Diabetologia (2012) 55:1128–1139
DOI 10.1007/s00125-012-2454-z
ARTICLE
Contractile activity of human skeletal muscle cells prevents
insulin resistance by inhibiting pro-inflammatory
signalling pathways
S. Lambernd & A. Taube & A. Schober & B. Platzbecker &
S. W. Görgens & R. Schlich & K. Jeruschke & J. Weiss &
K. Eckardt & J. Eckel
Received: 5 May 2011 / Accepted: 8 December 2011 / Published online: 27 January 2012
# Springer-Verlag 2012
Abstract
Aims/hypothesis Obesity is closely associated with muscle
insulin resistance and is a major risk factor for the pathogenesis of type 2 diabetes. Regular physical activity not only
prevents obesity, but also considerably improves insulin
sensitivity and skeletal muscle metabolism. We sought to
establish and characterise an in vitro model of human skeletal muscle contraction, with a view to directly studying the
signalling pathways and mechanisms that are involved in
the beneficial effects of muscle activity.
Methods Contracting human skeletal muscle cell cultures
were established by applying electrical pulse stimulation.
To induce insulin resistance, skeletal muscle cells were
incubated with human adipocyte-derived conditioned medium, monocyte chemotactic protein (MCP)-1 and chemerin.
Results Similarly to in exercising skeletal muscle in vivo,
electrical pulse stimulation induced contractile activity in
human skeletal muscle cells, combined with the formation
of sarcomeres, activation of AMP-activated protein kinase
Electronic supplementary material The online version of this article
(doi:10.1007/s00125-012-2454-z) contains peer-reviewed but unedited
supplementary material, which is available to authorised users.
S. Lambernd : A. Taube : S. W. Görgens : R. Schlich :
K. Eckardt : J. Eckel (*)
Paul-Langerhans-Group, Integrative Physiology,
German Diabetes Center,
Auf´m Hennekamp 65,
40225 Duesseldorf, Germany
e-mail: eckel@uni-duesseldorf.de
A. Schober : B. Platzbecker : K. Jeruschke : J. Weiss
Institute of Clinical Biochemistry and Pathobiochemistry,
German Diabetes Center,
Duesseldorf, Germany
(AMPK) and increased IL-6 secretion. Insulin-stimulated
glucose uptake was substantially elevated in contracting
cells compared with control. The incubation of skeletal
muscle cells with adipocyte-conditioned media, chemerin
and MCP-1 significantly reduced the insulin-stimulated
phosphorylation of Akt. This effect was abrogated by concomitant pulse stimulation of the cells. Additionally, proinflammatory signalling by adipocyte-derived factors was
completely prevented by electrical pulse stimulation of the
myotubes.
Conclusions/interpretation We showed that the effects of
electrical pulse stimulation on skeletal muscle cells were
similar to the effect of exercise on skeletal muscle in vivo
in terms of enhanced AMPK activation and IL-6 secretion.
In our model, muscle contractile activity eliminates insulin
resistance by blocking pro-inflammatory signalling pathways. This novel model therefore provides a unique tool
for investigating the molecular mechanisms that mediate the
beneficial effects of muscle contraction.
Keywords Electrical pulse stimulation . Exercise . Glucose
uptake . Inflammation . Insulin resistance . Muscle .
Myotubes
Abbreviations
AMPK AMP-activated protein kinase
CM
Adipocyte-conditioned medium
EPS
Electrical pulse stimulation
GSK
Glycogen synthase kinase
hSkMC Human skeletal muscle cells
IKK
IκB kinase
MAPK Mitogen-activated protein kinase
MCP
Monocyte chemotactic protein
αMEM α-Modified Eagle’s medium
Diabetologia (2012) 55:1128–1139
MHC
MTT
NFκB
SSC
VEGF
Myosin heavy chain
3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide
Nuclear factor ‘kappa-light-chain-enhancer’ of
activated B cells
Saline-sodium citrate
Vascular endothelial growth factor
Introduction
Obesity in combination with a lack of exercise is a strong
risk factor for the development of type 2 diabetes. It is well
established that physical inactivity causes accumulation of
visceral fat and that the health consequences of both are
related to systemic low-grade inflammation [1, 2]. Importantly, the visceral fat compartment is a major secretory and
endocrine-active tissue producing numerous cytokines that
regulate energy metabolism and insulin sensitivity [3–5].
Adipocytes from obese persons are characterised by altered
endocrine function, leading to increased secretion of proinflammatory adipokines, such as TNFα, chemerin, monocyte chemotactic protein (MCP)-1 and resistin [6–9]. The
activation of inflammatory pathways leads to insulin resistance [10] in peripheral tissues such as skeletal muscle and
adipose tissue, constituting an early defect in the pathogenesis of type 2 diabetes [11]. Insulin-resistant and type 2
diabetic patients display impaired insulin action on wholebody glucose uptake, in part due to impaired insulinstimulated glucose uptake in skeletal muscle [11].
