Biochemical interactions in the wnt pathway

Essential role for integrin linked kinase in Akt-mediated integrin survival signaling in hippocampal neurons

Activation of integrin receptors in neurons can promote cell survival and synaptic plasticity, but the underlying signal transduction pathway(s) is unknown. We report that integrin signaling prevents apoptosis of embryonic hippocampal neurons by a mechanism involving integrin-linked kinase (ILK) that activates Akt kinase. Activation of integrins using a peptide containing the amino acid sequence EIKLLIS derived from the alpha chain of laminin protected hippocampal neurons from apoptosis induced by glutamate or staurosporine, and increased Akt activity in a beta1 integrin-dependent manner. Transfection of neurons with a plasmid encoding dominant negative Akt blocked the protective effect of the integrin-activating peptide, as did a chemical inhibitor of Akt. Although inhibitors of phosphoinositide-3 (PI3) kinase blocked the protective effect of the peptide, we found no increase in PI3 kinase activity following integrin stimulation suggesting that PI3 kinase was necessary for Akt activity but was not sufficient for the increase in Akt activity following integrin activation. Instead, we show a requirement for ILK in integrin receptor-induced Akt activation. ILK was activated following integrin stimulation and dominant negative ILK blocked integrin-mediated Akt activation and cell survival. Activation of ILK and Akt were also required for neuroprotection by substrate-associated laminin. These results establish a novel pathway that signals cell survival in neurons in response to integrin receptor activation.

Activation of beta-catenin-LEF/TCF signal pathway in chondrocytes stimulates ectopic endochondral ossification

OBJECTIVE

Members of the Wnt signaling protein family are expressed during cartilage development and skeletogenesis, but their roles and mechanisms of action in those processes remain unclear. Recently, we found that beta-catenin-LEF/TCF-dependent Wnt signaling stimulates chondrocyte maturation and hypertrophy and extracellular matrix calcification in vitro, events normally associated with cartilage-to-bone transition during skeletogenesis. Thus, we tested here whether activation of this pathway promotes endochondral ossification.

DESIGN

Chick chondrocytes were infected with avian retroviral expression vectors encoding constitutive-active (CA) or dominant-negative (DN) forms of LEF, which activate or block beta-catenin-dependent Wnt signaling respectively. These cells and companion uninfected control cells were seeded into type I collagen gels and transplanted intramuscularly into nude mice. The resulting ectopic tissue masses forming over time in vivo were subjected to histological and molecular biological analyses.

RESULTS

Transplantation of chick chondrocytes induced de novo endochondral bone formation. In situ hybridization and RT-PCR using species-specific probes and primers showed that the ectopic cartilaginous tissue was avian and thus donor-derived, whereas the bone tissue was mouse and thus host-derived. CA-LEF-expressing ectopic tissue masses contained abundant bone and marrow, while DN-LEF-expressing masses contained little bone and lacked marrow.

CONCLUSIONS

Activation of beta-catenin-LEF/TCF-dependent Wnt signaling accelerates chondrocyte maturation and replacement of cartilage by bone.

PTP1B modulates the association of beta-catenin with N-cadherin through binding to an adjacent and partially overlapping target site

The nonreceptor tyrosine phosphatase PTP1B associates with the cytoplasmic domain of N-cadherin and may regulate cadherin function through dephosphorylation of beta-catenin. We have now identified the domain on N-cadherin to which PTP1B binds and characterized the effect of perturbing this domain on cadherin function. Deletion constructs lacking amino acids 872-891 fail to bind PTP1B. This domain partially overlaps with the beta-catenin binding domain. To further define the relationship of these two sites, we used peptides to compete in vitro binding. A peptide representing the most NH(2)-terminal 8 amino acids of the PTP1B binding site, the region of overlap with the beta-catenin target, effectively competes for binding of beta-catenin but is much less effective in competing PTP1B, whereas two peptides representing the remaining 12 amino acids have no effect on beta-catenin binding but effectively compete for PTP1B binding. Introduction into embryonic chick retina cells of a cell-permeable peptide mimicking the 8 most COOH-terminal amino acids in the PTP1B target domain, the region most distant from the beta-catenin target site, prevents binding of PTP1B, increases the pool of free, tyrosine-phosphorylated beta-catenin, and results in loss of N-cadherin function. N-cadherin lacking this same region of the PTP1B target site does not associate with PTP1B or beta-catenin and is not efficiently expressed at the cell surface of transfected L cells. Thus, interaction of PTP1B with N-cadherin is essential for its association with beta-catenin, stable expression at the cell surface, and consequently, cadherin function.

Protein kinase CKII regulates the interaction of beta-catenin with alpha-catenin and its protein stability

beta-Catenin is a multi-functional cellular component and a substrate for several protein kinases. Here we investigated the interaction of protein kinase CKII (casein kinase II) and beta-catenin. We show that CKII phosphorylates the N-terminal region of beta-catenin and we identified Ser29, Thr102, and Thr112 as substrates for the enzyme. We provide evidence that CKII regulates the cytoplasmic stability of beta-catenin and acts synergistically with GSK-3beta in the multi-protein complex that controls the degradation of beta-catenin. In comparing wild-type and Ser/Thr-mutant beta-catenin, a decreased affinity of the mutant protein to alpha-catenin was observed. Moreover, kinase assays in vitro demonstrate a CKII-dependent increase in the binding of wild-type beta-catenin with alpha-catenin. In line with that, cells expressing Ser/Thr-mutant beta-catenin exhibit an increased migratory potential, which correlates with an enhanced cytosolic localization and a reduced association with the cytoskeleton of the mutant protein. From these results we conclude that CKII regulates the function of beta-catenin in the cadherin adhesion complex as well as its cytoplasmic stability.

Apc modulates embryonic stem-cell differentiation by controlling the dosage of beta-catenin signaling

The Wnt signal-transduction pathway induces the nuclear translocation of membrane-bound beta-catenin (Catnb) and has a key role in cell-fate determination. Tight somatic regulation of this signal is essential, as uncontrolled nuclear accumulation of beta-catenin can cause developmental defects and tumorigenesis in the adult organism. The adenomatous polyposis coli gene (APC) is a major controller of the Wnt pathway and is essential to prevent tumorigenesis in a variety of tissues and organs. Here, we have investigated the effect of different mutations in Apc on the differentiation potential of mouse embryonic stem (ES) cells. We provide genetic and molecular evidence that the ability and sensitivity of ES cells to differentiate into the three germ layers is inhibited by increased doses of beta-catenin by specific Apc mutations. These range from a severe differentiation blockade in Apc alleles completely deficient in beta-catenin regulation to more specific neuroectodermal, dorsal mesodermal and endodermal defects in more hypomorphic alleles. Accordingly, a targeted oncogenic mutation in Catnb also affects the differentiation potential of ES cells. Expression profiling of wildtype and Apc-mutated teratomas supports the differentiation defects at the molecular level and pinpoints a large number of downstream structural and regulating genes. Chimeric experiments showed that this effect is cell-autonomous. Our results imply that constitutive activation of the Apc/beta-catenin signaling pathway results in differentiation defects in tissue homeostasis, and possibly underlies tumorigenesis in the colon and other self-renewing tissues.

