Role of myokines in the maintenance of whole body metabolic homeostasis pdf

MINERVA ENDOCRINOLOGICA
EDIZIONI MINERVA MEDICA
This provisional PDF corresponds to the article as it appeared upon acceptance.
A copyedited and fully formatted version will be made available soon.
The final version may contain major or minor changes.

Role of myokines in the maintenance of whole-body
metabolic homeostasis
Tatiana Y KOSTROMINOVA
Minerva Endocrinol 2016 Mar 22 [Epub ahead of print]

MINERVA ENDOCRINOLOGICA
Rivista sulle Malattie del Sistema Endocrino
pISSN 0391-1977 - eISSN 1827-1634
Article type: Review Article
The online version of this article is located at

Subscription: Information about subscribing to Minerva Medica journals is online at:
/>Reprints and permissions: For information about reprints and permissions send an email to:
- -

COPYRIGHT© 2016 EDIZIONI MINERVA MEDICA


Role of myokines in the maintenance of whole-body metabolic homeostasis

Tatiana Y. Kostrominova

Department of Anatomy and Cell Biology, Indiana University School of
Medicine-Northwest, Gary, IN, USA.

Corresponding author: Tatiana Y. Kostrominova
Department of Anatomy and Cell Biology, Indiana University School of MedicineNorthwest, Gary, IN, USA.
E-mail:

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

1


ABSTRACT
Obesity is reaching epidemic proportions in developed countries and is on the
rise in developing countries. Obesity-related changes in lipid and glucose
metabolism predispose to the development of metabolic syndrome and type 2
diabetes. Skeletal muscle constitutes about 40 percent of total body weight and is
unique compared to other muscle types since it is one of the most important organs
for insulin-dependent glucose metabolism in humans. Abnormalities in skeletal
muscle lipid and glucose metabolism as well as abnormal accumulation of
intramyocellular lipids could predispose for the development of type 2 diabetes.
Skeletal muscle synthesizes and secretes factors with
autocrine/paracrine/endocrine functions that can regulate skeletal muscle
metabolism as well as affect other organs. These factors secreted by skeletal
muscle are called myokines. Secretion and action of myokines is regulated by
physiological conditions. Some myokines have positive effect on metabolism,
improving functions of multiple organs. Yet, other myokines are released under
pathological conditions and might exacerbate abnormal metabolic functions.
Expression and/or secretion of a number of myokines are regulated by exercise and
therefore might mediate positive effects of physical activity on whole-body

metabolism. In the current review we summarized current knowledge on some of
the myokines with important physiological functions in lipid and glucose metabolism.
A better understanding of the effects of myokines on whole-body metabolism can
aid in development of the future pharmacologic therapies for counteracting the
current worldwide obesity epidemic and obesity-mediated abnormalities.

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

2


Key words: skeletal muscle, myokines, obesity, metabolism.

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

3


TEXT
Introduction
Obesity has reached epidemic proportions worldwide and the rate of obesity
continues to increase. According to the World Health Organization, in 2014 about
39% (1.9 billion) of adults worldwide were overweight and 13% (over 600 million)
were obese. The World Obesity Federation estimates that if current trends continue,

around 3 billion adults worldwide will be overweight by the 2025. In order to combat
the increasing rate of obesity it is essential to understand molecular mechanisms
and factors that mediate the development of obesity-induced abnormalities. Due to
the extensive studies performed in the last two decades the critical role of factors
secreted by adipose tissue (adipokines) in regulation of the whole-body lipid and
glucose metabolism is now well established.
There are more than 600 factors secreted by adipocytes and circulating in the
blood that could have autocrine/paracrine/endocrine functions [1]. Some adipokines
have beneficial effects on metabolism and improve insulin sensitivity, as well as lipid
and glucose metabolism (adiponectin, vaspin, FGF21). Their secretion usually is
decreased by obesity. Other adpokines have negative effect on metabolism,
promoting insulin resistance and dyslipidemia (resistin, visfatin, RBP4) and their
secretion is increased by obesity. Current medical textbooks describe adipose
tissue as an adipokine-secreting endocrine organ, with a well-established role in
whole-body metabolism. Adipokines circulating in the blood can affect the function
of many organs, including skeletal muscle. A number of previous and current

