Purification and characterization of a plasminogen activator inhibitor 1 binding protein from human plasma. Identification as a multimeric form of S protein (vitronectin)

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.

Growth differentiation factor myostatin regulates epithelial-mesenchymal transition genes and enhances invasion by increasing serine protease inhibitors E1 and E2 in human trophoblast cells

Placental insufficiency disorders, including preeclampsia and intrauterine growth restriction, are major obstetric complications that can have devastating effects on both the mother and the fetus. These syndromes have underlying poor placental trophoblast cell invasion into uterine tissues. Placental invasion is controlled by many hormones and growth factors. Myostatin (MSTN) is a transforming growth factor-β superfamily member recognized for its important role in muscle growth control. MSTN has also been shown to be secreted and functioning in the placenta, and its serum and/or placental levels were found to be upregulated in preeclampsia and intrauterine growth restriction. Considering that the mechanistic role of MSTN in placentation remains poorly understood, we hypothesized that MSTN uses ALK4/5-SMAD2/3/4 signaling to increase human trophoblast invasion through a group of epithelial-mesenchymal transition genes including SERPINE2, PAI-1, and SOX4. mRNA sequencing of control and MSTN-treated primary human trophoblast cells (n = 5) yielded a total of 610 differentially expressed genes (false discovery rate <0.05) of which 380 genes were upregulated and 230 were downregulated. These differentially expressed genes were highly enriched in epithelial-mesenchymal transition genes, and a subset including SERPINE2, PAI-1, and SOX4 was investigated for its role in MSTN-induced trophoblast cell invasion. We found that MSTN induced upregulation of SERPINE2 via ALK4/5-SMAD2/3/4 signaling; however, SMAD2 was not involved in MSTN-induced PAI-1 upregulation. SOX4 was involved in MSTN-induced upregulation of SERPINE2, but not PAI-1. Collectively, this study discovers novel molecular mechanisms of MSTN-induced human trophoblast cell invasion and provides insight into the functional consequences of its dysregulation in placental insufficiency disorders.

Past, Present, and Future Perspectives of Plasminogen Activator Inhibitor 1 (PAI-1)

Plasminogen activator inhibitor 1 (PAI-1), a SERPIN inhibitor, is primarily known for its regulation of fibrinolysis. However, it is now known that this inhibitor functions and contributes to many (patho)physiological processes including inflammation, wound healing, cell adhesion, and tumor progression.This review discusses the past, present, and future roles of PAI-1, with a particular focus on the discovery of this inhibitor in the 1970s and subsequent characterization in health and disease. Throughout the past few decades diverse functions of this serpin have unraveled and it is now considered an important player in many disease processes. PAI-1 is expressed by numerous cell types, including megakaryocytes and platelets, adipocytes, endothelial cells, hepatocytes, and smooth muscle cells. In the circulation PAI-1 exists in two pools, within plasma itself and in platelet α-granules. Platelet PAI-1 is secreted following activation with retention of the inhibitor on the activated platelet membrane. Furthermore, these anucleate cells contain PAI-1 messenger ribonucleic acid to allow de novo synthesis.Outside of the traditional role of PAI-1 in fibrinolysis, this serpin has also been identified to play important roles in metabolic syndrome, obesity, diabetes, and most recently, acute respiratory distress syndrome, including coronavirus disease 2019 disease. This review highlights the complexity of PAI-1 and the requirement to ascertain a better understanding on how this complex serpin functions in (patho)physiological processes.

Laboratory evaluation of a new integrative assay to phenotype plasma fibrinolytic system

BACKGROUND

There is currently no universal and standardized test available to phenotype plasma fibrinolytic system.

AIMS

Our main aims were to evaluate the performances of the 'global fibrinolysis capacity' assay (GFC) performed with the Lysis Timer® instrument, and to study the influence of some preanalytical conditions.

METHOD

Euglobulin clot lysis time (ECLT) and GFC were performed under several preanalytical conditions.

RESULTS

GFC showed satisfactory intra- and inter-run precision. Frozen controls and reagents showed stability over the studied period. There was no statistically significant difference between GFC assessed in plasma samples processed at 4 °C or at 20 °C. GFC assessed with frozen-thawed plasma samples was prolonged when compared to fresh samples (p = 0.014). The centrifugation scheme had no influence on PAI-1 activity levels, GFC and ECLT. Reference interval for GFC ranges from 29.3 (C I90% = 26.9-31.9) to 49.5 (90% CI = 45.9-52.2) minutes. In addition, a preliminary study in 40 healthy volunteers and 43 adult patients referred for investigation of a bleeding disorder was conducted to compare GFC and ECLT assays in their ability to classify samples with shortened or prolonged clot lysis times. Disagreements between ECLT and GFC were observed for 23 samples (out of 83), most of them minor.

CONCLUSION

GFC is suitable and convenient for a broad clinical use and can be performed with frozen-thawed plasma samples. Unlike ECLT, GFC is designed to take into account the balance between inhibitors and activators of the fibrinolytic system and could detect both hypo- and hyperfibrinolytic states. Whether it is as suitable as or even better than ECLT to detect a bleeding tendency due to a hyperactive fibrinolytic system deserves to be properly investigated.

Deep mutational scanning and massively parallel kinetics of plasminogen activator inhibitor-1 functional stability to probe its latency transition

Plasminogen activator inhibitor-1 (PAI-1), a member of the serine protease inhibitor superfamily of proteins, is unique among serine protease inhibitors for exhibiting a spontaneous conformational change to a latent or inactive state. The functional half-life for this transition at physiologic temperature and pH is ∼1 to 2 h. To better understand the molecular mechanisms underlying this transition, we now report on the analysis of a comprehensive PAI-1 variant library expressed on filamentous phage and selected for functional stability after 48 h at 37 °C. Of the 7201 possible single amino acid substitutions in PAI-1, we identified 439 that increased the functional stability of PAI-1 beyond that of the WT protein. We also found 1549 single amino acid substitutions that retained inhibitory activity toward the canonical target protease of PAI-1 (urokinase-like plasminogen activator), whereas exhibiting functional stability less than or equal to that of WT PAI-1. Missense mutations that increase PAI-1 functional stability are concentrated in highly flexible regions within the PAI-1 structure. Finally, we developed a method for simultaneously measuring the functional half-lives of hundreds of PAI-1 variants in a multiplexed, massively parallel manner, quantifying the functional half-lives for 697 single missense variants of PAI-1 by this approach. Overall, these findings provide novel insight into the mechanisms underlying the latency transition of PAI-1 and provide a database for interpreting human PAI-1 genetic variants.

