Arginine deficiency augments inflammatory mediator production by airway epithelial cells in vitro

Previously we showed that reduced availability of the essential amino acid tryptophan per se attenuates post-transcriptional control of interleukin (IL)-6 and IL-8 leading to hyperresponsive production of these inflammatory mediators by airway epithelial cells. Availability of the non-essential amino acid arginine in the inflamed airway mucosa of patients with asthma is reduced markedly, but it is not known whether this can also lead to an exaggerated production of IL-6 and IL-8.

IL-6 and IL-8 were determined by ELISA in culture supernatants of NCI-H292 airway epithelial-like cells and normal bronchial epithelial (NHBE) cells that were exposed to TNF-alpha, LPS or no stimulus, in medium with or without arginine. Arginine deficiency may also result from exposure to poly-L-arginine or major basic protein (MBP), which can block arginine uptake. Epithelial cells were exposed to these polycationic proteins and L-(14)C-arginine uptake was assessed as well as IL-6 and IL-8 production. To determine the mode of action, IL-6 and IL-8 mRNA profiles over time were assessed as were gene transcription and post-transcriptional mRNA degradation.

For both NCI-H292 and NHBE cells, low arginine concentrations enhanced basal epithelial IL-6 and IL-8 production and synergized with TNF-alpha-induced IL-6 and IL-8 production. Poly-L-arginine enhanced the stimulus-induced IL-6 and IL-8 production, however, blocking arginine uptake and the enhanced IL-6 and IL-8 production appeared unrelated. The exaggerated IL-6 and IL-8 production due to arginine deficiency and to poly-L-arginine depend on a post-transcriptional and a transcriptional process, respectively.

We conclude that both reduced arginine availability per se and the presence of polycationic proteins may promote airway inflammation by enhanced pro-inflammatory mediator production in airway epithelial cells, but due to distinct mechanisms.