Murine Nr4a1 and Herpud1 are up-regulated by Wnt-1, but the homologous human genes are independent from beta-catenin activation

The Wnt signal transduction pathway regulates morphogenesis and mitogenesis of cells in multicellular organisms. A major downstream consequence of Wnt-1 signalling is the activation of beta-catenin/T-cell factor (TCF)-mediated transcription. We compared Wnt-1-transformed murine mammary epithelial cells with control cells by subtractive hybridization. We found the two genes Nr4a1 and Herpud1 to be overexpressed in Wnt-1-transformed cells. Remarkably, the transcription levels of the two homologous human genes NR4A1 and HERPUD1 are neither activated in cells with activated beta-catenin/TCF-mediated transcription nor can be induced by beta-catenin transfection. These results indicate different regulation mechanisms of the two genes in murine and human cells.

Axin negatively affects tau phosphorylation by glycogen synthase kinase 3beta

Glycogen synthase kinase 3beta (GSK3beta) is an essential protein kinase that regulates numerous functions within the cell. One critically important substrate of GSK3beta is the microtubule-associated protein tau. Phosphorylation of tau by GSK3beta decreases tau-microtubule interactions. In addition to phosphorylating tau, GSK3beta is a downstream regulator of the wnt signaling pathway, which maintains the levels of beta-catenin. Axin plays a central role in regulating beta-catenin levels by bringing together GSK3beta and beta-catenin and facilitating the phosphorylation of beta-catenin, targeting it for ubiquitination and degradation by the proteasome. Although axin clearly facilitates the phosphorylation of beta-catenin, its effects on the phosphorylation of other GSK3beta substrates are unclear. Therefore in this study the effects of axin on GSK3beta-mediated tau phosphorylation were examined. The results clearly demonstrate that axin is a negative regulator of tau phosphorylation by GSK3beta. This negative regulation of GSK3beta-mediated tau phosphorylation is due to the fact that axin efficiently binds GSK3beta but not tau and thus sequesters GSK3beta away from tau, as an axin mutant that does not bind GSK3beta did not inhibit tau phosphorylation by GSK3beta. This is the first demonstration that axin negatively affects the phosphorylation of a GSK3beta substrate, and provides a novel mechanism by which tau phosphorylation and function can be regulated within the cell.

The LIM-only protein FHL2 interacts with beta-catenin and promotes differentiation of mouse myoblasts

FHL2 is a LIM-domain protein expressed in myoblasts but down-regulated in malignant rhabdomyosarcoma cells, suggesting an important role of FHL2 in muscle development. To investigate the importance of FHL2 during myoblast differentiation, we performed a yeast two-hybrid screen using a cDNA library derived from myoblasts induced for differentiation. We identified beta-catenin as a novel interaction partner of FHL2 and confirmed the specificity of association by direct in vitro binding tests and coimmunoprecipitation assays from cell lysates. Deletion analysis of both proteins revealed that the NH2-terminal part of beta-catenin is sufficient for binding in yeast, but addition of the first armadillo repeat is necessary for binding FHL2 in mammalian cells, whereas the presence of all four LIM domains of FHL2 is needed for the interaction. Expression of FHL2 counteracts beta-catenin-mediated activation of a TCF/LEF-dependent reporter gene in a dose-dependent and muscle cell-specific manner. After injection into Xenopus embryos, FHL2 inhibited the beta-catenin-induced axis duplication. C2C12 mouse myoblasts stably expressing FHL2 show increased myogenic differentiation reflected by accelerated myotube formation and expression of muscle-specific proteins. These data imply that FHL2 is a muscle-specific repressor of LEF/TCF target genes and promotes myogenic differentiation by interacting with beta-catenin.

Mammalian Rcd1 is a novel transcriptional cofactor that mediates retinoic acid-induced cell differentiation

Rcd1, initially identified as a factor essential for the commitment to nitrogen starvation-invoked differentiation in fission yeast, is one of the most conserved proteins found across eukaryotes, and its mammalian homolog is expressed in a variety of differentiating tissues. Here we show that mammalian Rcd1 is a novel transcriptional cofactor and is critically involved in the commitment step in the retinoic acid-induced differentiation of F9 mouse teratocarcinoma cells, at least in part, via forming complexes with retinoic acid receptor and activation transcription factor-2 (ATF-2). In addition, antisense oligonucleotide treatment of embryonic mouse lung explants suggests that Rcd1 also plays a role in retinoic acid-controlled lung development.

Glycogen synthase kinase-3beta inhibitors prevent cellular polyglutamine toxicity caused by the Huntington's disease mutation

Huntington's disease is one of nine known neurodegenerative disorders caused by an expanded polyglutamine (poly(Q)) tract in the disease protein. These diseases are associated with intraneuronal protein aggregates. Heat-inducible chaperones like HSP70 and HSP27 suppress poly(Q) aggregation and/or toxicity/cell death. Heat shock transcription factors, including HSF-1, regulate HSP70 and HSP27 expression. HSF-1 activity is reduced by glycogen synthase kinase-3 (GSK-3) and enhanced by GSK-3 inhibitors, like lithium. Thus, we hypothesized that lithium treatment may partially rescue death in Huntington's disease cell models. LiCl reduced poly(Q) toxicity in neuronal and nonneuronal cell lines, but this was not associated with elevation of HSP70 or HSP27. The protective effect of lithium involved GSK-3beta inhibition, since poly(Q) toxicity was also reduced by SB216763, a GSK-3beta inhibitor, and by overexpression of a dominant-negative GSK-3beta mutant. LiCl and SB216763 increased beta-catenin-dependent T-cell factor-mediated transcription. Since beta-catenin overexpression protected cells from poly(Q) toxicity, we tested whether this pathway was impaired by a poly(Q) expansion mutation. Cells expressing expanded repeats had reduced beta-catenin levels associated with a parallel decrease in T-cell factor-mediated transcription, compared with cells expressing wild type constructs. Since LiCl can protect against polyglutamine toxicity in cell lines, it is an excellent candidate for further in vivo therapeutic trials.

Wnt-3A/beta-catenin signaling induces transcription from the LEF-1 promoter

Members of the Wnt family of secreted molecules have been established as key factors in determining cell fate and morphogenic signaling. It has long been recognized that Wnt induces morphogenic signaling through the Tcf/LEF-1 cascade by regulating free intracellular levels of beta-catenin, a co-factor for Tcf/LEF-1 transcription factors. In the present study, we have demonstrated that Wnt-3A can also directly induce transcription from the LEF-1 promoter. This induction was dependent on glycogen synthase kinase 3beta inactivation, a rise in free intracellular beta-catenin, and a short 110-bp Wnt-responsive element (WRE) in the LEF-1 promoter. Linear and internal deletion of this WRE led to a dramatic increase in constitutive LEF-1 promoter activity and loss of Wnt-3A responsiveness. In isolation, the 110-bp WRE conferred context-independent Wnt-3A or beta-catenin(S37A) responsiveness to a heterologous SV40 promoter. Studies expressing dominant active and negative forms of LEF-1, beta-catenin, GSK-3beta, and beta-catenin/LEF-1 fusions suggest that Wnt-3A activates the LEF-1 promoter through a beta-catenin-dependent and LEF-1-independent process. Wnt-3A expression also induced multiple changes in the binding of factors to the WRE and suggests that regulatory mechanisms may involve modulation of a multiprotein complex. In summary, these results provide evidence for transcriptional regulation of the LEF-1 promoter by Wnt and enhance the mechanistic understanding of Wnt/beta-catenin signaling in the regulation of LEF-1-dependent developmental processes.

