Lung cancer-derived galectin-1 enhances tumorigenic potentiation of tumor-associated dendritic cells by expressing heparin-binding EGF-like growth factor

The interaction between cancer cells and their microenvironment is a vicious cycle that enhances the survival and progression of cancer, resulting in metastasis. This study is the first to indicate that lung cancer-derived galectin-1 secretion is responsible for stimulating tumor-associated dendritic cells (TADCs) production of mature heparin-binding EGF-like growth factor (HB-EGF), which, in turn, increases cancer progression. Treatment of galectin-1, present in large amounts in lung cancer conditioned medium and lung cancer patient sera, mimicked the inductive effect of lung cancer conditioned medium on the expression and ectodomain shedding of HB-EGF by TNFα-converting enzyme/a disintegrin and metalloproteinase 9 (ADAM9) and ADAM17. Significant up-regulation of HB-EGF has been seen in tumor-infiltrating CD11c(+) dendritic cells in human lung cancer samples. Active cleavage of HB-EGF in TADCs by ADAM9 and ADAM17 is associated with increased protein kinase C δ and Lyn signaling. Enhancement of HB-EGF production in TADCs increased the proliferation, migration, and epithelial-to-mesenchymal transition abilities of lung cancer. In contrast, inhibiting HB-EGF by siRNA suppressed TADC-mediated cancer progression. Moreover, mice injected with galectin-1 knockdown Lewis lung carcinoma showed decreased expression and ectodomain shedding of HB-EGF and reduced incidence of cancer development, resulting in increased survival rates. We demonstrate here for the first time that human and mouse DCs are a source of HB-EGF, an EGFR ligand with tumorigenic properties. Antagonists of the effect of lung cancer-derived galectin-1 on DCs and anti-HB-EGF blocking antibodies could, therefore, have therapeutic potential as antitumor agents.     

A Highly Unusual Thioester Bond in a Pilus Adhesin Is Required for Efficient Host Cell Interaction

Many bacterial pathogens present adhesins at the tips of long macromolecular filaments known as pili that are often important virulence determinants. Very little is known about how pili presented by Gram-positive pathogens mediate host cell binding. The crystal structure of a pilus adhesin from the important human pathogen Streptococcus pyogenes reveals an internal thioester bond formed between the side chains of a cysteine and a glutamine residue. The presence of the thioester was verified using UV-visible spectroscopy and mass spectrometry. This unusual bond has only previously been observed in thioester domains of complement and complement-like proteins where it is used to form covalent attachment to target molecules. The structure also reveals two intramolecular isopeptide bonds, one of these formed through a Lys/Asp residue pair, which are strategically positioned to confer protein stability. Removal of the internal thioester by allele-replacement mutagenesis in S. pyogenes severely compromises bacterial adhesion to model host cells. Although current paradigms of bacterial/host cell interaction envisage strong non-covalent interactions, the present study suggests cell adhesion could also involve covalent bonds.     

Zonadhesin Is Essential for Species Specificity of Sperm Adhesion to the Egg Zona Pellucida

Interaction of rapidly evolving molecules imparts species specificity to sperm-egg recognition in marine invertebrates, but it is unclear whether comparable interactions occur during fertilization in any vertebrate species. In mammals, the sperm acrosomal protein zonadhesin is a rapidly evolving molecule with species-specific binding activity for the egg zona pellucida (ZP). Here we show using null mice produced by targeted disruption of Zan that zonadhesin confers species specificity to sperm-ZP adhesion. Sperm capacitation selectively exposed a partial von Willebrand D domain of mouse zonadhesin on the surface of living, motile cells. Antibodies to the exposed domain inhibited adhesion of wild-type spermatozoa to the mouse ZP but did not inhibit adhesion of spermatozoa lacking zonadhesin. Zan^−/− males were fertile, and their spermatozoa readily fertilized mouse eggs in vitro. Remarkably, however, loss of zonadhesin increased adhesion of mouse spermatozoa to pig, cow, and rabbit ZP but not mouse ZP. We conclude that zonadhesin mediates species-specific ZP adhesion, and Zan^−/− males are fertile because their spermatozoa retain adhesion capability that is not species-specific. Mammalian sperm-ZP adhesion is therefore molecularly robust, and species-specific egg recognition by a protein in the sperm acrosome is conserved between invertebrates and vertebrates, even though the adhesion molecules themselves are unrelated.     

