Nepalolide A inhibits the expression of inducible nitric oxide synthase by modulating the degradation of IkappaB-alpha and IkappaB-beta in C6 glioma cells and rat primary astrocytes.

1 The effects of nepalolide A on the expression of inducible nitric oxide synthase (iNOS) caused by incubation with lipopolysaccharide/interferon-gamma (LPS/IFN-gamma) or tumour necrosis factor-alpha/interleukin-1beta/IFN-gamma (TNF-alpha/IL-1beta/IFN-gamma, mixed cytokines) in C6 glioma cells and primary astrocytes of rat were investigated. The mechanisms by which nepalolide A confers its effect on iNOS expression were also elucidated. 2 Treatment with LPS/IFN-gamma and mixed cytokines for 24 h elicited the induction of iNOS activity as determined by nitrite accumulation in the culture medium and assay of enzyme activity. Nepalolide A at 10 microM abrogated the LPS/IFN-gamma- and mixed cytokines-mediated induction of iNOS by more than 90% in C6 glioma cells, and by 80% for mixed cytokines-induced induction of iNOS in primary astrocytes. The effect of nepalolide A (2-10 microM) was concentration-dependent. 3 The inhibition of iNOS induction by nepalolide A was attributed to decreases in the content of iNOS protein and the level of iNOS mRNA, as measured by immunoblotting and reverse transcriptase-polymerase chain reaction. 4 Electrophoretic mobility shift assay was used to evaluate the effect of nepalolide A on the activation of nuclear factor-kappaB (NF-kappaB). Results showed that nepalolide A diminished the LPS/IFN-gamma-mediated association of NF-kappaB with consensus oligonucleotide in a concentration-dependent manner. The activation of NF-kappaB by mixed cytokines was modulated both in the extent of activation and in its time-course by nepalolide A. 5 The ability of nepalolide A to inhibit NF-kappaB activation was further confirmed by studies on the degradation of the inhibitor of NF-kappaB, IkappaB, as measured by immunoblotting. 6 The present study demonstrates that the attenuation of NF-kappaB activation by nepalolide A was mediated by blockade of the degradation of IkappaB, leading to suppression of the expression of iNOS.