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

T-cell generation by lymph node resident progenitor cells

In the thymus, 2 types of Lin–Sca-1+ (lineage-negative stem cell antigen-1–positive) progenitors can generate T-lineage cells: c-Kithi interleukin-7 receptor α–negative (c-KithiIL-7Rα–) and c-KitloIL-7Rα+. While c-KithiIL-7Rα– progenitors are absent, c-KitloIL-7Rα+ progenitors are abundant in the lymph nodes (LNs). c-KitloIL-7Rα+ progenitors undergo abortive T-cell commitment in the LNs and become arrested in the G1 phase of the cell cycle because they fail both to up-regulate c-myb, c-myc, and cyclin D2 and to repress junB, p16INK4a, and p21Cip1/WAF. As a result, development of LN c-KitloIL-7Rα+ progenitors is blocked at an intermediate CD44+CD25lo development stage in vivo, and LN-derived progenitors fail to generate mature T cells when cultured with OP9-DL1 stromal cells. LN stroma can provide key signals for T-cell development including IL-7, Kit ligand, and Delta-like–1 but lacks Wnt4 and Wnt7b transcripts. LN c-KitloIL-7Rα+ progenitors are able to generate mature T cells when cultured with stromal cells producing wingless-related MMTV integration site 4 (Wnt4) or upon in vivo exposure to oncostatin M whose signaling pathway intersects with Wnt. Thus, supplying Wnt signals to c-KitloIL-7Rα+ progenitors may be sufficient to transform the LN into a primary T-lymphoid organ. These data provide unique insights into the essence of a primary T-lymphoid organ and into how a cryptic extrathymic T-cell development pathway can be amplified.

Inducible changes in cell size and attachment area due to expression of a mutant SWI/SNF chromatin remodeling enzyme

The SWI/SNF enzymes belong to a family of ATP-dependent chromatin remodeling enzymes that have been functionally implicated in gene regulation, development, differentiation and oncogenesis. BRG1, the catalytic core subunit of some of the SWI/SNF enzymes, can interact with known tumor suppressor proteins and can act as a tumor suppressor itself. We report that cells that inducibly express ATPase-deficient versions of BRG1 increase in cell volume, area of attachment and nuclear size upon expression of the mutant BRG1 protein. Examination of focal adhesions reveals qualitative changes in paxillin distribution but no difference in the actin cytoskeletal structure. Increases in cell size and shape correlate with over-expression of two integrins and the urokinase-type plasminogen activator receptor (uPAR), which is also involved in cell adhesion and is often over-expressed in metastatic cancer cells. These findings demonstrate that gene expression pathways affected by chromatin remodeling enzymes can regulate the physical dimensions of mammalian cell morphology.

The Brm gene suppressed at the post-transcriptional level in various human cell lines is inducible by transient HDAC inhibitor treatment, which exhibits antioncogenic potential

The mammalian SWI/SNF chromatin remodeling complex is composed of more than 10 protein subunits, and plays important roles in epigenetic regulation. Each complex includes a single BRG1 or Brm molecule as the catalytic subunit. We previously reported that loss of Brm, but not BRG1, causes transcriptional gene silencing of murine leukemia virus-based retrovirus vectors. To understand the biological function and biogenesis of Brm protein, we examined seven cell lines derived from various human tumors that do not produce Brm protein. We show here that these Brm-deficient cell lines transcribe the Brm genes efficiently as detected by nuclear run-on transcription assay, whereas Brm mRNA and Brm hnRNA were undetectable by reverse transcription-polymerase chain reaction analysis. These results indicate that expression of Brm is strongly and promptly suppressed at the post-transcriptional level, through processing and transport of the primary transcript or through stability of mature Brm mRNA. This suppression was attenuated by transient treatment of these cell lines with HDAC inhibitors probably through indirect mechanism. Importantly, all of the treated cells showed prolonged induction of Brm expression after the removal of HDAC inhibitors, and acquired the ability to maintain retroviral gene expression. These results indicate that these Brm-deficient human tumor cell lines carry a functional Brm gene. Treatment with HDAC inhibitors or introduction of exogenous Brm into Brm-deficient cell lines significantly reduced the oncogenic potential as assessed by colony-forming activity in soft agar or invasion into collagen gel, indicating that, like BRG1, Brm is involved in tumor suppression.

Brg1, the ATPase subunit of the SWI/SNF chromatin remodeling complex, is required for myeloid differentiation to granulocytes

Many mammalian SWI/SNF complexes use Brahma-related gene 1 (Brg1) as a catalytic subunit to remodel nucleosomes for transcription regulation. In several mesenchymal cells and tissues, expression of a defective Brg1 protein negates the normal activity of the SWI/SNF complex and delays or blocks differentiation. To investigate the role of SWI/SNF complexes during myelopoiesis, we stably expressed a dominant negative (dn) Brg1 mutant in the myeloid lineage. Forced expression of dnBrg1 in IL-3-dependent murine 32Dcl3 myeloid progenitor cells results in a profound delay in the granulocyte-colony stimulating factor (G-CSF) induced granulocytic maturation. These cells also exhibit a significant decrease in the expression of both CD11b and Gr-1 surface receptors, which are normally upregulated during granulopoiesis, and show sustained expression of myeloperoxidase, which is synthesized primarily during the promyelocytic (blast) stage of myeloid development. Thus, dnBrg1 expression causes a developmental block at the promyelocytic/metamyelocytic stage of myeloid differentiation. Our findings indicate that the normal chromatin remodeling function of Brg1 is necessary for the G-CSF dependent differentiation of myeloid cells towards the granulocytic lineage. This dependency on Brg1 may reflect a stringent requirement for chromatin remodeling at a critical stage of hematopoietic cell maturation.

The expression of the SWI/SNF ATPase subunits BRG1 and BRM in normal human tissues

SWI/SNF is a chromatin-remodeling complex important in gene regulation, cytokine responses, tumorigenesis, differentiation, and development. As a multitude of signaling pathways require SWI/SNF, loss of SWI/SNF function is expected to have an impact on cellular phenotypes. The SWI/SNF ATPase subunits, BRG1 and BRM, have been shown to be lost in a subset of human cancer cell lines and human primary cancers and may represent tumor suppressor proteins. To better understand the biology of these proteins, the authors examined the expression pattern of BRG1 and BRM in a variety of normal tissues. BRG1 expression was predominantly seen in cell types that constantly undergo proliferation or self-renewal; in contrast, BRM was preferentially expressed in brain, liver, fibromuscular stroma, and endothelial cell types, cell types not constantly engaged in proliferation or self-renewal. This differential expression suggests that these proteins serve distinct functions in human tissues.

Wnt signaling in the intestinal epithelium: from endoderm to cancer

The Wnt pathway controls cell fate during embryonic development. It also persists as a key regulator of homeostasis in adult self-renewing tissues. In these tissues, mutational deregulation of the Wnt cascade is closely associated with malignant transformation. The intestinal epithelium represents the best-understood example for the closely linked roles of Wnt signaling in homeostatic self-renewal and malignant transformation. In this review, we outline current understanding of the physiological role of Wnt signaling in intestinal biology. From this perspective, we then describe how mutational subversion of the Wnt cascade leads to colorectal cancer.

