Regulation of Coagulation by the Fibrinolytic System: Expecting the Unexpected
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Bleeding disorders Regulation of coagulation by the fibrinolytic system: expecting the unexpected K.A. Hajjar ABSTRACT Department of Cell and Recent investigations into fibrinolysis and coagulation have yielded exciting and unexpected findings. Developmental Biology, Of note, deficiencies of the major components of the classical fibrinolytic system, namely plasminogen, Department of Pediatrics tissue plasminogen activator, and urokinase, are now recognized to be rare causes of macrovascular Department of Medicine, thrombosis. At the same time, both plasminogen activator excess and fibrinolytic inhibitor deficiencies Weill Cornell Medical College, are associated with clinically significant bleeding, while elevated inhibitor levels, particularly thrombin New York, New York, USA activatable fibrinolysis inhibitor, appear to confer clinically significant risk for thrombosis. Receptor- mediated cell surface fibrinolysis adds a new dimension to the regulation of fibrin balance. Exaggerated expression of the annexin A2 complex, for example, is associated with hemorrhage in acute promyelo- Hematology Education: cytic leukemia, whereas autoantibodies directed against A2 correlate with thrombotic disease in the education program for the patients with antiphospholipid syndrome. The contributions of the urokinase receptor and a diverse annual congress of the European array of plasminogen receptors to fibrin homeostasis remain to be defined. Future studies are likely to Hematology Association reveal novel mechanisms that co-regulate coagulation and fibrinolysis. 2012;6:49-56 Introduction involving compartmentalization, functional redundancies, and multiple points of cross Fibrinolysis is a finely tuned process in regulation with the coagulation system. which cross-linked fibrin is proteolyzed, giv - ing rise to a defined series of degradation products 1-3 (Figure 1). In the major pathway, Classical fibrinolysis either of two serine proteases, tissue plas - minogen activation (tPA) or urokinase (uPA), Plasminogen deficiency cleaves a single peptide bond within the inac - In both humans and rodents, plasminogen- tive zymogen plasminogen to yield active deficiency is more likely to be associated with plasmin. Serine protease inhibitors, primarily ligneous mucositis, especially conjunctivitis, 12-14 a2antiplasmin ( a2AP), serve to dampen plas - rather than thrombosis. (Table 1) Ligneous min activity, while plasminogen activator mucositis in humans, which usually presents inhibitor-1 (PAI-1) blocks plasmin formation in infancy (mean age 9.5 months), is charac - by inhibiting tPA- or uPA-mediated plasmino - terized by the formation of fibrin-containing gen activation. Other key regulators include pseudomembranes usually of the conjunctiva, fibrin, the most potent known cofactor for but occasionally involving other mucous tPA-dependent plasminogen activation, and membranes, such as the gingiva, upper and thrombin activatable fibrinolysis inhibitor lower respiratory tract, middle ear, gastroin - (TAFI), a plasma carboxypeptidase. TAFI testinal tract, female genital tract, and kid - cleaves C-terminal lysine residues on plas - neys. Fibrin deposition in this disorder can be min-modified fibrin, eliminating binding sites prevented with the use of anticoagulants, and, for plasminogen and tPA, and impeding fur - thus, the defect in ligneous conjunctivitis ther plasmin generation. A number of cell sur - appears to represent unopposed procoagulant face receptors, including the urokinase recep - activity within the microvasculature, possibly tor (uPAR), the annexin A2/p11 complex, and in association with a localized vascular leak. a variety of plasminogen-binding proteins, Thus, while plasmin may be uniquely impor - may contribute to fibrinolytic balance by tant for clearing fibrin associated with mucos - stimulating the catalytic efficiency of plasmin al microvessels, other proteases, such as generation on the surface of blood vessels. membrane type 1 matrix metalloproteinase, Understanding the precise contributions of kallikrein, factor XIa, and factor XIIa may the fibrinolytic system to hemostatic balance replace its function, at least partially, within has proven challenging. For example, while larger vessels. 15-18 fibrinolytic deficiency rarely seems to cause Mice deficient in plasminogen have surpris - clinical thrombosis, 4-9 loss of fibrinolytic ingly normal embryonic development and fer - inhibitors is a major cause of clinical bleed - tility, but display significant runting, and ing. 10,11 This apparent paradox invites intra- and extravascular fibrin accumulation reassessment of the intricacies of fibrinolysis, in multiple tissues. 