Title page
Acute systemic insulin intolerance does not alter the response of the Akt/GSK-3 pathway to
environmental hypoxia in human skeletal muscle
Gommaar D'Hulst1, Lykke Sylow2, Peter Hespel1, Louise Deldicque1
1
Department of Kinesiology, Exercise Physiology Research Group, FaBeR, KU Leuven,
Tervuursevest 101, 3001 Leuven, Belgium.
2
Molecular Physiology Group, Department of Nutrition, Exercise and Sports, August Krogh
Centre, University of Copenhagen, Universitetsparken 13, 2100 Copenhagen, Denmark.
Corresponding author:
Louise Deldicque
Exercise Physiology Research Group
FaBeR-KU Leuven
Tervuursevest 101
3001 Leuven, Belgium
E-mail: louise.deldicque@faber.kuleuven.be
Fax: +32 16 329091; Tel: +32 16 329088
1
Abstract
Purpose: To investigate how acute environmental hypoxia regulates blood glucose and
downstream intramuscular insulin signaling after a meal in healthy humans.
Methods: Fifteen subjects were exposed for 4h to normoxia (NOR) or to normobaric
hypoxia (HYP, FiO2=0.11) in a randomized order 40 min after consumption of a high glycemic
meal. A muscle biopsy from m. vastus lateralis and a blood sample were taken before (T0), after
1h (T60) and 4h (T240) in NOR or HYP and blood glucose levels were measured before
exposure and every 30min.
Results: In HYP, blood glucose was reduced 100min (110.1±5.4 in NOR vs. 89.5±4.7
mg·dl-1 in HYP) and 130min (98.7±3.8 in NOR vs. 85.6±4.9 mg·dl-1 in HYP) after completion of
a meal, which resulted in a 83% lower AUC in HYP compared to NOR (p=0.006). This
coincided with 40% lower GLUT4 protein in the cytosolic fraction (p=0.013) and a tendency to
increase in the crude membrane fraction (p=0.070) in HYP compared to NOR. At T240, blood
glucose concentration was similar between HYP and NOR whereas plasma insulin as well as
phosphorylation of muscle Akt and GSK-3 were ~2-fold higher in HYP compared to NOR
(p<0.05). In contrast, Rac1 protein was less abundant in the membrane fraction in HYP compared
to NOR (p=0.003), reflecting lower activation.
Conclusion: Acute environmental hypoxia initially reduced blood glucose response to a
meal, possibly via an increase in GLUT4 abundance at the sarcolemmal membrane. Later on,
whole body insulin intolerance developed independently of defects in conventional insulin
signaling in skeletal muscle.
Key words: Rac1, GLUT4, TBC1D4, glucose, muscle biopsy, insulin, hypoxia
2
Abbreviation list:
Akt/PKB
AMPK
ANOVA
AS160/TBC1D4
BSA
CaM
CaMK
DTT
EDTA
eEF2
EGTA
ELISA
G/I
GAPs
GLUT4
GS
GSK
HCl
HIF-1α
HOMA
HYP
LIMK
NAC
NOR
PAK
PI3K
PLN
PVDF
Rac1
ROS
SDS-PAGE
Ser
Thr
TRIS
Protein Kinase B
5' adenosine monophosphate-activated protein Kinase
Analysis Of Variance
Akt substrate of 160 kDa
Bovine Serum Albumin
Calmodulin
Calcium/Calmodulin-dependent protein kinase
Dithiothreitol
Ethylene-Diamine-Tetra-Acetic acid
Eukaryotic Elongation Factor 2
Ethylene glycol bis(2-aminoethyl ether)tetraacetic acid
Enzyme-linked immunosorbent assay
Glucose-to-insulin ratio
Rab-GTPase-activating proteins
Glucose Transporter 4
Glycogen Synthase
Glycogen Synthase Kinase
Hydrochloric Acid
Hypoxia-Inducible Factor-1alpha
Homeostatic Model Assessment
Hypoxic trial
LIM Kinase
N-Acetylcysteine
Normoxic trial
p21 protein-activated kinase
phosphatidylinositol 3-kinase
Phospholamban
Polyvinylidene Fluoride
GTPase Ras-related C3 botulinum toxin 1substrate
Reactive Oxygen Species
Sodium Dodecyl Sulfate - PolyAcrylamide Gel
Serine
Electrophoresis
Threonine
Tris(hydroxymethyl)aminomethane
3
Introduction
Peripheral insulin resistance contributes to the development of type 2 diabetes. Fortunately,
factors such as muscle contraction or exercise augment postprandial glucose uptake in skeletal
muscles and thereby improve the outcome of the disease (Sigal et al., 2006). Interestingly, lower
blood glucose levels and prevalence of type 2 diabetes have been observed in high altitude
populations (Castillo et al., 2007) or after long term hypoxic exposure (Marquez et al., 2013),
suggesting that environmental hypoxia could modulate glucose uptake and insulin sensitivity as
well depending on the severity and duration of the exposure. During acute exposure to hypoxia,
whole body insulin sensitivity transiently declines after 30min to 48h partially due to a
concomitant increase in catecholamines and cortisol levels (Peltonen et al., 2012; Larsen et al.,
1997; Oltmanns et al., 2004). Thereafter insulin sensitivity is restored and glucose homeostasis is
improved, likely because of a transient normalization to pre hypoxic values of these circulating
sympathic hormones (Lecoultre et al., 2013; Larsen et al., 1997).
