Activation of Glut1 glucose transporter in response to inhibition of oxidative phosphorylation

PURA and GLUT1: Sweet partners for brain health

PURA, also known as Pur-alpha, is an evolutionarily conserved DNA/RNA-binding protein crucial for various cellular processes, including DNA replication, transcriptional regulation, and translational control. Comprising three PUR domains, it engages with nucleic acids and has a role in protein-protein interactions. The manifestation of PURA syndrome, arising from mutations in the PURA gene, presents neurologically with developmental delay, hypotonia, and seizures. In our prior work from 2018, we highlighted the unique case of a PURA patient displaying hypoglycorrhachia, suggesting a potential association with GLUT1 dysfunction in this syndrome. In this current study, we expand the patient cohort with PURA mutations exhibiting hypoglycorrhachia and aim to unravel the molecular basis of this phenomenon. We established an in vitro model in HeLa cells to modulate PURA expression and investigated GLUT1 function and expression. Our findings indicate that PURA levels directly impact glucose uptake through the functioning of GLUT1, without influencing significantly GLUT1 expression. Moreover, our study reveals evidence for a possible physical interaction between PURA and GLUT1, demonstrated by colocalization and co-immunoprecipitation of both proteins. Computational analyses, employing molecular dynamics, further corroborates these findings, demonstrating that PURA:GLUT1 interactions are plausible, and that the stability of the complex is altered when PURA is truncated and/or mutated. In conclusion, our results suggest that PURA plays a pivotal role in driving the function of GLUT1 for glucose uptake, potentially forming a regulatory complex. Additional investigations are warranted to elucidate the precise mechanisms governing this complex and its significance in ensuring proper GLUT1 function.

Nucleosides are overlooked fuels in central carbon metabolism

From our daily nutrition and synthesis within cells, nucleosides enter the bloodstream and circulate throughout the body and tissues. Nucleosides and nucleotides are classically viewed as precursors of nucleic acids, but recently they have emerged as a novel energy source for central carbon metabolism. Through catabolism by nucleoside phosphorylases, the ribose sugar group is released and can provide substrates for lower steps in glycolysis. In environments with limited glucose, such as at sites of infection or in the tumor microenvironment (TME), cells can use, and may even require, this alternative energy source. Here, we discuss the implications of these new findings in health and disease and speculate on the potential new roles of nucleosides and nucleic acids in energy metabolism.

Salvage of ribose from uridine or RNA supports glycolysis in nutrient-limited conditions

Glucose is vital for life, serving as both a source of energy and carbon building block for growth. When glucose is limiting, alternative nutrients must be harnessed. To identify mechanisms by which cells can tolerate complete loss of glucose, we performed nutrient-sensitized genome-wide genetic screens and a PRISM growth assay across 482 cancer cell lines. We report that catabolism of uridine from the medium enables the growth of cells in the complete absence of glucose. While previous studies have shown that uridine can be salvaged to support pyrimidine synthesis in the setting of mitochondrial oxidative phosphorylation deficiency1, our work demonstrates that the ribose moiety of uridine or RNA can be salvaged to fulfil energy requirements via a pathway based on: (1) the phosphorylytic cleavage of uridine by uridine phosphorylase UPP1/UPP2 into uracil and ribose-1-phosphate (R1P), (2) the conversion of uridine-derived R1P into fructose-6-P and glyceraldehyde-3-P by the non-oxidative branch of the pentose phosphate pathway and (3) their glycolytic utilization to fuel ATP production, biosynthesis and gluconeogenesis. Capacity for glycolysis from uridine-derived ribose appears widespread, and we confirm its activity in cancer lineages, primary macrophages and mice in vivo. An interesting property of this pathway is that R1P enters downstream of the initial, highly regulated steps of glucose transport and upper glycolysis. We anticipate that 'uridine bypass' of upper glycolysis could be important in the context of disease and even exploited for therapeutic purposes.

Differential Effects of Human Adenovirus E1A Protein Isoforms on Aerobic Glycolysis in A549 Human Lung Epithelial Cells

Viruses alter a multitude of host-cell processes to create a more optimal environment for viral replication. This includes altering metabolism to provide adequate substrates and energy required for replication. Typically, viral infections induce a metabolic phenotype resembling the Warburg effect, with an upregulation of glycolysis and a concurrent decrease in cellular respiration. Human adenovirus (HAdV) has been observed to induce the Warburg effect, which can be partially attributed to the adenovirus protein early region 4, open reading frame 1 (E4orf1). E4orf1 regulates a multitude of host-cell processes to benefit viral replication and can influence cellular metabolism through the transcription factor avian myelocytomatosis viral oncogene homolog (MYC). However, E4orf1 does not explain the full extent of Warburg-like HAdV metabolic reprogramming, especially the accompanying decrease in cellular respiration. The HAdV protein early region 1A (E1A) also modulates the function of the infected cell to promote viral replication. E1A can interact with a wide variety of host-cell proteins, some of which have been shown to interact with metabolic enzymes independently of an interaction with E1A. To determine if the HAdV E1A proteins are responsible for reprogramming cell metabolism, we measured the extracellular acidification rate and oxygen consumption rate of A549 human lung epithelial cells with constitutive endogenous expression of either of the two major E1A isoforms. This was followed by the characterization of transcript levels for genes involved in glycolysis and cellular respiration, and related metabolic pathways. Cells expressing the 13S encoded E1A isoform had drastically increased baseline glycolysis and lower maximal cellular respiration than cells expressing the 12S encoded E1A isoform. Cells expressing the 13S encoded E1A isoform exhibited upregulated expression of glycolysis genes and downregulated expression of cellular respiration genes. However, tricarboxylic acid cycle genes were upregulated, resembling anaplerotic metabolism employed by certain cancers. Upregulation of glycolysis and tricarboxylic acid cycle genes was also apparent in IMR-90 human primary lung fibroblast cells infected with a HAdV-5 mutant virus that expressed the 13S, but not the 12S encoded E1A isoform. In conclusion, it appears that the two major isoforms of E1A differentially influence cellular glycolysis and oxidative phosphorylation and this is at least partially due to the altered regulation of mRNA expression for the genes in these pathways.

