Heptanoic and medium branched-chain fatty acids as anaplerotic treatment for medium chain acyl-CoA dehydrogenase deficiency

Triheptanoin (triheptanoylglycerol) has shown value as anaplerotic therapy for patients with long chain fatty acid oxidation disorders but is contraindicated in medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. In search for anaplerotic therapy for patients with MCAD deficiency, fibroblasts from three patients homozygous for the most common mutation, ACADMG985A/G985A, were treated with fatty acids hypothesized not to require MCAD for their metabolism, including heptanoic (C7; the active component of triheptanoin), 2,6-dimethylheptanoic (dMC7), 6-amino-2,4-dimethylheptanoic (AdMC7), or 4,8-dimethylnonanoic (dMC9) acids. Their effectiveness as anaplerotic fatty acids was assessed in live cells by monitoring changes in cellular oxygen consumption rate (OCR) and mitochondrial protein lysine succinylation, which reflects cellular succinyl-CoA levels, using immunofluorescence (IF) staining. Krebs cycle intermediates were also quantitated in these cells using targeted metabolomics. The four fatty acids induced positive changes in OCR parameters, consistent with their oxidative catalysis and utilization. Increases in cellular IF staining of succinylated lysines were observed, indicating that the fatty acids were effective sources of succinyl-CoA in the absence of media glucose, pyruvate, and lipids. The ability of MCAD deficient cells to metabolize C7 was confirmed by the ability of extracts to enzymatically utilize C7-CoA as substrate but not C8-CoA. To evaluate C7 therapeutic potential in vivo, Acadm-/- mice were treated with triheptanoin for seven days. Dose dependent increase in plasma levels of heptanoyl-, valeryl-, and propionylcarnitine indicated efficient metabolism of the medication. The pattern of the acylcarnitine profile paralleled resolution of liver pathology including reversing hepatic steatosis, increasing hepatic glycogen content, and increasing hepatocyte protein succinylation, all indicating improved energy homeostasis in the treated mice. These results provide the impetus to evaluate triheptanoin and the medium branched chain fatty acids as potential therapeutic agents for patients with MCAD deficiency.

Odd- and even-numbered medium-chained fatty acids protect against glutathione depletion in very long-chain acyl-CoA dehydrogenase deficiency

Recent trials have reported the ability of triheptanoin to improve clinical outcomes for the severe symptoms associated with long-chain fatty acid oxidation disorders, including very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency. However, the milder myopathic symptoms are still challenging to treat satisfactorily. Myopathic pathogenesis is multifactorial, but oxidative stress is an important component. We have previously shown that metabolic stress increases the oxidative burden in VLCAD-deficient cell lines and can deplete the antioxidant glutathione (GSH). We investigated whether medium-chain fatty acids provide protection against GSH depletion during metabolic stress in VLCAD-deficient fibroblasts. To investigate the effect of differences in anaplerotic capacity, we included both even-(octanoate) and odd-numbered (heptanoate) medium-chain fatty acids. Overall, we show that modulation of the concentration of medium-chain fatty acids in culture media affects levels of GSH retained during metabolic stress in VLCAD-deficient cell lines but not in controls. Lowered glutamine concentration in the culture media during metabolic stress led to GSH depletion and decreased viability in VLCAD deficient cells, which could be rescued by both heptanoate and octanoate in a dose-dependent manner. Unlike GSH levels, the levels of total thiols increased after metabolic stress exposure, the size of this increase was not affected by differences in cell culture medium concentrations of glutamine, heptanoate or octanoate. Addition of a PPAR agonist further exacerbated stress-related GSH-depletion and viability loss, requiring higher concentrations of fatty acids to restore GSH levels and cell viability. Both odd- and even-numbered medium-chain fatty acids efficiently protect VLCADdeficient cells against metabolic stress-induced antioxidant depletion.

Rates of pyruvate carboxylase, glutamate and GABA neurotransmitter cycling, and glucose oxidation in multiple brain regions of the awake rat using a combination of [2-13C]/[1-13C]glucose infusion and 1H-[13C]NMR ex vivo

Anaplerosis occurs predominately in astroglia through the action of pyruvate carboxylase (PC). The rate of PC (Vpc) has been reported for cerebral cortex (or whole brain) of awake humans and anesthetized rodents, but regional brain rates remain largely unknown and, hence, were subjected to investigation in the current study. Awake male rats were infused with either [2-¹³C]glucose or [1-¹³C]glucose (n = 27/30) for 8, 15, 30, 60 or 120 min, followed by rapid euthanasia with focused-beam microwave irradiation to the brain. Blood plasma and extracts of cerebellum, hippocampus, striatum, and cerebral cortex were analyzed by ¹H-[¹³C]-NMR to establish ¹³C-enrichment time courses for glutamate-C4,C3,C2, glutamine-C4,C3, GABA-C2,C3,C4 and aspartate-C2,C3. Metabolic rates were determined by fitting a three-compartment metabolic model (glutamatergic and GABAergic neurons and astroglia) to the eighteen time courses. Vpc varied by 44% across brain regions, being lowest in the cerebellum (0.087 ± 0.004 µmol/g/min) and highest in striatum (0.125 ± 0.009) with intermediate values in cerebral cortex (0.106 ± 0.005) and hippocampus (0.114 ± 0.005). Vpc constituted 13-19% of the oxidative glucose consumption rate. Combination of cerebral cortical data with literature values revealed a positive correlation between Vpc and the rates of glutamate/glutamine-cycling and oxidative glucose consumption, respectively, consistent with earlier observations.

The anaplerotic pyruvate carboxylase from white shrimp Litopenaeus vannamei: Gene structure, molecular characterization, protein modelling and expression during hypoxia

Hypoxic zones are spreading worldwide in marine environments affecting many organisms. Shrimp and other marine crustaceans can withstand environmental hypoxia using several strategies, including the regulation of energy producing metabolic pathways. Pyruvate carboxylase (PC) catalyzes the first reaction of gluconeogenesis to produce oxaloacetate from pyruvate. In mammals, PC also participates in lipogenesis, insulin secretion and other processes, but this enzyme has been scarcely studied in marine invertebrates. In this work, we characterized the gene encoding PC in the white shrimp Litopenaeus vannamei, modelled the protein structure and evaluated its gene expression in hepatopancreas during hypoxia, as well as glucose and lactate concentrations. The PC gene codes for a mitochondrial protein and has 21 coding exons and 4 non-coding exons that generate three transcript variants with differences only in the 5'-UTR. Total PC expression is more abundant in hepatopancreas compared to gills or muscle, indicating tissue-specific expression. Under hypoxic conditions of 1.53 mg/L dissolved oxygen, PC expression is maintained in hepatopancreas, indicating its key role even in energy-limited conditions. Finally, both glucose and lactate concentrations were maintained under hypoxia for 24-48 h in hepatopancreas.

Autophosphorylation Inhibits RcCDPK1, a Dual-Specificity Kinase that Phosphorylates Bacterial-Type Phosphoenolpyruvate Carboxylase in Castor Oil Seeds

Phosphoenolpyruvate carboxylase (PEPC) is a tightly regulated enzyme that plays a crucial anaplerotic role in central plant metabolism. Bacterial-type PEPC (BTPC) of developing castor oil seeds (COS) is highly expressed as a catalytic and regulatory subunit of a novel Class-2 PEPC heteromeric complex. Ricinus communis Ca2+-dependent protein kinase-1 (RcCDPK1) catalyzes in vivo inhibitory phosphorylation of COS BTPC at Ser451. Autokinase activity of recombinant RcCDPK1 was detected and 42 autophosphorylated Ser, Thr or Tyr residues were mapped via liquid chromatography-tandem mass spectrometry. Prior autophosphorylation markedly attenuated the ability of RcCDPK1 to transphosphorylate its BTPC substrate at Ser451. However, fully dephosphorylated RcCDPK1 rapidly autophosphorylated during the initial stages of a BTPC transphosphorylation assay. This suggests that Ca2+-dependent binding of dephospho-RcCDPK1 to BTPC may trigger a structural change that leads to rapid autophosphorylation and subsequent substrate transphosphorylation. Tyr30 was identified as an autophosphorylation site via LC-MS/MS and immunoblotting with a phosphosite-specific antibody. Tyr30 occurs at the junction of RcCDPK1's N-terminal variable (NTVD) and catalytic domains and is widely conserved in plant and protist CDPKs. Interestingly, a reduced rate and extent of BTPC transphosphorylation occurred with a RcCDPK1Y30F mutant. Prior research demonstrated that RcCDPK1's NTVD is essential for its Ca2+-dependent autophosphorylation or BTPC transphosphorylation activities but plays no role in target recognition. We propose that Tyr30 autophosphorylation facilitates a Ca2+-dependent interaction between the NTVD and Ca2+-activation domain that primes RcCDPK1 for transphosphorylating BTPC at Ser451. Our results provide insights into links between the post-translational control of COS anaplerosis, Ca2+-dependent signaling and the biological significance of RcCDPK1 autophosphorylation.

