Promegapoietin, a family of chimeric growth factors, supports megakaryocyte development through activation of IL-3 and c-Mpl ligand signaling pathways

Objective. The signaling pathways induced by promegapoietin (PMP), a family of chimeric growth factors that activate the human IL-3 and c-Mp1 receptor s, were investigated. Methods. The biological activity of PMP was examined by receptor binding, c ell proliferation, ex vivo expansion of hematopoietic progenitor cells, and in vivo production of platelets. The activation of signaling pathways was examined by Western blot and Northern blot analyses. Results. Two PMP molecules, PMP-1 and PMP-1a, induced proliferation of cell s expressing the IL-3 receptor, c-Mp1, or both receptors and bound to the I L-3 receptor and c-Mp1 with high affinity. Ex vivo expansion assays using h uman bone marrow CD34(+) cells suggested that PMP-1 induced greater total c ellular expansion as well as expansion of CD41(+) megakaryocytic precursor cells than IL-3 or c-Mp1 ligand alone. Subcutaneous administration of 50 mu g/kg of PMP-1 for 10 days to rhesus monkeys resulted in increased platelet production in vivo from a baseline of 357 +/- 45 x 10(3) cells/mL to 1376 /- 151 x 10(3) cell/mL. PMP-1 induced phosphorylation of the betac subunit of IL-3 receptor and c-Mp1, JAK2, and STAT5b, but not STAT3. PMP-1 induced greater expression of Pim-1, c-Myc, and cyclin D2 than did either an IL-3 r eceptor agonist or c-Mp1 receptor agonist alone. The magnitude of induction of early response genes was similar for PMP and the coaddition of IL-3 rec eptor agonist and c-Mp1 receptor agonist. Conclusion. PMP combines the biological activities of IL-3 and c-Mp1 ligand in a single molecule that can simultaneously activate signaling pathways i nduced by both these ligands. (C) 2001 International Society for Experiment al Hematology. Published by Elsevier Science Inc.