Function Of Wnt Signaling Research Articles

Wnt/β-catenin signaling directly regulates Foxj1 expression and ciliogenesis in zebrafish Kupffer’s vesicle

Cilia are essential for normal development. The composition and assembly of cilia has been well characterized, but the signaling and transcriptional pathways that govern ciliogenesis remain poorly studied. Here, we report that Wnt/β-catenin signaling directly regulates ciliogenic transcription factor foxj1a expression and ciliogenesis in zebrafish Kupffer's vesicle (KV). We show that Wnt signaling acts temporally and KV cell-autonomously to control left-right (LR) axis determination and ciliogenesis. Specifically, reduction of Wnt signaling leads to a disruption of LR patterning, shorter and fewer cilia, a loss of cilia motility and a downregulation of foxj1a expression. However, these phenotypes can be rescued by KV-targeted overexpression of foxj1a. In comparison to the FGF pathway that has been previously implicated in the control of ciliogenesis, our epistatic studies suggest a more downstream function of Wnt signaling in the regulation of foxj1a expression and ciliogenesis in KV. Importantly, enhancer analysis reveals that KV-specific expression of foxj1a requires the presence of putative Lef1/Tcf binding sites, indicating that Wnt signaling activates foxj1a transcription directly. We also find that impaired Wnt signaling leads to kidney cysts and otolith disorganization, which can be attributed to a loss of foxj1 expression and disrupted ciliogenesis in the developing pronephric ducts and otic vesicles. Together, our data reveal a novel role of Wnt/β-catenin signaling upstream of ciliogenesis, which might be a general developmental mechanism beyond KV. Moreover, our results also prompt a hypothesis that certain developmental effects of the Wnt/β-catenin pathway are due to the activation of Foxj1 and cilia formation.

19th Acta Physiologica international symposium ‘WNT Signaling in Physiology and Disease’– a long way from developmental biology to physiology

In 1996, a series of Acta Physiologica international symposia sponsored by the Scandinavian Physiological Society was initiated. This issue of Acta Physiologica contains 10 review articles from participants at the 19th Acta Physiologica International Symposium ‘WNT2010 – WNT Signaling in Physiology and Disease’ held at the Karolinska Institutet, Stockholm, Sweden from 27 to 30 October 2010. Prof Hans Clevers (The Netherlands) and Prof Akira Kikuchi (2011) (Japan) summarized their work in keynote lectures opening the meeting. With 13 invited speakers, almost 200 participants (Fig.1) and about 50 oral presentations, the meeting collected some of the leading researchers in the WNT field including most of the pioneers in this research area. Participants of the WNT2010 meeting held in late October 2010 at the Karolinska Institutet, Stockholm, Sweden. WNTs form a large family of secreted lipoglycoproteins, which was discovered almost 30 years ago as inducers of oncogenic transformation of mammary cells. Since then it emerged that WNTs, their receptors and the downstream signalling networks are important for embryonic development, the regulation of embryonic and adult stem cells, cancer and other diseases. Only more recently it became apparent that WNT signalling is also important for basic physiology in adult organisms. Thus, we see it as a great opportunity for the WNT field to cooperate with Acta Physiologica in the publication of recent overviews to manifest a general role for WNT signalling in human physiology. The articles published in the present issue of Acta Physiologica cover various topics touched upon at the recent WNT meeting from molecular aspects over cellular signalling to physiology and the treatment of human disease. In the first article, Rik Korswagen (2011) and Martin Harterink summarize the recent progress in the mechanisms of WNT maturation and release from WNT producing cells providing a detailed map of involved components and secretory pathways. Thereafter Akira Kikuchi, Gustavo Miranda-Carboni and Robert Kypta (2011) with coworkers provide detailed views of the roles of WNT-5A, -10B and -11, respectively, in physiology and disease and the signalling pathways involved. Important relays in WNT signaling ate the disheveled (DVL) phosphoproteins. Recent efforts from many groups have led to an increased understanding of these versatile proteins. Craig Malbon (2011) and Hsien Yu Wang provide here their view of the importance of the DVL C terminal domain and some novel insights employing in silico analysis. One of the major pathways in WNT signalling culminates in the β-catenin-mediated modulation of transcriptional regulation. Ken Cadigan (2011) therefore summarizes the current status of underlying mechanisms. The remaining four reviews are devoted to biological and physiological roles of WNT signaling. Matthijs Blankesteijn (2011) and coworkers provide a current view of possibilities to target WNT signalling for the treatment of cardiac hypertrophy and heart failure. Furthermore, Thierry Jarde and Trevor Dale (2011) address the importance of WNT signalling for postnatal mammary gland development providing a timely summary of historic and up-to-date information. Patricia Salinas (2011) has worked extensively to map WNT signalling pathways activated during synaptogenesis. Together with her postdoc Juan P. Henriquez she summarizes data on the dual function of WNTs in establishment of the neuromuscular junction. Last but not least Owen Sansom (2011) and Julia B. Cordero elaborate on the important function of WNT signaling for stem cell regulation focusing on intestinal regeneration and hyperplasia. We hope that the readers of Acta Physiologica will enjoy this potpourri of topics providing snapshots from the WNT world. For members of the WNT community we hope that the review collection will serve as a useful summary of the present status of the field. Organization of the WNT2010 meeting at the Karolinska Institutet was generously supported from the Scandinavian Physiological Society as the main sponsor. Acta Physiologica provided the possibility to publish this WNT special issue. The Swedish Research Council, the Swedish Foundation for International Cooperation and Higher Education (STINT), the Wenner-Gren-Foundations, the Company of Biologists, the Swedish Society for Drug Research, and Merck-Serono provided additional funding. Further, we like to thank the staff of our research groups who helped to plan, prepare and implement the meeting.

