Inducible nitric oxide synthase activity and expression in a human colonic epithelial cell line, HT‐29

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Abstract
1 We have determined which cytokines regulate the expression of human inducible nitric oxide synthase (iNOS) mRNA and nitrite generation in the human colonic epithelial cell line HT-29. 2 Growth arrested cell cultures were stimulated with the human recombinant cytokines interleukin-lα (IL-lα), tumour necrosisfactor-α (TNF-α), interferon γ (IFN-γ) or vehicle added alone or in combination. Human iNOS mRNA was determined by Northern blot analysis and nitrite generation by the use of a fluorometric assay. 3 Unstimulated cells produced a small time-dependent increase in nitrite generation of 50 ± 4, 75 ± 8, and 103 ±8 nM per 106 cells at 24 h, 48 h, and 72 h respectively. This nitrite generation was unaffected by cycloheximide (5 μg ml−1) pretreatment and iNOS mRNA was not detected. 4 None of cytokines alone induced either iNOS mRNA expression or an increase in nitrite generation. The combination of IL-lα/IFN-γ produced a highly significant (P< 0.001) 4 fold increase in nitrite production at 48 h, compared to basal values, while no other pair of cytokines was effective. 5 Time course studies with IL-lα/IFN-γ combination revealed significant (P< 0.001) increases in nitrite at 24 h (153 ±7), 48 h (306 ±24), and 72 h (384 ±15) compared to basal values of 50 ±4, 75 ±8, and 103 ±8 nM per 106 cells respectively. 6 Studies with IL-lα/IFN-γ combination demonstrated a time dependent expression of iNOS mRNA, first observed at 6 h, peaked at 24 h and was undetectable by 72 h. IL-lα (0.3–10 ng ml−1) and IFN-γ (10–300 u ml−1) in combination induced a concentration-dependent expression of iNOS mRNA at 24 h. 7 Pretreatment with cycloheximide before IL-lα/IFN-γ stimulation reduced nitrite levels to basal values. These data suggest that the IL-lα/IFN-γ-induced nitrite production by HT-29 cells is dependent on de novo protein synthesis, probably the iNOS enzyme. 8 The addition of TNF-α produced a significant (P< 0.001) 3 fold increase of IL-lα/IFN-γ-induced nitrite generation. In marked contrast the presence of TNF-α had no effect on IL-lα/IFN-γ-induced iNOS mRNA expression by HT-29 cells. These findings suggest that the up-regulation by TNF-α of IL-lα/IFN-γ-induced nitrite generation is at the post-transcriptional level. 9 These data suggest that pro-inflammatory cytokines induce NO production in colonic epithelial cells probably due to the induction of iNOS and these cells may be a major source of NO generation in inflammatory bowel disease.