A role of Pygopus as an anti-repressor in facilitating Wnt-dependent transcription

Pygo-F773W Mutation Reveals Novel Functions beyond Wnt Signaling in Drosophila

Pygopus (Pygo) has been identified as a specific nuclear co-activator of the canonical Wingless (Wg)/Wnt signaling pathway in Drosophila melanogaster. Pygo proteins consist of two conserved domains: an N-terminal homologous domain (NHD) and a C-terminal plant homologous domain (PHD). The PHD's ability to bind to di- and trimethylated lysine 4 of histone H3 (H3K4me2/3) appears to be independent of Wnt signaling. There is ongoing debate regarding the significance of Pygo's histone-binding capacity. Drosophila Pygo orthologs have a tryptophan (W) > phenylalanine (F) substitution in their histone pocket-divider compared to vertebrates, leading to reduced histone affinity. In this research, we utilized CRISPR/Cas9 technology to introduce the Pygo-F773W point mutation in Drosophila, successfully establishing a viable homozygous Pygo mutant line for the first time. Adult mutant flies displayed noticeable abnormalities in reproduction, locomotion, heart function, and lifespan. RNA-seq and cluster analysis indicated that the mutation primarily affected pathways related to immunity, metabolism, and posttranslational modification in adult flies rather than the Wnt signaling pathway. Additionally, a reduction in H3K9 acetylation levels during the embryonic stage was observed in the mutant strains. These findings support the notion that Pygo plays a wider role in chromatin remodeling, with its involvement in Wnt signaling representing only a specific aspect of its chromatin-related functions.

Dissecting Molecular Genetic Mechanisms of 1q21.1 CNV in Neuropsychiatric Disorders

Pathogenic copy number variations (CNVs) contribute to the etiology of neurodevelopmental/neuropsychiatric disorders (NDs). Increased CNV burden has been found to be critically involved in NDs compared with controls in clinical studies. The 1q21.1 CNVs, rare and large chromosomal microduplications and microdeletions, are detected in many patients with NDs. Phenotypes of duplication and deletion appear at the two ends of the spectrum. Microdeletions are predominant in individuals with schizophrenia (SCZ) and microcephaly, whereas microduplications are predominant in individuals with autism spectrum disorder (ASD) and macrocephaly. However, its complexity hinders the discovery of molecular pathways and phenotypic networks. In this review, we summarize the recent genome-wide association studies (GWASs) that have identified candidate genes positively correlated with 1q21.1 CNVs, which are likely to contribute to abnormal phenotypes in carriers. We discuss the clinical data implicated in the 1q21.1 genetic structure that is strongly associated with neurodevelopmental dysfunctions like cognitive impairment and reduced synaptic plasticity. We further present variations reported in the phenotypic severity, genomic penetrance and inheritance.

Pharmacological Wnt ligand inhibition overcomes key tumor-mediated resistance pathways to anti-PD-1 immunotherapy

While immune checkpoint blockade is associated with prolonged responses in multiple cancers, most patients still do not benefit from this therapeutic strategy. The Wnt-β-catenin pathway is associated with diminished T cell infiltration; however, activating mutations are rare, implicating a role for autocrine/paracrine Wnt ligand-driven signaling in immune evasion. In this study, we show that proximal mediators of the Wnt signaling pathway are associated with anti-PD-1 resistance, and pharmacologic inhibition of Wnt ligand signaling supports anti-PD-1 efficacy by reversing dendritic cell tolerization and the recruitment of granulocytic myeloid-derived suppressor cells in autochthonous tumor models. We further demonstrate that the inhibition of Wnt signaling promotes the development of a tumor microenvironment that is more conducive to favorable responses to checkpoint blockade in cancer patients. These findings support a rationale for Wnt ligand-focused treatment approaches in future immunotherapy clinical trials and suggest a strategy for selecting those tumors more responsive to Wnt inhibition.

The WNT/β‐catenin dependent transcription: A tissue‐specific business

β‐catenin‐mediated Wnt signaling is an ancient cell‐communication pathway in which β‐catenin drives the expression of certain genes as a consequence of the trigger given by extracellular WNT molecules. The events occurring from signal to transcription are evolutionarily conserved, and their final output orchestrates countless processes during embryonic development and tissue homeostasis. Importantly, a dysfunctional Wnt/β‐catenin pathway causes developmental malformations, and its aberrant activation is the root of several types of cancer. A rich literature describes the multitude of nuclear players that cooperate with β‐catenin to generate a transcriptional program. However, a unified theory of how β‐catenin drives target gene expression is still missing. We will discuss two types of β‐catenin interactors: transcription factors that allow β‐catenin to localize at target regions on the DNA, and transcriptional co‐factors that ultimately activate gene expression. In contrast to the presumed universality of β‐catenin's action, the ensemble of available evidence suggests a view in which β‐catenin drives a complex system of responses in different cells and tissues. A malleable armamentarium of players might interact with β‐catenin in order to activate the right “canonical” targets in each tissue, developmental stage, or disease context. Discovering the mechanism by which each tissue‐specific β‐catenin response is executed will be crucial to comprehend how a seemingly universal pathway fosters a wide spectrum of processes during development and homeostasis. Perhaps more importantly, this could ultimately inform us about which are the tumor‐specific components that need to be targeted to dampen the activity of oncogenic β‐catenin.

