We describe the molecular features of the interferon (IFN)-γ-mediated transcription of the human intercellular adhesion molecule (ICAM-1) gene. We identified putative IFN-γ-activated sites (GAS) distributed throughout a large segment of the ICAM-1 promoter (4.0 kb region). Using computer-assisted search, these sequences were similar to potential IFN-γ responsive elements that have a core sequence 5'-TTNCNNNAA-3'. In this report we show that in the ICAM-1 promoter a GAS site is located at -115 from the translation initiation site, and binds with strong affinity to IFN-y-activated Signal Transducers and Activators of Transcription (STAT1) homodimers. The same sequence is responsible for the IFN-γ-mediated transcription of the ICAM-1 gene. Moreover, we present evidence that a more distal GAS element that maps at - 2787 from the translation initiation site, binds IFN-γ-activated STAT1 dimers with lower affinity. Multimeric copies of such GAS sequence inserted into a tkCAT minimal promoter can drive transcription, demonstrating that the -2787 bp GAS element has an independent functional activity upon binding of IFN-γ-activated STAT1 proteins as documented by in vitro binding assays. Furthermore, using recombinant ICAM-CAT mutants, we show that, in vivo, the - 2787 GAS, but not a mutagenized -2787 GAS site, when coupled to the more proximal -115 GAS element, has an additive effect in enhancing the IFN-γ- mediated transcription of ICAM-1 promoter. Nevertheless, using a recombinant construct bearing the wild type -2787 GAS element and a mutagenized -115 GAS element, we could not detect any transcription after transfection of U937 recipient cells, suggesting that the -2787 bp GAS element is not sufficient as such for gene activation, but can cooperate with its cognate proximal sequence to give full function to the ICAM-1 promoter during the IFN-γ response. Taken together these data provide evidence that two GAS sites are required for the full potential activity in the mechanism of ICAM-1 gene activation by IFN-γ.
|Number of pages||8|
|Journal||European Journal of Biochemistry|
|Publication status||Published - Dec 15 1998|
- Intercellular adhesion molecule 1
- Interferon-γ-activation sites
- Signal transducer and activators of transcription
ASJC Scopus subject areas