A retrovirus vector expressing the putative mammary oncogene int-1 causes partial transformation of a mammary epithelial cell line

Nuclear Dishevelled: An enigmatic role in governing cell fate and Wnt signaling

The Dishevelled gene was first identified in Drosophila mutants with disoriented hair and bristle polarity and subsequent work has now demonstrated its importance in critical and diverse aspects of biology. Since those early discoveries, Dishevelled has been shown to coordinate a plethora of developmental and cellular processes that range from controlling cell polarity during gastrulation to partnering with chromatin modifying enzymes to regulate histone methylation at genomic loci. While the role of DVL in development is well-respected and the cytosolic function of DVL has been studied more extensively, its nuclear role continues to remain murky. In this review we highlight some of the seminal discoveries that have contributed to the field, but the primary focus is to discuss recent advances with respect to the nuclear role of Dishevelled. This nuclear function of Dishevelled is a dimension which is proving to be increasingly important yet remains enigmatic.

PLEKHA4/kramer Attenuates Dishevelled Ubiquitination to Modulate Wnt and Planar Cell Polarity Signaling

Wnt signaling pathways direct key physiological decisions in development. Here, we establish a role for a pleckstrin homology domain-containing protein, PLEKHA4, as a modulator of signaling strength in Wnt-receiving cells. PLEKHA4 oligomerizes into clusters at PI(4,5)P2-rich regions of the plasma membrane and recruits the Cullin-3 (CUL3) E3 ubiquitin ligase substrate adaptor Kelch-like protein 12 (KLHL12) to these assemblies. This recruitment decreases CUL3-KLHL12-mediated polyubiquitination of Dishevelled, a central intermediate in canonical and non-canonical Wnt signaling. Knockdown of PLEKHA4 in mammalian cells demonstrates that PLEKHA4 positively regulates canonical and non-canonical Wnt signaling via these effects on the Dishevelled polyubiquitination machinery. In vivo knockout of the Drosophila melanogaster PLEKHA4 homolog, kramer, selectively affects the non-canonical, planar cell polarity (PCP) signaling pathway. We propose that PLEKHA4 tunes the sensitivities of cells toward the stimulation of Wnt or PCP signaling by sequestering a key E3 ligase adaptor controlling Dishevelled polyubiquitination within PI(4,5)P2-rich plasma membrane clusters.

Dishevelled: A masterful conductor of complex Wnt signals

The Dishevelled gene was first identified in Drosophila mutants with disoriented hair and bristle polarity [1-3]. The Dsh gene (Dsh/Dvl, in Drosophila and vertebrates respectively) gained popularity when it was discovered that it plays a key role in segment polarity during early embryonic development in Drosophila [4]. Subsequently, the vertebrate homolog of Dishevelled genes were identified in Xenopus (Xdsh), mice (Dvl1, Dvl2, Dvl3), and in humans (DVL1, DVL2, DVL3) [5-10]. Dishevelled functions as a principal component of Wnt signaling pathway and governs several cellular processes including cell proliferation, survival, migration, differentiation, polarity and stem cell renewal. This review will revisit seminal discoveries and also summarize recent advances in characterizing the role of Dishevelled in both normal and pathophysiological settings.

Establishing estrogen-responsive mouse mammary organoids from single Lgr5+ cells

Recent evidence suggests that mammary cells expressing R-spondin receptor and Wnt pathway regulator Lgr5, regarded as a stem cell marker in multiple tissues, might represent mammary stem cells (MaSCs). Whether L gr5 marks a multipotent subpopulation of Lin-CD24^low/medCD49f^high MaSCs remains controversial. To some extent the differing results reflect different assays used to assess properties of stemness, including lineage tracing in vivo, mammosphere culture, and mammary fat pad transplantation assays. To address this issue directly, we isolated Lgr5⁺ cells from mammary glands of Lgr5-lacZ mice and established organoids based on principles adapted from studies of Wnt-driven Lgr5⁺ cell populations in other organs. Mammary organoids were grown from single Lgr5⁺ mammary cells in Matrigel, the substratum of choice for intestinal organoids, and in a growth factor cocktail containing EGF, Wnt3a and R-spondin, designed to optimally activate the endogenous Wnt signaling program of stem cells. Colonies derived from single Lgr5⁺ cells manifest at least four distinct cell populations: Lgr5⁺ and Lgr5^- basal cells and c-Kit⁺ and c-Kit^- luminal cells that spontaneously organize into a ductal structure with basal cells around the periphery and luminal cells lining an interior cavity, reminiscent of normal mammary duct structure. Lgr5⁺ cell-derived organoids were sustainable during prolonged passaging. In contrast, although Lgr5^- cells expand into primary colonies, colony-forming efficiency immediately dissipated upon passaging. Furthermore, reproductive hormones induce epithelial cell proliferation resulting in marked increases in lumen diameter accompanied by squamous transdifferentiation. We propose this estrogen-responsive, self-organizing duct-like structure derived from single murine Lgr5⁺ mammary cells represents a "mini-breast" organoid.

MiR-23a/-24-induced gene silencing results in mesothelial cell integration of pancreatic cancer

Background:

Invasion of the surrounding tissue is part of the metastatic cascade. Here, we examined the invasion of pancreatic ductal adenocarcinoma (PDAC) cells into the mesothelial barrier and identified the related microRNA (miRNA) expression profiles.

Methods:

The interactions between PDAC cells and mesothelial monolayers were characterised and quantified using a specific time-lapse videomicroscopy assay. Pancreatic ductal adenocarcinoma cells were further evaluated using the adhesion assay, and miRNA, mRNA and protein expressions were determined using microarray, q-RT–PCR and western blots, respectively. These data were correlated with in vivo dissemination scores.

Results:

Two groups of PDAC cell lines were distinguished by their integration capacity into the mesothelial monolayer using mean elongation factors (MEFs). Adhesion assays showed a concordant relation between adhesive properties and integration capacity. The distant metastases scores were reverse correlated with MEFs. Microarray analysis of these groups revealed that miR-23a and/or miR-24 target for FZD5, HNF1B and/or TMEM92, respectively, and that they are significantly deregulated.

Conclusions:

MiR-23a and/or miR-24 overexpression leads to gene silencing of FZD5, TMEM92 and/or HNF1B. Their downregulation induces deregulated expression and degradation of E-cadherin and β-catenin causing destabilisation of the cadherin/catenin complex, and altered the expression of Wnt-related genes. We propose a molecular (epi)genetic mechanism by which increased EMT-like cell shape transformation and integration into mesothelial monolayers of PDAC cells can be observed.

