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Looking beyond PGC-1α: Emerging regulators of exercise-induced skeletal muscle mitochondrial biogenesis and their
activation by dietary compounds
Journal: Applied Physiology, Nutrition, and Metabolism
Manuscript ID apnm-2019-0069.R2
Manuscript Type: Review
Date Submitted by the Author: 23-May-2019
Complete List of Authors: Islam, Hashim; Queen's University, School of Kinesiology and Health StudiesHood, David; York University, Muscle Health Research Centre, School of Kinesiology and Health ScienceGurd, Brendon; Queens University, School of Kinesiology and Health Studies
Keyword: mitochondria, Nrf2, ERRγ, PPARβ, LRP130, sulforaphane, quercetin, epicatechin
Is the invited manuscript for consideration in a Special
Issue? :Not applicable (regular submission)
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Applied Physiology, Nutrition, and Metabolism
DraftLooking beyond PGC-1: Emerging regulators of exercise-induced skeletal muscle
mitochondrial biogenesis and their activation by dietary compounds
Hashim Islam1, David A. Hood2 and Brendon J. Gurd1
1 School of Kinesiology and Health Studies, Queen’s University, Kingston, Ontario, Canada2 Muscle Health Research Centre, School of Kinesiology and Health Science, York University,
Toronto, Ontario, Canada
Corresponding Author:
Brendon J. Gurd, PhD Telephone: 613-533-6000 x79023
Fax: 613-533-6000Email: gurdb@queensu.ca
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Abstract
Despite its widespread acceptance as the “master regulator” of mitochondrial biogenesis
(i.e. the expansion of the mitochondrial reticulum), peroxisome proliferator-activated receptor
(PPAR) gamma coactivator-1 alpha (PGC-1) appears to be dispensable for the training-induced
augmentation of skeletal muscle mitochondrial content and respiratory function. In fact, a number
of regulatory protein have emerged as important players in skeletal muscle mitochondrial
biogenesis and many of these protein share key attributes with PGC-1. In an effort to move past
the simplistic notion of a “master regulator” of mitochondrial biogenesis, we highlight the
regulatory mechanisms by which nuclear factor erythroid 2-related factor 2 (Nrf2), estrogen-
related receptor gamma (ERRγ), PPARβ and leucine-rich pentatricopeptide repeat-containing
protein (LRP130) may contribute to the control of skeletal muscle mitochondrial biogenesis. We
also present evidence supporting/refuting the ability of sulforaphane, quercetin, and epicatechin to
promote skeletal muscle mitochondrial biogenesis and their potential to augment mitochondrial
training adaptations. Targeted activation of specific pathways by these compounds may allow for
greater mechanistic insight into the molecular pathways controlling mitochondrial biogenesis in
human skeletal muscle. Dietary activation of mitochondrial biogenesis may also be useful in
clinical populations with basal reductions in mitochondrial protein content, enzyme activities,
and/or respiratory function as well as individuals who exhibit a blunted skeletal muscle
responsiveness to contractile activity.
Key points:
The existence of redundant pathways leading to mitochondrial biogenesis refutes the
simplistic notion of a “master regulator” of mitochondrial biogenesis.
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Dietary activation of specific pathways may provide greater mechanistic insight into
the exercise-induced mitochondrial biogenesis in human skeletal muscle.
Keywords: mitochondria, Nrf2, ERRγ, PPARβ, LRP130, sulforaphane, quercetin, epicatechin
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Introduction
Peroxisome proliferator-activated receptor (PPAR) gamma coactivator-1 alpha (PGC-1)
has gained widespread acceptance as the “master regulator” of skeletal muscle mitochondrial
biogenesis (Olesen et al. 2010; Fernandez-Marcos and Auwerx 2011; Scarpulla 2011). Although
several knockout and overexpression studies have revealed the importance of PGC-1 for the
maintenance of basal mitochondrial content, enzyme activities, and/or respiratory function (Lin et
al. 2005; Olesen et al. 2010; Scarpulla 2011), PGC-1 appears to be dispensable for training-
induced mitochondrial adaptations in rodent skeletal muscle (Hood et al. 2016). PGC-1’s non-
obligatory role in training-induced mitochondrial remodelling is perhaps unsurprising given the
complex nature of mitochondrial biogenesis, which involves the upregulated expression of two
genomes, and the subsequent synthesis, import, and assembly of newly formed proteins into the
mitochondrial reticulum (Hood et al. 2016). Therefore, the notion that a number of overlapping
and redundant pathways exist seems much more logical than the concept of a single “master
regulator” (Islam et al. 2018), particularly when considering the inherent redundancies in adaptive
pathways that defend homeostasis.
In an effort to move past the simplistic notion of a “master regulator” of mitochondrial
biogenesis and to provide avenues for future research, this review highlights several alternative
regulators of exercise-induced mitochondrial biogenesis. We also discuss a number of dietary
compounds that are believed to target some of these regulators and have the potential to augment
mitochondrial adaptations to exercise training. The latter discussion may be useful for studies
involving human skeletal muscle where targeted activation of a specific pathway could provide
greater mechanistic insight into the molecular regulation of mitochondrial biogenesis. Dietary
compounds that augment mitochondrial adaptations to training may also be of interest in clinical
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populations with basal mitochondrial defects (i.e. impairments in basal mitochondrial protein
content, morphology, enzyme activities, and/or respiratory function) and/or individuals who
exhibit blunted adaptive responses to exercise.
In the absence of a universally accepted definition, we have adopted the definition of
mitochondrial biogenesis proposed by Granata and coworkers (Granata et al. 2018) (i.e. “the
making of new components of the mitochondrial reticulum”) for the purpose of this paper. Thus,
transcriptional and post-transcriptional mechanisms that contribute to the expansion of the
mitochondrial reticulum (as reflected by an increase in mitochondrial protein content, volume
density, enzymatic activities, and/or mtDNA copy number) are presented to support/refute the
ability of a regulatory protein or dietary compound to promote mitochondrial biogenesis.
Emerging regulators of exercise-induced skeletal muscle mitochondrial biogenesis
Previously explored regulatory protein
Although we will highlight several emerging regulators of mitochondrial biogenesis in the
review below, the list of protein examined here is not comprehensive and many other regulatory
proteins are undoubtedly involved. For example, it is becoming increasingly clear that the tumour
suppressor protein p53 plays a key role in skeletal muscle remodelling in both rodents and humans
and several excellent reviews have been published on p53’s role in exercise-induced mitochondrial
biogenesis (Hood et al. 2016; Smiles and Camera 2018; Granata et al. 2018). Similarly, numerous
in vitro and ex vivo experiments have implicated sirtuin 1 (SIRT1) as a key mediator of
mitochondrial biogenesis due to its ability to deacetylate and activate nuclear PGC-1 (Gurd
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2011), though in vivo support for this paradigm is limited and largely contradictory to work
conducted in cell models (Gurd et al. 2012; Philp and Schenk 2013).
Unlike PGC-1, p53 and SIRT1, there is relatively limited information regarding the
regulatory mechanisms by which nuclear factor erythroid 2-related factor 2 (NFE2L2 or Nrf2),
estrogen-related receptor gamma (ERRγ), PPARβ and leucine-rich pentatricopeptide repeat-
containing protein (LRPPRC or LRP130) contribute to the control of mitochondrial biogenesis.
