Distribution of glucose transporter messenger RNA transcripts in tissues of rat and man

Transcellular Barriers to Glucose Delivery in the Body

Glucose is the universal fuel of most mammalian cells, and it is largely replenished through dietary intake. Glucose availability to tissues is paramount for the maintenance of homeostatic energetics and, hence, supply should match demand by the consuming organs. In its journey through the body, glucose encounters cellular barriers for transit at the levels of the absorbing intestinal epithelial wall, the renal epithelium mediating glucose reabsorption, and the tight capillary endothelia (especially in the brain). Glucose transiting through these cellular barriers must escape degradation to ensure optimal glucose delivery to the bloodstream or tissues. The liver, which stores glycogen and generates glucose de novo, must similarly be able to release it intact to the circulation. We present the most up-to-date knowledge on glucose handling by the gut, liver, brain endothelium, and kidney, and discuss underlying molecular mechanisms and open questions. Diseases associated with defects in glucose delivery and homeostasis are also briefly addressed. We propose that the universal problem of sparing glucose from catabolism in favor of translocation across the barriers posed by epithelia and endothelia is resolved through common mechanisms involving glucose transfer to the endoplasmic reticulum, from where glucose exits the cells via unconventional cellular mechanisms.

Functional importance of glucose transporters and chromatin epigenetic factors in Glioblastoma Multiforme (GBM): possible therapeutics

Glioblastoma Multiforme (GBM) is an aggressive brain cancer affecting glial cells and is chemo- and radio-resistant. Glucose is considered the most vital energy source for cancer cell proliferation. During metabolism, hexose molecules will be transported into the cells via transmembrane proteins known as glucose transporter (GLUT). Among them, GLUT-1 and GLUT-3 play pivotal roles in glucose transport in GBM. Knockdown studies have established the role of GLUT-1, and GLUT-3 mediated glucose transport in GBM cells, providing insight into GLUT-mediated cancer signaling and cancer aggressiveness. This review focussed on the vital role of GLUT-1 and GLUT-3 proteins, which regulate glucose transport. Recent studies have identified the role of GLUT inhibitors in effective cancer prevention. Several of them are in clinical trials. Understanding and functional approaches towards glucose-mediated cell metabolism and chromatin epigenetics will provide valuable insights into the mechanism of cancer aggressiveness, cancer stemness, and chemo-resistance in Glioblastoma Multiforme (GBM). This review summarizes the role of GLUT inhibitors, micro-RNAs, and long non-coding RNAs that aid in inhibiting glucose uptake by the GBM cells and other cancer cells leading to the identification of potential therapeutic, prognostic as well as diagnostic markers. Furthermore, the involvement of epigenetic factors, such as microRNAs, in regulating glycolytic genes was demonstrated.

Decrease in glucose transporter 1 levels and translocation of glucose transporter 3 in the dentate gyrus of C57BL/6 mice and gerbils with aging

In the present study, we compared the cell-specific expression and changes protein levels in the glucose transporters (GLUTs) 1 and 3, the major GLUTs in the mouse and gerbil brains using immunohistochemistry and Western blot analysis. In both mouse and gerbils, GLUT1 immunoreactivity was mainly found in the blood vessels in the dentate gyrus, while GLUT3 immunoreactivity was detected in the subgranular zone and the molecular layer of the dentate gyrus. GLUT1-immunoreactivity in blood vessels and GLUT1 protein levels were significantly decreased with age in the mice and gerbils, respectively. In addition, few GLUT3-immunoreactive cells were found in the subgranular zone in aged mice and gerbils, but GLUT3-immunoreactivity was abundantly found in the polymorphic layer of dentate gyrus in mice and gerbils with a dot-like pattern. Based on the double immunofluorescence study, GLUT3-immunoreactive structures in gerbils were localized in the glial fibrillary acidic protein-immunoreactive astrocytes in the dentate gyrus. Western blot analysis showed that GLUT3 expression in the hippocampal homogenates was slightly, although not significantly, decreased with age in mice and gerbils, respectively. These results indicate that the reduction in GLUT1 in the blood vessels of dentate gyrus and GLUT3 in the subgranular zone of dentate gyrus may be associated with the decrease in uptake of glucose into brain and neuroblasts in the dentate gyrus. In addition, the expression of GLUT3 in the astrocytes in polymorphic layer of dentate gyrus may be associated with metabolic changes in glucose in aged hippocampus.

Targeting Warburg Effect in Cancers with PEGylated Glucose

In highly proliferative cancer cells, energy is predominantly produced by a high rate of glycolysis, followed by lactic acid fermentation, despite the availability of oxygen - an observation known as the Warburg effect. As a consequence, cells employing this glycolytic pathway require high uptake of glucose and increased metabolic rates to maintain their proliferation. It has been hypothesized that by blocking glucose uptake using modified glucose molecules, apoptosis in the cancer cells can be induced. In this study, it has been showed that several poly(ethylene glycol) (PEG)-modified glucose compounds could reduce cell proliferation in various cancer cell lines by a phenomenon that blocked the availability of the glucose transporters and reduced AKT1 (serine/threonine-specific protein kinase) activation. Xenograft cancer models that are intravenously administered with glucose-conjugated branched PEG (GBrP) daily for 14 d show little tumor development, as compared to the control group without GBrP treatment. The toxicological effects and the pharmacokinetics of the PEGylated glucose are studied in rodents. The PEGylated glucose exerts no systemic toxicity at 40 mg kg(-1) dosage. However, doses above 80 mg kg(-1) show dose-dependent toxicity in all the organs analyzed. The present results suggest PEGylated glucose as a promising "metabolic therapy" approach for the treatment of cancer.

Novel therapeutic targets of tumor metabolism

The study of tumor metabolism has resulted in new understandings of how cancer cells modify metabolic pathways that control cellular energetics to allow increased proliferation and survival. Tumor cells have been shown to alter metabolic pathways involved in glucose, glutamine, and mitochondrial metabolism to generate raw materials needed for rapid cellular proliferation, maintain favorable cellular redox environments, modify cellular epigenetics, and even promote and maintain oncogenic transformation. As a consequence, there has been intense scientific and clinical interest in targeting metabolic alterations that are commonly adopted by tumor cells for therapeutic purposes. In this review, we describe common metabolic alterations seen in tumor cells and discuss how these alterations are being investigated as potential targets for pharmacological intervention in preclinical and clinical settings. We also discuss some of the challenges associated with using tumor metabolism as a therapeutic target in cancer therapy, along with potential avenues to overcome these challenges.

