Nucleo-cytoplasmic shuttling of Axin, a negative regulator of the Wnt-beta-catenin Pathway

BIRC6 Modulates the Protein Stability of Axin to Regulate the Growth, Stemness, and Resistance of Renal Cancer Cells via the β-Catenin Pathway

The mechanism underlying the development of renal cell carcinoma (RCC) remains unclear, and effective prevention and therapeutic measures are lacking. BIRC6, a protein inhibitor of apoptosis, has attracted great interest. Our data indicated that overexpression of BIRC6 elevated cell growth, colony formation, migration, and invasion of cultured RCC cells, while siRNA knockdown of BIRC6 suppressed these processes. Additionally, BIRC6 was highly expressed in RCC clinical samples along with a downregulated level of Axin. Immunoprecipitation assays found that BIRC6 interacted with Axin and the two proteins colocalized within the cytoplasm of RCC cells. Overexpression of BIRC6 promoted the ubiquitination modification of Axin, while genetic knockdown of BIRC6 suppressed it. Furthermore, overexpression of BIRC6 significantly promoted the turnover of Axin, suggesting BIRC6's inhibitory effect on Axin protein stability. BIRC6 was also upregulated in cancer stem-like cells of RCC and increased the drug resistance of RCC cells against sunitinib. Western blotting assays showed that the overexpression of BIRC6 upregulated CXCR4 protein expression and activated the β-catenin pathway. Two cell lines were then constructed with BIRC6 overexpressed by lentiviruses. Pharmacological administration of a Wnt/β-catenin inhibitor, XAV-939, or genetic knockdown of β-catenin inhibited cell growth, tumor sphere formation, colony formation, migration, and invasion of BIRC6-overexpressed cells. In vivo administration of XAV-939 markedly suppressed the tumorigenesis of BIRC6-overexpressed RCC cells in nude mice. In conclusion, we propose that BIRC6 activates the β-catenin signaling pathway via mediating the ubiquitination and degradation of Axin, promoting the growth, stemness, and drug resistance of RCC cells. This project aims to elucidate the role of BIRC6 as a potential therapeutic target and provide new insights into the clinical treatment of RCC.

The scaffold protein AXIN1: gene ontology, signal network, and physiological function

AXIN1, has been initially identified as a prominent antagonist within the WNT/β-catenin signaling pathway, and subsequently unveiled its integral involvement across a diverse spectrum of signaling cascades. These encompass the WNT/β-catenin, Hippo, TGFβ, AMPK, mTOR, MAPK, and antioxidant signaling pathways. The versatile engagement of AXIN1 underscores its pivotal role in the modulation of developmental biological signaling, maintenance of metabolic homeostasis, and coordination of cellular stress responses. The multifaceted functionalities of AXIN1 render it as a compelling candidate for targeted intervention in the realms of degenerative pathologies, systemic metabolic disorders, cancer therapeutics, and anti-aging strategies. This review provides an intricate exploration of the mechanisms governing mammalian AXIN1 gene expression and protein turnover since its initial discovery, while also elucidating its significance in the regulation of signaling pathways, tissue development, and carcinogenesis. Furthermore, we have introduced the innovative concept of the AXIN1-Associated Phosphokinase Complex (AAPC), where the scaffold protein AXIN1 assumes a pivotal role in orchestrating site-specific phosphorylation modifications through interactions with various phosphokinases and their respective substrates.

Inhibition of PI3K/Akt signaling suppresses epithelial-to-mesenchymal transition in hepatocellular carcinoma through the Snail/GSK-3/beta-catenin pathway

Background/Aims

Patients with advanced hepatocellular carcinoma (HCC) have a poor prognosis due to the lack of effective systemic therapies. Epithelial-to-mesenchymal transition (EMT) is a pivotal event in tumor progression, during which cancer cells acquire invasive properties. In this study, we investigated the effects of phosphatidylinositol 3-kinase (PI3K) inhibitors, including LY294002 and idelalisib, on the EMT features of HCC cells in vitro.

Methods

Human HCC cell lines, including Huh-BAT and HepG2, were used in this study. Cell proliferation was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide, and cell cycle distributions were evaluated using a flow cytometer by propidium iodide staining. Immunofluorescence staining, quantitative real-time polymerase chain reaction, and immunoblotting were performed to detect EMT-associated changes.

Results

PI3K inhibitors suppressed the proliferation and invasion of HCC cells and deregulated the expression of EMT markers, as indicated by increased expression of E-cadherin, an epithelial marker, and decreased expression of N-cadherin, a mesenchymal marker, and Snail, a transcription factor implicated in EMT regulation. Furthermore, LY294002 and idelalisib inhibited the phosphorylation of GSK-3β and induced the nuclear translocation of GSK-3β, which corresponded to the downregulation of Snail and β-catenin expressions in Huh-BAT and HepG2 cells.

Conclusions

The inhibition of PI3K/Akt signaling decreases Snail expression by enhancing the nuclear translocation of GSK-3β, which suppresses EMT in HCC cells, suggesting the potential clinical application of PI3K inhibitors for HCC treatment.

Pleiotropic roles of tankyrase/PARP proteins in the establishment and maintenance of human naïve pluripotency

Tankyrase 1 (TNKS1; PARP-5a) and Tankyrase 2 (TNKS2; PARP-5b) are poly-ADP-ribosyl-polymerase (PARP)-domain-containing proteins that regulate the activities of a wide repertoire of target proteins via post-translational addition of poly-ADP-ribose polymers (PARylation). Although tankyrases were first identified as regulators of human telomere elongation, important and expansive roles of tankyrase activity have recently emerged in the development and maintenance of stem cell states. Herein, we summarize the current state of knowledge of the various tankyrase-mediated activities that may promote human naïve and 'extended' pluripotency'. We review the putative role of tankyrase and PARP inhibition in trophectoderm specification, telomere elongation, DNA repair and chromosomal segregation, metabolism, and PTEN-mediated apoptosis. Importantly, tankyrases possess PARP-independent activities that include regulation of MDC1-associated DNA repair by homologous recombination (HR) and autophagy/pexophagy, which is an essential mechanism of protein synthesis in the preimplantation embryo. Additionally, tankyrases auto-regulate themselves via auto-PARylation which augments their cellular protein levels and potentiates their non-PARP tankyrase functions. We propose that these non-PARP-related activities of tankyrase proteins may further independently affect both naïve and extended pluripotency via mechanisms that remain undetermined. We broadly outline a hypothetical framework for how inclusion of a tankyrase/PARP inhibitor in small molecule cocktails may stabilize and potentiate naïve and extended pluripotency via pleiotropic routes and mechanisms.

