Colon Carcinoma Cell Line Interferon and 5-Fluorouracil in the H 630 Human γ Interaction of Updated

The antiproliferative effects and pharmacological interactions of 5fluorouracil (5-FU) in combination with 7 interferon (IFN-7) were deter mined against the human colon carcinoma H630 cell line in vitro. H630 was 9-fold more resistant to 5-FU, as compared to a relatively sensitive human colon line (Cl). IFN-7 showed modest antiproliferative activity against the H630 line, with a 50% inhibitory concentration of 440 units/ ml. Simultaneous treatment of H630 with subinhibitory concentrations of IFN-7 and 5-FU produced a significant enhancement of the 5-FUassociated growth inhibition. The growth-inhibitory activity of the com bination against H630 was prevented by the addition of 20 MMthymidine. Thymidylate synthase (TS) activity was measured by both the 5-fluoro2'-deoxyuridine-5'-monophosphate binding and catalytic assays, using cytosolic extracts. A 24-h exposure to 1 MM5-FU in the H630 line resulted in a 3.1-fold increase in the total amount of TS, while in the 5FU/IFN-7-treated cells TS remained unchanged from non-drug-treated control levels. Moreover, we found that free thymidylate synthase in the 5-FU/IFN-7-treated cells was significantly decreased, as compared to the cells treated with 5-FU alone. Incorporation of 5-FU into both the RNA and DNA fractions did not change with the addition of IFN-7. Accumulation of the fluoropyrimidine metabolites 5-fluoro-2'-deoxyuridine-5 '-monophosphate and 5-fluorouridine-5'-triphosphate remained the same for 5-FU alone and the combination treatment. These findings suggest that acute TS induction by 5-FU may provide an important mechanism by which human colon carcinoma cells express decreased sensitivity to 5-FU and that IFN-7 can reverse the development of resistance to 5-FU in the H630 line by inhibiting the overexpression of TS that results from 5-FU exposure. These studies contribute to a growing understanding of the complex interaction between 5-FU and IFN-7.