INTERFERON-GAMMA DOWN-REGULATES STEM-CELL FACTOR AND ERYTHROPOIETIN RECEPTORS BUT NOT INSULIN-LIKE GROWTH-FACTOR-I RECEPTORS IN HUMAN ERYTHROID COLONY-FORMING CELLS

Interferon gamma (IFN gamma) has been shown to inhibit proliferation a nd differentiation of erythroid progenitor cells and to produce apopto sis of erythroid cells, whereas stem cell factor (SCF), erythropoietin (EP), and insulin-like growth factor-I (IGF-I) have distinct roles in enhancing erythroid cell production and preventing apoptosis. The mec hanism by which IFN gamma exerts an inhibitory effect on the positive roles of these growth factors is unknown. Although some inhibitory cyt okines including IFN gamma have been shown to downregulate growth fact or receptors, the effect of IFN gamma on SCF, EP, and IGF-I receptors of human erythroid progenitor cells has not been defined. We obtained highly purified day-5 or day-6 erythroid colony-forming cells (ECFCs) from human blood in sufficient quantity and purity for radiolabeled cy tokine binding studies and analysis of mRNA. When day-5 ECFCs were inc ubated with increasing concentrations of recombinant human (rh) IFN ga mma for 24 hours at 37 degrees C, specific binding of I-125-rhSCF to S CF receptors was significantly decreased by 25% to 40% in a dose-depen dent fashion, with the maximum effect at 2,500 to 5,000 U/mL of IFN ga mma. The decrease was apparent by 12 hours of incubation and was only slightly lower by 24 hours. The numbers of SCF and EP receptors, but n ot of IGF-I receptors, per ECFC, calculated by Scatchard analysis, wer e significantly decreased by 30% and 23% to 25%, respectively, after i ncubation with 2,500 U/mL rhIFN gamma for 24 hours at 37 degrees C, wh ereas the binding affinities were not affected. This decrease in SCF r eceptors was confirmed by flow cytometry using an anti-c-kit mouse mon oclonal antibody. Northern blot analysis showed that the mRNAs for the SCF and EP receptors, but not for the IGF-I receptors, were decreased by 50% to 60% after 3 hours of incubation at 37 degrees C with 2,500 U/mL of rhIFN gamma. This persisted for 24 hours without alteration of the stability of the SCF and EP receptor mRNAs. These observations su ggest that one means by which IFN gamma inhibits erythroid cell prolif eration and differentiation and produces apoptosis may be through the reduction of the number of target receptors for SCF and EP and that th is occurs through transcriptional inhibition of the corresponding mRNA s. (C) 1997 by The American Society of Hematology.