Metabolic effects of D-glyceraldehyde in isolated hepatocytes

L-Glyceraldehyde Inhibits Neuroblastoma Cell Growth via a Multi-Modal Mechanism on Metabolism and Signaling

Glyceraldehyde (GA) is a three-carbon monosaccharide that can be present in cells as a by-product of fructose metabolism. Bruno Mendel and Otto Warburg showed that the application of GA to cancer cells inhibits glycolysis and their growth. However, the molecular mechanism by which this occurred was not clarified. We describe a novel multi-modal mechanism by which the L-isomer of GA (L-GA) inhibits neuroblastoma cell growth. L-GA induces significant changes in the metabolic profile, promotes oxidative stress and hinders nucleotide biosynthesis. GC-MS and 13C-labeling was employed to measure the flow of carbon through glycolytic intermediates under L-GA treatment. It was found that L-GA is a potent inhibitor of glycolysis due to its proposed targeting of NAD(H)-dependent reactions. This results in growth inhibition, apoptosis and a redox crisis in neuroblastoma cells. It was confirmed that the redox mechanisms were modulated via L-GA by proteomic analysis. Analysis of nucleotide pools in L-GA-treated cells depicted a previously unreported observation, in which nucleotide biosynthesis is significantly inhibited. The inhibitory action of L-GA was partially relieved with the co-application of the antioxidant N-acetyl-cysteine. We present novel evidence for a simple sugar that inhibits cancer cell proliferation via dysregulating its fragile homeostatic environment.

Sedoheptulose kinase regulates cellular carbohydrate metabolism by sedoheptulose 7-phosphate supply

Dynamic carbon re-routing between catabolic and anabolic metabolism is an essential element of cellular transformation associated with tumour formation and immune cell activation. Such bioenergetic adaptations are important for cellular function and therefore require tight control. Carbohydrate phosphorylation has been proposed as a rate-limiting step of several metabolic networks. The recent identification of a sedoheptulose kinase indicated that free sedoheptulose is a relevant and accessible carbon source in humans. Furthermore, the bioavailability of its phosphorylated form, sedoheptulose 7-phosphate, appears to function as a rheostat for carbon-flux at the interface of glycolysis and the pentose phosphate pathway. In the present paper, we review reports of sedoheptulose metabolism, compare it with glucose metabolism, and discuss the regulation of sedoheptulose kinase as mechanism to achieve bioenergetic reprogramming in cells.

AMP-activated protein kinase-independent inhibition of hepatic mitochondrial oxidative phosphorylation by AICA riboside

AICA riboside (5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside) has been extensively used in cells to activate the AMPK (AMP-activated protein kinase), a metabolic sensor involved in cell energy homoeostasis. In the present study, we investigated the effects of AICA riboside on mitochondrial oxidative; phosphorylation. AICA riboside was found to dose-dependently inhibit the oligomycin-sensitive JO2 (oxygen consumption rate) of isolated rat hepatocytes. A decrease in P(i) (inorganic phosphate), ATP, AMP and total adenine nucleotide contents was also observed with AICA riboside concentrations >0.1 mM. Interestingly, in hepatocytes from mice lacking both alpha1 and alpha2 AMPK catalytic subunits, basal JO2 and expression of several mitochondrial proteins were significantly reduced compared with wild-type mice, suggesting that mitochondrial biogenesis was perturbed. However, inhibition of JO2 by AICA riboside was still present in the mutant mice and thus was clearly not mediated by AMPK. In permeabilized hepatocytes, this inhibition was no longer evident, suggesting that it could be due to intracellular accumulation of Z nucleotides and/or loss of adenine nucleotides and P(i). ZMP did indeed inhibit respiration in isolated rat mitochondria through a direct effect on the respiratory-chain complex I. In addition, inhibition of JO2 by AICA riboside was also potentiated in cells incubated with fructose to deplete adenine nucleotides and P(i). We conclude that AICA riboside inhibits cellular respiration by an AMPK-independent mechanism that likely results from the combined intracellular P(i) depletion and ZMP accumulation. Our data also demonstrate that the cellular effects of AICA riboside are not necessarily caused by AMPK activation and that their interpretation should be taken with caution.

