The role of fructose 2,6-bisphosphate in the regulation of carbohydrate metabolism

Hyperglycemia induces PFKFB3 overexpression and promotes malignant phenotype of breast cancer through RAS/MAPK activation

BACKGROUND

Breast cancer is the most common tumor in women worldwide. Diabetes mellitus is a global chronic metabolic disease with increasing incidence. Diabetes mellitus has been reported to positively regulate the development of many tumors. However, the specific mechanism of hyperglycemic environment regulating breast cancer remains unclear. PFKFB3 (6-phosphofructose-2-kinase/fructose-2, 6-bisphosphatase 3) is a key regulatory factor of the glycolysis process in diabetes mellitus, as well as a promoter of breast cancer. So, we want to explore the potential link between PFKFB3 and the poor prognosis of breast cancer patients with hyperglycemia in this study.

METHODS

Cell culture was utilized to construct different-glucose breast cancer cell lines. Immunohistochemistry was adopted to analyze the protein level of PFKFB3 in benign breast tissues, invasive ductal carcinoma with diabetes and invasive ductal carcinoma without diabetes. The Kaplan-Meier plotter database and GEO database (GSE61304) was adopted to analyze the survival of breast cancer patients with different PFKFB3 expression. Western blot was adopted to analyze the protein level of PFKFB3, epithelial-mesenchymal transition (EMT)-related protein and extracellular regulated protein kinases (ERK) in breast cancer cells. Gene Set Cancer Analysis (GSCA) was utilized to investigate the potential downstream signaling pathways of PFKFB3. TargetScan and OncomiR were utilized to explore the potential mechanism of PFKFB3 overexpression by hyperglycemia. Transfections (including siRNAs and miRNA transfection premiers) was utilized to restrain or mimic the expression of the corresponding RNA. Cell functional assays (including cell counting, MTT, colony formation, wound-healing, and cell migration assays) were utilized to explore the proliferation and migration of breast cancer cells.

RESULTS

In this study, we demonstrated that the expression of PFKFB3 in breast cancer complicated with hyperglycemia was higher than that in breast cancer with euglycemia through cell experiment in vitro and histological experiment. PFKFB3 overexpression decreased the survival period of breast cancer patients and was correlated with a number of clinicopathological parameters of breast cancer complicated with diabetes. PFKFB3 promoted the proliferation and migration of breast cancer in a hyperglycemic environment and might be regulated by miR-26. In addition, PFKFB3 stimulated epithelial-mesenchymal transition of breast cancer in a hyperglycemic environment. In terms of downstream mechanism exploration, we predicted and verified the cancer-promoting effect of PFKFB3 in breast cancer complicated with hyperglycemia through RAS/MAPK pathway.

CONCLUSIONS

In conclusion, PFKFB3 could be overexpressed by hyperglycemia and might be a potential therapeutic target for breast cancer complicated with diabetes.

Molecular basis of the fructose-2,6-bisphosphatase reaction of PFKFB3: transition state and the C-terminal function

The molecular basis of fructose-2,6-bisphosphatase (F-2,6-P(2)ase) of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFKFB) was investigated using the crystal structures of the human inducible form (PFKFB3) in a phospho-enzyme intermediate state (PFKFB3-P•F-6-P), in a transition state-analogous complex (PFKFB3•AlF(4)), and in a complex with pyrophosphate (PFKFB3•PP(i)) at resolutions of 2.45, 2.2, and 2.3 Å, respectively. Trapping the PFKFB3-P•F-6-P intermediate was achieved by flash cooling the crystal during the reaction, and the PFKFB3•AlF(4) and PFKFB3•PP(i) complexes were obtained by soaking. The PFKFB3•AlF(4) and PFKFB3•PP(i) complexes resulted in removing F-6-P from the catalytic pocket. With these structures, the structures of the Michaelis complex and the transition state were extrapolated. For both the PFKFB3-P formation and break down, the phosphoryl donor and the acceptor are located within ~5.1 Å, and the pivotal point 2-P is on the same line, suggesting an "in-line" transfer with a direct inversion of phosphate configuration. The geometry suggests that NE2 of His253 undergoes a nucleophilic attack to form a covalent N-P bond, breaking the 2O-P bond in the substrate. The resulting high reactivity of the leaving group, 2O of F-6-P, is neutralized by a proton donated by Glu322. Negative charges on the equatorial oxygen of the transient bipyramidal phosphorane formed during the transfer are stabilized by Arg252, His387, and Asn259. The C-terminal domain (residues 440-446) was rearranged in PFKFB3•PP(i), implying that this domain plays a critical role in binding of substrate to and release of product from the F-2,6-P(2) ase catalytic pocket. These findings provide a new insight into the understanding of the phosphoryl transfer reaction.

Endogenous activation of glucokinase by 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase is glucose dependent

Glucokinase (GK) plays a crucial role as glucose sensor in glucose-induced insulin secretion in pancreatic β-cells. The bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) acts as an endogenous GK activator. Therefore, the goal of this study was the analysis of GK-PFK-2/FBPase-2 complex formation and its effect on metabolic stimulus-secretion coupling in β-cells in dependence upon glucose. The interaction between GK and PFK-2/FBPase-2 was analyzed in insulin-secreting MIN6 cells with a new fluorescence-based mammalian two-hybrid system. In contrast to the commonly used mammalian two-hybrid systems that require sampling before detection, the system used allows monitoring of the effects of environmental changes on protein-protein interactions on the single-cell level. Increasing the glucose concentration in the cell culture medium from 3 to 10 and 25 mmol/liter amplified the interaction between the enzymes stepwise. Importantly, in line with these results, overexpression of PFK-2/FBPase-2 in MIN6 cells evoked only at 10 and 25 mmol/liter, an increase in insulin secretion. Furthermore, a PFK-2/FBPase-2 mutant with an abolished GK-binding motif neither showed a glucose-dependent GK binding nor was able to increase insulin secretion. The results obtained with the mammalian two-hybrid system could be confirmed by fluorescence resonance energy transfer experiments in COS cells. Furthermore, the established interaction between GK and the liver GRP served in all experiments as a control. Thus, this study clearly showed that binding and activation of GK by PFK-2/FBPase-2 in β-cells is promoted by glucose, resulting in an enhancement of insulin secretion at stimulatory glucose concentrations, without affecting basal insulin secretion.

