Evidence that activation of 2-deoxy-D-glucose transport in rat thymocyte suspensions results from enhanced coupling between transport and hexokinase activity

Discrimination of GLUTs by Fructose Isomers Enables Simultaneous Screening of GLUT5 and GLUT2 Activity in Live Cells

Facilitative carbohydrate transporters (GLUTs, SLC2 gene family) are transmembrane proteins transporting hexoses and other sugars based on cellular metabolic demands. While a direct link between GLUTs and metabolic disorders has framed them as important biological and medicinal targets, targeting disease-relevant GLUTs remains challenging. In this study, we aimed to identify substrate-GLUT interactions that would discriminate between major fructose transporters. We examined the uptake distribution for conformational and configurational isomers of fructose using the corresponding conformationally locked fluorescently labeled mimetics as probes for assessing GLUT preferences in real time. Through comparative analysis of the uptake of the probes in the yeast-based single GLUT expression systems and the multi-GLUT mammalian cell environment, we established the ability of fructose transporters to discriminate between fructose conformers and epimers. We demonstrated that recreating the conformational and configurational mixture of fructose with molecular probes allows for the specific probe distribution, with fructofuranose mimetic being taken up preferentially through GLUT5 and β-d-fructopyranose mimetic passing through GLUT2. The uptake of α-d-fructopyranose mimetic was found to be independent of GLUT5 or GLUT2. The results of this study provide a new approach to analyzing GLUT5 and GLUT2 activity in live cells, and the findings can be used as a proof-of-concept for multi-GLUT activity screening in live cells. The research also provides new knowledge on substrate-GLUT interactions and new tools for monitoring alterations in GLUT activities.

Late-Stage Functionalization through Click Chemistry Provides GLUT5-Targeting Glycoconjugate as a Potential PET Imaging Probe

The targeting of facilitative sugar transporters (GLUTs) has been utilized in the development of tools for diagnostics and therapy. The interest in this area is promoted by the phenomenon of alterations in cellular metabolic processes that are linked to multitudes of metabolic disorders and diseases. However, nonspecific targeting (e.g., glucose-transporting GLUTs) leads to a lack of disease detection efficiency. Among GLUTs, GLUT5 stands out as a prominent target for developing specific molecular tools due to its association with metabolic diseases, including cancer. This work reports a non-radiolabeled fluoride (19F) coumarin-based glycoconjugate of 2,5-anhydro-D-mannitol as a potential PET imaging probe that targets the GLUT5 transporter. Inherent fluorescent properties of the coumarin fluorophore allowed us to establish the probe's uptake efficiency and GLUT5-specificity in a GLUT5-positive breast cell line using fluorescence detection techniques. The click chemistry approach employed in the design of the probe enables late-stage functionalization, an essential requirement for obtaining the radiolabeled analog of the probe for future in vivo cancer imaging applications. The high affinity of the probe to GLUT5 allowed for the effective uptake in nutrition-rich media.

The synergism of high-intensity intermittent exercise and every-other-day intermittent fasting regimen on energy metabolism adaptations includes hexokinase activity and mitochondrial efficiency

Visceral lipid accumulation, organ hypertrophy and a reduction in skeletal muscle strength are all signs associated with the severity of obesity-related disease. Intermittent fasting (IF) and high-intensity intermittent exercise (HIIE) are natural strategies that, individually, can prevent and help treat obesity along with metabolic syndrome and its associated diseases. However, the combinatorial effect of IF and HIIE on energetic metabolism is currently not well understood. We hypothesized that their combination could have a potential for more than strictly additive benefits. Here, we show that two months of every-other-day intermittent fasting regimen combined with a high-intensity intermittent exercise protocol (IF/HIIE) produced a synergistic effect, enhancing physical endurance (vs. control, HIIE and IF) and optimizing metabolic pathways of energy production in male Wistar rats. The IF/HIIE group presented enhanced glucose tolerance (vs. control, HIIE and IF), lower levels of plasma insulin (vs. control and HIIE), and a global activation of low Km hexokinases in liver (vs. control, HIIE and IF), heart (vs. control and HIIE) and skeletal muscle (vs. control, HIIE and IF). The IF/HIIE synergism, rather than a simply additive effect, is evidenced by increase in muscle mass and cross-section area, activation of the FoF1 ATP synthase, and the gain of characteristics suggestive of augmented mitochondrial mass and efficiency observed in this group. Finally, important reductions in plasma oxidative stress markers were present preferentially in IF/HIIE group. These findings provide new insights for the implementation of non-pharmaceutical strategies to prevent/treat metabolic syndrome and associated diseases.

