Association of the cytotoxic T lymphocyte-associated antigen 4 gene with type 1 diabetes: evidence for independent effects of two polymorphisms on the same haplotype block

A recent study mapped the known association of type 1 diabetes with the cytotoxic T lymphocyte-associated antigen 4 gene to a polymorphism at the 3'end (+6230G>A), but could not rule out additional contribution from the 5' end of the gene. To examine this possibility, we analyzed four polymorphisms at the 5'-flanking region for effects independent of +6230G>A. We confirm, by the transmission disequilibrium test, in 496 family trios overtransmission of the susceptibility allele (G) at +6230 (217/168; P = 0.013). Of the four promoter polymorphisms, one (-319C>T) showed overtransmission of the C allele (97/58; P = 0.0017). Because the undertransmitted T at the promoter is in linkage disequilibrium with the overtransmitted G at +6230G>A, the effect observed at the promoter cannot be accounted for by linkage disequilibrium with the +6230G>A. We confirm this by showing that parents heterozygous at the promoter but homozygous at +6230 overtransmit the C promoter allele even more significantly (53/24; P = 9 x 10(-4)). In vitro, the T promoter allele directs higher luciferase expression in Jurkat cells by 42% (P = 0.006), a difference also found in lymphocyte mRNA from eight individuals heterozygous at the promoter, but homozygous at +6230 (P = 1.3 x 10(-4)). Thus, the +6230G>A cannot be the sole functional variant. Either the two polymorphisms define a haplotype carrying the (yet unexamined) functional variant or the -319C>T contributes to the genetic association independently, a possibility suggested by the functional evidence we present.

Glucocorticoids inhibit the transcriptional activity of LEF/TCF in differentiating osteoblasts in a glycogen synthase kinase-3beta-dependent and -independent manner

Glucocorticoids, widely used as immune suppressors, cause osteoporosis by inhibiting bone formation. In MC3T3-E1 osteoblast-like cultures, dexamethasone (DEX) activates glycogen synthase kinase-3beta (GSK3beta) and inhibits a differentiation-related cell cycle that occurs at a commitment stage immediately after confluence. Here we show that DEX inhibition of the differentiation-related cell cycle is associated with a decrease in beta-catenin levels and inhibition of LEF/TCF-mediated transcription. These inhibitory activities are no longer observed in the presence of lithium, a GSK3beta inhibitor. DEX decreased the serum-responsive phosphorylation of protein kinase B/Akt-Ser(473) within minutes, and this inhibition was also observed after 12 h. When the phosphatidylinositol 3-kinase (PI3K)/Akt pathway was inhibited by wortmannin, DEX no longer inhibited beta-catenin levels. Furthermore, DEX-mediated inhibition of LEF/TCF transcriptional activity was attenuated in the presence of dominant negative forms of either PI3K or protein kinase B/Akt. These results suggest cross-talk between the PI3K/Akt and Wnt signaling pathways. Consistent with a role for Wnt signaling in the osteoblast differentiation-related cell cycle, wortmannin partially negated the DEX inhibition of this cell cycle. DEX also induced histone deacetylase (HDAC) 1, which is known to inhibit LEF/TCF transcriptional activity. Overexpression of HDAC1 negated the inhibitory effect of DEX on LEF/TCF transcriptional activity. In the presence of trichostatin A, a deacetylase inhibitor, DEX-mediated inhibition of the differentiation-related cell cycle was partially negated. When administered together, wortmannin and trichostatin A completely negated the inhibitory effect of DEX on the differentiation-related cell cycle. These results suggest that inhibition of a PI3K/Akt/GSK3beta/beta-catenin/LEF axis and stimulation of HDAC1 cooperate to mediate the inhibitory effect of DEX on Wnt signaling and the osteoblast differentiation-related cell cycle.

