Nutrient control of GLUT1 processing and turnover in 3T3-L1 adipocytes

A non-catalytic function of carbonic anhydrase IX contributes to the glycolytic phenotype and pH regulation in human breast cancer cells

The most aggressive and invasive tumor cells often reside in hypoxic microenvironments and rely heavily on rapid anaerobic glycolysis for energy production. This switch from oxidative phosphorylation to glycolysis, along with up-regulation of the glucose transport system, significantly increases the release of lactic acid from cells into the tumor microenvironment. Excess lactate and proton excretion exacerbate extracellular acidification to which cancer cells, but not normal cells, adapt. We have hypothesized that carbonic anhydrases (CAs) play a role in stabilizing both intracellular and extracellular pH to favor cancer progression and metastasis. Here, we show that proton efflux (acidification) using the glycolytic rate assay is dependent on both extracellular pH (pH_e) and CA IX expression. Yet, isoform-selective sulfonamide-based inhibitors of CA IX did not alter proton flux, which suggests that the catalytic activity of CA IX is not necessary for this regulation. Other investigators have suggested the CA IX co-operates with the MCT transport family to excrete protons. To test this possibility, we examined the expression patterns of selected ion transporters and show that members of this family are differentially expressed within the molecular subtypes of breast cancer. The most aggressive form of breast cancer, triple-negative breast cancer, appears to co-ordinately express the monocarboxylate transporter 4 (MCT4) and carbonic anhydrase IX (CA IX). This supports a possible mechanism that utilizes the intramolecular H⁺ shuttle system in CA IX to facilitate proton efflux through MCT4.

Role of hypoxia and EGF on expression, activity, localization and phosphorylation of carbonic anhydrase IX in MDA-MB-231 breast cancer cells

Carbonic anhydrase IX (CAIX) is a zinc metalloenzyme that catalyzes the reversible hydration of CO(2). CAIX is overexpressed in many types of cancer, including breast cancer, but is most frequently absent in corresponding normal tissues. CAIX expression is strongly induced by hypoxia and is significantly associated with tumor grade and poor survival. Herein, we show that hypoxia induces a significant increase in CAIX protein in MDA-MB-231 breast cancer cells. Using a unique mass spectrophotometric assay, we demonstrate that CAIX activity in plasma membranes isolated from MDA-MB-231 is correlated with CAIX content. We also show that CAIX exists predominantly as a dimeric, high-mannose N-linked glycoprotein. While there is some evidence that the dimeric form resides specifically in lipid rafts, our data do not support this hypothesis. EGF, alone, did not affect the distribution of CAIX into lipid rafts. However, acute EGF treatment in the context of hypoxia increased the amount of CAIX in lipid rafts by about 5-fold. EGF did not stimulate tyrosine phosphorylation of CAIX, although EGFR and down-stream signaling pathways were activated by EGF. Interestingly, hypoxia activated Akt independent of EGF action. Together, these data demonstrate that the active form of CAIX in the MDA-MB-231 breast cancer cell line is dimeric but that neither lipid raft localization nor phosphorylation are likely required for its dimerization or activity.

Impaired glucose transporter-1 degradation and increased glucose transport and oxidative stress in response to high glucose in chondrocytes from osteoarthritic versus normal human cartilage

Introduction

Disorders that affect glucose metabolism, namely diabetes mellitus (DM), may favor the development and/or progression of osteoarthritis (OA). Thus far, little is known regarding the ability of chondrocytes to adjust to variations in the extracellular glucose concentration, resulting from hypoglycemia and hyperglycemia episodes, and so, to avoid deleterious effects resulting from deprivation or intracellular accumulation of glucose. The aim of this study was to compare the ability of normal and OA chondrocytes to regulate their glucose transport capacity in conditions of insufficient or excessive extracellular glucose and to identify the mechanisms involved and eventual deleterious consequences, namely the production of reactive oxygen species (ROS).

Methods

Chondrocytes, isolated from normal and OA human cartilage, were maintained in high-density monolayer cultures, in media without or with 10 or 30 mM glucose. Glucose transport was measured as the uptake of 2-deoxy-D-glucose (2-DG). Glucose transporter-1 (GLUT-1) mRNA and protein content were evaluated by real-time RT-PCR and western blot, respectively. ROS production was measured with 2',7'-dichlorodihydrofluorescein diacetate.

Results

Basal and IL-1β-induced 2-DG uptake, including the affinity (1.066 ± 0.284 and 1.49 ± 0.59 mM) and maximal velocity (0.27 ± 0.08 and 0.33 ± 0.08 nmol/μg protein/hour), and GLUT-1 content were identical in normal and OA chondrocytes. Glucose deprivation increased 2-DG uptake and GLUT-1 protein both in normal and OA chondrocytes. Exposure to high glucose (30 mM) for 18 or 48 hours decreased those parameters in normal but not in OA chondrocytes. GLUT-1 mRNA levels were unaffected by high glucose, either in normal or OA chondrocytes. The high glucose-induced reduction in GLUT-1 protein in normal chondrocytes was reversed by treatment with a lysosome inhibitor. High glucose induced ROS production, which lasted significantly longer in OA than in normal chondrocytes.

Conclusions

Normal human chondrocytes adjust to variations in the extracellular glucose concentration by modulating GLUT-1 synthesis and degradation which involves the lysosome pathway. Although capable of adjusting to glucose deprivation, OA chondrocytes exposed to high glucose were unable downregulate GLUT-1, accumulating more glucose and producing more ROS. Impaired GLUT-1 downregulation may constitute an important pathogenic mechanism by which conditions characterized by hyperglycemia, like DM, can promote degenerative changes in chondrocytes that can facilitate the progression of OA.

