Adenomatous polyposis coli (APC) protein regulates epithelial cell migration and morphogenesis via PDZ domain-based interactions with plasma membranes

Force transmission by retrograde actin flow-induced dynamic molecular stretching of Talin

Force transmission at integrin-based adhesions is important for cell migration and mechanosensing. Talin is an essential focal adhesion (FA) protein that links F-actin to integrins. F-actin constantly moves on FAs, yet how Talin simultaneously maintains the connection to F-actin and transmits forces to integrins remains unclear. Here we show a critical role of dynamic Talin unfolding in force transmission. Using single-molecule speckle microscopy, we found that the majority of Talin are bound only to either F-actin or the substrate, whereas 4.1% of Talin is linked to both structures via elastic transient clutch. By reconstituting Talin knockdown cells with Talin chimeric mutants, in which the Talin rod subdomains are replaced with the stretchable β-spectrin repeats, we show that the stretchable property is critical for force transmission. Simulations suggest that unfolding of the Talin rod subdomains increases in the linkage duration and work at FAs. This study elucidates a force transmission mechanism, in which stochastic molecular stretching bridges two cellular structures moving at different speeds.

A DLG1-ARHGAP31-CDC42 axis is essential for the intestinal stem cell response to fluctuating niche Wnt signaling

A central factor in the maintenance of tissue integrity is the response of stem cells to variations in the levels of niche signals. In the gut, intestinal stem cells (ISCs) depend on Wnt ligands for self-renewal and proliferation. Transient increases in Wnt signaling promote regeneration after injury or in inflammatory bowel diseases, whereas constitutive activation of this pathway leads to colorectal cancer. Here, we report that Discs large 1 (Dlg1), although dispensable for polarity and cellular turnover during intestinal homeostasis, is required for ISC survival in the context of increased Wnt signaling. RNA sequencing (RNA-seq) and genetic mouse models demonstrated that DLG1 regulates the cellular response to increased canonical Wnt ligands. This occurs via the transcriptional regulation of Arhgap31, a GTPase-activating protein that deactivates CDC42, an effector of the non-canonical Wnt pathway. These findings reveal a DLG1-ARHGAP31-CDC42 axis that is essential for the ISC response to increased niche Wnt signaling.

Force-dependent remodeling of cytoplasmic ZO-1 condensates contributes to cell-cell adhesion through enhancing tight junctions

The physiological importance of biomolecular condensates is widely recognized, but how it is controlled in time and space during development is largely unknown. Here, we show that a tight junction protein ZO-1 forms cytoplasmic condensates in the trophectoderm (TE) of the mouse embryo before E4.0. These disappear via dissolution, and ZO-1 accumulates at the cell junction as the blastocyst cavity grows and internal pressure on TE cells increases. In contrast, this dissolution was less evident in TE cells attached to the inner cell mass because they receive weaker tensile forces. Furthermore, analyses using MDCK cells demonstrated that the ZO-1 condensates are generated and maintained by liquid-liquid phase separation. Our study also highlights that the dynamics of these condensates depends on the physical environment via an interaction between ZO-1 and F-actin. We propose that the force-dependent regulation of ZO-1 condensation contributes to the establishment of robust cell-cell adhesion during early development.

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ZO-1 forms cytoplasmic droplets via liquid-liquid phase separation

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In hatching mouse embryos, ZO-1 droplets dissolve and it localizes to cell junctions

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In MDCK cells, ZO-1 forms droplets in response to mechanical environments

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Interaction with F-actin negatively regulates ZO-1 phase separation

Identification of Endogenous Adenomatous Polyposis Coli Interaction Partners and β-Catenin-Independent Targets by Proteomics

