Inhibition of inducible nitric oxide synthase in the human intestinal epithelial cell line, DLD-1, by the inducers of heme oxygenase 1, bismuth salts, heme, and nitric oxide donors

Background-The inducible isoform of nitric oxide synthase (iNOS) may be inv olved in the mucosal injury associated with inflammatory bowel disease (IBD ). In contrast with iNOS, the inducible heme oxygenase 1 (HO-1) is consider ed to act as a protective antioxidant system. Aims-To evaluate the effects of the known HO-1 inducers, cadmium and bismut h salts, heme, and nitric oxide (NO) donors, on iNOS activity, and expressi on in the human intestinal epithelial cell line DLD-1. Methods-iNOS activity was assessed by the Griess reaction and the radiochem ical L-arginine conversion assay. iNOS mRNA and iNOS protein expression wer e determined by northern and western blotting, respectively. Results-Cytokine exposure led to induction of iNOS activity, iNOS mRNA, and iNOS protein expression. Preincubation of DLD-1 cells with heme (1-50 muM) inhibited cytokine induced iNOS activity in a concentration dependent mann er. This inhibitory effect was abolished by the HO-1 specific inhibitor tin protoporphyrin. Preincubation with NO donors sodium nitroprusside (SNP 1-1 000 muM) or S-nitroso-acetyl-penicillamine (SNAP 1-1000 PM), or with the he avy metals cadmium chloride (10-40 muM), bismuth citrate, or ranitidine bis muth citrate (10-3000 muM) inhibited iNOS activity in a concentration depen dent manner. Moreover, SNP and heme abolished cytokine induced iNOS protein as well as iNOS mRNA expression, whereas cadmium chloride did not modify i NOS protein expression. Conclusions-Heme, the heavy cadmium and bismuth, as well as NO donors, are potent inhibitors of cytokine induced iNOS activity. Heme and NO donors act at the transcriptional level inhibiting iNOS mRNA expression. Such finding s suggest the potential for interplay between the iNOS and HO-1 systems, wh ich may modulate the progress of IBD.