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

4


studies are focused on using adipokines for the development of the treatment for
obesity-induced abnormalities in humans.
Currently it is much less appreciated that skeletal muscle also can secrete many
of the same factors with autocrine/paracrine/endocrine functions that are secreted
by adipocytes, as well as some muscle-specific factors. These factors secreted by

skeletal muscle are called myokines. This term, derived from Greek words for
“muscle” and “motion” was proposed to describe factors with endocrine function that
are produced and secreted by skeletal muscle cells [2]. Skeletal muscle is the
second largest organ of the human body. It is the largest target organ for insulinstimulated glucose utilization. Therefore, abnormalities in skeletal muscle glucose
and lipid metabolism can lead to pronounced whole-body metabolic abnormalities.
Skeletal muscle-secreted myokines can regulate metabolism by endocrine
mechanism acting on distant organs such as liver and adipose tissue (Figure 1).
Myokines are also capable of regulating muscle functions by an autocrine
mechanism as well as function of the located nearby tendon and bone tissues by a
paracrine mechanism (Figure 1). A number of myokines have already been
identified. For many of myokines we have only limited knowledge. We know that
they potentially can be myokines but their functions are not yet studied. Analysis of
the secretory profile of human skeletal muscle cells has identified 305 proteins as
potential myokines [3]. Expression of some of these myokines is regulated by
exercise. Fifteen out of the 236 proteins first identified in cultured skeletal muscle
cells were later shown to be significantly up-regulated in human skeletal muscle in
vivo in response to strength training [4].

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

5


A number of myokines have been previously described in detail (FGF21,
adiponectin, LIF, IL-6, CTRPs, irisin, apelin, lipocalins, etc.) although the
mechanism regulating their secretion by skeletal muscle and their effect on other
organs and whole-body metabolism are not yet completely elucidated. Multiple

studies suggest that myokine secretion is regulated by diverse physiological
changes including obesity, cancer cachexia, insulin resistance and exercise. Unlike
adipokines, myokines are not yet mentioned in medical textbooks and are not
discussed in medical school curricula. As a result, the role of myokines as regulators
of the whole-body lipid and glucose metabolism is currently underappreciated in the
medical community. A better understanding of the mechanisms by which myokines
can affect obesity-induced abnormalities will help to develop future therapies.
In this review we will focus on selected myokines that, in our opinion, play crucial
roles in whole-body lipid and glucose metabolism and have a potential to be used in
the future as diagnostic or therapeutic tools to diminish obesity-induced
abnormalities and prevent type 2 diabetes.

Myostatin, also known as GDF-8, is a member of TGF beta family and is one of the
first described myokines [5]. Binding of myostatin to the activin receptor type IIB
promotes skeletal muscle atrophy in many physiological conditions, including
denervation and fasting. Myostatin also inhibits activation of skeletal muscle-specific
stem cells, satellite cells, in response to injury and slows down muscle differentiation
[6]. Myostatin mediates skeletal muscle atrophy through activation of the forkhead
box O transcription factors (FoxO) leading to increased expression of muscle-

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

6


specific E3 ligases, atrogin-1 and MuRF1 (muscle RING-finger protein 1). E3 ligases
target muscle myofibrillar and intracellular proteins for degradation through the

ubiquitin–proteasome system. Myostatin also increases skeletal muscle oxidative
stress via NF-κB- and NADPH-mediated signaling cascades [7]. Myostatin-null mice
have significantly increased lean skeletal muscle mass when compared with wild
type littermates [5]. Increased muscle mass results from both hypertrophy (increase
in muscle fiber size) and hyperplasia (proliferation of muscle stem cells to increase
the number of muscle fibers) [5]. Myostatin-null mice also are protected from dietinduced obesity, they have enhanced peripheral tissue fatty acid oxidation and
increased thermogenesis [8] . Myostatin-induced catabolic processes mediate
skeletal muscle atrophy in cancer cachexia [9]. Myostatin expression can be
regulated by endocrine hormones. In rats hypothyroidism is associated with
increased myostatin mRNA expression [10]. It is currently unclear whether
hypothyroidism in humans is also associated with increased myostatin levels in
skeletal muscle. The endocrine function of myostatin could be mediated by its
effects on adipose tissue. Myostatin treatment has been shown to increase
proliferation of adipocytes and inhibit their differentiation [11]. Myostatin treatment
also influenced the expression and secretion of adiponectin, resistin, and visfatin by
adipocytes [11].

Brain-derived neurotrophic factor (BDNF) is a myokine that affects
myogenesis and skeletal muscle regeneration [12]. The expression of BDNF is high
in myoblasts but it is decreased in differentiated myotubes [13]. BDNF effects are

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

7


mediated by binding to specific cell surface receptors. Human myocytes express

p75NTR and not TrkB BDNF receptors [14]. BDNF is expressed at different levels in
fast and slow muscle fibers. Mostly slow rat soleus muscle has 2-fold higher levels
of BDNF expression than mostly fast gastrocnemius muscle [15]. BDNF has
beneficial effects on nerve growth and regeneration via paracrine mechanism [16].
Following denervation skeletal muscle increases BDNF expression and retrograde
transport of BDNF to spinal cord [16]. BDNF also affects whole-body metabolism via
autocrine and endocrine mechanisms. Systemic treatment of mice with BDNF
reduces total food intake and inhibits weight gain [17]. These effects are mediated
by increased GLUT4 expression in skeletal muscle [17]. In obese diabetic mice
BDNF treatment enhances glucose utilization in skeletal muscle and brown adipose
tissue [18].