Acute stimulation of glucose influx upon mitoenergetic dysfunction requires LKB1, AMPK, Sirt2 and mTOR-RAPTOR

Mitochondria play a central role in cellular energy production, and their dysfunction can trigger a compensatory increase in glycolytic flux to sustain cellular ATP levels. Here, we studied the mechanism of this homeostatic phenomenon in C2C12 myoblasts. Acute (30 min) mitoenergetic dysfunction induced by the mitochondrial inhibitors piericidin A and antimycin A stimulated Glut1-mediated glucose uptake without altering Glut1 (also known as SLC2A1) mRNA or plasma membrane levels. The serine/threonine liver kinase B1 (LKB1; also known as STK11) and AMP-activated protein kinase (AMPK) played a central role in this stimulation. In contrast, ataxia-telangiectasia mutated (ATM; a potential AMPK kinase) and hydroethidium (HEt)-oxidizing reactive oxygen species (ROS; increased in piericidin-A- and antimycin-A-treated cells) appeared not to be involved in the stimulation of glucose uptake. Treatment with mitochondrial inhibitors increased NAD⁺ and NADH levels (associated with a lower NAD⁺:NADH ratio) but did not affect the level of Glut1 acetylation. Stimulation of glucose uptake was greatly reduced by chemical inhibition of Sirt2 or mTOR-RAPTOR. We propose that mitochondrial dysfunction triggers LKB1-mediated AMPK activation, which stimulates Sirt2 phosphorylation, leading to activation of mTOR-RAPTOR and Glut1-mediated glucose uptake.

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.

Interactions between mitochondrial reactive oxygen species and cellular glucose metabolism

Mitochondrial reactive oxygen species (ROS) production and detoxification are tightly balanced. Shifting this balance enables ROS to activate intracellular signaling and/or induce cellular damage and cell death. Increased mitochondrial ROS production is observed in a number of pathological conditions characterized by mitochondrial dysfunction. One important hallmark of these diseases is enhanced glycolytic activity and low or impaired oxidative phosphorylation. This suggests that ROS is involved in glycolysis (dys)regulation and vice versa. Here we focus on the bidirectional link between ROS and the regulation of glucose metabolism. To this end, we provide a basic introduction into mitochondrial energy metabolism, ROS generation and redox homeostasis. Next, we discuss the interactions between cellular glucose metabolism and ROS. ROS-stimulated cellular glucose uptake can stimulate both ROS production and scavenging. When glucose-stimulated ROS production, leading to further glucose uptake, is not adequately counterbalanced by (glucose-stimulated) ROS scavenging systems, a toxic cycle is triggered, ultimately leading to cell death. Here we inventoried the various cellular regulatory mechanisms and negative feedback loops that prevent this cycle from occurring. It is concluded that more insight in these processes is required to understand why they are (un)able to prevent excessive ROS production during various pathological conditions in humans.

Chronic ethanol consumption disrupts diurnal rhythms of hepatic glycogen metabolism in mice

Chronic ethanol consumption has been shown to significantly decrease hepatic glycogen content; however, the mechanisms responsible for this adverse metabolic effect are unknown. In this study, we examined the impact chronic ethanol consumption has on time-of-day-dependent oscillations (rhythms) in glycogen metabolism processes in the liver. For this, male C57BL/6J mice were fed either a control or ethanol-containing liquid diet for 5 wk, and livers were collected every 4 h for 24 h and analyzed for changes in various genes and proteins involved in hepatic glycogen metabolism. Glycogen displayed a robust diurnal rhythm in the livers of mice fed the control diet, with the peak occurring during the active (dark) period of the day. The diurnal glycogen rhythm was significantly altered in livers of ethanol-fed mice, with the glycogen peak shifted into the inactive (light) period and the overall content of glycogen decreased compared with controls. Chronic ethanol consumption further disrupted diurnal rhythms in gene expression (glycogen synthase 1 and 2, glycogenin, glucokinase, protein targeting to glycogen, and pyruvate kinase), total and phosphorylated glycogen synthase protein, and enzyme activities of glycogen synthase and glycogen phosphorylase, the rate-limiting enzymes of glycogen metabolism. In summary, these results show for the first time that chronic ethanol consumption disrupts diurnal rhythms in hepatic glycogen metabolism at the gene and protein level. Chronic ethanol-induced disruption in these daily rhythms likely contributes to glycogen depletion and disruption of hepatic energy homeostasis, a recognized risk factor in the etiology of alcoholic liver disease.

Modulation of glucose metabolism by CD44 contributes to antioxidant status and drug resistance in cancer cells

An increased glycolytic flux accompanied by activation of the pentose phosphate pathway (PPP) is implicated in chemoresistance of cancer cells. In this study, we found that CD44, a cell surface marker for cancer stem cells, interacts with pyruvate kinase M2 (PKM2) and thereby enhances the glycolytic phenotype of cancer cells that are either deficient in p53 or exposed to hypoxia. CD44 ablation by RNA interference increased metabolic flux to mitochondrial respiration and concomitantly inhibited entry into glycolysis and the PPP. Such metabolic changes induced by CD44 ablation resulted in marked depletion of cellular reduced glutathione (GSH) and increased the intracellular level of reactive oxygen species in glycolytic cancer cells. Furthermore, CD44 ablation enhanced the effect of chemotherapeutic drugs in p53-deficient or hypoxic cancer cells. Taken together, our findings suggest that metabolic modulation by CD44 is a potential therapeutic target for glycolytic cancer cells that manifest drug resistance.

GLUT-1 glucose transporters in the blood-brain barrier: differential phosphorylation

Glucose is the primary metabolic fuel for the mammalian brain, and a continuous supply is required to maintain normal CNS function. The transport of glucose across the blood-brain barrier (BBB) into the brain is mediated by the facilitative glucose transporter GLUT-1. Prior studies (Simpson et al. [2001] J Biol Chem 276:12725-12729) had revealed that the conformations of the GLUT-1 transporter were different in luminal (blood facing) and abluminal (brain facing) membranes of bovine cerebral endothelial cells, based on differential antibody recognition. This study has extended these observations and, by using a combination of 2D-PAGE/Western blotting and immunogold electron microscopy, determined that these different conformations are exhibited in vivo and arise from differential phosphorylation of GLUT-1 and not from alternative splicing or altered O- or N-linked glycosylation.

Downregulation of adipose glutathione S-transferase A4 leads to increased protein carbonylation, oxidative stress, and mitochondrial dysfunction

OBJECTIVE

Peripheral insulin resistance is linked to an increase in reactive oxygen species (ROS), leading in part to the production of reactive lipid aldehydes that modify the side chains of protein amino acids in a reaction termed protein carbonylation. The primary enzymatic method for lipid aldehyde detoxification is via glutathione S-transferase A4 (GSTA4) dependent glutathionylation. The objective of this study was to evaluate the expression of GSTA4 and the role(s) of protein carbonylation in adipocyte function.