Wnt signaling in disease and in development

The highly conserved Wnt secreted proteins are critical mediators of cell-to-cell signaling during development of animals. Recent biochemical and genetic analyses have led to significant insight into understanding how Wnt signals work. The catalogue of Wnt signaling components has exploded. We now realize that multiple extracellular, cytoplasmic, and nuclear components modulate Wnt signaling. Moreover, receptor-ligand specificity and multiple feedback loops determine Wnt signaling outputs. It is also clear that Wnt signals are required for adult tissue maintenance. Perturbations in Wnt signaling cause human degenerative diseases as well as cancer.

A Vertebrate Homolog of the Cell Cycle Regulator Dbf4 Is an Inhibitor of Wnt Signaling Required for Heart Development

Early stages of vertebrate heart development have been linked to Wnt signaling. Here we show in both gain- and loss-of-function experiments that XDbf4, a known regulator of Cdc7 kinase, is an inhibitor of the canonical Wnt signaling pathway. Depletion of endogenous XDbf4 protein did not disturb gastrulation movements or early organizer genes but resulted in embryos with morphologically defective heart and eyes and suppressed cardiac markers. These markers were restored by overexpressed XDbf4, or an XDbf4 mutant that inhibits Wnt signaling but lacks the ability to regulate Cdc7. This indicates that the function of XDbf4 in heart development is independent of its role in the cell cycle. Moreover, our data suggest that XDbf4 acts through the physical and functional interaction with Frodo, a context-dependent regulator of Wnt signaling. These findings establish an unexpected function for a vertebrate Dbf4 homolog and demonstrate the requirement for Wnt inhibition in early cardiac specification.

Beta-catenin directly displaces Groucho/TLE repressors from Tcf/Lef in Wnt-mediated transcription activation

Wnt growth factors mediate cell fate determination during embryogenesis and in the renewal of tissues in the adult. Wnts act by stabilizing cellular levels of the transcriptional coactivator beta-catenin, which forms complexes with sequence-specific DNA-binding Tcf/Lef transcription factors. In the absence of nuclear beta-catenin, Tcf/Lefs act as transcriptional repressors by binding to Groucho/TLE proteins. The molecular basis of the switch from transcriptional repression to activation during Wnt signaling has not been clear, in particular whether factors other than beta-catenin are required to disrupt the interaction between Groucho/TLE and Tcf/Lef. Using highly purified proteins, we demonstrate that beta-catenin displaces Groucho/TLE from Tcf/Lef by binding to a previously unidentified second, low-affinity binding site on Lef-1 that includes sequences just N-terminal to the DNA-binding domain, and that overlaps the Groucho/TLE-binding site.

BRCA1 interacts with dominant negative SWI/SNF enzymes without affecting homologous recombination or radiation-induced gene activation of p21 or Mdm2

BRCA1 is a tumor suppressor gene linked to familial breast and ovarian cancer. The BRCA1 protein has been implicated in a diverse set of cellular functions, including activation of gene expression by the p53 tumor suppressor and control of homologous recombination (HR) during DNA repair. Prior reports have demonstrated that BRCA1 can exist in cells in a complex with the BRG1-based SWI/SNF ATP-dependent chromatin remodeling enzymes and that SWI/SNF components contribute to p53-mediated gene activation. To investigate the link between SWI/SNF function and BRCA1 mediated effects on p53-mediated gene activation and on mechanisms of homologous recombination, we have utilized mammalian cells that inducibly express an ATPase-deficient, dominant negative SWI/SNF enzymes. Mutant SWI/SNF ATPases retain the ability to interact with BRCA1 in cells. We report that expression of dominant negative SWI/SNF enzymes does not affect p53-mediated induction of the p21 cyclin dependent kinase inhibitor or the Mdm2 E3 ubiquitin ligase that regulates p53 in cells exposed to UV or gamma irradiation. Similarly, integration of a reporter that monitors homologous recombination by gene conversion into these cells demonstrated no change in the recombination rate in the absence of functional SWI/SNF enzyme. We conclude that the SWI/SNF chromatin remodeling enzymes may contribute to but are not required for these processes.

Transcriptional targets of the chromatin-remodelling factor SMARCA4/BRG1 in lung cancer cells

BRG1, also called SMARCA4, is the catalytic subunit of the SWI/SNF chromatin-remodelling complex and influences transcriptional regulation by disrupting histone-DNA contacts in an ATP-dependent manner. BRG1 and other members of the SWI/SNF complex become inactivated in tumours, implying a role in cancer development. To understand the contribution of BRG1 to lung tumourigenesis, we restored BRG1 in H1299 lung cancer cells and used cDNA microarray analysis to identify changes in gene expression. Forty-three transcripts became activated, whereas two were repressed. Chromatin immunoprecipitation of resulting candidate genes revealed that the CYP3A4 and ZNF185 promoters recruited BRG1 and that recruitment to the CYP3A4 promoter was specific to this gene and did not involve the CYP3A5 or CYP3A7 family members. Moreover, specifically BRG1 but not its homologue BRM was recruited to the CYP3A4 and ZNF185 promoters. To explore their potential relevance in lung tumours, levels of CYP3A4 and ZNF185 transcripts were evaluated in seven additional lung cancer cell lines. CYP3A4 was undetectable in any of the lung cancer cells tested, and only the CYP3A5 family member was present in the A549 and Calu-3 cells. In contrast, the amount of ZNF185 transcript clearly varied among lung cancer cell lines and severely reduced levels were observed in BRG1-deficient cells, except those of A427. We extended these observations to 27 lung primary tumours using real-time RT-PCR (TaqMan) and observed that four (15%) and 14 (52%) of them had BRG1 and ZNF185 downregulation, respectively, when compared with normal lung. No significant correlation between reduced levels of BRG1 and ZNF185 was observed, indicating that additional mechanisms to BRG1 inactivation may contribute to the loss of ZNF185 expression in lung tumours. In conclusion, our results provide evidence that transcriptional activation of ZNF185 and CYP3A4 is mediated by direct association of BRG1 with their promoters and also indicate that a decreased level of ZNF185 is a common feature of lung tumours and may be of biological relevance in lung carcinogenesis.

WNT signalling and haematopoiesis: a WNT-WNT situation

The evolutionarily conserved WNT-signalling pathway has pivotal roles during the development of many organ systems, and dysregulated WNT signalling is a key factor in the initiation of various tumours. Recent studies have implicated a role for WNT signal transduction at several stages of lymphocyte development and in the self-renewal of haematopoietic stem cells. Here, we outline new insights into the WNT-signalling pathway, review its role in the self-renewal of haematopoietic stem cells and in the development of T and B cells, and discuss controversies and future developments with regard to WNT signalling in the thymus.

The Wnt signaling pathway in development and disease

Tight control of cell-cell communication is essential for the generation of a normally patterned embryo. A critical mediator of key cell-cell signaling events during embryogenesis is the highly conserved Wnt family of secreted proteins. Recent biochemical and genetic analyses have greatly enriched our understanding of how Wnts signal, and the list of canonical Wnt signaling components has exploded. The data reveal that multiple extracellular, cytoplasmic, and nuclear regulators intricately modulate Wnt signaling levels. In addition, receptor-ligand specificity and feedback loops help to determine Wnt signaling outputs. Wnts are required for adult tissue maintenance, and perturbations in Wnt signaling promote both human degenerative diseases and cancer. The next few years are likely to see novel therapeutic reagents aimed at controlling Wnt signaling in order to alleviate these conditions.

