreased survival and timing of tumor onsets, and significantly higher overall tumor incidences and prevalence of particular tumors, including stomach tumors and squamous cell carcinomas, associated with the homozygous loss of Brca2, independent of p53 status. The addition of a p53 mutation had a further impact on overall survival, incidence of osteosarcomas and stomach tumors, and tumor latency. The spectrum of tumors observed for this Brca2 germline mouse model suggest that it faithfully recapitulates some human disease phenotypes associated with BRCA2 loss. In addition, these findings include extensive in vivo data demonstrating that germline Brca2 and p53 mutations cooperatively affect animal survivals, tumor susceptibilities, and tumor onsets.

Prolactin potentiates transforming growth factor alpha induction of mammary neoplasia in transgenic mice

Prolactin influences mammary development and carcinogenesis through endocrine and autocrine/paracrine mechanisms. In virgin female mice, pro-lactin overexpression under control of a mammary selective nonhormonally responsive promoter, neu-related lipocalin, results in estrogen receptor alpha (ERalpha)-positive and ERalpha-negative adenocarcinomas. However, disease in vivo occurs in the context of dysregulation of multiple pathways. In this study, we investigated the ability of prolactin to modulate carcinogenesis when co-expressed with the potent oncogene transforming growth factor alpha (TGFalpha) in bitransgenic mice. Prolactin and TGFalpha cooperated to reduce dramatically the latency of mammary macrocyst development, the principal lesion type induced by TGFalpha. In combination, prolactin and TGFalpha also increased the incidence and reduced the latency of other preneoplastic lesions and increased cellular turnover in structurally normal alveoli and ducts compared with single transgenic females. Bitransgenic glands contained higher levels of phosphorylated ERK1/2 compared with single TGFalpha transgenic glands, suggesting that this kinase may be a point of signaling crosstalk. Furthermore, transgenic prolactin also reversed the decrease in ERalpha induced by neu-related lipocalin-TGFalpha. Our findings demonstrate that locally produced prolactin can strikingly potentiate the carcinogenic actions of another oncogene and modify ovarian hormone responsiveness, suggesting that prolactin signaling may be a potential therapeutic target.

Mammary ductal morphogenesis requires paracrine activation of stromal EGFR via ADAM17-dependent shedding of epithelial amphiregulin

Epithelial-mesenchymal crosstalk is essential for tissue morphogenesis, but incompletely understood. Postnatal mammary gland development requires epidermal growth factor receptor (EGFR) and its ligand amphiregulin (AREG), which generally must be cleaved from its transmembrane form in order to function. As the transmembrane metalloproteinase ADAM17 can process AREG in culture and Adam17(-/-) mice tend to phenocopy Egfr(-/-) mice, we examined the role of each of these molecules in mammary development. Tissue recombination and transplantation studies revealed that EGFR phosphorylation and ductal development occur only when ADAM17 and AREG are expressed on mammary epithelial cells, whereas EGFR is required stromally, and that local AREG administration can rescue Adam17(-/-) transplants. Several EGFR agonists also stimulated Adam17(-/-) mammary organoid growth in culture, but only AREG was expressed abundantly in the developing ductal system in vivo. Thus, ADAM17 plays a crucial role in mammary morphogenesis by releasing AREG from mammary epithelial cells, thereby eliciting paracrine activation of stromal EGFR and reciprocal responses that regulate mammary epithelial development.

Retinoids arrest breast cancer cell proliferation: retinoic acid selectively reduces the duration of receptor tyrosine kinase signaling

Retinoic acid (RA) induces cell cycle arrest of hormone-dependent human breast cancer (HBC) cells. Previously, we demonstrated that RA-induced growth arrest of T-47D HBC cells required the activity of the RA-induced protein kinase, protein kinase Calpha (PKCalpha) [J. Cell Physiol. 172 (1997) 306]. Here, we demonstrate that RA treatment of T-47D cells interfered with growth factor signaling to downstream, cytoplasmic and nuclear targets. RA treatment did not inhibit epidermal growth factor (EGF) receptor activation but resulted in rapid inactivation. The lack of sustained EGFR activation was associated with transient rather than sustained association of the EGFR with the Shc adaptor proteins and activation of Erk 1/2 and with compromised induction of expression of immediate early response genes. Inhibiting the activity of PKCalpha, a retinoic acid-induced target gene, prevented the effects of RA on cell proliferation and EGF signaling. Constitutive expression of PKCalpha, in the absence of RA, decreased cell proliferation and decreased EGF signaling. RA treatment increased steady-state levels of the protein tyrosine phosphatase PTP-1C and all measured effects of RA on EGF receptor function were reversed by the tyrosine phosphate inhibitor orthovanadate. These results indicate that RA-induced target genes, particularly PKCalpha, prevent sustained growth factor signaling, uncoupling activated receptor tyrosine kinases and nuclear targets that are required for cell cycle progression.

