Mechanisms of conversion of plasminogen activator inhibitor 1 from a suicide inhibitor to a substrate by monoclonal antibodies

The fibrinolysis renaissance

Fibrinolysis is the system primarily responsible for removal of fibrin deposits and blood clots in the vasculature. The terminal enzyme in the pathway, plasmin, is formed from its circulating precursor, plasminogen. Fibrin is by far the most legendary substrate, but plasmin is notoriously prolific and is known to cleave many other proteins and participate in the activation of other proteolytic systems. Fibrinolysis is often overshadowed by the coagulation system and viewed as a simplistic poorer relation. However, the primordial plasminogen activators evolved alongside the complement system, approximately 70 million years before coagulation saw the light of day. It is highly likely that the plasminogen activation system evolved with its roots in primordial immunity. Almost all immune cells harbor at least one of a dozen plasminogen receptors that allow plasmin formation on the cell surface that in turn modulates immune cell behavior. Similarly, numerous pathogens express their own plasminogen activators or contain surface proteins that provide binding sites for host plasminogen. The fibrinolytic system has been harnessed for clinical medicine for many decades with the development of thrombolytic drugs and antifibrinolytic agents. Our refined understanding and appreciation of the fibrinolytic system and its alliance with infection and immunity and beyond are paving the way for new developments and interest in novel therapeutics and applications. One must ponder as to whether the nomenclature of the system hampered our understanding, by focusing on fibrin, rather than the complex myriad of interactions and substrates of the plasminogen activation system.

A Serpin With a Finger in Many PAIs: PAI-1's Central Function in Thromboinflammation and Cardiovascular Disease

Plasminogen activator inhibitor 1 (PAI-1) is a member of the serine protease inhibitor (serpin) superfamily. PAI-1 is the principal inhibitor of the plasminogen activators, tissue plasminogen activator (tPA), and urokinase-type plasminogen activator (uPA). Turbulence in the levels of PAI-1 tilts the balance of the hemostatic system resulting in bleeding or thrombotic complications. Not surprisingly, there is strong evidence that documents the role of PAI-1 in cardiovascular disease. The more recent uncovering of the coalition between the hemostatic and inflammatory pathways has exposed a distinct role for PAI-1. The storm of proinflammatory cytokines liberated during inflammation, including IL-6 and TNF-α, directly influence PAI-1 synthesis and increase circulating levels of this serpin. Consequently, elevated levels of PAI-1 are commonplace during infection and are frequently associated with a hypofibrinolytic state and thrombotic complications. Elevated PAI-1 levels are also a feature of metabolic syndrome, which is defined by a cluster of abnormalities including obesity, type 2 diabetes, hypertension, and elevated triglyceride. Metabolic syndrome is in itself defined as a proinflammatory state associated with elevated levels of cytokines. In addition, insulin has a direct impact on PAI-1 synthesis bridging these pathways. This review describes the key physiological functions of PAI-1 and how these become perturbed during disease processes. We focus on the direct relationship between PAI-1 and inflammation and the repercussion in terms of an ensuing hypofibrinolytic state and thromboembolic complications. Collectively, these observations strengthen the utility of PAI-1 as a viable drug target for the treatment of various diseases.

Targeting PAI-1 in Cardiovascular Disease: Structural Insights Into PAI-1 Functionality and Inhibition

Plasminogen activator inhibitor-1 (PAI-1), a member of the serine protease inhibitor (serpin) superfamily with antiprotease activity, is the main physiological inhibitor of tissue-type (tPA) and urokinase-type (uPA) plasminogen activators (PAs). Apart from being crucially involved in fibrinolysis and wound healing, PAI-1 plays a pivotal role in various acute and chronic pathophysiological processes, including cardiovascular disease, tissue fibrosis, cancer, and age-related diseases. In the prospect of treating the broad range of PAI-1-related pathologies, many efforts have been devoted to developing PAI-1 inhibitors. The use of these inhibitors, including low molecular weight molecules, peptides, antibodies, and antibody fragments, in various animal disease models has provided ample evidence of their beneficial effect in vivo and moved forward some of these inhibitors in clinical trials. However, none of these inhibitors is currently approved for therapeutic use in humans, mainly due to selectivity and toxicity issues. Furthermore, the conformational plasticity of PAI-1, which is unique among serpins, poses a real challenge in the identification and development of PAI-1 inhibitors. This review will provide an overview of the structural insights into PAI-1 functionality and modulation thereof and will highlight diverse approaches to inhibit PAI-1 activity.

Structural Insights into the Mechanism of a Nanobody That Stabilizes PAI-1 and Modulates Its Activity

Plasminogen activator inhibitor-1 (PAI-1) is the main physiological inhibitor of tissue-type (tPA) and urokinase-type (uPA) plasminogen activators (PAs). Apart from being critically involved in fibrinolysis and wound healing, emerging evidence indicates that PAI-1 plays an important role in many diseases, including cardiovascular disease, tissue fibrosis, and cancer. Targeting PAI-1 is therefore a promising therapeutic strategy in PAI-1 related pathologies. Despite ongoing efforts no PAI-1 inhibitors were approved to date for therapeutic use in humans. A better understanding of the molecular mechanisms of PAI-1 inhibition is therefore necessary to guide the rational design of PAI-1 modulators. Here, we present a 1.9 Å crystal structure of PAI-1 in complex with an inhibitory nanobody VHH-s-a93 (Nb93). Structural analysis in combination with biochemical characterization reveals that Nb93 directly interferes with PAI-1/PA complex formation and stabilizes the active conformation of the PAI-1 molecule.

