Cell surface labeling of glucose transporter isoform GLUT4 by bis-mannose photolabel. Correlation with stimulation of glucose transport in rat adipose cells by insulin and phorbol ester

ATP-sensitive binding of a 70-kDa cytosolic protein to the glucose transporter in rat adipocytes

We have identified a 70-kDa cytosolic protein (GTBP70) in rat adipocytes that binds to glutathione S-transferase fusion proteins corresponding to the cytoplasmic domains of the facilitative glucose transporter isoforms Glut1, Glut2, and Glut4. GTBP70 did not bind to irrelevant fusion proteins, indicating that the binding is specific to the glucose transporter. GTBP70 binding to the glucose transporter showed little isoform specificity but was significantly subdomain-specific; it bound to the C-terminal domain and the central loop, but not to the N-terminal domain of Glut4. The GTBP70 binding to Glut4 was not affected by the presence of 2 mM EDTA, 2.4 mM Ca2+, or 150 mM K+. The binding was inhibited by ATP in a dose-dependent manner, with 50% inhibition at 10 mM ATP. This inhibition was specific to ATP, as ADP and AMP-PCP (adenosine 5'-(beta, gamma-methylenetriphosphate)) were without effect. GTBP70 did not react with antibodies against phosphotyrosine, phosphothreonine, or phosphoserine, suggesting that it is not a phosphoprotein. The binding of GTBP70 to Glut4 was not affected by the pretreatment of adipocytes with insulin. When these experiments were repeated using rat hepatocyte cytosols, no ATP-sensitive 70-kDa protein binding to the glucose transporter fusion proteins was evident, suggesting that either GTBP70 expression or its function is cell-specific. These findings strongly suggest the possibility that GTBP70 may play a key role in glucose transporter regulation in insulin target cells such as adipocytes.

Translocation of two glucose transporters in heart: effects of rotenone, uncouplers, workload, palmitate, insulin and anoxia

Our previous studies on the acute regulation of glucose transport in perfused rat hearts were extended to explore further the mechanism of regulation by anoxia; to test the effects of palmitate, a transport inhibitor; and to compare the translocation of two glucose transporter isoforms (GLUT1 and GLUT4). Following heart perfusions under various conditions, glucose transporters in intracellular membranes were quantitated by reconstitution of transport activity and by Western blotting. Rotenone stimulated glucose uptake and decreased the intracellular contents of glucose transporters. This indicates that it activates glucose transport via net outward translocation, similarly to anoxia. However, two uncouplers of oxidative phosphorylation produced little or no effect. Increased workload (which stimulates glucose transport) reduced the intracellular contents of transporters, while palmitate increased the contents, indicating that these factors cause net translocation from or to the intracellular pool, respectively. Relative changes in GLUT1 were similar to those in GLUT4 for most factors tested. A plot of changes in total intracellular transporter content vs. changes in glucose uptake was roughly linear, with a slope of -0.18. This indicates that translocation accounts for most of the changes in glucose transport, and the basal pool of intracellular transporters is five times as large as the plasma membrane pool.

Expression of human GLUT4 in mice results in increased insulin action

Glucose metabolism was evaluated in transgenic mice expressing the human GLUT 4 glucose transporter. Fed GLUT 4 transgenic mice exhibited a 32% and 56% reduction in serum glucose and insulin and a 69% and 33% increase in non-esterified fatty acid and lactate levels, respectively. Transgenic mice exhibited a significant increase in whole-body glucose disposal during a euglycaemic-hyperinsulinaemic clamp. Insulin-stimulated glucose uptake in isolated soleus muscles and adipocytes was greater in transgenic compared to control mice due to increased basal glucose uptake. Transgenic mice displayed increased glycogen levels in liver and gastrocnemius muscle, and increased insulin-stimulated 14C-glycogen accumulation in isolated soleus muscle. We conclude that over-expression of the GLUT 4 glucose transporter in mice results in 1) an increase in whole-body glucose disposal and storage, and 2) an increase in both basal and insulin-stimulated glucose uptake and disposal in vitro. These changes resulted in the reduction of serum glucose and insulin levels. These results provide direct evidence that glucose transport (and GLUT 4 per se) plays a significant role in regulating whole-body glucose homeostasis. Additionally, these data support the idea that pharmacological strategies directed at increasing the expression of GLUT 4 protein may have beneficial (hypoglycaemic) effects in the diabetic state.