How low oxygen levels affect insulin signaling in various tissues is less clear. Recent reports
have shown a direct decrease in downstream insulin signaling by hypoxia in human and murine
adipocytes (Regazzetti et al., 2009). In contrast, a stabilization of hypoxia-inducible transcription
factors (HIF)-1α or HIF-2α resulted in improved insulin-induced Akt activation via elevated
expression of insulin receptor substrate (IRS) in mice liver (Taniguchi et al., 2013) and human
HepG2 cells (Dongiovanni et al., 2008). In C2C12 myogenic cells, deletion of HIF-1α abrogated
insulin-stimulated glucose uptake due to reduced Akt substrate of 160 kDa (AS160/TBC1D4)
phosphorylation and impaired glucose transporter 4 (GLUT4) translocation to the plasma
membrane, whereas constitutively active HIF-1α increased insulin-induced GLUT4 translocation
(Sakagami et al., 2014). Furthermore, chronic hypoxia (4 weeks, 10% O2) increased insulin-
4
induced Akt phosphorylation in incubated mice m.soleus (Gamboa et al., 2011). All together
those results indicate that the effects of lowered oxygen availability on Akt signaling are tissue
specific and depend on the duration and severity of hypoxia.
Interestingly, hypoxia is able to increase glucose uptake in skeletal muscle both in vitro (Cartee et
al., 1991) and in vivo (Roberts et al., 1996) independently of insulin by using very similar, or at
least overlapping, signaling pathways as those of contraction-induced glucose uptake. Hypoxia
increases calcium efflux from the sarcoplasmic reticulum (Cartee et al., 1991; Brozinick, Jr. et
al., 1999), thereby triggering calmodulin and its downstream kinase calcium/calmodulindependent protein kinase (CaMK) to facilitate GLUT4 translocation in skeletal muscle. In turn,
CaMK phosphorylates a small protein called phospholamban (PLN) at Thr17, which leads to an
activation of the SERCA2a pump to increase calcium influx into the sarcoplasmic reticulum
(Vittone et al., 2008), thereby restoring calcium homeostasis. However, while the molecular
mechanisms by which insulin and contraction increase glucose uptake in skeletal muscle are quite
well described (Richter and Hargreaves, 2013), those triggered by hypoxia remain to be largely
defined.
Finally, the small Rho GTPase Ras-related C3 botulinum toxin substrate 1 (Rac1) is another
downstream target of PI3K controlling glucose uptake. In mature skeletal muscle, it acts in
parallel, but independently of Akt and is not activated by 5-aminoimidazole-4-carboxamide
ribonucleotide, suggesting it is AMP-activated protein kinase (AMPK)-independently regulated
(Sylow et al., 2013a). Rac1 activates p21 protein-activated kinase (PAK) by releasing the latter
from its autoinhibitory domain, thereby allowing PAK1 to be phosphorylated at Thr423 and PAK2
at Thr402, although other mechanisms of regulation have been found as well (Radu et al., 2014).
Rac1 has been linked to insulin resistance in obese and diabetic patients (Sylow et al., 2013b), but
5
has never been studied after a normal meal with physiological plasma insulin concentrations or in
response to severe hypoxia in humans.
In summary, acute hypoxia seems to induce whole body insulin resistance against the face of
enhanced rate of glucose uptake in skeletal muscle. The latter regulation seems therefore
independent of insulin but the molecular mechanisms are not defined yet. Based on preliminary
results showing a reduced blood glucose peak after a meal due to exposure to environmental
hypoxia, we hypothesize that the blunted peak could be explained by enhanced GLUT4
translocation in hypoxic conditions. The aim of the present work was therefore to investigate
intracellular mechanisms known to regulate GLUT4 translocation in human skeletal muscle of
subjects exposed to environmental hypoxia and to determine how hypoxic conditions alter blood
insulin levels and downstream skeletal muscle signaling.
