The fibrinolytic system of the vessel wall and its role in the control of thrombosis

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.

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.

Therapeutic strategies for thrombosis: new targets and approaches

Antiplatelet agents and anticoagulants are a mainstay for the prevention and treatment of thrombosis. However, despite advances in antithrombotic therapy, a fundamental challenge is the side effect of bleeding. Improved understanding of the mechanisms of haemostasis and thrombosis has revealed new targets for attenuating thrombosis with the potential for less bleeding, including glycoprotein VI on platelets and factor XIa of the coagulation system. The efficacy and safety of new agents are currently being evaluated in phase III trials. This Review provides an overview of haemostasis and thrombosis, details the current landscape of antithrombotic agents, addresses challenges with preventing thromboembolic events in patients at high risk and describes the emerging therapeutic strategies that may break the inexorable link between antithrombotic therapy and bleeding risk.

The effects of residual platelets in plasma on plasminogen activator inhibitor-1 and plasminogen activator inhibitor-1-related assays

Due to controversial evidence in the literature pertaining to the activity of plasminogen activator inhibitor-1 in platelets, we examined the effects of residual platelets present in plasma (a potential pre-analytical variable) on various plasminogen activator inhibitor-1 and plasminogen activator inhibitor-1-related assays. Blood samples were collected from 151 individuals and centrifuged at 352 and 1500 g to obtain plasma with varying numbers of platelet. In a follow-up study, blood samples were collected from an additional 23 individuals, from whom platelet-poor (2000 g), platelet-containing (352 g) and platelet-rich plasma (200 g) were prepared and analysed as fresh-frozen and after five defrost-refreeze cycles (to determine the contribution of in vitro platelet degradation). Plasminogen activator inhibitor-1 activity, plasminogen activator inhibitor-1 antigen, tissue plasminogen activator/plasminogen activator inhibitor-1 complex, plasma clot lysis time, β-thromboglobulin and plasma platelet count were analysed. Platelet α-granule release (plasma β-thromboglobulin) showed a significant association with plasminogen activator inhibitor-1 antigen levels but weak associations with plasminogen activator inhibitor-1 activity and a functional marker of fibrinolysis, clot lysis time. Upon dividing the study population into quartiles based on β-thromboglobulin levels, plasminogen activator inhibitor-1 antigen increased significantly across the quartiles while plasminogen activator inhibitor-1 activity and clot lysis time tended to increase in the 4^th quartile only. In the follow-up study, plasma plasminogen activator inhibitor-1 antigen was also significantly influenced by platelet count in a concentration-dependent manner. Plasma plasminogen activator inhibitor-1 antigen levels increased further after complete platelet degradation. Residual platelets in plasma significantly influence plasma plasminogen activator inhibitor-1 antigen levels mainly through release of latent plasminogen activator inhibitor-1 with limited effects on plasminogen activator inhibitor-1 activity, tissue plasminogen activator/plasminogen activator inhibitor-1 complex or plasma clot lysis time. Platelets may however also have functional effects on plasma fibrinolytic potential in the presence of high platelet counts, such as in platelet-rich plasma.

Association between the SERPINE1 (PAI-1) 4G/5G insertion/deletion promoter polymorphism (rs1799889) and pre-eclampsia: a systematic review and meta-analysis

The SERPINE1 -675 4G/5G promoter region insertion/deletion polymorphism (rs1799889) has been implicated in the pathogenesis of pre-eclampsia (PE), but the genetic association has been inconsistently replicated. To derive a more precise estimate of the association, a systematic review and meta-analysis was conducted. This study conformed to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. PubMed (MEDLINE), Scopus and HuGE Literature Finder literature databases were systematically searched for relevant studies. Summary odds ratios (ORs) and 95% confidence intervals (CIs) were calculated for the allelic comparison (4G versus 5G) and genotypic comparisons following the co-dominant (4G/4G versus 5G/5G and 4G/5G versus 5G/5G), dominant (4G/4G+4G/5G versus 5G/5G) and recessive (4G/4G versus 4G/5G+5G/5G) genetic models. Between-study heterogeneity was quantified by I(2) statistics and publication bias was appraised with funnel plots. Sensitivity analysis was conducted to evaluate the robustness of meta-analysis findings. Meta-analysis of 11 studies involving 1297 PE cases and 1791 controls found a significant association between the SERPINE1 -675 4G/5G polymorphism and PE for the recessive genetic model (OR = 1.36, 95% CI: 1.13-1.64, P = 0.001), a robust finding according to sensitivity analysis. A low level of between-study heterogeneity was detected (I(2) = 20%) in this comparison, which may be explained by ethnic differences. Funnel plot inspection did not reveal evidence of publication bias. In conclusion, this study provides a comprehensive examination of the available literature on the association between SERPINE1 -675 4G/5G and PE. Meta-analysis results support this polymorphism as a likely susceptibility variant for PE.

