Characterization of rat Glut4 glucose transporter expressed in the yeast Saccharomyces cerevisiae: comparison with Glut1 glucose transporter

GLUT4 dispersal at the plasma membrane of adipocytes: a super-resolved journey

In adipose tissue, insulin stimulates glucose uptake by mediating the translocation of GLUT4 from intracellular vesicles to the plasma membrane. In 2010, insulin was revealed to also have a fundamental impact on the spatial distribution of GLUT4 within the plasma membrane, with the existence of two GLUT4 populations at the plasma membrane being defined: (1) as stationary clusters and (2) as diffusible monomers. In this model, in the absence of insulin, plasma membrane-fused GLUT4 are found to behave as clusters. These clusters are thought to arise from exocytic events that retain GLUT4 at their fusion sites; this has been proposed to function as an intermediate hub between GLUT4 exocytosis and re-internalisation. By contrast, insulin stimulation induces the dispersal of GLUT4 clusters into monomers and favours a distinct type of GLUT4-vesicle fusion event, known as fusion-with-release exocytosis. Here, we review how super-resolution microscopy approaches have allowed investigation of the characteristics of plasma membrane-fused GLUT4 and further discuss regulatory step(s) involved in the GLUT4 dispersal machinery, introducing the scaffold protein EFR3 which facilitates localisation of phosphatidylinositol 4-kinase type IIIα (PI4KIIIα) to the cell surface. We consider how dispersal may be linked to the control of transporter activity, consider whether macro-organisation may be a widely used phenomenon to control proteins within the plasma membrane, and speculate on the origin of different forms of GLUT4-vesicle exocytosis.

EFR3 and phosphatidylinositol 4-kinase IIIα regulate insulin-stimulated glucose transport and GLUT4 dispersal in 3T3-L1 adipocytes

Insulin stimulates glucose transport in muscle and adipocytes. This is achieved by regulated delivery of intracellular glucose transporter (GLUT4)-containing vesicles to the plasma membrane where they dock and fuse, resulting in increased cell surface GLUT4 levels. Recent work identified a potential further regulatory step, in which insulin increases the dispersal of GLUT4 in the plasma membrane away from the sites of vesicle fusion. EFR3 is a scaffold protein that facilitates localization of phosphatidylinositol 4-kinase type IIIα to the cell surface. Here we show that knockdown of EFR3 or phosphatidylinositol 4-kinase type IIIα impairs insulin-stimulated glucose transport in adipocytes. Using direct stochastic reconstruction microscopy, we also show that EFR3 knockdown impairs insulin stimulated GLUT4 dispersal in the plasma membrane. We propose that EFR3 plays a previously unidentified role in controlling insulin-stimulated glucose transport by facilitating dispersal of GLUT4 within the plasma membrane.

GLUT3 inhibitor discovery through in silico ligand screening and in vivo validation in eukaryotic expression systems

The passive transport of glucose and related hexoses in human cells is facilitated by members of the glucose transporter family (GLUT, SLC2 gene family). GLUT3 is a high-affinity glucose transporter primarily responsible for glucose entry in neurons. Changes in its expression have been implicated in neurodegenerative diseases and cancer. GLUT3 inhibitors can provide new ways to probe the pathophysiological role of GLUT3 and tackle GLUT3-dependent cancers. Through in silico screening of an ~ 8 million compounds library against the inward- and outward-facing models of GLUT3, we selected ~ 200 ligand candidates. These were tested for in vivo inhibition of GLUT3 expressed in hexose transporter-deficient yeast cells, resulting in six new GLUT3 inhibitors. Examining their specificity for GLUT1-5 revealed that the most potent GLUT3 inhibitor (G3iA, IC₅₀ ~ 7 µM) was most selective for GLUT3, inhibiting less strongly only GLUT2 (IC₅₀ ~ 29 µM). None of the GLUT3 inhibitors affected GLUT5, three inhibited GLUT1 with equal or twofold lower potency, and four showed comparable or two- to fivefold better inhibition of GLUT4. G3iD was a pan-Class 1 GLUT inhibitor with the highest preference for GLUT4 (IC₅₀ ~ 3.9 µM). Given the prevalence of GLUT1 and GLUT3 overexpression in many cancers and multiple myeloma’s reliance on GLUT4, these GLUT3 inhibitors may discriminately hinder glucose entry into various cancer cells, promising novel therapeutic avenues in oncology.

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.

Targeting Colorectal Cancer with Conjugates of a Glucose Transporter Inhibitor and 5-Fluorouracil

Overexpression of glucose transporters (GLUTs) in colorectal cancer cells is associated with 5-fluorouracil (1, 5-FU) resistance and poor clinical outcomes. We designed and synthesized a novel GLUT-targeting drug conjugate, triggered by glutathione in the tumor microenvironment, that releases 5-FU and GLUTs inhibitor (phlorizin (2) and phloretin (3)). Using an orthotopic colorectal cancer mice model, we showed that the conjugate exhibited better antitumor efficacy than 5-FU, with much lower exposure of 5-FU during treatment and without significant side effects. Our study establishes a GLUT-targeting theranostic incorporating a disulfide linker between the targeting module and cytotoxic payload as a potential antitumor therapy.

Functional Expression of the Human Glucose Transporters GLUT2 and GLUT3 in Yeast Offers Novel Screening Systems for GLUT-Targeting Drugs

Human GLUT2 and GLUT3, members of the GLUT/SLC2 gene family, facilitate glucose transport in specific tissues. Their malfunction or misregulation is associated with serious diseases, including diabetes, metabolic syndrome, and cancer. Despite being promising drug targets, GLUTs have only a few specific inhibitors. To identify and characterize potential GLUT2 and GLUT3 ligands, we developed a whole-cell system based on a yeast strain deficient in hexose uptake, whose growth defect on glucose can be rescued by the functional expression of human transporters. The simplicity of handling yeast cells makes this platform convenient for screening potential GLUT2 and GLUT3 inhibitors in a growth-based manner, amenable to high-throughput approaches. Moreover, our expression system is less laborious for detailed kinetic characterization of inhibitors than alternative methods such as the preparation of proteoliposomes or uptake assays in Xenopus oocytes. We show that functional expression of GLUT2 in yeast requires the deletion of the extended extracellular loop connecting transmembrane domains TM1 and TM2, which appears to negatively affect the trafficking of the transporter in the heterologous expression system. Furthermore, single amino acid substitutions at specific positions of the transporter sequence appear to positively affect the functionality of both GLUT2 and GLUT3 in yeast. We show that these variants are sensitive to known inhibitors phloretin and quercetin, demonstrating the potential of our expression systems to significantly accelerate the discovery of compounds that modulate the hexose transport activity of GLUT2 and GLUT3.

