title: Enhancement of 5-fluorouracil-induced cytotoxicity by leucovorin in 5-fluorouracil-resistant gastric cancer cells with upregulated expression of thymidylate synthase.
abstract: BACKGROUND
  Elucidation of the mechanisms by which gastric cancer cells acquire resistance to 5-fluorouracil (5FU) may provide important clues to the development of effective chemotherapy for 5FU-resistant gastric cancer
  
  
  METHODS
  Four 5FU-resistant cell lines (MKN45/5FU, MKN74/5FU, NCI-N87/5FU, and KATOIII/5FU) were established by continuous exposure of the cells to progressively increasing concentrations of 5FU for about 1 year. Then, mRNA expression levels of four genes associated with 5FU metabolism, i.e., thymidylate synthase (TS), dihydropyrimidine dehydrogenase, thymidine phosphorylase, and orotate phosphoribosyltransferase, were quantitatively evaluated by real-time reverse transcriptase-polymerase chain reaction. In addition, TS protein expression was measured by Western blot analysis.
  
  
  RESULTS
  As compared with the parent cell lines, the 5FU-resistant cell lines showed 3.8- to 11.6-fold higher resistance to 5FU, as well as 1.9- to 3.5-fold higher TS mRNA expression and 1.6- to 7.1-fold higher TS protein expression. In contrast, the expressions of other genes did not differ significantly among the cell lines. The cytotoxicity of 5FU was enhanced 2.3- to 2.8 fold by leucovorin (LV) against three of the four 5FU-resistant cell lines.
  
  
  CONCLUSIONS
  Collectively, LV enhanced the cytotoxicity of 5FU not only against the parent gastric cancer cell lines, but also against the 5FU-resistant cell lines, even those with elevated TS expression levels. These results suggest that clinical studies of a combination of 5FU and LV are warranted in patients who have recurrent gastric cancer after 5FU-based therapy.
authors.affiliation: Field of Chemotherapy on Digestive Organs Division of Gastrointestinal Surgery, Tokyo Women's Medical University Graduate School of Medicine, 8-1 Kawada-cho, Shinjuku-ku, Tokyo, 162-8666, Japan.
authors.forename: Ayako
authors.initials: A
authors.lastname: Nakamura
authors.forename: Go
authors.initials: G
authors.lastname: Nakajima
authors.forename: Ryuji
authors.initials: R
authors.lastname: Okuyama
authors.forename: Hidekazu
authors.initials: H
authors.lastname: Kuramochi
authors.forename: Yurin
authors.initials: Y
authors.lastname: Kondoh
authors.forename: Toshinori
authors.initials: T
authors.lastname: Kanemura
authors.forename: Teiji
authors.initials: T
authors.lastname: Takechi
authors.forename: Masakazu
authors.initials: M
authors.lastname: Yamamoto
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Hayashi
mesh_terms: D045744:Cell Line, Tumor; D019008:Drug Resistance, Neoplasm / Q000187:drug effects*; D005472:Fluorouracil / Q000494:pharmacology*; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D006801:Humans; D002955:Leucovorin / Q000494:pharmacology*; D013274:Stomach Neoplasms / Q000188:drug therapy* / Q000235:genetics / Q000378:metabolism; D013940:Thymidylate Synthase / Q000235:genetics / Q000378:metabolism*; D015854:Up-Regulation / Q000187:drug effects
chemical_list: D013940:Thymidylate Synthase; D002955:Leucovorin; D005472:Fluorouracil