Thrombopoietin stimulates cortactin translocation to the cytoskeleton independently of tyrosine phosphorylation

Cortactin is an F-actin-binding protein expressed in platelets. During aggr egation by thrombin, cortactin associates with Src, is tyrosine phosphoryla ted, and then translocates to the cytoskeleton. It is also found to associa te with Syk during platelet shape change. Since cortactin undergoes tyrosin e phosphorylation in platelets activated by thrombopoietin (TPO) that exhib it neither shape change nor aggregation, we investigated whether it could a lso relocalize to the detergent-insoluble fraction. We demonstrate that cor tactin was present as a tyrosine-phosphorylated protein and co-localized wi th Syk in the Triton X-100-insoluble fraction of TPO-activated platelets. T PO stimulated Syk activation and association with cortactin. Conversely, co rtactin associated with the kinases, Syk and Src, Cortactin tyrosine phosph orylation was blocked by Syk kinase inhibitor, piceatannol or Src family ki nase inhibitor, PP2, suggesting that it depends on these two kinases. Howev er, piceatannol or PP2 did not prevent cortactin translocation to the deter gent-insoluble fraction. These data suggest that tyrosine phosphorylation i s not required for cortactin translocation to the detergent-insoluble compa rtment. Furthermore, TPO activates, through its receptor c-Mp1, a signallin g pathway to the cytoskeleton.