Activation of plasminogen by pro-urokinase. I. Mechanism

Fibrin and Fibrinolytic Enzyme Cascade in Thrombosis: Unravelling the Role

Blood clot formation in blood vessels (thrombosis) is a major cause of life-threatening cardiovascular diseases. These clots are formed by αA-, βB-, and ϒ-peptide chains of fibrinogen joined together by isopeptide bonds with the help of blood coagulation factor XIIIa. These clot structures are altered by various factors such as thrombin, platelets, transglutaminase, DNA, histones, and red blood cells. Various factors are used to dissolve the blood clot, such as anticoagulant agents, antiplatelets drugs, fibrinolytic enzymes, and surgical operations. Fibrinolytic enzymes are produced by microorganisms (bacteria, fungi, etc.): streptokinase of Streptococcus hemolyticus, nattokinase of Bacillus subtilis YF 38, bafibrinase of Bacillus sp. AS-S20-I, longolytin of Arthrobotrys longa, versiase of Aspergillus versicolor ZLH-1, etc. They act as a thrombolytic agent by either enhancing the production of plasminogen activators (tissue or urokinase types), which convert inactive plasminogen to active plasmin, or acting as plasmin-like proteins themselves, forming fibrin degradation products which cause normal blood flow again in blood vessels. Fibrinolytic enzymes may be classified in two groups, as serine proteases and metalloproteases, based on their catalytic properties, consisting of a catalytic triad responsible for their fibrinolytic activity having different physiochemical properties (such as molecular weight, pH, and temperature). The analysis of fibrinolysis helps to detect hyperfibrinolysis (menorrhagia, renal failure, etc.) and hypofibrinolysis (diabetes, obesity, etc.) with the help of various fibrinolytic assays such as a fibrin plate assay, fibrin microplate assay, the viscoelastic method, etc. These fibrinolytic activities serve as a key aspect in the recognition of numerous cardiovascular diseases and can be easily produced on a large scale with a short generation time by microbes and are less expensive.

Urokinase-type plasminogen activator promotes N-cadherin-mediated synaptic recovery in the ischemic brain

Urokinase-type plasminogen activator (uPA) is a serine proteinase that catalyzes the generation of plasmin on the cell surface and activates cell signaling pathways that promote remodeling and repair. Neuronal cadherin (NCAD) is a transmembrane protein that in the mature brain mediates the formation of synaptic contacts in the II/III and V cortical layers. Our studies show that uPA is preferentially found in the II/III and V cortical laminae of the gyrencephalic cortex of the non-human primate. Furthermore, we found that in murine cerebral cortical neurons and induced pluripotent stem cell (iPSC)-derived neurons prepared from healthy human donors, most of this uPA is associated with pre-synaptic vesicles. Our in vivo experiments revealed that in both, the gyrencephalic cortex of the non-human primate and the lissecephalic murine brain, cerebral ischemia decreases the number of intact synaptic contacts and the expression of uPA and NCAD in a band of tissue surrounding the necrotic core. Additionally, our in vitro data show that uPA induces the synthesis of NCAD in cerebral cortical neurons, and in line with these observations, intravenous treatment with recombinant uPA three hours after the onset of cerebral ischemia induces NCAD-mediated repair of synaptic contacts in the area surrounding the necrotic core.

Urokinase-type plasminogen activator is a modulator of synaptic plasticity in the central nervous system: implications for neurorepair in the ischemic brain

The last two decades have witnessed a rapid decrease in mortality due to acute cerebral ischemia that paradoxically has led to a rapid increase in the number of patients that survive an acute ischemic stroke with various degrees of disability. Unfortunately, the lack of an effective therapeutic strategy to promote neurological recovery among stroke survivors has led to a rapidly growing population of disabled patients. Thus, understanding the mechanisms of neurorepair in the ischemic brain is a priority with wide scientific, social and economic implications. Cerebral ischemia has a harmful effect on synaptic structure associated with the development of functional impairment. In agreement with these observations, experimental evidence indicates that synaptic repair underlies the recovery of neurological function following an ischemic stroke. Furthermore, it has become evident that synaptic plasticity is crucial not only during development and learning, but also for synaptic repair after an ischemic insult. The plasminogen activating system is assembled by a cascade of enzymes and their inhibitors initially thought to be solely involved in the generation of plasmin. However, recent work has shown that in the brain this system has an important function regulating the development of synaptic plasticity via mechanisms that not always require plasmin generation. Urokinase-type plasminogen activator (uPA) is a serine proteinase and one of the plasminogen activators, that upon binding to its receptor (uPAR) not only catalyzes the conversion of plasminogen into plasmin on the cell surface, but also activates cell signaling pathways that promote cell migration, proliferation and survival. The role of uPA is the brain is not fully understood. However, it has been reported while uPA and uPAR are abundantly found in the developing central nervous system, in the mature brain their expression is restricted to a limited group of cells. Remarkably, following an ischemic injury to the mature brain the expression of uPA and uPAR increases to levels comparable to those observed during development. More specifically, neurons release uPA during the recovery phase from an ischemic injury, and astrocytes, axonal boutons and dendritic spines recruit uPAR to their plasma membrane. Here we will review recent evidence indicating that binding of uPA to uPAR promotes the repair of synapses damaged by an ischemic injury, with the resultant recovery of neurological function. Furthermore, we will discuss data indicating that treatment with recombinant uPA is a potential therapeutic strategy to promote neurological recovery among ischemic stroke survivors.

Urokinase-type plasminogen activator (uPA) regulates the expression and function of growth-associated protein 43 (GAP-43) in the synapse

Growth-associated protein 43 (GAP-43) plays a central role in the formation of presynaptic terminals, synaptic plasticity, and axonal growth and regeneration. During development, GAP-43 is found in axonal extensions of most neurons. In contrast, in the mature brain, its expression is restricted to a few presynaptic terminals and scattered axonal growth cones. Urokinase-type plasminogen activator (uPA) is a serine proteinase that, upon binding to its receptor (uPAR), catalyzes the conversion of plasminogen into plasmin and activates signaling pathways that promote cell migration, proliferation, and survival. In the developing brain, uPA induces neuritogenesis and neuronal migration. In contrast, the expression and function of uPA in the mature brain are poorly understood. However, recent evidence reveals that different forms of injury induce release of uPA and expression of uPAR in neurons and that uPA/uPAR binding triggers axonal growth and synapse formation. Here we show that binding of uPA to uPAR induces not only the mobilization of GAP-43 from the axonal shaft to the presynaptic terminal but also its activation in the axonal bouton by PKC-induced calcium-dependent phosphorylation at Ser-41 (pGAP-43). We found that this effect requires open presynaptic N-methyl-d-aspartate receptors but not plasmin generation. Furthermore, our work reveals that, following its activation by uPA/uPAR binding, pGAP-43 colocalizes with presynaptic vesicles and triggers their mobilization to the synaptic release site. Together, these data reveal a novel role of uPA as an activator of the synaptic vesicle cycle in cerebral cortical neurons via its ability to induce presynaptic recruitment and activation of GAP-43.

