Glutamine deficiency shifts the asthmatic state toward neutrophilic airway inflammation

Glutamine depletion was achieved by using the glutamine transporter ASCT2 inhibitor l-?-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.

AbstractBackground

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-?-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.