Molecular mechanism of two nanobodies that inhibit PAI-1 activity reveals a modulation at distinct stages of the PAI-1/plasminogen activator interaction

BACKGROUND

Plasminogen activator inhibitor-1 (PAI-1), a key inhibitor of plasminogen activators (PAs) tissue-type PA (tPA) and urokinase-type PA (uPA) plays a crucial role in many (patho)physiological processes (e.g., cardiovascular disease, tissue fibrosis) as well as in many age-related pathologies. Therefore, much effort has been put into the development of small molecule or antibody-based PAI-1 inhibitors.

OBJECTIVE

To elucidate the molecular mechanism of nanobody-induced PAI-1 inhibition.

METHODS AND RESULTS

Here we present the first crystal structures of PAI-1 in complex with two neutralizing nanobodies (Nbs). These structures, together with biochemical and biophysical characterization, reveal that Nb VHH-2g-42 (Nb42) interferes with the initial PAI-1/PA complex formation, whereas VHH-2w-64 (Nb64) redirects the PAI-1/PA interaction to PAI-1 deactivation and regeneration of active PA. Furthermore, whereas vitronectin does not have an impact on the inhibitory effect of Nb42, it strongly potentiates the inhibitory effect of Nb64, which may contribute to a strong inhibitory potential of Nb64 in vivo.

CONCLUSIONS

These findings illuminate the molecular mechanisms of PAI-1 inhibition. Nb42 and Nb64 can be used as starting points to engineer further improved antibody-based PAI-1 inhibitors or guide the rational design of small molecule inhibitors to treat a wide range of PAI-1-related pathophysiological conditions.

A novel solution phase PAI-1/uPA-biotin complex assay for the measurement of active PAI-1 in plasma

We devised a new assay procedure to use biotinylated uPA to trace the active PAI-1 levels in the plasma. We show here that the potency of inhibitory monoclonal antibody 33B8 measured with the new assay is consistent with its in vivo efficacy in PAI-1 inactivation. We also found that among the three monoclonal antibodies tested, the traditional solid phase assay caused mechanism dependent significant right shift of IC₅₀ values. As our new assay avoids the use of non-physiological large quantities of uPA, we conclude that it is a better measure of pharmacodynamic effects of anti-PAI-1 antibodies in vivo.

Fibrin turnover and pleural organization: bench to bedside

Recent studies have shed new light on the role of the fibrinolytic system in the pathogenesis of pleural organization, including the mechanisms by which the system regulates mesenchymal transition of mesothelial cells and how that process affects outcomes of pleural injury. The key contribution of plasminogen activator inhibitor-1 to the outcomes of pleural injury is now better understood as is its role in the regulation of intrapleural fibrinolytic therapy. In addition, the mechanisms by which fibrinolysins are processed after intrapleural administration have now been elucidated, informing new candidate diagnostics and therapeutics for pleural loculation and failed drainage. The emergence of new potential interventional targets offers the potential for the development of new and more effective therapeutic candidates.

In Situ Imaging of Tissue Remodeling with Collagen Hybridizing Peptides

Collagen, the major structural component of nearly all mammalian tissues, undergoes extensive proteolytic remodeling during developmental states and a variety of life-threatening diseases such as cancer, myocardial infarction, and fibrosis. While degraded collagen could be an important marker of tissue damage, it is difficult to detect and target using conventional tools. Here, we show that a designed peptide (collagen hybridizing peptide: CHP), which specifically hybridizes to the degraded, unfolded collagen chains, can be used to image degraded collagen and inform tissue remodeling activity in various tissues: labeled with 5-carboxyfluorescein and biotin, CHPs enabled direct localization and quantification of collagen degradation in isolated tissues within pathologic states ranging from osteoarthritis and myocardial infarction to glomerulonephritis and pulmonary fibrosis, as well as in normal tissues during developmental programs associated with embryonic bone formation and skin aging. The results indicate the general correlation between the level of collagen remodeling and the amount of denatured collagen in tissue and show that the CHP probes can be used across species and collagen types, providing a versatile tool for not only pathology and developmental biology research but also histology-based disease diagnosis, staging, and therapeutic screening. This study lays the foundation for further testing CHP as a targeting moiety for theranostic delivery in various animal models.

Targeting plasminogen activator inhibitor-1 in tetracycline-induced pleural injury in rabbits

