Formation, characterization and detection of a ternary complex between S protein, thrombin and antithrombin III in serum

Pregnancy-associated changes in the hemostatic system

Pregnancy, from implantation to parturition, presents unique and profound challenges to a women's hemostatic system. During pregnancy, potentially catastrophic bleeding can occur during implantation and endovascular trophoblast invasion of the maternal spiral arteries. The risk of hemorrhage reaches a peak during the third stage of labor when the placenta is shorn from the decidua basalis exposing 120 spiral arteries largely denuded of their smooth muscle, and thus, their ability to constrict in response to injury. These challenges are met by dramatic changes in the local uterine, and systemic hemostatic systems. The net effect of these changes is to increase the efficiency of clotting and to impair fibrinolysis. Unfortunately, they also lead to an increase in the prevalence of venous thromboembolism, which is otherwise uncommon in reproductive age women.

Activated vitronectin as a target for anticancer therapy with human antibodies

The formation of a provisional extracellular matrix represents an important step during tumor growth and angiogenesis. Proteins that participate in this process become activated and undergo conformational changes that expose biologically active cryptic sites. Activated matrix proteins express epitopes not found on their native counterparts. We hypothesized that these epitopes may have a restricted tissue distribution, rendering them suitable targets for therapeutic human monoclonal antibodies (huMabs). In this study, we exploited phage antibody display technology and subtractive phage selection to generate human monoclonal antibody fragments that discriminate between the activated and native conformation of the extracellular matrix protein vitronectin. One of the selected antibody fragments, scFv VN18, was used to construct a fully human IgG/kappa monoclonal antibody with an affinity of 9.3 nM. In immunohistochemical analysis, scFv and huMab VN18 recognized activated vitronectin in tumor tissues, whereas hardly any activated vitronectin was detectable in normal tissues. Iodine 123-radiolabeled huMabVN18 was shown to target to Rous sarcoma virus-induced tumors in chickens, an animal model in which the epitope for huMab VN18 is exposed during tumor development. Our results establish activated vitronectin as a potential target for tumor therapy in humans.

Heritable coagulopathies in pregnancy

Heritable coagulopathies are leading causes of maternal thromboembolism and are associated with an increased risk of maternal and perinatal morbidity and mortality. The most common of these disorders are antithrombin III deficiency, protein C deficiency, protein S deficiency, activated protein C resistance resulting from the factor V Leiden mutation, elevated prothrombin activity associated with a mutation in the prothrombin gene, and hyperhomocystinemia. The maternal risk of a thromboembolic episode is increased by a factor of eight in the presence of any of these heritable states. In addition, the relative risk for a stillbirth in the presence of one of these disorders is 3.6. These conditions are also associated with intrauterine growth retardation and preeclampsia. Proper management of heritable coagulopathies during pregnancy is essential to reduce the risk of these serious sequelae. Patients with newly diagnosed deep-vein thromboses or pulmonary emboli should be treated with therapeutic levels of unfractionated or low molecular weight heparin, followed by subsequent prophylactic heparin therapy. All patients with a history of thromboembolism before pregnancy or evidence of any of these coagulopathies may be offered prophylactic therapy with low molecular weight heparin. Patients with antithrombin III deficiency should receive full therapeutic heparin therapy for the entire pregnancy, irrespective of their thromboembolic history. Postpartum therapy with either heparin or warfarin is required in all cases.

TARGET AUDIENCE

Obstetricians & Gynecologists, Family Physicians

LEARNING OBJECTIVES

After completion of this article, the reader will be able to describe the various heritable coagulopathies that can complicate pregnancy, to state the potential adverse effects of heritable coagulopathies in pregnancy, and to explain the management of heritable coagulopathies during pregnancy.

