Effect of exercise intensity and AICAR on isoform-specific expressions of murine skeletal muscle PGC-1α mRNA: a role of β₂-adrenergic receptor activation

Peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) family in physiological and pathophysiological process and diseases

Peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) family (PGC-1s), consisting of three members encompassing PGC-1α, PGC-1β, and PGC-1-related coactivator (PRC), was discovered more than a quarter-century ago. PGC-1s are essential coordinators of many vital cellular events, including mitochondrial functions, oxidative stress, endoplasmic reticulum homeostasis, and inflammation. Accumulating evidence has shown that PGC-1s are implicated in many diseases, such as cancers, cardiac diseases and cardiovascular diseases, neurological disorders, kidney diseases, motor system diseases, and metabolic disorders. Examining the upstream modulators and co-activated partners of PGC-1s and identifying critical biological events modulated by downstream effectors of PGC-1s contribute to the presentation of the elaborate network of PGC-1s. Furthermore, discussing the correlation between PGC-1s and diseases as well as summarizing the therapy targeting PGC-1s helps make individualized and precise intervention methods. In this review, we summarize basic knowledge regarding the PGC-1s family as well as the molecular regulatory network, discuss the physio-pathological roles of PGC-1s in human diseases, review the application of PGC-1s, including the diagnostic and prognostic value of PGC-1s and several therapies in pre-clinical studies, and suggest several directions for future investigations. This review presents the immense potential of targeting PGC-1s in the treatment of diseases and hopefully facilitates the promotion of PGC-1s as new therapeutic targets.

Cooling of male rat skeletal muscle during endurance-like contraction attenuates contraction-induced PGC-1α mRNA expression

This study aimed to determine effects of cooling on contraction-induced peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) and vascular endothelial growth factor (VEGF) gene expression, phosphorylations of its related protein kinases, and metabolic responses. Male rats were separated into two groups; room temperature (RT) or ice-treated (COLD) on the right tibialis anterior (TA). The TA was contracted isometrically using nerve electrical stimulation (1-s stimulation × 30 contractions, with 1-s intervals, for 10 sets with 1-min intervals). The TA was treated before the contraction and during 1-min intervals with an ice pack for the COLD group and a water pack at RT for the RT group. The muscle temperature of the COLD group decreased to 19.42 ± 0.44°C (p < 0.0001, -36.4%) compared with the RT group after the experimental protocol. An increase in mRNA expression level of PGC-1α, not VEGF, after muscle contractions was significantly lower in the COLD group than in the RT group (p < 0.0001, -63.0%). An increase in phosphorylated AMP-activated kinase (AMPK) (p = 0.0037, -28.8%) and a decrease in glycogen concentration (p = 0.0231, +106.3%) after muscle contraction were also significantly inhibited by cooling. Collectively, muscle cooling attenuated the post-contraction increases in PGC-1α mRNA expression coinciding with decreases in AMPK phosphorylation and glycogen degradation.

Transient changes to metabolic homeostasis initiate mitochondrial adaptation to endurance exercise

Endurance exercise is well established to increase mitochondrial content and function in skeletal muscle, a process termed mitochondrial biogenesis. Current understanding is that exercise initiates skeletal muscle mitochondrial remodeling via modulation of cellular nutrient, energetic and contractile stress pathways. These subtle changes in the cellular milieu are sensed by numerous transduction pathways that serve to initiate and coordinate an increase in mitochondrial gene transcription and translation. The result of these acute signaling events is the promotion of growth and assembly of mitochondria, coupled to a greater capacity for aerobic ATP provision in skeletal muscle. The aim of this review is to highlight the acute metabolic events induced by endurance exercise and the subsequent molecular pathways that sense this transient change in cellular homeostasis to drive mitochondrial adaptation and remodeling.

Impacts of altered exercise volume, intensity, and duration on the activation of AMPK and CaMKII and increases in PGC-1α mRNA

The purpose of this review is to explore and discuss the impacts of augmented training volume, intensity, and duration on the phosphorylation/activation of key signaling protein - AMPK, CaMKII and PGC-1α - involved in the initiation of mitochondrial biogenesis. Specifically, we explore the impacts of augmented exercise protocols on AMP/ADP and Ca²⁺ signaling and changes in post exercise PGC - 1α gene expression. Although AMP/ADP concentrations appear to increase with increasing intensity and during extended durations of higher intensity exercise AMPK activation results are varied with some results supporting and intensity/duration effect and others not. Similarly, CaMKII activation and signaling results following exercise of different intensities and durations are inconsistent. The PGC-1α literature is equally inconsistent with only some studies demonstrating an effect of intensity on post exercise mRNA expression. We present a novel meta-analysis that suggests that the inconsistency in the PGC-1α literature may be due to sample size and statistical power limitations owing to the effect of intensity on PGC-1α expression being small. There is little data available regarding the impact of exercise duration on PGC-1α expression. We highlight the need for future well designed, adequately statistically powered, studies to clarify our understanding of the effects of volume, intensity, and duration on the induction of mitochondrial biogenesis by exercise.

Exercise modulates APOE expression in brain cortex of female APOE3 and APOE4 targeted replacement mice

Apolipoprotein E (ApoE) is the main cholesterol carrier of the brain and the ε4 gene variant (APOE4) is the most prevalent genetic risk factor for Alzheimer's disease (AD), increasing risk up to 15-fold. Several studies indicate that APOE4 modulates critical factors for neuronal function, including brain-derived neurotrophic factor (BDNF) and peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α). Both proteins show exercise-induced upregulation, which is presumed to mediate many of the beneficial effects of physical activity including improved cognition; however, there is variability in results between individuals potentially in-part due to genetic variations including APOE isoform. This study aimed to determine if the two most prevalent human APOE isoforms influence adaptive responses to exercise-training. Targeted replacement mice, homozygous for either APOE3 or APOE4 were randomized into exercised and sedentary groups. Baseline locomotor function and voluntary wheel-running behavior was reduced in APOE4 mice. Exercised groups were subjected to daily treadmill running for 8 weeks. ApoE protein in brain cortex was significantly increased by exercise in both genotypes. PGC-1α mRNA levels in brain cortex were significantly lower in APOE4 mice, and only tended to increase with exercise in both genotypes. Hippocampal BDNF protein were similar between genotypes and was not significantly modulated by treadmill running. Behavioral and biochemical variations between APOE3 and APOE4 mice likely contribute to the differential risk for neurological and vascular diseases and the exercise-induced increase in ApoE levels suggests an added feature of the potential efficacy of physical activity as a preventative and therapeutic strategy for neurogenerative processes in both genotypes.

Baicalin ameliorates CUMS-induced depression-like behaviors through activating AMPK/PGC-1α pathway and enhancing NIX-mediated mitophagy in mice

Mitochondrial dysfunction has been reported to be involved in the pathogenesis of depression, and mitophagy is a key pathway for mitochondrial quality control. This study aimed to investigate the effect of baicalin on mitophagy in the hippocampus of mice exposed to chronic unpredictable mild stress (CUMS) and explore its potential mechanism. After exposure to CUMS for 6 weeks, mice were given baicalin (20 mg/kg) or fluoxetine (20 mg/kg) by oral gavage for 4 weeks, and HT22 cells were injured by corticosterone (CORT) in vitro. Depression-like behaviors were assessed by sucrose preference test and tail suspension test. The mitochondrial structure was observed by transmission electron microscopy. Detection of mitophagy and mitophagy-related protein by mitophagy kit and Western blot. The results showed that baicalin improved depressive-like behaviors in CUMS mice, and ameliorated mitochondrial structural impairment in the hippocampus neuron. Baicalin significantly down-regulated light chain 3(LC3)II/I, protein sequestosome 1 (P62), and translocase of the outer membrane 20 (TOM20), and up-regulated Nip-like protein (NIX), Adenylate activated protein kinase (AMPK), and Peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1α. Furthermore, molecular docking showed that baicalin interacts with AMPK through hydrogen bonding. Baicalin increased NIX and AMPK, and improved mitophagy level and mitochondrial function in HT22 cells. Treatment with Phorbol 12-Myristate 13-acetate demonstrated that up-regulation of NIX ameliorated CORT-induced mitochondrial dysfunction in HT22 cells. In conclusion, the present study suggested that the antidepressant effect of baicalin may be related to the enhancement of NIX-mediated mitophagy through activating the AMPK/PGC-1α pathway by directly binding to AMPK.

