Pyruvate Dehydrogenase Complex Flux Research Articles

Pharmacological activation of PDC flux reverses lipid-induced inhibition of insulin action in muscle during recovery from exercise.

Insulin resistance is a risk factor for type 2 diabetes and exercise can improve insulin sensitivity. However, following exercise high circulating fatty acid (FA) levels might counteract this. We hypothesized that such inhibition would be reduced by forcibly increasing carbohydrate oxidation through pharmacological activation of the pyruvate dehydrogenase complex (PDC). Insulin-stimulated glucose uptake was examined with a cross-over design in healthy young men (n = 8) in a previously exercised and a rested leg during a hyperinsulinemiceuglycemic clamp five hours after one-legged exercise with: 1) infusion of saline, 2) infusion of intralipid imitating circulating FA levels during recovery from whole-body exercise, and 3) infusion of intralipid + oral PDC-activator, dichloroacetate (DCA). Intralipid infusion reduced insulin-stimulated glucose uptake by 19% in the previously exercised leg, which was not observed in the contralateral rested leg. Interestingly, this effect of intralipid in the exercised leg was abolished by DCA, which increased muscle PDC activity (130%) and flux (acetylcarnitine 130%) and decreased inhibitory phosphorylation of PDC on Ser293 (∼40%) and Ser300 (∼80%). Novel insight is provided into the regulatory interaction between glucose and lipid metabolism during exercise recovery. Coupling exercise and PDC flux activation upregulated the capacity for both glucose transport (exercise) and oxidation (DCA), which seems necessary to fully stimulate insulin-stimulated glucose uptake during recovery.

Detailed evaluation of pyruvate dehydrogenase complex inhibition in simulated exercise conditions

The mammalian pyruvate dehydrogenase complex (PDC) is a mitochondrial multienzyme complex that connects glycolysis to the tricarboxylic acid cycle by catalyzing pyruvate oxidation to produce acetyl-CoA, NADH, and CO2. This reaction is required to aerobically utilize glucose, a preferred metabolic fuel, and is composed of three core enzymes: pyruvate dehydrogenase (E1), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3). The pyruvate-dehydrogenase-specific kinase (PDK) and pyruvate-dehydrogenase-specific phosphatase (PDP) are considered the main control mechanism of mammalian PDC activity. However, PDK and PDP activity are allosterically regulated by several effectors fully overlapping PDC substrates and products. This collection of positive and negative feedback mechanisms confounds simple predictions of relative PDC flux, especially when all effectors are dynamically modulated during metabolic states that exist in physiologically realistic conditions, such as exercise. Here, we provide, to our knowledge, the first globally fitted, pH-dependent kinetic model of the PDC accounting for the PDC core reaction because it is regulated by PDK, PDP, metal binding equilibria, and numerous allosteric effectors. The model was used to compute PDH regulatory complex flux as a function of previously determined metabolic conditions used to simulate exercise and demonstrates increased flux with exercise. Our model reveals that PDC flux in physiological conditions is primarily inhibited by product inhibition (∼60%), mostly NADH inhibition (∼30–50%), rather than phosphorylation cycle inhibition (∼40%), but the degree to which depends on the metabolic state and PDC tissue source.

Maximal-intensity exercise does not fully restore muscle pyruvate dehydrogenase complex activation after 3 days of high-fat dietary intake

