Axin forms a complex with MEKK1 and activates c-Jun NH(2)-terminal kinase/stress-activated protein kinase through domains distinct from Wnt signaling

Axin formation inhibitor 1 aggravates hepatic ischemia‒reperfusion injury by promoting the ubiquitination and degradation of PPARβ

Hepatic ischemia‒reperfusion injury (HIRI) is a common pathological phenomenon after hepatectomy and liver transplantation. Here, we aim to explore the role of Axin formation inhibitor 1 (Axin1) in HIRI. In this work, we find that the expression of Axin1 is upregulated after HIRI. Cellular experiments confirme that Axin1 knockdown alleviated hypoxia/reoxygenation (H/R)-induced inflammation and apoptosis. Subsequently, we construct a HIRI model based on transgenic hepatocellular-specific Axin1 knockout and overexpression male mice and find that Axin1 deletion alleviated inflammation and apoptosis. Transcriptome sequencing reveal that the genes whose expression differed after Axin1 overexpression are significantly enriched in the PPAR signaling pathway. Furthermore, we demonstrate that Axin1 negatively regulates the expression of PPARβ, thereby activating the NF-κB pathway. Mechanistically, Axin1 binds to PPARβ to enhance the ubiquitination-mediated degradation of PPARβ by the E3 ubiquitin ligase RBBP6. Notably, adenovirus-mediated Axin1 knockdown block I/R damage in mice. Our study results demonstrate that Axin1 exacerbates HIRI by promoting the ubiquitination and degradation of PPARβ, which in turn activates the NF-κB signaling pathway. These results suggest that Axin1 may be a potential therapeutic target for HIRI.

Intestinal Epithelial Axin1 Deficiency Protects Against Colitis via Altered Gut Microbiota

Intestinal homeostasis is maintained by specialized host cells and the gut microbiota. Wnt/β-catenin signaling is essential for gastrointestinal development and homeostasis, and its dysregulation has been implicated in inflammation and colorectal cancer. Axin1 negatively regulates activated Wnt/β-catenin signaling, but little is known regarding its role in regulating host-microbial interactions in health and disease. Here, we aim to demonstrate that intestinal Axin1 determines gut homeostasis and host response to inflammation. Axin1 expression was analyzed in human inflammatory bowel disease datasets. To explore the effects and mechanism of intestinal Axin1 in regulating intestinal homeostasis and colitis, we generated new mouse models with Axin1 conditional knockout in intestinal epithelial cell (IEC; Axin1 ΔIEC) and Paneth cell (PC; Axin1 ΔPC) to compare with control (Axin1 LoxP; LoxP: locus of X-over, P1) mice. We found increased Axin1 expression in the colonic epithelium of human inflammatory bowel disease (IBD). Axin1 ΔIEC mice exhibited altered goblet cell spatial distribution, PC morphology, reduced lysozyme expression, and enriched Akkermansia muciniphila (A. muciniphila). The absence of intestinal epithelial and PC Axin1 decreased susceptibility to dextran sulfate sodium (DSS)-induced colitis in vivo. Axin1 ΔIEC and Axin1 ΔPC mice became more susceptible to DSS-colitis after cohousing with control mice. Treatment with A. muciniphila reduced DSS-colitis severity. Antibiotic treatment did not change the IEC proliferation in the Axin1 Loxp mice. However, the intestinal proliferative cells in Axin1 ΔIEC mice with antibiotic treatment were reduced compared with those in Axin1 ΔIEC mice without treatment. These data suggest non-colitogenic effects driven by the gut microbiome. In conclusion, we found that the loss of intestinal Axin1 protects against colitis, likely driven by epithelial Axin1 and Axin1-associated A. muciniphila. Our study demonstrates a novel role of Axin1 in mediating intestinal homeostasis and the microbiota. Further mechanistic studies using specific Axin1 mutations elucidating how Axin1 modulates the microbiome and host inflammatory response will provide new therapeutic strategies for human IBD.

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

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

The Role of MAP3K1 in the Development of the Female Reproductive Tract

Mitogen-Activated Protein 3 Kinase 1 (MAP3K1) is a dynamic signaling molecule with a plethora of cell-type specific functions, most of which are yet to be understood. Here we describe a role for MAP3K1 in the development of female reproductive tract (FRT). MAP3K1 kinase domain-deficient ( Map3k1 ^ΔKD ) females exhibit imperforate vagina, labor failure, and infertility. These defects correspond to a shunted Müllerian duct (MD), the principle precursor of the FRT, in embryos, while they manifest as a contorted caudal vagina with abrogated vaginal-urogenital sinus fusion in neonates. In epithelial cells, MAP3K1 acts through JNK and ERK to activate WNT, yet in vivo MAP3K1 is crucial for WNT activity in mesenchyme associated with the caudal MD. Expression of Wnt7b is high in wild type, but low in Map3k1 knockout MD epithelium and MAP3K1-deficient keratinocytes. Correspondingly, conditioned media derived from MAP3K1-competent epithelial cells activate TCF/Lef-luciferase reporter in fibroblasts, suggesting that MAP3K1-induced factors released from epithelial cells trans-activate WNT signaling in fibroblasts. Our results reveal a temporal-spatial and paracrine MAP3K1-WNT crosstalk contributing to MD caudal elongation and FRT development.

Highlights

MAP3K1 deficient female mice exhibit imperforate vagina and infertilityLoss of MAP3K1 kinase activity impedes Müllerian duct (MD) caudal elongation and fusion with urogenital sinus (UGS) in embryogenesisThe MAP3K1-MAPK pathway up-regulates WNT signaling in epithelial cellsMAP3K1 deficiency down-regulates Wnt7b expression in the MD epithelium and prevents WNT activity in mesenchyme of the caudal MD.

Axin1: A novel scaffold protein joins the antiviral network of interferon

Acute respiratory infection by influenza virus is a persistent and pervasive public health problem. Antiviral innate immunity initiated by type I interferon (IFN) is the first responder to pathogen invasion and provides the first line of defense. We discovered that Axin1, a scaffold protein, was reduced during influenza virus infection. We also found that overexpression of Axin1 and the chemical stabilizer of Axin1, XAV939, reduced influenza virus replication in lung epithelial cells. This effect was also observed with respiratory syncytial virus and vesicular stomatitis virus. Axin1 boosted type I IFN response to influenza virus infection and activated JNK/c-Jun and Smad3 signaling. XAV939 protected mice from influenza virus infection. Thus, our studies provide new mechanistic insights into the regulation of the type I IFN response and present a new potential therapeutic of targeting Axin1 against influenza virus infection.

A MEKK1 - JNK mitogen activated kinase (MAPK) cascade module is active in Echinococcus multilocularis stem cells

Background

The metacestode larval stage of the fox-tapeworm Echinococcus multilocularis causes alveolar echinococcosis by tumour-like growth within the liver of the intermediate host. Metacestode growth and development is stimulated by host-derived cytokines such as insulin, fibroblast growth factor, and epidermal growth factor via activation of cognate receptor tyrosine kinases expressed by the parasite. Little is known, however, concerning signal transmission to the parasite nucleus and cross-reaction with other parasite signalling systems.

Methodology/Principal findings

Using bioinformatic approaches, cloning, and yeast two-hybrid analyses we identified a novel mitogen-activated kinase (MAPK) cascade module that consists of E. multilocularis orthologs of the tyrosine kinase receptor interactor Growth factor receptor-bound 2, EmGrb2, the MAPK kinase kinase EmMEKK1, a novel MAPK kinase, EmMKK3, and a close homolog to c-Jun N-terminal kinase (JNK), EmMPK3. Whole mount in situ hybridization analyses indicated that EmMEKK1 and EmMPK3 are both expressed in E. multilocularis germinative (stem) cells but also in differentiated or differentiating cells. Treatment with the known JNK inhibitor SP600125 led to a significantly reduced formation of metacestode vesicles from stem cells and to a specific reduction of proliferating stem cells in mature metacestode vesicles.

