The chromatin remodelling factor Brg-1 interacts with beta-catenin to promote target gene activation

Selective gene regulation by SWI/SNF-related chromatin remodeling factors

Chromatin is a highly dynamic structure that plays a key role in the orchestration of gene expression patterns during cellular differentiation and development. The packaging of DNA into chromatin generates a barrier to the transcription machinery. The two main strategies by which cells alleviate chromatin-mediated repression are through the action of ATP-dependent chromatin remodeling complexes and enzymes that covalently modify the histones. Various signaling pathways impinge upon the targeting and activity of these enzymes, thereby controlling gene expression in response to physiological and developmental cues. Chromatin structure also underlies many so-called epigenetic phenomena, leading to the mitotically stable propagation of differential expression of genetic information. Here, we will focus on the role of SWI/SNF-related ATP-dependent chromatin remodeling complexes in developmental gene regulation. First, we compare different models for how remodelers can act in a gene-selective manner, and either cooperate or antagonize other chromatin-modulating systems in the cell. Next, we discuss their functioning during the control of developmental gene expression programs.

The Wnt signaling pathway and its role in tumor development

Cancer development depends on the aberrant activation of signal transduction pathways that control cell growth and survival and play important roles in normal embryonic development. This review will focus on one of the most powerful pathways, the canonical Wnt signal transduction cascade, which has been originally described in vertebrate and non-vertebrate embryogenesis and subsequently associated with the development of a multitude of different tumor types, mainly of gastrointestinal origin. In recent years, a variety of novel interacting components and functions have been identified in the Wnt pathway revealing not only the complexity of Wnt signaling but also its potency. Here we will concentrate on the role of the Wnt pathway in cancer development with emphasis placed on the molecular defects known to promote neoplastic transformation in humans and in animal models.

Armadillo/beta-catenin signals in the nucleus--proof beyond a reasonable doubt?

Wnt signalling results in transcriptional stimulation of genes controlling normal and malignant development. A key effector of the canonical Wnt pathway is beta-catenin (also known as Drosophila melanogaster Armadillo (Arm)), thought to function as a nuclear co-activator of TCF transcription factors. This has been challenged by unexpected observations of membrane-bound Arm/beta-catenin signalling activity. Plausible explanations allow these observations to be reconciled with the large body of evidence supporting a nuclear function of Arm/beta-catenin.

Identification of the LIM protein FHL2 as a coactivator of beta-catenin

Beta-catenin is a key mediator of the Wnt pathway, which plays a critical role in embryogenesis and oncogenesis. As a transcriptional activator, beta-catenin binds the transcription factors, T-cell factor and lymphoid enhancer factor, and regulates gene expression in response to Wnt signaling. Abnormal activation of beta-catenin has been linked to various types of cancer. In a yeast two-hybrid screen, we identified the four and a half of LIM-only protein 2 (FHL2) as a novel beta-catenin-interacting protein. Here we show specific interaction of FHL2 with beta-catenin, which requires the intact structure of FHL2 and armadillo repeats 1-9 of beta-catenin. FHL2 cooperated with beta-catenin to activate T-cell factor/lymphoid enhancer factor-dependent transcription from a synthetic reporter and the cyclin D1 and interleukin-8 promoters in kidney and colon cell lines. In contrast, coexpression of beta-catenin and FHL2 had no synergistic effect on androgen receptor-mediated transcription, whereas each of these two coactivators independently stimulated AR transcriptional activity. Thus, the ability of FHL2 to stimulate the trans-activating function of beta-catenin might be dependent on the promoter context. The detection of increased FHL2 expression in hepatoblastoma, a liver tumor harboring frequent beta-catenin mutations, suggests that FHL2 might enforce beta-catenin transactivation activity in cancer cells. These findings reveal a new function of the LIM coactivator FHL2 in transcriptional activation of Wnt-responsive genes.

