ATF4 differentially regulates transcriptional activation of myeloid-specific genes by C/EBPε and C/EBPα

Dimerization between different basic region lencine zipper (ZIP) transcription factors is regarded as an important mechanism for integrating various extracellular signals to control specific patterns of gene expression in cells. The activating transcription factor 4 (ATF4) protein was identified as a principal partner for the myeloid-specific transcriptional factor C/EBPE. Dimerization required the ZIP motif of each protein and redirected DNA binding of C/EBPE and ATF4 from their respective symmetric consensus sites to asymmetric C/EBP and cAMP response element sites. The C/EBP epsilon:ATF4 heterodimer bound to the C/EBP sites in the promoters of the myeloid-specific genes encoding nentrophil elastase (NE) and the G-CSF receptor (G-CSFR). Also, the heterodimer bound a previously uncharacterized site in the promoter of the mim-1 gene at nucleotide -174. Coexpression of ATF4 and C/EBPE in the presence of c-Myb synergistically activated the mim-1 and NE promoters compared with C/EBP epsilon plus e-Myb alone. Synergistic activation was not observed for the G-CSFR promoter and only occurred in the presence of c-myb with the NE or mim-1 promoters. In contrast, ATF4:C/EBP alpha dimers bound to the C/EBP sites in the G-CSFR and NE promoters, but transcriptional activation was inhibited by 30-80% in the presence or absence of c-Myb. We propose that ATF4 may regulate myeloid gene expression differentially by potentiating C/EBPE but inhibiting C/EBP alpha-mediated transcriptional activation.