Inhibition of extracellular signal-regulated kinase suppresses endotoxin-induced nitric oxide synthesis in mouse macrophages and in human colon epithelial cells

Macrophages produce large amounts of nitric oxide (NO) in response to proin flammatory cytokines and lipopolysaccharide (LPS) by expressing inducible i soform of NO synthase (iNOS). We examined the role of extracellular signal- regulated kinase p42/44(MAPK) (Erk1/2) in signal transduction pathways lead ing to induction of NO synthesis in response to LPS in J774 mouse macrophag es and T-84 human colon epithelial cells. LPS activated Erk1/2 and induced iNOS and subsequent NO production. Erk1/2 activation was inhibited by PD 98 059, a specific inhibitor of mitogen-activated protein kinase kinase (Mek) that is an upstream activator of Erk1/2. At corresponding concentrations PD 98059 reduced LPS-induced NO formation by 40 to 50% by inhibiting iNOS exp ression in J774 and T-84 cells. Inhibition of iNOS expression was not media ted by nuclear factor-kappa B because PD 98059 had no effect on nuclear fac tor-kappa B activity in J774 macrophages, In addition, PD 98059 reduced LPS -induced L-arginine transport into the cells as measured in J774 macrophage s, whereas the availability of tetrahydrobiopterin was not a limiting facto r in NO production after PD 98059. Our results indicate that Erk1/2 activat ion mediates up-regulation but is not essential for LPS-induced iNOS expres sion.