Hexose metabolism in pancreatic islets. The phosphorylation of fructose

A luminescence-based protocol for assessing fructose metabolism via quantification of ketohexokinase enzymatic activity in mouse or human hepatocytes

Ketohexokinase (KHK) catalyzes the first step of fructose metabolism. Inhibitors of KHK enzymatic activity are being evaluated in clinical trials for the treatment of non-alcoholic fatty liver disease (NAFLD) and diabetes. Here, we present a luminescence-based protocol to quantify KHK activity. The accuracy of this technique has been validated using knockdown and overexpression of KHK in vivo and in vitro. The specificity of the assay has been verified using 3-O-methyl-D-fructose, a non-metabolizable analog of fructose, heat inactivation of hexokinases, and depletion of potassium. For complete details on the use of this protocol, please refer to Damen et al. (2021).

Tissue-Specific Fructose Metabolism in Obesity and Diabetes

PURPOSE OF REVIEW

The objective of this review is to provide up-to-date and comprehensive discussion of tissue-specific fructose metabolism in the context of diabetes, dyslipidemia, and nonalcoholic fatty liver disease (NAFLD).

RECENT FINDINGS

Increased intake of dietary fructose is a risk factor for a myriad of metabolic complications. Tissue-specific fructose metabolism has not been well delineated in terms of its contribution to detrimental health effects associated with fructose intake. Since inhibitors targeting fructose metabolism are being developed for the management of NAFLD and diabetes, it is essential to recognize how inability of one tissue to metabolize fructose may affect metabolism in the other tissues. The primary sites of fructose metabolism are the liver, intestine, and kidney. Skeletal muscle and adipose tissue can also metabolize a large portion of fructose load, especially in the setting of ketohexokinase deficiency, the rate-limiting enzyme of fructose metabolism. Fructose can also be sensed by the pancreas and the brain, where it can influence essential functions involved in energy homeostasis. Lastly, fructose is metabolized by the testes, red blood cells, and lens of the eye where it may contribute to infertility, advanced glycation end products, and cataracts, respectively. An increase in sugar intake, particularly fructose, has been associated with the development of obesity and its complications. Inhibition of fructose utilization in tissues primary responsible for its metabolism alters consumption in other tissues, which have not been traditionally regarded as important depots of fructose metabolism.

Glucose induces MafA expression in pancreatic beta cell lines via the hexosamine biosynthetic pathway

MafA is a basic leucine zipper transcription factor that regulates gene expression in both the neuroretina and pancreas. Within the pancreas, MafA is exclusively expressed in the beta cells and is involved in insulin gene transcription, insulin secretion, and beta cell survival. The expression of the mafA gene within beta cells is known to increase in response to high glucose levels by an unknown mechanism. In this study, we demonstrate that pyruvate, which is produced by glycolysis from glucose, is not sufficient to induce mafA gene expression compared with high glucose. This suggests that the signal for MafA induction is independent of ATP levels and that a metabolic event occurring upstream of pyruvate production leads to the induction of MafA. Furthermore, insulin secretion mediated by high glucose is not important for MafA expression. However, the addition of glucosamine to beta cell lines stimulates MafA expression in the absence of high glucose, and inhibition of the hexosamine biosynthetic pathway in the presence of high glucose abolishes MafA induction. Moreover, we demonstrate that the expression of UDP-N-acetylglucosaminyl transferase, the enzyme mediating O-linked glycosylation of cytosolic and nuclear proteins, is essential for glucose-dependent MafA expression. Consistent with this observation, inhibition of N-acetylglucosaminidase, the enzyme involved in the removal of the O-GlcNAc modification from proteins, with O-(2-acetamido-2-deoxy-d-glucopyranosylidene)amino-N-phenylcarbamate stimulates MafA expression under low glucose conditions. The presented data suggest that MafA expression mediated by high glucose requires flux through the hexosamine biosynthetic pathway and the O-linked glycosylation of an unknown protein(s) by UDP-N-acetylglucosaminyl transferase.