It is well accepted that physical activity exerts major
beneficial effects on the prevention of chronic diseases like
type 2 diabetes, cardiovascular disease, dementia and depression [12, 13]. Regular physical activity not only prevents obesity and reduces adipose tissue mass, but is also
known to increase insulin-stimulated glucose uptake in the
immediate post-exercise period [14], while chronic physical
activity enhances insulin sensitivity in human skeletal muscle [15, 16]. Acute exercise increases glucose uptake in
skeletal muscle by an insulin-independent mechanism that
bypasses the insulin signalling defects associated with pathophysiological conditions [17]. Additionally, exercise activates AMP-activated protein kinase (AMPK), which
phosphorylates and thereby inhibits acetyl-CoA carboxylase, resulting in reduced malonyl CoA content and hence
enhanced fatty acid oxidation [18].
At present, the molecular mechanisms mediating the
health-promoting effects of physical activity are not entirely
understood. In the last decade, it became evident that skeletal muscle is an endocrine organ that produces and releases
myokines in response to contraction [19]; these myokines
probably mediate the health-promoting effects of physical
activity. Myokines such as IL-6 and brain derived
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neurotrophic factor are released by skeletal muscle cells
after exercise and lead to enhanced fatty acid oxidation in
an AMPK-dependent fashion [20, 21].
We have previously shown that primary human skeletal
muscle cells (hSkMC) incubated with adipocyte-conditioned
medium (CM) from primary human adipocytes or treated with
adipokines like MCP-1 and chemerin are characterised by
impaired insulin signalling and glucose uptake [8, 9, 22].
Our aim here was to establish and characterise an in vitro
contraction model of hSkMC, which mimics exercise; the
overall goal was to analyse the signalling pathways and mechanisms involved in the beneficial effects of muscle activity.
We also combined this contraction model with the insulin
resistance model to analyse the cross-talk between adipocytes
and contracting hSkMC. We show here that the contractile
activity of hSkMC exerts an anti-inflammatory action, which
prevents the induction of insulin resistance.
Methods
Materials Reagents for SDS-PAGE were supplied by GE
Healthcare (Munich, Germany) and Sigma (Munich, Germany), and rotiphorese was supplied by Carl Roth (Karlsruhe, Germany). The following antibodies were used: antiphospho glycogen synthase kinase (GSK) 3α/β (Ser21/9),
anti-phospho Akt (Ser473, Thr308), anti-Akt, anti-phospho
nuclear factor ‘kappa-light-chain-enhancer’ of activated B
cells (NFκB) (Ser536), anti-NFκB, anti-IκB kinase (IKK)α/
ß, anti-IκBα, anti-phospho AMPKα (Thr172) and antiAMPKα (Cell Signalling Technology, Frankfurt, Germany);
anti-tubulin (Calbiochem, Merck Biosciences, Schmalbach,
Germany); sarcomeric α-actinin (Sigma); anti-myosin
heavy chain (MHC) (Upstate, San Diego, CA, USA);
and mitochondria oxidative phosphorylation antibody
cocktail and anti-GLUT12 antibody (Acris, Herford, Germany). Horseradish peroxidase-conjugated goat antirabbit and anti-mouse IgG were purchased from Promega
(Mannheim, Germany). Collagenase NB4 standard grade
was obtained from Serva (Heidelberg, Germany) and
culture media from Gibco (Berlin, Germany). Recombinant human chemerin was supplied by R&D Systems
(Wiesbaden-Nordenstadt, Germany) and MCP-1 by
PeproTech (Hamburg, Germany). Primary hSkMC and
the supplement pack for growth medium were obtained
from PromoCell (Heidelberg, Germany). Horse serum for
the differentiation medium was provided by Gibco. All
other chemicals were of the highest analytical grade
commercially available and purchased from Sigma.
Culture of hSkMC Primary hSkMC from five healthy white
donors (three males, 16, 21 and 47 years old; two females,
33 and 37 years old) were supplied as proliferating
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myoblasts and cultured as described earlier [23]. For an
individual experiment, myoblasts were seeded in six-well
culture dishes at a density of 1×105 cells/well and cultured
to near-confluence in α-modified Eagle’s medium
(αMEM)/Ham’s F-12 medium containing skeletal muscle
cell growth medium supplement. The cells were then differentiated in αMEM containing 2% (vol./vol.) horse serum
until day 5 of differentiation, followed by overnight starvation in αMEM without serum. Differentiated cells were
electrically stimulated and incubated as indicated with
CM, chemerin, MCP-1 and TNFα, respectively. Afterwards,
cells were stimulated with 100 nmol/l insulin for 10 min.
Electrical pulse stimulation Electrical pulse stimulation
(EPS) was applied to fully differentiated myotubes in sixwell dishes using a C-Dish combined with a pulse generator
(C-Pace 100; IonOptix, Milton, MA, USA). The instrument
emits bipolar stimuli to the carbon electrodes of the C-dish,
which are placed in the cell culture medium. The myotubes
were stimulated at 1 Hz, 2 ms and 11.5 V for 2 to 24 h. The
medium was changed directly before stimulation. To document the contraction of the stimulated myotubes, a series of
two images per s was taken using a microscope (DM RBE;
Leica, Heidelberg, Germany) and camera (HV-C20; Hitachi,
Tokyo, Japan). The images, shown at a rate of two images
per s, can be viewed in the electronic supplementary material [ESM] Video 1.
