Wnt signalling: variety at the core

Alternative splicing of Tcf7l2 transcripts generates protein variants with differential promoter-binding and transcriptional activation properties at Wnt/beta-catenin targets

Alternative splicing can produce multiple protein products with variable domain composition from a single gene. The mouse Tcf7l2 gene is subject to alternative splicing. It encodes TCF4, a member of the T-cell factor (TCF) family of DNA-binding proteins and a nuclear interaction partner of β-catenin which performs essential functions in Wnt growth factor signalling. Multiple TCF4 isoforms, potentially exhibiting cell-type-specific distribution and differing in gene regulatory properties, could strongly influence tissue-specific Wnt responses. Therefore, we have examined mouse Tcf7l2 splice variants in neonatal tissues, embryonic stem cells and neural progenitors. By polymerase chain reaction amplification, cloning and sequencing, we identify a large number of alternatively spliced transcripts and report a highly flexible combinatorial repertoire of alternative exons. Many, but not all of the variants exhibit a broad tissue distribution. Moreover, two functionally equivalent versions of the C-clamp, thought to represent an auxiliary DNA-binding domain, were identified. Depending upon promoter context and precise domain composition, TCF4 isoforms exhibit strikingly different transactivation potentials at natural Wnt/β-catenin target promoters. However, differences in C-clamp-mediated DNA binding can only partially explain functional differences among TCF4 variants. Still, the cell-type-specific complement of TCF4 isoforms is likely to be a major determinant for the context-dependent transcriptional output of Wnt/β-catenin signalling.

Omega-3 polyunsaturated fatty acids inhibit hepatocellular carcinoma cell growth through blocking beta-catenin and cyclooxygenase-2

Hepatocellular carcinoma (HCC) is a common human cancer with high mortality, and currently, there is no effective chemoprevention or systematic treatment. Recent evidence suggests that cyclooxygenase-2 (COX-2)-derived PGE(2) and Wnt/beta-catenin signaling pathways are implicated in hepatocarcinogenesis. Here, we report that omega-3 polyunsaturated fatty acids (PUFA), docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA) inhibit HCC growth through simultaneously inhibition of COX-2 and beta-catenin. DHA and EPA treatment resulted in a dose-dependent reduction of cell viability with cleavage of poly ADP ribose polymerase, caspase-3, and caspase-9 in three human HCC cell lines (Hep3B, Huh-7, HepG2). In contrast, AA, a omega-6 PUFA, exhibited no significant effect. DHA and EPA treatment caused dephosphorylation and thus activation of GSK-3beta, leading to beta-catenin degradation in Hep3B cells. The GSK-3beta inhibitor, LiCl, partially prevented DHA-induced beta-catenin protein degradation and apoptosis. Additionally, DHA induced the formation of beta-catenin/Axin/GSK-3beta binding complex, which serves as a parallel mechanism for beta-catenin degradation. Furthermore, DHA inhibited PGE(2) signaling through downregulation of COX-2 and upregulation of the COX-2 antagonist, 15-hydroxyprostaglandin dehydrogenase. Finally, the growth of HCC in vivo was significantly reduced when mouse HCCs (Hepa1-6) were inoculated into the Fat-1 transgenic mice, which express a Caenorhabditis elegans desaturase converting omega-6 to omega-3 PUFAs endogenously. These findings provide important preclinical evidence and molecular insight for utilization of omega-3 PUFAs for the chemoprevention and treatment of human HCC.

Omega-3 polyunsaturated fatty acids inhibit hepatocellular carcinoma cell growth through blocking beta-catenin and cyclooxygenase-2

Hepatocellular carcinoma (HCC) is a common human cancer with high mortality, and currently, there is no effective chemoprevention or systematic treatment. Recent evidence suggests that cyclooxygenase-2 (COX-2)-derived PGE(2) and Wnt/beta-catenin signaling pathways are implicated in hepatocarcinogenesis. Here, we report that omega-3 polyunsaturated fatty acids (PUFA), docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA) inhibit HCC growth through simultaneously inhibition of COX-2 and beta-catenin. DHA and EPA treatment resulted in a dose-dependent reduction of cell viability with cleavage of poly ADP ribose polymerase, caspase-3, and caspase-9 in three human HCC cell lines (Hep3B, Huh-7, HepG2). In contrast, AA, a omega-6 PUFA, exhibited no significant effect. DHA and EPA treatment caused dephosphorylation and thus activation of GSK-3beta, leading to beta-catenin degradation in Hep3B cells. The GSK-3beta inhibitor, LiCl, partially prevented DHA-induced beta-catenin protein degradation and apoptosis. Additionally, DHA induced the formation of beta-catenin/Axin/GSK-3beta binding complex, which serves as a parallel mechanism for beta-catenin degradation. Furthermore, DHA inhibited PGE(2) signaling through downregulation of COX-2 and upregulation of the COX-2 antagonist, 15-hydroxyprostaglandin dehydrogenase. Finally, the growth of HCC in vivo was significantly reduced when mouse HCCs (Hepa1-6) were inoculated into the Fat-1 transgenic mice, which express a Caenorhabditis elegans desaturase converting omega-6 to omega-3 PUFAs endogenously. These findings provide important preclinical evidence and molecular insight for utilization of omega-3 PUFAs for the chemoprevention and treatment of human HCC.

Cloning of the human GLI2 Promoter: transcriptional activation by transforming growth factor-beta via SMAD3/beta-catenin cooperation

GLI2 (GLI-Kruppel family member 2), a zinc finger transcription factor that mediates Hedgehog signaling, is implicated in the progression of an ever-growing number of human malignancies, including prostate and pancreatic cancer, as well as basal cell carcinoma of the skin. Its expression is up-regulated by transforming growth factor-beta (TGF-beta) in a variety of cell types, both normal and transformed. We report herein that TGF-beta-driven GLI2 expression is transcriptional and does not result from stabilization of GLI2 transcripts. We describe the characterization of the 5'-flanking sequence of human GLI2 mRNA, the identification of a transcription start site, the cloning of approximately 1,600 bp of the regulatory promoter region and the identification and functional analysis of a TGF-beta-responsive region mapped to a 91-bp sequence between nucleotides -119 and -29 of the promoter. This region harbors SMAD and lymphoid enhancer factor/T cell factor binding sites that allow functional cooperation between SMAD3 and beta-catenin, recruited to the promoter in response to TGF-beta to drive GLI2 gene transcription.