Long-range allostery mediates the regulation of plasminogen activator inhibitor-1 by cell adhesion factor vitronectin

The serpin plasminogen activator inhibitor 1 (PAI-1) spontaneously undergoes a massive structural change from a metastable and active conformation, with a solvent-accessible reactive center loop (RCL), to a stable, inactive, or latent conformation, with the RCL inserted into the central β-sheet. Physiologically, conversion to the latent state is regulated by the binding of vitronectin, which hinders the latency transition rate approximately twofold. The molecular mechanisms leading to this rate change are unclear. Here, we investigated the effects of vitronectin on the PAI-1 latency transition using all-atom path sampling simulations in explicit solvent. In simulated latency transitions of free PAI-1, the RCL is quite mobile as is the gate, the region that impedes RCL access to the central β-sheet. This mobility allows the formation of a transient salt bridge that facilitates the transition; this finding rationalizes existing mutagenesis results. Vitronectin binding reduces RCL and gate mobility by allosterically rigidifying structural elements over 40 Å away from the binding site, thus blocking transition to the latent conformation. The effects of vitronectin are propagated by a network of dynamically correlated residues including a number of conserved sites that were previously identified as important for PAI-1 stability. Simulations also revealed a transient pocket populated only in the vitronectin-bound state, corresponding to a cryptic drug-binding site identified by crystallography. Overall, these results shed new light on PAI-1 latency transition regulation by vitronectin and illustrate the potential of path sampling simulations for understanding functional protein conformational changes and for facilitating drug discovery.

The suboptimal fibrinolytic response in COVID-19 is dictated by high PAI-1

BACKGROUND

Severe COVID-19 disease is associated with thrombotic complications and extensive fibrin deposition. This study investigates whether the hemostatic complications in COVID-19 disease arise due to dysregulation of the fibrinolytic system.

METHODS

This prospective study analyzed fibrinolytic profiles of 113 patients hospitalized with COVID-19 disease with 24 patients with non-COVID-19 respiratory infection and healthy controls. Antigens were quantified by Ella system or ELISA, clot lysis by turbidimetric assay, and plasminogen activator inhibitor-1 (PAI-1)/plasmin activity using chromogenic substrates. Clot structure was visualized by confocal microscopy.

RESULTS

PAI-1 and its cofactor, vitronectin, are significantly elevated in patients with COVID-19 disease compared with those with non-COVID-19 respiratory infection and healthy control groups. Thrombin activatable fibrinolysis inhibitor and tissue plasminogen activator were elevated in patients with COVID-19 disease relative to healthy controls. PAI-1 and tissue plasminogen activator (tPA) were associated with more severe COVID-19 disease severity. Clots formed from COVID-19 plasma demonstrate an altered fibrin network, with attenuated fiber length and increased branching. Functional studies reveal that plasmin generation and clot lysis were markedly attenuated in COVID-19 disease, while PAI-1 activity was elevated. Clot lysis time significantly correlated with PAI-1 levels. Stratification of COVID-19 samples according to PAI-1 levels reveals significantly faster lysis when using the PAI-1 resistant (tPA) variant, tenecteplase, over alteplase lysis.

CONCLUSION

This study shows that the suboptimal fibrinolytic response in COVID-19 disease is directly attributable to elevated levels of PAI-1, which attenuate plasmin generation. These data highlight the important prognostic potential of PAI-1 and the possibility of using pre-existing drugs, such as tenecteplase, to treat COVID-19 disease and potentially other respiratory diseases.

Vitronectin and Its Interaction with PAI-1 Suggests a Functional Link to Vascular Changes in AMD Pathobiology

The pathogenesis of age-related macular degeneration (AMD), a frequent disorder of the central retina, is incompletely understood. Genome-wide association studies (GWAS) suggest a strong contribution of genomic variation in AMD susceptibility. Nevertheless, little is known about biological mechanisms of the disease. We reported previously that the AMD-associated polymorphism rs704C > T in the vitronectin (VTN) gene influences protein expression and functional aspects of encoded vitronectin, a human blood and extracellular matrix (ECM) protein. Here, we refined the association of rs704 with AMD in 16,144 cases and 17,832 controls and noted that rs704 is carried exclusively by the neovascular AMD subtype. Interaction studies demonstrate that rs704 affects the ability of vitronectin to bind the angiogenic regulator plasminogen activator inhibitor 1 (PAI-1) but has no influence on stabilizing its active state. Western blot analysis and confocal imaging reveal a strong enrichment of PAI-1 in the ECM of cultured endothelial cells and RPE cell line ARPE-19 exposed to vitronectin. Large-scale gene expression of VTN and PAI-1 showed positive correlations and a statistically significant increase in human retinal and blood tissues aged 60 years and older. Our results suggest a mechanism by which the AMD-associated rs704 variant in combination with ageing may contribute to the vascular complications in AMD.

Dysregulation of complement and coagulation pathways: emerging mechanisms in the development of psychosis

Early identification and treatment significantly improve clinical outcomes of psychotic disorders. Recent studies identified protein components of the complement and coagulation systems as key pathways implicated in psychosis. These specific protein alterations are integral to the inflammatory response and can begin years before the onset of clinical symptoms of psychotic disorder. Critically, they have recently been shown to predict the transition from clinical high risk to first-episode psychosis, enabling stratification of individuals who are most likely to transition to psychotic disorder from those who are not. This reinforces the concept that the psychosis spectrum is likely a central nervous system manifestation of systemic changes and highlights the need to investigate plasma proteins as diagnostic or prognostic biomarkers and pathophysiological mediators. In this review, we integrate evidence of alterations in proteins belonging to the complement and coagulation protein systems, including the coagulation, anticoagulation, and fibrinolytic pathways and their dysregulation in psychosis, into a consolidated mechanism that could be integral to the progression and manifestation of psychosis. We consolidate the findings of altered blood proteins relevant for progression to psychotic disorders, using data from longitudinal studies of the general population in addition to clinical high-risk (CHR) individuals transitioning to psychotic disorder. These are compared to markers identified from first-episode psychosis and schizophrenia as well as other psychosis spectrum disorders. We propose the novel hypothesis that altered complement and coagulation plasma levels enhance their pathways' activating capacities, while low levels observed in key regulatory components contribute to excessive activation observed in patients. This hypothesis will require future testing through a range of experimental paradigms, and if upheld, complement and coagulation pathways or specific proteins could be useful diagnostic or prognostic tools and targets for early intervention and preventive strategies.

The Fibrinolytic System: Mysteries and Opportunities

The deposition and removal of fibrin has been the primary role of coagulation and fibrinolysis, respectively. There is also little doubt that these 2 enzyme cascades influence each other given they share the same serine protease family ancestry and changes to 1 arm of the hemostatic pathway would influence the other. The fibrinolytic system in particular has also been known for its capacity to clear various non-fibrin proteins and to activate other enzyme systems, including complement and the contact pathway. Furthermore, it can also convert a number of growth factors into their mature, active forms. More recent findings have extended the reach of this system even further. Here we will review some of these developments and also provide an account of the influence of individual players of the fibrinolytic (plasminogen activating) pathway in relation to physiological and pathophysiological events, including aging and metabolism.

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.

A Narrative Review on Plasminogen Activator Inhibitor-1 and Its (Patho)Physiological Role: To Target or Not to Target?

Plasminogen activator inhibitor-1 (PAI-1) is the main physiological inhibitor of plasminogen activators (PAs) and is therefore an important inhibitor of the plasminogen/plasmin system. Being the fast-acting inhibitor of tissue-type PA (tPA), PAI-1 primarily attenuates fibrinolysis. Through inhibition of urokinase-type PA (uPA) and interaction with biological ligands such as vitronectin and cell-surface receptors, the function of PAI-1 extends to pericellular proteolysis, tissue remodeling and other processes including cell migration. This review aims at providing a general overview of the properties of PAI-1 and the role it plays in many biological processes and touches upon the possible use of PAI-1 inhibitors as therapeutics.