Metabolic activation of glucose low-responsive beta-cells by glyceraldehyde correlates with their biosynthetic activation in lower glucose concentration range but not at high glucose

Insulin synthesis and release activities of beta-cells can be acutely regulated by glucose through its glycolytic and mitochondrial breakdown involving a glucokinase-dependent rate-limiting step. Isolated beta-cell populations are composed of cells with intercellular differences in acute glucose responsiveness that have been attributed to differences in glucokinase (GK) expression and activity. This study first shows that glyceraldehyde can be used as GK-bypassing oxidative substrate and then examines whether the triose can metabolically activate beta-cells with low glucose responsiveness. Glyceraldehyde 1 mm induced a similar cellular (14)CO(2) output and metabolic redox state as glucose 4 mM. Using flow cytometric analysis, glyceraldehyde (0.25-2 mM) was shown to concentration-dependently increase the percent metabolically activated cells at all tested glucose concentrations (2.5-20 mM). Its ability to activate beta-cells that are unresponsive to the prevailing glucose level was further illustrated in glucose low-responsive cells that were isolated by flow sorting. Metabolic activation by glyceraldehyde was associated with an activation of nutrient-driven translational control proteins and an increased protein synthetic response to glucose, however not beyond the maximal rates that are inducible by glucose alone. It is concluded that glucose low-responsive beta-cells can be metabolically activated by the GK-bypassing glyceraldehyde, increasing their acute biosynthetic response to glucose but not their maximal glucose-inducible biosynthetic capacity, which is considered subject to chronic regulation.

Chloroacetaldehyde-induced hepatocyte cytotoxicity. Mechanisms for cytoprotection

2-Chloroacetaldehyde (CAA)-induced cytotoxicity in isolated hepatocytes was enhanced markedly if hepatocyte alcohol or aldehyde dehydrogenase was inhibited prior to CAA addition. Hepatocyte GSH depletion, ATP depletion and lipid peroxidation by CAA were also enhanced markedly. Furthermore, CAA was about 10- and 70-fold more cytotoxic than its oxidative or reductive metabolite chloroacetate or chloroethanol, respectively. Nutrients such as lactate, xylitol, sorbitol or glycerol, which increase cytosolic NADH levels, prevented CAA cytotoxicity in normal hepatocytes but further enhanced cytotoxicity toward alcohol dehydrogenase inactivated hepatocytes, suggesting that increased cytosolic NADH reduces CAA via alcohol dehydrogenase in normal hepatocytes but prevents CAA oxidation in alcohol dehydrogenase inactivated hepatocytes. However, increasing cytosolic NADH levels with ethanol or NADH-generating nutrients after CAA had been metabolized also prevented cytotoxicity and caused a partial ATP recovery, whereas oxidation of cytosolic NADH with pyruvate markedly increased cytotoxicity. This indicates that cytotoxic CAA concentrations cause oxidative stress and that ATP levels can be restored if cellular redox homeostasis is normalized with reductants. Furthermore, except for fructose, nutrients that did not increase NADH did not affect CAA-induced cytotoxicity. Fructose also caused a partial ATP recovery, and its protection was prevented by the glycolytic inhibitor fluoride. Hepatocytes isolated from fasted animals were 4- to 6-fold more susceptible to CAA-induced ATP depletion and cytotoxicity. No lipid peroxidation occurred at these lower CAA concentrations. Furthermore, all nutrients, including alanine, glutamine and glucose, prevented cytotoxicity toward hepatocytes isolated from fasted animals. The susceptibility of hepatocytes to CAA cytotoxicity, therefore, depends on both cellular redox homeostasis and cellular energy supply.

