Mechanisms underlying regulation of the expression and activities of the mammalian pyruvate dehydrogenase kinases

Stimulating pyruvate dehydrogenase complex reduces itaconate levels and enhances TCA cycle anabolic bioenergetics in acutely inflamed monocytes

The pyruvate dehydrogenase complex (PDC)/pyruvate dehydrogenase kinase (PDK) axis directs the universal survival principles of immune resistance and tolerance in monocytes by controlling anabolic and catabolic energetics. Immune resistance shifts to immune tolerance during inflammatory shock syndromes when inactivation of PDC by increased PDK activity disrupts the tricarboxylic acid (TCA) cycle support of anabolic pathways. The transition from immune resistance to tolerance also diverts the TCA cycle from citrate-derived cis-aconitate to itaconate, a recently discovered catabolic mediator that separates the TCA cycle at isocitrate and succinate dehydrogenase (SDH). Itaconate inhibits succinate dehydrogenase and its anabolic role in mitochondrial ATP generation. We previously reported that inhibiting PDK in septic mice with dichloroacetate (DCA) increased TCA cycle activity, reversed septic shock, restored innate and adaptive immune and organ function, and increased survival. Here, using unbiased metabolomics in a monocyte culture model of severe acute inflammation that simulates sepsis reprogramming, we show that DCA-induced activation of PDC restored anabolic energetics in inflammatory monocytes while increasing TCA cycle intermediates, decreasing itaconate, and increasing amino acid anaplerotic catabolism of branched-chain amino acids (BCAAs). Our study provides new mechanistic insight that the DCA-stimulated PDC homeostat reconfigures the TCA cycle and promotes anabolic energetics in monocytes by reducing levels of the catabolic mediator itaconate. It further supports the theory that PDC is an energy sensing and signaling homeostat that restores metabolic and energy fitness during acute inflammation.

Mice deficient in pyruvate dehydrogenase kinase 4 are protected against acetaminophen-induced hepatotoxicity

Though mitochondrial oxidant stress plays a critical role in the progression of acetaminophen (APAP) overdose-induced liver damage, the influence of mitochondrial bioenergetics on this is not well characterized. This is important, since lifestyle and diet alter hepatic mitochondrial bioenergetics and an understanding of its effects on APAP-induced liver injury is clinically relevant. Pyruvate dehydrogenase (PDH) is critical to mitochondrial bioenergetics, since it controls the rate of generation of reducing equivalents driving respiration, and pyruvate dehydrogenase kinase 4 (PDK4) regulates (inhibits) PDH by phosphorylation. We examined APAP-induced liver injury in PDK4-deficient (PDK4^-/-) mice, which would have constitutively active PDH and hence elevated flux through the mitochondrial electron transport chain. PDK4^-/- mice showed significant protection against APAP-induced liver injury when compared to wild type (WT) mice as measured by ALT levels and histology. Deficiency of PDK4 did not alter APAP metabolism, with similar APAP-adduct levels in PDK4^-/- and WT mice, and no difference in JNK activation and translocation to mitochondria. However, subsequent amplification of mitochondrial dysfunction with release of mitochondrial AIF, peroxynitrite formation and DNA fragmentation were prevented. Interestingly, APAP induced a rapid decline in UCP2 protein levels in PDK4-deficient mice. These data suggest that adaptive changes in mitochondrial bioenergetics induced by enhanced respiratory chain flux in PDK4^-/- mice render them highly efficient in handling APAP-induced oxidant stress, probably through modulation of UCP2 levels. Further investigation of these specific adaptive mechanisms would provide better insight into the control exerted by mitochondrial bioenergetics on cellular responses to an APAP overdose.

Metabolic plasticity mediates differential responses to spring and autumn migrations: Evidence from gene expression patterns in migratory buntings

NEW FINDINGS

What is the central question of this study? What are the molecular underpinnings of seasonal metabolic plasticity during spring and autumn migrations in songbirds? What is the main finding and its importance? We report differences in mRNA levels of genes involved in the regulation of glucose and fat metabolism between photoinduced non-migratory and migratory states and between the spring and autumn migratory states. Higher expression of genes associated with fat mobilization and energy generation in the spring than in the autumn migration suggests differential activation of the metabolic pathways or alteration in the efficiency of existing functional machinery during annual journeys between nearly fixed destinations.

ABSTRACT

The molecular underpinnings of metabolic plasticity underlying differential responses to spring and autumn migrations are not well understood. We investigated this by examining the differences in mRNA levels of metabolic genes in the liver, muscle and adipose tissues of night-migratory red-headed buntings between photostimulated non-migratory and migratory states and between spring and autumn migratory states. Buntings accumulated more subcutaneous fat and hepatic lipid, had higher body mass, larger adipose cells and higher circulating triglyceride and free fatty acid levels and exhibited more intense Zugunruhe in the spring migratory state than in the autumn migratory state. More importantly, we found differences in the hepatic expression of pdc and pdk genes, indicating a differential acetyl-CoA requirement, and of the mdh and ogdh genes, suggesting differential oxidative phosphorylation between the non-migratory and migratory states and between the spring and autumn migratory states. Differences in fasn, bmal1 and glut1 mRNA levels were consistent with this and suggested seasonal differences in lipogenesis and/or glucose uptake. Likewise, differences in mRNA levels of genes coding for lipases (atgl and lpl) suggested that adipose triglycerides and free fatty acids serve largely as the metabolic substrate. Furthermore, changes in mRNA levels of genes coding for the fatty acid binding protein (fabp3) and fatty acid translocases (cd36) were consistent with differential fat fuel supply (via circulating free fatty acids) to aerobically exercising flight muscles between the spring and autumn migrations. These results show seasonal adaptation of genetic pathway(s) underlying seasonal metabolic plasticity that seems to mediate differential responses to spring and autumn migrations in latitudinal migratory songbirds.

Mito-Nuclear Communication in Hepatocellular Carcinoma Metabolic Rewiring

As the main metabolic and detoxification organ, the liver constantly adapts its activity to fulfill the energy requirements of the whole body. Despite the remarkable adaptive capacity of the liver, prolonged exposure to noxious stimuli such as alcohol, viruses and metabolic disorders results in the development of chronic liver disease that can progress to hepatocellular carcinoma (HCC), which is currently the second leading cause of cancer-related death worldwide. Metabolic rewiring is a common feature of cancers, including HCC. Altered mito-nuclear communication is emerging as a driving force in the metabolic reprogramming of cancer cells, affecting all aspects of cancer biology from neoplastic transformation to acquired drug resistance. Here, we explore relevant aspects (and discuss recent findings) of mito-nuclear crosstalk in the metabolic reprogramming of hepatocellular carcinoma.

Adipose Tissue Exosomal Proteomic Profile Reveals a Role on Placenta Glucose Metabolism in Gestational Diabetes Mellitus

CONTEXT

Molecules produced by adipose tissue (AT) function as an endocrine link between maternal AT and fetal growth by regulating placental function in normal women and women with gestational diabetes mellitus (GDM).

OBJECTIVE

We hypothesized that AT-derived exosomes (exo-AT) from women with GDM would carry a specific set of proteins that influences glucose metabolism in the placenta.

DESIGN

Exosomes were isolated from omental AT-conditioned media from normal glucose tolerant (NGT) pregnant women (n = 65) and pregnant women with GDM (n = 82). Sequential window acquisition of all theoretical fragment ion spectra mass spectrometry was used to construct a small ion library from AT and exosomal proteins, followed by ingenuity pathway analysis to determine the canonical pathways and biofunctions. The effect of exosomes on human placental cells was determined using a Human Glucose Metabolism RT2 Profiler PCR array.

RESULTS

The number of exosomes (vesicles/μg of tissue/24 hours) was substantially (1.7-fold) greater in GDM than in NGT, and the number of exosomes correlated positively with the birthweight Z score. Ingenuity pathway analysis of the exosomal proteins revealed differential expression of the proteins targeting the sirtuin signaling pathway, oxidative phosphorylation, and mechanistic target of rapamycin signaling pathway in GDM compared with NGT. GDM exo-AT increased the expression of genes associated with glycolysis and gluconeogenesis in placental cells compared with the effect of NGT exo-AT.

CONCLUSIONS

Our findings are consistent with the possibility that AT exosomes play an important role in mediating the changes in placental function in GDM and might be responsible for some of the adverse consequences in this pregnancy complication, such as fetal overgrowth.

