Transcriptional Control of the Pgc-1α Gene in Skeletal Muscle In Vivo

Stay Fit, Stay Young: Mitochondria in Movement: The Role of Exercise in the New Mitochondrial Paradigm

Skeletal muscles require the proper production and distribution of energy to sustain their work. To ensure this requirement is met, mitochondria form large networks within skeletal muscle cells, and during exercise, they can enhance their functions. In the present review, we discuss recent findings on exercise-induced mitochondrial adaptations. We emphasize the importance of mitochondrial biogenesis, morphological changes, and increases in respiratory supercomplex formation as mechanisms triggered by exercise that may increase the function of skeletal muscles. Finally, we highlight the possible effects of nutraceutical compounds on mitochondrial performance during exercise and outline the use of exercise as a therapeutic tool in noncommunicable disease prevention. The resulting picture shows that the modulation of mitochondrial activity by exercise is not only fundamental for physical performance but also a key point for whole-organism well-being.

Mitochondrial dynamics in cancer-induced cachexia

Cancer-induced cachexia has a negative impact on quality of life and adversely affects therapeutic outcomes and survival rates. It is characterized by, often severe, loss of muscle, with or without loss of fat mass. Insight in the pathophysiology of this complex metabolic syndrome and direct treatment options are still limited, which creates a research demand. Results from recent studies point towards a significant involvement of muscle mitochondrial networks. However, data are scattered and a comprehensive overview is lacking. This paper aims to fill existing knowledge gaps by integrating published data sets on muscle protein or gene expression from cancer-induced cachexia animal models. To this end, a database was compiled from 94 research papers, comprising 11 different rodent models. This was combined with four genome-wide transcriptome datasets of cancer-induced cachexia rodent models. Analysis showed that the expression of genes involved in mitochondrial fusion, fission, ATP production and mitochondrial density is decreased, while that of genes involved ROS detoxification and mitophagy is increased. Our results underline the relevance of including post-translational modifications of key proteins involved in mitochondrial functioning in future studies on cancer-induced cachexia.

Unravelling the mechanisms regulating muscle mitochondrial biogenesis

Skeletal muscle is a tissue with a low mitochondrial content under basal conditions, but it is responsive to acute increases in contractile activity patterns (i.e. exercise) which initiate the signalling of a compensatory response, leading to the biogenesis of mitochondria and improved organelle function. Exercise also promotes the degradation of poorly functioning mitochondria (i.e. mitophagy), thereby accelerating mitochondrial turnover, and preserving a pool of healthy organelles. In contrast, muscle disuse, as well as the aging process, are associated with reduced mitochondrial quality and quantity in muscle. This has strong negative implications for whole-body metabolic health and the preservation of muscle mass. A number of traditional, as well as novel regulatory pathways exist in muscle that control both biogenesis and mitophagy. Interestingly, although the ablation of single regulatory transcription factors within these pathways often leads to a reduction in the basal mitochondrial content of muscle, this can invariably be overcome with exercise, signifying that exercise activates a multitude of pathways which can respond to restore mitochondrial health. This knowledge, along with growing realization that pharmacological agents can also promote mitochondrial health independently of exercise, leads to an optimistic outlook in which the maintenance of mitochondrial and whole-body metabolic health can be achieved by taking advantage of the broad benefits of exercise, along with the potential specificity of drug action.

Exercise and the Regulation of Mitochondrial Turnover

Exercise is a well-known stimulus for the expansion of the mitochondrial pool within skeletal muscle. Mitochondria have a remarkable ability to remodel their networks and can respond to an array of signaling stimuli following contractile activity to adapt to the metabolic demands of the tissue, synthesizing proteins to expand the mitochondrial reticulum. In addition, when they become dysfunctional, these organelles can be recycled by a specialized intracellular system. The signals regulating this mitochondrial life cycle of synthesis and degradation during exercise are still an area of great research interest. As mitochondrial turnover has valuable consequences in physical performance, in addition to metabolic health, disease, and aging, consideration of the signals which control this cycle is vital. This review focuses on the regulation of mitochondrial turnover in skeletal muscle and summarizes our current understanding of the impact that exercise has in modulating this process.

