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At least 163 records · Page 9Linked to original sources
PubMed1999

Cloning and expression of a hexose transporter gene expressed during the ripening of grape berry.

The ripening of grape (Vitis vinifera L.) is characterized by massive sugar import into the berries. The events triggering this process and the pathways of assimilate transport are still poorly known. A genomic clone Vvht1 (Vitis vinifera hexose transporter1) and the corresponding cDNA encoding a hexose transporter whose expression is induced during berry ripening have been isolated. Vvht1 is expressed mainly in the berries, with a first peak of expression at anthesis, and a second peak about 5 weeks after véraison (a viniculture term for the inception of ripening). Vvht is strictly conserved between two grape cultivars (Pinot Noir and Ugni-Blanc). The organization of the Vvht1 genomic sequence is homologous to that of the Arabidopsis hexose transporter, but differs strongly from that of the Chlorella kessleri hexose transporter genes. The Vvht1 promoter sequence contains several potential regulating cis elements, including ethylene-, abscisic acid-, and sugar-responsive boxes. Comparison of the Vvht1 promoter with the promoter of grape alcohol dehydrogenase, which is expressed at the same time during ripening, also allowed the identification of a 15-bp consensus sequence, which suggests a possible co-regulation of the expression of these genes. The expression of Vvht1 during ripening indicates that sucrose is at least partially cleaved before uptake into the flesh cells.

Alcohol Dehydrogenase↗
PubMed1980

Amino acid and hexose transport by cultured crypt cells from rat small intestine.

The characteristics of amino acid and sugar transport in intestinal crypt epithelial cells have been examined by measuring substrate uptake in an established epithelial cell line. These cells (IEC-6 cells) have been characterized as derived from rat small intestinal crypt cells on the basis of morphological criteria (J. Cell. Biol. 80: 248-265, 1979). Amino acid transport appeared to be mediated by both Na+-dependent and Na+-independent systems. Hexose uptake was stereospecific and Na+ independent, and was markedly inhibited by phloretin and cytochalasin B. Since glucocorticoids are known to have profound effects on maturation of the intestinal epithelium in vivo, their effects on transport properties of the cultured crypt cells were studied. Hydrocortisone, while completely inhibiting cell growth, increased the initial uptake rates of various hexoses, while having little or nor effect on the initial rate of amino acid uptake. The increased hexose uptake appeared to be due to a change in Vmax rather than Km. Appearance of the Na+-dependent hexose transport system, which is present in differentiated enterocytes, was not elicited by in vitro treatment with glucocortcoids.

Amino Acids↗
PubMed1983

Kinetic parameters of hexose transport in hybrids between malignant and nonmalignant cells.

Matched pairs of isogeneic hybrid cells, in which one member of the pair was malignant and the other not, were used to examine the linkage between malignancy and functional alterations in hexose transport. The kinetic parameters of uptake of 2-deoxy-D-glucose were measured in a range of such hybrids, both human and murine. Some other malignant cell lines were also examined and were compared with non-tumorigenic derivatives of tumour cells selected by exposure to the lectin, wheat-germ agglutinin. In every case, malignancy, as defined by the ability of cells to grow progressively in vivo, was found to be linked to a decrease in the Michaelis constant of hexose uptake. Independent measurement of the transport and phosphorylation reactions involved in hexose uptake revealed that this decrease was determined by the membrane transport system. The difference in Michaelis constant between malignant and non-malignant cells was observed with 3-O-methylglucose, a hexose that is transported into the cell but not further metabolized. The activity of hexokinase in cell homogenates was higher than the level that would be required to cope with transport and showed no correlation with tumorigenicity. Measurement of the uptake of D-glucose itself, by a rapid filtration centrifugation method, gave results similar to those obtained with 2-deoxy-D-glucose.

3-O-Methylglucose↗
PubMed1989

Insulin-stimulated hexose transport and glucose oxidation in rat adipocytes is inhibited by sphingosine at a step after insulin binding.