Conclusions/Significance

We provide evidence for the expression of a MEKK1-JNK MAPK cascade module which, in mammals, is crucially involved in stress responses, cytoskeletal rearrangements, and apoptosis, in E. multilocularis stem cells. Inhibitor studies indicate an important role of JNK signalling in E. multilocularis stem cell survival and/or maintenance. Our data are relevant for molecular and cellular studies into crosstalk signalling mechanisms that govern Echinococcus stem cell function and introduce the JNK signalling cascade as a possible target of chemotherapeutics against echinococcosis.

The metacestode larva of the tapeworm E. multilocularis grows infiltrative, like a malignant tumour, within the liver of the host thus causing the lethal disease alveolar echinococcosis. Previous work established that the metacestode senses signals of host hormones and cytokines by expressing surface receptors that share high homology with respective host receptors. However, little is known how these signals are transmitted from the parasite cell surface to the nucleus to alter gene expression. In this work, the authors present a module of several protein kinases that typically transmit cytokine signals from surface receptors to central regulators called mitogen-activated protein kinases (MAPK). The authors demonstrate that this module is active in parasite stem cells, which drive the development of metacestode larva. They also show that inhibitors directed against one component of the module, EmMPK3, affect maintenance and/or survival of stem cells in the metacestode and prevent the formation of metacestode larva from parasite cell cultures. This information facilitates molecular and cellular studies to unravel the complex signalling network that regulate Echinococcus stem cell proliferation in response to host signals. Furthermore, these data could open new ways of anti-parasitic chemotherapy by introducing EmMPK3 as a possible drug target.

Wnt/β-catenin signaling acts cell-autonomously to promote cardiomyocyte regeneration in the zebrafish heart

Zebrafish can achieve scar-free healing of heart injuries, and robustly replace all cardiomyocytes lost to injury via dedifferentiation and proliferation of mature cardiomyocytes. Previous studies suggested that Wnt/β-catenin signaling is active in the injured zebrafish heart, where it induces fibrosis and prevents cardiomyocyte cell cycling. Here, via targeting the destruction complex of the Wnt/β-catenin pathway with pharmacological and genetic tools, we demonstrate that Wnt/β-catenin activity is required for cardiomyocyte proliferation and dedifferentiation, as well as for maturation of the scar during regeneration. Using cardiomyocyte-specific conditional inhibition of the pathway, we show that Wnt/β-catenin signaling acts cell-autonomously to promote cardiomyocyte proliferation. Our results stand in contrast to previous reports and rather support a model in which Wnt/β-catenin signaling plays a positive role during heart regeneration in zebrafish.

A non-translational role of threonyl-tRNA synthetase in regulating JNK signaling during myogenic differentiation

Aminoacyl-tRNA synthetases (aaRSs) are house-keeping enzymes that are essential for protein synthesis. However, it has become increasingly evident that some aaRSs also have non-translational functions. Here we report the identification of a non-translational function of threonyl-tRNA synthetase (ThrRS) in myogenic differentiation. We find that ThrRS negatively regulates myoblast differentiation in vitro and injury-induced skeletal muscle regeneration in vivo. This function is independent of amino acid binding or aminoacylation activity of ThrRS, and knockdown of ThrRS leads to enhanced differentiation without affecting the global protein synthesis rate. Furthermore, we show that the non-catalytic new domains (UNE-T and TGS) of ThrRS are both necessary and sufficient for the myogenic function. In searching for a molecular mechanism of this new function, we find the kinase JNK to be a downstream target of ThrRS. Our data further reveal MEKK4 and MKK4 as upstream regulators of JNK in myogenesis and the MEKK4-MKK4-JNK pathway to be a mediator of the myogenic function of ThrRS. Finally, we show that ThrRS physically interacts with Axin1, disrupts Axin1-MEKK4 interaction and consequently inhibits JNK signaling. In conclusion, we uncover a non-translational function for ThrRS in the maintenance of homeostasis of skeletal myogenesis and identify the Axin1-MEKK4-MKK4-JNK signaling axis to be an immediate target of ThrRS action.

Mutations in MAP3K1 that cause 46,XY disorders of sex development disrupt distinct structural domains in the protein

Missense mutations in the gene, MAP3K1, are a common cause of 46,XY gonadal dysgenesis, accounting for 15-20% of cases [Ostrer, 2014, Disorders of sex development (DSDs): an update. J. Clin. Endocrinol. Metab., 99, 1503-1509]. Functional studies demonstrated that all of these mutations cause a protein gain-of-function that alters co-factor binding and increases phosphorylation of the downstream MAP kinase pathway targets, MAPK11, MAP3K and MAPK1. This dysregulation of the MAP kinase pathway results in increased CTNNB1, increased expression of WNT4 and FOXL2 and decreased expression of SRY and SOX9. Unique and recurrent pathogenic mutations cluster in three semi-contiguous domains outside the kinase region of the protein, a newly identified N-terminal domain that shares homology with the Guanine Exchange Factor (residues Met164 to Glu231), a Plant HomeoDomain (residues Met442 to Trp495) and an ARMadillo repeat domain (residues Met566 to Glu862). Despite the presence of the mutation clusters and clinical data, there exists a dearth of mechanistic insights behind the development imbalance. In this paper, we use structural modeling and functional data of these mutations to understand alterations of the MAP3K1 protein and the effects on protein folding, binding and downstream target phosphorylation. We show that these mutations have differential effects on protein binding depending on the domains in which they occur. These mutations increase the binding of the RHOA, MAP3K4 and FRAT1 proteins and generally decrease the binding of RAC1. Thus, pathologies in MAP3K1 disrupt the balance between the pro-kinase activities of the RHOA and MAP3K4 binding partners and the inhibitory activity of RAC1.

Tankyrase Inhibitors Target Colorectal Cancer Stem Cells via AXIN-Dependent Downregulation of c-KIT Tyrosine Kinase

Cancer stem cells (CSC) constitute heterogeneous cell subpopulations of a tumor. Although targeting CSCs is important for cancer eradication, no clinically approved drugs that target CSCs have been established. Tankyrase poly(ADP-ribosyl)ates and destabilizes AXIN, a negative regulator of β-catenin, and promotes β-catenin signaling. Here, we report that tankyrase inhibitors downregulate c-KIT tyrosine kinase and inhibit the growth of CD44-positive colorectal CSCs. c-KIT expression in CD44-positive subpopulations of colorectal cancer COLO-320DM cells is associated with their tumor-initiating potential in vivo Tankyrase inhibitors downregulate c-KIT expression in established cell lines, such as COLO-320DM and DLD-1, and colorectal cancer patient-derived cells. These effects of tankyrase inhibitors are caused by reducing the recruitment of SP1 transcription factor to the c-KIT gene promoter and depend on AXIN2 stabilization but not β-catenin downregulation. Whereas c-KIT knockdown inhibits the growth of CD44-positive COLO-320DM cells, c-KIT overexpression in DLD-1 cells confers resistance to tankyrase inhibitors. Combination of a low-dose tankyrase inhibitor and irinotecan significantly inhibited the growth of COLO-320DM tumors in a mouse xenograft model. These observations suggest that tankyrase inhibitors target c-KIT-positive colorectal CSCs and provide a novel therapeutic strategy for cancer.

Axin Family of Scaffolding Proteins in Development: Lessons from C. 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.