Runx2 and bone morphogenic protein 2 regulate the expression of an alternative Lef1 transcript during osteoblast maturation

Lymphoid Enhancer Binding Factor (Lef) 1 is a transcriptional effector of the Wnt/Lrp5/beta-catenin signaling cascade, which regulates osteoblast differentiation, bone density, and skeletal strength. In this study, we describe the expression and function of an alternative Lef1 isoform in osseous cells. Lef1DeltaN is a naturally occurring isoform driven by a promoter (p2) within the intron between exons 3 and 4 of Lef1. Lef1DeltaN is induced during late osteoblast differentiation. This is converse to the expression pattern of the full-length Lef1 protein, which as we previously showed, decreases during differentiation. Agonists of osteoblast maturation differentially affected Lef1DeltaN expression. BMP2 stimulated Lef1DeltaN expression, whereas Wnt3a blocked basal and BMP2-induced expression of Lef1DeltaN transcripts during osteoblast differentiation. We determined that the Lef1DeltaN p2 promoter is active in osteoblasts and Runx2 regulates its activity. Stable overexpression of Lef1DeltaN in differentiating osteoblasts induced the expression of osteoblast differentiation genes, osteocalcin and type 1 collagen. Taken together, our results suggest Lef1DeltaN is a crucial regulator of terminal differentiation in osseous cells.

Wnt/beta-catenin signaling: components, mechanisms, and diseases

Signaling by the Wnt family of secreted glycolipoproteins via the transcriptional coactivator beta-catenin controls embryonic development and adult homeostasis. Here we review recent progress in this so-called canonical Wnt signaling pathway. We discuss Wnt ligands, agonists, and antagonists, and their interactions with Wnt receptors. We also dissect critical events that regulate beta-catenin stability, from Wnt receptors to the cytoplasmic beta-catenin destruction complex, and nuclear machinery that mediates beta-catenin-dependent transcription. Finally, we highlight some key aspects of Wnt/beta-catenin signaling in human diseases including congenital malformations, cancer, and osteoporosis, and discuss potential therapeutic implications.

Wnt/beta-catenin signaling: components, mechanisms, and diseases

Signaling by the Wnt family of secreted glycolipoproteins via the transcriptional coactivator beta-catenin controls embryonic development and adult homeostasis. Here we review recent progress in this so-called canonical Wnt signaling pathway. We discuss Wnt ligands, agonists, and antagonists, and their interactions with Wnt receptors. We also dissect critical events that regulate beta-catenin stability, from Wnt receptors to the cytoplasmic beta-catenin destruction complex, and nuclear machinery that mediates beta-catenin-dependent transcription. Finally, we highlight some key aspects of Wnt/beta-catenin signaling in human diseases including congenital malformations, cancer, and osteoporosis, and discuss potential therapeutic implications.

Wnt/beta-catenin signaling: components, mechanisms, and diseases

Signaling by the Wnt family of secreted glycolipoproteins via the transcriptional coactivator beta-catenin controls embryonic development and adult homeostasis. Here we review recent progress in this so-called canonical Wnt signaling pathway. We discuss Wnt ligands, agonists, and antagonists, and their interactions with Wnt receptors. We also dissect critical events that regulate beta-catenin stability, from Wnt receptors to the cytoplasmic beta-catenin destruction complex, and nuclear machinery that mediates beta-catenin-dependent transcription. Finally, we highlight some key aspects of Wnt/beta-catenin signaling in human diseases including congenital malformations, cancer, and osteoporosis, and discuss potential therapeutic implications.

IGF-I activates the mouse type IIb myosin heavy chain gene

IGF-I increases skeletal muscle mass, but whether IGF-I increases type IIb myosin heavy chain (MyHC) transcriptional activity is not known. C2C12 myotubes were cultured with or without IGF-I to determine whether IGF-I increases type IIb MyHC promoter activity, and if so, what region of the promoter might IGF-I signaling regulate. At differentiation days 3 and 4, IGF-I increased type IIb MyHC mRNA and mouse 3.0-kb type IIb MyHC promoter activity. Deletion construct studies identified a potential IGF-I-responsive region between 1.25 and 1.2 kb of the type IIb MyHC promoter, which contained an exact 6-bp T-cell factor/lymphoid enhancer factor (Tcf/Lef) binding site at position -1206 to -1201. Site-specific mutation of the putative Tcf/Lef binding site reduced IGF-I-induced 1.3-kb type IIb MyHC promoter activity. To identify potential IGF-I signaling molecules, the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY-294002 were both found to markedly attenuate IGF-I activation of the 1.3-kb type IIb MyHC promoter. Downstream signaling of IGF-I can phosphorylate and inactivate GSK-3beta, thereby enhancing beta-catenin protein. The GSK-3beta inhibitor, LiCl, dramatically enhanced IGF-I induction of the 1.3-kb type IIb MyHC promoter, and constitutively active GSK-3beta attenuated IGF-I-induced 1.3-kb type IIb MyHC promoter activity. Finally, IGF-I increased nuclear beta-catenin protein, and small interfering RNA knockdown of beta-catenin attenuated IGF-I-induced 1.3-kb type IIb MyHC promoter activity and type IIb MyHC mRNA. In summary, IGF-I stimulation of C2C12 myotubes increases mouse type IIb MyHC promoter activity, likely through signaling of PI3K, GSK-3beta, beta-catenin, and a Tcf/Lef binding site at -1,206 to -1,201 bp in the promoter.

Expression and molecular diversity of Tcf7l2 in the developing murine cerebellum and brain

The Wingless family of secreted proteins impinges on multiple aspects of vertebrate nervous system development, from early global patterning and cell fate decision to synaptogenesis. Here, we mapped the developmental expression of the Tcf7l2, which is key to the canonical Wingless signaling cascade, in the developing cerebellum. The exclusive and transient expression of Tcf7l2 in ventricular and Olig2-defined precursor cells within the cerebellar anlage, and its predominant expression in postmitotic neurons in the midbrain/inferior colliculus allowed us to ask whether cell type-specific differences are also reflected in splice isoform variability. We also included in this analysis intestinal epithelia, where Tcf7l2 function has been intensively studied. Our data reveal extensive variability of Tcf7l2 splicing in the central nervous system. Additional variability in brain-expressed Tcf7l2 is generated by a length polymorphism of expressed mRNAs in a stretch of normally nine adenines found at the beginning of exon 18, reminiscent of variability observed at the same site in cancers with microsatellite instability. A consensus emerging from our data is that the expression of isoforms comprising or lacking the C-clamp motif, which has been linked by in vitro studies to the regulation of cell growth, is indeed tightly correlated with the proliferative status in vivo.

Genomic response to Wnt signalling is highly context-dependent--evidence from DNA microarray and chromatin immunoprecipitation screens of Wnt/TCF targets

Wnt proteins are important regulators of embryonic development, and dysregulated Wnt signalling is involved in the oncogenesis of several human cancers. Our knowledge of the downstream target genes is limited, however. We used a chromatin immunoprecipitation-based assay to isolate and characterize the actual gene segments through which Wnt-activatable transcription factors, TCFs, regulate transcription and an Affymetrix microarray analysis to study the global transcriptional response to the Wnt3a ligand. The anti-beta-catenin immunoprecipitation of DNA-protein complexes from mouse NIH3T3 fibroblasts expressing a fusion protein of beta-catenin and TCF7 resulted in the identification of 92 genes as putative TCF targets. GeneChip assays of gene expression performed on NIH3T3 cells and the rat pheochromocytoma cell line PC12 revealed 355 genes in NIH3T3 and 129 genes in the PC12 cells with marked changes in expression after Wnt3a stimulus. Only 2 Wnt-regulated genes were shared by both cell lines. Surprisingly, Disabled-2 was the only gene identified by the chromatin immunoprecipitation approach that displayed a marked change in expression in the GeneChip assay. Taken together, our approaches give an insight into the complex context-dependent nature of Wnt pathway transcriptional responses and identify Disabled-2 as a potential new direct target for Wnt signalling.

