The roles of cell adhesion molecules in tumor suppression and cell migration

In addition to mediating cell adhesion, many cell adhesion molecules act as tumor suppressors. These proteins are capable of restricting cell growth mainly through contact inhibition. Alterations of these cell adhesion molecules are a common event in cancer. The resulting loss of cell-cell and/or cell-extracellular matrix adhesion promotes cell growth as well as tumor dissemination. Therefore, it is conventionally accepted that cell adhesion molecules that function as tumor suppressors are also involved in limiting tumor cell migration. Paradoxically, in 2005, we identified an immunoglobulin superfamily cell adhesion molecule hepaCAM that is able to suppress cancer cell growth and yet induce migration. Almost concurrently, CEACAM1 was verified to co-function as a tumor suppressor and invasion promoter. To date, the reason and mechanism responsible for this exceptional phenomenon remain unclear. Nevertheless, the emergence of these intriguing cell adhesion molecules with conflicting roles may open a new chapter to the biological significance of cell adhesion molecules.     

Integrins and other cell adhesion molecules

Cell-cell and cell-substratum interactions are mediated through several different families of receptors. In addition to targeting cell adhesion to specific extracellular matrix proteins and ligands on adjacent cells, these receptors influence many diverse processes including cellular growth, differentiation, junction formation, and polarity. Several families of adhesion receptors have been identified. These include: 1) the integrins, heterodimeric molecules that function both as cell-substratum and cell-cell adhesion receptors; 2) the adhesion molecules of the immunoglobulin superfamily, which are involved in cell-cell adhesion and especially important during embryo-genesis, wound healing, and the inflammatory response; 3) the cadherins, developmentally regulated, calcium-dependent homophilic cell-cell adhesion proteins; 4) the LEC-CAMs, cell adhesion molecules with lectin-like domains that mediate white blood cell/endothelial cell adhesion; and 5) homing receptors that target lymphocytes to specific lymphoid tissue. In this review we summarize recent data describing the structure and function of some of these cell adhesion molecules (with special emphasis on the integrin family) and discuss the possible role of these molecules in development, inflammation, wound healing, coagulation, and tumor metastasis.     

The CCN family of proteins: structure-function relationships

The CCN proteins are key signalling and regulatory molecules involved in many vital biological functions, including cell proliferation, angiogenesis, tumourigenesis and wound healing. How these proteins influence such a range of functions remains incompletely understood but is probably related to their discrete modular nature and a complex array of intra- and inter-molecular interactions with a variety of regulatory proteins and ligands. Although certain aspects of their biology can be attributed to the four individual modules that constitute the CCN proteins, recent results suggest that some of their biological functions require cooperation between modules. Indeed, the modular structure of CCN proteins provides important insight into their structure-function relationships.     

Proteins on the catwalk: modelling the structural domains of the CCN family of proteins

The CCN family of proteins (CCN1, CCN2, CCN3, CCN4, CCN5 and CCN6) are multifunctional mosaic proteins that play keys roles in crucial areas of physiology such as angiogenesis, skeletal development tumourigenesis, cell proliferation, adhesion and survival. This expansive repertoire of functions comes through a modular structure of 4 discrete domains that act both independently and in concert. How these interactions with ligands and with neighbouring domains lead to the biological effects is still to be explored but the molecular structure of the domains is likely to play an important role in this. In this review we have highlighted some of the key features of the individual domains of CCN family of proteins based on their biological effects using a homology modelling approach.     

The echinoderm adhesome

Although the development of sea urchin embryos has been studied extensively and clearly involves both cell adhesion and cell migration, rather little is known about the adhesion receptors and extracellular matrix molecules involved. The completion of the genome of Strongylocentrotus purpuratus allows a comprehensive survey of the complement of cell-cell and cell-matrix adhesion molecules in this organism. Furthermore, the phylogenetic position of echinoderms offers the opportunity to compare the complement of adhesion proteins between protostome and deuterostome invertebrates and between invertebrate and vertebrate deuterostomes. Many aspects of development and cell interactions differ among these different taxa and it is likely that analysis of the spectrum of adhesion receptors and extracellular matrix proteins can open up new insights into which molecules have evolved to suit particular developmental processes. In this paper, we report the results of an initial analysis along these lines. The echinoderm adhesome (complement of adhesion-related genes/proteins) is similar overall to that of other invertebrates although there are significant deuterostome-specific innovations and some interesting features previously thought to be chordate or vertebrate specific.     

