Progenipoietins: Biological characterization of a family of dual agonists of fetal liver tyrosine kinase-3 and the granulocyte colony-stimulating factor receptor

Objective. The progenipoietins, a class of engineered proteins containing b oth fetal liver tyrosine kinase-3 and granulocyte colony-stimulating factor receptor agonist activities, were functionally characterized in vitro and in vivo. Materials and Methods. Four representative progenipoietins were evaluated f or receptor binding, receptor-dependent cell proliferation, colony-forming unit activity, and their effects on hematopoiesis in the C57BL/6 mouse. Results. The progenipoietins bound to fetal liver tyrosine kinase-3 and the granulocyte colony-stimulating factor receptor with affinities within twof old to threefold of the native ligands, and each progenipoietin bound simul taneously to both fetal liver tyrosine kinase-3 and the granulocyte colony- stimulating factor receptor. The progenipoietins exhibited different levels of activity in receptor-dependent cell proliferation assays. The fetal liv er tyrosine kinase-3-dependent cell proliferation activity of three of four progenipoietins was decreased sixfold to 33-fold relative to native fetal liver tyrosine kinase-3 ligand, while granulocyte colony-stimulating factor receptor-dependent activity of the progenipoietins was within twofold to t hreefold of native granulocyte colony-stimulating factor, At nonsaturating concentrations, the progenipoietins stimulated colony formation to a greate r extent than the equimolar combination of fetal liver tyrosine kinase-3 an d granulocyte colony-stimulating factor. Treatment of mice with the progeni poietins yielded dramatic increases in peripheral blood and splenic white b lood cells, polymorphonuclear leukocytes, and dendritic cells. Conclusion. These preclinical results demonstrate that the progenipoietins are potent hematopoietic growth factors that stimulate cells in a receptor- dependent manner. When administered in vivo, the progenipoietins effectivel y promote the generation of multiple cell lineages. Thus, in both in vitro and in vivo settings, the progenipoietins as single molecules exhibit the s ynergistic activity of the combination of fetal liver tyrosine kinase-3 and granulocyte colony-stimulating factor, (C) 2001 international Society for Experimental Hematology, Published by Elsevier Science Inc.