title: Effects of 5-fluorouracil on dihydrofolate reductase and dihydrofolate reductase mRNA from methotrexate-resistant KB cells.
abstract: Growth of methotrexate-resistant dihydrofolate reductase gene-amplified KB cells in the presence of 5-fluorouracil results in an increase in dihydrofolate reductase mRNA. This increase can be solely attributed to a species of RNA of approximately 3.5 kilobase pairs in size. Although dihydrofolate reductase enzyme activity increases per cell with increasing 5-fluorouracil, there is a decrease of enzyme activity per mg of protein (Dolnick, B. J., and Pink, J. J. (1983) J. Biol. Chem. 258, 13299-13306). The rate of in vivo enzyme synthesis, as assayed by immunoprecipitation and supported by gel electrophoresis, does not decrease and may in fact increase with increasing 5-fluorouracil. Translation of purified dihydrofolate reductase mRNA in vitro shows that the rate of translation is unaffected by 5-fluorouracil incorporation into mRNA. The inhibition of dihydrofolate reductase by a monospecific polyclonal antiserum is reduced with extracts from 5-fluorouracil-treated cells. Inhibition of dihydrofolate reductase by methotrexate is significantly reduced in extracts from 5-fluorouracil-treated cells compared to control extracts. Tight binding of [3H]methotrexate is also different in extracts from 5-fluorouracil-treated cells. This data supports the hypothesis of translational miscoding during protein synthesis as a major mechanism of 5-fluorouracil-mediated cytotoxicity and suggests a new mechanism of 5-fluorouracil-methotrexate antagonism.
mesh_terms: D000818:Animals; D004351:Drug Resistance; D005470:Fluorometry; D005472:Fluorouracil / Q000494:pharmacology*; D005784:Gene Amplification; D007624:KB Cells / Q000201:enzymology; D008727:Methotrexate / Q000494:pharmacology*; D012333:RNA, Messenger / Q000378:metabolism*; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics / Q000378:metabolism*
chemical_list: D012333:RNA, Messenger; D013762:Tetrahydrofolate Dehydrogenase; D005472:Fluorouracil; D008727:Methotrexate