The transcriptional coactivator WBP2 primes triple-negative breast cancer cells for responses to Wnt signaling via the JNK/Jun kinase pathway

The transcriptional coactivator WW domain-binding protein 2 (WBP2) is an emerging oncogene and serves as a node between the signaling protein Wnt and other signaling molecules and pathways, including epidermal growth factor receptor, estrogen receptor/progesterone receptor, and the Hippo pathway. The upstream regulation of WBP2 is well-studied, but its downstream activity remains unclear. Here, we elucidated WBP2's role in triple-negative breast cancer (TNBC), in which Wnt signaling is predominantly activated. Using RNAi coupled with RNA-Seq and MS analyses to identify Wnt/WBP2- and WBP2-dependent targets in MDA-MB-231 TNBC cells, we found that WBP2 is required for the expression of a core set of genes in Wnt signaling. These included AXIN2, which was essential for Wnt/WBP2-mediated breast cancer growth and migration. WBP2 also regulated a much larger set of genes and proteins independently of Wnt, revealing that WBP2 primes cells to Wnt activity by up-regulating G protein pathway suppressor 1 (GPS1) and TRAF2- and NCK-interacting kinase (TNIK). GPS1 activated the c-Jun N-terminal kinase (JNK)/Jun pathway, resulting in a positive feedback loop with TNIK that mediated Wnt-induced AXIN2 expression. WBP2 promoted TNBC growth by integrating JNK with Wnt signaling, and its expression profoundly influenced the sensitivity of TNBC to JNK/TNIK inhibitors. In conclusion, WBP2 links JNK to Wnt signaling in TNBC. GPS1 and TNIK are constituents of a WBP2-initiated cascade that primes responses to Wnt ligands and are also important for TNBC biology. We propose that WBP2 is a potential drug target for JNK/TNIK-based precision medicine for managing TNBC.

Repression of MAP3K1 expression and JNK activity by canonical Wnt signaling

Morphogenesis is a complex and highly coordinated process orchestrated by temporal spatial activity of developmental pathways. How the different pathways interact to guide the developmental program remains an intriguing and open question. MAP3K1-JNK and Wnt are signaling pathways crucial for embryonic eyelid closure, an epithelial morphogenetic event conserved in mammals. Here we used a mouse model of eyelid development and genetic and biochemistry tools to investigate the relationships between the two pathways. We found that Wnt activation repressed MAP3K1 expression. Using Axin-LacZ reporter mice, spatial Wnt activity was detected in the leading edge of the developing eyelid. Conditional knockout of Wntless (Wls) in ocular surface ectoderm blocked eyelid formation, and significantly increased MAP3K1 expression in eyelid cells at the nasal canthus region. Conversely, knockout of Dkk2, encoding a canonical Wnt antagonist, resulted in an increase of Wnt activity in cells at the upper eyelid margin near the nasal canthus. Up-regulation of Wnt signaling in the Dkk2-knockout embryos corresponded to down-regulation of MAP3K1 expression. In vitro data showed that Wnt3a treatment decreased MAP3K1 promoter activity, whereas activation of Wnt by lithium chloride inhibited MAP3K1 expression, and attenuated MAP3K1-mediated JNK activity. Our data identify a unique signal crosstalk between Wnt signaling and the MAP3K1-JNK pathway in epithelial morphogenesis.

AXIN1 protects against testicular germ cell tumors via the PI3K/AKT/mTOR signaling pathway

Axis inhibition protein 1 (AXIN1) is characterized as a tumor suppressor in numerous types of cancer. However, the functional role of AXIN1 in the testicular germ cell tumors (TGCTs) remains unclear. The human embryonal carcinoma-derived cell line NTera2 was transfected with a recombinant AXIN1 expression vector (pcDNA3.1-AXIN1) and/or a small interfering RNA (siRNA) directed against AXIN1 (siAXIN). Following transfection, the mRNA and protein levels of AXIN1 were determined via reverse transcription-quantitative polymerase chain reaction analysis and western blotting, respectively. In addition, cell viability, apoptosis and the expression of apoptosis-associated proteins [apoptosis regulator Bax (Bax) and B-cell lymphoma (Bcl)-2] and phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) signaling pathway proteins [phosphorylated (p)-mTOR, mTOR, p-AKT, AKT, P-70S ribosomal protein S6 (S6) and S6] were assessed. AXIN1 mRNA and protein levels were increased following transfection with pcDNA3.1-AXIN1 and decreased following transfection with siAXIN1 compared with their respective control groups. After overexpression of AXIN1, NTera2 cell viability and expression of Bcl-2, p-mTOR p-AKT and p-S6 protein was decreased, while apoptosis and Bax protein levels were increased, compared with the control group. However, there was no significant difference in AXIN1 mRNA expression, apoptosis or Bax/Bcl-2 protein expression when NTera2 cells were simultaneously transfected with pcDNA3.1-AXIN1+siAXIN1. In conclusion, the results of the present study indicate that overexpression of AXIN1 protects against TGCTs via inhibiting the PI3K/AKT/mTOR signaling pathway, suggesting that AXIN1 may be a potential target for gene therapy in TGCTs.

Relationship between RUNX1 and AXIN1 in ER-negative versus ER-positive Breast Cancer

RUNX1 plays opposing roles in breast cancer: a tumor suppressor in estrogen receptor-positive (ER⁺) disease and an oncogenic role in ER-negative (ER^-) tumors. Potentially mediating the former, we have recently reported that RUNX1 prevents estrogen-driven suppression of the mRNA encoding the tumor suppressor AXIN1. Accordingly, AXIN1 protein expression was diminished upon RUNX1 silencing in ER⁺ breast cancer cells and was positively correlated with AXIN1 protein expression across tumors with high levels of ER. Here we report the surprising observation that RUNX1 and AXIN1 proteins are strongly correlated in ER^- tumors as well. However, this correlation is not attributable to regulation of AXIN1 by RUNX1 or vice versa. The unexpected correlation between RUNX1, playing an oncogenic role in ER^- breast cancer, and AXIN1, a well-established tumor suppressor hub, may be related to a high ratio between the expression of variant 2 and variant 1 (v2/v1) of AXIN1 in ER^- compared with ER⁺ breast cancer. Although both isoforms are similarly regulated by RUNX1 in estrogen-stimulated ER⁺ breast cancer cells, the higher v2/v1 ratio in ER^- disease is expected to weaken the tumor suppressor activity of AXIN1 in these tumors.

Axin-1 Regulates Meiotic Spindle Organization in Mouse Oocytes

Axin-1, a negative regulator of Wnt signaling, is a versatile scaffold protein involved in centrosome separation and spindle assembly in mitosis, but its function in mammalian oogenesis remains unknown. Here we examined the localization and function of Axin-1 during meiotic maturation in mouse oocytes. Immunofluorescence analysis showed that Axin-1 was localized around the spindle. Knockdown of the Axin1 gene by microinjection of specific short interfering (si)RNA into the oocyte cytoplasm resulted in severely defective spindles, misaligned chromosomes, failure of first polar body (PB1) extrusion, and impaired pronuclear formation. However, supplementing the culture medium with the Wnt pathway activator LiCl improved spindle morphology and pronuclear formation. Downregulation of Axin1 gene expression also impaired the spindle pole localization of γ-tubulin/Nek9 and resulted in retention of the spindle assembly checkpoint protein BubR1 at kinetochores after 8.5 h of culture. Our results suggest that Axin-1 is critical for spindle organization and cell cycle progression during meiotic maturation in mouse oocytes.