Exercise activates muscle pyruvate dehydrogenase complex (PDC), but moderate intensity exercise fails to fully activate muscle PDC after high-fat diet [1]. We investigated whether maximal intensity exercise overcomes this inhibition. Quadriceps femoris muscle biopsy samples were obtained from healthy males at rest, and after 46 and 92 electrically-evoked maximal intermittent isometric contractions, which were preceded by 3 days of either low- (18%) or high- (69%) isocaloric dietary fat intake (LFD and HFD, respectively). The ratio of PDCa (active form) to total PDCt (fully activated) at rest was 50% less after HFD (0.32±0.01 vs 0.15±0.01; P<0.05). This ratio increased to 0.77±0.06 after 46 contractions (P<0.001) and to 0.98±0.07 after 92 contractions (P<0.001) in LFD. The corresponding values after HFD were less (0.54±0.06; P<0.01 and 0.70±0.07; P<0.01, respectively). Resting muscle acetyl-CoA and acetylcarnitine content was greater after HFD than LFD (both P<0.05), but their rate of accumulation in the former was reduced during contraction. Muscle lactate content after 92 contractions was 30% greater after HFD (P<0.05). Muscle force generation during contraction was no different between interventions, but HFD lengthened muscle relaxation time (P<0.05). Daily urinary total carnitine excretion after HFD was 2.5-fold greater than after LFD (P<0.01). A bout of maximal intense exercise did not overcome dietary fat-mediated inhibition of muscle pyruvate dehydrogenase complex activation, and was associated with greater muscle lactate accumulation, as a result of lower PDC flux, and increased muscle relaxation time.

Tetrameric Acetyl-CoA Acetyltransferase 1 Is Important for Tumor Growth

Mitochondrial acetyl-CoA acetyltransferase 1 (ACAT1) regulates pyruvate dehydrogenase complex (PDC) by acetylating pyruvate dehydrogenase (PDH) and PDH phosphatase. How ACAT1 is "hijacked" to contribute to the Warburg effect in human cancer remains unclear. We found that active, tetrameric ACAT1 is commonly upregulated in cells stimulated by EGF and in diverse human cancer cells, where ACAT1 tetramers, but not monomers, are phosphorylated and stabilized by enhanced Y407 phosphorylation. Moreover, we identified arecoline hydrobromide (AH) as a covalent ACAT1 inhibitor that binds to and disrupts only ACAT1 tetramers. The resultant AH-bound ACAT1 monomers cannot reform tetramers. Inhibition of tetrameric ACAT1 by abolishing Y407 phosphorylation or AH treatment results in decreased ACAT1 activity, leading to increased PDC flux and oxidative phosphorylation with attenuated cancer cell proliferation and tumor growth. These findings provide a mechanistic understanding of how oncogenic events signal through distinct acetyltransferases to regulate cancer metabolism and suggest ACAT1 as an anti-cancer target.

Dual effects of fibroblast growth factor 21 on hepatic energy metabolism.

The aim of this study was to investigate the mechanisms by which fibroblast growth factor 21 (FGF21) affects hepatic integration of carbohydrate and fat metabolism in Siberian hamsters, a natural model of adiposity. Twelve aged matched adult male Siberian hamsters maintained in their long-day fat state since birth were randomly assigned to one of two treatment groups and were continuously infused with either vehicle (saline; n=6) or recombinant human FGF21 protein (1 mg/kg per day; n=6) for 14 days. FGF21 administration caused a 40% suppression (P<0.05) of hepatic pyruvate dehydrogenase complex (PDC), the rate-limiting step in glucose oxidation, a 34% decrease (P<0.05) in hepatic acetylcarnitine accumulation, an index of reduced PDC flux, a 35% increase (P<0.05) in long-chain acylcarnitine content (an index of flux through β-oxidation) and a 47% reduction (P<0.05) in hepatic lipid content. These effects were underpinned by increased protein abundance of PD kinase-4 (PDK4, a negative regulator of PDC), the phosphorylated (inhibited) form of acetyl-CoA carboxylase (ACC, a negative regulator of delivery of fatty acids into the mitochondria) and the transcriptional co-regulators of energy metabolism peroxisome proliferator activated receptor gamma co-activator alpha (PGC1α) and sirtuin-1. These findings provide novel mechanistic basis to support the notion that FGF21 exerts profound metabolic benefits in the liver by modulating nutrient flux through both carbohydrate (mediated by a PDK4-mediated suppression of PDC activity) and fat (mediated by deactivation of ACC) metabolism, and therefore may be an attractive target for protection from increased hepatic lipid content and insulin resistance that frequently accompany obesity and diabetes.