Identification of a promoter-specific transcriptional activation domain at the C terminus of the Wnt effector protein T-cell factor 4

Wnt growth factors control numerous cell fate decisions in development by altering specific gene expression patterns through the activity of heterodimeric transcriptional activators. These consist of beta-catenin and one of the four members of the T-cell factor (TCF) family of DNA-binding proteins. How can the Wnt/beta-catenin pathway control various sets of target genes in distinct cellular settings with such a limited number of nuclear effectors? Here we asked whether different TCF proteins could perform specific, nonredundant functions at natural beta-catenin/TCF-regulated promoters. We found that TCF4E but not LEF1 supported beta-catenin-dependent activation of the Cdx1 promoter, whereas LEF1 specifically activated the Siamois promoter. Deletion of a C-terminal domain of TCF4E prevented Cdx1 promoter induction. A chimeric protein consisting of LEF1 and the C terminus of TCF4E was fully functional. Therefore, the TCF4E C terminus harbors a promoter-specific transactivation domain. This domain influences the DNA binding properties of TCF4 and additionally mediates an interaction with the transcriptional coactivator p300. Apparently, the C terminus of TCF4E cooperates with beta-catenin and p300 to form a specialized transcription factor complex that specifically supports the activation of the Cdx1 promoter.

Fibroblast growth factor-2 induces Lef/Tcf-dependent transcription in human endothelial cells

Lef/Tcf proteins belong to a family of architectural transcription factors that control developmental processes and play an important role in oncogenesis. Classical activators of Lef/Tcf-dependent transcription comprise the Wnt family of proteins, which translocate beta-catenin into the nucleus and allow the formation of transactivation-competent Lef/Tcf-beta-catenin complexes. Here we show that in human endothelial cells fibroblast growth factor-2 (FGF-2) reduces GSK-3 activity and augments nuclear levels of beta-catenin. FGF-2 induced Lef/Tcf-dependent transcription of a cyclin D1-luciferase construct. Gel shift assays revealed binding of Tcf-4 as the only Lef/Tcf family member and of beta-catenin to the Lef/Tcf site in the cyclin D1 promoter. Cotransfection with a dominant negative Tcf-4 construct inhibited the FGF-2-induced cyclin D1 promoter activity. Overexpression of an uninhibitable GSK-3beta mutant resulted in partial inhibition of FGF-2-mediated cyclin D1 induction. The importance for cyclin D1 in FGF-2-induced angiogenesis in vivo is shown in cyclin D1(-/-) mice, where FGF-2-induced new vessel formation was significantly reduced compared with FGF-2-induced angiogenesis in cyclin D1(+/+) mice. In conclusion, FGF-2 is a novel modulator of Lef/Tcf-beta-catenin signaling in endothelial cells, suggesting that angiogenic properties of FGF-2 are at least in part mediated by Lef/Tcf-beta-catenin activation.

ATP-dependent nucleosome remodeling

It has been a long-standing challenge to decipher the principles that enable cells to both organize their genomes into compact chromatin and ensure that the genetic information remains accessible to regulatory factors and enzymes within the confines of the nucleus. The discovery of nucleosome remodeling activities that utilize the energy of ATP to render nucleosomal DNA accessible has been a great leap forward. In vitro, these enzymes weaken the tight wrapping of DNA around the histone octamers, thereby facilitating the sliding of histone octamers to neighboring DNA segments, their displacement to unlinked DNA, and the accumulation of patches of accessible DNA on the surface of nucleosomes. It is presumed that the collective action of these enzymes endows chromatin with dynamic properties that govern all nuclear functions dealing with chromatin as a substrate. The diverse set of ATPases that qualify as the molecular motors of the nucleosome remodeling process have a common history and are part of a superfamily. The physiological context of their remodeling action builds on the association with a wide range of other proteins to form distinct complexes for nucleosome remodeling. This review summarizes the recent progress in our understanding of the mechanisms underlying the nucleosome remodeling reaction, the targeting of remodeling machines to selected sites in chromatin, and their integration into complex regulatory schemes.