Oxygen: a master regulator of pancreatic development?

Beyond its role as an electron acceptor in aerobic respiration, oxygen is also a key effector of many developmental events. The oxygen-sensing machinery and the very fabric of cell identity and function have been shown to be deeply intertwined. Here we take a first look at how oxygen might lie at the crossroads of at least two of the major molecular pathways that shape pancreatic development. Based on recent evidence and a thorough review of the literature, we present a theoretical model whereby evolving oxygen tensions might choreograph to a large extent the sequence of molecular events resulting in the development of the organ. In particular, we propose that lower oxygenation prior to the expansion of the vasculature may favour HIF (hypoxia inducible factor)-mediated activation of Notch and repression of Wnt/beta-catenin signalling, limiting endocrine cell differentiation. With the development of vasculature and improved oxygen delivery to the developing organ, HIF-mediated support for Notch signalling may decline while the beta-catenin-directed Wnt signalling is favoured, which would support endocrine cell differentiation and perhaps exocrine cell proliferation/differentiation.

Mouse cofactor of BRCA1 (Cobra1) is required for early embryogenesis

Background

Negative elongation factor (NELF) is a four-subunit protein complex conserved from Drosophila to humans. In vitro biochemical and tissue culture-based studies have demonstrated an important role of NELF in controlling RNA polymerase II (Pol II) pausing in transcription. However, the physiological significance of NELF function is not clear due to the lack of any genetic systems for studying NELF.

Principal Findings

Here we show that disruption of the mouse B subunit of NELF (NELF-B), also known as cofactor of BRCA1 (Cobra1), causes inner cell mass (ICM) deficiency and embryonic lethality at the time of implantation. Consistent with the phenotype of the Cobra1 knockout (KO) embryos, knockdown of Cobra1 in mouse embryonic stem cells (ESCs) reduces the efficiency of colony formation and increases spontaneous differentiation. Cobra1-depleted ESCs maintain normal levels of Oct4, Nanog, and Sox2, master regulators of pluripotency in ESCs. However, knockdown of Cobra1 leads to precocious expression of developmental regulators including lymphoid enhancer-binding factor 1 (Lef1). Chromatin immunoprecipitation (ChIP) indicates that Cobra1 binds to the Lef1 promoter and modulates the abundance of promoter-bound RNA polymerase.

Conclusions

Cobra1 is essential for early embryogenesis. Our findings also indicate that Cobra1 helps maintain the undifferentiated state of mESCs by preventing unscheduled expression of developmental genes.

Out, in and back again: PtdIns(4,5)P(2) regulates cadherin trafficking in epithelial morphogenesis

The morphogenesis of epithelial cells in the tissue microenvironment depends on the regulation of the forces and structures that keep cells in contact with their neighbours. The formation of cell-cell contacts is integral to the establishment and maintenance of epithelial morphogenesis. In epithelial tissues, the misregulation of the signalling pathways that control epithelial polarization induces migratory and invasive cellular phenotypes. Many cellular processes influence cadherin targeting and function, including exocytosis, endocytosis and recycling. However, the localized generation of the lipid messenger PtdIns(4,5)P(2) is emerging as a fundamental signal controlling all of these processes. The PtdIns(4,5)P(2)-generating enzymes, PIPKs (phosphatidylinositol phosphate kinases) are therefore integral to these pathways. By the spatial and temporal targeting of PIPKs via the actions of its functional protein associates, PtdIns(4,5)P(2) is generated at discrete cellular locales to provide the cadherin-trafficking machinery with its required lipid messenger. In the present review, we discuss the involvement of PtdIns(4,5)P(2) and the PIPKs in the regulation of the E-cadherin (epithelial cadherin) exocytic and endocytic machinery, the modulation of actin structures at sites of adhesion, and the direction of cellular pathways which determine the fate of E-cadherin and cell-cell junctions. Recent work is also described that has defined phosphoinositide-mediated E-cadherin regulatory pathways by the use of organismal models.

Endostatin induces autophagy in endothelial cells by modulating Beclin 1 and beta-catenin levels

Endostatin is a well-characterized endogenous inhibitor of angiogenesis that affects cell proliferation and migration by inhibiting integrin and Wnt-mediated signalling pathways. Here, we show that endothelial cells treated with native and P125A-endostatin activate autophagy. Because autophagy can either be protective or induce programmed cell death, experiments were carried out to understand the signalling pathways leading to autophagy in endothelial cells. P125A-endostatin treatment increased the levels of Beclin 1, a crucial molecule in vesicle nucleation and autophagy. The treatment also reduced the levels of Bcl-2, Bcl-x_L and β-catenin; however, progressively increasing amounts of Bcl-2 and Bcl-x_L were found to be complexed with Beclin 1. Increased β-catenin and Wnt-mediated signalling reduced Beclin 1 levels and rescued endothelial cells from endostatin-induced autophagy. Finally, knocking down Beclin 1 levels by RNA interference decreased autophagy and accelerated caspase activation in endostatin-treated cells. These studies suggest that endothelial cells may initiate autophagy as a survival response to limit the effects of angiogenesis inhibitors. Thus, interfering with autophagy can potentiate the effects of endostatin by promoting a switch to apoptosis.

Molecular development of the lateral geniculate nucleus in the absence of retinal waves during the time of retinal axon eye-specific segregation

When retinal waves are inhibited binocularly, eye-specific segregation of retinal axons is disrupted, and retinal axons from the two eyes remain intermingled in the lateral geniculate nucleus (LGN). This effect of binocular retinal wave inhibition is mediated by the lack of activity-dependent competition between retinal axons from the two eyes, but it is unknown whether this effect is also mediated by the developmental arrest of the LGN in an immature state. Here we find developmental markers of the LGN during eye-specific segregation. The expression levels of Purkinje cell protein 4 (PCP4/PEP19), transcription factor 7-like 2 (TCF7L2/TCF4) and LIM homeobox protein 9 (Lhx9) in the LGN change significantly during eye-specific segregation. Using PCP4, TCF7L2 and Lhx9 as developmental markers of the LGN, we examine whether LGN development is affected by binocular disruption of retinal waves during eye-specific segregation. Binocular injection of epibatidine strongly inhibits eye-specific segregation, whereas it does not affect the expression of PCP4, TCF7L2 and Lhx9. Furthermore, the expression of PCP4, TCF7L2 and Lhx9 is normal in binocularly enucleated animals and in mice treated with the monoamine oxidase A (MAOA) inhibitor, clorgyline. In addition, our experiments using LGN slice cultures show that the expression of PCP4 and TCF7L2 in LGN slices changes as in vivo. Our results suggest that LGN development proceeds, at least in part, even in the absence of retinal inputs. PCP4, TCF7L2 and Lhx9 should be useful to examine LGN development during eye-specific segregation in mice and in ferrets.