Current understanding of glucose transporter 4 expression and functional mechanisms

Glucose is used aerobically and anaerobically to generate energy for cells. Glucose transporters (GLUTs) are transmembrane proteins that transport glucose across the cell membrane. Insulin promotes glucose utilization in part through promoting glucose entry into the skeletal and adipose tissues. This has been thought to be achieved through insulin-induced GLUT4 translocation from intracellular compartments to the cell membrane, which increases the overall rate of glucose flux into a cell. The insulin-induced GLUT4 translocation has been investigated extensively. Recently, significant progress has been made in our understanding of GLUT4 expression and translocation. Here, we summarized the methods and reagents used to determine the expression levels of Slc2a4 mRNA and GLUT4 protein, and GLUT4 translocation in the skeletal muscle, adipose tissues, heart and brain. Overall, a variety of methods such real-time polymerase chain reaction, immunohistochemistry, fluorescence microscopy, fusion proteins, stable cell line and transgenic animals have been used to answer particular questions related to GLUT4 system and insulin action. It seems that insulin-induced GLUT4 translocation can be observed in the heart and brain in addition to the skeletal muscle and adipocytes. Hormones other than insulin can induce GLUT4 translocation. Clearly, more studies of GLUT4 are warranted in the future to advance of our understanding of glucose homeostasis.

Structure guided design and synthesis of furyl thiazolidinedione derivatives as inhibitors of GLUT 1 and GLUT 4, and evaluation of their anti-leukemic potential

Cancer cells increase their glucose uptake and glycolytic activity to meet the high energy requirements of proliferation. Glucose transporters (GLUTs), which facilitate the transport of glucose and related hexoses across the cell membrane, play a vital role in tumor cell survival and are overexpressed in various cancers. GLUT1, the most overexpressed GLUT in many cancers, is emerging as a promising anti-cancer target. To develop GLUT1 inhibitors, we rationally designed, synthesized, structurally characterized, and biologically evaluated in-vitro and in-vivo a novel series of furyl-2-methylene thiazolidinediones (TZDs). Among 25 TZDs tested, F18 and F19 inhibited GLUT1 most potently (IC50 11.4 and 14.7 μM, respectively). F18 was equally selective for GLUT4 (IC50 6.8 μM), while F19 was specific for GLUT1 (IC50 152 μM in GLUT4). In-silico ligand docking studies showed that F18 interacted with conserved residues in GLUT1 and GLUT4, while F19 had slightly different interactions with the transporters. In in-vitro antiproliferative screening of leukemic/lymphoid cells, F18 was most lethal to CEM cells (CC50 of 1.7 μM). Flow cytometry analysis indicated that F18 arrested cell cycle growth in the subG0-G1 phase and lead to cell death due to necrosis and apoptosis. Western blot analysis exhibited alterations in cell signaling proteins, consistent with cell growth arrest and death. In-vivo xenograft study in a CEM model showed that F18 impaired tumor growth significantly.

Permuted 2,4-thiazolidinedione (TZD) analogs as GLUT inhibitors and their in-vitro evaluation in leukemic cells

Cancer is a heterogeneous disease, and its treatment requires the identification of new ways to thwart tumor cells. Amongst such emerging targets are glucose transporters (GLUTs, SLC2 family), which are overexpressed by almost all types of cancer cells; their inhibition provides a strategy to disrupt tumor metabolism selectively, leading to antitumor effects. Here, novel thiazolidinedione (TZD) derivatives were designed, synthesized, characterized, and evaluated for their GLUT1, GLUT4, and GLUT5 inhibitory potential, followed by in-vitro cytotoxicity determination in leukemic cell lines. Compounds G5, G16, and G17 inhibited GLUT1, with IC₅₀ values of 5.4 ± 1.3, 26.6 ± 1.8, and 12.6 ± 1.2 μM, respectively. G17 was specific for GLUT1, G16 inhibited GLUT4 (IC₅₀ = 21.6 ± 4.5 μM) comparably but did not affect GLUT5. The most active compound, G5, inhibited all three GLUT types, with GLUT4 IC₅₀ = 9.5 ± 2.8 μM, and GLUT5 IC₅₀ = 34.5 ± 2.4 μM. Docking G5, G16, and G17 to the inward- and outward-facing structural models of GLUT1 predicted ligand binding affinities consistent with the kinetic inhibition data and implicated E380 and W388 of GLUT1 vs. their substitutions in GLUT5 (A388 and A396, respectively) in inhibitor preference for GLUT1. G5 inhibited the proliferation of leukemia CEM cells at low micromolar range (IC₅₀ = 13.4 μM) while being safer for normal blood cells. Investigation of CEM cell cycle progression after treatment with G5 showed that cells accumulated in the G2/M phase. Flow cytometric apoptosis studies revealed that compound G5 induced both early and late-stage apoptosis in CEM cells.

Identification and characterisation of two high-affinity glucose transporters from the spoilage yeast Brettanomyces bruxellensis

The yeast Brettanomyces bruxellensis (syn. Dekkera bruxellensis) is an emerging and undesirable contaminant in industrial low-sugar ethanol fermentations that employ the yeast Saccharomyces cerevisiae. High-affinity glucose import in B. bruxellensis has been proposed to be the mechanism by which this yeast can outcompete S. cerevisiae. The present study describes the characterization of two B. bruxellensis genes (BHT1 and BHT3) believed to encode putative high-affinity glucose transporters. In vitro-generated transcripts of both genes as well as the S. cerevisiae HXT7 high-affinity glucose transporter were injected into Xenopus laevis oocytes and subsequent glucose uptake rates were assayed using 14C-labelled glucose. At 0.1 mM glucose, Bht1p was shown to transport glucose five times faster than Hxt7p. pH affected the rate of glucose transport by Bht1p and Bht3p, indicating an active glucose transport mechanism that involves proton symport. These results suggest a possible role for BHT1 and BHT3 in the competitive ability of B. bruxellensis.

Discovery of novel N-substituted thiazolidinediones (TZDs) as HDAC8 inhibitors: in-silico studies, synthesis, and biological evaluation

Epigenetics plays a fundamental role in cancer progression, and developing agents that regulate epigenetics is crucial for cancer management. Among Class I and Class II HDACs, HDAC8 is one of the essential epigenetic players in cancer progression. Therefore, we designed, synthesized, purified, and structurally characterized novel compounds containing N-substituted TZD (P1-P25). Cell viability assay of all compounds on leukemic cell lines (CEM, K562, and KCL22) showed the cytotoxic potential of P8, P9, P10, P12, P19, and P25. In-vitro screening of different HDACs isoforms revealed that P19 was the most potent and selective inhibitor for HDAC8 (IC50 - 9.3 μM). Thermal shift analysis (TSA) confirmed the binding of P19 to HDAC8. In-vitro screening of all compounds on the transport activity of GLUT1, GLUT4, and GLUT5 indicated that P19 inhibited GLUT1 (IC50 - 28.2 μM). P10 and P19 induced apoptotic cell death in CEM cells (55.19% and 60.97% respectively) and P19 was less cytotoxic on normal WBCs (CC50 - 104.2 μM) and human fibroblasts (HS27) (CC50 - 105.0 μM). Thus, among this novel series of TZD derivatives, compound P19 was most promising HDAC8 inhibitor and cytotoxic on leukemic cells. Thus, P19 could serve as a lead for further development of optimized molecules with enhanced selectivity and potency.