Vitronectin binds to the gonococcal adhesin OpaA through a glycosaminoglycan molecular bridge

Several bacterial pathogens including Neisseria gonorrhoeae bind the human serum glycoprotein vitronectin. We aimed at defining the gonococcal receptor for vitronectin. Ligand blots demonstrated that vitronectin bound specifically to the heparin-binding outer-membrane protein OpaA, but that coating OpaA with the sulphated polysaccharide heparin was required for the interaction to occur. Bound vitronectin could be dissociated from OpaA-heparin-vitronectin complexes by the addition of excess heparin, indicating that sulphated polysaccharides provided the main linkage between the two proteins. Binding assays with intact micro-organisms substantiated the requirement of sulphated polysaccharides such as heparin and dextran sulphate for the efficient binding of vitronectin to OpaA+ gonococci. This was underscored by the increased binding of vitronectin to gonococci that had been preincubated with saturating concentrations of dextran sulphate, as opposed to the inhibition of vitronectin binding observed when bacteria were incubated simultaneously with vitronectin and saturating concentrations of dextran sulphate. Binding assays with dextran sulphates of various sizes indicated that vitronectin binding correlated with the size of the polysaccharide rather than with the amount of OpaA produced by the bacteria. The inability of zero-length cross-linking agents to couple vitronectin to OpaA provided further evidence that sulphated polysaccharides formed the linkage between vitronectin and OpaA. Infection experiments demonstrated that proteoglycan-deficient Chinese hamster ovary cells efficiently internalized dextran sulphate/vitronectin-coated gonococci, suggesting that soluble sulphated polysaccharides could substitute for cell surface glycosaminoglycans in the internalization process. On the basis of our results, we propose a novel mechanism of vitronectin binding in which sulphated polysaccharides act as molecular bridges, linking the glycosaminoglycan-binding sites of vitronectin and gonococcal OpaA.

The cluster of basic amino acids in vitronectin contributes to its binding of plasminogen activator inhibitor-1: evidence from thrombin-, elastase- and plasmin-cleaved vitronectins and anti-peptide antibodies

Derivatives of vitronectin obtained by specific cleavage at its cluster of basic amino acids with thrombin, elastase and plasmin are shown to have a decreased ability to bind plasminogen activator inhibitor-1 (PAI-1). The identification and localization of the segment involved in the binding of PAI-1 (Lys348-Arg379) were carried out by purification of these cleaved vitronectins and their subsequent structural characterization (sequence analysis, phosphorylation of Ser378 with cAMP-dependent protein kinase and immunostaining with peptide-specific antibodies), then measurement of the vitronectin-PAI-1 interaction by (a) a two-phase system (ELISA); (b) co-precipitation of the vitronectin-PAI-1 complex out of solution, and (c) analysis of the stereospecific interaction between the active conformation of PAI-1 and a peptide derived from the above-mentioned cluster; this interaction occurs when the peptide is composed of all-l-amino acids but not when it is composed of all-d-amino acids. Our results explain why workers who have used immobilized vitronectin to study this interaction could not have observed the involvement of the cluster of basic amino acids in PAI-1 binding, since the immobilization of vitronectin is shown to render this cluster inaccessible for interaction. We propose that vitronectin binds active PAI-1 by interaction via amino acid residues that originate from distal locations in the N- and C-termini of vitronectin.

Internalization of vitronectin-thrombin-antithrombin complex by endothelial cells leads to deposition of the complex into the subendothelial matrix