GLUT3 and PKM2 regulate OCT4 expression and support the hypoxic culture of human embryonic stem cells

Human embryonic stem cells (hESCs) have the capacity to differentiate into all cell types and thus have great potential for regenerative medicine. hESCs cultured at low oxygen tensions are more pluripotent and display an increased glycolytic rate but how this is regulated is unknown. This study therefore aimed to investigate the regulation of glucose metabolism in hESCs and whether this might impact OCT4 expression. In contrast to the glucose transporter GLUT1, GLUT3 was regulated by environmental oxygen and localised to hESC membranes. Silencing GLUT3 caused a reduction in glucose uptake and lactate production as well as OCT4 expression. GLUT3 and OCT4 expression were correlated suggesting that hESC self-renewal is regulated by the rate of glucose uptake. Surprisingly, PKM2, a rate limiting enzyme of glycolysis displayed a nuclear localisation in hESCs and silencing PKM2 did not alter glucose metabolism suggesting a role other than as a glycolytic enzyme. PKM2 expression was increased in hESCs cultured at 5% oxygen compared to 20% oxygen and silencing PKM2 reduced OCT4 expression highlighting a transcriptional role for PKM2 in hESCs. Together, these data demonstrate two separate mechanisms by which genes regulating glucose uptake and metabolism are involved in the hypoxic support of pluripotency in hESCs.

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

Environmental oxygen tension regulates the energy metabolism and self-renewal of human embryonic stem cells

Energy metabolism is intrinsic to cell viability but surprisingly has been little studied in human embryonic stem cells (hESCs). The current study aims to investigate the effect of environmental O2 tension on carbohydrate utilisation of hESCs. Highly pluripotent hESCs cultured at 5% O2 consumed significantly more glucose, less pyruvate and produced more lactate compared to those maintained at 20% O2. Moreover, hESCs cultured at atmospheric O2 levels expressed significantly less OCT4, SOX2 and NANOG than those maintained at 5% O2. To determine whether this difference in metabolism was a reflection of the pluripotent state, hESCs were cultured at 5% O2 in the absence of FGF2 for 16 hours leading to a significant reduction in the expression of SOX2. In addition, these cells consumed less glucose and produced significantly less lactate compared to those cultured in the presence of FGF2. hESCs maintained at 5% O2 were found to consume significantly less O2 than those cultured in the absence of FGF2, or at 20% O2. GLUT1 expression correlated with glucose consumption and using siRNA and chromatin immunoprecipitation was found to be directly regulated by hypoxia inducible factor (HIF)-2α at 5% O2. In conclusion, highly pluripotent cells associated with hypoxic culture consume low levels of O2, high levels of glucose and produce large amounts of lactate, while at atmospheric conditions glucose consumption and lactate production are reduced and there is an increase in oxidative metabolism. These data suggest that environmental O2 regulates energy metabolism and is intrinsic to the self-renewal of hESCs.

Drug transport across the blood-brain barrier

The blood-brain barrier (BBB) prevents the brain uptake of most pharmaceuticals. This property arises from the epithelial-like tight junctions within the brain capillary endothelium. The BBB is anatomically and functionally distinct from the blood-cerebrospinal fluid barrier at the choroid plexus. Certain small molecule drugs may cross the BBB via lipid-mediated free diffusion, providing the drug has a molecular weight <400 Da and forms <8 hydrogen bonds. These chemical properties are lacking in the majority of small molecule drugs, and all large molecule drugs. Nevertheless, drugs can be reengineered for BBB transport, based on the knowledge of the endogenous transport systems within the BBB. Small molecule drugs can be synthesized that access carrier-mediated transport (CMT) systems within the BBB. Large molecule drugs can be reengineered with molecular Trojan horse delivery systems to access receptor-mediated transport (RMT) systems within the BBB. Peptide and antisense radiopharmaceuticals are made brain-penetrating with the combined use of RMT-based delivery systems and avidin-biotin technology. Knowledge on the endogenous CMT and RMT systems expressed at the BBB enable new solutions to the problem of BBB drug transport.

Molecular cloning of the bovine blood-brain barrier glucose transporter cDNA and demonstration of phylogenetic conservation of the 5'-untranslated region

The brain-type glucose transporter (GLUT-1) is a blood-brain barrier (BBB)-specific gene. The isolation of BBB-specific genes from total brain cDNA libraries is difficult because the brain capillary endothelium, which makes up the BBB in vivo, constitutes <0.2% of brain volume. Therefore, the present studies describe the preparation of a bovine brain capillary cDNA library in the lambdagt11 vector and the cloning of the BBB GLUT-1 cDNA. The cDNA sequenced was full length, as confirmed by primer extension analysis. The 5'-untranslated region (UTR) of the bovine GLUT-1 was 67 and 75% homologous with 5'-UTR sequences of the rabbit and human GLUT-1 through nucleotide overlaps of 116 and 159, respectively. A unique proline-rich sequence near the N-terminus of the bovine GLUT-1 was not found in other species and this correlated with marked immunoreactivity of the bovine, but not the human or rat, BBB GLUT-1 protein with an antiserum directed against the 15 amino acids at the N-terminus. In conclusion, these studies describe the cloning of the GLUT-1 cDNA from a BBB cDNA library. The extensive phylogenetic conservation of the 5'-UTR suggests the GLUT-1 gene may be subject to translational control in the regulation of BBB glucose transport.

Glucose transporter gene expression in rat brain: Pretranslational changes associated with chronic insulin-induced hypoglycemia, fasting, and diabetes

Steady-state levels of the major glucose transporter gene (GLUT-1) of the brain were evaluated under three conditions that induced chronic changes in plasma glucose and insulin in adult rats: (i) repeated injection of insulin for 5 days, resulting in plasma glucose levels of 60-70 mg/dl for at least 3 days; (ii) fasting for 3 days; and (iii) moderate streptozotocin-induced diabetes of 1 week duration. Brain GLUT-1 mRNA was measured by dot blot hybridization with a HepG2/erythrocyte (GLUT1) [(32)P]cRNA probe, and GLUT-1 protein by immunoblot analysis with a polyclonal antibody (11493). Insulin injection resulted in hypoglycemia, increased GLUT-1 mRNA (143 +/- 15%, P < 0.05), and increased GLUT-1 protein (141 +/- 6%, P < 0.05). The increase in GLUT-1 mRNA was specific for brain, as no change was observed in liver or kidney. Fasting resulted in mild hypoglycemia, lower plasma insulin, increased GLUT-1 mRNA (131 +/- 17%, P < 0.05 vs control), and no change in GLUT-1 protein (125 +/- 9%, N.S.). Mild streptozotocin diabetes resulted in hyperglycemia, undetectable plasma insulin, decreased GLUT-1 mRNA (65 +/- 6%, P < 0.05 vs control), and no change in GLUT-1 protein (84 +/- 9%, N.S.). A negative correlation (r = -0.61, P < .0001) between GLUT-1 mRNA levels in brain and plasma glucose concentrations was observed among the three experimental groups and control animals, suggesting that the plasma glucose concentration may be at least one determinant of GLUT-1 levels in rat brain. The importance of these results is the finding that GLUT-1 gene expression in rat brain is regulated in vivo by the nutritional and endocrine status of the animal.

