NF-Y cooperates with USF1/2 to induce the hematopoietic expression of HOXB4

The transcription factor homeobox B4 (HOXB4) is preferentially expressed in immature hematopoietic cells and implicated in the transition from primitive hematopoiesis to definitive hematopoiesis as well as in immature hematopoietic cell proliferation and differentiation. We previously identified Hox response element 1 (HxRE-1) and HxRE-2/E-box as 2 critical DNA-binding sites of the HOXB4 promoter active in hematopoietic cells and demonstrated that upstream stimulating factor 1 and 2 (USF1/2) activate HOXB4 transcription through their binding to the E-box site. Here we report that the trimeric regulatory complex nuclear factor Y (NF-Y) is the factor that recognizes HxRE-1 and activates the HOXB4 promoter in hematopoietic cells. We further show that NF-Y interacts biochemically with USF1/2 on the HOXB4 promoter, and that the formation of this NF-Y/USF1/2 complex is required for the full activity of the HOXB4 promoter. Most important, NF-Ya subunit protein levels are found to be lower in c-Kit-Gr-1+ granulocytic bone marrow (BM) cells than in c-Kit+ immature BM cells, in parallel with a reduction of NF-Y occupancy on the HOXB4 promoter as shown by chromatin immunoprecipitation (ChIP) assay. These results suggest that NF-Y is a developmentally regulated inducer of the HOXB4 gene in hematopoietic cells.