Wnt Target Gene Promoters Research Articles

Highlights of This Issue

The tight control of wild-type p53 by MDM2 in normal cells is permanently lost in tumors with mutant p53 that exhibit dramatic constitutive hyperstabilization, critical for mutp53's oncogenic gain‐of‐function. Li and coworkers show that mutp53 stabilization is caused by stable complexes with the HSP90 chaperone machinery that inhibits the MDM2 and CHIP E3 ligase activity. Interference with HSP90 destroys the complex, liberates mutp53 and reactivates MDM2 and CHIP to robustly degrade mutp53. HSP90 interference with 17AAG also induces stronger cell death in mutp53 than in wtp53 cancer cells. This suggests that pharmacologic suppression of mutp53 levels in established cancers might achieve clinically significant effects.Tumor-associated fibroblasts constitute an important part of the tumor stroma. It has been shown that they are essential in stimulation of the metastatic spread of tumor cells from the primary site. Here we show that fibroblasts are also instrumental in stimulating metastasis formation during organ colonization. Gene expression profiling of fibroblasts identified genes whose expression is modulated by factors derived from tumor cells. One of them encodes for the matricellular protein Fibulin-5, which inhibits metastatic organ colonization by regulating the ability of fibroblasts to invade the extracellular matrix, hence generating a favorable microenvironment for metastasis development.The lack of biomarkers that specify cancers with different cellular properties is a critical unmet need in cancer diagnosis. Cain and colleagues report surface proteome signature (SPS) technology. SPS uses a phage display library directed to the surface of HT-1080 fibrosarcoma cells for a differential immunocytochemistry screen to detect differences between closely related cancer cell lines. SPS identified CD44, the hyaluronan receptor, as a biomarker differentially expressed between two cell lines thought to differ in drug resistance due solely to the expression of the drug efflux pump, P-Glycoprotein. Additionally, they showed that CD44 increases drug resistance independent of P-Glycoprotein.Aberrant activation of Wnt/β-catenin signaling promotes oncogenesis by increasing expression of oncogenes and is, therefore, critical for the etiology of colorectal cancer. Occupancy of β-catenin at promoters of Wnt target genes drives transcription, but the mechanism of β- catenin action remains poorly understood. Here, Ou and coworkers show that the arginine methyltransferase and transcriptional coactivator CARM1 is recruited by β-catenin to Wnt target genes as an important positive modulator of Wnt/β-catenin transcription and neoplastic transformation. Abnormal expression of CARM1 was previously linked to human colorectal cancers. Thus, CARM1 is a potential new target for treatment of colorectal cancers involving aberrantly activated Wnt/β-catenin signaling.

The kinase TNIK is an essential activator of Wnt target genes

Wnt signalling maintains the undifferentiated state of intestinal crypt/progenitor cells through the TCF4/beta-catenin-activating transcriptional complex. In colorectal cancer, activating mutations in Wnt pathway components lead to inappropriate activation of the TCF4/beta-catenin transcriptional programme and tumourigenesis. The mechanisms by which TCF4/beta-catenin activate key target genes are not well understood. Using a proteomics approach, we identified Tnik, a member of the germinal centre kinase family as a Tcf4 interactor in the proliferative crypts of mouse small intestine. Tnik is recruited to promoters of Wnt target genes in mouse crypts and in Ls174T colorectal cancer cells in a beta-catenin-dependent manner. Depletion of TNIK and expression of TNIK kinase mutants abrogated TCF-LEF transcription, highlighting the essential function of the kinase activity in Wnt target gene activation. In vitro binding and kinase assays show that TNIK directly binds both TCF4 and beta-catenin and phosphorylates TCF4. siRNA depletion of TNIK followed by expression array analysis showed that TNIK is an essential, specific activator of Wnt transcriptional programme. This kinase may present an attractive candidate for drug targeting in colorectal cancer.

Interactions between SOX factors and Wnt/β‐catenin signaling in development and disease

The SOX family of transcription factors have emerged as modulators of canonical Wnt/beta-catenin signaling in diverse development and disease contexts. There are over 20 SOX proteins encoded in the vertebrate genome and recent evidence suggests that many of these can physically interact with beta-catenin and modulate the transcription of Wnt-target genes. The precise mechanisms by which SOX proteins regulate beta-catenin/TCF activity are still being resolved and there is evidence to support a number of models including: protein-protein interactions, the binding of SOX factors to Wnt-target gene promoters, the recruitment of co-repressors or co-activators, modulation of protein stability, and nuclear translocation. In some contexts, Wnt signaling also regulates SOX expression resulting in feedback regulatory loops that fine-tune cellular responses to beta-catenin/TCF activity. In this review, we summarize the examples of Sox-Wnt interactions and examine the underlying mechanisms of this potentially widespread and underappreciated mode of Wnt-regulation.