Internalization of the ternary vitronectin-thrombin-antithrombin (VN-TAT) complex by human umbilical vein endothelial cells was investigated. Radiolabeled VN-TAT was bound to the cell surface at 4 degrees C, and internalization was initiated by increasing the temperature to 37 degrees C. After 30 min about half of the VN-TAT complex disappeared from the cell surface and accumulated in the subendothelial matrix. Translocation of VN-TAT complex from the luminal to the basolateral side was confirmed by electron microscopic evaluation of cross-sections of endothelial cells incubated with gold-conjugated VN-TAT complex. Furthermore, cells cultured in VN-TAT deficient serum, incubated with purified VN-TAT, and subsequently assayed for fluorescent staining using a monoclonal antibody directed against thrombin-modified antithrombin and a polyclonal antibody against vitronectin showed co-localization of both antibodies in punctates. Punctates were randomly distributed in both the xy and xz plane of endothelial cells as evidenced by confocal laser scanning microscopy. Trichloroacetic acid precipitation and SDS-polyacrylamide gel electrophoresis showed that VN-TAT was not degraded during translocation and inhibition of the microfilament system reduced release of VN-TAT to the matrix, indicating that transcytosis was responsible for translocation. These findings emphasize that VN-TAT complex is taken up by endothelial cells, not only leading to the removal of inactivated thrombin from the circulation but also to deposition of VN into the subendothelial matrix.

Vitronectin diversity in evolution but uniformity in ligand binding and size of the core polypeptide

We isolated vitronectins from the plasma or sera of 14 animal species including mouse and rat by heparin affinity chromatography. They cross-reacted with anti-vitronectin antibody and their amino terminal sequences showed strong homology. They also promoted spreading of BHK cells and were bound to heparin and collagen in the same way. Therefore, these properties appear to be essential for vitronectin function. However, the apparent molecular weights of these vitronectins varied considerable from 59 to 78 kDa in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). In addition, the number of bands also varied from 1 to 3. To search for the uniformity of vitronectin polypeptide, vitronectins were deglycosylated and examined by Ferguson plot analysis. The size of the polypeptide portion of vitronectins was estimated to range from 40 to 57 kDa which was 19-26 kDa smaller than original values. Supposing a possible cleavage site at 5-13 kDa far from the carboxyl terminus, all vitronectin polypeptides were speculated to be synthesized de novo in the size range of 50-57 kDa. Proteins reacting with anti-vitronectin antibody were also detected on the immunoblot of 13 more species including Drosophila and Physarum. Almost all of these vitronectin-like proteins showed marked species-specific variations in their apparent molecular weights from 51 to 96 kDa in SDS-PAGE.

Characterization of the cellular binding domain and the effects of monoclonal antibodies and thrombin inhibitors on the binding and internalization of the antithrombin-III--thrombin complex by cultured cells

Antithrombin III (AT) binds to cultured cells mainly as a complex with thrombin or other serine proteases rather than in its free form. This implies that, upon complex formation, a new determinant appears on the AT molecule which is recognized by the cells. Fragmentation of AT by cyanogen bromide exposes this determinant and an 8-kDa fragment is recognized by cultured cells. The binding of this fragment to cultured cells is inhibited by antithrombin-III-thrombin (AT-T) complex, but not by free AT. The putative cellular binding domain of AT-T is located over amino acid residues 253-314 of AT, in the large loop close to the carboxy-terminus of the molecule. The cell-associated AT-T is internalized and degraded, forming the thrombin-modified AT (ATM). This process is inhibited by lysosomal degradation inhibitors, such as chloroquine or benzamidine. Thus, the appearance of ATM in cells is not a result of its binding to the cell surface, but probably a result of lysosomal degradation of cell-associated AT-T. Another degradation product, with a molecular mass of 43 kDa, appears in cells along with the appearance of ATM. Hirudin, a specific inhibitor of thrombin, inhibits the cellular internalization of AT-T complexes, indicating a possible role for thrombin activity in the internalization process of complexes. Monoclonal antibodies (mAb) against AT, whose epitopes on the AT molecule have been located, affect the binding of AT-T to cultured cells. Two mAb, A10 whose epitope is found outside of the cellular binding site, and B108 whose epitope may overlap this sequence, totally inhibit the binding of the complex to cells. Inhibition of binding results either from steric hindrance or induced changes in the binding site.

