Activation of macrophage nuclear factor-κB and induction of inducible nitric oxide synthase by LPS

BACKGROUND: Chronic lung disease (CLD) of prematurity is a major problem of neonatal care. Bacterial infection and inflammatory response have been thought to play an important role in the development of CLD and steroids have been given, with some benefit, to neonates with this disease. In the present study, we assessed the ability of lipopolysaccharide (LPS) to stimulate rat alveolar macrophages to produce nitric oxide (NO), express inducible nitric oxide synthase (iNOS) and activate nuclear factor-κB (NF-κB) in vitro. In addition, we investigated the impact of dexamethasone and budesonide on these processes. METHODS: Griess reaction was used to measure the nitrite level. Western blot and a semi-quantitative RT-PCR were performed to detect iNOS expression. Electrophoretic mobility shift assay (EMSA) was performed to analyze the activation of NF-κB. RESULTS: We found that LPS stimulated the rat alveolar macrophages to produce NO in a dose (≥10 ng/ml) and time dependent manner (p < 0.05). This effect was further enhanced by IFN-γ (≥10 IU/ml, p < 0.05), but was attenuated by budesonide (10(-4)–10(-10) M) and dexamethasone (10(-4)–10(-6) M) (p < 0.05). The mRNA and protein levels of iNOS were also induced in response to LPS and attenuated by steroids. LPS triggered NF-κB activation, a mechanism responsible for the iNOS expression. CONCLUSION: Our findings imply that Gram-negative bacterial infection and the inflammatory responses are important factors in the development of CLD. The down-regulatory effect of steroids on iNOS expression and NO production might explain the beneficial effect of steroids in neonates with CLD.