Multiple signaling pathways regulate contractile activity-mediated PGC-1α gene expression and activity in skeletal muscle cells

PGC‐1α is an important transcriptional coactivator that plays a key role in mediating mitochondrial biogenesis. Within seconds of the onset of contractile activity, a number of rapid cellular events occur that form part of the initial signaling processes involved in PGC‐1α gene regulation, such as elevations in cytoplasmic calcium, AMPK and p38 activation, and elevated ROS production. We observed that basal levels of PGC‐1α promoter activity were more sensitive to resting Ca²⁺ levels, compared to ROS, p38 or, AMPK signaling. Moreover, enhanced PGC‐1α transcription and post‐translational activity on DNA were a result of the activation of multiple signal transduction pathways during contractile activity of myotubes. AMPK, ROS, and Ca²⁺ appear to be necessary for the regulation of contractile activity‐induced PGC‐1α gene expression, governed partly through p38 MAPK and CaMKII activity. Whether these signaling pathways are arranged as a linear sequence of events, or as largely independent pathways during contractile activity, remains to be determined.

When we exercise regularly, one of the most pronounced adaptations is the increase in mitochondrial content in skeletal muscle. This adaptation allows us to exercise longer with less fatigue, and it stimulates to breakdown of fat as an energy source. In this study, we sought to investigate some of the molecular signals which underlie this adaptation. Using a cell culture model of muscle contractile activity, we investigated the expression and regulation of one of the most important proteins that controls mitochondria: PGC‐1alpha. This protein is a transcription factor that regulates the expression of numerous genes which mediate the synthesis of mitochondria. Our results indicate that there are several redundant signaling pathways, activated by exercise, that control the transcription and activity of PGC‐1alpha. This is likely beneficial to insure that exercise adaptations in muscle can take place, even if one pathway is deficient.

Vitamin C supplementation does not alter high-intensity endurance training-induced mitochondrial biogenesis in rat epitrochlearis muscle

The purpose of this study was to investigate whether vitamin C supplementation prevents high-intensity intermittent endurance training-induced mitochondrial biogenesis in the skeletal muscle. Male Wistar-strain rats were assigned to one of five groups: a control group, training group, small dose vitamin C supplemented training group, middle dose vitamin C supplemented training group, and large dose vitamin C supplemented training group. The rats of the trained groups were subjected to intense intermittent swimming training. The vitamin C supplemented groups were administrated vitamin C for the pretraining and training periods. High-intensity intermittent swimming training without vitamin C supplementation significantly increased peroxisome proliferator-activated receptor-γ coactivator-1α protein content and citrate synthase activity in the epitrochlearis muscle. The vitamin C supplementation did not alter the training-induced increase of these regardless of the dose of vitamin C supplementation. The results demonstrate that vitamin C supplementation does not prevent high-intensity intermittent training-induced mitochondrial biogenesis in the skeletal muscle.

Mechanisms of exercise-induced mitochondrial biogenesis in skeletal muscle: implications for health and disease

Mitochondria have paradoxical functions within cells. Essential providers of energy for cellular survival, they are also harbingers of cell death (apoptosis). Mitochondria exhibit remarkable dynamics, undergoing fission, fusion, and reticular expansion. Both nuclear and mitochondrial DNA (mtDNA) encode vital sets of proteins which, when incorporated into the inner mitochondrial membrane, provide electron transport capacity for ATP production, and when mutated lead to a broad spectrum of diseases. Acute exercise can activate a set of signaling cascades in skeletal muscle, leading to the activation of the gene expression pathway, from transcription, to post-translational modifications. Research has begun to unravel the important signals and their protein targets that trigger the onset of mitochondrial adaptations to exercise. Exercise training leads to an accumulation of nuclear- and mtDNA-encoded proteins that assemble into functional complexes devoted to mitochondrial respiration, reactive oxygen species (ROS) production, the import of proteins and metabolites, or apoptosis. This process of biogenesis has important consequences for metabolic health, the oxidative capacity of muscle, and whole body fitness. In contrast, the chronic muscle disuse that accompanies aging or muscle wasting diseases provokes a decline in mitochondrial content and function, which elicits excessive ROS formation and apoptotic signaling. Research continues to seek the molecular underpinnings of how regular exercise can be used to attenuate these decrements in organelle function, maintain skeletal muscle health, and improve quality of life.

