Protein phosphatase 1 regulates assembly and function of the beta-catenin degradation complex

Co-encapsulation of norcantharidin prodrugs and lomitapide in nanoparticles to regulate CCL4 expression by inhibiting Wnt/β-catenin pathway for improved anti-tumor immunotherapy

In the absence of tumor antigen specificity, direct chemokine administration carries the risk of significant "on-target, off-tumor" toxicities, highlighting the need for small-molecule approaches with reduced immunogenicity. This study investigates the synergistic potential of norcantharidin (NCTD) and lomitapide (lomi) in selectively restoring CCL4 expression by deactivating the tumor intrinsic β-catenin pathway. Due to its similar lipophilicity to lomi and potential to suppress β-catenin, NCTD prodrug (C12) was selected to be co-encapsulated with lomi in a nanoparticle-mediated co-delivery system (NP"C12 + lomi"). The NP"C12 + lomi" formulation exhibited a high encapsulation rate, uniform particle size, and suitability for therapeutic use. It effectively inhibited the proliferation of 4T1 cells and restored CCL4 expression. In both primary breast tumor and surgically resected tumor mouse models, NP"C12 + lomi" significantly increased the proportion of CD8+ cells in primary tumors, blood, and lung metastases, approximately doubling their presence. This led to a prolongation of median survival in mice to 59 days. Furthermore, when combined with an immune checkpoint inhibitor, NP"C12 + lomi" substantially inhibited tumor growth and lung metastasis without affecting body weight or causing major tissue or organ damage. This was attributed to the controlled dissociation of the nanoparticle and the subsequent modulation of C12 and lomi, which mitigated CCL4-related toxicity. This study provides valuable insights into the safe production of chemokines using a small-molecule pair through a nanosystem and presents a robust chemo-immunological cascade therapy strategy, demonstrating significant efficacy against malignant metastatic tumors.

Wnt signalosome assembly is governed by conformational flexibility of Axin and by the AP2 clathrin adaptor

Wnt signal transduction relies on the direct inhibition of GSK3 by phosphorylated PPPSPxS motifs within the cytoplasmic tail of the LRP6 co-receptor. How GSK3 is recruited to LRP6 remains unclear. Here, we use nuclear magnetic resonance spectroscopy to identify the membrane-proximal PPPSPxS motif and its flanking sequences as the primary binding site for both Axin and GSK3, and an intrinsically disordered segment of Axin as its LRP6-interacting region (LIR). Co-immunoprecipitation and CRISPR-engineered mutations in endogenous Axin indicate that its docking at LRP6 is antagonized by a phospho-dependent foldback within LIR and by a PRTxR motif that allows Axin and GSK3 to form a multi-pronged interaction which favors their detachment from LRP6. Crucially, signaling by LRP6 also depends on its binding to the AP2 clathrin adaptor. We propose that the Wnt-driven clustering of LRP6 within clathrin-coated locales allows the Axin-GSK complex to dock at adjacent LRP6 molecules, while also exposing it to co-targeted kinases that change its activity in Wnt signal transduction.

Wnt signaling inhibits casein kinase 1α activity by modulating its interaction with protein phosphatase 2A

The mechanism by which Wnt signaling, an essential pathway controlling development and disease, stabilizes β-catenin has been a subject of debate over the last four decades. Casein kinase 1α (CK1α) functions as a pivotal negative regulator of this signaling pathway, initiating the events that destabilize β-catenin. However, whether and how CK1α activity is regulated in Wnt-off and Wnt-on states remains poorly understood. We now show that CK1α activity requires its association with the α catalytic subunit of protein phosphatase 2A (PPP2CA) on AXIN, the scaffold protein of the β-catenin destruction complex. Wnt stimulation induces the dissociation of PPP2CA from CK1α, resulting in CK1α autophosphorylation and its consequent inactivation. Moreover, autophosphorylated CK1α is enriched in a subset of colorectal cancers (CRCs) harboring constitutive Wnt activation. Our findings identify a mechanism by which Wnt stimulation inactivates CK1α, filling a critical gap in our understanding of Wnt signaling, with relevance for CRC.

MerlinS13 phosphorylation regulates meningioma Wnt signaling and magnetic resonance imaging features

Meningiomas are associated with inactivation of NF2/Merlin, but approximately one-third of meningiomas with favorable clinical outcomes retain Merlin expression. Biochemical mechanisms underlying Merlin-intact meningioma growth are incompletely understood, and non-invasive biomarkers that may be used to guide treatment de-escalation or imaging surveillance are lacking. Here, we use single-cell RNA sequencing, proximity-labeling proteomic mass spectrometry, mechanistic and functional approaches, and magnetic resonance imaging (MRI) across meningioma xenografts and patients to define biochemical mechanisms and an imaging biomarker that underlie Merlin-intact meningiomas. We find Merlin serine 13 (S13) dephosphorylation drives meningioma Wnt signaling and tumor growth by attenuating inhibitory interactions with β-catenin and activating the Wnt pathway. MRI analyses show Merlin-intact meningiomas with S13 phosphorylation and favorable clinical outcomes are associated with high apparent diffusion coefficient (ADC). These results define mechanisms underlying a potential imaging biomarker that could be used to guide treatment de-escalation or imaging surveillance for patients with Merlin-intact meningiomas.

Unraveling Cancer's Wnt Signaling: Dynamic Control through Protein Kinase Regulation

Since the initial identification of oncogenic Wnt in mice and Drosophila, the Wnt signaling pathway has been subjected to thorough and extensive investigation. Persistent activation of Wnt signaling exerts diverse cancer characteristics, encompassing tumor initiation, tumor growth, cell senescence, cell death, differentiation, and metastasis. Here we review the principal signaling mechanisms and the regulatory influence of pathway-intrinsic and extrinsic kinases on cancer progression. Additionally, we underscore the divergences and intricate interplays of the canonical and non-canonical Wnt signaling pathways and their critical influence in cancer pathophysiology, exhibiting both growth-promoting and growth-suppressing roles across diverse cancer types.

Splice variants of CK1α and CK1α-like: Comparative analysis of subcellular localization, kinase activity, and function in the Wnt signaling pathway

Members of the casein kinase 1 (CK1) family are important regulators of multiple signaling pathways. CK1α is a well-known negative regulator of the Wnt/β-catenin pathway, which promotes the degradation of β-catenin via its phosphorylation of Ser45. In contrast, the closest paralog of CK1α, CK1α-like, is a poorly characterized kinase of unknown function. In this study, we show that the deletion of CK1α, but not CK1α-like, resulted in a strong activation of the Wnt/β-catenin pathway. Wnt-3a treatment further enhanced the activation, which suggests there are at least two modes, a CK1α-dependent and Wnt-dependent, of β-catenin regulation. Rescue experiments showed that only two out of ten naturally occurring splice CK1α/α-like variants were able to rescue the augmented Wnt/β-catenin signaling caused by CK1α deficiency in cells. Importantly, the ability to phosphorylate β-catenin on Ser45 in the in vitro kinase assay was required but not sufficient for such rescue. Our compound CK1α and GSK3α/β KO models suggest that the additional nonredundant function of CK1α in the Wnt pathway beyond Ser45-β-catenin phosphorylation includes Axin phosphorylation. Finally, we established NanoBRET assays for the three most common CK1α splice variants as well as CK1α-like. Target engagement data revealed comparable potency of known CK1α inhibitors for all CK1α variants but not for CK1α-like. In summary, our work brings important novel insights into the biology of CK1α, including evidence for the lack of redundancy with other CK1 kinases in the negative regulation of the Wnt/β-catenin pathway at the level of β-catenin and Axin.

