Distinct ligand-binding modes for integrin alpha(v)beta(3)-mediated adhesion to fibronectin versus vitronectin

Molecular mechanisms of catch bonds and their implications for platelet hemostasis

Adhesive molecular bonds between blood cells are essential for thrombosis and hemostasis as they provide means for platelet adhesion, aggregation, and signaling in flowing blood. According to the nowadays conventional definition, a "catch" bond is a type of non-covalent bio-molecular bridge, whose dissociation lifetime counter-intuitively increases with applied tensile force. Following recent experimental findings, such receptor-ligand protein bonds are vital to the blood cells involved in the prevention of bleeding (hemostatic response) and infection (immunity). In this review, we examine the up-to-date experimental discoveries and theoretical insights about catch bonds between the blood cells, their biomechanical principles at the molecular level, and their role in platelet thrombosis and hemostasis.

Cell adhesion strength and tractions are mechano-diagnostic features of cellular invasiveness

The adhesion of cells to substrates occurs via integrin clustering and binding to the actin cytoskeleton. Oncogenes modify anchorage-dependent mechanisms in cells during cancer progression. Fluid shear devices provide a label-free way to characterize cell-substrate interactions and heterogeneities in cell populations. We quantified the critical adhesion strengths of MCF-7, MDAMB-231, A549, HPL1D, HeLa, and NIH3T3 cells using a custom fluid shear device. The detachment response was sigmoidal for each cell type. A549 and MDAMB-231 cells had significantly lower critical adhesion strengths (τ₅₀) than their non-invasive counterparts, HPL1D and MCF-7. Detachment dynamics inversely correlated with cell invasion potentials. A theoretical model, based on τ₅₀ values and the distribution of cell areas on substrates, provided good fits to results from de-adhesion experiments. Quantification of cell tractions, using the Reg-FTTC method on 10 kPa polyacrylamide gels, showed highest values for invasive, MDAMB-231 and A549, cells compared to non-invasive cells. Immunofluorescence studies show differences in vinculin distributions; non-invasive cells have distinct vinculin puncta, whereas invasive cells have more dispersed distributions. The cytoskeleton in non-invasive cells was devoid of well-developed stress fibers, and had thicker cortical actin bundles in the boundary. Fluorescence intensity of actin was significantly lower in invasive cells as compared to non invasive cells. These correlations in adhesion strengths and traction stresses with cell invasiveness may be useful in cancer diagnostics and other pathologies featuring mis-regulation in adhesion.

Endothelial superoxide dismutase 2 is decreased in sickle cell disease and regulates fibronectin processing

Sickle cell disease (SCD) is a genetic red blood cell disorder characterized by increased reactive oxygen species (ROS) and a concordant reduction in antioxidant capacity in the endothelium. Superoxide dismutase 2 (SOD2) is a mitochondrial-localized enzyme that catalyzes the dismutation of superoxide to hydrogen peroxide. Decreased peripheral blood expression of SOD2 is correlated with increased hemolysis and cardiomyopathy in SCD. Here, we report for the first time that endothelial cells exhibit reduced SOD2 protein expression in the pulmonary endothelium of SCD patients. To investigate the impact of decreased SOD2 expression in the endothelium, SOD2 was knocked down in human pulmonary microvascular endothelial cells (hPMVECs). We found that SOD2 deficiency in hPMVECs results in endothelial cell dysfunction, including reduced cellular adhesion, diminished migration, integrin protein dysregulation, and disruption of permeability. Furthermore, we uncover that SOD2 mediates changes in endothelial cell function via processing of fibronectin through its inability to facilitate dimerization. These results demonstrate that endothelial cells are deficient in SOD2 expression in SCD patients and suggest a novel pathway for SOD2 in regulating fibronectin processing.

Evolving roles and dynamics for catch and slip bonds during adhesion cluster maturation

Focal adhesions are the loci of cellular adhesion to the extracellular matrix. At these sites, various integrins forge connections between the intracellular cytoskeleton and the outside world; large patches of multiple types of integrins together grip hold of collagen, fibronectin, and other extracellular matrix components. A single focal adhesion will likely contain bonds whose lifetime increases with applied load (catch bonds), and bonds whose lifetime decreases with applied load (slip bonds). Prior work suggests that the combination of different types of integrins is essential for focal adhesion stability and mechanosensory functionality. In the present work, we investigate numerically the interplay between two distinct types of bonds, and we ask how the presence of slip bonds, in the same focal integrin cluster, augments the collective behavior of the catch bonds. We show that mixing these two components may increase the low-force mechanical integrity that may be lacking in pure-catch adhesions, while preserving the potential to strengthen the entire adhesion when a force is applied. We investigate the spatial distribution in mixed-integrin focal adhesions, and we show that the differential response to loading leads, via an excluded volume interaction, to a dependence of the individual integrin diffusivities on the applied load, an effect that has been reported in experiments.

A Penicillin Derivative Exerts an Anti-Metastatic Activity in Melanoma Cells Through the Downregulation of Integrin αvβ3 and Wnt/β-Catenin Pathway

The synthetic triazolylpeptidyl penicillin derivative, named TAP7f, has been previously characterized as an effective antitumor agent in vitro and in vivo against B16-F0 melanoma cells. In this study, we investigated the anti-metastatic potential of this compound on highly metastatic murine B16-F10 and human A375 melanoma cells. We found that TAP7f inhibited cell adhesion, migration and invasion in a dose-dependent manner. Additionally, we demonstrated that TAP7f downregulated integrin αvβ3 expression and Wnt/β-catenin pathway, a signaling cascade commonly related to tumor invasion and metastasis. Thus, TAP7f reduced both the enzymatic activity and the expression levels of matrix-metalloproteinases-2 and -9 in a time dependent manner. Moreover, TAP7f inhibited the expression of the transcription factor Snail and the mesenchymal markers vimentin, and N-cadherin, and up-regulated the expression of the epithelial marker E-cadherin, suggesting that the penicillin derivative affects epithelial-mesenchymal transition. Results obtained in vitro were supported by those obtained in a B16-F10-bearing mice metastatic model, that showed a significant TAP7f inhibition of lung metastasis. These findings suggest the potential of TAP7f as a chemotherapeutic agent for the treatment of metastatic melanoma.

Hantavirus entry: Perspectives and recent advances

Hantaviruses are important zoonotic pathogens of public health importance that are found on all continents except Antarctica and are associated with hemorrhagic fever with renal syndrome (HFRS) in the Old World and hantavirus pulmonary syndrome (HPS) in the New World. Despite the significant disease burden they cause, no FDA-approved specific therapeutics or vaccines exist against these lethal viruses. The lack of available interventions is largely due to an incomplete understanding of hantavirus pathogenesis and molecular mechanisms of virus replication, including cellular entry. Hantavirus Gn/Gc glycoproteins are the only viral proteins exposed on the surface of virions and are necessary and sufficient to orchestrate virus attachment and entry. In vitro studies have implicated integrins (β_1-3), DAF/CD55, and gC1qR as candidate receptors that mediate viral attachment for both Old World and New World hantaviruses. Recently, protocadherin-1 (PCDH1) was demonstrated as a requirement for cellular attachment and entry of New World hantaviruses in vitro and lethal HPS in vivo, making it the first clade-specific host factor to be identified. Attachment of hantavirus particles to cellular receptors induces their internalization by clathrin-mediated, dynamin-independent, or macropinocytosis-like mechanisms, followed by particle trafficking to an endosomal compartment where the fusion of viral and endosomal membranes can occur. Following membrane fusion, which requires cholesterol and acid pH, viral nucleocapsids escape into the cytoplasm and launch genome replication. In this review, we discuss the current mechanistic understanding of hantavirus entry, highlight gaps in our existing knowledge, and suggest areas for future inquiry.

