Investigating how cancer cells maintain redox homeostasis to support biomass production - Project Summary To proliferate, cells must synthesize sufficient biomass, including nucleotides, proteins, and lipids. This is particularly important for rapidly proliferating cancer cells, which reprogram metabolism to meet the increased biosynthetic demands of proliferation. However, biosynthetic demands differ across cancer types and physiological environments. How cancer cells alter metabolism in response to specific biosynthetic demands is not well characterized, and better understanding these alterations will reveal metabolic liabilities that can be targeted to inhibit tumor growth. To synthesize biomass, cells require electron acceptors to oxidize nutrients like glucose and glutamine into biomass precursors, such as amino acids for protein synthesis and citrate for fatty acid synthesis. One essential electron acceptor is the redox cofactor NAD+. The ratio of oxidized NAD+ to reduced NADH (NAD+/NADH) influences the cellular redox state and reflects NAD+ availability for oxidative biosynthetic reactions. Thus, a sufficiently oxidized NAD+/NADH ratio must be maintained to enable synthesis of oxidized biomass and can limit cancer cell proliferation. We have found that increased demands for the amino acid serine and the lipid precursor citrate elevate the NAD+ demand, yet cancer cells surprisingly maintain a similar NAD+/NADH ratio, indicating that cells preserve NAD+/NADH homeostasis despite increased demands for oxidized biomass synthesis. The goal of the proposed research is to determine how cancer cells maintain NAD+/NADH homeostasis to support increased oxidative biosynthetic demands and tumor growth. I hypothesize that cancer cells maintain the NAD+/NADH ratio by modulating NAD+ regeneration, consumption, and synthesis to support biomass production. To test this hypothesis, I will use both cancer cell lines and mouse models to investigate how cancer cells and tumors maintain NAD+/NADH homeostasis in lipid-depleted conditions, a nutrient environment that increases cellular NAD+ demand for citrate production and is a physiological condition faced by cancer cells in the body. In Aim 1, I will investigate whether cancer cells increase three mechanisms of NAD+ regeneration to maintain redox homeostasis for lipid synthesis and tumor growth: fermentation, mitochondrial respiration, and uncoupling mitochondrial respiration from ATP synthesis. In Aim 2, I will investigate whether decreased flux through select NAD+ consuming redox reactions increases NAD+ availability for lipid synthesis. In Aim 3, I will investigate if NAD+ salvage synthesis from nicotinamide helps maintain a sufficiently oxidized NAD+/NADH ratio for increased lipid synthesis and tumor growth. The results of this study will lead to a better understanding of how cancer cells adapt to different nutrient and tissue environments and provide insight into a fundamental process in which cancer cells must engage to maintain redox homeostasis for proliferation. This will improve therapeutic approaches that target cancer metabolism and lead to the development of more effective and selective cancer treatments.
