Pygopus and the Wnt signaling pathway: a diverse set of connections

Automated profiling of gene function during embryonic development

Systematic functional profiling of the gene set that directs embryonic development is an important challenge. To tackle this challenge, we used 4D imaging of C. elegans embryogenesis to capture the effects of 500 gene knockdowns and developed an automated approach to compare developmental phenotypes. The automated approach quantifies features-including germ layer cell numbers, tissue position, and tissue shape-to generate temporal curves whose parameterization yields numerical phenotypic signatures. In conjunction with a new similarity metric that operates across phenotypic space, these signatures enabled the generation of ranked lists of genes predicted to have similar functions, accessible in the PhenoBank web portal, for ∼25% of essential development genes. The approach identified new gene and pathway relationships in cell fate specification and morphogenesis and highlighted the utilization of specialized energy generation pathways during embryogenesis. Collectively, the effort establishes the foundation for comprehensive analysis of the gene set that builds a multicellular organism.

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.

Complex Genomic Rearrangements Involving ETV6::ABL1 Gene Fusion in an Individual with Myeloid Neoplasm

ETV6::ABL1 gene fusion is a rare recurrent genomic rearrangement associated with hematologic malignancies, and frequently occurs with additional anomalies. Due to the opposite chromosome orientations of the ETV6 and ABL1 genes, an oncogenic in-frame ETV6::ABL1 gene fusion cannot be formed by a simple translocation. The molecular mechanism of the ETV6::ABL1 fusion and the significance of co-occurring anomalies are not fully understood. We characterized genomic alterations in an individual with ETV6::ABL1 gene-fusion-positive myeloid neoplasm using various genomic technologies. Our findings uncovered a molecular mechanism of the ETV6::ABL1 fusion, in which a paracentric inversion within the short arm of chromosome 12 (12p) and a translocation between the long arm of a chromosome 9 and the 12p with the inversion were involved. In addition, we detected multiple additional anomalies in the individual, and our findings suggested that the ETV6::ABL1 fusion occurred as a secondary event in a subset of cells with the additional anomalies. We speculate that the additional anomalies may predispose to further pathogenic changes, including ETV6::ABL1 fusion, leading to neoplastic transformation.

Structural basis of the interaction between BCL9-Pygo and LDB-SSBP complexes in assembling the Wnt enhanceosome

The Wnt enhanceosome is responsible for transactivation of Wnt-responsive genes and a promising therapeutic target for treatment of numerous cancers with Adenomatous Polyposis Coli (APC) or β-catenin mutations. How the Wnt enhanceosome is assembled remains poorly understood. Here we show that B-cell lymphoma 9 protein (BCL9), Pygopus (Pygo), LIM domain-binding protein 1 (LDB1) and single-stranded DNA-binding protein (SSBP) form a stable core complex within the Wnt enhanceosome. Their mutual interactions rely on a highly conserved N-terminal asparagine proline phenylalanine (NPF) motif of Pygo, through which the BCL9-Pygo complex binds to the LDB-SSBP core complex. Our crystal structure of a ternary complex comprising the N-terminus of human Pygo2, LDB1 and SSBP2 reveals a single LDB1-SSBP2 complex binding simultaneously to two Pygo2 molecules via their NPF motifs. These interactions critically depend on the NPF motifs which bind to a deep groove formed between LDB1 and SSBP2, potentially constituting a binding site for drugs blocking Wnt/β-catenin signaling. Analysis of human cell lines lacking LDB or Pygo supports the functional relevance of the Pygo-LDB1-SSBP2 interaction for Wnt/β-catenin-dependent transcription.

Crosstalk Between β-CATENIN-Mediated Cell Adhesion and the WNT Signaling Pathway

Cell adhesion and stable signaling regulation are fundamental ways of maintaining homeostasis. Among them, the Wnt/β-CATENIN signaling plays a key role in embryonic development and maintenance of body dynamic homeostasis. At the same time, the key signaling molecule β-CATENIN in the Wnt signaling can also function as a cytoskeletal linker protein to regulate tissue barriers, cell migration, and morphogenesis. Dysregulation of the balance between Wnt signaling and adherens junctions can lead to disease. How β-CATENIN maintains the independence of these two functions, or mediates the interaction and balance of these two functions, has been explored and debated for a long time. In this study, we will focus on five aspects of β-CATENIN chaperone molecules, phosphorylation of β-CATENIN and related proteins, epithelial mesenchymal transition, β-CATENIN homolog protein γ-CATENIN and disease, thus deepening the understanding of the Wnt/β-CATENIN signaling and the homeostasis between cell adhesion and further addressing related disease problems.

The role of Wnt signaling in male reproductive physiology and pathology

Accumulating evidence has shown that Wnt signaling is deeply involved in male reproductive physiology, and malfunction of the signal path can cause pathological changes in genital organs and sperm cells. These abnormalities are diverse in manifestation and have been constantly found in the knockout models of Wnt studies. Nevertheless, most of the research solely focused on a certain factor in the Wnt pathway, and there are few reports on the overall relation between Wnt signals and male reproductive physiology. In our review, Wnt findings relating to the reproductive system were sought and summarized in terms of Wnt ligands, Wnt receptors, Wnt intracellular signals and Wnt regulators. By sorting out and integrating relevant functions, as well as underlining the controversies among different reports, our review aims to offer an overview of Wnt signaling in male reproductive physiology and pathology for further mechanistic studies.

