Chounghun Kang

Exercise activation of muscle peroxisome proliferator-activated receptor-γ coactivator-1α signaling is redox sensitive

The peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha)-activated signal transduction pathway has previously been shown to stimulate mitochondrial biogenesis in skeletal muscle in response to endurance exercise. In vitro data indicate that PGC-1alpha signaling may be sensitive to reactive oxygen species (ROS) but its role in vivo is not clear. The objectives of this study were (1) to investigate whether the PGC-1alpha pathway could be activated by a single bout of anaerobic exercise in rats, wherein a major portion of ROS was generated via the cytosolic xanthine oxidase (XO), and (2) to examine whether allopurinol (ALP), a specific XO inhibitor, would attenuate PGC-1alpha expression and signaling owing to decreased ROS generation. Female Sprague-Dawley rats were randomly divided into three groups: (1) subjected to sprinting on a treadmill at 35 m/min, 15% grade, for 3 min followed by 3 min slow running at 15 m/min, 0% grade, repeated until exhaustion (88 +/- 4 min; Exer; N= 9); (2) subjected to the same exercise protocol (88 +/- 4 min) but injected with two doses of ALP (0.4 mmol/kg, ip) 24 and 1 h before the experiment (Exer+ ALP; N= 9); and (3) rested control (C; N= 9). Exercise increased XO activity and ROS generation in the Exer rat vastus lateralis muscle (P< 0.05), whereas the Exer+ ALP group displayed only 7% XO activity and similar ROS level compared with the C group. PGC-1alpha protein content showed a 5.6-fold increase (P< 0.01) in Exer vs C, along with a 200% (P< 0.01) increase in both nuclear respiratory factor (NRF)-1 and mitochondrial transcription factor A (Tfam) content. ALP treatment decreased PGC-1alpha, NRF-1, and Tfam levels by 45, 19, and 20% (P< 0.05), respectively. Exercise doubled the content of the phosphorylated cAMP-responsive element-binding protein in the Exer group (P< 0.01) and tripled phosphorylated p38 mitogen-activated protein kinase (P< 0.01), whereas these effects were reduced by 60 and 30% (P< 0.01, P< 0.05), respectively, in Exer+ ALP rats. Nuclear factor-kappaB binding and phospho-IkappaB content were also increased in Exer rats (P< 0.01) and these increases were abolished by ALP treatment. The data indicate that contraction-activated PGC-1alpha signaling pathways in skeletal muscle are redox sensitive and that nonmitochondrial ROS play an important role in stimulating mitochondrial biogenesis.

Exercise training attenuates aging-associated mitochondrial dysfunction in rat skeletal muscle: Role of PGC-1α

Aged skeletal muscle demonstrates declines in muscle mass and deterioration of mitochondrial content and function. Peroxisome proliferator-activated receptor γ co-activator 1α (PGC-1α) plays an important role in promoting muscle mitochondrial biogenesis in response to exercise training, but its role in senescent muscle is not clear. In the present study we hypothesize that a downregulation of the PGC-1α signaling pathway contributes to mitochondrial deterioration in aged muscle whereas endurance training ameliorates the deficits. Three groups of Fischer 344/BNF1 rats were used: young, sedentary (Y, 4 months); old, sedentary (O, 22 months); and old trained (OT, 22 months), subjected to treadmill running at 17.5 m/min, 10% grade for 45 min/day, 5 days/week for 12-weeks. PGC-1α mRNA and nuclear PGC-1α protein content in the soleus muscle were both decreased in O vs. Y rats, whereas OT rats showed a 2.3 and 1.8-fold higher PGC-1α content than O and Y rats, respectively (P<0.01). Mitochondrial transcription factor A (Tfam), cytochrome c (Cyt c) and mitochondrial (mt) DNA contents were significantly decreased in O vs. Y rats, but elevated by 2.2 (P<0.01), 1.4 (P<0.05) and 2.4-fold (P<0.01), respectively, in OT vs. O rats. In addition, Tfam and mtDNA showed 1.6 and 1.8-fold (P<0.01) higher levels, respectively, in OT vs. Y rats. These adaptations were accompanied by significant increases in the expression of the phosphorylated form of AMP-activated kinase (AMPK) (P<0.01), p38 mitogen-activated kinase (MAPK) (P<0.05) and silent mating type information regulator 2 homolog 1 (SIRT1) (P<0.01) in OT rats. Furthermore, OT rats showed great levels of phosphorylation in cAMP responsive element binding protein (p-CREB) and DNA binding compared to O and Y rats. These data indicate that endurance training can attenuate aging-associated decline in mitochondrial protein synthesis in skeletal muscle partly due to upregulation of PGC-1α signaling.

