Direct evidence for ATP modulation of sugar transport in human erythrocyte ghosts

Identification of cytoskeletal elements enclosing the ATP pools that fuel human red blood cell membrane cation pumps

The type of metabolic compartmentalization that occurs in red blood cells differs from the types that exist in most eukaryotic cells, such as intracellular organelles. In red blood cells (ghosts), ATP is sequestered within the cytoskeletal-membrane complex. These pools of ATP are known to directly fuel both the Na(+)/K(+) and Ca(2+) pumps. ATP can be entrapped within these pools either by incubation with bulk ATP or by operation of the phosphoglycerate kinase and pyruvate kinase reactions to enzymatically generate ATP. When the pool is filled with nascent ATP, metabolic labeling of the Na(+)/K(+) or Ca(2+) pump phosphoproteins (E(Na)-P and E(Ca)-P, respectively) from bulk [γ-(32)P]-ATP is prevented until the pool is emptied by various means. Importantly, the pool also can be filled with the fluorescent ATP analog trinitrophenol ATP, as well as with a photoactivatable ATP analog, 8-azido-ATP (N(3)-ATP). Using the fluorescent ATP, we show that ATP accumulates and then disappears from the membrane as the ATP pools are filled and subsequently emptied, respectively. By loading N(3)-ATP into the membrane pool, we demonstrate that membrane proteins that contribute to the pool's architecture can be photolabeled. With the aid of an antibody to N(3)-ATP, we identify these labeled proteins by immunoblotting and characterize their derived peptides by mass spectrometry. These analyses show that the specific peptides that corral the entrapped ATP derive from sequences within β-spectrin, ankyrin, band 3, and GAPDH.

Hexose transporter GLUT1 harbors several distinct regulatory binding sites for flavones and tyrphostins

The facilitative hexose transporter GLUT1 activity is blocked by tyrosine kinase inhibitors that include natural products such as flavones and isoflavones and synthetic compounds such as tyrphostins, molecules that are structurally unrelated to the transported substrates [Vera, et al. (2001) Biochemistry, 40, 777-790]. Here we analyzed the interaction of GLUT1 with quercetin (a flavone), genistein (an isoflavone), and tyrphostin A47 and B46 to evaluate if they share one common or have several binding sites on the protein. Kinetic assays showed that genistein, quercetin, and tyrphostin B46 behave as competitive inhibitors of equilibrium exchange and zero-trans uptake transport and noncompetitive inhibitors of net sugar exit out of human red cells, suggesting that they interact with the external surface of the GLUT1 molecule. In contrast, tyrphostin A47 was a competitive inhibitor of equilibrium exchange and zero-trans exit transport and a noncompetitive inhibitor of net sugar entry into red cells, suggesting that it interacts with the cytoplasmic surface of the transporter. Genistein protected GLUT1 against iodide-elicited fluorescence quenching and also decreased the affinity of d-glucose for its external binding site, while quercetin and tyrphostins B46 and A47 promoted fluorescence quenching and did not affect the external d-glucose binding site. These findings are explained by a carrier that presents at least three binding sites for tyrosine kinase inhibitors, in which (i) genistein interacts with the transporter in a conformation that binds glucose on the external surface (outward-facing conformation), in a site which overlaps with the external binding site for d-glucose, (ii) quercetin and tyrphostin B46 interact with the GLUT1 conformation which binds glucose by the internal side of the membrane (inward-facing conformation), but to a site accessible from the external surface of the protein, and (iii) the binding site for tyrphostin A47 is accessible from the inner surface of GLUT1 by binding to the inward-facing conformation of the transporter. These data provide groundwork for a molecular understanding of how the tyrosine kinase inhibitors directly affect glucose transport in animal cells.

Molecular dynamics simulation studies of GLUT4: substrate-free and substrate-induced dynamics and ATP-mediated glucose transport inhibition

Background

Glucose transporter 4 (GLUT4) is an insulin facilitated glucose transporter that plays an important role in maintaining blood glucose homeostasis. GLUT4 is sequestered into intracellular vesicles in unstimulated cells and translocated to the plasma membrane by various stimuli. Understanding the structural details of GLUT4 will provide insights into the mechanism of glucose transport and its regulation. To date, a crystal structure for GLUT4 is not available. However, earlier work from our laboratory proposed a well validated homology model for GLUT4 based on the experimental data available on GLUT1 and the crystal structure data obtained from the glycerol 3-phosphate transporter.