IRE1 signaling is essential for ischemia-induced vascular endothelial growth factor-A expression and contributes to angiogenesis and tumor growth in vivo

In solid tumors, cancer cells subjected to ischemic conditions trigger distinct signaling pathways contributing to angiogenic stimulation and tumor development. Characteristic features of tumor ischemia include hypoxia and glucose deprivation, leading to the activation of hypoxia-inducible factor-1-dependent signaling pathways and to complex signaling events known as the unfolded protein response. Here, we show that the activation of the endoplasmic reticulum stress sensor IRE1 is a common determinant linking hypoxia- and hypoglycemia-dependent responses to the up-regulation of vascular endothelial growth factor-A (VEGF-A). Tumor cells expressing a dominant-negative IRE1 transgene as well as Ire1alpha-null mouse embryonic fibroblasts were unable to trigger VEGF-A up-regulation upon either oxygen or glucose deprivation. These data correlated with a reduction of tumor angiogenesis and growth in vivo. Our results therefore suggest an essential role for IRE1-dependent signaling pathways in response to ischemia and identify this protein as a potential therapeutic target to control both the angiogenic switch and tumor development.

Uncoupling of 3T3-L1 gene expression from lipid accumulation during adipogenesis

Adipocyte differentiation comprises altered gene expression and increased triglyceride storage. To investigate the interdependency of these two events, 3T3-L1 cells were differentiated in the presence of glucose or pyruvate. All adipocytic proteins examined were similarly increased between the two conditions. In contrast, 3T3-L1 adipocytes differentiated with glucose exhibited significant lipid accumulation, which was largely suppressed in the presence of pyruvate. Subsequent addition of glucose to the latter cells restored lipid accumulation and acute rates of insulin-stimulated lipogenesis. These data indicate that extracellular energy is required for induction of adipocytic proteins, while only glucose sustained the parallel increase in triglyceride storage.

Differential Activation of Glucose Transport in Cultured Muscle Cells by Polyphenolic Compounds from Canna indica L. Root

Effects of extracts of a plant, which has been used as a traditional medicine for treating diabetes on glucose transport activity was evaluated in cultured L8 muscle cells. The aqueous extract of Canna indica root (CI) at doses of 0.1-0.5 mg/ml, which contains total phenolic compounds equivalent to 6-30 microg of catechin caused a dose- and time-dependent induction of 2-deoxy-[3H]glucose (2-DG) uptake activity. The induced 2-DG uptake was significantly increased within 8 h and reached a maximum by 16 h. The CI extract increased the amount of glucose transporter isoforms 1 (GLUT1) and 4 (GLUT4) at the cell surface and enhanced expression of GLUT1 protein. Cycloheximide treatment almost completely reversed CI-induced 2-DG uptake to the basal level. Exposure of muscle cells to wortmannin and SB203580 diminished CI-mediated glucose uptake by 38 and 14%, respectively. The effect of CI and insulin was partially additive. Phytochemical analysis detected the presence of flavonoids and catechol in the CI. Taken together, these data provide evidence for differential effects of CI on regulated-glucose transport in muscle cells. Our findings suggest that GLUT1 protein synthesis and the activation of phosphatidylinositol 3-kinase (PI3K) are critical for the increase in glucose transporter activity at the plasma membrane and essential for the maximal induction of glucose transport by CI in L8 muscle cells.

Dehydroascorbate transport in human chondrocytes is regulated by hypoxia and is a physiologically relevant source of ascorbic acid in the joint

OBJECTIVE

To evaluate the dehydroascorbate (DHA) transport mechanisms in human chondrocytes.

METHODS

The transport of L-(14)C-DHA in human chondrocytes was analyzed under various conditions, including the use of RNA interference (RNAi), to determine the role of glucose transporter 1 (GLUT-1) and GLUT-3 in L-14C-DHA transport and to evaluate the effects of physiologically relevant oxygen tensions on L-14C-DHA transport. In order to estimate the contributions of reduced ascorbic acid (AA) and DHA to intracellular ascorbic acid (Asc), the quantities of AA and DHA were measured in synovial fluid samples from osteoarthritis (OA) patients and compared with the reported levels in rheumatoid arthritis (RA) patients.

RESULTS

DHA transport in human chondrocytes was glucose-sensitive, temperature-dependent, cytochalasin B-inhibitable, modestly stereoselective for L-DHA, and up-regulated by low oxygen tension. Based on the RNAi results, GLUT-1 mediated, at least in part, the uptake of DHA, whereas GLUT-3 had a minimal effect on DHA transport. DHA constituted a mean 8% of the total Asc in the synovial fluid of OA joints, in contrast to 80% of the reported total Asc in RA joints.

CONCLUSION

We provide the first evidence that chondrocytes transport DHA via the GLUTs and that this transport mechanism is modestly selective for L-DHA. In the setting of up-regulated DHA transport at low oxygen tensions, DHA would contribute 26% of the total intracellular Asc in OA chondrocytes and 94% of that in RA chondrocytes. These results demonstrate that DHA is a physiologically relevant source of Asc for chondrocytes, particularly in the setting of an inflammatory arthritis, such as RA.

Human T Cell Leukemia Virus Envelope Binding and Virus Entry Are Mediated by Distinct Domains of the Glucose Transporter GLUT1

The glucose transporter GLUT1, a member of the multimembrane-spanning facilitative nutrient transporter family, serves as a receptor for human T cell leukemia virus (HTLV) infection. Here, we show that the 7 amino acids of the extracellular loop 6 of GLUT1 (ECL6) placed in the context of the related GLUT3 transporter were sufficient for HTLV envelope binding. Glutamate residue 426 in ECL6 was identified as critical for binding. However, binding to ECL6 was not sufficient for HTLV envelope-driven infection. Infection required two additional determinants located in ECL1 and ECL5, which otherwise did not influence HTLV envelope binding. Moreover the single N-glycosylation chain located in ECL1 was not required for HTLV infection. Therefore, binding involves a discrete determinant in the carboxyl terminal ECL6, whereas post-binding events engage extracellular sequences in the amino and carboxyl terminus of GLUT1.

