HIF prolyl hydroxylases (PHD1-3) are oxygen sensors that regulate the stability of the hypoxia-inducible factors (HIFs) in an oxygen-dependent manner. Here, we show that loss of Phd1 lowers oxygen consumption in skeletal muscle by reprogramming glucose metabolism from oxidative to more anaerobic ATP production through activation of a Pparalpha pathway. This metabolic adaptation to oxygen conservation impairs oxidative muscle performance in healthy conditions, but it provides acute protection of myofibers against lethal ischemia. Hypoxia tolerance is not due to HIF-dependent angiogenesis, erythropoiesis or vasodilation, but rather to reduced generation of oxidative stress, which allows Phd1-deficient myofibers to preserve mitochondrial respiration. Hypoxia tolerance relies primarily on Hif-2alpha and was not observed in heterozygous Phd2-deficient or homozygous Phd3-deficient mice. Of medical importance, conditional knockdown of Phd1 also rapidly induces hypoxia tolerance. These findings delineate a new role of Phd1 in hypoxia tolerance and offer new treatment perspectives for disorders characterized by oxidative stress.

To establish an accurate molecular model of human branched-chain amino acid (BCAA) metabolism, the distribution, activity, and expression of the first 2 enzymes in the catabolic pathway--branched-chain-amino-acid aminotransferase (BCAT) and branched-chain alpha-keto acid dehydrogenase (BCKD) complex--were determined in human tissues. The same enzyme activities were measured in rat and African green monkey tissues. Overall, the activities of BCAT and BCKD were higher in rat than in human and monkey tissues; nevertheless, the ratio of the 2 activities was similar in most tissues in the 3 species. Total oxidative capacity was concentrated in skeletal muscle and liver (> 70%) with muscle having a higher proportion of the total in humans and monkeys. In humans, brain (10-20%) and kidney (8-13%) may contribute significantly to whole-body BCAA metabolism. Furthermore, in primates the high ratio of transaminase to oxidative capacity in the entire gastrointestinal tract serves to prevent loss of essential BCAA carbon and raises the possibility that the gastrointestinal tract contributes to the plasma branched-chain alpha-keto acid pool. Quantitative polymerase chain reaction was used to examine expression of human branched-chain alpha-keto acid dehydrogenase kinase (BCKDK), the key enzyme that regulates the activity state of the human BCKD complex and human BCAT isoenzymes. To design the primers for the polymerase chain reaction, human BCKDK was cloned. BCKDK message was found in all human tissues tested, with the highest amount in human muscle. As in rats, there was ubiquitous expression of mitochondrial BCAT, whereas mRNA for the cytosolic enzyme was at or below the limit of detection outside the brain. Finally, the role of BCAA in body nitrogen metabolism is discussed.

Regulation of the activity of the pyruvate dehydrogenase complex in skeletal muscle plays an important role in fuel selection and glucose homeostasis. Activation of the complex promotes disposal of glucose, whereas inactivation conserves substrates for hepatic glucose production. Starvation and diabetes induce a stable increase in pyruvate dehydrogenase kinase activity in skeletal muscle mitochondria that promotes phosphorylation and inactivation of the complex. The present study shows that these metabolic conditions induce a large increase in the expression of PDK4, one of four pyruvate dehydrogenase kinase isoenzymes expressed in mammalian tissues, in the mitochondria of gastrocnemius muscle. Refeeding starved rats and insulin treatment of diabetic rats decreased pyruvate dehydrogenase kinase activity and also reversed the increase in PDK4 protein in gastrocnemius muscle mitochondria. Starvation and diabetes also increased the abundance of PDK4 mRNA in gastrocnemius muscle, and refeeding and insulin treatment again reversed the effects of starvation and diabetes. These findings suggest that an increase in amount of this enzyme contributes to hyperphosphorylation and inactivation of the pyruvate dehydrogenase complex in these metabolic conditions. It was further found that feeding rats WY-14,643, a selective agonist for the peroxisome proliferator-activated receptor-alpha (PPAR-alpha), also induced large increases in pyruvate dehydrogenase kinase activity, PDK4 protein, and PDK4 mRNA in gastrocnemius muscle. Since long-chain fatty acids activate PPAR-alpha endogenously, increased levels of these compounds in starvation and diabetes may signal increased expression of PDK4 in skeletal muscle.