Effect of endurance training and PGC-1α overexpression on calculated lactate production volume during exercise based on blood lactate concentration

Lactate production is an important clue for understanding metabolic and signal responses to exercise but its measurement is difficult. Therefore, this study aimed (1) to develop a method of calculating lactate production volume during exercise based on blood lactate concentration and compare the effects between endurance exercise training (EX) and PGC-1α overexpression (OE), (2) to elucidate which proteins and enzymes contribute to changes in lactate production due to EX and muscle PGC-1α OE, and (3) to elucidate the relationship between lactate production volume and signaling phosphorylations involved in mitochondrial biogenesis. EX and PGC-1α OE decreased muscle lactate production volume at the absolute same-intensity exercise, but only PGC-1α OE increased lactate production volume at the relative same-intensity exercise. Multiple linear regression revealed that phosphofructokinase, monocarboxylate transporter (MCT)1, MCT4, and citrate synthase equally contribute to the lactate production volume at high-intensity exercise within physiological adaptations, such as EX, not PGC-1α OE. We found that an exercise intensity-dependent increase in the lactate production volume was associated with a decrease in glycogen concentration and an increase in P-AMPK/T-AMPK. This suggested that the calculated lactate production volume was appropriate and reflected metabolic and signal responses but further modifications are needed for the translation to humans.

Chronic treatment with terbutaline increases glucose and oleic acid oxidation and protein synthesis in cultured human myotubes

Objective

In vivo studies have reported several beneficial metabolic effects of β-adrenergic receptor agonist administration in skeletal muscle, including increased glucose uptake, fatty acid metabolism, lipolysis and mitochondrial biogenesis. Although these effects have been widely studied in vivo, the in vitro data are limited to mouse and rat cell lines. Therefore, we sought to discover the effects of the β₂-adrenergic receptor agonist terbutaline on metabolism and protein synthesis in human primary skeletal muscle cells.

Methods

Human cultured myotubes were exposed to terbutaline in various concentrations (0.01–30 ​μM) for 4 or 96 ​h. Thereafter uptake of [¹⁴C]deoxy-D-glucose, oxydation of [¹⁴C]glucose and [¹⁴C]oleic acid were measured. Incorporation of [¹⁴C]leucine, gene expression by qPCR and proteomics analyses by mass spectrometry by the STAGE-TIP method were performed after 96 ​h exposure to 1 and 10 ​μM of terbutaline.

Results

The results showed that 4 ​h treatment with terbutaline in concentrations up to 1 ​μM increased glucose uptake in human myotubes, but also decreased both glucose and oleic acid oxidation along with oleic acid uptake in concentrations of 10–30 ​μM. Moreover, administration of terbutaline for 96 ​h increased glucose uptake (in terbutaline concentrations up to 1 ​μM) and oxidation (1 ​μM), as well as oleic acid oxidation (0.1–30 ​μM), leucine incorporation into cellular protein (1–10 ​μM) and upregulated several pathways related to mitochondrial metabolism (1 ​μM). Data are available via ProteomeXchange with identifier PXD024063.

Conclusion

These results suggest that β₂-adrenergic receptor have direct effects in human skeletal muscle affecting fuel metabolism and net protein synthesis, effects that might be favourable for both type 2 diabetes and muscle wasting disorders.

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The metabolic effects of terbutaline were studied in human primary myotubes.

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Acute treatment with terbutaline increased glucose uptake.

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Chronic treatment with terbutaline increased glucose and oleic acid oxidation.

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Chronic treatment with terbutaline increased protein synthesis.

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Proteomics analysis revealed an increase in mitochondrial proteins.

Exercise-Induced Lactate Release Mediates Mitochondrial Biogenesis in the Hippocampus of Mice via Monocarboxylate Transporters

Regular exercise training induces mitochondrial biogenesis in the brain via activation of peroxisome proliferator-activated receptor gamma-coactivator 1α (PGC-1α). However, it remains unclear whether a single bout of exercise would increase mitochondrial biogenesis in the brain. Therefore, we first investigated whether mitochondrial biogenesis in the hippocampus is affected by a single bout of exercise in mice. A single bout of high-intensity exercise, but not low- or moderate-intensity, increased hippocampal PGC-1α mRNA and mitochondrial DNA (mtDNA) copy number at 12 and 48h. These results depended on exercise intensity, and blood lactate levels observed immediately after exercise. As lactate induces mitochondrial biogenesis in the brain, we examined the effects of acute lactate administration on blood and hippocampal extracellular lactate concentration by in vivo microdialysis. Intraperitoneal (I.P.) lactate injection increased hippocampal extracellular lactate concentration to the same as blood lactate level, promoting PGC-1α mRNA expression in the hippocampus. However, this was suppressed by administering UK5099, a lactate transporter inhibitor, before lactate injection. I.P. UK5099 administration did not affect running performance and blood lactate concentration immediately after exercise but attenuated exercise-induced hippocampal PGC-1α mRNA and mtDNA copy number. In addition, hippocampal monocarboxylate transporters (MCT)1, MCT2, and brain-derived neurotrophic factor (BDNF) mRNA expression, except MCT4, also increased after high-intensity exercise, which was abolished by UK5099 administration. Further, injection of 1,4-dideoxy-1,4-imino-D-arabinitol (glycogen phosphorylase inhibitor) into the hippocampus before high-intensity exercise suppressed glycogen consumption during exercise, but hippocampal lactate, PGC-1α, MCT1, and MCT2 mRNA concentrations were not altered after exercise. These results indicate that the increased blood lactate released from skeletal muscle may induce hippocampal mitochondrial biogenesis and BDNF expression by inducing MCT expression in mice, especially during short-term high-intensity exercise. Thus, a single bout of exercise above the lactate threshold could provide an effective strategy for increasing mitochondrial biogenesis in the hippocampus.

Protective effects of Enterococcus faecium strain R30 supplementation on decreased muscle endurance under disuse in rats

NEW FINDINGS

What is the central question of this study? Does Enterococcus faecium strain R30 (R30), a new lactic acid bacterial strain for supplementation, attenuate shifts in the typology of whole muscle fibres from slow- to fast-twitch by altering the autonomic nervous system in atrophied skeletal muscles? What is the main finding and its importance? R30 supplementation may attenuate the shifts in the typology of whole muscle fibres from slow- to fast-twitch fibres by upregulating peroxisome proliferator-activated receptor-γ coactivator-1α and activating the calcineurin-nuclear factor of activated T-cells signalling pathway, thus ameliorating the decrease in muscle endurance associated with disuse.

ABSTRACT

Enterococcus faecium strain R30 (R30), a new lactic acid bacterial strain for supplementation, was hypothesized to attenuate shifts in the typology of whole muscle fibres from slow- to fast-twitch fibres in atrophied skeletal muscles. We further postulated that the prevention of slow-to-fast fibre shifts would suppress the decreased muscle endurance associated with atrophy. To evaluate the protective effects of R30, we analysed slow-to-fast fibre shifts and disuse-associated reduced muscle endurance. R30 was administered to rats with an acclimation period of 7 days before hindlimb unloading (HU) for 2 weeks. The composition ratio of the fibre type and the expression levels of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), calcineurin and nuclear factor of activated T-cells (NFAT) were measured. Muscle endurance was evaluated at the end of the 2-week HU period in an in situ environment. R30 supplementation suppressed the slow-to-fast fibre switch and decreased the HU-induced expression of PGC-1α proteins and the deactivation of the calcineurin-NFAT pathway. Furthermore, R30 prevented a decrease in HU-associated muscle endurance in calf muscles. These results indicate that R30 supplementation may attenuate the shifts in the typology of whole muscle fibres from slow- to fast-twitch fibres via the upregulation of PGC-1α and the activation of the calcineurin-NFAT signalling pathway, thereby ameliorating the decrease in muscle endurance associated with disuse.

Olive leaf extract prevents obesity, cognitive decline, and depression and improves exercise capacity in mice

Obesity is a risk factor for development of metabolic diseases and cognitive decline; therefore, obesity prevention is of paramount importance. Neuronal mitochondrial dysfunction induced by oxidative stress is an important mechanism underlying cognitive decline. Olive leaf extract contains large amounts of oleanolic acid, a transmembrane G protein-coupled receptor 5 (TGR5) agonist, and oleuropein, an antioxidant. Activation of TGR5 results in enhanced mitochondrial biogenesis, which suggests that olive leaf extract may help prevent cognitive decline through its mitochondrial and antioxidant effects. Therefore, we investigated olive leaf extract’s effects on obesity, cognitive decline, depression, and endurance exercise capacity in a mouse model. In physically inactive mice fed a high-fat diet, olive leaf extract administration suppressed increases in fat mass and body weight and prevented cognitive declines, specifically decreased working memory and depressive behaviors. Additionally, olive leaf extract increased endurance exercise capacity under atmospheric and hypoxic conditions. Our study suggests that these promising effects may be related to oleanolic acid’s improvement of mitochondrial function and oleuropein’s increase of antioxidant capacity.

