THROMBIN PROMOTES ANGIOGENESIS BY A MECHANISM INDEPENDENT OF FIBRIN FORMATION

The role of thrombin in angiogenesis was investigated in the chick cho rioallantoic membrane (CAM) system. Alpha-thrombin promoted angiogenes is in a dose-dependent fashion and at 8.4 pmol/disk reached a maximum of 78% above the control. At a higher dose of alpha-thrombin (25 pmol/ disk) the angiogenic effect declines and this can be explained by dese nsitization of the thrombin receptor. The promotion of angiogenesis by alpha-thrombin is specific as evidenced by the reversal of this effec t by hirudin, which binds both the catalytic and the anion-binding exo site of thrombin or by heparin, which binds thrombin and accelerates i ts inactivation by antithrombin III. Gamma-thrombin, which is catalyti cally active but lacks the anion-binding exosite required for clotting activity, promotes angiogenesis in the CAM in the same fashion and to the same extent as alpha-thrombin, at doses up to 130 pmol/disk. Phen ylalanyl-propyl-arginine chloromethyl ketone (P-PACK)-thrombin, the ca talytically inactive analogue of alpha-thrombin that retains the anion -binding exosite, had no significant effect on angiogenesis in the CAM . When combined with alpha-thrombin, P-PACK-thrombin abolished the ang iogenesis-promoting effect of alpha-thrombin. These results suggest th at alpha-thrombin can promote angiogenesis in the CAM through interact ion with its catalytic site without the requirement for fibrin formati on.