THE ROLE OF MEGAKARYOCYTE GROWTH AND DEVELOPMENT FACTOR IN TERMINAL STAGES OF THROMBOPOIESIS

Thrombopoietin (TPO), the ligand for the c-Mpl cytokine receptor, is a recently identified cytokine with potent effects on platelet producti on, The receptor-binding portion of c-Mpl ligand is encompassed in ano ther molecule known as megakaryocyte growth and development factor, or MGDF. Although it is clear that the administration of TPO or MGDF to animals dramatically increases the platelet count, the specific stage( s) of thrombopoiesis during which these molecules are principally acti ve have not been unambiguously determined, Pharmacology studies admini stering MGDF at doses ranging from 0.1 to 630 mu g/kg/d to mice reveal ed a biphasic response in platelet production. Administration of the d rug at concentrations from 6 to 60 mu g/kg/d resulted in platelet coun ts 5-fold above normal. However, doses >60 mu g/kg/d resulted in less- than-optimal platelet production, This phenomenon was investigated in vitro, Using an established culture system for the generation of human megakaryocytes and platelets, MGDF was shown to be optimally and equi valently active in the generation of mature megakaryocytes at concentr ations from 10 to 1000 ng/ml. However, the cytokine was not required f or proplatelet formation and in fact was inhibitory to that process in a dose-dependent manner. When MGDF was added to human megakaryocytes at concentrations of 200 ng/ml or greater, proplatelet formation was i nhibited to 30% of control values, MGDF-mediated inhibition was specif ic, since the addition of the truncated form of the c-Mpl receptor rev ersed the inhibition in a dose-dependent manner, Other recombinant fac tors, interleukin-6, interleukin-11 and erythropoietin had no signific ant positive or negative effects in this human proplatelet assay, Toge ther, these data suggest that although TPO and MGDF promote the full s pectrum of megakaryocyte growth and development, they are not necessar y for proplatelet formation, and may in part regulate platelet sheddin g by their absence.