Methodology/Principal Findings

In the present study, the dynamic behavior of GLUT4 in a membrane environment was analyzed using three forms of GLUT4 (apo, substrate and ATP-substrate bound states). Apo form simulation analysis revealed an extracellular open conformation of GLUT4 in the membrane favoring easy exofacial binding of substrate. Simulation studies with the substrate bound form proposed a stable state of GLUT4 with glucose, which can be a substrate-occluded state of the transporter. Principal component analysis suggested a clockwise movement for the domains in the apo form, whereas ATP substrate-bound form induced an anti-clockwise rotation. Simulation studies suggested distinct conformational changes for the GLUT4 domains in the ATP substrate-bound form and favor a constricted behavior for the transport channel. Various inter-domain hydrogen bonds and switching of a salt-bridge network from E345-R350-E409 to E345-R169-E409 contributed to this ATP-mediated channel constriction favoring substrate occlusion and prevention of its release into cytoplasm. These data are consistent with the biochemical studies, suggesting an inhibitory role for ATP in GLUT-mediated glucose transport.

Conclusions/Significance

In the absence of a crystal structure for any glucose transporter, this study provides mechanistic details of the conformational changes in GLUT4 induced by substrate and its regulator.

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

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

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.

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.

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.

Gleevec inhibits beta-amyloid production but not Notch cleavage

Amyloid-beta (Abeta) peptides, consisting mainly of 40 and 42 aa (Abeta40 and Abeta42, respectively), are metabolites of the amyloid precursor protein and are believed to be major pathological determinants of Alzheimer's disease. The proteolytic cleavages that form the Abeta N and C termini are catalyzed by beta-secretase and gamma-secretase, respectively. Here we demonstrate that gamma-secretase generation of Abeta in an N2a cell-free system is ATP dependent. In addition, the Abl kinase inhibitor imatinib mesylate (Gleevec, or STI571), which targets the ATP-binding site of Abl and several other tyrosine kinases, potently reduces Abeta production in the N2a cell-free system and in intact N2a cells. Both STI571 and a related compound, inhibitor 2, also reduce Abeta production in rat primary neuronal cultures and in vivo in guinea pig brain. STI571 does not inhibit the gamma-secretase-catalyzed S3 cleavage of Notch-1. Furthermore, production of Abeta and its inhibition by STI571 were demonstrated to occur to similar extents in both Abl-/- and WT mouse fibroblasts, indicating that the effect of STI571 on Abeta production does not involve Abl kinase. The efficacy of STI571 in reducing Abeta without affecting Notch-1 cleavage may prove useful as a basis for developing novel therapies for Alzheimer's disease.

Molecular determinants of sugar transport regulation by ATP

Intracellular ATP inhibits human erythrocyte net sugar transport by binding cooperatively to the glucose transport protein (GluT1). ATP binding produces altered transporter affinity for substrate and promotes substrate occlusion within a post-translocation vestibule formed by GluT1 cytosolic domains. The accompanying paper (Cloherty, E. K., Levine, K. B., Graybill, C., and Carruthers, A. (2002) Biochemistry 41, 12639-12651) demonstrates that reduced intracellular pH promotes high-affinity ATP binding to GluT1 but inhibits ATP-modulation of GluT1-mediated sugar transport. The present study explores the role of GluT1 residues 326-343 (a proposed GluT1 ATP-binding site subdomain) in GluT1 ATP binding by using alanine scanning mutagenesis. Cos-7 and HEK cells were transfected with a cDNA encoding full-length human GluT1 terminating in a carboxyl-terminal hemagglutinin (HA)-His6 epitope. The transporter (GluT1.HA.H6) is expressed at the surface of both cell-types and is catalytically active. In HEK cells, both parental GluT1- and GluT1.HA.H6-mediated sugar transport are acutely sensitive to cellular metabolic inhibition. Isolated, detergent-solubilized GluT1.HA.H6 is photolabeled by [gamma-32P]-azidoATP in an ATP-protectable manner. Alanine substitution of E329 or G332/R333/R334 enhances GluT1.HA.H6 [gamma-32P]azidoATP photoincorporation but blocks acute modulation of net sugar transport by cellular metabolic inhibition. These actions resemble those of reduced pH on ATP binding to and modulation of red cell GluT1. It is proposed that cooperative nucleotide binding to GluT1 and nucleotide modulation of GluT1-mediated sugar transport are regulated by a proton-sensitive saltbridge (Glu329-Arg333/334).