I-mfa domain proteins interact with Axin and affect its regulation of the Wnt and c-Jun N-terminal kinase signaling pathways

I-mfa has been identified as an inhibitor of myogenic basic helix-loop-helix transcription factors, and a related human I-mfa domain-containing protein (HIC) also has been identified as a protein that regulates Tat- and Tax-mediated expression of viral promoters. HIC and I-mfa represent a family of proteins that share a highly conserved cysteine-rich domain, termed the I-mfa domain. We show here that both I-mfa domain proteins, HIC and I-mfa, interacted in vivo with the Axin complex through their C-terminal I-mfa domains. This interaction inhibited Axin-mediated downregulation of free levels of cytosolic beta-catenin. I-mfa and HIC also both directly interacted with lymphocyte enhancer factor (LEF); however, I-mfa but not HIC significantly inhibited reporter constructs regulated by beta-catenin. The overexpression of HIC but not I-mfa decreased the inhibitory effects of Axin on beta-catenin-regulated reporter constructs, while both HIC and I-mfa decreased Axin-mediated c-Jun N-terminal kinase (JNK) activation. These data reveal for the first time that I-mfa domain proteins interact with the Axin complex and affect Axin regulation of both the Wnt and the JNK activation pathways. Interestingly, HIC differs from I-mfa in that I-mfa affects both Axin function and T-cell factor- or LEF-regulated transcription in the Wnt signaling pathway while HIC affects primarily Axin function.

The crystal structure of the beta-catenin/ICAT complex reveals the inhibitory mechanism of ICAT

Beta-catenin is a multifunctional protein involved in both cell adhesion and transcriptional activation. Transcription mediated by the beta-catenin/Tcf complex is involved in embryological development and is upregulated in various cancers. We have determined the crystal structure at 2.5 A resolution of a complex between beta-catenin and ICAT, a protein that prevents the interaction between beta-catenin and Tcf/Lef family transcription factors. ICAT contains a 3-helix bundle that binds armadillo repeats 10-12 and a C-terminal tail that, similar to Tcf and E-cadherin, binds in the groove formed by armadillo repeats 5-9 of beta-catenin. We show that ICAT selectively inhibits beta-catenin/Tcf binding in vivo, without disrupting beta-catenin/cadherin interactions. Thus, it should be possible to design cancer therapeutics that inhibit beta-catenin-mediated transcriptional activation without interfering with cell adhesion.

Intracellular signal propagation in a two-dimensional autocatalytic reaction model

We study a simple reaction scheme in a two-dimensional lattice of particles or molecules with a refractory state. We analyze the dynamics of the propagating front as a function of physical-chemical properties of the host medium. The anisotropy of the medium significantly affects the smoothness of the wave front. Similarly, if particles or molecules may diffuse slowly to neighboring sites, then the front wave is more likely to be irregular. Both situations affect the ability of the whole system to relax to the original state, which is a required feature in the biological cells. Attempts to map this simple reaction scheme to reactions involved in the intracellular pathways suggest that, in some cases, signal transduction might take both connotation of a random walk and a propagating wave, depending on the local density of the medium. In particular, a sufficient condition for the appearance of waves in high-density regions of the media, is the existence of at least one autocatalytic reaction in the chain of reactions characterizing the pathway.

Wnt-3A enhances bone morphogenetic protein-2-mediated chondrogenesis of murine C3H10T1/2 mesenchymal cells

We have recently reported the chondrogenic effect of bone morphogenetic protein-2 (BMP-2) in high density cultures of the mouse multipotent mesenchymal C3H10T1/2 cell line and have shown the functional requirement of the cell-cell adhesion molecule N-cadherin in BMP-2-induced chondrogenesis in vitro (Denker, A. E., Nicoll, S. B., and Tuan, R. S. (1995) Differentiation 59, 25-34; Haas, A. R., and Tuan, R. S. (1999) Differentiation 64, 77-89). Furthermore, BMP-2 treatment also results in an increased protein level of beta-catenin, a known N-cadherin-associated Wnt signal transducer (Fischer, L., Haas, A., and Tuan, R. S. (2001) Signal Transduction 2, 66-78), suggesting functional cross-talk between the BMP-2 and Wnt signaling pathways. We have observed previously that BMP-2 treatment up-regulates expression of Wnt-3A in high density cultures of C3H10T1/2 cells. To assess the contribution of Wnt-3A to BMP-2-mediated chondrogenesis, we have generated C3H10T1/2 cell lines overexpressing Wnt-3A and various forms of glycogen synthase kinase-3beta (GSK-3beta), an immediate cytosolic component of the Wnt signaling pathway, and examined their response to BMP-2. We show that overexpression of either Wnt-3A or kinase-dead GSK-3beta enhances BMP-2-mediated chondrogenesis. Furthermore, Wnt-3A overexpression results in decreases in both N-cadherin and GSK-3beta protein levels, whereas Wnt-3A as well as kinase-dead GSK-3beta overexpression increase total and nuclear levels of both beta-catenin and LEF-1. Direct cross-talk between Wnts and BMP-2 was also indicated by the up-regulated interaction between beta-catenin and SMAD-4 in response to BMP-2. These results suggest that Wnt-3A acts in a manner opposite to that of other Wnts, such as Wnt-7A, which were previously identified as inhibitory to chondrogenesis, and is the first BMP-2-regulated, chondrogenesis-enhancing member of the Wnt family.

The polycystic kidney disease-1 promoter is a target of the beta-catenin/T-cell factor pathway

Polycystic kidney disease (PKD) results from loss-of-function mutations in the PKD1 gene. There are also reports showing abnormally high levels of PKD1 expression in cystic epithelial cells. At present, nothing is known about the molecular mechanisms regulating the normal expression of the PKD1 gene or whether transcriptional disregulation of the PKD1 gene has a role in cyst formation. We have analyzed a 3.3-kb 5'-proximal portion of the human PKD1 gene. Sequence analysis revealed the presence of consensus sequences for numerous transactivating factors, including four T-cell factor (TCF) binding elements (TBEs). Transcriptional activity of the 3.3-kb fragment and a series of deletion constructs was assayed in HEK293T cells. A 2.0-kb proximal promoter region containing one of the four TBEs (TBE1) was inducible up to 6-fold by cotransfection with beta-catenin. beta-catenin-mediated induction was inhibited by dominant-negative TCF and by deletion of the TBE1 sequence. 15- or 109-bp sequences containing the TBE1 site, when cloned upstream of a minimal promoter, were shown to respond to beta-catenin induction. Gel shift assays confirmed that the TBE1 site is capable of forming complexes with TCF and beta-catenin. To determine whether expression of the endogenous PKD1 gene responds to beta-catenin, HT1080 cells were treated with LiCl, and HeLa cells were stably transfected with beta-catenin. In both cases, endogenous PKD1 mRNA levels were elevated in response to these treatments. Taken together, these studies define an active PKD1 promoter region and suggest that the PKD1 gene is a target of the beta-catenin/TCF pathway.

