title: Reduced activity of anabolizing enzymes in 5-fluorouracil-resistant human stomach cancer cells.
abstract: The mechanism of resistance to 5-fluorourcil (5-FU) was studied with NUGC-3/5FU/L, a human stomach cancer cell line which had acquired resistance as a consequence of repeated 5-day exposures to stepwise-increasing concentrations of 5-FU in vitro. NUGC-3/5FU/L was 200-fold and over 16-fold resistant to 96-h and 1-h exposures to 5-FU, respectively. NUGC-3/5FU/L incorporated less 5-FU into RNA, indicating resistance to the RNA-directed action of 5-FU. On the other hand, NUGC-3/5/5FU/L also showed resistance to in situ thymidylate synthase (TS) inhibition by 5-FU. Polymerase chain reaction-single-strand conformation polymorphism analysis of TS cDNA and a FdUMP ligand binding assay showed that quantitative and qualitative alterations of TS are not responsible for this resistance. In contrast, the ability to metabolize 5-FU to its active metabolites, FUTP and FdUMP, was reduced in NUGC-3/5FU/L. We found that not only the activities of uridine phosphorylase/kinase and orotate phosphoribosyl-transferase (OPRT), but also the level of phosphoribosyl pyrophosphate, a cosubstrate for OPRT, were significantly lower in NUGC-3/5FU/L than in the parent NUGC-3. These results indicated that resistance to 5-FU in NUGC-3/5FU/L is due to reduced activities of 5-FU-anabolizing enzymes, but not to an alteration of TS. 2'-Deoxyinosine effectively enhanced TS inhibition by 5-FU in the resistant cells, thus markedly sensitizing them to 5-FU.
authors.affiliation: Cancer Chemotherapy Center, Japanese Foundation for Cancer Research, Tokyo, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Inaba
authors.forename: J
authors.initials: J
authors.lastname: Mitsuhashi
authors.forename: H
authors.initials: H
authors.lastname: Sawada
authors.forename: N
authors.initials: N
authors.lastname: Miike
authors.forename: Y
authors.initials: Y
authors.lastname: Naoe
authors.forename: A
authors.initials: A
authors.lastname: Daimon
authors.forename: K
authors.initials: K
authors.lastname: Koizumi
authors.forename: H
authors.initials: H
authors.lastname: Tsujimoto
authors.forename: M
authors.initials: M
authors.lastname: Fukushima
mesh_terms: D000964:Antimetabolites, Antineoplastic / Q000378:metabolism; D004273:DNA, Neoplasm / Q000235:genetics; D004351:Drug Resistance; D005472:Fluorouracil / Q000378:metabolism*; D005796:Genes; D006801:Humans; D010754:Phosphoribosyl Pyrophosphate / Q000378:metabolism; D018807:Polymorphism, Single-Stranded Conformational; D012334:RNA, Neoplasm / Q000737:chemistry; D013274:Stomach Neoplasms / Q000201:enzymology*; D013940:Thymidylate Synthase / Q000378:metabolism*; D014407:Tumor Cells, Cultured
chemical_list: D000964:Antimetabolites, Antineoplastic; D004273:DNA, Neoplasm; D012334:RNA, Neoplasm; D010754:Phosphoribosyl Pyrophosphate; D013940:Thymidylate Synthase; D005472:Fluorouracil