Cell-specific transcriptional regulation of human leukotriene B-4 receptorgene

Leukotriene B-4 (LTB4) is a lipid mediator that activates leukocytes and is involved in host defense and inflammation. BLT1, a high-affinity receptor for LTB4 (originally termed BLT), is expressed exclusively in inflammatory cells and is inducible in macrophages upon activation. The mechanisms of ti ssue-specific expression and induction of BLT1 are important for the unders tanding of mechanism of onset and the potential treatment of inflammatory d isorders. Here, we report the genomic structure and a promoter analysis of the human BLT1 gene, with an emphasis on the mechanism of cell-specific tra nscription No TATA or CAAT elements exist around the transcription initiati on sites, but a GC-rich sequence is observed in this region. A reporter gen e assay revealed that a region similar to 80 basepair upstream from the ini tiator sequence is required for the basal transcription of the BLT1 gene. S p1 was found to be a major activator of basal transcription by electrophore tic mobility shift assays and site-directed mutagenesis. The CpG sites of t he BLT1 promoter region were highly methylated in BLT1-nonexpressing cells, but not methylated in BLT1-expressing cells. Further, methylation of this region in vitro inhibited the promoter activity to similar to 15% of the co ntrol. Thus, methylation at CpG sites in the promoter region is important f or cell-specific transcription of the BLT1. gene. The promoter region of th e BLT-1 gene is localized within the open reading frame (ORF) of the BLT2 g ene, which encodes a low-affinity receptor for LTB4 (Yokomizo, T., K. Kato, K. Terawaki, T. Izumi, and T. Shimizu. 2000. J. Exp. Med. 392:421-431). To our knowledge, this is the first example of "promoter in ORF" in higher eu karyotes.