Involvement of the c-Src-Crk-C3G-Rap1 Signaling in the Nectin-induced Activation of Cdc42 and Formation of Adherens Junctions

The Rap1 guanine nucleotide exchange factor C3G is required for preservation of larval muscle integrity in Drosophila melanogaster

C3G is a guanine nucleotide exchange factor (GEF) and modulator of small G-protein activity, which primarily acts on members of the Rap GTPase subfamily. Via promotion of the active GTP bound conformation of target GTPases, C3G has been implicated in the regulation of multiple cellular and developmental events including proliferation, differentiation and apoptosis. The Drosophila C3G orthologue exhibits a domain organization similar to that of vertebrate C3G. Through deletion of the C3G locus, we have observed that loss of C3G causes semi-lethality, and that escaping adult flies are characterized by a reduction in lifespan and general fitness. In situ hybridization reveals C3G expression in the developing embryonic somatic and visceral muscles, and indeed analysis of C3G mutants suggests essential functions of C3G for normal body wall muscle development during larval stages. C3G mutants display abnormal muscle morphology and attachment, as well as failure to properly localize βPS integrins to muscle attachment sites. Moreover, we show that C3G stimulates guanine nucleotide exchange on Drosophila Rap GTPases in vitro. Taken together, we conclude that Drosophila C3G is a Rap1-specific GEF with important functions in maintaining muscle integrity during larval stages.

Localization of nectin-free afadin at the leading edge and its involvement in directional cell movement induced by platelet-derived growth factor

Afadin is an actin-filament-binding protein that binds to nectin, an immunoglobulin-like cell-cell adhesion molecule, and plays an important role in the formation of adherens junctions. Here, we show that afadin, which did not bind to nectin and was localized at the leading edge of moving cells, has another role: enhancement of the directional, but not random, cell movement. When NIH3T3 cells were stimulated with platelet-derived growth factor (PDGF), afadin colocalized with PDGF receptor, αvβ3 integrin and nectin-like molecule-5 at the leading edge and facilitated the formation of leading-edge structures and directional cell movement in the direction of PDGF stimulation. However, these phenotypes were markedly perturbed by knockdown of afadin, and were dependent on the binding of afadin to active Rap1. Binding of Rap1 to afadin was necessary for the recruitment of afadin and the tyrosine phosphatase SHP-2 to the leading edge. SHP-2 was previously reported to tightly regulate the activation of PDGF receptor and its downstream signaling pathway for the formation of the leading edge. These results indicate that afadin has a novel role in PDGF-induced directional cell movement, presumably in cooperation with active Rap1 and SHP-2.

Regulation by afadin of cyclical activation and inactivation of Rap1, Rac1, and RhoA small G proteins at leading edges of moving NIH3T3 cells

Cyclical activation and inactivation of Rho family small G proteins, such as Rho, Rac, and Cdc42, are needed for moving cells to form leading edge structures in response to chemoattractants. However, the mechanisms underlying the dynamic regulation of their activities are not fully understood. We recently showed that another small G protein, Rap1, plays a crucial role in the platelet-derived growth factor (PDGF)-induced formation of leading edge structures and activation of Rac1 in NIH3T3 cells. We showed here that knockdown of afadin, an actin-binding protein, in NIH3T3 cells resulted in a failure to develop leading edge structures in association with an impairment of the activation of Rap1 and Rac1 and inactivation of RhoA in response to PDGF. Overexpression of a constitutively active mutant of Rap1 (Rap1-CA) and knockdown of SPA-1, a Rap1 GTPase-activating protein that was negatively regulated by afadin by virtue of binding to it, in afadin-knockdown NIH3T3 cells restored the formation of leading edge structures and the reduction of the PDGF-induced activation of Rac1 and inactivation of RhoA, suggesting that the inactivation of Rap1 by SPA-1 is responsible for inhibition of the formation of leading edge structures. The effect of Rap1-CA on the restoration of the formation of leading edge structures and RhoA inactivation was diminished by additional knockdown of ARAP1, a Rap-activated Rho GAP, which localized at the leading edges of moving NIH3T3 cells. These results indicate that afadin regulates the cyclical activation and inactivation of Rap1, Rac1, and RhoA through SPA-1 and ARAP1.

Crk and CrkL adaptor proteins: networks for physiological and pathological signaling

The Crk adaptor proteins (Crk and CrkL) constitute an integral part of a network of essential signal transduction pathways in humans and other organisms that act as major convergence points in tyrosine kinase signaling. Crk proteins integrate signals from a wide variety of sources, including growth factors, extracellular matrix molecules, bacterial pathogens, and apoptotic cells. Mounting evidence indicates that dysregulation of Crk proteins is associated with human diseases, including cancer and susceptibility to pathogen infections. Recent structural work has identified new and unusual insights into the regulation of Crk proteins, providing a rationale for how Crk can sense diverse signals and produce a myriad of biological responses.

E-cadherin dis-engagement activates the Rap1 GTPase

E-cadherin based adherens junctions are finely regulated by multiple cellular signaling events. Here we show that the Ras-related Rap1 GTPase is enriched in regions of nascent cell-cell contacts and strengthens E-cadherin junctions: constitutively active Rap1 expressing MDCK cells exhibit increased junctional contact and resisted calcium depletion-induced cell-cell junction disruption. E-cadherin disengagement activated Rap1 and this correlated with E-cadherin association with the Rap GEFs, C3G and PDZ-GEF I. PDZ-GEF I associated with E-cadherin and beta-catenin whereas C3G interaction with E-cadherin did not involve beta-catenin. Knockdown of PDZ-GEF I in MDCK cells decreased Rap1 activity following E-cadherin junction disruption. We hereby show that Rap1 plays a role in the maintenance and repair of E-cadherin junctions and is activated via an "outside-in" signaling pathway initiated by E-cadherin and mediated at least in part by PDZ-GEF I.

