Enhancement by Uridine of the Anabolism of 5-Fluorouracil in Mouse T-Lymphoma (S-49) Cells1

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 /¿M) was increased more than two-fold by simultaneous exposure to 10 /IM undine. Studies were undertaken to explain the mechanism by which undine enhanced the cytotoxicity of FUra in S-49 cells. Uridine (10 UM) increased by about 50% both the anabolism of 1.0 pM [3H]FUra to acid-soluble metabolites and the incorporation of 1.0 ^M [3H]FUra into RNA. However, the incorporation of 1.0 fiM [3H]FUra into these fractions was less than that seen with 2.4 UM [3H]FUra, a dose which was equitoxic to 1.0 tiM [3H]FUra plus 10 ^M undine. 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 ^M uridine. In addition, 5-fluoro-2'-deoxyuridine monophosphate levels and the amount of [3H]FUra which was incorporated into the alkali-stable, acidinsoluble fraction were not increased by uridine. Uridine (10 ¿<M) inhibited de novo pyrimidine biosynthesis by 70%, while 5phosphoribosyl-1-pyrophosphate levels were unchanged. Pre sumably, the inhibition of de novo pyrimidine biosynthesis de creased orotic acid levels and allowed more FUra to be anabolized to 5-fluorouridine monophosphate via orotate phosphoribosyl transferase. Furthermore, 2.4 /¿MFUra inhibited the incor poration of [3H]deoxyguanosine into DNA by 50% after 24 h of incubation. In contrast, 1.0 I¿MFUra plus 10 UM 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 UM FUra plus 10 MMuridine killed S-49 cells as compared to 2.4 ¿¿M FUra alone, and that the enhance ment by uridine of the cytotoxicity of FUra was due, in part, to the increased anabolism of FUra to ribonucleotides.