Localization and quantification of Wnt-2 gene expression in mouse mammary development

Wnt Signaling in the Breast: From Development to Disease

The Wnt cascade is a primordial developmental signaling pathway that plays a myriad of essential functions throughout development and adult homeostasis in virtually all animal species. Aberrant Wnt activity is implicated in embryonic and tissue morphogenesis defects, and several diseases, most notably cancer. The role of Wnt signaling in mammary gland development and breast cancer initiation, maintenance, and progression is far from being completely understood and is rather shrouded in controversy. In this review, we dissect the fundamental role of Wnt signaling in mammary gland development and adult homeostasis and explore how defects in its tightly regulated and intricated molecular network are interlinked with cancer, with a focus on the breast.

The Mammary Gland: Basic Structure and Molecular Signaling during Development

The mammary gland is a compound, branched tubuloalveolar structure and a major characteristic of mammals. The mammary gland has evolved from epidermal apocrine glands, the skin glands as an accessory reproductive organ to support postnatal survival of offspring by producing milk as a source of nutrition. The mammary gland development begins during embryogenesis as a rudimentary structure that grows into an elementary branched ductal tree and is embedded in one end of a larger mammary fat pad at birth. At the onset of ovarian function at puberty, the rudimentary ductal system undergoes dramatic morphogenetic change with ductal elongation and branching. During pregnancy, the alveolar differentiation and tertiary branching are completed, and during lactation, the mature milk-producing glands eventually develop. The early stages of mammary development are hormonal independent, whereas during puberty and pregnancy, mammary gland development is hormonal dependent. We highlight the current understanding of molecular regulators involved during different stages of mammary gland development.

Breast cancer development and progression: Risk factors, cancer stem cells, signaling pathways, genomics, and molecular pathogenesis

As the most commonly occurring cancer in women worldwide, breast cancer poses a formidable public health challenge on a global scale. Breast cancer consists of a group of biologically and molecularly heterogeneous diseases originated from the breast. While the risk factors associated with this cancer varies with respect to other cancers, genetic predisposition, most notably mutations in BRCA1 or BRCA2 gene, is an important causative factor for this malignancy. Breast cancers can begin in different areas of the breast, such as the ducts, the lobules, or the tissue in between. Within the large group of diverse breast carcinomas, there are various denoted types of breast cancer based on their invasiveness relative to the primary tumor sites. It is important to distinguish between the various subtypes because they have different prognoses and treatment implications. As there are remarkable parallels between normal development and breast cancer progression at the molecular level, it has been postulated that breast cancer may be derived from mammary cancer stem cells. Normal breast development and mammary stem cells are regulated by several signaling pathways, such as estrogen receptors (ERs), HER2, and Wnt/β-catenin signaling pathways, which control stem cell proliferation, cell death, cell differentiation, and cell motility. Furthermore, emerging evidence indicates that epigenetic regulations and noncoding RNAs may play important roles in breast cancer development and may contribute to the heterogeneity and metastatic aspects of breast cancer, especially for triple-negative breast cancer. This review provides a comprehensive survey of the molecular, cellular and genetic aspects of breast cancer.

Signaling Networks in Epithelial Tube Formation

Epithelial tubes are crucial to the function of organ systems including the excretory, gastrointestinal, cardiovascular, and pulmonary. Studies in the last two decades using in vitro organotypic systems and a variety of animal models have substantiated a large number of the morphogenetic mechanisms required to form epithelial tubes in development and regeneration. Many of these mechanisms modulate the differentiation and proliferation events necessary for generating the cell movements and changes in cell shape to delineate the wide variety of epithelial tube sizes, lengths, and conformations. For instance, when coupled with oriented cell division, proliferation itself plays a role in changes in tube shape and their directed expansion. Most of these processes are regulated in response to signaling inputs from adjacent cells or soluble factors from the environment. Despite the great deal of recent investigation in this direction, the knowledge we have about the signaling pathways associated with all epithelial tubulogenesis in development and regeneration is still very limited.

The Terminal End Bud: the Little Engine that Could

The mammary gland is one of the most regenerative organs in the body, with the majority of development occurring postnatally and in the adult mammal. Formation of the ductal tree is orchestrated by a specialized structure called the terminal end bud (TEB). The TEB is responsible for the production of mature cell types leading to the elongation of the subtending duct. The TEB is also the regulatory control point for basement membrane deposition, branching, angiogenesis, and pattern formation. While the hormonal control of TEB growth is well characterized, the local regulatory factors are less well understood. Recent studies of pubertal outgrowth and ductal elongation have yielded surprising details in regards to ongoing processes in the TEB. Here we summarize the current understanding of TEB biology, discuss areas of future study, and discuss the use of the TEB as a model for the study of breast cancer.

Mammary Development and Breast Cancer: A Wnt Perspective

The Wnt pathway has emerged as a key signaling cascade participating in mammary organogenesis and breast oncogenesis. In this review, we will summarize the current knowledge of how the pathway regulates stem cells and normal development of the mammary gland, and discuss how its various components contribute to breast carcinoma pathology.

Essential Roles of Cyclin Y-Like 1 and Cyclin Y in Dividing Wnt-Responsive Mammary Stem/Progenitor Cells

Cyclin Y family can enhance Wnt/β-catenin signaling in mitosis. Their physiological roles in mammalian development are yet unknown. Here we show that Cyclin Y-like 1 (Ccnyl1) and Cyclin Y (Ccny) have overlapping function and are crucial for mouse embryonic development and mammary stem/progenitor cell functions. Double knockout of Ccnys results in embryonic lethality at E16.5. In pubertal development, mammary terminal end buds robustly express Ccnyl1. Depletion of Ccnys leads to reduction of Lrp6 phosphorylation, hampering β-catenin activities and abolishing mammary stem/progenitor cell expansion in vitro. In lineage tracing experiments, Ccnys-deficient mammary cells lose their competitiveness and cease to contribute to mammary development. In transplantation assays, Ccnys-deficient mammary cells fail to reconstitute, whereas constitutively active β-catenin restores their regeneration abilities. Together, our results demonstrate the physiological significance of Ccnys-mediated mitotic Wnt signaling in embryonic development and mammary stem/progenitor cells, and reveal insights in the molecular mechanisms orchestrating cell cycle progression and maintenance of stem cell properties.

