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The mammary gland is a dynamic organ that undergoes extensive morphogenesis during the different stages of embryonic development, puberty, estrus, pregnancy, lactation and involution. Systemic and local cues underlie this constant tissue remodeling and act by eliciting an intricate pattern of responses in the mammary epithelial and stromal cells. Decades of studies utilizing methods such as transplantation and lineage-tracing have identified a complex hierarchy of mammary stem cells, progenitors and differentiated epithelial cells that fuel mammary epithelial development. Importantly, these studies have extended our understanding of the molecular crosstalk between cell types and the signaling pathways maintaining normal homeostasis that often are deregulated during tumorigenesis. While several questions remain, this research has many implications for breast cancer. Fundamental among these are the identification of the cells of origin for the multiple subtypes of breast cancer and the understanding of tumor heterogeneity. A deeper understanding of these critical questions will unveil novel breast cancer drug targets and treatment paradigms. In this review, we provide a current overview of normal mammary development and tumorigenesis from a stem cell perspective., (© 2015 Society for Endocrinology.)

MicroRNAs are differentially expressed in breast cancer cells and have been implicated in cancer formation, tumour invasion and metastasis. We investigated the miRNA expression profiles in the developing mammary gland. MiR-137 was expressed prominently in the developing mammary gland. When the miR-137 was over-expressed in the embryo, the mammary epithelium became thickened. Moreover, genes associated with mammary gland formation such as Tbx3 and Lef1 were not expressed. This suggests that miR-137 induces gland formation and invasion. When miR-137 was over-expressed in MDA-MB-231 cells, their ability to form tumours in adult mice was significantly reduced. These data support miR-137 decides epithelial cell behavior in the human breast cancer. It also suggests that miR-137 is a potential therapeutic target for amelioration of breast cancer progression.

Extracellular matrix proteins from embryonic mesenchyme have a normalizing effect on cancer cells in vitro and slow tumor growth in vivo. This concept is suggestive of a new method for controlling the growth and spread of existing cancer cells in situ and indicates the possibility that extracellular proteins and/or embryonic mesenchymal fibroblasts may represent a fertile subject for study of new anti-cancer treatments.

Some epithelial cancers can be induced to revert to quiescent differentiated tissue when combined with embryonic mesenchyme; however, the mechanism of this induction is unknown. Here we combine tissue engineering, developmental biology, biochemistry and proteomics approaches to attack this problem. Using a synthetic reconstitution system, we show that co-culture of breast cancer cells with embryonic mesenchyme from early stage (E12.5-13.5) mammary glands decreases tumor cell proliferation while stimulating acinus differentiation, whereas cancer-associated fibroblasts (CAFs) fail to produce these normalizing effects. When insoluble extracellular matrices (ECMs) were isolated from cultured early stage (E12.5-13.5) embryonic mammary mesenchyme cells or E10 tooth mesenchyme and recombined with mammary tumor cells, they were found to be sufficient to induce breast cancer normalization including enhanced expression of estrogen receptor-α (ER-α). In contrast, ECM from later stage (E14.5) mammary mesenchyme and conditioned medium isolated from mesenchymal cell cultures were ineffective. Importantly, when the inductive ECMs produced by early stage embryonic mammary mesenchyme were scraped from dishes and injected into fast-growing breast tumors in mice, they significantly inhibited cancer expansion. Proteomics analysis of the detergent insoluble ECM material revealed several matrix components that were preferentially expressed in the embryonic ECMs. Analysis of two of these molecules previously implicated in cancer regulation--biglycan and tenascin C--revealed that addition of biglyan can mimic the tumor normalization response, and that siRNA knockdown of its expression in cultured embryonic mesenchyme results in loss of the ECM's inductive activity. These studies confirm that embryonic mesenchyme retains the ability to induce partial breast cancer reversion, and that its inductive capability resides at least in part in the ECM protein biglycan that it produces.