Molecular genetic analysis of malignant melanomas for aberrations of the WNT signaling pathway genes CTNNB1, APC, ICAT and BTRC

Aberrant activation of the Wnt signaling pathway has been reported in different human tumor types, including malignant melanomas. We investigated 37 malignant melanomas (15 primary tumors and 22 metastases) for alterations of 4 genes encoding members of this pathway, i.e., CTNNB1 (beta-catenin gene, 3p22.1), APC (adenomatous polyposis coli gene, 5q22.2), BTRC (beta-transducin repeat-containing protein gene, 10q24.3) and ICAT (inhibitor of beta-catenin and Tcf-4, 1p36.2). Mutational analysis of CTNNB1 identified somatic mutations in 1 primary melanoma and 1 melanoma metastasis from 2 different patients (5%). Both mutations affected the N-terminal degradation box of beta-catenin, which is important for the regulation of beta-catenin homeostasis. Another primary melanoma carried a somatic APC missense mutation within the known mutation cluster region in exon 15. Fourteen tumors (40%) showed LOH at microsatellite markers on 1p36. None of the tumors had lost both copies of the ICAT gene, but 1 melanoma metastasis carried a somatic point mutation altering the translation start codon of ICAT. Real-time RT-PCR showed markedly reduced ICAT transcript levels (<or=20% relative to normal skin and benign melanocytic nevi) in 28/36 malignant melanomas (78%), including 13/14 tumors with LOH on 1p36. Allelic loss on 10q was detected in 15 tumors (44%). We found neither mutations nor complete loss of expression of the BTRC gene in our melanoma series. Taken together, our results indicate that the Wnt pathway may be altered in malignant melanomas by different mechanisms, including rare somatic mutations in CTNNB1, APC or ICAT, as well as low or absent expression of ICAT transcripts.

Familial medulloblastoma: case report of one family and review of the literature

OBJECTIVE AND IMPORTANCE

Medulloblastoma is the most common malignant brain tumor and the most common malignant solid tumor in children. Most medulloblastomas are sporadic, but rare familial forms have been described. To the best of our knowledge, only 10 case reports of familial medulloblastoma have been published. A variety of candidate genes have been suggested to be involved in familial medulloblastomas. However, the exact pathogenesis and genetics involved in familial medulloblastoma remain unknown.

CLINICAL PRESENTATION

We describe the presentation of medulloblastoma in two siblings (one of each sex) and their great-uncle. The three cases differ with regard to age at onset and pathological subtype of medulloblastoma. INTERVENTION OR TECHNIQUE: Immunostaining of tissue blocks for gene products involved in medulloblastoma differed in the two siblings for beta-catenin and was similar with staining for gli.

CONCLUSION

This article is only the second report in the literature to address the genetics of familial medulloblastoma in the absence of characterized conditions such as Li-Fraumeni's cancer syndrome and basal cell nevus, Rubinstein-Taybi's, and Turcot's syndromes. The discrepancy in beta-catenin staining in the two siblings suggests that the two tumors differentiated through divergent pathways. We briefly summarize all published cases of familial medulloblastoma and review the literature on the genes involved in medulloblastoma formation.

Gene expression profile of adipocyte differentiation and its regulation by peroxisome proliferator-activated receptor-gamma agonists

PPAR gamma is an adipocyte-specific nuclear hormone receptor. Agonists of PPAR gamma, such as thiazolidinediones (TZDs), promote adipocyte differentiation and have insulin-sensitizing effects in animals and diabetic patients. Affymetrix oligonucleotide arrays representing 6347 genes were employed to profile the gene expression responses of mature 3T3-L1 adipocytes and differentiating preadipocytes to a TZD PPAR gamma agonist in vitro. The expression of 579 genes was significantly up- or down-regulated by more than 1.5-fold during differentiation and/or by treatment with TZD, and these genes were organized into 32 clusters that demonstrated concerted changes in expression of genes controlling cell growth or lipid metabolism. Quantitative PCR was employed to further characterize gene expression and led to the identification of beta-catenin as a new PPAR gamma target gene. Both mRNA and protein levels for beta-catenin were down-regulated in 3T3-L1 adipocytes compared with fibroblasts and were further decreased by treatment of adipocytes with PPAR gamma agonists. Treatment of db/db mice with a PPAR gamma agonist also resulted in reduction of beta-catenin mRNA levels in adipose tissue. These results suggest that beta-catenin plays an important role in the regulation of adipogenesis. Thus, the transcriptional patterns revealed in this study further the understanding of adipogenesis process and the function of PPAR gamma activation.

Wnt signaling during BMP-2 stimulation of mesenchymal chondrogenesis

Members of both the Wnt and bone morphogenetic protein (BMP) families of signaling molecules have been implicated in the regulation of cartilage development. A key component of the Wnt signaling pathway is the cytosolic protein, beta-catenin. We have recently shown that the chondrogenic activity of BMP-2 in vitro involves the action of the cell-cell adhesion protein, N-cadherin, which functionally complexes with beta-catenin. The aim of this study is to test the hypothesis that Wnts may be involved in BMP-2 induced chondrogenesis, using an in vitro model of high-density micromass cultures of the murine multipotent mesenchymal cell line, C3H10T1/2. Expression of a number of Wnt members was detected in these cultures, including Wnt-3A and Wnt-7A, whose levels were up- and downregulated, respectively, by BMP-2. To assess the functional involvement of Wnt signaling in BMP-2 induced chondrogenesis, cultures were treated with lithium chloride, a Wnt-7A mimetic that acts by inhibiting the serine/threonine phosphorylation activity of glycogen synthase kinase-3beta (GSK-3beta). Lithium treatment significantly inhibited BMP-2 stimulation of chondrogenesis as well as GSK-3beta enzymatic activity, and decreased the levels of N-cadherin protein and mRNA. Furthermore, lithium decreased BMP-2 upregulation of total and nuclear levels of LEF-1 and beta-catenin as well as their interaction during later chondrogenesis; similarly, the interaction of beta-catenin with N-cadherin was also decreased. Interestingly, lithium treatment did not affect the ability of BMP-2 to decrease ubiquitination of beta-catenin, although it did reduce the interaction of beta-catenin with GSK-3beta during late chondrogenesis (days 9-13). We suggest that the chondro-inhibitory effect of lithium on BMP-2 induced chondrogenesis indicates antagonism between lithium-like Wnts and BMP-2 during mesenchymal condensation.

Turn-off, drop-out: functional state switching of cadherins

The classic cadherins are a group of calcium dependent, homophilic cell-cell adhesion molecules that drive morphogenetic rearrangements and maintain the integrity of cell groups through the formation of adherens junctions. The formation and maintenance of cadherin-mediated adhesions is a multistep process and mechanisms have evolved to regulate each step. This suggests that functional state switching plays an important role in development. Among the many challenges ahead is to determine the developmental role that functional state switching plays in tissue morphogenesis and to define the roles of each of the several regulatory interactions that participate in switching. One correlate of the loss of cadherin-mediated adhesion, the "turn-off" of cadherin function, is the exit, or "drop-out" of cells from neural and epithelial layers and their conversion to a motile phenotype. We suggest that epithelial mesenchymal conversions may be initiated by signaling pathways that result in the loss of cadherin function. Tyrosine phosphorylation of beta-catenin is one such mechanism. Enhanced phosphorylation of tyrosine residues on beta-catenin is almost invariably associated with loss of the cadherin-actin connection concomitant with loss of adhesive function. There are several tyrosine kinases and phosphatases that have been shown to have the potential to alter the phosphorylation state of beta-catenin and thus the function of cadherins. Our laboratory has focused on the role of the nonreceptor tyrosine phosphatase PTP1B in regulating the phosphorylation of beta-catenin on tyrosine residues. Our data suggest that PTP1B is crucial for maintenance of N-cadherin-mediated adhesions in embryonic neural retina cells. By using an L-cell model system constitutively expressing N-cadherin, we have worked out many of the molecular interactions essential for this regulatory interaction. Extracellular cues that bias this critical regulatory interaction toward increased phosphorylation of beta-catenin may be a critical component of many developmental events.