Identification of Uncharacterized Components of Prokaryotic Immune Systems and Their Diverse Eukaryotic Reformulations

Nucleotide-activated effector deployment, prototyped by interferon-dependent immunity, is a common mechanistic theme shared by immune systems of several animals and prokaryotes. Prokaryotic versions include CRISPR-Cas with the CRISPR polymerase domain, their minimal variants, and systems with second messenger oligonucleotide or dinucleotide synthetase (SMODS). Cyclic or linear oligonucleotide signals in these systems help set a threshold for the activation of potentially deleterious downstream effectors in response to invader detection. We establish such a regulatory mechanism to be a more general principle of immune systems, which can also operate independently of such messengers. Using sensitive sequence analysis and comparative genomics, we identify 12 new prokaryotic immune systems, which we unify by this principle of threshold-dependent effector activation. These display regulatory mechanisms paralleling physiological signaling based on 3'-5' cyclic mononucleotides, NAD⁺-derived messengers, two- and one-component signaling that includes histidine kinase-based signaling, and proteolytic activation. Furthermore, these systems allowed the identification of multiple new sensory signal sensory components, such as a tetratricopeptide repeat (TPR) scaffold predicted to recognize NAD⁺-derived signals, unreported versions of the STING domain, prokaryotic YEATS domains, and a predicted nucleotide sensor related to receiver domains. We also identify previously unrecognized invader detection components and effector components, such as prokaryotic versions of the Wnt domain. Finally, we show that there have been multiple acquisitions of unidentified STING domains in eukaryotes, while the TPR scaffold was incorporated into the animal immunity/apoptosis signal-regulating kinase (ASK) signalosome.IMPORTANCE Both prokaryotic and eukaryotic immune systems face the dangers of premature activation of effectors and degradation of self-molecules in the absence of an invader. To mitigate this, they have evolved threshold-setting regulatory mechanisms for the triggering of effectors only upon the detection of a sufficiently strong invader signal. This work defines general templates for such regulation in effector-based immune systems. Using this, we identify several previously uncharacterized prokaryotic immune mechanisms that accomplish the regulation of downstream effector deployment by using nucleotide, NAD⁺-derived, two-component, and one-component signals paralleling physiological homeostasis. This study has also helped identify several previously unknown sensor and effector modules in these systems. Our findings also augment the growing evidence for the emergence of key animal immunity and chromatin regulatory components from prokaryotic progenitors.

PYGO2 as an independent diagnostic marker expressed in a majority of colorectal cancers

Colorectal cancer (CRC) is one of the most common gastrointestinal cancers worldwide. Detection of CRC at the early stages of disease can play an important role in decrease of associated mortality rates. The Wnt signaling pathway is crucial for the progression of different cellular and developmental processes and Wnt pathway deregulation has been well characterized in a variety of cancers, particularly in CRC. The aim of this study was to analyze protein expression of Pygopus2 (PYGO2), the main transcription factor of Wnt pathway, in CRC tissues and evaluate its probable correlation with clinicopathological features of the patients. The expression pattern of PYGO2 was evaluated by immunohistochemistry in tumor tissues and their margin normal which is the piece of normal, or unaffected tissue excised from the surrounding the visible tumors in 46 CRC patients. A defined scoring system was applied to analyze immunostaining results. The expression of PYGO2 protein was detected in all tumor tissues. Furthermore, this expression was significantly higher in CRC samples than in normal tissues. There was a significant association between PYGO2 protein expression in CRC and tumor cell metastasis to the lymph nodes. Considering the significant expression of PYGO2 protein in colorectal tumor cells and its correlation with lymph node metastasis, this protein may be used as a biomarker for metastatic CRC as well as a putative therapeutic target to inhibit aggressiveness and metastasis of CRC.

The WNT Framework in Shaping Immune Cell Responses During Bacterial Infections

A large proportion of the world is inflicted with health concerns arising from infectious diseases. Moreover, there is a widespread emergence of antibiotic resistance among major infectious agents, partially stemming from their continuous dialog with the host, and their enormous capacity to remodel the latter toward a secure niche. Among the several infection-driven events, moderation of WNT signaling pathway has been identified to be strategically tuned during infections to govern host-pathogen interactions. Primarily known for its role in arbitrating early embryonic developmental events; aberrant activation of the WNT pathway has also been associated with immunological consequences during diverse patho-physiological conditions. Here, we review the different mechanisms by which components of WNT signaling pathways are exploited by discrete bacterial agents for their pathogenesis. Furthermore, recent advances on the cross-talk of WNT with other signaling pathways, the varied modes of WNT-mediated alteration of gene expression, and WNT-dependent post-transcriptional and post-translational regulation of the immune landscape during distinct bacterial infections would be highlighted.

Immunohistochemistry analysis of Pygo2 expression in central nervous system tumors

Pygo2 as a Wnt signaling pathway component has been detected in multiple cancer types. In this study, we identified Pygo2 expression features by immunohistochemistry in 73 central nervous system tumor specimens, in comparison with 14 normal brain tissues and surrounding non-tumorous tissues of tumor. Our study indicated that 59% of the patient tumor specimens exhibited positive Pygo2-staining and increases intensity with the grade of malignancy, especially for WHO grade III and IV gliomas, was observed high level expression, compared with normal brain tissues. Five out of nine WHO grade III anaplastic astrocytomas and seven out of nine WHO grade IV glioblastomas showed Pygo2-positive staining. The analysis of Pygo2 gene expression by quantitative real-time PCR of additional ten fresh patient samples yielded similar results. Further studies performed with stable cell lines in vitro demonstrated that Pygo2 render cells higher proliferation rate, migration and anchorage-independent colony-forming ability in soft agar. Taken together, our studies suggest an important role of Pygo2 in brain tumor progression.

Pygopus2 inhibits the efficacy of paclitaxel-induced apoptosis and induces multidrug resistance in human glioma cells

Anti-microtubule drugs, such as paclitaxel (PTX), are extensively used for the treatment of numerous cancers. However, growing evidence has shown that PTX resistance, either intrinsic or acquired, frequently occurs in patients and results in the failure of treatment, contributing to the high cancer mortality rate. Therefore, it is necessary to identify the genes or pathways involved in anti-microtubule drug resistance for future successful treatment of cancers. Pygopus2 (Pygo2), which contains a Zn-coordinated plant homeodomain (PHD) finger domain, is critical for β-catenin-dependent transcriptional switches in normal and malignant tissues and is over-expressed in various cancers, including human brain glioma. In this study, we report that over-expression of Pygo2 inhibited the efficacy of PTX and contributed to cell multidrug resistance in two different ways. First, over-expression of Pygo2 inhibited the PTX-induced phosphorylation of B-cell lymphoma 2 (Bcl-2), suppressing the proteolytic cleavage of procaspase-8/9 and further inhibiting the activation of caspase-3, which also inhibits the activation of the JNK/SAPK pathway, ultimately inhibiting cell apoptosis. Second, over-expression of Pygo2 facilitated the expression of P-glycoprotein, which acts as a drug efflux pump, by promoting the transcription of Multi-drug resistance 1 (MDR1) at the MDR1 promoter loci, resulting in acceleration of the efflux of PTX.

