REVIEWS REGULATED TRANSPORT OF THE GLUCOSE TRANSPORTER GLUT4 Nia J. Bryant, Roland Govers and David E. James In muscle and fat cells, insulin stimulates the delivery of the glucose transporter GLUT4 from an intracellular location to the cell surface, where it facilitates the reduction of plasma glucose levels. Understanding the molecular mechanisms that mediate this translocation event involves integrating our knowledge of two fundamental processes — the signal transduction pathways that are triggered when insulin binds to its receptor and the membrane transport events that need to be modified to divert GLUT4 from intracellular storage to an active plasma membrane shuttle service. FACILITATIVE SUGAR Glucose is a fundamental source of energy for all glucose-transport system, the activity of which can be TRANSPORTER eukaryotic cells. In humans, although all cells use glu- rapidly upregulated to allow these tissues to increase A polytopic membrane protein cose for their energy needs, the main consumer under their rate of glucose transport by 10–40-fold within that transports sugars down a basal conditions is the brain, which accounts for as minutes of exposure to a particular stimulus (reviewed concentration gradient in an energy-independent manner. much as 80% of whole-body consumption. The energy in REF.1). This system is crucial during exercise, when is provided by the breakdown of endogenous glycogen the metabolic demands of skeletal muscle can increase TYPE II DIABETES stores that are primarily in the liver. These whole-body more than 100-fold, and during the absorptive period Also known as non-insulin- energy stores are replenished from glucose in the diet, (after a meal), to facilitate the rapid insulin-dependent dependent diabetes or maturity onset diabetes. which, after being digested and absorbed across the gut storage of glucose in muscle and adipose tissue, so pre- wall, is distributed among the various tissues of the body venting large fluctuations in blood glucose levels. (reviewed in REF.1). This distribution process involves a Dysfunctional glucose uptake into muscle and fat cells family of transport proteins — called GLUTs — which contributes to the onset of TYPE II DIABETES (BOX 1). act as shuttles to move sugar across the cell surface. In 1980, it was reported that, in rat adipocytes, These polytopic membrane proteins (FIG. 1) form an insulin triggers the movement of the sugar transporter aqueous pore across the membrane through which that is found in these cells from an intracellular store to glucose can move. A large family of FACILITATIVE SUGAR the plasma membrane2,3. This translocation hypothesis TRANSPORTERS exists in mammals, the individual mem- was later confirmed when GLUT4 was identified as the bers of which differ in their tissue distribution and main glucose transporter in these cells. GLUT4, which kinetic properties, as well as in their intracellular local- is expressed primarily in muscle and fat cells, is found in ization. The latter property is of particular interest for a complex intracellular tubulo–vesicular network that is glucose transport in specialized cell types, and provides connected to the endosomal–trans-Golgi network the basis for polarized glucose transport in epithelial (TGN) system. In the absence of stimulation, GLUT4 is cells, such as those in the gut, or for acute regulation of almost completely excluded from the plasma mem- Garvan Institute of Medical glucose transport, as is observed in muscle and fat cells brane (FIG. 2). The addition of insulin, or exercise in the Research, 384 Victoria Road, after a meal. Many mammalian tissues, such as the case of muscle cells, causes GLUT4 to shift from its Darlinghurst, New South brain, have a constitutively high glucose requirement intracellular location to the plasma membrane (FIG. 2). Wales 2010,Australia. and have been endowed with transporters that are con- Several observations indicate that GLUT4 has a crucial Correspondence to D.E.J. e-mail: stitutively targeted to the cell surface (for example, role in whole-body glucose homeostasis. First, insulin- [email protected] GLUTs 1–3). By contrast, certain tissues, such as muscle stimulated glucose transport is an important rate-limit- DOI: 10.1038/nrm782 and adipose tissue, have acquired a highly specialized ing step for glucose metabolism in both muscle and fat NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 3 | APRIL 2002 | 267 REVIEWS Sugar moiety an insulin-regulated step(s). Although many important signalling molecules that are integral to the insulin reg- ulation of GLUT4 translocation have been identified (BOX 2), any convergence between these two approaches remains to be achieved. In this review, we focus on our Plasma membrane cell-biological understanding of GLUT4 transport, and highlight potential regulatory sites of the insulin- NH Cytoplasm 2 signalling cascade. COOHH GLUT4 transport GLUT4 is found in many organelles, including the plasma membrane, sorting endosomes, recycling endo- somes, the TGN and vesicles that mediate the