Muscles, exercise and obesity: skeletal muscle as a secretory organ

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Muscles, exercise and obesity: skeletal muscle
as a secretory organ
Bente K. Pedersen and Mark A. Febbraio
Abstract | During the past decade, skeletal muscle has been identified as a secretory organ. Accordingly, we
have suggested that cytokines and other peptides that are produced, expressed and released by muscle
fibres and exert either autocrine, paracrine or endocrine effects should be classified as myokines. The finding
that the muscle secretome consists of several hundred secreted peptides provides a conceptual basis and
a whole new paradigm for understanding how muscles communicate with other organs, such as adipose
tissue, liver, pancreas, bones and brain. However, some myokines exert their effects within the muscle itself.
Thus, myostatin, LIF, IL‑6 and IL‑7 are involved in muscle hypertrophy and myogenesis, whereas BDNF and IL‑6
are involved in AMPK-mediated fat oxidation. IL‑6 also appears to have systemic effects on the liver, adipose
tissue and the immune system, and mediates crosstalk between intestinal L cells and pancreatic islets. Other
myokines include the osteogenic factors IGF‑1 and FGF‑2; FSTL‑1, which improves the endothelial function of the
vascular system; and the PGC‑1α-dependent myokine irisin, which drives brown-fat-like development. Studies
in the past few years suggest the existence of yet unidentified factors, secreted from muscle cells, which may
influence cancer cell growth and pancreas function. Many proteins produced by skeletal muscle are dependent
upon contraction; therefore, physical inactivity probably leads to an altered myokine response, which could
provide a potential mechanism for the association between sedentary behaviour and many chronic diseases.
Pedersen, B. K. & Febbraio, M. A. Nat. Rev. Endocrinol. advance online publication 3 April 2012; doi:10.1038/nrendo.2012.49
Introduction
The views on hormonal regulation of metabolism in
health and disease have markedly changed over the
past 20 years, principally owing to extensive research
into adipose tissue. Initially considered an inert storage
compartment for triglycerides, pioneering work in the
mid 1980s demonstrated that adipocytes are an abundant source of a specific secretory protein called complement factor D or adipsin.1 A little over a decade ago,
in a landmark finding, Zhang et al.2 identified leptin as
a fat-cell-specific secretory factor—lacking in the obese
ob/ob mouse—that mediates a canonical hormonal
signal between adipose tissue and the brain. Since then,
adiponectin, resistin, nicotinamide phosphoribosyltransferase (also known as visfatin) and retinol-binding protein
4 have joined the growing list of adipokines.3 Although
adipokines have been the focus of much research in
terms of their role as circulatory factors with effects on
metabolically active tissue, it should be noted that during
contractions, muscle cells undergo one of the most
marked alterations to cellular quiescence in physio­logy
and indeed pathophysiology. In addition, exercise influences the metabolism and function of several organs.
Muscles, therefore, could represent an important source of
se­cretory molecules with either local or en­docrine effects.
Apart from adiponectin, 4 most of the factors that
are produced by adipocytes are proinflammatory—for
Competing interests
The authors declare no competing interests.
example, TNF, CCL2 and PAI‑1—and are potentially
harmful with regard to the development of obesityinduced metabolic and cardiovascular diseases. This
understanding raises the important question of which
tissue or tissues could be protective and provide a
counter­balance to the proinflammatory factors that are
produced by adipocytes.
Even short periods of physical inactivity are associated with metabolic changes, including decreased insulin
sensitivity, attenuation of postprandial lipid metabolism, loss of muscle mass and accumulation of visceral
adipose tissue.5,6 Such abnormalities probably represent
a link between reduced exercise and increased risks of
the progression of chronic disorders and premature
mor­tality.7 Physical inactivity increases the risk of type 2
diabetes mellitus (T2DM), 8 cardiovascular diseases, 9
colon cancer, 10 postmenopausal breast cancer 11 and
osteo­porosis.12 A reasonable suggestion is that skeletal
muscle might mediate some of the well-established protective effects of exercise via secretion of proteins that
could counteract the harmful effects of proinflammatory
ad­ipokines (Figure 1).