6
Materials and Methods
Subjects and study design
Experimental session 1: Eight healthy young men (age 24.0 ± 0.8 years; BMI 21.1 ± 0.6 kg·m-2)
participated in this study, which was approved by the local Ethics Committee (KU Leuven) and
was in conformity with The Helsinki Declaration. Subjects were asked to refrain from vigorous
physical activity for 2 days, as well as to abstain from alcohol consumption the day before the
experiments. Furthermore, they were not exposed for more than 7 days to an altitude higher than
1500 m within a period of 3 months preceding the start of the study. A written consent was
obtained from all subjects after explaining all potential risks of the study. A pre-study medical
checkup showed no anti-diabetic drug usage and HOMA-IR ranged from 0.4 to 0.9 indicating no
signs of pre-diabetes.
All subjects underwent 2 trials (Fig. 1), one in normoxia (NOR) and one in hypoxia (HYP) in a
randomized cross-over designed order, with a 4-week interval period in between. After a
standardized dinner (58 % carbohydrates, 28 % fat and 14 % protein) and 8h overnight fast, the
subjects reported to the laboratory between 6:00 – 8:30 am where they received a standardized
high-glycemic meal (65 % carbohydrates, 29 % fat and 6 % protein, glycemic load range: 49 –
67). A period of exactly 20 min was provided to consume the meal and an additional 40-min rest
was given. The subjects were then transferred to an air-conditioned (~21°C) hypoxic facility
(Sporting Edge, Leicestershire, UK) where they remained seated for 4 h. The chamber was either
maintained at 20.93 % O2 (NOR), or at 11.1 % O2 (HYP) corresponding to ~5000 m of altitude.
Capillary blood glucose was measured with an Glucocard X-meter, (Arkray, Kyoto, Japan),
which has been validated by Freckmann et al. (2010). Measurements were taken in the fasted
state (T-60), 20 min after meal completion (T-20), 40 min after meal completion (T0,
7
immediately before entering the room), one minute after entering the room (T1) and thereafter
every 30 min until the end of the protocol (T240). Another capillary blood sample was taken at
T-60 and T240 for insulin determination. Subjects remained seated during the whole
experimental period, while reading a book or watching a movie.
Experimental session 2: Based on the results from experimental session 1 showing a reduced
blood glucose peak after a meal due to exposure to environmental hypoxia and knowing the
essential role of skeletal muscle in blood glucose homeostasis, we aimed to determine the
contribution of skeletal muscle to our preliminary observations. We therefore repeated a similar
experiment including muscle biopsies. Samples from experimental session 2 have already been
used in a previous work where the protocol has been extensively described (D'Hulst et al., 2013).
In the present study, we used the samples to generate new data except for plasma insulin levels
which have been published in our previous work. Briefly, fifteen healthy young men (age 21.3 ±
0.4 years; BMI 21.8 ± 0.5 kg·m-2) participated in this experimental session 2. The protocol was
the same as in experimental session 1 except for blood sampling and muscle biopsies. Forty min
after meal completion (T0), a first biopsy sample, with the needle pointing proximally (Van
Thienen et al., 2014), was taken from the left m. vastus lateralis under local anesthesia (1-2 ml
Lidocaine) through a 5-mm incision in the skin. Immediately after, a blood sample was taken
from an antecubital vein. After those first measurements in normoxia, subjects were transferred to
the hypoxic facility, which was either maintained at 20.93 % O2 (NOR) or set at 11.1 % O2
(HYP). Immediately after entering the room (T1), capillary blood glucose was measured and this
was repeated every 30 min until the end of the trial. One hour after the first biopsy (T60), a
second biopsy was taken through the same incision as the first, yet with the needle pointing
distally. Furthermore, a venous blood sample was taken. Finally, at T240 the last biopsy and
8
blood sample were taken. The last biopsy was taken with the needle pointing distally and through
a new incision in the skin 3 cm distally to the first incision.
Cellular fractionation
Details of the fractionation protocol are described elsewhere (Gualano et al., 2011). Briefly, ~15
mg of frozen muscle sample was homogenized 3 x 5 s with a polytron mixer in an ice cold buffer
(1:15, w/v) [20 mM Tris–HCl at pH 7.5, 250 mM sucrose, 2 mM EDTA,10 mM EGTA, and a
protease inhibitor cocktail (#04693124001, Complete Mini, Roche Applied Science, Vilvoorde,
Belgium)]. The homogenate was centrifuged at 900 g for 10 min (4°C). The obtained supernatant
(s1) was further centrifuged at 100 000g for 30 min (4°C). This resulted in a pellet and a second
supernatant (s2). The supernatant (s2) was used as a cytosolic lysate and the pellet was carefully
re-dissolved in a 1% Triton X-100 lysis buffer and centrifuged at 100 000g for 30 min (4°C) to
obtain the crude membrane fraction (supernatant, s3).
