Quantitation of insulin-like growth factor-I binding to highly purified human erythroid colony-forming units

A method has been developed by which erythroid colony-forming units (CFU-E) may be obtained from human blood in sufficient number and purity for quantitative studies of growth factor binding. Studies in serum-free medium have shown that CFU-E require the addition of only two growth factors, erythropoietin (EP) and insulin-like growth factor-I (IGF-I), for growth and differentiation. The IGF-I may be replaced by higher (100-fold) concentrations of insulin. Incubation of CFU-E with 125I recombinant human IGF-I (rhIGF-I) at 4 degrees C has demonstrated specific binding that is directly proportional to the cell concentration. Competition with unlabeled rhIGF-I markedly decreased binding, whereas other growth factors such as granulocyte-monocyte colony-stimulating factor (GM-CSF), interleukin 3 (IL-3), and epidermal growth factor (EGF) had no significant effect on the binding of [125I]rhIGF-I. The binding was saturable at an [125I]rhIGF-I concentration of 10 ng/ml (1.2 nM). Scatchard analysis revealed two classes of IGF-I receptors present on the CFU-E cell surface: a low-affinity class of 549 receptors with Kd = 0.44 nM and a high-affinity class of 341 receptors with Kd = 0.04 nM.