HIF-1alpha in endurance training: suppression of oxidative metabolism

PGC-1β regulates angiogenesis in skeletal muscle

Aerobic metabolism requires oxygen and carbon sources brought to tissues via the vasculature. Metabolically active tissues such as skeletal muscle can regulate blood vessel density to match metabolic needs; however, the molecular cues that coordinate these processes remain poorly understood. Here we report that the transcriptional coactivator peroxisome proliferator-activated receptor-γ coactivator-1β (PGC-1β), a potent regulator of mitochondrial biology, induces angiogenesis in skeletal muscle. PGC-1β induces the expression of vascular endothelial growth factor (VEGF) in cell culture and in vivo. The induction of VEGF by PGC-1β requires coactivation of the orphan nuclear receptor estrogen-related receptor-α (ERRα) and is independent of the hypoxia-inducible factor (HIF) pathway. In coculture experiments, overexpression of PGC-1β in skeletal myotubes increases the migration of adjacent endothelial cells, and this depends on VEGF signaling. Transgenic expression of PGC-1β in skeletal myocytes dramatically increases muscular vessel density. Taken together, these data indicate that PGC-1β is a potent regulator of angiogenesis, thus providing a novel link between the regulations of oxidative metabolism and vascular density.

HIF signaling and overall gene expression changes during hypoxia and prolonged exercise differ considerably

Exercise as well as hypoxia cause an increase in angiogenesis, changes in mitochondrial density and alterations in metabolism, but it is still under debate whether the hypoxia inducible factor (HIF) is active during both situations. In this study gene expression analysis of zebrafish larvae that were raised under normoxic, hypoxic, or training conditions were compared, using microarray analysis, quantitative real-time PCR and protein data. Although HIF expression is posttranslationally regulated, mRNA expression levels of all three isoforms ( HIF-1α, HIF-2α, and HIF-3α) differed in each of the experimental groups, but the changes observed in hypoxic animals were much smaller than in trained larvae. Prominent changes were seen for Hif-2α expression, which significantly increased after the first day of exercise and then decreased down to values significantly below control values. HIF-3α mRNA expression in turn increased significantly, and at the end of the training period (9–15 days postfertilization) it was elevated three times. At the protein level a transient increase in HIF-1α was observed in hypoxic larvae, whereas in the exercise group the amount of HIF-1α protein even decreased below the level of control animals. The analyzed transcriptome was more affected in hypoxic zebrafish larvae, and hardly any genes were similarly altered by both treatments. These results clearly showed that HIF proteins played different roles in trained and hypoxic zebrafish larvae and that the exercise-induced transition to a more aerobic phenotype was not achieved by persistent activation of the hypoxic signaling pathway.

Exercise and PGC-1α-independent synchronization of type I muscle metabolism and vasculature by ERRγ

How type I skeletal muscle inherently maintains high oxidative and vascular capacity in the absence of exercise is unclear. We show that nuclear receptor ERRγ is highly expressed in type I muscle and, when transgenically expressed in anaerobic type II muscles (ERRGO mice), dually induces metabolic and vascular transformation in the absence of exercise. ERRGO mice show increased expression of genes promoting fat metabolism, mitochondrial respiration, and type I fiber specification. Muscles in ERRGO mice also display an activated angiogenic program marked by myofibrillar induction and secretion of proangiogenic factors, neovascularization, and a 100% increase in running endurance. Surprisingly, the induction of type I muscle properties by ERRγ does not involve PGC-1α. Instead, ERRγ genetically activates the energy sensor AMPK in mediating the metabovascular changes in ERRGO mice. Therefore, ERRγ represents a previously unrecognized determinant that specifies intrinsic vascular and oxidative metabolic features that distinguish type I from type II muscle.

Ancient atmospheres and the evolution of oxygen sensing via the hypoxia-inducible factor in metazoans

Metazoan diversification occurred during a time when atmospheric oxygen levels fluctuated between 15 and 30%. The hypoxia-inducible factor (HIF) is a primary regulator of the adaptive transcriptional response to hypoxia. Although the HIF pathway is highly conserved, its complexity increased during periods when atmospheric oxygen concentrations were increasing. Thus atmospheric oxygen levels may have provided a selection force on the development of cellular oxygen-sensing pathways.