Adipocyte isolation and culture Adipose tissue samples
were obtained from subcutaneous fat of normal or moderately overweight women (BMI 27.9±0.9 kg/m2 [mean±
SEM], age 26–44 years). The procedure for obtaining adipose tissue was approved by the Ethics Committee of
Heinrich-Heine-University, Duesseldorf, Germany. All tissue donors were healthy, free of medication and had no
evidence of diabetes according to routine laboratory tests.
Pre-adipocytes were isolated by collagenase digestion and
differentiated as previously described [9, 24]. After 15 days,
70–90% of the seeded pre-adipocytes developed to differentiated adipocytes, as defined by accumulation of lipid
droplets. These mature adipocytes were then used to generate CM by incubation with αMEM for 48 h, as previously
described [25].
Immunofluorescence staining hSkMC were seeded on glass
coverslips, differentiated and stimulated by EPS. Afterwards
cells were fixed with 2% (wt/vol.) paraformaldehyde dissolved in PBS for 15 min at room temperature. Cells were
washed twice and permeabilised on ice for 5 min with 0.2%
(vol./vol.) Triton-X in buffer containing 20 mmol/l HEPES,
300 mmol/l saccharose, 50 mmol/l NaCl and 3 mmol/
l MgCl2. After blocking with 5% (wt/vol.) non-fat dry milk
in PBS, myotubes were incubated with anti-sarcomeric α-
Diabetologia (2012) 55:1128–1139
actinin, washed and incubated with a secondary rhodamineconjugated antibody. Myotubes were washed with 2×
saline-sodium citrate (SSC) buffer (0.3 mol/l NaCl,
0.03 mol/l sodium citrate, pH 7.0) and incubated with
100 μg/ml RNAse in 2× SSC buffer for 20 min at 37°C.
After washing with 2× SSC buffer, the nuclei were stained
with 5 μmol/l Syto13 green for 5 min at room temperature.
Electron microscopy Embedding of hSkMC in Epon 812 was
performed as described by Luft [26] and modified by Reale
[27]. In brief, cells were fixed in 2.5% (vol./vol.) glutaraldehyde/190 mmol/l cacodylate buffer, pH 7.4, and post-fixed in
1% (wt/vol.) osmium tetroxide. We used 1% (wt/vol.) uranyl
acetate and lead citrate [28] to stain ultra-thin sections. Sections were investigated using a transmission electron microscope (TEM910; Zeiss, Oberkochem, Germany).
Measurement of IL-6 and vascular endothelial growth
factor The cytokine concentration in the supernatant fractions was determined by IL-6 and vascular endothelial
growth factor (VEGF) ELISA (Biovendor, Heidelberg, Germany), respectively, both assays used according to the manufacturer’s protocol.
Cell viability assays hSkMC were differentiated and electrically stimulated for 2–24 h.
ATP assay Changes of relative ATP level were analysed
using an ATP cell viability assay kit (ApoSENSOR; BioVision, Heidelberg, Germany) according to the instructions.
Lactate assay L-(+)-lactate was detected in the supernatant
fraction with a kit (Lactate Assay Kit II; BioVision).
MTT assay The NADH content in the cells was determined
using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium
bromide (MTT). Myotubes were incubated for 2 h at 37°C
with 1.2 mmol/l MTT solution in medium. Afterwards, cells
were washed with PBS and lysed with 500 μl DMSO.
Absorption at 540 nm was determined using a plate reader
(InfiniteM200; Tecan, Maennedorf, Switzerland).
Cytotoxicity assay The supernatant fraction was collected
and lactate dehydrogenase release into the medium measured with a kit (Cytotoxicity Detection Kit Plus; Roche
Applied Science, Mannheim, Germany) used according to
the manufacturer’s protocol.
Marker of mitochondrial function hSkMC were incubated
with 1 μmol/l JC-1 dye for 30 min under culture conditions
after the indicated time points. Afterwards, hSkMC were
washed and analysed using a plate reader (InfiniteM200;
Tecan). JC-1 monomers were assessed using excitation/
Diabetologia (2012) 55:1128–1139
emission wavelengths of 485/530 nm, while J-aggregates
were measured at 560/595 nm.
RNA-isolation and quantitative real-time PCR Total RNA
was isolated and reverse-transcribed using kits (RNeasy
Mini, Omniscript Reverse Transcription; Qiagen, Hilden,
Germany) according to the manufacturer’s instructions.
Gene expression was determined by quantitative real-time
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PCR using QuantiTect primer assays and SYBR green
reagents (Qiagen) with 0.016 to 20.00 ng cDNA on a cycler
(Step One Plus; Applied Biosystems, Carlsbad, CA, USA).
Expression of the genes investigated was normalised to
actin. Gene expression was analysed via the ΔΔCt method.
Glucose uptake hSkMC were electrically stimulated for 7.5 h
on day 6 of differentiation. During the last 30 min of EPS, cells
were treated with insulin (100 nmol/l) and uptake of 2-deoxyglucose was measured for 2 h as described above [9, 29].
