5-FLUOROURACIL SUPPRESSES NITRIC-OXIDE BIOSYNTHESIS IN COLON-CARCINOMA CELLS

Nitric oxide is an important cellular mediator that plays a role in re gulating 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-flu orouracil (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 N-G- monomethyl-L-arginine, nitroarginine, and aminoguanidine, Production o f nitric oxide by DLD-1 cells was due to the expression of the inducib le (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 regu lated posttranscriptionally. Treatment of DLD-1 cells with concentrati ons of 5-FUra that are not growth inhibitory or cytotoxic strongly inh ibited their ability to express nitric oxide synthase and produce nitr ic oxide in response to IFN-gamma, This effect was not reversed with t hymidine, indicating that inhibition of nitric oxide production was du e to incorporation of 5-FUra into RNA, However, pretreatment of DLD-1 cells with 5-FUra before stimulation with IFN-gamma also suppressed ni tric 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.