Effect of acetic acid feeding on the circadian changes in glycogen and metabolites of glucose and lipid in liver and skeletal muscle of rats

The aim of the present study is to investigate the effect of acetic acid feeding on the circadian changes in glycogen concentration in liver and skeletal muscle. Rats were provided meal once daily (09.00–13.00 hours) for 10d. On the 11th day, they were either killed immediately or given 9g diet containing either 0 (control) or 0·7g/kg-diet acetic acid beginning at 09.00 hours for 4h, as in the previous regimen. Rats in the fed group were killed at 4, 8 or 24h after the start of feeding. At 4h after the start of feeding, the acetic acid group had significantly greater liver and gastrocnemius muscle glycogen concentrations (P<0·05). Also, at this same point, liver xylulose-5-phosphate, a key stimulator of glycolysis, the ratio of fructose-1,6-bisphosphate to fructose-6-phosphate in skeletal muscle, which reflects phosphofructokinase-1 activity, and liver malonyl-CoA, an allosteric inhibitor of carnitine palmitoyl-transferase, were significantly lower in the acetic acid group than in the control group (P<0·05). In addition, the acetic acid group had a significantly lower serum lactate concentration and lower ratio of insulin to glucagon than the control group at the same point (P<0·05). We conclude that a diet containing acetic acid may enhance glycogen repletion but not induce supercompensation, a large increase in the glycogen level that is beneficial in improving performance, in liver and skeletal muscle by transitory inhibition of glycolysis. Further, we indicate the possibility of a transient enhancement of fatty acid oxidation in liver by acetic acid feeding.

Metabolic control of glucose degradation in yeast and tumor cells

Regulation of glucose degradation in both yeasts and tumor cells is very similar in many respects. In both cases it leads to excretion of intermediary metabolites (e.g., ethanol, lactate) in those cell types where uptake of glucose is unrestricted (Saccharomyces cerevisiae, Bowes melanoma cells). The similarities between glucose metabolism observed in yeast and tumor cells is explained by the fact that cell transformation of animal cells leads to inadequate expression of (proto-)oncogenes, which force the cell to enter the cell cycle. These events are accompanied by alterations at the signal transduction level, a marked increase of glucose transporter synthesis, enhancement of glycolytic key enzyme activities, and slightly reduced respiration of the tumor cell. In relation to homologous glucose degradation found in yeast and tumor cells there exist strong similarities on the level of cell division cycle genes, signal transduction and regulation of glycolytic key enzymes. It has been demonstrated that ethanol and lactate excretion in yeast and tumor cells, respectively, result from an overflow reaction at the point of pyruvate that is due to a carbon flux exceeding the capacity of oxidative breakdown. Therefore, the respiratory capacity of a cell determines the amount of glycolytic breakdown products if ample glucose is available. This restricted flux is also referred to as the respiratory bottleneck. The expression "catabolite repression", which is often used in textbooks to explain ethanol and acid excretion, should be abandoned, unless specific mechanisms can be demonstrated. Furthermore, it was shown that maximum respiration and growth rates are only obtained under optimum culture conditions, where the carbon source is limiting.

Mimicry of a cellular low energy status blocks tumor cell anabolism and suppresses the malignant phenotype

Aggressive cancer cells typically show a high rate of energy-consuming anabolic processes driving the synthesis of lipids, proteins, and DNA. Here, we took advantage of the ability of the cell-permeable nucleoside 5-aminoimidazole-4-carboxamide (AICA) riboside to increase the intracellular levels of AICA ribotide, an AMP analogue, mimicking a low energy status of the cell. Treatment of cancer cells with AICA riboside impeded lipogenesis, decreased protein translation, and blocked DNA synthesis. Cells treated with AICA riboside stopped proliferating and lost their invasive properties and their ability to form colonies. When administered in vivo, AICA riboside attenuated the growth of MDA-MB-231 tumors in nude mice. These findings point toward a central tie between energy, anabolism, and cancer and suggest that the cellular energy sensing machinery in cancer cells is an exploitable target for cancer prevention and/or therapy.

Effects of hyperglycemia on hepatic gluconeogenic flux during glycogen phosphorylase inhibition in the conscious dog

The aim of these studies was to investigate the effect of hyperglycemia with or without hyperinsulinemia on hepatic gluconeogenic flux, with the hypothesis that inhibition would be greatest with combined hyperglycemia/hyperinsulinemia. A glycogen phosphorylase inhibitor (BAY R3401) was used to inhibit glycogen breakdown in the conscious overnight-fasted dog, and the effects of a twofold rise in plasma glucose level (HI group) accompanied by 1) euinsulinemia (HG group) or 2) a fourfold rise in plasma insulin were assessed over a 5-h experimental period. Hormone levels were controlled using somatostatin with portal insulin and glucagon infusion. In the HG group, net hepatic glucose uptake and net hepatic lactate output substantially increased. There was little or no effect on the net hepatic uptake of gluconeogenic precursors other than lactate (amino acids and glycerol) or on the net hepatic uptake of free fatty acids compared with the control group. Consequently, whereas hyperglycemia had little effect on gluconeogenic flux to glucose 6-phosphate (G-6-P), net hepatic gluconeogenic flux was reduced because of increased hepatic glycolytic flux during hyperglycemia. Net hepatic glycogen synthesis was increased by hyperglycemia. The effect of hyperglycemia on gluconeogenic flux to G-6-P and net hepatic gluconeogenic flux was similar. We conclude that, in the absence of appreciable glycogen breakdown, the increase in glycolytic flux that accompanies hyperglycemia results in decreased net carbon flux to G-6-P but no effect on gluconeogenic flux to G-6-P.