Adenomatous Polyposis Coli (APC) is the most frequently mutated gene in colorectal cancer. APC negatively regulates the Wnt signaling pathway by promoting the degradation of β-catenin, but the extent to which APC exerts Wnt/β-catenin-independent tumor-suppressive activity is unclear. To identify interaction partners and β-catenin-independent targets of endogenous, full-length APC, we applied label-free and multiplexed tandem mass tag-based mass spectrometry. Affinity enrichment-mass spectrometry identified more than 150 previously unidentified APC interaction partners. Moreover, our global proteomic analysis revealed that roughly half of the protein expression changes that occur in response to APC loss are independent of β-catenin. Combining these two analyses, we identified Misshapen-like kinase 1 (MINK1) as a putative substrate of an APC-containing destruction complex. We validated the interaction between endogenous MINK1 and APC and further confirmed the negative, and β-catenin-independent, regulation of MINK1 by APC. Increased Mink1/Msn levels were also observed in mouse intestinal tissue and Drosophila follicular cells expressing mutant Apc/APC when compared with wild-type tissue/cells. Collectively, our results highlight the extent and importance of Wnt-independent APC functions in epithelial biology and disease. IMPLICATIONS: The tumor-suppressive function of APC, the most frequently mutated gene in colorectal cancer, is mainly attributed to its role in β-catenin/Wnt signaling. Our study substantially expands the list of APC interaction partners and reveals that approximately half of the changes in the cellular proteome induced by loss of APC function are mediated by β-catenin-independent mechanisms.

Oncogenic transformation of normal breast epithelial cells co-cultured with cancer cells

The heterogeneity in human breast cancer poses a challenge for effective treatment. Better understanding of tumor initiation and development will help to resolve this problem. Current models explaining intratumoral diversity include cancer stem cells, clonal evolution and cancer cell dedifferentiation and reprogramming. Herein, a new model, cancer transmission, is proposed to explain cancer heterogeneity. We found breast cancer cells (MCF10A.NeuT) were capable of transforming normal mammary epithelial cells (MCF10A). The transformed cells exhibited cancerous properties including enhanced proliferation and migration, loss of apical-basal polarity and depolarized acini structure associated with epithelial-mesenchymal transition (EMT). The transformed MCF10A cells displayed distinct EMT characteristics compared to parental cells. We further showed that cancer cell-secreted factors were sufficient to induce cancerous transformation of normal cells. Furthermore, transformed cells were resistant to radiation treatment, providing new insights into mechanisms underlying therapeutic resistance.

An Infrared Actin Probe for Deep-Cell Electroporation-Based Single-Molecule Speckle (eSiMS) Microscopy

Single-molecule speckle (SiMS) microscopy is a powerful method to directly elucidate biochemical reactions in live cells. However, since the signal from an individual fluorophore is extremely faint, the observation area by epi-fluorescence microscopy is restricted to the thin cell periphery to reduce autofluorescence, or only molecules near the plasma membrane are visualized by total internal reflection fluorescence (TIRF) microscopy. Here, we introduce a new actin probe labeled with near infrared (NIR) emissive CF680R dye for easy-to-use, electroporation-based SiMS microscopy (eSiMS) for deep-cell observation. CF680R-labeled actin (CF680R-actin) incorporated into actin structures and showed excellent brightness and photostability suitable for single-molecule imaging. Importantly, the intensity of autofluorescence with respect to SiMS brightness was reduced to approximately 13% compared to DyLight 550-labeled actin (DL550-actin). CF680R-actin enabled the monitoring of actin SiMS in actomyosin bundles associated with adherens junctions (AJs) located at 3.5–4 µm above the basal surfaces of epithelial monolayers. These favorable properties of CF680R-actin extend the application of eSiMS to actin turnover and flow analyses in deep cellular structures.

Adenomatous polyposis coli (APC) regulates miR17-92 cluster through β-catenin pathway in colorectal cancer

Adenomatous polyposis coli (APC) mutation is the most common genetic change in sporadic colorectal cancer (CRC). Although deregulations of miRNAs have been frequently reported in this malignancy, APC-regulated miRNAs have not been extensively documented. Here, by using an APC-inducible cell line and array analysis, we identified a total of 26 deregulated miRNAs. Among them, members of miR-17-92 cluster were dramatically inhibited by APC and induced by enforced expression of β-catenin. Furthermore, we demonstrate that activated β-catenin resulted from APC loss binds to and activates the miR-17-92 promoter. Notably, enforced expression of miR-19a overrides APC tumor suppressor activity, and knockdown of miR-19a in cancer cells with compromised APC function reduced their aggressive features in vitro. Finally, we observed that expression of miR-19a significantly correlates with β-catenin levels in colorectal cancer specimens, and it is associated to the aggressive stage of tumor progression. Thus, our study reveals that miR-17-92 cluster is directly regulated by APC/β-catenin pathway and could be a potential therapeutic target in colon cancers with aberrant APC/β-catenin signaling.

