A critical role of the Erk1/2 downstream targets c-Fos, Egr-1, and Egr-2 in PU.1-driven monocyte development

The Ets family transcription factor PU.1 plays multiple roles in hematopoiesis. PU.1 acts in a graded manner to regulate myeloid and lymphoid development with high expression levels promoting monopoiesis and low expression levels instructing B cell fate. It remains incompletely understood how PU.1 promotes monocyte development. In this study, we show that PU.1 activation upregulated, whereas PU.1 knockdown downregulated, the expression of c-Fos, Egr-1, and Egr-2, which are the immediate early response genes of the Erk1/2 pathway and have been shown to promote monopoiesis. A PU.1 mutant defective in c-Jun interaction was unable to upregulate the expression of c-Fos, Egr-1, and Egr-2, indicating that interaction with c-Jun is required for PU.1-mediated upregulation of c-Fos, Egr-1, and Egr-2. We further demonstrate that PU.1 bound to and transcriptionally activated the promoters of c-Fos, Egr-1, and Egr-2. Knockdown of c-Fos or Egr-1 favored neutrophil over monocyte development in response to PU.1 activation, which was associated with increased Gfi1 expression. Notably, PU.1-mediated upregulation of c-Fos, Egr-1, and Egr-2 was blocked upon inhibition of Erk1/2 signaling, which was also associated with an increase in Gfi1 expression and a shift toward neutrophil development at the expense of monopoiesis. Together, these results reveal a novel mechanism by which PU.1 acts to promote monopoiesis and a critical role of the Erk1/2 downstream targets c-Fos, Egr-1 and Egr-2 in PU.1-driven monocyte development.