Structural Insight into the Two-Step Mechanism of PAI-1 Inhibition by Small Molecule TM5484

Plasminogen activator inhibitor-1 (PAI-1), a key regulator of the fibrinolytic system, is the main physiological inhibitor of plasminogen activators. By interacting with matrix components, including vitronectin (Vn), PAI-1 plays a regulatory role in tissue remodeling, cell migration, and intracellular signaling. Emerging evidence points to a role for PAI-1 in various pathological conditions, including cardiovascular diseases, cancer, and fibrosis. Targeting PAI-1 is therefore a promising therapeutic strategy in PAI-1-related pathologies. A class of small molecule inhibitors including TM5441 and TM5484, designed to bind the cleft in the central β-sheet A of PAI-1, showed to be potent PAI-1 inhibitors in vivo. However, their binding site has not yet been confirmed. Here, we report two X-ray crystallographic structures of PAI-1 in complex with TM5484. The structures revealed a binding site at the flexible joint region, which is distinct from the presumed binding site. Based on the structural analysis and biochemical data we propose a mechanism for the observed dose-dependent two-step mechanism of PAI-1 inhibition. By binding to the flexible joint region in PAI-1, TM5484 might restrict the structural flexibility of this region, thereby inducing a substrate form of PAI-1 followed by a conversion to an inert form.

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.

Dissecting molecular details and functional effects of the high-affinity copper binding site in plasminogen activator Inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) is the primary inhibitor for plasminogen activators, tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). As a unique member in the serine protease inhibitor (serpin) family, PAI-1 is metastable and converts to an inactive, latent structure with a half-life of 1-2 hr under physiological conditions. Unusual effects of metals on the rate of the latency conversion are incompletely understood. Previous work has identified two residues near the N-terminus, H2 and H3, which reside in a high-affinity copper-binding site in PAI-1 [Bucci JC, McClintock CS, Chu Y, Ware GL, McConnell KD, Emerson JP, Peterson CB (2017) J Biol Inorg Chem 22:1123-1,135]. In this study, neighboring residues, H10, E81, and H364, were tested as possible sites that participate in Cu(II) coordination at the high-affinity site. Kinetic methods, gel sensitivity assays, and isothermal titration calorimetry (ITC) revealed that E81 and H364 have different roles in coordinating metal and mediating the stability of PAI-1. H364 provides a third histidine in the metal-coordination sphere with H2 and H3. In contrast, E81 does not appear to be required for metal ligation along with histidines; contacts made by the side-chain carboxylate upon metal binding are perturbed and, in turn, influence dynamic fluctuations within the region encompassing helices D, E, and F and the W86 loop that are important in the pathway for the PAI-1 latency conversion. This investigation underscores a prominent role of protein dynamics, noncovalent bonding networks and ligand binding in controlling the stability of the active form of PAI-1.

Altered Protein Function Caused by AMD-associated Variant rs704 Links Vitronectin to Disease Pathology

Purpose

Vitronectin, a cell adhesion and spreading factor, is suspected to play a role in the pathogenesis of age-related macular degeneration (AMD), as it is a major component of AMD-specific extracellular deposits (e.g., soft drusen, subretinal drusenoid deposits). The present study addressed the impact of AMD-associated non-synonymous variant rs704 in the vitronectin-encoding gene VTN on vitronectin functionality.

Methods

Effects of rs704 on vitronectin expression and processing were analyzed by semi-quantitative sequencing of VTN transcripts from retinal pigment epithelium (RPE) cells generated from human induced pluripotent stem cells (hiPSCs) and from human neural retina, as well as by western blot analyses on heterologously expressed vitronectin isoforms. Binding of vitronectin isoforms to retinal and endothelial cells was analyzed by western blot. Immunofluorescence staining followed extracellular matrix (ECM) deposition in cultured RPE cells heterologously expressing the vitronectin isoforms. Adhesion of fluorescently labeled RPE or endothelial cells in dependence of recombinant vitronectin or vitronectin-containing ECM was investigated fluorometrically or microscopically. Tube formation and migration assays addressed effects of vitronectin on angiogenesis-related processes.

Results

Variant rs704 affected expression, secretion, and processing but not oligomerization of vitronectin. Cell binding and influence on RPE-mediated ECM deposition differed between AMD-risk-associated and non-AMD-risk-associated protein isoforms. Finally, vitronectin affected adhesion and endothelial tube formation.

Conclusions

The AMD-risk-associated vitronectin isoform exhibits increased expression and altered functionality in cellular processes related to the sub-RPE aspects of AMD pathology. Although further research is required to address the subretinal disease aspects, this initial study supports an involvement of vitronectin in AMD pathogenesis.

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

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

AS3288802, a highly selective antibody to active plasminogen activator inhibitor-1 (PAI-1), exhibits long efficacy duration in cynomolgus monkeys

Antibodies have strong affinity to their target molecules, a characteristic that is utilized in antibody drugs. For antibody drugs, target molecule specificity and long duration pharmacokinetics, along with strong affinity to the target molecule are important characteristics. Plasminogen activator inhibitor-1 (PAI-1) is one of the key regulators of the fibrinolysis system, and the benefits of PAI-1 activity inhibition have been widely reported for multiple thrombosis and fibrosis-related diseases. Here, we generated a novel antibody, AS3288802, with high selectivity for active PAI-1. AS3288802 exhibited prolonged and strong inhibition of PAI-1 activity in cynomolgus monkey blood in vivo. Given that AS3288802 showed prolonged antigen inhibition activity due to its high target molecule selectivity, we propose that increasing target molecule selectivity may be a key strategy for lengthening the efficacy duration of antibody drugs. AS3288802 may be a promising anti-PAI-1 antibody drug with multiple clinical applications including thrombosis and fibrosis-related diseases.

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.

Characterization of an Extensive Interface on Vitronectin for Binding to Plasminogen Activator Inhibitor-1: Adoption of Structure in an Intrinsically Disordered Region

Small-angle neutron scattering (SANS) measurements were pursued to study human vitronectin, a protein found in tissues and the circulation that regulates cell adhesion/migration and proteolytic cascades that govern hemostasis and pericellular proteolysis. Many of these functions occur via interactions with its binding partner, plasminogen activator inhibitor-1 (PAI-1), the chief inhibitor of proteases that lyse and activate plasminogen. We focused on a region of vitronectin that remains uncharacterized from previous X-ray scattering, nuclear magnetic resonance, and computational modeling approaches and which we propose is involved in binding to PAI-1. This region, which bridges the N-terminal somatomedin B (SMB) domain with a large central β-propeller domain of vitronectin, appears unstructured and has characteristics of an intrinsically disordered domain (IDD). The effect of osmolytes was evaluated using circular dichroism and SANS to explore the potential of the IDD to undergo a disorder-to-order transition. The results suggest that the IDD favors a more ordered structure under osmotic pressure; SANS shows a smaller radius of gyration (R_g) and a more compact fold of the IDD upon addition of osmolytes. To test whether PAI-1 binding is also coupled to folding within the IDD structure, a set of SANS experiments with contrast variation were performed on the complex of PAI-1 with a vitronectin fragment corresponding to the N-terminal 130 amino acids (denoted the SMB-IDD because it contains the SMB domain and IDD in linear sequence). Analysis of the SANS data using the Ensemble Optimization Method confirms that the SMB-IDD adopts a more compact configuration when bound to PAI-1. Calculated structures for the PAI-1:SMB-IDD complex suggest that the IDD provides an interaction surface outside of the primary PAI-1-binding site located within the SMB domain; this binding is proposed to lead to the assembly of higher-order structures of vitronectin and PAI-1 commonly found in tissues.