VANGL2 protein stability is regulated by integrin αv and the extracellular matrix

Vang-like 2 (VANGL2) is a four-pass transmembrane protein required for a variety of polarized cell behaviors underlying embryonic development. Recent data show human VANGL2 interacts with integrin αv to control cell adhesion to extracellular matrix proteins. The goal of this study was to further define the functional relationship between integrin αv and VANGL2. We demonstrate integrin αv regulates VANGL2 protein levels both in vitro and in the zebrafish embryo. While integrin αv knockdown reduces VANGL2 expression at membrane compartments, it does not affect VANGL2 transcription. Knockdown of integrin β5, but not β1 or β3, also decreases VANGL2 protein levels. Inhibition of protein translation using cycloheximide demonstrates that integrin αv knockdown cells have increased VANGL2 degradation while interference with either proteasome or lysosome function restores VANGL2. We further show integrin activation and stimulation of cell-matrix adhesion using MnCl₂ fails to influence VANGL2. However, MnCl₂ treatment stabilizes VANGL2 protein expression levels in the presence of cycloheximide. In the converse experiment, blockage of integrin-mediated cell-matrix adhesion using a cyclic RGD peptide causes a reduction in VANGL2 protein levels. Together, our findings support a model where integrin αv and cellular interactions with the extracellular matrix are required to maintain VANGL2 protein levels and thus function at the plasma membrane.

Inhibition of the signaling pathway of syndecan-1 by synstatin: A promising anti-integrin inhibitor of angiogenesis and proliferation in HCC in rats

AIM OF WORK

The study was conducted for evaluation of the antitumor activity of SSTN_92-119 against HCC induced by thioacetamide in rats.

METHODS

Sixty male Sprague-Dawley rats were randomized into four equal groups: Control, SSTN_92-119, HCC, and HCC + SSTN_92-119. Liver function tests were measured in serum. Liver homogenate was used for determination of: i) integrinαѴβ3 (ITGαѴβ3), insulin like growth factor-1 receptor (IGF-1R), vascular endothelial growth factor (VEGF), fibroblast growth factor-2 (FGF-2) and alpha-fetoprotein (AFP) levels by ELISA, ii) syndecan-1 (CD-138), IGF-1R and VEGF genes expressions by qRT-PCR, iii) MDA, NO, GSH concentrations and SOD activity. Histopathological and immunohistochemical examination of liver tissue was performed.

RESULTS

SSTN_92-119 decreased HCC-induced elevation in ALT, AST, ALP and GGT activities and reversed HCC-induced reduction in total protein and albumin concentrations significantly. SSTN_92-119 significantly elevated hepatic SOD and GSH and reduced both NO and MDA levels. Protein levels of ITGαѴβ3, IGF-1R, VEGF, FGF-2 and AFP were decreased in HCC- SSTN_92-119 group as well as gene expression of CD-138, IGF-1R and VEGF compared with HCC group.

CONCLUSIONS

SSTN_92-119 down regulates ITGαѴβ3 receptor and subsequently reduces the activation of angiogenic growth factors VEGF and FGF-2. Therefore, SSTN_92-119 is becoming a promising anti-integrin αѴβ3 that inhibits angiogenesis and proliferation in HCC.

The cytoskeleton regulates cell attachment strength

Quantitative information about adhesion strength is a fundamental part of our understanding of cell-extracellular matrix (ECM) interactions. Adhesion assays should measure integrin-ECM bond strength, but reports now suggest that cell components remain behind after exposure to acute force for radial shear assays in the presence of divalent cations that increase integrin-ECM affinity. Here, we show that focal adhesion proteins FAK, paxillin, and vinculin but not the cytoskeletal protein actin remain behind after shear-induced detachment of HT1080 fibrosarcoma cells. Cytoskeletal stabilization increased attachment strength by eightfold, whereas cross-linking integrins to the substrate only caused a 1.5-fold increase. Reducing temperature-only during shear application-also increased attachment strength eightfold, with detachment again occurring between focal adhesion proteins and actin. Detachment at the focal adhesion-cytoskeleton interface was also observed in mouse and human fibroblasts and was ligand-independent, highlighting the ubiquity of this mode of detachment in the presence of divalent cations. These data show that the cytoskeleton and its dynamic coupling to focal adhesions are critically important for cell adhesion in niche with divalent cations.

Neural Cell Chip Based Electrochemical Detection of Nanotoxicity

Development of a rapid, sensitive and cost-effective method for toxicity assessment of commonly used nanoparticles is urgently needed for the sustainable development of nanotechnology. A neural cell with high sensitivity and conductivity has become a potential candidate for a cell chip to investigate toxicity of environmental influences. A neural cell immobilized on a conductive surface has become a potential tool for the assessment of nanotoxicity based on electrochemical methods. The effective electrochemical monitoring largely depends on the adequate attachment of a neural cell on the chip surfaces. Recently, establishment of integrin receptor specific ligand molecules arginine-glycine-aspartic acid (RGD) or its several modifications RGD-Multi Armed Peptide terminated with cysteine (RGD-MAP-C), C(RGD)₄ ensure farm attachment of neural cell on the electrode surfaces either in their two dimensional (dot) or three dimensional (rod or pillar) like nano-scale arrangement. A three dimensional RGD modified electrode surface has been proven to be more suitable for cell adhesion, proliferation, differentiation as well as electrochemical measurement. This review discusses fabrication as well as electrochemical measurements of neural cell chip with particular emphasis on their use for nanotoxicity assessments sequentially since inception to date. Successful monitoring of quantum dot (QD), graphene oxide (GO) and cosmetic compound toxicity using the newly developed neural cell chip were discussed here as a case study. This review recommended that a neural cell chip established on a nanostructured ligand modified conductive surface can be a potential tool for the toxicity assessments of newly developed nanomaterials prior to their use on biology or biomedical technologies.