Insulin stimulated GLUT4 translocation - Size is not everything!

Insulin-regulated trafficking of the facilitative glucose transporter GLUT4 has been studied in many cell types. The translocation of GLUT4 from intracellular membranes to the cell surface is often described as a highly specialised form of membrane traffic restricted to certain cell types such as fat and muscle, which are the major storage depots for insulin-stimulated glucose uptake. Here, we discuss evidence that favours the argument that rather than being restricted to specialised cell types, the machinery through which insulin regulates GLUT4 traffic is present in all cell types. This is an important point as it provides confidence in the use of experimentally tractable model systems to interrogate the trafficking itinerary of GLUT4.

Solute carrier transporters: the metabolic gatekeepers of immune cells

Solute carrier (SLC) transporters meditate many essential physiological functions, including nutrient uptake, ion influx/efflux, and waste disposal. In its protective role against tumors and infections, the mammalian immune system coordinates complex signals to support the proliferation, differentiation, and effector function of individual cell subsets. Recent research in this area has yielded surprising findings on the roles of solute carrier transporters, which were discovered to regulate lymphocyte signaling and control their differentiation, function, and fate by modulating diverse metabolic pathways and balanced levels of different metabolites. In this review, we present current information mainly on glucose transporters, amino-acid transporters, and metal ion transporters, which are critically important for mediating immune cell homeostasis in many different pathological conditions.

Plant-derived glucose transport inhibitors with potential antitumor activity

Glucose, a key nutrient utilized by human cells to provide cellular energy and a carbon source for biomass synthesis, is internalized in cells via glucose transporters that regulate glucose homeostasis throughout the human body. Glucose transporters have been used as important targets for the discovery of new drugs to treat cancer, diabetes, and heart disease, owing to their abnormal expression during these disease conditions. Thus far, several glucose transport inhibitors have been used in clinical trials, and increasing numbers of natural products have been characterized as potential anticancer agents targeting glucose transport. The present review focuses on natural product glucose transport inhibitors of plant origin, including alkaloids, flavonoids and other phenolic compounds, and isoprenoids, with their potential antitumor properties also discussed.

Fructose and prostate cancer: toward an integrated view of cancer cell metabolism

Activation of glucose transporter-1 (Glut-1) gene expression is a molecular feature of cancer cells that increases glucose uptake and metabolism. Increased glucose uptake is the basis for the clinical localization of primary tumors using positron emission tomography (PET) and 2-deoxy-2-[18F]-fluoro-D-glucose (FDG) as a radiotracer. However, previous studies have demonstrated that a considerable number of cancers, which include prostate cancer (CaP), express low to undetectable levels of Glut-1 and that FDG-PET has limited clinical applicability in CaP. This observation could be explained by a low metabolic activity of CaP cells that may be overcome using different hexoses, such as fructose, as the preferred energy source. However, these hypotheses have not been examined critically in CaP. This review article summarizes what is currently known about transport and metabolism of hexoses, and more specifically fructose, in CaP and provides experimental evidences indicating that CaP cells may have increased capacity to transport and metabolize fructose in vitro and in vivo. Moreover, this review highlights recent findings that allow better understanding of how metabolism of fructose may regulate cancer cell proliferation and how fructose uptake and metabolism, through the de novo lipogenesis pathway, may provide new opportunities for CaP early diagnosis, staging, and treatment.

A Growth-Based Screening System for Hexose Transporters in Yeast

As the simplest eukaryotic model system, the unicellular yeast Saccharomyces cerevisiae is ideally suited for quick and simple functional studies as well as for high-throughput screening. We generated a strain deficient for all endogenous hexose transporters, which has been successfully used to clone, characterize, and engineer carbohydrate transporters from different source organisms. Here we present basic protocols for handling this strain and characterizing sugar transporters heterologously expressed in it.

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.

Establishing a yeast-based screening system for discovery of human GLUT5 inhibitors and activators

Human GLUT5 is a fructose-specific transporter in the glucose transporter family (GLUT, SLC2 gene family). Its substrate-specificity and tissue-specific expression make it a promising target for treatment of diabetes, metabolic syndrome and cancer, but few GLUT5 inhibitors are known. To identify and characterize potential GLUT5 ligands, we developed a whole-cell system based on a yeast strain deficient in fructose uptake, in which GLUT5 transport activity is associated with cell growth in fructose-based media or assayed by fructose uptake in whole cells. The former method is convenient for high-throughput screening of potential GLUT5 inhibitors and activators, while the latter enables detailed kinetic characterization of identified GLUT5 ligands. We show that functional expression of GLUT5 in yeast requires mutations at specific positions of the transporter sequence. The mutated proteins exhibit kinetic properties similar to the wild-type transporter and are inhibited by established GLUT5 inhibitors N-[4-(methylsulfonyl)-2-nitrophenyl]-1,3-benzodioxol-5-amine (MSNBA) and (−)-epicatechin-gallate (ECG). Thus, this system has the potential to greatly accelerate the discovery of compounds that modulate the fructose transport activity of GLUT5.

GLUT4 Is Not Necessary for Overload-Induced Glucose Uptake or Hypertrophic Growth in Mouse Skeletal Muscle

GLUT4 is necessary for acute insulin- and contraction-induced skeletal muscle glucose uptake, but its role in chronic muscle loading (overload)-induced glucose uptake is unknown. Our goal was to determine whether GLUT4 is required for overload-induced glucose uptake. Overload was induced in mouse plantaris muscle by unilateral synergist ablation. After 5 days, muscle weights and ex vivo [³H]-2-deoxy-d-glucose uptake were assessed. Overload-induced muscle glucose uptake and hypertrophic growth were not impaired in muscle-specific GLUT4 knockout mice, demonstrating that GLUT4 is not necessary for these processes. To assess which transporters mediate overload-induced glucose uptake, chemical inhibitors were used. The facilitative GLUT inhibitor cytochalasin B, but not the sodium-dependent glucose cotransport inhibitor phloridzin, prevented overload-induced uptake demonstrating that GLUTs mediate this effect. To assess which GLUT, hexose competition experiments were performed. Overload-induced [³H]-2-deoxy-d-glucose uptake was not inhibited by d-fructose, demonstrating that the fructose-transporting GLUT2, GLUT5, GLUT8, and GLUT12 do not mediate this effect. To assess additional GLUTs, immunoblots were performed. Overload increased GLUT1, GLUT3, GLUT6, and GLUT10 protein levels twofold to fivefold. Collectively, these results demonstrate that GLUT4 is not necessary for overload-induced muscle glucose uptake or hypertrophic growth and suggest that GLUT1, GLUT3, GLUT6, and/or GLUT10 mediate overload-induced glucose uptake.

A multiscale study of the role of dynamin in the regulation of glucose uptake

Cells- and organisms-on-a-chip strategies were used to highlight the role of the molecular motor dynamin in regulating the translocation of specific glucose transporters.

