NH2-Terminal Sequences of Mammalian Plasminogens and Plasmin S-Carboxymethyl Heavy (A) and Light (B) Chain Derivatives

Role of Shear Stress and tPA Concentration in the Fibrinolytic Potential of Thrombi

The resolution of arterial thrombi is critically dependent on the endogenous fibrinolytic system. Using well-established and complementary whole blood models, we investigated the endogenous fibrinolytic potential of the tissue-type plasminogen activator (tPA) and the intra-thrombus distribution of fibrinolytic proteins, formed ex vivo under shear. tPA was present at physiologically relevant concentrations and fibrinolysis was monitored using an FITC-labelled fibrinogen tracer. Thrombi were formed from anticoagulated blood using a Chandler Loop and from non-anticoagulated blood perfused over specially-prepared porcine aorta strips under low (212 s⁻¹) and high shear (1690 s⁻¹) conditions in a Badimon Chamber. Plasminogen, tPA and plasminogen activator inhibitor-1 (PAI-1) concentrations were measured by ELISA. The tPA–PAI-1 complex was abundant in Chandler model thrombi serum. In contrast, free tPA was evident in the head of thrombi and correlated with fibrinolytic activity. Badimon thrombi formed under high shear conditions were more resistant to fibrinolysis than those formed at low shear. Plasminogen and tPA concentrations were elevated in thrombi formed at low shear, while PAI-1 concentrations were augmented at high shear rates. In conclusion, tPA primarily localises to the thrombus head in a free and active form. Thrombi formed at high shear incorporate less tPA and plasminogen and increased PAI-1, thereby enhancing resistance to degradation.

uPA-mediated plasminogen activation is enhanced by polyphosphate

Tissue plasminogen activator (tPA) and urokinase (uPA) differ in their modes of action. Efficient tPA-mediated plasminogen activation requires binding to fibrin. In contrast, uPA is fibrin independent and activates plasminogen in solution or when associated with its cellular receptor uPAR. We have previously shown that polyphosphate (polyP), alters fibrin structure and attenuates tPA and plasminogen binding to fibrin, thereby down-regulating fibrinolysis. Here we investigate the impact of polyP on uPA-mediated fibrinolysis. As previously reported polyP of an average chain length of 65 (polyP65) delays tPA-mediated fibrinolysis. The rate of plasmin generation was also delayed and reduced 1.6-fold in polyP65-containing clots (0.74 ± 0.06 vs. 1.17 ± 0.14 pM/s in P < 0.05). Analysis of tPA-mediated fibrinolysis in real-time by confocal microscopy was significantly slower in polyP65-containing clots. In marked contrast, polyP65 augmented the rate of uPA-mediated plasmin generation 4.7-fold (3.96 ± 0.34 vs. 0.84 ± 0.08 pM/s; P < 0.001) and accelerated fibrinolysis (t1/2 64.5 ± 1.7 min vs. 108.2 ± 3.8 min; P < 0.001). Analysis of lysis in real-time confirmed that polyP65 enhanced uPA-mediated fibrinolysis. Varying the plasminogen concentration (0.125 to 1 μM) in clots dose-dependently enhanced uPA-mediated fibrinolysis, while negligible changes were observed on tPA-mediated fibrinolysis. The accelerating effect of polyP65 on uPA-mediated fibrinolysis was overcome by additional plasminogen, while the down-regulation of tPA-mediated lysis and plasmin generation was largely unaffected. PolyP65 exerts opposing effects on tPA- and uPA-mediated fibrinolysis, attenuating the fibrin cofactor function in tPA-mediated plasminogen activation. In contrast, polyP may facilitate the interaction between fibrin-independent uPA and plasminogen thereby accelerating plasmin generation and downstream fibrinolysis.