Cholinergic activation of glucose transport in bovine chromaffin cells involves calmodulin and protein kinase Czeta signaling

The aim of the present study was to delineate possible signaling pathways involved in acetylcholine (Ach)-induced glucose transport in chromaffin cells, a widely applied model system for sympathetic neurons. Acute Ach stimulation (10 min) enhanced the rate of glucose transport through activation of both nicotinic and muscarinic receptors. The calmodulin antagonist, W13, and the protein kinase C (PKC) inhibitor, staurosporine, each partially depressed Ach-induced glucose transport, with staurosporine exhibiting the stronger inhibitory effect. Pretreating the cells with phorbol 12-myristate 13-acetate (PMA) to downregulate PKC activity did not affect the nicotine-induced glucose transport, but completely attenuated that activated by muscarine, suggesting that Ach activation of transport involved both diacylglycerol-independent (PKCzeta) and diacylglycerol-dependent PKCs (PKCalpha/PKCepsilon). The PI 3-kinase inhibitor, wortmannin, diminished the Ach response, consistent with activation of the PKCs by the upstream PI 3-kinase-dependent phosphoinositide-dependent kinase, PDK1. Cholinergic activation strongly activated the ERK1/ERK2 cascade and p38 MAP kinase, but only p38 MAP kinase appeared to play a role, however minor, in nicotine-induced glucose uptake. The results are consistent with PKCs being more important than calmodulin in coupling cholinergic activation to glucose transport in chromaffin cells, but additional, yet unidentified, signaling pathways appear to be needed to obtain full activation of glucose transport in response to Ach.

The red blood cell glucose transporter presents multiple, nucleotide-sensitive sugar exit sites

At any instant, the human erythrocyte sugar transporter presents at least one sugar export site but multiple sugar import sites. The present study asks whether the transporter also presents more than one sugar exit site. We approached this question by analysis of binding of [3H]cytochalasin B (an export conformer ligand) to the human erythrocyte sugar transporter and by analysis of cytochalasin B modulation of human red blood cell sugar uptake. Phloretin-inhibitable cytochalasin B binding to human red blood cells, to human red blood cell integral membrane proteins, and to purified human red blood cell glucose transport protein (GluT1) displays positive cooperativity at very low cytochalasin B levels. Cooperativity between sites and K(d(app)) for cytochalasin B binding are reduced in the presence of intracellular ATP. Red cell sugar uptake at subsaturating sugar levels is inhibited by high concentrations of cytochalasin B but is stimulated by lower (<20 nM) concentrations. Increasing concentrations of the e1 ligand forskolin also first stimulate then inhibit sugar uptake. Cytochalasin D (a cytochalasin B analogue that does not interact with GluT1) is without effect on sugar transport over the same concentration range. Cytochalasin B and ATP binding are synergistic. ATP (but not AMP) enhances [3H]cytochalasin B photoincorporation into GluT1 while cytochalasin B (but not cytochalasin D) enhances [gamma-32P]azidoATP photoincorporation into GluT1. We propose that the red blood cell glucose transporter is a cooperative tetramer of GluT1 proteins in which each protein presents a translocation pathway that alternates between uptake (e2) and export (e1) states but where, at any instant, two subunits must present uptake (e2) and two subunits must present exit (e1) states.

The predicted ATP-binding domains in the hexose transporter GLUT1 critically affect transporter activity

The glucose transporter GLUT1 has three short amino acid sequences (domains I-III) with homology to typical ATP-binding domains. GLUT1 is a facilitative transporter, however, and transports its substrates down a concentration gradient without a specific requirement for energy or hydrolysis of ATP. Therefore, we assessed the functional role of the predicted ATP-binding domains in GLUT1 by site-directed mutagenesis and expression in Xenopus oocytes. For each mutant, we determined the level of protein expression and the kinetics of transport under zero-trans influx, zero-trans efflux, and equilibrium exchange conditions. Although all five mutants were expressed at levels similar to that of the wild-type GLUT1, each single amino acid change in domains I or III profoundly affected GLUT1 function. The mutants Gly116-->Ala in domain I and Gly332-->Ala in domain III exhibited only 10-20% of the transport activity of the wild-type GLUT1. The mutants Gly111-->Ala in domain I and Leu336-->Ala in domain III showed altered kinetic properties; neither the apparent Km nor the Vmax for 3-methylglucose transport were increased under equilibrium exchange conditions, and they did not show the expected level of countertransport acceleration. The mutant Lys117-->Arg in domain I showed a marked increase in the apparent Km for 3-methylglucose transport under zero-trans efflux and equilibrium exchange conditions while maintaining countertransport acceleration. These results indicate that the predicted ATP-binding domains I and III in GLUT1 are important components of the region in GLUT1 involved in transport of the substrate and that their integrity is critical for maintaining the activity and kinetic properties of the transporter.