Identification of a PAI-1-binding site within an intrinsically disordered region of vitronectin

The serine protease inhibitor, plasminogen activator inhibitor Type-1 (PAI-1) is a metastable protein that undergoes an unusual transition to an inactive conformation with a short half-life of only 1-2 hr. Circulating PAI-1 is bound to a cofactor vitronectin, which stabilizes PAI-1 by slowing this latency conversion. A well-characterized PAI-1-binding site on vitronectin is located within the somatomedin B (SMB) domain, corresponding to the first 44 residues of the protein. Another PAI-1 recognition site has been identified with an engineered form of vitronectin lacking the SMB domain, yet retaining PAI-1 binding capacity (Schar, Blouse, Minor, Peterson. J Biol Chem. 2008;283:28487-28496). This additional binding site is hypothesized to lie within an intrinsically disordered domain (IDD) of vitronectin. To localize the putative binding site, we constructed a truncated form of vitronectin containing 71 amino acids from the N-terminus, including the SMB domain and an additional 24 amino acids from the IDD region. This portion of the IDD is rich in acidic amino acids, which are hypothesized to be complementary to several basic residues identified within an extensive vitronectin-binding site mapped on PAI-1 (Schar, Jensen, Christensen, Blouse, Andreasen, Peterson. J Biol Chem. 2008;283:10297-10309). Steady-state and stopped-flow fluorescence measurements demonstrate that the truncated form of vitronectin exhibits the same rapid biphasic association as full-length vitronectin and that the IDD hosts the elusive second PAI-1 binding site that lies external to the SMB domain of vitronectin.

Thrombin Provokes Degranulation of Platelet α-Granules Leading to the Release of Active Plasminogen Activator Inhibitor-1 (PAI-1)

BACKGROUND

The balance of fibrinolytic mediators is crucial to the survival of the critically ill patient, with tissue plasminogen activator (t-PA) and plasminogen activator inhibitor-1 (PAI-1) playing significant roles. While elevated levels of PAI-1 are associated with increased morbidity and mortality, the source of this PAI-1 remains elusive. Platelets contain 90% of circulating plasma PAI-1, however, their ability to release active PAI-1 is controversial. We hypothesize platelets contain active PAI-1 in α-granules capable of immediate degranulation when exposed to high concentrations of thrombin.

METHODS

In vitro apheresis platelets were stimulated with thrombin (1 IU/mL, 5 IU/mL) followed by the collection of supernatant (5-120 min). Supernatant and lysate PAI-1 was measured by ELISA. The experiment was repeated in the presence of t-PA followed by measurement of t-PA:PAI-1 complex measurement by ELISA. Finally, healthy whole blood underwent dilution with control and thrombin-treated platelet lysate followed by thrombelastography (TEG) in a t-PA-stimulated TEG.

RESULTS

Thrombin provoked immediate near-complete degranulation of PAI-1 from α-granules (median 5m 5 IU/mL thrombin 125.1 ng/mL, 1 IU/mL thrombin 114.9 ng/mL, control 9.9 ng/mL). The released PAI-1 rapidly complexed with t-PA, with a 4-fold increase in complex formation in the thrombin-treated supernatant. Conversely, PAI-1 in the control lysate demonstrated a 6-fold increase in complex formation compared with thrombin lysate. Last, control platelet lysate inhibited t-PA-induced fibrinolysis by TEG (median LY30 control 15m 7.9%), while thrombin-treated platelet lysates, after PAI-1 degranulation, were unable to affect the fibrinolysis profile (median LY30 5 IU/mL 28.5%, 1 IU/mL 12.4%).

CONCLUSION

Thrombin provokes rapid α-degranulation of active PAI-1, capable of complexing with t-PA and neutralizing t-PA-induced fibrinolysis by TEG.

The Immune Functions of α1 Acid Glycoprotein

α1-acid glycoprotein (orosomucoid, AGP) is an Acute Phase Protein produced by liver and peripheral tissues in response to systemic reaction to inflammation. AGP functions have been studied mostly in human, cattle and fish, although the protein has been also found in many mammalian species and birds. AGP fulfils at least two set of functions, which are apparently different from each other but in fact intimately linked. On one hand, AGP is an immunomodulatory protein. On the other hand, AGP is one of the most important binding proteins in plasma and, beside modulating pharmacokinetics and pharmacodynamics of many drugs, it is also able to bind and transport several endogen ligands related to inflammation. The focus of this review is the immunomodulatory activity of AGP. This protein regulates every single event related to inflammation, including binding of pathogens and modulating white blood cells activity throughout the entire leukocyte attacking sequence. The regulation of AGP activity is complex: the inflammation induces not only an increase in AGP serum concentration, but also a qualitative change in its carbohydrate moiety, generating a multitude of glycoforms, each of them with different, and sometimes opposite and contradictory, activities. We also present the most recent findings about the relationship between AGP and adipose tissue: AGP interacts with leptin receptor and, given its immunomodulatory function, it may be included among the potential players in the field of immunometabolism.

Prospective assessment of fibrinolysis in morbid obesity: tissue plasminogen activator resistance improves after bariatric surgery

BACKGROUND

Morbid obesity is associated with an increased risk of thrombotic events, which has been attributed to increased thrombotic activity. Multiple mechanisms have been proposed to explain this increased risk, including an inflammatory state with upregulation of procoagulant and antifibrinolytic proteins. We therefore hypothesize that patients with morbid obesity are hypercoagulable and will revert to normal after bariatric surgery.

OBJECTIVES

To evaluate changes in the hypercoagulable state after bariatric surgery.

SETTING

University Hospital, Bariatric Center of Excellence, United States.

METHODS

Thromboelastography (TEG) data were collected on 72 subjects with morbid obesity, with 36 who had 6 months of follow-up after bariatric surgery. TEG data of 75 healthy subjects (HS) without obesity, recent trauma or surgery, acute infection, or chronic conditions (e.g., liver, cardiovascular, or kidney disease; cancer; diabetes; autoimmune or inflammatory disorders; and disorders of coagulation) were used for comparison. TEG was performed alone and with the addition of 75 and 150 ng/mL tissue plasminogen activator (tPA) to quantify fibrinolysis resistance (tPA-challenged TEG).