An AXIN2 Mutant Allele Associated With Predisposition to Colorectal Neoplasia Has Context-Dependent Effects on AXIN2 Protein Function

Heterozygous, germline nonsense mutations in AXIN2 have been reported in two families with oligodontia and colorectal cancer (CRC) predisposition, including an AXIN2 1989G>A mutation. Somatic AXIN2 mutations predicted to generate truncated AXIN2 (trAXIN2) proteins have been reported in some CRCs. Our studies of cells from an AXIN2 1989G>A mutation carrier showed that the mutant transcripts are not significantly susceptible to nonsense-mediated decay and, thus, could encode a trAXIN2 protein. In transient transfection assays, trAXIN2 was more abundant than wild-type AXIN2 protein, and in contrast to AXIN2, glycogen synthase kinase 3β inhibition did not increase trAXIN2 levels. Like AXIN2, the trAXIN2 protein interacts with β-catenin destruction complex proteins. When ectopically overexpressed, trAXIN2 inhibits β-catenin/T-cell factor-dependent reporter gene activity and SW480 CRC cell colony formation. These findings suggest the trAXIN2 protein may retain some wild-type functions when highly expressed. However, when stably expressed in rat intestinal IEC-6 cells, the trAXIN2 protein did not match AXIN2's activity in inhibiting Wnt-mediated induction of Wnt-regulated target genes, and SW480 cells with stable expression of trAXIN2 but not AXIN2 could be generated. Our data suggest the AXIN2 1989G>A mutation may not have solely a loss-of-function role in CRC. Rather, its contribution may depend on context, with potential loss-of-function when AXIN2 levels are low, such as in the absence of Wnt pathway activation. However, given its apparent increased stability in some settings, the trAXIN2 protein might have gain-of-function in cells with substantially elevated AXIN2 expression, such as Wnt pathway-defective CRC cells.

Ifit1 Protects Against Lipopolysaccharide and D-galactosamine-Induced Fatal Hepatitis by Inhibiting Activation of the JNK Pathway

Treatment of mice with lipopolysaccharide (LPS) and the liver-specific transcriptional inhibitor D-(+)-galactosamine (GalN) induces fatal hepatitis, which is mediated by tumor necrosis factor α (TNF-α) and characterized by massive hepatic apoptosis. Previous studies suggest that GalN increases the sensitivity to LPS/TNF-α, probably by blocking the transcription of protective factors, but the identity of most of these factors is still unclear. Here, we report that Ifit1 protects against LPS/GalN-induced fatal hepatitis. Forced expression of Ifit1 in hepatocytes significantly diminished TNF-α-mediated apoptosis. Moreover, targeted expression of Ifit1 in the liver by recombinant adeno-associated virus serotype 8 protected mice from LPS/GalN-induced lethal hepatitis, which was associated with the inhibition of TNF-α-mediated activation of the c-Jun N-terminal kinase (JNK)-Bim cascade. Furthermore, Ifit1 bound to a scaffolding protein Axin and inhibited its function to mediate JNK activation. Together, our data demonstrate that Ifit1 is a novel protective factor that inhibits LPS/GalN-induced (TNF-α-mediated) fatal hepatitis, suggesting that Ifit1 is a potential therapeutic target for treatment of inflammatory liver diseases.

Alternative splicing within the Wnt signaling pathway: role in cancer development

BACKGROUND

The Wnt signaling cascade plays a fundamental role in embryonic development, adult tissue regeneration, homeostasis and stem cell maintenance. Abnormal Wnt signaling has been found to be prevalent in various human cancers. Also, a role of Wnt signaling in the regulation of alternative splicing of several cancer-related genes has been established. In addition, accumulating evidence suggests the existence of multiple splice isoforms of Wnt signaling cascade components, including Wnt ligands, receptors, components of the destruction complex and transcription activators/suppressors. The presence of multiple Wnt signaling-related isoforms may affect the functionality of the Wnt pathway, including its deregulation in cancer. As such, specific Wnt pathway isoform components may serve as therapeutic targets or as biomarkers for certain human cancers. Here, we review the role of alternative splicing of Wnt signaling components during the onset and progression of cancer.

CONCLUSIONS

Splice isoforms of components of the Wnt signaling pathway play distinct roles in cancer development. Isoforms of the same component may function in a tissue- and/or cancer-specific manner. Splice isoform expression analyses along with deregulated Wnt signaling pathway analyses may be of help to design efficient diagnostic and therapeutic strategies.

A RING to rule them all? Insights into the Map3k1 PHD motif provide a new mechanistic understanding into the diverse roles of Map3k1

Despite the sizable number of components that comprise Mapk cascades, Map3k1 is the only element that contains both a kinase domain and a plant homeodomain (PHD) motif, allowing Map3k1 to regulate the protein phosphorylation and ubiquitin proteasome systems. As such, Map3k1 has complex roles in the regulation of cell death, survival, migration and differentiation. Numerous mouse and human genetic analyses have demonstrated that Map3k1 is of critical importance for the immune system, cardiac tissue, testis, wound healing, tumorigenesis and cancer. Recent gene knockin of Map3k1 to mutate the E2 binding site within the Map3k1 PHD motif and high throughput ubiquitin protein array screening for Map3k1 PHD motif substrates provide critical novel insights into Map3k1 PHD motif signal transduction and bring a brand-new understanding to Map3k1 signaling in mammalian biology.

AXIN1 and AXIN2 variants in gastrointestinal cancers

Mutations in the APC (adenomatous polyposis coli) gene, which encodes a multi-functional protein with a well-defined role in the canonical Wnt pathway, underlie familial adenomatous polypsosis, a rare, inherited form of colorectal cancer (CRC) and contribute to the majority of sporadic CRCs. However, not all sporadic and familial CRCs can be explained by mutations in APC or other genes with well-established roles in CRC. The AXIN1 and AXIN2 proteins function in the canonical Wnt pathway, and AXIN1/2 alterations have been proposed as key defects in some cancers. Here, we review AXIN1 and AXIN2 sequence alterations reported in gastrointestinal cancers, with the goal of vetting the evidence that some of the variants may have key functional roles in cancer development.

MAP3K1: Genomic Alterations in Cancer and Function in Promoting Cell Survival or Apoptosis

MAP3K1 is a member of the mitogen-activated protein kinase kinase kinase (MAP3K) family of serine/threonine kinases. MAP3K1 regulates JNK activation and is unique among human kinases in that it also encodes an E3 ligase domain that ubiquitylates c-Jun and ERK1/2. Full length MAP3K1 regulates cell migration and contributes to pro-survival signaling while its caspase 3-mediated cleavage generates a C-terminal kinase domain that promotes apoptosis. The critical function of MAP3K1 in cell fate decisions suggests that it may be a target for deregulation in cancer. Recent large-scale genomic studies have revealed that MAP3K1 copy number loss and somatic missense or nonsense mutations are observed in a significant number of different cancers, being most prominent in luminal breast cancer. The alteration of MAP3K1 in diverse cancer types demonstrates the importance of defining phenotypes for possible therapeutic targeting of tumor cell vulnerabilities created when MAP3K1 function is lost or gained.

MDM2 Inhibits Axin-Induced p53 Activation Independently of its E3 Ligase Activity

MDM2 plays a crucial role in negatively regulating the functions of tumor suppressor p53. Here we show that MDM2 can inhibit Axin-stimulated p53-dependent apoptosis by suppressing p53 phosphorylation at Ser 46 and apoptosis-related p53 transactivational activity. Interestingly, the ubiquitin E3 ligase activity of MDM2 is not required for this inhibitory effect. Mechanically, either wildtype MDM2 or its E3-dead mutant, disrupts the Axin-based HIPK2/p53 complex formation by blocking the binding of p53 and HIPK2 to Axin. MDM2Δp53, a deletion mutant that lacks p53 binding domain fails to exert the inhibitory effect, demonstrating that the interaction of MDM2 and p53, but not its E3 ligase activity toward p53 plays key role in suppressing Axin-stimulated p53 activation. Our results thus have revealed a novel aspect of the mechanism by which MDM2 regulates p53 activities.

