Thrombopoietin Modulates Platelet Activation in Vitro through Protein‐Tyrosine Phosphorylation

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.