Expression of WNT Signaling Genes in the Dorsolateral Prefrontal Cortex in Schizophrenia

Gene expression alterations in postmortem schizophrenia tissue are well-documented and are influenced by genetic, medication, and epigenetic factors. The Wingless/Integrated (WNT) signaling pathway, critical for cell growth and development, is involved in various cellular processes including neurodevelopment and synaptic plasticity. Despite its importance, WNT signaling remains understudied in schizophrenia, a disorder characterized by metabolic and bioenergetic defects in cortical regions. In this study, we examined the gene expression of 10 key WNT signaling pathway transcripts: IQGAP1, CTNNβ1, GSK3β, FOXO1, LRP6, MGEA5, TCF4, βTRC, PPP1Cβ, and DVL2 in the dorsolateral prefrontal cortex (DLPFC) using postmortem tissue from schizophrenia subjects (n = 20, 10 males, 10 females) compared to age, pH, and postmortem interval (PMI)-matched controls (n = 20, 10 males, 10 females). Employing the R-shiny application Kaleidoscope, we conducted in silico "lookup" studies from published transcriptomic datasets to examine cell- and region-level expression of these WNT genes. In addition, we investigated the impact of antipsychotics on the mRNA expression of the WNT genes of interest in rodent brain transcriptomic datasets. Our findings revealed no significant changes in region-level WNT transcript expression; however, analyses of previously published cell-level datasets indicated alterations in WNT transcript expression and antipsychotic-specific modulation of certain genes. These results suggest that WNT signaling transcripts may be variably expressed at the cellular level and influenced by antipsychotic treatment, providing novel insights into the role of WNT signaling in the pathophysiology of schizophrenia.

Semaphorin Receptors Antagonize Wnt Signaling Through Beta-Catenin Degradation

Precise control of morphogen signaling levels is essential for proper development. An outstanding question is: what mechanisms ensure proper morphogen activity and correct cellular responses? Previous work has identified Semaphorin (SEMA) receptors, Neuropilins (NRPs) and Plexins (PLXNs), as positive regulators of the Hedgehog (HH) signaling pathway. Here, we provide evidence that NRPs and PLXNs antagonize Wnt signaling in both fibroblasts and epithelial cells. Further, Nrp1/2 deletion in fibroblasts results in elevated baseline Wnt pathway activity and increased maximal responses to Wnt stimulation. Notably, and in contrast to HH signaling, SEMA receptor-mediated Wnt antagonism is independent of primary cilia. Mechanistically, PLXNs and NRPs act downstream of Dishevelled (DVL) to destabilize β-catenin (CTNNB1) in a proteosome-dependent manner. Further, NRPs, but not PLXNs, act in a GSK3β/CK1-dependent fashion to antagonize Wnt signaling, suggesting distinct repressive mechanisms for these SEMA receptors. Overall, this study identifies SEMA receptors as novel Wnt pathway antagonists that may also play larger roles integrating signals from multiple inputs.

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.

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

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

The WNT/β-catenin system in chronic kidney disease-mineral bone disorder syndrome

BACKGROUND

The WNT/β-catenin system is an evolutionarily conserved signaling pathway that plays a crucial role in morphogenesis and cell tissue formation during embryogenesis. Although usually suppressed in adulthood, it can be reactivated during organ damage and regeneration. Transient activation of the WNT/β-catenin pathway stimulates tissue regeneration after acute kidney injury, while persistent (uncontrolled) activation can promote the development of chronic kidney disease (CKD). CKD-MBD is a clinical syndrome that develops with systemic mineral and bone metabolism disorders caused by CKD, characterized by abnormal bone mineral metabolism and/or extraosseous calcification, as well as cardiovascular disease associated with CKD, including vascular stiffness and calcification.

OBJECTIVE

This paper aims to comprehensively review the WNT/β-catenin signaling pathway in relation to CKD-MBD, focusing on its components, regulatory molecules, and regulatory mechanisms. Additionally, this review highlights the challenges and opportunities for using small molecular compounds to target the WNT/β-catenin signaling pathway in CKD-MBD therapy.

METHODS

We conducted a comprehensive literature review using various scientific databases, including PubMed, Scopus, and Web of Science, to identify relevant articles. We searched for articles that discussed the WNT/β-catenin signaling pathway, CKD-MBD, and their relationship. We also reviewed articles that discussed the components of the WNT/β-catenin signaling pathway, its regulatory molecules, and regulatory mechanisms.

RESULTS

The WNT/β-catenin signaling pathway plays a crucial role in CKD-MBD by promoting vascular calcification and bone mineral metabolism disorders. The pathway's components include WNT ligands, Frizzled receptors, and LRP5/6 co-receptors, which initiate downstream signaling cascades leading to the activation of β-catenin. Several regulatory molecules, including GSK-3β, APC, and Axin, modulate β-catenin activation. The WNT/β-catenin signaling pathway also interacts with other signaling pathways, such as the BMP pathway, to regulate CKD-MBD.

CONCLUSIONS

The WNT/β-catenin signaling pathway is a potential therapeutic target for CKD-MBD. Small molecular compounds that target the components or regulatory molecules of the pathway may provide a promising approach to treat CKD-MBD. However, more research is needed to identify safe and effective compounds and to determine the optimal dosages and treatment regimens.

A novel RIP1-mediated canonical WNT signaling pathway that promotes colorectal cancer metastasis via β -catenin stabilization-induced EMT

RIP1 (receptor-interacting protein kinase 1) is an important component of TNF-α signaling that contributes to various pathological effects. Here, we revealed new potential roles of RIP1 in controlling WNT/β-catenin canonical signaling to enhance metastasis of colorectal cancer (CRC). First, we showed that WNT3A treatment sequentially increased the expression of RIP1 and β-catenin. Immunohistochemical analyses of human CRC tissue arrays consisting of normal, primary, and metastatic cancers indicated that elevated RIP1 expression might be related to β-catenin expression, carcinogenesis, and metastasis. Intravenous injection of RIP1 over-expressed CRC cells into mice has demonstrated that RIP1 may promote metastasis. Immunoprecipitation (IP) results indicated that WNT3A treatment induces direct binding between RIP1 and β-catenin, and that this stabilizes the β-catenin protein in a manner that depends on the regulation of RIP1 ubiquitination via downregulation of the E3 ligase, cIAP1/2. Elimination of cIAP1/2 expression and inhibition of its ubiquitinase activity enhance WNT3A-induced RIP1 and β-catenin protein expression and binding, which stimulates endothelial-mesenchymal transition (EMT) induction to enhance the migration and invasion of CRC cells in vitro. The results of the in vitro binding assay and IP of exogenous RIP1-containing CRC cells additionally verified the direct binding of RIP1 and β-catenin. RIP1 expression can destroy the β-catenin-β-TrCP complex. Taken together, these results suggest a novel EMT-enhancing role of RIP1 in the WNT pathway and suggest a new canonical WNT3A-RIP1-β-catenin pathway that contributes to CRC malignancy by promoting EMT.