Modulation of β-catenin signaling by the inhibitors of MAP kinase, tyrosine kinase, and PI3-kinase pathways

Aberrant activation of β-catenin signaling plays an important role in human tumorigenesis. However, molecular mechanisms behind the β-catenin signaling deregulation are mostly unknown because genetic alterations in this pathway only account for a small fraction of tumors. Here, we investigator if other major pathways can regulate β-catenin signaling activity. By employing a panel of chemical activators and/or inhibitors of several cellular signaling pathways, we assess these modulators' effects on luciferase reporter driven by β-catenin/TCF4-responsive elements. We find that lithium-stimulated β-catenin activity is synergistically enhanced by protein kinase C activator PMA. However, β-catenin-regulated transcriptional (CRT) activity is significantly inhibited by casein kinase II inhibitor DRB, MEK inhibitor PD98059, G-proteins and their receptor uncoupling agent suramin, protein tyrosine kinase inhibitor genistein, and PI-3 kinase inhibitor wortmannin, suggesting that these cellular pathways may participate in regulating β-catenin signaling. Interestingly, the Ca⁺⁺/calmodulin kinase II inhibitor HDBA is shown to activate β-catenin activity at low doses. Furthermore, Wnt3A-stimulated and constitutively activated CRT activities, as well as the intracellular accumulation of β-catenin protein in human colon cancer cells, are effectively suppressed by PD98059, genistein, and wortmannin. We further demonstrate that EGF can activate TCF4/β-catenin activity and induce the tyrosine phosphorylation of β-catenin protein. Thus, our results should provide important insights into the molecular mechanisms underlying Wnt/β-catenin activation. This knowledge should facilitate our efforts to develop efficacious and novel therapeutics by targeting these pathways.

PLK1 interacts and phosphorylates Axin that is essential for proper centrosome formation

Abnormal amplification of centrosomes could lead to improper chromosome segregation and aneuploidy and is implicated in cancer development. Here, we demonstrate that Axin, a scaffolding protein in Wnt signaling, is phosphorylated by PLK1 during mitosis. Phosphorylation of Axin Ser-157 by PLK1 abolished Axin association with γ-tubulin, while substitution of Ser-157 with alanine exhibited sustained interaction with γ-tubulin. In addition, overexpression of Axin-S157A significantly increased the number of cells with multi-centrosomes. These results suggest that the phosphorylation status of Axin, mediated by PLK1, dynamically regulates its association with γ-tubulin and centrosome formation and segregation.

The roles of intrinsic disorder in orchestrating the Wnt-pathway

The canonical Wnt-pathway plays a number of crucial roles in the development of organism. Malfunctions of this pathway lead to various diseases including cancer. In the inactivated state, this pathway involves five proteins, Axin, CKI-α, GSK-3β, APC, and β-catenin. We analyzed these proteins by a number of computational tools, such as PONDR(r)VLXT, PONDR(r)VSL2, MoRF-II predictor and Hydrophobic Cluster Analysis (HCA) to show that each of the Wnt-pathway proteins contains several intrinsically disordered regions. Based on a comprehensive analysis of published data we conclude that these disordered regions facilitate protein-protein interactions, post-translational modifications, and signaling. The scaffold protein Axin and another large protein, APC, act as flexible concentrators in gathering together all other proteins involved in the Wnt-pathway, emphasizing the role of intrinsically disordered regions in orchestrating the complex protein-protein interactions. We further explore the intricate roles of highly disordered APC in regulation of β-catenin function. Intrinsically disordered APC helps the collection of β-catenin from cytoplasm, facilitates the b-catenin delivery to the binding sites on Axin, and controls the final detachment of β-catenin from Axin.

Differential role of Axin RGS domain function in Wnt signaling during anteroposterior patterning and maternal axis formation

Axin is a critical component of the β-catenin destruction complex and is also necessary for Wnt signaling initiation at the level of co-receptor activation. Axin contains an RGS domain, which is similar to that of proteins that accelerate the GTPase activity of heterotrimeric Gα/Gna proteins and thereby limit the duration of active G-protein signaling. Although G-proteins are increasingly recognized as essential components of Wnt signaling, it has been unclear whether this domain of Axin might function in G-protein regulation. This study was performed to test the hypothesis that Axin RGS-Gna interactions would be required to attenuate Wnt signaling. We tested these ideas using an axin1 genetic mutant (masterblind) and antisense oligo knockdowns in developing zebrafish and Xenopus embryos. We generated a point mutation that is predicted to reduce Axin-Gna interaction and tested for the ability of the mutant forms to rescue Axin loss-of-function function. This Axin point mutation was deficient in binding to Gna proteins in vitro, and was unable to relocalize to the plasma membrane upon Gna overexpression. We found that the Axin point mutant construct failed to rescue normal anteroposterior neural patterning in masterblind mutant zebrafish, suggesting a requirement for G-protein interactions in this context. We also found that the same mutant was able to rescue deficiencies in maternal axin1 loss-of-function in Xenopus. These data suggest that maternal and zygotic Wnt signaling may differ in the extent of Axin regulation of G-protein signaling. We further report that expression of a membrane-localized Axin construct is sufficient to inhibit Wnt/β-catenin signaling and to promote Axin protein turnover.

JNK-dependent AP-1 activation is required for aristolochic acid-induced TGF-β1 synthesis in human renal proximal epithelial cells

Chronic aristolochic acid nephropathy (CAAN) is a chronic and progressive tubulointerstitial nephropathy characterized by extensive interstitial fibrosis. Aristolochic acid (AA) could induce overexpression of transforming growth factor-β1 (TGF-β1) in a human renal proximal tubule epithelial cells line (HKC), which has been implicated in the pathogenesis of CAAN. The present studies in HKC cells showed 1) AA could activate JNK in time- and dose-dependent manners and JNK inhibitor SP600125 could inhibit AA-induced TGF-β1 promoter activity and TGF-β1 synthesis; 2) AA-induced JNK activation and TGF-β1 synthesis were significantly inhibited by kinase-inactive mutants of MEKK4, MKK4, or MKK7; 3) AA could upregulate luciferase activity derived by a wild-type TGF-β1 promoter, but not by an AP-1 binding-deficient TGF-β1 promoter; and 4) AA could upregulate expression of c-Fos, phospho-c-Jun, and phospho-ATF2. The above data suggest AA-induced TGF-β1 overexpression in HKC cells may be mainly mediated by the JNK signaling pathway. Both the upstream kinases of JNK including MEKK4, MKK4, and MKK7, and the downstream transcription factor of JNK, AP-1, may also participate in this process.

Wnt signalling pathway parameters for mammalian cells

Wnt/β-catenin signalling regulates cell fate, survival, proliferation and differentiation at many stages of mammalian development and pathology. Mutations of two key proteins in the pathway, APC and β-catenin, have been implicated in a range of cancers, including colorectal cancer. Activation of Wnt signalling has been associated with the stabilization and nuclear accumulation of β-catenin and consequential up-regulation of β-catenin/TCF gene transcription. In 2003, Lee et al. constructed a computational model of Wnt signalling supported by experimental data from analysis of time-dependent concentration of Wnt signalling proteins in Xenopus egg extracts. Subsequent studies have used the Xenopus quantitative data to infer Wnt pathway dynamics in other systems. As a basis for understanding Wnt signalling in mammalian cells, a confocal live cell imaging measurement technique is developed to measure the cell and nuclear volumes of MDCK, HEK293T cells and 3 human colorectal cancer cell lines and the concentrations of Wnt signalling proteins β-catenin, Axin, APC, GSK3β and E-cadherin. These parameters provide the basis for formulating Wnt signalling models for kidney/intestinal epithelial mammalian cells. There are significant differences in concentrations of key proteins between Xenopus extracts and mammalian whole cell lysates. Higher concentrations of Axin and lower concentrations of APC are present in mammalian cells. Axin concentrations are greater than APC in kidney epithelial cells, whereas in intestinal epithelial cells the APC concentration is higher than Axin. Computational simulations based on Lee's model, with this new data, suggest a need for a recalibration of the model.A quantitative understanding of Wnt signalling in mammalian cells, in particular human colorectal cancers requires a detailed understanding of the concentrations of key protein complexes over time. Simulations of Wnt signalling in mammalian cells can be initiated with the parameters measured in this report.

