A novel member of the immunoglobulin gene superfamily expressed in rat carcinoma cell lines

Emerging roles of the nectin family of cell adhesion molecules in tumour-associated pathways

Tumour cells achieve maximum survival by modifying cellular machineries associated with processes such as cell division, migration, survival, and apoptosis, resulting in genetically complex and heterogeneous populations. While nectin and nectin-like cell adhesion molecules control development and maintenance of multicellular organisation in higher vertebrates by mediating cell-cell adhesion and related signalling processes, recent studies indicate that they also critically regulate growth and development of different types of cancers. In this review, we detail current knowledge about the role of nectin family members in various tumours. Furthermore, we also analyse the seemingly opposing roles of some members of nectin family in tumour-associated pathways, as they function as both tumour suppressors and oncogenes. Understanding this functional duality of nectin family in tumours will further our knowledge of molecular mechanisms regulating tumour development and progression, and contribute to the advancement of tumour diagnosis and therapy.

Inflammatory monocytes and NK cells play a crucial role in DNAM-1-dependent control of cytomegalovirus infection

Jonjic et al. show that inflammatory macrophages play an essential role in the control of murine CMV (MCMV) infection through a DNAM-1–PVR pathway.

The poliovirus receptor (PVR) is a ubiquitously expressed glycoprotein involved in cellular adhesion and immune response. It engages the activating receptor DNAX accessory molecule (DNAM)-1, the inhibitory receptor TIGIT, and the CD96 receptor with both activating and inhibitory functions. Human cytomegalovirus (HCMV) down-regulates PVR expression, but the significance of this viral function in vivo remains unknown. Here, we demonstrate that mouse CMV (MCMV) also down-regulates the surface PVR. The m20.1 protein of MCMV retains PVR in the endoplasmic reticulum and promotes its degradation. A MCMV mutant lacking the PVR inhibitor was attenuated in normal mice but not in mice lacking DNAM-1. This attenuation was partially reversed by NK cell depletion, whereas the simultaneous depletion of mononuclear phagocytes abolished the virus control. This effect was associated with the increased expression of DNAM-1, whereas TIGIT and CD96 were absent on these cells. An increased level of proinflammatory cytokines in sera of mice infected with the virus lacking the m20.1 and an increased production of iNOS by inflammatory monocytes was observed. Blocking of CCL2 or the inhibition of iNOS significantly increased titer of the virus lacking m20.1. In this study, we have demonstrated that inflammatory monocytes, together with NK cells, are essential in the early control of CMV through the DNAM-1–PVR pathway.

Nectins and nectin-like molecules in development and disease

Nectins and nectin-like molecules (Necls)/Cadms are Ca(2+)-independent immunoglobulin superfamily cell adhesion molecules, expressed in most cell types. Nectins mediate not only homotypic but also heterotypic cell-cell adhesion, in contrast to classic cadherins which participate only in homophilic adhesion. Nectins and Necls function in organogenesis of the eye, inner ear, tooth, and cerebral cortex and in a variety of developmental processes including spermatogenesis, axon guidance, synapse formation, and myelination. They are also involved in various diseases, such as viral infection, hereditary ectodermal dysplasia, Alzheimer's disease, autism spectrum disorder, and cancer. Thus, nectins and Necls are crucial for both physiology and pathology. This review summarizes recent advances in research on these cell adhesion molecules in development and pathogenesis.

CEACAM2-L on spermatids interacts with poliovirus receptor on Sertoli cells in mouse seminiferous epithelium

The removal of excess cytoplasm from elongated spermatids by Sertoli cells is the last essential step in spermatogenesis. It requires cell-to-cell recognition between a Sertoli cell and an elongating spermatid through protein-protein interactions. CEACAM2-L, an adhesion molecule of the immunoglobulin superfamily (IgSF), is present at the plasma membrane covering the excess cytoplasm of elongated spermatids, and is possibly involved in the cell-to-cell recognition. In this study, we investigated the interaction between CEACAM2-L and Poliovirus receptor (PVR), which is also from the IgSF and is expressed by Sertoli cells. Immunohistochemical analysis showed that CEACAM2-L expressed on elongated spermatids was in close contact with PVR-positive cell processes of Sertoli cells. Immunoprecipitation experiments both in vivo and in vitro demonstrated a direct heterophilic interaction between CEACAM2-L and PVR. We show that the N-terminal Ig domain of CEACAM2-L was critical for its interaction with PVR. In addition, we found that CEACAM2-L formed heterophilic trans-tetramers with PVR in transfected COS-7 cells. From these data, we propose that Sertoli cells recognize the excess cytoplasm of elongated spermatids through the PVR-CEACAM2-L interaction in mouse testis.

Poliomyelitis in transgenic mice expressing CD155 under the control of the Tage4 promoter after oral and parenteral poliovirus inoculation

An important step in poliovirus (PV) infection by the oral route in humans is replication of the virus in lymphatic tissues of the gastrointestinal (GI) tract, thought to be mainly in the Peyer's patches of the small intestine. No immunocompetent transgenic (tg) mice that express human PV receptor (CD155) under the control of different promoters can be infected orally. The mouse orthologue of human CD155 is Tage4, a protein expressed at the surface of enterocytes and in the Peyer's patches. We describe here the generation of a tg mouse model in which the Tage4 promoter was used to drive expression of the human PV receptor-coding region (Tage4-CD155tg mice). In this model, CD155 expression was observed by immunostaining in different regions in the Peyer's patches but not in their germinal centres. Although a similar pattern of staining was observed between 3- and 6-week-old Tage4-CD155tg mice, poliomyelitis was only seen in the younger mice after PV infection by the oral route. When compared with TgPVR21 mice that expressed CD155 driven by its human promoter, 3-week-old Tage4-CD155tg mice were more susceptible to gut infection and paralysis following feeding with PV. Also, Tage4-CD155tg mice exhibited higher susceptibility to poliomyelitis after parenteral inoculation of PV. Remarkably, the LD50 after intracerebral inoculation of PV was similar in both CD155 tg mouse strains. The CD155 tg mouse model reported here, although moderately susceptible to oral infection, may be suitable to study mechanisms of PV replication in the gastrointestinal tract and to dissect important aspects of PV neuroinvasiveness.