Capturing Human Naïve Pluripotency in the Embryo and in the Dish

Although human embryonic stem cells (hESCs) were first derived almost 20 years ago, it was only recently acknowledged that they share closer molecular and functional identity to postimplantation lineage-primed murine epiblast stem cells than to naïve preimplantation inner cell mass-derived mouse ESCs (mESCs). A myriad of transcriptional, epigenetic, biochemical, and metabolic attributes have now been described that distinguish naïve and primed pluripotent states in both rodents and humans. Conventional hESCs and human induced pluripotent stem cells (hiPSCs) appear to lack many of the defining hallmarks of naïve mESCs. These include important features of the naïve ground state murine epiblast, such as an open epigenetic architecture, reduced lineage-primed gene expression, and chimera and germline competence following injection into a recipient blastocyst-stage embryo. Several transgenic and chemical methods were recently reported that appear to revert conventional human PSCs to mESC-like ground states. However, it remains unclear if subtle deviations in global transcription, cell signaling dependencies, and extent of epigenetic/metabolic shifts in these various human naïve-reverted pluripotent states represent true functional differences or alternatively the existence of distinct human pluripotent states along a spectrum. In this study, we review the current understanding and developmental features of various human pluripotency-associated phenotypes and discuss potential biological mechanisms that may support stable maintenance of an authentic epiblast-like ground state of human pluripotency.

Targeting Tankyrase to Fight WNT-dependent Tumours

Aberrant WNT signalling activity is linked to various diseases due to the WNT dependency of fundamental processes during development and in adult tissue homeostasis. Mutations in components of the multi-protein β-catenin destruction complex promote excessive amounts of the main transcriptional activator β-catenin and are particularly common in colorectal cancer (CRC). The tankyrase enzymes were recently implicated as negative regulators of destruction complex activity by mediating degradation of the scaffolding protein AXIN. Indeed, tankyrase inhibitors (TNKSi) have emerged as promising therapeutics by restoring functional signal-limiting destruction complexes in CRCs. Furthermore, as TNKSi-induced destruction complexes (so-called degradasomes) can be visualized by microscopy, they have served as a valuable experimental model system to address unresolved aspects regarding the structure, function and composition of the β-catenin destruction complex. This MiniReview provides an overview of the current knowledge on the regulatory mechanisms and interactions that govern the β-catenin destruction complex activity. It further highlights the potential of TNKSi as anticancer drugs and as a novel research tool to dissect the WNT signalling pathway.

N-terminal nesprin-2 variants regulate β-catenin signalling

The spatial compartmentalisation of biochemical signalling pathways is essential for cell function. Nesprins are a multi-isomeric family of proteins that have emerged as signalling scaffolds, herein, we investigate the localisation and function of novel nesprin-2 N-terminal variants. We show that these nesprin-2 variants display cell specific distribution and reside in both the cytoplasm and nucleus. Immunofluorescence microscopy revealed that nesprin-2 N-terminal variants colocalised with β-catenin at cell-cell junctions in U2OS cells. Calcium switch assays demonstrated that nesprin-2 and β-catenin are lost from cell-cell junctions in low calcium conditions whereas emerin localisation at the NE remained unaltered, furthermore, an N-terminal fragment of nesprin-2 was sufficient for cell-cell junction localisation and interacted with β-catenin. Disruption of these N-terminal nesprin-2 variants, using siRNA depletion resulted in loss of β-catenin from cell-cell junctions, nuclear accumulation of active β-catenin and augmented β-catenin transcriptional activity. Importantly, we show that U2OS cells lack nesprin-2 giant, suggesting that the N-terminal nesprin-2 variants regulate β-catenin signalling independently of the NE. Together, these data identify N-terminal nesprin-2 variants as novel regulators of β-catenin signalling that tether β-catenin to cell-cell contacts to inhibit β-catenin transcriptional activity.

Characterization of a beta-catenin nuclear localization defect in MCF-7 breast cancer cells

Beta-catenin plays a key role in transducing Wnt signals from the plasma membrane to the nucleus. Here we characterize an unusual subcellular distribution of beta-catenin in MCF-7 breast cancer cells, wherein beta-catenin localizes to the cytoplasm and membrane but atypically did not relocate to the nucleus after Wnt treatment. The inability of Wnt or the Wnt agonist LiCl to induce nuclear localization of beta-catenin was not due to defective nuclear transport, as the transport machinery was intact and ectopic GFP-beta-catenin displayed rapid nuclear entry in living cells. The mislocalization is explained by a shift in the retention of beta-catenin from nucleus to cytoplasm. The reduced nuclear retention is caused by unusually low expression of lymphoid enhancer factor/T-cell factor (LEF/TCF) transcription factors. The reconstitution of LEF-1 or TCF4 expression rescued nuclear localization of beta-catenin in Wnt treated cells. In the cytoplasm, beta-catenin accumulated in recycling endosomes, golgi and beta-COP-positive coatomer complexes. The peripheral association with endosomes diminished after Wnt treatment, potentially releasing β-catenin into the cytoplasm for nuclear entry. We propose that in MCF-7 and perhaps other breast cancer cells, beta-catenin may contribute to cytoplasmic functions such as ER-golgi transport, in addition to its transactivation role in the nucleus.

Mathematical and Statistical Techniques for Systems Medicine: The Wnt Signaling Pathway as a Case Study

The last decade has seen an explosion in models that describe phenomena in systems medicine. Such models are especially useful for studying signaling pathways, such as the Wnt pathway. In this chapter we use the Wnt pathway to showcase current mathematical and statistical techniques that enable modelers to gain insight into (models of) gene regulation and generate testable predictions. We introduce a range of modeling frameworks, but focus on ordinary differential equation (ODE) models since they remain the most widely used approach in systems biology and medicine and continue to offer great potential. We present methods for the analysis of a single model, comprising applications of standard dynamical systems approaches such as nondimensionalization, steady state, asymptotic and sensitivity analysis, and more recent statistical and algebraic approaches to compare models with data. We present parameter estimation and model comparison techniques, focusing on Bayesian analysis and coplanarity via algebraic geometry. Our intention is that this (non-exhaustive) review may serve as a useful starting point for the analysis of models in systems medicine.

Tankyrases: structure, function and therapeutic implications in cancer

Several cellular signaling pathways are regulated by ADP-ribosylation, a posttranslational modification catalyzed by members of the ARTD superfamily. Tankyrases are distinguishable from the rest of this family by their unique domain organization, notably the sterile alpha motif responsible for oligomerization and ankyrin repeats mediating protein-protein interactions. Tankyrases are involved in various cellular functions, such as telomere homeostasis, Wnt/β-catenin signaling, glucose metabolism, and cell cycle progression. In these processes, Tankyrases regulate the interactions and stability of target proteins by poly (ADP-ribosyl)ation. Modified proteins are subsequently recognized by the E3 ubiquitin ligase RNF146, poly-ubiquitinated and predominantly guided to 26S proteasomal degradation. Several small molecule inhibitors have been described for Tankyrases; they compete with the co-substrate NAD+ for binding to the ARTD catalytic domain. The recent, highly potent and selective inhibitors possess several properties of lead compounds and can be used for proof-of-concept studies in cancer and other Tankyrase linked diseases.

