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

Assays to quantify fibrinolysis: strengths and limitations. Communication from the International Society on Thrombosis and Haemostasis Scientific and Standardization Committee on fibrinolysis

Fibrinolysis is a series of enzymatic reactions that degrade insoluble fibrin. Plasminogen activators convert the zymogen plasminogen to the active serine protease plasmin, which cleaves and solubilizes crosslinked fibrin clots into fibrin degradation products. The quantity and quality of fibrinolytic enzymes, their respective inhibitors, and clot structure determine overall fibrinolysis. The quantity of protein can be measured by antigen-based assays, and both quantity and quality can be assessed using functional assays. Furthermore, variations of commonly used assays have been reported, which are tailored to address the role(s) of specific fibrinolytic factors and cellular elements (eg, platelets, neutrophils, and red blood cells). Although the concentration and/or activity of a protein can be quantified, how these individual components contribute to the overall fibrinolysis outcome can be challenging to determine. This difficulty is due to temporal changes within and around the thrombi during the clot breakdown, particularly the fibrin matrix structure, and composition. Furthermore, terms such as "fibrinolytic activity/potential," "plasminogen activation," and "plasmin activity" are often used interchangeably despite having different definitions. The purpose of this review is to 1) summarize the assays measuring fibrinolysis activity and potential, 2) facilitate the interpretation of data generated by these assays, and 3) summarize the strengths and limitations of these assays.

Longitudinal Assessment of Cytokine Expression and Plasminogen Activation in Hantavirus Cardiopulmonary Syndrome Reveals Immune Regulatory Dysfunction in End-Stage Disease

Pathogenic New World orthohantaviruses cause hantavirus cardiopulmonary syndrome (HCPS), a severe immunopathogenic disease in humans manifested by pulmonary edema and respiratory distress, with case fatality rates approaching 40%. High levels of inflammatory mediators are present in the lungs and systemic circulation of HCPS patients. Previous studies have provided insights into the pathophysiology of HCPS. However, the longitudinal correlations of innate and adaptive immune responses and disease outcomes remain unresolved. This study analyzed serial immune responses in 13 HCPS cases due to Sin Nombre orthohantavirus (SNV), with 11 severe cases requiring extracorporeal membrane oxygenation (ECMO) treatment and two mild cases. We measured viral load, levels of various cytokines, urokinase plasminogen activator (uPA), and plasminogen activator inhibitor-1 (PAI-1). We found significantly elevated levels of proinflammatory cytokines and PAI-1 in five end-stage cases. There was no difference between the expression of active uPA in survivors' and decedents' cases. However, total uPA in decedents' cases was significantly higher compared to survivors'. In some end-stage cases, uPA was refractory to PAI-1 inhibition as measured by zymography, where uPA and PAI-1 were strongly correlated to lymphocyte counts and IFN-γ. We also found bacterial co-infection influencing the etiology and outcome of immune response in two cases. Unsupervised Principal Component Analysis and hierarchical cluster analyses resolved separate waves of correlated immune mediators expressed in one case patient due to a sequential co-infection of bacteria and SNV. Overall, a robust proinflammatory immune response, characterized by an imbalance in T helper 17 (Th17) and regulatory T-cells (Treg) subsets, was correlated with dysregulated inflammation and mortality. Our sample size is small; however, the core differences correlated to survivors and end-stage HCPS are instructive.

Functional plasminogen activator inhibitor 1 is retained on the activated platelet membrane following platelet activation

Platelets harbor the primary reservoir of circulating plasminogen activator inhibitor 1 (PAI-1), but the reportedly low functional activity of this pool of inhibitor has led to debate over its contribution to thrombus stability. Here we analyze the fate of PAI-1 secreted from activated platelets and examine its role in maintaining thrombus integrity. Activation of platelets results in translocation of PAI-1 to the outer leaflet of the membrane, with maximal exposure in response to strong dual agonist stimulation. PAI-1 is found to co-localize in the cap of PS-exposing platelets with its cofactor, vitronectin, and fibrinogen. Inclusion of tirofiban or Gly-Pro-Arg-Pro significantly attenuated exposure of PAI-1, indicating a crucial role for integrin αIIbβ3 and fibrin in delivery of PAI-1 to the activated membrane. Separation of platelets post-stimulation into soluble and cellular components revealed the presence of PAI-1 antigen and activity in both fractions, with approximately 40% of total platelet-derived PAI-1 remaining associated with the cellular fraction. Using a variety of fibrinolytic models we found that platelets produce a strong stabilizing effect against tPA-mediated clot lysis. Platelet lysate, as well as soluble and cellular fractions stabilize thrombi against premature degradation in a PAI-1 dependent manner. Our data show for the first time that a functional pool of PAI-1 is anchored to the membrane of stimulated platelets and regulates local fibrinolysis. We reveal a key role for integrin αIIbβ3 and fibrin in delivery of PAI-1 from platelet α-granules to the activated membrane. These data suggest that targeting platelet-associated PAI-1 may represent a viable target for novel profibrinolytic agents.

