Identification of a flexible loop region (297-313) of urokinase-type plasminogen activator, which helps determine its catalytic activity

Structural Biology and Protein Engineering of Thrombolytics

Myocardial infarction and ischemic stroke are the most frequent causes of death or disability worldwide. Due to their ability to dissolve blood clots, the thrombolytics are frequently used for their treatment. Improving the effectiveness of thrombolytics for clinical uses is of great interest. The knowledge of the multiple roles of the endogenous thrombolytics and the fibrinolytic system grows continuously. The effects of thrombolytics on the alteration of the nervous system and the regulation of the cell migration offer promising novel uses for treating neurodegenerative disorders or targeting cancer metastasis. However, secondary activities of thrombolytics may lead to life-threatening side-effects such as intracranial bleeding and neurotoxicity. Here we provide a structural biology perspective on various thrombolytic enzymes and their key properties: (i) effectiveness of clot lysis, (ii) affinity and specificity towards fibrin, (iii) biological half-life, (iv) mechanisms of activation/inhibition, and (v) risks of side effects. This information needs to be carefully considered while establishing protein engineering strategies aiming at the development of novel thrombolytics. Current trends and perspectives are discussed, including the screening for novel enzymes and small molecules, the enhancement of fibrin specificity by protein engineering, the suppression of interactions with native receptors, liposomal encapsulation and targeted release, the application of adjuvants, and the development of improved production systems.

Characterising the Subsite Specificity of Urokinase-Type Plasminogen Activator and Tissue-Type Plasminogen Activator using a Sequence-Defined Peptide Aldehyde Library

Urokinase-type plasminogen activator (uPA) and tissue-type plasminogen activator (tPA) are two serine proteases that contribute to initiating fibrinolysis by activating plasminogen. uPA is also an important tumour-associated protease due to its role in extracellular matrix remodelling. Overexpression of uPA has been identified in several different cancers and uPA inhibition has been reported as a promising therapeutic strategy. Although several peptide-based uPA inhibitors have been developed, the extent to which uPA tolerates different tetrapeptide sequences that span the P1-P4 positions remains to be thoroughly explored. In this study, we screened a sequence-defined peptide aldehyde library against uPA and tPA. Preferred sequences from the library screen yielded potent inhibitors for uPA, led by Ac-GTAR-H (K_i =18 nm), but not for tPA. Additionally, synthetic peptide substrates corresponding to preferred inhibitor sequences were cleaved with high catalytic efficiency by uPA but not by tPA. These findings provide new insights into the binding specificity of uPA and tPA and the relative activity of tetrapeptide inhibitors and substrates against these enzymes.

Highly effective fibrinolysis by a sequential synergistic combination of mini-dose tPA plus low-dose mutant proUK

Results of thrombolysis by monotherapy with either tPA or proUK have not lived up to expectations. Since these natural activators are inherently complementary, this property can be utilized to a synergistic advantage; and yet, this has undergone little evaluation. ProUK is no longer available because at pharmacological concentrations it converts to UK in plasma. Therefore, a single site proUK mutant, M5, was developed to address this problem and was used in this study. Fibrinolysis was measured using preformed fluoresceinated 24 h old clots in a plasma milieu rather than by the standard automated method, because proUK/M5 is sensitive to inactivation by thrombin and activation by plasmin. The shortest 50% clot lysis time that could be achieved by tPA or M5 alone was determined: mean times were 55 and 48 minutes respectively. These bench marks were matched by 6% of the tPA monotherapy dose combined with 40% that of M5: mean lysis time 47 minutes with less associated fibrinogenolysis. Results showed that the tPA effect was limited to initiating fibrinolysis which was completed by M5 and then tcM5. Plasma C1-inhibitor inhibited fibrinogenolysis by M5, providing protection from side effects not available for proUK. In conclusion, by utilizing the complementary properties and sequential modes of action of each activator, more efficient fibrinolysis with less non-specific effects can be achieved than with traditional monotherapy. In vivo validation is needed, but in a previous clinical trial using a similar combination of tPA and proUK (5% and 50% monotherapy doses) very promising results have already been obtained.

