The roles of cell adhesion molecules in tumor suppression and cell migration: A new paradox

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.Key words: hepaCAM, cell adhesion molecules, tumor suppressor, migration, E-cadherin, CADM1, integrin α7, CEACAM1It is well known that many cell adhesion molecules function as tumor suppressors (reviewed in ref. ¹). These molecules exert their tumor suppressive effect mainly through cell-adhesion-mediated contact inhibition. Cell adhesion molecules allow cells to communicate with one another or to the extracellular environment by mediating cell-cell or cell-extracellular matrix (ECM) interactions (reviewed in refs. ² and ³). Broadly, these proteins can be classified into five families including immunoglobulin superfamily, integrins, cadherins, selectins and CD44. Apart from participating in the development and maintenance of tissue architecture, cell adhesion molecules serve as cell surface receptors critical for capturing, integrating and transmitting signals from the extracellular milieu to the cell interior (reviewed in refs. ² and ³). These signaling events are vital for the regulation of a wide variety of cellular functions including embryogenesis, immune and inflammatory responses, tissue repair, cell migration, differentiation, proliferation and apoptosis. Alterations of these cell adhesion molecules are a common event in cancer (reviewed in refs. ^{1, 2, 4} and ⁵). The disrupted cell-cell or cell-ECM adhesion significantly contributes to uncontrolled cell proliferation and progressive distortion of normal tissue architecture. More importantly, changes in cell adhesion molecules play a causal role in tumor dissemination. Loss of cell adhesion contacts allows malignant cells to detach and to escape from the primary mass. Gaining a more motile and invasive phenotype, these cells break down the ECM and eventually invade and metastasize to distal organs.Based on the above understanding, it is conventionally accepted that cell adhesion molecules with tumor suppressor activity, when expressed in cancer cells, are able to exert inhibitory effect on cell motility. The ability of cells in migration/motility is a prerequisite for cancer invasion and metastasis (reviewed in refs. ¹ and ⁵). Indeed, a number of cell adhesion molecule-tumor suppressors have been reported to be capable of reducing cell migration. The most classical example is E-cadherin, a calcium-dependent cell adhesion molecule. E-cadherin is expressed exclusively in epithelial cells and its expression is commonly suppressed in tumors of epithelial origins. The cytoplasmic domain of E-cadherin interacts with catenins to establish an intracellular linkage with the actin cytoskeleton (reviewed in ref. ⁶). The assembly of E-cadherin with the cytoskeleton via catenins at the sites of adherens junctions is important for the stabilization of cell-cell adhesions. Disruption of E-cadherin-mediated cell-cell adhesion, due to loss of expression or function of E-cadherin and/or catenins, is assocated with tumor development and progression (reviewed in ref. ⁷). Forced expression of E-cadherin in several cancer cell lines not only slows down cell growth^8,9 but also significantly reduces the invasiveness of the cells.^10,11 On the other hand, inhibition of E-cadherin by function-blocking antibodies and antisense RNA restores the invasiveness in non-invasive transformed cells.¹¹ Furthermore, using a transgenic mouse model of pancreatic beta-cell carcinogenesis, it has been demonstrated that E-cadherin-mediated cell adhesion is important in preventing the transition from well differentiated adenoma to invasive carcinoma.¹²Cell adhesion molecule 1 (CADM1), another example, has also been implicated in cancer progression. CADM1 is a member of the immunoglobulin superfamily and mediates cell-cell adhesion.¹³ The molecule associates with the actin cytoskeleton via the differentially expressed in adenocarcinoma of the lung (DAL1) protein; and the formation of CADM1-DAL1 complex is dependent on the integrity of actin cytoskeleton.¹⁴ Inactivation of the CADM1 and/or DAL1 gene usually through methylation has been reported in diverse human cancers.^15,16 A paper by Ito et al. showed that restoration of CADM1 expression in esophageal squamous cell carcinoma cells not only suppresses cell growth, but also retards cell motility and invasion.