Decreased FANCJ caused by 5FU contributes to the increased sensitivity to oxaliplatin in gastric cancer cells

Oxaliplatin is effective against many types of cancer, and the combination of 5-fluorouracil (5FU) and oxaliplatin is synergistically effective against gastric cancer, as well as colon cancer. The FANCJ protein is one of the Fanconi anemia (FA) gene products, and its interaction with the tumor suppressor BRCA1 is required for DNA double-strand break (DSB) repair. FANCJ also functions in interstrand crosslinks (ICLs) repair by linking to mismatch repair protein complex MLH1-PMS2 (MutL alpha). While oxaliplatin causes ICLs, 5FU is considered to cause DSBs. Therefore, we investigated the importance of FANCJ in the synergistic effects of oxaliplatin and 5FU in MKN45 gastric cancer cells and the derived 5FU-resistant cell line, MKN45/F2R. MKN1, TMK1, MKN45, and MKN45/F2R (5FU-resistant) gastric cancer cells were treated with 5FU and/or oxaliplatin. The signaling pathway was evaluated by a western blotting analysis and reverse transcription polymerase chain reaction (RT-PCR). Drug resistance was evaluated by the 3-(4,5-dimethyl-2-tetrazolyl)-2,5-diphenyl-2H tetrazolium bromide (MTT) assay. In MKN45 cells, the combination of 5FU and oxaliplatin had synergistic effects. DSBs appeared when the cells were treated with 5FU. FANCJ was down-regulated, and BRCA1 was induced in a dose- and time-dependent manner. MKN45 cells showed increased sensitivity to oxaliplatin when FANCJ was knocked down by short interfering (si) RNA. However, these findings were not observed in MKN45/F2R 5FU-resistant cells. These results strongly suggest that the decrease in FANCJ caused by 5FU treatment leads to an increase in sensitivity to oxaliplatin, thus indicating that the FANCJ protein plays an important role in the synergism of the combination of 5FU and oxaliplatin.