Integrins: versatility, modulation, and signaling in cell adhesion

Multiple functional forms of the integrin VLA-2 can be derived from a single alpha 2 cDNA clone: interconversion of forms induced by an anti-beta 1 antibody

The integrin VLA-2 was previously found to bind to either collagen alone, or collagen plus laminin, but the mechanism for this cell-specific functional difference was unknown. Here we transfected VLA-2 alpha 2 subunit cDNA into K562 cells and obtained VLA-2 (called Form-O) which bound to neither collagen nor laminin. We then used a Matrigel selection procedure to enrich for a minor subpopulation of K562 cells stably expressing a form of VLA-2 (Form-C) that bound to collagen but not laminin. In contrast, the same alpha 2 cDNA transfected into RD cells yielded VLA-2 (Form-CL) which bound to both collagen and laminin. These Form-O, -C, and -CL activities were stably expressed during extended cell culture, and could not be qualitatively altered by adding phorbol esters or by exchaning the resident divalent cations. However, addition of stimulatory anti-beta 1 antibodies (TS2/16, A-1A5) rapidly converted VLA-2 Form-O and Form-C into Form-CL. Anti-beta 1 antibody stimulation of VLA-2 activity was observed not only on whole cells, but also with solubilized receptors. These results suggest (a) that the ligand binding specificity of VLA-2 can be determined by its cellular environment, rather than by variations in the primary sequence of the alpha 2 subunit, (b) that stably inactive or partly active VLA-2 can be rapidly converted to a fully active form through conformational changes initiated at a nonligand binding site on the beta 1 subunit, and (c) that the mechanisms for VLA-2 stimulation by phorbol ester and by antibody are quite distinct, because the latter does not require an intact cell.

Smooth muscle cells and the pathogenesis of the lesions of atherosclerosis

In this review we have tried to identify the characteristic features of SMCs in developing lesions of atherosclerosis and the extracellular factors that may be involved in regulating these altered features. Though the list seems long and complex there is probably a great deal of interplay among the different regulatory mechanisms. The function and activities of SMCs in the artery are dependent on the milieu created by the surrounding cells and the components of the extracellular matrix. In the normal, uninjured media of the artery, SMC phenotype and function seem to be in large part determined by the extracellular matrix in which they are embedded and by diffusible factors, in particular from endothelial cells. Endothelial cell injury, infiltration of monocytes and lymphocytes, and ultimately, thrombosis and platelet release, as seen in developing lesions of atherosclerosis, dramatically alter the balance of growth-regulatory and vasoactive factors present in the local environment. These extracellular factors (table and figure) can alter both SMC phenotype, and thus responsiveness, and SMC migration, proliferation, and synthesis of the extracellular matrix. A better understanding of how specific factors mediate these responses, should make it possible to determine the ways in which the SMC response can be modulated. Though growth regulatory molecules seem to be key to this process, the challenge for the future is to understand their regulation in the environment of the artery wall and the interplay between growth-regulatory molecules, extracellular matrix, and vasoactive agents.

Exploitation of host signal transduction pathways and cytoskeletal functions by invasive bacteria

Many bacteria that cause disease have the capacity to enter into and live within eukaryotic cells such as epithelial cells and macrophages. The mechanisms used by these organisms to achieve and maintain this intracellular lifestyle vary considerably, but most mechanisms involve subversion and exploitation of host cell functions. Entry into non-phagocytic cells involves triggering host signal transduction mechanisms to induce rearrangement of the host cytoskeleton, thereby facilitating bacterial uptake. Once inside the host cell, intracellular pathogens either remain within membrane bound inclusions or escape to the cytoplasm. Those living in the cytoplasm can further pirate the host actin system, using actin as a mechanism to facilitate movement within and between host cells. Organisms remaining within the vacuole have specialized mechanisms for intracellular survival and growth which involve additional communication with the host cell. Some of the processes involved in the various steps of facultative intracellular parasitism are discussed in the context of subverting the host cell cytoskeleton and signal transduction pathways for bacterial benefit.

Compartmentalization of the peripheral immune system

The periphery of the immune system--as opposed to the central lymphoid organs--contains inhomogeneously distributed B and T cells whose phenotype, repertoire, developmental origin, and function are highly divergent. Nonconventional lymphocytes bearing a phenotype that is rare in the blood, spleen, or lymph nodes of undiseased individuals are encountered at high frequency in different localizations, e.g., alpha/beta TCR+CD4-CD8- cells in the bone marrow and gut epithelium, particular invariant gamma/delta TCR+CD4-CD8 alpha+CD8 beta- and gamma/delta TCR+CD4-CD8 alpha-CD8 beta- T cells in various epithelia, or CD5+ B cells in the peritoneum. The antigen receptor repertoire is different in each localization. Thus, different gamma/delta TCR gene products dominant in each site, and the proportion of cells expressing transgenic and endogenous alpha/beta TCR and immunoglobulin gene products follows a gradient, with a maximum of endogenous gene expression in the peritoneum, intermediate values in other peripheral lymphoid organs (spleen, lymph nodes), and minimum values in thymus and bone marrow. Forbidden T cells that bear self-superantigen-reactive V beta gene products are physiologically detected among alpha/beta TCR+CD4-CD8- lymphocytes of the bone marrow, as well as in the gut. Violating previous ideas on self-tolerance preservation, self-peptide-specific gamma/delta T cells are present among intestinal intraepithelial lymphocytes, and CD5+ B cells produce low-affinity crossreactive autoantibodies in a physiological fashion. It appears that, in contrast to the bulk of T and B lymphocytes, certain gamma/delta and alpha/beta T cells found in the periphery, as well as most CD5+ B cells, do not depend on the thymus or bone marrow for their development, respectively, but arise from different, nonconventional lineages. In addition to divergent lineages that are targeted to different organs guided by a spatiotemporal sequence of tissue-specific homing receptors, local induction or selection processes may be important in the diversification of peripheral lymphocyte compartments. Selection may be exerted by local antigens, antigen-presenting cells whose function varies in each anatomical localization, cytokines, and cell-matrix interactions, thus leading to the expansion and maintenance of some clones, whereas others are diluted out or deleted. The spatial compartmentalization of lymphocytes in different microenvironments has major functional consequences and leads to a partial fragmentation of immunoregulatory circuits at the local level. Lymphocytes residing in certain antigen-exposed compartments are likely to combat tissue-specific pathogens or self-proteins.(ABSTRACT TRUNCATED AT 400 WORDS)

