Mitochondrial metabolism supports resistance to IDH mutant inhibitors in acute myeloid leukemia.

Mutations in IDH induce epigenetic and transcriptional reprogramming, differentiation bias, and susceptibility to mitochondrial inhibitors in cancer cells. Here, we first show that cell lines, PDXs, and patients with acute myeloid leukemia (AML) harboring an IDH mutation displayed an enhanced mitochondrial oxidative metabolism. Along with an increase in TCA cycle intermediates, this AML-specific metabolic behavior mechanistically occurred through the increase in electron transport chain complex I activity, mitochondrial respiration, and methylation-driven CEBPα-induced fatty acid ß-oxidation of IDH1 mutant cells. While IDH1 mutant inhibitor reduced 2-HG oncometabolite and CEBPα methylation, it failed to reverse FAO and OxPHOS. These mitochondrial activities were maintained through the inhibition of Akt and enhanced activation of peroxisome proliferator-activated receptor-γ coactivator-1 PGC1α upon IDH1 mutant inhibitor. Accordingly, OxPHOS inhibitors improved anti-AML efficacy of IDH mutant inhibitors in vivo. This work provides a scientific rationale for combinatory mitochondrial-targeted therapies to treat IDH mutant AML patients, especially those unresponsive to or relapsing from IDH mutant inhibitors.

Resistance Is Futile: Targeting Mitochondrial Energetics and Metabolism to Overcome Drug Resistance in Cancer Treatment.

Metabolism is a key regulator of cancer biology; however, its role in therapeutic resistance has remained largely unresolved. Several new studies disclose that mitochondrial metabolism and oxidative phosphorylation at least in part drive chemoresistance in cancer and thus have important implications for targeted and more effective chemotherapies.

Chemotherapy-Resistant Human Acute Myeloid Leukemia Cells Are Not Enriched for Leukemic Stem Cells but Require Oxidative Metabolism.

Chemotherapy-resistant human acute myeloid leukemia (AML) cells are thought to be enriched in quiescent immature leukemic stem cells (LSC). To validate this hypothesis in vivo, we developed a clinically relevant chemotherapeutic approach treating patient-derived xenografts (PDX) with cytarabine (AraC). AraC residual AML cells are enriched in neither immature, quiescent cells nor LSCs. Strikingly, AraC-resistant preexisting and persisting cells displayed high levels of reactive oxygen species, showed increased mitochondrial mass, and retained active polarized mitochondria, consistent with a high oxidative phosphorylation (OXPHOS) status. AraC residual cells exhibited increased fatty-acid oxidation, upregulated CD36 expression, and a high OXPHOS gene signature predictive for treatment response in PDX and patients with AML. High OXPHOS but not low OXPHOS human AML cell lines were chemoresistant in vivo. Targeting mitochondrial protein synthesis, electron transfer, or fatty-acid oxidation induced an energetic shift toward low OXPHOS and markedly enhanced antileukemic effects of AraC. Together, this study demonstrates that essential mitochondrial functions contribute to AraC resistance in AML and are a robust hallmark of AraC sensitivity and a promising therapeutic avenue to treat AML residual disease.Significance: AraC-resistant AML cells exhibit metabolic features and gene signatures consistent with a high OXPHOS status. In these cells, targeting mitochondrial metabolism through the CD36-FAO-OXPHOS axis induces an energetic shift toward low OXPHOS and strongly enhanced antileukemic effects of AraC, offering a promising avenue to design new therapeutic strategies and fight AraC resistance in AML. Cancer Discov; 7(7); 716-35. ©2017 AACR.See related commentary by Schimmer, p. 670This article is highlighted in the In This Issue feature, p. 653.

Eltrombopag modulates reactive oxygen species and decreases acute myeloid leukemia cell survival.

