Dynamic expression of Lef/Tcf family members and β-catenin during chick gastrulation, neurulation, and early limb development

Members of the Lef/Tcf family of HMG-box transcription factors mediate the response to Wnt as part of the canonical Wnt signaling cascade. Positive and negative cofactors, including beta-catenin, CtBP, and Smad3, regulate the activity of Lef/Tcf transcription complexes. Interaction of Lef/Tcfs with beta-catenin results in target gene activation or repression, depending on the context. Here, we report the cloning of a novel chick Tcf-1 splice variant and of a partial cDNA for chick Tcf-3. We describe their expression patterns during early development and have compared them with the expression profiles of Lef-1 and beta-catenin. We found restricted patterns during gastrulation, neurulation, somitogenesis, and early limb development. beta-catenin and Lef/Tcf expression did not always coincide, indicating developmental contexts in which Lef/Tcf proteins may interact with other cofactors and conversely, the areas in which beta-catenin may interact with other coregulators, or be involved in regulating adhesive properties of cells.

E2a/Pbx1 induces the rapid proliferation of stem cell factor-dependent murine pro-T cells that cause acute T-lymphoid or myeloid leukemias in mice

Oncoprotein E2a/Pbx1 is produced by the t(1;19) chromosomal translocation of human pre-B acute lymphoblastic leukemia. E2a/Pbx1 blocks differentiation of primary myeloid progenitors but, paradoxically, induces apoptosis in established pre-B-cell lines, and no transforming function of E2a/Pbx1 has been reported in cultured lymphoid progenitors. Here, we demonstrate that E2a/Pbx1 induces immortal proliferation of stem cell factor (SCF)-dependent pro-T thymocytes by a mechanism dependent upon both its transactivation and DNA-binding functions. E2a-Pbx1 cooperated with cytokines or activated signaling oncoproteins to induce cell division, as inactivation of conditional E2a/Pbx1 in either factor-dependent pro-T cells or pro-T cells made factor independent by expression of Bcr/Abl resulted in pro-T-cell quiescence, while reactivation of E2a/Pbx1 restored cell division. Infusion of E2a/Pbx1 pro-T cells in mice caused T lymphoblastic leukemia and, unexpectedly, acute myeloid leukemia. The acute lymphoblastic leukemia did not evidence further maturation, suggesting that E2a/Pbx1 establishes an early block in pro-T-cell development that cannot be overcome by marrow or thymic microenvironments. In an E2a/Pbx1 pro-T thymocyte clone that induced only pro-T acute lymphoblastic leukemia, coexpression of Bcr/Abl expanded its leukemic phenotype to include acute myeloid leukemia, suggesting that unique functions of cooperating signaling oncoproteins can influence the lymphoid versus myeloid character of E2a/Pbx1 leukemia and may cooperate with E2a/Pbx1 to dictate the pre-B-cell phenotype of human leukemia containing t(1;19).

Wnt signaling mutants have decreased dentate granule cell production and radial glial scaffolding abnormalities

LRP6 mutant mice have generalized defects in the Wnt/beta-catenin signaling pathway because of the crucial function of LRP6 as a Wnt signaling co-receptor (Pinson et al., 2000). We examined the hippocampal phenotype of single LRP6 mutant mice as well as LRP6/Lef1 double mutant mice. LRP6 mutants had reduced production of dentate granule neurons and abnormalities of the radial glial scaffolding in the forming dentate gyrus. These defects were more severe with the addition of a single Lef1 null allele to an LRP6 null background. Pyramidal cell fields were unaffected in the LRP6, Lef1, or double mutants. The dentate defects were accompanied by decreased numbers of mitotic precursors in the migratory pathway to the dentate and in the displaced proliferative zone in the dentate itself. At earlier gestational ages, there was a reduction in the number of dentate granule cell progenitors in the dentate ventricular zone before the emigration of the earliest differentiated granule neurons and precursors to form the dentate anlage.

