Collagen receptors α(1)β(1) and α(2)β(1) integrins are involved in transmigration of peripheral blood eosinophils, but not mononuclear cells through human microvascular endothelial cells monolayer

Asthma development may be driven by T helper lymphocytes with eosinophils playing the role of major effector cells. Recruitment of the inflammatory cells from blood to the airways is mediated by adhesive molecules, e.g. selectins and integrins. The most important in cell trafficking are integrins containing α(4) and β(2) subunits. We hypothesized that also collagen receptors: α(1)β(1) and α(2)β(1), may be involved in cell migration to the inflammatory site in asthma. The aim of the study was to determine whether the inhibition of α(1)β(1) or α(2)β(1) integrins, affects transmigration of eosinophils and peripheral blood mononuclear cells (PBMC) through human microvascular endothelial cells monolayer (HMVEC) seeded on collagen IV coated wells in moderate persistent atopic asthmatics.

METHODS

PBMC from 9 asthmatics were separated by gradient centrifugation followed by negative magnetic separation of eosinophils. Snake venom derived anti-adhesive proteins: viperistatin and VP12 (potent and selective inhibitors of α(1)β(1) and α(2)β(1) integrins, respectively) as well as VLO4 (a non-selective inhibitor of α(4)β(1), α(5)β(1) and α(v)β(3) - used as a positive control), were used for inhibition studies. All anti-adhesive proteins studied, inhibited eosinophils, but only VLO4 affected PBMC transmigration through HMVEC. In bronchial asthma both collagen receptors α(1)β(1) and α(2)β(1) are likely to be involved in eosinophil transmigration to the inflammatory site. The role of α(2)β(1) on lymphocytes is probably different. As the α(2)β(1) integrin has been described as a stimulator of collagen accumulation, it might be, at least in part, responsible for asthma airway remodelling.

Increased expression of collagen receptors: alpha1beta1 and alpha2beta1 integrins on blood eosinophils in bronchial asthma

BACKGROUND

Eosinophils are one of the major effector cells in bronchial asthma. Their infiltration of airways correlates with the asthma severity. Recruitment and activation of eosinophils are partially mediated by integrins alpha4beta1 and alpha4beta7. Collagens type I and IV constitute important components of extracellular matrix and vascular basement membrane, respectively. Therefore, collagen-binding integrins (alpha1beta1 and alpha2beta1) may also play a role in eosinophil lung infiltration.

OBJECTIVE

To evaluate the possible presence of alpha1beta1 and alpha2beta1 integrins on peripheral blood eosinophils from asthmatic subjects.

METHODS

Collagen receptors were studied on eosinophils separated by immunomagnetic CD16-negative method from healthy donors (n=13) and patients with moderate persistent atopic bronchial asthma (n=15). Surface receptor identification was performed by flow cytometry and cell adhesion assay.

RESULTS

Eosinophils isolated from the patients showed increased expression of both alpha1beta1 and alpha2beta1 integrins as compared with healthy controls. Moreover, adhesive function of eosinophils to collagen type IV was inhibited by snake venom disintegrins: viperistatin and obtustatin. These disintegrins contain KTS active motif and are specific inhibitors of alpha1beta1 integrin.

CONCLUSION

We demonstrated for the first time that collagen receptors: alpha1beta1 and alpha2beta1 integrins are overexpressed on the surface of peripheral blood eosinophils of asthmatic subjects. Further studies may reveal potential application of KTS-disintegrins or their structural analogs for therapy of bronchial asthma.

Integrins in pulmonary inflammatory diseases

Integrins are a family of heterodimeric transmembrane glycoproteins that mediate cell-cell and cell-matrix interactions. They participate in inflammatory reactions mainly by regulation of leukocyte migration, activation and survival. Elevated expression of the cell adhesion molecules, such as VCAM, ICAM and MAdCAM on the lumenal surface of vascular endothelial cells is a critical early event in organ inflammatory processes - including the lung. Adhesive interactions with their counter-receptors on leukocytes, selectins and integrins, result in migration of the leukocytes to the inflammed tissues. Integrins also participate in physiological and pathological reorganization of the lung structure during e.g. pneumonia healing, airway remodeling, angiogenesis, emphysema and pulmonary fibrosis. Agents that could inhibit the function of one or more of these integrins could provide a novel therapeutic strategy targeted to inhibit inflammatory and immune phenomena in the lung.

Extracellular matrix molecules regulate endothelial cell migration stimulated by lysophosphatidic acid

BACKGROUND

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are lipids that bind G-protein coupled receptors and differentially promote transmigration of endothelial cells.

OBJECTIVE

To determine if endothelial cell transmigration stimulated by LPA, not S1P, is dependent on the extracellular matrix.

METHODS

Bovine pulmonary artery (BPAE) endothelial cell transmigration and locomotion were measured using a modified-Boyden chamber and video microscopy, respectively. Results were related to strength of adhesion and characteristics of cell adhesive contacts.