Role of PGC‐1α signaling in skeletal muscle health and disease

This paper reviews the current understanding of the molecular basis of the peroxisome proliferator‐activated receptor‐γ coactivator‐1α (PGC‐1α)–mediated pathway and discusses the role of PGC‐1α in skeletal muscle atrophy caused by immobilization. PGC‐1α is the master transcription regulator that stimulates mitochondrial biogenesis, by upregulating nuclear respiratory factors (NRF‐1, 2) and mitochondrial transcription factor A (Tfam), which leads to increased mitochondrial DNA replication and gene transcription. PGC‐1α also regulates cellular oxidant–antioxidant homeostasis by stimulating the gene expression of superoxide dismutase‐2 (SOD2), catalase, glutathione peroxidase 1 (GPx1), and uncoupling protein (UCP). Recent reports from muscle‐specific PGC‐1α overexpression underline the importance of PGC‐1α in atrophied skeletal muscle, demonstrate enhancement of the PGC‐1α mitochondrial biogenic pathway, and reduced oxidative damage. Thus, PGC‐1α appears to play a protective role against atrophy‐linked skeletal muscle deterioration.

Muscle immobilization and remobilization downregulates PGC-1α signaling and the mitochondrial biogenesis pathway

Prolonged immobilization (IM) results in skeletal muscle atrophy accompanied by increased reactive oxygen species (ROS) generation, inflammation, and protein degradation. However, the biological consequence of remobilizing such muscle has been studied only sparsely. In this study, we examined the peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α)-controlled mitochondrial biogenesis pathway and inflammatory response in mice subjected to 2 wk of hindlimb IM followed by 5 days of remobilization (RM). We hypothesized that ROS generation and activation of redox-sensitive signaling pathways play important roles in the etiology of muscle injury. FVB/N mice (age 2 mo) were randomly assigned to either 14 days of IM by casting one of the hindlimbs (n = 7), IM followed by 5 days of RM with casting removed (n = 7), or to a control group (Con; n = 7). Muscle to body weight ratios of three major leg muscles were significantly decreased as a result of IM. Two ubiquitin-proteasome pathway enzymes, muscle atrophy F-box (MAFb or atrogin-1) and muscle ring finger-1 (MuRF-1), were upregulated with IM and maintained at high levels during RM. Protein contents of PGC-1α and nuclear respiratory factors 1 and 2 in tibialis anterior (TA) muscle were reduced by 50% (P < 0.01) in IM vs. Con, with no recovery observed during RM. IM suppressed mitochondrial transcription factor A and cytochrome-c content by 57% and 63% (P < 0.01), respectively, and cytochrome-c oxidase activity by 58% (P < 0.05). Furthermore, mitochondrial DNA content was reduced by 71% (P < 0.01) with IM. None of these changes were reversed after RM. With RM, TA muscle showed a 2.3-fold (P < 0.05) higher H2O2 content and a 4-fold (P < 0.01) higher 8-isoprostane content compared with Con, indicating oxidative stress. Tumor necrosis factor-α and interleukin-6 levels in TA muscle were 4- and 3-fold higher (P < 0.05), respectively, in IM and RM vs. CON. The nuclear factor-κB (NF-κB) pathway activation was observed only after RM, but not after IM alone. These data indicate an increase in ROS generation during the initial phase of muscle RM that could activate the NF-κB pathway, and elicit inflammation and oxidative stress. These events may hinder muscle recovery from IM-induced mitochondrial deterioration and protein loss.

