A proximal element within the human α2 (I) collagen (COL1A2) promoter, distinct from the tumor necrosis factor-α response element, mediates transcriptional repression by interferon-γ

Abstract Previous studies have shown that interferon-γ (IFN-γ) inhibits type I collagen gene expression through both transcriptional and post-transcriptional mechanisms (Kahari et al., 1990). In the present study, using transient cell transfections of human dermal fibroblast cultures with a series of 5′ deletion promoter/CAT reporter gene constructs, we have identified the IFN-γ-response element of the human α2(I) collagen gene (COL1A2) promoter. Specifically, we have identified a segment of the proximal promoter region, located between nucleotides −161 and −125 relative to the transcription start site, as critical for down-regulation of COL1A2 promoter activity by IFN-γ. This IFN-γ response element (IgRE) is clearly distinct from the previously described tumor necrosis factor-α response element (TaRE) located between nucleotides −265 and −241 of the COL1A2 promoter, a difference which is likely to explain the additive inhibitory effect of these two cytokines. The inhibitory effect of IFN-γ was dose-dependent and rapidly induced, requiring less than 5 min exposure of fibroblast cultures. Gel mobility shift assays indicated that a highly specific nuclear protein complex bound to this 37-base pair region of promoter. Competition experiments with oligonucleotides spanning discrete segments of this promoter region mapped the binding element within a distinctive pyrimidine-rich sequence. Point mutations within the latter revealed that this element plays a crucial role not only in the IFN-γ response, but also in the basal activity of the proximal promoter. Substitution mutations within the IgRE of the −161/CAT construct attenuated the promoter response to IFN-γ, as measured in transient cell transfections, and eliminated specific DNA/protein complex formation, as measured by gel mobility shift assay. UV-crosslinking experiments indicated that two DNA/protein complexes were formed with the IgRE, with molecular weights around 55 kDa and 30 kDa, corresponding to proteins of −30 kDa and −6 kDa, respectively. Our results further clarify the molecular mechanisms involved in the regulation of type I collagen gene expression by IFN-γ.