CD44 and its interaction with extracellular matrix

It is now generally accepted that CD44 is a cell adhesion receptor and that hyaluronan is one of its ligands. Like many cell adhesion receptors, CD44 is broadly distributed, and its ligand, hyaluronan, is a common component of extracellular matrices and extracellular fluids. Yet a great variety of responses has been reported to result from CD44 ligation. These include cell adhesion, cell migration, induction (or at least support) of hematopoietic differentiation, effects on other cell adhesion mechanisms, and interaction with cell activation signals. This diversity of responses indicates that downstream events following ligand binding by CD44 may vary depending on the cell type expressing CD44 and on the environment of that cell. CD44 is expressed on cells in the early stages of hematopoiesis and has been shown to participate in at least some aspects of the hematopoietic process. In mature lymphocytes, CD44 is upregulated in response to antigenic stimuli and may participate in the effector stage of immunological responses. Along with other adhesion receptors that show alterations in expression after activation, CD44 probably contributes to differences in the recirculation patterns of different lymphocyte subpopulations. CD44 ligand-binding function on lymphocytes is strictly regulated, such that most CD44-expressing cells do not constitutively bind ligand. Ligand-binding function may be activated as a result of differentiation, inside-out signaling, and/or extracellular stimuli. This regulation, which in some situations can be rapid and transient, potentially provides exquisite specificity to what would otherwise be a common interaction. CD44 is not a single molecule, but a diverse family of molecules generated by alternate splicing of multiple exons of a single gene and by different posttranslational modifications in different cell types. It is not yet clear how these modifications influence ligand-binding function. The significance of the multiple isoforms of CD44 is not understood, but association of some isoforms with malignancies has been observed. And in at least some experimental systems, a contribution of CD44 isoforms to metastatic behavior has been demonstrated.

The role of adhesion molecules in the pathogenesis of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disease characterized by infiltration of mononuclear cells, mainly T lymphocytes, into the synovial membrane (SM). The interaction of peripheral blood T cells with the different components of the rheumatoid synovium is mediated by cell surface proteins such as selectins, integrins, members of the immunoglobulin superfamily and homing receptors. T lymphocytes infiltrating the rheumatoid SM show an activated phenotype and display an increased avidity of their adhesion receptors that results in an enhanced interaction of these cells with both extracellular matrix proteins (ECM) and cellular ligands (VCAM-1, ICAMs). The interaction of T cell integrins with their ligands, besides an additional antigenic stimulus, could trigger a mitogenic response on these cells, a phenomenon that can contribute to increased cellularity observed into the rheumatoid SM. Moreover, cell attachment to ECM through integrins induces the secretion of several proteases that can contribute to the tissue damage observed in RA. The increased knowledge about the role of adhesion receptors in the pathogenesis of RA and other inflammatory diseases will allow the introduction of a new therapeutic approach by: the use of specific blocking reagents designed to interfere with the function of adhesion molecules.

Migration of bovine bronchial epithelial cells to extracellular matrix components

Migration of epithelial cells is an important feature of wound healing. Components of extracellular matrix stimulate migration of other cells, and we hypothesized that basement membrane components, laminin and type IV collagen, stimulate migration of bovine bronchial epithelial cells (BBEC). BBEC cultured for 3 days were used in migration assays using the blindwell chamber technique. BBEC migrated in a concentration-dependent manner to laminin, type IV collagen, and fibronectin. "Checkerboard" analysis demonstrated that the migration was directional for each of the components studied. We also evaluated the stimulatory activity of fibronectin fragments. A fragment containing the RGDS peptide was stimulatory of migration while other fragments were much less so, suggesting a role for RGDS-sensitive, integrin-mediated mechanisms. In order to evaluate whether bound or soluble matrix components were required to direct cell migration, we performed haptotaxis assays with precoated filters in the migration chambers. Fibronectin-precoated filters demonstrated significant stimulation of migration, suggesting that some of the migration in our original assays with fibronectin present in the lower chambers could be termed haptotactic. laminin- and type IV collagen-precoated filters were less active. When insulin was used as a chemoattractant, the fibronectin-precoated filters were more facilitatory of migration than the other matrix coatings. In summary, BBEC can chemotactically migrate to extracellular matrix components. Thus, the composition of the provisional matrix that forms at sites of epithelial injury may play an important role in the repair processes that occur after injury to the bronchial epithelium.