Transcriptional activation of interleukin-8 by beta-catenin-Tcf4

Nuclear translocation of beta-catenin and its association with Tcf/Lef factors are key steps in transduction of the Wnt signal, which is aberrantly activated in a variety of human cancers. In a search for new beta-catenin-Tcf target genes, we analyzed beta-catenin-induced alterations of gene expression in primary human hepatocytes, after transduction of either dominant stable beta-catenin or its truncated, transactivation-deficient counterpart by means of a lentiviral vector. cDNA microarray analysis revealed a limited set of up-regulated genes, including known Wnt targets such as matrilysin and keratin-1. In this screen, we identified the CXC chemokine interleukin 8 (IL-8) as a direct target of beta-catenin-Tcf4. IL-8 is constitutively expressed in various cancers, and it has been implicated in tumor progression through its mitogenic, motogenic, and angiogenic activities. The IL-8 promoter contains a unique consensus Tcf/Lef site that is critical for IL-8 activation by beta-catenin. We show here that the p300 coactivator was required for efficient transactivation of beta-catenin on this promoter. Ectopic expression of beta-catenin in hepatoma cells promoted IL-8 secretion, which stimulated endothelial cell migration. These data define IL-8 as a Wnt target and suggest that IL-8 induction by beta-catenin might be implicated in developmental and tumorigenic processes.

The Drosophila BRM complex facilitates global transcription by RNA polymerase II

Drosophila brahma (brm) encodes the ATPase subunit of a 2 MDa complex that is related to yeast SWI/SNF and other chromatin-remodeling complexes. BRM was identified as a transcriptional activator of Hox genes required for the specification of body segment identities. To clarify the role of the BRM complex in the transcription of other genes, we examined its distribution on larval salivary gland polytene chromosomes. The BRM complex is associated with nearly all transcriptionally active chromatin in a pattern that is generally non-overlapping with that of Polycomb, a repressor of Hox gene transcription. Reduction of BRM function dramatically reduces the association of RNA polymerase II with salivary gland chromosomes. A few genes, such as induced heat shock loci, are not associated with the BRM complex; transcription of these genes is not compromised by loss of BRM function. The distribution of the BRM complex thus correlates with a dependence on BRM for gene activity. These data suggest that the chromatin remodeling activity of the BRM complex plays a general role in facilitating transcription by RNA polymerase II.

Chromatin-remodelling factor BRG1 selectively activates a subset of interferon-α-inducible genes

Brahma-related gene 1 (BRG1 ) is a key component of the ATP-dependent chromatin-remodelling SWI2-SNF2 complex and has been implicated in regulating gene expression, cell-cycle control and tumorigenesis. Here we report that BRG1 interacts with signal transducer and activator of transcription 2 (STAT2) - a transcription factor that regulates gene expression mediated by interferon-alpha (IFN-alpha). BRG1 enhances the IFN-alpha-induced expression of 9-27 and IFI27 but not that of four other target genes tested, showing that the activation of different target genes by STAT2 may involve alternative chromatin modifiers. Our results also suggest that the recruitment and activation of BRG1 may require other cis-acting and trans-acting elements in addition to STAT2. Our study links the SWI2-SNF2 complex to the regulation of cytokine-induced gene expression and may identify a molecular mechanism of BRG1-mediated gene activation and tumorigenesis.

ICAT inhibits beta-catenin binding to Tcf/Lef-family transcription factors and the general coactivator p300 using independent structural modules

In the canonical Wnt signaling pathway, beta-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 beta-catenin-mediated transcription, bound to the armadillo repeat domain of beta-catenin, has been determined. ICAT contains an N-terminal helilical domain that binds to repeats 11 and 12 of beta-catenin, and an extended C-terminal region that binds to repeats 5-10 in a manner similar to that of Tcfs and other beta-catenin ligands. Full-length ICAT dissociates complexes of beta-catenin, Lef-1, and the transcriptional coactivator p300, whereas the helical domain alone selectively blocks binding to p300. The C-terminal armadillo repeats of beta-catenin may be an attractive target for compounds designed to disrupt aberrant beta-catenin-mediated transcription associated with various cancers.

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.

A developmental conundrum: a stabilized form of beta-catenin lacking the transcriptional activation domain triggers features of hair cell fate in epidermal cells and epidermal cell fate in hair follicle cells

Wnt signaling orchestrates morphogenetic processes in which changes in gene expression are associated with dramatic changes in cell organization within developing tissue/organs. Upon signaling, excess beta-catenin not utilized at cell-cell junctions becomes stabilized, where it can provide the transcriptional activating domain for Lef/Tcf DNA binding proteins. In skin epithelium, forced stabilization of beta-catenin in epidermis promotes hair follicle morphogenesis, whereas conditional removal of beta-catenin in hair progenitor cells specifies an epidermal fate. We now report that a single protein, a stabilized version of beta-catenin lacking the COOH-terminal transactivation domain, acts in epidermis to promote hair fates and in hair cells to promote epidermal fate. This reveals fundamental differences in ways that epidermal and hair cells naturally respond to beta-catenin signaling. In exploring the phenotype, we uncovered mechanistic insights into the complexities of Lef1/Tcf/beta-catenin signaling. Importantly, how a cell will respond to the transgene product, where it will be localized, and whether it can lead to activation of endogenous beta-catenin/Tcf/Lef complexes is specifically tailored to skin stem cells, their particular lineage and their relative stage of differentiation. Finally, by varying the level of beta-catenin signaling during a cell fate program, the skin cell appears to be pliable, switching fates multiple times.