Previous studies have demonstrated that the small molecule thrombopoietin (TPO) mimetic, eltrombopag (E), induces apoptosis in acute myeloid leukemia (AML) cells. Here, we sought to define the mechanism of the anti-leukemic effect of eltrombopag. Our studies demonstrate that, at a concentration of 5 µM E in 2% serum, E induces apoptosis in leukemia cells by triggering PARP cleavage and activation of caspase cascades within 2-6 hours. The induction of apoptotic enzymes is critically dependent on drug concentration and the concentration of serum. This effect is not associated with an alteration in mitochondrial potential but is associated with a rapid decrease in a reactive oxygen species (ROS) in particular hydrogen peroxide (H2O2). Interestingly, E also decreases mitochondrial maximal and spare respiratory capacities suggesting an induced mitochondrial dysfunction that may not be readily apparent under basal conditions but becomes manifest only under stress. Co-treatment of MOLM14 AML cells with E plus Tempol or H2O2 provides a partial rescue of cell toxicity. Ferric ammonioum citrate (FAC) also antagonized the E induced toxicity, by inducing notable increase in ROS level. Overall, we propose that E dramatically decreases ROS levels leading to a disruption of AML intracellular metabolism and rapid cell death.

Mitochondrial energy metabolism and apoptosis regulation in glioblastoma.

Glioblastoma is the most aggressive form of gliomas and is associated with short survival. Recent advancements in molecular genetics resulted in the identification of glioma genomic, epigenomic and transcriptomic hallmarks, and multidimensional data allowed clustering of glioblastomas into molecular subtypes. Parallel with these developments, much scientific attention has been attracted by the exploration of two functional processes linked to mitochondrial regulation. One of these processes involves genomic and mitochondrial gene mutations, mitochondrial protein expression modifications and altered metabolic regulation that define glioblastoma. The second mitochondrially-centered process involves complex molecular interactions and pathways that influence the extrinsic or the intrinsic mechanisms of apoptosis regulation and may underlie the uncontrolled spreading, recurrence and drug resistance of glioblastoma. While the available data are not yet comprehensive, these two complex processes represent important aspects of tumor cell biology, which may provide complementary opportunities for therapeutic manipulations of this highly resistant tumor type.

Mitochondrial dysfunction in peripheral blood mononuclear cells in pediatric septic shock.

OBJECTIVES: Mitochondrial dysfunction in peripheral blood mononuclear cells has been linked to immune dysregulation and organ failure in adult sepsis, but pediatric data are limited. We hypothesized that pediatric septic shock patients exhibit mitochondrial dysfunction within peripheral blood mononuclear cells which in turn correlates with global organ injury. DESIGN: Prospective observational study. SETTING: Academic PICU. PATIENTS: Thirteen pediatric patients with septic shock and greater than or equal to two organ failures and 11 PICU controls without sepsis or organ failure. INTERVENTIONS: Ex vivo measurements of mitochondrial oxygen consumption and membrane potential (ΔΨm) were performed in intact peripheral blood mononuclear cells on day 1-2 and day 5-7 of septic illness and in controls. The Pediatric Logistic Organ Dysfunction score, inotrope score, and organ failure-free days were determined from medical records. MEASUREMENTS AND MAIN RESULTS: Spare respiratory capacity, an index of bioenergetic reserve, was lower in septic peripheral blood mononuclear cells on day 1-2 (median, 1.81; interquartile range, 0.52-2.09 pmol O2/s/10 cells) compared with controls (5.55; 2.80-7.21; p = 0.03). Spare respiratory capacity normalized by day 5-7. Patients with sepsis on day 1-2 exhibited a higher ratio of LEAK to maximal respiration than controls (17% vs < 1%; p = 0.047) with normalization by day 5-7 (1%; p = 0.008), suggesting mitochondrial uncoupling early in sepsis. However, septic peripheral blood mononuclear cells exhibited no differences in basal or adenosine triphosphate-linked oxygen consumption or ΔΨm. Oxygen consumption did not correlate with Pediatric Logistic Organ Dysfunction score, inotrope score, or organ failure-free days (all p > 0.05). Although there was a weak overall association between ΔΨm on day 1-2 and organ failure-free days (Spearman ρ = 0.56, p = 0.06), patients with sepsis with normal organ function by day 7 exhibited higher ΔΨm on day 1-2 compared with patients with organ failure for more than 7 days (p = 0.04). CONCLUSIONS: Mitochondrial dysfunction was present in peripheral blood mononuclear cells in pediatric sepsis, evidenced by decreased bioenergetic reserve and increased uncoupling. Mitochondrial membrane potential, but not respiration, was associated with duration of organ injury.

