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Mammalian TCF protein interactions

In a yeast two-hybrid screen, the architectural transcription factor LEF-1 (for lymphoid enhancer-binding factor) interacts with beta-catenin. In mammalian cells, coexpressed LEF-1 and beta-catenin form a complex that is localized to the nucleus and can be detected by immunoprecipitation. LEF-1 and beta-catenin form a ternary complex with DNA that displays an altered DNA bend. Microinjection of LEF-1 into Xenopus embryos induces axis duplication, which is augmented by interaction with beta-catenin. Thus beta-catenin regulates gene expression by direct interaction with transcription factors such as LEF-1, providing a molecular mechanism for the transmission of signals to the nucleus either from cell-adhesion components or from WNT protein (Behrens, 1996).

The integrin-linked kinase (ILK) is an ankyrin repeat containing serine-threonine protein kinase that can interact directly with the cytoplasmic domains of the beta1 and beta3 integrin subunits (Drosophila homolog: Myospheroid) and whose kinase activity is modulated by cell-extracellular matrix interactions. Overexpression of constitutively active ILK results in loss of cell-cell adhesion, anchorage-independent growth, and tumorigenicity in nude mice. Modest overexpression of ILK in intestinal epithelial cells as well as in mammary epithelial cells results in an invasive phenotype concomitant with a down-regulation of E-cadherin expression, translocation of beta-catenin to the nucleus, formation of a complex between beta-catenin and the high mobility group transcription factor, LEF-1, and transcriptional activation by this LEF-1/beta-catenin complex. LEF-1 protein expression is rapidly modulated by cell detachment from the extracellular matrix, and LEF-1 protein levels are constitutively up-regulated at ILK overexpression. These effects are specific for ILK, because transformation by activated H-ras or v-src oncogenes do not result in the activation of LEF-1/beta-catenin. The results demonstrate that the oncogenic properties of ILK involve activation of the LEF-1/beta-catenin signaling pathway, and also suggest ILK-mediated cross-talk between cell-matrix interactions and cell-cell adhesion as well as components of the Wnt signaling pathway (Novak, 1998).

Control of the nuclear localization of specific proteins is an important mechanism for regulating many signal transduction pathways. When the Wnt signaling pathway is activated, beta-catenin localizes to the nucleus and interacts with TCF/LEF-1 (T-cell factor/lymphocyte enhancer factor-1) transcription factors, triggering activation of downstream genes. The role of regulated nuclear localization in beta-catenin signaling is still unclear. beta-catenin has no nuclear localization sequence (NLS). Although it has been reported that beta-catenin can piggyback into the nucleus by binding to TCF/LEF-1, there is evidence that in vivo its import is independent of TCF/LEF-1. Therefore, the mechanism for beta-catenin nuclear localization remains to be established. Beta-catenin nuclear import has been analyzed in an in vitro assay using permeabilized cells. Beta-catenin docks specifically onto the nuclear envelope in the absence of other cytosolic factors. Docking is not inhibited by an NLS peptide and does not require importins/karyopherins, the receptors for classical NLS substrates. Rather, docking is specifically competed by importin-beta/beta-karyopherin, indicating that beta-catenin and importin-beta/beta-karyopherin both interact with common nuclear pore components. Nuclear translocation of beta-catenin is energy dependent and is inhibited by nonhydrolyzable GTP analogs and by a dominant-negative mutant form of the Ran GTPase. Cytosol preparations contain inhibitory activities for beta-catenin import that are distinct from the competition by importin-beta/beta-karyopherin and may be involved in the physiological regulation of the pathway. Beta-catenin is imported into the nucleus by binding directly to the nuclear pore machinery, similar to importin-beta/beta-karyopherin or other importin-beta-like import factors, such as transportin. These findings provide an explanation for how beta-catenin localizes to the nucleus without an NLS, independent of its interaction with TCF/LEF-1. This is a new and unusual mechanism for the nuclear import of a signal transduction protein. The lack of beta-catenin import activity in the presence of normal cytosol suggests that its import may be regulated by upstream events in the Wnt signaling pathway (Fagotto, 1998).