The idea that muscle cells might produce and release
a humoral factor dates back many years before the identification of adipose tissue as an endocrine organ. For
nearly half a century, researchers had hypothesized that
skeletal muscle cells possess a ‘humoral’ factor that is
released in response to increased glucose demand during
con­t raction.13 To date, owing to lack of more precise
NATURE REVIEWS | ENDOCRINOLOGY The Centre of
Inflammation and
Metabolism,
Department of
Infectious Diseases
and CMRC,
Rigshospitalet, Section
7641, Faculty of Health
Sciences, University
of Copenhagen,
Blegdamsvej 9,
DK‑2100, Copenhagen,
Denmark
(B. K. Pedersen).
Cellular and Molecular
Metabolism Laboratory,
Baker IDI Heart and
Diabetes Institute,
75 Commercial Road,
Melbourne, VIC 3004,
Australia
(M. A. Febbraio).
Correspondence to:
B. K. Pedersen
bkp@rh.dk
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Key points
■■ Myokines are cytokines or other peptides that are produced, expressed
and released by muscle fibres
■■ Myokines may exert autocrine, paracrine or endocrine effects
■■ Myokines may balance and counteract the effects of adipokines
■■ The muscle–cell secretome consists of several hundred secreted products
■■ Identified myokines include myostatin, LIF, IL‑6, IL‑7, BDNF, IGF‑1, FGF‑2, FSTL‑1
and irisin
■■ Myokines may mediate protective effects of muscular exercise, with regard
to diseases associated with a physically inactive lifestyle
Myokines
Identification of skeletal muscle as a secretory organ
has created a new paradigm: muscles produce and release
myokines, which work in a hormone-like fashion and
exert specific endocrine effects on distant organs. Other
proteins produced by skeletal muscle that are not released
into the circulation, could work via autocrine or para­crine
mechanisms, exerting their effects on signalling pathways
within the muscle itself.20–25 Myokines could, therefore,
be involved in mediating the multiple health benefits of
exercise. This Review provides an update on some of the
muscle-derived cytokines that have been identified to
date (Figure 2). Furthermore, the identification of skeletal
muscle as an endocrine organ has clinical implications,
which are highlighted in the Review, such as the central
part that skeletal muscle plays in organ crosstalk, in­cluding
muscle–liver and muscle–adipose tissue crosstalk.
Myostatin
Myokines
Proinflammatory
adipokines
Type 2 diabetes mellitus,
cardiovascular disease, cancer,
osteoporosis
Figure 1 | Interplay between adipokines and myokines represent a yin–yang
balance. Especially under conditions of obesity, adipose tissue secretes
adipokines, which contribute to establish a chronic inflammatory environment that
promotes pathological processes such as atherosclerosis and insulin resistance.
Skeletal muscles are capable of producing myokines that confer some of the
health benefits of exercise. Such myokines might counteract the harmful effects
of proinflammatory adipokines.
knowledge, the unidentified contraction-induced factor
has been named ‘the work stimulus’, ‘the work factor’ or
‘the exercise factor’.14
In our view, the plural form ‘exercise factors’ would be
more applicable, given the fact that multiple metabolic
and physiologic changes are induced by exercise. The
early view on the exercise factor concept was predicated
by the fact that contracting skeletal muscle mediates meta­
bolic and physiologic responses in other organs that are
not mediated via the nervous system. Namely, electrical
stimulation of paralysed muscles in patients with spinal
cord injury with no afferent or efferent impulses induces
many of the same physiological changes as in uninjured
individuals.15,16 Contracting skeletal muscles must, therefore, be able to communicate to other organs via humoral
factors, which are released into the circulation during
physical activity. Such factors might directly or indirectly
influence the function of other organs such as adipose
tissue, liver, the cardiovascular system and the brain.
Skeletal muscle represents approximately 40% of the
body weight in lean men and women and, therefore, constitutes the largest organ in nonobese humans. During the
past decade, muscle cells have been identified as cells with
a high secretory capacity, in support of the concept of
adipocytes being major endocrine cells. Muscle cells are
thought to have the capacity to produce several hundred
secreted factors.17–19 In 2003, Pedersen et al. suggested that
cytokines or other peptides that are produced, expressed
and released by muscle fibres and exert endocrine effects
should be classified as myokines.14
Myostatin (also known as growth/differentiation fac­tor 8),
was the first secreted muscle factor to fulfil the cri­teria of
a myokine. This protein is secreted into the circulation.