Releasing or expression modulating mediator involved in hemostasis by Berythractivase and Jararhagin (SVMPs)

PIII snake venom metalloproteases (SVMPs) are structurally related to ADAMs (a disintegrin and metalloprotease human family of proteins). Berythractivase and Jararhagin are PIII SVMPs with 69% homology with different hemostatic properties. In order to clarify these differences and further characterize the biological effects of these proteins, we compared the effect of both proteases on human umbilical vein endothelial cell (HUVEC) for evaluating the release and modulation of coagulation and fibrinolysis mechanisms as well as the expression of their correlated genes. We found that both proteins increase the von Willebrand factor liberation, but did not modulate gene expression. Berythractivase, differently from Jararhagin increased the expression of tissue factor. Our results showed that both SVMPs (Berythractivase and Jararhagin) activate HUVEC releasing or modulating mediators involved in hemostasis. Meanwhile, we can suggest through the up-regulation of TF gene that the studied SVMP acts in a specific manner, suggesting that Jararhagin has preferentially a local action, while Berythractivase can be assumed as a systemic pro-coagulant protein with activity on the surface of HUVECs.

Massive Transfusion Coagulopathy

Coagulopathy following massive transfusion is a consequence of post-traumatic and surgical hemorrhage. Bleeding following massive transfusion can occur due to hypothermia, dilutional coagulopathy, platelet dysfunction, fibrinolysis, or hypofibrinogenemia. Transfusion of 15 to 20 units of blood products causes dilutional thrombocytopenia, and both antiplatelet agents (eg, clopidogrel [Plavix, Sanofi, Bridgewater, NJ]) and hemostatic inhibitors (eg, low-molecular-weight heparins, pentasaccharides, and direct thrombin inhibitors) are contributing factors to bleeding. Tests for platelet dysfunction are not readily available. Excessive fibrinolysis and low fibrinogen are also causes of bleeding in these patients. Currently, however, there are several agents that have been reported to be effective for the prophylaxis of hemorrhage in surgical patients, including aprotinin for cardiac surgery, orthopedic surgery, and hepatic transplantation, and the off-label use of recombinant activated factor VII (NovoSeven, Novo Nordisk, Bagsvaerd, Denmark) as rescue therapy for life-threatening hemorrhage.

A truncated plasminogen activator inhibitor-1 protein blocks the availability of heparin-binding vascular endothelial growth factor A isoforms

We have made deletions of the porcine plasminogen activator inhibitor-1 (PAI-1) gene to obtain recombinant truncated PAI-1 proteins to examine functions of the PAI-1 isoforms. We previously reported that one recombinant truncated protein, rPAI-1(23), induces the formation of angiostatin by cleaving plasmin. The rPAI-1(23) protein is also able to bind urokinase plasminogen activator and plasminogen and then reduce the amount of plasmin that is formed. We have now prepared three different truncated rPAI-1 proteins and demonstrate that PAI-1 conformations control the release of heparin-binding vascular endothelial growth factor (VEGF) isoforms. The rPAI-1(23) isoform can regulate the functional activity of heparan sulfate-binding VEGF-A isoforms by blocking the activation of VEGF from heparan sulfate. The rPAI-1(23) conformation induced extensive apoptosis in cultured endothelial cells and thus reduced the number of proliferating cells. The rPAI-1(23) isoform inhibited migration of VEGF-stimulated sprouting from chick aortic rings by 65%, thus displaying a role in anti-angiogenic mechanisms. This insight into anti-angiogenic functions related to PAI-1 conformational changes could provide potential intervention points in angiogenesis associated with atherosclerotic plaques and cancer.

High accumulation of plasminogen and tissue plasminogen activator at the flow surface of mural fibrin in the human arterial system

PURPOSE

We assessed the fibrinolytic activity of the organized mural thrombus lining of aneurysms and prosthetic grafts.

METHODS

Between May 1995 and April 1998, the full-thickness mural thrombi of aneurysms and the pseudointima lining of vascular grafts were obtained from 12 patients, ranging from 55 to 78 years in age, who underwent elective surgery. These included five aortic arch aneurysms, four abdominal aortic aneurysms, and three patent synthetic vascular grafts. The specimens were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)/immunoblot and immunohistochemistry for human plasmin/plasminogen, tissue plasminogen activator (tPA), and fibrin degradation product (D-dimer).

