De novo and salvage purine synthesis pathways across tissues and tumors.

Purine nucleotides are vital for RNA and DNA synthesis, signaling, metabolism, and energy homeostasis. To synthesize purines, cells use two principal routes: the de novo and salvage pathways. Traditionally, it is believed that proliferating cells predominantly rely on de novo synthesis, whereas differentiated tissues favor the salvage pathway. Unexpectedly, we find that adenine and inosine are the most effective circulating precursors for supplying purine nucleotides to tissues and tumors, while hypoxanthine is rapidly catabolized and poorly salvaged in vivo. Quantitative metabolic analysis demonstrates comparative contribution from de novo synthesis and salvage pathways in maintaining purine nucleotide pools in tumors. Notably, feeding mice nucleotides accelerates tumor growth, while inhibiting purine salvage slows down tumor progression, revealing a crucial role of the salvage pathway in tumor metabolism. These findings provide fundamental insights into how normal tissues and tumors maintain purine nucleotides and highlight the significance of purine salvage in cancer.

Crystal structure of human NADK2 reveals a dimeric organization and active site occlusion by lysine acetylation.

NAD+ kinases (NADKs) are metabolite kinases that phosphorylate NAD+ molecules to make NADP+, a limiting substrate for the generation of reducing power NADPH. NADK2 sustains mitochondrial NADPH production that enables proline biosynthesis and antioxidant defense. However, its molecular architecture and mechanistic regulation remain undescribed. Here, we report the crystal structure of human NADK2, revealing a substrate-driven mode of activation. We find that NADK2 presents an unexpected dimeric organization instead of the typical tetrameric assemblage observed for other NADKs. A specific extended segment (aa 325-365) is crucial for NADK2 dimerization and activity. Moreover, we characterize numerous acetylation events, including those on Lys76 and Lys304, which reside near the active site and inhibit NADK2 activity without disrupting dimerization, thereby reducing mitochondrial NADP(H) production, proline synthesis, and cell growth. These findings reveal important molecular insight into the structure and regulation of a vital enzyme in mitochondrial NADPH and proline metabolism.

BMAL1 drives muscle repair through control of hypoxic NAD+ regeneration in satellite cells.

The process of tissue regeneration occurs in a developmentally timed manner, yet the role of circadian timing is not understood. Here, we identify a role for the adult muscle stem cell (MuSC)-autonomous clock in the control of muscle regeneration following acute ischemic injury. We observed greater muscle repair capacity following injury during the active/wake period as compared with the inactive/rest period in mice, and loss of Bmal1 within MuSCs leads to impaired muscle regeneration. We demonstrate that Bmal1 loss in MuSCs leads to reduced activated MuSC number at day 3 postinjury, indicating a failure to properly expand the myogenic precursor pool. In cultured primary myoblasts, we observed that loss of Bmal1 impairs cell proliferation in hypoxia (a condition that occurs in the first 1-3 d following tissue injury in vivo), as well as subsequent myofiber differentiation. Loss of Bmal1 in both cultured myoblasts and in vivo activated MuSCs leads to reduced glycolysis and premature activation of prodifferentiation gene transcription and epigenetic remodeling. Finally, hypoxic cell proliferation and myofiber formation in Bmal1-deficient myoblasts are restored by increasing cytosolic NAD+ Together, we identify the MuSC clock as a pivotal regulator of oxygen-dependent myoblast cell fate and muscle repair through the control of the NAD+-driven response to injury.

Compartmentalized metabolism supports midgestation mammalian development.

Mammalian embryogenesis requires rapid growth and proper metabolic regulation1. Midgestation features increasing oxygen and nutrient availability concomitant with fetal organ development2,3. Understanding how metabolism supports development requires approaches to observe metabolism directly in model organisms in utero. Here we used isotope tracing and metabolomics to identify evolving metabolic programmes in the placenta and embryo during midgestation in mice. These tissues differ metabolically throughout midgestation, but we pinpointed gestational days (GD) 10.5-11.5 as a transition period for both placenta and embryo. Isotope tracing revealed differences in carbohydrate metabolism between the tissues and rapid glucose-dependent purine synthesis, especially in the embryo. Glucose's contribution to the tricarboxylic acid (TCA) cycle rises throughout midgestation in the embryo but not in the placenta. By GD12.5, compartmentalized metabolic programmes are apparent within the embryo, including different nutrient contributions to the TCA cycle in different organs. To contextualize developmental anomalies associated with Mendelian metabolic defects, we analysed mice deficient in LIPT1, the enzyme that activates 2-ketoacid dehydrogenases related to the TCA cycle4,5. LIPT1 deficiency suppresses TCA cycle metabolism during the GD10.5-GD11.5 transition, perturbs brain, heart and erythrocyte development and leads to embryonic demise by GD11.5. These data document individualized metabolic programmes in developing organs in utero.

