Cytokine Modulation of Na+-dependent Glutamine Transport Across the Brush Border Membrane of Monolayers of Human Intestinal Caco-2 Cells

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Abstract
Previous studies have demonstrated that Na+-dependent brush border glutamine transport is diminished in septic patients. To examine the potential regulation of this decreased transport by endotoxin, cytokines, or glucocorticoids, the human intestinal Caco-2 cell line was studied in vitro. Na+-dependent glutamine transport across the apical brush border membrane was assayed in confluent monolayers of differentiated cells that were 10 days old. Uptake of 50 μM glutamine was determined after a 12-hour incubation with varying doses (10 to 1000 U/mL) of tumor necrosis factor-aP, interleukin-1, interleukin-6, interferon-Γ, and various combinations of these cytokines. Studies were also done in cells incubated with E. coli endotoxin (1 μg/mL) or dexamethasone (1 and 10 μM). Endotoxin, tumor necrosis factor, interleukin-1, and interleukin-6 alone and in combination did not significantly reduce Na+-dependent glutamine transport across the brush border of Caco-2 cells. Dexamethasone decreased glutamine transport by 20%, but this decrease was not apparent for 48 hours. Interferon consistently decreased glutamine transport by 30%; this was due to a reduction in carrier maximal transport velocity (3427 ± 783 pmol/mg protein/minute in controls versus 2279 ± 411 in interferon, p < 0.05) rather than a change in Km (276 ± 29 μM in controls versus 333 ± 74 in interferon, p = not significant). The combination of interferon + dexamethasone + tumor necrosis factor + interleukin-1 resulted in a 38% decrease in transport activity. Cytokines and glucocorticoids may work independently and synergistically in regulating Na+-dependent brush border glutamine transport in human intestinal cells. Whether these signal molecules play a central role in the cause of the diminished brush border glutamine transport that occurs in septic patients requires further study.