Expression of polarity genes in human cancer

Polarity protein complexes are crucial for epithelial apical–basal polarity and directed cell migration. Since alterations of these processes are common in cancer, polarity proteins have been proposed to function as tumor suppressors or oncogenic promoters. Here, we review the current understanding of polarity protein functions in epithelial homeostasis, as well as tumor formation and progression. As most previous studies focused on the function of single polarity proteins in simplified model systems, we used a genomics approach to systematically examine and identify the expression profiles of polarity genes in human cancer. The expression profiles of polarity genes were distinct in different human tissues and classified cancer types. Additionally, polarity expression profiles correlated with disease progression and aggressiveness, as well as with identified cancer types, where specific polarity genes were commonly altered. In the case of Scribble, gene expression analysis indicated its common amplification and upregulation in human cancer, suggesting a tumor promoting function.

An oncologist׳s friend: How Xenopus contributes to cancer research

One of the most striking features of the Xenopus system is the versatility in providing a unique range of both in vitro and in vivo models that are rapid, accessible and easily manipulated. Here we present an overview of the diverse contribution that Xenopus has made to advance our understanding of tumour biology and behaviour; a contribution that goes beyond the traditional view of Xenopus as a developmental model organism. From the utility of the egg and oocyte extract system to the use of whole embryos as developmental or induced tumour models, the Xenopus system has been fundamental to investigation of cell cycle mechanisms, cell metabolism, cell signalling and cell behaviour, and has allowed an increasing appreciation of the parallels between early development and the pathogenesis of tumour progression and metastasis. Although not the prototypical oncological model system, we propose that Xenopus is an adaptable and multifunctional tool in the oncologist׳s arsenal.

Dissecting the nanoscale distributions and functions of microtubule-end-binding proteins EB1 and ch-TOG in interphase HeLa cells

Recently, the EB1 and XMAP215/TOG families of microtubule binding proteins have been demonstrated to bind autonomously to the growing plus ends of microtubules and regulate their behaviour in in vitro systems. However, their functional redundancy or difference in cells remains obscure. Here, we compared the nanoscale distributions of EB1 and ch-TOG along microtubules using high-resolution microscopy techniques, and also their roles in microtubule organisation in interphase HeLa cells. The ch-TOG accumulation sites protruded ∼100 nm from the EB1 comets. Overexpression experiments showed that ch-TOG and EB1 did not interfere with each other’s localisation, confirming that they recognise distinct regions at the ends of microtubules. While both EB1 and ch-TOG showed similar effects on microtubule plus end dynamics and additively increased microtubule dynamicity, only EB1 exhibited microtubule-cell cortex attachment activity. These observations indicate that EB1 and ch-TOG regulate microtubule organisation differently via distinct regions in the plus ends of microtubules.

Leucine-rich repeat-containing G protein-coupled receptor 5 regulates epithelial cell phenotype and survival of hepatocellular carcinoma cells

The leucine-rich repeat containing G protein-coupled receptor 5 (LGR5), also known as GPR49, is a seven-transmembrane receptor that is expressed in stem cells of the intestinal crypts and hair follicles of mice. LGR5 is overexpressed in some types of human cancer, and is one of the target genes of the Wnt signaling pathway. To explore the function of LGR5 in cancer cells, stable hepatocellular carcinoma (HCC) cell lines expressing FLAG-tagged LGR5 were established. Overexpression of LGR5 resulted in changes in cell shape from an extended flat (mesenchymal) phenotype to a round aggregated (stem cell-like) phenotype. Cells transfected with LGR5 showed higher colony forming activity, and were more resistant to a cytotoxic drug than cells transfected with empty vector. Overexpression of LGR5 inhibited cell motility. LGR5-transfected cells formed nodule type tumors in the livers of immunodeficient mice, whereas empty vector-transfected cells formed more invasive tumors. Down-regulation of LGR5 changed the morphology of HCC cells from the aggregated phenotype to an extended spindle phenotype, and cell motility was increased. This is the first study reporting the functional role of LGR5 in the biology of HCC cells, and the results suggest that aberrant expression of LGR5 regulates epithelial cell phenotype and survival.