Wnt signalling pathway parameters for mammalian cells

Wnt/β-catenin signalling regulates cell fate, survival, proliferation and differentiation at many stages of mammalian development and pathology. Mutations of two key proteins in the pathway, APC and β-catenin, have been implicated in a range of cancers, including colorectal cancer. Activation of Wnt signalling has been associated with the stabilization and nuclear accumulation of β-catenin and consequential up-regulation of β-catenin/TCF gene transcription. In 2003, Lee et al. constructed a computational model of Wnt signalling supported by experimental data from analysis of time-dependent concentration of Wnt signalling proteins in Xenopus egg extracts. Subsequent studies have used the Xenopus quantitative data to infer Wnt pathway dynamics in other systems. As a basis for understanding Wnt signalling in mammalian cells, a confocal live cell imaging measurement technique is developed to measure the cell and nuclear volumes of MDCK, HEK293T cells and 3 human colorectal cancer cell lines and the concentrations of Wnt signalling proteins β-catenin, Axin, APC, GSK3β and E-cadherin. These parameters provide the basis for formulating Wnt signalling models for kidney/intestinal epithelial mammalian cells. There are significant differences in concentrations of key proteins between Xenopus extracts and mammalian whole cell lysates. Higher concentrations of Axin and lower concentrations of APC are present in mammalian cells. Axin concentrations are greater than APC in kidney epithelial cells, whereas in intestinal epithelial cells the APC concentration is higher than Axin. Computational simulations based on Lee's model, with this new data, suggest a need for a recalibration of the model.A quantitative understanding of Wnt signalling in mammalian cells, in particular human colorectal cancers requires a detailed understanding of the concentrations of key protein complexes over time. Simulations of Wnt signalling in mammalian cells can be initiated with the parameters measured in this report.

Apc mutation enhances PyMT-induced mammary tumorigenesis

The Adenomatous Polyposis Coli (APC) tumor suppressor gene is silenced by hypermethylation or mutated in up to 70% of human breast cancers. In mouse models, Apc mutation disrupts normal mammary development and predisposes to mammary tumor formation; however, the cooperation between APC and other mutations in breast tumorigenesis has not been studied. To test the hypothesis that loss of one copy of APC promotes oncogene-mediated mammary tumorigenesis, Apc^Min/+ mice were crossed with the mouse mammary tumor virus (MMTV)-Polyoma virus middle T antigen (PyMT) or MMTV-c-Neu transgenic mice. In the PyMT tumor model, the Apc^Min/+ mutation significantly decreased survival and tumor latency, promoted a squamous adenocarcinoma phenotype, and enhanced tumor cell proliferation. In tumor-derived cell lines, the proliferative advantage was a result of increased FAK, Src and JNK signaling. These effects were specific to the PyMT model, as no changes were observed in MMTV-c-Neu mice carrying the Apc^Min/+ mutation. Our data indicate that heterozygosity of Apc enhances tumor development in an oncogene-specific manner, providing evidence that APC-dependent pathways may be valuable therapeutic targets in breast cancer. Moreover, these preclinical model systems offer a platform for dissection of the molecular mechanisms by which APC mutation enhances breast carcinogenesis, such as altered FAK/Src/JNK signaling.

c-Jun, at the crossroad of the signaling network

c-Jun, the most extensively studied protein of the activator protein-1 (AP-1) complex, is involved in numerous cell activities, such as proliferation, apoptosis, survival, tumorigenesis and tissue morphogenesis. Earlier studies focused on the structure and function have led to the identification of c-Jun as a basic leucine zipper (bZIP) transcription factor that acts as homo- or heterodimer, binding to DNA and regulating gene transcription. Later on, it was shown that extracellular signals can induce post-translational modifications of c-Jun, resulting in altered transcriptional activity and target gene expression. More recent work has uncovered multiple layers of a complex regulatory scheme in which c-Jun is able to crosstalk, amplify and integrate different signals for tissue development and disease. One example of such scheme is the autocrine amplification loop, in which signal-induced AP-1 activates the c-Jun gene promoter, while increased c-Jun expression feedbacks to potentiate AP-1 activity. Another example of such scheme, based on recent characterization of gene knockout mice, is that c-Jun integrates signals of several developmental pathways, including EGFR-ERK, EGFR-RhoA-ROCK, and activin B-MAP3K1-JNK for embryonic eyelid closure. After more than two decades of extensive research, c-Jun remains at the center stage of a molecular network with mysterious functional properties, some of which are yet to be discovered. In this article, we will provide a brief historical overview of studies on c-Jun regulation and function, and use eyelid development as an example to illustrate the complexity of c-Jun crosstalking with signaling pathways.

Mechanistic insight into Myc stabilization in breast cancer involving aberrant Axin1 expression

High expression of the oncoprotein Myc has been linked to poor outcome in human tumors. Although MYC gene amplification and translocations have been observed, this can explain Myc overexpression in only a subset of human tumors. Myc expression is in part controlled by its protein stability, which can be regulated by phosphorylation at threonine 58 (T58) and serine 62 (S62). We now report that Myc protein stability is increased in a number of breast cancer cell lines and this correlates with increased phosphorylation at S62 and decreased phosphorylation at T58. Moreover, we find this same shift in phosphorylation in primary breast cancers. The signaling cascade that controls phosphorylation at T58 and S62 is coordinated by the scaffold protein Axin1. We therefore examined Axin1 in breast cancer and report decreased AXIN1 expression and a shift in the ratio of expression of two naturally occurring AXIN1 splice variants. We demonstrate that this contributes to increased Myc protein stability, altered phosphorylation at S62 and T58, and increased oncogenic activity of Myc in breast cancer. Thus, our results reveal an important mode of Myc activation in human breast cancer and a mechanism contributing to Myc deregulation involving unique insight into inactivation of the Axin1 tumor suppressor in breast cancer.

Novelty of Axin 2 and lack of Axin 1 gene mutation in colorectal cancer: a study in Kashmiri population

Colorectal cancer is (CRC) one of the leading causes of mortality and morbidity. Various genetic factors have been reported to be involved in the development of colorectal cancers including Axin gene. Axin, a major scaffold protein, plays an important role in various bio signaling pathways. We aim to study mutational pattern of Axin gene in colorectal cancer patients of Kashmiri population. The paired tumor and adjacent normal tissue specimens of 50 consecutive patients with CRC were used in our study. The DNA preparations were evaluated for the occurrence of Axin 1 and Axin 2 gene mutations by direct DNA sequencing. We analyzed exon 1a, 1b, 1c, 2, 4, 6, and 10 of Axin 1 and exon 7 of Axin 2. In this study, we found a novel mutation of G>T (GCT>TCT) transversion in exon 7 of Axin 2 gene at codon G695T (p.alanine > serine) at a frequency of 6% (3/50). In the same exon of Axin 2 gene a single nucleotide polymorphism (SNP) was detected in codon L688L (CCT>CTT) at a frequency of 36% (18/50). In exon 1c of Axin 1 a SNP was detected at codon D726D (GAT>GAC) at a frequency of 62.5% (31/50). Both the SNPs were synonymous hence do not lead to change of amino acid. Although Axin 1 and Axin 2 gene mutations have been found to be involved in the development of colorectal cancers, it seems to be a relatively rare event in Kashmiri population. However, an interesting finding of this study is the novelty of Axin 2 gene mutations which may be a predisposing factor in ethnic Kashmiri population to CRC.