Streptococcus uberis plasminogen activator (SUPA) activates human plasminogen through novel species-specific and fibrin-targeted mechanisms

Bacterial plasminogen (Pg) activators generate plasmin to degrade fibrin blood clots and other proteins that modulate the pathogenesis of infection, yet despite strong homology between mammalian Pgs, the activity of bacterial Pg activators is thought to be restricted to the Pg of their host mammalian species. Thus, we found that Streptococcus uberis Pg activator (SUPA), isolated from a Streptococcus species that infects cows but not humans, robustly activated bovine but not human Pg in purified systems and in plasma. Consistent with this, SUPA formed a higher avidity complex (118-fold) with bovine Pg than with human Pg and non-proteolytically activated bovine but not human Pg. Surprisingly, however, the presence of human fibrin overrides the species-restricted action of SUPA. First, human fibrin enhanced the binding avidity of SUPA for human Pg by 4-8-fold in the presence and absence of chloride ion (a negative regulator). Second, although SUPA did not protect plasmin from inactivation by α(2)-antiplasmin, fibrin did protect human plasmin, which formed a 31-fold higher avidity complex with SUPA than Pg. Third, fibrin significantly enhanced Pg activation by reducing the K(m) (4-fold) and improving the catalytic efficiency of the SUPA complex (6-fold). Taken together, these data suggest that indirect molecular interactions may override the species-restricted activity of bacterial Pg activators; this may affect the pathogenesis of infections or may be exploited to facilitate the design of new blood clot-dissolving drugs.

Small-molecule modulators of zymogen activation in the fibrinolytic and coagulation systems

The coagulation and fibrinolytic systems are central to the hemostatic mechanism, which works promptly on vascular injury and tissue damage. The rapid response is generated by specific molecular interactions between components in these systems. Thus, the regulation mechanism of the systems is programmed in each component, as exemplified by the elegant processes in zymogen activation. This review describes recently identified small molecules that modulate the activation of zymogens in the fibrinolytic and coagulation systems.

Activation of single-chain urokinase-type plasminogen activator by platelet-associated plasminogen: a mechanism for stimulation of fibrinolysis by platelets

BACKGROUND AND OBJECTIVE

Platelets are essential for hemostasis, and they cause resistance to fibrinolysis by tissue-type plasminogen activator. In contrast, platelets enhance fibrinolysis mediated by single-chain urokinase-type plasminogen activator (scu-PA). This study investigated the mechanism behind this profibrinolytic role of platelets.

METHODS AND RESULTS

Platelets enhanced scu-PA activity, but not urokinase-type plasminogen activator (u-PA) activity, in plasma clot lysis and chromogenic assays. We established, using the non-cleavable scu-PA mutant (Lys158-->Glu) and protease inhibitors, that platelets increased activation to u-PA by a serine protease. Activation of scu-PA was platelet-dependent, even in plasma. It occurred in platelet-rich but not in platelet-poor plasma, as assessed by sodium dodecylsulfate polyacrylamide gel electrophoresis and zymography after addition of plasminogen activator inhibitor-1. Candidate proteases that are known to activate scu-PA and are present in platelet preparations were investigated. Factor VII activating protease was detected in platelet preparations by western blotting, but its inhibition by antibodies did not inhibit activation of scu-PA by platelets. Plasmin and plasma kallikrein both mimicked the platelet effect, but were distinguished by their responses to a range of inhibitors. Analysis of platelet-associated protease activity and the time course of scu-PA activation pointed towards plasminogen, and the data were consistent with a mechanism of reciprocal activation. The essential role of plasminogen was revealed using platelets from plasminogen-deficient mice, which could not activate scu-PA. Local plasminogen on platelet membranes was markedly more effective than solution-phase plasminogen in activation of scu-PA.

CONCLUSIONS

Platelets enhance fibrinolysis by scu-PA through reciprocal activation of scu-PA and platelet-associated plasminogen, a system that is potentially important in the lysis of platelet-rich thrombi.

Polyamine-promoted autoactivation of plasma hyaluronan-binding protein

BACKGROUND

Plasma hyaluronan-binding protein (PHBP), a protease implicated in extracellular proteolysis, consists of multiple domains: an N-terminal region (NTR), three epidermal growth factor (EGF)-like domains, a kringle domain, and a protease domain. PHBP circulates as a single-chain proenzyme (pro-PHBP), which is converted to an active, two-chain form through autoproteolysis.

OBJECTIVE

To understand the mechanism of autoactivation. Here, we report that polyamine induces the formation of pro-PHBP autoactivation complex, in which an intermolecular interaction between NTR and the third EGF-like domain (E3) plays a role.

METHODS

Using a series of pro-PHBP mutants that partially lack functional domains, polyamine-induced pro-PHBP autoactivation was investigated in terms of enzyme activity, protein interaction, and inhibition by carminic acid, an anthraquinone compound identified in this study.

RESULTS

Polyamine enhanced intermolecular binding of pro-PHBP, but not of mutant pro-PHBP that partially lacked NTR (DeltaN). Carminic acid inhibited intermolecular pro-PHBP binding and specifically abolished polyamine-induced autoactivation. NTR bound to pro-PHBP and DeltaN, but its binding was minimal to a mutant that lacked E3. The NTR-DeltaN binding was inhibited by a combination of polyamine and carminic acid, but each compound alone was ineffective.

CONCLUSIONS

We infer from the data that (i) polyamine modulates intramolecular NTR-E3 interaction to allow intermolecular binding between NTR and E3 in another pro-PHBP molecule to form an autoactivation complex, and (ii) carminic acid inhibits polyamine-modulated intermolecular NTR-E3 binding. Polyamine concentrations are higher in cells and tissues with inflammation and malignancy. Polyamine leakage from legions through cell death or tissue injury may account for physiologically relevant pro-PHBP activation.

Plasminogen-mediated activation and release of hepatocyte growth factor from extracellular matrix

Interventions that enhance plasminogen activation within the lung consistently limit the fibrosis that follows alveolar injury. However, this protective effect cannot be attributed solely to accelerated clearance of fibrin that forms as a provisional matrix after lung injury. To explore other mechanisms, we considered interactions between the plasminogen activation system and hepatocyte growth factor (HGF). HGF is known to have antifibrotic activity, but to do so, it must be both released from its sites of sequestration within extracellular matrix (ECM) and activated by proteolytic cleavage. A recent study using bleomycin-exposed mice showed that manipulations of the plasminogen activation system influenced the amount of free HGF within bronchoalveolar lavage fluid without affecting total lung HGF mRNA or protein. To elucidate the mechanisms, we studied the role of plasminogen activation in fibroblast-mediated HGF release and activation. We found that NIH3T3 and mouse lung fibroblasts release ECM-bound HGF in a plasminogen-dependent fashion. The plasminogen effect was lost when lung fibroblasts from urokinase-type plasminogen activator (uPA)-deficient mice were used, and was increased by fibroblasts from plasminogen activator inhibitor (PAI)-1-deficient mice. Plasminogen addition to NIH3T3 or mouse lung fibroblasts increased conversion of pro-HGF to its active form. The plasminogen effect on activation was lost when uPA-deficient fibroblasts were used and accentuated by PAI-1-deficient fibroblasts. In conjunction with the previous in vivo study, these results suggest that plasminogen activation can protect the lung against fibrosis by increasing the availability of active HGF.

Molecular mechanisms of fibrinolysis

The molecular mechanisms that finely co-ordinate fibrin formation and fibrinolysis are now well defined. The structure and function of all major fibrinolytic proteins, which include serine proteases, their inhibitors, activators and receptors, have been characterized. Measurements of real time, dynamic molecular interactions during fibrinolysis of whole blood clots can now be carried out in vitro. The development of gene-targeted mice deficient in one or more fibrinolytic protein(s) has demonstrated expected and unexpected roles for these proteins in both intravascular and extravascular settings. In addition, genetic analysis of human deficiency syndromes has revealed specific mutations that result in human disorders that are reflective of either fibrinolytic deficiency or excess. Elucidation of the fine control of fibrinolysis under different physiological and pathological haemostatic states will undoubtedly lead to novel therapeutic interventions. Here, we review the fundamental features of intravascular plasmin generation, and consider the major clinical syndromes resulting from abnormalities in fibrinolysis.