Interaction of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) with glucokinase activates glucose phosphorylation and glucose metabolism in insulin-producing cells

The bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) was recently identified as a new intracellular binding partner for glucokinase (GK). Therefore, we studied the importance of this interaction for the activity status of GK and glucose metabolism in insulin-producing cells by overexpression of the rat liver and pancreatic islet isoforms of PFK-2/FBPase-2. PFK-2/FBPase-2 overexpression in RINm5F-GK cells significantly increased the GK activity by 78% in cells expressing the islet isoform, by 130% in cells expressing the liver isoform, and by 116% in cells expressing a cAMP-insensitive liver S32A/H258A double mutant isoform. Only in cells overexpressing the wild-type liver PFK-2/FBPase-2 isoform was the increase of GK activity abolished by forskolin, apparently due to the regulatory site for phosphorylation by a cAMP-dependent protein kinase. In cells overexpressing any isoform of the PFK-2/FBPase-2, the increase of the GK enzyme activity was antagonized by treatment with anti-FBPase-2 antibody. Increasing the glucose concentration from 2 to 10 mmol/l had a significant stimulatory effect on the GK activity in RINm5F-GK cells overexpressing any isoform of PFK-2/FBPase-2. The interaction of GK with PFK-2/FBPase-2 takes place at glucose concentrations that are physiologically relevant for the activation of GK and the regulation of glucose-induced insulin secretion. This new mechanism of posttranslational GK regulation may also represent a new site for pharmacotherapeutic intervention in type 2 diabetes treatment.

Pyruvate kinase from Chlamydia trachomatis is activated by fructose-2,6-bisphosphate

Pyruvate kinase is the final regulatory point in the catabolic Embden-Meyerhoff-Parnas pathway, which controls the carbon flux of glycolytic intermediates and regulates the level of ATP in the cell. In a previous study, we identified, cloned and sequenced pyruvate kinase from the obligate intracellular bacterium Chlamydia trachomatis and demonstrated that the enzyme was active in crude extract. Here, we report the kinetic properties of highly purified C. trachomatis pyruvate kinase. The results indicate that C. trachomatis pyruvate kinase is 53.5 kDa with a pH optima of 7.3. Kinetic studies show that C. trachomatis pyruvate kinase requires both K+ and Mg2+ ions for activity, exhibits sigmoidal kinetics with respect to phosphoenolpyruvate and Michaelis-Menten kinetics with respect to ADP. In addition, C. trachomatis pyruvate kinase is able to use alternative nucleoside diphosphates as phosphate acceptors, although it shows the greatest activity with ADP. In contrast to other bacterial pyruvate kinases that are activated by AMP, our data show that AMP, in addition to ATP and GTP, inhibits C. trachomatis pyruvate kinase. Surprisingly, unlike any other known bacterial pyruvate kinase, C. trachomatis pyruvate kinase was allosterically activated by fructose-2,6-bisphosphate, an important regulatory metabolite that has only been reported in eukaryotes.

Acetic acid feeding enhances glycogen repletion in liver and skeletal muscle of rats

To investigate the efficacy of the ingestion of vinegar in aiding recovery from fatigue, we examined the effect of dietary acetic acid, the main component of vinegar, on glycogen repletion in rats. Rats were allowed access to a commercial diet twice daily for 6 d. After 15 h of food deprivation, they were either killed immediately or given 2 g of a diet containing 0 (control), 0.1, 0.2 or 0.4 g acetic acid/100 g diet for 2 h. The 0.2 g acetic acid group had significantly greater liver and gastrocnemius muscle glycogen concentration than the control group (P < 0.05). The concentrations of citrate in this group in both the liver and skeletal muscles were >1.3-fold greater than in the control group (P > 0.1). In liver, the concentration of xylulose-5-phosphate in the control group was significantly higher than in the 0.2 and 0.4 g acetic acid groups (P < 0.01). In gastrocnemius muscle, the concentration of glucose-6-phosphate in the control group was significantly lower and the ratio of fructose-1,6-bisphosphate/fructose-6-phosphate was significantly higher than in the 0.2 g acetic acid group (P < 0.05). This ratio in the soleus muscle of the acetic acid fed groups was <0.8-fold that of the control group (P > 0.1). In liver, acetic acid may activate gluconeogenesis and inactivate glycolysis through inactivation of fructose-2,6-bisphosphate synthesis due to suppression of xylulose-5-phosphate accumulation. In skeletal muscle, acetic acid may inhibit glycolysis by suppression of phosphofructokinase-1 activity. We conclude that a diet containing acetic acid may enhance glycogen repletion in liver and skeletal muscle.

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.