A sense phosphorothioate oligonucleotide directed to the initiation codon of transcription factor NF-kappa B p65 causes sequence-specific immune stimulation

Antisense oligonucleotides have proved effective in achieving targeted inhibition of gene expression. In such experiments, sense oligonucleotides have frequently been used as a control for nonspecific effects, but the results have been variable, raising questions about the reliability of sense oligomers as a control. It is possible that some of the effects of sense oligonucleotides may be specific. We have shown that phosphorothioate antisense oligonucleotides to the p65 subunit of NF-kappa B, a transcription factor, cause a block in cell adhesion. In our efforts to test the efficacy of NF-kappa B p65 oligonucleotides in vivo, we unexpectedly observed that the control p65-sense, but not the p65-antisense, oligonucleotides caused massive splenomegaly in mice. In the current study we demonstrate a sequence-specific stimulation of splenic cell proliferation, both in vivo and in vitro, by treatment with p65-sense oligonucleotides. Cells expanded by this treatment are primarily B-220+, sIg+ B cells. The secretion of immunoglobulins by the p65-sense oligonucleotide-treated splenocytes is also enhanced. In addition, the p65-sense-treated splenocytes, but not several other cell lines, showed an upregulation of NF-kappa B-like activity in the nuclear extracts, an effect not dependent on new protein or RNA synthesis. These results demonstrate that phosphorothioate oligonucleotides can exert sequence-specific effects in vivo, irrespective of sense or antisense orientation.

Neutrophil sequestration and migration in localized pulmonary inflammation. Capillary localization and migration across the interalveolar septum

Recent studies have raised the possibility that biophysical properties of the neutrophil, specifically cell size and deformability, along with the unique hemodynamic properties of the pulmonary microvasculature may be important determinants of neutrophil transit through the lung and lead to significant retention in capillaries. Additionally, accumulating evidence supports the concept that leukocytes may emigrate into inflammatory lesions through the pulmonary capillary wall, quite unlike the situation in the systemic circulation where the predominant site of margination and emigration is the postcapillary venule. To confirm and extent this suggestion, morphometric techniques were employed to determine the location and kinetics of neutrophil movement from the vascular compartment to the alveolar space in response to an inflammatory reaction initiated by local instillation of fragments of the activated form of the fifth component of complement (C5f). We found that the site of neutrophil sequestration and probable migration was almost exclusively the capillary. Neutrophil sequestration appeared first in the capillaries of the interalveolar septae, with negligible accumulation in arterioles and venules (less than that in interalveolar septae by a factor of 10(4) when corrected for the relevant surface areas). Accumulation in the airspaces lagged behind that in the interalveolar septa by about 60 min, placing an upper time-limit on the emigration process across the epithelial barriers. Interestingly, neutrophils samples by bronchoalveolar lavage represented only 1 to 2% of those neutrophils shown to be present in the airspaces of the lung by morphometric assessment, suggesting that bronchoalveolar lavage may sample only a subpopulation of the emigrated cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Adhesion proteins on human microglial cells and modulation of their expression by IL1 alpha and TNF alpha

Expression of adhesion proteins on human microglial cells was studied by immunocytochemistry. Both microglial cells and peripheral blood monocytes expressed beta 2 integrins and molecules of the immunoglobulin superfamily at similar levels whereas the expression of the beta 1 integrins (alpha 2-VLA (very late antigen), alpha 4-VLA, alpha 5-VLA, alpha 6-VLA) was higher on microglial cells than on monocytes. Stimulation of microglial cells with interleukin-1 alpha and tumour necrosis factor-alpha, the main cytokines detected in HIV1-infected central nervous system (CNS), increased the microglial expression of alpha 1-VLA, intercellular adhesion molecule-1, vascular cell adhesion molecule-1 and beta 2-LFA-1 (leukocyte-function-associated molecule-1) but not of alpha L-LFA-1. Such an induction of adhesion molecules could facilitate penetration of HIV1-infected monocytes into brain parenchyma and their adhesion to CNS cells, and could maintain a chronic inflammation during human immunodeficiency virus-1 (HIV1) encephalopathy.

Rapid effects of EGF on cytoskeletal structures and adhesive properties of highly metastatic rat mammary adenocarcinoma cells

In the highly metastatic rat mammary adenocarcinoma cell clone MTLn3, EGF induced increased adhesion to fibronectin while in the human epidermoid carcinoma cell line A431 EGF induced diminished adhesive properties. Flattening of cells with extensive formation of filopodia was observed in MTLn3 cells within 5 min of EGF addition, while in A431 cells EGF induced rounding up and only occasional formation of filopodia. Immunofluorescent analysis revealed extension of microtubules (MT) into the filopodia and Western blot analysis demonstrated an EGF-induced 2- to 3-fold increase in the amount of assembled tubulin in MTLn3 but not in A431 cells. In MTLn3, but only marginally in A431 cells, EGF treatment resulted in phosphorylation of a 280 kD cytoskeleton-associated protein, which was rapid and dose-dependent. These results suggest differential signal transduction pathway of cytoskeleton-associated EGFRs in highly metastatic MTLn3 as compared with A431 cells.