Subcellular localization and trafficking of the GLUT4 glucose transporter isoform in insulin-responsive cells

The rate-limiting step in the uptake and metabolism of D-glucose by insulin target cells is thought to be glucose transport mediated by glucose transporters (primarily the GLUT4 isoform) localized to the plasma membrane. However, subcellular fractionation, photolabelling and immunocytochemical studies have shown that the pool of GLUT4 present in the plasma membrane is only one of many subcellular pools of this protein. GLUT4 has been found in occluded vesicles at the plasma membrane, clathrin-coated pits and vesicles, early endosomes, and tubulo-vesicular structures; the latter are analogous to known specialized secretory compartments. Tracking the movement of GLUT4 through these compartments, and defining the mechanism and site of action of insulin in stimulating this subcellular trafficking, are major topics of current investigation. Recent evidence focuses attention on the exocytosis of GLUT4 as the major site of insulin action. Increased exocytosis may be due to decreased retention of glucose transporters in an intracellular pool, or possibly to increased assembly of a vesicle docking and fusion complex. Although details are unknown, the presence in GLUT4 vesicles of a synaptobrevin homologue leads us to propose that a process analogous to that occurring in synaptic vesicle trafficking is involved in the assembly of GLUT4 vesicles into a form suitable for fusion with the plasma membrane. Evidence that the pathways of signalling from the insulin receptor and of GLUT4 vesicle exocytosis may converge at the level of the key signalling enzyme, phosphatidylinositol 3-kinase, is discussed.

Insulin action, thermogenesis and obesity

The case for obesity per se being a major cause of insulin resistance has been made. There is evidence that each of the control points of insulin on glucose metabolism are negatively influenced by lipid oversupply, a characteristic of the obese state. The answer to the corollary, whether insulin resistance (a universal concomitant of obesity) can in turn lead to obesity via a decrease in thermogenesis, is more complex. Overall, the answer would appear to be no. On a population basis, obese individuals would not appear to have lower metabolic rates, whether expressed on a lean tissue or any other basis, than lean individuals. Even in the subpopulation of hypometabolic obese, there are no convincing data that the reduced metabolic rate is linked to particularly severe insulin resistance. Further, improving insulin action by weight loss would not appear to increase thermogenesis as would be predicted if insulin resistance impaired thermogenesis. A case can be made for reductions in a specific aspect of energy expenditure in obesity, that of meal-induced or glucose-induced thermogenesis, and this may be due to insulin resistance. However, meal-induced thermogenesis is a small component of total energy expenditure and total energy expenditure is not different between lean and obese. That leaves the intriguing possibility that a relative failure of prandial thermogenesis has an impact upon energy balance via impairment of satiety (related to reduced metabolic flux) and thus by increasing intake. While a potentially fruitful research avenue, too few data exist on this possibility for it to be anything more than speculative at this stage.

Accelerated net efflux of 3-O-methylglucose from rat adipocytes: a reevaluation

In a study of 3-O-methylglucose transport in insulin-stimulated rat adipocytes (catalyzed primarily by the GLUT4 isoform), it was reported that at 37 degrees C the Km and Vmax were 2.8-fold higher for net efflux than for equilibrium exchange (Vinten, J. (1984) Biochim. Biophys. Acta 772, 244-250). Because of its implications for the relative sizes of steps in the transport cycle, we reinvestigated this phenomenon. Accelerated net efflux was apparent when the extracellular methylglucose was diluted 26-fold but not when it was diluted 11-fold. When analyzed according to the one-site alternating conformation model, the data indicate about a 1.7-fold higher Vmax for efflux than for exchange, only about 40% of the difference reported previously. Together with other results in the literature, the accelerated net flux indicates that the conformational change of the loaded transporter from its outward-facing to its inward-facing form is likely the slowest step in the transport cycle, in contrast to the case for GLUT1. Experiments at 25 degrees C indicate a lower degree of accelerated net flux than at 37 degrees C. This is consistent with the above conformational change being the step with the lowest activation energy, as for GLUT1.