Examination of physiological function and biochemical disorders in a rat model of prolonged asphyxia-induced cardiac arrest followed by cardio pulmonary bypass resuscitation.

BACKGROUND: Cardiac arrest induces whole body ischemia, which causes damage to multiple organs particularly the heart and the brain. There is clinical and preclinical evidence that neurological injury is responsible for high mortality and morbidity of patients even after successful cardiopulmonary resuscitation. A better understanding of the metabolic alterations in the brain during ischemia will enable the development of better targeted resuscitation protocols that repair the ischemic damage and minimize the additional damage caused by reperfusion. METHOD: A validated whole body model of rodent arrest followed by resuscitation was utilized; animals were randomized into three groups: control, 30 minute asphyxial arrest, or 30 minutes asphyxial arrest followed by 60 min cardiopulmonary bypass (CPB) resuscitation. Blood gases and hemodynamics were monitored during the procedures. An untargeted metabolic survey of heart and brain tissues following cardiac arrest and after CPB resuscitation was conducted to better define the alterations associated with each condition. RESULTS: After 30 min cardiac arrest and 60 min CPB, the rats exhibited no observable brain function and weakened heart function in a physiological assessment. Heart and brain tissues harvested following 30 min ischemia had significant changes in the concentration of metabolites in lipid and carbohydrate metabolism. In addition, the brain had increased lysophospholipid content. CPB resuscitation significantly normalized metabolite concentrations in the heart tissue, but not in the brain tissue. CONCLUSION: The observation that metabolic alterations are seen primarily during cardiac arrest suggests that the events of ischemia are the major cause of neurological damage in our rat model of asphyxia-CPB resuscitation. Impaired glycolysis and increased lysophospholipids observed only in the brain suggest that altered energy metabolism and phospholipid degradation may be a central mechanism in unresuscitatable brain damage.

Mitochondrial DNA variant in COX1 subunit significantly alters energy metabolism of geographically divergent wild isolates in Caenorhabditis elegans.

Mitochondrial DNA (mtDNA) sequence variation can influence the penetrance of complex diseases and climatic adaptation. While studies in geographically defined human populations suggest that mtDNA mutations become fixed when they have conferred metabolic capabilities optimally suited for a specific environment, it has been challenging to definitively assign adaptive functions to specific mtDNA sequence variants in mammals. We investigated whether mtDNA genome variation functionally influences Caenorhabditis elegans wild isolates of distinct mtDNA lineages and geographic origins. We found that, relative to N2 (England) wild-type nematodes, CB4856 wild isolates from a warmer native climate (Hawaii) had a unique p.A12S amino acid substitution in the mtDNA-encoded COX1 core catalytic subunit of mitochondrial complex IV (CIV). Relative to N2, CB4856 worms grown at 20°C had significantly increased CIV enzyme activity, mitochondrial matrix oxidant burden, and sensitivity to oxidative stress but had significantly reduced lifespan and mitochondrial membrane potential. Interestingly, mitochondrial membrane potential was significantly increased in CB4856 grown at its native temperature of 25°C. A transmitochondrial cybrid worm strain, chpIR (M, CB4856>N2), was bred as homoplasmic for the CB4856 mtDNA genome in the N2 nuclear background. The cybrid strain also displayed significantly increased CIV activity, demonstrating that this difference results from the mtDNA-encoded p.A12S variant. However, chpIR (M, CB4856>N2) worms had significantly reduced median and maximal lifespan relative to CB4856, which may relate to their nuclear-mtDNA genome mismatch. Overall, these data suggest that C. elegans wild isolates of varying geographic origins may adapt to environmental challenges through mtDNA variation to modulate critical aspects of mitochondrial energy metabolism.

Increased platelet storage time is associated with mitochondrial dysfunction and impaired platelet function.

