Alpha v beta 1 is a receptor for vitronectin and fibrinogen, and acts with alpha 5 beta 1 to mediate spreading on fibronectin

A Review of Integrin-Mediated Endothelial Cell Phenotype in the Design of Cardiovascular Devices

Sustained biomaterial thromboresistance has long been a goal and challenge in blood-contacting device design. Endothelialization is one of the most successful strategies to achieve long-term thromboresistance of blood-contacting devices, with the endothelial cell layer providing dynamic hemostatic regulation. It is well established that endothelial cell behavior is influenced by interactions with the underlying extracellular matrix (ECM). Numerous researchers have sought to exploit these interactions to generate improved blood-contacting devices by investigating the expression of hemostatic regulators in endothelial cells on various ECM coatings. The ability to select substrates that promote endothelial cell-mediated thromboresistance is crucial to advancing material design strategies to improve cardiovascular device outcomes. This review provides an overview of endothelial cell regulation of hemostasis, the major components found within the cardiovascular basal lamina, and the interactions of endothelial cells with prominent ECM components of the basement membrane. A summary of ECM-mimetic strategies used in cardiovascular devices is provided with a focus on the effects of key adhesion modalities on endothelial cell regulators of hemostasis.

Improvement of paracellular transport in the Caco-2 drug screening model using protein-engineered substrates

The Caco-2 assay has achieved wide popularity among pharmaceutical companies in the past two decades as an in vitro method for estimation of in vivo oral bioavailability of pharmaceutical compounds during preclinical characterization. Despite its popularity, this assay suffers from a severe underprediction of the transport of drugs which are absorbed paracellularly, that is, which pass through the cell-cell tight junctions of the absorptive cells of the small intestine. Here, we propose that simply replacing the collagen I matrix employed in the standard Caco-2 assay with an engineered matrix, we can control cell morphology and hence regulate the cell-cell junctions that dictate paracellular transport. Specifically, we use a biomimetic engineered extracellular matrix (eECM) that contains modular protein domains derived from two ECM proteins found in the small intestine, fibronectin and elastin. This eECM allows us to independently tune the density of cell-adhesive RGD ligands presented to Caco-2 cells as well as the mechanical stiffness of the eECM. We observe that lower amounts of RGD ligand presentation as well as decreased matrix stiffness results in Caco-2 morphologies that more closely resemble primary small intestinal epithelial cells than Caco-2 cells cultured on collagen. Additionally, these matrices result in Caco-2 monolayers with decreased recruitment of actin to the apical junctional complex and increased expression of claudin-2, a tight junction protein associated with higher paracellular permeability that is highly expressed throughout the small intestine. Consistent with these morphological differences, drugs known to be paracellularly transported in vivo exhibited significantly improved transport rates in this modified Caco-2 model. As expected, permeability of transcellularly transported drugs remained unaffected. Thus, we have demonstrated a method of improving the physiological accuracy of the Caco-2 assay that could be readily adopted by pharmaceutical companies without major changes to their current testing protocols.

Combination scaffolds of salmon fibrin, hyaluronic acid, and laminin for human neural stem cell and vascular tissue engineering

Human neural stem/progenitor cells (hNSPCs) are good candidates for treating central nervous system (CNS) trauma since they secrete beneficial trophic factors and differentiate into mature CNS cells; however, many cells die after transplantation. This cell death can be ameliorated by inclusion of a biomaterial scaffold, making identification of optimal scaffolds for hNSPCs a critical research focus. We investigated the properties of fibrin-based scaffolds and their effects on hNSPCs and found that fibrin generated from salmon fibrinogen and thrombin stimulates greater hNSPC proliferation than mammalian fibrin. Fibrin scaffolds degrade over the course of a few days in vivo, so we sought to develop a novel scaffold that would retain the beneficial properties of fibrin but degrade more slowly to provide longer support for hNSPCs. We found combination scaffolds of salmon fibrin with interpenetrating networks (IPNs) of hyaluronic acid (HA) with and without laminin polymerize more effectively than fibrin alone and generate compliant hydrogels matching the physical properties of brain tissue. Furthermore, combination scaffolds support hNSPC proliferation and differentiation while significantly attenuating the cell-mediated degradation seen with fibrin alone. HNSPCs express two fibrinogen-binding integrins, αVβ1 and α5β1, and several laminin binding integrins (α7β1, α6β1, α3β1) that can mediate interaction with the scaffold. Lastly, to test the ability of scaffolds to support vascularization, we analyzed human cord blood-derived endothelial cells alone and in co-culture with hNSPCs and found enhanced vessel formation and complexity in co-cultures within combination scaffolds. Overall, combination scaffolds of fibrin, HA, and laminin are excellent biomaterials for hNSPCs.

STATEMENT OF SIGNIFICANCE

Interest has increased recently in the development of biomaterials as neural stem cell transplantation scaffolds to treat central nervous system (CNS) injury since scaffolds improve survival and integration of transplanted cells. We report here on a novel combination scaffold composed of fibrin, hyaluronic acid, and laminin to support human neural stem/progenitor cell (hNSPC) function. This combined biomaterial scaffold has appropriate physical properties for hNSPCs and the CNS, supports hNSPC proliferation and differentiation, and attenuates rapid cell-mediated scaffold degradation. The hNSPCs and scaffold components synergistically encourage new vessel formation from human endothelial cells. This work marks the first report of a combination scaffold supporting human neural and vascular cells to encourage vasculogenesis, and sets a benchmark for biomaterials to treat CNS injury.