A functional network of the tumor suppressors APC, hDlg, and PTEN, that relies on recognition of specific PDZ-domains

APC and PTEN are tumor suppressor proteins that bind through their C-termini to the PDZ domain containing-hDlg scaffolding protein. We have found that co-expression of PTEN and hDlg enhanced the negative regulation of the PI3K/Akt pathway by PTEN, indicating the physiologic importance of these interactions. APC and PTEN share other PDZ domain containing-interacting partners, including the MAGI scaffolding proteins and the MAST family of protein kinases. Mutational analysis revealed that the C-terminal PDZ-binding motifs from APC and PTEN were differentially recognized by distinct PDZ domains. APC bound to the three PDZ domains from hDlg, whereas PTEN mainly bound to PDZ-2/hDlg. This indicates the existence of overlapping, but distinct PDZ-domain recognition patterns by APC and PTEN. Furthermore, a ternary complex formed by APC, PTEN, and hDlg was detected, suggesting that hDlg may serve as a platform to bring in proximity APC and PTEN tumor suppressor activities. In line with this, tumor-related mutations targeting the PDZ-2/hDlg domain diminished its interaction with APC and PTEN. Our results expand the PDZ-domain counterparts for the tumor suppressor APC, show that APC and PTEN share PDZ-domain partners but have individual molecular determinants for specific recognition of PDZ domains, and suggest the participation of the tumor suppressors APC, PTEN, and hDlg in PDZ-domain interaction networks which may be relevant in oncogenesis.

Shaping microtubules into diverse patterns: molecular connections for setting up both ends

Microtubules serve as rails for intracellular trafficking and their appropriate organization is critical for the generation of cell polarity, which is a foundation of cell differentiation, tissue morphogenesis, ontogenesis and the maintenance of homeostasis. The microtubule array is not just a static railway network; it undergoes repeated collapse and reassembly in diverse patterns during cell morphogenesis. In the last decade much progress has been made toward understanding the molecular mechanisms governing complex microtubule patterning. This review first revisits the basic principle of microtubule dynamics, and then provides an overview of how microtubules are arranged in highly shaped and functional patterns in cells changing their morphology by factors controlling the fate of microtubule ends.

Molecular basis for the recognition of adenomatous polyposis coli by the Discs Large 1 protein

The human Discs Large 1 (DLG1) protein uses two of its three PDZ domains to interact with the C-terminal peptide of the Adenomatous Polyposis Coli (APC) tumor suppressor protein. The DLG1/APC complex inhibits the cell cycle progression from the G0/G1 to the S phase, regulates epithelial cell migration and morphogenesis, and is required for polarization of the microtubule cytoskeleton. However, the molecular details of how DLG1 recognizes APC is not clear. In this study, we performed biochemical and biophysical assays to investigate the interactions between PDZ domains of DLG1 and the C-terminal peptide of APC. In addition, we determined the crystal structures of the PDZ1 and PDZ2 domains of DLG1 each in complex with the C-terminal 11-residue peptide of APC. Our biochemical, biophysical, and structural results revealed structural elements and residues on PDZ1 and PDZ2 domains of DLG1 and on APC crucial for their mutual interaction. In particular, our results show that the β2/β3 loops of PDZ1 and PDZ2 play important roles in contributing to the binding affinities between PDZ domains and APC, through interacting with the residues upstream of the canonical PDZ-binding S/T-X-V motif. The results provide new insights into the binding mode of a defined C-terminal segment of APC by the PDZ domains of DLG1.

