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Human muscle fiber type specific insulin signaling – Impact of obesity and type 2 diabetes
Peter H. Albers1,2, Andreas J.T. Pedersen3, Jesper B. Birk1, Dorte E. Kristensen1, Birgitte F. Vind3,
Otto Baba4, Jane Nøhr2, Kurt Højlund3, Jørgen F.P. Wojtaszewski1
1Section of Molecular Physiology, Department of Nutrition, Exercise and Sports, August Krogh
Centre, University of Copenhagen, Denmark
2Diabetes Research Unit, Novo Nordisk A/S, Maaloev, Denmark
3Diabetes Research Center, Department of Endocrinology, Odense University Hospital, Odense,
Denmark
4Section of Biology, Department of Oral Function & Molecular Biology, School of Dentistry, Ohu
University, Koriyama, Japan
*Corresponding Author: Jørgen F.P. Wojtaszewski, PhD. Universitetsparken 13, DK 2100
Copenhagen Ø, Denmark. Phone no: (+45) 28751625, e mail: [email protected]
Running title: Muscle fiber types and insulin signaling
Word count (abstract): 197
Word count (main text): 3979
References: 48
Number of tables+figures: 2+6
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ABSTRACT
Skeletal muscle is a heterogeneous tissue composed of different fiber types. Studies suggest that insulin mediated glucose metabolism is different between muscle fiber types. We hypothesized that differences are due to fiber type specific expression/regulation of insulin signaling elements and/or metabolic enzymes. Pools of type I and II fibers were prepared from biopsies of the vastus lateralis muscles from lean, obese and type 2 diabetic subjects before and after a hyperinsulinemic euglycemic clamp. Type I fibers compared to type II fibers have higher protein levels of the insulin receptor, GLUT4, hexokinase II, glycogen synthase (GS), pyruvate dehydrogenase (PDH E1α) and a lower protein content of Akt2, TBC1D4 and TBC1D1. In type I fibers compared to type II fibers, the phosphorylation response to insulin was similar (TBC1D4, TBC1D1 and GS) or decreased (Akt and PDH E1α). Phosphorylation responses to insulin adjusted for protein level were not different between fiber types. Independently of fiber type, insulin signaling was similar (TBC1D1, GS and
PDH E1α) or decreased (Akt and TBC1D4) in muscle from patients with type 2 diabetes compared to lean and obese subjects. We conclude that human type I muscle fibers compared to type II fibers have a higher glucose handling capacity but a similar sensitivity for phosphor regulation by insulin.
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Keywords
Skeletal muscle • Insulin sensitivity • Glucose disposal rate • Indirect calorimetric • Insulin signaling •
Myosin heavy chain composition • Glycogen • GLUT4 • Glycogen synthase • TBC1 domain family member
• Pyruvate dehydrogenase • Akt
Abbreviations
BMI Body mass index EDL Extensor digitorum longus GDR Glucose disposal rate GS Glycogen synthase GSK Glycogen synthase kinase HK Hexokinase HRP Horseradish peroxidase mTOR mammalian target of rapamycin mTORC mammalian target of rapamycin complex MHC Myosin heavy chain NDRG N myc downstream regulated gene NOGM Non oxidative glucose uptake PDC Pyruvate dehydrogenase complex PDH E1α Pyruvate dehydrogenase E1 alpha subunit RT Room temperature T2D Type 2 diabetic TBC1 Tre 2/USP6, BUB2, cdc16 TBC1D TBC1 domain family member
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INTRODUCTION
Skeletal muscle is important for whole body insulin stimulated glucose disposal (1), and skeletal muscle insulin resistance is a common phenotype of obesity and type 2 diabetes (2). Skeletal muscle is a heterogeneous tissue composed of different fiber types, which can be divided according to myosin heavy chain (MHC) isoform expression. Studies in rodents show that insulin stimulated glucose uptake in the oxidative type I fiber dominant muscles is higher than in muscles with a high degree of glycolytic type II fibers (3 6). Whether this phenomenon is due to differences in locomotor activity of individual muscles or a direct consequence of the fiber type composition is largely unknown. In incubated rat muscle, insulin induced glucose uptake was higher (~100%) in type IIa (oxidative/glycolytic) compared to IIx and IIb (glycolytic) fibers (7;8), suggesting that insulin mediated glucose uptake is related to the oxidative capacity of the muscle fiber. In humans, a positive correlation between proportions of type I fibers in muscle and whole body insulin sensitivity has been demonstrated (9 11). Furthermore, insulin stimulated glucose transport in human muscle strips was associated with the relative type I fiber content (12). Thus, it is likely that human type I fibers are more important than type II fibers for maintaining glucose homeostasis in response to insulin. Indeed, a decreased proportion of type I fibers has been found in various insulin resistant states such as the metabolic syndrome (9), obesity (13;14), type 2 diabetes in some
(10;13;14) but not all (12;15) studies and following bedrest (16) as well as in tetraplegic patients
(17) and subjects with an insulin receptor gene mutation (18).
Mechanisms for a fiber type dependent regulation of glucose uptake could involve altered abundance/regulation of insulin signaling elements and/or metabolic enzymes. In rats, insulin receptor content and Akt and GLUT4 protein abundance are higher in type I compared to type II fiber dominated muscles (4;5;19 21). Furthermore, in rats, Akt phosphorylation under insulin stimulationare highest in type I compared to type II fiber dominant muscles (20). In humans,
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GLUT4 protein levels are higher in type I compared to type IIa and IIx muscle fibers (14;22).
Overall, these findings suggest that insulin signaling to and effect on glucose transport is highest in
type I fibers. Thus, a shift towards reduced type I and hence higher type II fiber content in obesity
and type 2 diabetes (10;13;14) could negatively influence muscle insulin action on glucose
metabolism. Insulin resistance in obesity and type 2 diabetes is characterized by a decreased ability
of insulin to induce signaling proteins proposed to mediate GLUT4 translocation by i.e.
phosphorylation/activation of Akt (23 25) and/or TBC1 domain family member (TBC1D) 4 (23;25).
Whether this relates to differences in the response to insulin between fiber types is unknown.
Intracellular glucose metabolism could also be different between muscle fiber types. Glucose
entering the muscle cell is initially phosphorylated by hexokinase (HK) and predominantly stored as
glycogen or oxidized in the mitochondria, through processes regulated by glycogen synthase (GS)
and the pyruvate dehydrogenase complex (PDC), respectively. HKII content is higher in human
soleus muscle (~70% type I fibers) compared to gastrocnemius and vastus lateralis muscle (~50%
type I fibers) (26). Also, the content of the PDC subunit PDH E1α is decreased in muscle of
proliferator activated receptor gamma coactivator 1α knock out mice (27), concomitant with a
switch towards reduced type I fiber abundance (28). Furthermore, mitochondrial density is higher in
human type I compared to type II fibers (29). In contrast, no fiber type specific expression pattern
of GS has been shown (30). All together these observations suggest that glucose phosphorylation
and oxidation but not storage rate capacity are enhanced in type I compared to type II fibers.
