Characterization of the mouse liver fructose-1,6-bisphosphatase gene

Hepatic but not Intestinal FBP1 Is Required for Fructose Metabolism and Tolerance

Fructose intolerance in mammals is caused by defects in fructose absorption and metabolism. Fructose-1,6-bisphosphatase 1 (FBP1) is a key enzyme in gluconeogenesis, and its deficiency results in hypoglycemia as well as intolerance to fructose. However, the mechanism about fructose intolerance caused by FBP1 deficiency has not been fully elucidated. Here, we demonstrate that hepatic but not intestinal FBP1 is required for fructose metabolism and tolerance. We generated inducible knockout mouse models specifically lacking FBP1 in adult intestine or liver. Intestine-specific deletion of Fbp1 in adult mice does not compromise fructose tolerance, as evidenced by no significant body weight loss, food intake reduction, or morphological changes of the small intestine during 4 weeks of exposure to a high-fructose diet. By contrast, liver-specific deletion of Fbp1 in adult mice leads to fructose intolerance, as manifested by substantial weight loss, hepatomegaly, and liver injury after exposure to a high-fructose diet. Notably, the fructose metabolite fructose-1-phosphate is accumulated in FBP1-deficient liver after fructose challenge, which indicates a defect of fructolysis, probably due to competitive inhibition by fructose-1,6-bisphosphate and may account for the fructose intolerance. In conclusion, these data have clarified the essential role of hepatic but not intestinal FBP1 in fructose metabolism and tolerance.

Molecular characterization of fructose-1,6-bisphosphatase 1b in blunt snout bream Megalobrama amblycephala and the transcriptional response to glucose loading after the adaptation to high-carbohydrate diets

This study aimed to characterize the full-length complementary DNA (cDNA) of fructose-1,6-bisphosphatase 1b (FBP1b) from fish Megalobrama amblycephala, and investigate its transcriptional response to glucose administration after the adaptation to high-carbohydrate diets. The cDNA obtained covered 1435 bp with an open reading frame of 1014 bp. Sequence alignment and phylogenetic analysis revealed a high degree of conservation (76-96%) among most fish and other vertebrates, retaining one N-linked glycosylation site, one N-terminal acetylation site, 13 phosphorylation sites, one fructose-1,6-bisphosphatase (FBPase) active site, five metal-binding sites, four substrate-binding sites, and several AMP-binding sites. The highest messenger RNA (mRNA) level of FBP1b was observed in liver followed by intestine, whereas relatively low values were detected in heart, gill, and eye. Then, the mRNA levels of FBP1b and the FBPase activity were both determined in the liver of fish injected intraperitoneally with 1.67 g glucose per kilogram body weight after being fed two dietary carbohydrate levels (30 and 42%) for 11 weeks. After the glucose load, the mRNA levels of FBP1b in both treatments decreased significantly to the basal value at 8 h and showed a slight increase afterward. However, the enzymatic activity showed no statistical difference during the first 4 h, but increased remarkably with further increasing times. In addition, both the mRNA levels and activities decreased significantly with increasing dietary carbohydrate levels. The results indicated that the FBP1b of M. amblycephala shared a high similarity with that of the other vertebrates. Its mRNA expression in liver was downregulated remarkably by a glucose administration, as also held true after the long-term adaptation of a carbohydrate-rich diet.

Antidiabetic activity of Ganoderma lucidum polysaccharides F31 down-regulated hepatic glucose regulatory enzymes in diabetic mice

ETHNOPHARMACOLOGICAL RELEVANCE

Ganoderma lucidum (Lin Zhi) has been used to treat diabetes in Chinese folk for centuries. Our laboratory previously demonstrated that Ganoderma lucidum polysaccharides (GLPs) had hypoglycemic effects in diabetic mice. Our aim was to identify the main bioactives in GLPs and corresponding mechanism of action.

