Cytokines Stimulate Lung Epithelial Cells to Release Nitric Oxide

Cytokine release from alveolar macrophages and subsequent interaction of these cytokines with the bronchial epithelium can induce epithelial cells to release inflammatory mediators. Nitric oxide(NO), a highly reactive gas formed from arginine by nitric oxide synthase(NOS), is known to be involved in inflammation and edema formation, and the inducible form of NOS(iNOS) can be increased by cytokines. In this context, we hypothesized that lung epithelial cells could be stimulated by cytokines released by alveolar macrophages to express iNOS. To test this hypothesis, the murine lung epithelial cell line, LA-4, or the human lung epithelial cell line, A549, were stimulated with culture supernatant fluids from alveolar macrophages. NO production was assessed by evaluating the culture supernatant fluids for nitrite and nitrate, the stable end products of NO. Both murine and human cell culture supernatant fluids demonstrated an increase in nitrite and nitrate which were time- and dose-dependent and attenuated by $TNF{\alpha}$ and IL-$1{\beta}$ antibodies(p<0.05, all comparisons). Consistent with these observations, cytomix a combination of $TNF{\alpha}$, IL-$1{\beta}$, and $\gamma$-interferon, stimulated the lung epithelial cell lines as well as primary cultures of human bronchial epithelial cells to increase their NO production as evidenced by an increase in nitrite and nitrate in their culture supernatant fluids, an increase in the iNOS staining by immunocytochemistry, and an increase in iNOS mRNA by Northern blottin(p<0.05, all comparisons). The cytokine effects on iNOS were all attenuated by dexamethasone. To determine if these in vitro observations are reflected in vivo, exhaled NO was measured and found to be increased in asthmatics not receiving corticosteroids. These data demonstrate that alveolar macrophage derived cytokines increase iNOS expression in lung epithelial cells and that these in vitro observations are mirrored by increased exhaled NO levels in asthmatics. Increased NO in the lung may contribute to edema formation and airway narrowing.