5-Formyluracil and its nucleoside derivatives confer toxicity and mutagenicity to mammalian cells by interfering with normal RNA and DNA metabolism

Oxidation of the methyl group of thymine yields 5-(hydroxymethyl)uracil (5- hmU) and 5-formyluracil (5-foU) as major products. Whereas 5-hmU appears to have normal base pairing properties, the biological effects of 5-foU are r ather poorly characterised. Here, we show that the colony forming ability o f Chinese hamster fibroblast (CHF) cells is greatly reduced by addition of 5-foU, 5-formyluridine (5-foUrd) and 5-formyl-2'-deoxyuridine (5-fodUrd) to the growth medium. There are no toxic effects of 5-fodUrd on cells defecti ve in thymidine kinase or thymidylate synthetase, suggesting that the toxic ity may be caused by 5-fodUrd phosphorylation and subsequent inhibition of thymidylate synthetase. Whereas 5-fodUrd was the most effective 5-foU deriv ative causing cell growth inhibition, the corresponding ribonucleoside 5-fo Urd was more effective in inhibiting [H-3]uridine incorporation in non-divi ding rat nerve cells in culture, suggesting that 5-foUrd exerts its toxicit y through interference with RNA rather than DNA synthesis. Addition of 5-fo U and 5-fodUrd was also found to promote mutagenicity at the hypoxanthine-g uanine phosphoribosyltransferase (HPRT) locus of CHF cells; 5-fodUrd being three orders of magnitude more potent than 5-foU. In contrast, neither 5-hm U nor 5-(hydroxymethyl)-2'-deoxyuridine induced HPRT mutations. The mutatio n induction indicates that 5-foU will be incorporated into DNA and has base pairing properties different from that of thymine. These results suggest t hat 5-foU residues, originating from incorporation of oxidised bases, nucle osides or nucleotides or by oxidation of DNA, may contribute significantly to the damaging effects of oxygen radical species in mammalian cells. (C) 2 001 Elsevier Science Ireland Ltd. All rights reserved.