RESULTS AND CONCLUSIONS

BPAEs responded to LPA by transmigration through gelatin- or collagen-coated filters, but not through fibronectin-, vitronectin-, or fibrinogen-coated filters. Fewer cells adhered to collagen or gelatin than to fibronectin in a static cell adhesion assay or after application of a g-force to detach cells. Video microscopy revealed that S1P stimulates large lamellipodia on two-dimensional fibronectin substrate. LPA stimulated lamellipodia on fibronectin, but the trailing edge remained attached, resulting in sting ray-shaped cells in video microscopy. LPA-treated cells on gelatin released the trailing edge. To understand how the extracellular matrix may regulate endothelial cell shape during movement, we surveyed changes in focal adhesion proteins. More Hic-5, a paxillin homolog, was detected in the detergent insoluble fraction of BPAEs attached to gelatin than fibronectin. No such difference was found in paxillin. In BPAEs, Hic-5 was localized to smaller punctate structures on fibronectin and longer, thinner focal adhesions on gelatin. These results indicated that localization of Hic-5 and strength of adhesion correlate with endothelial cell transmigration stimulated by LPA, but not with transmigration stimulated by S1P.

Extracellular signal-regulated kinase (ERK) activation is required for GP Ibalpha-dependent endothelial cell migration

The GP Ib complex can participate in endothelial cell (EC) migration on von Willebrand factor (vWF) or the mixed matrix of vWF and type I collagen (vWF/collagen). In this study, viper venom proteins alboaggregin (albo) A or B blocked GP Ibalpha, and echistatin inhibited alphavbeta3 binding. Albo A, B and echistatin inhibited EC migration on vWF and vWF/collagen. Albo B or the anti-GP Ibalpha monoclonal antibody (mAb) 1b1 did not affect the migration of smooth muscle cells or fibroblasts, which lack GP Ib. EC also migrate on albo A- or albo B-coated dishes. PD98059, which blocks ERK activation, abolished EC migration on vWF, vWF/collagen, collagen or albo B. Soluble albo A or 1b1 dramatically inhibited ERK activation during EC migration on vWF or albo B. Echistatin inhibited ERK activation on vWF and vitronectin (VN), but not albo B. Thus, in addition to alphavbeta3, EC GP Ibalpha initiates ERK activation, and regulates ERK-induced EC migration on vWF.

Regional differences in integrin expression: role of alpha(5)beta(1) in regulating smooth muscle cell functions

There is increasing evidence to suggest that coronary smooth muscle cells (SMCs) differ from noncoronary SMCs. As integrin adhesion molecules regulate many SMC functions, we hypothesized that differences in integrin expression on coronary and noncoronary SMCs may account for cellular differences. Analysis of integrin expression on freshly isolated porcine coronary and noncoronary SMCs revealed that coronary SMCs express significantly less alpha(5)beta(1) than noncoronary SMCs, whereas the expression of total beta(1) and that of alpha(v)beta(3) are similar. Consistent with these findings, coronary SMCs demonstrated significantly less adhesion to fibronectin, compared with carotid artery SMCs. As alpha(5)beta(1)-mediated signaling has been associated with cellular proliferation, the effects of differential alpha(5)beta(1) expression on cell proliferation were examined by comparing primary coronary and carotid artery SMC proliferation. Coronary SMC growth was significantly lower than that of carotid artery SMCs when plated on fibronectin or type I collagen. Blocking alpha(5)beta(1) function on carotid artery SMCs produced a significant decrease in cellular proliferation, resulting in growth similar to that of coronary SMCs. Furthermore, blocking alpha(5)beta(1), but not alpha(v)beta(3), inhibited loss of alpha-smooth muscle actin in proliferating SMCs. Proliferating coronary SMCs were found to upregulate alpha(5)beta(1) expression, further indicating a role for alpha(5)beta(1) in SMC growth. These results suggest that dissimilar alpha(5)beta(1) integrin expression may mediate regional differences in phenotype of vascular SMCs.

Inhibitory effects of MLDG-containing heterodimeric disintegrins reveal distinct structural requirements for interaction of the integrin alpha 9beta 1 with VCAM-1, tenascin-C, and osteopontin

The integrin alpha9beta1 is expressed on epithelial cells, smooth muscle cells, skeletal muscle, and neutrophils and recognizes at least three distinct ligands: vascular cell adhesion molecule 1 (VCAM-1), tenascin-C, and osteopontin. The alpha9 subunit is structurally similar to the integrin alpha4 subunit, and alpha9beta1 and alpha4beta1 both recognize VCAM-1 as a ligand. We therefore examined whether the disintegrin EC3, which we have recently shown specifically inhibits the binding of alpha4 integrins to ligands, would also be a functional inhibitor of alpha9beta1. EC3 and a novel heterodimeric disintegrin that we identified, EC6, both were potent inhibitors of alpha9beta1-mediated adhesion to VCAM-1 and of neutrophil migration across tumor necrosis factor-activated endothelial cells. A peptide containing a novel MLDG motif shared by both of these disintegrins also inhibited alpha9beta1- and alpha4beta1-mediated adhesion to VCAM-1. Surprisingly though, concentrations of EC3 that completely inhibited adhesion of alpha9-transfected cells to VCAM-1 had little or no effect on adhesion to either of the other alpha9beta1 ligands, osteopontin and tenascin-C. Furthermore, peptides AEIDGIEL and SVVYGLR, which we have previously shown inhibit binding of alpha9beta1-expressing cells to tenascin-C and osteopontin, respectively, had no effect on adhesion to VCAM-1. These data suggest that there are structurally distinct requirements for interactions of the alpha9beta1 integrin with VCAM-1 and the extracellular matrix ligands osteopontin and tenascin-C.