¹⁶In contrast to E-cadherin and CADM1, integrin α7 is a cell-ECM adhesion molecule which also possesses tumor suppressor activity. Ren et al. showed that integrin α7 gene is mutated in several human malignances; and the mutations are associated with an increase in cancer recurrence.¹⁷ Forced expression of integrin α7 in integrin α7-deficient leiomyosarcoma cells results in decreased colony formation and slower cell motility. Conversely, knockdown of integrin α7 in lung cancer cells expressing wild-type integrin α7 increases the colony number and cell motility rate. In addition, the researchers revealed that mice bearing xenograft tumors overexpressing integrin α7 have reduced tumor size with no obvious metastasis.In 2005, we first reported the identification of a cell adhesion molecule belonging to the immunoglobulin superfamily, designated as hepaCAM.¹⁸ To date, we have shown that the gene is frequently downregulated in a variety of human cancers.^18,19 Re-expression of hepaCAM in the hepatocellular carcinoma HepG2 cells¹⁸ and breast cancer MCF7 cells¹⁹ inhibits colony formation and retards cell proliferation. In addition, expression of hepaCAM in MCF7 cells results in cell cycle arrest at the G₂/M phase and cellular senescence. Concurrently, the expression of several senescence-associated proteins including p53, p21 and p27 is enhanced. Moreover, downregulation of p53 by p53-specific small interfering RNA in cells expressing hepaCAM clearly reduces p21 without changing p27 and alleviates senescence, indicating that hepaCAM induces senescence through a p53/p21-dependent pathway.¹⁹ Together, the data suggest that hepaCAM is a tumor suppressor. Interestingly, the expression of hepaCAM in both HepG2 and MCF7 cells stimulates both cell-ECM adhesion and cell migration.^18,20,21 The function of hepaCAM as a tumor suppressor in cell migration is contradictory to other cell adhesion molecule-tumor suppressors. Noteworthily, hepaCAM-mediated cell motility is evidenced by its direct interaction with the actin cytoskeleton.²¹Evidences are currently emerging to support the contradictory roles of cell adhesion molecules that both inhibit cell growth and promote cell motility when restored in cancer cells. In addition to hepaCAM, the immunoglobulin superfamily carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) is implicated to function as a tumor suppressor and a metastasis promoter. The characteristics and functions of CEACAM1 have been demonstrated in individual reports. CEACAM1 is frequently downregulated or dysregulated in multiple human tumors,^22–25 and is capable of suppressing cell growth and inducing apoptosis.^26–28 Ebrahimnejad et al. demonstrated that exogenous expression of CEACAM1 enhances melanoma cell invasion and migration; and this enhanced motility can be reverted by anti-CEACAM antibodies.²⁹ The ability of CEACAM to co-stimulate tumor suppression and invasion was finally established by Liu et al. in restricting thyroid cancer growth but promoting invasiveness.³⁰ Introduction of CEACAM1 into CEACAM1-deficient thyroid cancer cells results in G₁/S phase cell cycle arrest accompanied by elevated p21 expression and diminished Rb phosphorylation. Overexpression of CEACAM1 also increases cell-ECM adhesion and promotes cell migration and tumor invasiveness. In xenografted mice, CEACAM1 expression results in reduced tumor growth but increased tumor invasiveness. Conversely, silencing of endogenous CEACAM1 accelerates tumor growth and suppresses invasiveness.³⁰It is an exciting issue to address why a cell adhesion molecule is able to suppress tumor growth yet promote tumor progression. Could there be a molecular switch that controls the functions of the gene between a tumor suppressor and a migration promoter in cancer or are the functions executed simultaneously? The expression level, the extracellular cues as well as the interacting partners of the cell adhesion molecules may likely play a critical role in regulating its functions. The question is under what circumstances these factors come into play. To answer all these questions, and maybe more, on the intriguing findings of these proteins, more extensive and intensive experimentation is required. Nevertheless, it is obvious that the emergence of these cell adhesion molecules that function in a contradictory manner opens a new chapter to the biological significance of cell adhesion molecules.     

