Plasminogen activator inhibitor type-1 interacts exclusively with the proteinase domain of tissue plasminogen activator

Hemorrhagic shock and tissue injury provoke distinct components of trauma-induced coagulopathy in a swine model

INTRODUCTION

Tissue injury (TI) and hemorrhagic shock (HS) are the major contributors to trauma-induced coagulopathy (TIC). However, the individual contributions of these insults are difficult to discern clinically because they typically coexist. TI has been reported to release procoagulants, while HS has been associated with bleeding. We developed a large animal model to isolate TI and HS and characterize their individual mechanistic pathways. We hypothesized that while TI and HS are both drivers of TIC, they provoke different pathways; specifically, TI reduces time to clotting, whereas, HS decreases clot strength stimulates hyperfibrinolysis.

METHODS

After induction of general anesthesia, 50 kg male, Yorkshire swine underwent isolated TI (bilateral muscle cutdown of quadriceps, bilateral femur fractures) or isolated HS (controlled bleeding to a base excess target of - 5 mmol/l) and observed for 240 min. Thrombelastography (TEG), calcium levels, thrombin activatable fibrinolysis inhibitor (TAFI), protein C, plasminogen activator inhibitor 1 (PAI-1), and plasminogen activator inhibitor 1/tissue-type plasminogen activator complex (PAI-1-tPA) were analyzed at pre-selected timepoints. Linear mixed models for repeated measures were used to compare results throughout the model.

RESULTS

TI resulted in elevated histone release which peaked at 120 min (p = 0.02), and this was associated with reduced time to clot formation (R time) by 240 min (p = 0.006). HS decreased clot strength at time 30 min (p = 0.003), with a significant decline in calcium (p = 0.001). At study completion, HS animals had elevated PAI-1 (p = 0.01) and PAI-1-tPA (p = 0.04), showing a trend toward hyperfibrinolysis, while TI animals had suppressed fibrinolysis. Protein C, TAFI and skeletal myosin were not different among the groups.

CONCLUSION

Isolated injury in animal models can help elucidate the mechanistic pathways leading to TIC. Our results suggest that isolated TI leads to early histone release and a hypercoagulable state, with suppressed fibrinolysis. In contrast, HS promotes poor clot strength and hyperfibrinolysis resulting in hypocoagulability.

Tranexamic Acid for Prevention of Hemorrhage in Elective Repeat Cesarean Delivery - A Randomized Study

BACKGROUND

The American College of Obstetricians and Gynecologists states that data is insufficient to recommend Tranexamic acid (TXA) prophylaxis for postpartum hemorrhage.

OBJECTIVE

This study's objective was to evaluate if prophylactic TXA reduces calculated blood loss versus placebo in women undergoing elective repeat cesarean delivery.

STUDY DESIGN

A double-blind, randomized, placebo-controlled trial, examining calculated blood loss with prophylactic doses of 1-gram of TXA given before skin incision and after placental delivery and standard uterotonics in women with singleton pregnancies at least 37 weeks' gestation, presenting for their second or third cesarean delivery under neuraxial anesthesia. The primary outcome was calculated blood loss at 24 hours. The calculation was based on the participant's height, weight, and the difference in hematocrit before the start of surgery and 24 hours after delivery. Prespecified secondary outcomes were quantification of maternal coagulation activity during the perioperative course. A sample size of 50 women per group was planned (N=100), based on a meta-analysis of mean reduction in blood loss after TXA.

RESULTS

723 women were screened, and 110 women were randomized as follows: 55 to TXA and 55 to placebo. The primary outcome of mean calculated blood for TXA (2274 ± 469 mL) and the placebo group (2407 ± 388 mL), p > 0.05. In the secondary outcomes, D-dimer levels were lower in the TXA group than the placebo group 24 hours after delivery (2.1 ± 1.2 µg/mL versus 4.3 ± 2.4 µg/mL), p < 0.001.

CONCLUSIONS

Prophylactic tranexamic acid did not decrease mean calculated blood loss. Significantly less participants had calculated blood loss greater than 2000 mL in the tranexamic acid group compared to the placebo group with lower levels of D-dimer at 24 hours.