High lactate generation and low glucose oxidation, despite normal oxygen conditions, are commonly seen in cancer cells and tumors. Historically known as the Warburg effect, this altered metabolic phenotype has long been correlated with malignant progression and poor clinical outcome. However, the mechanistic relationship between altered glucose metabolism and malignancy remains poorly understood. Here we show that inhibition of pyruvate dehydrogenase complex (PDC) activity contributes to the Warburg metabolic and malignant phenotype in human head and neck squamous cell carcinoma. PDC inhibition occurs via enhanced expression of pyruvate dehydrogenase kinase-1 (PDK-1), which results in inhibitory phosphorylation of the pyruvate dehydrogenase ␣ (PDH␣) subunit. We also demonstrate that PDC inhibition in cancer cells is associated with normoxic stabilization of the malignancy-promoting transcription factor hypoxia-inducible factor-1␣ (HIF-1␣) by glycolytic metabolites. Knockdown of PDK-1 via short hairpin RNA lowers PDH␣ phosphorylation, restores PDC activity, reverts the Warburg metabolic phenotype, decreases normoxic HIF-1␣ expression, lowers hypoxic cell survival, decreases invasiveness, and inhibits tumor growth. PDK-1 is an HIF-1-regulated gene, and these data suggest that the buildup of glycolytic metabolites, resulting from high PDK-1 expression, may in turn promote HIF-1 activation, thus sustaining a feed-forward loop for malignant progression. In addition to providing anabolic support for cancer cells, altered fuel metabolism thus supports a malignant phenotype. Correction of metabolic abnormalities offers unique opportunities for cancer treatment and may potentially synergize with other cancer therapies.Cancer is a disease whereby genetic mutation results in uncontrolled cell growth combined with malignancy. High lactate accumulation, despite adequate oxygen availability, is a metabolic pattern commonly associated with malignant transformation of the uncontrolled dividing cell. This metabolic phenotype, termed aerobic glycolysis and historically known as the Warburg effect, is characterized by high glycolytic rates and reduced mitochondrial oxidation (1, 2), features that may favor cell survival in the hypoxic microenvironments found in tumors. This phenotype also favors the routing of key metabolic intermediates away from oxidative destruction and toward anabolic processes required by rapidly dividing cells (2). Hypoxia and growth factors may select for this phenotype by activating hypoxia-inducible transcription factor-1 (HIF-1), 3 which induces transcription of glucose transporters, glycolytic enzymes, and many other genes associated with hypoxic survival, angiogenesis, and tissue invasion (3). Hypoxia, HIF-1 activation, and high lactate levels in tumors are all independently correlated with poor clinical outcome for many human cancers (3-5). A causative role for hypoxia and HIF-1 stabilization in tumor formation and progression has been demonstrated (reviewed in Ref. 3). However, the buildup of glycoly...

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Post-translational modification of mitochondrial proteins by phosphorylation or dephosphorylation plays an essential role in numerous cell signaling pathways involved in regulating energy metabolism and in mitochondria-induced apoptosis. Here we present a phosphoproteomic screen of the mitochondria matrix proteins and begin to establish the protein phosphorylations acutely associated with calcium ions (Ca 2+ ) signaling in porcine heart mitochondria. Forty-five phosphorylated proteins were detected by gel electrophoresis/mass spectrometry of Pro-Q Diamond staining while many more Pro-Q Diamond stained proteins were below mass spectrometry detection. Time dependent 32 P incorporation in intact mitochondria confirmed the extensive matrix protein phosphoryation and revealed the dynamic nature of this process. Classes of proteins detected included all of the mitochondrial respiratory chain complexes, as well as enzymes involved in intermediary metabolism, such as pyruvate dehydrogenase (PDH), citrate synthase and acyl-CoA dehydrogenases. These data demonstrate that the phosphoproteome of the mitochondria matrix is extensive and dynamic. Ca 2+ has previously been shown to activate various dehydrogenases, promote reactive oxygen species (ROS) generation, and initiate apoptosis via cytochrome c release. To evaluate the Ca 2+ signaling network, the effects of a Ca 2+ challenge sufficient to release cytochrome c were evaluated on the mitochondrial phosphoproteome. Novel Ca 2+ -induced dephosphorylation was observed in manganese superoxide dismutase (MnSOD) as well as the previously characterized PDH. A Ca 2+ dose dependent dephosphorylation of MnSOD was associated with a ∼2-fold maximum increase in activity; neither the dephosphorylation nor activity changes were induced by ROS production in the absence of Ca 2+ . These data demonstrate the use of a phosphoproteome screen in determining mitochondrial signaling pathways and reveal new pathways for Ca 2+ modification of mitochondrial function at the level of MnSOD.Mitochondria are thought to be the result of an early interaction of two lines of cellular life, the bacterium and eukaryotic cell (1;2). At this point in time, mitochondria play a critical role in energy metabolism, apoptosis and cell signaling pathways in the cell. However, the acute and chronic regulatory mechanisms of this organelle remain poorly defined. One approach to assessing the function and regulation of the mitochondrion is an evaluation of the mitochondrial Address correspondence to: Robert S. Balaban, Laboratory of Cardiac Energetics, National Heart Lung and Blood Institute, National Institutes of Health, 10 Center Drive Room B1D416, Bethesda, MD 20892-1061. Tel. 301 496-3658; Fax. 301 402-2389; E-mail: rsb@nih.gov. proteome. Estimates predict up to 3000 proteins (3;4) in mitochondria, however, recent largescale screening studies by Taylor (5) and Mootha (6) identified only about 600 distinct mitochondrial proteins. Many have used proteomic approaches to evaluate differential protein expr...

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Autism spectrum disorders are a genetically heterogeneous constellation of syndromes characterized by impairments in reciprocal social interaction. Available somatic treatments have limited efficacy. We have identified inactivating mutations in the gene BCKDK (Branched Chain Ketoacid Dehydrogenase Kinase) in consanguineous families with autism, epilepsy, and intellectual disability. The encoded protein is responsible for phosphorylation-mediated inactivation of the E1α subunit of branched-chain ketoacid dehydrogenase (BCKDH). Patients with homozygous BCKDK mutations display reductions in BCKDK messenger RNA and protein, E1α phosphorylation, and plasma branched-chain amino acids. Bckdk knockout mice show abnormal brain amino acid profiles and neurobehavioral deficits that respond to dietary supplementation. Thus, autism presenting with intellectual disability and epilepsy caused by BCKDK mutations represents a potentially treatable syndrome.