Insights into the role of cell-cell junctions in physiology and disease

Contacting cells establish different classes of intricate structures at the cell-cell junctions. These structures are of increasing research interest as they regulate a broad variety of processes in development and disease. Further, in vitro studies are revealing that various cell-cell interaction proteins are involved not only in cell-cell processes but also in many additional aspects of physiology, such as migration and apoptosis. This chapter reviews the basic classification of cell-cell junctional structures and some of their representative proteins. Their roles in development and disease are briefly outlined, followed by a section on contemporary methods for probing cell-cell interactions and some recent developments. This chapter concludes with a few suggestions for potential research directions to further develop this promising area of study.

Three decades of Wnts: a personal perspective on how a scientific field developed

Wnt genes and components of Wnt signalling pathways have been implicated in a wide spectrum of important biological phenomena, ranging from early organismal development to cell behaviours to several diseases, especially cancers. Emergence of the field of Wnt signalling can be largely traced back to the discovery of the first mammalian Wnt gene in 1982. In this essay, we mark the thirtieth anniversary of that discovery by describing some of the critical scientific developments that led to the flowering of this field of research.

An RNAi-based chemical genetic screen identifies three small-molecule inhibitors of the Wnt/wingless signaling pathway

Misregulated β-catenin responsive transcription (CRT) has been implicated in the genesis of various malignancies, including colorectal carcinomas, and it is a key therapeutic target in combating various cancers. Despite significant effort, successful clinical implementation of CRT inhibitory therapeutics remains a challenging goal. This is, in part, because of the challenge of identifying inhibitory compounds that specifically modulate the nuclear transcriptional activity of β-catenin while not affecting its cytoskeletal function in stabilizing adherens junctions at the cell membrane. Here, we report an RNAi-based modifier screening strategy for the identification of CRT inhibitors. Our data provide support for the specificity of these inhibitory compounds in antagonizing the transcriptional function of nuclear β-catenin. We show that these inhibitors efficiently block Wnt/β-catenin-induced target genes and phenotypes in various mammalian and cancer cell lines. Importantly, these Wnt inhibitors are specifically cytotoxic to human colon tumor biopsy cultures as well as colon cancer cell lines that exhibit deregulated Wnt signaling.

Effect of Wnt-1 antisense RNA on the outgrowth of a mammary adenocarcinoma cell line expressing that oncogene

Aims-To investigate the effect of Wnt-1 antisense RNA on the outgrowth of a mammary tumour cell line expressing that oncogene.Methods-A plasmid (pMT 70), containing Wnt-1 cDNA, was cut with appropriate enzymes and inserted into a eukaryotic expression vector (pMAMneo). A mammary tumour cell line (CAC-L153) was transfected with the expression vector and cells with the vector in sense and antisense orientation were selected.Results-Tumour cells with the expression vector in the antisense orientation had a notable reduction in expression of Wnt-1 protein and a considerable reduction in tumour outgrowth compared with controls.Conclusions-The results indicate that the Wnt-1 proto-oncogene may be a possible target for antisense therapy.

Striking the target in Wnt-y conditions: intervening in Wnt signaling during cancer progression

Wnt signaling can be divided into three pathways, namely the canonical Wnt/beta-catenin pathway, and the non-canonical (or heretical) Wnt/Ca(2+) and planar cell polarity (PCP) pathways. Although the canonical Wnt/beta-catenin pathway is the best described in cancer, increasing data points to the importance of the heretical Wnt pathways in several aspects of tumor progression. The recent advances in understanding the players and mechanisms by which these Wnt pathways contribute to cancer progression have led to the identification of numerous molecules that are already, or could be considered, targets for cancer therapy.

Wnt therapy for bone loss: golden goose or Trojan horse?

The Wnt pathway has been found to play a role in the development of many tissues and to spur growth and differentiation of adult osteoblasts, sparking interest in its potential clinical application for bone growth. However, when deregulated, this pathway can be oncogenic in some tissues. In this issue of the JCI, Kansara and colleagues reveal that Wnt inhibitory factor 1 is epigenetically silenced in human osteosarcomas and that its absence augments osteosarcoma formation in mice (see the related article beginning on page 837). These observations suggest the need for caution in stimulating the Wnt pathway for therapeutic bone growth.

Cadherins and cancer: how does cadherin dysfunction promote tumor progression?

It has long been recognized that the cell-cell adhesion receptor, E-cadherin, is an important determinant of tumor progression, serving as a suppressor of invasion and metastasis in many contexts. Yet how the loss of E-cadherin function promotes tumor progression is poorly understood. In this review, we focus on three potential underlying mechanisms: the capacity of E-cadherin to regulate beta-catenin signaling in the canonical Wnt pathway; its potential to inhibit mitogenic signaling through growth factor receptors and the possible links between cadherins and the molecular determinants of epithelial polarity. Each of these potential mechanisms provides insights into the complexity that is likely responsible for the tumor-suppressive action of E-cadherin.

Cadherins and cancer: how does cadherin dysfunction promote tumor progression?

It has long been recognized that the cell-cell adhesion receptor, E-cadherin, is an important determinant of tumor progression, serving as a suppressor of invasion and metastasis in many contexts. Yet how the loss of E-cadherin function promotes tumor progression is poorly understood. In this review, we focus on three potential underlying mechanisms: the capacity of E-cadherin to regulate beta-catenin signaling in the canonical Wnt pathway; its potential to inhibit mitogenic signaling through growth factor receptors and the possible links between cadherins and the molecular determinants of epithelial polarity. Each of these potential mechanisms provides insights into the complexity that is likely responsible for the tumor-suppressive action of E-cadherin.

Transgenic mice and host cell mutants resistant to transformation as model systems for identifying multiple components in oncogenesis

Tumorigenesis appears to be a multistep process involving mutations of conventional, dominantly acting proto-oncogenes, mutations of other genes that may act in a recessive manner, and interactions (or a lack of interactions) between the products of mutant and wild-type genes. Our laboratory is using a few well-established, dominant oncogenes to pose experimental questions that could lead to a better understanding of the more elusive genetic interactions which occur during tumour development. Two such situations are described: (1) We have created a line of transgenic mice that carry the int-1 proto-oncogene under the control of the enhancer element in the mouse mammary tumour virus long terminal repeat. Such mice express the transgene in mammary glands, salivary glands and male reproductive tract; mammary glands from both male and female animals are grossly hyperplastic, yet tumours arise rarely in the males and sporadically in the females (80% of female mice have one or a few tumours by six months of age). Thus expression of int-1 in these mice appears to place a large number of mammary cells at risk for secondary events that lead to carcinogenesis, providing a provocative experimental context for identifying such secondary events. (2) We have isolated a rat cell line that lacks most of the characteristics of transformed cells, despite the expression of two wild-type copies of the v-src gene of Rous sarcoma virus. This line harbours what appears to be a dominant mutation in an unidentified gene that renders the cell resistant to transformation by v-src and several other oncogenes. Isolation of the mutant gene responsible for suppressing transformation in this line should provide new insights into the interactions between oncoproteins and other cellular proteins.