Physico-Chemical and Biological Properties of Biosimilar and Reference Tissue Plasminogen Activator Products

Recombinant tissue plasminogen activator (international nonproprietary name — alteplase) which was developed by «GENERIUM» (Russia) and received a marketing authorisation in Russia is completely analogous to Actilyse® which is used to treat medical conditions accompanied by thrombosis, such as acute myocardial infarction, pulmonary embolism, and ischemic stroke. The aim of the study was to carry out a comprehensive comparison of physico-chemical and biological properties of Revelyse® and the reference product Actilyse® in order to assess their biosimilarity. Materials and Methods: comparative peptide mapping and determination of comparability of chromatographic profiles of tryptic hydrolysates was performed using RP-HPLC and massspectrometry; the molecular weight distribution was determined by mass-spectrometry and polyacrylamide gel electrophoresis (Laemmli method). The purity and homogeneity of products as well as the content of related impurities (oligomers and fragments) were determined using gel filtration; N-glycosylation profile was analysed by hydrophilic HPLC, total sialic acid was quantified by the Svennerholm resorcinol method. Protein binding to fibrin and human fibrinogen was assessed by surface plasmon resonance, and the specific activity was compared by fibrin clot lysis. Results: the research demonstrated a complete overlap of the products’ peptide maps, which indicates the identity of аlteplase amino acid sequences in the two medicines being compared. The authors of the study also determined the molecular weight and the content of the intact single-stranded form of the protein, and quantified post-translational modifications, the content of sialic acids and neutral sugars. The analysis of the N-glycosylation profile revealed insignificant differences in the percentage of multiantenna complex glycans. The specificity of alteplase was evaluated by analysing the formation of protein complexes with natural alteplase ligands – fibrin and plasminogen activator inhibitor-1, but no significant differences were found. The comparison of specific activation of plasminogen fibrinolytic activity was performed based on the results of the assay analysing the fibrin clot lysis rate, and it demonstrated comparability of Revelyse® and Actilyse®. Conclusions: comparative experimental studies have shown no differences in the structure, charge distribution heterogeneity, impurities content, and specific activity of alteplase as a component of Revelyse® and the reference product Actilyse®, which leads to the conclusion that they are similar in terms of physicochemical and biological properties.

Overwhelming tPA release, not PAI-1 degradation, is responsible for hyperfibrinolysis in severely injured trauma patients

BACKGROUND

Trauma-induced coagulopathy (TIC) is associated with a fourfold increased risk of mortality. Hyperfibrinolysis is a component of TIC, but its mechanism is poorly understood. Plasminogen activation inhibitor (PAI-1) degradation by activated protein C has been proposed as a mechanism for deregulation of the plasmin system in hemorrhagic shock, but in other settings of ischemia, tissue plasminogen activator (tPA) has been shown to be elevated. We hypothesized that the hyperfibrinolysis in TIC is not the result of PAI-1 degradation but is driven by an increase in tPA, with resultant loss of PAI-1 activity through complexation with tPA.

METHODS

Eighty-six consecutive trauma activation patients had blood collected at the earliest time after injury and were screened for hyperfibrinolysis using thrombelastography (TEG). Twenty-five hyperfibrinolytic patients were compared with 14 healthy controls using enzyme-linked immunosorbent assays for active tPA, active PAI-1, and PAI-1/tPA complex. Blood was also subjected to TEG with exogenous tPA challenge as a functional assay for PAI-1 reserve.

RESULTS

Total levels of PAI-1 (the sum of the active PAI-1 species and its covalent complex with tPA) are not significantly different between hyperfibrinolytic trauma patients and healthy controls: median, 104 pM (interquartile range [IQR], 48-201 pM) versus 115 pM (IQR, 54-202 pM). The ratio of active to complexed PAI-1, however, was two orders of magnitude lower in hyperfibrinolytic patients than in controls. Conversely, total tPA levels (active + complex) were significantly higher in hyperfibrinolytic patients than in controls: 139 pM (IQR, 68-237 pM) versus 32 pM (IQR, 16-37 pM). Hyperfibrinolytic trauma patients displayed increased sensitivity to exogenous challenge with tPA (median LY30 of 66.8% compared with 9.6% for controls).