A Slow-Digesting Carbohydrate Diet during Rat Pregnancy Protects Offspring from Non-Alcoholic Fatty Liver Disease Risk through the Modulation of the Carbohydrate-Response Element and Sterol Regulatory Element Binding Proteins

High-fat (HF) and rapid digestive (RD) carbohydrate diets during pregnancy promote excessive adipogenesis in offspring. This effect can be corrected by diets with similar glycemic loads, but low rates of carbohydrate digestion. However, the effects of these diets on metabolic programming in the livers of offspring, and the liver metabolism contributions to adipogenesis, remain to be addressed. In this study, pregnant insulin-resistant rats were fed high-fat diets with similar glycemic loads but different rates of carbohydrate digestion, High Fat-Rapid Digestive (HF–RD) diet or High Fat-Slow Digestive (HF–SD) diet. Offspring were fed a standard diet for 10 weeks, and the impact of these diets on the metabolic and signaling pathways involved in liver fat synthesis and storage of offspring were analyzed, including liver lipidomics, glycogen and carbohydrate and lipid metabolism key enzymes and signaling pathways. Livers from animals whose mothers were fed an HF–RD diet showed higher saturated triacylglycerol deposits with lower carbon numbers and double bond contents compared with the HF–SD group. Moreover, the HF–RD group exhibited enhanced glucose transporter 2, pyruvate kinase (PK), acetyl coenzyme A carboxylase (ACC) and fatty acid (FA) synthase expression, and a decrease in pyruvate carboxylase (PyC) expression leading to an altered liver lipid profile. These parameters were normalized in the HF–SD group. The changes in lipogenic enzyme expression were parallel to changes in AktPKB phosphorylation status and nuclear expression in carbohydrate-response element and sterol regulatory element binding proteins. In conclusion, an HF–RD diet during pregnancy translates to changes in liver signaling and metabolic pathways in offspring, enhancing liver lipid storage and synthesis, and therefore non-alcoholic fatty liver disease (NAFLD) risk. These changes can be corrected by feeding an HF–SD diet during pregnancy.

Diets contaminated with Bisphenol A and Di-isononyl phtalate modify skeletal muscle composition: A new target for environmental pollutant action

In the last years, an increasing number of studies reported that food pollution represents a significant route of exposure to environmental toxicants, able to cause mild to severe food illnesses and health problems, including hormonal and metabolic diseases. Pollutants can accumulate in organisms and biomagnify along the food web, finally targeting top consumers causing health and economic problems. In this study, adults of gilthead sea bream, Sparus aurata, were fed with diets contaminated with Bisphenol A (BPA) (4 and 4000 μg BPA kg^-1 bw day^-1) and Di-isononyl phthalate (DiNP) (15 and 1500 μg DiNP kg^-1 bw day^-1), to evaluate the effects of the contamination on the muscle macromolecular composition and alterations of its texture. The analysis conducted in the muscle using infrared microspectroscopy, molecular biology and biochemical assays, showed, in fish fed BPA contaminated diets, a decrease of unsaturated lipids and an increase of triglycerides and saturated alkyl chains. Conversely, in fish fed DiNP, a decrease of lipid content, caused by a reduction of both saturated and unsaturated chains and triglycerides was measured. Protein content was decreased by both xenobiotics evidencing a novel macromolecular target affected by these environmental contaminants. In addition, in all treated groups, proteins resulted more phosphorylated than in controls. Calpain and cathepsin levels, orchestrating protein turnover, were deregulated by both xenobiotics, evidencing alterations of muscle composition and texture. In conclusion, the results obtained suggest the ability of BPA and DiNP to modify the muscle macromolecular building, advising this tissue as a target of Endocrine-Disrupting Chemicals (EDCs) and providing a set of biomarkers as possible monitoring endpoints to develop novel OEDC test guidelines.

PDK4 drives metabolic alterations and muscle atrophy in cancer cachexia

Cachexia is frequently accompanied by severe metabolic derangements, although the mechanisms responsible for this debilitating condition remain unclear. Pyruvate dehydrogenase kinase (PDK)4, a critical regulator of cellular energetic metabolism, was found elevated in experimental models of cancer, starvation, diabetes, and sepsis. Here we aimed to investigate the link between PDK4 and the changes in muscle size in cancer cachexia. High PDK4 and abnormal energetic metabolism were found in the skeletal muscle of colon-26 tumor hosts, as well as in mice fed a diet enriched in Pirinixic acid, previously shown to increase PDK4 levels. Viral-mediated PDK4 overexpression in myotube cultures was sufficient to promote myofiber shrinkage, consistent with enhanced protein catabolism and mitochondrial abnormalities. On the contrary, blockade of PDK4 was sufficient to restore myotube size in C2C12 cultures exposed to tumor media. Our data support, for the first time, a direct role for PDK4 in promoting cancer-associated muscle metabolic alterations and skeletal muscle atrophy.-Pin, F., Novinger, L. J., Huot, J. R., Harris, R. A., Couch, M. E., O'Connell, T. M., Bonetto, A. PDK4 drives metabolic alterations and muscle atrophy in cancer cachexia.

Cardiac Specific Knockout of p53 Decreases ER Stress-Induced Mitochondrial Damage

Endoplasmic reticulum (ER) stress contributes to cardiovascular disease including heart failure. Interactions between the ER and mitochondria during ER stress can impair the mitochondrial respiratory chain and increase cell injury. p53 is a tumor suppressor protein that regulates apoptosis. p53 contributes to the regulation of mitochondrial and ER interactions, especially during the progression of ER stress. The knockout (KO) of p53 leads to decreased injury in hearts following ischemia-reperfusion. We asked if KO of p53 can protect mitochondria during the induction of ER stress and decrease cell injury. Floxed p53 mice were crossed with mice carrying an α-myosin heavy chain cre to generate cardiac specific p53 KO mice. Thapsigargin (THAP) was used to induce ER stress in wild type (WT) and p53 KO mice. Mice were euthanized after 48 h THAP treatment. Cardiac mitochondria were isolated for functional measurement. TUNEL staining was used to assess myocyte death. In WT mice, THAP treatment decreased the rate of oxidative phosphorylation using pyruvate + malate as complex I substrates compared to vehicle-treated control. Complex I activity was also decreased in the THAP-treated WT mice. The rate of oxidative phosphorylation and complex I activity were not altered in THAP-treated p53 KO mice. The content of pyruvate dehydrogenase (PDH) α1 subunit was decreased in THAP-treated WT mice but not in p53 KO mice. ER stress led to a release of cytochrome c and apoptosis inducing factor from mitochondria into cytosol in WT but not in KO mice. Knockout of p53 also preserved mitochondrial bcl-2 content in THAP-treated mice. In WT mice, THAP treatment markedly increased cell death compared to vehicle treated hearts. In contrast, cell injury was decreased in THAP-treated p53 KO mice compared to corresponding wild type. Thus, KO of p53 decreased cell injury by protecting mitochondria during the ER stress.

FOXK1 and FOXK2 regulate aerobic glycolysis

Adaptation to the environment and extraction of energy are essential for survival. Some species have found niches and specialized in using a particular source of energy, whereas others-including humans and several other mammals-have developed a high degree of flexibility¹. A lot is known about the general metabolic fates of different substrates but we still lack a detailed mechanistic understanding of how cells adapt in their use of basic nutrients². Here we show that the closely related fasting/starvation-induced forkhead transcription factors FOXK1 and FOXK2 induce aerobic glycolysis by upregulating the enzymatic machinery required for this (for example, hexokinase-2, phosphofructokinase, pyruvate kinase, and lactate dehydrogenase), while at the same time suppressing further oxidation of pyruvate in the mitochondria by increasing the activity of pyruvate dehydrogenase kinases 1 and 4. Together with suppression of the catalytic subunit of pyruvate dehydrogenase phosphatase 1 this leads to increased phosphorylation of the E1α regulatory subunit of the pyruvate dehydrogenase complex, which in turn inhibits further oxidation of pyruvate in the mitochondria-instead, pyruvate is reduced to lactate. Suppression of FOXK1 and FOXK2 induce the opposite phenotype. Both in vitro and in vivo experiments, including studies of primary human cells, show how FOXK1 and/or FOXK2 are likely to act as important regulators that reprogram cellular metabolism to induce aerobic glycolysis.

Restoring mitochondrial biogenesis with metformin attenuates β-GP-induced phenotypic transformation of VSMCs into an osteogenic phenotype via inhibition of PDK4/oxidative stress-mediated apoptosis

Mitochondrial abnormalities have long been observed in the development of vascular calcification. Metformin, a member of the biguanide class of antidiabetic drugs, has recently received attention owing to new findings regarding its protective role in cardiovascular disease. Since the precise control of mitochondrial quantity and quality is critical for the survival and function of vascular smooth muscle cells (VSMCs), maintaining mitochondrial homeostasis may be a potential protective factor for VSMCs against osteoblast-like phenotypic transition. However, limited studies have been reported in this area. Here, we investigated the role of metformin in the phenotypic transformation of VSMCs, as well as its intracellular signal transduction pathways. We demonstrated that supplementation with metformin restored the β-glycerophosphate (β-GP)-mediated impairment of mitochondrial biogenesis in VSMCs, as evidenced by an increased mitochondrial DNA copy number, a restored mitochondrial membrane potential (MMP), and upregulated mitochondrial biogenesis-related gene expression, whereas the AMP-activated protein kinase (AMPK) inhibitor compound C suppressed these effects. We also observed that overexpression of pyruvate dehydrogenase kinase 4 (PDK4), an important mitochondrial matrix enzyme in cellular energy metabolism, exacerbated β-GP-induced oxidative stress and subsequent apoptosis in VSMCs but that these effects were suppressed by dichloroacetate, a widely reported PDK4 inhibitor. More importantly, enhanced mitochondrial biogenesis attenuated the β-GP-induced phenotypic transformation of VSMCs into an osteogenic phenotype through inhibition of the PDK4/oxidative stress-mediated apoptosis pathway, whereas disruption of mitochondrial biogenesis by zidovudine aggravated β-GP-induced apoptosis in VSMCs. In addition, inhibition of autophagy by small interfering RNA targeting Atg5 reduced mitochondrial biogenesis in VSMCs. In summary, we uncovered a novel mechanism by which metformin attenuates the phenotypic transformation of VSMCs into an osteogenic phenotype via inhibition of the PDK4/oxidative stress-mediated apoptosis pathway, and mitochondrial homeostasis is involved in this process.