Distinct molecular forms of beta-catenin are targeted to adhesive or transcriptional complexes

β-Catenin plays essential roles in both cell–cell adhesion and Wnt signal transduction, but what precisely controls β-catenin targeting to cadherin adhesive complexes, or T-cell factor (TCF)-transcriptional complexes is less well understood. We show that during Wnt signaling, a form of β-catenin is generated that binds TCF but not the cadherin cytoplasmic domain. The Wnt-stimulated, TCF-selective form is monomeric and is regulated by the COOH terminus of β-catenin, which selectively competes cadherin binding through an intramolecular fold-back mechanism. Phosphorylation of the cadherin reverses the TCF binding selectivity, suggesting another potential layer of regulation. In contrast, the main cadherin-binding form of β-catenin is a β-catenin–α-catenin dimer, indicating that there is a distinct molecular form of β-catenin that can interact with both the cadherin and α-catenin. We propose that participation of β-catenin in adhesion or Wnt signaling is dictated by the regulation of distinct molecular forms of β-catenin with different binding properties, rather than simple competition between cadherins and TCFs for a single constitutive form. This model explains how cells can control whether β-catenin is used independently in cell adhesion and nuclear signaling, or competitively so that the two processes are coordinated and interrelated.

Lymphoid enhancer factor/T cell factor expression in colorectal cancer

Genetic inactivation of key components of the Wnt signal transduction system is a frequent event in colorectal cancer. These genetic mutations lead to stabilization of beta-catenin, a cytoplasmic-nuclear shuttling protein with a potent transcription activation domain. Stabilization and subsequent nuclear localization of beta-catenin produces aberrant, Wnt-independent signals to target genes, an activity tightly linked to the genesis of colon cancers. In the nucleus, the transcription factor family of LEF/TCF proteins transmits Wnt signals by binding to beta-catenin and recruiting it to target genes for activation. Such activities are carried out by full-length LEF/TCFs that are thought to be mostly interchangeable and redundant. However, truncated forms of LEF-1 and TCF-1 that do not bind to beta-catenin function as dominant negatives and an alternatively spliced TCF isoform with a unique activation function has recently been discovered. The dominant negative forms block Wnt signals because they occupy Wnt target genes and limit beta-catenin access; the alternatively spliced TCF isoform activates certain Wnt target promoters whereas other TCF isoforms and LEF-1 do not. A study of LEF/TCF expression and activity in normal intestine and colon carcinomas suggests that the relative amounts of LEF/TCF isoforms may change as tumors progress and this may influence the strength and specificity of Wnt signals in the nucleus. While the underlying mechanism for a change in the LEF/TCF isoform expression is not yet known, recent evidence implicates the Wnt signaling pathway itself as a potential modulator.

Cloning of the rat beta-catenin gene (Ctnnb1) promoter and its functional analysis compared with the Catnb and CTNNB1 promoters

Considerable recent interest has focused on the stabilization and accumulation of beta-catenin protein in human and animal tumors and the corresponding activation of downstream beta-catenin/TCF/LEF target genes. However, there is only sparse information on the regulation of beta-catenin expression at the transcriptional level and its possible involvement in physiological and pathophysiological processes. We report here the cloning and characterization of a 3.6-kb promoter fragment from the rat beta-catenin gene, Ctnnb1, and its comparison with corresponding promoters from the mouse and human genes, Catnb and CTNNB1. Several AP1 binding sites were confirmed in the promoters of all three species using mobility shift and reporter assays, and one putative TCF/LEF site also was observed in the promoter of CTNNB1. Subsequently, protein/DNA array analyses identified numerous other transcription factors through their high-affinity binding to the Ctnnb1 promoter, including E2F1, NFkappaB, MEF1, Pax5, ISRE2, Smad3/4, GATA, and ZIC. The strong binding of E2F1 and NFkappaB is especially noteworthy, because the former transcription factor is regulated by cyclin D1, a downstream target of beta-catenin/TCF/LEF signaling, whereas the latter transcription factor has been implicated in "cross talk" between the Wnt and the NFkappaB signaling pathways. These results are discussed in terms of their implications for human cancer development and specifically the various tumors in which beta-catenin mRNA is overexpressed, as well as for embryonic development, when reversible changes in beta-catenin expression occur in response to secreted Wnt ligands. The findings reported here should provide important avenues for further research focused on the regulation of Ctnnb1 activity, including the ability of beta-catenin/Tcf downstream targets to modulate beta-catenin expression at the transcriptional level.