RESEARCH DESIGN AND METHODS

GSTA4-silenced 3T3-L1 adipocytes and GSTA4-null mice were evaluated for metabolic processes, mitochondrial function, and reactive oxygen species production. GSTA4 expression in human obesity was evaluated using microarray analysis.

RESULTS

GSTA4 expression is selectively downregulated in adipose tissue of obese insulin-resistant C57BL/6J mice and in human obesity-linked insulin resistance. Tumor necrosis factor-alpha treatment of 3T3-L1 adipocytes decreased GSTA4 expression, and silencing GSTA4 mRNA in cultured adipocytes resulted in increased protein carbonylation, increased mitochondrial ROS, dysfunctional state 3 respiration, and altered glucose transport and lipolysis. Mitochondrial function in adipocytes of lean or obese GSTA4-null mice was significantly compromised compared with wild-type controls and was accompanied by an increase in superoxide anion.

CONCLUSIONS

These results indicate that downregulation of GSTA4 in adipose tissue leads to increased protein carbonylation, ROS production, and mitochondrial dysfunction and may contribute to the development of insulin resistance and type 2 diabetes.

Protonophore- and pH-insensitive glucose and sucrose accumulation detected by FRET nanosensors in Arabidopsis root tips

Although soil contains only traces of soluble carbohydrates, plant roots take up glucose and sucrose efficiently when supplied in artificial media. Soluble carbohydrates and other small metabolites found in soil are in part products from exudation from plant roots. The molecular nature of the transporters for uptake and exudation is unknown. Here, fluorescence resonance energy transfer (FRET) glucose and sucrose sensors were used to characterize accumulation and elimination of glucose and sucrose in Arabidopsis roots tips. Using an improved image acquisition set-up, FRET responses to perfusion with carbohydrates were detectable in roots within less than 10 sec and over a wide concentration range. Accumulation was fully reversible within 10-180 sec after glucose or sucrose had been withdrawn; elimination may be caused by metabolism and/or efflux. The rate of elimination was unaffected by pre-incubation with high concentrations of glucose, suggesting that elimination is not due to accumulation in a short-term buffer such as the vacuole. Glucose and sucrose accumulation was insensitive to protonophores, was comparable in media differing in potassium levels, and was similar at pH 5.8, 6.8 and 7.8, suggesting that both influx and efflux may be mediated by proton-independent transport systems. High-resolution expression mapping in root tips showed that only a few proton-dependent transport of the STP (Sugar Transport Protein) and SUT/SUC (Sucrose Transporter/Carrier) families are expressed in the external cell layers of root tips. The root expression maps may help to pinpoint candidate genes for uptake and release of carbohydrates from roots.

Stimulation of glucose transport in response to activation of distinct AMPK signaling pathways

AMP-activated protein kinase (AMPK) plays a critical role in the stimulation of glucose transport in response to hypoxia and inhibition of oxidative phosphorylation. In the present study, we examined the signaling pathway(s) mediating the glucose transport response following activation of AMPK. Using mouse fibroblasts of AMPK wild type and AMPK knockout, we documented that the expression of AMPK is essential for the glucose transport response to both azide and 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR). In Clone 9 cells, the stimulation of glucose transport by a combination of azide and AICAR was not additive, whereas there was an additive increase in the abundance of phosphorylated AMPK (p-AMPK). In Clone 9 cells, AMPK wild-type fibroblasts, and H9c2 heart cells, azide or hypoxia selectively increased p-ERK1/2, whereas, in contrast, AICAR selectively stimulated p-p38; phosphorylation of JNK was unaffected. Azide's effect on p-ERK1/2 abundance and glucose transport in Clone 9 cells was partially abolished by the MEK1/2 inhibitor U0126. SB 203580, an inhibitor of p38, prevented the phosphorylation of p38 and the glucose transport response to AICAR and, unexpectedly, to azide. Hypoxia, azide, and AICAR all led to increased phosphorylation of Akt substrate of 160 kDa (AS160) in Clone 9 cells. Employing small interference RNA directed against AS160 did not inhibit the glucose transport response to azide or AICAR, whereas the content of P-AS160 was reduced by approximately 80%. Finally, we found no evidence for coimmunoprecipitation of Glut1 and p-AS160. We conclude that although azide, hypoxia, and AICAR all activate AMPK, the downstream signaling pathways are distinct, with azide and hypoxia stimulating ERK1/2 and AICAR stimulating the p38 pathway.

Identification of poly(ADP-ribose) polymerase-1 as the OXPHOS-generated ATP sensor of nuclei of animal cells

Our results show that in the intact normal animal cell mitochondrial ATP is directly connected to nuclear PARP-1 by way of a specific adenylate kinase enzymatic path. This mechanism is demonstrated in two models: (a) by its inhibition with a specific inhibitor of adenylate kinase, and (b) by disruption of ATP synthesis through uncoupling of OXPHOS. In each instance the de-inhibited PARP-1 is quantitatively determined by enzyme kinetics. The nuclear binding site of PARP-1 is Topo I, and is identified as a critical "switchpoint" indicating the nuclear element that connects OXPHOS with mRNA synthesis in real time. The mitochondrial-nuclear PARP-1 pathway is not operative in cancer cells.

6-Fluoro-6-deoxy-D-glucose as a tracer of glucose transport

Glucose transport rates are estimated noninvasively in physiological and pathological states by kinetic imaging using PET. The glucose analog most often used is (18)F-labeled 2FDG. Compared with glucose, 2FDG is poorly transported by intestine and kidney. We examined the possible use of 6FDG as a tracer of glucose transport. Lacking a hydroxyl at its 6th position, 6FDG cannot be phosphorylated as 2FDG is. Prior studies have shown that 6FDG competes with glucose for transport in yeast and is actively transported by intestine. Its uptake by muscle has been reported to be unresponsive to insulin, but that study is suspect. We found that insulin stimulated 6FDG uptake 1.6-fold in 3T3-L1 adipocytes and azide stimulated the uptake 3.7-fold in Clone 9 cells. Stimulations of the uptake of 3OMG, commonly used in transport assays, were similar, and the uptakes were inhibited by cyclochalasin B. Glucose transport is by GLUT1 and GLUT4 transporters in 3T3-L1 adipocyte and by the GLUT1 transporter in Clone 9 cells. Cytochalasin B inhibits those transporters. Rats were also imaged in vivo by PET using 6(18)FDG. There was no excretion of (18)F into the urinary bladder unless phlorizin, an inhibitor of active renal transport, was also injected. (18)F activity in brain, liver, and heart over the time of scanning reached a constant level, in keeping with the 6FDG being distributed in body water. In contrast, (18)F from 2(18)FDG was excreted in relatively large amounts into the bladder, and (18)F activity rose with time in heart and brain in accord with accumulation of 2(18)FDG-6-P in those organs. We conclude that 6FDG is actively transported by kidney as well as intestine and is insulin responsive. In trace quantity, it appears to be distributed in body water unchanged. These results provide support for its use as a valid tracer of glucose transport.

