Page 1979 - Williams Hematology ( PDFDrive )

1954 Part XII: Hemostasis and Thrombosis Chapter 114: Control of Coagulation Reactions 1955

Inflammatory mediators induce EPCR ectodomain shedding from the generate anticoagulant activity because of APC. 136,137 Interestingly,
endothelial cell surface by metalloproteinase TNF-α converting enzyme thrombin infusion into hyperlipidemic monkeys with atherosclerosis
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(known as “TACE”). The soluble EPCR ectodomain is found in nor- generates less APC and causes a poorer ex vivo response to APC com-
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mal human plasma at 100 ng/mL; however, carriers of the H3 EPCR pared with normolipidemic control monkeys, showing that hyperlipi-
haplotype that includes a Ser219Gly polymorphism (rs867186) have demia and vascular disease can affect protein C activation.
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threefold higher soluble EPCR plasma levels. Higher plasma levels Ischemia causes protein C activation in vivo. A brief occlusion
of soluble EPCR are found in patients with disseminated intravascular of the left anterior descending coronary artery in pigs results in APC
coagulation or systemic lupus erythematosus, although plasma EPCR generation. During cerebral ischemia in humans undergoing routine
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levels are not correlated with pathology-related alterations in circulating endarterectomy, APC increases in the venous cerebral blood. Protein
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thrombomodulin levels. Soluble EPCR binds the protein C and APC C is significantly activated during cardiopulmonary bypass, mainly dur-
via their GLA domains with an affinity similar to the membrane-bound ing the minutes immediately after aortic unclamping in the ischemic
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receptor. Because binding of the APC GLA domain to negatively charged vascular beds. Streptokinase therapy for acute myocardial infarction
phospholipid membranes is required for its anticoagulant activity, solu- increases circulating APC. 142
ble EPCR at relatively high levels in purified reaction mixtures inhibits Circulating APC concentration in normal human subjects is
the anticoagulant action of APC against factor Va, although it does not highly correlated with circulating levels of protein C zymogen. Based
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block the reaction of APC with protease inhibitors. 10,86,99,115 on protein C infusion studies in protein C-deficient subjects, the level of
The EPCR crystal structure surprisingly revealed a single phos- circulating APC is strongly determined by the concentration of protein
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pholipid molecule bound in a surface groove on the protein. Secreted C. EPCR appears to be required for normal protein C activation in
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phospholipase A group V can modify the lipid in EPCR and cause response to thrombin infusions in experimental animals. EPCR and
2
EPCR to lose its ability to bind protein C and APC. 116,117 The presence thrombomodulin must be in close proximity on cell surfaces (see Fig.
of functional EPCR on the cell surface has important implications for 114–2), although this has yet to be experimentally demonstrated.
thrombotic and inflammatory vascular disease because EPCR inacti- Thrombomodulin and EPCR appear to differ markedly in their rel-
vation in vivo increases susceptibility to thrombotic and inflammatory ative distribution densities on blood vessels as the former is abundantly
diseases. 10,86,97 present in the small blood vessels but less so in large vessels, whereas the
The physiologic requirement for EPCR in mice was established by the latter is more abundant in large vessels than in small vessels. 85,86,146,147 Low
embryonic lethality observed for knockout of the murine EPCR gene. 118 levels of thrombomodulin are expressed in brain, and brain-specific
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EPCR has functionally important interactions with multiple mol- activation of protein C in humans occurs during carotid occlusion. 140
ecules beyond protein C and APC. It binds factor VII and factor VIIa. Proteolytic cleavage and activation of protein C can also be effected
10
Furthermore, EPCR was recently implicated to play a potentially impor- by meizothrombin, plasmin, or factor Xa. 149–153 On the surface of cul-
tant role in the pathogenesis of severe malaria. 119–122 tured endothelial cells, negatively charged sulfated polysaccharides in
the presence of phospholipid vesicles containing phosphatidyletha-
Endothelial Protein C Receptor Gene nolamine can enhance the rate of protein C activation by factor Xa to
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The EPCR gene, comprising four exons and three introns, is located on approach the protein C activation rate of thrombin:thrombomodulin.
human chromosome 20q11.2 and spans 6 kb (see Table 114–1). 123 No data yet indicate whether protein C activation by meizothrombin,
plasmin, or factor Xa is physiologically relevant.
Protein C activation is stimulated by platelet factor 4. Both in vitro
PROTEASE-ACTIVATED RECEPTOR-1 and in vivo data imply that platelet factor 4 may play a physiologic role
PAR-1, discovered as a high-affinity human platelet receptor for in enhancing APC generation and influencing the activities of the pro-
thrombin, is the prototype of a four-member subfamily of G-protein– tein C system. 154–157
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coupled receptors that share an unusual mechanism of activation,
namely activation by proteases. 124–130 Each PAR contains seven trans-
membrane helical domains and an extracellular N-terminal tail that ACTIVATED PROTEIN C ACTIVITIES
is cleaved by an activating protease such that the newly generated The clinical phenotype of severe protein C deficiency in neonatal pur-
aminoterminus is a tethered ligand that triggers activation of the cou- pura fulminans implies that APC exerts multiple physiologically essen-
pled G-protein. Human platelets employ PAR-1 and PAR-4 for activa- tial activities, including potent anticoagulant and antiinflammatory
tion by thrombin whereas, curiously, murine platelets that are devoid actions (Chap. 130). Recent advances establish that APC’s antiinflam-
of PAR-1 require PAR-3 and PAR-4 for thrombin’s normal effects. 125,131 matory actions are but one manifestation of its ability to interact directly
PAR-1 is activated by various plasma proteases 132–135 and is generally with cell receptors to provide multiple cytoprotective activities. 7,8,110,111
required for APC’s cytoprotective activities (see “Cellular Receptors for These two distinct types of activities of APC—intravascular anticoag-
Physiologic Effects of Activated Protein C on Cells” below). 7,8,110,111 ulant activity and initiation of cell signaling—are mediated by different
sets of molecular interactions, and both types of activities are clinically
Protease-Activated Receptor-1 Gene relevant.
The PAR-1 gene contains only two introns, is located on chromosome
5q13, and spans 25 kb (see Table 114–1). Much is known about many
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factors that can either upregulate or downregulate the PAR-1 gene. 124–130 ACTIVATED PROTEIN C ANTICOAGULANT
ACTIVITY
ACTIVATION OF PROTEIN C Mechanisms for APC’s direct anticoagulant activity involve factors V
and VIII, the two homologous coagulation cofactors that circulate as
Protein C is activated from zymogen to active protease as a result of inactive molecules and that are converted to active cofactors by lim-
cleavage by thrombin at the Arg169–Leu170 peptide bond in a reaction ited proteolysis (see Chap. 113, Figs. 113–11 and 113–13). APC circu-
that is accelerated by thrombomodulin and EPCR (see Fig. 114–2 and lates at 40 pM (picomolars) in normal humans, and there is an inverse
“Thrombomodulin” above). 5,7,8,76,81,86 Thrombin infusions into animals correlation between fibrinopeptide A, the product that is cleaved

Kaushansky_chapter 114_p1949-1966.indd 1954 9/18/15 10:05 AM