Transgenic reporter tools tracing endogenous canonical Wnt signaling in Xenopus

Activation of the canonical Wnt pathway leads to the transcriptional activation of a particular subset of downstream Wnt target genes. To track this localized cellular output in a living organism, reporter constructs can be designed containing multimerized consensus lymphoid enhancer binding factor (LEF)-1/T cell factor (TCF) transcription factor binding sites, generally referred to as TCF optimal promoter (TOP) sites. In Xenopus, several Wnt-responsive reporter systems have been designed containing a number of these TOP sites that, in combination with a minimal promoter, drive the expression of a reporter gene. Following transgenic integration in Xenopus embryos, a Wnt reporter tool reveals the spatiotemporal delineation of endogenous Wnt pathway activities throughout development. Assumed to be a general readout of the Wnt pathway, such reporters can assist in elucidating unknown functional implications in developing Xenopus embryos.

Inducible gene expression in transient transgenic Xenopus embryos

Xenopus laevis has for many years been successfully used to study Wnt signaling during early development. However, because loss of function and gain of function experiments generally involve injecting RNA, DNA, or morpholinos into early embryos (1- to 32-cell), major phenotypes are often observed before the embryo has reached later stages of development. The combined use of transgenics and a heat shock inducible system has overcome these problems and enables investigations of Wnt signaling at later stages of Xenopus embryonic development, including organogenesis.

Myocilin is a modulator of Wnt signaling

It is well documented that mutations in the MYOCILIN gene may lead to juvenile- and adult-onset primary open-angle glaucoma. However, the functions of wild-type myocilin are still not well understood. To study the functions of human myocilin and its two proteolytic fragments, these proteins were expressed in HEK293 cells. Conditioned medium from myocilin-expressing cells, as well as purified myocilin, induced the formation of stress fibers in primary cultures of human trabecular meshwork or NIH 3T3 cells. Stress fiber-inducing activity of myocilin was blocked by antibodies against myocilin, as well as secreted inhibitors of Wnt signaling, secreted Frizzled-related protein 1 (sFRP1) or sFRP3, and beta-catenin small interfering RNA. Interaction of myocilin with sFRP1, sFRP3, and several Frizzled receptors was confirmed by immunoprecipitation experiments and by binding of myocilin to the surface of cells expressing cysteine-rich domains of different Frizzled and sFRPs. Treatment of NIH 3T3 cells with myocilin and its fragments induced intracellular redistribution of beta-catenin and its accumulation on the cellular membrane but did not induce nuclear accumulation of beta-catenin. Overexpression of myocilin in the eye angle tissues of transgenic mice stimulated accumulation of beta-catenin in these tissues. Myocilin and Wnt proteins may perform redundant functions in the mammalian eye, since myocilin modulates Wnt signaling by interacting with components of this signaling pathway.

Helicobacter pylori virulence and the diversity of gastric cancer in Asia

Infection with cagPAI positive strains of Helicobacter pylori is recognized as being associated with an increased risk of gastric cancer. This article reviews the current knowledge on the structures and pathological functions of cagPAI and the CagA protein, focusing especially on the molecular mechanism through which CagA may be involved in gastric carcinogenesis. The possible link between the geographical distribution of cagPAI and cagA variations and gastric cancer diversity in Asia is also discussed.

Transcriptional regulation of SM22alpha by Wnt3a: convergence with TGFbeta(1)/Smad signaling at a novel regulatory element

The role of canonical Wnt signaling in myofibroblast biology has not been fully investigated. The C3H10T1/2 mesenchymal cell line recapitulates myofibroblast differentiation in vitro and in vivo, including SM22alpha expression. Using this model, we find that Wnt3a upregulates SM22alpha in concert with TGFbeta(1). Wnt1, Wnt5a and BMP2 could not replace Wnt3a and TGFbeta(1) signals. Chromatin immunoprecipitation identified that Wnt3a enhances both genomic SM22alpha histone H3 acetylation and beta-catenin association, hallmarks of transcriptional activation. By analyzing a series of SM22alpha promoter-luciferase (LUC) reporter constructs, we mapped Wnt3a-regulated DNA transcriptional activation to nucleotides -213 to -192 relative to the transcription initiation site. In gel shift assays, DNA-protein complexes assembled on this element were disrupted with antibodies to beta-catenin, Smad2/3, and TCF7, confirming the participation of known Wnt3a and TGFbeta transcriptional mediators. Mutation of a CAGAG motif within this region abrogated recognition by these DNA binding proteins. Wnt3a treatment increased Smad2/3 binding to this element. Mutation of the cognate within the context of the native 0.44 kb SM22alpha promoter resulted in a 70% decrease in transcription, and reduced Wnt3a+TGFbeta(1) induction. A concatamer of SM22alpha [-213 to -192] conveyed Wnt3a+TGFbeta(1) activation to the unresponsive RSV promoter. Dominant negative TCF inhibited SM22alpha [-213 to -192] x 6 RSVLUC activation. Moreover, ICAT (inhibitor of beta-catenin and TCF) decreased while TCF7L2 and beta-catenin enhanced 0.44 kb SM22alpha promoter induction by Wnt3a+TGFbeta(1). RNAi "knockdown" of beta-catenin inhibited Wnt3a induction of SM22alpha. Thus, Wnt/beta-catenin signaling interacts with TGFbeta/Smad pathways to control SM22alpha gene transcription.

Periodic activation of Wnt/beta-catenin signaling enhances somatic cell reprogramming mediated by cell fusion

Reprogramming of nuclei allows the dedifferentiation of differentiated cells. Somatic cells can undergo epigenetic modifications and reprogramming through their fusion with embryonic stem cells (ESCs) or after overexpression of a specific blend of ESC transcription factor-encoding genes. We show here that cyclic activation of Wnt/beta-catenin signaling in ESCs with Wnt3a or the glycogen synthase kinase-3 (GSK-3) inhibitor 6-bromoindirubin-3'-oxime (BIO) strikingly enhances the ability of ESCs to reprogram somatic cells after fusion. In addition, we show that reprogramming is triggered by a dose-dependent accumulation of active beta-catenin. Reprogrammed clones express ESC-specific genes, lose somatic differentiation markers, become demethylated on Oct4 and Nanog CpG islands, and can differentiate into cardiomyocytes in vitro and generate teratomas in vivo. Our data thus demonstrate that in ESCs, periodic beta-catenin accumulation via the Wnt/beta-catenin pathway provides a specific threshold that leads to the reprogramming of somatic cells after fusion.