Stem cell self-renewal has two essential elements, cell division and at least of one of the daughter cells retaining stem cell properties, so-called stemness. The interconnections between cell cycle and cell fate specification have been explored in embryonic stem cells. However, less is known about how cell cycle affects the cell fate decision in tissue stem cells. In this study, we explore the function of particular mitotic factors Ccny and Ccnyl1 in regulating the dividing tissue stem cells. The development of the mammary gland occurs mostly in postnatal pubertal stage. At the time, the robustly dividing stem/progenitor cells reside at the forefront of the mammary epithelium extension, underlining the mammary gland as a good model to study the interconnection of cell cycle and tissue stem cells. In this study, we show that in dividing mammary stem/progenitor cells, Ccny and Ccnyl1 enhance Wnt signaling activities in mitosis. The signaling enhancement in this time window is essential for the stem/progenitor cell property maintenance during division. Deletion of Ccnys results in diminishing their competitiveness and developmental potential.

Recurrence and metastasis of breast cancer is influenced by ovarian hormone's effect on breast cancer stem cells

Cancer stem cells (CSCs) have recently attracted great interest because of their emerging role in initiation, progression and metastasis, combined with their intrinsic resistance to chemotherapy and radiation therapy. CSCs and its interaction with hormones in breast cancer are currently being investigated with the aim of uncovering the molecular mechanisms by which they evade conventional treatment regimens. In this review, we discuss recent experimental data and new perspectives in the area of steroid hormones and their cross-talk with breast CSCs. We have covered literature associated with biomarkers, hormone receptors and hormone responsive signaling pathways in breast CSC. In addition, we also discuss the role of miRNAs in hormone mediated regulation of breast CSCs.

A Ror recruit to mammary gland development

Study shows how noncanonical Wnt signaling promotes differentiation and morphogenesis of mammary epithelia.

Ror2 regulates branching, differentiation, and actin-cytoskeletal dynamics within the mammary epithelium

Intricate cross-talk between classical and alternative Wnt signaling pathways includes an essential role for Ror2 in mammary epithelial development and differentiation.

Wnt signaling encompasses β-catenin–dependent and –independent networks. How receptor context provides Wnt specificity in vivo to assimilate multiple concurrent Wnt inputs throughout development remains unclear. Here, we identified a refined expression pattern of Wnt/receptor combinations associated with the Wnt/β-catenin–independent pathway in mammary epithelial subpopulations. Moreover, we elucidated the function of the alternative Wnt receptor Ror2 in mammary development and provided evidence for coordination of this pathway with Wnt/β-catenin–dependent signaling in the mammary epithelium. Lentiviral short hairpin RNA (shRNA)-mediated depletion of Ror2 in vivo increased branching and altered the differentiation of the mammary epithelium. Microarray analyses identified distinct gene level alterations within the epithelial compartments in the absence of Ror2, with marked changes observed in genes associated with the actin cytoskeleton. Modeling of branching morphogenesis in vitro defined specific defects in cytoskeletal dynamics accompanied by Rho pathway alterations downstream of Ror2 loss. The current study presents a model of Wnt signaling coordination in vivo and assigns an important role for Ror2 in mammary development.

In-silico QTL mapping of postpubertal mammary ductal development in the mouse uncovers potential human breast cancer risk loci

Genetic background plays a dominant role in mammary gland development and breast cancer (BrCa). Despite this, the role of genetics is only partially understood. This study used strain-dependent variation in an inbred mouse mapping panel, to identify quantitative trait loci (QTL) underlying structural variation in mammary ductal development, and determined if these QTL correlated with genomic intervals conferring BrCa susceptibility in humans. For about half of the traits, developmental variation among the complete set of strains in this study was greater (P < 0.05) than that of previously studied strains, or strains in current common use for mammary gland biology. Correlations were also detected with previously reported variation in mammary tumor latency and metastasis. In-silico genome-wide association identified 20 mammary development QTL (Mdq). Of these, five were syntenic with previously reported human BrCa loci. The most significant (P = 1 × 10(-11)) association of the study was on MMU6 and contained the genes Plxna4, Plxna4os1, and Chchd3. On MMU5, a QTL was detected (P = 8 × 10(-7)) that was syntenic to a human BrCa locus on h12q24.5 containing the genes Tbx3 and Tbx5. Intersection of linked SNP (r(2) > 0.8) with genomic and epigenomic features, and intersection of candidate genes with gene expression and survival data from human BrCa highlighted several for further study. These results support the conclusion that mammary tumorigenesis and normal ductal development are influenced by common genetic factors and that further studies of genetically diverse mice can improve our understanding of BrCa in humans.

SOX9 regulates low density lipoprotein receptor-related protein 6 (LRP6) and T-cell factor 4 (TCF4) expression and Wnt/β-catenin activation in breast cancer

Gene expression profiling has identified breast cancer (BCa) subtypes, including an aggressive basal-like (BL) subtype. The molecular signals underlying the behavior observed in BL-BCa group are largely unknown, although recent results indicate a prevalent increase in Wnt/β-catenin activity. Our immunohistochemistry study confirmed that SOX9, one of the BL-BCa signature genes, was expressed by most BL-BCa, and its expression correlated with indicators of poor prognosis. Importantly, BCa gene expression profiling strongly associated SOX9 with the expression of Wnt/β-catenin pathway components, LRP6 and TCF4. In cancer cell lines, SOX9 silencing reduced cell proliferation and invasion, LRP6 and TCF4 transcription, and decreased Wnt/β-catenin activation. SOX9 expression was also increased by Wnt, indicating that SOX9 is at the center of a positive feedback loop that enhances Wnt/β-catenin signaling. Consistently, SOX9 overexpression in BCa cell lines and transgenic SOX9 expression in breast epithelium caused increased LRP6 and TCF4 expression and Wnt/β-catenin activation. These results identify SOX9-mediated Wnt/β-catenin activation as one of the molecular mechanisms underlying aberrant Wnt/β-catenin activity in BCa, especially in the BL-BCa subgroup.

Wnt signaling in mammary glands: plastic cell fates and combinatorial signaling

The mouse mammary gland is an outstanding developmental model that exemplifies the activities of many of the effector pathways known to organize mammalian morphogenesis; furthermore, there are well-characterized methods for the specific genetic manipulation of various mammary epithelial cell components. Among these signaling pathways, Wnt signaling has been shown to generate plasticity of fate determination, expanding the genetic programs available to cells in the mammary lineage. It is responsible first for the appearance of the mammary fate in embryonic ectoderm and then for maintaining bi-potential basal stem cells in adult mammary ductal trees. Recent technical developments have led to the separate analysis of various mammary epithelial cell subpopulations, spurring the investigation of Wnt-dependent interactions. Although Wnt signaling was shown to be oncogenic for mouse mammary epithelium even before being identified as the principle oncogenic driver for gut epithelium, conclusive data implicating this pathway as a tumor driver for breast cancer lag behind, and we examine potential reasons.