CONCLUSION

Depletion of PAI-1 in TIC is driven by an increase in tPA, not PAI-1 degradation. The tPA-challenged TEG, based on this principle, is a functional test for PAI-1 reserves. Exploration of the mechanism of up-regulation of tPA is critical to an understanding of hyperfibrinolysis in trauma.

LEVEL OF EVIDENCE

Prognostic and epidemiologic study, level II.

Characterization of a small molecule inhibitor of plasminogen activator inhibitor type 1 that accelerates the transition into the latent conformation

A novel class of small molecule inhibitors for plasminogen activator inhibitor type 1 (PAI-1), represented by AZ3976, was identified in a high throughput screening campaign. AZ3976 displayed an IC(50) value of 26 μm in an enzymatic chromogenic assay. In a plasma clot lysis assay, the compound was active with an IC(50) of 16 μm. Surprisingly, AZ3976 did not bind to active PAI-1 but bound to latent PAI-1 with a K(D) of 0.29 μm at 35 °C and a binding stoichiometry of 0.94, as measured by isothermal calorimetry. Reversible binding was confirmed by surface plasmon resonance direct binding experiments. The x-ray structure of AZ3976 in complex with latent PAI-1 was determined at 2.4 Å resolution. The inhibitor was bound in the flexible joint region with the entrance to the cavity located between α-helix D and β-strand 2A. A set of surface plasmon resonance experiments revealed that AZ3976 inhibited PAI-1 by enhancing the latency transition of active PAI-1. Because AZ3976 only had measurable affinity for latent PAI-1, we propose that its mechanism of inhibition is based on binding to a small fraction in equilibrium with active PAI-1, a latent-like prelatent form, from which latent PAI-1 is then generated more rapidly. This mode of action, with induced accelerated latency transition of active PAI-1 may, together with supporting x-ray data, provide improved opportunities for small molecule drug design in the hunt for therapeutically useful PAI-1 inhibitors.

Ensembles of uncertain mathematical models can identify network response to therapeutic interventions

The role of mechanistic modeling and systems biology in molecular medicine remains unclear. In this study, we explored whether uncertain models could be used to understand how a network responds to a therapeutic intervention. As a proof of concept, we modeled and analyzed the response of the human coagulation cascade to recombinant factor VIIa (rFVIIa) and prothrombin (fII) addition in normal and hemophilic plasma. An ensemble of parametrically uncertain human coagulation models was developed (N = 437). Each model described the time evolution of 193 proteins and protein complexes interconnected by 301 interactions under quiescent flow. The 467 unknown model parameters were estimated, using multiobjective optimization, from published in vitro coagulation studies. The model ensemble was validated using published in vitro thrombin measurements and thrombin measurements taken from coronary artery disease patients. Sensitivity analysis was then used to rank-order the importance of model parameters as a function of experimental or physiological conditions. A novel strategy for the systematic comparison of ranks identified a family of fX/FXa and fII/FIIa interactions that became more sensitive with decreasing fVIII/fIX. The fragility of these interactions was preserved following the addition of exogenous rFVIIa and fII. This suggested that exogenous rFVIIa did not alter the qualitative operation of the cascade. Rather, exogenous rFVIIa and fII took advantage of existing fluid and interfacial fX/FXa and fII/FIIa sensitivity to restore normal coagulation in low fVIII/fIX conditions. The proposed rFVIIa mechanism of action was consistent with experimental literature not used in model training. Thus, we demonstrated that an ensemble of uncertain models could unravel key facets of the mechanism of action of a focused intervention. Whereas the current study was limited to coagulation, perhaps the general strategy used could be extended to other molecular networks relevant to human health.

Heterogeneous glycosylation patterns of human PAI-1 may reveal its cellular origin

