Regulation of GLUT1-mediated sugar transport by an antiport/uniport switch mechanism

Do Amino Acid Antiporters Have Asymmetric Substrate Specificity?

Amino acid antiporters mediate the 1:1 exchange of groups of amino acids. Whether substrate specificity can be different for the inward and outward facing conformation has not been investigated systematically, although examples of asymmetric transport have been reported. Here we used LC-MS to detect the movement of ¹²C- and ¹³C-labelled amino acid mixtures across the plasma membrane of Xenopus laevis oocytes expressing a variety of amino acid antiporters. Differences of substrate specificity between transporter paralogs were readily observed using this method. Our results suggest that antiporters are largely symmetric, equalizing the pools of their substrate amino acids. Exceptions are the antiporters y⁺LAT1 and y⁺LAT2 where neutral amino acids are co-transported with Na⁺ ions, favouring their import. For the antiporters ASCT1 and ASCT2 glycine acted as a selective influx substrate, while proline was a selective influx substrate of ASCT1. These data show that antiporters can display non-canonical modes of transport.

Pleiotropic effects of Syntaxin16 identified by gene editing in cultured adipocytes

Adipocytes play multiple roles in the regulation of glucose metabolism which rely on the regulation of membrane traffic. These include secretion of adipokines and serving as an energy store. Central to their energy storing function is the ability to increase glucose uptake in response to insulin, mediated through translocation of the facilitative glucose transporter GLUT4 to the cell surface. The trans-Golgi reticulum localized SNARE protein syntaxin 16 (Sx16) has been identified as a key component of the secretory pathway required for insulin-regulated trafficking of GLUT4. We used CRISPR/Cas9 technology to generate 3T3-L1 adipocytes lacking Sx16 to understand the role of the secretory pathway on adipocyte function. GLUT4 mRNA and protein levels were reduced in Sx16 knockout adipocytes and insulin stimulated GLUT4 translocation to the cell surface was reduced. Strikingly, neither basal nor insulin-stimulated glucose transport were affected. By contrast, GLUT1 levels were upregulated in Sx16 knockout cells. Levels of sortilin and insulin regulated aminopeptidase were also increased in Sx16 knockout adipocytes which may indicate an upregulation of an alternative GLUT4 sorting pathway as a compensatory mechanism for the loss of Sx16. In response to chronic insulin stimulation, Sx16 knockout adipocytes exhibit elevated insulin-independent glucose transport and significant alterations in lactate metabolism. We further show that the adipokine secretory pathways are impaired in Sx16 knockout cells. Together this demonstrates a role for Sx16 in the control of glucose transport, the response to elevated insulin, cellular metabolic profiles and adipocytokine secretion.

A mathematical model of GLUT1 modulation in rods and RPE and its differential impact in cell metabolism

We present a mathematical model of key glucose metabolic pathways in two cells of the human retina: the rods and the retinal pigmented epithelium (RPE). Computational simulations of glucose transporter 1 (GLUT1) inhibition in the model accurately reproduce experimental data from conditional knockout mice and reveal that modification of GLUT1 expression levels of both cells differentially impacts their metabolism. We hypothesize that, under glucose scarcity, the RPE's energy producing pathways are altered in order to preserve its functionality, impacting the photoreceptors' outer segment renewal. On the other hand, when glucose is limited in the rods, aerobic glycolysis is preserved, which maintains the lactate contribution to the RPE.

Comparative antimalarial, toxicity and mito-protective effects of Diospyros mespiliformis Hochst. ex A. DC. and Mondia whitei (Hook.f.) Skeels on Plasmodium berghei infection in mice

ETHNOPHARMACOLOGICAL RELEVANCE

Diospyros mespiliformis Hochst. ex A. DC. and Mondia whitei (Hook.f.) Skeels are traditionally used in Africa for the treatment of malaria. However, scientific evidence to substantiate this folkloric claim and their effects on liver mitochondria during malaria treatment have not been reported.

AIM OF THE STUDY

This study investigated the efficacy of D. mespiliformis and M. whitei against chloroquine-sensitive and resistant strains of malarial parasites in mice. It also investigated the toxicity and protection against cellular organelles like mitochondria.