Generation of Axin1 conditional mutant mice

Axin1 is a critical negative regulator of the canonical Wnt-signaling pathway. It is a concentration-limiting factor in the β-catenin degradation complex. Axin1 null mutant mouse embryos died at embryonic day 9.5, precluding direct genetic analysis of the roles of Axin1 in many developmental and physiological processes using these mutant mice. In this study, we have generated mice carrying two directly repeated loxP sites flanking the exon 2 region of the Axin1 gene. We show that floxed-allele-carrying mice (Axin1( fx/fx) ) mice appear normal and fertile. Upon crossing the Axin1( fx/fx) mice to the CMV-Cre transgenic mice, the loxP-flanked exon 2 region that encodes the N-terminus and the conserved regulation of G-protein signaling domain was efficiently deleted by Cre-mediated excision in vivo. Moreover, we show that mouse embryos homozygous for the Cre/loxP-mediated deletion of exon 2 of the Axin1 gene display embryonic lethality and developmental defects similar to those reported for Axin1(-/-) mice. Thus, this Axin1(fx/fx) mouse model will be valuable for systematic tissue-specific dissection of the roles of Axin1 in embryonic and postnatal development and diseases.

The WD40-repeat protein Han11 functions as a scaffold protein to control HIPK2 and MEKK1 kinase functions

Protein kinases are organized in hierarchical networks that are assembled and regulated by scaffold proteins. Here, we identify the evolutionary conserved WD40-repeat protein Han11 as an interactor of the kinase homeodomain-interacting protein kinase 2 (HIPK2). In vitro experiments showed the direct binding of Han11 to HIPK2, but also to the kinases DYRK1a, DYRK1b and mitogen-activated protein kinase kinase kinase 1 (MEKK1). Han11 was required to allow coupling of MEKK1 to DYRK1 and HIPK2. Knockdown experiments in Caenorhabditis elegans showed the relevance of the Han11 orthologs Swan-1 and Swan-2 for the osmotic stress response. Downregulation of Han11 in human cells lowered the threshold and amplitude of HIPK2- and MEKK1-triggered signalling events and changed the kinetics of kinase induction. Han11 knockdown changed the amplitude and time dependence of HIPK2-driven transcription in response to DNA damage and also interfered with MEKK1-triggered gene expression and stress signalling. Impaired signal transmission also occurred upon interference with stoichiometrically assembled signalling complexes by Han11 overexpression. Collectively, these experiments identify Han11 as a novel scaffold protein regulating kinase signalling by HIPK2 and MEKK1.

A quantitative trait locus for a primary antibody response to keyhole limpet hemocyanin on chicken chromosome 14--confirmation and candidate gene approach

A QTL involved in the primary antibody response toward keyhole limpet hemocyanin (KLH) was detected on chicken chromosome 14 in the experimental population, which was created by crossing commercial White Leghorn and a Polish native chicken breed (green-legged partridgelike). The current QTL location is a validation of previous experiments pointing to the same genomic location for the QTL linked to a primary antibody response to KLH. An experimental population was typed with microsatellite markers distributed over the chicken chromosome 14. Titers of antibodies binding KLH were measured for all individuals by ELISA. Statistical models applied in the Grid QTL Web-based software were used to analyze the data: a half-sib model, a line-cross model, and combined analysis in a linkage disequilibrium and linkage analysis model. Candidate genes that have been proposed were genotyped with SNP located in genes exons. Statistical analyses of single SNP associations were performed pointing out 2 SNP of an axis inhibitor protein (AXIN1) gene as significantly associated with the trait of an interest.

The various roles of ubiquitin in Wnt pathway regulation

Wnt signaling mediates key developmental and homeostatic processes including stem cell maintenance, growth and cell fate specification, cell polarity and migration. Inappropriate activation of Wnt signaling is linked to a range of human disorders, most notably cancer and neurodegenerative diseases. In the Wnt/β-catenin cascade, signaling events converge on the regulation of ubiquitin-mediated degradation of the crucial transcriptional regulator β-catenin. The emerging mechanisms by which ubiquitin modification of proteins controls cellular pathways comprise both proteolytic and nonproteolytic functions. In nonproteolytic functions, ubiquitin acts as a signaling device in the control of protein activity, subcellular localization and complex formation. Here, we review and discuss recent developments that implicate ubiquitin-mediated mechanisms at multiple steps of Wnt pathway activation.

MAP3K1 functionally interacts with Axin1 in the canonical Wnt signalling pathway

A central point of regulation in the Wnt/β-catenin signalling pathway is the formation of the β-catenin destruction complex. Axin1, an essential negative regulator of Wnt signalling, serves as a scaffold within this complex and is critical for rapid turnover of β-catenin. To examine the mechanism by which Wnt signalling disables the destruction complex, we used an immunoprecipitation-coupled proteomics approach to identify novel endogenous binding partners of Axin1. We found mitogen-activated protein kinase kinase kinase 1 (MAP3K1) as an Axin1 interactor in Ls174T colorectal cancer (CRC) cells. Importantly, confirmation of this interaction in HEK293T cells indicated that the Axin1-MAP3K1 interaction is induced and modulated by Wnt stimulation. siRNA depletion of MAP3K1 specifically abrogated TCF/LEF-driven transcription and Wnt3A-driven endogenous gene expression in both HEK293T as well as DLD-1 CRC. Expression of ubiquitin ligase mutants of MAP3K1 abrogated TCF/LEF transcription, whereas kinase mutants had no effect in TCF-driven activity, highlighting the essential role of the MAP3K1 E3 ubiquitin ligase activity in regulation of the Wnt/β-catenin pathway. These results suggest that MAP3K1, previously reported as an Axin1 inter-actor in c-Jun NH2-terminal kinase pathway, is also involved in the canonical Wnt signalling pathway and positively regulates expression of Wnt target genes.

The links between transcription, beta-catenin/JNK signaling, and carcinogenesis

Interactions between transcription and signaling are fundamentally important for understanding both the structure and function of genetic pathways and their role in diseases such as cancer. The finding that beta-catenin/TCF4 and JNK/c-Jun cooperate has important implications in carcinogenesis. Previously, we found that binding of c-Jun and beta-catenin/TCF4 to the c-jun promoter is dependent upon JNK activity, thus one role for this complex is to contribute to the repression and/or activation of genes that may mediate cell maintenance, proliferation, differentiation, and death, whereas deregulation of these signals may contribute to carcinogenesis. Here we address the functional links reported between activated beta-catenin/JNK signaling pathways, their component genes, and their common targets, and discuss how alterations in the properties of these genes lead to the development of cancer.

Reduced axin protein expression is associated with a poor prognosis in patients with squamous cell carcinoma of esophagus

AIMS

Our study investigates the significance of the expression of Wnt pathway proteins including beta-catenin, Axin, beta-transducin-repeat-containing protein (beta-TrCP), and adenomatous polyposis coli (APC) in squamous cell carcinoma of the esophagus (ESCC).

METHODS

Immunohistochemical analysis was performed on paraffin-embedded tissue specimens from 128 resected ESCC tumors to detect the expression of beta-catenin, Axin, beta-TrCP, and APC. Correlation between immunoexpression, clinicopathological parameters, and patient survival was analyzed.