The main inhibitor of intravascular fibrinolysis is plasminogen activator inhibitor 1 (PAI-1) which binds to and irreversibly inhibits tissue plasminogen activator (tPA). PAI-1 is present in blood, both in platelets and in plasma, and PAI-1 levels are associated with risk of atherothrombosis. Several tissues express PAI-1 but the source of plasma PAI-1 is not known. We recently found that platelets can de novo synthesize PAI-1 and the amount synthesized in vitro in 24 hours is 35-fold higher than required to maintain normal plasma levels. Recombinant human PAI-1 expressed in different cell types or secreted naturally by human cell lines, exhibit heterogeneous glycosylation patterns. The aim of this study was to investigate the hypothesis that platelets might be the source of plasma PAI-1 and that the cellular source of PAI-1 can be determined by its tissue-specific glycosylation pattern. PAI-1 was isolated from platelets, macrophages, endothelial cells, adipose tissue, as well as plasma from lean and obese subjects. The glycosylation was analyzed by nanoLC-MS/MS. PAI-1 isolated from cell lysates and conditioned media from macrophages, endothelial cells, and adipose tissue expressed heterogeneous glycosylation patterns. By contrast, no glycans were detected on PAI-1 isolated from plasma or platelets from healthy lean individuals. Hence, our data suggest that platelets may be the main source of plasma PAI-1 in lean individuals. Interestingly, plasma PAI-1 from obese subjects had a glycan composition similar to that of adipose tissue suggesting that obese subjects with elevated PAI-1 levels may have a major contribution from other tissues.

Characterization of a small molecule PAI-1 inhibitor, ZK4044

Plasminogen activator inhibitor-1 (PAI-1) is a key negative regulator of the fibrinolytic system. In animal studies, inhibition of PAI-1 activity prevents arterial and venous thrombosis, indicating that PAI-1 inhibitors may be used as a new class of antithrombotics. In this study, we characterize a small molecule PAI-1 inhibitor, ZK4044, which was identified by high throughput screening and chemically optimized. In a chromogenic substrate-based urokinse (uPA)/PAI-1 assay and a tissue-type plasminogen activator (tPA)-mediated clot lysis assay, ZK4044 inhibited human PAI-1 activity with IC50 values of 644+/-255 and 100+/-90 nM, respectively. ZK4044 had no detectable inhibitory activity toward other serpins such as antithrombin III, alpha1-antitrypsin and alpha2-antiplasmin, indicating that ZK4044 is a specific PAI-1 inhibitor. ZK4044 was shown to bind directly to PAI-1 and prevent the binding of PAI-1 to tPA in a dose-dependent manner in surface plasmon resonance Biacore-based experiments. ZK4044 also prevented PAI-1/tPA complex formation, as analyzed by SDS/PAGE. ZK4044 had little effect on elastase-mediated cleavage of active PAI-1, indicating that the primary mode of action of ZK4044 is most likely to directly block the PAI-1/tPA interaction rather than to convert active PAI-1 to latent PAI-1. In the chromogenic substrate-based uPA/PAI-1 assay, ZK4044 was approximately 2-fold less potent against a mutant PAI-1 (14B-1), which contains four mutations at N150H, K154T, Q319L and M354I, compared with wild-type PAI-1, suggesting that the ZK4044 binding site on the surface of PAI-1 is close to these mutant residues. Together, our data show that ZK4044 represents a new class of small molecule PAI-1 inhibitors with anti-thrombotic potential.

Protonation State of a Single Histidine Residue Contributes Significantly to the Kinetics of the Reaction of Plasminogen Activator Inhibitor-1 with Tissue-type Plasminogen Activator

Stopped-flow fluorometry was used to study the kinetics of the reactive center loop insertion occurring during the reaction of N-((2-(iodoacetoxy)ethyl)-N-methyl)amino-7-nitrobenz-2-oxa-3-diazole (NBD) P9 plasminogen activator inhibitor-1 (PAI-1) with tissue-(tPA) and urokinase (uPA)-type plasminogen activators and human pancreatic elastase at pH 5.5-8.5. The limiting rate constants of reactive center loop insertion (k(lim)) and concentrations of proteinase at half-saturation (K(0.5)) for tPA and uPA and the specificity constants (k(lim)/K(0.5)) for elastase were determined. The pH dependences of k(lim)/K(0.5) reflected inactivation of each enzyme due to protonation of His57 of the catalytic triad. However, the specificity of the inhibitory reaction with tPA and uPA was notably higher than that for the substrate reaction catalyzed by elastase. pH dependences of k(lim) and K(0.5) obtained for tPA revealed an additional ionizable group (pKa, 6.0-6.2) affecting the reaction. Protonation of this group resulted in a significant increase in both k(lim) and K(0.5) and a 4.6-fold decrease in the specificity of the reaction of tPA with NBD P9 PAI-1. Binding of monoclonal antibody MA-55F4C12 to PAI-1 induced a decrease in k(lim) and K(0.5) at any pH but did not affect either the pKa of the group or an observed decrease in k(lim)/K(0.5) due to protonation of the group. In contrast to tPA, the k(lim) and K(0.5) for the reactions of uPA with NBD P9 PAI-1 or its complex with the monoclonal antibody were independent of pH in the 6.5-8.5 range. Since slightly acidic pH is a feature of a number of malignant tumors, alterations in PAI-1/tPA kinetics could play a role in the cancerogenesis. Changes in the protonation state of His(188), which is placed closely to the S1 site and is unique for tPA, has been proposed to contribute to the observed pH dependences of k(lim) and K(0.5).