Lymph protects metastasizing melanoma cells from ferroptosis.

Cancer cells, including melanoma cells, often metastasize regionally through the lymphatic system before metastasizing systemically through the blood1-4; however, the reason for this is unclear. Here we show that melanoma cells in lymph experience less oxidative stress and form more metastases than melanoma cells in blood. Immunocompromised mice with melanomas derived from patients, and immunocompetent mice with mouse melanomas, had more melanoma cells per microlitre in tumour-draining lymph than in tumour-draining blood. Cells that metastasized through blood, but not those that metastasized through lymph, became dependent on the ferroptosis inhibitor GPX4. Cells that were pretreated with chemical ferroptosis inhibitors formed more metastases than untreated cells after intravenous, but not intralymphatic, injection. We observed multiple differences between lymph fluid and blood plasma that may contribute to decreased oxidative stress and ferroptosis in lymph, including higher levels of glutathione and oleic acid and less free iron in lymph. Oleic acid protected melanoma cells from ferroptosis in an Acsl3-dependent manner and increased their capacity to form metastatic tumours. Melanoma cells from lymph nodes were more resistant to ferroptosis and formed more metastases after intravenous injection than did melanoma cells from subcutaneous tumours. Exposure to the lymphatic environment thus protects melanoma cells from ferroptosis and increases their ability to survive during subsequent metastasis through the blood.

Metabolic heterogeneity confers differences in melanoma metastatic potential.

Metastasis requires cancer cells to undergo metabolic changes that are poorly understood1-3. Here we show that metabolic differences among melanoma cells confer differences in metastatic potential as a result of differences in the function of the MCT1 transporter. In vivo isotope tracing analysis in patient-derived xenografts revealed differences in nutrient handling between efficiently and inefficiently metastasizing melanomas, with circulating lactate being a more prominent source of tumour lactate in efficient metastasizers. Efficient metastasizers had higher levels of MCT1, and inhibition of MCT1 reduced lactate uptake. MCT1 inhibition had little effect on the growth of primary subcutaneous tumours, but resulted in depletion of circulating melanoma cells and reduced the metastatic disease burden in patient-derived xenografts and in mouse melanomas. In addition, inhibition of MCT1 suppressed the oxidative pentose phosphate pathway and increased levels of reactive oxygen species. Antioxidants blocked the effects of MCT1 inhibition on metastasis. MCT1high and MCT1-/low cells from the same melanomas had similar capacities to form subcutaneous tumours, but MCT1high cells formed more metastases after intravenous injection. Metabolic differences among cancer cells thus confer differences in metastatic potential as metastasizing cells depend on MCT1 to manage oxidative stress.

The cardiac-enriched microprotein mitolamban regulates mitochondrial respiratory complex assembly and function in mice.

Emerging evidence indicates that a subset of RNA molecules annotated as noncoding contain short open reading frames that code for small functional proteins called microproteins, which have largely been overlooked due to their small size. To search for cardiac-expressed microproteins, we used a comparative genomics approach and identified mitolamban (Mtlbn) as a highly conserved 47-amino acid transmembrane protein that is abundantly expressed in the heart. Mtlbn localizes specifically to the inner mitochondrial membrane where it interacts with subunits of complex III of the electron transport chain and with mitochondrial respiratory supercomplexes. Genetic deletion of Mtlbn in mice altered complex III assembly dynamics and reduced complex III activity. Unbiased metabolomic analysis of heart tissue from Mtlbn knockout mice further revealed an altered metabolite profile consistent with deficiencies in complex III activity. Cardiac-specific Mtlbn overexpression in transgenic (TG) mice induced cardiomyopathy with histological, biochemical, and ultrastructural pathologic features that contributed to premature death. Metabolomic analysis and biochemical studies indicated that hearts from Mtlbn TG mice exhibited increased oxidative stress and mitochondrial dysfunction. These findings reveal Mtlbn as a cardiac-expressed inner mitochondrial membrane microprotein that contributes to mitochondrial electron transport chain activity through direct association with complex III and the regulation of its assembly and function.