Isolation and characterization of EMS16, a C-lectin type protein from Echis multisquamatus venom, a potent and selective inhibitor of the alpha2beta1 integrin

We have isolated and characterized EMS16, a potent and selective inhibitor of the alpha2beta1 integrin, from Echis multisquamatus venom. It belongs to the family of C-lectin type of proteins (CLPs), and its amino acid sequence is homologous with other members of this protein family occurring in snake venoms. EMS16 (M(r) approximately 33K) is a heterodimer composed of two distinct subunits linked by S-S bonds. K562 cells transfected with alpha2 integrin selectively adhere to immobilized EMS16, but not to two other snake venom-derived CLPs, echicetin and alboaggregin B. EMS16 inhibits adhesion of alpha2beta1-expressing cells to immobilized collagen I at picomolar concentrations, and the platelet/collagen I interaction in solution at nanomolar concentrations. EMS16 inhibits binding of isolated, recombinant I domain of alpha2 integrin to collagen in an ELISA assay, but not the interaction of isolated I domain of alpha1 integrin with collagen IV. Studies with monoclonal antibodies suggested that EMS16 binds to the alpha2 subunit of the integrin. EMS16 inhibits collagen-induced platelet aggregation, but has no effect on aggregation induced by other agonists such as ADP, thromboxane analogue (U46619), TRAP, or convulxin. EMS16 also inhibits collagen-induced, but not convulxin-induced, platelet cytosolic Ca(2+) mobilization. In addition, EMS16 inhibits HUVEC migration in collagen I gel. In conclusion, we report a new, potent viper venom-derived inhibitor of alpha2beta1 integrin, which does not belong to the disintegrin family.

Disulphide-bond pattern and molecular modelling of the dimeric disintegrin EMF-10, a potent and selective integrin alpha5beta1 antagonist from Eristocophis macmahoni venom

The disulphide-bond pattern of the heterodimeric disintegrin EMF-10, a potent and selective integrin alpha(5)beta(1) antagonist from Eristocophis macmahoni venom, was established by combination of amino-acid analysis, N-terminal sequencing and collision-induced dissociation by nanoelectrospray ionization quadrupole ion-trap MS of fragments isolated by reversed-phase HPLC after degradation of EMF-10 with oxalic acid. Each EMF-10 subunit contains four intrachain disulphide bonds. Two interchain cystine residues join the EMF-10 polypeptides. The intrachain linkages are conserved in monomeric disintegrins. A molecular model of EMF-10 was built using averaged NMR co-ordinates of flavoridin as a template. The active hairpin loops of the EMF-10 subunits occupy opposite locations at the ends of an elongated disulphide-bond ladder. In the EMF-10 model the N-terminal polypeptide of EMF-10B is close to the RGD-loop of the EMF-10A subunit, suggesting that the N-terminal region of the B-subunit could potentially influence the biological activity of the A-subunit.

EC3, a heterodimeric disintegrin from Echis carinatus, inhibits human and murine alpha4 integrin and attenuates lymphocyte infiltration of Langerhans islets in pancreas and salivary glands in nonobese diabetic mice

The venom of Echis carinatus suchoreki contains a monomeric disintegrin echistatin (Mr 5,500 Da) that strongly inhibits alphaIIbbeta3, alphavbeta3, and alpha5beta1 integrins and a heterodimeric disintegrin called EC3 (M(r) 14,762 Da). At nanomolar concentration, EC3 inhibits adhesion of human cell lines expressing alpha4beta1 and alpha4beta7 to immobilized VCAM-1; it has a lower inhibitory effect on alpha5beta1-mediated cell adhesion. In this study, we demonstrated that EC3, in contrast to echistatin, inhibited binding of monoclonal anti-alpha4 and anti-alpha5 antibodies to cells expressing alpha4beta7. In a dose-dependent manner and to the same extent, EC3 inhibited adhesion of Jurkat cells and murine splenic lymphocytes to immobilized VCAM-1, whereas echistatin was not active. EC3 injected intraperitoneally into nonobese diabetic (NOD mice) suppressed development of insulitis and sialoadenitis, whereas echistatin had no significant effect. We propose that the effect of EC3 is mediated, at least, in part, by blocking alpha4beta1 and alpha4beta7 on murine lymphocytes.

