Improved understanding of the molecular mechanisms of fibrinolysis has led to major advances in fibrinolytic and antifibrinolytic therapy. Characterization of the genes for all the major fibrinolytic proteins revealed the structure of the relevant serine proteases, their inhibitors, and their receptors. The development of genetically engineered animals deficient in one or more fibrinolytic protein(s) revealed both expected and unexpected functions, including those relating to tissue remodeling, inflammation, and vascular integrity. In addition, we now have a catalog of acquired and inherited disorders reflective of either fibrinolytic deficiency with thrombosis or fibrinolytic excess with hemorrhage. These advances have led to development of more effective and safer protocols for both profibrinolytic and antifibrinolytic therapy in a variety of circumstances.
Acronyms and Abbreviations
α2-PI, alpha-2 plasmin inhibitor; APL, acute promyelocytic leukemia; CI, confidence interval; CT, computed tomography; DIC, disseminated intravascular coagulation; HC, homocysteine; IL, interleukin; LDL, low-density lipoprotein; MMP, matrix metalloproteinase; MRI, magnetic resonance imaging; Plg, plasminogen; PAI, plasminogen activator inhibitor; PLG-RKT, plasminogen receptor KT; TAFI, thrombin-activatable fibrinolysis inhibitor; TGF-β, transforming growth factor beta; t-PA, tissue-type plasminogen activator; u-PA, urokinase-type plasminogen activator; u-PAR, urokinase-type plasminogen activator receptor.
BASIC CONCEPTS OF FIBRINOLYSIS
In response to vascular injury, fibrin, the insoluble end-product of the action of thrombin on fibrinogen, is deposited in blood vessels, thus stemming the flow of blood. Once the vessel has healed, the fibrinolytic system is activated, converting fibrin to its soluble degradation products through the action of the serine protease, plasmin (Fig. 135–1A). Fibrinolysis is subject to precise control because of the actions of multiple activators, inhibitors, and cofactors.1 In addition, receptors expressed by endothelial, monocytoid, and myeloid cells provide specialized, protected environments where plasmin can be generated without compromise by circulating inhibitors (Fig. 135–1B).2,3 Beyond its more traditional role in fibrin degradation, the fibrinolytic system also supports a variety of processes, including tissue remodeling, inflammation, and vascular integrity. This chapter reviews the fundamental features of plasmin generation, considers the major clinical syndromes resulting from abnormalities in fibrinolysis, and discusses approaches to fibrinolytic and antifibrinolytic therapy.
Overview of the fibrinolytic system. A. Fibrin-based plasminogen activation. The zymogen plasminogen (Plg) is converted to the active serine protease, plasmin (PN), through the action of tissue plasminogen activator (t-PA) or urokinase plasminogen activator (u-PA). The activity of t-PA is greatly enhanced by its assembly with Plg through lysine residues (K) on a fibrin-containing thrombus. u-PA acts independently of fibrin. Both t-PA and u-PA can be inhibited by plasminogen activator inhibitor-1 (PAI-1). In addition, thrombin-activatable fibrinolysis inhibitor (TAFI), an arginine-lysine carboxypeptidase, removes C-terminal lysine residues, thereby limiting the binding of Plg and t-PA to fibrin. By binding to fibrin, PN is protected from its major inhibitor, α2-plasmin inhibitor (α2-PI). Fibrin-bound plasmin degrades crosslinked fibrin, giving rise to soluble fibrin degradation products (FDPs). B. Cell ...