Dominant negative Ras enhances lactogenic hormone-induced differentiation by blocking activation of the Raf-Mek-Erk signal transduction pathway

Epidermal growth factor (EGF) and Ras mitogenic signal transduction pathways are frequently activated in breast carcinoma and inhibit mammary differentiation and apoptosis. HC11 mouse mammary epithelial cells, which differentiate and synthesize beta-casein following growth to confluency and stimulation with lactogenic hormones, were used to study EGF-dependent signaling during differentiation. Blocking Mek-Erk or phosphotidylinositol-3-kinase (PI-3 kinase) signaling with specific chemical inhibitors enhanced beta-casein promotor-driven luciferase activity. Because EGF stimulation of HC11 cells resulted in the activation of Ras, the effect of activated Ras (RasV12) or dominant negative (DNRasN17) on lactogen induced differentiation was examined. HC11 cell lines expressing RasV12 or DNRasN17 under the control of a tetracycline (tet)-responsive promotor were constructed. Activated RasV12 expression resulted in reduced tyrosine phosphorylation of Stat5 and a delay in beta-casein expression in response to prolactin. However, the expression of tet-regulated DNRasN17 and adenovirus-encoded DNRasN17 enhanced Stat5 tyrosine phosphorylation, Stat5 DNA binding, and beta-casein transcription. The expression of DNRasN17 blocked the activation of the Mek-Erk pathway by EGF but did not prevent the phosphorylation of AKT, a measure of activation of the PI-3-kinase pathway. Moreover, the expression of DNRasN17 prevented the block to lactogenic differentiation induced by EGF. Stimulation of HC11 cells with prolactin resulted in the association of the SHP2 phosphatase with Stat5, and this association was prevented by DNRasN17 expression. These results demonstrate that in HC11 cells DNRas inhibits the Mek-Erk pathway and enhances lactogenic hormone-induced differentiation. This occurs, in part, by inhibiting the association of the SHP2 phosphatase with Stat5.

Bax regulates c-Myc-induced mammary tumour apoptosis but not proliferation in MMTV-c-myc transgenic mice

The expression of the proto-oncogene c-myc is frequently deregulated, via multiple mechanisms, in human breast cancers. Deregulated expression of c-myc contributes to mammary epithelial cell transformation and is causally involved in mammary tumorigenesis in MMTV-c-myc transgenic mice. c-Myc is known to promote cellular proliferation, apoptosis, genomic instability and tumorigenesis in several distinct tissues, both in vivo and in vitro. Expression of the proapoptotic regulatory gene bax is reduced or absent in human breast cancers, and c-Myc has been shown to regulate the expression of Bax, as well as cooperate with Bax in controlling apoptosis in a fibroblast model. Additionally, loss of bax reduces c-Myc-induced apoptosis in lymphoid cells and increases c-Myc-mediated lymphomagenesis in vivo. In order to assess whether loss of bax could influence c-Myc-induced apoptosis and tumorigenesis in the mammary gland in vivo, we generated MMTV-c-myc transgenic mice in which neither, one, or both wild-type alleles of bax were eliminated. Haploid loss of bax in MMTV-c-myc transgenic mice resulted in significantly reduced mammary tumour apoptosis. As anticipated for an apoptosis-regulatory gene, loss of the wild-type bax alleles did not significantly alter cellular proliferation in either mammary adenocarcinomas or dysplastic mammary tissues. However, in contrast to c-Myc-mediated lymphomagenesis, loss of one or both alleles of bax in MMTV-c-myc transgenic mice did not significantly enhance mammary tumorigenesis, despite evidence that haploid loss of bax might modestly increase mammary tumour multiplicity. Our results demonstrate that Bax contributes significantly to c-Myc-induced apoptosis in mammary tumours. In addition, they suggest that in contrast to c-Myc-induced lymphomagenesis, mammary tumorigenesis induced by deregulated c-myc expression requires some amount of Bax expression.

Role of transforming growth factor β in breast carcinogenesis

Transforming growth factor (TGF) beta is a pre-eminent negative growth regulator that has antiproliferative effects on a range of epithelial cells. This ability has evoked interest in this growth factor as a tumour suppressor with potential clinical significance. In the early stages of breast carcinogenesis, a growth-inhibitory response to TGFbeta is maintained, which depends on an intact TGFbeta signalling pathway. Tumour development and progression of cells along a neoplastic continuum is accompanied by loss of this growth-inhibitory response to TGFbeta, which might instead promote tumour growth indirectly through a combination of permissive effects on stromal tissue, angiogenesis, and the immune system. This review discusses the complexity of functional pleiotropy and the continually changing roles of TGFbeta as a tumour evolves, along with competing therapeutic strategies. The boosting of local endogenous amounts of TGFbeta in conjunction with enhancement of cellular responsiveness might be appropriate in early malignant disease, and anti-TGFbeta approaches could yield a therapeutic gain in metastatic states.

