C-MPL LIGAND IS A HUMORAL REGULATOR OF MEGAKARYOCYTOPOIESIS

MEGAKARYOCYTOPOIESIS is the cellular developmental process that leads to platelet production. At least two humoral growth factors mag be necessary for megakaryocyte proliferation and maturation. One is a megakaryocyte-colony stimulating factor (MK-CSF) which induces the proliferation and differentiation of megakaryocyte progenitors(1,2), and the second, thrombopoietin, is a megakaryocyte maturation factor(3). Neither of these factors has been fully characterized. The proto-oncogene c-mpl, an orphan member of the haematopoietin receptor family(4-6), is specifically involved in megakaryocyte regulation(7). Here we present evidence that the c-mpl-encoded receptor binds a ligand (c-Mpl ligand) which is a humoral factor implicated in platelet homeostasis. Our results suggest that c-Mpl ligand, thrombopoietin and MK-CSF might be the same molecule.