As such, it is the goal of this review is to discuss these relatively underexplored regulatory protein
in an attempt to continue and expand recent discussion of the intricacies of the mitochondrial
biogenic response to exercise. We chose to focus on these specific protein based on compelling
support for their involvement in the control of mitochondrial content and/or respiratory function
in animal and cell models, but a limited amount of evidence regarding their role in exercised human
skeletal muscle. Further, as several of the protein discussed in this article share key attributes with
PGC-1 (e.g. responsiveness to exercise and ability to coordinate the expression of nuclear- and
mitochondrial-encoded genes), they represent appropriate candidates for alternative and/or
redundant pathways by which exercise promotes mitochondrial biogenesis in skeletal muscle. We
hope the following review will promote future study of the protein discussed below thereby
pushing our understanding of exercise-induced skeletal muscle mitochondrial biogenesis forward.
Nuclear factor erythroid 2-related factor 2 (Nrf2)
I. Regulation
The transcription factor Nrf2 is a known regulator of cellular redox homeostasis and its
role in a variety of cellular processes has been investigated in various rodent models and cell lines
(see (Tebay et al. 2015) for an extensive review on this topic). Mechanisms of Nrf2 activation by
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upstream signals as well as its downstream actions are summarized in Figure 1. Basally, Nrf2’s
physical interaction with the cytosolic Kelch-like ECH-associated protein (Keap1) continuously
targets Nrf2 for proteasomal degradation (Tebay et al. 2015). During periods of cellular stress,
modifications of cysteine residues on Keap1 by reactive oxygen species (ROS) and nitric oxide
(NO) disrupt Keap1’s inhibitory interaction with Nrf2 resulting in the nuclear accumulation of
Nrf2 (Tebay et al. 2015). Once in the nucleus, Nrf2 heterodimerizes with small
musculoaponeurotic fibrosarcoma (Maf) proteins on the antioxidant response elements (ARE) of
target genes (Itoh et al. 1997) to upregulate antioxidant and detoxification enzyme gene expression
(Tebay et al. 2015) as well as its own expression (Kwak et al. 2002).
Nrf2 activity can also be modified independent of Keap1 via signalling kinases that are
activated by growth factors and intracellular stressors (Bryan et al. 2013; Tebay et al. 2015). For
instance, AMP-activated protein kinase (AMPK) directly phosphorylates Nrf2 in liver cells,
thereby promoting Nrf2’s nuclear accumulation and activation of ARE-driven gene expression
(Joo et al. 2016). Additionally, inhibition of the Nrf2 inhibitor glycogen synthase kinase-3 beta
(GSK-3β) by numerous other upstream kinases (see Figure 1) promotes Nrf2 stabilization in
various cell lines (Bryan et al. 2013; Tebay et al. 2015) as well as nuclear Nrf2 import and ARE
binding in murine cardiomyocytes (Piantadosi et al. 2008). Consistent with the ability of exercise
to activate a number of pathways involved in Nrf2 regulation (e.g. AMPK, MAPK, mTOR) (Egan
and Zierath 2013), increases in Nrf2 mRNA expression (and supposedly its activity (Kwak et al.
2002)) (Ballmann et al. 2014; Li et al. 2015; Merry and Ristow 2016), protein content (Wang et
al. 2016), and Nrf2-ARE binding (Crilly et al. 2016) have been observed in murine and/or human
skeletal muscle following acute exercise.
II. Control of mitochondrial biogenesis
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At present, the most compelling evidence for the involvement of Nrf2 in the control of
mitochondrial biogenesis is the observation that Nrf2 directly interacts with the nuclear respiratory
factor-1 (NRF-1) in mouse cardiac muscle (Piantadosi et al. 2008). NRF-1 is a transcription factor
that controls the expression of many nuclear-encoded mitochondrial genes, and the expression of
the mitochondrial transcription factors A (TFAM), B1 (TFB1M) and B2 (TFB2M) (Wu et al. 1999;
Gleyzer et al. 2005; Scarpulla 2008), allowing for the coordinated expression of nuclear- and
mitochondrial-encoded genes. Initial evidence supporting NRF-1 as a direct Nrf2 target came from
the observation that ROS production in murine cardiomyocytes results in the transcriptional
activation of mitochondrial biogenesis subsequent to an increase in Nrf2 binding to the NRF-1
promotor and the nuclear accumulation of NRF-1 protein (Piantadosi et al. 2008). Further, Nrf2
silencing in murine skeletal muscle and C2C12 cells prevents exercise- and ROS/NO – induced
increases in NRF-1 and TFAM mRNA expression (Merry and Ristow 2016). Nrf2 ablation also
prevents increases in PGC-1 and NRF-1 mRNA expression in liver tissue from mice challenged
with E. coli induced sepsis (Piantadosi et al. 2011) and blunts increases in NRF-1, TFAM and
PGC-1 protein content, mtDNA copy number, and citrate synthase (CS) activity in murine lung
tissue during pneumonia (Athale et al. 2012). Together, these findings support Nrf2’s role as a
mediator of mitochondrial biogenesis in transgenic animal and cell models in response to exercise
and oxidative/inflammatory stress.
III. Evidence for involvement in exercise-induced skeletal muscle mitochondrial biogenesis
Although Nrf2 is not required for the maintenance of basal mitochondrial content and
exercise capacity, mitochondrial respiration is impaired in skeletal muscle of whole body Nrf2
knockout mice (Crilly et al. 2016). Further, training-induced increases in skeletal muscle
mitochondrial content are blunted (Crilly et al. 2016) or completely abolished (Merry and Ristow
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2016) in Nrf2 knockout animals. Paradoxically, Nrf2 protein content decreases following training
in mice suggesting that Nrf2 may be more important for initiating mitochondrial biogenesis than
the maintenance of mitochondrial content (Crilly et al. 2016). However, it is also possible that
Nrf2 activity can increase independently from changes in protein content during training in a
fashion to similar to that of SIRT1 (Gurd et al. 2010) and p53 (Beyfuss et al. 2018). Also
noteworthy is the inverse relationship between tissue oxidative capacity and Nrf2 protein content
(tibialis anterior>soleus>heart) in mice (Crilly et al. 2016), which contrasts the tissue-specific
distribution of PGC-1 (Irrcher et al. 2003) but resembles that of SIRT1 (Gurd et al. 2009). Despite
a lack of data in exercised human skeletal muscle, nuclear Nrf2 accumulation in peripheral blood
mononuclear cells occurs in an exercise-intensity dependent manner in recreationally young males
(Done et al. 2017) and is impaired in sedentary older individuals (Done et al. 2016), highlighting
the potential impact of exercise intensity, fitness level, and/or age-group on Nrf2 activation
following exercise. Thus, preliminary evidence from training studies in mice supports Nrf2’s
importance for optimal skeletal muscle adaptation to exercise training, warranting the examination
of Nrf2’s role in the regulation of mitochondrial biogenesis in exercised human skeletal muscle.
IV. Summary
Taken together, the aforementioned studies provide evidence that: 1) Nrf2 is a stress-
responsive transcription factor that activates mitochondrial biogenesis by directly targeting NRF-
1 in mouse cardiac muscle; 2) Nrf2 is activated in rat and murine skeletal muscle in response to
acute exercise in a ROS/NO dependent manner; and 3) Nrf2 is required for basal mitochondrial
respiratory function and optimal mitochondrial adaptations to training in mouse skeletal muscle.