Growth factor-induced stimulation of hexose transport in 3T3-L1 adipocytes: evidence that insulin-induced translocation of GLUT4 is independent of activation of MAP kinase

We have examined the effect of growth factors on the rate of hexose transport in 3T3-L1 adipocytes. Epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) were found to stimulate deoxyglucose transport by about 2-fold. The concentrations of EGF and PDGF which elicited half maximal responses were 100 and 350 pM, respectively. The increases in transport rate were acute effects; the stimulations were evident within minutes of exposure to growth factors. By contrast, insulin stimulated deoxyglucose transport approximately 16-fold over similar time periods. We have measured the appearance of both the insulin-responsive glucose transporter (GLUT4) and the erythrocyte-type glucose transporter (GLUT1) at the cell surface in response to insulin, EGF and PDGF. We show that both EGF and PDGF induce a 2-fold increase in GLUT1 at the cell surface, but both these growth factors were without effect on GLUT4 levels at the cell surface. In contrast, insulin induced a 13-fold increase in cell surface GLUT4. We further show that insulin, EGF and PDGF all activate MAP kinase as determined by a shift in electrophoretic mobility of this protein on SDS-PAGE. However, since the large translocation of GLUT4 to the cell surface is specific for insulin, we suggest that activation of MAP kinase is not the sole requisite for this process.

Glucose transporter localization in rat skeletal muscle. Autoradiographic study using ATB-[2-3H]BMPA photolabel

Surface glucose transporters of intact muscles were photolabeled with the membrane impermeant ATB-[2-3H]BMPA reagent and localized by autoradiography. We found sparse labeling of the glucose transporters by ATB-[2-3H]BMPA on the sarcolemmal membrane around the muscle fiber. The majority of label was on the interior of the muscle fiber, at a discrete site which matched the distribution of AI junctions and which was presumed to be on the exterior surface of T-tubules. The amount of photolabel on the T-tubule was increased in response to insulin and was blocked by cytochalasin B. These results support the concept that glucose transport may occur predominantly across the T-tubule membrane under basal and insulin-stimulated conditions.

Facilitative glucose transporters

Facilitative glucose transport is mediated by members of the Glut protein family that belong to a much larger superfamily of 12 transmembrane segment transporters. Six members of the Glut family have been described thus far. These proteins are expressed in a tissue- and cell-specific manner and exhibit distinct kinetic and regulatory properties that reflect their specific functional roles. Glut1 is a widely expressed isoform that provides many cells with their basal glucose requirement. It also plays a special role in transporting glucose across epithelial and endothelial barrier tissues. Glut2 is a high-Km isoform expressed in hepatocytes, pancreatic beta cells, and the basolateral membranes of intestinal and renal epithelial cells. It acts as a high-capacity transport system to allow the uninhibited (non-rate-limiting) flux of glucose into or out of these cell types. Glut3 is a low-Km isoform responsible for glucose uptake into neurons. Glut4 is expressed exclusively in the insulin-sensitive tissues, fat and muscle. It is responsible for increased glucose disposal in these tissues in the postprandial state and is important in whole-body glucose homeostasis. Glut5 is a fructose transporter that is abundant in spermatozoa and the apical membrane of intestinal cells. Glut7 is the transporter present in the endoplasmic reticulum membrane that allows the flux of free glucose out of the lumen of this organelle after the action of glucose-6-phosphatase on glucose 6-phosphate. This review summarizes recent advances concerning the structure, function, and regulation of the Glut proteins.

