Date of Award
Doctor of Philosophy (PhD)
Drug resistance limits the clinical efficacy of many cancer treatment modalities. Mutations, as a result of genomic instability within tumours, are thought to generate subpopulations of tumour cells which are less responsive to cytotoxic agents. This thesis investigates the hypothesis that drug resistance is associated with alterations in the mitochondria of cancer cells and these mitochondrial alterations provide a means to restore drug sensitivity in drug-resistant cell populations. A human ovarian carcinoma model was used to further characterize mitochondrial changes associated with cisplatin resistance (Chapter 1). It was observed that mitochondria in a cisplatin-resistant variant, C13*, accumulated and retained more of a lipophilic cation, rhodamine 123 (Rh123), compared to its parental line, 2008. This extended mitochondrial retention of lipophilic cations rendered C13* cells sensitive to another lipophilic cation, dequalinium chloride (Deca), compared to 2008 cells. Moreover, combinations of Deca and cisplatin induced synergistic cell kill in both cell types. These data suggest that disruption of mitochondrial function may sensitize tumour cells, including cisplatin-resistant tumour cells, to the cytotoxic effects of cisplatin. An association between mitochondrial alterations and resistance to another type of cancer therapy was also examined (Chapter 2). Photofrin II-mediated photodynamic therapy (PDT) is thought to destroy tumour cells, in vitro, by disrupting mitochondria. It was observed that a variant resistant to Photofrin II-mediated PDT, RIF-8A, contained mitochondria that differed structurally and functionally compared to its parental line, RIF-1. This was the first report to associate mitochondrial changes with PDT resistance. Similarities between RIF-8A mitochondria and C13* mitochondria at the ultrastructural level, suggested that RIF-8A may be resistant to cisplatin compared to RIF-1 cells. This hypothesis was supported by the observations that RIF-8A cells were cross-resistant to cisplatin (Chapter 3). These data indicate that mitochondrial characteristics may modulate the sensitivity of tumour cells to certain cytotoxic agents. Since cisplatin's cytotoxicity is believed to be mediated through its interactions with nuclear DNA, the mechanisms through which mitochondria influence cisplatin sensitivity, are unclear. Several of the steps involved in the repair of cisplatin-DNA lesions require ATP hydrolysis. Therefore, an increase in mitochondrial activity may augment the production of ATP, which can then be used for DNA repair. A host cell reactivation (HCR) assay indicated that C13* cells did not have an enhanced capacity to repair cisplatin-damaged DNA compared to either 2008 or RH4 cells suggesting that changes in the mitochondrial membrane potential do not influence the repair of cisplatin-DNA lesions. Independent observations have implicated the proto-oncogene, c-fos, as a potential regulator of mitochondrial activity and/or cisplatin sensitivity. This hypothesis was examined in three different model systems (Chapter 5). In both instances where there was an overexpression of the c-fos gene there was no corresponding increase in mitochondrial membrane potential. Similarly, in C13* cells which have an elevated mitochondrial membrane potential compared to 2008 cells, there was no significant difference in either c-fos mRNA or protein levels. Cells that overexpressed c-fos were resistant to cisplatin however, reducing c-fos expression did not sensitize cells to cisplatin. These data suggest that alterations in both mitochondria and c-fos expression may modulate cisplatin sensitivity but these two characteristics are not interdependent.
Moorehead, Roger A., "Mitochondrial alterations in tumour cells" (1997). Open Access Dissertations and Theses. Paper 3360.