Impaired anaplerosis is a major contributor to glycolysis inhibitor toxicity in glioma

BACKGROUND

Reprogramming of metabolic pathways is crucial to satisfy the bioenergetic and biosynthetic demands and maintain the redox status of rapidly proliferating cancer cells. In tumors, the tricarboxylic acid (TCA) cycle generates biosynthetic intermediates and must be replenished (anaplerosis), mainly from pyruvate and glutamine. We recently described a novel enolase inhibitor, HEX, and its pro-drug POMHEX. Since glycolysis inhibition would deprive the cell of a key source of pyruvate, we hypothesized that enolase inhibitors might inhibit anaplerosis and synergize with other inhibitors of anaplerosis, such as the glutaminase inhibitor, CB-839.

METHODS

We analyzed polar metabolites in sensitive (ENO1-deleted) and resistant (ENO1-WT) glioma cells treated with enolase and glutaminase inhibitors. We investigated whether sensitivity to enolase inhibitors could be attenuated by exogenous anaplerotic metabolites. We also determined the synergy between enolase inhibitors and the glutaminase inhibitor CB-839 in glioma cells in vitro and in vivo in both intracranial and subcutaneous tumor models.

RESULTS

Metabolomic profiling of ENO1-deleted glioma cells treated with the enolase inhibitor revealed a profound decrease in the TCA cycle metabolites with the toxicity reversible upon exogenous supplementation of supraphysiological levels of anaplerotic substrates, including pyruvate. ENO1-deleted cells also exhibited selective sensitivity to the glutaminase inhibitor CB-839, in a manner rescuable by supplementation of anaplerotic substrates or plasma-like media Plasmax^TM. In vitro, the interaction of these two drugs yielded a strong synergistic interaction but the antineoplastic effects of CB-839 as a single agent in ENO1-deleted xenograft tumors in vivo were modest in both intracranial orthotopic tumors, where the limited efficacy could be attributed to the blood-brain barrier (BBB), and subcutaneous xenografts, where BBB penetration is not an issue. This contrasts with the enolase inhibitor HEX, which, despite its negative charge, achieved antineoplastic effects in both intracranial and subcutaneous tumors.

CONCLUSION

Together, these data suggest that at least for ENO1-deleted gliomas, tumors in vivo-unlike cells in culture-show limited dependence on glutaminolysis and instead primarily depend on glycolysis for anaplerosis. Our findings reinforce the previously reported metabolic idiosyncrasies of in vitro culture and suggest that cell culture media nutrient composition more faithful to the in vivo environment will more accurately predict in vivo efficacy of metabolism targeting drugs.

New routes to eradicating chronic myelogenous leukemia stem cells by targeting metabolism

Chronic myelogenous leukemia (CML) stem cells are the cellular source of the vast majority of mature CML cells and responsible for relapse of CML disease post-tyrosine kinase inhibitor (TKI) therapy. Although mature CML cells, whose active division is driven by BCR-ABL1 oncogene-dependent signaling, are reduced by TKI therapy, CML stem cells are resistant because they become quiescent via a heretofore elusive mechanism that is independent of oncogene signaling. Recent advances in highly sensitive metabolomics analyses, however, have unveiled new metabolic pathways that are essential for the survival of CML stem cells. With respect to glucose metabolism, CML stem cells elevate anaplerosis to sustain the TCA cycle. Blast crisis (BC)-CML stem cells increase their branched-chained amino acid (BCAA) metabolism. Recently, we showed that CML stem cell quiescence in vivo is regulated by lysophospholipid metabolism that is specific to these cells, namely cooperation between the stemness factors FOXO and β-catenin. These findings reveal biologically significant links between CML stemness and novel metabolic mechanisms. In this review, I describe these links in the contexts of glucose, amino acid, and lipid metabolism, and speculate on how innovative therapeutics might be designed to eradicate CML stem cells in vivo and overcome disease relapse post-TKI therapy.

Deficient astrocyte metabolism impairs glutamine synthesis and neurotransmitter homeostasis in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) leads to cerebral accumulation of insoluble amyloid-β plaques causing synaptic dysfunction and neuronal death. Neurons rely on astrocyte-derived glutamine for replenishment of the amino acid neurotransmitter pools. Perturbations of astrocyte glutamine synthesis have been described in AD, but whether this functionally affects neuronal neurotransmitter synthesis is not known. Since the synthesis and recycling of neurotransmitter glutamate and GABA are intimately coupled to cellular metabolism, the aim of this study was to provide a functional investigation of neuronal and astrocytic energy and neurotransmitter metabolism in AD. To achieve this, we incubated acutely isolated cerebral cortical and hippocampal slices from 8-month-old female 5xFAD mice, in the presence of ¹³C isotopically enriched substrates, with subsequent gas chromatography-mass spectrometry (GC-MS) analysis. A prominent neuronal hypometabolism of [U-¹³C]glucose was observed in the hippocampal slices of the 5xFAD mice. Investigating astrocyte metabolism, using [1,2-¹³C]acetate, revealed a marked reduction in glutamine synthesis, which directly hampered neuronal synthesis of GABA. This was supported by an increased metabolism of exogenously supplied [U-¹³C]glutamine, suggesting a neuronal metabolic compensation of the reduced astrocytic glutamine supply. In contrast, astrocytic metabolism of [U-¹³C]GABA was reduced, whereas [U-¹³C]glutamate metabolism was unaffected. Finally, astrocyte de novo synthesis of glutamate and glutamine was hampered, whereas the enzymatic capacity of glutamine synthetase for ammonia fixation was maintained. Collectively, we demonstrate that deficient astrocyte metabolism leads to reduced glutamine synthesis, directly impairing neuronal GABA synthesis in the 5xFAD brain. These findings suggest that astrocyte metabolic dysfunction may be fundamental for the imbalances of synaptic excitation and inhibition in the AD brain.

New perspectives on butyrate assimilation in Rhodospirillum rubrum S1H under photoheterotrophic conditions

BACKGROUND

The great metabolic versatility of the purple non-sulfur bacteria is of particular interest in green technology. Rhodospirillum rubrum S1H is an α-proteobacterium that is capable of photoheterotrophic assimilation of volatile fatty acids (VFAs). Butyrate is one of the most abundant VFAs produced during fermentative biodegradation of crude organic wastes in various applications. While there is a growing understanding of the photoassimilation of acetate, another abundantly produced VFA, the mechanisms involved in the photoheterotrophic metabolism of butyrate remain poorly studied.

RESULTS

In this work, we used proteomic and functional genomic analyses to determine potential metabolic pathways involved in the photoassimilation of butyrate. We propose that a fraction of butyrate is converted to acetyl-CoA, a reaction shared with polyhydroxybutyrate metabolism, while the other fraction supplies the ethylmalonyl-CoA (EMC) pathway used as an anaplerotic pathway to replenish the TCA cycle. Surprisingly, we also highlighted a potential assimilation pathway, through isoleucine synthesis and degradation, allowing the conversion of acetyl-CoA to propionyl-CoA. We tentatively named this pathway the methylbutanoyl-CoA pathway (MBC). An increase in isoleucine abundance was observed during the early growth phase under butyrate condition. Nevertheless, while the EMC and MBC pathways appeared to be concomitantly used, a genome-wide mutant fitness assay highlighted the EMC pathway as the only pathway strictly required for the assimilation of butyrate.