Elevated active plasminogen activator inhibitor-1 (PAI-1) has an adverse effect on the outcomes of intrapleural fibrinolytic therapy (IPFT) in tetracycline-induced pleural injury in rabbits. To enhance IPFT with prourokinase (scuPA), two mechanistically distinct approaches to targeting PAI-1 were tested: slowing its reaction with urokinase (uPA) and monoclonal antibody (mAb)-mediated PAI-1 inactivation. Removing positively charged residues at the "PAI-1 docking site" (¹⁷⁹RHRGGS¹⁸⁴→¹⁷⁹AAAAAA¹⁸⁴) of uPA results in a 60-fold decrease in the rate of inhibition by PAI-1. Mutant prourokinase (0.0625-0.5 mg/kg; n = 12) showed efficacy comparable to wild-type scuPA and did not change IPFT outcomes ( P > 0.05). Notably, the rate of PAI-1-independent intrapleural inactivation of mutant uPA was 2 times higher ( P < 0.05) than that of the wild-type enzyme. Trapping PAI-1 in a "molecular sandwich"-type complex with catalytically inactive two-chain urokinase with Ser¹⁹⁵Ala substitution (S195A-tcuPA; 0.1 and 0.5 mg/kg) did not improve the efficacy of IPFT with scuPA (0.0625-0.5 mg/kg; n = 11). IPFT failed in the presence of MA-56A7C10 (0.5 mg/kg; n = 2), which forms a stable intrapleural molecular sandwich complex, allowing active PAI-1 to accumulate by blocking its transition to a latent form. In contrast, inactivation of PAI-1 by accelerating the active-to-latent transition mediated by mAb MA-33B8 (0.5 mg/kg; n = 2) improved the efficacy of IPFT with scuPA (0.25 mg/kg). Thus, under conditions of slow (4-8 h) fibrinolysis in tetracycline-induced pleural injury in rabbits, only the inactivation of PAI-1, but not a decrease in the rate of its reaction with uPA, enhances IPFT. Therefore the rate of fibrinolysis, which varies in different pathologic states, could affect the selection of PAI-1 inhibitors to enhance fibrinolytic therapy.

Targeting of plasminogen activator inhibitor 1 improves fibrinolytic therapy for tetracycline-induced pleural injury in rabbits

Endogenous active plasminogen activator inhibitor 1 (PAI-1) was targeted in vivo with monoclonal antibodies (mAbs) that redirect its reaction with proteinases to the substrate branch. mAbs were used as an adjunct to prourokinase (single-chain [sc] urokinase [uPA]) intrapleural fibrinolytic therapy (IPFT) of tetracycline-induced pleural injury in rabbits. Outcomes of scuPA IPFT (0.25 or 0.0625 mg/kg) with 0.5 mg/kg of mouse IgG or mAbs (MA-33H1F7 and MA-8H9D4) were assessed at 24 hours. Pleural fluid (PF) was collected at 0, 10, 20, and 40 minutes and 24 hours after IPFT and analyzed for plasminogen activating (PA), uPA, fibrinolytic activities, levels of total plasmin/plasminogen, α-macroglobulin (αM), mAbs/IgG antigens, free active uPA, and αM/uPA complexes. Anti-PAI-1 mAbs, but not mouse IgG, delivered with an eightfold reduction in the minimal effective dose of scuPA (from 0.5 to 0.0625 mg/kg), improved the outcome of IPFT (P < 0.05). mAbs and IgG were detectable in PFs at 24 hours. Compared with identical doses of scuPA alone or with IgG, treatment with scuPA and anti-PAI-1 mAbs generated higher PF uPA amidolytic and PA activities, faster formation of αM/uPA complexes, and slower uPA inactivation. However, PAI-1 targeting did not significantly affect intrapleural fibrinolytic activity or levels of total plasmin/plasminogen and αM antigens. Targeting PAI-1 did not induce bleeding, and rendered otherwise ineffective doses of scuPA able to improve outcomes in tetracycline-induced pleural injury. PAI-1-neutralizing mAbs improved IPFT by increasing the durability of intrapleural PA activity. These results suggest a novel, well-tolerated IPFT strategy that is tractable for clinical development.

Single fluorescence probes along the reactive center loop reveal site-specific changes during the latency transition of PAI-1

The serine protease inhibitor (serpin), plasminogen activator inhibitor-1 (PAI-1), is an important biomarker for cardiovascular disease and many cancers. It is therefore a desirable target for pharmaceutical intervention. However, to date, no PAI-1 inhibitor has successfully reached clinical trial, indicating the necessity to learn more about the mechanics of the serpin. Although its kinetics of inhibition have been extensively studied, less is known about the latency transition of PAI-1, in which the solvent-exposed reactive center loop (RCL) inserts into its central β-sheet, rendering the inhibitor inactive. This spontaneous transition is concomitant with a large translocation of the RCL, but no change in covalent structure. Here, we conjugated the fluorescent probe, NBD, to single positions along the RCL (P13-P5') to detect changes in solvent exposure that occur during the latency transition. The results support a mousetrap-like RCL-insertion that occurs with a half-life of 1-2 h in accordance with previous reports. Importantly, this study exposes unique transitions during latency that occur with a half-life of ∼5 and 25 min at the P5' and P8 RCL positions, respectively. We hypothesize that the process detected at P5' represents s1C detachment, while that at P8 results from a steric barrier to RCL insertion. Together, these findings provide new insights by characterizing multiple steps in the latency transition.

