Phosphoprotein phosphatase-2A docks to Dishevelled and counterregulates Wnt3a/beta-catenin signaling

PPM1G dephosphorylates eIF4E in control of mRNA translation and cell proliferation

The mRNA 5'cap-binding eukaryotic translation initiation factor 4E (eIF4E) plays a critical role in the control of mRNA translation in health and disease. One mechanism of regulation of eIF4E activity is via phosphorylation of eIF4E by MNK kinases, which promotes the translation of a subset of mRNAs encoding pro-tumorigenic proteins. Work on eIF4E phosphatases has been paltry. Here, we show that PPM1G is the phosphatase that dephosphorylates eIF4E. We describe the eIF4E-binding motif in PPM1G that is similar to 4E-binding proteins (4E-BPs). We demonstrate that PPM1G inhibits cell proliferation by targeting phospho-eIF4E-dependent mRNA translation.

Neuroretinal Cell Culture Model as a Tool for the Development of New Therapeutic Approaches for Oxidative Stress-Induced Ocular Diseases, with a Focus on Glaucoma

Glaucoma is a heterogeneous group of optic neuropathies characterized by a progressive degeneration of the retinal ganglion cells (RGCs), leading to irreversible vision loss. Nowadays, the traditional therapeutic approach to glaucoma consists of lowering the intraocular pressure (IOP), which does not address the neurodegenerative features of the disease. Besides animal models of glaucoma, there is a considerable need for in vitro experimental models to propose new therapeutic strategies for this ocular disease. In this study, we elucidated the pathological mechanisms leading to neuroretinal R28 cell death after exposure to glutamate and hydrogen peroxide (H2O2) in order to develop new therapeutic approaches for oxidative stress-induced retinal diseases, including glaucoma. We were able to show that glutamate and H2O2 can induce a decrease in R28 cell viability in a concentration-dependent manner. A cell viability of about 42% was found after exposure to 3 mM of glutamate and about 56% after exposure to 100 µM of H2O2 (n = 4). Label-free quantitative mass spectrometry analysis revealed differential alterations of 193 and 311 proteins in R28 cells exposed to 3 mM of glutamate and 100 µM of H2O2, respectively (FDR < 1%; p < 0.05). Bioinformatics analysis indicated that the protein changes were associated with the dysregulation of signaling pathways, which was similar to those observed in glaucoma. Thus, the proteomic alteration induced by glutamate was associated with the inhibition of the PI3K/AKT signaling pathway. On the other hand, H2O2-induced toxicity in R28 cells was linked to the activation of apoptosis signaling and the inhibition of the mTOR and ERK/MAPK signaling pathways. Furthermore, the data show a similarity in the inhibition of the EIF2 and AMPK signaling pathways and the activation of the sumoylation and WNT/β-catenin signaling pathways in both groups. Our findings suggest that the exposure of R28 cells to glutamate and H2O2 could induce glaucoma-like neurodegenerative features and potentially provide a suitable tool for the development of new therapeutic strategies for retinal diseases.

Wnt signaling in colorectal cancer: pathogenic role and therapeutic target

Background

The Wnt signaling pathway is a complex network of protein interactions that functions most commonly in embryonic development and cancer, but is also involved in normal physiological processes in adults. The canonical Wnt signaling pathway regulates cell pluripotency and determines the differentiation fate of cells during development. The canonical Wnt signaling pathway (also known as the Wnt/β-catenin signaling pathway) is a recognized driver of colon cancer and one of the most representative signaling pathways. As a functional effector molecule of Wnt signaling, the modification and degradation of β-catenin are key events in the Wnt signaling pathway and the development and progression of colon cancer. Therefore, the Wnt signaling pathway plays an important role in the pathogenesis of diseases, especially the pathogenesis of colorectal cancer (CRC).

Objective

Inhibit the Wnt signaling pathway to explore the therapeutic targets of colorectal cancer.

Methods

Based on studying the Wnt pathway, master the biochemical processes related to the Wnt pathway, and analyze the relevant targets when drugs or inhibitors act on the Wnt pathway, to clarify the medication ideas of drugs or inhibitors for the treatment of diseases, especially colorectal cancer.

Results

Wnt signaling pathways include: Wnt/β-catenin or canonical Wnt signaling pathway, planar cell polarity (Wnt-PCP) pathway and Wnt-Ca²⁺ signaling pathway. The Wnt signaling pathway is closely related to cancer cell proliferation, stemness, apoptosis, autophagy, metabolism, inflammation and immunization, microenvironment, resistance, ion channel, heterogeneity, EMT/migration/invasion/metastasis. Drugs/phytochemicals and molecular preparations for the Wnt pathway of CRC treatment have now been developed. Wnt inhibitors are also commonly used clinically for the treatment of CRC.

