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

Why Is Wnt/β-Catenin Not Yet Targeted in Routine Cancer Care?

Despite significant progress in cancer prevention, screening, and treatment, the still limited number of therapeutic options is an obstacle towards increasing the cancer cure rate. In recent years, many efforts were put forth to develop therapeutics that selectively target different components of the oncogenic Wnt/β-catenin signaling pathway. These include small molecule inhibitors, antibodies, and more recently, gene-based approaches. Although some of them showed promising outcomes in clinical trials, the Wnt/β-catenin pathway is still not targeted in routine clinical practice for cancer management. As for most anticancer treatments, a critical limitation to the use of Wnt/β-catenin inhibitors is their therapeutic index, i.e., the difficulty of combining effective anticancer activity with acceptable toxicity. Protecting healthy tissues from the effects of Wnt/β-catenin inhibitors is a major issue due to the vital role of the Wnt/β-catenin signaling pathway in adult tissue homeostasis and regeneration. In this review, we provide an up-to-date summary of clinical trials on Wnt/β-catenin pathway inhibitors, examine their anti-tumor activity and associated adverse events, and explore strategies under development to improve the benefit/risk profile of this therapeutic approach.

LEF1 enhances β-catenin transactivation through IDR-dependent liquid-liquid phase separation

Wnt/β-catenin signaling plays a crucial role in cancer development, primarily activated by β-catenin forming a transcription complex with LEF/TCF in the nucleus and initiating the transcription of Wnt target genes. Here, we report that LEF1, a member of the LEF/TCF family, can form intrinsically disordered region (IDR)-dependent condensates with β-catenin both in vivo and in vitro, which is required for β-catenin-dependent transcription. Notably, LEF1 with disrupted IDR lost its promoting activity on tumor proliferation and metastasis, which can be restored by substituting with FUS IDR. Our findings provide new insight into the essential role of liquid-liquid phase separation in Wnt/β-catenin signaling and present a potential new target for cancer therapy.

Hepatocyte nuclear factor 1β suppresses canonical Wnt signaling through transcriptional repression of lymphoid enhancer-binding factor 1

Hepatocyte nuclear factor-1β (HNF-1β) is a tissue-specific transcription factor that is required for normal kidney development and renal epithelial differentiation. Mutations of HNF-1β produce congenital kidney abnormalities and inherited renal tubulopathies. Here, we show that ablation of HNF-1β in mIMCD3 renal epithelial cells results in activation of β-catenin and increased expression of lymphoid enhancer-binding factor 1 (LEF1), a downstream effector in the canonical Wnt signaling pathway. Increased expression and nuclear localization of LEF1 are also observed in cystic kidneys from Hnf1b mutant mice. Expression of dominant-negative mutant HNF-1β in mIMCD3 cells produces hyperresponsiveness to exogenous Wnt ligands, which is inhibited by siRNA-mediated knockdown of Lef1. WT HNF-1β binds to two evolutionarily conserved sites located 94 and 30 kb from the mouse Lef1 promoter. Ablation of HNF-1β decreases H3K27 trimethylation repressive marks and increases β-catenin occupancy at a site 4 kb upstream to Lef1. Mechanistically, WT HNF-1β recruits the polycomb-repressive complex 2 that catalyzes H3K27 trimethylation. Deletion of the β-catenin-binding domain of LEF1 in HNF-1β-deficient cells abolishes the increase in Lef1 transcription and decreases the expression of downstream Wnt target genes. The canonical Wnt target gene, Axin2, is also a direct transcriptional target of HNF-1β through binding to negative regulatory elements in the gene promoter. These findings demonstrate that HNF-1β regulates canonical Wnt target genes through long-range effects on histone methylation at Wnt enhancers and reveal a new mode of active transcriptional repression by HNF-1β.

[Effects of ICAT silencing in Wnt signaling pathway and NSC67657 induced cell differentiation of HL-60 cells]

目的

探讨β-catenin相关蛋白1（ICAT）沉默对Wnt信号通路及甾醇类新药NSC67657诱导HL-60细胞单核系分化的影响。

方法

NSC67657诱导HL-60细胞分化，流式细胞术检测细胞表面分化抗原CD14的表达，检测细胞分化程度；构建慢病毒LV-ICAT-RNAi载体，感染HL-60细胞，采用荧光实时定量PCR和Western blot技术检测感染前后ICAT基因和蛋白表达情况，判断干扰效果；采用免疫共沉淀技术检测β-catenin与ICAT蛋白在细胞内的相互作用；采用Western blot技术分析NSC67657诱导非感染HL-60细胞（HL-60v组）和LV-ICAT-RNAi载体感染HL-60细胞（HL-60i组）前后Wnt/β-catenin通路下游靶点Cyclin D1、TCF-1和c-Jun的表达情况；NSC67657分别作用HL-60v组和HL-60i组细胞24 h，采用瑞氏染色、透射电子显微镜、流式细胞术观察细胞分化情况。

结果

10 µmol/L NSC67657可以诱导HL-60细胞向单核系分化，连续诱导5 d后，CD14⁺细胞比例为（92.30±5.14）%；HL-60i细胞ICAT mRNA表达（0.07±0.01）明显低于HL-60v组（1.00±0.08）（P=0.002）（平均敲减效率为93.2%），Western blot结果与PCR结果一致（P=0.001）；免疫共沉淀结果显示，ICAT与β-catenin蛋白在细胞分化前后都存在相互作用，药物诱导细胞分化后两者相互作用条带吸光度明显增加。药物作用HL-60i细胞Wnt信号通路下游靶蛋白Cyclin D1、TCF-1和c-Jun表达明显高于HL-60v组，但低于非药物处理组。NSC67657作用HL-60i细胞CD14⁺细胞比例为（8.33±3.14）%，明显低于HL-60v组的（19.08±4.73）%，但仍高于非药物处理非感染HL-60细胞组（0.60±0.03）%（F=119.24，P=0.010），细胞形态和超微结果符合细胞表面分化抗原检测结论。

结论

ICAT蛋白参与了NSC67657诱导HL-60细胞的单核系分化，Wnt/β-catenin信号通路可能起到桥梁作用。

Interaction of LEF1 with TAZ is necessary for the osteoblastogenic activity of Wnt3a

Canonical Wnt signalling plays an important role in osteoblast differentiation and bone formation. However, the molecular mechanisms by which canonical Wnt signalling exerts its osteoblastogenic effect remain elusive. Here, we investigated the relationship between lymphoid enhancer-binding factor 1 (LEF1) and transcriptional co-activator with PDZ-binding motif (TAZ), both of which are transcriptional regulators that mediate canonical Wnt signalling during osteoblast differentiation. Reporter assay and co-immunoprecipitation experiments revealed functional and physical interaction between LEF1 and TAZ. Overexpression of dominant-negative forms of either LEF1 or TAZ markedly inhibited Wnt3a-dependent osteoblast differentiation. Moreover, we found that LEF1 and TAZ formed a transcriptional complex with runt-related transcription factor 2 (Runx2) and that inhibition of LEF1 or TAZ by their dominant-negative forms dramatically suppressed the osteoblastogenic activity of Ruxn2. Additionally, Wnt3a enhanced osteoblast differentiation induced by bone morphogenetic protein 2 (BMP2), which stimulates osteoblast differentiation by regulating Runx2. Collectively, these findings suggest that interaction between LEF1 and TAZ is crucial for the osteoblastogenic activity of Wnt3a and that LEF1 and TAZ contribute to the cooperative effect of Wnt3a and BMP2 on osteoblast differentiation through association with Runx2.