Biochemical and structural analyses suggest that plasminogen activators coevolved with their cognate protein substrates and inhibitors

Protein sequences of members of the plasminogen activation system are present throughout the entire vertebrate phylum. This important and well-described proteolytic cascade is governed by numerous protease-substrate and protease-inhibitor interactions whose conservation is crucial to maintaining unchanged protein function throughout evolution. The pressure to preserve protein-protein interactions may lead to either co-conservation or covariation of binding interfaces. Here, we combined covariation analysis and structure-based prediction to analyze the binding interfaces of urokinase (uPA):plasminogen activator inhibitor-1 (PAI-1) and uPA:plasminogen complexes. We detected correlated variation between the S3-pocket-lining residues of uPA and the P3 residue of both PAI-1 and plasminogen. These residues are known to form numerous polar interactions in the human uPA:PAI-1 Michaelis complex. To test the effect of mutations that correlate with each other and have occurred during mammalian diversification on protein-protein interactions, we produced uPA, PAI-1, and plasminogen from human and zebrafish to represent mammalian and nonmammalian orthologs. Using single amino acid point substitutions in these proteins, we found that the binding interfaces of uPA:plasminogen and uPA:PAI-1 may have coevolved to maintain tight interactions. Moreover, we conclude that although the interaction areas between protease-substrate and protease-inhibitor are shared, the two interactions are mechanistically different. Compared with a protease cleaving its natural substrate, the interaction between a protease and its inhibitor is more complex and involves a more fine-tuned mechanism. Understanding the effects of evolution on specific protein interactions may help further pharmacological interventions of the plasminogen activation system and other proteolytic systems.

The role of plasminogen-plasmin system in cancer

Components of the plasminogen-plasmin system participate in a wide variety of physiologic and pathologic processes, including tumor growth, invasion and metastasis, through their effect on angiogenesis and cell migration. These components are found in most tumors and their expression not only signifies their function but also carries a prognostic value. Their expression is in turn modulated by cytokines and growth factors, many of which are up-regulated in cancer. Though both tPA and uPA are expressed in tumor cells, uPA with its receptor (uPAR) is mostly involved in cellular functions, while tPA with its receptor Annexin II on endothelial surface, regulates intravascular fibrin deposition. Among the inhibitors of fibrinolysis, PAI-1 is a major player in the pathogenesis of many vascular diseases as well as in cancer. Therapeutic interventions, either using plasminogen activators or experimental inhibitor agents against PAI-1, have shown encouraging results in experimental tumors but not been verified clinically.

Effects of a fucoidan on the activation of plasminogen by u-PA and t-PA

The effect of an anticoagulant fucoidan (C-I-H) from the brown seaweed Ecklonia kurome on the fibrinolytic system was studied in vitro using S-2251 as a substrate of plasmin. C-I-H enhanced the activation of Glu- and Lys-plasminogen by high molecular weight urokinase-type plasminogen activator (HMW u-PA) very effectively, but the activation by low molecular weight u-PA was hardly enhanced with C-I-H. C-I-H also potentiated moderately the activation by single- and two-chain tissue-type plasminogen activators (sct- and tct-PA). These effects of C-I-H were higher than those of heparin used. But C-I-H had no effect on the amidolytic activity of plasmin to S-2251. These results indicate that C-I-H promotes the generation of plasmin in the plasminogen activation by HMW u-PA and t-PA, but not the activity of generated plasmin. Kinetic analyses suggest that C-I-H enhances the HMW u-PA-mediated plasminogen activation by increasing the affinity of the activator for Glu- and Lys-plasminogen and by increasing the molecular activity of the activator. On the other hand, C-I-H had no effect on the affinity of tct-PA for both plasminogens. The catalytic efficiencies of HMW u-PA and tct-PA for the activation of both plasminogens were increased with C-I-H about 8- and 2-fold, respectively. The present results suggest that C-I-H has the fibrinolytic activity by stimulating the plasminogen activation by HMW u-PA and t-PA. The mechanism of the enhancement effect of C-I-H on the activation is presumed to be that C-I-H binds to plasminogen, thereby inducing a structural change of plasminogen susceptible to the action of plasminogen activators.

