Reconciling contradictory findings: Glucose transporter 1 (GLUT1) functions as an oligomer of allosteric, alternating access transporters

Identification of Structural Determinants of the Transport of the Dehydroascorbic Acid Mediated by Glucose Transport GLUT1

GLUT1 is a facilitative glucose transporter that can transport oxidized vitamin C (i.e., dehydroascorbic acid) and complements the action of reduced vitamin C transporters. To identify the residues involved in human GLUT1's transport of dehydroascorbic acid, we performed docking studies in the 5 Å grid of the glucose-binding cavity of GLUT1. The interactions of the bicyclic hemiacetal form of dehydroascorbic acid with GLUT1 through hydrogen bonds with the -OH group of C3 and C5 were less favorable than the interactions with the sugars transported by GLUT1. The eight most relevant residues in such interactions (i.e., F26, Q161, I164, Q282, Y292, and W412) were mutated to alanine to perform functional studies for dehydroascorbic acid and the glucose analog, 2-deoxiglucose, in Xenopus laevis oocytes. All the mutants decreased the uptake of both substrates to less than 50%. The partial effect of the N317A mutant in transporting dehydroascorbic acid was associated with a 30% decrease in the V_max compared to the wildtype GLUT1. The results show that both substrates share the eight residues studied in GLUT1, albeit with a differential contribution of N317. Our work, combining docking with functional studies, marks the first to identify structural determinants of oxidized vitamin C's transport via GLUT1.

Structural Determinants and Functional Significance of Dimerization for Osmosensing Transporter ProP in Escherichia coli

Osmosensing transporter ProP forestalls cellular dehydration by detecting environments with high osmotic pressure and mediating the accumulation of organic osmolytes by bacterial cells. It is composed of 12 transmembrane helices with cytoplasmic N- and C-termini. In Escherichia coli, dimers form when the C-terminal domains of ProP molecules form homodimeric, antiparallel, α-helical coiled coils. No dominant negative effect was detected when inactive and active ProP molecules formed heterodimers in vivo. Purification of ProP in detergent dodecylmaltoside yielded monomers, which were functional after reconstitution in proteoliposomes. With other evidence, this suggests that ProP monomers function independently whether in the monomeric or dimeric state. Amino acid replacements that disrupted or reversed the coiled coil did not prevent in vivo dimerization of ProP detected with a bacterial two-hybrid system. Maleimide labeling detected no osmolality-dependent variation in the reactivities of cysteine residues introduced to transmembrane helix (TM) XII. In contrast, coarse-grained molecular dynamic simulations detected deformation of the lipid around TMs III and VI, on the lipid-exposed protein surface opposite to TM XII. This suggests that the dimer interface of ProP includes the surfaces of TMs III and VI, not of TM XII as previously suggested by crosslinking data. Homology modeling suggested that coiled-coil formation and dimerization via such an interface are not mutually exclusive. In previous work, alterations to the C-terminal coiled coil blocked co-localization of ProP with phospholipid cardiolipin at E. coli cell poles. Thus, dimerization may contribute to ProP targeting, adjust its lipid environment, and hence indirectly modify its osmotic stress response.

An appraisal of the current status of inhibition of glucose transporters as an emerging antineoplastic approach: Promising potential of new pan-GLUT inhibitors

Neoplastic cells displayed altered metabolism with accelerated glycolysis. Therefore, these cells need a mammoth supply of glucose for which they display an upregulated expression of various glucose transporters (GLUT). Thus, novel antineoplastic strategies focus on inhibiting GLUT to intersect the glycolytic lifeline of cancer cells. This review focuses on the current status of various GLUT inhibition scenarios. The GLUT inhibitors belong to both natural and synthetic small inhibitory molecules category. As neoplastic cells express multiple GLUT isoforms, it is necessary to use pan-GLUT inhibitors. Nevertheless, it is also necessary that such pan-GLUT inhibitors exert their action at a low concentration so that normal healthy cells are left unharmed and minimal injury is caused to the other vital organs and systems of the body. Moreover, approaches are also emerging from combining GLUT inhibitors with other chemotherapeutic agents to potentiate the antineoplastic action. A new pan-GLUT inhibitor named glutor, a piperazine-one derivative, has shown a potent antineoplastic action owing to its inhibitory action exerted at nanomolar concentrations. The review discusses the merits and limitations of the existing GLUT inhibitory approach with possible future outcomes.

