Physiological role of alpha 2-plasmin inhibitor in rats

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.

Fibrin(ogen) as a Therapeutic Target: Opportunities and Challenges

Fibrinogen is one of the key molecular players in haemostasis. Thrombin-mediated release of fibrinopeptides from fibrinogen converts this soluble protein into a network of fibrin fibres that form a building block for blood clots. Thrombin-activated factor XIII further crosslinks the fibrin fibres and incorporates antifibrinolytic proteins into the network, thus stabilising the clot. The conversion of fibrinogen to fibrin also exposes binding sites for fibrinolytic proteins to limit clot formation and avoid unwanted extension of the fibrin fibres. Altered clot structure and/or incorporation of antifibrinolytic proteins into fibrin networks disturbs the delicate equilibrium between clot formation and lysis, resulting in either unstable clots (predisposing to bleeding events) or persistent clots that are resistant to lysis (increasing risk of thrombosis). In this review, we discuss the factors responsible for alterations in fibrin(ogen) that can modulate clot stability, in turn predisposing to abnormal haemostasis. We also explore the mechanistic pathways that may allow the use of fibrinogen as a potential therapeutic target to treat vascular thrombosis or bleeding disorders. Better understanding of fibrinogen function will help to devise future effective and safe therapies to modulate thrombosis and bleeding risk, while maintaining the fine balance between clot formation and lysis.

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.

Proteases and protease inhibitors in cerulein-induced acute pancreatitis in rats

BACKGROUND

Proteases and protease inhibitors are important in acute pancreatitis (AP), although little is known about the time course in cerulein-induced AP in the rat.

MATERIALS AND METHODS

AP was induced by supramaximal stimulation of cerulein, 10 microgram/kg/h, and during 72 h we measured lipase, amylase, albumin, prekallikrein, factor X, alpha(1)-protease inhibitor, alpha(1)-macroglobulin, alpha(2)-antiplasmin, antithrombin III (all in plasma) and macroscopic and histologic variables.

RESULTS

Within 12 h an edematous pancreatitis was evident with peak values of peritoneal exudate, pancreatic wet weight ratio, and plasma amylase and lipase activities. Histologically, edema and vacuolization were prominent already after 3 and 6 h, respectively, while inflammation, necrosis, and total histological score gradually increase to reach peak levels at 48 h. Proenzymes and most plasma protease inhibitors decreased to low levels after 6-12 h followed by a gradual increase. The sequential changes over time indicate that kallikrein - kinin activation, and plasminogen activation are probably early events in cerulein-induced AP in rats. alpha(1)-Macroglobulin and alpha(1)-protease inhibitor gradually decreased during the whole study period, probably being "second line" defense inhibitors. Levels above normal were seen for alpha(2)-antiplasmin and factor X at 48 h, normalizing at 72 h.

CONCLUSIONS

These results suggest that protease activation and protease inhibitor consumption occur in cerulein-induced AP in the rat.

Plasmin inhibitory effect of rat tissues: comparison with antiplasmin activity of plasma

Plasmin inhibition by rat tissue extracts was compared with rat plasma plasmin inhibition. Nine different tissues were extracted with an acid acetate-buffer (lung, heart, brain, adrenal, aorta, skeletal muscle, spleen and liver) or with KSCN-buffer (kidney), and antiplasmin activity was evaluated by a spectrophotometric method. All tissue extracts inhibited plasmin dose-dependently; the amount of plasmin inhibited by a fixed amount of extract depended upon the tissue. In rat plasma, antiplasmin activity was acid-labile, but acid treatment did not affect the plasmin inhibitory effect of tissues extracts.

An ibuprofen-antagonized plasmin inhibitor released by human endothelial cells

Serum-free culture medium harvested from endothelial cell monolayer cultures derived from human scars and dermis was examined for inhibition of fibrinolysis using a fibrin plate assay. Human cultured fibroblasts and smooth muscle cells did not produce any detectable inhibitory activity. The inhibitor is spontaneously released from the cultured endothelial cells over time. In the fibrin plate assay of plasmin-induced fibrinolysis, one nonsteroidal antiinflammatory (NSAI) drug, ibuprofen, was demonstrated to antagonize the inhibition of fibrinolysis. The antagonistic activity of ibuprofen appears unrelated to its NSAI drug activity because other NSAI drugs such as indomethacin and tolmetin have minimal antagonistic activity. Heating the cultured endothelial cells to 42 degrees C stimulates greater release of the inhibitor in a shorter period of time. This plasmin inhibitor, which is produced by endothelial cells, may contribute to postburn vascular occlusion, leading to secondary progressive necrosis in burn-traumatized patients.

