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Abstract

Uncontrolled proliferation is one of the characteristic hallmarks of cancer and underlines the aberrant nutrient cycling seen in tumors. Cancer cells, therefore, need to rewire their metabolic pathways to meet the biosynthetic and bioenergetics demands of rapid growth. Metabolic reprogramming is thus, crucial for oncogenesis. The ‘Warburg effect’ is the most commonly reported metabolic characteristic in tumors, where cancer cells consume high amounts of glucose which then get shunted mostly into lactate production. Glutamine addiction is commonly observed in several cancers making it the second most important carbon source which fuels the citric acid (TCA) cycle to provide both energy and biosynthetic precursors. Breast cancer is a heterogeneous disease and despite successes in targeted therapy of certain subtypes, treatment still remains a clinical challenge. Investigating metabolic changes in breast cancer, therefore, provides an opportunity to overcome these therapeutic challenges. The aim of this project was to understand the mechanisms by which tumors efficiently utilize their carbon and nitrogen sources and to identify potential targets for future therapeutic interventions. To this end, I identified alanine aminotransferase 2(also known as glutamate pyruvate transaminase 2 or GPT2) as a highly deregulated enzyme in breast cancer patients who have a poor prognosis. GPT2 catalyzes the reversible reaction that generates alanine from glutamate and pyruvate which, in turn, are produced by glutaminolysis and glycolysis, respectively. I found RNAi and chemical inhibition of GPT2 to suppress proliferation of several breast cancer cell lines. MDA MB 468 cells, which produce the highest levels of GPT2 and alanine, show a decrease in the supply of TCA cycle intermediates as well as a reduction in glutamine uptake when alanine production is blocked. Furthermore, GPT2 inhibition rewires glucose carbon cycling in the cell lines resulting in increased glucose carbon flow into the TCA cycle. I found the expression of GPT2 in these cancer cells to be regulated by the transcription factors, c-Myc and ATF4. In conclusion, my results demonstrate the role of GPT2 in connecting glucose and glutamine anaplerosis, thereby driving carbon atoms into key biosynthetic pathways of cancer cells. This study underlines the importance of investigating the balance of nutrient cycling in cancer to identify potential targets for therapeutic intervention.