Hypothalamus-skeletal muscle crosstalk during exercise and its role in metabolism modulation

Physical inactivity is a major public health problem that contributes to the development of several pathologies such as obesity, type 2 diabetes and cardiovascular diseases. Regular exercise mitigates the progression of these metabolic problems and contributes positively to memory and behavior. Therefore, public health agencies have incorporated exercise in the treatment of widespread disorders. The hypothalamus, specifically the ventromedial and the arcuate nuclei, responds to exercise activity and modulates energy metabolism through stimulation of the sympathetic nervous system and catecholamine secretion into the circulation. In addition, physical performance enhances cognitive functions and memory, mediated mostly by an increase in brain-derived neurotrophic factor levels in brain. During exercise training, skeletal muscle myofibers remodel their biochemical, morphological and physiological state. Moreover, skeletal muscle interacts with other organs by the release into the circulation of myokines, molecules that exhibit autocrine, paracrine and endocrine functions. Several studies have focused on the role of skeletal muscle and tissues in response to physical activity. However, how the hypothalamus could influence the skeletal muscle task in the context of exercise is less studied. Here, we review recent data about hypothalamus-skeletal muscle crosstalk in response to physical activity and focus on specific aspects such as the neuroendocrinological effects of exercise and the endocrine functions of skeletal muscle, to provide a perspective for future study directions.

Molecular regulation of skeletal muscle mitochondrial biogenesis following blood flow-restricted aerobic exercise: a call to action

Blood flow-restricted (BFR) exercise can induce training adaptations comparable to those observed following training in free flow conditions. However, little is known about the acute responses within skeletal muscle following BFR aerobic exercise (AE). Moreover, although preliminary evidence suggests chronic BFR AE may augment certain training adaptations in skeletal muscle mitochondria more than non-BFR AE, the underlying mechanisms are poorly understood. In this review, we summarise the acute BFR AE literature examining mitochondrial biogenic signalling pathways and provide insight into mechanisms linked to skeletal muscle remodelling following BFR AE. Specifically, we focus on signalling pathways potentially contributing to augmented peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) mRNA following work-rate-matched BFR AE compared with non-BFR AE. We present evidence suggesting reductions in muscle oxygenation during acute BFR AE lead to increased intracellular energetic stress, AMP-activated protein kinase (AMPK) activation and PGC-1α mRNA. In addition, we briefly discuss mitochondrial adaptations to BFR aerobic training, and we assess the risk of bias using the Cochrane Collaboration risk of bias assessment tool. We ultimately call for several straightforward modifications to help minimise bias in future BFR AE studies.

Sex-based differences after a single bout of exercise on PGC1α isoforms in skeletal muscle: A pilot study

To date, there are limited and incomplete data on possible sex-based differences in fiber-types of skeletal muscle and their response to physical exercise. Adult healthy male and female mice completed a single bout of endurance exercise to examine the sex-based differences of the peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC1α), heat shock protein 60 (Hsp60), interleukin 6 (IL-6) expression, as well as the Myosin Heavy Chain (MHC) fiber-type distribution in soleus and extensor digitorum longus (EDL) muscles. Our results showed for the first time that in male soleus, a muscle rich of type IIa fibers, endurance exercise activates specifically genes involved in mitochondrial biogenesis such as PGC1 α1 isoform, Hsp60 and IL-6, whereas the expression of PGC1 α2 and α3 was significantly upregulated in EDL muscle, a fast-twitch skeletal muscle, independently from the gender. Moreover, we found that the acute response of different PGC1α isoforms was muscle and gender dependent. These findings add a new piece to the huge puzzle of muscle response to physical exercise. Given the importance of these genes in the physiological response of the muscle to exercise, we strongly believe that our data could support future research studies to personalize a specific and sex-based exercise training protocol.

Bufei Jianpi Formula Improves Mitochondrial Function and Suppresses Mitophagy in Skeletal Muscle via the Adenosine Monophosphate-Activated Protein Kinase Pathway in Chronic Obstructive Pulmonary Disease

Skeletal muscle dysfunction, a striking systemic comorbidity of chronic obstructive pulmonary disease (COPD), is associated with declines in activities of daily living, reductions in health status and prognosis, and increases in mortality. Bufei Jianpi formula (BJF), a traditional Chinese herbal formulation, has been shown to improve skeletal muscle tension and tolerance via inhibition of cellular apoptosis in COPD rat models. This study aimed to investigate the mechanisms by which BJF regulates the adenosine monophosphate-activated protein kinase (AMPK) pathway to improve mitochondrial function and to suppress mitophagy in skeletal muscle cells. Our study showed that BJF repaired lung function and ameliorated pathological impairment in rat lung and skeletal muscle tissues. BJF also improved mitochondrial function and reduced mitophagy via the AMPK signaling pathway in rat skeletal muscle tissue. In vitro, BJF significantly improved cigarette smoke extract-induced mitochondrial functional impairment in L6 skeletal muscle cells through effects on mitochondrial membrane potential, mitochondrial permeability transition pores, adenosine triphosphate production, and mitochondrial respiration. In addition, BJF led to upregulated expression of mitochondrial biogenesis markers, including AMPK-α, PGC-1α, and TFAM and downregulation of mitophagy markers, including LC3B, ULK1, PINK1, and Parkin, with increased expression of downstream markers of the AMPK pathway, including mTOR, PPARγ, and SIRT1. In conclusion, BJF significantly improved skeletal muscle and mitochondrial function in COPD rats and L6 cells by promoting mitochondrial biogenesis and suppressing mitophagy via the AMPK pathway. This study suggests that BJF may have therapeutic potential for prophylaxis and treatment of skeletal muscle dysfunction in patients with COPD.

Embryonic exposure to hyper glucocorticoids suppresses brown fat development and thermogenesis via REDD1

Maternal stress during pregnancy is prevailing worldwide, which exposes fetuses to intrauterine hyper glucocorticoids (GC), programming offspring to obesity and metabolic diseases. Despite the importance of brown adipose tissue (BAT) in maintaining long-term metabolic health, impacts of prenatal hyper GC on postnatal BAT thermogenesis and underlying regulations remain poorly defined. Pregnant mice were administrated with synthetic GC dexamethasone (DEX) at levels comparable to fetal GC exposure of stressed mothers. Prenatal GC exposure dose-dependently reduced BAT thermogenic activity, contributing to lower body temperature and higher mortality of neonates; such difference was abolished under thermoneutrality, underscoring BAT deficiency was the major contributor to adverse changes in postnatal thermogenesis due to excessive GC. Prenatal GC exposure highly activated Redd1 expression and reduced Ppargc1a transcription from the alternative promoter (Ppargc1a-AP) in neonatal BAT. During brown adipocyte differentiation, ectopic Redd1 expression reduced Ppargc1a-AP expression and mitochondrial biogenesis; and the inhibitory effects of GC on mitochondrial biogenesis and Ppargc1a-AP expression were blocked by Redd1 ablation. Redd1 reduced protein kinase A phosphorylation and suppressed cyclic adenosine monophosphate (cAMP) -responsive element-binding protein (CREB) binding to the cAMP regulatory element (CRE) in Ppargc1a-AP promoter, leading to Ppargc1a-AP inactivation. In summary, excessive maternal GC exposure during pregnancy dysregulates Redd1-Ppargc1a-AP axis, which impairs fetal BAT development, hampering postnatal thermogenic adaptation and metabolic health of offspring.

