A small molecule compound 759 inhibits the wnt/beta-catenin signaling pathway via increasing the Axin protein stability

Wnt/β-catenin signaling plays important role in cancers. Compound 759 is one of the compounds previously screened to identify inhibitors of the Wnt/β-catenin pathway in A549 cells [Lee et al. in Bioorg Med Chem Lett 20:5900-5904, 2010]. However, the mechanism by which Compound 759 induces the inhibition of the Wnt/β-catenin pathway remains unknown. In our study, we employed various assays to comprehensively evaluate the effects of Compound 759 on lung cancer cells. Our results demonstrated that Compound 759 significantly suppressed cell proliferation and Wnt3a-induced Topflash activity and arrested the cell cycle at the G1 stage. Changes in Wnt/β-catenin signaling-related protein expression, gene activity, and protein stability including Axin, and p21, were achieved through western blot and qRT-PCR analysis. Compound 759 treatment upregulated the mRNA level of p21 and increased Axin protein levels without altering the mRNA expression in A549 cells. Co-treatment of Wnt3a and varying doses of Compound 759 dose-dependently increased the amounts of Axin1 in the cytosol and inhibited β-catenin translocation into the nucleus. Moreover, Compound 759 reduced tumor size and weight in the A549 cell-induced tumor growth in the in vivo tumor xenograft mouse model. Our findings indicate that Compound 759 exhibits potential anti-cancer activity by inhibiting the Wnt/β-catenin signaling pathway through the increase of Axin1 protein stability.

APC mutations disrupt β-catenin destruction complex condensates organized by Axin phase separation

The Wnt/β-catenin pathway is critical to maintaining cell fate decisions. Recent study showed that liquid-liquid-phase separation (LLPS) of Axin organized the β-catenin destruction complex condensates in a normal cellular state. Mutations inactivating the APC gene are found in approximately 80% of all human colorectal cancer (CRC). However, the molecular mechanism of the formation of β-catenin destruction complex condensates organized by Axin phase separation and how APC mutations impact the condensates are still unclear. Here, we report that the β-catenin destruction complex, which is constructed by Axin, was assembled condensates via a phase separation process in CRC cells. The key role of wild-type APC is to stabilize destruction complex condensates. Surprisingly, truncated APC did not affect the formation of condensates, and GSK 3β and CK1α were unsuccessfully recruited, preventing β-catenin phosphorylation and resulting in accumulation in the cytoplasm of CRCs. Besides, we propose that the phase separation ability of Axin participates in the nucleus translocation of β-catenin and be incorporated and concentrated into transcriptional condensates, affecting the transcriptional activity of Wnt signaling pathway.

R-spondin-1 induces Axin degradation via the LRP6-CK1ε axis

R-spondins (RSPOs) are secreted signaling molecules that potentiate the Wnt/β-catenin pathway by cooperating with Wnt ligands. RSPO1 is crucial in tissue development and tissue homeostasis. However, the molecular mechanism by which RSPOs activate Wnt/β-catenin signaling remains elusive. In this study, we found that RSPOs could mediate the degradation of Axin through the ubiquitin-proteasome pathway. The results of Co-IP showed that the recombinant RSPO1 protein promoted the interaction between Axin1 and CK1ε. Either knockout of the CK1ε gene or treatment with the CK1δ/CK1ε inhibitor SR3029 caused an increase in Axin1 protein levels and attenuated RSPO1-induced degradation of the Axin1 protein. Moreover, we observed an increase in the number of associations of LRP6 with CK1ε and Axin1 following RSPO1 stimulation. Overexpression of LRP6 further potentiated Axin1 degradation mediated by RSPO1 or CK1ε. In addition, recombinant RSPO1 and Wnt3A proteins synergistically downregulated the protein expression of Axin1 and enhanced the transcriptional activity of the SuperTOPFlash reporter. Taken together, these results uncover the novel mechanism by which RSPOs activate Wnt/β-catenin signaling through LRP6/CK1ε-mediated degradation of Axin.

Auxin/cytokinin antagonism in shoot development: from moss to seed plants

This article comments on:

Cammarata J, Roeder AHK, Scanlon MJ. 2023. The ratio of auxin to cytokinin controls leaf development and meristem initiation in Physcomitrium patens. Journal of Experimental Botany 74, 6541–6550.

Phosphorylation of axin within biomolecular condensates counteracts its tankyrase-mediated degradation

Axin (also known as AXIN1) is a central negative regulator of the proto-oncogenic Wnt/β-catenin signaling pathway, as axin condensates provide a scaffold for the assembly of a multiprotein complex degrading β-catenin. Axin, in turn, is degraded through tankyrase. Consequently, tankyrase small-molecule inhibitors block Wnt signaling by stabilizing axin, revealing potential for cancer therapy. Here, we discovered that axin is phosphorylated by casein kinase 1 alpha 1 (CSNK1A1, also known as CK1α) at an N-terminal casein kinase 1 consensus motif, and that this phosphorylation is antagonized by the catalytic subunit alpha of protein phosphatase 1 (PPP1CA, hereafter referred to as PP1). Axin condensates promoted phosphorylation by enriching CK1α over PP1. Importantly, the phosphorylation took place within the tankyrase-binding site, electrostatically and/or sterically hindering axin-tankyrase interaction, and counteracting tankyrase-mediated degradation of axin. Thus, the presented data propose a novel mechanism regulating axin stability, with implications for Wnt signaling, cancer therapy and self-organization of biomolecular condensates.

When You Come to a Fork in the Road, Take It: Wnt Signaling Activates Multiple Pathways through the APC/Axin/GSK-3 Complex

The Wnt signaling pathway is a highly conserved regulator of metazoan development and stem cell maintenance. Activation of Wnt signaling is an early step in diverse malignancies. Work over the past four decades has defined a "canonical" Wnt pathway that is initiated by Wnt proteins, secreted glycoproteins that bind to a surface receptor complex and activate intracellular signal transduction by inhibiting a catalytic complex composed of the classical tumor suppressor Adenomatous Polyposis Coli (APC), Axin, and Glycogen Synthase Kinase-3 (GSK-3). The best characterized effector of this complex is β-catenin, which is stabilized by inhibition of GSK-3, allowing β-catenin entrance to the nucleus and activation of Wnt target gene transcription, leading to multiple cancers when inappropriately activated. However, canonical Wnt signaling through the APC/Axin/GSK-3 complex impinges on other effectors, independently of β-catenin, including the mechanistic Target of Rapamycin (mTOR), regulators of protein stability, mitotic spindle orientation, and Hippo signaling. This review focuses on these alternative effectors of the canonical Wnt pathway and how they may contribute to cancers.

Axin1 Protects Colon Carcinogenesis by an Immune-Mediated Effect

BACKGROUND & AIMS

Axin1 is a negative regulator of wingless-type MMTV integration site family, member 1 (Wnt)/β-catenin signaling with tumor-suppressor function. The Wnt pathway has a critical role in the intestine, both during homeostasis and cancer, but the role of Axin1 remains elusive.

METHODS

We assessed the role of Axin1 in normal intestinal homeostasis, with control, epithelial-specific, Axin1-knockout mice (Axin1^ΔIEC) and Axin2-knockout mice. We evaluated the tumor-suppressor function of Axin1 during chemically induced colorectal tumorigenesis and dextran sulfate sodium-induced colitis, and performed comparative gene expression profiling by whole-genome RNA sequencing. The clinical relevance of the Axin1-dependent gene expression signature then was tested in a database of 2239 clinical colorectal cancer (CRC) samples.