Neuroserpin Differentiates Between Forms of Tissue Type Plasminogen Activator via pH Dependent Deacylation

Tissue-type plasminogen activator (t-PA), initially characterized for its critical role in fibrinolysis, also has key functions in both physiologic and pathologic processes in the CNS. Neuroserpin (NSP) is a t-PA specific serine protease inhibitor (serpin) found almost exclusively in the CNS that regulates t-PA's proteolytic activity and protects against t-PA mediated seizure propagation and blood-brain barrier disruption. This report demonstrates that NSP inhibition of t-PA varies profoundly as a function of pH within the biologically relevant pH range for the CNS, and reflects the stability, rather than the formation of NSP: t-PA acyl-enzyme complexes. Moreover, NSP differentiates between the zymogen-like single chain form (single chain t-PA, sct-PA) and the mature protease form (two chain t-PA, tct-PA) of t-PA, demonstrating different pH profiles for protease inhibition, different pH ranges over which catalytic deacylation occurs, and different pH dependent profiles of deacylation rates for each form of t-PA. NSP's pH dependent inhibition of t-PA is not accounted for by differential acylation, and is specific for the NSP-t-PA serpin-protease pair. These results demonstrate a novel mechanism for the differential regulation of the two forms of t-PA in the CNS, and suggest a potential specific regulatory role for CNS pH in controlling t-PA proteolytic activity.

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.

Regulation of intrapleural fibrinolysis by urokinase-alpha-macroglobulin complexes in tetracycline-induced pleural injury in rabbits

The proenzyme single-chain urokinase plasminogen activator (scuPA) more effectively resolved intrapleural loculations in rabbits with tetracycline (TCN)-induced loculation than a range of clinical doses of two-chain uPA (Abbokinase) and demonstrated a trend toward greater efficacy than single-chain tPA (Activase) (Idell S et al., Exp Lung Res 33: 419, 2007.). scuPA more slowly generates durable intrapleural fibrinolytic activity than Abbokinase or Activase, but the interactions of these agents with inhibitors in pleural fluids (PFs) have been poorly understood. PFs from rabbits with TCN-induced pleural injury treated with intrapleural scuPA, its inactive Ser195Ala mutant, Abbokinase, Activase, or vehicle, were analyzed to define the mechanism by which scuPA induces durable fibrinolysis. uPA activity was elevated in PFs of animals treated with scuPA, correlated with the ability to clear pleural loculations, and resisted (70-80%) inhibition by PAI-1. Alpha-macroglobulin (alphaM) but not urokinase receptor complexes immunoprecipitated from PFs of scuPA-treated rabbits retained uPA activity that resists PAI-1 and activates plasminogen. Conversely, little plasminogen activating or enzymatic activity resistant to PAI-1 was detectable in PFs of rabbits treated with Abbokinase or Activase. Consistent with these findings, PAI-1 interacts with scuPA much slower than with Activase or Abbokinase in vitro. An equilibrium between active and inactive scuPA (k(on) = 4.3 h(-1)) limits the rate of its inactivation by PAI-1, favoring formation of complexes with alphaM. These observations define a newly recognized mechanism that promotes durable intrapleural fibrinolysis via formation of alphaM/uPA complexes. These complexes promote uPA-mediated plasminogen activation in scuPA-treated rabbits with TCN-induced pleural injury.