PGC-1α alternative promoter (Exon 1b) controls augmentation of total PGC-1α gene expression in response to cold water immersion and low glycogen availability

This investigation sought to determine whether post-exercise cold water immersion and low glycogen availability, separately and in combination, would preferentially activate either the Exon 1a or Exon 1b Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) promoter. Through a reanalysis of sample design, we identified that the systemic cold-induced augmentation of total PGC-1α gene expression observed previously (Allan et al. in J Appl Physiol 123(2):451–459, 2017) was largely a result of increased expression from the alternative promoter (Exon 1b), rather than canonical promoter (Exon 1a). Low glycogen availability in combination with local cooling of the muscle (Allan et al. in Physiol Rep 7(11):e14082, 2019) demonstrated that PGC-1α alternative promoter (Exon 1b) expression continued to rise at 3 h post-exercise in all conditions; whilst, expression from the canonical promoter (Exon 1a) decreased between the same time points (post-exercise–3 h post-exercise). Importantly, this increase in PGC-1α Exon 1b expression was reduced compared to the response of low glycogen or cold water immersion alone, suggesting that the combination of prior low glycogen and CWI post-exercise impaired the response in gene expression versus these conditions individually. Data herein emphasise the influence of post-exercise cooling and low glycogen availability on Exon-specific control of total PGC-1 α gene expression and highlight the need for future research to assess Exon-specific regulation of PGC-1α.

Exercise- and Cold-Induced Human PGC-1α mRNA Isoform Specific Responses

Cold exposure in conjunction with aerobic exercise stimulates gene expression of PGC-1α, the master regulator of mitochondrial biogenesis. PGC-1α can be expressed as multiple isoforms due to alternative splicing mechanisms. Among these isoforms is NT-PGC-1α, which produces a truncated form of the PGC-1α protein, as well as isoforms derived from the first exon of the transcript, PGC-1α-a, PGC-1α-b, and PGC-1α-c. Relatively little is known about the individual responses of these isoforms to exercise and environmental temperature. Therefore, we determined the expression of PGC-1α isoforms following an acute bout of cycling in cold (C) and room temperature (RT) conditions. Nine male participants cycled for 1h at 65% Wmax at −2 °C and 20 °C. A muscle biopsy was taken from the vastus lateralis before and 3h post-exercise. RT-qPCR was used to analyze gene expression of PGC-1α isoforms. Gene expression of all PGC-1α isoforms increased due to the exercise intervention (p < 0.05). Exercise and cold exposure induced a greater increase in gene expression for total PGC-1α (p = 0.028) and its truncated isoform, NT-PGC-1α (p = 0.034), but there was no temperature-dependent response in the other PGC-1α isoforms measured. It appears that NT-PGC-1α may have a significant contribution to the reported alterations in the exercise- and temperature-induced PGC-1α response.

Astaxanthin stimulates mitochondrial biogenesis in insulin resistant muscle via activation of AMPK pathway

BACKGROUND

Skeletal muscle is mainly responsible for insulin-stimulated glucose disposal. Dysfunction in skeletal muscle metabolism especially during obesity contributes to the insulin resistance. Astaxanthin (AX), a natural antioxidant, has been shown to ameliorate hepatic insulin resistance in obese mice. However, its effects in skeletal muscle are poorly understood. The current study aimed to investigate the molecular target of AX in ameliorating skeletal muscle insulin resistance.

METHODS

We fed 6-week-old male C57BL/6J mice with normal chow (NC) or NC supplemented with AX (NC+AX) and high-fat-diet (HFD) or HFD supplemented with AX for 24 weeks. We determined the effect of AX on various parameters including insulin sensitivity, glucose uptake, inflammation, kinase signaling, gene expression, and mitochondrial function in muscle. We also determined energy metabolism in intact C2C12 cells treated with AX using the Seahorse XFe96 Extracellular Flux Analyzer and assessed the effect of AX on mitochondrial oxidative phosphorylation and mitochondrial biogenesis.

RESULTS

AX-treated HFD mice showed improved metabolic status with significant reduction in blood glucose, serum total triglycerides, and cholesterol (p< 0.05). AX-treated HFD mice also showed improved glucose metabolism by enhancing glucose incorporation into peripheral target tissues, such as the skeletal muscle, rather than by suppressing gluconeogenesis in the liver as shown by hyperinsulinemic-euglycemic clamp study. AX activated AMPK in the skeletal muscle of the HFD mice and upregulated the expressions of transcriptional factors and coactivator, thereby inducing mitochondrial remodeling, including increased mitochondrial oxidative phosphorylation component and free fatty acid metabolism. We also assessed the effects of AX on mitochondrial biogenesis in the siRNA-mediated AMPK-depleted C2C12 cells and showed that the effect of AX was lost in the genetically AMPK-depleted C2C12 cells. Collectively, AX treatment (i) significantly ameliorated insulin resistance and glucose intolerance through regulation of AMPK activation in the muscle, (ii) stimulated mitochondrial biogenesis in the muscle, (iii) enhanced exercise tolerance and exercise-induced fatty acid metabolism, and (iv) exerted antiinflammatory effects via its antioxidant activity in adipose tissue.

CONCLUSIONS

We concluded that AX treatment stimulated mitochondrial biogenesis and significantly ameliorated insulin resistance through activation of AMPK pathway in the skeletal muscle.

PGC-1α isoforms coordinate to balance hepatic metabolism and apoptosis in inflammatory environments

Objective

The liver is regularly exposed to changing metabolic and inflammatory environments. It must sense and adapt to metabolic need while balancing resources required to protect itself from insult. Peroxisome proliferator activated receptor gamma coactivator-1 alpha (PGC-1α) is a transcriptional coactivator expressed as multiple, alternatively spliced variants transcribed from different promoters that coordinate metabolic adaptation and protect against inflammation. It is not known how PGC-1α integrates extracellular signals to balance metabolic and anti-inflammatory outcomes.

Methods

Primary mouse hepatocytes were used to evaluate the role(s) of different PGC-1α proteins in regulating hepatic metabolism and inflammatory signaling downstream of tumor necrosis factor alpha (TNFα). Gene expression and signaling analysis were combined with biochemical measurement of apoptosis using gain- and loss-of-function in vitro and in vivo.

Results

Hepatocytes expressed multiple isoforms of PGC-1α, including PGC-1α4, which microarray analysis showed had common and isoform-specific functions linked to metabolism and inflammation compared with canonical PGC-1α1. Whereas PGC-1α1 primarily impacted gene programs of nutrient metabolism and mitochondrial biology, TNFα signaling showed several pathways related to innate immunity and cell death downstream of PGC-1α4. Gain- and loss-of-function models illustrated that PGC-1α4 uniquely enhanced expression of anti-apoptotic gene programs and attenuated hepatocyte apoptosis in response to TNFα or lipopolysaccharide (LPS). This was in contrast to PGC-1α1, which decreased the expression of a wide inflammatory gene network but did not prevent hepatocyte death in response to cytokines.

Conclusions

PGC-1α variants have distinct, yet complementary roles in hepatic responses to metabolism and inflammation, and we identify PGC-1α4 as an important mitigator of apoptosis.

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Multiple isoforms of PGC-1α are expressed in hepatocytes, including PGC-1α4.

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PGC-1α1 and PGC-1α4 share many metabolic targets, but PGC-1α4 has unique functions linked to hepatic inflammatory signalling.

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PGC-1α4 attenuates hepatocyte apoptosis in response to TNFα and LPS in vitro and in vivo.

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Inflammatory signaling influences PGC-1α4 localization in hepatocytes.

High intensity interval training and molecular adaptive response of skeletal muscle

Increased cardiovascular fitness, V˙O_2max, is associated with enhanced endurance capacity and a decreased rate of mortality. High intensity interval training (HIIT) is one of the best methods to increase V˙O_2max and endurance capacity for top athletes and for the general public as well. Because of the high intensity of this type of training, the adaptive response is not restricted to Type I fibers, as found for moderate intensity exercise of long duration. Even with a short exercise duration, HIIT can induce activation of AMPK, PGC-1α, SIRT1 and ROS pathway as well as by the modulation of Ca²⁺ homeostasis, leading to enhanced mitochondrial biogenesis, and angiogenesis. The present review summarizes the current knowledge of the adaptive response of HIIT.