Enhanced plasminogen activation by staphylokinase in the presence of streptokinase β/βγ domains: Plasminogen kringles play a role

Presence of isolated beta or betagamma domains of streptokinase (SK) increased the catalytic activity of staphylokinase (SAK)-plasmin (Pm) complex up to 60%. In contrast, fusion of SK beta or betagamma domains with the C-terminal end of SAK drastically reduced the catalytic activity of the activator complex. The enhancement effect mediated by beta or betagamma domain on Pg activator activity of SAK-Pm complex was reduced greatly (45%) in the presence of isolated kringles of Pg, whereas, kringles did not change cofactor activity of SAK fusion proteins (carrying beta or betagamma domains) significantly. When catalytic activity of SAK-microPm (catalytic domain of Pm lacking kringle domains) complex was examined in the presence of isolated beta and betagamma domains, no enhancement effect on Pg activation was observed, whereas, enzyme complex formed between microplasmin and SAK fusion proteins (SAKbeta and SAKbetagamma) displayed 50-70% reduction in their catalytic activity. The present study, thus, suggests that the exogenously present beta and betagamma interact with Pg/Pm via kringle domains and elevate catalytic activity of SAK-Pm activator complex resulting in enhanced substrate Pg activation. Fusion of beta or betagamma domains with SAK might alter these intermolecular interactions resulting in attenuated functional activity of SAK.

Expression of an antibody-targeted plasminogen activator in Escherichia coli using chemically induced stress responses

A recombinant gene coding for an antibody-targeted urokinase-type plasminogen activator was constructed for the purpose of enhancing the thrombolytic specificity of urokinase. The recombinant gene was cloned into prokaryotic expression vector pTrcHisA, and transformed into Escherichia coli strain Rosetta (DE3). Less than 4 mg of the desired protein l(-1) could be obtained in the form of inclusion bodies. Of various inducers and enhancers of stress responses, the heat-shock enhances, streptomycin, the osmotic stress inducers, D-arabinose and sucrose, and the cold-shock enhancer, tetracycline, simulated the expression of the antibody-targeted plasminogen activator by 2-5-fold.

Regulation of the single-chain urokinase-urokinase receptor complex activity by plasminogen and fibrin: novel mechanism of fibrin specificity

Activation of plasminogen by urokinase plasminogen activator (uPA) plays important roles in several physiologic and pathologic conditions. Cells secrete uPA as a single-chain molecule (scuPA). scuPA can be activated by proteolytic cleavage to a 2-chain enzyme (tcuPA). scuPA is also activated when it binds to its receptor (uPAR). The mechanism by which the enzymatic activity of the scuPA/suPAR complex is regulated is only partially understood. We now report that the plasminogen activator activity of the scuPA/suPAR complex is inhibited by Glu- and Lys-plasminogen, but not by mini-plasminogen. In contrast, neither Glunor Lys-plasminogen inhibits the activation of plasminogen by 2-chain uPA. Inhibition of scuPA/suPAR activity was evident at a Glu-plasminogen concentration of approximately 100 nM, and at physiologic plasma concentrations inhibition was nearly complete. A plasminogen fragment containing kringles 1-3 inhibited the enzymatic activity of scuPA/suPAR with an inhibition constant (Ki) equal to 1.9 microM, increased the Michaelis constant (Km) of scuPA/suPAR from 18 nM to 49 nM, and decreased the catalytic constant (Kcat) approximately 3-fold from 0.035 sec(-1) to 0.011 sec(-1). Inhibition of scuPA/suPAR by plasminogen was completely abolished in the presence of fibrin clots. These studies provide insight into the regulation of uPA-mediated plasminogen activation and identify a novel mechanism for its fibrin specificity.

Glucosyldiacylglycerol enhances reciprocal activation of prourokinase and plasminogen

Reciprocal activation of prourokinase (pro-u-PA) and plasminogen is an important mechanism in the initiation and propagation of local fibrinolytic activity. We found that glucosyldiacylglycerol (GDG) enhanced the reciprocal activation by 1.5- to 2-fold at 0.7-16 microM, accompanying increased conversions of both zymogens to active two-chain forms. The reciprocal activation system consists of (i) plasminogen activation by pro-u-PA to form plasmin, (ii) pro-u-PA activation by the resulting plasmin to form two-chain u-PA (tcu-PA), and (iii) plasminogen activation by the resulting tcu-PA. Whereas GDG minimally affected steps (ii) and (iii) in isolated systems, it markedly enhanced step (i) in the absence of the conversion of pro-u-PA to tcu-PA. GDG significantly increased the intrinsic fluorescence of pro-u-PA (6.7%), but not that of tcu-PA or plasminogen. The large change in intrinsic fluorescence suggests that GDG selectively affects pro-u-PA to alter its conformation, and this mechanism may account for enhancement of its intrinsic plasminogen activator activity.

The role of recombinant lysine-plasminogen and recombinant urokinase and sulfur hexafluoride combination in inducing posterior vitreous detachment

PURPOSE

To determine the optimal method of generating plasmin in vitreous using recombinant lysine-plasminogen and recombinant urokinase and to determine its efficacy in inducing posterior vitreous detachment when combined with sulfur hexafluoride.

METHODS

Plasmin concentration of the rabbit vitreous after separate and combined intravitreal administrations of recombinant lysine-plasminogen and recombinant urokinase was tested in 78 rabbits to determine the optimal method of administration. The safety and efficacy of these agents and sulfur hexafluoride in inducing complete posterior vitreous detachment (total separation of the vitreous apart from vitreous base) were also evaluated.

RESULTS

The highest plasmin concentration in vitreous was measured 10 minutes after injection. Intravitreal administration of recombinant lysine-plasminogen and recombinant urokinase did not cause any toxicity findings up to concentrations of 100 microg and 200 IU, respectively, on funduscopy, electroretinography, and histopathologic studies. When combined with sulfur hexafluoride injection, separate intravitreal administrations of 75 microg/0.1 mL of recombinant lysine-plasminogen and 15 IU/0.1 mL of recombinant urokinase induced complete posterior vitreous detachment in 75% of the eyes compared with 13% in eyes that received sulfur hexafluoride injection combined with balanced salt solution, recombinant lysine-plasminogen, or recombinant urokinase.

CONCLUSIONS

Plasmin was effectively generated in the vitreous after separate intravitreal administrations of recombinant lysine-plasminogen and recombinant urokinase. When combined with intravitreal gas injection, this method of plasmin production induced complete posterior vitreous detachment in 75% of the eyes.

Construction and characterization of a recombinant plasminogen activator composed of an anti-fibrin single-chain antibody and low-molecular-weight urokinase

BACKGROUND

Targeting of plasminogen activators to the fibrin component of a thrombus by antibodies directed against human fibrin can enhance their thrombolytic potency and clot specificity.

OBJECTIVES

To overcome the disadvantages of chemical conjugation, we investigated whether the recombinant fusion of a single-chain antibody and a plasminogen activator results in an active bifunctional molecule that might be useful as a therapeutic agent.

