Parallel induction of nitric oxide and tetrahydrobiopterin synthesis in alveolar macrophages

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.

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.

We compared the induction by lipopolysaccharide (LPS), interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha), 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.

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-gamma to stimulate the production of both NO and BH4.

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.