Myostatin is a highly conserved member of the TGF‑β
superfamily, and inactivation of the myo­statin gene
(knockout) results in extensive skeletal muscle hyper­
trophy in mice,26 cattle and humans.27 In addition to the
regulatory roles of myostatin on skeletal muscle growth,
this myokine is also involved in the maintenance of meta­
bolic homeostasis and in modulation of adipose tissue
function and mass.28–31 Deletion of myostatin in mice
produces concomitant skeletal muscle hypertrophy and
reduction in total body adipose tissue.32,33
In humans and rodents, aerobic and strength exercise attenuate myostatin expression, whereas myostatin
inactiva­tion seems to potentiate the beneficial effects
of endurance exercise on metabolism.34 Several lines of
evidence demonstrate that obesity is associated with
increased myostatin expression. Muscle and circulating
myostatin protein levels are increased in indivi­duals with
obesity; furthermore, myostatin secretion from myotubes
derived from myoblasts isolated from muscle biopsy
samples is increased in women with obesity compared
with lean women.35
Follistatin, another member of the TGF‑β super­
family, is a naturally occurring inhibitor of myostatin
with regard to its regulatory role in skeletal muscle.
Plasma follistatin levels increased in healthy individuals
following acute bicycle exercise, with a peak increase of
seven­fold. However, there was no net release of follistatin
from the exercising limb, which suggests that contracting skeletal muscle was not the source of follistatin. In
mice performing a bout of swimming exercise, a marked
increase occurred both in plasma follistatin levels as
well as folli­statin mRNA and protein expression in the
liver. The marked increase in systemic levels of folli­
statin could, in principle, contribute to the regulation of
muscular expression of myostatin in relation to exercise.
Although follistatin should probably be classified as a
hepatokine rather than a myokine, these data suggest the
existence of possible muscle–liver crosstalk during and
following exercise.36
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IL‑6
The cytokine IL‑6 was the first myokine found to be
secreted into the bloodstream in response to muscle
contractions.23 The cytokine was serendipitously discovered as a myokine because of the observation that its
levels increased in an exponential fashion proportional
to the length of exercise and the amount of muscle mass
engaged in the exercise. Thus, plasma IL‑6 levels can
increase up to 100-fold in response to exercise, although
less dramatic increases are more frequent. 37 IL‑6 is
expressed by human myoblasts,38,39 human cultured myotubes,40 growing murine myofibres and associated muscle
stem cells (satel­lite cells).41 In addition, IL‑6 is released
from human primary muscle cell cultures from healthy
indivi­duals and from patients with T2DM.42,43
Interestingly, the increase in IL‑6 levels in the circulation occurs during exercise without any sign of muscle
damage.37 Until the beginning of this millennium, it was
commonly thought that the increase in IL‑6 levels during
exercise was a consequence of an immune response
owing to local damage in the working muscles,44 and
macro­phages were hypothesized to be responsible for
this increase.45 However, the IL6 mRNA level in monocytes does not increase as a result of exercise.46 This
finding was confirmed at the protein level.47,48 Today,
muscle cells are known to be the dominant source of IL‑6
production during exercise. Furthermore, the hepatosplanchnic viscera remove IL‑6 from the circulation in
humans during exercise.49 The removal of IL‑6 by the
liver could constitute a mechanism that limits the negative metabolic effects of chronically elevated levels of
circulating IL‑6.
Several pieces of evidence support the notion that IL‑6
is produced by muscle cells during exercise. The nuclear
transcription rate for IL‑6 and IL6 mRNA levels increase
rapidly and markedly within 30 min of the onset of exercise.50,51 which suggests that a factor associated with contraction is involved in the regulation of IL‑6 transcription
within the nuclei of muscle cells. Further evidence that
contracting muscle fibres are themselves a source of IL6
mRNA and protein has been gained from analysis of
biopsy samples from the human vastus lateral­is muscle
using in situ hybridization and immunohistochemistry
techniques.52 Microdialysis studies suggest that the concentration of IL‑6 within the contracting skeletal muscle
could be fivefold to 100-fold higher than the levels
found in the circulation and supports the idea that IL‑6
accumulates within muscle fibres as well as in the inter­
stitium during and following exercise.53 The simulta­neous
measure­ment of arteriovenous IL‑6 concentrations and
blood flow across an exercising leg has demonstrated that
large amounts of IL‑6 are also released into the circulation
from the exercising muscle.54
Human skeletal muscle is unique in that it can produce
IL‑6 during contraction in a strictly TNF-independent
fashion. 40 This finding suggests that muscular IL‑6
has a role in metabolism rather than in inflammation.