Expression Profiling of Wnt Signaling Genes during Gonadal Differentiation and Gametogenesis in Rainbow Trout

Wnt signaling plays major roles in various processes, including ovarian differentiation and development in mammals. In order to explore its potential implication during gonadal development in a nonmammalian vertebrate species, expression of Wnt signaling genes was investigated in rainbow trout during gonadal differentiation and gametogenesis. Multiple Wnt pathway genes were expressed and exhibited distinct expression patterns. In ovary, tcf7 was highly expressed during early differentiation, whereas no sexually dimorphic expression of rspo1 was detected. During later ovarian development, wnt11 was highly expressed in granulosa cells and oocytes suggesting an implication in folliculogenesis and oogenesis, whereas wnt9b was principally detected in granulosa cells. In testis, Wnt pathway genes were mostly expressed during early spermatogenesis. Overall, these present results suggest that Wnt signaling is implicated in multiple processes of male and female gonadal development and provide basis for future studies on Wnt signaling functions in teleost fish gonads.

Waif1/5T4 Inhibits Wnt/β-Catenin Signaling and Activates Noncanonical Wnt Pathways by Modifying LRP6 Subcellular Localization

Wnt proteins can activate distinct signaling pathways, but little is known about the mechanisms regulating pathway selection. Here we show that the metastasis-associated transmembrane protein Wnt-activated inhibitory factor 1 (Waif1/5T4) interferes with Wnt/β-catenin signaling and concomitantly activates noncanonical Wnt pathways. Waif1 inhibits β-catenin signaling in zebrafish and Xenopus embryos as well as in mammalian cells, and zebrafish waif1a acts as a direct feedback inhibitor of wnt8-mediated mesoderm and neuroectoderm patterning during zebrafish gastrulation. Waif1a binds to the Wnt coreceptor LRP6 and inhibits Wnt-induced LRP6 internalization into endocytic vesicles, a process that is required for pathway activation. Thus, Waif1a modifies Wnt/β-catenin signaling by regulating LRP6 subcellular localization. In addition, Waif1a enhances β-catenin-independent Wnt signaling in zebrafish embryos and Xenopus explants by promoting a noncanonical function of Dickkopf1. These results suggest that Waif1 modulates pathway selection in Wnt-receiving cells.

