Tumour cells with defective mitochondria are found to use glutamine-dependent reductive carboxylation, rather than oxidative metabolism, as the major pathway of citrate and lipid formation. Oxidative metabolism of glucose has long been considered to be the major provider of carbon for lipid synthesis in animal cells. Two papers in this issue of Nature demonstrate that reductive carboxylation of glutamine is an alternative. Metallo et al. show that various normal and cancerous human cell lines proliferating in hypoxic conditions produce the acetyl-coenzyme A required as a precursor for fatty acid synthesis by the reductive metabolism of glutamine-derived α-ketoglutarate through a pathway requiring isocitrate dehydrogenase 1. Mullen et al. show that tumour cells with defective mitochondria use glutamine-dependent reductive carboxylation as the major pathway of citrate formation. As well as adding a new dimension to our understanding of cell carbohydrate metabolism, this work suggests that there may be potential therapeutic targets along the reductive carboxylation and glutamine catabolic pathways that could prevent hypoxic tumour growth. Mitochondrial metabolism provides precursors to build macromolecules in growing cancer cells1,2. In normally functioning tumour cell mitochondria, oxidative metabolism of glucose- and glutamine-derived carbon produces citrate and acetyl-coenzyme A for lipid synthesis, which is required for tumorigenesis3. Yet some tumours harbour mutations in the citric acid cycle (CAC) or electron transport chain (ETC) that disable normal oxidative mitochondrial function4,5,6,7, and it is unknown how cells from such tumours generate precursors for macromolecular synthesis. Here we show that tumour cells with defective mitochondria use glutamine-dependent reductive carboxylation rather than oxidative metabolism as the major pathway of citrate formation. This pathway uses mitochondrial and cytosolic isoforms of NADP+/NADPH-dependent isocitrate dehydrogenase, and subsequent metabolism of glutamine-derived citrate provides both the acetyl-coenzyme A for lipid synthesis and the four-carbon intermediates needed to produce the remaining CAC metabolites and related macromolecular precursors. This reductive, glutamine-dependent pathway is the dominant mode of metabolism in rapidly growing malignant cells containing mutations in complex I or complex III of the ETC, in patient-derived renal carcinoma cells with mutations in fumarate hydratase, and in cells with normal mitochondria subjected to acute pharmacological ETC inhibition. Our findings reveal the novel induction of a versatile glutamine-dependent pathway that reverses many of the reactions of the canonical CAC, supports tumour cell growth, and explains how cells generate pools of CAC intermediates in the face of impaired mitochondrial metabolism.

Recent epidemiological and laboratory-based studies suggest that the anti-diabetic drug metformin prevents cancer progression. How metformin diminishes tumor growth is not fully understood. In this study, we report that in human cancer cells, metformin inhibits mitochondrial complex I (NADH dehydrogenase) activity and cellular respiration. Metformin inhibited cellular proliferation in the presence of glucose, but induced cell death upon glucose deprivation, indicating that cancer cells rely exclusively on glycolysis for survival in the presence of metformin. Metformin also reduced hypoxic activation of hypoxia-inducible factor 1 (HIF-1). All of these effects of metformin were reversed when the metformin-resistant Saccharomyces cerevisiae NADH dehydrogenase NDI1 was overexpressed. In vivo, the administration of metformin to mice inhibited the growth of control human cancer cells but not those expressing NDI1. Thus, we have demonstrated that metformin's inhibitory effects on cancer progression are cancer cell autonomous and depend on its ability to inhibit mitochondrial complex I.

Mitochondria produce reactive oxygen species (mROS) as a natural by-product of electron transport chain activity. While initial studies focused on the damaging effects of reactive oxygen species, a recent paradigm shift has shown that mROS can act as signaling molecules to activate pro-growth responses. Cancer cells have long been observed to have increased production of ROS relative to normal cells, although the implications of this increase were not always clear. This is especially interesting considering cancer cells often also induce expression of antioxidant proteins. Here, we discuss how cancer-associated mutations and microenvironments can increase production of mROS, which can lead to activation of tumorigenic signaling and metabolic reprogramming. This tumorigenic signaling also increases expression of antioxidant proteins to balance the high production of ROS to maintain redox homeostasis. We also discuss how cancer-specific modifications to ROS and antioxidants may be targeted for therapy.

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.