The int genes in mouse mammary tumorigenesis and in normal development

In mice, the mouse mammary tumour virus causes tumours by insertional activation of host cell oncogenes. By the application of transposon tagging techniques, several cellular oncogenes, called int, have been discovered. The int-1 gene encodes a cysteine-rich protein with a signal peptide, suggesting that it may act as an extracellular growth or differentiation factor. Normally, the int-1 gene is expressed in early embryogenesis of the mouse, in particular in the developing nervous system. The essential role of int-1 in embryogenesis is underscored by its high degree of homology with the Drosophila segment polarity gene wingless, a gene involved in pattern formation in segments of the developing fly. In Drosophila, the int-1/wingless gene appears to encode a secreted factor, as concluded from antibody staining experiments. The int-4 gene is not yet fully characterized at the molecular level. From its expression pattern, however, we have concluded that int-4 may also act in the control of embryogenesis: the gene is expressed only during specific time intervals in mouse embryos and it is highly conserved in evolution.

Wnt1 and Wnt5a affect endothelial proliferation and capillary length; Wnt2 does not

Blood vessel growth is critical for embryonic development and contributes to pathologies including cancer and diabetic retinopathy. A growing body of evidence suggests that signaling via the Wnt/beta-catenin pathway contributes to angiogenesis, and that paracrine Wnt signaling might alter endothelial cell function. To test the hypothesis that Wnt signaling promotes endothelial cell proliferation and vessel growth, we treated bovine aortic endothelial cells with Wnt1, Wnt2 and Wnt5a derived from coculture with Wnt-expressing fibroblasts. Endothelial cells cultured in the presence of Wnt1 displayed increased Wnt/beta-catenin signaling, proliferation and capillary stability in vitro. Wnt5a, which primarily signals via an alternate Wnt pathway, the Wnt/Ca(++) pathway, decreased both cell number and capillary length. Wnt2, which in other cell types activates the Wnt/beta-catenin pathway, did not activate signaling, affect cell number or increase capillary length. These results suggest that Wnt/beta-catenin and Wnt/Ca(++) signals might have opposing effects on angiogenesis.

Differential inhibition of Wnt-3a by Sfrp-1, Sfrp-2, and Sfrp-3

Secreted frizzled related proteins (Sfrps) are extracellular attenuators of Wnt signaling that play important roles in both embryogenesis and oncogenesis. Although Sfrps are generally thought to bind and sequester Wnts away from active receptor complexes, very little is known about the specificity of Sfrp family members for various Wnts. In the developing chick neural tube, sfrp-1, 2, and 3 transcripts are expressed in and adjacent to the dorsal neural tube, where Wnt-1 and Wnt-3a are expressed. To better define the possible roles of Sfrp-1, 2, and 3 in the neural tube, we first tested the ability of purified Sfrps to inhibit Wnt-3a-induced accumulation of beta-catenin in L cells. We find that both Sfrp-1 and Sfrp-2 can inhibit Wnt-3a activity while Sfrp-3 cannot. To determine where Sfrp-1 and Sfrp-2 impinge on the Wnt signaling pathway, we tested the ability of these Sfrps to inhibit Wnt signaling induced by the addition of LiCl, an inhibitor of GSK-3. Sfrp-1 and Sfrp-2 are unable to inhibit the accumulation of beta-catenin in LiCl-treated cells, suggesting that the ability of Sfrps to inhibit the accumulation of beta-catenin is GSK-3 dependent. We have further shown that Sfrp-2 inhibits the ability of ectopic Wnt-3a to stimulate proliferation in the developing chick neural tube. These results provide the framework for understanding how Sfrps function to regulate Wnt-3a activity in developing embryos and in cancer.

EN2 is a candidate oncogene in human breast cancer

Only a few critical oncogenes have been identified in the more commonly occurring cases of sporadic breast cancer. We provide evidence that EN2 is ectopically expressed in a subset of human breast cancer and may have a causal role in mammary tumorigenesis. Nontumorigenic mammary cell lines engineered to ectopically express En-2 have a marked reduction in their cycling time, lose cell contact inhibition, become sensitive to 17-AAG treatment, fail to differentiate when exposed to lactogenic hormones and induce mammary tumors when transplanted into cleared mammary glands of syngeneic hosts. RNA interference studies suggest that EN2 expression is required for the maintenance of the transformed phenotype of a human breast tumor cell line.

Cyclooxygenase-2 overexpression in MCF-10F human breast epithelial cells inhibits proliferation, apoptosis and differentiation, and causes partial transformation

To investigate the effects of cyclooxygenase-2 (COX-2) overexpression on breast cancer development, we stably transfected MCF-10F human breast epithelial cells with an expression vector containing human COX-2 cDNA oriented in the sense (10F-S) or antisense (10F-AS) direction. As expected, 10F-S cells expressed elevated levels of COX-2 protein, whereas this protein was undetectable in the 10F-AS cells. Prostaglandin E(2) production in these cells reflected COX-2 levels. The 10F-S cells had a significantly decreased rate of proliferation compared to 10F-AS or parental cells, and a delay in progression through the G(1) phase of the cell cycle. COX-2 overexpression also caused resistance to detachment-induced apoptosis (anoikis) as well as an inhibition of differentiation in cells cultured in Matrigel. Furthermore, after approximately 20 passages in culture, 10F-S cells developed fibroblast-like features, expressed vimentin, and formed foci of dense growth when cultured at confluence, suggesting that the cells were undergoing epithelial to mesenchymal transition (EMT). The 10F-S cells, however, were unable to grow in soft agar or form tumors in nude mice, suggesting that they were only partially transformed. Our observations suggest that COX-2 overexpression in human breast epithelial cells will predispose the mammary gland to carcinogenesis.