Low-intensity exercise induces acute shifts in liver and skeletal muscle substrate metabolism but not chronic adaptations in tissue oxidative capacity

Adaptations in hepatic and skeletal muscle substrate metabolism following acute and chronic (6 wk; 5 days/wk; 1 h/day) low-intensity treadmill exercise were tested in healthy male C57BL/6J mice. Low-intensity exercise maximizes lipid utilization; therefore, we hypothesized pathways involved in lipid metabolism would be most robustly affected. Acute exercise nearly depleted liver glycogen immediately postexercise (0 h), whereas hepatic triglyceride (TAG) stores increased in the early stages after exercise (0-3 h). Also, hepatic peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) gene expression and fat oxidation (mitochondrial and peroxisomal) increased immediately postexercise (0 h), whereas carbohydrate and amino acid oxidation in liver peaked 24-48 h later. Alternatively, skeletal muscle exhibited a less robust response to acute exercise as stored substrates (glycogen and TAG) remained unchanged, induction of PGC-1α gene expression was delayed (up at 3 h), and mitochondrial substrate oxidation pathways (carbohydrate, amino acid, and lipid) were largely unaltered. Peroxisomal lipid oxidation exhibited the most dynamic changes in skeletal muscle substrate metabolism after acute exercise; however, this response was also delayed (peaked 3-24 h postexercise), and expression of peroxisomal genes remained unaffected. Interestingly, 6 wk of training at a similar intensity limited weight gain, increased muscle glycogen, and reduced TAG accrual in liver and muscle; however, substrate oxidation pathways remained unaltered in both tissues. Collectively, these results suggest changes in substrate metabolism induced by an acute low-intensity exercise bout in healthy mice are more rapid and robust in liver than in skeletal muscle; however, training at a similar intensity for 6 wk is insufficient to induce remodeling of substrate metabolism pathways in either tissue. NEW & NOTEWORTHY Effects of low-intensity exercise on substrate metabolism pathways were tested in liver and skeletal muscle of healthy mice. This is the first study to describe exercise-induced adaptations in peroxisomal lipid metabolism and also reports comprehensive adaptations in mitochondrial substrate metabolism pathways (carbohydrate, lipid, and amino acid). Acute low-intensity exercise induced shifts in mitochondrial and peroxisomal metabolism in both tissues, but training at this intensity did not induce adaptive remodeling of metabolic pathways in healthy mice.

The effect of physical stimuli on the expression level of key elements in mitochondrial biogenesis

Proper mitochondrial function is crucial for intact cellular homeostasis. Mitochondrial dysfunction is clearly involved in the pathogenesis of most neurodegenerative- and age-related chronic disorders. The aim of this study is to stimulate cellular production of important compounds of mitochondrial biogenesis, namely in the peroxisome proliferator-activated receptor-gamma coactivator (PGC)- and Sirtuin (SIRT)-systems. We studied the effect of cold challenge and training on the mRNA expression levels of some compounds of these systems in different brain areas of mice. With regard to the PGC-system, the mRNA levels of the full- and N-truncated isoforms, and those of the two promoters (brain-specific, reference) were measured. In case of Sirtuins, the mRNA levels of SIRT1 and SIRT3-M1/M2/M3 were assessed. We found the following expression level alterations: cooling resulted in the elevation of cortical SIRT3-M1 levels and the decrease of cerebellar SIRT3-M3 levels after 200 min. 900 min of cold exposure resulted in the reduction of cortical SIRT1 and striatal SIRT3-M1 levels. A prominent elevation could be observed in the levels of all PGC-1α isoforms in the cerebellum after 12 days of training. The 12 days of exercise resulted in increased cerebellar SIRT3-M1 and SIRT3-M2 mRNA levels as well. Although the efficacy of cooling core body and brain temperature is questionable, we found that training exerted a clear effect. The cause of the prominent cerebellar elevation of PGC-, and Sirtuin isoforms could be an increase in synaptic plasticity between Purkinje cells, which facilitates better motor coordination and more precise movement integration. We propose that these systems may serve as promising targets for future therapeutic studies in neurodegenerative diseases.

Autophagy and aging: Maintaining the proteome through exercise and caloric restriction

Accumulation of dysfunctional and damaged cellular proteins and organelles occurs during aging, resulting in a disruption of cellular homeostasis and progressive degeneration and increases the risk of cell death. Moderating the accrual of these defunct components is likely a key in the promotion of longevity. While exercise is known to promote healthy aging and mitigate age‐related pathologies, the molecular underpinnings of this phenomenon remain largely unclear. However, recent evidences suggest that exercise modulates the proteome. Similarly, caloric restriction (CR), a known promoter of lifespan, is understood to augment intracellular protein quality. Autophagy is an evolutionary conserved recycling pathway responsible for the degradation, then turnover of cellular proteins and organelles. This housekeeping system has been reliably linked to the aging process. Moreover, autophagic activity declines during aging. The target of rapamycin complex 1 (TORC1), a central kinase involved in protein translation, is a negative regulator of autophagy, and inhibition of TORC1 enhances lifespan. Inhibition of TORC1 may reduce the production of cellular proteins which may otherwise contribute to the deleterious accumulation observed in aging. TORC1 may also exert its effects in an autophagy‐dependent manner. Exercise and CR result in a concomitant downregulation of TORC1 activity and upregulation of autophagy in a number of tissues. Moreover, exercise‐induced TORC1 and autophagy signaling share common pathways with that of CR. Therefore, the longevity effects of exercise and CR may stem from the maintenance of the proteome by balancing the synthesis and recycling of intracellular proteins and thus may represent practical means to promote longevity.

AMPK Mediates Muscle Mass Change But Not the Transition of Myosin Heavy Chain Isoforms during Unloading and Reloading of Skeletal Muscles in Mice

5′AMP-activated protein kinase (AMPK) plays an important role in the regulation of skeletal muscle mass and fiber-type distribution. However, it is unclear whether AMPK is involved in muscle mass change or transition of myosin heavy chain (MyHC) isoforms in response to unloading or increased loading. Here, we checked whether AMPK controls muscle mass change and transition of MyHC isoforms during unloading and reloading using mice expressing a skeletal-muscle-specific dominant-negative AMPKα1 (AMPK-DN). Fourteen days of hindlimb unloading reduced the soleus muscle weight in wild-type and AMPK-DN mice, but reduction in the muscle mass was partly attenuated in AMPK-DN mice. There was no difference in the regrown muscle weight between the mice after 7 days of reloading, and there was concomitantly reduced AMPKα2 activity, however it was higher in AMPK-DN mice after 14 days reloading. No difference was observed between the mice in relation to the levels of slow-type MyHC I, fast-type MyHC IIa/x, and MyHC IIb isoforms following unloading and reloading. The levels of 72-kDa heat-shock protein, which preserves muscle mass, increased in AMPK-DN-mice. Our results indicate that AMPK mediates the progress of atrophy during unloading and regrowth of atrophied muscles following reloading, but it does not influence the transition of MyHC isoforms.

Regulation of PPARGC1A gene expression in trained and untrained human skeletal muscle

Promoter‐specific expression of the PPARGC1A gene in untrained and trained human skeletal muscle was investigated. Ten untrained males performed a one‐legged knee extension exercise (for 60 min) with the same relative intensity both before and after 8 weeks of cycling training. Samples from the m. vastus lateralis of each leg were taken before and after exercise. Postexercise PPARGC1A gene expression via the canonical promoter increased by ~100% (P < 0.05) in exercised and nonexercised untrained muscles, but did not change in either leg after training program. In untrained and trained exercised muscle, PPARGC1A gene expression via the alternative promoter increased by two orders of magnitude (P < 0.01). We found increases in postexercise content of dephosphorylated (activated) CRTC2, a coactivator of CREB1, in untrained exercised muscle and in expression of CREB1‐related genes in untrained and trained exercised muscle (P < 0.01–0.05); this may partially explain the increased expression of PPARGC1A via the alternative promoter. In addition, comparison of the regulatory regions of both promoters revealed unique conserved motifs in the alternative promoter that were associated with transcriptional repressors SNAI1 and HIC1. In conclusion, in untrained muscle, exercise‐induced expression of the PPARGC1A gene via the canonical promoter may be regulated by systemic factors, while in trained muscle the canonical promoter shows constitutive expression at rest and after exercise. Exercise‐induced expression of PPARGC1A via the alternative promoter relates to intramuscular factors and associates with activation of CRTC2‐CREB1. Apparently, expression via the alternative promoter is regulated by other transcription factors, particularly repressors.

