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

Transient gain of function of cannabinoid CB1 receptors in the control of frontocortical glucose consumption in a rat model of Type-1 diabetes

Here we aimed to unify some previous controversial reports on changes in both cannabinoid CB₁ receptor (CB₁R) expression and glucose metabolism in the forebrain of rodent models of diabetes. We determined how glucose metabolism and its modulation by CB₁R ligands evolve in the frontal cortex of young adult male Wistar rats, in the first 8 weeks of streptozotocin-induced type-1 diabetes (T1D). We report that frontocortical CB₁R protein density was biphasically altered in the first month of T1D, which was accompanied with a reduction of resting glucose uptake ex vivo in acute frontocortical slices that was normalized after eight weeks in T1D. This early reduction of glucose uptake in slices was also restored by ex vivo treatment with both the non-selective CB₁R agonists, WIN55212-2 (500 nM) and the CB₁R-selective agonist, ACEA (3 μM) while it was exacerbated by the CB₁R-selective antagonist, O-2050 (500 nM). These results suggest a gain-of-function for the cerebrocortical CB₁Rs in the control of glucose uptake in diabetes. Although insulin and IGF-1 receptor protein densities remained unaffected, phosphorylated GSKα and GSKβ levels showed different profiles 2 and 8 weeks after T1D induction in the frontal cortex. Altogether, the biphasic response in frontocortical CB₁R density within a month after T1D induction resolves previous controversial reports on forebrain CB₁R levels in T1D rodent models. Furthermore, this study also hints that cannabinoids may be useful to alleviate impaired glucoregulation in the diabetic cortex.

A pairwise chemical genetic screen identifies new inhibitors of glucose transport

Oxidative phosphorylation (OXPHOS) and glycolysis are the two main pathways that control energy metabolism of a cell. The Warburg effect, in which glycolysis remains active even under aerobic conditions, is considered a key driver for cancer cell proliferation, malignancy, metastasis, and therapeutic resistance. To target aerobic glycolysis, we exploited the complementary roles of OXPHOS and glycolysis in ATP synthesis as the basis for a chemical genetic screen, enabling rapid identification of novel small-molecule inhibitors of facilitative glucose transport. Blocking mitochondrial electron transport with antimycin A or leucascandrolide A had little effect on highly glycolytic A549 lung carcinoma cells, but adding known glycolytic inhibitors 2-deoxy-D-glucose, iodoacetate or cytochalasin B, rapidly depleted intracellular ATP, displaying chemical synthetic lethality. Based on this principle, we exposed antimycin A-treated A549 cells to a newly synthesized 955 member diverse scaffold small-molecule library, screening for compounds that rapidly depleted ATP levels. Two compounds potently suppressed ATP synthesis, induced G1 cell-cycle arrest and inhibited lactate production. Pathway analysis revealed that these novel probes inhibited GLUT family of facilitative transmembrane transporters but, unlike cytochalasin B, had no effect on the actin cytoskeleton. Our work illustrated the utility of a pairwise chemical genetic screen for discovery of novel chemical probes, which would be useful not only to study the system-level organization of energy metabolism but could also facilitate development of drugs targeting upregulation of aerobic glycolysis in cancer.

Berberine improves glucose metabolism through induction of glycolysis

Berberine, a botanical alkaloid used to control blood glucose in type 2 diabetes in China, has recently been reported to activate AMPK. However, it is not clear how AMPK is activated by berberine. In this study, activity and action mechanism of berberine were investigated in vivo and in vitro. In dietary obese rats, berberine increased insulin sensitivity after 5-wk administration. Fasting insulin and HOMA-IR were decreased by 46 and 48%, respectively, in the rats. In cell lines including 3T3-L1 adipocytes, L6 myotubes, C2C12 myotubes, and H4IIE hepatocytes, berberine was found to increase glucose consumption, 2-deoxyglucose uptake, and to a less degree 3-O-methylglucose (3-OMG) uptake independently of insulin. The insulin-induced glucose uptake was enhanced by berberine in the absence of change in IRS-1 (Ser307/312), Akt, p70 S6, and ERK phosphorylation. AMPK phosphorylation was increased by berberine at 0.5 h, and the increase remained for > or =16 h. Aerobic and anaerobic respiration were determined to understand the mechanism of berberine action. The long-lasting phosphorylation of AMPK was associated with persistent elevation in AMP/ATP ratio and reduction in oxygen consumption. An increase in glycolysis was observed with a rise in lactic acid production. Berberine exhibited no cytotoxicity, and it protected plasma membrane in L6 myotubes in the cell culture. These results suggest that berberine enhances glucose metabolism by stimulation of glycolysis, which is related to inhibition of glucose oxidation in mitochondria. Berberine-induced AMPK activation is likely a consequence of mitochondria inhibition that increases the AMP/ATP ratio.