Exercise-induced hormesis and skeletal muscle health

Hormesis refers to the phenomenon that an exposure or repeated exposures of a toxin can elicit adaptive changes within the organism to resist to higher doses of toxin with reduced harm. Skeletal muscle shows considerable plasticity and adaptions in response to a single bout of acute exercise or chronic training, especially in antioxidant defense capacity and metabolic functions mainly due to remodeling of mitochondria. It has thus been hypothesized that contraction-induced production of reactive oxygen species (ROS) may stimulate the hormesis-like adaptations. Furthermore, there has been considerable evidence that select ROS such as hydrogen peroxide and nitric oxide, or even oxidatively degraded macromolecules, may serve as signaling molecules to stimulate such hermetic adaptations due to the activation of redox-sensitive signaling pathways. Recent research has highlighted the important role of nuclear factor (NF) κB, mitogen-activated protein kinase (MAPK), and peroxisome proliferator-activated receptor γ co-activator 1α (PGC-1α), along with other newly discovered signaling pathways, in some of the most vital biological functions such as mitochondrial biogenesis, antioxidant defense, inflammation, protein turnover, apoptosis, and autophagy. The inability of the cell to maintain proper redox signaling underlies mechanisms of biological aging, during which inflammatory and catabolic pathways prevail. Research evidence and mechanisms connecting exercise-induced hormesis and redox signaling are reviewed.

Exercise-Induced Hormesis May Help Healthy Aging

Hormesis plays a critical role in producing some major benefits derived from physical exercise. However whether these known cellular mechanisms are applicable to ameliorate age-related deterioration of muscle function is not entirely clear. The present communication proposes that antioxidant adaptation, the peroxisome proliferator-activated receptor gamma coactivator (PGC)-1α activated mitochondrial biogenesis, and eccentric contraction-induced, cytokine-propelled muscle inflammation could be important redox-sensitive pathways by which exercise-induced disturbance in oxidant-antioxidant hemeostasis may serve as a heretic stimulus to promote adaptations that help healthy aging and improve the quality of life.

PGC-1α overexpression via local transfection attenuates mitophagy pathway in muscle disuse atrophy

Loss of mitochondrial structural and functional integrity plays a critical role in the pathogenesis of muscle disuse atrophy. Peroxisome proliferator-activated receptor γ coactivator 1-α (PGC-1α) has been suggested to modulate autophagy-lysosome pathway (mitophagy) during muscle atrophy, but clear evidence is still lacking. In the current study, we tested the hypothesis that overexpression of PGC-1α via in vivo transfection would ameliorate mitophagy in mouse tibialis anterior muscle subjected to two weeks of immobilization (IM), followed by remobilization (RM). While mitochondrial biogenesis and antioxidant enzymes are decreased, all autophagic and mitophagic protein markers such as Beclin-1, Bnip3, PINK1, parkin, Mul1 and the LC3II/LC3I ratio were increased in IM-RM muscle together with activation of FoxO pathway. Overexpression of PGC-1α significantly increased mitochondrial DNA proliferation and oxidative enzyme activity, whereas it mitigated oxidative stress and mitochondrial ubiquination in IM-RM muscle. Protein contents of PINK1, parkin and Mul1 in mitochondria decreased by approximately 50% with PGC-1α treatment. Furthermore, PGC-1α overexpression suppressed FoxO1 and FoxO3 activation along with a decreased LC3II/LC3I ratio. Importantly, PGC-1α attenuated IM-RM-induced ubiquination and degradation of mitofusion protein Mfn2. Muscle apoptotic tendency, measured by Bax/Bcl2 ratio and caspase-3 activity, were elevated with IM-RM, but unaffected by PGC-1α. We conclude that overexpression of PGC-1α by in vivo transfection can inhibit activation of mitophagy pathway in the atrophying muscle caused by immobilization.