Scaffold composition affects cytoskeleton organization, cell-matrix interaction and the cellular fate of human mesenchymal stem cells upon chondrogenic differentiation

The stem cell niche, or microenvironment, consists of soluble, matrix, cell and mechanical factors that together determine the cellular fates and/or differentiation patterns of stem cells. Collagen and glycosaminoglycans (GAGs) are important scaffolding materials that can mimic the natural matrix niche. Here, we hypothesize that imposing changes in the scaffold composition or, more specifically, incorporating GAGs into the collagen meshwork, will affect the morphology, cytoskeletal organization and integrin expression profiles, and hence the fate of human mesenchymal stem cells (MSCs) upon the induction of differentiation. Using chondrogenesis as an example, we microencapsulated MSCs in three scaffold systems that had varying matrix compositions: collagen alone (C), aminated collagen (AC) and aminated collagen with GAGs (ACG). We then induced the MSCs to differentiate toward a chondrogenic lineage, after which, we characterized the cell viability and morphology, as well as the level of cytoskeletal organization and the integrin expression profile. We also studied the fate of the MSCs by evaluating the major chondrogenic markers at both the gene and protein level. In C, MSC chondrogenesis was successfully induced and MSCs that spread in the scaffolds had a clear actin cytoskeleton; they expressed integrin α2β1, α5 and αv; promoted sox9 nuclear localization transcription activation; and upregulated the expression of chondrogenic matrix markers. In AC, MSC chondrogenesis was completely inhibited but the scaffold still supported cell survival. The MSCs did not spread and they had no actin cytoskeleton; did not express integrin α2 or αv; they failed to differentiate into chondrogenic lineage cells even on chemical induction; and there was little colocalization or functional interaction between integrin α5 and fibronectin. In ACG, although the MSCs did not express integrin α2, they did express integrin αv and there was strong co-localization and hence functional binding between αv and fibronectin. In addition, vimentin was the dominant cytoskeletal protein in these cells, and the chondrogenic marker genes were expressed but at a much lower level than in the MSCs encapsulated in C alone. This work suggests the importance of controlling the matrix composition as a strategy to manipulate cell-matrix interactions (through changes in the integrin expression profile and cytoskeleton organization), and hence stem cell fates.

Analysis of SAT type foot-and-mouth disease virus capsid proteins and the identification of putative amino acid residues affecting virus stability

Foot-and-mouth disease virus (FMDV) initiates infection by adhering to integrin receptors on target cells, followed by cell entry and disassembly of the virion through acidification within endosomes. Mild heating of the virions also leads to irreversible dissociation into pentamers, a characteristic linked to reduced vaccine efficacy. In this study, the structural stability of intra- and inter-serotype chimeric SAT2 and SAT3 virus particles to various conditions including low pH, mild temperatures or high ionic strength, was compared. Our results demonstrated that while both the SAT2 and SAT3 infectious capsids displayed different sensitivities in a series of low pH buffers, their stability profiles were comparable at high temperatures or high ionic strength conditions. Recombinant vSAT2 and intra-serotype chimeric viruses were used to map the amino acid differences in the capsid proteins of viruses with disparate low pH stabilities. Four His residues at the inter-pentamer interface were identified that change protonation states at pH 6.0. Of these, the H145 of VP3 appears to be involved in interactions with A141 in VP3 and K63 in VP2, and may be involved in orientating H142 of VP3 for interaction at the inter-pentamer interfaces.

Extracellular matrix proteins secreted from both the endometrium and the embryo are required for attachment: a study using a co-culture model of rat blastocysts and Ishikawa cells

Integrins are expressed in a highly regulated manner at the maternal-fetal interface during implantation. However, the significance of extracellular matrix (ECM) ligands during the integrin-mediated embryo attachment to the endometrium is not fully understood. Thus, the distribution of fibronectin in the rat uterus and blastocyst was studied at the time of implantation. Fibronectin was absent in the uterine luminal epithelial cells but was intensely expressed in the trophoblast cells and the inner cell mass suggesting that fibronectin secreted from the blastocyst may be a possible bridging ligand for the integrins expressed at the maternal-fetal interface. An Arg-Gly-Asp (RGD) peptide was used to block the RGD recognition sites on integrins, and the effect on rat blastocyst attachment to Ishikawa cells was examined. There was a significant reduction in blastocyst attachment when either the blastocysts or the Ishikawa cells were pre-incubated with the RGD-blocking peptide. Thus, successful attachment of the embryo to the endometrium requires the interaction of integrins on both the endometrium and the blastocyst with the RGD sequence of ECM ligands, such as fibronectin. Pre-treatment of both blastocysts and Ishikawa cells with the RGD peptide also inhibited blastocyst attachment, but not completely, suggesting that ECM bridging ligands that do not contain the RGD sequence are also involved in embryo attachment.

β(1) and β(3) integrins disassemble from basal focal adhesions and β(3) integrin is later localised to the apical plasma membrane of rat uterine luminal epithelial cells at the time of implantation

The present study investigated the expression of integrin subunits that are known to be associated with focal adhesions, namely β(1) and β(3) integrins in rat uterine luminal epithelial cells during early pregnancy. The β(1) and β(3) integrins were concentrated along the basal cell surface and were colocalised and structurally interacted with talin, a principal focal adhesion protein, on Day 1 of pregnancy. At the time of implantation, β(1) and β(3) integrins disassembled from the site of focal adhesions, facilitating the removal of uterine luminal epithelial cells for embryo invasion. Also at this time, β(3) integrin markedly increased along the apical membrane, suggesting a role in embryo attachment. This distributional change in β(1) and β(3) integrins seen at the time of implantation was predominantly under the influence of progesterone. Taken together, β(1) and β(3) integrin disassembly from focal adhesions and the increase in β(3) integrin apically are key components of hormonally regulated endometrial receptivity.

Integrin β3 in rat blastocysts and epithelial cells is essential for implantation in vitro: studies with Ishikawa cells and small interfering RNA transfection

BACKGROUND

Integrins are involved in the process of embryo-endometrium interaction during implantation. We investigated the localization of integrin β3 in the rat blastocyst and Ishikawa cells using an in vitro co-culture model of implantation.

METHODS

Zona pellucida-free rat blastocysts were co-cultured with the Ishikawa cells (endometrial adenocarcinoma cell line) to observe the attachment between the embryo and endometrium. Immunofluorescence staining was used to investigate the localization of integrin β3 in rat embryos at different stages of development (each n= 3 embryos) and at the embryo/endometrium interface, observed by confocal microscopy. The Ishikawa cells were transfected with integrin β3 small interfering RNA (siRNA) for 48 h and then co-cultured with Day 5 rat blastocysts to observe the effect on attachment.

RESULTS

Integrin β3 staining in the rat embryos increased at the blastocyst stage being highly concentrated in the cytoplasm of trophoblast cells (n= 9 embryos). Integrin β3 was localized on the apical surface of the Ishikawa cells (n= 3 experiments). However, integrin β3 relocated to the apical membrane of trophoblast cells in response to attachment to Ishikawa cells (n= 6 embryos). Moreover, when Ishikawa cells were transfected with integrin β3 siRNA, blastocyst attachment was significantly reduced compared with those transfected with control siRNA (16.7 versus 92.3%, respectively, P< 0.05).

CONCLUSIONS

Integrin β3, localized apically in the blastocyst and the Ishikawa cells, is important during initial attachment of the blastocyst to endometrial cells. This study provides further evidence of the importance of integrins during implantation and may aid in elucidating the molecular mechanism of implantation failure and infertility in women.

