GLUCOCORTICOIDS REGULATE RAT GLUTAMINE-SYNTHETASE EXPRESSION IN A TISSUE-SPECIFIC MANNER

During stress states, organismal glutamine production is augmented sec ondary to an increase in the activity of glutamine synthetase (GS) in the lung and skeletal muscle. Because glucocorticoids are key regulato rs of the metabolic response to stress, we undertook a survey of gluco corticoid induction of GS expression in rat organs in response to dexa methasone. Male adult rats were injected with glucocorticoid or vehicl e and 4 hr later, 10 organs were assayed for GS messenger RNA and prot ein contents by Northern and Western blotting. We observed a 20-fold r ange of GS mRNA levels in organs of control animals. Blotting detected two GS RNA species of approximately 2.8- and 1.4-kb sizes in all tiss ue except testis, where an additional 2-kb RNA species was observed, G lyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA levels were also assayed and used as a normalization factor. An approximately 10-fold r ange of GAPDH mRNA levels was observed. Four hours after dexamethasone injection, a nearly a 5-fold increase in glutamine synthetase mRNA le vels in lung and muscle, as well as an approximately a-fold increase i n heart were observed. Relative to GAPDH mRNA, a significant decrease in GS mRNA levels was observed in the liver. A wide range of glutamine synthetase protein contents were observed in rat organs. Comparison o f Northern and Western blotting results revealed a dichotomy in the ra tio of relative GS mRNA and protein level in rat organs, suggesting th at tissue-specific posttranscriptional processes determine GS protein levels. Four hours after dexamethasone injection, an apparent increase in GS protein was observed in the lung, muscle, and thymus; however, significant induction of GS protein was demonstrated only in lung. The differential response of GS mRNA and protein levels to dexamethasone suggests that GS expression is induced by glucocorticoids in a tissue- specific manner. (C) 1995 Academic Press, Inc.