title: Transcription and activity of 5-fluorouracil converting enzymes in fluoropyrimidine resistance in colon cancer in vitro.
abstract: Cellular resistance to 5-fluorouracil (5-FU) is not completely understood. Since 5-FU shares the pyrimidine pathway with the physiological pyrimidines, we investigated the relationship between fluoropyrimidine metabolism, nucleic acid uptake and cytotoxicity of 5-FU in eight colon tumour cell lines including 5-FU-resistant subclones. The cytotoxicity of 5-FU was increased up to 423-fold when the anabolites 5-fluorouridine (FUrd), 5-fluorodeoxyuridine (FdUrd), and 5-fluorodeoxyuridine monophosphate (FdUMP) were compared with the parent drug in vitro. The enzymes uridine phosphorylase and thymidine phosphorylase were predictive for the cytotoxicity of 5-FU in 5/7 cell lines. Inhibition of uridine phosphorylase and thymidine phosphorylase by antisense strategies effectively antagonised 5-FU, abolishing 84% and 79% of its toxicity. The importance of thymidine phosphorylase was supported by a highly restricted enzyme activity in 5-FU-resistant cells. In 5-FU naive cells, a stimulating effect of 5-FU on thymidylate synthase mRNA and ribonucleotide reductase mRNA expression was observed. In these cells, antisense oligonucleotides to ribonucleotide reductase significantly reduced cell growth. Downregulation of ribonucleotide reductase mRNA in 5-FU-resistant subclones suggests different mechanisms in primary and secondary resistance to 5-FU. Most of the intracellular 5-FU was selectively incorporated into RNA (range: 45-91%) and generally spared DNA (range: 0.2-11%). In synthesising our data, we conclude that drug resistance could be overwhelmed through bypassing limiting steps in the activation of 5-FU. In the majority of colonic tumours, the activity of uridine phosphorylase and thymidine phosphorylase may have prognostic relevance for the cytotoxicity of 5-FU in vitro.
authors.affiliation: Department of Internal Medicine I, University of Vienna, Austria. robert.mader@akh-wien.ac.at
authors.forename: R M
authors.initials: RM
authors.lastname: Mader
authors.forename: A E
authors.initials: AE
authors.lastname: Sieder
authors.forename: J
authors.initials: J
authors.lastname: Braun
authors.forename: B
authors.initials: B
authors.lastname: Rizovski
authors.forename: M
authors.initials: M
authors.lastname: Kalipciyan
authors.forename: M W
authors.initials: MW
authors.lastname: Mueller
authors.forename: R
authors.initials: R
authors.lastname: Jakesz
authors.forename: H
authors.initials: H
authors.lastname: Rainer
authors.forename: G G
authors.initials: GG
authors.lastname: Steger
mesh_terms: D000964:Antimetabolites, Antineoplastic / Q000378:metabolism*; D003110:Colonic Neoplasms / Q000188:drug therapy*; D004247:DNA / Q000378:metabolism; D019008:Drug Resistance, Neoplasm; D005472:Fluorouracil / Q000378:metabolism* / Q000494:pharmacology; D006801:Humans; D016376:Oligonucleotides, Antisense / Q000494:pharmacology; D014176:Protein Biosynthesis / Q000187:drug effects; D012313:RNA / Q000378:metabolism; D014158:Transcription, Genetic; D014407:Tumor Cells, Cultured
chemical_list: D000964:Antimetabolites, Antineoplastic; D016376:Oligonucleotides, Antisense; D012313:RNA; D004247:DNA; D005472:Fluorouracil