Cathepsin G activates protease-activated receptor-4 in human platelets

Of the four known protease-activated receptors (PARs), PARI and PARE are ex pressed by human platelets and mediate thrombin signaling. Whether these re ceptors are redundant, interact, or play at least partially distinct roles is unknown. It is possible that PAR1 and/or PAR4 might confer responsivenes s to proteases other than thrombin. The neutrophil granule protease, cathep sin G, is known to cause platelet secretion and aggregation. We now report that this action of cathepsin G is mediated by PAR4. Cathepsin G triggered calcium mobilization in PAR4-transfected fibroblasts, PAR4-expressing Xenop us oocytes, and washed human platelets. An antibody raised against the PAR4 thrombin cleavage site blocked platelet activation by cathepsin G but not other agonists, Desensitization with a PAR4 activating peptide had a simila r effect. By contrast, inhibition of PAR1 function had no effect on platele t responses to cathepsin G. When neutrophils were present, the neutrophil a gonist fMet-Leu-Phe triggered calcium signaling in Fura-2-loaded platelets. Strikingly, this neutrophil-dependent platelet activation was blocked by t he PAR4 antibody. These data show that PARI mediates platelet responses to cathepsin G and support the hypothesis that cathepsin G might mediate neutr ophil-platelet interactions at sites of vascular injury or inflammation.