Fig. 4

The resistance to the 5-FU is linked to altered expression of the enzymes playing role in 5-FC and 5-FU metabolism. a Quantitative PCR showed different expression of enzymes involved in 5-FC and 5-FU metabolism between parental and HT-29/EGFP/FUR “high” cells. The expression in the HT-29/EGFP cells was set as a reference, GAPDH and HPRT1 served as internal controls. The data are expressed as means of triplicates ± SD. Student’s t-test was used for statistical analysis. ***p < 0.001; ****p < 0.0001. b The dose-response curve results show arisen resistance of HT-29/EGFP/FUR cells to specific inhibitor of the TS enzyme raltitrexed even without the previous exposure of the cells to this drug. This resistance correlates with the increased expression of TS in chemoresistant cells. Values were expressed as means of sextaplicates ± SD. c The selective inhibition of the TP and (d) DPD enzymes by TPI and GIM, respectively, led to increased 5-FU chemoresistance in the HT-29/EGFP cells. Tumor cells were treated with the 5-FU for 5 days in the presence or absence of specific inhibitors with subsequent viability evaluation. Both TPI and GIM inhibitors significantly reversed 5-FU resistance in HT-29/EGFP cell population. Values were expressed as means of sextaplicates ± SD. Mann–Whitney U-test was used for statistical analysis. **p < 0.01. TS thymidylate synthase, TP thymidine phosphorylase, OPRT orotate phosphoribosyl transferase, DPD dihydropyrimidine dehydrogenase, ns not significant