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Title: Potentiation of Topoisomerase I inhibitors by Hsp90 inhibitors: Mechanistic and Functional studies
Author: McNamara, Anne
ISNI:       0000 0001 3388 5851
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2007
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Purpose: Inhibitors of topoisomerase I are of considerable biomedical importance as they are the sole target of the camptothecin family of anticancer drugs. Camptothecin derivatives are routinely used in cancer treatment however these drugs are highly toxic and tumour cells can develop drug resistance. It is therefore essential that new treatment strategies are investigated. The aim of this study was to use a rational approach previously developed by our group to enhance the efficacy of topoisomerase I inhibitors. Objectives: Previous work in our group identified Hsp90 as a topoisomerase II interacting protein. The combination of a topoisomerase II poison and . Hsp90 inhibitor were found to act synergistically in human cancer cell lines. We proposed that topoisomerase I and Hsp90 inhibitors would also. synergistically interact. The p53 isogenic pair HCT116 human colon adenomacarcinoma cell line were used to dissect the mechanism behind the synergy. Results: Hsp90 inhibitors interact synergistically with topoisomerase I inhibitors in causing inhibition of cell growth and survival in the p53-isogenic pair HCT116 human colon adenocarcinoma cell line. The mode of cell death induced by the combination treatment was determined as apoptotic which was synergistically enhanced in the combined treatment compared to the drugs alone. p53 deficient cells were significantly more sensitive to the combined treatment initiating apoptosis'16 hours post treatment compared to 24 hours in p53 WT cells. The synergistic increase in apoptosis was not caused by increased topoisomerase mediated-DNA damage in either the p53 KO or WT cell lines. Dual Hsp90 and topoisomerase I inhibition selectively abrogated the G2/M checkpoint in p53 deficient cells. p53 WT cells maintained both Sand G2/M checkpoint integrity. We propose that abrogation of the G2/M checkpoint was responsible for the enhanced sensitivity of p53 KO cells to the combined treatment, and that an underlying mechanism was responsible for the synergy observed in both p53 WT and KO cells. Inhibition of Hsp90 caused levels of the anti-apoptotic protein, Bcl2 to decline. Bcl2 loss was associated with a corresponding increase in caspase-3 activation following combined treatment with Hsp90 and topoisomerase I inhibitors. This phenomenon was observed in both p53 WT and KO cell lines. We propose that inhibition of Hsp90 mediated loss of Bcl2 function, resulting in enhanced caspase-3 activation and apoptosis, and that this underlying general mechanism of synergy applies to both p53 WT and KO cell lines. Conclusion: We have developed a rational, mechanism based anticancer drug therapy using Hsp90 and topoisomerase I inhibitors in combination. p53 deficient cells were significantly more sensitive to this treatment caused by selective abrogation of the G2IM checkpoint in these cells. We therefore predict that this treatment may be more effective in patients with p53 deficient or mutant tumours. Based on this study and previous studies in our group, combined Hsp90 and topoisomerase I inhibitor therapy has been put forward as a clinical based trial.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available