MATERIALS AND METHODS

Four polysaccharide-enriched fraction were isolated from GLPs and the antidiabetic activities were evaluated by type 2 diabetic mice. Fasting serum glucose (FSG), fasting serum insulin (FSI) and epididymal fat/BW ratio were measured at the end of the experiment. In liver, the mRNA levels of hepatic glucose regulatory enzymes were determined by quantitative polymerase chain reaction (qPCR) and the protein levels of phospho-AMP-activated protein kinase (p-AMPK)/AMPK were determined by western blotting test. In epididymal fat tissue, the mRNA and protein levels GLUT4, resistin, fatty acid synthase (FAS) and acetyl-CoA carboxylase (ACC1) were determined by qPCR and immuno-histochemistry. The structure of polysaccharide F31 was obtained from GPC, FTIR NMR and GC-MS spectroscopy, RESULTS: F31 significantly decreased FSG (P<0.05), FSI and epididymal fat/BW ratio (P<0.01). In liver, F31 decreased the mRNA levels of hepatic glucose regulatory enzymes, and up-regulated the ratio of phospho-AMP-activated protein kinase (p-AMPK)/AMPK. In epididymal fat tissue, F31 increased the mRNA levels of GLUT4 but decreased fatty acid synthase (FAS), acetyl-CoA carboxylase (ACC1) and resistin. Immuno-histochemistry results revealed F31 increased the protein levels of GLUT4 and decreased resistin.

CONCLUSION

Data suggested that the main bioactives in GLPs was F31, which was determined to be a β-heteropolysaccharide with the weight-average molecular weight of 15.9kDa. The possible action mechanism of F31 may be associated with down-regulation of the hepatic glucose regulated enzyme mRNA levels via AMPK activation, improvement of insulin resistance and decrease of epididymal fat/BW ratio. These results strongly suggest that F31 has antidiabetic potential.

Effects of dietary glucose and dextrin on activity and gene expression of glucokinase and fructose-1,6-bisphosphatase in liver of turbot Scophthalmus maximus

Glucokinase (GK) and fructose-1,6-bisphosphatase (FBPase) play crucial role in glucose metabolism. In the present study, the cDNA encoding GK and FBPase was cloned from the liver of turbot Scophthalmus maximus by rapid amplification of cDNA end technique. Effects of dietary glucose and dextrin on the activities and gene expressions of these two enzymes were also studied. Results showed that the full length of GK cDNA was 2226 bp, consisting of an open reading frame (ORF) of 1434 bp. The full-length cDNA coding FBPase was 1314 bp with a 1014 bp ORF encoding 337 amino acids. Analyses of gene expression of GK and FBPase were conducted in gill, liver, the whole intestine, the whole kidney, heart, the dorsal white muscle and brain. The highest expression of GK was found in liver, followed by muscle. The expression of FBPase was found higher in liver than heart and gill. Both hepatic GK activity and mRNA expression were highly induced in turbot after being fed with dietary carbohydrates (p < 0.05). However, the GK activity and mRNA expression in the group with dietary glucose did not significantly differ from those in the group with dietary dextrin (p > 0.05). Compared with the control group, there were no significant differences in FBPase activity and mRNA expression in the glucose as well as dextrin group (p > 0.05). The increased hepatic GK activity and gene expression indicated that the first step of glycolysis was activated in turbot by dietary carbohydrates.

Biochemical characterization and functional analysis of fructose-1,6-bisphosphatase from Clonorchis sinensis

