Molecular mechanisms underlying dexamethasone inhibition of iNOS expression and activity in C6 glioma cells

Abstract

The synthetic glucocorticoid dexamethasone is routinely used to stabilize patients with malignant gliomas. One putative target for glucocorticoid action is inducible nitric oxide synthase (iNOS), which is produced by the tumor cells as well as the host immune cells. In this study, we characterize the stimulatory effects of lipopolysaccharide (LPS) and the cytokine, tumor necrosis factor-alpha (TNFalpha), as well as the inhibitory effect of glucocorticoids, on iNOS gene expression and activity in C6 glioma cells cultured in vitro. LPS significantly increased iNOS mRNA expression, peaking at 6 h, while nitrite formation increased with time up to 72 h. Although TNFalpha alone induced neither iNOS mRNA expression nor nitrite formation, it significantly potentiated the effect of LPS on both. iNOS activity induced by LPS with or without TNFalpha was dose-dependently inhibited by dexamethasone, reaching a maximum of approximately 83% inhibition. This was completely reversed by the addition of RU38486, an antagonist of glucocorticoid receptors (GR). Dexamethasone inhibited iNOS mRNA expression; however, the maximum inhibition obtained was only 10%. These results suggest that as for induction of iNOS activity in C6 cells in vitro, the stimulatory effect of LPS is mainly due to an action at the transcriptional level. TNFalpha does not have intrinsic inducing activity, but has potentiating effects at the transcriptional and possibly at the posttranscriptional levels in the presence of LPS. The inhibitory effect of dexamethasone is GR-mediated and is mainly due to action at the posttranscriptional level.