In support of this hypothesis, both intramuscular IL6
mRNA expression55 and IL-6 protein release56 are markedly enhanced when intramuscular glycogen levels
Lipolysis
UCP-1
IL-6
Irisin
IL-6
IL-6
BDNF
IL-6
Follistatin
LIF
IL-4
IL-6
IL-7
IL-15 Myostatin Unknown
exercise
Lipolysis
Hypertrophy
Hepatic glucose
production
during exercise
Hepatic CXCL-1
production
stimulus
IL-6
AMPK
Glucose
uptake
Fat oxidation
Angiogenesis
IGF-1
FGF-2
FGF-21
IL-8?
IL-6
Insulin secretion CXCL-1?
via GLP-1
Follistatin-related
protein 1
Promotes endothelial
function and
revascularization
Figure 2 | Skeletal muscle is a secretory organ. LIF, IL‑4, IL‑6, IL‑7 and IL-15
promote muscle hypertrophy. Myostatin inhibits muscle hypertrophy and exercise
provokes the release of a myostatin inhibitor, follistatin, from the liver. BDNF and
IL‑6 are involved in AMPK-mediated fat oxidation and IL‑6 enhances insulinstimulated glucose uptake. IL‑6 appears to have systemic effects on the liver and
adipose tissue and increases insulin secretion via upregulation of GLP‑1. IGF‑1
and FGF‑2 are involved in bone formation, and follistatin-related protein 1 improves
endothelial function and revascularization of ischaemic vessels. Irisin has a role in
‘browning’ of white adipose tissue.
are low, which suggests that IL‑6 works as an energy
sensor.57–60 This idea is supported by numerous studies
showing that glucose ingestion during exercise attenuates the exercise-induced increase in plasma IL‑623 and
inhibits the IL‑6 release from contracting skeletal muscle
in humans.23,61
Skeletal muscle is a tissue that is capable of altering
the type and amount of protein in response to regular
exercise. Exercise-induced adaptation in skeletal muscle
increases pre-exercise skeletal muscle glycogen content,
enhances activity of key enzymes involved in β‑oxidation,
increases sensitivity of adipose tissue to epinephrinestimulated lipolysis, and increases oxidation of intramuscular triglycerides. As a consequence, the trained
skeletal muscle can utilize fat as a substrate and is less
dependent on plasma glucose and muscle glycogen as
substrates during exercise.23,62 Several epidemiological
studies have reported a negative association between the
amount of regular physical activity and resting plasma
IL‑6 levels: the more physical activity, the lower the basal
plasma IL‑6 level.37 By contrast, high basal plasma levels
of IL‑6 are closely associated with physical inactivity and
the metabolic syndrome. Moreover, basal levels of IL‑6
are reduced after endurance training.37 In addition, the
exercise-induced increase in plasma IL‑6 and muscular
IL6 mRNA levels is diminished by endurance training.63
Interestingly, although resting plasma IL‑6 levels are
downregulated by endurance training, the resting muscular expression of IL‑6Rα is upregulated. In response
to endurance training, the basal IL6Rα mRNA content
in muscle is increased by ~100%.55 Thus, with exercise
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training, the downregulation of IL‑6 is partially counter­
acted by an enhanced expression of IL‑6Rα, such that
sensitivity to IL‑6 is increased. Our hypothesis is that
muscle disuse leads to IL‑6 resistance and elevated circulating levels of IL‑6, which would parallel the insulin
resistance that is accompanied by hyperinsulinaemia and
the leptin re­sistance that reflects chronic, high circulating
levels of leptin.
