5-Fluorouracil suppresses nitric oxide biosynthesis in colon carcinoma cells

Nitric oxide is an important cellular mediator that plays a role in regulating cellular proliferation of both normal and tumor cells. In the present study, we characterized nitric oxide production by the human colon adenocarcinoma cell line DLD-1 and examined the effects of 5-fluorouracil (5-FUra), an antimetabolite effective against colon tumors, on nitric oxide production. IFN-gamma was found to be a potent inducer of nitric oxide production in DLD-1 cells. This effect was dependent on L-arginine and blocked by the nitric oxide synthase inhibitors NG-monomethyl-L-arginine, nitroarginine, and aminoguanidine. Production of nitric oxide by DLD-1 cells was due to the expression of the inducible (type II) form of nitric oxide synthase. mRNA for the nitric oxide synthase was present in both untreated and IFN-gamma-stimulated cells, as determined by RT-PCR, suggesting that expression of enzyme is regulated posttranscriptionally. Treatment of DLD-1 cells with concentrations of 5-FUra that are not growth inhibitory or cytotoxic strongly inhibited their ability to express nitric oxide synthase and produce nitric oxide in response to IFN-gamma. This effect was not reversed with thymidine, indicating that inhibition of nitric oxide production was due to incorporation of 5-FUra into RNA. However, pretreatment of DLD-1 cells with 5-FUra before stimulation with IFN-gamma also suppressed nitric oxide production. Thus, inhibition of nitric oxide production was not due directly to incorporation of 5-FUra into the mRNA for nitric oxide synthase. Taken together, these data suggest that inhibition of nitric oxide biosynthesis in colon tumor cells by 5-FUra may underlie, at least in part, the efficacy of this antitumor agent.