Granulocyte colony-stimulating factor-induced dephosphorylation of a 45 kD cytosolic protein in HL-60 cells differentiating into neutrophils

Granulocyte colony-stimulating factor (G-CSF)-induced alteration of phosphoprotein during differentiation of HL-60 cells was studied. From the two-dimensional gel electrophoresis analysis of phosphoproteins, a 45 kD phosphoprotein in the cytosolic fraction of DMSO-pretreated HL-60 cells was rapidly dephosphorylated by the addition of G-CSF. This 45 kD phosphoprotein migrated into four or five spots between 4.5 and 5.5 pI. The dephosphorylation of 45 kD protein was observed within at least 10 min and reached a maximum at 60 min. Phosphoamino acid analysis showed that only serine residue of 45 kD phosphoprotein was phosphorylated, suggesting that G-CSF induced an activation of serine phosphatase. Furthermore, Staurosporine and calphostin C inhibited the phosphorylation of 45 kD protein, suggesting that protein kinase C or its downstream kinase(s) is involved in the phosphorylation of 45 kD protein. These results indicate that G-CSF causes dephosphorylation of a 45 kD cytosolic phosphoprotein which may play a role in signal transduction of G-CSF.