Acute treatment of rat L6 muscle cells in vitro with
IL‑6 increased basal glucose uptake and the translocation of the glucose transporter GLUT4.64 Moreover, IL‑6
increased insulin-stimulated glucose uptake in muscle
cells in vitro. The findings appear to be of clinical rele­
vance, as infusion of recombinant human IL‑6 into
healthy individuals during a hyperinsulinaemic, euglycaemic clamp procedure enhanced whole-body insulin sensitivity. Treatment with IL‑6 increased the glucose infusion
rate without having any influence on the total suppression
of endogenous glucose production.64 In vitro, the effects of
IL‑6 on glucose uptake appeared to be mediated by activation of AMPK, as the results were abolished in cells
infected with a recombinant adeno­v irus ex­pressing
­dominant-negative AMPK.64
Several studies have reported that IL‑6 might also
increase intramyocellular 64–66 or whole-body 67 fatty acid
oxidation via AMPK.64,68 IL‑6 acutely mediates signalling
through the receptor IL‑6Rβ (also known as glyco­protein
130 [gp130]) and exhibits many leptin-like actions, such
as activation of AMPK69–71 and insulin signalling.72
Interestingly, IL‑6 knockout mice develop late-onset
obesity and glucose intolerance,73 which supports the
notion that IL‑6 exerts beneficial effects on metabolism.
Importantly, IL‑6 is a myokine with cardinal biological activity, as it contributes to hepatic glucose production during exercise.74 The mechanisms that mediate the
tightly controlled production and clearance of glucose
during muscular work are unclear. An unidentified
‘work factor’ has been suggested to exist that influences
the contraction-induced increase in endogenous glucose
production. Acute administration of recombinant human
IL‑6 infused at physiological concentrations into resting
human individuals has no effect on whole-body glucose
disposal, glucose uptake or endogenous glucose production.66,75,76 By contrast, IL‑6 contributes to the c­ontractioninduced increase in endogenous glucose production.
When recombinant human IL‑6 was infused into healthy
volunteers during low-intensity exercise, to mimic the
circulating concentration of IL‑6 observed during highintensity exercise, the glucose output was as high as
during high-intensity exercise. The study demonstrated
a direct muscle–liver crosstalk. IL‑6 appeared to have a
role in endogenous glucose production during exercise
in humans; however, its action on the liver was dependent
on a yet unidentified muscle contraction-induced factor.74
Infusion of recombinant human IL‑6 into healthy
indivi­duals also caused an increase in lipolysis in the
absence of hypertriglyceridaemia or changes in catecholamines, glucagon, insulin or any adverse effects.66,67,76
These findings combined with cell culture experiments
show that IL‑6 has direct effects on both lipolysis and
fatty acid oxidation and identify IL‑6 as a lipolytic factor.66
Infusion of IL‑6 into healthy humans at a physiological
level primarily stimulates lipolysis in skeletal muscle,
whereas adipose tissue is unaffected.77
IL‑6 probably also mediates some of the anti-­
inflammatory and immunoregulatory effects of exercise.78,79 IL‑6 inhibits lipopolysaccharide-induced TNF
production in cultured human monocytes,80 and levels
of TNF are markedly elevated in mice treated with an
anti-IL‑6 antibody and in IL6 knockout mice,81 which
suggests that circulating IL‑6 is involved in the regulation of TNF levels. In addition, both recombinant human
IL‑6 infusion and exercise inhibit the endotoxin-induced
increase in circulating levels of TNF in healthy indivi­
duals.82 The anti-inflammatory effects of IL‑6 are also
demonstrated by IL‑6 stimulating the production of the
classic anti-inflammatory cytokines IL‑1ra and IL‑10.83
IL‑7
Haugen et al. identified IL‑7 as a myokine.42 IL‑7 is a
cytokine that is required for T‑cell and B‑cell development, whereas possible biological functions of IL‑7 in
nonimmune cells have not been explored. IL7 mRNA
and protein were detected in conditioned media from
primary cultures of human myotubes as well as inside the
myotube. The amount of IL‑7 in the medium increased
with incubation time.42 Incubations with recombinant
IL‑7 during differentiation of human myoblasts induced
a reduction in mRNA levels of the terminal myogenic
markers myosin heavy chain 2 and myogenin. This
finding suggests that IL‑7 might act on satellite cells,
which are small mononuclear progenitor cells found in
mature muscle. Haugen and co-workers also demonstrated that the muscular expression of IL7 mRNA was
increased several fold in biopsy samples from resting
vastus lateralis and trapezius muscles taken from male
individuals undergoing a strength training program.42
IL‑8 and CXCL‑1
IL‑8 belongs to a large family of chemokines. This myo­
kine is mainly produced by macrophages and endothelial cells and exerts marked chemotactic activity towards
leukocytes, in addition to being an angiogenic factor.
