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

Structural advances for the major facilitator superfamily (MFS) transporters

The major facilitator superfamily (MFS) is one of the largest groups of secondary active transporters conserved from bacteria to humans. MFS proteins selectively transport a wide spectrum of substrates across biomembranes and play a pivotal role in multiple physiological processes. Despite intense investigation, only seven MFS proteins from six subfamilies have been structurally elucidated. These structures were captured in distinct states during a transport cycle involving alternating access to binding sites from either side of the membrane. This review discusses recent progress in MFS structure analysis and focuses on the molecular basis for substrate binding, co-transport coupling, and alternating access.

Crystal structure of a bacterial homologue of glucose transporters GLUT1-4

Glucose transporters are essential for metabolism of glucose in cells of diverse organisms from microbes to humans, exemplified by the disease-related human proteins GLUT1, 2, 3 and 4. Despite rigorous efforts, the structural information for GLUT1-4 or their homologues remains largely unknown. Here we report three related crystal structures of XylE, an Escherichia coli homologue of GLUT1-4, in complex with d-xylose, d-glucose and 6-bromo-6-deoxy-D-glucose, at resolutions of 2.8, 2.9 and 2.6 Å, respectively. The structure consists of a typical major facilitator superfamily fold of 12 transmembrane segments and a unique intracellular four-helix domain. XylE was captured in an outward-facing, partly occluded conformation. Most of the important amino acids responsible for recognition of D-xylose or d-glucose are invariant in GLUT1-4, suggesting functional and mechanistic conservations. Structure-based modelling of GLUT1-4 allows mapping and interpretation of disease-related mutations. The structural and biochemical information reported here constitutes an important framework for mechanistic understanding of glucose transporters and sugar porters in general.

Involvement of TNF-alpha in abnormal adipocyte and muscle sortilin expression in obese mice and humans

AIMS/HYPOTHESIS

Insulin resistance is caused by numerous factors including inflammation. It is characterised by defective insulin stimulation of adipocyte and muscle glucose transport, which requires the glucose transporter GLUT4 translocation towards the plasma membrane. Defects in insulin signalling can cause insulin resistance, but alterations in GLUT4 trafficking could also play a role. Our goal was to determine whether proteins controlling GLUT4 trafficking are altered in insulin resistance linked to obesity.

METHODS

Using real-time RT-PCR, we searched for selected transcripts that were differentially expressed in adipose tissue and muscle in obese mice and humans. Using various adipocyte culture models and in vivo mice treatment, we searched for the involvement of TNF-alpha in these alterations in obesity.

RESULTS

Sortilin mRNA and protein were downregulated in adipose tissue from obese db/db and ob/ob mice, and also in muscle. Importantly, sortilin mRNA was also decreased in morbidly obese human diabetic patients. Sortilin and TNF-alpha (also known as TNF) mRNA levels were inversely correlated in mice and human adipose tissues. TNF-alpha decreased sortilin mRNA and protein levels in cultured mouse and human adipocytes, an effect partly prevented by the peroxisome proliferator-activated receptor gamma activator rosiglitazone. TNF-alpha also inhibited adipocyte and muscle sortilin mRNA when injected to mice.

CONCLUSIONS/INTERPRETATION

Sortilin, an essential player in adipocyte and muscle glucose metabolism through the control of GLUT4 localisation, is downregulated in obesity and TNF-alpha is likely to be involved in this defect. Chronic low-grade inflammation in obesity could thus contribute to insulin resistance by modulating proteins that control GLUT4 trafficking.

Glucose transporter GLUT12-functional characterization in Xenopus laevis oocytes

We have recently identified and cloned the cDNA of a new member of the glucose transporter family that has been designated GLUT12. GLUT12 possesses the structural features critical to facilitative transport of glucose but the key to understanding the possible physiological roles of this novel protein requires analysis of functional glucose transport. In the current study, we have utilized the Xenopus laevis oocyte expression system to assay transport of the glucose analog 2-deoxy-D-glucose and characterize the glucose transport properties and hexose affinities of GLUT12. Our results demonstrate that GLUT12 facilitates transport of glucose with an apparent preferential substrate affinity for glucose over other hexoses assayed. The results are significant to understanding the potential role and importance of GLUT12 in insulin-sensitive tissues and also cells with high glucose utilization such as cancer cells.