Purification of soluble and membrane-bound proteases with substrate-analogous inhibitors by affinity chromatography

Specific modified substrate-analogous amino acids and peptides have been used as affinity ligands in the affinity chromatography of proteases. Alanine methyl ketone-Sepharose (AMK-Sepharose) is introduced as affinity support for the purification of a bacterial alanyl aminopeptidase (AAP) from a membrane protein extract and Arginine-Agarose as support for the preparation of a membrane-bound proteinase of myeloma cells (MP-1). Peptidyl methyl ketones as affinity ligands have been used to separate subtilisin enzymes and the cysteine proteases cathepsin B, L, and S. As a new type of ligands, spacer-bound peptidyl chloromethyl ketones are presented for a specific and oriented immobilization of proteinases. Oriented-immobilized cathepsin B was used to isolate antibodies against this enzyme.

Acute phase protein alpha 1-acid glycoprotein interacts with plasminogen activator inhibitor type 1 and stabilizes its inhibitory activity

alpha(1)-Acid glycoprotein, one of the major acute phase proteins, was found to interact with plasminogen activator inhibitor type 1 (PAI-1) and to stabilize its inhibitory activity toward plasminogen activators. This conclusion is based on the following observations: (a) alpha(1)-acid glycoprotein was identified to bind PAI-1 by a yeast two-hybrid system. Three of 10 positive clones identified by this method to interact with PAI-1 contained almost the entire sequence of alpha(1)-acid glycoprotein; (b) this protein formed complexes with PAI-1 that could be immunoprecipitated from both the incubation mixtures and blood plasma by specific antibodies to either PAI-1 or alpha(1)-acid glycoprotein. Such complexes could be also detected by a solid phase binding assay; and (c) the real-time bimolecular interactions monitored by surface plasmon resonance indicated that the complex of alpha(1)-acid glycoprotein with PAI-1 is less stable than that formed by vitronectin with PAI-1, but in both cases, the apparent K(D) values were in the range of strong interactions (4.51 + 1.33 and 0.58 + 0.07 nm, respectively). The on rate for binding of PAI-1 to alpha(1)-glycoprotein or vitronectin differed by 2-fold, indicating much faster complex formation by vitronectin than by alpha(1)-acid glycoprotein. On the other hand, dissociation of PAI-1 bound to vitronectin was much slower than that from the alpha(1)-acid glycoprotein, as indicated by 4-fold lower k(off) values. Furthermore, the PAI-1 activity toward urokinase-type plasminogen activator and tissue-type plasminogen activator was significantly prolonged in the presence of alpha(1)-acid glycoprotein. These observations suggest that the complex of PAI-1 with alpha(1)-acid glycoprotein can play a role as an alternative reservoir of the physiologically active form of the inhibitor, particularly during inflammation or other acute phase reactions.

Protein movement during complex-formation between tissue plasminogen activator and plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) rapidly inactivates tissue plasminogen activator (tPA). After initial binding and cleavage of the reactive-centre loop of PAI-1, this complex is believed to undergo a major rearrangement. Using surface plasmon resonance and SDS-PAGE, we have studied the influence of a panel of monoclonal antibodies on the reaction leading to the final covalent complex. On the basis of these data, we suggest the mechanisms for the action of different classes of inhibitory antibodies. We propose that the antibodies which convert PAI-1 into a substrate for tPA do this by means of preventing the conversion of the initial PAI-1/tPA complex into the final complex by sterical intervention. Moreover, the localisation of the binding epitopes on free PAI-1, as well as on the PAI-1/tPA complex, suggests that tPA in the final complex cannot be located near helices E and F, as has previously been proposed.

