Enhancement by uridine of the anabolism of 5-fluorouracil in mouse T-lymphoma (S-49) cells.

Uridine enhances the growth inhibition due to 5-fluorouracil (FUra) in a cultured mouse T-cell lymphoma (S-49). Using colony formation assays we found that cytotoxicity produced by 24-h continuous exposure to FUra (0.5 to 3.5 microM) was increased more than two-fold by simultaneous exposure to 10 microM uridine. Studies were undertaken to explain the mechanism by which uridine enhanced the cytotoxicity of FUra in S-49 cells. Uridine (10 microM) increased by about 50% both the anabolism of 1.0 microM [3H]FUra to acid-soluble metabolites and the incorporation of 1.0 microM [3H]-FUra into RNA. However, the incorporation of 1.0 microM [3H]FUra into these fractions was less than that seen with 2.4 microM [3H]-FUra, a dose which was equitoxic to 1.0 microM [3H]FUra plus 10 microM uridine. High-pressure liquid chromatography analysis of the acid-soluble metabolites of FUra did not show any selective change in specific FUra nucleotides, which could explain the increased cytotoxicity associated with 10 microM uridine. In addition, 5-fluoro-2'-deoxyuridine monophosphate levels and the amount of [3H]FUra which was incorporated into the alkali-stable, acid-insoluble fraction were not increased by uridine. Uridine (10 microM) inhibited de novo pyrimidine biosynthesis by 70%, while 5-phosphoribosyl-1-pyrophosphate levels were unchanged. Presumably, the inhibition of de novo pyrimidine biosynthesis decreased orotic acid levels and allowed more FUra to be anabolized to 5-fluorouridine monophosphate via orotate phosphoribosyl transferase. Furthermore, 2.4 microM FUra inhibited the incorporation of [3H]deoxyguanosine into DNA by 50% after 24 h of incubation. In contrast, 1.0 microM FUra plus 10 microM uridine did not inhibit the incorporation of [3H]deoxyguanosine into DNA. The data suggested that there was a qualitative difference in the mechanism by which 1.0 microM FUra plus 10 microM uridine killed S-49 cells as compared to 2.4 microM FUra alone, and that the enhancement by uridine of the cytotoxicity of FUra was due, in part, to the increased anabolism of FUra to ribonucleotides.