Oxygen homeostasis

Metazoan life is dependent upon the utilization of O(2) for essential metabolic processes and oxygen homeostasis is an organizing principle for understanding metazoan evolution, ontology, physiology, and pathology. Hypoxia-inducible factor 1 (HIF-1) is a transcription factor that is expressed by all metazoan species and functions as a master regulator of oxygen homeostasis. Recent studies have elucidated complex mechanisms by which HIF-1 activity is regulated and by which HIF-1 regulates gene expression, with profound consequences for prenatal development, postnatal physiology, and disease pathogenesis.

Hypoxia inducible factor 1 links fast-patterned muscle activity and fast muscle phenotype in rats

Exercise influences muscle phenotype by the specific pattern of action potentials delivered to the muscle, triggering intracellular signalling pathways. PO2 can be reduced by an order of magnitude in working muscle. In humans, carriers of a hyperactive polymorphism of the transcription factor hypoxia inducible factor 1α (HIF-1α) have 50% more fast fibres, and this polymorphism is prevalent among strength athletes. We have investigated the putative role of HIF-1α in mediating activity changes in muscle.When rat muscles were stimulated with short high frequency bursts of action potentials known to induce a fast muscle phenotype, HIF-1α increased by about 80%. In contrast, a pattern consisting of long low frequency trains known to make fast muscles slow reduced the HIF-1α level of the fast extensor digitorum longus (EDL) muscle by 44%. Nuclear protein extracts from normal EDL contained 2.3-fold more HIF-1α and 4-fold more HIF-1β than the slow soleus muscle, while von-Hippel-Lindau protein was 4.8-fold higher in slow muscles. mRNA displayed a reciprocal pattern; thus FIH-1 mRNA was almost 2-fold higher in fast muscle, while the HIF-1α level was half, and consequently protein/mRNA ratio for HIF-1α was more than 4-fold higher in the fast muscle, suggesting that HIF-1α is strongly suppressed post-transcriptionally in slow muscles.When HIF-1α was overexpressed for 14 days after somatic gene transfer in adult rats, a slow-to-fast transformation was observed, encompassing an increase in fibre cross sectional area, oxidative enzyme activity and myosin heavy chain. The latter was shown to be regulated at the mRNA level in C2C12 myotubes.

PGC-1alpha regulates a HIF2alpha-dependent switch in skeletal muscle fiber types

The coactivator peroxisome proliferator-activated receptor-gamma coactivator 1 α (PGC-1α) coordinates a broad set of transcriptional programs that regulate the response of skeletal muscle to exercise. However, the complete transcriptional network controlled by PGC-1α has not been described. In this study, we used a qPCR-based screen of all known transcriptional components (Quanttrx) to identify transcription factors that are quantitatively regulated by PGC-1α in cultured skeletal muscle cells. This analysis identified hypoxia-inducible factor 2 α (HIF2α) as a major PGC-1α target in skeletal muscle that is positively regulated by both exercise and β-adrenergic signaling. This transcriptional regulation of HIF2α is completely dependent on the PGC-1α/ERRα complex and is further modulated by the action of SIRT1. Transcriptional profiling of HIF2α target genes in primary myotubes suggested an unexpected role for HIF2α in the regulation of muscle fiber types, specifically enhancing the expression of a slow twitch gene program. The PGC-1α-mediated switch to slow, oxidative fibers in vitro is dependent on HIF2α, and mice with a muscle-specific knockout of HIF2α increase the expression of genes and proteins characteristic of a fast-twitch fiber-type switch. These data indicate that HIF2α acts downstream of PGC-1α as a key regulator of a muscle fiber-type program and the adaptive response to exercise.

Hypoxia. 2. Hypoxia regulates cellular metabolism

Adaptation to lowering oxygen levels (hypoxia) requires coordinated downregulation of metabolic demand and supply to prevent a mismatch in ATP utilization and production that might culminate in a bioenergetic collapse. Hypoxia diminishes ATP utilization by downregulating protein translation and the activity of the Na-K-ATPase. Hypoxia diminishes ATP production in part by lowering the activity of the electron transport chain through activation of the transcription factor hypoxia-inducible factor-1. The decrease in electron transport limits the overproduction of reactove oxygen species during hypoxia and slows the rate of oxygen depletion to prevent anoxia. In this review, we discuss these mechanisms that diminish metabolic supply and demand for adaptation to hypoxia.

