Dexamethasone stimulation of glutaminase expression in mesenteric lymph nodes

Lymphocytes in the mesenteric lymph nodes (MLNs) play a key role in protecting the body from the translocation of bacteria through the bowel during catabolic states. Lymphocytes use glutamine for energy and for DNA synthesis and have high levels of the glutaminase (GA) enzyme that regulates intracellular glutamine metabolism. This study tested the hypothesis that the increase in circulating glucocorticoid hormones that occurs in response to stress states regulates MLN lymphocyte GA expression at the molecular level. Adult male rats (n = 60) received a single dose of dexamethasone (DEX 0.5 mg/kg intraperitoneally) or saline vehicle (CONT). Ileocolic lymph nodes were excised from anesthetized rats via laparotomy at 2, 4, 12, 24, 48, and 72 hours after injection, and GA activity was assayed in MLN homogenates. A second group of rats received repeated doses of DEX (0.5 mg/kg/d for 4 days). GA activity was assayed in MLN homogenates, and total RNA was extracted for quantitation by Northern hybridization using a phosphorus 32-labeled rat GA cDNA probe. GA activity was increased within 2 hours after a single dose of DEX, with a peak after 4 hours, Kinetic analysis at the 4-hour time point showed an increase in the maximum GA activity (maximal transport velocity [Vmax]: 619 +/- 107 nmol/mg protein/hr in DEX versus 380 +/- 53 nmol/mg protein/hr in CONT, p < 0.05), with no change in GA affinity (Michaelis constant [Km]: 2.28 +/- 0.39 mM in DEX versus 2.20 +/- 0.36 mM in CONT, p = NS). Repeated doses of DEX resulted in a twofold increase in GA activity (550 +/- 125 nmol/mg protein/hr versus 1,175 +/- 40, p < 0.01). Simultaneously, GA mRNA levels were increased by 70%. Glucocorticoids stimulate GA activity and gene expression in lymphocytes that reside in MLNs.