Predominant role of alpha 4-integrins for distinct steps of lymphoma metastasis

Regulation of cell migration by α4 and α9 integrins

Integrins are heterodimeric transmembrane receptors that play an essential role in enabling cells to sense and bind to extracellular ligands. Activation and clustering of integrins leads to the formation of focal adhesions at the plasma membrane that subsequently initiate signalling pathways to control a broad range of functional endpoints including cell migration, proliferation and survival. The α4 and α9 integrins form a small sub-family of receptors that share some specific ligands and binding partners. Although relatively poorly studied compared with other integrin family members, emerging evidence suggests that despite restricted cell and tissue expression profiles, these integrins play a key role in the regulation of signalling pathways controlling cytoskeletal remodelling and migration in both adherent and non-adherent cell types. This review summarises the known shared and specific roles for α4 and α9 integrins and highlights the importance of these receptors in controlling cell migration within both homeostatic and disease settings.

Identification of critical genes and gene interaction networks that mediate osteosarcoma metastasis to the lungs

Osteosarcoma (OS) is the most commonly diagnosed bone tumor in young adults under the age of 20. Metastasis is considered an important factor underlying cancer-associated morbidity and mortality, and, as a result, the survival rate of patients with metastatic OS is low. In spite of this, the mechanisms underlying metastasis in OS are currently not well understood. The present study compared gene expression levels between five non-metastatic and four metastatic OS tumor samples, using an Affymetrix microarray. A total of 282 genes were differentially expressed in the metastatic samples, as compared with the non-metastatic samples. Of these differentially expressed genes (DEGs), 212 were upregulated and 70 were downregulated. The following DEGs were associated with metastasis: Homeobox only protein; lysosomal-associated membrane protein-3; chemokine (C-C motif) ligand-18; carcinoembryonic antigen-related cell adhesion molecule-6; keratin-19; prostaglandin-endoperoxide synthase-2; clusterin; and nucleoside diphosphate kinase-1. Subsequently, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment pathway analyses were conducted, which identified 529 biological processes (P<0.01) and 10 KEGG pathways (P<0.05) that were significantly over-represented in the metastatic samples, as compared with the non-metastatic samples. Interaction networks for the DEGs were constructed using the corresponding GO terms and KEGG pathways, and these identified numerous genes that may contribute to OS metastasis. Among the enriched biological processes, four DEGs were consistently over-represented: Jun proto-oncogene, caveolin-1, nuclear factor-κB-inhibitor-α and integrin alpha-4; thus suggesting that they may have key roles in OS metastasis, and may be considered potential therapeutic targets in the treatment of patients with OS.

Aberrant DNA methylation of integrin α4 in human breast cancer

Integrins are cell surface receptors that mediate cell-cell/extracellular interactions and have shown an association with metastasis or transformation in several cancers. However, the correlation between the clinicopathological status of breast cancer and the altered integrin α4 status is not clear. In this study, we investigated DNA methylation of integrin α4 in breast cancer. We retrieved 351 cases of surgically resected breast cancer (290 invasive carcinoma and 61 intraductal carcinoma). Methylation-specific polymerase chain reaction was performed to determine integrin α4 methylation status. Integrin α4 methylation was frequently observed in breast cancer specimens (145/351 cases, 41.3 %). In addition, DNA methylation of integrin α4 showed statistical correlation with HER2 positivity and higher histologic grade (p = 0.001, 0.008 in ductal carcinoma in situ and 0.003 in invasive ductal carcinoma). However, other clinicopathological data such as estrogen receptor, progesterone receptor, metastasis, and TNM status showed no statistical correlation. Moreover, we found that the downregulated expression of integrin α4 in a heavily methylated breast cancer cell line (ZR-75) was restored by treatment with 5-aza-2'deoxycytidine, a DNA methyltransferase inhibitor, implying transcriptional silencing by DNA methylation. We observed that integrin α4 methylation is associated with the histologic grade of tumors and lymph node metastasis. Also, this data supports a previous study that suggested integrin α4 and HER2 are involved in the same signaling pathway. DNA methylation of integrin α4 may be a poor prognostic factor which affects undifferentiated histologic change of breast cancer.

Engagement of αIIbβ3 (GPIIb/IIIa) with ανβ3 integrin mediates interaction of melanoma cells with platelets: a connection to hematogenous metastasis

A mutual relationship exists between metastasizing tumor cells and components of the coagulation cascade. The exact mechanisms as to how platelets influence blood-borne metastasis, however, remain poorly understood. Here, we used murine B16 melanoma cells to observe functional aspects of how platelets contribute to the process of hematogenous metastasis. We found that platelets interfere with a distinct step of the metastasis cascade, as they promote adhesion of melanoma cells to the endothelium in vitro under shear conditions. Constitutively active platelet receptor GPIIb/IIIa (integrin αIIbβ3) expressed on Chinese hamster ovary cells promoted melanoma cell adhesion in the presence of fibrinogen, whereas blocking antibodies to aνβ3 integrin on melanoma cells or to GPIIb/IIIa significantly reduced melanoma cell adhesion to platelets. Furthermore, using intravital microscopy, we observed functional platelet-melanoma cell interactions, as platelet depletion resulted in significantly reduced melanoma cell adhesion to the injured vascular wall in vivo. Using a mouse model of hematogenous metastasis to the lung, we observed decreased metastasis of B16 melanoma cells to the lung by treatment with a mAb blocking the aν subunit of aνβ3 integrin. This effect was significantly reduced when platelets were depleted in vivo. Thus, the engagement of GPIIb/IIIa with aνβ3 integrin interaction mediates tumor cell-platelet interactions and highlights how this interaction is involved in hematogenous tumor metastasis.

