Gαi2-induced conductin/axin2 condensates inhibit Wnt/ß-catenin signaling and suppress cancer growth.

Conductin/axin2 is a scaffold protein negatively regulating the pro-proliferative Wnt/ß-catenin signaling pathway. Accumulation of scaffold proteins in condensates frequently increases their activity, but whether condensation contributes to Wnt pathway inhibition by conductin remains unclear. Here, we show that the Gαi2 subunit of trimeric G-proteins induces conductin condensation by targeting a polymerization-inhibiting aggregon in its RGS domain, thereby promoting conductin-mediated ß-catenin degradation. Consistently, transient Gαi2 expression inhibited, whereas knockdown activated Wnt signaling via conductin. Colorectal cancers appear to evade Gαi2-induced Wnt pathway suppression by decreased Gαi2 expression and inactivating mutations, associated with shorter patient survival. Notably, the Gαi2-activating drug guanabenz inhibited Wnt signaling via conductin, consequently reducing colorectal cancer growth in vitro and in mouse models. In summary, we demonstrate Wnt pathway inhibition via Gαi2-triggered conductin condensation, suggesting a tumor suppressor function for Gαi2 in colorectal cancer, and pointing to the FDA-approved drug guanabenz for targeted cancer therapy.

An aggregon in conductin/axin2 regulates Wnt/ß-catenin signaling and holds potential for cancer therapy.

The paralogous scaffold proteins axin and conductin/axin2 are key factors in the negative regulation of the Wnt pathway transcription factor ß-catenin, thereby representing interesting targets for signaling regulation. Polymerization of axin proteins is essential for their activity in suppressing Wnt/ß-catenin signaling. Notably, conductin shows less polymerization and lower activity than axin. By domain swapping between axin and conductin we here identify an aggregation site in the conductin RGS domain which prevents conductin polymerization. Induction of conductin polymerization by point mutations of this aggregon results in enhanced inhibition of Wnt/ß-catenin signaling. Importantly, we identify a short peptide which induces conductin polymerization via masking the aggregon, thereby enhancing ß-catenin degradation, inhibiting ß-catenin-dependent transcription and repressing growth of colorectal cancer cells. Our study reveals a mechanism for regulating signaling pathways via the polymerization status of scaffold proteins and suggests a strategy for targeted colorectal cancer therapy.

Sulforaphane inhibits growth and blocks Wnt/ß-catenin signaling of colorectal cancer cells.

The naturally occurring isothiocyanate sulforaphane (SFN) from cruciferous vegetables is associated with growth inhibition of various cancer types, including colorectal cancer. Colorectal cancer is most frequently driven by hyperactive Wnt/ß-catenin signaling. Here, we show that SFN treatment reduced growth of three unrelated colorectal cancer cell lines (SW480, DLD1 and HCT116) via induction of cell death and inhibition of proliferation. Importantly, SFN inhibits Wnt/ß-catenin signaling in colorectal cancer cells as shown by inhibition of ß-catenin-dependent luciferase reporters and repression of ß-catenin target genes (AXIN2, LGR5). SFN inhibits Wnt signaling downstream of ß-catenin degradation and induces the formation of nuclear ß-catenin structures associated with closed chromatin. Co-expression of the transcription factors LEF1 or TCF4 prevented formation of these structures and rescued inhibition of Wnt/ß-catenin signaling by SFN. Our findings provide a molecular basis explaining SFN effects in colorectal cancer cells and underline its potential for prevention and therapy of colorectal cancer.

Feedback regulation of mitochondrial homeostasis via Wnt/ß-catenin signaling.

Cellular abundance of mitochondria is dynamically regulated. We could recently show that dysfunctional mitochondria release the phosphatase PGAM family member 5 (PGAM5) into the cytosol, where it interacts with the Wnt signaling-component AXIN1 and dephosphorylates AXIN1-bound ß-catenin (CTNNB1) thereby activating Wnt/ß-catenin signaling. Because Wnt/ß-catenin signaling induces mitochondrial biogenesis dysfunctional mitochondria trigger their own replacement by releasing PGAM5.

IFN-γ-response mediator GBP-1 represses human cell proliferation by inhibiting the Hippo signaling transcription factor TEAD.