METHODS

The cDNA of low-molecular-weight single-chain urokinase-type plasminogen activator, comprising amino acids Leu144-Leu411 (scuPA(LMW)), was cloned from human endothelial cells and fused to a single-chain antibody specific for the 7 N-terminal amino acids (beta(15-22)) in the beta-chain of human fibrin (scFv(59D8)). The fusion protein was purified using affinity chromatography with the beta(15-22)-peptide of human fibrin.

RESULTS

Purified scFv(59D8)-scuPA(LMW) migrated as a 60-kDa band, which is consistent with a molecule composed of one scFv(59D8) and one scuPA(LMW) moiety. Both functions of the fusion molecule, fibrin-specific binding and plasminogen activation, were fully preserved. In human plasma clots, thrombolysis by scFv(59D8)-scuPA(LMW) is significantly faster and more potent compared with the clinically used urokinase.

CONCLUSIONS

ScFv(59D8)-scuPA(LMW) constitutes a new recombinant chimeric plasminogen activator with a significantly enhanced thrombolytic potency and relative fibrin selectivity, that can be produced with modern methods at low cost, large quantities and reproducible activity in Escherichia coli.

Enhancement of plasminogen activation by surfactin C: augmentation of fibrinolysis in vitro and in vivo

The reciprocal activation of plasminogen and prourokinase (pro-u-PA) is an important mechanism in the initiation and propagation of local fibrinolytic activity. We have found that a bacterial lipopeptide compound, surfactin C (3-20 microM), enhances the activation of pro-u-PA in the presence of plasminogen. This effect accompanied increased conversions of both pro-u-PA and plasminogen to their two-chain forms. Surfactin C also elevated the rate of plasminogen activation by two-chain urokinase (tcu-PA) while not affecting plasmin-catalyzed pro-u-PA activation and amidolytic activities of tcu-PA and plasmin. The intrinsic fluorescence of plasminogen was increased, and molecular elution time of plasminogen in size-exclusion chromatography was shortened in the presence of surfactin C. These results suggested that surfactin C induced a relaxation of plasminogen conformation, thus leading to enhancement of u-PA-catalyzed plasminogen activation, which in turn caused feedback pro-u-PA activation. Surfactin C was active in enhancing [125I]fibrin degradation both by pro-u-PA/plasminogen and tcu-PA/plasminogen systems. In a rat pulmonary embolism model, surfactin C (1 mg/kg, i.v.) elevated 125I plasma clot lysis when injected in combination with pro-u-PA. The present results provide first evidence that pharmacological relaxation of plasminogen conformation leads to enhanced fibrinolysis in vivo.

Characterization of hK4 (prostase), a prostate-specific serine protease: activation of the precursor of prostate specific antigen (pro-PSA) and single-chain urokinase-type plasminogen activator and degradation of prostatic acid phosphatase

hK4 (prostase, KLK4), a recently cloned prostate-specific serine protease and a member of the tissue kallikrein family, is a zymogen composed of 228 amino acid residues including an amino-terminal propiece, Ser-Cys-Ser-Gln-. A chimeric form of hK4 (ch-hK4) was constructed in which the propiece of hK4 was replaced by that of prostate-specific antigen (PSA) to create an activation site susceptible to trypsin-type proteases. ch-hK4 was expressed in Escherichia coli, isolated from inclusion bodies, refolded, and purified with an overall yield of 25%. The zymogen was readily self-activated during the refolding process to generate an active form (21 kDa) of hK4 (rhK4). rhK4 cleaved the chromogenic substrates Val-Leu-Arg-pNA (S-2266), Pro-Phe-Arg-pNA (S-2302), Ile-Glu-Gly-Arg-pNA (S-2222), and Val-Leu-Lys-pNA (S-2251), indicating that rhK4 has a trypsin-type substrate specificity. The rhK4 was inhibited by aprotinin (6 kDa), forming an equimolar 27 kDa complex. rhK4 readily activated both the precursor of PSA (pro-PSA) and single chain urokinase-type plasminogen activator (scuPA, pro-uPA). rhK4 also completely degraded prostatic acid phosphatase but failed to cleave serum albumin, another protein purified from human seminal plasma. These results indicate that hK4 may have a role in the physiologic processing of seminal plasma proteins such as pro-PSA, as well as in the pathogenesis of prostate cancer through its activation of pro-uPA.

Construction and characterization of a recombinant chimeric plasminogen activator consisting of a fibrin peptide and a low molecular mass single-chain urokinase

A recombinant chimeric plasminogen activator (f beta/scuPA-32k), with a fibrin beta-chain peptide (comprising Gly15 through Arg 42) linked to the N-terminal of a low molecular mass (32 kDa) single-chain urokinase (scuPA-32k, comprising Leu144 through Leu 411) via a 50 amino acid linker sequence, was produced by expression the corresponding chimeric cDNA in Escherichia coli cells. After refolding in vitro, the chimeric protein was purified to homogeneity by zinc chelate-Sepharose chromatography, Sephacryl S200 chromatography and benzamidine-Sepharose chromatography in sequence. The apparent molecular mass was 36 kDa shown by SDS-PAGE analysis. The special activity was 87,000 IU/mg detected by fibrin plate determination. F beta/scuPA-32k could directly activate plasminogen following Michaelis-Menten kinetics with K(m) = 0.52 microM and k(2) = 0.0024 s(-1). Mediated by plasmin, the single-chain molecule could be converted to the active two-chain molecule. The chimeric protein had 3.3 times higher fibrin affinity than scuPA-32k in the fibrin concentration of 3.2 mg/mL, while the chimeric protein inhibited the fibrin clotting and platelet aggregation. F beta/scuPA-32k showed a higher thrombolytic potency in vitro plasma clot lysis than scuPA-32k and depleted less fibrinogen in plasma. These results showed that the chimeric protein had not only higher fibrinolytic activity but also anti-thrombus activity. Further evaluation of the thrombolytic potential in appropriate animal models is required.

Activation of hepatocyte growth factor (HGF) by endogenous HGF activator is required for metanephric kidney morphogenesis in vitro

The interaction of hepatocyte growth factor (HGF) with c-Met has been implicated in morphogenesis of the kidney, lung, mammary gland, liver, placenta, and limb bud. HGF is secreted as an inactive zymogen and must be cleaved by a serine protease to initiate Met signaling. We show here that a serine protease specific for HGF, HGF activator (HGFA), is expressed and activated by the ureteric bud of the developing kidney in vivo and in vitro. Inhibition of HGFA activity with serine protease inhibitors reduced ureteric bud branching and inhibited glomerulogenesis and nephrogenesis. Activated HGF rescued developing kidneys from the effects of inhibitors. HGFA was localized around the tips of the ureteric bud in developing kidneys, while HGF was expressed diffusely throughout the mesenchyme. These data show that expression of HGF is not sufficient for development, but that its activation is also required. The localization of HGFA to the ureteric bud and the mesenchyme immediately adjacent to it suggests that HGFA creates a gradient of HGF activity in the developing kidney. The creation and shape of gradients of activated HGF by the localized secretion of HGF activators could play an important role in pattern formation by HGF responsive tissues.