Regulation and role of the PP2A-B56 holoenzyme family in cancer

Protein phosphatase 2A (PP2A) inactivation is common in cancer, leading to sustained activation of pro-survival and growth-promoting pathways. PP2A consists of a scaffolding A-subunit, a catalytic C-subunit, and a regulatory B-subunit. The functional complexity of PP2A holoenzymes arises mainly through the vast repertoire of regulatory B-subunits, which determine both their substrate specificity and their subcellular localization. Therefore, a major challenge for developing more effective therapeutic strategies for cancer is to identify the specific PP2A complexes to be targeted. Of note, the development of small molecules specifically directed at PP2A-B56α has opened new therapeutic avenues in both solid and hematological tumors. Here, we focus on the B56/PR61 family of PP2A regulatory subunits, which have a central role in directing PP2A tumor suppressor activity. We provide an overview of the mechanisms controlling the formation and regulation of these complexes, the pathways they control, and the mechanisms underlying their deregulation in cancer.

Identification of protein phosphatase 4 catalytic subunit as a Wnt promoting factor in pan-cancer and Xenopus early embryogenesis

Protein Phosphatase 4 Catalytic Subunit (PPP4C) is an evolutionarily conserved protein involved in multiple biological and pathological events, including embryogenesis, organogenesis, cellular homeostasis, and oncogenesis. However, the detailed mechanisms underlying these processes remain largely unknown. Thus, we investigated the potential correlation between PPP4C and biological processes (BPs) and canonical Wnt signaling using pan-cancer analysis and Xenopus laevis (X. laevis) embryo model. Our results indicate that PPP4C is a potential biomarker for specific cancer types due to its high diagnostic accuracy and significant prognostic correlation. Furthermore, in multiple cancer types, PPP4C-related differentially expressed genes (DEGs) were significantly enriched in pattern specification, morphogenesis, and canonical Wnt activation. Consistently, perturbation of Ppp4c in X. laevis embryos interfered with normal embryogenesis and canonical Wnt responses. Moreover, biochemical analysis of X. laevis embryos demonstrated that both endogenous and exogenous Ppp4c negatively regulated AXIN1 (Wnt inhibitor) abundance. This study provides novel insights into PPP4C roles in pattern specification and Wnt activation. The similarities in BPs and Wnt signaling regulation regarding PPP4C support the intrinsic link between tumorigenesis and early embryogenesis.

MerlinS13 phosphorylation controls meningioma Wnt signaling and magnetic resonance imaging features

Meningiomas are the most common primary intracranial tumors and are associated with inactivation of the tumor suppressor NF2/Merlin, but one-third of meningiomas retain Merlin expression and typically have favorable clinical outcomes. Biochemical mechanisms underlying Merlin-intact meningioma growth are incompletely understood, and non-invasive biomarkers that predict meningioma outcomes and could be used to guide treatment de-escalation or imaging surveillance of Merlin-intact meningiomas are lacking. Here we integrate single-cell RNA sequencing, proximity-labeling proteomic mass spectrometry, mechanistic and functional approaches, and magnetic resonance imaging (MRI) across meningioma cells, xenografts, and human patients to define biochemical mechanisms and an imaging biomarker that distinguish Merlin-intact meningiomas with favorable clinical outcomes from meningiomas with unfavorable clinical outcomes. We find Merlin drives meningioma Wnt signaling and tumor growth through a feed-forward mechanism that requires Merlin dephosphorylation on serine 13 (S13) to attenuate inhibitory interactions with β-catenin and activate the Wnt pathway. Meningioma MRI analyses of xenografts and human patients show Merlin-intact meningiomas with S13 phosphorylation and favorable clinical outcomes are associated with high apparent diffusion coefficient (ADC) on diffusion-weighted imaging. In sum, our results shed light on Merlin posttranslational modifications that regulate meningioma Wnt signaling and tumor growth in tumors without NF2/Merlin inactivation. To translate these findings to clinical practice, we establish a non-invasive imaging biomarker that could be used to guide treatment de-escalation or imaging surveillance for patients with favorable meningiomas.

WNT/β-catenin pathway is a key regulator of cardiac function and energetic metabolism

The WNT/β-catenin pathway is a master regulator of cardiac development and growth, and its activity is low in healthy adult hearts. However, even this low activity is essential for maintaining normal heart function. Acute activation of the WNT/β-catenin signaling cascade is considered to be cardioprotective after infarction through the upregulation of prosurvival genes and reprogramming of metabolism. Chronically high WNT/β-catenin pathway activity causes profibrotic and hypertrophic effects in the adult heart. New data suggest more complex functions of β-catenin in metabolic maturation of the perinatal heart, establishing an adult pattern of glucose and fatty acid utilization. Additionally, low basal activity of the WNT/β-catenin cascade maintains oxidative metabolism in the adult heart, and this pathway is reactivated by physiological or pathological stimuli to meet the higher energy needs of the heart. This review summarizes the current state of knowledge of the organization of canonical WNT signaling and its function in cardiogenesis, heart maturation, adult heart function, and remodeling. We also discuss the role of the WNT/β-catenin pathway in cardiac glucose, lipid metabolism, and mitochondrial physiology.

Inhibitory effect of selected Indian honey on colon cancer cell growth by inducing apoptosis and targeting the β-catenin/Wnt pathway

Colon cancer is the most prevalent cause of death from cancer across the globe. Although chemotherapy drugs are predominantly used, their toxicity always remains a cause of concern. As an alternative to synthetic drugs, natural compounds or nutraceuticals are comparatively less toxic. Honey is widely used across different cultures as an alternative form of medicine. It represents a prominent source of plant-phenolic compounds and there is demonstrable evidence of its anti-oxidant and anti-microbial activities. The aim of the present work was to investigate the anti-proliferative effect of some Indian honeys and analyze their mechanism of action in colon cancer. In order to establish the composition-activity relationship, we evaluated the bioactive components present in selected honey samples by GC-MS and HPLC analysis. Indian honey samples showed a significant inhibitory impact on cell growth by restricting cell proliferation, causing apoptosis, and restricting the cell cycle in the G₂/M phase specifically for colon cancer cells. The apoptotic activities, as imparted by the honey samples, were established by Annexin V/PI staining, real-time PCR, and immunoblot analyses. The treated cells showed increased expressions of p53 and caspases 3, 8, and 9, thus indicating the involvement of both extrinsic and intrinsic apoptotic pathways. The honey samples were also found to inhibit the β-catenin/Wnt pathway. In the next phase of the study, the efficacy of these honey samples was evaluated in colon carcinoma induced SD-rats. Overall, these findings demonstrated that selected Indian honeys could be established as effective nutraceuticals for the prevention as well as cure of colon cancer.