Prophylactic thrombolysis by thrombin-activated latent prourokinase targeted to PECAM-1 in the pulmonary vasculature

A recombinant prodrug, single-chain urokinase-type plasminogen activator (scuPA) fused to an anti-PECAM-1 antibody single-chain variable fragment (anti-PECAM scFv/scuPA) targets endothelium and augments thrombolysis in the pulmonary vasculature.(1) To avoid premature activation and inactivation and to limit systemic toxicity, we replaced the native plasmin activation site in scFv/low-molecular-weight (lmw)-scuPA with a thrombin activation site, generating anti-PECAM scFv/uPA-T that (1) is latent and activated by thrombin instead of plasmin; (2) binds to PECAM-1; (3) does not consume plasma fibrinogen; (4) accumulates in mouse lungs after intravenous injection; and (5) resists PA inhibitor PAI-1 until activated by thrombin. In mouse models of pulmonary thrombosis caused by thromboplastin and ischemia-reperfusion (I/R), scFv/uPA-T provided more potent thromboprophylaxis and greater lung protection than plasmin-sensitive scFv/uPA. Endothelium-targeted thromboprophylaxis triggered by a prothrombotic enzyme illustrates a novel approach to time- and site-specific regulation of proteolytic reactions that can be modulated for therapeutic benefit.

Plasminogen activator inhibitor-1 is an inhibitor of factor VII-activating protease in patients with acute respiratory distress syndrome

Factor VII-activating protease (FSAP) is a novel plasma-derived serine protease structurally homologous to tissue-type and urokinase-type plasminogen activators. We demonstrate that plasminogen activator inhibitor-1 (PAI-1), the predominant inhibitor of tissue-type and urokinase-type plasminogen activators in plasma and tissues, is an inhibitor of FSAP as well. We detected PAI-1·FSAP complexes in addition to high levels of extracellular RNA, an important FSAP cofactor, in bronchoalveolar lavage fluids from patients with acute respiratory distress syndrome. Hydrolytic activity of FSAP was inhibited by PAI-1 with a second-order inhibition rate constant (K_a) of 3.38 ± 1.12 × 10⁵m^–1·s^–1. Residue Arg³⁴⁶ was a critical recognition element on PAI-1 for interaction with FSAP. RNA, but not DNA, fragments (>400 nucleotides in length) dramatically enhanced the reactivity of PAI-1 with FSAP, and 4 μg·ml^–1 RNA increased the K_a to 1.61 ± 0.94 × 10⁶m^–1·s^–1. RNA also stabilized the active conformation of PAI-1, increasing the half-life for spontaneous conversion of active to latent PAI-1 from 48.4 ± 8 min to 114.6 ± 5 min. In contrast, little effect of DNA on PAI-1 stability was apparent. Residues Arg⁷⁶ and Lys⁸⁰ in PAI-1 were key elements mediating binding of nucleic acids to PAI-1. FSAP-driven inhibition of vascular smooth muscle cell proliferation was antagonized by PAI-1, suggesting functional consequences for the FSAP-PAI-1 interaction. These data indicate that extracellular RNA and PAI-1 can regulate FSAP activity, thereby playing a potentially important role in hemostasis and cell functions under various pathophysiological conditions, such as acute respiratory distress syndrome.

Production and purification of urokinase: A comprehensive review

An increased emphasis on prevention of fatalities due to thrombovascular disorders is broadening opportunities for several cardiovascular agents, especially plasminogen activators, for preventing strokes and heart attacks. Hence, urokinase, as one of the most potent plasminogen activators is attracting a great deal of attention. Developments in cell lines and bioprocess technology have made it possible to produce urokinase from in vitro cell culture. Attempts are now underway to enhance urokinase production from cell culture through media manipulation, bioreactor cultivation, and innovative purification techniques. Downstream processing also poses an intricate problem due to the complexity of cell culture extracts, susceptibility of urokinase to autocatalytic and proteolytic degradation and due to the presence of plasminogen activator inhibitors in the culture media. Hence, enhancing cellular productivity and downstream product recovery continue to be major challenges as discussed in this review. Furthermore, an approach for integrated upstream and downstream processing is needed to develop an economically viable technology. In the present review the emerging trends in urokinase production and purification have been discussed in detail.