Parameter-free methods distinguish Wnt pathway models and guide design of experiments

The canonical Wnt signaling pathway, mediated by β-catenin, is crucially involved in development, adult stem cell tissue maintenance, and a host of diseases including cancer. We analyze existing mathematical models of Wnt and compare them to a new Wnt signaling model that targets spatial localization; our aim is to distinguish between the models and distill biological insight from them. Using Bayesian methods we infer parameters for each model from mammalian Wnt signaling data and find that all models can fit this time course. We appeal to algebraic methods (concepts from chemical reaction network theory and matroid theory) to analyze the models without recourse to specific parameter values. These approaches provide insight into aspects of Wnt regulation: the new model, via control of shuttling and degradation parameters, permits multiple stable steady states corresponding to stem-like vs. committed cell states in the differentiation hierarchy. Our analysis also identifies groups of variables that should be measured to fully characterize and discriminate between competing models, and thus serves as a guide for performing minimal experiments for model comparison.

Regulation of nuclear-cytoplasmic shuttling and function of Family with sequence similarity 13, member A (Fam13a), by B56-containing PP2As and Akt

Recent genome-wide association studies reveal that the FAM13A gene is associated with human lung function and a variety of lung diseases, including chronic obstructive pulmonary disease, asthma, lung cancer, and pulmonary fibrosis. The biological functions of Fam13a, however, have not been studied. In an effort to identify novel substrates of B56-containing PP2As, we found that B56-containing PP2As and Akt act antagonistically to control reversible phosphorylation of Fam13a on Ser-322. We show that Ser-322 phosphorylation acts as a molecular switch to control the subcellular distribution of Fam13a. Fam13a shuttles between the nucleus and cytoplasm. When Ser-322 is phosphorylated by Akt, the binding between Fam13a and 14-3-3 is enhanced, leading to cytoplasmic sequestration of Fam13a. B56-containing PP2As dephosphorylate phospho-Ser-322 and promote nuclear localization of Fam13a. We generated Fam13a-knockout mice. Fam13a-mutant mice are viable and healthy, indicating that Fam13a is dispensable for embryonic development and physiological functions in adult animals. Intriguingly, Fam13a has the ability to activate the Wnt pathway. Although Wnt signaling remains largely normal in Fam13a-knockout lungs, depletion of Fam13a in human lung cancer cells causes an obvious reduction in Wnt signaling activity. Our work provides important clues to elucidating the mechanism by which Fam13a may contribute to human lung diseases.

Nucleoporin 62-like protein activates canonical Wnt signaling through facilitating the nuclear import of β-catenin in zebrafish

Nucleoporin p62 (Nup62) localizes in the central channel of nuclear pore complexes (NPCs) and regulates nuclear pore permeability and nucleocytoplasmic transport. However, the developmental roles of Nup62 in vertebrates remain largely unclear. Zebrafish Nup62-like protein (Nup62l) is a homolog of mammalian Nup62. The nup62l gene is maternally expressed, but its transcripts are ubiquitously distributed during early embryogenesis and enriched in the head, pharynx, and intestine of developing embryos. Activation of the Wnt/β-catenin pathway positively modulates nup62l transcription, while Bmp signaling acts downstream of Wnt/β-catenin signaling to negatively regulate nup62l expression. Overexpression of nup62l dorsalized embryos and enhanced gastrula convergence and extension (CE) movements. In contrast, knockdown of Nup62l led to ventralized embryos, an impediment to CE movements, and defects in specification of midline organ progenitors. Mechanistically, Nup62l acts as an activator of Wnt/β-catenin signaling through interaction with and facilitation of nuclear import of β-catenin-1/2 in zebrafish. Thus, Nup62l regulates dorsoventral patterning, gastrula CE movements, and proper specification of midline organ precursors through mediating the nuclear import of β-catenins in zebrafish.

different Roles for the axin interactions with the SAMP versus the second twenty amino acid repeat of adenomatous polyposis coli

Wnt signalling is prevented by the proteosomal degradation of β-catenin, which occurs in a destruction complex containing adenomatous polyposis coli (APC), APC-like (APCL), Axin and Axin2. Truncating mutations of the APC gene result in the constitutive stabilisation of β-catenin and the initiation of colon cancer, although tumour cells tolerate the expression of wild-type APCL. Using the colocalisation of overexpressed Axin, APC and APCL constructs as a readout of interaction, we found that Axin interacted with the second twenty amino acid repeat (20R2) of APC and APCL. This interaction involved a domain adjacent to the C-terminal DIX domain of Axin. We identified serine residues within the 20R2 of APCL that were involved in Axin colocalisation, the phosphorylation of truncated APCL and the down-regulation of β-catenin. Our results indicated that Axin, but not Axin2, displaced APC, but not APCL, from the cytoskeleton and stimulated its incorporation into bright cytoplasmic dots that others have recognised as β-catenin destruction complexes. The SAMP repeats in APC interact with the N-terminal RGS domain of Axin. Our data showed that a short domain containing the first SAMP repeat in truncated APC was required to stimulate Axin oligomerisation. This was independent of Axin colocalisation with 20R2. Our data also suggested that the RGS domain exerted an internal inhibitory constraint on Axin oligomerisation. Considering our data and those from others, we discuss a working model whereby β-catenin phosphorylation involves Axin and the 20R2 of APC or APCL and further processing of phospho-β-catenin occurs upon the oligomerisation of Axin that is induced by binding the SAMP repeats in APC.

Analysis of Wnt signaling β-catenin spatial dynamics in HEK293T cells

Background

Wnt/β-catenin signaling is involved in different stages of mammalian development and implicated in various cancers (e.g. colorectal cancer). Recent experimental and computational studies have revealed characteristics of the pathway, however a cell-specific spatial perspective is lacking. In this study, a novel 3D confocal quantitation protocol is developed to acquire spatial (two cellular compartments: nucleus and cytosol-membrane) and temporal quantitative data on target protein (e.g. β-catenin) concentrations in Human Epithelial Kidney cells (HEK293T) during perturbation (with either cycloheximide or Wnt3A). Computational models of the Wnt pathway are constructed and interrogated based on this data.

Results

A single compartment Wnt pathway model is compared with a simple β-catenin two compartment model to investigate Wnt3A signaling in HEK293T cells. When protein synthesis is inhibited, β-catenin decreases at the same rate in both cellular compartments, suggesting diffusional transport is fast compared to β-catenin degradation in the cytosol. With Wnt3A stimulation, the total amount of β-catenin rises throughout the cell, however the increase is initially (~first hour) faster in the nuclear compartment. While both models were able to reproduce the whole cell changes in β-catenin, only the compartment model reproduced the Wnt3A induced changes in β-catenin distribution and it was also the best fit for the data obtained when active transport was included alongside passive diffusion transport.

Conclusions

This integrated 3D quantitation imaging protocol and computational modeling approach allowed cell-specific compartment models of the signaling pathways to be constructed and analyzed. The Wnt models constructed in this study are the first for HEK293T and have suggested potential roles of inter-compartment transport to the dynamics of signaling.

