Current understanding of glucose transporter 4 expression and functional mechanisms

Tiannan Wang,Jing Wang,Xinge Hu,Xian-Ju Huang,Guo-Xun Chen

Abstract Glucose is used aerobically and anaerobically to generate energy for cells.Glucose transporters (GLUTs) are transmembrane proteins that transport glucose across the cell membrane.Insulin promotes glucose utilization in part through promoting glucose entry into the skeletal and adipose tissues.This has been thought to be achieved through insulin-induced GLUT4 translocation from intracellular compartments to the cell membrane,which increases the overall rate of glucose flux into a cell.The insulin-induced GLUT4 translocation has been investigated extensively.Recently,significant progress has been made in our understanding of GLUT4 expression and translocation.Here,we summarized the methods and reagents used to determine the expression levels of Slc2a4 mRNA and GLUT4 protein,and GLUT4 translocation in the skeletal muscle,adipose tissues,heart and brain.Overall,a variety of methods such real-time polymerase chain reaction,immunohistochemistry,fluorescence microscopy,fusion proteins,stable cell line and transgenic animals have been used to answer particular questions related to GLUT4 system and insulin action.It seems that insulininduced GLUT4 translocation can be observed in the heart and brain in addition to the skeletal muscle and adipocytes.Hormones other than insulin can induce GLUT4 translocation.Clearly,more studies of GLUT4 are warranted in the future to advance of our understanding of glucose homeostasis.

Key Words:Glucose transporter 4;Insulin;Skeletal muscle;Adipocytes;Brain;Heart;Antibodies

INTRODUCTION

Currently,diabetes is a problem of public health[1].Based on the American Diabetes Association definition,diabetes is a serious chronic health condition of your body that causes blood glucose levels to rise higher than normal,which will lead to multiple complications if hyperglycemia is left untreated or mismanaged[2].Diabetes occurs when your body cannot make insulin or cannot effectively respond to insulin to regulate blood glucose level.There are two type of diabetes,insulin-dependent type 1 diabetes mellitus (T1DM) and -independent type 2 diabetes mellitus (T2DM).T2DM accounts for about 90% to 95% of all diagnosed cases of diabetes,and is due to the lack of responses to insulin in the body[3].Insulin resistance is a characteristic of T2DM.For a person with diabetes,a major challenge is to control or manage blood glucose level.Glucose is a common molecule used for production of energy or other metabolites in cells.As a quick energy source,glucose can be metabolized aerobically or anaerobically depending on the availability of oxygen or cell characteristics[4].Glucose is a hydrophilic molecule,and cannot diffuse into or out of a cell freely.It needs transporters to cross the cell membrane.Glucose transporters (GLUTs) are proteins that serve this purpose.

GLUTs are members of the major facilitator superfamily (MFS) transporters,which are responsible for the transfer of a large array of small molecules such as nutrients,metabolites and toxins across the cell membrane[5].Multiple members have been identified in each family of MFS transporters,and changes of their functions have been associated with a number of diseases[5].Members of MFS transporters have 12 transmembrane helices,and transport their substrates as uniporters,symporters or antiporters[5].Upon binding of the substrates on side of the membrane,a conformation change occurs,which is achieved through coordinative interactions of those helices through a “clamp-and-switch” mechanism.Structural studies have shown that the substrate specificity is achieved through the conserved amino acid residues within each family[5].Thus,it is important to understand GLUT functions,expressions and regulations for the control of blood glucose homeostasis.