The plasminogen-plasmin system in malignancy

The study of the plasminogen-plasmin system has, in the past, contributed much to the understanding of fibrinolysis and thrombolysis. Attention is now focused on the role of the components of this system in many biologic functions. Findings of uPA, its receptor and its inhibitor in many tumor tissues and tumor cell lines, strongly implicate their involvement in tumor invasion, tumor cell proliferation and metastasis. The characteristics of the plasminogen activators, the uPA receptor and the plasminogen activator inhibitors as well as their expression and regulation in tumors and tumor cell lines are reviewed.

A spectrophotometric solid-phase fibrin-tissue plasminogen activator activity assay (SOFIA-tPA) for high-fibrin-affinity tissue plasminogen activators

A new spectrophotometric solid-phase fibrin-tissue plasminogen activator activity assay (SOFIA-tPA), specific for the quantitation of tissue plasminogen activators, is described. The method is based on (1) the high-affinity binding (Kp = 1.4 +/- 2 nM) of tPA to a solid-phase fibrin network constructed by thrombin proteolysis of fibrinogen covalently coupled to polyglutaraldehyde-activated polyvinyl chloride microtiter plates, and (2) the subsequent development of PA activity by the fibrin-tPA complex and its measurement with a coupled assay using a chromogenic substrate highly selective for plasmin. Conditions were chosen such that the rate of para-nitroaniline release from the substrate is directly proportional to the concentration of tPA. The support is able to isolate tPA from the bulk of proteins present in any biological fluid allowing the assay to specifically detect tPA activity (range: 0.01 to 50 IU/ml) even in the presence of other activators, proteases, and inhibitors. Since the assay is done in a well-defined reaction mixture (the fibrin-tPA complex, plasminogen, and the synthetic substrate), kinetics studies using pure or crude tPA can be performed. Standard curves (rate measurement and endpoint methods) were made using the international standard (preparation 83/517) for tPA.

Transcriptional induction of urokinase in cultured human kidney carcinoma cells by tetradecanoyl-phorbol-acetate

A cell line derived from a human kidney carcinoma produces in vitro the urinary type of plasminogen activator (urokinase). The synthesis of plasminogen activator is enhanced by 12-O-tetradecanoyl-phorbol-13-acetate(TPA); the increase can be followed both in the cell lysate and in the culture medium. The effect requires RNA and protein synthesis as well as the continuous presence of the inducer. Immunofluorescence and immunoprecipitation experiments with monospecific antiurokinase IgG show that kidney carcinoma cells synthesize a 50,000-dalton urokinase and TPA enhances the synthesis of the same molecular species. Hybridization of the total cellular RNA to a human urokinase cDNA probe shows that TPA increases the urokinase mRNA level.

Comparative activation kinetics of mammalian plasminogens

Five native mammalian plasminogen species, namely, cat, dog, bovine, rabbit and horse, were studied and compared to native human plasminogen with respect to their substrate and enzymatic properties in various activated forms. These studies are an extension of previous work and were designed to confirm our previously proposed mechanism of plasminogen activation, using a series of native, but different, plasminogen substrates. The plasminogen activator species used were high molecular weight urokinase, streptokinase, human Glu-plasminogen-streptokinase complex, human plasmin-derived light(B)-chain-streptokinase complex, and the equimolar streptokinase activator complexes prepared from cat and dog plasmins. The peptidase parameters of the plasmins, plasmin-streptokinase and plasminogen-streptokinase complexes were determined with H-D-valyl-L-leucyl-L-lysyl-p-nitroanilide and Tos-glycyl-L-prolyl-L-lysyl-p-nitroanilide. Activation kinetics were measured with the same substrates. The peptidase parameters of all plasmin species were found to be similar, but with minor variations. The equimolar streptokinase mixtures of bovine, rabbit and horse plasminogens and plasmins did not form complexes and did not form active sites with plasminogen, under the conditions used. The second-order rate constants of activation revealed great differences (as much as 1400-fold), presumably expressing differences in the tertiary structure of the various plasminogen scissile bonds. The catalytic rate constants of activation, kplg, varied by as much as a 100-fold, while differences in Kplg were relatively small. The results of this study confirm the activation mechanism we have postulated previously, namely, that rapid-equilibrium rather than steady-state conditions prevail and that k2 (acylation) is the catalytic rate constant and the rate-determining step, while KS is a true dissociation constant. Calculations of the free energy of interaction of the peptidase and plasminogen activation reactions showed -4.4 to -5.6 kcal/mol for peptidase and -6.5 to -10 kcal/mol for the activation reaction. These values indicate 1-3 subsite binding interactions for the peptidase activity and 3-5 subsite binding interactions for the activation catalytic event. Streptokinase activator complexes have at least one more interacting subsite than the urokinase active site.