Spingosine, a naturally occurring inhibitor of protein kinase C, has recently been shown to have potent bioregulatory effects on a variety of cellular processes involving signal transduction mechanisms. In the present studies, we have investigated its effects on activation by insulin of hexose transport and glucose oxidation in isolated rat adipocytes. Preincubation of cells with this long-chain base blocked both the marked activation of these processes by insulin and the smaller activation by phorbol myristate acetate. Inhibition of both insulin and phorbol 12-myristate 13-acetate activation showed the same sphingosine concentration dependence, suggesting a common locus of action. The effectiveness of sphingosine was inversely proportional to the lipid content in the incubation (which was a function of both the age of the animal and the number of cells used) presumably due to dilution of the lipophilic long-chain base into the cellular triglycerides. Sphingosine did not affect either insulin binding to its receptor or the half-maximal concentration of the hormone required to activate hexose transport, but reduced the maximal responses. Thus, the inhibition was at a step distal to the binding of insulin to its receptor. Basal transport activity was not inhibited, suggesting a locus of action prior to the glucose transporter. The inhibitor was also effective when added following activation by insulin of hexose transport and resulted in a rapid reversal of activation (t 1/2 for inhibition was 2-4 min.). Sphingosine and its analogs showed a parallel potency for inhibition both of isolated protein kinase C and of insulin activation in adipocytes, consistent with an essential role for protein kinase C in the activation of hexose transport by insulin.

Adipose Tissue↗
PubMed1982

Cyclic AMP does not modulate hexose uptake of serum-deprived BHK-21 fibroblasts.

We have studied the uptakes of the glucose analogs, 2-deoxyglucose (2DG), and 3-O-methylglucose (3OMG) in serum-deprived BHK-21 fibroblasts. Incubation for 4 hours with 0.25 mM 1-methyl-3-isobutylxanthine (MIX) increased cellular cyclic AMP 3 to 15 fold, without affecting the initial rates of uptake of these sugars. Incubation with 100 nM insulin, doubled hexose uptake without affecting cyclic AMP. Insulin did not modify the cyclic AMP response to MIX; nor did MIX alter the hexose uptake response to insulin. There was no effect of 0.1 mM L-isoproterenol (ISO) or 0.1 mM prostaglandin E1 (PGE1) on cyclic AMP. PGE1 slightly stimulated uptake of 2DG but not 3OMG; ISO did not affect hexose uptake. We conclude that cyclic AMP does not directly regulate hexose uptake in these cells.

1-Methyl-3-isobutylxanthine↗
PubMed1986

Stress induces an increased hexose uptake in cultured cells.

Temperature-sensitive mutants have revealed a region of the herpes simplex virus 1 genome that affects both the uptake of hexose and the synthesis of heat shock proteins. Other inducers of heat-shock proteins, namely heat shock itself and arsenite, likewise induce an increased uptake of hexose. The increased uptake, like that induced by insulin, is insensitive to the presence of actinomycin D or cycloheximide. It is concluded that an increased hexose uptake, reflecting an activation or relocation of existing hexose transport protein, is a general biochemical response of stressed cells.

3-O-Methylglucose↗
PubMed1987

Effect of hypertonicity on hexose transporter regulation in chicken embryo fibroblasts.

The regulation of hexose transporters of cultured fibroblasts was investigated by exposing chicken embryo fibroblasts (CEF) to hypertonic culture medium, a condition known to enhance hexose transport activity. The effects of hypertonicity and the role of protein synthesis were examined with CEF in the basal (glucose fed) and transport enhanced (glucose starved) states. Glucose-fed CEF exposed to hypertonic conditions developed four-fold enhancement of hexose transport activity within 4 hrs; this declined in the following 20 hrs to a level slightly higher than the fed control. Protein synthesis was required in part for this effect, since the presence of cycloheximide during hypertonic exposure of fed CEF blocked the increase in of transport by almost 50%. Although the increased transport produced by glucose starvation was not further enhanced by hypertonicity, hypertonic treatment of starved CEF during glucose refeeding largely prevented the loss of transport activity to the basal, fed state. The hypertonic effects were concentration dependent (240mOsm optimal) and could be elicited with NaCl, KCl, or sucrose. Hypertonic treatment typically led to a greater than 50% decline in the incorporation of [3H]leucine into acid-insoluble fractions. The changes in transport were evident at the plasma membrane level, and studies of membrane vesicles prepared from hypertonically treated fed CEF showed a doubling of both [3H]cytochalasin B binding and the Vmax of D-glucose transport. These findings indicate that exposure of CEF to hypertonic conditions has some effects similar to those produced by glucose starvation and suggest that protein synthesis is to some extent involved in the regulation of hexose transporters in CEF.

