Lucas B. Sullivan

Reductive carboxylation supports growth in tumour cells with defective mitochondria

Mitochondrial metabolism provides precursors to build macromolecules in growing cancer cells. 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 tumorigenesis. Yet some tumours harbour mutations in the citric acid cycle (CAC) or electron transport chain (ETC) that disable normal oxidative mitochondrial function, 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.

Metformin inhibits mitochondrial complex I of cancer cells to reduce tumorigenesis

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 metformins inhibitory effects on cancer progression are cancer cell autonomous and depend on its ability to inhibit mitochondrial complex I. DOI: http://dx.doi.org/10.7554/eLife.02242.001

Supporting Aspartate Biosynthesis Is an Essential Function of Respiration in Proliferating Cells

Mitochondrial respiration is important for cell proliferation; however, the specific metabolic requirements fulfilled by respiration to support proliferation have not been defined. Here, we show that a major role of respiration in proliferating cells is to provide electron acceptors for aspartate synthesis. This finding is consistent with the observation that cells lacking a functional respiratory chain are auxotrophic for pyruvate, which serves as an exogenous electron acceptor. Further, the pyruvate requirement can be fulfilled with an alternative electron acceptor, alpha-ketobutyrate, which provides cells neither carbon nor ATP. Alpha-ketobutyrate restores proliferation when respiration is inhibited, suggesting that an alternative electron acceptor can substitute for respiration to support proliferation. We find that electron acceptors are limiting for producing aspartate, and supplying aspartate enables proliferation of respiration deficient cells in the absence of exogenous electron acceptors. Together, these data argue a major function of respiration in proliferating cells is to support aspartate synthesis.

Mitochondrial reactive oxygen species and cancer

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.

Environment Impacts the Metabolic Dependencies of Ras-Driven Non-Small Cell Lung Cancer

Cultured cells convert glucose to lactate, and glutamine is the major source of tricarboxylic acid (TCA)-cycle carbon, but whether the same metabolic phenotype is found in tumors is less studied. We infused mice with lung cancers with isotope-labeled glucose or glutamine and compared the fate of these nutrients in tumor and normal tissue. As expected, lung tumors exhibit increased lactate production from glucose. However, glutamine utilization by both lung tumors and normal lung was minimal, with lung tumors showing increased glucose contribution to the TCA cycle relative to normal lung tissue. Deletion of enzymes involved in glucose oxidation demonstrates that glucose carbon contribution to the TCA cycle is required for tumor formation. These data suggest that understanding nutrient utilization by tumors can predict metabolic dependencies of cancers in vivo. Furthermore, these data argue that the in vivo environment is an important determinant of the metabolic phenotype of cancer cells.

The proto-oncometabolite fumarate binds glutathione to amplify ROS dependent signaling

The tricarboxylic acid cycle enzyme fumarate hydratase (FH) has been identified as a tumor suppressor in a subset of human renal cell carcinomas. Human FH-deficient cancer cells display high fumarate concentration and ROS levels along with activation of HIF-1. The underlying mechanisms by which FH loss increases ROS and HIF-1 are not fully understood. Here, we report that glutamine-dependent oxidative citric acid cycle metabolism is required to generate fumarate and increase ROS and HIF-1 levels. Accumulated fumarate directly bonds the antioxidant glutathione in vitro and in vivo to produce the metabolite succinated glutathione (GSF). GSF acts as an alternative substrate to glutathione reductase to decrease NADPH levels and enhance mitochondrial ROS and HIF-1 activation. Increased ROS also correlates with hypermethylation of histones in these cells. Thus, fumarate serves as a proto-oncometabolite by binding to glutathione which results in the accumulation of ROS.

Altered metabolite levels in cancer: implications for tumour biology and cancer therapy

Altered cell metabolism is a characteristic feature of many cancers. Aside from well-described changes in nutrient consumption and waste excretion, altered cancer cell metabolism also results in changes to intracellular metabolite concentrations. Increased levels of metabolites that result directly from genetic mutations and cancer-associated modifications in protein expression can promote cancer initiation and progression. Changes in the levels of specific metabolites, such as 2-hydroxyglutarate, fumarate, succinate, aspartate and reactive oxygen species, can result in altered cell signalling, enzyme activity and/or metabolic flux. In this Review, we discuss the mechanisms that lead to changes in metabolite concentrations in cancer cells, the consequences of these changes for the cells and how they might be exploited to improve cancer therapy.

A PHGDH inhibitor reveals coordination of serine synthesis and one-carbon unit fate

Serine is a both a proteinogenic amino acid and the source of one-carbon units essential for de novo purine and deoxythymidine synthesis. In the canonical glucose-derived serine synthesis pathway, Homo sapiens phosphoglycerate dehydrogenase (PHGDH) catalyzes the first, rate-limiting step. Genetic loss of PHGDH is toxic towards PHGDH-overexpressing breast cancer cell lines even in the presence of exogenous serine. Here, we use 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 suggest that one-carbon unit wasting may contribute to the efficacy of PHGDH inhibitors in vitro and in vivo.

EGLN1 Inhibition and Rerouting of α-Ketoglutarate Suffice for Remote Ischemic Protection

Ischemic preconditioning is the phenomenon whereby brief periods of sublethal ischemia protect against a subsequent, more prolonged, ischemic insult. In remote ischemic preconditioning (RIPC), ischemia to one organ protects others organs at a distance. We created mouse models to ask if inhibition of the alpha-ketoglutarate (αKG)-dependent dioxygenase Egln1, which senses oxygen and regulates the hypoxia-inducible factor (HIF) transcription factor, could suffice to mediate local and remote ischemic preconditioning. Using somatic gene deletion and a pharmacological inhibitor, we found that inhibiting Egln1 systemically or in skeletal muscles protects mice against myocardial ischemia-reperfusion (I/R) injury. Parabiosis experiments confirmed that RIPC in this latter model was mediated by a secreted factor. Egln1 loss causes accumulation of circulating αKG, which drives hepatic production and secretion of kynurenic acid (KYNA) that is necessary and sufficient to mediate cardiac ischemic protection in this setting.

Understanding the complex-I-ty of metformin action: limiting mitochondrial respiration to improve cancer therapy

Metformin has been a first-line treatment for type II diabetes mellitus for decades and is the most widely prescribed antidiabetic drug. Retrospective studies have found that metformin treatment is associated with both reduced cancer diagnoses and cancer-related deaths. Despite the prevalence of metformin use in the clinic, its molecular mechanism of action remains controversial. In a recent issue of Cancer & Metabolism, Andrzejewski et al. present evidence that metformin acts directly on mitochondria to inhibit complex I and limits the ability of cancer cells to cope with energetic stress. Here, we discuss evidence that supports the role of metformin as a cancer therapeutic.See research article: http://www.cancerandmetabolism.com/content/2/1/12.