Background
Glucagon-like peptide-1 (GLP-1) is a gut hormone secreted by intestinal L cells into the bloodstream in response to nutrient ingestion and, together with the glucose-dependent insulinotropic peptide (GIP), belongs to the incretins family [
1,
2]. Exenatide (EXE) is the synthetic analog of exendin-4, a GLP-1 homologous originally purified from the salivary glands of the lizard
Heloderma suspectum, and it is currently employed in the treatment of type 2 diabetes (T2D) [
3,
4]. In contrast to GLP-1, which is rapidly cleaved by the dipeptidyl peptidase IV (DPP-IV) into its inactive form, EXE is resistant to the proteolytic effect of DPP-IV, has comparable effects on insulin secretion and exerts a more potent blood glucose lowering effect [
3‐
7].
GLP-1 and EXE stimulate glucose-dependent insulin secretion and decrease glucagon release after binding to the GLP-1 Receptor (GLP-1R) present on pancreatic endocrine β- and α-cells [
8,
9]. The GLP-1R is coupled to a G protein that, once activated, increases intracellular cyclic AMP (cAMP) and induces activation of protein kinase A (PKA), extracellular signal-regulated kinase (ERK)1/2 and phosphoinositol 3 kinase (PI3K)/protein kinase B (PKB) [
10‐
12]. It has been previously shown that GLP-1 exerts also an insulin-sensitizing effect by stimulating the uptake of glucose via the activation of PI3K, AKT and p70s6 k, whereas EXE seems to share only in part this capability in human myocytes [
13‐
16].
The heterotrimeric protein AMPK, consisting of a catalytic α and regulatory β and γ subunits, is ubiquitously expressed and it is regulated by a variety of physiological stimuli and drugs such as metformin and thiazolidinediones and plays a pivotal role in the regulation of mitochondrial biogenesis, fatty acid synthesis and glucose uptake [
17]. Experiments performed in human skeletal muscle cells have shown that AMPK activation, through phosphorylation at threonine 172 (T172) of the α subunit and subsequent phosphorylation of TBC1D4 (AS160) and TBC1D1, induces the translocation of the glucose transporter Glut-4 to the plasma membrane [
18‐
22].
To learn more about the influence of GLP-1R agonists administration on peripheral glucose utilization, we here studied the effects of EXE and liraglutide (lira) on the molecular mechanisms leading to glucose uptake in rat L6 myotubes.
Methods
Reagents
Media, sera and antibiotics for cell culture were from Lonza (Walkersville, MD, USA). Protein electrophoresis and western blot reagents were from Bio-Rad (Richmond, VA, USA) and electrochemiluminescence reagents from Pierce (Rockford, IL, USA). Insulin was from Eli Lilly (Florence, Italy). The antibodies used were: anti-GLP-1 receptor (kind gift of Drs. Wanda Dolci and Bernard Thorens [
10,
23], anti-insulin receptor (IR), anti-p-tyrosine (Biosource, Camarillo, CA), anti-IRS-1 (Millipore, Billerica, MA, USA), anti-p85 subunit of PI3-kinase, anti-Akt, anti-p-Akt S473, anti-p-AMPK T172, anti-AMPK, anti-p-ACC S78/80, anti-ACC, anti-p-JNK T183/Y185, anti-JNK, anti-p-AS160T642, anti-AS160 (Cell Signaling Technology, Beverly, MA, USA), anti-ERK1/2, anti-p-ERK1/2 (Santa Cruz, CA, USA), anti-GLUT4 antibodies [
24], and anti-αTubulin (Sigma, St Louis, MO, USA). 5-amino-4-imidazole carboxamide riboside (AICAR), Compound C, and LY294002 were from Calbiochem (La Jolla, CA). SiRNAs were from Riboxx (Radebeul, Germany). Methylglyoxal (MGO) was from SIGMA-Aldrich (St Louis, MO, USA). Exenatide and liraglutide were kind gifts from Amylin and Novo Nordisk, respectively.
Cell culture, treatment and transfection of siRNAs
Rat L6 skeletal muscle myoblasts were grown in complete high glucose DMEM (with
l-glutamine) supplemented with 10 % (vol/vol) inactivated fetal calf serum and 1 % (vol/vol) antibiotics-antimycotics and maintained in a humidified atmosphere of 5 % CO2 in air at 37 °C. L6 myotubes were allowed to differentiate as described previously [
25,
26]. Myotubes were treated with EXE 100 nmol/l for 20 min and 2, 4, 24 and 48 h, or with insulin 100 nmol/l for 30 min. Where indicated, cells were treated with AICAR 2 mmol/l for 2 h, or pre-treated with CC 40 μmol/l for 30 min, with LY 25 μmol/l for 30 min, methylglioxal 2.5 mmol/l for 30 min. L6 myotubes were also transfected with siAMPK-1, -2 and -3 targeting both AMPK α 1 and 2, using Oligofectamine Reagent from Life Technologies (Carlsbad, CA) according to the manufacturer’s instructions as previously described [
27]. SiNR was used as negative control.