Next generation sequencing of RNA reveals novel targets of resveratrol with possible implications for Canavan disease

Resveratrol (RSV) is a small compound first identified as an activator of sirtuin 1 (SIRT1), a key factor in mediating the effects of caloric restriction. Since then, RSV received great attention for its widespread beneficial effects on health and in connection to many diseases. RSV improves the metabolism and the mitochondrial function, and more recently it was shown to restore fatty acid β-oxidation (FAO) capacities in patient fibroblasts harboring mutations with residual enzyme activity. Many of RSV's beneficial effects are mediated by the transcriptional coactivator PGC-1α, a direct target of SIRT1 and a master regulator of the mitochondrial fatty acid oxidation. Despite numerous studies RSV's mechanism of action is still not completely elucidated. Our aim was to investigate the effects of RSV on gene regulation on a wide scale, possibly to detect novel genes whose up-regulation by RSV may be of interest with respect to disease treatment. We performed Next Generation Sequencing of RNA on normal fibroblasts treated with RSV. To investigate whether the effects of RSV are mediated through SIRT1 we expanded the analysis to include SIRT1-knockdown fibroblasts. We identified the aspartoacylase (ASPA) gene, mutated in Canavan disease, to be strongly up-regulated by RSV in several cell lines, including Canavan disease fibroblasts. We further link RSV to the up-regulation of other genes involved in myelination including the glial specific transcription factors POU3F1, POU3F2, and myelin basic protein (MBP). We also observe a strong up-regulation by RSV of the riboflavin transporter gene SLC52a1. Mutations in SLC52a1 cause transient multiple acyl-CoA dehydrogenase deficiency (MADD). Our analysis of alternative splicing identified novel metabolically important genes affected by RSV, among which is particularly interesting the α subunit of the stimulatory G protein (Gsα), which regulates the cellular levels of cAMP through adenylyl cyclase. We conclude that in fibroblasts RSV stimulates the PGC-1α and p53 pathways, and up-regulates genes affecting the glucose metabolism, mitochondrial β-oxidation, and mitochondrial biogenesis. We further confirm that RSV might be a relevant treatment in the correction of FAO deficiencies and we suggest that treatment in other metabolic disorders including Canavan disease and MADD might be also beneficial.

Tissue-specific kinase expression and activity regulate flux through the pyruvate dehydrogenase complex

The pyruvate dehydrogenase complex (PDC) is a multienzyme assembly that converts pyruvate to acetyl-CoA. As pyruvate and acetyl-CoA play central roles in cellular metabolism, understanding PDC regulation is pivotal to understanding the larger metabolic network. The activity of mammalian PDC is regulated through reversible phosphorylation governed by at least four isozymes of pyruvate dehydrogenase kinase (PDK). Deciphering which kinase regulates PDC in organisms at specific times or places has been challenging. In this study, we analyzed mouse strains carrying targeted mutations of individual isozymes to explore their role in regulating PDC activity. Analysis of protein content of PDK isozymes in major metabolic tissues revealed that PDK1 and PDK2 were ubiquitously expressed, whereas PDK3 and PDK4 displayed a rather limited tissue distribution. Measurement of kinase activity showed that PDK1 is the principal isozyme regulating hepatic PDC. PDK2 was largely responsible for inactivation of PDC in tissues of muscle origin and brown adipose tissue (BAT). PDK3 was the principal kinase regulating pyruvate dehydrogenase activity in kidney and brain. In a well-fed state, the tissue levels of PDK4 protein were fairly low. In most tissues tested, PDK4 ablation had little effect on the overall rates of inactivation of PDC in kinase reaction. Taken together, these data strongly suggest that the activity of PDC is regulated by different isozymes in different tissues. Furthermore, it appears that the overall flux through PDC in a given tissue largely reflects the properties of the PDK isozyme that is principally responsible for the regulation of PDC activity in that tissue.

Cardiac Glucolipotoxicity and Cardiovascular Outcomes

Cardiac insulin signaling can be impaired due to the altered fatty acid metabolism to induce insulin resistance. In diabetes and insulin resistance, the metabolic, structural and ultimately functional alterations in the heart and vasculature culminate in diabetic cardiomyopathy, coronary artery disease, ischemia and eventually heart failure. Glucolipotoxicity describes the combined, often synergistic, adverse effects of elevated glucose and free fatty acid concentrations on heart structure, function, and survival. The quality of fatty acid shapes the cardiac structure and function, often influencing survival. A healthy fatty acid balance is therefore critical for maintaining cardiac integrity and function.

PDK4 Deficiency Suppresses Hepatic Glucagon Signaling by Decreasing cAMP Levels

In fasting or diabetes, gluconeogenic genes are transcriptionally activated by glucagon stimulation of the cAMP-protein kinase A (PKA)-CREB signaling pathway. Previous work showed pyruvate dehydrogenase kinase (PDK) inhibition in skeletal muscle increases pyruvate oxidation, which limits the availability of gluconeogenic substrates in the liver. However, this study found upregulation of hepatic PDK4 promoted glucagon-mediated expression of gluconeogenic genes, whereas knockdown or inhibition of hepatic PDK4 caused the opposite effect on gluconeogenic gene expression and decreased hepatic glucose production. Mechanistically, PDK4 deficiency decreased ATP levels, thus increasing phosphorylated AMPK (p-AMPK), which increased p-AMPK-sensitive phosphorylation of cyclic nucleotide phosphodiesterase 4B (p-PDE4B). This reduced cAMP levels and consequently p-CREB. Metabolic flux analysis showed that the reduction in ATP was a consequence of a diminished rate of fatty acid oxidation (FAO). However, overexpression of PDK4 increased FAO and increased ATP levels, which decreased p-AMPK and p-PDE4B and allowed greater accumulation of cAMP and p-CREB. The latter were abrogated by the FAO inhibitor etomoxir, suggesting a critical role for PDK4 in FAO stimulation and the regulation of cAMP levels. This finding strengthens the possibility of PDK4 as a target against diabetes.

Advanced glycation end products accelerate calcification in VSMCs through HIF-1α/PDK4 activation and suppress glucose metabolism

Arterial media calcification is associated with diabetes mellitus. Previous studies have shown that advanced glycation end products (AGEs) are responsible for vascular smooth muscle cell (VSMC) calcification, but the underlying mechanisms remain unclear. Hypoxia-inducible factor-1α (HIF-1α), one of the major factors during hypoxia, and pyruvate dehydrogenase kinase 4 (PDK4), an important mitochondrial matrix enzyme in cellular metabolism shift, have been reported in VSMC calcification. The potential link among HIF-1α, PDK4, and AGEs-induced vascular calcification was investigated in this study. We observed that AGEs elevated HIF-1α and PDK4 expression levels in a dose-dependent manner and that maximal stimulation was attained at 24 h. Two important HIF-1α-regulated genes, vascular endothelial growth factor A (VEGFA) and glucose transporter 1 (GLUT-1), were significantly increased after AGEs exposure. Stabilization or nuclear translocation of HIF-1α increased PDK4 expression. PDK4 inhibition attenuated AGEs-induced VSMC calcification, which was evaluated by measuring the calcium content, alkaline phosphatase (ALP) activity and runt-related transcription factor 2 (RUNX2) expression levels and by Alizarin red S staining. In addition, the glucose consumption, lactate production, key enzymes of glucose metabolism and oxygen consumption rate (OCR) were decreased during AGEs-induced VSMC calcification. In conclusion, this study suggests that AGEs accelerate vascular calcification partly through the HIF-1α/PDK4 pathway and suppress glucose metabolism.

Metabolic Flexibility as an Adaptation to Energy Resources and Requirements in Health and Disease

The ability to efficiently adapt metabolism by substrate sensing, trafficking, storage, and utilization, dependent on availability and requirement, is known as metabolic flexibility. In this review, we discuss the breadth and depth of metabolic flexibility and its impact on health and disease. Metabolic flexibility is essential to maintain energy homeostasis in times of either caloric excess or caloric restriction, and in times of either low or high energy demand, such as during exercise. The liver, adipose tissue, and muscle govern systemic metabolic flexibility and manage nutrient sensing, uptake, transport, storage, and expenditure by communication via endocrine cues. At a molecular level, metabolic flexibility relies on the configuration of metabolic pathways, which are regulated by key metabolic enzymes and transcription factors, many of which interact closely with the mitochondria. Disrupted metabolic flexibility, or metabolic inflexibility, however, is associated with many pathological conditions including metabolic syndrome, type 2 diabetes mellitus, and cancer. Multiple factors such as dietary composition and feeding frequency, exercise training, and use of pharmacological compounds, influence metabolic flexibility and will be discussed here. Last, we outline important advances in metabolic flexibility research and discuss medical horizons and translational aspects.