Conclusion

The development of drugs/phytochemicals and molecular inhibitors targeting the Wnt pathway can effectively treat colorectal cancer clinically.

Regulation of Dishevelled protein activity and stability by post-translational modifications and autophagy

As a key component of Wnt signaling, Dishevelled (Dvl/Dsh) plays essential roles in development processes and adult tissue homeostasis in multicellular organisms, and its deregulation results in human development disorders and other diseases. Dvl integrates and relays complex Wnt signals by acting as a branch-point of β-catenin-dependent canonical and β-catenin-independent noncanonical pathways. It dynamically interacts with multiple proteins to modulate Wnt signaling, while its activity and stability are tightly controlled by other proteins. This Review summarizes the current understanding of regulation of Dvl activity, localization, and stability by post-translational modifications, aggregation, and autophagy, and the impacts on Dvl function in both Wnt signaling and biological processes.

Cancer stem cell phosphatases

Cancer stem cells (CSCs) are involved in the initiation and progression of human malignancies by enabling cancer tissue self-renewal capacity and constituting the therapy-resistant population of tumor cells. However, despite the exhausting characterization of CSC genetics, epigenetics, and kinase signaling, eradication of CSCs remains an unattainable goal in most human malignancies. While phosphatases contribute equally with kinases to cellular phosphoregulation, our understanding of phosphatases in CSCs lags severely behind our knowledge about other CSC signaling mechanisms. Many cancer-relevant phosphatases have recently become druggable, indicating that further understanding of the CSC phosphatases might provide novel therapeutic opportunities. This review summarizes the current knowledge about fundamental, but yet poorly understood involvement of phosphatases in the regulation of major CSC signaling pathways. We also review the functional roles of phosphatases in CSC self-renewal, cancer progression, and therapy resistance; focusing particularly on hematological cancers and glioblastoma. We further discuss the small molecule targeting of CSC phosphatases and their therapeutic potential in cancer combination therapies.

Review of PP2A Tumor Biology and Antitumor Effects of PP2A Inhibitor LB100 in the Nervous System

Simple Summary

Central and peripheral nervous system tumors represent a heterogenous group of neoplasms which often demonstrate resistance to treatment. Given that these tumors are often refractory to conventional therapy, novel pharmaceutical regimens are needed for successfully treating this pathology. One such therapeutic is the serine/threonine phosphatase inhibitor, LB100. LB100 is a water-soluble competitive protein phosphtase inhibitor that has demonstrated antitumor effects in preclinical and clinical trials. In this review, we aim to summarize current evidence demonstrating the efficacy of LB100 as an inhibitor of nervous system tumors. Furthermore, we review the involvement of the well-studied phosphatase, protein phosphatase 2A, in oncogenic cell signaling pathways, neurophysiology, and neurodevelopment.

Abstract

Protein phosphatase 2A (PP2A) is a ubiquitous serine/threonine phosphatase implicated in a wide variety of regulatory cellular functions. PP2A is abundant in the mammalian nervous system, and dysregulation of its cellular functions is associated with myriad neurodegenerative disorders. Additionally, PP2A has oncologic implications, recently garnering attention and emerging as a therapeutic target because of the antitumor effects of a potent PP2A inhibitor, LB100. LB100 abrogation of PP2A is believed to exert its inhibitory effects on tumor progression through cellular chemo- and radiosensitization to adjuvant agents. An updated and unifying review of PP2A biology and inhibition with LB100 as a therapeutic strategy for targeting cancers of the nervous system is needed, as other reviews have mainly covered broader applications of LB100. In this review, we discuss the role of PP2A in normal cells and tumor cells of the nervous system. Furthermore, we summarize current evidence regarding the therapeutic potential of LB100 for treating solid tumors of the nervous system.