Regulated expression of sFRP-1 protein by the GeneSwitch system

The GeneSwitch system is a mifepristone-inducible expression system that provides exceptionally low uninduced and high-induced protein expression in mammalian cells. We have developed an adenovirus recombinant containing GeneSwitch protein driven by the GAL4-tk promoter, as well as recombinants containing sFRP-1 and luciferase reporter under the control of the GAL4-E1b promoter. Luciferase activity in A549 cells infected with the GeneSwitch and Luciferase viruses is very low in ethanol-treated cells, while the level of luciferase activity increases 200-fold in cells treated with mifepristone. Conditional expression of functional sFRP-1 is demonstrated in A549, human osteoblast, and CHO cell lines by either the co-infection of cells with sFRP-1 and GeneSwitch viruses or the infection of GeneSwitch expressing cell lines with sFRP-1 virus and subsequent treatment with mifepristone. The expression of sFRP-1 is seen as early as 4 h post-mifepristone treatment, reaching the highest levels at 20 h. The sFRP-1 protein is present in conditioned media, and the protein is functional based upon its ability to inhibit the Wnt-mediated activation of TCF-Luciferase reporter activity. The regulated expression of sFRP-1 utilizing adenovirus vectors provides an opportunity to address the contribution of sFRP-1 in the regulation of stem cell differentiation, maturation, and their function by modulating the Wnt signaling.

Mechanisms involved in prostaglandin E2-mediated neuroprotection against TNF-α: possible involvement of multiple signal transduction and β-catenin/T-cell factor

Cerebrospinal fluid prostaglandin E2 (PGE2) and tumor necrosis factor-alpha (TNF-alpha) levels are elevated in patients with Alzheimer's disease (AD), which suggests that they are involved in neurodegeneration. We previously reported that TNF-alpha derived from human macrophages, in response to beta-amyloid or amyloidogenic C-terminal peptide, is a main mediator of inflammatory neurotoxicity. In a continuation of this work, the present study investigated the direct effect of PGE2, one of the major prostaglandins produced in the brain, on cell viability in SH-SY5Y neuronal cells treated with TNF-alpha. PGE2 did not promote neurotoxicity, but rather had a strong protective effect against TNF-alpha by ameliorating TNF-alpha-induced apoptosis and also by rescuing the intracellular level of beta-catenin, a key transducer of the Wnt signaling pathway. PGE2-mediated stabilization of beta-catenin was accompanied by T-cell factor/lymphoid enhancer factor (Tcf/Lef)-mediated transcriptional activation, which was followed by an increase in the cyclinD1 level. Pharmacological studies provided further evidence supporting the notion that PGE2-mediated neuroprotection against TNF-alpha involves the stimulation of Tcf/Lef signaling through EP1-, EP2-, and EP4-mediated increases of beta-catenin in SH-SY5Y cells. In addition, this PGE2 effect appears to be dependent on the activation of protein kinase A, phosphatidylinositol 3-kinase, phospholipase C, and to a lesser extent protein kinase C. Thus, the molecular mechanism governing the inhibitory effect of PGE2 against TNF-alpha may involve the activation and cross talk of multiple signal transduction and play an important role in regulating the survival of neurons during the neurotoxic inflammatory response associated with neurodegenerative diseases including AD.

Mammalian Ryk Is a Wnt Coreceptor Required for Stimulation of Neurite Outgrowth

The Ryk receptor belongs to the atypical receptor tyrosine kinase family. It is a new member of the family of Wnt receptor proteins. However, the molecular mechanisms by which the Ryk receptor functions remain unknown. Here, we report that mammalian Ryk, unlike the Drosophila Ryk homolog Derailed, functions as a coreceptor along with Frizzled for Wnt ligands. Ryk also binds to Dishevelled, through which it activates the canonical Wnt pathway, providing a link between Wnt and Dishevelled. Transgenic mice expressing Ryk siRNA exhibit defects in axon guidance, and Ryk is required for neurite outgrowth induced by Wnt-3a and in the activation of T cell factor (TCF) induced by Wnt-1. Thus, Ryk appears to play a crucial role in Wnt-mediated signaling.

