Ligand-mediated induction of thymidylate synthase occurs by enzyme stabilization. Implications for autoregulation of translation

Thymidylate synthase (TS) is indispensable in the de novo synthesis of dTMP. As such, it has been an important target at which anti-neoplastic drugs are directed. The fluoropyrimidines 5-fluorouracil and 5-fluoro-2'-deoxyuridine are cytotoxic as a consequence of inhibition of TS by the metaboli...

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Bibliographic Details
Published in:The Journal of biological chemistry 1999-04, Vol.274 (18), p.12544
Main Authors: Kitchens, M E, Forsthoefel, A M, Rafique, Z, Spencer, H T, Berger, F G
Format: Article
Language:English
Subjects:
Base Sequence
DNA Primers
Enzyme Stability - drug effects
Floxuridine - pharmacology
Gene Expression Regulation, Enzymologic
Humans
Ligands
Mutagenesis, Site-Directed
Protein Binding
Protein Biosynthesis
RNA, Messenger - genetics
Thymidylate Synthase - biosynthesis
Thymidylate Synthase - genetics
Thymidylate Synthase - metabolism
Tumor Cells, Cultured
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Summary:Thymidylate synthase (TS) is indispensable in the de novo synthesis of dTMP. As such, it has been an important target at which anti-neoplastic drugs are directed. The fluoropyrimidines 5-fluorouracil and 5-fluoro-2'-deoxyuridine are cytotoxic as a consequence of inhibition of TS by the metabolite 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP). This inhibition occurs through formation of a stable ternary complex among the enzyme, the nucleotide analog, and the co-substrate N5, N10-methylenetetrahydrofolate. Numerous studies have shown that cellular concentrations of TS undergo about a 2-4-fold induction following treatment with TS inhibitors. An extensive body of in vitro studies has led to the proposal that this induction occurs because of relief of the translational repression brought on by the binding of TS to its own mRNA. In the current study, we have tested several predictions of this autoregulatory translation model. In contrast to expectations, we find that fluoropyrimidines do not cause a change in the extent of ribosome binding to TS mRNA. Furthermore, mutations within the mRNA that abolish its ability to bind TS have no effect on the induction. Finally, enzyme turnover measurements show that the induction is associated with an increase in the stability of the TS polypeptide. Our results, in total, indicate that enzyme stabilization, rather than translational derepression, is the primary mechanism of TS induction by fluoropyrimidines and call into question the general applicability of the autoregulatory translation model.
ISSN:0021-9258
DOI:10.1074/jbc.274.18.12544