Specific interaction of vitronectin with the cell-secreted protease inhibitor glia-derived nexin and its thrombin complex

Interaction of vitronectin with glia-derived nexin (GDN), thrombin, and the complex GDN-thrombin was demonstrated in direct binding assays that indicated the formation of binary and ternary complexes. The concentration of vitronectin necessary to obtain 50% saturation of the immobilized GDN-thrombin complex binding sites (EC50) was about 1 nM. Under similar experimental conditions, the EC50 of vitronectin for the immobilized antithrombin-III-thrombin complex was about fivefold higher. A tight complex was also formed between vitronectin and immobilized GDN (EC50 approximately 1.5 nM) but when vitronectin was immobilized, GDN displayed a reduced affinity for vitronectin (EC50 approximately 10 nM). These results suggest differences between the immobilized and free conformations of GDN and/or vitronectin. In contrast, vitronectin displayed negligible affinity for antithrombin III. Biotinylated GDN was used to characterize further the binding of GDN or the GDN-thrombin complex to vitronectin. The interaction of the biotinylated GDN-thrombin complex with immobilized vitronectin (EC50 approximately 2 nM) was completely blocked by nonbiotinylated complexes of thrombin with either GDN or antithrombin III, whereas free GDN, free thrombin and the GDN-trypsin complex were only weak competitors. Active-site-blocked urokinase and the complex GDN-urokinase also strongly competed for binding of the biotinylated GDN-thrombin complex to vitronectin. Binding of biotinylated GDN to immobilized vitronectin was specific, saturable and was competed with decreasing efficiency by the GDN-thrombin complex, free GDN and free antithrombin III. These interactions between the adhesive component vitronectin and the serine protease inhibitor GDN may relate to localized control of thrombin and/or urokinase action at certain extravascular sites. These results are discussed in terms of binding sites for vitronectin on GDN, thrombin, and the GDN-thrombin complex.

Specific binding of plasminogen to vitronectin. Evidence for a modulatory role of vitronectin on fibrin(ogen)-induced plasmin formation by tissue plasminogen activator

Vitronectin immobilized onto polystyrene microtiter wells was demonstrated to specifically bind plasminogen in a concentration-dependent manner, yielding an estimated KD = 0.4 microM. Heparin only moderately interfered with the vitronectin-plasminogen interaction, whereas high concentrations of 6-amino-hexanoic acid inhibited binding. Utilizing a ligand-blotting procedure in which plasminogen was reacted with proteolytic fragments of vitronectin, transblotted onto nitrocellulose, the plasminogen-binding site of vitronectin was localized to the heparin-binding domain of the adhesive protein. Moreover, vitronectin was found to inhibit in a dose-dependent fashion the fibrin(ogen)-induced activation of plasminogen by tissue plasminogen activator. These results provide the first evidence for a novel vitronectin-mediated control of plasminogen activation potentially relevant for directional clot-lysis and plasmin-dependent proteolysis in extracellular matrices.

Sulfation of two tyrosine-residues in human complement S-protein (vitronectin)

Human S-protein (vitronectin) and hemopexin, two structurally related plasma proteins of similar molecular mass and abundance, were analyzed for tyrosine sulfation. Both proteins were synthesized and secreted by the human hepatoma-derived cell line Hep G2, as shown by immunoprecipitation from the culture medium of [35S]methionine-labelled cells. When Hep G2 cells were labelled with [35S]sulfate, S-protein, but not hemopexin, was found to be sulfated. Half of the [35S]sulfate incorporated into S-protein was recovered as tyrosine sulfate. The stoichiometry of tyrosine sulfation was approximately two mol tyrosine sulfate/mol S-protein. Examination of the S-protein sequence for the presence of the known consensus features for tyrosine sulfation revealed three potential sulfation sites at positions 56, 59 and 401. Tyrosine 56 is the most probable site for stoichiometric sulfation, followed by tyrosine 59 which appears more likely to become sulfated than tyrosine 401. Tyrosines 56 and 59 are located in the anionic region of S-protein which has no homologous counterpart in hemopexin. We discuss the possibility that tyrosine sulfation of the anionic region of S-protein may stabilize the conformation of S-protein in the absence of thrombin-antithrombin III complexes and may play a role in its binding to thrombin-antithrombin III complexes during coagulation.