Characterization of a Glomus intraradices gene encoding a putative Zn transporter of the cation diffusion facilitator family

A full-length cDNA (GintZnT1) encoding a putative Zn transporter was isolated from the extraradical mycelium of Glomus intraradices. Based on its sequence analysis, GintZnT1 was classified as a member of the cation diffusion facilitator (CDF) family of heavy metal transporters. Functional analysis of GintZnT1 was performed by heterologous expression in yeast mutants defective in different CDFs. Although Zn sensitivity of the mutants was not reverted, an effect of GintZnT1 on the labile regulatory Zn pool was detected by using a Zn-regulated beta-galactosidase reporter gene. GintZnT1 expression was studied in the extraradical mycelium obtained from a symbiotic root organ culture. Gin +/- ZnT1 was up-regulated in the extraradical mycelium of G. intraradices upon short-time exposure to Zn and when the mycelia were developed in 75 microM Zn supplemented plates. These data suggest a role of GintZnT1 in Zn compartmentalization and in the protection of G. intraradices against Zn stress.

Soluble fibroin enhances insulin sensitivity and glucose metabolism in 3T3-L1 adipocytes

Type 2 diabetes is characterized by hyperglycemia and hyperinsulinemia, features of insulin resistance. In vivo treatment of ob/ob mice with hydrolyzed fibroin reverses these pathological attributes. To explore the mechanism underlying this effect, we used the murine, 3T3-L1 adipocyte cell line, which has been used extensively to model adipocyte function. Chronic exposure of 3T3-L1 adipocytes to insulin leads to a 50% loss of insulin-stimulated glucose uptake. Chronic exposure to different preparations of fibroin partially blocked the response to insulin but also increased the sensitivity of control cells to the acute action of insulin. The latter effect was most robust at physiologic concentrations of insulin. Fibroin did not prevent the insulin-induced downregulation of the insulin receptor or the tyrosine kinase activity associated with the receptor. Further, fibroin had no effect on the activity of the insulin-sensitive downstream kinase, Akt. Interestingly, fibroin accelerated glucose metabolism and glycogen turnover independent of insulin action. In addition, fibroin upregulated glucose transporter (GLUT)1, which increased its expression at the cell surface and enhanced GLUT4 translocation. Together, these phenomena may underlie the improvement in diabetic hyperglycemia noted in vivo in response to fibroin.

Glucose deprivation enhances targeting of GLUT1 to lipid rafts in 3T3-L1 adipocytes

Glucose deprivation dramatically increases glucose transport activity in 3T3-L1 adipocytes without changing the concentration of GLUT1 in the plasma membrane (PM). Recent data suggest that subcompartments within the PM, specifically lipid rafts, may sequester selected proteins and alter their activity. To evaluate this possibility, we examined the distribution of GLUT1 in Triton X-100-soluble and -insoluble fractions. Our data show that 77% of the GLUT1 pool in PMs isolated from control 3T3-L1 adipocytes was extracted by 0.2% Triton X-100. After glucose deprivation for 12 h, only 56% of GLUT1 was extracted by detergent. In contrast, there was a twofold increase in the GLUT1 content of the detergent-resistant fraction. To evaluate whether GLUT1 interacts with a specific protein within lipid rafts, we focused on stomatin, recently shown to interact with and inhibit GLUT1 activity. Stomatin is distributed about equally between the PM and the biosynthetic compartments, and its expression is not affected by glucose deprivation. Nearly 90% of the PM pool of stomatin is in detergent-resistant lipid rafts. In normal 3T3-L1 adipocytes, we were unable to demonstrate an interaction between GLUT1 and stomatin in coimmunoprecipitation experiments. However, in stomatin-overexpressing cells, there was clear coprecipitation of stomatin with GLUT1 antibodies. Glucose deprivation increased this interaction threefold, which may reflect the increase of GLUT1 in lipid rafts. Despite this, there was little change in transport activity in glucose-deprived, stomatin-overexpressing cells vs. that in control cells. Thus GLUT1 interacts with stomatin in lipid rafts, but this interaction per se does not alter transport activity. Rather, stomatin may serve as an anchor for GLUT1 in lipid rafts, the environment of which favors activation.

Distinct regulation of glucose transport and GLUT1/GLUT3 transporters by glucose deprivation and IGF-I in chromaffin cells

Effects of prolonged metabolic (glucose deprivation) and hormonal [insulin-like growth factor I (IGF-I)] challenge on regulation of glucose transporter (GLUT) expression, glucose transport rate and possible signaling pathways involved were studied in the neuroendocrine chromaffin cell. The results show that bovine chromaffin cells express both GLUT1 and GLUT3. Glucose deprivation and IGF-I activation led to an elevation of GLUT1 and GLUT3 mRNA, the strongest effect being that of IGF-I on GLUT3 mRNA. Both types of stimulus increased the GLUT1 protein content in a cycloheximide (CHX)-sensitive manner, and the glucose transport rate was elevated by 3- to 4-fold after 48 h under both experimental conditions. IGF-I-induced glucose uptake was totally suppressed by CHX. In contrast, only approximately 50% of transport activation in glucose-deprived cells was sensitive to the protein synthesis inhibitor. Specific inhibitors of mTOR/FRAP and p38 MAPK each partially blocked IGF-I-stimulated glucose transport, but had no effect on transport rate in glucose-deprived cells. The results are consistent with IGF-I-activated transport being completely dependent on new GLUT protein synthesis while the enhanced transport in glucose-deprived cells was partially achieved independent of new synthesis of proteins, suggesting a mechanism relying on preexisting transporters.