Prevention of aneurysm development and rupture by local overexpression of plasminogen activator inhibitor-1

BACKGROUND

Arterial aneurysms exhibit a loss of elastin and an increase in the plasminogen activators urokinase plasminogen activator (u-PA) and tissue plasminogen activator (t-PA). Because u-PA, t-PA, and plasmin have a limited proteolytic activity against elastin, the role of plasminogen activators in the aneurysmal disease is unclear. To investigate this question, we overexpressed plasminogen activator inhibitor-1 (PAI-1), an inhibitor of t-PA and u-PA, in a rat model of aortic aneurysm.

METHODS AND RESULTS

Guinea pig-to-rat aortic xenografts were seeded with syngeneic Fischer 344 rat smooth muscle cells retrovirally transduced with the rat PAI-1 gene (LPSN group) or the vector alone (LXSN group). Some grafts were not seeded with cells (NO group). Western blots showed increased PAI-1 in grafts from the LPSN group compared with LXSN and NO groups. All grafts in the NO group (n=8) and 40% in the LXSN group ruptured between days 4 and 14. At 4 weeks in the LXSN group, the remaining unruptured grafts (n=6) were aneurysmal (diameter increase > or =100%), whereas in the LPSN group (n=6) none of the grafts had ruptured or were aneurysmal. Elastin was preserved in the LPSN group. t-PA, the major PA expressed in the model, was decreased in the LPSN group compared with the other groups, as determined by zymography. Quantitative zymography showed decreased levels of two matrix metalloproteinases (MMPs), a 28-kD caseinase, and activated MMP-9 in the LPSN group.

CONCLUSIONS

The blockade of plasminogen activators prevents formation of aneurysms and arterial rupture by inhibiting MMP activation.

Interfering with the inhibitory mechanism of serpins: crystal structure of a complex formed between cleaved plasminogen activator inhibitor type 1 and a reactive-centre loop peptide

BACKGROUND

Plasminogen activator inhibitor type 1 (PAI-1) is an important endogenous regulator of the fibrinolytic system. Reduction of PAI-1 activity has been shown to enhance dissolution of blood clots. Like other serpins, PAI-1 binds covalently to a target serine protease, thereby irreversibly inactivating the enzyme. During this process the exposed reactive-centre loop of PAI-1 is believed to undergo a conformational change becoming inserted into beta sheet A of the serpin. Incubation with peptides from the reactive-centre loop transform serpins into a substrate for their target protease. It has been hypothesised that these peptides bind to beta sheet A, thereby hindering the conformational rearrangement leading to loop insertion and formation of the stable serpin-protease complex.

RESULTS

We report here the 1.95 A X-ray crystal structure of a complex of a glycosylated mutant of PAI-1, PAI-1-ala335Glu, with two molecules of the inhibitory reactive-centre loop peptide N-Ac-TVASS-NH2. Both bound peptide molecules are located between beta strands 3A and 5A of the serpin. The binding kinetics of the peptide inhibitor to immobilised PAI-1-Ala335Glu, as monitored by surface plasmon resonance, is consistent with there being two different binding sites.

CONCLUSIONS

This is the first reported crystal structure of a complex formed between a serpin and a serpin inhibitor. The localisation of the inhibitory peptide in the complex strongly supports the theory that molecules binding in the space between beta strands 3A and 5A of a serpin are able to prevent insertion of the reactive-centre loop into beta sheet A, thereby abolishing the ability of the serpin to irreversibly inactivate its target enzyme. The characterisation of the two binding sites for the peptide inhibitor provides a solid foundation for computer-aided design of novel, low molecular weight PAI-1 inhibitors.