PYGOPUS2 expression in prostatic adenocarcinoma is a potential risk stratification marker for PSA progression following radical prostatectomy

AIMS

Prostate cancer (PrCa) is the most frequently diagnosed non-cutaneous cancer in men. Without clear pathological indicators of disease trajectory at diagnosis, management of PrCa is challenging, given its wide-ranging manifestation from indolent to highly aggressive disease. This study examines the role in PrCa of the Pygopus (PYGO)2 chromatin effector protein as a risk stratification marker in PrCa.

METHODS

RNA expression was performed in PrCa cell lines using Northern and RT-PCR analyses. Protein levels were assessed using immunoblot and immunofluorescence. Immunohistochemistry was performed on tissue microarrays constructed from radical prostatectomies with 5-year patient follow-up data including Gleason score tumour staging, margin and lymph node involvement and prostate serum antigen (PSA) levels. Biochemical recurrence (BR) was defined as a postoperative PSA level of >0.2 nL. Univariate and multivariate analyses were performed using SAS and Kaplan-Meier curves using graphPad (Prism).

RESULTS

In vitro depletion of PYGO2 by RNAi in both androgen receptor positive and negative PrCa cell lines attenuated growth and reduced Ki67 and 47S rRNA expression, while PYGO2 protein was localised to the nuclei of tumours as determined by immunohistochemistry. High expression levels of PYGO2 in tumours (n=156) were correlated with BR identified as PSA progression, after 7-year follow-up independent of other traditional risk factors. Most importantly, high PYGO2 levels in intermediate grade tumours suggested increased risk of recurrence over those with negative or weak expression.

CONCLUSION

Our data suggest that elevated PYGO2 expression in primary prostate adenocarcinoma is a potential risk factor for BR.

Reversal of hyperactive Wnt signaling-dependent adipocyte defects by peptide boronic acids

Deregulated Wnt signaling and altered lipid metabolism have been linked to obesity, diabetes, and various cancers, highlighting the importance of identifying inhibitors that can modulate Wnt signaling and aberrant lipid metabolism. We have established a Drosophila model with hyperactivated Wnt signaling caused by partial loss of axin, a key component of the Wnt cascade. The Axin mutant larvae are transparent and have severe adipocyte defects caused by up-regulation of β-catenin transcriptional activities. We demonstrate pharmacologic mitigation of these phenotypes in Axin mutants by identifying bortezomib and additional peptide boronic acids. We show that the suppressive effect of peptide boronic acids on hyperactive Wnt signaling is dependent on α-catenin; the rescue effect is completely abolished with the depletion of α-catenin in adipocytes. These results indicate that rather than targeting the canonical Wnt signaling pathway directly, pharmacologic modulation of β-catenin activity through α-catenin is a potentially attractive approach to attenuating Wnt signaling in vivo.

Intestinal stem cell overproliferation resulting from inactivation of the APC tumor suppressor requires the transcription cofactors Earthbound and Erect wing

Wnt/β-catenin signal transduction directs intestinal stem cell (ISC) proliferation during homeostasis. Hyperactivation of Wnt signaling initiates colorectal cancer, which most frequently results from truncation of the tumor suppressor Adenomatous polyposis coli (APC). The β-catenin-TCF transcription complex activates both the physiological expression of Wnt target genes in the normal intestinal epithelium and their aberrantly increased expression in colorectal tumors. Whether mechanistic differences in the Wnt transcription machinery drive these distinct levels of target gene activation in physiological versus pathological states remains uncertain, but is relevant for the design of new therapeutic strategies. Here, using a Drosophila model, we demonstrate that two evolutionarily conserved transcription cofactors, Earthbound (Ebd) and Erect wing (Ewg), are essential for all major consequences of Apc1 inactivation in the intestine: the hyperactivation of Wnt target gene expression, excess number of ISCs, and hyperplasia of the epithelium. In contrast, only Ebd, but not Ewg, mediates the Wnt-dependent regulation of ISC proliferation during homeostasis. Therefore, in the adult intestine, Ebd acts independently of Ewg in physiological Wnt signaling, but cooperates with Ewg to induce the hyperactivation of Wnt target gene expression following Apc1 loss. These findings have relevance for human tumorigenesis, as Jerky (JRK/JH8), the human Ebd homolog, promotes Wnt pathway hyperactivation and is overexpressed in colorectal, breast, and ovarian cancers. Together, our findings reveal distinct requirements for Ebd and Ewg in physiological Wnt pathway activation versus oncogenic Wnt pathway hyperactivation following Apc1 loss. Such differentially utilized transcription cofactors may offer new opportunities for the selective targeting of Wnt-driven cancers.

Integrative genomic and transcriptomic analysis for pinpointing recurrent alterations of plant homeodomain genes and their clinical significance in breast cancer

A wide range of the epigenetic effectors that regulate chromatin modification, gene expression, genomic stability, and DNA repair contain structurally conserved domains called plant homeodomain (PHD) fingers. Alternations of several PHD finger-containing proteins (PHFs) due to genomic amplification, mutations, deletions, and translocations have been linked directly to various types of cancer. However, little is known about the genomic landscape and the clinical significance of PHFs in breast cancer. Hence, we performed a large-scale genomic and transcriptomic analysis of 98 PHF genes in breast cancer using TCGA and METABRIC datasets and correlated the recurrent alterations with clinicopathological features and survival of patients. Different subtypes of breast cancer had different patterns of copy number and expression for each PHF. We identified a subset of PHF genes that was recurrently altered with high prevalence, including PYGO2 (pygopus family PHD finger 2), ZMYND8 (zinc finger, MYND-type containing 8), ASXL1 (additional sex combs like 1) and CHD3 (chromodomain helicase DNA binding protein 3). Copy number increase and overexpression of ZMYND8 were more prevalent in Luminal B subtypes and were significantly associated with shorter survival of breast cancer patients. ZMYND8 was also involved in a positive feedback circuit of the estrogen receptor (ER) pathway, and the expression of ZMYND8 was repressed by the bromodomain and extra terminal (BET) inhibitor in breast cancer. Our findings suggest a promising avenue for future research-to focus on a subset of PHFs to better understand the molecular mechanisms and to identify therapeutic targets in breast cancer.