Importantly, the concept of NRF-1 as a common downstream target for both Nrf2 and PGC-1
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supports the existence of parallel pathways that converge on key transcription factors controlling
mitochondrial biogenesis.
Estrogen-related receptor gamma (ERR)
I. Regulation
ERR, the most recently discovered isoform of the ERR nuclear receptor family, is highly
expressed in metabolically active tissues including skeletal muscle (Narkar et al. 2011; Giguère
2008) and controls the expression of genes involved in various aspects of oxidative metabolism in
the mouse heart (Dufour et al. 2007; Alaynick et al. 2007). ERR’s transcriptional activity is
largely dependent on its interactions with various co-regulators that activate or repress ERR’s
transcriptional function (Giguère 2008). For instance, PGC-1 interacts with the activating
function (AF)-2 domain of the ERR protein in various cell models and is a major coactivator of
ERR - mediated transcription (Giguère 2008). On the other hand, receptor interacting protein 140
(RIP140) binding to the AF-2 domain represses ERR’s transcriptional activity on the promoters
of certain genes (Giguère 2008), but coactivates the ERR - mediated transcription of other genes
in HeLa cells (Castet et al. 2006). Intriguingly, ERR transactivates the PGC-1 promoter in
brown adipocytes (Wang et al. 2005), raising the possibility of an autoregulatory feedforward loop
where ERR activates its own transcriptional function via enhanced PGC-1 expression (Misra et
al. 2017). In addition to co-regulation, ERR’s activity is also modulated via post-translational
modifications by several upstream kinases in liver, kidney, and breast cancer cells (Misra et al.
2017), but it is presently unclear if ERR is phosphorylated directly within the nucleus or imported
into the nucleus after phosphorylation (see Figure 2).
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ERR gene expression is highly inducible in cellular models following increases in
intracellular stress and involves the binding of cAMP response element-binding protein (CREB),
hypoxia-inducible factor 1-alpha (HIF-1), and activating transcription factor 6-alpha (ATF-6)
to the ERR promoter (Misra et al. 2017). Consistent with the ability of exercise to activate several
upstream regulators of ERR (e.g. PGC-1, CREB, HIF-1) (Egan and Zierath 2013), ERR
mRNA expression is increased in murine skeletal muscle following exercise (Rangwala et al.
2010; Fan et al. 2018).
II. Control of mitochondrial biogenesis
Basal ERR gene expression parallels oxidative capacity in murine skeletal muscle
(Rangwala et al. 2010; Narkar et al. 2011) and whole-body ERR deletion in mice is postnatally
lethal due to defective oxidative phosphorylation (OXPHOS) in the heart (Alaynick et al. 2007).
Muscle-specific ERR overexpression in mice augments skeletal muscle mitochondrial protein
content, enzyme activities, and/or respiration (Rangwala et al. 2010; Badin et al. 2016) presumably
due to ERR’s ability to upregulate the expression of genes encoding mitochondrial transcription
factors, electron transport chain (ETC) proteins, and components of the mitochondrial import and
translational machineries in rodent heart and skeletal muscle (Dufour et al. 2007; Alaynick et al.
2007; Narkar et al. 2011; Fan et al. 2018). Importantly, ERR overexpression in muscle-specific
PGC-1/ knockout mice promotes mitochondrial biogenesis and significantly improves
reductions in ETC complex activities, mtDNA copy number, and mitochondrial protein content
associated with the loss of PGC-1/, likely due to a large overlap in PGC-1/ and ERR target
genes (Fan et al. 2018).
III. Evidence for involvement in exercise-induced skeletal muscle mitochondrial biogenesis
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In addition to the rescue of basal mitochondrial defects, overexpression of ERR in muscle-
specific PGC-1/ knockout mice augments training-induced increases in exercise performance,
skeletal muscle mitochondrial protein content, and OXPHOS transcript levels (Fan et al. 2018).
Although transgenic muscle-specific ERR overexpression robustly enhances basal exercise
performance in mice (Rangwala et al. 2010; Narkar et al. 2011; Fan et al. 2018), ERR
overexpression in skeletal muscle does not potentiate training adaptations in mice with normal
levels of PGC-1/ (Fan et al. 2018), raising the possibility that ERR’s role in exercise-induced
mitochondrial remodelling may become important only in the absence of other regulators of
mitochondrial biogenesis. Although intriguing, the relevance of this scenario in humans, where
PGC-1 and other regulators of mitochondrial biogenesis are present is not readily apparent but
appears to warrant further investigation.
IV. Summary
Evidence from the aforementioned rodent studies allows for the following conclusions: 1)
ERR is required for the maintenance of mitochondrial protein content, enzyme activities, and
respiration and its basal expression closely parallels skeletal muscle oxidative capacity; 2) ERR
is responsive to exercise and directly activates genes involved in multiple aspects of skeletal
muscle mitochondrial biogenesis; and 3) ERR can promote mitochondrial biogenesis in PGC-
1/ deficient muscle. Collectively, these findings highlight the functional redundancy of ERR
and PGC-1/ in skeletal muscle and implicate ERR as an additional player in the transcriptional
control of mitochondrial biogenesis.
Peroxisome proliferator-activated receptor-beta (PPAR)
I. Regulation
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The PPAR family of nuclear hormone receptors (PPAR, PPAR, and PPAR)
heterodimerize with retinoic acid receptors (RARs) and bind to PPAR response elements (PPRE)
on target genes predominantly involved in fat metabolism (Fan et al. 2013). In the absence of a
ligand (e.g. fatty acids), PPAR interaction with co-repressors (e.g. nuclear receptor corepressor 1;
cryptochrome 1/2) inhibits their transcriptional activity, whereas PPAR-ligand binding and/or
post-translational modification (Figure 3) leads to the recruitment of co-activators (e.g. PGC-1)
and the subsequent activation of target gene transcription in cultured muscle and non-muscle cells
and/or rodent skeletal muscle (Wang et al. 2003; Dressel et al. 2003; Perez-Schindler et al. 2012;
Jordan et al. 2017). Consistent with the regulation of other PPAR family members (Burns and
Vanden Heuvel 2007), experiments involving PPAR agonists in cultured human myoblasts
indicate that AMPK and MAPK signaling cascades are involved in the control of PPAR activity
(Krämer et al. 2005). In addition, stimulation of endogenous cyclic AMP production (Hansen et
al. 2001) and protein kinase A (PKA) activation (Lazennec et al. 2000) enhances basal and ligand-
dependent PPAR activation in various cell lines. Like ERR, it is not clear if the post-translational
modulation of PPAR’s activity is an exclusively nuclear event or if modifications occurring in
the cytosol induce nuclear translocation of PPARβ.
Although PPAR mRNA expression is exercise-inducible in human skeletal muscle (Watt
et al. 2004; Perry et al. 2010; Barrès et al. 2012), the upstream events that regulate PPAR gene
expression are not well-understood. Intriguingly, a model where PPAR activates its own
promoter has been proposed (Neels and Grimaldi 2014) and is consistent with the observation of
increased endogenous PPAR protein levels with ectopic PPAR expression in mouse skeletal
muscle (Koh et al. 2017). Support for the positive feedback regulation of PPAR expression
comes from recent experiments in C2C12 and embryonic kidney cells where PPAR and AMPK
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cooperatively activate the myocyte enhancer factor 2A (MEF2A), which then binds to the PPAR
promoter to increase its activity (Koh et al. 2019). Collectively, PPAR’s activity appears to be
modulated by various intracellular signaling cascades that are responsive to exercise (Egan and
Zierath 2013) and the increase PPAR mRNA expression (and presumably its activity (Koh et al.