Mutant prourokinase with adjunctive C1-inhibitor is an effective and safer alternative to tPA in rat stroke

A single-site mutant (M5) of native urokinase plasminogen activator (prouPA) induces effective thrombolysis in dogs with venous or arterial thrombosis with a reduction in bleeding complications compared to tPA. This effect, related to inhibition of two-chain M5 (tcM5) by plasma C1-inhibitor (C1I), thereby preventing non-specific plasmin generation, was augmented by the addition of exogenous C1I to plasma in vitro. In the present study, tPA, M5 or placebo +/- C1I were administered in two rat stroke models. In Part-I, permanent MCA occlusion was used to evaluate intracranial hemorrhage (ICH) by the thrombolytic regimens. In Part II, thromboembolic occlusion was used with thrombolysis administered 2 h later. Infarct and edema volumes, and ICH were determined at 24 h, and neuroscore pre (2 h) and post (24 h) treatment. In Part I, fatal ICH occurred in 57% of tPA and 75% of M5 rats. Adjunctive C1I reduced this to 25% and 17% respectively. Similarly, semiquantitation of ICH by neuropathological examination showed significantly less ICH in rats given adjunctive C1I compared with tPA or M5 alone. In Part-II, tPA, M5, and M5+C1I induced comparable ischemic volume reductions (>55%) compared with the saline or C1I controls, indicating the three treatments had a similar fibrinolytic effect. ICH was seen in 40% of tPA and 50% of M5 rats, with 1 death in the latter. Only 17% of the M5+C1I rats showed ICH, and the bleeding score in this group was significantly less than that in either the tPA or M5 group. The M5+C1I group had the best Benefit Index, calculated by dividing percent brain salvaged by the ICH visual score in each group. In conclusion, adjunctive C1I inhibited bleeding by M5, induced significant neuroscore improvement and had the best Benefit Index. The C1I did not compromise fibrinolysis by M5 in contrast with tPA, consistent with previous in vitro findings.

C1-inhibitor prevents non-specific plasminogen activation by a prourokinase mutant without impeding fibrin-specific fibrinolysis

BACKGROUND

Prourokinase (prouPA) is unstable in plasma at therapeutic concentrations. A mutant form, M5, made more stable by reducing its intrinsic activity was therefore developed. Activation to two-chain M5 (tcM5) induced a higher catalytic activity than two-chain urokinase plasminogen activator (tcuPA), implicating an active site functional difference. Consistent with this, an unusual tcM5 complex with plasma C1-inhibitor was recently described in dog and human plasma. The effect of C1-inhibitor on fibrinolysis and fibrinogenolysis by M5 is the subject of this study.

METHODS AND RESULTS

Zymograms of tcM5 and tcuPA incubated in plasma revealed prominent tcM5-C1-inhibitor complexes, which formed within 5 min. The inhibition rate by purified human C1-inhibitor (250 microg mL(-1)) was about 7-fold faster for tcM5 than it was for tcuPA (10 microg mL(-1)). The effect of the inhibitor on the stability of M5 and prouPA was determined by incubating them in plasma at high concentrations (10-20 microg mL(-1)) +/- C1-inhibitor supplementation. Above 10 microg mL(-1), depletion of all plasma plasminogen occurred, indicating plasmin generation and tcM5/tcuPA formation. With supplemental C1-inhibitor, M5 stability was restored but not prouPA stability. Clot lysis by M5 +/- supplemental C1-inhibitor showed no attenuation of the rate of fibrinolysis, whereas fibrinogenolysis was prevented by C1-inhibitor. Moreover, because of higher dose-tolerance, the rate of fibrin-specific lysis reached that achievable by non-specific fibrinolysis without inhibitor.

CONCLUSIONS

Plasma C1-inhibitor stabilized M5 in its proenzyme configuration in plasma by inhibiting tcM5 and thereby non-specific plasminogen activation. At the same time, fibrin-specific plasminogen activation remained unimpaired. This unusual dissociation of effects has significant implications for improving the safety and efficacy of fibrinolysis.