PGC-1α is dispensable for exercise-induced mitochondrial biogenesis in skeletal muscle

Exercise confers numerous health benefits, many of which are thought to stem from exercise-induced mitochondrial biogenesis (EIMB) in skeletal muscle. The transcriptional coactivator PGC-1α, a potent regulator of metabolism in numerous tissues, is widely believed to be required for EIMB. We show here that this is not the case. Mice engineered to lack PGC-1α specifically in skeletal muscle (Myo-PGC-1αKO mice) retained intact EIMB. The exercise capacity of these mice was comparable to littermate controls. Induction of metabolic genes after 2 weeks of in-cage voluntary wheel running was intact. Electron microscopy revealed no gross abnormalities in mitochondria, and the mitochondrial biogenic response to endurance exercise was as robust in Myo-PGC-1αKO mice as in wildtype mice. The induction of enzymatic activity of the electron transport chain by exercise was likewise unperturbed in Myo-PGC-1αKO mice. These data demonstrate that PGC-1α is dispensable for exercise-induced mitochondrial biogenesis in skeletal muscle, in sharp contrast to the prevalent assumption in the field.

Localization and regulation of the N terminal splice variant of PGC-1α in adult skeletal muscle fibers

The transcriptional coactivator peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) regulates expression of genes for metabolism and muscle fiber type. Recently, a novel splice variant of PGC-1α (NT-PGC-1α, amino acids 1-270) was cloned and found to be expressed in muscle. Here we use Flag-tagged NT-PGC-1α to examine the subcellular localization and regulation of NT-PGC-1α in skeletal muscle fibers. Flag-NT-PGC-1α is located predominantly in the myoplasm. Nuclear NT-PGC-1α can be increased by activation of protein kinase A. Activation of p38 MAPK by muscle activity or of AMPK had no effect on the subcellular distribution of NT-PGC-1α. Inhibition of CRM1-mediated export only caused relatively slow nuclear accumulation of NT-PGC-1α, indicating that nuclear export of NT-PGC-1α may be mediated by both CRM1-dependent and -independent pathways. Together these results suggest that the regulation of NT-PGC-1α in muscle fibers may be very different from that of the full-length PGC-1α, which is exclusively nuclear.

Effects of a 300 mT static magnetic field on human umbilical vein endothelial cells

This study describes the effects of a static magnetic field (SMF) on cell growth and DNA integrity of human umbilical vein endothelial cells (HUVECs). Fast halo assay was used to investigate nuclear damage; quantitative polymerase chain reaction (QPCR), standard PCR, and real-time PCR were used to evaluate mitochondrial DNA integrity, content, and gene expression. HUVECs were continually exposed to a 300 mT SMF for 4, 24, 48, and 72 h. Compared to control samples (unexposed cultures) the SMF-exposed cells did not show a statistically significant change in their viability. Conversely, the static field was shown to be significant after 4 h of exposure, inducing damage on both the nuclear and mitochondrial levels, reducing mitochondrial content and increasing reactive oxygen species. Twenty-four hours of exposure increased mitochondrial DNA content as well as expression of one of the main genes related to mitochondrial biogenesis. No significant differences between exposed and sham cultures were found after 48 and 72 h of exposure. The results suggest that a 300 mT SMF does not cause permanent DNA damage in HUVECs and stimulates a transient mitochondrial biogenesis.

Down-expression of PGC-1alpha partially mediated by JNK/c-Jun through binding to CRE site during apoptotic procedure in cerebellar granule neurons

In eukaryotes, mitochondria are critical for cellular bioenergetics and mediating apoptosis. The transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) is an important regulator of mitochondrial biogenesis and function. However, the role of PGC-1alpha in neuronal apoptosis and its regulation by apoptotic pathway are still unknown. We demonstrated that PGC-1alpha expression was down-regulated in cerebellar granule neurons(CGNs) after activation of the JNK/c-Jun pathway by potassium deprivation. Overexpression of PGC-1alpha partially protected CGNs from potassium deprivation-induced apoptosis. JNK-specific inhibitors, SP600125 and CEP11004, partially blocked the inhibitory effects of JNK on PGC-1alpha expression and its promoter activity. Furthermore, ChIP assays revealed that c-Jun was able to bind to the CRE site (-188 to -180) in the PGC-1alpha promoter. In conclusion, these results suggest that down-expression of PGC-1alpha partially mediated by activation of JNK/c-Jun may be through the binding of c-Jun to the CRE site in the PGC-1alpha promoter, and it might be involved in potassium deprivation-induced apoptosis in CGNs.

