Platelet inhibition by hypochlorous acid involves cAMP signalling

Hypochlorous acid (HOCl), made by neutrophil-derived myeloperoxidase, has been suggested to inhibit platelets, however, the mechanisms involved have not been described. Here we confirm that HOCl exposure changes platelet morphology and inhibits platelet spreading and aggregation. HOCl effects could be reversed by glutathione suggesting a role for cysteine oxidation. Mass spectrometry-based proteomics of HOCl-exposed platelets revealed oxidised cysteine residues in 37 proteins including adenylate cyclase 6 and Rap1B. Adenylate cyclase is involved in the inhibitory cAMP pathway triggered by endothelium-derived prostacyclin and Rap1 is a small G protein required for integrin αIIbβ3 activation and platelet aggregation. We show that HOCl exposure stimulates cAMP production and phosphorylation of the cAMP-dependent protein kinase substrate VASP in platelets and transfected HEK293T cells. In addition, HOCl inhibited Rap1-GTP formation. These data suggest that HOCl inhibits platelets at least in part through the cAMP pathway and by regulating Rap1. Thus, this study provides new insights into HOCl mediated crosstalk between neutrophils and platelets.