High Fat Diet Upregulates Fatty Acid Oxidation and Ketogenesis via Intervention of PPAR-γ

Background/Aims:

Systemic hyperlipidemia and intracellular lipid accumulation induced by chronic high fat diet (HFD) leads to enhanced fatty acid oxidation (FAO) and ketogenesis. The present study was aimed to determine whether activation of peroxisome proliferator-activated receptor-γ (PPAR-γ) by surplus free fatty acids (FA) in hyperlipidemic condition, has a positive feedback regulation over FAO and ketogenic enzymes controlling lipotoxicity and cardiac apoptosis.

Methods:

8 weeks old C57BL/6 wild type (WT) or PPAR-γ^−/− mice were challenged with 16 weeks 60% HFD to induce obesity mediated type 2 diabetes mellitus (T2DM) and diabetic cardiomyopathy. Treatment course was followed by echocardiographic measurements, glycemic and lipid profiling, immunoblot, qPCR and immunohistochemistry (IHC) analysis of PPAR-γ and following mitochondrial metabolic enzymes 3-hydroxy-3- methylglutaryl-CoA synthase (HMGCS2), mitochondrial β-hydroxy butyrate dehydrogenase (BDH1) and pyruvate dehydrogenase kinase isoform 4 (PDK4). In vivo model was translated in vitro, with neonatal rat cardiomyocytes (NRCM) treated with PPAR-γ agonist/antagonist and PPAR-γ overexpression adenovirus in presence of palmitic acid (PA). Apoptosis was determined in vivo from left ventricular heart by TUNEL assay and immunoblot analysis.

Results:

We found exaggerated circulating ketone bodies production and expressions of the related mitochondrial enzymes HMGCS2, BDH1 and PDK4 in HFD-induced diabetic hearts and in PA-treated NRCM. As a mechanistic approach we found HFD mediated activation of PPAR-03B3 is associated with the above-mentioned mitochondrial enzymes. HFD-fed PPAR-γ^−/− mice display decreased hyperglycemia, hyperlipidemia associated with increased insulin responsiveness as compared to HFD-fed WT mice PPAR-γ^−/−−HFD mice demonstrated a more robust functional recovery after diabetes induction, as well as significantly reduced myocyte apoptosis and improved cardiac function.

Conclusions:

PPAR-γ has been described previously to regulate lipid metabolism and adipogenesis. The present study suggests for the first time that increased PPAR-γ expression by HFD is responsible for cardiac dysfunction via upregulation of mitochondrial enzymes HMGCS2, BDH1 and PDK4. Targeting PPAR-γ and its downstream mitochondrial enzymes will provide novel strategies in preventing metabolic and myocardial dysfunction in diabetes mellitus.

The Role of Pyruvate Dehydrogenase Kinase-4 (PDK4) in Bladder Cancer and Chemoresistance

Advanced bladder cancer remains a major source of mortality, with poor treatment options. Cisplatin-based chemotherapy is the standard treatment, however many patients are or become resistant. One potential cause of chemoresistance is the Warburg effect, a metabolic switch to aerobic glycolysis that occurs in many cancers. Upregulation of the pyruvate dehydrogenase kinase family (PDK1-PDK4) is associated with aerobic glycolysis and chemoresistance through inhibition of the pyruvate dehydrogenase complex (PDH). We have previously observed upregulation of PDK4 in high-grade compared with low-grade bladder cancers. We initiated this study to determine if inhibition of PDK4 could reduce tumor growth rates or sensitize bladder cancer cells to cisplatin. Upregulation of PDK4 in malignant bladder cancer cell lines as compared with benign transformed urothelial cells was confirmed using qPCR. Inhibition of PDK4 with dichloroacetate (DCA) resulted in increased PDH activity, reduced cell growth, and G₀-G₁ phase arrest in bladder cancer cells. Similarly, siRNA knockdown of PDK4 inhibited bladder cancer cell proliferation. Cotreatment of bladder cancer cells with cisplatin and DCA did not increase caspase-3 activity but did enhance overall cell death in vitro Although daily treatment with 200 mg/kg DCA alone did not reduce tumor volumes in a xenograft model, combination treatment with cisplatin resulted in dramatically reduced tumor volumes as compared with either DCA or cisplatin alone. This was attributed to substantial intratumoral necrosis. These findings indicate inhibition of PDK4 may potentiate cisplatin-induced cell death and warrant further studies investigating the mechanism through which this occurs. Mol Cancer Ther; 17(9); 2004-12. ©2018 AACR.

Coenzyme A-mediated degradation of pyruvate dehydrogenase kinase 4 promotes cardiac metabolic flexibility after high-fat feeding in mice

Cardiac energy is produced primarily by oxidation of fatty acids and glucose, with the relative contributions of each nutrient being sensitive to changes in substrate availability and energetic demand. A major contributor to cardiac metabolic flexibility is pyruvate dehydrogenase (PDH), which converts glucose-derived pyruvate to acetyl-CoA within the mitochondria. PDH is inhibited by phosphorylation dependent on the competing activities of pyruvate dehydrogenase kinases (PDK1-4) and phosphatases (PDP1-2). A single high-fat meal increases cardiac PDK4 content and subsequently inhibits PDH activity, reducing pyruvate utilization when abundant fatty acids are available. In this study, we demonstrate that diet-induced increases in PDK4 are reversible and characterize a novel pathway that regulates PDK4 degradation in response to the cardiac metabolic environment. We found that PDK4 degradation is promoted by CoA (CoASH), the levels of which declined in mice fed a high-fat diet and normalized following transition to a control diet. We conclude that CoASH functions as a metabolic sensor linking the rate of PDK4 degradation to fatty acid availability in the heart. However, prolonged high-fat feeding followed by return to a low-fat diet resulted in persistent in vitro sensitivity of PDH to fatty acid-induced inhibition despite reductions in PDK4 content. Moreover, increases in the levels of proteins responsible for β-oxidation and rates of palmitate oxidation by isolated cardiac mitochondria following long-term consumption of high dietary fat persisted after transition to the control diet. We propose that these changes prime PDH for inhibition upon reintroduction of fatty acids.

Plasma proteomic study of acute mountain sickness susceptible and resistant individuals

Although extensive studies have focused on the development of acute mountain sickness (AMS), the exact mechanisms of AMS are still obscure. In this study, we used isobaric tags for relative and absolute quantitation (iTRAQ) proteomic analysis to identify novel AMS−associated biomarkers in human plasma. After 9 hours of hypobaric hypoxia the abundance of proteins related to tricarboxylic acid (TCA) cycle, glycolysis, ribosome, and proteasome were significantly reduced in AMS resistant (AMS−) group, but not in AMS susceptible (AMS+) group. This suggested that AMS− individuals could reduce oxygen consumption via repressing TCA cycle and glycolysis, and reduce energy consumption through decreasing protein degradation and synthesis compared to AMS+ individuals after acute hypoxic exposure. The inflammatory response might be decreased resulting from the repressed TCA cycle. We propose that the ability for oxygen consumption reduction may play an important role in the development of AMS. Our present plasma proteomic study in plateau of the Han Chinese volunteers gives new data to address the development of AMS and potential AMS correlative biomarkers.

The Destiny of Glucose from a MicroRNA Perspective

Glucose serves as a primary, and for some tissues the unique, fuel source in order to generate and maintain the biological functions. Hyperglycemia is a hallmark of type 2 diabetes and is the direct consequence of perturbations in the glucose homeostasis. Insulin resistance, referred to as a reduced response of target tissues to the hormone, contributes to the development of hyperglycemia. The molecular mechanisms responsible for the altered glucose homeostasis are numerous and not completely understood. MicroRNAs (miRNAs) are now recognized as regulators of the lipid and glucose metabolism and are involved in the onset of metabolic diseases. Indeed, these small non-coding RNA molecules operate in the RNA silencing and posttranscriptional regulation of gene expression and may modulate the levels of kinases and enzymes in the glucose metabolism. Therefore, a better characterization of the function of miRNAs and a deeper understanding of their role in disease may represent a fundamental step toward innovative treatments addressing the causes, not only the symptoms, of hyperglycemia, using approaches aimed at restoring either miRNAs or their specific targets. In this review, we outline the current understanding regarding the impact of miRNAs in the glucose metabolism and highlight the need for further research focused on altered key kinases and enzymes in metabolic diseases.

Dioxygen and Metabolism; Dangerous Liaisons in Cardiac Function and Disease

The heart must consume a significant amount of energy to sustain its contractile activity. Although the fuel demands are huge, the stock remains very low. Thus, in order to supply its daily needs, the heart must have amazing adaptive abilities, which are dependent on dioxygen availability. However, in myriad cardiovascular diseases, “fuel” depletion and hypoxia are common features, leading cardiomyocytes to favor low-dioxygen-consuming glycolysis rather than oxidation of fatty acids. This metabolic switch makes it challenging to distinguish causes from consequences in cardiac pathologies. Finally, despite the progress achieved in the past few decades, medical treatments have not improved substantially, either. In such a situation, it seems clear that much remains to be learned about cardiac diseases. Therefore, in this review, we will discuss how reconciling dioxygen availability and cardiac metabolic adaptations may contribute to develop full and innovative strategies from bench to bedside.