Regulation of p53 and cell proliferation by resveratrol and its derivatives in breast cancer cells: an in silico and biochemical approach targeting integrin αvβ3

Resveratrol is a grape polyphenol with cancer preventative activities in tissue culture and animal model studies. Potential of resveratrol as a broad-based chemopreventive agent have been questioned by its limited bioavailability. The bioefficacy of resveratrol was compared with its derivatives, triacetyl-resveratrol (trans-3,5,4'-triacetylstilbene) and trimethoxy-resveratrol (trans-3,5,4'-trimethoxystilbene) in both estrogen receptor-α (ERα)-positive MCF-7 and ERα-negative MDA-MB-231 breast cancer cells. Binding to integrin αvβ3 and control of cell proliferation and p53 were chosen as targets for comparative analysis using an in silico and biochemical approach. Resveratrol and triacetyl-resveratrol interacted avidly and specifically with integrin αvβ3 through binding at the site targeted by the high affinity cyclic Arg-Gly-Asp (RGD) peptide. In contrast, binding of trimethoxy-resveratrol to this site was substantially less robust. Moreover, the different stilbenes also elicited diverse cellular and signaling responses in MCF-7 and MDA-MB-231 cells, as evidenced by analysis of colony formation, cell proliferation, cell cycle phase transition, the extent of phosphorylation of p53 at Ser15 and p53-inducible proteins, p21 and p53R2, respectively. Further, stilbene-elicited signaling cascade leading to p53 activation was examined in MCF-7 cells and results showed that resveratrol and triacetyl-resveratrol induced both ERK and p38 phosphorylation, whereas only marginal changes in state of phosphorylation in these two kinases were observed in trimethoxy-resveratrol-treated cells. Taken together, these results support that resveratrol and triacetyl-resveratrol regulate proliferation and gene expression in breast cancer cells by utilizing largely similar signaling molecules and pathways and cellular events, which appear quite distinct from those targeted by trimethoxy-resveratrol.

Cross-talk between the insulin-like growth factor (IGF) axis and membrane integrins to regulate cell physiology

The biology of cross-talk between activated growth factor receptors and cell-surface integrins is an area which has attracted much interest in recent years (Schwartz and Ginsberg, 2002). This review discusses the relationship between the insulin-like growth factor (IGF) axis and cell-surface integrin receptors in the regulation of various aspects of cell physiology. Key to these interactions are signals transmitted between integrins and the IGF-I receptor (IGF-IR) when either or both are bound to their cognate ligands and we will review the current state of knowledge in this area. The IGF axis comprises many molecular components and we will also discuss the potential role of these species in cross-talk with the integrin receptor. With respect to integrin ligands, we will mainly focus on the well-characterized interactions of the two extracellular matrix (ECM) glycoproteins fibronectin (FN) and vitronectin (VN) with cell-surface ligands, and, how this affects activity through the IGF axis. However, we will also highlight the importance of other integrin activation mechanisms and their impact on IGF activity.

Integral roles of a guanine nucleotide exchange factor, FARP2, in osteoclast podosome rearrangements

Podosomes are recently rediscovered highly dynamic actin-rich structural and functional modules that form close contact with the surrounding substrate. They play a role in the control of migration, tissue invasion, and matrix remodeling of highly motile cells, including lymphocytes, macrophages, dendritic cells, and osteoclasts. In osteoclasts, the compaction of podosomes induces the formation of a tight adhesive contact, the sealing zone, which defines a subosteoclastic environment specialized for bone resorption. Integrins and the Rho family small GTPases are key regulators of podosome rearrangements. However, it remains to be determined how the activation of integrins and Rho family GTPases is regulated during osteoclast podosome rearrangements. Here, we demonstrate a crucial role for the FERM domain-containing guanine nucleotide exchange factor (GEF), FARP2, in osteoclast podosome rearrangements and resorbing activity. We determine by live cell imaging and biochemical assays that FARP2 is required for localized activation of GTP-bound Rac1 into podosome-ring like structures. In addition, FARP2 is relevant to integrin β3 activity during osteoclastogenesis. Furthermore, FARP2 deficiency results in reduced formation of multinucleated osteoclasts and resorption pits compared to wild-type osteoclasts (controls). Collectively, our findings reveal an integral role of FARP2 for regulation of Rac1 and integrin β3 throughout podosome rearrangement in osteoclastogenesis.

Integral roles of a guanine nucleotide exchange factor, FARP2, in osteoclast podosome rearrangements

Podosomes are recently rediscovered highly dynamic actin-rich structural and functional modules that form close contact with the surrounding substrate. They play a role in the control of migration, tissue invasion, and matrix remodeling of highly motile cells, including lymphocytes, macrophages, dendritic cells, and osteoclasts. In osteoclasts, the compaction of podosomes induces the formation of a tight adhesive contact, the sealing zone, which defines a subosteoclastic environment specialized for bone resorption. Integrins and the Rho family small GTPases are key regulators of podosome rearrangements. However, it remains to be determined how the activation of integrins and Rho family GTPases is regulated during osteoclast podosome rearrangements. Here, we demonstrate a crucial role for the FERM domain-containing guanine nucleotide exchange factor (GEF), FARP2, in osteoclast podosome rearrangements and resorbing activity. We determine by live cell imaging and biochemical assays that FARP2 is required for localized activation of GTP-bound Rac1 into podosome-ring like structures. In addition, FARP2 is relevant to integrin β3 activity during osteoclastogenesis. Furthermore, FARP2 deficiency results in reduced formation of multinucleated osteoclasts and resorption pits compared to wild-type osteoclasts (controls). Collectively, our findings reveal an integral role of FARP2 for regulation of Rac1 and integrin β3 throughout podosome rearrangement in osteoclastogenesis.

p21-activated kinase 4 phosphorylation of integrin beta5 Ser-759 and Ser-762 regulates cell migration

Modulation of integrin alphavbeta5 regulates vascular permeability, angiogenesis, and tumor dissemination. In addition, we previously found a role for p21-activated kinase 4 (PAK4) in selective regulation of integrin alphavbeta5-mediated cell motility (Zhang, H., Li, Z., Viklund, E. K., and Strömblad, S. (2002) J. Cell Biol. 158, 1287-1297). This report focuses on the molecular mechanisms of this regulation. We here identified a unique PAK4-binding membrane-proximal integrin beta5-SERS-motif involved in controlling cell attachment and migration. We also mapped the integrin beta5-binding site within PAK4. We found that PAK4 binding to integrin beta5 was not sufficient to promote cell migration, but that PAK4 kinase activity was required for PAK4 promotion of cell motility. Importantly, PAK4 specifically phosphorylated the integrin beta5 subunit at Ser-759 and Ser-762 within the beta5-SERS-motif. Point mutation of these two serine residues abolished the PAK4-induced cell migration, indicating a functional role for these phosphorylations in migration. Our results may give important leads to the functional regulation of integrin alphavbeta5, with implications for vascular permeability, angiogenesis, and cancer dissemination.

