Objective— Besides having a key role in fibrinolysis, the plasminogen system has been implicated in cell migration and angiogenesis. A common mechanism is the binding of plasminogen to carboxy-terminal lysine residues in partially degraded fibrin or on cellular surfaces. Here we examined the involvement of thrombin activatable fibrinolysis inhibitor (TAFI) and pancreatic carboxypeptidase B (CPB) in an in vitro capillary tube formation system, which is largely plasminogen-dependent.

Methods and Results— Human microvascular endothelial cells (hMVECs) were seeded on a 3D plasma clot matrix and subsequently stimulated with bFGF/tumor necrosis factor (TNF)-α. Tube formation was analyzed and fibrin degradation products (FbDP) were determined in the medium. Supplementation of the matrix with additional TAFI or CPB produced a reduction in tube formation. Pretreatment of hMVECs with CPB before seeding resulted in a similar effect. FbDP-levels indicated a concomitant reduction in matrix proteolysis. A TAFIa inhibitor increased tube formation and FbDP release into the medium. In separate assays, CPB impaired the migration of hMVECs in a dose-dependent manner, whereas proliferation and adhesion remained unaffected.

Conclusions— Overall, these results demonstrate that TAFI and CPB in these systems modulate the plasminogen system both in the matrix and on the cell surface, thus leading to the inhibition of endothelial cell movement and tube formation.