Differential expression of GLUT12 in breast cancer and normal breast tissue

Increased and deregulated expression of glucose transporters is a characteristic of cancer cells. We previously identified a novel glucose transporter protein, GLUT12, in the MCF7 malignant breast epithelial cell line. Here we present the first demonstration of GLUT12 expression in human breast tumours. Using immunohistochemistry and reverse transcription polymerase chain reaction, GLUT12 was detected in eight of ten invasive tumours. Ductal cell carcinoma in situ cells also stained strongly for GLUT12. Immunohistochemical staining for GLUT12 in benign ducts was less intense, with few positively stained cells. GLUT12 may have a role in hexose supply to breast cancer cells.

Identification of a novel glucose transporter-like protein-GLUT-12

Facilitative glucose transporters exhibit variable hexose affinity and tissue-specific expression. These characteristics contribute to specialized metabolic properties of cells. Here we describe the characterization of a novel glucose transporter-like molecule, GLUT-12. GLUT-12 was identified in MCF-7 breast cancer cells by homology to the insulin-regulatable glucose transporter GLUT-4. The GLUT-12 cDNA encodes 617 amino acids, which possess features essential for sugar transport. Di-leucine motifs are present in NH(2) and COOH termini at positions similar to the GLUT-4 FQQI and LL targeting motifs. GLUT-12 exhibits 29% amino acid identity with GLUT-4 and 40% to the recently described GLUT-10. Like GLUT-10, a large extracellular domain is predicted between transmembrane domains 9 and 10. Genomic organization of GLUT-12 is highly conserved with GLUT-10 but distinct from GLUTs 1-5. Immunofluorescence showed that, in the absence of insulin, GLUT-12 is localized to the perinuclear region in MCF-7 cells. Immunoblotting demonstrated GLUT-12 expression in skeletal muscle, adipose tissue, and small intestine. Thus GLUT-12 is potentially part of a second insulin-responsive glucose transport system.

Syntaxin 4 heterozygous knockout mice develop muscle insulin resistance

To investigate the physiological function of syntaxin 4 in the regulation of GLUT4 vesicle trafficking, we used homologous recombination to generate syntaxin 4-knockout mice. Homozygotic disruption of the syntaxin 4 gene results in early embryonic lethality, whereas heterozygous knockout mice, Syn4(+/-), had normal viability with no significant impairment in growth, development, or reproduction. However, the Syn4(+/-) mice manifested impaired glucose tolerance with a 50% reduction in whole-body glucose uptake. This defect was attributed to a 50% reduction in skeletal muscle glucose transport determined by 2-deoxyglucose uptake during hyperinsulinemic-euglycemic clamp procedures. In parallel, insulin-stimulated GLUT4 translocation in skeletal muscle was also significantly reduced in these mice. In contrast, Syn4(+/-) mice displayed normal insulin-stimulated glucose uptake and metabolism in adipose tissue and liver. Together, these data demonstrate that syntaxin 4 plays a critical physiological role in insulin-stimulated glucose uptake in skeletal muscle. Furthermore, reduction in syntaxin 4 protein levels in this tissue can account for the impairment in whole-body insulin-stimulated glucose metabolism in this animal model.

Regulation of the Na+/K+-ATPase by insulin: why and how?

The sodium-potassium ATPase (Na+/K+-ATPase or Na+/K+-pump) is an enzyme present at the surface of all eukaryotic cells, which actively extrudes Na+ from cells in exchange for K+ at a ratio of 3:2, respectively. Its activity also provides the driving force for secondary active transport of solutes such as amino acids, phosphate, vitamins and, in epithelial cells, glucose. The enzyme consists of two subunits (alpha and beta) each expressed in several isoforms. Many hormones regulate Na+/K+-ATPase activity and in this review we will focus on the effects of insulin. The possible mechanisms whereby insulin controls Na+/K+-ATPase activity are discussed. These are tissue- and isoform-specific, and include reversible covalent modification of catalytic subunits, activation by a rise in intracellular Na+ concentration, altered Na+ sensitivity and changes in subunit gene or protein expression. Given the recent escalation in knowledge of insulin-stimulated signal transduction systems, it is pertinent to ask which intracellular signalling pathways are utilized by insulin in controlling Na+/K+-ATPase activity. Evidence for and against a role for the phosphatidylinositol-3-kinase and mitogen activated protein kinase arms of the insulin-stimulated intracellular signalling networks is suggested. Finally, the clinical relevance of Na+/K+-ATPase control by insulin in diabetes and related disorders is addressed.