An in vitro model of epithelial cell growth stimulation in the rodent mammary gland

Mouse mammary epithelial cell cultures previously described bring about extensive proliferation and a cell population with the appropriate markers for luminal ductal epithelial cells, and also the ability to form normal tissue after implantation into mice. This success may result from a culture environment that resembles certain aspects of the environment in the mammary gland. Mouse mammary epithelial cells, whose proliferation is limited when plated alone, can be stimulated to multiply by contact with lethally irradiated cells of the LA7 rat mammary tumour line. Most of the proliferative stimulus is imparted by direct cell contact between LA7 and mouse mammary cells. Junctions, including adherens junctions, form among all cells in the culture, much as junctions form in the mammary gland. LA7 cells secrete TGFalpha and bFGF, factors found in the mammary gland, and factors to which mouse mammary cells respond in culture. Mouse mammary cells express keratins 8 and 18, markers for luminal cells of the mammary duct. LA7 cells express keratin 14 and vimentin, markers for myoepithelial cells. These facts, taken together, fit a model of cell replacement in an epithelial tissue and also imitate the relationship between luminal ductal cells and myoepithelial cells in the mammary gland. This method of culturing cells is useful, not only for in vitro-in vivo carcinogenesis studies, but also for the study of mechanisms by which growth signals are imparted from one cell to another.

Effect of exogenous epidermal-like growth factors on mammary gland development and differentiation in the estrogen receptor-alpha knockout (ERKO) mouse

The development of the mouse mammary gland requires the interaction between several different ovarian and pituitary hormones such as estrogen, progesterone and prolactin as well as several locally-derived growth factors in the mammary gland such as epidermal growth factor (EGF), transforming growth factor alpha (TGFalpha), amphiregulin (AR) and heregulin (HRG). The focus of this study was to investigate the degree of mammary growth and differentiation in the adult, virgin mammary gland of wild type (wt) and estrogen receptor knockout (ERKO) females that lack estrogen receptor alpha (ERalpha) after reciprocal transplantation into the cleared mammary fat pad of virgin wt or ERKO mice. In addition, we assessed the local response of ERKO mammary tissue to TGFalpha or HRGbeta1 delivered from slow release-Elvax pellets. Our initial results indicated that when we transplanted virgin wt mammary tissue into ERKO mammary fat pads, mammary morphogenesis failed to occur. However, when transplanted virgin ERKO mammary tissue was transplanted into fat pads of virgin or pregnant wt mice, the development and differentiation of lobuloalveoli was readily observed. In addition, treatment of the virgin ERKO mammary gland with TGFalpha or HRGbeta1 stimulated ducts to undergo localized branching and growth and both growth factors induced secretory differentiation as evidenced by the production of milk proteins, caseins and/or whey acidic protein (WAP). The results from this study imply that in ERKO mammary tissue. ERKO ductal epithelium has the capacity to proliferate and differentiate in response to non-estrogenic, morphogenic stimuli.

The role of hepatocyte growth factor (scatter factor) in epithelial-mesenchymal transition and breast cancer

North American women have a one in eight lifetime risk of developing breast cancer, and approximately one in three women with breast cancer will die of metastases. We, and others, have recently shown that high levels of expression of hepatocyte growth factor (HGF) and its receptor Met are associated with invasive human breast cancer and may be causally linked to metastasis. This high level of HGF and Met expression has been considered as a possible indicator of earlier recurrence and shortened survival in breast cancer patients. In contrast, HGF expression (but not Met) is strongly suppressed in normal breast epithelial cells. HGF and Met are therefore candidate targets for therapeutic intervention in the treatment of breast cancer. We have recently demonstrated that sustained activation or hyper-activation of c-Src and Stat3, which occurs in invasive breast cancer, can stimulate strong expression of HGF in carcinoma cells. In contrast, transient induction of Stat3 occurs in normal epithelium and promotes mammary tubulogenesis. We hypothesize that increased autocrine HGF-Met signaling is a critical downstream function of c-Src-Stat3 activation in mammary tumorigenesis. Future studies will identify novel Stat3 consensus sites that regulate HGF promoter activity and HGF expression preferentially in carcinoma cells and could lead to novel therapeutic drugs that specifically block HGF expression in mammary carcinoma cells, and which could be used in combined treatments to abrogate metastasis.

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.

Detection and localization of Cripto-1 binding in mouse mammary epithelial cells and in the mouse mammary gland using an immunoglobulin-cripto-1 fusion protein

Human Cripto-1 (CR-1), a member of the epidermal growth factor-CFC (EGF-CFC) family of peptides, is expressed in the developing mouse mammary gland and can modulate mammary epithelial cell migration, branching morphogenesis and milk protein expression in vitro. In order to screen for a CR-1 receptor and to identify potential CR-1 target tissues, we constructed a fusion protein comprising the EGF-like domain of CR-1 and the Fc domain of a human IgG1. The recombinant CR-1 fusion protein (CR-1-Fc) was biologically active as it was able to activate the ras/raf/mitogen activated protein kinase (MAPK) pathway and to inhibit transcription of the milk protein beta-casein in NMuMG and HC-11 mouse mammary epithelial cells. By using immunocytochemistry and by an in situ enzyme-linked immunosorbent assay (ELISA), CR-1-Fc was found to specifically bind to NMuMG and HC-11 cells. Finally, immunohistochemical analysis using CR-1-Fc showed a specific localization of CR-1 binding to tissue sections from mouse mammary gland. In particular, more than 60% of the epithelial cells were intensely stained with the CR-1-Fc fusion protein in the lactating mouse mammary gland, whereas approximately 25% of the mammary epithelial cells were stained in the gland from pregnant mouse. Since expression of mouse cripto-1 (Cr-1) in the pregnant and lactating mouse mammary gland as well as its presence in milk has been previously demonstrated, these data strongly suggest that an autocrine pathway involving Cr-1 and its putative receptor is operating in the mouse mammary gland during pregnancy and lactation.