Direct inhibition of the hexose transporter GLUT1 by tyrosine kinase inhibitors

The facilitative hexose transporter GLUT1 is a multifunctional protein that transports hexoses and dehydroascorbic acid, the oxidized form of vitamin C, and interacts with several molecules structurally unrelated to the transported substrates. Here we analyzed in detail the interaction of GLUT1 with a group of tyrosine kinase inhibitors that include natural products of the family of flavones and isoflavones and synthetic compounds such as the tyrphostins. These compounds inhibited, in a dose-dependent manner, the transport of hexoses and dehydroascorbic acid in human myeloid HL-60 cells, in transfected Chinese hamster ovary cells overexpressing GLUT1, and in normal human erythrocytes, and blocked the glucose-displaceable binding of cytochalasin B to GLUT1 in erythrocyte ghosts. Kinetic analysis of transport data indicated that only tyrosine kinase inhibitors with specificity for ATP binding sites inhibited the transport activity of GLUT1 in a competitive manner. In contrast, those inhibitors that are competitive with tyrosine but not with ATP failed to inhibit hexose uptake or did so in a noncompetitive manner. These results, together with recent evidence demonstrating that GLUT1 is a nucleotide binding protein, support the concept that the inhibitory effect on transport is related to the direct interaction of the inhibitors with GLUT1. We conclude that predicted nucleotide-binding motifs present in GLUT1 are important for the interaction of the tyrosine kinase inhibitors with the transporter and may participate directly in the binding transport of substrates by GLUT1.

ATP-dependent substrate occlusion by the human erythrocyte sugar transporter

Human erythrocyte sugar transport presents a functional complexity that is not explained by existing models for carrier-mediated transport. It has been suggested that net sugar uptake is the sum of three serial processes: sugar translocation, sugar interaction with an intracellular binding complex, and the release from this complex into bulk cytosol. The present study was carried out to identify the erythrocyte sugar binding complex, to determine whether sugar binding occurs inside or outside the cell, and to determine whether this binding complex is affected by cytosolic ATP or transporter quaternary structure. Sugar binding assays using cells and membrane protein fractions indicate that sugar binding to erythrocytes is quantitatively accounted for by sugar binding to the hexose transport protein, GluT1. Kinetic analysis of net sugar fluxes indicates that GluT1 sugar binding sites are cytoplasmic. Intracellular ATP increases GluT1 sugar binding capacity from 1 to 2 mol of 3-O-methylglucose/mol GluT1 and inhibits the release of bound sugar into cytosol. Reductant-mediated, tetrameric GluT1 dissociation into dimeric GluT1 is associated with the loss of ATP and 3-O-methylglucose binding. We propose that sugar uptake involves GluT1-mediated, extracellular sugar translocation into an ATP-dependent cage formed by GluT1 cytoplasmic domains. Caged or occluded sugar has three possible fates: (1) transport out of the cell (substrate cycling); (2) interaction with sugar binding sites within the cage, or (3) release into bulk cytosol. We show how this hypothesis can account for the complexity of erythrocyte sugar transport and its regulation by cytoplasmic ATP.

Sodium-potassium-chloride cotransport

Obligatory, coupled cotransport of Na(+), K(+), and Cl(-) by cell membranes has been reported in nearly every animal cell type. This review examines the current status of our knowledge about this ion transport mechanism. Two isoforms of the Na(+)-K(+)-Cl(-) cotransporter (NKCC) protein (approximately 120-130 kDa, unglycosylated) are currently known. One isoform (NKCC2) has at least three alternatively spliced variants and is found exclusively in the kidney. The other (NKCC1) is found in nearly all cell types. The NKCC maintains intracellular Cl(-) concentration ([Cl(-)](i)) at levels above the predicted electrochemical equilibrium. The high [Cl(-)](i) is used by epithelial tissues to promote net salt transport and by neural cells to set synaptic potentials; its function in other cells is unknown. There is substantial evidence in some cells that the NKCC functions to offset osmotically induced cell shrinkage by mediating the net influx of osmotically active ions. Whether it serves to maintain cell volume under euvolemic conditons is less clear. The NKCC may play an important role in the cell cycle. Evidence that each cotransport cycle of the NKCC is electrically silent is discussed along with evidence for the electrically neutral stoichiometries of 1 Na(+):1 K(+):2 Cl- (for most cells) and 2 Na(+):1 K(+):3 Cl(-) (in squid axon). Evidence that the absolute dependence on ATP of the NKCC is the result of regulatory phosphorylation/dephosphorylation mechanisms is decribed. Interestingly, the presumed protein kinase(s) responsible has not been identified. An unusual form of NKCC regulation is by [Cl(-)](i). [Cl(-)](i) in the physiological range and above strongly inhibits the NKCC. This effect may be mediated by a decrease of protein phosphorylation. Although the NKCC has been studied for approximately 20 years, we are only beginning to frame the broad outlines of the structure, function, and regulation of this ubiquitous ion transport mechanism.

