We observed that platelets, which mediate thrombus formation, also enhance fibrinolysis by single-chain urokinase (scuPA). Preliminary data suggested that this enhancement was due to platelet thrombospondin (TSP), which, depending upon the specifics of the environment, changed conformation, influencing its role in the fibrinolytic system. Results showed that the activity of scuPA was enhanced by platelets, regardless of platelet treatment or protein release, and that TSP could not explain the platelet effect. Investigation of the underlying mechanism, using the non-cleavable mutant scuPA (K158E) and protease inhibitors, showed that the platelet-dependent enhancement of scuPA arose from the activation to uPA by a serine protease. Factor VII activating protease (FSAP) was not the protease responsible, since inhibition of platelet FSAP with function-blocking antibodies did not inhibit the platelet effect. Comparison of plasminogen and plasma kallikrein, using an array of inhibitors, showed that both candidates mirrored the platelet effect. Further results, including those from assessment of protease activity on platelets against chromogenic substrates and from the evaluation of uPA formation over time, were consistent with the involvement of plasminogen. In addition, experiments with platelets from plasminogen-deficient mice showed that platelets lacking plasminogen no longer activated scuPA. The enhancement of scuPA was found to be platelet-dependent, even in plasma; scuPA activation was more efficient when plasminogen was associated with the platelet membrane, compared to in solution; and the presence of membranes was essential to induce rapid lysis by scuPA. Our findings indicate that platelets stimulate fibrinolysis by scuPA via local activation to uPA by platelet-associated plasminogen, which is activated by scuPA, consistent with a system of reciprocal activation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:499298 |
| Date | January 2009 |
| Creators | Baeten, Kim Marieke |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=25495 |
Page generated in 0.0026 seconds