The role of thrombin receptors PAR1 and PAR4 for PAI-1 storage, synthesis and secretion by human platelets

Thromb Res. 2012 Apr;129(4):e51-8. doi: 10.1016/j.thromres.2011.12.021. Epub 2012 Jan 26.


Introduction: Arterial thrombi contain more platelets than venous thrombi and are more resistant to fibrinolysis. This resistance could partly be due to plasminogen activator inhibitor 1 (PAI-1) secreted by platelets. The aim of this study was to elucidate differences between thrombin receptors protease-activated receptor (PAR) 1 and 4 and platelet storage, secretion and synthesis of platelet PAI-1, as compared to other platelet α-granule proteins such as VEGF and endostatin.

Materials and methods: Human isolated platelets were incubated with thrombin (0.5 U/ml), PAR1-activating peptide (AP) (0.4-30 μM) or PAR4-AP (1.5-300 μM) for up to 24 hours. ELISA, western blot and fluorescence microscopy were used to measure secretion, contents and localization of PAI-1, VEGF and endostatin.

Results: Our results show that PAI-1 and VEGF might be co-localized and that endostatin does not co-localize with either PAI-1 or VEGF. PAI-1 and VEGF show a similar secretion pattern, being more sensitive to low grade PAR1 activation, but secretion was also observed with higher concentrations of PAR4-APs. PAI-1 is secreted in an active form. PAI-1 mRNA was found in platelets, and elevated levels of PAI-1 were detected after 24 hours incubation of platelets.

Conclusions: PAI-1 and VEGF, but not endostatin, might be stored in the same α-granule in human platelets. PAI-1 and VEGF also show a similar secretion pattern, being more sensitive to PAR1 than to PAR4 activation, but the secretion is not exclusively selective. Our results also show that platelet PAI-1 is increased if incubated for 24 hours, both with addition of PAR1-activating peptide and without activation, which could indicate de novo synthesis.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Blood Platelets / metabolism*
  • Cells, Cultured
  • Endostatins / metabolism*
  • Humans
  • Plasminogen Activator Inhibitor 1 / metabolism*
  • Receptor, PAR-1 / metabolism*
  • Receptors, Thrombin / metabolism*
  • Thrombin / pharmacokinetics*


  • Endostatins
  • Plasminogen Activator Inhibitor 1
  • Receptor, PAR-1
  • Receptors, Thrombin
  • SERPINE1 protein, human
  • Thrombin
  • protease-activated receptor 4