Post-transcriptional inhibition of glutamine synthetase induction in rat liver epithelial cells exerted by conditioned medium from rat hepatocytes

Recently, a soluble factor produced by primary periportal hepatocytes and different from glutamine was found to completely block induction of glutamine synthetase (GS) by dexamethasone (DEX) in the liver cell line RL-ET-1G. Using Northern and Western blotting we investigated whether this block is regulated on the transcriptional or the post-transcriptional level. Three different species of GS mRNA were detected that all increased (though in different proportions), when the cells were exposed to DEX for 24h. Further maintenance for another 24 h in normal DEX-containing culture medium, conditioned medium from primary periportal hepatocytes or medium containing glutamine did not result in significant differences in GS mRNA levels. In contrast, GS protein and specific GS-activity remained high only under normal culture conditions, whereas both returned to basal levels in conditioned and glutamine-supplemented culture medium. Throughout, GS protein content and specific GS-activity strongly correlated (r = 0,98) excluding that GS-activity is regulated by chemical modification. These data suggest that the decrease in enzyme activity caused by cultivation in CM is controlled on the post-transcriptional level. (C) 2000 Elsevier Science Inc. All rights reserved.