Survey of fructose 1,6-bisphosphatase isoenzyme in rat organs and ontogenic expression of the enzyme in rat fetus

Fructose-1,6-Bisphosphatase 2 Inhibits Sarcoma Progression by Restraining Mitochondrial Biogenesis

The remarkable cellular and genetic heterogeneity of soft tissue sarcomas (STSs) limits the clinical benefit of targeted therapies. Here, we show that expression of the gluconeogenic isozyme fructose-1,6-bisphosphatase 2 (FBP2) is silenced in a broad spectrum of sarcoma subtypes, revealing an apparent common metabolic feature shared by diverse STSs. Enforced FBP2 expression inhibits sarcoma cell and tumor growth through two distinct mechanisms. First, cytosolic FBP2 antagonizes elevated glycolysis associated with the "Warburg effect," thereby inhibiting sarcoma cell proliferation. Second, nuclear-localized FBP2 restrains mitochondrial biogenesis and respiration in a catalytic-activity-independent manner by inhibiting the expression of nuclear respiratory factor and mitochondrial transcription factor A (TFAM). Specifically, nuclear FBP2 colocalizes with the c-Myc transcription factor at the TFAM locus and represses c-Myc-dependent TFAM expression. This unique dual function of FBP2 provides a rationale for its selective suppression in STSs, identifying a potential metabolic vulnerability of this malignancy and possible therapeutic target.

Intrinsic gluconeogenesis is enhanced in renal proximal tubules of Zucker diabetic fatty rats

Recent studies indicate that renal gluconeogenesis is substantially stimulated in patients with type 2 diabetes, but the mechanism that is responsible for such stimulation remains unknown. Therefore, this study tested the hypothesis that renal gluconeogenesis is intrinsically elevated in the Zucker diabetic fatty rat, which is considered to be an excellent model of type 2 diabetes. For this, isolated renal proximal tubules from diabetic rats and from their lean nondiabetic littermates were incubated in the presence of physiologic gluconeogenic precursors. Although there was no increase in substrate removal and despite a reduced cellular ATP level, a marked stimulation of gluconeogenesis was observed in diabetic relative to nondiabetic rats, with near-physiologic concentrations of lactate (38%), glutamine (51%) and glycerol (66%). This stimulation was caused by a change in the fate of the substrate carbon skeletons resulting from an increase in the activities and mRNA levels of the key gluconeogenic enzymes that are common to lactate, glutamine, and glycerol metabolism, i.e., mainly of phosphoenolpyruvate carboxykinase and, to a lesser extent, of glucose-6-phosphatase and fructose-1,6-bisphosphatase. Experimental evidence suggests that glucocorticoids and cAMP were two factors that were responsible for the long-term stimulation of renal gluconeogenesis observed in the diabetic rats. These data provide the first demonstration in an animal model that renal gluconeogenesis is upregulated by a long-term mechanism during type 2 diabetes. Together with the increased renal mass (38%) observed, they lend support to the view so far based only on in vivo studies performed in humans that renal gluconeogenesis may be stimulated by and crucially contribute to the hyperglycemia of type 2 diabetes.

Kinetic properties of pig (Sus scrofa domestica) and bovine (Bos taurus) D-fructose-1,6-bisphosphate 1-phosphohydrolase (F1,6BPase): liver-like isozymes in mammalian lung tissue

F1,6BPases from porcine and bovine lung were isolated and their kinetic properties were determined. Ks, Kis and beta were determined assuming partial-noncompetitive inhibition (simple intersecting hyperbolic noncompetitive inhibition) of the enzyme by the substrate. Values for Ks were 4.1 and 4.4 microM for porcine and bovine F1,6BPase, respectively and values for 1 were close to 0.55 in both cases. Kis were 9 and 15 microM for porcine and bovine F1,6BPase, respectively. I0.5 for AMP were determined as 7 microM for pig enzyme and 14 microM for F1,6BPase from bovine lung. The enzymes were inhibited by F2,6BP with Ki's of 0.19 and 0.21 microM for porcine and bovine enzymes, respectively. In the presence of AMP concentration equal to I0.5, the Ki values for pig and bovine enzymes were 0.07 and 0.09 microM, respectively. The levels of F2,6BP, AMP and antioxidant enzymes activities in pig and bovine lung tissues were also determined. The cDNA coding sequence of pig lung F1,6BPase1 showed a high homology with pig liver enzyme, differing only in four positions (G/C-63, T/A-808, G/C-884 and T/A-1005) resulting in a single amino acid substitution (Gly-295 for Ala-295). It is hypothesized that the lung F1,6BPase participates in gluconeogenesis, surfactant synthesis and antioxidant reactions.

