TYROSINE PHOSPHORYLATION AND RELOCATION OF SHIP ARE INTEGRIN-MEDIATEDIN THROMBIN-STIMULATED HUMAN BLOOD-PLATELETS

The SH2 domain-containing inositol 5-phosphatase, SHIP, known to depho sphorylate inositol 1,3,4,5-tetrakisphosphate and phosphatidylinositol 3,4,5-trisphosphate has recently been shown to be expressed in a vari ety of hemopoietic cells. This 145-kDa protein is induced to associate with Shc by multiple cytokines and may play an important role in the negative regulation of immunocompetent cells mediated by Fc gamma RIIB receptor. We report here that SHIP is present in human blood platelet s and may be involved in platelet activation evoked by thrombin. Plate let SHIP was identified by Western blotting as a single 145-kDa protei n, Both phosphatidylinositol 3,4,5-trisphosphate and inositol 1,3,4,5- tetrakisphosphate 5-phosphatase activities could be demonstrated in an ti-SHIP immunoprecipitates of platelet lysate. Thrombin stimulation in duced a tyrosine phosphorylation of SHIP, this effect being pre vented if platelets were not shaken or if RGD-containing peptides were prese nt, indicating an aggregation-dependent, integrin-mediated event. More over, although the intrinsic phosphatase activity of SHIP did not appe ar to be significantly increased, tyrosine-phosphorylated SHIP was rel ocated to the actin cytoskeleton upon activation in an aggregation-and integrin engagement-dependent manner. Finally, the striking correlati on observed between phosphatidylinositol 3,4-bisphosphate production a nd the tyrosine phosphorylation of SHIP, as well as its relocation to the cytoskeleton upon thrombin stimulation, suggest a role for SHIP in the aggregation-dependent and GpIIb-IIIa-mediated accumulation of thi s important phosphoinositide.