Assembly and analysis of the mouse immunoglobulin kappa gene sequence

The mechanisms regulating V gene usage leading to the immunoglobulin (Ig) repertoire have been of interest for many years but are only partially defined. To gain insight into these processes, we have assembled the nucleotide sequence of the Mus musculus Igkappa locus using data recently made available from genome-wide sequencing efforts. We found the locus to be 3.21 Mb in length and mapped all known functional, pseudo- and relic V gene segments onto the sequence, along with known regulatory elements. We corrected errors in former gene assignments, positions and orientations and identified a novel Vkappa4 gene segment. This assembly allowed the establishment of a unified nomenclature for the V genes based on their relative positions similar to the nomenclature system adopted for the human Ig loci. The 5' boundary of the locus is defined by the presence of the tumor-associated calcium-signal transducer-2 gene located 19 kb upstream of Vkappa24-140, the most distal V gene. No non- Vkappa genes were found in the sequence of the locus. Detailed analysis of the sequences 0.5 kb upstream, within, and 0.5 kb downstream of each potentially functional V gene revealed interesting patterns of statistically significant clustering of transcription factor consensus binding sites, generally specific to a particular family. We found E boxes were clustered not only in promoter regions, but also nearby recombination signal sequences. Family members of Vkappa4/5 genes exhibit a conserved pattern of octamer sites in their downstream regions, as well as Ebf sites in their introns, and Lef-1 sites in their upstream regions. We discuss potential functional implications of these findings in the context of possible combinatorial mechanisms for targeting V genes for rearrangement. The assembled sequence and its analyses are available as a resource to the scientific community.

Hydrogen peroxide negatively modulates Wnt signaling through downregulation of beta-catenin

The Wnt signal transduction pathway plays an important role in organogenesis and carcinogenesis. In an effort to better understand the action of oxidative stress-induced cellular signaling, we investigate the effect of exogenous H2O2 on the Wnt signal pathway. H2O2 decreases the amount of nuclear beta-catenin and Tcf/Lef-dependent transcription. Overexpression of Dvl-1 abrogated H2O2-induced downregulation of beta-catenin. Pretreatment with LiCl or Wnt-3a conditioned medium completely inhibited H2O2-induced release of mitochondrial cytochrome c and DNA fragmentation. These results suggest that H2O2 negatively modulates the Wnt signal pathway through downregulation of beta-catenin.

IL-7 receptor signals inhibit expression of transcription factors TCF-1, LEF-1, and RORgammat: impact on thymocyte development

Intrathymic T cell development depends on signals transduced by both T cell receptor and cytokine receptors. Early CD4⁻CD8⁻ (double negative) thymocytes require interleukin (IL)-7 receptor (IL-7R) signals for survival and proliferation, but IL-7R signals are normally extinguished by the immature single positive (ISP) stage of thymocyte development. We now demonstrate that IL-7R signals inhibit expression of transcription factors TCF-1, LEF-1, and RORγt that are required for the ISP to double positive (DP) transition in the thymus. In addition, we demonstrate that IL-7R signals also inhibit TCF-1 and LEF-1 expression in mature peripheral T cells. Thus, the present work has identified several important downstream target genes of IL-7R signaling in T cells and thymocytes that provide a molecular mechanism for the inhibitory influence of IL-7R signaling on DP thymocyte development. We conclude that IL-7R signals down-regulate transcription factors required for the ISP to DP transition and so must be terminated by the ISP stage of thymocyte development.

Interactions between FGF and Wnt signals and Tbx3 gene expression in mammary gland initiation in mouse embryos