The balance of tissue repair and remodeling in chronic arthritis

The introduction of targeted therapies has dramatically changed the prognosis of patients with chronic joint diseases such as rheumatoid arthritis (RA) and ankylosing spondylitis (AS). As control of inflammation, and hence of symptoms of disease, is increasingly achieved, more attention is given towards the long-term consequences of these disorders, to the structural damage in the skeletal tissues and to the resulting disability. In AS, bone remodeling with new cartilage and bone formation leading to ankylosis is a striking feature. Clinically successful TNF antagonists do not inhibit radiographic progression of disease. New insights into the molecules involved in ankylosis (such as bone morphogenetic proteins and Wnts) have suggested that the classical paradigm linking inflammation and ankylosis can be challenged, and new concepts of disease onset and progression, with a focus on cell stress and damage, are rapidly evolving. In RA, inhibition of Wnt signaling and defective osteoblast function have been associated with lack of repair. As restoration of tissue integrity and homeostasis is the ultimate goal of therapy, these findings suggest new roads for therapeutic intervention. For patients with AS or RA, such strategies will be critically dependent on further research that defines individual risk factors and need for interventions.

P58. The biphasic function of Wnt signaling during neural differentiation of mouse embryonic stem cells

Activation of Wnt signaling enhances self-renewal of mouse embryonic and neural stem/progenitor cells. However, contrary to these results, undifferentiated ES cells shows very low level of endogenous Wnt signaling and ectopic activation of Wnt signaling blocks neural differentiation. Therefore it is not clear whether or when endogenous Wnt/β-catenin signaling is necessary for self-renewal or neural differentiation of ES cells. When we examined the expression profile of Wnt signaling components, expressions of most Wnts are very low in undifferentiated ES cells while some of Wnts are increased during differentiation. We generated stable ES cell lines that probe Wnt signaling activity and these cells showed that Wnt signaling was very low in undifferentiated ES cells while increased during embryonic body formation or neural differentiation. However, conditioned media from differentiated ES cells did not induce higher reporter activity than that of undifferentiated ES cells, which suggests that ES cells may change the sensitivity to Wnt signaling during differentiation whereas the levels of secreted Wnts are same. Changes in the composition of Tcfs or Lef1 in ES cells at different conditions may be a possible mechanism. In addition, we observed that activation of Wnt/β-catenin signaling at different time periods resulted in differential effects on neural differentiation of 46C ES cells that show increased Sox1 promoter driven GFP during neural differentiation; activation of Wnt signaling by the treatment with BIO (a GSK3β inhibitor) in early or late period blocks or enhances neural differentiation, respectively. Consistent with these results, inactivation of Wnt/β-catenin signaling in late period by the treatment with endo-IWR1 (an Axin stabilizer) blocked neural differentiation. Overall, our data suggest that Wnt/β-catenin signaling plays biphasic roles during neural differentiation of mouse embryonic stem cells. Supported by the National Research Foundation of Korea (2006-2004046) and Brain Korea 21 program.

Wnt signaling controls temporal identities of seam cells in Caenorhabditis elegans

The Wnt signaling pathway regulates multiple aspects of the development of stem cell-like epithelial seam cells in Caenorhabditis elegans, including cell fate specification and symmetric/asymmetric division. In this study, we demonstrate that lit-1, encoding the Nemo-like kinase in the Wnt/β-catenin asymmetry pathway, plays a role in specifying temporal identities of seam cells. Loss of function of lit-1 suppresses defects in retarded heterochronic mutants and enhances defects in precocious heterochronic mutants. Overexpressing lit-1 causes heterochronic defects opposite to those in lit-1(lf) mutants. LIT-1 exhibits a periodic expression pattern in seam cells within each larval stage. The kinase activity of LIT-1 is essential for its role in the heterochronic pathway. lit-1 specifies the temporal fate of seam cells likely by modulating miRNA-mediated silencing of target heterochronic genes. We further show that loss of function of other components of Wnt signaling, including mom-4, wrm-1, apr-1, and pop-1, also causes heterochronic defects in sensitized genetic backgrounds. Our study reveals a novel function of Wnt signaling in controlling the timing of seam cell development in C. elegans.