Epithelial-to-mesenchymal transition (EMT) is a critical multistep process that converts epithelial cells to more motile and invasive mesenchymal cells, contributing to body patterning and morphogenesis during embryonic development. In addition, both epithelial plasticity and increased motility and invasiveness are essential for the branching morphogenesis that occurs during development of the mammary gland and during tumor formation, allowing cancer cells to escape from the primary tumor. Cripto-1, a member of the epidermal growth factor-Cripto-1/FRL-1/Cryptic (EGF/CFC) gene family, together with the transforming growth factor (TGF)-β family ligand Nodal, regulates both cell movement and EMT during embryonic development. During postnatal development, Cripto-1 regulates the branching morphogenesis of the mouse mammary gland and enhances both the invasive and migratory properties of mammary epithelial cells in vitro. Furthermore, transgenic mouse models have shown that Cripto-1 promotes the formation of mammary tumors that display properties of EMT, including the down-regulation of the cell surface adherens junctional protein E-cadherin and the up-regulation of mesenchymal markers, such as vimentin, N-cadherin, and Snail. Interestingly, Cripto-1 is enriched in a subpopulation of embryonal, melanoma, prostate, and pancreatic cancer cells that possess stem-like characteristics. Therefore, Cripto-1 may play a role during developmental EMT, and it may also be involved in the reprogramming of differentiated tumor cells into cancer stem cells through the induction of an EMT program., (Published by Elsevier Inc.)

The Mediator subunit MED1 is essential for mammary gland development and lactation, whose contribution through direct interaction with estrogen receptors (ERs) is restricted to involvement in pubertal mammary gland development and luminal cell differentiation. Here, we provide evidence that the MED24-containing submodule of Mediator functionally communicates specifically with MED1 in pubertal mammary gland development. Mammary glands from MED1/MED24 double heterozygous knockout mice showed profound retardation in ductal branching during puberty, while single haploinsufficient glands developed normally. DNA synthesis of both luminal and basal cells were impaired in double mutant mice, and the expression of ER-targeted genes encoding E2F1 and cyclin D1, which promote progression through the G(1)/S phase of the cell cycle, was attenuated. Luciferase reporter assays employing double mutant mouse embryonic fibroblasts showed selective impairment in ER functions. Various breast carcinoma cell lines expressed abundant amounts of MED1, MED24, and MED30, and attenuated expression of MED1 and MED24 in breast carcinoma cells led to attenuated DNA synthesis and growth. These results indicate functional communications between the MED1 subunit and the MED24-containing submodule that mediate estrogen receptor functions and growth of both normal mammary epithelial cells and breast carcinoma cells.

P-cadherin is a cell-cell adhesion molecule, whose expression is highly associated with undifferentiated cells in normal adult epithelial tissues, as well as with poorly differentiated carcinomas. Its expression has been already reported in human embryonic stem cells and it is presumed to be a marker of stem or progenitor cells of some epithelial tissues. In normal breast, P-cadherin has an essential role during ductal mammary branching, being expressed by the monolayer of epithelial cap cells at the end buds. In mature mammary tissue, its expression is restricted to the myoepithelium; it has been postulated that it may also be present in early luminal progenitor cells. In breast cancer, P-cadherin is frequently overexpressed in high-grade tumours, being a well-established indicator of poor patient prognosis. It has been reported as an important inducer of cancer cell migration and invasion, with underlying molecular mechanisms involving the signalling mediated by its juxtamembrane domain, the secretion of matrix metalloproteases to the extracellular media, and the cleavage of a P-cadherin soluble form with pro-invasive activity. Intracellularly, this protein interferes with the endogenous cadherin/catenin complex, inducing p120-catenin delocalization to the cytoplasm, and the consequent activation of Rac1/Cdc42 and associated alterations in the actin cytoskeleton. Considering P-cadherin's role in cancer cell invasion and metastasis formation, a humanized monoclonal antibody was recently produced to antagonize P-cadherin-associated signalling pathways, which is currently under Phase I clinical trials. In this review, the most important findings about the role of P-cadherin in normal breast development and cancer will be illustrated and discussed, with emphasis on the most recent data.