Opposing roles of ICAT and Wnt/β-catenin signaling in NSC67657-induced monocytic differentiation

NSC67657 is a new steroid drug that induces monocytic differentiation of acute myeloid leukemia cells. Here, we demonstrate that NSC67657 has opposing effects on expression of downstream targets of inhibitor of β-catenin and TCF (ICAT) and Wnt signaling in HL60 cells. ICAT binds to β-catenin, and this interaction is further increased in NSC67657-differentiated cells. ICAT overexpression decreases expression of Wnt downstream targets and increases sensitivity of HL60 cells to NSC67657, while ICAT silencing increases Wnt signaling and delays the NSC67657-induced cell differentiation. In addition, pharmacological inhibition of Wnt/β-catenin signaling increases the NSC67657-induced cell differentiation, while activation of Wnt/β-catenin signaling inhibits the differentiation, indicating Wnt/β-catenin signaling inhibits NSC67657-induced monocytic differentiation of HL60 cells. Our data demonstrate the opposing roles of ICAT and Wnt signaling in the NSC67657-induced monocytic differentiation, and suggest that ICAT and Wnt signaling may serve as therapeutic targets for leukemia chemotherapy.

Βeta-catenin N-terminal domain: An enigmatic region prone to cancer causing mutations

The Wnt/β-catenin is a highly conserved signaling pathway involved in cell fate decisions during various stages of development. Dysregulation of canonical Wnt/β-catenin signaling has been associated with various diseases including cancer. β-Catenin, the central component of canonical Wnt signaling pathway, is a multi-functional protein playing both structural and signaling roles. β-Catenin is composed of three distinct domains: N-terminal domain, C-terminal domain and a central armadillo repeat domain. N-terminal domain of β-catenin harbours almost all of the cancer causing mutations, thus deciphering its critical structural and functional roles offers great potential in cancer detection and therapy. Here, in this review, we have collected information from pharmacological analysis, bio-physical and structural studies, molecular modeling, in-vivo and in-vitro assays, and transgenic animal experiments employing various N-terminal domain variants of β-catenin to discuss the interaction of β-catenin with its binding partners that specifically interact with this domain and the implications of these interactions on signaling, cell fate determination, and in tumorigenesis. A thorough understanding of interactions between β-catenin and its binding partners will enable us to more effectively understand how β-catenin switches between its multiple roles, and will lead to the development of specific assays for the identification of small molecules as chemotherapeutic agents to treat diseases, including cancer and neurological disorders, where Wnt/β-catenin signaling is dysregulated.

Ubiquitin C-terminal hydrolase37 regulates Tcf7 DNA binding for the activation of Wnt signalling

The Tcf/Lef family of transcription factors mediates the Wnt/β-catenin pathway that is involved in a wide range of biological processes, including vertebrate embryogenesis and diverse pathogenesis. Post-translational modifications, including phosphorylation, sumoylation and acetylation, are known to be important for the regulation of Tcf/Lef proteins. However, the importance of ubiquitination and ubiquitin-mediated regulatory mechanisms for Tcf/Lef activity are still unclear. Here, we newly show that ubiquitin C-terminal hydrolase 37 (Uch37), a deubiquitinase, interacts with Tcf7 (formerly named Tcf1) to activate Wnt signalling. Biochemical analyses demonstrated that deubiquitinating activity of Uch37 is not involved in Tcf7 protein stability but is required for the association of Tcf7 to target gene promoter in both Xenopus embryo and human liver cancer cells. In vivo analyses further revealed that Uch37 functions as a positive regulator of the Wnt/β-catenin pathway downstream of β-catenin stabilization that is required for the expression of ventrolateral mesoderm genes during Xenopus gastrulation. Our study provides a new mechanism for chromatin occupancy of Tcf7 and uncovers the physiological significance of Uch37 during early vertebrate development by regulating the Wnt/β-catenin pathway.

The WNT-less wonder: WNT-independent β-catenin signaling

β-catenin is known as an Armadillo protein that regulates gene expression following WNT pathway activation. However, WNT-independent pathways also activate β-catenin. During the establishment of the melanocyte lineage, β-catenin plays an important role. In the context of physiopathology, β-catenin is activated genetically or transiently in various cancers, including melanoma, where it can be found in the nucleus of tumors. In this review, we discuss alternative pathways that activate β-catenin independent of WNTs and highlight what is known regarding these pathways in melanoma. We also discuss the role of β-catenin as a transcriptional regulator in various cell types, with emphasis on the different transcription factors it associates with independent of WNT induction. Finally, the role of WNT-independent β-catenin in melanocyte development and melanomagenesis is also discussed.

Frequent inactivation of MCC/CTNNBIP1 and overexpression of phospho-beta-catenin(Y654) are associated with breast carcinoma: Clinical and prognostic significance

Transcriptional activation of β-catenin is a hallmark of Wnt/β-catenin pathway activation. The MCC (Mutated in colorectal cancers) and CTNNBIP1 (catenin, beta interacting protein 1) are two candidate genes which inhibit the transcriptional activity of nuclear β-catenin. The importance of MCC and CTNNBIP1 in breast cancer (BC) development has not yet been studied in detail. For this reason, in present study, the alterations (deletion/methylation/mutation/expression) of MCC and CTNNBIP1 were analyzed in BC of Indian patients (N=120) followed by expression/mutation analysis of β-catenin. Then transcriptional activity of β-catenin was checked by expression analysis of its target genes (EGFR, C-MYC and CCND1) in the same set of samples. Frequent methylation (44-45%) than deletion (20-32%) with overall alterations of 52-55% was observed in MCC/CTNNBIP1 in the BC samples. The alterations of MCC/CTNNBIP1 showed significant correlation with increased nuclear β-catenin/p-β-catenin(Y654) expression. Also, a significant correlation was seen between nuclear β-catenin expression and overexpression of its target genes like EGFR, MYC and CCND1 in the BC samples (P<0.0001). An upregulation of MCC and CTNNBIP1 expression by 5-Aza-2'-deoxycytidine treatment of MCF7 and MDA-MB-231 cell lines lead to downregulation of β-catenin and its target genes. The expression of nuclear p-β-catenin(Y654), EGFR, MYC and CCND1 were significantly high in TNBC (Triple negative BC) and Her2+ compared to Luminal A/B+ subtypes. The TNBC patients in stage III/IV having reduced expression of MCC in the tumors showed poor prognosis. Thus, our data suggests that inactivation of MCC/CTNNBIP1 could be an important event in activation of β-catenin mediated transcription of target genes in BC.