PDZD8 is a novel moesin-interacting cytoskeletal regulatory protein that suppresses infection by herpes simplex virus type 1

The host cytoskeleton plays a central role in the life cycle of many viruses yet our knowledge of cytoskeletal regulators and their role in viral infection remains limited. Recently, moesin and ezrin, two members of the ERM (Ezrin/Radixin/Moesin) family of proteins that regulate actin and plasma membrane cross-linking and microtubule (MT) stability, have been shown to inhibit retroviral infection. To further understand how ERM proteins function and whether they also influence infection by other viruses, we identified PDZD8 as a novel moesin-interacting protein. PDZD8 is a poorly understood protein whose function is unknown. Exogenous expression of either moesin or PDZD8 reduced the levels of stable MTs, suggesting that these proteins functioned as part of a cytoskeletal regulatory complex. Additionally, exogenous expression or siRNA-mediated knockdown of either factor affected Herpes Simplex Virus type 1 (HSV-1) infection, identifying a cellular function for PDZD8 and novel antiviral properties for these two cytoskeletal regulatory proteins.

Methods for expressing and analyzing GFP-tubulin and GFP-microtubule-associated proteins

Important advances in our understanding of the organization and dynamics of the cytoskeleton have been made by direct observations of fluorescently tagged cytoskeletal proteins in living cells. In early experiments, the cytoskeletal protein of interest was purified, covalently modified with a fluorescent dye, and microinjected into living cells. In the mid-1990s, a powerful new technology arose: Researchers developed methods for expressing chimeric proteins consisting of the gene of interest fused to green fluorescent protein (GFP). This approach has become a standard method for characterizing protein localization and dynamics. More recently, a profusion of "XFP" (spectral variants of GFP) has been developed, allowing researchers straightforwardly to perform experiments ranging from simultaneous co-observation of protein dynamics to fluorescence recovery after photobleaching (FRAP), fluorescence resonance energy transfer (FRET), and subresolution techniques such as stimulated emission-depletion microscopy (STED) and photoactivated localization microscopy (PALM). In this article, the methods used to express and analyze GFP- and/or XFP-tagged tubulin and microtubule-associated proteins (MAPs) are discussed. Although some details may be system-specific, the methods and considerations outlined here can be adapted to a wide variety of proteins and organisms.

LKB1-mediated spatial control of GSK3beta and adenomatous polyposis coli contributes to centrosomal forward movement and neuronal migration in the developing neocortex

Neuronal migration is an essential process for the development of the cerebral cortex. We have previously shown that LKB1, an evolutionally conserved polarity kinase, plays a critical role in neuronal migration in the developing neocortex. Here we show that LKB1 mediates Ser9 phosphorylation of GSK3beta to inactivate the kinase at the leading process tip of migrating neurons in the developing neocortex. This enables the microtubule plus-end binding protein adenomatous polyposis coli (APC) to localize at the distal ends of microtubules in the tip, thereby stabilizing microtubules near the leading edge. We also show that LKB1 activity, Ser9 phosphorylation of GSK3beta, and APC binding to the distal ends of microtubules are required for the microtubule stabilization in the leading process tip, centrosomal forward movement, and neuronal migration. These findings suggest that LKB1-induced spatial control of GSK3beta and APC at the leading process tip mediates the stabilization of microtubules within the tip and is critical for centrosomal forward movement and neuronal migration in the developing neocortex.

Laminin-based cell adhesion anchors microtubule plus ends to the epithelial cell basal cortex through LL5alpha/beta

A newly discovered interaction between LL5s, laminins, and integrins reveals how the extracellular matrix directs microtubule polarity in epithelial tissues.

LL5β has been identified as a microtubule-anchoring factor that attaches EB1/CLIP-associating protein (CLASP)–bound microtubule plus ends to the cell cortex. In this study, we show that LL5β and its homologue LL5α (LL5s) colocalize with autocrine laminin-5 and its receptors, integrins α3β1 and α6β4, at the basal side of fully polarized epithelial sheets. Depletion of both laminin receptor integrins abolishes the cortical localization of LL5s, whereas LL5 depletion reduces the amount of integrin α3 at the basal cell cortex. Activation of integrin α3 is sufficient to initiate LL5 accumulation at the cell cortex. LL5s form a complex with the cytoplasmic tails of these integrins, but their interaction might be indirect. Analysis of the three-dimensional distribution of microtubule growth by visualizing EB1-GFP in epithelial sheets in combination with RNA interference reveals that LL5s are required to maintain the density of growing microtubules selectively at the basal cortex. These findings reveal that signaling from laminin–integrin associations attaches microtubule plus ends to the epithelial basal cell cortex.