Shh expression is required for embryonic hair follicle but not mammary gland development

The embryonic mammary gland and hair follicle are both derived from the ventral ectoderm, and their development depends on a number of common fundamental developmental pathways. While the Hedgehog (Hh) signaling pathway is required for hair follicle morphogenesis, the role of this pathway during embryonic mammary gland development remains undetermined. We demonstrate here that, unlike the hair follicle, both Shh and Ihh are expressed in the developing embryonic mouse mammary rudiment as early as E12.5. In Shh(-/-) embryos, hair follicle development becomes arrested at an early stage, while the mammary rudiment, which continues to express Ihh, develops in a manner indistinguishable from that of wild-type littermates. The five pairs of mammary buds in Shh(-/-) female embryos exhibit normal branching morphogenesis at E16.5, forming a rudimentary ductal structure identical to wild-type embryonic mammary glands. We further demonstrate that loss of Hh signaling causes altered cyclin D1 expression in the embryonic dermal mesenchyme. Specifically, cyclin D1 is expressed at E14.5 principally in the condensed mesenchymal cells of the presumptive hair follicles and in both mesenchymal and epithelial cells of the mammary rudiments in wild-type and Shh-deficient embryos. By E18.5, robust cyclin D1 expression is maintained in mammary rudiments of both wild-type and Shh-deficient embryos. In hair follicles of wild-type embryos by E18.5, cyclin D1 expression switches to follicular epithelial cells. In contrast, strong cyclin D1 expression is observed principally in the mesenchymal cells of arrested hair follicles in Shh(-/-) embryos at E18.5. These data reveal that, despite the common embryonic origin of hair follicles and mammary glands, distinct patterns of Hh-family expression occur in these two tissues. Furthermore, these data suggest that cyclin D1 expression in the embryonic hair follicle is mediated by both Hh-independent and Hh-dependent mechanisms.

Defining BMP functions in the hair follicle by conditional ablation of BMP receptor IA

Using conditional gene targeting in mice, we show that BMP receptor IA is essential for the differentiation of progenitor cells of the inner root sheath and hair shaft. Without BMPRIA activation, GATA-3 is down-regulated and its regulated control of IRS differentiation is compromised. In contrast, Lef1 is up-regulated, but its regulated control of hair differentiation is still blocked, and BMPRIA-null follicles fail to activate Lef1/β-catenin–regulated genes, including keratin genes. Wnt-mediated transcriptional activation can be restored by transfecting BMPRIA-null keratinocytes with a constitutively activated β-catenin. This places the block downstream from Lef1 expression but upstream from β-catenin stabilization. Because mice lacking the BMP inhibitor Noggin fail to express Lef1, our findings support a model, whereby a sequential inhibition and then activation of BMPRIA is necessary to define a band of hair progenitor cells, which possess enough Lef1 and stabilized β-catenin to activate the hair specific keratin genes and generate the hair shaft.

Wnt Target Genes Identified by DNA Microarrays in Immature CD34+Thymocytes Regulate Proliferation and Cell Adhesion

The thymus is seeded by very small numbers of progenitor cells that undergo massive proliferation before differentiation and rearrangement of TCR genes occurs. Various signals mediate proliferation and differentiation of these cells, including Wnt signals. Wnt signals induce the interaction of the cytoplasmic cofactor beta-catenin with nuclear T cell factor (TCF) transcription factors. We identified target genes of the Wnt/beta-catenin/TCF pathway in the most immature (CD4-CD8-CD34+) thymocytes using Affymetrix DNA microarrays in combination with three different functional assays for in vitro induction of Wnt signaling. A relatively small number (approximately 30) of genes changed expression, including several proliferation-inducing transcription factors such as c-fos and c-jun, protein phosphatases, and adhesion molecules, but no genes involved in differentiation to mature T cell stages. The adhesion molecules likely confine the proliferating immature thymocytes to the appropriate anatomical sites in the thymus. For several of these target genes, we validated that they are true Wnt/beta-catenin/TCF target genes using real-time quantitative PCR and reporter gene assays. The same core set of genes was repressed in Tcf-1-null mice, explaining the block in early thymocyte development in these mice. In conclusion, Wnt signals mediate proliferation and cell adhesion, but not differentiation of the immature thymic progenitor pool.

Sublytic terminal complement attack induces c-fos transcriptional activation in myotubes

Sublytic C5b-9 alters the molecular phenotype of myotubes by inhibiting muscle-specific gene expression. Here, we showed that C5b-9 induced c-fos mRNA and transcription. Using c-fos promoter-CAT constructs and electrophoretic mobility shift assay (EMSA), the minimal c-fos promoter activity was shown to increase within 30-min exposure to serum C5b-9, which also induced the binding of serum response factor (SRF), along with ternary complex factor (TCF) Elk1 and Sap1a to the serum response element. C5b-9 activated ERK1, which in turn activated Elk1 in myotubes. We propose that c-fos gene transcription associated with myotube dedifferentiation is induced by C5b-9 through ERK1-mediated assembly of serum response factor-ternary complex.