Adverse Intrauterine Environment and Cardiac miRNA Expression

Placental insufficiency, high altitude pregnancies, maternal obesity/diabetes, maternal undernutrition and stress can result in a poor setting for growth of the developing fetus. These adverse intrauterine environments result in physiological changes to the developing heart that impact how the heart will function in postnatal life. The intrauterine environment plays a key role in the complex interplay between genes and the epigenetic mechanisms that regulate their expression. In this review we describe how an adverse intrauterine environment can influence the expression of miRNAs (a sub-set of non-coding RNAs) and how these changes may impact heart development. Potential consequences of altered miRNA expression in the fetal heart include; Hypoxia inducible factor (HIF) activation, dysregulation of angiogenesis, mitochondrial abnormalities and altered glucose and fatty acid transport/metabolism. It is important to understand how miRNAs are altered in these adverse environments to identify key pathways that can be targeted using miRNA mimics or inhibitors to condition an improved developmental response.

Nε-carboxymethyl-lysine promotes calcium deposition in VSMCs via intracellular oxidative stress-induced PDK4 activation and alters glucose metabolism

Diabetes and vascular calcification are intrinsically linked. We previously reported that advanced glycation end products (AGEs) accelerate calcium deposition in vascular smooth muscle cells (VSMCs) via excessive oxidative stress. However, the underlying mechanism remains poorly understood. Pyruvate dehydrogenase kinase 4 (PDK4) is an important mitochondrial matrix enzyme in cellular energy metabolism. Since hyperactivation of PDK4 has been reported in calcified vessels and in patients with diabetes mellitus, inhibition of PDK4 expression may be a strategy for the prevention of diabetic vascular calcification. In this study, we used a rat VSMC model to investigate the role of PDK4 in diabetic vascular calcification and further explore the underlying mechanisms. We observed that Nε-carboxymethyl-lysine (CML), which is a major immunogen of AGEs, accelerated calcium deposition in VSMCs through PDK4 activation. An elevated level of reactive oxygen species (ROS) acted as a signal transduction intermediate to increase PDK4 expression. Either inhibition of PDK4 expression or RAGE (receptor for AGEs) blockade attenuated CML-induced VSMC calcification, as shown by decreased alkaline phosphatase (ALP) activity and runt-related transcription factor 2 (RUNX2) expression. Glucose consumption and lactate production were increased during CML-induced VSMC calcification. Importantly, CML accelerates glycolysis in VSMCs via a PDK4-dependent pathway. In conclusion, this study demonstrates a novel mechanism by which CML promotes VSMC calcification via PDK4 activation and alters glucose metabolism in VSMCs.

Therapeutic Targeting of the Pyruvate Dehydrogenase Complex/Pyruvate Dehydrogenase Kinase (PDC/PDK) Axis in Cancer

The mitochondrial pyruvate dehydrogenase complex (PDC) irreversibly decarboxylates pyruvate to acetyl coenzyme A, thereby linking glycolysis to the tricarboxylic acid cycle and defining a critical step in cellular bioenergetics. Inhibition of PDC activity by pyruvate dehydrogenase kinase (PDK)-mediated phosphorylation has been associated with the pathobiology of many disorders of metabolic integration, including cancer. Consequently, the PDC/PDK axis has long been a therapeutic target. The most common underlying mechanism accounting for PDC inhibition in these conditions is post-transcriptional upregulation of one or more PDK isoforms, leading to phosphorylation of the E1α subunit of PDC. Such perturbations of the PDC/PDK axis induce a "glycolytic shift," whereby affected cells favor adenosine triphosphate production by glycolysis over mitochondrial oxidative phosphorylation and cellular proliferation over cellular quiescence. Dichloroacetate is the prototypic xenobiotic inhibitor of PDK, thereby maintaining PDC in its unphosphorylated, catalytically active form. However, recent interest in the therapeutic targeting of the PDC/PDK axis for the treatment of cancer has yielded a new generation of small molecule PDK inhibitors. Ongoing investigations of the central role of PDC in cellular energy metabolism and its regulation by pharmacological effectors of PDKs promise to open multiple exciting vistas into the biochemical understanding and treatment of cancer and other diseases.

Assessment of growth and metabolism characteristics in offspring of dehydroepiandrosterone-induced polycystic ovary syndrome adults

Polycystic ovary syndrome (PCOS) is a common reproductive disorder that has many characteristic features including hyperandrogenemia, insulin resistance and obesity, which may have significant implications for pregnancy outcomes and long-term health of women. Daughters born to PCOS mothers constitute a high-risk group for metabolic and reproductive derangements, but no report has described potential growth and metabolic risk factors for such female offspring. Hence, we used a mouse model of dehydroepiandrosterone (DHEA)-induced PCOS to study the mechanisms underlying the pathology of PCOS by investigating the growth, developmental characteristics, metabolic indexes and expression profiles of key genes of offspring born to the models. We found that the average litter size was significantly smaller in the DHEA group, and female offspring had sustained higher body weight, increased body fat and triglyceride content in serum and liver; they also exhibited decreased energy expenditure, oxygen consumption and impaired glucose tolerance. Genes related to glucolipid metabolism such as Pparγ, Acot1/2, Fgf21, Pdk4 and Inhbb were upregulated in the liver of the offspring in DHEA group compared with those in controls, whereas Cyp17a1 expression was significantly decreased. However, the expression of these genes was not detected in male offspring. Our results show that female offspring in DHEA group exhibit perturbed growth and glucolipid metabolism that were not observed in male offspring.

Effects of training status on PDH regulation in human skeletal muscle during exercise

Pyruvate dehydrogenase (PDH) is the gateway enzyme for carbohydrate-derived pyruvate feeding into the TCA cycle. PDH may play a central role in regulating substrate shifts during exercise, but the influence of training state on PDH regulation during exercise is not fully elucidated. The purpose of this study was to investigate the impact of training state on post-translational regulation of PDHa activity during submaximal and exhaustive exercise. Eight untrained and nine endurance exercise-trained healthy male subjects performed incremental exercise on a cycle ergometer: 40 min at 50% incremental peak power output (IPPO), 10 min at 65% (IPPO), followed by 80% (IPPO) until exhaustion. Trained subjects had higher (P < 0.05) PDH-E1α, PDK1, PDK2, PDK4, and PDP1 protein content as well as PDH phosphorylation and PDH acetylation. Exercising at the same relative intensity led to similar muscle PDH activation in untrained and trained subjects, whereas PDHa activity at exhaustion was higher (P < 0.05) in trained than untrained. Furthermore, exercise induced similar PDH dephosphorylation in untrained and trained subjects, while PDH acetylation was increased (P < 0.05) only in trained subjects. In conclusion, PDHa activity and PDH dephosphorylation were well adjusted to the relative exercise intensity during submaximal exercise. In addition, higher PDHa activity in trained than untrained at exhaustion seemed related to differences in glycogen utilization rather than differences in PDH phosphorylation and acetylation state, although site-specific contributions cannot be ruled out.

Pyruvate dehydrogenase kinase 4 exhibits a novel role in the activation of mutant KRAS, regulating cell growth in lung and colorectal tumour cells

RAS signalling is involved in the control of several metabolic pathways including glycolysis, mitochondrial respiration and glutamine metabolism. Importantly, we have found here that loss of PDHK4, a key regulator of the pyruvate dehydrogenase complex, caused a profound cell growth inhibition in tumour cells harbouring KRAS mutations. Using isogenic cells and a panel of colorectal and lung cell lines we demonstrated that KRAS mutant cells showed a dependency on PDHK4 whereas KRAS wild-type cells were significantly resistant to PDHK4 knockdown. We have found that PDHK4 plays a role in the post-translational regulation of mutant KRAS activity. Depletion of PDHK4 causes disruption of KRAS cellular localization, a reduction in KRAS activity which, in turn, results in reduced MAPK signalling. Interestingly, PDHK4 and KRAS depletion resulted in a similar metabolic phenotype consisting of a reduction of glucose and fatty acid oxidation. Moreover, stable expression of PDHK4 increased localization of activated KRAS at the plasma membrane and induced tumour cell growth in vitro and in vivo. Taken together these data support a model where PDHK4 regulates KRAS signalling and its tumorigenic properties and suggest that inhibition of PDHK4 could represent a novel therapeutic strategy to target KRAS mutant colorectal and lung cancers.

Hypothalamic Regulation of Liver and Muscle Nutrient Partitioning by Brain-Specific Carnitine Palmitoyltransferase 1C in Male Mice

Carnitine palmitoyltransferase (CPT) 1C, a brain-specific protein localized in the endoplasmic reticulum of neurons, is expressed in almost all brain regions. Based on global knockout (KO) models, CPT1C has demonstrated relevance in hippocampus-dependent spatial learning and in hypothalamic regulation of energy balance. Specifically, it has been shown that CPT1C is protective against high-fat diet-induced obesity (DIO), and that CPT1C KO mice show reduced peripheral fatty acid oxidation (FAO) during both fasting and DIO. However, the mechanisms mediating CPT1C-dependent regulation of energy homeostasis remain unclear. Here, we focus on the mechanistic understanding of hypothalamic CPT1C on the regulation of fuel selection in liver and muscle of male mice during energy deprivation situations, such as fasting. In CPT1C-deficient mice, modulation of the main hypothalamic energy sensors (5' adenosine monophosphate-activated protein kinase, Sirtuin 1, and mammalian target of rapamycin) was impaired and plasma catecholamine levels were decreased. Consequently, CPT1C-deficient mice presented defective fasting-induced FAO in liver, leading to higher triacylglycerol accumulation and lower glycogen levels. Moreover, muscle pyruvate dehydrogenase activity was increased, which was indicative of glycolysis enhancement. The respiratory quotient did not decrease in CPT1C KO mice after 48 hours of fasting, confirming a defective switch on fuel substrate selection under hypoglycemia. Phenotype reversion studies identified the mediobasal hypothalamus (MBH) as the main area mediating CPT1C effects on fuel selection. Overall, our data demonstrate that CPT1C in the MBH is necessary for proper hypothalamic sensing of a negative energy balance and fuel partitioning in liver and muscle.