Regulation of signaling pathways in hair follicle stem cells

Hair follicle stem cells (HFSCs) reside in the bulge region of the outer root sheath of the hair follicle. They are considered slow-cycling cells that are endowed with multilineage differentiation potential and superior proliferative capacity. The normal morphology and periodic growth of HFSCs play a significant role in normal skin functions, wound repair and skin regeneration. The HFSCs involved in these pathophysiological processes are regulated by a series of cell signal transduction pathways, such as lymphoid enhancer factor/T-cell factor, Wnt/β-catenin, transforming growth factor-β/bone morphogenetic protein, Notch and Hedgehog. The mechanisms of the interactions among these signaling pathways and their regulatory effects on HFSCs have been previously studied, but many mechanisms are still unclear. This article reviews the regulation of hair follicles, HFSCs and related signaling pathways, with the aims of summarizing previous research results, revealing the regulatory mechanisms of HFSC proliferation and differentiation and providing important references and new ideas for treating clinical diseases.

Transgenic Drosophila lines for LexA-dependent gene and growth regulation

Conditional expression of short hairpin RNAs with binary genetic systems is an indispensable tool for studying gene function. Addressing mechanisms underlying cell-cell communication in vivo benefits from simultaneous use of 2 independent gene expression systems. To complement the abundance of existing Gal4/UAS-based resources in Drosophila, we and others have developed LexA/LexAop-based genetic tools. Here, we describe experimental and pedagogical advances that promote the efficient conversion of Drosophila Gal4 lines to LexA lines, and the generation of LexAop-short hairpin RNA lines to suppress gene function. We developed a CRISPR/Cas9-based knock-in system to replace Gal4 coding sequences with LexA, and a LexAop-based short hairpin RNA expression vector to achieve short hairpin RNA-mediated gene silencing. We demonstrate the use of these approaches to achieve targeted genetic loss-of-function in multiple tissues. We also detail our development of secondary school curricula that enable students to create transgenic flies, thereby magnifying the production of well-characterized LexA/LexAop lines for the scientific community. The genetic tools and teaching methods presented here provide LexA/LexAop resources that complement existing resources to study intercellular communication coordinating metazoan physiology and development.

Functional requirements of protein kinases and phosphatases in the development of the Drosophila melanogaster wing

Protein kinases and phosphatases constitute a large family of conserved enzymes that control a variety of biological processes by regulating the phosphorylation state of target proteins. They play fundamental regulatory roles during cell cycle progression and signaling, among other key aspects of multicellular development. The complement of protein kinases and phosphatases includes approximately 326 members in Drosophila, and they have been the subject of several functional screens searching for novel components of signaling pathways and regulators of cell division and survival. These approaches have been carried out mostly in cell cultures using RNA interference to evaluate the contribution of each protein in different functional assays, and have contributed significantly to assign specific roles to the corresponding genes. In this work we describe the results of an evaluation of the Drosophila complement of kinases and phosphatases using the wing as a system to identify their functional requirements in vivo. We also describe the results of several modifying screens aiming to identify among the set of protein kinases and phosphatases additional components or regulators of the activities of the Epidermal Growth Factor and Insulin receptors signaling pathways.

The Wnt signaling pathway in tumorigenesis, pharmacological targets, and drug development for cancer therapy

Wnt signaling was initially recognized to be vital for tissue development and homeostasis maintenance. Further studies revealed that this pathway is also important for tumorigenesis and progression. Abnormal expression of signaling components through gene mutation or epigenetic regulation is closely associated with tumor progression and poor prognosis in several tissues. Additionally, Wnt signaling also influences the tumor microenvironment and immune response. Some strategies and drugs have been proposed to target this pathway, such as blocking receptors/ligands, targeting intracellular molecules, beta-catenin/TCF4 complex and its downstream target genes, or tumor microenvironment and immune response. Here we discuss the roles of these components in Wnt signaling pathway in tumorigenesis and cancer progression, the underlying mechanisms that is responsible for the activation of Wnt signaling, and a series of drugs targeting the Wnt pathway provide multiple therapeutic values. Although some of these drugs exhibit exciting anti-cancer effect, clinical trials and systematic evaluation should be strictly performed along with multiple-omics technology.

Wnt/β-catenin signaling in cancers and targeted therapies

Wnt/β-catenin signaling has been broadly implicated in human cancers and experimental cancer models of animals. Aberrant activation of Wnt/β-catenin signaling is tightly linked with the increment of prevalence, advancement of malignant progression, development of poor prognostics, and even ascendence of the cancer-associated mortality. Early experimental investigations have proposed the theoretical potential that efficient repression of this signaling might provide promising therapeutic choices in managing various types of cancers. Up to date, many therapies targeting Wnt/β-catenin signaling in cancers have been developed, which is assumed to endow clinicians with new opportunities of developing more satisfactory and precise remedies for cancer patients with aberrant Wnt/β-catenin signaling. However, current facts indicate that the clinical translations of Wnt/β-catenin signaling-dependent targeted therapies have faced un-neglectable crises and challenges. Therefore, in this study, we systematically reviewed the most updated knowledge of Wnt/β-catenin signaling in cancers and relatively targeted therapies to generate a clearer and more accurate awareness of both the developmental stage and underlying limitations of Wnt/β-catenin-targeted therapies in cancers. Insights of this study will help readers better understand the roles of Wnt/β-catenin signaling in cancers and provide insights to acknowledge the current opportunities and challenges of targeting this signaling in cancers.

Tau: A Signaling Hub Protein

Over four decades ago, in vitro experiments showed that tau protein interacts with and stabilizes microtubules in a phosphorylation-dependent manner. This observation fueled the widespread hypotheses that these properties extend to living neurons and that reduced stability of microtubules represents a major disease-driving event induced by pathological forms of tau in Alzheimer’s disease and other tauopathies. Accordingly, most research efforts to date have addressed this protein as a substrate, focusing on evaluating how specific mutations, phosphorylation, and other post-translational modifications impact its microtubule-binding and stabilizing properties. In contrast, fewer efforts were made to illuminate potential mechanisms linking physiological and disease-related forms of tau to the normal and pathological regulation of kinases and phosphatases. Here, we discuss published work indicating that, through interactions with various kinases and phosphatases, tau may normally act as a scaffolding protein to regulate phosphorylation-based signaling pathways. Expanding on this concept, we also review experimental evidence linking disease-related tau species to the misregulation of these pathways. Collectively, the available evidence supports the participation of tau in multiple cellular processes sustaining neuronal and glial function through various mechanisms involving the scaffolding and regulation of selected kinases and phosphatases at discrete subcellular compartments. The notion that the repertoire of tau functions includes a role as a signaling hub should widen our interpretation of experimental results and increase our understanding of tau biology in normal and disease conditions.