Interactions between Wnts, Fgfs and Tbx genes are involved in limb initiation and the same gene families have been implicated in mammary gland development. Here we explore how these genes act together in mammary gland initiation. We compared expression of Tbx3, the gene associated with the human condition ulnar-mammary syndrome, expression of the gene encoding the dual-specificity MAPK phosphatase Pyst1/MKP3, which is an early response to FGFR1 signalling (as judged by sensitivity to the SU5402 inhibitor), and expression of Lef1, encoding a transcription factor mediating Wnt signalling and the earliest gene so far known to be expressed in mammary gland development. We found that Tbx3 is expressed earlier than Lef1 and that Pyst1 is also expressed early but only transiently. Patterns of expression of Tbx3, Pyst1 and Lef1 in different glands suggest that the order of mammary gland initiation is 3, 4, 1, 2 and 5. Consistent with expression of Pyst1 in the mammary gland, we detected expression of Fgfr1b, Fgf8 and Fgf9 in both surface ectoderm and mammary bud epithelium, and Fgf4 and Fgf17 in mammary bud epithelium. Beads soaked in FGF-8 applied to the flank of mouse embryos, at a stage just prior to mammary bud initiation, induce expression of Pyst1 and Lef1 and maintain Tbx3 expression in flank tissue surrounding the bead. Grafting beads soaked in the FGFR1 inhibitor, SU5402, abolishes Tbx3, Pyst1 and Lef1 expression, supporting the idea that FGFR1 signalling is required for early mammary gland initiation. We also showed that blocking Wnt signalling abolishes Tbx3 expression but not Pyst1 expression. These data, taken together with previous findings, suggest a model in which Tbx3 expression is induced and maintained in early gland initiation by both Wnt and Fgf signalling through FGFR1.

A small molecule inhibitor of beta-catenin/CREB-binding protein transcription [corrected]

Inherited and somatic mutations in the adenomatous polyposis coli occur in most colon cancers, leading to activation of beta-catenin-responsive genes. To identify small molecule antagonists of this pathway, we challenged transformed colorectal cells with a secondary structure-templated chemical library, looking for compounds that inhibit a beta-catenin-responsive reporter. We identified ICG-001, a small molecule that down-regulates beta-catenin/T cell factor signaling by specifically binding to cyclic AMP response element-binding protein. ICG-001 selectively induces apoptosis in transformed cells but not in normal colon cells, reduces in vitro growth of colon carcinoma cells, and is efficacious in the Min mouse and nude mouse xenograft models of colon cancer.

Characterization of Lef-1 promoter segments that facilitate inductive developmental expression in skin

Lymphoid Enhancer Factor 1 (Lef-1) is an important developmental transcription factor required for the inductive formation of several epithelial-derived organs including hair follicles. Inductive expression of Lef-1 mRNA is tightly regulated during embryo development, suggesting the involvement of a highly regulated promoter. In vitro analysis of the Lef-1 gene has demonstrated the existence of at least two spatially distinct promoters with multiple transcriptional start sites that are responsive to the canonical Wnt/beta-catenin pathway. Regions of the Lef-1 promoter required for inductive regulation in vivo, however, have yet to be determined. To this end, we utilized LacZ-reporter transgenic mice to define segments of the human Lef-1 promoter capable of reproducing mesenchymal- or epithelial-restricted transcriptional patterns of Lef-1 expression during hair and vibrissa follicle development. These studies have revealed that a 110 bp Wnt/beta-catenin-responsive element, contained within a minimal 2.5 kb Lef-1 promoter, plays an important role in regulating mesenchymal, and potentially epithelial, expression during follicle development in mouse embryos. This 2.5 kb Lef-1 promoter also demonstrated inductive mesenchymal expression during postnatal anagen stage hair-follicle cycling. Additionally, analysis of Lef-1 promoter expression revealed previously uncharacterized regions of endogenous Lef-1 expression seen in the sebaceous glands of vibrissa and hair follicles in transgenic lines harboring the minimal Lef-1 promoter and additional intronic sequences. In summary, these studies have begun to dissect the transcriptional diversity of the human Lef-1 promoter during the hair/vibrissa follicle and sebaceous gland formation.

Granulocyte-macrophage progenitors as candidate leukemic stem cells in blast-crisis CML

BACKGROUND

The progression of chronic myelogenous leukemia (CML) to blast crisis is supported by self-renewing leukemic stem cells. In normal mouse hematopoietic stem cells, the process of self-renewal involves the beta-catenin-signaling pathway. We investigated whether leukemic stem cells in CML also use the beta-catenin pathway for self-renewal.

