Thymidine phosphorylase mediates the sensitivity of human colon carcinoma cells to 5-fluorouracil

Interferon-α (IFNα) potentiates the antitumor activity of 5- fluorouracil (FUra) in colon cancer in vitro, in vivo, and clinically. A likely mechanism for this action is the induction by IFNα of thymidine phosphorylase (TP), the first enzyme in one pathway for the metabolic activation of FUra to fluorodeoxyribonucleotides. To test this hypothesis, an expression vector containing the TP cDNA was transfected into HT-29 human colon carcinoma cells. Five stable transfectants were selected and analyzed. All showed increased sensitivity to FUra cytotoxicity, ranging from a 2-fold to a 19-fold decrease in the IC₅₀ for FUra, compared to wild-type cells. Levels of TP mRNA, protein, and enzyme activity were elevated in the transfectants, and there was a significant correlation between the relative increase in sensitivity to FUra and both the increase in both TP mRNA levels and TP activity. Transfected cells exhibited increased formation of FdUMP, but not the ribonucleotides FUDP and FUTP, from FUra when compared to wild- type cells. The changes in TP activity, FdUMP formation, and FUra sensitivity in the transfected cells were comparable with those seen after treatment of wild-type cells with IFNα. These studies provide direct evidence for the role of TP in mediating the sensitivity of colon carcinoma cells to FUra, and further support the importance of the induction of TP in the biomodulating action of IFNα on FUra chemosensitivity.