Interferon-gamma (IFN-γ) is a pleiotropic cytokine that exerts important functions in inflammation, infectious diseases, and cancer. The large GTPase human guanylate-binding protein 1 (GBP-1) is among the most strongly IFN-γ-induced cellular proteins. Previously, it has been shown that GBP-1 mediates manifold cellular responses to IFN-γ including the inhibition of proliferation, spreading, migration, and invasion and through this exerts anti-tumorigenic activity. However, the mechanisms of GBP-1 anti-tumorigenic activities remain poorly understood. Here, we elucidated the molecular mechanism of the human GBP-1-mediated suppression of proliferation by demonstrating for the first time a cross-talk between the anti-tumorigenic IFN-γ and Hippo pathways. The α9-helix of GBP-1 was found to be sufficient to inhibit proliferation. Protein-binding and molecular modeling studies revealed that the α9-helix binds to the DNA-binding domain of the Hippo signaling transcription factor TEA domain protein (TEAD) mediated by the 376VDHLFQK382 sequence at the N-terminus of the GBP-1-α9-helix. Mutation of this sequence resulted in abrogation of both TEAD interaction and suppression of proliferation. Further on, the interaction caused inhibition of TEAD transcriptional activity associated with the down-regulation of TEAD-target genes. In agreement with these results, IFN-γ treatment of the cells also impaired TEAD activity, and this effect was abrogated by siRNA-mediated inhibition of GBP-1 expression. Altogether, this demonstrated that the α9-helix is the proliferation inhibitory domain of GBP-1, which acts independent of the GTPase activity through the inhibition of the Hippo transcription factor TEAD in mediating the anti-proliferative cell response to IFN-γ.

Pgam5 released from damaged mitochondria induces mitochondrial biogenesis via Wnt signaling.

Mitochondrial abundance is dynamically regulated and was previously shown to be increased by Wnt/ß-catenin signaling. Pgam5 is a mitochondrial phosphatase which is cleaved by the rhomboid protease presenilin-associated rhomboid-like protein (PARL) and released from membranes after mitochondrial stress. In this study, we show that Pgam5 interacts with the Wnt pathway component axin in the cytosol, blocks axin-mediated ß-catenin degradation, and increases ß-catenin levels and ß-catenin-dependent transcription. Pgam5 stabilized ß-catenin by inducing its dephosphorylation in an axin-dependent manner. Mitochondrial stress triggered by carbonyl cyanide m-chlorophenyl hydrazone (CCCP) treatment led to cytosolic release of endogenous Pgam5 and subsequent dephosphorylation of ß-catenin, which was strongly diminished in Pgam5 and PARL knockout cells. Similarly, hypoxic stress generated cytosolic Pgam5 and led to stabilization of ß-catenin, which was abolished by Pgam5 knockout. Cells stably expressing cytosolic Pgam5 exhibit elevated ß-catenin levels and increased mitochondrial numbers. Our study reveals a novel mechanism by which damaged mitochondria might induce replenishment of the mitochondrial pool by cell-intrinsic activation of Wnt signaling via the Pgam5-ß-catenin axis.

Genetic and Molecular Insights Into Genotype-Phenotype Relationships in Osteopathia Striata With Cranial Sclerosis (OSCS) Through the Analysis of Novel Mouse Wtx Mutant Alleles.

The X-linked WTX/AMER1 protein constitutes an important component of the ß-catenin destruction complex that can both enhance and suppress canonical ß-catenin signaling. Somatic mutations in WTX/AMER1 have been found in a proportion of the pediatric kidney cancer Wilms' tumor. By contrast, germline mutations cause the severe sclerosing bone dysplasia osteopathia striata congenita with cranial sclerosis (OSCS), a condition usually associated with fetal or perinatal lethality in male patients. Here we address the developmental and molecular function of WTX by generating two novel mouse alleles. We show that in addition to the previously reported skeletal abnormalities, loss of Wtx causes severe midline fusion defects including cleft palate and ectopic synostosis at the base of the skull. By contrast, deletion of the C-terminal part of the protein results in only mild developmental abnormalities permitting survival beyond birth. Adult analysis, however, revealed skeletal defects including changed skull morphology and an increased whole-body bone density, resembling a subgroup of male patients carrying a milder, survivable phenotype. Molecular analysis in vitro showed that while ß-catenin fails to co-immunoprecipitate with the truncated protein, partial recruitment appears to be achieved in an indirect manner using AXIN/AXIN2 as a molecular bridge. Taken together our analysis provides a novel model for WTX-caused bone diseases and explains on the molecular level how truncation mutations in this gene may retain some of WTX-protein functions. © 2018 American Society for Bone and Mineral Research.