Whole cell lysate
Frozen muscle tissue (~20 mg) was homogenized 3 x 5 s with a Polytron mixer in an ice-cold
buffer (1:10, w/v) [50 mM Tris-HCl pH 7.0, 270 mM sucrose, 5 mM EGTA, 1 mM EDTA, 1
mM sodium orthovanadate, 50 mM glycerophosphate, 5 mM sodium pyrophosphate, 50 mM
sodium fluoride, 1 mM DTT, 1 % Triton-X 100 and the same protease inhibitor cocktail as
described above]. Homogenates were then centrifuged at 10 000g for 10 min at 4°C. The
supernatant was collected and immediately stored at -80°C. The protein concentration was
measured using the DC protein assay kit (Bio-Rad laboratories, Nazareth, Belgium).
Western blot
9
Proteins (15-30 µg) were separated by SDS-PAGE (7.5 – 12.5 %) and semi-dry transferred to
PVDF
membranes
(Immobilon
Transfer
Membrane;
Millipore,
Hellerup,
Denmark).
Subsequently, membranes were blocked with 3 % non-fat milk or 3 % BSA for 1 h and
afterwards incubated overnight (4°C) with the following antibodies: p-LIMK1/2 Thr508/Thr505
(#3841), LIMK1 tot (#)3842, p-PAK1/2 Thr423/Thr402 (#2601), PAK1tot (#2602), p-GSK-3α/β
Ser21/9 (#9331), p-TBC1D4 Thr642 (#8881), p-CAMKII Thr286 (12716), CAMKII pan (#4436),
eEF2 tot (#2332) (Cell Signaling Technology, Leiden, The Netherlands), Akt Thr308 (#05-802R),
AS160/TBC1D4 tot (#07-741), Rac1 (#05-389) (Millipore, Hellerup, Denmark), p-PLN (#sc17024-R) (Santa Cruz, Huissen, The Netherlands), GLUT4 (#PA1-1065) (Thermo Scientific,
Roskilde, Denmark), p-GS3+3a en p-TBC1D4 Ser588 were homemade. Appropriate horseradish
peroxidase-conjugated secondary anti-bodies (DAKO) were used for chemiluminescent detection
of proteins. Bands were visualized using the BioRad ChemiDocTM MP Imaging system (BioRad, Copenhagen, Denmark) and enhanced chemiluminescence (ECL+; Amersham Biosciences,
Brondby, Denmark). Then, membranes were stripped and re-probed with the antibody against the
respective total protein to ascertain the relative amount of the phosphorylated protein compared
to total amount of protein throughout the whole experiment. The results are presented as the ratio
phosphorylated/total amount of the proteins, except for p-PLN which was reported to eEF2,
which was found not to be modified by any condition during preliminary experiments. For pGSK-3α/β and p-GS site 3a+b, we used coomassie blue staining as loading control, because
intramuscular glycogen levels can lead to variations in the total form of these proteins (Nielsen
and Wojtaszewski, 2004). A value of 1 was arbitrarily assigned for each subject to the respective
T0 conditions (NOR and HYP) to which the values obtained at T60 and T240 were reported.
Analysis of blood samples
10
Experimental session 1: plasma insulin was assayed by ELISA (EIA1825, DRG International,
Inc., USA). Experimental session 2: plasma insulin was assayed by chemiluminescence using the
Siemens DPC kit (Brussels, Belgium) according to the instructions of the manufacturer; the
interassay coefficient of variation of this measurement has been reported to be less than 10%
(Gkourogianni et al., 2014). Serum C-Peptide was measured by ELISA (ALPCO Diagnostics,
Salem, USA), all samples were analyzed in duplicate with an averaged coefficient of variation
within duplicates of 5.3%.
Indices obtained from blood samples
Using the data from experimental session 2, we calculated glucose-to-insulin ratio (G/I) and Cpeptide-to-insulin molar ratio at T240 as well as delta insulin and delta C-peptide (∆T240-T0 and
∆T240-T60) as surrogate markers of insulin resistance and hepatic insulin clearance (Singh and
Saxena, 2010).
Statistical analyses
A repeated measures ANOVA design was used to assess the statistical significance of differences
between mean values over time and between conditions. When appropriate, Bonferroni pairwise
multiple comparison test was used as post-hoc. A paired student t-test was used to determine
significant differences for the area under the curve of blood glucose, glucose-to-insulin ratio, Cpeptide-to-insulin molar ratio, delta insulin and delta C-peptide. Area under the glucose curve
was calculated using the trapezoid method. Threshold of significance was set at p=0.05. Results
are expressed as the means ± SEM. SEM at T0 represents inter-subject variability and SEM at
T60 and T240 represent intra-subject variability.
11
Results
Acute environmental hypoxia decreases the blood glucose response to a high glycemic meal
Experimental session 1. The blood glucose response to the standardized high glycemic meal
peaked 20 min after completion (p=0.001), corresponding to 20 min before entering the room,
with no difference between NOR and HYP (Fig. 2a). Immediately after entering the room (T1),
blood glucose dropped by 12% in NOR and by 21% in HYP but only the decrease in HYP was
significant (p=0.008, T1 vs T0 in HYP). Sixty minutes after entering the room (T60), blood
glucose increased a second time in NOR compared to fasted levels (p=0.023) although the peak
was much less than 20 min after completion of the meal. This increase was not observed in HYP,
resulting in a 18% difference in glucose levels between NOR and HYP at T60 (p=0.002). Plasma
insulin level did not differ between NOR and HYP at basal but was 25% higher in HYP
compared to NOR at T240 (p=0.034, Fig. 2b).
