Effect of phenylpyruvate on enzymes involved in fatty acid synthesis in rat brain

Plasma metabonomic study on the effect of Para‑hydroxybenzaldehyde intervention in a rat model of transient focal cerebral ischemia

Gastrodia elata Blume has been widely used to treat various central and peripheral nerve diseases, and Para‑hydroxybenzaldehyde (PHBA) is one of the indicated components suggested to provide a neuroprotective effect. In our previous, it was shown that PHBA protected mitochondria against cerebral ischemia‑reperfusion (I/R) injury in rats. In the present study, how PHBA regulated the metabolic mechanism in blood following cerebral I/R was assessed to identify an effective therapeutic target for the prevention and treatment of ischemic stroke (IS). First, a rat model of cerebral ischemia‑reperfusion injury was established via middle cerebral artery occlusion/reperfusion (MCAO/R). The therapeutic effect of PHBA on brain I/R was evaluated by assessing the neurological function score, triphenyl tetrazolium chloride, hematoxylin and eosin, and Nissl staining. Next, a non‑targeted metabolomic based on high‑performance liquid chromatography quadrupole time‑of‑flight mass spectrometry was established to identify differential metabolites. Finally, a targeted metabolic spectrum was analyzed and the potential therapeutic targets were verified by Western blotting. The results showed that the neurological function score, cerebral infarction area, hippocampal morphology, and the number of neurons in the PHBA group were significantly improved compared with the model group. Metabonomic analysis showed that 13 different metabolites were identified between the model and PHBA group, which may be involved in the 'tricarboxylic acid cycle', 'glutathione metabolism', and 'mutual transformation of pentose and glucuronates', amongst others. Among these, the levels of the most significant differential metabolite, dGMP, decreased significantly following PHBA treatment. Western blotting was used to verify the expression of membrane‑associated guanosine kinase PSD‑95 and the subunit of glutamate AMPA receptor GluA1, which significantly increased after PHBA treatment. In addition, it was also found that PHBA increased the expression of the light chain‑3 protein and autophagy effector protein 1, whilst the expression of sequestosome‑1 decreased, indicating that PHBA promoted autophagy. Similarly, in TUNEL staining and detection of apoptosis‑related proteins, it was found that MCAO/R upregulated the expression of Bax and cleaved‑caspase‑3 whilst downregulating the expression of Bcl‑2 and increasing the apoptosis of hippocampal neurons; PHBA reversed this situation. These results suggest that cerebral I/R causes postsynaptic dysfunction by disrupting the interaction between PSD‑95 and AMPARs, and the inhibition of the autophagy system eventually leads to the apoptosis of hippocampal neurons.

Mechanism of action of Imeglimin: A novel therapeutic agent for type 2 diabetes

Imeglimin is an investigational first-in-class novel oral agent for the treatment of type 2 diabetes (T2D). Several pivotal phase III trials have been completed with evidence of statistically significant glucose lowering and a generally favourable safety and tolerability profile, including the lack of severe hypoglycaemia. Imeglimin's mechanism of action involves dual effects: (a) amplification of glucose-stimulated insulin secretion (GSIS) and preservation of β-cell mass; and (b) enhanced insulin action, including the potential for inhibition of hepatic glucose output and improvement in insulin signalling in both liver and skeletal muscle. At a cellular and molecular level, Imeglimin's underlying mechanism may involve correction of mitochondrial dysfunction, a common underlying element of T2D pathogenesis. It has been observed to rebalance respiratory chain activity (partial inhibition of Complex I and correction of deficient Complex III activity), resulting in reduced reactive oxygen species formation (decreasing oxidative stress) and prevention of mitochondrial permeability transition pore opening (implicated in preventing cell death). In islets derived from diseased rodents with T2D, Imeglimin also enhances glucose-stimulated ATP generation and induces the synthesis of nicotinamide adenine dinucleotide (NAD+ ) via the 'salvage pathway'. In addition to playing a key role as a mitochondrial co-factor, NAD+ metabolites may contribute to the increase in GSIS (via enhanced Ca++ mobilization). Imeglimin has also been shown to preserve β-cell mass in rodents with T2D. Overall, Imeglimin appears to target a key root cause of T2D: defective cellular energy metabolism. This potential mode of action is unique and has been shown to differ from that of other major therapeutic classes, including biguanides, sulphonylureas and glucagon-like peptide-1 receptor agonists.