BACKGROUND: Hemorrhagic shock is a leading cause of death following severe trauma, and platelet transfusions are frequently necessary to achieve hemostasis. Platelets, however, require special storage conditions, and storage time has been associated with loss of platelet quality. We hypothesized that standard storage conditions have a deleterious effect on platelet mitochondrial function and platelet activation. MATERIALS AND METHODS: Platelet donations were collected from healthy donors (n = 5) and stored in gas-permeable collection bags according to American Association of Blood Bank recommendations. Platelet units were sampled from day of collection (day 0) until day 7. High-resolution respirometry was used to assess baseline mitochondrial respiration, maximal oxygen utilization, and individual mitochondrial complex-dependent respiration. Fluorescence-activated cell sorting was performed to analyze mitochondrial content, mitochondrial reactive oxygen species, the expression of P-selectin (both before and after challenge with thrombin receptor-activating peptide), and apoptosis. Data were analyzed using analysis of variance and Pearson correlation (P < 0.05 significant). RESULTS: Mitochondrial respiration decreased significantly in platelets stored longer than 2 d (P < 0.05). Platelets also demonstrated a persistent decrease in response to stimulation with thrombin receptor-activating peptide by the third day of storage (P < 0.05) as well as an increase in mitochondrial reactive oxygen species and apoptosis (P < 0.05). Mitochondrial respiration significantly correlated with platelet capacity to activate (r = 0.8, P < 0.05). CONCLUSIONS: Platelet mitochondrial respiratory function and activation response decrease significantly in platelets stored for 3 d or more. Because platelet transfusions almost universally occur between the third and fifth day of storage, our findings may have significant clinical importance and warrant further in vivo analysis.

Hemorrhagic shock and resuscitation are associated with peripheral blood mononuclear cell mitochondrial dysfunction and immunosuppression.

BACKGROUND: Trauma and hypovolemic shock are associated with mitochondrial dysfunction and septic complications. We hypothesize that hypovolemic shock and resuscitation results in peripheral blood mononuclear cell (PBMC) mitochondrial dysfunction that is linked to immunosuppression. METHODS: With the use of a decompensated shock model, Long-Evans rats were bled to a mean arterial pressure of 40 mm Hg until the blood pressure could no longer be maintained without fluid infusion. Shock was sustained by incremental infusion of lactated Ringer's solution until 40% of the shed volume had been returned (severe shock). Animals were resuscitated with four times the shed volume in lactated Ringer's solution over 60 minutes (resuscitation). Control animals underwent line placement but were not hemorrhaged. Animals were randomized to control (n = 5), severe shock (n = 5), or resuscitation (n = 6) groups. At each time point, PBMC were isolated for mitochondrial function analysis using flow cytometry and high-resolution respirometry. Immune function was evaluated by quantifying serum interleukin 6 (IL-6) and tumor necrosis factor (TNF-α) after PBMC stimulation with lipopolysaccharide. The impact of plasma on mitochondrial function was evaluated by incubating PBMCs harvested following severe shock with control plasma. PBMCs from control animals were likewise mixed with plasma collected following resuscitation. Student's t test and Pearson correlations were performed (significance, p < 0.05). RESULTS: Following resuscitation, PBMCs demonstrated significant bioenergetic failure with a marked decrease in basal, maximal, and adenosine triphosphate-linked respiration. Mitochondrial membrane potential also decreased significantly by 50% following resuscitation. Serum IL-6 increased, while lipopolysaccharide stimulated TNF-α production decreased dramatically following shock and resuscitation. Observed mitochondrial dysfunction correlated significantly with IL-6 and TNF-α levels. PBMCs demonstrated significant mitochondrial recovery when incubated in control serum, whereas control PBMCs developed depressed function when incubated with serum collected following severe shock. CONCLUSION: Mitochondrial dysfunction following hemorrhagic shock and resuscitation was associated with the inhibition of PBMC response to endotoxin that may lead to an immunosuppressed state.

Blood cells from Friedreich ataxia patients harbor frataxin deficiency without a loss of mitochondrial function.