Glucose uptake saturation explains glucose kinetics profiles measured by different tests

It is known that for a given insulin level glucose clearance depends on glucose concentration. However, a quantitative representation of the concomitant effects of hyperinsulinemia and hyperglycemia on glucose clearance, necessary to describe heterogeneous tests such as euglycemic and hyperglycemic clamps and oral tests, is lacking. Data from five studies (123 subjects) using a glucose tracer and including all the above tests in normal and diabetic subjects were collected. A mathematical model was developed in which glucose utilization was represented as a Michaelis-Menten function of glucose with constant Km and insulin-controlled Vmax, consistently with the basic notions of glucose transport. Individual values for the model parameters were estimated using a population approach. Tracer data were accurately fitted in all tests. The estimated Km was 3.88 (2.83-5.32) mmol/l [median (interquartile range)]. Median model-derived glucose clearance at 600 pmol/l insulin was reduced from 246 to 158 ml·min(-1)·m(-2) when glucose was raised from 5 to 10 mmol/l. The model reproduced the characteristic lack of increase in glucose clearance when moderate hyperinsulinemia was accompanied by hyperglycemia. In all tests, insulin sensitivity was inversely correlated with BMI, as expected (R(2) = 0.234, P = 0.0001). In conclusion, glucose clearance in euglycemic and hyperglycemic clamps and oral tests can be described with a unifying model, consistent with the notions of glucose transport and able to reproduce the suppression of glucose clearance due to hyperglycemia observed in previous studies. The model may be important for the design of reliable glucose homeostasis simulators.

Local Anesthetics and Antipsychotic Phenothiazines Interact Nonspecifically with Membranes and Inhibit Hexose Transporters in Yeast

Action mechanisms of anesthetics remain unclear because of difficulty in explaining how structurally different anesthetics cause similar effects. In Saccharomyces cerevisiae, local anesthetics and antipsychotic phenothiazines induced responses similar to those caused by glucose starvation, and they eventually inhibited cell growth. These drugs inhibited glucose uptake, but additional glucose conferred resistance to their effects; hence, the primary action of the drugs is to cause glucose starvation. In hxt(0) strains with all hexose transporter (HXT) genes deleted, a strain harboring a single copy of HXT1 (HXT1s) was more sensitive to tetracaine than a strain harboring multiple copies (HXT1m), which indicates that quantitative reduction of HXT1 increases tetracaine sensitivity. However, additional glucose rather than the overexpression of HXT1/2 conferred tetracaine resistance to wild-type yeast; therefore, Hxts that actively transport hexoses apparently confer tetracaine resistance. Additional glucose alleviated sensitivity to local anesthetics and phenothiazines in the HXT1m strain but not the HXT1s strain; thus, the glucose-induced effects required a certain amount of Hxt1. At low concentrations, fluorescent phenothiazines were distributed in various membranes. At higher concentrations, they destroyed the membranes and thereby delocalized Hxt1-GFP from the plasma membrane, similar to local anesthetics. These results suggest that the aforementioned drugs affect various membrane targets via nonspecific interactions with membranes. However, the drugs preferentially inhibit the function of abundant Hxts, resulting in glucose starvation. When Hxts are scarce, this preference is lost, thereby mitigating the alleviation by additional glucose. These results provide a mechanism that explains how different compounds induce similar effects based on lipid theory.

Facilitative glucose transporters: Implications for cancer detection, prognosis and treatment

It is long recognized that cancer cells display increased glucose uptake and metabolism. In a rate-limiting step for glucose metabolism, the glucose transporter (GLUT) proteins facilitate glucose uptake across the plasma membrane. Fourteen members of the GLUT protein family have been identified in humans. This review describes the major characteristics of each member of the GLUT family and highlights evidence of abnormal expression in tumors and cancer cells. The regulation of GLUTs by key proliferation and pro-survival pathways including the phosphatidylinositol 3-kinase (PI3K)-Akt, hypoxia-inducible factor-1 (HIF-1), Ras, c-Myc and p53 pathways is discussed. The clinical utility of GLUT expression in cancer has been recognized and evidence regarding the use of GLUTs as prognostic or predictive biomarkers is presented. GLUTs represent attractive targets for cancer therapy and this review summarizes recent studies in which GLUT1, GLUT3, GLUT5 and others are inhibited to decrease cancer growth.

Expression of SGLT1 in Human Hearts and Impairment of Cardiac Glucose Uptake by Phlorizin during Ischemia-Reperfusion Injury in Mice

Objective

Sodium-glucose cotransporter 1 (SGLT1) is thought to be expressed in the heart as the dominant isoform of cardiac SGLT, although more information is required to delineate the subtypes of SGLTs in human hearts. Moreover, the functional role of SGLTs in the heart remains to be fully elucidated. We herein investigated whether SGLT1 is expressed in human hearts and whether SGLTs significantly contribute to cardiac energy metabolism during ischemia-reperfusion injury (IRI) via enhanced glucose utilization in mice.

Methods and Results

We determined that SGLT1 was highly expressed in both human autopsied hearts and murine perfused hearts, as assessed by immunostaining and immunoblotting with membrane fractionation. To test the functional significance of the substantial expression of SGLTs in the heart, we studied the effects of a non-selective SGLT inhibitor, phlorizin, on the baseline cardiac function and its response to ischemia-reperfusion using the murine Langendorff model. Although phlorizin perfusion did not affect baseline cardiac function, its administration during IRI significantly impaired the recovery in left ventricular contractions and rate pressure product, associated with an increased infarct size, as demonstrated by triphenyltetrazolium chloride staining and creatine phosphokinase activity released into the perfusate. The onset of ischemic contracture, which indicates the initiation of ATP depletion in myocardium, was earlier with phlorizin. Consistent with this finding, there was a significant decrease in the tissue ATP content associated with reductions in glucose uptake, as well as lactate output (indicating glycolytic flux), during ischemia-reperfusion in the phlorizin-perfused hearts.

Conclusions

Cardiac SGLTs, possibly SGLT1 in particular, appear to provide an important protective mechanism against IRI by replenishing ATP stores in ischemic cardiac tissues via enhancing availability of glucose. The present findings provide new insight into the significant role of SGLTs in optimizing cardiac energy metabolism, at least during the acute phase of IRI.

