Abstract
Purpose
Thymidylate synthase (TS) over-expression is widely accepted as a major molecular mechanism responsible for 5-fluorouracil (5-FU) and tomudex (TDX) resistance. In this study, the importance of TS in 5-FU and TDX resistance was evaluated.
Methods
The sensitivity of TS-over-expressing 5-FU (3) and TDX (3) resistant cell lines to 5-FU and TDX was analysed. The cross-resistance between 5-FU and TDX resistant cell lines was determined. The relationship between p53 and NF-κB status and the sensitivity to 5-FU and TDX was evaluated.
Results
Compared to relevant parental sensitive cell lines, the 5-FU resistant cell lines were highly cross-resistant to TDX (over 20,000-fold). In contrast, over-expression of TS did not significantly confer 5-FU resistance on the TDX resistant cell lines (0.8- to 1.3-fold). Thymidine (20 μM) rescue induced TDX resistance in TDX sensitive cell lines (over 10,000-fold) but only moderately influenced 5-FU sensitivity in 5-FU sensitive cell lines (1.1- to 2.4-fold). Uridine moderately protected one cancer cell line (RKO) from 5-FU-induced, but not TDX-induced, cytotoxicity. NF-κB transfected MCF-7 and p53 knockout HCT116 cells were resistant to 5-FU (4.4- and 2.4-fold, respectively) but not to TDX. TS protein expression in NF-κB transfected and p53 knockout cell lines was comparable to the relevant parental cell lines.
Conclusion
In some cancer cell lines, TS-independent molecular events may play a key role in 5-FU resistance. Loss of p53 function and NF-κB over-expression may be involved in TS-independent 5-FU chemoresistance in some cancer cell lines.


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Abbreviations
- 5-FU:
-
5-Fluorouracil
- dTMP:
-
Deoxythymidine monophosphate
- dTTP:
-
Deoxythymidine triphosphate
- dUMP:
-
Deoxyuridine monophosphate
- dUTP:
-
Deoxyuridine triphosphate
- FdUMP:
-
5-Fluorodeoxyuridine monophosphate
- MTT:
-
Tetrazolium-based semiautomated colorimetric
- NF-κB:
-
Nuclear factor-kappa B
- TdR:
-
Thymidine
- TDX:
-
Tomudex
- TS:
-
Thymidylate synthase
- Urd:
-
Uridine
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Acknowledgments
This work is supported by North Glasgow University Hospital NHS Trust West Research Endowment.
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Wang, W., McLeod, H.L., Cassidy, J. et al. Mechanisms of acquired chemoresistance to 5-fluorouracil and tomudex: thymidylate synthase dependent and independent networks. Cancer Chemother Pharmacol 59, 839–845 (2007). https://doi.org/10.1007/s00280-006-0384-5
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DOI: https://doi.org/10.1007/s00280-006-0384-5