Extracellular matrix destruction by invasive tumor cells

The invasion of normal tissues and penetration of basement membranes by malignant cells is likely to require the active participation of hydrolytic enzymes. The four major groups of connective tissue proteins, glycoproteins, proteoglycans, collagen and elastin, vary in their quantitative distributions between different tissues. With the exception of elastin, they also vary qualitatively within each class, so that there are no 'typical' connective tissue barriers to tumor cell penetration. The matrix constituents are stabilized and organized by a variety of covalent and noncovalent interactions between the connective tissue proteins. These interactions play important roles in matrix integrity and may alter the susceptibilities of the constituents to degradative enzymes. It is likely that the complete degradation of the matrix will require the action of more than one enzyme because of differing susceptibilities to tissue proteinases. Primary and transplantable tumors produce well-characterized enzymes which may participate in invasion. These enzymes may also be involved in connective tissue turnover in other normal and pathological situations. The use of long-term tumor cell cultures has verified that tumor cells themselves are capable of producing these enzymes. However, there are many potential modulating influences operative in vivo which are absent in culture so that details of actual mechanisms and control of digestion of complex substrates are not well understood. Recent work on the degradation by tumor cells of extracellular matrices previously produced by cultured cells is likely to shed more light on pathways of tissue destruction in vivo. Experiments with tumor cell variants of defined metastatic potentials will also be useful, but invasive and metastatic abilities are not necessarily correlated. It is unlikely that simple correlations can be drawn between the production of one particular degradative enzyme by all tumor cells and the complex biological mechanisms operative during tumor invasion.

Human low-molecular-weight urinary urokinase. Partial characterization and preliminary sequence data of the two polypeptide chains

Low-molecular-weight urokinase (molecular weight 33100) was separated by analytical and preparative isoelectric focusing into five major subforms with isoelectric points between 8.7 and 9.6. These subforms are very similar in molecular weight, specific activity, amino acid composition and content of amino sugar and their N-terminal sequence constellation is identical. Low-molecular-weight urokinase consists of two polypeptide chains connected by a single disulfide bridge. The N-terminal region of the heavy chain (calculated Mr 30700) exhibits homology within the first 46 residues analyzed, with the known primary structure of other serine proteases. The mini chain (Mr 2426), whose complete sequence was determined, consists of 21 residues which show homology with the primary structure of the C-terminal region of the plasmin heavy chain. Based on sequence data and homology criteria with serine proteases a single-chain urokinase precursor is postulated having a peptide bond constellation between heavy and light chain region compatible with the requirements for serine protease activation.

