Hybrid peptide containing RGDF (Arg-Gly-Asp-Phe) coupled with the carboxy terminal part of alpha 2-antiplasmin capable of inhibiting platelet aggregation and promoting fibrinolysis

Plasmin Inhibitor in Health and Diabetes: Role of the Protein as a Therapeutic Target

The vascular obstructive thrombus is composed of a mesh of fibrin fibers with blood cells trapped in these networks. Enhanced fibrin clot formation and/or suppression of fibrinolysis are associated with an increased risk of vascular occlusive events. Inhibitors of coagulation factors and activators of plasminogen have been clinically used to limit fibrin network formation and enhance lysis. While these agents are effective at reducing vascular occlusion, they carry a significant risk of bleeding complications. Fibrin clot lysis, essential for normal hemostasis, is controlled by several factors including the incorporation of antifibrinolytic proteins into the clot. Plasmin inhibitor (PI), a key antifibrinolytic protein, is cross-linked into fibrin networks with higher concentrations of PI documented in fibrin clots and plasma from high vascular risk individuals. This review is focused on exploring PI as a target for the prevention and treatment of vascular occlusive disease. We first discuss the relationship between the PI structure and antifibrinolytic activity, followed by describing the function of the protein in normal physiology and its role in pathological vascular thrombosis. Subsequently, we describe in detail the potential use of PI as a therapeutic target, including the array of methods employed for the modulation of protein activity. Effective and safe inhibition of PI may prove to be an alternative and specific way to reduce vascular thrombotic events while keeping bleeding risk to a minimum. Key Points Plasmin inhibitor (PI) is a key protein that inhibits fibrinolysis and stabilizes the fibrin network.This review is focused on discussing mechanistic pathways for PI action, role of the molecule in disease states, and potential use as a therapeutic target.

α2-Antiplasmin as a Potential Therapeutic Target for Systemic Sclerosis

Systemic sclerosis is a connective tissue disease of unknown origin that is characterized by immune system abnormalities, vascular damage, and extensive fibrosis of the skin and visceral organs. α2-antiplasmin is known to be the main plasmin inhibitor and has various functions such as cell differentiation and cytokine production, as well as the regulation of the maintenance of the immune system, endothelial homeostasis, and extracellular matrix metabolism. The expression of α2-antiplasmin is elevated in dermal fibroblasts from systemic sclerosis patients, and the blockade of α2-antiplasmin suppresses fibrosis progression and vascular dysfunction in systemic sclerosis model mice. α2-antiplasmin may have promise as a potential therapeutic target for systemic sclerosis. This review considers the role of α2-antiplasmin in the progression of systemic sclerosis.

Fibrinogen and Antifibrinolytic Proteins: Interactions and Future Therapeutics

Thrombus formation remains a major cause of morbidity and mortality worldwide. Current antiplatelet and anticoagulant therapies have been effective at reducing vascular events, but at the expense of increased bleeding risk. Targeting proteins that interact with fibrinogen and which are involved in hypofibrinolysis represents a more specific approach for the development of effective and safe therapeutic agents. The antifibrinolytic proteins alpha-2 antiplasmin (α2AP), thrombin activatable fibrinolysis inhibitor (TAFI), complement C3 and plasminogen activator inhibitor-2 (PAI-2), can be incorporated into the fibrin clot by FXIIIa and affect fibrinolysis by different mechanisms. Therefore, these antifibrinolytic proteins are attractive targets for the development of novel therapeutics, both for the modulation of thrombosis risk, but also for potentially improving clot instability in bleeding disorders. This review summarises the main properties of fibrinogen-bound antifibrinolytic proteins, their effect on clot lysis and association with thrombotic or bleeding conditions. The role of these proteins in therapeutic strategies targeting the fibrinolytic system for thrombotic diseases or bleeding disorders is also discussed.