Lymphoid Enhancer Factor-1 Links Two Hereditary Leukemia Syndromes through Core-binding Factor α Regulation of ELA2

Two hereditary human leukemia syndromes are severe congenital neutropenia (SCN), caused by mutations in the gene ELA2, encoding the protease neutrophil elastase, and familial platelet disorder with acute myelogenous leukemia (AML), caused by mutations in the gene AML1, encoding the transcription factor core-binding factor alpha (CBFalpha). In mice, CBFalpha regulates the expression of ELA2, suggesting a common link for both diseases. However, gene-targeted mouse models have failed to reproduce either human disease, thus prohibiting further in vivo studies in mice. Here we investigate CBFalpha regulation of the human ELA2 promoter, taking advantage of bone marrow obtained from patients with either illness. In particular, we have identified novel ELA2 promoter substitutions (-199 C to A) within a potential motif for lymphoid enhancer factor-1 (LEF-1), a transcriptional mediator of Wnt/beta-catenin signaling, in SCN patients. The LEF-1 motif lies adjacent to a potential CBFalpha binding site that is in a different position in human compared with mouse ELA2. We find that LEF-1 and CBFalpha co-activate ELA2 expression. In vitro, the high mobility group domain of LEF-1 interacts with the runt DNA binding and proline-, serine-, threonine-rich activation domains of CBFalpha. ELA2 transcript levels are up-regulated in bone marrow of an SCN patient with the -199 C to A substitution. Conversely, a mutation of the CBFalpha activation domain, found in a patient with familial platelet disorder with AML, fails to stimulate the ELA2 promoter in vitro, and bone marrow correspondingly demonstrates reduced ELA2 transcript. Observations in these complementary patients indicate that LEF-1 cooperates with CBFalpha to activate ELA2 in vivo and also suggest the possibility that up-regulating promoter mutations can contribute to SCN. Two hereditary AML predisposition syndromes may therefore intersect via LEF-1, potentially linking them to more generalized cancer mechanisms.

Tcf binding sequence and position determines ?-catenin and Lef-1 responsiveness of MMP-7 promoters

The matrix metalloproteinase-7 (MMP-7) gene is a target of beta-catenin transactivation. Expression of the T-cell factor, Lef-1, enhances transcriptional activation of the human MMP-7 promoter by beta-catenin, but represses activation of the mouse MMP-7 promoter, both activities through consensus Tcf binding sites. The mouse promoter has a single Tcf binding element (mTBE) located downstream of the transcriptional start site, while the human promoter has two Tcf binding elements (hTBE1, hTBE2), both located upstream of the transcriptional start. hTBE1 and hTBE2 also differ in sequence from mTBE. Here we demonstrate that positioning of mTBE, upstream or downstream of the transcriptional start site dictated whether Lef-1 functioned as an activator or repressor, respectively. Sequence differences between mTBE and hTBE sites determined the potency of these activities, with hTBE sites being weaker. Mutational analysis of mTBE showed that increased Lef-1 activity mapped to G . C base pairings at 5' and 3' ends, and correlated with a threefold increase in Lef-1 binding affinity in vitro. Heterologous promoters with high affinity binding sites were 115-fold more responsive to beta-catenin than those with low affinity sites. Converting low affinity Tcf binding sites to high affinity sites increased beta-catenin responsiveness of the mouse and human promoters by 2-3 fold, and ectopic expression of Lef-1 increased beta-catenin responsiveness for promoters with low affinity binding sequences. We concluded that sequence and position of Tcf binding sites can determine the extent of beta-catenin-Lef-1 responsiveness for beta-catenin target genes.