Role of Pygo2 in tumors

Pygo2, a vital component of the Wnt signaling pathway that was recently discovered, has been reported to be closely related to the tumorigenesis of several types of malignant tumors. There are two highly conservative domains named NHD in the N terminus and PHD in the C terminus of Pygo2. Previous studies have shown that the Pygo2 PHD domain can act as a protein code reader to link the chromatin remodeling complex to specific changes in gene transcription, as demonstrated for the Wnt target genes. Furthermore, the activity of the chromatin remodeling is further facilitated by the recruiting of histone methyltransferase and acetyltransferase through the interaction with the Pygo2 NHD domain. However, the molecular mechanism of Pygo2 in the tumor development is still poorly understood. In the present study, we intend to review the structure and role of Pygo2 in tumor progression.

Pygopus-2 promotes invasion and metastasis of hepatic carcinoma cell by decreasing E-cadherin expression

Pygopus-2 over-expression has been reported in several malignancies, such as ovarian, breast, lung and liver cancers. Here we demonstrated that down-regulation of Pygopus-2 by shRNA inhibited hepatic carcinoma cell invasion in vitro and metastasis in xenograft tumor models, which were promoted when Pygopus-2 was over-expressed. Pygopus-2 increased hepatic carcinoma cell invasion and metastasis, by decreasing E-cadherin. Pygopus-2 could bind to the E-cadherin promoter, increasing its methylation, and also indirectly decreased zeb2 expression. In turn these effects caused down-regulation of E-cadherin, potentiating invasion and metastasis. We suggest that targeting Pygopus-2 may potentially inhibit metastasis of hepatic carcinoma.

Targeting self-renewal pathways in cancer stem cells: clinical implications for cancer therapy

Extensive cancer research in the past few decades has identified the existence of a rare subpopulation of stem cells in the grove of cancer cells. These cells are known as the cancer stem cells marked by the presence of surface biomarkers, multi-drug resistance pumps and deregulated self-renewal pathways (SRPs). They have a crucial role in provoking cancer cells leading to tumorigenesis and its progressive metastasis. Cancer stem cells (CSCs) are much alike to normal stem cells in their self-renewal mechanisms. However, deregulations in the SRPs are seen in CSCs, making them resistant to conventional chemotherapeutic agents resulting in the tumor recurrence. Current treatment strategies in cancer fail to detect and differentiate the CSCs from their non-tumorigenic progenies owing to absence of specific biomarkers. Now, it has become imperative to understand complex functional biology of CSCs, especially the signaling pathways to design improved treatment strategies to target them. It is hopeful that the SRPs in CSCs offer a promising target to alter their survival strategies and impede their tumorigenic potential. However, there are many perils associated with the direct targeting method by conventional therapeutic agents such as off targets, poor bioavailability and poor cellular distribution. Recent evidences have shown an increased use of small molecule antagonists directly to target these SRPs may lead to severe side-effects. An alternative to solve these issues could be an appropriate nanoformulation. Nanoformulations of these molecules could provide an added advantage for the selective targeting of the pathways especially Hedgehog, Wnt, Notch and B-cell-specific moloney murine leukemia virus integration site 1 in the CSCs while sparing the normal stem cells. Hence, to achieve this goal a complete understanding of the molecular pathways corroborate with the use of holistic nanosystem (nanomaterial inhibition molecule) could possibly be an encouraging direction for future cancer therapy.

Akt Phosphorylates Wnt Coactivator and Chromatin Effector Pygo2 at Serine 48 to Antagonize Its Ubiquitin/Proteasome-mediated Degradation

Pygopus 2 (Pygo2/PYGO2) is an evolutionarily conserved coactivator and chromatin effector in the Wnt/β-catenin signaling pathway that regulates cell growth and differentiation in various normal and malignant tissues. Although PYGO2 is highly overexpressed in a number of human cancers, the molecular mechanism underlying its deregulation is largely unknown. Here we report that Pygo2 protein is degraded through the ubiquitin/proteasome pathway and is posttranslationally stabilized through phosphorylation by activated phosphatidylinositol 3-kinase/Akt signaling. Specifically, Pygo2 is stabilized upon inhibition of the proteasome, and its intracellular level is regulated by Cullin 4 (Cul4) and DNA damage-binding protein 1 (DDB1), components of the Cul4-DDB1 E3 ubiquitin ligase complex. Furthermore, Pygo2 is phosphorylated at multiple residues, and Akt-mediated phosphorylation at serine 48 leads to its decreased ubiquitylation and increased stability. Finally, we provide evidence that Akt and its upstream growth factors act in parallel with Wnt to stabilize Pygo2. Taken together, our findings highlight chromatin regulator Pygo2 as a common node downstream of oncogenic Wnt and Akt signaling pathways and underscore posttranslational modification, particularly phosphorylation and ubiquitylation, as a significant mode of regulation of Pygo2 protein expression.