RESULTS

We found that Axin1 was dispensable for normal intestinal homeostasis and redundant with Axin2 for Wnt pathway down-regulation. Axin1 deficiency in intestinal epithelial cells rendered mice more susceptible to chemically induced colon carcinogenesis, but reduced dextran sulfate sodium-induced colitis by attenuating the induction of a proinflammatory program. RNA-seq analyses identified an interferon γ/T-helper1 immune program controlled by Axin1 that enhances the inflammatory response and protects against CRC. The Axin1-dependent gene expression signature was applied to human CRC samples and identified a group of patients with potential vulnerability to immune checkpoint blockade therapies.

CONCLUSIONS

Our study establishes, in vivo, that Axin1 has redundant function with Axin2 for Wnt down-regulation and infers a new role for Axin1. Physiologically, Axin1 stimulates gut inflammation via an interferon γ/Th1 program that prevents tumor growth. Linked to its T-cell-mediated effect, the colonic Axin1 signature offers therapeutic perspectives for CRC.

Glycogen Synthase Kinase-3 Interaction Domain Enhances Phosphorylation of SARS-CoV-2 Nucleocapsid Protein

A structural protein of SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2), nucleocapsid (N) protein is phosphorylated by glycogen synthase kinase (GSK)-3 on the serine/arginine (SR) rich motif located in disordered regions. Although phosphorylation by GSK-3β constitutes a critical event for viral replication, the molecular mechanism underlying N phosphorylation is not well understood. In this study, we found the putative alpha-helix L/FxxxL/AxxRL motif known as the GSK-3 interacting domain (GID), found in many endogenous GSK-3β binding proteins, such as Axins, FRATs, WWOX, and GSKIP. Indeed, N interacts with GSK-3β similarly to Axin, and Leu to Glu substitution of the GID abolished the interaction, with loss of N phosphorylation. The N phosphorylation is also required for its structural loading in a virus-like particle (VLP). Compared to other coronaviruses, N of Sarbecovirus lineage including bat RaTG13 harbors a CDK1-primed phosphorylation site and Gly-rich linker for enhanced phosphorylation by GSK-3β. Furthermore, we found that the S202R mutant found in Delta and R203K/G204R mutant found in the Omicron variant allow increased abundance and hyper-phosphorylation of N. Our observations suggest that GID and mutations for increased phosphorylation in N may have contributed to the evolution of variants.

The nonredundant nature of the Axin2 regulatory network in the canonical Wnt signaling pathway

The mystery of two functionally redundant Axin genes in all vertebrates can now be explained by the demonstration that they form a nested proteostatic and transcriptional feedback system that confers regulatory options in different developmental settings, a form of dynamic versatility that may explain the widespread occurrence of closely related seemingly redundant genes with similar functions.

Axin is one of two essential scaffolds in the canonical Wnt pathway that converts signals at the plasma membrane to signals inhibiting the degradation of β-catenin, leading to its accumulation and specific gene activation. In vertebrates, there are two forms of Axin, Axin1 and Axin2, which are similar at the protein level and genetically redundant. We show here that differential regulation of the two genes on the transcriptional and proteostatic level confers differential responsiveness that can be used in tissue-specific regulation. Such subtle features may distinguish other redundant gene pairs that are commonly found in vertebrates through gene knockout experiments.

Design, synthesis and structure-activity relationship optimization of phenanthridine derivatives as new anti-vitiligo compounds

Humans have been suffering from vitiligo for a long time. Target vitiligo drugs have yet been approved. Activation of Wnt/β-catenin signalling has potential in the therapeutic use of vitiligo, so exploring new drugs that specifically directly activate Wnt is worthwhile to obtain new anti-vitiligo agents. In this work, two portions design and synthesis were put into effect. firstly, 17 phenanthridine derivatives with C-4 substitutes were designed and synthesized, which compounds 4, 6, 12, 13 served as H-acceptor with protein showed enhance melanogenesis activity; Secondly, 7 hybrid new scaffolds of compounds were designed and synthesized, scaffold hopping compound 36 that aromatic benzene was replaced pyrazole on ring C showed enhance melanogenesis and tyrosinase activity; The last and most important, a comprehensive optimization and SARs of compound 36 were carried out, compounds 41 and 43 shared phenolic hydroxyl or 3-methyl-pyridine substitutes at C-7 position remarkably improved the capacity of melanogenesis and tyrosinase activity. Compound 43 were identified as new anti-vitiligo agents that specifically activate the Wnt/β-catenin signalling pathway by targeting Axin. Structure-activity relationship analysis implied that H-acceptor substitutions at the C-4 position and phenolic hydroxyl or pyridine substitutions at the C-7 position would improve the activities of the compounds. These findings reveal a new therapeutic strategy for vitiligo, and compounds 41 and 43 may represent potential compounds for vitiligo treatment.

Liquid-liquid phase separation drives the β-catenin destruction complex formation

The intracellular multiprotein complex β-catenin destruction complex plays a key role in Wnt/β-catenin signaling. Wnt stimulation induces the assembly of the receptor-associated signalosome and the inactivation of the destruction complex, leading to β-catenin accumulation and transcriptional activation of the target genes. The core components of the destruction complex include Axin, APC, GSK3β, CK1α and other proteins. Recent studies demonstrated that Axin and APC undergo liquid-liquid phase separation (LLPS), which is critical for their function to regulate Wnt/β-catenin signaling. Here, we discuss the possible roles of LLPS in Wnt/β-catenin signaling and regulation of Axin LLPS by post-translational modifications.

Tissue-Specific Regulation of the Wnt/β-Catenin Pathway by PAGE4 Inhibition of Tankyrase

Spatiotemporal control of Wnt/β-catenin signaling is critical for organism development and homeostasis. The poly-(ADP)-ribose polymerase Tankyrase (TNKS1) promotes Wnt/β-catenin signaling through PARylation-mediated degradation of AXIN1, a component of the β-catenin destruction complex. Although Wnt/β-catenin is a niche-restricted signaling program, tissue-specific factors that regulate TNKS1 are not known. Here, we report prostate-associated gene 4 (PAGE4) as a tissue-specific TNKS1 inhibitor that robustly represses canonical Wnt/β-catenin signaling in human cells, zebrafish, and mice. Structural and biochemical studies reveal that PAGE4 acts as an optimal substrate decoy that potently hijacks substrate binding sites on TNKS1 to prevent AXIN1 PARylation and degradation. Consistently, transgenic expression of PAGE4 in mice phenocopies TNKS1 knockout. Physiologically, PAGE4 is selectively expressed in stromal prostate fibroblasts and functions to establish a proper Wnt/β-catenin signaling niche through suppression of autocrine signaling. Our findings reveal a non-canonical mechanism for TNKS1 inhibition that functions to establish tissue-specific control of the Wnt/β-catenin pathway.