Ribose 1,5-bisphosphate is a putative regulator of fructose 6-phosphate/fructose 1,6-bisphosphate cycle in liver

6-Phosphofructo-1-kinase and fructose-1,6-bisphosphatase are rate-limiting enzymes for glycolysis and gluconeogenesis respectively, in the fructose 6-phosphate/fructose 1,6-bisphosphate cycle in the liver. The effect of ribose 1,5-bisphosphate on the enzymes was investigated. Ribose 1,5-bisphosphate synergistically relieved the ATP inhibition and increased the affinity of liver 6-phosphofructo-1-kinase for fructose 6-phosphate in the presence of AMP. Ribose 1,5-bisphosphate synergistically inhibited fructose-1,6-bisphosphatase in the presence of AMP. The activating effect on 6-phosphofructo-1-kinase and the inhibitory effect on fructose-1,6-bisphosphatase suggest ribose 1,5-bisphosphate is a potent regulator of the fructose 6-phosphate/fructose 1,6-bisphosphate cycle in the liver.

Ribose 1,5-bisphosphate inhibits fructose-1,6-bisphosphatase in rat kidney cortex

Fructose-1,6-bisphosphatase is one of the regulatory enzymes of gluconeogenesis in kidney cortex. The effect of ribose 1,5-bisphosphate on fructose-1,6-bisphosphatase purified from rat kidney cortex was studied. Rat kidney cortex, fructose-1,6-bisphosphatase exhibited hyperbolic kinetics with regard to its substrate, but the activity was inhibited by ribose 1,5-bisphosphate at nanomolar concentrations. The inhibitory effect of ribose 1,5-bisphosphate on the fructose-1,6-bisphosphatase was enhanced in the presence of AMP, one of the inhibitors of fructose-1,6-bisphosphatase. Fructose-2,6-bisphosphate, which is an inhibitor of fructose-1,6-bisphosphatase, inhibited rat kidney cortex fructose-1,6-bisphosphatase activities at a low concentration of fructose-1,6-bisphosphate but a high concentration of fructose-1,6-bisphosphate relieved fructose-1,6-bisphosphatase from fructose-2,6-bisphosphate-dependent inhibition. On the contrary, fructose-1,6-bisphosphate was not effective for the recovery of fructose-1,6-bisphosphatase from ribose 1,5-bisphosphate-dependent inhibition. These results suggest that ribose 1,5-bisphosphate is a potent inhibitor and is involved in the regulation of fructose-1,6-bisphosphatase in rat kidney cortex.

Rat muscle fructose-1,6-bisphosphatase: cloning of the cDNA, expression of the recombinant enzyme, and expression analysis in different tissues

The 1282 bp cDNA of an isoenzyme of fructose-1,6-bisphosphatase was cloned from rat muscle. It shows 70% positional identity to the cDNA of rat liver fructose-1,6-bisphosphatase and is clearly the product of a gene different from that coding for the liver enzyme. After cloning of the coding region of the rat muscle fructose-1,6-bisphosphatase cDNA in an expression vector, the recombinant enzyme could be detected in E. coli cell-free extracts by activity determination and Western blotting. Overexpressed fructose-1,6-bisphosphatase was found to be allosterically inhibited by AMP comparably to the enzyme isolated from rat muscle. Analysis of steady-state mRNA levels of various rat tissues with reverse-transcriptase polymerase chain reaction (RT-PCR) and Northern blotting revealed one or the two fructose-1,6-bisphosphatase isoenzyme mRNAs in most tissues tested with significant quantitative differences. Quantitative PCR using a homologous competitor showed that 1 microg of total RNA of rat muscle contains 1.7 x 10(6) molecules of rat muscle fructose-1,6-bisphosphatase mRNA. 3 x 10(4) copies of this message were found per microg total RNA of heart and kidney, respectively.

cDNA sequence and kinetic properties of human lung fructose(1, 6)bisphosphatase

A cDNA encoding fructose(1,6)bisphosphatase was isolated from total human lung RNA. The cDNA contained an open reading frame encoding 337 amino acids. The determined nucleotide sequence of the lung cDNA was significantly different from muscle cDNA and slightly differed from human liver cDNA in a single mutation (Gly-336 for Ala-336) and a T for C substitution in position 648. The human lung fructose(1, 6)bisphosphatase [Fru(1,6)Pase] was isolated and its kinetic parameters were compared with liver and muscle isoenzymes. Values of kcat for the lung Fru(1,6)Pase were lower than for the liver and muscle enzyme. Like the liver isoenzyme, lung Fru(1,6)Pase is significantly less inhibited by AMP than the muscle enzyme. The values of I0.5 were 9.5, 9.8, and 0.3 microM for the liver, lung, and muscle enzyme, respectively. The lung enzyme was slightly more sensitive to fructose(2,6)bisphosphate [Fru(2,6)P2] inhibition than the liver enzyme. Ki was 75 microM for the lung and 96 microM for the liver enzyme. The synergistic effect of AMP and Fru(2,6)P2 on the lung and liver Fru(1,6)Pase was also observed. In the presence of AMP the corresponding values of Ki for Fru(2,6)P2 were 16 microM for the lung and 10 microM for the liver enzyme.