Fructose-1,6-bisphosphatase (FBPase), a key regulatory enzyme of gluconeogenesis, plays an essential role in metabolism and development of most organisms. To the wealth of available knowledge about FBPase from Clonorchis sinensis (CsFBPase), in this study, the characteristics of CsFBPase and its potential role in pathogenesis of clonorchiasis were investigated. The Km value of CsFBPase was calculated to be 41.9 uM. The optimal temperature and pH of CsFBPase were 37 °C and pH 7.5-8.0, respectively. In addition, Mg(2+) or K(+) played a regulatory role in enzyme activity of CsFBPase. Both transcriptional and translational level of CsFBPase were higher in metacercariae (one of larva stages) than those in adult worm (P < 0.05). CsFBPase were observed to extensively express in the intestine, vitellaria and tegument of adult worms and ubiquitously in metacercariae. Moreover, CsFBPase was confirmed as a component of excretory/secretory products. Consequently, the translocation of CsFBPase could be detected on epithelial cells of bile duct in liver of C. sinensis infected rat. Recombinant CsFBPase can specifically bind to the membrane of human hepatic stellate cell line LX-2 by immunofluorescence analysis and stimulated proliferation and activation of LX-2 which demonstrated by Cell Counting Kit-8 and upregulation of key fibrosis-related factors, such as α-smooth muscle actin, collagen I and collagen III using qRT-PCR. Thus, we predicated that CsFBPase might be a multifunctional enzyme which played as both regulatory enzyme and virulence factor in pathogenesis of C. sinensis infection.

Hypoglycemic effects of Ganoderma lucidum polysaccharides in type 2 diabetic mice

Our aims were to investigate the hypoglycemic effects and mechanisms of action of Ganoderma lucidum polysaccharides (GLPs) administered for 7 days in type 2 diabetic mice. The mice were randomly divided into four groups (8 mice/group): normal control group, diabetic control group, low-dose GLP-treated diabetic group (50 mg/kg/d), and high-dose GLP-treated diabetic group (100 mg/kg/d). Diabetes was induced by streptozotocin injection and high-fat dietary feeding. At the end of the study, fasting serum glucose, insulin, body weight (BW) and epididymal white adipose tissue weight were measured. The hepatic mRNA levels of glycogen phosphorylase (GP), fructose-1,6-bisphosphatase (FBPase), phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) genes were determined by real-time polymerase chain reaction. Both doses of GLPs significantly decreased fasting serum glucose, insulin and epididymal fat/BW ratio compared with the diabetic control group (p < 0.05). The hepatic mRNA levels of GP, FBPase, PEPCK and G6Pase were significantly lower in both GLP-treated groups compared with the diabetic control group. Taken together, GLPs significantly decrease fasting serum glucose levels in type 2 diabetic mice in a dose-dependent manner. The decreases in fasting serum glucose levels may be associated with decreased mRNA expression levels of several key enzymes involved in gluconeogenesis and/or glycogenolysis.

Gene expression profiles in developing nephrons using Lim1 metanephric mesenchyme-specific conditional mutant mice

BACKGROUND

Lim1 is a homeobox gene that is essential for nephrogenesis. During metanephric kidney development, Lim1 is expressed in the nephric duct, ureteric buds, and the induced metanephric mesenchyme. Conditional ablation of Lim1 in the metanephric mesenchyme blocks the formation of nephrons at the nephric vesicle stage, leading to the production of small, non-functional kidneys that lack nephrons.

METHODS

In the present study, we used Affymetrix probe arrays to screen for nephron-specific genes by comparing the expression profiles of control and Lim1 conditional mutant kidneys. Kidneys from two developmental stages, embryonic day 14.5 (E14.5) and 18.5 (E18.5), were examined.

RESULTS

Comparison of E18.5 kidney expression profiles generated a list of 465 nephron-specific gene candidates that showed a more than 2-fold increase in their expression level in control kidney versus the Lim1 conditional mutant kidney. Computational analysis confirmed that this screen enriched for kidney-specific genes. Furthermore, at least twenty-eight of the top fifty (56%) candidates (or their vertebrate orthologs) were previously reported to have a nephron-specific expression pattern. Our analysis of E14.5 expression data yielded 41 candidate genes that are up-regulated in the control kidneys compared to the conditional mutants. Three of them are related to the Notch signaling pathway that is known to be important in cell fate determination and nephron patterning.

CONCLUSION

Therefore, we demonstrate that Lim1 conditional mutant kidneys serve as a novel tissue source for comprehensive expression studies and provide a means to identify nephron-specific genes.