2019)) following acute exercise supports its potential involvement in contractile-activity induced
activation of gene transcription.
II. Control of mitochondrial biogenesis
In murine skeletal muscle, transgenic muscle-specific PPAR overexpression promotes a
glycolytic-to-oxidative fibre-type shift and increases markers of mitochondrial content (Luquet et
al. 2003; Wang et al. 2004; Gan et al. 2011, 2013; Koh et al. 2017), whereas PPAR
knockdown/deletion is associated with reductions in PGC-1, NRF-1, TFAM, and ETC subunits
at the mRNA and/or protein level (Schuler et al. 2006; Koh et al. 2017). The control of
mitochondrial content by PPAR appears to be mediated by its transcriptional control of PGC-1
expression in C2C12 myocytes (Schuler et al. 2006) and its post-translational control of PGC-1
stability in murine skeletal muscle (Koh et al. 2017). However, the subcellular compartment where
the PPAR - PGC-1 interaction occurs is currently unclear. PPAR also cooperates with the
transcription factor Sp1 to increase SIRT1 promoter activity in cultured human liver cells (Okazaki
et al. 2010), thereby raising the possibility that enhanced SIRT1 expression is an additional
pathway by which PPAR modulates PGC-1 activity to promote mitochondrial biogenesis.
Recent experiments involving C2C12 muscle cells have also identified PPAR as a
transcription factor targeting NRF-1, providing a direct mechanism for PPAR-mediated induction
of mitochondrial biogenesis (Koh et al. 2017). Because NRF-1 is also a transcription factor for the
upstream AMPK activator, calcium calmodulin-dependent protein kinase kinase beta (CaMKK)
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(Koh et al. 2017), this PPAR - NRF-1 pathway may help explain the increased AMPK
phosphorylation observed with muscle-specific PPAR overexpression in mice (Gan et al. 2011;
Koh et al. 2017). Experiments in cell models also indicate that PPAR controls lactate
dehydrogenase B expression by forming a transcriptional activation complex with AMPK and
MEF2A (Gan et al. 2011), both of which are established regulators of PGC-1 (Fernandez-Marcos
and Auwerx 2011) and mitochondrial biogenesis (Scarpulla 2008, 2011; Ramachandran et al.
2008). Interestingly, PPAR expression in skeletal muscle is much greater than other PPAR
isoforms and is highest in oxidative muscle groups in mice (Wang et al. 2004). Taken together,
loss- and gain-of-function studies and the abundance of PPARβ in oxidative muscle highlight the
importance of PPAR in the control of skeletal muscle mitochondrial content through multiple
PGC-1 - dependent and independent pathways.
III. Evidence for involvement in exercise-induced skeletal muscle mitochondrial biogenesis
Transgenic muscle-specific PPAR overexpression (Gan et al. 2011; Fan et al. 2017) and
synthetic ligand-dependent PPAR activation (Narkar et al. 2008) robustly enhance exercise
performance in mice (Fan et al. 2017). In various rodent and/or human models, skeletal muscle
PPAR expression increases at the mRNA and protein level in response to acute (Watt et al. 2004;
Perry et al. 2010; Barrès et al. 2012) and chronic (Luquet et al. 2003; Koh et al. 2017) exercise,
respectively. The training-induced augmentation of PPAR protein content parallels increases in
mitochondrial protein content in rats (Koh et al. 2017) and humans (Perry et al. 2010), which
supports PPAR’s potential involvement in the adaptive response to exercise training, though
causative relationships between these variables have not been established. However, given the
importance of PPAR for the maintenance of basal mitochondrial content (Schuler et al. 2006;
Koh et al. 2017), it is reasonable to speculate that higher levels of PPAR protein and/or increased
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activity of existing PPAR protein would be required to sustain the increased demands associated
with a higher mitochondrial content after training. Consistent with the notion that PPAR activates
mitochondrial biogenesis through a dual mechanism involving PGC-1 and NRF-1, partial
knockdown of PPAR in rat skeletal muscle diminishes increases in PGC-1 and NRF-1 protein
content and markers of mitochondrial biogenesis following two weeks of swimming (Koh et al.
2017). Thus, preliminary evidence in rodents and humans supports the importance of PPAR in
skeletal muscle adaptation to training and further investigation of PPAR’s role in exercise-
induced mitochondrial biogenesis is warranted.
IV. Summary
Based on the evidence presented above, it can be concluded that: 1) PPAR is required for
the maintenance of basal mitochondrial content in murine skeletal muscle and its expression
pattern parallels skeletal muscle oxidative capacity; 2) PPAR induces mitochondrial biogenesis
through a dual mechanism involving PGC-1 and NRF-1, and also interacts with AMPK and
MEF2A to control genes involved in glucose oxidation as well as its own expression; and 3)
PPAR protein content increases following training in rodents and humans and is required for
optimal mitochondrial adaptations to training in rat skeletal muscle. It is also noteworthy that
PPAR’s ability to stimulate a fast-to-slow twitch fibre-type shift is partly dependent on ERR
(Gan et al. 2013), which (as discussed in in the previous section) directly targets genes involved
in various aspects of oxidative metabolism (Narkar et al. 2011) and can promote mitochondrial
biogenesis in PGC-1/ knockout mice (Fan et al. 2018). Collectively, PPAR’s ability to
promote oxidative remodelling in skeletal muscle both independently and cooperatively with other
putative regulators of mitochondrial content highlights its potential as a viable alternative regulator
of skeletal muscle mitochondrial biogenesis.
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Leucine-rich pentatricopeptide repeat motif-containing protein (LRP130)
I. Regulation
LRP130 is implicated in the control of mitochondrial-encoded gene expression, OXPHOS,
and fat oxidation in the liver (Cooper et al. 2006; Liu et al. 2014; Cuillerier et al. 2017). In murine
liver, LRP130 is deacetylated by sirtuin 3 (SIRT3), and is required for the SIRT3-mediated
induction of mitochondrial-encoded gene expression in response to fasting (Liu et al. 2014). In
addition, LRP130 complexes with nuclear PGC-1 in murine liver (Cooper et al. 2006), and PGC-
1/ knockdown reduces LRP130 mRNA expression in brown fat cells (Cooper et al. 2008)
implicating PGC-1/ as upstream regulators of LRP130 expression. Based on the presence of
several serine and threonine residues, LRP130 activity may also depend on phosphorylation by
ATP-dependent kinases (Hou et al. 1994), though direct evidence supporting this notion does not
presently exist. Nevertheless, LRP130’s interactions with SIRT3 and PGC-1 raises the possibility
that LRP130’s activity and/or expression may be modulated by upstream signals that are sensitive
to contraction-induced changes in the intracellular milieu (Figure 4).