Glucose: an energy currency and structural precursor in articular cartilage and bone with emerging roles as an extracellular signaling molecule and metabolic regulator

In the skeletal system glucose serves as an essential source of energy for the development, growth, and maintenance of bone and articular cartilage. It is particularly needed for skeletal morphogenesis during embryonic growth and fetal development. Glucose is vital for osteogenesis and chondrogenesis, and is used as a precursor for the synthesis of glycosaminoglycans, glycoproteins, and glycolipids. Glucose sensors are present in tissues and organs that carry out bulk glucose fluxes (i.e., intestine, kidney, and liver). The beta cells of the pancreatic islets of Langerhans respond to changes in blood glucose concentration by varying the rate of insulin synthesis and secretion. Neuronal cells in the hypothalamus are also capable of sensing extracellular glucose. Glucosensing neurons use glucose as a signaling molecule to alter their action potential frequency in response to variations in ambient glucose levels. Skeletal muscle and adipose tissue can respond to changes in circulating glucose but much less is known about glucosensing in bone and cartilage. Recent research suggests that bone cells can influence (and be influenced by) systemic glucose metabolism. This focused review article discusses what we know about glucose transport and metabolism in bone and cartilage and highlights recent studies that have linked glucose metabolism, insulin signaling, and osteocalcin activity in bone. These new findings in bone cells raise important questions about nutrient sensing, uptake, storage and processing mechanisms and how they might contribute to overall energy homeostasis in health and disease. The role of glucose in modulating anabolic and catabolic gene expression in normal and osteoarthritic chondrocytes is also discussed. In summary, cartilage and bone cells are sensitive to extracellular glucose and adjust their gene expression and metabolism in response to varying extracellular glucose concentrations.

Acute modulation of sugar transport in brain capillary endothelial cell cultures during activation of the metabolic stress pathway

GLUT1-catalyzed equilibrative sugar transport across the mammalian blood-brain barrier is stimulated during acute and chronic metabolic stress; however, the mechanism of acute transport regulation is unknown. We have examined acute sugar transport regulation in the murine brain microvasculature endothelial cell line bEnd.3. Acute cellular metabolic stress was induced by glucose depletion, by potassium cyanide, or by carbonyl cyanide p-trifluoromethoxyphenylhydrazone, which reduce or deplete intracellular ATP within 15 min. This results in a 1.7-7-fold increase in V(max) for zero-trans 3-O-methylglucose uptake (sugar uptake into sugar-free cells) and a 3-10-fold increase in V(max) for equilibrium exchange transport (intracellular [sugar] = extracellular [sugar]). GLUT1, GLUT8, and GLUT9 mRNAs are detected in bEnd.3 cells where GLUT1 mRNA levels are 33-fold greater than levels of GLUT8 or GLUT9 mRNA. Neither GLUT1 mRNA nor total protein levels are affected by acute metabolic stress. Cell surface biotinylation reveals that plasma membrane GLUT1 levels are increased 2-3-fold by metabolic depletion, although cell surface Na(+),K(+)-ATPase levels remain unaffected by ATP depletion. Treatment with the AMP-activated kinase agonist, AICAR, increases V(max) for net 3-O-methylglucose uptake by 2-fold. Glucose depletion and treatment with potassium cyanide, carbonyl cyanide p-trifluoromethoxyphenylhydrazone, and AICAR also increase AMP-dependent kinase phosphorylation in bEnd.3 cells. These results suggest that metabolic stress rapidly stimulates blood-brain barrier endothelial cell sugar transport by acute up-regulation of plasma membrane GLUT1 levels, possibly involving AMP-activated kinase activity.

The impact of fluorodeoxyglucose-positron emission tomography in primary staging and patient management in lymphoma patients

Fully diagnostic positron emission tomography (PET)/CT scans acquired during oral and intravenous contrast can be provided to patients and referring physicians in a single imaging session. Although FDG uptake varies, most low-grade lymphomas exhibit sufficient FDG avidity to also be staged reliably with FDG PET/CT. PET/CT imaging is more accurate for lymphoma staging than PET or CT alone and has substantial impact on patient management. This accurate whole-body glucose metabolic survey should serve as the baseline for subsequent treatment response evaluations. PET/CT has evolved to become the modality of choice for staging of nodal and extranodal lymphoma, for assessing therapeutic response, and for establishing patient prognosis.

The prognostic value of endogenous hypoxia-related markers for head and neck squamous cell carcinomas treated with ARCON

BACKGROUND AND PURPOSE

Hypoxic radioresistance is an important cause for treatment failure in a number of tumor types including head and neck cancers. Recent studies suggest that outcome can be improved by oxygenation modifying treatments such as ARCON. A robust endogenous marker of hypoxia might be a valuable aid to select patients for such treatments. The aim of this investigation was to study associations between the putative endogenous hypoxia markers CA-IX, Glut-1 and Glut-3 and clinical tumor and patient characteristics and to evaluate the prognostic value of these markers.

PATIENTS AND METHODS

Tumor biopsies from 58 patients treated with ARCON in a phase II trial were included. Tumor sections were immunohistochemically stained for CA-IX, Glut-1 and Glut-3. Sections were scored for relative tumor area stained by the markers (CA-IX and Glut-3) and for intensity of staining (Glut-1 and Glut-3). Further, sections were stained for CD34, an endothelial marker to assess microvascular density.

RESULTS

Staining of CA-IX and Glut-3 was observed at some distance from vessels and adjacent to necrosis. Glut-1 staining was generally very diffuse. The distribution of clinical characteristics was equal between tumors with high and low marker expression. Significant differences were found for locoregional control (P = 0.04) and for freedom of distant metastases (P = 0.02) in favour of patients with high CA-IX positivity (>25% of tumor area). High Glut-3 expression was associated with a better locoregional control (P = 0.04). Higher Glut-1 intensity was associated with an increased rate of distant metastases (P = 0.0005) and a worse overall survival (P = 0.001).

CONCLUSIONS

The inconsistent associations with outcome of CA-IX and the glucose transporters indicate that different factors play a role in up-regulation of these markers. Compared to studies with conventional treatment, the correlation between CA-IX expression and Glut-3 expression and outcome was reversed after treatment with ARCON. This does not support the potential of any of these proteins as very specific and robust hypoxia markers.

Differential subcellular distribution of glucose transporters GLUT1–6 and GLUT9 in human cancer: Ultrastructural localization of GLUT1 and GLUT5 in breast tumor tissues

It has been proposed that the enhanced metabolic activity of tumor cells is accompanied by an increased expression of facilitative hexose transporters (GLUTs). However, a previous immunohistochemical analysis of GLUT1 expression in 154 malignant human neoplasms failed to detect the GLUT1 isoform in 87 tumors. We used 146 normal human tissues and 215 tumor samples to reassess GLUT1 expression. A similar number of samples were used to compare the expression of GLUT2-6 and 9. The classical expression of GLUT1-5 in different normal human tissues was confirmed, however, we were unable to detect GLUT2 in human pancreatic islet cells. GLUT6 was principally detected in testis germinal cells and GLUT9 was localized in kidney, liver, heart, and adrenal. In tumor samples, GLUT1, 2, and 5 were the main transporters detected. GLUT1 was the most widely expressed transporter, however, 42% of the samples had very low-to-negative expression levels. GLUT2 was detected in 31% of the samples, being mainly expressed in breast, colon, and liver carcinoma. GLUT5 was detected in 27% of breast and colon adenocarcinoma, liver carcinoma, lymphomas, and testis seminoma samples. In situ RT-PCR and ultrastructural immunohistochemistry confirmed GLUT5 expression in breast cancer. GLUT6 and 9 are not clearly over-expressed in human cancer. The extensive expression of GLUT2 and 5 (glucose/fructose and fructose transporters, respectively) in malignant human tissues indicates that fructose may be a good energy substrate in tumor cells. Our functional data obtained in vitro in different tumor cells support this hypothesis. Additionally, these results suggest that fructose uptake could be used for positron emission tomography imaging and, may possibly represent a novel target for the development of therapeutic agents in different human cancers.