The time course of resolution of adhesions during fibrinolytic therapy in tetracycline-induced pleural injury in rabbits

The time required for the effective clearance of pleural adhesions/organization after intrapleural fibrinolytic therapy (IPFT) is unknown. Chest ultrasonography and computed tomography (CT) were used to assess the efficacy of IPFT in a rabbit model of tetracycline-induced pleural injury, treated with single-chain (sc) urokinase plasminogen activators (scuPAs) or tissue PAs (sctPA). IPFT with sctPA (0.145 mg/kg; n = 10) and scuPA (0.5 mg/kg; n = 12) was monitored by serial ultrasonography alone (n = 12) or alongside CT scanning (n = 10). IPFT efficacy was assessed with gross lung injury scores (GLIS) and ultrasonography scores (USS). Pleural fluids withdrawn at 0-240 min and 24 h after IPFT were assayed for PA and fibrinolytic activities, α-macroglobulin/fibrinolysin complexes, and active PA inhibitor 1 (PAI-1). scuPA and sctPA generated comparable steady-state fibrinolytic activities by 20 min. PA activity in the scuPA group decreased slower than the sctPA group (kobs = 0.016 and 0.042 min(-1)). Significant amounts of bioactive uPA/α-macroglobulin (but not tPA; P < 0.05) complexes accumulated at 0-40 min after IPFT. Despite the differences in intrapleural processing, IPFT with either fibrinolysin was effective (GLIS ≤ 10) in animals imaged with ultrasonography only. USS correlated well with postmortem GLIS (r(2) = 0.85) and confirmed relatively slow intrapleural fibrinolysis after IPFT, which coincided with effective clearance of adhesions/organization at 4-8 h. CT scanning was associated with less effective (GLIS > 10) IPFT and higher levels of active PAI-1 at 24 h following therapy. We concluded that intrapleural fibrinolysis in tetracycline-induced pleural injury in rabbits is relatively slow (4-8 h). In CT-scanned animals, elevated PAI-1 activity (possibly radiation induced) reduced the efficacy of IPFT, buttressing the major impact of active PAI-1 on IPFT outcomes.

Three monoclonal antibodies against the serpin protease nexin-1 prevent protease translocation

Protease nexin-1 (PN-1) belongs to the serpin family and is an inhibitor of thrombin, plasmin, urokinase-type plasminogen activator, and matriptase. Recent studies have suggested PN-1 to play important roles in vascular-, neuro-, and tumour-biology. The serpin inhibitory mechanism consists of the serpin presenting its so-called reactive centre loop as a substrate to its target protease, resulting in a covalent complex with the inactivated enzyme. Previously, three mechanisms have been proposed for the inactivation of serpins by monoclonal antibodies: steric blockage of protease recognition, conversion to an inactive conformation or induction of serpin substrate behaviour. Until now, no inhibitory antibodies against PN-1 have been thoroughly characterised. Here we report the development of three monoclonal antibodies binding specifically and with high affinity to human PN-1. The antibodies all abolish the protease inhibitory activity of PN-1. In the presence of the antibodies, PN-1 does not form a complex with its target proteases, but is recovered in a reactive centre cleaved form. Using site-directed mutagenesis, we mapped the three overlapping epitopes to an area spanning the gap between the loop connecting α-helix F with β-strand 3A and the loop connecting α-helix A with β-strand 1B. We conclude that antibody binding causes a direct blockage of the final critical step of protease translocation, resulting in abortive inhibition and premature release of reactive centre cleaved PN-1. These new antibodies will provide a powerful tool to study the in vivo role of PN-1's protease inhibitory activity.

Remarkable stabilization of plasminogen activator inhibitor 1 in a "molecular sandwich" complex

Plasminogen activator inhibitor 1 (PAI-1) levels are elevated in a number of life-threatening conditions and often correlate with unfavorable outcomes. Spontaneous inactivation due to active to latent transition limits PAI-1 activity in vivo. While endogenous vitronectin (Vn) stabilizes PAI-1 by 1.5-2.0-fold, further stabilization occurs in a "molecular sandwich" complex (MSC) in which a ligand that restricts the exposed reactive center loop is bound to PAI-1/Vn. The effects of S195A two-chain urokinase (tcuPA) and Vn on inactivation of wild-type (wt) glycosylated (Gl-PAI-1), nonglycosylated (rPAI-1), and nonglycosylated Q123K PAI-1 (lacks Vn binding) forms were studied. S195A tcuPA decreased the rate constant (kL) for spontaneous inactivation at 37 °C for rPAI-1, Q123K, and Gl-PAI-1 by 6.7-, 3.4-, and 7.8-fold, respectively, and both S195A tcuPA and Vn by 66.7-, 5.5-, and 103.3-fold, respectively. Analysis of the temperature dependences of kL revealed a synergistic increase in the Gibbs free activation energy for spontaneous inactivation of wt Gl-PAI-1 and rPAI-1 in MSC from 99.8 and 96.1 to 111.3 and 107.0 kJ/mol, respectively, due to an increase in the activation enthalpy and a decrease in the activation entropy. Anti-PAI-1 monoclonal antibodies (mAbs) competing with proteinase also stabilize PAI-1/Vn. The rate of inhibition of target proteinases by MSCs, with a stoichiometry close to unity, was limited by the dissociation (k = 10(-4) to 10(-3) s(-1)) of S195A tcuPA or mAb. The stabilization of PAI-1 in MSCs in vivo may potentiate uncontrolled thrombosis or extravascular fibrin deposition, suggesting a new paradigm for using PAI-1 inhibitors and novel potential targets for therapy.

