Regulation of inducible nitric oxide synthase in proinflammatory cytokine-stimulated human primary astrocytes

The present study was undertaken to investigate the mechanism of expression of inducible nitric oxide synthase (iNOS) in human primary astrocytes. Among IL-1β, TNF-α, and IFN-γ, only IL-1β alone was capable of inducing iNOS. Similarly, among different cytokine combinations, the combinations involving only IL-1β as a partner were capable of inducing iNOS. The combination of IL-1β and IFN-γ (IL-IF) induced the expression of iNOS at the highest level. All three cytokines alone induced the activation of AP-1 while IL-1β and TNF-α but not IFN-γ induced the activation of NF-κB. However, among the three cytokines, only IL-1β was capable of inducing the activation of CCAAT/enhancer-binding proteinβ (C/EBPβ), suggesting an essential role of C/EBPβ in the expression of iNOS in astrocytes. Although IL-1β and IFN-γ alone induced the activation of AP-1, the combination of these two cytokines (IL-IF) markedly inhibited the activation of AP-1. Consistently, JNK-I, a specific inhibitor of JNK, inhibited IL-1β-mediated activation of AP-1 and expression of iNOS. On the other hand, JNK-I had no effect on (IL-IF)-induced expression of iNOS, suggesting that the activation of AP-1 is involved only during the low level of iNOS induction by IL-1β but not during the high level of induction by IL-IF. In contrast, the activation of γ-activation site (GAS) was involved only during the high level of induction by IL-IF but not during the low level of induction by IL-1β. However, the activation of NF-κB and C/EBPβ was involved in the induction of iNOS by IL-1β as well as by IL-IF.