OLIVEIRA L, DE VICENTE LG, FERREIRA GN, DE FREITAS EC, DA SILVA ASR. Uphill Running Excessive Training Increases Gastrocnemius Glycogen Content in C57BL/6 Mice

The main aim of the present investigation was to verify the effects of three overtraining (OT) protocols performed in downhill (OTR/down), uphill (OTR/up) and without inclination (OTR) on the protein levels of Akt (Ser473), AMPKα (Thr172), PGC-1α, plasma membrane GLUT-1 and GLUT-4 as well as on the glycogen contents in mice gastrocnemius. A trained (TR) protocol was used as positive control. Rodents were divided into naïve (N, sedentary mice), control (CT, sedentary mice submitted to the performance evaluations), TR, OTR/down, OTR/up and OTR groups. At the end of the experimental protocols, gastrocnemius samples were removed and used for immunoblotting analysis as well as for glycogen measurements. There was no significant difference between the experimental groups for the protein levels of pAkt (Ser473), pAMPKα (Thr172), PGC-1α, plasma membrane GLUT-1 and GLUT-4. However, the OTR/up protocol exhibited higher contents of glycogen compared to the CT and TR groups. In summary, the OTR/up group increased the gastrocnemius glycogen content without significant changes of pAkt (Ser473), pAMPKα (Thr172), PGC-1α, plasma membrane GLUT-1 and GLUT-4.

Nutritional Ketosis and Mitohormesis: Potential Implications for Mitochondrial Function and Human Health

Impaired mitochondrial function often results in excessive production of reactive oxygen species (ROS) and is involved in the etiology of many chronic diseases, including cardiovascular disease, diabetes, neurodegenerative disorders, and cancer. Moderate levels of mitochondrial ROS, however, can protect against chronic disease by inducing upregulation of mitochondrial capacity and endogenous antioxidant defense. This phenomenon, referred to as mitohormesis, is induced through increased reliance on mitochondrial respiration, which can occur through diet or exercise. Nutritional ketosis is a safe and physiological metabolic state induced through a ketogenic diet low in carbohydrate and moderate in protein. Such a diet increases reliance on mitochondrial respiration and may, therefore, induce mitohormesis. Furthermore, the ketone β-hydroxybutyrate (BHB), which is elevated during nutritional ketosis to levels no greater than those resulting from fasting, acts as a signaling molecule in addition to its traditionally known role as an energy substrate. BHB signaling induces adaptations similar to mitohormesis, thereby expanding the potential benefit of nutritional ketosis beyond carbohydrate restriction. This review describes the evidence supporting enhancement of mitochondrial function and endogenous antioxidant defense in response to nutritional ketosis, as well as the potential mechanisms leading to these adaptations.

Impact of β-adrenergic signaling in PGC-1α-mediated adaptations in mouse skeletal muscle

PGC-1α has been suggested to regulate exercise training-induced metabolic adaptations and autophagy in skeletal muscle. The factors regulating PGC-1α, however, have not been fully resolved. The aim was to investigate the impact of β-adrenergic signaling in PGC-1α-mediated metabolic adaptations in skeletal muscle with exercise training. Muscle was obtained from muscle-specific PGC-1α knockout (MKO) and lox/lox mice 1) 3 h after a single exercise bout with or without prior injection of propranolol or 3 h after a single injection of clenbuterol and 2) after 5 wk of wheel running exercise training with or without propranolol treatment or after 5 wk of clenbuterol treatment. A single clenbuterol injection and an acute exercise bout similarly increased the mRNA content of both N-terminal and full-length PGC-1α isoforms, and prior propranolol treatment reduced the exercise-induced increase in mRNA of all isoforms. Furthermore, a single clenbuterol injection elicited a PGC-1α-dependent increase in cytochrome c and vascular endothelial growth factor mRNA, whereas prolonged clenbuterol treatment increased fiber size but reduced capillary density. Exercise training increased the protein content of OXPHOS, LC3I, and Parkin in a PGC-1α-dependent manner without effect of propranolol, while an exercise training-induced increase in Akt2 and p62 protein required PGC-1α and was blunted by prolonged propranolol treatment. This suggests that β-adrenergic signaling is not required for PGC-1α-mediated exercise training-induced adaptations in mitochondrial proteins, but contributes to exercise training-mediated adaptations in insulin signaling and autophagy regulation through PGC-1α. Furthermore, changes observed with acute stimulation of compounds like clenbuterol and propranolol may not lead to corresponding adaptations with prolonged treatment.

Induction of glucose uptake in skeletal muscle by central leptin is mediated by muscle β2-adrenergic receptor but not by AMPK

Leptin increases glucose uptake and fatty acid oxidation (FAO) in red-type skeletal muscle. However, the mechanism remains unknown. We have investigated the role of β₂-adrenergic receptor (AR), the major β-AR isoform in skeletal muscle, and AMPK in leptin-induced muscle glucose uptake of mice. Leptin injection into the ventromedial hypothalamus (VMH) increased 2-deoxy-D-glucose (2DG) uptake in red-type skeletal muscle in wild-type (WT) mice accompanied with increased phosphorylation of the insulin receptor (IR) and Akt as well as of norepinephrine (NE) turnover in the muscle. Leptin-induced 2DG uptake was not observed in β-AR-deficient (β-less) mice despite that AMPK phosphorylation was increased in the muscle. Forced expression of β₂-AR in the unilateral hind limb of β-less mice restored leptin-induced glucose uptake and enhancement of insulin signalling in red-type skeletal muscle. Leptin increased 2DG uptake and enhanced insulin signalling in red-type skeletal muscle of mice expressing a dominant negative form of AMPK (DN-AMPK) in skeletal muscle. Thus, leptin increases glucose uptake and enhances insulin signalling in red-type skeletal muscle via activation of sympathetic nerves and β₂-AR in muscle and in a manner independent of muscle AMPK.

PGC-1α and exercise intensity dependent adaptations in mouse skeletal muscle

The aim of the present study was to examine the role of PGC-1α in intensity dependent exercise and exercise training-induced metabolic adaptations in mouse skeletal muscle. Whole body PGC-1α knockout (KO) and littermate wildtype (WT) mice performed a single treadmill running bout at either low intensity (LI) for 40 min or moderate intensity (MI) for 20 min. Blood and quadriceps muscles were removed either immediately after exercise or at 3h or 6h into recovery from exercise and from resting controls. In addition PGC-1α KO and littermate WT mice were exercise trained at either low intensity (LIT) for 40 min or at moderate intensity (MIT) for 20 min 2 times pr. day for 5 weeks. In the first and the last week of the intervention period, mice performed a graded running endurance test. Quadriceps muscles were removed before and after the training period for analyses. The acute exercise bout elicited intensity dependent increases in LC3I and LC3II protein and intensity independent decrease in p62 protein in skeletal muscle late in recovery and increased LC3II with exercise training independent of exercise intensity and volume in WT mice. Furthermore, acute exercise and exercise training did not increase LC3I and LC3II protein in PGC-1α KO. In addition, exercise-induced mRNA responses of PGC-1α isoforms were intensity dependent. In conclusion, these findings indicate that exercise intensity affected autophagy markers differently in skeletal muscle and suggest that PGC-1α regulates both acute and exercise training-induced autophagy in skeletal muscle potentially in a PGC-1α isoform specific manner.

Transcriptional coactivator NT-PGC-1α promotes gluconeogenic gene expression and enhances hepatic gluconeogenesis

The transcriptional coactivator PGC‐1α plays a central role in hepatic gluconeogenesis. We previously reported that alternative splicing of the PGC‐1α gene produces an additional transcript encoding the truncated protein NT‐PGC‐1α. NT‐PGC‐1α is co‐expressed with PGC‐1α and highly induced by fasting in the liver. NT‐PGC‐1α regulates tissue‐specific metabolism, but its role in the liver has not been investigated. Thus, the objective of this study was to determine the role of hepatic NT‐PGC‐1α in the regulation of gluconeogenesis. Adenovirus‐mediated expression of NT‐PGC‐1α in primary hepatocytes strongly stimulated the expression of key gluconeogenic enzyme genes (PEPCK and G6Pase), leading to increased glucose production. To further understand NT‐PGC‐1α function in hepatic gluconeogenesis in vivo, we took advantage of a previously reported FL‐PGC‐1α^−/− mouse line that lacks full‐length PGC‐1α (FL‐PGC‐1α) but retains a slightly shorter and functionally equivalent form of NT‐PGC‐1α (NT‐PGC‐1α²⁵⁴). In FL‐PGC‐1α^−/− mice, NT‐PGC‐1α²⁵⁴ was induced by fasting in the liver and recruited to the promoters of PEPCK and G6Pase genes. The enrichment of NT‐PGC‐1α²⁵⁴ at the promoters was closely associated with fasting‐induced increase in PEPCK and G6Pase gene expression and efficient production of glucose from pyruvate during a pyruvate tolerance test in FL‐PGC‐1α^−/− mice. Moreover, FL‐PGC‐1α^−/− primary hepatocytes showed a significant increase in gluconeogenic gene expression and glucose production after treatment with dexamethasone and forskolin, suggesting that NT‐PGC‐1α²⁵⁴ is sufficient to stimulate the gluconeogenic program in the absence of FL‐PGC‐1α. Collectively, our findings highlight the role of hepatic NT‐PGC‐1α in stimulating gluconeogenic gene expression and glucose production.