Regulation of Wnt/beta-catenin pathway by cPLA2alpha and PPARdelta

Cytosolic phospholipase A(2)alpha (cPLA(2)alpha) is a rate-limiting key enzyme that releases arachidonic acid (AA) from membrane phospholipid for the production of biologically active lipid mediators including prostaglandins, leukotrienes and platelet-activating factor. cPLA(2)alpha is translocated to nuclear envelope in response to intracellular calcium increase and the enzyme is also present inside the cell nucleus; however, the biological function of cPLA(2)alpha in the nucleus remains unknown. Here we show a novel role of cPLA(2)alpha for activation of peroxisome proliferator-activated receptor-delta (PPARdelta) and beta-catenin in the nuclei. Overexpression of cPLA(2)alpha in human cholangiocarcinoma cells induced the binding of PPARdelta to beta-catenin and increased their association with the TCF/LEF response element. These effects are inhibited by the cPLA(2)alpha siRNA and inhibitors as well as by siRNA knockdown of PPARdelta. Overexpression of PPARdelta or treatment with the selective PPARdelta ligand, GW501516, also increased beta-catenin binding to TCF/LEF response element and increased its reporter activity. Addition of AA and GW501516 to nuclear extracts induced a comparable degree of beta-catenin binding to TCF/LEF response element. Furthermore, cPLA(2)alpha protein is present in the PPARdelta and beta-catenin binding complex. Thus the close proximity between cPLA(2)alpha and PPARdelta provides a unique advantage for their efficient functional coupling in the nucleus, where AA produced by cPLA(2)alpha becomes immediately available for PPARdelta binding and subsequent beta-catenin activation. These results depict a novel interaction linking cPLA(2)alpha, PPARdelta and Wnt/beta-catenin signaling pathways and provide insight for further understanding the roles of these key molecules in human cells and diseases.

The assembly and maintenance of epithelial junctions in C. elegans

The epithelial tissues of the C. elegans embryo provide a "minimalist" system for examining phylogenetically conserved proteins that function in epithelial polarity and cell-cell adhesion in a multicellular organism. In this review, we provide an overview of three major molecular complexes at the apical surface of epithelial cells in the C. elegans embryo: the cadherin-catenin complex, the more basal DLG-1/AJM-1 complex, and the apical membrane domain, which shares similarities with the subapical complex in Drosophila and the PAR/aPKC complex in vertebrates. We discuss how the assembly of these complexes contributes to epithelial polarity and adhesion, proteins that act as effectors and/or regulators of each subdomain, and how these complexes functionally interact during embryonic morphogenesis. Although much remains to be clarified, significant progress has been made in recent years to clarify the role of these protein complexes in epithelial morphogenesis, and suggests that C. elegans will continue to be a fruitful system in which to elucidate functional roles for these proteins in a living embryo.

Genetic interaction between the homeobox transcription factors HESX1 and SIX3 is required for normal pituitary development

Hesx1 has been shown to be essential for normal pituitary development. The homeobox gene Six3 is expressed in the developing pituitary gland during mouse development but its function in this tissue has been precluded by the fact that in the Six3-deficient embryos the pituitary gland is not induced. To gain insights into the function of Six3 during pituitary development we have generated Six3+/- ;Hesx1Cre/+ double heterozygous mice. Strikingly, these mice show marked dwarfism, which is first detectable around weaning, and die by the 5th-6th week of age. Thyroid and gonad development is also impaired in these animals. Analysis of Six3+/- ;Hesx1Cre/+ compound embryos indicates that hypopituitarism is the likely cause of these defects since pituitary development is severely impaired in these mutants. Similar to the Hesx1-deficient embryos, Rathke's pouch is initially expanded in Six3+/- ;Hesx1Cre/+ compound embryos due to an increase in cell proliferation. Subsequently, the anterior pituitary gland appears bifurcated, dysmorphic and occasionally ectopically misplaced in the nasopharyngeal cavity, but cell differentiation is unaffected. Our research has revealed a role for Six3 in normal pituitary development, which has likely been conserved during evolution as SIX3 is also expressed in the pituitary gland of the human embryo.

Identification of WNT/beta-CATENIN signaling pathway components in human cumulus cells

Signaling via the conserved WNT/beta-CATENIN pathway controls diverse developmental processes. To explore its potential role in the ovary, we investigated the expression of WNTs, frizzled (FZD) receptors and other pathway components in human cumulus cells obtained from oocytes collected for in vitro fertilization. Proteins were detected in cultured cells using immunofluorescence microscopy. Protein-protein interactions were analyzed by means of immunoprecipitation. WNT2, FZD2, FZD3 and FZD9 were identified but WNT1, WNT4 and FZD4 were not detected. WNT2 is co-expressed with FZD2, FZD3 and FZD9. Co-immunoprecipitation using WNT2 antibody demonstrated that WNT2 interacts with both FZD3 and FZD9, but only FZD9 antibody precipitated WNT2. We also identified DVL (disheveled), AXIN, GSK-3beta (glycogen synthase kinase-3beta) and beta-CATENIN. beta-CATENIN is concentrated in the plasma membranes. DVL co-localizes with FZD9 and AXIN in the membranes, but GSK-3beta has little co-localization with AXIN and beta-CATENIN. Interestingly, beta-CATENIN is highly co-localized with FZD9 and AXIN. CDH1 (E-cadherin) was also detected in the plasma membranes and cytoplasm, co-localized with beta-CATENIN, and CDH1 antibody precipitated beta-CATENIN. The results suggest that WNT2 could act through its receptor FZD9 to regulate the beta-CATENIN pathway in cumulus cells, recruiting beta-CATENIN into plasma membranes and promoting the formation of adherens junctions involving CDH1.

Cold-inducible RNA binding protein (CIRP), a novel XTcf-3 specific target gene regulates neural development in Xenopus

BACKGROUND

As nuclear mediators of wnt/beta-catenin signaling, Lef/Tcf transcription factors play important roles in development and disease. Although it is well established, that the four vertebrate Lef/Tcfs have unique functional properties, most studies unite Lef-1, Tcf-1, Tcf-3 and Tcf-4 and reduce their function to uniformly transduce wnt/beta-catenin signaling for activating wnt target genes. In order to discriminate target genes regulated by XTcf-3 from those regulated by XTcf-4 or Lef/Tcfs in general, we performed a subtractive screen, using neuralized Xenopus animal cap explants.

RESULTS

We identified cold-inducible RNA binding protein (CIRP) as novel XTcf-3 specific target gene. Furthermore, we show that knockdown of XTcf-3 by injection of an antisense morpholino oligonucleotide results in a general broadening of the anterior neural tissue. Depletion of XCIRP by antisense morpholino oligonucleotide injection leads to a reduced stability of mRNA and an enlargement of the anterior neural plate similar to the depletion of XTcf-3.

CONCLUSION

Distinct steps in neural development are differentially regulated by individual Lef/Tcfs. For proper development of the anterior brain XTcf-3 and the Tcf-subtype specific target XCIRP appear indispensable. Thus, regulation of anterior neural development, at least in part, depends on mRNA stabilization by the novel XTcf-3 target gene XCIRP.