Conserved mechanism of Wnt signaling function in the specification of vulval precursor fates in C. elegans and C. briggsae

The C. elegans hermaphrodite vulva serves as a paradigm for understanding how signaling pathways control organ formation. Previous studies have shown that Wnt signaling plays important roles in vulval development. To understand the function and evolution of Wnt signaling in Caenorhabditis nematodes we focused on C. briggsae, a species that is substantially divergent from C. elegans in terms of the evolutionary time scale yet shares almost identical morphology. We isolated mutants in C. briggsae that display multiple pseudo-vulvae resulting from ectopic VPC induction. We cloned one of these loci and found that it encodes an Axin homolog, Cbr-PRY-1. Our genetic studies revealed that Cbr-pry-1 functions upstream of the canonical Wnt pathway components Cbr-bar-1 ( β-catenin) and Cbr-pop-1( tcf/lef) as well as the Hox target Cbr-lin-39 (Dfd/Scr). We further characterized the pry-1 vulval phenotype in C. briggsae and C. elegans using 8 cell fate markers, cell ablation, and genetic interaction approaches. Our results show that ectopically induced VPCs in pry-1 mutants adopt 2° fates independently of the gonad-derived inductive and LIN-12/Notch-mediated lateral signaling pathways. We also found that Cbr-pry-1 mutants frequently show a failure of P7.p induction. A similar, albeit low penetrant, defect is also observed in C. elegans pry-1 mutants. The genetic analysis of the P7.p induction defect revealed that it was caused by altered regulation of lin-12 and its transcriptional target lip-1 (MAP kinase phosphatase). Thus, our results provide evidence for LIN-12/Notch-dependent and independent roles of Wnt signaling in promoting 2° VPC fates in both nematode species.

Re-programming of C. elegans male epidermal precursor fates by Wnt, Hox, and LIN-12/Notch activities

In C aenorhabditis elegans males, different subsets of ventral epidermal precursor (Pn.p) cells adopt distinct fates in a position-specific manner: three posterior cells, P(9–11).p, comprise the hook sensillum competence group (HCG) with three potential fates (1°, 2°, or 3°), while eight anterior cells, P(1–8).p, fuse with the hyp7 epidermal syncytium. Here we show that activation of the canonical BAR-1 β-catenin pathway of Wnt signaling alters the competence of P(3–8).p and specifies ectopic HCG-like fates. This fate transformation requires the Hox gene mab-5. In addition, misexpression of mab-5 in P(1–8).p is sufficient to establish HCG competence among these cells, as well as to generate ectopic HCG fates in combination with LIN-12 or EGF signaling. While increased Wnt signaling induces predominantly 1° HCG fates, increased LIN-12 or EGF signaling in combination with MAB-5 overexpression promotes 2° HCG fates in anterior Pn.p cells, suggesting distinctive functions of Wnt, LIN-12, and EGF signaling in specification of HCG fates. Lastly, wild-type mab-5 function is necessary for normal P(9–11).p fate specification, indicating that regulation of ectopic HCG fate formation revealed in anterior Pn.p cells reflect mechanisms of pattern formation during normal hook development.

Wnt Target Genes in CNS Development

We are interested in the function of Wnt signaling and Lef/Tcf factors in the developing zebrafish central nervous system. In the spinal cord we have evidence that Tcf3 acts to maintain the multipotent progenitor state, both through direct transcriptional regulation and through a novel post‐transcriptional pathway. In the hypothalamus we have identified a role for Tcf7 and Lef1 as Wnt‐mediated activators regulating the development and maintenance of committed neural precursors. We have also identified Lef/Tcf target genes in both processes. Together our data fit with established roles for Lef/Tcf factors in other epithelial tissues, where Tcf3‐mediated repression acts to maintain the stem cell state, while Wnt‐mediated activation supports the progression of cells into a differentiation pathway.