Should I stay or should I go? Shedding of RPTPs in cancer cells switches signals from stabilizing cell-cell adhesion to driving cell migration

Dissolution of cell-cell adhesive contacts and increased cell-extracellular matrix adhesion are hallmarks of the migratory and invasive phenotype of cancer cells. These changes are facilitated by growth factor binding to receptor protein tyrosine kinases (RTKs). In normal cells, cell-cell adhesion molecules (CAMs), including some receptor protein tyrosine phosphatases (RPTPs), antagonize RTK signaling by promoting adhesion over migration. In cancer, RTK signaling is constitutive due to mutated or amplified RTKs, which leads to growth factor independence, or autonomy. An alternative route for a tumor cell to achieve autonomy is to inactivate cell-cell CAMs such as RPTPs. RPTPs directly mediate cell adhesion and regulate both cadherin-dependent adhesion and signaling. In addition, RPTPs antagonize RTK signaling by dephosphorylating molecules activated following ligand binding. Both RPTPs and cadherins are downregulated in tumor cells by cleavage at the cell surface. This results in shedding of the extracellular, adhesive segment and displacement of the intracellular segment, altering its subcellular localization and access to substrates or binding partners. In this commentary we discuss the signals that are altered following RPTP and cadherin cleavage to promote cell migration. Tumor cells both step on the gas (RTKs) and disconnect the brakes (RPTPs and cadherins) during their invasive and metastatic journey.     

Paradoxical roles of FAK in tumor cell migration and metastasis

Focal adhesion kinase (FAK), as a key mediator of signaling induced by integrins, plays an instrumental role in many cellular functions, including cell survival and proliferation. Many studies have reported that FAK is a positive regulator of normal cell migration and cancer cell metastasis. However, emerging evidence shows that FAK—under certain oncogenic signaling, such as that initiated by activated Ras and some growth factor receptor kinases—negatively regulates cancer cell migration. Activated Ras may promote tumor cell migration by dephosphorylation of FAK at Y397 and facilitation of focal adhesion turnover at the leading edge of cells.     

Collective dynamics of cancer cells confined in a confluent monolayer of normal cells

Tumorigenesis often involves specific changes in cell motility and intercellular adhesion. Understanding the collective cancer cell behavior associated with these specific changes could facilitate the detection of malignant characteristics during tumor growth and invasion. In this study, a cellular vertex model is developed to investigate the collective dynamics of a disk-like aggregate of cancer cells confined in a confluent monolayer of normal cells. The effects of intercellular adhesion and cell motility on tumor progression are examined. It is found that the stresses in both the cancer cells and the normal cells increase with tumor growth, resulting in a crowded environment and enhanced cell apoptosis. The intercellular adhesion between cancer cells and normal cells is revealed to promote tumor growth and invasion. The tumor invasion dynamics hinges on the motility of cancer cells. The cancer cells could orchestrate into different collective migration modes, e.g., directional migration and rotational oscillations, dictated by the competition between cell persistence and local coordination. Phase diagrams are established to reveal the competitive mechanisms. This work highlights the role of mechanics in regulating tumor growth and invasion.     

Junctional adhesion molecule C (JAM-C) dimerization aids cancer cell migration and metastasis

Most cancer deaths result from metastasis, which is the dissemination of cells from a primary tumor to distant organs. Metastasis involves changes to molecules that are essential for tumor cell adhesion to the extracellular matrix and to endothelial cells. Junctional Adhesion Molecule C (JAM-C) localizes at intercellular junctions as homodimers or more affine heterodimers with JAM-B. We previously showed that the homodimerization site (E66) in JAM-C is also involved in JAM-B binding. Here we show that neoexpression of JAM-C in a JAM-C-negative carcinoma cell line induced loss of adhesive property and pro-metastatic capacities. We also identify two critical structural sites (E66 and K68) for JAM-C/JAM-B interaction by directed mutagenesis of JAM-C and studied their implication on tumor cell behavior. JAM-C mutants did not bind to JAM-B or localize correctly to junctions. Moreover, mutated JAM-C proteins increased adhesion and reduced proliferation and migration of lung carcinoma cell lines. Carcinoma cells expressing mutant JAM-C grew slower than with JAM-C WT and were not able to establish metastatic lung nodules in mice. Overall these data demonstrate that the dimerization sites E66-K68 of JAM-C affected cell adhesion, polarization and migration and are essential for tumor cell metastasis.     