An early global role for Axin is required for correct patterning of the anterior-posterior axis in the sea urchin embryo

Activation of Wnt/β-catenin (cWnt) signaling at the future posterior end of early bilaterian embryos is a highly conserved mechanism for establishing the anterior-posterior (AP) axis. Moreover, inhibition of cWnt at the anterior end is required for development of anterior structures in many deuterostome taxa. This phenomenon, which occurs around the time of gastrulation, has been fairly well characterized, but the significance of intracellular inhibition of cWnt signaling in cleavage-stage deuterostome embryos for normal AP patterning is less well understood. To investigate this process in an invertebrate deuterostome, we defined Axin function in early sea urchin embryos. Axin is ubiquitously expressed at relatively high levels in early embryos and functional analysis revealed that Axin suppresses posterior cell fates in anterior blastomeres by blocking ectopic cWnt activation in these cells. Structure-function analysis of sea urchin Axin demonstrated that only its GSK-3β-binding domain is required for cWnt inhibition. These observations and results in other deuterostomes suggest that Axin plays a crucial conserved role in embryonic AP patterning by preventing cWnt activation in multipotent early blastomeres, thus protecting them from assuming ectopic cell fates.

Summary: Axin function is required in the early sea urchin embryo to regulate nuclear β-catenin levels and prevent ectopic cell fates in multipotent early blastomeres, and to ensure correct anterior-posterior axis patterning.

Identification and Mechanistic Characterization of a Peptide Inhibitor of Glycogen Synthase Kinase (GSK3β) Derived from the Disrupted in Schizophrenia 1 (DISC1) Protein

Glycogen synthase kinase 3-beta (GSK3β) is a critical regulator of several cellular pathways involved in neurodevelopment and neuroplasticity and as such is a potential focus for the discovery of new neurotherapeutics toward the treatment of neuropsychiatric and neurodegenerative diseases. The majority of efforts to develop inhibitors of GSK3β have been focused on developing small molecule inhibitors that compete with adenosine triphosphate (ATP) through direct interaction with the ATP binding site. This strategy has presented selectivity challenges due to the evolutionary conservation of this domain within the kinome. The disrupted in schizophrenia 1 (DISC1) protein has previously been shown to bind and inhibit GSK3β activity. Here, we report the characterization of a 44-mer peptide derived from human DISC1 (hDISCtide) that is sufficient to both bind and inhibit GSK3β in a noncompetitive mode distinct from classical ATP competitive inhibitors. Based on multiple independent biochemical and biophysical assays, we propose that hDISCtide interacts at two distinct regions of GSK3β: an inhibitory region that partially overlaps with the binding site of FRATide, a well-known GSK3β binding peptide, and a specific binding region that is unique to hDISCtide. Taken together, our findings present a novel avenue for developing a peptide-based selective inhibitor of GSK3β.

The Scaffold Protein Axin Promotes Signaling Specificity within the Wnt Pathway by Suppressing Competing Kinase Reactions

Scaffold proteins are thought to promote signaling specificity by accelerating reactions between bound kinase and substrate proteins. To test the long-standing hypothesis that the scaffold protein Axin accelerates glycogen synthase kinase 3β (GSK3β)-mediated phosphorylation of β-catenin in the Wnt signaling network, we measured GSK3β reaction rates with multiple substrates in a minimal, biochemically reconstituted system. We observed an unexpectedly small, ∼2-fold Axin-mediated rate increase for the β-catenin reaction when measured in isolation. In contrast, when both β-catenin and non-Wnt pathway substrates are present, Axin accelerates the β-catenin reaction by preventing competition with alternative substrates. At high competitor concentrations, Axin produces >10-fold rate effects. Thus, while Axin alone does not markedly accelerate the β-catenin reaction, in physiological settings where multiple GSK3β substrates are present, Axin may promote signaling specificity by suppressing interactions with competing, non-Wnt pathway targets. This mechanism for scaffold-mediated control of competition enables a shared kinase to perform distinct functions in multiple signaling networks.

Limited dishevelled/Axin oligomerization determines efficiency of Wnt/β-catenin signal transduction

In Wnt/β-catenin signaling, the transcriptional coactivator β-catenin is regulated by its phosphorylation in a complex that includes the scaffold protein Axin and associated kinases. Wnt binding to its coreceptors activates the cytosolic effector Dishevelled (Dvl), leading to the recruitment of Axin and the inhibition of β-catenin phosphorylation. This process requires interaction of homologous DIX domains present in Dvl and Axin, but is mechanistically undefined. We show that Dvl DIX forms antiparallel, double-stranded oligomers in vitro, and that Dvl in cells forms oligomers typically <10 molecules at endogenous expression levels. Axin DIX (DAX) forms small single-stranded oligomers, but its self-association is stronger than that of DIX. DAX caps the ends of DIX oligomers, such that a DIX oligomer has at most four DAX binding sites. The relative affinities and stoichiometry of the DIX-DAX interaction provide a mechanism for efficient inhibition of β-catenin phosphorylation upon Axin recruitment to the Wnt receptor complex.

elegans

Scaffold proteins serve important roles in cellular signaling by integrating inputs from multiple signaling molecules to regulate downstream effectors that, in turn, carry out specific biological functions. One such protein, Axin, represents a major evolutionarily conserved scaffold protein in metazoans that participates in the WNT pathway and other pathways to regulate diverse cellular processes. This review summarizes the vast amount of literature on the regulation and functions of the Axin family of genes in eukaryotes, with a specific focus on Caenorhabditis elegans development. By combining early studies with recent findings, the review is aimed to serve as an updated reference for the roles of Axin in C. elegans and other model systems.

Role of PRY-1/Axin in heterochronic miRNA-mediated seam cell development

Background

Caenorhabditis elegans seam cells serve as a good model to understand how genes and signaling pathways interact to control asymmetric cell fates. The stage-specific pattern of seam cell division is coordinated by a genetic network that includes WNT asymmetry pathway components WRM-1, LIT-1, and POP-1, as well as heterochronic microRNAs (miRNAs) and their downstream targets. Mutations in pry-1, a negative regulator of WNT signaling that belongs to the Axin family, were shown to cause seam cell defects; however, the mechanism of PRY-1 action and its interactions with miRNAs remain unclear.

Results

We found that pry-1 mutants in C. elegans exhibit seam cell, cuticle, and alae defects. To examine this further, a miRNA transcriptome analysis was carried out, which showed that let-7 (miR-48, miR-84, miR-241) and lin-4 (lin-4, miR-237) family members were upregulated in the absence of pry-1 function. Similar phenotypes and patterns of miRNA overexpression were also observed in C. briggsae pry-1 mutants, a species that is closely related to C. elegans. RNA interference-mediated silencing of wrm-1 and lit-1 in the C. elegans pry-1 mutants rescued the seam cell defect, whereas pop-1 silencing enhanced the phenotype, suggesting that all three proteins are likely important for PRY-1 function in seam cells. We also found that these miRNAs were overexpressed in pop-1 hypomorphic animals, suggesting that PRY-1 may be required for POP-1-mediated miRNA suppression. Analysis of the let-7 and lin-4-family heterochronic targets, lin-28 and hbl-1, showed that both genes were significantly downregulated in pry-1 mutants, and furthermore, lin-28 silencing reduced the number of seam cells in mutant animals.

Conclusions

Our results show that PRY-1 plays a conserved role to maintain normal expression of heterochronic miRNAs in nematodes. Furthermore, we demonstrated that PRY-1 acts upstream of the WNT asymmetry pathway components WRM-1, LIT-1, and POP-1, and miRNA target genes in seam cell development.

Electronic supplementary material

The online version of this article (10.1186/s12861-019-0197-5) contains supplementary material, which is available to authorized users.

