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

Abstract

IntroductionArterial 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 methodsHuman isolated platelets were incubated with thrombin (0.5U/ml), PAR1-activating peptide (AP) (0.4-30μM) or PAR4-AP (1.5-300μM) for up to 24hours. ELISA, western blot and fluorescence microscopy were used to measure secretion, contents and localization of PAI-1, VEGF and endostatin. ResultsOur 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 24hours incubation of platelets. ConclusionsPAI-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 24hours, both with addition of PAR1-activating peptide and without activation, which could indicate de novo synthesis.