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COAGULATION CASCADE | Fibrinogen and Fibrin
COAGULATION CASCADE / Fibrinogen and Fibrin 503
is a broad-spectrum serine protease inhibitor whose reactivity is enhanced by heparin. TFPI targets the fVIIa-TF activation of fX, but is also a potent fXa inhibitor in the absence of fVIIa-TF. APC inactivates fVa thus attenuating the procoagulant activity of fXa. Each of these inhibitors has demonstrated beneficial reductions in ALI in various animal models. Unfortunately, these results have not been reproduced in clinical studies. While TFPI was successful in phase II clinical trials, the results in phase III trials (OPTIMIST study) were disappointing and showed no beneficial effect above placebo. APC, however, has demonstrated a significant reduction in mortality in cases of severe sepsis (PROWESS study), which is the most common cause of ALI and ARDS. See also: Acute Respiratory Distress Syndrome. Anticoagulants. Chemokines. Coagulation Cascade: Overview; Antithrombin III; Factor V; Factor VII; Fibrinogen and Fibrin; Intrinsic Factors; Protein C and Protein S; Thrombin; Tissue Factor. Complement. Endothelial Cells and Endothelium. G-Protein-Coupled Receptors. Interleukins: IL-1 and IL-18; lL-4; lL-5; lL-6; lL-7; lL-9; IL-10; IL-12; IL-13; IL-15; IL-16; IL-17; IL-23 and IL-27. Leukocytes: Monocytes. Platelets. ProteinaseActivated Receptors. Pulmonary Fibrosis. Pulmonary Thromboembolism: Deep Venous Thrombosis.
Further Reading Abraham E (2000) Coagulation abnormalities in acute lung injury and sepsis. American Journal of Respiratory Cell and Molecular Biology 22: 401–404. Altieri DC (1995) Proteases and protease receptors in modulation of leukocyte effector functions. Journal of Leukocyte Biology 58: 120–127. Altieri DC (1995) Xa receptor EPR-1. FASEB Journal 9: 860–865. Bretschneider E and Schror K (2001) Cellular effects of factor Xa on vascular smooth muscle cells – inhibition by heparins? Seminars in Thrombosis and Hemostasis. 27: 489–493. Chapman HA (2004) Disorders of lung matrix remodeling. Journal of Clinical Investigation 113: 148–157. Dugina TN, Kiseleva EV, Chistov IV, Umarova BA, and Strukova SM (2002) Receptors of the PAR family as a link between blood coagulation and inflammation. Biochemistry (Moscow) 67: 74–75. Ichinose A and Davie EW (1994) The blood coagulation factors: Their cDNAs, genes, and expression. In: Colman RW, Hirsh J, Marder VJ, and Salzman EW (eds.) Hemostasis and Thrombosis: Basic Principles and Clinical Practice, 3rd edn., pp. 19–54. Philadelphia: J.B. Lippincott Company. Idell S (2001) Anticoagulants for acute respiratory distress syndrome: can they work? American Journal of Respiratory and Critical Care Medicine 164: 517–520. Laterre P-F, Wittebole X, and Dhainaut J-F (2003) Anticoagulant therapy in acute lung injury. Critical Care Medicine 31: S329– S336. Levi M, Schultz MJ, Rijneveld AW, and van der Poll T (2003) Bronchoalveolar coagulation and fibrinolysis in endotoxemia and pneumonia. Critical Care Medicine 31: S238–S242.
Mann KG, Gaffney D, and Bovill EG (1995) Molecular biology, biochemistry, and lifespan of plasma coagulation factors. In: Beutler E, Lichtman MA, Coller BS, and Kipps TJ (eds.) Williams Hematology, 5th edn., pp. 1206–1226. New York: McGraw-Hill. Riewald M and Ruf W (2002) Orchestration of coagulation protease signaling by tissue factor. Trends in Cardiovascular Medicine 12: 149–154. Ruf W, Dorfleutner A, and Riewald M (2003) Specificity of coagulation factor signaling. Journal of Thrombosis and Haemostasis 1: 1495–1503. Ware LB and Matthay MA (2000) The acute respiratory distress syndrome. New England Journal of Medicine 342: 1334–1349. Welty-Wolf KE, Carraway MS, Ortel TL, and Piantadosi CA (2002) Coagulation and inflammation in acute lung injury. Thrombosis and Haemostasis 88: 17–25.
Fibrinogen and Fibrin A Gu¨nther and C Ruppert, University of Giessen Lung Center (UGLC), Giessen, Germany & 2006 Elsevier Ltd. All rights reserved.
Abstract Fibrinogen is a complex multifunctional glycoprotein that plays a key role during blood clotting. Thrombin-mediated conversion of fibrinogen into fibrin and the subsequent cross-linking by factor XIII results in the formation of the three-dimensional framework of the thrombus. In addition, fibrinogen promotes platelet adhesion and aggregation via interaction with a specific integrin receptor. Next to its essential function in normal hemostasis and wound repair fibrin(ogen) also exerts multiple regulatory effects and is involved in pathological processes such as thrombosis and atherosclerosis. Abnormal fibrin turnover has also been implicated in a number of respiratory diseases including acute lung injury (ALI), acute respiratory distress syndrome (ARDS), and chronic interstitial lung diseases such as pulmonary fibrosis.
Introduction In 1666 the Italian anatomist Marcello Malphigi described fibrin as a white fibrous substance in a hemostatic plug and thus confirmed the much earlier observation of the Roman physician Claudius Galenos (AD 129–199) who proposed the presence of ‘fibers’ (Latin: fibrae) in circulating and clotted blood. In 1832 the German anatomist and physiologist Johannes Mu¨ller provided evidence that blood contains ‘soluble fibrin’, and in 1859 Denis de Commercy proposed the existence of a soluble fibrin precursor that could be salted out of plasma. In the second half of the nineteenth century Rudolf Virchow, a German pathologist, identified fibrinogen as being the precursor of fibrin, and Alexander Schmidt, a German physiologist, identified the so-called ‘fibrin-ferment’ (i.e., thrombin) as fibrinogen-converting factor. At the
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same time the Swedish physiologist Olof Hammarsten advanced Schmidt’s theory and suggested that preceding fibrin formation, activation of fibrinogen by thrombin occurred by limited proteolysis. These observations led to Paul Morawitz’s classical four-factor theory of blood coagulation according to which coagulation is initiated when the clot-promoting factor thrombokinase (today referred to as thromboplastin or tissue factor) is released by destroyed tissues or liberated by platelets and leukocytes. Thrombokinase in the presence of calcium converts prothrombin into thrombin, which in turn converts fibrinogen into fibrin. In subsequent years identification of numerous clotting factors resulted in the proposal of the ‘waterfall’ or ‘cascade’ theory of blood clotting. In the final step of this classical paradigm of blood coagulation soluble fibrinogen is cleaved by thrombin forming soluble fibrin monomers, which then polymerize to make an insoluble fibrin clot. It took several decades to elucidate the exact activation mechanism of fibrinogen. In the early 1950s Bailey found that thrombin removed small peptides from the N-terminus of fibrinogen and hypothesized that the newly exposed N-termini must be the contact sites for fibrin polymerization. As new analysis tools became available in the second half of the twentieth century, new insights in fibrin(ogen) structure were given. Fibrinogen and fibrin are multifunctional proteins. Fibrinogen binds via a glycoprotein receptor (GPIIb/ IIIa) to platelets mediating platelet adhesion and aggregation during hemostasis. Fibrin is an essential matrix of wound repair by stabilizing wound fields and supporting local cell proliferation and migration.
