GLUT4

Structure

Like all proteins, the unique amino acid arrangement in the primary sequence of GLUT4 is what allows it to transport glucose across the plasma membrane. In addition to the phenylalanine on the N-terminus, two Leucine residues and acidic motifs on the COOH-terminus are believed to play a key role in the kinetics of endocytosis and exocytosis.

Other GLUT proteins

There are 14 total GLUT proteins separated into 3 classes based on sequence 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 the cytoplasm of the cell, and they all have 12 transmembrane segments.

Tissue distribution

Skeletal muscle

In striated skeletal 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 to ATP to be used as energy. As G-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 this facilitated diffusion.

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.

Cardiac muscle

Cardiac muscle is slightly different from skeletal muscle. At rest, they prefer to utilize fatty 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.

An analysis of mRNA levels of GLUT1 and GLUT4 in cardiac muscles show that GLUT1 plays a larger role in cardiac muscles than it does in skeletal muscles. GLUT4, however, is still believed to be the primary transporter for glucose.

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.

Adipose tissue

Adipose tissue, commonly known as fat, is a depository for energy in order to conserve metabolic homeostasis. As the body takes in energy in the form of glucose, some is expended, and the rest is stored as glycogen (primarily in the liver, muscle cells), or as triglyceride in adipose tissue.

An imbalance in glucose intake and energy expenditure has been shown to lead to both adipose cell hypertrophy and hyperplasia, which lead to obesity. In addition, mutations in GLUT4 genes in adipocytes 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.

Regulation

Insulin

Insulin is released from the pancreas and into the bloodstream in response to increased glucose concentration in the blood. Insulin is stored in beta cells in the pancreas. When glucose in the blood binds to glucose receptors on the beta cell membrane, a signal cascade is initiated inside the cell that results in insulin stored in vesicles in these cells being released into the blood stream. Increased insulin levels cause the uptake of glucose into the cells. GLUT4 is stored in the cell in transport vesicles, and is quickly incorporated into the plasma membrane of the cell when insulin binds to membrane receptors.

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. 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.

The mechanism for GLUT4 is an example of a cascade effect, where binding of a ligand to a membrane receptor amplifies the signal and causes a cellular response. In this case, insulin binds to the insulin receptor in its dimeric form and activates the receptor's tyrosine-kinase domain. The receptor then recruits Insulin Receptor Substrate, or IRS-1, which binds the enzyme PI-3 kinase. PI-3 kinase converts the membrane lipid PIP2 to PIP3. 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 phosphorylates TBC1D4, which inhibits the GTPase-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.

RAC1 is a GTPase also activated by insulin. Rac1 stimulates reorganization of the cortical Actin cytoskeleton which allows for the GLUT4 vesicles to be inserted into the plasma membrane. A RAC1 Knockout mouse has reduced glucose uptake in muscle tissue.

Knockout mice that are heterozygous for GLUT4 develop insulin resistance in their muscles as well as diabetes.

Muscle contraction

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. In skeletal muscle, muscle contractions substantially increase GLUT4 translocation, which is regulated by RAC1 and AMP-activated protein kinase (AMPK). Contraction-induced glucose uptake involves the phosphorylation of RabGaps, TBC1D1 and TBC1D4, by AMPK and other kinases such as SNARK. This mechanism remains functional in insulin-resistant states, establishing the muscle-contraction pathway's independence from insulin stimulation. 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.

AMPK plays a crucial role in the contraction pathway. ATP is known as an energy-sensing enzyme, as it's highly responsive to an increase in the AMP to ATP ratio. ATP is hydrolyzed to ADP during muscle contraction by actomyosin ATPase. Adenylate kinase subsequently converts ADP through the following reaction: 2ADP→ATP+AMP. This ensures rapid replenishment of ATP, while increasing AMP concentration. 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. 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. This ultimately results in the phosphorylation and activation of AMPK by LKB1 and triggers a cascade of signaling events driven by AMPK, leading to the translocation of GLUT4.

Muscle stretching also stimulates GLUT4 translocation and glucose uptake in rodent muscle via RAC1.

Interactions

GLUT4 has been shown to interact with death-associated protein 6, also known as Daxx. Daxx, which is used to regulate apoptosis, 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. So this interaction aids in the translocation of Daxx within the cell.

In addition, recent reports demonstrated the presence of GLUT4 gene in central nervous system such as the hippocampus. 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.