Imeglimin amplifies glucose-stimulated insulin release from diabetic islets via a distinct mechanism of action

Pancreatic islet β-cell dysfunction is characterized by defective glucose-stimulated insulin secretion (GSIS) and is a predominant component of the pathophysiology of diabetes. Imeglimin, a novel first-in-class small molecule tetrahydrotriazine drug candidate, improves glycemia and GSIS in preclinical models and clinical trials in patients with Type 2 diabetes; however, the mechanism by which it restores β-cell function is unknown. Here, we show that imeglimin acutely and directly amplifies GSIS in islets isolated from rodents with Type 2 diabetes via a mode of action that is distinct from other known therapeutic approaches. The underlying mechanism involves increases in the cellular nicotinamide adenine dinucleotide (NAD+) pool-potentially via the salvage pathway and induction of nicotinamide phosphoribosyltransferase (NAMPT) along with augmentation of glucose-induced ATP levels. Further, additional results suggest that NAD+ conversion to a second messenger, cyclic ADP ribose (cADPR), via ADP ribosyl cyclase/cADPR hydrolase (CD38) is required for imeglimin's effects in islets, thus representing a potential link between increased NAD+ and enhanced glucose-induced Ca2+ mobilization which-in turn-is known to drive insulin granule exocytosis. Collectively, these findings implicate a novel mode of action for imeglimin that explains its ability to effectively restore-β-cell function and provides for a new approach to treat patients suffering from Type 2 diabetes.

Phosphate depletion impairs insulin secretion by pancreatic islets

Phosphate depletion (PD) is associated with resistance to the peripheral action of insulin and with glucose intolerance. However, data on the effect of PD on insulin secretion are not consistent, and were derived indirectly by measurements of blood levels of insulin during intravenous glucose tolerance test (IVGTT) or with hyperglycemic clamp technique. Direct evidence for an effect of PD on insulin secretion by pancreatic islets is not available, and the potential mechanisms through which PD may affect insulin secretion are not known. We performed IVGTT, examined in vitro insulin secretion by pancreatic islets, and evaluated various factors involved in insulin secretion in PD and pair weighed (PW) rats. PD animals had fasting hyperglycemia and normal plasma insulin levels, and displayed abnormal IVGTT as compared to PW rats. Both initial and late phases of D-glucose-induced insulin secretion from islets were markedly and significantly (P less than 0.01) lower than from islets of PW rats. In contrast, D-glyceraldehyde-induced insulin release in PD rats was similar to that of PW rats. [H3]2-deoxyglucose uptake by islets and their cyclic AMP content after exposure to D-glucose, D-glyceraldehyde or forskolin were not different among the two groups of animals. Insulin content in PD islets was modestly but significantly (P less than 0.01) higher than PW islets. In PD islets, ATP content and the ATP/ADP ratio at basal state and after incubation with 16.7 mM D-glucose were significantly (P less than 0.01) lower and resting cytosolic calcium was significantly (P less than 0.01) higher than in PW islets.(ABSTRACT TRUNCATED AT 250 WORDS)

HPLC analysis of nucleotide profiles in glucose-stimulated perifused rat islets

Nucleotide concentrations were measured in isolated pancreatic islets from rats using HPLC. This method was used to study the role of nucleotides in islets, which were perifused for several hours with different substrates and exhibited signs of exhaustion. Using islet tissue extracts, it was possible to determine 11 nucleotides. It was hoped that alterations in islet nucleotide concentrations would provide clues for the biochemical basis for the exhaustion phenomenon; however, changes in nucleotide concentrations that were observed in exhausted islet tissue did not correlate with corresponding changes in insulin release. Therefore, they are unlikely to be directly related to islet exhaustion.