Insulin and GLUTs

Dietary starch is first digested into glucose before being absorbed into the body and utilized[4].The first transporter identified is GLUT1,which is expressed universally in all cells,and responsible for basal glucose transport[6].Insulin stimulates glucose utilization in the body.This is in part through the insulin-induced glucose uptake in the muscle and adipose tissues.In addition to insulin stimulation,physical activity can also increase glucose entering into the skeletal muscle cells[7].The observation that insulin promotes the redistribution of GLUTs from intracellular locations to the plasma membrane in adipocytes began in the 1980s[8-10].Few years later,the insulininduced glucose transport was also found in muscle cells[11,12].To understand the underlying mechanism of insulin-stimulated glucose uptake,antibodies against membrane glucose transport proteins were created[13].Subcellular fractionation,cytokinin B (glucose-sensitive ligand),and glucose absorption into isolated vesicles were used to study the phenomenon.It was proposed that these GLUTs are moved from intracellular components to the plasma membrane of adipocytes and muscle cells upon insulin stimulation[6].In 1988,a specific antibody against a GLUT sample preparation was created,which eventually led to the identification of a molecular clone that encodes an insulin-induced GLUT from mouse adipocytes[6].It was named GLUT4.Since the 1990s,fluorescent-labeled fusion proteins,GLUT4-specific antibodies,photoaffinity labeling reagents,immunofluorescence microscopy,and high-resolution electron microscope have been used to confirm the insulin-induced translocation and underlying mechanisms[6]

It has been widely accepted that insulin mainly stimulates transfer of GLUT4 from intracellular storage vesicles to the plasma membrane.Insulin stimulation accelerates the movement rate of GLUT4 containing vesicles to the cell membrane[14].When more GLUT4 is on the plasma membrane,more glucose enters the cells without any change of the GLUT4 specific activity.During insulin stimulation,GLUT4 is not statically maintained in the plasma membrane but continuously recycled[6].After insulin is removed,the amount of GLUT4 on the plasma membrane drops and the rate of movement returns to basal level.

Since identification and cloning of GLUT1,13 additional GLUTs have been cloned using recombinant DNA techniques[15].Based on their phylogeny or genetic and structural similarities,GLUTs are classified into three classes.Class I includes GLUTs 1-4,and GLUT14 which are responsible for glucose transfer.Class II consists of GLTUs 5,7,9 and GLUT 11 which are considered as fructose transporters.Class III contains GLTUs 6,8,10,12 and GLUT 13[16].All GLUTs have nearly 500 amino acid residues that form 12 transmembrane helices[15].

Each GLUT has its own unique affinity and specificity for its substrate,tissue distribution,intracellular location,regulatory mechanisms and physiological functions[17].The most well studied and known members are GLUTs 1-6.GLUT1 is found evenly distributed in the fetal tissues.In human adults,GLUT1 Level is high in erythrocytes and endothelial cells.It is responsible for basal glucose uptake[18].GLUT2 is expressed in the liver and pancreas,and contributes to glucose sensing and homeostasis[17].In enterocytes,GLUT2 is responsible to transport the absorbed glucose,fructose and galactose out of the basolateral membrane to enter into the blood circulation through the portal vein[19].GLUT3 just like GLUT1 is expressed in almost all mammalian cells and is responsible for the basal uptake of glucose.GLUT3 is considered as the main GLUT isoform expressed in neurons and the placenta,but has also been detected in the testis,placenta,and skeletal muscle[20-22].GLUT5 is specific for uptake of fructose in a passive diffusion manner,and is expressed in the small intestine,testes and kidney[17].GLUT6 is expressed in the spleen,brain,and leukocytes as well as in muscle and adipose tissue[15,23].GLUT6 has been shown to move from the intracellular locations and plasma membrane of rat adipocytes in a dynamindependent manner[23].Table 1[24-57]summarizes names,numbers of amino acids,Kms,expression profiles and potential functions of those GLUTs.