Wnt signalling in murine postnatal mammary gland development

The mammary gland undergoes numerous developmental processes postnatally, from the elongation of the ductal tree-like structure to pregnancy-induced lobulo-alveolar development. Mammary epithelial stem cells have been suggested to be central to the control of enormous tissue expansion and remodelling during phases of mammary development. The Wnt signalling pathway plays a critical role in these biological steps and is suggested to be involved in the maintenance of the stem cell population. This review provides insight into recent findings on the activity of Wnt signalling during ductal and lobular mammary development and discusses the potential interplay between Wnt signals and mammary stem cells in mice.

Shared signaling pathways in normal and breast cancer stem cells

Recent advances in our understanding of breast cancer biology have led to the identification of a subpopulation of cells within tumors that appear to be responsible for initiating and propagating the cancer. These tumor initiating cells are not only unique in their ability to generate tumors, but also share many similarities with elements of normal adult tissue stem cells, and have therefore been termed cancer stem cells (CSCs). These CSCs often inappropriately use many of the same signaling pathways utilized by their normal stem cell counterparts which may present a challenge to the development of CSC specific therapies. Here, we discuss three major stem cell signaling pathways (Notch, Wnt, and Hedgehog); with a focus on their function in normal mammary gland development and their misuse in breast cancer stem cell fate determination.

Wnt and mammary stem cells: hormones cannot fly wingless

The mammary stem cell and its local microenvironment are central for the maintenance of proper tissue homeostasis during normal development. Defining the hierarchical organization of the epithelial subtypes in the mammary gland and the molecular pathways guiding their development has begun to provide a framework for understanding how cancer stem cells sustain the progression and heterogeneity of breast cancers. The Wnt pathway plays a fundamental role in multiple adult stem cells, as well as in orchestrating proper mammary gland development and maintenance. These processes are intricately guided by the influence of systemic hormones and local factors. Alterations in Wnt signaling can skew the homeostatic balance of the mammary epithelium to drive malignant progression; however, complexities of Wnt pathway components present a challenge in understanding their physiological function.

Wnt proteins are self-renewal factors for mammary stem cells and promote their long-term expansion in culture

Adult stem cells have the ability to self-renew and to generate specialized cells. Self-renewal is dependent on extrinsic niche factors but few of those signals have been identified. In addition, stem cells tend to differentiate in the absence of the proper signals and are therefore difficult to maintain in cell culture. The mammary gland provides an excellent system to study self-renewal signals, because the organ develops postnatally, arises from stem cells, and is readily generated from transplanted cells. We show here that adult mammary glands contain a Wnt-responsive cell population that is enriched for stem cells. In addition, stem cells mutant for the negative-feedback regulator Axin2 and therefore sensitized to Wnt signals have a competitive advantage in mammary gland reconstitution assays. In cell culture experiments, exposure to purified Wnt protein clonally expands mammary stem cells for many generations and maintains their ability to generate functional glands in transplantation assays. We conclude that Wnt proteins serve as rate-limiting self-renewal signals acting directly on mammary stem cells.

Key signaling nodes in mammary gland development and cancer: β-catenin

β-Catenin plays important roles in mammary development and tumorigenesis through its functions in cell adhesion, signal transduction and regulation of cell-context-specific gene expression. Studies in mice have highlighted the critical role of β-catenin signaling for stem cell biology at multiple stages of mammary development. Deregulated β-catenin signaling disturbs stem and progenitor cell dynamics and induces mammary tumors in mice. Recent data showing deregulated β-catenin signaling in metaplastic and basal-type tumors suggest a similar link to reactivated developmental pathways and human breast cancer. The present review will discuss β-catenin as a central transducer of numerous signaling pathways and its role in mammary development and breast cancer.

The embryonic transcription cofactor LBH is a direct target of the Wnt signaling pathway in epithelial development and in aggressive basal subtype breast cancers

Limb-bud and heart (LBH) is a novel key transcriptional regulator of vertebrate development. However, the molecular mechanisms upstream of LBH and its role in adult development are unknown. Here we show that in epithelial development, LBH expression is tightly controlled by Wnt signaling. LBH is transcriptionally induced by the canonical Wnt pathway, as evident by the presence of conserved functional T-cell factor (TCF)/lymphoid enhancer-binding factor (LEF) binding sites in the LBH locus and rapid beta-catenin-dependent upregulation of endogenous LBH by Wnt3a. In contrast, LBH induction by Wnt/beta-catenin signaling is inhibited by Wnt7a, which in limb development signals through a noncanonical pathway involving Lmx1b. Furthermore, we show that LBH is aberrantly overexpressed in mammary tumors of mouse mammary tumor virus (MMTV)-Wnt1-transgenic mice and in aggressive basal subtype human breast cancers that display Wnt/beta-catenin hyperactivation. Deregulation of LBH in human basal breast cancer appears to be Wnt/beta-catenin dependent, as DKK1 and Wnt7a inhibit LBH expression in breast tumor cells. Overexpression studies indicate that LBH suppresses mammary epithelial cell differentiation, an effect that could contribute to Wnt-induced tumorigenesis. Taken together, our findings link LBH for the first time to the Wnt signaling pathway in both development and cancer and highlight LBH as a potential new marker for therapeutically challenging basal-like breast cancers.

The Wnt receptor, Lrp5, is expressed by mouse mammary stem cells and is required to maintain the basal lineage

BACKGROUND

Ectopic Wnt signaling induces increased stem/progenitor cell activity in the mouse mammary gland, followed by tumor development. The Wnt signaling receptors, Lrp5/6, are uniquely required for canonical Wnt activity. Previous data has shown that the absence of Lrp5 confers resistance to Wnt1-induced tumor development.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we show that all basal mammary cells express Lrp5, and co-express Lrp6 in a similar fashion. Though Wnt dependent transcription of key target genes is relatively unchanged in mammary epithelial cell cultures, the absence of Lrp5 specifically depletes adult regenerative stem cell activity (to less than 1%). Stem cell activity can be enriched by >200 fold (over 80% of activity), based on high Lrp5 expression alone. Though Lrp5 null glands have apparent normal function, the basal lineage is relatively reduced (from 42% basal/total epithelial cells to 22%) and Lrp5-/- mammary epithelial cells show enhanced expression of senescence-associated markers in vitro, as measured by expression of p16(Ink4a) and TA-p63.