Wnt signaling is thought to be mediated via interactions between beta-catenin and members of the LEF-1/TCF family of transcription factors. The mechanism of transcriptional regulation by LEF-1 in response to a Wnt-1 signal has been studied under conditions of endogenous beta-catenin in NIH 3T3 cells, and an examination made as to whether association with beta-catenin is obligatory for the function of LEF-1. Wnt-1 signaling confers transcriptional activation potential on LEF-1 by association with beta-catenin in the nucleus. By mutagenesis, specific residues in LEF-1 have been identified that are important for interaction with beta-catenin. The amino-terminal 56 residues of LEF-1 are necessary and sufficient for mediating the interaction with beta-catenin. Two transcriptional activation domains in beta-catenin have been delineated whose function is augmented in specific association with LEF-1. To examine whether the carboxyl terminus of beta-catenin is the sole determinant for transcriptional activation in association with LEF-1, the deletion mutant delta C was generated. Expression of this mutant beta-catenin with LEF-1 in transfected Neuro2A cells stimulates the activity of the LEF-CAT reporter gene to a level similar to that obtained with the wild-type beta-catenin. Since this deletion removes the epitope recognized by the anti-beta-catenin antibody, the accumulation of the mutant protein cannot be compared to that of the wt protein. Nevertheless, the data suggest that beta-catenin may contain additional sequences that contribute to transcriptional activation (Hsu, 1998).

Alternatively, overexpression of exogenous forms of beta-catenin may result in an increase in the pool of free cytosolic endogenous beta-catenin, which would obscure the mapping of transcriptional activation domains. To identify transcriptional activation domains in beta-catenin, fusion proteins were generated in which various portions of beta-catenin were linked to the GAL4 DNA-binding domain. Expression of GAL4 fusion proteins containing either the amino- or carboxyl-terminal region of beta-catenin stimulate transcription of a GAL-CAT reporter gene in transfected Neuro2A cells by a factor of either 8 or 14, respectively. In comparison, a 50-fold activation of the reporter gene is observed with a GAL4-VP16 expression plasmid in which the transcriptional activation domain of the viral protein VP16 is linked to GAL4 as a positive control. A similar accumulation of both GAL4-beta-catenin fusion proteins was confirmed by electrophoretic mobility shift assays with nuclear extracts from COS cells transfected with the corresponding expression plasmids. Thus, beta-catenin contains two distinct transcription activation domains that can function in a heterologous context. However, the levels of transcriptional activation by the GAL4-beta-catenin fusion proteins are significantly lower than those observed in cotransfections of Neuro2A cells with LEF-1 and beta-catenin, raising the possibility that the transcriptional activation domains of beta-catenin collaborate with one another or with LEF-1. A Wnt-1 signal and beta-catenin association are not required for the architectural function of LEF-1 in the regulation of the T-cell receptor alpha enhancer, which involves association of LEF-1 with a different cofactor, ALY. Thus, LEF-1 can assume diverse regulatory functions by association with different proteins (Hsu, 1998).

The effect of N-cadherin, and its free or membrane-anchored cytoplasmic domain, was studied to determine the level and localization of beta-catenin and to assess its ability to induce lymphocyte enhancer-binding factor 1 (LEF-1)-responsive transactivation. These cadherin derivatives form complexes with beta-catenin and protect it from degradation. N-cadherin directs beta-catenin into adherens junctions, and the chimeric protein induces diffuse distribution of beta-catenin along the membrane whereas the cytoplasmic domain of N-cadherin colocalizes with beta-catenin in the nucleus. Cotransfection of beta-catenin and LEF-1 into Chinese hamster ovary cells induces transactivation of a LEF-1 reporter, which is blocked by the N-cadherin-derived molecules. Expression of N-cadherin and an interleukin 2 receptor/cadherin chimera in SW480 cells relocates beta-catenin from the nucleus to the plasma membrane and reduces transactivation. The cytoplasmic tails of N- or E-cadherin colocalize with beta-catenin in the nucleus, and suppress the constitutive LEF-1-mediated transactivation, by blocking beta-catenin-LEF-1 interaction. Moreover, the 72 C-terminal amino acids of N-cadherin stabilize beta-catenin and reduce its transactivation potential. These results indicate that beta-catenin binding to the cadherin cytoplasmic tail either in the membrane, or in the nucleus, can inhibit beta-catenin degradation and efficiently block its transactivation capacity (Sadot, 1998).

