Glucocorticoids regulate glutamine synthetase expression in lung epithelial cells

During septic states efflux of glutamine from the lung increases, a response sustained by an increase in glutamine synthetase (GS) activity. We have used a cell culture model employing a rat epithelial cell line of pulmonary origin (L2 cells) to study the effect of several hormones and cytokines which mediate the septic shock response on GS expression. We found that GS mRNA and GS protein contents increased rapidly and severalfold in response to physiologically relevant levels of the synthetic glucocorticoid dexamethasone (Dex). In contrast, GS expression was not markedly induced by Escherichia coli lipopolysaccharide (LPS), cytokines, activated complement C5a, or prostaglandins. Dex did not alter the kinetics of GS mRNA decay in the presence of actinomycin D. The increase in GS mRNA in response to Dex was completely blocked by RU-38486 and by actinomycin D, but not by cycloheximide (CHX). CHX together with Dex caused a superinduction of GS mRNA. GS mRNA decay kinetics suggested that this superinduction is at least in part caused by an approximately twofold increase in GS mRNA half-life caused by CHX. In addition, actinomycin D was found to increase GS mRNA half-life by similar to 50%. Actinomycin D plus CHX acted synergistically to cause a profound inhibition of GS mRNA decay. Our results are consistent with regulation of lung GS expression via a direct glucocorticoid receptor-mediated response. In addition, GS mRNA decay in L2 cells seems to be regulated by two independent mechanisms, one which is sensitive to CHX and one which is sensitive to actinomycin D.