Wnt-induced deubiquitination FoxM1 ensures nucleus β-catenin transactivation

A key step of Wnt signaling activation is the recruitment of β-catenin to the Wnt target-gene promoter in the nucleus, but its mechanisms are largely unknown. Here, we identified FoxM1 as a novel target of Wnt signaling, which is essential for β-catenin/TCF4 transactivation. GSK3 phosphorylates FoxM1 on serine 474 which induces FoxM1 ubiquitination mediated by FBXW7. Wnt signaling activation inhibits FoxM1 phosphorylation by GSK3-Axin complex and leads to interaction between FoxM1 and deubiquitinating enzyme USP5, thereby deubiquitination and stabilization of FoxM1. FoxM1 accumulation in the nucleus promotes recruitment of β-catenin to Wnt target-gene promoter and activates the Wnt signaling pathway by protecting the β-catenin/TCF4 complex from ICAT inhibition. Subsequently, the USP5-FoxM1 axis abolishes the inhibitory effect of ICAT and is required for Wnt-mediated tumor cell proliferation. Therefore, Wnt-induced deubiquitination of FoxM1 represents a novel and critical mechanism for controlling canonical Wnt signaling and cell proliferation.

A Simple Method to Assess Abundance of the β-Catenin Signaling Pool in Cells

β-catenin (CTNNB1) is a dual-function cell-cell adhesion/transcriptional co-activator protein and an essential transducer of canonical Wnt signals. Although a number of established techniques and reagents are available to quantify the nuclear signaling activity of β-catenin (e.g., TCF-dependent reporter assays, nuclear accumulation of β-catenin, and generation of N-terminally hypophosphorylated β-catenin), there are cell-type and context-dependent limitations of these methods. Since the posttranscriptional stabilization of β-catenin outside of the cadherin complex appears universally required for β-catenin signaling, the following method allows for simple assessment of the cadherin-free fraction of β-catenin in cells, using a GST-tagged form of ICAT (Inhibitor of β-Catenin and Tcf) as an affinity matrix. This method is more sensitive and quantitative than immunofluorescence and may be useful in studies that implicate TCF-independent signaling events.

Positive regulation of β-catenin-PROX1 signaling axis by DBC1 in colon cancer progression

Aberrant activation of Wnt/β-catenin pathway contributes to colorectal cancer (CRC) progression. However, little is known about regulatory mechanisms of the β-catenin activity in cancer progression. Here we investigated the role of DBC1, which was recently reported as a negative regulator of SIRT1 and a transcriptional coactivator, in the regulation of Wnt/β-catenin signaling. We identified the genome-wide targets of DBC1 and found that loss of DBC1 inhibits the expression of β-catenin target genes including PROX1, a transcription factor linked to CRC progression. Mechanistically, DBC1 stabilizes LEF1-β-catenin interaction by inhibiting SIRT1-mediated β-catenin deacetylation, thereby enhancing LEF1-β-catenin complex formation and long-range chromatin looping at the PROX1 locus. Furthermore, DBC1 is also required for the transcriptional activity of PROX1, suggesting that DBC1 has a dual function in regulating β-catenin-PROX1 signaling axis: as a coactivator for both β-catenin and PROX1. Importantly, loss of DBC1 inhibited growth and tumorigenic potential of colon cancer cells, and DBC1 expression correlated with shorter relapse-free survival in patients with advanced CRC. Our results firmly establish DBC1 as a critical positive regulator of β-catenin-PROX1 signaling axis and a key factor in β-catenin-PROX1-mediated CRC progression.

A conserved phosphorylation switch controls the interaction between cadherin and β-catenin in vitro and in vivo

In metazoan adherens junctions, β-catenin links the cytoplasmic tail of classical cadherins to the F-actin-binding protein α-catenin. Phosphorylation of a Ser/Thr-rich region in the cadherin tail dramatically enhances affinity for β-catenin and promotes cell-cell adhesion in cell culture systems, but its importance has not been demonstrated in vivo. Here, we identify a critical phosphorylated serine in the C. elegans cadherin HMR-1 required for strong binding to the β-catenin homolog HMP-2. Ablation of this phosphoserine interaction produces developmental defects that resemble full loss-of-function (Hammerhead and Humpback) phenotypes. Most metazoans possess a single gene for β-catenin, which is also a transcriptional coactivator in Wnt signaling. Nematodes and planaria, however, have a set of paralogous β-catenins; for example, C. elegans HMP-2 functions only in cell-cell adhesion, whereas SYS-1 mediates transcriptional activation through interactions with POP-1/Tcf. Our structural data define critical sequence differences responsible for the unique ligand specificities of these two proteins.

SMADs and YAP compete to control elongation of β-catenin:LEF-1-recruited RNAPII during hESC differentiation

The Wnt3a/β-catenin and Activin/SMAD2,3 signaling pathways synergize to induce endodermal differentiation of human embryonic stem cells; however, the underlying mechanism is not well understood. Using ChIP-seq and GRO-seq analyses, we show here that Wnt3a-induced β-catenin:LEF-1 enhancers recruit cohesin to direct enhancer-promoter looping and activate mesendodermal (ME) lineage genes. Moreover, we find that LEF-1 and other hESC enhancers recruit RNAPII complexes (eRNAPII) that are highly phosphorylated at Ser5, but not Ser7. Wnt3a signaling further increases Ser5P-RNAPII at LEF-1 sites and ME gene promoters, indicating that elongation remains limiting. However, subsequent Activin/SMAD2,3 signaling selectively increases transcription elongation, P-TEFb occupancy, and Ser7P-RNAPII levels at these genes. Finally, we show that the Hippo regulator, YAP, functions with TEAD to regulate binding of the NELF negative elongation factor and block SMAD2,3 induction of ME genes. Thus, the Wnt3a/β-catenin and Activin/SMAD2,3 pathways act in concert to counteract YAP repression and upregulate ME genes during early hESC differentiation.

β-Catenin inactivation is a pre-requisite for chick retina regeneration

In the present study we explored the role of β-catenin in mediating chick retina regeneration. The chick can regenerate its retina by activating stem/progenitor cells present in the ciliary margin (CM) of the eye or via transdifferentiation of the retinal pigmented epithelium (RPE). Both modes require fibroblast growth factor 2 (FGF2). We observed, by immunohistochemistry, dynamic changes of nuclear β-catenin in the CM and RPE after injury (retinectomy). β-catenin nuclear accumulation was transiently lost in cells of the CM in response to injury alone, while the loss of nuclear β-catenin was maintained as long as FGF2 was present. However, nuclear β-catenin positive cells remained in the RPE in response to injury and were BrdU-/p27+, suggesting that nuclear β-catenin prevents those cells from entering the cell cycle. If FGF2 is present, the RPE undergoes dedifferentiation and proliferation concomitant with loss of nuclear β-catenin. Moreover, retinectomy followed by disruption of active β-catenin by using a signaling inhibitor (XAV939) or over-expressing a dominant negative form of Lef-1 induces regeneration from both the CM and RPE in the absence of FGF2. Our results imply that β-catenin protects cells of the CM and RPE from entering the cell cycle in the developing eye, and specifically for the RPE during injury. Thus inactivation of β-catenin is a pre-requisite for chick retina regeneration.