An integrin-alpha4-14-3-3zeta-paxillin ternary complex mediates localised Cdc42 activity and accelerates cell migration

Alpha4 integrins are used by leukocytes and neural crest derivatives for adhesion and migration during embryogenesis, immune responses and tumour invasion. The pro-migratory activity of alpha4 integrin is mediated in part through the direct binding of the cytoplasmic domain to paxillin. Here, using intermolecular FRET and biochemical analyses, we report a novel interaction of the alpha4 integrin cytoplasmic domain with 14-3-3zeta. This interaction depends on serine phosphorylation of alpha4 integrin at a site (S978) distinct from that which regulates paxillin binding (S988). Using a combination of metabolic labelling and targeted mass spectrometry by multiple reaction monitoring we demonstrate the low stoichiometry phosphorylation of S978. The interaction between alpha4 integrin and 14-3-3zeta is enhanced by the direct association between 14-3-3zeta and paxillin, resulting in the formation of a ternary complex that stabilises the recruitment of each component. Although pair-wise interaction between alpha4 integrin and paxillin is sufficient for normal Rac1 regulation, the integrity of the ternary complex is essential for focused Cdc42 activity at the lamellipodial leading edge and directed cell movement. Taken together, these data identify a key signalling nexus mediating alpha4 integrin-dependent migration.

Immunoscreening of a cutaneous T-cell lymphoma library for plasma membrane proteins

Cutaneous T-cell lymphomas (CTCL) belong to non-Hodgkin lymphomas, which are primarily manifested in the skin and mostly exhibit a T-helper memory phenotype. Mycosis fungoides (MF) and the leukemic variant Sézary syndrome (SzS) are the most common forms of CTCL. The aim of this study was to identify CTCL surface proteins with a tumor specific expression profile. A plasma membrane enriched fraction of the CTCL cell line HuT78 was used for immunization of two rabbits. Subsequently, a CTCL cDNA phage library was screened by a new variant of the SEREX method (serological identification of antigens by recombinant expression cloning) using the polyspecific rabbit antisera instead of patients' sera. Isolated reactive transfectants were sequenced and 42 different genes identified including four known plasma membrane proteins: Ligatin, HLA-A, integrin alpha4 and MT5-MMP. The level of transcripts of the matrix metalloproteinase MT5-MMP was diminished in MF tumor specimens. MT5-MMP normally occurs in several different protein variants. Western blot analysis revealed that activated MT5-MMPs were reduced in tumor specimens, whereas the amounts of most of the inactivated variants were unchanged. The amount of mRNA coding for the adhesion protein integrin alpha4 was not altered in tumor specimens in comparison to controls when analyzed by quantitative real-time PCR analysis. Ku86, known to be predominantly located in the nucleus and cytosol, was frequently detected during the SEREX screening. Western blot analysis revealed higher protein amounts of Ku86 in HuT78 than in control cells. In addition, we could show, that Ku86 can also be detected in lipid rafts of CTCL cells as it has been described for other tumor types. Thus, Ku86 might be involved in homo- and heterotypic adhesion steps of CTCL tumor cells and might protect these cells against apoptosis triggered by irradiation as it was suggested for multiple myeloma cells. The design of this study enabled screening for all proteins on the plasma membrane, irrespectively of whether these are directly anchored within the membrane or associated with other membrane proteins. Further analysis will unravel whether the list of identified proteins harbors candidates, which might be accessible for antibodies from outside the cell.

VCAM-1 inhibits TGFβ stimulated epithelial–mesenchymal transformation by modulating Rho activity and stabilizing intercellular adhesion in epicardial mesothelial cells

Regulation of epithelial-mesenchymal transformation (EMT) is of central importance both in normal development and in disease. During heart development, cells of the superficial epicardial mesothelium undergo EMT to give rise to precursor cells of the coronary vasculature and cardiac fibroblasts. Here we report that the alpha(4)beta(1) integrin ligand, VCAM-1, inhibits EMT of chick epicardial mesothelial cells stimulated by TGFbeta isoforms. We further investigated the molecular basis of this inhibition using cultured chick embryonic and rat adult epicardial mesothelial cells. We observed that VCAM-1 increased cortical actin filaments at intercellular junctions and reduced stress fibers across epicardial cells. VCAM-1 inhibited stress fiber formation by TGFbeta1, TGFbeta2, TGFbeta3 and lysophosphatidic acid and altered Rho activity stimulated by TGFbeta3. This was accompanied by an increase in tyrosine phosphorylation of p190RhoGAP. All three TGFbeta isoforms weakened intercellular adhesion, reduced membrane localization of beta-catenin and E-cadherin and stimulated epicardial EMT in chick hearts. Each of these effects was restricted by simultaneous VCAM-1 treatment. Our data support the hypothesis that VCAM-1 can alter epicardial EMT at two key points: it limits Rho-dependent events such as stress fiber formation and it maintains the association of beta-catenin and E-cadherin with the adherens junction.

Constitutive integrin activation on tumor cells contributes to progression of leptomeningeal metastases

Leptomeningeal metastases are a serious neurological complication in cancer patients and associated with a dismal prognosis. Tumor cells that enter the subarachnoid space adhere to the leptomeninges and form tumor deposits. It is largely unknown which adhesion molecules mediate tumor cell adhesion to leptomeninges. We studied the role of integrin expression and activation in the progression of leptomeningeal metastases. For this study, we used a mouse acute lymphocytic leukemic cell line that was grown in suspension (L1210-S cell line) to develop an adherent L1210 cell line (L1210-A) by selectively culturing the few adherent cells in the cell culture. beta1, beta2, and beta3 integrins were in a constitutively high active state on L1210-A cells and in a low, but inducible, active state on L1210-S cells. Expression levels of these integrins were comparable in the two cell lines. Static adhesion levels of L1210-A cells on a leptomeningeal cell layer were significantly higher than those of L1210-S cells. All mice that were injected intrathecally with L1210-A cells died rapidly of leptomeningeal leukemia. In contrast, 45% long-term survival was seen after intrathecal injection of mice with L1210-S cells. Our data indicate that constitutive integrin activation on leukemic cells promotes progression of leptomeningeal leukemia by increased tumor cell adhesion to the leptomeninges. We argue that an aberrantly regulated inside-out signaling pathway underlies constitutive integrin activation on the adherent leukemic cell population.

