Coagulation-dependent inhibition of fibrinolysis: role of carboxypeptidase-U and the premature lysis of clots from hemophilic plasma

Abstract

Coagulation is initiated by the binding of factor VIIa to tissue factor, with resultant limited factor IX and X activation and thrombin production. Owing to the feedback inhibition of the factor VIIa/tissue factor complex by tissue factor pathway inhibitor (TFPI), additional factor X activation and thrombin generation must proceed through a pathway involving factors VIII, IX, and XI. Experiments designed to elucidate the requirement for amplified factor Xa and thrombin generation in normal hemostasis show that the resistance of plasma clots to tissue plasminogen activator (tPA)- and urokinase-induced fibrinolysis is related to the extent of thrombin generation. Inhibition of fibrinolysis is mediated in part by plasma carboxypeptidase-U ([CPU] carboxypeptidase-R, procarboxypeptidase-B, thrombin-activatable fibrinolysis inhibitor), a proenzyme that is proteolytically activated by thrombin in a process enhanced dramatically by the cofactor thrombomodulin. A clot induced in factor IX-deficient plasma with limited amounts of tissue factor in the presence of urokinase (100 U/mL) lyses prematurely, and this defect is corrected by supplementation of the deficient plasma with factor IX (5 micrograms/mL) or thrombomodulin (20 ng/mL). These additions enhance the rate and extent of CPU activation: in the case of factor IX, presumably by permitting amplified generation of factor Xa and thrombin, and in the case of thrombomodulin, presumably by increasing the degree of CPU activation produced by the low levels of thrombin generated in the absence of factor IX. Pretreatment of the factor IX-deficient plasma with specific anti-CPU antibodies prevents the increased resistance to fibrinolysis produced by addition of factor IX and thrombomodulin. Likewise, when coagulation is induced by thrombin (2 U/mL) in the presence of tPA (60 U/mL), clots formed from plasmas deficient in factors VIII, IX, X, or XI lyse prematurely unless the missing factor is replaced or thrombomodulin (20 ng/mL) is added.

Thrombin activatable fibrinolysis inhibitor activation and bleeding in haemophilia A

“…The hypothesis that bleeding in haemophilia is partially because of enhanced fibrinolysis, was first suggested by Broze and Higuchi in 1996 [12]. The potential mechanisms for aberrant fibrinolysis in haemophilia include impaired cross-linking [9], lower TAFI levels [22] and impaired TAFI activation [14].…”

Considerations in the evaluation of haemophilia patients for short‐term prophylactic therapy: a paediatric and adult case study

“…The tenase complexes generate additional factor Xa that, with its cofactor Va, dramatically amplify thrombin generation. If tenase complexes are absent or inadequate, the level of thrombin generation is insufficient to produce substantial platelet recruitment and robust fibrin formation or to enhance clot stability through its activation of the fibrin crosslinker factor XIII and thrombin-inactivatable fibrinolysis inhibitor (TAFI) [36].…”

A review of hyperfibrinolysis in cats and dogs

“…The critical role of factors VIII and FIX in coagulation is best illustrated by the cell-based model of coagulation (Smith 2009). In people with haemophilia, haemorrhage occurs due to both defective coagulation and up-regulated fibrinolysis (Broze & Higuchi 1996, Mosnier et al 2001, Foley & Nesheim 2009. A more intensely haemorrhagic phenotype has been reported in human haemophiliacs with hyperfibrinolysis (Grünewald et al 2002).…”

“…Thrombin activates FXIII, which cross-links fibrin monomers to stabilise clots, so decreased FXIIIa results in the formation of fragile clots susceptible to lysis (Lorand et al 1981, Muszbek et al 1999. Finally, thrombin is required for activation of TAFI, and insufficient TAFIa is associated with premature clot lysis (Broze & Higuchi 1996, Foley & Nesheim 2009). Haemophiliac dogs treated with low-dose soluble thrombomodulin to increase TAFIa produce clots that are more resistant to fibrinolysis (Foley et al 2012).…”