Kinase-inactive glycogen synthase kinase 3beta promotes Wnt signaling and mammary tumorigenesis

Recent studies have implicated ectopic activation of the Wnt pathway in many human cancers, including breast cancer. beta-catenin is a critical coactivator in this signaling pathway and is regulated in a complex fashion by phosphorylation, degradation, and nuclear translocation. Glycogen synthase kinase 3beta (GSK3beta) phosphorylation of the NH2-terminal domain of beta-catenin targets it for ubiquitination and proteosomal degradation. We hypothesized that expression of kinase-inactive GSK3beta (KI-GSK3beta) in mammary glands would function in a dominant-negative fashion by antagonizing the endogenous activity of GSK3beta and promoting breast cancer development. Consistent with this, we find that KI-GSK3beta stabilizes beta-catenin expression, catalyzes its localization to the nucleus, and up-regulates the downstream target gene, cyclin D1, in vitro. In vivo, transgenic mice overexpressing the KI-GSK3beta under the control of the mouse mammary tumor virus-long terminal repeat develop mammary tumors with overexpression of beta-catenin and cyclin D1. Thus, antagonism of GSK3beta activity is oncogenic in the mammary epithelium; mutation or pharmacologic down-regulation of GSK3beta could promote mammary tumors.

Wnt-1 expression in PC12 cells induces exon 15 deletion and expression of L-APP

The metabolism of amyloid precursor protein (APP) is central to Alzheimer's disease pathogenesis. Recent data have linked APP and presenilin to the Wnt/wingless signaling pathway. To assess affects of Wnt stimulation on APP isoform expression, we infected PC12 cells and C57MG cells with a retrovirus containing murine Wnt-1. In PC12 cells, Wnt-1 expression is associated with induction of exon 15 deletion from APP mRNA and expression of L-APP. Our data suggest that APP isoform expression is regulated, in part, by the Wnt/wingless signaling pathway.

Wnt proteins induce dishevelled phosphorylation via an LRP5/6- independent mechanism, irrespective of their ability to stabilize beta-catenin

Wnt glycoproteins play essential roles in the development of metazoan organisms. Many Wnt proteins, such as Wnt1, activate the well-conserved canonical Wnt signaling pathway, which results in accumulation of beta-catenin in the cytosol and nucleus. Other Wnts, such as Wnt5a, activate signaling mechanisms which do not involve beta-catenin and are less well characterized. Dishevelled (Dvl) is a key component of Wnt/beta-catenin signaling and becomes phosphorylated upon activation of this pathway. In addition to Wnt1, we show that several Wnt proteins, including Wnt5a, trigger phosphorylation of mammalian Dvl proteins and that this occurs within 20 to 30 min. Unlike the effects of Wnt1, phosphorylation of Dvl in response to Wnt5a is not concomitant with beta-catenin stabilization, indicating that Dvl phosphorylation is not sufficient to activate canonical Wnt/beta-catenin signaling. Moreover, neither Dickkopf1, which inhibits Wnt/beta-catenin signaling by binding the Wnt coreceptors LRP5 and -6, nor dominant-negative LRP5/6 constructs could block Wnt-mediated Dvl phosphorylation. We conclude that Wnt-induced phosphorylation of Dvl is independent of LRP5/6 receptors and that canonical Wnts can elicit both LRP-dependent (to beta-catenin) and LRP-independent (to Dvl) signals. Our data also present Dvl phosphorylation as a general biochemical assay for Wnt protein function, including those Wnts that do not activate the Wnt/beta-catenin pathway.

Regulation of E-cadherin expression and beta-catenin/Tcf transcriptional activity by the integrin-linked kinase

Integrin-linked kinase (ILK) is a serine/threonine protein kinase which interacts with the cytoplasmic domains of beta1 and beta3 integrins. ILK structure and its localization at the focal adhesion allows it not only to interact with different structural proteins, but also to mediate many different signalling pathways. Extracellular matrices (ECM) and growth factors each stimulate ILK signalling. Constitutive activation of ILK in epithelial cells results in oncogenic phenotypes such as disruption of cell extracellular matrix and cell to cell interactions, suppression of suspension-induced apoptosis, and induction of anchorage independent cell growth and cell cycle progression. More specifically, pathological overexpression of ILK results in down-regulation of E-cadherin expression, and nuclear accumulation of beta-catenin, leading to the subsequent activation of the beta-catenin/Tcf transcription complex, the downstream components of the Wnt signalling pathway. Here we review the data implicating ILK in the regulation of these two signalling pathways, and discuss recent novel insights into the molecular basis and requirement of ILK in the process of epithelial to mesenchymal transformation (EMT).

Identification of Wnt responsive genes using a murine mammary epithelial cell line model system

Background

The Wnt/Wg pathway plays an important role in the developmental program of many cells and tissues in a variety of organisms. In addition, many Wnts and components of their downstream signaling pathways, such as β-catenin and APC, have been implicated in tumorigenesis. Over the past years, several genes have been identified as Wnt responsive, including c-myc, siamois, and cyclin D1.

Results

In order to identify additional genes responsive to Wnt signaling that contribute to the transformed phenotype, we performed a cDNA subtractive hybridization screen between a mouse mammary epithelial cell line that overexpresses Wnt-1 (C57MG/Wnt-1) and the parental cell line (C57MG). The screen identified a total of 67 genes to be up-regulated in response to Wnt signaling. Of these 67 genes, the up-regulation of 62 was subsequently confirmed by Northern and dot blot analyses (and, for a subset, semi-quantitative PCR) of RNA isolated from C57MG cells subjected to (1) an independent Wnt-1 retroviral infection, and (2) co-culture with Wnt-1 expressing cells. Among the confirmed Wnt-1 responsive genes, we further characterized a mouse homolog of the human transcription factor Basic Transcription Element Binding protein 2 (BTEB2), Wnt-1 Responsive Cdc42 homolog (Wrch-1), and Wnt-1 Induced Secreted Protein (WISP-1).

Conclusion

Several novel genes were identified in this screen, as well as others that have been shown previously to be regulated by Wnt signaling, such as connexin43. The results indicate that cDNA subtractive hybridization is a useful method for identifying genes involved in the process of Wnt-1-induced transformation.

Cell polarity and epithelial oncogenesis

Pathologists have long recognized that tumour formation in epithelia leads to disruption of normal epithelial cell polarity. Despite this, few studies have taken advantage of new information on the biogenesis of cell polarity to analyse the process of epithelial oncogenesis. Recent studies of epithelial cell lines now indicate that the pattern of breakdown of polarity during oncogenesis may reflect the way in which normal epithelial cells achieve polarity. These results suggest not only a novel way to study the development of polarity in vitro, but also new ideas for the early detection of cancer.