Extracellular vesicle-mediated crosstalk between NPCE cells and TM cells result in modulation of Wnt signalling pathway and ECM remodelling

Primary open-angle glaucoma is a leading cause of irreversible blindness, often associated with increased intraocular pressure. Extracellular vesicles (EVs) carry a specific composition of proteins, lipids and nucleotides have been considered as essential mediators of cell-cell communication. Their potential impact for crosstalk between tissues responsible for aqueous humour production and out-flow is largely unknown. The study objective was to investigate the effects of EVs derived from non-pigmented ciliary epithelium (NPCE) primary cells on the expression of Wnt proteins in a human primary trabecular meshwork (TM) cells and define the mechanism underlying exosome-mediated regulation that signalling pathway. Consistent with the results in TM cell line, EVs released by both primary NPCE cells and NPCE cell line showed diminished pGSK3β phosphorylation and decreased cytosolic levels of β-catenin in primary TM cells. At the molecular level, we showed that NPCE exosome treatment downregulated the expression of positive GSKβ regulator-AKT protein but increased the levels of GSKβ negative regulator-PP2A protein in TM cells. NPCE exosome protein analysis revealed 584 miRNAs and 182 proteins involved in the regulation of TM cellular processes, including WNT/β-catenin signalling pathway, cell adhesion and extracellular matrix deposition. We found that negative modulator of Wnt signalling miR-29b was abundant in NPCE exosomal samples and treatment of TM cells with NPCE EVs significantly decreased COL3A1 expression. Suggesting that miR-29b can be responsible for decreased levels of WNT/β-catenin pathway. Overall, this study highlights a potential role of EVs derived from NPCE cells in modulating ECM proteins and TM canonical Wnt signalling.

Atypical function of a centrosomal module in WNT signalling drives contextual cancer cell motility

Centrosomes control cell motility, polarity and migration that is thought to be mediated by their microtubule-organizing capacity. Here we demonstrate that WNT signalling drives a distinct form of non-directional cell motility that requires a key centrosome module, but not microtubules or centrosomes. Upon exosome mobilization of PCP-proteins, we show that DVL2 orchestrates recruitment of a CEP192-PLK4/AURKB complex to the cell cortex where PLK4/AURKB act redundantly to drive protrusive activity and cell motility. This is mediated by coordination of formin-dependent actin remodelling through displacement of cortically localized DAAM1 for DAAM2. Furthermore, abnormal expression of PLK4, AURKB and DAAM1 is associated with poor outcomes in breast and bladder cancers. Thus, a centrosomal module plays an atypical function in WNT signalling and actin nucleation that is critical for cancer cell motility and is associated with more aggressive cancers. These studies have broad implications in how contextual signalling controls distinct modes of cell migration.

Centrosomes function in cell migration by organizing microtubules. Here, Luo et al. surprisingly show that centrosome proteins also control migration after recruitment by Wnt-PCP proteins to the cell cortex, leading to actin remodelling and protrusive activity relevant to aggressive cancer motility.

Okadaic acid activates Wnt/β-catenin-signaling in human HepaRG cells

The lipophilic phycotoxin okadaic acid (OA) occurs in the fatty tissue and hepatopancreas of filter-feeding shellfish. The compound provokes the diarrhetic shellfish poisoning (DSP) syndrome after intake of seafood contaminated with high levels of the DSP toxin. In animal experiments, long-term exposure to OA is associated with an elevated risk for tumor formation in different organs including the liver. Although OA is a known inhibitor of the serine/threonine protein phosphatase 2A, the mechanisms behind OA-induced carcinogenesis are not fully understood. Here, we investigated the influence of OA on the β-catenin-dependent Wnt-signaling pathway, addressing a major oncogenic pathway relevant for tumor development. We analyzed OA-mediated effects on β-catenin and its biological function, cellular localization, post-translational modifications, and target gene expression in human HepaRG hepatocarcinoma cells treated with non-cytotoxic concentrations up to 50 nM. We detected concentration- and time-dependent effects of OA on the phosphorylation state, cellular redistribution as well as on the amount of transcriptionally active β-catenin. These findings were confirmed by quantitative live-cell imaging of U2OS cells stably expressing a green fluorescent chromobody which specifically recognize hypophosphorylated β-catenin. Finally, we demonstrated that nuclear translocation of β-catenin mediated by non-cytotoxic OA concentrations results in an upregulation of Wnt-target genes. In conclusion, our results show a significant induction of the canonical Wnt/β-catenin-signaling pathway by OA in human liver cells. Our data contribute to a better understanding of the molecular mechanisms underlying OA-induced carcinogenesis.