Friedreich ataxia (FRDA) is an autosomal recessive neurodegenerative disorder caused by GAA triplet expansions or point mutations in the FXN gene on chromosome 9q13. The gene product called frataxin, a mitochondrial protein that is severely reduced in FRDA patients, leads to mitochondrial iron accumulation, Fe-S cluster deficiency and oxidative damage. The tissue specificity of this mitochondrial disease is complex and poorly understood. While frataxin is ubiquitously expressed, the cellular phenotype is most severe in neurons and cardiomyocytes. Here, we conducted comprehensive proteomic, metabolic and functional studies to determine whether subclinical abnormalities exist in mitochondria of blood cells from FRDA patients. Frataxin protein levels were significantly decreased in platelets and peripheral blood mononuclear cells from FRDA patients. Furthermore, the most significant differences associated with frataxin deficiency in FRDA blood cell mitochondria were the decrease of two mitochondrial heat shock proteins. We did not observe profound changes in frataxin-targeted mitochondrial proteins or mitochondrial functions or an increase of apoptosis in peripheral blood cells, suggesting that functional defects in these mitochondria are not readily apparent under resting conditions in these cells.

Neonatal exendin-4 leads to protection from reperfusion injury and reduced rates of oxidative phosphorylation in the adult rat heart.

PURPOSE: Glucagon like peptide-1 (7-36) amide (GLP-1) is an incretin hormone with multiple salutary cardiovascular effects. A short course of the GLP-1 analogue Exendin-4 (Ex-4) in the neonatal period prevents the development of mitochondrial dysfunction and oxidative stress in a rat prone to obesity and diabetes. We sought to evaluate whether neonatal Ex-4 can exert the same effect in the normal rat heart, as well as whether Ex-4 could affect susceptibility to cardiac reperfusion injury. METHODS: After birth, Sprague Dawley rat pups were given either Ex-4 (1 nmole/kg body weight) or vehicle (1% BSA in 0.9% saline) subcutaneously for 6 days. Animals were studied at juvenile (4-6 weeks) and adult (8-9 months) ages. Using the Langendorff isolated perfused heart, cardiovascular function was assessed at baseline and following ischemia-reperfusion. Mitochondria were isolated from fresh heart tissue, and oxidative phosphorylation and calcium sequestration were analyzed. TBARS, MnSOD activity, and non-enzymatic anti-oxidant capacity were measured to assess the degree of oxidative stress present in the two groups. RESULTS: Both at the juvenile and adult age, Ex-4 treated rats demonstrated improved recovery from an ischemic insult. Rates of oxidative phosphorylation were globally reduced in adult, but not juvenile Ex-4 treated animals. Furthermore, mitochondria isolated from adult Ex-4 treated rats sequestered less calcium before undergoing the mitochondrial permeability transition. Oxidative stress did not differ between groups at any time point. CONCLUSION: A short course of Exendin-4 in the neonatal period leads to protection from ischemic injury and a preconditioned mitochondrial phenotype in the adult rat.

Mechanisms underlying metabolic and neural defects in zebrafish and human multiple acyl-CoA dehydrogenase deficiency (MADD).

In humans, mutations in electron transfer flavoprotein (ETF) or electron transfer flavoprotein dehydrogenase (ETFDH) lead to MADD/glutaric aciduria type II, an autosomal recessively inherited disorder characterized by a broad spectrum of devastating neurological, systemic and metabolic symptoms. We show that a zebrafish mutant in ETFDH, xavier, and fibroblast cells from MADD patients demonstrate similar mitochondrial and metabolic abnormalities, including reduced oxidative phosphorylation, increased aerobic glycolysis, and upregulation of the PPARG-ERK pathway. This metabolic dysfunction is associated with aberrant neural proliferation in xav, in addition to other neural phenotypes and paralysis. Strikingly, a PPARG antagonist attenuates aberrant neural proliferation and alleviates paralysis in xav, while PPARG agonists increase neural proliferation in wild type embryos. These results show that mitochondrial dysfunction, leading to an increase in aerobic glycolysis, affects neurogenesis through the PPARG-ERK pathway, a potential target for therapeutic intervention.

Erythroid dysplasia, megaloblastic anemia, and impaired lymphopoiesis arising from mitochondrial dysfunction.