Identification of the binding site for a low-molecular-weight inhibitor of plasminogen activator inhibitor type 1 by site-directed mutagenesis

A novel low-molecular-weight inhibitor, AR-H029953XX, was developed from a known fibrinolytic compound, flufenamic acid, which prevented complex formation of human plasminogen activator inhibitor type 1 (PAI-1) with tissue plasminogen activator (tPA) by inhibition of PAI-1. To explore the binding site for AR-H029953XX, mutants of human PAI-1 were constructed by site-directed mutagenesis and were then expressed in CHO cells, purified, activated, and characterized. (1) PAI-1 with mutations in the reactive center loop: L1-PAI-1 (P10, Ser337Glu) had stability and activity similar to those of wild-type PAI-1 (wt-PAI-1), and L2-PAI-1 (P12, Ala335Glu) was highly stable but was a substrate for tPA. (2) PAI-1 with mutations near the binding epitope for the strongly inhibiting monoclonal antibody CLB-2C8: C1-PAI-1 (Phe114Glu), C2-PAI-1 (Val121Phe), C3-PAI-1 (Arg76Glu/Arg115Glu/Arg118Glu), and C4-PAI-1 (Arg115Glu) were all comparable in activity and stability to wt-PAI-1. AR-H029953XX (Ki = 25 microM) prevented complex formation between tPA and active wt-PAI-1 as well as that with mutants L1-, L2-, C1-, C2-, and C4-PAI-1. AR-H029953XX also inhibited binding of these PAI-1 variants to the antibody CLB-2C8, as measured by surface plasmon resonance. In contrast, AR-H029953XX had almost no inhibitory effect on the complex formation of tPA with C3-PAI-1. Moreover, AR-H029953XX had no effect on the binding rate of CLB-2C8 to C3-PAI-1, or on the binding to latent PAI-1 or to cleaved L2-PAI-1. The binding site of AR-H029953XX thus appears to be located in the neighborhood of the postulated epitope for CLB-2C8, near residues Arg76 and/or Arg118. This specific domain of the PAI-1 molecule might thus also be important for the mechanism of inhibitory activity toward tPA. Moreover, the structure of this region in active PAI-1 has to be different from the corresponding regions in latent and cleaved PAI-1.

Assessment of the interaction between urokinase and reactive site mutants of protein C inhibitor

Urokinase-type plasminogen activator (uPA) is a serine protease involved in pericellular proteolysis and tumor cell metastasis via plasmin-mediated degradation of extracellular matrix proteins. Plasma uPA is inhibited by the serine protease inhibitor protein C inhibitor (PCI) by the insertion of PCI's reactive site loop into the active site of the protease. To better understand the structural aspects of this inhibition, 15 reactive-site mutants of recombinant PCI (rPCI) were assayed for differences in uPA inhibition. These assays revealed that substitutions at the P1 Arg354 and P3 Thr352 sites of rPCI were detrimental to inhibitory activity, while P3' Arg357 mutations had little effect upon the inhibition rate. However, replacement of the P2 Phe353 with small residues like Ala and Gly increased the effectiveness of rPCI three- to four fold. To explain these altered rates of inhibition, a computer-derived molecular model of uPA was generated and docked to a model of PCI to simulate complex formation. The changes made by mutagenesis were then recreated in the model of uPA-PCI. In accordance with the kinetic data, the poor performance of P3 variants is primarily attributable to charge repulsion, while alleviation of steric hindrance at P2 produces the observed increase in uPA inhibition. In the model, residues at P3' interact with PCI rather than uPA, consistent with P3' variants demonstrating that little variation from wild-type activity. Ultimately, this combination of mutagenesis and molecular modeling will further refine our understanding of the interaction between PCI and uPA.

The Suicide Substrate Reaction Between Plasminogen Activator Inhibitor 1 and Thrombin Is Regulated by the Cofactors Vitronectin and Heparin