Protein Phosphatase 2A in the Regulation of Wnt Signaling, Stem Cells, and Cancer

Protein phosphorylation is a ubiquitous cellular process that allows for the nuanced and reversible regulation of protein activity. Protein phosphatase 2A (PP2A) is a heterotrimeric serine-threonine phosphatase—composed of a structural, regulatory, and catalytic subunit—that controls a variety of cellular events via protein dephosphorylation. While much is known about PP2A and its basic biochemistry, the diversity of its components—especially the multitude of regulatory subunits—has impeded the determination of PP2A function. As a consequence of this complexity, PP2A has been shown to both positively and negatively regulate signaling networks such as the Wnt pathway. Wnt signaling modulates major developmental processes, and is a dominant mediator of stem cell self-renewal, cell fate, and cancer stem cells. Because PP2A affects Wnt signaling both positively and negatively and at multiple levels, further understanding of this complex dynamic may ultimately provide insight into stem cell biology and how to better treat cancers that result from alterations in Wnt signaling. This review will summarize literature that implicates PP2A as a tumor suppressor, explore PP2A mutations identified in human malignancy, and focus on PP2A in the regulation of Wnt signaling and stem cells so as to better understand how aberrancy in this pathway can contribute to tumorigenesis.

PP5 (PPP5C) is a phosphatase of Dvl2

Dishevelled (Dvl) family proteins are key mediators of Wnt signalling and function in both canonical and noncanonical branches. Dvl2, the most studied Dvl protein, is extensively regulated by phosphorylation. Several kinases were found to be critical for Dvl2 localisation, stability control and functional segregation. For example, S143-phosphorylated Dvl2 was detected, together with CK1δ/ε, at the centrosome and basal body of primary cilia and plays pivotal roles during ciliogenesis. However, relatively less is known about Dvl dephosphorylation and the phosphatases involved. Here, we identified PP5 (PPP5C) as a phosphatase of Dvl2. PP5 interacts with and can directly dephosphorylate Dvl2. Knockdown of PP5 caused elevated Dvl2 phosphorylation both at the basal level and upon Wnt stimulation. In the Dvl2 protein, S143, the 10B5 cluster and other sites were dephosphorylated by PP5. Interestingly, comparison of PP5 with PP2A, another known Dvl2 phosphatase, revealed that PP5 and PP2A are not fully redundant in the regulation of Dvl2 phosphorylation status. In hTERT-RPE1 cells, PP5 was found at the basal body of cilia, where S143-phosphorylated Dvl2 also resides. Functional assays revealed modest effects on ciliogenesis after PP5 depletion or over-expression. Taken together, our results provided evidence to suggest PP5 as a new phosphatase for Dvl2.

The way Wnt works: components and mechanism

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

Assembly of Dishevelled 3-based supermolecular complexes via phosphorylation and Axin

Background

Dishevelled-3 (Dvl3) is a multivalent scaffold essential to cell signaling in development. Dsh/Dvls enable a myriad of protein-protein interactions in Wnt signaling. In the canonical Wnt/β-catenin pathway specifically, Dvl3 polymerizes to form dynamic protein aggregates, so-called “signalsomes”, which propagate signals from the Wnt receptor Frizzled to downstream elements.

Results

Very large Dvl3-based supermolecular complexes form in response to Wnt3a. These complexes are identified by steric-exclusion chromatography, affinity pull-downs, proteomics, and fluorescence correlation microscopy (fcs). In the current work, the roles of Dvl3 phosphorylation and of Axin in the assembly of Dvl3-based supermolecular complexes in response to Wnt3a are probed in totipotent mouse F9 teratocarcinoma cells. Point mutations of phosphorylation sites of Dvl3 which interfere with Lef/Tcf-sensitive transcriptional activation by Wnt3a are shown to interfere more proximally with the assembly of Dvl3-based supermolecular complexes. Axin, a Dvl-interacting protein, plays a central role in organizing the beta-catenin destruction complex. The assembly of Dvl3-based supermolecular complexes is blocked either by depletion of Axin or by mutation of Axin sites necessary for polymerization in response to Wnt3a.

Conclusion

These data demonstrate that Wnt3a activation of the canonical pathway requires specific phosphorylation events as well as Axin to assemble very large, Dvl3-based supermolecular complexes; these complexes are a prerequisite to activation of Lef/Tcf-sensitive transcription.