RESULTS

The bariatric surgery cohort had a median age of 40.5 years, a median body mass index of 44.6 kg/m², and 90% female patients. Median body mass index reduced significantly 6 months post surgery but remained elevated compared with the HS group (31.4 versus 25.4 kg/m², P < .0001). At 6 months post surgery, subjects had longer reaction time (mean difference, 1.3; P = .02), lower maximum amplitude (-2.4, P = .01), and increased fibrinolysis with low-dose (3.1, P < .0001) and high-dose tPA-challenged TEG (9, P < .0001). Compared with HS, the postsurgery TEG values were still more likely to be abnormal (all P < .05).

CONCLUSIONS

Patients with morbid obesity form stronger clots more rapidly and are more resistant to fibrinolysis than subjects without obesity. Bariatric surgery significantly improved the hypercoagulable profile and fibrinolysis resistance of morbid obesity.

Thrombin stimulates increased plasminogen activator inhibitor-1 release from liver compared to lung endothelium

BACKGROUND

Plasminogen activator inhibitor-1 (PAI-1) is a major regulator of the fibrinolytic system, covalently binding to tissue plasminogen activator and blocking its activity. Fibrinolysis shutdown is evident in the majority of severely injured patients in the first 24 h and is thought to be due to PAI-1. The source of this PAI-1 is thought to be predominantly endothelial cells, but there are known organ-specific differences, with higher levels thought to be in the liver. Thrombin generation is also elevated in injured patients and is a potent stimulus for PAI-1 release in human umbilical endothelial cells. We hypothesize that thrombin induces liver endothelial cells to release increased amounts of PAI-1, versus pulmonary endothelium, consisting of both stored PAI-1 and a larger contribution from de novo PAI-1 synthesis.

METHODS

Human liver sinusoidal endothelial cells (LSECs) and human microvascular lung endothelial cells (HMVECs) were stimulated in vitro ± thrombin (1 and 5 IU/mL) for 15-240 min, the supernatants were collected, and PAI-1 was measured by enzyme-linked immunosorbent assays. To elucidate the PAI-1 contribution from storage versus de novo synthesis, cycloheximide (10 μg/mL) was added before thrombin in separate experiments.

RESULTS

While both LSECs and HMVECs rapidly stimulated PAI-1 release, LSECs released more PAI-1 than HMVECs in response to high-dose thrombin, whereas low-dose thrombin did not provoke immediate release. LSECs continued to release PAI-1 over the ensuing 240 min, whereas HMVECs did not. Cycloheximide did not inhibit early PAI-1 release from LSECs but did at the later time points (30-240 min).

CONCLUSIONS

Thrombin elicits increased amounts of PAI-1 release from liver endothelium compared with lung, with a small presynthesized stored contribution and a later, larger increase in PAI-1 release via de novo synthesis. This study suggests that the liver may be an important therapeutic target for inhibition of the hypercoagulable surgical patient and the associated complications that result.

Association of vitronectin and plasminogen activator inhibitor-1 levels with the risk of metabolic syndrome and type 2 diabetes mellitus

It was the objective of this study to investigate the relation between vitronectin and plasminogen activator inhibitor (PAI)-1 plasma levels with nine-year incidences of the metabolic syndrome (MetS) and of type 2 diabetes mellitus (T2DM). Baseline plasma concentrations of vitronectin and PAI-1 were measured in 627 healthy participants from the prospective D.E.S.I.R. cohort who subsequently developed MetS (n=487) and T2DM (n=182) over a nine-year follow-up (42 presented both) and who were matched with two healthy control subjects each by use of a nested case-control design. Parameters composing the MetS explained about 20% of plasma vitronectin levels. An increase of one standard deviation of vitronectin was associated with increased risks of both the MetS (odds ratio [OR] = 1.21 [1.07 – 1.37], p = 0.003) and T2DM (OR = 1.24 [1.01 – 1.53], p = 0.045). Corresponding ORs for PAI-1 levels were 1.46 [1.27 – 1.68] (p < 10−4) and 1.40 [1.14 – 1.72] (p = 0.0012). However, the effects of vitronectin and PAI-1 levels on outcomes were not independent. The vitronectin–MetS association was restricted to individuals with low to modest PAI-1 levels (OR = 1.33 [1.14 – 1.54], p = 0.0003) while no association was observed in individuals with high PAI-1 levels (OR = 0.87 [0.68 – 1.10], p = 0.24), the test for interaction being highly significant (p = 0.0009). In conclusion, baseline plasma vitronectin is a marker of incident MetS at nine years. Its predictive ability for MetS and T2DM should not be assessed independently of PAI-1 levels.

Latency transition of plasminogen activator inhibitor type 1 is evolutionarily conserved

Plasminogen activator inhibitor type 1 (PAI-1) is a central regulator of fibrinolysis and tissue remodelling. PAI-1 belongs to the serpin superfamily and unlike other inhibitory serpins undergoes a spontaneous inactivation process under physiological conditions, termed latency transition. During latency transition the solvent exposed reactive centre loop is inserted into the central β-sheet A of the molecule, and is no longer accessible to reaction with the protease. More than three decades of research on mammalian PAI-1 has not been able to clarify the evolutionary advantage and physiological relevance of latency transition. In order to study the origin of PAI-1 latency transition, we produced PAI-1 from Spiny dogfish shark (Squalus acanthias) and African lungfish (Protopterus sp.), which represent central species in the evolution of vertebrates. Although human PAI-1 and the non-mammalian PAI-1 variants share only approximately 50 % sequence identity, our results showed that all tested PAI-1 variants undergo latency transition with a similar rate. Since the functional stability of PAI-1 can be greatly increased by substitution of few amino acid residues, we conclude that the ability to undergo latency transition must have been a specific selection criterion for the evolution of PAI-1. It appears that all PAI-1 molecules must harbour latency transition to fulfil their physiological function, stressing the importance to further pursue a complete understanding of this molecular phenomenon with possible implication to pharmacological intervention. Our results provide the next step in understanding how the complete role of this important protease inhibitor evolved along with the fibrinolytic system.

Characterization of the Annonaceous acetogenin, annonacinone, a natural product inhibitor of plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) is the main inhibitor of the tissue type and urokinase type plasminogen activators. High levels of PAI-1 are correlated with an increased risk of thrombotic events and several other pathologies. Despite several compounds with in vitro activity being developed, none of them are currently in clinical use. In this study, we evaluated a novel PAI-1 inhibitor, annonacinone, a natural product from the Annonaceous acetogenins group. Annonacinone was identified in a chromogenic screening assay and was more potent than tiplaxtinin. Annonacinone showed high potency ex vivo on thromboelastography and was able to potentiate the thrombolytic effect of tPA in vivo in a murine model. SDS-PAGE showed that annonacinone inhibited formation of PAI-1/tPA complex via enhancement of the substrate pathway. Mutagenesis and molecular dynamics allowed us to identify annonacinone binding site close to helix D and E and β-sheets 2A.

