Increased Sensitivity to Long‐term 5‐Fluorouracil Exposure of Human Colon Cancer HT‐29 Cells Resistant to Short‐term Exposure

A 5‐fluorouracil (5‐FU)‐resistant subline of human colon cancer HT‐29 cells was developed by repeated 1‐h exposure in vitro to 5‐FU. This subline (HT‐29/5‐FU/S) had 8‐fold resistance to 5‐FU in a 1‐h exposure assay. However, it had rather increased sensitivity to 5‐FU when assayed after a continuous 96‐h exposure to it. Significantly less 5‐fluorouridine‐5′‐triphosphate was produced in the resistant cells, leading to a lower level of 5‐FU incorporation into the cellular RNA. The reduced activity of orotate phosphoribosyltransferase might explain these results. In contrast, the HT‐29/5‐FU/S cells were more sensitive to the inhibition of in situ thymidylate synthase (TS) by 5‐FU than were the parent cells. The lower in situ TS activity may have made HT‐29/5‐FU/S cells more sensitive to TS inhibition by 5‐FU as compared with the parent cells. The fact that HT‐29/5‐FU/S was more resistant to short‐term 5‐FU exposure but more sensitive to long‐term exposure than the parent line confirmed the existence of different modes of action of 5‐FU, depending on the exposure time.