The interaction of thrombin with plasminogen activator inhibitor 1 (PAI-1) is shown to result in the simultaneous formation of both cleaved PAI-1 and a sodium dodecyl sulfate-stable thrombin-PAI-1 complex. The kinetics of this reaction can be described by a “suicide substrate” mechanism that includes a branched reaction pathway, which terminates in either the stable inhibitor-enzyme complex or the cleaved inhibitor plus free enzyme. Because of the branched pathway, approximately three moles of PAI-1 are needed to completely inhibit one mole of thrombin. Heparin and vitronectin enhance the rate of inhibition from 9.8 × 102 L mol−1 s−1 to 6.2 × 104 L mol−1 s−1 and 2.1 × 105 L mol−1 s−1, respectively, under optimal conditions. In addition to enhancing the rate of inhibition, both cofactors increase the apparent stoichiometry of the PAI-1–thrombin interaction, with cofactor concentration dependencies similar to the inhibition reaction. Thus, at 37°C approximately six cleavage reactions occur per inhibition reaction. Therefore, thrombin will efficiently inactivate PAI-1 in the presence of either vitronectin or heparin, unless a sufficient excess of the inhibitor is present. These results show that physiological cofactors are able to switch a protease-serpin inhibition reaction to a substrate reaction, depending on the local concentrations of each of the components.

Epitopes on plasminogen activator inhibitor type-1 important for binding to tissue plasminogen activator

The molecular details of the rapid complex formation between tissue plasminogen activator (tPA, E.C. 3.4.21.68) and plasminogen activator inhibitor type-1 (PAI-1) are still not fully elucidated. We have used surface plasmon resonance (SPR), the BIAcore, to characterize the binding of a large panel of monoclonal antibodies to four forms of recombinant human PAI-1, including active and latent PAI-1 as well as the complex between PAI-1 and recombinant human tc tPA or the protease part of tPA, the B-chain. Antibodies that discriminate between these different forms of PAI-1 have been identified, which is reflected by differences in k(a), k(d) as well as in Kd. In addition, in a chromogenic assay with PAI-1 and tPA we determined the IC50-values for these antibodies, i.e., studied their ability to inhibit the decrease in tPA-activity caused by PAI-1. In a competition assay using SPR, we have also been able to study whether concurrent binding of these antibodies to PAI-1 was possible. We could thereby assign the antibodies to five groups according to their binding areas. Furthermore, by using this technique, we have for the first time been able to identify three distinct epitopes on PAI-1, which are all of importance for the interaction and complex-formation with tPA. Since the antibodies that bind to one of these areas all have very poor affinity for the complex between PAI-1 and tPA, we suggest that this not previously described epitope must be located near the final binding site for tPA in this complex. Altogether, this also supports the theory of a multistep reaction between PAI-1 and tPA, in which tPA interacts with different parts of the PAI-1-molecule.

Characterisation of the complex of plasminogen activator inhibitor type 1 with tissue-type plasminogen activator by mass spectrometry and size-exclusion chromatography

Glycosylated human plasminogen activator inhibitor type 1 (PAI-1), produced in Chinese hamster ovary (CHO) cells, showed a variety of compounds with different molecular weights when subjected to electrospray mass spectrometry (ES-MS), owing to the heterogeneity of the carbohydrate chains. However, non-glycosylated human PAI-1, produced in E. coli, gave rise to a prominent species with a molecular weight of 42,774, consistent with the amino-acid sequence. A non-glycosylated mutant of the proteinase domain (B-chain) of tissue-type plasminogen activator (tPA) produced in C 127 cells, had a molecular weight of 28,168. Full-length, glycosylated, tPA showed a large heterogeneity in molecular mass. For a mass study, a tPA-PAI-1 complex was formed, composed of non-glycosylated PAI-1 and non-glycosylated B-chain. This complex was remarkably stable at room temperature in buffer with a neutral pH. The mass spectrum of the complex provided two main species, a peptide with a mass of 3803 and a dominating species of 67,133. These masses are consistent with a complex where PAI-1 is cleaved at the P1-P1' position. A trace of a species with a molecular mass of 70,942 was also found, corresponding to the complete, non-dissociated complex with PAI-1. Separation of the cleaved peptide, corresponding to the hydrophobic C-terminal 33 amino-acid residues of PAI-1, from the complex, was achieved by size-exclusion chromatography in the presence of 30% acetonitrile. Thus, in the complex between tPA and PAI-1, the proteins are held together by a tight covalent bond, but the C-terminal cleaved peptide of PAI-1 is only bound to the complex by hydrophobic forces. To assess whether this is specific to the tPA B-chain alone, experiments with the complex of full-length, glycosylated tPA and glycosylated PAI-1 were also performed, and it was possible to demonstrate the release of the C-terminal PAI-1 peptide by chromatography, mass spectrometry, as well as by SDS-PAGE.