The role of vitronectin as multifunctional regulator in the hemostatic and immune systems

Vitronectin (= complement S-protein) belongs to the group of structurally and functionally homologous adhesive proteins (fibrinogen, fibronectin, von Willebrand factor) which are essential in the procoagulant phase of the hemostatic system, interacting with platelets and the vessel wall. In addition to a structural motif in vitronectin responsible for this interaction (cell attachment domain) other functional domains in the protein molecule exist that contribute to its multifunctional role as regulator in the immune system (complement) as well as in fibrinolysis. These various activities and the ubiquitous distribution of vitronectin in the organism are discussed with regard to structure-function relationships of the protein molecule. Vitronectin may thus provide a conceptual molecular link between cell adhesion, humoral immune response and the hemostatic system, particularly at the blood-vessel wall interphase.

Conformational lability of vitronectin: induction of an antigenic change by alpha-thrombin-serpin complexes and by proteolytically modified thrombin

We previously showed that the alpha-thrombin-antithrombin III complex causes antigenic change in vitronectin as monitored by the monoclonal anti-vitronectin antibody 8E6 (Tomasini & Mosher, 1988). We have extended these studies to other protease-serpin complexes and to gamma-thrombin, a proteolytic derivative of alpha-thrombin. In the presence of heparin, recognition of vitronectin by 8E6 was increased 64- or 52-fold by interaction with the complex of alpha-thrombin and heparin cofactor II or the Pittsburgh mutant (Met358----Arg) of alpha 1-protease inhibitor, respectively. This was comparable to the value obtained with the alpha-thrombin-antithrombin III complex. Factor Xa-serpin complexes were approximately 4-fold less effective than the corresponding thrombin complexes. alpha-Thrombin-serpin complexes but not Xa-serpin complexes formed disulfide-bonded complexes with vitronectin. Antigenic changes and disulfide-bonded complexes were not detected when trypsin- or chymotrypsin-serpin complexes were incubated with vitronectin. gamma-Thrombin caused 7- and 34-fold increases in recognition of vitronectin by MaVN 8E6 in the absence and presence of heparin, respectively. In contrast, alpha-thrombin by itself had no effect. The antigenic change induced by gamma-thrombin was maximal when gamma-thrombin and vitronectin were equimolar, was not dependent on cleavage of vitronectin, and was abolished by inhibition of gamma-thrombin with Phe-Pro-Arg-chloromethyl ketone but not with diisopropyl fluorophosphate. These data indicate that alpha-thrombin is the component in alpha-thrombin-serpin complexes that induces the antigenic change in vitronectin, probably via a region that is preferentially exposed in gamma-thrombin.

Identification of the beta-endorphin-binding subunit of the SC5b-9 complement complex: S protein exhibits specific beta-endorphin-binding sites upon complex formation with complement proteins

Beta-Endorphin has been reported to specifically interact with SC5b-9 complement complexes via non-opioid binding sites. Covalent cross-linking of [125I]beta H-endorphin to SC5b-9 and analysis of the cross-linking products by gel electrophoresis and subsequent autoradiography revealed a single specifically labelled species which was identical with the S protein subunit of the complement complex. In contrast to SC5b-9, no cross-linking of labelled beta-endorphin to subunits of C5b-9(m) could be observed, indicating that beta-endorphin binding to SC5b-9 was mediated exclusively via S protein. Beta-Endorphin binding to SC5b-9 was compared with binding to purified S protein. Whereas beta-endorphin binding to purified S protein was only modest, complex formation of S protein with complement proteins led to a strong increase in beta-endorphin-binding site concentration, compatible with the exposure of primarily cryptic beta-endorphin-binding sites on S protein.