Cell-cell junction formation: the role of Rap1 and Rap1 guanine nucleotide exchange factors

Rap proteins are Ras-like small GTP-binding proteins that amongst others are involved in the control of cell-cell and cell-matrix adhesion. Several Rap guanine nucleotide exchange factors (RapGEFs) function to activate Rap. These multi-domain proteins, which include C3G, Epacs, PDZ-GEFs, RapGRPs and DOCK4, are regulated by various different stimuli and may function at different levels in junction formation. Downstream of Rap, a number of effector proteins have been implicated in junctional control, most notably the adaptor proteins AF6 and KRIT/CCM1. In this review, we will highlight the latest findings on the Rap signaling network in the control of epithelial and endothelial cell-cell junctions.

The immunoglobulin-like cell adhesion molecule nectin and its associated protein afadin

Nectins are immunoglobulin-like cell adhesion molecules (CAMs) that compose a family of four members. Nectins homophilically and heterophilically interact in trans with each other to form cell-cell adhesions. In addition, they heterophilically interact in trans with other immunoglobulin-like CAMs. Nectins bind afadin, an actin filament (F-actin)-binding protein, at its cytoplasmic tail and associate with the actin cytoskeleton. Afadin additionally serves as an adaptor protein by further binding many scaffolding proteins and F-actin-binding proteins and contributes to the association of nectins with other cell-cell adhesion and intracellular signaling systems. Nectins and afadin play roles in the formation of a variety of cell-cell junctions cooperatively with, or independently of, cadherins. Cooperation between nectins and cadherins is required for the formation of cell-cell junctions; cadherins alone are not sufficient. Additionally, nectins regulate many other cellular activities (such as movement, proliferation, survival, differentiation, polarization, and the entry of viruses) in cooperation with other CAMs and cell surface membrane receptors.

Roles of cell adhesion molecules nectin and nectin-like molecule-5 in the regulation of cell movement and proliferation

In response to chemoattractants, migrating cells form protrusions, such as lamellipodia and filopodia, and structures, such as ruffles over lamellipodia, focal complexes and focal adhesions at leading edges. The formation of these leading edge structures is essential for directional cell movement. Nectin-like molecule-5 (Necl-5) interacts in cis with PDGF receptor and integrin alpha(v)beta(3), and enhances the activation of signalling molecules associated with these transmembrane proteins, which results in the formation of leading edge structures and enhancement of directional cell movement. When migrating cells come into contact with each other, cell-cell adhesion is initiated, resulting in reduced cell velocity. Necl-5 first interacts in trans with nectin-3. This interaction is transient and induces down-regulation of Necl-5 expression at the cell surface, resulting in reduced cell movement. Cell proliferation is also suppressed by the down-regulation of Necl-5, because the inhibitory effect of Necl-5 on Sprouty2, a negative regulator of the Ras signalling, is diminished. PDGF receptor and integrin alpha(v)beta(3), which have interacted with Necl-5, then form a complex with nectin, which initiates cell-cell adhesion and recruits cadherin to the nectin-based cell-cell adhesion sites to form stable adherens junctions. The formation of adherens junctions stops cell movement, in part through inactivation of integrin alpha(v)beta(3) caused by the trans-interaction of nectin. Thus, nectin and Necl-5 play key roles in the regulation of cell movement and proliferation.

The guanine nucleotide exchange factor, C3G regulates differentiation and survival of human neuroblastoma cells

Neuronal differentiation involving neurite growth is dependent on environmental cues which are relayed by signalling pathways to actin cytoskeletal remodelling. C3G, the exchange factor for Rap1, functions in pathways leading to actin reorganization and filopodia formation, processes required during neurite growth. In the present study, we have analyzed the function of C3G, in regulating neuronal cell survival and plasticity. Human neuroblastoma cells, IMR-32 induced to differentiate by serum starvation or by treatment with nerve growth factor (NGF) or forskolin showed enhanced C3G protein levels. Transient over-expression of C3G stimulated neurite growth and also increased responsiveness to NGF and serum deprivation induced differentiation. C3G-induced neurite growth was dependent on both its catalytic and N-terminal regulatory domains, and on the functions of Cdc42 and Rap1. Knockdown of C3G using small hairpin RNA inhibited forskolin and NGF-induced morphological differentiation of IMR-32 cells. Forskolin-induced differentiation was dependent on catalytic activity of C3G. Forskolin and NGF treatment resulted in phosphorylation of C3G at Tyr504 predominantly in the Golgi. C3G expression induced the cell cycle inhibitor p21 and C3G knockdown enhanced cell death in response to serum starvation. These findings demonstrate a novel function for C3G in regulating survival and differentiation of human neuroblastoma cells.

Changes in regulation of cell-cell adhesion during tumor transformation

Cadherin-mediated cell-cell adhesion defines the integrity of most tissues. Cell-cell adherens junctions are dynamic structures whose functional state is regulated by interactions of cadherin with beta-catenin, p120, and actin cytoskeleton structures. Small GTPases of the Rho family and GTPase Rap1 play the central role in the formation and maintenance of cell-cell adhesion. Aberrant activation of signaling pathways, transcriptional repression of the E-cadherin gene, ectopic expression of N-cadherin, and disturbances in regulation of adhesive and transcriptional functions of beta-catenin stimulate tumor progression.

Nectins and nectin-like molecules: roles in contact inhibition of cell movement and proliferation

Nectins and nectin-like molecules (Necls) are immunoglobulin-like transmembrane cell adhesion molecules that are expressed in various cell types. Homophilic and heterophilic engagements between family members provide cells with molecular tools for intercellular communications. Nectins primarily regulate cell-cell adhesions, whereas Necls are involved in a greater variety of cellular functions. Recent studies have revealed that nectins and NECL-5, in cooperation with integrin alphavbeta3 and platelet-derived growth factor receptor, are crucial for the mechanisms that underlie contact inhibition of cell movement and proliferation; this has important implications for the development and tissue regeneration of multicellular organisms and the phenotypes of cancer cells.