Structural requirements of echistatin for the recognition of alpha(v)beta(3) and alpha(5)beta(1) integrins

There are key differences between the amino acid residues of the RGD loops and the C termini of echistatin, a potent antagonist of alpha(IIb)beta(3), alpha(v)beta(3) and alpha(5)beta(1), and eristostatin, a similar disintegrin selectively inhibiting alpha(IIb)beta(3). In order to identify echistatin motifs required for selective recognition of alpha(v)beta(3) and alpha(5)beta(1) integrins, we expressed recombinant echistatin, eristostatin, and 15 hybrid molecules. We tested them for their ability to inhibit adhesion of different cell lines to fibronectin and von Willebrand factor and to express ligand-induced binding site epitope. The results showed that Asp(27) and Met(28) support recognition of both alpha(v)beta(3) and alpha(5)beta(1). Replacement of Met(28) with Asn completely abolished echistatin's ability to recognize each of the integrins, while replacement of Met(28) with Leu selectively decreased echistatin's ability to recognize alpha(5)beta(1) only. Eristostatin in which C-terminal WNG sequence was substituted with HKGPAT exhibited new activity with alpha(5)beta(1), which was 10-20-fold higher than that of wild type eristostatin. A hypothesis is proposed that the C terminus of echistatin interacts with separate sites on beta(1) and beta(3) integrin molecules.

Structural and functional characterization of EMF10, a heterodimeric disintegrin from Eristocophis macmahoni venom that selectively inhibits alpha 5 beta 1 integrin

Alpha5beta1, a major fibronectin receptor, is a widely distributed integrin that is essential for cell growth and organ development. Here, we describe a novel heterodimeric disintegrin named EMF10, isolated from the Eristocophis macmahoni venom, that is an extremely potent and selective inhibitor of alpha5beta1. EMF10 inhibited adhesion of cells expressing alpha5beta1 to fibronectin (IC(50) = 1-4 nM) and caused expression of a ligand-induced binding site (LIBS) on the beta1 subunit of alpha5beta1 integrin. It partially inhibited adhesion of cells expressing alphaIIbbeta3, alphavbeta3, and alpha4beta1 to appropriate ligands only at concentration higher than 500 nM. Guinea pig megakaryocytes expressing alpha5beta1 adhered to immobilized EMF10 and showed extensive spreading and cytoskeletal mobilization. As determined by electrospray mass spectrometry, EMF10 is composed of two species with molecular masses of 14 575 and 14 949 Da, respectively. EMF10 is a heterodimer containing two subunits: EMF10A (Mr 7544 Da) and EMF10B (Mr 7405 and 7032 Da) linked covalently by S-S bonds. Subunit B showed heterogeneity and may be present as EMF10B1 (Mr 7032) and EMF10B2 (Mr 7405). In putative hairpin loops, EMF10A and EMF10B contained CKKGRGDNLNDYC and CWPAMGDWNDDYC motifs, respectively. The reduced and alkylated subunit B of EMF10 inhibited adhesion of K562 cells to fibronectin in a dose-dependent, saturable manner with IC(50) of 3 microM. The synthetic, cyclic CKKGRGDNLNDYC and CWPAMGDWNDDYC peptides expressed their inhibitory activity in the same system with IC(50) of 100 microM. We propose that alpha5beta1 recognition of EMF10 is associated with the MGDW motif located in a putative hairpin loop of the B subunit and that the expression of activity may also depend on the RGDN motif in the subunit A and on the C-termini of both subunits.

EC3, a novel heterodimeric disintegrin from Echis carinatus venom, inhibits alpha4 and alpha5 integrins in an RGD-independent manner

EC3, a heterodimeric disintegrin (Mr = 14,762) isolated from Echis carinatus venom is a potent antagonist of alpha4 integrins. Two subunits called EC3A and EC3B were isolated from reduced and alkylated EC3 by reverse-phase high performance liquid chromatography. Each subunit contained 67 residues, including 10 cysteines, and displayed a high degree of homology to each other and to other disintegrins. EC3 inhibited adhesion of cells expressing alpha4beta1 and alpha4beta7 integrins to natural ligands vascular cell adhesion molecule 1 (VCAM-1) and mucosal addressin cell adhesion molecule 1 (MadCAM-1) with IC50 = 6-30 nM, adhesion of K562 cells (alpha5beta1) to fibronectin with IC50 = 150 nM, and adhesion of alphaIIbbeta3 Chinese hamster ovary cells to fibrinogen with IC50 = 500 nM; it did not inhibit adhesion of alphavbeta3 Chinese hamster ovary cells to vitronectin. Ethylpyridylethylated EC3B inhibited adhesion of Jurkat cells to immobilized VCAM-1 (IC50 = 6 microM), whereas EC3A was inactive in this system. The MLDG motif appeared to be essential for activity of EC3B. Linear MLDG peptide inhibited the adhesion of Jurkat to VCAM-1 in a dose-dependent manner (IC50 = 4 mM), whereas RGDS peptide was not active at the same concentration. MLDG partially inhibited adhesion of K562 cells to fibronectin (5-10 mM) in contrast to RGDS peptide (IC50 = 3 mM), inhibiting completely at 10 mM.

Interaction of leukocytes with platelet microparticles derived from outdated platelet concentrates

Platelet microparticles (PMP) were isolated from outdated platelets by a combination of differential centrifugation and gel filtration, and the concentration of PMP was expressed in the equivalent of GPIIb/IIIa complex measured by captured ELISA. PMP bound to isolated neutrophils and macrophages in a dose-dependent manner, but they did not bind to lymphocytes. Incubation of PMP with neutrophils did not activate these cells as measured by up-regulation of Mac-1, release of human granulocyte elastase, and calcium mobilization. Incubation of PMP with macrophages did not enhance IL-8 production and the oxygen burst but slightly and significantly increased production of MCP-1. After 10 min incubation of PMP with macrophages, an increase of GPIIb/IIIa antigen was observed suggesting that PMP may be endocytosed by macrophages. In conclusion, PMP bind to leukocytes, but, in contrast to activated platelets, do not play a significant role in leukocyte activation.