Regulation of PPARγ Coactivator-1α Function and Expression in Muscle: Effect of Exercise

PPARγ coactivator-1α (PGC-1α) is considered to be a major regulator of mitochondrial biogenesis. Though first discovered in brown adipose tissue, this coactivator has emerged as a coordinator of mitochondrial biogenesis in skeletal muscle via enhanced transcription of many nuclear genes encoding mitochondrial proteins. Stimuli such as exercise provoke the activation of signalling cascades that lead to the induction of PGC-1α. Posttranslational modifications also regulate the function of PGC-1α, with a multitude of upstream molecules targeting the protein to modify its activity and/or expression. Previous research has established a positive correlation between resistance to fatigue and skeletal muscle mitochondrial content. Recently, studies have begun to elucidate the specific role of PGC-1α in exercise-related skeletal muscle adaptations, with several studies identifying it as a dominant regulator of organelle synthesis. This paper will highlight the function, regulation, and expression of PGC-1α, as well as the role of the coactivator during exercise adaptations.

Similar expression of oxidative genes after interval and continuous exercise

PURPOSE

There is a debate whether interval or traditional endurance training is the most effective stimulus of mitochondrial biogenesis. Here, we compared the effects of acute interval exercise (IE) or continuous exercise (CE) on the muscle messenger RNA (mRNA) content for several genes involved in mitochondrial biogenesis and lipid metabolism.

METHODS

Nine sedentary subjects cycled for 90 min with two protocols: CE (at 67% VO2max) and IE (12 s at 120% and 18 s at 20% of VO2max). The duration of exercise and work performed with CE and IE was identical. Muscle biopsies were taken before and 3 h after exercise.

RESULTS

There were no significant differences between the two exercise protocols in the increases in VO2 and HR, the reduction in muscle glycogen (35%-40% with both protocols) or the changes in blood metabolites (lactate, glucose, and fatty acids). The mRNA content for major regulators of mitochondrial biogenesis [peroxisome proliferator-activated receptor (PPAR) gamma coactivator 1alpha (PGC-1alpha), PGC-1-related coactivator, PPARbeta/delta] and of lipid metabolism [pyruvate dehydrogenase kinase isozyme 4 (PDK4)] increased after exercise, but there was no significant difference between IE and CE. However, the mRNA content for several downstream targets of PGC-1alpha increased significantly only after CE, and mRNA content for nuclear respiratory factor 2 was significantly higher after CE (P < 0.025 vs IE).

CONCLUSIONS

The present findings demonstrate that, when the duration of exercise and work performed is the same, IE and CE influence the transcription of genes involved in oxidative metabolism in a similar manner.

Changes in contraction-induced phosphorylation of AMP-activated protein kinase and mitogen-activated protein kinases in skeletal muscle after ovariectomy

Recent evidence suggests that ovarian hormones contribute to altered function of skeletal muscle, however the signaling processes thought to regulate muscle function remain undefined in females. Thus, the purpose of this investigation is to determine if ovarian hormone status is critical for contraction-induced activation of AMPK or MAPK in skeletal muscle. Female mice were divided into two groups, ovariectomy (OVX) and SHAM, which were then subjected to in situ isometric contractile protocols. AMPK, ERK 1/2, p38, and JNK phosphorylation were measured in the control and contracting limb. In the in situ protocol, OVX muscles were significantly more resistant to fatigue compared to the SHAM animals. In addition, the muscles from OVX mice demonstrated significantly lower levels of normalized AMPK phosphorylation at rest. AMPK phosphorylation was not increased in the muscles from SHAM mice after the in situ contractile protocol, while the OVX demonstrated significant increases in AMPK phosphorylation. After contraction, normalized ERK2 phosphorylation was significantly higher in the OVX group compared to the SHAM group. Both p38 and JNK phosphorylation increased in response to contraction; but no group differences were detected. A second set of SHAM and OVX animals were subjected to fatigue stimulated under in vitro conditions. Significant increases in AMPK and ERK2 phosphorylation were detected, but no differences were found between groups. In conclusion, removal of the ovaries results in different responses to contraction-induced changes in phosphorylation of AMPK and ERK2 in female mice and suggests hormones secreted from the ovaries significantly impacts cellular signaling in skeletal muscle.