Insulin-like growth factor 1 (IGF-1) is released by skeletal muscle and affects
skeletal muscle growth. Mutant mice with reduced IGF-1 content in muscle are
~30% smaller, have reduced levels of circulating IGF-1, have smaller muscles and
decreased bone mineral density [19]. Growth defects are diminished by
administration of recombinant IGF-1 [19]. IGF-1 regulates whole-body metabolism.
Skeletal muscle-specific inactivation of IFG-1 receptors leads to the development of
insulin resistance [20]. High fat diet-induced obesity in mice reduces expression of
IGF-1 in intact muscles and diminishes up-regulation of IGF-1 in response to muscle
injury [21].

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

8

Fibroblast Growth Factor 2 (FGF-2), also known as basic FGF, is a myokine that
has pleiotropic effects in a range of cell types. During myogenic differentiation
expression of FGFs and FGF receptors are downregulated [22]. FGF-2 secreted by
muscle cells may act as a paracrine and autocrine regulator of skeletal muscle
development in vivo [23]. During ischemia, increased expression of FGF-2 in
skeletal muscle promotes angiogenesis via its paracrine effects on blood vessels
[24]. Treatment with FGF-2 stimulates proliferation of tendon cells [25] suggesting
that skeletal muscle released FGF-2 may have paracrine effects on tendon. FGF-2
also plays an important role in bone formation and remodeling. Mechanical
wounding of muscle cells increases release of FGF-2 into media [26] and may have
paracrine effect on tendon and bone.

Fibroblast growth factor 21 (FGF21) is a well described adipocytokine/
hepatokine/ myokine with glucose and lipid-lowering properties. The liver is
considered the major source of the plasma FGF21. Under normal physiological
conditions the level of hepatic FGF21 expression is low but it increases during
prolonged fasting, liver injury, exposure to toxic chemicals and viral infections [27].
In mice chronic infusion of FGF21 improves insulin responsiveness due to the
reduced diacylglycerol content and reduced protein kinase C activation in skeletal
muscle [28]. In human clinical trials FGF21 treatment of obese type 2 diabetic
patients improved total body weight, dyslipidemia, insulin sensitivity and adiponectin
levels [29] [27]. FGF21 works through the FGF receptors (FGFR1 and FGFR2) and
requires transmembrane protein beta-Klotho (KLB) as a co-receptor for activation of

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

9

the intracellular signaling pathways [27]. Recent studies have focused on the
development of novel FGF21 mimetics with improved stability and pharmacokinetics
that could be applied for improving lipid and glucose metabolism [30].
Izumiya and colleagues reported that FGF21 is a myokine regulated through Aktdependent mechanisms [31]. Levels of FGF21 in skeletal muscle were increased in
response to insulin stimulation and to hyperinsulinemia [32]. Interestingly, the
expression of FGF21 in human skeletal muscle does not correlate with FGF21
levels in the plasma [32]. mRNA expression of FGF21 in skeletal muscle is
significantly lower than in liver but similar to the expression in subcutaneous fat [33].
These might explain the lack of correlation between plasma and skeletal muscle
FGF21 levels. Nevertheless, FGF21 at low levels could have considerable local
autocrine effects on skeletal muscle metabolism. This hypothesis is supported by
studies showing direct effects of FGF21 on increased glucose uptake in cultured
human skeletal muscle mediated by increased glucose transporter 1 (GLUT1)
expression [34].
Skeletal muscle-specific overexpression of uncoupling protein 1 (UCP1)
increases expression of mRNA for FGF21 (several–hundred-fold) as well as its coreceptor KLB (~2-fold) but not FGFR1 [33]. Similarly, in a mouse model of diabesity
on a NZO background, skeletal muscle-specific UCP1 overexpression diminished
obesity and insulin resistance via induced expression of FGF21 [35]. These suggest
that under certain physiological conditions when expression of uncoupling proteins
in skeletal muscle is increased [33] or the cellular stress response activating
transcription factor (ATF) 4 cascade is activated [33], the expression of FGF21 in