Rapid and delayed effects of epidermal growth factor on gluconeogenesis

Most reports on the effects of epidermal growth factor (EGF) on gluconeogenesis have indicated that such effects depend on the substrate used and are only observable after a lag time of 30-40 min. Recently, an immediate and transient effect of EGF on glucose synthesis was described in a perfused liver system. Here we extend the study of the effect of EGF on gluconeogenesis to isolated hepatocytes from fasted rats. The delayed effect of EGF on gluconeogenesis was studied by adding the substrate 40 min after the peptide. Under these conditions EGF increased glucose synthesis from pyruvate, decreased it when the substrate was lactate or glycerol and did not modify gluconeogensis from fructose or dihydroxyacetone. EGF did not affect the metabolic flux through glycolysis, determined as the production of lactate+pyruvate from 30 mM glucose. Furthermore, EGF did not modify the metabolic flux through pyruvate kinase, determined as the production of lactate+pyruvate from 1 mM dihydroxyacetone. The differing effects of EGF on gluconeogenesis depending on the substrate used can be explained by the effects of EGF on the cytosolic redox state (measured as the lactate/pyruvate ratio). About 20 min after the addition of EGF, the mitochondrial redox state (measured as the 3-hydroxybutyrate/acetoacetate ratio) decreased. This effect of EGF was blocked by ammonium, which also abolished the effect of the peptide on gluconeogenesis. Thus the effect of EGF at the mitochondrial level appears to be necessary for its effects on gluconeogenesis. Taken together, our results indicate that the delayed effects of EGF on gluconeogenesis are secondary to the effects of the peptide at both the mitochondrial and cytosolic levels. In addition to these delayed effects, we observed that EGF rapidly and transiently stimulated glucose synthesis from lactate, decreased the cytosolic redox state and increased oxygen consumption. All of these rapid effects required the presence of extracellular calcium and disappeared in the presence of rotenone, suggesting that this rapid effect of EGF on gluconeogenesis is secondary to the stimulation of mitochondrial respiration.

Role of mitochondria in hepatic fructose metabolism

During metabolism of fructose at concentrations exceeding 5 mM, isolated liver cells accumulate fructose 1-phosphate and lose ATP. At added bicarbonate concentrations below 10 mM in the incubation medium, the addition of atractyloside (or carboxyatractyloside) causes a significant net accumulation of 2-phosphoglycerate, resulting in an increase in the ratio 2-phosphoglycerate: 3-phosphoglycerate from below 1 to greater than 5. Digitonin fractionation revealed that virtually all this 2-phosphoglycerate is associated with the mitochondrial fraction, where it achieves a concentration estimated to be about 40 mM. The amount of 2-phosphoglycerate that accumulates is directly related to the initial concentration of fructose. With DL-glyceraldehyde in place of fructose, an even greater accumulation of 2-phosphoglycerate occurs, and this is also dependent upon both the presence of atractyloside and low bicarbonate. Formation of 2-phosphoglycerate is also observed when isolated mitochondria from rat liver are incubated together with glyceraldehyde and an energy source. The obligatory role of atractyloside for the accumulation of 2-phosphoglycerate within intact cells indicates the involvement of the mitochondrial adenylate translocator in this process, possibly as a carrier directly responsible for 2-phosphoglycerate egress from the mitochondrial matrix. If this is so, competition between 2-phosphoglycerate and ATP for egress from the matrix would be predicted to further exaggerate the fructose-induced depletion of cytosolic ATP.

Fructose 1-phosphate and the regulation of glucokinase activity in isolated hepatocytes

Fructose 1-phosphate kinase was partially purified from Clostridium difficile and used to develop specific assays of fructose 1-phosphate and fructose. The concentration of fructose 1-phosphate was below the detection limit of the assay (25 pmol/mg protein) in hepatocytes incubated in the presence of glucose as sole carbohydrate. Addition of fructose (0.05-1 mM) caused a concentration-dependent and transient increase in the fructose 1-phosphate content. Glucagon (1 microM) and ethanol (10 mM) caused a severalfold decrease in the concentration of fructose 1-phosphate in cells incubated with fructose, whereas the addition of 0.1 microM vasopressin or 10 mM glycerone, or raising the concentration of glucose from 5 mM to 20 mM had the opposite effect. All these agents caused changes in the concentration of triose phosphates that almost paralleled those of the fructose 1-phosphate concentration. Sorbitol had a similar effect to fructose in causing the formation of fructose 1-phosphate. D-Glyceraldehyde was much less potent in this respect than the ketose and its effect disappeared earlier. The effect of D-glyceraldehyde was reinforced by an increase in the glucose concentration and decreased by glucagon. Both fructose and D-glyceraldehyde stimulated the phosphorylation of glucose as estimated by the release of 3H2O from [2-3H]glucose, but the triose was less potent in this respect than fructose and its effect disappeared earlier. Glucagon and ethanol antagonised the effect of low concentrations of fructose or D-glyceraldehyde on the detritiation of glucose. These results support the proposal that fructose 1-phosphate mediates the effects of fructose, D-glyceraldehyde and sorbitol by relieving the inhibition exerted on glucokinase by a regulatory protein.