Different role of insulin in GLUT-1 and -4 regulation in heart and skeletal muscle during perinatal hypothyroidism

Two groups of hypothyroid rats were used; one group was given 2-mercapto-1-methylimidazole (MMI) treatment in the drinking water of the mothers and was killed at 2 and 4 days of life, and the other group was given similar MMI treatment and then was thyroidectomized at 5 days of life and killed at 8 or 20 days. Serum insulin, growth hormone (GH), and insulin-like growth factor I (IGF-I) were decreased in MMI-treated rats but increased in MMI-treated plus thyroidectomized rats. No significant reduction of thyroid hormones was observed in 2-day-old MMI rats. Protein and mRNA expression of GLUT-1 increased, and those of GLUT-4 decreased, in the heart in all populations independent of changes in insulin, GH, and IGF-I levels. However, GLUT-4 protein and mRNA expression in quadriceps and gastrocnemius skeletal muscles decreased at 4 days and increased at 8 and 20 days of life in parallel with insulin, GH, and IGF-I levels. GLUT-1 in the skeletal muscles seemed regulated posttranscriptionally and presented a decrease of mRNA expression in all stages studied. A differential sensitivity to insulin regulation of GLUT-1 and GLUT-4 glucose transporters seems to be one of the causes for the tissue-specific regulation of these glucose transporters in heart and skeletal muscles during the perinatal period.

GLUT-3 expression in human skeletal muscle

Muscle biopsy homogenates contain GLUT-3 mRNA and protein. Before these studies, it was unclear where GLUT-3 was located in muscle tissue. In situ hybridization using a midmolecule probe demonstrated GLUT-3 within all muscle fibers. Fluorescent-tagged antibody reacting with affinity-purified antibody directed at the carboxy-terminus demonstrated GLUT-3 protein in all fibers. Slow-twitch muscle fibers, identified by NADH-tetrazolium reductase staining, possessed more GLUT-3 protein than fast-twitch fibers. Electron microscopy using affinity-purified primary antibody and gold particle-tagged second antibody showed that the majority of GLUT-3 was in association with triads and transverse tubules inside the fiber. Strong GLUT-3 signals were seen in association with the few nerves that traversed muscle sections. Electron microscopic evaluation of human peripheral nerve demonstrated GLUT-3 within the axon, with many of the particles related to mitochondria. GLUT-3 protein was found in myelin but not in Schwann cells. GLUT-1 protein was not present in nerve cells, axons, myelin, or Schwann cells but was seen at the surface of the peripheral nerve in the perineurium. These studies demonstrated that GLUT-3 mRNA and protein are expressed throughout normal human skeletal muscle, but the protein is predominantly found in the triads of slow-twitch muscle fibers.

Glucose transporter expression in human skeletal muscle fibers

The present study was initiated to investigate GLUT-1 through -5 expression in developing and mature human skeletal muscle. To bypass the problems inherent in techniques using tissue homogenates, we applied an immunocytochemical approach, employing the sensitive enhanced tyramide signal amplification (TSA) technique to detect the localization of glucose transporter expression in human skeletal muscle. We found expression of GLUT-1, GLUT-3, and GLUT-4 in developing human muscle fibers showing a distinct expression pattern. 1) GLUT-1 is expressed in human skeletal muscle cells during gestation, but its expression is markedly reduced around birth and is further reduced to undetectable levels within the first year of life; 2) GLUT-3 protein expression appears at 18 wk of gestation and disappears after birth; and 3) GLUT-4 protein is diffusely expressed in muscle cells throughout gestation, whereas after birth, the characteristic subcellular localization is as seen in adult muscle fibers. Our results show that GLUT-1, GLUT-3, and GLUT-4 seem to be of importance during muscle fiber growth and development. GLUT-5 protein was undetectable in fetal and adult skeletal muscle fibers. In adult muscle fibers, only GLUT-4 was expressed at significant levels. GLUT-1 immunoreactivity was below the detection limit in muscle fibers, indicating that this glucose transporter is of minor importance for muscle glucose supply. Thus we hypothesize that GLUT-4 also mediates basal glucose transport in muscle fibers, possibly through constant exposure to tonal contraction and basal insulin levels.

In vivo upregulation of the blood-brain barrier GLUT1 glucose transporter by brain-derived peptides

Glucose is the critical metabolic fluid for the brain, and the transport of this nutrient from blood to brain is limited by the blood-brain barrier (BBB) GLUT1 glucose transporter. The expression of the BBB-GLUT1 gene is augmented in brain endothelial cultured cells incubated with brain-derived trophic factors and the brain-derived peptide preparation Cerebrolysin (C1, EBEWE, Austria). The aim of the present investigation was to determine if C1 induces similar changes in the expression of the BBB-GLUT1 gene following its administration to rats in vivo. The BBB glucose transporter activity was investigated with the intracarotid artery perfusion technique using [3H]diazepam as cerebral blood flow marker. The acute or chronic administration of C1 markedly increased the brain permeability surface area of D-[14C]glucose compared to controls (D-[14C]glucose/[3H]diazepam ratio, 1.6- to 1.9-fold increase in frontal cortex, P < 0.05). Increased activity of the BBB glucose transporter was correlated with a significant rise in the abundance of the BBB-GLUT1 protein measured by both Western blot analysis and immunocytochemistry, and with a decrease in the transcript levels of this transporter. Data presented here demonstrate that the in vivo administration of Cl increases the transport of glucose from blood to brain via BBB-GLUT1 gene expression.