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

10

skeletal muscle and in plasma could be significantly up-regulated. Mitochondrial
stress and dysfunction is characterized by increased production of reactive oxygen
species, activation of p38 MAPK and ATF2, leading to the increased expression of
FGF21 in skeletal muscle [36]. FGF21 expression is controlled by myogenic factor
MyoD [36]. Constitutive activation of the mammalian target of rapamycin complex 1
(mTORC1) in mice due to the muscle-specific depletion of tuberous sclerosis
complex 1 (TSC1) triggers endoplasmic reticulum stress and activation of protein
kinase RNA-like ER kinase, eukaryotic translation initiation factor 2! and ATF4 [37].
As a consequence skeletal muscle increases synthesis and release of FGF21
leading to increased whole-body insulin sensitivity and lipid oxidation [37]. Similarly,
in skeletal muscle, enhanced activity of eukaryotic translation initiation factor 4Ebinding protein 1 (4E-BP1), a key downstream effector of mTORC1, increases
FGF21 synthesis [38].
High levels of plasma FGF21 correlating with increased FGF21 expression in
skeletal muscle were detected in patients with mitochondrial dysfunctions in skeletal
muscles due to a deficiency of iron-sulfur clusters [39]. Treatment of cultured human
skeletal muscle cells with FGF21 in a time-dependent manner increased mRNA
expression of KLB and at the same time decreased palmitate-induced insulin
resistance by inhibiting activation of Akt and NF-κB [40]. This phenomenon could
represent a protective mechanism activated in response to metabolic and oxidative
stress.
Insulin-sensitizing effects of FGF21 were reported to be mediated by adiponectin
[41]. Skeletal muscle–specific delivery of adenoviral FGF21 vector increased

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

11


plasma FGF21 as well as adiponectin levels [42]. These correlate with the
decreased negative effects of myocardial infarction in mice [42] suggesting
protective effects of FGF21 and adiponectin against infarction-induced stress.
There might be significant differences in the whole-body response to FGF21
based on the tissue source of the secreted FGF21 as well as duration of the FGF21
exposure. A recent report by Berti and colleagues [43] showed that unhealthy
(insulin-insensitive) obese subjects had two-fold higher levels of plasma FGF21 than
body fat-matched healthy obese subjects. Fatty liver might be the predominant
source of plasma FGF21 in unhealthy obese subjects [43]. Prolonged treatment of
cultured human pre-adipocytes with FGF21 reduced adiponectin expression and
release and increased release of leptin and interleukin-6 [43]. These studies
suggest that short-term treatment with FGF21 (once a day injections) could have
beneficial effects on whole-body lipid and glucose metabolism. To the contrary,
chronically present in the plasma high levels of FGF21 could potentially promote
hyperlipidemia and insulin resistance.

Adiponectin is well known adipocytokine that belongs to the complement
component 1q (C1q) family. In the plasma, adiponectin can be found in high-,
middle- and low-molecular weight multimeric forms. Reduced levels of highmolecular-weight adiponectin are highly associated with increased risk for
cardiovascular abnormalities [44]. Adiponectin has insulin sensitizing, antiinflammatory, anti-atherogenic and anti-apoptotic effects and therefore could
alleviate some of the negative effects of obesity. Adiponectin mRNA and protein

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

12


expression are increased during skeletal muscle differentiation [45]. Skeletal muscle
can increase adiponectin synthesis in response to physiological stress. For
example, increased inflammation and cytokine exposure in response to injection of
lipopolysaccharides causes increased adiponectin mRNA and protein expression in
mice [46] . Adiponectin works through two receptors AdipoR1 and AdipoR2 both of
which are expressed in skeletal muscle [47]. AdipoR1 expression is significantly
higher in muscle while and AdipoR2 is highly expressed in liver [47]. In muscle,
adiponectin promotes the interaction of AdipoR with APPL1 (adaptor protein
containing pleckstrin homology domain, phosphotyrosine binding domain and
leucine zipper motif) potentiating adiponectin signaling and promoting adiponectininduced GLUT4 membrane translocation, glucose uptake and lipid oxidation [48].
Expression of AdipoR1, AdipoR2 and APPL1 is increased during skeletal muscle
differentiation [45]. Skeletal muscle unloading causes muscle atrophy via increased
proteolysis and is associated with decreased expression of AdipoR1 but not
AdipoR2 [45]. To the contrary, skeletal muscle functional overloading is associated
with increased expression of adiponectin and APPL1 [45].
The link between obesity and alterations in adiponectin expression is well
described. Obesity, increased visceral fat accumulation, type 2 diabetes and
cardiovascular disease are highly correlated with down-regulated adiponectin
secretion by adipocytes, decreased adiponectin signaling in liver and skeletal
muscle and lower plasma adiponectin levels [49]. Polymorphisms at the adiponectin
locus may be associated with low levels of circulating adiponectin levels, insulin