METHODS

We used fluorescence-activated cell sorting to isolate hematopoietic stem cells, common myeloid progenitors, granulocyte-macrophage progenitors, and megakaryocyte-erythroid progenitors from marrow during several phases of CML and from normal marrow. BCR-ABL, beta-catenin, and LEF-1 transcripts were compared by means of a quantitative reverse-transcriptase-polymerase-chain-reaction assay in normal and CML hematopoietic stem cells and granulocyte-macrophage progenitors. Confocal fluorescence microscopy and a lymphoid enhancer factor/T-cell factor reporter assay were used to detect nuclear beta-catenin in these cells. In vitro replating assays were used to identify self-renewing cells as candidate leukemic stem cells, and the dependence of self-renewal on beta-catenin activation was tested by lentiviral transduction of hematopoietic progenitors with axin, an inhibitor of the beta-catenin pathway.

RESULTS

The granulocyte-macrophage progenitor pool from patients with CML in blast crisis and imatinib-resistant CML was expanded, expressed BCR-ABL, and had elevated levels of nuclear beta-catenin as compared with the levels in progenitors from normal marrow. Unlike normal granulocyte-macrophage progenitors, CML granulocyte-macrophage progenitors formed self-renewing, replatable myeloid colonies, and in vitro self-renewal capacity was reduced by enforced expression of axin.

CONCLUSIONS

Activation of beta-catenin in CML granulocyte-macrophage progenitors appears to enhance the self-renewal activity and leukemic potential of these cells.

Fibroblast growth factor-2 induces osteoblast survival through a phosphatidylinositol 3-kinase-dependent, -beta-catenin-independent signaling pathway

Fibroblast growth factor-2 (FGF-2) is an important molecule that controls bone formation through activation of osteoblastic cell replication and differentiation. The role of FGF-2 on human osteoblast survival and the signaling pathway that mediates its effect are not known. We studied the effect of FGF-2 on apoptosis induced by low serum concentration and the signal transduction pathway involved in this effect in human primary calvaria osteoblasts and immortalized osteoblastic cells. Treatment with FGF-2 for 24-48 h protected against osteoblast apoptosis induced by low serum concentration, through specific inhibition of caspase-2 and caspase-3 activity. Pharmacological inhibition of MEK-1 and p38 MAPK had no effect on the inhibition of caspases-2 and -3 induced by FGF-2. In contrast, inhibition of PI3K with LY294002 abolished the FGF-2-induced inhibition of caspases-2 and -3. FGF-2 increased PI3K activity but did not induce phosphorylation of Akt or the downstream effector p70 S6 kinase. FGF-2 also induced GSK-3alpha and beta phosphorylation in osteoblastic cells, which however did not result in beta-catenin accumulation or Lef/Tcf transcriptional activity. In contrast, lithium induced beta-catenin accumulation, Lef/Tcf transcriptional activation and increased caspase-2 and -3 activity. The results indicate that the immediate protective effect of FGF-2 on human osteoblastic cell apoptosis involves PI3K and inhibition of downstream caspases, independently of GSK-3 and beta-catenin-Lef/Tcf-mediated transcription.

Disruption of STAT3 signaling leads to tumor cell invasion through alterations of homotypic cell-cell adhesion complexes

STAT3 is frequently overexpressed and constitutively activated by tyrosine phosphorylation during malignant transformation. Despite the clear importance of STAT3 in cell proliferation and survival in diverse human cancers, its possible contribution to tumor cell adhesion, motility and invasion remains hypothetical. We therefore compared the transforming properties of STAT3wt, its constitutively activated dimeric form STAT3C, and the dominant negative mutant STAT3-Y705F in human colorectal HCT8/S11 cancer cells. Both STAT3wt and STAT3C exert a permissive action to the proinvasive activity of the scatter factor HGF in HCT8/S11 cells. In contrast, the monomeric and cytoplasmic mutant Y705F induces a constitutive invasive phenotype through Wnt/Rho-independent and EGFR/PI3-kinase-dependent pathways. Accordingly, Y705F decreases cell-cell homotypic adhesions, and increases cell motility and scattering, as well as lamellipodia-type cellular extensions. STAT3-Y705F-transfected HCT8/S11 cells display an increased tyrosine phosphorylation of the cell-cell adhesion regulator beta-catenin and its dissociation from the invasion suppressor E-cadherin at cell-cell contacts. Our data imply that both invasion promoter and repressor genes are controlled by the canonical STAT3 transcription pathways. Disruption of this cascade by Y705F reveals the proinvasive potential of altered forms of STAT3 as a persistent signaling adaptor in cytokine/transforming growth factor receptor scaffolds and oncogenic pathways.