Lymphoid enhancer factor-1 blocks adenomatous polyposis coli-mediated nuclear export and degradation of beta-catenin. Regulation by histone deacetylase 1

The oncogenic protein beta-catenin is overexpressed in many cancers, frequently accumulating in nuclei where it forms active complexes with lymphoid enhancer factor-1 (LEF-1)/T-cell transcription factors, inducing genes such as c-myc and cyclin D1. In normal cells, nuclear beta-catenin levels are controlled by the adenomatous polyposis coli (APC) protein through nuclear export and cytoplasmic degradation. Transient expression of LEF-1 is known to increase nuclear beta-catenin levels by an unknown mechanism. Here, we show that APC and LEF-1 compete for nuclear beta-catenin with opposing consequences. APC can export nuclear beta-catenin to the cytoplasm for degradation. In contrast, LEF-1 anchors beta-catenin in the nucleus by blocking APC-mediated nuclear export. LEF-1 also prevented the APC/CRM1-independent nuclear export of beta-catenin as revealed by in vitro assays. Importantly, LEF-1-bound beta-catenin was protected from degradation by APC and axin in SW480 colon cancer cells. The ability of LEF-1 to trap beta-catenin in the nucleus was down-regulated by histone deacetylase 1, and this correlated with a decrease in LEF1 transcription activity. Our findings identify LEF-1 as key regulator of beta-catenin nuclear localization and stability and suggest that overexpression of LEF-1 in colon cancer and melanoma cells may contribute to the accumulation of oncogenic beta-catenin in the nucleus.

The class II transactivator requires brahma-related gene 1 to activate transcription of major histocompatibility complex class II genes

The class II transactivator (CIITA) is the key regulator of major histocompatibility complex (MHC) class II gene transcription. We demonstrate here that CIITA requires the ATPase subunit of an hSWI/SNF complex, brahma-related gene 1 (BRG-1), to activate transcription. When introduced into a cell line lacking BRG-1, CIITA was unable to activate cellular MHC class II genes. Reexpression of the wild-type but not an ATP-binding-deficient BRG-1 protein in this cell line restored the ability of CIITA to transactivate transcription of MHC class II genes. Interestingly, when the activity of CIITA was assayed in the BRG-1-deficient cell line by using a plasmid-based reporter assay, BRG-1 was not required for transcriptional activation, suggesting that the chromatin structure on the plasmid is such that BRG-1 is not necessary. Coimmunoprecipitation experiments were performed to determine if BRG-1 and CIITA proteins associate with each other in cells. We found that the two proteins coimmunoprecipitate and that amino acids 1 to 140 of CIITA are sufficient for binding. Taken together, these data suggest that BRG-1 and, very likely, an hSWI/SNF complex are required for transcription of MHC class II genes. The complex is likely recruited to MHC class II promoters, at least in part, by interaction with CIITA.

Activation of AXIN2 expression by beta-catenin-T cell factor. A feedback repressor pathway regulating Wnt signaling