MATERIALS AND METHODS

Male Swiss mice were infected with a chloroquine resistant (ANKA) strain of Plasmodium berghei and were treated via oral gavage with methanol extracts of D. mespiliformis and M. whitei reconstituted in diluted dimethylsulfoxide as vehicle (DMSO, 5% v/v) for five consecutive days. Percentage parasite load and clearance were assessed by microscopy. The infected control was treated with the vehicle. Hematological indices were assessed using standard procedures. Alanine aminotransferase (ALT), aspartate aminotransferase (AST) and alkaline phosphatase (ALP) were determined using assay kits. Hepatic mitochondria were isolated via centrifugation, and their permeability transition (mPT), ATPase (mATPase) activity and lipid peroxidation (mLPO) were determined spectroscopically. Liver tissue histology was carried out by standard laboratory procedures. Phytochemical analysis of both extracts were performed using LC-MS to identify the most prominent compounds from each of the extracts.

RESULTS

After treatment on day 5, D. mespiliformis and M. whitei at 400 mg/kg decreased mean values for: percentage parasitemia (5.0 ± 1.0, 2.0 ± 0.2), increased Packed Cell Volume (PCV) (36.0 ± 1.4, 36.0 ± 0.0%) and platelets (2.0 ± 1.4, 2.0 ± 2.8 × 10⁵mm³) relative to the untreated control (20.0 ± 5.2; 30.0 ± 0.0%; 1.4 ± 1.4 × 10⁵ mm³, respectively). At the same dose, D. mespiliformis and M. whitei decreased ALT (8.0 ± 3.8, 24.2 ± 4.0U/L), AST (6.2 ± 0.8, 8.0 ± 0.9U/L) and ALP (56.0 ± 0.7, 51.0 ± 1.0U/L) activities compared to the infected control (77.0 ± 10.9U/L, 14.0 ± 0.7U/L and 76.0 ± 6.0U/L, respectively). Both D. mespiliformis and M. whitei reversed mPT opening, decreased mATPase enhancement and mLPO, relative to the control. Histopathology of the liver showed extensive hemorrhagic lesions and severe disseminated congestion in the infected control while both D. mespiliformis and M. whitei were well tolerated at the highest dose. The LC-MS analysis of D. mespiliformis showed the presence of betulinic acid, tocopherol and kaempferol with antimalarial and antioxidant properties while the M. whitei sample contained coumarin and chlorogenic acid that have antimalarial and hepato-protective properties.

CONCLUSIONS

D.mespiliformis and M. whitei show antimalarial effects against resistant Plasmodium berghei infection, enhanced cell viability, mito-protection and are not toxic in mice.

mTOR-dependent dysregulation of autophagy contributes to the retinal ganglion cell loss in streptozotocin-induced diabetic retinopathy

BACKGROUND

Neurodegeneration, an early event in the pathogenesis of diabetic retinopathy (DR), precedes clinically detectable microvascular damage. Autophagy dysregulation is considered a potential cause of neuronal cell loss, however underlying mechanisms remain unclear. The mechanistic target of rapamycin (mTOR) integrates diverse environmental signals to coordinate biological processes, including autophagy. Here, we investigated the role of mTOR signaling in neuronal cell death in DR.

METHODS

Diabetes was induced by a single intraperitoneal injection of streptozotocin and tissue samples were harvested at 1, 2, 3, 4, and 6 months of diabetes. Early-stage of DR was investigated in 1-month-diabetic mice treated with phlorizin (two daily subcutaneous injections at a dose of 200 mg/kg of body weight during the last 7 full days of the experiment and the morning of the 8th day, 3 h before sacrifice) or rapamycin (daily intraperitoneal injections, at a dose of 3 mg/kg for the same period as for phlorizin treatment). The effect of autophagy modulation on retinal ganglion cells was investigated in 3-months-diabetic mice treated with phlorizin (two daily subcutaneous injections during the last 10 full days of the experiment and the morning of the 11th day, 3 h before sacrifice) or MHY1485 (daily i.p. injections, at a dose of 10 mg/kg for the same period as for phlorizin treatment). Tissue samples obtained from treated/untreated diabetic mice and age-matched controls were used for Western blot and histologic analysis.

RESULTS

mTOR-related proteins and glucose transporter 1 (GLUT1) was upregulated at 1 month and downregulated in the following period up to 6 months. Diabetes-induced neurodegeneration was characterized by an increase of apoptotic marker-cleaved caspase 3, a decrease of the total number of cells, and NeuN immunoreactivity in the ganglion cell layer, as well as an increase of autophagic protein. Insulin-independent glycemic control restored the mTOR pathway activity and GLUT1 expression, along with a decrease of autophagic and apoptotic proteins in 3-months-diabetic mice neuroretina. However, blockade of autophagy using MHY1485 resulted in a more protective effect on ganglion cells compared with phlorizin treatment.

CONCLUSION

Collectively, our study describes the mechanisms of neurodegeneration through the hyperglycemia/ mTOR/ autophagy/ apoptosis pathway. Video Abstract.