Wnt signaling and its downstream target N-myc regulate basal progenitors in the developing neocortex

Basal progenitors (also called non-surface dividing or intermediate progenitors) have been proposed to regulate the number of neurons during neocortical development through expanding cells committed to a neuronal fate, although the signals that govern this population have remained largely unknown. Here, we show that N-myc mediates the functions of Wnt signaling in promoting neuronal fate commitment and proliferation of neural precursor cells in vitro. Wnt signaling and N-myc also contribute to the production of basal progenitors in vivo. Expression of a stabilized form of beta-catenin, a component of the Wnt signaling pathway, or of N-myc increased the numbers of neocortical basal progenitors, whereas conditional deletion of the N-myc gene reduced these and, as a likely consequence, the number of neocortical neurons. These results reveal that Wnt signaling via N-myc is crucial for the control of neuron number in the developing neocortex.

Lentiviral Vectors to Probe and Manipulate the Wnt Signaling Pathway

BackgroundThe Wnt signaling pathway plays key roles in development, adult tissue homeostasis and stem cell maintenance. Further understanding of the function of Wnt signaling in specific cell types could benefit from lentiviral vectors expressing reporters for the Wnt pathway or vectors interfering with signaling.Methodology/Principal FindingsWe have developed a set of fluorescent and luminescent lentiviral vectors that report Wnt signaling activity and discriminate between negative and uninfected cells. These vectors possess a 7xTcf-eGFP or 7xTcf-FFluc (Firefly Luciferase) reporter cassette followed by either an SV40-mCherry or SV40-PuroR (puromycin N-acetyltransferase) selection cassette. We have also constructed a vector that allows drug-based selection of cells with activated Wnt signaling by placing PuroR under the control of the 7xTcf promoter. Lastly, we have expressed dominant-negative Tcf4 (dnTcf4) or constitutively active beta-catenin (β-catenin4A) from the hEF1α promoter in a SV40-PuroR or SV40-mCherry backbone to create vectors that inhibit or activate the Wnt signaling pathway. These vectors will be made available to the scientific community through Addgene.ConclusionsThese novel lentiviruses are efficient tools to probe and manipulate Wnt signaling. The use of a selection cassette in Wnt-reporter viruses enables discriminating between uninfected and non-responsive cells, an important requirement for experiments where selection of clones is not possible. The use of a chemiluminescent readout enables quantification of signaling. Finally, selectable vectors can be used to either inhibit or activate the Wnt signaling pathway. Altogether, these vectors can probe and modulate the Wnt signaling pathway in experimental settings where persistence of the transgene or gene transfer cannot be accomplished by non-viral techniques.

Increased Dickkopf‐1 expression in transgenic mouse models of neurodegenerative disease

To investigate the role of the Wnt inhibitor Dickkopf-1 (DKK-1) in the pathophysiology of neurodegenerative diseases, we analysed DKK-1 expression and localization in transgenic mouse models expressing familial Alzheimer's disease mutations and a frontotemporal dementia mutation. A significant increase of DKK-1 expression was found in the diseased brain areas of all transgenic lines, where it co-localized with hyperphosphorylated tau-bearing neurons. In TgCRND8 mice, DKK-1 immunoreactivity was detected in neurons surrounding amyloid deposits and within the choline acetyltransferase-positive neurons of the basal forebrain. Active glycogen synthase kinase-3 (GSK-3) was found to co-localize with DKK-1 and phospho-tau staining. Downstream to GSK-3, a significant reduction in beta-catenin translocation to the nucleus, indicative of impaired Wnt signaling functions, was found as well. Cumulatively, our findings indicate that DKK-1 expression is associated with events that lead to neuronal death in neurodegenerative diseases and support a role for DKK-1 as a key mediator of neurodegeneration with therapeutic potential.

Characterization and In Vivo Pharmacological Rescue of a Wnt2-Gata6 Pathway Required for Cardiac Inflow Tract Development

Little is understood about the molecular mechanisms underlying the morphogenesis of the posterior pole of the heart. Here we show that Wnt2 is expressed specifically in the developing inflow tract mesoderm, which generates portions of the atria and atrio-ventricular canal. Loss of Wnt2 results in defective development of the posterior pole of the heart, resulting in a phenotype resembling the human congenital heart syndrome complete common atrio-ventricular canal. The number and proliferation of posterior second heart field progenitors is reduced in Wnt2(-/-) mutants. Moreover, these defects can be rescued in a temporally restricted manner through pharmacological inhibition of Gsk-3beta. We also show that Wnt2 works in a feedforward transcriptional loop with Gata6 to regulate posterior cardiac development. These data reveal a molecular pathway regulating the posterior cardiac mesoderm and demonstrate that cardiovascular defects caused by loss of Wnt signaling can be rescued pharmacologically in vivo.