Destruction complex dynamics: Wnt/β-catenin signaling alters Axin-GSK3β interactions in vivo

The central regulator of the Wnt/β-catenin pathway is the Axin/APC/GSK3β destruction complex (DC), which, under unstimulated conditions, targets cytoplasmic β-catenin for degradation. How Wnt activation inhibits the DC to permit β-catenin-dependent signaling remains controversial, in part because the DC and its regulation have never been observed in vivo Using bimolecular fluorescence complementation (BiFC) methods, we have now analyzed the activity of the DC under near-physiological conditions in Drosophila By focusing on well-established patterns of Wnt/Wg signaling in the developing Drosophila wing, we have defined the sequence of events by which activated Wnt receptors induce a conformational change within the DC, resulting in modified Axin-GSK3β interactions that prevent β-catenin degradation. Surprisingly, the nucleus is surrounded by active DCs, which principally control the degradation of β-catenin and thereby nuclear access. These DCs are inactivated and removed upon Wnt signal transduction. These results suggest a novel mechanistic model for dynamic Wnt signal transduction in vivo.

Divergent Axin and GSK-3 paralogs in the beta-catenin destruction complexes of tapeworms

The Wnt/beta-catenin pathway has many key roles in the development of animals, including a conserved and central role in the specification of the primary (antero-posterior) body axis. The posterior expression of Wnt ligands and the anterior expression of secreted Wnt inhibitors are known to be conserved during the larval metamorphosis of tapeworms. However, their downstream signaling components for Wnt/beta-catenin signaling have not been characterized. In this work, we have studied the core components of the beta-catenin destruction complex of the human pathogen Echinococcus multilocularis, the causative agent of alveolar echinococcosis. We focused on two Axin paralogs that are conserved in tapeworms and other flatworm parasites. Despite their divergent sequences, both Axins could robustly interact with one E. multilocularis beta-catenin paralog and limited its accumulation in a heterologous mammalian expression system. Similarly to what has been described in planarians (free-living flatworms), other beta-catenin paralogs showed limited or no interaction with either Axin and are unlikely to function as effectors in Wnt signaling. Additionally, both Axins interacted with three divergent GSK-3 paralogs that are conserved in free-living and parasitic flatworms. Axin paralogs have highly segregated expression patterns along the antero-posterior axis in the tapeworms E. multilocularis and Hymenolepis microstoma, indicating that different beta-catenin destruction complexes may operate in different regions during their larval metamorphosis.

Axin proteolysis by Iduna is required for the regulation of stem cell proliferation and intestinal homeostasis in Drosophila

Self-renewal of intestinal stem cells is controlled by Wingless/Wnt-β catenin signaling in both Drosophila and mammals. As Axin is a rate-limiting factor in Wingless signaling, its regulation is essential. Iduna is an evolutionarily conserved ubiquitin E3 ligase that has been identified as a crucial regulator for degradation of ADP-ribosylated Axin and, thus, of Wnt/β-catenin signaling. However, its physiological significance remains to be demonstrated. Here, we generated loss-of-function mutants of Iduna to investigate its physiological role in Drosophila Genetic depletion of Iduna causes the accumulation of both Tankyrase and Axin. Increase of Axin protein in enterocytes non-autonomously enhanced stem cell divisions in the Drosophila midgut. Enterocytes secreted Unpaired proteins and thereby stimulated the activity of the JAK-STAT pathway in intestinal stem cells. A decrease in Axin gene expression suppressed the over-proliferation of stem cells and restored their numbers to normal levels in Iduna mutants. These findings suggest that Iduna-mediated regulation of Axin proteolysis is essential for tissue homeostasis in the Drosophila midgut.

A Context-Dependent Role for the RNF146 Ubiquitin Ligase in Wingless/Wnt Signaling in Drosophila

Aberrant activation of the Wnt signal transduction pathway triggers the development of colorectal cancer. The ADP-ribose polymerase Tankyrase (TNKS) mediates proteolysis of Axin-a negative regulator of Wnt signaling-and provides a promising therapeutic target for Wnt-driven diseases. Proteolysis of TNKS substrates is mediated through their ubiquitination by the poly-ADP-ribose (pADPr)-dependent RING-domain E3 ubiquitin ligase RNF146/Iduna. Like TNKS, RNF146 promotes Axin proteolysis and Wnt pathway activation in some cultured cell lines, but in contrast with TNKS, RNF146 is dispensable for Axin degradation in colorectal carcinoma cells. Thus, the contexts in which RNF146 is essential for TNKS-mediated Axin destabilization and Wnt signaling remain uncertain. Herein, we tested the requirement for RNF146 in TNKS-mediated Axin proteolysis and Wnt pathway activation in a range of in vivo settings. Using null mutants in Drosophila, we provide genetic and biochemical evidence that Rnf146 and Tnks function in the same proteolysis pathway in vivo Furthermore, like Tnks, Drosophila Rnf146 promotes Wingless signaling in multiple developmental contexts by buffering Axin levels to ensure they remain below the threshold at which Wingless signaling is inhibited. However, in contrast with Tnks, Rnf146 is dispensable for Wingless target gene activation and the Wingless-dependent control of intestinal stem cell proliferation in the adult midgut during homeostasis. Together, these findings demonstrate that the requirement for Rnf146 in Tnks-mediated Axin proteolysis and Wingless pathway activation is dependent on physiological context, and suggest that, in some cell types, functionally redundant pADPr-dependent E3 ligases or other compensatory mechanisms promote the Tnks-dependent proteolysis of Axin in both mammalian and Drosophila cells.

Wnt/Beta-Catenin Signaling Regulation and a Role for Biomolecular Condensates

Wnt/β-Catenin signaling plays key roles in tissue homeostasis and cell fate decisions in embryonic and post-embryonic development across the animal kingdom. As a result, pathway mutations are associated with developmental disorders and many human cancers. The multiprotein destruction complex keeps signaling off in the absence of Wnt ligands and needs to be downregulated for pathway activation. We discuss new insights into destruction complex activity and regulation, highlighting parallels to the control of other cell biological processes by biomolecular condensates that form by phase separation to suggest that the destruction complex acts as a biomolecular condensate in Wnt pathway regulation.

Forskolin exerts anticancer roles in non-Hodgkin's lymphomas via regulating Axin/β-catenin signaling pathway

Background

Non-Hodgkin’s lymphomas (NHLs) account for 85% of lymphomas, which are characterized by high-degree malignancy, rapid progress, and even invasion into central nervous system in pediatric patients. Although the cure rate of pediatric NHL has improved, some patients have still underwent recurrence or death. This study focuses on the effects and mechanism of forskolin on the progression of NHL, aiming to find efficient therapy methods for pediatric NHL.

Methods

MTT, flow cytometry and mice tumor bearing experiments were used to evaluate the effects of forskolin on NHL cell proliferation, apoptosis and tumorigenesis. Western blotting and RT-PCR assays were used to detect protein and mRNA expression. Immunohistochemistry technology was recruited to analyze Ki-67 expression in tumor tissues.

