TAFIa is a Novel Inhibitor of Tumour Angiogenesis in Breast Cancer

Thrombomodulin (TM) is known mostly for its role in the regulation of hemostasis, but it has also been shown to play an antimetastatic role in cancer. A previous study showed that the anitmetastatic effects of TM were dependent on thrombin binding. However, it is not known which substrate of thrombin-TM, protein C or TAFI, is responsible for these antimetastatic effects. We hypothesized that TAFI is responsible for the antimetastatic effects of TM, specifically tumour angiogenesis, due to its role in the inhibition of pericellular plasminogen activation. We determined the effect of TAFIa on tumour angiogenesis through inhibition of TAFIa and manipulation of cofactor ability of TM. We found inhibition of TAFIa resulted in increased proliferation, invasion, tube formation, and ECM proteolysis. We also found an inhibitory effect of TAFI activation by thrombin-TM on tube formation and ECM proteolysis. Our results suggest that TAFIa inhibits tumour angiogenesis in breast cancer cells

Activated thrombin-activatable fibrinolysis inhibitor attenuates the angiogenic potential of endothelial cells: potential relevance to the breast tumour microenvironment

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a basic carboxypeptidase zymogen present in blood plasma. Proteolytic activation of TAFI by thrombin, thrombin in complex with the endothelial cell cofactor thrombomodulin, or plasmin results in an enzyme (TAFIa) that removes carboxyl-terminal lysine residues from protein and peptide substrates, including cell-surface plasminogen receptors. TAFIa is therefore capable of inhibiting plasminogen activation in the pericellular milieu. Since plasminogen activation has been linked to angiogenesis, TAFIa could therefore have anti-angiogenic properties, and indeed TAFIa has been shown to inhibit endothelial tube formation in a fibrin matrix. In this study, the TAFI pathway was manipulated by providing exogenous TAFI or TAFIa or by adding a potent and specific inhibitor of TAFIa. We found that TAFIa elicited a series of anti-angiogenic responses by endothelial cells, including decreased endothelial cell proliferation, cell invasion, cell migration, tube formation, and collagen degradation. Moreover, TAFIa decreased tube formation and proteolysis in endothelial cell culture grown alone and in co-culture with breast cancer cell lines. In accordance with these findings, inhibition of TAFIa increased secretion of matrix metalloprotease proenzymes by endothelial and breast cancer cells. Finally, treatment of endothelial cells with TAFIa significantly inhibited plasminogen activation. Taken together our results suggest a novel role for TAFI in inhibiting tumour angiogenic behaviors in breast cancer