Fatty acid and glucose oxidation hSkMC were seeded on
10 mm coverslips in six-well culture dishes and electrically
stimulated as described above. Subsequently, coverslips
were transferred to 48 well culture dishes, and 11.1 kBq/
well of [14C]oleic acid and [14C]palmitic acid supplemented
with 1 μmol/l L-carnitine, or 7.4 kBq/well [U-14C]glucose
supplemented with 0.35 mmol/l glucose were added to
hSkMC. Culture dishes were incubated for 4 h in an oxidation chamber, which allows gas exchange between two
neighbouring wells. Filter papers soaked with NaOH were
placed in the empty neighbouring wells. Oxidation was
stopped, and CO2 was liberated via acidification of culture
media by injecting 1 mol/l HCl and trapped in filter paper.
Radioactivity was counted in a liquid scintillation counter
(Beckman, Munich, Germany).
Immunoblotting hSkMC were treated as indicated and lysed
in a buffer containing 50 mmol/l HEPES (pH 7.4), 1% (vol./
Fig. 1 Process of selecting the conditions of EPS protocol. Based on
previously published conditions of EPS, frequencies of 0.1, 1 and
10 Hz were tested [21, 30, 42]. The conditions were selected with
regard to optimised AMPK activation and IL-6 secretion. Effects of
variation of frequency (a, b) and test of differentiation with horse
serum (c, d) were quantified. a, b Myotubes were differentiated in
αMEM serum-free medium and subjected to EPS at 0.1, 1 and 10 Hz,
2 ms and 11.5 V. Total cell lysates (a) were obtained, resolved by SDSPAGE and immunoblotted with phospho-specific (p) AMPK (Thr172)
antibody; n03. b IL-6 secretion was determined in supernatant fractions; n04–5; **p<0.01 vs control. c Myotubes were differentiated in
αMEM serum-free medium containing 2% (vol./vol.) horse serum
during differentiation, in combination with overnight starvation. Total
cell lysates were obtained and treated as above (a); n≥3; *p<0.05.
White symbols, control; black symbols, EPS. d IL-6 secretion was
determined in supernatant fractions; n≥3; ***p<0.001 vs control
Fig. 2 Effect of EPS on sarcomere structure assembly in human
skeletal myotubes. a The cells were fixed and analysed for localisation
of sarcomeric α-actinin without (control), and after 8 (b) and 24 h (c)
of EPS (1 Hz, 2 ms, 11.5 V) by immunofluorescence staining, as
described. Red, sarcomeric α-actinin; green, sytogreen. Magnification×100. Sarcomeric α-actinin was evenly distributed in the cytoplasm without EPS (a). Sarcomeres became rapidly visible after EPS,
with (b, c) Z-lines appearing as a series of red lines. These findings
were confirmed by electron microscopy (d–f). Magnification×10,000.
MT, mitochondria; N, nucleus
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vol.) Triton-X, PhosStop and complete protease inhibitor
cocktail (Roche). Western blot analysis was performed as
described before [9]. Signals were visualised and evaluated
on a work station (VersaDoc 4000 MP; BioRad, Munich,
Germany) and analysed with an analysis software package
(Quantity One, version 4.6.7, BioRad, Munich, Germany).
Presentation of data and statistics Data are presented as
means±SEM. Unpaired two-tailed Student’s t test or oneway ANOVA (post hoc test Bonferroni’s multiple comparison test) were used to determine statistical significance. All
statistical analyses were performed using Prism5 (GraphPad, La Jolla, CA, USA), with a value of p<0.05 considered
to be statistically significant. Corresponding significance
levels are as indicated.
Results
EPS induces de novo sarcomere structure assembly in
human skeletal myotubes After 6 days of differentiation,
Fig. 3 Effect of EPS on the abundance of muscle proteins, secretion of
IL-6, and on VEGF and AMPK activation. hSkMC were differentiated
and exposed to EPS for the indicated times at 1 Hz, 2 ms and 11.5 V.
a Total cell lysates were resolved by SDS-PAGE; representative blots of
proteins as labelled are shown. b Relative gene expression of MYH1 and
MYH2 was measured by real-time PCR as described; n04–5; ***p<
0.001. c IL-6 and (d) VEGF secretion of human myotubes with and
without EPS was measured after indicated time points by ELISA; n≥3;
Diabetologia (2012) 55:1128–1139
most of the hSkMC fused and formed typical multinucleated
myotubes. As reported in our earlier study [22], differentiating hSkMC display increased protein abundance of myogenin, myoblast determination protein and MHC, which are
typical markers of myogenesis, but cells did not contract
spontaneously. The conditions of 1 Hz frequency, 2 ms
pulse duration and 11.5 V intensity were selected with
regard to most marked AMPK activation and IL-6 secretion
after differentiation with 2% horse serum (Fig. 1). Using this
protocol, we observed that after a few hours of continuous
EPS, a subset of myotubes showed noticeable, vigorous
contraction, with most of the myotubes contracting after
8 h EPS (ESM Video 1). Immunofluorescence staining of
sarcomeric α-actinin showed uniform distribution of the
protein in the cytoplasm of unstimulated cells. After EPS,
a reorganisation of the cytoskeleton and de novo formation
of sarcomeric structures with their typical striated pattern
were observed (Fig. 2a–c). These findings were confirmed
by electron microscopy (Fig. 2d–f). The typical Z-lines
appeared as a series of dark lines as described for crossstriated muscles.