Mercuric compounds induce pancreatic islets dysfunction and apoptosis in vivo

Mercury is a toxic heavy metal that is an environmental and industrial pollutant throughout the world. Mercury exposure leads to many physiopathological injuries in mammals. However, the precise toxicological effects of mercury on pancreatic islets in vivo are still unclear. Here, we investigated whether mercuric compounds can induce dysfunction and damage in the pancreatic islets of mice, as well as the possible mechanisms involved in this process. Mice were treated with methyl mercuric chloride (MeHgCl, 2 mg/kg) and mercuric chloride (HgCl₂, 5 mg/kg) for more than 2 consecutive weeks. Our results showed that the blood glucose levels increased and plasma insulin secretions decreased in the mice as a consequence of their exposure. A significant number of TUNEL-positive cells were revealed in the islets of mice that were treated with mercury for 2 consecutive weeks, which was accompanied by changes in the expression of the mRNA of anti-apoptotic (Bcl-2, Mcl-1, and Mdm-2) and apoptotic (p53, caspase-3, and caspase-7) genes. Moreover, plasma malondialdehyde (MDA) levels increased significantly in the mice after treatment with mercuric compounds for 2 consecutive weeks, and the generation of reactive oxygen species (ROS) in the pancreatic islets also markedly increased. In addition, the mRNA expression of genes related to antioxidation, including Nrf2, GPx, and NQO1, were also significantly reduced in these islets. These results indicate that oxidative stress injuries that are induced by mercuric compounds can cause pancreatic islets dysfunction and apoptosis in vivo.

Functional role of sodium glucose transporter in high glucose-mediated angiotensin type 1 receptor downregulation in human proximal tubule cells

Previously, we have demonstrated human angiotensin type 1 receptor (hAT(1)R) promoter architecture with regard to the effect of high glucose (25 mM)-mediated transcriptional repression in human proximal tubule epithelial cells (hPTEC; Thomas BE, Thekkumkara TJ. Mol Biol Cell 15: 4347-4355, 2004). In the present study, we investigated the role of glucose transporters in high glucose-mediated hAT(1)R repression in primary hPTEC. Cells were exposed to normal glucose (5.5 mM) and high glucose (25 mM), followed by determination of hyperglycemia-mediated changes in receptor expression and glucose transporter activity. Exposure of cells to high glucose resulted in downregulation of ANG II binding (4,034 ± 163.3 to 1,360 ± 154.3 dpm/mg protein) and hAT(1)R mRNA expression (reduced 60.6 ± 4.643%) at 48 h. Under similar conditions, we observed a significant increase in glucose uptake (influx) in cells exposed to hyperglycemia. Our data indicated that the magnitude of glucose influx is concentration and time dependent. In euglycemic cells, inhibiting sodium-glucose cotransporters (SGLTs) with phlorizin and facilitative glucose transporters (GLUTs) with phloretin decreased glucose influx by 28.57 ± 0.9123 and 54.33 ± 1.202%, respectively. However, inhibiting SGLTs in cells under hyperglycemic conditions decreased glucose influx by 53.67 ± 2.906%, while GLUT-mediated glucose uptake remained unaltered (57.67 ± 3.180%). Furthermore, pretreating cells with an SGLT inhibitor reversed high glucose-mediated downregulation of the hAT(1)R, suggesting an involvement of SGLT in high glucose-mediated hAT(1)R repression. Our results suggest that in hPTEC, hyperglycemia-induced hAT(1)R downregulation is largely mediated through SGLT-dependent glucose influx. As ANG II is an important modulator of hPTEC transcellular sodium reabsorption and function, glucose-mediated changes in hAT(1)R gene expression may participate in the pathogenesis of diabetic renal disease.

Reduction of glucose uptake through inhibition of hexose transporters and enhancement of their endocytosis by methylglyoxal in Saccharomyces cerevisiae

Diabetes mellitus is characterized by an impairment of glucose uptake even though blood glucose levels are increased. Methylglyoxal is derived from glycolysis and has been implicated in the development of diabetes mellitus, because methylglyoxal levels in blood and tissues are higher in diabetic patients than in healthy individuals. However, it remains to be elucidated whether such factors are a cause, or consequence, of diabetes. Here, we show that methylglyoxal inhibits the activity of mammalian glucose transporters using recombinant Saccharomyces cerevisiae cells genetically lacking all hexose transporters but carrying cDNA for human GLUT1 or rat GLUT4. We found that methylglyoxal inhibits yeast hexose transporters also. Glucose uptake was reduced in a stepwise manner following treatment with methylglyoxal, i.e. a rapid reduction within 5 min, followed by a slow and gradual reduction. The rapid reduction was due to the inhibitory effect of methylglyoxal on hexose transporters, whereas the slow and gradual reduction seemed due to endocytosis, which leads to a decrease in the amount of hexose transporters on the plasma membrane. We found that Rsp5, a HECT-type ubiquitin ligase, is responsible for the ubiquitination of hexose transporters. Intriguingly, Plc1 (phospholipase C) negatively regulated the endocytosis of hexose transporters in an Rsp5-dependent manner, although the methylglyoxal-induced endocytosis of hexose transporters occurred irrespective of Plc1. Meanwhile, the internalization of hexose transporters following treatment with methylglyoxal was delayed in a mutant defective in protein kinase C.

Antioxidant properties of rare sugar D-allose: Effects on mitochondrial reactive oxygen species production in Neuro2A cells

The anti-oxidative activity of the rare sugar D-allose has recently been reported, but the mechanism is largely unclear. In this study, we evaluated the reactive oxygen species (ROS) scavenging activities of D-allose and then examined the effects of D-allose on ROS production in mitochondria to clarify the antioxidant properties of D-allose. While D-allose did not scavenge hydrogen peroxide and superoxide anions, it eliminated hydroxyl radicals to the same extent as D-glucose. Rotenone, an uncoupler of mitochondrial respiratory complex I, induces ROS production in mouse neuroblastoma Neuro2A cells in the presence of D-glucose. However, in the presence of D-allose, there was no change in the ROS levels in Neuro2A cells following rotenone treatment. Furthermore, treatment with D-allose attenuated the D-glucose-dependent ROS generation induced by rotenone. Whereas treatment with D-glucose enhanced ATP synthesis in Neuro2A cells, D-allose was less effective in producing intracellular ATP than D-glucose. Treatment with D-allose inhibited the ATP synthesis stimulated by D-glucose. These results suggest that D-allose suppresses ROS production in the mitochondria due to competition with D-glucose at the cellular level.

Functional properties and genomics of glucose transporters

Glucose is the major energy source for mammalian cells as well as an important substrate for protein and lipid synthesis. Mammalian cells take up glucose from extracellular fluid into the cell through two families of structurallyrelated glucose transporters. The facilitative glucose transporter family (solute carriers SLC2A, protein symbol GLUT) mediates a bidirectional and energy-independent process of glucose transport in most tissues and cells, while the NaM(+)/glucose cotransporter family (solute carriers SLC5A, protein symbol SGLT) mediates an active, Na(+)-linked transport process against an electrochemical gradient. The GLUT family consists of thirteen members (GLUT1-12 and HMIT). Phylogenetically, the members of the GLUT family are split into three classes based on protein similarities. Up to now, at least six members of the SGLT family have been cloned (SGLT1-6). In this review, we report both the genomic structure and function of each transporter as well as intra-species comparative genomic analysis of some of these transporters. The affinity for glucose and transport kinetics of each transporter differs and ranges from 0.2 to 17mM. The ability of each protein to transport alternative substrates also differs and includes substrates such as fructose and galactose. In addition, the tissue distribution pattern varies between species. There are different regulation mechanisms of these transporters. Characterization of transcriptional control of some of the gene promoters has been investigated and alternative promoter usage to generate different protein isoforms has been demonstrated. We also introduce some pathophysiological roles of these transporters in human.