The mammary gland, the unique organ that primarily form at puberty, is an ideal model to study the functions of homeobox (HB) genes in both development and tumorigenesis. HB genes comprise a large family of developmental regulators that have a critical role in cell growth and differentiation. In the normal mammary gland, homeobox genes are involved in ductal formation, epithelial branching, and lobulo-alveolar development by regulating epithelial proliferation and differentiation. The HB genes are controlled in a spatial and temporal manner in both stromal and epithelial cells. They are coordinately regulated by hormones and extracellular matrix, suggesting that many signaling pathways are involved in homeobox gene functions. When homeobox genes are misexpressed in animal models, different defects are displayed in mammary gland development. Aberrant expression of homeobox genes, overexpressed or downregulated, is found in primary carcinomas and in breast cancer. The Otx1 HB gene is a classic regulatory of nervous system development during embryogenesis. Postnatally Otx1 is transcribed in the anterior pituitary gland, where activates transcription of the pituitary hormones, and plays a role in hematopoiesis, enhancing pluripotent cells, and erythroid differentiation. Otx1 can still be detected in mature cells of the erythroid and megacaryocytic lineage. During cyclical development of mammary gland, the Otx1 gene is overexpressed in lactation, confirming a role of this transcription factor in cell differentiation. Recent studies report that Otx1 is overexpressed in breast cancer. Otx1 is expressed during embryogenesis, and it is expressed again during carcinogenesis, implying its possible function in differentiation of neoplastic cells., (© 2010 Wiley Periodicals, Inc.)

Activation of the ErbB family of receptor tyrosine kinases via cognate Epidermal Growth Factor (EGF)-like peptide ligands constitutes a major group of related signaling pathways that control proliferation, survival, angiogenesis and metastasis of breast cancer. In this respect, clinical trials with various ErbB receptor blocking antibodies and specific tyrosine kinase inhibitors have proven to be partially efficacious in the treatment of this heterogeneous disease. Induction of an embryonic program of epithelial-to-mesenchymal transition (EMT) in breast cancer, whereupon epithelial tumor cells convert to a more mesenchymal-like phenotype, facilitates the migration, intravasation, and extravasation of tumor cells during metastasis. Breast cancers which exhibit properties of EMT are highly aggressive and resistant to therapy. Activation of ErbB signaling can regulate EMT-associated invasion and migration in normal and malignant mammary epithelial cells, as well as modulating discrete stages of mammary gland development. The purpose of this review is to summarize current information regarding the role of ErbB signaling in aspects of EMT that influence epithelial cell plasticity during mammary gland development and tumorigenesis. How this information may contribute to the improvement of therapeutic approaches in breast cancer will also be addressed.

The extracellular matrix (ECM), once thought to solely provide physical support to a tissue, is a key component of a cell's microenvironment responsible for directing cell fate and maintaining tissue specificity. It stands to reason, then, that changes in the ECM itself or in how signals from the ECM are presented to or interpreted by cells can disrupt tissue organization; the latter is a necessary step for malignant progression. In this review, we elaborate on this concept using the mammary gland as an example. We describe how the ECM directs mammary gland formation and function, and discuss how a cell's inability to interpret these signals -- whether as a result of genetic insults or physicochemical alterations in the ECM -- disorganizes the gland and promotes malignancy. By restoring context and forcing cells to properly interpret these native signals, aberrant behavior can be quelled and organization re-established. Traditional imaging approaches have been a key complement to the standard biochemical, molecular, and cell biology approaches used in these studies. Utilizing imaging modalities with enhanced spatial resolution in live tissues may uncover additional means by which the ECM regulates tissue structure, on different length scales, through its pericellular organization (short-scale) and by biasing morphogenic and morphostatic gradients (long-scale).

