title: Cathepsin G and thrombin: evidence for two different platelet receptors.
abstract: Neutrophil cathepsin G and thrombin, the only platelet agonists that are proteases, exhibit a mandatory requirement for catalytic activity to induce platelet aggregation and signal transduction. The thrombin receptor is a G-protein-coupled receptor which undergoes proteolysis to generate a tethered ligand that causes self-activation. Since cathepsin G strongly resembles thrombin in its ability to activate platelets, we have attempted to determine whether cathepsin G and thrombin function through the same or different receptors. Evidence that thrombin and cathepsin G act at different receptors was as follows: (a) an antibody directed against the thrombin receptor blocked thrombin-induced but not cathepsin G-induced platelet responses; (b) human fibroblasts responded to thrombin and to a synthetic thrombin receptor peptide (comprising residues 42-55 of the thrombin receptor) by exhibiting an elevation in cytosolic Ca2+ concentration but did not respond to cathepsin G; and (c) platelets pretreated with neutrophil elastase failed to respond to thrombin but responded when rechallenged by cathepsin G. Thrombin and cathepsin G exhibit heterologous desensitization that is potentiated by okadaic acid and is attenuated by staurosporine, indicating that phosphorylation of serine/threonine residues is important for desensitization and that protein kinase C may be involved. Since catalytic activity of cathepsin G is required for platelet stimulation, it is probable that platelet activation by cathepsin G requires receptor proteolysis and that a tethered ligand mechanism is involved, suggesting that platelets may possess a family of protease receptors.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of New Hampshire, Durham 03824.
authors.forename: M A
authors.initials: MA
authors.lastname: Selak
mesh_terms: D000595:Amino Acid Sequence; D001792:Blood Platelets / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D056649:Cathepsin G; D002403:Cathepsins / Q000378:metabolism*; D015536:Down-Regulation; D006801:Humans; D066298:In Vitro Techniques; D019272:Leukocyte Elastase / Q000378:metabolism; D008969:Molecular Sequence Data; D010196:Pancreatic Elastase / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D015539:Platelet Activation; D011956:Receptors, Cell Surface / Q000378:metabolism*; D018179:Receptors, Thrombin / Q000378:metabolism*; D012697:Serine Endopeptidases; D015398:Signal Transduction; D013917:Thrombin / Q000378:metabolism*
chemical_list: D010750:Phosphoproteins; D011956:Receptors, Cell Surface; D018179:Receptors, Thrombin; D002403:Cathepsins; D012697:Serine Endopeptidases; C534094:CTSG protein, human; D056649:Cathepsin G; D010196:Pancreatic Elastase; D019272:Leukocyte Elastase; D013917:Thrombin; D002118:Calcium