GLUT4 gene,its tissue distribution,and physiological functions

Human GLUT4 has 509 amino acid residues and is encoded bySLC2A4gene in the human genome.It is mainly expressed in adipocytes and skeletal muscle.The unique N-terminal and COOH terminal sequences are responsible for GLUT4's response to insulin signaling and membrane transport[58].The Km of GLUT4 is about 5 mmol/L.In response to insulin stimulation,intracellular vesicles containing GLUT4 are moved from cytosol to the cell membrane.As shown in Figure 1,insulin receptor is a tetramer with two alpha-subunits and two beta-subunits linked by disulfide bonds[59].When insulin binds to its receptor on the cell membrane,insulin receptor beta subunits that contain tyrosine kinase domain autophosphorylate each other.The phosphorylated βsubunits recruit insulin receptor substrates (IRS) and phosphorylate them.Then phosphorylated IRSs bind to and activate phosphatidylinositol 3-kinase (PI3K) which is recruited to the plasma membrane and converts PIP2 to PIP3.On the plasma membrane,PI3K activates PIP3 dependent protein kinase,which phosphorylates and activates AKT (also referred to as protein kinase B,PKB).Akt activation triggers vesicle fusion,which results in the translocation of GLUT4 containing vesicles from intracellular compartments to the plasma membrane.The elevation of GLUT4 on the membrane leads to increase of glucose entry into the cell.

Upon refeeding,elevated glucose levels in the blood stimulates insulin secretion from pancreatic beta cells.Insulin stimulates GLUT4 translocation to the cell membrane,which increases glucose uptake in cells,and achieves glucose homeostasis[60,61].After the insulin stimulation disappears,GLUT4 is transferred back into the cytosol from the plasma membrane.More than 90% of GLUT4 is located in the intracellular body,trans-Golgi network,and heterogeneous tube-like vesicle structure,etc.,which constitute the GLUT4 storage vesicle (GSV).In an unstimulated state,most GLUT4 is in the intracellular vesicles of muscle and adipocytes[62].

The amount of GLUT4 on the cell membrane is determined by the rate of the movement from intracellular GSV to the cell membrane.In adipocytes and skeletal muscle cells,insulin increases the rate of GLUT4 translocation from GSVs to membrane and decreases the rate of GLUT4 movement from membrane back to the vesicles,which lead to elevation of GLUT4 content on the cell membrane by 2-3 times[63].Moreover,in adipocytes,insulin increases the GLUT4 recirculation to maintain a stable and releasable vesicle[64].

So far,insulin-induced GLUT4 translocation has been studied extensively.However,questions still remain.Methods and reagents used to determine the expression levels of GLUT4 and its translocation mechanism deserve to be summarized and analyzed.Therefore,we searched the relevant articles in PubMed and investigated the methods and reagents used in the studies.“Glucose transporter 4” and “GLUT4” as the protein and “SLC2A4” as the gene name were used as keywords in the search.In order to have a more clearly overview,we further divided and focused on the search into three parts,GLUT4 in the skeletal muscle,GLUT4 in adipose tissues,and GLUT4 in heart and brain.

Table 1 Summary of glucose transporter family members

GLUT4 IN THE SKELETAL MUSCLE

The term "muscle" covers a variety of cell types.Mammals have four main types of muscle cells:skeletal,heart,smooth,and myoepithelial cells.They are different in function,structure and development[65].The skeletal muscle mass accounts for 40% of the total body mass,and the regulation of skeletal muscle glucose metabolism will significantly affect the body's glucose homeostasis[66,67].Skeletal muscle is composed of many muscle fibers connected by collagen and reticular fibers.Each skeletal muscle fiber is a syncytium that derives from the fusion of many myoblasts.Myoblasts proliferate in large quantities,but once fused,they no longer divide.The fusion usually follows the onset of myoblast differentiation[65].Different fiber types have distinct contractile and metabolic properties[68].The skeletal muscle maintains skeletal structure and essential daily activities[69].Also,it is a source of proteins that can be broken down into amino acids for the body to use.

Figure 1 Schematic of insulin-induced translocation of glucose transporter 4 from cytosol to the cell membrane.

Insulin stimulates glucose uptake and utilization in the skeletal muscle.GLUT4 plays a key role in the uptake process.Glucose can be stored as glycogen,which is used as a quick source of energy in physical activity[70].In the skeletal muscle,exercise helps increase insulin sensitivity and stimulatesSLC2A4gene transcription[60].Physiological factors such as the type of muscle fibers can also affect the GLUT4 Level.An increase in physical activity will induce the GLUT4 Levels,whereas a decrease in activity level will reduce GLUT4[68].The skeletal muscle not only maintains the activities,but also regulates the glucose homeostasis in the body,which plays a key role in the development of metabolic diseases[69].Obesity and T2DM have a negative impact on skeletal muscle glucose metabolism[71].