Advances in kinetic analysis of insulin-stimulated GLUT-4 translocation in adipose cells

GLUT-4 is the major insulin-sensitive glucose transporter in muscle and adipose tissue. Regulation of GLUT-4 is an important component of whole body glucose homeostasis. Abnormalities in the regulation of insulin-stimulated reversible translocation of glucose transporters have been observed in various pathological states, including diabetes. Recently, the development of specific photolabels for glucose transporters and the availability of antibodies against the various transporter isoforms have presented the opportunity for detailed kinetic analysis of GLUT-4 regulation. A kinetic analysis of some of the most recent data is presented to demonstrate how this approach can advance the understanding of GLUT-4 regulation. Some areas in which the currently available data limit the ability to resolve certain mechanistic questions are noted. Using a two-compartment model, we show that the mechanism of insulin-stimulated GLUT-4 translocation is likely to involve a large increase in the exocytosis rate of GLUT-4 with a minimal decrease in the endocytosis rate. Mathematical models based on these kinetic analyses are helpful for testing hypotheses and designing experiments to elucidate the molecular and cellular mechanisms of GLUT-4 regulation under normal and pathological conditions. This type of approach may be useful for evaluating the contribution of GLUT-4 regulation to the pathogenesis of diabetes.

GLUT-4 content in plasma membrane of muscle from patients with non-insulin-dependent diabetes mellitus

The abundance of GLUT-4 protein in both total crude membrane and plasma membrane fractions of vastus lateralis muscle from 13 obese non-insulin-dependent diabetes mellitus (NIDDM) patients and 14 healthy subjects were examined in the fasting state and after supraphysiological hyperinsulinemia. In the basal state the immunoreactive mass of GLUT-4 protein both in the crude membrane preparation and in the plasma membrane fraction was similar in NIDDM patients and control subjects. Moreover, in vivo insulin exposure neither for 30 min nor for 4 h had any impact on the content of GLUT-4 protein in plasma membranes. With the use of the same methodology, antibody, and achieving the same degree of plasma membrane purification and recovery, we found, however, that intraperitoneal administration of insulin to 7-wk-old rats within 30 min increased the content of GLUT-4 protein more than twofold (P < 0.01) in the plasma membrane from red gastrocnemius and soleus muscle. In conclusion, when the subcellular fractionation method was applied to human muscle biopsies taken in the basal state, no difference could be found in the plasma membrane content of immunoreactive GLUT-4 protein between NIDDM patients and normal subjects. With this technique, we were unable to show evidence for a regulatory effect of insulin on the plasma membrane level of GLUT-4 protein in human muscle.

Glut 4 content in the plasma membrane of rat skeletal muscle: comparative studies of the subcellular fractionation method and the exofacial photolabelling technique using ATB-BMPA

Employing subcellular membrane fractionation methods it has been shown that insulin induces a 2-fold increase in Glut 4 protein content in the plasma membrane of skeletal muscle from rats. Data based upon this technique are, however, impeded by poor plasma membrane recovery and cross-contamination with intracellular membrane vesicles. The present study was undertaken to compare the subcellular fractionation technique with the technique using [3H]ATB-BMPA exofacial photolabelling and immunoprecipitation of Glut 4 on soleus muscles from 3-week-old Wistar rats. Maximal insulin stimulation resulted in a 6-fold increase in 3-O-methylglucose uptake, and studies based on the subcellular fractionation method showed a 2-fold increase in Glut 4 content in the plasma membrane, whereas the exofacial photolabelling demonstrated a 6- to 7-fold rise in cell surface associated Glut 4 protein. Glucose transport activity was positively correlated with cell surface Glut 4 content as estimated by exofacial labelling.