β-catenin inhibitor ICAT modulates the invasive motility of melanoma cells

Inhibitor of β-catenin and TCF (ICAT) inhibits β-catenin transcriptional activity by competing with T-cell factor/lymphoid enhancer factor. We documented high ICAT levels in human melanoma cells, in which β-catenin signaling is frequently deregulated, finding a correlation with the capacity to form metastases in nude mice. Ectopic expression of ICAT in melanoma cells did not affect their proliferation but increased cell motility and Matrigel invasion of metastatic cells in a manner relying upon stable ICAT-β-catenin interaction. This effect was associated with conversion of an elongated/mesenchymal phenotype to a round/amoeboid phenotype in the absence of similar effects on elongated morphology of nonmetastatic melanoma cells. Transition from mesenchymal to amoeboid movement was associated with decreased levels of NEDD9 and activated Rac1, a positive regulator of mesenchymal movement. Ectopic ICAT promoted colonization of melanoma cells in the lungs of nude mice, suggesting an increase in metastatic potential. Together, our results showed that by downregulating Rac signaling in metastatic melanoma cells, ICAT increased their invasive motility by promoting a morphologic variation that facilitates a favorable adaptation to their microenvironment.

Regulation of Tcf7l1 DNA binding and protein stability as principal mechanisms of Wnt/β-catenin signaling

Wnt/β-catenin signal transduction requires direct binding of β-catenin to Tcf/Lef proteins, an event that is classically associated with stimulating transcription by recruiting coactivators. This molecular cascade plays critical roles throughout embryonic development and normal postnatal life by affecting stem cell characteristics and tumor formation. Here, we show that this pathway utilizes a fundamentally different mechanism to regulate Tcf7l1 (formerly named Tcf3) activity. β-catenin inactivates Tcf7l1 without a switch to a coactivator complex by removing it from DNA, which leads to Tcf7l1 protein degradation. Mouse genetic experiments demonstrate that Tcf7l1 inactivation is the only required effect of the Tcf7l1-β-catenin interaction. Given the expression of Tcf7l1 in pluripotent embryonic and adult stem cells, as well as in poorly differentiated breast cancer, these findings provide mechanistic insights into the regulation of pluripotency and the role of Wnt/β-catenin in breast cancer.

Intrinsic properties of Tcf1 and Tcf4 splice variants determine cell-type-specific Wnt/β-catenin target gene expression

T-cell factor (Tcf)/lymphoid-enhancer factor (Lef) proteins are a structurally diverse family of deoxyribonucleic acid-binding proteins that have essential nuclear functions in Wnt/β-catenin signalling. Expression of Wnt/β-catenin target genes is highly dependent on context, but the precise role of Tcf/Lef family members in the generation and maintenance of cell-type-specific Wnt/β-catenin responses is unknown. Herein, we show that induction of a subset of Wnt/β-catenin targets in embryonic stem cells depends on Tcf1 and Tcf4, whereas other co-expressed Tcf/Lef family members cannot induce these targets. The Tcf1/Tcf4-dependent gene responses to Wnt are primarily if not exclusively mediated by C-clamp-containing Tcf1E and Tcf4E splice variants. A combined knockdown of Tcf1/Tcf4 abrogates Wnt-inducible transcription but does not affect the active chromatin conformation of their targets. Thus, the transcriptionally poised state of Wnt/β-catenin targets is maintained independent of Tcf/Lef proteins. Conversely, ectopically overexpressed Tcf1E cannot invade silent chromatin and fails to initiate expression of inactive Wnt/β-catenin targets even if repressive chromatin modifications are abolished. The observed non-redundant functions of Tcf1/Tcf4 isoforms in acute transcriptional activation demonstrated that the cell-type-specific complement of Tcf/Lef proteins is a critical determinant of context-dependent Wnt/β-catenin responses. Moreover, the apparent inability to cope with chromatin uncovers an intrinsic property of Tcf/Lef proteins that prevents false ectopic induction and ensures spatiotemporal stability of Wnt/β-catenin target gene expression.

Prostaglandin E2 promotes the nuclear accumulation of lymphoid enhancer factor-1 in poorly differentiated breast cancer cells

Products of the COX reaction are frequently elevated in solid tumors and their roles in the malignant phenotype have been extensively investigated. We have shown that COX-2 is essential for the growth of MDA-MB-231 cells in the fat pad of SCID mice and for their extrapulmonary colonization following injection in the tail vein of SCID mice. The molecular changes that follow shRNA-mediated silencing of COX-2 include a significant downregulation of LEF-1, a transcription factor normally activated during development following the Wnt-induced nuclear translocation of β-catenin. We also report that COX-2-silenced cells have reduced nuclear accumulation of LEF-1 protein and that the COX-2 product PGE(2) partially restored nuclear LEF-1 expression in COX-2-silenced cells. Further, we demonstrate that, like parental COX-2 containing MDA-MB-231 cells, COX-2-silenced cells maintain nuclear localization of β-catenin.

The beta-catenin binding protein ICAT modulates androgen receptor activity

Androgens have important roles in the development of the prostate gland and in prostate cancer. Since the finding that β-catenin is a cofactor of the androgen receptor (AR) and can augment AR signaling, several proteins have been found to affect AR signaling through their interaction with β-catenin. Here, we investigated inhibitor of β-catenin and T-cell factor (ICAT), a β-catenin binding protein that inhibits the canonical Wnt/β-catenin signaling pathway, in AR signaling. We demonstrated that expression of ICAT in two AR positive prostate cancer cell lines, LNCaP and LAPC4, augments ligand-dependent AR-mediated transcription. In contrast, short hairpin RNA knockdown of ICAT and β-catenin specifically blocks enhanced AR-mediated transcription by ICAT. Using both stable expression of ICAT and short hairpin RNA knockdown of ICAT expression approaches, we further showed that ICAT enhances expression of endogenous PSA and KLK2, two androgen response genes, and ligand-induced cell growth. In addition, we identified that ICAT and AR can form a ternary complex with β-catenin using in vitro glutathione S-transferase protein pulldown assays. Moreover, we detected the endogenous protein complex containing ICAT, AR, and β-catenin in prostate cancer cells using immunoprecipitation assays. Recruitment of endogenous ICAT onto the promoter region of the human PSA gene, an AR downstream target promoter, was also identified in LNCaP cells. Finally, using in vitro protein binding assays, we examined the effect of full-length and truncated ICAT on the AR-β-catenin interaction and observed that addition of full-length ICAT retained the interaction between β-catenin and AR proteins. Intriguingly, the truncated ICAT comprising the N-terminal helical domain showed a more pronounced effect on β-catenin binding to AR proteins. Our findings suggest a novel molecular mechanism underlying the cross talk between androgen and Wnt signaling pathways.

FoxOs, Wnts and oxidative stress-induced bone loss: new players in the periodontitis arena?