To review the methods and reagents of GLUT4 studies in skeletal muscle,"GLUT4,skeletal muscle" and "SLC2A4,skeletal muscle" as keywords were used to search the PubMed database to retrieve relevant articles.The skeletal muscle is a highly specialized tissue made of well-organized muscle fibers.The unique structural characteristic of muscle inherits difficulties to be lysed for biochemical studies.Therefore,we want to focus on the sample preparation of skeletal muscle in GLUT4 studies.The retrieved articles were screened mainly according to the research methods and reagents used for skeletal muscle preparation,experimental groups included,Slc2a4mRNA and GLUT4 protein measurements and the source of GLUT4 antibodies obtained.In the end,10 representative articles were selected for analysis and summary as shown in Table 2[72-81].

Overall,the current research methods of GLUT 4 studies in skeletal muscle are listed below:(1) Samples were homogenized to prepare membrane fractions for analysis of GLUT 4 in western blot using monoclonal or polyclonal antibodies;(2)Real-time polymerase chain reaction (PCR) was used to determine the mRNA abundance ofSlc2a4;(3) Immunocytochemical staining was used to detect GLUT4in situ.The fibers were labeled for GLUT4 by a preembedding technique and observed as whole mounts by immunofluorescence microscopy or after sectioning,by immunogold electron microscopy.Preembedding is a technique to label GLUT4 immediately after tissues or cells are collected,which allows that the antibody interacts with the antigen before denaturation;(4) Muscle cell lines stably expressing tagged GLUT4 were established to study the translocation;and (5) Radiolabeled 2-deoxyglucose was used to determine the glucose uptake in muscle tissue slices.

The antibodies used in these articles were from Santa Cruz,Millipore,East Acres,Biogenesis and other sources not specified.Only two of the publications have a positive control group of GLUT4 expression using overexpression of a fusion protein and tissue preparation as a standard for determination.Positive controls are important when Western blot and fusion protein immunofluorescence methods are used to determine the GLUT4 protein levels.

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

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.

According to studies summarized in Table 2,the following key points can be obtained.Insulin and muscle contraction increase glucose uptake in the skeletal muscle,which is associated with increases of GLUT4 content and its translocation.Neomycin,a phospholipase D inhibitor,blocks GLUT4 translocation.In the skeletal muscle isolated from GLUT1 transgenic mice,insulin-induced GLUT4 translocation response is lost,which is not due to down-regulation of GLUT4 expression.Conjugated linoleic acid in the diet does not affect theSlc2a4mRNA expression in the skeletal muscle.Indinavir,an HIV-1 protease inhibitor,can block the glucose uptake mediated by GLUT4 in normal skeletal muscle and adipocytes without or with insulin stimulation.More studies are anticipated to elucidate how insulin resistance and T2DM affect the functions of GLUT4 system and whether overnutrition plays a role in it.

GLUT4 IN ADIPOSE TISSUES

The ability for an organism to store excessive amount of energy in the form of fat is helpful for it to navigate a condition of an uncertain supply of food[82].Adipocytes,the main type of cells in adipose tissue,are not only a place for fat storage,but also endocrine cells to secrete cytokines for the regulation of whole body energy homeostasis[82].Based on the mitochondrial content and physiological functions,adipocytes are divided into white,beige and brown fat cells.Structurally,90% of the cell volume of a white adipocyte is occupied by lipid droplets.In normal-weight adults,white adipose tissue accounts for 15% to 20% of body weight[83,84].Excessive accumulation of body fat results in the development of obesity,which can lead to the development of T2DM if not managed[85,86].In addition,adipose tissues secrete cytokines such as leptin and adiponectin with abilities to regulate food intake and insulin sensitivity[87].GLUT4 is expressed in adipocytes,where insulin stimulates its translocation from intracellular locations to the cell membrane,which leads to increase of glucose uptake[88].High expression levels of GLUT4 in adipose tissue can enhance insulin sensitivity and glucose tolerance[89].