Tolbutamide inhibits glucagon-induced phosphorylation of 6PF-2-K/Fru-2,6-P2ase in rat hepatocytes

In previous studies, we demonstrated that tolbutamide inhibits a phosphorylation of hepatic 6-phosphofructo-2-kinase (6PF-2-K)/fructose-2,6-bisphosphatase (Fru-2,6-P2ase) catalyzed by the adenosine 3',5'-cyclic monophosphate-dependent protein kinase in a reconstruction system using the purified enzyme from the rat liver. In the current study, to assess a role of tolbutamide on hepatic 6PF-2-K/Fru-2,6-P2ase physiologically, we used intact rat hepatocytes and examined effects of tolbutamide on a phosphorylation of the bifunctional enzyme in the presence of glucagon. Glucagon induced a rapid phosphorylation of hepatic 6PF-2-K/Fru-2,6-P2ase accompanied by an inhibition of 6PF-2-K activity and a stimulation of Fru-2,6-P2ase activity in a dose-dependent manner. Tolbutamide inhibited glucagon-induced phosphorylation of the bifunctional enzyme protein in a dose-dependent manner. By adding 2 mM tolbutamide, reduced activity of 6PF-2-K and increased activity of Fru-2,6-P2ase in the presence of 10(-9) M glucagon were partially restored. The present results suggest the possibility that tolbutamide modulates the activity of hepatic 6PF-2-K/Fru-2,6-P2ase through inhibiting a phosphorylation of the enzyme protein. The counterregulatory influence of tolbutamide on the effect of glucagon suggests a possible mechanism for the extrapancreatic effect of sulfonylurea drugs.

Regional distribution of glycogen, glucose and phosphorylated sugars in rat brain after intoxicating doses of ethanol

Ethanol and anaesthetics increase glycogen levels in the brain. However, no data have been reported about the effect of ethanol on glycogen and glucose metabolism in specific brain regions. We have studied the concentrations of glycogen, glucose, glucose 6-P, glucose 1,6-P2 and fructose 2,6-P2 and the activities of glycogen synthase, glycogen phosphorylase and glycogen phosphorylase kinase in seven brain regions of starved rats following treatment with a single dose or several doses of ethanol. Our results show that: (1) the effect of ethanol on glucose metabolism depends on whether it is given in one single dose or in a series of doses; (2) glycogen concentration increases after a single dose of ethanol but not after long exposure; (3) glucose, glucose 6-P in some areas, and the bisphosphorylated sugar, fructose 2,6-P2 significantly increase after prolonged exposure to ethanol; and (4) the enzymatic activities of glycogen metabolism are not modified after a long exposure to ethanol. In summary, these data show that ethanol may modify the use of glycogen, glucose and derivatives in brain. Moreover, the changes produced depend on the pattern of ethanol intake and the brain area considered.

Citrate inhibition of rat-kidney cortex phosphofructokinase

The regulatory properties of citrate on the activity of phosphofructokinase (PFK) purified from rat-kidney cortex has been studied. Citrate produces increases in the K0.5 for Fru-6-P and in the Hill coefficient as well as a decrease in the Vmax of the reaction without affecting the kinetic parameters for ATP as substrate. ATP potentiates synergistically the effects of citrate as an inhibitor of the enzyme. Fru-2,6-P2 and AMP at concentrations equal to Ka were not able to completely prevent citrate inhibition of the enzyme. Physiological concentrations of ATP and citrate produce a strong inhibition of renal PFK suggesting that may participate in the control of glycolysis in vivo.

Glycolysis revisited

Glycolysis is usually considered as a paradigm metabolic pathway, due to the fact that it is present in most organisms, and also because it is the pathway by which an important nutrient, glucose, is consumed. Far from being completely understood, the regulation of this pathway witnessed several important progresses during the last few years. One of these is the discovery of fructose 2,6-bisphosphate, a potent stimulator of phosphofructokinase and inhibitor of fructose-1,6-bisphosphatase. Originally found in the liver during the course of a study on the mechanism by which glucagon acts on gluconeogenesis, this compound is now recognized as a major element in the control of glycolysis and/or gluconeogenesis in many cell types and in various organisms. The other finding is that of a regulatory protein that modulates the activity of glucokinase, the enzyme that phosphorylates glucose in the liver and in the beta cells of pancreatic islets.

Fructose 2,6-bisphosphate: changes during neonatal maturation and aging of rat and potential role in regulation of glucose utilization

During the 6 days following birth, tissue levels of fructose-2,6-P2 in rat brain, liver, muscle, heart and kidney did not significantly change. However, by the tenth day postpartum fructose-2,6-P2 levels in brain, heart, and skeletal muscle increased approximately 50% and attained adult values. During maturation of liver, adult levels of fructose-2,6-P2 were not achieved until 3-4 weeks after birth or approximately at the time of maximum rates of gluconeogenesis. Renal fructose-2,6-P2 levels in the neonate were initially elevated and 2-3 weeks after birth decreased approximately 2.5-fold to adult values. With the exception of the pons-medulla, which showed no significant changes in fructose-2,6-P2 amounts, levels of this regulatory sugar from aging brain regions were generally decreased. The fructose-2,6-P2 levels from heart atria of old rats (24-30 month) were also significantly decreased. In diaphragm, the fructose-2,6-P2 levels were increased at 12 months of age and at 27 months of age were twice the level at 3 months. The fructose-2,6-P2 levels during the aging of liver, skeletal muscle (EDL and soleus), spleen, thymus, kidney, testis and lung were not significantly altered.