Molecular resolution of cell adhesion forces

Recently, AFM-based force spectroscopy has been used to quantify single-molecule adhesion forces on living ameboid cells. Force spectroscopy was used to measure the rupture forces of single receptor-ligand bonds which can occur rapidly between the cell types used, a metastasising B16 melanoma cell and a vascular bEnd.3 endothelial cell. Parameters which influence the critical experimental conditions are discussed to discriminate between multiple bond ruptures and single bonds. Under physiological conditions of temperature and pH the force measurements show an average rupture force of 33 pN (SD = 12 pN) for single bonds. Single-molecule force spectroscopy will be very useful to study the regulation of cell adhesion on a molecular level in normal processes, such as leukocyte homing, and in major human disorders, including tumor metastasis, autoimmune diseases and atherosclerosis.

Anti-idiotype x anti-CD44 bispecific antibodies inhibit invasion of lymphoid organs by B cell lymphoma

The demonstration that Abs to adhesion molecules can block tumor metastasis suggested their use for therapy. However, such Abs affect nonmalignant cells as well. To circumvent this adverse effect, we proposed the use of bispecific Abs that bind simultaneously to an adhesion receptor and to a tumor-specific Ag. Such bifunctional Abs bind more avidly to tumor cells that coexpress both target Ags than to normal cells. The Id of the surface Ig of malignant B lymphocytes is a tumor-specific Ag. Therefore, we produced bispecific Abs with specificity to the adhesion molecule, CD44, and to an idiotypic determinant of the murine B cell lymphoma, 38C-13. These anti-Id x anti-CD44 bispecific Abs blocked 38C-13 cell adhesion to hyaluronic acid, while not affecting adhesion of Id-negative cells. In vivo studies demonstrated that the bispecific Abs inhibited lymphoma cell dissemination to the lymph nodes, bone marrow, and spleen, and prolonged survival of tumor-bearing mice. Migration of 38C-13 cells to the lymphoid organs was inhibited by the bispecific Abs. Thus, the bispecific Ab-mediated reduction in metastasis resulted, at least in part, from reduced homing to these organs. In contrast to anti-CD44 monospecific Abs, the anti-Id x anti-CD44 bispecific Abs did not affect immune responses such as delayed-type hypersensitivity. Hence, bispecific Abs against adhesion molecules and tumor-specific Ags may selectively block tumor metastasis in a way which may leave at least part of the immune system intact.

Aberrant methylation of integrin alpha4 gene in human gastric cancer cells

Integrins are adhesion receptors that mediate both cell-extracellular matrix and cell-cell interactions. It has also been reported that the loss of integrin alpha4 expression might be associated with metastasis in several cancers. However, the molecular mechanism for loss of their expression in cancers has not been explored. In the present study, we found that the integrin alpha4 expression is lost in human gastric cancer cell lines and that this is recovered by treatment with DNA methyltransferase inhibitor, implying transcriptional silencing by DNA methylation. Methylation-specific PCR (MSP) and bisulfite genomic DNA sequencing demonstrated the CpG methylation-dependent silencing of integrin alpha4 expression in eight of nine (88.8%) gastric cancer cell lines and in 84.7% of 46 primary tumors. We also investigated whether the restoration of integrin alpha4 in integrin alpha4-inactivated cells affects their ability to invade extracellular matrix, using matrigel assays. Interestingly, integrin alpha4-stable transfectants had markedly less invasive ability than the parental cells. Taken together, these results suggest that the transcriptional repression of the integrin alpha4 gene is caused by aberrant DNA methylation, and that this may play an important role in human gastric carcinogenesis.

Inhibition of alpha4-integrin stimulates epicardial-mesenchymal transformation and alters migration and cell fate of epicardially derived mesenchyme

Epithelial-mesenchymal transformation of the embryonic epicardium produces the subepicardial mesenchyme that is essential for normal coronary vascular development. Gene targeting experiments in mice have demonstrated an essential role for alpha4-integrin in normal epicardial development, but the precise cellular consequences of alpha4-integrin loss remain uncertain. To better understand the function of alpha4-integrin in epicardial development, we constructed a replication-incompetent adenovirus (AdlacZalpha4AS) that expresses antisense chicken alpha4-integrin as the 3' untranslated region of a lacZ reporter gene. This construct effectively labeled cells while greatly reducing levels of alpha4-integrin mRNA and protein. In quail chick chimeras, transplanted epicardial cells infected with AdlacZalpha4AS adhered to the heart and were incorporated into the epicardium, but 4 days after grafting, were largely absent from the epicardial epithelium, recapitulating the defect in alpha4-null mice. This did not result from epicardial cell apoptosis or anomalous migration of epicardial cells to extracardiac sites. Rather, AdlacZalpha4AS-infected epicardial cells were particularly invasive, being three to four times more likely to migrate to the interstitium of the myocardium than AdlacZ-infected epicardial cells. Accelerated epicardial-mesenchymal transformation and migration of alpha4-negative epicardium was observed in an organ culture system that does not require prior culture of epicardial cells. Remarkably, AdlacZalpha4AS infection also prevented targeting of epicardially derived mesenchyme to the media of developing coronary vasculature in the myocardial interstitium. This study provides evidence that epicardial alpha4-integrin normally restrains epicardial-mesenchymal transformation, invasion, and migration and is essential for correct targeting of epicardially derived mesenchyme to the developing coronary vasculature.

