5-Aminosalicylic acid (5ASA), 4ASA, their N-acetylated metabolites N-acetyl-5ASA and N-acetyl-4ASA, olsalazine, and colchicine impair interferon-gamma (IFN gamma) induced HLA-DR expression on a colonic cell line, HT-29. The mechanism of this effect is now reported. HT-29 cells were cultured with 50 U/ml IFN gamma with or without drug, and northern blot analysis was performed using a probe for the beta chain of the DR molecule. IFN gamma led to a noticeable increase in HLA-DR mRNA which was attenuated by the drugs. Analysis of the specific binding of increasing concentrations of 125I-IFN gamma by non-linear regression showed a Kd of 1.35 x 10(-10) M and 2.3 x 10(5) binding sites per HT-29 cell. Binding of 125I-IFN gamma was reduced by incubation with increasing concentrations of unlabelled IFN gamma but not with IFN alpha. Incubation with therapeutic concentrations of drugs led to the following reductions in binding: 10 mM 5ASA, 20% (p < 0.001); 10 mM N-acetyl-5ASA, 24% (p < 0.01); 10 mM 4ASA, 21% (p < 0.005); 10 mM N-acetyl-4ASA, 29% (p < 0.001); and 1 mM olsalazine, 29% (p < 0.001). Colchicine (10(-7) M) and 10(-5) M prednisolone had no effect. Incubation with higher concentrations of the drugs revealed a dose-response effect on binding with complete inhibition by 100 mM 4ASA and 10 mM olsalazine, and lesser degrees of inhibition by 100 mM 5ASA, N-acetyl-5ASA, and N-acetyl-4ASA. At concentrations found in the rectal lumen, the salicylates used in inflammatory bowel disease impair the binding of IFN gamma to its receptor on colonic epithelial cells.
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