ROS production induced by BRAF inhibitor treatment rewires metabolic processes affecting cell growth of melanoma cells

Background

Most melanoma patients with BRAF^V600E positive tumors respond well to a combination of BRAF kinase and MEK inhibitors. However, some patients are intrinsically resistant while the majority of patients eventually develop drug resistance to the treatment. For patients insufficiently responding to BRAF and MEK inhibitors, there is an ongoing need for new treatment targets. Cellular metabolism is such a promising new target line: mutant BRAF^V600E has been shown to affect the metabolism.

Methods

Time course experiments and a series of western blots were performed in a panel of BRAF^V600E and BRAF^WT/NRAS^mut human melanoma cells, which were incubated with BRAF and MEK1 kinase inhibitors. siRNA approaches were used to investigate the metabolic players involved. Reactive oxygen species (ROS) were measured by confocal microscopy and AZD7545, an inhibitor targeting PDKs (pyruvate dehydrogenase kinase) was tested.

Results

We show that inhibition of the RAS/RAF/MEK/ERK pathway induces phosphorylation of the pyruvate dehydrogenase PDH-E1α subunit in BRAF^V600E and in BRAF^WT/NRAS^mut harboring cells. Inhibition of BRAF, MEK1 and siRNA knock-down of ERK1/2 mediated phosphorylation of PDH. siRNA-mediated knock-down of all PDKs or the use of DCA (a pan-PDK inhibitor) abolished PDH-E1α phosphorylation. BRAF inhibitor treatment also induced the upregulation of ROS, concomitantly with the induction of PDH phosphorylation. Suppression of ROS by MitoQ suppressed PDH-E1α phosphorylation, strongly suggesting that ROS mediate the activation of PDKs. Interestingly, the inhibition of PDK1 with AZD7545 specifically suppressed growth of BRAF-mutant and BRAF inhibitor resistant melanoma cells.

Conclusions

In BRAF^V600E and BRAF^WT/NRAS^mut melanoma cells, the increased production of ROS upon inhibition of the RAS/RAF/MEK/ERK pathway, is responsible for activating PDKs, which in turn phosphorylate and inactivate PDH. As part of a possible salvage pathway, the tricarboxylic acid cycle is inhibited leading to reduced oxidative metabolism and reduced ROS levels. We show that inhibition of PDKs by AZD7545 leads to growth suppression of BRAF-mutated and -inhibitor resistant melanoma cells. Thus small molecule PDK inhibitors such as AZD7545, might be promising drugs for combination treatment in melanoma patients with activating RAS/RAF/MEK/ERK pathway mutations (50% BRAF, 25% NRAS^mut, 11.9% NF1^mut).

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-017-0667-y) contains supplementary material, which is available to authorized users.

PPARs: regulators of metabolism and as therapeutic targets in cardiovascular disease. Part I: PPAR-α

This article provides a comprehensive review about the molecular and metabolic actions of PPAR-α. It describes its structural features, ligand specificity, gene transcription mechanisms, functional characteristics and target genes. In addition, recent progress with the use of loss of function and gain of function mouse models in the discovery of diverse biological functions of PPAR-α, particularly in the vascular system and the status of the development of new single, dual, pan and partial PPAR agonists (PPAR modulators) in the clinical management of metabolic diseases are presented. This review also summarizes the clinical outcomes from a large number of clinical trials aimed at evaluating the atheroprotective actions of current clinically used PPAR-α agonists, fibrates and statin-fibrate combination therapy.

Histone Deacetylase Inhibitors Protect Against Pyruvate Dehydrogenase Dysfunction in Huntington's Disease

Transcriptional deregulation and changes in mitochondrial bioenergetics, including pyruvate dehydrogenase (PDH) dysfunction, have been described in Huntington's disease (HD). We showed previously that the histone deacetylase inhibitors (HDACIs) trichostatin A and sodium butyrate (SB) ameliorate mitochondrial function in cells expressing mutant huntingtin. In this work, we investigated the effect of HDACIs on the regulation of PDH activity in striatal cells derived from HD knock-in mice and YAC128 mice. Mutant cells exhibited decreased PDH activity and increased PDH E1alpha phosphorylation/inactivation, accompanied by enhanced protein levels of PDH kinases 1 and 3 (PDK1 and PDK3). Exposure to dichloroacetate, an inhibitor of PDKs, increased mitochondrial respiration and decreased production of reactive oxygen species in mutant cells, emphasizing PDH as an interesting therapeutic target in HD. Treatment with SB and sodium phenylbutyrate, another HDACI, recovered cell viability and overall mitochondrial metabolism in mutant cells. Exposure to SB also suppressed hypoxia-inducible factor-1 (HIF-1α) stabilization and decreased the transcription of the two most abundant PDK isoforms, PDK2 and PDK3, culminating in increased PDH activation in mutant cells. Concordantly, PDK3 knockdown improved mitochondrial function, emphasizing the role of PDK3 inactivation on the positive effects achieved by SB treatment. YAC128 mouse brain presented higher mRNA levels of PDK1-3 and PDH phosphorylation and decreased energy levels that were significantly ameliorated after SB treatment. Furthermore, enhanced motor learning and coordination were observed in SB-treated YAC128 mice. These results suggest that HDACIs, particularly SB, promote the activity of PDH in the HD brain, helping to counteract HD-related deficits in mitochondrial bioenergetics and motor function.SIGNIFICANCE STATEMENT The present work provides a better understanding of mitochondrial dysfunction in Huntington's disease (HD) by showing that the pyruvate dehydrogenase (PDH) complex is a promising therapeutic target. In particular, the histone deacetylase inhibitor sodium butyrate (SB) may indirectly (through reduced hypoxia-inducible factor 1 alpha stabilization) decrease the expression of the most abundant PDH kinase isoforms (e.g., PDK3), ameliorating PDH activity and mitochondrial metabolism and further affecting motor behavior in HD mice, thus constituting a promising agent for HD neuroprotective treatment.

PPARs and Mitochondrial Metabolism: From NAFLD to HCC

Metabolic related diseases, such as type 2 diabetes, metabolic syndrome, and nonalcoholic fatty liver disease (NAFLD), are widespread threats which bring about a significant burden of deaths worldwide, mainly due to cardiovascular events and cancer. The pathogenesis of these diseases is extremely complex, multifactorial, and only partially understood. As the main metabolic organ, the liver is central to maintain whole body energetic homeostasis. At the cellular level, mitochondria are the metabolic hub connecting and integrating all the main biochemical, hormonal, and inflammatory signaling pathways to fulfill the energetic and biosynthetic demand of the cell. In the liver, mitochondria metabolism needs to cope with the energetic regulation of the whole body. The nuclear receptors PPARs orchestrate lipid and glucose metabolism and are involved in a variety of diseases, from metabolic disorders to cancer. In this review, focus is placed on the roles of PPARs in the regulation of liver mitochondrial metabolism in physiology and pathology, from NAFLD to HCC.

Regulation of Pyruvate Dehydrogenase Kinase 4 in the Heart through Degradation by the Lon Protease in Response to Mitochondrial Substrate Availability

Cardiac metabolic inflexibility is driven by robust up-regulation of pyruvate dehydrogenase kinase 4 (PDK4) and phosphorylation-dependent inhibition of pyruvate dehydrogenase (PDH) within a single day of feeding mice a high fat diet. In the current study, we have discovered that PDK4 is a short lived protein (t_½ ∼ 1 h) and is specifically degraded by the mitochondrial protease Lon. Lon does not rapidly degrade PDK1 and -2, indicating specificity toward the PDK isoform that is a potent modulator of metabolic flexibility. Moreover, PDK4 degradation appears regulated by dissociation from the PDH complex dependent on the respiratory state and energetic substrate availability of mouse heart mitochondria. Finally, we demonstrate that pharmacologic inhibition of PDK4 promotes PDK4 degradation in vitro and in vivo These findings reveal a novel strategy to manipulate PDH activity by selectively targeting PDK4 content through dissociation and proteolysis.