Studies on biosynthesis of alpha 2-antiplasmin in rat liver cells

Biosynthesis, intracellular processing and secretion of alpha 2-antiplasmin (AP) were studied in primary cultures of rat hepatocytes by the pulse-chase technique. The experiments demonstrated that AP is present intracellularly as a 73 kD form and is secreted as a 79 kD form with affinity for human plasminogen kringles. Addition of human plasmin to the culture medium resulted in the formation of a 141 kD protein, probably a plasmin-AP complex, concomitantly with the disappearance of the 79 kD protein. The presence of tunicamycin, a blocker of N-linked glycosylation in the endoplasmic reticulum, during culture resulted in expression of a 63 kD form in medium and cell lysates. This latter form was unchanged after reduction and displayed affinity for human plasminogen kringles. The 79 kD form in the culture medium was resistant to treatment with endo-beta-N-acetylglucosaminidase-H while the intracellular 73 kD form was degraded to 63 kD, indicating glycosylation in the Golgi complex. It is concluded that rat AP is synthesized in a 63 kD form which is glycosylated in the endoplasmic reticulum to a 73 kD form; and that this in turn is trimmed and glycosylated further in the Golgi complex to the 79 kD form, which is secreted. The glycosylation is apparently not necessary for the secretion or for the affinity for human plasminogen kringles.

Effects of kringles derived from human plasminogen on fibrinolysis in vitro

Plasminogen kringle 1+2+3 (K1-3) containing lysine-binding sites inhibited the reaction of plasmin with alpha 2-plasmin inhibitor (alpha 2PI), in a rate assay using a synthetic chromogenic substrate, S-2251. However, K1-3 did not inhibit the reaction to any degree between alpha 2PI and mini-plasmin which lacked the kringle 1 to 4 portion of plasmin. These results suggest that K1-3 blocked the binding of alpha 2PI to the lysine-binding site of plasmin. In the urokinase (UK)-induced fibrinolysis, K1-3 shortened the human plasma clot lysis time at low concentration (0.5-6 microM), and prolonged the lysis time at a high concentration (20 microM). Similar results were obtained in the lysis time of a fibrin clot consisting of plasminogen, fibrinogen and alpha 2PI isolated from human plasma. The kringle 4 (K4) of human plasminogen did not accelerate human plasma clot lysis at any concentration (1.2-24.1 microM). Furthermore, in the tissue plasminogen activator (TPA)-induced fibrinolysis, K1-3 also shortened both the lysis time of human plasma clot and fibrin clot as observed in UK-induced fibrinolysis, but K4 did not. The above findings indicate that the reaction of alpha 2PI with the lysine-binding site of plasmin is involved in the inhibition of plasmin activity by alpha 2PI, and in the presence of an inhibitor of this reaction, the balance of coagulofibrinolytic activity in plasma will be shifted towards the fibrinolytic side.

Ibuprofen as an antagonist of inhibitors of fibrinolysis in wound fluid

Fibrin plate assay revealed that rat serum and wound fluid harvested from seven day subcutaneously implanted wound chambers prevented fibrinolysis. Samples of wound fluid from one to four hour burns displayed greater inhibiting activity than unburned or 24 hour old burns. Ibuprofen, a non-steroid anti-inflammatory drug, reversed the blocking of fibrinolysis in wound fluid, but it had no action on rat serum. The activity of ibuprofen appears unrelated to the synthesis of prostanoids. Fractionation of wound fluid and serum by column chromatography showed differences in elutions of inhibitors of fibrinolysis. Serum fractions having molecular weights greater than 60,000 prevented fibrinolysis and they were unaffected by the addition of ibuprofen. Fractionations of wound fluid produced a number of inhibitors, some of which had molecular weights of approximately 40,000. This inhibitor(s) was not detected in serum and was reversed by adding ibuprofen. Wound fluid has a fibrinolytic inhibitor which differs from that in the circulatory system, and which may be critical to the vascular changes of burn trauma.

Release of alpha 2-plasmin inhibitor from plasma fibrin clots by activated coagulation factor XIII. Its effect on fibrinolysis