Full-Length Fibronectin Drives Fibroblast Accumulation at the Surface of Collagen Microtissues during Cell-Induced Tissue Morphogenesis

Generating and maintaining gradients of cell density and extracellular matrix (ECM) components is a prerequisite for the development of functionality of healthy tissue. Therefore, gaining insights into the drivers of spatial organization of cells and the role of ECM during tissue morphogenesis is vital. In a 3D model system of tissue morphogenesis, a fibronectin-FRET sensor recently revealed the existence of two separate fibronectin populations with different conformations in microtissues, i.e. 'compact and adsorbed to collagen' versus 'extended and fibrillar' fibronectin that does not colocalize with the collagen scaffold. Here we asked how the presence of fibronectin might drive this cell-induced tissue morphogenesis, more specifically the formation of gradients in cell density and ECM composition. Microtissues were engineered in a high-throughput model system containing rectangular microarrays of 12 posts, which constrained fibroblast-populated collagen gels, remodeled by the contractile cells into trampoline-shaped microtissues. Fibronectin's contribution during the tissue maturation process was assessed using fibronectin-knockout mouse embryonic fibroblasts (Fn-/- MEFs) and floxed equivalents (Fnf/f MEFs), in fibronectin-depleted growth medium with and without exogenously added plasma fibronectin (full-length, or various fragments). In the absence of full-length fibronectin, Fn-/- MEFs remained homogenously distributed throughout the cell-contracted collagen gels. In contrast, in the presence of full-length fibronectin, both cell types produced shell-like tissues with a predominantly cell-free compacted collagen core and a peripheral surface layer rich in cells. Single cell assays then revealed that Fn-/- MEFs applied lower total strain energy on nanopillar arrays coated with either fibronectin or vitronectin when compared to Fnf/f MEFs, but that the presence of exogenously added plasma fibronectin rescued their contractility. While collagen decoration of single fibronectin fibers enhanced the non-persistent migration of both Fnf/f and Fn-/- MEFs, the migration speed was increased for Fn-/- MEFs on plasma fibronectin fibers compared to Fnf/f MEFs. In contrast, the average speed was the same for all cells on collagen-coated Fn fibers. A Fn-FRET sensor revealed that fibronectin on average was more extended on the microtissue surface compared to fibronectin in the core. Gradients of collagen-to-fibronectin ratios and of the fraction of collagen-adsorbed to stretched fibrillar fibronectin conformations might thereby provide critical cell migration cues. This study highlights a dominant role for fibronectin in tissue morphogenesis and the development of tissue heterogeneities.

What Kind of Signaling Maintains Pluripotency and Viability in Human-Induced Pluripotent Stem Cells Cultured on Laminin-511 with Serum-Free Medium?

Xeno-free medium contains no animal-derived components, but is composed of minimal growth factors and is serum free; the medium may be supplemented with insulin, transferrin, and selenium (ITS medium). Serum-free and xeno-free culture of human-induced pluripotent stem cells (hiPSCs) uses a variety of components based on ITS medium and Dulbecco's modified Eagle's medium/Ham's nutrient mixture F12 (DMEM/F12) that contain high levels of iron salt and glucose. Culture of hiPSCs also requires scaffolding materials, such as extracellular matrix, collagen, fibronectin, laminin, proteoglycan, and vitronectin. The scaffolding component laminin-511, which is composed of α5, β1, and γ1 chains, binds to α3β1, α6β1, and α6β4 integrins on the cell membrane to induce activation of the PI3K/AKT- and Ras/MAPK-dependent signaling pathways. In hiPSCs, the interaction of laminin-511/α6β1 integrin with the cell–cell adhesion molecule E-cadherin confers protection against apoptosis through the Ras homolog gene family member A (RhoA)/Rho kinase (ROCK) signaling pathway (the major pathways for cell death) and the proto-oncogene tyrosine-protein kinase Fyn (Fyn)-RhoA-ROCK signaling pathway. The expression levels of α6β1 integrin and E-cadherin on cell membranes are controlled through the activation of insulin receptor/insulin, FGF receptor/FGF2, or activin-like kinase 5 (ALK5)-dependent TGF-β signaling. A combination of growth factors, medium constituents, cell membrane-located E-cadherin, and α6β1 integrin-induced signaling is required for pluripotent cell proliferation and for optimal cell survival on a laminin-511 scaffold. In this review, we discuss and explore the influence of growth factors on the cadherin and integrin signaling pathways in serum-free and xeno-free cultures of hiPSCs during the preparation of products for regenerative medicinal therapies. In addition, we suggest the optimum serum-free medium components for use with laminin-511, a new scaffold for hiPSC culture.

Atomic basis for the species-specific inhibition of αV integrins by monoclonal antibody 17E6 is revealed by the crystal structure of αVβ3 ectodomain-17E6 Fab complex

The function-blocking, non-RGD-containing, and primate-specific mouse monoclonal antibody 17E6 binds the αV subfamily of integrins. 17E6 is currently in phase II clinical trials for treating cancer. To elucidate the structural basis of recognition and the molecular mechanism of inhibition, we crystallized αVβ3 ectodomain in complex with the Fab fragment of 17E6. Protein crystals grew in presence of the activating cation Mn(2+). The integrin in the complex and in solution assumed the genuflected conformation. 17E6 Fab bound exclusively to the Propeller domain of the αV subunit. At the core of αV-Fab interface were interactions involving Propeller residues Lys-203 and Gln-145, with the latter accounting for primate specificity. The Propeller residue Asp-150, which normally coordinates Arg of the ligand Arg-Gly-Asp motif, formed contacts with Arg-54 of the Fab that were expected to reduce soluble FN10 binding to cellular αVβ3 complexed with 17E6. This was confirmed in direct binding studies, suggesting that 17E6 is an allosteric inhibitor of αV integrins.