Characterization of a recombinant chimeric plasminogen activator with enhanced fibrin binding

A recombinant chimeric plasminogen activator (GHRP-scu-PA-32K), consisting of the tetrapeptide Gly-His-Arg-Pro fused to the N-terminus of the low-molecular single-chain urokinase-type plasminogen activator (Leu144-Leu411), was produced by expression in CHO cells. The stable expression cell line was selected for large-scale expression. The product was purified by antibody-Sepharose affinity chromatography with a recovery of 67%. The apparent molecular weight of purified GHRP-scu-PA-32K was 33 kDa according to SDS-PAGE. Its specific activity was 150000 IU/mg protein according to fibrin plate determination. The conversion of single-chain to two-chain molecules mediated by plasmin was comparable for GHRP-scu-PA-32K (K(m)=4.9 microM, k(2)=0.35 s(-1)) and scu-PA-32K. The activation of plasminogen by GHRP-scu-PA-32K (K(m)=1.02 microM, k(2)=0.0028 s(-1)) was also similar to that of scu-PA-32K. The fibrin binding of GHRP-scu-PA-32K was 2.5 times higher than that of scu-PA-32K at a fibrin concentration of 3.2 mg/ml. In contrast to scu-PA-32K in vitro 125I-fibrin-labeled plasma clot lysis, GHRP-scu-PA had a higher thrombolytic potency, whereas it depleted less fibrinogen in plasma. These results show that GHRP-scu-PA-32K as expected is a potential thrombolytic agent.

Activation of prostate-specific antigen precursor (pro-PSA) by prostin, a novel human prostatic serine protease identified by degenerate PCR

A novel serine protease was found in human prostate by degenerate oligonucleotide PCR amplification and cloned. The zymogen form of this enzyme, named prostinogen, is composed of 240 amino acid residues with an amino-terminal propiece of 5 residues and a 235-residue mature enzyme. The transcript has a signal peptide of 15 amino acid residues. The mature enzyme has 41% sequence identity with prostate specific antigen (PSA). Prostinogen was expressed in Escherichia coli and refolded from inclusion bodies. The zymogen, with a molecular mass of 28 kDa, was readily activated by agarose-immobilized trypsin to generate prostin, a serine protease, which cleaves the chromogenic substrate (N-benzoyl-L-Ile-L-Glu-L-Gly-L-Arg-p-nitroaniline hydrochloride) (S-2222). Recombinant prostin readily activates the precursor of PSA (pro-PSA) by cleavage of the amino terminal Arg(7)-Ile(8) peptide bond. These results indicate that prostin may be a physiological activator of pro-PSA following its own proteolytic activation, as part of a cascade system involving a series of serine protease precursor proteins in the prostate.

A Double-Blind Multicenter Comparison of the Efficacy and Safety of Saruplase and Urokinase in the Treatment of Acute Myocardial Infarction: Report of the SUTAMI Study Group

Background: Urokinase or two-chain urokinase-type plasminogen activator has been shown to be effective in the treatment of acute myocardial infarction. Its parent molecule, single-chain urokinase-type plasminogen activator (scu-PA), unlike urokinase, can selectively activate fibrin-bound plasminogen. The induced clot lysis is amplified by plasmin-triggered conversion of scu-PA to urokinase and by further plasmin generation. The aim of our study was to compare the efficacy and safety of recombinant unglycosylated scu-PA, or saruplase, and urokinase at doses considered optimal in patients with acute myocardial infarction within 6 hours of onset of pain. Methods and results: In a double-blind trial 543 patients were randomized to saruplase (20 mg bolus + 60 mg/hr) or urokinase (1.5 million unit bolus + 1.5 million units/hr). Primary endpoint: The patency rates at 24-72 hours were 75.4% (95% CI 70.3-80.5%) for saruplase and 74.2% (95% CI 69.0-79.4%; P = 0.77) for urokinase. Secondary endpoint: The incidence of bleeding events in both groups was 10.7%. There were three hemorrhagic strokes in the saruplase group (ns). Other efficacy and safety evaluations: Apart from the generation of more fibrinogen degradation products under saruplase, the changes in hemostatic parameters did not differ. Hospital mortality was 4.4% for saruplase and 8.1% for urokinase. This nonsignificant difference was maintained for 1 year. Conclusion: The efficacy and safety of saruplase and urokinase in the regimens used are very similar.

Two distinct urokinase-serpin interactions regulate the initiation of cell surface-associated plasminogen activation

Single-chain urokinase-type plasminogen activator (scu-PA) possesses enzymatic activity that increases by 2-3 orders of magnitude upon binding to its cellular cofactor, the u-PA receptor (u-PAR), hence activating an enzymatic cascade initially composed of zymogens. The present study demonstrates that plasminogen activator inhibitor type 3 (PAI-3) reversibly inhibits scu-PA in solution, maintaining the system "off." Because the scu-PA/PAI-3 interaction is reversible, cellular expression of u-PAR allows partitioning of scu-PA from PAI-3 to u-PAR with resultant expression of full enzymatic activity. PAI-3 that was originally complexed to scu-PA remains in solution, retaining its functional activity. Importantly, the scu-PA on cell surface u-PAR is protected from PAI-3 inhibition, remaining an effective activator in a PAI-rich environment. Plasmin formed as a result of scu-PA activity then cleaves scu-PA to the mature protease, two-chain u-PA (tcu-PA), which is efficiently and irreversibly inhibited by PAI-3 via the standard serpin mechanism, even on u-PAR. This data generates a new hypothesis which, in contrast to the previous paradigm, holds that receptor bound scu-PA is the initiating enzyme and that tcu-PA is generated not to augment enzymatic activity but rather to allow for inhibition and therefore appropriate regulation of the process.

Interventional use of plasminogen activators in central nervous system diseases

Still an experimental approach, the direct intra-arterial infusion of plasminogen activators in the setting of acute thrombotic stroke has received impetus from successful clinical trials of intravenous infusion therapy. Direct therapy, employing catheter delivery, has successfully produced evidence of recanalization in carotid artery territory and vertebrobasilar artery territory thrombotic occlusions. One very recent prospective randomized study has demonstrated the success and limitations of this approach. Attention to safety concerns will be important to the future success of direct intra-arterial delivery of plasminogen activators in acute thrombotic stroke.

Characterization of the precursor of prostate-specific antigen. Activation by trypsin and by human glandular kallikrein

The precursor or zymogen form of prostate-specific antigen (pro-PSA) is composed of 244 amino acid residues including an amino-terminal propiece of 7 amino acids. Recombinant pro-PSA was expressed in Escherichia coli, isolated from inclusion bodies, refolded, and purified. The zymogen was readily activated by trypsin at a weight ratio of 50:1 to generate PSA, a serine protease that cleaves the chromogenic chymotrypsin substrate 3-carbomethoxypropionyl-L-arginyl-L-prolyl-L-tyrosine-p-nitroanili ne- HCl (S-2586). In this activation, the amino-terminal propiece Ala-Pro-Leu-Ile-Leu-Ser-Arg was released by cleavage at the Arg-Ile peptide bond. The recombinant pro-PSA was also activated by recombinant human glandular kallikrein, another prostate-specific serine protease, as well as by a partially purified protease(s) from seminal plasma. The recombinant PSA was inhibited by alpha1-antichymotrypsin, forming an equimolar complex with a molecular mass of approximately 100 kDa. The recombinant PSA failed to activate single chain urokinase-type plasminogen activator, in contrast to the recombinant hK2, which readily activated single chain urokinase-type plasminogen activator. These results indicate that pro-PSA is converted to an active serine protease by minor proteolysis analogous to the activation of many of the proteases present in blood, pancreas, and other tissues. Furthermore, PSA is probably generated by a cascade system involving a series of precursor proteins. These proteins may interact in a stepwise manner similar to the generation of plasmin during fibrinolysis or thrombin during blood coagulation.