chromosome 4 (4q28). For the g-chain an elongated splice variant (427 amino acid residues) exists in human plasma accounting for B8% of the total fibrinogen g pool. The major form of fibrinogen circulating in human plasma is that of a homodimeric g/g molecule (‘fibrinogen 1’), whereas heterodimeric g/g0 molecules (‘fibrinogen 2’) and homodimeric g0 /g0 molecules account for B15% and less than 1%, respectively. The central E-domain contains the fibrinopeptides (FP)-A and -B, which are released from the N-terminus of the Aa (Aa 1–16) and the Bb (Bb 1–14) chains upon enzymatic cleavage by thrombin. The C-terminal ends of the g-chain are important for interaction with GP-IIb/IIIa receptors on platelets, polymerization of fibrin, and cross-linking by FXIIIa. They also contain two self-association sites (gXL and D:D, see Figure 1), which participate in the fibrin(ogen) assembly and cross-linking process, and a high specific binding site for tPA. The C-terminal Aa-chain protuberances contain sites for interaction with platelets (integrin binding to arginine–glycine–aspartic acid (RGD) sequences). Two sets of high-affinity binding sites for both tPA and plasminogen, which play an important role in the initiation of fibrinolysis, are located on the a-C-domain and the distal D-domain. Calcium binding sites are integral parts of the fibrinogen molecule. Among them there are three high-affinity binding sites, two of which are associated with the D-domain and one located at the E-domain. Ca2 þ is important for stabilizing fibrinogen against heat denaturation and proteolytic digestion by trypsin or plasmin, and for fibrin monomer polymerization.
Structure
Fibrinogen Biosynthesis, Fibrin Assembly, Cross-Linking, and Fibrin(ogen)olysis
Fibrinogen is a trinodular disulfide-bridged molecule, composed of two symmetrical half molecules, each half consisting of three different polypeptide chains, termed Aa (610 amino acids, 67 kDa), Bb (461 amino acids, 56 kDa), and g (411 amino acids, 47 kDa) (Figure 1). The two halves are covalently joined in an antiparallel orientation in the central Edomain by five disulfide bridges. At the two ends of the cylindrical fibrinogen molecule, the globular Bband g-C-termini and the Aa-C-terminal random coil form the distal D-domains. The three polypetide ˚ -long coiled-coil rechains interweave forming 160 A gions, which connect the central E-domain to the outer D-domains. Inter-chain disulfide bonds further stabilize the structure. The overall molecular weight of fibrinogen is 340 kDa and its overall length is B450 A˚ (45 nm). Each of the polypeptide chains is a product of a single-copy gene located on the long arm of
Fibrinogen is constitutively produced in the liver and circulates in human plasma at a concentration of 150–400 mg dl 1, and has a half-life of 3–5 days. About 75% of the total fibrinogen pool circulates in plasma whereas the rest distributes to tissues, interstitial fluids, and lymph. Plasma fibrinogen levels are regulated by genetic (e.g., Bb-chain polymorphisms) as well as environmental factors. Each gene is separately transcribed and translated into a precursor polypeptide, which is then processed and assembled into the mature protein. The rate-limiting step in the fibrinogen biosynthesis is the synthesis of the fibrinogen Bb-chain. In addition to the liver, fibrinogen biosynthesis has been found in lung epithelial cells under inflammatory conditions. The thrombin-catalyzed release of the fibrinopeptides FP-A and FP-B exposes two types of binding site in the N-terminal regions of the fibrin molecules,
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Figure 1 Structural model and domains of fibrin(ogen). The major domains (E- and D-domains) of fibrin(ogen) and the constitutive Ddomain association sites participating in fibrin polymerization and cross-linking are shown. EA/EB – polymerization sites that are exposed upon cleavage of the fibrinopeptides FPA and FPB. Da/Db – association sites that interact with available EA/EB sites in fibrin. gA/g0 – normal (gA) and elongated (g0 ) variant of the g-chains with FXIIIa-cross-linking site in the C-terminal region. gXL/D:D – self-association sites. ac – domain in the C-terminal region of the Aa-chain, which dissociates from its noncovalent association with the E-domain upon thrombin cleavage of FPB. Adapted from Mosesson MW (1998) Fibrinogen structure and fibrin clot assembly. Seminars in Thrombosis and Hemostasis 24(2): 169–174, with permission from Thieme New York.
termed EA and EB, which combine with constitutive complementary binding pockets in the D-domains (Da, Db) of neighboring molecules. The initial release of FP-A from the Aa-chain and (noncovalent) assembly of Da:EA associations results in the formation of double-stranded, twisted fibrils. The molecules within a strand are joined by longitudinal contacts between the D-domains (D:D interaction) (Figure 2). Subsequently and concomitantly, fibrils
undergo branching and lateral fibril association resulting in a network of fibers with increased thickness. The EB:Db interaction is not absolutely required for lateral fibril/fiber association, but it contributes to this process through cooperative interactions resulting from alignment of D-domains forming so-called trimolecular and tetramolecular branch points in the fibrin polymer (Figure 2). The fibrin clot, initially held together only by noncovalent
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interactions, becomes further stabilized by the incorporation of covalent bonds between C-terminal gXL sites of fibrin units. FXIIIa (plasma transglutaminase) catalyzes the formation of covalent e-(g-glutamyl)lysine isopeptide bonds in the presence of Ca2 þ resulting in the formation of g-dimers and higherorder forms of cross-linked g-chains (g-trimers, g-tetramers). Plasmin-catalyzed degradation of fibrinogen and fibrin occurs similarly; however, degradation of cross-linked fibrin results in somewhat different macromolecular intermediates and end product fragments due to its g-g cross-links. When cross-linked fibrin is cleaved, the a-chain polymers are removed first and degraded to low-molecular-weight fragments. Next the coiled-coil region joining the D- and E-domains are cleaved resulting in macromolecular intermediates such as fragment DD/E, fragment YD/ DY, or fragment YX/XD. Terminal split products of cross-linked fibrin are FsE, D-dimer, as well as D-trimer and D-tetramer derived from tri- and tetramolecular branch points (Figure 2).
