Granulocyte colony-stimulating factor regulates myeloid differentiation through CCAAT/enhancer-binding protein epsilon

Granulocyte colony-stimulating factor (G-CSF) is a major cytokine that regu lates proliferation and differentiation of myeloid cells, although the unde rlying mechanisms by which G-CSF controls myeloid differentiation are large ly unknown. Differentiation of hematopoietic cells is regulated by lineage- specific transcription factors, and gene-targeting studies previously revea led the critical roles of CCAAT/enhancer-binding protein (C/EBP) alpha and C/EBP epsilon, respectively, in the early and mid-late stages of granulocyt e differentiation, The expression of C/EBP epsilon in 32Dcl3 cells and FDCP 1 cells expressing mutant G-CSF receptors was examined and it was found tha t G-CSF up-regulates C/EBP epsilon. The signal for this expression required the region containing the first tyrosine residue of G-CSF receptor. Domina nt-negative signal transducers and activators of transcription 3 blocked G- CSF-induced granulocytic differentiation in 32D cells but did not block ind uction of C/EBP epsilon, indicating that these proteins work in different p athways. It was also found that overexpression of C/EBP epsilon greatly fac ilitated granulocytic differentiation by G-CSF and, surprisingly, that expr ession of C/EBP epsilon alone was sufficient to make cells differentiate in to morphologically and functionally mature granulocytes. Overexpression of c-myc inhibits differentiation of hematopoietic cells, but the molecular me chanisms of this inhibition are not fully understood. In 32Dcl3 cells overe xpressing c-myc that do not differentiate by means of G-CSF, induction of C /EBP epsilon is completely abrogated. Ectopic expression of C/EBP epsilon i n these cells induced features of differentiation, including changes in nuc lear morphologic characteristics and the appearance of granules. These data show that C/EBP epsilon constitutes a rate-limiting step in G-CSF-regulate d granulocyte differentiation and that c-myc antagonizes G-CSF-induced myel oid differentiation, at least partly by suppressing induction of C/EBP epsi lon. (C) 2001 by The American Society of Hematology.