Pygo2 siRNA Inhibit the Growth and Increase Apoptosis of U251 Cell by Suppressing Histone H3K4 Trimethylation

The development of novel therapeutic strategies for glioma requires the identification of molecular targets involved in malignancy. Pygopus (Pygo) is a new discovered and specific downstream component of canonical Wnt signaling. Our previous study has demonstrated that Pygo2 is highly expressed in and promotes the growth of glioma cells. However, the role of Pygo2 in glioma remains to be elucidated. In the current study, we investigated the role of Pygo2 in human glioma U251 cells and showed that knocking down of the expression of Pygo2 in U251 cells using lentivirally expressed siRNA have inhibited cell proliferation and increased apoptosis through decreasing H3K4me3 expression. Moreover, we found Pygo2 was enriched in U251 glioma cancer stem-like cells and Pygo2 siRNA resulted in a reduced number as well as size of tumor spheres. According to our result, this paper now links mechanistically Pygo2's role in histone modification to its enhancement/reduction of proliferation/apoptosis in glioma cells and indicate that Pygo2 may play an important role in self-renew and proliferation in U251 glioma cancer stem-like cells.

Abnormal nuclear expression of Pygopus-2 in human primary hepatocellular carcinoma correlates with a poor prognosis

AIMS

Pygopus-2 (Pygo2) is a critical element of the canonical Wnt/β-catenin transcriptional complex. The aim of the present study was to investigate the expression patterns and clinicopathological significance of Pygo2 in human primary hepatocellular carcinoma (HCC).

METHODS AND RESULTS

Real-time polymerase chain reaction (PCR) analysis of the mRNA levels of Pygo2 in 50 paired HCC cancer/adjacent non-cancerous tissues showed that Pygo2 mRNA expression was significantly higher in cancerous tissues (P = 0.009). Immunohistochemical analysis showed that abnormal Pygo2 protein expression in HCC patients was associated with age (P = 0.025), tumour size (P = 0.005), intra- or extra-hepatic metastasis (P = 0.029), vascular invasion (P = 0.026) and tumour differentiation (P = 0.004). Patients with normal Pygo2 protein expression showed a longer survival time (P = 0.031) and a higher 1-year survival rate (P = 0.032) than those with abnormal Pygo2 expression. Cox's proportional hazard regression model showed that abnormal Pygo2 protein expression was a risk factor associated with the prognosis of HCC patients (P = 0.043).

CONCLUSION

To the best of our knowledge, this is the first report investigating Pygo2 expression patterns and their clinicopathological significance in HCC. Our findings suggest that Pygo2 may be an important predictor of poor outcome in HCC patients, and could serve as a novel biomarker for HCC.

Neural crest stem cell-specific deletion of the Pygopus2 gene modulates hair follicle development

We show that neural crest stem cells affect mouse hair follicle development. During embryogenesis hair follicle induction is regulated by complex reciprocal and functionally redundant signals between epidermis and dermis, which remain to be fully understood. Canonical Wnt signalling is a hallmark of neural crest cells and also a prerequisite for hair follicle induction prior to hair placode formation in the epidermis. As neural crest stem cells invade the epidermis during early embryonic development we aimed at determining whether neural crest cells affect hair follicle development. To attenuate, but not silence, canonical Wnt signalling specifically in neural crest cells, we analyzed Wnt1-cre(+/−)::Pygo2(−/−) mice in which the β-catenin co-activator gene, Pygopus 2 (Pygo2), is deleted specifically in neural crest cells. Both, hair density and hair thickness were reduced in mutant mice. Furthermore, hair development was delayed and the relative ratio of hair types was affected. There was a decrease in zig-zag hairs and an increase in awl hairs. Mouse neural crest stem cells expressed ectodysplasin, an essential effector in the formation of zig-zag hair. Taken together, our data support the novel notion that neural crest cells are involved in the earliest stages of hair follicle development.

Pygo2 regulates β-catenin-induced activation of hair follicle stem/progenitor cells and skin hyperplasia

Understanding the epigenetic mechanisms that control the activation of adult stem cells holds the promise of tissue and organ regeneration. Hair follicle stem cells have emerged as a prime model to study stem cell activation. Wnt/β-catenin signaling controls multiple aspects of skin epithelial regeneration, with its excessive activity promoting the hyperactivation of hair follicle stem/progenitor cells and tumorigenesis. The contribution of chromatin factors in regulating Wnt/β-catenin pathway function in these processes is unknown. Here, we show that chromatin effector Pygopus homolog 2 (Pygo2) produced by the epithelial cells facilitates depilation-induced hair regeneration, as well as β-catenin-induced activation of hair follicle stem/early progenitor cells and trichofolliculoma-like skin hyperplasia. Pygo2 maximizes the expression of Wnt/β-catenin targets, but is dispensable for β-catenin-mediated expansion of LIM/homeobox protein Lhx2(+) cells, in the stem/early progenitor cell compartment of the hair follicle. Moreover, β-catenin and Pygo2 converge to induce the accumulation and acetylation of tumor suppressor protein p53 upon the cell cycle entry of hair follicle early progenitor cells and in cultured keratinocytes. These findings identify Pygo2 as an important regulator of Wnt/β-catenin function in skin epithelia and p53 activation as a prominent downstream event of β-catenin/Pygo2 action in stem cell activation.

Integrative ChIP-seq/microarray analysis identifies a CTNNB1 target signature enriched in intestinal stem cells and colon cancer

Background

Deregulation of canonical Wnt/CTNNB1 (beta-catenin) pathway is one of the earliest events in the pathogenesis of colon cancer. Mutations in APC or CTNNB1 are highly frequent in colon cancer and cause aberrant stabilization of CTNNB1, which activates the transcription of Wnt target genes by binding to chromatin via the TCF/LEF transcription factors. Here we report an integrative analysis of genome-wide chromatin occupancy of CTNNB1 by chromatin immunoprecipitation coupled with high-throughput sequencing (ChIP-seq) and gene expression profiling by microarray analysis upon RNAi-mediated knockdown of CTNNB1 in colon cancer cells.

Results

We observed 3629 CTNNB1 binding peaks across the genome and a significant correlation between CTNNB1 binding and knockdown-induced gene expression change. Our integrative analysis led to the discovery of a direct Wnt target signature composed of 162 genes. Gene ontology analysis of this signature revealed a significant enrichment of Wnt pathway genes, suggesting multiple feedback regulations of the pathway. We provide evidence that this gene signature partially overlaps with the Lgr5⁺ intestinal stem cell signature, and is significantly enriched in normal intestinal stem cells as well as in clinical colorectal cancer samples. Interestingly, while the expression of the CTNNB1 target gene set does not correlate with survival, elevated expression of negative feedback regulators within the signature predicts better prognosis.