Insulin-induced GLUT4 translocation in adipose tissue and skeletal muscle has been studied extensively.Overall,in recent years,great progress has been made in the understanding of GLUT4 vesicles movement,the fusion of the vesicles with the cell membrane,and the translocation mechanism in response to insulin.As shown in Figure 2,in adipocytes of an adipose depot,GLUT4 vesicles move from the specialized intracellular compartment to the cell periphery (near cell membrane),which is followed by tethering and docking.Tethering is the interaction between GLUT4 vesicles and the plasma membrane.Docking is the assembly of the soluble Nethylmaleimide-sensitive factor-attachment protein (SNAP) receptor (SNARE)complex.Fusion occurs when the lipid bilayers of the vesicles with GLUT4 and the cell membrane merge[90].The actin cytoskeleton system plays an important role in retaining GLUT4 vesicles in adipocytes.After insulin stimulation,remodeling of cortical actin causes the release GLUT4 vesicles to the plasma membrane[90-92].β-catenin plays an important role in regulating the transport of synaptic vesicles.The amount of GLUT4 within the insulin sensitive pool is determined by the β-catenin levels in adipocytes,which allows GLUT4 translocate to the cell membrane in response to insulin stimulation[93,94].

To summarize the current methods and reagents used for GLUT4 analysis in adipose tissues and adipocytes,“GLUT4,3T3-L1”,and “GLUT4,adipocytes” were used to search the literature published in the past 15 years.We ignored those studies that only measuredSlc4a2mRNA,lacked the focus on adipocytes,and did not have full text versions.The resulted 30 articles were analyzed and summarized in Tables 3[95-112]and 4[113-124].Table 3 contains 18 articles and summarizes the effects of drugs or bioactive compounds onSlc2a4mRNA and GLUT4 protein expressions,and GLUT4 translocation.Table 4 contains 12 articles and summarizes studies of the regulatory mechanisms of GLUT4 system.

There are 11 and 2 articles respectively using real-time PCR and Northern blot to evaluateSlc2a4mRNA levels.Western blot and ELISA are used in 24 articles to detect GLUT4 protein content.The antibodies were from Santa Cruz (6),Millipore (3),Cell Signaling Technology (4),Chemicon (4),Abcam (3),Pierce (1),Oxford (1) and Signalway (1).Two articles did not indicate the antibody sources.One article used antibodies from two companies.Nine articles indicated the dilutions of antibodies,and only 3 articles included the production catalog number.Immunofluorescence was used to detect the content and translocation of GLUT4 protein in 9 articles.Three articles used flow cytometry to detect GLUT4 protein.Twenty four of these 30 articles directly assessed levels ofSlc2a4mRNA or GLUT4 protein.The remaining 6 of the 30 articles measured GLUT4 protein using fusion proteins.No article has a positive control group that uses overexpression of GLUT4viaa recombinant construct or purified recombinant GLUT4 protein.

As shown in Table 3,GLUT4 translocation,andSlc2a4mRNA and GLUT4 expression levels in 3T3-L1 cells can be regulated by bioactive compounds,crude extract of herbs,agonists of nuclear receptors,proteins and chemical drugs.Sl2a4mRNA or/and GLUT4 expressions in 3T3-L1 cells or adipose tissues can be increased by kaempferitrin,GW9662,inhibitor of p38 kinase,estradiol,crude extract of stevia leaf,fargesin,phillyrin,selenium-enriched exopolysaccharide,aspalathin-enriched green rooibos extract,bone morphogenetic proteins 2 and 6,and glucose pulse.On the other hand,Slc2a4mRNA and GLUT4 protein levels can be reduced by luteolin,and shilianhua extract in 3T3-L1 cells.GLUT4 translocation can be enhanced by kaempferitrin,curculigoside and ethyl acetate fractions,gallic acid,6-hydroxydaidzein and ginsenoside Re,and reduced by green tea epigallocatechin gallate.