The murine chemokine CXC ligand 1 (CXCL‑1) shares
the highest sequence homology with human CXCL‑1,
but it is often mentioned as the functional homologue
to human IL‑8.84 CXCL‑1 and IL‑8 possess neutrophil
chemoattractant activity. In addition, they are involved
in the processes of angiogenesis.85 The capacity of IL‑8
to induce angiogenesis is distinct from its capacity to
induce inflammation.86 IL‑8 binds to the CXC chemo­kine
receptors CXCR‑1 and CXCR‑2.87 CXCR‑2 is expressed by
human microvascular endothelial cells and is the receptor
responsible for IL‑8-induced angiogenesis.88
The production of different CXC chemokines is induced
by IL‑6.89 The role of exercise and IL‑6 in the regulation of
murine CXCL‑1 has, therefore, been studied.90 Following
a single bout of exercise, CXCL1 mRNA levels increased
in serum, muscle and liver. The exercise-induced regulation of liver CXCL1 mRNA expression was completely
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blunted in IL6 knockout mice. When IL‑6 was over­
expressed in murine muscles, a marked increase in serum
CXCL‑1 and liver CXCL1 mRNA expression occurred.
These data demonstrate a robust muscle–liver crosstalk
during exercise, in which exercise-induced IL‑6 production stimulates the liver to produce CXCL‑1. The study
found a higher expression of CXCL‑1 in liver compared
with muscle. However, muscular IL8 mRNA levels are
enhanced by exercise,91 and IL‑8 is released by human
primary cultured myotubes.42 The biological role of both
liver-derived and muscle-cell-derived IL‑8 remains to
be defined.
LIF
LIF was identified in 1988 as a protein secreted from
ascites tumour cells.92 This myokine belongs to the IL‑6
cytokine superfamily, which consists of structurally and
functionally related proteins named neuropoietins (or
gp130 cytokines).93 LIF has multiple biological functions,
being a stimulus for platelet formation, proliferation of
haematopoietic cells, bone formation, neural survival and
formation, and acute phase production by hepatocytes.94
Moreover, LIF induces satellite cell proliferation, which
is considered essential for proper muscle hypertrophy
and regeneration.95
LIF mRNA expression is induced in human skeletal
muscle following resistance exercise, and LIF protein is
secreted from electrically stimulated human cultured
myotubes. 96 In addition, chemical inhibition of the
signal­ling molecules PI3K and mTor and small interfering RNA (siRNA) knockdown (silencing) of Akt1 were
independently sufficient to downregulate LIF. Moreover,
LIF stimulated proliferation of human myoblasts and
induced expression of jun‑B and c‑Myc in human myotubes. By contrast, siRNA knockdown of the LIF receptor
resulted in a reduction of proliferation. These findings
suggest that LIF is a contraction-induced myokine that
exerts its effects in an autocrine and/or paracrine fashion
to promote satellite cell proliferation.
IL‑15
IL‑15 belongs to the IL‑2 superfamily and is expressed in
human skeletal muscle. In addition to its anabolic effects
on skeletal muscle, IL‑15 may have a role in lipid metabo­
lism.97 IL‑15 decreases lipid deposition in preadipo­
cytes and decreases the mass of white adipose tissue.98,99
Consequently, a negative association has been found in
humans between plasma IL‑15 levels and total adipose
tissue mass, trunk adipose tissue mass and percent
adipose tissue mass.100
Physical inactivity leads to loss of muscle mass and
accumulation of visceral fat,5 and some evidence points
to IL‑15 being involved in the regulation of abdominal
adiposity. In support, we demonstrated a decrease in visceral fat mass, but not subcutaneous fat mass, when IL‑15
was overexpressed in murine muscle.100 Although, IL‑15
has been suggested to play a part in muscle–adipose
tissue crosstalk, secretion of IL‑15 from muscle cells
has not been described and it is, therefore, premature to
classi­fy IL‑15 as a true myokine.