This article is categorized under:

Cancer > Molecular and Cellular Physiology

Cancer > Genetics/Genomics/Epigenetics

Cancer > Stem Cells and Development

Structure and function of Pygo in organ development dependent and independent Wnt signalling

Pygo is a nuclear protein containing two conserved domains, NHD and PHD, which play important roles in embryonic development and carcinogenesis. Pygo was first identified as a core component of the Wnt/β-catenin signalling pathway. However, it has also been reported that the function of Pygo is not always Wnt/β-catenin signalling dependent. In this review, we summarise the functions of both domains of Pygo and show that their functions are synergetic. The PHD domain mainly combines with transcription co-factors, including histone 3 and Bcl9/9l. The NHD domain mainly recruits histone methyltransferase/acetyltransferase (HMT/HAT) to modify lysine 4 of the histone 3 tail (H3K4) and interacts with Chip/LIM-domain DNA-binding proteins (ChiLS) to form enhanceosomes to regulate transcriptional activity. Furthermore, we summarised chromatin modification differences of Pygo in Drosophila (dPygo) and vertebrates, and found that Pygo displayes a chromatin silencing function in Drosophila, while in vertebates, Pygo has a chromatin-activating function due to the two substitution of two amino acid residues. Next, we confirmed the relationship between Pygo and Bcl9/9l and found that Pygo-Bcl/9l are specifically partnered both in the nucleus and in the cytoplasm. Finally, we discuss whether transcriptional activity of Pygo is Wnt/β-catenin dependent during embryonic development. Available information indications that the transcriptional activity of Pygo in embryonic development is either Wnt/β-catenin dependent or independent in both tissue-specific and cell-specific-modes.

Low BCL9 expression inhibited ovarian epithelial malignant tumor progression by decreasing proliferation, migration, and increasing apoptosis to cancer cells

Background

Abnormal activation of the classic Wnt signaling pathway is closely related to the occurrence of epithelial cancers. B-cell lymphoma 9 (BCL9), a transcription factor, is a novel oncogene discovered in the classic Wnt pathway and promotes the occurrence and development of various tumors. Ovarian cancer is the gynecological malignant tumor with the highest mortality because it is difficult to diagnose early, and easy to relapse and metastasis. The expression and role of BCL9 in epithelial ovarian cancer (EOC) have not been studied. Thus, in this research, we aimed to investigate the expression and clinical significance of BCL9 in EOC tissues and its effect on the malignant biological behavior of human ovarian cancer cells.

Methods

We detect the expression of BCL9 in ovarian epithelial tumor tissues and normal ovarian tissues using immunohistochemistry and analyzed the relationship between it and clinicopathological parameters and patient prognosis. The expression of proteins was detected by Western blot. The MTT assay, flow cytometry, the scratch assay, and the transwell assay were used to detect cell proliferation, apoptosis, migration, and invasion, respectively. A total of 374 ovarian cancer tissue samples were collected using TCGA database. A gene set enrichment analysis of BCL9 was performed.

Results

BCL9 was overexpressed in EOC tissues. The level of BCL9 expression was correlated with the 5-year progression-free survival rate and overall survival rate in ovarian cancer patients and independently predicted the risk of ovarian cancer recurrence. Low BCL9 expression inhibited proliferation, invasion and migration of EOC cells, decreased MMP2 and MMP9 expression of ES-2 cell line, increased the BAX/BCL2 ratio and promoted apoptosis of EOC cells.

Conclusion

BCL9 is overexpressed in epithelial ovarian tumors, resulting in a poor prognosis for ovarian cancer patients. Low BCL9 expression can promote ovarian cancer cell apoptosis, inhibit proliferation and migration. BCL9 promotes the development of ovarian cancer.

USP9X-mediated deubiquitination of B-cell CLL/lymphoma 9 potentiates Wnt signaling and promotes breast carcinogenesis

Hyperactivation of the canonical Wnt-signaling pathway is a prominent feature of a number of human malignancies. Transcriptional activation of this signaling cascade depends on the formation of the β-catenin-B-cell CLL/lymphoma 9 (BCL9)-pygopus (PYGO) family plant homeodomain finger 1 complex, yet how the assembly of this complex is regulated remains to be investigated. Here, using MCF-7, HeLa, HEK293T, MDA-MB-231, and Sf9 cells, along with immunoblotting and immunofluorescence, nano-HPLC-MS/MS, deubiquitination, immunoprecipitation, and chromatin immunoprecipitation (ChIP) assays, we report that BCL9 physically associates with a protein deubiquitinase, ubiquitin-specific peptidase 9, X-linked (USP9X), and that USP9X removes Lys-63-linked polyubiquitin on Lys-212 of BCL9. Importantly, the USP9X-mediated BCL9 deubiquitination facilitated the formation of the β-catenin-BCL9-PYGO complex, thereby potentiating the transcriptional activation of Wnt/β-catenin target genes. We also show that USP9X-mediated BCL9 deubiquitination promotes the proliferation and invasion of breast cancer cells. Together, these results uncover USP9X as a deubiquitinase of BCL9, implicating USP9X in Wnt/β-catenin signaling and breast carcinogenesis.