Mitoenergetic Dysfunction Triggers a Rapid Compensatory Increase in Steady-State Glucose Flux

ATP can be produced in the cytosol by glycolytic conversion of glucose (GLC) into pyruvate. The latter can be metabolized into lactate, which is released by the cell, or taken up by mitochondria to fuel ATP production by the tricarboxylic acid cycle and oxidative phosphorylation (OXPHOS) system. Altering the balance between glycolytic and mitochondrial ATP generation is crucial for cell survival during mitoenergetic dysfunction, which is observed in a large variety of human disorders including cancer. To gain insight into the kinetic properties of this adaptive mechanism we determined here how acute (30 min) inhibition of OXPHOS affected cytosolic GLC homeostasis. GLC dynamics were analyzed in single living C2C12 myoblasts expressing the fluorescent biosensor FLII(12)Pglu-700μδ6 (FLII). Following in situ FLII calibration, the kinetic properties of GLC uptake (V1) and GLC consumption (V2) were determined independently and used to construct a minimal mathematical model of cytosolic GLC dynamics. After validating the model, it was applied to quantitatively predict V1 and V2 at steady-state (i.e., when V1 = V2 = Vsteady-state) in the absence and presence of OXPHOS inhibitors. Integrating model predictions with experimental data on lactate production, cell volume, and O2 consumption revealed that glycolysis and mitochondria equally contribute to cellular ATP production in control myoblasts. Inhibition of OXPHOS induced a twofold increase in Vsteady-state and glycolytic ATP production flux. Both in the absence and presence of OXPHOS inhibitors, GLC was consumed at near maximal rates, meaning that GLC consumption is rate-limiting under steady-state conditions. Taken together, we demonstrate here that OXPHOS inhibition increases steady-state GLC uptake and consumption in C2C12 myoblasts. This activation fully compensates for the reduction in mitochondrial ATP production, thereby maintaining the balance between cellular ATP supply and demand.

GLUT1-mediated glucose uptake plays a crucial role during Plasmodium hepatic infection

Intracellular pathogens have evolved mechanisms to ensure their survival and development inside their host cells. Here, we show that glucose is a pivotal modulator of hepatic infection by the rodent malaria parasite Plasmodium berghei and that glucose uptake via the GLUT1 transporter is specifically enhanced in P. berghei‐infected cells. We further show that ATP levels of cells containing developing parasites are decreased, which is known to enhance membrane GLUT1 activity. In addition, GLUT1 molecules are translocated to the membrane of the hepatic cell, increasing glucose uptake at later stages of infection. Chemical inhibition of GLUT1 activity leads to a decrease in glucose uptake and the consequent impairment of hepatic infection, both in vitro and in vivo. Our results reveal that changes in GLUT1 conformation and cellular localization seem to be part of an adaptive host response to maintain adequate cellular nutrition and energy levels, ensuring host cell survival and supporting P. berghei hepatic development.

Pituitary Adenylate Cyclase Activating Polypeptide, A Potential Therapeutic Agent for Diabetic Retinopathy in Rats: Focus on the Vertical Information Processing Pathway

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neurotrophic and neuroprotective peptide that has been shown to exert protective effects in different neuronal injuries, such as retinal degenerations. Diabetic retinopathy (DR), the most common complication of diabetes, affects the microvasculature and neuronal architecture of the retina. We have proven earlier that PACAP is also protective in a rat model of DR. In this study, streptozotocin-induced DR was treated with intravitreal PACAP administration in order to further analyze the synaptic structure and proteins of PACAP-treated diabetic retinas, primarily in the vertical information processing pathway. Streptozotocin-treated Wistar rats received intravitreal PACAP injection three times into the right eye 2 weeks after the induction of diabetes. Morphological and molecular biological (qRT-PCR; Western blot) methods were used to analyze retinal synapses (ribbons, conventional) and related structures. Electron microscopic analysis revealed that retinal pigment epithelium, the ribbon synapses and other synaptic profiles suffered alterations in diabetes. However, in PACAP-treated diabetic retinas more bipolar ribbon synapses were found intact in the inner plexiform layer than in DR animals. The ribbon synapse was marked with C-terminal binding protein 2/Bassoon and formed horseshoe-shape ribbons, which were more retained in PACAP-treated diabetic retinas than in DR rats. These results are supported by molecular biological data. The selective degeneration of related structures such as bipolar and ganglion cells could be ameliorated by PACAP treatment. In summary, intravitreal administration of PACAP may have therapeutic potential in streptozotocin-induced DR through maintaining synapse integrity in the vertical pathway.