CONCLUSIONS/SIGNIFICANCE

This is the first single biomarker that has been demonstrated to be functionally involved in stem cell maintenance. Together, these results demonstrate that Wnt signaling through Lrp5 is an important component of normal mammary stem cell function.

MMTV-Wnt1 and -DeltaN89beta-catenin induce canonical signaling in distinct progenitors and differentially activate Hedgehog signaling within mammary tumors

Canonical Wnt/β-catenin signaling regulates stem/progenitor cells and, when perturbed, induces many human cancers. A significant proportion of human breast cancer is associated with loss of secreted Wnt antagonists and mice expressing MMTV-Wnt1 and MMTV-ΔN89β-catenin develop mammary adenocarcinomas. Many studies have assumed these mouse models of breast cancer to be equivalent. Here we show that MMTV-Wnt1 and MMTV-ΔN89β-catenin transgenes induce tumors with different phenotypes. Using axin2/conductin reporter genes we show that MMTV-Wnt1 and MMTV-ΔN89β-catenin activate canonical Wnt signaling within distinct cell-types. ΔN89β-catenin activated signaling within a luminal subpopulation scattered along ducts that exhibited a K18⁺ER⁻PR⁻CD24^highCD49f^low profile and progenitor properties. In contrast, MMTV-Wnt1 induced canonical signaling in K14⁺ basal cells with CD24/CD49f profiles characteristic of two distinct stem/progenitor cell-types. MMTV-Wnt1 produced additional profound effects on multiple cell-types that correlated with focal activation of the Hedgehog pathway. We document that large melanocytic nevi are a hitherto unreported hallmark of early hyperplastic Wnt1 glands. These nevi formed along the primary mammary ducts and were associated with Hedgehog pathway activity within a subset of melanocytes and surrounding stroma. Hh pathway activity also occurred within tumor-associated stromal and K14⁺/p63⁺ subpopulations in a manner correlated with Wnt1 tumor onset. These data show MMTV-Wnt1 and MMTV-ΔN89β-catenin induce canonical signaling in distinct progenitors and that Hedgehog pathway activation is linked to melanocytic nevi and mammary tumor onset arising from excess Wnt1 ligand. They further suggest that Hedgehog pathway activation maybe a critical component and useful indicator of breast tumors arising from unopposed Wnt1 ligand.

The role of Wnt5a in prostate gland development

The Wnt genes encode a large family of secreted glycoproteins that play important roles in controlling tissue patterning, cell fate and proliferation during development. Currently, little is known regarding the role(s) of Wnt genes during prostate gland development. The present study examines the role of the noncanonical Wnt5a during prostate gland development in rat and murine models. In the rat prostate, Wnt5a mRNA is expressed by distal mesenchyme during the budding stage and localizes to periductal mesenchymal cells with an increasing proximal-to-distal gradient during branching morphogenesis. Wnt5a protein is secreted and localizes to periductal stroma, extracellular matrix and epithelial cells in the distal ducts. While Wnt5a expression is high during active morphogenesis in all prostate lobes, ventral prostate (VP) expression declines rapidly following morphogenesis while dorsal (DP) and lateral lobe (LP) expression remains high into adulthood. Steroids modulate prostatic Wnt5a expression during early development with testosterone suppressing Wnt5a and neonatal estrogen increasing expression. In vivo and ex vivo analyses of developing mouse and rat prostates were used to assess the functional roles of Wnt5a. Wnt5a(-/-) murine prostates rescued by organ culture exhibit disturbances in bud position and directed outgrowth leading to large bulbous sacs in place of elongating ducts. In contrast, epithelial cell proliferation, ductal elongation and branchpoint formation are suppressed in newborn rat prostates cultured with exogenous Wnt5a protein. While renal grafts of Wnt5a(-/-) murine prostates revealed that Wnt5a is not essential for cyto- and functional differentiation, a role in luminal cell polarity and lumenization of the ducts was indicated. Wnt5a suppresses prostatic Shh expression while Shh stimulates Wnt5a expression in a lobe-specific manner during early development indicating that Wnt5a participates in cross-talk with other members of the gene regulatory network that control prostate development. Although Wnt5a does not influence prostatic expression of other Wnt morphogens, it suppresses Wif-1 expression and can thus indirectly modulate Wnt signaling. In summary, the present finds demonstrate that Wnt5a is essential for normal prostate development where it regulates bud outgrowth, ductal elongation, branching, cell polarity and lumenization. These findings contribute to the growing body of knowledge on regulatory mechanisms involved in prostate gland development which are key to understanding abnormal growth processes associated with aging.

WNT10B functional dualism: beta-catenin/Tcf-dependent growth promotion or independent suppression with deregulated expression in cancer

We found aberrant DNA methylation of the WNT10B promoter region in 46% of primary hepatocellular carcinoma (HCC) and 15% of colon cancer samples. Three of 10 HCC and one of two colon cancer cell lines demonstrated low or no expression, and 5-aza-2'deoxycytidine reactivated WNT10B expression with the induction of demethylation, indicating that WNT10B is silenced by DNA methylation in some cancers, whereas WNT10B expression is up-regulated in seven of the 10 HCC cell lines and a colon cancer cell line. These results indicate that WNT10B can be deregulated by either overexpression or silencing in cancer. We found that WNT10B up-regulated beta-catenin/Tcf activity. However, WNT10B-overexpressing cells demonstrated a reduced growth rate and anchorage-independent growth that is independent of the beta-catenin/Tcf activation, because mutant beta-catenin-transduced cells did not suppress growth, and dominant-negative hTcf-4 failed to alleviate the growth suppression by WNT10B. Although WNT10B expression alone inhibits cell growth, it acts synergistically with the fibroblast growth factor (FGF) to stimulate cell growth. WNT10B is bifunctional, one function of which is involved in beta-catenin/Tcf activation, and the other function is related to the down-regulation of cell growth through a different mechanism. We suggest that FGF switches WNT10B from a negative to a positive cell growth regulator.

Human breast epithelial stem cells and their regulation

This review summarizes the current evidence for the existence of human breast stem cells and the pathways involved in their regulation, and discusses how the disruption of these pathways may result in the generation of a population of cells with the capacity for unlimited self-renewal. Relevant data from mouse model systems are also discussed where appropriate. By understanding the molecular pathways that regulate self-renewal of normal mammary stem cells, it may be possible to target the activation of these pathways in breast tumours.