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.

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

Avian erythrocyte sugar transport is stimulated during anoxia and during exposure to inhibitors of oxidative phosphorylation. This stimulation results from catalytic desuppression of the cell surface glucose transporter GLUT1 [Diamond, D., & Carruthers, A. (1993) J. Biol. Chem. 268, 6437-6444]. The present study was undertaken to investigate the mechanisms of GLUT1 suppression/desuppression. Sugar uniport (sugar uptake or exit in the absence of sugar at the opposite side of the membrane) is absent in normoxic avian erythrocytes, but sugar antiport (sugar uptake coupled to sugar exit) is present. Exposure to cyanide and/or to FCCP (mitochondrial inhibitors) stimulates erythrocyte sugar uniport but not sugar antiport. K(m)(app) for 3-O-methylglucose uniport and antiport are unaffected by metabolic poisoning. Ki(app) for inhibitions of 3-O-methylglucose uniport by cytochalasin B and forskolin (sugar export site ligands) are unaffected by progressive stimulation of sugar uniport. Cyanide and FCCP stimulation of 3-O-methylglucose uniport are associated with increased AMP-activated protein kinase activity. Purified human GLUT1 is not phosphorylated by exposure to cytosol extracted from poisoned avian erythrocytes. FCCP does not stimulate GLUT1-mediated 3-O-methylglucose uptake in K562 cells but does increase K562 AMP-activated protein kinase activity. FCCP stimulation of 3-O-methylglucose uniport in resealed erythrocyte ghosts requires cytosolic ATP and/or glutathione. The nonmetabolizable ATP analog AMP-PNP cannot be substituted for ATP in this action. These results are contrasted with allosteric regulation of human erythrocyte sugar transport and suggest that avian erythrocyte sugar transport suppression results from inhibition of carrier uniport function. Uniport suppression is not mediated by interaction with cytosolic molecular species that bind to the sugar export site. The antiport to uniport switch mechanism requires ATP hydrolysis, is associated with elevated AMP-activated kinase function, and, if triggered by this kinase, is mediated by factors absent in K562 cells and downstream from the kinase.

Genistein is a natural inhibitor of hexose and dehydroascorbic acid transport through the glucose transporter, GLUT1

Genistein is a dietary-derived plant product that inhibits the activity of protein-tyrosine kinases. We show here that it is a potent inhibitor of the mammalian facilitative hexose transporter GLUT1. In human HL-60 cells, which express GLUT1, genistein inhibited the transport of dehydroascorbic acid, deoxyglucose, and methylglucose in a dose-dependent manner. Transport was not affected by daidzein, an inactive genistein analog that does not inhibit protein-tyrosine kinase activity, or by the general protein kinase inhibitor staurosporine. Genistein inhibited the uptake of deoxyglucose and dehydroascorbic acid in Chinese hamster ovary (CHO) cells overexpressing GLUT1 in a similar dose-dependent manner. Genistein also inhibited the uptake of deoxyglucose in human erythrocytes indicating that its effect on glucose transporter function is cell-independent. The inhibitory action of genistein on transport was instantaneous, with no additional effect observed in cells preincubated with it for various periods of time. Genistein did not alter the uptake of leucine by HL-60 cells, indicating that its inhibitory effect was specific for the glucose transporters. The inhibitory effect of genistein was of the competitive type, with a Ki of approximately 12 microM for inhibition of the transport of both methylglucose and deoxyglucose. Binding studies showed that genistein inhibited glucose-displaceable binding of cytochalasin B to GLUT1 in erythrocyte ghosts in a competitive manner, with a Ki of 7 microM. These data indicate that genistein inhibits the transport of dehydroascorbic acid and hexoses by directly interacting with the hexose transporter GLUT1 and interfering with its transport activity, rather than as a consequence of its known ability to inhibit protein-tyrosine kinases. These observations indicate that some of the many effects of genistein on cellular physiology may be related to its ability to disrupt the normal cellular flux of substrates through GLUT1, a hexose transporter universally expressed in cells, and is responsible for the basal uptake of glucose.