Evaluation of 2-deoxy-D-glucose as a chemotherapeutic agent: mechanism of cell death

Nutrient deprivation has been shown to cause cancer cell death. To exploit nutrient deprivation as anti-cancer therapy, we investigated the effects of the anti-metabolite 2-deoxy-D-glucose on breast cancer cells in vitro. This compound has been shown to inhibit glucose metabolism. Treatment of human breast cancer cell lines with 2-deoxy-D-glucose results in cessation of cell growth in a dose dependent manner. Cell viability as measured by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide conversion assay and clonogenic survival are decreased with 2-deoxy-D-glucose treatment indicating that 2-deoxy-D-glucose causes breast cancer cell death. The cell death induced by 2-deoxy-D-glucose was found to be due to apoptosis as demonstrated by induction of caspase 3 activity and cleavage of poly (ADP-ribose) polymerase. Breast cancer cells treated with 2-deoxy-D-glucose express higher levels of Glut1 transporter protein as measured by Western blot analysis and have increased glucose uptake compared to non-treated breast cancer cells. From these results we conclude that 2-deoxy-D-glucose treatment causes death in human breast cancer cell lines by the activation of the apoptotic pathway. Our data suggest that breast cancer cells treated with 2-deoxy-D-glucose accelerate their own demise by initially expressing high levels of glucose transporter protein, which allows increased uptake of 2-deoxy-D-glucose, and subsequent induction of cell death. These data support the targeting of glucose metabolism as a site for chemotherapeutic intervention by agents such as 2-deoxy-D-glucose.

C2C12 myocytes lack an insulin-responsive vesicular compartment despite dexamethasone-induced GLUT4 expression

Insulin regulates the uptake of glucose into skeletal muscle and adipocytes by redistributing the tissue-specific glucose transporter GLUT4 from intracellular vesicles to the cell surface. To date, GLUT4 is the only protein involved in insulin-regulated vesicular traffic that has this tissue distribution, thus raising the possibility that its expression alone may allow formation of an insulin-responsive vesicular compartment. We show here that treatment of differentiating C2C12 myoblasts with dexamethasone, acting via the glucocorticoid receptor, causes a >or=10-fold increase in GLUT4 expression but results in no significant change in insulin-stimulated glucose transport. Signaling from the insulin receptor to its target, Akt2, and expression of the soluble N-ethylmaleimide-sensitive factor-attachment protein receptor, or SNARE, proteins syntaxin 4 and vesicle-associated membrane protein are normal in dexamethasone-treated C2C12 cells. However, these cells show no insulin-dependent trafficking of the insulin-responsive aminopeptidase or the transferrin receptor, respective markers for intracellular GLUT4-rich compartments and endosomes that are insulin responsive in mature muscle and adipose cells. Therefore, these data support the hypothesis that GLUT4 expression by itself is insufficient to establish an insulin-sensitive vesicular compartment.

Soaking turkey meat in salt-glucose syrup solutions--an experimental study of mass transfers

Turkey meat can be salted and dried in one step by soaking in a concentrated salt-glucose syrup solution at low temperature. Sugar impregnation is minimal; only low molecular weight sugars generally penetrate the product. Glucose uptake is very quick, suggesting the possible involvement of passive glucose transporters. The operational scope of this process, depending on the targeted end-product features, was determined for turkey meat on the basis of clearly characterized mass transport phenomena between the product and the soaking solution. With 2 cm thick meat fillets processed at 10 C it is thus possible to obtain salted-dried end-products containing 2 to 10% salt and 35 to 70% water, ranges that are compatible with a broad range of commercial cured products.

An unusual subcellular localization of GLUT1 and link with metabolism in oocytes and preimplantation mouse embryos

Although mouse oocytes and cleavage-stage embryos prefer pyruvate and lactate for metabolic fuels, they do take up and metabolize glucose. Indeed, presentation of glucose during the cleavage stages is required for subsequent blastocyst formation, which normally relies on uptake and metabolism of large amounts of glucose. Expression of the facilitative glucose transporter GLUT1 was examined using immunohistochemistry and Western blotting, and in polyspermic oocytes, metabolism of glucose was measured and compared with that of pyruvate and glutamine. GLUT1 was observed in all oocytes and embryos, and membrane and vesicular staining was present. Additionally, however, in polyspermic oocytes, the most intense staining was in the pronuclei, and this nuclear staining persisted in cleaving normal embryos. Furthermore, GLUT1 expression appeared to be up-regulated both in nuclei and plasma membranes following culture of oocytes in the absence of glucose. In polyspermic oocytes, the metabolism of glucose, but not of pyruvate or glutamine, was directly proportional to the number of pronuclei formed. After compaction, nuclear staining diminished, and GLUT1 localized to basolateral membranes of the outer cells and trophectoderm. In blastocysts, a weak but uniform staining of inner-cell-mass plasma membranes was apparent. The results are discussed in terms of potential roles for GLUT1 in pronuclei of oocytes and zygotes, nuclei of cleavage-stage embryos, and a transepithelial transport function for GLUT1, probably coupled with GLUT3, in compacted embryos and blastocysts.