Targeted activation of beta-catenin signaling in basal mammary epithelial cells affects mammary development and leads to hyperplasia

Wnt/beta-catenin signaling pathway is involved in the maintenance of the progenitor cell population in the skin, intestine and other tissues, and its aberrant activation caused by stabilization of beta-catenin contributes to tumorigenesis. In the mammary gland, constitutive activation of Wnt/beta-catenin signaling in luminal secretory cells results in precocious lobuloalveolar differentiation and induces adenocarcinomas, whereas the impact of this signaling pathway on the function of the second major mammary epithelial cell lineage, the basal myoepithelial cells, has not been analyzed. We have used the keratin (K) 5 promoter to target the expression of stabilized N-terminally truncated beta-catenin to the basal cell layer of mouse mammary epithelium. The transgenic mice presented an abnormal mammary phenotype: precocious lateral bud formation, increased proliferation and premature differentiation of luminal epithelium in pregnancy, persistent proliferation in lactation and accelerated involution. Precocious development in pregnancy was accompanied by increased Myc and cyclin D1 transcript levels, and a shift in p63 variant expression towards the DeltaNp63 form. The expression of ECM-degrading proteinases and their inhibitors was altered in pregnancy and involution. Nulliparous transgenic females developed mammary hyperplasia that comprised undifferentiated basal (K5/14-positive, K8- and alpha-smooth muscle-actin-negative) cells. Multiparous mice, in addition, developed invasive basal-type carcinomas. Thus, activation of beta-catenin signaling in basal mammary epithelial cells affects the entire process of mammary gland development and induces amplification of basal-type cells that lack lineage markers, presumably, a subpopulation of mammary progenitors able to give rise to tumors.

Beta-catenin and Tcfs in mammary development and cancer

Beta-catenin regulates cell-cell adhesion and transduces signals from many pathways to regulate the transcriptional activities of Tcf/Lef DNA binding factors. Gene ablation and transgenic expression studies strongly support the concept that beta-catenin together with Lef/Tcf factors act as a switch to determine cell fate and promote cell survival and proliferation at several stages during mammary gland development. Mice expressing the negative regulator of Wnt/beta-catenin signaling (K14-Dkk) fail to form mammary buds, and those lacking Lef-1 show an early arrest in this process at stage E13.5. Stabilized deltaN89beta-catenin initiates precocious alveologenesis during pubertal development, and negative regulators of endogenous beta-catenin signaling suppress normal alveologenesis during pregnancy. Stabilized beta-catenin induces hyperplasia and mammary tumors in mice. Each of the beta-catenin-induced phenotypes is accompanied by upregulation of the target genes cyclin D1 and c-myc. Cyclin D1, however, is dispensable for tumor formation and the initiation of alveologenesis but is essential for later alveolar expansion.

Primary breast myoepithelial cells exert an invasion-suppressor effect on breast cancer cells via paracrine down-regulation of MMP expression in fibroblasts and tumour cells

In normal breast and ductal carcinoma in situ, myoepithelial cells form an incomplete layer separating the epithelial compartment from the stromal environment. Transition to invasive disease is marked by penetration of the myoepithelial-basement membrane (BM) interface. One mechanism involved in tumour invasion is breakdown of extracellular matrices by matrix metalloproteinases (MMPs). It was hypothesized that myoepithelial cells may modulate tumour invasion by controlling MMP gene expression, both in tumour cells and in peri-ductal fibroblasts. To investigate this, myoepithelial cells from normal breast were purified and characterized and their effect on tumour cell invasive potential was assessed. The effect on MMP gene expression of breast cancer cells cultured alone or in combination with primary normal breast fibroblasts was also analysed using RT-PCR with ELISA quantitation, with zymographic analysis to measure enzyme activity. Normal breast myoepithelial cells significantly reduced invasion by the breast cancer cell lines MCF-7, T47D, MDA-MB 231, and MDA-MB 468 when they were cultured alone or in the presence of a fibroblast population. Reduced invasion was associated with changes in MMP gene expression. In those tumour cells expressing MMP, there was a significant down-regulation of MMP-2 (MDA-MB 468, p<0.001), MMP-9 (MDA-MB 231, p=0.05; MDA-MB 468, p<0.001), and MT1-MMP (p<0.001 for both MDA-MB 231 and MDA-MB 468). Myoepithelial cells also caused a significant decrease in MMP gene expression in co-cultured fibroblasts. Furthermore, this was associated with reduced gelatinolytic activity as identified by zymography. This study demonstrates for the first time that primary myoepithelial cells from normal breast reduce breast cancer cell invasion and that this is mediated via modulation of both tumour cell and fibroblast function. This emphasizes the importance of the myoepithelial cell in controlling the breast microenvironment and focuses on the potential significance of the loss of this population with disease progression.

Mouse strain-specific patterns of mammary epithelial ductal side branching are elicited by stromal factors

Variations in mammary ductal side branching patterns are known to occur between different strains of mice and this is related to the rate of spontaneous mammary cancers, which are increased in those strains which show highly side-branched mammary architecture. The cause of the variation in ductal side branching between mouse strains is unknown, but epithelial, stromal, and endocrine factors have been implicated. To define the mammary elements responsible for controlling strain-specific ductal side branching patterns, we formed recombined mammary glands from epithelial and stroma elements taken from highly side-branched 129 and poorly side-branched C57BL/6J mammary glands and transplanted them to Rag1(-/-) hosts on the inbred C57BL/6J background. When 129 epithelium was recombined with C57BL/6J stroma the poorly side-branched C57BL/6J pattern was observed. C57BL/6J epithelium recombined with 129 stroma resulted in development of the highly side- branched pattern, as did 129 epithelium recombined with 129 stroma. All transplants used the same C57BL/6J endocrine background, demonstrating that strain differences in the mammary stroma are responsible for the strain-specific ductal side branching patterns and that strain differences in epithelium or endocrine background play no part. Genes currently known to influence side branching by means of the stroma include activin/inhibin, epidermal growth factor receptor (EGFR), Wnt-2, Wnt-5a, and Wnt-6. Of these, Wnt-5a mRNA expression was decreased in 129 mammary glands compared with C57BL/6J mammary glands, but in F2 129:C57BL/6J animals Wnt-5a mRNA expression level did not correlate with the highly variable side branching patterns observed. These experiments exclude variation in the expression level of known candidate genes as the mechanism responsible. Regardless of underlying mechanism, transplantation without regard to the genetic background of the stromal donor, whether inbred or mixed, will compromise experiments with side branching and associated gene expression endpoints.

