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-day (HCT-8/7dR) repeated exposures to fluorouracil (FUra). The HCT-8/7dR cells were studied in more detail to determine their mechanism of resistance to FUra. Thymidylate synthase activity, binding of 5-fluorodeoxyuridylate to thymidylate synthase, and incorporation of FUra into RNA were not different between the parental and resistant sublines. However, folylpolyglutamate synthetase activity was markedly decreased in this subline using tetrahydrofolate, 5,10-methylenetetrahydrofolate, and methotrexate as substrates. Northern blot analysis revealed decreased folylpolyglutamate synthetase mRNA expression in HCT-8/7dR cells as compared to HCT-8 cells. These findings indicate that low-dose continuous exposure schedules to FUra are cytotoxic primarily due to inhibition of thymidylate synthase and underscores the role of 5,10-methylenetetrahydrofolate polyglutamates in this inhibition.