Immunoglobulin superfamily receptors: cis-interactions, intracellular adapters and alternative splicing regulate adhesion.

The immunoglobulin domain is a module found in vertebrates and invertebrates. Its ability to form linear rods when deployed in series, combined with its propensity to bind specifically to other proteins has made it ideal for building cell surface receptors and cell adhesion molecules. These features have resulted in the incorporation of immunoglobulin domains into many hundreds of cell surface molecules. Recently three major advances have been made in understanding immunoglobulin receptors. One is the recognition that their intracellular binding partners are likely to link to multiple cell surface molecules, allowing cross-talk or oligomeric complex formation. A second, but related phenomenon, is their participation in cis-interactions on the extracellular surface that regulate signaling or adhesion. The third is the dramatic ability to form dozens to thousands of different isoforms via alternative splicing. Although antibodies may have been the first example of immunoglobulin-domain-containing proteins using cis-interactions to form receptor like molecules, and the grandest instance of diversity production from limited genetic material, these are clearly old ideas in this superfamily.     

Physiology and pathophysiology of selectins, integrins, and IgSF cell adhesion molecules focusing on inflammation. A paradigm model on infectious endocarditis

The development of adhesion bonds, either among cells or among cells and components of the extracellular matrix, is a crucial process. These interactions are mediated by some molecules collectively known as adhesion molecules (CAMs). CAMs are ubiquitously expressed proteins playing a central role in controlling cell migration, proliferation, survival, and apoptosis. Besides their key function in physiological maintenance of tissue integrity, CAMs play an eminent role in various pathological processes such as cardiovascular disorders, atherogenesis, atherosclerotic plaque progression and regulation of the inflammatory response. CAMs such as selectins, integrins, and immunoglobulin superfamily take part in interactions between leukocyte and vascular endothelium (leukocyte rolling, arrest, firm adhesion, migration). Experimental data and pathologic observations support the assumption that pathogenic microorganisms attach to vascular endothelial cells or sites of vascular injury initiating intravascular infections. In this review a paradigm focusing on cell adhesion molecules pathophysiology and infective endocarditis development is given.     

Catenin association with E-cadherin changes with the state of polarity of HT-29 cells

Cadherins are transmembrane glycoproteins that mediate the calcium-dependent adhesion of cells to one another. It has been reported that at least two and probably more proteins associate with cadherins in various systems. These proteins have been called catenins. HT-29 cells can be manipulated to express either a polar or a nonpolar phenotype, depending on the growth conditions. We have taken advantage of this feature of HT-29 cells to explore the role catenins may play in cadherin-mediated adhesion. In this paper we report that several catenins co-immune-precipitate with E-cadherin in cultured human cells (HT-29 and JAR PR497) and that the nature of the complex of proteins varies with the physiological state of the HT-29 cells. In addition, we show data suggesting that the proteins that associate with calcium-dependent adhesion molecules may represent a group of proteins, some of which are present in all cells and some of which are cell-type specific.     

Approaches in extracellular matrix engineering for determination of adhesion molecule mediated single cell function

The native extracellular matrix (ECM) and the cells that comprise human tissues are together engaged in a complex relationship; cells alter the composition and structure of the ECM to regulate the material and biologic properties of the surrounding environment while the composition and structure of the ECM modulates cellular processes that maintain healthy tissue and repair diseased tissue. This reciprocal relationship occurs via cell adhesion molecules (CAMs) such as integrins, selectins, cadherins and IgSF adhesion molecules. To study these cell-ECM interactions, researchers use two-dimensional substrates or three-dimensional matrices composed of native proteins or bioactive peptide sequences to study single cell function. While two-dimensional substrates provide valuable information about cell-ECM interactions, three-dimensional matrices more closely mimic the native ECM; cells cultured in three-dimensional matrices have demonstrated greater cell movement and increased integrin expression when compared to cells cultured on two-dimensional substrates. In this article we review a number of cellular processes (adhesion, motility, phagocytosis, differentiation and survival) and examine the cell adhesion molecules and ECM proteins (or bioactive peptide sequences) that mediate cell functionality.     

Lectin cell adhesion molecules (LEC-CAMs): a new family of cell adhesion proteins involved with inflammation.