Whether HKII and PDH E1α abundance as well as GS and PDH E1α regulation by insulin is
different between human muscle fiber types is unknown.
We investigated whether proteins involved in glucose metabolism were expressed and/or regulated
by insulin in a fiber type specific manner in human skeletal muscle. This was achieved by creating
pools of single fibers expressing either MHC I (type I) or II (type II). These fibers were dissected
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from vastus lateralis muscle biopsies obtained from lean and obese normal glucose tolerant subjects as well as type 2 diabetic patients.
RESEARCH DESIGN AND METHODS
Subjects. 10 lean healthy, 11 obese non diabetic and 11 obese type 2 diabetic (T2D) subjects were randomly chosen from two studies conducted at Odense University Hospital, Odense, Denmark.
One fraction (8 lean, 7 obese, 6 T2D) were from an already published study (31), while the remaining subjects were from an unpublished study, in which subjects were investigated with an identical experimental protocol as previously described (31). Both studies were approved by the regional ethics committee and carried out in accordance with the Declaration of Helsinki II. Subject medication is detailed in supplemental materials.
Experimental protocol. Detailed explanation of the in vivo study protocol has been published elsewhere (31). In short, all subjects were instructed to refrain from strenuous physical activity 48 h before the experimental day. After an overnight fast, subjects underwent a 2 h basal tracer equilibration period followed by a 4 h hyperinsulinemic euglycemic clamp at an insulin (Actrapid,
Novo Nordisk, Denmark) infusion rate of 40 mU m 2 min 1 combined with tracer glucose and indirect calorimetry. A primed constant [3 3H]glucose infusion was used throughout the 6 h study, and [3 3H]glucose was added to the glucose infusates to maintain plasma specific activity constant at baseline levels during the 4 h clamp period as described in detail previously (32). Vastus lateralis muscle biopsies were obtained before and after the clamp under local anesthesia (1% lidocaine) using a modified Bergström needle with suction. Muscle biopsies were immediately frozen in liquid nitrogen and stored below 80⁰C.
Dissection of individual muscle fibers. Muscle fibers were prepared as previously described (33) but with minor modifications. 20 60 mg of muscle tissue were freeze dried for 48 h before
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dissection of individual muscle fibers in a climate controlled room (20⁰C, <35% humidity) using a
dissection microscope (in total n= 5384 fibers from 64 biopsies). The length of each fiber was
estimated under the microscope (1.5±0.4 mm, mean±SD) before being carefully placed in a PCR
tube and stored on dry ice. On the day of dissection 5 µl of ice cooled Laemmli sample buffer (125
mM Tris HCl, pH 6.8, 10% glycerol, 125 mM SDS, 200 mM DTT, 0.004% Bromophenol Blue)
was added to each tube. During method optimization addition of protease and phosphatase
inhibitors were found to be unnecessary for preservation of either protein content or protein
phosphorylation for this type of sample preparation (data not shown). After thorough mixing at 4⁰C,
each tube was inspected under a microscope to confirm that the fiber was properly dissolved (if not,
the tube was discarded). Each sample was then heated for 10 minutes at 70⁰C and stored at 80⁰C.
Preparation of pooled muscle fiber samples. A small fraction (1/5) of the solubilized fiber was
used for identification of MHC expression using Western Blotting and specific antibodies against
MHC I or II (see section on immunoblotting). Hybrid fibers (~5%) expressing more than one MHC
isoform were discarded. Pools of type I and II fibers from each biopsy were prepared (128 pools in
total). The average number of type I and II fibers per muscle biopsy included in each pool were 20
(range: 9 36) and 42 (range: 22 147), respectively.
Estimation of protein content and test of purity. Protein content of the fiber specific samples was
estimated using 4 20% Mini PROTEAN TGX stain free gels (BioRad, CA), which allowed for gel
protein imaging following UV activation on a ChemiDoc MP Imaging System (BioRad, CA). The
intensity of visualized protein bands (from 37 260 kDa) was compared to a standard curve from 3
different pools of human muscle homogenates with a known protein concentration (supplementary
figure 1). After gel imaging, the purity of each pooled sample was re evaluated using Western
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Blotting and MHC I and II specific antibodies (see section on Immunoblotting). All fiber specific samples were diluted with Laemmli sample buffer to a protein concentration of 0.2 mg/ml.
Glycogen determination in muscle fiber pools. Glycogen content in the fiber specific pools was measured by dot blotting using a specific antibody against glycogen (34;35). Briefly, 150 ng of protein was spotted onto a PVDF membrane. After air drying, the membrane was re activated in ethanol before blocking, incubation in primary and secondary antibody and visualization as described in the section on Immunoblotting. The intensity of each dot was compared to a standard curve (supplementary figure 2) from a muscle homogenate with an glycogen content pre determined biochemically as previously described (31) and expressed accordingly.
MHC determination. For MHC determination in muscle biopsies, lysates were prepared and protein content was measured as previously described (31). Muscle lysates were diluted 1:3 with
100% glycerol/Laemmli sample buffer (50/50) and run on 8% self cast stain free gels, containing
0.5% 2,2,2 Trichloroethanol (36). 3 µg of lysate protein was separated for ~16 h at 140 V as previously described (37). Protein bands were visualized by UV activation of the stain free gel on a
ChemiDoc MP Imaging System (Biorad, CA) and quantified as stated below. Coomassie staining of the gel and the use of muscle homogenates provided similar results as stain free gel imaging and muscle lysates, respectively (data not shown).
Immunoblotting. For MHC determination of single muscle fibers and evaluation of total and phosphorylated levels of relevant proteins, equal amounts of sample volume (for MHC determination) or protein amount were separated using either pre cast (Biorad, CA) or self cast 7.5% gels. On each gel, an internal control (muscle lysate) was loaded two times per gel in order to minimize assay variation. Muscle fiber pool values were divided by the average of the internal control sample from the corresponding gel. Furthermore, on one gel a standard curve of muscle
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homogenate was loaded to ensure that quantification of each protein probed for was within the
linear range. Following separation, proteins were transferred (semidry) from multiple gels to a
single PVDF membrane which was incubated with blocking agent (0.05% Tween 20 and 2%
skimmed milk in TBS) for 45 minutes at room temperature (RT), followed by incubation in primary
antibody solution overnight at 4⁰C (for antibody details, see supplementary table 1). Membranes
were incubated with appropriate secondary antibodies (Jackson ImmunoResearch, PA) which were
conjugated to either horseradish peroxidase (HRP) or biotin for 1 h at RT. Membranes incubated
with biotin conjugated antibody were further treated with HRP conjugated streptavidin. Protein
bands were visualized using a ChemiDoc MP imaging system (BioRad, CA) and enhanced
chemiluminescence (SuperSignal West Femto, Pierce, IL). Band densitometry was performed using
Image Lab (version 4.0). Membranes were re probed with an alternate antibody according to the
scheme given in supplementary table 2.