Multicolor GLUT5-permeable fluorescent probes for fructose transport analysis

The specificity of carbohydrate transporters towards their substrates poses a significant challenge for the development of molecular probes to monitor sugar uptake in cells for biochemical and biomedical applications. Herein we report a new set of coumarin-based fluorescent sugar conjugates applicable for the analysis of fructose uptake due to their free passage through the fructose-specific transporter GLUT5. The reported probes cover a broad range of the fluorescence spectrum providing essential tools for the evaluation of fructose transport capacity in live cells.

Molecular Tools for Facilitative Carbohydrate Transporters (Gluts)

Facilitative carbohydrate transporters-Gluts-have received wide attention over decades due to their essential role in nutrient uptake and links with various metabolic disorders, including diabetes, obesity, and cancer. Endeavors directed towards understanding the mechanisms of Glut-mediated nutrient uptake have resulted in a multidisciplinary research field spanning protein chemistry, chemical biology, organic synthesis, crystallography, and biomolecular modeling. Gluts became attractive targets for cancer research and medicinal chemistry, leading to the development of new approaches to cancer diagnostics and providing avenues for cancer-targeting therapeutics. In this review, the current state of knowledge of the molecular interactions behind Glut-mediated sugar uptake, Glut-targeting probes, therapeutics, and inhibitors are discussed.

δ-Opioid receptors stimulate GLUT1-mediated glucose uptake through Src- and IGF-1 receptor-dependent activation of PI3-kinase signalling in CHO cells

BACKGROUND AND PURPOSE

Although opioids have been reported to affect glucose homeostasis, relatively little is known on the role of δ-opioid receptors. We have investigated the regulation of glucose transport by human δ-opioid receptors expressed in Chinese hamster ovary cells.

EXPERIMENTAL APPROACH

The uptake of [(3)H]-2-deoxy-D-glucose and 3-O-[methyl-[(3)H]]-D-glucose in response to δ-opioid receptor ligands and the expression of GLUT1, GLUT3 and GLUT4 glucose transporters were examined. Moreover, the effects of intracellular signal transduction inhibitors on δ-opioid receptor-regulated [(3)H]-2-deoxy-D-glucose uptake and protein phosphorylation were investigated.

KEY RESULTS

Activation of δ-opioid receptors rapidly stimulated [(3)H]-2-deoxy-D-glucose and 3-O-[methyl-[(3)H]]-D-glucose uptakes, which were blocked by the GLUT inhibitors cytochalasin B and phloretin. The stimulation of [(3)H]-2-deoxy-D-glucose uptake that occurred without a change in plasma membrane GLUT1 - required the coupling to G(i) /G(o) proteins - was independent of cAMP and extracellular signal-regulated protein kinases, and was suppressed by blockade of Src and insulin-like growth factor-1 receptor (IGF-1R) tyrosine kinases. Inhibition of phosphatidylinositol 3-kinase (PI3K) by wortmannin or LY294002 and by PI3Kα, but not γ, isoform-selective inhibitors greatly reduced the δ-opioid receptor stimulation of glucose uptake. Moreover, the response was attenuated by overexpressing a dominant-negative kinase-deficient Akt form and by chemical inhibition of Akt. Stimulation of δ-opioid receptors increased protein kinase Cζ/λ (PKCζ/λ) phosphorylation and a selective PKCζ/λ inhibitor slightly reduced opioid stimulation of glucose uptake.