Mechanisms of talin-dependent integrin signaling and crosstalk

Cells undergo dynamic remodeling of the cytoskeleton during adhesion and migration on various extracellular matrix (ECM) substrates in response to physiological and pathological cues. The major mediators of such cellular responses are the heterodimeric adhesion receptors, the integrins. Extracellular or intracellular signals emanating from different signaling cascades cause inside-out signaling of integrins via talin, a cystokeletal protein that links integrins to the actin cytoskeleton. Various integrin subfamilies communicate with each other and growth factor receptors under diverse cellular contexts to facilitate or inhibit various integrin-mediated functions. Since talin is an essential mediator of integrin activation, much of the integrin crosstalk would therefore be influenced by talin. However, despite the existence of an extensive body of knowledge on the role of talin in integrin activation and as a stabilizer of ECM-actin linkage, information on its role in regulating inter-integrin communication is limited. This review will focus on the structure of talin, its regulation of integrin activation and discuss its potential role in integrin crosstalk. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

β1 integrin targeting potentiates antiangiogenic therapy and inhibits the growth of bevacizumab-resistant glioblastoma

Antiangiogenic therapies like bevacizumab offer promise for cancer treatment, but acquired resistance, which often includes an aggressive mesenchymal phenotype, can limit the use of these agents. Upregulation of β1 integrin (ITGB1) occurs in some bevacizumab-resistant glioblastomas (BRG) whereby, mediating tumor-microenvironment interactions, we hypothesized that it may mediate a mesenchymal-type resistance to antiangiogenic therapy. Immunostaining analyses of β1 integrin and its downstream effector kinase FAK revealed upregulation in 75% and 86% of BRGs, respectively, compared with pretreatment paired specimens. Furthermore, flow cytometry revealed eight-fold more β1 integrin in primary BRG cells compared with cells from bevacizumab-naïve glioblastomas (BNG). Fluorescence recovery after photobleaching of cells engineered to express a β1-GFP fusion protein indicated that the mobile β1 integrin fraction was doubled, and half-life of β1 integrin turnover in focal adhesions was reduced markedly in BRG cells compared with bevacizumab-responsive glioblastoma multiforme cells. Hypoxia, which was increased with acquisition of bevacizumab resistance, was associated with increased β1 integrin expression in cultured BNG cells. BRGs displayed an aggressive mesenchymal-like phenotype in vitro. We found that growth of BRG xenograft tumors was attenuated by the β1 antibody, OS2966, allowing a 20-fold dose reduction of bevacizumab per cycle in this model. Intracranial delivery of OS2966 through osmotic pumps over 28 days increased tumor cell apoptosis, decreased tumor cell invasiveness, and blunted the mesenchymal morphology of tumor cells. We concluded that β1 integrin upregulation in BRGs likely reflects an onset of hypoxia caused by antiangiogenic therapy, and that β1 inhibition is well tolerated in vivo as a tractable strategy to disrupt resistance to this therapy.

RGD-Binding Integrins in Prostate Cancer: Expression Patterns and Therapeutic Prospects against Bone Metastasis

Prostate cancer is the third leading cause of male cancer deaths in the developed world. The current lack of highly specific detection methods and efficient therapeutic agents for advanced disease have been identified as problems requiring further research. The integrins play a vital role in the cross-talk between the cell and extracellular matrix, enhancing the growth, migration, invasion and metastasis of cancer cells. Progression and metastasis of prostate adenocarcinoma is strongly associated with changes in integrin expression, notably abnormal expression and activation of the β₃ integrins in tumour cells, which promotes haematogenous spread and tumour growth in bone. As such, influencing integrin cell expression and function using targeted therapeutics represents a potential treatment for bone metastasis, the most common and debilitating complication of advanced prostate cancer. In this review, we highlight the multiple ways in which RGD-binding integrins contribute to prostate cancer progression and metastasis, and identify the rationale for development of multi-integrin antagonists targeting the RGD-binding subfamily as molecularly targeted agents for its treatment.

A feeder-free, human plasma-derived hydrogel for maintenance of a human embryonic stem cell phenotype in vitro

Background

Human embryonic stem cells (hESCs) represent a tremendous resource for cell therapies and the study of human development; however to maintain their undifferentiated state in vitro they routinely require the use of mouse embryonic fibroblast (MEF) feeder-layers and exogenous protein media supplementation.

Results

These well established requirements can be overcome and in this study, it will be demonstrated that phenotypic stability of hESCs can be maintained using a novel, human plasma protein-based hydrogel as an extracellular culture matrix without the use of feeder cell co-culture. hESCs were resuspended in human platelet poor plasma (PPP), which was gelled by the addition of calcium containing DMEM-based hESC culture medium. Phenotypic and genomic expression of the pluripotency markers OCT4, NANOG and SOX2 were measured using immunohistochemistry and qRT-PCR respectively. Typical hESC morphology was demonstrated throughout in vitro culture and both viability and phenotypic stability were maintained throughout extended culture, up to 25 passages.