Toll-like receptor ligands induce expression of the costimulatory molecule CD155 on antigen-presenting cells

Genotoxic stress and RAS induce the expression of CD155, a ligand for the immune receptors DNAM-1, CD96 and TIGIT. Here we show that antigen-presenting cells upregulate CD155 expression in response to Toll-like receptor activation. Induction of CD155 by Toll-like receptors depended on MYD88, TRIF and NF-κB. In addition, IRF3, but not IRF7, modulated CD155 upregulation in response to TLR3 signals. Immunization of CD155-deficient mice with OVA and the TLR9 agonist CpG resulted in increased OVA-specific IgG2a/c titers when compared to wild type mice. Splenocytes of immunized CD155-deficient mice secreted lower levels of IL-4 and fewer IL-4 and GATA-3 expressing CD4⁺ T cells were present in the spleen of Cd155^−/− mice. Our data suggest that CD155 regulates T_h2 differentiation. Targeting of CD155 in immunization protocols using peptides may represent a promising new approach to boost protective humoral immunity in viral vaccines.

The role of Necl-5 in the invasive activity of lung adenocarcinoma

Nectin-like molecule-5 (Necl-5) is an immunoglobulin-like molecule that was originally identified as a poliovirus receptor and is often upregulated in cancer cells. It has been said that Necl-5 plays a role in not only cell-cell adhesion, but also cell migration, proliferation, and metastasis. In this study, we used a bronchioloalveolar carcinoma (BAC) cell line and fibroblasts to assess the expression of Necl-5 in the development of cancer-stroma communication by using an easy-to-prepare double-layered collagen gel hemisphere (DL-CGH) system that enables visualization of cell migration during invasion. The expression of Necl-5 was higher in BAC cells than in fibroblasts. This tendency didn't change even when the BAC cells were mixed with fibroblasts. To assess the role of Necl-5 in the invasive activity of the BAC cells, we knocked down its expression using RNA interference (RNAi). The invasion assay with DL-CGH revealed that inhibition of Necl-5 expression in the BAC cells was associated with suppressed invasiveness. In addition, Necl-5 knockdown inhibited the movement and proliferation of the BAC cells. Necl-5 expression in lung cancer cells is crucial for their invasiveness in the cancer-stromal interaction, suggesting that Necl-5 could be a favorable molecular target for the suppression of invasiveness in lung adenocarcinoma.

Necl-5/poliovirus receptor interacts with VEGFR2 and regulates VEGF-induced angiogenesis

RATIONALE

Vascular endothelial growth factor (VEGF), a major proangiogenic agent, exerts its proangiogenic action by binding to VEGF receptor 2 (VEGFR2), the activity of which is regulated by direct interactions with other cell surface proteins, including integrin α(V)β(3). However, how the interaction between VEGFR2 and integrin α(V)β(3) is regulated is not clear.

OBJECTIVE

To investigate whether Necl-5/poliovirus receptor, an immunoglobulin-like molecule that is known to bind integrin α(V)β(3), regulates the interaction between VEGFR2 and integrin α(V)β(3), and to clarify the role of Necl-5 in the VEGF-induced angiogenesis.

METHODS AND RESULTS

Necl-5-knockout mice displayed no obvious defect in vascular development; however, recovery of blood flow after hindlimb ischemia and the VEGF-induced neovascularization in implanted Matrigel plugs were impaired in Necl-5-knockout mice. To clarify the mechanism of the regulation of angiogenesis by Necl-5, we investigated the roles of Necl-5 in the VEGF-induced angiogenic responses in vitro. Knockdown of Necl-5 by siRNAs in human umbilical vein endothelial cells (HUVECs) inhibited the VEGF-induced capillary-like network formation on Matrigel, migration, and proliferation, and conversely, enhanced apoptosis. Coimmunoprecipitation assays showed the interaction of Necl-5 with VEGFR2, and knockdown of Necl-5 prevented the VEGF-induced interaction of integrin α(V)β(3) with VEGFR2. Knockdown of Necl-5 suppressed the VEGFR2-mediated activation of downstream proangiogenic and survival signals, including Rap1, Akt, and endothelial nitric oxide synthase.

CONCLUSIONS

These results demonstrate the critical role of Necl-5 in angiogenesis and suggest that Necl-5 may regulate the VEGF-induced angiogenesis by controlling the interaction of VEGFR2 with integrin α(v)β(3), and the VEGFR2-mediated Rap1-Akt signaling pathway.

Necl-5/PVR enhances PDGF-induced attraction of growing microtubules to the plasma membrane of the leading edge of moving NIH3T3 cells

Microtubules (MTs) search for and grow toward the leading edge of moving cells, followed by their stabilization at a specific structure at the rear site of the leading edge. This dynamic re-orientation of MTs is critical to directional cell movement. We previously showed that Necl-5/poliovirus receptor (PVR) interacts with platelet-derived growth factor (PDGF) receptor and integrin α(v) β(3) at the leading edge of moving NIH3T3 cells, resulting in an enhancement of their directional movement. We studied here the role of Necl-5 in the PDGF-induced attraction of growing MTs to the leading edge of NIH3T3 cells. Necl-5 enhanced the PDGF-induced growth of MTs and attracted them near to the plasma membrane of the leading edge of NIH3T3 cells in an integrin α(v) β(3) -dependent manner. Furthermore, Necl-5 enhanced the PDGF-induced attraction of the plus-end-tracking proteins (+TIPs), including EB1, CLIP170, an intermediate chain subunit of cytoplasmic dynein, and p150(Glued) , a subunit of dynactin, near to the plasma membrane of the leading edge. Thus, Necl-5 plays a role in the attraction of growing MTs to the plasma membrane of the leading edge of moving cells.