Involvement of the nectin-afadin complex in PDGF-induced cell survival

The nectin-afadin complex is involved in the formation of cell-cell junctions, such as adherens junctions (AJs) and tight junctions (TJs). Nectins are Ca2+-independent immunoglobulin-like cell-cell adhesion molecules, whereas afadin is an intracellular nectin-binding protein that connects nectins to the cadherin-catenin system at AJs and to the claudin–zona-occludens (ZO) protein system at TJs. Afadin–/– mice show embryonic lethality, resulting from impaired migration and improper differentiation of cells due to disorganization of cell-cell junctions during gastrulation. However, it remains to be elucidated whether disruption of afadin affects apoptosis. In the present study, we first found that embryoid bodies derived from afadin-knockout embryonic stem (ES) cells contained many more apoptotic cells than those derived from wild-type ES cells. We also revealed that apoptosis induced by serum starvation or Fas-ligand stimulation was increased in cultured NIH3T3 cells when afadin or nectin-3 was knocked down. The nectin-afadin complex was involved in the platelet-derived growth factor (PDGF)-induced activation of phosphatidylinositol 3-kinase (PI3K)-Akt signaling for cell survival. This complex was associated with PDGF receptor on the plasma membrane at cell-cell adhesion sites. Thus, the nectin-afadin complex is involved in PDGF-induced cell survival, at least through the PI3K-Akt signaling pathway.

The RapGEF PDZ-GEF2 is required for maturation of cell-cell junctions

The small G-protein Rap1 is a critical regulator of cell-cell contacts and is activated by the remodeling of adherens junctions. Here we identify the Rap1 guanine nucleotide exchange factor PDZ-GEF2 as an upstream activator of Rap1 required for the maturation of adherens junctions in the lung carcinoma cells A549. Knockdown of PDZ-GEF2 results in the persistence of adhesion zippers at cell-cell contacts. Activation of Rap1A rescues junction maturation in absence of PDZ-GEF2, demonstrating that Rap1A is downstream of PDZ-GEF2 in this process. Moreover, depletion of Rap1A, but not Rap1B, impairs adherens junction maturation. siRNA for PDZ-GEF2 also lowers the levels of E-cadherin, an effect that can be mimicked by Rap1B, but not Rap1A siRNA. Since junctions in Rap1B depleted cells have a mature appearance, these data suggest that PDZ-GEF2 activates Rap1A and Rap1B to perform different functions. Our results present the first direct evidence that PDZ-GEF2 plays a critical role in the maturation of adherens junctions.

Cross-talk among integrin, cadherin, and growth factor receptor: roles of nectin and nectin-like molecule

Integrin, cadherin, and growth factor receptor are key molecules for fundamental cellular functions including cell movement, proliferation, differentiation, adhesion, and survival. These cell surface molecules cross-talk with each other in the regulation of such cellular functions. Nectin and nectin-like molecule (Necl) have been identified as cell adhesion molecules that belong to the immunoglobulin superfamily. Nectin and Necl play important roles in the integration of integrin, cadherin, and growth factor receptor at the cell-cell adhesion sites of contacting cells and at the leading edges of moving cells, and thus are also involved in the fundamental cellular functions together with integrin, cadherin, and growth factor receptor. This chapter describes how newly identified cell adhesion molecules, nectin and Necl, modulate the cross-talk among integrin, cadherin, and growth factor receptor and how these integrated molecules act in the regulation of fundamental cellular functions.

Frabin and other related Cdc42-specific guanine nucleotide exchange factors couple the actin cytoskeleton with the plasma membrane

Frabin, together with, at least, FGD1, FGD2, FGD3 and FGD1-related Cdc42-GEF (FRG), is a member of a family of Cdc42-specific gua-nine nucleotide exchange factors (GEFs). These proteins have multiple phosphoinositide-binding domains, including two pleckstrin homology (PH) domains and an FYVE or FERM domain. It is likely that they couple the actin cytoskeleton with the plasma membrane. Frabin associates with a specific actin structure(s) and induces the direct activation of Cdc42 in the vicinity of this structure(s), resulting in actin reorganization. Furthermore, frabin associates with a specific membrane structure(s) and induces the indirect activation of Rac in the vicinity of this structure(s), resulting in the reorganization of the actin cytoskeleton. This reorganization of the actin cytoskeleton induces cell shape changes such as the formation of filopodia and lamellipodia.

Organization of multiprotein complexes at cell-cell junctions

The formation of stable cell–cell contacts is required for the generation of barrier-forming sheets of epithelial and endothelial cells. During various physiological processes like tissue development, wound healing or tumorigenesis, cellular junctions are reorganized to allow the release or the incorporation of individual cells. Cell–cell contact formation is regulated by multiprotein complexes which are localized at specific structures along the lateral cell junctions like the tight junctions and adherens junctions and which are targeted to these site through their association with cell adhesion molecules. Recent evidence indicates that several major protein complexes exist which have distinct functions during junction formation. However, this evidence also indicates that their composition is dynamic and subject to changes depending on the state of junction maturation. Thus, cell–cell contact formation and integrity is regulated by a complex network of protein complexes. Imbalancing this network by oncogenic proteins or pathogens results in barrier breakdown and eventually in cancer. Here, I will review the molecular organization of the major multiprotein complexes at junctions of epithelial cells and discuss their function in cell–cell contact formation and maintenance.

Integrin regulation of caveolin function

Caveolae are unique organelles that are found in the plasma membrane of many cell types. They participate in various processes such as lipid recycling, cellular signalling and endocytosis. A variety of signalling molecules localize to caveolae in response to various stimuli, providing a potential mechanism for the spatial regulation of signal transduction pathways. Caveolin-1, a constitutive protein of caveolae, has been implicated in the regulation of cell growth, lipid trafficking, endocytosis and cell migration. Phosphorylation of caveolin-1 on Tyr 14 is involved in integrin-regulated caveolae trafficking and also in signalling at focal adhesions in migrating cells. In this review, we focus on recent studies that describe the role of caveolin-1 in integrin signal transduction, and how this interplay links extracellular matrix anchorage to cell proliferation, polarity and directional migration.