Molecular cloning and initial characterization of the MG61/PORC gene, the human homologue of the Drosophila segment polarity gene Porcupine

Insect and vertebrate Porcupine genes encode multi-pass endoplasmic reticulum proteins involved in the processing of Wnt (wingless and int homologue) proteins, a class of secreted glycoprotein factors homologous to the Drosophila melanogaster segment polarity gene Wingless (Wg). Here we report the cloning of cDNAs encoding the human homologue of the Drosophila gene Porcupine (Porc), the characterization of its genomic structure and the quantitative analysis of its expression in a comprehensive panel of human tissues. The human Porcupine locus (MG61/PORC) spans 15 exons over approximately 12 kb of genomic sequence on Xp11.23. Real-time quantitative expression analysis reveals that MG61/PORC transcripts are expressed in multiple tissues, but are particularly abundant in the brain. Like its mouse and Xenopus homologues, MG61/PORC encodes four protein isoforms (A-D) generated through alternative splicing and expressed in a tissue-specific fashion. Finally, we present evidence indicating that MG61/PORC can influence the activity of a human Wnt7A expression construct in a T-cell factor-responsive reporter assay.

Nuclear localization of Duplin, a beta-catenin-binding protein, is essential for its inhibitory activity on the Wnt signaling pathway

Duplin binds to beta-catenin and inhibits the Wnt signaling pathway, thereby leading to repression of the beta-catenin-mediated transactivation and Xenopus axis formation. To find an additional function of Duplin, yeast two-hybrid screening was carried out. Importin alpha was isolated as a binding protein of Duplin. Importin alpha bound directly to basic amino acid clusters of Duplin. Although Duplin was present in the nucleus, deletion of the basic amino acid clusters (Duplin(Delta 500-584)) retained Duplin in the cytoplasm. Duplin(Delta 500-584) bound to beta-catenin as efficiently as wild-type Duplin, but it neither repressed Wnt-dependent Tcf transcriptional activation in mammalian cells nor showed ventralization in Xenopus embryos. The Duplin mutant without a beta-catenin-binding region lost the ability to inhibit the Wnt-dependent Tcf activation, but retained its ventralizing activity. Furthermore, Duplin not only suppressed beta-catenin-dependent axis duplication and expression of siamois, a Wnt-regulated gene, but also inhibited siamois-dependent axis duplication. These results indicate that Duplin is translocated to the nucleus by interacting with importin alpha, and that nuclear localization is essential for the function of Duplin. Moreover, Duplin has an additional activity of inhibiting the Wnt signaling pathway by affecting the downstream beta-catenin target genes.

A role for the beta-catenin/T-cell factor signaling cascade in vascular remodeling

Beta-catenin and T cell factor (Tcf) are distal components of the highly conserved Wnt pathway that govern cell fate and proliferation in lower organisms. Thus, we hypothesized that the regulation of beta-catenin and Tcf played a critical role in vascular remodeling. The first objective was to define beta-catenin expression in vascular smooth muscle cells (VSMCs) after balloon injury. Indeed, beta-catenin mRNA and protein were significantly elevated 7 days after balloon injury in the rat carotid artery. We hypothesized that beta-catenin accumulation in response to vascular injury inhibited VSMC apoptosis. In line with our hypothesis, transfection of a degradation-resistant beta-catenin transgene into rat VSMCs significantly inhibited apoptosis. Accumulation of beta-catenin also resulted in a 10-fold increase in the activation of Tcf. To test if Tcf was necessary to confer beta-catenin-induced survival, loss of function studies were carried out with a dominant negative Tcf-4 transgene lacking the beta-catenin binding domain, Tcf4(N31). Indeed, loss of Tcf-4 activity abolished beta-catenin-induced survival. We further postulated that beta-catenin and Tcf promoted cell cycle progression by activating cyclin D1, a target gene of Tcf-4. Beta-catenin activated cyclin D1, and this activation was partially blocked with loss of Tcf-4. In parallel, blockade of Tcf-4 resulted in inhibition of [3H]thymidine incorporation and partial blockade of the G1-S phase transition. In conclusion, beta-catenin and Tcf-4 play a dual role in vascular remodeling by inhibiting VSMC apoptosis and promoting proliferation.

Negative feedback loop of Wnt signaling through upregulation of conductin/axin2 in colorectal and liver tumors

Activation of Wnt signaling through beta-catenin/TCF complexes is a key event in the development of various tumors, in particular colorectal and liver tumors. Wnt signaling is controlled by the negative regulator conductin/axin2/axil, which induces degradation of beta-catenin by functional interaction with the tumor suppressor APC and the serine/threonine kinase GSK3beta. Here we show that conductin is upregulated in human tumors that are induced by beta-catenin/Wnt signaling, i.e., high levels of conductin protein and mRNA were found in colorectal and liver tumors but not in the corresponding normal tissues. In various other tumor types, conductin levels did not differ between tumor and normal tissue. Upregulation of conductin was also observed in the APC-deficient intestinal tumors of Min mice. Inhibition of Wnt signaling by a dominant-negative mutant of TCF downregulated conductin but not the related protein, axin, in DLD1 colorectal tumor cells. Conversely, activation of Wnt signaling by Wnt-1 or dishevelled increased conductin levels in MDA MB 231 and Neuro2A cells, respectively. In time course experiments, stabilization of beta-catenin preceded the upregulation of conductin by Wnt-1. These results demonstrate that conductin is a target of the Wnt signaling pathway. Upregulation of conductin may constitute a negative feedback loop that controls Wnt signaling activity.

Glycogen synthase kinase 3 phosphorylates kinesin light chains and negatively regulates kinesin-based motility

Membrane-bounded organelles (MBOs) are delivered to different domains in neurons by fast axonal transport. The importance of kinesin for fast antero grade transport is well established, but mechanisms for regulating kinesin-based motility are largely unknown. In this report, we provide biochemical and in vivo evidence that kinesin light chains (KLCs) interact with and are in vivo substrates for glycogen synthase kinase 3 (GSK3). Active GSK3 inhibited anterograde, but not retrograde, transport in squid axoplasm and reduced the amount of kinesin bound to MBOs. Kinesin microtubule binding and microtubule-stimulated ATPase activities were unaffected by GSK3 phosphorylation of KLCs. Active GSK3 was also localized preferentially to regions known to be sites of membrane delivery. These data suggest that GSK3 can regulate fast anterograde axonal transport and targeting of cargos to specific subcellular domains in neurons.

Cell adhesion molecules and adhesion abnormalities in prostate cancer

Prostate cancer, the leading male cancer in Western countries, has accelerated in its incidence in the past decade. Patients with prostate cancer frequently have a poor prognosis as a result of local or distant spread of cancer. This review summarises some of the recent progress made in understanding the biology of cancer metastasis with a special emphasis on the role of cell adhesion molecules and adhesion abnormalities. The molecular and cellular function of cell adhesion molecules, their role in cancer and cancer progression, the clinical impact of these molecules, and therapeutic considerations are also discussed.

Insight into hepatocellular carcinogenesis at transcriptome level by comparing gene expression profiles of hepatocellular carcinoma with those of corresponding noncancerous liver

Human hepatocellular carcinoma (HCC) is one of the most common cancers worldwide. In this work, we report on a comprehensive characterization of gene expression profiles of hepatitis B virus-positive HCC through the generation of a large set of 5'-read expressed sequence tag (EST) clusters (11,065 in total) from HCC and noncancerous liver samples, which then were applied to a cDNA microarray system containing 12,393 genes/ESTs and to comparison with a public database. The commercial cDNA microarray, which contains 1,176 known genes related to oncogenesis, was used also for profiling gene expression. Integrated data from the above approaches identified 2,253 genes/ESTs as candidates with differential expression. A number of genes related to oncogenesis and hepatic function/differentiation were selected for further semiquantitative reverse transcriptase-PCR analysis in 29 paired HCC/noncancerous liver samples. Many genes involved in cell cycle regulation such as cyclins, cyclin-dependent kinases, and cell cycle negative regulators were deregulated in most patients with HCC. Aberrant expression of the Wnt-beta-catenin pathway and enzymes for DNA replication also could contribute to the pathogenesis of HCC. The alteration of transcription levels was noted in a large number of genes implicated in metabolism, whereas a profile change of others might represent a status of dedifferentiation of the malignant hepatocytes, both considered as potential markers of diagnostic value. Notably, the altered transcriptome profiles in HCC could be correlated to a number of chromosome regions with amplification or loss of heterozygosity, providing one of the underlying causes of the transcription anomaly of HCC.

