Endothelial protease-activated receptor-2 induces tissue factor expression and von Willebrand factor release

A second protease-activated receptor (PAR-2) that could be activated by trypsin or more physiologically by mast cell tryptase has been recently cloned. Both the structure and activation mechanism of PAR-2 was similar to the functional thrombin receptor (PAR-1). Although many effects of the coagulation protease thrombin on the vascular endothelium could be attributed to PAR-1 activation, very little is known about the physiological and pathophysiological role of PAR-2. We investigated whether stimulation of PAR-2 on endothelial cells induced two cellular responses that play a central role in primary and secondary haemostasis: the release of high molecular weight von Willebrand factor (hmw-VWF) from Weibel-Palade bodies and the de novo synthesis of tissue factor (TF) mRNA and protein. Human umbilical vein endothelial cells (HUVEC) were incubated with agonists for PAR-2 at 37 degrees C. Both trypsin and SLIGKV increased TF mRNA and activity and induced the release of hmw-VWF due to elevated levels of cytosolic Ca2+. Trypsin (10 nm) induced a 6-fold increase of TF mRNA and reduced time until fibrin clot formation to 37%, indicating trebling of the cell surface located TF activity. Stimulation of HUVEC with the PAR-2 agonist peptide SLIGKV induced a dose-dependent increase of TF mRNA up to 6 times and TF activity up to 3 times. Release of hmw-VWF was achieved both after incubation of HUVEC with trypsin and SLIGKV and was directly depending on intracellular Ca2+ mobilization. To make results comparable to the functional thrombin receptor, homologous experiments were carried out using the PAR-1 agonists thrombin and SFLLRN.