Other myokines
Generation of skeletal-muscle-specific, inducible Akt1
transgenic mice, which can reversibly grow functional
type II muscle fibres by switching Akt1 signalling on and
off, has enabled identification of novel muscle-secreted
factors.101 They include follistatin-related protein 1, which
seems to have cardioprotective effects,102,103 and FGF‑21.104
Follistatin-related protein 1 promotes endothelial cell
function and revascularization in ischemic tissue through
a mechanism dependent on nitric oxide synthase.102
Studies in humans support the notion that FGF‑21 is a
myokine that is upregulated by insulin.105 Other musclecell-derived proteins include BDNF,106 calprotectin,107
erythropoietin 108 and IL‑4, which enhances muscle
regeneration by stimulating the fusion of myoblasts
with myotubes.109
A role for myokines in muscle–bone interactions
has been suggested on the basis that two well-known
osteogenic factors, IGF‑1 and FGF‑2, are abundant in
homogenized muscle tissue and secreted from cultured
myotubes in vitro.110 The receptors for these growth
factors are localized to the periosteum at the muscle–
bone interface,111 which suggests that IGF‑1 and FGF‑2
might be involved in muscle–bone crosstalk.
In the past few years, irisin was discovered as a myo­
kine that drives brown-fat-like development of white
adipose tissue. PGC‑1α expression in muscle was shown
to increase the expression of FNDC5, which encodes a
membrane protein that is cleaved and secreted as irisin.112
Mice were injected with FNDC5-expressing adenoviral
particles, whereby irisin levels increased by threefold
to fourfold, resulting in the induction of a programme
of development of brown-fat-like cells of white adipose
tissue and a concomitant increase in energy expenditure.
Basal plasma levels of irisin increased in response to
10 weeks of regular exercise in humans, which suggests
that irisin has a role in training adaptation to exercise.112
Bioinformatics and proteomic studies
Up to 10% of encoded human genes have the capacity
to express proteins that could potentially be secreted
from cells. Such secreted factors may be involved in the
cell–cell communication that is required for homeostasis
in a complex organism.22 A number of research groups
have contributed to the identification of the muscle cell
secretome. Bortoluzzi et al. screened 6,255 products of
genes expressed in normal human skeletal muscle.17
They reported that the resulting putative skeletal muscle
secretome consisted of 319 proteins, including 78 still
uncharacterized proteins. Yoon et al. studied differentiated L6 rat skeletal muscle cells and identified a total of
254 proteins, among which 153 were classified as secretory proteins.18 In a study by Henningsen et al., a quantitative proteomics platform was used to search for factors
secreted during the differentiation of murine C2C12
skeletal muscle cells. In total, 635 secreted proteins were
identified.19 The members of the CC chemokine family
of proteins showed a highly distinct pattern of secretion
during differentiation.113 Norheim and co-investigators
demonstrated that a total of 236 proteins were detected
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Physical inactivity
Loss of muscle mass and abdominal adiposity
Macrophage infiltration of visceral adipose tissue
Chronic systemic inflammation
Insulin resistance, atherosclerosis,
tumour growth, impaired bone formation
Type 2 diabetes mellitus, cardiovascular diseases,
cancer, osteoporosis
Figure 3 | Links between physical inactivity and disease
development. Loss of muscle mass and accumulation
of visceral adipose tissue are general consequences of
physical inactivity. Abdominal adiposity stimulates
macrophage infiltration of adipose tissue, whereby a
network of inflammatory pathways is activated.
Inflammation promotes development of insulin resistance,
atherosclerosis, neurodegeneration, tumour growth and
impaired bone formation and, consequently, the
development of a myriad of chronic diseases. During
physical activity, muscles release myokines, which
stimulate muscle growth and hypertrophy, increase fat
oxidation, enhance insulin sensitivity and induce antiinflammatory actions.
Physical activity
Myokines
Muscle hypertrophy (myostatin, LIF, IL-4, IL-6, IL-7, IL-15)
Adipose tissue oxidation (IL-6, BDNF)
Insulin sensitivity (IL-6)
Osteogenesis (IGF-1, FGF-2)
Anti-inflammation (IL-6)
Anti-tumour defence (unidentified secreted factor(s))
Pancreas function (unidentified secreted factor(s))
Browning of fat (Irisin)
Decreased risk of chronic diseases and premature mortality
Figure 4 | The finding that muscle produces and releases
myokines provides a conceptual basis for understanding
some of the molecular mechanisms that link physical
activity to protection against premature mortality.
by proteome analysis in medium conditioned by cultured
human myotubes.114 Reverse transcription PCR analyses showed that 15 of the secreted muscle proteins had
markedly enhanced mRNA expression in the vastus lateralis and/or trapezius muscles after 11 weeks of strength
t­raining among healthy volunteers.
