DECREASED FOLYLPOLYGLUTAMATE SYNTHETASE EXPRESSION - A NOVEL MECHANISM OF FLUOROURACIL RESISTANCE

HCT-8 cells become rapidly resistant to either 4-h (HCT-8/4hR) or 7-da y (HCT-8/7dR) repeated exposures to fluorouracil (FUra). The HCT-8/7dR cells were studied in more detail to determine their mechanism of res istance to FUra. Thymidylate synthase activity, binding of 5-fluorodeo xyuridylate to thymidylate synthase, and incorporation of FUra into RN A were not different between the parental and resistant sublines. Howe ver, folylpolyglutamate synthetase activity was markedly decreased in this subline using tetrahydrofolate, 5,10-methylenetetrahydrofolate, a nd methotrexate as substrates. Northern blot analysis revealed decreas ed folylpolyglutamate synthetase mRNA expression in HCT-8/7dR cells as compared to HCT-8 cells. These findings indicate that low-dose contin uous exposure schedules to FUra are cytotoxic primarily due to inhibit ion of thymidylate synthase and underscores the role of 5,10-methylene tetrahydrofolate polyglutamates in this inhibition.