Requirement for a nuclear function of beta-catenin in Wnt signaling

Wnt signaling stabilizes beta-catenin, which in turn influences the transcription of Wnt-responsive genes in conjunction with T-cell factor (TCF) transcription factors. At present, there are two models for the actions of beta-catenin. The conventional nuclear model suggests that beta-catenin acts in the nucleus to form a heterodimeric transcriptional factor complex with TCF, with TCF providing DNA-specific binding and the C and N termini of beta-catenin stimulating transcription. The alternative cytoplasmic model postulates that beta-catenin exports TCF from the nucleus to relieve its repressive activity or activates it in the cytoplasm. We have generated modified forms of beta-catenin and used RNA interference against endogenous beta-catenin to distinguish between these models in cultured mammalian and Drosophila cells. We show that the VP16 transcriptional activation domain can replace the C terminus of beta-catenin without loss of function and that the function of beta-catenin is compromised by fusion to a transcriptional repressor domain from histone deacetylase, favoring the direct effects of beta-catenin in the nucleus. Furthermore, membrane-tethered beta-catenin requires interaction with the adenomatous polyposis coli protein but not with TCF for its function, whereas untethered beta-catenin requires binding to TCF for its signaling activity. Importantly, by using RNA interference, we show that the signaling activity of membrane-tethered beta-catenin, but not free beta-catenin, requires the presence of endogenous beta-catenin, which is able to accumulate in the nucleus when stabilized by the binding of the beta-catenin degradation machinery to the membrane-tethered form. All of these data support a nuclear model for the normal function of beta-catenin.

Restoration of full-length adenomatous polyposis coli (APC) protein in a colon cancer cell line enhances cell adhesion

The APC tumour suppressor gene is mutated in most colon cancers. A major role of APC is the downregulation of the beta-catenin/T-cell factor (Tcf)/lymphoid enhancer factor (LEF) signalling pathway; however, there are also suggestions that it plays a role in the organization of the cytoskeleton, and in cell adhesion and migration. For the first time, we have achieved stable expression of wild-type APC in SW480 colon cancer cells, which normally express a truncated form of APC. The ectopically expressed APC is functional, and results in the translocation of beta-catenin from the nucleus and cytoplasm to the cell periphery, and reduces beta-catenin/Tcf/LEF transcriptional signalling. E-cadherin is also translocated to the cell membrane, where it forms functional adherens junctions. Total cellular levels of E-cadherin are increased in the SW480APC cells and the altered charge distribution in the presence of full-length APC suggests that APC is involved in post-translational regulation of E-cadherin localization. Changes in the location of adherens junction proteins are associated with tighter cell-cell adhesion in SW480APC cells, with consequent changes in cell morphology, the actin cytoskeleton and cell migration in a wound assay. SW480APC cells have a reduced proliferation rate, a reduced ability to form colonies in soft agar and do not grow tumours in a xenograft mouse tumour model. By regulating the intracellular transport of junctional proteins, we propose that APC plays a role in cell adhesion in addition to its known role in beta-catenin transcriptional signalling.

Identification of BMP and activin membrane-bound inhibitor (BAMBI), an inhibitor of transforming growth factor-beta signaling, as a target of the beta-catenin pathway in colorectal tumor cells

The Wnt signaling pathway is activated in most human colorectal tumors. Mutational inactivation in the tumor suppressor adenomatous polyposis coli (APC), as well as activation of beta-catenin, causes the accumulation of beta-catenin, which in turn associates with the T cell factor/lymphoid enhancer factor (TCF/LEF) family of transcription factors and activates transcription of their target genes. Here we show that beta-catenin activates transcription of the BMP and activin membrane-bound inhibitor (BAMBI)/NMA gene. The expression level of BAMBI was found to be aberrantly elevated in most colorectal and hepatocellular carcinomas relative to the corresponding non-cancerous tissues. Expression of BAMBI in colorectal tumor cell lines was repressed by a dominant-negative mutant of TCF-4 or by an inhibitor of beta-catenin-TCF interaction, suggesting that beta-catenin is responsible for the aberrant expression of BAMBI in colorectal tumor cells. Furthermore, overexpression of BAMBI inhibited the response of tumor cells to transforming growth factor-beta signaling. These results suggest that beta-catenin interferes with transforming growth factor-beta-mediated growth arrest by inducing the expression of BAMBI, and this may contribute to colorectal and hepatocellular tumorigenesis.