When blood coagulation takes place in the presence of calcium ions, alpha 2-plasmin inhibitor (alpha 2PI) is cross-linked to fibrin by activated coagulation Factor XIII (XIIIa) and thereby contributes to the resistance of fibrin to fibrinolysis. It was previously shown that the cross-linking reaction is a reversible one, since the alpha 2PI-fibrinogen cross-linked complex could be dissociated. In the present study we have shown that the alpha 2PI-fibrin cross-linking reaction is also a reversible reaction and alpha 2PI which had been cross-linked to fibrin can be released from fibrin by disrupting the equilibrium, resulting in a decrease of its resistance to fibrinolysis. When the fibrin clot formed from normal plasma in the presence of calcium ions was suspended in alpha 2PI-deficient plasma of buffered saline, alpha 2PI was gradually released from fibrin on incubation. When alpha 2PI was present in the suspending milieu, the release was decreased inversely to the concentrations of alpha 2PI in the suspending milieu. The release was accelerated by supplementing XIIIa or the presence of a high concentration of the NH2-terminal 12-residue peptide of alpha 2PI (N-peptide) which is cross-linked to fibrin in exchange for the release of alpha 2PI. When the release of alpha 2PI from fibrin was accelerated by XIIIa or N-peptide, the fibrin became less resistant to the fibrinolytic process, resulting in an acceleration of fibrinolysis which was proportional to the degree of the release of alpha 2PI. These results suggest the possibility that alpha 2PI could be released from fibrin in vivo by disrupting the equilibrium of the alpha 2PI-fibrin cross-linking reaction, and that the release would result in accelerated thrombolysis.

Antithrombotic and thrombolytic effects of galactan polysulfate (DH 6322) in rats

Anticoagulant, fibrinolytic and thrombolytic effects of galactan polysulfate (DH 6322), a new semi-synthetic polysaccharide sulfate of bacterial origin, were studied in the rat in vitro, ex vivo and in vivo. Addition of DH 6322 to citrated plasma gave a 50% reduction in euglobulin lysis time at around 50 micrograms/ml and a 2-fold prolongation in plasma recalcification time at 40 micrograms/ml. Intravenous injection of DH 6322 caused significant acceleration of euglobulin clot lysis and urokinase-induced plasma clot lysis at doses as low as 2 mg/kg, and also significant prolongation of plasma recalcification time. Oral application of the compound gave similar effects only at an extremely high dose of 3 g/kg. When injected intravenously before induction of pulmonary thrombosis by lactic acid infusion, DH 6322 was found to be effective in preventing thrombus formation. In progressive venous thrombosis induced by insertion of a steel coil into the inferior vena cava, DH 6322 treatment also caused a significant reduction of thrombus size together with increase in the serum levels of fibrin degradation products. The findings indicate that the antithrombotic effects of DH 6322 result from its enhancement of the fibrinolytic potential as well as its anticoagulative action.

Fibrinolytic action of a new semi-synthetic polysaccharide sulfate, galactan polysulfate (DH 6322), in the rat

Galactan polysulfate, DH 6322, when added to rat plasma, shortened euglobulin clot lysis time through both increase in the recovery of plasminogen activator activity in euglobulin precipitates and direct acceleration of fibrinolysis. DH 6322 treatment also resulted in accelerating plasma clot lysis induced by urokinase. The effect of DH 6322 was further studied with a purified system composed of bovine fibrinogen and plasminogen. In this system, DH 6322 was found to be a potent stimulator of both fibrinolysis and fibrinogenolysis induced by urokinase. The activation process of plasminogen was not affected by DH 6322 even in the presence of fibrin or fibrinogen. The accelerating effect of DH 6322 on fibrinolysis in the purified system was scarcely affected by addition of plasma antifibrinolytic factors. DH 6322 also stimulated fibrin clot lysis induced by a low concentration of purified plasmin in place of plasminogen and urokinase, and its accelerating effect was not observed in the presence of tranexamic acid. Formation of fibrin clot susceptible to generated plasmin was observed at relatively high concentrations of DH 6322 above 80 micrograms/ml. Additional experiments indicated that DH 6322 increased the amount of plasminogen bound to fibrin. These results suggested that DH 6322 stimulated fibrinolysis through increasing the binding of plasminogen or plasmin to fibrin.

Purification and some properties of rat alpha 2-plasmin inhibitor

alpha 2-Plasmin inhibitor was purified from rat plasma by the procedures including sequential lysine-Sepharose chromatography, ammonium sulfate precipitation and plasminogen-Sepharose, DEAE-Sephadex, concanavalin A-Sepharose, and Sephadex G-200 chromatographies. The final preparation was homogeneous as judged by sodium dodecyl sulfate polyacrylamide gel- and immuno-electrophoresis. It was a single polypeptide chain with a molecular weight of about 7.1 X 10(4) daltons and was found to possess fast-acting antiplasmin activity. There existed other molecular forms of alpha 2-plasmin inhibitor-related antigen with molecular weight of 6.3 X 10(4) and 6.1 X 10(4) daltons. The latter was found to be inactive in binding to plasminogen and in inhibiting plasmin activity. Normal plasma level of alpha 2-plasmin inhibitor in rats was immunologically determined to be 7.9 +/- 0.8 mg/dl. Indirect evidence was obtained that rat alpha 2-plasmin inhibitor was incorporated into cross-linked fibrin during blood clotting.