Viper venom disintegrins and related molecules

The term "disintegrin" was first used in 1990 to describe a group of viper venom-derived, nonenzymatic small proteins that shared numerous structural and functional properties. These proteins, which have been found in a great number of viper species studied since that time possess both a remarkable sequence homology and an equally notable variability in potency and selectivity in their interactions with integrin receptors. The discovery that small disintegrins may actually have been derived from much larger mosaic proteins possessing catalytic activity, and that species other than snakes (both plant and animal) produce proteins containing disintegrin-like domains, has led to much research related to both the proteins themselves and the receptors to which they bind. The purpose of this review is to discuss the literature and the authors' own data on the structure and function of disintegrins and their relevance to the studies on proteins containing disintegrin-like domains, such as hemorrhagins and ADAMs.

Integrins involved in the adhesion of megakaryocytes to fibronectin and fibrinogen

We studied integrins involved in the adhesion of resting and activated megakaryocytes (MK) to fibronectin (FN) and fibrinogen (FGN). Guinea pig MK were isolated and in some experiments were activated by thrombin. MK adhering to FN or FGN coated on coverslips were quantitated by a computerized image analysis program. The binding of soluble human FN to MK was detected by Western blotting. Anti-integrin antibodies, disintegrins, and cyclic RGD peptides were used to identify integrins involved in the adhesion of MK to FN or FGN. Resting MK adhered to coverslips with immobilized FN. The adhesion of MK to FN was primarily inhibited by an anti-alpha5 antibody and EMF-10, a distintegrin highly specific for alpha5 beta1. However, the adhesion of MK to FN was not blocked by agents that inhibit alphaIIb beta3, alphav beta3 or alpha4 beta1. A beta1 activating antibody increased the number of MK bound to FN due to the activation of alpha5 beta1. The binding of soluble FN was also primarily inhibited by agents that block alpha5 beta1. Resting MK did not adhere to FGN. However, MK activated by thrombin did adhere to FGN. This binding was mediated by alphaIIb beta3, because binding was inhibited by bitistatin, a disintegrin, and a cyclic RGD peptide that are known to block this integrin. The binding of thrombin-activated MK to FN was mediated by both alpha5 beta1 and alphaIIb beta3 based on the additive effect of agents that inhibit these integrins. The study indicates that resting MK bind to FN but not to FGN and that alpha5 beta1 is the major integrin involved in the binding of MK to FN. Activated MK bind to FGN primarily by alphaIIb beta3. However, the binding of activated MK to FN is due to both alpha5 beta1 and alphaIIb beta3. The demonstration that alpha5 beta1 and that alphaIIb beta3 are involved in MK adhesion indicates that these integrins may have a role in MK maturation and platelet production.

Measurement of platelet microparticles during cardiopulmonary bypass by means of captured ELISA for GPIIb/IIIa

A captured enzyme-linked immunosorbent assay (ELISA) using the disintegrin, kistrin, is described and used to measure platelet microparticles (PMP) generated during open heart surgery. This ELISA detects 75 ng/ml of glycoprotein IIb/IIIa (GPIIa/IIIa) in solution and is more sensitive and less variable than flow cytometry and radioimmunoassay. By ELISA, mean values of GPIIb/IIIa in PMP are 14.2 +/- 7.9 microg/ml for outdated platelets and 0.28 +/- 0.1 microg/ml in fresh blood from healthy donors. Normal washed platelets (10(8)) contain 8.8 microg of GPIIb/IIIa. In 12 cardiac surgical patients, PMP measured by ELISA significantly increased (p = 0.039) to 0.58 +/- 0.3 microg/ml at the end of cardiopulmonary bypass, but the increase measured by flow cytometry (1207 to 1447 events in PMP gate) was not significant. Neither heparin nor protamine alter PMP. After cardiopulmonary bypass, PMP concentrations return to baseline values before protamine is given. Concentrations of PMP in pericardial blood are greater than in simultaneous perfusate. This ELISA is more sensitive and accurate than alternate methods for measuring PMP and shows the PMP production and rapid clearance during open cardiac surgery.