Intra- and extracellular plasminogen activator inhibitor-1 regulate effect of vitronectin against radiation-induced endothelial cell death

Plasminogen activator inhibitor-1 (PAI-1) is induced by radiation resulting in endothelial cell impairment, potentially leading to multiple organ failure. Vitronectin (VN) is a 75-kDa glycoprotein (VN₇₅) cleaved into two forms (VN₇₅ or VN_65/10) by furin, which is regulated by intracellular PAI-1. VN protects against radiation-induced endothelial cell death, but the mechanisms involved in VN processing and its interactions with intra- and extracellular PAI-1 remain unclear. We examined these processes in cells in vitro using recombinant proteins or overexpression of VN and PAI-1 genes, including furin-susceptible (T³⁸¹) and furin-resistant VN (A³⁸¹). VN processing was analyzed using a mutant PAI-1 with relatively weaker binding to VN. VN function was evaluated by survival of radiation-damaged endothelial cells. Wild-type, but not mutant PAI-1 inhibited furin-dependent VN processing. Gene transfer revealed that furin-susceptible VN was processed more than the furin-resistant form, but processing of both was inhibited by PAI-1 overexpression. Intracellular PAI-1 formed a complex with VN₇₅ (T³⁸¹) in cells and media, and the VN₇₅ form was secreted preferentially. Only VN₇₅ protected against radiation-induced endothelial cell death, in which its effect was abolished by wild-type but not mutant PAI-1. These findings indicate that intracellular PAI-1 inhibits VN processing and protects against radiation-induced endothelial cell death.

Proteolysis of plasminogen activator inhibitor-1 by Yersinia pestis remodulates the host environment to promote virulence

Essentials Effect of plasminogen activator inhibitor (PAI)-1 on plague and its Y. pestis cleavage is unknown. An intranasal mouse model of infection was used to determine the role of PAI-1 in pneumonic plague. PAI-1 is cleaved and inactivated by the Pla protease of Y. pestis in the lung airspace. PAI-1 impacts both bacterial outgrowth and the immune response to respiratory Y. pestis infection. Click to hear Dr Bock discuss pathogen activators of plasminogen.

SUMMARY

Background The hemostatic regulator plasminogen activator inhibitor-1 (PAI-1) inactivates endogenous plasminogen activators and aids in the immune response to bacterial infection. Yersinia pestis, the causative agent of plague, produces the Pla protease, a virulence factor that is required during plague. However, the specific hemostatic proteins cleaved by Pla in vivo that contribute to pathogenesis have not yet been fully elucidated. Objectives To determine whether PAI-1 is cleaved by the Pla protease during pneumonic plague, and to define the impact of PAI-1 on Y. pestis respiratory infection in the presence or absence of Pla. Methods An intranasal mouse model of pneumonic plague was used to assess the levels of total and active PAI-1 in the lung airspace, and the impact of PAI-1 deficiency on bacterial pathogenesis, the host immune response and plasmin generation following infection with wild-type or ∆pla Y. pestis. Results We found that Y. pestis cleaves and inactivates PAI-1 in the lungs in a Pla-dependent manner. The loss of PAI-1 enhances Y. pestis outgrowth in the absence of Pla, and is associated with increased conversion of plasminogen to plasmin. Furthermore, we found that PAI-1 regulates immune cell recruitment, cytokine production and tissue permeability during pneumonic plague. Conclusions Our data demonstrate that PAI-1 is an in vivo target of the Pla protease in the lungs, and that PAI-1 is a key regulator of the pulmonary innate immune response. We conclude that the inactivation of PAI-1 by Y. pestis alters the host environment to promote virulence during pneumonic plague.

Role of heparin and non heparin binding serpins in coagulation and angiogenesis: A complex interplay

Pro-coagulant, anti-coagulant and fibrinolytic pathways are responsible for maintaining hemostatic balance under physiological conditions. Any deviation from these pathways would result in hypercoagulability leading to life threatening diseases like myocardial infarction, stroke, portal vein thrombosis, deep vein thrombosis (DVT) and pulmonary embolism (PE). Angiogenesis is the process of sprouting of new blood vessels from pre-existing ones and plays a critical role in vascular repair, diabetic retinopathy, chronic inflammation and cancer progression. Serpins; a superfamily of protease inhibitors, play a key role in regulating both angiogenesis and coagulation. They are characterized by the presence of highly conserved secondary structure comprising of 3 β-sheets and 7-9 α-helices. Inhibitory role of serpins is modulated by binding to cofactors, specially heparin and heparan sulfate proteoglycans (HSPGs) present on cell surfaces and extracellular matrix. Heparin and HSPGs are the mainstay of anti-coagulant therapy and also have therapeutic potential as anti-angiogenic inhibitors. Many of the heparin binding serpins that regulate coagulation cascade are also potent inhibitors of angiogenesis. Understanding the molecular mechanism of the switch between their specific anti-coagulant and anti-angiogenic role during inflammation, stress and regular hemostasis is important. In this review, we have tried to integrate the role of different serpins, their interaction with cofactors and their interplay in regulating coagulation and angiogenesis.

A Case of Unexplained Cerebral Sinus Thrombosis in a 22-Year-Old Obese Caucasian Woman

Herein, we present the case of a 22-year old obese Caucasian woman female with no acquired thrombophilic risk factors who was diagnosed with extensive cerebral sinus thrombosis. A detailed thrombophilia workup demonstrated persistently elevated plasminogen activator inhibitor 1 (PAI-1) activity levels, with an elevated PAI-1 antigen concentration and homozygosity for the PAI-1 4G allele (4G/4G genotype). The patient was treated with indefinite warfarin anticoagulation medication due to the unprovoked nature of her thrombotic event. Disturbances in the fibrinolytic system, in particular PAI-1, have been related to an increased risk of arterial and venous thrombosis. In this article, we discuss the pathophysiology of hypofibrinolysis associated with elevated PAI-1 levels and the PAI-1 4G/5G polymorphism.

Plasminogen Activator Inhibitor-1 in Cancer: Rationale and Insight for Future Therapeutic Testing

Despite its function as an inhibitor of urokinase and tissue-type plasminogen activator (PA), PA inhibitor-1 (PAI-1) has a paradoxical protumorigenic role in cancer, promoting angiogenesis and tumor cell survival. In this review, we summarize preclinical evidence in support of the protumorigenic function of PAI-1 that has led to the testing of small-molecule PAI-1 inhibitors, initially developed as antithrombotic agents, in animal models of cancer. The review discusses the challenges and the opportunities that lay ahead to the development of efficacious and nontoxic PAI-1 inhibitors as anticancer agents.

D-dimer: An Overview of Hemostasis and Fibrinolysis, Assays, and Clinical Applications

D-dimer is the smallest fibrinolysis-specific degradation product found in the circulation. The origins, assays, and clinical use of D-dimer will be addressed. Hemostasis (platelet and vascular function, coagulation, fibrinolysis, hemostasis) is briefly reviewed. D-dimer assays are reviewed. The D-dimer is very sensitive to intravascular thrombus and may be markedly elevated in disseminated intravascular coagulation, acute aortic dissection, and pulmonary embolus. Because of its exquisite sensitivity, negative tests are useful in the exclusion venous thromboembolism. Elevations occur in normal pregnancy, rising two- to fourfold by delivery. D-dimer also rises with age, limiting its use in those >80 years old. There is a variable rise in D-dimer in active malignancy and indicates increased thrombosis risk in active disease. Elevated D-dimer following anticoagulation for a thrombotic event indicates increased risk of recurrent thrombosis. These and other issues are addressed.