Regulation of the phosphorylation and nuclear import and export of β-catenin by APC and its cancer-related truncated form

We report the first direct analysis of the endogenous β-catenin phosphorylation activity in colon cancer SW480 cells. By comparing parental SW480 cells that harbor a typical truncated adenomatous polyposis coli (APC) form, cells expressing full-length APC and APC-depleted cells, we provide the formal demonstration that APC is necessary for β-catenin phosphorylation, both for priming of the protein at residue serine 45 and for the subsequent phosphorylation of residues 33, 37 and 41. Truncated APC still sustains a surprisingly high phosphorylation activity, which requires the protein to bind to β-catenin through the APC 20-amino-acid (20AA) repeats, thus providing a biochemical explanation for the precise truncations found in cancer cells. We also show that most of the β-catenin phosphorylation activity is associated with a dense insoluble fraction. We finally examine the impact of full-length and truncated APC on β-catenin nuclear transport. We observe that β-catenin is transported much faster than previously thought. Although this fast translocation is largely insensitive to the presence of wild-type or truncated APC, the two forms appear to limit the pool of β-catenin that is available for transport, which could have an impact on β-catenin nuclear activities in normal and cancer cells.

Wnt signaling proteins associate with the nuclear pore complex: implications for cancer

Several components of the Wnt signaling pathway have in recent years been linked to the nuclear pore complex. β-catenin, the primary transducer of Wnt signals from the plasma membrane to the nucleus, has been shown to transiently associate with different FG-repeat containing nucleoporins (Nups) and to translocate bidirectionally through pores of the nuclear envelope in a manner independent of classical transport receptors and the Ran GTPase. Two key regulators of β-catenin, IQGAP1 and APC, have also been reported to bind specific Nups or to locate at the nuclear pore complex. The interaction between these Wnt signaling proteins and different Nups may have functional implications beyond nuclear transport in cellular processes that include mitotic regulation, centrosome positioning and cell migration, nuclear envelope assembly/disassembly, and the DNA replication checkpoint. The broad implications of interactions between Wnt signaling proteins and Nups will be discussed in the context of cancer.

Expression and localization of axin 2 in colorectal carcinoma and its clinical implication

PURPOSE

Aberrant activation of the Wnt/β-catenin pathway plays a major role in the development of colorectal carcinoma (CRC). Axin 2 is a key protein of this pathway and is upregulated in CRC. Here, we investigated RNA- and protein expression of axin 2 in CRC tissues at the single cell level. Moreover, the association of axin 2 with prognosis and survival was investigated in a large cohort of CRC patients (n = 280).

METHODS

Localization and expression of axin 2 and β-catenin was investigated using in situ hybridization and immunohistochemical staining. The quantitative expression levels of axin 2 were determined using RT-qPCR. The association of axin 2 expression with prognosis and survival of the patients was determined by statistical analysis (logrank test, Kaplan-Meier).

RESULTS

Our results confirmed the upregulation of axin 2 in CRC and showed that it is broadly expressed in the cytoplasm of the tumor epithelial cells both, in the tumor center and at the invasion front. Axin 2 was rarely expressed by tumor stromal cells and only weakly by normal colonic epithelial cells. Staining of β-catenin and axin 2 in consecutive CRC tissue sections revealed that nuclear translocation of β-catenin in the tumor front was not associated with changes in the cytoplasmic localization of axin 2. Axin 2 did not show any association with proven prognostic factors or survival of the CRC patients.

CONCLUSION

The generally increased expression of axin 2 in all tumor stages as compared to normal tissue suggests an initiating pathogenic function in the development of CRC.

The Cdc42/Rac nucleotide exchange factor protein β1Pix (Pak-interacting exchange factor) modulates β-catenin transcriptional activity in colon cancer cells: evidence for direct interaction of β1PIX with β-catenin

Wnt/β-catenin signaling is highly regulated and critical for intestinal epithelial development and repair; aberrant β-catenin signaling is strongly associated with colon cancer. The small GTPase Rac1 regulates β-catenin nuclear translocation and signaling. Here we tested the hypothesis that β1Pix, a Cdc42/Rac guanine nucleotide exchange factor (GEF), regulates β-catenin-dependent transcriptional activity and cell function. We report the novel observations that β1Pix binds directly to β-catenin, an action requiring both the β1Pix DH and dimerization domains but not β1Pix GEF activity. In human colon cancer cells, activation of β-catenin signaling with LiCl decreased β1Pix/β-catenin association in the cytosol and increased nuclear binding of β-catenin to β1Pix. Nuclear association of β1Pix and β-catenin was independent of Rac1 expression and activation; down- and up-regulating Rac1 expression levels did not alter nuclear β1Pix/β-catenin association. Ectopic β1Pix expression enhanced LiCl-induced β-catenin transcriptional activity. Conversely, siRNA knockdown of β1Pix attenuated both LiCl-induced β-catenin transcriptional activity and colon cancer cell proliferation. Ectopic expression of β1Pix stimulated β-catenin transcriptional activity, whereas β1PixΔ(602-611), which is unable to bind β-catenin, had no effect. Altogether, these findings suggest that β1Pix functions as a transcriptional regulator of β-catenin signaling through direct interaction with β-catenin, an action that may be functionally relevant to colon cancer biology.

Analysing the impact of nucleo-cytoplasmic shuttling of β-catenin and its antagonists APC, Axin and GSK3 on Wnt/β-catenin signalling

The canonical Wnt signalling pathway plays a critical role in development and disease. The key player of the pathway is β-catenin. Its activity is mainly regulated by the destruction complex consisting of APC, Axin and GSK3. In the nucleus, the complex formation of β-catenin and TCF initiates target gene expression. Our study provides a comprehensive analysis of the role of nucleo-cytoplasmic shuttling of APC, Axin, and GSK3 and the inactivation of β-catenin by the destruction complex in Wnt/β-catenin signalling. We address the following questions: Can nucleo-cytoplasmic shuttling of APC, Axin and GSK3 increase the [β-catenin/TCF] concentration? And, how is the [β-catenin/TCF] concentration influenced by phosphorylation and subsequent degradation of nuclear β-catenin? Based on experimental findings, we develop a compartmental model and conduct several simulation experiments. Our analysis reveals the following key findings: 1) nucleo-cytoplasmic shuttling of β-catenin and its antagonists can yield a spatial separation between the said proteins, which results in a breakdown of β-catenin degradation, followed by an accumulation of β-catenin and hence leads to an increase of the [β-catenin/TCF] concentration. Our results strongly suggest that Wnt signalling can benefit from nucleo-cytoplasmic shuttling of APC, Axin and GSK3, although they are in general β-catenin antagonising proteins. 2) The total robustness of the [β-catenin/TCF] output is closely linked to its absolute concentration levels. We demonstrate that the compartmental separation of β-catenin and the destruction complex does not only lead to a maximization, but additionally to an increased robustness of [β-catenin/TCF] signalling against perturbations in the cellular environment. 3) A nuclear accumulation of the destruction complex renders the pathway robust against fluctuations in Wnt signalling and against changes in the compartmental distribution of β-catenin. 4) Elucidating the impact of destruction complex inhibition, we show that the [β-catenin/TCF] concentration is more effectively enhanced by inhibition of the kinase GSK3 rather than the binding of β-catenin to the destruction complex.