CONCLUSION

Photoheterotrophic growth of Rs. rubrum with butyrate as sole carbon source requires a functional EMC pathway. In addition, a new assimilation pathway involving isoleucine synthesis and degradation, named the methylbutanoyl-CoA (MBC) pathway, could also be involved in the assimilation of this volatile fatty acid by Rs. rubrum.

c-Myc directly targets an over-expression of pyruvate carboxylase in highly invasive breast cancer

Here we showed that the c-Myc oncogene is responsible for overexpression of pyruvate carboxylase (PC) in highly invasive MDA-MB-231 cells. Pharmacological inhibition of c-Myc activity with 10074-G5 compound, resulted in a marked reduction of PC mRNA and protein, concomitant with reduced cell growth, migration and invasion. This growth inhibition but not migration and invasion can be partly restored by overexpression of PC, indicating that PC is a c-Myc-regulated pro-proliferating enzyme. Analysis of chromatin immunoprecipitation sequencing of c-Myc bound promoters revealed that c-Myc binds to two canonical c-Myc binding sites, locating at nucleotides -417 to -407 and -301 to -291 in the P2 promoter of human PC gene. Mutation of either c-Myc binding site in the P2 promoter-luciferase construct resulted in 50-60% decrease in luciferase activity while double mutation of c-Myc binding sites further decreased the luciferase activity in MDA-MB-231 cells. Overexpression of c-Myc in HEK293T cells that have no endogenous c-Myc resulted in 250-fold increase in luciferase activity. Mutation of either E-boxes lowered luciferase activity by 50% and 25%, respectively while double mutation of both sites abolished the c-Myc transactivation response. An electrophoretic mobility shift assay using nuclear proteins from MDA-MB-231 confirmed binding of c-Myc to both c-Myc binding sites in the P2 promoter. Bioinformatic analysis of publicly available transcriptomes from the cancer genome atlas (TCGA) dataset revealed an association between expression of c-Myc and PC in primary breast, as well as in lung and colon cancer tissues, suggesting that overexpression of PC is deregulated by c-Myc in these cancers.

Tricarboxylic acid cycle enzyme activities in a mouse model of methylmalonic aciduria

Methylmalonic acidemia (MMA) is a propionate pathway disorder caused by dysfunction of the mitochondrial enzyme methylmalonyl-CoA mutase (MMUT). MMUT catalyzes the conversion of methylmalonyl-CoA to succinyl-CoA, an anaplerotic reaction which feeds into the tricarboxylic acid (TCA) cycle. As part of the pathological mechanisms of MMA, previous studies have suggested there is decreased TCA activity due to a "toxic inhibition" of TCA cycle enzymes by MMA related metabolites, in addition to reduced anaplerosis. Here, we have utilized mitochondria isolated from livers of a mouse model of MMA (Mut-ko/ki) and their littermate controls (Ki/wt) to examine the amounts and enzyme functions of most of the TCA cycle enzymes. We have performed mRNA quantification, protein semi-quantitation, and enzyme activity quantification for TCA cycle enzymes in these samples. Expression profiling showed increased mRNA levels of fumarate hydratase in the Mut-ko/ki samples, which by contrast had reduced protein levels as detected by immunoblot, while all other mRNA levels were unaltered. Immunoblotting also revealed decreased protein levels of 2-oxoglutarate dehydrogenase and malate dehydrogenase 2. Interesting, the decreased protein amount of 2-oxoglutarate dehydrogenase was reflected in decreased activity for this enzyme while there is a trend towards decreased activity of fumarate hydratase and malate dehydrogenase 2. Citrate synthase, isocitrate dehydrogenase 2/3, succinyl-CoA synthase, and succinate dehydrogenase are not statistically different in terms of quantity of enzyme or activity. Finally, we found decreased activity when examining the function of methylmalonyl-CoA mutase in series with succinate synthase and succinate dehydrogenase in the Mut-ko/ki mice compared to their littermate controls, as expected. This study demonstrates decreased activity of certain TCA cycle enzymes and by corollary decreased TCA cycle function, but it supports decreased protein quantity rather than "toxic inhibition" as the underlying mechanism of action. SUMMARY: Methylmalonic acidemia (MMA) is an inborn metabolic disorder of propionate catabolism. In this disorder, toxic metabolites are considered to be the major pathogenic mechanism for acute and long-term complications. However, despite optimized therapies aimed at reducing metabolite levels, patients continue to suffer from late complications, including metabolic stroke and renal insufficiency. Since the propionate pathway feeds into the tricarboxylic acid (TCA) cycle, we investigated TCA cycle function in a constitutive MMA mouse model. We demonstrated decreased amounts of the TCA enzymes, Mdh2 and Ogdh as semi-quantified by immunoblot. Enzymatic activity of Ogdh is also decreased in the MMA mouse model compared to controls. Thus, when the enzyme amounts are decreased, we see the enzymatic activity also decreased to a similar extent for Ogdh. Further studies to elucidate the structural and/or functional links between the TCA cycle and propionate pathways might lead to new treatment approaches for MMA patients.

Differential contribution of pyruvate carboxylation to anaplerosis and cataplerosis during non-gluconeogenic and gluconeogenic conditions in HepG2 cells

Pyruvate carboxylase (PC) is an anaplerotic enzyme that supplies oxaloacetate to mitochondria enabling the maintenance of other metabolic intermediates consumed by cataplerosis. Using liquid chromatography mass spectrometry (LC-MS) to measure metabolic intermediates derived from uniformly labeled ¹³C₆-glucose or [3-¹³C]l-lactate, we investigated the contribution of PC to anaplerosis and cataplerosis in the liver cell line HepG2. Suppression of PC expression by short hairpin RNA lowered incorporation of ¹³C glucose incorporation into tricarboxylic acid cycle intermediates, aspartate, glutamate and sugar derivatives, indicating impaired cataplerosis. The perturbation of these biosynthetic pathways is accompanied by a marked decrease of cell viability and proliferation. In contrast, under gluconeogenic conditions where the HepG2 cells use lactate as a carbon source, pyruvate carboxylation contributed very little to the maintenance of these metabolites. Suppression of PC did not affect the percent incorporation of ¹³C-labeled carbon from lactate into citrate, α-ketoglutarate, malate, succinate as well as aspartate and glutamate, suggesting that under gluconeogenic condition, PC does not support cataplerosis from lactate.

Remodeling of substrate consumption in the murine sTAC model of heart failure

BACKGROUND

Energy metabolism and substrate selection are key aspects of correct myocardial mechanical function. Myocardial preference for oxidizable substrates changes in both hypertrophy and in overt failure. Previous work has shown that glucose oxidation is upregulated in overpressure hypertrophy, but its fate in overt failure is less clear. Anaplerotic flux of pyruvate into the tricarboxylic acid cycle (TCA) has been posited as a secondary fate of glycolysis, aside from pyruvate oxidation or lactate production.

METHODS AND RESULTS

A model of heart failure that emulates both valvular and hypertensive heart disease, the severe transaortic constriction (sTAC) mouse, was assayed for changes in substrate preference using metabolomic and carbon-13 flux measurements. Quantitative measures of O₂ consumption in the Langendorff perfused mouse heart were paired with ¹³C isotopomer analysis to assess TCA cycle turnover. Since the heart accommodates oxidation of all physiological energy sources, the utilization of carbohydrates, fatty acids, and ketones were measured simultaneously using a triple-tracer NMR method. The fractional contribution of glucose to acetyl-CoA production was upregulated in heart failure, while other sources were not significantly different. A model that includes both pyruvate carboxylation and anaplerosis through succinyl-CoA produced superior fits to the data compared to a model using only pyruvate carboxylation. In the sTAC heart, anaplerosis through succinyl-CoA is elevated, while pyruvate carboxylation was not. Metabolomic data showed depleted TCA cycle intermediate pool sizes versus the control, in agreement with previous results.

CONCLUSION

In the sTAC heart failure model, the glucose contribution to acetyl-CoA production was significantly higher, with compensatory changes in fatty acid and ketone oxidation not reaching a significant level. Anaplerosis through succinyl-CoA is also upregulated, and is likely used to preserve TCA cycle intermediate pool sizes. The triple tracer method used here is new, and can be used to assess sources of acetyl-CoA production in any oxidative tissue.

