Regulation of iNOS mRNA levels in endothelial cells by glutathione, a double-edged sword

Both inducible nitric oxide synthase (iNOS) and glutathione are important mediators in various physiological and pathological conditions in humans. In human endothelial cells the intracellular glutathione levels were modulated by N-acetyl-L-cysteine (NAC), a precursor of glutathione and 1,3-bis(chloroethyl)-1-nitrosouresea (BCNU), an inhibitor of glutathione reductase. BCNU significantly decreased reduced glutathione (GSH) but increased oxidized glutathione (GSSG) whereas NAC markedly elevated GSH with a relatively small increase in GSSG. Appropriate concentrations of GSH and GSSG increase the expression of iNOS gene. However, either GSH or GSSG at a too high concentration inhibits its expression, indicating that iNOS gene is fine tuned by the metabolites of glutathione cycle. The changes of iNOS mRNA steady state levels by the glutathione metabolites were associated with a similar alteration in its gene transcription and NF-kappa B activity. BCNU at high concentrations also shortens the half-life of iNOS mRNA, suggesting a role of GSSG in the stability of the iNOS gene. Thus, the change of glutathione levels in vitro can regulate iNOS mRNA steady state levels in a bi-phasic manner in human endothelial cells.