beta-catenin is a multifunctional protein found in three cell compartments: the plasma membrane, the cytoplasm and the nucleus. The cell has developed elaborate ways of regulating the level and localization of beta-catenin to assure its specific function in each compartment. One aspect of this regulation is inherent in the structural organization of beta-catenin itself; most of its protein-interacting motifs overlap so that interaction with one partner can block binding of another at the same time. Using recombinant proteins, it was found that E-cadherin and lymphocyte-enhancer factor-1 (LEF-1) form mutually exclusive complexes with beta-catenin; the association of beta-catenin with LEF-1 is competed out by the E-cadherin cytoplasmic domain. Similarly, LEF-1 and adenomatous polyposis coli (APC) form separate, mutually exclusive complexes with beta-catenin. In Wnt-1-transfected C57MG cells, free beta-catenin accumulates and is able to associate with LEF-1. The absence of E-cadherin in E-cadherin minus embryonic stem (ES) cells also leads to an accumulation of free beta-catenin and its association with LEF-1, thereby mimicking Wnt signaling. beta-catenin/LEF-1-mediated transactivation in these cells is antagonized by transient expression of wild-type E-cadherin, but not of E-cadherin lacking the beta-catenin binding site. The potent ability of E-cadherin to recruit beta-catenin to the cell membrane and prevent its nuclear localization and transactivation has also been demonstrated using SW480 colon carcinoma cells (Orsulic, 1999).

The transforming growth factor-beta (TGFbeta) and Wnt/wingless pathways play pivotal roles in tissue specification during development. Activation of Smads, the effectors of TGFbeta superfamily signals, results in Smad translocation from the cytoplasm into the nucleus where they act as transcriptional comodulators to regulate target gene expression. Wnt/wingless signals are mediated by the DNA-binding HMG box transcription factors lymphoid enhancer binding factor 1/T cell-specific factor (LEF1/TCF) and their coactivator beta-catenin. Smad3 is shown to physically interact with the HMG box domain of LEF1 and TGFbeta and Wnt pathways synergize to activate transcription of the Xenopus homeobox gene twin (Xtwn). Disruption of specific Smad and LEF1/TCF DNA-binding sites in the promoter abrogates synergistic activation of the promoter. Consistent with this observation, introduction of Smad sites into a TGFbeta-insensitive LEF1/TCF target gene confers cooperative TGFbeta and Wnt responsiveness to the promoter. Furthermore, TGFbeta-dependent activation of LEF1/TCF target genes can occur in the absence of beta-catenin binding to LEF1/TCF and requires both Smad and LEF1/TCF DNA-binding sites in the Xtwn promoter. Thus, these results show that TGFbeta and Wnt signaling pathways can independently or cooperatively regulate LEF1/TCF target genes and suggest a model for how these pathways can synergistically activate target genes (Labbe, 2000).

In certain cancers, constitutive Wnt signaling results from mutation in one or more pathway components. The result is the accumulation and nuclear localization of ß-catenin, which interacts with the lymphoid enhancer factor-1 (LEF)/T-cell factor (TCF) family of HMG-box transcription factors, which activate important growth regulatory genes, including cyclin D1 and c-myc. As exemplified by APC and axin, the negative regulation of ß-catenin is important for tumor suppression. Another potential mode of negative regulation is transcriptional repression of cyclin D1 and other Wnt target genes. In mammals, the transcriptional repressors in the Wnt pathway are not well defined. HBP1 is an HMG-box repressor and a cell cycle inhibitor. HBP1 is a known repressor of the cyclin D1 gene and inhibits the Wnt signaling pathway. HBP1 has been shown to interact with retinoblastoma family members. A subset of HBP1 DNA binding sites are similar to DNA sequences for LEF/TCF proteins and suggest that HBP1 might repress LEF/TCF target genes. The inhibition of Wnt signaling and growth requires a common domain of HBP1. The apparent mechanism is an inhibition of TCF/LEF DNA binding through a physical interaction with HBP1. These data suggest that the suppression of Wnt signaling by HBP1 may be a mechanism to prevent inappropriate proliferation (Sampson, 2001).