Conclusions

PPP-derived hydrogel has demonstrated to be an efficacious alternative to MEF co-culture with its hydrophilicity allowing for this substrate to be delivered via minimally invasive procedures in a liquid phase with polymerization ensuing in situ. Together this provides a novel technique for the study of this unique group of stem cells in either 2D or 3D both in vitro and in vivo.

Two cDNAs coding for the porcine CD51 (αv) integrin subunit: cloning, expression analysis, adhesion assays and chromosomal localization

CD51 (α(v)) is an integrin chain that associates with multiple β integrin chains to form different receptor complexes that mediate important human processes. Pigs show substantial physiological, immunological and anatomical similarities to humans, and are therefore a good model system to study immunological and pathological processes. Here we report the cloning and characterization of two cDNAs produced by alternative splicing that encode two different porcine CD51 proteins that differ in five amino acid residues. Pig CD51 cDNAs encode polypeptides of 1046 or 1041 amino acid residues, respectively, that share with other mammalian homologous proteins a high percentage amino acid identity and the functional domains. Expression analysis of CD51 was carried out at two different levels. RT-PCR analysis revealed that both CD51 transcripts were expressed ubiquitously but heterogeneously, with the exception of some platelets in which only the smallest CD51 transcript was detected. A specific monoclonal antibody against a pig CD51 recombinant protein was made and used in the immunohistochemical localization of CD51 proteins. It showed that CD51 was mainly expressed in hematopoietic cells of myeloid linage, epithelial and endothelial cells, osteoclasts, nervous fibers and smooth muscle. Adhesion assays showed that in the presence of Mn(++) pig α(v)-CHO-B2 transfected cells increased their attachment to fibronectin and vitonectin, but not to fibrinogen. Finally, we localized the CD51 gene on the porcine chromosome 15 (SSC15), q23-q26.

Alpha-v integrins as therapeutic targets in oncology

Integrins are heterodimeric cell adhesion receptors that mediate intercellular communication through cell-extracellular matrix interactions and cell-cell interactions. Integrins have been demonstrated to play a direct role in cancer progression, specifically in tumor cell survival, tumor angiogenesis, and metastasis. Therefore, agents targeted against integrin function have potential as effective anticancer therapies. Numerous anti-integrin agents, including monoclonal antibodies and small-molecule inhibitors, are in clinical development for the treatment of solid and hematologic tumors. This review focuses on the role of alpha(v) integrins in cancer progression, the current status of integrin-targeted agents in development, and strategies for the clinical development of anti-integrin therapies.

Macrophage Adhesion on Gelatin-Based Interpenetrating Networks Grafted with PEGylated RGD

Human blood-derived macrophage adhesion on interpenetrating networks (IPNs) composed of PEGylated RGD-modified gelatin and poly(ethylene glycol) diacrylate was studied. The interaction between biomaterial immobilized with biofunctional peptides such as RGD and macrophages is central in the design of tissue-engineering scaffolds. PEGylated RGD-modified gelatin was synthesized via several steps involving PEG derivations and characterized by high-performance liquid chromatography, mass spectroscopy, gel permeation chromatography, and the trinitrobenzenesulfonic acid method. IPNs containing modified or unmodified gelatin were cultured with human macrophages and monitored at 2, 24, 96, and 168 h. At each time point, IPNs containing gelatin modified with PEGylated RGD showed a comparable adherent macrophage density as tissue culture polystyrene and a significantly higher cell density than other IPN formulations containing unmodified gelatin or gelatin modified with PEGylated triglycine. Although surface-immobilized RGD can serve to mediate the adhesion of different cell types on the biomaterial surface, the interaction of RGD with immune/inflammatory cells such as macrophages should also be considered when assessing the potential host response of tissue-engineering scaffolds.

Latent effects of fibronectin, alpha5beta1 integrin, alphaVbeta5 integrin and the cytoskeleton regulate pancreatic carcinoma cell IL-8 secretion

Interactions between tumour cells and the microenvironment are increasingly recognised to have an influence on cancer progression. In pancreatic carcinoma, a highly desmoplastic stroma with abnormal extracellular matrix (ECM) protein and interleukin-8 (IL-8) expression is seen. To investigate whether the ECM may further contribute to abnormalities in the microenvironment by influencing IL-8 secretion, we cultured the Mia PaCa2 pancreatic carcinoma cell line on fibronectin. This resulted in a dose-dependent increase in IL-8 secretion, which was RGD dependent and accompanied by cell spreading and proliferation. The role of spreading was assessed by disruption of the cytoskeleton with cytochalasin D, resulting in a large increase in IL-8 secretion, which was reduced from 31- to 24-fold by fibronectin. This remarkable response was associated with inhibition of spreading and proliferation and represents a novel cytoskeletal function. To investigate whether it could be accounted for by the loss of integrin-mediated signalling, the expressed α5β1, αVβ5 and α3β1 integrins were inhibited. α5β1 inhibition prevented spreading and proliferation but produced a much smaller rise in IL-8 secretion than cytochalasin D. αVβ5 inhibition alone had only minor effects but when inhibited in combination with α5β1 completely abolished the response to fibronectin. These results reveal latent stimulatory effects of the αVβ5 integrin on IL-8 secretion and suggest that integrin crosstalk may limit the induction of IL-8 secretion by fibronectin. However, the magnitude of IL-8 secretion induced by cytochalasin cannot be accounted for by integrin signalling and may reflect the influence of another signalling pathway or a novel, intrinsic cytoskeletal function.