Adenosine 5'-triphosphate modulation of nitrobenzylthioinosine binding sites in plasma membranes of bovine chromaffin cells

Nitrobenzylthioinosine (NBTI) is a high affinity probe for facilitated diffusion nucleoside transporters. Kinetic analysis of the binding of [3H]NBTI to plasma membranes of chromaffin cells was conducted in the presence or absence of adenosine 5'-triphosphate (ATP). Similar curvilinear plots with a Hill number of 1.32 were obtained in both conditions. ATP significantly increased the number of NBTI binding sites in these preparations showing Bmax values of 1.62 +/- 0.20 pmol/mg protein for controls and 3.22 +/- 0.31 pmol/mg protein in the presence of ATP. However, the affinity constant (KD) was not significantly modified. The non-metabolizable ATP analogue, 5'-adenylyl imidodiphosphate (AMP-PNP) and diadenosine tetraphosphate (Ap4A) can mimic the stimulatory ATP effect, but adenosine monophosphate (AMP) has no effect on the NBTI binding to plasma membranes. These results indicate a modulatory role for ATP, non-hydrolysis dependent, on nucleoside transport in chromaffin cells. Therefore, a nucleotide binding site on the nucleoside transporter similar to that described for glucose transporter could be suggested.

The human erythrocyte sugar transporter is also a nucleotide binding protein

We have previously shown that ATP interacts with an intracellular, stereoselective, regulatory site(s) on the human erythrocyte sugar transport system to modify transport function in a hydrolysis-independent manner. This present study examines the nucleotide binding properties of the human erythrocyte sugar transport system. We demonstrate by transport studies in ghosts, by nucleotide binding studies with purified transport protein by measurements of nucleotide inhibition of 8-azidoadenosine 5'-[gamma-32P]triphosphate (azido-ATP) photoincorporation into purified carrier, and by analysis of nucleotide inhibition of carboxyl-terminal peptide antisera binding to purified glucose carrier than the glucose transport protein binds (with increasing order of affinity) AMP, ADP, ATP, 5'-adenylyl imidodiphosphate (AMP-PNP), and 1,N6-ethenoadenosine 5'-triphosphate (EATP) at a single site. The carrier lacks detectable ATPase activity and GTP binding capacity. While AMP and ADP bind to the carrier protein and act as competitive inhibitors of ATP binding, these nucleotides are unable to mimic the ability of ATP, AMP-PNP, and EATP to modify the catalytic properties of the sugar transport system. Limited tryptic digestion of azido-ATP-photolabeled carrier suggests that the region of the glucose transport protein containing the intracellular cytochalasin B binding and extracellular bis(mannose) binding domains [residues 270-456; Holman, G. D., & Rees, W. D. (1987) Biochim. Biophys. Acta 897, 395-405] may also contain the intracellular ATP binding site.

Modulation of red blood cell sugar transport by lyso-lipid

The in vitro presentation to red blood cells of specific lysolipids in amounts comparable to lysolipid levels in serum is shown to markedly influence protein-mediated glucose transport. Lysolipids were introduced exogenously into cell membranes by incubating erythrocytes in buffer containing varying concentrations of lysolipid (under 3.2 microM). The transport-modulating potency of the lysolipids was found to be dependent both on headgroup and hydrocarbon chain. MPL (monopalmitoyl lecithin, L-alpha-lysopalmitoylphosphatidylcholine) had the greatest influence on sugar transport. 15 min incubation of red cells in MPL suspensions sufficed for 99% association of the lysolipid with the cell membranes. This association correlated with altered red-cell sugar transport. At MPL/bilayer lipid molar ratios as low as 0.03%, MPL was found to act as a reversible, hyperbolic, mixed-type inhibitor of exchange D-glucose exit (both Km(app) and Vmax for transport are reduced). Dissociation of MPL from the membrane results in the recovery of original transport activity. MPL at 1.5.10(-17) mol MPL/red cell was found to reduce Ki(app) for D-glucose inhibition of cytochalasin B binding to the glucose carrier protein in red cell ghost membranes. Our findings demonstrate that red-cell membrane-exogenous lysolipid associations can significantly modify protein mediated sugar transport. The simplest explanation of our findings is a direct interaction of lysolipid with the transport protein.