Comparison between D-[3-3H]- and D-[5-3H]glucose and fructose utilization in pancreatic islets from control and hereditarily diabetic rats

The metabolism of D-glucose and/or D-fructose was investigated in pancreatic islets from control rats and hereditarily diabetic GK rats. In the case of both D-glucose and D-fructose metabolism, a preferential alteration of oxidative events was observed in islets from GK rats. The generation of 3HOH from D-[5-3H]glucose (or D-[5-3H]fructose) exceeded that from D-[3-3H]glucose (or D-[3-3H]fructose) in both control and GK rats. This difference, which is possibly attributable to a partial escape from glycolysis of tritiated dihydroxyacetone phosphate, was accentuated whenever the rate of glycolysis was decreased, e.g., in the absence of extracellular Ca(2+) or presence of exogenous D-glyceraldehyde. D-Mannoheptulose, which inhibited D-glucose metabolism, exerted only limited effects upon D-fructose metabolism. In the presence of both hexoses, the paired ratio between D-[U-14C]fructose oxidation and D-[3-3H]fructose or D-[5-3H]fructose utilization was considerably increased, this being probably attributable, in part at least, to a preferential stimulation by the aldohexose of mitochondrial oxidative events. Moreover, this coincided with the fact that D-mannoheptulose now severely inhibited the catabolism of D-[5-3H]fructose and D-[U-14C]fructose. The latter situation is consistent with both the knowledge that D-glucose augments D-fructose phosphorylation by glucokinase and the findings that D-mannoheptulose, which fails to affect D-fructose phosphorylation by fructokinase, inhibits the phosphorylation of D-fructose by glucokinase.

Hexose metabolism in pancreatic islets: effect of D-glucose upon D-fructose metabolism

In the light of recent findings on the effect of D-glucose upon D-fructose phosphorylation by human B-cell glucokinase, the influence of the aldohexose upon the metabolism of the ketohexose was investigated in rat pancreatic islets. D-glucose, although slightly decreasing D-[5-(3)H]fructose utilization, augmented the oxidation of the ketohexose, indicating that the aldohexose stimulates preferentially the oxidative, as distinct from anaerobic, modality of glycolysis. Such was not the case in parotid cells, taken as representative of functionally nonglucose-responsive cells. In the islets exposed to D-fructose, D-glucose also decreased the fractional contribution of the pentose shunt to the generation of CO2 and D-glyceraldehyde 3-phosphate from the ketohexose, and increased the inflow into the Krebs cycle of dicarboxylic metabolites relative to that of fructose-derived acetyl-CoA. This glucose-induced remodeling of D-fructose metabolism may optimize the insulin secretory response of islet cells to these hexoses, e.g. after food intake.

The effect of fructose metabolism on the accumulation of inositol phosphates in rat pancreatic islets

The mechanism by which glucose recognition of B cells results in the release of inositol 1,4,5-trisphosphate is not known at present. In pancreatic islets, fructose shares a common metabolic pathway with glucose from the second step of glycolysis and can augment insulin secretion at stimulatory glucose levels. To evaluate the impact of glycolysis on the release of inositol 1,4,5-trisphosphate, we studied the effect of glucose and fructose metabolism on insulin secretion and the activation of inositol-specific phospholipase C, using collagenase digested rat pancreatic islets incorporated with 3H-labelled myo-inositol. Inositol phosphates, generated by the cleavage of phosphatidyl inositol by inositol phospholipase C, were analyzed using fast protein liquid chromatography. The islets were exposed to 3.3, 5.5 and 12 mmol 1(-1) glucose for 45 min in the absence or presence of 10, 20 or 30 mmol 1(-1) fructose, and the amount of insulin released into the medium was measured. Intracellular inositol phosphate accumulation was measured under the same glucose concentrations with 0, 10 and 30 mmol 1(-1) fructose. As expected, fructose alone had no insulinotropic effect, but potentiated the glucose-induced (5.5 and 12 mmol 1(-1)) insulin secretion at concentrations of 10-30 mmol 1(-1). Glucose (12 vs. 3.3 mmol 1(-1)) significantly increased both intracellular content of inositol 1,4,5-trisphosphate, as well as its metabolite inositol 1,3,4-trisphosphate. Fructose, however, had no potentiating effects on the accumulation of inositol phosphates. It is therefore supposed that glucose does not activate inositol-specific phospholipase C via the glycolysis. Further, since fructose did not activate inositol-specific phospholipase C, this stimulation is likely to be induced by glucose as such.