Integrins and their ligands

Integrins are expressed on almost every cell type and are responsible for the linkage of the extracellular matrix with the cytoskeleton. In this review I have focused on the intra- and extracellular proteins that bind to integrins. Although many integrins bind to the same extracellular ligand, they mostly recognize different sites on these ligands. Some integrins interact with the same site but then there are requirements for different additional sequences to obtain high affinity. By modulating the expression and activity of integrins in the plasma membrane, cells can adapt their capacity of binding to the matrix. How integrins become activated is as yet not clear, but interaction with other proteins or lipids may be critical. Binding to ligands could also be modulated by alternative splicing of mRNAs for ligand binding sites in the extracellular domain. In Drosophila, the mRNA for the extracellular domain of the PS2 integrin is spliced near a site implicated in ligand binding. In humans, however, there are no indications that alternative splicing contributes to the regulation of function of the extracellular domain of integrins. The only splice variant of the extracellular domain of an integrin identified so far concerns are alpha subunit of the alpha IIb beta 3 complex, but the splicing occurs in a region that has not been implicated in cell adhesion. There is also no evidence as yet that integrin function can be modulated by alternative splicing of mRNA for the cytoplasmic domain of integrin subunits. However, the loss of function seen with some deletion mutants of the cytoplasmic domains of integrin subunits suggests that such a mechanism may well exist. In a different way the binding capacity of a given cell can be influenced by regulating the expression of its ligand or by alternative mRNA splicing of sequences encoding the cell binding domain in their ligands. In the case of fibronectin, the mRNA for one of the integrin binding sites is subject to alternative splicing. The mRNAs for the three chains of laminin appear not to be subject to alternative splicing but, by combining different variant chains of laminin, isoforms can be generated which may have different affinities for integrins. Binding of cells to the matrix therefore does not only depend on the expression and activity of the correct integrin but also of the correct variant of the ligand.(ABSTRACT TRUNCATED AT 400 WORDS)

Fibronectin receptors are functional on mitotic Chinese hamster ovary cells

In this paper, evidence is provided indicating that the blockade of presynchronized CHO 15B cells in prometaphase by nocodazole is fully reversible and efficient enough to allow us to analyze the function of the integrin receptors. Flow cytometry analysis using a specific antibody raised against the fibronectin receptor, and binding studies of the radiolabeled fibronectin on the cell membrane, indicated a stable number of receptors at the cell surface during mitosis. Furthermore, in the mean time, only a slight increase in the Kd value of the fibronectin-receptor interaction was detected. A binding assay designed to test the affinity of the receptor for its extracellular ligand in an insoluble form was used. No difference was observed between mitotic and interphasic cells. Taken together, these results indicate that the rounding up of the cells observed during mitosis is not due to a loss of the receptor affinity for its extracellular ligand.

Stimulation of neutrophil adhesion by antibodies recognizing CD15 (Le(X)) and CD15-expressing carcinoembryonic antigen-related glycoprotein NCA-160

The carbohydrate antigen, CD15 (Le(X)), and its sialylated derivative have recently been shown to be involved in the binding of neutrophils to the endothelial lectins, E-selectin and P-selectin. Neutrophil NCA-160, a carcinoembryonic antigen (CEA)-related glycoprotein, is the major carrier of CD15, which is also expressed on the common beta 2 chain of leucocyte integrins. Rabbit IgG antibodies directed against CEA, which cross-react with neutrophil NCAs, increase the adhesion of neutrophils to plastic. This effect is also observed with F(ab')2 and Fab antibody fragments and a monoclonal antibody (mAb) recognizing the same antigen. Anti-CD15 mAbs inhibit adhesion at higher concentrations, but augment adhesion at lower concentrations. Anti-CEA and anti-CD15 antibodies cause the homotypic adhesion of neutrophils demonstrable by light microscopy and flow cytometry. Anti-(integrin beta 2 chain) mAbs inhibit both adhesion to plastic and homotypic adhesion. These results suggest that binding of ligand to NCA-160 is able to trigger neutrophil adhesion events which have been shown to be integrin mediated. Anti-CD15 mAbs do not, however, induce a respiratory burst from neutrophils.

Expression of tissue factor by melanoma cells promotes efficient hematogenous metastasis

Metastasis is a multistep process which requires highly adapted interactions of tumor cells with host target organs. Compared with nonmetastatic cells, metastatic human melanoma cells express 1000-fold higher level of tissue factor (TF), the major cellular initiator of the plasma coagulation protease cascades. To explore whether TF may contribute to metastatic tumor dissemination, we analyzed the effect of specific inhibition of TF function on human melanoma metastasis in severe combined immunodeficient (SCID) mice. Using species-specific antibodies to TF, we demonstrate that initial adherence in insufficient for successful tumor cell implantation in a target organ. Rapid arrest of human tumor cells in the lungs of mice was not diminished by inhibition of TF. However, inhibition of TF receptor function and consequent reduction in local protease generation abolished prolonged adherence of tumor cells, resulting in significantly reduced numbers of tumor cells retained in the vasculature of the lungs. The growth of pulmonary metastases was also significantly inhibited by a blocking anti-TF monoclonal antibody and Fab fragments thereof, whereas a noninhibitory antibody lacked antimetastatic effects. Cell surface expression of functional TF thus contributes to melanoma progression by allowing metastatic cells to provide requisite signals for prolonged adhesive interactions and/or transmigration of tumor cells across the endothelium, resulting in successful metastatic tumor implantation.