Does glyceraldehyde enter pancreatic islet metabolism via both the triokinase and the glyceraldehyde phosphate dehydrogenase reactions? A study of these enzymes in islets

Glyceraldehyde has been known to be an insulin secretagogue for more than 15 years. It has been (reasonably) assumed that glyceraldehyde enters the glycolytic pathway via its phosphorylation by ATP to form glyceraldehyde phosphate, a reaction catalyzed by the enzyme triokinase, and that subsequent metabolism is identical to that of glucose. glucose. However, up to now there have been no studies verifying the presence of triokinase in the pancreatic beta cell. We report here that (1) the activity of triokinase in pancreatic islets is very low, indicating that the activity is intrinsically low and/or the enzyme was rapidly inactivated during the preparation of tissue for assay; (2) the activity is much lower than glucose phosphorylating activity (hexokinase plus glucokinase) in islets, even though glyceraldehyde is a more efficient insulin secretagogue than glucose; (3) glyceraldehyde phosphate dehydrogenase from pancreatic islets can use glyceraldehyde as a substrate in place of glyceraldehyde phosphate (the Vmax of glyceraldehyde phosphate dehydrogenase from islets when glyceraldehyde is the substrate is 20-fold that of triokinase when glyceraldehyde is the substrate); and (4) the Km of glyceraldehyde phosphate dehydrogenase with respect to glyceraldehyde (4.8 mM) is similar to the concentration of glyceraldehyde that gives one-half maximal rates of insulin release from pancreatic islets, whereas the Km of triokinase with respect to glyceraldehyde is much lower (less than 50 microM). These data suggest that besides stimulating insulin release in islets via its entering metabolism by phosphorylation to glyceraldehyde phosphate in the triokinase reaction, glyceraldehyde could be phosphorylated by Pi in the glyceraldehyde phosphate dehydrogenase reaction to form glycerate 1-phosphate which is probably unmetabolizable in islets. The second reaction could drastically increase the NADH/NAD ratio in islets without providing substrates for hydrogen shuttles that reoxidize cytosolic NADH. Since an increased NAD(P)H/NAD(P) ratio is believed to be a key part of the signal for insulin release, such a mechanism would explain the potent insulinotropism of glyceraldehyde in short-term experiments. In addition, the formation of unmetabolizable acids may explain the toxic effects of long-term exposure of islets to glyceraldehyde and why glyceraldehyde causes the beta cell to become acidic, whereas glucose does not.

Stimulation of glucose phosphorylation by fructose in isolated rat hepatocytes

The phosphorylation of glucose was measured by the formation of [3H]H2O from [2-3H]glucose in suspensions of freshly isolated rat hepatocytes. Fructose (0.2 mM) stimulated 2-4-fold the rate of phosphorylation of 5 mM glucose although not of 40 mM glucose, thus increasing the apparent affinity of the glucose phosphorylating system. A half-maximal stimulatory effect was observed at about 50 microM fructose. Stimulation was maximal 5 min after addition of the ketose and was stable for at least 40 min, during which period 60% of the fructose was consumed. The effect of fructose was reversible upon removal of the ketose. Sorbitol and tagatose were as potent as fructose in stimulating the phosphorylation of 5 mM glucose. D-Glyceraldehyde also had a stimulatory effect but at tenfold higher concentrations. In contrast, dihydroxyacetone had no significant effect and glycerol inhibited the detritiation of glucose. Oleate did not affect the phosphorylation of glucose, even in the presence of fructose, although it stimulated the formation of ketone bodies severalfold, indicating that it was converted to its acyl-CoA derivative. These results allow the conclusion that fructose stimulates glucokinase in the intact hepatocyte. They also suggest that this effect is mediated through the formation of fructose 1-phosphate, which presumably interacts with a competitive inhibitor of glucokinase other than long-chain acyl-CoAs.