IN CONCLUSION

(1) the increase in glucose uptake in muscle after insulin exposure is caused by an augmented concentration of Glut 4 protein on the cell surface membrane, (2) at maximal insulin stimulation (20 mU/ml) approximately 40% of the muscle cell content of Glut 4 is at the cell surface, and (3) the exofacial labelling technique is more sensitive than the subcellular fractionation technique in measuring the amount of glucose transporters on muscle cell surface.

Immunolocalization of GLUT-1 glucose transporter in rat skeletal muscle and in normal and hypoxic cardiac tissue

We compared the expression and cell-type localization of GLUT-1 mRNA and protein between cardiac and skeletal muscle of normal rats. Also, since we recently showed that cardiac GLUT-1 is upregulated in rats exposed to hypobaric hypoxia, we examined the cellular localization of GLUT-1 in cardiac tissue of normal and hypoxic rats. Confocal light microscopy and double immunofluorescent labeling revealed intense localization of GLUT-1 around neurofilament immunoreactivity within gastrocnemius muscle consistent with the previously described localization of large amounts of GLUT-1 in perineurial sheaths of skeletal muscle. However, using the same methods, we were unable to visualize GLUT-1 adjacent to nerve fibers in numerous sections of right or left ventricles or atria. Compared with skeletal myoctes, however, GLUT-1 immunofluorescence among cardiomyocytes was much more intense, particularly along the plasma membrane and especially intercalated discs. GLUT-1 immunofluorescence was also seen within the walls of arterioles within the heart. The predominant localization of GLUT-1 expression to cardiomyocytes in heart tissue was confirmed by in situ mRNA hybridization to digoxigenin-conjugated GLUT-1 cDNA. Northern blot analysis demonstrated that GLUT-1 mRNA was increased severalfold in the cardiac tissues compared with skeletal muscle. Although we detected GLUT-1 protein by immunoblotting of detergent extracts of the heart, we could not detect GLUT-1 in similar extracts of skeletal muscle. The cell type distribution of GLUT-1 in hearts of hypoxic rats was not different by immunohistochemistry from normals. These data indicate that 1) the cell-type distribution of GLUT-1 in the heart differs markedly from that in skeletal muscle. GLUT-1 in cardiac tissue, unlike skeletal muscle, is predominantly expressed within myocytes. 2) Cardiac GLUT-1 is not located along nerve fibers. 3) GLUT-1 mRNA and protein levels in cardiac tissue are considerably greater than in skeletal muscle. 4) The hypoxia-induced increase in cardiac GLUT-1 that we previously reported must occur within cardiomyocytes.

Abundance and subcellular distribution of GTP-binding proteins in 3T3-L1 cells before and after differentiation to the insulin-sensitive phenotype

The abundance and the subcellular distribution of GTP-binding proteins was studied in membrane fractions (plasma membranes and low-density microsomes) from 3T3-L1 cells before and after differentiation to the insulin-sensitive phenotype. After differentiation, the abundance of alpha i (alpha subunit of GTP-binding protein Gi), alpha o (alpha subunit of GTP-binding protein G(o)), and of a 47-kDa alpha s (alpha subunit of GTP-binding protein Gs) as detected by immunoblotting with specific antisera was reduced by 10-50% when normalized per membrane protein. In contrast, a 43-kDa alpha s was increased about threefold after differentiation. Furthermore, cholera-toxin-catalyzed ADP-ribosylation of both 43-kDa and 47-kDa alpha s was disproportionately increased ninefold and threefold, respectively, possibly reflecting the increased production of an ADP-ribosylation factor in the differentiated cells. The small GTP-binding protein Ha-ras was reduced by 50%, whereas rab1 and other small GTP-binding proteins tentatively identified as rab-isoforms (ras-homologous gene products from brain) were increased by 100% and 70%, respectively. Since the total protein content of 3T3-L1 cells was increased threefold after differentiation, the observed increase of the 43-kDa alpha s, rab1 and of the other rab isoforms was eightfold, sixfold and fivefold, respectively, when normalized/cell count. With the exception of the rab isoforms, all GTP-binding proteins were predominantly, if not exclusively, located in the plasma membrane; comparable amounts of the rab isoforms were found in plasma membranes and low-density microsomes. Insulin induced the characteristic redistribution of glucose transporters GLUT4 from low-density microsomes to the plasma membranes, but failed to alter the subcellular distribution of any of the GTP-binding proteins investigated. These data suggest that the increase in the abundance of the 43-kDa alpha s subunit and of several rab isoforms might be related to specific functions of the adipocyte-like phenotype, but that none of the investigated guanine-nucleotide-binding regulatory (G)-proteins appears to be tightly associated with the GLUT4.

