C/EBPα and DEK coordinately regulate myeloid differentiation

The transcription factor C/EBP alpha is a critical mediator of myeloid differentiation and is often functionally impaired in acute myeloid leukemia. Recent studies have suggested that oncogenic FLT3 activity disrupts wild-type C/EBP alpha function via phosphorylation on serine 21 (S21). Despite the apparent role of pS21 as a negative regulator of C/EBP alpha transcription activity, the mechanism by which phosphorylation tips the balance between transcriptionally competent and inhibited forms remains unresolved. In the present study, we used immuno-affinity purification combined with quantitative mass spectrometry to delineate the proteins associated with C/EBP alpha on chromatin. We identified DEK, a protein with genetic links to leukemia, as a member of the C/EBP alpha complexes, and demonstrate that this association is disrupted by S21 phosphorylation. We confirmed that DEK is recruited specifically to chromatin with C/EBP alpha to enhance GCSFR3 promoter activation. In addition, we demonstrated that genetic depletion of DEK reduces the ability of C/EBP alpha to drive the expression of granulocytic target genes in vitro and disrupts G-CSF-mediated granulocytic differentiation of fresh human BM-derived CD34(+) cells. Our data suggest that C/EBP alpha and DEK coordinately activate myeloid gene expression and that S21 phosphorylation on wild-type C/EBP alpha mediates protein interactions that regulate the differentiation capacity of hematopoietic progenitors. (Blood. 2012;119(21): 4878-4888)