Zooming in on the WNT/CTNNB1 Destruction Complex: Functional Mechanistic Details with Implications for Therapeutic Targeting

WNT/CTNNB1 signaling is crucial for balancing cell proliferation and differentiation in all multicellular animals. CTNNB1 accumulation is the hallmark of WNT/CTNNB1 pathway activation and the key downstream event in both a physiological and an oncogenic context. In the absence of WNT stimulation, the cytoplasmic and nuclear levels of CTNNB1 are kept low because of its sequestration and phosphorylation by the so-called destruction complex, which targets CTNNB1 for proteasomal degradation. In the presence of WNT proteins, or as a result of oncogenic mutations, this process is impaired and CTNNB1 levels become elevated.Here we discuss recent advances in our understanding of destruction complex activity and inactivation, focusing on the individual components and interactions that ultimately control CTNNB1 turnover (in the "WNT off" situation) and stabilization (in the "WNT on" situation). We especially highlight the insights gleaned from recent quantitative, image-based studies, which paint an unprecedentedly detailed picture of the dynamic events that control destruction protein complex composition and function. We argue that these mechanistic details may reveal new opportunities for therapeutic intervention and could result in the destruction complex re-emerging as a target for therapy in cancer.

Protein phosphatase 1 regulatory subunit 1A regulates cell cycle progression in Ewing sarcoma

Ewing sarcoma (ES) is a malignant pediatric bone and soft tissue tumor. Patients with metastatic ES have a dismal outcome which has not been improved in decades. The major challenge in the treatment of metastatic ES is the lack of specific targets and rational combinatorial therapy. We recently found that protein phosphatase 1 regulatory subunit 1A (PPP1R1A) is specifically highly expressed in ES and promotes tumor growth and metastasis in ES. In the current investigation, we show that PPP1R1A regulates ES cell cycle progression in G1/S phase by down-regulating cell cycle inhibitors p21^Cip1 and p27^Kip1, which leads to retinoblastoma (Rb) protein hyperphosphorylation. In addition, we show that PPP1R1A promotes normal transcription of histone genes during cell cycle progression. Importantly, we demonstrate a synergistic/additive effect of the combinatorial therapy of PPP1R1A and insulin-like growth factor 1 receptor (IGF-1R) inhibition on decreasing ES cell proliferation and migration in vitro and limiting xenograft tumor growth and metastasis in vivo. Taken together, our findings suggest a role of PPP1R1A as an ES specific cell cycle modulator and that simultaneous targeting of PPP1R1A and IGF-1R pathways is a promising specific and effective strategy to treat both primary and metastatic ES.

APC controls Wnt-induced β-catenin destruction complex recruitment in human colonocytes

Wnt/β-catenin signaling is essential for intestinal homeostasis and is aberrantly activated in most colorectal cancers (CRC) through mutation of the tumor suppressor Adenomatous Polyposis Coli (APC). APC is an essential component of a cytoplasmic protein complex that targets β-catenin for destruction. Following Wnt ligand presentation, this complex is inhibited. However, a role for APC in this inhibition has not been shown. Here, we utilized Wnt3a-beads to locally activate Wnt co-receptors. In response, the endogenous β-catenin destruction complex reoriented toward the local Wnt cue in CRC cells with full-length APC, but not if APC was truncated or depleted. Non-transformed human colon epithelial cells displayed similar Wnt-induced destruction complex localization which appeared to be dependent on APC and less so on Axin. Our results expand the current model of Wnt/β-catenin signaling such that in response to Wnt, the β-catenin destruction complex: (1) maintains composition and binding to β-catenin, (2) moves toward the plasma membrane, and (3) requires full-length APC for this relocalization.

Exploring major signaling cascades in melanomagenesis: a rationale route for targetted skin cancer therapy

Although most melanoma cases may be treated by surgical intervention upon early diagnosis, a significant portion of patients can still be refractory, presenting low survival rates within 5 years after the discovery of the illness. As a hallmark, melanomas are highly prone to evolve into metastatic sites. Moreover, melanoma tumors are highly resistant to most available drug therapies and their incidence have increased over the years, therefore leading to public health concerns about the development of novel therapies. Therefore, researches are getting deeper in unveiling the mechanisms by which melanoma initiation can be triggered and sustained. In this context, important progress has been achieved regarding the roles and the impact of cellular signaling pathways in melanoma. This knowledge has provided tools for the development of therapies based on the intervention of signal(s) promoted by these cascades. In this review, we summarize the importance of major signaling pathways (mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinase (PI3K)-Akt, Wnt, nuclear factor κ-light-chain-enhancer of activated B cell (NF-κB), Janus kinase (JAK)-signal transducer and activator of transcription (STAT), transforming growth factor β (TGF-β) and Notch) in skin homeostasis and melanoma progression. Available and developing melanoma therapies interfering with these signaling cascades are further discussed.

RIF1 promotes tumor growth and cancer stem cell-like traits in NSCLC by protein phosphatase 1-mediated activation of Wnt/β-catenin signaling

Wnt/β-catenin signaling is essential for proliferation and maintenance of cancer stem cell-like traits of various cancer cells. In non-small-cell lung carcinoma (NSCLC), the mechanisms underlying the hyperactivation of Wnt signaling remain unclear, as mutations in APC and β-catenin genes are rare in NSCLC. RIF1 has been shown upregulated in breast and cervical cancer, this study intends to find out the potential effects of the expression and biological functions of RIF1 in NSCLC. Here we revealed that RIF1 was highly expressed in NCSLC at both mRNA and protein levels. RIF1 expression was significantly associated with clinical stage (P < 0.05) and prognosis (P < 0.001) of NSCLC patients. RIF1 knockdown inhibited NSCLC cell growth in vitro and in vivo, whereas overexpression of RIF1 in NSCLC cell lines promoted cell growth, cell cycle progression and cancer stem cell (CSC)-like properties via promoting PP1-AXIN interaction and thereby activating Wnt/β-catenin signaling. Inhibition of PP1 in RIF1-overexpressed cells counteracted the effects of RIF1 on cell growth and CSC-like phenotype, as well as the Wnt/β-catenin signaling. RIF1 expression was positively correlated with β-catenin at the protein level in 32 NSCLC tissues. RIF1 expression closely related to MYC (r = 0.28, P < 0.001) and CCND1 (r = 0.14, P < 0.01) expression at the mRNA level in cohorts of The Cancer Genome Atlas (TCGA). These results indicated that RIF1 had an oncogenic role as a novel positive regulator of Wnt/β-catenin signaling by directing PP1 to dephosphorylate AXIN; this novel mechanism may present a new therapeutic target for NSCLC.

The impact of phosphatases on proliferative and survival signaling in cancer

The dynamic and stringent coordination of kinase and phosphatase activity controls a myriad of physiologic processes. Aberrations that disrupt the balance of this interplay represent the basis of numerous diseases. For a variety of reasons, early work in this area portrayed kinases as the dominant actors in these signaling events with phosphatases playing a secondary role. In oncology, these efforts led to breakthroughs that have dramatically altered the course of certain diseases and directed vast resources toward the development of additional kinase-targeted therapies. Yet, more recent scientific efforts have demonstrated a prominent and sometimes driving role for phosphatases across numerous malignancies. This maturation of the phosphatase field has brought with it the promise of further therapeutic advances in the field of oncology. In this review, we discuss the role of phosphatases in the regulation of cellular proliferation and survival signaling using the examples of the MAPK and PI3K/AKT pathways, c-Myc and the apoptosis machinery. Emphasis is placed on instances where these signaling networks are perturbed by dysregulation of specific phosphatases to favor growth and persistence of human cancer.