Characterization of Function of Three Domains in Dishevelled-1: DEP Domain is Responsible for Membrane Translocation of Dishevelled-1

Wnt signaling plays an important role in embryogenesis and tumorgenesis. Although the mechanism about how Wnts transduce their signaling from receptor frizzled (Fz) to cytosol has not been understood, dishevelled (Dvl) protein was considered as the intersection of Wnt signal traffic. In this study, we characterized the function of three domains (DIX, PDZ and DEP) of Dvl-1 in canonical Wnt signal transduction and Dvl-1 membrane translocation. It was found both DIX and DEP domain were sufficient to block Wnt-3a-induced LEF-1 transcriptional activity and free cytosol beta-catenin accumulation; whereas PDZ domain and a functional mutant form of DEP domain (DEP-KM) had no effect on canonical Wnt signaling. In addition, when cotransfected with Fz-7, DEP domain, but not DIX, PDZ or DEP-KM, translocated and co-localized with Fz-7 to the plasma membrane, which was similar to Dvl-1. Furthermore, it was DEP domain that could block Fz-7-induced membrane translocation of Dvl-1 via a possible competitive mechanism. These results strongly suggest that DEP domain is responsible for the membrane translocation of Dvl-1 protein upon Wnt signal stimulation.

Wnt and beta-catenin signaling target the expression of ecto-5'-nucleotidase and increase extracellular adenosine generation

Solid tumors, which routinely experience necrosis and ischemia, release and degrade adenine nucleotides. This process may lead, depending on the expression of enzymes that regulate adenosine, to the generation of extracellular adenosine. Since genes encoding ecto-5'-nucleotidase (eN) and adenosine deaminase (ADA) contain TCF/LEF consensus binding sites, we asked whether Wnt/beta-catenin signaling, a pathway that is deregulated in several human tumors, targets the expression of these genes and thus influence extracellular adenosine generation. Our results show that beta-catenin strongly increased the activity of the 969-bp promoter of eN and this increase depended on the presence of TCF-1 transcription factor. Reciprocally, the eN promoter activity was decreased by co-transfection of APC, a beta-catenin antagonist. The expression of endogenous eN mRNA was increased either in Cos-7 cells transfected with a mutated beta-catenin and TCF-1 or in Rat-1 cells transformed by the Wnt-1 oncogene. In Rat-1 cells, expression of Wnt-1 correlated with increased eN protein levels and enzymatic activity and a concomitant decrease of adenosine deaminase mRNA and enzymatic activity. This expression profile is accompanied by a threefold increase in the generation of extracellular adenosine. Our study demonstrates a link between the Wnt signaling and the regulation of two enzymes that control the metabolism of adenosine.

Signalling activity of beta-catenin targeted to different subcellular compartments

Beta-catenin plays a dual role as an adhesion molecule in adherens junctions at the plasma membrane and as a key intermediate in the canonical Wnt signalling pathway. The cytosolic soluble pool of beta-catenin, involved in the transmission of the Wnt signal, is normally subjected to rapid protein degradation. On activation of the Wnt cascade, beta-catenin becomes stabilized and then translocates into the nucleus where it co-activates transcription factors of the TCF (T-cell factor)/LEF (lymphoid enhancer factor) family. The expression of plasma membrane-targeted forms of beta-catenin has been shown to also activate TCF/LEF-dependent transcription and different mechanisms have been put forward. In the present study, we have undertaken a systematic analysis of the signalling capability of non-degradable forms of beta-catenin targeted to different cellular compartments. beta-Catenin targeted to the plasma membrane activated transcription to a greater extent compared with non-targeted beta-catenin, and led to a marked stabilization of cytosolic soluble beta-catenin. These effects were independent of the competition with endogenous beta-catenin for binding to E-cadherin at the plasma membrane, since targeting non-degradable beta-catenin to other cellular compartments, i.e. the outer mitochondrial membrane and the endoplasmic reticulum membrane, also resulted in the accumulation of cytosolic wild-type beta-catenin and activation of beta-catenin-dependent signalling. In contrast, nuclear-targeted beta-catenin was without significant effect on cytosolic wild-type beta-catenin and did not activate transcription. Our results suggest that cytosolic accumulation of beta-catenin is a prerequisite for the activation of TCF/LEF-dependent transcription in the nucleus.

