Roles of IKK-β, IRF1, and p65 in the Activation of Chemokine Genes by Interferon-γ

Activation of chemokine genes in response to interferon (IFN)-gamma or NF-kappa B is an important aspect of inflammation. Using the chemokine gene ip-10 in mouse embryonic fibroblast cells as an example, we show that the response to IFN-gamma is long lasting but secondary: initial STAT1 activation drives IRF1 synthesis, and IRF1 then binds to IFN-stimulated regulatory elements (ISREs) in the ip-10 promoter. The promoters of most IKK-beta-dependent IFN-stimulated genes (ISGs) also contain ISREs. In response to IFN-gamma, inhibitor of NF-kappa B (I kappa B) kinase beta (IKK-beta) is required to activate both newly synthesized IRF1 and the p65 subunit of NF-kappa B, which contributes to ip-10 expression by binding to kappa B sites in the ip-10 promoter, with little or no activation of classical NF-kappa B. In contrast to IFN-gamma, IL-1 beta induces ip-10 expression rapidly but transiently, by activating classical NF-kappa B and increasing the synthesis of IRF1. Together, IL-1 beta and IFN-gamma induce ip-10 synergistically. IFN-gamma does not affect the transient activation of classical NF-kappa B by IL-1 beta and synergistic induction of ip-10 expression by IFN-gamma and IL-1 beta occurs even after the activation of classical NF-kappa B has returned to basal levels. Therefore, IKK-kappa has a novel role in IFN-gamma-dependent activation of chemokine gene expression through its activation of IRF1 and p65.