The disintegrin eristostatin interferes with integrin alpha 4 beta 1 function and with experimental metastasis of human melanoma cells

Peptides containing the integrin recognition sequence, RGD, can inhibit experimental metastasis of mouse melanoma cells, but the integrin(s) affected in these experiments is unknown. Besides "classical" RGD-binding integrins such as alpha 5 beta 1 and alpha v beta 3, RGD has been reported to bind alpha 4 beta 1, and mAbs to alpha 4 beta 1 can inhibit melanoma metastasis. We investigated the mode of action of the disintegrin eristostatin, an RGD-containing peptide isolated from snake venom, in a human melanoma experimental metastasis model. Lung colonization following i.v. injection of MV3 cells in nude mice was strongly inhibited by eristostatin. MV3 cells bound FITC-eristostatin and adhered to eristostatin-coated wells. This adhesion was partially inhibited by a GRGDSP peptide and by alpha 4 mAb. Binding of FITC-eristostatin to Jurkat cells and adhesion of Jurkat (but not K562) cells to eristostatin-coated wells further suggested that eristostatin binds alpha 4 beta 1, even though, again, alpha 4 mAb only partially inhibited adhesion. Expression of alpha 4 beta 1 was enhanced in metastatic melanoma cells compared to normal melanocytes and nonmetastatic melanoma cells. Finally, eristostatin inhibited adhesion of both MV3 and CHO alpha 4 cells to the alpha 4 beta 1-ligand VCAM-1, while adhesion to other ligands via other integrins was not affected. These findings demonstrate that inhibition of melanoma cell metastasis by RGD-containing peptides such as eristostatin, may be due to interference with alpha 4 beta 1-VCAM binding, in addition to inhibition of the classical RGD-binding integrins.

A baboon model for hematologic studies of cardiopulmonary bypass

Objective investigation of new inhibitors of blood protein or cellular systems that are activated during cardiopulmonary bypass (CPB) is impeded by the absence of a satisfactory animal model. Because most baboon hematologic proteins immunologically cross-react with those used for human assays, we developed a robust, reusable baboon model of CPB. Blood samples were obtained from adult baboons at six time intervals before, during, and after 60 minutes of partial CPB at 37 degrees C with peripheral cannulas. Both membrane (n = 7) and bubble oxygenators (n = 7) were investigated. We measured platelet and white blood cell counts; platelet response to adenosine diphosphate and release of beta-thromboglobulin; fibrinopeptide A, prothrombin fragment F1.2, thrombin-antithrombin complex, D-dimer, and plasmin-antiplasmin complex; activated complement (C3b/c and C4b/c); elastase-alpha1 proteinase inhibitor complex; and bleeding times. Adherent glycoprotein IIIa antigen in Triton X-100 washes of the perfusion circuit was also measured. Markers of baboon platelet, complement, and neutrophil activation and thrombosis significantly increased during CPB with bubble oxygenator systems but did not change appreciably in membrane oxygenator circuits. Markers of fibrinolysis, D-dimer, and plasmin-antiplasmin complex did not change with either oxygenator. The baboon model of CPB, when a bubble oxygenator is used, is a robust, reusable animal model for evaluating inhibitors of platelet, complement, and neutrophil activation and thrombosis during and after CPB.

Significance of RGD loop and C-terminal domain of echistatin for recognition of alphaIIb beta3 and alpha(v) beta3 integrins and expression of ligand-induced binding site

Echistatin is a viper venom disintegrin containing RGD loop maintained by disulfide bridges. It binds with a high affinity to alpha(v) beta3 and alphaIIb beta3 and it induces extensive conformational changes in these integrins resulting in expression of ligand-induced binding site (LIBS) epitopes. We investigated the activities of echistatin and its three analogues (R24A, D27W, echistatin 1-41). R24A echistatin did not react with alphaIIb beta3 and alpha(v) beta3 integrins and did not cause LIBS effect. D27W echistatin showed increased binding to alphaIIb beta3 and decreased binding to alpha(v) beta3. This substitution impaired the ability of echistatin to induce LIBS in alpha(v) beta3 integrin. Deletion of nine C-terminal amino acids of echistatin decreased its ability to bind alphaIIb beta3 and inhibit platelet aggregation. Truncated echistatin failed to induce LIBS epitopes on cells transfected with alphaIIb beta3 and alpha(v) beta3 genes. The ability of echistatin 1-41 to compete with binding of vitronectin to immobilized alpha(v) beta3 and monoclonal antibody 7E3 to platelets and to VNRC3 cells was decreased, although this analogue, after immobilization, retained its ability to bind purified alpha(v) beta3. We propose a hypothesis in which echistatin's RGD loop determines selective recognition of alphaIIb beta3 and alpha(v) beta3 integrin, whereas the C-terminal domain supports its binding to resting integrin and significantly contributes to the expression of LIBS epitope and to conformational changes of the receptor, leading to a further increase of the binding affinity of echistatin and of the inhibitory effect.

Tirofiban provides "platelet anesthesia" during cardiopulmonary bypass in baboons

OBJECTIVE

Tirofiban (Aggrastat) is a reversible, nonpeptide inhibitor of platelet glycoprotein II/IIIa receptors. We tested the hypothesis that tirofiban preserves platelet number and function and shortens postoperative bleeding times in baboons after cardiopulmonary bypass.

METHODS

Four groups were studied: control, n = 12; low-dose tirofiban (0.1 microg/kg per minute), n = 7; high-dose tirofiban (0.3 microg/kg per minute), n = 7; and bolus tirofiban (15 microg/kg) followed by 0.1 microg/kg per minute during cardiopulmonary bypass, n = 7. After heparin, animals were perfused for 60 minutes at 50 ml/kg per minute and 37 degrees C with a bubble oxygenator, roller pump, and peripheral cannulation. Hemodynamics, platelet count, platelet aggregation to adenosine diphosphate, and release of beta-thromboglobulin were measured before tirofiban infusion, before heparin, after heparin before bypass, after 5 and 55 minutes of bypass, after protamine, and 60 minutes after protamine. Template bleeding times were measured at the same times except during cardiopulmonary bypass and 120 and 180 minutes after protamine administration. Platelet glycoprotein IIIa antigen was measured in Triton X-100 washes (Sigma Chemical Company) of the perfusion circuit after bypass.