Loss of glucose 6-phosphate dehydrogenase function increases oxidative stress and glutaminolysis in metastasizing melanoma cells.

The pentose phosphate pathway is a major source of NADPH for oxidative stress resistance in cancer cells but there is limited insight into its role in metastasis, when some cancer cells experience high levels of oxidative stress. To address this, we mutated the substrate binding site of glucose 6-phosphate dehydrogenase (G6PD), which catalyzes the first step of the pentose phosphate pathway, in patient-derived melanomas. G6PD mutant melanomas had significantly decreased G6PD enzymatic activity and depletion of intermediates in the oxidative pentose phosphate pathway. Reduced G6PD function had little effect on the formation of primary subcutaneous tumors, but when these tumors spontaneously metastasized, the frequency of circulating melanoma cells in the blood and metastatic disease burden were significantly reduced. G6PD mutant melanomas exhibited increased levels of reactive oxygen species, decreased NADPH levels, and depleted glutathione as compared to control melanomas. G6PD mutant melanomas compensated for this increase in oxidative stress by increasing malic enzyme activity and glutamine consumption. This generated a new metabolic vulnerability as G6PD mutant melanomas were more dependent upon glutaminase than control melanomas, both for oxidative stress management and anaplerosis. The oxidative pentose phosphate pathway, malic enzyme, and glutaminolysis thus confer layered protection against oxidative stress during metastasis.

Drugs for allosteric sites on receptors.

The presence of druggable, topographically distinct allosteric sites on a wide range of receptor families has offered new paradigms for small molecules to modulate receptor function. Moreover, ligands that target allosteric sites offer significant advantages over the corresponding orthosteric ligands in terms of selectivity, including subtype selectivity within receptor families, and can also impart improved physicochemical properties. However, allosteric ligands are not a panacea. Many chemical issues (e.g., flat structure-activity relationships) and pharmacological issues (e.g., ligand-biased signaling) that are allosteric centric have emerged. Notably, the fact that allosteric sites are less evolutionarily conserved leads to improved selectivity; however, this can also lead to species differences that can hinder safety assessment. Many allosteric ligands possess molecular switches, wherein a small structural change (chemical or metabolic) can modulate the mode of pharmacology or receptor subtype selectivity. As the field has matured, as described here, key principles and strategies have emerged for the design of ligands/drugs for allosteric sites.

High-Confidence Qualitative Identification of Organophosphorus Nerve Agent Adducts to Human Butyrylcholinesterase.

In this study, a data-dependent, high-resolution tandem mass spectrometry (ddHRMS/MS) method capable of detecting all organophosphorus nerve agent (OPNA) adducts to human butyrylcholinesterase (BChE) was developed. After an exposure event, immunoprecipitation from blood with a BChE-specific antibody and digestion with pepsin produces a nine amino acid peptide containing the OPNA adduct. Signature product ions of this peptic BChE nonapeptide (FGES*AGAAS) offer a route to broadly screen for OPNA exposure. Taking this approach on an HRMS instrument identifies biomarkers, including unknowns, with high mass accuracy. Using a set of pooled human sera exposed to OPNAs as quality control (QC) materials, the developed method successfully identified precursor ions with <1 ppm and tied them to signature product ions with <5 ppm deviation from their chemical formulas. This high mass accuracy data from precursor and product ions, collected over 23 independent immunoprecipitation preparations, established method operating limits. QC data and experiments with 14 synthetic reference peptides indicated that reliable qualitative identification of biomarkers was possible for analytes >15 ng/mL. The developed method was applied to a convenience set of 96 unexposed serum samples and a blinded set of 80 samples treated with OPNAs. OPNA biomarkers were not observed in convenience set samples and no false positive or negative identifications were observed in blinded samples. All biomarkers in the blinded serum set >15 ng/mL were correctly identified. For the first time, this study reports a ddHRMS/MS method capable of complementing existing quantitative methodologies and suitable for identifying exposure to unknown organophosphorus agents.