BACKGROUND AND OBJECTIVE

Chronic periodontitis is a widespread disease affecting tooth-supporting structures that can lead to extensive loss of periodontal ligament and bone, ultimately resulting in tooth loss. Extensive evidence has demonstrated a strong association between age, metabolic disorders such as type II diabetes, oxidative stress and alveolar bone loss. The molecular players controlling bone maintenance and underlying age-related bone loss and its links to the general metabolism are currently the object of intense research.

MATERIAL AND METHODS

Recent findings are summarized to elucidate the molecular mechanisms linking oxidative stress, bone loss and metabolic factors.

RESULTS

It is well known that reactive oxygen species are an inevitable consequence of cellular respiration and that organisms have developed an efficient array of defenses against them. The core of this complex defense line is a family of transcription factors, known as FoxOs, which can bind to β-catenin and initiate a transcriptional programme regulating cell apoptosis, DNA repair and degradation of reactive oxygen species. An increase in reactive oxygen species due, for example, to age or insulin resistance, generates a situation in which bone formation is impaired by activation of FoxO, and a decrease in Wnt signaling and bone resorption are promoted.

CONCLUSION

The balance between FoxO and the Wnt pathway is finely tuned by systemic and local factors, creating a far-reaching mechanism that dictates the fate of mesenchymal progenitors and regulates the homeostasis of bone, providing a rationale for the impairment of systemic and alveolar bone maintenance clinically observed with age and metabolic diseases.

Phosphorylation of TCF proteins by homeodomain-interacting protein kinase 2

Wnt pathways play essential roles in cell proliferation, morphogenesis, and cell fate specification during embryonic development. According to the consensus view, the Wnt pathway prevents the degradation of the key signaling component β-catenin by the protein complex containing the negative regulators Axin and glycogen synthase kinase 3 (GSK3). Stabilized β-catenin associates with TCF proteins and enters the nucleus to promote target gene expression. This study examines the involvement of HIPK2 (homeodomain-interacting protein kinase 2) in the regulation of different TCF proteins in Xenopus embryos in vivo. We show that the TCF family members LEF1, TCF4, and TCF3 are phosphorylated in embryonic ectoderm after Wnt8 stimulation and HIPK2 overexpression. We also find that TCF3 phosphorylation is triggered by canonical Wnt ligands, LRP6, and dominant negative mutants for Axin and GSK3, indicating that this process shares the same upstream regulators with β-catenin stabilization. HIPK2-dependent phosphorylation caused the dissociation of LEF1, TCF4, and TCF3 from a target promoter in vivo. This result provides a mechanistic explanation for the context-dependent function of HIPK2 in Wnt signaling; HIPK2 up-regulates transcription by phosphorylating TCF3, a transcriptional repressor, but inhibits transcription by phosphorylating LEF1, a transcriptional activator. Finally, we show that upon HIPK2-mediated phosphorylation, TCF3 is replaced with positively acting TCF1 at a target promoter. These observations emphasize a critical role for Wnt/HIPK2-dependent TCF phosphorylation and suggest that TCF switching is an important mechanism of Wnt target gene activation in vertebrate embryos.

Histone H4 Lys 20 monomethylation by histone methylase SET8 mediates Wnt target gene activation

Histone methylation has an important role in transcriptional regulation. However, unlike H3K4 and H3K9 methylation, the role of H4K20 monomethylation (H4K20me-1) in transcriptional regulation remains unclear. Here, we show that Wnt3a specifically stimulates H4K20 monomethylation at the T cell factor (TCF)-binding element through the histone methylase SET8. Additionally, SET8 is crucial for activation of the Wnt reporter gene and target genes in both mammalian cells and zebrafish. Furthermore, SET8 interacts with lymphoid enhancing factor-1 (LEF1)/TCF4 directly, and this interaction is regulated by Wnt3a. Therefore, we conclude that SET8 is a Wnt signaling mediator and is recruited by LEF1/TCF4 to regulate the transcription of Wnt-activated genes, possibly through H4K20 monomethylation at the target gene promoters. Our findings also indicate that H4K20me-1 is a marker for gene transcription activation, at least in canonical Wnt signaling.

Biochemical and structural characterization of β-catenin interactions with nonphosphorylated and CK2-phosphorylated Lef-1

In the Wnt/β-catenin signaling pathway, β-catenin activates target genes through its interactions with the T-cell factor/lymphoid enhancer-binding factor (TCF/Lef) family of transcription factors. The crystal structures of complexes between the β-catenin armadillo domain and the Lef-1 N-terminal domain show that the overall conformation and many of the interactions are similar to other published structures of TCFs bound to β-catenin. However, a second salt bridge in other TCF-β-catenin structures is absent in the structure of β-catenin-Lef-1 complex, indicating that this feature is not obligatory for β-catenin binding. Casein kinase II (CK2) has been shown to act as a positive regulator of Wnt signaling, and Lef-1 is a substrate of CK2. In vitro phosphorylation of purified Lef-1 was used to examine the effect of CK2 on the interaction of Lef-1 with β-catenin. Mass spectrometry data show that CK2 phosphorylation of Lef-1 N-terminal domain results in a single phosphorylation site at Ser40. Isothermal titration calorimetry revealed that β-catenin binds to nonphosphorylated or CK2-phosphorylated Lef-1 with the same affinity, which is consistent with the absence of phospho-Ser40 interactions in the crystal structure of phosphorylated Lef-1 N-terminal domain bound to β-catenin. These data indicate that the effect of CK2 on the Wnt/β-catenin pathway does not appear to be at the level of the Lef-1-β-catenin interaction.

beta-Catenin primes organizer gene expression by recruiting a histone H3 arginine 8 methyltransferase, Prmt2

An emerging concept in development is that transcriptional poising presets patterns of gene expression in a manner that reflects a cell's developmental potential. However, it is not known how certain loci are specified in the embryo to establish poised chromatin architecture as the developmental program unfolds. We find that, in the context of transcriptional quiescence prior to the midblastula transition in Xenopus, dorsal specification by the Wnt/beta-catenin pathway is temporally uncoupled from the onset of dorsal target gene expression, and that beta-catenin establishes poised chromatin architecture at target promoters. beta-catenin recruits the arginine methyltransferase Prmt2 to target promoters, thereby establishing asymmetrically dimethylated H3 arginine 8 (R8). Recruitment of Prmt2 to beta-catenin target genes is necessary and sufficient to establish the dorsal developmental program, indicating that Prmt2-mediated histone H3(R8) methylation plays a critical role downstream of beta-catenin in establishing poised chromatin architecture and marking key organizer genes for later expression.