Selective cyclooxygenase-2 inhibitors stimulate glucose transport in L6 myotubes in a protein kinase Cδ-dependent manner

Selective inhibitors of cyclooxygenase-2 (prostaglandin-endoperoxide synthase-2; COX-2) augment the rate of hexose uptake in myotubes by recruiting glucose transporter-4 (GLUT-4) to the plasma membrane in an insulin- and AMPKalpha-independent manner [Alpert E, Gruzman A, Lardi-Studler B, Cohen G, Reich R, Sasson S. Cyclooxygenase-2 (PTGS2) inhibitors augment the rate of hexose transport in L6 myotubes in an insulin- and AMPKalpha-independent manner. Diabetologia 2006;49:562-70]. We aimed at elucidating the molecular interactions that mediate this effect of COX-2 inhibitors in L6 myotubes. The effects of the inhibitors niflumic acid, nimesulide and rofecoxib on activities and phosphorylation state of key proteins in the insulin transduction pathway were determined. These inhibitors did not induce specific tyrosine phosphorylation in IRS-1, could not assemble a functional IRS-PI3K-PKB/Akt complex and did not activate GSK3alpha/beta, JNK1/2, ERK1/2, p38-MAPK or c-Cbl by site-specific phosphorylation(s). Yet, like insulin, they activated mTOR and induced downstream threonine phosphorylation in p70S6K and 4EBP1. However, rapamycin, which inhibits mTOR enzymatic activity, did not interfere with COX-2 inhibitor-induced stimulation of hexose uptake in myotube. Thus, mTOR activation was not required for COX-2 inhibitor-dependent augmentation of hexose transport in myotubes. Because PKCdelta has also been shown to activate mTOR, we asked whether COX-2 inhibitors activate mTOR by a prior activation of PKCdelta. Indeed, all three inhibitors induced tyrosine phosphorylation in PKCdelta and stimulated its kinase activity. Moreover, pharmacological inhibition of PKCdelta or the expression of a dominant-negative form of PKCdelta in myotubes completely abolished COX-2 inhibitor-dependent stimulation of hexose uptake. This study shows that selective COX-2 inhibitors activate a unique PKCdelta-dependent pathway to increase GLUT-4 abundance in the plasma membrane of myotubes and augment the rate of hexose transport.

Nonmetabolizable glucose compounds impart cryotolerance to primary rat hepatocytes

We herein report a novel method for the cryopreservation of hepatocytes using a non-metabolizable glucose derivative in an attempt to mimic the natural cryoprotective adaptations observed in freeze-tolerant frogs. Primary rat hepatocytes were loaded with 3-O-methyl glucose (3OMG) through endogenous glucose transporters without evident toxicity. The 3OMG-loaded hepatocytes were then frozen in a controlled rate freezer down to -80 degrees C and stored in liquid nitrogen at -196 degrees C. Hepatocytes cryopreserved with a relatively small amount of intracellular 3OMG (<0.2 M) showed high post-thaw viability and maintained long-term hepatospecific functions, including synthesis, metabolism, and detoxification. Metabolite uptake and secretion rates were also largely preserved in the cryopreserved hepatocytes. This is the first study to demonstrate the use of the non-metabolizable glucose derivative 3OMG in hepatocyte cryopreservation.

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

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

Role of 5'-AMP-activated protein kinase in stimulation of glucose transport in response to inhibition of oxidative phosphorylation

Glucose transport is stimulated in a variety of cells and tissues in response to inhibition of oxidative phosphorylation. However, the underlying mechanisms and mediating steps remain largely unknown. In the present study we first tested whether a decrease in the redox state of the cell per se and the resultant increase in generation of reactive oxygen species (ROS) lead to stimulation of glucose transport. Clone 9 cells (expressing the Glut1 isoform of facilitative glucose transporters) were exposed to azide, lactate, and ethanol for 1 h. Although all three agents stimulated glucose transport and increased cell NADH-to-NAD(+) ratio and phospho-ERK1/2, signifying increased ROS generation, the response to the stimuli was not blocked by N-acetyl-l-cysteine (an agent that counteracts ROS); moreover, the response to azide was not blocked by diamide (an intracellular sulfhydryl oxidizing agent). We then found that cell AMP-to-ATP and ADP-to-ATP ratios were increased and 5'-AMP-activated protein kinase (AMPK) was stimulated by all three agents, as evidenced by increased phosphorylation of AMPK and acetyl-CoA carboxylase. We conclude that although azide, lactate, and ethanol increase NADH-to-NAD(+) ratios and ROS production, their stimulatory effect on glucose transport is not mediated by increased ROS generation. However, all three agents increased cell AMP-to-ATP ratio and stimulated AMPK, making it likely that the latter pathway plays an important role in the glucose transport response.

Mitochondrial dysfunction induces triglyceride accumulation in 3T3-L1 cells: role of fatty acid beta-oxidation and glucose

Mitochondrial cytopathy has been associated with modifications of lipid metabolism in various situations, such as the acquisition of an abnormal adipocyte phenotype observed in multiple symmetrical lipomatosis or triglyceride (TG) accumulation in muscles associated with the myoclonic epilepsy with ragged red fibers syndrome. However, the molecular signaling leading to fat metabolism dysregulation in cells with impaired mitochondrial activity is still poorly understood. Here, we found that preadipocytes incubated with inhibitors of mitochondrial respiration such as antimycin A (AA) accumulate TG vesicles but do not acquire specific markers of adipocytes. Although the uptake of TG precursors is not stimulated in 3T3-L1 cells with impaired mitochondrial activity, we found a strong stimulation of glucose uptake in AA-treated cells mediated by calcium and phosphatidylinositol 3-kinase/Akt1/glycogen synthase kinase 3beta, a pathway known to trigger the translocation of glucose transporter 4 to the plasma membrane in response to insulin. TG accumulation in AA-treated cells is mediated by a reduced peroxisome proliferator-activated receptor gamma activity that downregulates muscle carnitine palmitoyl transferase-1 expression and fatty acid beta-oxidation, and by a direct conversion of glucose into TGs accompanied by the activation of carbohydrate-responsive element binding protein, a lipogenic transcription factor. Taken together, these results could explain how mitochondrial impairment leads to the multivesicular phenotype found in some mitochondria-originating diseases associated with a dysfunction in fat metabolism.