Abstract 239: Augmented Cardiac Pyruvate Dehydrogenase Complex Flux During Hypoxia Increases Post-hypoxic Damage

Modulation of cardiac pyruvate dehydrogenase complex (PDC) flux is known to alter post-ischemic myocardial injury. Hypoxia is intrinsic to ischemia, but its specific role in cardiac injury is unknown. We hypothesized that post-hypoxic cardiac injury would be attenuated with activation of the PDC complex during hypoxia. Mouse hearts were isolated and Langendorff perfused with 0.4mM palmitate and 11mM glucose. Following 30min of normoxia, hearts were subjected to 30min hypoxia (20% of normoxic pO2) followed by 30min of reoxygenation. PDC flux was increased by infusion of 1mM dichloroacetate (DCA), either from onset of hypoxia, or with onset of reoxygenation. Cardiac function was measured using an IV balloon, and fatty acid β-oxidation determined via 3H-labelling. Lactate efflux was assayed from timed buffer collections. Hearts were frozen at end-normoxia, end-hypoxia and end-reoxygenation, and assayed for PDC flux, and phosphate metabolites. Reactive oxygen species formation was assessed by Western blotting of protein carbonyls. End-hypoxic cardiac PDC flux fell to 41±5% of pre-hypoxic values (0.96 of 2.37 μmol/gdwt, p&lt;0.001) with β-oxidation also lower (25±11%, 0.06 of 0.24 μmol/min/gwwt, p&lt;0.025). Lactate efflux increased 2.3-fold (4.7 to 10.6 μmol/min/gwwt, p&lt;0.028) and AMP/ATP ratio 5.3-fold (0.04 to 0.21 p&lt;0.002). DCA infusion increased end-hypoxic PDC flux (170±16%, 1.64 of 0.96 μmol/gdwt, p&lt;0.003), reducing lactate efflux (47±8%, 5 of 11 μmol/min/gwwt, p&lt;0.021) and AMP/ATP ratio (43±12%, 0.09 of 0.21, p&lt;0.025). Control hearts were damaged by hypoxia, final recovery being 63±7% of pre-hypoxic function (18 of 29 bpm.mmHg.103). Hypoxic DCA infusion impaired recovery (49±4%, 15 of 31 bpm.mmHg.103), compared to DCA infusion during reoxygenation (63±3%, 19 of 30 bpm.mmHg.103, p&lt;0.023). Reactive oxygen species formation was not altered. Increasing PDC flux during hypoxia significantly modified myocardial substrate metabolism. In contrast to the reported beneficial effects of augmented PDC flux during ischemia, increasing PDC flux in the hypoxic heart exacerbated myocardial injury. These results indicate that the potentially therapeutic effects of increasing PDC flux are not dependent on the hypoxic aspect of myocardial ischemia.

Tyr-301 Phosphorylation Inhibits Pyruvate Dehydrogenase by Blocking Substrate Binding and Promotes the Warburg Effect

The mitochondrial pyruvate dehydrogenase complex (PDC) plays a crucial role in regulation of glucose homoeostasis in mammalian cells. PDC flux depends on catalytic activity of the most important enzyme component pyruvate dehydrogenase (PDH). PDH kinase inactivates PDC by phosphorylating PDH at specific serine residues, including Ser-293, whereas dephosphorylation of PDH by PDH phosphatase restores PDC activity. The current understanding suggests that Ser-293 phosphorylation of PDH impedes active site accessibility to its substrate pyruvate. Here, we report that phosphorylation of a tyrosine residue Tyr-301 also inhibits PDH α 1 (PDHA1) by blocking pyruvate binding through a novel mechanism in addition to Ser-293 phosphorylation. In addition, we found that multiple oncogenic tyrosine kinases directly phosphorylate PDHA1 at Tyr-301, and Tyr-301 phosphorylation of PDHA1 is common in EGF-stimulated cells as well as diverse human cancer cells and primary leukemia cells from human patients. Moreover, expression of a phosphorylation-deficient PDHA1 Y301F mutant in cancer cells resulted in increased oxidative phosphorylation, decreased cell proliferation under hypoxia, and reduced tumor growth in mice. Together, our findings suggest that phosphorylation at distinct serine and tyrosine residues inhibits PDHA1 through distinct mechanisms to impact active site accessibility, which act in concert to regulate PDC activity and promote the Warburg effect.