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

13


resistance, and atherosclerosis [50]. A high fat diet decreases skeletal muscle
AdipoR1 expression predisposing to the potential metabolic abnormalities [51].
A protective effect of adiponectin in muscle under diverse physiological
conditions is well documented. In cultured skeletal muscle cells adiponectin
increases Akt phosphorylation/activation and promotes glucose uptake [52].
Adiponectin can also promote mitochondrial biogenesis in skeletal muscle via PGC1 alpha signaling pathway and suppression of MAPK phosphatase-1 [53].
Adiponectin inhibits (NF)-κB-inducing kinase (NIK), an upstream kinase of NF-κB
pathway, reducing inflammation and promoting insulin sensitivity in skeletal muscle
[54]. Adiponectin exerts insulin-sensitizing effects in liver and skeletal muscle via
adenosine monophosphate-activated protein kinase and proliferator-activated
receptor alpha activation. Increased levels of adiponectin could also have a
beneficial effect during skeletal muscle injury and oxidative stress. A mouse model
of Duchenne muscular dystrophy (mdx mice) have muscle degeneration and
increased muscle inflammation, accompanied by the reduced levels of adiponectin
in the plasma [55]. When mdx mice were crossed with mice overexpressing
adiponectin muscles were protected from injury and displayed higher force and
endurance properties [55]. Obesity is associated with increased oxidative stress and
accumulation of damaged proteins and organelles. Despite decreased levels of
adiponectin in plasma, skeletal muscle of obese mice has increased levels of
adiponectin expression [56]. It was previously reported that beneficial metabolic
effects of adiponectin in skeletal muscle of high fat diet-treated mice are mediated
by autophagy [57], probably via removal of damaged/ nonfunctional organelles. In

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

14


mice, activation of adiponectin receptors by the small molecule agonist AdipoRon
improves insulin resistance and glucose uptake [58] and potentially could be
developed into a clinically-approved drug for treatment of obesity-induced
abnormalities.

CTRP proteins. Recently fifteen novel secreted proteins of the C1q family were
identified (C1q/TNF-related proteins; CTRP1–15) that share structural and
functional similarity with adiponectin [59]. CTRP proteins have in common four
distinct domains: a signal peptide at the N terminus, a variable region, a collagenous
domain (Gly-X-Y repeats), and a C-terminal globular domain C1q. The majority of
CTRPs are considered to be adipokines due to their preferential expression in
adipose tissue, but CTRP15/myonectin is a skeletal muscle-specific myokine that
regulates lipid metabolism in liver and adipose tissue [60]. Some other CTRPs are
also expressed in skeletal muscle (reviewed in [59] [61]) but their role as myokines
is not yet well studied. For example, it is reported that CTRP1, CTRP3, CTRP5,
CTRP12, CTRP15 are expressed in skeletal muscle but very little is known about
their contribution to the regulation of the whole-body lipid and glucose metabolism.
CTRP1 is a 35-kDa glycoprotein that functions as adipokine/myokine/hepatokine
and is most highly expressed in adipocytes. CTRP1 is highly up-regulated in
response to lipopolysaccharide treatment [62]. Expression of CTRP1is regulated by
TNF-alpha and interleukin-I beta [62]. CTRP1 has higher expression levels in
adipose tissue of genetically modified obese rats and mice when compared with
lean controls [62]. In the obese state, synthesis and secretion of adiponectin in

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute

the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

15


adipose tissue is highly down-regulated. Therefore a high expression of CTRP1 in
the obese state might represent a compensatory effect. In skeletal muscle cells
CTRP1 promotes fatty acid oxidation and inhibits acetyl-CoA carboxylase [63].
When CTRP1 overexpressing transgenic mice are placed on a high-fat or a highsucrose diet, they have increased glucose uptake and diminished insulin resistance
[63].
CTRP3, cartonectin, is an adipokine/myokine/hepatokine involved in multiple
physiological processes. It was initially identified as a TGF beta-responsive gene in
growing cartilage [64]. Expression of CTRP3 in muscle is regulated via the
transforming growth factor beta pathway and is increased during myogenic
differentiation of muscle cells in vitro [65]. CTRP3 can stimulate proliferation and
differentiation of chondrogenic precursor cells via an ERK signaling pathway,
playing an important role in cartilage development [66]. CTRP3 also promotes
angiogenesis by stimulation of proliferation and migration of endothelial cells [67].
Therefore, CTRP3 can potentially be involved in the regulation of blood vessels and
bone development by growing skeletal muscle during embryonic development and
physiological adaptations. Levels of CTRP3 in plasma are increased with fasting
and decreased in diet-induced obesity [68]. Treatment of mice with CTRP3 can
decrease plasma glucose levels but does not affect levels of insulin and adiponectin
[68]. CTRP3 also educes lipopolysaccharide-induced release of IL-5 and TNF in
monocytes from healthy controls but not from type 2 diabetic patients [69].
CTRP5 has significant effects on glucose metabolism and mitochondrial content in
skeletal muscle. CTRP5 expression is increased in insulin-resistant rodents and in