LEF1/beta-catenin complex regulates transcription of the Cav3.1 calcium channel gene (Cacna1g) in thalamic neurons of the adult brain

beta-Catenin, together with LEF1/TCF transcription factors, activates genes involved in the proliferation and differentiation of neuronal precursor cells. In mature neurons, beta-catenin participates in dendritogenesis and synaptic function as a component of the cadherin cell adhesion complex. However, the transcriptional activity of beta-catenin in these cells remains elusive. In the present study, we found that in the adult mouse brain, beta-catenin and LEF1 accumulate in the nuclei of neurons specifically in the thalamus. The particular electrophysiological properties of thalamic neurons depend on T-type calcium channels. Cav3.1 is the predominant T-type channel subunit in the thalamus, and we hypothesized that the Cacna1g gene encoding Cav3.1 is a target of the LEF1/beta-catenin complex. We demonstrated that the expression of Cacna1g is high in the thalamus and is further increased in thalamic neurons treated in vitro with LiCl or WNT3A, activators of beta-catenin. Luciferase reporter assays confirmed that the Cacna1G promoter is activated by LEF1 and beta-catenin, and footprinting analysis revealed four LEF1 binding sites in the proximal region of this promoter. Chromatin immunoprecipitation demonstrated that the Cacna1g proximal promoter is occupied by beta-catenin in vivo in the thalamus, but not in the hippocampus. Moreover, WNT3A stimulation enhanced T-type current in cultured thalamic neurons. Together, our data indicate that the LEF1/beta-catenin complex regulates transcription of Cacna1g and uncover a novel function for beta-catenin in mature neurons. We propose that beta-catenin contributes to neuronal excitability not only by a local action at the synapse but also by activating gene expression in thalamic neurons.

Osteocytes and WNT: the Mechanical Control of Bone Formation

Mechanical loading is of pivotal importance in the maintenance of skeletal homeostasis, but the players involved in the transduction of mechanical stimuli to promote bone maintenance have long remained elusive. Osteocytes, the most abundant cells in bone, possess mechanosensing appendices stretching through a system of bone canaliculi. Mechanical stimulation plays an important role in osteocyte survival and hence in the preservation of bone mechanical properties, through the maintenance of bone hydratation. Osteocytes can also control the osteoblastic differentiation of mesenchymal precursors in response to mechanical loading by modulating WNT signaling pathways, essential regulators of cell fate and commitment, through the protein sclerostin. Mutations of Sost, the sclerostin-encoding gene, have dramatic effects on the skeleton, indicating that osteocytes may act as master regulators of bone formation and localized bone remodeling. Moreover, the development of sclerostin inhibitors is opening new possibilities for bone regeneration in orthopedics and the dental field.

Lef-1 isoforms regulate different target genes and reduce cellular adhesion

The lymphoid enhancer factor 1 (Lef-1) belongs to the nuclear transducers of canonical Wnt-signalling in embryogenesis and cancer. Lef-1 acts, in cooperation with beta-catenin, as a context-dependent transcriptional activator or repressor, thereby influencing multiple cellular functions such as proliferation, differentiation and migration. Here we report that an increased Lef-1 expression in human pancreatic cancer correlates with advanced tumour stages. In pancreatic tumours, two different transcripts of Lef-1 have been detected in various stages, as demonstrated by RT-PCR analysis. One transcript was identified as the full length Lef-1 (Lef-1 FL), whereas the second, shorter transcript lacked exon VI (Lef-1 Deltaexon VI) compared to the published sequence. Comparative analysis of these two Lef-1 variants revealed that they exhibit different cellular effects after transient expression in pancreatic carcinoma cells. Forced expression of Lef-1 Deltaexon VI inhibited E-cadherin expression in a beta-catenin-independent way. Increased amounts of Lef-1 Deltaexon VI resulted in reduced cellular aggregation and increased cell migration. Expression of Lef-1 FL, but not the newly identified Lef-1 Deltaexon VI, induced the expression of the cell cycle regulating proteins c-myc and cyclin D1 in cooperation with beta-catenin and it enhanced cell proliferation. Our findings indicate that expression of alternatively spliced Lef-1 isoforms is involved in the determination of proliferative or migratory characteristics of pancreatic carcinoma cells.

Alternative splicing of Tcf7l2 transcripts generates protein variants with differential promoter-binding and transcriptional activation properties at Wnt/beta-catenin targets

Alternative splicing can produce multiple protein products with variable domain composition from a single gene. The mouse Tcf7l2 gene is subject to alternative splicing. It encodes TCF4, a member of the T-cell factor (TCF) family of DNA-binding proteins and a nuclear interaction partner of β-catenin which performs essential functions in Wnt growth factor signalling. Multiple TCF4 isoforms, potentially exhibiting cell-type-specific distribution and differing in gene regulatory properties, could strongly influence tissue-specific Wnt responses. Therefore, we have examined mouse Tcf7l2 splice variants in neonatal tissues, embryonic stem cells and neural progenitors. By polymerase chain reaction amplification, cloning and sequencing, we identify a large number of alternatively spliced transcripts and report a highly flexible combinatorial repertoire of alternative exons. Many, but not all of the variants exhibit a broad tissue distribution. Moreover, two functionally equivalent versions of the C-clamp, thought to represent an auxiliary DNA-binding domain, were identified. Depending upon promoter context and precise domain composition, TCF4 isoforms exhibit strikingly different transactivation potentials at natural Wnt/β-catenin target promoters. However, differences in C-clamp-mediated DNA binding can only partially explain functional differences among TCF4 variants. Still, the cell-type-specific complement of TCF4 isoforms is likely to be a major determinant for the context-dependent transcriptional output of Wnt/β-catenin signalling.

Non-cell-autonomous stimulation of stem cell proliferation following ablation of Tcf3

A combination of cell intrinsic factors and extracellular signals determine whether mouse embryonic stem cells (ESC) divide, self-renew, and differentiate. Here, we report a new interaction between cell intrinsic aspects of the canonical Wnt/Tcf/beta-catenin signaling pathway and extracellular Lif/Jak/Stat3 stimulation that combines to promote self-renewal and proliferation of ESC. Mutant ESC lacking the Tcf3 transcriptional repressor continue to self-renew in the absence of exogenous Lif and through pharmacological inhibition of Lif/Jak/Stat3 signaling; however, proliferation rates of TCF3-/- ESC were significantly decreased by inhibiting Jak/Stat3 activity. Cell mixing experiments showed that stimulation of Stat3 phosphorylation in TCF3-/- ESC was mediated through secretion of paracrine acting factors, but did not involve elevated Lif or LifR transcription. The new interaction between Wnt and Lif/Jak/Stat3 signaling pathways has potential for new insights into the growth of tumors caused by aberrant activity of Wnt/Tcf/beta-catenin signaling.

Runx2 and bone morphogenic protein 2 regulate the expression of an alternative Lef1 transcript during osteoblast maturation

Lymphoid Enhancer Binding Factor (Lef) 1 is a transcriptional effector of the Wnt/Lrp5/beta-catenin signaling cascade, which regulates osteoblast differentiation, bone density, and skeletal strength. In this study, we describe the expression and function of an alternative Lef1 isoform in osseous cells. Lef1DeltaN is a naturally occurring isoform driven by a promoter (p2) within the intron between exons 3 and 4 of Lef1. Lef1DeltaN is induced during late osteoblast differentiation. This is converse to the expression pattern of the full-length Lef1 protein, which as we previously showed, decreases during differentiation. Agonists of osteoblast maturation differentially affected Lef1DeltaN expression. BMP2 stimulated Lef1DeltaN expression, whereas Wnt3a blocked basal and BMP2-induced expression of Lef1DeltaN transcripts during osteoblast differentiation. We determined that the Lef1DeltaN p2 promoter is active in osteoblasts and Runx2 regulates its activity. Stable overexpression of Lef1DeltaN in differentiating osteoblasts induced the expression of osteoblast differentiation genes, osteocalcin and type 1 collagen. Taken together, our results suggest Lef1DeltaN is a crucial regulator of terminal differentiation in osseous 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.