Dishevelled C-terminus: prolyl and histidinyl motifs

The phosphoprotein scaffold Dishevelled is an essential component of both Wnt signalling and of the signalsome that constitutes the supermolecular 'punctae' of assembled proteins often observed in fluorescence microscopy. The C-terminal region beyond the DEP domain displays unique and interesting character, exploited herein by careful analysis of the primary structure. Human Dishevelled-1, -2, -3 and fly Dishevelled (Dsh) sequences were downloaded and interrogated in silico. The C-terminus of Dishevelled-3 is revealed by FoldIndex(®) to be rich in ordered structure. It displays primary sequence that is unique and divergent in important ways from vertebrate isoforms as well as from the fly Dsh. The region is amphipathic, high in prolyl content, and harbours polyprolines. Dishevelled-3 displays some regions, where the proline content is >40%. Polyprolyl sequences (2-4 residues) likely constitute important sites of interaction with other Dishevelled isoforms. Several histidine-single amino acid repeats are notable. The 637,638/647,648 repeats of Dvl3 are essential for Wnt non-canonical, but not canonical signalling. Mutagenesis reveals that the C-terminal sequence is essential for the formation of punctae, made visible by fluorescence microscopy. These Dvl3-based signalsomes are very large (25-35 MDa-MW), supermolecular complexes that display dynamic reorganization in response to Wnt stimulation. Dishevelled-3 C-terminus is rich in structure and unique motifs, worthy of detailed analysis with modern molecular tools.

Wnt-dependent assembly of supermolecular Dishevelled-3-based complexes

Dishevelled-3 (Dvl3) is a multivalent scaffold protein that is essential to Wnt signaling during development. Although Dvl-based punctae have been visualized by fluorescence microscopy; the physical nature and dynamic character of the such complexes are enigmatic. We use steric-exclusion chromatography, affinity pull-downs, proteomics and fluorescence correlation microscopy to characterize supermolecular Dvl3-based complexes of totipotent mouse F9 cells. The molecular mass of the complexes ranges from that of homodimeric Dvl3 to well-defined peaks harboring supermolecular complexes of 0.4 to 2.0 MDa. Addition of Wnt3a stimulates the formation of Dvl3-based complexes of greater molecular mass within 30 minutes. The presence of DKK1 and knockdown of Dishevelled proteins block formation of the 2 MDa Dvl3-based complexes and also block Wnt3a stimulation of the canonical pathway. Fluorescent correlation microscopy identified supermolecular Dvl3-based complexes with a molecular mass >30 MDa in live cells; these complexes were provoked to form structures with even greater molecular mass by Wnt3a. We establish for the first time the physical and functional nature of very large, supermolecular Dvl3-based complexes.

Dishevelled: The hub of Wnt signaling

Wnt signaling controls a variety of developmental and homeostatic events. As a key component of Wnt signaling, Dishevelled (Dvl/Dsh) protein relays Wnt signals from receptors to downstream effectors. In the canonical Wnt pathway that depends on the nuclear translocation of beta-catenin, Dvl is recruited by the receptor Frizzled and prevents the constitutive destruction of cytosolic beta-catenin. In the non-canonical Wnt pathways such as Wnt-Frizzled/PCP (planar cell polarity) signaling, Dvl signals via the Daam1-RhoA axis and the Rac1 axis. In addition, Dvl plays important roles in Wnt-GSK3beta-microtubule signaling, Wnt-calcium signaling, Wnt-RYK signaling, Wnt-atypical PKC signaling, etc. Dvl also functions to mediate receptor endocytosis. To fulfill its multifaceted functions, it is not surprising that Dvl associates with various kinds of proteins. Its activity is also modulated dynamically by phosphorylation, ubiquitination and degradation. In this review, we summarize the current understanding of Dvl functions in Wnt signal transduction and its biological functions in mouse development, and also discuss the molecular mechanisms of its actions.

Dishevelled-2 docks and activates Src in a Wnt-dependent manner

Wnt3a activates the ;canonical' signaling pathway, stimulating the nuclear accumulation of beta-catenin and activation of Lef/Tcf-sensitive transcription of developmentally important genes. Using totipotent mouse F9 teratocarcinoma cells expressing frizzled-1 (Fz1), we investigated roles of tyrosine kinase activity in Wnt/beta-catenin signaling. Treatment with either genistein or Src family kinase inhibitor PP2 attenuates Wnt3a-stimulated Lef/Tcf transcription activation and primitive endoderm formation. siRNA-induced knockdown of Src likewise attenuates Lef/Tcf transcription and primitive endoderm formation in response to Wnt3a, implicating Src as a positive regulator of Wnt/beta-catenin signaling. We discovered that Src binds dishevelled-2 (Dvl2), a key phosphoprotein in Wnt signaling, at two positions: an SH3-binding domain and a C-terminal domain. The Y18F mutant of Dvl2 attenuates the Wnt3a-stimulated Lef/Tcf-sensitive transcriptional response. Wnt3a stimulates Src docking to Dvl2 and activation of this tyrosine kinase. Activated Src, in turn, enhances Wnt activation of the canonical pathway. We show that Dvl2 and beta-catenin are crucially important substrates for tyrosine phosphorylation in the canonical Wnt/beta-catenin pathway.