ERBB3/HER3 is one of the four members of the epidermal growth factor receptor (ERBB) family. It is activated by binding to ligands Neuregulin-1 and Neuregulin-2. Since ERBB3 lacks intrinsic kinase activity, signal transduction occurs through formation of heterodimers with EGFR, ERBB2, and ERBB4. ERBB3 is a signaling specialist since it has six binding sites for the p85 SH2 adapter subunit of phosphoinositide 3' kinases. These lipid kinases coordinate regulation of metabolism, cell size, proliferation, survival, and angiogenesis. Not surprisingly, ERBB3 signaling has been linked to cancer etiology and progression. In breast cancer, the partnership of ERBB2 and ERBB3 may be crucial for the aggressive properties of cancers with ERBB2 amplification, and may contribute to pre-existing and acquired resistance to therapy. This partnership creates opportunities for improving efficacy of ERBB-targeted pharmaceuticals, by interfering with coupling of ERBB2 to ERBB3 through dimerization inhibitors, and by use of therapeutic compounds that target AKT-dependent pathways activated through ERBB3. Additional therapeutic opportunities may be identified through better understanding of how ERBBs are regulated and deployed in normal mammary gland processes. Work using mouse models has identified the main processes regulated by each of the four ERBBs, which has practical implications in understanding breast cancer etiology, and eventual development of better prognostic, predictive, and therapeutic tools.

Breast carcinoma cells and embryonic progenitors similarly implement stem cell-associated signaling pathways to sustain continued growth and plasticity. Indeed, recent studies have implicated signaling pathways, including those associated with the Notch, and Transforming Growth Factor-Beta (TGF-beta) superfamilies, as instrumental to both embryological development and breast cancer progression. In particular, Nodal, an embryonic morphogen belonging to the TGF-beta superfamily, and its co-receptor, Cripto-1, are requisite to both embryogenesis and mammary gland maturation. Moreover, these developmental proteins have been shown to promote breast cancer progression. Here, we review the role of Nodal and its co-receptor Cripto-1 during development and we describe how this signaling pathway may be involved in breast cancer tumorigenesis. Moreover, we emphasize the potential utility of this signaling pathway as a novel target for the treatment and diagnosis of breast cancer.

Specification of mammary epithelial cell fate occurs during embryogenesis as cells aggregate to form the mammary anlage. Within the embryonic mammary bud, a population of epithelial cells exists that will subsequently proliferate to form a ductal tree filling the stromal compartment, and which can produce milk upon terminal differentiation after birth. Subsequently, these structures can be remodelled and returned to a basal state after weaning before regenerating in future pregnancies. The plasticity of the mammary epithelial cell, and its responsiveness to hormone receptors, facilitates this amazing biological feat, but aberrant signalling may also result in unintended consequences in the form of frequent malignancies. Reflecting this intimate connection, a considerable number of signalling pathways have been implicated in both mammary gland morphogenesis and carcinogenesis., Competing Interests: Competing interests. The authors have declared that no competing interests exist.

Breast cancer is the most common non-skin cancer among women in this country. Breast cancer risk is significantly influenced by genetics, but over 70% of the women that are diagnosed have noninherited or sporadic cancer. The risk of breast cancer is thought to be modified by lifestyle and environment. Exposures to certain chemicals and hormone-mimicking or endocrine-disrupting compounds (EDCs) are suspected of contributing to increased breast cancer incidence as well as precocious puberty in the United States. Studies of EDC effects in rodents indicate that multiple toxicants can alter mammary gland development, with or without changing other markers of puberty. EDCs can cause transient and persistent effects on mammary gland development depending on dose, exposure parameters, and whether exposure was during critical periods of gland growth or differentiation. Adverse effects from these abnormal developmental patterns include the presence of carcinogen-sensitive structures in greater numbers or for longer periods in the gland and inhibited functional differentiation leading to malnutrition or increased mortality of their offspring. Developmental toxicants of the mammary gland could lead to an increase in the incidence of mammary tumors if they alter circulating or tissue-localized hormone levels, gland receptor expression patterns, hormone transport, or metabolism that results in altered response to endogenous hormones or growth factors. Environmental disruptors of rodent mammary gland development must be identified for informed decisions in epidemiological studies aimed at identification of environmental factors contributing to breast cancer risk, altered breast development during puberty, or inability to produce sufficient breast milk.