Crystal Structure of the cis-Dimer of Nectin-1: implications for the architecture of cell-cell junctions

In multicellular organisms, cells are interconnected by cell adhesion molecules. Nectins are immunoglobulin (Ig)-like cell adhesion molecules that mediate homotypic and heterotypic cell-cell adhesion, playing key roles in tissue organization. To mediate cell-cell adhesion, nectin molecules dimerize in cis on the surface of the same cell, followed by trans-dimerization of the cis-dimers between the neighboring cells. Previous cell biological studies deduced that the first Ig-like domain of nectin and the second Ig-like domain are involved in trans-dimerization and cis-dimerization, respectively. However, to understand better the steps involved in nectin adhesion, the structural basis for the dimerization of nectin must be determined. In this study, we determined the first crystal structure of the entire extracellular region of nectin-1. In the crystal, nectin-1 formed a V-shaped homophilic dimer through the first Ig-like domain. Structure-based site-directed mutagenesis of the first Ig-like domain identified four essential residues that are involved in the homophilic dimerization. Upon mutating the four residues, nectin-1 significantly decreased cis-dimerization on the surface of cultured cells and abolished the homophilic and heterophilic adhesion activities. These results indicate that, in contrast with the previous notion, our structure represents a cis-dimer. Thus, our findings clearly reveal the structural basis for the cis-dimerization of nectins through the first Ig-like domains.     

Phosphorylation of Actin-related Protein 2 (Arp2) Is Required for Normal Development and cAMP Chemotaxis in Dictyostelium

Phosphorylation of the actin-related protein 2 (Arp2) subunit of the Arp2/3 complex on evolutionarily conserved threonine and tyrosine residues was recently identified and shown to be necessary for nucleating activity of the Arp2/3 complex and membrane protrusion of Drosophila cells. Here we use the Dictyostelium diploid system to replace the essential Arp2 protein with mutants that cannot be phosphorylated at Thr-235/6 and Tyr-200. We found that aggregation of the resulting mutant cells after starvation was substantially slowed with delayed early developmental gene expression and that chemotaxis toward a cAMP gradient was defective with loss of polarity and attenuated F-actin assembly. Chemotaxis toward cAMP was also diminished with reduced cell speed and directionality and shorter pseudopod lifetime when Arp2 phosphorylation mutant cells were allowed to develop longer to a responsive state similar to that of wild-type cells. However, clathrin-mediated endocytosis and chemotaxis under agar to folate in vegetative cells were only subtly affected in Arp2 phosphorylation mutants. Thus, phosphorylation of threonine and tyrosine is important for a subset of the functions of the Arp2/3 complex, in particular an unexpected major role in regulating development.     

Rab13 Small G Protein and Junctional Rab13-binding Protein (JRAB) Orchestrate Actin Cytoskeletal Organization during Epithelial Junctional Development

During epithelial junctional development, both vesicle transport and reorganization of the actin cytoskeleton must be spatiotemporally regulated. Coordination of these cellular functions is especially important, but the precise mechanism remains elusive. Previously, we identified junctional Rab13-binding protein (JRAB)/molecules interacting with CasL-like 2 (MICAL-L2) as an effector of the Rab13 small G protein, and we found that the Rab13-JRAB system may be involved in the formation of cell-cell adhesions via transport of adhesion molecules. Here, we showed that JRAB interacts with two actin-binding proteins, actinin-1 and -4, and filamentous actin via different domains and regulates actin cross-linking and stabilization through these interactions. During epithelial junctional development, JRAB is prominently enriched in the actin bundle at the free border; subsequently, JRAB undergoes a Rab13-dependent conformational change that is required for maturation of cell-cell adhesion sites. These results suggest that Rab13 and JRAB regulate reorganization of the actin cytoskeleton throughout epithelial junctional development from establishment to maturation of cell-cell adhesion.     

A 130-kDa Protein 4.1B Regulates Cell Adhesion,Spreading, and Migration of Mouse Embryo Fibroblasts by Influencing Actin Cytoskeleton Organization

Protein 4.1B is a member of protein 4.1 family, adaptor proteins at the interface of membranes and the cytoskeleton. It is expressed in most mammalian tissues and is known to be required in formation of nervous and cardiac systems; it is also a tumor suppressor with a role in metastasis. Here, we explore functions of 4.1B using primary mouse embryonic fibroblasts (MEF) derived from wild type and 4.1B knock-out mice. MEF cells express two 4.1B isoforms: 130 and 60-kDa. 130-kDa 4.1B was absent from 4.1B knock-out MEF cells, but 60-kDa 4.1B remained, suggesting incomplete knock-out. Although the 130-kDa isoform was predominantly located at the plasma membrane, the 60-kDa isoform was enriched in nuclei. 130-kDa-deficient 4.1B MEF cells exhibited impaired cell adhesion, spreading, and migration; they also failed to form actin stress fibers. Impaired cell spreading and stress fiber formation were rescued by re-expression of the 130-kDa 4.1B but not the 60-kDa 4.1B. Our findings document novel, isoform-selective roles for 130-kDa 4.1B in adhesion, spreading, and migration of MEF cells by affecting actin organization, giving new insight into 4.1B functions in normal tissues as well as its role in cancer.     