SC5b-7, SC5b-8 and SC5b-9 complexes of complement: ultrastructure and localization of the S-protein (vitronectin) within the macromolecules

Purified terminal components of the complement system were used together with purified S-protein, the inhibitor of the membrane attack complex, to generate the soluble complexes SC5b-7, SC5b-8 and SC5b-9. These complexes were purified by ultracentrifugation in sucrose density gradients with 50-70% yield, exhibiting sedimentation coefficients of 20 S, 21 S and 23 S, respectively. In Ouchterlony double-diffusion analysis, the purified complexes gave a line of identity against all antisera of the precursor components indicating that complex formation had occurred. The identity of the complexes was also revealed by the appearance of all subunit components after polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Since the inhibitor function of S-protein in the terminal complement cascade should also be manifested in the morphology of the macromolecules generated, the ultrastructures of the three complexes were analyzed by electron microscopy. In contrast to aggregated (C5b-7)n and (C5b-8)n, negatively stained SC5b-7 and SC5b-8 imaged mostly as monomeric irregularly shaped cylindrical structures, whereas SC5b-9 less than 27 S) appeared as wedge-shaped structure lacking the tubular polymerized C9. (All three complexes were also generated in the presence of biotinyl-S-protein and labeled with avidin-gold conjugates as electron-dense marker). Analysis of the modified complexes in electron micrographs demonstrated that the complexes were marked exclusively at one site of their ultrastructures, suggesting this region to be the location of S-protein and the critical site for membrane binding of C5b-7 or C5b-8 and for initiation of C9 polymerization. These results support recent findings in which the function of S-protein as complement inhibitor was dependent on conformational changes of the protein molecule with concomitant exposure of the heparin-binding domain.

S protein/vitronectin in chronic liver diseases: correlations with serum cholinesterase, coagulation factor X and complement component C3

S protein/vitronectin plays an important role as a regulatory component in the terminal steps of the complement- and coagulation cascades. In patients suffering from chronic liver diseases, plasma S protein concentration was measured and compared with changes in serum cholinesterase activity, coagulation factor X activity and complement component C3 concentration. Significant decreases of all these proteins were seen in liver cirrhosis. Changes in S protein concentration correlated closely with those of cholinesterase, factor X and complement C3. The data give support for the liver as the main organ of plasma S protein/vitronectin synthesis.

Specific binding of the human S protein (vitronectin) to streptococci, Staphylococcus aureus, and Escherichia coli

Specific binding of the 125I-labeled human S protein (vitronectin) which has been shown to be identical with serum-spreading factor, was observed with group A, C, and G streptococci as well as with Staphylococcus aureus and Escherichia coli. The specific binding of S protein to group A, C, and G streptococci was high, whereas the binding to S. aureus and E. coli cultures was moderate. In contrast, group B streptococci and a number of other bacterial species tested did not interact with S protein. The binding of S protein to bacteria was saturable and could be inhibited only by unlabeled S protein but not by albumin. Trypsinization and heat treatment of bacteria destroyed the S-protein binding capacity for group G streptococci, S. aureus, and E. coli but not for group A and C streptococci. Likewise, unlabeled human fibronectin and heparin inhibited the binding of labeled S protein to group G streptococci, S. aureus, and E. coli, but did not influence the binding to group A and C streptococci. Double-reciprocal plots of S-protein binding to group G streptococci indicated that fibronectin inhibited the binding in a competitive manner, while heparin acts in a noncompetitive manner. Moreover, the binding of S protein to G streptococci could be partially by the synthetic peptide Gly-Arg-Gly-Asp-Ser, which contains the cell attachment site of S protein. Trypsin-treated S protein had similar binding activity as untreated S protein for group G streptococci, S. aureus, and E. coli, but showed reduced binding to group A and C streptococci. The present data are indicative of two different types of bacterial binding sites in S protein. The binding to group G streptococci, S. aureus, and E. coli is mediated in part through a domain in the S protein containing the sequence Arg-Gly-Asp, whereas a different site is responsible for the binding to group A and C streptococci.