Results

Forskolin significantly increased the expression of cleaved caspase-3/9 in both NHL Toledo and NK-92 cell lines, and inhibited cell growth. Besides, forskolin obviously reduced the expression of β-catenin protein, promoted its ubiquitination, enhanced its transportation from nuclear to cytoplasm, as well as decreased the expression of its downstream oncogenes c-myc and cyclin D1 through upregulating Axin expression and stability and inhibiting Axin ubiquitination. Moreover, forskolin enhanced the effects of SP600125, an inhibitor of c-Jun N-terminal kinase signaling on cell apoptosis promotion and tumorigenesis inhibition via Axin-induced β-catenin signaling repression.

Conclusion

The current study clarifies that forskolin can inhibit the progression of NHL through Axin-mediated inhibition of β-catenin signaling. Moreover, forskolin improves the effects of SP600125 on cell apoptosis enhancement and tumorigenesis inhibition of NHL cells. These findings provide theoretical foundation of serving forskolin as a new effective therapeutic drug for pediatric NHL.

Zbed3 promotes proliferation and invasion of lung cancer partly through regulating the function of Axin-Gsk3β complex

Our previous work showed that Zbed3 is overexpressed in nonsmall cell lung cancer and that down‐regulation of Zbed3 inhibited β‐catenin expression and cancer cell proliferation and invasiveness. Here, we investigated Zbed3's ability to promote lung cancer cell proliferation and invasion and the involvement of the Axin/TPC/glycogen synthase kinase 3β (Gsk‐3β) complex to the response. Coimmunoprecipitation assays showed that wild‐type Zbed3 bound to Axin but a Zbed3 mutant lacking the Axin binding site did not. In A549 and H1299 lung cancer cells, Zbed3 overexpression promoted cancer cell proliferation and invasiveness, as well as Wnt signalling and expression of downstream mediators, including β‐catenin, cyclin D1 and MMP7 (P < 0.05). In contrast, the Zbed3 mutant failed to enhance β‐catenin expression (P > 0.05), and its ability to promote cancer cell proliferation and invasiveness was much less than wild‐type Zbed3 (P < 0.05). The ability of Zbed3 to increase β‐catenin levels was abolished by Axin knockdown in A549 cells (P > 0.05). Similarly, treating the cells with a GSK‐3β inhibitor abolished Zbed3's ability to increase β‐catenin levels and Wnt signalling. These results indicate that Zbed3 enhances lung cancer cell proliferation and invasiveness at least in part by inhibiting Axin/adenomatous polyposis coli/GSK‐3β‐mediated negative regulation of β‐catenin levels.

Heterogeneous nuclear ribonucleoprotein M promotes the progression of breast cancer by regulating the axin/β-catenin signaling pathway

Despite significant progress in the treatment of breast cancer due to advances in surgery, cytotoxic agents, and endocrine therapy, the prognosis for patients has not improved much. Accumulated evidence indicates that heterogeneous nuclear ribonucleoprotein M (hnRNPM) and Wnt/β-catenin function as tumor oncogenes in the progression of many cancers. The present study aimed to explore whether HnRNPM/β-catenin signaling molecules might serve as a genetic target for breast cancer treatment. To shed light on this issue, quantitative real-time polymerase chain reaction (qRT-PCR) detection, Western blotting, and immunohistochemical staining were performed. The hnRNPM is expressed at a much higher level in breast cancer tissues and cell lines than in noncancerous tissues and cell lines. In vitro studies revealed that overexpressed hnRNPM promoted cell proliferation and colony formation but inhibited cell apoptosis. In vivo results demonstrated that upregulation of hnRNPM dramatically increased breast cancer xenograft tumor growth. Western blotting and immunofluorescence studies revealed that hnRNPM markedly activated the Wnt/β-catenin pathway and catalyzed its translocation from the cytoplasm to the nucleus by targeting axin, a negative regulator of Wnt/β-catenin signaling in MCF-7 and KPL-4 cells. Elevated levels of c-Myc and cyclin D1 were observed when MCF-7 and KPL-4 cells were transfected with a hnRNPM vector. These findings indicate that the hnRNPM/axin/β-catenin signaling pathway acts as an oncogenic promoter in the progression of breast cancer, suggesting that hnRNPM may be a potential target for the treatment of this disease.

The GSK3 kinase inhibitor lithium produces unexpected hyperphosphorylation of β-catenin, a GSK3 substrate, in human glioblastoma cells

Lithium salt is a classic glycogen synthase kinase 3 (GSK3) inhibitor. Beryllium is a structurally related inhibitor that is more potent but relatively uncharacterized. This study examined the effects of these inhibitors on the phosphorylation of endogenous GSK3 substrates. In NIH-3T3 cells, both salts caused a decrease in phosphorylated glycogen synthase, as expected. GSK3 inhibitors produce enhanced phosphorylation of Ser9 of GSK3β via a positive feedback mechanism, and both salts elicited this enhancement. Another GSK3 substrate is β-catenin, which has a central role in Wnt signaling. In A172 human glioblastoma cells, lithium treatment caused a surprising increase in phospho-Ser33/Ser37-β-catenin, which was quantified using an antibody-coupled capillary electrophoresis method. The β-catenin hyperphosphorylation was unaffected by p53 RNAi knockdown, indicating that p53 is not involved in the mechanism of this response. Lithium caused a decrease in the abundance of axin, a component of the β-catenin destruction complex that has a role in coordinating β-catenin ubiquitination and protein turnover. The axin and phospho-β-catenin results were reproduced in U251 and U87MG glioblastoma cell lines. These observations run contrary to the conventional view of the canonical Wnt signaling pathway, in which a GSK3 inhibitor would be expected to decrease, not increase, phospho-β-catenin levels.

This article has an associated First Person interview with the first author of the paper.

Summary: GSK3 inhibitors have potential use against Alzheimer's disease and other conditions. In this study, a classic inhibitor produced unexpected molecular effects on key components of the Wnt signaling pathway.

Reconstitution of the Cytoplasmic Regulation of the Wnt Signaling Pathway Using Xenopus Egg Extracts

The regulation of β-catenin turnover is the central mechanism governing activation of the Wnt signaling pathway. All components of the pathway are present in the early embryo of Xenopus laevis, and Xenopus egg extracts have been used to recapitulate complex biological reactions such as microtubule dynamics, DNA replication, chromatin assembly, and phases of the cell cycle. Herein, we describe a biochemical method for analyzing β-catenin degradation using radiolabeled and luciferase-fusion proteins in Xenopus egg extracts. We show that in such a biochemical system, cytoplasmic β-catenin degradation is regulated by soluble components of the Wnt pathway as well as small molecules.

The ADP-ribose polymerase Tankyrase regulates adult intestinal stem cell proliferation during homeostasis in Drosophila

Wnt/β-catenin signaling controls intestinal stem cell (ISC) proliferation, and is aberrantly activated in colorectal cancer. Inhibitors of the ADP-ribose polymerase Tankyrase (Tnks) have become lead therapeutic candidates for Wnt-driven cancers, following the recent discovery that Tnks targets Axin, a negative regulator of Wnt signaling, for proteolysis. Initial reports indicated that Tnks is important for Wnt pathway activation in cultured human cell lines. However, the requirement for Tnks in physiological settings has been less clear, as subsequent studies in mice, fish and flies suggested that Tnks was either entirely dispensable for Wnt-dependent processes in vivo, or alternatively, had tissue-specific roles. Here, using null alleles, we demonstrate that the regulation of Axin by the highly conserved Drosophila Tnks homolog is essential for the control of ISC proliferation. Furthermore, in the adult intestine, where activity of the Wingless pathway is graded and peaks at each compartmental boundary, Tnks is dispensable for signaling in regions where pathway activity is high, but essential where pathway activity is relatively low. Finally, as observed previously for Wingless pathway components, Tnks activity in absorptive enterocytes controls the proliferation of neighboring ISCs non-autonomously by regulating JAK/STAT signaling. These findings reveal the requirement for Tnks in the control of ISC proliferation and suggest an essential role in the amplification of Wnt signaling, with relevance for development, homeostasis and cancer.