Interaction and functional cooperation between the LIM protein FHL2, CBP/p300, and beta-catenin

Transcriptional activation of gene expression by Wnt signaling is driven by the association of beta-catenin with TCF/LEF factors and the recruitment of transcriptional coactivators. It has been shown that the LIM protein FHL2 and the acetyltransferase CBP/p300 individually stimulate beta-catenin transactivating activity and that beta-catenin is acetylated by p300. Here, we report that FHL2 and CBP/p300 synergistically enhanced beta-catenin/TCF-mediated transcription from Wnt-responsive promoters and that the acetyltransferase activity of CBP/p300 was involved in the cooperation. CBP/p300 interacted directly with FHL2, predominantly through the CH3 domain but not the histone acetyltransferase domain, and different regions of CBP/p300 were involved in FHL2 and beta-catenin binding. We provided evidence for the formation of a ternary complex by FHL2, CBP/p300, and beta-catenin and for colocalization of the three proteins in the nucleus. In murine FHL2(-/-) embryo fibroblasts, the transactivation activity of beta-catenin/TCF was markedly reduced, and this defect could be restored by exogenous expression of FHL2. However, CBP/p300 were still able to coactivate the beta-catenin/TCF complex in FHL2(-/-) cells, suggesting that FHL2 is dispensable for the coactivator function of CBP/p300 on beta-catenin. Furthermore, we found that FHL2 significantly increased acetylation of beta-catenin by p300 in vivo. Finally, we showed that FHL2, CBP/p300, and beta-catenin could synergistically activate androgen receptor-mediated transcription, indicating that the synergistic coactivator function is not restricted to TCF/LEF.

Baf60c is essential for function of BAF chromatin remodelling complexes in heart development

Tissue-specific transcription factors regulate several important aspects of embryonic development. They must function in the context of DNA assembled into the higher-order structure of chromatin. Enzymatic complexes such as the Swi/Snf-like BAF complexes remodel chromatin to allow the transcriptional machinery access to gene regulatory elements. Here we show that Smarcd3, encoding Baf60c, a subunit of the BAF complexes, is expressed specifically in the heart and somites in the early mouse embryo. Smarcd3 silencing by RNA interference in mouse embryos derived from embryonic stem cells causes defects in heart morphogenesis that reflect impaired expansion of the anterior/secondary heart field, and also results in abnormal cardiac and skeletal muscle differentiation. An intermediate reduction in Smarcd3 expression leads to defects in outflow tract remodelling reminiscent of human congenital heart defects. Baf60c overexpressed in cell culture can mediate interactions between cardiac transcription factors and the BAF complex ATPase Brg1, thereby potentiating the activation of target genes. These results reveal tissue-specific and dose-dependent roles for Baf60c in recruiting BAF chromatin remodelling complexes to heart-specific enhancers, providing a novel mechanism to ensure transcriptional regulation during organogenesis.

The APC Tumor Suppressor Binds to C-Terminal Binding Protein to Divert Nuclear β-Catenin from TCF

Adenomatous polyposis coli (APC) is an important tumor suppressor in the colon. APC antagonizes the transcriptional activity of the Wnt effector beta-catenin by promoting its nuclear export and its proteasomal destruction in the cytoplasm. Here, we show that a third function of APC in antagonizing beta-catenin involves C-terminal binding protein (CtBP). APC is associated with CtBP in vivo and binds to CtBP in vitro through its conserved 15 amino acid repeats. Failure of this association results in elevated levels of beta-catenin/TCF complexes and of TCF-mediated transcription. Notably, CtBP is neither associated with TCF in vivo nor does mutation of the CtBP binding motifs in TCF-4 alter its transcriptional activity. This questions the idea that CtBP is a direct corepressor of TCF. Our evidence indicates that APC is an adaptor between beta-catenin and CtBP and that CtBP lowers the availability of free nuclear beta-catenin for binding to TCF by sequestering APC/beta-catenin complexes.

Proteomic analysis identifies that 14-3-3zeta interacts with beta-catenin and facilitates its activation by Akt

beta-Catenin is a central effector of Wnt signaling in embryonic and stem cell development and in tumorigenesis. Here, through a mass spectrometric analysis of a beta-catenin protein complex, we identified 12 proteins as putative beta-catenin interactors. We show that one of them, 14-3-3zeta, enhances beta-catenin-dependent transcription by maintaining a high level of beta-catenin protein in the cytoplasm. More importantly, 14-3-3zeta facilitates activation of beta-catenin by the survival kinase Akt and colocalizes with activated Akt in intestinal stem cells. We propose that Akt phosphorylates beta-catenin, which results in 14-3-3zeta binding and stabilization of beta-catenin, and these interactions may be involved in stem cell development.

Distinct molecular forms of beta-catenin are targeted to adhesive or transcriptional complexes

β-Catenin plays essential roles in both cell–cell adhesion and Wnt signal transduction, but what precisely controls β-catenin targeting to cadherin adhesive complexes, or T-cell factor (TCF)-transcriptional complexes is less well understood. We show that during Wnt signaling, a form of β-catenin is generated that binds TCF but not the cadherin cytoplasmic domain. The Wnt-stimulated, TCF-selective form is monomeric and is regulated by the COOH terminus of β-catenin, which selectively competes cadherin binding through an intramolecular fold-back mechanism. Phosphorylation of the cadherin reverses the TCF binding selectivity, suggesting another potential layer of regulation. In contrast, the main cadherin-binding form of β-catenin is a β-catenin–α-catenin dimer, indicating that there is a distinct molecular form of β-catenin that can interact with both the cadherin and α-catenin. We propose that participation of β-catenin in adhesion or Wnt signaling is dictated by the regulation of distinct molecular forms of β-catenin with different binding properties, rather than simple competition between cadherins and TCFs for a single constitutive form. This model explains how cells can control whether β-catenin is used independently in cell adhesion and nuclear signaling, or competitively so that the two processes are coordinated and interrelated.

beta-Catenin and plakoglobin N- and C-tails determine ligand specificity

beta-Catenin and plakoglobin are related proteins involved in the regulation of adherens junctions and desmosomes. Moreover, by binding to Tcf-4, they can act as transcriptional modulators of genes involved in embryonic development and tumorigenesis. However, they associate to distinct Tcf-4 subdomains causing opposing effects on Tcf-4 binding to DNA: whereas beta-catenin does not affect this binding, plakoglobin prevents it. Both proteins are composed by two N- and C-tails and a central armadillo repeat domain. Interaction of Tcf-4, as well as other desmosomal or adherens junction components, with beta-catenin or plakoglobin takes place through the central armadillo domain. Here we show that, as reported for beta-catenin, plakoglobin terminal tails also interact with the central domain and regulate the ability of this region to bind to different cofactors. Moreover the specificity of the interaction of beta-catenin and plakoglobin with different subdomains in Tcf-4 or with other junctional components resides within the terminal tails and not in the armadillo domain. For instance, a chimeric protein in which the central domain of beta-catenin was replaced by that of plakoglobin presented the same specificity as wild-type beta-catenin. Therefore, the terminal tails of these proteins are responsible for discerning among binding of factors to the armadillo domain. These results contribute to the understanding of the molecular basis of the interactions established by these key regulators of epithelial tumorigenesis.