Anti-idiotype x anti-LFA-1 bispecific antibodies inhibit metastasis of B cell lymphoma

Abs to adhesion molecules can block tumor metastasis. However, they may also block the function of normal cells. To circumvent this adverse effect, we proposed the use of bispecific Abs that bind simultaneously to an adhesion receptor and to a tumor-specific Ag. Such Abs bind more avidly to tumor cells that coexpress both target Ags than to normal cells. The Id of the surface Ig of malignant B lymphocytes is a tumor-specific Ag. We therefore produced a bispecific Ab with specificity to the adhesion molecule LFA-1 and to the Id of the murine B cell lymphoma 38C-13. Here we demonstrate that this Ab blocked liver metastasis in mice carrying primary s.c. tumors and partially inhibited lymph node metastasis. Migration of 38C-13 cells to liver and lymph nodes was inhibited by the bispecific Ab, while migration to spleen was not affected. Hence, the bispecific Ab-mediated reduction in liver and lymph node metastasis resulted at least in part from reduced homing to these organs. In contrast to anti-LFA-1 monospecific Abs, the anti-Id x anti-LFA-1 bispecific Ab did not affect immune responses such as delayed-type hypersensitivity. Hence, bispecific Abs against adhesion molecules and against tumor-specific Ags may selectively block tumor metastasis in a way that may leave much of the immune system intact.

L1 adhesion molecule (CD 171) in development and progression of human malignant melanoma

The L1 adhesion molecule (CD171) plays an important role in axon guidance and cell migration in the nervous system. In the human, L1 is expressed on tumors derived from neurocrest and on certain carcinomas. We have analyzed immunohistochemically L1 expression on paraffin embedded specimens of acquired melanocytic nevi, primary cutaneous melanomas, and cutaneous and lymph node metastases of malignant melanomas. We found an increase in L1 immunoreactivity in malignant melanomas and metastases of malignant melanomas as compared to acquired melanocytic nevi that was statistically significant (P<0.05). Additionally, a correlation of L1 immunoreactivity with histological data of prognostic value such as Clark level and the expression of alphav-integrins was found. We detected soluble L1 in the conditioned medium of cultivated melanoma cells but only in 1/40 serum samples from a panel of melanoma patients representing various stages of disease. Our findings suggest that the presence of L1 might contribute to tumor progression by promoting cell adhesion and migration.

Inhibition of LFA-1/ICAM-1 and VLA-4/VCAM-1 as a therapeutic approach to inflammation and autoimmune diseases

This review focuses on providing insights into the structural basis and clinical relevance of LFA-1 and VLA-4 inhibition by peptides and small molecules as adhesion-based therapeutic strategies for inflammation and autoimmune diseases. Interactions of cell adhesion molecules (CAM) play central roles in mediating immune and inflammatory responses. Leukocyte function-associated antigen (LFA-1, alpha(L)beta(2), and CD11a/CD18) and very late antigen (VLA-4, alpha(4)beta(1), and CD49d/CD29) are members of integrin-type CAM that are predominantly involved in leukocyte trafficking and extravasation. LFA-1 is exclusively expressed on leukocytes and interacts with its ligands ICAM-1, -2, and -3 to promote a variety of homotypic and heterotypic cell adhesion events required for normal and pathologic functions of the immune systems. VLA-4 is expressed mainly on lymphocyte, monocytes, and eosinophils, but is not found on neutrophils. VLA-4 interacts with its ligands VCAM-1 and fibronectin (FN) CS1 during chronic inflammatory diseases, such as rheumatoid arthritis, asthma, psoriasis, transplant-rejection, and allergy. Blockade of LFA-1 and VLA-4 interactions with their ligands is a potential target for immunosuppression. LFA-1 and VLA-4 antagonists (antibodies, peptides, and small molecules) are being developed for controlling inflammation and autoimmune diseases. The therapeutic intervention of mostly mAb-based has been extensively studied. However, due to the challenging relative efficacy/safety ratio of mAb-based therapy application, especially in terms of systemic administration and immunogenic potential, strategic alternatives in the forms of peptide, peptide mimetic inhibitors, and small molecule non-peptide antagonists are being sought. Linear and cyclic peptides derived from the sequences of LFA-1, ICAM-1, ICAM-2, VCAM-1, and FN C1 have been shown to have inhibitory effects in vitro and in vivo. Finally, understanding the mechanism of LFA-1 and VLA-4 binding to their ligands has become a fundamental basis in developing therapeutic agents for inflammation and autoimmune diseases.

Differentiation-dependent and subset-specific recruitment of T-helper cells into murine liver

It has been suggested that the liver traps and deletes activated and potentially harmful T cells, especially of the CD8(+) subset, providing mechanisms to limit systemic immune responses. It is unknown whether this also applies to CD4(+) T cells. In this study, we show that activated stages of CD4(+) T cells were trapped in the liver on intraportal injection. Intravital microscopy showed an immediate adhesion of activated CD4(+) T cells within periportal sinusoids after intraportal injection. Furthermore, we detected high frequencies of interferon gamma (IFN-gamma)-- (Th1) and interleukin 4 (IL-4)-- (Th2) synthesizing effector cells in the liver. Transfer experiments were performed to identify those phenotypes showing specific retention in the liver. Our data show that effector stages and activated cells in general are more efficiently recruited into the liver than resting CD4(+) T cells, similar to what has previously been shown for CD45RB(low) memory cells. In addition, we observed a certain preference for Th1-polarized cells to be trapped by the liver. However, the actual cytokine-producing cells did not specifically enrich among the total population. In conclusion, these data indicate that the liver acts as a filter for activated and memory/effector cells. Cells trapped in the liver might subsequently undergo modulatory influences exerted by the postulated specific microenvironment of the liver.