Cdc42 regulates E-cadherin ubiquitination and degradation through an epidermal growth factor receptor to Src-mediated pathway

E-cadherins play an essential role in maintaining epithelial polarity by forming Ca2+-dependent adherens junctions between epithelial cells. Here, we report that Ca2+ depletion induces E-cadherin ubiquitination and lysosomal degradation and that Cdc42 plays an important role in regulating this process. We demonstrate that Ca2+ depletion induces activation of Cdc42. This in turn up-regulates epidermal growth factor receptor (EGFR) signaling to mediate Src activation, leading to E-cadherin ubiquitination and lysosomal degradation. Silencing Cdc42 blocks activation of EGFR and Src induced by Ca2+ depletion, resulting in a reduction in E-cadherin degradation. The role of Cdc42 in regulating E-cadherin ubiquitination and degradation is underscored by the fact that constitutively active Cdc42(F28L) increases the activity of EGFR and Src and significantly enhances E-cadherin ubiquitination and lysosomal degradation. Furthermore, we found that GTP-dependent binding of Cdc42 to E-cadherin is critical for Cdc42 to induce the dissolution of adherens junctions. Our data support a model that activation of Cdc42 contributes to mesenchyme-like phenotype by targeting of E-cadherin for lysosomal degradation.

Cooperative roles of Par-3 and afadin in the formation of adherens and tight junctions

Par-3 is a cell-polarity protein that regulates the formation of tight junctions (TJs) in epithelial cells, where claudin is a major cell-cell adhesion molecule (CAM). TJs are formed at the apical side of adherens junctions (AJs), where E-cadherin and nectin are major CAMs. We have revealed that nectin first forms cell-cell adhesions, and then recruits cadherin to nectin-based cell-cell adhesion sites to form AJs and subsequently recruits claudin to the apical side of AJs to form TJs. The cytoplasmic tail of nectin binds afadin and Par-3. Afadin regulates the formation of AJs and TJs cooperatively with nectin. Here, we studied the role of Par-3 in the formation of these junctions by using Par-3-knockdown MDCK cells. Par-3 was necessary for the formation of AJs and TJs but was not necessary for nectin-based cell-cell adhesion. Par-3 promoted the association of afadin with nectin, whereas afadin was not necessary for the association of Par-3 with nectin. However, the association of afadin with nectin alone was not sufficient for the formation of AJs or TJs, and Par-3 and afadin cooperatively regulated it. We describe here these novel roles of Par-3 in the formation of junctional complexes.

Nectin and nectin-like molecules: biology and pathology

Nectins and nectin-like molecules (Necls) are structurally related transmembrane proteins primarily involved in cell adhesion. Nectins and afadin, the adaptor or anchoring protein, stabilize the epithelium and endothelium and establish apical-basal polarity of epithelial cells, independently or in cooperation with other cell adhesion molecules. Necls facilitate cell-cell communication implicated in cell movement and proliferation, immune responses, and cancer cell phenotypes. Necls interact with nectins and specific ligands at cell-cell contacts, whereas Necls associate with integrin alpha v beta 3 and growth factor receptors on the same cell surface. Besides their roles in cell adhesion, nectins regulate the activities of Rho family small G proteins which play critical roles in maintaining the apical junctions of epithelial cells through reorganization of the actin cytoskeleton. Since mice lacking the Rho GDP-dissociation inhibitor (GDI)alpha show massive proteinuria and degeneration of renal epithelial cells, nectins and other cell adhesion molecules may play roles in the structural and functional aspects of renal diseases. Here we summarize our knowledge of nectins and Necls and discuss cell adhesion biology in the kidney.

Tyrosine phosphorylation of β2-chimaerin by Src-family kinase negatively regulates its Rac-specific GAP activity

beta2-Chimaerin, an intracellular receptor for the second messenger diacylglycerol and phorbol esters, is a GTPase-activating protein (GAP) specific for Rac. beta2-Chimaerin negatively controls many Rac-dependent pathophysiological events including tumor development. However, the regulatory mechanism of beta2-chimaerin remains largely unknown. Here we report that beta2-chimaerin is tyrosine-phosphorylated by Src-family kinases (SFKs) upon cell stimulation with epidermal growth factor (EGF). Mutational analysis identified Tyr-21 in the N-terminal regulatory region as a major phosphorylation site. Intriguingly, the addition of SFK inhibitor and the replacement of Tyr-21 with Phe (Y21F) markedly enhanced Rac-GAP activity of beta2-chimaerin in EGF-treated cells. Moreover, the Y21F mutant inhibited integrin-dependent cell spreading, in which Rac1 plays a critical role, more strongly than wild-type beta2-chimaerin. These results suggest Tyr-21 phosphorylation as a novel, SFK-dependent mechanism that negatively regulates beta2-chimaerin Rac-GAP activity.

Smooth muscle cell signal transduction: implications of vascular biology for vascular surgeons

Vascular smooth muscle cells exhibit varied responses after vessel injury and surgical interventions, including phenotypic switching, migration, proliferation, protein synthesis, and apoptosis. Although the source of the smooth muscle cells that accumulate in the vascular wall is controversial, possibly reflecting migration from the adventitia, from the circulating blood, or in situ differentiation, the intracellular signal transduction pathways that control these processes are being defined. Some of these pathways include the Ras-mitogen-activated protein kinase, phosphatidylinositol 3-kinase-Akt, Rho, death receptor-caspase, and nitric oxide pathways. Signal transduction pathways provide amplification, redundancy, and control points within the cell and culminate in biologic responses. We review some of the signaling pathways activated within smooth muscle cells that contribute to smooth muscle cell heterogeneity and development of pathology such as restenosis and neointimal hyperplasia.

Caveolin-1 regulates cell polarization and directional migration through Src kinase and Rho GTPases

Development, angiogenesis, wound healing, and metastasis all involve the movement of cells in response to changes in the extracellular environment. To determine whether caveolin-1 plays a role in cell migration, we have used fibroblasts from knockout mice. Caveolin-1–deficient cells lose normal cell polarity, exhibit impaired wound healing, and have decreased Rho and increased Rac and Cdc42 GTPase activities. Directional persistency of migration is lost, and the cells show an impaired response to external directional stimuli. Both Src inactivation and p190RhoGAP knockdown restore the wild-type phenotype to caveolin-1–deficient cells, suggesting that caveolin-1 stimulates normal Rho GTP loading through inactivation of the Src–p190RhoGAP pathway. These findings highlight the importance of caveolin-1 in the establishment of cell polarity during directional migration through coordination of the signaling of Src kinase and Rho GTPases.