The binding and processing of plasminogen by Balb/c 3T3 and SV3T3 cells

The binding and processing of plasminogen by Balb/c 3T3 and SV3T3 cells was studied using 125I-labeled canine plasinogen. Throughout a 3-day period, 125I-plasminogen in the incubation medium bound to the cells and was degraded, first to intermediate-sized macromolecules that were the same size as the large (74,600-dalton) and small (25,000-dalton) chains of active plasmin, and to smaller fragments including 3-iodo-L-tyrosine. Binding to SV3T3 cells was independent of the protease-dependent morphological change (PDMC) characteristic of these and many other transformed cells. The SV3T3, and to a somewhat lesser extent, the 3T3 cells, both accumulated and released into the incubation medium 3-iodo-L-tyrosine, a terminal lysozymal digestion product. The results of a sublethal cell-surface trypsinization assay suggest that the cell-associated plasminogen was primarily bound to the surfaces of the 3T3 and SV3T3 cells while the macromolecular degradation products including active plasmin were inside the cells. The rate of 125I plasminogen degradation exhibited by SV3T3 cells was approximately two time greater than that of 3T3 cells, which presumably reflects differences in endocytosis or lysosomal hydrolysis, or both. The rates were unaffected by addition of pancreatic or soybean trypsin inhibitor sufficient to inhibit PDMC. In the incubation medium, plasminogen was activated to plasmin by SV3T3, but not by 3T3 cells. However, 95-100% of plasmin covalently bound to a 47,000-dalton canine serum component, which could be dissociated from plasmin by hydroxylamine: 95-100% of the plasmin was inactive to reaction with DF32P. Thus the serum component is a plasmin inhibitor. The plasmin-containing complex in the medium had an apparent molecular weight of 212,000. Under denaturing conditions, the complex dissociated into two covalently modified plasmin-containing species of 153,000 and 127,000 daltons. In addition to forming a complex with a serum component, the plasmin is cleaved into two small fragments (approximately 10,000 and 12,000 daltons) by as-yet uncharacterized serum factors.

Purified proenzyme C1r. Some characteristics of its activation and subsequent proteolytic cleavage

1. Upon incubation for 1 h at 37 degrees C, proenzymic C1r was activated by a proteolytic cleavage comparable to that observed in vivo; after reduction and alkylation, two fragments of apparent molecular weights 57 000 and 35 000 were evident on sodium dodecyl sulphate (SDS)-polyacrylamide gel electrophoresis. The activation kinetics were slightly sigmoidal and nearly independent of C1r concentration. They were characterized by a marked thermal dependence (activation energy = 45 kcal/mol). The reaction was inhibited by calcium and p-nitrophenyl-p'-guanidinobenzoate, but poorly sensitive to di-isopropyl phosphorofluoridate. The dependence of the activation rate on pH was unusual; it decreased progressively in the acid range (pH 4.5-6.5) which coincides with the dissociation of the C1r-C1r dimer. Above pH 6.5, the rate increased slightly and showed no clear maximum. These results are consistent with an intramolecular autocatalytic activation mechanism involving the pro-site of each subunit of the C1r-C1r dimer. 2. During a 5 h incubation period at 37 degrees C, C1r underwent two proteolytic cleavages which led to the successive removal of two fragments, alpha (35 000) and beta (7000-11 000) from each subunit, leaving a dimeric molecule of reduced size (Mr = 110 000; s20,w = 6.1 S). The proteolytic process was nearly independent of C1r concentration and characterized by a pH optimum at 8.5-9.0, and a high activation energy (36.8 kcal/mol). Calcium and p-nitrophenyl-p'-guanidinobenzoate, and also di-isopropyl phosphorofluoridate and benzamidine were inhibitors of this reaction. The product, C1r II, retained the original antigenic properties of C1r and a functional active site, but lost the capacity to bind C1s. These results are consistent with an autocatalytic intramolecular proteolysis mediated by the active site of each subunit of the C1r-C1r dimer.

Plasminogen activator from cells transformed by an oncogenic virus: inhibitors of the activation reaction

This paper describes an assay for direct measurement of plasminogen activation and its application for determining the kinetic constants and for screening potential inhibitors of the reaction. The assay is based on the conversion of the single chain of 125I-labelled plasminogen to the two chains of 125I-labelled plasmin (EC 3.4.21.7), the latter then being separated from each other and from the plasminogen substrate by electrophoresis under reducing conditions in SDS-polyacrylamide gels. The Km of activator from transformed murine cells for human plasminogen was 180 nM. A broad range of compounds was tested as potential inhibitors of plasminogen activation and of plasmin-catalyzed fibrinolysis respectively, and the two reactions differed qualitatively and quantitatively in their response to previous agents. The principal qualitative difference was in the susceptibility of the reactions to a spectrum of naturally-occurring macromolecular inhibitors: all of the macromolecular inhibitors that blocked the action of plasmin were without effect on murine activator or human urokinase (EC 3.4.99.26). A variety of small molecules inhibited both of the reactions tested, and showed significant quantitative differences; some of these were active at micron concentrations. The exacting specificity of plasminogen activators for macromolecules, both substrates and inhibitors, encourages the expectation that effective inhibitors of great specificity may be isolated from as yet undiscovered natural sources.