CNTO 95, a fully human monoclonal antibody that inhibits alphav integrins, has antitumor and antiangiogenic activity in vivo

Integrins of the alphav family, such as alphavbeta3 and alphavbeta5, are implicated in tumor-induced angiogenesis; but their role in tumor growth has not been fully explored. CNTO 95 is a fully human antibody that recognizes the alphav family of integrins and is likely to be less immunogenic in humans compared to chimeric or humanized antibodies. CNTO 95 bound to purified alphavbeta3 and alphavbeta5 with a Kd of approximately 200 pM and to alphav integrin-expressing human cells with a Kd of 1-24 nM. In vitro, CNTO 95 inhibited human melanoma cell adhesion, migration and invasion at doses ranging 7-20 nM. In a rat aortic ring sprouting assay, CNTO 95 (approx. 70 nM) completely inhibited sprouting. Using a human melanoma xenograft model in nude mice wherein CNTO 95 recognized alphavbeta3 and alphavbeta5 on human tumor cells but not mouse angiogenic integrins, CNTO 95 (10 mg/kg, 3 times/week) inhibited growth of human melanoma tumors in nude mice by approximately 80% (p = 0.0005), suggesting that CNTO 95 inhibited human tumor growth independently of its antiangiogenic activity. In a nude rat human xenograft model where CNTO 95 binds and blocks both tumor and host integrins, this antibody (10 mg/kg once/week) reduced final tumor weight by >99% (p < 0.0001). Based on these preclinical data, a dose-escalating phase I clinical trial in cancer patients has been initiated. To our knowledge, CNTO 95 is the first fully human MAb to alphav integrins that has potent antitumor and antiangiogenic properties in in vivo preclinical models.

Alphav-integrin utilization in human beta-cell adhesion, spreading, and motility

The role of individual integrins in human beta-cell development and function is largely unknown. This study describes the contribution of alpha(v)-integrins to human beta-cell adhesion, spreading, and motility. Developmental differences in alpha(v)-integrin utilization are addressed by comparing the responses of adult and fetal beta-cells, and vitronectin is used as a substrate based on its unique pattern of expression in the developing pancreas. Fetal and adult beta-cells attached equally to vitronectin and integrin alpha(v)beta(5) was found to support the adhesion of both mature and immature beta-cell populations. Fetal beta-cells were also observed to spread and migrate on vitronectin, and integrin alpha(v)beta(1) was found to be essential for these responses. In contrast to their fetal counterparts, adult beta-cells failed to either spread or migrate and this deficit was associated with a marked down-regulation of alpha(v)beta(1) expression in adult islet preparations. The integrin alpha(v)beta(3) was not found to support significant beta-cell attachment or migration. Based on our findings, we conclude that integrins alpha(v)beta(5) and alpha(v)beta(1) are important mediators of human beta-cell adhesion and motility, respectively. By supporting fetal beta-cell migration, alpha(v)beta(1) could play an important role in early motile processes required for islet neogenesis.

A highly efficient electroporation method for the transfection of endothelial cells

Several approaches have been described for improving transfection efficiencies of endothelial cells but the general observations have indicated that yields of transfected endothelial cells are low, irrespective of the techniques used. Here we describe a transfection procedure performed by means of electroporation, with efficiencies up to 85%, by optimizing several parameters such as the electroporation buffer, number of cells, voltage, capacitance and pulse length. The protocol was applied to three endothelial cell types (HUVECs, HUAECs and HMEC-1) commonly used in 'in vitro' angiogenic assays. We did not observe functional impairment between transfected and non-transfected cells in their adhesion to different components of the extracellular matrix, migration, or the development of capillary-like structures. Our experiments show that this electroporation procedure does not alter the physiology of endothelial cells and can be applied to functional studies, as exemplified by the successful transfection of the isoform 1 of calcipressin 1 (CALP1L).

Epidermal growth factor enhances invasive activity of BeWo choriocarcinoma cells by inducing alpha2 integrin expression

The trophoblast, an important component of the mammalian placenta, has several essential biological roles in the maintenance of pregnancy. First, trophoblast cells must attach to the uterine endometrium, and then they must invade to a depth at which the vascular network exists. Here, we investigated the effects of epidermal growth factor (EGF) on alpha2 integrin expression, adhesiveness to collagen, and invasive activity using human BeWo choriocarcinoma cells. EGF induced the expression of alpha2 integrin mRNA and protein, as shown by Northern blotting, Western blotting, and flow cytometry. Human chorionic gonadotropin (hCG) secretion was enhanced by the addition of EGF, which suggests that the BeWo cells functionally differentiated similarly to normal trophoblasts. EGF also dose-dependently stimulated the invasiveness of BeWo cells. Antibody against alpha2 integrin inhibited this effect, suggesting that it may be mediated by an increase of cell surface integrin. EGF had no effect on the adhesiveness of BeWo cells to collagen, whereas it stimulated the chemokinetic activity in a dose-dependent manner. The increase of chemokinetic activity was suppressed by antibody against alpha2 integrin. These results suggest that EGF may induce alpha2 integrin expression in trophoblast cells, thereby enhancing their invasiveness into the endometrium via an increase of their chemokinetic activity.