CONCLUSIONS AND IMPLICATIONS

δ-Opioid receptors stimulated glucose transport probably by enhancing GLUT1 intrinsic activity through a signalling cascade involving G(i)/G(o), Src, IGF-1R, PI3Kα, Akt and, to a minor extent, PKCζ/λ. This effect may contribute to the opioid regulation of glucose homeostasis in physio-pathological conditions.

Relationship Between Glucose Transport and Metabolism in Isolated Bovine Mammary Epithelial Cells

Glucose transport by isolated bovine mammary epithelial cells involves translocation across the cell membrane into a compartment that exchanges slowly with the bulk cytosol. The significance to glucose metabolism of this compartmentalization was examined by generation, modeling, and analysis of transport and metabolism data. Net uptake of 5 mM 3-O-methyl-d-glucose by isolated bovine mammary epithelial cells was measured at 37 degrees C. Time-course curves were better fitted by a double exponential equation than a single exponential equation and were subjected to compartmental analysis to obtain glucose transport model parameters. Lactose synthesis and glucose oxidation rates and cellular concentrations of intermediary metabolites, glucose-6-phosphate and glucose-1-phosphate, were measured at varied media glucose concentrations. A model that integrates both glucose transport and metabolism under-predicted the rates of lactose synthesis and glucose oxidation by a factor of 3. To account for the observed glucose use rates, glucose must be available for phosphorylation once translocated across the cell membrane (intermediate compartmentalization of translocated glucose does not exclude access to hexokinase). Metabolic control analysis indicated that, at physiological glucose concentrations, phosphorylation by hexokinase exerts 80% of the control of glucose metabolism to lactose and CO(2), and transport exerts the remaining 20%.

Expression of hexokinase II and Glut-1 in untreated human breast cancer

Expressions of HKII and Glut-1 were studied in untreated primary human breast cancers by immunohistochemistry. 79% of the breast cancers were HKII-positive and 61% were Glut-1-positive. Average positive malignant cell areas were 66 +/- 41% for HKII and 29 +/- 36% for Glut-1. HKII staining was cytoplasmic, suggesting mitochondrial localization with no variations in staining intensities. Glut-1 staining was heterogeneous, cytoplasmic and membranous and varied with histology and tumor stage. Cells expressing HKII did not always express Glut-1 and vice versa. Increased FDG-uptake appeared to be associated with increased Glut-1 expression (P = 0.021), but not with HKII expression (p = 0.6).FDG uptake in breast cancer tissue appears to be associated with the extent of immunodetectable expression of Glut-1, but not that of HKII, and FDG uptake may differ between individual tumors depending on tumor stage and histology.

Regulation of mesangial cell hexokinase activity by PKC and the classic MAPK pathway

Phorbol esters increase glucose (Glc) uptake and utilization in a variety of cell types, and, in some cells, these changes have been attributed to increased Glc phosphorylation and better functional coupling of hexokinases (HKs) to facilitative Glc transporters. Phorbol esters are potent mesangial cell mitogens, but their effects on HK-catalyzed Glc phosphorylation and metabolism are unknown. When examined in murine mesangial cells, active, but not inactive, phorbol esters increased HK activity in a time- and dose-dependent manner. Maximal induction of HK activity at 12-24 h was accompanied by parallel increases in both Glc utilization and lactate production and was blocked by the specific MEK1/2 inhibitor PD-98059 (IC(50) approximately 3 microM). This effect involved early activation of protein kinase C (PKC), MEK1/2, and ERK1/2, and the prolonged time course of subsequent HK induction was attributable, in part, to requirements for ongoing gene transcription and de novo protein synthesis. Mesangial cell HK activity thus exhibits novel regulatory behavior involving both PKC and classic MAPK pathway activation, suggesting specific mechanisms whereby PKC activation may influence Glc metabolism.