Integrin receptors are required for cell survival and proliferation during development of the peripheral glial lineage

Proliferation and survival of Schwann cells are important for nerve development and for disease processes in peripheral nerves. We have analyzed embryos lacking alpha4- or alpha5-integrins and show here that these integrins contribute to the control of glial cell numbers. To overcome early embryonic lethality an explant and grafting system that allows the study of isolated glial progenitor cells both in vitro and in vivo was used. Schwann cells differentiate in the absence of alpha5 but their numbers and the proliferation rate of early progenitor cells are reduced, suggesting that alpha5 is essential for normal proliferation. Survival, rather than proliferation, is compromised in alpha4-deficient explants. Conditional immortalization allowed further characterization and revealed that alpha4 contributes to survival in a cell-density-dependent fashion. In addition, transplants into chicken embryos were used to analyze in vivo cell migration and showed that cell death occurs mainly in highly motile, individually migrating cells. The cell death patterns in vitro and in vivo argue that alpha4-integrins play a role in survival during cell migration. Neural crest migration has been suggested to require these integrins; however, no defects in migration were observed in the absence of alpha4 or alpha5. We conclude that integrins can complement growth factors in the control of glial cell numbers.

Functional cloning of ARM-1, an adhesion-regulating molecule upregulated in metastatic tumor cells

Interactions of tumor cells with the endothelium and tissue stroma are considered to be critical steps in metastasis formation and progression of cancer. To identify cellular receptors that mediate the binding of tumor cells to endothelium, a murine T cell lymphoma-derived expression library was screened for adhesion-inducing cDNA clones. We identified a novel cell adhesion-promoting molecule, termed ARM-1 (adhesion regulating molecule-1), which is homologous to a human Mr 110,000 tumor-associated antigen. The ARM-1 cDNA codes for a type I transmembrane protein of 407 amino acids with potential O- and N-glycosylation sites that does not belong to any of the known families of cell adhesion molecules. Overexpression of ARM-1 in 293T human embryonic kidney cells significantly increased adhesion to different endothelial cells. ARM-1 expression in 293T cells did not alter integrin expression or beta1-integrin-mediated cell adhesion. Northern blot analysis of human breast cancer cell lines revealed 3- to 5-fold elevated ARM-1 mRNA levels in metastatic as compared to non-metastatic cells. In conclusion, we have identified ARM-1 as a novel cell adhesion-promoting receptor that is upregulated in metastatic cancer cells.

Bidirectional induction of the cognate receptor-ligand α4/VCAM-1 pair defines a novel mechanism of tumor intravasation

Engagement of cell surface adhesion receptors with extracellular constituents and with cellular counter-receptors is crucial for the extravasation of blood-borne neoplastic cells and their seeding at distant sites; however, the early events of tumor dissemination—ie, the intravasation step(s)—have been largely neglected. A role for the 4β7 integrin was hypothesized to explain the high leukemogenicity exhibited by one (NQ22) among several T-cell lymphomas studied. To clarify the mechanisms of early aggressivity, the behavior of highly and poorly leukemogenic cell lines were compared in vitro. Cocultivation of physically separated leukemic cells with resting endothelial cells resulted in the up-regulation of VCAM-1 expression. NQ22 cells expressed mRNA of different cytokines that up-regulate VCAM-1 and at higher levels than cells of a nonaggressive lymphoma, and they migrated more efficiently through an activated endothelial cell layer. With the use of neutralizing antibodies against interferon-γ, granulocyte macrophage colony-stimulating factor, and tumor necrosis factor (TNF)-, it was determined that TNF- is one of the soluble factors released by NQ22 cells involved in the up-regulation of VCAM-1. The finding that vascular cells within the early local growth were strongly positive for VCAM-1 indicated that NQ22 cells could activate endothelial cells also in vivo. Finally, cocultivation of preleukemic 4−NQ22 cells with TNF--activated endothelial cells induced the expression of 4 integrins on the former cells. Reciprocal up-regulation and engagement of 4/VCAM-1 pairs determined the sequential transmigration and intravasation steps, and similar mechanisms might affect the aggressivity of human T lymphoblastic lymphomas.

Bidirectional induction of the cognate receptor-ligand α4/VCAM-1 pair defines a novel mechanism of tumor intravasation

Engagement of cell surface adhesion receptors with extracellular constituents and with cellular counter-receptors is crucial for the extravasation of blood-borne neoplastic cells and their seeding at distant sites; however, the early events of tumor dissemination—ie, the intravasation step(s)—have been largely neglected. A role for the 4β7 integrin was hypothesized to explain the high leukemogenicity exhibited by one (NQ22) among several T-cell lymphomas studied. To clarify the mechanisms of early aggressivity, the behavior of highly and poorly leukemogenic cell lines were compared in vitro. Cocultivation of physically separated leukemic cells with resting endothelial cells resulted in the up-regulation of VCAM-1 expression. NQ22 cells expressed mRNA of different cytokines that up-regulate VCAM-1 and at higher levels than cells of a nonaggressive lymphoma, and they migrated more efficiently through an activated endothelial cell layer. With the use of neutralizing antibodies against interferon-γ, granulocyte macrophage colony-stimulating factor, and tumor necrosis factor (TNF)-, it was determined that TNF- is one of the soluble factors released by NQ22 cells involved in the up-regulation of VCAM-1. The finding that vascular cells within the early local growth were strongly positive for VCAM-1 indicated that NQ22 cells could activate endothelial cells also in vivo. Finally, cocultivation of preleukemic 4−NQ22 cells with TNF--activated endothelial cells induced the expression of 4 integrins on the former cells. Reciprocal up-regulation and engagement of 4/VCAM-1 pairs determined the sequential transmigration and intravasation steps, and similar mechanisms might affect the aggressivity of human T lymphoblastic lymphomas.