Tumor suppressor interactions with microtubules: keeping cell polarity and cell division on track

Tumor suppressor proteins protect cells and tissues from malignant transformation. Among their diverse actions, many of these proteins interact with the microtubule cytoskeleton. This review focuses on the interactions of several tumor suppressors with microtubules and speculates on how disruption of microtubule-dependent processes may contribute to cancer development and spread. We conclude that several tumor suppressors stabilize microtubules and organize microtubule arrays, functions that are likely to be important in preventing tumorigenesis. How tumor suppressors link microtubule stability with cell fate, and how their mutation affects the response of cancer cells to anti-microtubule chemotherapy drugs, remains unclear; these should prove fertile areas for future research.

CASK deletion in intestinal epithelia causes mislocalization of LIN7C and the DLG1/Scrib polarity complex without affecting cell polarity

CASK is the mammalian ortholog of LIN2, a component of the LIN2/7/10 protein complex that targets epidermal growth factor receptor (EGFR) to basolateral membranes in Caenorhabditis elegans. A member of the MAGUK family of scaffolding proteins, CASK resides at basolateral membranes in polarized epithelia. Its interaction with LIN7 is evolutionarily conserved. In addition, CASK forms a complex with another MAGUK, the DLG1 tumor suppressor. Although complete knockout of CASK is lethal, the gene is X-linked, enabling us to generate heterozygous female adults that are mosaic for its expression. We also generated intestine-specific CASK knockout mice. Immunofluorescence analysis revealed that in intestine, CASK is not required for epithelial polarity or differentiation but is necessary for the basolateral localization of DLG1 and LIN7C. However, the subcellular distributions of DLG1 and LIN7C are independent of CASK in the stomach. Moreover, CASK and LIN7C show normal localization in dlg1(-/-) intestine. Despite the disappearance of basolateral LIN7C in CASK-deficient intestinal crypts, this epithelium retains normal localization of LIN7A/B, EGFR and ErbB-2. Finally, crypt-to-villus migration rates are unchanged in CASK-deficient intestinal epithelium. Thus, CASK expression and the appropriate localization of DLG1 are not essential for either epithelial polarity or intestinal homeostasis in vivo.

Serine protease HtrA1 associates with microtubules and inhibits cell migration

HtrA1 belongs to a family of serine proteases found in organisms ranging from bacteria to humans. Bacterial HtrA1 (DegP) is a heat shock-induced protein that behaves as a chaperone at low temperature and as a protease at high temperature to help remove unfolded proteins during heat shock. In contrast to bacterial HtrA1, little is known about the function of human HtrA1. Here, we report the first evidence that human HtrA1 is a microtubule-associated protein and modulates microtubule stability and cell motility. Intracellular HtrA1 is localized to microtubules in a PDZ (PSD95, Dlg, ZO1) domain-dependent, nocodazole-sensitive manner. During microtubule assembly, intracellular HtrA associates with centrosomes and newly polymerized microtubules. In vitro, purified HtrA1 promotes microtubule assembly. Moreover, HtrA1 cosediments and copurifies with microtubules. Purified HtrA1 associates with purified alpha- and beta-tubulins, and immunoprecipitation of endogenous HtrA1 results in coprecipitation of alpha-, beta-, and gamma-tubulins. Finally, downregulation of HtrA1 promotes cell motility, whereas enhanced expression of HtrA1 attenuates cell motility. These results offer an original identification of HtrA1 as a microtubule-associated protein and provide initial mechanistic insights into the role of HtrA1 in the regulation of cell motility by modulating microtubule stability.

