GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR (GM-CSF) REDUCES THEDENSITY OF STEM-CELL FACTOR RECEPTORS (C-KIT ONCOGENE PRODUCT) ON A GM-CSF-DEPENDENT HUMAN MYELOID CELL-LINE

By employing a monoclonal antibody against the stem cell factor recept or (SCF-R), c-kit oncogene product, we analysed in flow cytometric tec hnique the density of SCF-R on GM/SO cells which were incubated under various culture conditions. These experiments revealed that there is a n inverse correlation between the SCF-R density on the cells and the d oses of granulocyte-macrophage colony-stimulating factor(GM-CSF) in cu lture medium; the lower the dose, the higher the density of SCF-R on t he cells. More detailed analyses showed that, in contrast to SCF which rapidly downregulates its own receptor, GM-CSF does not alter the mea surable level of SCF-R in an exposition period of 60 minutes, which su ggests that the internalization or shedding of the receptor is not the mechanism of action. Since the most striking difference regarding den sity of SCF-R between GM-CSF-treated and untreated cells was observed on day 2, the modulation of c-kit oncogene protein by GM-CSF likely oc cur prior to expression of protein onto the cell surface. In order to exclude the possibility that altered cell viability due to insufficien t GM-CSF content in culture medium might be responsible for the increa sed SCF-R densities on GM-CSF-dependent cells, we subsequently generat ed a GM-CSF-independent subclone which still responded to GM-CSF as we ll as the dependent did. The experiments carried out with this subclon e confirmed the results presented above. Thus our data suggest that GM -CSF is directly involved in the regulation of SCF receptor density on GM/SO cells.