Lessons from development: A role for asymmetric stem cell division in cancer

Asymmetric stem cell division has emerged as a major regulatory mechanism for physiologic control of stem cell numbers. Reinvigoration of the cancer stem cell theory suggests that tumorigenesis may be regulated by maintaining the balance between asymmetric and symmetric cell division. Therefore, mutations affecting this balance could result in aberrant expansion of stem cells. Although a number of molecules have been implicated in regulation of asymmetric stem cell division, here, we highlight known tumor suppressors with established roles in this process. While a subset of these tumor suppressors were originally defined in developmental contexts, recent investigations reveal they are also lost or mutated in human cancers. Mutations in tumor suppressors involved in asymmetric stem cell division provide mechanisms by which cancer stem cells can hyperproliferate and offer an intriguing new focus for understanding cancer biology. Our discussion of this emerging research area derives insight from a frontier area of basic science and links these discoveries to human tumorigenesis. This highlights an important new focus for understanding the mechanism underlying expansion of cancer stem cells in driving tumorigenesis.     

Polysialylation promotes neural cell adhesion molecule-mediated cell migration in a fibroblast growth factor receptor-dependent manner, but independent of adhesion capability

Polysialic acid (PSA), a carbohydrate polymer mainly present in the neural cell adhesion molecule (NCAM), promotes neural plasticity; however, its mode of action in tumor malignancy remains largely unknown. In this study, we investigated the influence of polysialylation on cell migration. PSA consistently promoted cell migration on different extracellular matrices (ECMs) but differentially affected cell adhesion. All of these actions were reversed by endo-N-acetylneuraminidase treatment, and PSA-driven migration was inhibited by the specific fibroblast growth factor receptor (FGFR) inhibitor Su5402. Consistent with this latter observation, PSA-stimulated migration on different ECMs was paralleled by activation of the FGFR and its downstream signaling components, PLC-γ, focal adhesion kinase and extracellular signal-regulated kinase 1/2. In contrast, the pattern of p59(fyn) activation correlated with differential adhesion to different ECMs. Collectively, these results indicate that PSA-conjugated NCAM potentiates signal transduction by the FGFR pathway and thereby enhances cell migration independent of adhesion capability, providing additional insights into the role of PSA in cancer development.     

Should I stay or should I go?: Shedding of RPTPs in cancer cells switches signals from stabilizing cell-cell adhesion to driving cell migration

Dissolution of cell-cell adhesive contacts and increased cell-extracellular matrix adhesion are hallmarks of the migratory and invasive phenotype of cancer cells. These changes are facilitated by growth factor binding to receptor protein tyrosine kinases (RTKs). In normal cells, cell-cell adhesion molecules (CAMs), including some receptor protein tyrosine phosphatases (RPTPs), antagonize RTK signaling by promoting adhesion over migration. In cancer, RTK signaling is constitutive due to mutated or amplified RTKs, which leads to growth factor independence or autonomy. An alternative route for a tumor cell to achieve autonomy is to inactivate cell-cell CAMs such as RPTPs. RPTPs directly mediate cell adhesion and regulate both cadherin-dependent adhesion and signaling. In addition, RPTPs antagonize RTK signaling by dephosphorylating molecules activated following ligand binding. Both RPTPs and cadherins are downregulated in tumor cells by cleavage at the cell surface. This results in shedding of the extracellular, adhesive segment and displacement of the intracellular segment, altering its subcellular localization and access to substrates or binding partners. In this commentary we discuss the signals that are altered following RPTP and cadherin cleavage to promote cell migration. Tumor cells both step on the gas (RTKs) and disconnect the brakes (RPTPs and cadherins) during their invasive and metastatic journey.Key words: receptor protein tyrosine kinase, receptor-like protein tyrosine phosphatase, cadherins, cell adhesion, signal transduction, phospholipase C gamma, protein kinase C, catenins, IQGAP1 protein, regulated intramembrane proteolysis     