Coupled glucose transport and metabolism in cultured neuronal cells: determination of the rate-limiting step

In brain and nerves the phosphorylation of glucose, rather than its transport, is generally considered the major rate-limiting step in metabolism. Since little is known regarding the kinetic coupling between these processes in neuronal tissues, we investigated the transport and phosphorylation of [2-3H]glucose in two neuronal cell models: a stable neuroblastoma cell line (NCB20), and a primary culture of isolated rat dorsal root ganglia cells. When transport and phosphorylation were measured in series, phosphorylation was the limiting step, because intracellular glucose concentrations were the same as those outside of cells, and because the apparent Km for glucose utilization was lower than expected for the transport step. However, the apparent Km was still severalfold higher than the Km of hexokinase I. When [2-3H]glucose efflux and phosphorylation were measured from the same intracellular glucose pool in a parallel assay, rates of glucose efflux were three- to-fivefold greater than rates of phosphorylation. With the parallel assay, we observed that activation of glucose utilization by the sodium channel blocker veratridine caused a selective increase in glucose phosphorylation and was without effect on glucose transport. In contrast to results with glucose, both cell types accumulated 2-deoxy-D-[14C]glucose to concentrations severalfold greater than extracellular concentrations. We conclude from these studies that glucose utilization in neuronal cells is phosphorylation-limited, and that the coupling between transport and phosphorylation depends on the type of hexose used.

Does phosphorylation affect transport of inorganic phosphate?

1. This study investigated the effects of fructose and 2-deoxyglucose on the uptake and release of phosphate from everted intestinal sacs of mice. 2. Both the sugars significantly decreased the release of phosphate without affecting the uptake. 3. Succinate and fumarate were able to partially reverse the inhibition of phosphate release exerted by fructose but not that exerted by 2-deoxyglucose. 4. Pre-loading with mannoheptulose, a known inhibitor of hexokinase, improved the release of phosphate in the presence of either of these sugars. 5. Adrenaline, known to inhibit phosphorylation of 2-deoxyglucose, reduced the inhibition exerted by this sugar on phosphate release. 6. These results indicate that the inhibition of phosphate release caused by these sugars may be due to the trapping of free phosphate during their metabolism in the gut wall.

Interaction of facilitative glucose transporter with glucokinase and its modulation by ADP and glucose-6-phosphate

Bacterial glucokinase (GK) binds to purified, human erythrocyte glucose transporter (GT) reconstituted in vesicles. The binding is largely abolished if GT is predigested with trypsin, indicating that GK binds to the cytoplasmic domain of GT. The binding is a saturable function of GK concentration showing two distinct affinities with apparent KD of 0.33 and 5.1 microM. The binding is stimulated by an increasing concentration of ADP with the 50% maximal effect at 5 mM. Glucose-6-phosphate (G6P) also stimulates the binding with a distinct optimum at 25 mM. The binding is stimulated only slightly by ATP. D-glucose has no affect on the binding. KCl enhances the binding with the maximal effect at physiological intracellular concentrations. The binding is sensitive to changes in pH with an optimum at pH 4. The binding causes no detectable functional change in GT. However, the enzymatic activity of GK measured at nanomolar concentrations of GK is significantly greater in the presence of GT vesicles than in its absence or in the presence of protein-free vesicles, indicating that GK interacts with GT at this low concentration range with an apparent KD of 10 mM. Although its physiological significance is not known, the GK-GT interaction in vitro described here suggests that these two proteins may also interact in the cell and regulate carbohydrate metabolism.

The relationship between sugar metabolism, transport and superoxide radical production in rat peritoneal macrophages