*p<0.05. White symbols, control; black symbols, EPS. e Relative ATP
concentration of myotubes was measured after indicated time points of
EPS and in control, as described; n08, *p<0.05. f Total cell lysates were
obtained and analysed by SDS-PAGE and western blot using phosphospecific (p) AMPKα antibody. Quantified data were normalised to tubulin; n≥3; **p<0.01 vs corresponding basal level. Data (b–f) are mean±
SEM
Diabetologia (2012) 55:1128–1139
EPS increases IL-6 and VEGF secretion, as well as AMPK
activation EPS did not influence the abundance of motor
proteins like MHC and sarcomeric α-actinin, or that of the
glucose transporters GLUT1 and GLUT4 (Fig. 3a). However, in response to contraction, we observed an increase of
MYH1 mRNA, while no changes were measured for MYH2
mRNA (Fig. 3b). EPS-induced contraction upregulated secretion of IL-6 and VEGF, two known myokines. EPS
significantly upregulated IL-6 secretion after 8 and 24 h
compared with controls, reaching a concentration of 35±
8 pg/ml (mean±SEM) after 24 h EPS, compared with 14±
4 pg/ml in control (Fig. 3c). EPS immediately upregulated
VEGF secretion, reaching a maximum after 24 h of 274±
40 pg/ml (fivefold increase vs control) (Fig. 3d).
ATP content in hSkMC significantly increased during the
first 2 h of EPS (1.5-fold). Afterwards, the ATP level in
hSkMC decreased significantly (Fig. 3e). After 8 h, EPS
induced a 3.8-fold increase in AMPK (Thr172) phosphorylation compared with non-stimulated hSkMC (Fig. 3f). This
effect was diminished, but still significant after 24 h. However, significant changes in acetyl-CoA carboxylase phosphorylation or protein abundance were not observed upon
EPS (data not shown).
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in apoptotic cells. To rule out the possibility that EPS may
cause cell damage, several cell-based assays were performed. Cytotoxicity, as assessed by L-lactate dehydrogenase release, did not differ significantly between cells with
or without EPS (Fig. 4a). The NADH level in cells, as
assessed by MTT assay, was not changed by EPS compared
with control cells (Fig. 4b). The concentration of lactate in
the medium significantly increased from 8 to 24 h of culture
(Fig. 4c) from 187 ± 7 nmol/ml (mean ± SEM) to 336 ±
10 nmol/ml. EPS induced significant, higher lactate concentrations in the medium compared with controls. The polarisation of mitochondrial membranes was not disturbed,
since the ratio of JC-1 aggregates to monomers was not
changed during culture, either in unstimulated or in EPStreated cells (Fig. 4d). Protein abundance of the complexes
of the electron transport chain showed a significant increase
for complexes II, IV and V (Fig. 4e). In addition UCP3,
PGC1α (also known as PPARGC1A) and COX2 mRNA
were significantly increased compared with non-stimulated
cells (Fig. 4f).
EPS exerts no damaging effect on myotubes Decreased
levels of ATP and increased levels of ADP are recognised
EPS augments insulin-stimulated glucose uptake, but does
not change fatty acid oxidation To test whether EPS induces
changes in glucose homeostasis in hSkMC, we measured
glucose uptake. Insulin-stimulated glucose uptake was significantly increased over the basal level (2.6-fold; Fig. 5a),
Fig. 4 Determination of metabolic activity of skeletal muscle cells
after EPS. hSkMC were differentiated and stimulated for 8 and 24 h at
1 Hz, 2 ms and 11.5 V. a Lactate dehydrogenase (LDH) abundance in
the medium; n≥5, ***p<0.001. b The quantity of NADH in the cells
was measured using an MTT assay; n≥3, **p<0.01. c Lactate concentrations in the supernatant fractions of EPS-treated and untreated cells.
As positive control, lysis reagent (supplied with the kit) was used. n≥3,
***p< 0.001. d The ratio of JC-1 aggregates to monomers. As a
positive control, exposure to 100 μmol/l CCCP (carbonyl-cyanide mchlorophenyl hydrazone) for 45 min was used prior to JC-1 staining.
e Total cell lysates were obtained and analysed by SDS-PAGE and
western blot using an oxidative phosphorylation antibody cocktail.
