Targeting cancer metabolism for breast cancer therapy

Abstract

Cancer cells have a different metabolic profile compared to normal cells. The Warburg effect (increased aerobic glycolysis) and glutaminolysis (increased mitochondrial activity from glutamine catabolism) are well known hallmarks of cancer and are accompanied by increased lactate production, a hyperpolarised mitochondrial membrane and increased production of reactive oxygen species (ROS). Dichloroacetate (DCA) has recently been described as a novel and relatively non-toxic anticancer agent, and can reverse the glycolytic phenotype of cancer cells through the inhibition of pyruvate dehydrogenase kinase. A panel of breast cancer cells were tested for sensitivity to DCA, and their growth was found to be inhibited by DCA in vitro. Further examination of 13762 MAT rat mammary adenocarcinoma cells found DCA inhibited proliferation without any increase in cell death, which correlated with reversal of the glycolytic phenotype. In vivo, DCA caused a 58% reduction in the number of lung metastases observed macroscopically after injection of 13762 MAT cells into the tail vein of rats (p=0.0001, n{u2265}9 per group). These results demonstrate the anti-cancer potential of DCA and reversing the glycolytic phenotype. In a second study in breast cancer cells, the Warburg effect was targeted with dichloroacetate (DCA) and the increased mitochondrial activity of glutaminolysis was targeted with arsenic trioxide (ATO) (previously identified as a mitochondrial toxin). The combination of DCA and ATO showed cooperative effects inhibiting cell proliferation and inducing cell death. Examination of the effect of these treatments on mitochondrial membrane potential (MMP), ROS production and ATP levels identified new molecular mechanisms within the mitochondria for both ATO and DCA: ATO reduces mitochondrial function through the inhibition of cytochrome C oxidase (complex IV of the electron transport chain (ETC)) while DCA up-regulates ATP synthase {u03B2} subunit expression. The synergistic cytotoxicity of DCA and ATO is correlated with strong suppression of the expression of oncogenes c-Myc and HIF-1a, and decreased expression of the survival protein Bcl-2. This study is the first to demonstrate that targeting two key metabolic hallmarks of cancer is an effective anti-cancer strategy with therapeutic potential. Decreased expression of ATP synthase subunits and increased ROS production have been repeatedly identified in established tumours. Increased glutamine metabolism in the tricarboxylic acid (TCA) cycle coupled to increased ROS production indicates that the ETC is working at higher than normal capacity, however decreased ATP synthase subunits indicates that there is a down regulation of the ATP synthesis machinery within the mitochondria. It is proposed here that the ETC and ATP synthase, two coupled processes in normal cells, have become decoupled in cancer cells, with increased ETC activity and reduced ATP synthase activity. With decoupled ETC and ATP synthase, cancer cells can increase ROS production and increase MMP. In addition, this decoupling can enhance the Warburg effect and promote resistance to many apoptotic triggers. In an experimental model comparing non-cancerous MCF-10A breast cells and MCF-10AT1 breast cancer cells, it was clearly demonstrated that MCF-10AT1 cells have increased cytochrome C oxidase activity and reduced ATP{u03B2} expression compared to MCF-10A cells. However reducing ROS production and depolarisation of MMP via the inhibition of cytochrome C oxidase failed to inhibit cell proliferation or enhance the action of other cytotoxic drugs. Although this study identified a decoupling between cytochrome C oxidase and ATP synthase in cancer cells, the relationship between ETC, ATP synthase and malignant behaviour still requires further study. These studies confirmed the potential benefits of DCA as an anti-cancer drug both as a single therapy and in combination with other cytotoxic drugs. Furthermore an abnormal phenotype was identified in mitochondria that could be responsible for the increased MMP and ROS seen in most cancer cells.