Recent reports describe hematopoietic abnormalities in mice with targeted instability of the mitochondrial genome. However, these abnormalities have not been fully described. We demonstrate that mutant animals develop an age-dependent, macrocytic anemia with abnormal erythroid maturation and megaloblastic changes, as well as profound defects in lymphopoiesis. Mice die of severe fatal anemia at 15 months of age. Bone-marrow transplantation studies demonstrate that these abnormalities are intrinsic to the hematopoietic compartment and dependent upon the age of donor hematopoietic stem cells. These abnormalities are phenotypically similar to those found in patients with refractory anemia, suggesting that, in some cases, the myelodysplastic syndromes are caused by abnormalities of mitochondrial function.

Hypoxia-inducible factor 2 regulates hepatic lipid metabolism.

In mammals, the liver integrates nutrient uptake and delivery of carbohydrates and lipids to peripheral tissues to control overall energy balance. Hepatocytes maintain metabolic homeostasis by coordinating gene expression programs in response to dietary and systemic signals. Hepatic tissue oxygenation is an important systemic signal that contributes to normal hepatocyte function as well as disease. Hypoxia-inducible factors 1 and 2 (HIF-1 and HIF-2, respectively) are oxygen-sensitive heterodimeric transcription factors, which act as key mediators of cellular adaptation to low oxygen. Previously, we have shown that HIF-2 plays an important role in both physiologic and pathophysiologic processes in the liver. HIF-2 is essential for normal fetal EPO production and erythropoiesis, while constitutive HIF-2 activity in the adult results in polycythemia and vascular tumorigenesis. Here we report a novel role for HIF-2 in regulating hepatic lipid metabolism. We found that constitutive activation of HIF-2 in the adult results in the development of severe hepatic steatosis associated with impaired fatty acid beta-oxidation, decreased lipogenic gene expression, and increased lipid storage capacity. These findings demonstrate that HIF-2 functions as an important regulator of hepatic lipid metabolism and identify HIF-2 as a potential target for the treatment of fatty liver disease.

Ulk1 plays a critical role in the autophagic clearance of mitochondria and ribosomes during reticulocyte maturation.

Production of a red blood cell's hemoglobin depends on mitochondrial heme synthesis. However, mature red blood cells are devoid of mitochondria and rely on glycolysis for ATP production. The molecular basis for the selective elimination of mitochondria from mature red blood cells remains controversial. Recent evidence suggests that clearance of both mitochondria and ribosomes, which occurs in reticulocytes following nuclear extrusion, depends on autophagy. Here, we demonstrate that Ulk1, a serine threonine kinase with homology to yeast atg1p, is a critical regulator of mitochondrial and ribosomal clearance during the final stages of erythroid maturation. However, in contrast to the core autophagy genes such as atg5 and atg7, expression of ulk1 is not essential for induction of macroautophagy in response to nutrient deprivation or for survival of newborn mice. Together, these data suggest that the ATG1 homologue, Ulk1, is a component of the selective autophagy machinery that leads to the elimination of organelles in erythroid cells rather that an essential mechanistic component of autophagy.

Primary coenzyme Q deficiency in Pdss2 mutant mice causes isolated renal disease.

Coenzyme Q (CoQ) is an essential electron carrier in the respiratory chain whose deficiency has been implicated in a wide variety of human mitochondrial disease manifestations. Its multi-step biosynthesis involves production of polyisoprenoid diphosphate in a reaction that requires the enzymes be encoded by PDSS1 and PDSS2. Homozygous mutations in either of these genes, in humans, lead to severe neuromuscular disease, with nephrotic syndrome seen in PDSS2 deficiency. We now show that a presumed autoimmune kidney disease in mice with the missense Pdss2(kd/kd) genotype can be attributed to a mitochondrial CoQ biosynthetic defect. Levels of CoQ9 and CoQ10 in kidney homogenates from B6.Pdss2(kd/kd) mutants were significantly lower than those in B6 control mice. Disease manifestations originate specifically in glomerular podocytes, as renal disease is seen in Podocin/cre,Pdss2(loxP/loxP) knockout mice but not in conditional knockouts targeted to renal tubular epithelium, monocytes, or hepatocytes. Liver-conditional B6.Alb/cre,Pdss2(loxP/loxP) knockout mice have no overt disease despite demonstration that their livers have undetectable CoQ9 levels, impaired respiratory capacity, and significantly altered intermediary metabolism as evidenced by transcriptional profiling and amino acid quantitation. These data suggest that disease manifestations of CoQ deficiency relate to tissue-specific respiratory capacity thresholds, with glomerular podocytes displaying the greatest sensitivity to Pdss2 impairment.