Functional interactions between the noncovalently associated N- and C-terminal halves of mammalian Type I hexokinase

The 100 kDa Type I isozyme of mammalian hexokinase has evolved by duplication and fusion of a gene encoding an ancestral 50 kDa hexokinase. Although the N- and C-terminal halves are similar in sequence, they differ in function, catalytic activity being associated only with the C-terminal half while the N-terminal half serves a regulatory role. The N- and C-terminal halves of rat Type I hexokinase have been coexpressed in M + R 42 cells. The halves associate noncovalently to produce a 100 kDa form that exhibits characteristics seen with the intact Type I isozyme but not with the isolated catalytic C-terminal half, i.e., characteristics that are influenced by interactions between the halves. These include a decreased K(m) for the substrate ATP and the ability of P(i) to antagonize inhibition by Glc-6-P or its analog, 1-5-anhydroglucitol-6-P. Thus, functional interactions between the N- and C-terminal halves do not require their covalent linkage.

Kinetic and regulatory properties of HK I(+), a modified form of the type I isozyme of mammalian hexokinase in which interactions between the N- and C-terminal halves have been disrupted

A modified form (HK I(+)) of rat Type I hexokinase (HK I) has been expressed. HK I(+) contains a centrally located polyalanine insert which, along with the known helical propensity of adjacent sequence, was expected to lead to alpha-helix formation, with resulting distension of the molecule and disruption of interactions between the N- and C-terminal halves. The properties of HK I(+) are consistent with this expectation and with previous proposals that (1) inhibition of HK I by Glc-6-P or its analogs and antagonism of this inhibition by P(i) result from competition of these ligands for a binding site in the N-terminal half of HK I, with resulting conformational changes propagated through interactions with the catalytic C-terminal half, and (2) binding of Glc-6-P to a site in the C-terminal half of HK I is obstructed by interactions between the halves, present in HK I but not HK I(+).

Interaction of insulin-like growth factor binding protein-4, Miz-1, leptin, lipocalin-type prostaglandin D synthase, and granulin precursor with the N-terminal half of type III hexokinase

Insulin-like growth factor binding protein-4, Miz-1, leptin, prostaglandin D synthase, and granulin precursor were identified as proteins interacting with the N-terminal half of mammalian Type III hexokinase (HKIII) in the yeast two-hybrid method. These interactions were confirmed by in vitro binding studies. All five of these proteins, and their mRNAs, were present in PC12 cells, as shown by immunoblotting and RT-PCR, respectively. All were coimmunoprecipitated from PC12 extracts with an antibody against HKIII, but not with anti-Type I hexokinase. Moreover, all of these proteins were coimmunoprecipitated using antileptin as precipitating antibody, indicating the existence of a macromolecular complex including these five proteins and HKIII. Transfection of M+R 42 cells with HKIII-green fluorescent protein (GFP) reporter constructs gave a diffuse intracellular fluorescence. Cotransfection with leptin or Miz-1 resulted in distinctly different localization of the HKIII-GFP fusion protein, at intracellular sites coincident with localization of leptin-GFP or Miz-1-GFP reporter constructs.

A novel NH(2)-terminal, nonhydrophobic motif targets a male germ cell-specific hexokinase to the endoplasmic reticulum and plasma membrane

Although three germ cell-specific transcripts of type 1 hexokinase exist in murine male germ cells, only one form, HK1-sc, is found at the protein level. This single isoform localizes to three distinct structures in mouse spermatozoa: the membranes of the head, the mitochondria in the midpiece, and the fibrous sheath in the flagellum (Travis, A. J., Foster, J. A., Rosenbaum, N. A., Visconti, P. E., Gerton, G. L., Kopf, G. S., and Moss, S. B. (1998) Mol. Biol. Cell 9, 263-276). The mechanism by which one protein is targeted to multiple sites within this highly polarized cell poses important questions of protein targeting. Because the study of protein targeting in germ cells is hampered by the lack of established cell lines in culture, constructs containing different domains of the germ cell-specific hexokinase transcripts were linked to a green fluorescent protein and transfected into hexokinase-deficient M+R42 cells. Constructs containing a nonhydrophobic, germ cell-specific domain, present at the amino terminus of the HK1-SC protein, were targeted to the endoplasmic reticulum and the plasma membrane. Mutational analysis of this domain demonstrated that a complex motif, PKIRPPLTE (with essential residues italicized), represented a novel endoplasmic reticulum-targeting motif. Constructs based on another germ cell-specific hexokinase transcript, HK1-sa, demonstrated the specific proteolytic removal of an amino-terminal domain, resulting in a protein product identical to HK1-SC. Such processing might constitute a regulatory mechanism governing the spatial and/or temporal expression of the protein.