The Wnt pathway regulates cell fate, proliferation, and apoptosis, and defects in the pathway play a key role in many cancers. Although Wnts act to stabilize beta-catenin levels in the cytosol and nucleus, a multiprotein complex containing adenomatous polyposis coli, glycogen synthase kinase 3beta, and Axin1 or its homolog Axin2/Axil/conductin promotes beta-catenin phosphorylation and subsequent proteasomal degradation. We found that the rat Axil gene was strongly induced upon neoplastic transformation of RK3E cells by mutant beta-catenin or gamma-catenin or after ligand-induced activation of a beta-catenin-estrogen receptor fusion protein. Expression of Wnt1 in murine breast epithelial cells activated the conductin gene, and human cancers with defective beta-catenin regulation had elevated AXIN2 gene and protein expression. Expression of AXIN2/Axil was strongly repressed in cancer cells by restoration of wild type adenomatous polyposis coli function or expression of a dominant negative form of T cell factor (TCF)-4. TCF binding sites in the AXIN2 promoter played a key role in the ability of beta-catenin to activate AXIN2 transcription. In contrast to AXIN2/Axil, expression of human or rat Axin1 homologs was nominally affected by beta-catenin-TCF. Because Axin2 can inhibit beta-catenin abundance and function, the data implicate AXIN2 in a negative feedback pathway regulating Wnt signaling. Additionally, although Axin1 and Axin2 have been thought to have comparable functions, the observation that Wnt pathway activation elevates AXIN2 but not AXIN1 expression suggests that there may be potentially significant functional differences between the two proteins.

Pygopus, a nuclear PHD-finger protein required for Wingless signaling inDrosophila

The secreted glycoprotein Wingless (Wg) acts through a conserved signaling pathway to regulate target gene expression. Wg signaling causes nuclear translocation of Armadillo, the fly β-catenin, which then complexes with the DNA-binding protein TCF, enabling it to activate transcription. Though many nuclear factors have been implicated in modulating TCF/Armadillo activity, their importance remains poorly understood. This work describes a ubiquitously expressed protein, called Pygopus, which is required for Wg signaling throughout Drosophila development. Pygopus contains a PHD finger at its C terminus, a motif often found in chromatin remodeling factors. Overexpression of pygopus also blocks the pathway, consistent with the protein acting in a complex. The pygopus mutant phenotype is highly, though not exclusively, specific for Wg signaling. Epistasis experiments indicate that Pygopus acts downstream of Armadillo nuclear import, consistent with the nuclear location of heterologously expressed protein. Our data argue strongly that Pygopus is a new core component of the Wg signaling pathway that acts downstream or at the level of TCF.

Molecular causes of colon cancer

Cells in a developing embryo communicate with each other through a limited number of intercellular signalling pathways, of which the Wnt signalling pathway is one. Little is known about the function of Wnt signalling beyond that in embryogenesis. However, recent insights into the molecular etiology of colon cancer have implied a central role for the Wnt signalling pathway. The malignant transformation of colorectal epithelium is well defined, leading to adenoma and sequentially carcinoma formation. Several genes that regulate the Wnt pathway are mutated in cancer of the human colon and other organs. All of these mutations lead to the inappropriate activation of the pathway, which instructs the cell to divide unrestrictedly. These insights now allow the Wnt pathway to be exploited as a new target for drug development in colon cancer.

Cloning and characterization of hELD/OSA1, a novel BRG1 interacting protein

A highly conserved multisubunit enzymic complex, SWI/SNF, participates in the regulation of eukaryote gene expression through its ability to remodel chromatin. While a single component of SWI/SNF, Swi2 or a related protein, can perform this function in vitro, the other components appear to modulate the activity and specificity of the complex in vivo. Here we describe the cloning of hELD/OSA1, a 189 KDa human homologue of Drosophila Eld/Osa protein, a constituent of Drosophila SWI/SNF. By comparing conserved peptide sequences in Eld/Osa homologues we define three domains common to all family members. A putative DNA binding domain, or ARID (AT-rich DNA-interacting domain), may function in targetting SWI/SNF to chromatin. Two other domains unique to Eld/Osa proteins, EHD1 and EHD2, map to the C-terminus. We show that EHD2 mediates binding to Brahma-related gene 1 (BRG1), a human homologue of yeast Swi2. EHD1 and EHD2 also appear capable of interacting with each other. Using an antibody raised against EHD2 of hELD/OSA1, we detected Eld/Osa1 in endogenous SWI/SNF complexes derived from mouse brain.

When the embryonic genome flexes its muscles

During the development of multicellular organisms, both transient and stable gene expression patterns have to be established in a precisely orchestrated sequence. Evidence from diverse model organisms indicates that this epigenetic program involves not only transcription factors, but also the local structure, composition, and modification of chromatin, which define and maintain the accessibility and transcriptional competence of the nucleosomal DNA template. A paradigm for the interdependence of development and chromatin is constituted by the mechanisms controlling the specification and differentiation of the skeletal muscle cell lineage in vertebrates, which is the topic of this review.