Statistical analyses. Subject characteristics and blood parameters were evaluated by a one way
ANOVA. To compare fiber type, insulin and group effects, a three way ANOVA with repeated
measures for fiber type and insulin was used. If no triple interaction was present, a two way
ANOVA on the increment with insulin ( insulin basal values) was performed for fiber type and
group effects with repeated measures for fiber type. Main effects of group and significant
interactions were evaluated by Tukey post hoc testing. Statistical analyses were performed in
SigmaPlot (version 12.5, Systat Software, IL; one and two way ANOVA) and in SAS statistical
software (version 9.2, SAS Institute, NC; three way ANOVA). Unless otherwise stated n equals
number of subjects as indicated in table I. Differences were considered significant at p<0.05.
RESULTS
Clinical and metabolic characteristics. BMI and fat mass were higher in the obese and T2D
groups compared to the lean group (table 1). Patients with type 2 diabetes compared to lean and
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obese subjects had elevated HbA1c levels, increased fasting plasma glucose, insulin and triglyceride
(vs lean only) concentrations (table 2). During the hyperinsulinemic euglycemic clamp the glucose disposal rate (GDR) was decreased in type 2 diabetic vs lean and obese subjects (table 2). The decrease in GDR resulted from both lower glucose oxidation rates and reduced non oxidative glucose metabolism.
Fiber type composition. In muscle biopsies from lean and obese subjects MHC I, IIa and IIx constituted 45, 46 and 9% (totally 55% MHCII), respectively (Figure 1A). This fiber type composition is in accordance with previous observations using (immuno) histochemistry (9 11;13
15;26) and biochemically methods (18;22). In the T2D group MHC I, IIa and IIx constituted 35, 45 and 20% (totally 65% MHC II), respectively. In the T2D group compared to the lean and obese group, the relative number of type I muscle fibers was lower and the relative number of type IIx muscle fibers was higher. MHC IIa expression was similar between all three groups.
Insulin receptor, hexokinase II, GLUT4 and complex II. As represented in figure 1B, all fiber pools contained one MHC isoform only. Actin was used as reference protein and actin abundance was equal between fiber pools (supplementary figure 3). Higher protein levels of insulin receptorβ
(+16%), HKII (+470%), GLUT4 (+29%) and electron transport chain complex II (+35%) was found in type I vs II fibers (Figure 1C F). No differences between groups were observed except for a reduced ( 24%) insulin receptorβ level in the T2D compared to the lean and obese groups (Figure
1C F).
Akt, mTOR and NDRG1. Akt2 protein content was lower ( 27%) in type I vs II fibers (Figure 2C).
In the three groups, the average increases under insulin stimulation of p AktThr308 and p AktSer473 were 5.8 and 3.5 fold in type I fibers and 6.1 and 3.7 fold in type II fibers, respectively (Figure
2A+B). In lean and obese groups levels of insulin stimulated p AktThr308 were lower ( 25%) in type
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I vs II fibers. In the T2D group the insulin stimulated p AktThr308 and p AktSer473 were lower in both
fiber types compared to lean and obese groups. In response to insulin, phosphorylation of
AktSer473/Akt2 but not AktThr308/Akt2 was fiber type dependent, although the relative response to
insulin was similar between fiber types (supplementary figure 4A+B). In type I fibers, a higher
protein level of mammalian target of rapamycin (mTOR) (+20%) and its downstream target N myc
downstream regulated gene (NDRG) 1 (+68%) compared to type II fibers was evident (Figure
3B+D). Insulin had no effect on p mTOR2481 but increased p NDRG1Thr346 only in type I fibers
from obese (+86%) and T2D (+100%) groups (Figure 3A+C). No fiber type differences were
evident when p NDRG1Thr346 was adjusted for NDRG1 protein abundance (supplementary figure
4C).
TBC1D1 and TBC1D4. TBC1D1 and TBC1D4 protein levels were 45% and 16% in type I vs II
fibers, respectively (Figure 4B+G). Irrespective of fiber type, insulin stimulation increased p
TBC1D1Thr596 (+36%) and p TBC1D4 at all sites investigated (Ser318 (+122%), Ser588 (+59%),
Thr642 (+103%) and Ser704 (+113%)) (Figure 4A+C F). Statistically significant main effects of fiber
type were evident for the level of phosphorylation of both TBC1D1 and TBC1D4. More
specifically p TBC1D1Thr596 ( 62%), p TBC1D4Ser318 ( 21%), p TBC1D4Ser588 ( 21%), p
TBC1D4Thr642 ( 24%) and p TBC1D4Ser704 ( 24%) were lower in type I compared to type II fibers.
No significant group differences in protein abundance or protein phosphorylation of TBC1D1 and
TBC1D4 were evident, although the response to insulin of p TBC1D4Ser588 tended (p=0.07) to be
group dependent.
Glycogen content, GSK3 and glycogen synthase. In the basal state, glycogen content was lower (
29%) in type I vs II fibers in the obese (p=0.09) and type 2 diabetic (p=0.09) group; Figure 5A).
Insulin induced no significant changes in glycogen content in either of the fiber types. The protein
levels of glycogen synthase kinase (GSK) 3β was 14% less in type I vs type II, whereas GS protein
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was 53% higher in type I compared to type II fibers (Figure 5C+F). In all 3 groups and in both fiber types insulin induced a similar change in phosphorylation of GSK3βSer9 (+62%), GS2+2a ( 36%) and
GS3a+b ( 38%) (Figure 5B+D+E). Phosphorylation of GSK3βSer9 was lower ( 31%), whereas phosphorylation of GSsite2+2a and GSsite3a+b were respectively 68% and 51 % higher in type I vs II fibers. No significant differences were evident between individual groups in protein abundance and protein phosphorylation of GSK3β and GS.
Pyruvate dehydrogenase. PDH E1α protein content was 34% higher in type I vs II fibers (Figure
6C). Basal levels of PDH E1α site1 phosphorylation were similar between fiber types in all 3 groups (Figure 6A). After insulin the degree of phosphorylation was significantly lower in type II vs
I fibers in the obese and T2D groups only, indicating de phosphorylation by insulin in type II but not in type I fibers. In line, PDH E1α site2 phosphorylation was decreased by insulin, and this effect was dependent on fiber type towards a greater effect of insulin in type II vs I fibers (Figure
6B). Fiber type differences were not evident when p PDHsite1 and p PDHsite2 was adjusted for PDH
E1α content (supplementary figure 4D+E).