Effects of extracellular DNA on plasminogen activation and fibrinolysis

The increased levels of extracellular DNA found in a number of disorders involving dysregulation of the fibrinolytic system may affect interactions between fibrinolytic enzymes and inhibitors. Double-stranded (ds) DNA and oligonucleotides bind tissue-(tPA) and urokinase (uPA)-type plasminogen activators, plasmin, and plasminogen with submicromolar affinity. The binding of enzymes to DNA was detected by EMSA, steady-state, and stopped-flow fluorimetry. The interaction of dsDNA/oligonucleotides with tPA and uPA includes a fast bimolecular step, followed by two monomolecular steps, likely indicating slow conformational changes in the enzyme. DNA (0.1-5.0 μg/ml), but not RNA, potentiates the activation of Glu- and Lys-plasminogen by tPA and uPA by 480- and 70-fold and 10.7- and 17-fold, respectively, via a template mechanism similar to that known for fibrin. However, unlike fibrin, dsDNA/oligonucleotides moderately affect the reaction between plasmin and α(2)-antiplasmin and accelerate the inactivation of tPA and two chain uPA by plasminogen activator inhibitor-1 (PAI-1), which is potentiated by vitronectin. dsDNA (0.1-1.0 μg/ml) does not affect the rate of fibrinolysis by plasmin but increases by 4-5-fold the rate of fibrinolysis by Glu-plasminogen/plasminogen activator. The presence of α(2)-antiplasmin abolishes the potentiation of fibrinolysis by dsDNA. At higher concentrations (1.0-20 μg/ml), dsDNA competes for plasmin with fibrin and decreases the rate of fibrinolysis. dsDNA/oligonucleotides incorporated into a fibrin film also inhibit fibrinolysis. Thus, extracellular DNA at physiological concentrations may potentiate fibrinolysis by stimulating fibrin-independent plasminogen activation. Conversely, DNA could inhibit fibrinolysis by increasing the susceptibility of fibrinolytic enzymes to serpins.

Plasminogen activator inhibitor-1 inhibits factor VIIa bound to tissue factor

BACKGROUND AND OBJECTIVE

A growing body of experimental evidence supports broad inhibitory and regulatory activity of plasminogen activator inhibitor 1 (PAI-1). The present study was designed to investigate whether PAI-1 inhibits factor (F) VIIa complexed with tissue factor (TF), a well-known procoagulant risk factor.

METHODS AND RESULTS

The ability of PAI-1 to inhibit FVIIa-TF activity was evaluated in both clotting and factor X (FX) activation assays. PAI-1 and its complex with vitronectin inhibit: (i) clotting activity of FVIIa-TF (PAI-1(IC50) , 817 and 125 nm, respectively); (ii) FVIIa-TF-mediated FX activation (PAI-1(IC50) , 260 and 50 nm, respectively); and (iii) FVIIa bound to TF expressed on the surface of stimulated endothelial cells (PAI-1(IC50) , 260 and 120 nm, respectively). The association rate constant (k(a)) for PAI-1 inhibition of FVIIa-TF was determined using a chromogenic assay. K(a) for PAI-1 inhibition of FVIIa bound to relipidated TF is 3.3-fold higher than that for FVIIa bound to soluble TF (k(a) = 0.09 ± 0.01 and 0.027 ± 0.03 μm(-1) min(-1), respectively). Vitronectin increases k(a) for both soluble and relipidated TF by 3.5- and 30-fold, respectively (to 0.094 ± 0.020 and 2.7 ± 0.2 μm(-1) min(-1)). However, only a 3.5- to 5.0-fold increase in the acylated FVIIa was observed on SDS PAGE in the presence of vitronectin for both relipidated and soluble TF, indicating fast formation of PAI-1/vitronectin/FVIIa/relipidated TF non-covalent complex.

CONCLUSIONS

Our results demonstrate potential anticoagulant activity of PAI-1 in the presence of vitronectin, which could contribute to regulation of hemostasis under pathological conditions such as severe sepsis, acute lung injury and pleural injury, where PAI-1 and TF are overexpressed.

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.

Fibrinolysis is amplified by converting alpha-antiplasmin from a plasmin inhibitor to a substrate

alpha(2)-Antiplasmin (alpha(2)-AP) is the fast serpin inhibitor of plasmin and appears to limit the success of treatment for thrombosis. We examined the mechanisms through which monoclonal antibodies (mAbs) against alpha(2)-AP amplify fibrinolysis. The mAbs RWR, 49 and 77 interfered with the ability of alpha(2)-AP to inhibit plasmin, microplasmin and trypsin. In solution, mAbs 49 and 77 bound to alpha(2)-AP with 5-fold to 10-fold higher relative affinity than mAb-RWR, while mAb-RWR bound with greater avidity to immobilized or denatured alpha(2)-AP. Binding studies with chimeric alpha(2)-APs revealed that none of the mAbs bound to sites in alpha(2)-AP that form putative contacts with plasmin, namely the carboxy terminal lysines of alpha(2)-AP, or the reactive center loop in the serpin domain of alpha(2)-AP. Rather, mAb-RWR recognized an epitope in the amino-terminus of alpha(2)-AP (L(13)GNQEPGGQTALKSPPGVCS(32)) near the site at which alpha(2)-AP cross-links to fibrin. mAbs 49 and 77 bound to another conformational epitope in the serpin domain of alpha(2)-AP. mAbs 49 and 77 markedly increased the stoichiometry of plasmin inhibition by alpha(2)-AP (from 1.1 +/- 0.1 to 51 +/- 4 and 67 +/- 7) indicating that they convert alpha(2)-AP from an inhibitor to a substrate of plasmin. This was confirmed by sodium dodecylsulfate polyacrylamide gel electrophoresis analysis showing cleavage of alpha(2)-AP by plasmin in the presence of these mAbs. In summary, these mAbs appear to act at sites distinct from known alpha(2)-AP-plasmin contacts to enhance fibrinolysis by converting alpha(2)-AP from an inhibitor to a plasmin substrate.