Coactivation of Rac and Rho by Wnt/Frizzled signaling is required for vertebrate gastrulation

Wnt/Frizzled (Fz) signaling controls cell polarity/movements during vertebrate gastrulation via incompletely defined mechanisms. We demonstrated previously that Wnt/Fz activation of Rho, a GTPase and regulator of cytoskeletal architecture, is essential for vertebrate gastrulation. Here we report that in mammalian cells and Xenopus embryos, Wnt/Fz signaling coactivates Rho and Rac, another GTPase and distinct regulator of cytoskeletal architecture. Wnt/Fz activation of Rac is independent of Rho and mediates Wnt/Fz activation of Jun N-terminal kinase (JNK). Dishevelled (Dvl), a cytoplasmic protein downstream of Fz, forms a Wnt-induced complex with Rac independent of the Wnt-induced Dvl-Rho complex. Depletion or inhibition of Rac function perturbs Xenopus gastrulation without affecting Wnt/Fz activation of the Rho or beta-catenin pathway. We propose that parallel activation of Rac and Rho pathways by Wnt/Fz signaling is required for cell polarity and movements during vertebrate gastrulation.

Mouse Wnt9b transforming activity, tissue-specific expression, and evolution

The members of the Wnt family of secreted factors have oncogenic potential and important roles as developmental regulators. We report an analysis of mouse Wnt9b (also called Wnt15 and Wnt14b), including its cDNA sequence, chromosomal mapping, epithelial cell transforming activity, adult and embryonic tissue expression patterns, and evolution. We also deduced the full-length amino acid sequence of its close relative, Wnt9a (also called Wnt14), from unannotated genomic DNA sequences in GenBank. Full-length comparisons among Wnt amino acid sequences provide evidence that Wnt9b and Wnt9a are close paralogs of each other and are orthologs of Wnt9 genes from shark and hagfish. Mapping Wnt9b to The Jackson Laboratory BSS interspecific backcross panel places it at 63.0 cM on chromosome 11. Sequence comparisons of two pairs of linked Wnt genes (the Wnt9a-Wnt3a pair and the Wnt9b-Wnt3 pair) suggest that they arose from the relatively recent duplication of a single ancestral Wnt gene pair, confirming the close paralogous relationship of Wnt9a and Wnt9b. Wnt9b expression is primarily restricted to the kidney in the adult mouse, with lower levels detected in the preputial gland, liver, and mammary gland. Testing of staged whole mouse embryos from 9.5 to 17.5 days of gestation showed expression at all stages with a peak at day 10.5. In situ hybridization analysis showed expression in most but not all tissues of the 16.5-day embryo. No significant elevation of Wnt9b expression was detected in 29 mouse mammary tumor virus-induced tumors. Overexpression of Wnt9b in C57MG mammary epithelial cells caused small transformed foci in cell monolayers and a moderate morphological transformation in pooled colonies compared with Wnt1.

Wnt/beta-catenin signaling suppresses apoptosis in low serum medium and induces morphologic change in rodent fibroblasts

Wnt/beta-catenin signaling plays important roles in tumorigenesis in certain tumors as well as during development. However, the mechanism of tumorigenesis mediated by this signaling remains to be elucidated. We investigated the response of rodent fibroblasts to activation of Wnt/beta-catenin signaling by treatment with conditioned medium containing soluble Wnt-3a protein (W3a-CM) and by expression of a constitutive active beta-catenin gene harbored by an adenovirus vector. W3a-CM induced transcriptional activation of a beta-catenin/T-cell factor (Tcf)-responsive promoter in rodent fibroblasts such as NIH3T3, Rat-1, Swiss3T3 and Balb3T3 cells. In these cells, an increase in saturation density and an inhibition of apoptosis and/or promotion of growth in low-serum medium were induced by treatment with W3a-CM. In Rat-1 cells, morphologic changes were also induced. All these alterations were reversible. Moreover, the inhibition of apoptosis of NIH3T3 cells in low-serum medium and the morphologic changes in Rat-1 cells, but not the increase in saturation density, were also induced by ectopic expression of a constitutive active beta-catenin gene. These results suggested that activation of Wnt/beta-catenin signaling induces inhibition of apoptosis and morphologic changes in these cells.

Wnt signaling promotes oncogenic transformation by inhibiting c-Myc-induced apoptosis

Aberrant activation of the Wnt/beta-catenin signaling pathway is associated with numerous human cancers and often correlates with the overexpression or amplification of the c-myc oncogene. Paradoxical to the cellular transformation potential of c-Myc is its ability to also induce apoptosis. Using an inducible c-MycER expression system, we found that Wnt/beta-catenin signaling suppressed apoptosis by inhibiting c-Myc-induced release of cytochrome c and caspase activation. Both cyclooxygenase 2 and WISP-1 were identified as effectors of the Wnt-mediated antiapoptotic signal. Soft agar assays showed that neither c-Myc nor Wnt-1 alone was sufficient to induce cellular transformation, but that Wnt and c-Myc coordinated in inducing transformation. Furthermore, coexpression of Wnt-1 and c-Myc induced high-frequency and rapid tumor growth in nude mice. Extensive apoptotic bodies were characteristic of c-Myc-induced tumors, but not tumors induced by coactivation of c-Myc and Wnt-1, indicating that the antiapoptotic function of Wnt-1 plays a critical role in the synergetic action between c-Myc and Wnt-1. These results elucidate the molecular mechanisms by which Wnt/beta-catenin inhibits apoptosis and provide new insight into Wnt signaling-mediated oncogenesis.

A possible role for the WNT-1 pathway in oral carcinogenesis

Reductions in cell-cell adhesion and stromal and vascular invasion are essential steps in the progression from localized malignancy to metastatic disease for all cancers. Proteins involved in intercellular adhesion, such as E-cadherin and catenin, probably play an important role in metastatic processes and cellular differentiation. While E-cadherin and beta-catenin expression has been extensively studied in many forms of human cancers, less is known about the role of the Wingless-Type-1 (WNT-1) pathway in human tumors. A large body of genetic and biochemical evidence has identified beta-catenin as a key downstream component of the WNT signaling pathway, and recent studies of colorectal tumors have shown a functional link among beta-catenin, adenomatous polyposis coli gene product (APC), and other components of the WNT-1 pathway. WNT-1 pathway signaling is thought to be mediated via interactions between beta-catenin and members of the LEF-1/TCF family of transcription factors. The WNT signal stabilizes beta-catenin protein and promotes its accumulation in the cytoplasm and nucleus. In the nucleus, beta-catenin associates with TCF to form a functional transcription factor which mediates the transactivation of target genes involved in the promotion of tumor progression, invasion, and metastasis, such as C-Myc, cyclin D1, c-jun, fra-1, and u-PAR. There is a strong correlation between the ability of the WNT-1 gene to induce beta-catenin accumulation and its transforming potential in vivo, suggesting that the WNT-1 gene activates an intracellular signaling pathway that can induce the morphological transformation of cells. For these reasons, data obtained from the study of the WNT-1 pathway could be important in our understanding of the mechanisms of epithelial tumors, in general, and probably also of oral squamous cell carcinoma, in particular.

beta-catenin is a downstream effector of Wnt-mediated tumorigenesis in the mammary gland