RESULTS

Increased beta-catenin expression was noted in 22 (18.2%) of 121 tumor specimens. Reduced expression of Axin, beta-TrCP, and APC was observed in 57 (46.0%) of 124, 29 (24.4%) of 119, and 54 (48.2%) of 119 specimens, respectively. No correlation was found among these protein expressions. Axin protein expression was inversely correlated with tumor invasion depth (P = 0.033). Reduced Axin protein expression, lymph node involvement, and distant metastasis were significant negative predictors for overall survival and disease-free survival on univariate analysis. In multivariate analysis, reduced Axin expression remained a significant prognostic factor for patients with ESCC (P = 0.005).

CONCLUSIONS

Reduced Axin expression was observed in 46% of ESCC tumor specimens and was associated with poor prognosis in patients with ESCC. Further study is mandatory to elucidate the underlying mechanism responsible for loss of Axin expression and the role of Axin in ESCC tumorigenesis.

Both the RGS domain and the six C-terminal amino acids of mouse Axin are required for normal embryogenesis

Axin is a negative regulator of canonical Wnt signaling, which promotes the degradation of beta-catenin, the major effector in this signaling cascade. While many protein-binding domains of Axin have been identified, their significance has not been evaluated in vivo. Here, we report the generation and analysis of mice carrying modified Axin alleles in which either the RGS domain or the six C-terminal amino acids (C6 motif) were deleted. The RGS domain is required for APC-binding, while the C6 motif has been implicated in the activation of c-Jun N-terminal kinase, but is not required for the effects of Axin on the Wnt/beta-catenin pathway, in vitro. Both mutant Axin alleles caused recessive embryonic lethality at E9.5-E10.5, with defects indistinguishable from those caused by a null allele. As Axin-DeltaRGS protein was produced at normal levels, its inability to support embryogenesis confirms the importance of interactions between Axin and APC. In contrast, Axin-DeltaC6 protein was expressed at only 25-30% of the normal level, which may account for the recessive lethality of this allele. Furthermore, many Axin(DeltaC6/DeltaC6) embryos that were heterozygous for a beta-catenin null mutation survived to term, demonstrating that early lethality was due to failure to negatively regulate beta-catenin.

Co-opted JNK/SAPK signaling in Wnt/beta-catenin-induced tumorigenesis

Aberrant stimulation of the canonical Wnt pathway induces mammary tumorigenesis in mice. It has been well documented that two types of tumors, adenocarcinoma and adenocarcinoma with squamous metaplasia, develop in these mutants. However, the molecular mechanism underlying the induction of squamous transdifferentiation remains largely unknown. Here, we show that JNK/SAPK signaling plays an important role in Wnt-dependent mammary development and malignant transformation. The JNK/SAPK pathway is stimulated in pregnancy-mediated lobulo-alveolar morphogenesis, a process highly dependent on Wnt/beta-catenin signaling. Strong elevations of JNK/SAPK signaling are associated with squamous metaplasia of the Wnt-induced adenocarcinoma. Reconstitution of beta-catenin and JNK/SAPK signaling activities also promotes expression of the squamous cell marker in cultured epithelial cells. Furthermore, a synergistic activation of these two pathways can be identified in the malignant squamous cells of human endometrial and lung cancers. This is potentially a significant discovery in modern cancer therapy because of the effectiveness of an angiogenesis inhibitor, Avastin, for the treatment of adenocarcinoma, but not squamous cell carcinoma, in human lung cancers. Our finding may improve the usage of biomarkers to distinguish these two poorly differentiated tumor types, sharing similar histologic features.

The Axin1 scaffold protein promotes formation of a degradation complex for c-Myc

Expression of the c-Myc proto-oncoprotein is tightly regulated in normal cells. Phosphorylation at two conserved residues, threonine58 (T58) and serine62 (S62), regulates c-Myc protein stability. In cancer cells, c-Myc can become aberrantly stabilized associated with altered T58 and S62 phosphorylation. A complex signalling cascade involving GSK3beta kinase, the Pin1 prolyl isomerase, and the PP2A-B56alpha phosphatase controls phosphorylation at these sites. We report here a novel role for the tumour suppressor scaffold protein Axin1 in facilitating the formation of a degradation complex for c-Myc containing GSK3beta, Pin1, and PP2A-B56alpha. Although knockdown of Axin1 decreases the association of c-Myc with these proteins, reduces T58 and enhances S62 phosphorylation, and increases c-Myc stability, acute expression of Axin1 reduces c-Myc levels and suppresses c-Myc transcriptional activity. Moreover, the regulation of c-Myc by Axin1 is impaired in several tested cancer cell lines with known stabilization of c-Myc or loss of Axin1. This study provides critical insight into the regulation of c-Myc expression, how this can be disrupted in three cancer types, and adds to our knowledge of the tumour suppressor activity of Axin1.

Intrinsic disorder in scaffold proteins: getting more from less

Regulation, recognition and cell signaling involve the coordinated actions of many players. Signaling scaffolds, with their ability to bring together proteins belonging to common and/or interlinked pathways, play crucial roles in orchestrating numerous events by coordinating specific interactions among signaling proteins. This review examines the roles of intrinsic disorder (ID) in signaling scaffold protein function. Several well-characterized scaffold proteins with structurally and functionally characterized ID regions are used here to illustrate the importance of ID for scaffolding function. These examples include scaffolds that are mostly disordered, only partially disordered or those in which the ID resides in a scaffold partner. Specific scaffolds discussed include RNase, voltage-activated potassium channels, axin, BRCA1, GSK-3beta, p53, Ste5, titin, Fus3, BRCA1, MAP2, D-AKAP2 and AKAP250. Among the mechanisms discussed are: molecular recognition features, fly-casting, ease of encounter complex formation, structural isolation of partners, modulation of interactions between bound partners, masking of intramolecular interaction sites, maximized interaction surface per residue, toleration of high evolutionary rates, binding site overlap, allosteric modification, palindromic binding, reduced constraints for alternative splicing, efficient regulation via posttranslational modification, efficient regulation via rapid degradation, protection of normally solvent-exposed sites, enhancing the plasticity of interaction and molecular crowding. We conclude that ID can enhance scaffold function by a diverse array of mechanisms. In other words, scaffold proteins utilize several ID-facilitated mechanisms to enhance function, and by doing so, get more functionality from less structure.

SUMOylation target sites at the C terminus protect Axin from ubiquitination and confer protein stability

Axin is a scaffold protein for the beta-catenin destruction complex, and a negative regulator of canonical Wnt signaling. Previous studies implicated the six C-terminal amino acids (C6 motif) in the ability of Axin to activate c-Jun N-terminal kinase, and identified them as a SUMOylation target. Deletion of the C6 motif of mouse Axin in vivo reduced the steady-state protein level, which caused embryonic lethality. Here, we report that this deletion (Axin-DeltaC6) causes a reduced half-life in mouse embryonic fibroblasts and an increased susceptibility to ubiquitination in HEK 293T cells. We confirmed the C6 motif as a SUMOylation target in vitro, and found that mutating the C-terminal SUMOylation target residues increased the susceptibility of Axin to polyubiquitination and reduced its steady-state level. Heterologous SUMOylation target sites could replace C6 in providing this protective effect. These findings suggest that SUMOylation of the C6 motif may prevent polyubiquitination, thus increasing the stability of Axin. Although C6 deletion also caused increased association of Axin with Dvl-1, this interaction was not altered by mutating the lysine residues in C6, nor could heterologous SUMOylation motifs replace the C6 motif in this assay. Therefore, some other specific property of the C6 motif seems to reduce the interaction of Axin with Dvl-1.