Candidate genes for physical performance in the horse

Intense selection for speed, endurance or pulling power in the domestic horse (Equus caballus) has resulted in a number of adaptive changes in the phenotype required for elite athletic performance. To date, studies in humans have revealed a large number of genes involved in elite athletic performance, but studies in horses are rare. The horse genome assembly and bioinformation tools for genome analyses have been used to compare human performance genes with their equine orthologues, both to retrieve pathways for these genes and to investigate their chromosomal distribution. In this review, 28 candidate genes for equine performance are presented that have polymorphisms associated with human elite athletic performance and may have impact on athletic performance in horses. A significant accumulation of candidate genes was found on horse chromosomes 4 and 12. Genes involved in pathways for focal adhesion, regulation of actin cytoskeleton, neuroactive ligand-receptor interaction, and calcium signalling were over-represented. Genome-wide association studies for athletic performance in horses may benefit from the strong conserved synteny of the chromosomal arrangement of genes in humans and horses.

The hypoxia response pathway and β-cell function

β-cells sense glucose and secrete appropriate amounts of insulin by coupling glucose uptake and glycolysis with quantitative ATP production via mitochondrial oxidative pathways. Therefore, oxidative phosphorylation is essential for normal β-cell function. Multiple cell types adapt to hypoxia by inducing a transcriptional programme coordinated by the transcription factor hypoxia-inducible factor (HIF). HIF activity is regulated by the von Hippel-Lindau (Vhl) protein, which targets the HIFα subunit for proteasomal degradation in the presence of oxygen. Several recent studies have shown that Vhl deletion in β-cells results in Hif1α activation, impaired glucose-stimulated insulin secretion (GSIS) and glucose intolerance. This was found to be because of alterations in β-cell gene expression inducing a switch from aerobic glucose metabolism to anaerobic glycolysis, thus disrupting the GSIS triggering pathway. Situations in which islets may become hypoxic are discussed, in particular islet transplantation which has been reported to cause islet hypoxia because of an inadequate blood supply post-transplant. Aside from this principal role for HIF in negatively regulating β-cell glucose sensing, other aspects of hypoxia signalling are discussed including β-cell differentiation, development and vascularization. In conclusion, recent studies clearly show that hypoxia response mechanisms can negatively impact on glucose sensing mechanisms in the β-cell and this has the potential to impair β-cell function in a number of physiological and clinical situations.

Basal and exercise-induced regulation of skeletal muscle capillarization

Regulation of skeletal muscle capillarization involves distinct signaling pathways and growth factors including nitric oxide and vascular endothelial growth factor. Our understanding of this complex regulation continues to expand with the identification of new angiogenic growth factors. Future work needs to increase the use of advanced molecular techniques to expand our knowledge of the regulation of basal and exercise-induced capillarization.

Myocyte vascular endothelial growth factor is required for exercise-induced skeletal muscle angiogenesis

We have previously shown, using a Cre-LoxP strategy, that vascular endothelial growth factor (VEGF) is required for the development and maintenance of skeletal muscle capillarity in sedentary adult mice. To determine whether VEGF expression is required for skeletal muscle capillary adaptation to exercise training, gastrocnemius muscle capillarity was measured in myocyte-specific VEGF gene-deleted (mVEGF(-/-)) and wild-type (WT) littermate mice following 6 wk of treadmill running (1 h/day, 5 days/wk) at the same running speed. The effect of training on metabolic enzyme activity levels and whole body running performance was also evaluated in mVEGF(-/-) and WT mice. Posttraining capillary density was significantly increased by 59% (P < 0.05) in the deep muscle region of the gastrocnemius in WT mice but did not change in mVEGF(-/-) mice. Maximal running speed and time to exhaustion during submaximal running increased by 20 and 13% (P < 0.05), respectively, in WT mice after training but were unchanged in mVEGF(-/-) mice. Training led to increases in skeletal muscle citrate synthase (CS) and phosphofructokinase (PFK) activities in both WT and mVEGF(-/-) mice (P < 0.05), whereas β-hydroxyacyl-CoA dehydrogenase (β-HAD) activity was increased only in WT mice. These data demonstrate that skeletal muscle capillary adaptation to physical training does not occur in the absence of myocyte-expressed VEGF. However, skeletal muscle metabolic adaptation to exercise training takes place independent of myocyte VEGF expression.

