PU.1 (Spi-1) autoregulates its expression in myeloid cells

Abstract

PU.1 (Spi-1), a member of the Ets transcription factor family, is predominantly expressed in myeloid (granulocytes, monocytes and macrophages) and B cells. PU.1 is upregulated early during commitment of multipotential progenitors to the myeloid lineages and inhibition of PU.1 function in human CD34+ progenitors prior to this upregulation blocks myeloid colony formation. Since PU.1 expression appears to play a role in hematopoietic development, we characterized the PU.1 promoter. Here we report that the murine PU.1 promoter, as well as the human promoter, demonstrate tissue-specific reporter gene expression in myeloid cell lines but not in T cells and HeLa (non-hematopoietic cells) cells. Deletion analysis of the PU.1 promoter indicates that tissue-specific functional elements are encoded in the -61 to -39 bp and -7 to +34 bp regions. The first region contains a functional octamer (Oct) site at -54 bp and an Sp1 site at -39 bp. The second contains a binding site at +20 bp for both PU.1 itself and the related ets family member Spi-B. In vivo footprinting assays demonstrate that a hypersensitive band was detected at the PU.1 site in myeloid cells but not in HeLa. A mutation of the PU.1 site which abolished PU.1 binding caused a significant decrease in promoter activity. Mutation of the Oct and/or Sp1 site results in a lesser decrease of promoter activity in myeloid cells. Co-transfection of PU.1 or Spi-B in cells lacking PU.1 and Spi-B specifically transactivated a minimal promoter containing the PU.1 binding site, indicating that PU.1 can activate its own promoter elements in an autoregulatory loop. Positive autoregulation of the PU.1 promoter may play an important role in the function of PU.1 in myeloid cells.