II. Control of mitochondrial biogenesis
LRP130 is highly expressed in metabolically active tissues with a high mitochondrial
content (e.g. heart, brown adipose tissue, skeletal muscle) and its tissue-specific distribution
parallels that of PGC-1 in mice (Cooper et al. 2008). Although a predominantly inner
mitochondrial protein (Sasarman et al. 2015), in vitro experiments demonstrate that LRP130 is
also present in nuclear fractions and promotes mRNA stability via direct RNA-binding in the
nucleus (Mili and Piñol-Roma 2003) or through its interaction with the SRA stem-loop-interacting
RNA-binding protein (SLIRP) in the mitochondria of human fibroblasts (Sasarman et al. 2010)
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and HeLa cells (Chujo et al. 2012). LRP130 also appears to control mitochondrial-encoded gene
transcription in liver cells via direct interactions with mitochondrial RNA polymerase (POLRMT)
(Liu et al. 2011, 2014) but this aspect of LRP130’s function has been questioned (Harmel et al.
2013).
LRP130 deletion in transgenic animal and cell models leads to profound reductions in the
basal expression of nearly all mitochondrial-encoded genes in the liver (Cooper et al. 2006, 2008;
Liu et al. 2014; Cuillerier et al. 2017), whereas nuclear-encoded gene expression is affected to a
lesser extent (Sasarman et al. 2010; Nam et al. 2017). The assembly and activity of several
OXPHOS complexes is also severely impaired following LRP130 deletion resulting in large
reductions in mitochondrial enzyme activities and respiration rates in various tissues (e.g. heart,
liver, BAT) (Mourier et al. 2014; Nam et al. 2017; Cuillerier et al. 2017). These impairments are
consistent with the OXPHOS complex assembly/activity defects observed in human skeletal
muscle from individuals with mitochondrial disorders that are characterized by reduced steady-
state levels of LRP130 (e.g. French-Canadian Leigh Syndrome) (Sasarman et al. 2015; Oláhová et
al. 2015). Although LRP130 loss- and gain-of-function in the aforementioned studies induces
marked changes in mitochondrial gene expression, function, and morphology (e.g. cristae density,
super-complex formation) (Liu et al. 2011; Mourier et al. 2014; Cuillerier et al. 2017) indices of
mitochondrial content appear to be unaffected (Cooper et al. 2008; Liu et al. 2011; Nam et al.
2017). Thus, LRP130 appears to contribute to the maintenance of basal mitochondrial enzyme
activities, respiration, and morphology, presumably due its transcriptional and post-transcriptional
control of mitochondrial gene expression.
III. Evidence for involvement in exercise-induced skeletal muscle mitochondrial biogenesis
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To date, only two studies have investigated LRP130’s role in skeletal muscle adaptations
to exercise (Vechetti-Junior et al. 2016; Edgett et al. 2016). In rat skeletal muscle, endurance
exercise promotes a coordinated increase in LRP130 and PGC-1 protein expression following
short-term immobilization (Vechetti-Junior et al. 2016) though markers of mitochondrial content
were not assessed in this study. On the other hand, LRP130 protein expression remains unchanged
in human skeletal muscle in response to 2-6 weeks of sprint interval training (SIT) despite a
trained-induced increase in succinate dehydrogenase activity (Edgett et al. 2016). Despite the lack
of a systematic increase in LRP130 protein, training-induced changes in LRP130, SIRT3 and PGC-
1 protein are positively correlated in human skeletal muscle (Edgett et al. 2016), supporting the
possibility of an interaction between these proteins (Cooper et al. 2006; Liu et al. 2014). Although
speculative, it is possible that changes in LRP130 activity and/or subcellular localization may
occur following exercise without changes in whole-muscle LRP130 protein content or that
LRP130 may be more important for the maintenance of basal mitochondrial enzyme activities
and/or respiratory function rather than exercise-induced mitochondrial remodelling. Nevertheless,
the lack of evidence presently available makes it difficult to support or refute LRP130’s
involvement in skeletal muscle adaptations to exercise particularly in relation to markers of
mitochondrial biogenesis, highlighting the need for future work in this area.
IV. Summary
Based on the evidence presented from various transgenic animal/cell models and/or studies
involving patients with LRP130-related mitochondrial disorders, it can be concluded that: 1)
LRP130 interacts with SIRT3 and PGC-1 in the liver to control mitochondrial gene expression
and OXPHOS activity; 2) LRP130 controls mitochondrial-encoded gene expression via
transcriptional and post-transcriptional mechanisms; and 3) LRP130 is required for the
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maintenance of mitochondrial morphology, enzyme activities, and respiratory function but appears
to be less important in the control of mitochondrial content. Although compelling, significant
research is warranted to confirm these findings in healthy humans and to determine if LRP130 is
in fact an additional player in exercise-induced skeletal muscle mitochondrial biogenesis.
Dietary activators of skeletal muscle mitochondrial biogenesis
The in vitro and transgenic animal models discussed thus far have provided valuable insight
into the molecular pathways controlling mitochondrial biogenesis. However, as complete loss- or
gain- of function is not possible in humans, dietary compounds that activate specific pathways may
allow for greater mechanistic insight into exercise-induced remodelling of skeletal muscle
mitochondria. Further, as the phenotypic changes associated with the knockdown/overexpression
of a specific protein are far more pronounced than what occurs within exercised human skeletal
muscle in vivo, targeted activation of specific pathways through dietary agents may be a more
realistic approach for understanding the roles of regulatory protein involved in mitochondrial
biogenesis. Dietary agents that amplify signaling responses to acute exercise and/or augment
skeletal muscle adaptations to training without yielding undesirable off-target effects may also be
beneficial for individuals with baseline mitochondrial defects (e.g. individuals with mitochondrial
disorders or cardiometabolic disease) and/or blunted skeletal muscle responses to contractile
activity (e.g. elderly individuals). As such, the following section highlights dietary compounds
that may be useful for examining the importance of some of the regulatory protein discussed above
in human skeletal muscle and/or augmenting mitochondrial adaptations to training.
Sulforaphane
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The isothiocyanate sulforaphane is derived from cruciferous vegetables and has the
potential to activate Nrf2 via its known ability to modify cysteine residues on Keap1 (Dinkova-
Kostova et al. 2002). Consistent with the concept of a Nrf2 – NRF-1 pathway (Piantadosi et al.
2008), sulforaphane treatment upregulates NRF-1 and TFAM mRNA expression and increases
indices of mitochondrial content in vitro and ex vivo (Whitman et al. 2013; Fernandes et al. 2015;
Zhang et al. 2016; Lei et al. 2018). In rat hepatoma cells (Axelsson et al. 2017) and/or murine
skeletal muscle (Sun et al. 2015; Oh et al. 2017), the induction of mitochondrial biogenesis by
sulforaphane is accompanied by an increase in Nrf2 nuclear import and activation, and is abolished
with NRF-1 knockdown (Negrette-Guzmán et al. 2017), thereby supporting Nrf2 as a primary
transducer of sulforaphane’s effects on the mitochondria.
Although the in vivo effects of sulforaphane on mitochondrial biogenesis remain largely
unexplored, sulforaphane administration to rodents enhances exercise performance and increases
skeletal muscle AMPK phosphorylation and Nrf2 mRNA/protein expression (Malaguti et al. 2009;
Sun et al. 2015; Oh et al. 2017). However, in contrast to in vitro work, skeletal muscle PGC-1,
NRF-1, and TFAM protein content and mtDNA copy number remain unchanged in sulforaphane
treated mice (Oh et al. 2017). Presently, information regarding sulforaphane-induced skeletal
muscle remodelling is extremely limited and, to our knowledge, the impact of sulforaphane on
exercise-induced mitochondrial biogenesis in human skeletal has not been examined. However,
the apparent safety of long-term sulforaphane supplementation in humans (Shapiro et al. 2006)
and its ability to directly activate Nrf2 underscores the importance of future work examining the
impact of sulforaphane on mitochondrial remodelling in human skeletal muscle.