Vinculin Is a Dually Regulated Actin Filament Barbed End-capping and Side-binding Protein

The focal adhesion protein vinculin is an actin-binding protein involved in the mechanical coupling between the actin cytoskeleton and the extracellular matrix. An autoinhibitory interaction between the N-terminal head (Vh) and the C-terminal tail (Vt) of vinculin masks an actin filament side-binding domain in Vt. The binding of several proteins to Vh disrupts this intramolecular interaction and exposes the actin filament side-binding domain. Here, by combining kinetic assays and microscopy observations, we show that Vt inhibits actin polymerization by blocking the barbed ends of actin filaments. In low salt conditions, Vt nucleates actin filaments capped at their barbed ends. We determined that the interaction between vinculin and the barbed end is characterized by slow association and dissociation rate constants. This barbed end capping activity requires C-terminal amino acids of Vt that are dispensable for actin filament side binding. Like the side-binding domain, the capping domain of vinculin is masked by an autoinhibitory interaction between Vh and Vt. In contrast to the side-binding domain, the capping domain is not unmasked by the binding of a talin domain to Vh and requires the dissociation of an additional autoinhibitory interaction. Finally, we show that vinculin and the formin mDia1, which is involved in the processive elongation of actin filaments in focal adhesions, compete for actin filament barbed ends.     

The disintegrin-like and cysteine-rich domains of ADAM-9 mediate interactions between melanoma cells and fibroblasts

A characteristic of malignant cells is their capacity to invade their surrounding and to metastasize to distant organs. During these processes, proteolytic activities of tumor and stromal cells modify the extracellular matrix to produce a microenvironment suitable for their growth and migration. In recent years the family of ADAM proteases has been ascribed important roles in these processes. ADAM-9 is expressed in human melanoma at the tumor-stroma border where direct or indirect interactions between tumor cells and fibroblasts occur. To analyze the role of ADAM-9 for the interaction between melanoma cells and stromal fibroblasts, we produced the recombinant disintegrin-like and cysteine-rich domain of ADAM-9 (DC-9). Melanoma cells and human fibroblasts adhered to immobilized DC-9 in a Mn(2+)-dependent fashion suggesting an integrin-mediated process. Inhibition studies showed that adhesion of fibroblasts was mediated by several β1 integrin receptors independent of the RGD and ECD recognition motif. Furthermore, interaction of fibroblasts and high invasive melanoma cells with soluble recombinant DC-9 resulted in enhanced expression of MMP-1 and MMP-2. Silencing of ADAM-9 in melanoma cells significantly reduced cell adhesion to fibroblasts. Ablation of ADAM-9 in fibroblasts almost completely abolished these cellular interactions and melanoma cell invasion in vitro. In summary, these results suggest that ADAM-9 expression plays an important role in mediating cell-cell contacts between fibroblasts and melanoma cells and that these interactions contribute to proteolytic activities required during invasion of melanoma cells.     

Rapid Fusion and Syncytium Formation of Heterologous Cells upon Expression of the FGFRL1 Receptor

The fusion of mammalian cells into syncytia is a developmental process that is tightly restricted to a limited subset of cells. Besides gamete and placental trophoblast fusion, only macrophages and myogenic stem cells fuse into multinucleated syncytia. In contrast to viral cell fusion, which is mediated by fusogenic glycoproteins that actively merge membranes, mammalian cell fusion is poorly understood at the molecular level. A variety of mammalian transmembrane proteins, among them many of the immunoglobulin superfamily, have been implicated in cell-cell fusion, but none has been shown to actively fuse cells in vitro. Here we report that the FGFRL1 receptor, which is up-regulated during the differentiation of myoblasts into myotubes, fuses cultured cells into large, multinucleated syncytia. We used luciferase and GFP-based reporter assays to confirm cytoplasmic mixing and to identify the fusion inducing domain of FGFRL1. These assays revealed that Ig-like domain III and the transmembrane domain are both necessary and sufficient to rapidly fuse CHO cells into multinucleated syncytia comprising several hundred nuclei. Moreover, FGFRL1 also fused HEK293 and HeLa cells with untransfected CHO cells. Our data show that FGFRL1 is the first mammalian protein that is capable of inducing syncytium formation of heterologous cells in vitro.     