G Protein-coupled Receptor Kinases Phosphorylate LRP6 in the Wnt Pathway

Wnt ligands conduct their functions in canonical Wnt signaling by binding to two receptors, the single transmembrane low density lipoprotein receptor-related proteins 5 and 6 (LRP5/6) and seven transmembrane (7TM) Frizzled receptors. Subsequently, phosphorylation of serine/threonine residues within five repeating signature PPPSP motifs on LRP6 is responsible for LRP6 activation. GSK3β, a cytosolic kinase for phosphorylation of a downstream effector β-catenin, was proposed to participate in such LRP6 phosphorylation. Here, we report a new class of membrane-associated kinases for LRP6 phosphorylation. We found that G protein-coupled receptor kinases 5 and 6 (GRK5/6), traditionally known to phosphorylate and desensitize 7TM G protein-coupled receptors, directly phosphorylate the PPPSP motifs on single transmembrane LRP6 and regulate Wnt/LRP6 signaling. GRK5/6-induced LRP6 activation is inhibited by the LRP6 antagonist Dickkopf. Depletion of GRK5 markedly reduces Wnt3A-stimulated LRP6 phosphorylation in cells. In zebrafish, functional knock-down of GRK5 results in reduced Wnt signaling, analogous to LRP6 knock-down, as assessed by decreased abundance of β-catenin and lowered expression of the Wnt target genes cdx4, vent, and axin2. Expression of GRK5 rescues the diminished β-catenin and axin2 response caused by GRK5 depletion. Thus, our findings identify GRK5/6 as novel kinases for the single transmembrane receptor LRP6 during Wnt signaling.

Centrosomal localization of Diversin and its relevance to Wnt signaling

Wnt pathways regulate many developmental processes, including cell-fate specification, cell polarity, and cell movements during morphogenesis. The subcellular distribution of pathway mediators in specific cellular compartments might be crucial for the selection of pathway targets and signaling specificity. We find that the ankyrin-repeat protein Diversin, which functions in different Wnt signaling branches, localizes to the centrosome in Xenopus ectoderm and mammalian cells. Upon stimulation with Wnt ligands, the centrosomal distribution of Diversin is transformed into punctate cortical localization. Also, Diversin was recruited by Frizzled receptors to non-homogeneous Dishevelled-containing cortical patches. Importantly, Diversin deletion constructs, which did not localize to the centrosome, failed to efficiently antagonize Wnt signaling. Furthermore, a C-terminal construct that interfered with Diversin localization inhibited Diversin-mediated beta-catenin degradation. These observations suggest that the centrosomal localization of Diversin is crucial for its function in Wnt signaling.

Canonical Wnt signaling activity during synovial joint development

Wnt signaling plays important roles in skeletal development. However, the activation and function of canonical Wnt signaling in joint development remains unclear. We analyzed the lineage identity and developmental changes of the Wnt-responsive cells during synovial joint formation as well as adulthood in the Wnt signaling reporter TOPgal transgenic mice. At embryonic day (E) 12.5, we found that the TOPgal was inactivated in the presumptive joint forming interzone, but it was intensively activated in the cartilage anlage of developing long bones and digits. At E14.5, the TOPgal activity was found in a subgroup of the articular chondrocyte lineage cells, which were co-immunolabeled with Doublecortin intensively and with Vinculin weakly. At E18.5, the TOPgal/Doublecortin co-immunolabeled cells were found in the superficial layer of the developing articular cartilage. During postnatal development, the TOPgal(+) articular chondrocytes were abundant at P7 and decreased from P10. A small number of TOPgal(+) articular chondrocytes were also found in adult joints. Our study suggests an age- and lineage-specific role of canonical Wnt signaling in joint development and maintenance.