The Wnt signal transduction pathway has been implicated in mammary tumorigenesis in the mouse. beta-catenin, a key downstream effector of this pathway interacts with and thus activates the Tcf/Lef family of transcription factors. Elevated levels of beta-catenin have been found in many human tumors, notably colon carcinomas. Recently, elevated levels of beta-catenin have been associated with poor prognosis in human adenocarcinoma of the breast. In order to assess the possible role of beta-catenin in mammary carcinoma, we have created transgenic mice bearing the MMTV-LTR driving an activated form of beta-catenin. These mice develop mammary gland hyperplasia and mammary adenocarcinoma, a phenotype very similar to that of transgenic mice expressing an MMTV-driven Wnt gene. Indeed, the histopathology of the mammary tumors in Wnt-mediated adenocarcinoma is identical to that observed in our beta-catenin-mediated disease model. Furthermore, putative beta-catenin transcriptional targets, cyclin D1 and c-myc, are elevated in beta-catenin-mediated mammary tumors and cell lines. These observations support the notion that the oncogenic Wnt pathway operates via beta-catenin and its targets in the context of mammary hyperplasia and carcinoma.

Wnt signaling regulates hemopoiesis through stromal cells

Hemopoietic cells develop in a complex milieu that is made up of diverse components, including stromal cells. Wnt genes, which are known to regulate the fate of the cells in a variety of tissues, are expressed in hemopoietic organs. However, their roles in hemopoiesis are not well characterized. In this study, we examined the roles of Wnt proteins in hemopoiesis using conditioned medium containing Wnt-3a. This conditioned medium dramatically reduced the production of B lineage cells and myeloid lineage cells, except for macrophages in the long-term bone marrow cultures grown on stromal cells, although the sensitivity to the conditioned medium differed, depending on the hemopoietic lineage. In contrast, the same conditioned medium did not affect the generation of B lineage or myeloid lineage cells in stromal cell-free conditions. These results suggested that Wnt proteins exert their effects through stromal cells. Indeed, these effects were mimicked by the expression of a stabilized form of beta-catenin in stromal cells. In this study, we demonstrated that Wnt signaling regulates hemopoiesis through stromal cells with selectivity and different degrees of the effect, depending on the hemopoietic lineage in the hemopoietic microenvironment.

PEA3 is up-regulated in response to Wnt1 and activates the expression of cyclooxygenase-2

The inducible prostaglandin synthase cyclooxygenase-2 (COX-2) is aberrantly expressed in intestinal tumors resulting from APC mutation, and is also transcriptionally up-regulated in mouse mammary epithelial cells in response to Wnt1 expression. beta-Catenin stabilization is a consequence of both APC mutation and Wnt signaling. We have previously observed coordinate regulation of the matrilysin promoter by beta-catenin and Ets family transcription factors of the PEA3 subfamily. Here we show that while beta-catenin only weakly activates the COX-2 promoter, PEA3 family transcription factors are potent activators of COX-2 transcription. Consistent with this, PEA3 is up-regulated in Wnt1-expressing mouse mammary epithelial cells, and PEA3 factors are highly expressed in tumors from Wnt1 transgenic mice, in which Cox-2 is also up-regulated. Promoter mapping experiments suggest that the NF-IL6 site in the COX-2 promoter is important for mediating PEA3 responsiveness. The NF-IL6 site is also important for COX-2 transcription in some colorectal cancer lines (Shao, J., Sheng, H., Inoue, H., Morrow, J. D., and DuBois, R. N. (2000) J. Biol. Chem. 275, 33951-33956), and PEA3 factors are highly expressed in colorectal cancer cell lines. Therefore, we speculate that PEA3 factors may contribute to the up-regulation of COX-2 expression resulting from both APC mutation and Wnt1 expression.

Heterotypic signaling between epithelial tumor cells and fibroblasts in carcinoma formation

Tumors arise from cells that have sustained genetic mutations resulting in deregulation of several of their normal growth-controlling mechanisms. Much of the research concerning the origins of cancer has focused on the genetic mutations within tumor cells, treating tumorigenesis as a cell-autonomous process governed by the genes carried by the tumor cells themselves. However, it is increasingly apparent that the stromal microenvironment in which the tumor cells develop profoundly influences many steps of tumor progression. In various experimental tumor models, the microenvironment affects the efficiency of tumor formation, the rate of tumor growth, the extent of invasiveness, and the ability of tumor cells to metastasize. In carcinomas, the influences of the microenvironment are mediated, in large part, by paracrine signaling between epithelial tumor cells and neighboring stromal fibroblasts. In this review, we summarize recent advances in understanding the paracrine signaling interactions between epithelial cancer cells and associated fibroblasts and examine the effects of these bidirectional interactions on various aspects of carcinoma formation. We note, however, that paracrine signaling between other cell types within the carcinomas, such as endothelial cells and inflammatory cells, may play equally important roles in tumor formation and we will refer to these heterotypic interactions where relevant.

Development of multigene and regulated lentivirus vectors

Previously we described safe and efficient three-component human immunodeficiency virus type 1 (HIV-1)-based gene transfer systems for delivery of genes into nondividing cells (H. Mochizuki, J. P. Schwartz, K. Tanaka, R. O. Brady, and J. Reiser, J. Virol. 72:8873-8883, 1998). To apply such vectors in anti-HIV gene therapy strategies and to express multiple proteins in single target cells, we have engineered HIV-1 vectors for the concurrent expression of multiple transgenes. Single-gene vectors, bicistronic vectors, and multigene vectors expressing up to three exogenous genes under the control of two or three different transcriptional units, placed within the viral gag-pol coding region and/or the viral nef and env genes, were designed. The genes encoding the enhanced version of green fluorescent protein (EGFP), mouse heat-stable antigen (HSA), and bacterial neomycin phosphotransferase were used as models whose expression was detected by fluorescence-activated cell sorting, fluorescence microscopy, and G418 selection. Coexpression of these reporter genes in contact-inhibited primary human skin fibroblasts (HSFs) persisted for at least 6 weeks in culture. Coexpression of the HSA and EGFP reporter genes was also achieved following cotransduction of target cells using two separate lentivirus vectors encoding HSA and EGFP, respectively. For the regulated expression of transgenes, tetracycline (Tet)-regulatable lentivirus vectors encoding the reverse Tet transactivator (rtTA) and EGFP controlled by a Tet-responsive element (TRE) were constructed. A binary HIV-1-based vector system consisting of a lentivirus encoding rtTA and a second lentivirus harboring a TRE driving the EGFP reporter gene was also designed. Doxycycline-modulated expression of the EGFP transgene was confirmed in transduced primary HSFs. These versatile vectors can potentially be used in a wide range of gene therapy applications.