Phospholipase D1 Couples CD4+ T Cell Activation to c-Myc-Dependent Deoxyribonucleotide Pool Expansion and HIV-1 Replication.

Quiescent CD4+ T cells restrict human immunodeficiency virus type 1 (HIV-1) infection at early steps of virus replication. Low levels of both deoxyribonucleotide triphosphates (dNTPs) and the biosynthetic enzymes required for their de novo synthesis provide one barrier to infection. CD4+ T cell activation induces metabolic reprogramming that reverses this block and facilitates HIV-1 replication. Here, we show that phospholipase D1 (PLD1) links T cell activation signals to increased HIV-1 permissivity by triggering a c-Myc-dependent transcriptional program that coordinates glucose uptake and nucleotide biosynthesis. Decreasing PLD1 activity pharmacologically or by RNA interference diminished c-Myc-dependent expression during T cell activation at the RNA and protein levels. PLD1 inhibition of HIV-1 infection was partially rescued by adding exogenous deoxyribonucleosides that bypass the need for de novo dNTP synthesis. Moreover, the data indicate that low dNTP levels that impact HIV-1 restriction involve decreased synthesis, and not only increased catabolism of these nucleotides. These findings uncover a unique mechanism of action for PLD1 inhibitors and support their further development as part of a therapeutic combination for HIV-1 and other viral infections dependent on host nucleotide biosynthesis.

A high-throughput UHPLC-MS/MS method for the quantification of five aged butyrylcholinesterase biomarkers from human exposure to organophosphorus nerve agents.

Organophosphorus nerve agents (OPNAs) are toxic compounds that are classified as prohibited Schedule 1 chemical weapons. In the body, OPNAs bind to butyrylcholinesterase (BChE) to form nerve agent adducts (OPNA-BChE). OPNA-BChE adducts can provide a reliable, long-term protein biomarker for assessing human exposure. A major challenge facing OPNA-BChE detection is hydrolysis (aging), which can continue to occur after a clinical specimen has been collected. During aging, the o-alkyl phosphoester bond hydrolyzes, and the specific identity of the nerve agent is lost. To better identify OPNA exposure events, a high-throughput method for the detection of five aged OPNA-BChE adducts was developed. This is the first diagnostic panel to allow for the simultaneous quantification of any Chemical Weapons Convention Schedule 1 OPNA by measuring the aged adducts methyl phosphonate, ethyl phosphonate, propyl phosphonate, ethyl phosphoryl, phosphoryl and unadducted BChE. The calibration range for all analytes is 2.00-250. ng/mL, which is consistent with similar methodologies used to detect unaged OPNA-BChE adducts. Each analytical run is 3 min, making the time to first unknown results, including calibration curve and quality controls, less than 1 h. Analysis of commercially purchased individual serum samples demonstrated no potential interferences with detection of aged OPNA-BChE adducts, and quantitative measurements of endogenous levels of BChE were similar to those previously reported in other OPNA-BChE adduct assays.

Biomarkers of NAFLD progression: a lipidomics approach to an epidemic.

The spectrum of nonalcoholic fatty liver disease (NAFLD) includes steatosis, nonalcoholic steatohepatitis (NASH), and cirrhosis. Recognition and timely diagnosis of these different stages, particularly NASH, is important for both potential reversibility and limitation of complications. Liver biopsy remains the clinical standard for definitive diagnosis. Diagnostic tools minimizing the need for invasive procedures or that add information to histologic data are important in novel management strategies for the growing epidemic of NAFLD. We describe an "omics" approach to detecting a reproducible signature of lipid metabolites, aqueous intracellular metabolites, SNPs, and mRNA transcripts in a double-blinded study of patients with different stages of NAFLD that involves profiling liver biopsies, plasma, and urine samples. Using linear discriminant analysis, a panel of 20 plasma metabolites that includes glycerophospholipids, sphingolipids, sterols, and various aqueous small molecular weight components involved in cellular metabolic pathways, can be used to differentiate between NASH and steatosis. This identification of differential biomolecular signatures has the potential to improve clinical diagnosis and facilitate therapeutic intervention of NAFLD.

Chemical modulation of glycerolipid signaling and metabolic pathways.