Western blot analysis
Baboons tissues and human skeletal muscle proteins were extracted as previously described [
28,
29]. α-TC1 and β-TC3 cell lines were a kind gift of prof. Douglas Hanahan [
30]. L6 myotubes protein were obtained by lysing cells in buffer containing 50 mmol/L HEPES (pH 7.5), 150 mmol/L NaCl, 10 mmol/L EDTA, 1 % Triton X-100, 10 mmol/L Na
4P
2O
7, 100 mmol/L NaF, and 2 mmol/L sodium orthovanadate supplemented with protease inhibitors cocktail. Protein concentration was determined with the Bradford assay (DC Protein Assay; Bio-Rad, Hercules, CA) according to the manufacturer’s instructions. For the immunoprecipitation protocol, equal amounts of proteins (300 µg) were incubated at 4 °C overnight with anti-IRS1 antibody; immune complexes were collected by incubation with protein A-sepharose for 3 h at 4 °C and resuspended in Laemmli buffer. Equal amounts of proteins resolved by SDS-PAGE were electrophoretically transferred to nitrocellulose membrane (Amersham Biosciences, Piscataway, NJ). The membranes were incubated with primary antibodies followed by incubation with peroxidase-conjugated secondary antibodies. Proteins were detected by using enhanced chemiluminescence (Amersham Biosciences, Piscataway, NJ), and band densities were quantified by densitometry. To normalize the blots for protein levels, after being immunoblotted with antiphosphospecific antibodies, the same blots were stripped and re-probed with appropriate primary antibodies. To assess Glut 4 translocation to the plasma membrane upon EXE treatment, plasma membrane proteins were extracted from L6 myotubes by the use of “Mem-PER Plus Membrane Protein Extraction Kit” (Thermo Fisher Scientific, Rockford, IL, USA) following the manufacturer instructions. Equal amount of protein extracts were then analyzed by western blot as previously described in this section.
Determination of 2-deoxyglucose (2DG) uptake
2-DG uptake measurements were carried out as described previously [
25,
26]. Myotubes were starved for 24 h and, subsequently, treated with hormones or chemical compounds. Cells were supplemented during the final 10 min with 0.2 mmol/L 2-DG and then solubilized. 2-DG uptake was quantified by liquid scintillation counting.
Gene expression analysis
Total RNA was obtained from L6 myotubes, reverse transcribed using the high capacity cDNA Reverse Transcription Kit (Applied Biosystems, Foster City, CA) and analyzed by Real-Time quantitative PCR using a Power SYBR Green PCR Master Mix (Applied Biosystems, Foster City, CA) with 10 ng of cDNA and 0.3 uM of the following oligonucleotide primers: GLUT4 FW: 5′-CCACAGAAAGTGATTGAACAGAGC-3′ GLUT4 RV: 5′-AATGATGCCAATGAGAAAGGAG-3′. The rat beta-2 microglobulin (B2M) was amplified and used to normalize the results according to the Livak method, with the following oligonucleotide primers: B2M FW: 5′-ACTGAATTCACACCCACCGA-3′; B2 m rv: 5′-attacatgtctcggtcccaggt-3′.
Statistical analysis
All results are given as mean ± SD. Statistical differences were assessed by Student’s t test and ANOVA. A p ≤ 0.05 was considered statistically significant.
Ethics
All animal studies (non-human primate) were approved by the Texas Biomedical Research Institute Animal Care and the Use Committee as well as the University of Texas Health Science Center at San Antonio (San Antonio, TX, USA), Institutional Animal Care and Use Committee. Human studies were approved by the Institutional Review Board of University of Texas Health Science Center at San Antonio (San Antonio, TX, USA).
Discussion
T2D is a chronic disease characterized by hyperglycemia, insulin resistance, defective insulin secretion and islet of Langerhans remodeling [
28,
29,
32,
33]. The intestinal incretin GLP-1 also regulates glucose homeostasis in response to dietary fat and carbohydrate intake [
4] and GLP-1 receptor (GLP-1R) agonists have become valuable tools in T2D treatment [
34,
35].