IL-6 induces AGS gastric cancer cell invasion via activation of the c-Src/RhoA/ROCK signaling pathway

Interleukin-6 (IL-6) is a multifunctional cytokine that is associated with the disease status and outcomes of gastric cancer. Nonetheless, the underlying mechanism of how IL-6 promotes the spread of gastric cancer is still unclear. In this study, we used a modified Boyden chamber assay to test the invasion ability of different gastric cancer cell lines. Liposome-mediated transfection was used to introduce an IL-6 expression vector into AGS cells, and the transfectants were further examined for the expression of active RhoA and phosphorylated Src using a pull-down assay and coimmunoprecipitation/Western blot analysis. Furthermore, RhoA expression in gastric adenocarcinoma specimens was investigated immunohistochemically. We documented that IL-6 could promote AGS cell motility and invasiveness, and inhibition of RhoA expression by dominant negative RhoA, C3 transferase, or dominant negative Src expressing plasmids could effectively decrease the invasiveness of IL-6 transfectants. We also documented an interaction between active RhoA and phosphorylated-Src following IL-6 treatment. Gastric cancers displaying high expression of RhoA are highly correlated with aggressive lymph node metastasis, more advanced tumor stage, histologically diffuse type and poorer survival. In conclusion, IL-6 induces AGS gastric cancer cell invasion via activation of the c-Src/RhoA/ROCK signaling pathway and RhoA expression could be used as a prognostic factor in patients with gastric adenocarcinoma.

Involvement of integrin-induced activation of protein kinase C in the formation of adherens junctions

In epithelial cells, tight junctions (TJs) and adherens junctions (AJs) form junctional complexes. At AJs, cadherins and nectins are the major cell-cell adhesion molecules. Nectins first form cell-cell adhesions and then recruit cadherins to the nectin-based cell-cell adhesion sites to form AJs in coordination with the activation of integrin alpha(v)beta(3), followed by the formation of TJs. We previously demonstrated that when MDCK cells precultured at a low Ca(2+) concentration were treated with the protein kinase C (PKC) activator 12-O-tetradecanoyl-phorbol-13-acetate (TPA), incomplete AJs and a TJ-like structure were achieved. However, it remains unknown how PKC is activated and how it regulates the formation of cell-cell junctions. When MDCK cells precultured at a low Ca(2+) concentration were treated with TPA, incomplete AJs were formed without the activation of integrin alpha(v)beta(3). Treatment of cells with TPA also enhanced the phosphorylation of FAK, which transmits the outside-in signal of integrin and plays a role in the nectin-induced formation of AJs. In addition, inhibition of PKC suppressed the formation of AJs. These results indicate that the activation of PKC functions downstream of integrin alpha(v)beta(3) and upstream of FAK, and is important for the nectin-induced formation of AJs.

Up-regulation of loricrin expression by cell adhesion molecule nectin-1 through Rap1-ERK signaling in keratinocytes

Nectin is an immunoglobulin-like cell-cell adhesion molecule, which plays essential roles in the initial step of formation of adherens junctions and tight junctions. We demonstrate here the role of nectin-1 in the epidermis using nectin-1-/- mice. Newborn nectin-1-/- pups showed shiny and slightly reddish skin; the amount of loricrin, one of the differentiation markers and also a major component of cornified cell envelopes, was markedly reduced in the epidermis of nectin-1-/- mice. The amounts of repetin and SPRRP, other components of cornified cell envelopes, were markedly elevated probably due to a compensatory mechanism to overcome the impaired expression of loricrin. However, cornified cells from nectin-1-/- mice were sensitive to mechanical stress. Moreover, Ca2+-induced activation of ERK through Rap1 and expression of loricrin were reduced in primary cultured nectin-1-/- keratinocytes; in turn, the inhibition of ERK activation reduced the amount of loricrin in wild-type keratinocytes. These results indicate that nectin-1 plays a key role in the expression of loricrin in the epidermis.

A novel cyathane diterpene, cyrneine A, induces neurite outgrowth in a Rac1-dependent mechanism in PC12 cells

We examined the effects of cyrneine A, a novel diterpene isolated from the mushroom Sarcodon cyrneus, on morphology of rat pheochromocytoma cells (PC12). In response to cyrneine A, PC12 cells extended their neurites, an effect partially blocked by the extracellular signal-regulated kinase (ERK) kinase inhibitor PD98059, but not by the protein kinase C inhibitor GF109203X, nor the phosphatidylinositol-3-kinase inhibitor wortmannin. Cyrneine A did not activate ERK at any of the time points tested (5-120 min), indicating that only the basal activity of ERK is required for cyrneine A-induced neurite outgrowth. As transcriptional regulation is required for neurite extension, the activity of three major transcription factors was determined. Cyrneine A enhanced activation of the transcription factors activator protein-1 (AP-1) and nuclear factor-kappaB, but not CREB, and this was accompanied by enhanced c-fos expression. Moreover, we determined the role of Rac1, a small GTPase protein of the Rho family that regulates actin dynamics, in cyrneine A-induced neurite outgrowth. Treatment with cyrneine A led to actin translocation and subsequently, to accumulation of F-actin at the tip of neurites. Rac1 activity was increased by cyrneine A and expression of a dominant-negative Rac1 mutant significantly inhibited the cyrneine A-induced extension of neurites. These results suggest that cyrneine A induces neurite outgrowth in a Rac1-dependent mechanism.