Maternal beta-catenin and E-cadherin in mouse development

The oocyte to embryo transition in metazoans depends on maternal proteins and transcripts to ensure the successful initiation of development, and the correct and timely activation of the embryonic genome. We conditionally eliminated the maternal gene encoding the cell adhesion molecule E-cadherin and partially eliminated the beta-catenin gene from the mouse oocyte. Oocytes lacking E-cadherin, or expressing a truncated allele of beta-catenin without the N-terminal part of the protein, give rise to embryos whose blastomeres do not adhere. Blastomere adhesion is restored after translation of protein from the wild-type paternal alleles: at the morula stage in embryos lacking maternal E-cadherin, and at the late four-cell stage in embryos expressing truncated beta-catenin. This suggests that adhesion per se is not essential in the early cleavage stage embryos, that embryos develop normally if compaction does not occur until the morula stage, and that the zona pellucida suffices to maintain blastomere proximity. Although maternal E-cadherin is not essential for the completion of the oocyte-to-embryo transition, absence of wild-type beta-catenin in oocytes does statistically compromise developmental success rates. This developmental deficit is alleviated by the simultaneous absence of maternal E-cadherin, suggesting that E-cadherin regulates nuclear beta-catenin availability during embryonic genome activation.

Wnt/beta-catenin and estrogen signaling converge in vivo

Wnt and estrogen signaling represent important regulatory pathways, each controlling a wide range of biological processes. While an increasing number of observations suggest potential convergence between these pathways, no direct evidence of their functional interaction has been reported. Using human colon and breast cancer cells, we found that estrogen receptor (ER) alpha- and beta-catenin precipitated within the same immunocomplexes, reciprocally enhanced the transactivation of cognate reporter genes, and were reciprocally recruited to cognate response elements in the promoters of endogenous target genes. Using transgenic Drosophila that ectopically expressed human ERalpha alone or together with metabolically stable beta-catenin/Armadillo mutants, we demonstrated genetic interaction between these signal transducers in vivo. Thus, we present here the first direct evidence of cross-talk between Wnt and estrogen signaling pathways via functional interaction between beta-catenin and ERalpha.

Pygopus and Legless target Armadillo/beta-catenin to the nucleus to enable its transcriptional co-activator function

Wnt signalling controls the transcription of genes that function during normal and malignant development. Stimulation by canonical Wnt ligands activates beta-catenin (or Drosophila melanogaster Armadillo) by blocking its phosphorylation, resulting in its stabilization and translocation to the nucleus. Here, Armadillo/beta-catenin binds to TCF/LEF transcription factors and recruits chromatin-modifying and -remodelling complexes to transcribe Wnt target genes. The transcriptional activity of Armadillo/beta-catenin depends on two conserved nuclear proteins recently discovered in Drosophila, Pygopus (Pygo) and Legless/BCL-9 (Lgs). Lgs functions as an adaptor between Pygo and Armadillo/beta-catenin, but how Armadillo/beta-catenin is controlled by Pygo and Lgs is not known. Here, we show that the nuclear localization of Lgs entirely depends on Pygo, which itself is constitutively localized to the nucleus; thus, Pygo functions as a nuclear anchor. Pygo is also required for high nuclear Armadillo levels during Wingless signalling, and together with Lgs increases the transcriptional activity of beta-catenin in APC mutant cancer cells. Notably, linking Armadillo to a nuclear localization sequence rescues pygo and lgs mutant fly embryos. This indicates that Pygo and Lgs function in targeting Armadillo/beta-catenin to the nucleus, thus ensuring its availability to TCF during Wnt signalling.

Sall1, a causative gene for Townes–Brocks syndrome, enhances the canonical Wnt signaling by localizing to heterochromatin

The Spalt (sal) gene family plays an important role in regulating developmental processes of many organisms. Mutations of human SALL1 cause the autosomal dominant disorder, Townes-Brocks syndrome (TBS), and result in ear, limb, anal, renal, and heart anomalies. Targeted deletion of mouse Sall1 results in kidney agenesis or severe dysgenesis. Molecular mechanisms of Sall1, however, have remained largely unknown. Here we report that Sall1 synergistically activates canonical Wnt signaling. The transcriptional activity of Sall1 is related to its nuclear localization to punctate nuclear foci (pericentromeric heterochromatin), but not to its localization or association with beta-catenin, the nuclear component of Wnt signaling. In contrast, the RNA interference of Sall1 reduces reporter activities of canonical Wnt signaling. The N-terminal truncated Sall1, produced by mutations often found in TBS, disturbs localization of native Sall1 to heterochromatin, and also down-regulates the synergistic transcriptional enhancement for Wnt signal by native Sall1. Thus, we propose a new mechanism for Wnt signaling activation, that is the heterochromatin localization of Sall1.

Acetylation of beta-catenin by p300 regulates beta-catenin-Tcf4 interaction

Lysine acetylation modulates the activities of nonhistone regulatory proteins and plays a critical role in the regulation of cellular gene transcription. In this study, we showed that the transcriptional coactivator p300 acetylated beta-catenin at lysine 345, located in arm repeat 6, in vitro and in vivo. Acetylation of this residue increased the affinity of beta-catenin for Tcf4, and the cellular Tcf4-bound pool of beta-catenin was significantly enriched in acetylated form. We demonstrated that the acetyltransferase activity of p300 was required for efficient activation of transcription mediated by beta-catenin/Tcf4 and that the cooperation between p300 and beta-catenin was severely reduced by the K345R mutation, implying that acetylation of beta-catenin plays a part in the coactivation of beta-catenin by p300. Interestingly, acetylation of beta-catenin had opposite, negative effects on the binding of beta-catenin to the androgen receptor. Our data suggest that acetylation of beta-catenin in the arm 6 domain regulates beta-catenin transcriptional activity by differentially modulating its affinity for Tcf4 and the androgen receptor. Thus, our results describe a new mechanism by which p300 might regulate beta-catenin transcriptional activity.

The Tumor Suppressor hSNF5/INI1 Modulates Cell Growth and Actin Cytoskeleton Organization

hSNF5/INI1, which encodes a component of the ATP-dependent chromatin remodeling hSWI-SNF complex, is a tumor suppressor gene mutated in malignant rhabdoid tumors. We have developed a tetracycline-based hSNF5/INI1-inducible system in a hSNF5/INI1-deficient malignant rhabdoid tumor cell line and studied time course variation of 22,000 genes/expressed sequence tags upon hSNF5/INI1 induction. A total of 482 responsive genes were identified and further clustered into 9 groups of coregulated genes. Among genes with early and strong inductions, the use of a fusion protein with the hormone-binding domain of the estrogen receptor enabled the identification of a subset of direct targets regulated independently of de novo protein synthesis. We show that the G(1) arrest induced by hSNF5/INI1 is reversible and associated with the down-regulation of components of the DNA replication complex. We also identify an unsuspected role of hSNF5/INI1 in cytoskeleton organization. Indeed, induction of hSNF5/INI1 induces dramatic modifications of the cell shape including complete disruption of the actin stress fiber network and disappearance of focal adhesions associated with up-regulation of genes involved in the organization of the actin cytoskeleton. We document a strong decrease of Rho activity upon hSNF5/INI1 expression, suggesting that the regulation of this activity constitutes a crucial step of the hSNF5/INI1-induced reorganization of the actin network. This study identifies hSNF5/INI1 target genes and provides evidence that hSNF5/INI1 may modulate the cell cycle control and cytoskeleton organization through the regulation of the retinoblastoma protein-E2F and Rho pathways.

