5-fluorouracil modulation of dihydrofolate reductase RNA levels in methotrexate-resistant KB cells.

Cytotoxicity and growth inhibition by 5-fluorouracil in methotrexate-resistant dihydrofolate reductase gene-amplified KB cells in the presence of 30 microM thymidine correlates with incorporation of this fluorinated pyrimidine into RNA. Growth of these cells over several generations in the presence of inhibitory concentrations of 5-fluorouracil does not depress the steady state levels of either 18 or 28 S RNA but actually causes an increase. Similarly the rates of RNA and protein synthesis in 5-fluorouracil-treated cells are not decreased. The level of dihydrofolate reductase RNA from 5-fluorouracil-treated cells increases in a dose-dependent manner correlated with 5-fluorouracil incorporation into RNA. The qualitative size distribution of the dihydrofolate reductase RNA species is unaffected when examined by the Northern blotting technique indicating an RNA processing lesion is not induced by 5-fluorouracil incorporation into RNA. As the dose of dihydrofolate reductase RNA increases, there is no change in the level of dihydrofolate reductase specific activity, but the level of enzyme activity per cell increases. The relevance of these phenomena to the mechanism of 5-fluorouracil effect on RNA and relevance to combination chemotherapy with methotrexate are discussed.