Fibrinogen Thrombin FPA Fibril assembly, branching and lateral fibril associations
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Figure 2 Schematic model of fibrin assembly, cross-linking, and plasmin-catalyzed degradation. D:E – noncovalent interactions between EA and Da sites that form end-to-middle staggered overlapping double-stranded fibrils. D:D – longitudinal interaction between two D-domains within a strand. ac – C-terminal a-chain protuberances, which become untethered from its noncovalent association with the E domain upon cleavage of FP-B and promote lateral fibril associations by self-association with other acdomains. g-dimer, g-trimer, g-tetramer – FXIIIa-mediated crosslinks between C-terminal gXL sites of D-domains. DD/E, YD/DY, YX/XD – intermediate lysis fragments. FsE, D-dimer, D-trimer, Dtetramer – terminal lysis fragments of plasmin-catalyzed fibrinolysis. Adapted from Mosesson MW (1998) Fibrinogen structure and fibrin clot assembly. Seminars in Thrombosis and Hemostasis 24(2): 169–174, with permission from Thieme New York.
The key role of fibrin(ogen) during hemostasis is to form the framework for thrombus formation. Cellular and proteinaceous components of a thrombus are stabilized by the covalent cross-linking of fibrin resulting in a stable hemostatic plug of the platelets. Thus, fibrin stabilizes damaged tissues and serves as a primary matrix of wound repair. However, this function is not solely limited to hemostasis and wound healing but may also play a role in developmental processes. Fibrin also exerts regulatory effects including the initiation of fibrinolysis and the regulation of thrombin activity. Fibrin, but not fibrinogen, accelerates the activation of plasminogen by tPA. This is mediated through formation of a tenary complex between tPA and plasminogen on the surface of fibrin involving the above-mentioned specific binding sites (Aa148–160, g312–324, Aa392–610), which are cryptic in the fibrinogen molecule and become exposed during fibrin formation. Next to the initiation of fibrinolysis, fibrin is also involved in the modulation of prolonged fibrinolysis. The most important plasmin inhibitor a2-antiplasmin becomes cross-linked to the Aa-chain of fibrinogen and fibrin by activated factor XIII. This cross-linking process is reversible and a2-antiplasmin can also be released from degraded fibrin. By this, fibrin restricts plasmin activity to sites where it is required and protects fibrinogen and other plasma proteins from degradation.
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Fibrin contains nonsubstrate thrombin-binding sites (two low-affinity sites in the E-domain and one high-affinity site in the g0 -chain) which mediate an antithrombin activity (referred to as antithrombin I). Experimental evidences and clinical observation suggest that antithrombin I is an important factor for downregulating thrombin activity. By this mechanism diffusion of thrombin and the extent of clot propagation is limited. Fibrinogen and fibrin can interact with cells via integrin binding to RGD motifs in the Aa- and g-chain. Binding of fibrinogen to the platelet integrin aIIbb3 (also referred to as GPIIb/IIIa-receptor, CD41a) is of central importance during primary hemostasis. Fibrinogen acts as a bridging molecule and thus mediates platelet adhesion, aggregation, and formation of platelet-rich thrombi. Recently, a regulatory role for fibrinogen in inflammatory cell function was discovered. The finding that leukocyte engagement of fibrin(ogen) via the integrin receptor aMb2 is critical in the inflammatory response suggested a physiologically relevant role for fibrin(ogen) as an inflammatory mediator in innate immunity. Targeted deletion of the gene encoding the Aachain or g-chain in mice resulted in animals without detectable plasma fibrinogen levels. About 30% of Fbg / mice developed bleeding events shortly after birth; however, most of these animals showed a resolution of the bleeding events and survived the neonatal period. In addition, adult mice showed an increased risk for fatal abdominal hemorrhage, and pregnancy resulting in fatal uterine bleeding. A study investigating wound-healing defects in fibrinogennull mice showed no differences with respect to the time required to overtly heal wounds; however, a role for fibrin(ogen) in cellular migration and organization of wound fields could be demonstrated. Fibrinogen knockout mice crossed with atherosclerosissusceptible apolipoprotein E-deficient mice did not show a decreased extent of atherosclerosis despite the absence of fibrinogen.
Receptors The fibrinogen receptor on platelets has been identified as a membrane-bound heterodimeric glycoprotein complex, termed GPIIb/IIIa. The receptor belongs to the b3 subfamily of integrins (aIIbb3) and binds fibrinogen via a specific recognition site g400–441. Two other potential binding sites are located at Aa95–98 (RGDF) and at Aa572–575 (RGDS). The latter sequence is also found in other proteins that interact with the integrin receptor such as von Willebrand’s factor, vitronectin and fibronectin.
The fibrinogen receptor represents a target for pharmacological agents that specifically inhibit platelet aggregation. Three intravenous GPIIb/IIIa antagonists are currently marketed for the prevention of myocardial infarction in patients undergoing angioplasty or stenting: the monoclonal antibody Abciximab (mouse-human chimeric Fab fragments), and eptifibatide and tirofiban, two low molecular mass inhibitors. The leukocyte integrin aMb2 (CD11b/CD18, Mac1) is a further high-affinity receptor for fibrinogen on stimulated macrophages and neutrophils. This receptor belongs to the b2 (CD18) subfamily of integrins, which play a pivotal role in leukocyte function. Multiple binding sites, including g190–202 and g377–395, are implicated in this interaction.
Fibrin(ogen) in Respiratory Diseases A large number of congenital or acquired fibrin(ogen) disorders are described as being associated with a bleeding diathesis or thrombophilia. They include afibrinogenemia (essentially absent fibrinogen), hypofibrinogenemia (plasma levels less than 100 mg dl 1), or dysfibrinogenemia (structurally abnormal fibrin molecules). Elevated fibrinogen plasma levels were found to be strongly and independently related to cardiovascular disease such as coronary heart disease and acute myocardial infarction. The use of fibrinogen plasma levels as a marker predicting the cardiovascular risk has been widely accepted. In contrast, the role of fibrin(ogen) in respiratory disease is still not fully settled. However, there is increasing evidence that fibrin(ogen) plays a central role in the pathogenesis of inflammatory and chronic interstitial lung disease, including the acute respiratory distress syndrome (ARDS), severe pneumonia, and idiopathic pulmonary fibrosis (IPF). Hyaline membranes, that is, the accumulation of fibrin-rich material in the alveolar space, are commonly found in ARDS and other acute or chronic interstitial lung diseases. Such kind of extravascular fibrin deposition results from imbalanced coagulation and fibrinolysis. Analysis of bronchoalveolar lavage fluids from patients with ARDS demonstrated a prominent procoagulant (tissue factor and FVII) and antifibrinolytic (plasminogen activator inhibitor (PAI)-1, a2-antiplasmin) activity, while the fibrinolytic capacity (urokinase) was found to be markedly depressed. Thus, in concert with fibrinogen leakage into the alveolar space, rapid and pronounced fibrin formation is to be expected under these conditions (Figure 3). Furthermore, experimental data suggest a far-reaching incorporation of hydrophobic surfactant compounds into the growing fibrin matrix. As a
508 COAGULATION CASCADE / Fibrinogen and Fibrin
Phospholipids SP-B SP-C
Alveolar hemostatic balance:
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Fibrin oligomer
Persistent fibrin deposition Theory: Collapse induration: Persistent atelectasis/ fibrin deposition Alveolar wall apposition
Fibroblasts
Fibroblast activation Fibrosis
Deposition of fibrous tissue Fibrosis, honeycombing
Figure 3 Diagram illustrating the potential mechanisms by which fibrin(ogen) contributes to respiratory disease. Under physiological conditions the phospholipid lining layer at the air–water interface reduces the surface tension and thereby promotes lung inflation upon inspiration and prevents lung collapse during expiration. Under inflammatory conditions, fibrinogen, leaking into the alveoli, is rapidly converted into fibrin due to a high procoagulant and antifibrinolytic activity in the alveolar compartment. Surfactant function is greatly inhibited by incorporation of hydrophobic surfactant compounds (PL, SP-B/C) into polymerizing fibrin. Persistence or delayed clearance of this ‘specialized’ fibrin matrix promotes fibroproliferative processes (‘collapse induration’) finally resulting in a structural remodeling of the lung with loss of compliance and gas exchange properties.