The phosphatase Pgam5 antagonizes Wnt/ß-Catenin signaling in embryonic anterior-posterior axis patterning.

The scaffold protein Dishevelled is a central intracellular component of Wnt signaling pathways. Various kinases have been described that regulate and modulate Wnt signaling through phosphorylation of Dishevelled. However, besides general protein phosphatases 1 and 2 (PP1 and PP2), no specific protein phosphatases have been identified. Here, we report on the identification and functional characterization of the protein phosphatase Pgam5 in vitro and in vivo in Xenopus Pgam5 is a novel antagonist of Wnt/ß-Catenin signaling in human cells and Xenopus embryogenesis. In early development, Pgam5 is essential for head formation, and for establishing and maintaining the Wnt/ß-Catenin signaling gradient that patterns the anterior-posterior body axis. Inhibition of Wnt/ß-Catenin signaling and developmental function depend on Pgam5 phosphatase activity. We show that Pgam5 interacts with Dishevelled2 and that Dishevelled2 is a substrate of Pgam5. Pgam5 mediates a marked decrease in Dishevelled2 phosphorylation in the cytoplasm and in the nucleus, as well as decreased interaction between Dishevelled2, Tcf1 and ß-Catenin, indicating that Pgam5 regulates Dishevelled function upstream and downstream of ß-Catenin stabilization.

Wnt/ß-catenin signaling via Axin2 is required for myogenesis and, together with YAP/Taz and Tead1, active in IIa/IIx muscle fibers.

Canonical Wnt/ß-catenin signaling plays an important role in myogenic differentiation, but its physiological role in muscle fibers remains elusive. Here, we studied activation of Wnt/ß-catenin signaling in adult muscle fibers and muscle stem cells in an Axin2 reporter mouse. Axin2 is a negative regulator and a target of Wnt/ß-catenin signaling. In adult muscle fibers, Wnt/ß-catenin signaling is only detectable in a subset of fast fibers that have a significantly smaller diameter than other fast fibers. In the same fibers, immunofluorescence staining for YAP/Taz and Tead1 was detected. Wnt/ß-catenin signaling was absent in quiescent and activated satellite cells. Upon injury, Wnt/ß-catenin signaling was detected in muscle fibers with centrally located nuclei. During differentiation of myoblasts expression of Axin2, but not of Axin1, increased together with Tead1 target gene expression. Furthermore, absence of Axin1 and Axin2 interfered with myoblast proliferation and myotube formation, respectively. Treatment with the canonical Wnt3a ligand also inhibited myotube formation. Wnt3a activated TOPflash and Tead1 reporter activity, whereas neither reporter was activated in the presence of Dkk1, an inhibitor of canonical Wnt signaling. We propose that Axin2-dependent Wnt/ß-catenin signaling is involved in myotube formation and, together with YAP/Taz/Tead1, associated with reduced muscle fiber diameter of a subset of fast fibers.

GSK3ß-dependent dysregulation of neurodevelopment in SPG11-patient induced pluripotent stem cell model.

OBJECTIVE: Mutations in the spastic paraplegia gene 11 (SPG11), encoding spatacsin, cause the most frequent form of autosomal-recessive complex hereditary spastic paraplegia (HSP) and juvenile-onset amyotrophic lateral sclerosis (ALS5). When SPG11 is mutated, patients frequently present with spastic paraparesis, a thin corpus callosum, and cognitive impairment. We previously delineated a neurodegenerative phenotype in neurons of these patients. In the current study, we recapitulated early developmental phenotypes of SPG11 and outlined their cellular and molecular mechanisms in patient-specific induced pluripotent stem cell (iPSC)-derived cortical neural progenitor cells (NPCs). METHODS: We generated and characterized iPSC-derived NPCs and neurons from 3 SPG11 patients and 2 age-matched controls. RESULTS: Gene expression profiling of SPG11-NPCs revealed widespread transcriptional alterations in neurodevelopmental pathways. These include changes in cell-cycle, neurogenesis, cortical development pathways, in addition to autophagic deficits. More important, the GSK3ß-signaling pathway was found to be dysregulated in SPG11-NPCs. Impaired proliferation of SPG11-NPCs resulted in a significant diminution in the number of neural cells. The decrease in mitotically active SPG11-NPCs was rescued by GSK3 modulation. INTERPRETATION: This iPSC-derived NPC model provides the first evidence for an early neurodevelopmental phenotype in SPG11, with GSK3ß as a potential novel target to reverse the disease phenotype. Ann Neurol 2016;79:826-840.

