Inhibition of gluconeogenesis in isolated rat liver cells by methylenecyclopropylpyruvate (ketohypoglycin)

In vivo studies on the mechanism of methylene cyclopropyl acetic acid and methylene cyclopropyl glycine-induced hypoglycemia

Exposure to the toxins methylene cyclopropyl acetic acid (MCPA) and methylene cyclopropyl glycine (MCPG) of unripe ackee and litchi fruit can lead to hypoglycemia and death; however, the molecular mechanisms by which MCPA and MCPG cause hypoglycemia have not been established in vivo To determine the in vivo mechanisms of action of these toxins, we infused them into conscious rodents and assessed rates of hepatic gluconeogenesis and ketogenesis, hepatic acyl-CoA and hepatic acetyl-CoA content, and hepatocellular energy charge. MCPG suppressed rates of hepatic β-oxidation as reflected by reductions in hepatic ketogenesis, reducing both short- and medium-chain hepatic acyl-CoA concentrations. Hepatic acetyl-CoA content decreased, and hepatic glucose production was inhibited. MCPA also suppressed β-oxidation of short-chain acyl-CoAs, rapidly inhibiting hepatic ketogenesis and hepatic glucose production, depleting hepatic acetyl-CoA content and ATP content, while increasing other short-chain acyl-CoAs. Utilizing a recently developed positional isotopomer NMR tracer analysis method, we demonstrated that MCPA-induced reductions in hepatic acetyl-CoA content were associated with a marked reduction of hepatic pyruvate carboxylase (PC) flux. Taken together, these data reveal the in vivo mechanisms of action of MCPA and MCPG: the hypoglycemia associated with ingestion of these toxins can be ascribed mostly to MCPA- or MCPG-induced reductions in hepatic PC flux due to inhibition of β-oxidation of short-chain acyl-CoAs by MCPA or inhibition of both short- and medium-chain acyl-CoAs by MCPG with resultant reductions in hepatic acetyl-CoA content, with an additional contribution to hypoglycemia through reduced hepatic ATP stores by MCPA.

Normal rates of whole-body fat oxidation and gluconeogenesis after overnight fasting and moderate-intensity exercise in patients with medium-chain acyl-CoA dehydrogenase deficiency

BACKGROUND

Impairments in gluconeogenesis have been implicated in the pathophysiology of fasting hypoglycemia in medium-chain acyl-CoA dehydrogenase deficiency. However, whole body glucose and fat metabolism have never been studied in vivo.

METHODS

Stable isotope methodology was applied to compare fat and glucose metabolism between four adult patients with MCADD and four matched controls both at rest and during 1.5 h of moderate-intensity exercise. Additionally, intramyocellular lipid and glycogen content and intramyocellular acylcarnitines were assessed in muscle biopsies collected prior to and immediately after cessation of exercise.

RESULTS

At rest, plasma FFA turnover was significantly higher in patients with MCADD, whereas the plasma FFA concentrations did not differ between patients and controls. Blood glucose kinetics did not differ between groups both at rest and during exercise. Palmitate and FFA turnover, total fat and carbohydrate oxidation rates, the use of muscle glycogen and muscle derived triglycerides during exercise did not differ between patients and controls. Plasma FFA oxidation rates were significantly lower in patients at the latter stages of exercise. Free carnitine levels in muscle were lower in patients, whereas no differences were detected in muscle acetylcarnitine levels.

CONCLUSIONS

Whole-body or skeletal muscle glucose and fat metabolism were not impaired in adult patients with MCADD. This implies that MCADD is not rate limiting for energy production under the conditions studied. In addition, patients with MCADD have a higher FFA turnover rate after overnight fasting, which may stimulate ectopic lipid deposition and, as such, make them more susceptible for developing insulin resistance.