Experimental session 2. As there was no difference between NOR and HYP in the first part of the
glucose curve (from T-60 to T0) in the first experimental session, glucose measurements were
started when the subjects entered the room (T1) in the second experimental session. Compared to
NOR, blood glucose response in HYP was reduced 30, 60 and 90 min after entering the room
(corresponding to 70, 100 and 130 min after consumption of the meal) (p=0.056, p<0.001 and
p=0.015, respectively, Fig. 2c). As a result, the area under the curve was lower in HYP compared
to NOR (396.6±605.9 in HYP vs. 2340.9±603.9 mg·dl-1·240min-1 in NOR, p=0.006, Fig. 2d). At
40 min after the meal (T0) when the first blood sample was obtained, plasma insulin had
probably already peaked (Ostman et al., 2005). Thereafter plasma insulin decreased over time in
both NOR and HYP (p<0.001, Fig. 2e). However, the decrease from T60 to T240 was less in
HYP compared to NOR (-3.2±3.7 in HYP vs. -15.1± 3.4 µU·ml-1 in NOR, p=0.043, Table 1),
12
resulting in a ~2 fold higher insulin level in HYP at the end of the experimental trial (p=0.033,
Fig. 2e). Plasma C-peptide also decreased in both NOR and HYP over time (p<0.001, Fig. 2f) but
less in HYP than in NOR (-729.1±101.2 in HYP vs. -983.2±95.1 ng·ml-1 in NOR, p=0.046, Table
1). At T240, plasma C-peptide tended to be higher in HYP compared to NOR (+34%, p=0.092).
The C-peptide-to-insulin molar ratio, considered as an index of hepatic insulin clearance (Osei et
al., 1984), was 37% higher in NOR compared to HYP at T240 (p=0.039, Table 1). At the same
time point, glucose-to-insulin ratio was 91% (p=0.001) lower in HYP compared to NOR.
Intramuscular response to insulin is unaffected by hypoxia
To test whether acute hypoxia impairs postprandial downstream insulin signaling, we measured
phosphorylation of Akt and its downstream targets. Phospho-Akt at Thr308 and phospho-GSK-3α
at Ser21 decreased from T0 to T240 in NOR (p<0.05), but not in HYP, resulting in a 2-fold higher
phosphorylation in HYP compared to NOR at T240 (p<0.05, Fig. 3a and b) similar to the
phosphorylation pattern of p-Akt at Ser479 as published before (D’Hulst et al. 2013). GSK-3β
phosphorylation at Ser9 decreased in NOR only (p=0.015, Fig. 3c) and did not differ between
conditions at T240. Phosphorylation of GSK-3 decreases its activity resulting in decrease
phosphorylation of GS. Following its upstream kinase, GS phosphorylation at site 3a+3b
increased only in NOR (p=0.001, Fig. 3d) and tended to be lower in HYP compared with NOR at
T240 (p=0.055).
Environmental hypoxia transiently increases the presence of GLUT4 at the membrane
To further determine the mechanisms contributing to the lowered blood glucose response in
HYP, GLUT4 was quantified in crude membrane and cytosolic fractions. GLUT4 in the cytosolic
fraction was ~40% lower at T60 in HYP compared to NOR (p=0.013, Fig. 4a), whereas at the
same time point GLUT4 in the membrane fraction tended to display the inverse pattern, namely a
13
30% increase (p=0.070, Fig. 4b). As expected, no change was observed for whole cell GLUT4
over time or between conditions (Fig. 4c). In order to determine possible mechanisms, we
measured p-TBC1D4, as it acts distally of Akt and inhibits GLUT4 translocation when
dephosphorylated. A general decrease in TBC1D4 phosphorylation at Thr642 was observed over
time (p=0.010, Fig. 4d) with no difference between NOR and HYP. Phospho-TBC1D4 at Ser588
followed a similar pattern, but the decrease in phosphorylation was only present in NOR at T240
vs T60 (p<0.001, Fig. 4e). Similarly to Thr642, no difference between NOR and HYP was
observed on Ser588 at any time point, despite higher insulin concentrations in HYP at T240.