In vitro analysis of the glucose-transport system in GLUT4-null skeletal muscle

We have characterized the glucose-transport system in soleus muscle from female GLUT4-null mice to determine whether GLUT1, 3 or 5 account for insulin-stimulated glucose-transport activity. Insulin increased 2-deoxyglucose uptake 2.8- and 2.1-fold in soleus muscle from wild-type and GLUT4-null mice, respectively. Cytochalasin B, an inhibitor of GLUT1- and GLUT4-mediated glucose transport, inhibited insulin-stimulated 2-deoxyglucose uptake by >95% in wild-type and GLUT4-null soleus muscle. Addition of 35 mM fructose to the incubation media was without effect on insulin-stimulated 3-O-methylglucose transport activity in soleus muscle from either genotype, whereas 35 mM glucose inhibited insulin-stimulated (20 nM) 3-O-methylglucose transport by 65% in wild-type and 99% in GLUT4-null mice. We utilized the 2-N-4-1-(1-azi-2,2,2-triflu oroethyl)benzoyl-1, 3-bis(D-mannose-4-yloxy)-2-propylamine (ATB-BMPA) exofacial photolabel to determine if increased cell-surface GLUT1 or GLUT4 content accounted for insulin-stimulated glucose transport in GLUT4-null muscle. In wild-type soleus muscle, cell-surface GLUT4 content was increased by 2.8-fold under insulin-stimulated conditions and this increase corresponded to the increase in 2-deoxyglucose uptake. No detectable cell-surface GLUT4 was observed in soleus muscle from female GLUT4-null mice under either basal or insulin-stimulated conditions. Basal cell-surface GLUT1 content was similar between wild-type and GLUT4-null mice, with no further increase noted in either genotype with insulin exposure. Neither GLUT3 nor GLUT5 appeared to account for insulin-stimulated glucose-transport activity in wild-type or GLUT4-null muscle. In conclusion, insulin-stimulated glucose-transport activity in female GLUT4-null soleus muscle is mediated by a facilitative transport process that is glucose- and cytochalasin B-inhibitable, but which is not labelled strongly by ATB-BMPA.

GLUT3 protein and mRNA in autopsy muscle specimens

GLUT3 is expressed in rat muscle, but this glucose transporter protein has not been identified previously in adult human skeletal muscle. We quantified the rapidity of disappearance of mRNA and protein from human skeletal muscle at room temperature and at 4 degrees C. Fifty percent of the immunologically detectable GLUT3 protein disappeared by 1 hour at 20 degrees C and by 2 hours at 4 degrees C. mRNA for GLUT3 was decreased 50% by 2.2 hours at 20 degrees C and by 24 hours at 4 degrees C. Half of the measurable mRNAs for GLUT4, glyceraldehyde 3-phosphate dehydrogenase (GAPDH), alpha-actin, and beta-myosin disappeared by 0.8 to 2.1 hours at 20 degrees C and by 5.0 to 16.6 hours at 4 degrees C. Previous conclusions that GLUT3 is not expressed in human muscle were likely drawn because of artifacts related to degradation of GLUT3 protein in the specimens prior to study. Because of the rapid degradation of protein and mRNA, autopsy specimens of muscle must be obtained within 6 hours of death, and even then, protein and mRNA data will likely dramatically underestimate their expression in fresh muscle. Some previously published conclusions and recommendations regarding autopsy specimens are not stringent enough to consistently yield useful protein and mRNA.

Factors involved in GLUT-1 glucose transporter gene transcription in cardiac muscle

Glucose constitutes a major fuel for the heart, and high glucose uptake during fetal development is coincident with the highest level of expression of the glucose transporter GLUT-1 during life. We have previously reported that GLUT-1 is repressed perinatally in rat heart, and GLUT-4, which shows a low level of expression in the fetal stage, becomes the main glucose transporter in the adult. Here, we show that the perinatal expression of GLUT-1 and GLUT-4 glucose transporters in heart is controlled directly at the level of gene transcription. Transient transfection assays show that the -99/-33 fragment of the GLUT-1 gene is sufficient to drive transcriptional activity in rat neonatal cardiomyocytes. Electrophoretic mobility shift assays demonstrate that the transcription factor Sp1, a trans-activator of GLUT-1 promoter, binds to the -102/-82 region of GLUT-1 promoter during the fetal state but not during adulthood. Mutation of the Sp1 site in this region demonstrates that Sp1 is essential for maintaining a high transcriptional activity in cardiac myocytes. Sp1 is markedly down-regulated both in heart and in skeletal muscle during neonatal life, suggesting an active role for Sp1 in the regulation of GLUT-1 transcription. In all, these results indicate that the expression of GLUT-1 and GLUT-4 in heart during perinatal development is largely controlled at a transcriptional level by mechanisms that might be related to hyperplasia and that are independent from the signals that trigger cell hypertrophy in the developing heart. Furthermore, our results provide the first functional insight into the mechanisms regulating muscle GLUT-1 gene expression in a live animal.

Brain-derived peptides increase blood-brain barrier GLUT1 glucose transporter gene expression via mRNA stabilization

The present investigation studied the effect of the brain-derived peptide preparation Cerebrolysin (CI, EBEWE, Austria) on the turnover rate and gene expression of the blood-brain barrier (BBB) GLUT1 glucose transporter mRNA. Studies were performed in brain endothelial cultured cells transfected with the human (h) GLUT1 transcript. In control cells, the full length 2.8 Kb hGLUT1 mRNA was rapidly degraded following transfection, and the abundance of this transcript at 4 and 6 h was comparable to background mRNA levels seen in cells transfected without hGLUT1 mRNA. On the contrary, the decay of the hGLUT1 mRNA was stabilized in CI-treated cells resulting in a marked reduction in the fractional turnover rate (72.4 and 4.0%/h, control and CI, respectively). In parallel experiments, CI induced a significant increase in the levels of immunoreactive GLUT1 protein measured by enzyme-linked immunosorbent assay (ELISA). In conclusion, data presented here demonstrate that factors in CI increase BBB-GLUT1 transcript stability, and that this is associated with an induction of BBB-GLUT1 gene expression in brain endothelial cultured cells.

Molecular regulation of the blood-brain barrier GLUT1 glucose transporter by brain-derived factors

Glucose is the crucial metabolic fluid for the brain, and the transport of this nutrient from blood to brain is limited by the blood-brain barrier (BBB) GLUT1 glucose transporter. The activity of this transporter is altered in different pathophysiological conditions including Alzheimer's disease. The expression of the BBB-GLUT1 gene is directed by brain trophic factors, and the brain-derived peptide preparation Cerebrolysin (Cl, EBEWE, Austria), used in the treatment of Alzheimer's disease, increases the BBB-GLUT1 mRNA stability and the expression of the BBB-GLUT1 gene. In the present investigation, Cl markedly increased (p < 0.001) the expression of a BBB-GLUT1 reporter gene, named clone 753, that contains an important regulatory cis-acting element involved in the stabilization of this transcript in brain endothelial cultured cells (ECL). In experiments with a reporter gene lacking this regulatory element, Cl produced only a minimal fraction of the effect observed with clone 753. UV-cross linking/PAGE experiments showed that the GLUT1 transcript reacts with ECL cytosolic proteins to form a RNA/protein complex of approximately 80 kDa. The abundance of this cis/trans acting complex was found to be increased in Cl-treated cells. Overall, data presented here demonstrate that i) Cl increases the expression of a BBB-GLUT1-luciferase reporter gene containing a region of the 3'-untranslated region of BBB-GLUT1 mRNA with important regulatory cis-acting elements involved in the stabilization of this transcript, and ii) the increased expression of this BBB-GLUT1 reporter gene was associated with augmented abundance of a transacting factor that binds to the cis-acting element described in (i), suggesting that this association may be involved in the stabilization of GLUT1 mRNA induced by Cl.