Thrombin-thrombomodulin inhibits prourokinase-mediated pleural mesothelial cell-dependent fibrinolysis

Fibrin deposition is a hallmark of pleural inflammation and loculation but understanding of mechanisms by which mesothelial cells regulate intrapleural fibrinolysins remains incomplete. We speculated that pleural mesothelial cells regulate local fibrinolytic capacity via processing of single chain urokinase type plasminogen activator (scuPA). Pretreatment of human pleural mesothelial (MeT-5A) cells with TGF-beta or thrombin, either alone or in combination, inhibited urokinase (uPA)-mediated fibrinolysis by MeT-5A. Thrombin, unlike TGF-beta, inhibited fibrinolysis without induction of PAI-1, suggesting that thrombin-mediated cleavage of scuPA inhibits the fibrinolytic capacity of MeT-5A cells. Thrombin cleaves both purified scuPA as well as that secreted by MeT-5A cells and cell surface thrombomodulin accelerates thrombin-mediated cleavage of scuPA to inhibit cellular fibrinolytic activity. Molecular dynamics analyses demonstrated that thrombin-cleaved scuPA (uPAt) do not acquire a catalytically active conformation and that secondary plasminogen binding sites of uPA implicated in plasminogen activation are distorted in uPAt, explaining, at least in part, why uPAt is a poor enzyme. uPAt was detectable in transudative and exudative pleural effusions from patients. Intrapleural administration of scuPA generated increased levels of uPAt in PF of rabbits with pleural injury and loculation induced by tetracycline in vivo. This pathway is operative in diverse forms of pleural injury, restricts the urokinase-dependent fibrinolytic capacity of pleural mesothelial cells and contributes to local control of fibrinolytic activity via processing of endogenous or exogenous scuPA within the pleural compartment.

Thrombolysis vs. bleeding from hemostatic sites by a prourokinase mutant compared with tissue plasminogen activator

BACKGROUND

A single site mutant (M5) of prourokinase (proUK) was developed to make proUK less vulnerable to spontaneous activation in plasma. This was a problem that seriously compromised proUK in clinical trials, as it precluded proUK-mediated fibrinolysis at therapeutic concentrations.

METHODS AND RESULTS

After completing dose-finding studies, 12 anesthetized dogs with femoral artery thrombosis were given either M5 (2.0 mg kg(-1)) or tissue plasminogen activator (t-PA) (1.4 mg kg(-1)) by i.v. infusion over 60 min (20% administered as a bolus). Two pairs of standardized injuries were inflicted at which hemostasis was completed prior to drug administration. Blood loss was quantified by measuring the hemoglobin in blood absorbed from these sites. Thrombolysis was evaluated at 90 min and was comparably effective by both activators. Rethrombosis developed in one t-PA dog. The principal difference found was that blood loss was 10-fold higher with t-PA (mean approximately 40 mL) than with M5 (mean approximately 4 mL) (P = 0.026) and occurred at more multiple sites (mean 2.7 vs. 1.2). This effect was postulated to be related to differences in the mechanism of plasminogen activation by t-PA and M5 in which the latter is promoted by degraded rather than intact (hemostatic) fibrin. In addition, two-chain M5 was efficiently inactivated by plasma C1 inactivator, an exceptional property which helped contain its non-specific proteolytic effect.

CONCLUSIONS

Intravascular thrombolysis by M5 was accompanied by significantly less bleeding from hemostatic sites than by t-PA. This was attributed to the proUK paradigm of fibrinolysis being retained at therapeutic concentrations by the mutation.