Localization of chicken muscle FBPase in cardiomyocyte nuclei

Fructose 1,6-bisphosphatase (FBPase; EC 3.1.3.11) localization in cardiomyocyte nuclei has recently been investigated in mammals [FEBS Lett. 539 (2003) 51]. In this study, nuclear localization of FBPase in the cardiac muscle of the chicken was studied by immunohistochemistry and other methods. A result of the electron microscopic investigation was confirmed by immunoblotting analysis. Using MALDI Q-TOF mass spectrometry and Mascot program, the nuclear FBPase was identified as muscle chicken FBPase. FBPase activity in isolated cardiomyocyte nuclei was 5.9 mU/g. Nuclear FBPase was strongly inhibited by allosteric inhibitor AMP. I(0.5) for AMP was 0.16 microM and was the same as for the purified chicken muscle enzyme.

Kupffer cell cytokines interleukin-1beta and interleukin-10 combine to inhibit phosphoenolpyruvate carboxykinase and gluconeogenesis in cultured hepatocytes

BACKGROUND AND AIMS

Recent evidence suggests that inflammatory cytokines may mediate reduced hepatic glucose production and reduced blood glucose concentrations in sepsis. Therefore the aim of this study is to provide direct evidence of a cytokine-mediated interaction between Kupffer cells and hepatocytes by characterising the effects of lipopolysaccharide-stimulated Kupffer cells on hepatocyte gluconeogenesis, and the activity of key regulatory enzymes of this pathway.

METHODS AND RESULTS

Primary isolates of hepatocytes co-cultured with lipopolysaccharide-stimulated Kupffer cells in Transwell inserts showed a 48% inhibition of gluconeogenesis (P < 0.001). RNase protection assay and ELISA of Kupffer cells and the culture media following exposure to lipopolysaccharide showed increased levels of interleukin-1 alpha and beta, tumour necrosis factor alpha and IL-10. The addition of IL-1beta and IL-10 to hepatocyte cultures inhibited gluconeogenesis by 52% (P < 0.001), whereas each cytokine alone was ineffective. To determine whether altered production or activity of phosphoenolpyruvate carboxykinase or pyruvate kinase was responsible for the reduced glucose synthesis, their mRNA, protein levels and enzyme activities were measured. Primary hepatocytes co-cultured with lipopolysaccharide-stimulated Kupffer cells or cultured with a combination of IL-1beta and IL-10 displayed reduced levels of phosphoenolpyruvate carboxykinase mRNA, protein and enzyme activity. In contrast the mRNA, protein levels and enzyme activity of pyruvate kinase were not altered; suggesting that gluconeogenesis was suppressed by downregulation of phosphoenolpyruvate carboxykinase.

CONCLUSIONS

Therefore, hypoglycaemia, which is often observed in sepsis, may be mediated by Kupffer cell-derived IL-1beta and IL-10. In addition this study suggests these cytokines inhibit phosphoenolpyruvate carboxykinase production and thereby hepatic gluconeogenesis.

The interaction of FBPase with aldolase: a kinetic and fluorescence investigation on chicken muscle enzymes