Effects of dexamethasone in vivo and in vitro on hexose transport in brain microvasculature

Glucocorticoids induce hyperinsulinemia, hyperglycemia, and depress glucose transport by aortic endothelium. High glucocorticoid doses are used for many diseases, but with unknown effects on brain glucose transport or metabolism. This study tested the hypothesis that glucocorticoids affect glucose transport or metabolism by brain microvascular endothelium. Male rats received dexamethasone (DEX) s.c. with sucrose feeding for up to seven days. Cerebral microvessels from rats treated with DEX/sucrose demonstrated increased GLUT1 and brain glucose extraction compared to controls. Glucose transport in vivo correlated with hyperinsulinemia. Pre-treatment with low doses of streptozotocin blunted hyperinsulinemia and prevented increased glucose extraction induced by DEX. In contrast, isolated brain microvessels exposed to DEX in vitro demonstrated suppression of 2-deoxyglucose uptake and glucose oxidation. We conclude that DEX/sucrose treatment in vivo increases blood-brain glucose transport in a manner that requires the effects of chronic hyperinsulinemia. These effects override any direct inhibitory effects of either hyperglycemia or DEX.

Quantitative determinations of the steady state transcript levels of hexokinase isozymes and glucose transporter isoforms in normal rat tissues and the malignant tumor cell line AH130

The steady state transcript levels of the four hexokinase (HK) isozymes and four glucose transporter (GLUT) isoforms were determined quantitatively by Northern analysis of RNA samples from rat tissues using synthetic fragments of the RNAs encoding the HK isozymes and GLUT isoforms. Results showed that the levels of HK isozyme transcripts were low in rat tissues, the level of that most highly expressed, the type I isozyme (HKI), in the brain being 0.025% of the total poly(A)+ RNA. A good correlation was found between the reported HK activities and the total amounts of transcripts encoding all HK isozymes in various tissues, showing that the HK activities in tissues can be estimated from the total amount of transcripts encoding HK isozymes. The proposed associated expressions of HK isozymes and GLUT isoforms in particular tissues were confirmed at their transcript levels. The steady state transcript levels of type II HK and the type 1 GLUT isoform in the malignant tumor cell line AH130 were also determined quantitatively.

Insulin-like growth factors in central nervous system tumors

Insulin-like growth factors (IGFs) appear to play a role in the development of tumors in general and brain tumors in particular. Specific receptors for IGFs have been identified in normal human and rat brain, and evidence suggests that components of the IGF signal transduction system may play a role in the transformation process. Secretion of IGFs by a variety of human brain tumors has been confirmed, and these growth factors appear to have an autocrine stimulatory effect on these tumors. IGFs circulate in the blood stream bound to at least six distinct binding proteins which may modulate the effects of these growth factors on target tissues. Sex steroids may also regulate the behavior of certain brain tumors such as meningiomas at least in part through their effects on the expression of IGFs and their binding proteins. Recently, antisense gene technology against certain IGFs or their receptors have resulted in potent antitumor effects in the case of several gliomas, although the mechanism for this remains unclear.

Expression of the facilitated glucose transporters (GLUT1 and GLUT3) by a choriocarcinoma cell line (JAr) and cytotrophoblast cells in culture

The expression of GLUT1 and GLUT3 mRNA and protein in human placental trophoblast-derived cells was investigated. A dividing choriocarcinoma derived cell line (JAr) was compared to differentiating cytotrophoblast cells, isolated from human term placenta, following 18 (mononucleate) and 66 h (multinucleate) in culture. JAr cells treated with 8-bromoadenosine, which inhibits growth and induces differentiation, were also studied. GLUT1 mRNA and protein expression were similar in the four groups of cells. However, GLUT3 mRNA expression was significantly higher (six- to sevenfold) in both control and 8-bromoadenosine-treated JAr cells compared to cytotrophoblast cells and was also significantly higher in untreated versus treated JAr cells. Western blotting showed that GLUT3 protein was undetectable in either cytotrophoblast cell groups, but was abundant in both groups of JAr cells. GLUT3 protein in JAr cells treated with 8-bromoadenosine was also significantly lower than in untreated JAr cells, in agreement with the mRNA data. We conclude that GLUT1 expression is unaffected by either growth or differentiation of trophoblast cells whereas GLUT3 expression is associated with dividing cells. We propose that in the placenta, GLUT3 may be involved in maintaining metabolic requirements of dividing trophoblast cells, rather than having a direct role in transport of glucose to the fetus.

Forebrain endothelium expresses GLUT4, the insulin-responsive glucose transporter

The presence of GLUT4, the insulin-responsive glucose transporter, in microvascular endothelium and the responsiveness of glucose transport at the blood-brain barrier to insulin have been matters of controversy. To address these issues, we examined GLUT4 mRNA and protein expression in isolated brain microvessels and in cultured calf vascular cells derived from brain microvessels and aorta. We report here that GLUT4 mRNA can be detected in rat forebrain and its microvasculature using high stringency hybridization of poly(A)+ RNA isolated from these sources. This mRNA is identical to that found in adipose cells from rat. Immunoblot analysis of isolate brain microvessels reveals that GLUT4 protein is also present. Peptide preadsorption studies and absence of our antibody reaction to human red cells suggest these findings are specific. Immunohistochemical staining of cultured calf vascular cells reveals that GLUT4 is expressed in brain endothelial cells but not pericytes, nor in aortic endothelium or smooth muscle cells. The sensitivity of the methods required to detect GLUT4 in brain and comparison to its abundance in low density microsomes from rat adipose cells indicate that GLUT4 is expressed in relatively low abundance in brain microvascular endothelium. No significant differences are observed in steady state levels of GLUT4 mRNA in brain from streptozotocin diabetic compared to control rats. This last finding supports the concept of tissue-specific regulation of GLUT4. We conclude that brain microvascular endothelium specifically expressed GLUT4 while other vascular cells do not.

Transport of glucose across the blood-tissue barriers

In specialized parts of the body, free exchange of substances between blood and tissue cells is hindered by the presence of a barrier cell layer(s). Specialized milieu of the compartments provided by these "blood-tissue barriers" seems to be important for specific functions of the tissue cells guarded by the barriers. In blood-tissue barriers, such as the blood-brain barrier, blood-cerebrospinal fluid barrier, blood-nerve barrier, blood-retinal barrier, blood-aqueous barrier, blood-perilymph barrier, and placental barrier, endothelial or epithelial cells sealed by tight junctions, or a syncytial cell layer(s), serve as a structural basis of the barrier. A selective transport system localized in the cells of the barrier provides substances needed by the cells inside the barrier. GLUT1, an isoform of facilitated-diffusion glucose transporters, is abundant in cells of the barrier. GLUT1 is concentrated at the critical plasma membranes of cells of the barriers and thereby constitutes the major machinery for the transport of glucose across these barriers where transport occurs by a transcellular mechanism. In the barrier composed of double-epithelial layers, such as the epithelium of the ciliary body in the case of the blood-aqueous barrier, gap junctions appear to play an important role in addition to GLUT1 for the transfer of glucose across the barrier.