Dietary iron induces rapid changes in rat intestinal divalent metal transporter expression

The divalent metal transporter (DMT1, also known as NRAMP2 or DCT1) is the likely target for regulation of intestinal iron absorption by iron stores. We investigated changes in intestinal DMT1 expression after a bolus of dietary iron in iron-deficient Belgrade rats homozygous for the DMT1 G185R mutation (b/b) and phenotypically normal heterozygous littermates (+/b). Immunofluorescent staining with anti-DMT1 antisera showed that DMT1 was located in the brush-border membrane. Duodenal DMT1 mRNA and protein levels were six- and twofold higher, respectively, in b/b rats than in +/b rats. At 1.5 h after dietary iron intake in +/b and b/b rats, DMT1 was internalized into cytoplasmic vesicles. At 1.5 and 3 h after iron intake in +/b and b/b rats, there was a rapid decrease of DMT1 mRNA and a transient increase of DMT1 protein. The decrease of DMT1 mRNA was specific, because ferritin mRNA was unchanged. After iron intake, an increase in ferritin protein and decrease in iron-regulatory protein binding activity occurred, reflecting elevated intracellular iron pools. Thus intestinal DMT1 rapidly responds to dietary iron in both +/b and b/b rats. The internalization of DMT1 may be an acute regulatory mechanism to limit iron uptake. In addition, the results suggest that in the Belgrade rat DMT1 with the G185R mutation is not an absolute block to iron.

Glucose deprivation does not affect GLUT1 targeting in 3T3-L1 adipocytes

We have previously demonstrated that glucose deprivation alters the glycosylation of the GLUT1 glucose transporter in 3T3-L1 adipocytes. Many aberrantly glycosylated proteins are retained in the endoplasmic reticulum by interaction with chaperones. Herein, we use three independent procedures to show that GLUT1 is targeted to the plasma membrane, despite alterations in glycosylation. While earlier experiments revealed that plasma membrane targeting of aglyco GLUT 1 transporter was significantly reduced, our data show for the first time that altered glycosylation provides sufficient information to drive appropriate trafficking.

Rapid GLUT-1 mediated glucose transport in erythrocytes from the grey-headed fruit bat (Pteropus poliocephalus)

D-Glucose entry into erythrocytes from adult grey-headed flying fox fruit bats (Pteropus poliocephalus) was rapid and showed saturation at high substrate concentrations. Kinetic parameters were estimated from the concentration dependence of initial rates of zero-trans D-glucose entry at 5.5 degrees C as Michaelis constant (K(m)) 1. 64+/-0.56 mM, and maximal velocity (V(max)) 1162+/-152 micromol.l. cell water(-1).min(-1). D-Glucose entry was inhibited by cytochalasin B; mass law analysis of D-glucose-displaceable cytochalasin B binding gave values of K(d) 37.1+/-5.0 nM and B(max) 361.2+/-9.1 pmol/mg membrane protein. Entry of 2-deoxy-D-glucose, and 3-O-methyl-D-glucose, into P. poliocephalus red cells was rapid, entry of D-fructose was very slow. Glucose transporter polypeptides were identified on immunoblots as a band M(r) 47000-54000 and their identity confirmed by D-glucose-sensitive photolabeling of membranes with [3H]-cytochalasin B. Peptide-N-glycanase F digestion of both human and bat erythrocyte membranes generated GLUT-1-derived bands M(r) 37000. Trypsin digestion of human and fruit bat erythrocyte membranes generated fragmentation patterns consistent with similar GLUT-1 polypeptide structures in both species. Erythrocytes from adult Australian ghost bats (Macroderma gigas), a carnivorous microchiropteran bat, also expressed high levels of GLUT-1.

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.

High glucose concentrations inhibit glucose phosphorylation, but not glucose transport, in human endothelial cells

Glucose uptake is autoregulated in a variety of cell types and it is thought that glucose transport is the major step that is subjected to control by sugar availability. Here, we examined the effect of high glucose concentrations on the rate of glucose uptake by human ECV-304 umbilical vein-derived endothelial cells. A rise in the glucose concentration in the medium led a dose-dependent decrease in the rate of 2-deoxyglucose uptake. The effect of high glucose was independent of protein synthesis and the time-course analysis indicated that it was relatively slow. The effect was not due to inhibition of glucose transport since neither the expression nor the subcellular distribution of the major glucose transporter GLUT1, nor the rate of 3-O-methylglucose uptake was affected. The total in vitro assayed hexokinase activity and the expression of hexokinase-I were similar in cells treated or not with high concentrations of glucose. In contrast, exposure of cells to a high glucose concentration caused a marked decrease in phosphorylated 2-deoxyglucose/free 2-deoxyglucose ratio. This suggests the existence of alterations in the rate of in vivo glucose phosphorylation in response to high glucose. In summary, we conclude that ECV304 human endothelial cells reduce glucose utilization in response to enhanced levels of glucose in the medium by inhibiting the rate of glucose phosphorylation, rather than by blocking glucose transport. This suggests a novel metabolic effect of high glucose on cellular glucose utilization.

Regulation of brain glucose transporters by glucose and oxygen deprivation

Brain cells are dependent on glucose and oxygen for energy. We investigated the effects of hypoxia, glucose deprivation, and hypoxia plus glucose deprivation on mRNA and protein levels of glucose transporter (GLUT1) and GLUT3 and 2-deoxyglucose (2-DG) uptake in primary cultures of rat neurons and astroglia. Hypoxia for 24 hours did not significantly affect cell viability but increased neuronal GLUT1 and GLUT3 mRNA up to 40-fold and fivefold, respectively, above control levels. Similar changes in GLUT1 mRNA were measured in glia. The effects of hypoxia on GLUT1 and GLUT3 mRNA were reversible. The increase in GLUT1 mRNA could be detected within 20 minutes of hypoxia and was blocked by actinomycin D. Nuclear runoff transcription assays showed that hypoxia did not alter the transcription rate of GLUT1. However, hypoxia enhanced the stability of GLUT1 mRNA in neurons (half-life [t(l/2)] > 12 hours) compared with normoxic conditions (t(1/2) approximately 10.4 hours), suggesting the existence of a posttranscriptional mechanism for the regulation of GLUT1 transcript levels. Twenty-four hours of normoxia and 1.0 mmol/L glucose increased neuronal GLUT1 mRNA less than threefold above basal, but 24 hours of glucose and oxygen deprivation increased GLUT1 over 111-fold above basal. Induction of neuronal GLUT1 mRNA was temporally associated with increased levels of GLUT1 protein and with stimulation of intracellular 2-DG accumulation. We conclude that hypoxia reversibly increases the transcript levels of GLUT1 and GLUT3 in rat brain cells and stimulates GLUT1 transcript levels by posttranscriptional mechanisms. Although glucose deprivation alone produces minimal effects on GLUT mRNA levels, hypoxia plus glucose deprivation synergize to markedly increase GLUT gene expression.