The role of metabolism in the control of cyclic AMP efflux, cyclic AMP content, and insulin secretion from rat islets of Langerhans

Glucose, alpha-ketoisocaproate, and N-acetylglucosamine all initiate insulin release and stimulate cyclic AMP efflux and cyclic AMP accumulation. Mannoheptulose inhibited these islet responses to glucose but not to N-acetylglucosamine or alpha-ketoisocaproate. On the other hand, menadione inhibited these responses to all three secretagogues. Nicotinamide enhanced the insulin release due to glucose and alpha-ketoisocaproate and also enhanced cyclic AMP efflux and intracellular cyclic AMP accumulation. N-acetylglucosamine-mediated insulin release, cyclic AMP efflux, and cyclic AMP accumulation were unaffected by nicotinamide. These results suggest that the metabolic state of the islet is a primary determinant of the ability of a secretagogue to raise cyclic AMP efflux and intracellular cyclic AMP, and these actions may be mediated in part by alterations in the redox state of the pyridine nucleotide system.

Adrenergic receptors and the onset of streptozotocin-induced diabetes in mice

Treatment with yohimbine, an alpha 2-adrenergic blocker, prior to the injection of a subdiabetogenic dose of streptozotocin (STZ) produced hyperglycemia and hypoinsulinemia in mice 7 days later. Prazosin, an alpha 1-adrenergic blocker, was ineffective. The capacity of phentolamine and phenoxybenzamine to potentiate the diabetogenic effect of STZ was intermediate between that of yohimbine and prazosin. Propranolol and hexamethonium inhibited the potentiating action of yohimbine. Yohimbine enhanced the potentiating effect of isoproterenol on the STZ-induced diabetes. Acute changes in phase glucose and insulin levels induced by STZ were potentiated by yohimbine but not by prazosin. The insulin releasing ability of the pancreatic islets 7 days after STZ was all but lost in mice pretreated with yohimbine but not with prazosin. These results suggest that the beta- and alpha 2-, not alpha 1-adrenergic system which modulates insulin release from pancreatic islets influences the response to the diabetogenic action of STZ in mice.

Effects of glucose on the cytosolic ration of reduced/oxidized nicotinamide-adenine dinucleotide phosphate in rat islets of Langerhans

The maximal extractable activity of "malic" enzyme (EC 1.1.1.40) in rat islets of Langerhans was similar to that reported for liver. Thus "malic" enzyme may catalyse a near-equilibrium reaction in the cytosol of islets of Langerhans. Measurements of islet content of malate and pyruvate, the metabolite substrate and product of "malic" enzyme, were therefore used to calculate the cytosolic ration of [NADPH]/[NADP+]. This ratio was higher in islets incubated with 20 mM-glucose than in islets incubated with 2 mM-glucose.

The stimulus--secretion coupling 4-methyl-2-oxopentanoate-induced insulin release

1. Pancreatic islet insulin secretion and 45Ca uptake showed similar responses to variation in the extracellular concentration of 4-methyl-2-oxopentanoate with a threshold at 4 mM and a maximal response at a 25 mM concentration. 2. Islet respiration, acetoacetate production and rates of substrate utilization, oxidation and amination all changed as a simple hyperbolic function of 4-methyl-2-oxopentanoate concentration and exhibited a maximal response at 25 mM. 3. The responses of ATP content, [ATP]/[ADP] ratio, adenylate energy charge and [NADH]/[NAD+] ratio were also hyperbolic in nature but were maximally elevated at lower concentrations of the secretagogue. The islet [NADPH]/[NADP+] ratio, however, was tightly correlated with parameters of metabolic flux, 45Ca uptake and insulin release. 4. NH4+ and menadione, agents that promote a more oxidized state in islet NADP, did not affect islet ATP content or the rates of [U-14C]4-methyl-2-oxopentanoate oxidation or amination, but markedly inhibited islet 45Ca uptake and insulin release. 5. It is proposed that changes in the redox state of NADP and Ca transport may serve as mediators in the stimulus-secretion coupling mechanism of insulin release induced by 4-methyl-2-oxopentanoate.

