Sesquiterpene lactones inhibit inducible nitric oxide synthase gene expression in cultured rat aortic smooth muscle cells.

Nitric oxide (NO) is an important regulator and effector molecule in various inflammatory disease states. High output of NO during inflammation is generated by the inducible isoform of nitric oxide synthase (iNOS). Sesquiterpene lactones are derived from Mexican-Indian medicinal plants and are known to have potent anti-inflammatory properties. The mechanisms by which sesquiterpene lactones exert their anti-inflammatory effects are not fully understood. In the current studies we determined if the sesquiterpene lactones, parthenolide and isohelenin, modulate iNOS gene expression in cultured rat aortic smooth muscle cells (RASMC) treated with lipopolysaccharide and interferon-gamma. Treatment with parthenolide or isohelenin inhibited NO production and iNOS mRNA expression in a concentration-dependent manner. Transient transfection studies with an iNOS promoter-luciferase reporter plasmid demonstrated that parthenolide and isohelenin also inhibited activation of the iNOS promoter. Inhibition of iNOS promoter activation was associated with inhibition of both I-kappaBalpha degradation and nuclear translocation of NF-kappaB. Neither parthenolide nor isohelenin induced the heat shock response in RASMC. We conclude that sesquiterpene lactones inhibit iNOS gene expression by a mechanism involving stabilization of the I-kappaBalpha/NF-kappaB complex. This effect is not related to induction of the heat shock response. The ability of sesquiterpene lactones to inhibit iNOS gene expression may account, in part, for their anti-inflammatory effects.