The hypothalamic regulation of metabolic adaptations to exercise

Our modern lifestyle is characterised by easy access to nutrient dense foods combined with limited physical activity. A sedentary lifestyle is one of several factors that have contributed to the global obesity epidemic and it also predisposes to chronic illnesses such as diabetes and cardiovascular disease. Although many studies have focused on the benefits of exercise in peripheral tissues, the contributions of the central nervous system to these exercise-induced metabolic adaptations are relatively unknown. The present review highlights the role of the ventromedial hypothalamus in regulating the metabolic response to exercise.

β1- and β2-adrenergic receptor stimulation differ in their effects on PGC-1α and atrogin-1/MAFbx gene expression in chick skeletal muscle

Adrenaline changes expression of the genes encoding peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1α), which is known as a regulator of muscle size, and atrogin-1/muscle atrophy F-box (MAFbx), which is a muscle-specific ubiquitin ligase. However, the subtype of β-adrenergic receptor (β-AR) involved in regulating these genes in skeletal muscle is not yet well defined. In this study, the effects of intraperitoneal injection of adrenaline and three β_1-3-AR selective agonists on chick skeletal muscle metabolism were examined, to evaluate the functions of β-AR subtypes. Adrenaline decreased atrogin-1/MAFbx mRNA levels accompanied by an increase in PGC-1α mRNA and protein levels. However, among the three selective agonists, only the β₁-AR agonist, dobutamine, increased PGC-1α mRNA and protein levels, while the β₂-AR agonist, clenbuterol, suppressed atrogin-1/MAFbx mRNA levels. In addition, preinjection of the β₁-AR antagonist, acebutolol, and the β₂-AR antagonist, butoxamine, inhibited the adrenaline-induced increase in PGC-1α mRNA levels and the decrease in atrogin-1/MAFbx mRNA levels, respectively. Compared with adrenaline administration, the β₃-AR agonist, BRL37344, decreased PGC-1α mRNA levels and increased atrogin-1/MAFbx mRNA levels. These results suggest that, in chick skeletal muscle, PGC-1α is induced via the β₁-AR, while atrogin-1/MAFbx is suppressed via the β₂-AR.

Acute Response of PGC-1α and IGF-1 Isoforms to Maximal Eccentric Exercise in Skeletal Muscle of Postmenopausal Women

PGC-1α4, a novel isoform of the transcriptional coactivator PGC-1α, was recently postulated to modulate the expression of anabolic and catabolic genes and therefore regulate skeletal muscle hypertrophy. Resting levels of PGC-1α4 messenger RNA (mRNA) expression were found to increase in healthy adults after resistance training. However, the acute effect of resistance exercise (RE) on PGC-1α4 expression in populations prone to progressive muscle loss, such as postmenopausal women, has not been evaluated. Here, we investigated alterations in mRNA expression of PGC-1α4 and PGC-1α1, a regulator of muscle oxidative changes, in postmenopausal women after high-intensity eccentric RE and analyzed these findings with respect to changes in insulin-like growth factor (IGF)-1 and catabolic gene expression. Nine postmenopausal women (age, 57.9 ± 3.2 years) performed 10 sets of 10 maximal eccentric repetitions of single-leg extension with 20-second rest periods between sets. Muscle biopsies were obtained from the vastus lateralis of the exercised leg before and 4 hours after the RE bout with mRNA expression determined by quantitative real-time polymerase chain reaction. No significant changes in the mRNA expression of either PGC-1α isoform were observed after acute eccentric RE (p > 0.05). IGF-1Ea mRNA expression significantly increased (p ≤ 0.05), whereas IGF-1Eb and mechano-growth factor (MGF) did not significantly change (p > 0.05). PGC-1α4 mRNA expression was associated with reduced mRNA expression of the catabolic gene myostatin (R = -0.88, p < 0.01), whereas MGF mRNA expression was associated with reduced mRNA expression of the catabolic gene FOXO3A (R = -0.81, p ≤ 0.05). These data demonstrate an attenuated response of PGC-1α isoforms to an acute bout of maximal eccentric exercise with short rest periods in postmenopausal women.

Impact of adrenaline and metabolic stress on exercise-induced intracellular signaling and PGC-1α mRNA response in human skeletal muscle

This study tested the hypothesis that elevated plasma adrenaline or metabolic stress enhances exercise‐induced PGC‐1α mRNA and intracellular signaling in human muscle. Trained (VO₂‐max: 53.8 ± 1.8 mL min⁻¹ kg⁻¹) male subjects completed four different exercise protocols (work load of the legs was matched): C – cycling at 171 ± 6 W for 60 min (control); A – cycling at 171 ± 6 W for 60 min, with addition of intermittent arm exercise (98 ± 4 W). DS – cycling at 171 ± 6 W interspersed by 30 sec sprints (513 ± 19 W) every 10 min (distributed sprints); and CS – cycling at 171 ± 6 W for 40 min followed by 20 min of six 30 sec sprints (clustered sprints). Sprints were followed by 3:24 min:sec at 111 ± 4 W. A biopsy was obtained from m. vastus lateralis at rest and immediately, and 2 and 5 h after exercise. Muscle PGC‐1α mRNA content was elevated (P < 0.05) three‐ to sixfold 2 h after exercise relative to rest in C, A, and DS, with no differences between protocols. AMPK and p38 phosphorylation was higher (P < 0.05) immediately after exercise than at rest in all protocols, and 1.3‐ to 2‐fold higher (P < 0.05) in CS than in the other protocols. CREB phosphorylation was higher (P < 0.05) 2 and 5 h after exercise than at rest in all protocols, and higher (P < 0.05) in DS than CS 2 h after exercise. This suggests that neither plasma adrenaline nor muscle metabolic stress determines the magnitude of PGC‐1α mRNA response in human muscle. Furthermore, higher exercise‐induced changes in AMPK, p38, and CREB phosphorylation are not associated with differences in the PGC‐1α mRNA response.

Conjugated linoleic acid (CLA) promotes endurance capacity via peroxisome proliferator-activated receptor δ-mediated mechanism in mice

Previously, it was reported that conjugated linoleic acid (CLA) with exercise training potentially improved endurance capacity via the peroxisome proliferator-activated receptor δ (PPARδ)-mediated mechanism in mice. This study determined the role of exercise and/or CLA in endurance capacity and PPARδ-associated regulators. Male 129Sv/J mice were fed either control (soybean oil) or CLA (0.5%) containing diets for 4 weeks and were further divided into sedentary or training regimes. CLA supplementation significantly reduced body weight and fat mass independent of exercise during the experimental period. Endurance capacity was significantly improved by CLA supplementation, while no effect of exercise was observed. Similarly, CLA treatment significantly increased expressions of sirtuin 1 and PPARγ coactivator-1α, up-stream regulators of PPARδ, in both sedentary and trained animals. With respect to downstream markers of PPARδ, CLA up-regulated the key biomarker needed to stimulate mitochondrial biogenesis, nuclear respiratory factor 1. Moreover, CLA supplementation significantly induced overall genes associated with muscle fibers, such as type I (slow-twitch) and type II (fast twitch). Taken together, it suggests that CLA improves endurance capacity independent of mild-intensity exercise via PPARδ-mediated mechanism.

