Anti-Inflammatory Cytokine Interleukin-4 Inhibits Inducible Nitric Oxide Synthase Gene Expression in the Mouse Macrophage Cell Line RAW264.7 through the Repression of Octamer-Dependent Transcription

Inducible nitric oxide synthase (iNOS) is a signature molecule involved in the classical activation of M1 macrophages and is induced by theNos2gene upon stimulation with Th1-cell derived interferon-gamma (IFNγ) and bacterial lipopolysaccharide (LPS). Although the anti-inflammatory cytokine IL-4 is known to inhibitNos2gene expression, the molecular mechanism involved in the negative regulation ofNos2by IL-4 remains to be fully elucidated. In the present study, we investigated the mechanism of IL-4-mediatedNos2transcriptional repression in the mouse macrophage-like cell line RAW264.7. Signal transducer and activator of transcription 6 (Stat6) knockdown by siRNA abolished the IL-4-mediated inhibition ofNos2induced by IFNγ/LPS. Transient transfection of a luciferase reporter gene containing the 5′-flanking region of theNos2gene demonstrated that an octamer transcription factor (OCT) binding site in the promoter region is required for both positive regulation by IFNγ/LPS and negative regulation by IL-4. Although IL-4 had no inhibitory effect on the DNA-binding activity of constitutively expressed Oct-1, IL-4-inducedNos2-reporter transcriptional repression was partially attenuated by overexpression of the coactivator CREB-binding protein (CBP). These results suggest that a coactivator/cofactor that functionally interacts with Oct-1 is a molecular target for the IL-4-mediated inhibition ofNos2and that IL-4-activated Stat6 represses Oct-1-dependent transcription by competing with this coactivator/cofactor.