Overexpression of Necl-5 correlates with unfavorable prognosis in patients with lung adenocarcinoma

Nectin-like molecule-5 (Necl-5) is an immunoglobulin (Ig)-like molecule that is up-regulated in many types of cancer cells. It was shown experimentally that Necl-5 enhances cell migration, proliferation, and metastasis, but its clinical significance has not been documented. The aim of this study was to observe the expression of Necl-5 in surgically resected primary lung adenocarcinomas and to investigate its clinical significance. A total of 63 surgically resected primary pulmonary adenocarcinoma tissues were investigated by immunohistochemistry for the expression of Necl-5. The relationship between expression of Necl-5 and clinicopathological features was analyzed, and the influence of Necl-5 expression on outcomes in these patients was assessed. A strong expression of Necl-5 by cancer cells was observed in 43 of the 63 tumors. The overexpression of Necl-5 by cancer cells was significantly associated with lymph node metastasis (P = 0.0398), TNM staging (P = 0.0367), and the bronchioloalveolar carcinoma ratio of tumors (P = 0.0423). Furthermore, the disease-free survival rate in patients with positive Necl-5 overexpression was significantly lower than that in patients with negative Necl-5 overexpression (P = 0.0004). Multivariate survival analysis revealed Necl-5 expression to be an independent risk factor for an unfavorable outcome (P = 0.0294). Additionally, an analysis including only the stage I cases revealed that the disease-free survival rate of the Necl-5-positive group was significantly lower than that of the Necl-5-negative group (P = 0.0192). These results indicate that Necl-5 plays a role in mediating tumor cell invasion and that the overexpression of Necl-5 in cancer cells has clinical significance for prognostic evaluation of patients with primary pulmonary adenocarcinoma.

Silencing of ErbB3/ErbB2 signaling by immunoglobulin-like Necl-2

ErbB2 and ErbB3, members of the EGF receptor/ErbB family, form a heterodimer upon binding of a ligand, inducing the activation of Rac small G protein and Akt protein kinase for cell movement and survival, respectively. The enhanced ErbB3/ErbB2 signaling causes tumorigenesis, invasion, and metastasis. We found here that the ErbB3/ErbB2 signaling is regulated by immunoglobulin-like Necl-2, which is down-regulated in various cancer cells and serves as a tumor suppressor. The extracellular region of ErbB3, but not ErbB2, interacted in cis with that of Necl-2. This interaction reduced the ligand-induced, ErbB2-catalyzed tyrosine phosphorylation of ErbB3 and inhibited the consequent ErbB3-mediated activation of Rac and Akt, resulting in the inhibition of cancer cell movement and survival. These inhibitory effects of Necl-2 were mediated by the protein-tyrosine phosphatase PTPN13 which interacted with the cytoplasmic tail of Necl-2. We describe here this novel mechanism for silencing of the ErbB3/ErbB2 signaling by Necl-2.

Role of the spermatogenic-Sertoli cell interaction through cell adhesion molecule-1 (CADM1) in spermatogenesis

Endocrine and local secretory factors have long been known to be required for spermatogenesis. Evidence has been accumulating in recent years indicating that direct contact between spermatogenic and Sertoli cells is also required for spermatogenesis. Cell adhesion molecules of various types have been found in the mammalian testis that are expressed in spermatogenic and/or Sertoli cells and involved in homophilic and/or heterophilic binding. We have cloned a novel cell adhesion molecule, cell adhesion molecule-1 (CADM1), also known as immunoglobulin superfamily 4A or spermatogenic immunoglobulin superfamily, from the mouse testis. CADM1 belongs to the immunoglobulin superfamily and is composed of three immunoglobulin-like domains, a transmembrane domain, and a short intracellular domain. In the seminiferous epithelium, CADM1 is expressed in intermediate spermatogonia through to early pachytene spermatocytes as well as in elongating spermatids--but not in round spermatids, mature spermatozoa, or Sertoli cells. One of the heterophilic binding partners of CADM1 has proven to be a poliovirus receptor, another member of the immunoglobulin superfamily that is expressed in Sertoli cells. Knockout mice for CADM1 develop male infertility due to defective spermatogenesis. These findings suggest that cell adhesion molecules between spermatogenic and Sertoli cells play essential roles in spermatogenesis.

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.

Interaction and localization of Necl-5 and PDGF receptor beta at the leading edges of moving NIH3T3 cells: Implications for directional cell movement

It was previously shown that platelet-derived growth factor (PDGF) receptor physically and functionally interacts with integrin alpha(v)beta(3), effectively inducing cell movement. We previously showed that Necl-5, originally identified as a poliovirus receptor, interacts with integrin alpha(v)beta(3) and enhances its clustering and the formation of focal complexes at the leading edges of moving cells, resulting in an enhancement of cell movement. We showed here that Necl-5 additionally interacts with PDGF receptor in NIH3T3 cells and regulates the interaction between PDGF receptor and integrin alpha(v)beta(3), effectively inducing directional cell movement. PDGF receptor co-localized with Necl-5 and integrin alpha(v)beta(3) at peripheral ruffles over lamellipodia, which were formed at the leading edges of moving cells in response to PDGF, but not at the focal complexes under these ruffles, whereas Necl-5 and integrin alpha(v)beta(3) co-localized at these focal complexes. The clustering of these three molecules at peripheral ruffles required the activation of integrin alpha(v)beta(3) by vitronectin and the PDGF-induced activation of the small G protein Rac and subsequent re-organization of the actin cytoskeleton. These results indicate a key role of Necl-5 in directional cell movement by physically and functionally interacting with both integrin alpha(v)beta(3) and PDGF receptor.

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.

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.

The CD155/poliovirus receptor enhances the proliferation of ras-mutated cells

Stimulation of the CD155/poliovirus receptor, which localizes in the cell-matrix and at cell-cell junctions, inhibits cell adhesion and enhances cell migration. Necl-5, a mouse homolog of CD155, is implicated in the formation of adherence junctions. Recently, Necl-5 has also been found to enhance cell proliferation via the stimulation of serum and platelet-derived growth factor through the Ras-Raf-MEK-ERK signaling pathway. In our present study, we find that CD155 significantly enhances the serum-induced cell proliferation of NIH3T3 cells which have been transformed by an oncogenic Ras (V12Ras-NIH3T3), but not the parental cells. CD155 expression in V12Ras-NIH3T3 cells is also found to upregulate cyclin D2, downregulate p27(Kip1) and shorten the G0/G1 phase of the cell cycle. An inhibitor of focal adhesion kinase does not reduce this CD155-mediated enhancement of V12Ras-NIH3T3 cell proliferation. The expression of CD155DeltaCP, which lacks the cytoplasmic region including the immunoreceptor tyrosine-based inhibitory motif (ITIM), has a reduced ability to enhance the serum responsiveness of V12Ras-NIH3T3 cells, suggesting that the ITIM might be required for this effect of CD155. In addition, the overexpression of exogenous CD155 enhances the serum responsiveness of HT1080 cells, which harbor a mutant N-ras gene. On the other hand, siRNA-induced knockdown of endogenous CD155 and/or CD155DeltaCP expression significantly repress the serum responsiveness of DLD-1 cells, which express endogenous CD155 and harbor a mutant K-ras gene, suggesting that this mutant may function in a dominant negative manner. Taken together, our present data suggest that CD155, at least in part, enhances the proliferation of ras-mutated cells.