As shown in Table 4,a variety of methods have been used to study the regulatory mechanisms of GLUT4 system.3T3-L1 cells have been the major model in those studies.In addition to the insulin,pathways involved in theSlc2a4gene expression,GLUT4 protein expression and its translocation include cannabinoid receptor 1 (CB1),ADP-ribosylation factor-related protein 1,MiR-29 family,proteasome system,estrogen pathway,oxidative stressviaCCAAT/enhancer-binding protein alpha (C/EBPα),obesity development,differentially expressed in normal and neoplastic cells domaincontaining protein 4C,nuclear factor-κB,Akt and Akt substrate of 160 kDa,phosphatidylinositol-3,4,5-trisphosphate dependent Rac exchange factor 1,secreted protein acidic and rich in cysteine (SPARC),sterol regulatory element-binding protein 1 (SREBP-1),and AMP-activated protein kinase (AMPK) pathway.CB1 receptor antagonists increaseSlc2a4mRNA and GLUT4 protein expressions through NF-kB and SREBP-1 pathways.Akt pathway regulates the rate of vesicle tethering/fusion by controlling the concentration of primed,and fusion-competent GSVs with the plasma membrane.Inhibition of the SPARC expression reducesSlc2a4mRNA and GLUT4 expressions.The expressions of C/EBPα and δ alter the C/EBPdimer formation at theSlc2a4gene promoter,which regulates its transcription.Inhibition of differentially expressed in normal and neoplastic cells domain-containing protein 4C can block GLUT4 translocation.Rac exchange factor 1 activation seems to promote GLUT4 translocation via arrangement of actin cytoskeleton.The mechanism of AMPK-mediated GLUT4 translocation in 3T3-L1 adipocytes seems to be distinct from that of insulin-induced one.Future studies are needed to integrate the roles of all these players in the regulation of GLUT4 system in adipocytes.

Table 3 Recent studies of effects of bioactive compounds and chemical drugs on glucose transporter 4 expression and translocation in adipocytes

GLUT4:Glucose transporter 4;AMPK:AMP-activated protein kinase;CEBPA and C/EBP:CCAAT/enhancer-binding protein alpha;ESR1:Estrogen receptor 1.

GLUT4 IN THE HEART

The heart works constantly to support the blood circulation throughout the lifespan.Cardiomyocytes constantly contract to pump blood,oxygen,metabolic substrates,and hormones to other parts of the body.This requires continuous ATP production for energy supply.The primary fuel for the heart is fatty acids,whereas glucose and lactate contribute to 30% of energy for ATP production[125].In addition,glucose plays an important role in circumstances like ischemia,increased workload,and pressure overload hypertrophy.

Glucose is transported into cardiac myocytes through GLUTs.GLUT4 represents around 70% of the total glucose transport activities[15].GLUT4 protein expression can be found as early as 21 days of gestation in rats[126].The expression level of GLUT4 in the heart may increase or decrease depending on the different models.For example,GLUT4 protein content decreases along with aging in male Fischer rats,but increases 4-5 times in C57 Bl6 mice[127,128].In basal state,GLUT4 is found mainly in intracellular membrane compartments,and can be stimulated by insulin and ischemia to translocate to the cell membrane[129].The binding of myocyte enhancer factor-2 (MEF2)and thyroid hormone receptor alpha 1 is needed for transcription ofSlc2a4gene in cardiac and skeletal muscle in rats[130].In addition,Slc2a4gene expression can also be regulated by other transcription factors.For example,overexpression of peroxisome proliferator-activated receptor gamma coactivator 1 works with MEF2-C to induceSlc2a4mRNA expression in L6 muscle cells[131].Moreover,GLUT4 expression level can be affected by cardiovascular diseases,and myocardial sarcolemma,which reduce the expression and translocation of GLUT4[132].The development of T1DM decreases GLUT4 expression level and its translocation in the heart of mice[132].T2DM development also reduces GLUT4 content and translocation due to insulin resistance and impairments of insulin signaling pathway in human cardiomyocytes[133].

