A Fas-dependent component in 5-fluorouracil/leucovorin-induced cytotoxicity in colon carcinoma cells

We have shown previously ( J, A. Houghton et al,, Proc. Natl, head. Sci, US A, 94: 8144-8149, 1997) that thymineless death in thymidylate synthase-defi cient (TS-) colon carcinoma cells is mediated via Fas/FasL interactions aft er deoxythymidine (dThd) deprivation, and that Pas-dependent sensitivity of human colon carcinoma cell lines may be dependent upon the level of Pas ex pressed. The objective of this study was to elucidate whether a Fas-depende nt component exists in 5-fluorouracil (FUra)/leucovorin (LV)-induced cytoto xicity of colon carcinoma cells, and whether this may be potentiated by IFN -gamma-induced elevation in Pas expression, using the HT29 cell line as a m odel. The cytotoxic activity of FUra/LV was inhibited by dThd in HT29 cells and also, in part, by NOK-1 + NOK-2 MoAbs that prevent Fas/FasL interactio ns. FUra/LV-induced cytotoxicity was significantly potentiated by LFN-gamma , reversed by exposure to NOK-1 + NOK-2 antibodies, and correlated with a 4 -fold induction of Pas expression in the presence of IFN-gamma and signific ant elevation in expression of Fast. Using five additional human colon carc inoma cell lines, FUra/LV-induced cytotoxicity was dThd-depefident in GC(3) /cl, VRC5/cl, and Caco2 but not in HCT8 or HCT116 cells. Like HT29 cells, t his cytotoxicity was potentiated by IFN-gamma in GC(3)/cl and VRC5/cl but n ot in Caco2, which fails to express Pas, nor in HCT8 and HCT116, in which n o dThd-dependent FUra-nduced cytotoxicity was demonstrated. Data suggest th at a Pas-dependent component, potentiated by IFN-gamma, exists in FUra/LV-i nduced cytotoxicity but requires FUra/LV-nduced DNA damage for IFN-gamma-in duced potentiation to occur.