Resistance of ICAM-1–deficient mice to metastasis overcome by increased aggressiveness of lymphoma cells

Our recent finding that resistance to lymphoma cell metastasis in intercellular adhesion molecule-1-(ICAM-1)–deficient mice was manifested after homing suggested that the mechanism could involve the capacity of ICAM-1 to induce, via leukocyte function-associated antigen-1 (LFA-1) signaling, the expression of new genes necessary for migration and survival of lymphoma cells after homing. This hypothesis would imply that lymphoma cells, on repeated metastatic cycles, would acquire such a highly aggressive phenotype that they no longer require contact with ICAM-1 at later stages of metastasis. We addressed this question by generating highly aggressive lymphoma variants to determine if increased tumorigenicity would allow lymphoma cells to grow into tumors in ICAM-1–deficient mice. We found that on repeated in vivo passages, a selective pressure favored the lymphoma cells that constitutively express high levels of matrix metalloproteainse-9 (MMP-9), a gene associated with a poor clinical outcome in non-Hodgkins's lymphoma. We further found that although the parent lymphoma cells could not grow tumors in ICAM-1–deficient mice, the aggressive lymphoma variants could. This indicates that, at late stages of the disease, tumor cells with a high metastatic efficiency, encoded by the repertoire of selected genes, no longer require some of the signals normally delivered by cell adhesion molecules. In light of these findings, the possibility of inhibiting dissemination of lymphoma cells at the late stage of the disease by acting against cell adhesion molecules must be reconsidered. (Blood. 2000;95:314-319)

Resistance of ICAM-1–deficient mice to metastasis overcome by increased aggressiveness of lymphoma cells

Our recent finding that resistance to lymphoma cell metastasis in intercellular adhesion molecule-1-(ICAM-1)–deficient mice was manifested after homing suggested that the mechanism could involve the capacity of ICAM-1 to induce, via leukocyte function-associated antigen-1 (LFA-1) signaling, the expression of new genes necessary for migration and survival of lymphoma cells after homing. This hypothesis would imply that lymphoma cells, on repeated metastatic cycles, would acquire such a highly aggressive phenotype that they no longer require contact with ICAM-1 at later stages of metastasis. We addressed this question by generating highly aggressive lymphoma variants to determine if increased tumorigenicity would allow lymphoma cells to grow into tumors in ICAM-1–deficient mice. We found that on repeated in vivo passages, a selective pressure favored the lymphoma cells that constitutively express high levels of matrix metalloproteainse-9 (MMP-9), a gene associated with a poor clinical outcome in non-Hodgkins's lymphoma. We further found that although the parent lymphoma cells could not grow tumors in ICAM-1–deficient mice, the aggressive lymphoma variants could. This indicates that, at late stages of the disease, tumor cells with a high metastatic efficiency, encoded by the repertoire of selected genes, no longer require some of the signals normally delivered by cell adhesion molecules. In light of these findings, the possibility of inhibiting dissemination of lymphoma cells at the late stage of the disease by acting against cell adhesion molecules must be reconsidered. (Blood. 2000;95:314-319)

Dissemination of T cell lymphoma to target organs: a post-homing event implicating ICAM-1 and matrix metalloproteinases

There is increasing evidence that metastasis of a tumor cell (its ability to induce the "development of a tumor" at distant sites following intravasation) is manifested only after homing to distant site(s). All tumor cells, however, do not necessarily undergo uncontrolled cellular division to form secondary tumors once they have "homed" to a target site. One of the major rate-limiting steps in metastasis is in fact related to the ability of the extravasated tumor cells to find an appropriate "nest", where favorable growth conditions will allow them to form a secondary tumor upon massive cell division (1). But to establish such a favorable nest (referred herein as the "nidification" process), tumor cells must penetrate deep into the stroma of the target tissue. This process is facilitated when tumor cells produce of specific proteases, which degrade structural proteins of the extracellular matrix (2,3). The production of proteases by stromal cells can also occur; these enzymes will degrade stroma surrounding the tumor cells, resulting in a massive remodeling of the local parenchyma that may interfere with the vital functions of a target organ as well as help nidification (4). In this review, we focus our attention on post-extravasation events involving adhesion molecules and MMP in the metastatic process of lymphoma cells. We propose that during dissemination of LFA-1-positive lymphoma cells to peripheral organs, the interaction between lymphoma cells and vascular endothelial cells upregulates the local expression of MMP and TIMPs. Since control of lymphoma metastasis appears to occur at the post-extravasation level, we hypothesize that in addition to extravasation, adhesion molecules are implicated in the control of post-extravasation events.

High Affinity Very Late Antigen-4 Subsets Expressed on T Cells Are Mandatory for Spontaneous Adhesion Strengthening But Not for Rolling on VCAM-1 in Shear Flow

The very late Ag-4 (VLA-4) integrin supports both rolling and firm adhesion of leukocytes on VCAM-1 under shear flow. The molecular basis for the unique ability of a single adhesion molecule to mediate these versatile adhesive processes was investigated. VLA-4 occurs in multiple activation states, with different affinities to ligand. In this study we tested how these states regulate VLA-4 adhesiveness under shear flow in Jurkat T cells and PBL. VLA-4 on nonstimulated Jurkat cells supported rolling and spontaneous arrest on VCAM-1, whereas a Jurkat activation mutant with reduced VLA-4 affinity failed to spontaneously arrest after tethering to or during rolling on VCAM-1. The contribution of VLA-4 affinity for ligand to rolling and spontaneous arrests on immobilized VCAM-1 was dissected using soluble VLA-4 ligands, which selectively block high affinity states. VLA-4 saturation with ligand completely blocked spontaneous adhesion strengthening post-tethering to VCAM-1, but did not impair rolling on the endothelial ligand. High affinity VLA-4 was found to comprise a small subset of VLA-4 on resting Jurkat cells and PBL. This subset is essential for firm adhesion but not for tethering or rolling adhesions on VCAM-1. Interestingly, low and high affinity VLA-4 states were found to mediate similar initial tethering to ligand. High affinity VLA-4, constitutively expressed on circulating T cells, may control their early adhesion strengthening on VCAM-1-expressing endothelium before exposure to vascular chemokines and activation of additional integrins.