Hormone/growth factor interactions mediating epithelial/stromal communication in mammary gland development and carcinogenesis

Epithelial/mesenchymal interactions begin during embryonic development of the mammary gland and continue throughout mammary gland development into adult life. Stromal and epithelial growth factors that may mediate interactions between these compartments of the mammary gland are reviewed. Since mammogenic hormones are the primary regulators of mammary gland development, special consideration is given to hormonal regulation of growth factors in order to explore the integration of hormones and growth factors in the regulation of mammary gland growth and neoplasia. Examination of hormonal regulation of the fibroblast growth factor (FGF)-7/FGFR2-IIIb receptor system in the mammary gland reveals that mammogenic hormones differentially regulate the synthesis of stromal growth factors and their epithelial receptors. These effects serve to optimize the action of estrogen and progesterone on mammary gland development and illustrate that the ratio of these two hormones is critical in regulating this growth factor axis. The role of stromal/epithelial mitogenic microenvironments in modulating the genotype and phenotype of preneoplastic and neoplastic lesions by chemical carcinogens is discussed. Finally, changes in growth factor expression during mammary tumor progression are described to illustrate the relative roles that stromally-derived and epithelial-derived growth factors may play during progression to hormone independent tumor growth.

The importance of being a myoepithelial cell

The mammary myoepithelial cell was named the 'Cinderella of mammary cell biology' in light of the earlier focus on the luminal cell. Mammary myoepithelial cells have recently been described as 'natural tumour suppressors'. We now need to understand more about their origin and to reconsider their place in the complex process of mammary morphogenesis. In the present review, we discuss the lineage segregation of mammary myoepithelial cells and their functions in mammary gland development. These functions include their effects on luminal cell growth and differentiation, their key role in the establishment of the polarised mammary epithelial bilayer and the control of stromal invasion in breast cancer.

Wnts and the female reproductive system

Wnts are intercellular growth and differentiation factors that regulate several key developmental steps, such as gastrulation, neurulation, and organogenesis, including the development of the midbrain, central nervous system, kidney, and limbs. Wnts are also needed for a normal development of the reproductive system. Deficiency of Wnt-4, -5a, and -7a, for example, results in sex reversal, infertility, and/or malformation of the internal and external genitals. Here we focus on the importance of Wnts in the female reproductive system.

Delta N89 beta-catenin induces precocious development, differentiation, and neoplasia in mammary gland

To investigate the role of beta-catenin in mammary gland development and neoplasia, we expressed a stabilized, transcriptionally active form of beta-catenin lacking the NH(2)-terminal 89 amino acids (Delta N 89 beta-catenin) under the control of the mouse mammary tumor virus long terminal repeat. Our results show that Delta N 89 beta-catenin induces precocious lobuloalveolar development and differentiation in the mammary glands of both male and female mice. Virgin Delta N 89 beta-catenin mammary glands resemble those found in wild-type (wt) pregnant mice and inappropriately express cyclin D1 mRNA. In contrast to wt mammary glands, which resume a virgin appearance after cessation of lactation, transgenic mammary glands involute to a midpregnant status. All transgenic females develop multiple aggressive adenocarcinomas early in life. Surprisingly, the Delta N89 beta-catenin phenotype differs from those elicited by overexpression of Wnt genes in this gland. In particular, Delta N 89 beta-catenin has no effect on ductal side branching. This suggests that Wnt induction of ductal branching involves additional downstream effectors or modulators.

Developmental role of the SNF1-related kinase Hunk in pregnancy-induced changes in the mammary gland

The steroid hormones 17 beta-estradiol and progesterone play a central role in the pathogenesis of breast cancer and regulate key phases of mammary gland development. This suggests that developmental regulatory molecules whose activity is influenced by ovarian hormones may also contribute to mammary carcinogenesis. In a screen designed to identify protein kinases expressed in the mammary gland, we previously identified a novel SNF1-related serine/threonine kinase, Hunk (hormonally upregulated Neu-associated kinase). During postnatal mammary development, Hunk mRNA expression is restricted to a subset of mammary epithelial cells and is temporally regulated with highest levels of expression occurring during early pregnancy. In addition, treatment of mice with 17 beta-estradiol and progesterone results in the rapid and synergistic upregulation of Hunk expression in a subset of mammary epithelial cells, suggesting that the expression of this kinase may be regulated by ovarian hormones. Consistent with the tightly regulated pattern of Hunk expression during pregnancy, mammary glands from transgenic mice engineered to misexpress Hunk in the mammary epithelium manifest temporally distinct defects in epithelial proliferation and differentiation during pregnancy, and fail to undergo normal lobuloalveolar development. Together, these observations suggest that Hunk may contribute to changes in the mammary gland that occur during pregnancy in response to ovarian hormones.

Retroviral expression of Wnt-1 and Wnt-7b produces different effects in mouse mammary epithelium

Several Wnt genes are expressed in the postnatal mouse mammary gland and are thought to be involved in mammary gland development. Ectopic expression of Wnt-1, which is not normally expressed in the mammary gland, drives the formation of a pre-neoplastic hyperplasia. Cell culture-based assays have shown that Wnt-1 and some mammary-expressed Wnts transform C57MG cells. This has led to the suggestion that Wnt-1 functions as an oncogene through the inappropriate activation of developmental events that are normally controlled by the ‘transforming’ class of Wnts. In this study, Wnt-7b was expressed in vivo using recombinant retroviruses. Wnt-7b did not alter normal mammary gland development despite having similar effects to Wnt-1 in cell culture. We conclude that the in vitro classification of Wnts as ‘transforming’ does not correlate with the transformation in vivo. To facilitate the analysis of Wnt-expression, a lacZ-containing, bicistronic recombinant retrovirus was developed. Immunohistochemistry and electron microscopy identified retrovirally transduced myoepithelial and luminal epithelial cells in normal and hyperplastic tissues. The distribution of transduced cells in mammary outgrowths was consistent with current models of mammary stem cell identity.

Transient overexpression of murine dishevelled genes results in apoptotic cell death

The Dishevelled (Dvl) gene family encodes cytoplasmic proteins that are implicated in Wnt signal transduction. In mammals, the manner in which Wnt signals are transduced remains unclear. The biochemical and molecular mechanisms defining the Wnt-1 pathway are of great interest because of its important role in development and its activation in murine breast tumors. In order to elucidate Dvl's role in Wnt signaling, we attempted to overexpress Dvl in cells, but were unable to obtain stable cell lines. We show here that the overexpression of Dvl genes alters nuclear and cellular morphology of COS-1 and C57MG cells and causes cell death due to the induction of apoptosis. Deletion studies demonstrate that all three conserved domains of Dvl (DIX, PDZ, and DEP) are required for Dvl-mediated cell death. Coexpression of protein phosphatase 2Calpha, a Dvl-interacting protein identified in yeast two-hybrid studies, protects cells from the cell death observed in cells overexpressing Dvl alone. Furthermore, the adenomatous polyposis coli (APC) gene product appears to be required for Dvl-mediated cell death. The relevance of these findings to Wnt signal transduction, as well as to developmental processes and disease, are discussed.