Regulation of brain 6-phosphofructo-1-kinase: effects of aging, fructose-2,6-bisphosphate, and regional subunit distribution

Total 6-phosphofructo-1-kinase (PFK) activity, amounts of each type of PFK subunit, and levels of fructose-2,6-P2 in the cerebral cortex, midbrain, pons-medulla, and cerebellum of 3, 12, and 25 month rats were measured. Further, the role of fructose-2,6-P2 in the regulation of brain PFK activity was examined. A positive correlation was found to exist between the reported losses of glucose utilization as measured by 2-deoxy-D-glucose uptake and PFK activity in each region. That is, both parameters decreased to their lowest level by 12 months of age and remained decreased and fairly constant thereafter. Fructose-2,6-P2 levels did not appear to directly correlate with regional changes in glucose utilization. Also, region-specific and age-related alterations of the PFK subunits were found although these changes apparently did not correlate with decreased glucose utilization. Brain PFK is apparently saturated with fructose-2,6-P2 due to the high endogenous levels, and it contains a large proportion of the C-type subunit which dampens catalytic efficiency. Consequently, brain PFK could exist in a conformational state such that it can readily consume fructose-6-P rather than in an inhibited state requiring activation. This may explain, in part, the ability of brain to efficiently but conservatively utilize available glucose in energy production.

Substrates and the regulation of hepatic glycogen metabolism

Glycogen metabolism is a complex process which depends on the metabolic circumstances and the hormonal milieu. In this overview an intriguing new possibility has been emphasized--the possible central role of lactate in coordinating, with glucose, the net synthesis of glycogen. Since lactate changes acutely under many physiological circumstances, it would be a logical candidate for a signal which communicates to the liver the metabolic states of the periphery. It would then acutely determine the synthetic rate of glycogen synthesis within the range determined by the glucose concentrations which in turn could be said to reflect the nutritional state of the system. Interestingly, after oral glucose loading, portal glucose levels would be about 25% higher (Radziuk et al., 1978) relative to arterial. As seen from Figs 8 and 9 however the glycogen synthetic rate appears very sensitive to glucose (at a given lactate uptake). Everything else being assumed equal therefore, more glycogen would be synthesized than during intravenous loading with an equivalent peripheral concentration. This is indeed the case (Shulman and Rossetti, 1989). On the other hand, during equivalent loads, peripheral glucose levels are higher and the same quantity of glycogen is synthesized (Radziuk, 1989a, 1989b). If lactate is typical of other glucogenic substrates, then it is also logical that mixed meals with higher levels of portal substrate would maximize glycogen synthetic rates. Similarly, in diabetes where hyperglycemia and hyperlactatemia prevail, gluconeogenesis plays a predominant role in glycogen synthesis (Giaccari and Rossetti, 1992).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of tolbutamide on fructose-2,6-bisphosphate formation and ketogenesis in hepatocytes from diabetic rats

To assess the extrapancreatic action of sulfonylurea directly in the diabetic, effects of tolbutamide on hepatocyte fructose-2,6-bisphosphate (F-2,6-P2) formation and ketone production were investigated using isolated hepatocytes from streptozotocin (STZ)-induced diabetic rats. The basal level of hepatocyte F-2,6-P2 was significantly higher in diabetic rats within 2 weeks after STZ (40 mg/kg body weight) injection compared with that in the nondiabetic control group. Ultimately, a marked decrease in the F-2,6-P2 level was observed at 4 weeks after STZ administration (10% of the control). Although the addition of tolbutamide further increased the hepatocyte F-2,6-P2 level during the first week after STZ injection, no significant effect was observed after the second week and on from the initial STZ. Treatment of diabetes with insulin restored the stimulatory effect of tolbutamide on the hepatocyte F-2,6-P2 formation. Tolbutamide, independently of insulin treatment, lowered the ketone production of hepatocytes from diabetic rats. The present results indicate that insulin is necessary, in advance, for sulfonylurea to stimulate the liver F-2,6-P2 formation, while tolbutamide inhibition of hepatocyte ketone production is independent of insulin. These results provide further support for the role of sulfonylurea in regulating hepatic energy metabolism in the diabetic.

Identification and cloning of yeast phosphofructokinase 2

Fructose-6-phosphate 2-kinase ('phosphofructokinase 2') was purified from a strain of Saccharomyces cerevisiae lacking fructose-6-phosphate 1-kinase. After chromatography on DEAE-Sephacel, Sephacryl blue, CM-Sephadex and rechromatography on CM-Sephadex with fructose-6-phosphate elution, the specific activity was 1.6 U/mg protein. Although the latter value is high for fructose-6-phosphate 2-kinase, as was the purification factor of 3 x 10(4), staining with Coomassie blue showed the fraction to still contain many proteins. Incubation with [gamma-32P]ATP and the catalytic subunit of cAMP-dependent protein kinase gave a further increase in specific activity and labeling of, only, 96-kDa and 93-kDa polypeptides. Antiserum raised against these polypeptides recognized them in an immunoblot and could be used to remove the enzyme activity from crude extracts. Tryptic peptide profiles were obtained from about 10 pmol of the 96-kDa and 93-kDa polypeptides. The profiles were similar and sequencing allowed construction of mixed probes and identification of a putative single structural gene. Returned to yeast on a multicopy plasmid, phosphofructokinase 2 activity was considerably above the wild-type level, as was polypeptide revealed by immunoblotting.

Expression of rat liver fructose-1,6-bisphosphatase in Escherichia coli

Rat liver fructose-1,6-bisphosphatase was expressed in Escherichia coli using a T7 RNA polymerase-transcribed expression system. Maximum yields of soluble active enzyme were obtained when the bacterial host cell, BL21(DE3), carrying the expression plasmid was grown and transcription induced in LB medium at 37 degrees C. Approximately 20mg of fructose-1,6-bisphosphatase are synthesized per liter of culture after 4hr, of which about 10mg are soluble and enzymatically active. Expressed fructose-1,6-bisphosphatase, purified to homogeneity by substrate elution from a carboxymethyl Sephadex column, was indistinguishable from that purified from rat liver in terms of subunit size and kinetic properties. The in vitro expression of fructose-1,6-bisphosphatase in an heterologous system is a necessary preliminary step for future studies on site-directed mutant enzyme forms.