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.

Interaction of cancer cells with platelets mediated by Necl-5/poliovirus receptor enhances cancer cell metastasis to the lungs

Necl-5 is an immunoglobulin (Ig)-like molecule that was originally identified as a poliovirus receptor and is often upregulated in cancer cells. We recently found that it colocalizes with integrin alpha(v)beta(3) at the leading edges of moving cells and enhances growth factor-induced cell movement and proliferation. Upon cell-cell contact, Necl-5 is removed from the cell surface by its trans-interaction with the cell adhesion molecule nectin-3, resulting in reduced cell movement and proliferation. Here, we investigated the role of Necl-5 in the interaction of cancer cells with platelets. Necl-5 was upregulated in CT26 cells, a colon adenocarcinoma cell line. When CT26 cells were injected into the tail vein of mice, they were arrested in the pulmonary vessels by adhering to platelets and subsequently metastasized to the lungs. Overexpression of Necl-5 in CT26 cells enhanced this metastasis, while inhibition of the trans-interaction of Necl-5 with CD226 by an anti-Necl-5 monoclonal antibody reduced the metastasis. Depletion of platelets by treatment with a rabbit anti-mouse platelet serum reduced the Necl-5-enhanced metastasis in mice. Thus, the trans-interaction of upregulated Necl-5 in cancer cells with its counter-receptor in platelets, probably CD226, is critical for efficient metastasis of cancer cells to the lungs.

Necl-5/poliovirus receptor interacts in cis with integrin alphaVbeta3 and regulates its clustering and focal complex formation

Integrin alphavbeta3, which forms focal complexes at leading edges in moving cells, is up-regulated in cancer cells and so is implicated in their invasiveness. Necl-5, originally identified as a poliovirus receptor and also up-regulated in cancer cells, colocalizes with integrin alphavbeta3 at leading edges in moving cells and enhances growth factor-induced cell movement. Here, we show that Necl-5 interacts directly, in cis, with integrin alphavbeta3, and enhances integrin alphavbeta3 clustering and focal complex formation at leading edges in NIH3T3 cells. The extracellular region of Necl-5, but not the cytoplasmic region, is necessary for its interaction with integrin alphavbeta3; however, both regions are necessary for its action. An interaction between integrin alphavbeta3 and vitronectin and PDGF-induced activation of Rac are also necessary for integrin alphavbeta3 clustering. The interaction between Necl-5 and integrin alphavbeta3 enhances PDGF-induced Rac activation, facilitating integrin alphavbeta3 clustering presumably in a feedback amplification manner. Thus, Necl-5 has a critical role in integrin alphavbeta3 clustering and focal complex formation.

Regulation of platelet-derived growth factor-induced Ras signaling by poliovirus receptor Necl-5 and negative growth regulator Sprouty2

Necl-5, known as a poliovirus receptor and up-regulated in many cancer cells, enhances platelet-derived growth factor (PDGF)-induced activation of Ras-Raf-MEK-ERK signaling, but not PDGF-induced tyrosine phosphorylation of PDGF receptor, resulting in facilitation of cell proliferation. Here, we showed that Necl-5 interacted with Sprouty2, known to be a negative regulator of growth factor-induced signaling, and reduced the inhibitory effect of Sprouty2 on PDGF-induced Ras signaling. Necl-5 was reported to be down-regulated by its trans-interaction with nectin-3 upon cell-cell contact, initiating cooperative cell-cell adhesion with cadherin. This down-regulation of Necl-5 caused tyrosine phosphorylation of Sprouty2 by c-Src, which was activated by PDGF receptor in response to PDGF, and inhibited PDGF-induced Ras signaling. Thus, Necl-5 and Sprouty2 cooperatively regulate PDGF-induced Ras signaling. The roles of Necl-5 and Sprouty2 in contact inhibition for cell proliferation are also discussed.

Expression and possible role of PVR/CD155/Necl-5 in osteoclastogenesis

Osteoclasts, the bone-resorbing cells, are differentiated from hematopoietic precursors via two-step cell-cell interactions. One is the interaction between the osteoclast precursor and the stromal cell to initiate differentiation. The other is the interaction among osteoclast precursors to form multinucleated osteoclasts. Recently, the poliovirus receptor (PVR, CD155, Necl-5) was reported to play important roles in cell adhesion and migration. However, there are no reports of PVR in osteoclastogenesis. In this paper, we examined the expression of PVR and its ligand, DNAX accessory molecule-1 (DNAM-1, CD226), in osteoclast precursors, mature osteoclasts, and stromal cells. We found that the PVR was constitutively expressed in both osteoclast cells and stromal cells. The expression of PVR was not changed at various stages of osteoclast formation. In contrast, the expression of DNAM-1 was observed in mononuclear cells and was down-regulated during osteoclastogenesis. Moreover, multinucleated osteoclast formation was inhibited by treatment with the extracellular domain of DNAM-1 (ED-DNAM-1) as a soluble ligand for PVR, but mononuclear preosteoclast formation was not affected. Especially, during the 7-day cultivation, osteoclast formation was suppressed by the treatment with ED-DNAM-1 on days 6 and 7, when the mononuclear preosteoclasts fused into multinucleated osteoclasts. This suppression was abrogated partially by a small interfering RNA specific for PVR. These results suggest that, at least in part, the binding of PVR with DNAM-1 negatively regulates osteoclast formation. Furthermore, our results indicate that the cellular fusion process may be inhibited by the PVR-mediated signaling.