result, such incorporation may yield a further increase in alveolar surface tension, alveolar instability, and finally gas exchange disturbances (Figure 3). Indeed, pronounced impairment of gas exchange could
be provoked by formation of fibrin in the distal lung in otherwise healthy lungs. In addition, fibrin clots embedding natural surfactant display markedly altered mechanical properties
COAGULATION CASCADE / Intrinsic Factors
and reduced susceptibility to proteolytic degradation. These properties, persistent alveolar collapse, reduced susceptibility to proteolysis, and sustained suppression of fibrinolytic enzymes, may prevent rapid clearance of fibrin from the lungs. Thus, persistent deposition of fibrin in the distal lung may promote fibroblast invasion and replacement of the primary fibrin matrix by a secondary collagenous matrix (Figure 3). Additionally, the fibrinopeptides A/B and fibrin(ogen) scission products have, next to thrombin itself, been shown to serve as potent fibroblast mitogens. However, it has to be stated that some fibrotic response was also encountered in fibrinogen null mice, suggesting that at least other extracellular matric (ECM) proteins may replace fibrin to some extent. Consequently, targeting abnormalities of fibrin turnover by anticoagulant and fibrinolytic interventions has been shown to protect the lung in animal models of acute lung injury and reduced the fibroproliferative response in bleomycin-induced lung fibrosis. Currently, anticoagulants such as heparin and fibrinolysins are being tested in ongoing experimental and clinical studies. Finally, it should be mentioned that in recent studies in a mouse model of allergic asthma evidence was provided that fibrin accumulation contributes to airway hyperresponsiveness. See also: Acute Respiratory Distress Syndrome. Adhesion, Cell–Matrix: Integrins. Anticoagulants. Coagulation Cascade: Overview; Antithrombin III; iuPA, tPA, uPAR; Thrombin. Fibrinolysis: Overview; Plasminogen Activator and Plasmin. Interstitial Lung Disease: Overview; Alveolar Proteinosis; Amyloidosis; Cryptogenic Organizing Pneumonia; Hypersensitivity Pneumonitis; Idiopathic Pulmonary Fibrosis. Surfactant: Overview. Thrombolytic Therapy.
Further Reading Bennet JS (2001) Platelet–fibrinogen interactions. Annals of the New York Academy of Sciences 936: 340–354. Budzynski AZ (1998) Fibrinogen and fibrin: biochemistry and pathophysiology. Critical Reviews in Oncology/Hematology 6: 97–146. Everse SJ (2002) New insights into fibrin(ogen) structure and function. Vox Sanguinis 83(supplement 1): 375–382. Gu¨nther A, Ruppert C, Schmidt R, et al. (2001) Surfactant alteration and replacement in acute respiratory distress syndrome. Respiratory Research 2: 353–364. Idell S (2002) Endothelium and disordered fibrin turnover in the injured lung: newly recognized pathways. Critical Care Medicine 30(supplement): S274–S280. Idell S (2003) Coagulation, fibrinolysis, and fibrin deposition in acute lung injury. Critical Care Medicine 31(supplement): S213–S220. Koenig W (2003) Fibrin(ogen) in cardiovascular disease: an update. Thrombosis and Haemostasis 89: 601–609.
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Levi M, Schultz MJ, Rijneveld AW, and van der Poll T (2003) Bronchoalveolar coagulation and fibrinolysis in endotoxemia and pneumonia. Critical Care Medicine 31(supplement): S238–S242. Medved L and Nieuwenhuizen W (2003) Molecular mechanisms of initiation of fibrinolysis by fibrin. Thrombosis and Haemostasis 89: 409–419. Mosesson MW (1998) Fibrinogen structure and fibrin clot assembly. Seminars in Thrombosis and Hemostasis 24(2): 169–174. Mosesson MW (2003) Fibrinogen gamma chain functions. Journal of Thrombosis and Haemostasis 1: 231–238. Ploplis VA and Castellino FJ (2002) Gene targeting of components of the fibrinolytic system. Thrombosis and Haemostasis 87: 22–31. Wilberding JA, Ploplis VA, McLennan L, et al. (2001) Development of pulmonary fibrosis in fibrinogen-deficient mice. Annals of the New York Academy of Sciences 936: 542–548.
Intrinsic Factors P F Neuenschwander, University of Texas Health Center, Tyler, TX, USA & 2006 Elsevier Ltd. All rights reserved.
Abstract The intrinsic coagulation pathway consists of factors XI, IX, and VIII (fXI, fIX, fVIII). FXI and fIX are serine protease zymogens requiring proteolytic activation to express protease and procoagulant activity. FVIII is a non-enzymatic cofactor that also requires proteolytic activation. FXI can be activated by either fXIIa (contact activation) or by thrombin (positive feedback). The activated fXI (fXIa) subsequently activates fIX to generate fIXa that binds to thrombin-activated fVIII (fVIIIa) to generate a potent procoagulant complex on an anionic phospholipid surface. This complex activates fX to propagate and amplify the coagulant response. The activity of the intrinsic pathway is regulated by the activation status of fVIII, defined by the spontaneous decay of fVIIIa activity. The rapid activation and decay of fVIII/fVIIIa provides a pulse of activity allowing controlled fibrin deposition to occur without excessive clot formation. This pathway is activated during pathological coagulation in the lung in acute lung injury models. If left unregulated it can lead to excessive generation of fXa, resulting in the deposition of extravascular alveolar fibrin followed by respiratory failure.
