Comparison of thymidylate synthase (TS) protein up-regulation after exposure to TS inhibitors in normal and tumor cell lines and tissues

Thymidylate synthase (TS) is an important target for cancer chemotherapy, H owever, several mechanisms of resistance to TS inhibitors have been describ ed. One mechanism that may be relevant to short-term exposure to TS inhibit ors occurs as a result of disruption of the autoregulatory loop, which allo ws TS to control its own translation. This disruption leads to up-regulatio n of TS protein and is generally thought to decrease efficacy, This study h as investigated TS protein up-regulation using a range of TS inhibitors in both tumor and nonmalignant cell lines in vitro and in vivo. Up-regulation of TS protein showed a time-, dose-, and cell-type-specific r esponse to treatment with ZD9331, This response was observed in W1L2 cells treated for 24 h at equitoxic doses of raltitrexed (6-fold), ZD9331 (IO-fol d), fluorouracil (5-fold), LY231514 (7-fold), AG337 (7-fold), and BW1843U89 (3-fold). Up-regulation was observed over a range of doses. Elevation of T S protein only persisted up to 12 h after removal of drug. The extent of in duction does not depend on basal TS levels. Nontransformed human fibroblast s showed significantly greater up-regulation of TS protein than tumor cells exposed to an equitoxic dose of ZD9331, In vivo experiments using the L517 8Y thymidine kinase -/- mouse lymphoma implanted into DBA2 mice also showed greater up-regulation of TS protein in normal intestinal epithelial tells compared with tumor cells. These results confirm that TS up-regulation Is a common feature of TS inhib ition in tumor cells and that it may occur to a greater extent in normal ti ssues, although the clinical implications of these findings remain to he de termined.