ATP does not regulate the reconstituted glucose transporter

ATP has been reported to affect glucose transport in human erythrocytes and resealed erythrocyte ghosts [Jacquez, J. A. (1983) Biochim. Biophys. Acta 727, 367-378; Jensen, M. R., & Brahm, J. (1987) Biochim. Biophys. Acta 900, 282-290]. In more detailed studies, effects of micromolar levels of ATP on transport in ghosts and inside-out vesicles, and on the fluorescence of ghosts and the purified glucose transporter [Carruthers, A. (1986) Biochemistry 25, 3592-3602; Hebert, D. N., & Carruthers, A. (1986) J. Biol. Chem. 261, 10093-10099; Carruthers, A. (1986) J. Biol. Chem. 261, 11028-11037], have been interpreted as supporting a model in which ATP regulates the catalytic properties of the transporter. Both allosteric and covalent effects of ATP were proposed; among the allosteric effects was a 60% reduction in the Km for zero-trans uptake. In order to test whether allosteric ATP regulation of the transporter occurs, we reconstituted glucose transport activity into liposomes using erythrocyte membranes without detergent treatment. The effects of ATP, present either outside, inside, or both inside and outside the liposomes, on the transport activity were examined. Effects of ATP on trypsin-treated liposomes, which have only a single orientation of active transporters, were also tested. While the model predicts activation by ATP, only inhibition was observed. This was significant only at millimolar concentrations of ATP, in contrast to the previously reported effects at micromolar levels, and was primarily on the extracellular surface of the transporter. In addition, the ATP effects on reconstituted transport were nonspecific, with similar effects produced by tripolyphosphate.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of hexose transport in rat myoblasts during growth and differentiation

We report here the effects of growth conditions and myogenic differentiation on rat myoblast hexose transport activities. We have previously shown that in undifferentiated myoblasts the preferred substrates for the high (HAHT)- and low (LAHT)-affinity hexose transport systems are 2-deoxyglucose (2-DG) and 3-O-methyl-D-glucose (3-OMG), respectively. The present study shows that at cell density higher than 4.4 x 10(4) cells/cm2, the activities of both transport processes decrease with increasing cell densities of the undifferentiated myoblasts. Since the transport affinities are not altered, the observed decrease is compatible with the notion that the number of functional hexose transporters may be decreased in the plasma membrane. Myogenic differentiation is found to alter the 2-DG, but not the 3-OMG, transport affinity. The Km values of 2-DG uptake are elevated upon the onset of fusion and are directly proportional to the extent of fusion. This relationship between myogenesis and hexose transport is further explored by using cultures impaired in myogenesis. Treatment of cells with 5-bromo-2'-deoxyuridine abolishes not only myogenesis but also the myogenesis-induced change in 2-DG transport affinity. Similarly, alteration in 2-DG transport affinity cannot be observed in a myogenesis-defective mutant, D1. However, under myogenesis-permissive condition, the myogenesis of this mutant is also accompanied by changes in its 2-DG transport affinity. The myotube 2-DG transport system also differs from its myoblast counterpart in its response to sulfhydryl reagents and in its turnover rate. It may be surmised from the above observations that myogenesis results in the alteration of the turnover rate or in the modification of the 2-DG transport system. Although glucose starvation has no effect on myogenesis, it is found to alter the substrate specificity and transport capacity of HAHT. In conclusion, the present study shows that hexose transport in rat myoblasts is very sensitive to the growth conditions and the stages of differentiation of the cultures. This may explain why different hexose transport properties have been observed with myoblasts grown under different conditions.

Exercise and insulin stimulate skeletal muscle glucose transport through different mechanisms

This study was designed to examine the effects of acute exercise, insulin stimulation, and their combination on the kinetics of glucose transport in rat skeletal muscle. Sarcolemmal (SL) membranes were isolated from control (C), acute exercise (E), insulin-stimulated (I), and combined (E + I) rats. Michaelis-Menten kinetics indicated that the Vmax for glucose transport was increased after each perturbation compared with C but were not different from each other (E, 4,334 +/- 377; I, 4,424 +/- 668; E + I, 4,338 +/- 602; and C, 1,366 +/- 124 pmol.mg protein-1.s-1). The apparent Km was unchanged. Scatchard plots of cytochalasin B binding sites indicated that both I and E + I increased the number of binding sites compared both E and C (9.4 +/- 0.5 and 7.8 +/- 0.5 vs. 5.1 +/- 0.2 and 5.5 +/- 0.3 pmol/mg protein) without altering the dissociation constant. The increase in Vmax was greater than the increase in cytochalasin B binding sites indicating that both I and E + I caused an increase in the turnover rate of transport molecules as well as an increase in the total number of transport molecules. Because there was no change in the Km for glucose transport and no increase in cytochalasin B binding sites after exercise, the increase in Vmax was due solely to an increased turnover rate of existing transport molecules.

