Gfi-1 Represses PU.1 Activity To Promote Granulocyte over Macrophage Differentiation.

Mice lacking the zinc finger transcriptional repressor protein Gfi-1 are neutropenic. These mice generate abnormal immature myeloid cells exhibiting characteristics of both macrophages and granulocytes. Interestingly these immature Gfi-1^−/− myeloid cells overexpress target genes of the PU.1 transcription factor: M-CSF receptor (M-CSFR) and PU.1 itself. Because of this we sought to determine if Gfi-1 modulates the transcriptional activity of PU.1. Here we demonstrate that Gfi-1 physically associates with PU.1 and represses PU.1- dependent transcription from the M-CSFR promoter. This interaction is functionally relevant as Gfi-1 blocks PU.1- dependent macrophage differentiation, but not granulocyte differentiation of a multipotential hematopoietic progenitor cell line. Additionally infection of primary bone marrow progenitors with a Gfi-1 retrovirus promotes granulocyte differentiation at the expense of macrophage differentiation. In contrast, a Gfi-1 mutant lacking repressor activity strongly promotes macrophage differentiation of hematopoietic progenitors. In transient transfections this Gfi-1 mutant not only blocks Gfi-1 repression of PU.1 transcriptional activity but C/EBP alpha mediated repression as well. From this data we hypothesize that Gfi-1 along with other transcriptional regulators target PU.1 for repression in the granulocyte-macrophage progenitor (GMP) in order to direct granulocyte differentiation. If true decreasing the concentration of PU.1 may decrease the need for Gfi-1 in directing granulocyte development. We are currently testing this in vivo by examining myeloid development in PU.1^+/−Gfi-1^−/− mice.