Establishing a framework for the functional mammary gland: from endocrinology to morphology

From its embryonic origins, the mammary gland in females undergoes a course of ductal development that supports the establishment of alveolar structures during pregnancy prior to the onset of lactogenesis. This development includes multiple stages of proliferation and morphogenesis that are largely directed by concurrent alterations in key hormones and growth factors across various reproductive states. Ductal elongation is directed by estrogen, growth hormone, insulin-like growth factor-I, and epidermal growth factor, whereas ductal branching and alveolar budding is influenced by additional factors such as progesterone, prolactin, and thyroid hormone. The response by the ductal epithelium to various hormones and growth factors is influenced by epithelial-stromal interactions that differ between species, possibly directing species-specific morphogenesis. Evolving technologies continue to provide the opportunity to further delineate the regulation of ductal development. Defining the hormonal control of ductal development should facilitate a better understanding of the mechanisms underlying mammary gland tumorigenesis.

Expression, action, and regulation of transforming growth factor alpha and epidermal growth factor receptor during embryonic and perinatal rat testis development

The objective of the current study was to extend previous observations and examine the expression pattern and effects of transforming growth factor alpha (TGFalpha) and epidermal growth factor receptor (EGFR) on embryonic testis morphogenesis and growth. The expression of TGFalpha was determined after morphological sex determination (seminiferous cord formation at embryonic day 13 [ED13]) through perinatal testis development (postnatal day 5 [PD5]) with a quantitative reverse transcription-polymerase chain reaction procedure. Expression of messenger RNA (mRNA) for TGFalpha appeared to be more dynamic during testis development when compared with the expression of mRNA for EGFR. Message for TGFalpha was reduced at ED16 and PD4, and was elevated at PD0 during testis development. In contrast, EGFR mRNA levels were negligible at ED15 and were elevated constitutively from ED16 through PD5. Immunohistochemistry was conducted at ED14, ED16, ED19, PD0, PD3, and PD5 to localize cellular expression of both TGFalpha and EGFR. At ED16, positive staining for EGFR was localized to the cords, and by ED19, was mainly in the cords with slight expression in the interstitium. From PD0 to PD5, positive staining for EGFR was detected in the germ, Sertoli, and interstitial cells. Immunohistochemistry for TGFalpha detected localization at ED14 and ED16 to the Sertoli cells and to specific cells in the interstitium. From ED19 through PD5, TGFalpha was detected in the Sertoli, germ, and interstitial cells, and in endothelial cells within the interstitium. To determine the effects of TGFalpha on embryonic testis growth and seminiferous cord formation, ED13 testis organ cultures were treated with sense and antisense TGFalpha oligonucleotides. Antisense TGFalpha inhibited testis growth by 25%-30% in ED13 testis organ cultures when compared with sense oligonucleotide control pairs. To examine the effects of TGFalpha on perinatal testis growth, PD0 testis cultures were treated with different doses of TGFalpha. TGFalpha increased thymidine incorporation into DNA in PD0 testis cultures. Therefore, TGFalpha appears to have actions on both embryonic and perinatal testis growth. The regulation of TGFalpha and EGFR mRNA levels were examined using PD0 testis cultures treated with hormones that stimulate testis growth. Follicle-stimulating hormone (FSH) stimulated (P < .05) and testosterone tended to stimulate (P < .07) mRNA expression of EGFR. Epidermal growth factor stimulation of PD0 testis cultures did not affect levels of mRNA expression for EGFR, but did suppress expression of mRNA for TGFalpha. These results taken together demonstrate that TGFalpha can act to regulate early embryonic and perinatal testis growth. Furthermore, TGFalpha and EGFR expression can be regulated through growth stimulatory hormones such as FSH and testosterone.

Estrogen receptor alpha mediated induction of the transforming growth factor alpha gene by estradiol and 4-hydroxytamoxifen in MDA-MB-231 breast cancer cells

The selective estrogen receptor modulator, 4-hydroxytamoxifen (4-OHT) is a full agonist at the transforming growth factor (TGF) alpha gene in ER negative breast cancer cells stably transfected with ER alpha cDNA (Levenson et al., Br. J. Cancer 77 (1998) 1812-1819). E(2) and 4-OHT increase TGF alpha mRNA and protein in a concentration dependent manner. The responses to E(2) and 4-OHT are blocked by the pure antiestrogen ICI 182,780, which does not induce TGF alpha. Transfected MDA-MB-231 cells contain functional ER alpha but no ER beta function was detected. Neo transfected cells that did not express ER alpha or cells stably transfected with the DNA binding domain mutant C202R/E203V which prevents gene activation did not induce TGF alpha mRNA after either E(2) or 4-OHT treatment. An examination of the time course for either 10 nM E(2) or 1 microM 4-OHT for MDA-MB-231 cells stably transfected with cDNA for ER alpha showed increases in TGF alpha mRNA within 2 or 3 h respectively. Cells pretreated with cycloheximide (1 microg/ml) showed induced TGF alpha mRNA in response to E(2) or 4-OHT but TGF alpha mRNA induction was blocked by actinomycin D (1 microg/ml). We conclude that both E(2) and 4-OHT induce TGF alpha by direct interaction of ER alpha with DNA and that ER beta is not involved in the estrogen-like response to 4-OHT in the MDA-MB-231 cells.