Data are normalised to tubulin; n04, *p<0.05. f Relative gene expression of UCP3, PGC1α and COX2 as measured by real-time PCR;
n04–5; **p<0.01 for UCP3 and PGC1α, *p>0.05 for COX2. Values
(a–f) are mean±SEM
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Diabetologia (2012) 55:1128–1139
Fig. 5 Effect of EPS on glucose uptake and fatty acid oxidation in
hSkMC. a In the last 30 min of 7.5 h of EPS (1 Hz, 2 ms, 11.5 V) cells
were stimulated with 100 nmol/l insulin. Glucose uptake was assessed
for 2 h after acute insulin stimulation. n05; *p<0.05 vs basal control;
†p<0.05 vs basal control and EPS control. White bars, basal; black
bars, insulin-stimulated. b The increment of insulin-stimulated glucose
uptake was calculated as the difference between insulin-stimulated
glucose uptake (2-deoxy-glucose, 2-DOG) and basal control; n05;
*p<0.05. c Total cell lysates were obtained and analysed by SDSPAGE and western blot. Representative blots for proteins as indicated are
shown. d Glucose oxidation. Cells were treated with 100 nmol/l insulin as
indicated during incubation in the oxidation chamber; n04–5; *p<0.05
vs basal control; †p<0.05 vs basal control and EPS control. White bars,
basal; black bars, insulin-stimulated e Fatty acid oxidation as assessed
after 24 h EPS; n≥3; ‡p00.4. Data (a, b, d, e) are means±SEM. OA,
oleic acid; PA, palmitic acid
as reported in our earlier studies [22, 29]. Contraction of
hSkMC led to a significant 2.4-fold increase compared with
basal control. Importantly, hSkMC showed a marked increase (fivefold) in insulin-stimulated glucose uptake after
EPS compared with basal control. The incremental increase
in insulin-stimulated glucose uptake, which reflects the efficiency of insulin action, was profoundly augmented by
contractile activity of myotubes (Fig. 5b). However, levels
of insulin-sensitive glucose transporters, namely GLUT4
and GLUT12, were not affected (Fig. 5c). Oxidation of the
fatty acids, oleic acid and palmitic acid, and glucose oxidation were assessed by 14CO2 production. Glucose oxidation
was increased in response to insulin and EPS alone, while
the combined effect was additive (Fig. 5d). However, oxidation of both fatty acids was not changed after EPSinduced contraction (Fig. 5e).
phosphorylation (Ser473) in hSkMC, with a significant reduction of this response after incubation with CM (Fig. 6a).
When the cells were EPS-stimulated during incubation with
CM, this effect on insulin-stimulated Akt phosphorylation
was abrogated. Comparable effects were observed at the
level of GSK3α (Ser21) phosphorylation, with very marked
inhibition by CM and complete prevention by EPS. Chemerin and MCP-1 induced a significant reduction of insulinstimulated Akt phosphorylation at Ser473 and Thr308 sites,
respectively, as reported in our earlier studies [8, 9, 23]. The
application of EPS during treatment with chemerin and
MCP-1, respectively, restored insulin signalling in hSkMC
(Fig. 6b).
EPS protects hSkMC from impaired insulin signalling
induced by CM, MCP-1 and chemerin Under control
conditions, insulin induced a significant increase of Akt
EPS prevents activation of NFκB and p42/44 mitogenactivated protein kinase in hSkMC NFκB and IKKβ protein
abundance was significantly reduced in EPS-treated cells,
whereas IKKα and IκBα protein abundance were not affected (Fig. 7a). Incubation of cells with TNFα led to
activation of NFκB, reaching a maximum after 10 min
Diabetologia (2012) 55:1128–1139
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Fig. 6 Effect of CM, chemerin and MCP-1 on insulin signalling in
control and electrically stimulated hSkMC. a Myotubes were simultaneously incubated with CM for 8 h and exposed to EPS. After acute
stimulation with insulin, total cell lysates were obtained, resolved by
SDS-PAGE and immunoblotted with phospho-specific (p)Akt and
GSK3 antibodies. All data were normalised to the level of tubulin
and are expressed relative to insulin-stimulated control values. Data
are means±SEM; n≥5; *p<0.05. b Myotubes were incubated with
2 μg/ml chemerin (Chem) and 2 ng/ml MCP-1, respectively, and
simultaneously exposed to EPS. After acute stimulation with insulin,
total cell lysates were obtained, resolved by SDS-PAGE and immunoblotted with phospho-specific Akt antibodies for Ser473 and Thr308.
All data were normalised to the level of tubulin and are expressed
relative to the insulin-stimulated control value. Data are presented as
means±SEM; n≥4, *p<0.05 vs insulin-stimulated control. White bars,
basal; black bars, 100 nmol/l insulin
(3.2-fold; Fig. 7c). As shown by application of the selective
inhibitor of IKK, I229 (Fig. 7b), this effect is specifically
mediated by IKK downstream signalling, After EPS, NFκB
activation was diminished due to reduced NFκB protein
abundance. Most importantly, after EPS, maximal NFκB
activation in response to TNFα was substantially reduced
(1.7-fold after 10 min). Thus contractile activity exerts an
anti-inflammatory effect and interferes with TNFα-induced
NFκB signalling.
Incubation of hSkMC with TNFα led to a significant
decrease (59% after 20 min) of IKKβ protein levels
(Fig. 7d). In contrast, IKKβ protein abundance was significantly reduced after EPS, compared with quiescent controls
(Fig. 7a), but was not altered after TNFα treatment
(Fig. 7d). The incubation of myotubes with TNFα diminished IκBα protein abundance by more than 60% after
20 min, while TNFα had no effect on IκBα levels in contracting cells (Fig. 7e). CM and chemerin induced NFκB
activation (1.8-fold and 1.5-fold; Fig. 8a), while MCP-1
induced significant activation of p44/p42 mitogenactivated protein kinase (MAPK) (1.5-fold; Fig. 8c). Both
effects on NFκB and p44/p42 MAPK activation were diminished to control level after EPS (Fig. 8b, c).
