Abstract. The mechanism of 5-fluorouracil (5FU) resistance was investigated, focusing on the level of thymidylate synthase (TS) ternary complex formed with fluoro-deoxyuridine monophosphate (FdUMP). MKN45 and 5FU-resistant MKN45/F2R cells were treated with 5FU and fluoro-deoxyuridine (FdU) in combination with deoxyuridine (dU) and thymidine (dT). Subsequently, the levels of ternary complex were determined by western blotting and the cell viability was calculated using an MTT assay. MKN45/F2R cells exhibited 5FU resistance (56.2-fold relative to MKN45 cells), and demonstrated decreased orotate phosphoribosyltransferase (OPRT) and increased TS levels, requiring a higher concentration of 5FU to induce ternary complex formation than MKN45 cells. Following transfection of small interfering RNA against OPRT, MKN45 exhibited increased resistance to 5FU and decreased ternary complex formation subsequent to treatment with 5FU, indicating that decreased OPRT led to increased 5FU resistance. However, MKN45/F2R also exhibited resistance to FdU, which can be converted to FdUMP without OPRT, and there was decreased ternary complex formation after treatment with FdU, indicating that the 5FU-resistant cells had the ability to decrease intracellular FdUMP. The addition of dU and thymidine dT to 5FU promoted the formation of ternary complexes and reversed 5FU resistance in MKN45/F2R cells, although dT inhibited the efficacy of raltitrexed (another TS inhibitor). These results suggested that 5FU-resistant cells had the ability to reduce intracellular FdUMP irrespective of decreased OPRT, which led to resistance to 5FU. This resistance was then inhibited by treatment with dT or dU.
IntroductionDespite recent advances in the development of diagnostic tools for gastric cancer, many patients with gastric cancer continue to be diagnosed at a late stage, and even following curative surgery, recurrent tumors frequently occur. Therefore, gastric cancer remains a major cause for cancer-associated mortality worldwide (1) and the development of novel drug therapies for gastric cancer is important.5FU is currently a key drug for adjuvant therapy following curative surgery (2-4) and for the treatment of metastatic gastric cancer (5,6). Three mechanisms have been proposed for its action: Incorporation into RNA (7), incorporation into DNA (8) and the inhibition of thymidine synthase (TS) leading to the inhibition of DNA de novo synthesis by forming a ternary complex composed of TS, 5,10-methylenetetrahydrofolate (CH2THF) and fluoro-deoxyuridine monophosphate (FdUMP) (9). The first step in the activation of 5FU is the phosphorylation of 5FU by orotate phosphoribosyltransferase (OPRT), which metabolizes 5FU to 5-fluorouridine monophosphate (FUMP). FUMP is then metabolized into 5-fluorouridine triphosphate (FUTP), which can be incorporated into RNA and into 5-flurodeoxyuridine triphosphate (FdUTP), which can be incorporated into DNA (10). Although the incorporation into RNA and DNA is certainly an important aspect of the mechanism of action of 5FU,...