Screening panels of monoclonal antibodies using phage-displayed antigen

A procedure is described to screen panels of hybridomas or purified monoclonal antibodies using antigen displayed on the surface of filamentous bacteriophage. In this system, samples containing murine monoclonal antibodies are incubated with phage-displayed antigen in microtiter plates coated with rabbit anti-mouse IgG, and bound antibody-phage complex is detected with horseradish peroxidase-sheep anti-phage M13 conjugate. The assay has been validated with a panel of 16 monoclonal antibodies directed against human plasminogen, using phage-displayed miniplasmin-(ogen) (amino acids Ala444 through Asn791 comprising kringle 5 and the proteinase domain of plasminogen) or microplasminogen (amino acids Ala543 through Asn791 comprising the proteinase domain). Six monoclonal antibodies were identified directed against miniplasminogen and miniplasmin; this was confirmed using a microtiter plate coated with antigens. One of these monoclonal antibodies (MA-42B12) did not react with microplasminogen, suggesting that its epitope is comprised within the kringle 5 domain. This test is rapid and sensitive (detecting 10-20 ng/ml of monoclonal antibody), and screening can be performed using phage-displayed zymogens or active enzymes or selected domains thereof. The procedure eliminates the need for large amounts of purified antigen for screening. Furthermore, immunization can be performed with partially purified antigen because only antibodies raised against the antigen of interest will be identified with the use of phage-displayed antigen. Therefore, this test may offer distinct advantages over the classical one-site enzyme-linked immunosorbent assay using antigen-coated microtiter plates.

Characterization of single chain urokinase-type plasminogen activator with a novel amino-acid substitution in the kringle structure

ECV304 is a cell line established by a spontaneous transformation of endothelial cells of a human umbilical vein. It was shown that ECV304 secretes single chain urokinase-type plasminogen activator (scu-PA). A subclone, ECV304 clone 15, was obtained by acclimatization of parental clone to serum-free medium followed by limiting dilution. The clone was found to produce approximately five times as much scu-PA (approximately 20 IU/10(6) cells per day) as the parental clone after a 40 days' culture. Though the biochemical characteristics of the purified scu-PA were indistinguishable from those of the native scu-PA, it had a lower affinity for fibrin clots under the employed conditions. Molecular cloning of a cDNA encoding the scu-PA has identified a novel substitution from C to T in the nucleotide sequence encoding the kringle structure. The substitution resulted in an alteration from Pro (CCG) to Leu (CTG) at amino-acid position 121, which may be directly or indirectly involved in the decrease in the apparent affinity.

Fibrinolytic mechanism, biochemistry, and preclinical pharmacology of recombinant prourokinase

The purpose of this review is to provide a biochemical characterization of recombinant prourokinase (r-ProUK [ABT-187]), including a description of its clot-specific fibrinolytic mechanism. In addition, the preclinical data will be briefly reviewed, demonstrating the efficacy of r-ProUK as a potent therapeutic plasminogen activator. r-ProUK was purified to homogeneity from the culture medium of SP2/0 mouse hybridoma cells. The fibrinolytic potency of r-ProUK was characterized by both in vitro clot lysis experiments in human plasma and a canine femoral artery thrombosis model. In the in vitro clot lysis system, with use of clots prepared from fresh frozen human plasma, r-ProUK exhibits a lag phase to the onset of lysis and a concentration-dependent threshold effect due to the presence of the inhibitors alpha 2-antiplasmin and plasminogen activator inhibitor (PAI-1). Effective clot lysis can be achieved without degradation of the fibrinogen in the surrounding plasma. Over a dose range of 50,000-220,000 IU, the canine femoral artery thrombosis model shows a dose-dependent relationship for r-ProUK and effective clot lysis. The lytic activity of r-ProUK is significantly enhanced in this model by the concomitant administration of heparin as an adjunctive agent for thrombolytic treatment. Fibrinogen, plasminogen, and alpha 2-antiplasmin levels in the systemic circulation were unaltered during the 30-minute infusion period and a 4-hour observation period, in which 85% lysis was achieved with r-ProUK (100,000 IU) and heparin. Moreover, restoration of blood flow in the previously fully occluded femoral artery was achieved within minutes of the start of the r-ProUK infusion. The experimental findings presented here are consistent with the clot-specific fibrinolytic mechanism of r-ProUK. Effective clot lysis can be achieved without alteration of the systemic coagulation and fibrinolytic parameters.

Interaction of single-chain urokinase and plasminogen activator inhibitor type 1

Urokinase (u-PA) is synthesized and secreted as a single-chain polypeptide (single-chain u-PA, scu-PA), which has such little enzymatic activity in solution that it has been considered essentially enzymatically inert. We found that plasminogen activator inhibitor type 1 (PAI-1), the major PAI in plasma, demonstrated concentration-dependent inhibition of this solution-phase scu-PA enzymatic activity. 125I-scu-PA formed complexes with PAI-1 in a concentration- and time-dependent manner, as detected by SDS-polyacrylamide gel electrophoresis under reducing conditions. Among a given population of scu-PA molecules, all measurable enzymatic activity was inhibited by a 10-fold molar excess of PAI-1. However, at this stoichiometry, only a minority of 125I-scu-PA molecules formed SDS-stable complexes with PAI-1 (i.e. complexes that formed a covalent bond upon denaturation), even though the uncomplexed PAI-1 molecules remained competent to inhibit u-PA enzymatic activity. Neither the extent nor the time course of complex formation was altered by using PAI-1 that had been pre-incubated with native human vitronectin, compared with native PAI-1 alone. 125I-scu-PA.PAI-1 complexes that would form a covalent bond if denatured were reversible and existed in equilibrium with either non-complexed or loosely complexed reactants. These data suggest that scu-PA has more enzyme-like properties than previously appreciated and raises the possibility that it resembles single-chain tissue type-plasminogen activator in lacking a complete zymogen conformation.

Effects of changing liver blood flow by exercise and food on kinetics and dynamics of saruplase

OBJECTIVE

To investigate the influence of changes in liver blood flow on the pharmacokinetics and pharmacodynamics of single-chain unglycosylated urokinase-type plasminogen activator.

METHODS

This open, randomized, crossover trial was carried out in the clinical research unit. Infusions of 37.5 mg saruplase and 90 mg indocyanine green were administered over 150 minutes to 10 healthy male volunteers. After 60 minutes the subjects consumed a standardized meal to increase liver blood flow or performed an exercise test (20 minutes) to decrease liver blood flow. Indocyanine green concentrations, total urokinase-type plasminogen activator (u-PA) antigen, two-chain u-PA activity, fibrinogen, total degradation products, alpha 2-antiplasmin, and factor XII-dependent fibrinolytic activity were measured. Blood flow was measured after food intake in a portal vein branch with Doppler echography.

RESULTS

The weighted average indocyanine green concentration after exercise was increased by 29% compared with baseline (steady-state concentration) values (95% confidence intervals [CI]: +6%, +56%). After food, the concentration was 27% lower compared with baseline values (95% CI: -35%, -19%), and portal vein flow was increased by a maximum of 103% (95% CI: +71%, +136%). Average maximal concentrations of u-PA antigen after exercise were increased by 130 ng/ml compared with baseline concentrations (95% CI: +65, +195 ng/ml) and, unexpectedly, 156 ng/ml higher after food (95% CI: +59, +253 ng/ml). Although not significant, an increase in average u-PA antigen concentration compared with baseline values was detected after both exercise (7%) and food (13%). This tendency toward a larger effect after food compared with the effect after exercise was reflected by minor changes in the pharmacodynamics.