Characterization of a low affinity thyroid hormone receptor binding site within the rat GLUT4 gene promoter

Previous studies have demonstrated that thyroid hormone (T3) stimulates insulin-responsive glucose transporter (GLUT4) transcription and protein expression in rat skeletal muscle. The aim of the present study was to define a putative thyroid hormone response element (TRE) within the rat GLUT4 promoter and thus perhaps determine whether T3 acts directly to augment skeletal muscle GLUT4 transcription. To this end, electrophoretic mobility shift analyses were performed to analyze thyroid hormone receptor (TR) binding to a previously characterized 281-bp T3-responsive region of the rat GLUT4 promoter. Indeed, within this region, a TR-binding site of the standard DR + 4 TRE variety was located between bases -457/ -426 and was shown to posses a specific affinity for in vitro translated TRs. Interestingly, however, the GLUT4 TR-binding site demonstrated a significantly lower affinity compared to a consensus DR + 4 TRE, and only bound TRs appreciatively in the form of high affinity heterodimers, in this case with the cis-retinoic acid receptor. In conclusion, these data demonstrated the presence of a specific TR-binding site within a T3-responsive region of the rat GLUT4 promoter and thus support the supposition that thyroid hormone acts directly to stimulate GLUT4 transcription in rat skeletal muscle. Moreover, characterization of a novel TR-binding site with low affinity suggests an additional mechanism by which the intrinsic activity and responsiveness of thyroid hormone regulated genes may be modulated.

Two different repressors collaborate to restrict expression of the yeast glucose transporter genes HXT2 and HXT4 to low levels of glucose

Transcription of the yeast HXT2 and HXT4 genes, which encode glucose transporters, is induced only by low levels of glucose. This low-glucose-induced expression is mediated by two independent repression mechanisms: in the absence of glucose, transcription of both genes is prevented by Rgt1p, a C6 zinc cluster protein; at high levels of glucose, expression of HXT2 and HXT4 is repressed by Mig1p. Only at low glucose concentrations are both repressors inactive, leading to a 10- to 20-fold induction of gene expression. Mig1p and Rgt1p act directly on HXT2 and HXT4 by binding to their promoters. This transcriptional regulation is physiologically very important to the yeast cell because it causes these glucose transporters to be expressed only in low-glucose media, in which they are required for growth.

Islet transplantation under the kidney capsule fully corrects the impaired skeletal muscle glucose transport system of streptozocin diabetic rats

Chronic insulin therapy improves but does not restore impaired insulin-mediated muscle glucose uptake in human diabetes or muscle glucose uptake, transport, and transporter translocation in streptozocin diabetic rats. To determine whether this inability is due to inadequate insulin replacement, we studied fasted streptozocin-induced diabetic Lewis rats either untreated or after islet transplantation under the kidney capsule. Plasma glucose was increased in untreated diabetics and normalized by the islet transplantation (110 +/- 5, 452 +/- 9, and 102 +/- 3 mg/dl in controls, untreated diabetics, and transplanted diabetics, respectively). Plasma membrane and intracellular microsomal membrane vesicles were prepared from hindlimb skeletal muscle of basal and maximally insulin-stimulated rats. Islet transplantation normalized plasma membrane carrier-mediated glucose transport Vmax, plasma membrane glucose transporter content, and insulin-induced transporter translocation. There were no differences in transporter intrinsic activity (Vmax/Ro) among the three groups. Microsomal membrane GLUT4 content was reduced by 30% in untreated diabetic rats and normal in transplanted diabetics, whereas the insulin-induced changes in microsomal membrane GLUT4 content were quantitatively similar in the three groups. There were no differences in plasma membrane GLUT1 among the groups and between basal and insulin stimulated states. Microsomal membrane GLUT1 content was increased 60% in untreated diabetics and normalized by the transplantation. In conclusion, an adequate insulin delivery in the peripheral circulation, obtained by islet transplantation, fully restores the muscle glucose transport system to normal in streptozocin diabetic rats.