Effects of oral antihyperglycemic agents on extracellular matrix synthesis by mesangial cells

BACKGROUND

Increased expression of the glucose transporter GLUT1 in mesangial cells (MCs) markedly stimulates glucose transport and the formation of extracellular matrix (ECM), even when ambient glucose concentrations are low. Certain antihyperglycemic agents cause GLUT1 overexpression and increase glucose transport in various tissues. However, their effects on the kidney are unknown. Because diabetic glomerulosclerosis is characterized by the accumulation of mesangial matrix, was studied the effects of antihyperglycemic agents on matrix metabolism in MCs cultured either in 8 or 20 mM glucose.

METHODS

Membrane-associated GLUT1 was measured by immunoblotting. The initial rate of glucose transport was determined according to the 2-deoxy-D[14C(U)]glucose uptake. Collagen metabolism was studied by metabolic radiolabeling with [14C]-proline. Fibronectin in the medium was measured by ELISA. GLUT1 mRNA was estimated by Northern analysis.

RESULTS

The sulfonylurea tolazamide increased GLUT1 protein expression by 107 and 69% in 8 and 20 mM glucose-grown cells, respectively. However, GLUT1 mRNA levels remained unchanged. Transporter-dependent deoxyglucose uptake was increased by tolazamide up to 184% in a dose-dependent fashion and was evident at both glucose concentrations after three or five days of exposure to the drug. Tolazamide significantly stimulated transforming growth factor-beta 1 (TGF-beta 1) secretion and the total synthesis of collagen and collagen and fibronectin accumulation in the medium of MCs maintained in high or low glucose concentrations. The biguanide metformin did not alter GLUT1 expression, glucose transport, fibronectin formation, or collagen metabolism, except at high concentrations.

CONCLUSION

Tolazamide markedly enhances ECM synthesis and accumulation in MCs probably by stimulating GLUT1 expression, glucose transport and TGF-beta 1 secretion, irrespective of the ambient glucose concentration. This effect was dose-dependent and minimally inducible by metformin.

Hyperglycemia regulates the glucose-transport system of clonal choriocarcinoma cells in vitro. A potential molecular mechanism contributing to the adjunct effect of glucose in tumor therapy

Glucose is taken up by tumor cells via sodium-independent facilitated diffusion along a concentration gradient. To examine the regulation of this process by substrate concentration, we investigated the effect of hyperglycemia on the glucose-transport system of choriocarcinoma-derived JAR and JEG-3 cells by culturing them for 24, 48 and 96 hr in medium containing either 5.5 (normoglycemia) or 25 (hyperglycemia) mM D-glucose, respectively. Immunocytochemically, choriocarcinoma cells expressed the high-affinity glucose transporter isoforms GLUT1 and GLUT3. Based on initial uptake measurements using 3-O-[14C]methyl-D-glucose, kinetic parameters were calculated as Km = 15 mM and Vmax = 95 fmol/sec per cell for JAR and Km = 9 mM and Vmax = 64 fmol/sec per cell for JEG-3 cells. In JAR cells cultured under hyperglycemic conditions, uptake rates were significantly increased at 15, 20 and 25 mM exogenous D-glucose concentrations as compared with normoglycemic conditions. This effect was due to an increase in Vmax, whereas Km remained unchanged. Using Northern blotting, GLUT1 mRNA levels were higher but GLUT3 transcripts were reduced upon hyperglycemia. Western blotting revealed elevated GLUT1 and GLUT3 expression under hyperglycemic conditions. Hyperglycemia did not significantly influence the glucose-transport system of JEG-3 cells. We conclude that sustained hyperglycemia stimulates the glucose-transport system of JAR, but not of JEG-3, choriocarcinoma cells in vitro due to changes in GLUT1 and GLUT3 expression levels. We speculate that this mechanism may contribute to the beneficial effects of induced hyperglycemia as an adjuvant in tumor therapy.

Sustained hyperglycemia in vitro down-regulates the GLUT1 glucose transport system of cultured human term placental trophoblast: a mechanism to protect fetal development?

The trophoblast of human placenta is directly exposed to the maternal circulation. It forms the main barrier to maternal-fetal glucose transport. The present study investigated the effect of sustained hyperglycemia in vitro on the glucose transport system of these cells. Trophoblasts isolated from term placentas and immunopurified were cultured for 24, 48, and 96 h in DMEM containing either 5.5 (normoglycemia) or 25 mmol/l D-glucose (hyperglycemia), respectively. Initial uptake of glucose was measured using 3-O-[14C]methyl-D-glucose. Kinetic parameters were calculated as K(M) = 73 mmol/l and Vmax = 29 fmol s(-1) per trophoblast cell. Uptake rates of cells cultured under hyperglycemic conditions did not differ at exogenous D-glucose concentrations in the physiological range (1, 5.5, 10, and 15 mmol/l), but were significantly decreased by 25% (P<0.05) at diabetes-like concentrations (20 and 25 mmol/l) as compared to normoglycemic conditions. This effect was due to a decrease in Vmax (-50%), whereas K(M) remained virtually unaffected. GLUT1 mRNA levels were lower by 50% (P<0.05; Northern blotting) and GLUT1 protein was reduced by 16% (P<0.05; Western blotting) in trophoblast cells cultured under hyperglycemic vs. normoglycemic conditions. We conclude that prolonged hyperglycemia in vitro reduces trophoblast glucose uptake at substrate concentrations corresponding to blood levels of poorly controlled diabetic gravidas. This effect is due to diminished GLUT1 mRNA and protein expression in the trophoblast.