The means by which leukocytes, including lymphocytes, monocytes, and neutrophils, migrate from the circulation to sites of acute and chronic inflammation is an area of intense research interest. Although a number of soluble mediators of these important cellular interactions have been identified, a major site of great importance to the inflammatory response is the physical interface between the white cell and the endothelium. This critical association is mediated by an array of cell surface adhesion molecules. Previous data have demonstrated that the integrin subfamily of heterotypic adhesion molecules was a major component of these adhesive interactions, although it was clear that other, non-integrin-like molecules of unknown identity also seemed to be involved during the inflammatory process. A number of these other cell-surface glycoproteins which may be involved with inflammation have recently been characterized by molecular cloning. These glycoproteins, including the peripheral lymph node homing receptor (pln HR), the endothelial cell adhesion molecule (ELAM), and PADGEM/gmp140, are all members of a family of proteins which are unified by the inclusion of three characteristic protein motifs: a lectin or carbohydrate recognition domain, an epidermal growth factor (egf) domain, and a variable number of short consensus repeats (scr) which are also found in members of the complement regulatory proteins. The appearance of lectin domains in all of these adhesion molecules is consistent with the possibility that these glycoproteins function by binding to carbohydrates which are expressed in a cell and/or region specific manner, and the members of this adhesion family have been given the generic name LEC-CAM (lectin cell adhesion molecules).(ABSTRACT TRUNCATED AT 250 WORDS)     

Synaptic cell adhesion

Chemical synapses are asymmetric intercellular junctions that mediate synaptic transmission. Synaptic junctions are organized by trans-synaptic cell adhesion molecules bridging the synaptic cleft. Synaptic cell adhesion molecules not only connect pre- and postsynaptic compartments, but also mediate trans-synaptic recognition and signaling processes that are essential for the establishment, specification, and plasticity of synapses. A growing number of synaptic cell adhesion molecules that include neurexins and neuroligins, Ig-domain proteins such as SynCAMs, receptor phosphotyrosine kinases and phosphatases, and several leucine-rich repeat proteins have been identified. These synaptic cell adhesion molecules use characteristic extracellular domains to perform complementary roles in organizing synaptic junctions that are only now being revealed. The importance of synaptic cell adhesion molecules for brain function is highlighted by recent findings implicating several such molecules, notably neurexins and neuroligins, in schizophrenia and autism.     

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.     

Physiology and Pathophysiology of Selectins,Integrins, and IgSf Cell Adhesion Molecules Focusing on Inflammation. A Paradigm Model on Infectious Endocarditis

Abstract

The development of adhesion bonds, either among cells or among cells and components of the extracellular matrix, is a crucial process. These interactions are mediated by some molecules collectively known as adhesion molecules (CAMs). CAMs are ubiquitously expressed proteins playing a central role in controlling cell migration, proliferation, survival, and apoptosis. Besides their key function in physiological maintenance of tissue integrity, CAMs play an eminent role in various pathological processes such as cardiovascular disorders, atherogenesis, atherosclerotic plaque progression and regulation of the inflammatory response. CAMs such as selectins, integrins, and immunoglobulin superfamily take part in interactions between leukocyte and vascular endothelium (leukocyte rolling, arrest, firm adhesion, migration). Experimental data and pathologic observations support the assumption that pathogenic microorganisms attach to vascular endothelial cells or sites of vascular injury initiating intravascular infections. In this review a paradigm focusing on cell adhesion molecules pathophysiology and infective endocarditis development is given.     

Interaction of human lactoferrin with cell adhesion molecules through RGD motif elucidated by lactoferrin-binding epitopes

Lactoferrin (LF) is an iron-binding secretory protein, which is distributed in the secondary granules of polynuclear lymphocytes as well as in the milk produced by female mammals. Although it has multiple functions, for example antimicrobial, immunomodulatory, antiviral, and anti-tumor metastasis activities, the receptors responsible for these activities are not fully understood. In this study, the binding epitopes for human LF were first isolated from a hexameric random peptide library displayed on T7 phage. Interestingly, two of the four isolated peptides had a representative cell adhesion motif, Arg-Gly-Asp (RGD), implying that human LF interacts with proteins with the RGD motif. We found that human LF bound to the RGD-containing human extracellular matrix proteins, fibronectin and vitronectin. Furthermore, human LF inhibited cell adhesion to these matrix proteins in a concentration-dependent manner but not to the RGD-independent cell adhesion molecule like laminin or collagen. These results indicate that a function of human LF is to block the various interactions between the cell surface and adhesion molecules. This may explain the multifunctionality of LF.     