Thirty years ago, glycerolipids captured the attention of biochemical researchers as novel cellular signaling entities. We now recognize that these biomolecules occupy signaling nodes critical to a number of physiological and pathological processes. Thus, glycerolipid-metabolizing enzymes present attractive targets for new therapies. A number of fields-ranging from neuroscience and cancer to diabetes and obesity-have elucidated the signaling properties of glycerolipids. The biochemical literature teems with newly emerging small molecule inhibitors capable of manipulating glycerolipid metabolism and signaling. This ever-expanding pool of chemical modulators appears daunting to those interested in exploiting glycerolipid-signaling pathways in their model system of choice. This review distills the current body of literature surrounding glycerolipid metabolism into a more approachable format, facilitating the application of small molecule inhibitors to novel systems. This article is part of a Special Issue entitled Tools to study lipid functions.

Quantification of metabolites for assessing human exposure to soapberry toxins hypoglycin A and methylenecyclopropylglycine.

Ingestion of soapberry fruit toxins hypoglycin A and methylenecyclopropylglycine has been linked to public health challenges worldwide. In 1976, over 100 years after Jamaican vomiting sickness (JVS) was first reported, the cause of JVS was linked to the ingestion of the toxin hypoglycin A produced by ackee fruit. A structural analogue of hypoglycin A, methylenecyclopropylglycine (MCPG), was implicated as the cause of an acute encephalitis syndrome (AES). Much of the evidence linking hypoglycin A and MCPG to these diseases has been largely circumstantial due to the lack of an analytical method for specific metabolites. This study presents an analytical approach to identify and quantify specific urine metabolites for exposure to hypoglycin A and MCPG. The metabolites are excreted in urine as glycine adducts methylenecyclopropylacetyl-glycine (MCPA-Gly) and methylenecyclopropylformyl-glycine (MCPF-Gly). These metabolites were processed by isotope dilution, separated by reverse-phase liquid chromatography, and monitored by electrospray ionization tandem mass spectrometry. The analytical response ratio was linearly proportional to the concentration of MCPF-Gly and MCPA-Gly in urine from 0.10 to 20 µg/mL with a correlation coefficient of r > 0.99. The assay demonstrated accuracy ≥80% and precision ≤20% RSD across the calibration range. This method has been applied to assess exposure to hypoglycin A and MCPG as part of a larger public health initiative and was used to provide the first reported identification of MCPF-Gly and MCPA-Gly in human urine.

Regulation of phospholipase D activity and phosphatidic acid production after purinergic (P2Y6) receptor stimulation.

Phosphatidic acid (PA) is a lipid second messenger located at the intersection of several lipid metabolism and cell signaling events including membrane trafficking, survival, and proliferation. Generation of signaling PA has long been primarily attributed to the activation of phospholipase D (PLD). PLD catalyzes the hydrolysis of phosphatidylcholine into PA. A variety of both receptor-tyrosine kinase and G-protein-coupled receptor stimulations have been shown to lead to PLD activation and PA generation. This study focuses on profiling the PA pool upon P2Y6 receptor signaling manipulation to determine the major PA producing enzymes. Here we show that PLD, although highly active, is not responsible for the majority of stable PA being produced upon UDP stimulation of the P2Y6 receptor and that PA levels are tightly regulated. By following PA flux in the cell we show that PLD is involved in an initial increase in PA upon receptor stimulation; however, when PLD is blocked, the cell compensates by increasing PA production from other sources. We further delineate the P2Y6 signaling pathway showing that phospholipase Cß3 (PLCß3), PLCδ1, DGKζ and PLD are all downstream of receptor activation. We also show that DGKζ is a novel negative regulator of PLD activity in this system that occurs through an inhibitory mechanism with PKCα. These results further define the downstream events resulting in PA production in the P2Y6 receptor signaling pathway.

Metabolic signatures of thymomas: potential biomarkers and treatment targets.