Complexity of the Wnt/β‑catenin pathway: Searching for an activation model

Wnt signaling refers to a conserved signaling pathway, widely studied due to its roles in cellular communication, cell fate decisions, development and cancer. However, the exact mechanism underlying inhibition of the GSK phosphorylation towards β-catenin and activation of the pathway after biding of Wnt ligand to its cognate receptors at the plasma membrane remains unclear. Wnt target genes are widely spread over several animal phyla. They participate in a plethora of functions during the development of an organism, from axial specification, gastrulation and organogenesis all the way to regeneration and repair in adults. Temporal and spatial oncogenetic re-activation of Wnt signaling almost certainly leads to cancer. Wnt signaling components have been extensively studied as possible targets in anti-cancer therapies. In this review we will discuss one of the most intriguing questions in this field, that is how β-catenin, a major component in this pathway, escapes the destruction complex, gets stabilized in the cytosol and it is translocated to the nucleus where it acts as a co-transcription factor. Four major models have evolved during the past 20years. We dissected each of them along with current views and future perspectives on this pathway. This review will focus on the molecular mechanisms by which Wnt proteins modulate β-catenin cytoplasmic levels and the relevance of this pathway for the development and cancer.

E-cadherin expression increases cell proliferation by regulating energy metabolism through nuclear factor-κB in AGS cells

β-Catenin is a central player in Wnt signaling, and activation of Wnt signaling is associated with cancer development. E-cadherin in complex with β-catenin mediates cell-cell adhesion, which suppresses β-catenin-dependent Wnt signaling. Recently, a tumor-suppressive role for E-cadherin has been reconsidered, as re-expression of E-cadherin was reported to enhance the metastatic potential of malignant tumors. To explore the role of E-cadherin, we established an E-cadherin-expressing cell line, EC96, from AGS cells that featured undetectable E-cadherin expression and a high level of Wnt signaling. In EC96 cells, E-cadherin re-expression enhanced cell proliferation, although Wnt signaling activity was reduced. Subsequent analysis revealed that nuclear factor-κB (NF-κB) activation and consequent c-myc expression might be involved in E-cadherin expression-mediated cell proliferation. To facilitate rapid proliferation, EC96 cells enhance glucose uptake and produce ATP using both mitochondria oxidative phosphorylation and glycolysis, whereas AGS cells use these mechanisms less efficiently. These events appeared to be mediated by NF-κB activation. Therefore, E-cadherin re-expression and subsequent induction of NF-κB signaling likely enhance energy production and cell proliferation.

The SIAH E3 ubiquitin ligases promote Wnt/β-catenin signaling through mediating Wnt-induced Axin degradation

In this study, Ji et al. identify SIAH1/2 (SIAH) as the E3 ligase mediating Wnt-induced Axin degradation. Their results suggest that Wnt-induced dissociation of the Axin/GSK3 complex allows SIAH to interact with Axin and promote its degradation, which represents an important feed-forward mechanism to achieve sustained Wnt/β-catenin signaling.

The Wnt/β-catenin signaling pathway plays essential roles in embryonic development and adult tissue homeostasis. Axin is a concentration-limiting factor responsible for the formation of the β-catenin destruction complex. Wnt signaling itself promotes the degradation of Axin. However, the underlying molecular mechanism and biological relevance of this targeting of Axin have not been elucidated. Here, we identify SIAH1/2 (SIAH) as the E3 ligase mediating Wnt-induced Axin degradation. SIAH proteins promote the ubiquitination and proteasomal degradation of Axin through interacting with a VxP motif in the GSK3-binding domain of Axin, and this function of SIAH is counteracted by GSK3 binding to Axin. Structural analysis reveals that the Axin segment responsible for SIAH binding is also involved in GSK3 binding but adopts distinct conformations in Axin/SIAH and Axin/GSK3 complexes. Knockout of SIAH1 blocks Wnt-induced Axin ubiquitination and attenuates Wnt-induced β-catenin stabilization. Our data suggest that Wnt-induced dissociation of the Axin/GSK3 complex allows SIAH to interact with Axin not associated with GSK3 and promote its degradation and that SIAH-mediated Axin degradation represents an important feed-forward mechanism to achieve sustained Wnt/β-catenin signaling.

Modulating the wnt signaling pathway with small molecules

Wnt signaling is a critical component during embryonic development and also plays an important role in regulating adult tissue homeostasis. Abnormal activation of Wnt signaling has been implicated in many cancers, while reduced activity of Wnt signaling leads to poor wound healing and structural formations. Thus, extensive efforts have been focused on developing small molecules that have potential to either inhibit or activate the pathway, hoping these molecules can offer leads for novel approaches in treating different human diseases. Many small-molecule inhibitors specifically target various elements, such as Frizzled, Disheveled, Porcupine, or Tankyrase, within the Wnt signaling pathways. These small molecules not only have the potential to be further developed as therapeutic reagents, but they may also be used as chemical probes to dissect the underlying mechanism of the Wnt signaling pathways. Therefore, their respective mechanisms and effective dosages are highly pertinent. Aiming to provide an overview of those molecules in a concise, easy-to-use manner, we summarize and organize the current research on them so that it may be helpful for utilization in different studies.

Regulation of Wnt/β-catenin signaling by herpesviruses

The Wnt/β-catenin signaling pathway is instrumental in successful differentiation and proliferation of mammalian cells. It is therefore not surprising that the herpesvirus family has developed mechanisms to interact with and manipulate this pathway. Successful coexistence with the host requires that herpesviruses establish a lifelong infection that includes periods of latency and reactivation or persistence. Many herpesviruses establish latency in progenitor cells and viral reactivation is linked to host-cell proliferation and differentiation status. Importantly, Wnt/β-catenin is tightly connected to stem/progenitor cell maintenance and differentiation. Numerous studies have linked Wnt/β-catenin signaling to a variety of cancers, emphasizing the importance of Wnt/β-catenin pathways in development, tissue homeostasis and disease. This review details how the alpha-, beta-, and gammaherpesviruses interact and manipulate the Wnt/β-catenin pathway to promote a virus-centric agenda.

Gtpbp2 is a positive regulator of Wnt signaling and maintains low levels of the Wnt negative regulator Axin

Background

Canonical Wnt signals, transduced by stabilized β-catenin, play similar roles across animals in maintaining stem cell pluripotency, regulating cell differentiation, and instructing normal embryonic development. Dysregulated Wnt/β-catenin signaling causes diseases and birth defects, and a variety of regulatory processes control this pathway to ensure its proper function and integration with other signaling systems. We previously identified GTP-binding protein 2 (Gtpbp2) as a novel regulator of BMP signaling, however further exploration revealed that Gtpbp2 can also affect Wnt signaling, which is a novel finding reported here.