Kinetic characterization of phosphofructokinase isolated from rat kidney cortex

1. Phosphofructokinase from rat kidney cortex has been purified by affinity chromatography to a final specific activity of 15 units per mg of protein, measured at 25 degrees C and pH 8. 2. This lower spec. act., compared with that of the enzyme from other sources, shows the enzyme in proximal tubules to be less active, which would account for the main gluconeogenic role of these nephron sections. 3. The binding of fructose-6-phosphate to the enzyme is co-operative. ATP increases the Hill coefficient and produces a marked allosteric inhibition on the activity. 4. Fructose-2,6-bis-phosphate is a potent activator of the enzyme from this source. It reduces the Hill coefficient of the enzyme and the inhibition constant of ATP. A marked difference between this and the liver enzyme is that the activation is not co-operative.

Expression of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase and its kinase domain in Escherichia coli

The rat liver bifunctional enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (ATP:D-fructose-6-phosphate 2-phosphotransferase/D-fructose-2,6-bisphosphate 2-phosphohydrolase, EC 2.7.1.105/EC 3.1.3.46) and its separate kinase domain were expressed in Escherichia coli by using an expression system based on bacteriophage T7 RNA polymerase. The bifunctional enzyme (470 residues per subunit) was efficiently expressed as a protein that starts with the initiator methionine residue and ends at the carboxyl-terminal tyrosine residue. The expressed protein was purified to homogeneity by anion exchange and Blue Sepharose chromatography and had kinetic and physical properties similar to the purified rat liver enzyme, including its behavior as a dimer during gel filtration, activation of the kinase by phosphate and inhibition by alpha-glycerol phosphate, and mediation of the bisphosphatase reaction by a phosphoenzyme intermediate. The expressed 6-phosphofructo-2-kinase also started with the initiator methionine but ended at residue 257. The partially purified kinase domain was catalytically active, had reduced affinities for ATP and fructose 6-phosphate compared with the kinase of the bifunctional enzyme, and had no fructose-2,6-bisphosphatase activity. The kinase domain also behaved as an oligomeric protein during gel filtration. The expression of an active kinase domain and the previous demonstration of an actively expressed bisphosphatase domain provide strong support for the hypothesis that the hepatic enzyme consists of two independent catalytic domains encoded by a fused gene.

Protective role of fructose 1,6-bisphosphate during CCl4 hepatotoxicity in rats

Rats were injected intraperitoneally with CCl4 (2.5 ml/kg body wt.) and the hepatotoxicity was compared with that of rats receiving the same dose of CCl4 and an intraperitoneal injection of fructose 1,6-bisphosphate (2 g/kg body wt.). A 50-70% decrease in plasma aspartate aminotransferase and alanine aminotransferase activities was observed in the latter treatment, indicating a protective role of the sugar bisphosphate in CCl4 hepatotoxicity. The protection was accompanied by elevated hepatic activities of ornithine decarboxylase at 2, 6 and 24 h, S-adenosylmethionine decarboxylase at 6 h, and spermidine N1-acetyltransferase at 2 h. The increase in the enzymes involved in polyamine metabolism was shown in our previous work [Rao, Young & Mehendale (1989) J. Biochem. Toxicol. 4, 55-63] to correlate with increased polyamine synthesis or interconversion, which was related to the extent of hepatocellular regeneration. The hepatic contents of fructose 1,6-bisphosphate and ATP significantly decreased after CCl4 treatment, and administration of the sugar bisphosphate increased hepatic ATP. Fructose 1,6-bisphosphate, an intermediary metabolite of the glycolytic pathway, may decrease CCl4 toxicity by increasing the ATP in the hepatocytes. The ATP generated is useful for hepatocellular regeneration and tissue repair, events which enable the liver to overcome CCl4 injury.

Sequence of the 5'-flanking region of the rat 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase gene: regulation by glucocorticoids

Dexamethasone addition to cultured hepatocytes caused a 90-fold increase in mRNA for 6-phosphofructo 2-kinase/fructose-2,6-bisphosphatase. Glucocorticoid administration in vivo also increased the enzyme's mRNA in skeletal muscle by 3-4-fold. The sequence of the 5'-flanking region of the enzyme's gene revealed at least one consensus glucocorticoid response element. The amino acid sequence derived from a partial cDNA clone for the rat skeletal muscle bifunctional enzyme was identical to that of the liver isozyme except for an undetermined amount of N-terminal sequence. It is concluded that the rat muscle and liver isozymes, which are postulated to be identical except for the N-terminal region, are both regulated by glucocorticoids.

The regulation of 6-phosphofructo-1-kinase by insulin and glucagon in isolated hepatocytes of the American eel

Kinetic characteristics of American eel liver 6-phosphofructo-1-kinase (PFK-1) and the effects of porcine insulin, bovine glucagon, and dibutyryl-cAMP were studied. At 0.1 mM ATP, kinetics were sigmoidal with respect to fructose-6-phosphate (F-6-P) concentrations and the S0.5 (F-6-P) increased with higher ATP concentrations. At 2 mM F-6-P, optimal ATP concentrations were 0.1 mM, with maximal inhibition at 0.5 mM. Fructose 2,6-bisphosphate (Fru-2,6-P2) offset ATP inhibition and activated the enzyme, changing F-6-P kinetic curves from sigmoidal to hyperbolic. At 2 mM F-6-P and 0.1 mM ATP the Fru-2,6-P2 activation curve was hyperbolic with a Ka of approximately 1 microM. In isolated hepatocytes, porcine insulin decreased the sensitivity of PFK-1 to ATP, an effect that was offset when bovine glucagon was also present. Insulin, alone and with glucagon, increased the Fru-2,6-P2 activation ratio. In the presence of glucagon, insulin increased Fru-2,6-P2 concentrations in hepatocytes. These effects suggest that PFK-1 is a potential regulatory point for hormones in the control of carbohydrate metabolism in the American eel liver.

