Improved cytotoxicity of pyridyl-substituted thiosemicarbazones against MCF-7 when used as metal ionophores
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Zinc is the second most abundant transition metal in the human body, between 3 and 10 % of human genes encoding for zinc binding proteins. We have investigated the interplay of reactive oxygen species and zinc homeostasis on the cytotoxicity of the thiosemicarbazone chelators against the MCF-7 cell line. The cytotoxicity of thiosemicarbazone chelators against MCF-7 can be improved through supplementation of ionic zinc provided the zinc ion is at a level exceeding the thiosemicarbazone concentration. Elimination of the entire cell population can be accomplished with this regime, unlike the plateau of cytotoxicity observed on thiosemicarbazone monotherapy. The cytotoxic effects of copper complexes of the thiosemicarbazone are not enhanced by zinc supplementation, displacement of copper from the complex being disfavoured. Treatment of MCF-7 with uncomplexed thiosemicarbazone initiates post G1 blockade alongside the induction of apoptosis, cell death being abrogated through subsequent supplementation with zinc ion after drug removal. This would implicate a metal depletion mechanism in the cytotoxic effect of the un-coordinated thiosemicarbazone. The metal complexes of the species, however, fail to initiate similar G1 blockade and apparently exert their cytotoxic effect through generation of reactive oxygen species, suggesting that multiple mechanisms of cytotoxicity can be associated with the thiosemicarbazones dependant on the level of metal ion association.
KeywordsCopper Zinc Cytotoxicity Reactive oxygen species (ROS) Thiosemicarbazone
The authors acknowledge the contribution of Dr Gregg Maynard for assistance in setting up flow cytometry studies. F Akladios acknowledges the receipt of an Australian Postgraduate Award (APA). CJP wishes to thank the CSU Pharmacy Foundation for a grant partially funding this study. CJP and SDA thank the Kolling Institute (Royal North Shore Hospital) for the donation and characterization of the MCF-7 cell line employed in this study.
CJP and SDA designed the research programme. FNA conducted all research, analysed data and assembled the draft manuscript. All authors read and approved the final content.
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