Blockade of the P38 mitogen-activated protein kinase pathway inhibits inducible nitric oxide synthase and interleukin-6 expression in MC3T3E-1 osteoblasts

Treatment of MC3T3E-1 osteoblast cultures with combined interferon-γ (1FN-γ), lipopolysaccharide (LPS) and tumor necrosis factor-α (TNF-α) induces expressions of inducible nitric oxide synthase (iNOS) and interleukin-6 (IL-6), resulting in sustained releases of large amounts of nitric oxide and IL-6. However IFN-γ, LPS and TNF-α individually induces non-detectable or small amounts of NO and IL-6 in MC3T3E-1 osteoblasts. The role of mitogen-activated protein kinase (MAPK) activation in the early intracellular signal transduction involved in iNOS and IL-6 transcription in the combined agents-stimulated osteoblasts has been investigated. The p38 MAPK pathway is specifically involved in the combined agents-induced NO and IL-6 release, since NO and IL-6 release in the presence of a specific inhibitor of p38 MAPK, 4-(4-fluorophenyl)-2-(4-metylsulfinylphenyl)-5-(4-pyridyl)imidazole (SB203580), are significantly diminished. In contrast, PD98059, a specific inhibitor of MEK1, had no effect on NO and IL-6 release. Northern blot analysis showed that the p38 MAPK pathway controlled iNOS and IL-6 transcription levels. These data suggest that p38 MAPK plays an important role in the secretion of NO and IL-6 in LPS/IFN-γ or TNF-α/IFN-γ-treated MC3T3E-1 osteoblasts.