5-FLUOROURACIL-MEDIATED THYMIDYLATE SYNTHASE INDUCTION IN MALIGNANT AND NONMALIGNANT HUMAN-CELLS

Thymidylate synthase (TS, EC 2.1.1.45) is an important: target enzyme for the fluoropyrimidines used in cancer chemotherapy. Studies have do cumented a 2- to 4-fold induction of TS protein following 5-fluorourac il (5-FU) treatment of malignant cells. We measured the effect that 5- FU exposure had on TS protein expression in nonmalignant human breast (MCF-10 and HBL-100), colorectal (ATCC Co18, Co112, and Co33), and bon e marrow cells along with malignant breast (MCF-7) and colon (NCI-H630 ) cells. Twenty-four hours after plating, cells were treated with 0.01 to 10 mu M of 5-FU for a period of 24 hr. TS was quantitated by Weste rn immunoblot using monoclonal antibody TS106. Absolute levels of TS i n nonmalignant cells were substantially lower than in the malignant li nes, ranging from approximately 40% in HBL-100 cells to less than 10% in the colon lines. An approximately two-fold induction in the level o f TS was found for all cell lines examined, and there was a strong dep endence on 5-FU exposure concentration in free TS levels of MCF-WT, an d total TS levels of H630-WT, normal bone marrow, and MCF 10 cells. Th e induction of TS following 5-FU exposure is a generally observed phen omenon in both malignant and nonmalignant cells, suggesting that a sel ective means for inhibiting this induction may be critical for the dev elopment of alternative therapeutic strategies using 5-FU and the anti folate TS inhibitors. (C) 1998 Elsevier Science Inc.