Expression of miR-30c and BCL-9 in gastric carcinoma tissues and their function in the development of gastric cancer

microRNA-30c (miR-30c) is a member of the miR-30s family, which is known to serve important roles in the occurrence and development of numerous tumor types. Our previous microarray analysis of extracted RNA from tissue samples was conducted to examine the expression of miR-30c and predict miR-30c target genes. In the present study, it was determined that the expression of miR-30c was differentially expressed in 82 paired gastric cancer (GC) and paracancerous tissues. Cellular expression of miR-30c in two GC cell lines MKN-45, MKN-74 and one non-cancer cell line GES-1 was modified using the miR-30c-mimic and miR-30c-inhibitor reagents, in a series of transfection experiments. Following transfection of cancer and non-cancer cell lines with the miR-30c-mimic, cell proliferation and apoptosis rates were increased. Compared with the NC group, MKN-74 cell proliferation was significantly inhibited (P<0.05) following transfection with the miR-30c-mimic at 48 and 24 h, GES-1 was significantly inhibited (P<0.05) at 24 and 48 h, and apoptosis was significantly reduced in transfected MKN-74 cells (P<0.05). The clinicopathological data and the expression of BCL-9 and miR-30c in patients with GC were used to identify associations. The expression levels of miR-30c were associated with age. Western blot analysis demonstrated that the BCL-9 expression levels in MKN-74 cells were higher following transfection with the miR-30c-mimic, and were lower following transfection with the miR-30c-inhibitor, both compared with the negative control group. It was concluded that compared with the negative control group, the expression of miR-30c was low in GC tissues and may be involved in GC development via regulation of proliferation, apoptosis and the cell cycle.

The Wnt Transcriptional Switch: TLE Removal or Inactivation?

Many targets of the Wnt/β-catenin signaling pathway are regulated by TCF transcription factors, which play important roles in animal development, stem cell biology, and oncogenesis. TCFs can regulate Wnt targets through a "transcriptional switch," repressing gene expression in unstimulated cells and promoting transcription upon Wnt signaling. However, it is not clear whether this switch mechanism is a general feature of Wnt gene regulation or limited to a subset of Wnt targets. Co-repressors of the TLE family are known to contribute to the repression of Wnt targets in the absence of signaling, but how they are inactivated or displaced by Wnt signaling is poorly understood. In this mini-review, we discuss several recent reports that address the prevalence and molecular mechanisms of the Wnt transcription switch, including the finding of Wnt-dependent ubiquitination/inactivation of TLEs. Together, these findings highlight the growing complexity of the regulation of gene expression by the Wnt pathway.

miR-1301 inhibits hepatocellular carcinoma cell migration, invasion, and angiogenesis by decreasing Wnt/β-catenin signaling through targeting BCL9

Metastasis is the major cause of the poor prognosis of hepatocellular carcinoma (HCC), and increasing evidence supports the contribution of miRNAs to cancer progression. However, the exact relationship between the level of miR-1301 expression and HCC cell migration, invasion, and angiogenesis remains largely unknown. Quantitative PCR was used to evaluate the level of miR-1301 expression in HCC tissues and cell lines. Transwell and tube-formation assays were used to measure the effects of miR-1301 on HCC cell migration and invasion, and angiogenesis, respectively. Luciferase reporter assays and western blotting were used to confirm the miR-1301 target genes. We found that miR-1301 was significantly downregulated in HCC tissues and cell lines. Low miR-1301 expression was associated with tumor vascular invasion and Edmondson grade. Gain- and loss-of-function assays demonstrated that miR-1301 inhibited the migration, invasion, epithelial-mesenchymal transition, and angiogenesis of HCC cells in vitro and in vivo. BCL9, upregulated in HCC tissues compared with matched adjacent normal tissues, was inversely correlated to miR-1301 levels in HCC tissues. Through reporter gene and western blot assays, BCL9 was shown to be a direct miR-1301 target. BCL9 overexpression could partially reverse the effects of miR-1301 on HCC cell migration and invasion. Most importantly, miR-1301 overexpression markedly suppressed the death of xenograft mouse models of cancer by reducing tumor load, metastasis, and host angiogenesis by downregulating BCL9, β-catenin, and vascular endothelial growth factor expression in tumor cells. Our observations suggested that miR-1301 inhibits HCC migration, invasion, and angiogenesis via decreasing Wnt/β-catenin signaling through targeting BCL9, and might be a therapeutic target for HCC.

