Role of CCAAT/enhancer-binding protein site in transcription of human neutrophil peptide-1 and -3 defensin genes.

The human neutrophil defensins (human neutrophil peptides (HNPs)), major components of azurophilic granules, contribute to innate and acquired host immunities through their potent antimicrobial activities and ability to activate T cells. Despite being encoded by nearly identical genes, HNP-1 is more abundant in the granules than HNP-3. We investigated the regulation of HNP-1 and HNP-3 expression at the transcriptional level using a promyelocytic HL-60 cell line. Luciferase analysis showed that transcriptional levels of HNP-1 and HNP-3 promoters were equivalent and that an approximately 200-bp region identical between promoters was sufficient for transcriptional activity. Furthermore, overlapping CCAAT/enhancer-binding protein (C/EBP) and c-Myb sites in the region were found to be required for efficient transcription. Gel mobility shift assay demonstrated that C/EBPalpha predominantly bound to the C/EBP/c-Myb sites using HL-60 nuclear extracts. No specific binding to C/EBP/c-Myb sites was observed in nuclear extracts from mature neutrophils, which expressed neither C/EBPalpha protein nor HNP mRNAs. Taken together, these findings suggest that the difference in the amounts of HNP-1 and HNP-3 peptides in neutrophils is caused by posttranscriptional regulation and that C/EBPalpha plays an important role in the transcription of HNP genes in immature myeloid cells.