Chromatin-Remodeling-Factor ARID1B Represses Wnt/ß-Catenin Signaling.

The link of chromatin remodeling to both neurodevelopment and cancer has recently been highlighted by the identification of mutations affecting BAF chromatin-remodeling components, such as ARID1B, in individuals with intellectual disability and cancer. However, the underlying molecular mechanism(s) remains unknown. Here, we show that ARID1B is a repressor of Wnt/ß-catenin signaling. Through whole-transcriptome analysis, we find that in individuals with intellectual disability and ARID1B loss-of-function mutations, Wnt/ß-catenin target genes are upregulated. Using cellular models of low and high Wnt/ß-catenin activity, we demonstrate that knockdown of ARID1B activates Wnt/ß-catenin target genes and Wnt/ß-catenin-dependent transcriptional reporters in a ß-catenin-dependent manner. Reciprocally, forced expression of ARID1B inhibits Wnt/ß-catenin signaling downstream of the ß-catenin destruction complex. Both endogenous and exogenous ARID1B associate with ß-catenin and repress Wnt/ß-catenin-mediated transcription through the BAF core subunit BRG1. Accordingly, mutations in ARID1B leading to partial or complete deletion of its BRG1-binding domain, as is often observed in intellectual disability and cancers, compromise association with ß-catenin, and the resultant ARID1B mutant proteins fail to suppress Wnt/ß-catenin signaling. Finally, knockdown of ARID1B in mouse neuroblastoma cells leads to neurite outgrowth through ß-catenin. The data suggest that aberrations in chromatin-remodeling factors, such as ARID1B, might contribute to neurodevelopmental abnormalities and cancer through deregulation of developmental and oncogenic pathways, such as the Wnt/ß-catenin signaling pathway.

Structures of the APC-ARM domain in complexes with discrete Amer1/WTX fragments reveal that it uses a consensus mode to recognize its binding partners.

The tumor suppressor APC employs its conserved armadillo repeat (ARM) domain to recognize many of its binding partners, including Amer1/WTX, which is mutated in Wilms' tumor and bone overgrowth syndrome. The APC-Amer1 complex has important roles in regulating Wnt signaling and cell adhesion. Three sites A1, A2, and A3 of Amer1 have been reported to mediate its interaction with APC-ARM. In this study, crystal structures of APC-ARM in complexes with Amer1-A1, -A2, and -A4, which is newly identified in this work, were determined. Combined with our GST pull-down, yeast two-hybrid, and isothermal titration calorimetry (ITC) assay results using mutants of APC and Amer1 interface residues, our structures demonstrate that Amer1-A1, -A2, and -A4, as well as other APC-binding proteins such as Asef and Sam68, all employ a common recognition pattern to associate with APC-ARM. In contrast, Amer1-A3 binds to the C-terminal side of APC-ARM through a bipartite interaction mode. Composite mutations on either APC or Amer1 disrupting all four interfaces abrogated their association in cultured cells and impaired the membrane recruitment of APC by Amer1. Our study thus comprehensively elucidated the recognition mechanism between APC and Amer1, and revealed a consensus recognition sequence employed by various APC-ARM binding partners.

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/ß-catenin signalling that promote degradation of ß-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 ß-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.

Activin B antagonizes RhoA signaling to stimulate mesenchymal morphology and invasiveness of clear cell renal cell carcinomas.

Activin B belongs to the TGFß family of growth factors and is upregulated in clear cell renal cell carcinoma cells by hypoxia inducible factors. Expression of Activin B is required for tumor growth in vivo and tumor cell invasion in vitro. Here we show that activation of RhoA signaling counteracts Activin B mediated disassembly of actin stress fibers, mesenchymal cell morphology and invasiveness, whereas inhibition of RhoA rescues these effects in Activin B knockdown cells. Conversely, Activin B inhibits RhoA signaling suggesting that there is an antagonistic connection between both pathways. In addition we found that Rac1 plays an opposite role to RhoA, i.e. activation of Rac1 initiates loss of actin stress fibers, promotes a mesenchymal cell morphology and induces invasion in Activin B knockown cells, whereas inhibition of Rac1 abolishes these Activin B effects. Collectively, our data provide evidence that reduction of RhoA signaling by Activin B together with persistent Rac1 activity is a prerequisite for inducing an invasive phenotype in clear cell renal cell carcinoma.