Zyxin and cCRP: two interactive LIM domain proteins associated with the cytoskeleton

Interaction with extracellular matrix can trigger a variety of responses by cells including changes in specific gene expression and cell differentiation. The mechanism by which cell surface events are coupled to the transcriptional machinery is not understood, however, proteins localized at sites of cell-substratum contact are likely to function as signal transducers. We have recently purified and characterized a low abundance adhesion plaque protein called zyxin (Crawford, A. W., and M. C. Beckerle. 1991. J. Biol. Chem. 266:5847-5853; Crawford, A. W., J. W. Michelsen, and M. C. Beckerle. 1992. J. Cell Biol. 116:1381-1393). We have now isolated and sequenced zyxin cDNA and we report here that zyxin exhibits an unusual proline-rich NH2-terminus followed by three tandemly arrayed LIM domains. LIM domains have previously been identified in proteins that play important roles in transcriptional regulation and cellular differentiation. LIM domains have been proposed to coordinate metal ions and we have demonstrated by atomic absorption spectroscopy that purified zyxin binds zinc, a result consistent with the idea that zyxin has zinc fingers. In addition, we have discovered that zyxin interacts in vitro with a 23-kD protein that also exhibits LIM domains. Microsequence analysis has revealed that the 23-kD protein (or cCRP) is the chicken homologue of the human cysteine-rich protein (hCRP). By double-label indirect immunofluorescence, we found that zyxin and cCRP are extensively colocalized in chicken embryo fibroblasts, consistent with the idea that they interact in vivo. We conclude that LIM domains are zinc-binding sequences that may be involved in protein-protein interactions. The demonstration that two cytoskeletal proteins, zyxin and cCRP, share a sequence motif with proteins important for transcriptional regulation raises the possibility that zyxin and cCRP are components of a signal transduction pathway that mediates adhesion-stimulated changes in gene expression.

Localization of ahe alpha 6 and beta 4 integrin subunits in normal human non-lymphoid tissues

The alpha 6/beta 4 integrin, of undefined receptor activity, has been shown to be expressed in a variety of murine epithelial cells. To gain information on the role of this heterodimer in tissue architecture as well as in malignant transformation we have performed an extensive immunohistochemical analysis of normal human tissues using monoclonal antibodies to alpha 6 and beta 4 subunits. Because alpha 6 is known to associate also with the beta 1 subunit to form a non-promiscuous receptor for laminin, the expression of beta 1 chain was also evaluated. The results of this study have shown that the alpha 6 chain has a wide distribution in tissues, including small vessels and peripheral nerves. alpha 6 colocalizes with beta 4 and beta 1 in most epithelial cells at the basolateral or basal aspect abutting the basement membrane. In a minority of tissues lacking beta 4, the alpha 6 chain is coexpressed with beta 1. These findings demonstrate that the expression of alpha 6/beta 1 laminin receptor and alpha 6/beta 4 heterodimer is phylogenetically conserved, suggesting that they are likely to play an important role in cellular scaffolding through binding to laminin and to still uncharacterized ligand/s present in basement membranes.

Induction of alpha v beta 3 integrin-mediated attachment to extracellular matrix in beta 1 integrin (CD29)-negative B cell lines

beta 1 integrin containing complexes have been implicated as the primary adhesion structures in many lymphocyte extracellular matrix (ECM) interactions. However, many B lymphocytes lack surface expression of the beta 1 subunit, implying that this subpopulation of lymphoid cells must employ alternate adhesion structures if they are to maintain an interactive capacity with ECM. An examination of the adherence properties of the beta 1 integrin-negative B cell line JY indicated that these cells exhibit little or no basal adherence to any of the ECM components examined. However, these cells could be induced to adhere to the ECM components fibronectin, laminin, and vitronectin following treatment with PMA. Blocking studies with monoclonal antibodies indicated the alpha v beta 3 integrin complex was involved in the attachment to each of these ligands. However, the adherence to fibronectin displayed a complex pattern of inhibition suggesting the involvement of other ECM receptors. The utilization of the alpha v beta 3 complex was not unique to the JY cell line. Other B cell lines were observed to employ alpha v beta 3, and these lines similarly lacked expression of beta 1 integrin. These results indicate that alpha v beta 3 can act as a lymphoid ECM-adhesion structure which may provide an alternative means for lymphocytes to interact with ECM. Furthermore, these studies provide evidence for the presence of lymphoid-associated alpha v beta 3 integrins with regulatable activity, which contrasts with the constitutive adhesive potential of these complexes when present on other cell types.

Differences in chemotaxis to fibronectin in weakly and highly metastatic tumor cells

We have examined the chemotactic ability of tumor cell lines with different metastatic potential to plasma fibronectin in Transwell chamber assay. Human renal carcinoma cells with highly metastatic potential, SN12 C-2, chemotactically migrated to fibronectin (10 micrograms/ml) about three-fold more strongly than weakly metastatic SN12 C-4 cells. Similarly, murine melanoma B16-BL6 cells (highly metastatic) showed higher motility to soluble fibronectin in comparison with weakly metastatic B16-F1 cells. Anti-VLA-alpha 3 and beta 1 antibodies potently blocked the chemotaxis of both highly and weakly metastatic cells (SN12 C-2 and C-4) to fibronectin. This implies that the migration of both C-2 and C-4 cells to fibronectin is basically mediated by VLA-3 receptor. In contrast, the anti-VLA-alpha 5 antibody and RGDS peptide significantly inhibited the chemotaxis of SN12 C-2 cells to fibronectin, but did not affect weakly metastatic SN12 C-4 cells. These results suggest that the chemotactic ability to fibronectin positively correlates with the metastatic potential in SN12 and B16 cell lines, and that VLA-5 receptor is concerned in the motility of highly metastatic SN12 C-2 cells to soluble fibronectin.