Toward a quantitative understanding of the Wnt/β-catenin pathway through simulation and experiment

Wnt signaling regulates cell survival, proliferation, and differentiation throughout development and is aberrantly regulated in cancer. The pathway is activated when Wnt ligands bind to specific receptors on the cell surface, resulting in the stabilization and nuclear accumulation of the transcriptional co-activator β-catenin. Mathematical and computational models have been used to study the spatial and temporal regulation of the Wnt/β-catenin pathway and to investigate the functional impact of mutations in key components. Such models range in complexity, from time-dependent, ordinary differential equations that describe the biochemical interactions between key pathway components within a single cell, to complex, multiscale models that incorporate the role of the Wnt/β-catenin pathway target genes in tissue homeostasis and carcinogenesis. This review aims to summarize recent progress in mathematical modeling of the Wnt pathway and to highlight new biological results that could form the basis for future theoretical investigations designed to increase the utility of theoretical models of Wnt signaling in the biomedical arena.

p53 regulates nuclear GSK-3 levels through miR-34-mediated Axin2 suppression in colorectal cancer cells

p53 is a bona fide tumor suppressor gene whose loss of function marks the most common genetic alteration in human malignancy. Although the causal link between loss of p53 function and tumorigenesis has been clearly demonstrated, the mechanistic links by which loss of p53 potentiates oncogenic signaling are not fully understood. Recent evidence indicates that the microRNA-34 (miR-34) family, a transcriptional target of the p53, directly suppresses a set of canonical Wnt genes and Snail, resulting in p53-mediated suppression of Wnt signaling and the EMT process. In this study, we report that p53 regulates GSK-3β nuclear localization via miR-34-mediated suppression of Axin2 in colorectal cancer. Exogenous miR-34a decreases Axin2 UTR-reporter activity through multiple binding sites within the 5' and 3' UTR of Axin2. Suppression of Axin2 by p53 or miR-34 increases nuclear GSK-3β abundance and leads to decreased Snail expression in colorectal cancer cells. Conversely, expression of the non-coding UTR of Axin2 causes depletion of endogenous miR-34 via the miR-sponge effect together with increased Axin2 function, supporting that the RNA-RNA interactions with Axin2 transcripts act as an endogenous decoy for miR-34. Further, RNA transcripts of miR-34 target were correlated with Axin2 in clinical data set of colorectal cancer patients. Although the biological relevance of nuclear GSK-3 level has not been fully studied, our results demonstrate that the tumor suppressor p53/miR-34 axis plays a role in regulating nuclear GSK-3 levels and Wnt signaling through the non-coding UTR of Axin2 in colorectal cancer.

Looking beyond the Wnt pathway for the deep nature of β-catenin

After two decades of stardom, one would think that β-catenin has revealed all of its most intimate details. Yet the essence of its duality has remained mysterious--how can a single protein both be the core link between cadherins and the cytoskeleton, and the nuclear messenger for Wnt signalling? On the basis of the available evidence and on molecular and evolutionary considerations, I propose that β-catenin was a born nuclear transport receptor, which by interacting with adhesion molecules acquired the property to coordinate nuclear functions with cell-cell adhesion. While Wnt signalling diverted this activity, the original pathway might still function in modern eukaryotes.

A Comprehensive Overview of Skeletal Phenotypes Associated with Alterations in Wnt/β-catenin Signaling in Humans and Mice

The Wnt signaling pathway plays key roles in differentiation and development and alterations in this signaling pathway are causally associated with numerous human diseases. While several laboratories were examining roles for Wnt signaling in skeletal development during the 1990s, interest in the pathway rose exponentially when three key papers were published in 2001-2002. One report found that loss of the Wnt co-receptor, Low-density lipoprotein related protein-5 (LRP5), was the underlying genetic cause of the syndrome Osteoporosis pseudoglioma (OPPG). OPPG is characterized by early-onset osteoporosis causing increased susceptibility to debilitating fractures. Shortly thereafter, two groups reported that individuals carrying a specific point mutation in LRP5 (G171V) develop high-bone mass. Subsequent to this, the causative mechanisms for these observations heightened the need to understand the mechanisms by which Wnt signaling controlled bone development and homeostasis and encouraged significant investment from biotechnology and pharmaceutical companies to develop methods to activate Wnt signaling to increase bone mass to treat osteoporosis and other bone disease. In this review, we will briefly summarize the cellular mechanisms underlying Wnt signaling and discuss the observations related to OPPG and the high-bone mass disorders that heightened the appreciation of the role of Wnt signaling in normal bone development and homeostasis. We will then present a comprehensive overview of the core components of the pathway with an emphasis on the phenotypes associated with mice carrying genetically engineered mutations in these genes and clinical observations that further link alterations in the pathway to changes in human bone.

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

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

The way Wnt works: components and mechanism

The canonical Wnt/β-catenin pathway is an ancient and evolutionarily conserved signaling pathway that is required for the proper development of all metazoans, from the basal demosponge Amphimedon queenslandica to humans. Misregulation of Wnt signaling is implicated in many human diseases, making this pathway an intense area of research in industry as well as academia. In this review, we explore our current understanding of the molecular steps involved in the transduction of a Wnt signal. We will focus on how the critical Wnt pathway component, β-catenin, is in a "futile cycle" of constant synthesis and degradation and how this cycle is disrupted upon pathway activation. We describe the role of the Wnt pathway in major human cancers and in the control of stem cell self-renewal in the developing organism and in adults. Finally, we describe well-accepted criteria that have been proposed as evidence for the involvement of a molecule in regulating the canonical Wnt pathway.

The WNT signaling pathway from ligand secretion to gene transcription: molecular mechanisms and pharmacological targets

Wingless/integrase-1 (WNT) signaling is a key pathway regulating various aspects of embryonic development; however it also underlies several pathological conditions in man, including various cancers and fibroproliferative diseases in several organs. Investigating the molecular processes involved in (canonical) WNT signaling will open new avenues for generating new therapeutics to specifically target diseases in which WNT signaling is aberrantly regulated. Here we describe the complexity of WNT signal transduction starting from the processes involved in WNT ligand biogenesis and secretion by WNT producing cells followed by a comprehensive overview of the molecular signaling events ultimately resulting in enhanced transcription of specific genes in WNT receiving cells. Finally, the possible targets for therapeutic intervention and the available pharmacological inhibitors for this complex signaling pathway are discussed.

Specific armadillo repeat sequences facilitate β-catenin nuclear transport in live cells via direct binding to nucleoporins Nup62, Nup153, and RanBP2/Nup358

β-Catenin transduces the Wnt signal from the membrane to nucleus, and certain gene mutations trigger its nuclear accumulation leading to cell transformation and cancer. β-Catenin shuttles between the nucleus and cytoplasm independent of classical Ran/transport receptor pathways, and this movement was previously hypothesized to involve the central Armadillo (Arm) domain. Fluorescence recovery after photobleaching (FRAP) assays were used to delineate functional transport regions of the Arm domain in living cells. The strongest nuclear import/export activity was mapped to Arm repeats R10-12 using both in vivo FRAP and in vitro export assays. By comparison, Arm repeats R3-8 of β-catenin were highly active for nuclear import but displayed a comparatively weak export activity. We show for the first time using purified components that specific Arm sequences of β-catenin interact directly in vitro with the FG repeats of the nuclear pore complex (NPC) components Nup62, Nup98, and Nup153, indicating an independent ability of β-catenin to traverse the NPC. Moreover, a proteomics screen identified RanBP2/Nup358 as a binding partner of Arm R10-12, and β-catenin was confirmed to interact with endogenous and ectopic forms of Nup358. We further demonstrate that knock-down of endogenous Nup358 and Nup62 impeded the rate of nuclear import/export of β-catenin to a greater extent than that of importin-β. The Arm R10-12 sequence facilitated transport even when β-catenin was bound to the Arm-binding partner LEF-1, and its activity was stimulated by phosphorylation at Tyr-654. These findings provide functional evidence that the Arm domain contributes to regulated β-catenin transport through direct interaction with the NPC.

