Dexamethasone suppresses iNOS gene expression by upregulating I-κBα and inhibiting NF-κB

Cytokine-stimulated inducible nitric oxide synthase (iNOS) gene expression is dependent on nuclear factor-κB (NF-κB) activation and is suppressed by glucocorticoids (GC). In this study we examined the molecular mechanisms of GC inhibition of iNOS expression in rat hepatocytes. Combinations of tumor necrosis factor-α, interleukin-1β, and interferon-γ (cytokine mixture CM) induced high levels of iNOS mRNA and NO synthesis. The synthetic GC dexamethasone markedly repressed iNOS mRNA and protein expression, and nuclear run-on assays showed that this inhibition was occurring at the level of transcription. In addition, transfection studies showed that CM-stimulated activity of a 1.6-kb murine iNOS promoter fragment linked upstream of luciferase was suppressed by dexamethasone. Electromobility shift assays demonstrated that CM induced the appearance of an NF-κB complex composed of p50 and p65 subunits; the addition of dexamethasone markedly decreased this band shift. I-κBα expression was decreased by CM and upregulated in the presence of dexamethasone. Subsequently, nuclear p65 levels were decreased by dexamethasone compared with CM-treated cells. Thus GC repress NF-κB DNA-binding activity in rat hepatocytes in part through the upregulation of its inhibitor I-κBα. These data indicate that one mechanism by which GC block iNOS expression is through the inhibition of NF-κB activation resulting in decreased iNOS transcription.