19,20 Depending upon back - and suggests a far more complex system ground strain, they also may exhibit ligneous Hematology Education: the education programme for the annual congress of the European Hematology Association | 2012; 6(1) | 49 | 17 th Congress of the European Hematology Association conjunctivitis. 12 Plg –/– mice clear injury-induced thrombi ficially induced pulmonary thrombi. 29 Doubly deficient much less readily than wild-type mice, 21 a defect that can (tPA –/– ; uPA –/– ) mice, on the other hand, exhibit extensive be reversed by bolus intravenous injection of exogenous spontaneous fibrin accumulation in multiple organs, rec - plasminogen. 22 tal prolapse, runting, and cachexia, all reminiscent of the Plg –/– phenotype. Thus, while tPA and uPA are not essen - Plasminogen activator deficiency tial for normal embryologic development in the mouse, Clinically significant gene mutations in tPA and uPA they do seem to play overlapping roles in fibrinolytic sur - have not been well-documented in humans, and deficien - veillance and in the lysis of spontaneous and induced cies in these factors are not associated with thrombosis. 23 thrombi. Although the endothelium is the principal source of tPA in blood, tPA expression appears to be highly restricted to Plasminogen activator excess smaller vessels in specific anatomic locations. In the Occasionally, excessive uPA activity leads to hyperfib - baboon, for example, neither tPA antigen nor tPA mRNA rinolytic hemorrhage. In metastatic prostate cancer, for was detected in endothelial cells of femoral artery or vein, example, hyperfibrinolysis with hemorrhagic sequelae is carotid artery, or aorta, but both were clearly present in thought to reflect overproduction of urokinase. 30 precapillary arterioles, postcapillary venules, and the vasa Similarly, in Factor V Quebec, a rare, moderately severe, vasora. 24 In the mouse lung, similarly, pulmonary blood autosomal dominant bleeding disorder, 31-33 platelets harbor vessels were uniformly negative for tPA antigen while increased quantities of uPA due to a duplication of the bronchial blood vessels displayed strong signals, 25 espe - uPA gene. Excessive activation of plasmin leads to degra - cially at branch points. 26 dation of factor V, fibrinogen, and other procoagulant fac - In humans, deficiency of functional tPA has been tors stored within platelet granules. The resultant bleeding reported in association with livedoid vasculopathy of the usually occurs at 12-24 hours after surgery, dental extrac - lower extremities and ulceration-induced atrophic scars tion, or trauma. (atrophie blanche). Livedoid vasculopathy, an occlusive Exaggerated release of tPA from the endothelium, its microvascular disorder, can be due to hyperproduction of site of synthesis, may also initiate hyperfibrinolytic plasminogen activator inhibitor-1 (PAI-1) as a result of bleeding under extreme circumstances. In patients under - the 4G/4G promoter polymorphism, or due to a marked going cardiopulmonary bypass, blood contact with non- defect in post-venous occlusion release of tPA. 27,28 To date, endothelial cell surfaces may lead to generation of throm - there are no reported hemostatic disorders related to uPA bin, and subsequent endothelial cell release of tPA, lead - deficiency in humans. ing to excessive activation of plasmin. 34 Hyperfibrinolysis Mice deficient in uPA or tPA exhibit little spontaneous is also a contributor to the coagulopathy of heat stroke 35,36 thrombosis. uPA –/– mice exhibit occasional fibrin deposi - and to trauma-induced coagulopathy, which arises most tion, and tPA-deficient mice display impaired lysis of arti - likely from the combined effects of tPA release from Figure 1. Schematic model for classical and cell surface fibrinolysis. In classical, fibrin-based fibrinolysis, fibrin forms in response to vascular injury, and tPA and plasminogen (Plg) bind to lysine (K) residues on plasmin-modified fibrin. Fibrin increases the catalytic efficiency of tPA-dependent plasminogen activation by approximately 500-fold. uPA-mediated acti - vation of Plg is fibrin-independent. Assembly of tPA and Plg on fibrin is reduced in the presence of TAFIa, which eliminates their binding sites by cleaving off C-terminal K residues. The action of tPA and uPA on Plg is further dampened by the action of PAI-1, the principal plasminogen activator inhibitor. Once formed, plasmin can reduce fibrin to its fibrin degra - dation products (FDPs). The action of plasmin is inhibited by a2-antiplasmin ( a2AP). Cell surface fibrinolysis is mediated by cell surface receptors that bind plasminogen and/or its activators. The annexin (A2-p11) 2 heterotetrameric complex binds both tPA and Plg, thereby increasing the efficiency of plasmin generation, and promoting fibrin clearance. A series of plasminogen receptors (PlgR), as well as urokinase (uPA)