Cellular mechanisms of wingless/Wnt signal transduction

Wg/Wnt signaling regulates cell proliferation and differentiation in species as divergent as nematodes, flies, frogs, and humans. Many components of this highly conserved process have been characterized and work from a number of laboratories is beginning to elucidate the mechanism by which this class of secreted growth factor triggers cellular decisions. The Wg/Wnt ligand apparently binds to Frizzled family receptor molecules to initiate a signal transduction cascade involving the novel cytosolic protein Dishevelled and the serine/threonine kinase Zeste-white 3/GSK3. Antagonism of Zw3 activity leads to stabilization of Armadillo/beta-catenin, which provides a transactivation domain when complexed with the HMG box transcription factor dTCF/LEF-1 and thereby activates expression of Wg/Wnt-responsive genes. The Wg/Wnt ligands pass through the secretory pathway and associate with extracellular matrix components; recent work shows that sulfated glycosaminoglycans are essential for proper transduction of the signal. Mutant forms of Wg in Drosophila reveal separable aspects of Wg function and suggest that proper transport of the protein across cells is essential for cell fate specification. Complex interactions with the Notch and EGF/Ras signaling pathways also play a role in cell fate decisions during different phases of Drosophila development. These many facets of Wg/Wnt signaling have been elucidated through studies in a variety of species, each with powerful and unique experimental approaches. The remarkable conservation of this pathway suggests that Wg/Wnt signal transduction represents a fundamental mechanism for the generation of diverse cell fates in animal embryos.

Signaling through the stromal epidermal growth factor receptor is necessary for mammary ductal development

Stromal-epithelial interactions are critical in determining patterns of growth, development and ductal morphogenesis in the mammary gland, and their perturbations are significant components of tumorigenesis. Growth factors such as epidermal growth factor (EGF) contribute to these reciprocal stromal-epithelial interactions. To determine the role of signaling through the EGF receptor (EGFR) in mammary ductal growth and branching, we used mice with a targeted null mutation in the Egfr. Because Egfr−/− mice die perinatally, transplantation methods were used to study these processes. When we transplanted neonatal mammary glands under the renal capsule of immuno-compromised female mice, we found that EGFR is essential for mammary ductal growth and branching morphogenesis, but not for mammary lobulo-alveolar development. Ductal growth and development was normal in transplants of mammary epithelium from Egfr−/− mice into wild-type (WT) gland-free fat pads and in tissue recombinants prepared with WT stroma, irrespective of the source of epithelium (StromaWT/Epi−/−, StromaWT/EpiWT). However, ductal growth and branching was impaired in tissue recombinants prepared with Egfr−/− stroma (Stroma−/−/EpiWT, Stroma−/−/Epi−/−). Thus, for ductal morphogenesis, signaling through the EGFR is required only in the stromal component, the mammary fat pad. These data indicate that the EGFR pathway plays a key role in the stromal-epithelial interactions required for mammary ductal growth and branching morphogenesis. In contrast, signaling through the EGFR is not essential for lobulo-alveolar development. Stimulation of lobulo-alveolar development in the mammary gland grafts by inclusion of a pituitary isograft under the renal capsule as a source of prolactin resulted in normal alveolar development in both Egfr−/− and wild-type transplants. Through the use of tissue recombinants and transplantation, we have gained new insights into the nature of stromal-epithelial interactions in the mammary gland, and how they regulate ductal growth and branching morphogenesis.

Differential expression patterns of Wnt genes in the murine female reproductive tract during development and the estrous cycle

The murine female reproductive tract differentiates during postnatal development. This process of cytodifferentiation and morphogenesis is dependent upon specific mesenchymal-epithelial interactions as well as circulating steroid hormones (Cunha, G.R., 1976. Int. Rev. Cytol. 47, 137-194; Pavlova, A. et al., 1994. Development 120, 335-346). Members of the Wnt family of signaling molecules have been recently identified in this system (Pavlova, A. et al., 1994. Development 120, 335-346; Bui, T.D. et al., 1997. Br. J. Cancer 75, 1131-1136; Miller, C., Sassoon, D.A., 1998. Development, in press). We describe the expression patterns of Wnt genes in the developing and adult female reproductive tract. Additionally, we note that changes in the levels of expression occur during the estrous cycle. Wnt gene expression patterns are regulated by the presence of epithelium in tissue graft experiments, suggesting that Wnt genes may indeed play roles in the mesenchymal-epithelial interactions critical for female reproductive tract development and function.

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

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

Fibroblast growth factor receptor signalling has a role in lobuloalveolar development of the mammary gland

We have used the mouse mammary tumor virus promoter to express two dominant negative (DN) fibroblast growth factor receptor (FGFR) isoforms in the mammary epithelium of transgenic mice. While expression of DN-FGFR1(IIIc) showed no discernible phenotype, a similar kinase negative form of FGFR2(IIIb) caused a marked impairment of lobuloalveolar development. The growth retardation was apparent by mid-pregnancy and persisted in the post-partum glands. Despite the substantial underdevelopment of the mammary gland there was a measurable lactational response, but it was insufficient to properly sustain the new-born pups. These findings demonstrate that fibroblast growth factor signalling is necessary for pregnancy dependent lobuloalveolar development of the mammary gland.

Xwnt-2b is a novel axis-inducing Xenopus Wnt, which is expressed in embryonic brain

Xwnt-2b is a novel member of the Wnt gene family and is 73-74% similar to human and mouse Wnt-2 proteins. Starting from stage 15, Xwnt-2b transcripts are localized to a non-contiguous stripe in the anterior neural plate of the Xenopus embryo. In the tailbud, Xwnt-2b is expressed along the dorsoanterior side of the prosencephalon-mesencephalon boundary. At the tadpole stages, the brain-specific expression fades, but the total amount of Xwnt-2b mRNA does not decline due to activation of its expression in non-brain areas. Microinjection of Xwnt-2b mRNA into a ventral blastomere of 4-8-cell embryos results in the formation of complete secondary body axes. These results suggest that Xwnt-2b is a member of the axis-inducing Wnts and that it is involved in brain development and in later organogenesis.