The p85 subunit of phosphoinositide 3-kinase is associated with beta-catenin in the cadherin-based adhesion complex

Cell adhesion is fundamental to establishing and maintaining the discrete tissues in multicellular organisms. Adhesion must be sufficiently strong to preserve tissue architecture, whilst having the capacity to readily dissociate to permit fundamental processes, such as wound repair, to occur. However, very little is known about the signalling mechanisms involved in temporary down-regulation of cell adhesion to facilitate such processes. Cadherins are the principal mediators of cell-cell adhesion in a wide variety of tissues and species and form multi-protein complexes with cytosolic and cytoskeletal proteins to express their full adhesive capacity. In the present study we report that the p85 subunit of phosphoinositide 3-kinase (PI 3-kinase) is associated with the cadherin-based adhesion complex in human epithelial cells. The interaction of p85 with the complex is via beta-catenin. We also show that the interaction of p85 and beta-catenin is direct, involves the N-terminal Src homology domain 2 of p85 and is regulated by tyrosine phosphorylation. These data suggest that PI 3-kinase may play a role in the functional regulation of the cadherin-based adhesion complex.

Foxl1 controls the Wnt/beta-catenin pathway by modulating the expression of proteoglycans in the gut

Foxl1 is a winged helix transcription factor expressed in the mesenchyme of the gastrointestinal tract. Foxl1 null mice display severe structural defects in the epithelia of the stomach, duodenum, and jejunum. Here we addressed the molecular mechanisms by which Foxl1 controls gastrointestinal differentiation. First we showed that the abnormalities found in the epithelia of the null mice are the result of an increase in the number of proliferating cells and not a change in the rate of cell migration. Next we investigated the regulatory circuits affected by Foxl1. We focused on the Wnt/beta-catenin signaling pathway as a possible target of Foxl1 as it has been shown to play a central role in gastrointestinal proliferation. We demonstrated that Foxl1 activates the Wnt/beta-catenin pathway by increasing extracellular proteoglycans, which act as co-receptors for Wnt. Thus we establish that Foxl1 is involved in the regulation of the Wnt/beta-catenin pathway, providing a novel link in mesenchymal/epithelial cross-talk in the gut. Moreover, we provide the first example implicating proteoglycans in the regulation of cellular proliferation in the gastrointestinal tract.

The multifaceted roles of glycogen synthase kinase 3beta in cellular signaling

Glycogen synthase kinase-3beta (GSK3beta) is a fascinating enzyme with an astoundingly diverse number of actions in intracellular signaling systems. GSK3beta activity is regulated by serine (inhibitory) and tyrosine (stimulatory) phosphorylation, by protein complex formation, and by its intracellular localization. GSK3beta phosphorylates and thereby regulates the functions of many metabolic, signaling, and structural proteins. Notable among the signaling proteins regulated by GSK3beta are the many transcription factors, including activator protein-1, cyclic AMP response element binding protein, heat shock factor-1, nuclear factor of activated T cells, Myc, beta-catenin, CCAAT/enhancer binding protein, and NFkappaB. Lithium, the primary therapeutic agent for bipolar mood disorder, is a selective inhibitor of GSK3beta. This raises the possibility that dysregulation of GSK3beta and its inhibition by lithium may contribute to the disorder and its treatment, respectively. GSK3beta has been linked to all of the primary abnormalities associated with Alzheimer's disease. These include interactions between GSK3beta and components of the plaque-producing amyloid system, the participation of GSK3beta in phosphorylating the microtubule-binding protein tau that may contribute to the formation of neurofibrillary tangles, and interactions of GSK3beta with presenilin and other Alzheimer's disease-associated proteins. GSK3beta also regulates cell survival, as it facilitates a variety of apoptotic mechanisms, and lithium provides protection from many insults. Thus, GSK3beta has a central role regulating neuronal plasticity, gene expression, and cell survival, and may be a key component of certain psychiatric and neurodegenerative diseases.

The role of protein phosphorylation in human health and disease. The Sir Hans Krebs Medal Lecture

The reversible phosphorylation of proteins regulates almost all aspects of cell life, while abnormal phosphorylation is a cause or consequence of many diseases. Mutations in particular protein kinases and phosphatases gives rise to a number of disorders and many naturally occurring toxins and pathogens exert their effects by altering the phosphorylation states of intracellular proteins. In this lecture, I present an overview of the progress that is being made in developing specific inhibitors of protein kinases for the treatment of cancer and chronic inflammatory diseases and describe how recent advances in our understanding of the specificity and regulation of one particular protein kinase (GSK3) may facilitate the development of drugs to treat diabetes that would not have the potential to be oncogenic. I also discuss the exploitation of specific protein kinase inhibitors for the study of cell signalling and make recommendations for their effective use in cell-based assays.

GSK3 takes centre stage more than 20 years after its discovery

Identified originally as a regulator of glycogen metabolism, glycogen synthase kinase-3 (GSK3) is now a well-established component of the Wnt signalling pathway, which is essential for setting up the entire body pattern during embryonic development. It may also play important roles in protein synthesis, cell proliferation, cell differentiation, microtubule dynamics and cell motility by phosphorylating initiation factors, components of the cell-division cycle, transcription factors and proteins involved in microtubule function and cell adhesion. Generation of the mouse knockout of GSK3beta, as well as studies in neurons, also suggest an important role in apoptosis. The substrate specificity of GSK3 is unusual in that efficient phosphorylation of many of its substrates requires the presence of another phosphorylated residue optimally located four amino acids C-terminal to the site of GSK3 phosphorylation. Recent experiments, including the elucidation of its three-dimensional structure, have enhanced our understanding of the molecular basis for the unique substrate specificity of GSK3. Insulin and growth factors inhibit GSK3 by triggering its phosphorylation, turning the N-terminus into a pseudosubstrate inhibitor that competes for binding with the 'priming phosphate' of substrates. In contrast, Wnt proteins inhibit GSK3 in a completely different way, by disrupting a multiprotein complex comprising GSK3 and its substrates in the Wnt signalling pathway, which do not appear to require a 'priming phosphate'. These latest findings have generated an enormous amount of interest in the development of drugs that inhibit GSK3 and which may have therapeutic potential for the treatment of diabetes, stroke and Alzheimer's disease.

The renaissance of GSK3

Glycogen synthase kinase 3 (GSK3) was initially described as a key enzyme involved in glycogen metabolism, but is now known to regulate a diverse array of cell functions. The study of the substrate specificity and regulation of GSK3 activity has been important in the quest for therapeutic intervention.