Conclusion

Our data provide a genome-wide view of chromatin occupancy and gene regulation of Wnt/CTNNB1 signaling in colon cancer cells.

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

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

Pygopus 2: tilting the Wnt-Notch balance in mammary epithelial lineage determination

The mammary gland requires the coordinated efforts of multiple epithelial cell lineages to build an organized bilayered ductal network. How epigenetic regulators direct decisions of cell-fate and lineage determination in cooperation with intrinsic transcriptional factors and extrinsic signaling factors remains a fundamental question in the field of developmental biology. Recent work sheds new light on the role of the histone methylation reader Pygopus 2 in coordinating the balance of self-renewal Wnt signals with luminal-specific Notch signals in mammary epithelial lineage determination.

Chromatin effector Pygo2 mediates Wnt-notch crosstalk to suppress luminal/alveolar potential of mammary stem and basal cells

Epigenetic mechanisms regulating lineage differentiation of mammary stem cells (MaSCs) remain poorly understood. Pygopus 2 (Pygo2) is a histone methylation reader and a context-dependent Wnt/β-catenin coactivator. Here we provide evidence for Pygo2's function in suppressing luminal/alveolar differentiation of MaSC-enriched basal cells. We show that Pygo2-deficient MaSC/basal cells exhibit partial molecular resemblance to luminal cells, such as elevated Notch signaling and reduced mammary repopulating capability upon transplantation. Inhibition of Notch signaling suppresses basal-level and Pygo2-deficiency-induced luminal/alveolar differentiation of MaSC/basal cells, whereas activation of Wnt/β-catenin signaling suppresses luminal/alveolar differentiation and Notch3 expression in a Pygo2-dependent manner. We show that Notch3 is a direct target of Pygo2 and that Pygo2 is required for β-catenin binding and maintenance of a poised/repressed chromatin state at the Notch3 locus in MaSC/basal cells. Together, our data support a model where Pygo2-mediated chromatin regulation connects Wnt signaling and Notch signaling to restrict the luminal/alveolar differentiation competence of MaSC/basal cells.

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.

Association of PYGO2 and EGFR in esophageal squamous cell carcinoma

Wnt signaling is an important evolutionary conserved pathway that is not only involved in determination of cellular development, self-renewal, and fate, but also has significant roles in tumor development and progression. Deregulation of the Wnt/β-catenin signaling pathway and aberrant expression of its components is commonly observed in solid tumors. Such aberrant regulation of Wnt signaling is commonly related to either malfunction of its components or crosstalk with other cellular processes such as the epidermal growth factor receptor (EGFR) signaling cascade. Therefore, identification of the roles of major involved components may be useful to identify new therapeutic targets for cancer treatment. In this study, we assessed EGFR and PYGO2 mRNA expression in tumors and margin normal tissues from 55 esophageal squamous cell carcinoma (ESCC) patients using real-time qRT-PCR, and evaluated clinicopathology relative to the two genes' expression levels. Significant PYGO2 and EGFR overexpression was observed in 30.9 % (P = 0.017) and 38.2 % (P = 0.006) of tumors, respectively. PYGO2 and EGFR expression were significantly associated not only with each other (P < 0.001), but also with tumor staging and depth (P < 0.001). Furthermore, PYGO2 expression was significantly correlated with the tumor grade (P = 0.043) and size (P = 0.023). We identify PYGO2 as a new molecular marker of invasive tumors, introducing its probable oncogenic role in ESCC progression and aggressiveness. In line with other reports, we also illustrate the oncogenic function of EGFR in the development of ESCC through advance stages. We also observed a significant correlation between PYGO2 and EGFR in ESCC tumors, which reveals a mutual convergent influence of these factors in tumor progression and development. Considering aberrant expression, mutual positive feedback, and the significant clinical relevance of these genes in ESCC, we introduce them as appropriate therapeutic targets in adjuvant therapy of ESCC.

Human papilloma virus (HPV) E7-mediated attenuation of retinoblastoma (Rb) induces hPygopus2 expression via Elf-1 in cervical cancer

The human papillomavirus (HPV) is the etiologic agent of cervical cancer. In this study, we provide evidence for the human Pygopus (hPygo)2 gene as a cellular biomarker for HPV-related disease. In a tumor microarray of cervical cancer progression, hPygo2 levels were greater in high-grade lesions and squamous cell carcinomas than in normal epithelia. Similarly, hPygo2 mRNA and protein levels were greater in HPV-positive cervical cancer cells relative to uninfected primary cells. RNA interference (RNAi)-mediated depletion of HPV-E7 increased whereas E74-like factor (Elf)-1 RNAi decreased association of Retinoblastoma (Rb) tumor suppressor with the hPygo2 promoter in cervical cancer cell lines. Transfection of dominant-active Rb inhibited Elf-1-dependent activation of hPygo2, whereas Elf-1 itself increased hPygo2 expression. Chromatin immunoprecipitation assays showed that Rb repressed hPygo2 by inhibiting Elf-1 at the Ets-binding site in the hPygo2 promoter. These results suggested that abrogation of Rb by E7 resulted in derepression of Elf-1, which in turn stimulated expression of hPygo2. Thus, initiation of hPygo2 expression by Elf-1 was required for proliferation of cervical cancer cells and its expression therefore may act as a surrogate marker for dysplasia.

TCF/LEFs and Wnt signaling in the nucleus

T-cell factor/lymphoid enhancer factor (TCF/LEF) transcription factors are the major end point mediators of Wnt/Wingless signaling throughout metazoans. TCF/LEFs are multifunctional proteins that use their sequence-specific DNA-binding and context-dependent interactions to specify which genes will be regulated by Wnts. Much of the work to define their actions has focused on their ability to repress target gene expression when Wnt signals are absent and to recruit β-catenin to target genes for activation when Wnts are present. Recent advances have highlighted how these on/off actions are regulated by Wnt signals and stabilized β-catenin. In contrast to invertebrates, which typically contain one TCF/LEF protein that can both activate and repress Wnt targets, gene duplication and isoform complexity of the family in vertebrates have led to specialization, in which individual TCF/LEF isoforms have distinct activities.