Generally recognized mechanisms of GLP-1 and GLP-1R agonists action are potentiation of glucose-induced insulin secretion, reduction of glucagon secretion, inhibition of gastric emptying and decreased appetite [
3,
4,
8,
9], however, insulin sensitizing effects have been also described. Insulin-like effects of GLP-1 have been reported in adipose tissue [
36‐
38] and muscle cell lines [
39,
40] and insulin sensitizing effects of the GLP-1R agonist EXE have been studied in animal models [
41,
42] as well as in T2D subjects [
43].
Previous studies in rodent [
14,
44] and human cell lines [
16] have shown that GLP-1 and exendin-4 can stimulate glucose uptake. In L6 myotubes, exendin-4 augments insulin-stimulated glucose uptake via a molecular pathway that requires PI3 K activation [
14]. However, whether or not exendin-4 might also exert a direct stimulating effect on glucose uptake was never addressed in muscle cells. In order to address this question, we studied the effect of the exendin-4 analog EXE, given alone and in combination with insulin, on the uptake of glucose by L6 myotubes. We also explored the molecular mechanisms involved in EXE-induced glucose uptake. Our data demonstrate that EXE induces glucose uptake by utilizing a molecular pathway different from the insulin stimulated PI3K/Akt/AS160 signaling cascade. Indeed, we show that EXE has the capability to increase glucose uptake in muscle cells in the absence of insulin, and interestingly, to an extent comparable to insulin. By using CC, which is a potent chemical inhibitor of AMPK and a number of other protein kinases [
45], as well as a specific siRNA-mediated knockdown designed to reduce the protein expression of the α1 and α2 subunits of AMPK, we provide direct evidence that the stimulatory effect of EXE on glucose uptake is mediated by: (i) AMPK activation; (ii) activation of the Rab GTPase-activating protein TBC1D1, a paralog of AS160, highly expressed in skeletal muscle [
17,
21]; and (iii) translocation to the plasma membrane of the insulin-dependent glucose transporter Glut-4 [
15,
22].
The fact that EXE stimulates glucose transport via AMPK might appear in contrast with previous data obtained in other muscle cell lines stimulated with GLP-1 in which PI3K activation has been described [
5,
16]. The involvement of different pathways might be explained by the different relative abundance of PI3K and AMPK in human and mouse cell lines and the degree of cell differentiation.
AMPK is a heterotrimeric protein which mediates several cellular processes, including stimulation of glucose uptake, mitochondrial biogenesis and inhibition of fatty acid synthesis via phosphorylation of ACC [
19‐
22]. AMPK is known to regulate adipocyte metabolism, inflammation, and vascular function and defective AMPK activation is associated with insulin resistance and T2D [
46‐
48]. Glucose transport relies on insulin stimulated AKT phosphorylation and PI3K activation, as well as an alternative pathway involving AMPK/Cbl-CAP signaling [
49]. Studies in AMPK-α1 KO mice have shown that of the two catalytic subunits of AMPK, the isoform α1 is essential for stimulation of glucose uptake by the soleus muscle [
50]. The involvement of AMPK, resulting in nitric oxide synthase activation, also accounts for the improvements in endothelial dysfunction observed after EXE treatment [
51,
52]. Likewise EXE-mediated beneficial effects on cardiac function are associated with AMPK activation [
53].
Our findings are consistent with recent studies where EXE and Lira administration reduced fasting and postprandial blood glucose levels, and improved insulin sensitivity, in subjects with type 1 diabetes with absent endogenous insulin secretion [
5,
6]. Differently from a previous report where Lira enhanced insulin sensitivity [
54], we show a direct effect of EXE and Lira on glucose transport in myotubes and suggest that these GLP-1R agonists might regulate glucose homeostasis also in an insulin-independent manner through an AMPK-dependent pathway.
The lack of additive effects of EXE (and Lira) and insulin on glucose uptake can be explained by considering that the final step of their mechanism of action involves Glut-4 translocation to the plasma membrane which is already maximally stimulated by insulin. Our findings are in agreement with a previous study where Lira was reported to induce Glut-4 translocation via AMPK activation in C2C12 murine muscle cells over-expressing Glut-4, although in that study glucose transport was not evaluated [
55].
It is important to note that EXE increases glucose uptake also in L6 myotubes previously treated with methylglyoxal (MGO), which is increased in hyperglycemic conditions and induces insulin resistance in these cells [
31]. Altogether, our findings demonstrate that the GLP-1R agonists EXE and Lira increase glucose uptake by muscle cells in an insulin-independent manner.
Authors’ contributions
FA, FF designed the study, acquired, analyzed, and interpreted the data, and wrote the manuscript; GAR acquired, analyzed, and interpreted the data; GCM, TP, CN and RD analyzed the data and reviewed the manuscript; AMD, FB, GS, CM interpreted the data and reviewed the manuscript. All authors read and approved the final manuscript.