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

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 alpha-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 mu M) or PAR4-AP (1.5-300 mu M) 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 alpha-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. (C) 2012 Elsevier Ltd. All rights reserved.