Exploring the Role of Nitrogen Metabolism in Cancer

PROJECT SUMMARY/ABSTRACT There has been a renewed interest in how oncogenic driver mutations and tumor suppressor losses contribute to cancer-associated alterations in cellular metabolism. Much of the effort has been focused on the avidity with which most cancer cells take up glucose only to release most of the glucose carbon as lactate, a process known as aerobic glycolysis or the “Warburg Effect”. This seemingly wasteful metabolism has puzzled cancer biologists for decades. Nevertheless, aerobic glycolysis has been shown to be a sustainable way to support the continuous production of glycolytic intermediates that are utilized in de novo synthesis of proteins, lipids, and nucleic acids. Over a decade ago, the Thompson laboratory embarked on analyzing tumor utilization of glutamine, the second most common nutrient present in extracellular fluid. While glucose is metabolized by cancer cells primarily in the cytosol, we found that glutamine was metabolized primarily in the mitochondria. Similar to glucose, we found that the majority of the carbon taken up as glutamine was secreted as lactate, a process now known as glutaminolysis. Since that time, the study of glutaminolysis has focused on the role of glutamine as an anaplerotic substrate to maintain mitochondrial function as carbon is taken out of the TCA cycle in the form of citrate to fuel fatty acid biosynthesis and as aspartate to support nucleotide biosynthesis. Tumor cell avidity for glutamine in vivo and the ability of glutamine catabolism to maintain oxidative phosphorylation through TCA cycle anaplerosis has been confirmed in vivo. However, the role of glutaminolysis in supporting tumor nitrogen metabolism is less well understood. Although inhibitors of glutamine metabolism have been explored in cancer therapy, their success in the clinic has been limited in part because of our incomplete knowledge of tumor nitrogen metabolism. Understanding the role of nitrogen metabolism in supporting cancer cell survival and growth has become the central focus of the Thompson laboratory. We are currently exploring the hypothesis that glutamine-dependent mitochondrial glutamate accumulation provides the cell with an intracellular reserve of reduced nitrogen that can be directed toward mitochondrial support of de novo polyamine production, amino acid biosynthesis, and glutathione generation. We are also studying how the differential fates of mitochondrial glutamate are regulated by growth factors, as well as by oncogenes and tumor suppressors. While the normal pool of mitochondrial glutamate is fed by extracellular glutamine uptake, we also plan to test whether the combination of lactate and ammonia that accumulates in the tumor microenvironment (TME) under nutrient-poor conditions can be utilized to restore mitochondrial glutamate and cytosolic glutamine to levels that support adaptive translation and cell survival. These results will help clarify how cancer cell avidity for nitrogen is satisfied based on nutrient availability and the presence of specific oncogenic mutations and tumor suppressor losses. The insights gained will help to shape new approaches for the diagnosis and treatment of cancer.