The stimulus-secretion coupling of glucose-induced insulin release. XXXV. The links between metabolic and cationic events

When isolated rat islets were exposed to glucose, the concentrations of NADH and NADPH, and the NADH/NAD+ and NADPH/NADP+ ratios were increased. The dose-response curve resembled that characterising the glucose-induced secondary rise in 45Ca efflux, displaying a sigmoidal pattern with a half-maximal value at glucose 7.5 mmol/l. The glucose-induced increase in NAD(P)H was detectable within 1 min of exposure to the sugar. Except for the fall in ATP concentration and ATP/ADP ratio found at very low glucose concentrations (zero to 1.7 mmol/l) no effect of glucose (2.8-27.8 mmol/l) upon the steady-state concentration of adenine nucleotides was observed. However, a stepwise increase in glucose concentration provoked a dramatic and transient fall in the ATP concentration, followed by a sustained increase in both O2 consumption and oxidation of exogenous + endogenous nutrients. This may be essential to meet the energy requirements in the stimulated B-cell. Although no significant effect of glucose upon intracellular pH was detected by the 5,5-dimethyloxazolidine-2,4-dione method, the net release of H+ was markedly increased by glucose, with a hyperbolic dose-response curve (half-maximal response at glucose 2.9 mmol/l) similar to that characterising the glucose-induced initial fall in 45Ca efflux. It is proposed that the generation of both NAD(P)H and H+ participates in the coupling of glucose metabolism to distal events in the secretory sequence, especially the ionophoretic process of Ca2+ inward and outward transport, and that changes in these parameters occur in concert with an increased turn-over rate of high-energy phosphate intermediates.

Inosine-stimulated insulin release and metabolism of inosine in isolated mouse pancreatic islets

Inosine is a potent primary stimulus of insulin secretion from isolated mouse islets. The inosine-induced insulin secretion was totally depressed during starvation, but was completely restored by the addition of 5 mM-caffeine to the medium and partially restored by the addition of 5 mM-glucose. Mannoheptulose (3 mg/ml) potentiated the effect of 10 mM-inosine in islets from fed mice. The mechanism of the stimulatory effect of inosine was further investigated, and it was demonstrated that pancreatic islets contain a nucleoside phosphorylase capable of converting inosine into hypoxanthine and ribose 1-phosphate. Inosine at 10 mM concentration increased the lactate production and the content of ATP, glucose 6-phosphate (fructose 1,6-diphosphate + triose phosphates) and cyclic AMP in islets from fed mice. In islets from starved mice inosine-induced lactate production was decreased and no change in the concentration of cyclic AMP could be demonstrated, whereas the concentration of ATP and glucose 6-phosphate rose. Inosine (10 mM) induced a higher concentration of (fructose 1,6-diphosphate + triose phosphates) in islets from starved mice than in islets from fed mice suggesting that in starvation the activities of glyceraldehyde 3-phosphate dehydrogenase or other enzymes below this step in glycolysis are decreased. Formation of glucose from inosine was negligible. Inosine had no direct effect on adenylate cyclase activity in islet homogenates. The observed changes in insulin secretion and islet metabolism mimic what is seen when glucose and glyceraldehyde stimulate insulin secretion, and as neither ribose nor hypoxanthine-stimulated insulin release, the results are interpreted as supporting the substrate-site hypothesis for glucose-induced insulin secretion according to which glucose has to be metabolized in the beta-cells before secretion is initiated.