High fat diet-induced liver steatosis promotes an increase in liver mitochondrial biogenesis in response to hypoxia

Mitochondrial DNA (mtDNA) copy number plays a key role in the pathophysiology of metabolic syndrome-related phenotypes, but its role in non-alcoholic fatty liver disease (NAFLD) is not well understood. We evaluated the molecular mechanisms that may be involved in the regulation of liver mtDNA content in a high-fat-induced rat model of NAFLD. In particular, we tested the hypothesis that liver mtDNA copy number is associated with liver expression of HIF-1α. Rats were given either standard chow diet (SCD, n= 10) or high-fat diet (HFD, n= 15) for 20 weeks. Subsequently, mtDNA quantification using nuclear DNA (nDNA) as a reference was carried out using real time quantitative PCR. HFD induced a significant increase in liver mtDNA/nDNA ratio, which significantly correlated with the liver triglyceride content (R: 0.29, P < 0.05). The liver mtDNA/nDNA ratio significantly correlated with the hepatic expression of HIF-1α mRNA (R: 0.37, P < 0.001); liver HIF-1α mRNA was significantly higher in the HFD group. In addition, liver cytochrome c oxidase subunit IV isoform 1 (COX4I1) mRNA expression was also positively correlated with liver mtDNA content. The hepatic expression of mRNA of transcriptional factors that regulate mitochondrial biogenesis, including peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) and PGC-1β, nuclear respiratory factor-1 (NRF-1), peroxisome proliferator-activated receptor δ and Tfam, was not associated with the liver mtDNA content. Neither hepatocyte apoptosis nor oxidative stress was involved in the HIF-1α-mediated increase in mtDNA copy number. In conclusion, we found that HFD promotes an increase in liver mitochondrial biogenesis in response to hypoxia via HIF-1α, probably to enhance the mitochondrial function as well as to accommodate the metabolic load.

Regulation of human metabolism by hypoxia-inducible factor

The hypoxia-inducible factor (HIF) family of transcription factors directs a coordinated cellular response to hypoxia that includes the transcriptional regulation of a number of metabolic enzymes. Chuvash polycythemia (CP) is an autosomal recessive human disorder in which the regulatory degradation of HIF is impaired, resulting in elevated levels of HIF at normal oxygen tensions. Apart from the polycythemia, CP patients have marked abnormalities of cardiopulmonary function. No studies of integrated metabolic function have been reported. Here we describe the response of these patients to a series of metabolic stresses: exercise of a large muscle mass on a cycle ergometer, exercise of a small muscle mass (calf muscle) which allowed noninvasive in vivo assessments of muscle metabolism using (31)P magnetic resonance spectroscopy, and a standard meal tolerance test. During exercise, CP patients had early and marked phosphocreatine depletion and acidosis in skeletal muscle, greater accumulation of lactate in blood, and reduced maximum exercise capacities. Muscle biopsy specimens from CP patients showed elevated levels of transcript for pyruvate dehydrogenase kinase, phosphofructokinase, and muscle pyruvate kinase. In cell culture, a range of experimental manipulations have been used to study the effects of HIF on cellular metabolism. However, these approaches provide no potential to investigate integrated responses at the level of the whole organism. Although CP is relatively subtle disorder, our study now reveals a striking regulatory role for HIF on metabolism during exercise in humans. These findings have significant implications for the development of therapeutic approaches targeting the HIF pathway.