Reduced expression of FOXC2 and brown adipogenic genes in human subjects with insulin resistance

OBJECTIVE

We investigated subcutaneous adipose tissue expression of FOXC2 and selected genes involved in brown adipogenesis in adult human subjects in whom we have previously identified a reduced potential of precursor cell commitment to adipose-lineage differentiation in relation to insulin resistance.

RESEARCH METHODS AND PROCEDURE

Gene expression was studied using quantitative real time polymerase chain reaction. The relation between the expression of brown adipogenic genes and the genes involved in progenitor cell commitment, adipose cell size, and insulin sensitivity in vivo was analyzed.

RESULTS

The expression of FOXC2, MASK, MAP3K5, retinoblastoma protein (pRb), peroxisome proliferator-activated protein gamma (PPARgamma), and retinoid X receptor gamma (RXRgamma) was decreased in the insulin-resistant compared with insulin-sensitive subjects, whereas PPARgamma-2 and CCAAT/enhancer binding protein alpha (C/EBPalpha) showed no differential expression. The FOXC2 expression correlated with that of Notch and Wnt signaling genes, as well as of the genes studied participating in brown adipogenesis, including MASK, MAP3K5, PPARgamma, pRb, RXRgamma, and PGC-1. A second-level correlation between PPARgamma and UCP-1 was also significant. In addition, the expression of MASK, MAP3K5, pRb, RXRgamma, and PGC-1 inversely correlated with adipose cell mass and also correlated with the glucose disposal rate in vivo.

DISCUSSION

Our results suggest that a reduced brown adipose phenotype is associated with insulin resistance and that a basal brown adipose phenotype may be important for maintaining normal insulin sensitivity.

Direct binding of Lef1 to sites in the boz promoter may mediate pre-midblastula-transition activation of boz expression

The Nieuwkoop center provides signals essential for the establishment of the dorsal gastrula organizer in vertebrates. Activation of beta-catenin is one of the events in the Nieuwkoop center that lead to activation of dorsal-specific genes during blastula and early gastrula stages. Zebrafish bozozok (boz) mutant embryos have severe defects in axial mesoderm and anterior neuroectoderm. The boz gene is activated in the organizer in response to beta-catenin signaling, and Boz protein has been demonstrated to contribute to organizer formation by repression of ventralizing genes, including bmp2b, vega1, and vega2. Here, we investigate the timing and molecular mechanism by which boz expression is activated in the organizer. We demonstrate that boz is already expressed before midblastula transition (MBT). We further identify high-affinity binding sites for Tcf/Lef1 within the boz promoter region. These sites, together with the finding that beta-catenin induces boz expression, indicate that transcription of boz may be activated directly by beta-catenin/Lef1. We hypothesize that pre-MBT activation of boz may be important to build up a sufficiently strong antagonizing activity against zygotic ventralizing genes activated immediately post-MBT. Thus, the early onset of boz expression may be crucial for organizer establishment in the presence of ubiquitous maternal activators of ventralizing genes.

Failure of a medulloblastoma-derived mutant of SUFU to suppress WNT signaling

Germline mutations of APC in patients with Turcot syndrome (colon cancer and medulloblastoma), was well as somatic mutations of APC, beta-catenin, and Axin in sporadic medulloblastomas (MBs) have shown the importance of WNT signaling in the pathogenesis of MB. A subset of children with MB have germline mutations of SUFU, a known inhibitor of Hedgehog signal transduction. A recent report suggested that murine Sufu can bind beta-catenin, export it from the nucleus, and thereby repress beta-catenin/T-cell factor (Tcf)-mediated transcription. We show that an MB-derived mutant of SUFU has lost the ability to decrease nuclear levels of beta-catenin, and cannot inhibit beta-catenin/Tcf-mediated transcription as compared to wild type SUFU. Our results suggest that loss of function of SUFU results in overactivity of both the Sonic Hedgehog, and the WNT signaling pathways, leading to excessive proliferation and failure to differentiate resulting in MB.