Secreted proteins of the Wnt family play widespread roles in the regulation of embryonic development, and aberrant activation of the canonical Wnt/beta-catenin pathway is one of the most frequent signaling abnormalities known in human cancer. While the consequences of Wnt signaling in development are diverse at the cellular level, they are often concerned with cell fate determination. Recent data also indicate that Wnt proteins influence the self-renewal of stem cells in certain tissues. In the mammary gland, Wnt signals are strongly implicated in initial development of the mammary rudiments, and in the ductal branching and alveolar morphogenesis that occurs during pregnancy. Transgenic expression of Wnt1 or Wnt10b in the mouse mammary gland leads to lobuloalveolar hyperplasia with a major risk of progression to carcinoma. Recent evidence suggests that this phenotype is associated with expansion of a multipotent progenitor cell population. In human breast cancer, evidence of beta-catenin accumulation implies that the canonical Wnt signaling pathway is active in over 50% of carcinomas. However, specific mutations that might account for this activation of signaling have not yet been identified.

With the discovery of an activated Notch oncogene as a causative agent in mouse mammary tumor virus induced breast cancer in mice, the potential role for Notch signaling in normal and pathological mammary development was revealed. Subsequently, Notch receptors have been found to regulate normal development in many organ systems. In addition, inappropriate Notch signaling has been implicated in cancer of several tissues in humans and animal model systems. Here we review important features of the Notch system, and how it may regulate development and cancer in the mammary gland. A large body of literature from studies in Drosophila and C. elegans has not only revealed molecular details of how the Notch proteins signal to control biology, but shown that Notch receptor activation helps to define how other signaling pathways are interpreted. In many ways the Notch system is used to define the context in which other pathways function to control proliferation, differentiation, cell survival, branching morphogenesis, asymmetric cell division, and angiogenesis--all processes which are critical for normal development and function of the mammary gland.

TBX2 and TBX3 are members of a family of genes encoding developmental transcription factors, characterized by a 200 amino acid DNA binding domain (T-box). Tbx2 and Tbx3 are closely related T-box proteins that have been implicated in development of a number of different tissues including the mammary gland. TBX3 is required for normal mammary development in mouse models and in patients with ulnar-mammary syndrome (UMS). In addition to a role in development, TBX2 and TBX3 have been implicated in tumor development through downregulation of the alternative reading frame (ARF) tumor suppressor and an associated bypass of senescence. Here we review the current information on the roles of Tbx2 and Tbx3 in mammary gland development and tumorigenesis.

Members of the TGFbeta superfamily and EGF-CFC family, such as Nodal and Cripto, are important mediators of anterior-posterior and left-right axis specification during embryogenesis. In this paper, we review the role of Nodal and Cripto as critical morphogen-like molecules, with an emphasis on Nodal and EGF-CFC signaling during embryonic pattern formation. New evidence from gene expression and transgenic mouse studies have shown that both Nodal and Cripto-1 are expressed within the mammary duct and that modulation of these genes can disrupt normal branching morphogenesis resulting in epithelial disorganization and defective ductal architecture. We describe these new findings and propose that Cripto and Nodal are candidate mammary morphogens. Finally, the data linking overexpression of Cripto and perturbations of Cripto signaling to cell transformation and tumor formation are discussed. The fact that Cripto can modulate multiple pathways suggests it may act to deregulate growth inhibitors/homeostasis factors early in the cell transformation process and then activate prosurvival pathways dependent on MAPK and PI3K/Akt later in fully transformed phenotypes.

The zinc-finger transcription factors Snail and Slug are involved in different processes controlling cell differentiation and apoptosis. They also appear to be involved in tumor progression. Their putative involvement in mammary gland development has not been specifically examined so far. Slug is expressed at a significant level in normal breast, and indirect evidence suggests it could be implicated in tubulogenesis. As an antiapoptotic agent, it could also protect epithelial cells from death during ductal lumen formation and during breast involution. In breast carcinomas, Snail transcription factors have been linked to tumor progression and invasiveness. Possible mechanisms include repression of the E-cadherin gene by Snail or Slug. However, it is not clear how this transcriptional activity is implicated in vivo. Other possible mechanisms involve maintenance of a plastic phenotype by Slug that could participate in local invasion of ductal carcinomas, and interference with apoptotic pathways that could contribute to global tumor growth and radioresistance. These processes probably also involve interactions with estrogen, EGF, or c-kit pathways.