Enhanced cerebral branched-chain amino acid metabolism in R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is a hereditary and fatal disease causing profound neurodegeneration. Deficits in cerebral energy and neurotransmitter metabolism have been suggested to play a central role in the neuronal dysfunction and death associated with HD. The branched-chain amino acids (BCAAs), leucine, isoleucine and valine, are important for cerebral nitrogen homeostasis, neurotransmitter recycling and can be utilized as energy substrates in the tricarboxylic acid (TCA) cycle. Reduced levels of BCAAs in HD have been validated by several reports. However, it is still unknown how cerebral BCAA metabolism is regulated in HD. Here we investigate the metabolism of leucine and isoleucine in the R6/2 mouse model of HD. Acutely isolated cerebral cortical and striatal slices of control and R6/2 mice were incubated in media containing 15N- or 13C-labeled leucine or isoleucine and slice extracts were analyzed by gas chromatography-mass spectrometry (GC-MS) to determine isotopic enrichment of derived metabolites. Elevated BCAA transamination was found from incubations with [15N]leucine and [15N]isoleucine, in both cerebral cortical and striatal slices of R6/2 mice compared to controls. Metabolism of [U-13C]leucine and [U-13C]isoleucine, entering oxidative metabolism as acetyl CoA, was maintained in R6/2 mice. However, metabolism of [U-13C]isoleucine, entering the TCA cycle as succinyl CoA, was elevated in both cerebral cortical and striatal slices of R6/2 mice, suggesting enhanced metabolic flux via this anaplerotic pathway. To support the metabolic studies, expression of enzymes in the BCAA metabolic pathway was assessed from a proteomic resource. Several enzymes related to BCAA metabolism were found to exhibit augmented expression in the R6/2 brain, particularly related to isoleucine metabolism, suggesting an increase in the BCAA metabolic machinery. Our results show that the capacity for cerebral BCAA metabolism, predominantly of isoleucine, is amplified in the R6/2 brain and indicates that perturbations in cerebral BCAA homeostasis could have functional consequences for HD pathology.

The molecular rationale for therapeutic targeting of glutamine metabolism in pulmonary hypertension

INTRODUCTION

Pulmonary hypertension (PH) is a deadly enigmatic disease with increasing prevalence. Cellular pathologic hallmarks of PH are driven at least partly by metabolic rewiring, but details are just emerging. The discovery that vascular matrix stiffening can mechanically activate the glutaminase (GLS) enzyme and serve as a pathogenic mechanism of PH has advanced our understanding of the complex role of glutamine in PH. It has also offered a novel therapeutic target for development as a next-generation drug for this disease. Area covered: This review discusses the cellular contribution of glutamine metabolism to PH together with the possible therapeutic application of pharmacologic GLS inhibitors in this disease. Expert opinion: Despite advances in our understanding of glutamine metabolism in PH, questions remain unanswered regarding the development of therapies targeting glutamine in PH. The comprehensive mechanisms by which glutamine metabolism rewiring influences pulmonary vascular cell behavior to drive PH are incompletely understood. Because glutamine metabolism exhibits a variety of functions in organ repair and homeostasis, a better understanding of the overall risk-benefit ratio of these strategies with long-term follow-up is needed. This knowledge should pave the way for the design of new strategies to prevent and hopefully even regress PH.

Ketogenic and anaplerotic dietary modifications ameliorate seizure activity in Drosophila models of mitochondrial encephalomyopathy and glycolytic enzymopathy

Seizures are a feature not only of the many forms of epilepsy, but also of global metabolic diseases such as mitochondrial encephalomyopathy (ME) and glycolytic enzymopathy (GE). Modern anti-epileptic drugs (AEDs) are successful in many cases, but some patients are refractory to existing AEDs, which has led to a surge in interest in clinically managed dietary therapy such as the ketogenic diet (KD). This high-fat, low-carbohydrate diet causes a cellular switch from glycolysis to fatty acid oxidation and ketone body generation, with a wide array of downstream effects at the genetic, protein, and metabolite level that may mediate seizure protection. We have recently shown that a Drosophila model of human ME (ATP6¹) responds robustly to the KD; here, we have investigated the mechanistic importance of the major metabolic consequences of the KD in the context of this bioenergetics disease: ketogenesis, reduction of glycolysis, and anaplerosis. We have found that reduction of glycolysis does not confer seizure protection, but that dietary supplementation with ketone bodies or the anaplerotic lipid triheptanoin, which directly replenishes the citric acid cycle, can mimic the success of the ketogenic diet even in the presence of standard carbohydrate levels. We have also shown that the proper functioning of the citric acid cycle is crucial to the success of the KD in the context of ME. Furthermore, our data reveal that multiple seizure models, in addition to ATP6¹, are treatable with the ketogenic diet. Importantly, one of these mutants is TPI^sugarkill, which models human glycolytic enzymopathy, an incurable metabolic disorder with severe neurological consequences. Overall, these studies reveal widespread success of the KD in Drosophila, further cementing its status as an excellent model for studies of KD treatment and mechanism, and reveal key insights into the therapeutic potential of dietary therapy against neuronal hyperexcitability in epilepsy and metabolic disease.

Overview of Glutamine Dependency and Metabolic Rescue Protocols

Enhanced glutaminolysis and glycolysis are the two most remarkable biochemical features of cancer cell metabolism, reflecting increased utilization of glutamine and glucose in proliferating cells. Most solid tumors often outgrow the blood supply, resulting in a tumor microenvironment characterized by the depletion of glutamine, glucose, and oxygen. Whereas mechanisms by which cancer cells sense and metabolically adapt to hypoxia have been well characterized with a variety of cancer types, mechanisms by which different types of tumor cells respond to a dynamic change of glutamine availability and the underlying importance remains to be characterized. Here we describe the protocol, which uses cultured Hep3B cells as a model in determining glutamine-dependent proliferation, metabolite rescuing, and cellular responses to glutamine depletion. These protocols may be modified to study the metabolic roles of glutamine in other types of tumor or non-tumor cells as well.

A pilot study of add-on oral triheptanoin treatment for children with medically refractory epilepsy

AIM

Despite antiepileptic medication and dietary treatment options available about 45% of children with epilepsy still suffer from uncontrolled seizures. Triheptanoin is an anaplerotic treatment designed to improve energy generation via the Krebs cycle.

METHOD

For the first time, we evaluated the feasibility, tolerability and efficacy of add-on triheptanoin in 12 patients with medically refractory epilepsy (seven males, five females; min-max: 3-18yr, median 13.5 yr).

RESULTS

Eight out of a total of 12 children (67%), who tested the treatment, finished the trial and tolerated between 30 and 100 ml of triheptanoin per day for >12 weeks (median 55 ml, 20.5% caloric intake). The most common adverse effects were diarrhea and other gastro-intestinal effects in seven kids. One child experienced leaking and another child had an infected percutaneous endoscopic gastrostomy button. Five children (62.5%), who all had been on the ketogenic diet previously, showed sustained >50% reductions in seizure frequency, including one patient who became seizure free for 30 weeks. Four patients extended their treatment to a total of 201-909 days, until seizure frequency or severity increased.

INTERPRETATION

In this small trial, triheptanoin was safe and tolerable in children with epilepsy. As some children showed reductions in seizure numbers and/or severity, larger randomized controlled studies are now needed for further evaluation of safety and efficacy.

Including the mitochondrial metabolism of L-lactate in cancer metabolic reprogramming

Glucose avidity, high glycolysis and L-lactate production, regardless of oxygen availability, are the main traits of cancer metabolic reprogramming. The idea that mitochondria are dysfunctional in cancer, thus causing a glycolysis increase for ATP production and L-lactate accumulation as a dead-end product of glucose catabolism, has oriented cancer research for many years. However, it was shown that mitochondrial metabolism is essential for cancer cell proliferation and tumorigenesis and that L-lactate is a fundamental energy substrate with tumor growth-promoting and signaling capabilities. Nevertheless, the known ability of mitochondria to take up and oxidize L-lactate has remained ignored by cancer research. Beginning with a brief overview of the metabolic changes occurring in cancer, we review the present knowledge of L-lactate formation, transport, and intracellular oxidation and underline the possible role of L-lactate metabolism as energetic, signaling and anabolic support for cancer cell proliferation. These unexplored aspects of cancer biochemistry might be exploited for therapeutic benefit.

Pancreatic Cancer Metabolism: Molecular Mechanisms and Clinical Applications

PURPOSE OF REVIEW

Pancreatic adenocarcinoma is a leading cause of cancer mortality in western countries with a uniformly poor prognosis. Unfortunately, there has been little in the way of novel therapeutics for this malignancy over the last several decades. Derangements in metabolic circuitry favoring excess glycolysis are increasingly recognized as a key hallmark of cancer.

RECENT FINDINGS

The role of alterations in glutamine metabolism in pancreatic tumor progression has been elucidated in animal models and human cells lines, and there has been considerable interest in exploiting these aberrations for the treatment of pancreatic cancer. Other strategies targeting NQO1/GLS1 inhibition, NAD+ synthesis, and TCA cycle intermediates are being actively studied in the clinic. Aberrant metabolism in pancreatic cancer poses a unique therapeutic strategy. We review preclinical and clinical studies looking to exploit alterations in the metabolic circuitry of pancreatic cancer.