Table 4 Recent studies of mechanisms of glucose transporter 4 expression and translocation in adipocytes

To investigate the methods and reagents used for GLUT4 studies in the heart,“GLUT4,heart”,and “cardiomyocytes,GLUT4 expression” were used as key words to search PubMed for articles published after 2000.We went through all papers with cardiomyocytes and GLUT4 in titles or short descriptions and selected 9 of them that are mainly focused on GLUT4 expression and translocation in the heart as shown in Table 5[134-142].

Rats are used in all 9 studies.Various methods and reagents are used to analyzeSlc2a4mRNA and GLUT4 protein levels in the heart and cardiomyocytes.Real-time PCR was used in 4 of them to determineSlc2a4mRNA levels.Antibodies and Western blot were used to assess GLUT 4 protein in 8 of them.Immunohistochemistry was used in 1 of them.Two of them used immunofluorescence to track down GLUT4 translocation.

To determine the content of GLUT4 protein in the heart,Western blot and fusion protein immunofluorescence methods were used.As shown in Table 5,these studies do not include overexpressed GLUT4 or cell samples withSlc2a4deletion as controls.Several of them did not mention sources of anti GLUT4 antibodies used in Western blots.Some used polyclonal antibodies,which may need a positive control to indicate the correct size and location of GLUT4 protein.

From the papers listed above,GLUT4 expression and translocation in the heart and cardiomyocytes can be affected through activations of ERK and Akt pathways.Proteins like growth hormone,catestatin or pigment epithelium-derived factor can stimulate GLUT4 translocation and glucose uptake.Chemicals like sitagliptin and ethanol can up- and down-regulateSlc2a4’s mRNA expression levels,respectively.However,the underlying mechanisms responsible for these regulations of GLUT4 translocation andSlc2a4mRNA expression remain to be revealed.In addition,the research results summarized here are from tissue and cells of rats.It will be interesting to see whether same results will be observed when tissues and cells from other animal models are used.

GLUT4 IN THE BRAIN

The brain is a complex organ in the body and controls a variety of functions from emotions to metabolism.It consists of cerebrum,the brainstem,and the cerebellum[143].Brain cells utilize glucose constantly to produce energy in normal physiological conditions.The brain can consume about 120 g of glucose per day,which is about 420 kcal and accounts for 60% of glucose ingested in a human subject[125].The glucose influx and metabolism in the brain can be affected by multiple factors such as aging,T2DM and Alzheimer’s disease[144].Reduction of glucose metabolism in the brain can lead to cognitive deficits[145].Due to the critical relationship between cognitive performance and glucose metabolism,it is important to understand the regulatory mechanism of glucose metabolism in the brain.

Table 5 Recent studies of glucose transporter 4 expression and translocation in the heart

Studies have shown that insulin signaling can be impacted in both T2DM and Alzheimer’s disease[146,147].Insulin is a key component for hippocampal memory process,and specifically involved in regulating hippocampal cognitive processes and metabolism[148].Insulin-modulated glucose metabolism depends on regions in the brain.The cortex and hippocampus are the most sensitive areas in the brain[149].Hippocampus located deeply in temporal lobe plays an important role in learning and memory,and relates to diseases like Alzheimer’s disease,short term memory loss and disorientation[150].Hippocampal cognitive and metabolic impairments are relatively common in T2DM,which may be caused by diet-induced obesity and systemic insulin resistance[151].On the other hand,insulin stimulation can enhance memory and cognitive function[152].This enhancement may require the brain GLUT4 translocation as shown in rats[153].It is very important to determine the expression profile of GLUT4 in the brain,and factors that impact GLUT4 expression and translocation.

Figure 2 The movement of glucose transporter 4 in adipocytes.

To summarize methods and reagents used in brain GLUT4 studies,“brain,GLUT4 expression” were used as key words to search PubMed for articles that have brain and GLUT4 in their titles or short descriptions.Ten research articles published after 2000 were identified as representative ones,which are focused on GLUT4 translocations and content in the brain of rats and mice.We summarized the methods and reagents for GLUT4 analysis,and conclusions as shown in Table 6[154-163].

