Endogenous nitric oxide downregulates the Bcl-2 expression of eosinophils through mitogen-activated protein kinase in bronchial asthma

Background: Eosinophil apoptosis might play a crucial role in the maintenance of persistent airway inflammation in asthma. Nitric oxide (NO) synthase activity is upregulated in eosinophils of asthmatic patients. Mitogen-activated protein kinase (MAPK) is implicated in regulating eosinophil apoptosis by modulating Bcl-2 expression. NO-induced cell apoptosis is associated with an inhibition of Bcl-2 expression. Objective: We sought to study whether NO might induce eosinophil apoptosis through extracellular signal-regulated protein kinase (ERK) or p38 MAPK pathways and Bcl-2 expression. Methods: Eosinophils were freshly isolated from peripheral blood of 16 asthmatic patients and 12 healthy subjects and then cultured with or without the NO synthase inhibitor N-nitro-L-arginine methyl ester (L-NAME) at 1 and 10 mmol/L for 16 hours. The expression of Bcl-2 and induced NO synthase on eosinophils was analyzed by means of flow cytometry. Apoptosis was assessed by means of propidium iodide and DNA ladder. The activity of ERK and p38 MAPK was determined by means of Western blotting. Results: The induced NO synthase immunoreactivities and the spontaneous release of nitrite from the eosinophils of asthmatic patients were higher compared with those of healthy subjects. Eosinophils of asthmatic patients were found to express more highly constitutive Bcl-2 than those of healthy subjects. After incubation for 16 hours, the expression of Bcl-2 on eosinophils from patients with asthma was significantly enhanced by L-NAME. The percentage of apoptosis was decreased by the addition of 1 mmol/L L-NAME in patients with asthma. The activity of p38 MAPK and ERK in eosinophils from patients with asthma was enhanced in the presence of L-NAME. An inhibition of MAPK reduced the Bcl-2 expression and increased eosinophil apoptosis in patients with asthma. Conclusion: We concluded that inhibition of endogenous NO might increase the expression of Bcl-2 in eosinophils from patients with asthma through the signaling pathway of ERK or p38 MAPK, which in turn decrease the apoptosis.