Convergence of TNFα and lFNγ signalling pathways through synergistic induction of IRF-1/ISGF-2 is mediated by a composite GAS/κB promoter element

Abstract

The molecular basis for the well known synergistic biological effects of tumor necrosis factor α (TNFα) and interferon γ (IFNγ) is still poorly understood. This report demonstrates that expression of interferon-regulatory factor 1 (IRF-1), also known as interferon-stimulated-gene factor 2 (ISGF-2), is synergistically induced by these cytokines. The induction is a primary transcriptional response that occurs rapidly without a requirement for new protein synthesis. Synergism is mediated by a novel composite element in the IRF-1 promoter that includes an IFNγ-activation site (GAS) overlapped by a non-consensus site for nuclear factor kappa B (NFκB). These sequences are bound strongly by signal transducer and activator of transcription 1 (STAT-1) and weakly by the p50/p65 heterodimer form of NFκB, respectively. However, the binding of STAT-1 and NFκB to the GAS/κB element in vitro seems to be mutually exclusive and independent. Synergistic induction of IRF-1 is likely to be an important early step in regulatory networks critical to the synergism of TNFα and IFNγ. The GAS/κB element may mediate synergistic transcriptional induction of IRF-1 by other pairs of ligands that together activate NFκB and STAT family members. Other genes are likely to contain this motif and be regulated similarly.