Wnt-Dependent Inactivation of the Groucho/TLE Co-repressor by the HECT E3 Ubiquitin Ligase Hyd/UBR5

Extracellular signals are transduced to the cell nucleus by effectors that bind to enhancer complexes to operate transcriptional switches. For example, the Wnt enhanceosome is a multiprotein complex associated with Wnt-responsive enhancers through T cell factors (TCF) and kept silent by Groucho/TLE co-repressors. Wnt-activated β-catenin binds to TCF to overcome this repression, but how it achieves this is unknown. Here, we discover that this process depends on the HECT E3 ubiquitin ligase Hyd/UBR5, which is required for Wnt signal responses in Drosophila and human cell lines downstream of activated Armadillo/β-catenin. We identify Groucho/TLE as a functionally relevant substrate, whose ubiquitylation by UBR5 is induced by Wnt signaling and conferred by β-catenin. Inactivation of TLE by UBR5-dependent ubiquitylation also involves VCP/p97, an AAA ATPase regulating the folding of various cellular substrates including ubiquitylated chromatin proteins. Thus, Groucho/TLE ubiquitylation by Hyd/UBR5 is a key prerequisite that enables Armadillo/β-catenin to activate transcription.

Constitutive scaffolding of multiple Wnt enhanceosome components by Legless/BCL9

Wnt/β-catenin signaling elicits context-dependent transcription switches that determine normal development and oncogenesis. These are mediated by the Wnt enhanceosome, a multiprotein complex binding to the Pygo chromatin reader and acting through TCF/LEF-responsive enhancers. Pygo renders this complex Wnt-responsive, by capturing β-catenin via the Legless/BCL9 adaptor. We used CRISPR/Cas9 genome engineering of Drosophila legless (lgs) and human BCL9 and B9L to show that the C-terminus downstream of their adaptor elements is crucial for Wnt responses. BioID proximity labeling revealed that BCL9 and B9L, like PYGO2, are constitutive components of the Wnt enhanceosome. Wnt-dependent docking of β-catenin to the enhanceosome apparently causes a rearrangement that apposes the BCL9/B9L C-terminus to TCF. This C-terminus binds to the Groucho/TLE co-repressor, and also to the Chip/LDB1-SSDP enhanceosome core complex via an evolutionary conserved element. An unexpected link between BCL9/B9L, PYGO2 and nuclear co-receptor complexes suggests that these β-catenin co-factors may coordinate Wnt and nuclear hormone responses.

Vertebrate Axial Patterning: From Egg to Asymmetry

The emergence of the bilateral embryonic body axis from a symmetrical egg has been a long-standing question in developmental biology. Historical and modern experiments point to an initial symmetry-breaking event leading to localized Wnt and Nodal growth factor signaling and subsequent induction and formation of a self-regulating dorsal "organizer." This organizer forms at the site of notochord cell internalization and expresses primarily Bone Morphogenetic Protein (BMP) growth factor antagonists that establish a spatiotemporal gradient of BMP signaling across the embryo, directing initial cell differentiation and morphogenesis. Although the basics of this model have been known for some time, many of the molecular and cellular details have only recently been elucidated and the extent that these events remain conserved throughout vertebrate evolution remains unclear. This chapter summarizes historical perspectives as well as recent molecular and genetic advances regarding: (1) the mechanisms that regulate symmetry-breaking in the vertebrate egg and early embryo, (2) the pathways that are activated by these events, in particular the Wnt pathway, and the role of these pathways in the formation and function of the organizer, and (3) how these pathways also mediate anteroposterior patterning and axial morphogenesis. Emphasis is placed on comparative aspects of the egg-to-embryo transition across vertebrates and their evolution. The future prospects for work regarding self-organization and gene regulatory networks in the context of early axis formation are also discussed.

BCL9, a coactivator for Wnt/β-catenin transcription, is targeted by miR-30c and is associated with prostate cancer progression

B-cell lymphoma 9 (BCL9), a component of aberrantly activated Wnt signaling, is an important contributing factor to tumor progression. Our previous data indicated that downregulation of the tumor suppressor microRNA-30c (miR-30c) was a frequent pathogenetic event in prostate cancer (PCa). However, a functional link between miR-30c and BCL9/Wnt signaling, and their clinical and pathological significance in PCa, have not been well established. The present study demonstrated that miR-30c serves as a key negative regulator targeting BCL9 transcription in PCa cells. Ectopic expression of miR-30c was associated with reduced expression of Wnt pathway downstream targets, including c-Myc, cluster of differentiation 44 and sex determining region Y-box 9 in DU145 human PCa cells. Examination of clinical prostate specimens revealed higher levels of BCL9 expression in PCa compared with that in benign prostate tissues. After substantiating this finding by patient sample analysis, BCL9 expression or activity was observed to be closely correlated with PCa biochemical recurrence (BCR) and disease progression, whereas it was inversely associated with miR-30c. Furthermore, overexpression of BCL9 in PCa acted cooperatively with miR-30c low expression to predict earlier BCR in PCa. These findings indicate that inhibition of BCL9/Wnt signaling by miR-30c is important in the progression of PCa. Furthermore, the combined analysis of miR-30c and BCL9 may be valuable tool for prediction of BCR in PCa patients following radical prostatectomy.