Integration of computational modeling with membrane transport studies reveals new insights into amino acid exchange transport mechanisms

Uptake of system L amino acid substrates into isolated placental plasma membrane vesicles in the absence of opposing side amino acid (zero-trans uptake) is incompatible with the concept of obligatory exchange, where influx of amino acid is coupled to efflux. We therefore hypothesized that system L amino acid exchange transporters are not fully obligatory and/or that amino acids are initially present inside the vesicles. To address this, we combined computational modeling with vesicle transport assays and transporter localization studies to investigate the mechanisms mediating [¹⁴C]l-serine (a system L substrate) transport into human placental microvillous plasma membrane (MVM) vesicles. The carrier model provided a quantitative framework to test the 2 hypotheses that l-serine transport occurs by either obligate exchange or nonobligate exchange coupled with facilitated transport (mixed transport model). The computational model could only account for experimental [¹⁴C]l-serine uptake data when the transporter was not exclusively in exchange mode, best described by the mixed transport model. MVM vesicle isolates contained endogenous amino acids allowing for potential contribution to zero-trans uptake. Both L-type amino acid transporter (LAT)1 and LAT2 subtypes of system L were distributed to MVM, with l-serine transport attributed to LAT2. These findings suggest that exchange transporters do not function exclusively as obligate exchangers.—Widdows, K. L., Panitchob, N., Crocker, I. P., Please, C. P., Hanson, M. A., Sibley, C. P., Johnstone, E. D., Sengers, B. G., Lewis, R. M., Glazier, J. D. Integration of computational modeling with membrane transport studies reveals new insights into amino acid exchange transport mechanisms.

Facile preparation of cancer-specific polyelectrolyte nanogels from natural and synthetic sugar polymers

Glycosylated polyelectrolyte nanogels prepared from natural and synthetic sugar polymers with excellent colloidal stability, specific bioactivities and imaging ability.

Adolescents with clinical type 1 diabetes display reduced red blood cell glucose transporter isoform 1 (GLUT1)

Type 1 diabetic (T1D) adolescent children on insulin therapy suffer episodes of both hyper- and hypoglycemic episodes. Glucose transporter isoform GLUT1 expressed in blood-brain barrier (BBB) and red blood cells (RBC) compensates for perturbed circulating glucose toward protecting the supply to brain and RBCs. We hypothesized that RBC-GLUT1 concentration, as a surrogate for BBB-GLUT1, is altered in T1D children. To test this hypothesis, we measured RBC-GLUT1 by enzyme-linked immunosorbent assay (ELISA) in T1D children (n = 72; mean age 15.3 ± 0.2 yr) and control children (CON; n = 11; mean age 15.6 ± 0.9 yr) after 12 h of euglycemia and during a hyperinsulinemic-hypoglycemic clamp with a nadir blood glucose of ~3.3 mmol/L for 90 min (clamp I) or ~3 mmol/L for 45 min (clamp II). Reduced baseline RBC-GLUT1 was observed in T1D (2.4 ± 0.17 ng/ng membrane protein); vs. CON (4.2 ± 0.61 ng/ng protein) (p < 0.0001). Additionally, baseline RBC-GLUT1 in T1D negatively correlated with hemoglobin A1c (HbA1c) (R = -0.23, p < 0.05) but not in CON (R = 0.06, p < 0.9). Acute decline in serum glucose to 3.3 mmol/L (90 min) or 3 mmol/L (45 min) did not change baseline RBC-GLUT1 in T1D or CON children. We conclude that reduced RBC-GLUT1 encountered in T1D, with no ability to compensate by increasing during acute hypoglycemia over the durations examined, may demonstrate a vulnerability of impaired RBC glucose transport (serving as a surrogate for BBB), especially in those with the worst control. We speculate that this may contribute to the perturbed cognition seen in T1D adolescents.

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.

Determinants of ligand binding affinity and cooperativity at the GLUT1 endofacial site

