A NOVEL FUNCTION OF STAT1-ALPHA AND STAT3 PROTEINS IN ERYTHROPOIETIN-INDUCED ERYTHROID-DIFFERENTIATION OF A HUMAN LEUKEMIA-CELL LINE

We recently determined that erythropoietin (EPO) activates 3 members o f the signal transducer and activator of transcription (STAT) family, Stat1 alpha, Stat3, and Stat5, in the human EPO-dependent cell lines, UT-7 and UT-7/EPO (Kirito et al, J Biol Chem 272:16507, 1997). In addi tion, we have shown that Stat1 alpha, but not Stat3, is involved in EP O-induced cellular proliferation. In this study, we examined the roles of Stat1 alpha and Stat3 in EPO-induced erythroid differentiation. UT -7/GM was used as a model system, because this cell line can different iate into erythroid-lineage cells with EPO treatment (Komatsu et at, B lood 89:4021, 1997). We found that EPO did not activate Stat1 alpha or Stat3 in UT-7/GM cells. Transfection experiments showed that both Sta t1 alpha and Stat3 inhibited the induction by EPO of gamma-globin and erythroid-specific 5-aminolevulinate synthetase transcripts, resulting in a reduction of the percentage of hemoglobin-positive cells. Domina nt negative forms of Stat1 alpha or Stat3 promoted the EPO-induced ery throid differentiation of UT-7/GM cells, even in the presence of granu locyte-macrophage colony-stimulating factor, although this cytokine ne ver induced erythroid differentiation of the parent UT-7/GM cells with or without EPO. A cell cycle analysis showed that the constitutive ac tivation of Stat1 alpha, but not Stat3, shortened the period of G0/G1 prolongation caused by EPO stimulation. Taken together, our data sugge st that Stat1 alpha and Stat3 act as negative regulators in EPO-induce d erythroid differentiation. Specifically, Stat1 alpha may activate a cell cycle-associated gene(s), leading to the entry of cells into the cell cycle. (C) 1998 by The American Society of Hematology.