The interaction of factor XIa with activated platelets but not endothelial cells promotes the activation of factor IX in the consolidation phase of blood coagulation.
We have previously shown that the zymogen factor XI (FXI) binds to activated platelets but not to human umbilical vein endothelial cells (HUVEC), a conclusion that is in conflict with previous reports stating that FXI binds to 2.7-13 x 10(6) high affinity sites per HUVEC (Berrettini, M., Schleef, R. R., Heeb, M. J., Hopmeier, P., and Griffin, J. H. (1992) J. Biol. Chem. 267, 19833-19839; Shariat-Madar, Z., Mahdi, F., and Schmaier, A. H. (2001) Thromb. Haemostasis 85, 544-551). It has also been reported that activated FXI (FXIa) binds to 1.5 x 10(6) sites per HUVEC and promotes the activation of factor IX by cell bound FXIa (Berrettini, M., Schleef, R. R., Heeb, M. J., Hopmeier, P., and Griffin, J. H. (1992) J. Biol. Chem. 267, 19833-19839). Therefore, the binding of FXIa to activated platelets was compared with FXIa binding to HUVEC and HEK293 cells immobilized on microcarrier beads. Specific and saturable zinc-dependent FXIa binding was demonstrated to 250 +/- 48 sites per activated platelet (K(D) = 1.7 +/- 0.78 nm) and 6.5 +/- 0.4 x 10(4) sites per HUVEC (K(D) = 2.4 +/- 0.5 nm), whereas no binding to HEK293 cells was detected. A titration with high molecular weight kininogen had no effect on FXIa binding to platelets, but revealed a concentration-dependent decrease in the amount of FXIa bound to HUVEC. The rate of factor IXa generation catalyzed by FXIa was unaffected by the presence of surfaces; however only the activated platelet surface protected FXIa from inhibition by protease nexin 2. The results presented here confirm the conclusion that activated platelets are procoagulant while unstimulated endothelial cells are not.
Baird
T Regan
TR
The Sol Sherry Thrombosis Research Center, Department of Biochemistry, Temple University School of Medicine, Philadelphia, Pennsylvania 19140, USA.
Walsh
Peter N
PN
D016564:Amyloid beta-Protein Precursor; D001665:Binding Sites; D001777:Blood Coagulation / Q000502:physiology*; D001792:Blood Platelets / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism; D002478:Cells, Cultured; D004730:Endothelium, Vascular / Q000166:cytology / Q000378:metabolism*; D005164:Factor IX / Q000378:metabolism*; D015945:Factor XIa / Q000378:metabolism*; D006801:Humans; D015849:Plasminogen Inactivators / Q000378:metabolism; D015539:Platelet Activation; D058255:Protease Nexins; D011956:Receptors, Cell Surface
C545487:APP protein, human; D016564:Amyloid beta-Protein Precursor; D002352:Carrier Proteins; D015849:Plasminogen Inactivators; D058255:Protease Nexins; D011956:Receptors, Cell Surface; D005164:Factor IX; D015945:Factor XIa