Discussion
Exercise has been shown to have a positive impact on a number
of diseases in humans, including obesity and type 2 diabetes [1,
2, 13]. The pathogenesis of type 2 diabetes has been intensively
studied and is characterised by chronic hyperglycaemia, resulting in defects in insulin secretion, insulin action or both. However, knowledge of the interplay between different molecular
signalling pathways during exercise is still incomplete and
experimentally adequate models of exercise remain elusive.
In this study, we established and validated a model of contracting hSkMC and used it to further analyse the beneficial effect of
exercise in the context of insulin resistance.
It is well known that hSkMC display increased levels of
typical myogenesis markers like myogenin, myoblast determination protein, MHC and GLUT4 during differentiation in vitro
[20]. One limitation of the models used is the lack of contraction,
a central characteristic of muscle cells. Therefore, we stimulated
hSkMC with electrical pulses, resulting in vigorous contraction
and formation of striation patterns of sarcomeric structures, as
visualised by immunofluorescence staining of α-actinin
and electron microscopy. In line with this, Fujita et al.
observed de novo formation of sarcomeres after EPS in murine
1136
Diabetologia (2012) 55:1128–1139
Fig. 7 Prevention of TNFα-induced inflammatory signalling by EPS.
a Skeletal muscle cells were stimulated by EPS for 8 h, after which
total cell lysates were resolved by SDS-PAGE, and NFκB, IKKα,
IKKβ and IκBα levels determined by western blotting. White bar,
control; black bars, EPS. b Skeletal muscle cells were pre-incubated
with 10 μmol/l specific IKK inhibitor I229 and afterwards stimulated
with 50 pg/ml TNFα for 10 min. c–e Cells were treated with 50 pg/ml
TNFα for times as indicated after 8 h EPS. Total cell lysates were
resolved by SDS-PAGE and immunoblotted with (c) phospho-specific
(p) antibody for NFκB, and (d) specific antibodies for IKKß and (e)
IκBα. All data were normalised to the level of tubulin and are presented as means±SEM; n≥3; *p<0.05 vs basal level. White symbols,
control; black symbols, EPS
C2C12 cells, whereas calcium channel blockers like verapamil
and BAPTA-AM suppressed sarcomere structure development
[30]. Thus it can be speculated that manipulated Ca2+ transients,
achieved by applying an appropriate EPS to differentiated
hSkMC, are primarily necessary to accelerate de novo sarcomere assembly and to rapidly develop contractile activity. In
response to contraction, we observed an increase of MYHI
mRNA expression after EPS, in combination with an increase
of mitochondrial marker proteins and mitochondrial mRNA
expression. MYHI fibres are more efficient at using oxygen to
generate ATP for continuous, extended muscle contractions over
a long time, reflecting the experimental setup used by us with its
rather long EPS time of 24 h.
In skeletal muscle, three energy systems function to replenish ATP, namely creatine kinase, glycolysis and mitochondrial
respiration. All systems contribute to different degrees to the
replenishment of ATP on the basis of an interaction between the
intensity and duration of exercise [31]. Using EPS, we observed
enhanced glucose uptake and increased lactate concentrations.
During high-performance sports, glucose is catabolised and
pyruvate is substantially generated. When the mitochondrial
capacity is exceeded, pyruvate is reduced to lactate, resulting in
oxidation of NADH/H+ to NAD+ [31]. As we did not observe
increased fatty acid oxidation after 24 h of EPS, it is likely that
cells mainly catabolise glucose for ATP regeneration under the
applied conditions. However, after depletion of muscle glycogen by contraction, cells may replenish glycogen stores, as
well.
Human studies using the one-legged exercise model followed by a euglycaemic–hyperinsulinaemic clamp have
Diabetologia (2012) 55:1128–1139
1137
Fig. 8 Prevention of CM-, MCP-1- and chemerin-induced inflammatory signalling by EPS. a Skeletal muscle cells were pre-incubated with
or without 10 μmol/l of the specific IKK inhibitor I229. Afterwards,
cells were treated with CM and 2 μg/ml chemerin for 30 min. Total cell
lysates were resolved by SDS-PAGE and immunoblotted with a
phospho-specific (p) antibody for NFκB. b, c Skeletal muscle cells
were exposed to EPS for 8 h at 1 Hz, 2 ms and 11.5 V. Cells were
treated with CM and 2 μg/ml chemerin (b) or with 2 ng/ml MCP-1 (c)
for the last 30 min of stimulation. Total cell lysates were resolved by
SDS-PAGE and immunoblotted with a phospho-specific antibody for
NFκB (b) or MAPK (c). Blots are representative. All data were
normalised to the level of tubulin and are means±SEM; n≥4; *p<
0.05 vs basal level. b, c White bars, control (non-EPS); black bars, EPS
shown increased insulin sensitivity after exercise [32].
