Glucose transporter type 4 (GLUT4), also known assolute carrier family 2, facilitated glucose transporter member 4, is aprotein encoded, in humans, by theSLC2A4gene. GLUT4 is theinsulin-regulatedglucose transporter found primarily inadipose tissues andstriated muscle (skeletal and cardiac). GLUT4 is distinctive because it is predominantly stored within intracellular vesicles, highlighting the importance of its trafficking and regulation as a central area of research.[5] The first evidence for this glucose transport protein was provided byDavid James in 1988.[6] The gene that encodes GLUT4 was cloned[7][8] and mapped in 1989.[9]
At the cell surface, GLUT4 permits the facilitated diffusion of circulating glucose down its concentration gradient into muscle and fat cells. Once within cells, glucose is rapidlyphosphorylated byglucokinase in the liver andhexokinase in other tissues to formglucose-6-phosphate, which then entersglycolysis or is polymerized into glycogen. Glucose-6-phosphate cannot diffuse back out of cells, which also serves to maintain the concentration gradient for glucose to passively enter cells.[10]
Like all proteins, the unique amino acid arrangement in theprimary sequence of GLUT4 is what allows it to transport glucose across the plasma membrane. In addition to thephenylalanine on the N-terminus, twoLeucine residues and acidic motifs on the COOH-terminus are believed to play a key role in the kinetics ofendocytosis andexocytosis.[12]
There are 14 total GLUT proteins separated into 3 classes based onsequence similarities. Class 1 consists of GLUT 1-4 and 14, class 2 contains GLUT 5, 7, 9 and 11, and class 3 has GLUT 6, 8, 10, 12 and 13.
Although there are some sequence differences between all GLUT proteins, they all have some basic structural components. For example, both the N and C termini in GLUT proteins are exposed to thecytoplasm of the cell, and they all have 12 transmembrane segments.[13]
As muscles contract, they use ATP. The energy needed to make ATP comes from a variety of different pathways—such as glycolysis or oxidative phosphorylation—that ultimately use glucose as a starting material.[14]
In striatedskeletal muscle cells, GLUT4 concentration in the plasma membrane can increase as a result of either exercise or muscle contraction.
During exercise, the body needs to convert glucose toATP to be used as energy. AsG-6-P concentrations decrease,hexokinase becomes less inhibited, and the glycolytic and oxidative pathways that make ATP are able to proceed. This also means that muscle cells are able to take in more glucose as its intracellular concentrations decrease. In order to increase glucose levels in the cell, GLUT4 is the primary transporter used in thisfacilitated diffusion.[15]
Although muscle contractions function in a similar way and also induce the translocation of GLUT4 into the plasma membrane, the two skeletal muscle processes obtain different forms of intracellular GLUT4. The GLUT4 carrier vesicles are either transferrin positive or negative, and are recruited by different stimuli. Transferrin-positive GLUT4 vesicles are utilized during muscle contraction while the transferrin-negative vesicles are activated by insulin stimulation as well as by exercise.[16][17]
Cardiac muscle is slightly different from skeletal muscle. At rest, they prefer to utilizefatty acids as their main energy source. As activity increases and it begins to pump faster, the cardiac muscles begin to oxidize glucose at a higher rate.[18]
An analysis of mRNA levels ofGLUT1 and GLUT4 in cardiac muscles show that GLUT1 plays a larger role in cardiac muscles than it does in skeletal muscles.[19] GLUT4, however, is still believed to be the primary transporter for glucose.[20]
Much like in other tissues, GLUT4 also responds to insulin signaling, and is transported into the plasma membrane to facilitate the diffusion of glucose into the cell. [21][22]
Adipose tissue, commonly known as fat,[23] is a depository for energy in order to conserve metabolichomeostasis. As the body takes in energy in the form of glucose, some is expended, and the rest is stored asglycogen (primarily in the liver, muscle cells), or as triglyceride in adipose tissue.[24]
An imbalance in glucose intake and energy expenditure has been shown to lead to both adipose cellhypertrophy andhyperplasia, which lead to obesity.[25] In addition, mutations in GLUT4 genes inadipocytes can also lead to increased GLUT4 expression in adipose cells, which allows for increased glucose uptake and therefore more fat stored. If GLUT4 is over-expressed, it can actually alter nutrient distribution and send excess glucose into adipose tissue, leading to increased adipose tissue mass.[25]
Insulin is released from the pancreas and into the bloodstream in response to increased glucose concentration in the blood.[26] Insulin is stored inbeta cells in the pancreas. When glucose in the blood binds to glucose receptors on the beta cell membrane, asignal cascade is initiated inside the cell that results in insulin stored invesicles in these cells being released into the blood stream.[27] Increased insulin levels cause the uptake of glucose into the cells. GLUT4 is stored in the cell intransport vesicles, and is quickly incorporated into the plasma membrane of the cell when insulin binds tomembrane receptors.[24]
Under conditions of low insulin, most GLUT4 is sequestered in intracellular vesicles in muscle and fat cells. As the vesicles fuse with the plasma membrane, GLUT4 transporters are inserted and become available for transporting glucose, and glucose absorption increases.[28]The genetically engineered muscle insulin receptor knock‐out (MIRKO) mouse was designed to be insensitive to glucose uptake caused by insulin, meaning that GLUT4 is absent. Mice with diabetes or fasting hyperglycemia, however, were found to be immune to the negative effects of the insensitivity.[29]
The insulin signal transduction pathway begins when insulin binds to the insulin receptor proteins. Once the transduction pathway is completed, the GLUT-4 storage vesicles becomes one with the cellular membrane. As a result, the GLUT-4 protein channels become embedded into the membrane, allowing glucose to be transported into the cell.