C3G is required for c-Abl-induced filopodia and its overexpression promotes filopodia formation

The Rap1 guanine nucleotide exchange factor, C3G (also known as Rap1GEF-1) is involved in signaling from growth factors, cytokines and integrins and plays a role in cell adhesion and migration, but the mechanism by which C3G regulates various cellular functions is poorly understood. We, therefore, investigated the ability of C3G to affect actin cytoskeleton-dependent morphological changes in cells. Using RNA interference, we provide evidence that C3G is required for c-Abl-induced filopodia during cell spreading on fibronectin. C3G expression induces actin cytoskeletal reorganization and promotes filopodia formation independent of its catalytic activity. It showed enrichment at filopodia tips characteristic of molecules involved in filopodia dynamics. C3G-induced filopodia were not inhibited by dominant negative mutants of Rho, Rac and Cdc42, but required Abl catalytic activity. Coexpression of N-Wasp-Crib inhibited C3G induced as well as c-Abl-induced filopodia and wiskostatin, a pharmacological inhibitor of N-Wasp attenuates C3G-induced filopodia. Cellular C3G interacts with c-Abl and C3G expression results in enhanced localization of endogenous c-Abl in the cytoplasm. We suggest that C3G and c-Abl function in an interdependent manner, in linking external signals to remodeling the cytoskeleton to induce filopodia.

Rap1 controls cell adhesion and cell motility through the regulation of myosin II

We have investigated the role of Rap1 in controlling chemotaxis and cell adhesion in Dictyostelium discoideum. Rap1 is activated rapidly in response to chemoattractant stimulation, and activated Rap1 is preferentially found at the leading edge of chemotaxing cells. Cells expressing constitutively active Rap1 are highly adhesive and exhibit strong chemotaxis defects, which are partially caused by an inability to spatially and temporally regulate myosin assembly and disassembly. We demonstrate that the kinase Phg2, a putative Rap1 effector, colocalizes with Rap1–guanosine triphosphate at the leading edge and is required in an in vitro assay for myosin II phosphorylation, which disassembles myosin II and facilitates filamentous actin–mediated leading edge protrusion. We suggest that Rap1/Phg2 plays a role in controlling leading edge myosin II disassembly while passively allowing myosin II assembly along the lateral sides and posterior of the cell.

cAMP signaling in leukocyte transendothelial migration

The migration of leukocytes across the vascular endothelium is crucial for immunosurveillance as well as for inflammatory responses. Uncontrolled leukocyte transendothelial migration results in pathologies such as asthma, rheumatoid arthritis, and atherosclerosis. The molecular mechanisms that regulate leukocyte transendothelial migration involve signaling downstream of intracellular messengers such as cAMP, calcium, phosphoinositol lipids, or reactive oxygen species. Among these, cAMP is particularly intriguing because it is generated in both leukocytes and endothelial cells and regulates leukocyte chemotaxis as well as endothelial barrier function. In addition, physiological stimuli that induce cAMP production generate both pro- and antiinflammatory signals, underscoring the complexity of cAMP-driven signaling. This review discusses our current knowledge of the control of leukocyte transendothelial migration by two main cAMP effectors: protein kinase A and the Rap exchange factor Epac (Exchange protein directly activated by cAMP).

Rac is a dominant regulator of cadherin-directed actin assembly that is activated by adhesive ligation independently of Tiam1

Classic cadherins function as adhesion-activated cell signaling receptors. On adhesive ligation, cadherins induce signaling cascades leading to actin cytoskeletal reorganization that is imperative for cadherin function. In particular, cadherin ligation activates actin assembly by the actin-related protein (Arp)2/3 complex, a process that critically affects the ability of cells to form and extend cadherin-based contacts. However, the signaling pathway(s) that activate Arp2/3 downstream of cadherin adhesion remain poorly understood. In this report we focused on the Rho family GTPases Rac and Cdc42, which can signal to Arp2/3. We found that homophilic engagement of E-cadherin simultaneously activates both Rac1 and Cdc42. However, by comparing the impact of dominant-negative Rac1 and Cdc42 mutants, we show that Rac1 is the dominant regulator of cadherin-directed actin assembly and homophilic contact formation. To pursue upstream elements of the Rac1 signaling pathway, we focused on the potential contribution of Tiam1 to cadherin-activated Rac signaling. We found that Tiam1 or the closely-related Tiam2/STEF1 was recruited to cell-cell contacts in an E-cadherin-dependent fashion. Moreover, a dominant-negative Tiam1 mutant perturbed cell spreading on cadherin-coated substrata. However, disruption of Tiam1 activity with dominant-negative mutants or RNA interference did not affect the ability of E-cadherin ligation to activate Rac1. We conclude that Rac1 critically influences cadherin-directed actin assembly as part of a signaling pathway independent of Tiam1.

C-terminal Src kinase controls development and maintenance of mouse squamous epithelia

Carboxy-terminal Src kinase (Csk) is a negative regulator of Src family kinases, which play pivotal roles in controlling cell adhesion, migration, and cancer progression. To elucidate the in vivo role of Csk in epithelial tissues, we conditionally inactivated Csk in squamous epithelia using the keratin-5 promoter/Cre-loxP system in mice. The mutant mice developed apparent defects in the skin, esophagus, and forestomach, with concomitant hyperplasia and chronic inflammation. Histology of the mutant epidermis revealed impaired cell-cell adhesion in basal cell layers. Analysis of primary keratinocytes showed that the defective cell-cell adhesion was caused by cytoskeletal remodeling via activation of the Rac1 pathway. Mutant keratinocytes also showed elevated expression of mesenchymal proteins, matrix metalloproteinases (MMPs), and the proinflammatory cytokine TNF-alpha. Inhibition of the expression of TNF-alpha and MMP9 by the anti-inflammatory reagent FK506 could cure the epidermal hyperplasia, suggesting a causal link between inflammation and epidermal hyperplasia. These observations demonstrate that the Src/Csk circuit plays crucial roles in development and maintenance of epithelia by controlling cytoskeletal organization as well as phenotypic conversion linked to inflammatory events.

p120-ctn: A nexus for contextual signaling via Rho GTPases

p120 catenin (p120) is the prototypic member of a subfamily of armadillo repeat domain proteins involved in intercellular adhesion. Recent evidence indicates that p120 associates with classical cadherins and regulates their stability. Ectopic p120 expression results in a variety of morphological effects, and promotes cell migration. There is now strong evidence that p120 acts, at least in part, through regulation of Rho GTPases. The data suggest that p120 may act as a signaling nexus, conveying messages from the cellular micro- and macro-environment to the cell's interior. By regulating Rho GTPases in a context-dependent manner p120 can exert profound effects on cellular responses from synaptic plasticity to vesicle trafficking, as well as regulate the motile vs. sessile, and possibly the proliferative vs. quiescent phenotype of epithelial cells. Here, we review the new evidence on the relationship of p120 to Rho GTPases, and discuss potential roles for the p120-Rho connection in normal and malignant cells.