Estrogen dependent expression of the receptor tyrosine kinase axl in normal and malignant human breast

BACKGROUND

Axl, a member of a family of receptor tyrosine kinases characterized by an extracellular domain resembling cell adhesion molecules and an intracellular conserved tyrosine kinase domain has been reported to induce cell proliferation and transformation. In mice, axl is expressed in the normal mammary gland and over-expressed in aggressive mammary tumors.

PATIENTS AND METHODS

We have investigated the expression of axl immunohistochemically in 23 normal human breast samples and in 111 consecutive breast carcinomas. Expression of axl was correlated with tumour characteristics (lymph node involvement, stage, grade) and immunohistochemical expression of ER, PR, Ki-67 and c-erbB-2.

RESULTS

In normal tissue, axl localizes to the membrane of breast epithelial cells. Axl protein shows membrane associated staining in high correlation (P = 0.004) with the expression of the estrogen receptor (ER). Axl expression was found in a subset of breast carcinomas and was also correlated with high significance (P < 0.0001) with the presence of ER.

CONCLUSION

Our results suggest that axl may serve as a mediator of estrogen stimulation preventing the completion of the breast epithelial life cycle and that estrogen induced axl expression may give a survival signal to cancerous cells, preventing them from dying through apoptosis.

Characterization of bipotent mammary epithelial progenitor cells in normal adult human breast tissue

The purpose of the present study was to characterize primitive epithelial progenitor populations present in adult normal human mammary tissue using a combination of flow cytometry and in vitro colony assay procedures. Three types of human breast epithelial cell (HBEC) progenitors were identified: luminal-restricted, myoepithelial-restricted and bipotent progenitors. The first type expressed epithelial cell adhesion molecule (EpCAM), alpha6 integrin and MUC1 and generated colonies composed exclusively of cells positive for the luminal-associated markers keratin 8/18, keratin 19, EpCAM and MUC1. Bipotent progenitors produced colonies containing a central core of cells expressing luminal markers surrounded by keratin 14+ myoepithelial-like cells. Single cell cultures confirmed the bipotentiality of these progenitors. Their high expression of alpha6 integrin and low expression of MUC1 suggests a basal position of these cells in the mammary epithelium in vivo. Serial passage in vitro of an enriched population of bipotent progenitors demonstrated that only myoepithelial-restricted progenitors could be readily generated under the culture conditions used. These results support a hierarchical branching model of HBEC progenitor differentiation from a primitive uncommitted cell to luminal- and myoepithelial-restricted progenitors.

Hepatocyte growth factor and neuregulin in mammary gland cell morphogenesis

Organ culture and transplantation experiments in the early 1960s and 1970s have demonstrated that growth and morphogenesis of the epithelium of the mammary gland are controlled by mesenchymal-epithelial interactions. The identification of molecules that provide the essential signals exchanged in mesenchymal-epithelial interactions is an area of active research. Recent evidence suggests that morphogenic programs of epithelia can be triggered by mesenchymal factors that signal via tyrosine kinase receptors. This review concentrates on the effects of two mesenchymal factors, Hepatocyte Growth Factor/Scatter Factor and neuregulin, on morphogenesis and differentiation of mammary epithelial cells in vitro and signalling pathways involved during morphogenesis of mammary epithelial cells.

Prolactin, growth hormone, and epidermal growth factor activate Stat5 in different compartments of mammary tissue and exert different and overlapping developmental effects

Prolactin (Prl)-induced phosphorylation of Stat (signal transducer and activator of transcription) 5 is considered a key event in functional mammary development and differentiation. We now demonstrate that not only Prl, but also growth hormone (GH) and epidermal growth factor (EGF), can activate Stat5 in mammary tissue. We investigated the roles of these hormones in mammary development using mice in which the respective receptors had been inactivated. Although Prl receptor (PrlR)-null mice are infertile, we were able to maintain pregnancies in a few mice by treatment with progesterone. Mammary tissue in these mice was severely underdeveloped and exhibited limited differentiation as assessed by the phosphorylation status of Stat5 and the expression of milk protein genes. PrlR +/- mice showed impaired mammary development and alveolar differentiation during pregnancy, which corresponded with reduced phosphorylation levels of Stat5a and 5b, and impaired expression of milk protein genes. Development of the glands in these mice was arrested at around day 13 of pregnancy. While Prl activated Stat5 only in the epithelium, GH and EGF activated Stat5 preferentially in the stroma. To assess the relevance of the GH receptor (GHR) in the mammary gland, we transplanted GHR-null epithelium into cleared fat pads of wild-type mice. These experiments demonstrated that the GHR in the epithelium is not required for functional mammary development. Similarly, the EGFR in the epithelium is not required for alveolar development. In contrast, epithelial PrlR is required for mammary development and milk protein gene expression during pregnancy. Although GH is not required for alveolar development, we were able to demonstrate its lactogenic function in cultured mammary epithelium from PrlR-null mice. However, ductal development in GHR-null mice was impaired, supporting the notion that GH signals through the stromal compartment. Our findings demonstrate that GH, Prl, and EGF activate Stat5 in separate compartments, which in turn reflects their specific roles in ductal and alveolar development and differentiation.