CONCLUSIONS

u-PA plasma concentrations were increased by reduced liver blood flow induced by exercise. Food intake produced an unexpected increase in u-PA concentrations despite increases in liver blood flow.

Novel thrombolytic agents

The fibrinolytic system comprises an inactive pro-enzyme, plasminogen, that is converted by plasminogen activators to the active enzyme, plasmin, that degrades fibrin. Two immunologically distinct plasminogen activators have been identified: tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA). Plasminogen activation is regulated by specific molecular interactions between its main components, as well as by controlled synthesis and release of plasminogen activator inhibitors, primarily from endothelial cells. The observed association between abnormal fibrinolysis and a tendency toward bleeding or thrombosis demonstrates the (patho)physiological importance of the fibrinolytic system. Transgenic animals are a suitable experimental model to examine the in vivo impact of fibrinolytic components in thrombosis and thrombolysis. Inactivation, by homologous recombination, of the tissue-type plasminogen activator genes in mice impairs thrombolysis in a significant manner whereas inactivation of the plasminogen activator-1 gene enhances the rate of spontaneous lysis. Despite their widespread use all currently available thrombolytic agents suffer from a number of significant limitations, including resistance to reperfusion, the occurrence of acute coronary reocclusion and bleeding complications. Therefore, the quest for thrombolytic agents with a higher thrombolytic potency, specific thrombolytic activity and/or a better fibrin-selectivity continues. Several lines of research toward improvement of thrombolytic agents are being explored, including the construction of mutants and variants of plasminogen activators, chimeric plasminogen activators, conjugates of plasminogen activators with monoclonal antibodies, or plasminogen activators from animal or bacterial origin.

Inactivation of receptor-bound pro-urokinase-type plasminogen activator (pro-uPA) by thrombin and thrombin/thrombomodulin complex

Urokinase-type plasminogen activator (uPA) binds with high affinity to a specific cell surface glycosyl phosphatidyl-inositol (GPI)-anchored receptor, the urokinase receptor (uPAR). Pro-uPA, the enzymatically inactive single-chain form of uPA after having been activated by certain proteases, converts plasminogen into plasmin. This activation of pro-uPA to enzymatically active uPA can be prevented by the action of thrombin on pro-uPA. This inactivation process is accelerated in the presence of thrombomodulin (TM). The present study investigated whether pro-uPA bound to uPAR is still susceptible to inactivation by thrombin in the presence or absence of TM. A truncated soluble form of the uPAR lacking the GPI-anchor was cloned and expressed in CHO-cells (rec-uPAR277). Rec-uPAR277 efficiently inhibited the thrombin-mediated inactivation of pro-uPA up to 90% in a concentration dependent manner. The protective effect of rec-uPAR277 was far less pronounced when thrombin was complexed with TM. Enzyme kinetic experiments with varying concentrations of pro-uPA showed that in the presence of TM the catalytic efficiency (kcat/Km) of thrombin-mediated inactivation raised from 0.010 microM-1 s-1 to 0.50 microM-1 s-1 corresponding to a fifty-fold increase. In the presence of rec-uPAR277, however, the catalytic efficiency dropped by 4.1-fold (0.5 microM-1 s-1 to 0.122 microM-1 s-1). The inactivation kinetics of pro-uPA by thrombin (no TM added) in the presence of an excess of rec-uPAR277 could not be determined since virtually no inactivation occurred. Our data suggest that pro-uPA once bound to uPAR, is significantly protected from inactivation by thrombin.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired fibrinolysis in patients with Blackfoot disease

Blackfoot disease is a unique endemic and chronic progressive arteriosclerotic vascular disease in southwest area of Taiwan. In this study, we determined the plasma levels of tissue plasminogen activator (tPA), plasminogen activator inhibitor (PAI), urokinase plasminogen activator (uPA), and von Willebrand factor (vWF) antigen in Blackfoot disease patients, in comparison with normal controls from non-endemic areas and the endemic area, Putai. Blackfoot disease patients had mean tPA antigen level of 7.9 ng/ml (n = 27) which was significantly lower (p < 0.05) than both the normal controls with 11.0 ng/ml (n = 20) and the Putai normal controls with 9.7 ng/ml (n = 39). However, the mean PAI-1 antigen level in the patient group was 41.2 ng/ml (n = 28) which was significantly higher (p = 0.0001) than both the normal controls with 19.7 ng/ml (n = 23) and the Putai normal controls with 21.3 ng/ml (n = 40). Furthermore, in the patient group, a significantly lower (p < 0.005) mean uPA antigen level (2.3 ng/ml, n = 18) was noted as compared with that in the normal controls (3.2 ng/ml, n = 14). No significant difference was observed in vWF antigen level between patients and normal controls. This study suggests that a reduced capacity for fibrinolysis is associated with Blackfoot disease.

Endothelial cell proteases: physiological role and regulation

Endothelial cell-derived proteases can be classified according to their physiological role. The proteases involved in extracellular matrix degradation are important in endothelial cell migration and thereby in angiogenesis. They include the urokinase-type plasminogen activator (uPA) and the metalloproteases, collagenases, gelatinases and stromelysin. uPA secreted from endothelial cells remains associated with the cell membrane, on specific receptors localized in the vicinity of the receptors for plasminogen. This favours the local activation of plasminogen into plasmin. Plasmin, generated on the cell surface, is fully active as it is not inhibited by alpha 2-antiplasmin. Plasmin acts directly by degrading some components of the extracellular matrix and indirectly by activating the prometalloproteases. Secretion of PAI by migrating cells is generally stimulated by the same factors that induce uPA secretion, limiting the degradation of the matrix to the pericellular path. The degradation of the fibrin clot involves the tissue-type plasminogen activator tPA, which like the uPA activates plasminogen to plasmin. This system is also regulated by two different mechanisms. On the one hand, fibrin itself favours its own degradation by formation of a ternary complex, fibrin-plasminogen-tPA, in which the affinity of tPA for plasminogen is markedly increased, as compared to the affinity of unbound tPA. In addition, plasmin generated on the clot is protected from inhibition by alpha 2-antiplasmin. On the other hand, as for uPA, tPA is inhibited by PAI-1. The importance of the regulation of this system is illustrated by the thrombotic risk observed when there is either a decrease in tPA or an increase in PAI-1, and inversely by haemorrhages in the case of increase in tPA.

Contact factor mediated fibrinolysis is increased by the combined oral contraceptive pill

OBJECTIVE

To study the fibrinolytic pathways and their relationship with the contact system in women using combined oral contraceptives (COCs).

DESIGN

Serial plasma samples were collected from 18 women before treatment with COCs containing 30 micrograms oestrogen during treatment cycles 3 and 6, and 2 weeks after stopping treatment. Fibrinolysis was measured before and after dextran sulphate mediated contact activation using fibrin plates.

RESULTS

Fibrinolysis increased significantly during cycles 3 and 6 (from 77% to 100% and 113%, respectively, P < 0.01) and showed a further increase after dextran sulphate activation (from 134% to 158% and 167%, respectively, P < 0.01). Tissue-plasminogen activator, urokinase-plasminogen activator and plasminogen activator inhibitor did not change significantly. There were significant elevations of Factor XII (from 0.92 u/ml to 1.43 u/ml, P < 0.01) and prekallikrein (0.94 u/ml to 1.10 u/ml, P < 0.05) in cycle 3, which both remained high at cycle 6 (P < 0.01) and decreased after stopping the COC. Alpha-2-macroglobulin and C1-esterase inhibitor showed no significant change, but alpha-1-antitrypsin increased from 0.85 u/ml to 1.11 u/ml by cycle 3 (P < 0.01), and returned to near normal levels after stopping the COC.

