Enhanced Abdominal Aortic Aneurysm Formation in Thrombin-Activatable Procarboxypeptidase B–Deficient Mice

Abstract

Objective-To determine whether procarboxypeptidase B (pCPB)Ϫ/Ϫ mice are susceptible to accelerated abdominal aortic aneurysm (AAA) development secondary to unregulated OPN-mediated mural inflammation in the absence of CPB inhibition. Methods and Results-Thrombin/thrombomodulin cleaves thrombin-activatable pCPB or thrombin-activatable fibrinolysis inhibitor, activating CPB, which inhibits the generation of plasmin and inactivates proinflammatory mediators (complement C5a and thrombin-cleaved osteopontin [OPN] Key Words: thrombosis Ⅲ abdominal aortic aneurysm Ⅲ carboxypeptidase Ⅲ osteopontin Ⅲ plasmin A bdominal aortic aneurysm (AAA) is a common and potentially lethal disease, found in approximately 6% to 8% of men older than 60 years. AAA rupture is the most serious consequence of the disease and accounts for approximately 15 000 deaths in the United States annually. 1,2 The pathogenesis of AAA disease is complex, involving chronic transmural inflammation, characterized by macrophage and lymphocyte infiltration and proteolytic degradation of medial elastin and interstitial collagens by matrix metalloproteinases (MMPs). Structural degradation of the aortic wall in combination with hemodynamic stress results in tissue failure and aneurysmal dilatation. 3 Osteopontin (OPN) is a multifunctional protein that is both a component of the extracellular matrix (ECM) in mineralized tissues and a circulating proinflammatory cytokine. 4 OPN is a prominent component of human atherosclerotic tissues and has recently been demonstrated to play a key role in AAA pathogenesis. 5 Compared with apolipoprotein E (ApoE) Ϫ/Ϫ mice, ApoE Ϫ/Ϫ OPN Ϫ/Ϫ double-knockout mice are protected from angiotensin II-induced accelerated atherosclerosis and AAA formation. 6 Structurally, OPN contains an RGD site (159 -161) that mediates binding to a number of integrins, including ␣v␤1, ␣v␤3, ␣v␤5, and ␣5␤1. Thrombin cleavage of OPN exposes a hitherto cryptic C-terminus ( 162 SVVYGLR 168 ) that mediates the binding of thrombincleaved OPN (OPN-R) to a new subset of integrins, consisting of ␣4␤1 and ␣9␤1. 7-9 OPN-R demonstrates enhanced cell binding and cell migration compared with intact OPN in vitro, 10,11 suggesting that thrombin cleavage of OPN may augment its proinflammatory properties.When thrombin binds to thrombomodulin on the endothelial cell surface, it undergoes a remarkable change in its substrate recognition, switching from procoagulant factors, such as fibrinogen, to protein C and functionally inhibiting progressive thrombosis. 12 In addition to inhibiting coagulation via the activation of protein C, the thrombinthrombomodulin complex also influences fibrinolysis by activating a second physiological substrate, the plasma carboxypeptidase, thrombin-activatable procarboxypeptidase B (pCPB), or thrombin-activatable fibrinolysis inhibitor. 13,14

Membrane-Bound Thrombomodulin Regulates Macrophage Inflammation in Abdominal Aortic Aneurysm

“…Based on their observations and the well-known relationship between thrombin and TM, it is tempting to speculate that TM on the EC surface acts as a cofactor to activate TAFI that confers protection against AAA formation. 48 In the present study, however, we demonstrate the significance of membrane-bound TM that is specifically located in macrophages in AAA. In addition, our findings suggest the differential impact of membrane-bound TM in macrophages and VSMCs on AAA, implying that TM of different cells might function diversely during AAA formation.…”

“…48 Using mice with whole-body knockout of TAFI, the authors demonstrated that TAFI deficiency enhances AAA as the result of increased plasmin generation. Based on their observations and the well-known relationship between thrombin and TM, it is tempting to speculate that TM on the EC surface acts as a cofactor to activate TAFI that confers protection against AAA formation.…”

Plasma carboxypeptidase B downregulates inflammatory responses in autoimmune arthritis

“…Mice were scored daily for arthritis by using the visual scoring system and by measuring paw thickness. For the TA studies, mice received 15 mg TA (Pfizer) via subcutaneous injection 1 day before the injection of anti-collagen antibody, and then daily until the conclusion of the study, using previously described protocols (54). Mice were also injected with 40 μg LPS on days 3 and 8.…”

The Controversial Role of the Urokinase System in Abdominal Aortic Aneurysm Formation and Rupture

“…6 Moreover, Schultz et al found that deficiency of thrombin-activatable procarboxypeptidase B, an inhibitor of clot-bound plasmin generation, leads to enhanced AAA formation and rupture in a porcine pancreatic elastase-induced model of AAA. 7 The formation and rupture of these aneurysms was prevented by treatment with transexamic acid, an inhibitor of plasmin generation, suggesting that increased plasmin activity in the absence of procarboxypeptidase B has a role in inducing inflammation and degeneration of the aorta. 7 The current findings of Uchida et al 1 completely contradict the historical findings implicating uPA in the formation and progression of AAA.…”

“…7 The formation and rupture of these aneurysms was prevented by treatment with transexamic acid, an inhibitor of plasmin generation, suggesting that increased plasmin activity in the absence of procarboxypeptidase B has a role in inducing inflammation and degeneration of the aorta. 7 The current findings of Uchida et al 1 completely contradict the historical findings implicating uPA in the formation and progression of AAA. The original focus of the article was not uPA itself but its cognate receptor uPAR and the role of uPAR in AAA formation and progression.…”