Introduction The intrinsic pathway of blood coagulation is so named due to the presence of all the required reactants in the circulation, with no external protein source required. It is evidenced most clearly in vitro by the ability of blood or plasma to spontaneously clot upon its collection into a glass vessel. The trigger for this mode of coagulation is the autoactivation of factor XII (fXII; Hageman factor), which auto-hydrolyzes on anionic surfaces (contact activation) to form fXIIa. In the presence of high-molecular-weight kininogen the fXIIa proteolytically activates fXI to generate fXIa, which subsequently activates fIX
is a broad-spectrum serine protease inhibitor whose reactivity is enhanced by heparin. TFPI targets the fVIIa-TF activation of fX, but is also a potent fXa inhibitor in the absence of fVIIa-TF. APC inactivates fVa thus attenuating the procoagulant activity of fXa. Each of these inhibitors has demonstrated beneficial reductions in ALI in various animal models. Unfortunately, these results have not been reproduced in clinical studies. While TFPI was successful in phase II clinical trials, the results in phase III trials (OPTIMIST study) were disappointing and showed no beneficial effect above placebo. APC, however, has demonstrated a significant reduction in mortality in cases of severe sepsis (PROWESS study), which is the most common cause of ALI and ARDS. See also: Acute Respiratory Distress Syndrome. Anticoagulants. Chemokines. Coagulation Cascade: Overview; Antithrombin III; Factor V; Factor VII; Fibrinogen and Fibrin; Intrinsic Factors; Protein C and Protein S; Thrombin; Tissue Factor. Complement. Endothelial Cells and Endothelium. G-Protein-Coupled Receptors. Interleukins: IL-1 and IL-18; lL-4; lL-5; lL-6; lL-7; lL-9; IL-10; IL-12; IL-13; IL-15; IL-16; IL-17; IL-23 and IL-27. Leukocytes: Monocytes. Platelets. ProteinaseActivated Receptors. Pulmonary Fibrosis. Pulmonary Thromboembolism: Deep Venous Thrombosis.
Further Reading Abraham E (2000) Coagulation abnormalities in acute lung injury and sepsis. American Journal of Respiratory Cell and Molecular Biology 22: 401–404. Altieri DC (1995) Proteases and protease receptors in modulation of leukocyte effector functions. Journal of Leukocyte Biology 58: 120–127. Altieri DC (1995) Xa receptor EPR-1. FASEB Journal 9: 860–865. Bretschneider E and Schror K (2001) Cellular effects of factor Xa on vascular smooth muscle cells – inhibition by heparins? Seminars in Thrombosis and Hemostasis. 27: 489–493. Chapman HA (2004) Disorders of lung matrix remodeling. Journal of Clinical Investigation 113: 148–157. Dugina TN, Kiseleva EV, Chistov IV, Umarova BA, and Strukova SM (2002) Receptors of the PAR family as a link between blood coagulation and inflammation. Biochemistry (Moscow) 67: 74–75. Ichinose A and Davie EW (1994) The blood coagulation factors: Their cDNAs, genes, and expression. In: Colman RW, Hirsh J, Marder VJ, and Salzman EW (eds.) Hemostasis and Thrombosis: Basic Principles and Clinical Practice, 3rd edn., pp. 19–54. Philadelphia: J.B. Lippincott Company. Idell S (2001) Anticoagulants for acute respiratory distress syndrome: can they work? American Journal of Respiratory and Critical Care Medicine 164: 517–520. Laterre P-F, Wittebole X, and Dhainaut J-F (2003) Anticoagulant therapy in acute lung injury. Critical Care Medicine 31: S329– S336. Levi M, Schultz MJ, Rijneveld AW, and van der Poll T (2003) Bronchoalveolar coagulation and fibrinolysis in endotoxemia and pneumonia. Critical Care Medicine 31: S238–S242.
Mann KG, Gaffney D, and Bovill EG (1995) Molecular biology, biochemistry, and lifespan of plasma coagulation factors. In: Beutler E, Lichtman MA, Coller BS, and Kipps TJ (eds.) Williams Hematology, 5th edn., pp. 1206–1226. New York: McGraw-Hill. Riewald M and Ruf W (2002) Orchestration of coagulation protease signaling by tissue factor. Trends in Cardiovascular Medicine 12: 149–154. Ruf W, Dorfleutner A, and Riewald M (2003) Specificity of coagulation factor signaling. Journal of Thrombosis and Haemostasis 1: 1495–1503. Ware LB and Matthay MA (2000) The acute respiratory distress syndrome. New England Journal of Medicine 342: 1334–1349. Welty-Wolf KE, Carraway MS, Ortel TL, and Piantadosi CA (2002) Coagulation and inflammation in acute lung injury. Thrombosis and Haemostasis 88: 17–25.
Fibrinogen and Fibrin A Gu¨nther and C Ruppert, University of Giessen Lung Center (UGLC), Giessen, Germany & 2006 Elsevier Ltd. All rights reserved.
Abstract Fibrinogen is a complex multifunctional glycoprotein that plays a key role during blood clotting. Thrombin-mediated conversion of fibrinogen into fibrin and the subsequent cross-linking by factor XIII results in the formation of the three-dimensional framework of the thrombus. In addition, fibrinogen promotes platelet adhesion and aggregation via interaction with a specific integrin receptor. Next to its essential function in normal hemostasis and wound repair fibrin(ogen) also exerts multiple regulatory effects and is involved in pathological processes such as thrombosis and atherosclerosis. Abnormal fibrin turnover has also been implicated in a number of respiratory diseases including acute lung injury (ALI), acute respiratory distress syndrome (ARDS), and chronic interstitial lung diseases such as pulmonary fibrosis.
Introduction In 1666 the Italian anatomist Marcello Malphigi described fibrin as a white fibrous substance in a hemostatic plug and thus confirmed the much earlier observation of the Roman physician Claudius Galenos (AD 129–199) who proposed the presence of ‘fibers’ (Latin: fibrae) in circulating and clotted blood. In 1832 the German anatomist and physiologist Johannes Mu¨ller provided evidence that blood contains ‘soluble fibrin’, and in 1859 Denis de Commercy proposed the existence of a soluble fibrin precursor that could be salted out of plasma. In the second half of the nineteenth century Rudolf Virchow, a German pathologist, identified fibrinogen as being the precursor of fibrin, and Alexander Schmidt, a German physiologist, identified the so-called ‘fibrin-ferment’ (i.e., thrombin) as fibrinogen-converting factor. At the
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same time the Swedish physiologist Olof Hammarsten advanced Schmidt’s theory and suggested that preceding fibrin formation, activation of fibrinogen by thrombin occurred by limited proteolysis. These observations led to Paul Morawitz’s classical four-factor theory of blood coagulation according to which coagulation is initiated when the clot-promoting factor thrombokinase (today referred to as thromboplastin or tissue factor) is released by destroyed tissues or liberated by platelets and leukocytes. Thrombokinase in the presence of calcium converts prothrombin into thrombin, which in turn converts fibrinogen into fibrin. In subsequent years identification of numerous clotting factors resulted in the proposal of the ‘waterfall’ or ‘cascade’ theory of blood clotting. In the final step of this classical paradigm of blood coagulation soluble fibrinogen is cleaved by thrombin forming soluble fibrin monomers, which then polymerize to make an insoluble fibrin clot. It took several decades to elucidate the exact activation mechanism of fibrinogen. In the early 1950s Bailey found that thrombin removed small peptides from the N-terminus of fibrinogen and hypothesized that the newly exposed N-termini must be the contact sites for fibrin polymerization. As new analysis tools became available in the second half of the twentieth century, new insights in fibrin(ogen) structure were given. Fibrinogen and fibrin are multifunctional proteins. Fibrinogen binds via a glycoprotein receptor (GPIIb/ IIIa) to platelets mediating platelet adhesion and aggregation during hemostasis. Fibrin is an essential matrix of wound repair by stabilizing wound fields and supporting local cell proliferation and migration.