DISCUSSION
The current study is the first to evaluate changes in signaling events in response to insulin in fiber type specific pools from human muscle. Based on our findings we propose a model in which human type I fibers have a greater abundance of proteins to transport (+29% GLUT4), phosphorylate (+470% HKII) and oxidize (+35% ETC complex II and +34% PDH) glucose and to synthesize glycogen (+35% GS) compared to type II fibers. These observations are supported by significant positive correlations between the MHC I content in whole muscle lysates and insulin stimulated glucose disposal rate (r=0.53, p=0.002), glucose oxidation rate (r=0.52, p=0.003) and non oxidative glucose metabolism (r=0.44, p=0.01) (supplementary figure 5). Interestingly, even
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though insulin receptor content was higher (+16%) in type I fibers, phosphor regulation of TBC1D1,
TBC1D4 and GS by insulin was similar between fiber types (all normalized to actin). The apparent
fiber type differences in insulin stimulated phosphorylation of Akt, NDRG1 and PDH E1α (when
related to actin) were eliminated when adjusted for Akt2, NDRG1 and PDH E1α protein abundance.
These findings suggest a similar sensitivity of type I and II muscle fibers for regulation by insulin of
the proteins investigated.
Insulin stimulated glucose disposal rate, glucose oxidation rates and non oxidative glucose
metabolism were decreased in T2D compared to the lean and obese groups. This was accompanied
by lower insulin receptor content and altered response to insulin of p Akt308, p Akt473, p
TBC1D4Ser588 (p=0.07) and p NDRG1Thr346 in the muscle fiber specific pools from the T2D
compared to the lean and obese groups. In cells, NDRG1 phosphorylation has been suggested to be
a read out of mTOR complex (mTORC) 2 activities (38). mTORC2 is also a widely accepted
upstream kinase for AktSer473 (39). Since the response to insulin of p NDRG1Thr346/NDRG1 was
similar between groups, these data could imply a specific dysfunctional link between mTORC2 and
p AktSer473 as the latter was decreased in response to insulin in both type I and II fibers in T2D
compared to the lean and obese groups. In rat muscle, abundance and insulin stimulated
phosphorylation of Akt were higher (+660% and +160 180%, respectively) in soleus muscle
primarily containing type I fibers, as opposed to epitrochlearis and extensor digitorum longus (EDL)
muscles primarily consisting of type II fibers (20). In contrast, in human muscle, we report a
decreased Akt phosphorylation after insulin in type I vs II fibers, due to higher Akt2 levels in type
II fibers. Thus, findings in rat muscles with a diverse fiber type composition could simply result
from differences in locomotor activity, although species related differences cannot be excluded. For
instance, TBC1D4 and TBC1D1 protein abundance in the present study is only modestly lower ( 16
and 45%) in human type I vs II fibers. In mice, a high (>10 fold) TBC1D4 and a low (<20%)
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TBC1D1 content are evident in the type I fiber abundant soleus compared to the type II fiber abundant EDL muscle (40). In rats, no significant correlations between MHC isoform abundance in various muscles and either TBC1D1 or TBC1D4 protein content was found (21). These findings indicate that fiber type differences in TBC1D4 and TBC1D1 protein levels are highly dependent on the species investigated.
In the present study, no differences in the response to insulin were observed between fiber types in phosphorylation of TBC1D4 and TBC1D1. We previously reported a decreased response to insulin of p TBC1D4Ser318 and p TBC1D4Ser588 in skeletal muscle from obese type 2 diabetic subjects compared to weight matched controls (23). In the current study, insulin induced (delta values
(insulin minus basal) p TBC1D4Ser588 was borderline (p=0.07) group dependent. The average response to insulin was 62%, 96% and 19% in the lean, obese and T2D groups, respectively. It has been shown that exercise training normalizes defects in insulin action on TBC1D4 regulation in type 2 diabetic vs control subjects (23). Thus, in the present study, the lack of significant defects in
TBC1D4 regulation by insulin in the T2D group compared to control groups could be due to the physical fitness level of the groups studied. We found that p TBC1D1Thr596 was increased by insulin in agreement with another study (41) and that the relative increase was irrespective of fiber type and group. We conclude that the relative response to insulin of Akt, TBC1D4 and TBC1D1 is independent of fiber type, while the absolute amount of phosphorylated protein is lower in type I vs
II fibers. Whether a higher amount of phosphorylated protein is important for the regulation of glucose uptake is unknown. To investigate the impact of the present findings on glucose uptake in different human muscle fiber types, future studies need to examine the membrane bound fraction of
GLUT4 in different fiber types or even measure single muscle fiber glucose transport as performed in rat muscle (7).
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Interestingly, Gaster et al. (14) previously reported that GLUT4 abundance was significantly lower
in type I fibers only, in muscle from type 2 diabetic patients compared to lean and obese controls.
This was not evident in the current study. However, we found a non significantly lower GLUT4
content of the same magnitude (10 20%) as previously reported (14) in both type I and II fibers
from the T2D compared to the lean and obese groups. Also, GLUT4 levels were generally higher in
type I vs II fibers. Thus, fewer type I fibers in the T2D compared to the lean and obese groups
possibly lowers the glucose uptake capacity in diabetic skeletal muscle. In support, HKII content
was higher in type I compared to type II fibers. The influence of HKII protein levels on glucose
uptake is controversial and has recently been estimated to control ~10% of human skeletal muscle
glucose metabolism during insulin stimulated conditions (42). In the current study, fiber type
specific HKII levels were not different between groups investigated. Thus, it is likely that decreased
HKII levels reported in muscles from T2D subjects (43) are at least partly influenced by a lower
number of type I fibers in T2D vs control subjects as also shown in the present study. Interestingly,
in contrast to HKII, HKI protein abundance was lower ( 19%) among the three groups in type I vs
II fibers (supplementary figure 6). This observation could indicate a different role of HK isoforms
in type I and II muscle fibers.
A close correlation between the insulin stimulated increase in non oxidative glucose metabolism
and GS activity has been reported (44). In the present study, insulin stimulated non oxidative
glucose metabolism was decreased in the T2D compared to the lean and obese groups as shown by
others (23;31;41;45). Thus, we investigated the fiber type specific regulation of GS by insulin. We
were unable to detect any differences in the response to insulin between fiber types, although the
absolute amount of phosphorylated GS was highest in type I fibers. Increased phosphorylation of
GS in type I fibers could be accounted for by a higher GS protein level in type I vs II fibers.