Functional stability of plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) is the main inhibitor of plasminogen activators, such as tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA), and a major regulator of the fibrinolytic system. PAI-1 plays a pivotal role in acute thrombotic events such as deep vein thrombosis (DVT) and myocardial infarction (MI). The biological effects of PAI-1 extend far beyond thrombosis including its critical role in fibrotic disorders, atherosclerosis, renal and pulmonary fibrosis, type-2 diabetes, and cancer. The conversion of PAI-1 from the active to the latent conformation appears to be unique among serpins in that it occurs spontaneously at a relatively rapid rate. Latency transition is believed to represent a regulatory mechanism, reducing the risk of thrombosis from a prolonged antifibrinolytic action of PAI-1. Thus, relying solely on plasma concentrations of PAI-1 without assessing its function may be misleading in interpreting the role of PAI-1 in many complex diseases. Environmental conditions, interaction with other proteins, mutations, and glycosylation are the main factors that have a significant impact on the stability of the PAI-1 structure. This review provides an overview on the current knowledge on PAI-1 especially importance of PAI-1 level and stability and highlights the potential use of PAI-1 inhibitors for treating cardiovascular disease.

A role for tissue plasminogen activator in thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening disease characterized by generalized microvascular occlusion. TTP has been related to severe deficiency of ADAMTS13, an enzyme that cleaves von Willebrand factor multimers into less adhesive molecules. However, ADAMTS13 deficiency correlates poorly with severity of thrombocytopenia or microangiopathic hemolysis, with the frequency of neurologic complications or the response to plasma exchange. Also, some patients with severe hereditary ADAMTS13 deficiency consistently relapse every few weeks, whereas others remain asymptomatic into their forties. Taken together, these findings suggest that an additional element is missing in the pathophysiology of TTP. We postulate that both low ADAMTS13 activity and low tissue-plasminogen activator activity are required to trigger TTP attacks. Tissue-plasminogen activator end product, plasmin, extensively degrades von Willebrand factor, breaking-down the bonds between platelets and the blood vessel wall, so that low tissue-plasminogen activator activity prevents a mechanism similar to that of ADAMTS13. The hypothesis that low tissue-plasminogen activator activity plays an important role in TTP pathogenesis is further substantiated by TTP comorbidity. Problems prevalent in patients with TTP attacks or with long-term TTP remission, including increased body mass index, major depression, cognitive abnormalities, hypertension, and premature death, are somehow associated with low tissue-plasminogen activator activity.

Proteomic analysis of human plasma in chronic rheumatic mitral stenosis reveals proteins involved in the complement and coagulation cascade

BACKGROUND

Rheumatic fever in childhood is the most common cause of Mitral Stenosis in developing countries. The disease is characterized by damaged and deformed mitral valves predisposing them to scarring and narrowing (stenosis) that results in left atrial hypertrophy followed by heart failure. Presently, echocardiography is the main imaging technique used to diagnose Mitral Stenosis. Despite the high prevalence and increased morbidity, no biochemical indicators are available for prediction, diagnosis and management of the disease. Adopting a proteomic approach to study Rheumatic Mitral Stenosis may therefore throw some light in this direction. In our study, we undertook plasma proteomics of human subjects suffering from Rheumatic Mitral Stenosis (n = 6) and Control subjects (n = 6). Six plasma samples, three each from the control and patient groups were pooled and subjected to low abundance protein enrichment. Pooled plasma samples (crude and equalized) were then subjected to in-solution trypsin digestion separately. Digests were analyzed using nano LC-MS(E). Data was acquired with the Protein Lynx Global Server v2.5.2 software and searches made against reviewed Homo sapiens database (UniProtKB) for protein identification. Label-free protein quantification was performed in crude plasma only.

RESULTS

A total of 130 proteins spanning 9-192 kDa were identified. Of these 83 proteins were common to both groups and 34 were differentially regulated. Functional annotation of overlapping and differential proteins revealed that more than 50% proteins are involved in inflammation and immune response. This was corroborated by findings from pathway analysis and histopathological studies on excised tissue sections of stenotic mitral valves. Verification of selected protein candidates by immunotechniques in crude plasma corroborated our findings from label-free protein quantification.

CONCLUSIONS

We propose that this protein profile of blood plasma, or any of the individual proteins, could serve as a focal point for future mechanistic studies on Mitral Stenosis. In addition, some of the proteins associated with this disorder may be candidate biomarkers for disease diagnosis and prognosis. Our findings might help to enrich existing knowledge on the molecular mechanisms involved in Mitral Stenosis and improve the current diagnostic tools in the long run.

The LRP1-independent mechanism of PAI-1-induced migration in CpG-ODN activated macrophages

CpG-oligodeoxynucleotides (CpG-ODNs) induces plasminogen activator inhibitor type-1 (PAI-1) expression in macrophages, leading to enhanced migration through vitronectin. However, the precise role of low-density lipoprotein receptor-related protein 1 (LRP1) in PAI-1 induced migration of macrophages in the inflammatory environment is not known. In this study, we elucidated a novel mechanism describing the altered role of LRP1 in macrophage migration depending on the activation state of the cells. Experimental evidence clearly shows that the blocking of LRP1 function inhibited the PAI-induced migration of resting RAW 264.7 cells through vitronectin but exerted a pro-migratory effect on CpG-ODN-activated cells. We also demonstrate that CpG-ODN downregulates the protein and mRNA levels of LRP1 both in vivo and in vitro, a function that depends on the NF-κB signaling pathway, resulting in reduced internalization of PAI-1. This work illustrates the distinct mechanism that PAI-1-induced migration of CpG-ODN-activated cells through vitronectin depends on the interaction of PAI-1 with vitronectin but not LRP1 unlike in the resting cells, where the migration is LRP1 dependent and vitronectin independent. In conclusion, our experimental results demonstrate the altered function of LRP1 in the migration of resting and activated macrophages in the context of microenvironmental extracellular matrix components.

Effects of steroid ablation and progestin replacement on the transcriptome of the primate corpus luteum during simulated early pregnancy

Previous microarray analyses indicated that a portion of the transcriptome in the macaque corpus luteum (CL) of the menstrual cycle was regulated indirectly by luteinizing hormone via the local actions of steroid hormones, notably progesterone (P). The current study was designed to investigate this concept in the CL of early pregnancy by analyzing chorionic gonadotrophin (CG)-regulated genes that are dependent versus independent of local steroid action. Exogenous human chorionic gonadotropin treatment simulating early pregnancy (SEP) began on Day 9 of the luteal phase in female rhesus monkeys with and without concurrent administration of the 3-β-hydroxysteroid dehydrogenase inhibitor trilostane (TRL) with or without the synthetic progestin R5020. Compared with SEP treatment alone, TRL altered 50 mRNA transcripts on Day 10, rising to 95 on Day 15 (P<0.05, ≥2-fold change in gene expression). Steroid-sensitive genes were validated; notably effects of steroid ablation and P replacement varied by day. Expression of some genes previously identified as P-regulated in the macaque CL during the menstrual cycle were not significantly altered by steroid ablation and P replacement during CG exposure in SEP. These data indicate that the majority of CG-regulated luteal transcripts are differentially expressed independently of local steroid actions. However, the steroid-regulated genes in the macaque CL may be essential during early pregnancy, based on previous reports that TRL treatment initiates premature structural regression of the CL during SEP. These data reinforce the concept that the structure, function and regulation of the rescued CL in early pregnancy differs from the CL of the menstrual cycle in primates.