β-Catenin, a Transcription Factor Activated by Canonical Wnt Signaling, Is Expressed in Sensory Neurons of Calves Latently Infected with Bovine Herpesvirus 1

Like many Alphaherpesvirinae subfamily members, bovine herpesvirus 1 (BoHV-1) expresses an abundant transcript in latently infected sensory neurons, the latency-related (LR)-RNA. LR-RNA encodes a protein (ORF2) that inhibits apoptosis, interacts with Notch family members, interferes with Notch-mediated transcription, and stimulates neurite formation in cells expressing Notch. An LR mutant virus containing stop codons at the amino terminus of ORF2 does not reactivate from latency or replicate efficiently in certain tissues, indicating that LR gene products are important. In this study, β-catenin, a transcription factor activated by the canonical Wnt signaling pathway, was frequently detected in ORF2-positive trigeminal ganglionic neurons of latently infected, but not mock-infected, calves. Conversely, the lytic cycle regulatory protein (BoHV-1 infected cell protein 0, or bICP0) was not frequently detected in β-catenin-positive neurons in latently infected calves. During dexamethasone-induced reactivation from latency, mRNA expression levels of two Wnt antagonists, Dickkopf-1 (DKK-1) and secreted Frizzled-related protein 2 (SFRP2), were induced in bovine trigeminal ganglia (TG), which correlated with reduced β-catenin protein expression in TG neurons 6 h after dexamethasone treatment. ORF2 and a coactivator of β-catenin, mastermind-like protein 1 (MAML1), stabilized β-catenin protein levels and stimulated β-catenin-dependent transcription in mouse neuroblastoma cells more effectively than MAML1 or ORF2 alone. Neuroblastoma cells expressing ORF2, MAML1, and β-catenin were highly resistant to cell death following serum withdrawal, whereas most cells transfected with only one of these genes died. The Wnt signaling pathway interferes with neurodegeneration but promotes neuronal differentiation, suggesting that stabilization of β-catenin expression by ORF2 promotes neuronal survival and differentiation.

IMPORTANCE

Bovine herpesvirus 1 (BoHV-1) is an important pathogen of cattle, and like many Alphaherpesvirinae subfamily members establishes latency in sensory neurons. Lifelong latency and the ability to reactivate from latency are crucial for virus transmission. Maintaining the survival and normal functions of terminally differentiated neurons is also crucial for lifelong latency. Our studies revealed that BoHV-1 gene products expressed during latency stabilize expression of the transcription factor β-catenin and perhaps its cofactor, mastermind-like protein 1 (MAML1). In contrast to expression during latency, β-catenin expression in sensory neurons is not detectable following treatment of latently infected calves with the synthetic corticosteroid dexamethasone to initiate reactivation from latency. A viral protein (ORF2) expressed in a subset of latently infected neurons stabilized β-catenin and MAML1 in transfected cells. ORF2, β-catenin, and MAML1 also enhanced cell survival when growth factors were withdrawn, suggesting that these genes enhance survival of latently infected neurons.

An ancient Pygo-dependent Wnt enhanceosome integrated by Chip/LDB-SSDP

TCF/LEF factors are ancient context-dependent enhancer-binding proteins that are activated by β-catenin following Wnt signaling. They control embryonic development and adult stem cell compartments, and their dysregulation often causes cancer. β-catenin-dependent transcription relies on the NPF motif of Pygo proteins. Here, we use a proteomics approach to discover the Chip/LDB-SSDP (ChiLS) complex as the ligand specifically binding to NPF. ChiLS also recognizes NPF motifs in other nuclear factors including Runt/RUNX2 and Drosophila ARID1, and binds to Groucho/TLE. Studies of Wnt-responsive dTCF enhancers in the Drosophila embryonic midgut indicate how these factors interact to form the Wnt enhanceosome, primed for Wnt responses by Pygo. Together with previous evidence, our study indicates that ChiLS confers context-dependence on TCF/LEF by integrating multiple inputs from lineage and signal-responsive factors, including enhanceosome switch-off by Notch. Its pivotal function in embryos and stem cells explain why its integrity is crucial in the avoidance of cancer.

Pygopus 2 promotes kidney cancer OS-RC-2 cells proliferation and invasion in vitro and in vivo

Objective

Human Pygopus 2 (Pygo2) was recently discovered to be a component of the Wnt signaling pathway required for β-catenin/Tcf-mediated transcription. But the role of Pygo2 in malignant cell proliferation and invasion has not yet been determined.

Methods

Lentivirus-mediated small interfering RNA (siRNA) and vector-based overexpression were used to study the function of Pygo2 in OS-RC-2 cells. The resulted cells were subject to Western blotting assay, MTT assay, colony formation and cell invasion assays. Furthermore, renal cell carcinoma (RCC) models were established in BALB/c nude mice inoculated with OS-RC-2 cells. Immunohistochemistry (IHC) staining of matrix metalloproteinase-7 (MMP-7), matrix metalloproteinase-9 (MMP-9) and vascular endothelial growth factor (VEGF) was performed in tumor tissue.