Dexamethasone inhibits sugar-dependent phorbol myristate acetate (PMA)-stimulated superoxide production and 2-deoxy-D-glucose (2-dGlc) transport in rat peritoneal macrophages (Rist, R.J., Jones, G.E. and Naftalin, R.J. (1991) Biochem. J. 278, 119-128; Rist, R.J. and Naftalin, R.J. (1991) Biochem J. 278, 129-135). Here it is shown that with glucose as a substrate, dexamethasone (0.1 microM) acts as a non-competitive inhibitor of PMA-induced superoxide production; decreasing the maximal rate of superoxide production (P < 0.001) without altering the Km. In contrast, with 2-dGlc as a substrate, dexamethasone shows competitive inhibition of PMA-stimulated superoxide production; increasing the Km of superoxide production, (P < 0.001) without altering the Vmax. The maximal rate of PMA-stimulated superoxide production with glucose as substrate is 10-12-fold in excess of the maximal rate with 2-dGlc as substrate. Diphenylene iodonium (DPI) is a non-competitive inhibitor of PMA-stimulated glucose-dependent superoxide production in macrophages, (Ki = 1-5 microM) and significantly reduces the activity of the PMA-induced hexose monophosphate shunt, (HMPS) (P < 0.01). However, DPI (1 microM) has no significant effect on the PMA-induced increase in 2-dGlc uptake, suggesting that the stimulus for HMPS activity and superoxide production is separate from the stimulus for hexose transport. A model is described which explains the observed differences in hexose transport and glucose- and 2-dGlc-dependent superoxide production in terms of the differences in metabolism of the two sugars. Accumulation of free 2-dGlc within the cytosol leads to saturation of hexokinase and hence, the effects of PMA and dexamethasone, which alter the coupling between hexokinase and the transporter, are only observed at low concentrations of 2-dGlc, where it is accumulated to sub-saturating levels. Since glucose is completely metabolized within the cell, PMA and dexamethasone increase and decrease, respectively, net uptake of sugar and superoxide production at all glucose concentrations.

Yeast sugar transporters

Transport of sugars is a fundamental property of all eukaryotic cells. Of particular importance is the uptake of glucose, a preferred carbon and energy source. The rate of glucose utilization in yeast is often dictated by the activity and concentration of glucose transporters in the plasma membrane. Given the importance of transport as a site of control of glycolytic flux, the regulation of glucose transporters is necessarily complex. The molecular analysis of these transporters in Saccharomyces has revealed the existence of a multigene family of sugar carriers. Recent data have raised the question of the actual role of all of these proteins in sugar catabolism, as some appear to be lowly expressed, and point mutations of these genes may confer pleiotropic phenotypes, inconsistent with a simple role as catabolic transporters. The transporters themselves appear to be intimately involved in the process of sensing glucose, a model for which there is growing support.

Effects of macrophage colony-stimulating factor and phorbol myristate acetate on 2-D-deoxyglucose transport and superoxide production in rat peritoneal macrophages

2-D-Deoxyglucose (2-dGlc) uptake and accumulation into rat peritoneal macrophages was increased by colony-stimulating factor (mCSF) by stimulating the coupling between endofacial hexokinase activity and the sugar transporter. The evidence for this is as follows: (1) mCSF significantly decreased the Km for zero-trans uptake (P less than 0.05), without altering Vmax.; (2) the accumulation of free 2-dGlc was increased by mCSF (P less than 0.05); (3) mCSF retarded the rate of exit of accumulated free 2-dGlc. The mCSF-dependent increase in 2-dGlc uptake by macrophages was enhanced by preincubation of the cells in mCSF-free solution. The activity of the hexose monophosphate shunt (HMPS) measured by the differential uptake of 2-d[1-3H]Glc and 2-d[2,6-3H]Glc was not stimulated by mCSF. Also, in quiescent cells, superoxide production, as determined by cytochrome c reduction, was unaffected by mCSF. Phorbol myristate acetate (PMA; 40 nM) stimulated both the HMPS activity and superoxide production. Both these effects were dependent on the uptake of external sugar (2-dGlc). Incubation of the macrophages with mCSF enhanced the sugar transport and PMA-dependent stimulation of HMPS activity and superoxide production, indicating a role for mCSF in the 'priming' of macrophage functions. Both HMPS activity and superoxide production are entirely dependent on uptake of exogenous sugar, since the potent sugar-transport inhibitor cytochalasin B competitively inhibited 2-dGlc uptake, HMPS activity and superoxide generation in PMA-activated cells (Ki approximately 0.3 microM for all three processes). Over a wide range of 2-dGlc concentrations, 4 mol of superoxide were generated/mol of 2-dGlc metabolized in the HMPS pathway, indicating coupling between these processes. The Km of 2-d[2,6-3H]Glc uptake in PMA-treated cells was 0.45 +/- 0.07 mM, and Vmax. was 1.32 +/- 0.05 mumol.min-1.ml of cell water-1. It is evident that there is a large degree of slippage between HMPS activity and membrane-associated hexokinase activity, since the Km for HMPS activity was 0.06 +/- 0.02 mM and the Vmax. was 0.10 +/- 0.03 mumol.min-1.ml of cell water-1.