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one

copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

16


mitochondrial DNA-depleted muscle cells [70]. Treatment of myocytes with
recombinant CTRP5 induces AMPK phosphorylation and increases glucose uptake
through the stimulation of GLUT4 translocation to the plasma membrane [70]. This
is very similar to the effects produced by adiponectin. CTRP5 does not induce
phosphorylation of IRS-1 and Akt, suggesting that it works independently from the
insulin signaling pathway [70]. In response to aerobic exercise in humans, serum
CTRP5 levels are negatively correlated with maximal oxygen consumption,
mitochondrial density and adiponectin levels [71].
CTRP12, also called adipolin, is an adipokine/myokine that improves insulin
sensitivity and glycemic control in mouse models of obesity and diabetes [72].
CTRP12 is synthesized and secreted as full-length (fCTRP12) and cleaved
(gCTRP12) isoforms [72]. CTRP12 expression is decreased in leptin-deficient
ob/ob, and diet-induced obese mice. The expression of CTRP 12 in adipocytes was
restored in response to treatment with anti-diabetic drug rosiglitazone [72]. CTRP12
improves insulin sensitivity by enhancing insulin signaling in liver and adipose tissue
via a PI3K-Akt signaling pathway [72]. In adipocytes, obesity and inflammation
decreased expression of CTRP12 via JNK-dependent down-regulation of
transcription regulator Kruppel-like factor 15 (KLF15) [73]. Currently it is not known
whether the expression and/or secretion of CTRP12 in skeletal muscle are
regulated via PI3K-Akt or JNK/KLF15 signaling pathways.
CTRP15, also known as myonectin, is a myokine that promotes fatty acid uptake,
controls cellular autophagy and links skeletal muscle to the whole-body lipid
homeostasis, mostly through effects on lipid and glucose metabolism in liver and

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

17


adipose tissue [60]. Although CTRP15 is expressed by several tissues, the highest
mRNA expression levels are found in skeletal muscle [60]. Therefore CTRP15 is
considered primarily to be a myokine, predominantly expressed by skeletal muscle
and released into serum. The deduced mouse myonectin protein consists of five
domains: a signal peptide for secretion, an N-terminal domain-1, a short collagen
domain with six Gly-X-Y repeats, an N-terminal domain-2, and a C-terminal
C1q/TNF-like domain [60]. Secreted CTRP15 can form disulfide-linked oligomers
and is able to form heteromeric complexes with other CTRP molecules [60]. When
co-expressed in mammalian cells, CTRP15 forms heteromeric complexes with
CTRP2 and CTRP12, and, to a lesser extent, with CTRP5 and CTRP10 [60]. mRNA
expression of CTRP15 is increased during myogenic differentiation from
undifferentiated myoblasts to differentiated myotubes [60]. This suggests that
concentrations of CTRP15 in the serum are regulated during embryonic/fetal
development and correlate with the degree of differentiation of skeletal muscle. The
expression is up-regulated by increased calcium and cAMP concentrations but it is
not changed in response to insulin and AMP-activated protein kinase activator
(AICAR) treatment [60]. In response to exercise, levels of CTRP15 mRNA
expression in skeletal muscle are increased. Interestingly, the expression is more
highly up-regulated in mostly fast plantaris muscle when compared with mostly slow
soleus muscle [60]. During fasting, the CTRP15 level in skeletal muscle is low but it
is increased in response to glucose and lipids both in vivo and in vitro [60]. Upregulation of mRNA expression in response to re-feeding was ~twenty-fold higher in

slow soleus muscle when compared with fast plantaris muscle [60]. These data

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

18


suggest differential regulation of CTRP15 expression in response to exercise and
re-feeding between fast and slow muscles. Therefore changes in skeletal muscle
composition under physiological conditions, such as a relative decrease of slow
muscle fibers with prolonged obesity or a relative increase of slow fibers with aging,
can affect CTRP15 expression. Females appeared to have higher levels of CTRP15
in the serum during fasting and re-feeding, although this trend did not reach
statistically significant levels [60]. High fat diet significantly reduces CTRP15 mRNA
expression in skeletal muscle and levels of CTRP15 protein in the serum [60].
Treatment of cultured adipocytes and hepatocytes with CTRP15 increases palmitate
intake to the same degree as treatment with insulin [60]. In cultured adipocytes and
hepatocytes, recombinant CTRP15 enhances fatty acid uptake through
transcriptional upregulation of genes (e.g., CD36, FATP1, Fabp1 and Fabp4) known
to be involved in fatty acid uptake [60].