Maternal and prepubertal diet, mammary development and breast cancer risk

At present, we do not know what causes sporadic breast cancer. Environmental factors,particularly diet, appear to explain at least 70% of newly diagnosed breast cancers, but it is not clear what these factors are. We propose that the lack of progress in this area is due to a lack of considering the effect of timing of environmental and dietary exposures on the breast. The evidence provided above suggests that an in utero exposure to an estrogenic environment-including that caused by diet [high (n-6) PUFA or genistein]-increases breast cancer risk. This increase may be mediated by an increased presence of TEB in the mammary epithelial tree and increased ER-alpha levels, reduced ER-beta levels or both. Prepubertal estrogenic exposure, in contrast, reduces later risk of developing breast cancer. The protective effect of estrogens may be mediated by early epithelial differentiation, reduced presence of ER-alpha and increased levels of ER-beta in the mammary gland. The challenge we are now facing is to determine whether the data obtained mainly through the use of animal models is relevant to women and if so, how we might be able to modulate pregnancy and childhood estrogenic exposure by appropriate dietary modifications to reduce breast cancer risk in women.

Ductal growth is impeded in mammary glands of C-neu transgenic mice

The steroid hormone, estradiol, is essential for both the growth of normal breast and induction of mammary carcinomas. The growth promoting effects of estrogen are presumed to be mediated by growth factors, in particular, epidermal growth factor, which mediates its effects through erbB receptors, erbB1 and erbB2/C-neu. C-neu is amplified and over-expressed in a large number of human cancers and transgenic mice over-expressing C-neu also develop mammary tumors. However, as yet, the impact of C-neu over-expression on estrogen action during normal mammary development and hence, its precise role in carcinogenesis, remains unclear. In the present studies, we demonstrate that estradiol-dependent mammary ductal growth accompanying puberty is impaired in transgenic mice expressing wild type Cneu, and is intrinsic to the tissue. The impairment is not due to an overall impairment in estrogen action, since progesterone receptor expression is unaffected in C-neu mice. It is also not due to an intrinsic inability of the epithelial cells to proliferate, since impeded ductal growth co-exists with alveolar growth during pregnancy. Therefore, we propose that, depending on the physiological state, C-neu may either promote or inhibit the growth of mammary epithelial cells, and discuss its potential significance to carcinogenesis.

What role(s) for TGFalpha in the central nervous system?

Transforming growth factor alpha (TGFalpha) is a member of the epidermal growth factor (EGF) family with which it shares the same receptor, the EGF receptor (EGFR or erbB1). Identified since 1985 in the central nervous system (CNS), its functions in this organ have started to be determined during the past decade although numerous questions remain unanswered. TGFalpha is widely distributed in the nervous system, both glial and neuronal cells contributing to its synthesis. Although astrocytes appear as its main targets, mediating in part TGFalpha effects on different neuronal populations, results from different studies have raised the possibility for a direct action of this growth factor on neurons. A large array of experimental data have thus pointed to TGFalpha as a multifunctional factor in the CNS. This review is an attempt to present, in a comprehensive manner, the very diverse works performed in vitro and in vivo which have provided evidences for (i) an intervention of TGFalpha in the control of developmental events such as neural progenitors proliferation/cell fate choice, neuronal survival/differentiation, and neuronal control of female puberty onset, (ii) its role as a potent regulator of astroglial metabolism including astrocytic reactivity, (iii) its neuroprotective potential, and (iv) its participation to neuropathological processes as exemplified by astroglial neoplasia. In addition, informations regarding the complex modes of TGFalpha action at the molecular level are provided, and its place within the large EGF family is precised with regard to the potential interactions and substitutions which may take place between TGFalpha and its kindred.

Estrogen-induced activation of Erk-1 and Erk-2 requires the G protein-coupled receptor homolog, GPR30, and occurs via trans-activation of the epidermal growth factor receptor through release of HB-EGF

