Glucocorticoids regulate intestinal glutaminase expression.

Abstract

BACKGROUND: The metabolism of glutamine by the small intestinal mucosal cells is highly dependent on the glutaminase enzyme. Because mucosal glutamine utilization is increased after operation, we hypothesized that the elevated glucocorticoid hormones that occur after surgical stress regulate expression of mucosal glutaminase at the molecular level. METHODS: Adult rats received saline solution or dexamethasone (0.5 mg/kg, one dose) and were sacrificed at various times after treatment. Jejunal mucosal total RNA was extracted for Northern hybridization with an alpha-32P-labeled rat glutaminase cDNA. The mRNA of the constitutively expressed gene beta-actin was the control for RNA loading. Quantitation of glutaminase transcripts was determined by densitometric scanning and normalized to actin. Glutaminase activity (micromoles per milligram of protein per hour) and its kinetic parameters, maximal transport velocity (micromoles per milligram of protein per hour) and Michaelis-Menten constant (micromoles per liter), were also determined. RESULTS: Dexamethasone increased glutaminase mRNA (twofold at 4 hours, sixfold at 12 hours; p less than 0.01) and glutaminase-specific activity. The increase in message preceded the increase in activity by 4 hours, consistent with de novo RNA synthesis followed by protein synthesis. The increase in glutaminase activity was the result of a 21% increase in the maximal enzyme capacity (maximal transport velocity = 8.6 +/- 0.5 mumol/mg protein/hr in control rats vs 10.4 +/- 0.3 mumol/mg protein/hr in rats treated with dexamethasone; p less than 0.01) rather than a change in enzyme affinity (Michaelis-Menten constant). CONCLUSION: Glucocorticoids may accelerate intestinal glutamine utilization by increasing glutaminase expression, an adaptive response that could provide more energy for mucosal cells in stress states.