Demonstration of a functionally active tPA-like plasminogen activator in human platelets.
The mechanism of platelet-enhanced fibrinolysis is unclear. We therefore investigated the fibrinolytic activity of human platelets and demonstrated that they contain a tissue plasminogen activator (tPA)-like plasminogen activator, abbreviated as tPA-like-PA. This activator was detected by ELISA in platelet incubation medium and in platelet Triton extracts. Plasminogen activation assays showed that this tPA-like-PA could induce plasminogen activation to form plasmin. Western blots of Triton extracts incubated with anti-tPA antibody demonstrated a major 64-kD protein band, compared to a 70-kD band for standard single chain tPA, plus a minor 118-kD band corresponding to a complex of tPA-like-PA and plasminogen activator inhibitor (PAI-1). Western blots of Triton extracts incubated with anti-PAI-1 antibody produced an approximately similar high-molecular-weight (118 kD) protein band. Fibrin zymographic analysis of affinity-purified tPA-like-PA demonstrated a major and a minor fibrin lysis zone, which approximately corresponded to the tPA-like-PA and its complex with PAI-1 observed by Western blots. Immunogold labelling and electron microscopy demonstrated that platelet activator, either as the free form or co-localized with PAI-1, was present in granules and in channels of the open canalicular system. We conclude that platelets contain a functionally active tPA-like-PA, whose low fibrinolytic activity might be due to its readily forming a complex with PAI-1. This functionally active tPA-like-PA might contribute to the enhanced fibrinolytic activity of platelets observed in platelet-rich thrombi.
Wang
D L
DL
Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan, ROC.
Pan
Y T
YT
Wang
J J
JJ
Cheng
C H
CH
Liu
C Y
CY
D001792:Blood Platelets / Q000737:chemistry* / Q000648:ultrastructure; D017077:Culture Media, Conditioned; D004797:Enzyme-Linked Immunosorbent Assay; D006801:Humans; D016253:Microscopy, Immunoelectron; D008970:Molecular Weight; D017395:Plasminogen Activator Inhibitor 1 / Q000097:blood; D010960:Plasminogen Activators / Q000097:blood* / Q000302:isolation & purification
D017077:Culture Media, Conditioned; D017395:Plasminogen Activator Inhibitor 1; D010960:Plasminogen Activators