Thrombopoietin induces activation of the phosphatidylinositol-3′ kinase pathway and formation of a complex containing p85PI3K and the protooncoprotein p120CBL

Thrombopoietin (TPO) promotes megakaryocyte growth and development. Its receptor, c-MPL, is restricted to cells of megakaryocytic lineage and stem cells. We have previously shown that activation of c-MPL by thrombopoietin rapidly activates at least two cytoplasmic tyrosine kinases, JAK2 and TYK2, after ligand binding. Phosphatidylinositol-3′ kinase (PI3K) has been shown to play an important role in downstream signaling for many receptors. Thrombopoietin was found to also rapidly activate phosphatidylinositol-3′ kinase, and the phosphatidylinositol-3′ kinase inhibitor wortmannin decreased proliferation of thrombopoietin-stimulated cells, implying that phosphatidylinositol-3′ kinase may have a regulatory role in thrombopoietin signaling. In immunoprecipitation studies, the regulatory subunit of phosphatidylinositol-3′ kinase, p85^PI3K, associated with several tyrosine phosphoproteins, and the major phosphoprotein was a 120 kDa protein identified as p120^CBL. The phosphatidylinositol-3′ kinase-enzyme activity in p120^CBL immunoprecipitates was elevated in thrombopoietin-stimulated cells as compared to immunoprecipitates from unstimulated cells. p120^CBL may be involved in signaling pathways activated by c-MPL which involve phosphatidylinositol-3′ kinase. J. Cell. Physiol. 171:28–33, 1997. © 1997 Wiley-Liss, Inc.