A complex of Armadillo, Legless, and Pygopus coactivates dTCF to activate wingless target genes

BACKGROUND

Upon receiving a Wnt signal, cells accumulate beta-catenin (Armadillo in Drosophila), which binds directly to TCF transcription factors, leading to the transcription of Wnt target genes. It is generally thought that beta-catenin/Armadillo is a transcriptional coactivator when bound to TCF in the nucleus and that this function is mediated by its C terminus. However, recent findings in Drosophila indicated that Armadillo may activate dTCF in the cytoplasm.

RESULTS

Here, I reexamine the mechanism of Armadillo's signaling function in light of Legless and Pygopus, two nuclear factors recently discovered to be essential for this function. I show that Armadillo, in order to activate dTCF, must enter the nucleus and form a complex with Legless and Pygopus. The ability of this complex to stimulate TCF-mediated transcription can be altered by linkage of a strong transcriptional activator or repressor to Armadillo. Furthermore, Armadillo is a strong transcriptional activator when fused to the yeast GAL4 DNA binding domain-an activity that depends on regions of the Armadillo repeat domain that mediate binding to Legless and to chromatin modifying and remodeling factors. Finally, linkage of the N-terminal region of Pygopus, but not the C terminus of Armadillo, to dominant-negative dTCF can restore its signaling activity in transgenic flies.

CONCLUSIONS

My evidence argues in favor of a revised coactivator factor model in which Armadillo's coactivator function depends on regions within its Armadillo repeat domain to which Legless/Pygopus and other transcriptional coactivators can bind. In contrast, the C terminus of Armadillo plays a less direct role in this function.

A nuclear function for armadillo/beta-catenin

The Wnt signaling pathway provides key information during development of vertebrates and invertebrates, and mutations in this pathway lead to various forms of cancer. Wnt binding to its receptor causes the stabilization and nuclear localization of beta-catenin. Nuclear beta-catenin then functions to activate transcription in conjunction with the transcription factor TCF. A recent report has challenged this basic precept of the Wnt signaling field, arguing that the nuclear localization of beta-catenin may be unrelated to its function and that beta-catenin functions at the plasma membrane to activate this signaling pathway. Here we present evidence that the pathway in fact does depend on the nuclear localization of beta-catenin. We reexamine the functionality of various truncations of beta-catenin and find that only the most severe truncations are true signaling-null mutations. Further, we define a signaling-null condition and use it to show that membrane-tethered beta-catenin is insufficient to activate transcription. We also define two novel loss-of-function mutations that are not truncations, but are missense point mutations that retain protein stability. These alleles allow us to show that the membrane-bound form of activated beta-catenin does indeed depend on the endogenous protein. Further, this activity is dependent on the presence of the C-terminus-specific negative regulator Chibby. Our data clearly show that nuclear localization of beta-catenin is in fact necessary for Wnt pathway activation.

In vivo functions of catenins

The adhesion of cells to neighbor cells determines cellular and tissue morphogenesis and regulates major cellular processes including motility, growth, survival, and differentiation. Regions of cell-cell adhesion are adherens junctions, desmosomes, and tight junctions. Cadherins are transmembrane molecules whose extracellular domains transmit the direct interaction of two cells. The intracellular cadherin domains bind directly or indirectly to the submembranous catenins, which are linked to the cytoskeleton. Four types of catenins, alpha-catenin, beta-catenin, gamma-catenin, and p120 catenin are known. Three of them, beta-, gamma-, and p120 catenin, are structurally related and possess similar protein interaction domains, the so-called armadillo repeats. These catenins are also parts of signal transduction pathways and play a role in phenotypical changes of cells, e.g., during switches from adherent to migratory cells. The function of catenins in such basic cellular processes also determines a role of catenins in embryogenesis, adult tissue homeostasis, and disease. In particular, beta-catenin is known to be an important oncoprotein in human cancer development.

Requirement for a nuclear function of beta-catenin in Wnt signaling

Wnt signaling stabilizes beta-catenin, which in turn influences the transcription of Wnt-responsive genes in conjunction with T-cell factor (TCF) transcription factors. At present, there are two models for the actions of beta-catenin. The conventional nuclear model suggests that beta-catenin acts in the nucleus to form a heterodimeric transcriptional factor complex with TCF, with TCF providing DNA-specific binding and the C and N termini of beta-catenin stimulating transcription. The alternative cytoplasmic model postulates that beta-catenin exports TCF from the nucleus to relieve its repressive activity or activates it in the cytoplasm. We have generated modified forms of beta-catenin and used RNA interference against endogenous beta-catenin to distinguish between these models in cultured mammalian and Drosophila cells. We show that the VP16 transcriptional activation domain can replace the C terminus of beta-catenin without loss of function and that the function of beta-catenin is compromised by fusion to a transcriptional repressor domain from histone deacetylase, favoring the direct effects of beta-catenin in the nucleus. Furthermore, membrane-tethered beta-catenin requires interaction with the adenomatous polyposis coli protein but not with TCF for its function, whereas untethered beta-catenin requires binding to TCF for its signaling activity. Importantly, by using RNA interference, we show that the signaling activity of membrane-tethered beta-catenin, but not free beta-catenin, requires the presence of endogenous beta-catenin, which is able to accumulate in the nucleus when stabilized by the binding of the beta-catenin degradation machinery to the membrane-tethered form. All of these data support a nuclear model for the normal function of beta-catenin.

Synergistic effects of coactivators GRIP1 and beta-catenin on gene activation: cross-talk between androgen receptor and Wnt signaling pathways

The p160 coactivators, such as GRIP1, bind nuclear receptors and help to mediate transcriptional activation. beta-Catenin binds to and serves as a coactivator for the nuclear receptor, androgen receptor (AR), and the Lymphoid Enhancer Factor/T Cell Factor family member, Lef1. Here we report that GRIP1 and beta-catenin can bind strongly to each other through the AD2 domain of GRIP1. Furthermore, GRIP1 and beta-catenin can synergistically enhance the activity of both AR and Lef1, and both coactivators are recruited specifically to AR-driven and Lef1-driven promoters. However, the mechanism of beta-catenin-GRIP1 coactivator function and synergy is different with AR and Lef1. While beta-catenin can bind directly to both AR and Lef1, GRIP1 can only bind directly to AR; the ability of GRIP1 to associate with and function as a coactivator for Lef1 is entirely dependent on the presence of beta-catenin. Thus, whereas GRIP1 coactivator function involves direct binding to nuclear receptors and most other classes of DNA-binding transcriptional activator proteins, the coactivator function of GRIP1 with Lef1 follows a novel paradigm where GRIP1 is recruited indirectly to Lef1 through their mutual association with beta-catenin. The beta-catenin-GRIP1 interaction represents another potential point of cross-talk between the AR and Wnt signaling pathways.