Hypofibrinolysis in diabetes: a therapeutic target for the reduction of cardiovascular risk

An enhanced thrombotic environment and premature atherosclerosis are key factors for the increased cardiovascular risk in diabetes. The occlusive vascular thrombus, formed secondary to interactions between platelets and coagulation proteins, is composed of a skeleton of fibrin fibres with cellular elements embedded in this network. Diabetes is characterised by quantitative and qualitative changes in coagulation proteins, which collectively increase resistance to fibrinolysis, consequently augmenting thrombosis risk. Current long-term therapies to prevent arterial occlusion in diabetes are focussed on anti-platelet agents, a strategy that fails to address the contribution of coagulation proteins to the enhanced thrombotic milieu. Moreover, antiplatelet treatment is associated with bleeding complications, particularly with newer agents and more aggressive combination therapies, questioning the safety of this approach. Therefore, to safely control thrombosis risk in diabetes, an alternative approach is required with the fibrin network representing a credible therapeutic target. In the current review, we address diabetes-specific mechanistic pathways responsible for hypofibrinolysis including the role of clot structure, defects in the fibrinolytic system and increased incorporation of anti-fibrinolytic proteins into the clot. Future anti-thrombotic therapeutic options are discussed with special emphasis on the potential advantages of modulating incorporation of the anti-fibrinolytic proteins into fibrin networks. This latter approach carries theoretical advantages, including specificity for diabetes, ability to target a particular protein with a possible favourable risk of bleeding. The development of alternative treatment strategies to better control residual thrombosis risk in diabetes will help to reduce vascular events, which remain the main cause of mortality in this condition.

Natural heterogeneity of α2-antiplasmin: functional and clinical consequences

Human α2-antiplasmin (α2AP, also called α2-plasmin inhibitor) is the main physiological inhibitor of the fibrinolytic enzyme plasmin. α2AP inhibits plasmin on the fibrin clot or in the circulation by forming plasmin-antiplasmin complexes. Severely reduced α2AP levels in hereditary α2AP deficiency may lead to bleeding symptoms, whereas increased α2AP levels have been associated with increased thrombotic risk. α2AP is a very heterogeneous protein. In the circulation, α2AP undergoes both amino terminal (N-terminal) and carboxyl terminal (C-terminal) proteolytic modifications that significantly modify its activities. About 70% of α2AP is cleaved at the N terminus by antiplasmin-cleaving enzyme (or soluble fibroblast activation protein), resulting in a 12-amino-acid residue shorter form. The glutamine residue that serves as a substrate for activated factor XIII becomes more efficient after removal of the N terminus, leading to faster crosslinking of α2AP to fibrin and consequently prolonged clot lysis. In approximately 35% of circulating α2AP, the C terminus is absent. This C terminus contains the binding site for plasmin(ogen), the key component necessary for the rapid and efficient inhibitory mechanism of α2AP. Without its C terminus, α2AP can no longer bind to the lysine binding sites of plasmin(ogen) and is only a kinetically slow plasmin inhibitor. Thus, proteolytic modifications of the N and C termini of α2AP constitute major regulatory mechanisms for the inhibitory function of the protein and may therefore have clinical consequences. This review presents recent findings regarding the main aspects of the natural heterogeneity of α2AP with particular focus on the functional and possible clinical implications.

The C-terminus of ?2-antiplasmin interacts with endothelial cells

The serpin, alpha(2)-antiplasmin (alpha(2)AP), has an extended C-terminus relative to other inhibitors. This 51-residue region contains an RGD sequence; such sequences constitute a key recognition sequence for cell adhesion, mediated through integrins. In the present study, this sequence was expressed in Escherichia coli and its binding to endothelial cells and whether binding depends on the RGD sequence was investigated. Binding to the surface of human umbilical vein endothelial cells (HUVEC-C) was observed by flow cytometry and immunohistochemistry. Binding studies on immobilised cells showed specific and RGD-dependent binding of the peptides to HUVEC-C. The binding of the wild-type peptide to the HUVEC-C was significantly higher than that of a mutant peptide, in which RGD was replaced by SAA (P < 0.05, n = 4). Similarly, ethylenediaminetetraacetic acid decreased the binding of the wild-type peptide (P < 0.05, n = 4). The binding was competed out by full-length alpha(2)AP, fibronectin and anti-alpha(5)beta(1). This is the first evidence of binding of the C-terminus of alpha(2)AP to endothelial cells via its RGD sequence, with most but not all of the binding being integrin-mediated. We speculate that this interaction with alpha(2)AP may potentially play a role in the control of cellular fibrinolysis by regulating local plasmin activity on cell surfaces.