Combined Proteomic and In Silico Target Identification Reveal a Role for 5-Lipoxygenase in Developmental Signaling Pathways

Identification and validation of the targets of bioactive small molecules identified in cell-based screening is challenging and often meets with failure, calling for the development of new methodology. We demonstrate that a combination of chemical proteomics with in silico target prediction employing the SPiDER method may provide efficient guidance for target candidate selection and prioritization for experimental in-depth evaluation. We identify 5-lipoxygenase (5-LO) as the target of the Wnt pathway inhibitor Lipoxygenin. Lipoxygenin is a non-redox 5-LO inhibitor, modulates the β-catenin-5-LO complex and induces reduction of both β-catenin and 5-LO levels in the nucleus. Lipoxygenin and the structurally unrelated 5-LO inhibitor CJ-13,610 promote cardiac differentiation of human induced pluripotent stem cells and inhibit Hedgehog, TGF-β, BMP, and Activin A signaling, suggesting an unexpected and yet unknown role of 5-LO in these developmental pathways.

Axin phosphorylation in both Wnt-off and Wnt-on states requires the tumor suppressor APC

The aberrant activation of Wnt signal transduction initiates the development of 90% of colorectal cancers, the majority of which arise from inactivation of the tumor suppressor Adenomatous polyposis coli (APC). In the classical model for Wnt signaling, the primary role of APC is to act, together with the concentration-limiting scaffold protein Axin, in a “destruction complex” that directs the phosphorylation and consequent proteasomal degradation of the transcriptional activator β-catenin, thereby preventing signaling in the Wnt-off state. Following Wnt stimulation, Axin is recruited to a multiprotein “signalosome” required for pathway activation. Whereas it is well-documented that APC is essential in the destruction complex, APC’s role in this complex remains elusive. Here, we demonstrate in Drosophila that Axin exists in two distinct phosphorylation states in Wnt-off and Wnt-on conditions, respectively, that underlie its roles in the destruction complex and signalosome. These two Axin phosphorylation states are catalyzed by glycogen synthase kinase 3 (GSK3), and unexpectedly, completely dependent on APC in both unstimulated and Wnt-stimulated conditions. In a major revision of the classical model, we show that APC is essential not only in the destruction complex, but also for the rapid transition in Axin that occurs after Wnt stimulation and Axin’s subsequent association with the Wnt co-receptor LRP6/Arrow, one of the earliest steps in pathway activation. We propose that this novel requirement for APC in Axin regulation through phosphorylation both prevents signaling in the Wnt-off state and promotes signaling immediately following Wnt stimulation.

The Wnt signal transduction pathway directs fundamental cellular processes during development and in homeostasis. Wnt signaling is deregulated in 90% of colorectal cancers, most of which are triggered by inactivation of the tumor suppressor Adenomatous polyposis coli (APC). In the classical model, APC’s sole role in Wnt signaling is to target the transcriptional coactivator β-catenin for phosphorylation and subsequent degradation, and thereby to inhibit signaling in the unstimulated state. However, the mechanisms by which APC functions remain unknown. Herein, we provide evidence in Drosophila that supports a major role for APC in the direct regulation of the scaffold protein Axin in both Wnt-on and Wnt-off conditions. Our results indicate that APC promotes Axin phosphorylation, which is required not only to inhibit signaling in the unstimulated state, but also to activate signaling following Wnt stimulation. These unanticipated findings support a more active and multifaceted role for APC in Wnt signaling than previously known, and force revision of the current model for APC function.

Protein phosphatase 1 regulatory subunit 1A in ewing sarcoma tumorigenesis and metastasis

Protein phosphatase inhibitors are often considered as tumor promoters. Protein phosphatase 1 regulatory subunit 1A (PPP1R1A) is a potent protein phosphatase 1 (PP1) inhibitor; however, its role in tumor development is largely undefined. Here we characterize, for the first time, the functions of PPP1R1A in Ewing sarcoma (ES) pathogenesis. We found that PPP1R1A is one of the top ranked target genes of EWS/FLI, the master regulator of ES, and is upregulated by EWS/FLI via a GGAA microsatellite enhancer element. Depletion of PPP1R1A resulted in a significant decrease in oncogenic transformation and cell migration in vitro as well as xenograft tumor growth and metastasis in an orthotopic mouse model. RNA-sequencing and functional annotation analyses revealed that PPP1R1A regulates genes associated with various cellular functions including cell junction, adhesion and neurogenesis. Interestingly, we found a significant overlap of PPP1R1A-regulated gene set with that of ZEB2 and EWS, which regulates metastasis and neuronal differentiation in ES, respectively. Further studies for characterization of the molecular mechanisms revealed that activation of PPP1R1A by PKA phosphorylation at Thr35, and subsequent PP1 binding and inhibition, was required for PPP1R1A-mediated tumorigenesis and metastasis, likely by increasing the phosphorylation levels of various PP1 substrates. Furthermore, we found that a PKA inhibitor impaired ES cell proliferation, tumor growth and metastasis, which was rescued by the constitutively active PPP1R1A. Together, these results offered new insights into the role and mechanism of PPP1R1A in tumor development and identified an important kinase and phosphatase pathway, PKA/PPP1R1A/PP1, in ES pathogenesis. Our findings strongly suggest a potential therapeutic value of inhibition of the PKA/PPP1R1A/PP1 pathway in the treatment of primary and metastatic ES.

Wnt/β-catenin signaling links embryonic lung development and asthmatic airway remodeling

Embryonic lung development requires reciprocal endodermal-mesodermal interactions; mediated by various signaling proteins. Wnt/β-catenin is a signaling protein that exhibits the pivotal role in lung development, injury and repair while aberrant expression of Wnt/β-catenin signaling leads to asthmatic airway remodeling: characterized by hyperplasia and hypertrophy of airway smooth muscle cells, alveolar and vascular damage goblet cells metaplasia, and deposition of extracellular matrix; resulting in decreased lung compliance and increased airway resistance. The substantial evidence suggests that Wnt/β-catenin signaling links embryonic lung development and asthmatic airway remodeling. Here, we summarized the recent advances related to the mechanistic role of Wnt/β-catenin signaling in lung development, consequences of aberrant expression or deletion of Wnt/β-catenin signaling in expansion and progression of asthmatic airway remodeling, and linking early-impaired pulmonary development and airway remodeling later in life. Finally, we emphasized all possible recent potential therapeutic significance and future prospectives, that are adaptable for therapeutic intervention to treat asthmatic airway remodeling.

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

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

Molecular regulation and pharmacological targeting of the β-catenin destruction complex

The β‐catenin destruction complex is a dynamic cytosolic multiprotein assembly that provides a key node in Wnt signalling regulation. The core components of the destruction complex comprise the scaffold proteins axin and adenomatous polyposis coli and the Ser/Thr kinases casein kinase 1 and glycogen synthase kinase 3. In unstimulated cells, the destruction complex efficiently drives degradation of the transcriptional coactivator β‐catenin, thereby preventing the activation of the Wnt/β‐catenin pathway. Mutational inactivation of the destruction complex is a major pathway in the pathogenesis of cancer. Here, we review recent insights in the regulation of the β‐catenin destruction complex, including newly identified interaction interfaces, regulatory elements and post‐translationally controlled mechanisms. In addition, we discuss how mutations in core destruction complex components deregulate Wnt signalling via distinct mechanisms and how these findings open up potential therapeutic approaches to restore destruction complex activity in cancer cells.