Alterations in peroxisome proliferator-activated receptor mRNA expression in skeletal muscle after acute and repeated bouts of exercise

Peroxisome proliferator-activated receptors (PPAR) exist in three different forms, alpha (alpha), beta/delta (beta/delta), or gamma (gamma), all of which are expressed in skeletal muscle and play a critical role in the regulation of oxidative metabolism. The purpose of this investigation was to determine the mRNA expression pattern of the different PPARs and peroxisome proliferator-activated receptor alpha coactivator-1 alpha (PGC-1alpha) in muscles that largely rely on either glycolytic (plantaris) or oxidative (soleus) metabolism. Further, we also examined the alterations in the PPARs mRNA expression after one bout of endurance exercise or after 12 weeks of exercise training in the different muscles. Female Sprague-Dawley rats (5-8 months) were either run on the treadmill once or exercised trained for 12 weeks. The muscles were removed 24 h after the last bout of exercise. The results demonstrated with the exception of PPAR beta/delta, the PPAR mRNAs are expressed to a greater extent in the soleus muscle than in the plantaris muscle in sedentary animals. PPARgamma was the least abundantly expressed PPAR in either the soleus or the plantaris muscle. With respect to exercise training, only PPARgamma mRNA expression increased in the soleus muscle, while PPARbeta/delta and gamma mRNA levels increased in the plantaris muscle. Minimal changes were detected in any of the PPARs with one bout of exercise training. These results suggest that PPARgamma mRNA levels are the lowest in skeletal muscle among all of the PPARs and PPARgamma mRNA is the most responsive to changes in physical activity levels.

Mechanisms of exercise-induced mitochondrial biogenesis in skeletal muscleThis paper is one of a selection of papers published in this Special Issue, entitled 14th International Biochemistry of Exercise Conference – Muscles as Molecular and Metabolic Machines, and has undergone the Journal’s usual peer review process

Acute exercise initiates rapid cellular signals, leading to the subsequent activation of proteins that increase gene transcription. The result is a higher level of mRNA expression, often observed during the recovery period following exercise. These molecules are translated into precursor proteins for import into preexisting mitochondria. Once inside the organelle, the protein is processed to its mature form and either activates mitochondrial DNA gene expression, serves as a single subunit enzyme, or is incorporated into multi-subunit complexes of the respiratory chain devoted to electron transport and substrate oxidation. The result of this exercise-induced sequence of events is the expansion of the mitochondrial network within muscle cells and the capacity for aerobic ATP provision. An understanding of the molecular processes involved in this complex pathway of organelle synthesis is important for therapeutic purposes, and is a primary research undertaking in laboratories involved in the study of mitochondrial biogenesis. This pathway in muscle becomes impaired with chronic inactivity and aging, which leads to a reduced muscle aerobic capacity and an increased tendency for mitochondrially mediated apoptosis, a situation that can contribute to muscle atrophy. The resumption, or adoption, of an active lifestyle can ameliorate this metabolic dysfunction, improve endurance, and help maintain muscle mass.

Effect of training with different mechanical loadings on MyHC and GLUT4 changes

PURPOSE

There is an inverse relationship between insulin sensitivity and percentage of myosin heavy chain IIx (MyHC IIx) isoform in sedentary, obese, and type 2 diabetic humans. How different exercise conditions may reduce the proportion of MyHC IIx and in parallel elevate glucose transporter 4 (GLUT4) content is interesting in a therapeutic setting. This study investigates the nature of exercise signals regulating MyHC gene switching and whether it is accompanied by GLUT4 changes.

METHODS

Thirty-two subjects performed high loading (60% of 1 repetition maximum [RM]) or low loading (30% of 1 RM) elbow extensions in a training apparatus and exercised three times per week for either 5 wk (low volume) or 8 wk (high volume). MyHC and GLUT4 contents in the musculus triceps brachii were measured by Western blotting pre- and posttraining and after 8 wk of detraining.