Functional organization and evolution of mammalian hexokinases: mutations that caused the loss of catalytic activity in N-terminal halves of type I and type III isozymes

Mammalian hexokinases are believed to have evolved from a 100-kDa hexokinase which itself is a product of duplication and fusion of an ancestral gene encoding a 50-kDa glucose 6-phosphate-sensitive hexokinase. Type II hexokinase has been shown to possess two distinct functional active sites, one in each half, which functionally resemble the original 100-kDa hexokinase, whereas type I and III isozymes possess only one active site in the C-terminal halves. This study was conducted to identify which mutations caused the loss of catalytic activity in the N-terminal halves of type I and III isozymes. Arg 174 and Ser 447 in type I isozyme and Asp 244 in type III isozyme are speculated to be the cause, because they reside adjacent to the "catalytic" site and corresponding residues, Gly 174, Asp 447, and Gly 231, are conserved in the N-terminal half of type II isozyme as well as all other 50-kDa units that possess catalytic activity. Mutations G174R and D447S in the N-terminal half of type II isozyme reduced specific activity by approximately 79 and 57%, respectively. Therefore, neither mutation alone can account for the inactivation of the N-terminal active site in type I isozyme. Either mutation, G174R or D447S, had moderate effects on Michaelis constants, K(m), for glucose and ATP. Mg(2+). Intriguingly, mutation D447S introduced a novel inhibition by unchelated ATP (K(i) = 68 microM ATP, competitive vs ATP. Mg(2+)) to the N-terminal active site of type II isozyme. Mutation G231D caused instability to type II hexokinase and near complete loss of catalytic activity (95%), suggesting that mutation G231D not only hinders catalysis at the N-terminal active site but also leads to structural instability in type II hexokinase.

Allosteric regulation of type I hexokinase: A site-directed mutational study indicating location of the functional glucose 6-phosphate binding site in the N-terminal half of the enzyme

The Type I isozyme of mammalian hexokinase has evolved by a gene duplication-fusion mechanism, with resulting internal duplication of sequence and ligand binding sites. However, 1:1 binding stoichiometry indicates that only one of these is available for binding the product inhibitor, Glc-6-P; the location of that site, in the N- or C-terminal half, remains under debate. Recent structural studies (Aleshin et al., Structure 6, 39-50, 1998; Mulichak et al., Nature Struct. Biol. 5, 555-560, 1998) implicated Asp 84 or its analog in the C-terminal half, Asp 532, in binding of Glc-6-P. Zeng et al. (Biochemistry 35, 13157-13164, 1996) demonstrated that mutation of Asp 532 to Lys or Glu did not affect inhibition by the Glc-6-P analog, 1,5-anhydroglucitol-6-P. These same mutations, as well as mutation to Ala, at the Asp 84 position are now shown to result in increased Ki for 1,5-anhydroglucitol-6-P. The ability of Pi to antagonize inhibition by the Glc-6-P analog is severely diminished or abolished by these mutations, suggesting that antagonism is dependent on precise positioning of the inhibitory hexose 6-phosphate. The structure of the enzyme complexed with Glc and Pi has been determined, and shows that Pi occupies the same site as the 6-phosphate group in the complex with Glc-6-P. Thus, antagonism between these ligands results from competition for a common anion binding site in the N-terminal half.

Chinese hamster ovary cells with reduced hexokinase activity maintain normal GDP-mannose levels

Parental Chinese hamster ovary (CHO) cells were mutagenized and subjected first to a mannose suicide selection technique and second to a screen of individual colonies grown on polyester discs for reduced mannose incorporation into protein. The incorporation of radioactivity for the selection and the screen was conducted at 41.5 degrees C instead of the normal growth temperature of 34 degrees C in order to allow for the isolation of temperature-sensitive lesions. This selection/screening procedure resulted in the isolation of M15-4 cells, which had three- to five-fold lower incorporation of [2-3H]mannose into mannose 6-phosphate, mannose 1-phosphate, GDP-mannose, oligosaccharide-lipid, and glycoprotein at 41.5 degrees C. We detected no difference in the qualitative pattern of mannose-labeled lipid-linked oligosaccharides compared to parental cells. M15-4 cells synthesized dolichol. The defect of M15-4 cells was determined to be in hexokinase activity; crude cytosolic extracts were eight- to nine-fold lower in hexokinase activity in M15-4 cells compared to parental cells. As a result of this defect, incorporation of labeled mannose from the medium was significantly decreased. However, the level of GDP-mannose in M15-4 cells was 70% of normal. The phenotype of M15-4 was a lower specific activity of labeled GDP-mannose, not a substantial reduction in the level of GDP-mannose. Consistent with these results, no alterations in the glycosylation of a model glycoprotein, G protein of vesicular stomatitis virus, were observed. These cells grew slower than parental cells, especially in low-glucose medium.

