THROMBIN STIMULATION OF PLATELETS INDUCES PLASMINOGEN ACTIVATION MEDIATED BY ENDOGENOUS UROKINASE-TYPE PLASMINOGEN-ACTIVATOR

Gene knockout mice studies indicate that urokinase-type plasminogen ac tivator (U-PA) is importantly involved in fibrinolysis, but its physio logic mechanism of action remains poorly understood. We postulated tha t platelets may be involved in this mechanism, as they carry a novel r eceptor for u-PA and a portion of the single-chain u-PA (scu-PA) intri nsic to blood is tightly associated with platelets. Therefore, plasmin ogen activation by platelet-associated u-PA was studied. When washed p latelets were incubated with plasminogen, no plasmin was generated as detected by plasmin synthetic substrate (S2403) hydrolysis; however, a fter the addition of thrombin, but not other agonists, platelet-depend ent plasminogen activation occurred. Plasminogen activation was surfac e-related, being inhibited by blocking platelet fibrinogen receptors o r by preventing plasminogen binding to the thrombin-activated platelet surface. U-PA was identified as the only plasminogen activator respon sible and enrichment of platelets with exogenous scu-PA significantly augmented plasminogen activation. These findings appeared paradoxical because thrombin inactivates scu-PA. Indeed, zymograms showed inactiva tion of sera-PA during the first hour of incubation with even the lowe st dose of thrombin used (1 u/mL). However, this was followed by a thr ombin dose-dependent (1 to 10 u/mL) partial return of u-PA activity. R eactivation of u-PA was not due to the direct action of thrombin, but required platelets and was found to be related to a platelet lysosomal thiol protease, consistent with cathepsin C. In conclusion, a new pat hway of plasminogen activation by platelet-associated endogenous or ex ogenous scu-PA was demonstrated, which is specifically triggered by th rombin activation of platelets. These findings may help explain uPA-me diated physiological fibrinolysis and have implications for therapeuti c thrombolysis with scu-PA. (C) 1997 by The American Society of Hemato logy.