The mechanism for GLUT4 is an example of acascade effect, where binding of aligand to a membrane receptor amplifies the signal and causes a cellular response. In this case, insulin binds to theinsulin receptor in itsdimeric form and activates the receptor's tyrosine-kinase domain. The receptor then recruits Insulin Receptor Substrate, orIRS-1, which binds the enzyme PI-3 kinase. PI-3 kinase converts the membrane lipidPIP2 toPIP3. PIP3 is specifically recognized by PKB (protein kinase B) and by PDK1, which can phosphorylate and activate PKB. Once phosphorylated, PKB is in its active form and phosphorylatesTBC1D4, which inhibits theGTPase-activating domain associated with TBC1D4, allowing for Rab protein to change from its GDP to GTP bound state. Inhibition of the GTPase-activating domain leaves proteins next in the cascade in their active form, and stimulates GLUT4 to be expressed on the plasma membrane.[30]
RAC1 is aGTPase also activated by insulin. Rac1 stimulates reorganization of the corticalActin cytoskeleton[31] which allows for the GLUT4 vesicles to be inserted into the plasma membrane.[32][33] ARAC1Knockout mouse has reduced glucose uptake in muscle tissue.[33]
Intracellular signaling pathways for insulin- and contraction-stimulated glucose transporter 4 (GLUT4) translocation in skeletal muscle through the regulation of TBC1D1 and TBC1D4.
Muscle contraction stimulates muscle cells to translocate GLUT4 receptors to their surfaces. This is especially true in cardiac muscle, where continuous contraction increases the rate of GLUT4 translocation; but is observed to a lesser extent in increased skeletal muscle contraction.[35] In skeletal muscle, muscle contractions substantially increase GLUT4 translocation,[36] which is regulated byRAC1[37][38] andAMP-activated protein kinase (AMPK).[39] Contraction-induced glucose uptake involves the phosphorylation ofRabGaps,TBC1D1 andTBC1D4, by AMPK and other kinases such as SNARK.[40][41] This mechanism remains functional in insulin-resistant states, establishing the muscle-contraction pathway's independence from insulin stimulation.[41] The figure to the right demonstrates how insulin- and contraction-stimulated GLUT4 translocation differ but ultimately converge on TBC1D1/4. Phosphorylation of TBC1D1/4 inactivates it, allowing Rab proteins to load GTP and directly participate in the trafficking of GLUT4 to the membrane.[15]
AMPK plays a crucial role in the contraction pathway.[42] ATP is known as an energy-sensing enzyme, as it's highly responsive to an increase in the AMP to ATP ratio.[42] ATP is hydrolyzed to ADP during muscle contraction byactomyosin ATPase.[43]Adenylate kinase subsequently converts ADP through the following reaction: 2ADP→ATP+AMP.[43] This ensures rapid replenishment of ATP, while increasing AMP concentration.[43] ATP competes with AMP for coupling to the AMPK binding domain and thus inhibits AMPK activity, particularly when the muscle is at rest and ATP concentration is high.[42] AMP has a much stronger affinity for the binding domain (known as the Bateman domain) of AMPK, and will thus out-compete ATP as AMP concentration increases.[42] This ultimately results in the phosphorylation and activation of AMPK by LKB1[44] and triggers a cascade of signaling events driven by AMPK, leading to the translocation of GLUT4.[15]
Muscle stretching also stimulates GLUT4 translocation and glucose uptake in rodent muscle viaRAC1.[45]
GLUT4 has been shown to interact withdeath-associated protein 6, also known as Daxx. Daxx, which is used to regulateapoptosis, has been shown to associate with GLUT4 in the cytoplasm. UBX-domains, such as the one found in GLUT4, have been shown to associate with apoptotic signaling.[11] So this interaction aids in the translocation of Daxx within the cell.[46]
In addition, recent reports demonstrated the presence of GLUT4 gene in central nervous system such as thehippocampus. Moreover, impairment in insulin-stimulated trafficking of GLUT4 in the hippocampus result in decreased metabolic activities and plasticity of hippocampal neurons, which leads to depressive like behaviour and cognitive dysfunction.[47][48][49]
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^"Mouse PubMed Reference:".National Center for Biotechnology Information, U.S. National Library of Medicine.
