ABSTRACT
The Janus family of tyrosine kinases (JAKs) plays a critical role in signal transduction by members of the cytokine receptor superfamily. In response to ligand-receptor interaction, these nonreceptor tyrosine kinases are rapidly phosphorylated and activated, triggering tyrosine phosphorylation and activation of downstream signaling intermediates. Upon binding to its receptor, the product of the proto-oncogene c-mpl, thrombopoietin (TPO) activates both JAK2 and TYK2 in multiple cell lines as well as megakaryocytes and platelets. To study whether one or both of these kinases are essential for TPO signal transduction, we engineered a parental human sarcoma cell line (2C4) as well as sarcoma cell lines that are deficient in JAK2 expression (gamma2A) or TYK2 expression (U1A) to express the wild-type Mpl receptor. The ability of TPO to induce tyrosine phosphorylation of Mpl and multiple intracellular substrates in each cell line was then examined. Our results demonstrate that JAK2-deficient cells (gamma2A-Mpl) are unable to initiate TPO-mediated signaling. In contrast, cells that are TYK2-deficient (U1A-Mpl) are able to induce tyrosine phosphorylation of Mpl, JAK2, STAT3, and Shc as efficiently as parental cells (2C4-Mpl). These data indicate that JAK2 is an essential component of Mpl signaling and that, in the absence of JAK2, TYK2 is incapable of initiating TPO-induced tyrosine phosphorylation.