Muscle-specific expression of hypoxia-inducible factor in human skeletal muscle

Skeletal muscle is well known to exhibit a high degree of plasticity depending on environmental changes, such as various oxygen concentrations. Studies of the oxygen-sensitive subunit alpha of hypoxia-inducible factor-1 (HIF-1) are difficult owing to the large variety of functionally diverse muscle fibres that possess unique patterns of protein and gene expression, producing different capillarization and energy metabolism systems. In this work, we analysed HIF-1alpha mRNA and protein expression related to the fibre-type composition in untrained human skeletal muscle by obtaining muscle biopsies from triceps brachii (characterized by a high proportion of type II fibres), from soleus (characterized by a high proportion of type I fibres) and from vastus lateralis (characterized by an equal proportion of type I and II fibres). The hypothesis was that type I muscle fibres would have lower HIF-1alpha mRNA and protein owing to their higher oxidative capacity. We have shown, in normoxic conditions, a higher HIF-1alpha protein expression in predominantly oxidative muscles than in predominantly glycolytic muscles. However, the HIF-1alpha mRNA expression pattern was not in agreement with the HIF-1alpha protein level. Interestingly, none of the HIF-1alpha target genes, like the most studied angiogenic factor involved in muscle angiogenesis, vascular endothelial growth factor (VEGF), exhibited a muscle fibre-specific-related mRNA expression at rest in normoxia. However, soleus presented a significantly higher VEGF protein content than vastus lateralis and triceps muscle. In conclusion, we have shown that there are muscle-specific differences in HIF-1alpha and VEGF expression within human skeletal muscle at rest in normoxic conditions. Recent results, when combined with the findings described here, support a key role for HIF-1alpha for maintaining muscle homeostasis in non-hypoxic conditions.

Placental metabolic reprogramming: do changes in the mix of energy-generating substrates modulate fetal growth?

Insufficient oxygen leads to the cessation of growth in favor of cellular survival. Our unique model of high-altitude human pregnancy indicates that hypoxia-induced reductions in fetal growth occur at higher levels of oxygen than previously described. Fetal PO(2) is surprisingly high and fetal oxygen consumption unaffected by high altitude, whereas fetal glucose delivery and consumption decrease. Placental delivery of energy-generating substrates to the fetus is thus altered by mild hypoxia, resulting in maintained fetal oxygenation but a relative fetal hypoglycemia. Our data point to this altered mix of substrates as a potential initiating factor in reduced fetal growth, since oxygen delivery is adequate. These data support the existence, in the placenta, of metabolic reprogramming mechanisms, previously documented in tumor cells, whereby HIF-1 stimulates reductions in mitochondrial oxygen consumption at the cost of increased glucose consumption. Decreased oxygen consumption is not due to substrate (oxygen) limitation but rather results from active inhibition of mitochondrial oxygen utilization. We suggest that under hypoxic conditions, metabolic reprogramming in the placenta decreases mitochondrial oxygen consumption and increases anerobic glucose consumption, altering the mix of energy-generating substrates available for transfer to the fetus. Increased oxygen is available to support the fetus, but at the cost of less glucose availability, leading to a hypoglycemia-mediated decrease in fetal growth. Our data suggest that metabolic reprogramming may be an initiating step in the progression to more severe forms of fetal growth restriction and points to the placenta as the pivotal source of fetal programming in response to an adverse intrauterine environment.

Genetics and sports

INTRODUCTION

The limit of each individual to perform a given type of exercise depends on the nature of the task, and is influenced by a variety of factors, including psychology, environment and genetic make up. Genetics provide useful insights, as sport performances can be ultimately defined as a polygenic trait.

SOURCES OF DATA

We searched PubMed using the terms 'sports' and 'genetics' over the period 1990 to present.

AREAS OF AGREEMENT

The physical performance phenotypes for which a genetic basis can be suspected include endurance capacity, muscle performance, physiological attitude to train and ability of tendons and ligaments to withstand injury. Genetic testing in sport would permit to identify individuals with optimal physiology and morphology, and also those with a greater capacity to respond/adapt to training and a lesser chance of suffering from injuries.

AREAS OF CONTROVERSY

Ethical and practical caveats should be clearly emphasized. The translation of an advantageous genotype into a champion's phenotype is still influenced by environmental, psychological and sociological factors.

EMERGING AREAS FOR DEVELOPING RESEARCH

The current scientific evidence on the relationship between genetics and sports look promising. There is a need for additional studies to determine whether genome-wide genotyping arrays would be really useful and cost-effective. Since exercise training regulates the expression of genes encoding various enzymes in muscle and other tissues, genetic research in sports will help clarify several aspects of human biology and physiology, such as RNA and protein level regulation under specific circumstances.

Hypoglycemia and the origin of hypoxia-induced reduction in human fetal growth

BACKGROUND

The most well known reproductive consequence of residence at high altitude (HA >2700 m) is reduction in fetal growth. Reduced fetoplacental oxygenation is an underlying cause of pregnancy pathologies, including intrauterine growth restriction and preeclampsia, which are more common at HA. Therefore, altitude is a natural experimental model to study the etiology of pregnancy pathophysiologies. We have shown that the proximate cause of decreased fetal growth is not reduced oxygen availability, delivery, or consumption. We therefore asked whether glucose, the primary substrate for fetal growth, might be decreased and/or whether altered fetoplacental glucose metabolism might account for reduced fetal growth at HA.