Linked Articles

This article is part of a themed section on WNT Signalling: Mechanisms and Therapeutic Opportunities. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.24/issuetoc

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

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

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

p120-catenin in canonical Wnt signaling

Canonical Wnt signaling controls β-catenin protein stabilization, its translocation to the nucleus and the activation of β-catenin/Tcf-4-dependent transcription. In this review, we revise and discuss the recent results describing actions of p120-catenin in different phases of this pathway. More specifically, we comment its involvement in four different steps: (i) the very early activation of CK1ɛ, essential for Dvl-2 binding to the Wnt receptor complex; (ii) the internalization of GSK3 and Axin into multivesicular bodies, necessary for a complete stabilization of β-catenin; (iii) the activation of Rac1 small GTPase, required for β-catenin translocation to the nucleus; and (iv) the release of the inhibitory action caused by Kaiso transcriptional repressor. We integrate these new results with the previously known action of other elements in this pathway, giving a particular relevance to the responses of the Wnt pathway not required for β-catenin stabilization but for β-catenin transcriptional activity. Moreover, we discuss the possible future implications, suggesting that the two cellular compartments where β-catenin is localized, thus, the adherens junction complex and the Wnt signalosome, are more physically connected that previously thought.

Loss of tumor suppressor Merlin results in aberrant activation of Wnt/β-catenin signaling in cancer

The expression of the tumor suppressor Merlin is compromised in nervous system malignancies due to genomic aberrations. We demonstrated for the first time, that in breast cancer, Merlin protein expression is lost due to proteasome-mediated elimination. Immunohistochemical analysis of tumor tissues from patients with metastatic breast cancer revealed characteristically reduced Merlin expression. Importantly, we identified a functional role for Merlin in impeding breast tumor xenograft growth and reducing invasive characteristics. We sought to determine a possible mechanism by which Merlin accomplishes this reduction in malignant activity. We observed that breast and pancreatic cancer cells with loss of Merlin show an aberrant increase in the activity of β-catenin concomitant with nuclear localization of β-catenin. We discovered that Merlin physically interacts with β-catenin, alters the sub-cellular localization of β-catenin, and significantly reduces the protein levels of β-catenin by targeting it for degradation through the upregulation of Axin1. Consequently, restoration of Merlin inhibited β-catenin-mediated transcriptional activity in breast and pancreatic cancer cells. We also present evidence that loss of Merlin sensitizes tumor cells to inhibition by compounds that target β-catenin-mediated activity. Thus, this study provides compelling evidence that Merlin reduces the malignant activity of pancreatic and breast cancer, in part by suppressing the Wnt/β-catenin pathway. Given the potent role of Wnt/β-catenin signaling in breast and pancreatic cancer and the flurry of activity to test β-catenin inhibitors in the clinic, our findings are opportune and provide evidence for Merlin in restraining aberrant activation of Wnt/β-catenin signaling.

Synchronization by Daytime Restricted Food Access Modulates the Presence and Subcellular Distribution of β-Catenin and Its Phosphorylated Forms in the Rat Liver

β-catenin, the principal effector of the Wnt pathway, is also one of the cadherin cell adhesion molecules; therefore, it fulfills signaling and structural roles in most of the tissues and organs. It has been reported that β-catenin in the liver regulates metabolic responses such as gluconeogenesis and histological changes in response to obesity-promoting diets. The function and cellular location of β-catenin is finely modulated by coordinated sequences of phosphorylation-dephosphorylation events. In this article, we evaluated the levels and cellular localization of liver β-catenin variants, more specifically β-catenin phosphorylated in serine 33 (this phosphorylation provides recognizing sites for β-TrCP, which results in ubiquitination and posterior proteasomal degradation of β-catenin) and β-catenin phosphorylated in serine 675 (phosphorylation that enhances signaling and transcriptional activity of β-catenin through recruitment of different transcriptional coactivators). β-catenin phosphorylated in serine 33 in the nucleus shows day-night fluctuations in their expression level in the Ad Libitum group. In addition, we used a daytime restricted feeding (DRF) protocol to show that the above effects are sensitive to food access-dependent circadian synchronization. We found through western blot and immunohistochemical analyses that DRF protocol promoted (1) higher total β-catenins levels mainly associated with the plasma membrane, (2) reduced the presence of cytoplasmic β-catenin phosphorylated in serine 33, (3) an increase in nuclear β-catenin phosphorylated in serine 675, (4) differential co-localization of total β-catenins/β-catenin phosphorylated in serine 33 and total β-catenins/β-catenin phosphorylated in serine 675 at different temporal points along day and in fasting and refeeding conditions, and (5) differential liver zonation of β-catenin variants studied along hepatic acinus. In conclusion, the present data comprehensively characterize the effect food synchronization has on the presence, subcellular distribution, and liver zonation of β-catenin variants. These results are relevant to understand the set of metabolic and structural liver adaptations that are associated with the expression of the food entrained oscillator (FEO).

Anticancer activity of a monobenzyltin complex C1 against MDA-MB-231 cells through induction of Apoptosis and inhibition of breast cancer stem cells

In the present study, we examined the cytotoxic effects of Schiff base complex, [N-(3,5-dichloro-2-oxidobenzylidene)-4-chlorobenzyhydrazidato](o-methylbenzyl)aquatin(IV) chloride, and C1 on MDA-MB-231 cells and derived breast cancer stem cells from MDA-MB-231 cells. The acute toxicity experiment with compound C1 revealed no cytotoxic effects on rats. Fluorescent microscopic studies using Acridine Orange/Propidium Iodide (AO/PI) staining and flow cytometric analysis using an Annexin V probe confirmed the occurrence of apoptosis in C1-treated MDA-MB-231 cells. Compound C1 triggered intracellular reactive oxygen species (ROS) production and lactate dehydrogenase (LDH) releases in treated MDA-MB-231 cells. The Cellomics High Content Screening (HCS) analysis showed the induction of intrinsic pathways in treated MDA-MB-231 cells, and a luminescence assay revealed significant increases in caspase 9 and 3/7 activity. Furthermore, flow cytometric analysis showed that compound C1 induced G0/G1 arrest in treated MDA-MB-231 cells. Real time PCR and western blot analysis revealed the upregulation of the Bax protein and the downregulation of the Bcl-2 and HSP70 proteins. Additionally, this study revealed the suppressive effect of compound C1 against breast CSCs and its ability to inhibit the Wnt/β-catenin signaling pathways. Our results demonstrate the chemotherapeutic properties of compound C1 against breast cancer cells and derived breast cancer stem cells, suggesting that the anticancer capabilities of this compound should be clinically assessed.

