Synergistic cytokine-induced nitric oxide production in human alveolar epithelial cells

Nitric oxide (NO) is an important mediator molecule in regulating normal airway function, as well as in the pathophysiology of inflammatory airway diseases. In addition, cytokines are potent messenger molecules at sites of inflammation. The specific relationship among IL-1β, TNF-α, and IFN-γ on iNOS induction and NO synthesis in human alveolar epithelial cells has not been determined. In addition, rigorous methods to determine potential synergistic action between the cytokines have not been employed. We exposed monolayer cultures of A549 cells to a factorial combination of three cytokines (IL-1β, TNF-α, and IFN-γ) and three concentrations (0, 5, and 100 ng/mL). TNF-α alone does not induce NO production directly; however, it does have a stimulatory effect on IL-1β-induced NO production. IL-1β and INF-γ both induce NO production alone, yet at different concentration thresholds, and act synergistically when present together. In the presence of all three cytokines, the net effect of NO production exceeds the predicted additive effect of each individual cytokine and the two-way interactions. Several plausible mechanisms of synergy among IL-1β, TNF-α, and IFN-γ in NO production from human alveolar epithelial cells (A549) are proposed. In order to verify the proposed mechanisms of synergy, future experimental and theoretical studies must address several molecular steps through which the iNOS gene is expressed and regulated, as well as the expression and regulation of enzyme cofactors and substrates.