Mechanisms related to [18F]fluorodeoxyglucose uptake of human colon cancers transplanted in nude mice.
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[18F]Fluorodeoxyglucose ([18F]FDG), a glucose analogue, has been widely used for tumor imaging. To investigate the mechanisms related to [18F]FDG uptake by tumors, an experiment involving nude mice was performed. METHODS: Human colon cancer cell lines SNU-C2A, SNU-C4 and SNU-C5 were transplanted to nude mice. Using immunohistochemical staining and Western blot, the expression of glucose transporter (Glut) isoforms (Glut-1 through -5) in xenografted tumors was analyzed. For the analysis of messenger ribonucleic acid (mRNA) expression, reverse-transcription polymerase chain reaction and Northern blot were used and the enzyme activity of hexokinase in cancer tissues was measured by continuous spectrophotometric rate determination. RESULTS: [18F]FDG uptake in SNU-C4 and SNU-C5 cells was higher than in normal colon cells. Among these cells and xenografted tumors, SNU-C5 showed the highest level of [18F]FDG uptake, followed by SNU-C4 and SNU-C2A. An immunostaining experiment showed intense staining of Glut-1 in SNU-C5 tumors but somewhat faint staining in SNU-C4. SNU-C5 tumors also showed positive staining with Glut-3, although this was not the case with SNU-C2A and SNU-C4. Western blot analysis showed the expression of Glut-1 and Glut-3 in all tumors. Experiments involving Northern blot analysis and reverse-transcription polymerase chain reaction confirmed the overexpression of Glut-1 mRNA in all tumors, with the highest level in SNU-C5. The level of Glut-3 mRNA was also elevated in SNU-C5 tumors but not in SNU-C2A and SNU-C4. The enzyme activity of hexokinase did not vary among different tumors. CONCLUSION: Gluts, especially Glut-1, are responsible for [18F]FDG uptake in a nude mouse model of colon cancer rather than hexokinase activity. Increased numbers of glucose transporters at the plasma membrane of cancer cells is attributed to an increased level of transcripts of glucose transporter genes and may be a cause of increased [18F]FDG uptake, at least in colon cancer tumors. (+info)
Mannose inhibits Arabidopsis germination via a hexokinase-mediated step.
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Low concentrations of the glucose (Glc) analog mannose (Man) inhibit germination of Arabidopsis seeds. Man is phosphorylated by hexokinase (HXK), but the absence of germination was not due to ATP or phosphate depletion. The addition of metabolizable sugars reversed the Man-mediated inhibition of germination. Carbohydrate-mediated regulation of gene expression involving a HXK-mediated pathway is known to be activated by Glc, Man, and other monosaccharides. Therefore, we investigated whether Man blocks germination through this system. By testing other Glc analogs, we found that 2-deoxyglucose, which, like Man, is phosphorylated by HXK, also blocked germination; no inhibition was observed with 6-deoxyglucose or 3-O-methylglucose, which are not substrates for HXK. Since these latter two sugars are taken up at a rate similar to that of Man, uptake is unlikely to be involved in the inhibition of germination. Furthermore, we show that mannoheptulose, a specific HXK inhibitor, restores germination of seeds grown in the presence of Man. We conclude that HXK is involved in the Man-mediated repression of germination of Arabidopsis seeds, possibly via energy depletion. (+info)
Developmental regulation of genes mediating murine brain glucose uptake.
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We examined the molecular mechanisms that mediate the developmental increase in murine whole brain 2-deoxyglucose uptake. Northern and Western blot analyses revealed an age-dependent increase in brain GLUT-1 (endothelial cell and glial) and GLUT-3 (neuronal) membrane-spanning facilitative glucose transporter mRNA and protein concentrations. Nuclear run-on experiments revealed that these developmental changes in GLUT-1 and -3 were regulated posttranscriptionally. In contrast, the mRNA and protein levels of the mitochondrially bound glucose phosphorylating hexokinase I enzyme were unaltered. However, hexokinase I enzyme activity increased in an age-dependent manner suggestive of a posttranslational modification that is necessary for enzymatic activation. Together, the postnatal increase in GLUT-1 and -3 concentrations and hexokinase I enzymatic activity led to a parallel increase in murine brain 2-deoxyglucose uptake. Whereas the molecular mechanisms regulating the increase in the three different gene products may vary, the age-dependent increase of all three constituents appears essential for meeting the increasing demand of the maturing brain to fuel the processes of cellular growth, differentiation, and neurotransmission. (+info)
Glucose metabolism in Neurospora is altered by heat shock and by disruption of HSP30.