METHODS

Doppler and ultrasound were used to measure maternal uterine and fetal umbilical blood flows in 69 and 58 residents of 400 vs 3600 m. Arterial and venous blood samples from mother and fetus were collected at elective cesarean delivery and analyzed for glucose, lactate and insulin. Maternal delivery and fetal uptakes for oxygen and glucose were calculated.

PRINCIPAL FINDINGS

The maternal arterial - venous glucose concentration difference was greater at HA. However, umbilical venous and arterial glucose concentrations were markedly decreased, resulting in lower glucose delivery at 3600 m. Fetal glucose consumption was reduced by >28%, but strongly correlated with glucose delivery, highlighting the relevance of glucose concentration to fetal uptake. At altitude, fetal lactate levels were increased, insulin concentrations decreased, and the expression of GLUT1 glucose transporter protein in the placental basal membrane was reduced.

CONCLUSION/SIGNIFICANCE

Our results support that preferential anaerobic consumption of glucose by the placenta at high altitude spares oxygen for fetal use, but limits glucose availability for fetal growth. Thus reduced fetal growth at high altitude is associated with fetal hypoglycemia, hypoinsulinemia and a trend towards lactacidemia. Our data support that placentally-mediated reduction in glucose transport is an initiating factor for reduced fetal growth under conditions of chronic hypoxemia.

The transcriptional coactivator PGC-1alpha mediates exercise-induced angiogenesis in skeletal muscle

Peripheral arterial disease (PAD) affects 5 million people in the US and is the primary cause of limb amputations. Exercise remains the single best intervention for PAD, in part thought to be mediated by increases in capillary density. How exercise triggers angiogenesis is not known. PPARgamma coactivator (PGC)-1alpha is a potent transcriptional co-activator that regulates oxidative metabolism in a variety of tissues. We show here that PGC-1alpha mediates exercise-induced angiogenesis. Voluntary exercise induced robust angiogenesis in mouse skeletal muscle. Mice lacking PGC-1alpha in skeletal muscle failed to increase capillary density in response to exercise. Exercise strongly induced expression of PGC-1alpha from an alternate promoter. The induction of PGC-1alpha depended on beta-adrenergic signaling. beta-adrenergic stimulation also induced a broad program of angiogenic factors, including vascular endothelial growth factor (VEGF). This induction required PGC-1alpha. The orphan nuclear receptor ERRalpha mediated the induction of VEGF by PGC-1alpha, and mice lacking ERRalpha also failed to increase vascular density after exercise. These data demonstrate that beta-adrenergic stimulation of a PGC-1alpha/ERRalpha/VEGF axis mediates exercise-induced angiogenesis in skeletal muscle.

Regulation of hypoxia-inducible genes by PGC-1 alpha

Atmospheric oxygen appeared approximately 2.3 billion years ago and sustains most complex life on earth. As mitochondria evolved to harness the energy in oxygen, systems developed to sense and respond to local oxygen concentrations and metabolic conditions. For more than a decade, research has focused on hypoxia-inducible factor 1 (HIF1), a key component of the eukaryotic oxygen-response system. Recently, evidence for other systems has also surfaced. One of these systems involves the PGC-1 alpha coactivator, a powerful transcriptional regulator of mitochondria and oxidative metabolic programs. This brief review will focus on this burgeoning role for PGC-1 alpha and will highlight the many questions that remain unanswered.

Exercise alters SIRT1, SIRT6, NAD and NAMPT levels in skeletal muscle of aged rats