Modulation of serpin reaction through stabilization of transient intermediate by ligands bound to alpha-helix F

Mechanism-based inhibition of proteinases by serpins involves enzyme acylation and fast insertion of the reactive center loop (RCL) into the central beta-sheet of the serpin, resulting in mechanical inactivation of the proteinase. We examined the effects of ligands specific to alpha-helix F (alphaHF) of plasminogen activator inhibitor-1 (PAI-1) on the stoichiometry of inhibition (SI) and limiting rate constant (k(lim)) of RCL insertion for reactions with beta-trypsin, tissue-type plasminogen activator (tPA), and urokinase. The somatomedin B domain of vitronectin (SMBD) did not affect SI for any proteinase or k(lim) for tPA but decreased the k(lim) for beta-trypsin. In contrast to SMBD, monoclonal antibodies MA-55F4C12 and MA-33H1F7, the epitopes of which are located at the opposite side of alphaHF, decreased k(lim) and increased SI for every enzyme. These effects were enhanced in the presence of SMBD. RCL insertion for beta-trypsin and tPA is limited by different subsequent steps of PAI-1 mechanism as follows: enzyme acylation and formation of a loop-displaced acyl complex (LDA), respectively. Stabilization of LDA through the disruption of the exosite interactions between PAI-1 and tPA induced an increase in the k(lim) but did not affect the SI. Thus it is unlikely that LDA contributes significantly to the outcome of the serpin reaction. These results demonstrate that the rate of RCL insertion is not necessarily correlated with SI and indicate that an intermediate, different from LDA, which forms during the late steps of PAI-1 mechanism, and could be stabilized by ligands specific to alphaHF, controls bifurcation between the inhibitory and the substrate pathways.

Function-stabilizing mechanism of plasminogen activator inhibitor type 1 induced upon binding to alpha1-acid glycoprotein

Recently, we found that alpha(1)-acid glycoprotein (AGP), one of the major acute-phase proteins, forms a function-stabilizing complex with plasminogen activator inhibitor 1 (PAI-1). In this study we describe the mechanism by which AGP, as well as its recombinant fragment AGP(118)(-)(201), interacts exclusively with the active form of PAI-1 and stabilizes its conformation. The binding domain of PAI-1 for AGP was initially mapped by antibodies reacting with the well-defined PAI-1 epitopes and then verified in binding assays utilizing a library of PAI-1 mutants. The latter consisted of PAI-1 molecules with individual, tandem, or grouped mutations in the epitope region of MA-55F4C12, MA-33B8, MA-33H1F7, MA-44E4, and MA-8H9D4. Solid-phase binding experiments showed that only MA-8H9D4 did not bind to the PAI-1/AGP complex, indicating that its epitope is hidden upon binding of PAI-1 to AGP. Consistently, only PAI-1 mutants with substitutions in the region of R300-D305, constituting the MA-8H9D4 epitope, showed a lack of binding or severe deficit in both the capacity and affinity of binding to AGP. These results support a location of the binding site close to the epitope within the segment connecting the regions hI with S5A. In conclusion, our present data suggest that AGP binding stabilizes the active conformation of PAI-1 by restricting the movement of beta-sheet A and thereby preventing insertion of the reactive center loop.

Additivity in effects of vitronectin and monoclonal antibodies against alpha-helix F of plasminogen activator inhibitor-1 on its reactions with target proteinases

The serpin plasminogen activator inhibitor-1 (PAI-1) is a potential therapeutic target in cardiovascular and cancerous diseases. PAI-1 circulates in blood as a complex with vitronectin. A PAI-1 variant (N-((2-(iodoacetoxy)ethyl)-N-methyl)amino-7-nitrobenz-2-oxa-3-diazole (NBD) P9 PAI-1) with a fluorescent tag at the reactive center loop (RCL) was used to study the effects of vitronectin and monoclonal antibodies (mAbs) directed against alpha-helix F (Mab-2 and MA-55F4C12) on the reactions of PAI-1 with tissue-type and urokinase-type plasminogen activators. Both mAbs delay the RCL insertion and induce an increase in the stoichiometry of inhibition (SI) to 1.4-9.5. Binding of vitronectin to NBD P9 PAI-1 does not affect SI but results in a 2.0-6.5-fold decrease in the limiting rate constant (klim) of RCL insertion for urokinase-type plasminogen activator at pH 6.2-8.0 and for tissue-type plasminogen activator at pH 6.2. Binding of vitronectin to the complexes of NBD P9 PAI-1 with mAbs results in a decrease in klim and in a 1.5-22-fold increase in SI. Thus, vitronectin and mAbs demonstrated additivity in the effects on the reaction with target proteinases. The same step in the reaction mechanism remains limiting for the rate of RCL insertion in the absence and presence of Vn and mAbs. We hypothesize that vitronectin, bound to alpha-helix F on the side opposite to the epitopes of the mAbs, potentiates the mAb-induced delay in RCL insertion and the associated substrate behavior by selectively decreasing the rate constant for the inhibitory branch of PAI-1 reaction (ki). These results demonstrate that mAbs represent a valid approach for inactivation of vitronectin-bound PAI-1 in vivo.