Modeling cytoskeletal flow over adhesion sites: competition between stochastic bond dynamics and intracellular relaxation

In migrating cells, retrograde flow of the actin cytoskeleton is related to traction at adhesion sites located at the base of the lamellipodium. The coupling between the moving cytoskeleton and the stationary adhesions is mediated by the continuous association and dissociation of molecular bonds. We introduce a simple model for the competition between the stochastic dynamics of elastic bonds at the moving interface and relaxation within the moving actin cytoskeleton represented by an internal viscous friction coefficient. Using exact stochastic simulations and an analytical mean field theory, we show that the stochastic bond dynamics lead to biphasic friction laws as observed experimentally. At low internal dissipation, stochastic bond dynamics lead to a regime of irregular stick-and-slip motion. High internal dissipation effectively suppresses cooperative effects among bonds and hence stabilizes the adhesion.

Expression order of alpha-v and beta-3 integrin subunits in the N-methyl-N-nitrosourea-induced rat mammary tumor model

We investigated the developmental time course of molecular expression of alpha(v)beta(3) subunits in a carcinogen-induced rat mammary tumor model for human ductal carcinoma in situ (DCIS). Tumors during various stages of growth (from <0.1 to >2.0 cm) were analyzed immunohistochemically for the expression of the alpha(v)beta(3) integrin and its subunits. In general, the expression profiles of these integrin subunits were directly proportional to the size of the tumor. The pattern of immunostaining revealed that anti-alpha(v)beta(3) monoclonal antibody binds to specific sites of tumor sections, forming isolated stained patches. This isolated patch pattern was found in more developed larger tumors. This could be due to the fact that the integrin molecule might be involved in migration and nesting of tumor cells into specific regions to form DCIS or intraductal carcinoma. Results also showed that the alpha(v) subunit expresses earlier than the beta(3) subunit. These data provide insight into tumor cell biology and developmental characteristics that will guide the future construction and use of targeted contrast and therapeutic agents capable of tracking, imaging, or treating a tumor at the earliest stage of formation.

Absence of filamin A prevents cells from responding to stiffness gradients on gels coated with collagen but not fibronectin

Cell types from many tissues respond to changes in substrate stiffness by actively remodeling their cytoskeletons to alter spread area or adhesion strength, and in some cases changing their own stiffness to match that of their substrate. These cell responses to substrate stiffness are linked to substrate-induced changes in the state, localization, and amount of numerous proteins, but detailed evidence for the requirement of specific proteins in these distinct forms of mechanical response are scarce. Here we use microfluidics techniques to produce gels with a gradient of stiffness to show the essential function of filamin A in cell responses to mechanical stimuli and dissociate cell spreading and stiffening by contrasting responses of a pair of human melanoma-derived cell lines that differ in expression of this actin cross-linking protein. M2 melanoma cells null for filamin A do not alter their adherent area in response to increased substrate stiffness when they link to the substrate only through collagen receptors, but change adherent area normally when bound through fibronectin receptors. In contrast, filamin A-replete A7 cells change adherent area on both substrates and respond more strongly to collagen I-coated gels than to fibronectin-coated gels. Strikingly, A7 cells alter their stiffness, as measured by atomic force microscopy, to match the elastic modulus of the substrate immediately adjacent to them on the gradient. M2 cells, in contrast, maintain a constant stiffness on all substrates that is as low as that of A7 cells on the softest gels examined (1000 Pa). Comparison of cell spreading and cell stiffening on the same gradient substrates shows that cell spreading is uncoupled from stiffening. At saturating collagen and fibronectin concentrations, adhesion of M2 cells is reduced compared to that of A7 cells to an extent approximately equal to the difference in adherent area. Filamin A appears to be essential for cell stiffening on collagen, but not for cell spreading on fibronectin. These results have implications for different models of cell protrusion and adhesion and identify a key role for filamin A in altering cellular stiffness that cannot be compensated for by other actin cross-linkers in vivo.

Neisseria meningitidis Opc invasin binds to the cytoskeletal protein alpha-actinin

Neisseria meningitidis Opc protein is an effective invasin for human endothelial cells. We have investigated novel human endothelial receptors targeted by Opc and observed that Opc-expressing bacteria interacted with a 100 kDa protein in whole-cell lysates of human endothelial and epithelial cells. The identity of the protein was established as alpha-actinin by mass spectrometry. Opc expression was essential for the recognition of alpha-actinin whether provided in a purified form or in cell extracts. The interaction of the two proteins did not involve intermediate molecules. As there was no demonstrable expression of alpha-actinin on the surfaces of any of the eight cell lines studied, the likelihood of the interactions after meningococcal internalization was examined. Confocal imaging demonstrated considerable colocalization of N. meningitidis with alpha-actinin especially after a prolonged period of internalization. This may imply that bacteria and alpha-actinin initially occur in separate compartments and co-compartmentalization occurs progressively over the 8 h infection period used. In conclusion, these studies have identified a novel and an intracellular target for the N. meningitidis Opc invasin. Since alpha-actinin is a modulator of a variety of signalling pathways and of cytoskeletal functions, its targeting by Opc may enable bacteria to survive/translocate across endothelial barriers.

Phosphorylation and glycosylation interplay: protein modifications at hydroxy amino acids and prediction of signaling functions of the human beta3 integrin family

Protein functions are determined by their three-dimensional structures and the folded 3-D structure is in turn governed by the primary structure and post-translational modifications the protein undergoes during synthesis and transport. Defining protein functions in vivo in the cellular and extracellular environments is made very difficult in the presence of other molecules. However, the modifications taking place during and after protein folding are determined by the modification potential of amino acids and not by the primary structure or sequence. These post-translational modifications, like phosphorylation and O-linked N-acetylglucosamine (O-GlcNAc) modifications, are dynamic and result in temporary conformational changes that regulate many functions of the protein. Computer-assisted studies can help determining protein functions by assessing the modification potentials of a given protein. Integrins are important membrane receptors involved in bi-directional (outside-in and inside-out) signaling events. The beta3 integrin family, including, alpha(IIb)beta3 and alpha(v)beta3, has been studied for its role in platelet aggregation during clot formation and clot retraction based on hydroxyl group modification by phosphate and GlcNAc on Ser, Thr, or Tyr and their interplay on Ser and Thr in the cytoplasmic domain of the beta3 subunit. An antagonistic role of phosphate and GlcNAc interplay at Thr758 for controlling both inside-out and outside-in signaling events is proposed. Additionally, interplay of GlcNAc and phosphate at Ser752 has been proposed to control activation and inactivation of integrin-associated Src kinases. This study describes the multifunctional behavior of integrins based on their modification potential at hydroxyl groups of amino acids as a source of interplay.

