Mitochondrial Fuel Metabolic Differences in Triple Negative Breast Cancer

Abstract

Triple negative breast cancer (TNBC) is a hard-to-kill form of breast cancer that do not express estrogen (ER), progesterone (PR), and human epidermal growth factor (HER2) receptors. Recently, it was reported that TNBC is characterized by mitochondrial dysfunction, elevated glycolysis, and increased generation of reactive oxygen species (ROS) as compared with receptor positive breast cancer cells. Based on those biochemical differences between same type of cancer, we sought to elucidate these skewed differences in metabolism in detail. Elucidating these metabolic differences and behaviors is important to establish novel and targeted ways of treatment. In this work, we aimed to establish any differences in TNBC and non-TNBC mitochondrial fuel usage by measuring the dependency, capacity, and flexibility of cells to oxidize glucose, glutamine and fatty acids in real time using the XF Mito Fuel Test. To determine any mitochondrial fuel dependencies preferences, we determined the rate of oxidation of each “fuel source” by measuring the oxygen consumption rates (OCR) in the presence and absence of inhibitors UK5099 (glycolysis), BPTES (glutamine) and Etomoxir (fatty acids) in TNBC cells (MDA-MB-231 and MDA-MB-468) and non TNBC cells (MCF7). We found that in MDA-MB-231 there is a higher dependency for glycolysis and fatty acid oxidation for fueling mitochondria. In contrast, parent TNBC cell line MDA-MB-468 had an exclusive dependence on glycolysis with virtually no use of glutamine or fatty acid oxidation pathways. In addition, MDA-MB-231 had a restricted flexibility by favoring only an increase fatty acid and glutamine oxidation over glycolysis. Nevertheless, MDA-MB-468 showed no specific flexibility on switching to any of the fuels. We also assessed changes in OCR in cells treated with mitochondria-targeted nitroxide SG1. Treatment with Mito-SG1 prompts a switch in both TNBC baseline and stressed metabolic phenotypes, impairing the cells’ ability to meet their energetic demands for survival by targeting mitochondria. We found that Mito-SG1 does not affect MDA-MB-231’s dependency, capacity and flexibility for using the three fuel mitochondrial sources. However, in MDA-MB-468’s, treatment induces a dramatic increase in fatty acid oxidation. Our results demonstrate that TNBC cells have distinct mitochondrial fuel dependencies and these should be taken in consideration for establishing novel successful therapeutic approaches.