Involvement of up-regulated Necl-5/Tage4/PVR/CD155 in the loss of contact inhibition in transformed NIH3T3 cells

Normal cells show contact inhibition of cell movement and proliferation, but this is lost following transformation. We found that Necl-5, originally identified as a poliovirus receptor and up-regulated in many cancer cells, enhances growth factor-induced cell movement and proliferation. We showed that when cells contact other cells, Necl-5 interacts in trans with nectin-3 and is removed by endocytosis from the cell surface, resulting in a reduction of cell movement and proliferation. We show here that up-regulation of the gene encoding Necl-5 by the oncogene V12-Ki-Ras causes enhanced cell movement and proliferation. Upon cell-cell contact, de novo synthesis of Necl-5 exceeds the rate of Necl-5 endocytosis, eventually resulting in a net increase in the amount of Necl-5 at the cell surface. In addition, expression of the gene encoding nectin-3 is markedly reduced in transformed cells. Thus, up-regulation of Necl-5 following transformation contributes to the loss of contact inhibition in transformed cells.

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.

Tightly regulated induction of the adhesion molecule necl-5/CD155 during rat liver regeneration and acute liver injury

TuAg1/TagE4, the rat ortholog of the human poliovirus receptor CD155, is expressed on a high percentage of rat hepatocellular carcinomas. Recent studies have shown that TuAg1/TagE4/CD155 is a member of the nectin family of immunoglobulin (Ig)-like cell adhesion molecules, designated necl-5. Necl-5 is present at exceedingly low levels in adult epithelial tissues but is upregulated in primary cultures of rat hepatocytes, suggesting that disruption of liver architecture triggers its expression. To explore this possibility, we examined expression of necl-5 after two-thirds partial hepatectomy or carbon tetrachloride (CCl4)-induced acute injury. Using quantitative real-time polymerase chain reaction (QPCR), we found that necl-5 mRNA levels increased 15-fold by 9 hours, and decreased to 4-fold above baseline by 24 hours after partial hepatectomy. Necl-5 mRNA levels increased over 100-fold 6 hours after treatment with CCl4, reaching a peak of 140-fold above baseline by 10 hours, and thereafter rapidly declining. Necl-5 was localized at the membrane of midlobular and centrilobular hepatocytes 10 to 48 hours after CCl4 exposure. Northern blot analysis demonstrated a close correlation between the kinetics of necl-5 expression and the immediate-early response gene c-myc. Subconfluent cultures of the non-transformed liver epithelial cell line WB-F344 expressed high levels of necl-5, which was down-regulated as cells approached confluence. The transformed WB-F344 line GP7TB did not demonstrate density-dependent regulation of necl-5 expression. In conclusion, we report the in vivo induction of necl-5 in rat hepatocytes and provide evidence that both necl-5 mRNA and protein are tightly regulated in adult epithelial cells and tissue.

Inhibition of cell movement and proliferation by cell-cell contact-induced interaction of Necl-5 with nectin-3

Immunoglobulin-like Necl-5/Tage4/poliovirus receptor (PVR)/CD155, originally identified as the PVR, has been shown to be up-regulated in cancer cells and to enhance growth factor–induced cell movement and proliferation. In addition, Necl-5 heterophilically trans-interacts with nectin-3, a cell–cell adhesion molecule known to form adherens junctions in cooperation with cadherin. We show here that Necl-5 was down-regulated from cell surface upon cell–cell contacts in NIH3T3 cells. This down-regulation of Necl-5 was initiated by its interaction with nectin-3 and was mainly mediated by clathrin-dependent endocytosis. Then, the down-regulation of Necl-5 induced in this way reduced movement and proliferation of NIH3T3 cells. These results indicate that the down-regulation of Necl-5 induced by its interaction with nectin-3 upon cell–cell contacts may be at least one mechanism underlying contact inhibition of cell movement and proliferation.

Common signaling pathway is used by the trans-interaction of Necl-5/Tage4/PVR/CD155 and nectin, and of nectin and nectin during the formation of cell-cell adhesion

Nectin is a Ca2+-independent Ig-like cell-cell adhesion molecule that forms homo- and hetero-trans-dimers (trans-interaction). Nectin first forms cell-cell adhesions and then recruits cadherin to the nectin-based cell-cell adhesion sites to form AJ cooperatively with cadherin. In addition, the trans-interaction of nectin and nectin induces the activation of Cdc42 and Rac small G proteins, which enhances the formation of AJ. The activation of Cdc42 and Rac by the trans-interaction of nectin and nectin is mediated by c-Src, another small G protein, Rap1, a Cdc42-GEF, FRG, and a Rac-GEF, Vav2. Necl-5/Tage4/PVR/CD155 is another Ca2+-independent Ig-like molecule, which does not homophilically trans-interact, but heterophilically trans-interacts with nectin-3, one member of the nectin family. We show here that the trans-interaction of Necl-5 and nectin-3 bidirectionally induces the activation of Cdc42 and Rac. Similarly to the activation of Cdc42 and Rac by the trans-interaction of nectin and nectin, the trans-interaction of Necl-5 and nectin-3 first recruits and activates c-Src at the Necl-5/nectin-3-based cell-cell contact sites. c-Src then phosphorylates FRG and Vav2, and the tyrosine-phosphorylated FRG and Vav2 are recruited to the Necl-5/nectin-3-based cell-cell contact sites. The trans-interaction of Necl-5 and nectin-3 also activates Rap1 through C3G, a Rap-GEF, and this activation of Rap1 is required for the activation of Cdc42 and Rac. These results indicate that the trans-interactions of Necl-5 and nectin-3 and of nectin and nectin induce the activation of Cdc42 and Rac through the common signaling molecules c-Src, Rap1, FRG, and Vav2.

Transcriptional activation of the mouse Necl-5/Tage4/PVR/CD155 gene by fibroblast growth factor or oncogenic Ras through the Raf-MEK-ERK-AP-1 pathway

Necl-5/Tage4/poliovirus receptor/CD155 is the poliovirus receptor and upregulated in rodent and human carcinoma. We have recently shown that mouse Necl-5 is upregulated by an oncogenic Ki-Ras (V12Ki-Ras) in NIH3T3 cells and enhances cell movement induced by growth factors, including platelet-derived growth factor and fibroblast growth factor (FGF), in an integrin alpha(v)beta(3)-dependent manner in wild type and V12Ki-Ras-transformed NIH3T3 cells. In addition, it enhances the growth factor-induced cell proliferation. We examined here how mouse Necl-5 was upregulated by V12Ki-Ras in NIH3T3 cells. Expression of the luciferase reporter gene fused to the Necl-5 promoter was induced by V12Ki-Ras in NIH3T3 cells. This induction was mediated through the Raf-MEK-ERK pathway. The Necl-5 promoter has an AP-1-binding site and this site was required for the V12Ki-Ras-induced activation of the Necl-5 promoter. Expression of the luciferase reporter gene fused to the Necl-5 promoter was also induced by FGF through the Raf-MEK-ERK-AP-1 pathway in NIH3T3 cells. These results indicate that the expression of mouse Necl-5 is induced by FGF or V12Ki-Ras through the Raf-MEK-ERK-AP-1 pathway.