Identification of new GLUT2-selective inhibitors through in silico ligand screening and validation in eukaryotic expression systems

Glucose is an essential energy source for cells. In humans, its passive diffusion through the cell membrane is facilitated by members of the glucose transporter family (GLUT, SLC2 gene family). GLUT2 transports both glucose and fructose with low affinity and plays a critical role in glucose sensing mechanisms. Alterations in the function or expression of GLUT2 are involved in the Fanconi-Bickel syndrome, diabetes, and cancer. Distinguishing GLUT2 transport in tissues where other GLUTs coexist is challenging due to the low affinity of GLUT2 for glucose and fructose and the scarcity of GLUT-specific modulators. By combining in silico ligand screening of an inward-facing conformation model of GLUT2 and glucose uptake assays in a hexose transporter-deficient yeast strain, in which the GLUT1-5 can be expressed individually, we identified eleven new GLUT2 inhibitors (IC₅₀ ranging from 0.61 to 19.3 µM). Among them, nine were GLUT2-selective, one inhibited GLUT1-4 (pan-Class I GLUT inhibitor), and another inhibited GLUT5 only. All these inhibitors dock to the substrate cavity periphery, close to the large cytosolic loop connecting the two transporter halves, outside the substrate-binding site. The GLUT2 inhibitors described here have various applications; GLUT2-specific inhibitors can serve as tools to examine the pathophysiological role of GLUT2 relative to other GLUTs, the pan-Class I GLUT inhibitor can block glucose entry in cancer cells, and the GLUT2/GLUT5 inhibitor can reduce the intestinal absorption of fructose to combat the harmful effects of a high-fructose diet.

Cellular binding and uptake of fluorescent glucose analogs 2-NBDG and 6-NBDG occurs independent of membrane glucose transporters

The classical methods for determining glucose uptake rates in living cells involve the use of isotopically labeled 2-deoxy-d-glucose or 3-O-methyl-d-glucose, which enter cells via well-characterized membrane transporters of the SLC2A and SLC5A families, respectively. These classical methods, however, are increasingly being displaced by high-throughput assays that utilize fluorescent analogs of glucose. Among the most commonly used of these analogs are 2-NBDG and 6-NBDG, which contain a bulky 7-nitro-2,1,3-benzoxadiazol-4-yl-amino moiety in place of a hydroxy group on d-glucose. This fluorescent group significantly alters both the size and shape of these molecules compared to glucose, calling into question whether they actually enter cells by the same transport mechanisms. In this study, we took advantage of the well-defined glucose uptake mechanism of L929 murine fibroblasts, which rely exclusively on the Glut1/Slc2a1 membrane transporter. We demonstrate that neither pharmacologic inhibition of Glut1 nor genetic manipulation of its expression has a significant impact on the binding or uptake of 2-NBDG or 6-NBDG by L929 cells, though both approaches significantly impact [³H]-2-deoxyglucose uptake rates. Together these data indicate that 2-NBDG and 6-NBDG can bind and enter mammalian cells by transporter-independent mechanisms, which calls into question their utility as an accurate proxy for glucose transport.

GLUT1 biological function and inhibition: research advances

The GLUT is a key regulator of glucose metabolism and is widely expressed on the surface of most cells of the body. GLUT provides a variety of nutrients for the growth, proliferation and differentiation of cells. In recent years, the development of drugs affecting the energy intake of tumor cells has become a research hotspot. GLUT inhibitors are gaining increased attention because they can block the energy supply of malignant tumors. Herein, we elaborate on the structure and function of GLUT1, the structural and functional differences among GLUT1-4 transporters and the relationship between GLUT1 and tumor development, as well as GLUT1 transporter inhibitors, to provide a reference for the development of new GLUT1 inhibitors.

Oral glucose tolerance test does not affect degree of hemoglobin glycation as measured by routine assay

BACKGROUND

Hemoglobin A1c (HbA1c) is an accurate index of fluctuation in glycemia over the 2-3 months prior to quantitative assessment. During this time, hemoglobin (Hb) slowly glycates until it shows the properties of advanced glycation end-products. Glycation kinetics is intensified by prolonged glucose exposure. In subjects undergoing oral glucose tolerance testing (OGTT), immediately after ingestion, glucose is ostensibly transported by the glucose transporter 1 (GLUT1) to erythrocyte corpuscular hemoglobin. The earliest significant measurable level of hemoglobin glycation associated with this transportation is still not clear.