Glucose transporter function is controlled by transporter oligomeric structure. A single, intramolecular disulfide promotes GLUT1 tetramerization

The human erythrocyte glucose transporter is an allosteric complex of four GLUT1 proteins whose structure and substrate binding properties are stabilized by reductant-sensitive, noncovalent subunit interactions [Hebert, D. N., & Carruthers, A. (1992) J. Biol. Chem. 267, 23829-23838]. In the present study, we use biochemical and molecular approaches to isolate specific determinants of transporter oligomeric structure and transport function. When unfolded in denaturant, each subunit (GLUT1 protein) of the transporter complex exposes two sulfhydryl groups. Four additional thiol groups are accessible following subunit exposure to reductant. Assays of subunit disulfide bridge content suggest that two inaccessible sulfhydryl groups form an internal disulfide bridge. Differential alkylation/peptide mapping/N-terminal sequence analyses show that a GLUT1 carboxyl-terminal peptide (residues 232-492) contains three inaccessible sulfhydryl groups and that an N-terminal GLUT1 peptide (residues 147-261/299) contains two accessible thiols. The carboxyl-terminal peptide most likely contains the intramolecular disulfide bridge since neither its yield nor its electrophoretic mobility is altered by addition of reductant. Each GLUT1 cysteine was changed to serine by oligonucleotide-directed, in vitro mutagenesis. The resulting transport proteins were expressed in CHO cells and screened by immunofluorescence microscopy for their ability to expose tetrameric GLUT1-specific epitopes. Serine substitution at cysteine residues 133, 201, 207, and 429 does not inhibit exposure of tetrameric GLUT1-specific epitopes. Serine substitution at cysteines 347 or 421 prevents exposure of tetrameric GLUT1-specific epitopes. Hydrodynamic analysis of GLUT1/GLUT4 chimeras expressed in and subsequently solubilized from CHO cells indicates that GLUT1 residues 1-199 promote chimera dimerization and permit GLUT1/chimera heterotetramerization. This GLUT1 N-terminal domain is insufficient for chimera tetramerization which additionally requires GLUT1 residues 200-463. Extracellular reductants (dithiothreitol, beta-mercaptoethanol, or glutathione) reduce erythrocyte 3-O-methylglucose uptake by up to 15-fold. This noncompetitive inhibition of sugar uptake is reversed by the cell-impermeant, oxidized glutathione. Reductant is without effect on sugar exit from erythrocytes. Dithiothreitol doubles the cytochalasin B binding capacity of erythrocyte-resident glucose transporter, abolishes allosteric interactions between substrate binding sites on adjacent subunits, and occludes tetrameric GLUT1-specific GLUT1 epitopes in situ. CHO cell-resident GLUT1 structure and transport function are similarly affected by extracellular reductant. We conclude that each subunit of the glucose transporter contains an extracellular disulfide bridge (Cys347 and Cys421) that stabilizes transporter oligomeric structure and thereby accelerates transport function.

Rapid substrate translocation by the multisubunit, erythroid glucose transporter requires subunit associations but not cooperative ligand binding

The human erythroid glucose transporter is a GLUT1 homotetramer whose structure and function are stabilized by noncovalent, cooperative subunit interactions. The present study demonstrates that exofacial tryptic digestion of GLUT1 abolishes cooperative interactions between substrate binding sites on adjacent subunits under circumstances where subunit associations and high catalytic turnover are maintained. Extracellular trypsin produces rapid, quantitative cleavage of the human red cell-resident sugar transport protein, GLUT1. One major carboxyl-terminal peptide of M(r)(app) 25,000 is detected by immunoblot analysis. Endofacial tryptic digestion of GLUT1 results in the complete loss of GLUT1 carboxyl-terminal structure. GLUT1-mediated erythrocyte sugar uptake, transport inhibition by cytochalasin B, and GLUT1 oligomeric structure are unaffected by exofacial GLUT1 proteolysis. In contrast, the cytochalasin B binding capacity of GLUT1 and the Kd(app) for cytochalasin B binding to the transporter are doubled following exofacial tryptic digestion of GLUT1. Photoaffinity labeling experiments show that increased cytochalasin B binding results from increased ligand binding to the 25 kDa carboxyl-terminal GLUT1 peptide. Proteolysis abolishes allosteric interactions between sugar import (maltose binding) and sugar export (cytochalasin B binding) sites that normally exist on adjacent subunits within the transporter complex, but interact with negative cooperativity. Following exofacial proteolysis, these sites become mutually exclusive. Dithiothreitol disrupts GLUT1 quaternary structure, inhibits 3-O-methylglucose transport, and abolishes cooperative interactions between sugar import and export sites in control cells. Studies with reconstituted purified GLUT1 confirm that the action of trypsin on cytochalasin B binding is direct, show that proteolysis increases the apparent affinity of the sugar efflux site for transported sugars, and suggest that the membrane bilayer stabilizes GLUT1 noncovalent structure and catalytic function following GLUT1 proteolysis. Collectively, these findings demonstrate that GLUT1 does not require an intact polypeptide backbone for catalytic function. They show that the multisite sugar transporter mechanism is converted to a simple ping-pong carrier mechanism following exofacial GLUT1 proteolysis. They reveal that subunit cooperativity can be lost under circumstances where cohesive structural interactions between transporter subunits are maintained. They also refute the hypothesis [Hebert, D. N., & Carruthers, A. (1992) J. Biol. Chem. 267, 23829-23838] that rapid substrate translocation by the multisubunit erythroid glucose transporter requires cooperative interactions between subunit ligand binding sites.

