To elucidate the mechanism of resistance to
5-fluorouracil (5-FU) in human
gastric cancer cells, we established a cell line MKN45/F2R, which acquired
5-FU resistance as a result of continuous exposure to increasing dosages of
5-FU over a year. The cell line showed 157-fold elevated
5-FU resistance compared to the MKN45 human
gastric cancer parental cell line. Furthermore, the cells acquired crossresistance to
paclitaxel and
docetaxel. To identify the mechanism of
5-FU resistance, the expressions of
5-FU metabolic
enzymes were examined. Although
protein expression and activity of
thymidylate synthase and
dihydropyrimidine dehydrogenase did not change, orotate phosphoribosyl-
transferase (OPRT)
protein expression and activity significantly decreased in the
5-FU resistant MKN45/F2R cells. Interestingly, expression of
proteins related to
taxane resistance including
P-glycoprotein, class III
beta-tubulin and Bcl-2 increased in MKN45/F2R cells. OPRT-knockout MKN45 parent cells using
small interfering RNA demonstrated 15.8-fold increased resistance to
5-FU compared to the control cells. However, resistance to
paclitaxel and
docetaxel was not observed. These results strongly indicate that decreased activity of OPRT plays an important role in the acquired resistance of
gastric cancer cells towards 5-FU; however, it does not play a direct role in
paclitaxel and
docetaxel resistance. Further studies are now underway to identify genes related to crossresistance to these chemotherapeutic agents.