Effect of a synthetic carboxy-terminal peptide of alpha(2)-antiplasmin on urokinase-induced fibrinolysis

alpha(2)-Antiplasmin (alpha(2)AP) interferes with the binding of plasminogen to fibrin because lysine residues in its carboxy-terminal region compete with those in fibrin, presumably the same way that free lysine or epsilon-aminocaproic acid (EACA) inhibits plasminogen binding to fibrin. While this overall process causes an inhibition of fibrinolysis, the converse was observed with a 26-residue synthetic peptide (AP26) corresponding to the carboxy-terminal region of alpha(2)AP. The AP26 peptide, in fact, accelerated urokinase-induced lysis of (1) fully crosslinked fibrin with complete gamma-dimer and alpha-polymer formation; (2) partially crosslinked fibrin that had undergone only gamma-dimerization; and (3) noncrosslinked fibrin. The AP26 peptide also inhibited factor XIIIa-catalyzed crosslinking of fibrin alpha-chains, and this also accelerated lysis of fibrin. EACA had no effect. In the presence of noncrosslinked fibrin, AP26 promoted plasminogen activation by urokinase and fibrinolysis. EACA only slightly increased the rate of plasminogen activation, and as expected, it inhibited fibrinolysis. Since AP26 peptide enhanced the lysis of partially crosslinked and noncrosslinked fibrin, our results indicate that inhibition of factor XIIIa-catalyzed alpha-polymer formation by AP26, although associated with accelerated fibrinolysis, is not the primary mechanism. Instead, our data support the conclusion that AP26 enhances the conversion of plasminogen to plasmin approximately 5-fold, probably by inducing a conformational change in plasminogen structure just as occurs with low concentrations of lysine or EACA. At higher concentrations, however, AP26 apparently does not approach the avidity or affinity of lysine or EACA for the kringle structures of plasminogen or plasmin so that their binding to fibrin is blocked. Whether AP26 alone, or as part of another molecule, could have potential for enhancing thrombolysis will require further study.

Chimeric derivative of fibrolase, a fibrinolytic enzyme from southern copperhead venom, possesses inhibitory activity on platelet aggregation

Fibrolase, a metalloproteinase isolated from the venom of Agkistrodon contortrix contortrix (southern copperhead snake), is a direct acting fibrinolytic enzyme that has been used to digest occlusive blood clots in animal models. The snake venom enzyme directly degrades fibrin associated with platelet rich blood clots and does not rely on plasminogen activation. Rethrombosis is a serious complication that is experienced in a significant percentage of patients treated with thrombolytic agents to remove occlusive vascular thrombi. The involvement of platelets in the initiation of rethrombosis is well known. Arg-Gly-Asp-(RGD)-containing agents have been shown to inhibit rethrombosis following thrombus dissolution by plasminogen activators. In an effort to create a more effective fibrinolytic enzyme and to target the enzyme to platelet-rich thrombi, thereby decreasing the potential for rethrombosis, a chimeric derivative of fibrolase has been produced. This report describes the construction and biochemical characterization of the chimeric enzyme and an evaluation of its in vitro activities. The chimera was formed by covalently incorporating an RGD-like peptide into fibrolase. The site of peptide attachment was determined to be a single lysine residue remote from the enzymes active site. Covalent modification of fibrolase with the RGD-like peptide did not inhibit either fibrinolytic activity of the enzyme nor platelet aggregation inhibitory activity of the peptide. The chimera not only retained the same level of enzymatic activity as native fibrolase, but also acquired the ability to inhibit platelet aggregation by binding to the fibrinogen receptor (integrin alphaIIbbeta3) on platelets.

Effect of L-arginine on in vitro plasmin-generation and fibrinogenolysis

L-arginine ahs received much attention in numerous aspects of the regulation of vascular tone and haemostasis. L-arginine seems to be capable to bind to plasminogen, too. The aim of the present paper is to investigate the action of L-arginine on in vitro plasmin generation and fibrino(geno)lysis by chromogenic, kinetic plasmin generation assay and electrophoretic analysis. The acceleration of tPA-induced plasmin generation in the presence of low concentration of L-arginine, along with augmentation of in vitro fibrinogenolysis have been documented. L-arginine may have a role in the modification of fibrinogenolysis, and this role should be considered if arginine is used as an element of some novel antithrombotic agents.