transport Figure 1 | Schematic representation of the GLUT family of proteins. The GLUT family of of GLUT4 between these compartments (FIG. 3). proteins comprises 13 members at present, which are predicted to span the membrane 12 times Presumably this localization represents a complex and with both amino- and carboxyl-termini located in the cytosol. On the basis of sequence homology and structural similarity, three subclasses of sugar transporters have been defined: Class I (GLUTs dynamic transport itinerary, and it raises several impor- 1–4) are glucose transporters; Class II (GLUTs 5, 7, 9 and 11) are fructose transporters; and Class tant questions. How does GLUT4 transport from one III (GLUTs 6, 8, 10,12 and HMIT1) are structurally atypical members of the GLUT family, which are organelle to another, what is the relationship between poorly defined at present. The diagram shows a homology plot between GLUT1 and GLUT4. these pathways and the intracellular sequestration of Residues that are unique to GLUT4 are shown in red. GLUT4 in basal cells, and which of these steps does insulin influence to affect GLUT4 exocytosis? In non-stimulated adipocytes, the rate of GLUT4 exo- tissue, and is severely disrupted in type II diabetes1 (BOX 1); cytosis is 10-fold slower than that of the transferrin recep- second, disruption of GLUT4 expression in mice results tor (TfR) — one of the most well-studied constitutive in insulin resistance4; and overexpression of GLUT4 recycling proteins in mammalian cells6,7. To account for ameliorates diabetes in the DB/DB MOUSE model5. this, GLUT4 must be selectively retained in one of its Analysing the molecular and cellular regulation of intracellular locations, packaged into a specialized com- GLUT4 transport not only promises to provide new partment that remains static in the absence of insulin, or insights into protein sorting, but could also yield new involved in a dynamic intracellular transport loop that targets for therapeutic intervention in what could well excludes it from recycling endosomal vesicles. Current be one of the most prevalent diseases that we will have evidence favours a role for all three mechanisms, which to confront in the future. emphasizes the complexity of this process. Another pro- Understanding the regulation of GLUT4 and glucose tein, the insulin-responsive aminopeptidase (IRAP), transport has proved to be extremely challenging, prin- which was recently described as a receptor for angiotensin cipally because it involves several signal-transduction IV (REF.8), colocalizes with GLUT4 and is transported in a pathways that are superimposed on a complex series of very similar manner9. Below, we discuss studies concern- vesicle transport processes. Insulin binds to a surface ing the transport of either GLUT4 or IRAP,with a partic- receptor on muscle and fat cells and triggers a cascade of ular focus on adipocytes and muscle cells. We also com- signalling events (BOX 2) that culminates in GLUT4 pare this with transport in cells such as fibroblasts that are translocation. Studies of this process have been carried not, or only mildly, responsive to insulin, as key differ- out using two approaches.‘Outside–in’ approaches have ences have been identified that provide new insights into largely focused on mapping insulin-specific signalling our understanding of insulin action. pathways in muscle and fat cells with the view to identi- fying downstream targets that directly control GLUT4 Endosomal sorting of GLUT4 translocation. Conversely,‘inside–out’ approaches have Morphological studies in both muscle and fat cells used cell-biological studies to map the intracellular indicate that, although there is some overlap of GLUT4 transport itinerary of GLUT4 with the aim of identifying with markers of the endocytic system such as the TfR, a Box 1 | Type II diabetes The prevalence of type II diabetes is increasing at an alarming rate. In 1998, 143 million people worldwide suffered from this disease, and it is likely that this number will double over the next 20–30 years71. The incidence of type II diabetes increases sharply with age, and it is highly prevalent in certain ethnic groups. For example, 10–30% of Australian aborigines are currently thought to have type II diabetes, and this number is predicted to increase to more than 50% in the next ten years. The disease is characterized by defective insulin action, a condition that is referred to as insulin resistance. Insulin resistance is characterized by dysfunctional glucose uptake into muscle and adipose tissue, in DB/DB MOUSE conjunction with an oversupply of glucose from the liver, which results in high circulating plasma glucose levels. This A genetic mouse model of type causes many of the complications of type II diabetes, including eye, nerve and kidney disease. The highest contributor II diabetes and obesity. The to morbidity and mortality in type II diabetes is heart disease and, strikingly, type II diabetes is one of the main causes defect has been mapped to the of heart disease in the Western world.