Brain-derived peptides increase the expression of a blood-brain barrier GLUT1 glucose transporter reporter gene

The brain-derived peptide preparation Cerebrolysin (C1; EBEWE, Austria) increases the stability of blood-brain barrier (BBB)-GLUT1 transcript. To determine if the increase in BBB-GLUT1 mRNA stability is associated with an augmentation of gene expression, the present investigation studied the effect of C1 on the expression of a BBB-GLUT1-luciferase reporter gene in brain endothelial cultured (ECL) cells. Dose response studies showed that C1 markedly increased the expression of luciferase when the BBB-GLUT1-reporter gene was used. On the contrary, C1 produced no changes in the expression pattern of the control reporter gene, which lacks the GLUT1 regulatory sequence. Desensitization of the protein kinase C (PKC) receptor with the phorbol ester TPA, or inhibition with either 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7) or staurosporine, had no effect on the increased levels of luciferase induced by C1. Transfection efficiency was determined by measuring intracellular levels of the expression vector using a quantitative polymerase chain reaction (PCR) assay. The data presented here demonstrate that C1 increases BBB-GLUT1 gene expression in ECL cells through a mechanism that appears to be independent of activation of PKC.

Modulation of cultured brain, adrenal, and aortic endothelial cell glucose transport

Studies of glucose transporter activity and anti-glucose transporter (GLUT1) immunoblots were performed on different endothelial cell primary cultures (brain capillary, adrenal capillary and aortic) to determine their response to glucose deprivation. Cell cultures were exposed to glucose deprivation (0.5 mM) for 48 h periods and refed (11.0 mM) for 36 additional hours. Control cultures were kept in 11.0 mM glucose for the duration of these studies. Measurements of 2-[3H]deoxy-D-glucose uptake and membrane fraction purification were performed every 12 h during these timecourses. Baseline cytochalasin-B sensitive uptake of 2-deoxy-D-glucose was near three times larger in brain capillary endothelial cells than in adrenal or aortic endothelial cultures. In all three endothelial cell cultures, 2-deoxy-D-glucose uptake increased during glucose deprivation, and returned to control values upon refeeding. Aortic and adrenal cortical endothelia expressed the starvation induced increases 12 h sooner than brain capillary endothelia. Return to control values was also 12 h faster in these cultured endothelia. Immunoblot studies showed that in all three endothelial cell cultures the increases in transporter activity during glucose starvation correlate with increased membrane expression of GLUT1. Quantitative analysis of the anti-GLUT1 immunoblots indicated that induction of GLUT1 following glucose starvation was slower in brain capillary endothelia than in aortic or adrenal endothelia. The slower response by brain capillary endothelial cells may be related to the higher transport rate of glucose in these cells.

Brain-derived peptides regulate the steady state levels and increase stability of the blood-brain barrier GLUT1 glucose transporter mRNA

The blood-brain barrier (BBB) GLUT1 glucose transporter gene expression is known to be regulated by putative brain trophic factors. Therefore, the present study investigated the effect of a brain-derived peptide rich preparation containing a neurotrophic factor-like action [Cerebrolysin (Cl), EBEWE, Austria]. In cultures of brain capillary endothelial cells, Cl induced a transient increase in the abundance of BBB-GLUT1 relative to actin measured by reverse transcription-polymerase chain reaction during the first 2 h of incubation, whereas a significant reduction in the GLUT1 transcript was observed at 20 and 48 h. In addition, Cl abolished the fall in GLUT1 levels induced by actinomycin D. The present data suggest that brain-derived factors in Cl are able to modulate the expression of the BBB-GLUT1 gene increasing the BBB-GLUT1 transcript stability.

Ontogenic expression of the erythroid-type glucose transporter (Glut 1) in the telencephalon of the mouse: correlation to the tightening of the blood-brain barrier

Since Glut 1 was shown to be highly abundant in brain microvessels, its distribution during early developmental stages seems of importance in respect to the timing of blood-brain barrier (bbb) formation in the developing CNS. Here we have followed the temporal expression of the erythroid-type glucose transporter Glut 1 in the telencephalon of the embryonic and newborn mouse, beginning at the 9th intrauterine day. Glut 1 immunofluorescence staining was done on cryosections using a rabbit polyclonal antiserum to purified human erythrocyte glucose transporter. Endothelial cells resp. capillaries were detected by staining with a rhodamin-coupled Bandeiraea simplicifolia lectin (BSL). In parallel, the developmental tightening of the embryonic bbb was assessed by perfusion of mouse embryos with Trypan blue and horse radish-peroxidase. At E9, prior to the onset of intraneural neovascularization, strong Glut 1 immunoreactivity was found in the whole neuroectoderm but only minor staining was seen in the perineural domain. Glut 1 expression remained uniformly distributed in the intraneural tissue at E10, the beginning of intraneural neovascularization in the mouse. From E11 onwards, Glut 1 immunoreactivity was invisible in neuroepithelial cells, but appeared tightly associated with intraneural capillaries. Perfusion of E12 embryos using trypan blue solution and HRP revealed that most parts of the CNS and spinal cord were impermeable to the tracer substances at that stage. Thus, we suggest that the bbb is established very early in CNS development, probably in the course of intraneural neovascularization. In addition, our data indicate that the restriction of Glut 1 expression to the intraneural capillaries reflects the onset of bbb function in the mouse embryo.

Insulin induces translocation of GLUT-4 glucose transporters in human skeletal muscle

Understanding the molecular mechanisms involved in the regulation of glucose transport into human muscle is necessary to unravel possible defects in glucose uptake associated with insulin resistance in humans. Here we report a strategy to subfractionate human skeletal muscle biopsies (0.5 g) removed from vastus lateralis during a euglycemic insulinemic clamp procedure. A sucrose gradient separated total membranes into five fractions. Fraction 25 (25% sucrose) contained the plasma membrane markers alpha 1- and alpha 2-subunits of the Na(+)-K(+)-adenosinetriphosphatase and the GLUT-5 hexose transporter, recently immunolocalized to the cell surface of human skeletal muscle. The dihydropyridine receptor, a transverse tubule marker, was present exclusively in this fraction. The GLUT-4 glucose transporter was more concentrated in fraction 27.5 (27.5% sucrose) and largely diminished in plasma membrane markers. Open skeletal muscle biopsies were removed before and 30 min after clamping insulin to 550 pM. This increased GLUT-4 protein by 1.61-fold in fraction 25 and lowered it by 50% in fraction 27.5. Thus physiological concentrations of insulin induce translocation of glucose transporters from an internal membrane pool to surface membranes in human skeletal muscle.