Okadaic acid, insulin, and denervation effects on glucose and amino acid transport and glycogen synthesis in muscle

Effects of okadaic acid (OKA) and calyculin A, cell-permeating specific inhibitors of phosphoprotein phosphatases-1 and -2A, were studied in isolated rat hemidiaphragms. OKA stimulated glucose transport (half-maximum = approximately 0.1 microM; maximum = approximately 1 microM) but was less effective than 6 nM insulin. Insulin and OKA effects were not additive. OKA diminished or abolished glucose transport-stimulation by insulin. System A amino acid transport was also stimulated by OKA, insulin was more effective, and preexposure to OKA inhibited insulin stimulation. Calyculin A affected both transport systems similarly to OKA. OKA did not affect basal glycogen synthesis but abolished its stimulation by insulin. Denervated muscles develop post-receptor insulin resistance. Glucose transport and glycogen synthesis were essentially unresponsive to insulin 3 days postdenervation; however, glucose transport was stimulated by OKA similarly to controls. OKA did not affect glycogen synthesis in denervated muscle except for abolishing a small insulin effect. The data suggest similar acute regulation of glucose and system A amino acid transport in muscle. Enhanced Ser/Thr phosphorylation of unidentified protein(s) stimulates both processes but inhibits their full stimulation by insulin. Postdenervation insulin resistance likely reflects impaired signal transduction.

Role of transverse tubules in insulin stimulated muscle glucose transport

Although the strongest evidence for recruitment of glucose transporters in response to insulin comes from studies with adipocytes, studies in muscle seem in general to confirm that glucose transporters are also translocated to the cell membrane in muscle in response to insulin. However, the observation that transverse tubule (T-tubule) membranes contain approximately five times more glucose transporter than sarcolemma raised a question as to where glucose transport occurs in muscle. The T-tubule membrane system is continuous with the surface sarcolemma and is a tubule system in which extracellular fluid is in proximity with the interior of the muscle fiber. The purpose of this Prospects article is to evaluate the possibility that the T-tubule membrane may represent a major site of glucose transport in skeletal muscle. Using immunocytochemical techniques we have located GLUT4 glucose transporters on the T-tubule membrane and in vesicles near T-tubules. Since T-tubules form channels into the interior of the muscle fiber, glucose could diffuse or be moved by some peristaltic-like pumping action into the transverse tubules and then be transported across the membrane deep into the interior of the muscle fiber. This mode of transport directly into the interior of the cell would be advantageous over transport across the sarcolemma and subsequent diffusion around the myofibrils to reach the interior of the muscle. Thus, in addition to the role of the T-tubule in ion fluxes and contraction, this unique membrane system can also provide a pathway for the delivery of substrates into the center of the muscle cell where many glycolytic enzymes and glycogen deposits are located.

The molecular biology of glucose transport: relevance to insulin resistance and non-insulin-dependent diabetes mellitus

The structure of the glucose transporter and the characteristics of the identified members of the facilitative glucose transporter gene family (GLUT1-5) are reviewed. The role of glucose transport in insulin resistance and non-insulin-dependent diabetes mellitus (NIDDM) is discussed. The potential contributions of genetic mutation and disruption of short- or long-term regulation of glucose transporters, particularly GLUT4, in insulin-sensitive tissues to the etiology of NIDDM are examined.