Roles of pyruvate carboxylase in human diseases: from diabetes to cancers and infection

Pyruvate carboxylase (PC), an anaplerotic enzyme, plays an essential role in various cellular metabolic pathways including gluconeogenesis, de novo fatty acid synthesis, amino acid synthesis, and glucose-induced insulin secretion. Deregulation of PC expression or activity has long been known to be associated with metabolic syndrome in several rodent models. Accumulating data in the past decade clearly showed that deregulation of PC expression is associated with type 2 diabetes in humans, while targeted inhibition of PC expression in a mouse model reduced adiposity and improved insulin sensitivity in diet-induced type 2 diabetes. More recent studies also show that PC is strongly involved in tumorigenesis in several cancers, including breast, non-small cell lung cancer, glioblastoma, renal carcinoma, and gall bladder. Systems metabolomics analysis of these cancers identified pyruvate carboxylation as an essential metabolic hub that feeds carbon skeletons of downstream metabolites of oxaloacetate into the biosynthesis of various cellular components including membrane lipids, nucleotides, amino acids, and the redox control. Inhibition or down-regulation of PC expression in several cancers markedly impairs their growth ex vivo and in vivo, drawing attention to PC as an anti-cancer target. PC has also exhibited a moonlight function by interacting with immune surveillance that can either promote or block viral infection. In certain pathogenic bacteria, PC is essential for infection, replication, and maintenance of their virulence phenotype.

Targeting Glutamine Metabolism for Cancer Treatment

Rapidly proliferating cancer cells require energy and cellular building blocks for their growth and ability to maintain redox balance. Many studies have focused on understanding how cancer cells adapt their nutrient metabolism to meet the high demand of anabolism required for proliferation and maintaining redox balance. Glutamine, the most abundant amino acid in plasma, is a well-known nutrient used by cancer cells to increase proliferation as well as survival under metabolic stress conditions. In this review, we provide an overview of the role of glutamine metabolism in cancer cell survival and growth and highlight the mechanisms by which glutamine metabolism affects cancer cell signaling. Furthermore, we summarize the potential therapeutic approaches of targeting glutamine metabolism for the treatment of numerous types of cancer.

Evaluation of Metabolic and Synaptic Dysfunction Hypotheses of Alzheimer's Disease (AD): A Meta-Analysis of CSF Markers

BACKGROUND

Alzheimer's disease (AD) is currently incurable and a majority of investigational drugs have failed clinical trials. One explanation for this failure may be the invalidity of hypotheses focusing on amyloid to explain AD pathogenesis. Recently, hypotheses which are centered on synaptic and metabolic dysfunction are increasingly implicated in AD.

OBJECTIVE

Evaluate AD hypotheses by comparing neurotransmitter and metabolite marker concentrations in normal versus AD CSF.

METHODS

Meta-analysis allows for statistical comparison of pooled, existing cerebrospinal fluid (CSF) marker data extracted from multiple publications, to obtain a more reliable estimate of concentrations. This method also provides a unique opportunity to rapidly validate AD hypotheses using the resulting CSF concentration data. Hubmed, Pubmed and Google Scholar were comprehensively searched for published English articles, without date restrictions, for the keywords "AD", "CSF", and "human" plus markers selected for synaptic and metabolic pathways. Synaptic markers were acetylcholine, gamma-aminobutyric acid (GABA), glutamine, and glycine. Metabolic markers were glutathione, glucose, lactate, pyruvate, and 8 other amino acids. Only studies that measured markers in AD and controls (Ctl), provided means, standard errors/deviation, and subject numbers were included. Data were extracted by six authors and reviewed by two others for accuracy. Data were pooled using ratio of means (RoM of AD/Ctl) and random effects meta-analysis using Cochrane Collaboration's Review Manager software.

RESULTS

Of the 435 identified publications, after exclusion and removal of duplicates, 35 articles were included comprising a total of 605 AD patients and 585 controls. The following markers of synaptic and metabolic pathways were significantly changed in AD/controls: acetylcholine (RoM 0.36, 95% CI 0.24-0.53, p<0.00001), GABA (0.74, 0.58-0.94, p<0.01), pyruvate (0.48, 0.24-0.94, p=0.03), glutathione (1.11, 1.01- 1.21, p=0.03), alanine (1.10, 0.98-1.23, p=0.09), and lower levels of significance for lactate (1.2, 1.00-1.47, p=0.05). Of note, CSF glucose and glutamate levels in AD were not significantly different than that of the controls.

CONCLUSION

This study provides proof of concept for the use of meta-analysis validation of AD hypotheses, specifically via robust evidence for the cholinergic hypothesis of AD. Our data disagree with the other synaptic hypotheses of glutamate excitotoxicity and GABAergic resistance to neurodegeneration, given observed unchanged glutamate levels and decreased GABA levels. With regards to metabolic hypotheses, the data supported upregulation of anaerobic glycolysis, pentose phosphate pathway (glutathione), and anaplerosis of the tricarboxylic acid cycle using glutamate. Future applications of meta-analysis indicate the possibility of further in silico evaluation and generation of novel hypotheses in the AD field.

Pyruvate enhancement of cardiac performance: Cellular mechanisms and clinical application

Cardiac contractile function is adenosine-5'-triphosphate (ATP)-intensive, and the myocardium's high demand for oxygen and energy substrates leaves it acutely vulnerable to interruptions in its blood supply. The myriad cardioprotective properties of the natural intermediary metabolite pyruvate make it a potentially powerful intervention against the complex injury cascade ignited by myocardial ischemia-reperfusion. A readily oxidized metabolic substrate, pyruvate augments myocardial free energy of ATP hydrolysis to a greater extent than the physiological fuels glucose, lactate and fatty acids, particularly when it is provided at supra-physiological plasma concentrations. Pyruvate also exerts antioxidant effects by detoxifying reactive oxygen and nitrogen intermediates, and by increasing nicotinamide adenine dinucleotide phosphate reduced form (NADPH) production to maintain glutathione redox state. These enhancements of free energy and antioxidant defenses combine to augment sarcoplasmic reticular Ca²⁺ release and re-uptake central to cardiac mechanical performance and to restore β-adrenergic signaling of ischemically stunned myocardium. By minimizing Ca²⁺ mismanagement and oxidative stress, pyruvate suppresses inflammation in post-ischemic myocardium. Thus, pyruvate administration stabilized cardiac performance, augmented free energy of ATP hydrolysis and glutathione redox systems, and/or quelled inflammation in a porcine model of cardiopulmonary bypass, a canine model of cardiac arrest-resuscitation, and a caprine model of hypovolemia and hindlimb ischemia-reperfusion. Pyruvate's myriad benefits in preclinical models provide the mechanistic framework for its clinical application as metabolic support for myocardium at risk. Phase one trials have demonstrated pyruvate's safety and efficacy for intravenous resuscitation for septic shock, intracoronary infusion for heart failure and as a component of cardioplegia for cardiopulmonary bypass. The favorable outcomes of these trials, which argue for expanded, phase three investigations of pyruvate therapy, mirror findings in isolated, perfused hearts, underscoring the pivotal role of preclinical research in identifying clinical interventions for cardiovascular diseases. Impact statement This article reviews pyruvate's cardioprotective properties as an energy-yielding metabolic fuel, antioxidant and anti-inflammatory agent in mammalian myocardium. Preclinical research has shown these properties make pyruvate a powerful intervention to curb the complex injury cascade ignited by ischemia and reperfusion. In ischemically stunned isolated hearts and in large mammal models of cardiopulmonary bypass, cardiac arrest-resuscitation and hypovolemia, intracoronary pyruvate supports recovery of myocardial contractile function, intracellular Ca²⁺ homeostasis and free energy of ATP hydrolysis, and its antioxidant actions restore β-adrenergic signaling and suppress inflammation. The first clinical trials of pyruvate for cardiopulmonary bypass, fluid resuscitation and intracoronary intervention for congestive heart failure have been reported. Receiver operating characteristic analyses show remarkable concordance between pyruvate's beneficial functional and metabolic effects in isolated, perfused hearts and in patients recovering from cardiopulmonary bypass in which they received pyruvate- vs. L-lactate-fortified cardioplegia. This research exemplifies the translation of mechanism-oriented preclinical studies to clinical application and outcomes.

Anaplerotic therapy in propionic acidemia

BACKGROUND

Propionic acidemia is a rare metabolic disorder caused by a deficiency of propionyl- CoA carboxylase, the enzyme converting propionyl-CoA to methylmalonyl-CoA that subsequently enters the citric acid cycle as succinyl-CoA. Patients with propionic acidemia cannot metabolize propionic acid, which combines with oxaloacetate to form methylcitric acid. This, with the defective supply of succinyl-CoA, may lead to a deficiency in citric acid cycle intermediates.