Cytochalasin B (CB) and forskolin (FSK) inhibit GLUT1-mediated sugar transport in red cells by binding at or close to the GLUT1 endofacial sugar binding site. Paradoxically, very low concentrations of each of these inhibitors produce a modest stimulation of sugar transport [ Cloherty, E. K., Levine, K. B., and Carruthers, A. ((2001)) The red blood cell glucose transporter presents multiple, nucleotide-sensitive sugar exit sites. Biochemistry 40 ((51)) 15549-15561]. This result is consistent with the hypothesis that the glucose transporter contains multiple, interacting, endofacial binding sites for CB and FSK. The present study tests this hypothesis directly and, by screening a library of cytochalasin and forskolin analogues, asks what structural features of endofacial site ligands determine binding site affinity and cooperativity. Like CB, FSK competitively inhibits exchange 3-O-methylglucose transport (sugar uptake in cells containing intracellular sugar) but noncompetitively inhibits sugar uptake into cells lacking sugar at 4 °C. This refutes the hypothesis that FSK binds at GLUT1 endofacial and exofacial sugar binding sites. Some forskolin derivatives and cytochalasins inhibit equilibrium [(3)H]-CB binding to red cell membranes depleted of peripheral proteins at 4 °C. Others produce a moderate stimulation of [(3)H]-CB binding when introduced at low concentrations but inhibit binding as their concentration is increased. Yet other analogues modestly stimulate [(3)H]-CB binding at all inhibitor concentrations applied. These findings are explained by a carrier that presents at least two interacting endofacial binding sites for CB or FSK. We discuss this result within the context of models for GLUT1-mediated sugar transport and GLUT1 quaternary structure, and we evaluate the major determinants of ligand binding affinity and cooperativity.

Will the original glucose transporter isoform please stand up!

Monosaccharides enter cells by slow translipid bilayer diffusion by rapid, protein-mediated, cation-dependent cotransport and by rapid, protein-mediated equilibrative transport. This review addresses protein-mediated, equilibrative glucose transport catalyzed by GLUT1, the first equilibrative glucose transporter to be identified, purified, and cloned. GLUT1 is a polytopic, membrane-spanning protein that is one of 13 members of the human equilibrative glucose transport protein family. We review GLUT1 catalytic and ligand-binding properties and interpret these behaviors in the context of several putative mechanisms for protein-mediated transport. We conclude that no single model satisfactorily explains GLUT1 behavior. We then review GLUT1 topology, subunit architecture, and oligomeric structure and examine a new model for sugar transport that combines structural and kinetic analyses to satisfactorily reproduce GLUT1 behavior in human erythrocytes. We next review GLUT1 cell biology and the transcriptional and posttranscriptional regulation of GLUT1 expression in the context of development and in response to glucose perturbations and hypoxia in blood-tissue barriers. Emphasis is placed on transgenic GLUT1 overexpression and null mutant model systems, the latter serving as surrogates for the human GLUT1 deficiency syndrome. Finally, we review the role of GLUT1 in the absence or deficiency of a related isoform, GLUT3, toward establishing the physiological significance of coordination between these two isoforms.

Structural basis of GLUT1 inhibition by cytoplasmic ATP

Cytoplasmic ATP inhibits human erythrocyte glucose transport protein (GLUT1)–mediated glucose transport in human red blood cells by reducing net glucose transport but not exchange glucose transport (Cloherty, E.K., D.L. Diamond, K.S. Heard, and A. Carruthers. 1996. Biochemistry. 35:13231–13239). We investigated the mechanism of ATP regulation of GLUT1 by identifying GLUT1 domains that undergo significant conformational change upon GLUT1–ATP interaction. ATP (but not GTP) protects GLUT1 against tryptic digestion. Immunoblot analysis indicates that ATP protection extends across multiple GLUT1 domains. Peptide-directed antibody binding to full-length GLUT1 is reduced by ATP at two specific locations: exofacial loop 7–8 and the cytoplasmic C terminus. C-terminal antibody binding to wild-type GLUT1 expressed in HEK cells is inhibited by ATP but binding of the same antibody to a GLUT1–GLUT4 chimera in which loop 6–7 of GLUT1 is substituted with loop 6–7 of GLUT4 is unaffected. ATP reduces GLUT1 lysine covalent modification by sulfo-NHS-LC-biotin by 40%. AMP is without effect on lysine accessibility but antagonizes ATP inhibition of lysine modification. Tandem electrospray ionization mass spectrometry analysis indicates that ATP reduces covalent modification of lysine residues 245, 255, 256, and 477, whereas labeling at lysine residues 225, 229, and 230 is unchanged. Exogenous, intracellular GLUT1 C-terminal peptide mimics ATP modulation of transport whereas C-terminal peptide-directed IgGs inhibit ATP modulation of glucose transport. These findings suggest that transport regulation involves ATP-dependent conformational changes in (or interactions between) the GLUT1 C terminus and the C-terminal half of GLUT1 cytoplasmic loop 6–7.

