Effects of nitrogen dioxide on the expression of intercellular adhesion molecule-1, neutrophil adhesion, and cytotoxicity: Studies in human bronchial epithelial cells

Nitrogen Dioxide (NO2) is a product of high-temperature combustion and an environmental oxidant of concern. We have recently reported that early changes in NO2-exposed human bronchial epithelial cells are causally linked to increased generation of proinflammatory mediators, such as nitric oxide/nitrite and cytokines like interleukin (IL)-1 beta, tumor necrosis factor (TNF)-alpha and IL-8. The objective of the present in vitro study was to further delineate the cellular mechanisms of NO2-mediated toxicity, and to define the nature of cell death that ensues upon exposure of normal human bronchial epithelial (NHBE) cells to a brief high dose of NO2. Our results demonstrate that the NHBE cells undergo apoptotic cell death during the early post-NO2 period, but this is independent of any significant increase in caspase-3 activity. However, necrotic cell death was more prevalent at later time intervals. Interestingly, an increased expression of HO-1, a redox-sensitive stress protein, was observed in NO2-exposed NHBE cells at 24 h. Since neutrophils (PMNs) play an active role in acute lung inflammation and resultant oxidative injury, we also investigated changes in human PMN-NHBE cell interactions. As compared to normal cells, increased adhesion of PMNs to NO2-exposed cells was observed, which resulted in an increased NHBE cell death. The latter was also increased in the presence of IL-8 and TNF-alpha + interferon (IFN)-gamma, which correlated with upregulation of intercellular adhesion molecule-1 (ICAM-1). Our results confirmed an involvement of nitric oxide (NO) in NO2-induced cytotoxicity. By using NO synthase inhibitors such as L-NAME and 3-aminoguanidine (AG), a significant decrease in cell death, PMN adhesion, and ICAM-1 expression was observed. These findings indicate a role for the L-arginine/NO synthase pathway in the observed NO2-mediated toxicity in NHBE cells. Therapeutic strategies aimed at controlling excess generation of NO and/or inflammatory cytokines may be useful in alleviating NO2-mediated adverse effects on the bronchial epithelium.