Sp1 and C/EBP are necessary to activate the lactoferrin gene promoter during myeloid differentiation

In this study, we sought to identify factors responsible for the positive modulation of lactoferrin (LF), a neutrophil-specific, secondary-granule protein gene. Initial reporter gene transfection assays indicated that the first 89 base pairs of the LF promoter are capable of directing myeloid-specific LF gene expression. The presence of a C/EBP site flanked by 2 Sp1 sites within this segment of the LF promoter prompted us to investigate the possible role of these sites in LF expression. Cotransfection studies of LF-89luc plasmid with increasing concentrations of a C/EBP expression vector in myeloid cells resulted in a linear transactivation of luciferase reporter activity. Electrophoretic mobility shift assays found that the C/EBP site is recognized by C/EBP and that both LF Sp1 binding sites bind the Sp1 transcription factor specifically in myeloid cells. Mutation of either Sp1 site markedly reduced activity of the LF-89luc plasmid in myeloid cells, and neither Sp1 mutant plasmid was transactivated by a C/EBP expression plasmid to the same extent as wild-type LF-89luc. We also transfected LF-89luc into Drosophila Schneider cells, which do not express endogenous Sp1, and demonstrated up-regulation of luciferase activity in response to a cotransfected Sp1 expression plasmid, as well as to a C/EBP expression plasmid. Furthermore, cotransfection of LF-89luc plasmid simultaneously with C/EBP and Sp1 expression plasmids resulted in an increase in luciferase activity greater than that induced by either factor alone. Taken together, these observations indicate a functional interaction between C/EBP and Sp1 in mediating LF expression.