Endogenous protein kinase CK2 participates in Wnt signaling in mammary epithelial cells

Protein kinase CK2 (formerly casein kinase II) is a serine/threonine kinase overexpressed in many human tumors, transformed cell lines, and rapidly proliferating tissues. Recent data have shown that many cancers involve inappropriate reactivation of Wnt signaling through ectopic expression of Wnts themselves, as has been seen in a number of human breast cancers, or through mutation of intermediates in the Wnt pathway, such as adenomatous polyposis coli or beta-catenin, as described in colon and other cancers. Wnts are secreted factors that are important in embryonic development, but overexpression of certain Wnts, such as Wnt-1, leads to proliferation and transformation of cells. We report that upon stable transfection of Wnt-1 into the mouse mammary epithelial cell line C57MG, morphological changes and increased proliferation are accompanied by increased levels of CK2, as well as of beta-catenin. CK2 and beta-catenin co-precipitate with the Dvl proteins, which are Wnt signaling intermediates. A major phosphoprotein of the size of beta-catenin appears in in vitro kinase reactions performed on the Dvl immunoprecipitates. In vitro translated beta-catenin, Dvl-2, and Dvl-3 are phosphorylated by CK2. The selective CK2 inhibitor apigenin blocks proliferation of Wnt-1-transfected cells, abrogates phosphorylation of beta-catenin, and reduces beta-catenin and Dvl protein levels. These results demonstrate that endogenous CK2 is a positive regulator of Wnt signaling and growth of mammary epithelial cells.

Syndecan-1 is required for Wnt-1-induced mammary tumorigenesis in mice

Syndecan-1 is a cell-surface, heparan-sulphate proteoglycan (HSPG) predominantly expressed by epithelial cells. It binds specifically to many proteins, including oncoproteins. For example, it induces the assembly of a signalling complex between FGF ligands and their cognate receptors. But so far there has been no direct evidence that this proteoglycan contributes to tumorigenesis. Here we have examined the role of syndecan-1 (encoded by Sdc1) during mammary tumour formation in response to the ectopic expression of the proto-oncogene Wnt1. We crossed syndecan-1-deficient mice with transgenic mice that express Wnt1 in mammary gland (TgN(Wnt-1)1Hev; ref. 2). Ectopic Wnt-1 expression induces generalized mammary hyperplasia, followed by the development of solitary tumours (median time 22 weeks). We show that in Sdc1-/- mice, Wnt-1-induced hyperplasia in virgin mammary gland was reduced by 70%, indicating that the Wnt-1 signalling pathway was inhibited. Of the 39 tumours that developed in a test cohort of mice, only 1 evolved in the Sdc1-/- background. In addition, we show that soluble syndecan-1 ectodomain purified from mouse mammary epithelial cells stimulates the activity of a Wnt-1 homologue in a tissue culture assay. Our results provide both genetic and biochemical evidence that syndecan-1 can modulate Wnt signalling, and is critical for Wnt-1-induced tumorigenesis of the mouse mammary gland.

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.

Use of MMTV-Wnt-1 transgenic mice for studying the genetic basis of breast cancer

Wnt-1 was first identified as a protooncogene activated by viral insertion in mouse mammary tumors. Transgenic expression of this gene using a mouse mammary tumor virus LTR enhancer causes extensive ductal hyperplasia early in life and mammary adenocarcinomas in approximately 50% of the female transgenic (TG) mice by 6 months of age. Metastasis to the lung and proximal lymph nodes is rare at the time tumors are detected but frequent after the removal of the primary neoplasm. The potent mitogenic effect mediated by Wnt-1 expression does not require estrogen stimulation; tumors form after an increased latency in estrogen receptor alpha-null mice. Several genetic lesions, including inactivation of p53 and over-expression of Fgf-3, collaborate with Wnt-1 in leading to mammary tumors, but loss of Sky and inactivation of one allele of Rb do not affect the rate of tumor formation in Wnt-1 TG mice.

A REIC gene shows down-regulation in human immortalized cells and human tumor-derived cell lines

Normal human cells stop proliferation after a certain number of cell divisions. This phenomenon is called cellular aging. The fact that the senescence phenotype is dominant and the immortal one is recessive indicates that immortalization of human cells may be caused by loss of functions of certain genes in normal cells. Based on this evidence, several cDNA clones whose expression was down-regulated during the immortalization process of human cells were isolated by the representative difference analysis (RDA) system in our laboratory. One of them, which was named REIC, was expressed to a lower degree in three human immortalized cell lines as compared with their parental normal counterparts. In addition, the expression of REIC was markedly lower in eight human tumor-derived cell lines (Hep3B and HuH-7 hepatocellular carcinomas, HuH-6 Clone 5 hepatoblastoma, HuCCT-1 cholangiocarcinoma, A549 lung cancer, HaCaT immortalized keratinocyte, HeLa cervical carcinoma, and Saos-2 osteosarcoma). In contrast, among the human tissues examined, the heart and brain, which contain a large number of post-mitotic cells, showed the highest expression of REIC. The full-length REIC cDNA revealed that the predicted protein is 350 amino acids in length and possesses coiled-coil tertiary structures in each of the amino- and carboxyl-termini. Furthermore, a search of the protein database revealed a match of this gene product with Dkk-3, which is a novel inhibitor of Wnt oncogene. These results indicate that the REIC cloned by us may function as a tumor suppressor.

Mechanism and function of signal transduction by the Wnt/beta-catenin and Wnt/Ca2+ pathways

Communication between cells is often mediated by secreted signaling molecules that bind cell surface receptors and modulate the activity of specific intracellular effectors. The Wnt family of secreted glycoproteins is one group of signaling molecules that has been shown to control a variety of developmental processes including cell fate specification, cell proliferation, cell polarity and cell migration. In addition, mis-regulation of Wnt signaling can cause developmental defects and is implicated in the genesis of several human cancers. The importance of Wnt signaling in development and in clinical pathologies is underscored by the large number of primary research papers examining various aspects of Wnt signaling that have been published in the past several years. In this review, we will present a synopsis of current research with particular attention paid to molecular mechanism of Wnt signal transduction and how the mis-regulation of Wnt signaling leads to cancer.

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.