Heterophilic interactions of DM-GRASP: GRASP-NgCAM interactions involved in neurite extension

DM-GRASP is an immunoglobulin superfamily cell adhesion molecule that is expressed in both the developing nervous and immune system. Specific populations of neurons respond to DM-GRASP substrates appears to require homophilic interactions between DM-GRASP molecules. We were interested in determining whether DM-GRASP interacts heterophilically with other ligands as well. We have found that eleven proteins from embryonic chick brain membranes consistently bind to and elute from a DM-GRASP-Sepharose affinity column. One of these proteins is DM-GRASP itself, consistent with its known homophilic binding. Another protein, at 130 kD, is immunoreactive with monoclonal antibodies to NgCAM. Other neural cell adhesion molecules were not detected in the eluate. The DM- GRASP-Sepharose eluate also contains a potent neurite stimulating activity, which cannot be accounted for by either DM-GRASP or NgCAM. To investigate the interaction of DM-GRASP and NgCAM, antibodies against DM-GRASP were added to neuronal cultures extending neurites on an NgCAM substrate. The presence of antibodies to DM-GRASP decreased neurite extension on laminin, suggesting that the antibody is not toxic or generally inhibiting motility. We present two possible models for the DM-GRASP-NgCAM association and a hypothesis for neural cell adhesion function that features the dimerization of cell adhesion molecules.     

Adhesion molecules and their role in cancer metastasis

This article describes various adhesion molecules and reviews evidence to support a mechanistic role for adhesion molecules in the process of cancer metastasis. A variety of evidence supports the involvement of specific adhesion molecules in metastasis.

1. For example, some cancer cells metastasize to specific organs, irrespective of the first organ encountered by the circulating cancer cells. This ability to colonize a specific organ has been correlated with the preferential adhesion of the cancer cells to endothelial cells derived from the target organ. This suggests that cancer cell/endothelial cell adhesion is involved in cancer cell metastasis and that adhesion molecules are expressed on the endothelium in an organ-specific manner.
2. Further, inclusion of peptides that inhibit cell adhesion, such as the YIGSR- or RGD-containing peptides, is capable of inhibiting experimental metastasis.
3. Metastasis can be enhanced by acute or chronic inflammation of target vessels, or by treatment of animals with inflammatory cytokines, such as interleukin-1. In vitro, cancer cell/endothelial cell adhesion can be enhanced by pretreating the endothelial cell monolayer with cytokines, such as interleukin-1 or tumor necrosis factor-α. This suggests that, in addition to organ-specific adhesion molecules, a population of inducible endothelial adhesion molecules is involved and is relevant to metastasis.
4. Further support for this model is found in the comparison to leukocyte/endothelial adhesion during leukocyte trafficking. Convincing evidence exists, both in vivo and in vitro, to demonstrate an absolute requirement for leukocyte/endothelial adhesion before leukocyte extravasation can occur. The relevance of this comparison to metastasis is reinforced by the observation that some of the adhesion molecules involved in leukocyte/endothelial adhesion are also implicated in cancer cell/endothelial adhesion. The involvement of adhesion molecules suggests a potential therapy for metastasis based on interrupting adhesive interactions that would augment other treatments for primary tumors.

     

Distribution and possible interactions of actin-associated proteins and cell adhesion molecules of nerve growth cones

Actin filaments and their interactions with cell surface molecules have key roles in tissue cell behaviour. Axonal pathfinding during embryogenesis, an especially complex cell behaviour, is based on the migration of nerve growth cones. We have used fluorescence immunocytochemistry to examine the distribution in growth cones, their filopodia and lamellipodia of several actin-associated proteins and nerve cell adhesion molecules. The leading margins of chick dorsal root ganglion nerve growth cones and their protrusions stain strongly for f-actin, filamin, alpha-actinin, myosin, tropomyosin, talin and vinculin. MAP2 is absent from DRG growth cones, and staining for spectrin fodrin extends into growth cones, but not along filopodia. Thus, organization of the leading margins of growth cones may strongly resemble the leading lamella of migrating fibroblasts. The adhesion-mediating molecules integrin, L1, N-CAM and A-CAM are all found on DRG neurites and growth cones. However, filopodia stain relatively more strongly for integrin and L1 than for A-CAM or N-CAM. In fact, the 180 X 10(3) Mr form of N-CAM may be absent from most of the length of filopodia. DRG neurones cultured in cytochalasin B display differences in immunofluorescence staining which further emphasize that these adhesion molecules interact differentially with the actin filament system of migrating growth cones. Several models for neuronal morphogenesis emphasize the importance of regulation of the expression of adhesion molecules. Our results support hypotheses that cellular distribution and transmembrane interactions are key elements in the functions of these adhesion molecules during axonal pathfinding.