Hypoxic regulation of glucose transport, anaerobic metabolism and angiogenesis in cancer: novel pathways and targets for anticancer therapeutics

Cancer cells require a steady source of metabolic energy in order to continue their uncontrolled growth and proliferation. Accelerated glycolysis is one of the biochemical characteristics of cancer cells. Recent work indicates that glucose transport and metabolism are essential for the posttreatment survival of tumor cells, leading to poor prognosis. Glycolytic breakdown of glucose is preceded by the transport of glucose across the cell membrane, a rate-limiting process mediated by facilitative glucose transporter proteins belonging to the facilitative glucose transporter/solute carrier GLUT/SLC2A family. Tumors frequently show overexpression of GLUTs, especially the hypoxia-responsive GLUT1 and GLUT3 proteins. There are also studies that have reported associations between GLUT expression and proliferative indices, whilst others suggest that GLUT expression may be of prognostic significance. In this article we revisit Warburg's original hypothesis and review the recent clinical and basic research on the expression of GLUT family members in human cancers and in cell lines derived from human tumors. We also explore the links between hypoxia-induced genes, glucose transporters and angiogenic factors. Hypoxic tumors are significantly more malignant, metastatic, radio- and chemoresistant and have a poor prognosis. With the discovery the oxygen-sensitive transcription factor hypoxia-inducible factor (HIF-1) has come a new understanding of the molecular link between hypoxia and deregulated glucose metabolism. HIF-1 induces a number of genes integral to angiogenesis, e.g. vascular endothelial growth factor (VEGF), a process intimately involved with metastatic spread. This knowledge may enhance existing chemotherapeutic strategies so that treatment can be more rationally applied and personalized for cancer patients.

ATP-dependent sugar transport complexity in human erythrocytes

Human erythrocyte glucose sugar transport was examined in resealed red cell ghosts under equilibrium exchange conditions ([sugar](intracellular) = [sugar](extracellular), where brackets indicate concentration). Exchange 3-O-methylglucose (3MG) import and export are monophasic in the absence of cytoplasmic ATP but are biphasic when ATP is present. Biphasic exchange is observed as the rapid filling of a large compartment (66% cell volume) followed by the slow filling of the remaining cytoplasmic space. Biphasic exchange at 20 mM 3MG eliminates the possibility that the rapid exchange phase represents ATP-dependent 3MG binding to the glucose transport protein (GLUT1; cellular [GLUT1] of </=20 microM). Immunofluorescence-activated cell sorting analysis shows that biphasic exchange does not result from heterogeneity in cell size or GLUT1 content. Nucleoside transporter-mediated uridine exchange proceeds as rapidly as 3MG exchange but is monoexponential regardless of cytoplasmic [ATP]. This eliminates cellular heterogeneity or an ATP-dependent, nonspecific intracellular diffusion barrier as causes of biphasic exchange. Red cell ghost 3MG and uridine equilibrium volumes (130 fl) are unaffected by ATP. GLUT1 intrinsic activity is unchanged during rapid and slow phases of 3MG exchange. Two models for biphasic sugar transport are presented in which 3MG must overcome a sugar-specific, physical (diffusional), or chemical (isomerization) barrier to equilibrate with cell water. Partial transport inhibition with the use of cytochalasin B or maltose depresses both rapid and slow phases of transport, thereby eliminating the physical barrier hypothesis. We propose that biphasic 3MG transport results from ATP-dependent, differential transport of 3MG anomers in which V(max)/apparent K(m) for beta-3MG exchange transport is 19-fold greater than V(max)/apparent K(m) for alpha-3MG transport.

Modulation of monocarboxylic acid transporter-1 kinetic function by the cAMP signaling pathway in rat brain endothelial cells

MCT1 (monocarboxylic acid transporter 1) facilitates bidirectional monocarboxylic acid transport across membranes. MCT1 function and regulation have not been characterized previously in cerebral endothelial cells but may be important during normal cerebral energy metabolism and during brain diseases such as stroke. Here, by using the cytoplasmic pH indicator 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein-acetoxymethyl ester, the initial rates of monocarboxylate-dependent cytoplasmic acidification were measured as an indication of MCT1 kinetic function in vitro using the rat brain endothelial cell (RBE4) model of blood-brain transport. The initial rate of L-lactate-dependent acidification was significantly inhibited by 5-10-min incubations with agonists of intracellular cAMP-dependent cell signaling pathways as follows: dibutyryl cAMP, forskolin, and isoproterenol. Isoproterenol reduced V(max) but did not affect K(m) values. The effects of forskolin were completely reversed by the protein kinase A inhibitor H89, whereas H89 alone increased transport rates. Cytoplasmic cAMP levels, measured by radioimmunoassay, were increased by forskolin or isoproterenol, and the effect of isoproterenol was inhibited by propranolol. MCT1-independent intracellular pH control mechanisms did not contribute to the forskolin or H89 effects on MCT1 kinetic function as determined with amiloride, monocarboxylate-independent acid loading, or the transport inhibitor alpha-cyano-4-hydroxycinnamate. The data demonstrate the direct modulation of MCT1 kinetic function in cerebral endothelial cells by agents known to affect the beta-adrenergic receptor/adenylyl cyclase/cAMP/protein kinase A intracellular signaling pathway.