Ghrelin is a hormone that has a variety of functions in multiple organs. Ghrelin is
expressed in skeletal muscle [74] and it promotes differentiation of skeletal muscle
cells via p38 kinase signaling [75].Ghrelin inhibits skeletal muscle apoptosis and
enhances cellular signaling for autophagy, resulting in overall myoprotective effects
on doxorubicin-induced toxicity in skeletal muscle [76]. Due to their myoprotective
effects ghrelin [77] and ghrelin receptor agonists [78] emerge as a promising

treatment option for cancer cachexia. Ghrelin has positive effects on glucose
utilization. Treatment with unacylated ghrelin improves insulin signaling and glucose
uptake in skeletal muscle of diabetic mice [79]. Similarly, acyl-ghrelin treatment of

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

19


rats on a high fat diet decreases muscle inflammation and lipid accumulation [80].
Ghrelin treatment of rats induces lipogenic and glucogenic patterns of gene
expression and increases triglyceride content in the liver [81]. At the same time in
skeletal muscle mitochondrial oxidative enzyme activities are increased and
triglyceride content is reduced [81]. This suggests that ghrelin treatment has
opposite effects on lipid metabolism in liver and skeletal muscle. In various organs
ghrelin expression is also regulated differently by physiological stimuli. For example,
in rats twelve weeks of exercise increased acyl-ghrelin levels in skeletal muscle and
in plasma while content of ghrelin in the fundus of stomach was decreased [74].

Musclin is a myokine that is induced during differentiation of skeletal muscle cells
[82]. Insulin increases levels of musclin expression, while compounds known to
increase cellular content of cyclic AMP epinephrine, isoproterenol, and forskolin
decrease musclin expression levels [82]. Recombinant musclin significantly
reduces insulin-stimulated glucose uptake and glycogen synthesis in myocytes [82].
The levels of musclin expression are low in the fasting state but increase upon refeeding [82]. Musclin expression can be regulated by transcription factor FoxO1
which is a key regulator of muscle atrophy. Musclin mRNA level is markedly
downregulated in gastrocnemius muscle of Foxo1 transgenic mice. Musclin levels

are also much lower in skeletal muscle of streptozotocin-treated insulin-deficient
mice when compared with muscles from control mice [82]. Musclin protein contains
a region homologous to the natriuretic peptide family, and a putative serine protease
cleavage site, similar to the natriuretic peptide family [82]. Moreover, musclin

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

20


specifically binds to the natriuretic peptide receptor 3 and competes for receptor
binding with atrial natriuretic peptide [83]. These data suggest that musclin can
modulate effects of atrial natriuretic peptide.

Irisin is a recently described secreted myokine that could have potential for
normalization of lipid and glucose metabolism [84]. Irisin is a skeletal musclesecreted protein representing cleaved by proteolysis extracellular part of membrane
fibronectin type III domain–containing protein 5 (FNDC5). Although irisin is
expressed in adipose tissue, the levels of expression of irisin in human muscle are
200-fold higher than in adipocytes [85]. Expression of irisin is regulated via a PGC1alpha pathway [84]. Irisin transforms white adipocytes into brown adipocytes by
increasing expression of uncoupling proteins via p38 and Erk MAP kinase signaling
pathways [86]. Browning of adipocytes promotes thermogenesis and energy
expenditure, having beneficial effects on whole-body lipid metabolism. In skeletal
muscle cells irisin increases oxidative metabolism via a PGC-1 alpha pathway and
up-regulates irisin mRNA expression by autocrine mechanism [87]. Patients with
type 2 diabetes have decreased serum levels of irisin [88]. Levels of serum irisin
negatively correlate with body mass index, percent of fat mass and waist to hip ratio,
and positively correlate with insulin sensitivity [85]. Moreover, irisin can have

beneficial effects in cancer prevention. It has been reported that irisin promotes
apoptosis of malignant breast epithelial cells and enhances the cytotoxic effects of
doxorubicin in these cells [89].

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

21


Visfatin, is an adipokine/myokine implicated in glucose metabolism, cell
differentiation and tissue regeneration. In skeletal muscle visfatin expression is
regulated by TNF, IL-6, and glucocorticoids. Treatment with recombinant visfatin
modulates the expression of myogenic transcription factors required for
differentiation [90]. Treatment of muscle cells with visfatin also increases glucose
uptake via increased GLUT4 expression and translocation to the plasma membrane
[91]. The effects on glucose uptake are mediated by the AMPK and p38 MAPK
pathways [91]. In another study, visfatin treatment increased glucose transport in
muscle but had no effect on fatty acid oxidation [92].