chromosome 4 (4q28). For the g-chain an elongated splice variant (427 amino acid residues) exists in human plasma accounting for B8% of the total fibrinogen g pool. The major form of fibrinogen circulating in human plasma is that of a homodimeric g/g molecule (‘fibrinogen 1’), whereas heterodimeric g/g0 molecules (‘fibrinogen 2’) and homodimeric g0 /g0 molecules account for B15% and less than 1%, respectively. The central E-domain contains the fibrinopeptides (FP)-A and -B, which are released from the N-terminus of the Aa (Aa 1–16) and the Bb (Bb 1–14) chains upon enzymatic cleavage by thrombin. The C-terminal ends of the g-chain are important for interaction with GP-IIb/IIIa receptors on platelets, polymerization of fibrin, and cross-linking by FXIIIa. They also contain two self-association sites (gXL and D:D, see Figure 1), which participate in the fibrin(ogen) assembly and cross-linking process, and a high specific binding site for tPA. The C-terminal Aa-chain protuberances contain sites for interaction with platelets (integrin binding to arginine–glycine–aspartic acid (RGD) sequences). Two sets of high-affinity binding sites for both tPA and plasminogen, which play an important role in the initiation of fibrinolysis, are located on the a-C-domain and the distal D-domain. Calcium binding sites are integral parts of the fibrinogen molecule. Among them there are three high-affinity binding sites, two of which are associated with the D-domain and one located at the E-domain. Ca2 þ is important for stabilizing fibrinogen against heat denaturation and proteolytic digestion by trypsin or plasmin, and for fibrin monomer polymerization.
Structure
Fibrinogen Biosynthesis, Fibrin Assembly, Cross-Linking, and Fibrin(ogen)olysis
Fibrinogen is a trinodular disulfide-bridged molecule, composed of two symmetrical half molecules, each half consisting of three different polypeptide chains, termed Aa (610 amino acids, 67 kDa), Bb (461 amino acids, 56 kDa), and g (411 amino acids, 47 kDa) (Figure 1). The two halves are covalently joined in an antiparallel orientation in the central Edomain by five disulfide bridges. At the two ends of the cylindrical fibrinogen molecule, the globular Bband g-C-termini and the Aa-C-terminal random coil form the distal D-domains. The three polypetide ˚ -long coiled-coil rechains interweave forming 160 A gions, which connect the central E-domain to the outer D-domains. Inter-chain disulfide bonds further stabilize the structure. The overall molecular weight of fibrinogen is 340 kDa and its overall length is B450 A˚ (45 nm). Each of the polypeptide chains is a product of a single-copy gene located on the long arm of
Fibrinogen is constitutively produced in the liver and circulates in human plasma at a concentration of 150–400 mg dl 1, and has a half-life of 3–5 days. About 75% of the total fibrinogen pool circulates in plasma whereas the rest distributes to tissues, interstitial fluids, and lymph. Plasma fibrinogen levels are regulated by genetic (e.g., Bb-chain polymorphisms) as well as environmental factors. Each gene is separately transcribed and translated into a precursor polypeptide, which is then processed and assembled into the mature protein. The rate-limiting step in the fibrinogen biosynthesis is the synthesis of the fibrinogen Bb-chain. In addition to the liver, fibrinogen biosynthesis has been found in lung epithelial cells under inflammatory conditions. The thrombin-catalyzed release of the fibrinopeptides FP-A and FP-B exposes two types of binding site in the N-terminal regions of the fibrin molecules,
COAGULATION CASCADE / Fibrinogen and Fibrin 505
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Figure 1 Structural model and domains of fibrin(ogen). The major domains (E- and D-domains) of fibrin(ogen) and the constitutive Ddomain association sites participating in fibrin polymerization and cross-linking are shown. EA/EB – polymerization sites that are exposed upon cleavage of the fibrinopeptides FPA and FPB. Da/Db – association sites that interact with available EA/EB sites in fibrin. gA/g0 – normal (gA) and elongated (g0 ) variant of the g-chains with FXIIIa-cross-linking site in the C-terminal region. gXL/D:D – self-association sites. ac – domain in the C-terminal region of the Aa-chain, which dissociates from its noncovalent association with the E-domain upon thrombin cleavage of FPB. Adapted from Mosesson MW (1998) Fibrinogen structure and fibrin clot assembly. Seminars in Thrombosis and Hemostasis 24(2): 169–174, with permission from Thieme New York.
termed EA and EB, which combine with constitutive complementary binding pockets in the D-domains (Da, Db) of neighboring molecules. The initial release of FP-A from the Aa-chain and (noncovalent) assembly of Da:EA associations results in the formation of double-stranded, twisted fibrils. The molecules within a strand are joined by longitudinal contacts between the D-domains (D:D interaction) (Figure 2). Subsequently and concomitantly, fibrils
undergo branching and lateral fibril association resulting in a network of fibers with increased thickness. The EB:Db interaction is not absolutely required for lateral fibril/fiber association, but it contributes to this process through cooperative interactions resulting from alignment of D-domains forming so-called trimolecular and tetramolecular branch points in the fibrin polymer (Figure 2). The fibrin clot, initially held together only by noncovalent
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interactions, becomes further stabilized by the incorporation of covalent bonds between C-terminal gXL sites of fibrin units. FXIIIa (plasma transglutaminase) catalyzes the formation of covalent e-(g-glutamyl)lysine isopeptide bonds in the presence of Ca2 þ resulting in the formation of g-dimers and higherorder forms of cross-linked g-chains (g-trimers, g-tetramers). Plasmin-catalyzed degradation of fibrinogen and fibrin occurs similarly; however, degradation of cross-linked fibrin results in somewhat different macromolecular intermediates and end product fragments due to its g-g cross-links. When cross-linked fibrin is cleaved, the a-chain polymers are removed first and degraded to low-molecular-weight fragments. Next the coiled-coil region joining the D- and E-domains are cleaved resulting in macromolecular intermediates such as fragment DD/E, fragment YD/ DY, or fragment YX/XD. Terminal split products of cross-linked fibrin are FsE, D-dimer, as well as D-trimer and D-tetramer derived from tri- and tetramolecular branch points (Figure 2).