Interactive pathway map

References

References

1. (2001). "Intracellular Organization of Insulin Signaling and GLUT4 Translocation". Recent Progress in Hormone Research.
2. (May 1988). "Insulin-regulatable tissues express a unique insulin-sensitive glucose transport protein". Nature.
3. (March 1989). "Molecular cloning and characterization of an insulin-regulatable glucose transporter". Nature.
4. (April 1989). "Identification of a novel gene encoding an insulin-responsive glucose transporter protein". Cell.
5. (August 1989). "Polymorphic human insulin-responsive glucose-transporter gene on chromosome 17p13". Diabetes.
6. (April 2004). "Regulated membrane trafficking of the insulin-responsive glucose transporter 4 in adipocytes". Endocrine Reviews.
7. (March 2001). "The UBX domain: a widespread ubiquitin-like module". Journal of Molecular Biology.
8. (April 2007). "The GLUT4 glucose transporter". Cell Metabolism.
9. (2013). "The SLC2 (GLUT) family of membrane transporters". Molecular Aspects of Medicine.
10. (2000). "Molecular Cell Biology". W. H. Freeman.
11. (July 2013). "Exercise, GLUT4, and skeletal muscle glucose uptake". Physiological Reviews.
12. (September 1998). "Analysis of GLUT4 distribution in whole skeletal muscle fibers: identification of distinct storage compartments that are recruited by insulin and muscle contractions". The Journal of Cell Biology.
13. (April 2013). "Insulin- and contraction-induced glucose transporter 4 traffic in muscle: insights from a novel imaging approach". Exercise and Sport Sciences Reviews.
14. (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.
15. (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.
16. (January 1996). "Insulin-induced glucose transporter (GLUT1 and GLUT4) translocation in cardiac muscle tissue is mimicked by bradykinin". Diabetes.
17. (September 1991). "Translocation of the glucose transporter GLUT4 in cardiac myocytes of the rat". Proceedings of the National Academy of Sciences of the United States of America.
18. (August 2015). "Cardiac contraction-induced GLUT4 translocation requires dual signaling input". Trends in Endocrinology and Metabolism.
19. "Adipose tissue". ScienceDaily.
20. (2014-10-09). "GLUT4 defects in adipose tissue are early signs of metabolic alterations in Alms1GT/GT, a mouse model for obesity and insulin resistance". PLOS ONE.
21. (October 1993). "Adipose cell hyperplasia and enhanced glucose disposal in transgenic mice overexpressing GLUT4 selectively in adipose tissue". The Journal of Biological Chemistry.
22. "Insulin Synthesis and Secretion".
23. (January 2013). "Regulation of insulin synthesis and secretion and pancreatic Beta-cell dysfunction in diabetes". Current Diabetes Reviews.
24. (May 1980). "Potential mechanism of insulin action on glucose transport in the isolated rat adipose cell. Apparent translocation of intracellular transport systems to the plasma membrane". The Journal of Biological Chemistry.
25. (July 2000). "Insulin: understanding its action in health and disease". British Journal of Anaesthesia.
26. (May 2012). "Regulation of glucose transport by insulin: traffic control of GLUT4". Nature Reviews. Molecular Cell Biology.
27. (February 2007). "Ceramide- and oxidant-induced insulin resistance involve loss of insulin-dependent Rac-activation and actin remodeling in muscle cells". Diabetes.
28. (February 2014). "Akt and Rac1 signaling are jointly required for insulin-stimulated glucose uptake in skeletal muscle and downregulated in insulin resistance". Cellular Signalling.
29. (June 2013). "Rac1 signaling is required for insulin-stimulated glucose uptake and is dysregulated in insulin-resistant murine and human skeletal muscle". Diabetes.
30. (October 1997). "GLUT4 heterozygous knockout mice develop muscle insulin resistance and diabetes". Nature Medicine.
31. (June 1995). "Contraction stimulates translocation of glucose transporter GLUT4 in skeletal muscle through a mechanism distinct from that of insulin". Proceedings of the National Academy of Sciences of the United States of America.
32. (October 2014). "Contraction-stimulated glucose transport in muscle is controlled by AMPK and mechanical stress but not sarcoplasmatic reticulum Ca(2+) release". Molecular Metabolism.
33. (December 2014). "Rac1--a novel regulator of contraction-stimulated glucose uptake in skeletal muscle". Experimental Physiology.
34. (April 2013). "Rac1 is a novel regulator of contraction-stimulated glucose uptake in skeletal muscle". Diabetes.
35. (May 2001). "A role for AMP-activated protein kinase in contraction- and hypoxia-regulated glucose transport in skeletal muscle". Molecular Cell.
36. (2024-11-18). "NUAK: never underestimate a kinase". Essays in Biochemistry.
37. (January 2024). "AMPK and Beyond: The Signaling Network Controlling RabGAPs and Contraction-Mediated Glucose Uptake in Skeletal Muscle". International Journal of Molecular Sciences.
38. (November 2006). "Role of AMP-Activated Protein Kinase in the Molecular Adaptation to Endurance Exercise". Medicine & Science in Sports & Exercise.
39. (2023-07-01). "Advances in understanding the energetics of muscle contraction". Journal of Biomechanics.
40. (2007-04-04). "The GLUT4 Glucose Transporter". Cell Metabolism.
41. (February 2015). "Stretch-stimulated glucose transport in skeletal muscle is regulated by Rac1". The Journal of Physiology.
42. (May 2002). "The insulin-sensitive glucose transporter, GLUT4, interacts physically with Daxx. Two proteins with capacity to bind Ubc9 and conjugated to SUMO1". The Journal of Biological Chemistry.
43. (March 2014). "Urtica dioica extract attenuates depressive like behavior and associative memory dysfunction in dexamethasone induced diabetic mice". Metabolic Brain Disease.
44. (2007). "Corticosterone impairs insulin-stimulated translocation of GLUT4 in the rat hippocampus". Neuroendocrinology.
45. (2010). "The role of insulin receptor signaling in synaptic plasticity and cognitive function". Chang Gung Medical Journal.