Cell-permeating alpha-ketoglutarate derivatives alleviate pseudohypoxia in succinate dehydrogenase-deficient cells.

Succinate dehydrogenase (SDH) and fumarate hydratase (FH) are components of the tricarboxylic acid (TCA) cycle and tumor suppressors. Loss of SDH or FH induces pseudohypoxia, a major tumor-supporting event, which is the activation of hypoxia-inducible factor (HIF) under normoxia. In SDH- or FH-deficient cells, HIF activation is due to HIF1alpha stabilization by succinate or fumarate, respectively, either of which, when in excess, inhibits HIFalpha prolyl hydroxylase (PHD). To reactivate PHD, we focused on its substrate, alpha-ketoglutarate. We designed and synthesized cell-permeating alpha-ketoglutarate derivatives, which build up rapidly and preferentially in cells with a dysfunctional TCA cycle. This study shows that succinate- or fumarate-mediated inhibition of PHD is competitive and is reversed by pharmacologically elevating intracellular alpha-ketoglutarate. Introduction of alpha-ketoglutarate derivatives restores normal PHD activity and HIF1alpha levels to SDH-suppressed cells, indicating new therapy possibilities for the cancers associated with TCA cycle dysfunction.

TIGAR, a p53-inducible regulator of glycolysis and apoptosis.

The p53 tumor-suppressor protein prevents cancer development through various mechanisms, including the induction of cell-cycle arrest, apoptosis, and the maintenance of genome stability. We have identified a p53-inducible gene named TIGAR (TP53-induced glycolysis and apoptosis regulator). TIGAR expression lowered fructose-2,6-bisphosphate levels in cells, resulting in an inhibition of glycolysis and an overall decrease in intracellular reactive oxygen species (ROS) levels. These functions of TIGAR correlated with an ability to protect cells from ROS-associated apoptosis, and consequently, knockdown of endogenous TIGAR expression sensitized cells to p53-induced death. Expression of TIGAR may therefore modulate the apoptotic response to p53, allowing survival in the face of mild or transient stress signals that may be reversed or repaired. The decrease of intracellular ROS levels in response to TIGAR may also play a role in the ability of p53 to protect from the accumulation of genomic damage.

Redox stress is not essential for the pseudo-hypoxic phenotype of succinate dehydrogenase deficient cells.

HIFalpha prolyl hydroxylases (PHDs) are a family of enzymes that regulate protein levels of the alpha subunit of the hypoxia inducible transcription factor (HIF) under different oxygen levels. PHDs catalyse the conversion of a prolyl residue, molecular oxygen and alpha-ketoglutarate to hydroxy-prolyl, carbon dioxide and succinate in a reaction dependent on ferrous iron and ascorbate as cofactors. Recently it was shown that pseudo-hypoxia, HIF induction under normoxic conditions, is an important feature of tumours generated as a consequence of inactivation of the mitochondrial tumour suppressor 'succinate dehydrogenase' (SDH). Two models have been proposed to describe the link between SDH inhibition and HIF activation. Both models suggest that a mitochondrial-generated signal leads to the inhibition of PHDs in the cytosol, however, the models differ in the nature of the proposed messenger. The first model postulates that mitochondrial-generated hydrogen peroxide mediates signal transduction while the second model implicates succinate as the molecular messenger which leaves the mitochondrion and inhibits PHDs in the cytosol. Here we show that pseudo-hypoxia can be observed in SDH-suppressed cells in the absence of oxidative stress and in the presence of effective antioxidant treatment.