Calcium and Rac1 signaling
Neither hypoxia nor the meal modified CaMKII phosphorylation (Fig. 5a). Despite this, hypoxia
increased phosphorylation of PLN at Thr17 by 25% at T240 (p=0.046, Fig. 5b) and tended to
increase it at T60 (p=0.070). Upon activation Rac1 translocates to the sarcolemma and Rac1 is
necessary for normal insulin-stimulated regulation of glucose uptake (Satoh, 2014). Therefore we
investigated Rac1 localization and downstream signaling. Rac1 was more abundant in the plasma
membrane in NOR compared to HYP at T240 (p=0.003) and tended to be more elevated at T60
(p=0.081, Fig. 5d). Furthermore, PAK1, a well-known downstream target of Rac1 (Chiu et al.,
2011), showed a similar phosphorylation pattern. Phospho-PAK1/2 at Thr423/402 increased by 79%
from T0 to T240 in NOR only (p=0.005, Fig. 5e). Phospho-LIMK Thr508 was not modified by
hypoxia at any time point.
14
Discussion
In the present study, we show that after a high-glycemic meal, acute environmental hypoxia: 1)
decreases the blood glucose peak independently of insulin and possibly due to increased GLUT4
at the sarcolemmal membrane; 2) delays the decrease in plasma insulin level after a meal, which
results in proportional activation of Akt but not of Rac1 signaling in skeletal muscle.
Plasma insulin and C-peptide levels were higher after 4-h exposure to environmental hypoxia
compared to normoxia, as the postprandial decrease of both hormones was blunted in hypoxia. In
contrast, blood glucose was identical between conditions at that time point. As skeletal muscle
accounts for ~70% of peripheral glucose uptake under high-insulinemic conditions (DeFronzo et
al., 1981), we further sought to determine any defects in downstream intramuscular insulin
signaling in hypoxic conditions. To our surprise, phosphorylation of Akt, GSK-3α/β and GS as
well as S6K1 (D'Hulst et al., 2013), all followed plasma insulin almost completely, indicating
that insulin signaling is not impaired in response to acute hypoxia in human skeletal muscle, at
least not at the Akt-GSK signaling level. To our knowledge, this is the first study to compare the
acute activation of intramuscular insulin signaling in hypoxia to the activation in normoxia in
humans. Previous data come from in vitro studies or studies on chronic hypoxia. The main
conclusions of those studies is that even though basal downstream insulin signaling is likely
impaired with chronic hypoxia in skeletal muscle (Alvarez-Tejado et al., 2001; Favier et al.,
2010), its ability to be stimulated by insulin remains intact, at least to a certain extent and
duration.
In addition to modulate insulin sensitivity, hypoxia has been shown to regulate glucose uptake in
several tissues (Wojtaszewski et al., 1998; Kelly et al., 2010). We did not assess glucose uptake
directly but the lower blood glucose levels and the predominant role of skeletal muscle in glucose
15
disposal (DeFronzo et al., 1981) naturally led to the hypothesis that glucose uptake could indeed
be up-regulated in skeletal muscle in our hypoxic conditions. One argument in favor of this is the
decrease in cytosolic GLUT4 concomitantly with a tendency to an increase in sarcolemmal
GLUT4 after 1h, suggestive of a higher glucose influx into skeletal muscle. However, our
experimental setup cannot rule out a specific role of other insulin dependent tissues. Therefore
further research using glucose tracers is needed to determine the specific role of fat, liver and
skeletal muscle tissue in postprandial glucose uptake under hypoxic conditions.
Importantly, contrary to previous results in C2C12 cells (Sakagami et al., 2014), changes in
GLUT4 localization were not initiated by HIF-1α as neither its expression nor transcription of
downstream targets were increased (D'Hulst et al., 2013). We then seek to determine which HIF1α-independent molecular mechanisms could be responsible for the decreased blood glucose
levels and increased GLUT4 location at the sarcolemma. One key molecule in the latter process
is TBC1D4, which mediates both insulin-dependent and -independent translocation of GLUT4 in
skeletal muscle (Treebak et al., 2014), making it an interesting target to analyze in the present
study. However, phosphorylation of TBC1D4 at Ser588 and at Thr642 was not modified after 1h
either in normoxia or in hypoxia. Together with unchanged phosphorylation of Akt and AMPK
(D'Hulst et al., 2013), two upstream kinases of TBC1D4 (Treebak et al., 2014), our results rule
out a major role of TBC1D4 in the regulation of GLUT4 translocation in our conditions.
Confirming previous reports (Wojtaszewski et al., 1998; Hayashi et al., 2000), the decrease in
blood glucose by hypoxia was insulin-independent as insulin levels were identical between
normoxia and hypoxia after 1h, while glucose levels were significantly lower in hypoxia.