OBJECTIVES: A study of tumour metabolic reprogramming has revealed disease biomarkers and avenues for therapeutic intervention. Metabolic reprogramming in thymoma is currently understudied and largely unknown. This study utilized metabolomics and isotope tracing with 13C-glucose to metabolically investigate thymomas, adjacent thymic tissue and benign thymic lesions. METHODS: From 2017 to 2021, 20 patients with a suspected thymoma were recruited to this prospective Institutional Review Board approved clinical trial. At the time of surgery, 11 patients were infused with 13C-glucose, a stable, non-radioactive tracer which reports the flow of carbon through metabolic pathways. Samples were analysed by mass spectrometry to measure the abundance of >200 metabolites.13C enrichment was measured in patients who received 13C-glucose infusions. RESULTS: Histological analysis showed that 9 patients had thymomas of diverse subtypes and 11 patients had benign cysts. In our metabolomic analysis, thymomas could be distinguished from both adjacent thymus tissue and benign lesions by metabolite abundances. Metabolites in pyrimidine biosynthesis and glycerophospholipid metabolism were differentially expressed across these tissues.13C-glucose infusions revealed differential labelling patterns in thymoma compared to benign cysts and normal thymus tissue. The lactate/3PG labelling ratio, a metabolic marker in aggressive lung tumours correlated with lactate uptake, was increased in thymomas (1.579) compared to normal thymus (0.945) and benign masses (0.807) (thymic tissue versus tumour P = 0.021, tumour versus benign P = 0.013). CONCLUSIONS: We report metabolic biomarkers, including differential 13C labelling of metabolites from central metabolism, that distinguish thymomas from benign tissues. Altered glucose and lactate metabolism warrant further investigation and may provide novel therapeutic targets for thymoma.

Ascorbate depletion increases quiescence and self-renewal potential in hematopoietic stem cells and multipotent progenitors.

Ascorbate (vitamin C) limits hematopoietic stem cell (HSC) function and suppresses leukemia development by promoting the function of the Tet2 tumor suppressor. In humans, ascorbate is obtained from the diet while in mice it is synthesized in the liver. In this study, we show that deletion of the Slc23a2 ascorbate transporter severely depleted ascorbate from hematopoietic cells. Slc23a2 deficiency increased HSC reconstituting potential and self-renewal potential upon transplantation into irradiated mice. Slc23a2 deficiency also increased the reconstituting and self-renewal potential of multipotent hematopoietic progenitors (MPPs), conferring the ability to long-term reconstitute irradiated mice. Slc23a2-deficient HSCs and MPPs divided much less frequently than control HSCs and MPPs. Increased self-renewal and reconstituting potential were observed particularly in quiescent Slc23a2-deficient HSCs and MPPs. The effect of Slc23a2 deficiency on MPP self-renewal was not mediated by reduced Tet2 function. Ascorbate thus regulates quiescence and restricts self-renewal potential in HSCs and MPPs such that ascorbate depletion confers MPPs with long-term self-renewal potential.

Metabolic inflexibility promotes mitochondrial health during liver regeneration.

Mitochondria are critical for proper organ function and mechanisms to promote mitochondrial health during regeneration would benefit tissue homeostasis. We report that during liver regeneration, proliferation is suppressed in electron transport chain (ETC)-dysfunctional hepatocytes due to an inability to generate acetyl-CoA from peripheral fatty acids through mitochondrial ß-oxidation. Alternative modes for acetyl-CoA production from pyruvate or acetate are suppressed in the setting of ETC dysfunction. This metabolic inflexibility forces a dependence on ETC-functional mitochondria and restoring acetyl-CoA production from pyruvate is sufficient to allow ETC-dysfunctional hepatocytes to proliferate. We propose that metabolic inflexibility within hepatocytes can be advantageous by limiting the expansion of ETC-dysfunctional cells.

Electron transport chain inhibition increases cellular dependence on purine transport and salvage.

Mitochondria house many metabolic pathways required for homeostasis and growth. To explore how human cells respond to mitochondrial dysfunction, we performed metabolomics in fibroblasts from patients with various mitochondrial disorders and cancer cells with electron transport chain (ETC) blockade. These analyses revealed extensive perturbations in purine metabolism, and stable isotope tracing demonstrated that ETC defects suppress de novo purine synthesis while enhancing purine salvage. In human lung cancer, tumors with markers of low oxidative mitochondrial metabolism exhibit enhanced expression of the salvage enzyme hypoxanthine phosphoribosyl transferase 1 (HPRT1) and high levels of the HPRT1 product inosine monophosphate. Mechanistically, ETC blockade activates the pentose phosphate pathway, providing phosphoribosyl diphosphate to drive purine salvage supplied by uptake of extracellular bases. Blocking HPRT1 sensitizes cancer cells to ETC inhibition. These findings demonstrate how cells remodel purine metabolism upon ETC blockade and uncover a new metabolic vulnerability in tumors with low respiration.