A novel approach to increase human islet cell mass while preserving beta-cell function

Human islet expansion in monolayer culture leads to loss of function and senescence. By maintaining the 3-D configuration of islets in fibrin gels, it is feasible to expand beta-cells in response to hepatocyte growth factor (HGF) while preserving physiologic glucose responsiveness both in vitro and in vivo after transplantation into nude mice. Islets were cultured free floating with or without growth factors and nicotinamide and in fibrin gels with the same conditions. Proliferation was observed only in islets cultured in fibrin gels and the cocktail; total insulin increased by threefold, with a concomitant increase in beta-cell mass by morphometry. Insulin release after glucose challenge was also preserved. Islets in fibrin gels gave rise in vivo to large grafts rich in insulin and glucagon, and grafts from free-floating islets were smaller with fewer endocrine cells. Circulating human C-peptide levels were higher than in the mice receiving free-floating islets. In summary, fibrin allows for HGF-mediated cell proliferation while preserving glucose responsiveness in an environment that preserves cell-cell contacts. Limited islet ex vivo expansion under these conditions may improve recipient-donor tissue ratios to equal the functional results of whole-organ transplants.

Unique ability of integrin alpha(v)beta 3 to support tumor cell arrest under dynamic flow conditions

Shear-resistant arrest of circulating tumor cells is required for metastasis from the blood stream. Arrest during blood flow can be supported by tumor cell interaction with attached, activated platelets. This is mediated by tumor cell integrin alpha(v)beta3 and cross-linking plasma protein ligands. To analyze the mechanism of tumor cell ligand interactions under dynamic flow conditions, we used real-time video microscopy and tested human melanoma cell binding to fibrinogen, von Willebrand Factor, or fibronectin matrices in a buffer perfusion system. When perfused at venous flow, melanoma cells arrested abruptly and began to spread immediately. This was uniquely mediated by integrin alpha(v)beta3 on all tested ligands, and required alpha(v)beta3 activation and actin polymerization. Under static conditions, alpha(v)beta3 cooperated with alpha(v)beta1 and alpha5beta1 in supporting melanoma cell adhesion to fibronectin. But even when activated, beta1 integrins did not contribute to melanoma cell arrest during flow. Soluble ligand served as a cross-linker between attached and circulating tumor cells and enhanced melanoma cell arrest. Cohesion of activated melanoma cells was restricted to the matrix surface and did not occur in suspension. We conclude that the presence of alpha(v)beta3 in a functionally activated state provides a unique advantage for circulating tumor cells by promoting tumor cell arrest in the presence of flow-dependent shear forces.

Integrin alphavbeta1 is a receptor for foot-and-mouth disease virus

Infection by field strains of Foot-and-mouth disease virus (FMDV) is initiated by binding to certain species of arginine-glycine-aspartic acid (RGD)-dependent integrin including alphavbeta3 and the epithelial integrin alphavbeta6. In this report we show that the integrin alphavbeta1, when expressed as a human/hamster heterodimer on transfected CHOB2 cells, is a receptor for FMDV. Virus binding and infection mediated by alphavbeta1 was inefficient in the presence of physiological concentrations of calcium and magnesium but were significantly enhanced by reagents that activate the integrin and promote ligand binding. The ability of chimeric alpha5/alphav integrin subunits, in association with the beta1 chain, to bind FMDV and mediate infection matched the ligand binding specificity of alphavbeta1, not alpha5beta1, thus providing further evidence for the receptor role of alphavbeta1. In addition, data are presented suggesting that amino acid residues near the RGD motif may be important for differentiating between the binding specificities of alphavbeta1 and alphavbeta6.

Integrin alpha(v)beta1 is an adenovirus coreceptor

The human embryonic kidney (HEK293) cell line, commonly used for recombinant adenovirus (Ad) propagation, does not express the Ad coreceptor alpha(v)beta3 or alpha(v)beta5 integrins, yet these cells are efficiently infected by Ad vectors. Here we demonstrate that Ad binds to HEK293 cells via the fiber receptor CAR and is subsequently internalized via interaction with integrin alpha(v)beta1. Function-blocking antibodies directed against alpha(v) or beta1, but not beta3, beta5, or alpha5, integrin subunits block Ad infection and viral endocytosis. Therefore, alpha(v)beta1 serves as a coreceptor for Ad infection, and the lack of beta3 and/or beta5 but the relatively high expression of alpha(v)beta1 integrins on certain tumor cell types may explain why these cells are readily transduced by Ad vectors.

Role of beta1 and beta3 integrins in human smooth muscle cell adhesion to and contraction of fibrin clots in vitro

The degree of lumen narrowing in advanced lesions correlates poorly with the amount of intimal mass accumulated in the atherosclerotic plaque. As an alternate mechanism of stenosis, we propose that human smooth muscle cells bind to fibrin deposited in the matrix and exert contractile forces to cause a narrowing of the lumen. In the present study we demonstrated in vitro that human newborn aortic smooth muscle cell lines can contract and adhere to fibrin clots composed of either fibronectin-depleted plasma ("plasma") or recombinant fibrin. By using neutralizing antibodies and RGD peptides, we showed that members of the integrin family mediated the interaction between human newborn smooth muscle cells and fibrin. Neutralizing antibodies against the integrin alphavbeta3 (c7E3 Fab and LM609) did not inhibit either plasma clot contraction or recombinant fibrin clot contraction by human newborn smooth muscle cells. In contrast, antibodies against alpha5, beta1, and alpha5/beta1 inhibited contraction of clots composed of either plasma or recombinant fibrin. Anti-alphavbeta3, anti-alphav, anti-alpha5, anti-beta1, and anti-alpha5beta1 antibodies inhibited human newborn smooth muscle cell adhesion to plasma clots; however, only anti-alpha5, anti-beta1, and anti-alpha5beta1 antibodies significantly inhibited adhesion to recombinant fibrin. While the linear RGD peptides had no effect, the cyclic peptide penRGD inhibited adhesion to plasma clots and recombinant fibrin. However, it did not block contraction of recombinant fibrin clots. These results suggest that during the interaction of human newborn smooth muscle cell lines with fibrin, alpha5beta1 plays a significant role. This interaction is of potential interest as a target for efforts to block vascular contraction.