Mutagenesis at Pro309 of single-chain urokinase-type plasminogen activator alters its catalytic properties

The charge of Lys300(c143) located within a flexible loop(297-313) of sc-uPA has been identified as an important determinant for its high intrinsic activity. Mutations affecting the flexibility of the loop also modulate the intrinsic activity. Glu-plasminogen activation by sc-uPA is strongly promoted by fibrin fragment E but not fibrin fragment D-dimer, whereas plasminogen activation by t-PA is strongly promoted by fragment D-dimer but not fragment E. To further investigate the effect of conformation changes in the flexible loop on catalytic properties of sc-uPA, cassette mutations at Pro309(c152) were made and characterized. It was found that the activation of Pro309(c152) mutants by Lys-plasmin was only moderately affected. In contrast, the intrinsic and two-chain activities of Pro309(c152) mutants against S2444 were both significantly decreased. The two-chain activities of these mutants against Glu-plasminogen were also reduced in a range of 1.1- to 127-fold. The mutations of Pro309(c152) to Trp/Phe and Arg/Asp more significantly affected both intrinsic and two-chain activities, while only a moderate decrease in activity was found with mutations to Ala/Ser/Thr. In contrast to wild-type sc-uPA, plasminogen activation by Pro309(c152) mutants was found to be promoted by both fibrin fragment E and D-dimer. In the presence of 2.0 microM D-dimer, plasminogen activation by mutant Pro309(c152) --> His was promoted by 22-fold, while only 2.0-fold promotion was found with mutant Pro309(c152) --> Gly. In conclusion, these findings demonstrated that conformation changes in the flexible loop of sc-uPA not only affect its intrinsic and two-chain activity, but also extend its promotion of plasminogen activation by fragment E to D-dimer.

Prourokinase mutant that induces highly effective clot lysis without interfering with hemostasis

Prourokinase (proUK) is a zymogenic plasminogen activator that at pharmacological doses is prone to nonspecific activation to urokinase. This has handicapped therapeutic exploitation of its fibrin-specific physiological properties. To attenuate this susceptibility without compromising specific activation of proUK on a fibrin clot, a Lys300-->His mutation (M5) was developed. M5 had a lower intrinsic activity and, therefore, remained stable in plasma at a 4-fold higher concentration than did proUK. M5 had a higher 2-chain activity and induced more rapid plasminogen activation and fibrin-specific clot lysis in vitro. Sixteen dogs embolized with radiolabeled clots were infused with saline, proUK, tissue plasminogen activator, or M5. The lower intrinsic activity allowed a higher infusion rate with M5, which induced the most rapid and efficient clot lysis (50% clot lysis by approximately 600 microg/kg M5 versus approximately 1200 microg/kg proUK). In association with this, M5 caused neither a significant increase in the primary bleeding time nor secondary bleeding (total blood loss). By contrast, these measurements increased 4-fold and 5-fold, respectively, with proUK and >5-fold and 8-fold, respectively, with tissue plasminogen activator. Clot lysis by M5 and hemostasis were further evaluated in 6 rhesus monkeys. M5 again induced rapid clot lysis without a significant increase in the primary bleeding time, and secondary bleeding did not occur. In conclusion, a site-directed mutation designed to improve the stability of proUK in blood at therapeutic concentrations induced superior clot lysis in vitro and in vivo without causing significant interference with hemostasis.

Fibrinolysis: an unfinished agenda

There has been a recent decline in interest in fibrinolysis, suggesting that its physiological basis is sufficiently understood and that therapeutic thrombolysis has reached its limit. The importance of the subject has not diminished since cardiovascular disease is now a leading health problem even in developing countries. Certain highlights and inconsistencies are reviewed. The clinical trials of tissue plasminogen activator (t-PA) revealed a major discrepancy between its fibrinolytic efficacy and its clinical benefit (the 't-PA paradox') that is unexplained. Dose-finding studies also showed that the fibrinolytic efficacy of t-PA required significant nonspecific plasminogen activation. Furthermore, the longstanding belief that t-PA is responsible for physiological fibrinolysis and urokinase-type PA (u-PA) for pericellular plasminogen activation is belied by extensive experimental animal data, but these findings have had little impact on traditional thinking. As a result, the mechanisms responsible for the u-PA paradigm of fibrinolysis have received little attention. Clinical experience with pro-u-PA remains limited and most clinical trials have used infusion rates at which pro-u-PA is largely converted systemically to urokinase. This is due to the unanticipated instability of pro-u-PA in plasma at pharmacological concentrations. Insufficient understanding of basic mechanisms of fibrinolysis has handicapped the design of chimeric or mutant activators. It is submitted that physiological fibrinolysis remains to be better defined, and that it is premature to conclude that therapeutic thrombolysis will be inevitably accompanied by side effects that undermine this method of inducing reperfusion.