Multiple expression of glucose transporters in the lateral wall of the cochlear duct studied by quantitative real-time PCR assay

We have investigated the gene expression of the facilitated glucose transporter (GLUT), H+-coupled myo-inositol cotransporter (HMIT), and Na+ glucose cotransporter (SGLT) in the lateral wall of the cochlear duct by conventional RT-PCR and quantitative real-time PCR. The isoforms GLUT1, -3, -4, -5, -8, -10, -12 and HMIT were detected in both the stria vascularis and the spiral ligament, whereas no SGLT isoforms could be detected in these tissues. Quantitative real-time PCR analysis revealed significant differences in the gene expression of GLUT1, -4, -5, -10, and HMIT isoforms between the stria vascularis and the spiral ligament. This result reflects the tissue-dependent distributions of GLUT isoforms. These findings strongly suggest that a number of GLUT isoforms participate in glucose transport in the stria vascularis and the spiral ligament.

Insulin-regulated trafficking of GLUT4 requires ubiquitination

A major consequence of insulin binding its receptor on fat and muscle cells is translocation of the facilitative glucose transporter GLUT4 from an intracellular store to the cell surface where it serves to clear glucose from the bloodstream. Sorting of GLUT4 into its insulin-sensitive store requires the GGA [Golgi-localized, γ-ear-containing, ADP ribosylation factor (ARF)-binding] adaptor proteins, but the signal on GLUT4 to direct this sorting step is unknown. Here, we have identified a role for ubiquitination of GLUT4 in this process. We demonstrate that GLUT4 is ubiquitinated in 3T3-L1 adipocytes, and that a ubiquitin-resistant version fails to translocate to the cell surface of these cells in response to insulin. Our data support a model in which ubiquitination acts as a signal for the trafficking of GLUT4 from the endosomal/trans-Golgi network (TGN) system into its intracellular storage compartment, from where it is mobilized to the cell surface in response to insulin.

The protein family of glucose transport facilitators: It's not only about glucose after all

The protein family of facilitative glucose transporters comprises 14 isoforms that share common structural features such as 12 transmembrane domains, N- and C-termini facing the cytoplasm of the cell, and a N-glycosylation side either within the first or fifth extracellular loop. Based on their sequence homology, three classes can be distinguished: class I includes GLUT1-4 and GLUT14, class II the "odd transporters" GLUT5, 7, 9, 11, and class III the "even transporters" GLUT6, 8, 10, 12 and the proton driven myoinositol transporter HMIT (or GLUT13). With the cloning and characterization of the more recent class II and III isoforms, it became apparent that despite their structural similarities, the different isoforms not only show a distinct tissue-specific expression pattern but also show distinct characteristics such as alternative splicing, specific (sub)cellular localization, and affinities for a spectrum of substrates. This review summarizes the current understanding of the physiological role for the various transport facilitators based on human genetically inherited disorders or single-nucleotide polymorphisms and knockout mice models. The emphasis of the review will be on the potential functional role of the more recent isoforms.

GLUT1 as a therapeutic target in hepatocellular carcinoma

Primary hepatocellular carcinoma (HCC) is one of the most fatal cancers in humans with rising incidence in many regions around the world. Currently, no satisfactory curative pharmacological treatment is available, and the outcome is mostly poor. Recently, we have shown that the glucose transporter GLUT1 is increased in a subset of patients with HCC and functionally affects tumorigenicity. GLUT1 is a rate-limiting transporter for glucose uptake, and its expression correlates with anaerobic glycolysis. This phenomenon is also known as the Warburg effect and recently became of great interest, since it affects not only glucose uptake and utilization but also has an influence on tumorigenic features like metastasis, chemoresistance and escape from immune surveillance. Consistent with this, RNA-interference-mediated inhibition of GLUT1 expression in HCC cells resulted in reduced tumorigenicity. Together, these findings indicate that GLUT1 is a novel and attractive therapeutic target for HCC. This review summarizes our current knowledge on the expression and function of GLUT1 in HCC, available drugs/strategies to inhibit GLUT1 expression or function, and potential side effects of such therapeutic strategies.

The Glut1 and Glut4 glucose transporters are differentially expressed during perinatal and postnatal erythropoiesis

Glucose is a major source of energy for living organisms, and its transport in vertebrates is a universally conserved property. Of all cell lineages, human erythrocytes express the highest level of the Glut1 glucose transporter with more than 200 000 molecules per cell. However, we recently reported that erythrocyte Glut1 expression is a specific trait of vitamin C–deficient mammalian species, comprising only higher primates, guinea pigs, and fruit bats. Here, we show that in all other tested mammalian species, Glut1 was transiently expressed in erythrocytes during the neonatal period. Glut1 was up-regulated during the erythroblast stage of erythroid differentiation and was present on the vast majority of murine red blood cells (RBCs) at birth. Notably though, Glut1 was not induced in adult mice undergoing anemia-induced erythropoiesis, and under these conditions, the up-regulation of a distinct transporter, Glut4, was responsible for an increased glucose transport. Sp3 and Sp1 transcriptions factors have been proposed to regulate Glut1 transcription, and we find that the concomitant repression of Glut1 and induction of Glut4 was associated with a significantly augmented Sp3/Sp1 ratio. Glucose transporter expression patterns in mice and human erythrocytes are therefore distinct. In mice, there is a postnatal switch from Glut1 to Glut4, with Glut4 further up-regulated under anemic conditions.

Channels for water efflux and influx involved in volume regulation of murine spermatozoa

The nature of the membrane channels mediating water transport in murine spermatozoa adjusting to anisotonic conditions was investigated. The volume of spermatozoa subjected to physiologically relevant hypotonic conditions either simultaneously, or after isotonic pre-incubation, with putative water transport inhibitors was monitored. Experiments in which quinine prevented osmolyte efflux, and thus regulatory volume decrease (RVD), revealed whether water influx or efflux was being inhibited. There was no evidence that sodium-dependent solute transporters or facilitative glucose transporters were involved in water transport during RVD of murine spermatozoa since phloretin, cytochalasin B and phloridzin had no effect on volume regulation. However, there was evidence that Hg(2+)- and Ag(+)-sensitive channels were involved in water transport and the possibility that they include aquaporin 8 is discussed. Toxic effects of these heavy metals were ruled out by evidence that mitochondrial poisons had no such effect on volume regulation.