Deficiency of Mbd2 attenuates Wnt signaling

We have previously shown that deficiency of the methyl binding domain protein Mbd2 dramatically reduces adenoma burden on an Apc(Min/+) background. To investigate the mechanism underlying this phenomenon, we have determined the effect of Mbd2 deficiency upon the phenotypes imposed by the conditional deletion of Apc in the small intestine. Microarray analysis demonstrated a partial suppression of the Wnt pathway in the absence of Mbd2. Mbd2 deficiency also influenced one immediate cellular consequence of Apc loss, with normalization of Paneth cell positioning. From a mechanistic perspective, we show that deficiency of Mbd2 elevates levels of the known Wnt target Lect2, and we confirm here that Mbd2 binds the Lect2 promoter in association with NuRD. Furthermore, we show that Lect2 is capable of functioning as a Wnt pathway repressor. These results therefore provide a mechanistic basis for the epigenetic control of adenoma formation mediated through Mbd2.

Variable DNA methylation patterns associated with progression of disease in hepatocellular carcinomas

Hepatocellular carcinoma (HCC) most commonly arises from chronic inflammation due to viral infection, as a result of genetic and epigenetic abnormalities. A global picture of epigenetic changes in HCC is lacking. We used methylated CpG island amplification microarrays (MCAMs) to study 6458 CpG islands in HCC and adjacent preneoplastic tissues [chronic hepatitis (CH) or liver cirrhosis (LC)] in comparison with normal liver tissues where neither viral infection nor hepatitis has existed. MCAM identified 719 (11%) prominent genes of hypermethylation in HCCs. HCCs arising from LC had significantly more methylation than those arising from CH (1249 genes or 19% versus 444 genes or 7%, P < 0.05). There were four patterns of aberrant methylation: Type I (4%, e.g. matrix metalloproteinase 14) shows a substantially high methylation level in adjacent tissue and does not increase further in cancer. Type II (55%, e.g. RASSF1A) shows progressively increasing methylation from adjacent tissue to HCC. Type III (4%, e.g. GNA14) shows decreased methylation in adjacent tissue but either similar or increased methylation in HCC. Type IV (37%, e.g. CDKN2A) shows low levels of methylation in normal tissue and adjacent tissue but high levels in HCC. These DNA methylation changes were confirmed by quantitative pyrosequencing methylation analysis in representative 24 genes and were analyzed for correlation with clinicopathological parameters in 38 patients. Intriguingly, methylation in the Type IV genes is characteristic of moderately/poorly differentiated cancer. Our global epigenome analysis reveals distinct patterns of methylation that are probably to represent different pathophysiologic processes in HCCs.

Inhibition of N-cadherin and beta-catenin function reduces axon-induced Schwann cell proliferation

N-cadherin and beta-catenin are involved in cell adhesion and cell cycle in tumor cells and neural crest. Both are expressed at key stages of Schwann cell (SC) development, but little is known about their function in the SC lineage. We studied the role of these molecules in adult rat derived SC-embryonic dorsal root ganglion cocultures by using low-Ca(2+) conditions and specific blocking antibodies to interfere with N-cadherin function and by using small interfering RNA (siRNA) to decrease beta-catenin expression in both SC-neuron cocultures and adult rat-derived SC monocultures. N-cadherin blocking conditions decreased SC-axon association and reduced axon-induced SC proliferation. In SC monocultures, beta-catenin reduction diminished the proliferative response of SCs to the mitogen beta1-heregulin, and, in SC-DRG cocultures, beta-catenin reduction inhibited axon-contact-dependent SC proliferation. Stimulation of SC cultures with beta1-heregulin increased total beta-catenin protein amount, phosphorylation of GSK-3beta and beta-catenin presence in nuclear extracts. In conclusion, our findings suggest a previously unrecognized contribution of beta-catenin and N-cadherin to axon-induced SC proliferation.

Characterization of a transient TCF/LEF-responsive progenitor population in the embryonic mouse retina

PURPOSE

High mobility group (HMG) transcription factors of the T-cell-specific transcription factor/lymphoid enhancer binding factor (TCF/LEF) family are a class of intrinsic regulators that are dynamically expressed in the embryonic mouse retina. Activation of TCF/LEFs is a hallmark of the Wnt/beta-catenin pathway; however, the requirement for Wnt/beta-catenin and noncanonical Wnt signaling during mammalian retinal development remains unclear. The goal of the study was to characterize more fully a TCF/LEF-responsive retinal progenitor population in the mouse embryo and to correlate this with Wnt/beta-catenin signaling.

METHODS

TCF/LEF activation was analyzed in the TOPgal (TCF optimal promoter) reporter mouse at embryonic ages and compared to Axin2 mRNA expression, an endogenous readout of Wnt/beta-catenin signaling. Reporter expression was also examined in embryos with a retina-specific deletion of the beta-catenin gene (Ctnnb1), using Six3-Cre transgenic mice. Finally, the extent to which TOPgal cells coexpress cell cycle proteins, basic helix-loop-helix (bHLH) transcription factors, and other retinal cell markers was tested by double immunohistochemistry.

RESULTS

TOPgal reporter activation occurred transiently in a subpopulation of embryonic retinal progenitor cells. Axin2 was not expressed in the central retina, and TOPgal reporter expression persisted in the absence of beta-catenin. Although a proportion of TOPgal-labeled cells were proliferative, most coexpressed the cyclin-dependent kinase inhibitor p27/Kip1.

CONCLUSIONS

TOPgal cells give rise to the four earliest cell types: ganglion, amacrine, horizontal, and photoreceptor. TCF/LEF activation in the central retina does not correlate with Wnt/beta-catenin signaling, pointing to an alternate role for this transcription factor family during retinal development.

Cadherins in development and cancer

Proper embryonic development is guaranteed under conditions of regulated cell-cell and cell-matrix adhesion. The cells of an embryo have to be able to distinguish their neighbours as being alike or different. Cadherins, single-pass transmembrane, Ca(2+)-dependent adhesion molecules that mainly interact in a homophilic manner, are major contributors to cell-cell adhesion. Cadherins play pivotal roles in important morphogenetic and differentiation processes during development, and in maintaining tissue integrity and homeostasis. Changes in cadherin expression throughout development enable differentiation and the formation of various organs. In addition to these functions, cadherins have strong implications in tumourigenesis, since frequently tumour cells show deregulated cadherin expression and inappropriate switching among family members. In this review, I focus on E- and N-cadherin, giving an overview of their structure, cellular function, importance during development, role in cancer, and of the complexity of Ecadherin gene regulation.