Mutant HBP1 proteins that can not repress transcription also fail to inhibit TCF4 DNA binding. HBP1-DNA binding is not required for inhibition of either TCF4 DNA binding or of Wnt-ß-catenin transcriptional activation. An endogenous complex of HBP1 and TCF4 is detected in both colon and non-colon cells, suggesting that HBP1 interaction with TCF4 might disrupt TCF4 function. Within HBP1, the necessary functional region for interacting with TCF4, inhibiting TCF4 DNA binding and TCF4 transcriptional activity is defined to amino acids 192-400, which contains the repression domain. Within TCF4, two HBP1 interaction regions (amino acids 53-171 and 327-400) have been defined, of which the latter contains the HMG-box DNA binding region of TCF4. The relevance of the second region from amino acids 53 to 171 of TCF4 is unknown. Together, these data support a mechanism in which HBP1 blocks DNA binding in a physical interaction with the HMG box of TCF4. This study highlights the potential role of HBP1 in the repression of the Wnt-ß-catenin pathway (Sampson, 2001).

ß-catenin is a multifunctional protein involved in both cell adhesion and transcriptional activation. Transcription mediated by the ß-catenin/Tcf complex is involved in embryological development and is upregulated in various cancers. The crystal structure at 2.5 Å resolution has been determined of a complex between ß-catenin and ICAT, a protein that prevents the interaction between ß-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 ß-catenin. ICAT selectively inhibits ß-catenin/Tcf binding in vivo, without disrupting ß-catenin/cadherin interactions. Thus, it should be possible to design cancer therapeutics that inhibit ß-catenin-mediated transcriptional activation without interfering with cell adhesion (Graham, 2002).

In the canonical Wnt signaling pathway, ß-catenin activates target genes through its interactions with Tcf/Lef-family transcription factors and additional transcriptional coactivators. The crystal structure of ICAT, an inhibitor of ß-catenin-mediated transcription, bound to the armadillo repeat domain of ß-catenin, has been determined. ICAT contains an N-terminal helilical domain that binds to repeats 11 and 12 of ß-catenin, and an extended C-terminal region that binds to repeats 5-10 in a manner similar to that of Tcfs and other ß-catenin ligands. Full-length ICAT dissociates complexes of ß-catenin, Lef-1, and the transcriptional coactivator p300, whereas the helical domain alone selectively blocks binding to p300. The C-terminal armadillo repeats of ß-catenin may be an attractive target for compounds designed to disrupt aberrant ß-catenin-mediated transcription associated with various cancers (Daniels, 2002).

Frodo is a novel conserved regulator of Wnt signaling that has been identified by its association with Dishevelled, an intracellular component of Wnt signal transduction. To understand further how Frodo functions, its role in neural development was analyzed using specific morpholino antisense oligonucleotides. Frodo and the closely related Dapper synergistically regulate head development and morphogenesis. Both genes were cell-autonomously required for neural tissue formation, as defined by the pan-neural markers sox2 and nrp1. By contrast, ß-catenin is not required for pan-neural marker expression, but is involved in the control of the anteroposterior patterning. In the mesoderm, Frodo and Dapper were essential for the expression of the organizer genes chordin, cerberus and Xnr3, but they are not necessary for the expression of siamois and goosecoid, established targets of ß-catenin signaling. Embryos depleted of either gene showed a decreased transcriptional response to TCF3-VP16, a ß-catenin-independent transcriptional activator. Whereas the C terminus of Frodo binds Dishevelled, the conserved N-terminal domain associates with TCF3. Based on these observations, it is proposed that Frodo and Dapper link Dsh and TCF to regulate Wnt target genes in a pathway parallel to that of ß-catenin (Hikasa, 2004).

β-catenin is the central signalling molecule of the canonical Wnt pathway, where it activates target genes in a complex with LEF/TCF transcription factors in the nucleus. The regulation of β-catenin activity is thought to occur mainly on the level of protein degradation, but it has been suggested that β-catenin nuclear localization and hence its transcriptional activity may additionally be regulated via nuclear import by TCF4 and BCL9 and via nuclear export by APC and axin. Using live-cell microscopy and fluorescence recovery after photobleaching (FRAP), the impact of these factors on the subcellular localization of β-catenin, its nucleo-cytoplasmic shuttling and its mobility within the nucleus and the cytoplasm were directly analysed. TCF4 and BCL9/Pygopus recruit β-catenin to the nucleus, and APC, axin and axin2 enrich β-catenin in the cytoplasm. Importantly, however, none of these factors accelerates the nucleo-cytoplasmic shuttling of β-catenin, i.e. increases the rate of β-catenin nuclear import or export. Moreover, the cytoplasmic enrichment of β-catenin by APC and axin is not abolished by inhibition of CRM-1-dependent nuclear export. TCF4, APC, axin and axin2 move more slowly than β-catenin in their respective compartment, and concomitantly decrease β-catenin mobility. Together, these data indicate that β-catenin interaction partners mainly regulate β-catenin subcellular localization by retaining it in the compartment in which they are localized, rather than by active transport into or out of the nucleus (Krieghoff, 2006).