The urokinase receptor (uPAR) and the uPAR-associated protein (uPARAP/Endo180): membrane proteins engaged in matrix turnover during tissue remodeling

The breakdown of the barriers formed by extracellular matrix proteins is a pre-requisite for all processes of tissue remodeling. Matrix degradation reactions take part in specific physiological events in the healthy organism but also represent a crucial step in cancer invasion. These degradation processes involve a highly organized interplay between proteases and their cellular binding sites as well as specific substrates and internalization receptors. This review article is focused on two components, the urokinase plasminogen activator receptor (uPAR) and the uPAR-associated protein (uPARAP, also designated Endo180), that are considered crucially engaged in matrix degradation. uPAR and uPARAP have highly diverse functions, but on certain cell types they interact with each other in a process that is still incompletely understood. uPAR is a glycosyl-phosphatidylinositol-anchored glycoprotein on the surface of various cell types that serves to bind the urokinase plasminogen activator and localize the activation reactions in the proteolytic cascade system of plasminogen activation. uPARAP is an integral membrane protein with a pronounced role in the internalization of collagen for intracellular degradation. Both receptors have additional functions that are currently being unraveled. The present discussion of uPAR and uPARAP is centered on their protein structure and molecular and cellular function.

The pro-urokinase plasminogen-activation system in the presence of serpin-type inhibitors and the urokinase receptor: rescue of activity through reciprocal pro-enzyme activation

The reciprocal pro-enzyme activation system of plasmin, urokinase-type plasminogen activator (uPA) and their respective zymogens is a potent mechanism in the generation of extracellular proteolytic activity. Plasminogen activator inhibitor type 1 (PAI-1) acts as a negative regulator. This system is complicated by a poorly understood intrinsic reactivity of the uPA pro-enzyme (pro-uPA) before proteolytic activation, directed against both plasminogen and PAI-1. We have studied the integrated activation mechanism under the repression of PAI-1 in a purified system. A covalent reaction between pro-uPA and PAI-1 was positively demonstrated but the reaction of PAI-1 with two-chain uPA was found to be at least 1000-fold faster. However, in spite of this very fast inhibition, two-chain uPA still became the dominant plasminogen activator when plasminogen was incubated with pro-uPA and PAI-1. The activity pattern observed under these conditions revealed an initial lag phase, followed by a continuous generation of minute amounts of active two-chain uPA, this uPA having a short lifetime before inhibition but still succeeding to generate new plasmin activity, thus preventing a complete inactivation of the feedback system. This property of the activation system was retained even in the simultaneous presence of PAI-1 and alpha(2)-antiplasmin. Addition of soluble uPA receptor to the system did not change the role of pro-uPA and the same pattern was observed when pro-uPA was bound to the uPA receptor on U937 cells. The present mechanism maintains the system at standby level and may be triggered to increased activity without the need for an external initiating event.

Single-chain urokinase alone or complexed to its receptor in tetracycline-induced pleuritis in rabbits

Intrapleural loculation can increase morbidity in hemothoraces or parapneumonic effusions. Intrapleural fibrin precedes visceral-parietal pleural adhesions. We speculated that single-chain urokinase plasminogen activator alone or bound to its receptor could prevent these adhesions by their relative resistance to local inhibition by plasminogen activator inhibitors. We found that recombinant human single-chain urokinase-bound rabbit pleural mesothelial cells or lung fibroblasts with kinetics similar to that reported for human cells (kD of approximately 5 nM). The receptor-bound fibrinolysin maintained in vitro fibrinolytic activity in the presence of pleural fluids from rabbits with tetracycline-induced pleural injury over 24 hours. In rabbits given intrapleural single-chain urokinase 24 and 48 hours after intrapleural tetracycline (n = 10 animals), adhesions were prevented, whereas the receptor-complexed form (n = 12) attenuated adhesions versus vehicle/tetracycline-treated rabbits (n = 22, p <or= 0.005 in both cases). There were more adhesions in the complex than the single-chain urokinase group (p = 0.02). Residual antigenic but not functional evidence of the interventional agents remained in pleural fluids at 72 hours after tetracycline. No local or systemic bleeding occurred because of either interventional agent. The data demonstrate that single-chain urokinase inhibits, whereas lysin-receptor complexes attenuate, adhesion formation in tetracycline-induced pleural injury in rabbits.