Isolation and characterization of an allelic cDNA for human muscle fructose-1,6-bisphosphatase

By applying a newly developed method, cDNAs for the human muscle isoform of fructose-1,6-bisphosphatase were isolated from phage- and plasmid-derived libraries. From these cDNAs and an EST clone, a composite sequence (1302 bp) was deduced that contains an open reading frame encoding a polypeptide of 339 amino acids with an estimated molecular weight of 36 755. After overexpression in E. coli, recombinant human muscle fructose 2,6-bisphosphatase was found to be active in cel-free extracts and could be strongly inhibited by AMP and fructose 2,6-bisphosphate. Sequence comparisons revealed that (1) all amino acids thought to be in contact with substrate molecules, regulatory molecules or metal ions in mammalian liver fructose-1,6-bisphosphatases are, with one exception, conserved in the human muscle enzyme and (2) the human muscle isoform is more homologous to the mouse intestine fructose-1,6-bisphosphatase than to the mammalian liver isoform. This is the first report of the cloning and expression of a muscle fructose-1,6-bisphosphatase isoenzyme.

Induction and turnover of fructose 1,6-bisphosphatase in HL-60 leukemia cells by calcitriol

Fructose 1,6-bisphosphatase mRNA and enzyme activity in HL-60 cells were rapidly and markedly induced by calcitriol (formerly known as 1 alpha,25-dihydroxyvitamin D3). The activity reached 70-80 times the basal level after 96 h. The enzyme activity in the cells incubated for 96 h with calcitriol decreased immediately after its withdrawal but after a 24-h incubation the activity in the cells continued to increase slightly and then decreased slowly. Calcitriol increased the enzyme activity dose-dependently with maximal stimulation at 10 nM and half-maximal at 2.1 nM. The rate of synthesis of fructose 1,6-bisphosphatase almost paralleled the increase in mRNA level during treatment with calcitriol. When calcitriol was removed from media after incubation for either 24 h or 96 h, fructose-1,6-bisphosphatase mRNA and fructose-1,6-bisphosphatase synthesis decreased rapidly to the basal level. The enzyme was only slightly degraded in the cells incubated with calcitriol for 24 h followed by the subsequent culture without calcitriol but it was degraded with a half-life estimated to be approximately 64 h in the same cells followed by culturing with calcitriol. In the cells incubated for 96 h, the same degradation rate (i.e. half-life approximately 64 h) was observed irrespective of the following culture with or without calcitriol. Calcitriol did not affect the degradation rate of total soluble proteins.

Regulation of rat-kidney cortex fructose-1,6-bisphosphatase activity. II. Effects of adenine nucleotides

1. The native rat-kidney cortex Fructose-1,6-bisphosphatase is differentially regulated by adenine nucleotides in the presence of divalent cations. 2. Binding of AMP and ADP to the enzyme is co-operative. The inhibition by both nucleotides show an uncompetitive mechanism AMP being the most efficient inhibitor. 3. Mg2+ decreases the inhibition produced by AMP and ADP by enhancing their I0.5 and completely annulates the inhibitory effect of ATP. 4. In the presence of Mn2+ ADP behaves as an inhibitor but no inhibition is evident with AMP, suggesting the existence of different allosteric sites for each nucleotide.

Regulation of rat-kidney cortex fructose-1,6-bisphosphatase activity. I. Effects of fructose-2,6-bisphosphate and divalent cations

1. The native rat-kidney cortex Fructose-1,6-BPase is differentially regulated by Mg2+ and Mn2+. 2. Mg2+ binding to the enzyme is hyperbolic and large concentrations of the cation are non-inhibitory. 3. Mn2+ produces a 10-fold rise in Vmax higher than Mg2+. [Mn2+]0.5 is much larger than [Mg2+]0.5. At elevated [Mn2+] inhibition is observed. 4. Mg2+ and Mn2+ produce antagonistic effects on the inhibition of the enzyme by high substrate. 5. Fru-2,6-P2 inhibits the enzyme by rising the S0.5 and favouring a sigmoidal kinetics. 6. The inhibition by Fru-2,6-P2 is released by Mg2+ and more powerfully by Mn2+ increasing the I0.5.