Amblyomin-X,a recombinant Kunitz-type inhibitor,regulates cell adhesion and migration of human tumor cells

ABSTRACT

In a tumor microenvironment, endothelial cell migration and angiogenesis allow cancer to spread to other organs causing metastasis. Indeed, a number of molecules that are involved in cytoskeleton re-organization and intracellular signaling have been investigated for their effects on tumor cell growth and metastasis. Alongside that, Amblyomin-X, a recombinant Kunitz-type protein, has been shown to reduce metastasis and tumor growth in in vivo experiments. In the present report, we provide a mechanistic insight to these antitumor effects, this is, Amblyomin-X modulates Rho-GTPases and uPAR signaling, and reduces the release of MMPs, leading to disruption of the actin cytoskeleton and decreased cell migration of tumor cell lines. Altogether, our data support a role for Amblyomin-X as a novel potential antitumor drug.     

Shc and FAK differentially regulate cell motility and directionality modulated by PTEN.

Cell migration is modulated by regulatory molecules such as growth factors, oncogenes, and the tumor suppressor PTEN. We previously described inhibition of cell migration by PTEN and restoration of motility by focal adhesion kinase (FAK) and p130 Crk-associated substrate (p130(Cas)). We now report a novel pathway regulating random cell motility involving Shc and mitogen-activated protein (MAP) kinase, which is downmodulated by PTEN and additive to a FAK pathway regulating directional migration. Overexpression of Shc or constitutively activated MEK1 in PTEN- reconstituted U87-MG cells stimulated integrin- mediated MAP kinase activation and cell migration. Conversely, overexpression of dominant negative Shc inhibited cell migration; Akt appeared uninvolved. PTEN directly dephosphorylated Shc. The migration induced by FAK or p130(Cas) was directionally persistent and involved extensive organization of actin microfilaments and focal adhesions. In contrast, Shc or MEK1 induced a random type of motility associated with less actin cytoskeletal and focal adhesion organization. These results identify two distinct, additive pathways regulating cell migration that are downregulated by tumor suppressor PTEN: one involves Shc, a MAP kinase pathway, and random migration, whereas the other involves FAK, p130(Cas), more extensive actin cytoskeletal organization, focal contacts, and directionally persistent cell motility. Integration of these pathways provides an intracellular mechanism for regulating the speed and the directionality of cell migration.     

P-selectin activates integrin-mediated colon carcinoma cell adhesion to fibronectin

During hematogenous cancer metastasis, tumor cells separate from a primary mass, enter the bloodstream, disperse throughout the body, migrate across vessel walls, and generate distant colonies. The later steps of metastasis superficially resemble leukocyte extravasation, a process initiated by selectin-mediated cell tethering to the blood vessel wall followed by integrin-mediated arrest and transendothelial migration. Some cancer cells express P-selectin ligands and attach to immobilized P-selectin, suggesting that these cells can arrest in blood vessels using sequential selectin- and integrin-mediated adhesion, as do leukocytes. We hypothesize that selectin binding may regulate subsequent integrin-mediated steps in metastasis. Using a model system of cultured Colo 320 human colon adenocarcinoma cells incubated with soluble P-selectin-IgG chimeric protein, we have found that P-selectin can stimulate activation of the alpha(5)beta(1) integrin resulting in a specific increase of adhesion and spreading of these cells on fibronectin substrates. P-selectin binding also induced activation of p38 mitogen-activated protein kinase (p38 MAPK) and phosphatidylinositol 3-kinase (PI3-K). PI3-K inhibitors blocked P-selectin-mediated integrin activation, cell attachment, and cell spreading. Inhibition of p38 MAPK activation blocked cell spreading, but not cell attachment. P-selectin binding also resulted in formation of a signaling complex containing PI3-K and p38 MAPK. These results suggest that P-selectin binding to tumor cells can activate alpha(5)beta(1) integrin via PI3-K and p38 MAPK signaling pathways leading to increased cell adhesion. We propose that P-selectin ligands are important tumor cell signaling molecules that modulate integrin-mediated cell adhesion in the metastatic process.     