Chemotactic effect of urokinase plasminogen activator: a major role for mechanisms independent of its proteolytic or growth factor domains

Urokinase type plasminogen activator (uPA) converts plasminogen to plasmin and is highly chemotactic for many cell types. We examined, using recombinant wild type and mutated forms of uPA, the extent to which its proteolytic properties, its growth-like domain (GFD) and/or interactions with the specific receptor (uPAR) contribute to the chemotactic activity towards vascular smooth muscle cells (SMC). Recombinant wild type uPA (r-uPA) stimulated cell migration nearly 5.8-fold, inactive r-uPA, with a mutation in the catalitic domain (r-uPA(H/Q)), 3-fold, uPA without growth factor like domain (r-uPA(GFD )), 2.6-fold, and a form containing both mutations (r-uPA(H/Q, GFD ), 3.3-fold. All recombinant forms of uPA, wild type and those with mutations were equally and highly effective (IC50 approximately 20 nM) in displacing 125I-r-uPA bound to SMC. These results indicate that additional mechanisms, not dependent on uPA's proteolytic activity or the binding ability of its GFD to uPAR, are the major contributors to its chemotactic action on SMC.

Mutational analysis of loading of iron into rat liver ferritin by ceruloplasmin

Site-directed mutagenesis was used to investigate the loading of iron into rat liver ferritin by ceruloplasmin. Changes were made in the H chain to investigate the role of tyrosines involved in an inherent ferroxidase activity thought to be involved in the self-loading of iron into ferritin. Mutation Y34F affected the rate of iron loading by ceruloplasmin and incorporation of the oxidized iron into the core. Mutation Y29R (making it analogous to the L chain) had no effect on iron oxidation but slightly decreased core formation. A double mutation in the L chain, to open the alpha-helix bundle channel, and R25Y, making the protein more analogous to the H chain, increased the amount of iron incorporated into the core, again suggesting that this Tyr is involved in ligand exchange for core formation. Additional changes in the L chain involving the BC loop suggest that the entire BC loop is involved in the association of ferritin with ceruloplasmin, increasing its ferroxidase activity and the rate of iron loading into ferritin.

Studies on the interaction between ferritin and ceruloplasmin

We showed previously that ceruloplasmin associates with the H chain of rat liver ferritin during iron loading into ferritin such that the iron oxidized by ceruloplasmin was deposited into ferritin [S.-H. Juan et al. (1997) Arch. Biochem. Biophys. 341, 280-286]. Three synthetic decapeptides derived from domains 2, 4, and 6 of ceruloplasmin, referred to CP-2, CP-4, and CP-6, were utilized to identify a possible binding site on ceruloplasmin for ferritin. Two of the peptides, CP-4 and CP-6, were found to inhibit iron loading into the recombinant ferritin H chain homopolymer (rH-Ft) by ceruloplasmin. The extent of inhibition of iron loading into ferritin by ceruloplasmin by CP-6, but not CP-4, varied with pH, whereas the inhibitory effect remained constant in increasing concentrations of NaCl. The addition of rH-Ft quenched the fluorescence emission of CP-4 and CP-6, but not CP-2. The quenching of fluorescence was used to estimate dissociation constants for the peptides. Iron loading into ferritin in Hepes buffer was not affected in the presence of these peptides. In addition, synthetic peptides corresponding to the BC loop of ferritin H and L chains were utilized to localize an interaction site on ferritin for ceruloplasmin. The BC loop of H chain but not L chain of ferritin stimulated the ferroxidase activity of ceruloplasmin. Only the BC loop of ferritin H chain decreased the amount of iron loading into ferritin by ceruloplasmin.