The mammary gland represents a remarkable developmental system for the study of genetic programs underlying proliferation, differentiation and inductive tissue interactions. Mammary gland ontogeny occurs predominantly in the adult and involves a complex cycle of morphogenesis, with the continuous production, differentiation and apoptosis of mammary epithelial cells occurring with each pregnancy. Perturbations in pathways controlling any of these processes may lead to neoplasia. Substantial progress has been made in defining signaling pathways important for mammopoiesis, in particular those that mediate the effects of peptide and steroid hormones. However, much less is known about the transcriptional regulators that dictate cell specificity, proliferation and differentiation within the mammary gland. This review will discuss recent insights into the transcriptional networks important for mammary gland development and consider how deregulation of specific transcription factors contributes to the pathogenesis of breast cancer.

The role of homeobox-containing genes in embryogenesis and organogenesis is well documented. Also, a sizeable body of evidence has accumulated and supports the fact that homeobox genes, when dysregulated, are involved in tumorigenesis. However, the precise mechanisms of homeobox gene functions are largely unknown. The mammary gland, in which most maturation occurs postnatally, provides an ideal model for studying the functions of homeobox genes in both development and tumorigenesis. The expression of many homeobox genes has been detected in both normal mammary gland and neoplastic breast tissues. In the normal mammary gland, the expression of homeobox genes is coordinately regulated by hormone and extracellular matrix (ECM) and other unknown factors in a spatial and temporal manner in both stromal and epithelial cells. Animals with misexpressed homeobox genes displayed different extents of defects in ductal proliferation, side branching, and alveoli formation, implying that homeobox genes are important for normal mammary gland development. Recent studies of homeobox genes in breast cancer cells and primary tumors indicate that they may also play a contributory or causal role in tumorigenesis by regulating the cell cycle, apoptosis, angiogenesis, and/or metastasis.

The basic helix-loop-helix (bHLH) family of transcription factors functions in the coordinated regulation of gene expression, cell lineage commitment, and cell differentiation in most mammalian tissues. Helix-loop-helix Id (Inhibitor of DNA binding) proteins are distinct from bHLH transcription factors in that they lack the basic domain necessary for DNA binding. Id proteins thus function as dominant negative regulators of bHLH transcription factors. The inhibition of bHLH factor activity by forced constitutive expression of Id proteins is closely associated with the inhibition of differentiation in a number of different cell types, including mammary epithelial cells. Moreover, recent literature suggests important roles of HLH proteins in many normal and transformed tissues, including mammary gland. Therefore, future directions for prognosis or therapeutic treatments of breast cancer may be able to exploit bHLH and Id genes as useful molecular targets. The purpose of this review is to summarize the evidence implicating HLH proteins in the regulation of normal and transformed mammary epithelial cell phenotypes.

The CCAAT/enhancer binding protein (C/EBP) family of bZIP transcription factors control the proliferation and differentiation of a variety of tissues. While C/EBPalpha and -delta are also expressed in the mammary gland, the multiple protein isoforms of C/EBPbeta appear to play a critical role in mammary gland development and breast cancer. Targeted deletion of all the C/EBPbeta isoforms results in a severe inhibition of lobuloalveolar development and a block to functional differentiation, as well as more subtle changes in ductal morphogenesis. The altered expression of a number of molecular markers, including the progesterone, estrogen, and prolactin receptors, the transporter proteins (NKCC1 and aquaporin 5), and several markers of skin differentiation (Sprr2A and keratin 6), suggests that germline deletion of C/EBPbeta results in an altered cell fate. Thus, C/EBPbeta appears to play a role in the specification of progenitor cell fate not only in the mammary gland, but also in a number of other tissues.

Breast cancer manifests itself in the mammary epithelium, yet there is a growing recognition that mammary stromal cells also play an important role in tumorigenesis. During its developmental cycle, the mammary gland displays many of the properties associated with breast cancer, and many of the stromal factors necessary for mammary development also promote or protect against breast cancer. Here we review our present knowledge of the specific factors and cell types that contribute to epithelial-stromal crosstalk during mammary development. To find cures for diseases like breast cancer that rely on epithelial-stromal crosstalk, we must understand how these different cell types communicate with each other.