Animals↗
PubMed1987

Alternate models for shared carriers or a single maturing carrier in hexose uptake into rabbit jejunum in vitro.

The uptake (tissue accumulation) of three hexoses into rabbit jejunum was measured in a flux chamber in conditions of effective stirring. Glucose uptake was inhibited by galactose or 3-O-methylglucose: 1-40 mM galactose caused a progressive decline in glucose uptake; 1-5 mM 3-O-methylglucose inhibited glucose uptake but higher concentrations of 3-O-methylglucose had no further effect. When 1-40 mM 3-O-methylglucose was added to glucose plus galactose there was a further decrease in the uptake of glucose; adding 1-40 mM galactose to glucose plus 3-O-methylglucose also produced a decrease in glucose uptake. Both glucose and 3-O-methylglucose inhibited uptake of galactose but the pattern of inhibition varied between the two sugars. The uptake of 3-O-methylglucose was also inhibited by glucose and by galactose, but the uptake of 3-O-methylglucose in the presence of either galactose or glucose was no further reduced by adding the third hexose. Graphical analysis and analysis by non-linear regression both showed that neither the single Michaelis-Menten function, nor the single Michaelis-Menten-plus-competitive-inhibition function was appropriate for any of these data. The results are consistent with the hypothesis that either there are multiple (at least three) intestinal carriers for hexoses; alternatively that there is a single carrier whose transport properties for the three hexoses change differentially during cell maturation and migration up the villus.

3-O-Methylglucose↗
PubMed1990

Development of the Na(+)-dependent hexose carrier in LLC-PK1 cells is dependent on microtubules.

The Na(+)-dependent hexose carrier, an endogenous apical marker, develops during differentiation of LLC-PK1, an established cell line with characteristics of the proximal tubule. This development was inhibited by the microtubule-disrupting drugs, colchicine and nocodazole, while it was insensitive to lumicolchicine. This strongly suggests that microtubules are involved in the plasma membrane expression of the Na(+)-dependent hexose carrier. We also analyzed the increase in activity of endogenous apical and basolateral membrane proteins during the polarization process. The development of three apical (Na(+)-dependent hexose carrier, gamma-glutamyltransferase and alkaline phosphatase) and one basolateral membrane protein (Na+/K(+)-ATPase) was studied during the reorganization of LLC-PK1 cells into a polarized epithelium. Colchicine inhibited the rapid, transient increase in the expression of the Na(+)-dependent hexose carrier during this polarization process. A similar result was observed for the development of the other apical proteins, while the development of Na+/K(+)-ATPase seemed to be largely insensitive to colchicine. Our results are in agreement with the model that the vesicles containing the apical membrane proteins use microtubules as tracks to reach the plasma membrane. The transport of vesicles containing basolateral membrane proteins clearly occurs by a different pathway which is independent on an intact microtubular network. Since the inhibition by the microtubule-disrupting drugs was complete, it can be concluded that after disruption of microtubules, the apical vesicles do not use the basolateral pathway by default.

Alkaline Phosphatase↗
PubMed1981

Differential effects of the NADPH/NADP+ ratio on the activities of hexose-6-phosphate dehydrogenase and glucose-6-phosphate dehydrogenase.

