Molecular Mechanisms Underlying the Regulation of Alternate Macrophage and Neutrophil Gene Programs by the Egr Transcription Factors

Cell fate determination in the hematopoietic system involves the onset and resolution of mixed lineage patterns of gene expression. Molecular mechanisms underlying the concerted activation and repression of alternate lineage genes remain to be elucidated. We have proposed a gene regulatory network for macrophage development in which the transcription factors PU.1 and the Egr’s function in a feed forward loop to activate macrophage genes. In the network, the Egr’s counteract the neutrophil regulator Gfi-1 and also repress alternate lineage genes. Using mutant alleles of Egr-1 and Egr-2 we validate the model and demonstrate that the Egr’s are required for initiating but not maintaining the repression of neutrophil genes during macrophage differentiation. Employing a PU.1−/− progenitor cell line we show that PU.1 transiently binds to and activates both macrophage and neutrophil gene promoters thereby leading to the onset of mixed lineage gene activity. While inducing macrophage cell fate determination, PU.1 remains persistently bound to macrophage gene promoters aided by the Egr’s. Surprisingly, the Egr’s displace PU.1 from neutrophil gene promoters by interacting with a co-repressor Nab-2 thereby repressing the alternate lineage program. We propose that the Egr’s have evolved to molecularly discriminate distinct sets of target genes and delineate divergent patterns of gene activity.