Etk/Bmx, a member of the Tec family of nonreceptor protein-tyrosine kinases, is characterized by an N-terminal pleckstrin homology domain and has been shown to be a downstream effector of phosphatidylinositol 3-kinase. P21-activated kinase 1 (Pak1), another well characterized effector of phosphatidylinositol 3-kinase, has been implicated in the progression of breast cancer cells. In this study, we characterized the role of Etk in mammary development and tumorigenesis and explored the functional interactions between Etk and Pak1. We report that Etk expression is developmentally regulated in the mammary gland. Using transient transfection, coimmunoprecipitation and glutathione S-transferase-pull down assays, we showed that Etk directly associates with Pak1 via its N-terminal pleckstrin homology domain and also phosphorylates Pak1 on tyrosine residues. The expression of wild-type Etk in a non-invasive human breast cancer MCF-7 cells significantly increased proliferation and anchorage-independent growth of epithelial cancer cells. Conversely, expression of kinase-inactive mutant Etk-KQ suppressed the proliferation, anchorage-independent growth, and tumorigenicity of human breast cancer MDA-MB435 cells. These results indicate that Pak1 is a target of Etk and that Etk controls the proliferation as well as the anchorage-independent and tumorigenic growth of mammary epithelial cancer cells.

Growth and morphogenesis in the mammary gland depend on locally derived growth factors such as those in the epidermal growth factor (EGF) superfamily. Cripto-1 (CR-1, human; Cr-1, mouse)--also known as teratocarcinoma-derived growth factor-1--is a novel EGF-related protein that induces branching morphogenesis in mammary epithelial cells both in vitro and in vivo and inhibits the expression of various milk proteins. In the mouse, Cr-1 is expressed in the growing terminal end buds in the virgin mouse mammary gland and expression increases during pregnancy and lactation. Cr-1/CR-1 is overexpressed in mouse and human mammary tumors and inappropriate overexpression of Cr-1 in mouse mammary epithelial cells can lead to the clonal expansion of ductal hyperplasias. Taken together, this evidence suggests that Cr-1/CR-1 performs a role in normal mammary gland development and that it might contribute to the early stages of mouse mammary tumorigenesis and the pathobiology of human breast cancer.

An attempt has been made to put forward a unifying hypothesis explaining the role hormones play in the genesis of mammary cancers of different phenotypes and genotypes in mice, rats, and humans. Most mammary cancers in these species originate in luminal mammary epithelial cells lining the mammary ducts and alveoli. These cancers are histopathologically diverse and are classified on the basis of growth requirements as hormone-dependent or hormone-independent tumors. In most strains of mice, mammary cancers at the time of detection are largely of the hormone-independent type; in rats, almost all mammary cancers are hormone-dependent, while humans have both phenotypes. In spite of these differences, in vivo studies show that hormones (ovarian and pituitary) are essential for luminal mammary epithelial cell proliferation and also for the development of mammary cancers of both hormone-independent and hormone-dependent types. This article, based on our extensive in vivo and in vivo studies and on current literature, proposes a model to explain the central role of hormones in the genesis of all types of mammary cancers. The model attempts to address the following questions: (i) how hormones regulate luminal mammary epithelial cell proliferation, (ii) why hormones are required for the genesis of mammary cancers of all phenotypes and genotypes, including those which are always classified as hormone-independent tumors, and (iii) why the three species (mouse, rat, and human) have consistently different ratios of hormone-dependent to hormone-independent tumors.

We desperately need to know more of the biological details of the onset and progression of breast cancer. The disease is of startlingly high incidence (approaching 1 in 9 women), our current therapies for the disease are inadequate once it has metastasized, and the disease is characterized by excessive morbidity and mortality. Most of the growth and differentiation of the mammary gland occurs relatively late in life: during sexual maturation, and then cyclically during pregnancy and lactation. Normal as well as malignant growth is regulated by endocrine hormones as well as by local tissue factors, such as polypeptide growth factors. Cancer seems to progress as hyperplastic ductal or lobular epithelial growth, acquiring progressive genetic changes (including those of oncogenes and tumor suppressor genes) leading to clonal outgrowths of progressively more malignant cells. The nature of proliferative controls and the relevant genetic changes are the subjects of the current review.