CONCLUSIONS

The increase in fibrinolysis may be due to increased levels of Factor XII and prekallikrein without a corresponding increase in their natural inhibitors (C1-esterase inhibitor and alpha-2-macroglobulin). A parallel increase in the intrinsic pathway of coagulation may be limited by elevated alpha-1-antitrypsin at the level of activated Factor XI. The increase in fibrinolysis caused by oral contraceptives may balance any potential thrombotic risk due to increased fibrinogen or vitamin K dependent coagulation factors.

Biochemical characterization of single-chain chimeric plasminogen activators consisting of a single-chain Fv fragment of a fibrin-specific antibody and single-chain urokinase

K12G0S32 is a 57-kDa recombinant single-chain chimeric plasminogen activator consisting of scFv-K12Go, a single-chain variable-region antigen-binding fragment (Fv) of the monoclonal antibody MA-15C5, which is specific for fragment D-dimer of human cross-linked fibrin, and a low-molecular-mass (33 kDa) urokinase-type plasminogen activator (u-PA-33k) containing amino acids Ala132-Leu411 (Holvoet, P., Laroche, Y., Lijnen, H. R., Van Cauwenberghe, R., Demarsin, E., Brouwers, E., Matthyssens, G. & Collen D. (1991) J. Biol. Chem. 266, 19717-19724). In addition, the Arg156-Phe157 thrombin-cleavage site in the u-PA moiety of K12G0S32 is removed by substitution of Phe157 with Asp. In the present study, the fibrinolytic potency of K12G0S32, determined in a system composed of a 125I-fibrin-labeled human plasma clot submerged in citrated plasma, was found to be only twofold higher than that of intact single-chain u-Pa (rscu-PA), but 17-fold higher than that of rscu-PA(M), a variant of rscu-PA in which the thrombin-cleavage site was removed by substitution of Phe157 with Asp. The fibrinolytic potency of K12G0S32T, with an intact thrombin-cleavage site, was 6-15-fold higher than that of rscu-PA. Conversion of 1 microM single-chain K12G0S32 or rscu-PA(M) into their two-chain derivatives with plasmin occurred at a rate of 1.0 +/- 0.15 nmol.min-1.nmol plasmin-1 and 0.85 +/- 0.074 nmol.min-1.nmol plasmin-1, compared to 14 +/- 2.3 nmol.min-1.nmol plasmin-1 and 18 +/- 2.6 nM.min-1.nmol plasmin-1 for K12G0S32T and rscu-PA, respectively. Purified fragment D-dimer of human cross-linked fibrin inhibited the fibrinolytic potency of single-chain K12G0S32T, but not of two-chain K12G0S32T, in a dose-dependent manner. Furthermore, the fibrinolytic potencies of two-chain K12G0S32 and K12G0S32T were not significantly higher than those of recombinant two-chain u-PA (rtcu-PA) or of rtcu-PA(M). These findings suggest that the 59-fold increase in fibrinolytic potency of K12G0S32T, relative to that of rscu-PA(M), is due both to targeting of the activator to the clot via the single-chain Fv fragment (sixfold increase) and to a more efficient conversion of single-chain K12G0S32T to its two-chain derivative (eightfold increase). Thus, targeting to clots by means of fibrin-specific antibodies results in a significant increase of the fibrinolytic potency of single-chain but not of two-chain u-PA.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

The potential improvement of thrombolytic therapy by targeting with bispecific monoclonal antibodies: why they are used and how they are made

The generation of the proteolytic enzyme plasmin from its inactive precursor plasminogen, mediated by so called plasminogen activators, is the essential step in thrombolytic therapy. Plasmin is responsible for the degradation of the insoluble fibrin, the major component of a thrombus, to soluble fibrin degradation products. So far, the use of the more recently developed thrombolytic agents single-chain urokinase-type plasminogen activator (scu-PA) and tissue-type plasminogen activator (t-PA) were disappointing, mainly due to some of their negative properties in vivo, i.e., rapid inhibition and/or hepatic clearance. Besides some background information on the haemostatic balance; t-PA and scu-PA structure; and mechanisms of action, we here review some reported attempts to improve on these agents for thrombolytic therapy following various strategies. One of the more potential strategies, antibody-targeted thrombolytic therapy using bispecific monoclonal antibodies, is discussed somewhat more extensively, as are the several procedures that can be followed for bispecific antibody preparation.

Biochemical properties of recombinant mutants of nonglycosylated single chain urokinase-type plasminogen activator

The role of glycosylation on the enzymatic properties of single chain urokinase-type plasminogen activator (scu-PA) was investigated by site-specific mutagenesis of the glycosylated Asn-302 residu to Gln. In addition, the role of the NH2-terminal polypeptide chain and of the Cys-148 to Cys-279 interchain disulphide bond on the activity of non-glycosylated scu-PA was investigated. Therefore, variants of recombinant scu-PA (rscu-PA) were produced by transfecting Chinese hamster ovary cells with cDNA encoding rscu-PA N302Q (rscu-PA with Asn-302 to Gln mutation), rscu-PA C279A,N302Q (rscu-PA with Cys-279 to Ala and Asn-302 to Gln mutations) or rscu-PA del(N2-F157)C279A,N302Q (rscu-PA C279A,N302Q with deletion of Asn-2 through Phe-157). These mutants were purified to homogeneity from conditioned cell culture medium and were obtained essentially as single chain molecules with specific activities on fibrin plates of (mean +/- S.E.; n = 6) 45,000 +/- 5000. IU/mg, 19,000 +/- 800 IU/mg and < or = 100 IU/mg for rscu-PA N302Q, rscu-PA C279A,N302Q and rscu-PA del(N2-F157)C279A,N302Q, respectively, as compared to 64,000 +/- 2600 IU/mg for wild-type rscu-PA obtained in the same expression system. Plasmin quantitatively converts rscu-PA N302Q and rscu-PA C279A,N302Q to amidolytically active two-chain derivatives with a specific activity of 56,000 IU/mg and 32,000 IU/mg, respectively, as compared to 75,000 IU/mg for wild-type rscu-PA. Plasminogen activation as a function of time was comparable for rscu-PA N302Q and wild-type rscu-PA, and somewhat slower for rscu-PA C279A,N302Q. In a human plasma milieu in vitro, consisting of a 125I-fibrin labeled plasma clot submerged in plasma, 50 percent clot lysis in 2 h required 2.2 micrograms/ml rscu-PA N302Q and 6.0 micrograms/ml rscu-PA C279A,N302Q, as compared to 3.2 micrograms/ml wild-type rscu-PA. In contrast, rscu-PA del(N2-F157)C279A,N302Q was not converted to an amidolytically active two chain derivative by plasmin, and did not induce significant plasminogen activation in purified systems or clot lysis in a human plasma milieu. Following bolus injections in hamsters, the initial half-lives (1.8-2.6 min) and the plasma clearances (0.6-1.5 ml min-1) were comparable for wild-type rscu-PA and for the three rscu-PA mutants. These results suggest that the fibrinolytic activity in a plasma milieu in vitro and the in vivo turnover of rscu-PA are not markedly affected by the absence of carbohydrate.(ABSTRACT TRUNCATED AT 400 WORDS)