Ca(2+) regulates the subcellular localization of adenomatous polyposis coli tumor suppressor protein

Microtubule (MT) plus-end tracking proteins (+TIPs) are involved in the regulation of MT plus-end dynamics and stabilization. It was reported previously that an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) induced by disruption of the plasma membrane stimulates rearrangement of MTs [T. Togo, Disruption of the plasma membrane stimulates rearrangement of microtubules and lipid traffic toward the wound site, J. Cell Sci. 119 (2006) 2780-2786], suggesting that some +TIPs are regulated by Ca(2+). In the present study, the behavior of adenomatous polyposis coli (APC) following an increase in [Ca(2+)](i) was observed using Xenopus A6 epithelial cell expressing GFP-tagged APC. An increase in [Ca(2+)](i) by cell membrane disruption or by ionomycin treatment induced dissociation of APC without depolymerizing MTs. Inhibition of a tyrosine kinase and GSK-3beta suppressed APC dissociation upon an increase in [Ca(2+)](i). Western blotting analysis showed that Ca(2+) transients activated GSK-3beta through a tyrosine kinase. These results suggest that Ca(2+) stimulates redistribution of APC through a tyrosine kinase- and GSK-3beta-dependent pathway.

Cell polarity proteins: common targets for tumorigenic human viruses

Loss of polarity and disruption of cell junctions are common features of epithelial-derived cancer cells, and mounting evidence indicates that such defects have a direct function in the pathology of cancer. Supporting this idea, results with several different human tumor viruses indicate that their oncogenic potential depends in part on a common ability to inactivate key cell polarity proteins. For example, adenovirus (Ad) type 9 is unique among human Ads by causing exclusively estrogen-dependent mammary tumors in experimental animals and in having E4 region-encoded open reading frame 1 (E4-ORF1) as its primary oncogenic determinant. The 125-residue E4-ORF1 protein consists of two separate protein-interaction elements, one of which defines a PDZ domain-binding motif (PBM) required for E4-ORF1 to induce both cellular transformation in vitro and tumorigenesis in vivo. Most notably, the E4-ORF1 PBM mediates interactions with a selected group of cellular PDZ proteins, three of which include the cell polarity proteins Dlg1, PATJ and ZO-2. Data further indicate that these interactions promote disruption of cell junctions and a loss of cell polarity. In addition, one or more of the E4-ORF1-interacting cell polarity proteins, as well as the cell polarity protein Scribble, are common targets for the high-risk human papillomavirus (HPV) E6 or human T-cell leukemia virus type 1 (HTLV-1) Tax oncoproteins. Underscoring the significance of these observations, in humans, high-risk HPV and HTLV-1 are causative agents for cervical cancer and adult T-cell leukemia, respectively. Consequently, human tumor viruses should serve as powerful tools for deciphering mechanisms whereby disruption of cell junctions and loss of cell polarity contribute to the development of many human cancers. This review article discusses evidence supporting this hypothesis, with an emphasis on the human Ad E4-ORF1 oncoprotein.

One-hit effects in cancer: altered proteome of morphologically normal colon crypts in familial adenomatous polyposis

We studied patients with Familial Adenomatous Polyposis (FAP) because they are virtually certain to develop colon cancer, and because much is known about the causative APC gene. We hypothesized that the inherited heterozygous mutation itself leads to changes in the proteome of morphologically normal crypts and the proteins that changed may represent targets for preventive and therapeutic agents. We determined the differential protein expression of morphologically normal colon crypts of FAP patients versus those of individuals without the mutation, using two-dimensional gel electrophoresis, mass spectrometry, and validation by two-dimensional gel Western blotting. Approximately 13% of 1,695 identified proteins were abnormally expressed in the morphologically normal crypts of APC mutation carriers, indicating that a colon crypt cell under the one-hit state is already abnormal. Many of the expression changes affect pathways consistent with the function of the APC protein, including apoptosis, cell adhesion, cell motility, cytoskeletal organization and biogenesis, mitosis, transcription, and oxidative stress response. Thus, heterozygosity for a mutant APC tumor suppressor gene alters the proteome of normal-appearing crypt cells in a gene-specific manner, consistent with a detectable one-hit event. These changes may represent the earliest biomarkers of colorectal cancer development, potentially leading to the identification of molecular targets for cancer prevention.