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

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

In vivo imaging of the dynamics of glucose uptake in the cytosol of COS-7 cells by fluorescent nanosensors

Glucose homeostasis is a function of glucose supply, transport across the plasma membrane, and metabolism. To monitor glucose dynamics in individual cells, a glucose nanosensor was developed by flanking the Escherichia coli periplasmic glucose/galactose-binding protein with two different green fluorescent protein variants. Upon binding of substrate the FLIPglu-170n sensor showed a concentration-dependent decrease in fluorescence resonance energy transfer between the attached chromophores with a binding affinity for glucose of 170 nm. Fluorescence resonance energy transfer measurements with different sugars indicated a broad selectivity for monosaccharides. An affinity mutant with a Kd of approximately 600 microM was generated, which showed higher substrate specificity, and thus allowed specific monitoring of reversible glucose dynamics in COS-7 cells in the physiological range. At external glucose concentrations between 0.5 and 10 mM, reflecting typical blood levels, free cytosolic glucose concentrations remained at approximately 50% of external levels. The removal of glucose lead to reduced glucose levels in the cell, demonstrating reversibility and visualizing homeostasis. Glucose levels dropped even in the presence of the transport inhibitor cytochalasin B, indicating rapid metabolism. Consistently, the addition of 2-deoxyglucose, which is not recognized by the sensor, affects glucose uptake and metabolism rates. Within the physiological range, glucose utilization, i.e. hexokinase activity, was not limiting. Furthermore, the results show that in COS-7 cells, cytosolic glucose concentrations can vary over at least two orders of magnitude. The glucose nanosensor provides a novel tool with numerous scientific, medical, and environmental applications.

Coordinate modulation of Na-K-2Cl cotransport and K-Cl cotransport by cell volume and chloride

Na-K-2Cl cotransporter (NKCC) and K-Cl cotransporter (KCC) play key roles in cell volume regulation and epithelial Cl(-) transport. Reductions in either cell volume or cytosolic Cl(-) concentration ([Cl(-)](i)) stimulate a corrective uptake of KCl and water via NKCC, whereas cell swelling triggers KCl loss via KCC. The dependence of these transporters on volume and [Cl(-)](i) was evaluated in model duck red blood cells. Replacement of [Cl(-)](i) with methanesulfonate elevated the volume set point at which NKCC activates and KCC inactivates. The set point was insensitive to cytosolic ionic strength. Reducing [Cl(-)](i) at a constant driving force for inward NKCC and outward KCC caused the cells to adopt the new set point volume. Phosphopeptide maps of NKCC indicated that activation by cell shrinkage or low [Cl(-)](i) is associated with phosphorylation of a similar constellation of Ser/Thr sites. Like shrinkage, reduction of [Cl(-)](i) accelerated NKCC phosphorylation after abrupt inhibition of the deactivating phosphatase with calyculin A in vivo, whereas [Cl(-)] had no specific effect on dephosphorylation in vitro. Our results indicate that NKCC and KCC are reciprocally regulated by a negative feedback system dually modulated by cell volume and [Cl(-)]. The major effect of Cl(-) on NKCC is exerted through the volume-sensitive kinase that phosphorylates the transport protein.

Adenosine and adenosine triphosphate modulate the substrate binding affinity of glucose transporter GLUT1 in vitro

Evidence indicates that a large portion of the facilitative glucose transporter isoform GLUT1 in certain animal cells is kept inactive and activated in response to acute metabolic stresses. A reversible interaction of a certain inhibitor molecule with GLUT1 protein has been implicated in this process. In an effort to identify this putative GLUT1 inhibitor molecule, we studied here the effects of adenosine and adenosine triphosphate (ATP) on the binding of D-glucose to GLUT1 by assessing their abilities to displace cytochalasin B (CB), using purified GLUT1 in vesicles. At pH 7.4, adenosine competitively inhibited CB binding to GLUT1 and also reduced the substrate binding affinity by more than an order of magnitude, both with an apparent dissociation constant (K(D)) of 3.0 mM. ATP had no effect on CB and D-glucose binding to GLUT1, but reduced adenosine binding affinity to GLUT1 by 2-fold with a K(D) of 30 mM. At pH 3.6, however, ATP inhibited the CB binding nearly competitively, and increased the substrate binding affinity by 4--5-fold, both with an apparent K(D) of 1.22 mM. These findings clearly demonstrate that adenosine and ATP interact with GLUT1 in vitro and modulate its substrate binding affinity. They also suggest that adenosine and ATP may regulate GLUT1 intrinsic activity in certain cells where adenosine reduces the substrate-binding affinity while ATP increases the substrate-binding affinity by interfering with the adenosine effect and/or by enhancing the substrate-binding affinity at an acidic compartment.

