Current understanding of glucose transporter 4 expression and functional mechanisms

Table 2 Recent studies of glucose transporter 4 expression and translocation in the skeletal muscle

Methods	Materials	Comparisons	Observations/conclusions	Ref.
Western blot.	Cell fractions of rat L6 myotubes, 3T3-L1, and mouse muscle and adipose tissues. Anti-GLUT4 from Santa Cruz Biotechnology (1:1000).	Cell: Total cell lysate vs membrane fractions. Mouse tissues: Control vs high-fat diets.	Insulin treatments increases GLUT4 levels in membrane fractions without any change in the total cell lysate. GLUT4 levels in adipose tissue and muscle of mice fed a high-fat diet are lower in all fractions than that fed the control diet.	[72]
Western blot.	Whole cell and cell fractions from rat L6 and mouse C2C12 muscle cells, and soleus muscle of hind limb from mice. Anti-GLUT4 from Santa Cruz. Biotechnology (1:1000).	Whole cell lysate vs membrane fractions. Treatments without or with insulin or AICAR.	GLUT4 translocation occurs in L6 myotubes and 3T3-L1 adipocytes stimulated by insulin and AICAR. GLUT4 translocation occurs in muscle at 15 to 30 minutes and in adipose tissue at 15 minutes after glucose treatment.	[73]
Western blot.	Giant sarcolemmal vesicles from soleus muscles of Sprague-Dawley rats. Anti-GLUT4 from Millipore (1:4000).	Tissue samples without or with insulin released in the presence of glucose as a stimulant and lipid as a control.	A glucose-dependent insulinotropic polypeptide increases glucose transport and plasma membrane GLUT4 protein content.	[74]
Real-time PCR for Slc2a4 mRNA levels.	Total RNA of the skeletal muscle from male C57BL/6J and ICR mice fed different diets.	mRNA levels in muscle samples from mice fed the control or CLA supplement diet	Dietary CLA does not affect Slc2a4 mRNA levels in the mouse skeletal muscle	[75]
Western blot.	Preparations of sarcolemmal membrane fractions and crude lysates from male Muscovy ducklings. Anti-GLUT4 from East Acres (1:500).	GLUT4 from a unique crude membrane fraction of rat skeletal muscle was used as an arbitrary unit and from erythrocyte ghost as a negative control	Polyclonal antibodies detect a protein of similar size (approximately 45 kDa) of GLUT4 in the crude membrane preparations from rat (positive control) and duckling skeletal muscle. No signal was obtained for rat erythrocyte ghost membrane preparation.	[76]
ATB-BMPA-labelling of glucose transporters, Immunoprecipitation, liquid-scintillation counting, Western blot.	Tissue samples of isolated and perfused EDL or soleus muscle from GLUT1 transgenic C57BL’KsJ-Leprdbj and control mice. Anti-GLUT4 (R1184; C-terminal) from an unknown source.	Non-transgenic mice vs transgenic mice.	Basal levels of cell-surface GLUT4 in isolated or perfused EDL are similar in transgenic and non- transgenic mice. Insulin induces cell-surface GLUT4 by 2-fold in isolated EDL and by 6-fold in perfused EDL of both transgenic and non-transgenic mice. Western blot results were not shown.	[77]
Preembedding technique (immune reaction occurs prior to resin embedding to label GLUT4), and observations of whole mounts by immunofluorescence microscopy, or after sectioning by immunogold electron microscopy.	Muscle samples from male Wistar rats. Anti-GLUT4 (C-terminal, 1:1000), and anti-GLUT4 (13 N-terminal, 1:500) from unknown species.	Rats were divided in four groups: Control, contraction received saline, insulin and insulin plus contraction groups. They received glucose followed by insulin injection.	Two populations of intracellular GLUT4 vesicles are differentially recruited by insulin and muscle contractions. The increase in glucose transport by insulin and contractions in the skeletal muscle is due to an additive translocation to both the plasma membrane and T tubules. Unmasking of GLUT4 COOH-terminal epitopes and changes in T tubule diameters does not contribute to the increase in glucose transport.	[78]
Immunoprecipitation, and Western blot.	Membrane fractions from skeletal muscle of male Wistar rats treated without or with insulin. Anti-GLUT-4 from Genzyme, Anti-GLUT-4 from Santa Cruz Biotechnology.	Crude membrane preparations and cytosolic fractions in samples of rats treated without or with insulin.	In vitro activation of PLD in crude membranes results in movement of GLUT4 to vesicles/microsomes. This GLUT4 translocation is blocked by the PLD inhibitor, neomycin, which also reduces insulin-stimulated glucose transport in rat soleus muscle.	[79]
Western blot for GLUT4 protein in homogenates of epitrochlearis muscles. Tissue slices labeled with 2-[1,2-3H]-deoxy-d-glucose and counted in a gamma counter.	Muscle homogenate and slices from male Sprague-Dawley rats. Anti- GLUT4 from Dr. Osamu Ezaki.	Sedentary control vs a 5-day swimming training group.	The change of insulin responsiveness after detraining is directly related to muscle GLUT-4 protein content. The greater the increase in GLUT-4 protein content induced by training, the longer an effect on insulin responsiveness lasts after training.	[80]
Immunofluorescence for membrane preparations, and 2-Deoxyglucose uptake in isolated skeletal muscles.	Membrane preparations from L6 cells over-expressing GLUT4myc. Isolated skeletal muscle samples from mice. Anti-GLUT4 from Biogenesis.	L6 cells over-expressing GLUT4myc treated without or with Indinavir.	HIV-1 protease inhibitor indinavir at 100 µmol/l inhibits 80% of basal and insulin-stimulated 2-deoxyglucose uptake in L6 myotubes with stable expression of GLUT4myc.	[81]

AICAR: 5-aminoimidazole-4- carboxamide ribonucleotide; CLA: Conjugated linoleic acid; EDL: Extensor digitorum longus; HIV: Human immunodeficiency virus; ICR: Institute of Cancer Research; PLD: Phospholipase D; GLUT4: Glucose transporter 4.