A new nuclear component of the Wnt signalling pathway

The Wnt signalling pathway is pivotal in normal and malignant development. A key effector is Armadillo (Arm)/beta-catenin, which functions with TCF to transcribe Wnt target-genes. Here, we report the discovery of pygopus (pygo), whose mutant phenotypes specifically mimic loss-of-Wingless (Wg) signalling. pygo is required for dTCF-mediated transcription, but not for Wg-induced stabilization of Arm. Pygo is a nuclear protein that is found in a complex with Arm in vivo. Humans possess two Pygo proteins, both of which are required for TCF-mediated transcription in colorectal cancer cells. The presence of a PHD domain implicates Pygo proteins in a chromatin-related function, and we propose that they mediate chromatin access to TCF or Arm/beta-catenin.

Interferon-gamma-induced chromatin remodeling at the CIITA locus is BRG1 dependent

SWI/SNF regulates growth control, differentiation and tumor suppression, yet few direct targets of this chromatin-remodeling complex have been identified in mammalian cells. We report that SWI/SNF is required for interferon (IFN)-gamma induction of CIITA, the master regulator of major histocompatibility complex class II expression. Despite the presence of functional STAT1, IRF-1 and USF-1, activators implicated in CIITA expression, IFN-gamma did not induce CIITA in cells lacking BRG1 and hBRM, the ATPase subunits of SWI/SNF. Reconstitution with BRG1, but not an ATPase-deficient version of this protein (K798R), rescued CIITA induction, and enhanced the rate of induction of the IFN-gamma-responsive GBP-1 gene. Not ably, BRG1 inhibited the CIITA promoter in transient transfection assays, underscoring the importance of an appropriate chromosomal environment. Chromatin immunoprecipitation revealed that BRG1 interacts directly with the endogenous CIITA promoter in an IFN-gamma-inducible fashion, while in vivo DNase I footprinting and restriction enzyme accessibility assays showed that chromatin remodeling at this locus requires functional BRG1. These data provide the first link between a cytokine pathway and SWI/SNF, and suggest a novel role for this chromatin-remodeling complex in immune surveillance.

Wnt/wingless signaling requires BCL9/legless-mediated recruitment of pygopus to the nuclear beta-catenin-TCF complex

Wnt/Wingless signaling controls many fundamental processes during animal development. Wnt transduction is mediated by the association of beta-catenin with nuclear TCF DNA binding factors. Here we report the identification of two segment polarity genes in Drosophila, legless (lgs), and pygopus (pygo), and we show that their products are required for Wnt signal transduction at the level of nuclear beta-catenin. Lgs encodes the homolog of human BCL9, and we provide genetic and molecular evidence that these proteins exert their function by physically linking Pygo to beta-catenin. Our results suggest that the recruitment of Pygo permits beta-catenin to transcriptionally activate Wnt target genes and raise the possibility that a deregulation of these events may play a causal role in the development of B cell malignancies.

Androgen receptor (AR) coregulators: an overview

The biological action of androgens is mediated through the androgen receptor (AR). Androgen-bound AR functions as a transcription factor to regulate genes involved in an array of physiological processes, most notably male sexual differentiation and maturation, and the maintenance of spermatogenesis. The transcriptional activity of AR is affected by coregulators that influence a number of functional properties of AR, including ligand selectivity and DNA binding capacity. As the promoter of target genes, coregulators participate in DNA modification, either directly through modification of histones or indirectly by the recruitment of chromatin-modifying complexes, as well as functioning in the recruitment of the basal transcriptional machinery. Aberrant coregulator activity due to mutation or altered expression levels may be a contributing factor in the progression of diseases related to AR activity, such as prostate cancer. AR demonstrates distinct differences in its interaction with coregulators from other steroid receptors due to differences in the functional interaction between AR domains, possibly resulting in alterations in the dynamic interactions between coregulator complexes.