In vivo investigation of cardiac metabolism in the rat using MRS of hyperpolarized [1‐13C] and [2‐13C]pyruvate

Hyperpolarized (13)C MRS allows the in vivo assessment of pyruvate dehydrogenase complex (PDC) flux, which converts pyruvate to acetyl-coenzyme A (acetyl-CoA). [1-(13)C]pyruvate has been used to measure changes in cardiac PDC flux, with demonstrated increase in (13)C-bicarbonate production after dichloroacetate (DCA) administration. With [1-(13)C]pyruvate, the (13)C label is released as (13 CO2 /(13)C-bicarbonate, and, hence, does not allow us to follow the fate of acetyl-CoA. Pyruvate labeled in the C2 position has been used to track the (13)C label into the TCA (tricarboxylic acid) cycle and measure [5-(13)C]glutamate as well as study changes in [1-(13)C]acetylcarnitine with DCA and dobutamine. This work investigates changes in the metabolic fate of acetyl-CoA in response to metabolic interventions of DCA-induced increased PDC flux in the fed and fasted state, and increased cardiac workload with dobutamine in vivo in rat heart at two different pyruvate doses. DCA led to a modest increase in the (13)C labeling of [5-(13)C]glutamate, and a considerable increase in [1-(13)C]acetylcarnitine and [1,3-(13)C]acetoacetate peaks. Dobutamine resulted in an increased labeling of [2-(13)C]lactate, [2-(13)C]alanine and [5-(13)C]glutamate. The change in glutamate with dobutamine was observed using a high pyruvate dose but not with a low dose. The relative changes in the different metabolic products provide information about the relationship between PDC-mediated oxidation of pyruvate and its subsequent incorporation into the TCA cycle compared with other metabolic pathways. Using a high dose of pyruvate may provide an improved ability to observe changes in glutamate.

Skeletal Muscle Metabolic Gene Expression Is Not Affected by Dichloroacetate-Mediated Modulation of Substrate Utilisation

Aim: This study investigated whether changing fuel use, by increasing pyruvate dehydrogenase complex (PDC) flux, independently of plasma substrate availability and insulin signalling, would alter metabolic gene expression. Methods: The PDC activator, dichloroacetate (DCA), was administered as an intravenous infusion in healthy male subjects at a rate of 50 mg kg^–1 min^–1, for 90 min. Saline was infused as a control (CON) on a separate occasion in a randomised sequence. Muscle biopsies were taken from the vastus lateralis at 0 and 30 min into the infusion and 90 min after infusion. Gene expression was quantified using RT-qPCR, and immunoblotting was used to confirm that there were no changes in insulin signalling via the PI3K/Akt pathway. Results: Blood glucose concentrations fell during both trials but 3 h after the start of the infusion they were lower in DCA (p < 0.05) than CON. Blood lactate concentrations also declined in both trials (p < 0.01), however, this decrease was also more pronounced in DCA than CON (p < 0.001). Carbohydrate oxidation was increased by DCA, 0.037 ± 0.017 g min^–1 (p < 0.05) at 3 h with no change observed in CON. UCP3 and PGC1α mRNA expression were induced in CON (as a response to continued fasting) but this was attenuated by DCA. Akt phosphorylation and the expression of other metabolic genes and transcription factors were unchanged throughout the intervention. Conclusion: It is concluded that PDC flux can be increased independently of plasma substrate availability, without causing downstream alterations to metabolic gene expression in the short term.