Cdk5-mediated phosphorylation of Axin directs axon formation during cerebral cortex development

Axon formation is critical for the establishment of connections between neurons, which is a prerequisite for the development of neural circuitry. Kinases such as cyclin-dependent kinase 5 (Cdk5) and glycogen synthase kinase-3β (GSK-3β), have been implicated to regulate axon outgrowth. Nonetheless, the in vivo roles of these kinases in axon development and the underlying signaling mechanisms remain essentially unknown. We report here that Cdk5 is important for axon formation in mouse cerebral cortex through regulating the functions of axis inhibitor (Axin), a scaffold protein of the canonical Wnt pathway. Knockdown of Axin in utero abolishes the formation and projection of axons. Importantly, Axin is phosphorylated by Cdk5, and this phosphorylation facilitates the interaction of Axin with GSK-3β, resulting in inhibition of GSK-3β activity and dephosphorylation of its substrate collapsin response mediator protein-2 (CRMP-2), a microtubule-associated protein. Specifically, both phosphorylation of Axin and its interaction with GSK-3β are critically required for axon formation in mouse cortex development. Together, our findings reveal a new regulatory mechanism of axon formation through Cdk5-dependent phosphorylation of Axin.

Frat is a phosphatidylinositol 3-kinase/Akt-regulated determinant of glycogen synthase kinase 3β subcellular localization in pluripotent cells

Suppressing the activity of Gsk3β is critical for maintenance of murine pluripotent stem cells. In murine embryonic stem cells (mESCs), Gsk3β is inhibited by multiple mechanisms, including its inhibitory phosphorylation on serine 9 by protein kinase B (Akt), a major effector of the canonical phosphatidylinositol 3-kinase (PI3K) pathway. A second PI3K/Akt-regulated mechanism promotes the nuclear export of Gsk3β, thereby restricting its access to nuclear substrates such as c-myc and β-catenin. Although Gsk3β shuttles between the nucleus and cytoplasm under self-renewing conditions, its localization is primarily cytoplasmic because its rate of nuclear export exceeds its rate of nuclear import. In this report, we show that Gsk3β is exported from the nucleus in a complex with Frat. Loss of PI3K/Akt activity results in dissociation of this complex and retention of Gsk3β in the nucleus. Frat continues to shuttle between the nucleus and cytoplasm under these conditions and remains predominantly in the cytoplasm. These results indicate that Frat carries Gsk3β out of the nucleus under self-renewing conditions and that PI3K regulates this by promoting its association with Frat. These findings provide new links between PI3K/Akt signaling and regulation of Gsk3β activity by Frat, an oncogene previously shown to cooperate with Myc in tumorigenesis.

Disabled-2 and Axin are concurrently colocalized and underexpressed in lung cancers

Disabled-2 expression is reduced in many cancers, suggesting that it is a potential tumor suppressor protein. To elucidate the role of Disabled-2 in lung cancer, we examined the expression of Disabled-2, the Disabled-2-binding protein Axin, and DNA methyltransferase-1 in lung cancer tissues and corresponding normal lung tissues using immunohistochemistry and Western blots. We also determined the subcellular localization of Axin and Disabled-2 in A549 cells using confocal immunofluorescence. Disabled-2 expression was significantly reduced in lung cancers and was colocalized and coexpressed with Axin (correlation coefficient = 0.321, P < .001 for cytoplasmic expression; correlation coefficient = 0.393, P < .001 for nuclear expression). Reduced nuclear Disabled-2 expression was correlated with the differentiation (P = .048) and TNM stage (P = .048) of the tumor. The cytoplasmic expression of Axin was also correlated with differentiation (P = .042), whereas the nuclear expression of Axin was correlated with both histologic type (P = .001) and TNM stage (P < .001) of lung cancers. Expression of DNA methyltransferase-1 was negatively correlated with the cytoplasmic expression of Axin (correlation coefficient = -0.244, P = .012) but positively correlated with the histologic type (P = .004), differentiation (P = .036), TNM stage (P = .044), and lymphatic metastasis (P = .011). Expressions of Disabled-2 and Axin were concurrently reduced and correlated with the malignant phenotype of lung cancers. Enhanced expression of DNA methyltransferase-1 correlated with the reduced expression of Axin and could be a marker for lung cancer development and progression.

The ubiquitin-specific protease USP34 regulates axin stability and Wnt/β-catenin signaling

Wnt proteins control multiple cell behaviors during development and tissue homeostasis. However, pathological activation of Wnt signaling is the underlying cause of various human diseases. The ubiquitin-proteasome system plays important regulatory functions within the Wnt pathway by regulating the activity of several of its core components. Hence, multiple E3 ubiquitin ligases have been implicated in its regulation. Less is known, however, about the role of ubiquitin-specific proteases in Wnt signaling. Analysis of purified axin-containing protein complexes by liquid chromatography-tandem mass spectrometry revealed the presence of the ubiquitin protease USP34. Our results indicate that USP34 functions downstream of the β-catenin destruction complex to control the stability of axin and opposes its tankyrase-dependent ubiquitination. Reflecting on the requirement for tight control of axin homeostasis during Wnt signaling, interfering with USP34 function by RNA interference leads to the degradation of axin and to the inhibition of β-catenin-mediated transcription. Given the numerous human diseases exhibiting spurious Wnt pathway activation, the development of USP34 inhibitors may offer a novel therapeutic opportunity.

Cytokinesis proteins Tum and Pav have a nuclear role in Wnt regulation

Wg/Wnt signals specify cell fates in both invertebrate and vertebrate embryos and maintain stem-cell populations in many adult tissues. Deregulation of the Wnt pathway can transform cells to a proliferative fate, leading to cancer. We have discovered that two Drosophila proteins that are crucial for cytokinesis have a second, largely independent, role in restricting activity of the Wnt pathway. The fly homolog of RacGAP1, Tumbleweed (Tum)/RacGAP50C, and its binding partner, the kinesin-like protein Pavarotti (Pav), negatively regulate Wnt activity in fly embryos and in cultured mammalian cells. Unlike many known regulators of the Wnt pathway, these molecules do not affect stabilization of Arm/beta-catenin (betacat), the principal effector molecule in Wnt signal transduction. Rather, they appear to act downstream of betacat stabilization to control target-gene transcription. Both Tum and Pav accumulate in the nuclei of interphase cells, a location that is spatially distinct from their cleavage-furrow localization during cytokinesis. We show that this nuclear localization is essential for their role in Wnt regulation. Thus, we have identified two modulators of the Wnt pathway that have shared functions in cell division, which hints at a possible link between cytokinesis and Wnt activity during tumorigenesis.