Stimulation of glucose transport by guanine nucleotides in permeabilized rat adipocytes

Effects of guanine nucleotides on glucose transport were studied in permeabilized rat epididymal fat cells. GTP gamma S and Gpp(NH)p, but not App(NH)p, stimulated 3-O-methylglucose transport. Effect of GTP gamma S was dose-dependent, being detectable at 0.1 mM, and 1.0 mM GTP gamma S stimulated glucose transport to the same extent as insulin. GTP gamma S (0.3 mM) enhanced insulin-stimulated glucose transport while 1 mM GTP gamma S did not affect insulin-mediated transport. GDP beta S had no effect on glucose transport by itself but rather enhanced insulin action. NaF, which is known to activate trimeric G proteins, increased glucose transport to the same extent as insulin. Likewise, mastoparan augmented glucose transport. These results indicate that a certain type of trimeric G protein(s) is involved in the regulation of glucose transport.

Sodium-dependent phosphate transport in a rat kidney endosomal fraction

An endosome-enriched fraction was prepared from rat kidney cortex by a standard procedure employing centrifugation on a Percoll gradient. This fraction showed time-dependent accumulation of inorganic phosphate (Pi) which was stimulated two- to threefold during the initial phase by an inwardly directed Na+ gradient. Na+ gradient-dependent Pi accumulation decreased with increasing medium osmolality and Pi binding accounted for only 16% of the total accumulation at two minutes. Like the Pi transporter in the brush border membrane (BBM), the Na+ gradient-dependent Pi uptake (but not the Na(+)-independent component) by the endosomal fraction was stimulated by intravesicular Pi and by an outwardly directed proton gradient, and was inhibited by extravesicular arsenate. Unlike the Pi transporter in BBM, the endosomal Pi transporter was not changed by acidic pH under non-gradient conditions. Activation of the endosomal proton pump by extravesicular ATP, leading to acidification of the vesicle interior, was accompanied by stimulation of endosomal Na+ gradient-dependent Pi transport. Inhibition of the proton pump by deletion of chloride or addition of N-ethylmaleimide abolished the stimulation of Pi uptake by ATP. The data indicate that the Na(+)-dependent Pi transporter in renal endosomal fractions is an intrinsic endosomal component. It remains to be determined if the endosomal Pi transporter plays a role in regulation of renal Pi transport.

Time-dependent regulation of rat adipose tissue glucose transporter (GLUT4) mRNA and protein by insulin in streptozocin-diabetic and normal rats

Streptozocin (STZ) administration (125 mg/kg) to normal rats resulted in a rapid (24-hour) decrease in circulating insulin levels, marked hyperglycemia, and weight loss. Adipose tissue glucose transporter 4 (GLUT4) mRNA levels decreased approximately eightfold, whereas GLUT4 protein levels were unchanged. However, GLUT4 protein levels decreased approximately 30% by 48 hours and fivefold by 72 hours of insulin deficiency. Although GLUT4 mRNA levels were rapidly restored by insulin therapy (twofold above control levels within 12 hours), GLUT4 protein levels increased only gradually, reaching peak values of 1.5-fold control levels following 7 to 10 days of insulin treatment. Insulin treatment in normal rats increased adipose GLUT4 mRNA levels nearly 100% by 24 hours, while GLUT4 protein levels increased in a more gradual fashion. The delay in GLUT4 protein induction relative to its mRNA was shorter in normal rats treated with insulin than in insulin-treated diabetic rats. These data demonstrate that insulin-induced changes in adipose GLUT4 protein are considerably delayed relative to its mRNA, and that the diabetic state enhances this difference. The known in vivo time-dependent effects of insulin treatment on adipocyte glucose transport activity can be at least partly explained by altered specific expression of GLUT4 protein.