Heads or tails: Wnts and anterior-posterior patterning

Cell-cell communication is critical during embryogenesis for organizing the vertebrate body plan. Members of the Wnt family of secreted signaling molecules possess axis-inducing and posteriorizing activity when overexpressed. Wnt signals are modulated extracellularly by a diverse group of secreted Wnt antagonists and cofactors. Recent work has revealed that inhibition of posteriorly localized Wnt signaling by anteriorly localized Wnt antagonists is critical for inducing the anterior structures, forebrain and heart, from neural ectoderm and mesoderm, respectively. This review centers on the role that Wnts and Wnt antagonists play in the patterning of the vertebrate anterior-posterior axis.

Protein phosphatase 2A and its B56 regulatory subunit inhibit Wnt signaling in Xenopus

Wnt signaling increases beta-catenin abundance and transcription of Wnt-responsive genes. Our previous work suggested that the B56 regulatory subunit of protein phosphatase 2A (PP2A) inhibits Wnt signaling. Okadaic acid (a phosphatase inhibitor) increases, while B56 expression reduces, beta-catenin abundance; B56 also reduces transcription of Wnt-responsive genes. Okadaic acid is a tumor promoter, and the structural A subunit of PP2A is mutated in multiple cancers. Taken together, the evidence suggests that PP2A is a tumor suppressor. However, other studies suggest that PP2A activates Wnt signaling. We now show that the B56, A and catalytic C subunits of PP2A each have ventralizing activity in Xenopus embryos. B56 was epistatically positioned downstream of GSK3beta and axin but upstream of beta-catenin, and axin co-immunoprecipitated B56, A and C subunits, suggesting that PP2A:B56 is in the beta-catenin degradation complex. PP2A appears to be essential for beta-catenin degradation, since beta-catenin degradation was reconstituted in phosphatase-depleted Xenopus egg extracts by PP2A, but not PP1. These results support the hypothesis that PP2A:B56 directly inhibits Wnt signaling and plays a role in development and carcinogenesis.

Novel mechanism of Wnt signalling inhibition mediated by Dickkopf-1 interaction with LRP6/Arrow

Wnt signalling has an important role in cell fate determination, tissue patterning and tumorigenesis. Secreted antagonists of Wnt include Frizzled (Fz)-related proteins (FRPs), Cerberus, Wnt inhibitory factor (WIF) and Dickkopf (Dkk). FRPs, Cerberus and WIF have all been shown to act by binding and sequestering Wnt. We report a novel mechanism of Wnt-signalling inhibition by human Dkk-1. Dkk-1 demonstrated no interaction with Wnt but bound a single cell surface site with high affinity (K(D) = 0.39 nM). Its receptor was detectable in a complex with a relative molecular mass of 240,000 (M(r) 240K) with [(125)I] Dkk-1 by covalent affinity cross-linking. Wnt signalling through beta-catenin is mediated by the Fz receptor and a recently identified low-density-lipoprotein-receptor-related co-receptor, LRP6/Arrow. Overproduction of the 200K LRP6 protein, but not of Fz, strikingly increased Dkk-1 binding as well as the amount of the 240K cross-linked complex, which was shown to be composed of Dkk-1 and LRP6. Moreover, Dkk-1 function was completely independent of Fz but LRP6 dramatically interfered with the Dkk-1 inhibition of Wnt signalling. Thus, unlike Wnt antagonists, which exert their effects by molecular mimicry of Fz or Wnt sequestration through other mechanisms, Dkk-1 specifically inhibits canonical Wnt signalling by binding to the LRP6 component of the receptor complex.

Characterization of mouse Frizzled-3 expression in hair follicle development and identification of the human homolog in keratinocytes

Frizzled genes encode a family of Wnt ligand receptors, which have a conserved cysteine-rich Wnt binding domain and include both transmembrane and secreted forms. Work by others has shown that experimental perturbation of Wnt signaling results in aberrant hair formation, hair growth, and hair structure. To date, however, there is no information on the contribution of individual Frizzled proteins to hair development. We now report that Frizzled-3 expression in skin is restricted to the epidermis and to the developing hair follicle. Northern analysis on total mouse skin mRNA revealed a single Frizzled-3 transcript of 3.7 kb. Reverse transcription-polymerase chain reaction and in situ hybridization analysis revealed Frizzled-3 expression in epidermal and hair follicle keratinocytes. Frizzled-3 transcripts are first detected in discrete foci in the developing epidermis of 13 d embryos and later in the hair follicle placodes of 15 d embryos, suggesting a role for this Frizzled isoform in follicle development. In 17 d embryos and 1 d old newborn mice Frizzled-3 expression is limited to suprabasal keratinocytes and is not seen in pelage follicles until 3 d postpartum. In 7 d old neonatal skin, Frizzled-3 is expressed throughout the epidermis and in the outer cell layers of hair follicles. We have also identified the mRNA encoding human Frizzled-3 in epidermal keratinocytes and in the HaCaT keratinocyte cell line. Human Frizzled-3 mRNA encodes a 666 amino acid protein with 97.8% identity to the mouse protein. The human Frizzled-3 gene was mapped using a radiation-hybrid cell line panel to the short arm of chromosome 8 between the markers WI-1172 and WI-8496 near the loci for the Hypotrichosis of Marie Unna and Hairless genes.

E-cadherin suppresses cellular transformation by inhibiting beta-catenin signaling in an adhesion-independent manner

E-cadherin is a tumor suppressor protein with a well-established role in cell-cell adhesion. Adhesion could contribute to tumor suppression either by physically joining cells or by facilitating other juxtacrine signaling events. Alternatively, E-cadherin tumor suppressor activity could result from binding and antagonizing the nuclear signaling function of beta-catenin, a known proto-oncogene. To distinguish between an adhesion- versus a beta-catenin signaling-dependent mechanism, chimeric cadherin constructs were expressed in the SW480 colorectal tumor cell line. Expression of wild-type E-cadherin significantly inhibits the growth of this cell line. Growth inhibitory activity is retained by all constructs that have the beta-catenin binding region of the cytoplasmic domain but not by E-cadherin constructs that exhibit adhesive activity, but lack the beta-catenin binding region. This growth suppression correlates with a reduction in beta-catenin/T cell factor (TCF) reporter gene activity. Importantly, direct inhibition of beta-catenin/TCF signaling inhibits the growth of SW480 cells, and the growth inhibitory activity of E-cadherin is rescued by constitutively activated forms of TCF. Thus, the growth suppressor activity of E-cadherin is adhesion independent and results from an inhibition of the beta-catenin/TCF signaling pathway, suggesting that loss of E-cadherin expression can contribute to upregulation of this pathway in human cancers. E-cadherin-mediated growth suppression was not accompanied by overall depletion of beta-catenin from the cytosol and nucleus. This appears to be due to the existence of a large pool of cytosolic beta-catenin in SW480 cells that is refractory to both cadherin binding and TCF binding. Thus, a small pool of beta-catenin that can bind TCF (i.e., the transcriptionally active pool) can be selectively depleted by E-cadherin expression. The existence of functionally distinct pools of cytosolic beta-catenin suggests that there are mechanisms to regulate beta-catenin signaling in addition to controlling its level of accumulation.

