Fluoropyrimidine effects on RNA metabolism in murine tumor cells

The interaction of different fluoropyrimidines with RNA metabolism was compared in murine tumor cells in order to elucidate possible mechanisms of RNA-directed fluoropyrimidine cytotoxicity. In S-180 cells, a six hour exposure to 1 μM 5-fluorouridine (FUrd) and 200 μM 5'-deoxy-5-fluorouriding (5'-dFUrd) resulted in similar levels of 5-fluorouridine-triphosphate (FUTP) incorporation in total cellular RNA, 7.8 and 12.5 pmole per μg RNA, respectively. Yet, the resulting RNA-directed or thymidine (dThd)-nonreversible cytotoxicities were quite different with the more toxic FUrd resulting in 26% clonal growth and the less toxic 5'-dFUrd resulting in 89% clonal growth on a soft-agar cloning assay. Thus, the total level of incorporation in RNA was a poor determinant of the resulting RNA-directed cytotoxicity when different fluoropyrimidines were compared. A potential biochemical mechanism for this differential incorporation in RNA is that individual fluoropyrimidines may be metabolic channeled directly into different types of RNA. In order to test this theory, L5178Y cells were exposed to trace concentrations of 0.038 μM [3H]5-fluorouracil (FUra) and 0.056 μM [3H]FUrd for 4 hours and the relative distribution of [3H]FUTP in cytoplasmic ribosomal, transfer, and polyadenylated (poly (A)) RNA was determined