Fibrinogen Thrombin FPA Fibril assembly, branching and lateral fibril associations
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Figure 2 Schematic model of fibrin assembly, cross-linking, and plasmin-catalyzed degradation. D:E – noncovalent interactions between EA and Da sites that form end-to-middle staggered overlapping double-stranded fibrils. D:D – longitudinal interaction between two D-domains within a strand. ac – C-terminal a-chain protuberances, which become untethered from its noncovalent association with the E domain upon cleavage of FP-B and promote lateral fibril associations by self-association with other acdomains. g-dimer, g-trimer, g-tetramer – FXIIIa-mediated crosslinks between C-terminal gXL sites of D-domains. DD/E, YD/DY, YX/XD – intermediate lysis fragments. FsE, D-dimer, D-trimer, Dtetramer – terminal lysis fragments of plasmin-catalyzed fibrinolysis. Adapted from Mosesson MW (1998) Fibrinogen structure and fibrin clot assembly. Seminars in Thrombosis and Hemostasis 24(2): 169–174, with permission from Thieme New York.
The key role of fibrin(ogen) during hemostasis is to form the framework for thrombus formation. Cellular and proteinaceous components of a thrombus are stabilized by the covalent cross-linking of fibrin resulting in a stable hemostatic plug of the platelets. Thus, fibrin stabilizes damaged tissues and serves as a primary matrix of wound repair. However, this function is not solely limited to hemostasis and wound healing but may also play a role in developmental processes. Fibrin also exerts regulatory effects including the initiation of fibrinolysis and the regulation of thrombin activity. Fibrin, but not fibrinogen, accelerates the activation of plasminogen by tPA. This is mediated through formation of a tenary complex between tPA and plasminogen on the surface of fibrin involving the above-mentioned specific binding sites (Aa148–160, g312–324, Aa392–610), which are cryptic in the fibrinogen molecule and become exposed during fibrin formation. Next to the initiation of fibrinolysis, fibrin is also involved in the modulation of prolonged fibrinolysis. The most important plasmin inhibitor a2-antiplasmin becomes cross-linked to the Aa-chain of fibrinogen and fibrin by activated factor XIII. This cross-linking process is reversible and a2-antiplasmin can also be released from degraded fibrin. By this, fibrin restricts plasmin activity to sites where it is required and protects fibrinogen and other plasma proteins from degradation.
COAGULATION CASCADE / Fibrinogen and Fibrin 507
Fibrin contains nonsubstrate thrombin-binding sites (two low-affinity sites in the E-domain and one high-affinity site in the g0 -chain) which mediate an antithrombin activity (referred to as antithrombin I). Experimental evidences and clinical observation suggest that antithrombin I is an important factor for downregulating thrombin activity. By this mechanism diffusion of thrombin and the extent of clot propagation is limited. Fibrinogen and fibrin can interact with cells via integrin binding to RGD motifs in the Aa- and g-chain. Binding of fibrinogen to the platelet integrin aIIbb3 (also referred to as GPIIb/IIIa-receptor, CD41a) is of central importance during primary hemostasis. Fibrinogen acts as a bridging molecule and thus mediates platelet adhesion, aggregation, and formation of platelet-rich thrombi. Recently, a regulatory role for fibrinogen in inflammatory cell function was discovered. The finding that leukocyte engagement of fibrin(ogen) via the integrin receptor aMb2 is critical in the inflammatory response suggested a physiologically relevant role for fibrin(ogen) as an inflammatory mediator in innate immunity. Targeted deletion of the gene encoding the Aachain or g-chain in mice resulted in animals without detectable plasma fibrinogen levels. About 30% of Fbg / mice developed bleeding events shortly after birth; however, most of these animals showed a resolution of the bleeding events and survived the neonatal period. In addition, adult mice showed an increased risk for fatal abdominal hemorrhage, and pregnancy resulting in fatal uterine bleeding. A study investigating wound-healing defects in fibrinogennull mice showed no differences with respect to the time required to overtly heal wounds; however, a role for fibrin(ogen) in cellular migration and organization of wound fields could be demonstrated. Fibrinogen knockout mice crossed with atherosclerosissusceptible apolipoprotein E-deficient mice did not show a decreased extent of atherosclerosis despite the absence of fibrinogen.
Receptors The fibrinogen receptor on platelets has been identified as a membrane-bound heterodimeric glycoprotein complex, termed GPIIb/IIIa. The receptor belongs to the b3 subfamily of integrins (aIIbb3) and binds fibrinogen via a specific recognition site g400–441. Two other potential binding sites are located at Aa95–98 (RGDF) and at Aa572–575 (RGDS). The latter sequence is also found in other proteins that interact with the integrin receptor such as von Willebrand’s factor, vitronectin and fibronectin.
The fibrinogen receptor represents a target for pharmacological agents that specifically inhibit platelet aggregation. Three intravenous GPIIb/IIIa antagonists are currently marketed for the prevention of myocardial infarction in patients undergoing angioplasty or stenting: the monoclonal antibody Abciximab (mouse-human chimeric Fab fragments), and eptifibatide and tirofiban, two low molecular mass inhibitors. The leukocyte integrin aMb2 (CD11b/CD18, Mac1) is a further high-affinity receptor for fibrinogen on stimulated macrophages and neutrophils. This receptor belongs to the b2 (CD18) subfamily of integrins, which play a pivotal role in leukocyte function. Multiple binding sites, including g190–202 and g377–395, are implicated in this interaction.
Fibrin(ogen) in Respiratory Diseases A large number of congenital or acquired fibrin(ogen) disorders are described as being associated with a bleeding diathesis or thrombophilia. They include afibrinogenemia (essentially absent fibrinogen), hypofibrinogenemia (plasma levels less than 100 mg dl 1), or dysfibrinogenemia (structurally abnormal fibrin molecules). Elevated fibrinogen plasma levels were found to be strongly and independently related to cardiovascular disease such as coronary heart disease and acute myocardial infarction. The use of fibrinogen plasma levels as a marker predicting the cardiovascular risk has been widely accepted. In contrast, the role of fibrin(ogen) in respiratory disease is still not fully settled. However, there is increasing evidence that fibrin(ogen) plays a central role in the pathogenesis of inflammatory and chronic interstitial lung disease, including the acute respiratory distress syndrome (ARDS), severe pneumonia, and idiopathic pulmonary fibrosis (IPF). Hyaline membranes, that is, the accumulation of fibrin-rich material in the alveolar space, are commonly found in ARDS and other acute or chronic interstitial lung diseases. Such kind of extravascular fibrin deposition results from imbalanced coagulation and fibrinolysis. Analysis of bronchoalveolar lavage fluids from patients with ARDS demonstrated a prominent procoagulant (tissue factor and FVII) and antifibrinolytic (plasminogen activator inhibitor (PAI)-1, a2-antiplasmin) activity, while the fibrinolytic capacity (urokinase) was found to be markedly depressed. Thus, in concert with fibrinogen leakage into the alveolar space, rapid and pronounced fibrin formation is to be expected under these conditions (Figure 3). Furthermore, experimental data suggest a far-reaching incorporation of hydrophobic surfactant compounds into the growing fibrin matrix. As a
508 COAGULATION CASCADE / Fibrinogen and Fibrin
Phospholipids SP-B SP-C
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Persistent fibrin deposition Theory: Collapse induration: Persistent atelectasis/ fibrin deposition Alveolar wall apposition
Fibroblasts
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Deposition of fibrous tissue Fibrosis, honeycombing
Figure 3 Diagram illustrating the potential mechanisms by which fibrin(ogen) contributes to respiratory disease. Under physiological conditions the phospholipid lining layer at the air–water interface reduces the surface tension and thereby promotes lung inflation upon inspiration and prevents lung collapse during expiration. Under inflammatory conditions, fibrinogen, leaking into the alveoli, is rapidly converted into fibrin due to a high procoagulant and antifibrinolytic activity in the alveolar compartment. Surfactant function is greatly inhibited by incorporation of hydrophobic surfactant compounds (PL, SP-B/C) into polymerizing fibrin. Persistence or delayed clearance of this ‘specialized’ fibrin matrix promotes fibroproliferative processes (‘collapse induration’) finally resulting in a structural remodeling of the lung with loss of compliance and gas exchange properties.
