GLUT4 is the insulin-regulated glucose transporter found in heart tissue, skeletal muscle, and adipose tissues (6). It is responsible for insulin-regulated glucose disposal (6). GLUT4 is regulated by insulin; therefore, its role is defected when insulin is not functioning in the right way (3). The major physiological action of insulin is to increase glucose uptake and storage in adipose tissue, skeletal muscle, and heart. This action is accomplished through the recruitment of a specialized facilitative glucose transport integral protein to the cell surface under insulin action (8). This integral protein is GLUT4. When plasma insulin levels are low (basal condition), GLUT 4 stays in a small, intracellular vesicle in muscle (1). When insulin activates its cell surface receptor, a signal is generated which results in the stimulation of exocytosis of the intracellular GLUT4 vesicles to the plasma membrane and facilitates the uptake of the glucose from the blood stream (9). Type 2 diabetes, non insulin-dependent diabetes mellitus (NIDDM), is the most common form of diabetes; it accounts for more than 90% of diabetes. NIDDM is caused by two physiological defects: resistance to the action of insulin and deficiency in insulin secretion (7). Insulin resistance is instrumental in the pathogenesis of type 2 diabetes mellitus; it decreases the glucose transport activity in skeletal muscle (3).
In the early phase of the disease, insulin resistance is greatest in skeletal muscle, which makes up a large part of the total body mass and represents a major tissue with respect to blood glucose utilization (7). The muscle GLUT4 glucose transporters level are normal in type 2 diabetes, therefore, the insulin resistance is due to impaired translocation of intracellular GLUT4 (3). Consequently, this will lead to accumulation of GLUT4 in a dense membrane compartment from which insulin is unable to recruit GLUT 4 to the cell surface (6). The defect in GLUT4 translocation could be due to either impaired insulin signal transduction or it could lie intrinsic to the glucose transporter system (9). The skeletal muscle tissue uptakes blood glucose through GLUT4, which is transported from the intracellular pool to the plasma membrane by insulin stimulation. In NIDDM patients, the insulin-stimulated glucose transport of GLUT4 is downregulated with out affecting the GLUT4 contents (8).
In "Lehninger Principle of Biochemistry" text, Nelson and Cox stated that Protein Kinase B (PKB) is believed to trigger the movement of GLUT4 from internal vesicles to the plasma membrane, stimulating glucose uptake form the blood. When PKB is phosphorylated, it stimulates GLUT4 movement to the plasma membrane. Nelson and Cox demonstrated that glucose uptake mechanism in myocytes and adipocytes is regulated insulin.
The glucose transporters are stored within cells in the membrane vesicles. When insulin interacts with its receptor, vesicles move to the surface and fuse with the plasma membrane, increasing the number of glucose transporters in the membrane. When insulin level drops, glucose transporters are removed from the plasma membrane by endocytosis, forming small vesicles. After that, the smaller vesicles fuse with larger endosome. Finally, patches of the endosomes enriched for glucose transporter bud off to become small vesicles, ready to return to the surface when insulin levels rise again. With more GLUT4 molecules in action, the rate of glucose uptake is increased by 15 fold or more.
Since muscle GLUT4 glucose transporter levels are normal in type 2 diabetes, Garvey and his colleagues tested the hypothesis that insulin resistant is due to impaired translocation and trafficking of intracellular GLUT4 to plasma membrane. They studied insulin-sensitive and insulin-resistant nondiabetic subgroups, in addition to type 2 diabetic patients. They took biopsies from basal and insulin-stimulated muscle. These biopsies were subfractioned on discontinuous sucrose density gradients to equilibrium or under nonequilibrium conditions after a shortened centrifugation time. Based on this experiment's results, the researchers concluded that insulin alters the subcellular localization of GLUT4 vesicles in human muscle and this effect is impaired equally in insulin-resistant subjects with and without diabetes. This translocation defect is associated with abnormal accumulation of GLUT4 in the membrane compartments.
A study, conducted by Able and his colleagues, demonstrated that GLUT4-deficient mice developed cardiac hypertrophy and die permanently. In order for the heart tissues to meet their substantial energy requirements, they are capable of metabolizing a variety of substrates. Under resting conditions, the heart derives about 70% of its energy from the oxidation of lipids, and the remainder primarily from glycolysis and glucose oxidation (1). In some pathological conditions such as hyperthyroidism, ischemia, and hypertrophy, the heart becomes increasingly dependent upon glucose to meet its metabolic demands. To determine the role of GLUT4 in the heart, the researchers used cre-loxP recombination to generate mice in which GLUT4 expression is abolished in the heart but present in skeletal muscle and adipose tissue. These mice developed modest cardiac hypertrophy associated with increased myocyte size. Based on these results, the researchers concluded that selective ablation of GLUT4 in the heart initiates a series of events that result in cardiac hypertrophy. Physical exercise can be of great importance in the treatment of both insulin and non-insulin dependent diabetes mellitus patients (4). Similar to insulin, a single bout of exercise increases the rate of glucose uptake into the contracting skeletal muscle, the process that is regulated by the translocation of GLUT4 glucose transporters to the plasma membrane (4). Although insulin and physical exercising use different signaling pathways, both of them lead to activation of glucose transport (3).