Previously, a similar GS content in type I, IIa and IIx fiber pools was reported in muscle from
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young (23 yrs) subjects (30). Thus, the present findings of a higher GS content in type I vs II fibers in muscle from middle aged (~55 yrs) subjects, indicates an age dependent fiber type specific regulation of GS abundance. The functional consequence of a differentiated GS content between fiber types is unknown, since we were unable to detect any differences in basal and insulin stimulated glycogen content in both fiber types. This is likely due to the relatively small (<6%) increase in glycogen content during a clamp procedure (46). If glycogen levels were solely dependent on glycogen synthase, the activity of this enzyme would be expected to be lower in type I vs II fibers. However, our data cannot support this because the higher expression and phosphorylation of GS indicates that total GS activity is infact higher in type I vs II fibers. Thus, other factors than GS activity per se determines glycogen levels.
In a recent study, Nellemann et al. (47) did not find any changes in phosphorylation of PDH E1α in human skeletal muscle in response to insulin. Interestingly, in the present study, PDH E1α phosphorylation was decreased by insulin in type II fibers only. Thus, results by Nellemann et al. could have been influenced by a muscle fiber type dependent regulation not detected in their whole muscle biopsy preparation. An inverse relationship between PDH E1α phosphorylation and PDHa activity has been shown in human skeletal muscle during exercise (48). Thus, findings in the present study suggest an increased PDHa activity in response to insulin in type II fibers only.
Study limitations:
All fiber pools were prepared from vastus lateralis muscle, which expresses relatively small (<10%) amounts of type IIx fibers (26). No significant differences in the MHC IIx expression were observed between type II fiber pools among the three groups (supplementary figure 7). Thus, differences between type I and II fiber pools observed in the present study are likely not influenced by differences in protein abundance/regulation between type IIa and IIx fibers. No measure of
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physical activity was performed. It has been shown that training induced increases in GLUT4
content mainly occur in type I fibers (22). Thus, training status of the subjects in the present study
could potentially influence differences between muscle fibers and/or groups. All measures were
performed in muscle fibers from the vastus lateralis muscle. Whether fiber type specific differences
in protein expression can be extended to other muscles is unknown, but has been challenged by one
study (30), in which GLUT4 expression was higher in type I vs. IIa and IIx fibers from vastus
lateralis muscles but similar between fiber types in soleus and triceps brachii muscles. The present
study design did not allow exploration of this further. To evaluate the biological impact of fiber
specific signaling events further, the methods used in the present study could be combined with ex
vivo incubation of human muscle strips (12) and the recently described method of single fiber
glucose uptake measurements (7). Such design demands open surgical biopsies and was therefore
not applicable to the cohort of the present study.
In conclusion, based on protein level measures, the enzymatic capacities for glucose uptake,
phosphorylation and oxidation as well as for glycogen synthesis are higher in human type I
compared to type II muscle fibers. In response to insulin, most differences in phosphorylation
between fiber types were due to differences in protein levels. Thus, sensitivity for phosphor
regulation by insulin of these proteins is similar between fiber types. Even though insulin induced
glucose disposal rate was decreased in patients with type 2 diabetes compared to lean and obese
subjects, few group differences in the muscle fiber specific measurements were observed. However,
our observations favor the idea that fewer type I fibers and a higher number of type IIx fibers in
muscles from type 2 diabetic patients contributes to the reduced glucose disposal rate under insulin
stimulated conditions compared to lean and obese subjects.
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ACKNOWLEDGMENTS
Author contributions: Conception and design of research: P.H.A., J.F.P.W. Performed in vivo experiments: A.J.T.P., B.F.V. Performed analysis: P.H.A., A.J.T.P., J.B.B., D.E.K., B.F.V.
Interpreted results: P.H.A., J.B.B., K.H., J.F.P.W. Drafted manuscript: P.H.A., J.F.P.W. Edited and revised manuscript: All. Approved final version: All. Jørgen F.P. Wojtaszewski is the guarantor of this work and, as such, had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Assistance/donations: Maximilian Kleinert (University of Copenhagen, Denmark) is acknowledged for sharing his knowhow on the mTOR/NDRG1 analyses. Also, we are grateful for the kind donation of material essential for this work by the following scientists: LJ Goodyear (Joslin
Diabetes Center and Harvard medical school, Boston, MA), OB Pedersen (University of
Copenhagen, Denmark), J Hastie and DG Hardie (University of Dundee, UK). The monoclonal antibodies against MHC I and II isoforms (A4.840 and A4.74) were developed by H.M. Blau and antibody directed against MHC IIx (6H1) was developed by C. Lucas. All MHC antibodies were obtained from the Developmental Studies Hybridoma Bank developed under the auspices of the
NICHD and maintained by The University of Iowa, Department of Biology, Iowa City, IA 52242.
Funding/financial support: This work was carried out as a part of the research programs "Physical activity and nutrition for improvement of health" funded by the University of Copenhagen (UCPH)
Excellence Program for Interdisciplinary Research; and the UNIK project: Food, Fitness & Pharma for Health and Disease (see www.foodfitnesspharma.ku.dk) supported by the Danish Ministry of
Science, Technology and Innovation. This study was funded by the Danish Council for Independent
Research Medical Sciences (FSS), the Novo Nordisk Foundation and a Clinical Research Grant from the European Foundation for the study of Diabetes (EFSD). Disclosure statement: P.H.A. is financed as an industrial PhD student by the Danish Agency for Science, Technology and
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Innovation and Novo Nordisk A/S and owns stocks in Novo Nordisk A/S. J.N. is an employee at
Novo Nordisk A/S and owns stocks in Novo Nordisk A/S. A.J.T.P., J.B.B., D.E.K., B.F.V., O.B.,
K.H., J.F.P.W. have nothing to disclose.
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28. Handschin,C, Chin,S, Li,P, Liu,F, Maratos Flier,E, LeBrasseur,NK, Yan,Z, Spiegelman,BM: Skeletal muscle fiber type switching, exercise intolerance, and myopathy in PGC 1alpha muscle specific knock out animals. J Biol Chem 282:30014 30021, 2007
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31. Vind,BF, Birk,JB, Vienberg,SG, Andersen,B, Beck Nielsen,H, Wojtaszewski,JF, Hojlund,K: Hyperglycaemia normalises insulin action on glucose metabolism but not the impaired activation of AKT and glycogen synthase in the skeletal muscle of patients with type 2 diabetes. Diabetologia 55:1435 1445, 2012
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49. Jessen,N, An,D, Lihn,AS, Nygren,J, Hirshman,MF, Thorell,A, Goodyear,LJ: Exercise increases TBC1D1 phosphorylation in human skeletal muscle. Am J Physiol Endocrinol Metab 301:E164 E171, 2011
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Figure legends
Figure 1. Myosin heavy chain composition and muscle fiber type specific protein abundance in lean, obese and type 2 diabetic subjects. Myosin heavy chain composition measured in whole muscle biopsies from lean, obese and type 2 diabetic (T2D) subjects (A). The purity of each muscle fiber pool was checked by Western Blotting of myosin heavy chain (MHC) I and II (B).