Quercetin
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The polyphenol quercetin is abundant in many fruits and vegetables, making it one of the
main dietary flavonoids (Boots et al. 2008). In humans, quercetin supplementation promotes small
but significant improvements in exercise capacity (Kressler et al. 2011) and the performance
benefits of quercetin appear to be more pronounced than other polyphenol supplements
(Somerville et al. 2017). In addition to its performance benefits, quercetin treatment increases
AMPK phosphorylation, PGC-1, SIRT1, NRF-1/2, TFAM mRNA expression and/or protein
content, and indices of mitochondrial content in cultured dopaminergic neuronal cells (Ay et al.
2017), hepatocytes (Rayamajhi et al. 2013), and chondrocytes (Qiu et al. 2018). Mechanistically,
the induction of mitochondrial biogenesis by quercetin appears to be dependent on Nrf2, as a single
dose of quercetin promotes Nrf2 nuclear import in mouse brain (Li et al. 2016) and increases in
oxidative gene expression with quercetin treatment are abolished with Nrf2 knockdown in murine
hepatocytes (Kim et al. 2015). However, as little is presently known about quercetin’s impact on
the other emerging regulators of mitochondrial biogenesis discussed above, it is entirely possible
that quercetin – mediated mitochondrial biogenesis also involves the activation of additional
regulatory protein.
Evidence for the ability of quercetin to augment mitochondrial biogenesis in vivo,
particularly when combined with exercise, is currently limited and inconclusive. Although
quercetin supplementation increases PGC-1 and SIRT1 mRNA expression, and mitochondrial
content in untrained rodent skeletal muscle (Davis et al. 2009; Casuso et al. 2014), training-induced
increases in these variables are blunted with quercetin supplementation versus exercise alone
(Casuso et al. 2014). In humans, 2 weeks of quercetin supplementation (1 g/day) does not alter
mitochondrial transcript levels in trained skeletal muscle (Nieman et al. 2009), but does improve
these indices along with markers of mitochondrial content in untrained individuals (Nieman et al.
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2010). Taken together, evidence from cellular and transgenic animal models supports the ability
of quercetin to induce mitochondrial biogenesis in multiple tissues, but its effects in human skeletal
muscle and in conjunction with exercise are presently unclear.
Epicatechin
Although not as extensively studied as quercetin, a growing body of literature also supports
the efficacy of epicatechin, the primary flavanol found in dark chocolate, for inducing
mitochondrial biogenesis in various tissues. Epicatechin treatment in vitro or supplementation in
vivo increases the expression of key signaling proteins (AMPK, p38 MAPK, and SIRT1),
transcription factors and co-activators (PGC-1, MEF2A, NRF-1/2, and TFAM), mitochondrial
proteins (ETC/OXPHOS, mitofilin, porin) and/or indices of mitochondrial content/morphology
(CS activity, volume density, cristate abundance) in endothelial cells (Ramirez-Sanchez et al.
2010; Moreno-Ulloa et al. 2013), adipocytes (Varela et al. 2017), myotubes (Moreno-Ulloa et al.
2018) and/or rodent tissues (e.g. heart, brain, kidney, skeletal muscle) (Nogueira et al. 2011;
Hüttemann et al. 2013; Lee et al. 2015; Moreno-Ulloa et al. 2015). Similar effects are also observed
in skeletal muscle from sedentary humans (Taub et al. 2016), type 2 diabetics, and heart failure
patients (Taub et al. 2012, 2013) administered 100 mg of epicatechin orally for 3 months.
A limited amount of evidence also points to epicatechin’s ability to augment skeletal
muscle adaptations to training (Nogueira et al. 2011; Lee et al. 2015), though this is not a universal
finding (Schwarz et al. 2018). For example, epicatechin supplemented mice subjected to an
exercise program exhibit greater increases in exercise performance, skeletal muscle TFAM and
mitochondrial protein expression, and indices of mitochondrial content compared to trained mice
in the placebo group (Nogueira et al. 2011; Lee et al. 2015). On the other hand, a recent study
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reported that training-induced increases in aerobic fitness and skeletal muscle succinate
dehydrogenase (SDH) protein content are blunted in human participants administered 200 mg
epicatechin daily during a 4-week cycling program (Schwarz et al. 2018). Therefore, while a
relatively larger body of literature supports a favorable impact of epicatechin on skeletal muscle
mitochondrial biogenesis when compared to quercetin, much of the available evidence comes from
animal and cell models. Further, the exact mechanisms by which epicatechin activates
mitochondrial biogenesis are unclear, though NO-dependent pathways have been implicated
(Ramirez-Sanchez et al. 2010; Taub et al. 2012; Moreno-Ulloa et al. 2013) raising the possibility
of Nrf2 involvement (see Figure 1).
Conclusions and future directions
Since the first in vivo observation of skeletal muscle mitochondrial biogenesis in trained
rats (Holloszy 1967), significant advancements have been made in our understanding of the
molecular pathways that underpin this adaptive response (Perry and Hawley 2018). The discovery
of the transcriptional co-activator PGC-1 (Puigserver et al. 1998) was a major breakthrough and
a significant body of research has subsequently examined its role in mitochondrial biogenesis
(Olesen et al. 2010; Fernandez-Marcos and Auwerx 2011; Islam et al. 2018). In light of recent
work, we have highlighted several additional players with potential roles in the molecular
regulation of exercise-induced mitochondrial biogenesis. Indeed, many of these regulatory
proteins share key attributes with PGC-1, including the ability to respond to stress-activated
signaling cascades activated by exercise and the ability to upregulate the expression of nuclear-
and mitochondrial-encoded genes encoding various mitochondrial proteins (Figure 5). However,
given that the majority of the currently available evidence is derived from in vitro and/or rodent
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models, the extension of these findings to in vivo models, particularly human skeletal muscle, is
an important direction for future research. Further, as much of the available evidence is based on
transcriptional responses (i.e. changes in mRNA expression), the findings presented here should
be interpreted with caution as changes in mRNA expression do not always translate to alterations
in protein content and/or muscle phenotype (Miller et al. 2016; Robinson et al. 2017). Thus, a
greater emphasis must be placed on regulatory events distal to transcriptional/post-transcriptional
processes in future studies examining the importance of a protein or dietary compound for
promoting mitochondrial biogenesis.
Based on preliminary evidence from rodent studies highlighting Nrf2’s importance for the
transcriptional activation of mitochondrial biogenesis following acute exercise and optimal
mitochondrial remodelling following training (Crilly et al. 2016; Merry and Ristow 2016), the
examination of Nrf2’s role in the regulation of mitochondrial biogenesis in exercised human
skeletal muscle is a logical next step for future work. Further, the potential impact of various
exercise parameters (e.g. intensity, mode, duration), participant fitness level, and/or age-group on
Nrf2 activation should also be explored further given the influence of these factors on Nrf2
activation in human peripheral blood mononuclear cells (Done et al. 2016, 2017).