PDC deletion: the way to a man's heart disease

EDITORIAL FOCUSPDC deletion: the way to a man's heart diseaseMary C. SugdenMary C. SugdenPublished Online:01 Sep 2008https://doi.org/10.1152/ajpheart.00663.2008This is the final version - click for previous versionMoreSectionsPDF (52 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat the heart has continuous high-energy demands required to sustain efficient contraction. This is met by the metabolism of major circulating substrates (e.g., glucose, lactate, or lipids), according to availability, since the heart has a limited capacity for nutrient storage (reviewed in Ref. 16). Although fatty acid (FA) oxidation rates are invariably higher than glucose oxidation rates, glucose oxidation is more energy efficient than FA oxidation (with ∼15% more ATP/O2 molecule). Glucose and lipids compete as oxidative substrates for the heart, resulting in either a glucose-FA cycle in which lipid predominates as oxidative substrate or a reverse glucose-FA cycle in which lipid oxidation may be suppressed and glucose utilization favored. In addition, cardiac fuel selection is modified in disease states. Cardiac hypertrophy and heart failure, often characterized by reexpression of fetal genes, can be associated with a preference for glycolytic glucose utilization, whereas the diabetic heart exhibits a major preference for FA as oxidative fuel (reviewed in Ref. 17).Glucose oxidation is suppressed by phosphorylation (inactivation) of the pyruvate dehydrogenase complex (PDC) by the pyruvate dehydrogenase kinases (PDKs). The heart contains three PDK isoforms (PDK1, PDK2, and PDK4). PDK activity in cardiac myocytes is increased in a stable manner in response to increased lipid supply, uptake, and utilization and in response to insulin deficiency (reviewed in Ref. 18). PDK upregulation is likely to contribute to cardiac “metabolic inflexibility” in diabetes (reviewed in Ref. 12), where use of glucose as a metabolic fuel is greatly diminished due to a constraint on the ability to switch to glucose oxidation (2, 17). A diet high in saturated fat increases cardiac PDK4 expression (8). However, enhanced PDK4 expression in the heart is not necessarily maladaptive. Wistar rats fed on a “Western” diet, only moderately high in fat (45% of calories from fat), develop cardiac dysfunction after 8–12 mo, and it was proposed that this might reflect suboptimal induction of FA-responsive genes, as well as pdk4, due to inadequate activation of peroxisome proliferator-activated receptor-α (PPARα) compared with rats provided with a diet containing 60% of calories from fat (19). The existence of a circadian rhythm within the cardiomyocyte (23) affects the responsiveness of the heart to acute circadian changes in exogenous FA (5) and also influences pdk4 expression (reviewed in Ref. 22). Pressure overload-induced cardiac hypertrophy, associated with a reliance on glucose, abolishes this diurnal variation of metabolic gene expression, thereby impairing the ability of the heart to anticipate and respond to physiological alterations within its environment (23), again a situation of metabolic inflexibility.New research has highlighted potential mechanisms underlying altered regulation of PDKs, for example, hypoxia-induced upregulation of PDK1 expression by hypoxia-inducible factor-1 (1), acute control of PDK2 by phosphorylation by the diacylglycerol-activated and redox-sensitive protein kinase C isoform PKCδ (3), and PPARα- and FoxO1-mediated control of PDK4 gene expression (reviewed in Ref. 18). Whereas the lipooxidative transcription factor PPARα is activated by lipids, FoxO1 is repressed by insulin. As a consequence, cardiac PDK4 mRNA and protein expression in the heart are enhanced in starvation and diabetes (8, 20), situations that are associated with increased systemic FA delivery and insulin deficiency or resistance.PDK4 null mice have lower-than-normal blood glucose levels during starvation because a higher PDC activity lowers the availability of gluconeogenic substrates (9). A