Epstein-Barr virus latent membrane protein 2A activates beta-catenin signaling in epithelial cells

The Epstein-Barr virus (EBV) latent membrane protein 2A (LMP2A) functions to maintain latency in EBV-infected B lymphocytes. Although LMP2A is nonessential for the immortalization of B lymphocytes by EBV, its expression in B lymphocytes prevents viral reactivation by blocking B-cell receptor activation and signaling. LMP2A also provides an antiapoptotic signal in transgenic mice that express LMP2A in B lymphocytes. LMP2A activates phosphatidylinositol 3-kinase (PI3K) and the serine/threonine kinase Akt in lymphocytes and epithelial cells. Here we show that EBV LMP2A activates the PI3K and beta-catenin signaling pathways in telomerase-immortalized human foreskin keratinocytes (HFK). LMP2A activated Akt in a PI3K-dependent manner, and the downstream Akt targets glycogen synthase kinase 3beta (GSK3beta) and the Forkhead transcription factor FKHR were phosphorylated and inactivated in LMP2A-expressing HFK cells. GSK3beta is a negative regulator of the Wnt signaling pathway, and inactivation of GSK3beta by LMP2A signaling led to stabilization of beta-catenin, the central oncoprotein of Wnt signaling. In LMP2A-expressing cells, beta-catenin accumulated in the cytoplasm and translocated into the nucleus via a two-step mechanism. The cytoplasmic accumulation of beta-catenin downstream of LMP2A was independent of PI3K signaling, whereas its nuclear translocation was dependent on PI3K signaling. In the nucleus, beta-catenin activated a reporter responsive to T-cell factor, and this activation was augmented by LMP2A coexpression. The Wnt pathway is inappropriately activated in 90% of colon cancers and is dysregulated in several other cancers, and these data suggest that activation of this pathway by LMP2A may contribute to the generation of EBV-associated cancers.

Wnt-signalling pathway in ovarian epithelial tumours: increased expression of beta-catenin and GSK3beta

Beta-catenin is involved in both cell–cell adhesion and in transcriptional regulation by the Wingless/Wnt signalling pathway. Alterations of components of this pathway have been suggested to play a central role in tumorigenesis. The present study investigated, by immunohistochemistry and immunoblotting, the protein expression and localisation of β-catenin, adenomatous polyposis coli (APC), glycogen synthase kinase 3β (GSK3β) and lymphocyte enhancer factor-1 (Lef-1) in normal human ovaries and in epithelial ovarian tumours in vivo and in vitro. Immortalised human ovarian surface epithelium and ovarian cancer cell cells (OVCAR-3) expressed β-catenin, APC, GSK3β and Lef-1. Nuclear staining of β-catenin and Lef-1 were demonstrated only in OVCAR-3 cells. There were significant increases of β-catenin and GSK3β, while APC was reduced in ovarian cancer compared to the normal ovary. Beta-catenin and Lef-1 were coimmunoprecipitated in ovarian tumours, but not in the normal ovary. Nuclear localisation of β-catenin or Lef-1 could not be demonstrated in the normal ovary or in the ovarian tumours. The absence of nuclear localisation of β-catenin could be due to an increased binding to the cadherin–α-catenin cell adhesion complex. In fact, we have earlier reported an increased expression of E-cadherin in ovarian adenocarcinomas. In summary, this study demonstrates an increase in the expression of components of the Wingless/Wnt pathway in malignant ovarian tumours. The increase suggests a role for this signalling pathway in cell transformation and in tumour progression. However, it remains to be demonstrated whether it is an increased participation of β-catenin in transcriptional regulation, or in the stabilisation of cellular integrity, or both, that is the crucial event in ovarian tumorigenesis.

Synergistic effects of coactivators GRIP1 and beta-catenin on gene activation: cross-talk between androgen receptor and Wnt signaling pathways