Silent information regulators are potent NAD(+)-dependent protein deacetylases, which have been shown to regulate gene silencing, muscle differentiation and DNA damage repair. Here, changes in the level and activity of sirtuin 1 (SIRT1) in response to exercise in groups of young and old rats were studied. There was an age-related increase in SIRT1 level, while exercise training significantly increased the relative activity of SIRT1. A strong inverse correlation was found between the nuclear activity of SIRT1 and the level of acetylated proteins. Exercise training induced SIRT1 activity due to the positive effect of exercise on the activity of nicotinamide phosphoribosyltransferase (NAMPT) and thereby the production of sirtuin-fueling NAD(+). Exercise training normalized the age-associated shift in redox balance, since exercised animals had significantly lower levels of carbonylated proteins, expression of hypoxia-inducible factor-1 alpha and vascular endothelial growth factor. The age-associated increase in the level of SIRT6 was attenuated by exercise training. On the other hand, aging did not significantly increase the level of DNA damage, which was in line with the activity of 8-oxoguanine DNA glycosylase, while exercise training increased the level of this enzyme. Regular exercise decelerates the deleterious effects of the aging process via SIRT1-dependent pathways through the stimulation of NAD(+) biosynthesis by NAMPT.

The response of human skeletal muscle tissue to hypoxia

Hypoxia refers to environmental or clinical settings that potentially threaten tissue oxygen homeostasis. One unique aspect of skeletal muscle is that, in addition to hypoxia, oxygen balance in this tissue may be further compromised when exercise is superimposed on hypoxia. This review focuses on the cellular and molecular responses of human skeletal muscle to acute and chronic hypoxia, with emphasis on physical exercise and training. Based on published work, it is suggested that hypoxia does not appear to promote angiogenesis or to greatly alter oxidative enzymes in skeletal muscle at rest. Although the HIF-1 pathway in skeletal muscle is still poorly documented, emerging evidence suggests that muscle HIF-1 signaling is only activated to a minor degree by hypoxia. On the other hand, combining hypoxia with exercise appears to improve some aspects of muscle O(2) transport and/or metabolism.

mTOR in renal cell cancer: modulator of tumor biology and therapeutic target

Elucidation of the crucial role of the PI3K/Akt/mTOR pathway in the pathogenesis of cancer has led to the development of various drugs targeting this signaling cascade at distinct levels. mTOR, a serine/threonine kinase plays a pivotal role in coupling growth stimuli to cell cycle progression. There are two distinct macromolecular complexes of mTOR: mTORC1, which is rapamycin-sensitive and contains raptor; and mTORC2, which is rapamycin-insensitive and contains rictor. However, in recent preclinical studies a sustained exposure of cancer cells to rapamycin has been shown to inhibit the function of both mTORC1 and mTORC2 complexes. Downstream targets of these complexes, which involve HIF-1alpha and HIF-2alpha, cyclin D1 and PKC-alpha, are responsible for the activation of various intracellular processes leading to the activation of cell proliferation, and induction of angiogenesis, metastasis or chemoresistance. Since the biology of renal cell cancer (RCC) is tightly controlled by mTOR, targeted inhibition of mTOR function appeared to be a promising therapeutic approach for RCC patients. To date, results of two, large, Phase III clinical trials evaluating the efficacy of rapamycin derivatives (i.e., temsirolimus and everolimus) in the treatment of RCC have been published. First-line temsirolimus (CCI-779) administered to metastatic, poor-prognosis RCC patients significantly prolonged overall and progression-free survival when compared with IFN-alpha. Treatment of metastatic RCC patients refractory to tyrosine kinase inhibitors with everolimus (RAD-001) significantly prolonged progression-free survival when compared with placebo. Therapeutic strategies based on mTOR inhibition in RCC demonstrated a significant clinical activity. However, there are still patients refractory to mTOR inhibitors. Various molecular mechanisms of resistance to rapalogues have been identified and will have to be targeted simultaneously with mTOR in order to achieve a complete inhibition of signaling pathways crucial for the pathogenesis of RCC. Such clinical trials evaluating a combination of mTOR inhibitors with other targeted therapies are ongoing.

Region-specific adaptations in determinants of rat skeletal muscle oxygenation to chronic hypoxia

