THROMBOPOIETIN INDUCES TYROSINE PHOSPHORYLATION AND ACTIVATION OF THEJANUS KINASE, JAK2

Thrombopoietin (TPO) is a recently characterized growth and differenti ation factor for megakaryocytes and platelets that exerts its effects via the receptor, c-Mpl. This receptor is a member of the hematopoieti n receptor superfamily and is essential for megakaryocyte maturation; however, the molecular mechanisms of TPO and c-Mpl action have not bee n elucidated. Recently, the Janus kinases have emerged as important el ements in signaling via this family of receptors. In this report, we s how that, in the M07e megakaryocytic cell line, which expresses c-Mpl and proliferates in response to TPO, TPO induces phosphorylation of a number of substrates between 80 and 140 kD. Specifically, we show that stimulation with TPO induces the rapid tyrosine phosphorylation of a 130-kD protein that we identity as the Janus kinase, JAK2. However, no detectable tyrosine phosphorylation of JAK1, JAK3, or TYK2 was observ ed. TPO also induced activation of JAK2 phosphotransferase activity in vitro. Taken together, these data indicate that JAK2 likely plays a k ey role in TPO-mediated signal transduction.