Src Induces Podoplanin Expression to Promote Cell Migration

Nontransformed cells can force tumor cells to assume a normal morphology and phenotype by the process of contact normalization. Transformed cells must escape this process to become invasive and malignant. However, mechanisms underlying contact normalization have not been elucidated. Here, we have identified genes that are affected by contact normalization of Src-transformed cells. Tumor cells must migrate to become invasive and malignant. Src must phosphorylate the adaptor protein Cas (Crk-associated substrate) to promote tumor cell motility. We report here that Src utilizes Cas to induce podoplanin (Pdpn) expression to promote tumor cell migration. Pdpn is a membrane-bound extracellular glycoprotein that associates with endogenous ligands to promote tumor cell migration leading to cancer invasion and metastasis. In fact, Pdpn expression accounted for a major part of the increased migration seen in Src-transformed cells. Moreover, nontransformed cells suppressed Pdpn expression in adjacent Src-transformed cells. Of >39,000 genes, Pdpn was one of only 23 genes found to be induced by transforming Src activity and suppressed by contact normalization of Src-transformed cells. In addition, we found 16 genes suppressed by Src and induced by contact normalization. These genes encode growth factor receptors, adaptor proteins, and products that have not yet been annotated and may play important roles in tumor cell growth and migration.     

Epithelial protein lost in neoplasm (EPLIN) interacts with α-catenin and actin filaments in endothelial cells and stabilizes vascular capillary network in vitro

Adherens junctions are required for vascular endothelium integrity. These structures are formed by the clustering of the homophilic adhesive protein VE-cadherin, which recruits intracellular partners, such as β- and α-catenins, vinculin, and actin filaments. The dogma according to which α-catenin bridges cadherin·β-catenin complexes to the actin cytoskeleton has been challenged during the past few years, and the link between the VE-cadherin·catenin complex and the actin cytoskeleton remains unclear. Recently, epithelial protein lost in neoplasm (EPLIN) has been proposed as a possible bond between the E-cadherin·catenin complex and actin in epithelial cells. Herein, we show that EPLIN is expressed at similar levels in endothelial and epithelial cells and is located at interendothelial junctions in confluent cells. Co-immunoprecipitation and GST pulldown experiments provided evidence that EPLIN interacts directly with α-catenin and tethers the VE-cadherin·catenin complex to the actin cytoskeleton. In the absence of EPLIN, vinculin was delocalized from the junctions. Furthermore, suppression of actomyosin tension using blebbistatin triggered a similar vinculin delocalization from the junctions. In a Matrigel assay, EPLIN-depleted endothelial cells exhibited a reduced capacity to form pseudocapillary networks because of numerous breakage events. In conclusion, we propose a model in which EPLIN establishes a link between the cadherin·catenin complex and actin that is independent of actomyosin tension. This link acts as a mechanotransmitter, allowing vinculin binding to α-catenin and formation of a secondary molecular bond between the adherens complex and the cytoskeleton through vinculin. In addition, we provide evidence that the EPLIN clutch is necessary for stabilization of capillary structures in an angiogenesis model.     

The CC' and DE loops in Ig domains 1 and 2 of MAdCAM-1 play different roles in MAdCAM-1 binding to low- and high-affinity integrin alpha4beta7

Lymphocyte homing is regulated by the dynamic interaction between integrins and their ligands. Integrin α4β7 mediates both rolling and firm adhesion of lymphocytes by modulating its affinity to the ligand, mucosal addressin cell adhesion molecule-1 (MAdCAM-1). Although previous studies have revealed some mechanisms of α4β7-MAdCAM-1 binding, little is known about the different molecular bases of the low- and high-affinity α4β7-MAdCAM-1 interactions, which mediate rolling and firm adhesion of lymphocytes, respectively. Here, we found that two loops in immunoglobulin domains 1 and 2 (D1 and D2) of MAdCAM-1 played different roles in MAdCAM-1 binding to low-affinity (inactive) and high-affinity (activated) α4β7. The Asp-42 in the CC' loop of D1 was indispensable for MAdCAM-1 binding to both low-affinity and high-affinity α4β7. The other CC' loop residues except for Arg-39 and Ser-44 were essential for MAdCAM-1 binding to both inactive α4β7 and α4β7 activated by SDF-1α or talin, but not required for MAdCAM-1 binding to Mn2+-activated α4β7. Single amino acid substitution of the DE loop residues mildly decreased MAdCAM-1 binding to both inactive and activated α4β7. Notably, removal of the DE loop greatly impaired MAdCAM-1 binding to inactive and SDF-1α- or talin-activated α4β7, but only decreased 60% of MAdCAM-1 binding to Mn2+-activated α4β7. Moreover, DE loop residues were important for stabilizing the low-affinity α4β7-MAdCAM-1 interaction. Thus, our findings demonstrate the distinct roles of the CC' and DE loops in the recognition of MAdCAM-1 by low- and high-affinity α4β7 and suggest that the inactive α4β7 and α4β7 activated by different stimuli have distinct conformations with different structural requirements for MAdCAM-1 binding.     