A key function for alphav containing integrins in mesodermal cell migration during Pleurodeles waltl gastrulation

During cleavage of Pleurodeles waltl amphibian embryos, inner cells of the blastocoel roof (presumptive ectodermal and mesodermal cells) organize a fibrillar extracellular matrix (ECM) containing fibronectin on their basal surface by a beta1-integrin-dependent process. This matrix is used as a migratory substrate by mesodermal cells during gastrulation. While alpha5beta1 integrin is expressed on both ectodermal and mesodermal cell surface, we have shown previously that alphav containing integrins are essentially restricted to the surface of mesodermal cells (Alfandari, D., Whittaker, C. A., DeSimone, D. W., and Darribère, T., Dev. Biol. 170, 249-261, 1995). To investigate the function of alphav integrins during gastrulation, we have generated a function blocking antibody directed against the extracellular domain of the Pleurodeles integrin alphav subunit. The antibody did not prevent fibronectin fibril formation, whereas an antibody against the alpha5beta1 integrin did. When injected into the blastocoel, the antibody against integrin alphav subunit perturbed gastrulation and further development in a stage-dependent manner. Developmental defects were correlated to an abnormal positioning of the mesoderm layer. In vitro, the antibody blocked spreading of mesodermal cell to fibronectin or blastocoel roof ECM but not their attachment. In contrast, the antibody directed against the alpha5beta1 integrin inhibited both cell attachment and spreading to the same substrates. We propose that the alpha5beta1 integrin is required for fibronectin assembly into fibrils and mesodermal cell attachment to the blastocoel roof ECM, while the alphav containing integrins are necessary for cell spreading, and possibly migration, on this complex network.

A single immunoglobulin-like domain of the human neural cell adhesion molecule L1 supports adhesion by multiple vascular and platelet integrins

The neural cell adhesion molecule L1 has been shown to function as a homophilic ligand in a variety of dynamic neurological processes. Here we demonstrate that the sixth immunoglobulin-like domain of human L1 (L1-Ig6) can function as a heterophilic ligand for multiple members of the integrin superfamily including alphavbeta3, alphavbeta1, alpha5beta1, and alphaIIbbeta3. The interaction between L1-Ig6 and alphaIIbbeta3 was found to support the rapid attachment of activated human platelets, whereas a corresponding interaction with alphavbeta3 and alphavbeta1 supported the adhesion of umbilical vein endothelial cells. Mutation of the single Arg-Gly-Asp (RGD) motif in human L1-Ig6 effectively abrogated binding by the aforementioned integrins. A L1 peptide containing this RGD motif and corresponding flanking amino acids (PSITWRGDGRDLQEL) effectively blocked L1 integrin interactions and, as an immobilized ligand, supported adhesion via alphavbeta3, alphavbeta1, alpha5beta1, and alphaIIbbeta3. Whereas beta3 integrin binding to L1-Ig6 was evident in the presence of either Ca2+, Mg2+, or Mn2+, a corresponding interaction with the beta1 integrins was only observed in the presence of Mn2+. Furthermore, such Mn2+-dependent binding by alpha5beta1 and alphavbeta1 was significantly inhibited by exogenous Ca2+. Our findings suggest that physiological levels of calcium will impose a hierarchy of integrin binding to L1 such that alphavbeta3 or active alphaIIbbeta3 > alphavbeta1 > alpha5beta1. Given that L1 can interact with multiple vascular or platelet integrins it is significant that we also present evidence for de novo L1 expression on blood vessels associated with certain neoplastic or inflammatory diseases. Together these findings suggest an expanded and novel role for L1 in vascular and thrombogenic processes.

Integrin alpha5beta1-mediated adenovirus infection is enhanced by the integrin-activating antibody TS2/16

Adenovirus internalization generally has been accepted to involve an interaction of the adenoviral penton base protein with alpha(v)beta3 and alpha(v)beta5 cell surface integrins. In this study we show that exposure of a panel of melanoma cells to the beta1-activating antibody TS2/16 rendered such cells more susceptible to adenovirus infection. This increase in adenoviral infectivity paralleled effects on cell adhesion, and both these characteristics were mediated, in part, by the alpha5beta1 integrin. These observations suggest that alpha5beta1 may act as an alternative adenovirus receptor and that integrin-activating strategies may improve the efficacy of recombinant adenoviruses as vectors for gene therapy.