Lipopolysaccharide activates Akt in human alveolar macrophages resulting in nuclear accumulation and transcriptional activity of beta-catenin

Exposure of human alveolar macrophages to bacterial LPS results in activation of a number of signal transduction pathways. An early event after the alveolar macrophage comes in contact with LPS is activation of the phosphatidylinositol 3 kinase (PI 3-kinase). This study evaluates the downstream effects of that activation. We observed that LPS exposure results in phosphorylation of Akt (serine 473). We found this using both phosphorylation-specific Abs and also by in vivo phosphorylation with (32)P-loaded cells. AKT activation resulted in the phosphorylation-dependent inactivation of glycogen synthase kinase (GSK-3) (serine 21/9). We found that both of these events were linked to PI 3-kinase because the PI 3-kinase inhibitors, wortmannin and LY294002, inhibited LPS-induced phosphorylation of both AKT and GSK-3. Inactivation of GSK-3 has been shown to reduce the ubiquitination of beta-catenin, resulting in nuclear accumulation and transcriptional activity of beta-catenin. Consistent with this, we found that LPS caused an increase in the amounts of PI 3-kinase-dependent nuclear beta-catenin in human alveolar macrophages and expression of genes that require nuclear beta-catenin for their activation. This is the first demonstration that LPS exposure activates AKT, inactivates GSK-3, and causes accumulation and transcriptional activity of beta-catenin in the nucleus of any cell, including alveolar macrophages.

Thermodynamics of the high-affinity interaction of TCF4 with beta-catenin

The formation of a complex between beta-catenin and members of the TCF/LEF family of high-mobility group proteins is a key regulatory event in the wnt-signaling pathway, essential for embryonal development as well as the growth of normal and malignant colon epithelium. We have characterized the binding of TCF4 to human beta-catenin by steady-state intrinsic fluorescence quenching experiments, surface plasmon resonance (SPR) and isothermal titration calorimetry (ITC). Binding studies in solution and in heterogeneous phase showed that TCF4 binds reversibly to beta-catenin with an affinity (KB) of 3(+/-1) 10(8) M(-1). Site-directed mutagenesis, together with calorimetric measurements, revealed that residue D16 in TCF4 plays a crucial role in high-affinity binding. Mutation of this residue to alanine resulted in a decrease of KB by two orders of magnitude as well as a significant reduction in binding enthalpy. Binding of TCF4 to beta-catenin gave rise to a large negative enthalpy change at 25 degrees C (-29.7 kcal/mol). Binding enthalpies were strongly temperature dependent, which resulted in the determination of a large heat capacity change upon binding of -1.5 kcal/(mol K). The molecular events that take place upon complex formation are discussed using the measured thermodynamic data together with the crystal structure of the beta-catenin arm repeat region/TCF complex.

Binding of the adenomatous polyposis coli protein to microtubules increases microtubule stability and is regulated by GSK3 beta phosphorylation

Truncation mutations in the adenomatous polyposis coli protein (APC) are responsible for familial polyposis, a form of inherited colon cancer. In addition to its role in mediating beta-catenin degradation in the Wnt signaling pathway, APC plays a role in regulating microtubules. This was suggested by its localization to the end of dynamic microtubules in actively migrating areas of cells and by the apparent correlation between the dissociation of APC from polymerizing microtubules and their subsequent depolymerization [1, 2]. The microtubule binding domain is deleted in the transforming mutations of APC [3, 4]; however, the direct effect of APC protein on microtubules has never been examined. Here we show that binding of APC to microtubules increases microtubule stability in vivo and in vitro. Deleting the previously identified microtubule binding site from the C-terminal domain of APC does not eliminate its binding to microtubules but decreases the ability of APC to stabilize them significantly. The interaction of APC with microtubules is decreased by phosphorylation of APC by GSK3 beta. These data confirm the hypothesis that APC is involved in stabilizing microtubule ends. They also suggest that binding of APC to microtubules is mediated by at least two distinct sites and is regulated by phosphorylation.

Crystal structure of a beta-catenin/Tcf complex

The Wnt signaling pathway plays critical roles in embryonic development and tumorigenesis. Stimulation of the Wnt pathway results in the accumulation of a nuclear beta-catenin/Tcf complex, activating Wnt target genes. A crystal structure of beta-catenin bound to the beta-catenin binding domain of Tcf3 (Tcf3-CBD) has been determined. The Tcf3-CBD forms an elongated structure with three binding modules that runs antiparallel to beta-catenin along the positively charged groove formed by the armadillo repeats. Structure-based mutagenesis defines three sites in beta-catenin that are critical for binding the Tcf3-CBD and are differentially involved in binding APC, cadherin, and Axin. The structural and mutagenesis data reveal a potential target for molecular drug design studies.

Purification of GSK-3 by affinity chromatography on immobilized axin

Glycogen synthase kinase 3 (GSK-3), an element of the Wnt signalling pathway, plays a key role in numerous cellular processes including cell proliferation, embryonic development, and neuronal functions. It is directly involved in diseases such as cancer (by controlling apoptosis and the levels of beta-catenin and cyclin D1), Alzheimer's disease (tau hyperphosphorylation), and diabetes (as a downstream element of insulin action, GSK-3 regulates glycogen and lipid synthesis). We describe here a rapid and efficient method for the purification of GSK-3 by affinity chromatography on an immobilized fragment of axin. Axin is a docking protein which interacts with GSK-3ss, beta-catenin, phosphatase 2A, and APC. A polyhistidine-tagged axin peptide (residues 419-672) was produced in Escherichia coli and either immobilized on Ni-NTA agarose beads or purified and immobilized on CNBr-activated Sepharose 4B. These "Axin-His6" matrices were found to selectively bind recombinant rat GSK-3 beta and native GSK-3 from yeast, sea urchin embryos, and porcine brain. The affinity-purified enzymes displayed high kinase activity. This single step purification method provides a convenient tool to follow the status of GSK-3 (protein level, phosphorylation state, kinase activity) under various physiological settings. It also provides a simple and efficient way to purify large amounts of active recombinant or native GSK-3 for screening purposes.

E-cadherin complex and its abnormalities in human breast cancer

E-cadherin and its adhesion complex play an essential role in the adhesion of breast cancer cells and tissues. Members of the complex, such as beta-catenin, act as regulators on cell adhesion, and are involved in cell signalling and transcription regulation. The adhesion complex is a known structure in the control of tumour progression and metastasis. Recent years have seen a rapid expansion in the understanding of the biology and clinical relevance of the complex in breast cancer. The current article summarises recent progresses in the molecular/cellular biology of E-cadherin and its complex and the clinical, diagnostic, prognostic, and therapeutic value of this complex in breast cancer.

Plakoglobin and beta-catenin: protein interactions, regulation and biological roles

Beta-catenin can play different roles in the cell, including one as a structural protein at cell-cell adherens junctions and another as a transcriptional activator mediating Wnt signal transduction. Plakoglobin (gamma)-catenin), a close homolog of beta-catenin, shares with beta-catenin common protein partners and can fulfill some of the same functions. The complexing of catenins with various protein partners is regulated by phosphorylation and by intramolecular interactions. The competition between different catenin partners for binding to catenins mediates the cross-talk between cadherin-based adhesion, catenin-dependent transcription and Wnt signaling. Although plakoglobin differs from beta-catenin in its functions and is unable to compensate for defects in Wnt signaling resulting from lack of beta-catenin, recent evidence suggests that plakoglobin plays a unique role in Wnt signaling that is different from that of beta-catenin. The functional difference between catenins is reflected in their differential involvement in embryonic development and cancer progression.

Identification, gene structure, and expression of human frizzled-3 (FZD3)

We report the identification, genomic structure, chromosomal localization, and expression analysis of human frizzled-3 (FZD3), a 7-transmembrane receptor belonging to the frizzled family. The cDNA obtained from adult human brain shows 91% identity at the nucleotide level and 98% at the amino acid level to mouse frizzled-3 (fzd3). The FZD3 locus is located on chromosome 8p21, spans 48 Kb and its coding sequence is distributed in 6 exons intercalated by 5 introns. FZD3 is expressed in all analyzed human tissues, with quantitatively higher expression in the CNS and in urogenital structures.