Roles of nectins in cell adhesion, migration and polarization

Nectins are Ca2+-independent immunoglobulin (Ig)-like cell-cell adhesion molecules, which comprise a family consisting of four members. Nectins have five activities: (1) they show Ca2+-independent cell-cell adhesion activity by homo- and hetero-trans-interactions through their extracellular regions; (2) they bind afadin, an actin filament (F-actin)-binding protein, through their cytoplasmic tails and are connected to the actin cytoskeleton; (3) they induce activation of Cdc42 and Rac small G proteins through their cytoplasmic tails; (4) they bind Par-3, a cell polarity protein, through their cytoplasmic tails; and (5) they heterophilically trans-interact with Necls, nectin-like molecules, through their extracellular regions. Through these activities, nectins regulate a variety of cellular functions, including adhesion, migration, and polarization. Here we describe these activities and functions of nectins.

Biology and pathology of nectins and nectin-like molecules

Immunoglobulin-like nectins contribute to the formation of a variety of cell-cell junctions, acting cooperatively with, or independently of, cadherins. In addition, nectins heterophilically trans-interact with nectin-like molecules (Necls), which are involved in cell adhesion, migration, and proliferation, and assist or modify their functions. On the other hand, nectins and Necls serve as viral receptors and are associated with human diseases (including cancer) when mutated or upregulated.

Involvement of heterophilic trans-interaction of Necl-5/Tage4/PVR/CD155 with nectin-3 in formation of nectin- and cadherin-based adherens junctions

Nectins, Ca(2+)-independent immunoglobulin (Ig)-like cell-cell adhesion molecules and cadherins, Ca(2+)-dependent cell-cell adhesion molecules, are associated through their respective cytoplasmic tail-binding proteins, afadin and catenins and play roles in formation of adherens junctions (AJs) in epithelial cells and fibroblasts. Nectin-like molecule-5 (Necl-5) is a Ca(2+)-independent Ig-like molecule which does not homophilically trans-interact, but heterophilically trans-interacts with nectin-3, one member of the nectin family. Necl-5 does not directly bind afadin and therefore is not associated with cadherins. Necl-5 regulates cell motility and proliferation in cooperation with integrins and growth factor receptors, when it does not interact with nectin-3. We studied here a role of the heterophilic trans-interaction of Necl-5 with nectin-3 in cell-cell adhesion using L cells stably expressing Necl-5, nectin-3 and E-cadherin (Necl-5-nectin-3-EL cells). Afadin, E-cadherin and catenins were recruited to the nectin-3 side, but not to the Necl-5 side, of the contact sites formed by the heterophilic trans-interaction between Necl-5 and nectin-3. The anti-Necl-5 monoclonal antibody, which specifically inhibited the heterophilic trans-interaction of Necl-5 with nectin-3, inhibited the formation of the E-cadherin-based AJs in Necl-5-nectin-3-EL cells. These results indicate that Necl-5 plays roles not only in cell motility and proliferation but also in cell-cell adhesion in cooperation with nectin-3.

Ligand stimulation of CD155alpha inhibits cell adhesion and enhances cell migration in fibroblasts

CD155 (poliovirus receptor) localizes in cell-matrix adhesions and cell-cell junctions, but its role in the regulation of cell adhesion and cell motility has not been investigated. We identified a conserved immunoreceptor tyrosine-based inhibitory motif (ITIM) in the cytoplasmic domain of human CD155alpha. The ITIM was tyrosine-phosphorylated upon binding of anti-CD155 monoclonal antibody D171, poliovirus, and DNAM-1 (CD226) to human CD155alpha, and recruited SH2-domain-containing tyrosine phosphatase-2 (SHP-2). After CD155alpha stimulation with its ligands, cell adhesion was inhibited and cell motility was enhanced, effects that were associated with the phosphorylation of ITIM by Src kinases and accompanied by dephosphorylation of focal adhesion kinase and paxillin. These effects were abolished by introducing a point-mutation in Y398F into the ITIM of CD155alpha and by coexpression of a dominant negative SHP-2 mutant with CD155alpha. These results suggest that CD155alpha plays a role in the regulation of cell adhesion and cell motility.

Enhancement of serum- and platelet-derived growth factor-induced cell proliferation by Necl-5/Tage4/poliovirus receptor/CD155 through the Ras-Raf-MEK-ERK signaling

Necl-5/Tage4/poliovirus receptor/CD155 has been shown to be the poliovirus receptor and to be up-regulated in rodent and human carcinoma. We have found previously that mouse Necl-5 regulates cell motility. We show here that mouse Necl-5 is furthermore involved in the regulation of cell proliferation. Studies using a specific antibody against Necl-5 and a dominant negative mutant of Necl-5 revealed that Necl-5 enhanced the serum-induced proliferation of NIH3T3, Swiss3T3, and mouse embryonic fibroblast cells. Necl-5 enhanced the serum-induced activation of the Ras-Raf-MEK-ERK signaling, up-regulated cyclins D2 and E, and down-regulated p27(Kip1), eventually shortening the period of the G(0)/G(1) phase of the cell cycle in NIH3T3 cells. Necl-5 similarly enhanced the platelet-derived growth factor-induced activation of the Ras-Raf-MEK-ERK signaling and shortened the period of the G(0)/G(1) phase of the cell cycle in NIH3T3 cells. Necl-5 acted downstream of the platelet-derived growth factor receptor and upstream of Ras. Moreover, up-regulated Necl-5 was involved at least partly in the enhanced proliferation of transformed cells including NIH3T3 cells transformed by an oncogenic Ras or v-Src. These results indicate that Necl-5 plays roles not only in cell motility but also in cell proliferation.

