Abstract 491: Selective killing of oncogenically transformed cells by arsenic trioxide and trolox
Authors: Redstone Genevieve G. J. , Nichol Jessica N. , Faubert Brandon , Jones Russell G. , Mann Koren K. , Miller Wilson H. 
Published: May 2015

Background: A major disadvantage of cytotoxic chemotherapy is the difficulty in selectively targeting tumor cells. Although arsenic trioxide (ATO) is an effective therapy in acute promyelocytic leukemia (APL), its use in solid tumors is limited by the toxicity observed at concentrations required to induce apoptosis in non-APL cells. We previously found that trolox, a vitamin E derivative, enhances ATO-induced apoptosis in APL cells, while simultaneously protecting normal cells. The objective of the current study is to understand the mechanism(s) by which trolox switches from cytoprotective to cytotoxic upon the oncogenic transformation of cells.

Materials and methods: This study was conducted using two isogenic models: mouse embryonic fibroblast (MEFs) and MEFs stably-transfected with either E6/E7 or E1A/Ras oncogenes. Apoptosis was assessed via staining with Propidium Iodide (Cell Signaling) followed by FACS analysis. Cells were treated with various concentrations of ATO alone or in combination with 100μM trolox for 48 hours. SubG0 cell population results were obtained by staining the cells with PI, followed by analysis using CellQuest software. Mitochondrial ROS levels were measured with MitoSox (Life Technologies), followed by FACS analysis. Cells were treated with 5μM ATO alone or in combination with 100μM trolox for 18 hours. Mitochondrial fluorescence was examined by staining the cells with MitoSOX (Life Technologies). For the visualization and enumeration of mitochondria, cells were examined with Transmission Electron Microscopy (TEM). Intracellular pictures were taken with TEM at FEMR, McGill University. Changes in cellular metabolism were measured with an XF96 Extracellular Flux Analyzer (Seahorse Bioscience).

Results: In both the E6/E7 and E1A/Ras models, trolox enhances ATO-induced death. In contrast, trolox has a protective effect from ATO toxicity in their untransformed counterparts. In the transformed cell lines, cell death correlates with enhanced formation of mitochondrial superoxide when treated with arsenic in combination with trolox. The functionality of mitochondria is closely linked to their structure. Preliminary analysis of TEM images indicates that mitochondrial structural integrity in transformed cells deteriorates when treated with ATO in combination with trolox. Finally, these results are supported by changes in cellular metabolisms in the transformed cell lines.

Conclusion: Transformed cells are more sensitive to cytotoxicity induced by arsenic trioxide in combination with trolox than their wild-type counterparts. We hypothesize that the predominant mechanism of action by which transformed cells are sensitized to the actions of trolox and ATO is due to a dysfunction of mitochondrial activity. The results of this study will provide a means to expand the therapeutic spectrum of ATO beyond APL by enhancing tumor selectivity and protecting normal tissue from toxicity.

Citation Format: Genevieve G. J. Redstone, Jessica N. Nichol, Brandon Faubert, Russell G. Jones, Koren K. Mann, Wilson H. Miller. Selective killing of oncogenically transformed cells by arsenic trioxide and trolox. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 491. doi:10.1158/1538-7445.AM2014-491