The p160 coactivators, such as GRIP1, bind nuclear receptors and help to mediate transcriptional activation. beta-Catenin binds to and serves as a coactivator for the nuclear receptor, androgen receptor (AR), and the Lymphoid Enhancer Factor/T Cell Factor family member, Lef1. Here we report that GRIP1 and beta-catenin can bind strongly to each other through the AD2 domain of GRIP1. Furthermore, GRIP1 and beta-catenin can synergistically enhance the activity of both AR and Lef1, and both coactivators are recruited specifically to AR-driven and Lef1-driven promoters. However, the mechanism of beta-catenin-GRIP1 coactivator function and synergy is different with AR and Lef1. While beta-catenin can bind directly to both AR and Lef1, GRIP1 can only bind directly to AR; the ability of GRIP1 to associate with and function as a coactivator for Lef1 is entirely dependent on the presence of beta-catenin. Thus, whereas GRIP1 coactivator function involves direct binding to nuclear receptors and most other classes of DNA-binding transcriptional activator proteins, the coactivator function of GRIP1 with Lef1 follows a novel paradigm where GRIP1 is recruited indirectly to Lef1 through their mutual association with beta-catenin. The beta-catenin-GRIP1 interaction represents another potential point of cross-talk between the AR and Wnt signaling pathways.

Manipulation of stem cell proliferation and lineage commitment: visualisation of label-retaining cells in wholemounts of mouse epidermis

Mammalian epidermis is maintained by stem cells that have the ability to self-renew and generate daughter cells that differentiate along the lineages of the hair follicles, interfollicular epidermis and sebaceous gland. As stem cells divide infrequently in adult mouse epidermis, they can be visualised as DNA label-retaining cells (LRC). With whole-mount labelling, we can examine large areas of interfollicular epidermis and many hair follicles simultaneously, enabling us to evaluate stem cell markers and examine the effects of different stimuli on the LRC population. LRC are not confined to the hair follicle, but also lie in sebaceous glands and interfollicular epidermis. LRC reside throughout the permanent region of the hair follicle, where they express keratin 15 and lie in a region of high alpha6beta4 integrin expression. LRC are not significantly depleted by successive hair growth cycles. They can, nevertheless, be stimulated to divide by treatment with phorbol ester, resulting in near complete loss of LRC within 12 days. Activation of Myc stimulates epidermal proliferation without depleting LRC and induces differentiation of sebocytes within the interfollicular epidermis. Expression of N-terminally truncated Lef1 to block beta-catenin signalling induces transdifferentiation of hair follicles into interfollicular epidermis and sebocytes and causes loss of LRC primarily through proliferation. We conclude that LRC are more sensitive to some proliferative stimuli than others and that changes in lineage can occur with or without recruitment of LRC into cycle.

Development of artificial chimerical gene regulatory elements specific for cancer gene therapy

To achieve satisfactory outcome by the expression of therapeutic genes, it is of great importance to obtain efficient and high level of gene expression as well as minimizing inappropriate gene expression in non-target cells. To accomplish this goal for cancer gene therapy, we have evaluated the potential of cancer specific gene expression of functional promoter/enhancer elements in six putative cancer-specific genes (Tcf1alpha, C-Ha-Ras, CyclinE, Cdc25A, HK II, and hTert) using a luciferase reporter assay. Most of the reporter constructs showed higher activity in HepG2 cells than in non-transformed or stem cells, and, in particular, the hTert (E) or Tcf1alpha (T) regulatory element showed significantly higher activity. We have also constructed a series of artificial chimerical regulatory elements by combinatorial linking of E promoter and T enhancer. A dramatic decrease of activity was observed as the copy number of concatenated T/E regulatory elements increased. In contrast, in chimerical constructs containing two or three copies of regulatory elements of T/E, cell type preferential expression profiles were changed. Thus, both pGL3-TE and -TEE showed higher activity specifically in MCF7 breast cancer cells, whereas pGL3-TET showed moderate activity in several cancer cell lines of different origins. Our results demonstrate that although the transcriptional activities of synthetic promoters are weak, some cancer-specific regulatory elements are useful in developing optimized and systemic cancer-specific regulatory regions with potential application in targeted cancer cell therapy.

High-Resolution assessment of protein DNA binding affinity and selectivity utilizing a fluorescent intercalator displacement (FID) assay

Protein titration displacement of ethidium bromide bound to hairpin deoxyoligonucleotides containing any sequence of interest provides a well-defined titration curve (measuring the loss of fluorescence derived from the DNA bound ethidium bromide) that provides both absolute binding constants (K(a)) and stoichiometry of binding. This use of a fluorescent intercalator displacement (FID) assay for establishing protein DNA binding affinity and selectivity is demonstrated with the examination of the LEF-1 HMG domain binding to hairpin deoxyoligonucleotides containing its commonly accepted consensus sequence 5'-CTTTGWW (W=A or T) and those modified (5'-CTNTGWW) to examine sequences implicated in early studies (5'-CTNTG). The effectiveness of the FID assay coupled with its technically non-demanding experimental use makes it an attractive alternative or complement to selection screening, footprinting or affinity cleavage, and electrophoretic mobility shift assays for detecting, characterizing, and quantitating protein DNA binding affinity and selectivity.