Myokines: clinical aspects
Many of the myokines identified exert their effects within
the muscle itself. Thus, myostatin, LIF, IL‑4, IL‑6, IL‑7
and IL‑15 are involved in the regulation of muscle hypertrophy and myogenesis. BDNF and IL‑6 are involved in
AMPK-mediated fat oxidation and IL‑8 (or CXCL‑1)
might be involved in mediating training-induced angiogenesis. However, IL‑6 also appears to mediate systemic
effects, including effects on the liver, adipose tissue and
the immune system. Follistatin-related protein 1 has
been identified to play a role in promoting endothelial
cell function and revascularization under conditions
of ischaemic stress, and IGF‑1 and FGF‑2 appear to be
involved in muscle–bone crosstalk.
The myokine field is new and, to date, most of the
human studies have focused on the biological role of
IL‑6. The finding that muscle-derived IL‑6 has several
beneficial metabolic effects suggests that it has a role in
the association between a physically inactive lifestyle
and an increased risk of chronic diseases. Sadagurski
and colleagues have demonstrated that transgenic mice
with sustained elevated circulating levels of human IL‑6
display enhanced central leptin action and improved
nutrient homeostasis that leads to protection from dietinduced obesity.115 In addition, Wunderlich et al.116 have
shown that IL‑6 signalling is required for normal liver
metabolism in mice. Of note, ciliary neurotrophic factor
is a member of the IL‑6 family of cytokines and also
improves metabolic homeostasis in mice when insulin
resistance is induced either from a high-fat diet 70 or lipid
infusion.117 Interestingly, a ciliary neurotrophic factor
variant, axokine, was in clinical trials for the treatment
of T2DM, but failed to be approved owing to a lack of
a neutralizing effect of the antibody. 118 Nonetheless,
the finding that exercise has multiple beneficial effects,
which may be mediated by myokines, suggests that there
might be therapeutic potential in myokine research.
Apart from the effects of myokines on peripheral
insulin sensitivity via the activation of AMPK, evidence
is emerging that myokines might also play a major part
in pancreatic β‑cell metabolism and insulin secretion.
Bouzakri et al. showed that human myotubes express
and release a different panel of myokines depending on
their insulin sensitivity, with each panel exerting differential effects on β cells. These preliminary data suggest
a new route of communication between skeletal muscle
and β cells, which is modulated by insulin resistance.119
Moreover, Ellingsgaard and colleagues showed that
whereas exercise increased glucose tolerance in normal
mice, this phenomenon did not occur in mice with global
IL‑6 deficiency.120 Using several elegant models, the
researchers were able to show that the exercise-induced
GLP‑1 response was dependent upon muscle-derived
IL‑6. Hence, IL‑6 mediates crosstalk between two different insulin-sensitive tissues, the gut and pancreatic
islets, in order to adapt to changes in insulin demand by
increasing GLP‑1 secretion.
Epidemiological studies suggest an increased risk
of breast cancer in women with a sedentary lifestyle,
whereas regular physical activity protects against the
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development of breast cancer.121 Evidence exists that one
or several myokines might mediate some of the inhibitory effects of exercise on mammary cancer cell proliferation and a possible candidate is oncostatin M, a member
of the IL‑6 superfamily.122
In summary, physical inactivity and muscle disuse
lead to loss of muscle mass and accumulation of visceral adipose tissue and consequently to the activation
of a network of inflammatory pathways, which promote
development of insulin resistance, atherosclerosis,
neurodegeneration and tumour growth and, thereby,
promote the development of a cluster of chronic diseases (Figure 3).21 By contrast, the finding that muscles
produce and release myokines provides a molecular basis
for understanding how physical activity could protect
against premature mortality (Figure 4).
Conclusions
Given that muscle is the largest organ in the body, the
identification of the muscle secretome could set a new
agenda for the scientific community. To view skeletal
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Acknowledgements
B. K. Pedersen is supported by a grant from the
Danish National Research Foundation (#02‑512‑55).
M. A. Febbraio is supported by grants from the
National Health and Medical Research Council
(NHMRC), The Diabetes Australia Research Trust and
the Victorian Government Operational Infrastructure
Support Program.
Author contributions
Both authors contributed equally to all aspects of
the article.
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