Aerobic glycolysis, or preferential fermentation of glucose-derived pyruvate to lactate despite available oxygen, is associated with proliferation across many organisms and conditions. To better understand that association, we examined the metabolic consequence of activating the pyruvate dehydrogenase complex (PDH) to increase pyruvate oxidation at the expense of fermentation. We find that increasing PDH activity impairs cell proliferation by reducing the NAD⁺/NADH ratio. This change in NAD⁺/NADH is caused by increased mitochondrial membrane potential that impairs mitochondrial electron transport and NAD⁺ regeneration. Uncoupling respiration from ATP synthesis or increasing ATP hydrolysis restores NAD⁺/NADH homeostasis and proliferation even when glucose oxidation is increased. These data suggest that when demand for NAD⁺ to support oxidation reactions exceeds the rate of ATP turnover in cells, NAD⁺ regeneration by mitochondrial respiration becomes constrained, promoting fermentation, despite available oxygen. This argues that cells engage in aerobic glycolysis when the demand for NAD⁺ is in excess of the demand for ATP.

A quantitative high-throughput screen identified an inhibitor of phosphoglycerate dehydrogenase (PHGDH), a key enzyme for serine synthesis. This inhibitor limits one-carbon unit availability for nucleotide synthesis. Serine is both a proteinogenic amino acid and the source of one-carbon units essential for de novo purine and deoxythymidine synthesis. In the canonical pathway of glucose-derived serine synthesis, Homo sapiens phosphoglycerate dehydrogenase (PHGDH) catalyzes the first, rate-limiting step. Genetic loss of PHGDH is toxic toward PHGDH-overexpressing breast cancer cell lines even in the presence of exogenous serine. Here, we used a quantitative high-throughput screen to identify small-molecule PHGDH inhibitors. These compounds reduce the production of glucose-derived serine in cells and suppress the growth of PHGDH-dependent cancer cells in culture and in orthotopic xenograft tumors. Surprisingly, PHGDH inhibition reduced the incorporation into nucleotides of one-carbon units from glucose-derived and exogenous serine. We conclude that glycolytic serine synthesis coordinates the use of one-carbon units from endogenous and exogenous serine in nucleotide synthesis, and we suggest that one-carbon unit wasting thus may contribute to the efficacy of PHGDH inhibitors in vitro and in vivo.