Introduction

Among the numerous homeostatic events maintained by the human body, the blood glucose level is a significant physiologic aspect under persistent tight regulation.  Glucose is an essential energy source that requires careful regulation within the body as both too much or too little glucose can cause detrimental effects. Blood glucose level is impacted by carbohydrate ingestion and regulated by insulin. Insulin regulates peripheral glucose uptake and glucose production within the liver — a family of five transmembrane proteins, known as GLUT, transport glucose via facilitated diffusion across the cell plasma membrane. They differ in kinetics and tissue distribution.  The primary regulatory mechanism by which glucose uptake takes place is via insulin-stimulated transport of glucose into skeletal muscle and adipose tissue, primarily mediated by glucose transporter protein type-4 (GLUT4). GLUT4 is a key component in glucose homeostasis and the removal of glucose from circulation.[1][2]

Cellular Level

GLUT4 is part of a family of glucose transporter proteins containing 12-transmembrane domains.  It is expressed primarily in skeletal muscle and adipose tissue. Unique N-terminal and COOH-terminal sequences are responsible for GLUT4’s responsiveness to insulin signaling and membrane trafficking.  The transportation of glucose across the cell membrane occurs via GLUT4’s mechanism of ATP-independent facilitative diffusion. Once glucose influxes into the cell, it can be metabolized for energy or lipid synthesis, or stored as glycogen.[1]

GLUT4 shifts its location between the intracellular domain and the plasma membrane.  By being part of an intracellular tubulo-vesicular network connected to the endosomal-trans-Golgi network (TGN) system, it is able to switch locations based on the presence of stimulation.  In the absence of insulin or exercise, 90% of GLUT4 remains intracellular. In the presence of insulin or exercise, GLUT4 storage vesicles undergo exocytosis to the plasma membrane, as well as the sarcolemma and T-tubules of skeletal muscle cells, where it can carry out its function on glucose transport.  This increase in the number of GLUT4 molecules available at the cell surface increases the maximal velocity of glucose transport rate into cells. Once there is the removal of insulin stimulation, GLUT4 is endocytosis back into the cell by budding of vesicles on the plasma membrane containing clathrin. Upon internalization, GLUT4 becomes a part of early endosomes and re-sorted back into intracellular vesicles.[3][2]

Function

GLUT4 exists in skeletal muscle cells, adipocytes, and cardiomyocytes.  It is principally responsible for insulin-stimulated glucose uptake into muscle and adipose cells.  Approximately 80% of glucose gets transported into muscle cells. GLUT4’s glucose-transport system can be upregulated to meet elevated transport demands, such as during times of elevated blood glucose during a carbohydrate-containing meal, or during exercise when skeletal muscles have increased metabolic demand.[3]

Mechanism

Insulin-Mediated Stimulation

Insulin-regulated GLUT4 translocation can occur by two signaling pathways.  One pathway involves lipid kinase phosphatidylinositol 3-kinase (PI3K). Insulin binds to the insulin receptor found on the target cell surface, causing the receptor to undergo a conformational change which activates its tyrosine-kinase domain intracellularly.  Insulin receptor substrates (IRS) and c-Cbl (a proto-oncoprotein) are then phosphorylated. In muscle and adipose cells, IRS-1 and IRS-2 are the most important substrates. These substrates are located near the plasma membrane and recruit effector molecules to the area, such as PI3K which has been shown to take part in GLUT4 translocation to the plasma membrane.[3]

The other pathway involves proto-oncoprotein c-Cbl.  Insulin stimulates a dimeric complex of c-Cbl and c-Cbl associated protein (CAP) to move into lipid rafts on the cell surface.  Phosphorylation of c-Cbl recruits to the lipid rafts an adaptor protein complex (CrkII) and an exchange factor (C3G) for GTPase TC10.  TC10 specifically localizes to lipid rafts. Thus, activation of TC10 by C3G is an insulin-dependent process which subsequently translocated GLUT4.  If this pathway gets inhibited, inhibition will likewise occur for insulin-stimulated GLUT4 translocation in adipocytes.[3][4]

Non-Insulin Mediated Stimulation

Physical activity stimulates GLUT4 translocation to the plasma membrane in skeletal muscle.  This stimulation occurs via a mechanism independent from PI3K, which is necessary for the insulin-stimulated pathway.  Skeletal muscle contraction activates 5’-AMP-activated protein kinase (AMPK) which is believed to translocate exercise-responsive GLUT4-containing vesicles to the cell surface to mediate glucose transport; this occurs to meet the increased energy demands of skeletal muscle during exercise.[2][3]

Clinical Significance

Type 2 diabetes mellitus (T2DM) has increased dramatically over the years and continues to do so at an alarming rate.  It is a disease characterized by insulin resistance, meaning the insulin produced by the body is not enough to meet the glucose transport demands, leading to an elevated amount of glucose remaining in the body’s circulating plasma.  This chronic state of hyperglycemia can lead to a multitude of long-term complications, such as retinopathy, neuropathy, renal disease, and most lethally - heart disease. Insulin resistance has also been found to be a key component in obesity and metabolic syndrome (insulin resistance, dyslipidemia, and hypertension).  GLUT4 expression is severely disrupted in individuals with T2DM and heavily contributes to insulin resistance disease pathophysiology as it obstructs glucose transport from extracellular to intracellular uptake for storage and metabolism. Potential causes for resistance to insulin-stimulated glucose transport may be because of defective intracellular signaling of GLUT4 translocation in skeletal muscle from stored intracellular vesicles to active components of the plasma membrane, which may be due to an inherent impairment in the muscle cells as T2DM is a heritable disease. It may also be due to glucose toxicity from chronic hyperglycemia, or elevated levels of free fatty acids or TNF-alpha in the serum.  As a result of GLUT4 expression downregulation, adipocytes also exhibit impaired insulin-stimulated glucose uptake.[3][2]

Research shows that increasing intracellular concentrations of GLUT4 can improve or even reverse T2DM.  A non-pharmacological method of doing so is by incorporating exercise into an individual’s lifestyle. Skeletal muscle contractions activate exercise-responsive GLUT4-containing vesicles for exocytosis to the cell surface via a mechanism that functions independently from that of the insulin-stimulated pathway.  Individuals at high risk of developing T2DM may decrease their risk by regularly incorporating exercise into their routine. One study found that females who exercised at least once per week had a 33% decrease in the risk of developing T2DM than sedentary women.[2][5]

From a pharmacologic standpoint, drugs such as metformin,  thiazolidinediones, and sulfonylureas may be used to ameliorate glycemic control in T2DM individuals.   Sulfonylureas stimulate the release of insulin from pancreatic beta cells by inhibiting potassium-channels responsible for insulin uptake into the cell, thus blocking this process increases insulin availability in the serum.  Metformin is a biguanide which functions to primarily decrease hepatic glucose production, as well as decrease intestinal glucose absorption. Thiazolidinediones improve insulin sensitivity by inhibiting a nuclear receptor primarily in adipocytes, known as peroxisome proliferator-activated receptor (PPAR-gamma) which alters gene transcription involving glucose and fat metabolism.  Part of its mechanism of action is that it improves GLUT4 translocation by decreasing the level of TNF-alpha.[2][6][7][8]