RESULTS

All training regimes resulted in MyHC IIx changes of similar magnitude, and differences in training volume had no effect on the outcome. The reduction in MyHC IIx content after high loading, high volume was similar to low loading, matching volume of training. Thus, there was no effect of training load on MyHC changes. GLUT4 increased more after high than low loading (P < 0.0.1). In addition, the larger increases in the GLUT4 were associated with the larger reductions in MyHC IIx content (r = -0.56, P < 0.01). Detraining returned GLUT4 to baseline, but MyHC IIx content was still higher than baseline (P < 0.01).

CONCLUSION

Magnitude of loading is not important for suppression of MyHC IIx but for increases in GLUT4 content. The GLUT4 content responded, however, more rapidly to detraining than the MyHC IIx and IIa isoforms.

Mitochondrial trifunctional protein defects: clinical implications and therapeutic approaches

The mitochondrial trifunctional protein (MTP) is a heterotrimeric protein that consists of four alpha-subunits and four beta-subunits and catalyzes three of the four chain-shortening reactions in the mitochondrial beta-oxidation of long-chain fatty acids. Families with recessively inherited MTP defects display a spectrum of maternal and fetal phenotypes. Current management of patients with MTP defects include long-term dietary therapy of fasting avoidance, low-fat/high-carbohydrate diet with restriction of long-chain fatty acid intake and substitution with medium-chain fatty acids. These dietary approaches appear promising in the short-term, but the long-term outcome of patients treated with dietary intervention is largely unknown. Potential therapeutic approaches targeted at correcting the metabolic defect will be discussed. We will discuss the potential use of protein transduction domains that cross the mitochondrial membranes for the treatment of mitochondrial disorders. In addition, we discuss the phenotypes of MTP in a heterozygous state and potential ways to intervene to increase hepatic fatty acid oxidative capacity.

AMP-activated protein kinase phosphorylates transcription factors of the CREB family

AMP-activated protein kinase (AMPK) has been identified as a regulator of gene transcription, increasing mitochondrial proteins of oxidative metabolism as well as hexokinase expression in skeletal muscle. In mice, muscle-specific knockout of LKB1, a component of the upstream kinase of AMPK, prevents contraction- and 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR)-induced activation of AMPK in skeletal muscle, and the increase in hexokinase II protein that is normally observed with chronic AICAR activation of AMPK. Since previous reports show a cAMP response element in the promoter region of the hexokinase II gene, we hypothesized that the cAMP-response element (CRE) binding protein (CREB) family of transcription factors could be targets of AMPK. Using radioisotopic kinase assays, we found that recombinant and rat liver and muscle AMPK phosphorylated CREB1 at the same site as cAMP-dependent protein kinase (PKA). AMPK was also found to phosphorylate activating transcription factor 1 (ATF1), CRE modulator (CREM), and CREB-like 2 (CREBL2), but not ATF2. Treatment of HEK-293 cells stably transfected with a CREB-driven luciferase reporter with AICAR increased luciferase activity approximately threefold over a 24-h time course. This increase was blocked with compound C, an AMPK inhibitor. In addition, AICAR-induced activation of AMPK in incubated rat epitrochlearis muscles resulted in an increase in both phospho-acetyl-CoA carboxylase and phospho-CREB. We conclude that CREB and related proteins are direct downstream targets for AMPK and are therefore likely involved in mediating some effects of AMPK on expression of genes having a CRE in their promoters.

ATF2 on the double - activating transcription factor and DNA damage response protein

Signal transduction pathways play a key role in the regulation of key cellular processes, including survival and death. Growing evidence points to changes in signaling pathway that occur during skin tumor development and progression. Such changes impact the activity of downstream substrates, including transcription factors. The activating transcription factor 2 (ATF2) has been implicated in malignant and non-malignant skin tumor developments. ATF2 mediates both transcription and DNA damage control, through its phosphorylation by JNK/p38 or ATM/ATR respectively. Here, we summarize our present understanding of ATF2 regulation, function and contribution to malignant and non-malignant skin tumor development.