Submitted June 26, 2006
Accepted March 1, 2007
Human platelets synthesize and express functional tissue factor
Olga Panes, Valeria Matus, Claudia G. Saez, Teresa Quiroga, Jaime Pereira, and Diego Mezzano*
Department of Hematology-Oncology, School of Medicine, Pontificia Universidad Catolica de Chile, Santiago, Chile
Department of Clinical Laboratory, School of Medicine, Pontificia Universidad Catolica de Chile, Santiago, Chile
* Corresponding author; email: dmezzano{at}med.puc.cl.
The source and significance of blood-borne tissue factor (TF) are controversial. TF-mRNA, protein and TF-dependent procoagulant activity (PCA) have been detected in human platelets, but direct evidence of TF synthesis is missing. Non-stimulated, monocyte-free platelets from most individuals expressed TF-mRNA, which was enhanced or induced in all of them after platelet activation. Immunoprecipitation assays revealed TF protein (mainly of Mr ~47kDa with other bands of ~35 and ~60kDa) in non-stimulated platelet membranes, which also increased post-activation. This enhancement was concomitant with TF translocation to the plasma membrane, as demonstrated by immunofluorescence-confocal microscopy and biotinylation of membrane proteins. Platelet PCA, assessed by FXa generation, was induced post-activation and inhibited by 48% and 76% with anti-TF and anti-FVIIa, respectively, but not by intrinsic pathway inhibitors. Platelets incorporated [35S]-methionine into TF proteins of Mr ~47, ~35, and ~60kDa, more intensely after activation. Puromycin, but not actinomycin D or DRB (5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole) inhibited TF neosynthesis. Thus, human platelets not only assemble the clotting reactions on their membrane but also supply their own TF for thrombin generation in a timely and spatially circumscribed process. These observations simplify, unify and provide a more coherent formulation of the current cell-based model of hemostasis.