Smad4 and beta-catenin co-activators functionally interact with lymphoid-enhancing factor to regulate graded expression of Msx2

Recent in vivo evidence suggests that Wnt signaling plays a central role in determining the fate of stem cells in the ectoderm and in the neural crest by modulating bone morphogenetic protein (BMP) levels, which, in turn, influence Msx gene expression. However, the molecular mechanism regulating the expression of the Msx genes as key regulators of cell fate has not been elucidated. Here we show in murine embryonic stem cells that BMP-dependent activation of Msx2 is mediated via the cooperative binding of Smad4 at two Smad binding elements and of lymphoid enhancing factor (Lef1) at two Lef1/TCF binding sites. Lef1 can synergize with Smad4 and Smad1 to activate Msx2 promoter, and this transcriptional complex is assembled on the endogenous promoter in response to BMP2. The Wnt/beta-catenin signaling pathway can activate Msx2 via the binding of Lef1 to its promoter and synergizes with BMP2 to activate Msx2 expression, possibly via enhanced recruitment of the p300/cAMP-response element-binding protein-binding protein co-factor. Interestingly, the Wnt/beta-catenin-dependent activation of Msx2 was defective in Smad4-deficient embryonic stem cells or when Smad binding elements were mutated but persisted in the presence of various BMP antagonists, indicating that Smad4 was involved in transducing the Wnt/beta-catenin signals in the absence of a BMP autocrine loop. A chromatin immunoprecipitation analysis revealed that endogenous Smad4, but not Smad1, was part of the Lef1 transcriptional complex in response to beta-catenin activation, dismissing any implication of BMP signaling in this response. We propose that Wnt signaling pathway could dictate cell fate not only by modulating BMP levels but also by directly regulating cooperatively BMP-target genes.

Beta-catenin is critical for dendritic morphogenesis

Regulated growth and arborization of dendritic processes are critical to the formation of functional neuronal networks. Here we identify beta-catenin as a critical mediator of dendritic morphogenesis. We found that increasing the intracellular levels of beta-catenin and other members of the cadherin/catenin complex, namely N-cadherin and alphaN-catenin, enhances dendritic arborization in rat hippocampal neurons, an effect that does not require Wnt/beta-catenin-dependent transcription. Conversely, proteins that sequester beta-catenin decreased dendritic branch tip number and total dendritic branch length. Enhancement of dendritic growth elicited by depolarization requires beta-catenin and increased Wnt release. These results identify Wnt/beta-catenin signaling as an important mediator of dendritic development and suggest that the intracellular level of the cadherin/catenin complex is a limiting factor during critical stages of dendritic morphogenesis.

The lipid phosphatase LPP3 regulates extra-embryonic vasculogenesis and axis patterning

Bioactive phospholipids, which include sphingosine-1-phosphate, lysophosphatidic acid, ceramide and their derivatives regulate a wide variety of cellular functions in culture such as proliferation, apoptosis and differentiation. The availability of these lipids and their products is regulated by the lipid phosphate phosphatases (LPPs). Here we show that mouse embryos deficient for LPP3 fail to form a chorio-allantoic placenta and yolk sac vasculature. A subset of embryos also show a shortening of the anterior-posterior axis and frequent duplication of axial structures that are strikingly similar to the phenotypes associated with axin deficiency, a critical regulator of Wnt signaling. Loss of LPP3 results in a marked increase in beta-catenin-mediated TCF transcription, whereas elevated levels of LPP3 inhibit beta-catenin-mediated TCF transcription. LPP3 also inhibits axis duplication and leads to mild ventralization in Xenopus embryo development. Although LPP3 null fibroblasts show altered levels of bioactive phospholipids, consistent with loss of LPP3 phosphatase activity, mutant forms of LPP3, specifically lacking phosphatase activity, were able to inhibit beta-catenin-mediated TCF transcription and also suppress axis duplication, although not as effectively as intact LPP3. These results reveal that LPP3 is essential to formation of the chorio-allantoic placenta and extra-embryonic vasculature. LPP3 also mediates gastrulation and axis formation, probably by influencing the canonical Wnt signaling pathway. The exact biochemical roles of LPP3 phosphatase activity and its undefined effect on beta-catenin-mediated TCF transcription remain to be determined.