Glutathione peroxidase 4 inhibits Wnt/beta-catenin signaling and regulates dorsal organizer formation in zebrafish embryos

The Wnt/β-catenin signaling pathway plays pivotal roles in axis formation during embryogenesis and in adult tissue homeostasis. Glutathione peroxidase 4 (GPx4) is a selenoenzyme and participates in the reduction of peroxides. Its synthesis depends on the availability of the element selenium. However, the roles of GPx4 in vertebrate embryonic development and underlying mechanisms are largely unknown. This study shows that maternal loss of zebrafish gpx4b promotes embryonic dorsal organizer formation, whereas overexpression of GPx4b inhibits the development of the dorsal organizer. Depletion of GPx4/GPx4b increases, while GPx4/GPx4b overexpression decreases, Wnt/beta-catenin signaling in vivo and in vitro. Functional and epistatic studies showed that GPx4 functions at the Tcf/Lef (see Drosophila Pangolin) level, independently of selenocysteine activation. Mechanistically, GPx4 interacts with Tcf/Lefs and inhibits Wnt activity by preventing the binding of Tcf/Lefs to the promoters of Wnt target genes, resulting in inhibitory action in the presence of Wnt/β-catenin signaling. These findings unravel GPx4 as a suppressor of Wnt/beta-catenin signals, suggesting a possible relationship between the Wnt/β-catenin pathway and selenium via the association of Tcf/Lef family proteins with GPx4 (Rong, 2017).

Tcf HMG box transcription factors interact with Groucho-related co-repressors

Tcf/Lef family transcription factors are the downstream effectors of the Wingless/Wnt signal transduction pathway. Upon Wingless/Wnt signalling, beta-catenin translocates to the nucleus, interacts with Tcf and thus activates transcription of target genes. Tcf factors also interact with members of the Groucho (Grg/TLE) family of transcriptional co-repressors. All known mammalian Groucho family members have been tested for their ability to interact specifically with individual Tcf/Lef family members. Transcriptional activation by any Tcf could be repressed by Grg-1, Grg-2/TLE-2, Grg-3 and Grg-4 in a reporter assay. Specific interactions between Tcf and Grg proteins may be achieved in vivo by tissue- or cell type-limited expression. To address this, the expression of all Tcf and Grg/TLE family members were determined in a panel of cell lines. Within any cell line, several Tcfs and TLEs are co-expressed. Thus, redundancy in Tcf/Grg interactions appears to be the rule. The 'long' Groucho family members containing five domains are repressors of Tcf-mediated transactivation, whereas Grg-5, which only contains the first two domains, acts as a de-repressor. As previously shown for Drosophila Groucho, this study shows that long Grg proteins interact with histone deacetylase-1. Although Grg-5 contains the GP homology domain that mediates HDAC binding in long Grg proteins, Grg-5 fails to bind this co-repressor, explaining how it can de-repress transcription (Brantjes, 2001).

Molecular functions of the TLE tetramerization domain in Wnt target gene repression

Wnt signaling activates target genes by promoting association of the co-activator β-catenin with TCF/LEF transcription factors. In the absence of beta-catenin, target genes are silenced by TCF-mediated recruitment of TLE/Groucho proteins, but the molecular basis for TLE/TCF-dependent repression is unclear. This paper describes the unusual three-dimensional structure of the N-terminal Q domain of TLE1 that mediates tetramerization and binds to TCFs. Differences in repression potential of TCF/LEFs correlates with their affinities for TLE-Q, rather than direct competition between β-catenin and TLE for TCFs as part of an activation-repression switch. Structure-based mutation of the TLE tetramer interface shows that dimers cannot mediate repression, even though they bind to TCFs with the same affinity as tetramers. Furthermore, the TLE Q tetramer, not the dimer, binds to chromatin, specifically to K20 methylated histone H4 tails, suggesting that the TCF/TLE tetramer complex promotes structural transitions of chromatin to mediate repression (Chodaparambil, 2014).

Table of contents

pangolin: Biological Overview | Regulation | Protein Interactions | Developmental Biology | Effects of Mutation | References

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