The steady-state kinetics of rat liver hexose-6-phosphate dehydrogenase (beta-D-glucose: NAD(P)+ 1-oxidoreductase, EC 1.1.1.47) using glucose 6-phosphate and NADP+ as substrates is studied. NADPH has been found to inhibit the enzyme noncompetitively with respect to NADP+, and uncompetitively with respect to glucose 6-phosphate. At a given concentration of glucose 6-phosphate, the reaction follows the basic inhibition equation. This suggests the presence of the enzyme-NADP+-NADPH complex, and contrasts with the NADPH inhibition of glucose-6-phosphate dehydrogenase which is competitive with respect to NADP+. An attempt was made to estimate the in vivo activities of the two enzymes in rat liver in the presence of NADPH at various NADPH/NADP+ ratios. The results show that the two enzymes appear to be at about the same level of activity in normal rat liver where the coenzyme redox ratio is 110 and the glucose 6-phosphate concentration is 217 microM. Under the same conditions, but with 50 microM dehydroepiandrosterone, a potent inhibitor of glucose-6-phosphate dehydrogenase, but not of hexose-6-phosphate dehydrogenase, the latter enzyme is estimated to be 1.6-times as active as the former. Such differential effects of NADPH and steroids on the two enzymes may support our notion that hexose-6-phosphate dehydrogenase may have advantages over glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate: NADP+ 1-oxidoreductase, EC 1.1.1.49) in steroid-metabolizing tissues (the activity of hexose-6-phosphate dehydrogenase is not, or less, affected by steroids of NADPH).

Androgens↗
PubMed1989

Glutamine dependency of human skin fibroblasts: modulation by hexoses.

The combined effects of carbohydrates and glutamine were investigated in diploid strains of normal human skin fibroblasts cultured for 21 days under eight different culture conditions: hexose-free medium or medium containing D-glucose, D-galactose, or D-fructose, with or without added glutamine. Cell growth, hexose consumption, lactate production, intracellular glycogen content and extracellular amino acid levels were measured every third to fourth day. In the presence of glutamine, cells reached a higher saturation density in fructose medium than in glucose or galactose medium but per cell consumption of fructose and galactose was much less than that of glucose. Consumption of all three carbohydrates per unit cell growth exhibited three distinct phases: Days 1-3, 3-10, and 10-20, respectively. In the absence of glutamine the rate of cell growth was not altered in glucose or galactose medium, but slowed down considerably in fructose medium. Glutamine deprivation also led to changes in hexose consumption. In hexose-free media the cell growth rate at first was very slow, but rose after 2 or 3 weeks of culture. The levels of extracellular nonessential amino acids varied according to medium and growth phase. One of the most exciting findings was that human fibroblasts are able to maintain a slight excess of glutamine in all media not supplemented with glutamine and, more surprisingly, to synthesize it in a medium containing galactose and glutamine.

Amino Acids↗
PubMed1987

Photolabeling of the adipocyte hexose carrier with an aryl azide derivative of maltose.

A nitrophenyl azide derivative of maltose, N-(4-azido-2-nitrophenyl)-maltosylamine (NAP-maltosylamine), was synthesized as a potential photoaffinity label for the hexose carrier of the rat adipocyte. This derivative inhibited 3-O-methylglucose uptake with a Ki of 1.3 mM in the dark, while that of maltose was 10.0 mM. Carbon-14-labeled maltose and NAP-maltosylamine entered adipocytes via the hexose carrier, the latter in a concentrative fashion. Photolysis of NAP-[14C]maltosylamine in the presence of an adipocyte low density microsomal membrane fraction labeled several electrophoretic bands. Among these are a 45 kDa band which showed features expected of the hexose carrier: its labeling was decreased 40% by D- but not L-glucose and pretreatment of intact adipocytes with insulin decreased labeling of the 45 kDa band by 10-40%, as predicted by the translocation theory of insulin-stimulated transport activation. These studies show the suitability of using carbon-1-modified sugar photoaffinity labels as probes for the hexose carrier and possibly of its regulation in rat adipocytes.