Fructose-1,6-bisphosphatase (FBPase; EC 3.1.3.11) is strongly inhibited by AMP in vitro and, therefore, at physiological concentrations of substrate and AMP, FBPase should be completely inhibited. Desensitization of rabbit muscle FBPase against AMP inhibition was previously observed in the presence of rabbit muscle aldolase. In this study, we analysed the kinetics of an FBPase catalyzed reaction and interaction between chicken muscle FBPase and chicken muscle aldolase. The initial rate of FBPase reaction vs. substrate concentration shows a maximum activity at a concentration of 20 microM Fru-1,6P2 and then decreases. Assuming rapid equilibrium kinetics, the enzyme-catalyzed reaction was described by the substrate inhibition model, with Ks approximately 5 microM and Ksi approximately 39 microM and factor beta approximately 0.2, describing change in the rate constant (k) of product formation from the ES and ESSi complexes. Based on ultracentrifugation studies, aldolase and FBPase form a hetero-complex with approximately 1:1 stoichiometry with a dissociation constant (Kd) of 3.8 microM. The FBPase-aldolase interaction was confirmed via fluorescence investigation. The aldolase-FBPase interaction results in aldolase fluorescence quenching and its maximum emission spectrum shifting from 344 to 356 nm. The Kd of the FBPase-aldolase complex, determined on the basis of fluorescence changes, is 0.4 microM at 25 degrees C with almost 1:1 stoichiometry. This interaction increases the I(0.5) for the AMP inhibition of FBPase threefold, and slightly affects FBPase affinity to magnesium ions, increasing the Ka and Hill coefficient (n). No effect of aldolase on the FBPase pH optimum was observed. Thus, the decrease in FBPase sensitivity to AMP inhibition enables FBPase to function in vivo thanks to aldolase.

Different sensitivities of mutants and chimeric forms of human muscle and liver fructose-1,6-bisphosphatases towards AMP

AMP is an allosteric inhibitor of human muscle and liver fructose-1,6-bisphosphatase (FBPase). Despite strong similarity of the nucleotide binding domains, the muscle enzyme is inhibited by AMP approximately 35 times stronger than liver FBPase: I0.5 for muscle and for liver FBPase are 0.14 microM and 4.8 microM, respectively. Chimeric human muscle (L50M288) and chimeric human liver enzymes (M50L288), in which the N-terminal residues (1-50) were derived from the human liver and human muscle FBPases, respectively, were inhibited by AMP 2-3 times stronger than the wild-type liver enzyme. An amino acid exchange within the N-terminal region of the muscle enzyme towards liver FBPase (Lys20-->Glu) resulted in 13-fold increased I0.5 values compared to the wild-type muscle enzyme. However, the opposite exchanges in the liver enzyme (Glu20-->Lys and double mutation Glu19-->Asp/Glu20-->Lys) did not change the sensitivity for AMP inhibition of the liver mutant (I0.5 value of 4.9 microM). The decrease of sensitivity for AMP of the muscle mutant Lys20-->Glu, as well as the lack of changes in the inhibition by AMP of liver mutants Glu20-->Lys and Glu19-->Asp/Glu20-->Lys, suggest a different mechanism of AMP binding to the muscle and liver enzyme.

Fructose-1,6-bisphosphatase genes in animals

A comparison of the amino acid sequences of the liver and muscle fructose-1,6-bisphosphatase (FbPase) isoforms in primates and rodents suggested an ancient duplication event leading to the corresponding genes. We investigated the presence of both genes in the rabbit (order lagomorphs) and in species belonging to further distantly related metazoan taxa. By an analysis of the available complete genomes and proteomes of the nematode Caenorhabditis elegans and of Drosophila melanogaster only one sequence homologous to known FbPases was found in each species. The corresponding mRNAs were characterized by cDNA sequencing. We then carried out reverse transcription-polymerase chain reactions to amplify central fragments of the FbPase cDNAs from liver and muscle of Gallus gallus, Xenopus laevis, and Esox lucius, respectively. Their sequencing revealed that (i) the livers of chicken, frog, and fish contain mRNAs which are closely related to mammalian liver FbPase mRNAs, (ii) chicken muscle contains an mRNA which is most homologous to mammalian muscle FbPase mRNAs, (iii) frog muscle contains both a liver-type and a muscle-type FbPase mRNA, while (iv) in fish muscle no FbPase mRNA could be detected by our approach despite the doubtless presence of the enzyme in this organ. An alignment of the partial amino acid sequences of the different FbPases showed that the residues that are thought to be in contact with the substrate, fructose-2,6-bisphosphate, and Mg(2+) are totally conserved, while some amino acids having contact with adenosine monophosphate were found to vary among several species. The question of what might be the advantage of having more than one gene coding for FbPase per haploid genome is discussed.