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?

Dissociation of E-cadherin and beta-catenin in a mouse model of total parenteral nutrition: a mechanism for the loss of epithelial cell proliferation and villus atrophy

Total parenteral nutrition (TPN) leads a loss of epithelial barrier function, decline in epithelial cell (EC) proliferation, and decreased expression of E-cadherin. As a large portion of intracellular beta-catenin is tightly associated with E-cadherin, we hypothesized that the loss of E-cadherin would result in a redistribution of intracellular beta-catenin, and could be a contributing mechanism for this TPN-associated loss of EC proliferation. An assessment of small bowel epithelium was performed in mice given either enteral nutrition (Control) or intravenous nutrition (TPN). TPN significantly down-regulated E-cadherin and beta-catenin expression, and resulted in a loss of a colocalization of these factors. TPN also up-regulated phosphorylated (p)-beta-catenin (Ser31/33,Thr41) and down-regulated (p)-beta-catenin(Ser552) expression. To further address mechanisms driving this, we observed a significant decrease in the abundance of p-AKT and p-GSK3beta expression, and an associated decline in tcf-4 transcription factors (cyclin D1, c-myc and Axin2), as well as a loss of EC proliferation by 7 days. To address whether the mechanism driving these changes was the absence of nutritional factors, glutamine was added to the TPN solution. This resulted in a partial restoration of beta-catenin expression and EC proliferation, suggesting that an alteration in nutrient delivery may affect many of these changes. Based on these findings, the loss of EC proliferation with TPN may well be due to a loss of total beta-catenin, as well as a concomitant change in the differential expression of beta-catenin phosphorylation sites, and a reduction in beta-catenin mediated tcf-4 transcription. This potential pathway may well explain many of the findings of mucosal atrophy associated with TPN.

Epigenetic background of neuronal fate determination

The development of the central nervous system (CNS) starts from neural stem cells (NSCs). During this process, NSCs are specified in space- and time-related fashions, becoming spatially heterogeneous and generating a progressively restricted repertoire of cell types: neurons, astrocytes and oligodendrocytes. The processes of neurodevelopment are determined reciprocally by intrinsic and external factors which interface to program and re-program the profiling of fate-determination gene expression. Multiple signaling pathways act in a dynamic web mode to determine the fate of NSCs through modulating the activity of a distinct set of transcription factors which in turn trigger the transcription of neural fate-determination genes. Accumulating evidence reveals that during CNS development, multiple epigenetic factors regulate the activities of extracellular signaling and corresponding transcription factors in a coordinative manner, leading to the formation of a system with sophisticated structure and magic functions. This review aims to introduce recent advances in the epigenetic background of neural cell fate determination.

CHD8 is an ATP-dependent chromatin remodeling factor that regulates beta-catenin target genes

ATP-dependent chromatin remodeling by the CHD family of proteins plays an important role in the regulation of gene transcription. Here we report that full-length CHD8 interacts directly with beta-catenin and that CHD8 is also recruited specifically to the promoter regions of several beta-catenin-responsive genes. Our results indicate that CHD8 negatively regulates beta-catenin-targeted gene expression, since short hairpin RNA against CHD8 results in the activation of several beta-catenin target genes. This regulation is also conserved through evolution; RNA interference against kismet, the apparent Drosophila ortholog of CHD8, results in a similar activation of beta-catenin target genes. We also report the first demonstration of chromatin remodeling activity for a member of the CHD6-9 family of proteins, suggesting that CHD8 functions in transcription through the ATP-dependent modulation of chromatin structure.

An oncogenic hub: beta-catenin as a molecular target for cancer therapeutics

The Wnt/beta-catenin signaling pathway plays diverse roles in embryonic development and in maintenance of organs and tissues in adults. Activation of this signaling cascade inhibits degradation of the pivotal component beta-catenin, which in turn stimulates transcription of downstream target genes. Over the past two decades, intensive worldwide investigations have yielded considerable progress toward understanding the cellular and molecular mechanisms of Wnt signaling and its involvement in the pathogenesis of a range of human diseases. Remarkably, beta-catenin signaling is aberrantly activated in greater than 70% of colorectal cancers and to a lesser extent in other tumor types, promoting cancer cell proliferation, survival and migration. Accordingly, beta-catenin has gained recognition as an enticing molecular target for cancer therapeutics. Disruption of protein-protein interactions essential for beta-catenin activity holds immense promise for the development of novel anti-cancer drugs. In this review, we focus on the regulation of beta-catenin-dependent transcriptional activation and discuss potential therapeutic opportunities to block this signaling pathway in cancer.

Wingless signaling induces widespread chromatin remodeling of target loci

How signaling cascades influence gene regulation at the level of chromatin modification is not well understood. We studied this process using the Wingless/Wnt pathway in Drosophila. When cells sense Wingless ligand, Armadillo (the fly beta-catenin) becomes stabilized and translocates to the nucleus, where it binds to the sequence-specific DNA binding protein TCF to activate transcription of target genes. Here, we show that Wingless signaling induces TCF and Armadillo recruitment to a select subset of TCF binding site clusters that act as Wingless response elements. Despite this localized TCF/Armadillo recruitment, histones are acetylated over a wide region (up to 30 kb) surrounding the Wingless response elements in response to pathway activation. This widespread histone acetylation occurs independently of transcription. In contrast to Wingless targets, other active genes not regulated by the pathway display sharp acetylation peaks centered on their core promoters. Widespread acetylation of Wingless targets is dependent upon CBP, a histone acetyltransferase known to bind to Armadillo and is correlated with activation of target gene expression. These data suggest that pathway activation induces localized recruitment of TCF/Armadillo/CBP to Wingless response elements, leading to widespread histone acetylation of target loci prior to transcriptional activation.

Necdin mediates skeletal muscle regeneration by promoting myoblast survival and differentiation

Regeneration of muscle fibers that are lost during pathological muscle degeneration or after injuries is sustained by the production of new myofibers. An important cell type involved in muscle regeneration is the satellite cell. Necdin is a protein expressed in satellite cell–derived myogenic precursors during perinatal growth. However, its function in myogenesis is not known. We compare transgenic mice that overexpress necdin in skeletal muscle with both wild-type and necdin null mice. After muscle injury the necdin null mice show a considerable defect in muscle healing, whereas mice that overexpress necdin show a substantial increase in myofiber regeneration. We also find that in muscle, necdin increases myogenin expression, accelerates differentiation, and counteracts myoblast apoptosis. Collectively, these data clarify the function and mechanism of necdin in skeletal muscle and show the importance of necdin in muscle regeneration.