The brain within the tumor: new roles for axon guidance molecules in cancers

Slits, semaphorins and netrins are three families of proteins that can attract or repel growing axons and migrating neurons in the developing nervous system of vertebrates and invertebrates. Recent studies have shown that they are widely expressed outside the nervous system and that they may play important roles in cancers. Several of the genes encoding these proteins are localized on chromosomal region associated with frequent loss-of-heterozygosity in tumors and cancer cell lines and there is also significant hypermethylation of their promoter suggesting that they may act as tumor suppressors. In addition, proteins in all these families and their receptors appear to control the vascularization of the tumors. Last, many axon guidance molecules also regulate cell migration and apoptosis in normal and tumorigenic tissues. Overall, this suggests that molecules that could mimick or block the activity of axon guidance molecules may be used as therapeutic agents for the treatment of malignancy.     

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 promise of stem cell markers in the diagnosis and therapy of epithelial dysplasia and oral squamous cell carcinoma

Oral squamous cell carcinoma (OSCC) is the most common type of head and neck cancer. Epithelial dysplasia is often initiated in the cells and cell nuclei adjacent to the epithelial cell membrane. Reduced cell–cell adhesions enable cancer cells to detach from the tumor and disseminate to other organs. The mutations in epithelial dysplasia markers such as E‐cadherin and epithelial cell adhesion molecules (CD326) can lead to proliferation, growth and survival of the tumor cells and persistence of numerous malignancies that play a key role in epithelial dysplasia of OSCC. Accordingly, these genes can be considered prognostic markers or potential therapeutic targets for the tailored management of patients with OSCC. The gene expression profile of OSCC stem cells indicates a differential pattern that facilitates establishing a cell signature. Owing to the highly tumorigenic behavior of cancer stem cells and the role of these cells in tumor differentiation, treatment resistance, relapse, and metastasis, we reviewed the role of stem cell markers in epithelial dysplasia and OSCC.     

Lipid rafts control human melanoma cell migration by regulating focal adhesion disassembly

Tumor cell migration is a crucial step in the metastatic cascade, and interruption of this step is considered to be logically effective in preventing tumor metastasis. Lipid rafts, distinct liquid ordered plasma membrane microdomains, have been shown to influence cancer cell migration, but the underlying mechanisms are still not well understood. Here, we report that lipid rafts regulate the dynamics of actin cytoskeleton and focal adhesion in human melanoma cell migration. Disrupting the integrity of lipid rafts with methyl-β cyclodextrin enhances actin stress fiber formation and inhibits focal adhesion disassembly, accompanied with alterations in cell morphology. Furthermore, actin cytoskeleton, rather than microtubules, mediates the lipid raft-dependent focal adhesion disassembly by regulating the dephosphorylation of focal adhesion proteins and the internalization of β3 integrin. We also show that Src–RhoA–Rho kinase signaling pathway is responsible for lipid raft disruption-induced stress fiber formation. Taken together, these observations provide a new mechanism to further explain how lipid rafts regulate the migration of melanoma cell and suggest that lipid rafts may be novel and attractive targets for cancer therapy.     

Platelet endothelial cell adhesion molecule, PECAM-1, modulates cell migration.

Cell migration is an important process in such phenomena as growth, development, and wound healing. The control of cell migration is orchestrated in part by cell surface adhesion molecules. These molecules fall into two major categories: those that bind to extracellular matrix and those that bind to adjacent cells. Here, we report on the role of a cell-cell adhesion molecule, platelet-endothelial cell adhesion molecule-1, (PECAM-1), a member of the lg superfamily, in the modulation of cell migration and cell-cell adhesion. PECAM-1 is a 120-130 kDa integral membrane protein that resides on endothelial cells and localizes at sites of cell-cell contact. Since endothelial cells express PECAM-1 constitutively, we studied the effects of PECAM-1 on cell-cell adhesion and migration in a null-cell population. Specifically, we transfected NIH/3T3 cells with the full length PECAM-1 molecule (two independent clones). Transfected cells containing only the neomycin resistance gene, cells expressing a construct coding for the extracellular domain of the molecule, and cells expressing the neu oncogene were used as controls. The PECAM-1 transfectants appeared smaller and more polygonal and tended to grow in clusters. Indirect immunofluorescence of PECAM-1 transfectants showed peripheral staining at sites of cell-cell contact, while the extracellular domain transfectants and the control cells did not. In two quantitative migration assays, the full-length PECAM-1 transfectants migrated more slowly than control cells. Thus, PECAM-1 transfected into a null cell appears to localize to sites of cell-cell contact, promote cell-cell adhesion, and diminish the rate of migration. These findings suggest a role for this cell-cell adhesion molecule in the process of endothelial cell migration.     