Rat skeletal muscle, liver and brain have different fetal and adult forms of the glucose transporter

Rabbit antibodies made against the human erythrocyte glucose transporter were used to determine whether or not embryonic glucose transporters of rat skeletal muscle, liver and brain are identical to the transporters of adult animals. The results indicate that in both skeletal muscle and liver, the transporter switches from a highly antibody-reactive embryonic form to a low antibody-reactive adult form within 2 days of birth. This suggests that there are two different forms of glucose transporter in embryonic and adult skeletal muscle and liver. In contrast, these antibodies have equal reactivity toward the glucose transporters of embryonic and adult brain. In embryonic brain, two forms of the transporter coexist, with different molecular weights (Mr = 45,000 and 40,000), while in the adult brain the Mr = 40,000 form is predominant. The dissociation constant for glucose for the embryonic liver transporter was measured by displacement of bound cytochalasin B. The results indicate that the embryonic liver transporter has a low affinity for glucose and for cytochalasin B, similar to the adult liver transporter, even though the antibody reactivity toward these two transporters is different.

Cellular mechanisms of insulin resistance in non-insulin-dependent (type II) diabetes

Recent studies have led to an enhanced understanding of cellular alterations that may play an important role in the pathophysiology of non-insulin-dependent diabetes mellitus (NIDDM). The insulin receptor links insulin binding at the cell surface to intracellular activation of insulin's effects. This transducer function involves the tyrosine kinase property of the beta-subunit of the receptor. It was found that adipocytes from subjects with NIDDM had a 50 to 80 percent reduction in insulin-stimulated receptor kinase activity compared with their non-diabetic counterparts. This defect was relatively specific for the diabetic state since no decrease was observed in insulin-resistant non-diabetic obese subjects. The reduction in kinase activity was accounted for by changes in the ratio of two pools of receptors, both of which bind insulin but only one of which is capable of tyrosine autophosphorylation and subsequent kinase activation; 43 percent of the receptors from non-diabetic subjects were capable of autophosphorylation compared with only 14 percent in the NIDDM group. A major component of cellular insulin resistance in NIDDM involves the glucose transport system. Exposure of cells to insulin normally results in enhanced glucose transport mediated by translocation of glucose transporters from a low-density microsomal intracellular pool to the plasma membrane. It was found that cells from NIDDM subjects had a marked depletion of glucose transporters in both plasma membranes and low-density microsomes, relative to obese non-diabetic control participants. Obese non-diabetic persons had a normal number of plasma membrane transporters but a reduced number of low-density microsome transporters in the basal state compared with lean control volunteers; insulin induced the translocation of relatively fewer transporters from the low-density microsome to the plasma membrane in the obese subgroups. In addition to the diminished number of glucose transporters, cells from both NIDDM and obese subjects had impaired functional activity of glucose carriers since decreased whole-cell glucose transport rates could not be entirely explained by the magnitude of the decrement in the number of plasma membrane transporters. Thus, impaired glucose transport is due to both a numerical and functional defect in glucose transporters. The cellular content of high-density microsomal transporters was the same in lean and obese control volunteers and NIDDM subjects, suggesting that transporter synthesis is normal and that cellular depletion results from increased protein turnover once transporters leave the high-density microsomal subfraction.(ABSTRACT TRUNCATED AT 400 WORDS)

Control of placental glucose transfer

There is little evidence to suggest that the membrane transfer mechanism of the placenta for glucose becomes saturated until maternal blood glucose concentrations are quite high. Also, recent evidence suggests that the membrane transport system for glucose in the placenta is not stimulated by maternal or fetal insulin. Furthermore, there is no solid evidence that hormonal or non-hormonal factors function in vivo to limit membrane transport of glucose in the placenta. Therefore, the limited data which are available suggest that there are no specific mechanisms which acutely regulate placental membrane transport of glucose, and that this membrane transport mechanism operates to maximize maternal-to-fetal glucose transfer. The rate of maternal-to-fetal glucose transfer is a function of the transplacental concentration gradient. This gradient appears to be under the control of fetal insulin and placental lactogen. The available data suggest that both hormones act to increase this concentration gradient: insulin by decreasing fetal blood glucose, and placental lactogen by both decreasing fetal and increasing maternal blood glucose concentrations. Furthermore, high rates of glucose uptake by fetal erythrocytes tend to promote maintenance of this concentration gradient. Therefore, these influences of the maternal-fetal concentration gradient promote transplacental glucose flux to the fetus. As illustrated by the fetal complications associated with maternal hyperglycaemia, the cellular and organismic physiology of the fetus and placenta appears to maximize, rather than optimize, glucose availability to the fetus. It may be, however, that during normal pregnancy, maximal availability is optimal for fetal development.