RESULTS

High-dose tirofiban completely prevents platelet loss during cardiopulmonary bypass. beta-Thromboglobulin release and sensitivity to adenosine diphosphate are significantly less than control at the end of bypass in all tirofiban groups. Template bleeding times return to preoperative values in both the low- and high-dose tirofiban groups 180 minutes after protamine administration and are significantly less than control bleeding times at both 120 and 180 minutes after protamine. Surface glycoprotein IIIa antigen does not significantly differ between groups.

CONCLUSION

High-dose tirofiban completely preserves platelet number and improves platelet function during cardiopulmonary bypass in baboons and significantly accelerates restoration of normal template bleeding times after bypass.

The substances of plant origin that inhibit protein biosynthesis

Some plants were used for a long time in folk medicine as sources of anti-tumour remedies. Their effects on protein biosynthesis in vitro have been examined and described. The separate features of the peptide elongation system, isolated from tumoural cells, have been demonstrated. Some elongation factors or ribosomes have been shown to be a target site for the inhibition of protein biosynthesis caused by the substances isolated from various sources. The glycoside and caffeic acid, isolated from Melissa officinalis leaves, inhibited protein biosynthesis by direct influence the elongation factor eEF-2. The activity of this factor was also inhibited by aloin and aloeemodin. Saponin glycoside and its aglycon, isolated from Verbascum thapsiforme flowers, as well as digoxin, emetine and cepheline directly inactivated ribosomes. "Chagi" fraction, isolated from Inonotus obliquus, is responsible for the inhibitory effect caused by the aqueous tannin--less extract from this fungus. The target site for quercetin has been found to be the subunit form EF-1 alpha. It may be supposed that, the plant inhibitors of protein biosynthesis could be utilized for searching specific antitumoural preparations.

Importance of the structure of the RGD-containing loop in the disintegrins echistatin and eristostatin for recognition of alpha IIb beta 3 and alpha v beta 3 integrins

Echistatin and eristostatin are structurally homologous distintegrins which exhibit significant functional differences in interaction with various integrins. We hypothesized that this may reflect differences in the sequences of their RGD loops: 20CKRARGDDMDDYC32 AND 23CRVARGDWNDDYC35, respectively. Mapping of eristostatin peptides obtained by proteolytic digestion suggested that it has the same alignment of S-S bridges as echistatin. Synthetic echistatin D27W resembled eristostatin since it had increased platelet aggregation inhibitory activity, increased potency to block fibrinogen binding to alpha IIb beta 3, and decreased potency to block vitronectin binding to alpha v beta 3 as compared to wild-type echistatin. Since eristostatin and echistatin have a similar pattern of disulfide bridges, we constructed molecular models of eristostatin based on echistatin NMR coordinates. The RGD loops of eristostatin and echistatin D27W were wider than echistatin's due to the placement of tryptophan (rather than aspartic acid) immediately after the RGD sequence. We propose a hypothesis that the width and shape of the RGD loop are important ligand structural features that affect fitting of ligand to the binding pocket of alpha IIb beta 3 and alpha v beta 3.

Immunological characterization of eristostatin and echistatin binding sites on alpha IIb beta 3 and alpha V beta 3 integrins

Two disintegrins with a high degree of amino acid sequence similarity, echistatin and eristostatin, showed a low level of interaction with Chinese hamster ovary (CHO) cells, but they bound to CHO cells transfected with alpha IIb beta 3 genes (A5 cells) and to CHO cells transfected with alpha v beta 3 genes (VNRC3 cells) in a reversible and saturable manner. Scatchard analysis revealed that eristostatin bound to 816000 sites per A5 cell (Kd 28 nM) and to 200000 sites (Kd 14 nM) per VNRC3 cell respectively. However, VNRC3 cells did not bind to immobilized eristostatin. Echistatin bound to 495000 sites (Kd 53 nM) per A5 cell and to 443000 sites (Kd 20 nM) per VNRC3 cell. As determined by flow cytometry, radiobinding assay and adhesion studies, binding of both disintegrins to A5 cells and resting platelets and binding of echistatin to VNRC3 cells resulted in the expression of ligand-induced binding sites (LIBS) on the beta 3 subunit. Eristostatin inhibited, more strongly than echistatin, the binding of three monoclonal antibodies: OPG2 (RGD motif dependent), A2A9 (alpha IIb beta 3 complex dependent) and 7E3 (alpha IIb beta 3 and alpha v beta 3 complex dependent) to A5 cells, to resting and to activated platelets and to purified alpha IIb beta 3. Experiments in which echistatin and eristostatin were used alone or in combination to inhibit the binding of 7E3 and OPG2 antibodies to resting platelets suggested that these two disintegrins bind to different but overlapping sites on alpha IIb beta 3 integrin. Monoclonal antibody LM 609 and echistatin seemed to bind to different sites on alpha v beta 3 integrin. However, echistatin inhibited binding of 7E3 antibody to VNRC3 cells and to purified alpha v beta 3 suggesting that alpha v beta 3 and alpha IIb beta 3 might share the same epitope to which both echistatin and 7E3 bind. Eristostatin had no effect in these systems, providing further evidence that it binds to a different epitope on alpha v beta 3.

