TPO and IL-3 induce overlapping but distinct protein tyrosine phosphorylation in a myeloid precursor cell line

TPO is a recently cloned cytokine which appears to play a central role in megakaryopoiesis and platelet production. It has been shown to be the ligand for c-mpl, a member of the hematopoietic receptor superfamily. We examined intracellular tyrosine phosphorylation events induced by TPO and compared them with events stimulated by IL-3, a pleiotropic cytokine which also has activity on the megakaryocyte lineage. The overall pattern of tyrosine phosphorylation stimulated by TPO and IL-3 in myeloid precursor cells revealed an overlapping but not identical pattern reflecting their distinct but partially redundant biological effects. We identify Shc and Vav as intracellular targets of TPO-induced tyrosine phosphorylation. Moreover, we demonstrate that the tyrosine kinase Jak2 is phosphorylated after TPO stimulation. Whereas phosphorylation of these proteins was induced by both cytokines, phosphorylation of Jak1 was induced only by IL-3 and not by TPO, distinguishing the signal transduction of the two cytokines on a molecular level.