Regulation of glucokinase by a fructose-1-phosphate-sensitive protein in pancreatic islets

In the post-microsomal supernatant of pancreatic islets, prepared from fasted or fed rats, D-fructose 1-phosphate increased the activity of glucokinase by 20-30% as measured in the presence of D-glucose 6-phosphate and D-fructose 6-phosphate. Such an activation was less marked than that found in liver extracts. The islet cytosol was also found to inhibit purified liver glucokinase, and this effect was antagonized by D-fructose 1-phosphate. In the presence of hexose 6-phosphates, partially purified islet glucokinase was inhibited by the hepatic glucokinase regulatory protein in a D-fructose-1-phosphate-sensitive manner. In intact islets, D-glyceraldehyde stimulated the generation of 14C-labelled D-fructose 1-phosphate from D-[U-14C]glucose and increased the production of 3H2O from D-[5-3H]glucose. These findings suggest that the activity of glucokinase in islet cells may be regulated by a protein mediating the antagonistic effects of D-fructose 6-phosphate and D-fructose 1-phosphate in a manner qualitatively similar to that operating in hepatocytes, but with lower efficiency.

Presence of fructokinase in pancreatic islets

Homogenates of rat pancreatic islets that had been heated for 5 min at 70 degrees C to inactive hexokinases, catalyzed the ATP-dependent phosphorylation of D-fructose. This reaction was dependent on the presence of K+ and was inhibited by D-tagatose although not by D-glucose or D-glucose 6-phosphate. The phosphorylation product was identified as fructose 1-phosphate through its conversion to a bisphosphate ester by Clostridium difficile fructose 1-phosphate kinase. These findings allowed the conclusion that fructokinase (ketohexokinase) was responsible for this process. Similar results were observed with tumoral insulin-producing cells (RINm5F line). Fructokinase may account for a large share of fructose phosphorylation in intact islets, particularly in the presence of D-glucose.

Hexose metabolism in pancreatic islets. Metabolic and secretory responses to D-fructose

D-Fructose (3.3 to 33.0 mmol/liter) caused a concentration-related increase in insulin output from rat islets exposed to D-glucose (3.3 to 7.0 mmol/liter), such an increase not being more marked in mouse islets. The fructose-induced increment in insulin release, relative to that evoked by D-glucose, was two times higher in islets exposed to D-glucose than in islets stimulated by D-mannose, 2-ketoisocaproate, or nonnutrient secretagogs. Likewise, the metabolism of D-fructose in islet cells was significantly different in the absence or presence of D-glucose. Thus, the ketose was largely channeled into the pentose phosphate pathway in glucose-deprived, but not so in glucose-stimulated, islets. In both glucose-deprived and glucose-stimulated islets, however, the magnitude of the secretory response to D-fructose was commensurate with the increase in ATP production attributable to its catabolism. These findings indicate that the metabolic fate of hexoses--and, hence, their insulinotropic capacity--is not ruled solely at the level of their phosphorylation.