Sequence Motifs in the Integrin α4 Cytoplasmic Tail Required for Regulation of In Vivo Expansion of Murine Lymphoma Cells

The binding of integrins to cognate ligands is tightly controlled by intracellular signals. Conversely, integrin occupancy generates biochemical signals inside the cell. The present study examined whether concepts of integrin function established by in vitro analysis apply to regulation of receptor function in complex biologic settings in vivo using a mouse model of tumor metastasis. Integrin α4 subunits were truncated at amino acid Gln1014 (A4-1014), preserving the conserved GFFKR motif, and at position Glu1021 (A4-1021). In vitro adhesion assays revealed that cytoplasmic tail truncations did not affect constitutive ligand binding of α4 integrins, while agonist-induced adhesion was abolished by the A4-1014, but not by the A4-1021, mutation. Inducible ligand binding of α4 integrins was dependent on cytoskeletal function, whereas constitutive adhesion was not. In vivo metastasis formation assays demonstrated that expansion of murine T lymphoma cells in spleen is strongly inhibited by the wild-type α4 subunit and the A4-1021 mutant. In contrast, the in vivo phenotype of α4 integrin expression in lymphoma cells was completely abrogated by the A4-1014 mutation. Cross-linking of α4 integrins in vitro inhibited proliferation and induced apoptosis of LB cells expressing wild-type α4 subunits or the A4-1021 mutant, but not of LB-A4-1014 cells. In summary, these results demonstrate that sequence motifs regulating cytoskeleton-dependent α4 integrin activation in vitro are essential for the control of LB lymphoma cell expansion both in vitro and in vivo.

Protection from Lymphoma Cell Metastasis in ICAM-1 Mutant Mice: A Posthoming Event

It has been hypothesized that the intercellular adhesion receptors used by normal cells could also be operative in the spreading of circulating malignant cells to target organs. In the present work, we show that genetic ablation of the ICAM-1 gene confers resistance to T cell lymphoma metastasis. Following i.v. inoculation of LFA-1-expressing malignant T lymphoma cells, we found that ICAM-1-deficient mice were almost completely resistant to the development of lymphoid malignancy compared with wild-type control mice that developed lymphoid tumors in the kidneys, spleen, and liver. Histologic examinations confirmed that ICAM-1-deficient mice, in contrast to wild-type mice, had no evidence of lymphoid infiltration in these organs. The effect of ICAM-1 on T cell lymphoma metastasis was observed in two distinct strains of ICAM-1-deficient animals. Nonetheless, lymphoma cells migrated with the same efficiency to target organs in both normal and ICAM-1-deficient mice, indicating not only that ICAM-1 expression by the host is essential in lymphoma metastasis, but also that this is so at stages subsequent to homing and extravasation into target organs. These results point to posthoming events as a focus of future investigation on the control of metastasis mediated by ICAM-1.

Dissemination capacity of murine lymphoma cells is not dependent on efficient homing

The extravasation of normal lymphocytes from blood into tissues is controlled by adhesion molecules ("homing receptors") that mediate their interaction with endothelial cells. It is an intriguing question whether malignant cells use the same pathways for hematogenous dissemination and whether these molecules are involved in the organ-specific formation of metastasis. To analyze the migration behavior of lymphoma cells in vivo, we here used several lines and sublines which exhibit differential expression of the lymph node homing receptor L-selectin and the mucosa-specific integrin alpha4beta7. We demonstrate that the ability of the various types of cells tested to accumulate in lymph nodes within the first 24 hr after intravenous (i.v.) injection is negligible, independent of their homing receptor expression profile. Our data indicate that lymphoma cells have, in comparison with naive lymphocytes, an impaired capacity to extravasate via high endothelial venules (HEV). Instead they predominantly accumulate in lung and liver, similar to activated lymphocyte populations. Nevertheless, most of the lymphoma lines tested readily form lymph node metastases in vivo. In addition, blockade of L-selectin by continual treatment with an anti-L-selectin antibody did not prevent metastatic growth of TK-1 cells in peripheral lymph nodes. We conclude that the expression of homing receptors and a high extravasation efficiency of neoplastic cells is not a prerequisite for their dissemination into lymphatic tissue.

Enhancement of metastatic activity of colon cancer as influenced by expression of cell surface antigens

BACKGROUND

Cell surface antigens are contributory factors toward metastatic activity. There have been no detailed studies on changes in cell surface antigens of colon cancer cell lines. To control life-threatening metastasis, it is necessary to evaluate what types of changes in cell surface antigens exert an influence on metastatic activity.

MATERIALS AND METHODS

In vivo selection was performed using the human colon cancer-derived cell line KM12SM to obtain variants of metastatic activity. A murine spleen injection-liver metastasis procedure reflecting the latter half of the metastatic process was adopted and repeated four times. Flow cytometric analyses were carried out to detect expression of antigens: Lewis a (Lea), Lewis x (Lex), sialyl Lewis a (sLea), sialyl Lewis x (sLex), E-cadherin, CD44v6, integrin alpha2 (CD49b), integrin alpha3 (CD49c), integrin alpha4 (CD49d), integrin alpha5 (CD49e), and integrin beta1 (CD29).

RESULTS

In vivo selection produced variants with higher metastatic activity. In the original line KM12SM, sLea, E-cadherin, CD49b, CD49c, or CD29 were positive in more than 40% of the cells. After selection, the percentage of cells positive for Lea, sLea, and all examined integrins significantly increased. Lex, sLex, and CD44v6 increased slightly, while E-cadherin decreased slightly.

CONCLUSIONS

In vivo selection and flow cytometric analysis revealed that Lea, sLea, CD49b, CD49c, and CD29 appear to be involved in the increase of metastatic activity. The changes of integrin expression in this study suggest that integrins collaborate in the promotion of adhesion to an extracellular matrix.