Training-induced mitochondrial adaptation: role of peroxisome proliferator-activated receptor γ coactivator-1α, nuclear factor-κB and β-blockade

•  What is the central question of this study? The peroxisome proliferator‐activated receptor γ coactivator‐1α (PGC‐1α) signalling pathway plays an important role in mitochondrial biogenesis and has been shown to be activated both by an acute bout of exercise and by long‐term training. However, the upstream signals and control mechanisms causing the adaptation and its interaction with other signalling pathways during exercise are not clear. •  What is the main finding and its importance? Our main finding was that PGC‐1α‐controlled mitochondrial training adaptation was attenuated by pyrolidine dithiocarbamate, an antioxidant known to block nuclear factor‐κB signalling, which indicates that PGC‐1α signalling is redox sensitive and can be influenced by nuclear factor‐κB. We also found that the β‐adrenergic blocker propranolol did not prevent the training‐induced adaptation of muscle mitochondrial protein under our experimental conditions.

PGC-1α overexpression by in vivo transfection attenuates mitochondrial deterioration of skeletal muscle caused by immobilization

Prolonged immobilization (IM) causes skeletal muscle atrophy characterized by mitochondrial deterioration and proteolysis. Muscle remobilization (RM) increases reactive oxygen species generation, proinflammatory cytokine expression, and oxidative stress, preventing muscle from quick recovery. Thus, we hypothesized that overexpression of peroxisome proliferator‐activated receptor γ coactivator 1‐α (PGG1α) via in vivo transfection would promote mitochondrial biogenesis and antioxidant defense, thus ameliorating the aforementioned deteriorations in a mouse model with 14‐d IM followed by 5‐d RM. PGC‐1α transfection in tibialis anterior muscle resulted in a 7.2‐ and 4‐fold increase in PGC‐1α content in cytosol and nucleus, respectively. Mitochondrial biogenic (cytochrome c, mitochondrial transcription factor A), morphologic (mitochondrial density, mDNA/ nDNA ratio), and functional (cytochrome c oxidase activity, ATP synthesis rate) markers, as well as fiber cross‐sectional area, significantly increased in IM‐RM muscle by PGC‐1α overexpression. These effects were accompanied by an 18% decrease in H2O2, 30% decrease in nuclear factor‐κB‐DNA binding, and 25% reduction of IL‐1β and‐6 production in IM‐RM muscle. There was a 34% increase in superoxide dismutase‐2 activity, along with a 3.5‐fold increase in NAD‐dependent deacetylase sirtuin‐3 expression caused by enhanced PGC‐1α‐estrogen‐related receptor a binding. Our findings highlighted the importance of PGC‐la in protecting muscle from metabolic and redox disturbances caused by IM.—Kang, C., Goodman, C. A., Horberger, T. A., Ji, L. L. PGC‐1α overexpression by in vivo transfection attenuates mitochondrial deterioration of skeletal muscle caused by immobilization. FASEB J. 29, 4092‐4106 (2015). www.fasebj.org

Role of PGC-1α in sarcopenia: Etiology and potential intervention - A mini-review

Sarcopenia is age-associated deterioration of muscle mass and function caused by a wide scope of physiological and pathological changes ranging from hormonal disorders to loss of subcellular homeostasis. Recent research indicates that mitochondrial dysregulation with advanced age plays a central role in the development of sarcopenia due to the multifactorial functions of this organelle in energy supply, redox regulation, crosstalk with nuclear gene expression and apoptosis. In order to fulfill these roles, it is crucial that mitochondria maintain their own structural and functional integrity through biogenesis, antioxidant defense, fusion/fission dynamics and autophagy (mitophagy). Unfortunately, mitochondria undergo age-associated changes that compromise the above-mentioned properties that eventually contribute to the development of sarcopenia. The peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) is involved in the transcriptional regulation of numerous nuclear and mitochondrial gene products participating in the cellular events that control muscle mass and function. Thus, it is not surprising that maintaining an optimal intracellular PGC-1α level and signaling activity is crucial in protecting the muscle from many degradative and destructive processes, such as proteolysis, oxidative damage, inflammation, uncontrolled autophagy and apoptosis. Physical exercise is a powerful stimulus to PGC-1α expression and signaling. Recent research indicates that PGC-1α-controlled mitochondrial biogenesis is not limited by old age per se and that elderly individuals can still benefit from increased muscular activity in terms of skeletal muscle health that ultimately contributes to quality of life in old age.