SUBJECTS AND METHODS

We attempted to explore the early impact of short-term glucose load on HbA1c levels, because it is now known that transmembrane GLUT1-mediated glucose transport occurs immediately. A total of 88 participants (46 patients and 42 clinically healthy controls) underwent fasting plasma glucose quantitation during an OGTT. HbA1c, revealed by a monoclonal anti-glycation epitope antibody and adiponectin, was quantitated before (T0) and 2 hours (T120) after 80 g glucose ingestion.

RESULTS

Wilcoxon test revealed that the HbA1c values did not significantly vary (P=0.15) during the OGTT, whereas glucose concentration varied strongly between T0 and T120.

DISCUSSION

It is well known that quantitative estimation of HbA1c is informative for clinical care, independently of glucose level. The molecular mechanisms and dynamics by which glucose enters/exits red blood cells are incompletely known and may differ between individuals. We here show, for the first time, that HbA1c levels do not significantly increase during OGTT, supporting the view that non-enzymatic glycation of hemoglobin occurs slowly and that glycation during the 2 hours of an OGTT is insignificant.

Glut1 Deficiency Syndrome (Glut1DS): State of the art in 2020 and recommendations of the international Glut1DS study group

Glut1 deficiency syndrome (Glut1DS) is a brain energy failure syndrome caused by impaired glucose transport across brain tissue barriers. Glucose diffusion across tissue barriers is facilitated by a family of proteins including glucose transporter type 1 (Glut1). Patients are treated effectively with ketogenic diet therapies (KDT) that provide a supplemental fuel, namely ketone bodies, for brain energy metabolism. The increasing complexity of Glut1DS, since its original description in 1991, now demands an international consensus statement regarding diagnosis and treatment. International experts (n = 23) developed a consensus statement utilizing their collective professional experience, responses to a standardized questionnaire, and serial discussions of wide-ranging issues related to Glut1DS. Key clinical features signaling the onset of Glut1DS are eye-head movement abnormalities, seizures, neurodevelopmental impairment, deceleration of head growth, and movement disorders. Diagnosis is confirmed by the presence of these clinical signs, hypoglycorrhachia documented by lumbar puncture, and genetic analysis showing pathogenic SLC2A1 variants. KDT represent standard choices with Glut1DS-specific recommendations regarding duration, composition, and management. Ongoing research has identified future interventions to restore Glut1 protein content and function. Clinical manifestations are influenced by patient age, genetic complexity, and novel therapeutic interventions. All clinical phenotypes will benefit from a better understanding of Glut1DS natural history throughout the life cycle and from improved guidelines facilitating early diagnosis and prompt treatment. Often, the presenting seizures are treated initially with antiseizure drugs before the cause of the epilepsy is ascertained and appropriate KDT are initiated. Initial drug treatment fails to treat the underlying metabolic disturbance during early brain development, contributing to the long-term disease burden. Impaired development of the brain microvasculature is one such complication of delayed Glut1DS treatment in the postnatal period. This international consensus statement should facilitate prompt diagnosis and guide best standard of care for Glut1DS throughout the life cycle.

Structure, function and regulation of mammalian glucose transporters of the SLC2 family

The SLC2 genes code for a family of GLUT proteins that are part of the major facilitator superfamily (MFS) of membrane transporters. Crystal structures have recently revealed how the unique protein fold of these proteins enables the catalysis of transport. The proteins have 12 transmembrane spans built from a replicated trimer substructure. This enables 4 trimer substructures to move relative to each other, and thereby alternately opening and closing a cleft to either the internal or the external side of the membrane. The physiological substrate for the GLUTs is usually a hexose but substrates for GLUTs can include urate, dehydro-ascorbate and myo-inositol. The GLUT proteins have varied physiological functions that are related to their principal substrates, the cell type in which the GLUTs are expressed and the extent to which the proteins are associated with subcellular compartments. Some of the GLUT proteins translocate between subcellular compartments and this facilitates the control of their function over long- and short-time scales. The control of GLUT function is necessary for a regulated supply of metabolites (mainly glucose) to tissues. Pathophysiological abnormalities in GLUT proteins are responsible for, or associated with, clinical problems including type 2 diabetes and cancer and a range of tissue disorders, related to tissue-specific GLUT protein profiles. The availability of GLUT crystal structures has facilitated the search for inhibitors and substrates and that are specific for each GLUT and that can be used therapeutically. Recent studies are starting to unravel the drug targetable properties of each of the GLUT proteins.