Dexamethasone inhibits the hexose monophosphate shunt in activated rat peritoneal macrophages by reducing hexokinase-dependent sugar uptake

Dexamethasone decreases 2-D-deoxyglucose (2-dGlc) uptake and accumulation into rat peritoneal macrophages in vitro in a concentration- and time-dependent manner (Ki for 1 microM-dexamethasone after a 2 h exposure = 0.71 +/- 0.21 microM; Ki for 0.1 microM-dexamethasone after exposure for 4 h = 0.10 +/- 0.06 microM). The inhibition of 2-dGlc uptake is consistent with a decrease in the coupling between endofacial hexokinase activity and the sugar transporter. The evidence for this is: (1) the Km for zero-trans 2-dGlc uptake in quiescent macrophages was increased by dexamethasone, but there was no significant effect on the Vmax.; (2) dexamethasone increased the rate of exit of sugar from cells preloaded with 2-dGlc; (3). the free sugar accumulation within the cytosol of the cells above the external solution concentration was significantly decreased by dexamethasone. These effects of dexamethasone on 2-dGlc transport were antagonized by simultaneous exposure to the steroid RU 38486 (Ki = 0.04 +/- 0.01 microM; 4 h incubation). Although dexamethasone inhibited zero-trans uptake, the maximum rate of infinite-trans exchange uptake of 2-dGlc into cells preloaded with 3-O-methyl-D-glucose (40 mM) was unaltered by dexamethasone or RU 38486, indicating that the dexamethasone-dependent decrease in zero-trans uptake was not due to a change in the number of transporters in the plasma membrane. Dexamethasone also inhibited the phorbol myristate acetate-induced stimulation of hexose monophosphate shunt (HMPS) activity, and this was reversed by RU 38486. Cytochalasin B, the potent sugar-transport inhibitor, inhibited HMPS activity and 2-d[2,6-3H]Glc uptake equally, indicating a single site of action. By contrast, dexamethasone showed differential inhibition of HMPS activity and 2-d[2,6-3H]Glc uptake, suggesting that it not only acts by decreasing the coupling between hexokinase and sugar transport, but also at one or more additional points.

Glucocorticoids inhibit fructose 2,6-bisphosphate synthesis in rat thymocytes. Opposite effect of cycloheximide

The content of fructose 2,6-bisphosphate (Fru(2,6)P2) and lactate production in triamcinolone acetonide-treated rats thymocytes was studied. The effect in vitro of corticosterone and dexamethasone on normal thymocytes was also examined. Glucocorticoids produced a marked decrease in Fru(2,5)P2 content and lactate production. The largest effect was observed with triamcinolone acetonide (7.5 mg per kg body weight), which after 20 h of treatment produced over 90% of inhibition. This change was accompanied by the decrease of both phosphofructokinase-1 and -2 activities and ATP levels, without modifications of hexoses phosphate content. The inhibitory actions of glucocorticoids were abolished by cycloheximide, an inhibitor of protein synthesis. Furthermore this drug, by itself, increased Fru(2,6)P2 content by more than 50% compared with the controls.

Effects of phorbol, dexamethasone and starvation on 3-O-methyl-D-glucose transport by rat thymocytes. Modulation of transport by altered trans effects