further study from this group (10) described the effect of a global deficiency of PDK4 on glucose homeostasis in mice maintained on a high-fat diet to induce obesity. Whereas the wild-type mice showed increased PDK4 protein expression in skeletal muscle and diaphragm (although not in liver or kidney in this model), global PDK4 deficiency in this model of obesity lowered blood glucose, suggesting that increased expression of PDK4 (with the resultant suppression of glucose oxidation) in muscle in response to high-fat feeding contributes to the development of hyperglycemia in diet-induced obesity. An interesting observation was that exogenous FA inhibited glucose oxidation by diaphragms from wild-type but not PDK4−/− mice, indicating that PDK4 is the predominant “lipid-responsive” PDK isoform. This implies that a rise in FA supply is a necessary component in the upregulation of muscle PDK4 expression and suppression of glucose oxidation.Sidhu et al. (15) examined the effect of complete suppression of pyruvate oxidation in heart and skeletal muscle, achieved by muscle-selective knockout of the α-subunit of the pyruvate dehydrogenase component of PDC under control of the heart/skeletal muscle-specific creatine kinase promoter. Knockout mice (H/SM-PDCKO) of both genders grew normally until weaning, but the transition from maternal milk (which is high-fat) to the normal high-carbohydrate diet at weaning had profound effects on mortality of the homozygous males, which did not survive beyond ∼7 days. Female H/SM-PDCKO mice fared better, experiencing no mortality even if weaned on a normal high-carbohydrate diet. Remarkably, male H/SM-PDCKO mice survived when weaned on a high-fat diet but developed marked myocyte hypertrophy and left ventricular dysfunction. These outcomes reiterate those seen in mice with cardiac-specific overexpression of PPARα [myosin heavy chain (MHC-PPARα) mice], which, in the absence of alterations in lipid-fuel delivery, exhibit increased rates of myocardial lipid uptake and oxidation and decreased rates of glucose uptake and oxidation, the latter being associated with enhanced PDK4 mRNA expression (6, 7). Like the H/SM-PDCKO mice, MHC-PPARα mice develop a cardiomyopathy with enhanced sensitivity to ischemia (11, 14), ventricular dysfunction (21), and cardiac hypertrophy (21). Both of these studies highlight the vital importance of glucose oxidation (and active PDC) for optimal cardiac performance and, as suggested by Sidhu et al. (15) for the H/SM-PDCKO mice, upregulation of PDK4 or absence of active PDC results in absent or limited metabolic flexibility in substrate switching.Use of transgenic mice that express PDK4 under control of the MHC protometer showed that selective overexpression of PDK4 in the heart markedly suppresses glucose oxidation and increases FA oxidation by isolated perfused hearts (24). These findings support the concept that suppressing glucose oxidation “forces” fat oxidation. Despite the substantial change in substrate selection, hearts from young (8 wk old) PDK4 transgenic mice showed no overt cardiomyopathy or cardiac hypertrophy, suggesting that the restraint on glucose oxidation imposed by PDK4 could be overridden. Of interest, MHC-PDK4 mice show some residual PDC activity (∼10% that of wild type), suggesting that protection against the development of cardiomyopathy could be conferred by a relatively low level of PDC flux. Consistent with this concept, in the study by Sidhu et al. (15), heterozygous H/SM-PDCKO female mice, which express about one-half the normal level of PDC activity consistent with random inactivation of one of the two X chromosomes in females, survive on standard high-carbohydrate/low-fat rodent diet. Enhanced anaerobic glucose metabolism is observed in ischemia, cardiac hypertrophy, and mechanical unloading (reviewed in Ref. 4). An ability to increase glucose oxidation by inhibition of FA oxidation is important for the ischemic heart because it reduces production of lactate and protons that are generated under anaerobic conditions