6-Phosphofructo-2-kinase and fructose-2,6-bisphosphatase from Saccharomyces cerevisiae

In permeabilized yeast cells 6-phosphofructo-2-kinase and fructose-2,6-bisphosphatase are studied during growth. It is shown that in yeast at least two fructose 2,6-bisphosphate degrading enzyme activities occur, differing in pH profile and in their substrate affinities. The activities of 6-phosphofructo-2-kinase and of fructose-2,6-bisphosphatases drop in the exponential and the transition phase while the activity of the alkaline phosphatases steadily increases. In the stationary phase the activities of 6-phosphofructo-2-kinase and of the low Km fructose-2,6-bisphosphatase increase again. Yeast 6-phosphofructo-2-kinase and fructose-2,6-bisphosphatase were purified and separated from each other. The purified 6-phosphofructo-2-kinase was found to exhibit a very high specific activity (1.3 U/mg). The enzyme is efficiently inhibited by ATP. The ATP inhibition is most pronounced at low concentrations of magnesium and fructose-6-phosphate. Phosphoenolpyruvate and sn-glycerol 3-phosphate are inhibitors of the enzyme. The high-affinity yeast fructose-2,6-bisphosphatase releases inorganic phosphate from the 2-position of fructose 2,6-bisphosphate. It displays hyperbolic kinetics towards fructose 2,6-bisphosphate (Km = 0.3 microM) and is strongly inhibited by fructose 6-phosphate. The inhibition is counteracted by sn-glycerol 3-phosphate. The enzyme is shown to be inactivated by cAMP-dependent phosphorylation and reactivated by the action of protein phosphatase 2A.

Gluconeogenesis in the amphibian retina. Lactate is preferred to glutamate as the gluconeogenic precursor

The capacity for gluconeogenesis in the isolated amphibian retina was found to be approx. 70-fold greater with lactate than with glutamate as the gluconeogenic precursor, 1426 versus 21 pmol of glucose incorporated into glycogen/h per mg of protein. It was also found that 11-15% of the glucosyl units in glycogen are derived from C3 metabolites of the glycolytic pathway, suggesting that lactate is recycled within the retina. In concert with these metabolic observations, a full complement of the gluconeogenic enzymes was detected in retinal homogenates. These included: glucose-6-phosphatase, fructose-1,6-bisphosphatase, acetyl-CoA-dependent pyruvate carboxylase and phosphoenolpyruvate carboxykinase. Agents that regulate the rate of gluconeogenesis in hepatic tissue were tested on the retina. At concentrations of glutamate and lactate that are presumed to be relevant physiologically, it was found that vasoactive intestinal peptide, ionophore A23187 and elevated [K+] each enhanced the rate of gluconeogenesis in Ringer containing 50 microM-glutamate, whereas in Ringer containing 8.5 mM-lactate these agents inhibited the rate of gluconeogenesis. Further, it was found that the classic gluconeogenic hormone glucagon inhibited gluconeogenesis in both glutamate- and lactate-containing Ringer. Retinal energy metabolism was found to be altered in lactate-containing Ringer, in that lactate production was suppressed completely. In addition, glycogen metabolism appeared to be dependent on increased cytosolic Ca2+ and was insensitive to increased retinal cyclic AMP.

Sensitivity of fructose-1,6-biphosphatase from yeast, liver and skeletal muscle to fructose-2,6-biphosphate and 5'-adenosine monophosphate

As a prerequisite for future studies on the possible effect of sulphite, an anti-microbial agent, on gluconeogenesis in yeast, a comparative study of fructose-1,6-bisphosphatase (FBPase), a key enzyme of gluconeogenesis, from yeast, liver and skeletal muscle is reported. In contrast to FBPase from yeast or liver, FBPase from skeletal muscle is approximately 1000-fold more sensitive to inhibition by 5' adenosine monophosphate and 30 to 250-fold less sensitive to inhibition by fructose-2,6-bisphosphate. The kinetic properties of the FBPases, determined by the ratios R(Mg2+/Mn2+) and R (pH 7/9) of the enzyme activities, measured at 10 mM Mg2+ and 2 mM Mn2+ and at pH 7.0 and 9.0, respectively, show a drastic difference between the skeletal muscle and the yeast or liver enzymes. The data support the idea that the enzymes from yeast and liver function in gluconeogenesis, whereas the enzyme from skeletal muscle is involved in other biological functions.