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 ß-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 ß-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 ß-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 ß-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 ß-catenin phosphorylation involves Axin and the 20R2 of APC or APCL and further processing of phospho-ß-catenin occurs upon the oligomerisation of Axin that is induced by binding the SAMP repeats in APC.

Adenomatous polyposis coli (APC) membrane recruitment 3, a member of the APC membrane recruitment family of APC-binding proteins, is a positive regulator of Wnt-ß-catenin signalling.

The adenomatous polyposis coli (APC) membrane recruitment (Amer) family proteins Amer1/Wilms tumour gene on the X chromosome and Amer2 are binding partners of the APC tumour suppressor protein, and act as negative regulators in the Wnt signalling cascade. So far, nothing has been known about the third member of the family, Amer3. Here we show that Amer3 binds to the armadillo repeat domain of APC, similarly to Amer1 and Amer2. Amer3 also binds to the Wnt pathway regulator conductin/axin2. Furthermore, we identified Amer1 as binding partner of Amer3. Whereas Amer1 and Amer2 are linked to the plasma membrane by an N-terminal membrane localization domain, Amer3 lacks this domain. Amer3 localizes to the cytoplasm and nucleus of epithelial cells, and this is dependent on specific nuclear import and export sequences. Functionally, exogenous Amer3 enhances the expression of a ß-catenin/T-cell factor-dependent reporter gene, and knockdown of endogenous Amer3 reduces Wnt target gene expression in colorectal cancer cells. Thus, Amer3 acts as an activator of Wnt signalling, in contrast to Amer1 and Amer2, which are inhibitors, suggesting a nonredundant role of Amer proteins in the regulation of this pathway. Our data, together with those of previous studies, provide a comprehensive picture of similarities and differences within the Amer protein family.

Functional comparison of human adenomatous polyposis coli (APC) and APC-like in targeting beta-catenin for degradation.

Truncating mutations affect the adenomatous polyposis coli (APC) gene in most cases of colon cancer, resulting in the stabilization of ß-catenin and uncontrolled cell proliferation. We show here that colon cancer cell lines express also the paralog APC-like (APCL or APC2). RNA interference revealed that it controls the level and/or the activity of ß-catenin, but it is less efficient and binds less well to ß-catenin than APC, thereby providing one explanation as to why the gene is not mutated in colon cancer. A further comparison indicates that APCL down-regulates the ß-catenin level despite the lack of the 15R region known to be important in APC. To understand this discrepancy, we performed immunoprecipitation experiments that revealed that phosphorylated ß-catenin displays a preference for binding to the 15 amino acid repeats (15R) rather than the first 20 amino acid repeat of APC. This suggests that the 15R region constitutes a gate connecting the steps of ß-catenin phosphorylation and subsequent ubiquitination/degradation. Using RNA interference and domain swapping experiments, we show that APCL benefits from the 15R of truncated APC to target ß-catenin for degradation, in a process likely involving heterodimerization of the two partners. Our data suggest that the functional complementation of APCL by APC constitutes a substantial facet of tumour development, because the truncating mutations of APC in colorectal tumours from familial adenomatous polyposis (FAP) patients are almost always selected for the retention of at least one 15R.

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

PURPOSE: Aberrant activation of the Wnt/ß-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 ß-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 ß-catenin and axin 2 in consecutive CRC tissue sections revealed that nuclear translocation of ß-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.

Amer2 protein interacts with EB1 protein and adenomatous polyposis coli (APC) and controls microtubule stability and cell migration.

EB1 is key factor in the organization of the microtubule cytoskeleton by binding to the plus-ends of microtubules and serving as a platform for a number of interacting proteins (termed +TIPs) that control microtubule dynamics. Together with its direct binding partner adenomatous polyposis coli (APC), EB1 can stabilize microtubules. Here, we show that Amer2 (APC membrane recruitment 2), a previously identified membrane-associated APC-binding protein, is a direct interaction partner of EB1 and acts as regulator of microtubule stability together with EB1. Amer2 binds to EB1 via specific (S/T)xIP motifs and recruits it to the plasma membrane. Coexpression of Amer2 and EB1 generates stabilized microtubules at the plasma membrane, whereas knockdown of Amer2 leads to destabilization of microtubules. Knockdown of Amer2, APC, or EB1 reduces cell migration, and morpholino-mediated down-regulation of Xenopus Amer2 blocks convergent extension cell movements, suggesting that the Amer2-EB1-APC complex regulates cell migration by altering microtubule stability.