Stem cells and TCF proteins: a role for beta-catenin--independent functions

The Wnt signal transduction pathway has been shown to stimulate stem cell self renewal and has been shown to cause cancer in humans. One interesting aspect of Wnt signaling is that it utilizes downstream DNA-binding transcription factors, called Tcf proteins, which can activate transcription of target genes in the presence of a Wnt signal and repress the expression of target genes in the absence of a Wnt signal. Since Tcf proteins are present in Wnt-stimulated and unstimulated stem cells, understanding how Tcf proteins regulate target gene expression in each state offers the potential to understand how stem cells regulate their self-renewal, differentiation, and proliferation. In this article, we will review recent work elucidating the roles Tcf-protein interactions in the context of stem cells and cancer.

Beta-catenin binds to the downstream region and regulates the expression C-reactive protein gene

C-Reactive protein (CRP) is a major acute-phase response protein, which is activated by various cytokines. We investigated the mechanism of TNF-alpha-induced CRP expression and found that the p50 subunit of NF-kappaB was responsible for the transcriptional activation of CRP. Since the p50 protein acts as a positive regulator of CRP expression without an inherent transactivation domain, we looked for an interaction partner that could provide p50 with such a domain. We found that beta-catenin enhanced the expression of a CRP mRNA in concert with p50 subunit. Protein-protein interaction between p50 and beta-catenin was important for CRP expression and their interactions to CRP promoter were induced after TNF-a treatment. Since gene expression depends upon the proximity of promoters and distal regulatory sites, we explored the long-range genomic interaction at the CRP locus by chromosome conformation capture (3C). We identified a binding site for beta-catenin in the downstream of CRP gene by 3C and confirmed TNF-alpha-induced association of beta-catenin and p50 by chromatin immunoprecipitation and co-immunoprecipitation assays. Our findings provide evidence that transcription of the CRP gene depends upon p50 and beta-catenin proteins, which is accompanied by close proximity between promoter and the downstream region of CRP gene.

Molecular plasticity of beta-catenin: new insights from single-molecule measurements and MD simulation

The multifunctional protein, beta-catenin, has essential roles in cell adhesion and, through the Wnt signaling pathway, in controlling cell differentiation, development, and generation of cancer. Could distinct molecular forms of beta-catenin underlie these two functions? Our single-molecule force spectroscopy of armadillo beta-catenin, with molecular dynamics (MD) simulation, suggests a model in which the cell generates various forms of beta-catenin, in equilibrium. We find beta-catenin and the transcriptional factor Tcf4 form two complexes with different affinities. Specific cellular response is achieved by the ligand binding to a particular matching preexisting conformer. Our MD simulation indicates that complexes derive from two conformers of the core region of the protein, whose preexisting molecular forms could arise from small variations in flexible regions of the beta-catenin main binding site. This mechanism for the generation of the various forms offers a route to tailoring future therapeutic strategies.

Mammalian Nemo-like kinase enhances β-catenin-TCF transcription activity in human osteosarcoma and neuroblastoma cells

Nemo-like kinase (NLK) is an evolutionarily conserved serine/threonine kinase and has been considered to be a suppressor of Wnt signaling in mammalian cells. Our study, however, has raised the possibility that NLK also functions as a Wnt signaling activator. In human osteosarcoma and neuroblastoma cell lines, NLK specifically enhanced β-catenin-TCF complex transcription activity. The effect required kinase activity of NLK and co-expression of the β-catenin ΔN (constitutive active mutant of β-catenin). The nuclear localization of Lymphoid enhancer factor 1 (LEF1) and β-catenin ΔN was not altered by NLK overexpression regardless of its effect on β-catenin-TCF complex activity. Reporter analysis using LEF1 mutants at known NLK target sites indicated that NLK may have different activation targets for β-catenin-TCF complex. Mutations in the potential NLK phosphorylation sites in β-catenin did not change its transcription activity either. Our results suggest that NLK positively regulates Wnt/β-catenin signaling in a cell type dependent manner through an unidentified mechanism.

CK2 controls the recruitment of Wnt regulators to target genes in vivo

Nuclear beta-catenin is a transcriptional coactivator of LEF-1/TCF DNA-binding proteins in the Wnt/Wg signaling pathway. Casein Kinase 2 (CK2), a positive regulator of Wnt signaling, is present in beta-catenin complexes and activated in Wnt-signaling cells. We show here that CK2 enhances beta-catenin:LEF-1 transactivation in vivo and in vitro and that beta-catenin and CK2 cycle on and off the DNA in an alternating manner with the TLE1 corepressor at Wnt target genes. Interestingly, CK2 phosphorylates hLEF-1 directly and stimulates binding and transactivation of beta-catenin:LEF-1 complexes on chromatin templates in vitro. In vitro, CK2 phosphorylation of hLEF-1 strongly enhances its affinity for beta-catenin and reduces its affinity for TLE1. MALDI-TOF mass spectrometry (MS) identified two CK2 phosphorylation sites (S42, S61) within the amino terminus of hLEF-1, and mutation of these sites reduced binding to beta-catenin in vitro and transactivation in vivo. Remarkably, treatment of cells with TBB, a pharmaceutical inhibitor of CK2, blocked the recruitment and cycling of beta-catenin and TLE1 at Wnt target genes in vivo. Taken together, these data indicate that CK2 is required for the assembly and cycling of Wnt-enhancer complexes in vivo.

Expression of motility-related protein MRP1/CD9, N-cadherin, E-cadherin, alpha-catenin and beta-catenin in retinoblastoma

In our earlier study we showed that invasive retinoblastoma (RB) had down regulated tetraspanin protein KAI1/CD82, a family of cell surface glycoprotein. KAI1 may link to the cell surface molecules, such as integrins, E-cadherin, and other TM4SF members, and loss of KAI1 function may have a significant role in the progression of retinoblastoma. We also showed that epithelial cell adhesion molecule (EpCAM) is overexpressed in invasive RB. EpCAM expression decreases adhesion mediated by cadherins. Thus, we were further interested in studying the role of other adhesion molecules like cadherins and catenins in RB. We studied the expression of Motility-Related Protein 1 (MRP-1)/CD9, E-cadherin, N-cadherin, alpha-catenin and beta-catenin in RB and correlated clinicopathologically in 62 archival paraffin-embedded tumors by immunohistochemistry. There were 29 tumors with no invasion of choroids/optic nerve and 33 tumors with invasion of choroid/optic nerve/orbit. Western blotting was performed on 20 tumors using the same antibodies. We observed higher expression of CD9 (P<0.001), E-cadherin (P<0.001) and alpha-catenin (P<0.001) in the non-invasive RB and higher expression of N-cadherin (P<0.001) in invasive RB. The expression of beta-catenin was not significantly different between two groups of tumors. In Western blotting, we were able to see CD9 and E-cadherin expression in a minority of tumors while N-cadherin, alpha-catenin and beta-catenin were expressed with differing intensities in a majority of tumors. Thus, invasive tumors expressed increased N-cadherin, alpha-catenin and decreased E-cadherin and CD9. Thus, it appears that loss of E-cadherin and gain of N-cadherin expression are features of invasiveness. Further functional studies are required to evaluate the role of beta-catenin in RB.