Factors That Limit Coagulation. Once initiated, coag- ulation must be restricted to the site of vascular injury to prevent deleterious consequences. One limiting factor is simple dilution; blood flowing past the site of injury washes out activated coagulation factors, which are rapidly removed by the liver. A second is the requirement for negatively charged phospholipids, which, as men- tioned, are mainly provided by platelets that have been activated by contact with subendothelial matrix at sites of vascular injury. However, the most important counter- regulatory mechanisms involve factors that are expressed by intact endothelium adjacent to the site of injury (described later). #151 | | |
Activation of the coagulation cascade also sets into motion a fibrinolytic cascade that limits the size of the clot and contributes to its later dissolution (Fig. 4.10). Fibrino- lysis is largely accomplished through the enzymatic activ- ity of plasmin, which breaks down fibrin and interferes with its polymerization. An elevated level of breakdown prod- ucts of fibrinogen (often called fibrin split products), most notably fibrin-derived D-dimers, are a useful clinical markers of several thrombotic states (described later). Plasmin is generated by enzymatic catabolism of the inac- tive circulating precursor plasminogen, either by a factor XII–dependent pathway (possibly explaining the associa- tion of factor XII deficiency and thrombosis) or by plas- minogen activators. The most important plasminogen activator is t-PA; it is synthesized principally by endothe- lium and is most active when bound to fibrin. This charac- teristic makes t-PA a useful therapeutic agent, since its fibrinolytic activity is largely confined to sites of recent thrombosis. Once activated, plasmin is in turn tightly con- trolled by counterregulatory factors such as α2-plasmin inhibitor, a plasma protein that binds and rapidly inhibits free plasmin. #152 | | |
Fig. 4.9 Role of thrombin in hemostasis and cellular activation. Thrombin generates fibrin by cleaving fibrinogen, activates factor XIII (which is respon- sible for crosslinking fibrin into an insoluble clot), and also activates several other coagulation factors, thereby amplifying the coagulation cascade (Fig. 4.7). Through protease-activated receptors (PARs), thrombin activates (1) platelet aggregation and TxA2 secretion; (2) endothelium, which responds by generating leukocyte adhesion molecules and a variety of fibrinolytic (t-PA), vasoactive (NO, PGI2), or cytokine (PDGF) mediators; and (3) leukocytes, increasing their adhesion to activated endothelium. ECM, Extracellular matrix; NO, nitric oxide; PDGF, platelet-derived growth factor; PGI2, prosta- glandin I2 (prostacyclin); TXA2, thromboxane A2; t-PA, tissue-type plasmino- gen activator. See Fig. 4.11 for anticoagulant activities mediated by thrombin via thrombomodulin. (Courtesy of Shaun Coughlin, MD, PhD, Cardiovascular Research Institute, University of California at San Francisco, San Francisco, California.) #153 | | |
SUMMARY COAGULATION FACTORS #154 | | |
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Coagulation occurs via the sequential enzymatic conversion of a cascade of circulating and locally synthesized proteins. #156 | | |
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Tissue factor elaborated at sites of injury is the most impor- tant initiator of the coagulation cascade in vivo. #158 | | |
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At the final stage of coagulation, thrombin converts fibrinogen into insoluble fibrin that contributes to formation of the defini- tive hemostatic plug. #160 | | |
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Coagulation normally is restricted to sites of vascular injury by: #162 | | |
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limiting enzymatic activation to phospholipid surfaces pro- vided by activated platelets or endothelium, #164 | | |
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circulating inhibitors of coagulation factors, such as anti- thrombin III, whose activity is augmented by heparin-like molecules expressed on endothelial cells #166 | | |
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expression of thrombomodulin on normal endothelial cells, which bind thrombin and convert it into an anti-coagulant, #168 | | |
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activation of fibrinolytic pathways (e.g., by association of tissue plasminogen activator with fibrin). #170 | | |
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The balance between the anticoagulant and procoagulant #172 | | |
activities of endothelium often determines whether clot formation, propagation, or dissolution occurs (Fig. 4.11). Normal endothelial cells express a multitude of factors that inhibit the procoagulant activities of platelets and coagula- tion factors and that augment fibrinolysis. These factors act in concert to prevent thrombosis and to limit clotting to sites of vascular damage. However, if injured or exposed to proinflammatory factors, endothelial cells lose many of their antithrombotic properties. Here, we complete the dis- cussion of hemostasis by focusing on the antithrombotic activities of normal endothelium; we return to the “dark side” of endothelial cells later when discussing thrombosis. #173 | | |
The antithrombotic properties of endothelium can be divided into activities directed at platelets, coagulation factors, and fibrinolysis. #174 | | |
Platelet inhibitory effects. An obvious effect of intact endo- thelium is to serve as a barrier that shields platelets from subendothelial vWF and collagen. However, normal endothelium also releases a number of factors that inhibit platelet activation and aggregation. Among the most important are prostacyclin (PGI2), nitric oxide (NO), and adenosine diphosphatase; the latter degrades ADP, already discussed as a potent activator of platelet aggre- gation. Finally, endothelial cells bind and alter the activ- ity of thrombin, which is one of the most potent activators of platelets. #175 | | |
