Inhibition of iNOS expression and NO production by anti-inflammatory steroids. Reversal by histone deacetylase inhibitors.

In inflammation, nitric oxide (NO) is produced by inducible nitric oxide synthase (iNOS) induced by bacterial products and cytokines, and NO acts as a regulatory and pro-inflammatory mediator. Glucocorticoids are powerful anti-inflammatory agents that inhibit the expression of iNOS and various other inflammatory factors. Histone deacetylation has been recently described as a novel mechanism how glucocorticoids down-regulate transcriptional activation of some inflammatory genes. The aim of the present study was to investigate the effects of inhibitors of histone deacetylation on the suppressive effects of glucocorticoids on NO production and iNOS expression. Dexamethasone and a dissociated glucocorticoid RU24858 inhibited NO production, and iNOS protein and mRNA expression in macrophages exposed to bacterial lipopolysaccharide (LPS). In the presence of a glucocorticoid receptor (GR) antagonist mifepristone, dexamethasone and RU24858 had no effect on NO production. The role of histone deacetylation in the glucocorticoid effect was studied by using three structurally different inhibitors of histone deacetylases (HDACs): trichostatin A, apicidin and MC1293. HDAC inhibitors reversed the effects of dexamethasone and RU24858 on iNOS expression and NO production. Stably transfected A549/8 cells containing luciferase gene under the control of human iNOS promoter were used in promoter-activity studies. iNOS promoter activity induced by IL-1beta was inhibited by dexamethasone and the inhibitory effect was reversed by HDAC inhibitor trichostatin A. The results suggest that glucocorticoids inhibit iNOS expression and NO production by a GR-mediated and GRE-independent manner through histone deacetylation and transcriptional silencing.