Identification of glucocorticoid-responsive elements that control transcription of rat glutamine synthetase

Basal expression of glutamine synthetase (GS) is very low in rat lung and muscle and remarkably enhanced by glucocorticoid hormones during trauma and catabolic states. Although this response is believed to be transcriptionally regulated, the genetic elements responsible for tissue-specific glucocorticoid induction of GS expression have not been identified. A rat lung epithelial cell line (L2) and a glucocorticoid receptor-deficient human prostate cancer cell line (PC3), together with GS reporter gene constructs, were utilized in gene transfer experiments to identify two regions within the rat genomic clone gGS3 that imparted dexamethasone (Dex) responsiveness to both the homologous GS promoter and the heterologous herpes simplex virus thymidine kinase promoter in glucocorticoid receptor-dependent fashions. One region lies nearly 6 kb upstream of the GS transcription initiation site, and the other lies within the first intron of the GS gene. Dex responsiveness was localized to a 325-bp fragment of the intron region containing a canonical glucocorticoid response element and to a 225-bp fragment of the far-upstream region containing three separate glucocorticoid response element half-sites. The GS promoter exhibited relatively high basal activity that was repressed by inclusion of the far-upstream or the intron glucocorticoid-responsive region. Dex treatment negated this repression. A model is suggested in which the glucocorticoid-receptor unit causes derepression of lung and muscle GS transcription during trauma and catabolic states.