Urokinase-type plasminogen activator up-regulates its own expression by endothelial cells and monocytes via the u-PAR pathway

Signal transduction by urokinase-type plasminogen activator (u-PA) bound to its cell receptor has been well established. In the present study, we found, for the first time to our knowledge, that u-PA promotes its own synthesis by endothelial cells and monocytes. This phenomenon was characterized and shown to involve the u-PA receptor (u-PAR) pathway. The finding may be of general importance, since most cells that express u-PAR also produce u-PA. Human umbilical vein endothelial cells (HUVECs), U937 monocytes, and human peripheral blood monocytes (PFMCs) were incubated with diisopropylfluorophosphate (DFP)-pretreated u-PA, the amino-terminal fragment (ATF) of u-PA, or the kringle domain. A threefold up-regulation of u-PA secretion and synthesis by u-PA or ATF was found. The predominant effect was expressed in HUVECs, in which u-PA mRNA was also up-regulated. The u-PA kringle domain had no effect on u-PA synthesis, leading to the conclusion that the EGF domain was responsible. This was also consistent with the additional finding that the u-PAR, to which the EGF domain binds, was necessary for the up-regulation. The results indicate that u-PA up-regulates itself via its EGF domain and u-PAR. The possibilities that the results were related to displacement of receptor-bound u-PA or the blocking of u-PA incorporation into the cells were excluded. A modest up-regulation of u-PAR was also associated with this phenomenon.

Protease inhibitor 10 inhibits tumor necrosis factor alpha -induced cell death. Evidence for the formation of intracellular high M(r) protease inhibitor 10-containing complexes

Protease inhibitor 10 (PI10) is a member of the ovalbumin family of serine protease inhibitors (ov-serpin) that is expressed at elevated levels in patients with acute myeloid leukemia and chronic myelomonocytic leukemia. Based upon the ability of the related serpin plasminogen activator inhibitor 2 (PAI-2) to protect cells against tumor necrosis factor alpha (TNFalpha)-induced cell death, this study was initiated to investigate the potential cytoprotective activity of PI10. Two different expression systems (i.e. plasmids encoding either PI10 alone or PI10 fused to the tag: enhanced green fluorescent protein, EGFP) were utilized to stably transfect an eukaryotic model cell system (i.e. HeLa cells) that neither expresses PAI-2 nor PI10. The level of PI10 expression in the stable transfectants was found to correlate with their resistance to TNFalpha-induced cell death. Immunoprecipitation/immunoblotting experiments demonstrated that PI10 is able to form SDS-stable complexes (i.e. M(r) >100,000) with a cytosolic protein(s). Increased levels of the PI10-containing complexes can be detected by TNFalpha treatment by preventing intracellular degradative activities with the proteasome inhibitor N-carbobenzyloxy-leucine-leucine-norvalinal. PI10-containing complexes are dissociated with conditions known to separate classical protease-serpin complexes (i.e., 1.5 m ammonium hydroxide in the presence of SDS). These data support a role for the regulation of intracellular protease activities by ov-serpins.

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.

Expression and characterization of a novel plasminogen activator from Agkistrodon halys venom

A venom gland cDNA library of Agkistrodon halys was constructed and screened with a probe based on the consensus sequence of venomic serine proteases. Next, we determined the sequences of the entire open reading frames of two selected positives which were found to encode novel serine proteases of 234 and 233 amino acids in length and named as Haly-PA and Haly 2, respectively. Upon protein data base search, Haly-PA showed the highest similarity of 82% to the previously characterized plasminogen activator, TSV-PA (Zhang et al. 1995, J. Biol. Chem. 270, 10246- 10255). Haly 2 displayed a 78% similarity to beta-fibrinogenase (Hung et al. 1994, B. B. R. C., 205, 1707 1715). Haly-PA was successfully expressed using the baculovirus system and secreted into the culture media as a 32 kDa glycoprotein. In the western analysis of snake venom, anti-Haly-PA antibody detected the same size of band indicating that this enzyme is a component of snake venom. Recombinant Haly-PA was purified to homogeneity using the combination of anion exchange and gel filtration column. In the fibrino(geno)lytic assay, recombinant Haly-PA displayed an indirect fibrino(geno)lytic activity depending on the presence of plasminogen and cleaved the plasminogen to generate the active plasimin. These results indicate that Haly-PA is a plasminogen activator and displays fibrino(geno)lytic activity through conversion of plasminogen to plasmin.

