A novel function of Stat1 and Stat3 proteins in erythropoietin-induced erythroid differentiation of a human leukemia cell line.

We recently determined that erythropoietin (EPO) activates 3 members of the signal transducer and activator of transcription (STAT) family, Stat1alpha, 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 addition, we have shown that Stat1alpha, but not Stat3, is involved in EPO-induced cellular proliferation. In this study, we examined the roles of Stat1alpha and Stat3 in EPO-induced erythroid differentiation. UT-7/GM was used as a model system, because this cell line can differentiate into erythroid-lineage cells with EPO treatment (Komatsu et al, Blood 89:4021, 1997). We found that EPO did not activate Stat1alpha or Stat3 in UT-7/GM cells. Transfection experiments showed that both Stat1alpha 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. Dominant negative forms of Stat1alpha or Stat3 promoted the EPO-induced erythroid differentiation of UT-7/GM cells, even in the presence of granulocyte-macrophage colony-stimulating factor, although this cytokine never induced erythroid differentiation of the parent UT-7/GM cells with or without EPO. A cell cycle analysis showed that the constitutive activation of Stat1alpha, but not Stat3, shortened the period of G0/G1 prolongation caused by EPO stimulation. Taken together, our data suggest that Stat1alpha and Stat3 act as negative regulators in EPO-induced erythroid differentiation. Specifically, Stat1alpha may activate a cell cycle-associated gene(s), leading to the entry of cells into the cell cycle.