Results

Pygo2 gene was successful knocked down and overexpressed in RCC OS-RC-2 cells by using an shRNA and overexpressing vector, respectively. Overexpression of Pygo2 effectively promoted cell proliferation, colony formation and invasion in vitro. Knockdown of Pygo2 obviously inhibited xenograft tumor growth in nude mice. In addition, overexpression of Pygo2 increased the levels of MMP-7, MMP-9 and VEGF in the xenograft tumors.

Conclusion

Pygo2 has a role in promoting cell proliferation, invasion and metastasis, and may regulate angiogenesis via the Wnt/β-catenin signaling pathway.

Wnt some lose some: transcriptional governance of stem cells by Wnt/β-catenin signaling

In mammals, Wnt/β-catenin signaling features prominently in stem cells and cancers, but how and for what purposes have been matters of much debate. In this review, we summarize our current knowledge of Wnt/β-catenin signaling and its downstream transcriptional regulators in normal and malignant stem cells. We centered this review largely on three types of stem cells--embryonic stem cells, hair follicle stem cells, and intestinal epithelial stem cells--in which the roles of Wnt/β-catenin have been extensively studied. Using these models, we unravel how many controversial issues surrounding Wnt signaling have been resolved by dissecting the diversity of its downstream circuitry and effectors, often leading to opposite outcomes of Wnt/β-catenin-mediated regulation and differences rooted in stage- and context-dependent effects.

Evolutionary adaptation of the fly Pygo PHD finger toward recognizing histone H3 tail methylated at arginine 2

Pygo proteins promote Armadillo- and β-catenin-dependent transcription, by relieving Groucho-dependent repression of Wnt targets. Their PHD fingers bind histone H3 tail methylated at lysine 4, and to the HD1 domain of their Legless/BCL9 cofactors, linking Pygo to Armadillo/β-catenin. Intriguingly, fly Pygo orthologs exhibit a tryptophan > phenylalanine substitution in their histone pocket-divider which reduces their affinity for histones. Here, we use X-ray crystallography and NMR, to discover a conspicuous groove bordering this phenylalanine in the Drosophila PHD-HD1 complex--a semi-aromatic cage recognizing asymmetrically methylated arginine 2 (R2me2a), a chromatin mark of silenced genes. Our structural model of the ternary complex reveals a distinct mode of dimethylarginine recognition, involving a polar interaction between R2me2a and its groove, the structural integrity of which is crucial for normal tissue patterning. Notably, humanized fly Pygo derepresses Notch targets, implying an inherent Notch-related function of classical Pygo orthologs, disabled in fly Pygo, which thus appears dedicated to Wnt signaling.

Regeneration-associated WNT signaling is activated in long-term reconstituting AC133bright acute myeloid leukemia cells

Acute myeloid leukemia (AML) is a genetically heterogeneous clonal disorder characterized by two molecularly distinct self-renewing leukemic stem cell (LSC) populations most closely related to normal progenitors and organized as a hierarchy. A requirement for WNT/β-catenin signaling in the pathogenesis of AML has recently been suggested by a mouse model. However, its relationship to a specific molecular function promoting retention of self-renewing leukemia-initiating cells (LICs) in human remains elusive. To identify transcriptional programs involved in the maintenance of a self-renewing state in LICs, we performed the expression profiling in normal (n = 10) and leukemic (n = 33) human long-term reconstituting AC133(+) cells, which represent an expanded cell population in most AML patients. This study reveals the ligand-dependent WNT pathway activation in AC133(bright) AML cells and shows a diffuse expression and release of WNT10B, a hematopoietic stem cell regenerative-associated molecule. The establishment of a primary AC133(+) AML cell culture (A46) demonstrated that leukemia cells synthesize and secrete WNT ligands, increasing the levels of dephosphorylated β-catenin in vivo. We tested the LSC functional activity in AC133(+) cells and found significant levels of engraftment upon transplantation of A46 cells into irradiated Rag2(-/-)γc(-/-) mice. Owing to the link between hematopoietic regeneration and developmental signaling, we transplanted A46 cells into developing zebrafish. This system revealed the formation of ectopic structures by activating dorsal organizer markers that act downstream of the WNT pathway. In conclusion, our findings suggest that AC133(bright) LSCs are promoted by misappropriating homeostatic WNT programs that control hematopoietic regeneration.