Inducible progenitor-derived Wingless regulates adult midgut regeneration in Drosophila

The ability to regenerate following stress is a hallmark of self-renewing tissues. However, little is known about how regeneration differs from homeostatic tissue maintenance. Here, we study the role and regulation of Wingless (Wg)/Wnt signalling during intestinal regeneration using the Drosophila adult midgut. We show that Wg is produced by the intestinal epithelial compartment upon damage or stress and it is exclusively required for intestinal stem cell (ISC) proliferation during tissue regeneration. Reducing Wg or downstream signalling components from the intestinal epithelium blocked tissue regeneration. Importantly, we demonstrate that Wg from the undifferentiated progenitor cell, the enteroblast, is required for Myc-dependent ISC proliferation during regeneration. Similar to young regenerating tissues, ageing intestines required Wg and Myc for ISC hyperproliferation. Unexpectedly, our results demonstrate that epithelial but not mesenchymal Wg is essential for ISC proliferation in response to damage, while neither source of the ligand is solely responsible for ISC maintenance and tissue self-renewal in unchallenged tissues. Therefore, fine-tuning Wnt results in optimal balance between the ability to respond to stress without negatively affecting organismal viability.

Drugging Wnt signalling in cancer

Aberrant regulation of the Wnt signalling pathway has emerged as a prevalent theme in cancer biology. This chapter summarizes the research that provides a proof of concept for inhibiting Wnt signalling in cancer, the potential means by which this could be achieved, and some recent advances towards this goal. A brief discussion of molecular diagnostics and possible safety concerns is also provided.

Pygo2 regulates histone gene expression and H3 K56 acetylation in human mammary epithelial cells

Histone gene expression is tightly controlled during cell cycle. The epigenetic mechanisms underlying this regulation remain to be fully elucidated. Pygopus 2 (Pygo2) is a context-dependent co-activator of Wnt/β-catenin signaling and a chromatin effector that participates in histone modification. In this study, we show that Pygo2 is required for the optimal expression of multiple classes of histone genes in cultured human mammary epithelial cells. Using chromatin immunoprecipitation assay, we demonstrate that Pygo2 directly occupies the promoters of multiple histone genes and enhances the acetylation of lysine 56 in histone H3 (H3K56Ac), previously shown to facilitate yeast histone gene transcription at these promoters. Moreover, we report reduced global levels of H3K56Ac in Pygo2-depleted cells that occur in a cell cycle-independent manner. Together, our data uncover a novel regulator of mammalian histone gene expression that may act in part via modifying H3K56Ac.

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.

A WNTer revisit: new faces of β-catenin and TCFs in pluripotency

New evidence has revealed interesting aspects of how the Wnt-β-catenin pathway controls self-renewal and lineage differentiation of pluripotent embryonic stem cells. Although Wnt-β-catenin signaling is dispensable for the self-renewal of naive mouse embryonic stem cells, it facilitates their expansion and resistance to differentiation through an unconventional dual mechanism involving the transcriptional repressor T cell factor (TCF) 3 and the transcriptional activator TCF1.

Chromatin-based mechanisms of renal epithelial differentiation

Successful regenerative renal medicine depends on understanding the molecular mechanisms by which diverse phenotypes of epithelial cells differentiate from metanephric mesenchyme to populate nephrons. Whereas many genes are maintained in a poised state within the population of pluripotent progenitors, specialized epithelial functions reflect the selective expression of a subset of genes and the repression of all others. Here we highlight some common mechanisms of cell differentiation and epigenetic regulation to discuss their implications for renal epithelial development, repair, and disease.

Canonical wingless signaling regulates cone cell specification in the Drosophila retina

Correct tissue patterning during development involves multiple morphogenetic events that include specification of different cell fates, cell proliferation, cell death, and coordinated changes in cell shape, position, and adhesion. Here, we use the Drosophila retina to explore the molecular mechanisms that regulate and integrate these various events. In a previous report, we found that wingless (wg) was required to induce a previously unknown surge of cell death ("early death") in the pupal retina. Here, we show that wg is also required to induce the more widely studied mid-pupal cell death ("late death") in a process that involves regulation of DIAP1. Furthermore, our data suggest that wg has a previously unreported role in specifying the glial-like cone cells. This activity requires canonical Wg signaling and is linked with Notch pathway activity. Our work broadens the role of canonical Wg signaling to encompass multiple patterning steps in the emerging Drosophila retina.

Epithelial stem cells: an epigenetic and Wnt-centric perspective

Epithelial stem cells, such as those present in mammalian skin, intestine, or mammary gland, are tissue stem cells capable of both long-term self-renewal and multi-lineage differentiation. Here we review studies implicating epigenetic control mechanisms in mammalian epithelial stem cell development and homeostasis. We also provide an update of recent progresses in the involvement of canonical Wnt signaling and note an interesting link between the Wnt pathway and chromatin regulation in epithelial stem cells. We anticipate that epigenetic and epigenomic studies of these cells will increase exponentially in the near future.

Pygo2 associates with MLL2 histone methyltransferase and GCN5 histone acetyltransferase complexes to augment Wnt target gene expression and breast cancer stem-like cell expansion

Resent studies have identified Pygopus as a core component of the β-catenin/T-cell factor (TCF)/lymphoid-enhancing factor 1 (LEF) transcriptional activation complex required for the expression of canonical Wg/Wnt target genes in Drosophila. However, the biochemical involvement of mammalian Pygopus proteins in β-catenin/TCF/LEF gene activation remains controversial. In this study, we perform a series of molecular/biochemical experiments to demonstrate that Pygo2 associates with histone-modifying enzymatic complexes, specifically the MLL2 histone methyltransferase (HMT) and STAGA histone acetyltransferase (HAT) complexes, to facilitate their interaction with β-catenin and to augment Wnt1-induced, TCF/LEF-dependent transcriptional activation in breast cancer cells. We identify a critical domain in Pygo2 encompassing the first 47 amino acids that mediates its HMT/HAT interaction. We further demonstrate the importance of this domain in Pygo2's ability to transcriptionally activate both artificial and endogenous Wnt target genes and to expand breast cancer stem-like cells in culture. This work now links mechanistically Pygo2's role in histone modification to its enhancement of the Wnt-dependent transcriptional program and cancer stem-like cell expansion.