Age related obesity-induced shortening of GLUT4 mRNA poly(A) tail length in rat gastrocnemius skeletal muscle

Obese insulin resistant animals and humans have shown reduced GLUT4 gene expression. Yet, in skeletal muscle, discrepancy between mRNA and protein regulation has been frequently observed, suggesting a post-transcriptional modulation. We investigated the GLUT4 expression in adipose tissue and muscle of obese 12-month-old (12-mo) rats, comparing with lean 2-month-old (2-mo) animals. Obesity was accompanied by insulin resistance, and 65% reduction (P<0.01) in GLUT4 mRNA and protein in adipose tissue. However, in muscle, despite increased (P<0.05) mRNA content, GLUT4 protein was unchanged. RNase H and poly(A) test assays showed a reduction (P<0.01) of approximately 80 adenines in the GLUT4 mRNA poly(A) tail of muscle from 12-mo rats, recognizing that the poly(A) tail length correlates with translation efficiency. Concluding, age related obesity of 12-mo rats involves suppression of GLUT4 expression in adipose tissue; however, in muscle, GLUT4 mRNA content increases, but with a shorter poly(A) tail, thus unchanging the protein content.

Is non-insulin dependent glucose uptake a therapeutic alternative? Part 1: physiology, mechanisms and role of non insulin-dependent glucose uptake in type 2 diabetes

Several decades of research for treating type 2 diabetes have yielded new drugs but the actual experience with the available oral antidiabetic compounds clearly shows that therapeutic needs are not matched. This highlights the urgent need for exploring other pathways. All cell types have the capacity to take up glucose independently of insulin, whereby basal but also hyperglycaemia-promoted glucose supply is ensured. Although poorly explored, insulin-independent glucose uptake might nevertheless represent a therapeutic target, as an alternative to the clear limits of actual drug treatments. This review not only critically examines some major pathways not requiring insulin (although they may be influenced by the hormone) but importantly, this analysis extends to the clinical applicability of these potential therapeutic principles by also considering their predictable tolerability for long-term therapy. In particular vascular safety (the ultimate problem linked with diabetes) will be envisaged because of the ubiquitous distribution of glucose transporters and some linked mechanisms. Several mechanisms can be identified which do not require insulin for their functioning. The first part of this review deals with the description, the regulation and the limits of some mechanisms representing potential pharmacological targets capable of having a highly significant impact on glucose uptake. These selected topics are: a) unmasking and/or activation of glucose transporters in cell plasma membranes, b) insulin mimetics acting at postreceptor level, c) activation of AMPK, d) increasing nitric oxide and e) increasing glucose-6P and glycogen stores.

Glucose transport by English sparrow (Passer domesticus) skeletal muscle: have we been chirping up the wrong tree?

Glucose uptake by mammalian skeletal muscle has been extensively covered in the literature, whereas the uptake of glucose by avian skeletal muscle has yet to be examined. As skeletal muscle provides the majority of postprandial glucose uptake in mammals, this study was designed to characterize the glucose transport mechanisms and glycogen content of avian skeletal muscle. In addition, plasma glucose levels were measured. English sparrow extensor digitorum communis (EDC) skeletal muscles were used for this study to quantify in vitro radiolabeled-glucose uptake. Uptake of labeled glucose was shown to decrease in the presence of increasing unlabeled glucose and was maximal by 60 minutes of incubation. Various agents known to increase glucose transport in mammalian tissues, via the insulin and contraction-responsive pathways, were used to manipulate and characterize in vitro transport in birds. The typical effectors of the mammalian insulin pathway, insulin (2 ng/ml) and insulin-like growth factor-1 (48 ng/ml), did not increase skeletal muscle glucose transport. Likewise, inducers of the mammalian contraction-responsive pathway had no effect on glucose transport by in vitro avian skeletal muscle (5 mM caffeine, 2 mM AICAR (5'-aminoimidazole-4-carboxamide-1-b-D-ribofuranoside). Interestingly, 200 microM phloretin, an agent used to block glucose transport proteins, significantly inhibited its uptake (P<0.001). These results suggest that a glucose transporter is responsible for glucose uptake by avian skeletal muscle, albeit at unexpectedly low levels, considering the high plasma glucose concentrations (265.9+/-53.5 mg/dl) and low skeletal muscle glycogen content (9.1+/-4.11 nM glucose/mg) of English sparrows.

Effects of anti-GLUT antibodies on glucose transport into human erythrocyte ghosts

We have studied the effects of anti-GLUT1 antibodies on the uptake of glucose into erythrocytes. Glucose transport into human erythrocyte ghosts was measured directly using 3H-2-deoxy-glucose, or indirectly by monitoring associated volume changes using light scattering. The uptake of glucose was significantly inhibited in ghosts resealed in solutions containing specific antibodies against GLUT1. Such an effect was not observed when an antibody against the oestrogen receptor, lacking specificity towards GLUT1, was employed instead. The antibodies were also without effect on the efflux of preloaded glucose from erythrocyte ghosts. The demonstration that anti-GLUT antibodies can inhibit glucose uptake is support for the hypothesis that they exaggerate the cytoplasmic barrier to glucose uptake created by endofacial segments of GLUT1.

Effects of dietary monosaccharides on sympathetic nervous system activity in adipose tissues of male rats

Dietary carbohydrate activates the sympathetic nervous system (SNS). As the mechanisms underlying this response are not fully characterized, studies were undertaken to compare SNS responses to ingestion of glucose, fructose, and galactose. SNS activity was examined using techniques of [(3)H]norepinephrine ([(3)H]NE) turnover in brown and white fat. In addition, gene expression for several sympathetically related proteins was also analyzed in these tissues. [(3)H]NE turnover in interscapular brown adipose tissue (IBAT) and retroperitoneal fat increased in response to glucose and fructose in the diet, whereas [(3)H]NE turnover in epididymal fat did not respond to either monosaccharide. Galactose feeding, by contrast, decreased [(3)H]NE turnover in IBAT, but increased it in epididymal, though not retroperitoneal, fat. Expression of GLUT4 was more abundant in IBAT and retroperitoneal fat from glucose- and fructose-fed animals than from diet- or galactose-fed rats. Chemical sympathectomy abolished the GLUT4 response in retroperitoneal fat, but was without effect on GLUT4 in epididymal fat. These studies are consistent with activation of a neural pathway by oral glucose or fructose, leading to SNS activation in IBAT and retroperitoneal fat and enhanced GLUT4 expression.

The SLC2 family of facilitated hexose and polyol transporters

The SLC2 family of glucose and polyol transporters comprises 13 members, the glucose transporters (GLUT) 1-12 and the H(+)- myo-inositol cotransporter (HMIT). These proteins all contain 12 transmembrane domains with both the amino and carboxy-terminal ends located on the cytoplasmic side of the plasma membrane and a N-linked oligosaccharide side-chain located either on the first or fifth extracellular loop. Based on sequence comparison, the GLUT isoforms can be grouped into three classes: class I comprises GLUT1-4; class II, GLUT6, 8, 10, and 12 and class III, GLUT5, 7, 9, 11 and HMIT. Despite their sequence similarity and the presence of class-specific signature sequences, these transporters carry various hexoses and HMIT is a H(+)/ myo-inositol co-transporter. Furthermore, the substrate transported by some isoforms has not yet been identified. Tissue- and cell-specific expression of the well-characterized GLUT isoforms underlies their specific role in the control of whole-body glucose homeostasis. Numerous studies with transgenic or knockout mice indeed support an important role for these transporters in the control of glucose utilization, glucose storage and glucose sensing. Much remains to be learned about the transport functions of the recently discovered isoforms (GLUT6-13 and HMIT) and their physiological role in the metabolism of glucose, myo-inositol and perhaps other substrates.