Nuclear Dvl, c-Jun, β-catenin, and TCF form a complex leading to stabiLization of β-catenin–TCF interaction

In canonical Wnt signaling, Dishevelled (Dvl) is a critical cytoplasmic regulator that releases β-catenin from degradation. Here, we find that Dvl and c-Jun form a complex with β-catenin–T-cell factor 4 (TCF-4) on the promoter of Wnt target genes and regulate gene transcription. The complex forms via two interactions of nuclear Dvl with c-Jun and β-catenin, respectively, both of which bind to TCF. Disrupting the interaction of Dvl with either c-Jun or β-catenin suppresses canonical Wnt signaling–stimulated transcription, and the reduction of Dvl diminished β-catenin–TCF-4 association on Wnt target gene promoters in vivo. Expression of a TCF-Dvl fusion protein largely rescued the c-Jun knockdown Wnt signaling deficiency in mammalian cells and zebrafish. Thus, we confirm that c-Jun functions in canonical Wnt signaling and show that c-Jun functions as a scaffold in the β-catenin–TCFs transcription complex bridging Dvl to TCF. Our results reveal a mechanism by which nuclear Dvl cooperates with c-Jun to regulate gene transcription stimulated by the canonical Wnt signaling pathway.

TBL1–TBLR1 and β-catenin recruit each other to Wnt target-gene promoter for transcription activation and oncogenesis

Aberrant Wnt signalling promotes oncogenesis by increasing the nuclear accumulation of beta-catenin to activate downstream target genes. However, the mechanism of beta-catenin recruitment to the Wnt target-gene promoter, a critical step for removing the co-repressor complex, is largely unknown. Here, we report that transducin beta-like protein 1 (TBL1) and its highly related family member TBLR1 were required for Wnt-beta-catenin-mediated transcription. Wnt signalling induced the interaction between beta-catenin and TBL1-TBLR1, as well as their binding to Wnt target genes. Importantly, the recruitment of TBL1-TBLR1 and beta-catenin to Wnt target-gene promoters was mutually dependent on each other. Furthermore, the depletion of TBL1-TBLR1 significantly inhibited Wnt-beta-catenin-induced gene expression and oncogenic growth in vitro and in vivo. Our results unravel two new components required for nuclear beta-catenin function, and have important implications in developing new strategies for inhibiting Wnt-beta-catenin-mediated tumorigenesis.

Differential Control of Wnt Target Genes Involves Epigenetic Mechanisms and Selective Promoter Occupancy by T-Cell Factors

Canonical Wnt signaling and its nuclear effectors, beta-catenin and the family of T-cell factor (TCF) DNA-binding proteins, belong to the small number of regulatory systems which are repeatedly used for context-dependent control of distinct genetic programs. The apparent ability to elicit a large variety of transcriptional responses necessitates that beta-catenin and TCFs distinguish precisely between genes to be activated and genes to remain silent in a specific context. How this is achieved is unclear. Here, we examined patterns of Wnt target gene activation and promoter occupancy by TCFs in different mouse cell culture models. Remarkably, within a given cell type only Wnt-responsive promoters are bound by specific subsets of TCFs, whereas nonresponsive Wnt target promoters remain unoccupied. Wnt-responsive, TCF-bound states correlate with DNA hypomethylation, histone H3 hyperacetylation, and H3K4 trimethylation. Inactive, nonresponsive promoter chromatin shows DNA hypermethylation, is devoid of active histone marks, and additionally can show repressive H3K27 trimethylation. Furthermore, chromatin structural states appear to be independent of Wnt pathway activity. Apparently, cell-type-specific regulation of Wnt target genes comprises multilayered control systems. These involve epigenetic modifications of promoter chromatin and differential promoter occupancy by functionally distinct TCF proteins, which together determine susceptibility to Wnt signaling.

Mediator Is a Transducer of Wnt/β-Catenin Signaling

Signal transduction within the canonical Wnt/beta-catenin pathway drives development and carcinogenesis through programmed or unprogrammed changes in gene transcription. Although the upstream events linked to signal-induced activation of beta-catenin in the cytoplasm have been deciphered in considerable detail, much less is known regarding the mechanism by which beta-catenin stimulates target gene transcription in the nucleus. Here, we show that beta-catenin physically and functionally targets the MED12 subunit in Mediator to activate transcription. The beta-catenin transactivation domain bound directly to isolated MED12 and intact Mediator both in vitro and in vivo, and Mediator was recruited to Wnt-responsive genes in a beta-catenin-dependent manner. Disruption of the beta-catenin/MED12 interaction through dominant-negative interference- or RNA interference-mediated MED12 suppression inhibited beta-catenin transactivation in response to Wnt signaling. This study thus identifies the MED12 interface within Mediator as a new component and a potential therapeutic target in the Wnt/beta-catenin pathway.

The p300/CBP acetyltransferases function as transcriptional coactivators of beta-catenin in vertebrates

Wnt growth factors regulate a variety of developmental processes by altering specific gene expression patterns. In vertebrates beta-catenin acts as transcriptional activator, which is needed to overcome target gene repression by Groucho/TLE proteins, and to permit promoter activation as the final consequence of Wnt signaling. However, the molecular mechanisms of transcriptional activation by beta-catenin are only poorly understood. Here we demonstrate that the closely related acetyltransferases p300 and CBP potentiate beta-catenin-mediated activation of the siamois promoter, a known Wnt target. beta-catenin and p300 also synergize to stimulate a synthetic reporter gene construct, whereas activation of the cyclin D1 promoter by beta-catenin is refractory to p300 stimulation. Axis formation and activation of the beta-catenin target genes siamois and Xnr-3 in Xenopus embryos are sensitive to the E1A oncoprotein, a known inhibitor of p300/CBP. The C-terminus of beta-catenin interacts directly with a region overlapping the CH-3 domain of p300. p300 could participate in alleviating promoter repression imposed by chromatin structure and in recruiting the basal transcription machinery to promoters of particular Wnt target genes.