Quantitative measurements of integrin-mediated adhesion to extracellular matrix

Integrin-mediated adhesion is based on the binding of integrins to immobilized ligands either in the extracellular matrix or on the surface of adjacent cells. The strength of adhesion is determined primarily by the number of adhesive bonds that form. Integrins have also been described as signaling receptors. Like adhesion, signals from integrins receptors can depend on the number of integrins that are bound to substrate attached ligands. The common methods for measuring cell adhesion are only capable of measuring initial interactions because the multivalent nature of adhesive bonds when the cell is considered as a unit, these assays reach plateau levels. To measure adhesive integrin-ligand bonds, a spinning disc device is described in which the mean cell detachment force is proportional to the number of adhesive bonds. Particularly as the field has moved from identification of integrins and their ligands into the analysis of adhesion and its relationship to cell signaling, it is important to shift to assays that relate the extracellular binding to the intracellular signals. Specific plans, methods, and analytic strategies are provided to apply this technology to current problems in integrin biology. At present, this is the only published approach that will provide good measures of the number of adhesive bonds that can be generally applied.

AlphaVbeta3 integrin ligands enhance volume-sensitive calcium influx in mechanically stretched osteocytes

We propose that specific osteocyte-matrix interactions regulate the volume-sensitive calcium influx pathway, which we have shown is mediated by stretch-activated cation channels (SA-Cat) and is essential for the stretch-activated anabolic response in bone. The current study measured the hypotonic swelling-induced increase in cytosolic calcium concentration, [Ca(2+)](i), in rat osteocytes, and found that cells adherent to different matrices behave differently. Osteopontin and vitronectin, matrix molecules that bind the alpha(V)beta(3) integrin, induced larger responses to the hypotonic swelling than other matrix molecules that bind other integrins. Addition of echistatin, which is a soluble alpha(V)beta(3) ligand, significantly enhanced the hypotonic [Ca(2+)](i) increase in addition to inducing an immediate increase in [Ca(2+)](i) by itself. These results strongly support the contention that alpha(V)beta(3) integrin signaling in osteocytes interacts with that in mechanotransduction, which is downstream of SA-Cat.

(alpha)v(beta)3 integrins and Pyk2 mediate insulin-like growth factor I activation of Src and mitogen-activated protein kinase in 3T3-L1 cells

IGF-I stimulates cell growth through interaction of the IGF receptor with multiprotein signaling complexes. However, the mechanisms of IGF-I receptor-mediated signaling are not completely understood. We have previously shown that IGF-I-stimulated 3T3-L1 cell proliferation is dependent on Src activation of the ERK-1/2 MAPK pathway. We hypothesized that IGF-I activation of the MAPK pathway is mediated through integrin activation of Src-containing signaling complexes. The disintegrin echistatin decreased IGF-I phosphorylation of Src and MAPK, and blocking antibodies to (alpha)v and beta3 integrin subunits inhibited IGF-I activation of MAPK, suggesting that (alpha)v(beta)3 integrins mediate IGF-I mitogenic signaling. IGF-I increased ligand binding to (alpha)v(beta)3 as detected by immunofluorescent staining of ligand-induced binding site antibody and stimulated phosphorylation of the beta3 subunit, consistent with inside-out activation of (alpha)v(beta)3 integrins. IGF-I increased tyrosine phosphorylation of the focal adhesion kinase (FAK) Pyk2 (calcium-dependent proline-rich tyrosine kinase-2) to a much greater extent than FAK, and increased association of Src with Pyk2 but not FAK. The intracellular calcium chelator BAPTA prevented IGF-I phosphorylation of Pyk2, Src, and MAPK, suggesting that IGF-I activation of Pyk2 is calcium dependent. Transient transfection with a dominant-negative Pyk2, which lacks the autophosphorylation and Src binding site, decreased IGF-I activation of MAPK, but no inhibition was seen with transfected wild-type Pyk2. These results indicate that IGF-I signaling to MAPK is dependent on inside-out activation of (alpha)v(beta)3 integrins and integrin-facilitated multiprotein complex formation involving Pyk2 activation and association with Src.

The syndecan-1 ectodomain regulates alphavbeta3 integrin activity in human mammary carcinoma cells

The α_vβ₃ integrin participates in cell morphogenesis, growth factor signaling, and cell survival. Activation of the integrin is central to these processes and is influenced by specific ECM components, which engage both integrins and syndecans. This paper demonstrates that the α_vβ₃ integrin and syndecan-1 (S1) are functionally coupled. The integrin is dependent on the syndecan to become activated and to mediate signals required for MDA-MB-231 and MDA-MB-435 human mammary carcinoma cell spreading on vitronectin or S1-specific antibody. Coupling of the syndecan to α_vβ₃ requires the S1 ectodomain (ED), as ectopic expression of glycosylphosphatidylinositol-linked S1ED enhances α_vβ₃ recognition of vitronectin; and treatments that target this domain, including competition with recombinant S1ED protein or anti-S1ED antibodies, mutation of the S1ED, or down-regulation of S1 expression by small-interfering RNAs, disrupt α_vβ₃-dependent cell spreading and migration. Thus, S1 is likely to be a critical regulator of many cellular behaviors that depend on activated α_vβ₃ integrins.

Shear stress modulates the interaction of platelet-secreted matrix proteins with tumor cells through the integrin alphavbeta3

Interaction of tumor cells with the vascular wall is required for metastasis from the bloodstream. The precise interaction among metastatic cells, circulating platelets, the vessel wall, and physiological flow conditions remains to be determined. In this study, we investigated the interaction of shear on metastatic cell lines adherent to lipopolysaccharide (LPS)-treated endothelium. Tumor cells were perfused over LPS-treated human umbilical vein endothelial cells (HUVECs) at incremental venous shear rates from 50 to 800 s(-1). At a venous shear rate of 400 s(-1), 3% of adherent tumor cells formed pseudopodia under shear, a process we termed shear-induced activation. Because platelets promote tumor dissemination, we then investigated the effect of pretreating tumor cells with platelet releasate collected from activated platelet concentrate. We found that in the presence of platelet releasate, the number of tumor cells adhering to HUVECs increased and tumor "activation" occurred at a significantly lower shear rate of 50 s(-1). This was inhibited with acetylsalicylic acid. Depletion of fibronectin or vitronectin from the platelet releasate resulted in significantly less adhesion at higher venous shear rates of 600 and 800 s(-1). The integrin alphavbeta3 has been shown to mediate cell adhesion primarily through vitronectin and fibronectin proteins. Inhibition of alphavbeta3, followed by the addition of platelet releasate to the tumor cells, resulted in significantly less adhesion at higher venous shear rates of 600 and 800 s(-1). Collectively, our data suggest that alphavbeta3 promotes the metastatic phenotype of tumor cells through interactions with the secreted platelet proteins vitronectin and fibronectin under venous shear conditions.

A Computational Model of Transmembrane Integrin Clustering

The presented work describes a structural model for integrin homooligomerization, focusing on the transmembrane domains. The two noncovalently linked integrin subunits, alpha and beta, were previously shown to homodimerize or homotrimerize, respectively. Our work is based on published mutational work that induced homotrimerization of beta3 integrins. The mutations provided structural restraints for the creation of a structural model of the beta3 homotrimer by a computational search of the conformational space of homomeric interactions of the beta3 integrin. Additionally, we explored possible conformations of the alphaIIb integrin homodimer, for which no unique solution was found. Two possible models of signal transduction, involving two different alphaIIb conformations, are discussed. One of the possible homodimeric alphaIIb conformations is GpA like, which is in line with experimental evidence. Based on our here-presented structural models and on recent experiments, we will argue that most probably the heteromeric alpha/beta transmembrane complex separates in the course of clustering.