Attachment, spreading and migration of melanoma cells on vitronectin. The rôle of alpha V beta 3 and alpha V beta 5 integrins

Recent in situ studies suggest the alpha V beta 3 integrin is a tumour progression marker in melanoma. We analyzed 5 human melanoma cell lines for their expression of the vitronectin binding alpha V beta 3 and alpha V beta 5 integrins using flow cytometry. The role of these receptors in cell attachment, spreading and migration was investigated using attachment assays, video time lapse spreading and migration assays and with function blocking monoclonal antibodies. Cell lines derived from later stages of tumor progression exhibited high levels of alpha V beta 3 expression, whereas no similar correlation with alpha V beta 5 expression was identified. Cell attachment, spreading and migration response on vitronectin correlated well with the expression level of the alpha V beta 3 but not the alpha V beta 5 vitronectin receptor. Blocking of the alpha V beta 3 integrin resulted in a significant decrease in cell attachment, spreading and motility whereas the function blocking antibody against the alpha V beta 5 integrin only inhibited cell attachment in cell lines with the highest level of expression of this integrin. Taken together, our study indicates that the level of expression of the alpha V beta 3 and alpha V beta 5 integrins is heterogeneous in melanoma cell lines and that the alpha V beta 5 integrin, if present, may function only during the initial cell attachment whereas the alpha V beta 3 plays an important role in cell spreading and cell migration as well.

Integrins: role in cell adhesion and communication

Adhesive interactions are crucial for the integrity and function of all cells and tissues. As one of the major families of cell adhesion receptors, the integrins have been the focus of scientific interest for more than a decade. The resulting studies have tremendously enhanced the understanding of integrin-mediated adhesive interactions and have unveiled novel integrin functions in the cytoskeletal organization of microfilaments and in the activation of diverse signaling pathways. These functions are critically involved in the regulation of multiple processes, such as tissue development, inflammation, tumor cell growth and metastasis, and programmed cell death. The global view of integrin receptor biology has radically changed and has become much more subtle and elaborate. The enormous complexity of integrin function is determined by the heterodimeric formation of more than 20 functional integrin receptors, the cell type-specific distribution, the receptor activation state, the presence of different activation and deactivation signals, and the subsequent employment of distinct cytoskeletal and signaling complexes within a more dimensional network of time and space. This article summarizes the structural and functional properties of the integrin receptors and emphasizes some of the major achievements made in the past to enhance the understanding of integrin biology.

Differential mRNA regulation of integrin subunits alpha V, beta 1, beta 3, and beta 5 during mouse embryonic organogenesis

Cell interactions with extracellular matrices play important roles in morphogenetic processes during embryonic development. Extracellular matrix receptors of the integrin family have been implicated in these steps. Recent studies indicate that a variety of integrins can be differentially expressed during development, consistent with diverse roles for integrins in embryogenesis. The present study compares the expression patterns of several major members of the alpha V integrin subfamily, focusing on mRNA expression of alpha V, beta 1, beta 3, and beta 5 subunits during mouse embryonic organogenesis using Northern blot analysis and in situ hybridization. The alpha V and beta 1 subunits showed widespread tissue expression, although most tissues expressed alpha V at relatively low or basal levels. The mRNA for beta 5 was also expressed in a variety of embryonic organs and showed unusual localization patterns in certain organs. Striking, high-level expression of beta 5 transcripts was detected in the ependymal layer of the central nervous system, glomeruli of the kidney, epicardial region of the heart, and in the tooth germs, suggesting specific functions for this molecule during morphogenetic events in these organs. In contrast, few beta 3 transcripts were expressed during mid-gestation mouse embryogenesis except in megakaryocytes within the embryonic liver. These observations of differing expression spectra suggest that members of the alpha V integrin subfamily have distinct roles and also suggests that they have different transcriptional regulation. The beta 5 integrin is unique in its degree of tissue-specific mRNA regulation associated with morphogenesis of embryonic organs.

An anti-alpha v-integrin antibody that blocks integrin function inhibits the development of a human melanoma in nude mice

A series of murine monoclonal antibodies were raised against purified human alpha v beta 3 integrin and against M21 human melanoma cells. Five notable hybridomas were identified by ELISA on purified integrins, and the isolated antibodies bound the alpha v-chain. These antibodies, 17E6, 20A9, 23G5, 14D9.F8 and 10G2, recognised the extracellular domains of the integrin, and were shown to be reactive in FACS, immunoprecipitation, ELISA, and ELISA on fixed cells with M21, M21-L4, and UCLA-P3, but not with the alpha v-deficient M21-L or M21-L-IIb (M21-L transfected with GpIIb integrin). One antibody, 17E6, strongly perturbed cell attachment mediated by alpha v integrins, reacting at least with alpha v beta 3, alpha v beta 5, and alpha v beta 1, and strongly inhibiting cell attachment to alpha v-ligands vitronectin and fibronectin with an IC50 of approximately 0.1 microgram ml-1. Furthermore, 17E6 at this concentration could induce cell retraction from the substrate, while LM609 (anti-alpha v beta 3) and control antibody 14E2 (anti-200 kDa melanoma surface protein) at 1,000-fold higher concentrations had minimal effects on cell morphology. The action of 17E6 was reversible and was not due to toxic effects: in vitro 17E6 at 0.1 mg ml-1 did not affect either cell proliferation or DNA synthesis. In two nude-mouse tumour models, subcutaneous tumour development and a lung colonisation (‘experimental metastasis’) assay, injection of 17E6 strongly inhibited tumour development, while isotype-matched controls had no effect. There was no obvious mechanism of cell or of complement-mediated tumour cytotoxicity; the antibody did not mediate ADCC or AECDC, or complement fixation. The data strongly support previous studies which have indicated the importance of alpha v-integrins, and especially alpha v beta 3, in the tumour progression of human melanoma.