^Watson RT, Pessin JE (2001)."9"(PDF). Written at 51 Newton Road, Iowa City, Iowa 52242.Intracellular Organization of Insulin Signaling and GLUT4 Translocation. Department of Physiology & Biophysics. Vol. 56. The University of Iowa. pp. 175–193.doi:10.1210/rp.56.1.175.PMID11237212.{{cite book}}:|journal= ignored (help)CS1 maint: location (link) CS1 maint: location missing publisher (link)
^Bell GI, Murray JC, Nakamura Y, Kayano T, Eddy RL, Fan YS, et al. (August 1989). "Polymorphic human insulin-responsive glucose-transporter gene on chromosome 17p13".Diabetes.38 (8):1072–1075.doi:10.2337/diabetes.38.8.1072.PMID2568955.
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^Morgan HE, Henderson MJ, Regen DM, Park CR (September 1959). "Regulation of glucose uptake in heart muscle from normal and alloxan-diabetic rats: the effects of insulin, growth hormone, cortisone, and anoxia".Annals of the New York Academy of Sciences.82 (2):387–402.Bibcode:1959NYASA..82..387M.doi:10.1111/j.1749-6632.1959.tb44920.x.PMID14424107.S2CID32458568.
^Laybutt DR, Thompson AL, Cooney GJ, Kraegen EW (September 1997). "Selective chronic regulation of GLUT1 and GLUT4 content by insulin, glucose, and lipid in rat cardiac muscle in vivo".The American Journal of Physiology.273 (3 Pt 2):H1309 –H1316.doi:10.1152/ajpheart.1997.273.3.H1309.PMID9321820.
^Rett K, Wicklmayr M, Dietze GJ, Häring HU (January 1996). "Insulin-induced glucose transporter (GLUT1 and GLUT4) translocation in cardiac muscle tissue is mimicked by bradykinin".Diabetes. 45 Suppl 1 (Supplement 1):S66 –S69.doi:10.2337/diab.45.1.S66.PMID8529803.S2CID7766813.
^Leto D, Saltiel AR (May 2012). "Regulation of glucose transport by insulin: traffic control of GLUT4".Nature Reviews. Molecular Cell Biology.13 (6):383–396.doi:10.1038/nrm3351.PMID22617471.S2CID39756994.
^Sylow L, Kleinert M, Pehmøller C, Prats C, Chiu TT, Klip A, et al. (February 2014). "Akt and Rac1 signaling are jointly required for insulin-stimulated glucose uptake in skeletal muscle and downregulated in insulin resistance".Cellular Signalling.26 (2):323–331.doi:10.1016/j.cellsig.2013.11.007.PMID24216610.
^Patel SS, Udayabanu M (March 2014). "Urtica dioica extract attenuates depressive like behavior and associative memory dysfunction in dexamethasone induced diabetic mice".Metabolic Brain Disease.29 (1):121–130.doi:10.1007/s11011-014-9480-0.PMID24435938.S2CID10955351.
^Piroli GG, Grillo CA, Reznikov LR, Adams S, McEwen BS, Charron MJ, et al. (2007). "Corticosterone impairs insulin-stimulated translocation of GLUT4 in the rat hippocampus".Neuroendocrinology.85 (2):71–80.doi:10.1159/000101694.PMID17426391.S2CID38081413.
^Huang CC, Lee CC, Hsu KS (2010). "The role of insulin receptor signaling in synaptic plasticity and cognitive function".Chang Gung Medical Journal.33 (2):115–125.PMID20438663.
