PU.1 (SPI-1) AUTOREGULATES ITS EXPRESSION IN MYELOID CELLS

PU.1 (Spi-1), a member of the Ets transcription factor family, is pred ominantly expressed in myeloid (granulocytes, monocytes and macrophage s) and B cells. PU.1 is upregulated early during commitment of multipo tential progenitors to the myeloid lineages and inhibition of PU.1 fun ction in human CD34(+) progenitors prior to this upregulation blocks m yeloid 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 promote r, demonstrate tissue-specific reporter gene expression in myeloid cel l lines but not in T cells and HeLa (non-hematopoietic cells) cells. D eletion analysis of the PU.1 promoter indicates that tissue-specific f unctional elements are encoded in the -61 to -39 bp and -7 to +34 bp r egions. The first region contains a functional octamer (Oct) site at - 54 bp and an Spl 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 mutatio n of the PU.1 site which abolished PU.1 binding caused a significant d ecrease in promoter activity., Mutation of the Oct and/or Spl site res ults in a lesser decrease of promoter activity in myeloid cells. Cotra nsfection of PU.1 or Spi-B in cells lacking PU.1 and Spi-B specificall y transactivated a minimal promoter containing the PU.1 binding site, indicating that PU.1 can activate its own promoter elements in an auto regulatory loop. Positive autoregulation of the PU.1 promoter may play an important role in the function of PU.1 in myeloid cells.