Activation of the Wnt pathway interferes with serum response element-driven transcription of immediate early genes

Mutational activation of the Wnt signaling pathway is a common early event in colorectal tumorigenesis, and the identification of target genes regulated by this pathway will provide a better understanding of tumor progression. Gene expression profiling on oligonucleotide microarrays revealed reduced expression of the immediate early genes fos and fosB following stimulation of cells by Wnt-1. Further analysis demonstrated that serum or 12-O-tetradecanoylphorbol-13-acetate activation of several immediate early genes including fos, fosB, junB, and egr1 was inhibited by Wnt signaling. Wnt signaling inhibited transcriptional activation driven by the serum response element without altering the activation of the extracellular signal-regulated kinase cascade or ternary complex formation at the fos serum response element promoter. The Wnt-mediated repression of c-Fos, FosB, and JunB expression was consistent with a decrease in their binding to an AP-1 promoter element and decreased target gene transcription. The expression of fos, fosB, junB, and egr1 was also repressed in human colon tumors relative to patient matched normal tissue. By contrast, the fos family member fra-1 was up-regulated in the human colon tumors, suggesting a compensatory mechanism for the reduction in fos and fosB expression. The results indicate that Wnt signaling can repress the expression of certain immediate early genes, and that this effect is consistent with changes in gene expression observed in human colorectal tumors.

TCF: Lady Justice casting the final verdict on the outcome of Wnt signalling

The Wnt signalling cascade plays an important role during embryonic patterning and cell fate determination and is highly conserved throughout evolution. Factors of the TCF/LEF HMG domain family (Tcfs) are the downstream effectors of this signal transduction pathway. Upon Wnt signalling, a cascade is initiated that results in the translocation of beta-catenin to the nucleus, where it interacts with Tcf to generate a transcriptionally active complex. This bipartite transcription factor is targeted to the upstream regulatory regions of Tcf target genes. In the absence of Wnt signals, beta-catenin is degraded in the cytoplasm via the ubiquitin-proteasome pathway. Several proteins are instrumental in achieving this tight regulation of beta-catenin levels in the cell, including adenomatous polyposis coli (APC), GSK3 beta, and Axin/Conductin. Deregulation of the Wnt signalling pathway is implicated in several forms of cancer, such as colon carcinoma and melanoma. This deregulation is achieved via mutation of APC, beta-catenin or Axin, resulting in elevated beta-catenin levels and the presence of constitutively active Tcf-beta-catenin complexes in the nucleus. The accompanying inappropriate activation of target genes is considered to be a critical, early event in this carcinogenesis. In addition to regulating beta-catenin levels, normal healthy cells have evolved a second level of regulation, by manipulating the activity of the Tcf proteins themselves. In the absence of Wnt signalling, Tcf complexes with several transcriptional repressor proteins ensuring active repression of Tcf target genes. In this review the dual role of Tcf proteins in the Wnt signalling cascade will be discussed.

Chromatin-specific regulation of LEF-1-beta-catenin transcription activation and inhibition in vitro

Transcriptional activation of Wnt/Wg-responsive genes requires the stabilization and nuclear accumulation of beta-catenin, a dedicated coactivator of LEF/TCF enhancer-binding proteins. Here we report that recombinant beta-catenin strongly enhances binding and transactivation by LEF-1 on chromatin templates in vitro. Interestingly, different LEF-1 isoforms vary in their ability to bind nucleosomal templates in the absence of beta-catenin, owing to N-terminal residues that repress binding to chromatin, but not nonchromatin, templates. Transcriptional activation in vitro requires both the armadillo (ARM) repeats and the C terminus of beta-catenin, whereas the phosphorylated N terminus is inhibitory to transcription. A fragment spanning the C terminus (CT) and ARM repeats 11 and 12 (CT-ARM), but not the CT alone, functions as a dominant negative inhibitor of LEF-1-beta-cat activity in vitro and can block ATP-dependent binding of the complex to chromatin. LEF-1-beta-cat transactivation in vitro was also repressed by inhibitor of beta-catenin and Tcf-4 (ICAT), a physiological inhibitor of Wnt/Wg signaling that interacts with ARM repeats 11 and 12, and by the nonsteroidal anti-inflammatory compound, sulindac. None of these transcription inhibitors (CT-ARM, ICAT, or sulindac) could disrupt the LEF-1-beta-cat complex after it was stably bound to chromatin. We conclude that the CT-ARM region of beta-catenin functions as a chromatin-specific activation domain, and that several inhibitors of the Wnt/Wg pathway directly modulate LEF-1-beta-cat activity on chromatin.