Inhaled Nitric Oxide Prevents 3-Nitrotyrosine Formation in the Lungs of Neonatal Mice Exposed to >95% Oxygen

Inhaled nitric oxide is being evaluated as a preventative therapy for patients at risk for bronchopulmonary dysplasia (BPD). Nitric oxide (NO), in the presence of superoxide, forms peroxynitrite, which reacts with tyrosine residues on proteins to form 3-nitrotyrosine (3-NT). However, NO can also act as an antioxidant and was recently found to improve the oxidative balance in preterm infants. Thus, we tested the hypothesis that the addition of a therapeutically relevant concentration (10 ppm) of NO to a hyperoxic exposure would lead to decreased 3-NT formation in the lung. FVB mouse pups were exposed to either room air (21% O₂) or >95% O₂ with or without 10 ppm NO within 24 h of birth. In the first set of studies, body weights and survival were monitored for 7 days, and exposure to >95% O₂ resulted in impaired weight gain and near 100% mortality by 7 days. However, the mortality occurred earlier in pups exposed to >95% O₂ + NO than in pups exposed to >95% O₂ alone. In a second set of studies, lungs were harvested at 72 h. Immunohistochemistry of the lungs at 72 h revealed that the addition of NO decreased alveolar, bronchial, and vascular 3-NT staining in pups exposed to both room air and hyperoxia. The lung nitrite levels were higher in animals exposed to >95% oxygen + NO than in animals exposed to >95% oxygen alone. The protein levels of myeloperoxidase, monocyte chemotactic protein-1, and intracellular adhesion molecule-1 were assessed after 72 h of exposure and found to be greatest in the lungs of pups exposed to >95% O₂. This hyperoxia-induced protein expression was significantly attenuated by the addition of 10 ppm NO. We propose that in the presence of >95% O₂, peroxynitrite formation results in protein nitration; however, adding excess NO to the >95% O₂ exposure prevents 3-NT formation by NO reacting with peroxynitrite to produce nitrite and NO₂. We speculate that the decreased protein nitration observed with the addition of NO may be a potential mechanism limiting hyperoxic lung injury.