Representative blots of type I (MHC I) and type II (MHC II) muscle fiber pools from three subjects are shown (B). In muscle fiber pools, the protein content of the insulin receptorβ (C), hexokinase II
(D), GLUT4 (E) and electron transport complex II (F) was evaluated by Western Blotting.
Quantified values of each protein (C F) are related to the content of actin protein and the basal type
I fiber value in the lean group is set to 100. Representative blots from three individuals are shown above each bar in A+C F. White bars represent type I fibers (A) or type I fiber pools (C F), black bars type IIa fibers (A) or type II fiber pools (C F) and gray bars IIx fibers (A). Data are means±SEM. Post hoc testing was only performed when an interaction was evident . AU, arbitrary units; MHC, myosin heavy chain. †p<0.05, †††p<0.001 vs type I muscle fibers; ‡p<0.05, ‡‡p<0.01 main effect of group compared with lean; (§)p=0.06, §p<0.05, §§p<0.01 main effect of group compared with obese.
Figure 2. Akt in muscle fiber pools from lean, obese and type 2 diabetic subjects. Muscle fiber type specific regulation of Akt phosphorylation on site Thr308 (A) and Ser473 (B) and protein content of
Akt2 (C) was evaluated by Western Blotting. Two bands are apparent for human Akt2 when insulin stimulated (both being Akt2 (31)). Quantified values of each protein are related to the content of actin protein and the basal type I fiber value in the lean group is set to 100. Representative blots are shown above bars for each protein probed for. White bars represent type I and black bars type II muscle fiber pools. Data are means±SEM. Post hoc testing was only performed when an interaction
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was evident. AU, arbitrary units. ***p<0.001 vs basal conditions; ††p<0.01 vs type I muscle fibers;
‡p<0.05, ‡‡p<0.01, ‡‡‡p<0.001 vs lean group; §p<0.05, §§p<0.01, §§§p<0.001 vs obese group.
Figure 3. Mammalian target of rapamycin (mTOR) and N myc downstream regulated gene 1
(NDRG1) in muscle fiber pools from lean, obese and type 2 diabetic subjects. Muscle fiber type
specific regulation of mTOR phosphorylation on site Ser2481 (A) and NDRG1 phosphorylation on
site Thr346 (C) as well as protein content of mTOR (B) and NDRG1 (D) were evaluated by Western
Blotting. Two bands are apparent for both p NDRG1Thr346 and NDRG1 (both quantified).
Quantified values of each protein are related to the content of actin protein and the basal type I fiber
value in the lean group is set to 100. Representative blots are shown above bars for each protein
probed for. White bars represent type I and black bars type II muscle fiber pools. Data are
means±SEM. Post hoc testing was only performed when an interaction was evident. AU, arbitrary
units. *p<0.05, ***p<0.001 vs basal conditions; †††p<0.001 vs type I muscle fibers.
Figure 4. TBC1 domain family member 1 and 4 (TBC1D1 and TBC1D4) in muscle fiber pools from
lean, obese and type 2 diabetic subjects. Muscle fiber specific regulation of TBC1D1
phosphorylation at site Thr596 (A) and TBC1D4 phosphorylation on site Ser318 (C), Ser588 (D),
Thr642 (E) and Ser704 (F) as well as protein content of TBC1D1 (B) and TBC1D4 (G) was evaluated
by Western Blotting. Two bands are apparent for p TBC1D1Thr596 and TBC1D1 (long and
medium/short isoform of TBC1D1 protein (49)). Quantified values of each protein are related to the
content of actin protein and the basal type I fiber value in the lean group is set to 100.
Representative blots are shown above bars for each protein probed for. White bars represent type I
and black bars type II muscle fiber pools. Data are means±SEM. AU, arbitrary units.
Figure 5. Glycogen content, glycogen synthase kinase 3β (GSK3β), glycogen synthase (GS) in
muscle fiber pools from lean, obese and type 2 diabetic subjects. Muscle fiber specific glycogen
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content measured by dot blotting (A). Muscle fiber specific phosphorylation of GSK3β on site Ser9
(B) and GS phosphorylation on site 2+2a (D) and 3a+b (E) as well as protein abundance of GSK3β
(C) and GS (F) was evaluated by Western Blotting. Quantified values of each protein (B F) are related to the content of actin protein and the basal type I fiber value in the lean group is set to 100.
Representative blots are shown above bars for each protein probed for. White bars represent type I and black bars type II muscle fiber pools. Data are means±SEM. Post hoc testing was only performed when an interaction was evident. AU, arbitrary units. (†)p=0.09, †††p<0.001 vs type I muscle fibers.
Figure 6. Pyruvate dehydrogenase (PDH) E1α in muscle fiber pools from lean, obese and type 2 diabetic subjects. Muscle fiber type specific regulation of PDH E1α phosphorylation on site 1 (A) and site 2 (B) as well as PDH E1α protein content (C) were evaluated by Western Blotting.
Phospho specific PDH E1α antibodies were directed against the phosphorylation of sites Ser293 (site
1) and Ser300 (site 2) on the human PDH E1α isoform. Due to sample limitations, protein levels of
PDH E1α were evaluated in a subset of fiber pools, with the number of samples indicated in each bar. Quantified values of each protein are related to the content of actin protein and the basal type I fiber value in the lean group is set to 100. Representative blots are shown above bars for each protein probed for. White bars represent type I and black bars type II muscle fiber pools. Data are means±SEM. Post hoc testing was only performed when an interaction was evident. AU, arbitrary units. †††p<0.001 vs type I muscle fibers.
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Table 1 Subject characteristics at study entry
Lean Obese T2D
n (female/male) 10 (2/8) 11 (2/9) 11 (2/9)
Age (years) 54 ± 2 56 ± 2 55 ± 2
Height (m) 1.77 ± 0.03 1.77 ± 0.03 1.75 ± 0.03
BMI (kg/m2) 23.9 ± 0.4 30.5 ± 0.6*** 30.8 ± 1.0***
Fat free mass (kg) 59.3 ± 3.3 68.5 ± 3.5 63.3 ± 3.3
Fat mass (kg) 16.2 ± 0.6 28.1 ± 1.1*** 31.8 ± 2.7***
HbA1c (%) 5.4 ± 0.1 5.2 ± 0.1 6.8 ± 0.2***,†††
HbA1c (mmol/mol) 35 ± 1 34 ± 1 51 ± 3***,†††
Plasma cholesterol (mmol/l) 5.5 ± 0.3 5.6 ± 0.2 5.0 ± 0.2
Plasma LDL cholesterol (mmol/l) 3.6 ± 0.2 3.7 ± 0.2 2.9 ± 0.2†
Plasma HDL cholesterol (mmol/l) 1.6 ± 0.1 1.4 ± 0.1 1.0 ± 0.1**,†
Plasma triglycerides (mmol/l) 0.9 ± 0.1 1.4 ± 0.2 2.6 ± 0.6*
Diabetes duration (years) 4.0 ± 1.5
Values are means±SEM. *p<0.05, **p<0.01, ***p<0.001 vs lean group; †p<0.05, †††p<0.001 vs
obese group.