In utero exposure to tributyltin alters the expression of e-cadherin and localization of claudin-1 in intercellular junctions of the rat ventral prostate

Tributyltin (TBT) is an environmental contaminant, exhibiting well-established toxicity to reproductive systems in aquatic organisms. Little information exists regarding the effects of TBT on mammalian reproduction. Cellular junctions are crucial for sperm development and maturation. Intercellular tight junctions are formed by transmembrane proteins such as claudins (Cldns), while the formation of tight junctions involves signaling components of adhering junctions, comprised of cadherins. The objectives of this study were to determine the effects of in utero exposure to TBT on the rat ventral prostate. Pregnant Sprague-Dawley rats were given doses of TBT (2.5, 10, or 20 mg/kg) throughout gestation and sacrificed at Day 91. Ventral prostate weights of TBT-treated rats were decreased in all treatment groups. Results of gene expression macro-array analysis indicated that numerous genes related to cellular adhesion and cell polarity were affected. Cldn-1 mRNA levels decreased after exposure to TBT. Cldn-1 was immunolocalized to the apical lateral margins of adjacent prostatic epithelial cells in controls, but was increasingly dispersed along the lateral plasma membrane with increasing TBT dose, suggesting that the targeting of Cldn-1 or its localization to tight junctions was altered as a result of fetal TBT exposure. E-cadherin mRNA levels and immunolocalization were decreased in a dose-dependent manner. These data indicate that in utero TBT exposure results in permanent alterations in ventral prostate and that these are associated with alterations in the expression and distribution of cell adhesion and tight junctional proteins.

Rap1: a key regulator in cell-cell junction formation

Rap1 is a Ras-like small GTPase that is activated by many extracellular stimuli and strongly implicated in the control of integrin-mediated cell adhesion. Recent evidence indicates that Rap1 also plays a key role in formation of cadherin-based cell-cell junctions. Indeed, inhibition of Rap1 generates immature adherens junctions, whereas activation of Rap1 tightens cell-cell junctions. Interestingly, Rap1 guanine nucleotide exchange factors, such as C3G and PDZ-GEF, are directly linked to E-cadherin or to other junction proteins. Furthermore, several junction proteins, such as afadin/AF6 and proteins controlling the actin cytoskeleton, function as effectors of Rap1. These findings point to a role of Rap1 in spatial and temporal control of cell-cell junction formation.

Adherens junctions in Drosophila retinal morphogenesis

Adherens junctions and their core molecular components, classic cadherins, make major contributions to animal morphogenesis. Although the significance of cadherins in development is generally accepted, the mechanisms regulating adherens junction function during morphogenesis remain a subject of intense research. Adherens junctions are involved in the organization of simple cellular patterns, and more complex cell shape changes and cell movements that depend on the dynamic modulation of adherens junctions.

Cell-cell adhesion in lung endothelium

Homotypic cell-cell adhesion is essential for tissue and organ development, remodeling, regeneration, and physiological function. Whereas a significant number of homotypic cell-cell adhesion molecules have been identified, much more is known about those concentrated in epithelia than in endothelia. Among the endothelial cell-cell adhesion molecules, very little is known that is specific to endothelium in the pulmonary and bronchial circulations. This review focuses primarily on homotypic cell-cell adhesion molecules that are or are likely to be important in lung endothelium.

Nectins and nectin-like molecules: roles in cell adhesion, polarization, movement, and proliferation

Nectins and nectin-like molecules (Necls) are immunoglobulin-like cell adhesion molecules that constitute families containing four and five members, respectively. All members, except for Necl-5, trans-interact homophilically. Furthermore, all members, including Necl-5, trans-interact heterophilically with their respective specific partners among the members. Necl-5 regulates cell movement and proliferation cooperatively with integrin alphavbeta3 and growth factor receptors. Nectins function as cell-cell adhesion molecules at a variety of cell-cell junctions, including adherens junctions, and regulate the initial step of cell-cell junction formation. Nectins and integrin alphavbeta3 are further involved in the cross-talk between cell-matrix and cell-cell junctions. Thus, both nectin and Necl family members play important roles in fundamental cellular functions, including cell adhesion, polarization, movement, and proliferation.

The requirements for herpes simplex virus type 1 cell-cell spread via nectin-1 parallel those for virus entry

Herpes simplex virus type 1 (HSV-1) spreads from an infected cell to an uninfected cell by virus entry, virus-induced cell fusion, and cell-cell spread. The three forms of virus spread require the viral proteins gB, gD, and gH-gL, as well as a cellular gD receptor. The mutual requirement for the fusion glycoproteins and gD receptor suggests that virus entry, cell fusion, and cell-cell spread occur by a similar mechanism. The goals of this study were to examine the role of the nectin-1alpha transmembrane domain and cytoplasmic tail in cell-cell spread and to obtain a better understanding of the receptor-dependent events occurring at the plasma membrane during cell-cell spread. We determined that an intact nectin-1alpha V-like domain was required for cell-cell spread, while a membrane-spanning domain and cytoplasmic tail were not. Chimeric forms of nectin-1 that were non-functional for virus entry did not mediate cell-cell spread regardless of whether they could mediate cell fusion. Also, cell-cell spread of syncytial isolates was dependent upon nectin-1alpha expression and occurred through a nectin-1-dependent mechanism. Taken together, our results indicate that nectin-1-dependent events occurring at the plasma membrane during cell-cell spread were equivalent to those for virus entry.