Cytosolic [Ca(2+)] modulates basal GLUT1 activity and plays a permissive role in its activation by metabolic stress and insulin in rat epithelial cells

The aim of this work was to investigate the role of cytosolic free calcium ([Ca(2+)]c) in the stimulation of GLUT1 by metabolic stress and insulin. Chelation of [Ca(2+)]c with bapta, introduced in rat liver epithelial Clone 9 cells in the acetoxymethyl (AM) form, decreased their basal rate of 2-deoxyglucose uptake in a dose-dependent fashion. Maximal inhibition at 75 microM bapta was by 38 +/- 8% (n = 8). The effect was partially reversed by ionomycin. Basal sugar uptake was also decreased by lowering extracellular [Ca(2+)] in ionomycin-permeabilized cells. Increasing [Ca(2+)]c over its resting level of 168 +/- 32 (n = 27) had no affect on sugar uptake. Chelation of [Ca(2+)]c did not change the abundance of surface GLUT1 and had no significant effect on the affinity of GLUT1 for sugars. In addition, calcium chelation abolished the activation of GLUT1 by azide, arsenate, 2,4-dinitrophenol and insulin. However, [Ca(2+)]c did not increase in the presence of azide. We conclude that [Ca(2+)]c, near or below its resting level, modulates GLUT1 activity over a considerable range and plays a permissive role in the activation of the carrier by metabolic stress and insulin.

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.

Biosynthesis of novel exopolymers by Aureobasidium pullulans

Aureobasidium pullulans ATCC 42023 was cultured under aerobic conditions with glucose, mannose, and glucose analogs as energy sources. The exopolymer extracts produced under these conditions were composed of glucose and mannose. The molar ratio of glucose to mannose in the exopolymer extract and the molecular weight of the exopolymer varied depending on the energy source and culture time. The glucose content of exopolymer extracts formed with glucose and mannose as the carbon sources was between 91 and 87%. The molecular weight decreased from 3.5 x 10(6) to 2.12 x 10(6) to 0.85 x 10(6) to 0.77 x 10(6) with culture time. As the culture time increased, the glucose content of the exopolymer extract formed with glucosamine decreased from 55 +/- 3 to 29 +/- 2 mol%, and the molecular weight increased from 2.73 x 10(6) to 4.86 x 10(6). There was no evidence that glucosamine was directly incorporated into exopolymers. The molar ratios of glucose to mannose in exopolymer extracts ranged from 87 +/- 3:13 +/- 3 to 28 +/- 2:72 +/- 2 and were affected by the energy source added. On the basis of the results of an enzyme hydrolysis analysis of the exopolymer extracts and the compositional changes observed, mannose (a repeating unit) was substituted for glucose, which gave rise to a new family of exopolymer analogs.

The effect of ATP-depletion on the inhibition of glucose exits from human red cells

The effect of ATP-depletion or its consequence, by metabolic inhibition, on the inhibition of glucose transport by various inhibitors was studied in human red cells. In cells depleted of ATP, glucose exit times were longer than in normal cells and the times increased with the duration of depletion. The Km for external glucose was higher in ATP-depleted cells than in normal undepleted cells (3.0 mM c.f. 2.5 mM at 30 degrees C). In contrast, the apparent Ki for cytochalasin B decreased from 0.85 microM in the normal cells to 0.5 microM after ATP-depletion. Half-maximal rates of glucose exit in the absence, and in the presence of 2 microM cytochalasin B were found at ATP concentrations of 0.43 and 0.68 microM, respectively. Although glucose exits from ATP-depleted cells exposed to the irreversible inhibitor of glucose transport, 1-fluoro-2,4-dinitrobenzene (FDNB) were slower than in normal cells, the relative degrees of inhibition were not significantly different. However, normal and ATP-depleted cells responded differently to treatment with 1,2-cyclohexanedione, a modifier of arginine residues which inhibits glucose exit. While normal cells were markedly inhibited, depleted cells were much less affected and the inhibitory effect of cytochalasin B seen in normal cells was reduced. These findings demonstrate that the glucose transport system of human red cells is affected by intracellular ATP and that ATP alters the affinity of the transporter for certain inhibitors. The implications of these findings are discussed.