Parallel Induction of Nitric Oxide and Tetrahydrobiopterin Synthesis in Alveolar Macrophages

Abstract

Background: Nitric oxide (NO) and an essential cofactor for both constitutive and inducible NO synthase (NOS) activity, tetrahydrobiopterin (6R-L-erythro-1′,2′-dihydroxypropyl-2-amino-4-hydroxy-5,6,7,8-tetrahydropteridine; BH₄), are thought to be important modulators of function in normal and inflamed airways. However, the exact pathologic roles of NO and BH₄ remain obscure. Even less is known about the effects of cytokines on alveolar macrophages. Objective: This study was designed to determine whether NO and BH₄ are induced by cytokines in mouse alveolar macrophages and to investigate whether NO synthesis is affected by changes in intracellular BH₄ levels in alveolar macrophages. Methods: We compared the induction by lipopolysaccharide (LPS), interferon-γ (IFN-γ), tumor necrosis factor-α (TNF-α), and interleukin-2 (IL-2) of NO production and BH₄ synthesis in alveolar macrophages. To determine whether NO synthesis is affected by changes in intracellular BH₄ levels in alveolar macrophages, we used inhibitors of BH₄ biosynthesis. Results: Activation of alveolar macrophages induced parallel increases in NO and intracellular BH₄ levels, although induction of the latter appears to be somewhat more sensitive than that of the latter to diverse cytokines. Inducible NO production in alveolar macrophages was blocked by inhibitors of BH₄ biosynthesis. IL-2, an important component of the immunomodulatory system, was only a weak activator of alveolar macrophages by itself but potently synergized with IFN-γ to stimulate the production of both NO and BH₄. Conclusion: Our results suggest that BH₄ synthesis in alveolar macrophages is a potential target for therapeutic intervention in airway inflammatory diseases, such as asthma, cystic fibrosis, and acute bronchial infections whose pathology may be mediated by overproduction of NO.