because of a mismatch between glycolysis and glucose oxidation. Thus glucose oxidation not only improves energy efficiency but also reduces cellular acidification and damage.The findings that transgenic mice overexpressing PDK4 show no signs of cardiac hypertrophy or abnormal cardiac function, contrasting with both H/SM-PDCKO and MHC-PPARα mice, also deserves comment. When MHC-PDK4 transgenic mice are crossed with transgenic mice expressing a constitutively active form of the calcineurin catalytic subunit (CnA) to induce cardiac hypertrophy, mice bearing both the CnA and PDK4 transgenes showed substantial fibrosis and necrosis, a reduced heart rate, and increased mortality (24). The authors hypothesized that disturbances in contractile function can be compensated for by altered metabolism and vice versa, but simultaneous disruption of energy producing and energy consuming pathways causes cardiac maladaption and heart failure. The study of Sidhu et al. (15) lends support to this concept, particularly as it would be predicted that the cardiac hypertrophy seen in male H/SM-PDCKO mice would be concomitant with increased glycolysis but with an inability of suppressed FA oxidation to augment glucose oxidation.In summary, Sidhu et al. (15) demonstrate an effect of PDC knockout in males (rapid death on weaning unless provided with a high-fat diet). While this might suggest that high-fat junk food from an early age is good for you if you are male and PDC deficient, hope for the Homer Simpsons among us, potential survival is jeopardized by left ventricular hypertrophy and dysfunction. Female H/SM-PDCKO mice, which show 50% of normal PDC activity, can survive on a normal diet, highlighting the survival advantage conferred by the retention of an ability to oxidize glucose. Further questions that might be addressed included whether cardiac hypertrophy in this model is entirely maladaptive or, perhaps, adaptive, for example, to increase the supply of glycerol 3-phosphate for esterification of a proportion of incoming FA, or to maintain glycogen stores and mammalian target of rapamycin pathways (see Ref. 13).REFERENCES1 Aragones J, Schneider M, Van GK, Fraisl P, Dresselaers T, Mazzone M, Dirkx R, Zacchigna S, Lemieux H, Jeoung NH, Lambrechts D, Bishop T, Lafuste P, ez-Juan A, Harten SK, Van NP, De BK, Willam C, Tjwa M, Grosfeld A, Navet R, Moons L, Vandendriessche T, Deroose C, Wijeyekoon B, Nuyts J, Jordan B, Silasi-Mansat R, Lupu F, Dewerchin M, Pugh C, Salmon P, Mortelmans L, Gallez B, Gorus F, Buyse J, Sluse F, Harris RA, Gnaiger E, Hespel P, Van HP, Schuit F, Van VP, Ratcliffe P, Baes M, Maxwell P, Carmeliet P. Deficiency or inhibition of oxygen sensor Phd1 induces hypoxia tolerance by reprogramming basal metabolism. 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Sugden, Centre for Diabetes and Metabolic Medicine, Institute of Cell and Molecular Science, 4 Newark St., Whitechapel, London E1 2AT, U.K. (e-mail: [email protected]) Download PDF Previous Back to Top Next FiguresReferencesRelatedInformationCited ByLipoFNT: Lipoylation Sites Identification with Flexible Neural TreeComplexity, Vol. 2019Predicting lysine lipoylation sites using bi-profile bayes feature extraction and fuzzy support vector machine algorithmAnalytical Biochemistry, Vol. 561-562Protein lipoylation: an evolutionarily conserved metabolic regulator of health and diseaseCurrent Opinion in Chemical Biology, Vol. 42Loss of NHE1 activity leads to reduced oxidative stress in heart and mitigates high-fat diet-induced myocardial stressJournal of Molecular and Cellular Cardiology, Vol. 65Structural Basis for Inactivation of the Human Pyruvate Dehydrogenase Complex by Phosphorylation: Role of Disordered Phosphorylation LoopsStructure, Vol. 16, No. 12 More from this issue > Volume 295Issue 3September 2008Pages H917-H919 Copyright & PermissionsCopyright © 2008 by the American Physiological Societyhttps://doi.org/10.1152/ajpheart.00663.2008PubMed18641270History Published online 1 September 2008 Published in print 1 September 2008 Metrics