Neutrophil adhesion: A point for therapeutic intervention?

It has become increasingly clear over the last 20 years that the potential exists to modulate inflammatory responses with compounds that interfere with intercellular adhesion. This review highlights the adhesion interactions that occur during neutrophil extravasation and indicates some of the possible ways of disrupting these interactions.

A chimeric N-cadherin/beta 1-integrin receptor which localizes to both cell-cell and cell-matrix adhesions

To study the molecular mechanisms involved in formation of cell contacts, we have transfected cultured cells with a chimeric cDNA encoding the cytoplasmic and transmembrane domains of beta 1 integrin and the extracellular region of N-cadherin and determined the subcellular distribution of the chimeric molecule. We show that the chimeric receptor associates preferentially with cell-matrix focal contacts, suggesting that its distribution is directed by its beta 1 integrin segment, presumably via interactions of the cytoplasmic domain with cytoskeletal elements characteristic of focal contacts. Transfected cells which expressed relatively high levels of the cadherin/integrin chimera underwent an apparent epithelialization and contained the molecule both in cell-matrix and cell-cell contacts. Location in cell-cell contacts indicates competence of the cadherin extracellular domain to participate in formation of cell-cell junctions using a foreign cytoplasmic domain. Labeling of these cultures for talin, which is normally associated only with matrix adhesions, revealed specific labeling along the newly formed intercellular junctions. This suggests that the local association of talin with these sites is induced by the cytoplasmic tail of beta 1 integrin receptor presented by the chimeric protein. These results suggest that the formation of adherens-type junctions is driven by the cooperative interactions of the relevant adhesion molecules (cadherins and integrins) both with the respective extracellular ligands and with the cytoskeleton.

Tyrosine phosphorylation of membrane proteins mediates cellular invasion by transformed cells

Tyrosine phosphorylation of membrane-associated proteins is involved at two distinct sites of contact between cells and the extracellular matrix: adhesion plaques (cell adhesion and de-adhesion) and invadopodia (invasion into the extracellular matrix). Adhesion plaques from chicken embryonic fibroblasts or from cells transformed by Rous sarcoma virus contain low levels of tyrosine-phosphorylated proteins (YPPs) which were below the level of detection in 0.5-microns thin, frozen sections. In contrast, intense localization of YPPs was observed at invadopodia of transformed cells at sites of degradation and invasion into the fibronectin-coated gelatin substratum, but not in membrane extensions free of contact with the extracellular matrix. Local extracellular matrix degradation and formation of invadopodia were blocked by genistein, an inhibitor of tyrosine-specific kinases, but cells remained attached to the substratum and retained their free-membrane extensions. Invadopodia reduced or lost YPP labeling after treatment of the cells with genistein, but adhesion plaques retained YPP labeling. The plasma membrane contact fractions of normal and transformed cells have been isolated form cells grown on gelatin cross-linked substratum using a novel fractionation scheme, and analyzed by immunoblotting. Four major YPPs (150, 130, 81, and 77 kD) characterize invadopodial membranes in contact with the matrix, and are probably responsible for the intense YPP labeling associated with invadopodia extending into sites of matrix degradation. YPP150 may be an invadopodal-specific YPP since it is approximately 3.6-fold enriched in the invasive contact fraction relative to the cell body fraction and is not observed in normal contacts. YPP130 is enriched in transformed cell contacts but may also be present in normal contacts. The two major YPPs of normal contacts (130 and 71 kD) are much lower in abundance than the major tyrosine-phosphorylated bands associated with invadopodial membranes, and likely represent major adhesion plaque YPPs. YPP150, paxillin, and tensin appear to be enriched in the cell contact fractions containing adhesion plaques and invadopodia relative to the cell body fraction, but are also present in the soluble supernate fraction. However, vinculin, talin, and alpha-actinin that are localized at invadopodia, are equally concentrated in cell bodies and cell contacts as is the membrane-adhesion receptor beta 1 integrin. Thus, tyrosine phosphorylation of the membrane-bound proteins may contribute to the cytoskeletal and plasma membrane events leading to the formation and function of invadopodia that contact and proteolytically degrade the extracellular matrix; we have identified several candidate YPPs that may participate in the regulation of these processes.

Inhibition of cell attachment, invasion and metastasis of human carcinoma cells by anti-integrin beta 1 subunit antibody

We investigated the expression of beta 1 integrins in human carcinoma cell lines, and the anti-metastatic and anti-invasive effects of a newly established anti-human beta 1 subunit monoclonal antibody designated NCC-INT-7. All the examined carcinoma cell lines expressed beta 1 integrins upon immunoblot analysis. NCC-INT-7 completely inhibited the adhesion of carcinoma cells to laminin, fibronectin, collagens and acetone-fixed tissues including lung, liver and brain. In an in vitro invasion model, NCC-INT-7 inhibited the invasion of human bladder carcinoma cell line T24 and human gastric carcinoma cell lines TMK-1, MKN-45 and MKN-74 through an artificially reconstructed basement membrane. In an in vivo nude mouse peritoneal dissemination model using MKN-45 and TMK-1, NCC-INT-7 significantly reduced the number of tumor nodules in the mesentery. In an in vivo nude mouse liver metastasis model using a serially transplantable human colonic carcinoma, COL-2-JCK, NCC-INT-7 significantly reduced the number of tumor nodules in liver. These results indicate that beta 1 integrins play an important role in the tissue attachment, migration, invasion and metastasis of human carcinoma cells, and that this new monoclonal antibody is useful for studies aimed at prevention of metastasis.