Developmental expression of Brca2 colocalizes with Brca1 and is associated with proliferation and differentiation in multiple tissues

Germline mutations in the putative tumor suppressor gene, BRCA1, predispose women to dramatically elevated risks of breast cancer, while germline mutations in the structurally unrelated gene, BRCA2, predispose both men and women to breast cancer. Recent studies have suggested an important developmental role for the murine homologue of BRCA1 in the regulation of proliferation and differentiation. At the present time, however, little is known about the developmental role of BRCA2 or the regulation of its expression in vivo. We have determined the spatial and temporal pattern of expression of the murine homologue of BRCA2 during fetal development, in adult tissues, and in the mammary gland during postnatal development. Our results indicate that Brca2 mRNA expression is highest in proliferating cellular compartments, particularly those undergoing differentiation. In the breast, Brca2 expression is developmentally regulated and is induced during puberty and pregnancy and as a result of parity. Surprisingly, in multiple fetal and adult tissues the spatial and temporal pattern of Brca2 mRNA expression is virtually indistinguishable from that of Brca1, despite the fact that these genes display no homology. These observations suggest that Brca2 is involved in the processes of proliferation and differentiation in the mammary gland and other tissues, and that Brca1 and Brca2 mRNA expression may be regulated by similar pathways and stimuli in multiple cell types. Interestingly, however, our analysis reveals that Brca1 and Brca2 expression are differentially regulated during the development of specific endocrine target tissues, such as the testis during spermatogenesis and the breast during pregnancy. In addition, the ratio of mRNA expression in the mammary glands of adult females relative to adult males is significantly greater for Brca1 than for Brca2. These observations imply that Brca1 and Brca2 mRNA expression are differentially regulated by sex hormones. In order to test this hypothesis, we have analyzed the expression of these two breast cancer susceptibility genes in ovariectomized mice treated with 17beta-estradiol and progesterone. Our results demonstrate that the up-regulation of mRNA expression in the breast by ovarian hormones is significantly greater for Brca1 than for Brca2. These observations suggest that the gender-specific differences in phenotype associated with germline mutations in BRCA2 versus BRCA1 may be related to the differential regulation of these genes by sex hormones.

Expression and hormone regulation of Wnt2, 3, 4, 5a, 7a, 7b and 10b in normal human endometrium and endometrial carcinoma

Wnt genes are transforming to mouse breast epithelium and are hormonally regulated in vivo. To assess their role in another endocrine-responsive human cancer, the expression of seven Wnt genes (Wnt 2, 3, 4, 5a, 7a, 7b and 10b) in normal human endometrium and endometrial cells, and endometrial carcinoma tissues and cell lines was investigated by ribonuclease protection analysis. Wnt2, 3, 4 and 5a mRNAs but not Wnt7a, 7b or 10b mRNAs were expressed in primary culture of normal endometrial epithelial (NEE) and stromal (NES) cells. In contrast, in four endometrial carcinoma cell lines (RL95-2, HEC-1-A, AN3 CA and Ishikawa), Wnt2 and Wnt3 mRNAs were absent. Wnt4 was expressed in only one out of four cell lines (RL95-2), and Wnt5a was much lower. Wnt7a and Wnt7b mRNAs were expressed in three out of four cell lines (RL95-2, HEC-1-A and Ishikawa). Wnt10b mRNA was expressed in RL95-2 and AN3 CA. In fresh tissues, all Wnt genes apart from Wnt10b were expressed in normal endometrium and endometrial carcinoma. Similar to the cell lines, the level of Wnt4 mRNA expression was significantly higher in the normal endometrium than endometrial carcinoma. Wnt2, 3 and 5a mRNAs were also lower in endometrial carcinoma compared with normal endometrium. There was no difference in the level of Wnt2, 3, 4 and 5a mRNA expression between proliferative phase and secretory phase of the menstrual cycle, or between either menstrual phase and the first trimester of pregnancy. In vitro, progesterone and/or 17beta-oestradiol had no effect on Wnt2, 3, 4, 5a and 7b mRNA expression in NES and all endometrial carcinoma cell lines. The data indicate that all Wnt genes were expressed in vitro, six out of seven Wnt genes (Wnt 2, 3, 4, 5a, 7a and 7b) were expressed endogenously in the human endometrium, their mRNA expression was hormonally independent and Wnt4 gene down-regulation as well as down-regulation of Wnt 2, 3 and 5a may be associated with endometrial carcinoma.

Targeted disruption of the Wnt2 gene results in placentation defects

Wnt genes have been implicated in a range of developmental processes in the mouse including the patterning of the central nervous system and limbs. Reported here for the first time is the expression of Wnt2 in the early heart field of 7.5-8.5 dpc (days post-coitum) mouse embryos, making Wnt2 a potentially useful gene marker for the early stages of heart development. Expression was also detected in the allantois from 8.0 dpc and at later stages in the placenta and umbilicus. Mice deficient in Wnt2, generated by gene targeting, displayed runting and approximately 50% died perinatally. Histological analysis revealed alterations in the size and structure of placentas from these mice from 14.5 dpc. The placental defects were associated primarily with the labyrinthine zone and included oedema and tissue disruption and accumulation of maternal blood in large pools. There was also an apparent decrease in the number of foetal capillaries and an increase in the amount of fibrinoid material in the Wnt2 mutant placentas. These results suggest that Wnt2 is required for the proper vascularisation of the mouse placenta and the placental defects in Wnt2-deficient mice result in a reduction in birthweight and perinatal lethality.

Regulation of Msx-1, Msx-2, Bmp-2 and Bmp-4 during foetal and postnatal mammary gland development

Expression of the Msx-1 and Msx-2 homeobox genes have been shown to be coordinately regulated with the Bmp-2 and Bmp-4 ligands in a variety of developing tissues. Here we report that transcripts from all four genes are developmentally regulated during both foetal and postnatal mammary gland development. The location and time-course of the Bmp and Msx expression point to a role for Msx and Bmp gene products in the control of epithelial-mesenchymal interactions. Expression of Msx-2, but not Msx-1, Bmp-2 or Bmp-4 was decreased following ovariectomy, while expression of the human Msx-2 homologue was regulated by 17beta-oestradiol in the MCF-7 breast cancer cell line. The regulation of Msx-2 expression by oestrogen raises the possibility that hormonal regulation of mammary development is mediated through the control of epithelial-mesenchymal interactions.