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Thrombopoietin Modulates Platelet Activation in Vitro through Protein-Tyrosine Phosphorylation

^a Department of Clinical Laboratory Medicine and
^b First Department of Internal Medicine, Kagawa Medical School, Kagawa, Japan;
^c Kagawa Medical School, Kagawa, Japan

Key Words. Thrombopoietin • Platelet aggregation • ADP • Genistein • Tyrosine phosphorylation • c-Mpl • Cytokine receptor

Dr. Yoshitsugu Kubota, Department of Clinical Laboratory Medicine, Kagawa Medical School, 1750-1, Miki-cho, Kita-gun, Kagawa, 761-07, Japan.

To determine the roles of thrombopoietin (TPO) in platelet function in vitro, we examined the effects of TPO on platelet aggregation. Although several proteins in platelets were tyrosine-phosphorylated by TPO treatment, TPO alone was unable to induce platelet aggregation. However, the secondary wave of platelet aggregation induced by adenosine diphosphate (ADP) was enhanced by TPO in a dose-dependent manner. TPO in conjunction with ADP augmented tyrosine phosphorylation of platelet proteins, including tyrosine-phosphorylated proteins induced by TPO alone. Genistein inhibited protein-tyrosine phosphorylation in platelets induced by TPO with ADP and suppressed TPOenhanced platelet aggregation. Moreover, tyrosine phosphorylation of MAP-kinases induced by TPO alone and TPO with ADP was consistent with TPO-enhanced platelet aggregation. These findings in the present study suggest that signal transduction involved in TPO-enhanced platelet aggregation is mediated in part by tyrosine-phosphorylated proteins, including MAP-kinases, in platelets through TPO-stimulated c-Mpl, TPO receptor.

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