Neurobarrier coupling in the brain: a partner of neurovascular and neurometabolic coupling?

Neurovascular and neurometabolic coupling help the brain to maintain an appropriate energy flow to the neural tissue under conditions of increased neuronal activity. Both coupling phenomena provide us, in addition, with two macroscopically measurable parameters, blood flow and intermediate metabolite fluxes, that are used to dynamically image the functioning brain. The main energy substrate for the brain is glucose, which is metabolized by glycolysis and oxidative breakdown in both astrocytes and neurons. Neuronal activation triggers increased glucose consumption and glucose demand, with new glucose being brought in by stimulated blood flow and glucose transport over the blood-brain barrier. Glucose is shuttled over the barrier by the GLUT-1 transporter, which, like all transporter proteins, has a ceiling above which no further stimulation of the transport is possible. Blood-brain barrier glucose transport is generally accepted as a nonrate-limiting step but to prevent it from becoming rate-limiting under conditions of neuronal activation, it might be necessary for the transport parameters to be adapted to the increased glucose demand. It is proposed that the blood-brain barrier glucose transport parameters are dynamically adapted to the increased glucose needs of the neural tissue after activation according to a neurobarrier coupling scheme. This review presents neurobarrier coupling within the current knowledge on neurovascular and neurometabolic coupling, and considers arguments and evidence in support of this hypothesis.

Differential accumulation of Glut1 in the non-DRM domain of the plasma membrane in response to the inhibition of oxidative phosphorylation

Approximately 50% of Glut1 in the plasma membrane of Clone 9 cells is localized to the detergent-resistant membrane (DRM) fraction. Acute exposure (90 min) to 5mM azide stimulated glucose transport by approximately 4.7-fold and increased the abundance of Glut1 in the non-DRM fraction of the plasma membrane by approximately 2.9-fold while the abundance of Glut1 in the DRMs was not changed. In parallel experiments, approximately 17 h exposure to azide further increased the rate of glucose transport over that observed at 90 min by approximately 33% and increased plasma membrane Glut1 content by approximately 3.5-fold over control. The increase in total plasma membrane Glut1 reflected a approximately 4.7-fold increase of Glut1 content in the non-DRM fraction and a approximately 2.6-fold increase in the DRMs. We conclude that acute exposure to azide increases Glut1 content in the non-DRM fractions, while prolonged exposure to azide increases the Glut1 content in both non-DRM and DRM fractions. These changes may play an important role in the stimulation of glucose transport in response to the inhibition of oxidative phosphorylation.

4-Hydroxy tempol-induced impairment of mitochondrial function and augmentation of glucose transport in vascular endothelial and smooth muscle cells

The water-soluble and cell permeable nitroxide derivative 4-hydroxy tempol (TPL) has been shown to reduce or ameliorate oxidative stress-induced dysfunction and damage in vascular endothelial cells. We studied the effects of TPL on glucose transport and metabolism in bovine aortic endothelial (VEC) and smooth muscle cells (VSMC) under normal and high glucose conditions. Normally, these cells operate an autoregulatory protective mechanism that limits the rate of glucose transport under hyperglycemic conditions by decreasing the cell content of their typical glucose transporter GLUT-1 mRNA and protein as well as its plasma membrane abundance. TPL augmented the rate of glucose transport both under normo- and hyperglycemic conditions by increasing GLUT-1 mRNA and protein content and its plasma membrane abundance in both types of cells, leading to an increased flux of glucose into the cells. These effects were found related to ROS-generating and oxidant activities of TPL and to a decreased rate of mitochondrial ATP production under both normo- and hyperglycemic conditions. Since impaired mitochondrial functions, and in particular decreased rate of ATP production, augment the expression of GLUT-1 protein and glucose transport and metabolism, we suggest that the stimulatory effects of TPL in vascular cells results from its unfavorable interactions in the mitochondrion. It is therefore suggested that effects of TPL in cells of cardiovascular system be evaluated in parallel to its adverse effects on glucose and energy metabolism.

Methyl-beta-cyclodextrin stimulates glucose uptake in Clone 9 cells: a possible role for lipid rafts

An acute increase in the Vmax for glucose uptake occurs in many mammalian cell types after exposure to osmotic or metabolic stress. In the rat epithelial Clone 9 cell line, the glucose transporter isoform GLUT1 is responsible for this enhanced uptake. Although stimulation of transport in these cells is known to result from the unmasking of 'cryptic' exofacial permeant-binding sites in GLUT1 molecules resident in the plasma membrane, the mechanism of such unmasking remains unclear. One possibility involves changes in the lipid environment of the transporter: reconstitution experiments have shown that transport activity in vitro is acutely sensitive to the phospholipid and cholesterol composition of the membrane. In the current study we found that treatment of Clone 9 cells with methyl-beta-cyclodextrin, which removed >80% of the cell cholesterol, led to a 3.5-fold increase in the Vmax for 3-O-methyl-D-glucose transport while having little effect on the Km. In contrast to the metabolic stress induced by inhibition of oxidative phosphorylation, cholesterol depletion led neither to depletion of cellular ATP nor stimulation of AMP-activated protein kinase. Similarly, it did not result in stimulation of members of the stress- and mitogen-activated protein kinase families. In unstressed, cholesterol-replete cells, a substantial proportion of GLUT1 in detergent lysates co-fractionated with the lipid-raft proteins caveolin and stomatin on density-gradient centrifugation. Immunocytochemistry also revealed the presence of GLUT1-enriched domains, some of which co-localized with stomatin, in the plasma membrane. Both techniques revealed that the abundance of such putative GLUT1-containing domains was decreased not only by cholesterol depletion but also in cells subjected to metabolic stress. Taken together, these data suggest that a change in the lipid environment of GLUT1, possibly associated with its re-distribution between different microdomains of the plasma membrane, could play a role in its activation in response to stress.