A study, conducted by Shimokawa and his colleagues, examined the effect of YM-138552 (5-Chloro-N- (2-chloro-4-nitrophenyl)-2-hydroxy-3-methylbenz-amid) on the glucose uptake, gene expression, and transport activities of the insulin-regulatable glucose transporter isotype 4 (GLUT4) in skeletal muscle cells. The pervious experiments have shown that the insulin-stimulated glucose transport of GLUT4 is downregulated but the content of the GLUT4 is not affected in NIDDM patient (7). Therefore, there is not evidence of deterioration of the GLUT4 expression level in skeletal muscle tissue of NIDDM patients. In this study, the researchers used GLUT4 glucose transporter as a pharmacological target for therapy oriented toward enhancing glucose disposal by skeletal tissue. Their pharmacological intervention was done by manipulation of GLUT4 upregulation and transportation. The goal of this study was to ameliorate insulin resistant of the skeletal muscle tissue by specific GLUT4 overexperssion in mice. They established screening system, which determines the glucose consumption rate by determining the glucose concentration in medium, cultured skeletal muscle cells. They found that the YM-138552 compound stimulates glucose consumption in skeletal muscle. The results of this experiment suggested that YM-138552 has an insulin-like effect and this finding might lead to the development of new drugs for the treatment of NIDDM patients.
Another study, conducted by Zhidan and his colleagues, suggested that PPARg alone or in combination with C/EBPb and C/EBPd is capable of activating GLUT4 gene expression. The researchers have demonstrated that C/EBPb along with C/EBPd in the presence of dexamethasone induce PPARg, adipsin, and aP2 mRNA production. The enhanced expression of a ligand-activated form of PPARg in the fibroblasts stimulates the synthesis of GLUT4 protein and gives rise to a population of adipocytic cells that can take up glucose in direct response to insulin.
The GLUT4 glucose transporter responds to insulin by mobilizing from the intracellular tissue to the cell surface. Allen Volchuk has examined two aspects of this process: the mechanism by which GLUT4 vesicles may recognize, dock and fuse with the cell surface and the mechanism by which GLUT4 endocytosis is regulated. The intracellular GLUT4 vesicles in 3T3-L1 adipocytes are shown to contain the v-SNARE proteins and VAMP-2 protein. These two proteins are required for vesicle secretion in many systems and here; they are shown to be involved in GLUT4 exocytosis to the cell surface. VAMP proteins are shown to interact with the t-SNARE syntaxin 4, which is found to be expressed primarily at the cell surface of 3T3-L1 adipocytes. GLUT4 endocytosis form the cell surface is thought to occur by the clathrin-dependent endocytosis system and dynamin II may potentially have regulatory role in GLUT4 movement to the cell surface. The researcher concluded that SNARE protein, VAMP-2, and syntaxin 4 are involved in GLUT4 exocytosis while dynamin II involved in the endocytosis of GLUT4 transporter.
Fasshauer and his colleagues studied the role of insulin receptor substrate-2 (IRS) in insulin stimulation of GLUT4 translocation and glucose uptake in brown adipocytes. Insulin signals are mediated by phosphorylation of a family of IRS proteins, which serve as complementary and overlapping function in the cell. The researchers established brown adipocyte cell lines from wild type and various IRS knockout animals and characterized insulin action in these cells in vitro. The results provided evidence for the critical role of IRS-2 as a mediator of insulin-stimulated GLUT4 translocation and glucose uptake in adipocytes.
In conclusion, GLUT4 is a member of glucose transport proteins family. It is found in skeletal muscles, adipose tissue, and heart muscle. GLUT4 is responsible for insulin-regulated glucose disposal. In type 2 diabetes mellitus (NIDDM), insulin-resistance is developed as a result of impairment of GLUT4 trafficking and translocation in the skeletal muscle. Because GLUT4 content is not deteriorated, some researchers are targeting GLUT4 for pharmacological therapy by enhancing glucose disposal in the skeletal muscle. GLUT4-deficient mice developed cardiac hypertrophy; thus, GLUT4 is essential for energy production in heart tissues. Some researchers were able to activate GLUT4 gene expression by using PPARg alone or in combination with C/EBPb and C/EBPd. Many researchers have shown that physical exercise has an effective role in the treatment of NIDDM patients. Physical exercise increases the rate of glucose uptake into the contracting skeletal muscles, which is regulated by GLUT4.
Copyright © 2001 Balquees Rihan and Koni Stone
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