Identification of a link between the SAMP repeats of adenomatous polyposis coli tumor suppressor and the Src homology 3 domain of DDEF

The adenomatous polyposis coli (APC) tumor suppressor protein is a multifunctional protein with a well characterized role in the Wnt signal transduction pathway and in cytoskeletal regulation. The SAMP repeats region of APC, an Axin-binding site, is known to be important for tumor suppression and for the developmental function of APC. We performed a yeast two-hybrid screening using the first SAMP motif-containing region of Xenopus APC as bait and obtained several SAMP binding candidates including DDEF2 (development and differentiation enhancing factor 2), which is an ADP-ribosylation factor (Arf) GTPase-activating protein (GAP (ArfGAP)) involved in the regulation of focal adhesions. In vitro and in cells the Src homology 3 (SH3) domain of DDEF2 and its close homolog, DDEF1, are associated with the SAMP motif of APC competitively with Axin1. Moreover, NMR chemical shift perturbation experiments revealed that the SAMP motif interacts at the same surface of the SH3 domain of DDEF as the known SH3 binding motif, PXXP. When fluorescent protein-tagged APC and DDEF are expressed in Xenopus A6 cells, co-localization at microtubule ends is observed. Overexpression and RNA interference experiments indicate that APC and DDEFs cooperatively regulate the distributions of microtubules and focal adhesions. Our findings reveal that the SAMP motif of APC specifically binds to the SH3 domains of DDEFs, providing new insights into the functions of APC in cell migration.

Role of adenomatous polyposis coli (APC) and microtubules in directional cell migration and neuronal polarization

In response to extracellular signals during embryonic development, cells undergo directional movements to specific sites and establish proper connections to other cells to form organs and tissues. Cell extension and migration in the direction of extracellular cues is mediated by the actin and microtubule cytoskeletons, and recent results have shed new light on how these pathways are activated by neurotrophins, Wnt or extracellular matrix. These signals lead to modifications of microtubule-associated proteins (MAPs) and point to glycogen synthase kinase (GSK) 3beta as a key regulator of microtubule function during directional migration. This review will summarize these results and then focus on the role of microtubule-binding protein adenomatous polyposis coli (APC) in neuronal polarization and directed migration, and on its regulation by GSK3beta.

Microtubule plus-end tracking proteins in differentiated mammalian cells

Differentiated mammalian cells are often characterized by highly specialized and polarized structure. Its formation and maintenance depends on cytoskeletal components, among which microtubules play an important role. The shape and dynamic properties of microtubule networks are controlled by multiple microtubule-associated factors. These include molecular motors and non-motor proteins, some of which accumulate specifically at the growing microtubule plus-ends (the so-called microtubule plus-end tracking proteins). Plus-end tracking proteins can contribute to the regulation of microtubule dynamics, mediate the cross-talk between microtubule ends, the actin cytoskeleton and the cell cortex, and participate in transport and positioning of structural and regulatory factors and membrane organelles. Malfunction of these proteins results in various human diseases including some forms of cancer, neurodevelopmental disorders and mental retardation. In this article we discuss recent data on microtubule dynamics and activities of microtubule plus-end binding proteins important for the physiology and pathology of differentiated mammalian cells such as neurons, polarized epithelia, muscle and sperm cells.

AMER1 regulates the distribution of the tumor suppressor APC between microtubules and the plasma membrane

APC is a multifunctional tumor suppressor protein that negatively controls Wnt signaling, but also regulates cell adhesion and migration by interacting with the plasma membrane and the microtubule cytoskeleton. Although the molecular basis for the microtubule association of APC is well understood, molecular mechanisms that underlie its plasma membrane localization have remained elusive. We show here that APC is recruited to the plasma membrane by binding to APC membrane recruitment 1 (AMER1), a novel membrane-associated protein that interacts with the ARM repeat domain of APC. The N-terminus of AMER1 contains two distinct phosphatidylinositol(4,5)-bisphosphate [PtdIns(4,5)P(2)]-binding domains, which mediate its localization to the plasma membrane. Overexpression of AMER1 increases APC levels and redirects APC from microtubule ends to the plasma membrane of epithelial cells. Conversely, siRNA-mediated knockdown of AMER1 reduces the overall levels of APC, promotes its association with microtubule ends in cellular protrusions and disturbs intercellular junctions. These data indicate that AMER1 controls the subcellular distribution of APC between membrane- and microtubule-associated pools, and might thereby regulate APC-dependent cellular morphogenesis, cell migration and cell-cell adhesion.