Stem cell factor and H2O2 induce GLUT1 translocation in M07e cells

This work aims to elucidate the mechanisms involved in the early activation of glucose transport in hematopoietic M07e cells by stem cell factor (SCF) and a reactive oxygen species (ROS) as H2O2. SCF and H2O2 increase Vmax for glucose transport; this enhancement is due to a higher content in GLUT1 in plasma membranes, possibly through a translocation from intracellular stores. Inhibitors of tyrosine kinases or phospholipase C (PLC) remove glucose transport enhancement and prevent translocation. The inhibitory effect of STI-571 suggests a role for c-kit tyrosine kinase on glucose transport activation not only by SCF, but also by H2O2. On the other hand, neither protein kinase C nor phosphoinositide-3-kinase appear to be involved in the acute activation of glucose transport. Our data suggest that i) in M07e cells, SCF and exogenous H2O2 elicit a short-term activation of glucose transport through a translocation of GLUT1 from intracellular stores to plasma membranes; ii) both stimuli could share at least some signaling pathways leading to glucose uptake activation, involving protein tyrosine kinases and PLC iii) H2O2 could act increasing the level of tyrosine phosphorylation through the inhibition of tyrosine phosphatases and mimicking the regulation role of endogenous ROS.

Implications of glucose transporter protein type 1 (GLUT1)-haplodeficiency in embryonic stem cells for their survival in response to hypoxic stress

Glucose transporter protein type 1 (GLUT1) is a major glucose transporter of the fertilized egg and preimplantation embryo. Haploinsufficiency for GLUT1 causes the GLUT1 deficiency syndrome in humans, however the embryo appears unaffected. Therefore, here we produced heterozygous GLUT1 knockout murine embryonic stem cells (GT1+/-) to study the role of GLUT1 deficiency in their growth, glucose metabolism, and survival in response to hypoxic stress. GT1(-/-) cells were determined to be nonviable. Both the GLUT1 and GLUT3 high-affinity, facilitative glucose transporters were expressed in GT1(+/+) and GT1(+/-) embryonic stem cells. GT1(+/-) demonstrated 49 +/- 4% reduction of GLUT1 mRNA. This induced a posttranscriptional, GLUT1 compensatory response resulting in 24 +/- 4% reduction of GLUT1 protein. GLUT3 was unchanged. GLUT8 and GLUT12 were also expressed and unchanged in GT1(+/-). Stimulation of glycolysis by azide inhibition of oxidative phosphorylation was impaired by 44% in GT1(+/-), with impaired up-regulation of GLUT1 protein. Hypoxia for up to 4 hours led to 201% more apoptosis in GT1(+/-) than in GT1(+/+) controls. Caspase-3 activity was 76% higher in GT1(+/-) versus GT1(+/+) at 2 hours. Heterozygous knockout of GLUT1 led to a partial GLUT1 compensatory response protecting nonstressed cells. However, inhibition of oxidative phosphorylation and hypoxia both exposed their increased susceptibility to these stresses.

Glutamate triggers rapid glucose transport stimulation in astrocytes as evidenced by real-time confocal microscopy

Glutamate stimulates glycolysis in astrocytes, a phenomenon that couples astrocytic metabolism with neuronal activity. However, it is not known whether glutamate also affects glucose transporter-1 (GLUT1), the transporter responsible for glucose entry into astrocytes. To address this question, two different real-time single-cell hexose uptake assays were applied to cultured hippocampal astrocytes using confocal epifluorescence microscopy. Glutamate caused a twofold to threefold increase in the zero-trans uptake rates of the fluorescent hexoses 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxyglucose (2-NBDG) and 6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-6-deoxyglucose (6-NBDG). Galactose uptake, determined by the calcein volumetric assay, was stimulated to a similar extent, confirming the fluorescent hexose data, and also demonstrating that glutamate stimulation is a Vmax effect. Remarkably, glucose transport stimulation developed fully inside 10 sec, which is 100 times faster than acute stimulations of glucose transport in other cell types. Glutamate did not significantly affect the rate of 6-NBDG uptake by GLUT1-expressing epithelial Clone 9 cells, suggesting that an astrocyte-specific factor is required for transport stimulation. We conclude that glucose transport stimulation occurs early during astrocytic activation by glutamate, which provides a novel regulatory node to current models of brain energy metabolism. This mechanism should also be considered for the interpretation of functional imaging data based on hexoses.

Distribution of Glut1 in detergent-resistant membranes (DRMs) and non-DRM domains: effect of treatment with azide

We have previously shown that the acute stimulation of glucose transport in Clone 9 cells in response to azide is mediated by activation of Glut1 and that stomatin, a Glut1-binding protein, appears to inhibit Glut1 function. In Clone 9 cells under basal conditions, approximately 38% of Glut1, approximately 70% of stomatin, and the bulk of caveolin-1 was localized in the detergent-resistant membrane (DRM) fraction; a significant fraction of Glut1 is also present in DRMs of 3T3-L1 fibroblasts and human red blood cells (RBCs). Acute exposure to azide resulted in 40 and 50% decreases in the content of Glut1 in DRMs of Clone 9 cells and 3T3-L1 fibroblasts, respectively, whereas the distribution of stomatin and caveolin-1 in Clone 9 cells remained unchanged. In addition, treatment of Clone 9 cells with azide resulted in a approximately 50% decrease in the content of Glut1 in the DRM fraction of plasma membranes. We conclude that 1) a significant fraction of Glut1 is localized in DRMs, and 2) treatment of cells with azide results in a partial redistribution of Glut1 out of the DRM fraction.

Age-related changes in neuronal glucose uptake in response to glutamate and beta-amyloid

Energy supplies that may decline with age are crucial for cells to maintain ionic homeostasis and prevent neuron death. We examined baseline glucose transporter expression and rate of glucose uptake in cultured hippocampal neurons from embryonic, middle-age (12-month-old), and old (24-month-old) rats and exposed the neurons to glutamate, beta-amyloid, and mitochondrial inhibitors. Without stress, the rate of glucose uptake was similar in middle-age and old neurons, and the rate of glucose uptake in embryonic neurons was threefold greater than that in middle-age and old neurons. Glucose uptake increased in the presence of mitochondrial inhibitors (FCCP and oligomycin) for embryonic and middle-age neurons. The old neurons failed to increase glucose uptake. In the presence of glutamate, FCCP, and oligomycin, embryonic neurons showed a decrease in glucose uptake and the middle-age and old neurons showed no change in glucose uptake. Middle-age neurons took up significantly more glucose than old neurons when under mitochondrial and glutamate stress. In the presence of beta-amyloid, only embryonic neurons increased glucose uptake; middle-age and old neurons did not. Fluorescence imaging of immunoreactive glut3 in response to beta-amyloid demonstrated a 16-49% increase in glut3 immunoreactivity at the plasma membrane for the three ages. The results suggest that old neurons were not able to upregulate glucose uptake to ensure cell survival. Neuron aging does not indicate a defect in normal glut3 function; rather, our results suggest that mechanisms regulating glucose uptake under stress fail to react in time to ensure cell survival.