Adipose Tissue↗
PubMed1981

Alterations in the latency of hepatic microsomal hexose-6-phosphate dehydrogenase under various in vivo and in vitro conditions.

To gain insight into the cause of the latency of microsomal hexose-6-phosphate dehydrogenase activity, changes in the activity and latency of hexose-6-phosphate dehydrogenase were examined to determine whether they were parallel to those of other microsomal enzymes (UDP glucuronyltransferase, nucleoside diphosphatase and glucose-6-phosphatase) during development and after treatments of rats with phenobarbital and a high carbohydrate diet. We also examined whether the latency of hexose-6-phosphate dehydrogenase could be ascribed to changes in the cholesterol content and phospholipid composition of microsomes under various conditions. The results show that the activities and latencies of the four microsomal enzymes change independently during development and after various treatments, and that the phospholipid composition of microsomes does not have any direct effect on the latency of hexose-6-phosphate dehydrogenase.

Acid Anhydride Hydrolases↗
PubMed1982

Possible functional coupling of hexose-6-phosphate dehydrogenase to microsomal electron transport system in rat kidney and liver.

Intrarenal distributions of cytosolic glucose-6-phosphate dehydrogenase, microsomal hexose-6-phosphate dehydrogenase, NADPH-cytochrome c reductase, mixed-function oxidases and cytochrome P-450, and effects of phenobarbital, methylcholanthrene, fasting, castration and sex hormones on the hepatic and renal enzyme activities were examined in an attempt to elucidate the as yet unknown function of hexose-6-phosphate dehydrogenase. The results show that there is a similarity between hexose-6-phosphate dehydrogenase, and NADPH-cytochrome c reductase with respect to the intrarenal distribution and the responsiveness to various treatments, thus suggesting functional coupling of hexose-6-phosphate dehydrogenase to microsomal electron transport system.

Animals↗
PubMed1988

Phorbol ester stimulates hexose uptake by brain microvessel endothelial cells.

Glucose uptake into cultured endothelial cells (EC) derived from brain microvessels was determined in the absence and presence of 12-O-tetradecanoylphorbol-13-acetate (TPA), EGTA, the calcium ionophore A23187, and insulin. EC were obtained from dog and human (autopsy) brain microvessels and maintained in culture for up to four passages. Monolayers of EC were treated with TPA and other compounds immediately prior to harvesting for hexose uptake measurements using 3-O-[3H]methyl-D-glucose, 2-[3H]deoxy-D-glucose, or D-[3H]glucose. Typically, treatment with TPA (0.1-100 ng/ml) resulted in hexose uptake levels 2 to 3 times those of controls, although occasionally levels 5 to 10 times those of controls were observed. Similar stimulation was observed with all radiolabeled hexoses. Stimulation by TPA was greatest in primary or first passage cells and was greatly diminished in older cells. Neither chelation of extracellular calcium with EGTA nor the presence of both EGTA and A23187 in the culture medium prevented the stimulatory effect of TPA. Insulin (1200 ng/ml) failed to stimulate hexose uptake. Treatment with 100 ng/ml TPA did not alter the appearance of actin filaments in canine EC as visualized with rhodamine phalloidin. These results, in combination with other recent studies, suggest that blood-brain glucose transport may be regulated by phorbol ester-activated protein kinase C.

Animals↗
PubMed1988

Structure of yeast glucokinase, a strongly diverged specific aldo-hexose-phosphorylating isoenzyme.

