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.