Asp-698 and Asp-811 of the integrin alpha4-subunit are critical for the formation of a functional heterodimer

The amino acid motif LDV is the principal binding site for alpha4 integrins in fibronectin, and homologous motifs are recognized in vascular cell adhesion molecule-1 and MAdCAM-1. Three conserved LDV motifs (LDV-1 to 3) occur in the ectodomain of the human and mouse alpha4-subunit, the functions of which are unknown. We demonstrate here that alpha4-transfected fibroblasts with mutation in LDV-1 (D489N) behaved like alpha4-wild type but that LDV-2 (D698N) and LDV-3 (D811N) mutants were impaired in binding and spreading on alpha4-specific substrates. On the RGD-containing fibronectin fragment FN-120 there was an inverse behavior; now the alpha4-wild type and the LDV-1 mutant could not adhere whereas the two other mutants could. The beta1 chain was critical for the differential integrin response. Biochemical analysis demonstrated that the LDV-2 and -3 mutations reduced the strength of the alpha4beta1 association, favored the formation of alpha5beta1, and prevented the expression of alpha4beta7 on the cell surface. Our results indicate that LDV-2 and LDV-3 are critical for the formation of a functional heterodimer. The presence of similar amino acid motifs in ligands and the alpha4-subunit suggest that metal coordination plays an important role in integrin-ligand binding as well as for heterodimer formation.

alpha 4 integrins and tumor metastasis

Taken together, alpha 4 integrins may influence metastatic process at various stages (Fig. 1). The detachment of tumor cells from the primary tumor and the invasion of the surrounding tissue represent the onset of tumor metastasis. There is good experimental evidence that at the primary tumor site expression of alpha 4 integrins inhibits the ability of melanoma cells to break loose. This could be achieved either by strengthening of homotypic adhesion to adjacent tumor cells or by down regulation of matrix metalloproteases that are required for tumor cell migration through the extracellular matrix. After entering the blood circulation, alpha 4 integrins on tumor cells derived from melanomas, sarcomas or lymphomas rather promote than inhibit accumulation of disseminated cells in distant organs. The positive effects of alpha 4 integrins at this stage of metastasis formation appear to depend on alpha 4 integrin interactions with ligands expressed on the surface of endothelial cells. While VCAM-1 is expressed on endothelial cells exposed to inflammatory cytokines, MAdCAM-1 is constitutively expressed on mucosal endothelium. In addition, it is conceivable that tumor cell aggregates trapped in the microcirculation may trigger local inflammatory reactions that result in VCAM-1 up-regulation. Tumor cell-bound alpha 4 integrins may strengthen adhesion to endothelium and promote trans-endothelial migration (HAUZENBERGER et al. 1997; MEERSCHAERT and FURIE 1994). Successful formation of new tumor colonies in distant organs is the final step in the metastatic cascade. Interestingly, alpha 4 integrin dependent mechanisms may either promote or inhibit this process. Thus, it was observed that alpha 4 integrins may direct cancer cells like CHO and lymphoma cells to organ compartments, where ligands for alpha 4 integrins are expressed (e.g., bone marrow). Depending on the tumor type this event may result in enhanced metastasis formation. However, as was documented for murine lymphoma cells alpha 4 integrins may also inhibit tumor cell growth either by inducing apoptosis or by reducing the proliferation rate. Based on numerous studies on human cancers and experimental tumor models, alpha 4 integrins may represent attractive target molecules for therapeutic manipulation of tumor cell behavior. To this end, however, it will be of great importance to precisely define the molecular basis for the adverse effects of alpha 4 integrins on metastasis formation.

The Metastatic Characteristics of Murine Lymphoma Cell Lines In Vivo Are Manifested After Target Organ Invasion

The ability of a tumor cell to survive is critical for successful dissemination to sites distant from the primary tumor. Tumor cells must enter blood circulation, resist hemodynamic shear stress of the blood circulation, successfully extravasate, and then migrate through dense tissue stroma to a site favorable for tumor growth. Some tumor cells must therefore be endowed with peculiar abilities to successfully metastasize, whereas others, although capable of forming tumor in specific organs, cannot metastasize. This property has often been associated with the homing ability of a given tumor cell, likely through the expression of organ-specific homing receptors that are critical for the extravasation process. The present work was aimed at establishing the point at which metastatic and nonmetastatic lymphoma cells diverge. Although 164T2 and 267T2 lymphoma cell lines can successfully form thymic lymphoma when injected intrathymically, only the 164T2 clone can efficiently form tumor in kidneys, spleen, and liver after intravenous inoculation. Using the Indium-labeling technique to monitor the homing kinetic of both cell lines, we showed that the critical step for the successful metastasis of the lymphoma cell was determined in the final steps of the disseminating process, namely after homing. These results indicate that, whereas binding of tumor cells to vascular endothelium through specific adhesion mechanisms is a prerequisite for dissemination of tumor cells, the resistance of a tumor cell to the antagonist action of the host and/or its ability to grow tumor occurs only after homing to the target organ.

The Metastatic Characteristics of Murine Lymphoma Cell Lines In Vivo Are Manifested After Target Organ Invasion

The ability of a tumor cell to survive is critical for successful dissemination to sites distant from the primary tumor. Tumor cells must enter blood circulation, resist hemodynamic shear stress of the blood circulation, successfully extravasate, and then migrate through dense tissue stroma to a site favorable for tumor growth. Some tumor cells must therefore be endowed with peculiar abilities to successfully metastasize, whereas others, although capable of forming tumor in specific organs, cannot metastasize. This property has often been associated with the homing ability of a given tumor cell, likely through the expression of organ-specific homing receptors that are critical for the extravasation process. The present work was aimed at establishing the point at which metastatic and nonmetastatic lymphoma cells diverge. Although 164T2 and 267T2 lymphoma cell lines can successfully form thymic lymphoma when injected intrathymically, only the 164T2 clone can efficiently form tumor in kidneys, spleen, and liver after intravenous inoculation. Using the Indium-labeling technique to monitor the homing kinetic of both cell lines, we showed that the critical step for the successful metastasis of the lymphoma cell was determined in the final steps of the disseminating process, namely after homing. These results indicate that, whereas binding of tumor cells to vascular endothelium through specific adhesion mechanisms is a prerequisite for dissemination of tumor cells, the resistance of a tumor cell to the antagonist action of the host and/or its ability to grow tumor occurs only after homing to the target organ.