Promoter analysis of an interferon-inducible gene associated with macrophage activation

We have investigated the regulation of an activation-associated guanylate-binding protein gene (mGBP-1/mag-1) in murine macrophage cell lines in response to the cytokine IFN-gamma. One of the cell lines utilized (RAW 264.7) acquires the ability to kill tumor cells after IFN-gamma and LPS treatment, whereas the other (WEHI-3) does not. We previously have demonstrated that mGBP-1 is induced by IFN-gamma in RAW 264.7 but not WEHI-3 cells. Here we present information concerning the cloning, sequencing, and initial characterization of the upstream region of the mGBP-1 gene as a first step towards understanding the differential control of this gene in RAW 264.7 versus WEHI-3 cells. Genomic fragments encompassing a portion of the mGBP-1 5' flanking region were inserted into vectors containing a luciferase reporter gene. 928 bp of upstream sequence were found to be sufficient for IFN-gamma-mediated induction of luciferase activity in the RAW 264.7 cell line. Furthermore, sequences within 100 bp of the major transcription initiation site conferred strong IFN-gamma responsiveness to the reporter gene. A perfect match to the interferon-stimulated response element (ISRE) was present within this region, and was shown to be essential for interferon-induced expression. An oligonucleotide corresponding to the mGBP-1 ISRE bestowed interferon-inducible expression on a heterologous minimal promoter. Site-specific mutation of the ISRE within the 106-bp upstream region eliminated interferon inducibility of this construct. Taken together, the results indicate the ISRE is necessary and sufficient for IFN-gamma induction of the mGBP-1 gene. Transient transfection assays carried out with the WEHI-3 cell line indicated that all promoter constructs were transcriptionally inactive in these cells, including the ISRE-minimal promoter construct. The inability of the WEHI-3 cell line to utilize an ISRE after IFN-gamma induction may underlie the functional differences exhibited by the two cell lines after IFN-gamma stimulation.