Dual Roles for Membrane Association of Drosophila Axin in Wnt Signaling

Deregulation of the Wnt signal transduction pathway underlies numerous congenital disorders and cancers. Axin, a concentration-limiting scaffold protein, facilitates assembly of a “destruction complex” that prevents signaling in the unstimulated state and a plasma membrane-associated “signalosome” that activates signaling following Wnt stimulation. In the classical model, Axin is cytoplasmic under basal conditions, but relocates to the cell membrane after Wnt exposure; however, due to the very low levels of endogenous Axin, this model is based largely on examination of Axin at supraphysiological levels. Here, we analyze the subcellular distribution of endogenous Drosophila Axin in vivo and find that a pool of Axin localizes to cell membrane proximal puncta even in the absence of Wnt stimulation. Axin localization in these puncta is dependent on the destruction complex component Adenomatous polyposis coli (Apc). In the unstimulated state, the membrane association of Axin increases its Tankyrase-dependent ADP-ribosylation and consequent proteasomal degradation to control its basal levels. Furthermore, Wnt stimulation does not result in a bulk redistribution of Axin from cytoplasmic to membrane pools, but causes an initial increase of Axin in both of these pools, with concomitant changes in two post-translational modifications, followed by Axin proteolysis hours later. Finally, the ADP-ribosylated Axin that increases rapidly following Wnt stimulation is membrane associated. We conclude that even in the unstimulated state, a pool of Axin forms membrane-proximal puncta that are dependent on Apc, and that membrane association regulates both Axin levels and Axin’s role in the rapid activation of signaling that follows Wnt exposure.

Axin is a scaffold protein with essential roles in Wnt signal transduction. In the classical model, the transition from the unstimulated to stimulated state is thought to be achieved by recruitment of Axin from cytosol to plasma membrane. We find that a pool of endogenous Drosophila Axin is localized in puncta juxtaposed with the cell membrane even under basal conditions and is targeted for degradation by the ADP-ribose polymerase Tankyrase. Wnt stimulation initially results in increased Axin levels in both the cytosolic and membrane pools, which may enhance the robust activation of signaling.

Monobenzyltin Complex C1 Induces Apoptosis in MCF-7 Breast Cancer Cells through the Intrinsic Signaling Pathway and through the Targeting of MCF-7-Derived Breast Cancer Stem Cells via the Wnt/β-Catenin Signaling Pathway

Monobenzyltin Schiff base complex, [N-(3,5-dichloro-2-oxidobenzylidene)-4-chlorobenzyhydrazidato](o-methylbenzyl)aquatin(IV) chloride, C1, is an organotin non-platinum metal-based agent. The present study was conducted to investigate its effects on MCF-7 cells with respect to the induction of apoptosis and its inhibitory effect against MCF-7 breast cancer stem cells. As determined in a previous study, compound C1 revealed strong antiproliferative activity on MCF-7 cells with an IC₅₀ value of 2.5 μg/mL. Annexin V/propidium iodide staining coupled with flow cytometry indicated the induction of apoptosis in treated cells. Compound C1 induced apoptosis in MCF-7 cells and was mediated through the intrinsic pathway with a reduction in mitochondrial membrane potential and mitochondrial cytochrome c release to cytosol. Complex C1 activated caspase 9 as a result of cytochrome c release. Subsequently, western blot and real time PCR revealed a significant increase in Bax and Bad expression and a significant decrease in the expression levels of Bcl2 and HSP70. Furthermore, a flow cytometric analysis showed that treatment with compound C1 caused a significant arrest of MCF-7 cells in G0/G1 phase. The inhibitory analysis of compound C1 against derived MCF-7 stem cells showed a significant reduction in the aldehyde dehydrogenase-positive cell population and a significant reduction in the population of MCF-7 cancer stem cells in primary, secondary, and tertiary mammospheres. Moreover, treatment with C1 down-regulated the Wnt/β-catenin self-renewal pathway. These findings indicate that complex C1 is a suppressive agent of MCF-7 cells that functions through the induction of apoptosis, cell cycle arrest, and the targeting of MCF-7-derived cancer stem cells. This work may lead to a better treatment strategy for the reduction of breast cancer recurrence.

Wnt pathway activation by ADP-ribosylation

Wnt/β-catenin signalling directs fundamental processes during metazoan development and can be aberrantly activated in cancer. Wnt stimulation induces the recruitment of the scaffold protein Axin from an inhibitory destruction complex to a stimulatory signalosome. Here we analyse the early effects of Wnt on Axin and find that the ADP-ribose polymerase Tankyrase (Tnks)--known to target Axin for proteolysis-regulates Axin's rapid transition following Wnt stimulation. We demonstrate that the pool of ADP-ribosylated Axin, which is degraded under basal conditions, increases immediately following Wnt stimulation in both Drosophila and human cells. ADP-ribosylation of Axin enhances its interaction with the Wnt co-receptor LRP6, an essential step in signalosome assembly. We suggest that in addition to controlling Axin levels, Tnks-dependent ADP-ribosylation promotes the reprogramming of Axin following Wnt stimulation; and propose that Tnks inhibition blocks Wnt signalling not only by increasing destruction complex activity, but also by impeding signalosome assembly.

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

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

PP2A as a master regulator of the cell cycle

Protein phosphatase 2A (PP2A) plays a critical multi-faceted role in the regulation of the cell cycle. It is known to dephosphorylate over 300 substrates involved in the cell cycle, regulating almost all major pathways and cell cycle checkpoints. PP2A is involved in such diverse processes by the formation of structurally distinct families of holoenzymes, which are regulated spatially and temporally by specific regulators. Here, we review the involvement of PP2A in the regulation of three cell signaling pathways: wnt, mTOR and MAP kinase, as well as the G1→S transition, DNA synthesis and mitotic initiation. These processes are all crucial for proper cell survival and proliferation and are often deregulated in cancer and other diseases.

A p38 Mitogen-Activated Protein Kinase-Regulated Myocyte Enhancer Factor 2-β-Catenin Interaction Enhances Canonical Wnt Signaling

Canonical Wnt/β-catenin signaling plays a major role in various biological contexts, such as embryonic development, cell proliferation, and cancer progression. Previously, a connection between p38 mitogen-activated protein kinase (MAPK) signaling and Wnt-mediated activation of β-catenin was implied but poorly understood. In the present study, we investigated potential cross talk between p38 MAPK and Wnt/β-catenin signaling. Here we show that a loss of p38 MAPK α/β function reduces β-catenin nuclear accumulation in Wnt3a-stimulated primary vascular smooth muscle cells (VSMCs). Conversely, active p38 MAPK signaling increases β-catenin nuclear localization and target gene activity in multiple cell types. Furthermore, the effect of p38 MAPK α/β on β-catenin activity is mediated through phosphorylation of a key p38 MAPK target, myocyte enhancer factor 2 (MEF2). Here we report a p38 MAPK-mediated, phosphorylation-dependent interaction between MEF2 and β-catenin in multiple cell types and primary VSMCs that results in (i) increased β-catenin nuclear retention, which is reversed by small interfering RNA (siRNA)-mediated MEF2 gene silencing; (ii) increased activation of MEF2 and Wnt/β-catenin target genes; and (iii) increased Wnt-stimulated cell proliferation. These observations provide mechanistic insight into a fundamental level of cross talk between p38 MAPK/MEF2 signaling and canonical Wnt signaling.