Vitronectin--master controller or micromanager?

The concept that the mammalian glycoprotein vitronectin acts as a biological 'glue' and key controller of mammalian tissue repair and remodelling activity is emerging from nearly 50 years of experimental in vitro and in vivo data. Unexpectedly, the vitronectin-knockout (VN-KO) mouse was found to be viable and to have largely normal phenotype. However, diligent observation revealed that the VN-KO animal exhibits delayed coagulation and poor wound healing. This is interpreted to indicate that VN occupies a role in the earliest events of thrombogenesis and tissue repair. VN is the foundation upon which the thrombus grows in an organised structure. In addition to sealing the wound, the thrombus also serves to protect the underlying tissue from oxidation, is a reservoir of mitogens and tissue repair mediators, and provides a provisional scaffold for the repairing tissue. In the absence of VN (e.g., VN-KO animal), this cascade is disrupted before it begins. A wide variety of biologically active species associate with VN. Although initial studies were focused on mitogens, other classes of bioactives (e.g., glycosaminoglycans and metalloproteinases) are now also known to specifically interact with VN. Although some interactions are transient, others are long-lived and often result in multi-protein complexes. Multi-protein complexes provide several advantages: prolonging molecular interactions, sustaining local concentrations, facilitating co-stimulation of cell surface receptors and thereby enhancing cellular/biological responses. We contend that these, or equivalent, multi-protein complexes facilitate VN polyfunctionality in vivo. It is also likely that many of the species demonstrated to associate with VN in vitro, also associate with VN in vivo in similar multi-protein complexes. Thus, the predominant biological function of VN is that of a master controller of the extracellular environment; informing, and possibly instructing cells 'where' to behave, 'when' to behave and 'how' to behave (i.e., appropriately for the current circumstance).

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.

Plasminogen activator inhibitor-1 inhibitors: a patent review (2006-present)

INTRODUCTION

Plasminogen activator inhibitor-1 (PAI-1), the serine protease inhibitor (serpin), binds to and inhibits the plasminogen activators-tissue-type plasminogen activator (tPA) and the urokinase-type plasminogen activator (uPA). This results in both a decrease in plasmin production and a decrease in the dissolution of fibrin clots. Elevated levels of PAI-1 are correlated with an increased risk for cardiovascular disease and have been linked to obesity and metabolic syndrome. Consequently, the pharmacological suppression of PAI-1 might prevent or treat vascular disease.

AREAS COVERED

This article provides an overview of the patenting activity on PAI-1 inhibitors. Patents filed by pharmaceutical companies or individual research groups are described, and the biological and biochemical evaluation of the inhibitors, including in vitro and in vivo studies, is discussed. An overview of patents pertaining to using these inhibitors for treating various diseases is also included.

EXPERT OPINION

Although there is still no PAI-1 inhibitor being evaluated in a clinical setting or approved for human therapy, research in this field has progressed, and promising new compounds have been designed. Most research has focused on improving the pharmacological profile of these compounds, which will hopefully allow them to proceed to clinical studies. Despite the need for further testing and research, the potential use of PAI-1 inhibitors for treating cardiovascular disease appears quite promising.

Hydrogen/deuterium exchange mass spectrometry reveals specific changes in the local flexibility of plasminogen activator inhibitor 1 upon binding to the somatomedin B domain of vitronectin

The native fold of plasminogen activator inhibitor 1 (PAI-1) represents an active metastable conformation that spontaneously converts to an inactive latent form. Binding of the somatomedin B domain (SMB) of the endogenous cofactor vitronectin to PAI-1 delays the transition to the latent state and increases the thermal stability of the protein dramatically. We have used hydrogen/deuterium exchange mass spectrometry to assess the inherent structural flexibility of PAI-1 and to monitor the changes induced by SMB binding. Our data show that the PAI-1 core consisting of β-sheet B is rather protected against exchange with the solvent, while the remainder of the molecule is more dynamic. SMB binding causes a pronounced and widespread stabilization of PAI-1 that is not confined to the binding interface with SMB. We further explored the local structural flexibility in a mutationally stabilized PAI-1 variant (14-1B) as well as the effect of stabilizing antibody Mab-1 on wild-type PAI-1. The three modes of stabilizing PAI-1 (SMB, Mab-1, and the mutations in 14-1B) all cause a delayed latency transition, and this effect was accompanied by unique signatures on the flexibility of PAI-1. Reduced flexibility in the region around helices B, C, and I was seen in all three cases, which suggests an involvement of this region in mediating structural flexibility necessary for the latency transition. These data therefore add considerable depth to our current understanding of the local structural flexibility in PAI-1 and provide novel indications of regions that may affect the functional stability of PAI-1.

Promoter variants of VTN are associated with vascular disease

BACKGROUND

Vitronectin is involved in the whole process of atherosclerosis. Our aim is to determine the association of VTN functional promoter variants with different types of vascular disease, and conclude the roles of vitronectin involved in vascular disease.

METHODS

Gel shift assays and luciferase reporter assays were used to determine the impact of variants on promoter activity. The correlation of plasma vitronectin levels with the variant was assessed in normal controls. The association of the variant with vascular disease was determined in 3 case-control studies.

RESULTS

A strong linkage disequilibrium was found between rs2227721 and rs2227720 in VTN promoter in Chinese (r(2)=1.0). Both variants resulted in a decreased transcription activity, and rs2227721 decreased the binding efficiency of transcription factor YY1 to the region. The rs2227721 was correlated with plasma vitronectin levels in normal controls (r=-0.207, P=0.028). The rs2227721 was associated with susceptibility of vascular disease; the odds ratios among subjects carrying rs2227721-T allele were 1.298 (95% Confidence Interval-CI, 1.033-1.631) for non-MI CAD (P<0.05), 1.346 (95% CI, 1.068-1.695) for chronic MI (P<0.05), 1.486 (95% CI, 1.145-1.928) for acute MI (P<0.001), and 1.619 (95% CI, 1.108-2.366) for deep venous thrombosis (P<0.05).

CONCLUSION

VTN promoter haplotype would be a novel genetic marker for vascular disease.

Beyond fibrinolysis: the role of plasminogen activator inhibitor-1 and vitronectin in vascular wound healing

Plasminogen activator inhibitor-1 (PAI-1), as the name implies, is the primary in vivo inhibitor of both tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). PAI-1 also binds to other nonproteinase ligands, including the matrix protein vitronectin, glycosaminoglycans such as heparin, and the endocytic clearance receptor, the low-density-lipoprotein-receptor-related protein (LRP). PAI-1 belongs to the superfamily of serine proteinase inhibitors (serpins), and, like other serpins, it acts as "suicide inhibitor" that reacts only once with a target proteinase. The suicide mechanism results in irreversible modification of the serpin and an extensive change in its conformation. In the case of PAI-1, this conformational change is important not only for inhibition of the proteinase, but it also causes changes in affinity for vitronectin and LRP. These changes have important consequences for cell migration.