Platelets and Thrombopoiesis

Protein kinase C(PKC) is expressed in platelets and activated downstream of protease-activated receptors (PARs) and glycoprotein VI (GPVI) receptors. We have previously shown that PKCpositively regulates PAR-mediated dense granule secretion, whereas it negatively regulates GPVI-mediated dense granule secretion. We further investigated the mechanism of such differential regulation of dense granule release by PKCin platelets. SH2 domain–containing inositol phosphatase1 (SHIP-1) is phosphorylated on Y1020, a marker for its activation, upon stimulation of human platelets with PAR agonists SFLLRN and AYPGKF or GPVI agonist convulxin. GPVI-mediated SHIP-1 phosphorylation occurred rapidly at 15 seconds, whereas PAR-mediated phosphorylation was delayed, occurring at 1 minute. Lyn and SHIP-1, but not SHIP-2 or Shc, preferentially associated with PKCon stimulation of platelets with a GPVI agonist, but not with a PAR agonist. In PKC– null murine platelets, convulxin-induced SHIP-1 phosphorylation was inhibited. Furthermore, in Lyn null murine platelets, GPVI-mediated phosphorylations on Y-1020 of SHIP-1 and Y311 of PKCwere inhibited. In murine platelets lacking Lyn or SHIP-1, GPVI-mediated dense granule secretions are potentiated, whereas PARmediated dense granule secretions are inhibited. Therefore, we conclude that Lyn-mediated phosphorylations of PKCand SHIP-1 and their associations negatively regulate GPVI-mediated dense granule secretion in platelets. (Blood. 2009; 114:3056-3063)