DEXAMETHASONE INCREASES JEJUNAL GLUTAMINE-SYNTHETASE EXPRESSION VIA TRANSLATIONAL REGULATION

Glutamine provides energy and precursors for nucleotide biosynthesis f or the gut mucosa, and it is essential for intestinal metabolism and f unction. During stress states, glutamine uptake of circulating and lum inal glutamine may be diminished, but the ability of the gut mucosa to synthesize glutamine de novo in response to this decreased delivery r emains undefined. Since the glucocorticoids play an important role in regulating interorgan glutamine metabolism during catabolic states, we hypothesized that these hormones induce the expression of gut mucosal glutamine synthetase (GS), the enzyme that catalyzes the intracellula r biosynthesis of glutamine. Adult rats were treated with dexamethason e (DEX, 0.5 mg/kg intraperitoneally) or saline (controls). At various times after treatment (4, 12, 24, 48, and 72 hours), jejunal mucosal G S-specific activity was assayed, and total RNA was extracted. GS trans cripts were detected by Northern blot analysis, using a radiolabeled r at CS cDNA probe. Transcripts were quantitated by phospho-imaging and normalized to beta-actin. An anti-GS polyclonal antibody was used to q uantitate GS protein concentrations by Western blot analysis. The rela tive quantities of GS translated were measured using a cell-free prote in-synthesizing system (reticulocyte lysate assay). Data were analyzed using analysis of variance and were considered statistically signific ant for p < 0.05. DEX increased GS activity by 45% 12 hours after admi nistration. Western blot analysis revealed an increase in the concentr ation of the GS protein in the jejunum of DEX-treated animals. Norther n blot analysis demonstrated no significant change in CS mRNA levels a fter DEX treatment, indicating the possibility of posttranscriptional regulation. In vitro translational experiments demonstrated that the q uantity of CS translated was increased by 25% after the administration of DEX. These data suggest that glucocorticoids may increase jejunal mucosal GS levels by accelerating protein translation. This adaptive r esponse could provide glutamine for the gut mucosa during stress, when exogenous glutamine supplies may be rate limiting.