Specific binding, internalization, and degradation of human recombinant interleukin‐3 by cells of the acute myelogenous, leukemia line, KG‐1

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Abstract
We have studied the interaction of 35S-labeled recombinant IL-3 with the acute myelogenous leukemia cell line, KG-1. 35S-IL-3 bound to these cells in a time dependent, saturable, and specific manner at 4°C. Scatchard transformation of binding isotherms demonstrated the existence of a small number (200) of binding sites, with an apparent dissociation constant of 70-105 pM. After a temperature shift from 4°C to 37°C, surface-bound 35S-IL-3 was rapidly internalized and processed into a trichloroacetic acid soluble form that was released into the medium. Experiments to address the specificity of the IL-3 binding site revealed that neither human IL-2, M-CSF, erythropoietin, transferrin, bovine insulin, nor murine nerve growth factor compete with IL-3 for binding to KG-1 cells. Both human and gibbon recombinant IL-3 and, surprisingly, human recombinant GM-CSF effectively competed the binding of the labeled IL-3 to these cells at 4°C. The competition by GM-CSF was found to be concentration dependent, but much higher concentrations were required to achieve the levels obtained with IL-3. These results suggest that GM-CSF may also interact with the high-affinity IL-3 binding site on KG-1 cells or, alternatively, that GM-CSF binding to its own receptor may decrease the affinity of the IL-3 receptor for its ligand.