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We compared the metabolism of [1-13C]glucose by wild type cells of Neurospora crassa at normal growth temperature and at heat shock temperatures, using nuclear magnetic resonance analysis of cell extracts. High temperature led to increased incorporation of 13C into trehalose, relative to all other metabolites, and there was undetectable synthesis of glycerol, which was a prominent metabolite of glucose at normal temperature (30 degrees C). Heat shock strongly reduced formation of tricarboxylic acid cycle intermediates, approximately 10-fold, and mannitol synthesis was severely depressed at 46 degrees C, but only moderately reduced at 45 degrees C. A mutant strain of N. crassa that lacks the small alpha-crystallin-related heat shock protein, Hsp30, shows poor survival during heat shock on a nutrient medium with restricted glucose. An analysis of glucose metabolism of this strain showed that, unlike the wild type strain, Hsp30-deficient cells may accumulate unphosphorylated glucose at high temperature. This suggestion that glucose-phosphorylating hexokinase activity might be depressed in mutant cells led us to compare hexokinase activity in the two strains at high temperature. Hexokinase was reduced more than 35% in the mutant cell extracts, relative to wild type extracts. alpha-Crystallin and an Hsp30-enriched preparation protected purified hexokinase from thermal inactivation in vitro, supporting the proposal that Hsp30 may directly stabilize hexokinase in vivo during heat shock. (+info)
Metabolic regulation, activity state, and intracellular binding of glucokinase in insulin-secreting cells.
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Regulation of glucose-induced insulin secretion is crucially dependent on glucokinase function in pancreatic beta-cells. Glucokinase mRNA expression was metabolically regulated allowing continuous translation into enzyme protein. Glucokinase enzyme activity in the beta-cell was exclusively regulated by glucose. Using a selective permeabilization technique, different intracellular activity states of the glucokinase enzyme in bioengineered glucokinase-overexpressing RINm5F tissue culture cells were observed. These results could be confirmed in analogous experiments with dispersed islet cells. A diffusible glucokinase fraction with high enzyme activity could be distinguished from an intracellularly bound fraction with low activity. Glucose induced a significant long-term increase of the active glucokinase fraction. This effect was accomplished through the release of glucokinase enzyme protein from an intracellular binding site of protein character. The inhibitory function of this protein factor was abolished through proteolytic digestion or heat inactivation. Northern blot analyses revealed that this binding protein was not identical to the well-known liver glucokinase regulatory protein. This hitherto unknown new protein factor may have the function of a glucokinase regulatory protein in the pancreatic beta-cell, which may regulate glucokinase enzyme activity in a glucose-dependent manner. (+info)
Mutations that confer resistance to 2-deoxyglucose reduce the specific activity of hexokinase from Myxococcus xanthus.
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The glucose analog 2-deoxyglucose (2dGlc) inhibits the growth and multicellular development of Myxococcus xanthus. Mutants of M. xanthus resistant to 2dGlc, designated hex mutants, arise at a low spontaneous frequency. Expression of the Escherichia coli glk (glucokinase) gene in M. xanthus hex mutants restores 2dGlc sensitivity, suggesting that these mutants arise upon the loss of a soluble hexokinase function that phosphorylates 2dGlc to form the toxic intermediate, 2-deoxyglucose-6-phosphate. Enzyme assays of M. xanthus extracts reveal a soluble hexokinase (ATP:D-hexose-6-phosphotransferase; EC 2.7.1.1) activity but no phosphotransferase system activities. The hex mutants have lower levels of hexokinase activities than the wild type, and the levels of hexokinase activity exhibited by the hex mutants are inversely correlated with the ability of 2dGlc to inhibit their growth and sporulation. Both 2dGlc and N-acetylglucosamine act as inhibitors of glucose turnover by the M. xanthus hexokinase in vitro, consistent with the finding that glucose and N-acetylglucosamine can antagonize the toxic effects of 2dGlc in vivo. (+info)
Novel alleles of yeast hexokinase PII with distinct effects on catalytic activity and catabolite repression of SUC2.