Beta-catenin/Tcf-1-mediated transactivation of cyclin D1 promoter is negatively regulated by thyroid hormone

Cyclin D1 is an oncogenic cyclin frequently over-expressed in cancer. To examine the effect of thyroid hormone (T3) and its receptor (TR) on the transcription of cyclin D1 gene, we co-transfected the chloramphenicol acetyl-transferase (CAT) reporter plasmid containing cyclin D1 promoter together with the expression plasmids for TRbeta1 and wild-type or mutant beta-catenin (SA) into 293T cells. In the presence of T3, beta-catenin-dependent transactivation of cyclin D1 promoter was suppressed by co-transfection of TRbeta1. The suppression by T3/TRbeta1 was in a dose-dependent manner. The CAT reporter gene in which Tcf/Lef-1 sites were fused to heterologous promoter was also suppressed by T3/TRbeta1. Furthermore, inhibition of endogenous wild-type beta-catenin by T3/TRbeta1 was observed in SW480 colon carcinoma cells with mutation of the adenomatous polyposis coli gene. These results indicate that the T3-bound TR inhibits the transcription of cyclin D1 through the Tcf/Lef-1 site, which is positively regulated by the Wnt-signaling pathway.

Synergistic cooperation between the AP-1 and LEF-1 transcription factors in activation of the matrilysin promoter by the src oncogene: implications in cellular invasion

The matrix metalloprotease matrilysin is expressed in premalignant polyps and plays a key role in local invasion during the progression of digestive tumors. In the present work, we investigated the possible relationships between the activity of the mouse and human matrilysin promoters (Mp), endogenous matrilysin protein expression, and two early oncogenetic defects frequently observed in human colonic cancers, namely activation of the src oncogene and impairment of the Wnt/APC/beta-catenin pathway. Using transient transfection assays, we report here that src signaling and the HMG-box transcription factor LEF-1 act synergistically with the proximal (-61 to -67) AP-1 binding site to transactivate the Mp in premalignant and tumorigenic kidney and colonic epithelial cells, through beta-catenin- and axin-independent signaling pathways. This synergism involves the -109 and -194 Tcf/LEF-1 binding sites in the Mp and a physical interaction between LEF-1 and c-Jun. Furthermore, src coordinates accumulation of the c-Jun factor and matrilysin transcripts. Conversely, the c-Jun dominant negative mutant TAM67 and the src tyrosine kinase inhibitor M475271 impaired src-induced Mp activation, matrilysin protein accumulation, and invasion of type I collagen gels. This mechanism may thereby contribute to cellular invasion during the early-stage adenoma/adenocarcinoma conversion and the metastatic process of digestive tumors.

Inhibition of breast cancer cell growth by the Wilms' tumor suppressor WT1 is associated with a destabilization of beta-catenin

The Wilms' tumor suppressor gene, wt1, encodes a zinc-finger protein, WT1, that functions as a potent inhibitor of cell growth. The findings that expression levels of WT1 were down-regulated in breast cancer cell lines and in subsets of primary breast tumors led us to investigate the possible role of WT1 in tumorigenesis of breast cancer. We have established stable cell lines from a breast cancer cell line MDA-MB-231 to express exogenous WT1, and investigated the ability of WT1 to inhibit the transformed phenotype of MDA-MB-231 cells. We found that WT1 suppressed clonal growth of MDA-MB-231 cells in soft-agar and inhibited tumor growth of these cells in nude mice. We also found that the steady state levels of beta-catenin protein and the transcription activity of beta-catenin/Tcf signaling pathway were dramatically decreased in WT1-transfected cells. This decrease of beta-catenin was associated with increased levels of beta-catenin phosphorylation. Furthermore, the expression levels of GSK-3 beta, the kinase that phosphorylates beta-catenin and signals its degradation, were up-regulated in WT1-transfected cells. The results suggest that WT1 inhibits the transformed phenotype of breast cancer cells and down-regulates the beta-catenin/TCF signaling pathway through destabilization of beta-catenin.