C2C12 cells derived from mouse skeletal muscle are mostly
used to investigate muscle cell differentiation, sarcomere
development, myotube contraction and glucose uptake [30,
33]. However, these cells do not produce sufficient levels of
GLUT4 and their insulin responsiveness is reportedly minimal, even after differentiation [34]. In our model, GLUT12
and GLUT4 protein abundance was not altered in cells
during EPS, but human cells exhibited a profound insulinstimulated glucose uptake (2.6-fold greater), compared with
C2C12 cells (1.4-fold) [33]. We observed increased insulin
efficiency after EPS, resulting in substantially elevated glucose uptake in contracting myotubes. As insulin signalling,
GLUT4 and GLUT12 protein abundance remained unaltered after EPS, it may be that contractile activity affects
the GLUT4 trafficking machinery, mediating a more efficient mobilisation of the transporter by insulin. Future studies will be needed to address these issues.
One of the major achievements in obesity research is the
finding that adipose tissue is a major endocrine organ, which
secretes numerous adipokines. Indeed, several hundred adipokines have now been identified [35]. The secretion of
adipokines is changed dramatically in the obese state, affecting a wide range of physiological functions and leading to
insulin resistance in skeletal muscle, among other tissues [6, 7,
36]. A main finding of the present study was the improved
insulin sensitivity of contracting hSkMC in conditions of
insulin resistance. As previously shown by our group, CM
(which contains the whole secretory output of mature adipocytes, including adipokines like chemerin, pigment
epithelium-derived factor and MCP-1) induced insulin resistance in hSkMC at the level of Akt and GSK3, and reduced
insulin-stimulated glucose uptake by activating inflammatory
signalling pathways [8, 9, 23, 37]. Incubation of hSkMC with
CM or adipokines is a suitable model to dissect different
mechanisms leading to muscle insulin resistance. In this study,
we showed that EPS completely prevented insulin resistance
in hSkMC at the level of Akt and GSK3 during incubation
with CM, MCP-1 or chemerin. To date, we have only analysed the effects of EPS with regard to insulin signalling.
Future work will be required to assess its effects at effector
systems located further downstream from insulin.
One potential reason for the observation described above
might be the blocking effect of EPS on NFκB activation by
CM and several adipokines. Some adipokines involved in the
development of insulin resistance are known to activate inflammatory signalling pathways by activating IKK and its
downstream effector NFκB [38]. These proteins belong to a
family of transcription factors that controls production of proinflammatory proteins. In our model, CM, chemerin and
TNFα induced activation of NFκB, which is consistent with
the elevated NFκB activity observed in muscle of insulinresistant participants in comparison with lean control participants under basal conditions [39]. EPS, which mimics contraction of an active muscle, diminished NFκB activity and
prevented activation of NFκB by CM and chemerin. Additionally, the increment of NFκB activation by TNFα treatment
after EPS was profoundly diminished.
Thus contractile activity of hSkMC appears to directly
inhibit TNFα signalling and activation of NFκB. This may
involve: (1) downregulation of pro-inflammatory signalling
components like IKK and NFκB, as shown here; (2) Ca2+mediated activation of anti-inflammatory pathways; and (3)
probably the release of myokines by contracting cells. The
1138
anti-inflammatory effect of exercise is well established [40]
and muscular IL-6 is thought to play a key role in this process
[41]. However, the effect of IL-6 is related to its profound
increase in the circulation after exercise, as well as to the
inhibition of TNFα and IL-1 receptor alpha production [19].
Thus additional myokines exerting an autocrine action may be
involved in the anti-inflammatory effect of muscle contractile
activity. Very recent work in our laboratory suggests that
several hundred myokines are released from contracting
hSkMC (S. Lambernd, unpublished observations). Identification of these factors will be instrumental to understanding the
beneficial effects of muscle contraction.
In conclusion, using this model of contracting myotubes,
we observed that the risk of insulin resistance is directly
diminished, as insulin signalling was not disturbed after
incubation with CM and inflammatory signalling was not
activated. This model provides a unique tool for investigation of the mechanisms and underlying signalling pathways
that mediate the beneficial effects of muscle contraction, and
will help further clarify the potential of exercise as a way of
combating insulin resistance.
Acknowledgements We thank J. Liebau (Department of Plastic Surgery, Florence-Nightingale-Hospital, Duesseldorf, Germany) and
C. Andree (Department of Plastic Surgery, Sana-Hospital, DuesseldorfGerresheim, Germany) for support in obtaining adipose tissue samples.
The secretarial assistance of B. Hurow and the technical help of
A. Cramer, A. Horrighs and D. Herzfeld de Wiza are gratefully
acknowledged.
Funding This work was supported by the Ministerium für Wissenschaft und Forschung des Landes Nordrhein-Westfalen (Ministry of
Science and Research of the State of North Rhine-Westphalia), the
Bundesministerium für Gesundheit (Federal Ministry of Health), the
Commission of the European Communities (Collaborative Project
ADAPT, contract no. HEALTH-F2-2008-201100), the European
Union COST Action (BM0602) and the Jühling Foundation.
Contribution statement SL contributed to the concept, acquired,
analysed and interpreted data, wrote the manuscript and had the main
responsibility together with JE. AT, AS, BP, SWG, RS, KJ and JW
performed research and contributed to analysis and interpretation of
data. KE and JE contributed to the concept, analysis of the data, and the
discussion and revision of the manuscript. All authors approved the
final version of the manuscript.
Duality of interest The authors declare that there is no duality of
interest associated with this manuscript.
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