Induction of hematopoietic commitment and erythromyeloid differentiation in embryonal stem cells constitutively expressing c-myb

To provide insight into the mechanisms by which c-myb regulates hematopoiesis, we analyzed the expression of markers for multiple hematopoietic lineages in differentiating parental embryonic stem (ES) cells and in ES cells transfected with c-myb or with a mutant c-myb deficient in DNA binding and assessed the ability of these cells to undergo hematopoietic commitment and colony formation. Undifferentiated ES cells transfected with intact c-myb, but not cells transfected with mutant c-myb, expressed CD34, c-kit, GATA1, and flt3 mRNA as well as surface CD34, c-kit, and flt3 product. In contrast, the kinetics of GATA-2 mRNA expression was identical in parental and Myb-transfected ES cells. Transient expression assays suggested transactivation of gene expression dependent on interaction with Myb binding sites in the CD34 and GATA1 5′ flanking regions. Undifferentiated parental and c-myb mutant-transfected ES cells were not clonogenic, whereas c-myb transfectants formed erythromyeloid colonies in methylcellulose cultures in the absence of added hematopoietic growth factors and, at higher frequency, in the presence of kit and flt-3 ligands. Colony formation was suppressed by treatment with antisense oligodeoxynucleotides specifically downregulating c-kit and flt-3 expression. These findings indicate that c-myb regulates hematopoietic commitment and progenitor cell proliferation and differentiation through the activation of certain genes that define the stem/progenitor cell compartment.