Results

Knockdown of Gtpbp2 in Xenopus embryos causes severe axial defects and reduces expression of Spemann-Mangold organizer genes. Gtpbp2 knockdown blocks responses to ectopic Wnt8 ligand, such as organizer gene induction in ectodermal tissue explants and induction of secondary axes in whole embryos. However, organizer gene induction by ectopic Nodal2 is unaffected by Gtpbp2 knockdown. Epistasis tests, conducted by activating Wnt signal transduction at sequential points in the canonical pathway, demonstrate that Gtpbp2 is required downstream of Dishevelled and Gsk3β but upstream of β-catenin, which is similar to the previously reported effects of Axin1 overexpression in Xenopus embryos. Focusing on Axin in Xenopus embryos, we find that knockdown of Gtpbp2 elevates endogenous or exogenous Axin protein levels. Furthermore, Gtpbp2 fusion proteins co-localize with Dishevelled and co-immunoprecipitate with Axin and Gsk3b.

Conclusions

We conclude that Gtpbp2 is required for canonical Wnt/β-catenin signaling in Xenopus embryos. Our data suggest a model in which Gtpbp2 suppresses the accumulation of Axin protein, a rate-limiting component of the β-catenin destruction complex, such that Axin protein levels negatively correlate with Gtpbp2 levels. This model is supported by the similarity of our Gtpbp2-Wnt epistasis results and previously reported effects of Axin overexpression, the physical interactions of Gtpbp2 with Axin, and the correlation between elevated Axin protein levels and lost Wnt responsiveness upon Gtpbp2 knockdown. A wide variety of cancer-causing Wnt pathway mutations require low Axin levels, so development of Gtpbp2 inhibitors may provide a new therapeutic strategy to elevate Axin and suppress aberrant β-catenin signaling in cancer and other Wnt-related diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-016-0138-x) contains supplementary material, which is available to authorized users.

Wnt/Wingless Pathway Activation Is Promoted by a Critical Threshold of Axin Maintained by the Tumor Suppressor APC and the ADP-Ribose Polymerase Tankyrase

Wnt/β-catenin signal transduction directs metazoan development and is deregulated in numerous human congenital disorders and cancers. In the absence of Wnt stimulation, a multiprotein "destruction complex," assembled by the scaffold protein Axin, targets the key transcriptional activator β-catenin for proteolysis. Axin is maintained at very low levels that limit destruction complex activity, a property that is currently being exploited in the development of novel therapeutics for Wnt-driven cancers. Here, we use an in vivo approach in Drosophila to determine how tightly basal Axin levels must be controlled for Wnt/Wingless pathway activation, and how Axin stability is regulated. We find that for nearly all Wingless-driven developmental processes, a three- to fourfold increase in Axin is insufficient to inhibit signaling, setting a lower-limit for the threshold level of Axin in the majority of in vivo contexts. Further, we find that both the tumor suppressor adenomatous polyposis coli (APC) and the ADP-ribose polymerase Tankyrase (Tnks) have evolutionarily conserved roles in maintaining basal Axin levels below this in vivo threshold, and we define separable domains in Axin that are important for APC- or Tnks-dependent destabilization. Together, these findings reveal that both APC and Tnks maintain basal Axin levels below a critical in vivo threshold to promote robust pathway activation following Wnt stimulation.

The DNA damage-induced cell death response: a roadmap to kill cancer cells

Upon massive DNA damage cells fail to undergo productive DNA repair and trigger the cell death response. Resistance to cell death is linked to cellular transformation and carcinogenesis as well as radio- and chemoresistance, making the underlying signaling pathways a promising target for therapeutic intervention. Diverse DNA damage-induced cell death pathways are operative in mammalian cells and finally culminate in the induction of programmed cell death via activation of apoptosis or necroptosis. These signaling routes affect nuclear, mitochondria- and plasma membrane-associated key molecules to activate the apoptotic or necroptotic response. In this review, we highlight the main signaling pathways, molecular players and mechanisms guiding the DNA damage-induced cell death response.

Axin is expressed in mitochondria and suppresses mitochondrial ATP synthesis in HeLa cells

Many recent studies have revealed that axin is involved in numerous cellular functions beyond the negative regulation of β-catenin-dependent Wnt signaling. Previously, an association of ectopic axin with mitochondria was observed. In an effort to investigate the relationship between axin and mitochondria, we found that axin expression suppressed cellular ATP production, which was more apparent as axin expression levels increased. Also, mitochondrial expression of axin was observed using two axin-expressing HeLa cell models: doxycycline-inducible ectopic axin expression (HeLa-axin) and axin expression enhanced by long-term treatment with XAV939 (HeLa-XAV). In biochemical analysis, axin is associated with oxidative phosphorylation (OXPHOS) complex IV and is involved in defects in the assembly of complex IV-containing supercomplexes. Functionally, axin expression reduced the activity of OXPHOS complex IV and the oxygen consumption rate (OCR), suggesting axin-mediated mitochondrial dysfunction. Subsequent studies using various inhibitors of Wnt signaling showed that the reduction in cellular ATP levels was weaker in cases of ICAT protein expression and treatment with iCRT3 or NSC668036 compared with XAV939 treatment, suggesting that XAV939 treatment affects ATP synthesis in addition to suppressing Wnt signaling activity. Axin-mediated regulation of mitochondrial function may be an additional mechanism to Wnt signaling for regulation of cell growth.

BMP10 inhibited the growth and migration of gastric cancer cells

Bone morphogenetic protein 10 (BMP10), a novel member of BMP family, has been identified as an important regulator for angiogenesis. Dysregulation of BMP has been observed in several cancer types. However, its roles in gastric cancer (GC) remain unknown. In this study, the expression of BMP10 was found to be down-regulated in GC samples. Forced expression of BMP10 in GC cells inhibited its growth and migration, while knocking down the expression of BMP10 in GC cells promoted cell growth, migration, and metastasis. BMP10 was shown to negatively regulated beta-catenin/TCF signaling by up-regulating Axin protein level. Taken together, the present study revealed the suppressive function of BMP10 in gastric cancer.

A novel GSK3-regulated APC:Axin interaction regulates Wnt signaling by driving a catalytic cycle of efficient βcatenin destruction

APC, a key negative regulator of Wnt signaling in development and oncogenesis, acts in the destruction complex with the scaffold Axin and the kinases GSK3 and CK1 to target βcatenin for destruction. Despite 20 years of research, APC's mechanistic function remains mysterious. We used FRAP, super-resolution microscopy, functional tests in mammalian cells and flies, and other approaches to define APC's mechanistic role in the active destruction complex when Wnt signaling is off. Our data suggest APC plays two roles: (1) APC promotes efficient Axin multimerization through one known and one novel APC:Axin interaction site, and (2) GSK3 acts through APC motifs R2 and B to regulate APC:Axin interactions, promoting high-throughput of βcatenin to destruction. We propose a new dynamic model of how the destruction complex regulates Wnt signaling and how this goes wrong in cancer, providing insights into how this multiprotein signaling complex is assembled and functions via multivalent interactions.

DOI:http://dx.doi.org/10.7554/eLife.08022.001

An embryo starts off as a small ball of stem cells, each of which has the potential to become any type of cell in the body. Adult organs and tissues also contain small numbers of stem cells that can replace old or damaged cells. In both of these processes, stem cells need to ‘decide’ when they should start to change into a more specialized cell type, and which cell fate to choose (e.g., liver cell vs kidney cell). A signaling pathway involving Wnt proteins helps to direct many of these decisions. But if the ‘Wnt signaling pathway’ becomes activated at the wrong time, it can lead to cancer. For example, the first step in development of colon cancer is the inappropriate activation of Wnt signaling, and is most often caused by mutations in the gene that encodes a protein called APC. The APC protein is a tumor suppressor and normally inhibits Wnt signaling. However, even after over 20 years of effort, it remains largely mysterious how APC does this.