Adipose HIF-1α causes obesity by suppressing brown adipose tissue thermogenesis

Hypoxia-inducible factor-1α (HIF-1α) in adipose tissue is known to promote obesity. We hypothesized that HIF-1α interferes with brown fat thermogenesis, thus decreasing energy expenditure. To test this hypothesis, we compared transgenic mice constitutively expressing HIF-1α in adipose tissues (HIF-1α++) at usual temperature (22 °C), where brown fat is somewhat active, or at thermoneutrality (30 °C), where brown fat is minimally active. HIF-1α++ mice or control litter mates were separated into room temperature (22 °C) or thermoneutrality (30 °C) groups. We assessed weight gain, food intake, calorimetry, activity, and oxygen consumption and transcriptional changes in isolated white and brown adipocytes. At 22 °C, HIF-1α++ mice exhibited accelerated weight gain, cold and glucose intolerance, hyperglycemia, and decreased energy expenditure without changes in food intake or activity. These changes were absent or minimal at thermoneutrality. In brown adipocytes of HIF-1α++ mice, oxygen consumption decreased ~50 % in association with reduced mitochondrial content, uncoupling protein 2, and peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1α). In conclusion, adipose HIF-1α overexpression inhibits thermogenesis and cellular respiration in brown adipose tissue, promoting obesity in the setting of reduced ambient temperature.

KEY MESSAGE

Constitutive HIF-1α activation in adipose tissue promotes weight gain in mice. The weight gain is associated with reduced brown adipose tissue function and oxygen consumption. Reduced oxygen consumption may be mediated by reductions in mitochondria.

Glucose-dependent anaplerosis in cancer cells is required for cellular redox balance in the absence of glutamine

Cancer cells have altered metabolism compared to normal cells, including dependence on glutamine (GLN) for survival, known as GLN addiction. However, some cancer cell lines do not require GLN for survival and the basis for this discrepancy is not well understood. GLN is a precursor for antioxidants such as glutathione (GSH) and NADPH, and GLN deprivation is therefore predicted to deplete antioxidants and increase reactive oxygen species (ROS). Using diverse human cancer cell lines we show that this occurs only in cells that rely on GLN for survival. Thus, the preference for GLN as a dominant antioxidant source defines GLN addiction. We show that despite increased glucose uptake, GLN addicted cells do not metabolize glucose via the TCA cycle when GLN is depleted, as revealed by ¹³C-glucose labeling. In contrast, GLN independent cells can compensate by diverting glucose-derived pyruvate into the TCA cycle. GLN addicted cells exhibit reduced PDH activity, increased PDK1 expression, and PDK inhibition partially rescues GLN starvation-induced ROS and cell death. Finally, we show that combining GLN starvation with pro-oxidants selectively kills GLN addicted cells. These data highlight a major role for GLN in maintaining redox balance in cancer cells that lack glucose-dependent anaplerosis.

Phosphorylation of the pyruvate dehydrogenase complex precedes HIF-1-mediated effects and pyruvate dehydrogenase kinase 1 upregulation during the first hours of hypoxic treatment in hepatocellular carcinoma cells

The pyruvate dehydrogenase complex (PDC) is an important gatekeeper enzyme connecting glycolysis to the tricarboxylic acid (TCA) cycle and oxidative phosphorylation (OXPHOS). Thereby, it has a strong impact on the glycolytic flux as well as the metabolic phenotype of a cell. PDC activity is regulated via reversible phosphorylation of three serine residues on the pyruvate dehydrogenase (PDH) E1α subunit. Phosphorylation of any of these residues by the PDH kinases (PDKs) leads to a strong decrease in PDC activity. Under hypoxia, the inactivation of the PDC has been described to be dependent on the hypoxia-inducible factor 1 (HIF-1)-induced PDK1 protein upregulation. In this study, we show in two hepatocellular carcinoma cell lines (HepG2 and JHH-4) that, during the adaptation to hypoxia, PDH is already phosphorylated at time points preceding HIF-1-mediated transcriptional events and PDK1 protein upregulation. Using siRNAs and small molecule inhibitor approaches, we show that this inactivation of PDC is independent of HIF-1α expression but that the PDKs need to be expressed and active. Furthermore, we show that reactive oxygen species might be important for the induction of this PDH phosphorylation since it correlates with the appearance of an altered redox state in the mitochondria and is also inducible by H₂O₂ treatment under normoxic conditions. Overall, these results show that neither HIF-1 expression nor PDK1 upregulation is necessary for the phosphorylation of PDH during the first hours of the adaptation to hypoxia.

Lack of Skeletal Muscle IL-6 Affects Pyruvate Dehydrogenase Activity at Rest and during Prolonged Exercise

Pyruvate dehydrogenase (PDH) plays a key role in the regulation of skeletal muscle substrate utilization. IL-6 is produced in skeletal muscle during exercise in a duration dependent manner and has been reported to increase whole body fatty acid oxidation, muscle glucose uptake and decrease PDHa activity in skeletal muscle of fed mice. The aim of the present study was to examine whether muscle IL-6 contributes to exercise-induced PDH regulation in skeletal muscle. Skeletal muscle-specific IL-6 knockout (IL-6 MKO) mice and floxed littermate controls (control) completed a single bout of treadmill exercise for 10, 60 or 120 min, with rested mice of each genotype serving as basal controls. The respiratory exchange ratio (RER) was overall higher (P<0.05) in IL-6 MKO than control mice during the 120 min of treadmill exercise, while RER decreased during exercise independent of genotype. AMPK and ACC phosphorylation also increased with exercise independent of genotype. PDHa activity was in control mice higher (P<0.05) at 10 and 60 min of exercise than at rest but remained unchanged in IL-6 MKO mice. In addition, PDHa activity was higher (P<0.05) in IL-6 MKO than control mice at rest and 60 min of exercise. Neither PDH phosphorylation nor acetylation could explain the genotype differences in PDHa activity. Together, this provides evidence that skeletal muscle IL-6 contributes to the regulation of PDH at rest and during prolonged exercise and suggests that muscle IL-6 normally dampens carbohydrate utilization during prolonged exercise via effects on PDH.

GATA4 Regulates Blood-Testis Barrier Function and Lactate Metabolism in Mouse Sertoli Cells

Conditional deletion of Gata4 in Sertoli cells (SCs) of adult mice has been shown to increase permeability of the blood-testis barrier (BTB) and disrupt spermatogenesis. To gain insight into the molecular underpinnings of these phenotypic abnormalities, we assessed the impact of Gata4 gene silencing in cell culture models. Microarray hybridization identified genes dysregulated by siRNA-mediated inhibition of Gata4 in TM4 cells, an immortalized mouse SC line. Differentially expressed genes were validated by quantitative RT-PCR analysis of primary cultures of Gata4(flox/flox) mouse SCs that had been subjected to cre-mediated recombination in vitro. Depletion of GATA4 in TM4 cells and primary SCs was associated with altered expression of genes involved in key facets of BTB maintenance, including tight/adherens junction formation (Tjp1, Cldn12, Vcl, Tnc, Csk) and extracellular matrix reorganization (Lamc1, Col4a1, Col4a5, Mmp10, Mmp23, Timp2). Western blotting and immunocytochemistry demonstrated reduced levels of tight junction protein-1, a prototypical tight junction protein, in GATA4-depleted cells. These changes were accompanied by a loss of morphologically recognizable junctional complexes and a decline in epithelial membrane resistance. Furthermore, Gata4 gene silencing was associated with altered expression of Hk1, Gpi1, Pfkp, Pgam1, Gls2, Pdk3, Pkd4, and Ldhb, genes regulating the production of lactate, a key nutrient that SCs provide to developing germ cells. Comprehensive metabolomic profiling demonstrated impaired lactate production in GATA4-deficient SCs. We conclude that GATA4 plays a pivotal role in the regulation of BTB function and lactate metabolism in mouse SCs.

Proteomic and metabolomic changes driven by elevating myocardial creatine suggest novel metabolic feedback mechanisms

Mice over-expressing the creatine transporter have elevated myocardial creatine levels [Cr] and are protected against ischaemia/reperfusion injury via improved energy reserve. However, mice with very high [Cr] develop cardiac hypertrophy and dysfunction. To investigate these contrasting effects, we applied a non-biased hypothesis-generating approach to quantify global protein and metabolite changes in the LV of mice stratified for [Cr] levels: wildtype, moderately elevated, and high [Cr] (65-85; 100-135; 160-250 nmol/mg protein, respectively). Male mice received an echocardiogram at 7 weeks of age with tissue harvested at 8 weeks. RV was used for [Cr] quantification by HPLC to select LV tissue for subsequent analysis. Two-dimensional difference in-gel electrophoresis identified differentially expressed proteins, which were manually picked and trypsin digested for nano-LC-MS/MS. Principal component analysis (PCA) showed efficient group separation (ANOVA P ≤ 0.05) and peptide sequences were identified by mouse database (UniProt 201203) using Mascot. A total of 27 unique proteins were found to be differentially expressed between normal and high [Cr], with proteins showing [Cr]-dependent differential expression, chosen for confirmation, e.g. α-crystallin B, a heat shock protein implicated in cardio-protection and myozenin-2, which could contribute to the hypertrophic phenotype. Nuclear magnetic resonance (¹H-NMR at 700 MHz) identified multiple strong correlations between [Cr] and key cardiac metabolites. For example, positive correlations with α-glucose (r² = 0.45; P = 0.002), acetyl-carnitine (r² = 0.50; P = 0.001), glutamine (r² = 0.59; P = 0.0002); and negative correlations with taurine (r² = 0.74; P < 0.0001), fumarate (r² = 0.45; P = 0.003), aspartate (r² = 0.59; P = 0.0002), alanine (r² = 0.66; P < 0.0001) and phosphocholine (r² = 0.60; P = 0.0002). These findings suggest wide-ranging and hitherto unexpected adaptations in substrate utilisation and energy metabolism with a general pattern of impaired energy generating pathways in mice with very high creatine levels.