Chronic exposure to hypoxia is associated with muscle atrophy (i.e., a reduction in muscle fiber cross-sectional area), reduced oxidative capacity, and capillary growth. It is controversial whether these changes are muscle and fiber type specific. We hypothesized that different regions of the same muscle would also respond differently to chronic hypoxia. To investigate this, we compared the deep (oxidative) and superficial (glycolytic) region of the plantaris muscle of eight male rats exposed to 4 wk of hypobaric hypoxia (410 mmHg, Po(2): 11.5 kPa) with those of nine normoxic rats. Hematocrit was higher in chronic hypoxic than control rats (59% vs. 50%, P < 0.001). Using histochemistry, we observed 10% fiber atrophy (P < 0.05) in both regions of the muscle but no shift in the fiber type composition and myoglobin concentration of the fibers. In hypoxic rats, succinate dehydrogenase (SDH) activity was elevated in fibers of each type in the superficial region (25%, P < 0.05) but not in the deep region, whereas in the deep region but not the superficial region the number of capillaries supplying a fiber was elevated (14%, P < 0.05). Model calculations showed that the region-specific alterations in fiber size, SDH activity, and capillary supply to a fiber prevented the occurrence of anoxic areas in the deep region but not in the superficial region. Inclusion of reported acclimatization-induced increases in mean capillary oxygen pressure attenuated the development of anoxic tissue areas in the superficial region of the muscle. We conclude that the determinants of tissue oxygenation show region-specific adaptations, resulting in a marked differential effect on tissue Po(2).

PGC-1alpha is coupled to HIF-1alpha-dependent gene expression by increasing mitochondrial oxygen consumption in skeletal muscle cells

Mitochondrial biogenesis occurs in response to increased cellular ATP demand. The mitochondrial electron transport chain requires molecular oxygen to produce ATP. Thus, increased ATP generation after mitochondrial biogenesis results in increased oxygen demand that must be matched by a corresponding increase in oxygen supply. We found that overexpression of peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PGC-1alpha), which increases mitochondrial biogenesis in primary skeletal muscle cells, leads to increased expression of a cohort of genes known to be regulated by the dimeric hypoxia-inducible factor (HIF), a master regulator of the adaptive response to hypoxia. PGC-1alpha-dependent induction of HIF target genes under physiologic oxygen concentrations is not through transcriptional coactivation of HIF or up-regulation of HIF-1alpha mRNA but through HIF-1alpha protein stabilization. It occurs because of intracellular hypoxia as a result of increased oxygen consumption after mitochondrial biogenesis. Thus, we propose that at physiologic oxygen concentrations, PGC-1alpha is coupled to HIF signaling through the regulation of intracellular oxygen availability, allowing cells and tissues to match increased oxygen demand after mitochondrial biogenesis with increased oxygen supply.

AMPK regulates basal skeletal muscle capillarization and VEGF expression, but is not necessary for the angiogenic response to exercise

5'-AMP-activated protein kinase (AMPK) is a metabolic fuel sensor that monitors cellular energy charge, while the vasculature is important for maintaining cellular energy homeostasis. Mice with muscle-specific inactive AMPK (AMPK DN) were used to investigate if AMPK regulates skeletal muscle capillarization and the angiogenic responses to exercise. Two hours of the AMP analogue AICAR (1.0 g kg(-1)) or systemic hypoxia (6% O(2)) increased vascular endothelial growth factor (VEGF) mRNA in wild-type (WT), but not in AMPK DN mice. In contrast, the increase in VEGF mRNA with acute exercise (1 h at 20 m min(-1), 10% gradient) was greater in AMPK DN compared to WT mice. Nuclear run-on assay demonstrated that exercise increased VEGF transcription, while hypoxia decreased VEGF transcription. There was no difference in VEGF transcription between WT and AMPK DN. There was a strong correlation between VEGF transcription and VEGF mRNA at rest and with exercise. Resting capillarization was lower in AMPK DN compared to WT. Wheel running (28 days) increased capillarization and this response was AMPK independent. Significant correlations between VEGF protein and muscle capillarization are consistent with VEGF being an important determinant of skeletal muscle capillarization. These data are to our knowledge the first to demonstrate in skeletal muscle in vivo that: (1) AMPK is necessary for hypoxia-induced VEGF mRNA stabilization, (2) acute exercise increases VEGF transcription, (3) inhibition of AMPK augments the VEGF mRNA response to acute exercise, and (4) AMPK regulates basal VEGF expression and capillarization, but is not necessary for exercise-induced angiogenesis.

Muscle fatigue from losing your PHD

Prolyl hydroxylases (PHDs) sense oxygen, regulate levels of hypoxia-inducible factors (HIFs), and permit hypoxic adaptation. A new study by Aragones et al. (2008) demonstrates that mice lacking skeletal muscle PHD1 have decreased exercise tolerance and oxygen consumption but remarkably tolerate ischemia in a HIF-2alpha- and PPARalpha-dependent fashion.