Role for ICAT in beta-catenin-dependent nuclear signaling and cadherin functions

Inhibitor of beta-catenin and TCF-4 (ICAT) is a 9-kDa polypeptide that inhibits beta-catenin nuclear signaling by binding beta-catenin and competing its interaction with the transcription factor TCF (T cell factor), but basic characterization of the endogenous protein and degree to which it alters other beta-catenin functions is less well understood. At the subcellular level, we show that ICAT localizes to both cytoplasmic and nuclear compartments. In intestinal tissue, ICAT is upregulated in the mature, nondividing enterocyte population lining intestinal villi and is absent in the beta-catenin/TCF signaling-active crypt region, suggesting that its protein levels may be inversely related with beta-catenin signaling activity. However, ICAT protein levels are not altered by activation or inhibition of Wnt signaling in cultured cells, suggesting that ICAT expression is not a direct target of the Wnt/beta-catenin pathway. In cells where beta-catenin levels are elevated by Wnt, a fraction of this beta-catenin pool is associated with ICAT, suggesting that ICAT may buffer the cell from increased levels of beta-catenin. Distinct from TCF and cadherin, ICAT does not protect the soluble pool of beta-catenin from degradation by the adenomatous polyposis coli containing "destruction complex." Although ICAT inhibits beta-catenin binding to the cadherin as well as TCF in vitro, stable overexpression of ICAT in Madin-Darby canine kidney (MDCK) epithelial cells shows no obvious alterations in the cadherin complex, suggesting that the ability of ICAT to inhibit beta-catenin binding to the cadherin may be restricted in vivo. MDCK cells overexpressing ICAT do, however, exhibit enhanced cell scattering on hepatocyte growth factor treatment, suggesting a possible role in the regulation of dynamic rather than steady-state cell-cell adhesions. These findings confirm ICAT's primary role in beta-catenin signaling inhibition and further suggest that ICAT may have consequences for cadherin-based adhesive function in certain circumstances, implying a broader role than previously described.

Pygopus residues required for its binding to Legless are critical for transcription and development

Pygopus and Legless/Bcl-9 are recently discovered core components of the Wnt signaling pathway that are required for the transcriptional activity of Armadillo/beta-catenin and T cell factors. It has been proposed that they are part of a tri-partite adaptor chain (Armadillo>Legless>Pygopus) that recruits transcriptional co-activator complexes to DNA-bound T cell factor. Here, we identify four conserved residues at the putative PHD domain surface of Drosophila and mouse Pygopus that are required for their binding to Legless in vitro and in vivo. The same residues are also critical for the transactivation potential of DNA-tethered Pygopus in transfected mammalian cells and for rescue activity of pygopus mutant embryos. These residues at the Legless>Pygopus interface thus define a specific molecular target for blocking Wnt signaling during development and cancer.

P16INK4a is required for hSNF5 chromatin remodeler-induced cellular senescence in malignant rhabdoid tumor cells

The hSNF5 chromatin-remodeling factor is a tumor suppressor that is inactivated in malignant rhabdoid tumors (MRTs). A number of studies have shown that hSNF5 re-expression blocks MRT cell proliferation. However, the pathway through which hSNF5 acts remains unknown. To address this question, we generated MRT-derived cell lines in which restoration of hSNF5 expression leads to an accumulation in G(0)/G(1), induces cellular senescence and increased apoptosis. Following hSNF5 expression, we observed transcriptional activation of the tumor suppressor p16(INK4a) but not of p14(ARF), repression of several cyclins and CD44, a cell surface glycoprotein implicated in metastasis. Chromatin immunoprecipitations indicated that hSNF5 activates p16(INK4a) transcription and CD44 down-regulation by mediating recruitment of the SWI/SNF complex. Thus, hSNF5 acts as a dualistic co-regulator that, depending on the promoter context, can either mediate activation or repression. Three lines of evidence established that p16(INK4a) is an essential effector of hSNF5-induced cell cycle arrest. 1) Overexpression of p16(INK4a) mimics the effect of hSNF5 induction and leads to cellular senescence. 2) Expression of a p16(INK4a)-insensitive form of CDK4 obstructs hSNF5-induced cell cycle arrest. 3) Inhibition of p16(INK4a) activation by siRNA blocks hSNF5-mediated cellular senescence. Collectively, these results indicate that in human MRT cells, the p16(INK4a)/pRb, rather than the p14(ARF)/p53 pathway, mediates hSNF5-induced cellular senescence.

The emerging role of epigenetics in cellular and organismal aging

Genome modifications resulting from epigenetic changes appear to play a critical role in the development and/or progression of cancer. Scatter experimental evidence suggests that epigenetic changes could also be critical determinants of cellular senescence and organismal aging. Here we review the current evidence and discuss how imbalances in chromatin remodelers might trigger irreversible growth arrest in proliferating cells and tissues. Experimental data using drugs that target specific chromatin remodeling enzymes suggest that such approach could lead to the development of novel therapeutic modalities for the prevention or amelioration of some age-related dysfunctions.

Mi-2 beta associates with BRG1 and RET finger protein at the distinct regions with transcriptional activating and repressing abilities

Mi-2 beta is the main component of the nucleosome remodeling and deacetylase complex and plays an important role in epigenetic transcriptional repression. Here we show that the amino-terminal and carboxyl-terminal regions of Mi-2 beta have distinct transcriptional activities and bind to BRG1, a component of the SWI/SNF complex, and the RET finger protein (RFP), respectively. Analysis by luciferase reporter assay revealed that the amino-terminal region of Mi-2 beta has a strong transactivating ability, whereas its carboxyl-terminal region has transcriptional repressive activity. Co-localization and association of Mi-2, RFP, and histone deacetylase 1 suggested that these proteins cooperate in transcriptional repression. Furthermore, the functional importance of the association of Mi-2 beta and RFP was confirmed by using Rfp-/- fibroblasts. On the other hand, we demonstrated that Mi-2 and BRG1 were associated with each other and that the bromodomain region of BRG1 strongly suppressed transactivation by the amino-terminal region of Mi-2 beta. The findings that Mi-2 beta interacts with both transactivating and repressing proteins and directly associates with another chromatin remodeling protein, BRG1, provide new insight into the formation of multiprotein supercomplex involved in transcriptional regulation.

The PDZ protein tax-interacting protein-1 inhibits beta-catenin transcriptional activity and growth of colorectal cancer cells

Wnt signaling is essential during development while deregulation of this pathway frequently leads to the formation of various tumors including colorectal carcinomas. A key component of the pathway is beta-catenin that, in association with TCF-4, directly regulates the expression of Wnt-responsive genes. To identify novel binding partners of beta-catenin that may control its transcriptional activity, we performed a mammalian two-hybrid screen and isolated the Tax-interacting protein (TIP-1). The in vivo complex formation between beta-catenin and TIP-1 was verified by coimmunoprecipitation, and a direct physical association was revealed by glutathione S-transferase pull-down experiments in vitro. By using a panel of deletion mutants of both proteins, we demonstrate that the interaction is mediated by the PDZ (PSD-95/DLG/ZO-1 homology) domain of TIP-1 and requires primarily the last four amino acids of beta-catenin. TIP-1 overexpression resulted in a dose-dependent decrease in the transcriptional activity of beta-catenin when tested on the TOP/FOPFLASH reporter system. Conversely, siRNA-mediated knock-down of endogenous TIP-1 slightly increased endogenous beta-catenin transactivation function. Moreover, we show that overexpression of TIP-1 reduced the proliferation and anchorage-independent growth of colorectal cancer cells. These data suggest that TIP-1 may represent a novel regulatory element in the Wnt/beta-catenin signaling pathway.

Transdetermination in Drosophila imaginal discs: a model for understanding pluripotency and selector gene maintenance

Drosophila imaginal disc cells have the ability to undergo transdetermination, a process whereby determined disc cells change fate to that of another disc identity. For example, leg disc cells can transdetermine to develop as wing cells. Such events can occur after mechanical disc fragmentation and subsequent regeneration. A subset of transdetermination events can be induced in situ by misexpression of the signaling gene wingless. Both fragmentation and wingless induce transdetermination by altering the expression of selector genes, which drive disc-specific developmental programs. An important future goal is to address how signaling pathways interact with chromatin structure to regulate and maintain the proper expression of selector genes.