Transcription and post-transcriptional regulation of spermatogenesis

Spermatogenesis in mammals is achieved by multiple players that pursue a common goal of generating mature spermatozoa. The developmental processes acting on male germ cells that culminate in the production of the functional spermatozoa are regulated at both the transcription and post-transcriptional levels. This review addresses recent progress towards understanding such regulatory mechanisms and identifies future challenges to be addressed in this field. We focus on transcription factors, chromatin-associated factors and RNA-binding proteins necessary for spermatogenesis and/or sperm maturation. Understanding the molecular mechanisms that govern spermatogenesis has enormous implications for new contraceptive approaches and treatments for infertility.

The role of Pygopus 2 in rat glioma cell growth

Glioma is a common malignant tumor of the human neural system, and Wnt signaling activation is closely connected with glioma malignancy. Pygopus 2 (Pygo2) was recently discovered as a component of the Wnt signaling pathway regulating β-catenin/Tcf dependent transcription. However, the role of Pygo2 in glioma cells has not yet been defined. In the current study, we investigated the role of Pygo2 in rat glioma C6 cells for the first time. Our results showed that over-expression of Pygo2 promoted cell proliferation as well as enhanced cell cycle progression from G₁ to S phase associated with an increase in the expression of the Wnt target gene cyclin D1. In contrast, knockdown of Pygo2 suppressed cell proliferation with cell cycle block from G₁ to S phase and down-regulation of cyclin D1. In addition, the expression of Pygo2 and cyclin D1 in 67 glioma tissue samples was quantified by real-time reverse transcription polymerase chain reaction (RT-PCR) and immunochemistry. The data indicated that tumor grade was significantly associated with over-expression of Pygo2 and cyclin D1. We conclude that Pygo2 is highly expressed in and promotes the growth of glioma cells by an increase in the expression of cyclin D1 to improve G₁/S transition.

Decreased pygopus 2 expression suppresses glioblastoma U251 cell growth

Gliomas are common malignant tumors of the human neural system, and Wnt signaling activation is closely related to glioma malignancy. Human Pygopus 2 (Pygo2) was recently discovered to be a component of the Wnt signaling pathway, which is required for β-catenin/Tcf-dependent transcription. However, the role of Pygo2 in glioblastoma cell growth and survival remains uncertain. In the present study, Pygo2 expression was evaluated in 80 glioma tissue samples. Results demonstrated that tumor grade exhibited a positive correlation with overexpression of Pygo2. In addition, small hairpin RNA (shRNA) was used to specifically knockdown Pygo2 expression in human glioblastoma U251 cell lines. Results showed that inhibition of Pygo2 expression resulted in inhibited cell proliferation and invasiveness, as well as increased cell cycle arrest at the G(1) stage and decreased expression of the Wnt target gene cyclin D1. These results demonstrated that Pygo2 was highly expressed in glioma tissue and required for growth of glioblastoma cells.

The transcriptional activity of Pygopus is enhanced by its interaction with cAMP-response-element-binding protein (CREB)-binding protein

Pygopus is a core component of the beta-catenin/TCF (T-cell factor) transcriptional activation complex required for the expression of canonical Wnt target genes. Recent evidence suggests that Pygopus could interpret histone methylation associated with target genes and it was shown to be required for histone acetylation. The involvement of a specific acetyltransferase, however, was not determined. In this report, we demonstrate that Pygopus can interact with the HAT (histone acetyltransferase) CBP [CREB (cAMP-responsive-element-binding protein)-binding protein]. The interaction is via the NHD (N-terminal homology domain) of Pygopus, which binds to two regions in the vicinity of the HAT domain of CBP. Transfected and endogenous hPygo2 (human Pygopus2) and CBP proteins co-immunoprecipitate in HEK-293 (human embryonic kidney 293) cells and both proteins co-localize in SW480 colorectal cancer cells. The interaction with CBP also enhances both DNA-tethered and TCF/LEF1 (lymphoid enhancing factor 1)-dependent transcriptional activity of Pygopus. Furthermore, immunoprecipitated Pygopus protein complexes displayed CBP-dependent histone acetyltransferase activity. Our data support a model in which the NHD region of Pygopus is required to augment TCF/beta-catenin-mediated transcriptional activation by a mechanism that includes both transcriptional activation and histone acetylation resulting from the recruitment of the CBP histone acetyltransferase.

Pygo2 expands mammary progenitor cells by facilitating histone H3 K4 methylation

Recent studies have unequivocally identified multipotent stem/progenitor cells in mammary glands, offering a tractable model system to unravel genetic and epigenetic regulation of epithelial stem/progenitor cell development and homeostasis. In this study, we show that Pygo2, a member of an evolutionarily conserved family of plant homeo domain–containing proteins, is expressed in embryonic and postnatal mammary progenitor cells. Pygo2 deficiency, which is achieved by complete or epithelia-specific gene ablation in mice, results in defective mammary morphogenesis and regeneration accompanied by severely compromised expansive self-renewal of epithelial progenitor cells. Pygo2 converges with Wnt/β-catenin signaling on progenitor cell regulation and cell cycle gene expression, and loss of epithelial Pygo2 completely rescues β-catenin–induced mammary outgrowth. We further describe a novel molecular function of Pygo2 that is required for mammary progenitor cell expansion, which is to facilitate K4 trimethylation of histone H3, both globally and at Wnt/β-catenin target loci, via direct binding to K4-methyl histone H3 and recruiting histone H3 K4 methyltransferase complexes.