Glutamine deficiency shifts the asthmatic state toward neutrophilic
airway inflammation
Abstract
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 (si)RNA. 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 A 2 (cPLA 2)
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 cPLA
2 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 cPLA
2, resulting in the augmentation of neutrophilic, more
so than eosinophilic, airway inflammation.