Similar to Nrf2, mechanistic evidence supporting the involvement of ERR and PPAR in
exercise-induced mitochondrial biogenesis in human skeletal muscle is virtually non-existent and
should be examined. Importantly, the ambiguity surrounding the subcellular localization of
functional interactions involving these proteins (e.g. PPAR - mediated stabilization of PGC-1)
and their post-translational regulation should be addressed in future experiments involving cellular
and/or transgenic animal models and if possible human tissue. Finally, the preliminary work
examining the impact of SIT on LRP130 expression in human skeletal muscle (Edgett et al. 2016)
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should be extended to include other exercise protocols that elicit robust increases in mitochondrial
biogenic markers and/or additional tissue sampling time-points to discern if LRP130 is indeed
involved in the exercise-induced remodelling of human skeletal muscle mitochondria. Relatedly,
the determination of LRP130’s fibre-specific distribution pattern and subcellular localization in
relation to markers of mitochondrial content, morphology, and/or respiratory function under basal
and exercised conditions will provide additional insight into LRP130’s role in healthy human
skeletal muscle. Collectively, these avenues should help provide a more integrative view of the
molecular underpinnings of mitochondrial remodelling following exercise.
Although several dietary compounds presented here appear to have a favorable impact on
indices of mitochondrial content based on preliminary work in vitro, their efficacy remains to be
tested in vivo, particularly in human skeletal muscle. Further, the precise molecular pathways that
mediate the effects of these compounds in vivo remain largely unknown providing an important
area for future work. Elucidation of these mechanisms may also help explain some of the
discrepancies in the literature, such as quercetin effectiveness for promoting mitochondrial
biogenesis in untrained (Nieman et al. 2010) but not trained (Nieman et al. 2009) human skeletal
muscle. Similarly, epicatechin’s potential to enhance the adaptive response to training should be
explored further in humans of varying levels of fitness levels, as existing human studies have
involved sedentary (Taub et al. 2016), untrained (Schwarz et al. 2018), and/or clinical (Taub et al.
2012, 2013) populations. Importantly, the apparent safety of these compounds for oral
administration in humans provides an excellent opportunity to further explore their acute and
chronic effects on skeletal muscle remodelling in vivo. Indeed, there are many previous examples
of compounds that induce beneficial effects in vitro yet offer little to no benefits when applied to
a human model. Perhaps one of the best-known examples of this is resveratrol, a purported SIRT1
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activator that potently induces mitochondrial biogenesis in animal/cell models, (Hood et al. 2016),
but fails to enhance (and may even impair) training adaptations in human skeletal muscle
(Williams et al. 2014; Olesen et al. 2014; Scribbans et al. 2014). Thus, verification of the evidence
presented from in vitro and rodent models in human studies is critical to determine the utility of
these dietary compounds and their potential to advance our current understanding of the molecular
regulation of mitochondrial biogenesis in skeletal muscle.
Conflict of interest
None.
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Figure captions
Figure 1. Regulation of Nrf2 nuclear import and Nrf2 – mediated induction of nuclear-encoded
gene transcription. Disruption of Nrf2’s inhibitory interaction with Keap1 by ROS and NO, direct
phosphorylation of Nrf2 by AMPK, and inactivating phosphorylation of the Nrf2 inhibitor GSK-
3β by AMPK, MAPK, PI3K/Akt, and mTOR promotes translocation of Nrf2 from the cytosol to
the nucleus. Nuclear Nrf2 forms an obligatory heterodimer with small MAF proteins to bind AREs
on the promoters of genes encoding antioxidant and detoxification enzymes, Nrf2 protein, and
NRF-1. Note: Akt; protein kinase B: AMP: adenosine monophosphate; AMPK: AMP activated
protein kinase; ARE: antioxidant response element; GSK-3β: glycogen synthase kinase-3 beta;
Keap1: Kelch-like ECH-associated protein; MAF: small musculoaponeurotic fibrosarcoma
proteins; MAPK: mitogen-activated protein kinases; mTOR: mammalian target of rapamycin; NO:
nitric oxide; NRF-1: nuclear respiratory factor-1; Nrf2: nuclear factor erythroid 2-related factor 2;
PI3K: phosphoinositide 3-kinase; ROS: reactive oxygen species.
Figure 2. Post-translational control of ERRγ protein activity, transcriptional activation
of ERRγ gene expression, and ERRγ – mediated induction of nuclear-encoded gene transcription.
Phosphorylation of ERRγ by ERK/MAPK (and potentially by additional exercise-related
signalling proteins) increases the transcriptional activity of ERRγ protein whereas inactivating
phosphorylation by Akt promotes nuclear export of ERRγ. In the nucleus, ERRγ forms a
transcriptional activation complex with PGC-1α and other co-activators to binds ERREs on the
promoters of genes encoding NEMPs and PGC-1α. The ERRγ gene itself is highly inducible and
responsive to various exercise-related stimuli (e.g. ER stress, hypoxia, cAMP production). Note:
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Dotted lines indicate uncertainty regarding the subcellular compartment where the post-
translational modification depicted occurs. Akt: protein kinase B; ATF-6α: activating transcription
factor 6-alpha; cAMP: cyclic adenosine monophosphate; CREB: cAMP response element-binding
protein; ER: endoplasmic reticulum; ERK: extracellular-signal regulated kinases; ERRγ: estrogen-
related receptor gamma; ERRE: estrogen related receptor response element; HIF-1α: hypoxia-
inducible factor 1-alpha: MAPK: mitogen activated protein kinases; NEMPs: nuclear-encoded
mitochondrial proteins; PGC-1α: peroxisome proliferator- activated receptor gamma coactivator-
1 alpha; PKA: protein kinase A; TC: transcriptional co-activator.
Figure 3. Post-translational control of PPARβ protein activity and PPARβ - mediated induction
of nuclear- encoded gene transcription. Phosphorylation of PPARβ protein by AMPK, ERK1/2,
p38 MAPK, and PKA increases PPARβ’s transcriptional activity. In the nucleus, PPARβ
heterodimerizes with RARs to bind PPREs on the promoters of genes encoding PGC-1α, NRF-1,
NEMPs. Cooperation between PPARβ, AMPK, and MEF2A leads to an increase in the expression
of LDHB and PPARβ itself, whereas Sp1 mediates the PPARβ - induced increase in SIRT1 gene
expression. The physical interaction between PPARβ and PGC-1α stabilizes and protects PGC-1α
from ubiquitination and proteasomal degradation. Note: Dotted lines indicate uncertainty
surrounding the subcellular compartment where the post-translational modification and/or protein-
protein interaction depicted occurs. AMP: adenosine monophosphate; AMPK: AMP activated
protein kinase; ERK1/2: extracellular-signal regulated kinase 1/2; LDHB: lactate dehydrogenase
B; MEF2A: myocyte enhancer factor 2A; NEMPs: nuclear-encode mitochondrial proteins; NRF-
1: nuclear respiratory factor-1; PGC-1α: peroxisome proliferator-activated receptor gamma
coactivator-1 alpha; PPARβ: peroxisome proliferator-activated receptor beta; PKA: protein kinase
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A; PPRE: PPAR response element; RAR: retinoic acid receptor; ROS: reactive oxygen species;
SIRT1: sirtuin 1; Sp1: transcription factor Sp1.
