C-Myc is a critical target for C/EBP alpha in granulopoiesis

CCAAT/enhancer binding protein alpha (C/EBP alpha) is an integral factor in the granulocytic developmental pathway, as myeloblasts from C/EBP alpha -n ull mice exhibit an early block in differentiation. Since mice deficient fo r known C/EBP alpha target genes do not exhibit the same block in granulocy te maturation, we sought to identify additional C/EBP alpha target genes es sential for myeloid cell development. To identify such genes, we used both representational difference analysis and oligonucleotide array analysis wit h RNA derived from a C/EBP alpha -inducible myeloid cell line. From each of these independent screens, we identified c-Myc as a C/EBP alpha negatively regulated gene. We mapped an E2F binding site in the c-Myc promoter as the cis-acting element critical for C/EBP alpha negative regulation. The ident ification of c-Myc as a C/EBP alpha target gene is intriguing, as it has be en previously shown that down-regulation of c-Myc can induce myeloid differ entiation. Here we show that stable expression of c-Myc from an exogenous p romoter not responsive to C/EBP alpha -mediated down-regulation forces myel oblasts to remain in an undifferentiated state. Therefore, C/EBP alpha nega tive regulation of c-Myc is critical for allowing early myeloid precursors to enter a differentiation pathway. This is the first report to demonstrate that C/EBP alpha directly affects the level of c-Myc expression and, thus, the decision of myeloid blasts to enter into the granulocytic differentiat ion pathway.