Pretranslational regulation of two cardiac glucose transporters in rats exposed to hypobaric hypoxia

To investigate the mechanism by which cardiac glucose utilization increases during hypoxia and increased work load, we studied the effect of 2 and 14 days of hypobaric hypoxia on the expression of two subtypes of the facilitative D-glucose transporter, the GLUT-4 or "insulin-regulatable" isoform and the GLUT-1 isoform thought to mediate basal transport. Rats lose weight when exposed to hypobaric hypoxia, so fasting controls were used in the 2-day studies and pair-fed controls in the 14-day experiments. Hypobaric hypoxia (PO2 69 mmHg) resulted in right ventricular (RV), but not left ventricular (LV), hypertrophy. RV and LV GLUT-1 mRNA levels increased 2- to 3-fold after 2 days and 1.5- to 2-fold after 14 days of hypobaric hypoxia compared with both fasted rats and normal controls. RV GLUT-1 protein increased approximately 3-fold and LV GLUT-1 protein increased 1.5-fold after 14 days of hypobaric hypoxia vs. both pair-fed and normal controls. RV GLUT-4 mRNA decreased to 26% and RV GLUT-4 protein decreased to 54% of normal control levels as a result of 2 days of hypobaric hypoxia. RV GLUT-4 mRNA decreased to 64% of normal control levels with no change in RV GLUT-4 protein as a result of 2 days of fasting. We conclude that hypobaric hypoxia increases cardiac GLUT-1 expression at the pretranslational level in both ventricles. The greater increase in GLUT-1 protein on the right suggests an additive effect of pressure overload. GLUT-4 expression is reduced early in the development of RV hypertrophy.

Subcellular distribution and activity of glucose transporter isoforms GLUT1 and GLUT4 transiently expressed in COS-7 cells

In adipose and muscle cells, the glucose transporter isoform GLUT4 is mainly located in an intracellular, vesicular compartment from which it is translocated to the plasma membrane in response to insulin. In order to test the hypothesis that this preferential targeting of a glucose transporter to an intracellular storage site is conferred only by its primary sequence, we compared the subcellular distribution of the fat/muscle glucose transporter GLUT4 with that of the erythrocyte/brain-type glucose transporter GLUT1 after transient expression in COS-7 cells. Full-length cDNA was ligated into the expression vector pCMV that is driven by the cytomegalovirus promoter, and introduced into COS cells by the DEAE-dextran method. Cells were homogenized and fractionated by differential centrifugation to yield plasma membranes and a Golgi-enriched fraction of intracellular membranes (low-density microsomes). In these membrane fractions, the abundance of glucose transporters was assessed by immunoblotting with specific antibodies against GLUT1 and GLUT4, and their transport activity was assayed after solubilization and reconstitution into lecithin liposomes. Uptake rates of 2-deoxyglucose assayed in parallel samples were higher in cells expressing GLUT1 or GLUT4 as compared with control cells (transfection of pCMV without transporter cDNA). Reconstituted glucose transport activity in plasma membranes was about 5-fold higher after expression of GLUT1 and GLUT4 as compared with control cells. The relative amount of GLUT4 in the low-density microsomes as detected by reconstitution and immunoblotting exceeded that of the GLUT1, but was much lower than that observed in typical insulin-sensitive cells, e.g., rat fat cells or 3T3-L1 adipocytes. These data indicate that COS-7 cells transfected with glucose transporter cDNA express the active transport proteins and can be used for functional studies.

Complex regulation of simple sugar transport in insulin-responsive cells

Facilitated sugar entry into mammalian cells is catalysed by multiple isoforms of the glucose transporter and regulated by hormonal stimuli, nutritional status and oncogenesis. A large reserve of latent glucose transport capacity must be maintained by muscle and adipose cells that are sensitive to insulin, the primary activator of sugar uptake after feeding. Intracellular sequestration of sugar transporters accounts for a large part of this latent capacity, but new findings suggest that there is also reversible suppression of intrinsic catalytic activity of those glucose transporters residing at the cell surface. The mechanism of this suppression appears to be occlusion or disruption of the exofacial sugar-binding sites on the glucose-transporter proteins.