PURPOSE

The objective of this study was to determine whether supplements with glutamine (400mg/kg per day), citrate (7.5mEq/kg per day), or ornithine α-ketoglutarate (400mg/kg per day) (anaplerotic agents that could fill up the citric acid cycle) would affect plasma levels of glutamine and ammonia, the urinary excretion of Krebs cycle intermediates, and the clinical outcome in 3 patients with propionic acidemia.

METHODS

Each supplement was administered daily for four weeks with a two week washout period between supplements. The supplement that produced the most favorable changes was supplemented for 30 weeks following the initial study period and then for a 2 year extension.

RESULTS

The urinary excretion of the Krebs cycle intermediates, α-ketoglutarate, succinate, and fumarate increased significantly compared to baseline during citrate supplementation, but not with the other two supplements. For this reason, citrate supplements were continued in the second part of the study. The urinary excretion of methylcitric acid and 3-hydroxypropionic acid did not change with any intervention. No significant changes in ammonia or glutamine levels were observed with any supplement. However, supplementation with any anaplerotic agents normalized the physiological buffering of ammonia by glutamate, with plasma glutamate and alanine levels significantly increasing, rather than decreasing with increasing ammonia levels. No significant side effects were observed with any therapy and safety labs (blood counts, chemistry and thyroid profile) remained unchanged. Motor and cognitive development was severely delayed before the trial and did not change significantly with therapy. Hospitalizations per year did not change during the trial period, but decreased significantly (p<0.05) in the 2years following the study (when citrate was continued) compared to the 2years before and during the study.

CONCLUSIONS

These results indicate that citrate entered the Krebs cycle providing successful anaplerotic therapy by increasing levels of the downstream intermediates of the Krebs cycle: α-ketoglutarate, succinate and fumarate. Citrate supplements were safe and might have contributed to reduce hospitalizations in patients with propionic acidemia.

Mass spectrometry analysis shows the biosynthetic pathways supported by pyruvate carboxylase in highly invasive breast cancer cells

We recently showed that the anaplerotic enzyme pyruvate carboxylase (PC) is up-regulated in human breast cancer tissue and its expression is correlated with the late stages of breast cancer and tumor size [Phannasil et al., PloS One 10, e0129848, 2015]. In the current study we showed that PC enzyme activity is much higher in the highly invasive breast cancer cell line MDA-MB-231 than in less invasive breast cancer cell lines. We generated multiple stable PC knockdown cell lines from the MDA-MB-231 cell line and used mass spectrometry with ¹³C₆-glucose and ¹³C₅-glutamine to discern the pathways that use PC in support of cell growth. Cells with severe PC knockdown showed a marked reduction in viability and proliferation rates suggesting the perturbation of pathways that are involved in cancer invasiveness. Strong PC suppression lowered glucose incorporation into downstream metabolites of oxaloacetate, the product of the PC reaction, including malate, citrate and aspartate. Levels of pyruvate, lactate, the redox partner of pyruvate, and acetyl-CoA were also lower suggesting the impairment of mitochondrial pyruvate cycles. Serine, glycine and 5-carbon sugar levels and flux of glucose into fatty acids were decreased. ATP, ADP and NAD(H) levels were unchanged indicating that PC suppression did not significantly affect mitochondrial energy production. The data indicate that the major metabolic roles of PC in invasive breast cancer are primarily anaplerosis, pyruvate cycling and mitochondrial biosynthesis of precursors of cellular components required for breast cancer cell growth and replication.

Alternative Fuels in Epilepsy and Amyotrophic Lateral Sclerosis

This review summarises the recent findings on metabolic treatments for epilepsy and Amyotrophic Lateral Sclerosis (ALS) in honour of Professor Ursula Sonnewald. The metabolic impairments in rodent models of these disorders as well as affected patients are being discussed. In both epilepsy and ALS, there are defects in glucose uptake and reduced tricarboxylic acid (TCA) cycling, at least in part due to reduced amounts of C4 TCA cycle intermediates. In addition there are impairments in glycolysis in ALS. A reduction in glucose uptake can be addressed by providing the brain with alternative fuels, such as ketones or medium-chain triglycerides. As anaplerotic fuels, such as the triglyceride of heptanoate, triheptanoin, refill the TCA cycle C4/C5 intermediate pool that is deficient, they are ideal to boost TCA cycling and thus the oxidative metabolism of all fuels.

Hyperammonemia as a Presenting Feature in Two Siblings with FBXL4 Variants

Early-onset mitochondrial encephalomyopathy is a rare disorder that presents in the neonatal period with lactic acidosis, hypotonia, and developmental delay. Sequence variants in the nuclear-encoded gene FBXL4 have been previously demonstrated to be a cause of early-onset mitochondrial encephalomyopathy in several unrelated families. We have identified a pair of siblings with mutations in FBXL4 who each presented in the neonatal period with hyperammonemia, low plasma levels of aspartate, low urine levels of tricarboxylic acid cycle intermediates suggesting a defect in anaplerosis, and cerebellar hypoplasia in addition to lactic acidosis and other classic signs of mitochondrial encephalomyopathy. After initial clinical stabilization, both subjects continued to have episodic exacerbations characterized by lactic acidosis and hyperammonemia. Previously reported cases of FBXL4 mutations are reviewed and compared with these affected siblings. These two new cases add to the spectrum of disease caused by mutations in FBLX4 and suggest possible benefit from anaplerotic therapies.

Cessation of biomechanical stretch model of C2C12 cells models myocyte atrophy and anaplerotic changes in metabolism using non-targeted metabolomics analysis

Studies of skeletal muscle disuse, either in patients on bed rest or experimentally in animals (immobilization), have demonstrated that decreased protein synthesis is common, with transient parallel increases in protein degradation. Muscle disuse atrophy involves a process of transition from slow to fast myosin fiber types. A shift toward glycolysis, decreased capacity for fat oxidation, and substrate accumulation in atrophied muscles have been reported, as has accommodation of the liver with an increased gluconeogenic capacity. Recent studies have modeled skeletal muscle disuse by using cyclic stretch of differentiated myotubes (C2C12), which mimics the loading pattern of mature skeletal muscle, followed by cessation of stretch. We utilized this model to determine the metabolic changes using non-targeted metabolomics analysis of the media. We identified increases in amino acids resulting from muscle atrophy-induced protein degradation (largely sarcomere) that occurs with muscle atrophy that are involved in feeding the Kreb's cycle through anaplerosis. Specifically, we identified increased alanine/proline metabolism (significantly elevated proline, alanine, glutamine, and asparagine) and increased α-ketoglutaric acid, the proposed Kreb's cycle intermediate being fed by the alanine/proline metabolic anaplerotic mechanism. Additionally, several unique pathways not clearly delineated in previous studies of muscle unloading were seen, including: (1) elevated keto-acids derived from branched chain amino acids (i.e. 2-ketoleucine and 2-keovaline), which feed into a metabolic pathway supplying acetyl-CoA and 2-hydroxybutyrate (also significantly increased); and (2) elevated guanine, an intermediate of purine metabolism, was seen at 12h unloading. Given the interest in targeting different aspects of the ubiquitin proteasome system to inhibit protein degradation, this C2C12 system may allow the identification of direct and indirect alterations in metabolism due to anaplerosis or through other yet to be identified mechanisms using a non-targeted metabolomics approach.

Hepatic glucose and lipid metabolism

The liver has a central role in the regulation of systemic glucose and lipid fluxes during feeding and fasting and also relies on these substrates for its own energy needs. These parallel requirements are met by coordinated control of carbohydrate and lipid fluxes into and out of the Krebs cycle, which is highly tuned to nutrient availability and heavily regulated by insulin and glucagon. During progression of type 2 diabetes, hepatic carbohydrate and lipid biosynthesis fluxes become elevated, thus contributing to hyperglycaemia and hypertriacylglycerolaemia. Over this interval there are also significant fluctuations in hepatic energy state. To date, it is not known to what extent abnormal glucose and lipid fluxes are causally linked to altered energy states. Recent evidence that the glucose-lowering effects of metformin appear to be mediated by attenuation of hepatic energy generation places an additional spotlight on the interdependence of hepatic biosynthetic and oxidative fluxes. The transition from fasting to feeding results in a significant re-direction of hepatic glucose and lipid fluxes and may also incur a temporary hepatic energy deficit. At present, it is not known to what extent these variables are additionally modified by type 2 diabetes and/or non-alcoholic fatty liver disease. Thus, there is a compelling need to measure fluxes through oxidative, gluconeogenic and lipogenic pathways and determine their relationship with hepatic energy state in both fasting and fed conditions. New magnetic resonance-based technologies allow these variables to be non-invasively studied in animal models and humans. This review summarises a presentation given at the symposium entitled 'The liver in focus' at the 2015 annual meeting of the EASD. It is accompanied by two other reviews on topics from this symposium (by Kenneth Cusi, DOI: 10.1007/s00125-016-3952-1 , and by Hannele Yki-Järvinen, DOI: 10.1007/s00125-016-3944-1 ) and a commentary by the Session Chair, Michael Roden (DOI: 10.1007/s00125-016-3911-x ).