Effect of dehydroepiandrosterone on glucose uptake in cultured human fibroblasts

Dehydroepiandrosterone (DHEA) and its sulfate derivative (DHEA-S) reportedly have antidiabetic and antiobesity effects. The effect of DHEA on glucose uptake in cultured human fibroblasts was examined. Incubation of cells with supraphysiologic concentrations of DHEA (10(-5) mol/L) for > or = 10 hours enhanced 2-deoxyglucose (2-DG) uptake significantly (P < .05). Supraphysiologic concentrations of insulin (10(-7) mol/L) increased the sensitivity of glucose uptake to DHEA. Conversely, the sensitivity of glucose uptake to insulin was increased by incubating cells with 10(-6) mol/L DHEA. Both the abundance of transcripts encoding glucose transporter-1 (Glut-1) and the maximal velocity (Vmax) of 2-DG transport were increased in cultured fibroblasts incubated with DHEA. Cultured fibroblasts expressed a specific binding factor with low affinity for [3H]DHEA (maximal number of binding sites, 18,496 sites per cell; Kd, 298 nmol/L). Other androgen hormones exerted a less-marked effect on glucose uptake; DHEA-S had no effect. These results suggested that DHEA increases Glut-1 mRNA through binding to a specific factor in cultured human fibroblasts and thereby stimulates glucose uptake in these cells.

Use of hexose transport mutants to examine the expression and properties of the rat myoblast GLUT 1 transport process

Rat L6 myoblasts were recently shown to possess the GLUT 1, 3 and 4 transporters, and not the GLUT 2 isoform [1]. This investigation examined the expression and properties of the GLUT 1 isoform. GLUT 1 transcript level was significantly reduced in cells grown at high densities and during myogenic differentiation. A comparison of the GLUT 1 and 4 transcript levels in myogenesis-competent and impaired cells revealed an inverse relationship between these two isoforms. This relationship was confirmed by studies using two independent spontaneous GLUT 3- GLUT 4- mutants, M1 and M3. These mutants possessed very high level of the GLUT 1 isoform, but negligible amount of the GLUT 3 and 4 isoforms. GLUT 1 expression was also subject to positive regulation. Glucose starvation was found to increase not only the levels of the GLUT 1 transcript and transporter, but also the intrinsic activity of the GLUT 1 transporter. Studies with M1 and M3 mutants revealed that the GLUT 1 transporter was not functional in glucose-grown cells, even though it was present at a very high level in the plasma membrane. This transporter became functional when cells were starved for glucose. The functional GLUT 1 transporter had an apparent Km value of around 0.9 mM, and was sensitive to cytochalasin B, phloretin, phlorizin and pCMBS.

A sodium- and energy-dependent glucose transporter with similarities to SGLT1-2 is expressed in bovine cortical vessels

In order to investigate glucose transport at the blood-brain barrier (BBB), glucose transport properties were studied pharmacologically with a novel model system of inverted bovine brain cortical arteries. These vessels displayed glucose transport characteristics of facilitative glucose transporters (GLUT1-5) and of sodium- and energy-dependent glucose transporters (SGLT1-2). So far, glucose transport in the central nervous system (CNS) has only been shown to be achieved by facilitative transporters, in particular by GLUT1 at the blood-brain barrier and in glial cells and by GLUT3 in a subset of neurons. We report here that a SGLT-like transporter might partake in glucose transport at the bovine BBB, as indicated by immunocytochemical analysis and by Western immunoblot analysis of cultured bovine brain endothelial cells. A RNA protection assay revealed the presence of a SGLT-like gene fragment in rabbit cortex.

Gene expression of GLUT3 and GLUT1 glucose transporters in human brain tumors

GLUT3 glucose transporter gene expression is confined to neurons, while GLUT1 gene expression is limited to endothelial cells in normal brain. Thus far, neither of the GLUT genes has been shown to be consistently expressed in glial cells in adult brain in vivo under normal conditions. However, GLUT gene expression may be aberrant in human brain glial tumors. The present investigation shows that the GLUT1 and GLUT3 transcripts are differentially expressed in a series of 20 human brain tumors. The GLUT1/actin mRNA ratio increased in parallel to the astrocytoma grade, compared to a control human brain cortex, although no change in this ratio was seen in 5 meningiomas. Immunoreactive GLUT1 protein was not detectable in human brain tumors, including high-grade gliomas. Both 4.2 or 2.7 kb GLUT3/actin mRNA ratios showed a linear correlation with the glioma grade (P < 0.025), and the GLUT3-immunoreactive protein was also expressed in high grade gliomas. These studies provide evidence for induction of GLUT1 and GLUT3 gene expression in malignant glial cells, and the mRNA levels correlate with the biologic aggressiveness of the tumor. The detection of immunoreactive GLUT3, but not GLUT1, in the high grade gliomas suggest the GLUT3 isoform may be the predominant glucose transporter in highly malignant glial cells of human brain.

Elevated GLUT 1 level in crude muscle membranes from diabetic Zucker rats despite a normal GLUT 1 level in perineurial sheaths

Recently, we demonstrated that approximately 60% of GLUT 1 in a crude membrane fraction of rat skeletal muscle originates from perineurial sheaths. To study the in vivo regulation of GLUT 1 expression in different tissues in muscles, we measured the level of GLUT 1 in crude muscle membranes and in perineurial sheaths in diabetic (fa/fa) Zucker rats and lean controls, with and without metformin treatment. The GLUT 1 concentration in perineurial sheaths was identical in all four groups of rats, both when measured by quantitative immunofluorescence and by immunoblotting and densitometry. In a fraction of crude membranes of soleus muscles GLUT 1 expression was more than two-fold higher in (fa/fa) rats than in lean controls (p < 0.005). Metformin treatment significantly elevated GLUT 1 in control rats (p < 0.05) and tended to decrease GLUT 1 in diabetic rats (p < 0.075). The expressions of GLUT 1 and GLUT 4 in crude muscle membranes were inversely correlated (p < 0.01), and GLUT 1 expression correlated positively with fasting glucose (p < 0.05). In conclusion, GLUT 1 expression in perineurial sheaths is unaffected by alterations in glucose homeostasis and by the genes responsible for obesity and diabetes in the Zucker rat. GLUT 1 expression in a crude membrane fraction of soleus muscle is increased in the diabetic animals, likely due to an increased expression in muscle cells proper.

Glucose transporter 2 (GLUT 2): expression in specific brain nuclei

In the brain, certain neurons appear to be sensitive to changes in local and/or plasma glucose concentration. The alterations in the electrical activity of these neurons probably depend on the existence of 'glucose sensors', which may be one of the glucose transporters described so far. Because of suitable kinetic properties, we hypothesized that the glucose transporter 2 (GLUT 2) may well constitute one of the cerebral 'glucose sensors'. In this study, it was demonstrated, using the polymerase chain reaction, that GLUT 2 mRNAs are present in a limited number of brain nuclei, including the nucleus tractus solitarius, the motor nucleus of the vagus, the paraventricular hypothalamic nucleus, the lateral hypothalamic area, the arcuate nucleus and the olfactory bulbs. These localizations were confirmed by immunocytochemistry, but the cerebral distribution of GLUT 2-like immunoreactivity was far larger than initially expected. Furthermore, electron microscopic observations showed that, within the regions examined, GLUT 2 was localized to a restricted population of astrocytes. The localization of GLUT 2 in regions previously connected with feeding behavior supports an indirect role for GLUT 2 in 'glucose sensing' in these specific cerebral structures.