Effect of resistance exercise intensity on the expression of PGC-1α isoforms and the anabolic and catabolic signaling mediators, IGF-1 and myostatin, in human skeletal muscle

The purpose of this study was to investigate the acute messenger (mRNA) expression of the peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) isoforms, insulin-like growth factor-1Ea (IGF-1Ea), and myostatin in response to 2 resistance exercise intensities. In a uniform-balanced, crossover design, 10 participants performed 2 separate testing sessions involving a lower body resistance exercise component consisting of a lower intensity (50% of 1-repetition maximum; 1RM) protocol and a higher intensity (80% of 1RM) protocol of equal volumes. Muscle samples were obtained at before exercise, 45 min, 3 h, 24 h, and 48 h postexercise. Resistance exercise did not alter total PGC-1α mRNA expression; however, distinct responses of each PGC-1α isoform were observed. The response of each isoform was consistent between sessions, suggesting no effect of resistance exercise intensity on the complex transcriptional expression of the PGC-1α gene. IGF-1Ea mRNA expression significantly increased following the higher intensity session compared with pre-exercise and the lower intensity session. Myostatin mRNA expression was significantly reduced compared with pre-exercise values at all time points with no difference between exercise intensity. Further research is needed to determine the effects of the various isoforms of PGC-1α in human skeletal muscle on the translational level as well as their relation to the expression of IGF-1 and myostatin.

Sirt1 and the Mitochondria

Sirt1 is the most prominent and extensively studied member of sirtuins, the family of mammalian class III histone deacetylases heavily implicated in health span and longevity. Although primarily a nuclear protein, Sirt1's deacetylation of Peroxisome proliferator-activated receptor Gamma Coactivator-1α (PGC-1α) has been extensively implicated in metabolic control and mitochondrial biogenesis, which was proposed to partially underlie Sirt1's role in caloric restriction and impacts on longevity. The notion of Sirt1's regulation of PGC-1α activity and its role in mitochondrial biogenesis has, however, been controversial. Interestingly, Sirt1 also appears to be important for the turnover of defective mitochondria by mitophagy. I discuss here evidences for Sirt1's regulation of mitochondrial biogenesis and turnover, in relation to PGC-1α deacetylation and various aspects of cellular physiology and disease.

A single oral dose of flavan-3-ols enhances energy expenditure by sympathetic nerve stimulation in mice

Numerous clinical studies have found that ingestion of chocolate reduces the risk of metabolic syndrome, however, the mechanisms were remain unclear. We have reported that a single dose of a flavan-3-ol fraction derived from cocoa (FL) enhanced energy expenditure (EE) and increased the mRNA expression levels of uncoupling proteins (UCPs) and peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α), and the protein level of phosphorylated AMP-activated protein kinase (AMPK)α in tissues, along with plasma adrenaline level. In the present study, we examined whether the EE enhancing activity of FL is mediated by adrenergic effect using several adrenalin receptor (AR) blockers. In the first study, mice were butoxamine, as β2AR blocker, with vehicle or 10mg/kg FL orally. We found that pretreatment with butoxamine prevented the increases of EE, the mRNA expression of UCP-3, and phosphorylated AMPKα that were induced in the gastrocnemius muscle of mice by 10mg/kg FL. Secondly, mice were given SR52930, as β3AR blocker. Pretreatment with SR52930 prevented the increases of EE, the mRNA expression of UCP-3, and phosphorylated AMPKα that were induced in the gastrocnemius muscle of mice by 10mg/kg FL. Pretreatment with a combination of both blockers also reduced the increments in mRNA expression levels of UCPs and PGC-1α, however, phosphorylated AMPKα in skeletal muscle was rather increased. These results suggest that the ability of a single oral dose of FL to enhance metabolic activity is mediated by sympathetic nerve system (SNS).

Aerobic Exercise and Pharmacological Therapies for Skeletal Myopathy in Heart Failure: Similarities and Differences

Skeletal myopathy has been identified as a major comorbidity of heart failure (HF) affecting up to 20% of ambulatory patients leading to shortness of breath, early fatigue, and exercise intolerance. Neurohumoral blockade, through the inhibition of renin angiotensin aldosterone system (RAS) and β-adrenergic receptor blockade (β-blockers), is a mandatory pharmacological therapy of HF since it reduces symptoms, mortality, and sudden death. However, the effect of these drugs on skeletal myopathy needs to be clarified, since exercise intolerance remains in HF patients optimized with β-blockers and inhibitors of RAS. Aerobic exercise training (AET) is efficient in counteracting skeletal myopathy and in improving functional capacity and quality of life. Indeed, AET has beneficial effects on failing heart itself despite being of less magnitude compared with neurohumoral blockade. In this way, AET should be implemented in the care standards, together with pharmacological therapies. Since both neurohumoral inhibition and AET have a direct and/or indirect impact on skeletal muscle, this review aims to provide an overview of the isolated effects of these therapeutic approaches in counteracting skeletal myopathy in HF. The similarities and dissimilarities of neurohumoral inhibition and AET therapies are also discussed to identify potential advantageous effects of these combined therapies for treating HF.

Promoter-specific regulation of PPARGC1A gene expression in human skeletal muscle

The goal of this study was to identify unknown transcription start sites of the PPARGC1A (PGC-1α) gene in human skeletal muscle and investigate the promoter-specific regulation of PGC-1α gene expression in human skeletal muscle. Ten amateur endurance-trained athletes performed high- and low-intensity exercise sessions (70  min, 70% or 50% o2max). High-throughput RNA sequencing and exon-exon junction mapping were applied to analyse muscle samples obtained at rest and after exercise. PGC-1α promoter-specific expression and activation of regulators of PGC-1α gene expression (AMPK, p38 MAPK, CaMKII, PKA and CREB1) after exercise were evaluated using qPCR and western blot. Our study has demonstrated that during post-exercise recovery, human skeletal muscle expresses the PGC-1α gene via two promoters only. As previously described, the additional exon 7a that contains a stop codon was found in all samples. Importantly, only minor levels of other splice site variants were found (and not in all samples). Constitutive expression PGC-1α gene occurs via the canonical promoter, independent of exercise intensity and exercise-induced increase of AMPK(Thr172) phosphorylation level. Expression of PGC-1α gene via the alternative promoter is increased of two orders after exercise. This post-exercise expression is highly dependent on the intensity of exercise. There is an apparent association between expression via the alternative promoter and activation of CREB1.

Involvement of AMPK in regulating slow-twitch muscle atrophy during hindlimb unloading in mice

AMPK is considered to have a role in regulating skeletal muscle mass. However, there are no studies investigating the function of AMPK in modulating skeletal muscle mass during atrophic conditions. In the present study, we investigated the difference in unloading-associated muscle atrophy and molecular functions in response to 2-wk hindlimb suspension between transgenic mice overexpressing the dominant-negative mutant of AMPK (AMPK-DN) and their wild-type (WT) littermates. Male WT (n = 24) and AMPK-DN (n = 24) mice were randomly divided into two groups: an untreated preexperimental control group (n = 12 in each group) and an unloading (n = 12 in each group) group. The relative soleus muscle weight and fiber cross-sectional area to body weight were decreased by ∼30% in WT mice by hindlimb unloading and by ∼20% in AMPK-DN mice. There were no changes in puromycin-labeled protein or Akt/70-kDa ribosomal S6 kinase signaling, the indicators of protein synthesis. The expressions of ubiquitinated proteins and muscle RING finger 1 mRNA and protein, markers of the ubiquitin-proteasome system, were increased by hindlimb unloading in WT mice but not in AMPK-DN mice. The expressions of molecules related to the protein degradation system, phosphorylated forkhead box class O3a, inhibitor of κBα, microRNA (miR)-1, and miR-23a, were decreased only in WT mice in response to hindlimb unloading, and 72-kDa heat shock protein expression was higher in AMPK-DN mice than in WT mice. These results imply that AMPK partially regulates unloading-induced atrophy of slow-twitch muscle possibly through modulation of the protein degradation system, especially the ubiquitin-proteasome system.

The hitchhiker's guide to PGC-1α isoform structure and biological functions

Proteins of the peroxisome proliferator-activated receptor γ (PPARγ) coactivator 1 (PGC-1) family of transcriptional coactivators coordinate physiological adaptations in many tissues, usually in response to demands for higher nutrient and energy supply. Of the founding members of the family, PGC-1α (also known as PPARGC1A) is the most highly regulated gene, using multiple promoters and alternative splicing to produce a growing number of coactivator variants. PGC-1α promoters are selectively active in distinct tissues in response to specific stimuli. To date, more than ten novel PGC-1α isoforms have been reported to be expressed from a novel promoter (PGC-1α-b, PGC-1α-c), to undergo alternative splicing (NT-PGC-1α) or both (PGC-1α2, PGC-1α3, PGC-1α4). The resulting proteins display differential regulation and tissue distribution and, most importantly, exert specific biological functions. In this review we discuss the structural and functional characteristics of the novel PGC-1α isoforms, aiming to provide an integrative view of this constantly expanding system of transcriptional coactivators.