Mitochondria in Cancer Energy Metabolism: Culprits or Bystanders?

Cancer is a disease characterized by uncontrolled growth. Metabolic demands to sustain rapid proliferation must be compelling since aerobic glycolysis is the first as well as the most commonly shared characteristic of cancer. During the last decade, the significance of metabolic reprogramming of cancer has been at the center of attention. Nonetheless, despite all the knowledge gained on cancer biology, the field is not able to reach agreement on the issue of mitochondria: Are damaged mitochondria the cause for aerobic glycolysis in cancer? Warburg proposed the damaged mitochondria theory over 80 years ago; the field has been testing the theory equally long. In this review, we will discuss alterations in metabolic fluxes of cancer cells, and provide an opinion on the damaged mitochondria theory.

Anaplerosis for Glutamate Synthesis in the Neonate and in Adulthood

A central task of the tricarboxylic acid (TCA, Krebs, citric acid) cycle in brain is to provide precursors for biosynthesis of glutamate, GABA, aspartate and glutamine. Three of these amino acids are the partners in the intricate interaction between astrocytes and neurons and form the so-called glutamine-glutamate (GABA) cycle. The ketoacids α-ketoglutarate and oxaloacetate are removed from the cycle for this process. When something is removed from the TCA cycle it must be replaced to permit the continued function of this essential pathway, a process termed anaplerosis. This anaplerotic process in the brain is mainly carried out by pyruvate carboxylation performed by pyruvate carboxylase. The present book chapter gives an introduction and overview into this carboxylation and additionally anaplerosis mediated by propionyl-CoA carboxylase under physiological conditions in the adult and in the developing rodent brain. Furthermore, examples are given about pathological conditions in which anaplerosis is disturbed.

The odd-carbon medium-chain fatty triglyceride triheptanoin does not reduce hepatic steatosis

BACKGROUND & AIMS

Non-alcoholic fatty-liver disease (NAFLD) is the hepatic manifestation of the metabolic syndrome. Previously, we showed that a high-protein diet minimized diet-induced development of fatty liver and even reversed pre-existing steatosis. A high-protein diet leads to amino-acid catabolism, which in turn causes anaplerosis of the tricarboxylic-acid (TCA) cycle. Therefore, we hypothesized that anaplerosis of the TCA cycle could be responsible for the high-protein diet-induced improvement of NAFLD by channeling amino acids into the TCA cycle. Next we considered that an efficient anaplerotic agent, the odd-carbon medium-chain triglyceride triheptanoin (TH), might have similar beneficial effects.

METHODS

C57BL/6J mice were fed low-fat (8en%) or high-fat (42en%) oleate-containing diets with or without 15en% TH for 3 weeks.

RESULTS

TH treatment enhanced the hepatic capacity for fatty-acid oxidation by a selective increase in hepatic Ppara, Acox, and Cd36 expression, and a decline in plasma acetyl-carnitines. It also induced pyruvate cycling through an increased hepatic PCK1 protein concentration and it increased thermogenesis reflected by an increased Ucp2 mRNA content. TH, however, did not reduce hepatic lipid content.

CONCLUSION

The comparison of the present effects of dietary triheptanoin with a previous study by our group on protein supplementation shows that the beneficial effects of the high-protein diet are not mimicked by TH. This argues against anaplerosis as the sole explanatory mechanism for the anti-steatotic effect of a high-protein diet.

Anaplerotic treatment of long-chain fat oxidation disorders with triheptanoin: Review of 15 years Experience

BACKGROUND

The treatment of long-chain mitochondrial β-oxidation disorders (LC-FOD) with a low fat-high carbohydrate diet, a diet rich in medium-even-chain triglycerides (MCT), or a combination of both has been associated with high morbidity and mortality for decades. The pathological tableau appears to be caused by energy deficiency resulting from reduced availability of citric acid cycle (CAC) intermediates required for optimal oxidation of acetyl-CoA. This hypothesis was investigated by diet therapy with carnitine and anaplerotic triheptanoin (TH).

METHODS

Fifty-two documented LC-FOD patients were studied in this investigation (age range: birth to 51 years). Safety monitoring included serial quantitative measurements of routine blood chemistries, blood levels of carnitine and acylcarnitines, and urinary organic acids.

RESULTS

The average frequency of serious clinical complications were reduced from ~60% with conventional diet therapy to 10% with TH and carnitine treatment and mortality decreased from ~65% with conventional diet therapy to 3.8%. Carnitine supplementation was uncomplicated.

CONCLUSION

The energy deficiency in LC-FOD patients was corrected safely and more effectively with the triheptanoin diet and carnitine supplement than with conventional diet therapy. Safe intervention in neonates and infants will permit earlier intervention following pre-natal diagnosis or diagnosis by expanded newborn screening.

Metabolic Tracing Using Stable Isotope-Labeled Substrates and Mass Spectrometry in the Perfused Mouse Heart

There has been a resurgence of interest for the field of cardiac metabolism catalyzed by evidence demonstrating a role of metabolic dysregulation in the pathogenesis of heart disease as well as the increased need for new therapeutic targets for patients with these diseases. In this regard, measuring substrate fluxes is critical in providing insight into the dynamics of cellular metabolism and in delineating the regulation of metabolite production and utilization. This chapter provides a comprehensive description of concepts, guidelines, and tips to assess metabolic fluxes relevant to energy substrate metabolism using (13)C-labeled substrates and (13)C-isotopomer analysis by gas chromatography-mass spectrometry (GC-MS), and the ex vivo working heart as study model. The focus will be on the mouse and on flux parameters, which are commonly assessed in the field, namely, those relevant to substrate selection for energy metabolism, specifically the relative contribution of carbohydrate (glucose, lactate, and pyruvate) and fatty acid oxidation to acetyl-CoA formation for citrate synthesis, glycolysis, as well as anaplerosis. We provide detailed procedures for the heart isolation and perfusion in the working mode as well as for sample processing for metabolite extraction and analysis by GC-MS and subsequent data processing for calculation of metabolic flux parameters. Finally, we address practical considerations and discuss additional applications and future challenges.

Mitochondria as biosynthetic factories for cancer proliferation

Unchecked growth and proliferation is a hallmark of cancer, and numerous oncogenic mutations reprogram cellular metabolism to fuel these processes. As a central metabolic organelle, mitochondria execute critical biochemical functions for the synthesis of fundamental cellular components, including fatty acids, amino acids, and nucleotides. Despite the extensive interest in the glycolytic phenotype of many cancer cells, tumors contain fully functional mitochondria that support proliferation and survival. Furthermore, tumor cells commonly increase flux through one or more mitochondrial pathways, and pharmacological inhibition of mitochondrial metabolism is emerging as a potential therapeutic strategy in some cancers. Here, we review the biosynthetic roles of mitochondrial metabolism in tumors and highlight specific cancers where these processes are activated.

Ammonia metabolism and hyperammonemic disorders

Human adults produce around 1000 mmol of ammonia daily. Some is reutilized in biosynthesis. The remainder is waste and neurotoxic. Eventually most is excreted in urine as urea, together with ammonia used as a buffer. In extrahepatic tissues, ammonia is incorporated into nontoxic glutamine and released into blood. Large amounts are metabolized by the kidneys and small intestine. In the intestine, this yields ammonia, which is sequestered in portal blood and transported to the liver for ureagenesis, and citrulline, which is converted to arginine by the kidneys. The amazing developments in NMR imaging and spectroscopy and molecular biology have confirmed concepts derived from early studies in animals and cell cultures. The processes involved are exquisitely tuned. When they are faulty, ammonia accumulates. Severe acute hyperammonemia causes a rapidly progressive, often fatal, encephalopathy with brain edema. Chronic milder hyperammonemia causes a neuropsychiatric illness. Survivors of severe neonatal hyperammonemia have structural brain damage. Proposed explanations for brain edema are an increase in astrocyte osmolality, generally attributed to glutamine accumulation, and cytotoxic oxidative/nitrosative damage. However, ammonia neurotoxicity is multifactorial, with disturbances also in neurotransmitters, energy production, anaplerosis, cerebral blood flow, potassium, and sodium. Around 90% of hyperammonemic patients have liver disease. Inherited defects are rare. They are being recognized increasingly in adults. Deficiencies of urea cycle enzymes, citrin, and pyruvate carboxylase demonstrate the roles of isolated pathways in ammonia metabolism. Phenylbutyrate is used routinely to treat inherited urea cycle disorders, and its use for hepatic encephalopathy is under investigation.