Integrin αvβ3 binds a unique non-RGD site near the C-terminus of human tropoelastin

Tropoelastin is the soluble precursor of the essential resilient connective tissue protein elastin. We examined the binding of integrin alpha(v)beta(3) to tropoelastin. In quantitative colorimetric solid-phase assays, purified alpha(v)beta(3) demonstrated saturable, divalent cation-dependent, single-site binding behavior on tropoelastin with a dissociation constant of 3.8 +/- 0.9 nM in the presence of 1 mM Mn(2+) which increased to 23 +/- 5 nM in the presence of 1 mM Ca(2+). Association with alpha(v)beta(3) was localized to the C-terminal 16 residues of tropoelastin, encompassing the region encoded by exon 36. This region comprises a unique disulfide loop in tropoelastin that is not essential for the interaction. This is the first identification of a specific, single binding site on tropoelastin and the first observation of direct binding of an integrin to a tropoelastin domain.

Talin1 is critical for force-dependent reinforcement of initial integrin-cytoskeleton bonds but not tyrosine kinase activation

Cells rapidly transduce forces exerted on extracellular matrix contacts into tyrosine kinase activation and recruitment of cytoskeletal proteins to reinforce integrin–cytoskeleton connections and initiate adhesion site formation. The relationship between these two processes has not been defined, particularly at the submicrometer level. Using talin1-deficient cells, it appears that talin1 is critical for building early mechanical linkages. Deletion of talin1 blocked laser tweezers, force-dependent reinforcement of submicrometer fibronectin-coated beads and early formation of adhesion sites in response to force, even though Src family kinases, focal adhesion kinase, and spreading were activated normally. Recruitment of vinculin and paxillin to sites of force application also required talin1. FilaminA had a secondary role in strengthening fibronectin–integrin–cytoskeleton connections and no role in stretch-dependent adhesion site assembly. Thus, force-dependent activation of tyrosine kinases is independent of early force-dependent structural changes that require talin1 as part of a critical scaffold.

Effects of thrombin on interactions between beta3-integrins and extracellular matrix in platelets and vascular cells

The beta3-integrin family consists of alphaIIbbeta3 (also known as glycoprotein IIb/IIIa) and alpha(v)beta3. alphaIIbbeta3 is found on platelets and megakaryocytes and has an essential role in hemostasis. alpha(v)beta3 has a broader distribution, and it functions in angiogenesis, neointimal formation after vascular injury, and leukocyte trafficking. There are important interactions between thrombin and beta3-integrins relative to both "inside-out" (integrin activation) and "outside-in" (modification of cellular events by ligand binding to integrins) signaling. Thrombin, by binding to G protein-coupled, protease-activated receptors, is a potent activator of alphaIIbbeta3. Conversely, outside-in signaling through alphaIIbbeta3 amplifies events initiated by thrombin and is necessary for full platelet spreading, platelet aggregation, granule secretion, and the formation of a stable platelet thrombus. In smooth muscle cells, alpha(v)beta3-integrins influence various responses to thrombin, including proliferation, c-Jun NH2-terminal kinase-1 activation, and focal adhesion formation. Other interactions between beta3-integrins and thrombin include beta3-integrin promotion of the generation of thrombin by localizing prothrombin to cellular surfaces and/or enhancing the formation of procoagulant microparticles and the requirement of beta3-integrin function for platelet-dependent clot retraction. In summary, there is increasing evidence that interactions between beta3-integrins and thrombin play important roles in the regulation of hemostatic and vascular functions.

Quantitative analysis of platelet alpha v beta 3 binding to osteopontin using laser tweezers

To determine whether platelet adhesion to surfaces coated with the matrix protein osteopontin requires an agonist-induced increase in the affinity of the integrin alpha v beta 3 for this ligand, we used laser tweezers to measure the rupture force between single alpha v beta 3 molecules on the platelet surface and osteopontin-coated beads. Virtually all platelets stimulated with 10 microM ADP bound strongly to osteopontin, producing rupture forces as great as 100 piconewtons (pN) with a peak at 45-50 pN. By contrast, 90% of unstimulated, resting non-reactive platelets bound weakly to osteopontin, with rupture forces rarely exceeding 30-35 pN. However, approximately 10% of unstimulated platelets, resting reactive platelets, exhibited rupture force distributions similar to stimulated platelets. Moreover, ADP stimulation resulted in a 12-fold increase in the probability of detecting rupture forces >30 pN compared with resting non-reactive platelets. Pre-incubating stimulated platelets with the inhibitory prostaglandin E1, a cyclic RGD peptide, the monoclonal antibody abciximab, or the alpha v beta 3-specific cyclic peptide XJ735 returned force histograms to those of non-reactive platelets. These experiments demonstrate that ADP stimulation increases the strength of the interaction between platelet alpha v beta 3 and osteopontin. Furthermore, they indicate that platelet adhesion to osteopontin-coated surfaces requires an agonist-induced exposure of alpha v beta 3-binding sites for this ligand.

Localization of BCR-ABL to F-actin regulates cell adhesion but does not attenuate CML development

We have previously found that P210BCR-ABL increases the adhesion of hematopoietic cell lines to fibronectin by a mechanism that is independent of tyrosine kinase activity. To investigate the pathway(s) by which P210BCR-ABL influences cell adhesion, we used a quantitative cell adhesion device that can discern small changes in cell adhesion to assay P210BCR-ABL with mutations in several critical domains. We expressed P210BCR-ABL mutants in 32D myeloblast cells and found that binding to fibronectin is mediated primarily by the alpha5beta1 integrin. We performed a structure/function analysis to map domains important for cell adhesion. Increased adhesion was mediated by 3 domains: (1) the N-terminal coiled-coil domain that facilitates oligomerization and F-actin localization; (2) bcr sequences between aa 163 to 210; and (3) F-actin localization through the C-terminal actin-binding domain of c-abl. We compared our adhesion results with the ability of these mutants to cause a chronic myelogenous leukemia (CML)-like disease in a murine bone marrow transplantation assay and found that adhesion to fibronectin did not correlate with the ability of these mutants to cause CML. Together, our results suggest that F-actin localization may play a pivotal role in modulating adhesion but that it is dispensable for the development of CML.