The Pygo2-H3K4me2/3 interaction is dispensable for mouse development and Wnt signaling-dependent transcription

Pygopus has been discovered as a fundamental Wnt signaling component in Drosophila. The mouse genome encodes two Pygopus homologs, Pygo1 and Pygo2. They serve as context-dependent β-catenin coactivators, with Pygo2 playing the more important role. All Pygo proteins share a highly conserved plant homology domain (PHD) that allows them to bind di- and trimethylated lysine 4 of histone H3 (H3K4me2/3). Despite the structural conservation of this domain, the relevance of histone binding for the role of Pygo2 as a Wnt signaling component and as a reader of chromatin modifications remains speculative. Here we generate a knock-in mouse line, homozygous for a Pygo2 mutant defective in chromatin binding. We show that even in the absence of the potentially redundant Pygo1, Pygo2 does not require the H3K4me2/3 binding activity to sustain its function during mouse development. Indeed, during tissue homeostasis, Wnt/β-catenin-dependent transcription is largely unaffected. However, the Pygo2-chromatin interaction is relevant in testes, where, importantly, Pygo2 binds in vivo to the chromatin in a PHD-dependent manner. Its presence on regulatory regions does not affect the transcription of nearby genes; rather, it is important for the recruitment of the histone acetyltransferase Gcn5 to chromatin, consistent with a testis-specific and Wnt-unrelated role for Pygo2 as a chromatin remodeler.

Wnt/β-catenin signalling: from plasma membrane to nucleus

Wnt/β-catenin signalling plays essential roles in embryonic development as well as tissue homoeostasis in adults. Thus abnormal regulation of Wnt/β-catenin signalling is linked to a variety of human diseases, including cancer, osteoporosis and Alzheimer's disease. Owing to the importance of Wnt signalling in a wide range of biological fields, a better understanding of its precise mechanisms could provide fundamental insights for therapeutic applications. Although many studies have investigated the regulation of Wnt/β-catenin signalling, our knowledge remains insufficient due to the complexity and diversity of Wnt signalling. It is generally accepted that the identification of novel regulators and their functions is a prerequisite to fully elucidating the regulation of Wnt/β-catenin signalling. Recently, several novel modulators of Wnt signalling have been determined through multiple genetic and proteomic approaches. In the present review, we discuss the mechanistic regulation of Wnt/β-catenin signalling by focusing on the roles of these novel regulators.

The many faces and functions of β-catenin

β-Catenin (Armadillo in Drosophila) is a multitasking and evolutionary conserved molecule that in metazoans exerts a crucial role in a multitude of developmental and homeostatic processes. More specifically, β-catenin is an integral structural component of cadherin-based adherens junctions, and the key nuclear effector of canonical Wnt signalling in the nucleus. Imbalance in the structural and signalling properties of β-catenin often results in disease and deregulated growth connected to cancer and metastasis. Intense research into the life of β-catenin has revealed a complex picture. Here, we try to capture the state of the art: we try to summarize and make some sense of the processes that regulate β-catenin, as well as the plethora of β-catenin binding partners. One focus will be the interaction of β-catenin with different transcription factors and the potential implications of these interactions for direct cross-talk between β-catenin and non-Wnt signalling pathways.

TCFs and Wnt/β-catenin signaling: more than one way to throw the switch

Wnts are conserved, secreted signaling proteins that can influence cell behavior by stabilizing β-catenin. Accumulated β-catenin enters the nucleus, where it physically associates with T-cell factor (TCF) family members to regulate target gene expression in many developmental and adult tissues. Recruitment of β-catenin to Wnt response element (WRE) chromatin converts TCFs from transcriptional repressors to activators. This review will outline the complex interplay between factors contributing to TCF repression and coactivators working with β-catenin to regulate Wnt targets. In addition, three variations of the standard transcriptional switch model will be discussed. One is the Wnt/β-catenin symmetry pathway in Caenorhabditis elegans, where Wnt-mediated nuclear efflux of TCF is crucial for activation of targets. Another occurs in vertebrates, where distinct TCF family members are associated with repression and activation, and recent evidence suggests that Wnt signaling facilitates a "TCF exchange" on WRE chromatin. Finally, a "reverse switch" mechanism for target genes that are directly repressed by Wnt/β-catenin signaling occurs in Drosophila cells. The diversity of TCF regulatory mechanisms may help to explain how a small group of transcription factors can function in so many different contexts to regulate target gene expression.

Identification of a link between Wnt/β-catenin signalling and the cell fusion pathway

Cell fusion has a critical role in various developmental processes, immune response, tissue homeostasis and regeneration, and possibly, in cancer. However, the signals that regulate cell fusion remain poorly understood. In a screen for novel targets of Wnt/β-catenin signalling, we identified glial cells missing 1 (GCM1), which encodes a transcription factor that is involved in epigenetic regulation and is critical for the fusion of syncytiotrophoblast (ST) cells. Here we show that β-catenin/BCL9-Like (BCL9L)/T-cell factor 4 (TCF4) signalling directly targets the GCM1/syncytin pathway and thereby regulates the fusion of human choriocarcinoma cells. Furthermore, we show that the GCM1/syncytin-B pathway is significantly downregulated in the placenta of BCL9L-deficient mice and that the fusion and differentiation of ST-II cells are blocked. Our results demonstrate a signal transduction pathway that regulates cell fusion, and may provide intriguing perspectives into the various biological and pathological processes that involve cell fusion.