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).

Site-directed targeting of plasminogen activator inhibitor-1 as an example for a novel approach in rational drug design

As plasminogen activator inhibitor-1 (PAI-1), the physiological inhibitor of tissue-type plasminogen activator, is considered to be an important risk factor in several (patho)physiological conditions, many research activities focus on attempts to inhibit this serpin. The approach illustrated in the current study focuses on elucidating important interaction sites allowing the inhibition of PAI-1. Since monoclonal antibodies are in most cases not ideal for therapeutic use, the question of whether smaller molecules exert comparable effects is a hot issue. To answer this question, Cys residues were introduced in PAI-1 at positions previously identified as determining the epitope of a PAI-1-inhibiting antibody, MA-8H9D4, resulting in PAI-1-R300C, PAI-1-Q303C, and PAI-1-D305C. Subsequently, low molecular mass sulfhydryl-specific reagents (i.e. BODIPY 530/550 IA (molecular mass 626 Da) and BODIPY FL C(1)-IA (molecular mass 417 Da)) were allowed to react covalently with the cysteine. The functional distribution (inhibitory versus substrate) toward tissue-type plasminogen activator was determined for the labeled and the unlabeled samples. Labeling at position 300 leads to a 1.7- and 2.2-fold increase in SI value for BODIPY 530/550 IA and BODIPY FL C(1)-IA, respectively. Labeling at position 303 results in a 3.3- and 1.9-fold increase of the SI value for the large and the small label, respectively. At position 305, the SI values are 3.1-fold increased for both labels. The effect (on SI and on serpin activity) of the manipulations at these positions is in good agreement with the effect exerted by MA-8H9D4. In conclusion, our study provides proof of concept for the proposed approach in evaluating whether targeting a functional epitope with a small synthetic compound may be a feasible strategy in rational drug design.

Inhibition of plasminogen activator inhibitor-1: antibody fragments and their unique sequences as a tool for the development of profibrinolytic drugs

Physiological inhibition of plasminogen activator inhibitor-1 (PAI-1) might improve the prevention and treatment of various cardiovascular diseases. To date, a variety of monoclonal antibodies that neutralize PAI-1 have been generated. The current study presents the cloning, expression and characterization of four single-chain variable fragments (i.e. scFv-33B8, scFv-33H1F7, scFv-35A5 and scFv-55F4C12) from the corresponding PAI-1 neutralizing monoclonal antibodies. Surprisingly, affinity constants of scFv-33B8, scFv-33H1F7 and scFv-55F4C12 for PAI-1 (KA = 1.4 +/- 0.2 x 1010 m-1, 3.7 +/- 0.1 x 109 m-1, 1.0 +/- 0.2 x 109 m-1, respectively) were only 2- to 4-fold lower compared to those of the respective monoclonal antibodies (MAs). In contrast, scFv-35A5 exhibited a 6250-fold decrease in affinity (KA = 3.2 +/- 0.8 x 106 m-1 vs. 2.0 +/- 0.8 x 1010 m-1 observed for MA-35A5) with a concomitant absence of functional effects on PAI-1 activity. Evaluation of the dose-response curves of the PAI-1 neutralizing effect of the other scFvs revealed a shift towards slightly higher concentrations (in line with the small decrease in affinity) eventually resulting in a similar maximum effect as the corresponding MAs (i.e. 92 +/- 2%, 34 +/- 3% and 66 +/- 5% PAI-1 inhibition for scFv-33B8, scFv-33H1F7 and scFv-55F4C12, respectively). In conclusion, the sequence information of the scFvs allows to humanize MAs with PAI-1 inhibiting properties whereas the scFv constructs serve as an excellent starting point for structure based drug design, both aiming at the reduction of cardiovascular diseases.

Mutation of the highly conserved tryptophan in the serpin breach region alters the inhibitory mechanism of plasminogen activator inhibitor-1

We have demonstrated that interactions within the conserved serpin breach region play a direct role in the critical step of the serpin reaction in which the acyl-enzyme intermediate must first be exposed to hydrolyzing water and aqueous deacylation. Substitution of the breach tryptophan in PAI-1 (Trp175), a residue found in virtually all known serpins, with phenylalanine altered the kinetics of the reaction mechanism and impeded the ability of PAI-1 to spontaneously become latent without compromising the inherent rate of cleaved loop insertion or partitioning between the final inhibited serpin-proteinase complex and hydrolyzed serpin. Kinetic dissection of the PAI-1 inhibitory mechanism using multiple target proteinases made possible the identification of a single rate-limiting intermediate step coupled to the molecular interactions within the breach region. This step involves the initial insertion of the proximal reactive center loop hinge residue(s) into beta-sheet A and facilitates translocation of the distal P'-side of the cleaved reactive center loop from the substrate cleft of the proteinase. Substitution of the tryptophan residue raised the kinetic barrier restricting the initial loop insertion event, significantly retarding the rate-limiting step in tPA reactions in which strong exosite interactions must be overcome for the reaction to proceed.