Extracellular gating of glucose transport through GLUT 1

The ubiquitous glucose transporter 1 (GLUT1) is physiologically and pathologically relevant in energy metabolism of the CNS, skeletal muscles, cancer cells etc. Extensive experiments on GLUT1 produced thorough understandings of its expressions, functions, and structures which were recently resolved to atomic accuracy. However, theoretical understandings are still controversial about how GLUT1 facilitates glucose diffusion across the cell membrane. Molecular dynamics (MD) simulations of the current literature have GLUT1 embedded in a symmetric bilayer of a single lipid type. They provide atomistic illustrations of the alternating access theory (AAT), but the simulation results are inconsistent with the undisputed experimental data of kinetics showing rapid transport of glucose at near-physiological temperatures, high Arrhenius activation barrier in zero-trans uptake, and large trans-acceleration at sub-physiological temperatures. In this research, we embedded GLUT1 in an asymmetric bilayer of multiple lipids to better mimic the erythrocyte membrane. We ran unbiased MD simulations at 37 °C and at 5 °C and found a new mechanism of glucose transport via GLUT1: The extracellular (EC) gate opened wide for EC glucopyranose at 37 °C and, only in the presence of intracellular (IC) glucose, at 5 °C. In the absence of IC glucose at 5 °C, the EC gate opened narrowly for acyclic glucose, gating out glucopyranose. This EC-gating mechanism is simpler than AAT and yet it well explains for the rapid glucose transport at near-physiological temperatures and large trans-acceleration at sub-physiological temperatures. It also explains why zero-trans uptake (involving the pyranose-to-aldehyde transformation) has an Arrhenius barrier ∼20 kcal/mol higher than the equilibrium exchange transport.

New insights into GluT1 mechanics during glucose transfer

Glucose plays a crucial role in the mammalian cell metabolism. In the erythrocytes and endothelial cells of the blood-brain barrier, glucose uptake is mediated by the glucose transporter type 1 (GluT1). GluT1 deficiency or mutations cause severe physiological disorders. GluT1 is also an important target in cancer therapy as it is overexpressed in tumor cells. Previous studies have suggested that GluT1 mediates solute transfer through a cycle of conformational changes. However, the corresponding 3D structures adopted by the transporter during the transfer process remain elusive. In the present work, we first elucidate the whole conformational landscape of GluT1 in the absence of glucose, using long molecular dynamics simulations and show that the transitions can be accomplished through thermal fluctuations. Importantly, we highlight a strong coupling between intracellular and extracellular domains of the protein that contributes to the transmembrane helices reorientation during the transition. The conformations adopted during the simulations differ from the known 3D bacterial homologs structures resolved in similar states. In holo state simulations, we find that glucose transits along the pathway through significant rotational motions, while maintaining hydrogen bonds with the protein. These persistent motions affect side chains orientation, which impacts protein mechanics and allows glucose progression.

Transporter oligomerisation: roles in structure and function

Oligomerisation is a key feature of integral membrane transporters with roles in structure, function and stability. In this review, we cover some very recent advances in our understanding of how oligomerisation affects these key transporter features, with emphasis on a few groups of transporters, including the nucleobase ascorbate transporters, neurotransmitter sodium symporters and major facilitator superfamily members.

Chemical biology probes of mammalian GLUT structure and function

The structure and function of glucose transporters of the mammalian GLUT family of proteins has been studied over many decades, and the proteins have fascinated numerous research groups over this time. This interest is related to the importance of the GLUTs as archetypical membrane transport facilitators, as key limiters of the supply of glucose to cell metabolism, as targets of cell insulin and exercise signalling and of regulated membrane traffic, and as potential drug targets to combat cancer and metabolic diseases such as type 2 diabetes and obesity. This review focusses on the use of chemical biology approaches and sugar analogue probes to study these important proteins.