Hypoxia per sé has been postulated to increase glucose transport via AMPK (Mu et al., 2001) and
calcium (Wright et al., 2005) -dependent pathways. Although skeletal muscle oxygenation index
16
was decreased by 7.2% (D'Hulst et al., 2013), we did not find any changes in AMPK, or
downstream ACC phosphorylation (data not shown) in any condition. AMPK is a very labile
protein and absence of changes in AMPK phosphorylation at Thr172 does not mean that this protein is
not involved in glucose metabolism in the present study. Indeed, this is in contrast to a recently
published study from our laboratory wherein p-AMPK at Thr172 was elevated by 25% at ~5000m
(Masschelein et al., 2014), probably because of a slightly different sampling time, whereby we
likely missed variations in AMPK phosphorylation, hypoxic stimulus, nutritional state or reactive
oxygen species (ROS) production (Emerling et al., 2009). However, although we did not assess
ROS production, the latter probably does not play a critical role in hypoxia-induced glucose
uptake (Sandstrom et al., 2006).
Hypoxia also regulates calcium efflux out of the sarcoplasmic reticulum, resulting in increased
GLUT4 translocation in a CAMK-dependent and insulin-independent ways (Wright et al., 2005).
However, the phosphorylation state of CAMKII was not modified in hypoxia at 1h, suggesting
that calcium signaling was not activated at that time in our conditions and therefore did not
contribute to increase GLUT4 translocation and to decrease blood glucose levels. Interestingly, pPLN was increased after 4-h hypoxia without any alternations in CAMKII phosphorylation. Most
likely this can be accounted to the higher and concomitant activation of Akt in hypoxia, which is
sufficient to phosphorylate PLN at Thr17 independently of CaMKII (Catalucci et al., 2009) but
the physiological significance of the increase in p-PLN at the end of the 4-h exposure remains to
be determined.
Recently, the actin cytoskeleton-regulating GTP-ase Rac1 has been put forward as an important
regulator of GLUT4 translocation and glucose transport via activation of PAK and its
downstream target LIMK which finally results in cytoskeleton remodeling (Chiu et al., 2011).
17
Importantly, activation of Rac1 is regulated by both insulin and contraction in skeletal muscle
and is AMPK-independent (Sylow et al., 2013a; Sylow et al., 2014), which made it a particular
interesting target in this experimental setup, as there was no hypoxia-induced AMPK activation.
Hypoxia as well has been shown to activate Rac1 in hepatocytes (Mollen et al., 2007), in breast
cancer cell lines (Du et al., 2011) and in epithelial cells (Mattagajasingh et al., 2012), but whether
hypoxia regulates Rac1 in skeletal muscle was unknown prior to this study. In the present study,
Rac1 abundance in the membrane, a marker for its activation (Bustelo et al., 2007), as well as
phosphorylation of PAK1/2 were increased over time in normoxia only. Therefore, contrary to
our hypothesis, Rac1 does not mediate the presence of GLUT4 at the sarcolemmal membrane
under hypoxia in vivo as the activation patterns of Rac1 and GLUT4 do not fit each other. Those
results support the hypothesis that GLUT4 translocation in the present study is not primarily
regulated by insulin. Indeed, previous studies identified Rac1 as a crucial element in insulindependent GLUT4 translocation in skeletal muscle that could play a major role in the
development of insulin resistance (Sylow et al., 2013b; Satoh, 2014). Here, neither insulin nor
Rac1 did play a key role in the higher abundance of GLUT4 at the sarcolemmal membrane but
we cannot exclude any contribution of Rac1 to the development of insulin intolerance observed at
the end of the exposure to hypoxia. Insulin-induced activation of the Rac1/PAK pathway is
known to be down-regulated in skeletal muscle of insulin-resistant mice and humans (Sylow et
al., 2013b) but how this pathway is regulated after a meal in healthy people is currently unknown.
We show here that the Rac1/PAK pathway is activated over time after a meal in normoxia and
that this activation is blunted in hypoxia. Knowing the close relationship between the Rac1/PAK
pathway and insulin resistance, it is possible that the reduced activation of this pathway
contributes to insulin resistance during acute hypoxia but its definite involvement can only be
determined using KO or in vitro models.
18
In the present study, no glucose tracer or hyperinsulinemic euglycemic clamp was used, which
limits the interpretation of the data. Another limitation is the lack of a pre-meal biopsy but it was
deliberately chosen to focus on the response to a meal in hypoxic conditions as the effect of a
meal itself on blood glucose and plasma insulin has already been documented largely before
(Reaven, 1979; Wolever and Bolognesi, 1996)
Conclusions
In conclusion, our data show that acute environmental hypoxia reduced blood glucose response to
a high glycemic meal, possibly via an increase in GLUT4 abundance at the sarcolemmal
membrane. Later on, whole body insulin intolerance develops independently of defects in
conventional insulin signaling in skeletal muscle as the response of the downstream Akt/GSK-3
pathway paralleled changes in plasma insulin levels during exposure to hypoxia. Further
investigation will be required to determine whether alteration in Rac1 signaling in skeletal muscle
could contribute to insulin intolerance during acute hypoxia.
