Resistance to bromodeoxyuridine mutagenesis and toxicity in mammalian cells selected for resistance to hydroxyurea

Abstract

Mutant cell lines resistant to hydroxyurea (HU), an inhibitor of the enzyme ribonucleotide reductase, were selected from a line of Syrian hamster melanoma cells. Mutant lines were selected for resistance to 0.3 mM HU, and from these lines, second-step mutants were selected for resistance to 1.0 mM HU. The HU^r lines were tested for their responses to 5-bromodeoxyuridine (BrdU), in terms of toxicity, mutagenesis, and incorporation of BrdU into DNA. All of the HU^r lines showed increased resistance to the toxic effects of BrdU. In addition, the HU^r lines all were resistant to BrdU mutagenesis. Overall, there was good correlation among the levels of resistance to HU toxicity, BrdU toxicity, and BrdU mutagenesis in the HU^r lines. These tests were carried out under conditions such that the parental and HU^r cells incorporated equal amounts of BrdU into nuclear DNA. Therefore, the resistance of the HU^r cells to the effects of BrdU cannot be attributed to decreased incorporation of BrdU into DNA. These results suggest that the HU^r cells have an alteration in ribonucleotide reductase activity that simultaneously confers resistance to HU and BrdU. The properties of the HU^r cells suggest that the perturbation of deoxycytidine metabolism by BrdU triphosphate inhibition of ribonucleotide reductase activity plays a key role in the toxic and mutagenic effects of BrdU in mammalian cells.