Resistance of colorectal cancer cells to 5-FUdR and 5-FU caused by mycoplasma infection

Background: 5-Fluorouracil (5-FU) is an antineoplastic drug that targets thymidylate synthase (TS). Tumour cells can develop resistance to anti-TS drugs by a variety of mechanisms including up-regulation of TS protein and alterations in drug uptake and degradation. The possible mechanisms of the observed rapid development of resistance to the pyrimidine analogs 5-FUdR and 5-FU in cultured HCT116 colon cancer cells were investigated. Materials and Methods: Cell survival was determined in resistant and control HCT116 cells treated with 5-FUdR and 5-FU for 7 days. The ability of the cells to take up and metabolize these drugs was determined by Western blotting and [H-3]thymidine incorporation. Results and Conclusion: Resistant HCT116 cells were 5- and 100-fold more resistant to killing by 5-FU and 5-FUdR, respectively, than the parental cells and exhibited impaired uptake. Although the HCT116R cells were initially Mycoplasma free, a low level of Mycoplasma contamination was found in these cells after several weeks in culture. Sensitivity to 5-FUdR was restored by treatment with an anti-Mycoplasma antibiotic. Our observations emphasize the need for frequent testing for Mycoplasma contamination in any cell line under investigation for resistance to anti-TS drugs.