Estrogen rapidly activates the mitogen-activated protein kinases, Erk-1 and Erk-2, via an as yet unknown mechanism. Here, evidence is provided that estrogen-induced Erk-1/-2 activation occurs independently of known estrogen receptors, but requires the expression of the G protein-coupled receptor homolog, GPR30. We show that 17beta-estradiol activates Erk-1/-2 not only in MCF-7 cells, which express both estrogen receptor alpha (ER alpha) and ER beta, but also in SKBR3 breast cancer cells, which fail to express either receptor. Immunoblot analysis using GPR30 peptide antibodies showed that this estrogen response was associated with the presence of GPR30 protein in these cells. MDA-MB-231 breast cancer cells (ER alpha-, ER beta+) are GPR30 deficient and insensitive to Erk-1/-2 activation by 17beta-estradiol. Transfection of MDA-MB-231 cells with a GPR30 complementary DNA resulted in overexpression of GPR30 protein and conversion to an estrogen-responsive phenotype. In addition, GPR30-dependent Erk-1/-2 activation was triggered by ER antagonists, including ICI 182,780, yet not by 17alpha-estradiol or progesterone. Consistent with acting through a G protein-coupled receptor, estradiol signaling to Erk-1/-2 occurred via a Gbetagamma-dependent, pertussis toxin-sensitive pathway that required Src-related tyrosine kinase activity and tyrosine phosphorylation of tyrosine 317 of the Shc adapter protein. Reinforcing this idea, estradiol signaling to Erk-1/-2 was dependent upon trans-activation of the epidermal growth factor (EGF) receptor via release of heparan-bound EGF (HB-EGF). Estradiol signaling to Erk-1/-2 could be blocked by: 1) inhibiting EGF-receptor tyrosine kinase activity, 2) neutralizing HB-EGF with antibodies, or 3) down-modulating HB-EGF from the cell surface with the diphtheria toxin mutant, CRM-197. Our data imply that ER-negative breast tumors that continue to express GPR30 may use estrogen to drive growth factor-dependent cellular responses.

Hormone-dependent nuclear localization of the tyrosine kinase iyk in the normal human breast epithelium and loss of expression during carcinogenesis

iyk, a member of the frk family of non-receptor tyrosine kinases, was originally isolated from normal mouse mammary glands and is characterized by a nuclear localizing signal within the SH2 domain. We have investigated the expression and subcellular localization of iyk in the normal human breast and in malignant breast diseases. Immuno-histochemical analyses revealed that in normal tissue iyk localizes to both cytoplasmic and nuclear compartments of breast epithelial cells. The subcellular distribution was dependent on the hormonal state, being mostly cytoplasmic during the follicular, proliferative phase of the menstrual cycle, whereas frequent nuclear staining was observed in the resting stages during the luteal phase and, most prominently, after menopause. Strikingly, invasive carcinomas, irrespective of tumor type or hormonal status of the patient, exhibited almost complete loss of iyk expression in both the cytoplasm and the nucleus. In contrast, in situ breast carcinomas from post-menopausal patients showed a clear reduction of the nuclear iyk localization while retaining cytoplasmic staining. Our results indicate that iyk expression is gradually lost during carcinogenesis; thus, iyk may be classified as a tumor-suppressor gene.

Role of transforming growth factor-alpha and the epidermal growth factor receptor in embryonic rat testis development

Embryonic testis development requires the morphogenesis of cords and growth of all cell populations to allow organ formation. It is anticipated that coordination of the growth and differentiation of various cell types involves locally produced growth factors. The current study was an investigation of the hypothesis that transforming growth factor-alpha (TGF-alpha) is involved in regulating embryonic testis growth. TGF-alpha has previously been shown to function in the postnatal testis. TGF-alpha and other members of the epidermal growth factor (EGF) family act through the epidermal growth factor receptor (EGFR) to stimulate cell proliferation and tissue morphogenesis. To understand the potential actions of TGF-alpha in the embryonic testis, general cell proliferation was investigated. Characterization of cell proliferation in the rat testis throughout embryonic and postnatal development indicated that each cell type has a distinct pattern of proliferation. Germ cell growth was transiently suppressed around birth. Interstitial cell growth was high embryonically and decreased to low levels around birth. A low level of Sertoli cell proliferation was observed at the onset of testis cord formation. Sertoli cell proliferation in early embryonic development was low; the levels were high later in embryonic development and remained high until the onset of puberty. Both TGF-alpha and the EGFR were shown to be expressed in the embryonic and postnatal rat and mouse testis. Perturbation of TGF-alpha function using neutralizing antibodies to TGF-alpha on testis organ cultures dramatically inhibited the growth of both embryonic and neonatal testis. TGF-alpha antibodies had no effect on cord formation. The TGF-alpha antibody was found to be specific for TGF-alpha in Western blots when compared to EGF and heregulin. Testis growth was also inhibited by perturbation of EGFR signaling using an EGFR kinase inhibitor. Therefore, TGF-alpha appears to influence embryonic testis growth but not morphogenesis (i.e., cord formation). Treatment of embryonic testis organ cultures with exogenous TGF-alpha also perturbed development, leading to an increased proliferation of unorganized cells. Testis from EGFR and TGF-alpha knockout mice were analyzed for testis morphology. TGF-alpha knockout mice had no alterations in testis phenotype, while EGFR knockout mice had a transient decrease in the relative amount of interstitial cells before birth. Observations suggest that there may be alternate or compensatory factors that allow testis growth to occur in the apparent absence of TGF-alpha actions in the mutant mice. In summary, the results obtained suggest that TGF-alpha is an important factor in the regulation of embryonic testis growth, but other factors will also be involved in the process.