Apelin is an adipokine/myokine that is involved in the regulation of glucose
metabolism, lipolysis, blood pressure, cardiovascular and fluid homoeostasis, food
intake, cell proliferation, and angiogenesis. In cultured muscle cells apelin treatment
increases glucose uptake and Akt phosphorylation [93]. Apelin-null mice have
diminished insulin sensitivity, hyperinsulinemia and decreased adiponectin levels in
the blood. Soleus muscles of apelin knockout mice have decreased insulin-induced
Akt phosphorylation. Apelin and apelin receptor expression levels are regulated by
obesity. Administration of apelin to apelin-null and db/db mice results in improved

insulin sensitivity [93]. In mice, treatment with apelin increases glucose utilization in
skeletal muscle and adipose tissue [94]. This effect is regulated via endothelial NO
synthase, AMP-activated protein kinase, and Akt in the soleus muscle of apelintreated mice [94]. Apelin treatment also increases UCP3 expression levels in
skeletal muscle and serum adiponectin levels in plasma [95]. In rats on a high fat

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

22


diet apelin level in plasma, and apelin and apelin receptor expression is increased in
gastrocnemius (mostly fast) muscle and in adipose tissue [96]. Interestingly, in
mostly slow soleus muscle, apelin expression was not changed. This suggests
differences in regulation of apelin expression between slow and fast skeletal muscle
fibers. Apelin has recently been described as a novel exercise-regulated myokine
with positive effects on insulin sensitivity [97], [98]. Treadmill running ameliorates
high fat diet-induced insulin resistance, down-regulates apelin and apelin receptor
expression in adipocytes and apelin levels in plasma [96]. At the same time,
running increases levels of expression of apelin and apelin receptors in muscles
[96]. This suggests differences in regulation of apelin expression in different tissue
types. Patients with type 2 diabetes have significantly higher serum apelin levels,
compared to non-diabetic control subjects [99]. In non-diabetic male subjects, eight
weeks of endurance training had produced two-fold increase in apelin mRNA levels
in vastus lateralis muscle but not in adipose tissue [97], indicating differences in
exercise-induced regulation of apelin expression in different tissue types.

Lipocalins (LCNs) have recently emerged as a myokine family involved in the

regulation of insulin sensitivity [100]. LCNs can have both positive as well as
negative effects on the regulation of insulin sensitivity [100]. The recently described
circulating LCN13 is markedly reduced in mice with genetic or diet-induced type 2
diabetes. The restoration of LCN13 improves insulin resistance, hyperglycemia, and
glucose intolerance. LCN13 can directly enhance insulin actions in cultured
adipocytes and therefore acts as an endogenous insulin sensitizer to regulate

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

23


glucose metabolism [101]. Both LCN13 mRNA and protein are highly expressed by
skeletal muscle [101]. Factors regulating LCN13 expression in skeletal muscle are
currently unknown.

Interleukins (IL) are pro- and/or anti-inflammatory molecules highly expressed by
immune cells. Some of the interleukins are secreted by adipocytes and by muscle
cells in response to diverse physiological stimuli and therefore function as
adipokines and/or myokines, respectively. Obesity and type 2 diabetes are
associated with low grade inflammation and dysregulated secretion of interleukins.
Interleukins reported to function as myokines include IL-4, IL-6, IL-7, IL-8, IL-15,
leukemia inhibitory factor (LIF), ciliary neurotrophic factor (CNTF), cardiotrophin-1
(CT-1), and oncostatin M.
IL-6 is an adipokine/myokine with diverse physiological functions. Depending on the
physiological conditions and the source of IL-6 secretion, it can have either positive
or negative effects on metabolism and muscle catabolic/anabolic processes

(reviewed in [102]). IL-6 signaling is mediated by the IL-6 receptor alpha and
membrane glycoprotein 130 complex. Treatment of muscle cells with IL-6 promotes
myogenic differentiation and increases mRNA expression of GLUT4, MEF2, PPAR,
UCP2, and FATP4 [103]. IL-6 treatment also increases mRNA expression of IL-6
itself suggesting autoregulation of expression [103]. IL-6 increases glucose uptake
and fatty acid oxidation in skeletal muscle [103] [104]. Glucose uptake stimulated by
IL-6 requires the PI3-kinase signaling pathway, while fatty acid oxidation is mediated
by AMPK signaling [103]. Muscle-specific IL-6 deletion caused a sex-dependent

This document is protected by international copyright laws. No additional reproduction is authorized. It is permitted for personal use to download and save only one file and print only one
copy of this Article. It is not permitted to make additional copies (either sporadically or systematically, either printed or electronic) of the Article for any purpose. It is not permitted to distribute
the electronic copy of the article through online internet and/or intranet file sharing systems, electronic mailing or any other means which may allow access to the Article. The use of all or an
part of the Article for any Commercial Use is not permitted. The creation of derivative works from the Article is not permitted. The production of reprints for personal or commercial use is no

24