Fructose metabolism via the pentose cycle in tumoral islet cells

In tumoral islet cells (RINm5F line) the phosphorylation of D-fructose is catalyzed by hexokinase rather than fructokinase. Fructose 6-phosphate appears to be preferentially channelled into the pentose cycle, as suggested by a ratio of D-[1-14C]fructose/D-[U-14C]fructose oxidation close to 2.7, the failure to generate 14C-labelled lactate from D-[1-14C]fructose and a poor metabolic response to menadione. When the islet cells are exposed to both D-fructose and D-glucose, however, the metabolism of the former hexose is dramatically modified, fructose 6-phosphate being now formed at a lower rate and preferentially channelled into the glycolytic pathway. These findings illustrate the existence of regulatory steps in fructose catabolism located distally to its site of phosphorylation.

Secretagogues for pancreatic hormone release in the channel catfish (Ictalurus punctatus)

We investigated the effect of several potential carbohydrate secretagogues, amino acids, a ketoacid, and potassium chloride on insulin, glucagon, and somatostatin release from the in vitro perfused Brockmann body of channel catfish (Ictalurus punctatus). Mannose (15 mM) stimulated the release of insulin and somatostatin. Fructose (30 mM) induced only a small and transient release of somatostatin. Galactose (15 mM) was not a secretagogue. Likewise, glyceraldehyde failed to stimulate hormone release. Among the amino acids newly tested, alanine and leucine, and also alpha-ketoisocaproic acid were without effect. A high concentration of potassium (25 mEq/liter) induced a pronounced release of insulin and glucagon and a moderate release of somatostatin. In conclusion, a striking similarity exists between catfish and higher vertebrates in their pancreatic endocrine response to hexoses; on the other hand, the catfish Brockmann body appears to respond only to a few of the common stimuli of pancreatic hormone release in mammals.

Artefactual and true uptake of labelled sucrose by rat pancreatic islet cells

Labelled sucrose is currently used as an extracellular marker in pancreatic islets or isolated islet cells. Contamination of [6,6'(n)-3H]sucrose by tritiated hexose results in severe overestimation of the extracellular space. The relative magnitude of this overestimation depends on such factors as the temperature and length of incubation and the presence or absence of metabolized unlabelled hexose. Even with uncontamined [U-14C]sucrose, a restricted amount of the disaccharide is apparently taken up, hydrolyzed and further metabolized in the islet cells. The uptake of sucrose by islet cells may reflect the phylogenic maintenance of a physiological attribute of these primitively intestinal epithelial cells.

Glucose metabolism in insulin-producing tumoral cells

Homogenates of insulin-producing tumoral cells catalyzed the phosphorylation of glucose, mannose, and fructose. The kinetics of phosphorylation at increasing glucose concentrations, the inhibitory effect of glucose 6-phosphate, and the comparison of results obtained with distinct hexoses indicated the presence of both low-Km hexokinase-like and high-Km enzymatic activities, the results being grossly comparable to those collected in normal pancreatic islets. Relative to protein content, the glucose-phosphorylating enzymatic activity was higher in tumoral than normal islet cells. The activity of other enzymes was either lower (glutamate dehydrogenase), moderately higher (phosphoglucomutase, lactate dehydrogenase) or considerably greater (ornithine decarboxylase) in tumoral than in normal islet cells. In intact tumoral cells, incubated under increasing glucose concentrations, the oxidation of D-[U-14C]glucose and the output of lactic and pyruvic acids reached a close-to-maximal value at 2.8 mM glucose. The ratios for glucose oxidation/utilization and lactate/pyruvate output were much lower in tumoral than in normal islet cells. Although glucose caused a modest increase in insulin output from the tumoral cells, this effect was saturated at a low glucose concentration (2.8 mM) and less marked than that of other secretagogues (e.g., L-leucine, L-ornithine, or forskolin). Thus, despite a close-to-normal enzymatic equipment for glucose phosphorylation, the tumoral cells displayed severe abnormalities in the metabolism and secretory response to this hexose. These findings point to regulatory mechanisms distal to glucose phosphorylation in the control of glucose metabolism in insulin-producing cells.