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In the yeast Saccharomyces cerevisiae, glucose or fructose represses the expression of a large number of genes. The phosphorylation of glucose or fructose is catalysed by hexokinase PI (Hxk1), hexokinase PII (Hxk2) and a specific glucokinase (Glk1). The authors have shown previously that either Hxk1 or Hxk2 is sufficient for a rapid, sugar-induced disappearance of catabolite-repressible mRNAs (short-term catabolite repression). Hxk2 is specifically required and sufficient for long-term glucose repression and either Hxk1 or Hxk2 is sufficient for long-term repression by fructose. Mutants lacking the TPS1 gene, which encodes trehalose 6-phosphate synthase, can not grow on glucose or fructose. In this study, suppressor mutations of the growth defect of a tps1delta hxk1delta double mutant on fructose were isolated and identified as novel HXK2 alleles. All six alleles studied have single amino acid substitutions. The mutations affected glucose and fructose phosphorylation to a different extent, indicating that Hxk2 binds glucose and fructose via distinct mechanisms. The mutations conferred different effects on long- and short-term repression. Two of the mutants showed very similar defects in catabolite repression, despite large differences in residual sugar-phosphorylation activity. The data show that the long- and short-term phases of catabolite repression can be dissected using different hexokinase mutations. The lack of correlation between in vitro catalytic hexokinase activity, in vivo sugar phosphate accumulation and the establishment of catabolite repression suggests that the production of sugar phosphate is not the sole role of hexokinase in repression. Using the set of six hxk2 mutants it was shown that there is a good correlation between the glucose-induced cAMP signal and in vivo hexokinase activity. There was no correlation between the cAMP signal and the short- or long-term repression of SUC2, arguing against an involvement of cAMP in either stage of catabolite repression. (+info)
Viscoelastic properties of f-actin, microtubules, f-actin/alpha-actinin, and f-actin/hexokinase determined in microliter volumes with a novel nondestructive method.
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A nondestructive method to determine viscoelastic properties of gels and fluids involves an oscillating glass fiber serving as a sensor for the viscosity of the surrounding fluid. Extremely small displacements (typically 1-100 nm) are caused by the glass rod oscillating at its resonance frequency. These displacements are analyzed using a phase-sensitive acoustic microscope. Alterations of the elastic modulus of a fluid or gel change the propagation speed of a longitudinal acoustic wave. The system allows to study quantities as small as 10 microliters with temporal resolution >1 Hz. For 2-100 microM f-actin gels a final viscosity of 1.3-9.4 mPa s and a final elastic modulus of 2.229-2.254 GPa (corresponding to 1493-1501 m/s sound velocity) have been determined. For 10- to 100-microM microtubule gels (native, without stabilization by taxol), a final viscosity of 1.5-124 mPa s and a final elastic modulus of 2.288-2. 547 GPa (approximately 1513-1596 m/s) have been determined. During polymerization the sound velocity in low-concentration actin solutions increased up to +1.3 m/s (approximately 1.69 kPa) and decreased up to -7 m/s (approximately 49 kPa) at high actin concentrations. On polymerization of tubulin a concentration-dependent decrease of sound velocity was observed, too (+48 to -12 m/s approximately 2.3-0.1 MPa, for 10- to 100-microM tubulin). This decrease was interpreted by a nematic phase transition of the actin filaments and microtubules with increasing concentration. 2 mM ATP (when compared to 0.2 mM ATP) increased polymerization rate, final viscosity and elastic modulus of f-actin (17 microM). The actin-binding glycolytic enzyme hexokinase also accelerated the polymerization rate and final viscosity but elastic modulus (2.26 GPa) was less than for f-actin polymerized in presence of 0.2 mM ATP (2.28 GPa). (+info)