In these 10 papers,two of them used real-time PCR to determine theSlc2a4mRNA in the brain.Eight of them used anti-GLUT4 antibodies and Western blot to detect GLUT 4 protein.Five papers included β-actin as loading control in Western blot.Four used immunohistochemistry.One paper used electrophysiological technique,and one paper used fluorescent microscopy to identify GLUT 4 in neurons.One study used brain specificSlc2a4knockout and wild type mice to study the functions of GLUT4 in the brain.

In conclusion,results of Western blot and real-time PCR demonstrate that GLUT4 protein andSlc2a4mRNA can be detected in rat’s brain and central nervous system.Deletion ofSlc2a4in the brain causes insulin resistance,glucose intolerance,and impaired glucose sensing in the ventromedial hypothalamus.GLUT4 mediates the effects of insulin,or insulin-like growth factor on regulations of cognition,memory,behavior,motor activity and seizures.GLUT4 positive neurons are responsible for glucose sensing.Physical activity improves GLUT4 translocation in neurons,a process that needs Rab10 phosphorylation.Interestingly,27-OH cholesterol treatment seems to decrease GLUT4 expression in the brain.Studies ofSlc2a4mRNA and GLUT4 protein in the brain and central nervous system have begun to demonstrate the potential roles of GLUT4 expression and its translocation in the regulation of glucose metabolism in the brain and central nervous system.More studies of GLUT4 expression and translocation in the control of functions and metabolism in various region in the brain and central nervous system are expected in the future.

Table 6 Recent studies of glucose transporter 4 expression and translocation in the brain

CONCLUSION

GLUT4 is generally thought to contribute to insulin-stimulated glucose uptake in adipocytes and skeletal muscle.Studies summarized here seem to show that GLUT4 is also expressed in the brain,neurons,and heart.GLUT4 is expressed concurrently with other GLUTs in multiple tissues in a temporal and spatial specific manner such as during brain development[163].Hormones and cytokines other than insulin can also regulate the expression levels and translocation of GLUT4 in different tissues[141,142,163].In adipocytes alone,many bioactive compounds or chemical reagents have shown to affect GLUT4 pathways as shown in Table 3.All these seem to indicate that the regulatory mechanism of the GLUT4 pathway is complicated than we originally proposed.

So far,various methods from gene knockout to immunohistochemistry have been used to study the mechanisms ofSlc2a4mRNA and GLUT4 expressions,and its translocation in different cells.Every technique has its pros and cons.Based on the studies summarized here,anti-GLUT4 antibodies from a variety of sources have been used to study GLUT4 expression and translocation.The conclusions of these studies are based on the experimental results derived from the use of those antibodies.A positive control derived from a cell or tissue with unique overexpression or silencing of GLUT4 is critical to confirm the antibody’s specificity to pick up a right signaling in the study system.This is especially true for Western blot.It appears that some of the studies did not include control groups like this.Another challenge facing biochemical study of GLUT4 translocation in the muscle may be the sample processing.This probably explains why fusion proteins and stable cell lines are developed to enhance signals and specificities for detection.Confirmation of the antibody specificity in a particular system probably should be done first.

As glucose homeostasis is a complicate process involved in many players.It is anticipated to see that many proteins seem to play a role in the regulation of GLUT4 system.It will be interesting to see how GLUT4 in different regions of the brain contributes to the regulation of glucose metabolism,and what the roles of insulininduced GLUT4 translocation in those areas are.In addition,other GLUTs are also expressed in the same cells that GLUT4 are expressed.How GLUT4 works with other GLUTs to regulate metabolism also deserves to be investigated.Last,as glucose usage in the skeletal muscle is altered in insulin resistance and T2DM,how GLUT4 system contributes to progressions and interventions of these diseases still remains to be the focus in the future.Nevertheless,further understanding GLUT4 system will be very helpful for us to combat the development of T2DM.

ACKNOWLEDGEMENTS

We would like to thank the Department of Nutrition at the University of Tennessee at Knoxville for financial support to T.W.G.C.would like to thank Yantai Zestern Biotechnique Co.LTD for the research funding support.