Influence of anoxia on glucose metabolism in pancreatic islets: lack of correlation between fructose-1,6-diphosphate and apparent glycolytic flux

When equilibrated with O2-CO2 (95:5), pancreatic islets of non-inbred ob/ob-mice exhibited a sigmoidal dependence of 3H2O production on D-(5-3H)-glucose concentration; the rate was most sensitive to changes of glucose concentration around 5mM and tended to be maximum above about 15mM glucose. 3H2O production from more than 5 mM D-(5-3H)-glucose was about twice as fast as the production of 14CO2 from equimolar D-(U-14C)-glucose. Islets equilibrated with N2-CO2 (95:5) did not exhibit a sigmoidal dose-response curve for 3h2o production, the process being inhibited by anoxia at glucose concentrations above 5mM. Pieces of exocrine pancreas had a slower aerobic 3H2O production than the islets and showed a clear enhancement of the process during anoxia. In comparison with oxygenated islets, anoxic islets exhibited decreased concentrations of glucose-6-phosphate and increased concentrations of furctose-1,6-diphosphate. The concomitant inhibition of glycolytic flux may be due to a low lactate dehydrogenase activity in islets yielding a slow reoxidation of NADH and a slow phosphoglyceraldehyde oxidation under anaerobic conditions.

Influence of the mutation "diabetes" on insulin release and islet morphology in mice of different genetic backgrounds

Mice, 7-8-mo old, of the C57BL/KsJ-db strain and homozygotic for the mutant gene db, exhibited marked hyperglycemia and moderately elevated serum insulin levels. Light and electron microscopy provided evidence of a slightly decreased proportion of beta cells in the pancreatic islets, irregular islet architecture with intraislet ducts, and degenerative as well as hypertrophic changes in the individual beta cells. As a rule, islets microdissected from these mice did not release insulin in response to glucose, theophylline, iodoacetamide, or chloromercuribenzene-p-sulphonic acid. The absence of secretory responses was not simply due to lack of insulin. Although the islet content of insulin was decreased in C57BL/KsJ-db/db mice, the remaining amount was severalfold larger than that released from stimulated islets of normal controls. Another mutation, db(2J), an allele of db with identical phenotypic expressions in the C57BL/KsJ strain, was studied on the genetic background C57BL/6J. In contrast to the severely diabetic C57BL/KsJ-db/db animals, the C57BL/6J-db(2J)/db(2J) mice were characterized by highly elevated serum insulin levels and only moderate hyperglycemia. Their endocrine pancreas was enlarged and showed an increased proportion of beta cells. Like the islets of normal mice, those of C57BL/6J-db(2J)/db(2J) mice responded to glucose and chloromercuribenzene-p-sulphonic acid, the glucose-induced responses being potentiated by theophylline or iodoacetamide. C57BL/KsJ-db/db mice should provide a valuable model for studying defects in insulin secretion in relation to diabetes mellitus. Mice of the C57BL/6J strain offer a control material that may help to elucidate the dependence of the insulin secretory defect on the background genome.

Cytotoxic effects of streptozotocin and N-nitrosomethylurea on the pancreatic B cells with special regard to the role of nicotinamide-adenine dinucleotide

The effects on the pancreatic B cell of streptozotocin and its aglucone derivative N-nitrosomethylurea were investigated in obese-hyperglycaemic mice and their lean littermates. Both streptozotocin and N-nitrosomethylurea were found to be B-cytotoxic although N-nitrosomethylurea produced less islet damage. Both substances decreased the concentrations of NAD(+) in the islet cells to about 10% of the control values within 2h after injection. This NAD(+) depletion was prevented by injection of nicotinamide 10min after the administration of streptozotocin or N-nitrosomethylurea. In islets taken from animals 10min after injection of streptozotocin or N-nitrosomethylurea there was no stimulatory effect of glucose on the respiration or insulin release and the oxidation of glucose was markedly decreased. Addition of nicotinamide (10mm) to the incubated islets restored glucose stimulation of both the oxygen consumption and insulin release. It is concluded that islet NAD(+) depletion is probably important for the B-cytotoxin action of N-nitrosomethylurea and streptozotocin. The glucose residue in the streptozotocin molecule may potentiate the B-cytotoxic action of this drug in mice.