title: Platelet activation induces metalloproteinase-dependent GP VI cleavage to down-regulate platelet reactivity to collagen.
abstract: Glycoprotein (GP) VI, the primary collagen receptor on platelets, has been shown to have variable expression, possibly as a consequence of immune modulation. The present study was designed to determine the mechanism by which GP VI clearance occurs. We found that direct activation of GP VI both by a GP VI-specific antibody and by GP VI ligands (collagen and convulxin) reduced binding of biotinylated convulxin to the stimulated platelets. Analysis of immunoblots of platelets and supernatants showed that the stimulated platelets contained less GP VI, while the soluble fraction contained a 57-kDa cleavage product. Stimulation of platelets with PAR-1 agonists (TRAP peptide and thrombin) also caused GP VI cleavage, although the amount of GP VI loss was less than that observed with direct GP VI ligands. The metalloproteinase (MMP) inhibitors GM6001 and TAPI prevented both the clearance of GP VI from the platelet surface and the appearance of the soluble cleavage product. Induction of GP VI cleavage caused specific down-regulation of collagen-induced platelet aggregation, providing a mechanism for the modulation of platelet responsiveness to this important platelet agonist.
authors.affiliation: Portola Pharmaceuticals Inc, San Francisco, CA 94080, USA.
authors.forename: Gillian
authors.initials: G
authors.lastname: Stephens
authors.forename: Yibing
authors.initials: Y
authors.lastname: Yan
authors.forename: Martine
authors.initials: M
authors.lastname: Jandrot-Perrus
authors.forename: Jean-Luc
authors.initials: JL
authors.lastname: Villeval
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Clemetson
authors.forename: David R
authors.initials: DR
authors.lastname: Phillips
mesh_terms: D000906:Antibodies / Q000276:immunology; D001792:Blood Platelets / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D003094:Collagen / Q000494:pharmacology*; D015536:Down-Regulation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D008666:Metalloendopeptidases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010446:Peptide Fragments / Q000378:metabolism; D015539:Platelet Activation / Q000187:drug effects*; D010980:Platelet Membrane Glycoproteins / Q000276:immunology / Q000378:metabolism*
chemical_list: D000906:Antibodies; D004791:Enzyme Inhibitors; D010446:Peptide Fragments; D010980:Platelet Membrane Glycoproteins; C098341:platelet membrane glycoprotein VI; D003094:Collagen; D008666:Metalloendopeptidases