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Table 2 Metabolic characteristics during hyperinsulinemic euglycemic clamp
Lean Obese T2D
,††† Plasma glucosebasal (mmol/l) 5.6 ± 0.2 5.9 ± 0.1 9.0 ± 0.6***
Plasma glucoseclamp (mmol/l) 5.5 ± 0.1 5.3 ± 0.2 5.5 ± 0.1
,† Serum insulinbasal (pmol/l) 27 ± 3 44 ± 5 86 ± 15***
Serum insulinclamp (pmol/l) 408 ± 23 399 ± 12 422 ± 17
2 GDRbasal (mg/m /min) 76 ± 3 77 ± 2 80 ± 4
2 ,††† GDRclamp (mg/m /min) 388 ± 28 334 ± 20 161 ± 24***
2 Glucose oxidationbasal (mg/m /min) 50 ± 8 47 ± 4 46 ± 7
2 ,†† Glucose oxidationclamp (mg/m /min) 141 ± 14 126 ± 10 77 ± 7***
2 NOGMbasal (mg/m /min) 26 ± 8 30 ± 4 34 ± 9
2 ,†† NOGMclamp (mg/m /min) 247 ± 22 208 ± 23 84 ± 22***
2 Lipid oxidationbasal (mg/m /min) 28 ± 2 30 ± 2 34 ± 3
2 ,† Lipid oxidationclamp (mg/m /min) 1 ± 5 4 ± 3 19 ± 4**
RERbasal 0.82 ± 0.01 0.81 ± 0.01 0.80 ± 0.01
,† RERclamp 0.98 ± 0.03 0.95 ± 0.02 0.87 ± 0.02**
Plasma lactatebasal (mmol/l) 0.78 ± 0.09 0.80 ± 0.07 1.06 ± 0.11
Plasma lactateclamp (mmol/l) 1.36 ± 0.08 1.18 ± 0.08 0.93 ± 0.06***
Values are means±SEM. GDR, glucose disposal rate; NOGM, non oxidative glucose metabolism;
RER, respiratory exchange ratio. **p<0.01, ***p<0.001 vs lean group; †p<0.05, ††p<0.01,
†††p<0.001 vs obese group.
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Total protein expression 202x207mm (300 x 300 DPI)
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Akt phosphorylation and Akt 2 protein expression 200x376mm (300 x 300 DPI)
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Phosphorylation and Protein expression of mTor and NDRG1 138x96mm (300 x 300 DPI)
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Protein expression and phosphorylation of TBC1D1 and TBC1D4 266x360mm (300 x 300 DPI)
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Phosphorylation and protein expression of GSK3 beta and Glycogen synthase (GS) 202x206mm (300 x 300 DPI)
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Pyruvate dehydrogenase (PDH) expression and phosphorylation 201x380mm (300 x 300 DPI)
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Online supplemental materials
Subject medications. Patients with type 2 diabetes were treated either by diet alone (n=2) or diet in
combination with metformin (n=7), metformin and long acting insulin (n=1) or rosiglitazone and
sulfonylurea (n=1). Oral anti diabetics were withdrawn one week prior to the study together with
antihypertensive (n=5) and lipid lowering (n=7) drugs. In addition, one lean subject were treated with a
proton pump inhibitor, 2 obese subjects were treated with angiotensin converting enzyme (ACE) inhibitor
whereas 4 patients with type 2 diabetes were treated with ACE inhibitor (n=3) or AT2 blocker (n=1). Long
acting insulin was withdrawn one day before the study. The patients with type 2 diabetes were GAD65
antibody negative and without any signs of diabetic retinopathy, nephropathy, neuropathy or macro vascular
complications. All woman included were post menopausal. Lean and obese control subjects had no family
history of diabetes.
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Supplementary table 1. Primary antibodies used for Western Blotting and DOT Blotting Antibody Manufacturer/donator Cat. # MHC I Developmental Studies Hybridoma Bank (Australia) A4.840 MHC II Developmental Studies Hybridoma Bank (Australia) A4.74 MHC IIx Developmental Studies Hybridoma Bank (Australia) 6H1 Actin Sigma Aldrich (MO, USA) A2066 Insulin Rβ Santa Cruz Biotechnology (CA, USA) SC 711 Hexokinase I Cell Signaling Technology (MA, USA) 2024 Hexokinase II Cell Signaling Technology (MA, USA) 2867 GLUT4 Thermo Scientific (Pierce, IL, USA) PA1 1065 Complex 2 Molecular Probes, Invitrogen (CA, USA) A11142 p AktThr308 Cell Signaling Technology (MA, USA) 9275 p AktSer473 Cell Signaling Technology (MA, USA) 9271 Akt2 Cell Signaling Technology, MA, USA 3063 p mTORSer2481 Cell Signaling Technology, MA, USA 2974 mTOR Cell Signaling Technology, MA, USA 2972 p NDRG1Thr346 Cell Signaling Technology, MA, USA 3217 NDRG1 Cell Signaling Technology, MA, USA 9485 p TBC1D1Thr596 Cell Signaling Technology (MA, USA) 6927 TBC1D1 James Hastie and Grahame Hardie, University of Dundee, UK p TBC1D4Ser318 Cell Signaling Technology (MA, USA) 8619 p TBC1D4Ser588 Cell Signaling Technology (MA, USA) 8730 p TBC1D4Thr642 Symansis (New Zealand) 3028 P1 p TBC1D4Ser704 Laurie J Goodyear, Joslin Diabetes Center and Harvard medical school (Boston, MA, USA) TBC1D4 Upstate Biotechnology (Millipore) (MA, USA) 07 741 Glycogen Otto Baba, Tokyo Medical and Dental University, Tokyo, Japan Ser9 p GSK3β Cell Signaling Technology, MA, USA 9331
GSK3β BD Transduction Laboratories, NJ, USA 610202 p GSsite2+2a Grahame Hardie, University of Dundee, UK p GSsite3a+b Grahame Hardie, University of Dundee, UK GS Oluf B. Pedersen, University of Copenhagen, Denmark p PDHsite1 Grahame Hardie, University of Dundee, UK p PDHsite2 Grahame Hardie, University of Dundee, UK List of primary antibodies used for Western Blotting analysis of skeletal muscle fiber type specific pools. Commercial available antibodies are listed with company name and catalog number while non commercial antibodies were kindly donated by persons as stated in the table.