19
Acknowledgements
The authors would like to thank Erik Richter for financing the biochemical assays and for his
insightful comments while generating the manuscript. We also thank Irene Bech Nielsen, Betina
Bolmgren, Ruud Van Thienen and Monique Ramaekers for their technical support. This study
was not externally funded.
Conflicts of interest
The authors do not have any conflicts of interest.
Ethical standards
The authors declare that all experiments comply with the Belgian law.
20
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23
Tables
Table 1 Indices for insulin sensitivity and hepatic insulin clearance
Glucose-to-insulin ratio
NOR
18.5 ± 2.9
C-peptide-to-insulin molar ratio
0.28 ±
0.008
HYP
9.7 ±
1.89
0.20 ±
0.006
p-value
0.001
0.039
∆T240-T0 insulin (µU·ml-1)
-21.4 ±
3.2
-18.1 ±
3.8
Ns
∆T240-T60 insulin (µU·ml-1)
-15.1 ±
3.4
-3.2 ±
3.7
0.043
∆T240-T0 C-peptide (ng·ml-1)
-983.2 ±
95.1
-729.1 ±
101.2
0.046
∆T240-T60 C-peptide (ng·ml-1)
-886.2 ±
141.9
-763.5 ±
214.9
Ns
Glucose-to-insulin ratio and C-peptide-to-insulin molar ratio at T240, delta insulin and C-peptide
levels (∆T240-T0 and ∆T240-T60) in normoxia (NOR) and hypoxia (HYP). Values are means ±
SEM (n=15).
24
Figures captions
Fig. 1
Experimental protocols of session 1 and session 2.
Fig. 2
Effect of environmental hypoxia and a high glycemic meal on insulin and glucose levels. (a)
Blood glucose during experimental session 1 (see methods section). (b) Plasma insulin during
experimental session 1 at T-60 (fasted) and T240 (end). (c) Blood glucose experimental session
2. (d) Area under the curve for blood glucose experimental session 2. (e) Plasma insulin and (f)
serum C-peptide at T1, T60 and T240 during experimental session 2. Data shown are expressed
as means ± SEM (n=8 experimental session 1, n=15 experimental session 2). SEM at T0
represents inter-subject variability and SEM at T60 and T240 represent intra-subject variability.
$
p<0.05 vs T-60; *p<0.05 vs T1; §p<0.05 vs T60; £p<0.05 vs T0; ‡p<0.05 vs NOR.
Fig. 3
Effect of environmental hypoxia and a high glycemic meal on downstream insulin signaling. (a)
Akt phosphorylation at Thr308 (b) GSK-3α phosphorylation at Ser21, (c) GSK-3β phosphorylation
at Ser9 and (d) GS phosphorylation at site 3a+3b at basal (T0), after 1h (T60) and after 4h (T240)
in normoxia (NOR) or in hypoxia (HYP). (e) Representative blots. Directly compared bands were
all from the same subject. Data shown are expressed as means ± SEM (n=15). SEM at T0
represents inter-subject variability and SEM at T60 and T240 represent intra-subject variability.
*p<0.05 vs T1; §p<0.05 vs T60; ‡p<0.05 vs NOR; (‡)p=0.055 vs NOR.
25
Fig. 4
Effect of environmental hypoxia and a high glycemic meal on GLUT4 and downstream signaling.
(a) Cytosolic GLUT4 fraction, (b) membrane GLUT4 fraction, (c) whole cell GLUT4, (d)
TBC1D4 Thr642 phosphorylation and (e) TBC1D4 Ser588 phosphorylation at basal (T0), after 1h
(T60) and after 4h (T240) in normoxia (NOR) or in hypoxia (HYP). (f) Representative blots.
Directly compared bands were all from the same subject. Data shown are expressed as means ±
SEM (n=15 for TBC1D4 phosphorylation, n=11 for membrane and cytosolic fractions). SEM at
T0 represents inter-subject variability and SEM at T60 and T240 represent intra-subject
variability. *p<0.05 vs T1; ‡p<0.05 vs. NOR; §p<0.05 vs T60; (‡)p<0.10 vs NOR.
Fig. 5
Effect of environmental hypoxia and a high glycemic meal on downstream calcium and Rac1
signaling. (a) CaMKII Thr286, (b) PLN Thr17 phosphorylation, (d) membrane Rac1 fraction, (e)
PAK1 Thr423 and (f) LIMK1 Thr508 phosphorylation at basal (T0), after 1h (T60) and after 4h
(T240) in normoxia (NOR) or in hypoxia (HYP). (c) and (g) Representative blots. Directly
compared bands were all from the same subject. Data shown are expressed as means ± SEM
(n=15). SEM at T0 represents inter-subject variability and SEM at T60 and T240 represent intrasubject variability. *p<0.05 vs. T0; ‡p<0.05 vs. NOR; (‡)p=<0.10 vs. NOR.
26