Nuclear-cytoplasmic shuttling of Chibby controls beta-catenin signaling

Chibby (Cby) acts with 14-3-3 to regulate β-catenin localization in the canonical Wnt pathway. We show that Cby harbors functional NLS and NES motifs, and shuttles between the nucleus and cytoplasm. Cby distribution at steady state is controlled by an intricate cooperation between 14-3-3, CRM1 and importin-α, which impacts on β-catenin signaling.

In the canonical Wnt pathway, β-catenin acts as a key coactivator that stimulates target gene expression through interaction with Tcf/Lef transcription factors. Its nuclear accumulation is the hallmark of active Wnt signaling and is frequently associated with cancers. Chibby (Cby) is an evolutionarily conserved molecule that represses β-catenin–dependent gene activation. Although Cby, in conjunction with 14-3-3 chaperones, controls β-catenin distribution, its molecular nature remains largely unclear. Here, we provide compelling evidence that Cby harbors bona fide nuclear localization signal (NLS) and nuclear export signal (NES) motifs, and constitutively shuttles between the nucleus and cytoplasm. Efficient nuclear export of Cby requires a cooperative action of the intrinsic NES, 14-3-3, and the CRM1 nuclear export receptor. Notably, 14-3-3 docking provokes Cby binding to CRM1 while inhibiting its interaction with the nuclear import receptor importin-α, thereby promoting cytoplasmic compartmentalization of Cby at steady state. Importantly, the NLS- and NES-dependent shuttling of Cby modulates the dynamic intracellular localization of β-catenin. In support of our model, short hairpin RNA–mediated knockdown of endogenous Cby results in nuclear accumulation of β-catenin. Taken together, these findings unravel the molecular basis through which a combinatorial action of Cby and 14-3-3 proteins controls the dynamic nuclear-cytoplasmic trafficking of β-catenin.

Endosomal adaptor proteins APPL1 and APPL2 are novel activators of beta-catenin/TCF-mediated transcription

Canonical Wnt signaling regulates many aspects of cellular physiology and tissue homeostasis during development and in adult organisms. In molecular terms, stimulation by Wnt ligands leads to the stabilization of beta-catenin, its translocation to the nucleus, and stimulation of TCF (T-cell factor)-dependent transcription of target genes. This process is controlled at various stages by a number of regulatory proteins, including transcriptional activators and repressors. Here we demonstrate that the endosomal proteins APPL1 and APPL2 are novel activators of beta-catenin/TCF-mediated transcription. APPL proteins are multifunctional adaptors and effectors of the small GTPase Rab5, which localize to a subpopulation of early endosomes but are also capable of nucleocytoplasmic shuttling. Overexpression of APPL1 or APPL2 protein stimulates the activity of beta-catenin/TCF-dependent reporter construct, whereas silencing of APPL1 reduces it. Both APPL proteins interact directly with Reptin, a transcriptional repressor binding to beta-catenin and HDAC1 (histone deacetylase 1), and this interaction was mapped to the pleckstrin homology domain of APPL1. Moreover, APPL proteins are present in an endogenous complex containing Reptin, beta-catenin, HDAC1, and HDAC2. Overexpression of either APPL protein relieves Reptin-dependent transcriptional repression and correlates with the reduced amounts of HDACs and beta-catenin associated with Reptin as well as with the lower levels of Reptin and HDAC1 on the promoters of beta-catenin target genes. We propose that APPL proteins exert their stimulatory effects on beta-catenin/TCF-dependent transcription by decreasing the activity of a Reptin-containing repressive complex.

Subcellular localization of glycogen synthase kinase 3beta controls embryonic stem cell self-renewal

Phosphatidylinositol 3-kinase (PI3K), protein kinase B (AKT1), and c-myc have well-established roles in promoting the maintenance of murine embryonic stem cells (mESCs). In contrast, the activity of glycogen synthase kinase 3beta (GSK3beta), a negatively regulated target of AKT1 signaling, antagonizes self-renewal. Here, we show that PI3K/AKT1 signaling promotes self-renewal by suppressing GSK3beta activity and restricting its access to nuclear substrates such as c-myc. GSK3beta shuttles between the cytoplasm and nucleus in mESCs but accumulates in the cytoplasm in an inactive form due to AKT1-dependent nuclear export and inhibitory phosphorylation. When PI3K/AKT1 signaling declines following leukemia inhibitory factor withdrawal, active GSK3beta accumulates in the nucleus, where it targets c-myc through phosphorylation on threonine 58 (T58), promoting its degradation. Ectopic nuclear localization of active GSK3beta promotes differentiation, but this process is blocked by a mutant form of c-myc (T58A) that evades phosphorylation by GSK3beta. This novel mechanism explains how AKT1 promotes self-renewal by regulating the activity and localization of GSK3beta. This pathway converges on c-myc, a key regulator of mESC self-renewal.

Fine-tuning of nuclear-catenin by Chibby and 14-3-3

Chibby (Cby) is an evolutionarily conserved antagonist of beta-catenin, a central player of the canonical Wnt signaling pathway, which acts as a transcriptional coactivator. Cby physically interacts with the C-terminal activation domain of beta-catenin and blocks its transcriptional activation potential through competition with DNA-binding Tcf/Lef transcription factors. Our recent study revealed a second mechanism for Cby-mediated beta-catenin inhibition in which Cby cooperates with 14-3-3 adaptor proteins to facilitate nuclear export of beta-catenin, following phosphorylation of Cby by Akt kinase. Therefore, our findings unravel a novel molecular mechanism regulating the dynamic nucleo-cytoplasmic trafficking of beta-catenin and provide new insights into the cross-talk between the Wnt and Akt signaling pathways. Here, we review recent literature concerning Cby function and discuss our current understanding of the relationship between Wnt and Akt signaling.

Membrane bound axin is sufficient for Wingless signaling in Drosophila embryos

The Wingless signaling pathway controls various developmental processes in both vertebrates and invertebrates. Here I probe the requirement for nuclear localization of APC2 and Axin in the Wg signal transduction pathway during embryonic development of Drosophila melanogaster. I find that nuclear localization of APC2 appears to be required, but Axin can block signaling when tethered to the membrane. These results support the model where Axin regulates Armadillo localization and activity in the cytoplasm.

Nuclear APC

Mutational inactivation of the tumor suppressor gene APC (Adenomatous polyposis coli) is thought to be an initiating step in the progression of the vast majority ofcolorectal cancers. Attempts to understand APC function have revealed more than a dozen binding partners as well as several subcellular localizations including at cell-cell junctions, associated with microtubules at the leading edge of migrating cells, at the apical membrane, in the cytoplasm and in the nucleus. The present chapter focuses on APC localization and functions in the nucleus. APC contains two classical nuclear localization signals, with a third domain that can enhance nuclear import. Along with two sets of nuclear export signals, the nuclear localization signals enable the large APC protein to shuttle between the nucleus and cytoplasm. Nuclear APC can oppose beta-catenin-mediated transcription. This down-regulation of nuclear beta-catenin activity by APC most likely involves nuclear sequestration of beta-catenin from the transcription complex as well as interaction of APC with transcription corepressor CtBP. Additional nuclear binding partners for APC include transcription factor activator protein AP-2alpha, nuclear export factor Crm1, protein tyrosine phosphatase PTP-BL and perhaps DNA itself. Interaction of APC with polymerase beta and PCNA, suggests a role for APC in DNA repair. The observation that increases in the cytoplasmic distribution of APC correlate with colon cancer progression suggests that disruption of these nuclear functions of APC plays an important role in cancer progression. APC prevalence in the cytoplasm of quiescent cells points to a potential function for nuclear APC in control of cell proliferation. Clear definition of APC's nuclear function(s) will expand the possibilities for early colorectal cancer diagnostics and therapeutics targeted to APC.

