CCAAT/enhancer binding protein epsilon is preferentially up-regulated during granulocytic differentiation and its functional versatility is determined by alternative use of promoters and differential splicing

CCAAT/enhancer binding protein (C/EBP) epsilon is a recently cloned member of the C/EBP family of transcription factors and is expressed exclusively in cells of hematopoietic origin, The human C/EBP epsilon gene is transcribed by two alternative promoters, P alpha and P beta. A combination of differential splicing and alternative use of promoters generates four mRNA isoforms, of 2.6 kb and 1.3-1.5 kb in size, These transcripts can encode three proteins of calculated molecular mass 32.2 kDa, 27.8 kDa, and 14.3 kDa, Accordingly, Western blots with antibodies specific for the DNA-binding domain, that is common to all forms, identify multiple proteins, C/EBP epsilon mRNA was greatly induced during in vitro granulocytic differentiation of human primary CD34(+) cells, Retinoic acid treatment of HL60 promyelocytic leukemia cells for 24 hr induced C/EBP epsilon mRNA levels by 4-fold, while prolonged treatment gradually reduced mRNA expression to pretreatment levels. Transient transfection experiments with expression vectors for two of the isoforms demonstrated that the 32.2-kDa protein is an activator of transcription of granulocyte colony-stimulating factor receptor promoter, while the 14.3-kDa protein is not. Thus, C/EBP epsilon is regulated in a complex fashion and may play a role in the regulation of genes involved in myeloid differentiation.