Novel human glioma-associated oncogene 1 (GLI1) splice variants reveal distinct mechanisms in the terminal transduction of the hedgehog signal

Hedgehog (HH) signaling is one of the key pathways with major significance for embryogenesis, tumorigenesis, and stem cell maintenance. Glioma-associated oncogene 1 (GLI1) is a transcription factor that acts as the terminal signaling effector but also represents a pathway target gene. Here we report the identification and functional properties of novel GLI1 splice variants generated by skipping exons 2 and 3 and encoding an N-terminal truncated GLI1 protein (GLI1DeltaN). Analysis of the GLI1DeltaN mRNAs in adult human tissues revealed comparable expression levels to the full-length GLI1 (GLI1FL), whereas in tumor cell lines a generally lower and more variable expression pattern was observed. Furthermore, GLI1DeltaN is up-regulated by HH signaling to the same extent as GLI1FL but has a weaker capacity to activate transcription. However, in specific cellular contexts GLI1DeltaN may be more potent than GLI1FL in activating endogenous gene expression. Moreover, the dual-specificity tyrosine phosphorylation-regulated kinase 1 (Dyrk1) potentiates the transcriptional activity of GLI1FL but not GLI1DeltaN. Interestingly, GLI1FL, in contrast to GLI1DeltaN, is localized solely at the nucleus, in line with its increased transcriptional capacity. The negative regulator of the pathway, Suppressor of Fused (SUFU), elicits a cytoplasmic retention of the GLI1 isoforms, which is more pronounced for GLI1FL, as this contains an N-terminal SUFU binding domain. Collectively, our findings reveal that the activation mechanism of the terminal transducer of the pathway, GLI1, is mediated not only by GLI1FL but also by the GLI1DeltaN variant.

The functional consequences of alternative promoter use in mammalian genomes

We are beginning to appreciate the increasing complexity of mammalian gene structure. A phenomenon that adds an important dimension to this complexity is the use of alternative gene promoters that drive widespread cell type, tissue type or developmental gene regulation. Recent annotations of the human genome suggest that almost one half of the protein-coding genes contain alternative promoters, including those of many disease-associated genes. Aberrant use of one promoter over another has been found to be associated with various diseases, including cancer. Here we discuss the functional consequences of use and misuse of alternative promoters in normal and disease genomes and review the molecular mechanisms regulating alternative promoter use in mammalian genomes.

Lrig3 regulates neural crest formation in Xenopus by modulating Fgf and Wnt signaling pathways

Leucine-rich repeats and immunoglobulin-like domains 3 (Lrig3) was identified by microarray analysis among genes that show differential expression during gastrulation in Xenopus laevis. Lrig3 was expressed in the neural plate and neural crest (NC) at neurula stages, and in NC derivatives and other dorsal structures during tailbud stages. A prominent consequence of the morpholino-induced inhibition of Lrig3 expression was impaired NC formation, as revealed by the suppression of marker genes, including Slug, Sox9 and Foxd3. In the NC induction assay involving Chordin plus Wnt3a-injected animal caps, Lrig3 morpholino inhibited expression of Slug, Sox9 and Foxd3, but not of Pax3 and Zic1. In line with this, Lrig3 knockdown prevented NC marker induction by Pax3 and Zic1, suggesting that Lrig3 acts downstream of these two genes in NC formation. Injection of Lrig3 and Wnt3a led to low-level induction of NC markers and enhanced induction of Fgf3, Fgf4 and Fgf8 in animal caps, suggesting a positive role for Lrig3 in Wnt signaling. Lrig3 could attenuate Fgf signaling in animal caps, did interact with Fgf receptor 1 in cultured cells and, according to context, decreased or increased the induction of NC markers by Fgf. We suggest that Lrig3 functions in NC formation in Xenopus by modulating the Wnt and Fgf signaling pathways.

Cyclooxygenase-2-derived prostaglandin E2 activates beta-catenin in human cholangiocarcinoma cells: evidence for inhibition of these signaling pathways by omega 3 polyunsaturated fatty acids

Cholangiocarcinoma is a highly malignant neoplasm of the biliary tree. It has a high rate of mortality, and currently, there is no effective chemoprevention and treatment. This study was designed to investigate the potential effect of omega 3 polyunsaturated fatty acids (omega 3-PUFA) on human cholangiocarcinoma cell growth and to determine their mechanisms of actions. Treatment of three human cholangiocarcinoma cells (CCLP1, HuCCT1, SG231) with two omega 3-PUFAs, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), for 12 to 72 h resulted in a dose- and time-dependent inhibition of cell growth; in contrast, arachidonic acid, a omega 6-PUFA, had no significant effect. The omega 3-PUFA effect is due to the induction of apoptosis, given that DHA induced the cleaved form of PARP, caspase-3, and caspase-9. DHA and EPA treatment caused dephosphorylation (and hence, the activation) of glycogen synthase kinase-3beta (GSK-3beta) with a decline of beta-catenin protein. Accordingly, DHA treatment also decreased the beta-catenin-mediated T cell factor/lymphoid enhancer factor (TCF/LEF) reporter activity, and inhibited the expression of c-Met, a beta-catenin-controlled downstream gene implicated in cholangiocarcinogenesis. The GSK-3beta inhibitor, SB216763, partially prevented DHA-induced reduction of beta-catenin protein and TCF/LEF reporter activity, and restored cell growth, suggesting the involvement of GSK-3beta dephosphorylation in omega 3-PUFA-induced beta-catenin degradation. In parallel, DHA treatment also induced the formation of the beta-catenin/Axin/GSK-3beta binding complex, further leading to beta-catenin degradation. Moreover, DHA inhibited the expression of cyclooxygenase-2 (COX-2) and enhanced the expression of 15-hydroxyprostaglandin dehydrogenase, a physiologic COX-2 antagonist, in human cholangiocarcinoma cells. These findings suggest that omega 3-PUFAs block cholangiocarcinoma cell growth at least in part through inhibition of Wnt/beta-catenin and COX-2 signaling pathways. Thus, utilization of omega 3-PUFAs may represent an effective and safe therapeutic approach for the chemoprevention and treatment of human cholangiocarcinoma.

The glucose transporter (GLUT4) enhancer factor is required for normal wing positioning in Drosophila

Many of the transcription factors and target genes that pattern the developing adult remain unknown. In the present study, we find that an ortholog of the poorly understood transcription factor, glucose transporter (GLUT4) enhancer factor (Glut4EF, GEF) [also known as the Huntington's disease gene regulatory region-binding protein (HDBP) 1], plays a critical role in specifying normal wing positioning in adult Drosophila. Glut4EF proteins are zinc-finger transcription factors named for their ability to regulate expression of GLUT4 but nothing is known of Glut4EF's in vivo physiological functions. Here, we identify a family of Glut4EF proteins that are well conserved from Drosophila to humans and find that mutations in Drosophila Glut4EF underlie the wing-positioning defects seen in stretch mutants. In addition, our results indicate that previously uncharacterized mutations in Glut4EF are present in at least 11 publicly available fly lines and on the widely used TM3 balancer chromosome. These results indicate that previous observations utilizing these common stocks may be complicated by the presence of Glut4EF mutations. For example, our results indicate that Glut4EF mutations are also present on the same chromosome as two gain-of-function mutations of the homeobox transcription factor Antennapedia (Antp) and underlie defects previously attributed to Antp. In fact, our results support a role for Glut4EF in the modulation of morphogenetic processes mediated by Antp, further highlighting the importance of Glut4EF transcription factors in patterning and morphogenesis.

Endocytosis: A Positive or a Negative Influence on Wnt Signalling?