Discovering novel chemotherapeutic drugs for the third millennium

There is enormous potential for the discovery of innovative cancer drugs with improved efficacy and selectivity for the third millennium. In this review we show how novel mechanism-based agents are being discovered by focusing on the molecular targets and pathways that are causally involved in cancer formation, maintenance and progression. We also show how new technologies, from genomics through high through-put bioscience, combinatorial chemistry, rational drug design and molecular pharmacodynamic and imaging techniques, are accelerating the pace of cancer drug discovery. The process of contemporary small molecule drug discovery is described and progress and current issues are reviewed. New and potential targets and pathways for therapeutic intervention are illustrated. The first examples of a new generation of molecular therapeutics are now entering hypothesis-testing clinical trials and showing activity. The early years of the new millennium will see a range of exciting new agents moving from bench to bedside and beginning to impact on the management and cure of cancer.

Isolation and biochemical characterization of the human Dkk-1 homologue, a novel inhibitor of mammalian Wnt signaling

In an effort to isolate novel growth factors, we identified a human protein, designated Sk, that co-eluted with Neuregulin during chromatographic separation of conditioned medium from the SK-LMS-1 human leiomyosarcoma cell line. Degenerate oligonucleotides based on amino-terminal sequence analysis of the purified protein were used to isolate the corresponding cDNA from a library generated from this cell line. Sk is a novel 266-amino acid protein that contains a signal peptide sequence and two cysteine-rich domains with no similarity to other known growth factors. A single major 2-kilobase transcript was expressed in several embryonic tissues. Transfection of mammalian cells demonstrated that the protein was secreted and expressed as a doublet of approximately 35 kDa. In vitro translation and endoglycosylase analysis indicated that this doublet, which was also observed in cells expressing the endogenous protein, arises from posttranslational modification. A search of the GenBankTM data base revealed a match of Sk with Dkk-1, which is a novel secreted protein required for head induction in amphibian embryos and a potent Wnt inhibitor. When coexpressed with Wnt-2 in NIH3T3 cells, human Sk/Dkk-1 caused reversion of Wnt-2 induced morphological alterations and inhibited the Wnt-2 induced increase in uncomplexed beta-catenin levels. These results provide biochemical evidence that human Sk/Dkk-1 antagonizes Wnt signaling upstream of its effect on beta-catenin regulation.

Interaction of frizzled related protein (FRP) with Wnt ligands and the frizzled receptor suggests alternative mechanisms for FRP inhibition of Wnt signaling

Frizzled related proteins (FRPs) comprise a family of secreted molecules that contain an N-terminal cysteine-rich domain (CRD) highly similar to the CRDs of the frizzled family of membrane-anchored Wnt receptors. FRPs have been shown to interact with Wnt proteins and antagonize Wnt signaling in a Xenopus developmental model. We demonstrated that FRP antagonizes the Wnt-induced increase in uncomplexed beta-catenin in both transient cotransfection and stable transformation models, where Wnt-induced morphological alterations are inhibited as well. We showed further that FRP inhibits Wnt signaling in a paracrine mode using a T-cell factor luciferase reporter to measure Wnt function. Investigation of the mechanisms responsible for FRP inhibition revealed that FRP forms complexes with WNT-1 or WNT-2 through its CRD domain. Transfection analysis with FRPs containing different tags revealed that FRP itself forms complexes and that this ability is conferred by its CRD domain. Finally, we demonstrated by cotransfection that FRP forms complexes with a prototype frizzled. All of these findings are consistent with a model by which FRP inhibits Wnt signaling through interactions with Wnt and/or formation of nonfunctional complexes with the frizzled receptor.

Antisense downregulation of a mouse mammary tumor virus activated protooncogene in mouse mammary tumor cells reverses the malignant phenotype

Activation of the protooncogene Wnt-1 by insertion of the mouse mammary tumor virus (MMTV) is known to cause mammary tumors in mice. Wnt-1 expression in mammary glands has been postulated to confer direct local growth stimulation of mammary epithelial cells leading to their acquisition of a preneoplastic state. Wnt-1 expression also induces morphological alterations in cultured normal mammary cells. However, it has not been determined whether or not transformed mammary cells require continuous Wnt-1 expression for their ability to form tumors in vivo. To address this question, we constructed antisense and sense Wnt-1 expression vectors containing a synthetic promoter composed of five high-affinity glucocorticoid response elements (GRE5). This promoter is at least 50-fold more inducible by dexamethasone than the promoter contained in the long terminal repeats of MMTV. The vectors were introduced into a mouse mammary tumor cell line (R/Sa-MT) that expresses high levels of endogenous Wnt-1 mRNA and forms rapidly growing tumors when transplanted into syngeneic hosts. Of the 12 stably transfected cell lines established (9 with antisense and 3 with sense constructs), 2 antisense cell lines (R/Sa-MT/antisense) and 1 sense cell line (R/Sa-MT/sense) were examined for inducibility by dexamethasone of antisense and sense Wnt-1 RNAs, changes in endogenous Wnt-1 RNA expression, and changes in cell morphology. The growth patterns of the cells in vitro and in vivo were also examined. Our results show that (1) the levels of the expression of endogenous Wnt-1 mRNA and protein were reduced significantly (>80%) in those cells (R/Sa-MT/antisense) that expressed antisense Wnt-1 RNA at high levels following exposure to dexamethasone, compared to the R/Sa-MT/sense and R/Sa-MT control cells and (2) transplantation of the R/Sa-MT/antisense cells produced smaller tumors ( approximately 0.2 cm in 16 weeks) compared to the tumors ( approximately 2.0 cm in 8 weeks) that were produced by the R/Sa-MT/sense and R/Sa-MT cells. We therefore suggest that Wnt-1 expression is required not only for the transformation of normal mammary cells into tumor cells, but also for the maintenance of their tumorigenicity.

Chimeric Wnt proteins define the amino-terminus of Wnt-1 as a transformation-specific determinant

Wnt-1 induces morphological transformation of C57MG mammary epithelial cells and accumulation of cytosolic beta-catenin whereas Wnt-5a has no effect. To identify regions within the 370 amino acid Wnt-1 protein required for these functions we tested eleven chimeric genes that contained variable amounts of Wnt-1 and Wnt-5a sequence. Transformation and beta-catenin regulation in C57MG cells is controlled by amino acids that lie within 186 residues of the amino terminus of Wnt-1. Small substitutions between residues 186 and 292 reduced Wnt-1 activity. Replacement of the carboxy terminal 79 amino acids of Wnt-1 by Wnt-5a did not affect function. These results were supported by transient expression assays in 293 cells wherein beta-catenin accumulated in the cytoplasm in response to ectopic Wnt-1 expression. In 293 cells, a larger region of the amino-terminus of Wnt-1 was found to be required for beta-catenin regulation. Nonfunctional chimeras that contained at least 99 amino terminal Wnt-1 residues inhibited Wnt-1 stimulation of 293 cells. One of these chimeras inhibited both Wnt-1 and Wnt-3 activity suggesting that Wnt-1 and Wnt-3 interact with a common signaling component.