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

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

Activation of GLUT1 by metabolic and osmotic stress: potential involvement of AMP-activated protein kinase (AMPK)

In the rat liver epithelial cell line Clone 9, the Vmax for glucose uptake is actuely increased by inhibition of oxidative phosphorylation and by osmotic stress. By using a membrane-impermeant photoaffinity labelling reagent together with an isoform-specific antibody, we have, for the first time, provided direct evidence for the involvement of the GLUT1 glucose transporter isoform in this response. Transport stimulation was found to be associated with enhanced accessibility of GLUT1 to its substrate and with photolabelling of formerly `cryptic' exofacial substrate binding sites in GLUT1 molecules. The total amount of cell surface GLUT1 remained constant. The precise mechanism for this binding site `unmasking' is unclear but appears to involve AMP-activated protein kinase: in the current study, osmotic and metabolic stresses were found to result in activation of the α1 isoform of AMP-activated protein kinase, and transport stimulation could be mimicked both by 5-aminoimidazole-4-carboxamide ribonucleoside and by infection of cells with a recombinant adenovirus encoding constitutively active AMP-activated protein kinase. The effect of 5-aminoimidazole-4-carboxamide ribonucleoside, as for metabolic stress, was on the Vmax rather than on the Km for transport and did not affect the cell-surface concentration of GLUT1. The relevant downstream target(s) of AMP-activated protein kinase have not yet been identified, but stimulation of transport by inhibition of oxidative phosphorylation or by 5-aminoimidazole-4-carboxamide ribonucleoside was not prevented by either inhibitors of conventional and novel protein kinase C isoforms or inhibitors of nitric oxide synthase. These enzymes, which have been implicated in stress-regulated pathways in other cell types, are therefore unlikely to play a role in transport regulation by stress in Clone 9 cells.

ATP stimulates glucose transport through activation of P2 purinergic receptors in C(2)C(12) skeletal muscle cells

Extracellular ATP acts as a signal that regulates a variety of cellular processes via binding to P2 purinergic receptors (P2 receptors). We herein investigated the effects and signaling pathways of ATP on glucose uptake in C(2)C(12) skeletal muscle cells. ATP as well as P2 receptor agonists (ATP-gamma S) stimulated the rate of glucose uptake, while P2 receptor antagonists (suramin) inhibited the stimulatory effect of ATP, indicating that P2 receptors are involved. This ATP-stimulated glucose transport was blocked by specific inhibitors of Gi protein (pertusiss toxin), phospholipase C (U73122), protein kinase C (GF109203X), and phosphatidylinositol (PI) 3-kinase (LY294002). ATP stimulated PI 3-kinase activity and P2 receptor antagonists blocked this activation. In C(2)C(12) myotubes expressing glucose transporter GLUT4, ATP increased basal and insulin-stimulated glucose transport. Finally, ATP facilitated translocation of GLUT1 and GLUT4 into plasma membrane. These results together suggest that cells respond to extracellular ATP to increase glucose transport through P2 receptors.

Role of microvillar cell surfaces in the regulation of glucose uptake and organization of energy metabolism

Experimental evidence suggesting a type of glucose uptake regulation prevailing in resting and differentiated cells was surveyed. This type of regulation is characterized by transport-limited glucose metabolism and depends on segregation of glucose transporters on microvilli of differentiated or resting cells. Earlier studies on glucose transport regulation and a recently presented general concept of influx regulation for ions and metabolic substrates via microvillar structures provide the basic framework for this theory. According to this concept, glucose uptake via transporters on microvilli is regulated by changes in the structural organization of the microfilament bundle, which is acting as a diffusion barrier between the microvillar tip compartment and the cytoplasm. Both microvilli formation and the switch of glucose metabolism from "metabolic regulation" to "transport limitation" occur during differentiation. The formation of microvillar cell surfaces creates the essential preconditions to establish the characteristic functions of specialized tissue cells including the coordination between glycolysis and oxidative phosphorylation, regulation of cellular functions by external signals, and Ca(2+) signaling. The proposed concept integrates various aspects of glucose uptake regulation into a ubiquitous cellular mechanism involved in regulation of transmembrane ion and substrate fluxes.

Mouse GLUT8: genomic organization and regulation of expression in 3T3-L1 adipocytes by glucose

Glucose transporter 8 (GLUT8) is a class III sugar transport facilitator predominantly expressed in testis and insulin-regulated tissues. Here we describe its genomic organization, the identification of its promoter region, and the regulation of its expression in 3T3-L1 cells. The mouse Glut8 gene spans approximately 9 kb, consists of 10 exons, and is highly similar to the human GLUT6 gene. Its 5'-flanking region exhibits promoter activity when fused with a luciferase reporter construct and expressed in HEK-293T cells. A deletion analysis indicated that the critical promoter elements are located in a region between -381 and the transcription start. This region comprises a CAAT box and consensus binding sites for the transcription factors SRY and NF1 that were highly conserved in the mouse and in the human sequence. In 3T3-L1 cells, GLUT8 mRNA levels increased markedly during the differentiation of cells. In contrast to GLUT1, expression of GLUT8 mRNA was significantly reduced by glucose deprivation and by prolonged hypoxia. The present data suggest that the function of GLUT8 is related to the adipocyte-like phenotype of 3T3-L1 cells, and that its expression is controlled by the metabolism of the adipocyte.

Acute regulation of glucose transport in a human megakaryocytic cell line: difference between growth factors and H(2)O(2)

The present study was undertaken to: (i) compare the effect of some hematopoietic growth factors, like interleukine-3, thrombopoietin, granulocyte-megakaryocyte colony-stimulating factor, stem cell factor, and reactive oxygen species such as H(2)O(2) on glucose uptake in a human leukemic megakaryocytic cell line, M07; (ii) investigate the changes in kinetic parameters of the transport activity induced by these stimuli; and (iii) evaluate the effect of genistein, a tyrosine kinase inhibitor, on the glucose uptake activation by the cited agents. The results are as follows: (i) exposure of M07 cells to thrombopoietin, granulocyte-megakaryocyte colony-stimulating factor, and stem cell factor resulted in a rapid stimulation of glucose transport; interleukine-3-treated cells exhibited no increase in the rate of glucose uptake, although M07 proliferation is interleukine-3 dependent; a rapid glucose transport enhancement was also observed when M07 cells were exposed to low doses of H(2)O(2); (ii) the transport kinetic parameters point out that an important difference exists between the effect of cytokines and that of H(2)O(2): cytokines increased predominantly the affinity for glucose, while H(2)O(2) raised both the V(max) and K(m) values; (iii) the isoflavone genistein, at a very low concentration, inhibited the stem cell factor- or H(2)O(2)-induced stimulation of hexose transport, reversing the variations of K(m) and V(max), but it did not affect the transport activity of granulocyte-megakaryocyte colony-stimulating factor-treated cells; and (iv) catalase completely abolished the stimulatory action of H(2)O(2) on glucose transport and slightly prevented the effect of stem cell factor, while caffeic acid phenethyl ester was only able to affect the activation due to stem cell factor.