Saccharomyces cerevisiae glucokinase (GLK) is the only described hexose-phosphorylating enzyme specific for aldo-hexoses. The gene was cloned by complementation of a triple mutant lacking all hexose-phosphorylating isoenzymes. Restriction sites were confirmed by genomic hybridization and GLK1 was mapped on chromosome III by ROFAGE, a method derived from the orthogonal field alteration gel electrophoresis. The mapping data were in agreement with previous genetic data. The open reading frame was established by two transcription start points in front of the initial ATG codon and by C-terminal beta-galactosidase fusions. The mRNA is 1.75 kb long and codes for 500 amino acid (aa) residues. Diversity of GLK from hexokinases PI and PII is very marked, with only 26 and 28% overall aa homology. A central core of about 350 aa shows 39% homology. No cross-hybridization could be observed by Southern hybridization. However, strong homologies were found over a range of 11 aa between glucokinase, yeast hexokinases (PI, PII) and rat hexokinase with 8 aa in common. These strongly conserved homologies give support to the view that this aa region corresponds to the binding site for glucose. Unlike all other hexose-phosphorylating enzymes, there is no proline residue indicating a conformational turn next to this glucokinase region. This finding may explain the failure of fructose phosphorylation. In both GLK and the hexokinases, a lysine residue is also conserved at aa position 110 which probably corresponds to the ATP-binding site. Additionally, a consensus sequence of 8 aa residues which is common for ATP-binding enzymes is conserved within the C-terminal part of GLK. The codon bias index for GLK1 is 0.25, which is very low compared with other glycolytic enzymes described so far. The gene is moderately expressed and constitutive on different carbon sources investigated. GLK1 null alleles had no detectable effects on sporulation and growth. Hence, a physiological role for GLK, which might explain its preservation, could not be detected under our laboratory test conditions.

Amino Acid Sequence↗
PubMed2004

A putative high affinity hexose transporter, hxtA, of Aspergillus nidulans is induced in vegetative hyphae upon starvation and in ascogenous hyphae during cleistothecium formation.

Fungi employ different carbohydrate uptake systems to adapt to certain environmental conditions and to different carbon source concentrations. The hydrolysis of polymeric carbohydrates and the subsequent uptake of monomeric forms may also play a role in development. Aspergillus nidulans accumulates cell wall components during vegetative growth and degrades them during sexual development. We have identified the hxtA (high affinity hexose transporter) gene in a differential library, which was enriched for sexual-specific genes. The hxtA gene is disrupted by 6 introns and predicted to encode a 531 amino acid protein with high similarity to major facilitator superfamily members including the high affinity hexose transporter Gtt1 from Trichoderma harzianum. A. nidulans HxtA contains the 12 predicted transmembrane domains characteristic for this family. Deletion of hxtA did not impair growth of A. nidulans on a variety of carbon sources nor did it inhibit sexual development suggesting redundant sugar uptake systems. We found at least 17 putative hexose transporters in the genome of A. nidulans. Despite the high similarity of HxtA to fungal high affinity glucose transporters, the hxtA gene did not restore growth on glucose of a Saccharomyces cerevisiae mutant, in which all hexose transporters were deleted. Northern blot analysis revealed that the A. nidulans hxtA gene was repressed under high glucose conditions and expressed in vegetative hyphae upon carbon starvation and during sexual development. We found hxtA(p)::sgfp expression in developing cleistothecia specifically in ascogenous hyphae and propose that HxtA is a high affinity glucose transporter involved in sugar metabolism during sexual development.

Amino Acid Sequence↗
PubMed2000

Hexose transport in asexual stages of Plasmodium falciparum and kinetoplastidae.

The hexose sugar, glucose, is a vital energy source for most organisms and an essential nutrient for asexual stages of Plasmodium falciparum. Kinetoplastid organisms (e.g. Trypanosoma and Leishmania spp) also require glucose at certain critical stages of their life cycles. Although phylogenetically unrelated, these organisms share many common challenges during the mammalian stages of a parasitic life cycle, and possess hexose uptake mechanisms that are amenable to study using similar methods. Defining hexose permeation pathways into parasites might expose an Achilles' heel at which both antidisease and antiparasite measures can be aimed. Understanding the mode of entry of glucose also presents a good general model for substrate acquisition in multicompartment systems. In this review, Sanjeev Krishna and colleagues summarize current understanding of hexose transport processes in P. falciparum and provide a comparison with data obtained from kinetoplastids.

Animals↗