Endocytosis, with subsequent targeting to lysosomes for degradation, is traditionally seen as a way for cells to terminate signalling. However, in a few instances, endocytosis has been demonstrated to contribute positively to signalling. Here we review recent work on the role of endocytosis in Wnt signalling. Biochemical evidence suggests that the branch of Wnt signalling that controls planar cell polarity (PCP) does require endocytosis, although how endocytosis of Frizzled receptors is translated into PCP in vivo remains unknown. With respect to the main signalling branch (called the canonical or beta-catenin pathway), the literature is divided as to whether endocytosis is required. Results of in vivo experiments are inconclusive because of the toxic side-effects of blocking endocytosis. Some results with cultured cells suggest the need for endocytosis in canonical signalling; however, it remains unclear whether the ligand-receptor complex must enter the cell by clathrin-mediated or caveolae-mediated endocytosis in order to signal. Means of specifically altering Wnt trafficking as well as of tracking the internalization route in different cell types are needed.

SERKAL syndrome: an autosomal-recessive disorder caused by a loss-of-function mutation in WNT4

The WNT-signaling pathway plays a major role during mammalian embryogenesis. We report a novel autosomal-recessive syndrome that consists of female to male sex reversal and renal, adrenal, and lung dysgenesis and is associated with additional developmental defects. Using a candidate-gene approach, we identified a disease-causing homozygous missense mutation in the human WNT4 gene. The mutation was found to result in markedly reduced WNT4 mRNA levels in vivo and in vitro and to downregulate WNT4-dependent inhibition of beta-catenin degradation. Taken together with previous observations in animal models, the present data attribute a pivotal role to WNT4 signaling during organogenesis in humans.

An oncogenic hub: beta-catenin as a molecular target for cancer therapeutics

The Wnt/beta-catenin signaling pathway plays diverse roles in embryonic development and in maintenance of organs and tissues in adults. Activation of this signaling cascade inhibits degradation of the pivotal component beta-catenin, which in turn stimulates transcription of downstream target genes. Over the past two decades, intensive worldwide investigations have yielded considerable progress toward understanding the cellular and molecular mechanisms of Wnt signaling and its involvement in the pathogenesis of a range of human diseases. Remarkably, beta-catenin signaling is aberrantly activated in greater than 70% of colorectal cancers and to a lesser extent in other tumor types, promoting cancer cell proliferation, survival and migration. Accordingly, beta-catenin has gained recognition as an enticing molecular target for cancer therapeutics. Disruption of protein-protein interactions essential for beta-catenin activity holds immense promise for the development of novel anti-cancer drugs. In this review, we focus on the regulation of beta-catenin-dependent transcriptional activation and discuss potential therapeutic opportunities to block this signaling pathway in cancer.

Gluten affects epithelial differentiation-associated genes in small intestinal mucosa of coeliac patients

In coeliac disease gluten induces an immunological reaction in genetically susceptible patients, and influences on epithelial cell proliferation and differentiation in the small-bowel mucosa. Our aim was to find novel genes which operate similarly in epithelial proliferation and differentiation in an epithelial cell differentiation model and in coeliac disease patient small-bowel mucosal biopsy samples. The combination of cDNA microarray data originating from a three-dimensional T84 epithelial cell differentiation model and small-bowel mucosal biopsy samples from untreated and treated coeliac disease patients and healthy controls resulted in 30 genes whose mRNA expression was similarly affected. Nine of 30 were located directly or indirectly in the receptor tyrosine kinase pathway starting from the epithelial growth factor receptor. Removal of gluten from the diet resulted in a reversion in the expression of 29 of the 30 genes in the small-bowel mucosal biopsy samples. Further characterization by blotting and labelling revealed increased epidermal growth factor receptor and beta-catenin protein expression in the small-bowel mucosal epithelium in untreated coeliac disease patients compared to healthy controls and treated coeliac patients. We found 30 genes whose mRNA expression was affected similarly in the epithelial cell differentiation model and in the coeliac disease patient small-bowel mucosal biopsy samples. In particular, those genes involved in the epithelial growth factor-mediated signalling pathways may be involved in epithelial cell differentiation and coeliac disease pathogenesis. The epithelial cell differentiation model is a useful tool for studying gene expression changes in the crypt-villus axis.

Loss of ICAT gene function leads to arrest of ureteric bud branching and renal agenesis

ICAT, inhibitor of beta-catenin and T cell factor, or Ctnnbip1, is a negative regulator of the Wnt signaling pathway that interferes with the interaction between beta-catenin and T cell factor. Some ICAT-deficient (ICAT-/-) embryos exhibit unilateral or bilateral renal agenesis. In this study, we investigated developmental processes in the ICAT-/- kidney. ICAT was highly expressed in both the ureteric bud (UB) and the surrounding metanephric mesenchymal (MM) cells in the metanephros of embryonic day E11.5-E13.5 wild-type (ICAT+/+) mouse. In the E12.5-ICAT-/- metanephros, UB branching was delayed, and a T-shaped, bifurcated UB was frequently seen; this was never seen in the E12.5-ICAT+/+ metanephros. More apoptotic MM cells were detected in the ICAT-/- metanephros than in the ICAT+/+ metanephros. These results suggest that the loss of ICAT gene function causes the arrest of UB branching and the apoptotic death of MM cells, resulting in renal agenesis.

Regulatory factors for initial T lymphocyte lineage specification

PURPOSE OF REVIEW

Initiation of T lymphocyte development depends on balanced regulatory inputs from multiple essential transcription factors. This review highlights contributions of E2A, hematopoietic transcription factor PU.1, growth factor independence (Gfi)-1, T cell factor (TCF)-1, and Runx factors and their interactions with the Notch pathway to promote T cell development.

RECENT FINDINGS

E2A and Runx family factors have been implicated in establishing competent precursors in which Notch signaling can induce the T cell program. An early role was also indicated for PU.1. Later PU.1 activities are antagonistic to pro-T cell factors, however, including E proteins, Myb, Gfi-1, and TCF-1. Diversion to a non-T lineage can be promoted by PU.1, CCAAT/enhancer binding protein, or even GATA and TCF, but these diversion mechanisms are blocked by Notch signaling. An emergent gene network summarizes the cross-regulatory relationships among these factors.

SUMMARY

Entry into the T-cell pathway is controlled by a dynamic balance among essential regulatory factors that depend on Notch signaling not only to trigger initiation of the T-cell program but also to maintain the lineage fidelity of their collective action.

Molecular piracy: manipulation of the ubiquitin system by Kaposi's sarcoma-associated herpesvirus

Ubiquitination, one of several post-translational protein modifications, plays a key role in the regulation of cellular events, including protein degradation, signal transduction, endocytosis, protein trafficking, apoptosis and immune responses. Ubiquitin attachment at the lysine residue of cellular factors acts as a signal for endocytosis and rapid degradation by the 26S proteasome. It has recently been observed that viruses, especially oncogenic herpesviruses, utilise molecular piracy by encoding their own proteins to interfere with regulation of cell signalling. Kaposi's sarcoma- associated herpesvirus (KSHV) manipulates the ubiquitin system to facilitate cell proliferation, anti-apoptosis and evasion from immunity. In this review, we will describe the strategies used by KSHV at distinct stages of the viral life-cycle to control the ubiquitin system and promote oncogenesis and viral persistence.