The oligomeric state of CtBP determines its role as a transcriptional co-activator and co-repressor of Wingless targets

C-terminal-binding protein (CtBP) is a well-characterized transcriptional co-repressor that requires homo-dimerization for its activity. CtBP can both repress and activate Wingless nuclear targets in Drosophila. Here, we examine the role of CtBP dimerization in these opposing processes. CtBP mutants that cannot dimerize are able to promote Wingless signalling, but are defective in repressing Wingless targets. To further test the role of dimerization in repression, the positions of basic and acidic residues that form inter-molecular salt bridges in the CtBP dimerization interface were swapped. These mutants cannot homo-dimerize and are compromised for repression. However, their co-expression leads to hetero-dimerization and consequent repression of Wingless targets. Our results support a model where CtBP is a gene-specific regulator of Wingless signalling, with some targets requiring CtBP dimers for inhibition while other targets utilize CtBP monomers for activation of their expression. Functional interactions between CtBP and Pygopus, a nuclear protein required for Wingless signalling, support a model where monomeric CtBP acts downstream of Pygopus in activating some Wingless targets.

Wnt signaling from development to disease: insights from model systems

One of the early surprises in the study of cell adhesion was the discovery that beta-catenin plays dual roles, serving as an essential component of cadherin-based cell-cell adherens junctions and also serving as the key regulated effector of the Wnt signaling pathway. Here, we review our current model of Wnt signaling and discuss how recent work using model organisms has advanced our understanding of the roles Wnt signaling plays in both normal development and in disease. These data help flesh out the mechanisms of signaling from the membrane to the nucleus, revealing new protein players and providing novel information about known components of the pathway.

Revving the Throttle on an oncogene: CDK8 takes the driver seat

The Wnt/beta-catenin pathway plays an important role in initiation in most, if not all, colon cancers. Prior work has provided important insights into the regulation of beta-catenin stability in the cytoplasm; however, relatively little is known about the mechanism by which beta-catenin activates gene transcription in the nucleus. Using genetic approaches, studies in human colon cancers and Drosophila have identified CDK8 as a colon cancer oncogene that regulates beta-catenin transcriptional activity. These convergent observations provide new insights into the regulation of nuclear beta-catenin activity and identify a novel therapeutic target for beta-catenin-driven malignancies.

Wnt/beta-catenin signaling: components, mechanisms, and diseases

Signaling by the Wnt family of secreted glycolipoproteins via the transcriptional coactivator beta-catenin controls embryonic development and adult homeostasis. Here we review recent progress in this so-called canonical Wnt signaling pathway. We discuss Wnt ligands, agonists, and antagonists, and their interactions with Wnt receptors. We also dissect critical events that regulate beta-catenin stability, from Wnt receptors to the cytoplasmic beta-catenin destruction complex, and nuclear machinery that mediates beta-catenin-dependent transcription. Finally, we highlight some key aspects of Wnt/beta-catenin signaling in human diseases including congenital malformations, cancer, and osteoporosis, and discuss potential therapeutic implications.

Wnt/beta-catenin signaling: components, mechanisms, and diseases

Signaling by the Wnt family of secreted glycolipoproteins via the transcriptional coactivator beta-catenin controls embryonic development and adult homeostasis. Here we review recent progress in this so-called canonical Wnt signaling pathway. We discuss Wnt ligands, agonists, and antagonists, and their interactions with Wnt receptors. We also dissect critical events that regulate beta-catenin stability, from Wnt receptors to the cytoplasmic beta-catenin destruction complex, and nuclear machinery that mediates beta-catenin-dependent transcription. Finally, we highlight some key aspects of Wnt/beta-catenin signaling in human diseases including congenital malformations, cancer, and osteoporosis, and discuss potential therapeutic implications.

Wnt/beta-catenin signaling: components, mechanisms, and diseases

Signaling by the Wnt family of secreted glycolipoproteins via the transcriptional coactivator beta-catenin controls embryonic development and adult homeostasis. Here we review recent progress in this so-called canonical Wnt signaling pathway. We discuss Wnt ligands, agonists, and antagonists, and their interactions with Wnt receptors. We also dissect critical events that regulate beta-catenin stability, from Wnt receptors to the cytoplasmic beta-catenin destruction complex, and nuclear machinery that mediates beta-catenin-dependent transcription. Finally, we highlight some key aspects of Wnt/beta-catenin signaling in human diseases including congenital malformations, cancer, and osteoporosis, and discuss potential therapeutic implications.

Beta-catenin hits chromatin: regulation of Wnt target gene activation

The canonical Wnt pathway has gathered much attention in recent years owing to its fundamental contribution to metazoan development, tissue homeostasis and human malignancies. Wnt target gene transcription is regulated by nuclear beta-catenin, and genetic assays have revealed various collaborating protein cofactors. Their daunting number and diverse nature, however, make it difficult to arrange an orderly picture of the nuclear Wnt transduction events. Yet, these findings emphasize that beta-catenin-mediated transcription affects chromatin. How does beta-catenin cope with chromatin regulation to turn on Wnt target genes?