Effects of Nitrogen Dioxide on the Expression of Intercellular Adhesion Molecule-1, Neutrophil Adhesion, and Cytotoxicity: Studies in Human Bronchial Epithelial Cells

Abstract

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)-1β, tumor necrosis factor (TNF)-α 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-α + interferon (IFN)-γ, 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.