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Supplementary table 2. Protein analysis order for each Western Blotting performed MW (kDa) Primary antibody Western Blot #1 gel cut off 1. 2. 3. 4. 5. 6. 250 Membrane 1 p TBC1D4Thr642 †p TBC1D4Ser704 N/A N/A †TBC1D4 125 125 Membrane 2 p GSsite2+2a *HKII †p GSsite3a+b †Insulin Rβ 75
75 Membrane 3 p AKTSer473 N/A †p AKTThr308 †Akt2 N/A †Complex II 50 50 Membrane 4 Actin N/A N/A †GLUT4 †p PDHsite1 †p PDHsite2 37
MW (kDa) Primary antibody Western Blot #2 gel cut off 1. 2. 3. 4. 5. 6. 7. Membrane 1 p mTORSer2481 N/A †mTOR 250
250 Membrane 2 p TBC1D1Thr596 N/A †TBC1D1 N/A N/A †p TBC1D4Ser588 †p TBC1D4Ser318 125 125 Membrane 3 N/A N/A †GS 75 50 Membrane 5 p NDRG1Thr346 *Actin †NDRG1 †GSK3β †p GSK3βSer9 37 Overview of each Western Blotting analysis performed on muscle fiber specific pools from lean, obese and type 2 diabetic subjects. Two runs of Western Blotting (Top: Western blot #1 and bottom: Western blot #2) were performed. Each gel was cut into 4 5 strips where after proteins from corresponding gel strips were semi dry transferred to a single PVDF membrane, blocked and probed with primary antibodies as listed. On each membrane, 3 7 different antibodies were used with either a sodium azide treatment or a stripping procedure in between. The use of 0.02% sodium azide in the primary antibody solution irreversible inhibits the enzyme activity of horseradish peroxidase already present on the membrane. This procedure allowed for reprobing of the membrane with an antibody recognizing a protein with a different molecular weight than the previously detected antibody. The stripping procedure was performed by incubating the membrane for 1 h at 58⁰C in stripping buffer (63 mM Tris HCl (pH 6.7), 2% SDS, 0.8% β mercaptoethanol). Following each stripping procedure, membranes were incubated in appropriate secondary antibody in order to verify, that the primary antibody was lost during stripping. If not, membranes were stripped again. Use of the sodium azide procedure and the stripping procedure is marked with an asterisk (*) and a dagger (†) symbol, respectively, in front of the re probed antibody. N/A, not applicable due to unsuccessful re probing of the membrane. 3
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Supplementary figure 1. In order to quantify the protein content of individual muscle fiber pools Mini Protean TGX Stain free gels with an acryl amide gradient of 4 20% was used. 3 µl of pooled muscle fiber sample was loaded and compared against a standard curve. Individual gels were compared using the same internal gel standard sample (termed Std. in (A)). The standard curve consisted of data obtained from three different pools of human muscle homogenate samples. Each homogenate sample was loaded in triplicates in the range from 0.25 2 µg of protein. Pooled muscle fiber samples were expected to be and in fact were within this protein range. An example of a stain free image from one of the standard homogenate samples, loaded in triplicates is shown (A). Each protein lane from the stain free image was quantified from ~30 250 kDa, due to a higher variation in the bands appearing between type I and II muscle fiber pools outside this range (data not shown). The averaged variation coefficient of each triplicate measure was 2.9% (range: 0.5 5.5%). In (B) the x axis states the amount of muscle homogenate protein loaded on the gel, while the y axis states the quantified values of the stain free image in arbitrary units (AU). The value of each muscle homogenate is shown as well as the standard curve from which signal intensity from the muscle fiber pools were compared to. The coefficient of determination of the standard curve was R2=1.00.
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Supplementary figure 2. In order to quantify the glycogen content of individual muscle fiber pools, 4 standard curves (50 400 ng of protein) from 4 different muscle homogenate samples with a known (biochemical determined) glycogen concentration was spotted onto the same PVDF membrane as the pooled fiber samples. The x axis states how much glycogen was spotted. The y axis states the quantified values of the visualized dot blot in arbitrary units (AU). Each single value of the different homogenate samples is shown as well as the fitted linear curve (R2=0.90). All muscle fiber pools were compared against this standard curve in order to estimate the glycogen content.
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Supplementary figure 3. Actin content in muscle fiber pools from lean, obese and type 2 diabetic subjects. Muscle fiber specific expression of actin was evaluated in the two runs of Western Blotting. No significant differences were observed between muscle fibers, clamp conditions and between groups in western blot #1 (A) and #2 (B). Quantified values are raw data with the basal type I fiber value in the lean group being 100. A representative blot is shown for each protein probed for. White bars represent type I fibers and black bars type II muscle fiber pools. Primary antibody used is stated in supplementary table 1. Data are means±SEM. AU, arbitrary units.
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Supplementary figure 4. Phosphorylation of Akt on sites Thr308 (A) and Ser473 (B) related to the total protein expression of Akt2 as well as phosphorylation of NDRG1 on site Thr346 related to the total content of NDRG1 (C). Phosphorylation of PDH E1α on site 1 (Ser293) (D) and site 2 (Ser300) (E) was related to PDH E1α protein expression. Since PDH E1α expression was determined in a subset of samples the total number of samples is indicated in each bar (D+E). The basal type I fiber value in the lean group is set at 100. White bars represent type I fibers and black bars type II muscle fiber pools. Data are means±SEM. AU, arbitrary units. ***p<0.001 vs basal conditions.
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Supplementary figure 5. Correlation between the MHC 1 content in whole muscle lysate preparation and the glucose disposal rate (A; r=0.53, p=0.002), net glucose oxidation rates (B; r=0.52, p=0.003) and non oxidative glucose metabolism (C; r=0.44, p=0.01) during insulin stimulated conditions. Circles represent lean controls, squares represent obese controls and triangles represent type 2 diabetic subjects. A Pearson product moment correlation was performed for all values combined using SigmaPlot (version 12.5, Systat Software, IL).
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Supplementary figure 6. Hexokinase I protein expression was evaluated in a subgroup of type I and II muscle fiber specific pools from lean, obese and type 2 diabetic subjects. The number of samples is indicated in each bar. Quantified values are related to actin with the basal value in the lean group being 100. White bars represent type I fibers and black bars type II muscle fiber pools. Data are means±SEM. AU, arbitrary units.
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Supplementary figure 7. MHC IIx protein expression in type II muscle fiber specific pools from lean, obese and type 2 diabetic subjects. Quantified values are related to actin with the basal value in the lean group being 100. No significant differences in the MHC IIx expression between type II muscle fiber pools were observed. White bars represent basal and black bars insulin stimulated conditions. Primary antibody used is stated in supplementary table 1. Data are means±SEM. AU, arbitrary units.
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