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

1Department of Kinesiology, Exercise Physiology Research Group, FaBeR, KU Leuven,

Tervuursevest 101, 3001 Leuven, Belgium.

2Molecular 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

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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

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Abbreviation list:

Akt/PKB Protein Kinase BAMPK 5' adenosine monophosphate-activated protein KinaseANOVA Analysis Of VarianceAS160/TBC1D4 Akt substrate of 160 kDaBSA Bovine Serum AlbuminCaM CalmodulinCaMK Calcium/Calmodulin-dependent protein kinaseDTT DithiothreitolEDTA Ethylene-Diamine-Tetra-Acetic acideEF2 Eukaryotic Elongation Factor 2EGTA Ethylene glycol bis(2-aminoethyl ether)tetraacetic acidELISA Enzyme-linked immunosorbent assayG/I Glucose-to-insulin ratioGAPs Rab-GTPase-activating proteinsGLUT4 Glucose Transporter 4GS Glycogen SynthaseGSK Glycogen Synthase KinaseHCl Hydrochloric AcidHIF-1α Hypoxia-Inducible Factor-1alphaHOMA Homeostatic Model AssessmentHYP Hypoxic trialLIMK LIM KinaseNAC N-AcetylcysteineNOR Normoxic trialPAK p21 protein-activated kinasePI3K phosphatidylinositol 3-kinasePLN PhospholambanPVDF Polyvinylidene FluorideRac1 GTPase Ras-related C3 botulinum toxin 1substrateROS Reactive Oxygen SpeciesSDS-PAGE Sodium Dodecyl Sulfate - PolyAcrylamide Gel

ElectrophoresisSer SerineThr ThreonineTRIS Tris(hydroxymethyl)aminomethane

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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-

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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/calmodulin-

dependent 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

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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.

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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,

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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

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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

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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 (#sc-

17024-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 (Bio-

Rad, 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 p-

GSK-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

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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 C-

peptide-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, C-

peptide-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.

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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),

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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

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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.

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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

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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 HIF-

1α-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

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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, p-

PLN 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).

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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 insulin-

dependent 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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Tables

Table 1 Indices for insulin sensitivity and hepatic insulin clearance

NOR HYP p-value

Glucose-to-insulin ratio 18.5 ± 2.9 9.7 ± 1.89 0.001

C-peptide-to-insulin molar ratio 0.28 ± 0.008 0.20 ± 0.006 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 intra-

subject variability. *p<0.05 vs. T0; ‡p<0.05 vs. NOR; (‡)p=<0.10 vs. NOR.

26