Red wine and green tea flavonoids are cis-allosteric activators and competitive inhibitors of glucose transporter 1 (GLUT1)-mediated sugar uptake

The antioxidant- and flavonoid-rich contents of red wine and green tea are reported to offer protection against cancer, cardiovascular disease, and diabetes. Some studies, however, show that flavonoids inhibit GLUT1-mediated, facilitative glucose transport, raising the possibility that their interaction with GLUT1 and subsequent downstream effects on carbohydrate metabolism may also impact health. The present study explores the structure-function relationships of flavonoid-GLUT1 interactions. We find that low concentrations of flavonoids act as cis-allosteric activators of sugar uptake, whereas higher concentrations competitively inhibit sugar uptake and noncompetitively inhibit sugar exit. Studies with heterologously expressed human GLUT1, -3, or -4 reveal that quercetin-GLUT1 and -GLUT4 interactions are stronger than quercetin-GLUT3 interactions, that epicatechin gallate (ECG) is more selective for GLUT1, and that epigallocatechin gallate (EGCG) is less GLUT isoform-selective. Docking studies suggest that only one flavonoid can bind to GLUT1 at any instant, but sugar transport and ligand-binding studies indicate that human erythrocyte GLUT1 can bind at least two flavonoid molecules simultaneously. Quercetin and EGCG are each characterized by positive, cooperative binding, whereas ECG shows negative cooperative binding. These findings support recent studies suggesting that GLUT1 forms an oligomeric complex of interacting, allosteric, alternating access transporters. We discuss how modulation of facilitative glucose transporters could contribute to the protective actions of the flavonoids against diabetes and Alzheimer's disease.

Ligand Screening Systems for Human Glucose Transporters as Tools in Drug Discovery

Hexoses are the major source of energy and carbon skeletons for biosynthetic processes in all kingdoms of life. Their cellular uptake is mediated by specialized transporters, including glucose transporters (GLUT, SLC2 gene family). Malfunction or altered expression pattern of GLUTs in humans is associated with several widespread diseases including cancer, diabetes and severe metabolic disorders. Their high relevance in the medical area makes these transporters valuable drug targets and potential biomarkers. Nevertheless, the lack of a suitable high-throughput screening system has impeded the determination of compounds that would enable specific manipulation of GLUTs so far. Availability of structural data on several GLUTs enabled in silico ligand screening, though limited by the fact that only two major conformations of the transporters can be tested. Recently, convenient high-throughput microbial and cell-free screening systems have been developed. These remarkable achievements set the foundation for further and detailed elucidation of the molecular mechanisms of glucose transport and will also lead to great progress in the discovery of GLUT effectors as therapeutic agents. In this mini-review, we focus on recent efforts to identify potential GLUT-targeting drugs, based on a combination of structural biology and different assay systems.

Kinetic Basis of Cis- and Trans-Allostery in GLUT1-Mediated Sugar Transport

A growing body of evidence demonstrates that GLUT1-mediated erythrocyte sugar transport is more complex than widely assumed and that contemporary interpretations of emergent GLUT1 structural data are incompatible with the available transport and biochemical data. This study examines the kinetic basis of one such incompatibility-transport allostery-and in doing so suggests how the results of studies examining GLUT1 structure and function may be reconciled. Three types of allostery are observed in GLUT1-mediated, human erythrocyte sugar transport: (1) exofacial cis-allostery in which low concentrations of extracellular inhibitors stimulate sugar uptake while high concentrations inhibit transport; (2) endofacial cis-allostery in which low concentrations of intracellular inhibitors enhance cytochalasin B binding to GLUT1 while high concentrations inhibit binding, and (3) trans-allostery in which low concentrations of ligands acting at one cell surface stimulate ligand binding at or sugar transport from the other surface while high concentrations inhibit these processes. We consider several kinetic models to account for these phenomena. Our results show that an inhibitor can only stimulate then inhibit sugar uptake if (1) the transporter binds two or more molecules of inhibitor; (2) high-affinity binding to the first site stimulates transport, and (3) low-affinity binding to the second site inhibits transport. Reviewing the available structural, transport, and ligand binding data, we propose that exofacial cis-allostery results from cross-talk between multiple, co-existent ligand interaction sites present in the exofacial cavity of each GLUT1 protein, whereas trans-allostery and endofacial cis-allostery require ligand-induced subunit-subunit interactions.