Deficiency in integrin-mediated transmembrane signaling and microfilament stress fiber formation by aging dermal fibroblasts from normal and Down's syndrome patients

Previous evidence has shown a deficiency in microfilament stress fiber formation upon short-term cycloheximide treatment of cultured human dermal fibroblasts while cytoplasmic spreading appeared completely normal and other cytoskeletal networks organized normally. This deficiency applied to collagen substrata (not fibronectin substrata) and was specific for in vitro-aged normal fibroblasts and for fibroblasts from three different Down's syndrome patients at any passage level. To identify the mechanism(s) for matrix receptor deficiency in aging cells, cells were evaluated for amounts and distributions of several integrin subunits using specific monoclonal antibodies and two complementary experimental approaches. Flow cytometric analyses have shown that all these cells at all passage levels have large amounts of alpha 3 and beta 1 integrin subunits and smaller amounts of the alpha 5 subunit, directed to fibronectin, which are minimally affected in their cell surface availability by cycloheximide treatment. In contrast, cycloheximide treatment leads to the loss from surface availability of most of the alpha 2 subunit, directed to collagen, in late-passage papillary and reticular normal fibroblasts and in all three Down's patient cells at all passages. Prior growth of cells in ascorbate-supplemented medium, which overcomes the deficiency in stress fiber formation, conserves the large amounts of cell surface-available alpha 2 subunit detectable by flow cytometry. When amounts of integrin subunits were evaluated by immunoprecipitation of [35S]methionine-radiolabeled cells, there was no diminution of the alpha 2 subunit or any other subunit for any cells upon cycloheximide treatment; however, there was much less alpha 2 subunit complexed with beta 1 in aging normal and Down's cells. Therefore, cycloheximide treatment does not lead to loss in the amounts of the alpha 2 subunit but rather to its masking at the cell surface and inability to transmit signals across the plasma membrane to effect stress fiber formation. This aging-related deficiency in integrin-mediated signaling can now be studied mechanistically with a variety of approaches to determine the nature of cell-surface molecules interacting with integrins (cis- and/or trans-acting molecules) that discriminate functional from nonfunctional receptors.

HMEC-1: Establishment of an Immortalized Human Microvascular Endothelial Cell Line

The study of human microvascular endothelial cells has been limited, because these cells are difficult to isolate in pure culture, are fastidious in their in vitro growth requirements, and have a very limited lifespan. In order to overcome these difficulties, we have transfected human dermal microvascular endothelial cells (HMEC) with a PBR-322-based plasmid containing the coding region for the simian virus 40 A gene product, large T antigen, and succeeded in immortalizing them. These cells, termed CDC/EU.HMEC-1 (HMEC-1), have been passaged 95 times to date and show no signs of senescence, whereas normal microvascular endothelial cells undergo senescence at passages 8-10. HMEC-1 exhibit typical cobblestone morphology when grown in monolayer culture, express and secrete von Willebrand's Factor, take up acteylated low-density lipoprotein, and rapidly form tubes when cultured on matrigel. HMEC-1 grow to densities three to seven times higher than microvascular endothelial cells and require much less stringent growth medium. HMEC-1 will grow in the absence of human serum, whereas microvascular endothelial cells require culture medium supplemented with 30% human serum. These cells express other cell-surface molecules typically associated with endothelial cells, including CD31 and CD36 and epitopes identified by monoclonal antibodies EN4 and PAL-E. They also express the cell adhesion molecules ICAM-1 and CD44 and following stimulation with interferon-gamma express major histocompatibility complex class II antigens. HMEC-1 specifically bind lymphocytes in cell adhesion assays. Thus HMEC-1 is the first immortalized human microvascular endothelial cell line that retains the morphologic, phenotypic, and functional characteristics of normal human microvascular endothelial cells.

Retrovirally introduced antisense integrin RNA inhibits neuroblast migration in vivo

We used retrovirus-mediated gene transfer to ask whether integrins are involved in the development of neuroblasts in the chicken optic tectum. Vectors were constructed with the E. coli lacZ gene in the sense orientation and beta 1 integrin sequences in the antisense orientation. Tests in culture showed that the progeny of cells infected by these vectors were identifiable by expression of LacZ and had reduced levels of beta 1 integrins on their surfaces. We then injected these vectors into optic tecta on E3, at the height of neuronal production. Clones of LacZ-positive cells were analyzed 3-9 days later, as they migrated along radial glia to form the tectal plate. Antisense sequences had little effect on the proliferation of progenitors, or on the radial stacking of their progeny in the ventricular zone (E6). However, many antisense-bearing cells accumulated in the ventricular zone and failed to migrate into the tectal plate (E7.5 and E9). At later stages (E12), few antisense-bearing cells could be found. Thus, integrin appears to be required in the migratory process, and cells that fail to engage in integrin-mediated interactions may die.