Primary structure of the B-chain of human plasmin

The primary structure of the human plasmin B-chain has been determined. It consists of 230 residues divided in three cyanogen bromide fragments: The amino-terminal 24 residues, the carboxy-terminal three residues and the middle 203 residues. Sequence detemination was performed on the tryptic and the chymotryptic peptides obtained from the main cyanogen bromide fragment of this chain. Owing to similarities between some of the overlapping chymotryptic peptides, two different sequences were possible from these results. However, since the homologies with the pancreatic serine proteases and also the B-chains of thrombin and factor XA are pronounced, the arrangement still could be settled. By peptic digestion of partially reduced and S-carboxymethylated B-chain it was shown that there are two interchain disulphide bridges, which connect the A and B-chains of plasmin, involving Cys-5 and Cys-105 from the B-chain. The intrachain disulphides in the B-chain seem to be situated exactly as in chymotrypsin as partly judged from homologies.

Inhibition of urokinase by complex formation with human alpha1-antitrypsin

Human alpha1-antitrypsin was prepared from fresh human plasma by (NH4)-SO4-precipitation, gel filtration, affinity chromatography on concanavalin A, ion exchange chromatography and isotachophoresis. Human urokinase (EC 3.4.99.26) (plasminogen activator from urine) with M, 46 000 and 36 000 was further purified from Urokinase Leo reagent preparation by gel filtration on Sephadex G-100 Superfine. The hydrolytic activity of urokinase on acetyl-glycyl-L-lysine methyl ester acetate (Ac-Gly-Lys-OMeAc) was inhibited in a strong time-dependent manner by alpha1-antitrypsin. Complex formation between enzyme and inhibitor could be demonstrated in crossed immunoelectrophoresis against anti-alpha1-antitrypsin and anti-urokinase serum as well as by sodium dodecyl sulphate polyacrylamide gel electrophoresis. The latter method revealed the formation of 1:1 and 2:1 molar enzyme-inhibitor complexes.

Follicular plasminogen and plasminogen activator and the effect of plasmin on ovarian follicle wall

Plasminogen, plasminogen activator, protease inhibitors, and a proteolytic activity are shown to be present in bovine follicular fluid. Much of the proteolytic activity appears to be due to plasmin. In addition, plasminogen activator activity can be demonstrated in follicle wall homogenates. Evidence that plasmin decreases the tensile strength of follicle wall preparations is also reported. The potential for the involvement of these substances in ovulation is discussed.

Structural relationship between "glutamic acid" and "lysine" forms of human plasminogen and their interaction with the NH2-terminal activation peptide as studied by affinity chromatography

Urokinase digestion of maleinated plasminogen results in cleavage of the single peptide bond Arg-68-Met-69, which is one of the bonds normally cleaved during the first step of the activation procedure. The inactive intermediate compound formed in this way was subjected to NH2-terminal amino acid sequence analysis, which clearly demonstrates the structural relationship between the forms of plasminogen with different NH2-terminal amino acids. It is thus shown that lysine-78 and valine-79 in the "glutamic acid" plasminogen actually are the NH2-terminal amino acids in "lysine" and "valine" plasminogen respectively. The forms with glutamic acid in NH2-terminal position are called plasminogen A, while all other forms lacking the NH2-terminal part of the molecule and which can be activated in a single step are called plasminogen B. By affinity chromatographic studies of the NH2-terminal activation peptide on insolubilized plasminogen B, it was demonstrated that this peptide has specific affinity for plasminogen B. It was also shown that this noncovalent interaction is broken by 6-aminohexanoic acid in two concentration. The tryptic heptapeptide (Ala-Phe-Gln-Tyr-His-Ser-Lys) which occupies the positions number 45 to 51 in the NH2-terminal activation peptide (as well as in the intact plasminogen molecule) is importance for the conformational state of the plasminogen molecule.