New insights concerning the role of carnitine in the regulation of fuel metabolism in skeletal muscle

In skeletal muscle, carnitine plays an essential role in the translocation of long-chain fatty-acids into the mitochondrial matrix for subsequent beta-oxidation, and in the regulation of the mitochondrial acetyl-CoA/CoASH ratio. Interest in these vital metabolic roles of carnitine in skeletal muscle appears to have waned over the past 25 years. However, recent research has shed new light on the importance of carnitine as a regulator of muscle fuel selection. It has been established that muscle free carnitine availability may be limiting to fat oxidation during high intensity submaximal exercise. Furthermore, increasing muscle total carnitine content in resting healthy humans (via insulin-mediated stimulation of muscle carnitine transport) reduces muscle glycolysis, increases glycogen storage and is accompanied by an apparent increase in fat oxidation. By increasing muscle pyruvate dehydrogenase complex (PDC) activity and acetylcarnitine content at rest, it has also been established that PDC flux and acetyl group availability limits aerobic ATP re-synthesis at the onset of exercise (the acetyl group deficit). Thus, carnitine plays a vital role in the regulation of muscle fuel metabolism. The demonstration that its availability can be readily manipulated in humans, and impacts on physiological function, will result in renewed business and scientific interest in this compound.

Exercise with low muscle glycogen augments TCA cycle anaplerosis but impairs oxidative energy provision in humans.

We tested the hypotheses that: (i) exercise with low muscle glycogen would reduce pyruvate flux through the alanine aminotransferase (AAT) reaction and attenuate the increase in tricarboxylic acid (TCA) cycle intermediates, and (ii) attenuation of tricarboxylic acid cycle intermediate (TCAI) pool expansion would limit TCA cycle flux, thereby accelerating phosphocreatine (PCr) degradation. Eight men cycled for 10 min at 70 % of their (VO(2,max) on two occasions: (i) following their normal diet (CON) and (ii) after cycling to exhaustion and consuming a low carbohydrate diet for approximately 2 days (LG). Biopsies (m. vastus lateralis) confirmed that [glycogen] was lower in LG vs. CON at rest (257 +/- 18 vs. 611 +/- 54 mmol (kg dry mass)(-1); P 0.05); however, net glycogenolysis was not different after 1 or 10 min of exercise. PCr degradation from rest to 1 min was approximately 26 % higher in LG vs. CON (38 +/- 4 vs. 28 +/- 4 mmol (kg dry mass)(-1); P< or =0.05). The sum of five measured TCAIs (approximately 90 % of total pool) was not different between trials at rest and after 1 min, but was higher after 10 min in LG vs. CON (5.51 +/- 0.43 vs. 4.45 +/- 0.49 mmol (kg dry mass)(-1); P 0.05). Pyruvate dehydrogenase complex (PDC) activity was lower during exercise in LG vs. CON (2.2 +/- 0.2 vs. 1.4 +/- 0.2 mmol min(-1) (kg wet weight)(-1) after 10 min; P< or =0.05), and acetylcarnitine was approximately threefold less, implying increased pyruvate availability for flux through AAT. Resting muscle [glutamate] was higher in LG vs. CON (16.1 +/- 0.8 vs. 11.8 +/- 0.4 mmol (kg dry mass)(-1); P< or =0.05) and the net decrease in [glutamate] during exercise was approximately 30 % greater in LG vs. CON. These findings suggest that: (i) contrary to our hypotheses, LG increased anaplerosis by decreasing PDC flux and/or increasing the conversion of glutamate carbon to TCAIs, and (ii) accelerating the rate of muscle TCAI expansion did not affect oxidative energy provision during the initial phase of contraction, since changes in [TCAI] were not temporally related to PCr degradation.