The contribution of the exosite residues of plasminogen activator inhibitor-1 to proteinase inhibition

The binding of plasminogen activator inhibitor-1 (PAI-1) to serine proteinases, such as tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA), is mediated by the exosite interactions between the surface-exposed variable region-1, or 37-loop, of the proteinase and the distal reactive center loop (RCL) of PAI-1. Although the contribution of such interactions to the inhibitory activity of PAI-1 has been established, the specific mechanistic steps affected by interactions at the distal RCL remain unknown. We have used protein engineering, stopped-flow fluorimetry, and rapid acid quenching techniques to elucidate the role of exosite interactions in the neutralization of tPA, uPA, and beta-trypsin by PAI-1. Alanine substitutions at the distal P4' (Glu-350) and P5' (Glu-351) residues of PAI-1 reduced the rates of Michaelis complex formation (k(a)) and overall inhibition (k(app)) with tPA by 13.4- and 4.7-fold, respectively, whereas the rate of loop insertion or final acyl-enzyme formation (k(lim)) increased by 3.3-fold. The effects of double mutations on k(a), k(lim), and k(app) were small with uPA and nonexistent with beta-trypsin. We provide the first kinetic evidence that the removal of exosite interactions significantly alters the formation of the noncovalent Michaelis complex, facilitating the release of the primed side of the distal loop from the active-site pocket of tPA and the subsequent insertion of the cleaved reactive center loop into beta-sheet A. Moreover, mutational analysis indicates that the P5' residue contributes more to the mechanism of tPA inhibition, notably by promoting the formation of a final Michaelis complex.

Structural determinants in the stability of the serpin/proteinase complex

Serpins inhibit serine proteinases through formation of stable 1:1 complexes. In this study we have evaluated the effects of PAI-1 neutralizing antibodies (MA) on the stability of PAI-1/proteinase complexes, partially destabilized through prolongation of the reactive center loop. MA-8H9D4, reacting with residues Arg(300), Gln(303), and Asp(305), had no effect on the stability. In contrast, MA-33H1F7 and MA-55F4C12, reacting with alpha-helix F and the turn connecting hF with s3A, affected significantly and proteinase-dependently formed PAI-1/proteinase complexes. That is, MA-33H1F7 increased the stability of both PAI-1/t-PA and u-PA complexes (7- and 3-fold, respectively) whereas MA-55F4C12 stabilized PAI-1/t-PA complexes (3-fold) but destabilized PAI-1/u-PA complexes (2-fold). It is concluded that interference with the docking site of the cognate proteinase in the preformed serpin/proteinase complex may affect the intrinsic stability. We hypothesize that this is the consequence of a decreased or increased torsion of the RCL on the catalytic triad in the proteinase.

Elucidation of a novel epitope of a substrate-inducing monoclonal antibody against the serpin PAI-1

Plasminogen activator inhibitor-1 (PAI-1) is the most important physiological inhibitor of plasminogen activators. Inhibition of PAI-1 constitutes a putative strategy for the prevention of cardiovascular disease. The monoclonal antibody MA-8H9D4 inhibits PAI-1 activity by inducing a substrate behavior in PAI-1. To identify the epitope, a rational approach was used to design various PAI-1 alanine mutants (n = 16) for evaluation of their affinity. PAI-1-R300A, PAI-1-Q303A and PAI-1-D305A had affinities for MA-8H9D4 of < 10(5) M(-1), 2.0 x 10(8) M(-1) and 2.5 x 10(8) M(-1), respectively, whereas the affinity of wtPAI-1 is 3.3 x 10(9) M(-1). The epitope on the axis of arginine 300, glutamine 303 and aspartic acid 305, located on the loop between alpha-helix I and beta-strand 5A, demonstrates that MA-8H9D4 interferes with the final locking step in the serpin/proteinase interaction, thereby explaining its substrate inducing properties. The location of the epitope as well as the proposed mechanism of action is clearly different from that of other substrate inducing monoclonal antibodies against PAI-1. Elucidation of this novel epitope and the previously unidentified molecular mechanism opens new perspectives for the rational development of PAI-1-neutralizing compounds, as well as for the further exploration of synergistic effects between different PAI-1-inhibiting compounds.

Mutational analysis of plasminogen activator inhibitor-1

The serpin plasminogen activator inhibitor-1 (PAI-1) is a fast and specific inhibitor of the plasminogen activating serine proteases tissue-type and urokinase-type plasminogen activator and, as such, an important regulator in turnover of extracellular matrix and in fibrinolysis. PAI-1 spontaneously loses its antiproteolytic activity by inserting its reactive centre loop (RCL) as strand 4 in beta-sheet A, thereby converting to the so-called latent state. We have investigated the importance of the amino acid sequence of alpha-helix F (hF) and the connecting loop to s3A (hF/s3A-loop) for the rate of latency transition. We grafted regions of the hF/s3A-loop from antithrombin III and alpha1-protease inhibitor onto PAI-1, creating eight variants, and found that one of these reversions towards the serpin consensus decreased the rate of latency transition. We prepared 28 PAI-1 variants with individual residues in hF and beta-sheet A replaced by an alanine. We found that mutating serpin consensus residues always had functional consequences whereas mutating nonconserved residues only had so in one case. Two variants had low but stable inhibitory activity and a pronounced tendency towards substrate behaviour, suggesting that insertion of the RCL is held back during latency transition as well as during complex formation with target proteases. The data presented identify new determinants of PAI-1 latency transition and provide general insight into the characteristic loop-sheet interactions in serpins.