Nectin-like molecule-5/Tage4 enhances cell migration in an integrin-dependent, Nectin-3-independent manner

Cell migration plays roles in invasion of transformed cells and scattering of embryonic mesenchymal cells into surrounding tissues. We have found that Ig-like Necl-5/Tage4 is up-regulated in NIH3T3 cells transformed by an oncogenic Ras (V12Ras-NIH3T3 cells) and heterophilically trans-interacts with a Ca(2+)-independent Ig-like cell adhesion molecule nectin-3, eventually enhancing their intercellular motility. We show here that Necl-5 furthermore enhances cell migration in a nectin-3-independent manner. Studies using L fibroblasts expressing various mutants of Necl-5, NIH3T3 cells, and V12Ras-NIH3T3 cells have revealed that Necl-5 enhances serum- and platelet-derived growth factor-induced cell migration. The extracellular region of Necl-5 is necessary for directional cell migration, but not for random cell motility. The cytoplasmic region of Necl-5 is necessary for both directional and random cell movement. Necl-5 colocalizes with integrin alpha(V)beta(3) at leading edges of migrating cells. Analyses using an inhibitor or an activator of integrin alpha(V)beta(3) or a dominant negative mutant of Necl-5 have shown the functional association of Necl-5 with integrin alpha(V)beta(3) in cell motility. Cdc42 and Rac small G proteins are activated by the action of Necl-5 and required for the serum-induced, Necl-5-enhanced cell motility. These results indicate that Necl-5 regulates serum- and platelet-derived growth factor-induced cell migration in an integrin-dependent, nectin-3-independent manner, when cells do not contact other cells. We furthermore show here that enhanced motility and metastasis of V12Ras-NIH3T3 cells are at least partly the result of up-regulated Necl-5.

Characterization and identification of Tage4 as the murine orthologue of human poliovirus receptor/CD155

CD155 is a member of the immunoglobulin superfamily also known as the human receptor for poliovirus (PVR). Transmembrane glycoproteins related to CD155, the nectins, are well-characterized cell adhesion receptors displaying a high degree of sequence conservation across species. In contrast, CD155 belongs to the category of rapidly evolving genes wherefore a mouse CD155 gene distinguished by an affirmative extent of amino acid conservation as observed for nectins is absent. Consequently, the existing genetic evidence by itself is an inferior indicator to consider whether Tage4, a mouse orphan receptor, represents the murine orthologue of CD155. In the present study Tage4 cDNA was cloned from mouse lung and further characterized genetically. CD155 and Tage4 possess an identical genomic organization and reside in syntenic chromosomal regions. The Tage4 expression pattern was explored applying a newly generated antibody. Both receptors, CD155 in human and Tage4 in mouse, are expressed by intestinal epithelia as well as by follicle associated epithelium and follicular dendritic cells inside Peyer's patches of the gut associated lymphoid tissue. Furthermore, Tage4 lacks self-adhesion capacity but binds to vitronectin, two known features of CD155. These data indicate that Tage4 represents the functional orthologue of CD155 in mouse. Therefore, we suggest to rename Tage4 into rodent CD155.

Loss of nectin-2 at Sertoli-spermatid junctions leads to male infertility and correlates with severe spermatozoan head and midpiece malformation, impaired binding to the zona pellucida, and oocyte penetration

The members of the nectin/CD155 gene family represent a growing class of novel cell adhesion molecules of the immunoglobulin superfamily. In the present study, we describe the generation of a mouse line lacking a functional nectin-2 gene (nectin-2LacZ/LacZ) and analyze the resulting male-specific infertility phenotype. Although nectin-2LacZ/LacZ males produced normal amounts of motile spermatozoa, scanning electron microscopy revealed severe malformations of the spermatozoan head and midpiece. Besides a 4-fold reduction in migration of nectin-2LacZ/LacZ spermatozoa to the oviducts, in vitro binding to zona-intact mouse oocytes was reduced 6-fold. On the other hand, nectin-2LacZ/LacZ spermatozoa bound to zona-free hamster oocytes at near-wild type levels but, remarkably, failed to penetrate. In addition to the previously reported expression of nectin-2 and nectin-3 at Sertoli-spermatid junctions and of nectin-2 at inter-Sertoli cell junctions, we also found nectin-2 to localize at apical cell-cell junctions of the epididymal epithelium. Expression analysis of a LacZ knockin gene into the defunct nectin-2 gene in nectin-2LacZ/LacZ mice provided additional support for our earlier conjecture that in normal testis, nectin-2 is produced exclusively by Sertoli cells. Finally, we found Sertoli-spermatid junctions in nectin-2LacZ/LacZ mice to be virtually devoid of the actin-bundling protein espin, suggesting that ectoplasmic specializations fail to form in the absence of nectin-2. Our functional analyses indicate that the infertility phenotype of nectin-2-deficient male mice is caused by a combination of reduced migration to the oviduct, spermatozoa-zona binding, and sperm-oocyte fusion. We corroborate our previous description of a heterotypic adhesion complex between Sertoli cells and elongated spermatids that is maintained by nectin-2 and nectin-3, respectively.

Implications of nectin-like molecule-2/IGSF4/RA175/SgIGSF/TSLC1/SynCAM1 in cell-cell adhesion and transmembrane protein localization in epithelial cells

Nectins are Ca2+-independent immunoglobulin-like cell-cell adhesion molecules that play roles in organization of a variety of cell-cell junctions in cooperation with or independently of cadherins. Four nectins have been identified. Five nectin-like molecules, which have domain structures similar to those of nectins, have been identified, and we characterized here nectin-like molecule-2 (Necl-2)/IGSF4/RA175/SgIGSF/TSLC1/SynCAM1. Necl-2 showed Ca2+-independent homophilic cell-cell adhesion activity. It furthermore showed Ca2+-independent heterophilic cell-cell adhesion activity with Necl-1/TSLL1/SynCAM3 and nectin-3. Necl-2 was widely expressed in rat tissues examined. Necl-2 localized at the basolateral plasma membrane in epithelial cells of the mouse gall bladder, but not at specialized cell-cell junctions, such as tight junctions, adherens junctions, and desmosomes. Nectins bind afadin, whereas Necl-2 did not bind afadin but bound Pals2, a membrane-associated guanylate kinase family member known to bind Lin-7, implicated in the proper localization of the Let-23 protein in Caenorhabditis elegans, the homologue of mammalian epidermal growth factor receptor. These results indicate the unique localization of Necl-2 and its possible involvement in localization of a transmembrane protein(s) through Pals2.