Wnt-cadherin connections in normal and neoplastic mammary epithelium

Since the early recognition that the murine Wnt locus is frequently activated by insertion of the mouse mammary tumour virus, the Wnt pathway has become increasingly associated with both normal and abnormal mammary gland development. This link is further underlined by an emerging role for Wnt deregulation in human mammary neoplasia. The control of transcription through the Wnt signaling pathway is clearly a prime element of this role; however, components of the Wnt pathway possess many functions in addition to their signaling activity, interacting with multiple factors implicated in cellular control processes. Prominent amongst these are the cadherins, which have characterized roles in mediating cell-cell adhesion, and which have been independently implicated in mammary neoplasia. The close interplay between these 2 systems will be discussed in this review with reference both to the normal development of the mammary gland and to the onset of neoplasia in this tissue.

Pygopus is required for embryonic brain patterning in Xenopus

We have identified two Xenopus mRNAs that encode proteins homologous to a component of the Wnt/beta-catenin transcriptional machinery known as Pygopus. The predicted proteins encoded by both mRNAs share the same structural properties with human Pygo-2, but with Xpygo-2alpha having an additional 21 N-terminal residues. Xpygo-2alpha messages accumulate in the prospective anterior neural plate after gastrulation and then are localized to the nervous system, rostral to and including the hindbrain. Xpygo-2beta mRNA is expressed in oocytes and early embryos but declines in level before and during gastrulation. In late neurula, Xpygo-2beta mRNA is restricted to the retinal field, including eye primordia and prospective forebrain. A C-terminal truncated mutant of Xpygo-2 containing the N-terminal Homology Domain (NHD) caused both axis duplication when injected at the 2-cell stage and inhibition of anterior neural development when injected in the prospective head, mimicking the previously described effects of Wnt-signaling activators. Inhibition of Xpygo-2alpha and Xpygo-2beta by injection of gene-specific antisense morpholino oligonucleotides into prospective anterior neurectoderm caused brain defects that were prevented by coinjection of Xpygo-2 mRNA. Both Xpygo-2alpha and Xpygo-2beta morpholinos reduced the eye and forebrain markers Xrx-1, Xpax-6, and XBF-1, while the Xpygo-2alpha morpholino also eliminated expression of the mid-hindbrain marker En-2. The differential expression and regulatory activities of Xpygo-2alpha/beta in rostral neural tissue indicate that they represent essential components of a novel mechanism for Wnt signaling in regionalization of the brain.

A direct beta-catenin-independent interaction between androgen receptor and T cell factor 4

T cell factor (Tcf) proteins bind beta-catenin and are downstream effectors of Wnt/beta-catenin signals. A recently demonstrated interaction between beta-catenin and the androgen receptor (AR) ligand binding domain has suggested that AR may be a Tcf-independent Wnt/beta-catenin effector. This study demonstrates that there is a direct interaction between the AR DNA binding domain (DBD) and Tcf4. Tcf4 bound specifically to a glutathione S-transferase-ARDBD fusion protein and could be coimmunoprecipitated with beta-catenin and transfected AR or endogenous AR in prostate cancer cells. Transfected Tcf4 repressed the transcriptional activity of full-length AR and a VP16-ARDBD fusion protein, and this repression was only partially reversed by transfected beta-catenin. AR activation by cyproterone acetate, a partial agonist that did not support beta-catenin binding to the AR, was also repressed by Tcf4, further indicating that repression was not due to beta-catenin sequestration. Tcf4 could recruit beta-catenin to the AR DBD in vitro and to the cyproterone acetate-liganded AR in vivo. Chromatin immunoprecipitation experiments in LNCaP prostate cancer cells showed that endogenous AR was bound to a Tcf4-responsive element in the c-myc promoter. These findings indicate that AR and Tcf4 can interact directly and that this interaction may occur on the promoters or enhancers of particular genes. The direct AR-Tcf4 interaction, in conjunction AR- and Tcf4-beta-catenin binding, provides a mechanism for cooperative and selective gene regulation by AR and the Wnt/beta-catenin-Tcf pathway that may contribute to normal and neoplastic prostate growth.

Sequential roles of Brg, the ATPase subunit of BAF chromatin remodeling complexes, in thymocyte development

T cells develop through distinct stages directed by a series of signals. We explored the roles of SWI/SNF-like BAF chromatin remodeling complexes in this process by progressive deletion of the ATPase subunit, Brg, through successive stages of early T cell development. Brg-deficient cells were blocked at each of the developmental transitions examined. Bcl-xL overexpression suppressed cell death without relieving the developmental blockades, leading to the accumulation of Brg-deleted cells that were unexpectedly cell cycle arrested. These defects resulted partly from the disruptions of pre-TCR and potentially Wnt signaling pathways controlling the expression of genes such as c-Kit and c-Myc critical for continued development. Our studies indicate that BAF complexes dynamically remodel chromatin to propel sequential developmental transitions in response to external signals.

Initiation and limitation of Ly-49A NK cell receptor acquisition by T cell factor-1

The establishment of clonally variable expression of MHC class I-specific receptors by NK cells is not well understood. The Ly-49A receptor is used by approximately 20% of NK cells, whereby most cells express either the maternal or paternal allele and few express simultaneously both alleles. We have previously shown that NK cells expressing Ly-49A were reduced or almost absent in mice harboring a single or no functional allele of the transcription factor T cell factor-1 (TCF-1), respectively. In this study, we show that enforced expression of TCF-1 in transgenic mice yields an expanded Ly-49A subset. Even though the frequencies of Ly-49A(+) NK cells varied as a function of the TCF-1 dosage, the relative abundance of mono- and biallelic Ly-49A cells was maintained. Mono- and biallelic Ly-49A NK cells were also observed in mice expressing exclusively a transgenic TCF-1, i.e., expressing a fixed amount of TCF-1 in all NK cells. These findings suggest that Ly-49A acquisition is a stochastic event due to limiting TCF-1 availability, rather than the consequence of clonally variable expression of the endogenous TCF-1 locus. Efficient Ly-49A acquisition depended on the expression of a TCF-1 isoform, which included a domain known to associate with the TCF-1 coactivator beta-catenin. Indeed, the proximal Ly-49A promoter was beta-catenin responsive in reporter gene assays. We thus propose that Ly-49A receptor expression is induced from a single allele in occasional NK cells due to a limitation in the amount of a transcription factor complex requiring TCF-1.

The latency-associated nuclear antigen of Kaposi's sarcoma-associated herpesvirus manipulates the activity of glycogen synthase kinase-3beta

The latency-associated nuclear antigen (LANA) of Kaposi's sarcoma-associated herpesvirus (KSHV) is expressed in all KSHV-associated malignancies. LANA is essential for replication and maintenance of the viral episomes during latent infection. However, LANA also has a transcriptional regulatory role and can affect gene expression both positively and negatively. A previously performed yeast two-hybrid screen identified glycogen synthase kinase 3 (GSK-3) as a LANA-interacting protein. Interaction with both GSK-3alpha and GSK-3beta was confirmed in transfected cells with coprecipitation assays. GSK-3beta also interacted with the herpesvirus saimiri homolog ORF73. GSK-3beta is an intermediate in the Wnt signaling pathway and a negative regulator of beta-catenin. In transfected cells, LANA was shown to overcome GSK-3beta-mediated degradation of beta-catenin. Examination of primary effusion lymphoma (PEL) cells found increased levels of beta-catenin relative to KSHV-negative B cells, and this translated into increased activity of a beta-catenin-responsive reporter containing Tcf/Lef binding sites. In tetradecanoyl phorbol acetate-treated PEL cells, loss of LANA expression correlated temporally with loss of detectable beta-catenin. LANA was found to alter the intracellular distribution of GSK-3beta so that nuclear GSK-3beta was more readily detectable in the presence of LANA. Mapping experiments with coimmunoprecipitation assays revealed that both N-terminal and C-terminal LANA sequences were required for efficient GSK-3beta interaction. LANA mutants that were defective for GSK-3beta interaction were unable to mediate GSK-3beta relocalization or activate a beta-catenin-responsive Tcf-luciferase reporter. This study identified manipulation of GSK-3beta activity as a mechanism by which LANA may modify transcriptional activity and contribute to the phenotype of primary effusion lymphoma.

