Gfi-1 Represses C/EBPe and Is Required for the Survival and Proliferation of Myeloid Cells Stimulated by G-CSF.

Granulocyte colony-stimulating factor (G-CSF) is a hematopoietic cytokine that plays a major role in granulopoiesis. G-CSF supports the proliferation, differentiation and survival of myeloid progenitor cells. Treatment of cells with G-CSF activates multiple intracellular signal transduction pathways leading to alterations in the activities of transcription factors involved in myeloid development. Gfi-1 is a zinc-finger transcriptional repressor that is required for granulopoiesis. Mutations in Gfi-1 have been associated with severe congenital neutropenia (SCN). How Gfi-1 acts in myeloid cells is still poorly understood. Expression of Gfi-1 was upregulated during granulocytic differentiation induced by G-CSF in myeloid 32D cells. Truncation of the carboxy terminus of the G-CSF receptor, as seen in patients with acute myeloid leukemia (AML) evolving from SCN, disrupted the ability of the G-CSF receptor to upregulate Gfi-1. Ectopic expression of a naturally occurring dominant negative Gfi-1 mutant, N382S, identified in patients with SCN, resulted in premature apoptosis and reduced proliferation of cells induced to differentiate with G-CSF. The expression of neutrophil elastase (NE) and C/EBPe was significantly increased in 32D cells expressing N382S. In contrast, overexpression of wild type Gfi-1 abolished G-CSF-induced upregulation of C/EBPe, but had no apparent effect on NE upregulation by G-CSF. Notably, G-CSF-dependent proliferation and survival were inhibited upon overexpression of C/EBPe, but not NE. These data suggest that a key role of Gfi-1 in granulopoiesis is to repress C/EBPe and that overt C/EBPe overexpression may be deleterious to the proliferation and survival of myeloid cells. Our data may explain why loss of Gfi-1 function, either as a result of gene knockout in mice or gene mutations in human, results in a block of granulocytic differentiation and severe neutropenia.