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PubMed1994

Glucocorticoids regulate glutaminase gene expression in human intestinal epithelial cells.

Glutamine is essential for intestinal metabolism and function, but its circulating and luminal availability to the mucosa may be diminished during critical illness. We hypothesized that glucocorticoids, which are produced in increased amounts during critical illness, accelerate mucosal glutamine metabolism. We studied intestinal glutamine utilization by examining the regulation of glutaminase in vitro, the enterocyte's principal enzyme of glutamine metabolism. Differentiated confluent human enterocytic cells (Caco-2 cells) were incubated with dexamethasone. Glutaminase activity was assayed and mRNA was extracted. Glutaminase transcripts were labeled with a 32P-labeled glutaminase cDNA probe, quantitated by phosphoimaging, and normalized to beta-actin. Dose- and time-response studies were performed. Dexamethasone-treated cells were also incubated with actinomycin D and cycloheximide. Dexamethasone (DEX) increased mucosal glutaminase activity by 45%, with maximal response at 12 hr. This increase was dose-dependent and was significant at doses of 1 and 10 microM. The dexamethasone-mediated increase in glutaminase activity was associated with a 40% increase in glutaminase mRNA. The DEX-induced increase in glutaminase activity was inhibited by actinomycin D and cycloheximide, indicating the requirement for de novo RNA and protein synthesis. Glucocorticoids stimulate glutamine metabolism in these human enterocytic cells by increasing the activity of glutaminase, a response that is preceded by an increase in gene transcription. This glucocorticoid-mediated increase in glutaminase activity may be a mechanism by which gut glutamine metabolism is maintained during critical illness when blood glutamine levels are diminished and food intake is often interrupted.

Autoradiography↗

PubMed1994

Decrease of glutaminase expression by interferon-gamma in human intestinal epithelial cells.

BACKGROUND: Glutaminase, the principal enzyme of glutamine hydrolysis, breaks down glutamine to supply energy and intermediates for cell growth and is present in high concentrations in replicating tissues such as intestinal epithelium and malignant tumors. In the host with cancer, glutaminase activity in the gut mucosa diminishes as the tumor grows, but the regulation of this response is unknown. Because cytokines may regulate the altered glutamine metabolism that is characteristic of the host with cancer, we studied the effects of cytokines on gut mucosal glutaminase expression in vitro using the human enterocytic Caco-2 cell line. METHODS: Differentiated confluent cells were incubated with interleukin (IL)-1, IL-6, tumor necrosis factor, or interferon-gamma (IFN-gamma). After a 12-h incubation, glutaminase-specific activity and kinetic parameters (maximal enzyme activity [Vmax] and enzyme affinity [Km]) were determined. Glutaminase protein concentration was determined by Western blot analysis using a rabbit antirat polyclonal antibody. Total cellular RNA was extracted for Northern hybridization and radiolabeled with a glutaminase cDNA probe. RESULTS: Of the cytokines studied, only IFN-gamma altered glutaminase activity. Kinetic studies indicated a decrease in activity secondary to a 25% decrease in Vmax with no change in Km, consistent with a reduction in the number of glutaminase molecules rather than a change in enzyme affinity. Glutaminase protein was decreased 50% in IFN-gamma-treated cells when compared with controls. This decrease was dose-independent and was associated with a concomitant 75% decrease in glutaminase messenger RNA levels. These reductions in message and protein translated into a 60-80% decrease in functional glutaminase-specific activity. CONCLUSIONS: This IFN-gamma-mediated decrease in glutaminase activity may be one mechanism by which gut glutamine metabolism is diminished as the tumor grows and becomes the principal organ of glutamine use.

Adenocarcinoma↗