Identification of secreted CD155 isoforms

The CD155 gene is a member of the immunoglobulin superfamily. We first demonstrate the existence of soluble CD155 (sCD155) isoforms in culture medium conditioned by CD155-expressing cells, in human serum and in cerebrospinal fluid. sCD155 concentration was measured in human serum and cerebrospinal fluid using a specific ELISA. Analysis of conditioned media indicated that sCD155 release does not require protease activity. In order to determine which tissues are responsible for sCD155 expression, we have quantified CD155 mRNAs in human normal tissues. The highest expression was observed in liver. The CD155alpha transcript is the most abundant and the proportion of the CD155beta and CD155gamma variants was similar between the tissues. Finally, serum purified sCD155 reduces poliovirus entry mediated by membrane-bound CD155. The high level of CD155 synthesis in many tissues and the presence of sCD155 in biological fluids suggest the existence of an important role for the protein in cellular function.

Recruitment of nectin-3 to cell-cell junctions through trans-heterophilic interaction with CD155, a vitronectin and poliovirus receptor that localizes to alpha(v)beta3 integrin-containing membrane microdomains

Nectins present a novel class of Ig superfamily adhesion molecules that, cooperatively with cadherins, establish and maintain cell-cell adherens junctions. CD155, the cognate receptor for poliovirus, undergoes cell-matrix contacts by binding to the extracellular matrix protein vitronectin. The significant homology of nectins with CD155 prompted us to investigate the possibility of their interaction. We determined that nectin-3 binds CD155 and its putative mouse homologue Tage4 in cell-based ligand binding assays. Coculture of nectin-3- and CD155-expressing HeLa cells led to CD155-dependent recruitment of nectin-3 to cell-cell contacts. In a heterologous coculture system with CD155 expressing mouse neuroblastoma cells, HeLa cell-expressed nectin-3 was recruited to contact sites with CD155 bearing neurites. CD155 and nectin-3 colocalized to epithelial cell-cell junctions in renal proximal tubules and in the amniotic membrane. Efficient interaction depended on CD155 dimerization, which appears to be aided by cell type-specific cofactors. We furthermore found CD155 to codistribute with alpha(v) integrin microdomains on the surface of transfected mouse fibroblasts and at amniotic epithelial cell junctions. Our findings demonstrate the possible trans-interaction between the bona fide cell-cell adherens type adhesion system (cadherin/nectin) and the cell-matrix adhesion system (integrin/CD155) by virtue of their nectin-3 and CD155 components, respectively.

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

Nectins are a family of Ca(2+)-independent immunoglobulin-like cell-cell adhesion molecules consisting of four members, which homophilically and heterophilically trans-interact and cause cell-cell adhesion. Nectin-based cell-cell adhesion is involved in the formation of cadherin-based adherens junctions in epithelial cells and fibroblasts. The nectin-based cell-cell adhesion induces activation of Cdc42 and Rac small G proteins, which eventually regulate the formation of adherens junctions through reorganization of the actin cytoskeleton, gene expression through activation of a mitogen-activated protein kinase cascade, and cell polarization through cell polarity proteins. Five nectin-like molecules (necls), which have domain structures similar to those of nectins, have recently been identified and appear to play different roles from those of nectins. One of them, named necl-5, which does not homophilically trans-interact, but heterophilically trans-interacts with nectin-3, regulates cell migration and adhesion. In this article, the roles and modes of action of nectins and necls in cell adhesion, migration, and polarization are reviewed.

Tage4/Nectin-like molecule-5 heterophilically trans-interacts with cell adhesion molecule Nectin-3 and enhances cell migration

Malignant transformation of cells causes disruption of cell-cell adhesion, enhancement of cell motility, and invasion into surrounding tissues. Nectins have both homophilic and heterophilic cell-cell adhesion activities and organize adherens junctions in cooperation with cadherins. We examined here whether Tage4, which was originally identified to be a gene overexpressed in colon carcinoma and has a domain structure similar to those of nectins, is involved in cell adhesion and/or migration. Tage4 heterophilically trans-interacted with nectin-3, but not homophilically with Tage4. Expression of Tage4 was markedly elevated in NIH3T3 cells transformed by an oncogenic Ki-Ras (V12Ras-NIH3T3 cells) as compared with that of wild-type NIH3T3 cells. trans-Interaction of Tage4 with nectin-3 enhanced motility of V12Ras-NIH3T3 cells. Tage4 did not bind afadin, a nectin- and actin filament-binding protein that connects nectins to the actin cytoskeleton and cadherins through catenins. Thus, Tage4 heterophilically trans-interacts with nectin-3 and regulates cell migration. Tage4 is tentatively re-named here nectin-like molecule-5 (necl-5) on the basis of its function and domain structure similar to those of nectins.

Aggregated colon cancer cells have a higher metastatic efficiency in the liver compared with nonaggregated cells: an experimental study

BACKGROUND

It remains unclear whether aggregated colon cancer cells have a higher tendency for metastasis formation than nonaggregated cells. Also, the absolute number of cancer cells required for hepatic metastasis remains undefined. The aim of the present study was to compare in the liver the metastatic efficiency of viable nonaggregated colon cancer cells versus cell aggregates for equivalent numbers of cancer cells.

MATERIALS AND METHODS

DHD/K12/TRb colon cancer cells were administered through the portal vein in syngeneic male BD IX rats. Surgical exploration was performed 8 weeks after injection. Four groups of rats were injected with 0.25 or 0.5 x 10(6) DHD/K12/TRb viable cancer cells, either as single nonaggregated cells or as cell aggregates.

RESULTS

Hepatic metastases were observed in 81% of the rats after intraportal injection of cell aggregates equivalent to 0.5 x 10(6) cancer cells. A significant lower metastatic efficiency was found after the injection of 0.5 x 10(6) non-aggregated, and 0.25 x 10(6) aggregated or nonaggregated cancer cells i.e., 16%, 32%, and 27%, respectively.

CONCLUSION

Aggregated colon cancer cells have a higher metastatic efficiency in the liver compared with non-aggregated cells, although a critical number of cancer cells are necessary.