The promoter of IL-18 binding protein:: Activation by an IFN-γ-induced complex of IFN regulatory factor 1 and CCAAT/enhancer binding protein β

The IL-18 binding protein (IL-18BP) is a circulating inhibitor of the proinflammatory cytokine IL-18. It is constitutively expressed in mononuclear cells, and elevated expression is induced by IFN-gamma. In this study, we characterized the IL-18BP promoter. We first showed that induction is at the transcriptional level and requires de novo protein synthesis. The IL-18BP promoter resides within 1.6 kb DNA upstream of the first exon and includes at least six regulatory elements. We identified in the basal promoter a gamma-activated sequence (GAS) proximal to the transcription start site (base 1), followed by an IFN regulatory factor 1 response element (IRF-E) and two CCAAT/enhancer binding protein beta (C/EBPbeta) sites, all of which are essential for basal promoter activity. Furthermore, GAS and IRF-E were essential for IFN-gamma-induced transcription. Indeed, sera of IRF-1-deficient mice lacked basal and IFN-gamma-induced IL-18BP. We found that after induction of IRF-1 by IFN-gamma, it formed a complex with C/EBPbeta, which bound to the IRF-E and GAS-containing proximal DNA. In contrast, the IFN-gamma-induced signal transducer and activator of transcription 1 dimer did not associate with this GAS. In addition, we identified a silencer element and a distal enhancer at bases -1081 to -1272, which was also physically associated with IRF-1. The IRF-1-C/EBPbeta complex described here probably plays a fundamental role in regulating additional IFN-gamma-responsive genes.