Mechanisms and time course of impaired skeletal muscle glucose transport activity in streptozocin diabetic rats

Skeletal muscle glucose transport is altered in diabetes in humans, as well as in rats. To investigate the mechanisms of this abnormality, we measured glucose transport Vmax, the total transporter number, their average intrinsic activity, GLUT4 and GLUT1 contents in skeletal muscle plasma membrane vesicles from basal or insulin-stimulated streptozocin diabetic rats with different duration of diabetes, treated or not with phlorizin. The glucose transport Vmax progressively decreased with the duration of diabetes. In the basal state, this decrease was primarily associated with the reduction of transporter intrinsic activity, which appeared earlier than any change in transporter number or GLUT4 and GLUT1 content. In the insulin-stimulated state, the decrease of transport was mainly associated with severe defects in transporter translocation. Phlorizin treatment partially increased the insulin-stimulated glucose transport by improving the transporter translocation defects. In conclusion, in streptozocin diabetes (a) reduction of intrinsic activity plays a major and early role in the impairment of basal glucose transport; (b) a defect in transporter translocation is the mechanism responsible for the decrease in insulin-stimulated glucose transport; and (c) hyperglycemia per se affects the insulin-stimulated glucose transport by altering the transporter translocation.

Glucose entry into rat mesangial cells is mediated by both Na(+)-coupled and facilitative transporters

Since previous studies from our laboratory have demonstrated that increased glucose consumption by cultured rat mesangial cells is accompanied by an accelerated production of type IV and type VI collagen, we have now examined the manner by which glucose is transported into these cells. A progressive stimulation of glucose uptake by the mesangial cells was observed with increasing concentrations of NaCl so that at 145 mmol/l about twice as much glucose entered the cells as in its absence (substituted by choline chloride). Moreover, since phlorizin inhibited the NaCl-promoted uptake of glucose and this salt was found to increase the accumulation of alpha-methylglucoside in a manner which could not be duplicated by KCl or mannitol, both Na(+)-coupled and facilitative glucose transporters appeared to be present in the cells. Km values of 1.93 mmol/l and 1.36 mmol/l were determined for the co-transport and facilitated transport pathways, respectively, with their Vmax being 29.5 and 18.0 nmol.mg protein-1.h-1. Both uptake activities were found to be down-regulated by exposure of the cells to high glucose and furthermore the Na(+)-dependent transport could no longer be detected after about 12 passages of the cells. Hybridization of mesangial cell mRNA with cDNA probes revealed transcripts for the Na+/glucose co-transporter as well as GLUT1 and to a lesser extent GLUT4.(ABSTRACT TRUNCATED AT 250 WORDS)

Schizophrenia is a diabetic brain state: an elucidation of impaired neurometabolism

In this paper a detailed argument will be advanced in support of the notion that schizophrenia is fundamentally a diabetic brain state, henceforth referred to as 'cerebral diabetes'. Many extraneous features of cerebral diabetes have been observed, including positron emission tomography (PET) scans which reflect abnormal distribution patterns and diminished supplies of glucose in the brain. Equally, empirical research has demonstrated that plasma levels of essential fatty acids and prostaglandins are abnormally low, and low levels of glycoproteins in the urine of cerebral diabetics have also been observed. In addition, cerebral diabetics manifest a wide range of disturbing physical symptoms, such as, impaired sexual function, temperature control, low blood pressure, disrupted sleep patterns, excessive thirst, poor memory, insensitivity to pain, and chronic unhappiness, all of which can be attributed to disrupted neuroendocrine function. Thus, in order to persuasively assert the redefinition of schizophrenia as 'cerebral diabetes', we shall first explicate glucose regulation and transport in the brain and then outline how this interacts with essential fatty acids and prostaglandins, neurotransmission, and the neuroendocrine system. In so doing, we shall provide a metabolic explanation for all the prominent symptoms currently known to be associated with cerebral diabetes and indicate some future therapeutic interventions.