Mitochondria in White, Brown, and Beige Adipocytes

Mitochondria play a key role in energy metabolism in many tissues, including cardiac and skeletal muscle, brain, liver, and adipose tissue. Three types of adipose depots can be identified in mammals, commonly classified according to their colour appearance: the white (WAT), the brown (BAT), and the beige/brite/brown-like (bAT) adipose tissues. WAT is mainly involved in the storage and mobilization of energy and BAT is predominantly responsible for nonshivering thermogenesis. Recent data suggest that adipocyte mitochondria might play an important role in the development of obesity through defects in mitochondrial lipogenesis and lipolysis, regulation of adipocyte differentiation, apoptosis, production of oxygen radicals, efficiency of oxidative phosphorylation, and regulation of conversion of white adipocytes into brown-like adipocytes. This review summarizes the main characteristics of each adipose tissue subtype and describes morphological and functional modifications focusing on mitochondria and their activity in healthy and unhealthy adipocytes.

Impact of maternal undernutrition around the time of conception on factors regulating hepatic lipid metabolism and microRNAs in singleton and twin fetuses

We have investigated the effects of embryo number and maternal undernutrition imposed either around the time of conception or before implantation on hepatic lipid metabolism in the sheep fetus. We have demonstrated that periconceptional undernutrition and preimplantation undernutrition each resulted in decreased hepatic fatty acid β-oxidation regulators, PGC-1α (P < 0.05), PDK2 (P < 0.01), and PDK4 (P < 0.01) mRNA expression in singleton and twin fetuses at 135-138 days gestation. In singletons, there was also lower hepatic PDK4 (P < 0.01), CPT-1 (P < 0.01), and PKCζ (P < 0.01) protein abundance in the PCUN and PIUN groups and a lower protein abundance of PDPK-1 (P < 0.05) in the PCUN group. Interestingly, in twins, the hepatic protein abundance of p-AMPK (Ser(485)) (P < 0.01), p-PDPK-1 (Ser(41)) (P < 0.05), and PKCζ (P < 0.05) was higher in the PCUN and PIUN groups, and hepatic PDK4 (P < 0.001) and CPT-1 (P < 0.05) protein abundance was also higher in the PIUN twin fetus. We also found that the expression of a number of microRNAs was altered in response to PCUN or PIUN and that there is evidence that these changes may underlie the changes in the protein abundance of key regulators of hepatic fatty acid β-oxidation in the PCUN and PIUN groups. Therefore, embryo number and the timing of maternal undernutrition in early pregnancy have a differential impact on hepatic microRNA expression and on the factors that regulate hepatic fatty acid oxidation and lipid synthesis.

Mitochondrial resetting and metabolic reprogramming in induced pluripotent stem cells and mitochondrial disease modeling

BACKGROUND

Nuclear reprogramming with pluripotency factors enables somatic cells to gain the properties of embryonic stem cells. Mitochondrial resetting and metabolic reprogramming are suggested to be key early events in the induction of human skin fibroblasts to induced pluripotent stem cells (iPSCs).

SCOPE OF REVIEW

We review recent advances in the study of the molecular basis for mitochondrial resetting and metabolic reprogramming in the regulation of the formation of iPSCs. In particular, the recent progress in using iPSCs for mitochondrial disease modeling was discussed.

MAJOR CONCLUSIONS

iPSCs rely on glycolysis rather than oxidative phosphorylation as a major supply of energy. Mitochondrial resetting and metabolic reprogramming thus play crucial roles in the process of generation of iPSCs from somatic cells.

GENERAL SIGNIFICANCE

Neurons, myocytes, and cardiomyocytes are cells containing abundant mitochondria in the human body, which can be differentiated from iPSCs or trans-differentiated from fibroblasts. Generating these cells from iPSCs derived from skin fibroblasts of patients with mitochondrial diseases or by trans-differentiation with cell-specific transcription factors will provide valuable insights into the role of mitochondrial DNA heteroplasmy in mitochondrial disease modeling and serves as a novel platform for screening of drugs to treat patients with mitochondrial diseases.

Nutrient sensing and utilization: Getting to the heart of metabolic flexibility

A central feature of obesity-related cardiometabolic diseases is the impaired ability to transition between fatty acid and glucose metabolism. This impairment, referred to as "metabolic inflexibility", occurs in a number of tissues, including the heart. Although the heart normally prefers to metabolize fatty acids over glucose, the inability to upregulate glucose metabolism under energetically demanding conditions contributes to a pathological state involving energy imbalance, impaired contractility, and post-translational protein modifications. This review discusses pathophysiologic processes that contribute to cardiac metabolic inflexibility and speculates on the potential physiologic origins that lead to the current state of cardiometabolic disease in an obesogenic environment.

Novel molecular mechanisms involved in hormonal regulation of lactate production in Sertoli cells

The aim of the study was to analyze molecular mechanisms involved in FSH and basic fibroblast growth factor (bFGF) regulation of lactate production in rat Sertoli cells. The regulation of the availability of pyruvate, which is converted to lactate, could be a mechanism utilized by hormones to ensure lactate supply to germ cells. On one hand, the regulation of 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase (PFKFB) expression could result in increased glycolysis, while an increase in pyruvate availability may also result from a lower conversion to acetyl-CoA by negative regulation of pyruvate dehydrogenase complex (PDC) activity by phosphorylation. Sertoli cell cultures obtained from 20-day-old rats were used. Stimulation of the cultures with FSH or bFGF showed that FSH increases Pfkfb1 and Pfkfb3 expression while bFGF increases Pfkfb1 mRNA levels. Additionally, we observed that FSH-stimulated lactate production was inhibited in the presence of a PFKFB3 inhibitor, revealing the physiological relevance of this mechanism. As for the regulation of PDC, analysis of pyruvate dehydrogenase kinase (Pdk) expression showed that FSH increases Pdk3 and decreases Pdk4 mRNA levels while bFGF increases the expression of all Pdks. In addition, we showed that bFGF increases phosphorylated PDC levels and that bFGF-stimulated lactate production is partially inhibited in the presence of a PDK inhibitor. Altogether, these results add new information regarding novel molecular mechanisms involved in hormonal regulation of lactate production in Sertoli cells. Considering that lactate is essential for the production of energy in spermatocytes and spermatids, these mechanisms might be relevant in maintaining spermatogenesis and male fertility.

Pyruvate Dehydrogenase Kinases: Therapeutic Targets for Diabetes and Cancers

Impaired glucose homeostasis is one of the risk factors for causing metabolic diseases including obesity, type 2 diabetes, and cancers. In glucose metabolism, pyruvate dehydrogenase complex (PDC) mediates a major regulatory step, an irreversible reaction of oxidative decarboxylation of pyruvate to acetyl-CoA. Tight control of PDC is critical because it plays a key role in glucose disposal. PDC activity is tightly regulated using phosphorylation by pyruvate dehydrogenase kinases (PDK1 to 4) and pyruvate dehydrogenase phosphatases (PDP1 and 2). PDKs and PDPs exhibit unique tissue expression patterns, kinetic properties, and sensitivities to regulatory molecules. During the last decades, the up-regulation of PDKs has been observed in the tissues of patients and mammals with metabolic diseases, which suggests that the inhibition of these kinases may have beneficial effects for treating metabolic diseases. This review summarizes the recent advances in the role of specific PDK isoenzymes on the induction of metabolic diseases and describes the effects of PDK inhibition on the prevention of metabolic diseases using pharmacological inhibitors. Based on these reports, PDK isoenzymes are strong therapeutic targets for preventing and treating metabolic diseases.

Differing roles of pyruvate dehydrogenase kinases during mouse oocyte maturation

Pyruvate dehydrogenase kinases (PDKs) modulate energy homeostasis in multiple tissues and cell types, under various nutrient conditions, through phosphorylation of the α subunit (PDHE1α, also known as PDHA1) of the pyruvate dehydrogenase (PDH) complex. However, the roles of PDKs in meiotic maturation are currently unknown. Here, by undertaking knockdown and overexpression analysis of PDK paralogs (PDK1-PDK4) in mouse oocytes, we established the site-specificity of PDKs towards the phosphorylation of three serine residues (Ser232, Ser293 and Ser300) on PDHE1α. We found that PDK3-mediated phosphorylation of Ser293-PDHE1α results in disruption of meiotic spindle morphology and chromosome alignment and decreased total ATP levels, probably through inhibition of PDH activity. Unexpectedly, we discovered that PDK1 and PDK2 promote meiotic maturation, as their knockdown disturbs the assembly of the meiotic apparatus, without significantly altering ATP content. Moreover, phosphorylation of Ser232-PDHE1α was demonstrated to mediate PDK1 and PDK2 action in meiotic maturation, possibly through a mechanism that is distinct from PDH inactivation. These findings reveal that there are divergent roles of PDKs during oocyte maturation and indicate a new mechanism controlling meiotic structure.