N1-S1/FdUrd
Novikoff hepatoma cells, which lack
thymidine kinase activity, are resistant to
5-fluorouracil (FUra) as well as
5-fluorodeoxyuridine (FdUrd), suggesting that the pathway, FUra leads to FdUrd leads to
FdUMP, is utilized for the conversion of FUra to
FdUMP. However, the inhibition of
thymidylate synthetase activity, the presumed target of
FdUMP, by 1 X 10(-4) M FUra in intact N1-S1
Novikoff hepatoma cells, which have significant levels of
thymidine kinase activity, is completely eliminated by 5 X 10(-4) M
hydroxyurea, which is a potent inhibitor of
ribonucleotide reductase. These results imply that the formation of
ribonucleotides and does not involve
thymidine kinase. This apparent dichotomy can be explained by the fact that, in addition to the well known lack of
thymidine kinase activity, [14C]FUra conversion to
ribonucleotides is greatly depressed in the N1-S1/FdUrd cells. Hence, the formation of
FdUMP from FUra in
Novikoff hepatoma cells apparently proceeds primarily via the intermediate formation of
ribonucleotides. The decreased conversion of FUra to
ribonucleotides in N1-S1/FdUrd cells decreases not only the ability of the analog to inhibit
DNA synthesis, but also its effect on
RNA metabolism. FUra, at a concentration of 1 X 10(-5) M, inhibits rRNA maturation in N1-S1 cells, but not in N1-S1/FdUrd cells. Since N1-S1/FdUrd cells are completely resistant to 1 X 10(-5) M FUra, whereas N1-S1 cells are completely inhibited by 1 X 10(-5) M FUra, even in the presence of 1 X 10(-4) M
thymidine, the effects of FUra on
RNA metabolism appear to contribute significantly to the cytotoxicity of the analog at higher
drug concentrations.