APC is known to work with another protein called Axin as part of a large protein machine. This protein complex performs one of the first steps in a process that ultimately marks a key component of the Wnt signaling pathway for destruction. Pronobis et al. have now used a range of techniques to define APC's role in this so-called ‘destruction complex’. This analysis revealed the internal structure of a complex made from APC and Axin, and showed that cable- and sheet-like assemblies of Axin were intertwined with APC cables. Further experiments then revealed how APC and Axin proteins are added into or leave these complexes, and showed that this is critical for this protein machine to work.

Pronobis et al.'s data also suggest that APC plays two roles, which make the destruction complex more efficient. Firstly, it can interact with Axin via two separate interaction sites that help to assemble the destruction complex. Secondly, specific features in APC allow it to interact with a third protein (called GSK3), which can then regulate how APC interacts with Axin. One of the next challenges will be to uncover how APC helps to transfer the components of Wnt signaling to the next step of their destruction, and to clear up the role played by GSK3.

DOI:http://dx.doi.org/10.7554/eLife.08022.002

Emerging roles of Axin in cerebral cortical development

Proper functioning of the cerebral cortex depends on the appropriate production and positioning of neurons, establishment of axon–dendrite polarity, and formation of proper neuronal connectivity. Deficits in any of these processes greatly impair neural functions and are associated with various human neurodevelopmental disorders including microcephaly, cortical heterotopias, and autism. The application of in vivo manipulation techniques such as in utero electroporation has resulted in significant advances in our understanding of the cellular and molecular mechanisms that underlie neural development in vivo. Axin is a scaffold protein that regulates neuronal differentiation and morphogenesis in vitro. Recent studies provide novel insights into the emerging roles of Axin in gene expression and cytoskeletal regulation during neurogenesis, neuronal polarization, and axon formation. This review summarizes current knowledge on Axin as a key molecular controller of cerebral cortical development.

Negative-feedback regulation of the Wnt pathway by conductin/axin2 involves insensitivity to upstream signalling

Axin and conductin (also known as axin2) are structurally related inhibitors of Wnt/β-catenin signalling that promote degradation of β-catenin. Whereas axin is constitutively expressed, conductin is a Wnt target gene implicated in Wnt negative-feedback regulation. Here, we show that axin and conductin differ in their functional interaction with the upstream Wnt pathway component Dvl. Conductin shows reduced binding to Dvl2 compared to axin, and degradation of β-catenin by conductin is only poorly blocked by Dvl2. We propose that insensitivity to Dvl is an important feature of the role of conductin as a negative-feedback regulator of Wnt signalling.

IGF antagonizes the Wnt/β-Catenin pathway and promotes differentiation of extra-embryonic endoderm

Mouse F9 teratocarcinoma cells are an established model for the differentiation of extra-embryonic endoderm (ExEn). Primitive endoderm, parietal and visceral endoderm can be generated by stimulation of F9 cells with retinoic acid and dibutyryl cyclic adenosine monophosphate. Here we show that Wnt/β-Catenin signaling is down-regulated during ExEn differentiation in F9 cells and that the inhibition of the Wnt pathway promotes differentiation of the three extra-embryonic endoderm lineages. Wnt inhibition is achieved through the IGF pathway, which is up-regulated during differentiation. IGF signaling antagonizes the Wnt pathway by stimulating transcription of axin2 and by stabilizing Axin1 protein. Both Axin1 and Axin2 are components of the β-Catenin destruction complex and act as intra-cellular inhibitors of the Wnt/β-Catenin pathway. The data presented reveal a mechanism which restricts pluripotency of undifferentiated cells and directs them toward extra-embryonic lineages.

Wnt signaling in adult intestinal stem cells and cancer. Cell Signal. 2014;26:570-579. [PMID: 24308963 DOI: 10.1016/j.cellsig.2013.11.032]

Signaling initiated by secreted glycoproteins of the Wnt family regulates many aspects of embryonic development and it is involved in homeostasis of adult tissues. In the gastrointestinal (GI) tract the Wnt pathway maintains the self-renewal capacity of epithelial stem cells. The stem cell attributes are conferred by mutual interactions of the stem cell with its local microenvironment, the stem cell niche. The niche ensures that the threshold of Wnt signaling in the stem cell is kept in physiological range. In addition, the Wnt pathway involves various feedback loops that balance the opposing processes of cell proliferation and differentiation. Today, we have compelling evidence that mutations causing aberrant activation of the Wnt pathway promote expansion of undifferentiated progenitors and lead to cancer. The review summarizes recent advances in characterization of adult epithelial stem cells in the gut. We mainly focus on discoveries related to molecular mechanisms regulating the output of the Wnt pathway. Moreover, we present novel experimental approaches utilized to investigate the epithelial cell signaling circuitry in vivo and in vitro. Pivotal aspects of tissue homeostasis are often deduced from studies of tumor cells; therefore, we also discuss some latest results gleaned from the deep genome sequencing studies of human carcinomas of the colon and rectum.

The phosphorylation state of Axin controls its scaffold function in two Wnt pathway protein complexes

The precise orchestration of two opposing protein complexes - one in the cytoplasm (β-catenin destruction complex) and the other at the plasma membrane (LRP6 signaling complex) - is critical for controlling levels of the transcriptional co-factor β-catenin, and subsequent activation of the Wnt/β-catenin signal transduction pathway. The Wnt pathway component Axin acts as an essential scaffold for the assembly of both complexes. How the β-catenin destruction and LRP6 signaling complexes are modulated following Wnt stimulation remains controversial. A recent study in Science by He and coworkers reveals an underlying logic for Wnt pathway control in which Axin phosphorylation toggles a switch between the active and inactive states. This mini-review focuses on this and two other recent studies that provide insight into the initial signaling events triggered by Wnt exposure. We emphasize regulation of the β-catenin destruction and LRP6 signaling complexes and propose a framework for future work in this area.

APC2 and Axin promote mitotic fidelity by facilitating centrosome separation and cytoskeletal regulation

To ensure the accurate transmission of genetic material, chromosome segregation must occur with extremely high fidelity. Segregation errors lead to chromosomal instability (CIN), with deleterious consequences. Mutations in the tumor suppressor adenomatous polyposis coli (APC) initiate most colon cancers and have also been suggested to promote disease progression through increased CIN, but the mechanistic role of APC in preventing CIN remains controversial. Using fly embryos as a model, we investigated the role of APC proteins in CIN. Our findings suggest that APC2 loss leads to increased rates of chromosome segregation error. This occurs through a cascade of events beginning with incomplete centrosome separation leading to failure to inhibit formation of ectopic cleavage furrows, which result in mitotic defects and DNA damage. We test several hypotheses related to the mechanism of action of APC2, revealing that APC2 functions at the embryonic cortex with several protein partners, including Axin, to promote mitotic fidelity. Our in vivo data demonstrate that APC2 protects genome stability by modulating mitotic fidelity through regulation of the cytoskeleton.