Importance of glutamine metabolism in leukemia cells by energy production through TCA cycle and by redox homeostasis

Some cancer cells depend on glutamine despite of pronounced glycolysis. We examined the glutamine metabolism in leukemia cells, and found that HL-60 cells most depended on glutamine in the 4 acute myelogenous leukemia (AML) cell lines examined: growth of HL-60 cells was most suppressed by glutamine deprivation and by inhibition of glutaminolysis, which was rescued by tricarboxylic acid (TCA) cycle intermediate, oxaloacetic acid. Glutamine is also involved in antioxidant defense function by increasing glutathione. Glutamine deprivation suppressed the glutathione content and elevated reactive oxygen species most evidently in HL-60 cells. Glutamine metabolism might be a therapeutic target in some leukemia.

The mitochondrial isoform of phosphoenolpyruvate carboxykinase (PEPCK-M) and glucose homeostasis: has it been overlooked?

BACKGROUND

Plasma glucose levels are tightly regulated within a narrow physiologic range. Insulin-mediated glucose uptake by tissues must be balanced by the appearance of glucose from nutritional sources, glycogen stores, or gluconeogenesis. In this regard, a common pathway regulating both glucose clearance and appearance has not been described. The metabolism of glucose to produce ATP is generally considered to be the primary stimulus for insulin release from beta-cells. Similarly, gluconeogenesis from phosphoenolpyruvate (PEP) is believed to be the primarily pathway via the cytosolic isoform of phosphoenolpyruvate carboxykinase (PEPCK-C). These models cannot adequately explain the regulation of insulin secretion or gluconeogenesis.

SCOPE OF REVIEW

A metabolic sensing pathway involving mitochondrial GTP (mtGTP) and PEP synthesis by the mitochondrial isoform of PEPCK (PEPCK-M) is associated with glucose-stimulated insulin secretion from pancreatic beta-cells. Here we examine whether there is evidence for a similar mtGTP-dependent pathway involved in gluconeogenesis. In both islets and the liver, mtGTP is produced at the substrate level by the enzyme succinyl CoA synthetase (SCS-GTP) with a rate proportional to the TCA cycle. In the beta-cell PEPCK-M then hydrolyzes mtGTP in the production of PEP that, unlike mtGTP, can escape the mitochondria to generate a signal for insulin release. Similarly, PEPCK-M and mtGTP might also provide a significant source of PEP in gluconeogenic tissues for the production of glucose. This review will focus on the possibility that PEPCK-M, as a sensor for TCA cycle flux, is a key mechanism to regulate both insulin secretion and gluconeogenesis suggesting conservation of this biochemical mechanism in regulating multiple aspects of glucose homeostasis. Moreover, we propose that this mechanism may be important for regulating insulin secretion and gluconeogenesis compared to canonical nutrient sensing pathways.

MAJOR CONCLUSIONS

PEPCK-M, initially believed to be absent in islets, carries a substantial metabolic flux in beta-cells. This flux is intimately involved with the coupling of glucose-stimulated insulin secretion. PEPCK-M activity may have been similarly underestimated in glucose producing tissues and could potentially be an unappreciated but important source of gluconeogenesis.

GENERAL SIGNIFICANCE

The generation of PEP via PEPCK-M may occur via a metabolic sensing pathway important for regulating both insulin secretion and gluconeogenesis. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Toxicity generated through inhibition of pyruvate carboxylase and carnitine palmitoyl transferase-1 is similar to high glucose/palmitate-induced glucolipotoxicity in INS-1 beta cells

This work was initiated to determine whether toxicity generated through inhibition of mitochondrial fuel metabolism is similar to high glucose/palmitate (HG/PA)-induced glucolipotoxicity. Influx of glucose and free fatty acids into the tricarboxylic acid (TCA) cycle was inhibited by treatment with the pyruvate carboxylase (PC) inhibitor phenylacetic acid (PAA) and carnitine palmitoyl transferase-1 (CPT-1) inhibitor etomoxir (Eto), or knockdown of PC and CPT-1. Treatment of PAA/Eto or knockdown of PC/CPT-1 induced apoptotic death in INS-1 beta cells. Similar to HG/PA treatment, PAA/Eto increased endoplasmic reticulum stress responses but decreased the Akt signal. JNK inhibitor or chemical chaperone was protective against both PAA/Eto- and HG/PA-induced cell death. All attempts to reduce [Ca²⁺](i), stimulate lipid metabolism, and increase the TCA cycle intermediate pool protected PAA/Eto-induced death as well as HG/PA-induced death. These data suggest that signals induced from impaired mitochondrial fuel metabolism play a critical role in HG/PA-induced glucolipotoxicity.

Simultaneous steady-state and dynamic 13C NMR can differentiate alternative routes of pyruvate metabolism in living cancer cells

Metabolic reprogramming facilitates cancer cell growth, so quantitative metabolic flux measurements could produce useful biomarkers. However, current methods to analyze flux in vivo provide either a steady-state overview of relative activities (infusion of (13)C and analysis of extracted metabolites) or a dynamic view of a few reactions (hyperpolarized (13)C spectroscopy). Moreover, although hyperpolarization has successfully quantified pyruvate-lactate exchanges, its ability to assess mitochondrial pyruvate metabolism is unproven in cancer. Here, we combined (13)C hyperpolarization and isotopomer analysis to quantify multiple fates of pyruvate simultaneously. Two cancer cell lines with divergent pyruvate metabolism were incubated with thermally polarized [3-(13)C]pyruvate for several hours, then briefly exposed to hyperpolarized [1-(13)C]pyruvate during acquisition of NMR spectra using selective excitation to maximize detection of H[(13)C]O3(-) and [1-(13)C]lactate. Metabolites were then extracted and subjected to isotopomer analysis to determine relative rates of pathways involving [3-(13)C]pyruvate. Quantitation of hyperpolarized H[(13)C]O3(-) provided a single definitive metabolic rate, which was then used to convert relative rates derived from isotopomer analysis into quantitative fluxes. This revealed that H[(13)C]O3(-) appearance reflects activity of pyruvate dehydrogenase rather than pyruvate carboxylation followed by subsequent decarboxylation reactions. Glucose substantially altered [1-(13)C]pyruvate metabolism, enhancing exchanges with [1-(13)C]lactate and suppressing H[(13)C]O3(-) formation. Furthermore, inhibiting Akt, an oncogenic kinase that stimulates glycolysis, reversed these effects, indicating that metabolism of pyruvate by both LDH and pyruvate dehydrogenase is subject to the acute effects of oncogenic signaling on glycolysis. The data suggest that combining (13)C isotopomer analyses and dynamic hyperpolarized (13)C spectroscopy may enable quantitative flux measurements in living tumors.

Fuel-Stimulated Insulin Secretion Depends upon Mitochondria Activation and the Integration of Mitochondrial and Cytosolic Substrate Cycles

The pancreatic islet β-cell is uniquely specialized to couple its metabolism and rates of insulin secretion with the levels of circulating nutrient fuels, with the mitochondrial playing a central regulatory role in this process. In the β-cell, mitochondrial activation generates an integrated signal reflecting rates of oxidativephosphorylation, Kreb's cycle flux, and anaplerosis that ultimately determines the rate of insulin exocytosis. Mitochondrial activation can be regulated by proton leak and mediated by UCP2, and by alkalinization to utilize the pH gradient to drive substrate and ion transport. Converging lines of evidence support the hypothesis that substrate cycles driven by rates of Kreb's cycle flux and by anaplerosis play an integral role in coupling responsive changes in mitochondrial metabolism with insulin secretion. The components and mechanisms that account for the integrated signal of ATP production, substrate cycling, the regulation of cellular redox state, and the production of other secondary signaling intermediates are operative in both rodent and human islet β-cells.