Uptake of 3-O-methyl-D-glucoside (3-OMG) into thymocytes was studied to ascertain if it is modulated by endofacial hexokinase activity or by intracellular glucose. (1) The Vmax for net uptake of 3-OMG into rat thymocytes is increased by phorbol 12-myristate 13-acetate (PMA; 40 nM) or starvation for 4 h, and decreased by dexamethasone (1 microM). Starvation for 4 h abolishes the PMA-dependent increase in 3-OMG uptake; this effect is prevented by incubation in 2-deoxyglucose (2-dGlc; 1 mM). (2) Dexamethasone decreases 2-dGlc uptake, increases the rate of 2-dGlc exit and decreases accumulation of free 2-dGlc, consistent with decreased endofacial hexokinase activity. (3) 3-OMG uptake is decreased by preloading the cells with 2-dGlc or glucose, whereas preloading with 3-OMG (40 mM) increases uptake of 3-OMG. (4) The inhibitory effect of preloaded 2-dGlc or glucose on 3-OMG uptake is decreased by PMA. (5) Preloading cells with 3-OMG (40 mM) increases 2-dGlc influx in control and dexamethasone-treated cells, but not into PMA-treated cells. (6) The maximal rate of self-exchange of 3-OMG is similar in control, PMA- or dexamethasone-treated cells. These results are consistent with the following view: 3-OMG uptake is retarded by exchange with cytosolic glucose, or 2-dGlc. PMA, by increasing endofacial hexokinase activity, or starvation depletes glucose from the endofacial surface of the transporter, and hence increase 3-OMG uptake. Dexamethasone, by decreasing endofacial hexokinase activity, increases endofacial binding of glucose, and hence decreases 3-OMG uptake. Cytosolic 3-OMG competes with glucose for endofacial sites, and hence the maximal rates of exchange uptake of 3-OMG are similar in control, PMA- or dexamethasone-treated cells, as the activity of thymocyte glucose transporters is apparently unaltered.

Synergistic activation of 2-deoxy-D-glucose uptake in rat and murine peritoneal macrophages by human macrophage colony-stimulating factor-stimulated coupling between transport and hexokinase activity and phorbol-dependent stimulation of pentose phosphate-shunt activity

1. Transport and accumulation of 2-deoxy-D-glucose (2dGlc) in rat and murine peritoneal macrophages were investigated by using C-1-3H-labelled and C-2,6-3H-labelled 2dGlc. 2. There was active accumulation of both C-1- and C-2,6-labelled 2dGlc by quiescent rat and murine macrophages via a phloretin-inhibitable transport system. 3. The rate of uptake and accumulation of 2dGlc (C-1 label) was increased by exposure to human macrophage colony-stimulating factor (mCSF-1) (1000 units/ml) in both murine and rat macrophages. This indicates that mCSF-1 enhances coupling between hexokinase activity and glucose transport at the endofacial surface of the transporter. 4. Phorbol 12-myristate 13-acetate ('phorbol') at 40 nM stimulated 2dGlc in rat macrophages entirely by increasing the C-2,6 label uptake. This indicates that phorbol stimulates 2dGlc uptake mainly by increasing the activity of the pentose phosphate pathway. 5. Simultaneous exposure to phorbol and mCSF-1 stimulates 2dGlc uptake to a greater extent than found with either phorbol or mCSF-1 alone. This result is explained by a simultaneous enhancement of pentose phosphate-pathway activity and of hexokinase activity acting at the endofacial surface of the cell membrane. The dual activation of these serial processes coupled to the loss of the reaction products of the pentose phosphate-shunt pathway from the cells in the form of reactive oxygen intermediates, protons and CO2 could explain the synergistic action of phorbol and mCSF-1 in activation of sugar transport in macrophages.

Transport and accumulation of 2-deoxy-D-glucose in wild-type and hexokinase-deficient cultured Chinese-hamster ovary (CHO) cells

Hexokinase-deficient mutants and wild-type Chinese-hamster ovary cells have been used to investigate the role of hexokinase in uptake and accumulation of 2-D-deoxyglucose (2-dGlc). The evidence for a specific sugar transport system in both types of cells is that there is similar saturable phloretin-sensitive uptake of 2-dGlc and 3-O-methyl-D-glucose (3-OMG) in both types of cell. In wild-type cells, 2-dGlc is accumulated to a tissue:medium ratio of 10- and in the mutant only 3-fold; 3-OMG is not accumulated by either mutant or wild-type cells. The evidence that hexokinase affects the membrane transport process is that the rate of exit of free 2-dGlc from wild-type cells is 5-fold less than from mutant cells, whereas there is no difference in the rate of loss of 3-OMG between mutant and wild-type cells.