Dual regulation of proliferation and apoptosis: c-myc in bitransgenic murine mammary tumor models

Recent progress in the study of c-Myc has convincingly demonstrated that it possesses a dual role in regulating both proliferation and apoptosis; however, the manner in which c-Myc influences these cellular response pathways remains incompletely characterized. Deregulation of c-Myc expression, via many mechanisms, is a common feature of multiple cancers and is an especially prominent feature of many breast cancers. Of significant interest to those who study mammary gland development and neoplasia is the unresolved nature and contribution of apoptosis to breast tumorigenesis. Recently, the use of transgenic mice and gene-knockout mice has allowed investigators to evaluate the pathological mechanisms by which different genes influence tumor development and progression. In this review, we address two distinct c-myc-containing bitransgenic murine mammary tumor models and discuss the contribution and possible future directions for resolution of cancer-relevant molecular pathways influenced by c-Myc.

Transforming growth factor alpha and mouse models of human breast cancer

Transforming growth factor alpha (TGFalpha) is a principal molecule in the normal and neoplastic development of the mammary gland. Binding of TGFalpha to the epidermal growth factor receptor (EGFR), activates the EGFRs' endogenous tyrosine kinase activity and stimulates growth of the epithelium in the virgin and pregnant mouse mammary gland. TGFalpha expression can be detected in breast cancer cells in vivo and in vitro and overexpression can elicit partial transformation or immortalized human and rodent mammary epithelial cells. Despite evidence implicating TGFalpha in the development of mammary neoplasia, the actual mechanism of TGFalpha-induced transformation is unclear. Transgenic mouse models targeting heterologus TGFalpha to the mammary gland have established TGFalpha overexpression can induce hyperproliferation, hyperplasia and occasional carcinoma. These transgenic studies demonstrated a facilitating, proliferative role for TGFalpha in the development of neoplasia and implicated several oncogenes that can cooperate with TGFalpha to transform the mammary epithelium. From studies of EGFR signaling pathways, inhibitory and modulating agents such as anti-EGFR antibodies and specific kinases inhibitors have been used to block the action of this pathway and prevent the development of TGFalpha-induced neoplasia and tumor formation. Studies in Stat5a knockout mice have established that the JAK2/Stat5a pathway can facilitate the survival of the mammary epithelium and can impact the progression of TGFalpha-mandated mammary tumorigenesis. Together these experiments indicate that TGFalpha and the EGFR signaling pathway are potentially amenable to therapies for treatment of human breast disease.

Laminin and beta1 integrins are crucial for normal mammary gland development in the mouse

We have examined the role of integrin-extracellular matrix interactions in the morphogenesis of ductal structures in vivo using the developing mouse mammary gland as a model. At puberty, ductal growth from terminal end buds results in an arborescent network that eventually fills the gland, whereupon the buds shrink in size and become mitotically inactive. End buds are surrounded by a basement membrane, which we show contains laminin-1 and collagen IV. To address the role of cell-matrix interactions in gland development, pellets containing function-perturbing anti-beta1 integrin, anti-alpha6 integrin, and anti-laminin antibodies respectively were implanted into mammary glands at puberty. Blocking beta1 integrins dramatically reduced both the number of end buds per gland and the extent of the mammary ductal network, compared with controls. These effects were specific to the end buds since the rest of the gland architecture remained intact. Reduced development was still apparent after 6 days, but end buds subsequently reappeared, indicating that the inhibition of beta1 integrins was reversible. Similar results were obtained with anti-laminin antibodies. In contrast, no effect on morphogenesis in vivo was seen with anti-alpha6 integrin antibody, suggesting that alpha6 is not the important partner for beta1 in this system. The studies with beta1 integrin were confirmed in a culture model of ductal morphogenesis, where we show that hepatocyte growth factor (HGF)-induced tubulogenesis is dependent on functional beta1 integrins. Thus integrins and HGF cooperate to regulate ductal morphogenesis. We propose that both laminin and beta1 integrins are required to permit cellular traction through the stromal matrix and are therefore essential for maintaining end bud structure and function in normal pubertal mammary gland development.

Human mammary epithelial cells rapidly exchange empty EGFR between surface and intracellular pools

Binding of ligand to the epidermal growth factor receptor (EGFR) initiates a series of processes including activation of the intrinsic EGFR tyrosine kinase, receptor autophosphorylation, and the assembly of active signaling complexes at the plasma membrane. Concomitantly, receptor trafficking is initiated, and the receptor is ultimately delivered to the lysosome, where it is degraded. Virtually all studies on EGFR trafficking have used fibroblasts and transformed cells. Because EGFR exerts a potent effect on the physiology of epithelial cells, we examined the regulation of EGFR activity and trafficking in nontransformed human mammary epithelial cells (HMEC). We found that HMEC that displayed a luminal phenotype were largely unresponsive to EGF and maintained a majority of their EGFR at the cell surface. In contrast, HMEC with a basal phenotype were highly responsive to EGF and, at steady state in the absence of exogenous ligand, distributed empty EGFR into intracellular pools. Maintenance of the intracellular pools was a direct consequence of specific and rapid endocytosis of the empty EGFR. The trafficking pattern was EGFR specific, used coated pits, and did not require receptor tyrosine kinase activity. Such an mechanism redistributes EGFR signaling potential among different membrane domains and into vesicles with unique biochemical microenviroments. In addition, our data show that EGFR endocytosis can be regulated in the absence of ligand binding and receptor activation in a cell-type-specific manner.