Unique mechanism of GLUT3 glucose transporter regulation by prolonged energy demand: increased protein half-life

L6 muscle cells survive long-term (18 h) disruption of oxidative phosphorylation by the mitochondrial uncoupler 2,4-dinitrophenol (DNP) because, in response to this metabolic stress, they increase their rate of glucose transport. This response is associated with an elevation of the protein content of glucose transporter isoforms GLUT3 and GLUT1, but not GLUT4. Previously we have reported that the rise in GLUT1 expression is likely to be a result of de novo biosynthesis of the transporter, since the uncoupler increases GLUT1 mRNA levels. Unlike GLUT1, very little is known about how interfering with mitochondrial ATP production regulates GLUT3 protein expression. Here we examine the mechanisms employed by DNP to increase GLUT3 protein content and glucose uptake in L6 muscle cells. We report that, in contrast with GLUT1, continuous exposure to DNP had no effect on GLUT3 mRNA levels. DNP-stimulated glucose transport was unaffected by the protein-synthesis inhibitor cycloheximide. The increase in GLUT3 protein mediated by DNP was also insensitive to cycloheximide, paralleling the response of glucose uptake, whereas the rise in GLUT1 protein levels was blocked by the inhibitor. The GLUT3 glucose transporter may therefore provide the majority of the glucose transport stimulation by DNP, despite elevated levels of GLUT1 protein. The half-lives of GLUT3 and GLUT1 proteins in L6 myotubes were determined to be about 15 h and 6 h respectively. DNP prolonged the half-life of both proteins. After 24 h of DNP treatment, 88% of GLUT3 protein and 57% of GLUT1 protein had not turned over, compared with 25% in untreated cells. We conclude that the long-term stimulation of glucose transport by DNP arises from an elevation of GLUT3 protein content associated with an increase in GLUT3 protein half-life. These findings suggest that disruption of the oxidative chain of L6 muscle cells leads to an adaptive response of glucose transport that is distinct from the insulin response, involving specific glucose transporter isoforms that are regulated by different mechanisms.

Regulation of the zinc transporter ZnT-1 by dietary zinc

The understanding of mechanisms controlling zinc absorption and metabolism at the molecular level has advanced recently. Kinetics of zinc transport have been investigated for many years, but only recently have genes coding for proteins thought to be involved in the transport process been cloned. Four putative zinc transporters, known as ZnT-1 through ZnT-4, have now been described. Among these transporters, only ZnT-1 is ubiquitously expressed. In this report, we examine the pattern of ZnT-1 expression in the intestine and analyze the regulation of ZnT-1 by dietary zinc in both the intestine and liver. Immunofluorescence demonstrated that intestinal ZnT-1 was most abundant at the basolateral surface of enterocytes lining the villi of the duodenum and jejunum. By Western blot analysis, intestinal and liver ZnT-1 protein migrated as a 42- and 36-kDa protein, respectively. Dietary zinc supplementation elevated the level of intestinal ZnT-1 mRNA and protein approximately 50% and 10%, respectively, but had no effect in the liver. In response to an acute oral zinc dose, the level of intestinal ZnT-1 mRNA was up-regulated 8-fold, without a corresponding increase in ZnT-1 protein. Conversely, the acute oral dose did not affect liver ZnT-1 mRNA, but resulted in a 5-fold increase in liver ZnT-1 protein. These results represent studies on the expression of intestinal and hepatic ZnT-1 in an intact animal model. The data suggest that ZnT-1 is at least part of the mechanism by which dietary zinc is absorbed and that, despite the zinc responsiveness of the ZnT-1 gene, additional factors may be regulating the steady-state level of ZnT-1 transporter protein.

Transcriptional activation of the Glut1 gene in response to oxidative stress in L6 myotubes

Exposure of L6 myotubes to prolonged low grade oxidative stress results in increased Glut1 expression at both the protein and mRNA levels, leading to elevated glucose transport activity. To further understand the cellular mechanisms responsible for this adaptive response, the Glut1 transcription rate and mRNA stability were assessed. Nuclear run-on assays revealed 2.0- and 2.4-fold increases in Glut1 transcription rates in glucose oxidase- and xanthine/xanthine oxidase-pretreated cells, respectively. Glut1 mRNA stability was increased with both treatments compared with the control (t1/2 = 7.8 +/- 1.3, 6.0 +/- 2.0, and 2.4 +/- 0.5 h, respectively). The serum-responsive element and AP-1 (but not the cAMP-responsive element) showed increased binding capacity following oxidative stress. Both activation of AP-1 binding and elevation of Glut1 mRNA were prevented by cycloheximide. The involvement of enhancer 1 of the Glut1 gene was demonstrated using transfected 293 cells. Induction of Glut1 mRNA in response to oxidative stress differed from its activation by chronic insulin exposure as demonstrated by the ability of rapamycin to inhibit the latter without an effect on the former. In conclusion, oxidative stress increases the Glut1 transcription rate by mechanisms that may involve activation of AP-1 binding to enhancer 1 of the Glut1 gene.

