Document Detail

Serine/threonine protein phosphatase inhibition enhances the effect of thymidylate synthase inhibition.
MedLine Citation:
PMID:  14648018     Owner:  NLM     Status:  MEDLINE    
PURPOSE: The serine/threonine protein phosphatases 1 (PP1) and 2A (PP2A) are key enzymes in regulating entry into the cell cycle, mitosis and apoptosis. Inhibition of PP1 and PP2A is associated with enhanced S-phase entry culminating in G(2)/M arrest and apoptotic cell death. Thymidylate synthase (TS) is a key regulatory enzyme in DNA synthesis, inhibition of which is often a first-line treatment for colorectal carcinoma. In this study the effect of combining PP inhibition with TS inhibition in two colorectal cell lines was examined. METHODS: Cantharidin and nolatrexed were used to inhibit PP and TS activity, respectively. The MTT cytotoxicity assay and cell cycle analysis were performed following single-drug treatment of HT29 and HCT116 colorectal cell lines. The median effect method was used to determine a combination index (CI), where drug antagonism was indicated by a CI>1.1, additivity by a CI between 0.9 and 1.1, and synergism by a CI<0.9. RESULTS: Both cell lines were equally sensitive to cantharidin alone (GI(50) values 5.4 and 7.3 micro M), which induced a significant increase in the S-phase population of both cell lines within 6 h with a concomitant increase in DNA synthesis. This response culminated in G(2)/M cell cycle arrest within 24 h and subsequent cell death. In response to nolatrexed alone, HT29 cells were more sensitive than HCT116 cells (GI(50) 1.9 micro M vs 9.8 micro M), with G(1)/S-phase cell cycle arrest occurring within 24 h in both cell lines. In HT29 cells, this was followed by cell death, whereas in HCT116 cells, a proportion of cells died following arrest but the predominant event was re-entry into the cell cycle. The simultaneous exposure of HT29 cells to the combination of nolatrexed and cantharidin in drug molar ratios of 1:1 and 1:2.5 for 72 h was synergistic producing composite CIs of 0.88 and 0.87, respectively. The sequence of nolatrexed followed by cantharidin 24 h later resulted in greater synergism (CI values of 0.75, 0.52, 0.55, 0.68 for molar ratios of 10:1, 1:1, 1:2.5, 1:10), whereas the reverse sequence was antagonistic, suggesting that the point of interaction is downstream of TS inhibition. In HCT116 cells only additive and antagonistic interactions were observed for any of the treatment combinations. The lack of synergism in these cells may be caused by the reduced sensitivity of these cells to nolatrexed as a single agent. CONCLUSION: The effect of TS inhibition can be enhanced by the inhibition of serine/threonine protein phosphatases.
Jennette A Sakoff; Ian J Howitt; Stephen P Ackland; Adam McCluskey
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Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't     Date:  2003-11-25
Journal Detail:
Title:  Cancer chemotherapy and pharmacology     Volume:  53     ISSN:  0344-5704     ISO Abbreviation:  Cancer Chemother. Pharmacol.     Publication Date:  2004 Mar 
Date Detail:
Created Date:  2004-02-03     Completed Date:  2004-03-15     Revised Date:  2007-11-15    
Medline Journal Info:
Nlm Unique ID:  7806519     Medline TA:  Cancer Chemother Pharmacol     Country:  Germany    
Other Details:
Languages:  eng     Pagination:  225-32     Citation Subset:  IM    
Department of Medical Oncology, Newcastle Mater Misericordiae Hospital, Edith Street, 2298 Waratah, NSW, Australia.
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MeSH Terms
Antimetabolites, Antineoplastic / pharmacology*,  therapeutic use
Cantharidin / pharmacology*,  therapeutic use
Carcinoma / drug therapy*,  metabolism,  pathology
Cell Cycle / drug effects
Cell Division / drug effects
Cell Line, Tumor
Colorectal Neoplasms / drug therapy*,  metabolism,  pathology
DNA, Neoplasm / biosynthesis
Drug Synergism
Enzyme Inhibitors / pharmacology*,  therapeutic use
HT29 Cells
Phosphoprotein Phosphatases / antagonists & inhibitors*
Quinazolines / pharmacology*,  therapeutic use
Thymidylate Synthase / antagonists & inhibitors*
Reg. No./Substance:
0/Antimetabolites, Antineoplastic; 0/DNA, Neoplasm; 0/Enzyme Inhibitors; 0/Quinazolines; 152946-68-4/nolatrexed; 56-25-7/Cantharidin; EC Synthase; EC Phosphatases

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