Crystal structure at 2.8 A resolution of a soluble form of the cell adhesion molecule CD2

The crystal structure of a soluble form of the T lymphocyte antigen CD2 provides the first complete view of the extracellular region of a cell adhesion molecule. The topology of the molecule, which comprises two immunoglobulin-like domains, is the same as that of the first two domains of CD4 but the relative domain orientation is altered by a fairly flexible linker region. The putative ligand-binding beta-sheet forms a flat surface towards the top of the molecule. Crystal contacts between these surfaces suggest a plausible model for the adhesive interaction.

Role of fibrinogen alpha and gamma chain sites in platelet aggregation

Fibrinogen (Fbg) mediates platelet aggregation by its interaction with the platelet glycoprotein IIb-IIIa (integrin alpha IIb beta 3). Peptides containing the amino acid sequence RGD derived from the alpha chain (residues alpha 95-97 and residues alpha 572-574) and the sequence HHLGGAKQAGDV derived from the carboxyl terminus of the gamma chain of Fbg (residues gamma 400-411) inhibit these interactions. To determine the role of these sequences in intact Fbg, recombinant human Fbg (rFbg), mutant rFbgs with an RGD-->RGE substitution at either position alpha 97 or alpha 574, and a rFbg gamma'-containing variant that has a carboxyl-terminal interruption in the HHLGGAKQAGDV sequence have been expressed in transfected BHK cells. Purified rFbg and the two RGE mutant Fbgs were similar to plasma Fbg in platelet aggregation assays. In contrast, the gamma' variant Fbg was markedly defective in platelet aggregation. These data support the proposals that the carboxyl-terminal region of the gamma chain of Fbg is essential for optimal platelet aggregation and that the alpha-chain RGD sequences are neither necessary nor sufficient for platelet aggregation.

The three-dimensional structure of the tenth type III module of fibronectin: an insight into RGD-mediated interactions

The solution structure of the tenth type III module of fibronectin has been determined using nuclear magnetic resonance techniques. The molecule has a fold similar to that of immunoglobulin domains, with seven beta strands forming two antiparallel beta sheets, which pack against each other. Both beta sheets contribute conserved hydrophobic residues to a compact core. The topology is more similar to that of domain 2 of CD4, PapD, and the extracellular domain of the human growth hormone receptor than to that of immunoglobulin C domains. The module contains an Arg-Gly-Asp sequence known to be involved in cell adhesion. This tripeptide is solvent exposed and lies on a conformationally mobile loop between strands F and G, consistent with its cell adhesion function.

Laminins and other strange proteins

Laminins are large multidomain proteins of the extracellular matrix (ECM) with important functions in the development and maintenance of cellular organization and supramolecular structure, in particular in basement membranes. Each molecule is composed of three polypeptide chains, A (300-400 kDa) and B1 and B2 (180-200 kDa), which together form the characteristic cross-shaped laminin structure with three short arms and one long arm. Many different domains have been identified in laminin by sequence analysis, structural investigations, and functional studies. Each short arm is formed by homologous N-terminal portions of one of the three chains. Structurally, each short arm contains two or three globular domains which are connected by rows of manyfold-repeated Cys-rich "EGF-like" domains. In all three chains this region is followed by a long heptad repeat region similar to those found in many alpha-helical coiled-coil proteins. These parts of the three laminin chains constitute a triple-stranded coiled-coil domain, which forms the extended rodlike structure of the long arm. This is the only domain in the protein which is made up of more than one chain and consequently serves the function of chain assembly. The two B chains are terminated by the coiled-coil domain, but the A chain contains an additional C-terminal segment which accounts for five globular domains located at the tip of the long arm. Several important functions of laminin have been assigned to individual domains in either the short arms or terminal regions of the long arm.(ABSTRACT TRUNCATED AT 250 WORDS)

Selectins: interpreters of cell-specific carbohydrate information during inflammation

Although a bewildering array of cell surface carbohydrate structures have been described, the physiological relevance of any of these complex molecules has often eluded biologists. A family of cell surface glycoproteins, the "selectins," has a characteristic ability to use some of these carbohydrate structures in adhesive mechanisms that help localize leukocytes to regions of inflammation. This article will review the biology of these carbohydrate-binding adhesive proteins and discuss the potential for developing anti-inflammatory antagonists that could inhibit binding events that are selectin-mediated.

Cytoskeleton--plasma membrane interactions

Proteins at the boundary between the cytoskeleton and the plasma membrane control cell shape, delimit specialized membrane domains, and stabilize attachments to other cells and to the substrate. These proteins also regulate cell locomotion and cytoplasmic responses to growth factors and other external stimuli. This diversity of cellular functions is matched by the large number of biochemical mechanisms that mediate the connections between membrane proteins and the underlying cytoskeleton, the so-called membrane skeleton. General organizational themes are beginning to emerge from examination of this biochemical diversity.

Structure of a fibronectin type III domain from tenascin phased by MAD analysis of the selenomethionyl protein

Fibronectin type III domains are found in many different proteins including cell surface receptors and cell adhesion molecules. The crystal structure of one such domain from the extracellular matrix protein tenascin was determined. The structure was solved by multiwavelength anomalous diffraction (MAD) phasing of the selenomethionyl protein and has been refined to 1.8 angstrom resolution. The folding topology of this domain is identical to that of the extracellular domains of the human growth hormone receptor, the second domain of CD4, and PapD. Although distinct, this topology is similar to that of immunoglobulin constant domains. An Arg-Gly-Asp (RGD) sequence that can function for cell adhesion is found in a tight turn on an exposed loop.