Urine glyceraldehyde excretion is elevated in the renal Fanconi syndrome

We analyzed urinary constituents using GC/MS in 16 children with the renal Fanconi syndrome and 13 normal individuals. Urine glyceraldehyde levels were strikingly elevated in the renal Fanconi syndrome group (mean 5.1 +/- 4.8 mg/mg creatinine) compared to levels in the normal group (mean 0.04 +/- 0.04 mg/mg creatinine, P less than 0.001). Urine lactate levels were also elevated in the renal Fanconi syndrome group (mean 2.3 +/- 2.6 mg/mg creatinine) compared to normals (mean 0.01 +/- 0.01 mg/mg creatinine, P less than 0.003). Only small elevations of glyceraldehyde and lactate were found in urine from children with other renal disorders. Serum levels of glyceraldehyde and lactate were no greater in individuals with the Fanconi syndrome than in the normals. The fractional reabsorption of both glyceraldehyde and lactate was virtually complete in the normals, but was markedly impaired in the Fanconi syndrome patients where, in some cases, glyceraldehyde excretion greatly exceeded the excretion of creatinine. We conclude that marked glyceraldehyde excretion is a previously unrecognized feature of the renal Fanconi syndrome which may result from disordered proximal tubular glycolytic metabolism. Further studies will be required to determine the role of glyceraldehyde loss in the pathogenesis of this generalized disturbance of proximal tubular function.

Effect of glycerol on gluconeogenesis in isolated rabbit kidney cortex tubules

In renal tubules isolated from fed rabbits glycerol is not utilized as a glucose precursor, probably due to the rate-limiting transfer of reducing equivalents from cytosol to mitochondria. Pyruvate and glutamate stimulated an incorporation of [14C]glycerol to glucose by 50- and 10-fold, respectively, indicating that glycerol is utilized as a gluconeogenic substrate under these conditions. Glycerol at concentration of 1.5 mM resulted in an acceleration of both glucose formation and incorporation of [14C]pyruvate and [14C]glutamate into glucose by 2- and 9-fold, respectively, while it decreased the rates of these processes from lactate as a substrate. In the presence of fructose, glycerol decreased the ATP level, limiting the rate of fructose phosphorylation and glucose synthesis. As concluded from the 'cross-over' plots, the ratios of both 3-hydroxybutyrate/acetoacetate and glycerol 3-phosphate/dihydroxyacetone phosphate, as well as from experiments performed with methylene blue and acetoacetate, the stimulatory effect of glycerol on glucose formation from pyruvate and glutamate may result from an acceleration of fluxes through the first steps of gluconeogenesis as well as glyceraldehyde-3-phosphate dehydrogenase. As inhibition by glycerol of gluconeogenesis from lactate is probably due to a marked elevation of the cytosolic NADH/NAD+ ratio resulting in a decline of flux through lactate dehydrogenase.

Glycogen synthase activation by sugars in isolated hepatocytes

We have investigated the activation by sugars of glycogen synthase in relation to (i) phosphorylase a activity and (ii) changes in the intracellular concentration of glucose 6-phosphate and adenine nucleotides. All the sugars tested in this work present the common denominator of activating glycogen synthase. On the other hand, phosphorylase a activity is decreased by mannose and glucose, unchanged by galactose and xylitol, and increased by tagatose, glyceraldehyde, and fructose. Dihydroxyacetone exerts a biphasic effect on phosphorylase. These findings provide additional evidence proving that glycogen synthase can be activated regardless of the levels of phosphorylase a, clearly establishing that a nonsequential mechanism for the activation of glycogen synthase occurs in liver cells. The glycogen synthase activation state is related to the concentrations of glucose 6-phosphate and adenine nucleotides. In this respect, tagatose, glyceraldehyde, and fructose deplete ATP and increase AMP contents, whereas glucose, mannose, galactose, xylitol, and dihydroxyacetone do not alter the concentration of these nucleotides. In addition, all these sugars, except glyceraldehyde, increase the intracellular content of glucose 6-phosphate. The activation of glycogen synthase by sugars is reflected in decreases on both kinetic constants of the enzyme, M0.5 (for glucose 6-phosphate) and S0.5 (for UDP-glucose). We propose that hepatocyte glycogen synthase is activated by monosaccharides by a mechanism triggered by changes in glucose 6-phosphate and adenine nucleotide concentrations which have been described to modify glycogen synthase phosphatase activity. This mechanism represents a metabolite control of the sugar-induced activation of hepatocyte glycogen synthase.