Dapper1 is a nucleocytoplasmic shuttling protein that negatively modulates Wnt signaling in the nucleus

Wnt signaling, via the activation of the canonical beta-catenin and lymphoid enhancer factor (LEF)/T-cell factor pathway, plays an important role in embryogenesis and cancer development by regulating the expression of genes involved in cell proliferation, differentiation, and survival. Dapper (Dpr), as a Dishevelled interactor, has been suggested to modulate Wnt signaling by promoting Dishevelled degradation. Here, we provide evidence that Dpr1 shuttles between the cytoplasm and the nucleus. Although overexpressed Dpr1 was mainly found in the cytoplasm, endogenous Dpr1 was localized over the cell, and Wnt1 induced its nuclear export. Treatment with leptomycin B induced nuclear accumulation of both endogenous and overexpressed Dpr1. We further identified the nuclear localization signal and the nuclear export signal within Dpr1. Using reporter assay and in vivo zebrafish embryo assay, we demonstrated that the forced nuclearly localized Dpr1 possessed the ability to antagonize Wnt signaling. Dpr1 interacted with beta-catenin and LEF1 and disrupted their complex formation. Furthermore, Dpr1 could associate with histone deacetylase 1 (HDAC1) and enhance the LEF1-HDAC1 interaction. Together, our findings suggest that Dpr1 negatively modulates the basal activity of Wnt/beta-catenin signaling in the nucleus by keeping LEF1 in the repressive state. Thus, Dpr1 controls Wnt/beta-catenin signaling in both the cytoplasm and the nucleus.

Recruitment of adenomatous polyposis coli and beta-catenin to axin-puncta

The adenomatous polyposis coli (APC) tumour suppressor is a multifunctional protein involved in the regulation of Wnt signalling and cytoskeletal dynamics. Little is known about how APC controls these disparate functions. In this study, we have used APC- and axin-fluorescent fusion proteins to examine the interactions between these proteins and show that the functionally distinct populations of APC are also spatially separate. Axin-RFP forms cytoplasmic punctate structures, similar to endogenous axin puncta. Axin-RFP recruits beta-catenin destruction complex proteins, including APC, beta-catenin, glycogen synthase kinase-3-beta (GSK3-beta) and casein kinase-1-alpha (CK1-alpha). Recruitment into axin-RFP puncta sequesters APC from clusters at cell extensions and this prevents its microtubule-associated functions. The interaction between APC-GFP and axin-RFP within the cytoplasmic puncta is direct and dramatically alters the dynamic properties of APC-GFP. However, recruitment of APC to axin puncta is not absolutely required for beta-catenin degradation. Instead, formation of axin puncta, mediated by the DIX domain, is required for beta-catenin degradation. An axinDeltaDIX mutant did not form puncta, but still mediated recruitment of destruction complex proteins and phosphorylation of beta-catenin. We conclude that there are distinct pools of APC and that the formation of axin puncta, rather than the axin/APC complex, is essential for beta-catenin destruction.

Chibby cooperates with 14-3-3 to regulate beta-catenin subcellular distribution and signaling activity

β-Catenin functions in both cell–cell adhesion and as a transcriptional coactivator in the canonical Wnt pathway. Nuclear accumulation of β-catenin is the hallmark of active Wnt signaling and is frequently observed in human cancers. Although β-catenin shuttles in and out of the nucleus, the molecular mechanisms underlying its translocation remain poorly understood. Chibby (Cby) is an evolutionarily conserved molecule that inhibits β-catenin–mediated transcriptional activation. Here, we identified 14-3-3ε and 14-3-3ζ as Cby-binding partners using affinity purification/mass spectrometry. 14-3-3 proteins specifically recognize serine 20 within the 14-3-3–binding motif of Cby when phosphorylated by Akt kinase. Notably, 14-3-3 binding results in sequestration of Cby into the cytoplasm. Moreover, Cby and 14-3-3 form a stable tripartite complex with β-catenin, causing β-catenin to partition into the cytoplasm. Our results therefore suggest a novel paradigm through which Cby acts in concert with 14-3-3 proteins to facilitate nuclear export of β-catenin, thereby antagonizing β-catenin signaling.

Beta-catenin can bind directly to CRM1 independently of adenomatous polyposis coli, which affects its nuclear localization and LEF-1/beta-catenin-dependent gene expression

Nuclear beta-catenin affects the developmental process and progression of tumors. However, the precise mechanism for the nuclear export of beta-catenin is not completely understood. We found that beta-catenin can bind directly to CRM1 through its central armadillo (ARM) repeats region, independently of the adenomatous polyposis coli (APC) protein. CRM1 overexpression transports nuclear beta-catenin into the cytoplasm and decreases LEF-1/beta-catenin-dependent transcriptional activity, which is also affected by the co-overexpression of E-cadherin. CRM1 competed with E-cadherin and LEF-1 for binding to beta-catenin. beta-catenin could interact directly with APC through its essential sequences between amino acids 342 and 350. The site-directed beta-catenin mutant (NES2(-)), which could interact with CRM1, but not with APC, still retained its ability to export from the nucleus and its transactivational activity. This suggests that CRM1 can function as an efficient nuclear exporter for beta-catenin independently of APC. These results strongly suggest that the CRM1-mediated pathway is involved in the efficient transport of nuclear beta-catenin in the nucleus of cells.

Drosophila Naked cuticle (Nkd) engages the nuclear import adaptor Importin-alpha3 to antagonize Wnt/beta-catenin signaling

Precise control of Wnt/beta-catenin signaling is critical for animal development, stem cell renewal, and prevention of disease. In the fruit fly Drosophila melanogaster, the naked cuticle (nkd) gene limits signaling by the Wnt ligand Wingless (Wg) during embryo segmentation. Nkd is an intracellular protein that is composed of separable membrane- and nuclear-localization sequences (NLS) as well as a conserved EF-hand motif that binds the Wnt receptor-associated scaffold protein Dishevelled (Dsh), but the mechanism by which Nkd inhibits Wnt signaling remains a mystery. Here we identify a second NLS in Nkd that is required for full activity and that binds to the canonical nuclear import adaptor Importin-alpha3. The Nkd NLS is similar to the Importin-alpha3-binding NLS in the Drosophila heat-shock transcription factor (dHSF), and each Importin-alpha3-binding NLS required intact basic residues in similar positions for nuclear import and protein function. Our results provide further support for the hypothesis that Nkd inhibits nuclear step(s) in Wnt/beta-catenin signaling and broaden our understanding of signaling pathways that engage the nuclear import machinery.