Glutamine deficiency shifts the asthmatic state toward neutrophilic airway inflammation

Background The administration of L‐glutamine (Gln) suppresses allergic airway inflammation via the rapid upregulation of MAPK phosphatase (MKP)‐1, which functions as a negative regulator of inflammation by deactivating p38 and JNK mitogen‐activated protein kinases (MAPKs). However, the role of endogenous Gln remains to be elucidated. Therefore, we investigated the mechanism by which endogenous Gln regulates MKP‐1 induction and allergic airway inflammation in an ovalbumin‐based murine asthma model. Methods We depleted endogenous Gln levels using L‐γ‐glutamyl‐p‐nitroanilide (GPNA), an inhibitor of the Gln transporter ASCT2 and glutamine synthetase small interfering siRNA. Lentivirus expressing MKP‐1 was injected to achieve overexpression of MKP‐1. Asthmatic phenotypes were assessed using our previously developed ovalbumin‐based murine model, which is suitable for examining sequential asthmatic events, including neutrophil infiltration. Gln levels were analyzed using a Gln assay kit. Results GPNA or glutamine synthetase siRNA successfully depleted endogenous Gln levels. Importantly, homeostatic MKP‐1 induction did not occur at all, which resulted in prolonged p38 MAPK and cytosolic phospholipase A2 (cPLA2) phosphorylation in Gln‐deficient mice. Gln deficiency augmented all examined asthmatic reactions, but it exhibited a strong bias toward increasing the neutrophil count, which was not observed in MKP‐1‐overexpressing lungs. This neutrophilia was inhibited by a cPLA2 inhibitor and a leukotriene B4 inhibitor but not by dexamethasone. Conclusion Gln deficiency leads to the impairment of MKP‐1 induction and activation of p38 MAPK and cPLA2, resulting in the augmentation of neutrophilic, more so than eosinophilic, airway inflammation. Glutamine depletion was achieved by using the glutamine transporter ASCT2 inhibitor l‐γ‐glutamyl‐p‐nitroanilide or glutamine synthetase siRNA. In glutamine deficiency, no MKP‐1 induction and prolonged p38 and cPLA2 phosphorylation were observed in response to airway ovalbumin challenge. As a result, dominant neutrophilic Th1 response, rather than eosinophilic Th2 response occurred. Abbreviations: ASCT2, solute carrier 1A5; cPLA2, cytosolic phospholipase A2; EOS, eosinophil; Gln, glutamine; GPNA, l‐γ‐glutamyl‐p‐nitroanilide; GS, glutamine synthetase; MKP‐1, MAPK phosphatase‐1; NEU, neutrophil; OVA, ovalbumin; siRNA, small interfering RNA.