Increased glucose transport in ras-transformed fibroblasts: a possible role for N-glycosylation of GLUT1

2-Deoxyglucose uptake was enhanced in ts371 KiMuSV-NRK cells when growing at the permissive temperature to allow the expression of a transforming p21 ras protein. This change is due to a decrease in the K(m) by approximately 2.5-fold without affecting the V(max) of the transporter. The amount of the GLUT1 glucose transporter dit not increase as deduced from immunoblot experiments on total membranes. Nevertheless, ras-transformed GLUT1 displays a higher molecular mass due to an increased N-glycosylation of the protein. Experiments made in tunicamycin-treated cells indicates that a higher glycosylation is responsible for the increase in 2-deoxyglucose uptake in ras-transformed cells.

Ectopic expression of Hel-N1, an RNA-binding protein, increases glucose transporter (GLUT1) expression in 3T3-L1 adipocytes

3T3-L1 preadipocytes ectopically expressing the mammalian RNA-binding protein Hel-N1 expressed up to 10-fold more glucose transporter (GLUT1) protein and exhibited elevated rates of basal glucose uptake. Hel-N1 is a member of the ELAV-like family of proteins associated with the induction and maintenance of differentiation in various species. ELAV proteins are known to bind in vitro to short stretches of uridylates in the 3' untranslated regions (3'UTRs) of unstable mRNAs encoding growth-regulatory proteins involved in transcription and signal transduction. GLUT1 mRNA also contains a large 3'UTR with a U-rich region that binds specifically to Hel-N1 in vitro. Analysis of the altered GLUT1 expression at the translational and posttranscriptional levels suggested a mechanism involving both mRNA stabilization and accelerated formation of translation initiation complexes. These findings are consistent with the hypothesis that the Hel-N1 family of proteins modulate gene expression at the level of mRNA in the cytoplasm.

Differential regulation of GRP78 and GLUT1 expression in 3T3-L1 adipocytes

We tested the hypothesis that the constitutive glucose transporter (GLUT1) in 3T3-L1 adipocytes belongs to the family of glucose-regulated proteins which are transcriptionally regulated by glucose deprivation. Using cDNA probes for both GRP78 (BiP) and GLUT1, we show that the level of GRP78 mRNA increased by 15-fold within 24 h of glucose deprivation with little change in GLUT1 mRNA. The elevated GRP78 mRNA in turn led to a time-dependent increase in GRP78 protein. While glucose deprivation did not alter the expression of the normal glycoform of GLUT1, a lower molecular weight glycoform accumulated with extended deprivation. Mannose and fructose, but not galactose, prevented the induction of GRP78 and accumulation of the abnormal GLUT1. Because GRP78 acts as a chaperone in other cell systems, we also sought evidence to support this activity in 3T3-L1 adipocytes. Using the technique of co-immunoprecipitation, we demonstrate that GRP78 bound several proteins unique to the glucose-deprived state. No deprivation-specific proteins could be detected in association with GLUT1. These data lead us to conclude that GLUT1 does not display characteristics of the glucose-regulated proteins, at least in 3T3-L1 adipocytes, a widely used model for differentiation, hormone action, and nutrient control. However, the mechanisms for activating traditional members of this family appear intact.

Studies of renal injury. II. Activation of the glucose transporter 1 (GLUT1) gene and glycolysis in LLC-PK1 cells under Ca2+ stress

Injury to the renal proximal tubule is common and may be followed by either recovery or cell death. The survival of injured cells is supported by a transient change in cellular metabolism that maintains life even when oxygen tension is reduced. This adaptive process involves the activation of the gene encoding the glucose transporter GLUT1, which is essential to maintain the high rates of glucose influx demanded by glycolysis. We hypothesized that after cell injury increases of cell Ca2+ (Ca2+i) initiate the flow of information that culminates with the upregulation of the stress response gene GLUT1. We found that elevations of Ca2+i caused by the calcium ionophore A23187 activated the expression of the GLUT1 gene in LLC-PK1 cells. The stimulatory effect of Ca2+i on GLUT1 gene expression was, at least in part, transcriptional and resulted in higher levels of GLUT1 mRNA, cognate protein, cellular hexose transport activity, glucose consumption, and lactate production. This response was vital to the renal cells, as its interruption severely increased Ca2+-induced cytotoxicity and cell mortality. We propose that increases of Ca2+i initiate stress responses, represented in part by activation of the GLUT1 gene, and that disruption to the flow of information originating from Ca2+-induced stress, or to the coordinated expression of the stress response, prevents cell recovery after injury and may be an important cause of permanent renal cell injury and cell death.

Glycogen: a carbohydrate source for GLUT-1 glycosylation during glucose deprivation of 3T3-L1 adipocytes

In 3T3-L1 adipocytes, the glycosylation of the GLUT-1 transporter is altered beyond 12 h of glucose deprivation. To determine whether glycogen degradation provides substrate for normal protein glycosylation during this delay, we measured the glycogen content of 3T3-L1 adipocytes. From an initial value of 0.537 +/- 0.097 mumol glucose/10(6) cells, glycogen was depleted in a time-dependent manner in response to glucose deprivation, exhibiting a half-time of 6 h. Surprisingly, fructose did not prevent glycogen depletion. However, in such glycogen-depleted adipocytes, the alteration of GLUT-1 glycosylation in response to glucose deprivation was more rapid than in normal adipocytes. Chinese hamster ovary (CHO) cells, which synthesize abbreviated dolichol-linked oligosaccharides within minutes of glucose deprivation (J. I. Rearick, A. Chapman, and S. Kornfeld. J. Biol. Chem. 256: 6255-6261, 1981), contained only 1% of the level of glycogen found in 3T3-L1 adipocytes. Glycosylation of GLUT-1 was altered in CHO cells within 3 h of glucose deprivation. These data demonstrate that, during glucose stress, glycogen may serve as a buffer for oligosaccharide biosynthesis and provide a potential explanation for varying sensitivities of different cell types to glucose deprivation.