Inhibition of urokinase activity by the antiangiogenic factor 16K prolactin: activation of plasminogen activator inhibitor 1 expression

The N-terminal fragment of PRL (16K PRL) is an antiangiogenic factor that, in vitro, inhibits several components of angiogenesis including basic fibroblast growth factor (bFGF)-induced cell division, migration, and organization of capillary endothelial cells. An essential step in the regulation of angiogenesis is the activation of urokinase (urokinase type plasminogen activator, uPA), which in turn activates a cascade of proteases that play essential roles in endothelial cell migration and tissue remodeling. Treatment of bovine capillary endothelial cells (BBEC) with 16K PRL inhibited bFGF-stimulated urokinase activity in BBEC as detected by plasminogen substrate gel assay. 16K PRL did not appear to be acting via an effect on uPA expression because no change in messenger RNA levels were observed. However, protein levels of plasminogen activator inhibitor-1 (PAI-1), a specific inhibitor of urokinase, were increased by 16K PRL independent of the action of bFGF. The 16K PRL-induced increase in PAI-1 protein levels appear to be the result of increased expression of the PAI-1 gene. Increased production of PAI-1 induced by 16K PRL results in the formation of inactive PAI-1/uPA complexes, consistent with the observed decrease in uPA activity.

Autoactivation of avian urokinase-type plasminogen activator (uPA). A novel mode of initiation of the uPA/plasmin cascade

In contrast to mammalian urokinase-type plasminogen activator (uPA), which is produced and maintained in zymogen form, avian uPA is found in the active two-chain form in cultures of normal and transformed chicken cells in the absence of plasmin, the putative natural activator of pro-uPA. Recombinant chicken uPA (ch-uPAwt) synthesized in two distinct expression systems also presents in the active two-chain form. In addition, conversion to the active uPA in both natural and recombinant expression systems could be prevented by uPA-specific inhibitors including a monoclonal antibody that uniquely inhibits the catalytic activity of ch-uPA. Most significantly, an active site mutant of avian uPA (ch-uPAS353A) that lacks catalytic activity is produced and maintained in single-chain form. Furthermore, the single-chain ch-uPAS353A mutant can be converted to the two-chain form by purified active ch-uPAwt. These results strongly indicate an autocatalytic mechanism of activation of ch-uPA. Autoactivation appears to be an intrinsic property of ch-uPA and may be the initiating molecular event in uPA-mediated proteolytic cascades.

Analysis of the forces which stabilize the active conformation of urokinase-type plasminogen activator

It was recently proposed that hydrophobic interactions control the active conformation of serine proteases in the trypsin family (Hedstrom et al. (1996) Biochemistry 35, 4515-23) rather than a charge interaction with Asp next to the active site Ser, as formerly believed. In the present study, certain site-directed mutants of the serine protease zymogen pro-urokinase (pro-UK) and its two-chain enzymatic derivative urokinase (UK) were characterized. The results provide information on the structure-function of the catalytic domain of pro-UK/UK, which is relevant to this controversy. Mutations at Asp355(c194), which eliminated its charge, induced a 6250-fold reduction in the catalytic activity of UK. By contrast, reducing the hydrophobicity at the neoterminal Ile159(c16) of UK had relatively little effect. However, when both the hydrophobicity and the size of the side chain were reduced by a glycine substitution at this position, a major reduction (9090-fold) in the catalytic efficiency of UK occurred. This effect was related to the smaller side chain increasing the cavity and the flexibility of the N-terminus and thereby interfering with its charge interaction with Asp355(c194). A similar mechanism, rather than a change in hydrophobicity, is believed also to explain the reduction in the stabilization energy of the activation domain observed in a trypsin mutant by Hedstrom et al. (1996). Although hydrophobic interaction facilitated the charge interaction with Asp355(c194), the latter was the primary force which stabilized the active conformation of UK. The charge interaction with Asp355(c194) was also found to be the principal determinant of the intrinsic catalytic activity of single-chain pro-UK. Additionally, the findings confirmed that the KM of pro-UK for its natural substrate was significantly lower than that of UK. Since this same phenomenon was also seen with each of the mutants, the substrate binding pocket of these single-chain zymogens was better formed than that of their two-chain, enzymatic derivatives.

