STEEL FACTOR INDUCES SERINE PHOSPHORYLATION OF STAT3 IN HUMAN GROWTH FACTOR-DEPENDENT MYELOID CELL-LINES

Steel factor (SLF) acts synergistically with various hematopoietic gro wth factors that use the Jak-Stat pathways in vivo and in vitro, altho ugh the contribution by SLF to this pathway is unknown. We show here t hat SLF induces time- and dose-dependent phosphorylation of Stat3 in t he human growth factor-dependent cell lines MO7e and TF-1. This phosph orylation occurs exclusively on serine residues. Simultaneous stimulat ion with SLF plus other cytokines that induce tyrosine phosphorylation of Stat3, such as interleukin-9 (IL-9) in MO7e cells or IL-6 in TF-1 cells, resulted in tyrosine phosphorylation and enhanced serine phosph orylation of Stat3, Serine phosphorylation alone did not promote nucle ar translocation or DNA binding activity to the sis-inducible element of Stat3. However, costimulation with SLF plus IL-9 in MO7e cells resu lted in the nuclear translocation of serine-hyperphosphorylated Stat3. Serine phosphorylation of Stat3 was also observed by the stimulation of cells with granulocyte-macrophage colony-stimulating factor and IL- 3, which do not induce tyrosine phosphorylation of Stat3., These resul ts suggest that SLF might modulate the Jak-Stat3 pathway by serine pho sphorylation and that the Jak-Stat pathway may be differentially regul ated by the combinational stimulation of two or more cytokines. (C) 19 96 by The American Society of Hematology.