Caught up in a Wnt storm: Wnt signaling in cancer

The Wnt signaling pathway, named for its most upstream ligands, the Wnts, is involved in various differentiation events during embryonic development and leads to tumor formation when aberrantly activated. Molecular studies have pinpointed activating mutations of the Wnt signaling pathway as the cause of approximately 90% of colorectal cancer (CRC), and somewhat less frequently in cancers at other sites, such as hepatocellular carcinoma (HCC). Ironically, Wnts themselves are only rarely involved in the activation of the pathway during carcinogenesis. Mutations mimicking Wnt stimulation-generally inactivating APC mutations or activating beta-catenin mutations-result in nuclear accumulation of beta-catenin which subsequently complexes with T-cell factor/lymphoid enhancing factor (TCF/LEF) transcription factors to activate gene transcription. Recent data identifying target genes has revealed a genetic program regulated by beta-catenin/TCF controlling the transcription of a suite of genes promoting cellular proliferation and repressing differentiation during embryogenesis, carcinogenesis, and in the post-embryonic regulation of cell positioning in the intestinal crypts. This review considers the spectra of tumors arising from active Wnt signaling and attempts to place perspective on recent data that begin to elucidate the mechanisms prompting uncontrolled cell growth following induction of Wnt signaling.

HMG box transcription factor TCF-4's interaction with CtBP1 controls the expression of the Wnt target Axin2/Conductin in human embryonic kidney cells

Members of the Tcf/Lef family of the HMG box transcription factors are nuclear effectors of the Wnt signal transduction pathway. Upon Wnt signaling, TCF/LEF proteins interact with beta-catenin and activate transcription of target genes, while, in the absence of the Wnt signal, TCFs function as transcriptional repressors. All vertebrate Tcf/Lef transcription factors associate with TLE/Groucho-related co-repressors, and here we provide evidence for an interaction between the C-terminus of the TCF-4 HMG box protein and the C-terminal binding protein 1 (CtBP1) transcriptional co-repressor. Using Wnt-1-stimulated human embryonic kidney 293 cells, we show that CtBP1 represses the transcriptional activity of a Tcf/beta-catenin-dependent synthetic promoter and, furthermore, decreases the expression of the endogenous Wnt target, Axin2/Conductin. The CtBP1-mediated repression was alleviated by trichostatin A treatment, indicating that the CtBP inhibitory mechanism is dependent on the activity of histone deacetylases.

Wnt/beta-catenin/tcf signaling: a critical pathway in gastrointestinal tumorigenesis

Cancers of the gastrointestinal tract, including the liver, bile ducts, and pancreas, constitute the largest group of malignant tumors. Colorectal cancer is one of the most common neoplastic diseases in Western countries and one of the leading causes of cancer-related deaths. Inactivation of the adenomatous polyposis coli (APC) tumor-suppressor gene during early adenoma formation is thought to be the first genetic event in the process of colorectal carcinogenesis followed by mutations in oncogenes like K-Ras and tumor-suppressor genes like p53. Identification of the interaction of APC with the proto-oncogene beta-catenin has linked colorectal carcinogenesis to the Wnt-signal transduction pathway. The main function of APC is thought to be the regulation of free beta-catenin in concert with the glycogen synthase kinase 3beta (GSK-3beta) and Axin proteins. Loss of APC function, inactivation of Axin or activating beta-catenin mutations result in the cellular accumulation of beta-catenin. Upon translocation to the nucleus beta-catenin serves as an activator of T-cell factor (Tcf)-dependent transcription leading to an increased expression of several specific target genes including c-Myc, cyclin D1, MMP-7, and ITF-2. While APC mutations are almost exclusively found in colorectal cancers, deregulation of Wnt/beta-catenin/Tcf signaling is also common in other gastrointestinal and extra-gastrointestinal human cancers. In a fraction of hepatocellular carcinomas the Wnt pathway is deregulated by inactivation of Axin or stabilizing mutations of beta-catenin. The majority of hepatoblastomas and a group of gastric cancers also carry beta-catenin mutations. Clearly, this pathway harbors great potential for future applications in cancer diagnostics, staging, and therapy.

Negative regulation of beta-catenin signaling by tyrosine phosphatase SHP-1 in intestinal epithelial cells

Protein-tyrosine phosphatase SHP-1 is expressed at high levels in hematopoietic cells and at moderate levels in many other cell types including epithelial cells. Although SHP-1 has been shown to be a negative regulator of multiple signaling pathways in hematopoietic cells, very little is known about the biological role of SHP-1 in epithelial cells. In order to elucidate the mechanism(s) responsible for the loss of proliferative potential once committed intestinal epithelial cells begin to differentiate, the role and regulation of SHP-1 were analyzed in both intact epithelium as well as in well established intestinal cell models recapitulating the crypt-villus axis in vitro. Results show that SHP-1 was expressed in the nuclei of all intestinal epithelial cell models as well as in epithelial cells of intact human fetal jejunum and colon. Expression and phosphatase activity levels of SHP-1 were much more elevated in confluent growth-arrested intestinal epithelial cells and in differentiated enterocytes as well. Overexpression of SHP-1 in intestinal epithelial crypt cells significantly inhibited dhfr, c-myc, and cyclin D1 gene expression but did not interfere with c-fos gene expression. In contrast, a mutated inactive form of SHP-1 had no effect on these genes. SHP-1 expression significantly decreased beta-catenin/TCF-dependent transcription in intestinal epithelial crypt cells. Immunoprecipitation experiments revealed that beta-catenin is one of the main binding partners and a substrate for SHP-1. Taken together, our results indicate that SHP-1 may be involved in the regulation of beta-catenin transcriptional function and in the negative control of intestinal epithelial cell proliferation.

The SWI/SNF family of ATP-dependent chromatin remodelers: similar mechanisms for diverse functions

The SWI/SNF family of complexes utilizes the energy of ATP hydrolysis to remodel chromatin structures, thereby allowing transcription factors to gain access to DNA. Recent studies suggest that these remodelers also participate in other DNA metabolic reactions such as replication and viral integration, and even in control of cell growth and tumor suppression. The SWI/SNF remodelers can be classified into at least two distinct subfamilies: one includes human BAF (also known as hSWI/SNF-A) and yeast SWI/SNF; the other comprises human PBAF (hSWI/SNF-B) and yeast RSC. Although both types of complexes have similar subunit composition and chromatin remodeling activity in vitro, they cannot replace each other during transcription mediated by specific activators. Thus, each remodeler probably works with a specific set of activators during gene activation. The availability of distinct types of remodelers can allow cells to regulate expression of a specific group of genes by modulating the activity of corresponding remodelers.