Vasodilator-stimulated Phosphoprotein (VASP) Induces Actin Assembly in Dendritic Spines to Promote Their Development and Potentiate Synaptic Strength

Dendritic spines are small actin-rich structures that receive the majority of excitatory synaptic input in the brain. The actin-based dynamics of spines are thought to mediate synaptic plasticity, which underlies cognitive processes, such as learning and memory. However, little is known about the molecular mechanisms that regulate actin dynamics in spines and synapses. In this study we show the multifunctional actin-binding protein vasodilator-stimulated phosphoprotein (VASP) regulates the density, size, and morphology of dendritic spines by inducing actin assembly in these structures. Knockdown of endogenous VASP by siRNA led to a significant decrease in the density of spines and synapses, whereas expression of siRNA-resistant VASP rescued this defect. The ability of VASP to modulate spine and synapse formation, maturation, and spine head enlargement is dependent on its actin binding Ena/VASP homology 2 (EVH2) domain and its EVH1 domain, which contributes to VASP localization to actin-rich structures. Moreover, VASP increases the amount of PSD-scaffolding proteins and the number of surface GluR1-containing α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) in spines. VASP knockdown results in a reduction in surface AMPAR density, suggesting a role for this protein in regulating synaptic strength. Consistent with this, VASP significantly enhances the retention of GluR1 in spines as determined by fluorescence recovery after photobleaching and increases AMPAR-mediated synaptic transmission. Collectively, our results suggest that actin polymerization and bundling by VASP are critical for spine formation, expansion, and modulating synaptic strength.     

Dynamic Phosphorylation of Tyrosine 665 in Pseudopodium-enriched Atypical Kinase 1 (PEAK1) Is Essential for the Regulation of Cell Migration and Focal Adhesion Turnover

Pseudopodium-enriched atypical kinase 1 (PEAK1) is a recently described tyrosine kinase that associates with the actin cytoskeleton and focal adhesion (FA) in migrating cells. PEAK1 is known to promote cell migration, but the responsible mechanisms remain unclear. Here, we show that PEAK1 controls FA assembly and disassembly in a dynamic pathway controlled by PEAK1 phosphorylation at Tyr-665. Knockdown of endogenous PEAK1 inhibits random cell migration. In PEAK1-deficient cells, FA lifetimes are decreased, FA assembly times are shortened, and FA disassembly times are extended. Phosphorylation of Tyr-665 in PEAK1 is essential for normal PEAK1 localization and its function in the regulation of FAs; however, constitutive phosphorylation of PEAK1 Tyr-665 is also disruptive of its function, indicating a requirement for precise spatiotemporal regulation of PEAK1. Src family kinases are required for normal PEAK1 localization and function. Finally, we provide evidence that PEAK1 promotes cancer cell invasion through Matrigel by a mechanism that requires dynamic regulation of Tyr-665 phosphorylation.     

Protein Kinase D Controls Actin Polymerization and Cell Motility through Phosphorylation of Cortactin

We here identify protein kinase D (PKD) as an upstream regulator of the F-actin-binding protein cortactin and the Arp actin polymerization machinery. PKD phosphorylates cortactin in vitro and in vivo at serine 298 thereby generating a 14-3-3 binding motif. In vitro, a phosphorylation-deficient cortactin-S298A protein accelerated VCA-Arp-cortactin-mediated synergistic actin polymerization and showed reduced F-actin binding, indicative of enhanced turnover of nucleation complexes. In vivo, cortactin co-localized with the nucleation promoting factor WAVE2, essential for lamellipodia extension, in the actin polymerization zone in Heregulin-treated MCF-7 cells. Using a 3-dye FRET-based approach we further demonstrate that WAVE2-Arp and cortactin prominently interact at these structures. Accordingly, cortactin-S298A significantly enhanced lamellipodia extension and directed cell migration. Our data thus unravel a previously unrecognized mechanism by which PKD controls cancer cell motility.