CD44 cross-linking induces integrin-mediated adhesion and transendothelial migration in breast cancer cell line by up-regulation of LFA-1 (alpha L beta2) and VLA-4 (alpha4beta1)

CD44, a widely expressed cell surface glycoprotein, plays a major role in cell-cell adhesion, cell-substrate interaction, lymphocyte homing, and tumor metastasis. For tumor metastasis to occur through the blood vessel and lymphatic vessel pathway, the tumor cells must first adhere to endothelial cells. Recent studies have shown that high expression of CD44 in certain types of tumors is associated with the hematogenic spread of cancer cells. However, the functional relevance of CD44 to tumor cell metastasis remains unknown. In this study, we investigated the mechanisms of CD44 cross-linking-induced adhesion and transendothelial migration of tumor cells using MDA-MB-435S breast cancer cell line. Breast cancer cells were found to express high levels of CD44. Using flow cytometric analysis and immunofluorescence staining, we demonstrated that cross-linking of CD44 resulted in a marked induction of the expression of lymphocyte function-associated antigen-1 (LFA-1) and very late antigen-4 (VLA-4) by exocytosis. These results were also observed with the Hs578T breast cancer cell line. Furthermore, LFA-1- and VLA-4-mediated adhesion and transendothelial cancer cell migration were also studied. Anti-LFA-1 mAb or anti-VLA-4 mAb alone had no effect on adhesion or transendothelial cancer cell migration, but were able to inhibit both of these functions when added together. This shows that CD44 cross-linking induces LFA-1 and VLA-4 expression in MDA-MB-435S cells and increases integrin-mediated adhesion to endothelial cells, resulting in the transendothelial migration of breast cancer cells. These observations provide direct evidence of a new function for CD44 that is involved in the induction of LFA-1 and VLA-4 expression by exocytosis in MDA-MB-435S cells. Because these induced integrins promote tumor cell migration into the target tissue, it may be possible to suppress this by pharmacological means, and thus potentially cause a reduction in invasive capability and metastasis.     

Mismatch in mechanical and adhesive properties induces pulsating cancer cell migration in epithelial monolayer

The mechanical and adhesive properties of cancer cells significantly change during tumor progression. Here we assess the functional consequences of mismatched stiffness and adhesive properties between neighboring normal cells on cancer cell migration in an epithelial-like cell monolayer. Using an in vitro coculture system and live-cell imaging, we find that the speed of single, mechanically soft breast carcinoma cells is dramatically enhanced by surrounding stiff nontransformed cells compared with single cells or a monolayer of carcinoma cells. Soft tumor cells undergo a mode of pulsating migration that is distinct from conventional mesenchymal and amoeboid migration, whereby long-lived episodes of slow, random migration are interlaced with short-lived episodes of extremely fast, directed migration, whereas the surrounding stiff cells show little net migration. This bursty migration is induced by the intermittent, myosin II-mediated deformation of the soft nucleus of the cancer cell, which is induced by the transient crowding of the stiff nuclei of the surrounding nontransformed cells, whose movements depend directly on the cadherin-mediated mismatched adhesion between normal and cancer cells as well as α-catenin-based intercellular adhesion of the normal cells. These results suggest that a mechanical and adhesive mismatch between transformed and nontransformed cells in a cell monolayer can trigger enhanced pulsating migration. These results shed light on the role of stiff epithelial cells that neighbor individual cancer cells in early steps of cancer dissemination.