Thrombin Stimulation of Platelets Induces Plasminogen Activation Mediated by Endogenous Urokinase-Type Plasminogen Activator

Gene knockout mice studies indicate that urokinase-type plasminogen activator (u-PA) is importantly involved in fibrinolysis, but its physiologic mechanism of action remains poorly understood. We postulated that platelets may be involved in this mechanism, as they carry a novel receptor for u-PA and a portion of the single-chain u-PA (scu-PA) intrinsic to blood is tightly associated with platelets. Therefore, plasminogen activation by platelet-associated u-PA was studied. When washed platelets were incubated with plasminogen, no plasmin was generated as detected by plasmin synthetic substrate (S2403) hydrolysis; however, after the addition of thrombin, but not other agonists, platelet-dependent plasminogen activation occurred. Plasminogen activation was surface-related, being inhibited by blocking platelet fibrinogen receptors or by preventing plasminogen binding to the thrombin-activated platelet surface. U-PA was identified as the only plasminogen activator responsible and enrichment of platelets with exogenous scu-PA significantly augmented plasminogen activation. These findings appeared paradoxical because thrombin inactivates scu-PA. Indeed, zymograms showed inactivation of scu-PA during the first hour of incubation with even the lowest dose of thrombin used (1 u/mL). However, this was followed by a thrombin dose-dependent (1 to 10 u/mL) partial return of u-PA activity. Reactivation of u-PA was not due to the direct action of thrombin, but required platelets and was found to be related to a platelet lysosomal thiol protease, consistent with cathepsin C. In conclusion, a new pathway of plasminogen activation by platelet-associated endogenous or exogenous scu-PA was demonstrated, which is specifically triggered by thrombin activation of platelets. These findings may help explain u-PA–mediated physiological fibrinolysis and have implications for therapeutic thrombolysis with scu-PA.

Regulation of single chain urokinase binding, internalization, and degradation by a plasminogen activator inhibitor 1-derived peptide

The internalization and degradation of cell-associated urokinase type plasminogen activator (uPA) through the alpha2-macroglobulin receptor/low density lipoprotein-related receptor (alpha2MR/LRP) represent important steps in the control of plasmin formation. Complexes between two chain urokinase (tcuPA) and plasminogen activator type 1 are degraded rapidly whereas single chain urokinase (scuPA) is not, suggesting that alpha2MR/LRP requires specific epitopes in the serpin for effective function. We report an alternative mechanism that may contribute to this process. The binding of scuPA to LM-TK- cells that lack the uPA receptor was stimulated by the hexapeptide EEIIMD, corresponding to amino acids 350-355 of plasminogen activator type 1, which contacts the sequence RHRGGS, corresponding to amino acids 179-184 in uPA. EEIIMD increased the Bmax of scuPA binding 4-fold with the half-maximal effect achieved at a peptide concentration of 50 microM. Stimulation was dependent on the charge on the COOH-terminal amino acid but not on the NH2 terminus of the peptide. EEIIMD also stimulated the internalization and degradation of scuPA. Both the binding and internalization of scuPA in the presence of EEIIMD were blocked by recombinant, 39-kDa alpha2MR/LRP-associated protein as well as by an anti-alpha2MR/LRP antibody. EEIIMD also stimulated the binding of scuPA to purified alpha2MR/LRP. EEIIMD had no effect on the binding of tcuPA or of complexes between scuPA and its receptor. These results suggest that EEIIMD regulates the binding of scuPA with alpha2MR/LRP. These findings also suggest a potential mechanism by which scuPA can be cleared which is independent of activation by plasmin or binding to uPA receptor.

Functional analysis of the cellular receptor for urokinase in plasminogen activation. Receptor binding has no influence on the zymogenic nature of pro-urokinase

Plasminogen activation catalyzed by the urokinase-type plasminogen activator (uPA) constitutes a reciprocal zymogen activation system, as plasmin can efficiently activate pro-uPA, the single-chain zymogenic form of the protease. We have previously shown that the overall efficiency of this plasminogen activation system is greatly enhanced by its assembly on the cell surface, involving binding of pro-uPA to its cellular binding site uPAR, and the concurrent cellular binding of plasminogen. We have now studied the effect of a recombinant soluble form of uPAR (residues 1-277) on the proteolytic reactions of this system. In contrast to the increased efficiencies of plasminogen activation and pro-uPA activation observed with cell-surface uPAR, soluble uPAR had an inhibitory effect on both of these individual reactions. Soluble uPAR also caused no increase in the low, but discernible, intrinsic activity of pro-uPA. Consistent with the observations on the isolated reactions, the overall activity of the pro-uPA-mediated plasminogen activation system was significantly inhibited. These observations confirm the previous interpretation of the observations made with cell-surface uPAR that the mechanism of the enhanced plasmin generation is due to the catalytically favorable interaction of uPAR-bound uPA/pro-uPA with cell-bound plasminogen/plasmin, rather than direct effects on the properties of uPA or pro-uPA on binding to uPAR.