Characterization of phosphofructokinase 2 and of enzymes involved in the degradation of fructose 2,6-bisphosphate in yeast

Phosphofructokinase 2 from Saccharomyces cerevisiae was purified 8500-fold by chromatography on blue Trisacryl, gel filtration on Superose 6B and chromatography on ATP-agarose. Its apparent molecular mass was close to 600 kDa. The purified enzyme could be activated fivefold upon incubation in the presence of [gamma-32P]ATP-Mg and the catalytic subunit of cyclic-AMP-dependent protein kinase from beef heart; there was a parallel incorporation of 32P into a 105-kDa peptide and also, but only faintly, into a 162-kDa subunit. A low-Km (0.1 microM) fructose-2,6-bisphosphatase could be identified both by its ability to hydrolyze fructose 2,6-[2-32P]bisphosphate and to form in its presence an intermediary radioactive phosphoprotein. This enzyme was purified 300-fold, had an apparent molecular mass of 110 kDa and was made of two 56-kDa subunits. It was inhibited by fructose 6-phosphate (Ki = 5 microM) and stimulated 2-3-fold by 50 mM benzoate or 20 mM salicylate. Remarkably, and in deep contrast to what is known of mammalian and plant enzymes, phosphofructokinase 2 and the low-Km fructose-2,6-bisphosphatase clearly separated from each other in all purification procedures used. A high-Km (approximately equal to 100 microM), apparently specific, fructose 2,6-bisphosphatase was separated by anion-exchange chromatography. This enzyme could play a major role in the physiological degradation of fructose 2,6-bisphosphate, which it converts to fructose 6-phosphate and Pi, because it is not inhibited by fructose 6-phosphate, glucose 6-phosphate or Pi. Several other phosphatases able to hydrolyze fructose 2,6-bisphosphate into a mixture of fructose 2-phosphate, fructose 6-phosphate and eventually fructose were identified. They have a low affinity for fructose 2,6-bisphosphate (Km greater than 50 microM), are most active at pH 6 and are deeply inhibited by inorganic phosphate and various phosphate esters.

Intracellular mechanism of action of sympathetic hepatic nerves on glucose and lactate balance in perfused rat liver

In rat liver perfused in situ stimulation of the nerve plexus around the hepatic artery and the portal vein caused an increase in glucose output and a shift from lactate uptake to output. The effects of nerve stimulation on some key enzymes, metabolites and effectors of carbohydrate metabolism were determined and compared to the actions of glucagon, which led to an increase not only of glucose output but also of lactate uptake. 1. Nerve stimulation caused an enhancement of the activity of glycogen phosphorylase a to 300% and a decrease of the activity of glycogen synthase I to 40%, while it left the activity of pyruvate kinase unaltered. Glucagon, similarly to nerve action, led to a strong increase of glycogen phosphorylase and to a decrease of glycogen synthase; yet in contrast to the nerve effect it lowered pyruvate kinase activity clearly. 2. Nerve stimulation increased the levels of glucose 6-phosphate and of fructose 6-phosphate to 200% and 170%, respectively; glucagon enhanced the levels to about 400% and 230%, respectively. The levels of ATP and ADP were not altered, those of AMP were increased slightly by nerve stimulation. 3. Nerve stimulation enhanced the levels of the effectors fructose 2,6-bisphosphate and cyclic AMP only slightly to 140% and 125%, respectively; glucagon lowered the level of fructose 2,6-bisphosphate to 15% and increased the level of cyclic AMP to 300%. 4. In calcium-free perfusions the metabolic responses to nerve stimulation showed normal kinetics, if calcium was re-added 3 min before, but delayed kinetics, if it was re-added 2 min after the onset of the stimulus. The delay may be due to the time required to refill intracellular calcium stores. The hemodynamic alterations dependent on extracellular calcium were normal in both cases. The activation of glycogen phosphorylase, the inhibition of glycogen synthase and the increase of glucose 6-phosphate can well explain the enhancement of glucose output following nerve stimulation. The unaltered activity of pyruvate kinase and the marginal increase of fructose 2,6-bisphosphate cannot be the cause of the nerve-stimulation-dependent shift from lactate uptake to output. The very slight increase of the level of cyclic AMP after nerve stimulation cannot elicit the observed activation of glycogen phosphorylase.(ABSTRACT TRUNCATED AT 400 WORDS)

Complete nucleotide sequence coding for rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase derived from a cDNA clone

cDNA clones for 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase were isolated from rat liver expression libraries in lambda gt11 by antibody, oligonucleotide, and cDNA screening. One 1860 bp long clone contained a full-length nucleotide sequence coding for the 470 amino acids of each of the two identical subunits of the bifunctional enzyme. This clone also contained untranslated sequences, one 173 bp long upstream from the ATG start codon and one 271 bp long downstream from the TGA stop codon. The clone was terminated by a poly(A) tail of 29 nucleotides.

Inhibition of hexokinase activity by a fructose 2,6-bisphosphate-dependent cytosolic protein from liver

Mammalian and yeast hexokinases were found to be reversibly inhibited by fructose 2,6-bisphosphate, an effect requiring the presence of a cytosolic protein factor. Experimental evidence suggests that this factor (inhibitor) is a regulatory protein, the interactions of which with hexokinases are modulated by fructose 2,6-bisphosphate. The Vmax of hexokinase D was decreased, and no changes on other kinetic parameters were observed. The inhibitor was present in fresh liver cytosol filtered through Sephadex G-25 and was partially isolated by negative absorption on DEAE-cellulose followed by ammonium sulfate fractionation. The inhibitor was also present in brain and kidney, but not in muscle. A molecular mass of 200,000 was determined by gel filtration. The inhibition was dependent on the concentrations of both the inhibitory protein and fructose 2,6-bisphosphate. No delay in fructose 2,6-bisphosphate inhibition was observed. Several other hexose phosphates were tested and were not effective. In the presence of amounts of inhibitor sufficient to produce complete inhibition of hexokinase D, the concentration of fructose 2,6-bisphosphate required to produce 50% inhibition was about 0.5 microM. The inhibitor was unstable and was stabilized by the presence of fructose 2,6-bisphosphate.