One-step affinity purification of recombinant alphavbeta3 integrin from transfected cells

Vitronectin receptor (alphavbeta3 integrin) is present on the surface of many types of cells. We describe a simple, fast, and reliable method of purification of recombinant human alphavbeta3 from Chinese hamster ovary (CHO) cells transfected with alphavbeta3 (VNRC3 cells). The method consists of two steps: lysis of the cells and affinity chromatography of the lysate on a GRGDSPK-Sepharose column. The yield of the procedure was about 79%. The purified receptor migrated as two bands on a silver stained SDS-polyacrylamide gel, corresponding to the alphav and beta3 subunits, and was recognized by monoclonal antibodies directed against alphav and the alphavbeta3 complex, but not by monoclonal antibody specific for the alphaIIbbeta3 complex. This receptor also bound to immobilized vitronectin, von Willebrand factor, and echistatin. However, binding to immobilized fibrinogen was not observed. Purified recombinant alphavbeta3 demonstrated greater immunoreactivity with LM 609, an alphavbeta3 complex-specific monoclonal antibody, than alphavbeta3 purified from placenta. As visualized by SDS-polyacrylamide gel electrophoresis, preparations of placenta-derived alphavbeta3 contained several contaminating proteins that were not present in preparations of recombinant alphavbeta3 purified from the transfected CHO cells.

Disintegrin interaction with alpha V beta 3 integrin on human umbilical vein endothelial cells: expression of ligand-induced binding site on beta 3 subunit

The effect of seven disintegrins (albolabrin, barbourin, bitistatin, echistatin, eristostatin, flavoridin, and kistrin) and the neurotoxin analogue, mambin, on the adhesion of human umbilical vein endothelial cells (HUVEC) to immobilized vitronectin and fibronectin has been studied. Adhesion to vitronectin was significantly inhibited by echistatin, kistrin, flavoridin, and mambin. Echistatin, flavoridin, and kistrin bound with high affinity to immobilized alpha V beta 3 in solid phase assay; other disintegrins bound at a much lower level. Echistatin and flavoridin had a modest inhibitory effect on HUVEC adhesion to fibronectin. HUVEC adhered to disintegrins with a high selectivity toward bitistatin, echistatin, flavoridin, kistrin, and mambin. Adhesion of HUVEC to fibronectin and vitronectin resulted in cell spreading, whereas cells adhering to immobilized echistatin remained globular and cells adhering to kistrin showed abnormal morphology. Echistatin and kistrin potently inhibited the binding of monoclonal antibody (Mab) 7E3, which recognizes the alpha V beta 3 complex, to HUVEC. Echistatin and kistrin also induced the binding to HUVEC of Mab 62, which recognizes the ligand-induced binding site (LIBS) epitope on the beta 3 subunit, enhancing HUVEC binding to immobilized Mab 62. Similar results with both antibodies were obtained in Chinese hamster ovary cells transfected with alpha V beta 3 genes. In conclusion, disintegrin interaction with HUVEC appears to be selectively mediated by alpha V beta 3 receptors, and it results in an expression of LIBS epitope that may play a role in the regulation of ligand-binding affinity and intracellular signaling.

EF-1 alpha is a target site for an inhibitory effect of quercetin in the peptide elongation process

The effect of quercetin (3,3',4',5,7-pentahydroxyflavone) on the polypeptide elongation system isolated from rat liver cells, was investigated. Quercetin inhibited [14C]leucine incorporation into proteins in vitro and the inhibitory effect is being directed towards the elongation factor eEF-1, but not to eEF-2 and ribosomes. Quercetin was found to form a complex with EF-1 alpha, which was inactive in GTP-dependent binding to ribosomes. It can be suggested that quercetin can block the total or the part of the domain of EF-1 alpha structure that is responsible for formation of the ternary complex EF-1 alpha-GTP-[14C]Phe-tRNA and therefore preventing formation of the quaternary complex with ribosomes.

Isolation and properties of the subunit form EF-1C of elongation factor 1 from Guerin epithelioma cells

EF-1C is a component of the aggregate EF-1B, consisting of the subunit forms EF-1A.EF-1C; it was isolated by dissociation of this aggregate in the presence of GTP. The subunit form EF-1C stimulates binding of aminoacyl-tRNA to ribosomes, catalysed by EF-1A, similarly as EF-1 beta gamma which stimulates the activity of EF-1 in other eukaryotic cells. EF-1C in the presence of 6 M urea was separated into two polypeptides. Polypeptide of molecular mass 32,000 Da is responsible for regeneration of the EF-1A.GTP active complex. Thermal sensitivity of EF-1A was much higher than that of EF-1B, thus a protective role of EF-1C in the EF-1A.EF-1C complex is suggested.