Folate depletion increases sensitivity of solid tumor cell lines to 5-fluorouracil and antifolates

Cancer cell lines in standard cell culture medium or in animal models are s urrounded by an environment with relatively high folate (HF) levels, compar ed with folate levels in human plasma. In the present study we adapted 4 co lon cancer (C26-A, C26-10, C26-G and WiDr) and 3 squamous cell carcinoma of the head and neck (HNSCC) cell lines (11B, 14C and 22B) to culture medium with low folate (LF) levels (2.5, 1.0 and 0.5 nM, respectively) and investi gated whether folate depletion had an effect on sensitivity to antifolates and which mechanisms were involved. All LF cell lines showed a higher sensi tivity to 5-fluorouracil (5-FU) alone or in combination with leucovorin (LV ) (2-5-fold), to the thymidylate synthase (TS) inhibitors, AG337 (2-7-fold) , ZD1694 (3-49-fold), ZD9331 (3-40-fold), LY231514 (2-21-fold) or GW1843U89 (4-29-fold) or to the dihydrofolate reductase (DHFR) inhibitor PT523 (2-50 -fold) compared with their HF variants cultured in standard medium containi ng up to 8 mu M folic acid. LV could only increase sensitivity to 5-FU in H NSCC cell lines 14C and 14C/F. The differences in sensitivity could partial ly be explained by a 2-7-fold increased transport activity of the reduced f olate carrier (RFC) in LF cell lines, whereas no significant change in foly lpolyglutamate synthetase (FPGS) activity was observed. Furthermore, the pr otein expression and catalytic activity of the target enzyme TS were up to 7-fold higher in HF colon cancer cells compared with the LF variants (p < 0 .05). Although the TS protein expression in LF HNSCC cells was also lower t han in hip variants, the TS catalytic activity and FdUMP binding sites were up to 3-fold higher (p < 0.05). Thus, changes in TS levels were associated with differences in sensitivity. These results indicate that folate deplet ion was associated with changes in TS and RFC levels which resulted in an i ncrease in sensitivity to 5-FU and antifolates. The folate levels in LF med ium used in this study are more representative for folate levels in human p lasma and therefore these data could be more predictive for the activity of 5-FU and antifolates in a clinical setting than results obtained from cell lines cultured in HF medium or in animal models. Int. J. Cancer 87:771-778 , 2000. (C) 2000 Wiley-Liss, Inc.