Structural and functional evidence for the interaction of insulin-like growth factors (IGFs) and IGF binding proteins with vitronectin

Previous studies demonstrated that IGF-II binds directly to vitronectin (VN), whereas IGF-I binds poorly. However, binding of VN to integrins has been demonstrated to be essential for a range of IGF-I-stimulated biological effects, including IGF binding protein (IGFBP)-5 production, IGF type-1 receptor autophosphorylation, and cell migration. Thus, we hypothesized that a link between IGF-I and VN must occur and may be mediated through IGFBPs. This was tested using competitive binding assays with VN and (125)iodine-labeled IGFs in the absence and presence of IGFBPs. IGFBP-4, IGFBP-5, and nonglycosylated IGFBP-3 were shown to significantly enhance binding of IGF-I to VN, whereas IGFBP-2 and glycosylated IGFBP-3 had a smaller effect. Furthermore, binding studies with analogs indicate that glycosylation status and the heparin-binding domain of IGFBP-3 are important in this interaction. To examine the functional significance of IGFs binding to VN, cell migration in MCF7 cells was measured and found to be enhanced when VN was prebound to IGF-I in the presence of IGFBP-5. The effect required IGF:IGFBP:VN complex formation; this was demonstrated by use of a non-IGFBP-binding IGF-I analog. Together, these data indicate the importance of IGFBPs in modulating IGF-I binding to VN and that this binding has functional consequences in cells.

Ligand-dependent activation of integrin alpha vbeta 3

The ability of leukocytes to self-regulate adhesion during transendothelial and extravascular migration is fundamental to the performance of immune surveillance in complex extracellular matrices. Leukocyte adhesion is regulated through the modulation of integrin receptors such as alpha(v)beta(3). In this study, we examined the activation of alpha(v)beta(3) resulting from attachment to vitronectin or fibronectin. In K562 cells stably expressing transfected alpha(v)beta(3), adhesion to vitronectin required tyrosine phosphorylation of the beta(3) subunit and activation of phosphoinositide 3-kinase and protein kinase C. In contrast, adhesion to fibronectin proceeded without beta(3)-tyrosine phosphorylation or the activities of phosphoinositide 3-kinase or protein kinase C. Firm adhesion to both ligands and actin stress fiber formation required both Syk and Rho activity, suggesting that each ligand employs unique signaling pathways to achieve an active integrin complex, likely merging at a common RhoGEF such as Vav. Distinct signaling by a single integrin species interacting with different ligands permits initiation of additional cellular processes specific to the current task and provides an explanation for what has been described as promiscuous ligand specificity among integrins.

Integrin alpha V beta 3 as a target for treatment of rheumatoid arthritis and related rheumatic diseases

A substantial and persuasive body of data now exists that supports the view that integrin alpha V beta 3 plays a critical part in activated macrophage dependent inflammation, osteoclast development, migration, and bone resorption, and inflammatory angiogenesis. All of these processes play an important part in the pathogenesis of rheumatoid arthritis (RA) and related arthropathies. Animal arthritis model data further support these concepts and also suggest that therapeutic antagonism of integrin alpha V beta 3 is worthy of further investigation in RA and related arthropathies. To this end, Vitaxin, also known as MEDI-522, has been developed. Vitaxin is a humanised monoclonal IgG1 antibody that specifically binds a conformational epitope formed by both the integrin alpha V and beta 3 subunits. It blocks the interaction of alpha V beta 3 with various ligands such as osteopontin and vitronectin. Clinical trials with Vitaxin in patients with RA are in progress.

BCR-ABL-induced adhesion defects are tyrosine kinase-independent

The t(9;22) chromosomal translocation results in expression of P210(BCR-ABL), a fusion protein necessary for the development of chronic myelogenous leukemia (CML). The constitutive activation of the P210(BCR-ABL) tyrosine kinase results in phosphorylation of multiple signaling pathways leading to the transformed phenotype. Additionally, extracellular interactions between P210(BCR-ABL)-expressing progenitor cells and bone marrow stroma may provide external signals that facilitate CML development. In contrast to the intracellular signaling pathways involved in CML, little is known about how P210(BCR-ABL) expression modifies cell-cell and cell-substratum interactions. To investigate the role of P210(BCR-ABL) in modulating cellular adhesion, we used a highly sensitive and quantitative cell detachment apparatus that measures the strength of association between a population of cells and an adhesive matrix. Our findings show that P210(BCR-ABL) expression increased adhesion nearly 2-fold between the myeloblastic cell line, 32D, and fibronectin compared to a control vector. We then investigated whether abnormal adhesion due to P210(BCR-ABL) expression was caused by its tyrosine kinase activity. A quantitative analysis of cell-fibronectin adhesion found that neither expression of a kinase-inactive P210(BCR-ABL) mutant in 32D cells or attenuation of kinase activity by STI571 (imatinib mesylate) in 32D cells transduced with wild-type P210(BCR-ABL) could correct the nearly 2-fold increase in cell-fibronectin adhesion. Similarly, STI571 treatment of Meg-01 cells, a P210(BCR-ABL)-expressing cell line derived from a patient in blast crisis, failed to inhibit adhesion to fibronectin. Together, our results indicate that changes in adhesion induced by P210(BCR-ABL) are independent of its tyrosine kinase activity.

Processing of integrin alpha(v) subunit by membrane type 1 matrix metalloproteinase stimulates migration of breast carcinoma cells on vitronectin and enhances tyrosine phosphorylation of focal adhesion kinase

Recently, we have shown that membrane type 1 matrix metalloproteinase (MT1-MMP) exhibits integrin convertase activity. Similar to furin-like proprotein convertases, MT1-MMP directly processes a single chain precursor of alpha(v) integrin subunit (pro-alpha(v)) into the heavy and light alpha-chains connected by a disulfide bridge. To evaluate functionality of MT1-MMP-processed integrins, we examined breast carcinoma MCF7 cells co-expressing alpha(v)beta(3) integrin with either the wild type or mutant MT1-MMP in a variety of migration and adhesion tests. Specific inhibitors of proprotein convertases and MMP were employed in our cell system to attenuate the individual pathways of pro-alpha(v) maturation. We present evidence that MT1-MMP cleavage of pro-alpha(v) in the cells did not affect RGD-ligand binding of the resulting alpha(v)beta(3) integrin but enhanced outside-in signal transduction through a focal adhesion kinase pathway. Enhanced tyrosine phosphorylation of focal adhesion kinase in cells co-expressing MT1-MMP and alpha(v)beta(3) integrin contributed to efficient adhesion and, especially, migration of cells on vitronectin, a ligand of alpha(v)beta(3) integrin. These mechanisms underscore the significance of a coordinated interplay between MT1-MMP and alpha(v)beta(3) integrin in tumor cells and identify downstream signaling pathways resulting from their interactions. Regulation of integrin maturation and functionality may be an important role of MT1-MMP in tumor cells.

Phosphorylation of beta3 integrin controls ligand binding strength

The cytoplasmic domain of beta(3) integrin contains tyrosines at positions 747 and 759 in domains that have been implicated in regulation of alpha(v)beta(3) function and that serve as potential substrates for Src family kinases. The phosphorylation level of beta(3) integrin was modulated using a temperature-sensitive v-Src kinase. Increased beta(3) phosphorylation abolished alpha(v)beta(3)- but not alpha(5)beta(1)-mediated adhesion to fibronectin. alpha(v)beta(3)-Mediated cell adhesion was restored by the expression of beta(3) containing Y747F or Y759F mutations but not by wild type beta(3) integrin. Thus, phosphorylation of the cytoplasmic domain of beta(3) is a negative regulator of alpha(v)beta(3)-fibronectin binding strength.