RESULTS

In the SDS-PAGE/immunoblot, 25- and 27-kd bands appeared specifically in experimental fibrin plates after limited digestion by plasmin and were also recognized in the mural thrombi. The presence of bands at 25 and 27 kd, which were most prominent in sections near the flow surface layer, was consistent with the hypothesis that the mural fibrin was digested by the endogenous plasmin. Apparent immunoreactivity was found at the flow surface of the masses at a thickness of 10 to 400 micrometer suggesting the presence of a plasminogen and tPA-rich layer, with D-dimer as a consequential product of fibrinolysis.

CONCLUSION

The hypothesis that fibrin surfaces in the arterial system acquire fibrinolytic activity because of digestion by circulating endogenous plasmin was confirmed; this may contribute to the antithrombogenicity of these flow surfaces.

Fibrinolytic proteins in apoptotic human umbilical vein endothelial cells

Endothelial cells express fibrinolytic proteins including: urokinase (u-PA) and tissue type (t-PA) plasminogen activators, type-1 (PAI-1) and 2 (PAI-2) plasminogen activator inhibitors, and u-PA receptor (u-PAR). Apoptotic endothelial cells detach, potentially forming both local and circulating microthrombi in vivo. In this article, apoptotic human umbilical vein endothelium was obtained by serum starvation and compared with nonapoptotic cells rescued from death with fresh medium containing serum. Antigen levels for t-PA, PAI-1, PAI-2, and u-PAR were reduced greatly in apoptosis (p< 0.05). In contrast, u-PA levels were similar in apoptotic as compared with rescued cells (p<0.05). Radioactive amino acids were used to determine absolute levels of protein synthesis and degradation in these cells. Reduced antigen levels likely were due to proteolysis as there was 98% total protein degradation and very little protein synthesis in apoptotic endothelial cells. Also, u-PA levels in apoptotic endothelial cells were not affected by the protein synthesis inhibitor cycloheximide. Endothelial cells in inflammatory sites are exposed to cytokines, which increase both apoptosis and u-PA levels. Data from this article support the idea that maintained u-PA levels in apoptotic endothelium may protect from micro-thrombosis in inflammatory sites as well as in the circulation.

The pathogenesis of atherosclerosis: a perspective for the 1990s

Atherosclerosis, the principal cause of heart attack, stroke and gangrene of the extremities, is responsible for 50% of all mortality in the USA, Europe and Japan. The lesions result from an excessive, inflammatory-fibroproliferative response to various forms of insult to the endothelium and smooth muscle of the artery wall. A large number of growth factors, cytokines and vasoregulatory molecules participate in this process. Our ability to control the expression of genes encoding these molecules and to target specific cell types provides opportunities to develop new diagnostic and therapeutic agents to induce the regression of the lesions and, possibly, to prevent their formation.

Gamma-interferon counteracts interleukin-1 alpha stimulated expression of urokinase-type plasminogen activator in human endothelial cells in vitro

The effect of gamma-interferon (gamma-IFN) on the interleukin-1 alpha (IL-1 alpha) induced stimulation of urokinase-type plasminogen activator (u-PA) expression in human foreskin microvascular endothelial cells (HFMEC) and in human umbilical vein endothelial cells (HUVEC) was investigated. When gamma-IFN and IL-1 alpha were added to the cells simultaneously, gamma-IFN inhibited the IL-1 alpha induced increase in u-PA antigen production in both HFMEC and HUVEC in a dose dependent fashion, with a maximum inhibitory effect achieved between 2.0 and 20.0 U/ml of gamma-IFN. Pretreatment of HFMEC with gamma-IFN for 1 hour before addition of IL-1 alpha resulted in a significant reduction in u-PA synthesis. However, when HFMEC were pretreated for 8 hours with gamma-IFN before the addition of IL-1 alpha the reduction in u-PA production was even more significant. When gamma-IFN was added to HFMEC 1 hour after IL-1 alpha, a significant inhibition in u-PA synthesis was seen. In contrast only a slight inhibition in IL-1 alpha induced u-PA production was seen when gamma-IFN was added to the cells 8 hours after IL-1 alpha. gamma-IFN also inhibited significantly the IL-1 alpha induced increase in u-PA specific mRNA in HUVEC and HFMEC.