Anticoagulant effects. Normal endothelium shields coag- ulation factors from tissue factor in vessel walls and expresses multiple factors that actively oppose coag- ulation, most notably thrombomodulin, endothelial protein C receptor, heparin-like molecules, and tissue factor pathway inhibitor. Thrombomodulin and endothe- lial protein C receptor bind thrombin and protein C, respectively, in a complex on the endothelial cell surface. When bound in this complex, thrombin loses its ability to activate coagulation factors and platelets, and instead cleaves and activates protein C, a vitamin K–dependent protease that requires a cofactor, protein S. Activated protein C/protein S complex is a potent inhibitor of coagulation factors Va and VIIIa. Heparin-like molecules on the surface of endothelium bind and activate anti- thrombin III, which then inhibits thrombin and factors IXa, Xa, XIa, and XIIa. The clinical utility of heparin and related drugs is based on their ability to stimulate anti- thrombin III activity. Tissue factor pathway inhibitor (TFPI), like protein C, requires protein S as a cofactor and, as the name implies, binds and inhibits tissue factor/factor VIIa complexes. #176 | | |
Fibrinolytic effects. Normal endothelial cells synthesize t-PA, already discussed, as a key component of the fibri- nolytic pathway. #177 | | |
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The primary abnormalities that lead to intravascular thrombosis are (1) endothelial injury, (2) stasis or turbu- lent blood flow, and (3) hypercoagulability of the blood (the so-called “Virchow triad”) (Fig. 4.12). Thrombosis is one of the scourges of modern man, because it underlies the most serious and common forms of cardiovascular disease. Here, the focus is on its causes and consequences; its role in cardiovascular disorders is discussed in detail in Chapters 10 and 11. #179 | | |
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Endothelial injury leading to platelet activation almost inevitably underlies thrombus formation in the heart and the arterial circulation, where the high rates of blood flow impede clot formation. Notably, cardiac and arterial clots are typically rich in platelets, and it is believed that platelet adherence and activation is a necessary prereq- uisite for thrombus formation under high shear stress, such as exists in arteries. This insight provides part of the reasoning behind the use of aspirin and other platelet inhibitors in coronary artery disease and acute myocardial infarction. #181 | | |
Fig. 4.11 Anti-coagulant properties of normal endothelium (top) and procoagulant properties of injured or activated endothelium (bottom). NO, Nitric oxide; #182 | | |
PGI2, prostaglandin I2 (prostacyclin); t-PA, tissue plasminogen activator; VWF, von Willebrand factor. Thrombin receptors are also called protease-activated receptors (PARs). #183 | | |
Fig. 4.12 Virchow’s triad in thrombosis. Endothelial integrity is the most important factor. Abnormalities of procoagulants or anti-coagulants can tip the balance in favor of thrombosis. Abnormal blood flow (stasis or turbu- lence) can lead to hypercoagulability directly and also indirectly through endothelial dysfunction. #184 | | |
Obviously, severe endothelial injury may trigger thrombosis by exposing VWF and tissue factor. However, inflam- mation and other noxious stimuli also promote thrombosis by shifting the pattern of gene expression in endothelium #185 | | |
to one that is “prothrombotic.” This change is sometimes referred to as endothelial activation or dysfunction and can be produced by diverse exposures, including physical injury, infectious agents, abnormal blood flow, inflammatory mediators, metabolic abnormalities, such as hypercholes- terolemia or homocystinemia, and toxins absorbed from cigarette smoke. Endothelial activation is believed to have an important role in triggering arterial thrombotic events. The role of endothelial cell activation and dysfunction #186 | | |
in arterial thrombosis is also discussed in Chapters 10 and #187 | | |
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Here it suffices to mention several of the major pro- #189 | | |
thrombotic alterations: #190 | | |
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Procoagulant changes. Endothelial cells activated by cyto- kines downregulate the expression of thrombomodulin, already described as a key modulator of thrombin activ- ity. This may result in sustained activation of throm- bin, which can in turn stimulate platelets and augment inflammation through PARs expressed on platelets and inflammatory cells. In addition, inflamed endothe- lium also downregulates the expression of other anti- coagulants, such as protein C and tissue factor protein inhibitor, changes that further promote a procoagulant state. #192 | | |
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Anti-fibrinolytic effects. Activated endothelial cells secrete Plasminogen activator inhibitors (PAI), which limit fibrinolysis and downregulate the expression of t-PA, alterations that also favor the development of thrombi. #194 | | |
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Turbulence (chaotic blood flow) contributes to arterial #196 | | |
and cardiac thrombosis by causing endothelial injury or dysfunction, as well as by forming countercurrents and local pockets of stasis. Stasis is a major factor in the devel- opment of venous thrombi. Under conditions of normal laminar blood flow, platelets (and other blood cells) are found mainly in the center of the vessel lumen, separated from the endothelium by a slower-moving layer of plasma. By contrast, stasis and turbulence have the following del- eterious effects: #197 | | |
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Both promote endothelial cell activation and enhanced procoagulant activity, in part through flow-induced changes in endothelial gene expression. #199 | | |
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