result, such incorporation may yield a further increase in alveolar surface tension, alveolar instability, and finally gas exchange disturbances (Figure 3). Indeed, pronounced impairment of gas exchange could
be provoked by formation of fibrin in the distal lung in otherwise healthy lungs. In addition, fibrin clots embedding natural surfactant display markedly altered mechanical properties
COAGULATION CASCADE / Intrinsic Factors
and reduced susceptibility to proteolytic degradation. These properties, persistent alveolar collapse, reduced susceptibility to proteolysis, and sustained suppression of fibrinolytic enzymes, may prevent rapid clearance of fibrin from the lungs. Thus, persistent deposition of fibrin in the distal lung may promote fibroblast invasion and replacement of the primary fibrin matrix by a secondary collagenous matrix (Figure 3). Additionally, the fibrinopeptides A/B and fibrin(ogen) scission products have, next to thrombin itself, been shown to serve as potent fibroblast mitogens. However, it has to be stated that some fibrotic response was also encountered in fibrinogen null mice, suggesting that at least other extracellular matric (ECM) proteins may replace fibrin to some extent. Consequently, targeting abnormalities of fibrin turnover by anticoagulant and fibrinolytic interventions has been shown to protect the lung in animal models of acute lung injury and reduced the fibroproliferative response in bleomycin-induced lung fibrosis. Currently, anticoagulants such as heparin and fibrinolysins are being tested in ongoing experimental and clinical studies. Finally, it should be mentioned that in recent studies in a mouse model of allergic asthma evidence was provided that fibrin accumulation contributes to airway hyperresponsiveness. See also: Acute Respiratory Distress Syndrome. Adhesion, Cell–Matrix: Integrins. Anticoagulants. Coagulation Cascade: Overview; Antithrombin III; iuPA, tPA, uPAR; Thrombin. Fibrinolysis: Overview; Plasminogen Activator and Plasmin. Interstitial Lung Disease: Overview; Alveolar Proteinosis; Amyloidosis; Cryptogenic Organizing Pneumonia; Hypersensitivity Pneumonitis; Idiopathic Pulmonary Fibrosis. Surfactant: Overview. Thrombolytic Therapy.
Further Reading Bennet JS (2001) Platelet–fibrinogen interactions. Annals of the New York Academy of Sciences 936: 340–354. Budzynski AZ (1998) Fibrinogen and fibrin: biochemistry and pathophysiology. Critical Reviews in Oncology/Hematology 6: 97–146. Everse SJ (2002) New insights into fibrin(ogen) structure and function. Vox Sanguinis 83(supplement 1): 375–382. Gu¨nther A, Ruppert C, Schmidt R, et al. (2001) Surfactant alteration and replacement in acute respiratory distress syndrome. Respiratory Research 2: 353–364. Idell S (2002) Endothelium and disordered fibrin turnover in the injured lung: newly recognized pathways. Critical Care Medicine 30(supplement): S274–S280. Idell S (2003) Coagulation, fibrinolysis, and fibrin deposition in acute lung injury. Critical Care Medicine 31(supplement): S213–S220. Koenig W (2003) Fibrin(ogen) in cardiovascular disease: an update. Thrombosis and Haemostasis 89: 601–609.
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Levi M, Schultz MJ, Rijneveld AW, and van der Poll T (2003) Bronchoalveolar coagulation and fibrinolysis in endotoxemia and pneumonia. Critical Care Medicine 31(supplement): S238–S242. Medved L and Nieuwenhuizen W (2003) Molecular mechanisms of initiation of fibrinolysis by fibrin. Thrombosis and Haemostasis 89: 409–419. Mosesson MW (1998) Fibrinogen structure and fibrin clot assembly. Seminars in Thrombosis and Hemostasis 24(2): 169–174. Mosesson MW (2003) Fibrinogen gamma chain functions. Journal of Thrombosis and Haemostasis 1: 231–238. Ploplis VA and Castellino FJ (2002) Gene targeting of components of the fibrinolytic system. Thrombosis and Haemostasis 87: 22–31. Wilberding JA, Ploplis VA, McLennan L, et al. (2001) Development of pulmonary fibrosis in fibrinogen-deficient mice. Annals of the New York Academy of Sciences 936: 542–548.
Intrinsic Factors P F Neuenschwander, University of Texas Health Center, Tyler, TX, USA & 2006 Elsevier Ltd. All rights reserved.
Abstract The intrinsic coagulation pathway consists of factors XI, IX, and VIII (fXI, fIX, fVIII). FXI and fIX are serine protease zymogens requiring proteolytic activation to express protease and procoagulant activity. FVIII is a non-enzymatic cofactor that also requires proteolytic activation. FXI can be activated by either fXIIa (contact activation) or by thrombin (positive feedback). The activated fXI (fXIa) subsequently activates fIX to generate fIXa that binds to thrombin-activated fVIII (fVIIIa) to generate a potent procoagulant complex on an anionic phospholipid surface. This complex activates fX to propagate and amplify the coagulant response. The activity of the intrinsic pathway is regulated by the activation status of fVIII, defined by the spontaneous decay of fVIIIa activity. The rapid activation and decay of fVIII/fVIIIa provides a pulse of activity allowing controlled fibrin deposition to occur without excessive clot formation. This pathway is activated during pathological coagulation in the lung in acute lung injury models. If left unregulated it can lead to excessive generation of fXa, resulting in the deposition of extravascular alveolar fibrin followed by respiratory failure.
Introduction The intrinsic pathway of blood coagulation is so named due to the presence of all the required reactants in the circulation, with no external protein source required. It is evidenced most clearly in vitro by the ability of blood or plasma to spontaneously clot upon its collection into a glass vessel. The trigger for this mode of coagulation is the autoactivation of factor XII (fXII; Hageman factor), which auto-hydrolyzes on anionic surfaces (contact activation) to form fXIIa. In the presence of high-molecular-weight kininogen the fXIIa proteolytically activates fXI to generate fXIa, which subsequently activates fIX