Intestinal gluconeogenesis and glucose transport according to body fuel availability in rats

Intestinal hexose absorption and gluconeogenesis have been studied in relation to refeeding after two different fasting phases: a long period of protein sparing during which energy expenditure is derived from lipid oxidation (phase II), and a later phase characterized by a rise in plasma corticosterone triggering protein catabolism (phase III). Such a switch in body fuel uses, leading to changes in body reserves and gluconeogenic precursors, could modulate intestinal gluconeogenesis and glucose transport. The gene and protein levels, and the cellular localization of the sodium-glucose cotransporter SGLT1, and of GLUT5 and GLUT2, as well as that of the key gluconeogenic enzymes phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (Glc6Pase) were measured. PEPCK and Glc6Pase activities were also determined. In phase III fasted rats, SGLT1 was up-regulated and intestinal glucose uptake rates were higher than in phase II fasted and fed rats. PEPCK and Glc6Pase mRNA, protein levels and activities also increased in phase III. GLUT5 and GLUT2 were down-regulated throughout the fast, but increased after refeeding, with GLUT2 recruited to the apical membrane. The increase in SGLT1 expression during phase III may allow glucose absorption at low concentrations as soon as food is available. Furthermore, an increased epithelial permeability due to fasting may induce a paracellular movement of glucose. In the absence of intestinal GLUT2 during fasting, Glc6Pase could be involved in glucose release to the bloodstream via membrane trafficking. Finally, refeeding triggered GLUT2 and GLUT5 synthesis and apical recruitment of GLUT2, to absorb larger amounts of hexoses.

[Results of clinical trials with the gluconeogenesis inhibitor 2-(3-methylcinnamylhydrazono)-propionate (MCHP)]

Many compounds which directly or indirectly inhibit gluconeogenesis have been described. Some of them showed hypoglycemic action in animal experiments. 2-(3-Methylcinnamylhydrazono)-Propionat (MCHP) a derivative of hydrazine demonstrated marked hypoglycemic action in animal experiments. Nontheless the administration of MCHP to type II diabetic patients showed no hypoglycemic action. Clinical or biochemical sideeffects could not be observed.

A nondestructive method for the measurement of radioactive 14CO2 in blood

A method for estimation of 14CO2 present in blood and tissular samples is described. It is basically based on the introduction of large amounts of a gas mixture (95% O2, 5% CO2) in the samples which serves to remove the CO2 label by gas dilution. The gas phase is later captured in scintillation vials containing an organic-soluble base that retains the carbon label. The results obtained by means of this methodology show much better recoveries for blood samples than those obtained when the classic acid-diffusion method is used. In addition, it is a very fast procedure which does not alter the pH or protein integrity of the biological sample.

Measurement of 14CO2 evolution during tissue oxidation in vitro; an alternative to the Kontes well

1. An alternative method to the use of the disposable Kontes well for trapping 14CO2 produced in the course of biological oxidations is described. 2. A polyethylene miniature scintillation vial was used to contain the hyamine hydroxide-impregnated filter paper wick. 3. The two methods are compared in their abilities to trap 14CO2 produced directly by acidification of sodium [14C]bicarbonate and during beta-oxidation of 1[14C] palmitic acid. 4. The miniature vial and Kontes well methods showed similar efficiencies in the trapping of 14CO2 (97% and 95%, respectively, on average) the radioactivity of which was determined in the miniature vial using 5 ml only of scintillation fluid compared with a minimum of 10 ml required by the standard scintillation vial used to accommodate the Kontes well. 5. The technical advantages of the suggested miniature vial system, during both incubation and counting stages, are discussed.

Decrease of fatty acid oxidation, ketogenesis and gluconeogenesis in isolated perfused rat liver by phenylalkyl oxirane carboxylate (B 807-27) due to inhibition of CPT I (EC 2.3.1.21)