Crosstalk between different adhesion molecules

Cell adhesion molecules mediate cell-cell and cell-extracellular matrix adhesions, and coordination between these molecules is essential for tissue formation and morphogenesis. Crosstalk between integrins and cadherins may result from a physical response to integrin-mediated adhesion, complex cell differentiation processes, or direct signaling pathways linking the two adhesion systems. Nectins have recently been shown to regulate the organization of cadherins into adherens junctions and the formation of tight junctions by several processes. Furthermore, protocadherins can interact with extracellular matrix proteins or function by regulating classical cadherins.

Junctional adhesion molecule-a participates in the formation of apico-basal polarity through different domains

Junctional adhesion molecule (JAM)-A is an integral membrane protein at tight junctions of epithelial cells which associates with the cell polarity protein PAR-3. Here, we demonstrate that downregulation of JAM-A impairs the ability of MDCK II cells to form cysts in a three-dimensional matrix indicating the requirement of JAM-A for the development of apico-basal polarity. To define the regions of JAM-A important for this function, we have generated MDCK II cell lines stably expressing inducible JAM-A mutants. Mutants of JAM-A which were designed to mislocalize strongly impaired the development of cysts and the formation of functional tight junctions. Surprisingly, similar mutants that lacked the PDZ domain-binding motif at the C-terminus were still impaired in apico-basal polarity formation suggesting that additional regions within the cytoplasmic tail of JAM-A are important for the function of JAM-A. A JAM-A mutant lacking the first Ig-like domain necessary for homophilic binding localized to cell-cell contacts similar to wild-type JAM-A. However, despite this same localization, this mutant interfered with cell polarity and tight junction formation. Together our findings suggest an important role for JAM-A in the development of apico-basal polarity in epithelial cells and identify regions in JAM-A which are critical for this role.

Interaction of integrin alpha(v)beta3 with nectin. Implication in cross-talk between cell-matrix and cell-cell junctions

Cell-matrix and cell-cell junctions cross-talk together, and these two junctions cooperatively regulate cell movement, proliferation, adhesion, and polarization. However, the mechanism of this cross-talk remains unknown. An immunoglobulin-like cell-cell adhesion molecule nectin first trans-interacts with each other to form cell-cell adhesion and induces activation of Rap1, Cdc42, and Rac small G proteins through c-Src. Trans-interacting nectin then recruits another cell-cell adhesion molecule cadherin to the nectin-based cell-cell adhesion sites and forms adherens junctions (AJs). Here, we show that integrin alpha(v)beta3 functionally and physically associates with nectin. Integrin alpha(v)beta3 colocalized with nectin at the nectin-based cell-cell adhesion sites. The association of integrin alpha(v)beta3 with nectin was direct and was mediated through their extracellular regions. This interaction was necessary for the nectin-induced signaling. Focal adhesion kinase, which relays the integrin-initiated outside-in signals to the intracellular signaling molecules, was also involved in the nectin-induced signaling. During the formation of AJs, the high affinity form of integrin alpha(v)beta3 co-localized with nectin at the primordial cell-cell contact sites, and then after the establishment of AJs, this high affinity form of integrin alpha(v)beta3 was converted to the low affinity form, which continued to co-localize with nectin. Thus, integrin alpha(v)beta3 and nectin play pivotal roles in the cross-talk between cell-matrix and cell-cell junctions and the formation of cadherin-based AJs.

Requirement of nectin, but not cadherin, for formation of claudin-based tight junctions in annexin II-knockdown MDCK cells

Adherens junctions (AJs) and tight junctions (TJs) comprise a junctional complex which plays key roles not only in cell adhesion and polarization but also in regulation of cell movement and proliferation in epithelial cells. E-Cadherin and nectin are major cell-cell adhesion molecules (CAMs) at AJs, whereas claudin is a major CAM at TJs. We have shown that the cadherin-based cell-cell adhesion is not formed in MDCK cells in which annexin II, a Ca(2+)- and phospholipid-binding protein, is knocked down. Here, we found that TJs and the nectin-based cell-cell adhesions were formed in annexin II-knockdown cells. The formation of TJs in annexin II-knockdown MDCK cells required the nectin-based cell-cell adhesion and afadin, a nectin- and actin-filament-binding protein. In addition, it required the activation of Cdc42 and Rac small G proteins and subsequent reorganization of the IQGAP1-dependent actin cytoskeleton which were induced by the nectin-based cell-cell adhesion. These results indicate that the nectin-based cell-cell adhesion and afadin, but not the cadherin-based cell-cell adhesion, are necessary for the formation of TJs and that the signaling by nectin and the subsequent reorganization of the actin cytoskeleton are also necessary for the formation of TJs under certain conditions.

Regulation of the Assembly and Adhesion Activity of E-cadherin by Nectin and Afadin for the Formation of Adherens Junctions in Madin-Darby Canine Kidney Cells

The Ca2+-independent immunoglobulin-like molecule nectin first forms cell-cell adhesion and then assembles cadherin at nectin-based cell-cell adhesion sites, resulting in the formation of adherens junctions (AJs). Afadin is a nectin- and actin filament-binding protein that connects nectin to the actin cytoskeleton. Here, we studied the roles and modes of action of nectin and afadin in the formation of AJs in cultured MDCK cells. The trans-interaction of nectin assembled E-cadherin, which associated with p120(ctn), beta-catenin, and alpha-catenin, at the nectin-based cell-cell adhesion sites in an afadin-independent manner. However, the assembled E-cadherin showed weak cell-cell adhesion activity and might be the non-trans-interacting form. This assembly was mediated by the IQGAP1-dependent actin cytoskeleton, which was organized by Cdc42 and Rac small G proteins that were activated by the action of trans-interacting nectin through c-Src and Rap1 small G protein in an afadin-independent manner. However, Rap1 bound to afadin, and this Rap1-afadin complex then interacted with p120(ctn) associated with non-trans-interacting E-cadherin, thereby causing the trans-interaction of E-cadherin. Thus, nectin regulates the assembly and cell-cell adhesion activity of E-cadherin through afadin, nectin signaling, and p120(ctn) for the formation of AJs in Madin-Darby canine kidney cells.