Structural determinants for the intracellular localization of the isozymes of mammalian hexokinase: intracellular localization of fusion constructs incorporating structural elements from the hexokinase isozymes and the green fluorescent protein

Fusion constructs incorporating structural elements from mammalian isozymes of hexokinase, Types I-IV, in frame with sequence encoding the green fluorescent protein (GFP) have been made and expressed in hexokinase-deficient M + R 42 cells. Fusion proteins incorporating catalytically active regions from the Type II isozyme, or the entire Type IV sequence, were expressed in catalytically active form. The intracellular localization of the fusion proteins was determined using confocal microscopy. Fusion proteins including the N-terminal halves of the Type I or Type II isozymes were targeted to mitochondria, while the N-terminal half of the Type III isozyme did not confer mitochondrial targeting. The mitochondrial targeting signal was represented by the hydrophobic sequence at the extreme N-termini ("binding domain") of the Type I and Type II isozymes. Inclusion of the binding domain from the Type I isozyme was sufficient to confer mitochondrial binding on GFP itself as well as on constructs including the N-terminal half of Type III hexokinase. However, the Type I hexokinase binding domain was not sufficient to cause mitochondrial targeting of a construct containing the Type IV sequence. These results suggest that, although the binding domain is critical for mitochondrial targeting, other interactions involving an adjacent structure might also play a role. Fusion proteins including the N-terminal half of Type I hexokinase became dissociated from mitochondria under conditions favorable for accumulation of intracellular Glc-6-P. The 2-deoxy analog was much less effective than Glc in causing mitochondrial dissociation of the fusion construct, in accord with previous studies showing 2-deoxy-Glc-6-P to be much less effective than Glc-6-P at promoting release of Type I hexokinase from mitochondria. Dissociation, induced by formation of Glc-6-P or 2-deoxy-Glc-6-P, did not occur with the fusion protein including only the binding domain of Type I hexokinase. This is consistent with previous studies indicating that Glc-6-P-dependent dissociation results from binding of this ligand to a site in the N-terminal half of the enzyme, but which is not likely to be present in the small segment represented by the binding domain. These studies demonstrate the usefulness of this approach in defining structural elements involved in targeting hexokinase isozymes to specific subcellular locations and modulation of that intracellular location by perturbations of metabolic status.

Functional organization of mammalian hexokinases: characterization of the rat type III isozyme and its chimeric forms, constructed with the N- and C-terminal halves of the type I and type II isozymes

Previous studies have shown that catalytic function is associated with both halves of the Type II isozyme of mammalian hexokinase, while the Type I isozyme is functionally differentiated into a catalytic C-terminal half and regulatory N-terminal half. The Type III isozyme has now been shown to be similar to the Type I isozyme in its functional organization. Chimeras composed of the N-terminal half of Type III hexokinase and the C-terminal half of either Type I or Type II hexokinase have activities that can be attributed to the C-terminal half and are similar in activity to chimeras composed of the C-terminal half of Type III and the intrinsically inactive N-terminal domain of Type I or the inactivated (by site-directed mutation) N-terminal half of Type II hexokinase. Virtually no activity was seen with chimeras constructed with the N-terminal half of the Type III isozyme and catalytically inactive (by site-directed mutation) C-terminal halves of Type I or Type II hexokinase. Substrate inhibition by Glc is seen only with the Type III isozyme and with chimeric forms containing the C-terminal half of Type III hexokinase and the N-terminal half of Type I or Type II isozyme, the latter inactivated by site-directed mutation; this is attributed to conformational changes induced by binding of Glc to a low affinity site in the N-terminal half, with subsequent effect on catalytic activity of the C-terminal half. These results also provide further insight into the role of interactions (or lack of interactions) between the N- and C-terminal halves in the inhibition of the Type I-III isozymes by Glc-6-P, its antagonism by low concentrations of Pi, and the inhibition seen at higher concentrations of Pi.

Characterization of cDNAs coding for glucose phosphate isomerase and phosphoglycerate kinase in Chinese hamster ovary cell line CHO-K1 and identification of defects in R1.1.7, a glycolysis-deficient variant of CHO-K1

Full-length cDNAs for glucose phosphate isomerase (GPI) and phosphoglycerate kinase (PGK) of the Chinese hamster ovary cell line CHO-K1 have been characterized using RT-PCR and cycle sequencing of the PCR-amplified templates. Mutations in both genes have been identified in a glycolysis-deficient Chinese hamster ovary cell line, R1.1.7, derived from CHO-K1 cells.

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.

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.