Figure 4. Post-translational control of LRP130 protein activity, transcriptional activation of
LRP130 gene expression, and LRP130 actions in the nucleus and mitochondria. In the nucleus,
LRP130 complexes with PGC-1α to regulate PGC-1α’s co-activator activity and knockdown of
PGC-1α/β reduces LRP130 gene expression. In the mitochondria, LRP130 is deacetylated by
SIRT3 and interacts with POLRMT. LRP130 stabilizes mRNAs via direct RNA binding in the
nucleus or as part of a ribonucleoprotein complex with SLIRP in the mitochondria. Note: PGC-
1α/β: peroxisome proliferator- activated receptor gamma coactivator-1 alpha/beta; MEPs:
mitochondrial-encoded proteins; NEMPs: nuclear-encoded mitochondrial proteins; POLRMT:
mitochondrial RNA polymerase; SIRT3: sirtuin 3; SLIRP: SRA stem-loop-interacting RNA-
binding protein; TF: transcription factor.
Figure 5. Redundancies in the transcriptional and post-transcriptional control of mitochondrial
biogenesis by PGC-1α, Nrf2, ERRγ, PPARβ, and LRP130. Note: See Figure legends 1-4 and
manuscript text for abbreviation definitions. Dotted lines indicate uncertainty surrounding the
subcellular compartment where the post-translational modification and/or protein-protein
interaction depicted occurs.
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Figure 1. Regulation of Nrf2 nuclear import and Nrf2 – mediated induction of nuclear-encoded gene transcription. Disruption of Nrf2’s inhibitor interaction with Keap1 by ROS and NO, direct phosphorylation of Nrf2 by AMPK, and inactivating phosphorylation of the Nrf2 inhibitor GSK-3β by AMPK, MAPK, PI3K/Akt, and
mTOR promotes translocation of Nrf2 from the cytosol to the nucleus. Nuclear Nrf2 forms an obligatory heterodimer with small Maf proteins to bind AREs on the promoters of genes encoding antioxidant and
detoxification enzymes, Nrf2 protein, and NRF-1. Note: Akt; protein kinase B: AMP: adenosine monophosphate; AMPK: AMP activated protein kinase; ARE: antioxidant response element; GSK-3β:
glycogen synthase kinase-3 beta; Keap1: Kelch-like ECH-associated protein; MAF: small musculoaponeurotic fibrosarcoma proteins; MAPK: mitogen-activated protein kinases; mTOR: mammalian target of rapamycin;
NO: nitric oxide; NRF-1: nuclear respiratory factor-1; Nrf2: nuclear factor erythroid 2-related factor 2; PI3K: phosphoinositide 3-kinase; ROS: reactive oxygen species.
337x189mm (300 x 300 DPI)
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Figure 2. Post-translational control of ERRγ protein activity, transcriptional activation of ERRγ gene expression, and ERRγ – mediated induction of nuclear-encoded gene transcription. Phosphorylation of ERRγ by ERK/MAPK (and potentially by additional exercise-related signalling proteins) increases the transcriptional
activity of ERRγ protein whereas inactivating phosphorylation by Akt promotes nuclear export of ERRγ. In the nucleus, ERRγ forms a transcriptional activation complex with PGC-1α and other co-activators to binds
ERREs on the promoters of genes encoding NEMPs and PGC-1α. The ERRγ gene itself is highly inducible and responsive to various exercise-related stimuli (e.g. ER stress, hypoxia, cAMP production). Note: Dotted lines
indicate uncertainty regarding the subcellular compartment where the post-translational modification depicted occurs. Akt: protein kinase B; ATF-6α: activating transcription factor 6-alpha; cAMP: cyclic
adenosine monophosphate; CREB: cAMP response element-binding protein; ER: endoplasmic reticulum; ERK: extracellular-signal regulated kinases; ERRγ: estrogen-related receptor gamma; ERRE: estrogen
related receptor response element; HIF-1α: hypoxia- inducible factor 1-alpha: MAPK: mitogen activated protein kinases; NEMPs: nuclear-encoded mitochondrial proteins; PGC-1α: peroxisome proliferator-
activated receptor gamma coactivator-1 alpha; PKA: protein kinase A; TC: transcriptional co-activator.
337x189mm (300 x 300 DPI)
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Figure 3. Post-translational control of PPARβ protein activity and PPARβ - mediated induction of nuclear- encoded gene transcription. Phosphorylation of PPARβ protein by AMPK, ERK1/2, p38 MAPK, and PKA
increases PPARβ’s transcriptional activity. In the nucleus, PPARβ heterodimerizes with RARs to bind PPREs on the promoters of genes encoding PGC-1α, NRF-1, NEMPs. Cooperation between PPARβ, AMPK, and MEF2A leads to an increase in the expression of LDHB and PPARβ itself, whereas Sp1 mediates the PPARβ - induced
increase in SIRT1 gene expression. The physical interaction between PPARβ and PGC-1α stabilizes and protects PGC-1α from ubiquitination and proteasomal degradation. Note: Dotted lines indicate uncertainty surrounding the subcellular compartment where the post-translational modification and/or protein-protein
interaction depicted occurs. AMP: adenosine monophosphate; AMPK: AMP activated protein kinase; ERK1/2: extracellular-signal regulated kinase 1/2; LDHB: lactate dehydrogenase B; MEF2A: myocyte enhancer factor
2A; NEMPs: nuclear-encode mitochondrial proteins; NRF-1: nuclear respiratory factor-1; PGC-1α: peroxisome proliferator-activated receptor gamma coactivator-1 alpha; PPARβ: peroxisome proliferator-
activated receptor beta; PKA: protein kinase A; PPRE: PPAR response element; RAR: retinoic acid receptor; ROS: reactive oxygen species; SIRT1: sirtuin 1; Sp1: transcription factor Sp1.
337x189mm (300 x 300 DPI)
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Figure 4. Post-translational control of LRP130 protein activity, transcriptional activation of LRP130 gene expression, and LRP130 actions in the nucleus and mitochondria. In the nucleus, LRP130 complexes with
PGC-1α to regulate PGC-1α’s co-activator activity and knockdown of PGC-1α/β reduces LRP130 gene expression. In the mitochondria, LRP130 is deacetylated by SIRT3 and interacts with POLRMT. LRP130
stabilizes mRNAs via direct RNA binding in the nucleus or as part of a ribonucleoprotein complex with SLIRP in the mitochondria. Note: PGC-1α/β: peroxisome proliferator- activated receptor gamma coactivator-1 alpha/beta; MEPs: mitochondrial-encoded proteins; NEMPs: nuclear-encoded mitochondrial proteins;
POLRMT: mitochondrial RNA polymerase; SIRT3: sirtuin 3; SLIRP: SRA stem-loop-interacting RNA-binding protein; TF: transcription factor.
337x189mm (300 x 300 DPI)
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Figure 5. Redundancies in the transcriptional and post-transcriptional control of mitochondrial biogenesis by PGC-1α, Nrf2, ERRγ, PPARβ, and LRP130. Note: See Figure legends 1-4 and manuscript text for abbreviation
definitions. Dotted lines indicate uncertainty surrounding the subcellular compartment where the post-translational modification and/or protein-protein interaction depicted occurs.
337x189mm (300 x 300 DPI)
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