Sodium 2-[5-(4-chlorophenyl)-pentyl]-oxirane-2-carboxylate (B 807-27 or POCA) inhibits ketogenesis from endogenous and exogenous long-chain fatty acids and 14CO2 production from [U-14 C]palmitate, but not from [U-14 C]palmitoylcarnitine or octanoate, and inhibits gluconeogenesis from lactate and pyruvate in perfused livers of starved rats. Inhibition of ketogenesis, 14CO2 production and gluconeogenesis was complete at concentrations of 10 mumol/l POCA, but onset was more rapid for inhibition of ketogenesis and CO2 production than for gluconeogenesis. Infusion of octanoate abolished inhibition of all three processes. Experiments with isolated rat liver mitochondria showed that carnitine palmitoyltransferase I (EC 2.3.1.21) is inhibited by POCA-CoA. The inhibitory process is dependent on time and concentration, and more pronounced in mitochondria from fed than from fasted rats. Concentrations required for 50% inhibition after 20 min preincubation with POCA-CoA are 0.02, 0.06 and 0.1 mumol/l in liver mitochondria from fed, 24-h-fasted and 48-h-fasted rats, respectively. The inhibitor appears to be tightly bound to the enzyme. The extent of inhibition of carnitine palmitoyltransferase I correlates well with the hypoglycaemic and hypoketonaemic effects of the compounds in fasted rats. We conclude that specific inhibition of the enzyme leads, due to inhibition of long-chain fatty acid utilization, to depressed ketogenesis and gluconeogenesis and, in consequence, to hypoglycaemic and hypoketonaemia in vivo under gluconeogenic and ketogenic conditions.

The control of fatty acid metabolism in liver cells from fed and starved sheep

Isolated liver cells prepared from starved sheep converted palmitate into ketone bodies at twice the rate seen with cells from fed animals. Carnitine stimulated palmitate oxidation only in liver cells from fed sheep, and completely abolished the difference between fed and starved animals in palmitate oxidation. The rates of palmitate oxidation to CO2 and of octanoate oxidation to ketone bodies and CO2 were not affected by starvation or carnitine. Neither starvation nor carnitine altered the ratio of 3-hydroxybutyrate to acetoacetate or the rate of esterification of [1-14C]palmitate. Propionate, lactate, pyruvate and fructose inhibited ketogenesis from palmitate in cells from fed sheep. Starvation or the addition of carnitine decreased the antiketogenic effectiveness of gluconeogenic precursors. Propionate was the most potent inhibitor of ketogenesis, 0.8 mM producing 50% inhibition. Propionate, lactate, fructose and glycerol increased palmitate esterification under all conditions examined. Lactate, pyruvate and fructose stimulated oxidation of palmitate and octanoate to CO2. Starvation and the addition of gluconeogenic precursors stimulated apparent palmitate utilization by cells. Propionate, lactate and pyruvate decreased cellular long-chain acylcarnitine concentrations. Propionate decreased cell contents of CoA and acyl-CoA. It is suggested that propionate may control hepatic ketogenesis by acting at some point in the beta-oxidation sequence. The results are discussed in relation to the differences in the regulation of hepatic fatty acid metabolism between sheep and rats.

Phenylalkyloxirane carboxylic acids, a new class of hypoglycaemic substances: hypoglycaemic and hypoketonaemic effects of sodium 2-[5-(4-chlorophenyl)-pentyl]-oxirane-2-carboxylate (B 807-27) in fasted animals

Phenylalkyloxirane carboxylic acids and esters are a new class of potent hypoglycaemic substances. Sodium 2-[5-(4-chlorophenyl)-pentyl]-oxirane-2-carboxylate (B 807-27) produces a dose-dependent hypoglycaemic effect when administered orally or intravenously to several fasted laboratory animals, i.e. rats (with and without adrenalectomy), guinea pigs, dogs, streptozotocin-treated diabetic pigs and db/db-mice. In addition, the substances have a more pronounced lowering effect on ketone bodies in the blood than any other known substance. The minimal dose for lowering blood glucose significantly in rats is 15 mumol/kg. The corresponding dose for a significant lowering of ketone bodies in the blood is less than 2.4 mumol/kg. Hence, the substance B 807-27 is approximately five times more potent than tolbutamide and 30 times more potent than the biguanide buformin with respect to lowering blood glucose levels. B 807-27 differs from the sulphonylureas in that it fails to stimulate insulin secretion. In contrast to the biguanides, the substance decreases rather than increases blood lactate concentration and blocks both fatty acid oxidation and gluconeogenesis. The therapeutic usefulness of these compounds in diabetic man remains to be elucidated.