Sugar transport regulation: comparative characterization of the effect of NADH CoQ reductase deficiency in two cell culture systems

In this report, we have characterized the upregulation of glucose transport in two different respiration-deficient fibroblast cell cultures. We have demonstrated that glucose transport increases in respiration-deficient cells as measured by 2 deoxy D-glucose transport and is readily observed in both the WG750 human and G14 Chinese hamster fibroblast respiration-deficient cell lines when compared with the MCH55 normal human and V79 parental Chinese hamster cell lines, respectively. Using subcellular fractionation techniques, the GLUT 1 glucose transporter was found located predominantly in the plasma membrane-enriched fraction of the human and hamster cell lines. In human cells, the expression of the GLUT 1 glucose transporter was elevated three-fold in the plasma membrane-enriched fraction of the WG750 respiration-deficient mutant cells. In the Chinese hamster cell lines, the respiration-deficient G14 cells exhibited no such GLUT 1 glucose transporter elevation in the plasma membrane-enriched fraction, yet expressed a >2-fold increase in glucose transport. Furthermore, the G14 cells had a similar content of GLUT 1 glucose transporter in the plasma membrane fraction when compared with the V79 parental cell line. Using Western blot analysis, the GLUT 1 glucose transporter in G14 cells exhibited a different mobility on a polyacrylamide gel when compared with the mobility of the GLUT 1 glucose transporter of the V79 cell line. This differential mobility of the glucose transporters in the hamster cells appeared to be related to glycosylation differences of the glucose transporters. Although normal human and hamster cell lines exhibited significant increases in insulin-stimulated sugar transport (P < 0.05), the two respective respiration-deficient cell lines exhibited no significant increases in insulin-stimulated sugar transport (P > 0.05). Additionally, the expression of the GLUT 1 mRNA in the human WG750 mutant cells was elevated when compared with GLUT 1 mRNA in normal cells. Insulin exposure significantly increased GLUT 1 mRNA in human cells (P < 0.05). No differences in the GLUT 1 mRNA were observed between both hamster cell lines. Thus, both respiration-deficient cell lines are insulin resistant (i.e., regarding their insulin-stimulated sugar transport). The respiration-deficient mutation results in an increased sugar transport in the human and hamster cells; however, the human cells adapt to the mutation by increasing their levels of GLUT 1 mRNA and eventually membrane-located glucose transporters. On the other hand, the hamster cells adapt by apparently modifying their glucose transporters' intrinsic activity via glycosylation. We feel that these cell systems can be effective models to study the multiple factors involved in sugar transport regulation in vertebrate cells.

Stimulation of AMP-activated protein kinase (AMPK) is associated with enhancement of Glut1-mediated glucose transport

In cells expressing only the Glut1 isoform of glucose transporters, we have shown that glucose transport is markedly stimulated in response to hypoxia or inhibition of oxidative phosphorylation, conditions that would be expected to cause a stimulation of AMP-activated protein kinase (AMPK) activity. In the present study we tested the hypothesis that the stimulation of AMPK activity might be accompanied by an enhancement of Glut1-mediated glucose transport. Exposure of Clone 9 cells, 3T3-L1 preadipocytes, and C(2)C(12) myoblasts (cells that express only the Glut1 isoform) to 5-aminoimidazole-4-carboxamideribonucleoside (AICAR), an adenosine analog that stimulates AMPK activity, resulted in a marked increase in the rate of glucose transport (ranging from four- to sixfold) that was accompanied by activation of AMPK. This stimulation of AMPK activity was associated with an increase in the phosphorylation of threonine 172 on the activation loop of its alpha subunit, with the predominant change being in the alpha-2 isoform. Exposure of Clone 9 cells to 5-iodotubercidin, an inhibitor of adenosine kinase, abolished the accumulation of AICAR-5'-monophosphate (ZMP), stimulation of AMPK, and the enhancement of glucose transport in response to AICAR. There was no significant increase in the content of Glut1 in plasma membranes of Clone 9 cells exposed to AICAR. We conclude that stimulation of AMPK activity is associated with enhancement of Glut1-mediated glucose transport, and that the glucose transport response is mediated by activation of Glut1 transporters preexisting in the plasma membrane.

Association of stomatin (band 7.2b) with Glut1 glucose transporter

Employing a monoclonal antibody directed against the C-terminal peptide of glucose transporter molecule 1 (Glut1), we identified a approximately 30-kDa polypeptide which coimmunoprecipitated with Glut1 from sample of human red blood cells (RBC) membranes. The approximately 30-kDa polypeptide reacted with an antibody directed against stomatin, an integral plasma membrane protein which is also present at a high abundance in the human RBC plasma membrane. Likewise, employing anti-stomatin antibody, we found that Glut1 coimmunoprecipitated with stomatin from samples of RBC membranes. However, neither band 3, which is the most abundant integral membrane protein in the RBC, nor actin coimmunoprecipitated with Glut1, indicating a specific interaction between Glut1 and stomatin. Similar to the results obtained in the RBC, Glut1 and stomatin immunoprecipitated with each other in lysates of Clone 9 cells, a rat liver cell line in which Glut1 is expressed at approximately 1/200 the level present in RBC. Employing conditions that resulted in immunoprecipitation of approximately 10% of Glut1 in RBC membranes led to a approximately 3% coimmunoprecipitation of stomatin. A mixed population of Clone 9 cells stably transfected with a plasmid overexpressing the mouse stomatin exhibited 30 +/- 3% reduction in the basal rate of glucose transport compared to control cells or cells stably transfected with the empty vector. The above results suggest that stomatin is closely associated with Glut1 in the plasma membrane and that overexpression of stomatin results in a depression in the basal rate of glucose transport.