Plasticity and regulatory mechanisms of Hox gene expression in mouse neural crest cells

In amniotes, the developmental potentials of neural crest cells differ between the cranium and the trunk. These differences may be attributable to the different expression patterns of Hox genes between cranial and trunk neural crest cells. However, little is known about the factors that control Hox genes expression in neural crest cells. The present data demonstrate that retinoic acid (RA) treatment and the activation of Wnt signaling induce Hoxa2 and Hoxd9 expression, respectively, in mouse mesencephalic neural crest cells, which never express Hox genes in vivo. Furthermore, Wnt signaling suppresses the induction of Hoxa2. We also demonstrate that these factors participate in the maintenance of Hoxa2 and Hoxd9 expression in mouse trunk neural crest cells. Our results suggest that RA and Wnt signaling function as environmental factors that regulate the expression of Hoxa2 and Hoxd9 in mouse neural crest cells.

Threonine 393 of β‐catenin regulates interaction with Axin

CK2 is a regulatory kinase implicated in embryonic development and in cancer. Among the CK2 substrates is beta-catenin, a protein with dual function in Wnt signaling and cell adhesion. Previously, we reported that CK2 activity is required for beta-catenin stability and we identified threonine (T) 393 as a major CK2 phosphorylation site in beta-catenin. However, it is not known whether phosphorylation at T393 increases beta-catenin stability and if so, what is the mechanism. In this study we investigate the molecular mechanism of beta-catenin stabilization through phosphorylation at T393. We found that pseudophosphorylation of beta-catenin at T393 resulted in a stable activated form of beta-catenin with decreased affinity for Axin in vitro. This phosphomimetic mutant also displayed decreased regulation by Axin in vivo in a bioassay in Xenopus laevis embryos. In contrast, the binding of T393 pseudophosphorylated beta-catenin to E-cadherin was unaffected. Further analysis showed that pseudophosphorylation at T393 did not prevent beta-catenin phosphorylation by GSK3beta. Interestingly, we found that in the presence of pseudophophorylated beta-catenin and another activated form of beta-catenin, the recruitment of GSK3beta to Axin is enhanced. These findings indicate that phosphorylation of T393 by CK2 may affect the stability of beta-catenin through decreased binding to Axin. In addition, the increased recruitment of GSK3beta to the destruction complex in the presence of activated beta-catenin mutants could be a feedback mechanism to suppress overactive Wnt signaling.

The constitutive active/androstane receptor facilitates unique phenobarbital-induced expression changes of genes involved in key pathways in precancerous liver and liver tumors.

Our overall goal is to elucidate progressive changes, in expression and methylation status, of genes which play key roles in phenobarbital (PB)-induced liver tumorigenesis, with an emphasis on their potential to affect signaling through critical pathways involved in the regulation of cell growth and differentiation. PB-elicited unique expression changes of genes, including some of those identified previously as exhibiting regions of altered DNA methylation, were discerned in precancerous liver tissue and/or individual liver tumors from susceptible constitutive active/androstane receptor (CAR) wild-type (WT) compared with resistant CAR knockout (KO) mice. Many of these function in crucial cancer-related processes, for example, angiogenesis, apoptosis, cell cycle, DNA methylation, Hedgehog signaling, invasion/metastasis, Notch signaling, and Wnt signaling. Furthermore, a subset of the uniquely altered genes contained CAR response elements (CAREs). This included Gadd45b, a coactivator of CAR and inhibitor of apoptosis, and two DNA methyltransferases (Dnmt1, Dnmt3a). The presence of CAREs in Dnmts suggests a potential direct link between PB and altered DNA methylation. The current data are juxtaposed with the effects of PB on DNA methylation and gene expression which occurred uniquely in liver tumor-prone B6C3F1 mice, as compared with the resistant C57BL/6, following 2 or 4 weeks of treatment. Collectively, these data reveal a comprehensive view of PB-elicited molecular alterations (i.e., changes in gene expression and DNA methylation) that can facilitate hepatocarcinogenesis. Notably, candidate genes for initial "fingerprints" of early and late stages of PB-induced tumorigenesis are proposed.