Functional characterisation of glucose transport in bovine articular chondrocytes

The adequate provision of glucose to articular chondrocytes is essential to sustain their predominantly anaerobic metabolism; glucose is also a precursor for the extracellular matrix macromolecules which these cells synthesise. Impaired glucose uptake would compromise cell function and potentially result in an imbalance of matrix synthesis and degradation, leading to osteoarthritis. We studied the glucose influx pathway into bovine articular chondrocytes using 2-deoxy- d-[(3)H]-glucose (DOG). Uptake occurs via an extracellular pH (pH(o))-insensitive, phloretin- and cytochalasin B-sensitive pathway, hallmarks of the GLUT family of facilitative glucose transporters, with a K(m) of 0.35+/-0.11 mM. Uptake was affected by a number of physiologically relevant factors: (1) raised hydrostatic pressure (1-30 MPa) inhibited DOG uptake by up to 30%; (2) interleukin-1 (IL-1beta) reduced uptake via an increase in transporter affinity; (3) glucosamine inhibited glucose uptake in a manner consistent with the actions of a competitive inhibitor. Given the involvement of IL-1beta in osteoarthritis and the protective role assigned to glucosamine, these findings implicate an important role for glucose delivery in chondrocyte energy production and matrix metabolism, which, therefore, may potentially affect the maintenance of cartilage integrity.

Intracellular transport of a heterologous membrane protein, the human transferrin receptor, in Saccharomyces cerevisiae

We have analyzed the intracellular behavior of the human transferrin receptor (TfR) in Saccharomyces cerevisiae. The major part of the heterologously expressed TfR, which has previously been used as a model for heterologous expression of membrane proteins in yeast, is localized in the endoplasmic reticulum (ER) membranes; a minor fraction is present in the plasma membrane (PM). The stability of the TfR depends on vacuolar proteases, implying that it is degraded in the vacuolar compartment. Degradation is further dependent on favorable transport conditions to this compartment. The main bottleneck of transport seems to be the transition from the ER to the PM. The chaperone Cne1p, which is involved in quality control in the ER, plays a role in regulating the amount of heterologous TfR, as deletion of CNE1 leads to significant accumulation of the protein. This is the first demonstration of the involvement of CNE1 in regulating the level of heterologous membrane proteins.

Effects of rearing temperature on sympathoadrenal activity in young adult rats

Animals reared at 18 degrees C exhibit enhanced innervation of brown adipose tissue (BAT) and greater cold tolerance as adults, yet gain more weight when fed an enriched diet compared with rats reared at 30 degrees C. To explore this paradox, sympathoadrenal activity was examined using techniques of [(3)H]norepinephrine ([(3)H]NE) turnover and urinary catecholamine excretion in male and female rats reared until 2 mo of age at 18 or 30 degrees C. Gene expression in BAT was also analyzed for several sympathetically related proteins. Although [(3)H]NE turnover in heart did not differ between groups, [(3)H]NE turnover in BAT was consistently elevated in the 18 degrees C-reared animals, even 2 mo after removal from the cool environment. Gene expression for uncoupling proteins 1 and 3, GLUT-4, leptin, and the alpha(1A)-adrenergic receptor was more abundant in BAT and the increase in epinephrine excretion with fasting suppressed in 18 degrees C-reared animals. These studies demonstrate that obesity consequent to exposure to 18 degrees C in early life occurs despite tonic elevation of sympathetic input to BAT. Diminished adrenal epinephrine responsiveness to fasting may play a contributory role.

Transepithelial glucose transport and metabolism in BeWo choriocarcinoma cells

Glucose transporters in the placental, epithelial syncytiotrophoblast barrier are asymmetrically arranged (microvillous>basal), leading to the hypothesis of a rate-limiting role for the basal membrane in transepithelial transport. This is significant since the changes which have been observed in basal membrane glucose transporter expression over gestation and in conditions such as diabetes would generate changes in maternal-to-foetal glucose transport. This study was designed to test whether the basal membrane of the syncytiotrophoblast is the rate-limiting step in transepithelial transport and to investigate the effects of metabolism on transpithelial transport. In the absence of a transporting syncytiotrophoblast monolayer, the BeWo choriocarcinoma cell line, derived from trophoblast and plated on a permeable support, was used as a model since it has an asymmetric distribution of glucose transporter activity, similar to the syncytiotrophoblast. Inhibition of basal membrane glucose transport with p -chloromercuribenzene-sulfonate (p CMBS) produced a proportional change in transepithelial transport, whereas this latter parameter was relatively insensitive to inhibition of microvillous membrane glucose transporters. These data demonstrate that the basal membrane is the rate-limiting step in transepithelial glucose transport. Experiments involving stimulation and inhibition of cellular glucose consumption demonstrated that there is a single intracellular glucose pool in BeWo cells, supplying both metabolism and transcellular transport.

Induction of ependymal, glial, and neuronal transactivation by intraventricular administration of the SGLT1 Na+-D-glucose cotransporter inhibitor phlorizin

Reports that food intake is stimulated by intracerebroventricular (i.c.v.) administration of the SGLT1 Na+-D-glucose cotransport inhibitor, phlorizin, suggest that decreased central glucose uptake is a stimulus for compensatory motor activity underlying restoration of energy imbalance. In order to identify central cell populations that are functionally responsive to decreased SGLT1 function in the brain, the present study utilized immunocytochemical techniques to demonstrate cellular expression of the inducible activator protein-1 transcription factor, Fos, following i.c.v. delivery of phlorizin. Groups of adult male rats were treated with phlorizin at a dose of 10, 50, or 250 microg/animal, then sacrificed 2 hr later by transcardial perfusion. Ependymal expression of Fos-immunoreactivity (-ir) was observed throughout most of the cerebroventricular system, except the medullary fourth ventricle, at each dose examined. Higher doses of the transport inhibitor elicited immunostaining of periventricular glia, characterized by cytoplasmic glial fibrillary acidic protein-ir, underlying the lateral, third, and rostral fourth ventricles and cerebral aqueduct. These doses also resulted in the transcriptional activation of neurons in discrete brain sites, including the rostral medial preoptic area, median preoptic n., preoptic and hypothalamic periventricular n., subfornical organ, thalamic medial habenular and paraventricular n., hypothalamic paraventricular, ventromedial, and arcuate n., and n. of the solitary tract. These results show that nonexcitable cells located throughout much of the central neuroaxis and discrete populations of neurons in the brain are genomically responsive to pharmacological inhibition of central SGLT1 function. Evidence for the functional responsiveness of these cell types to manipulation of energy-dependent glucose transport suggests that cellular uptake of this metabolic fuel may serve as an indicator of central energy substrate availability, and that alterations in glucose uptake via this specific mechanism may be the source of regulatory signals involved in the maintenance of energy homeostasis.