Georges Van den Berghe

Inhibition by AICA Riboside of Gluconeogenesis in Isolated Rat Hepatocytes

5-Amino-4-imidazolecarboxamide (AICA) riboside, the nucleoside corresponding to AICA ribotide (AICAR or ZMP), an intermediate of the de novo pathway of purine biosynthesis, was found to exert a dose-dependent inhibition on gluconeogenesis in isolated rat hepatocytes. Production of glucose from lactate-pyruvate mixtures was half-maximally inhibited by ∼100 μM and completely suppressed by 500 μM AICA riboside. AICA riboside also inhibited the production of glucose from all other gluconeogenic precursors investigated, i.e., fructose, dihydroxyacetone, and L-proline. Measurements of intermediates of the glycolytic-gluconeogenic pathway showed that AICA riboside provoked elevations of triose phosphates and fructose-1,6-bisphosphate and decreases in fructose-6-phosphate and glucose-6-phosphate. The effects of AICA riboside persisted when the cells were washed 10 min after its addition but were suppressed by 5-iodotubercidin, an inhibitor of adenosine kinase. AICA riboside provoked a dose-dependent buildup of normally undetectable Z nucleotides. After 20 min of incubation with 500 μM AICA riboside, ZMP, ZTP, and ZDP reached 3, 0.3, and 0.1 μmol/g cells, respectively. Concentrations of ATP were not significantly modified by addition of up to 500 μM AICA riboside when the cells were incubated with lactate-pyruvate but decreased with fructose or dihydroxyacetone. The activity of rat liver fructose-1,6-bisphosphatase was inhibited by ZMP with an apparent Ki of 370 μM. It is concluded that AICA riboside exerts a suppressive effect on gluconeogenesis because it provokes an accumulation of ZMP, which inhibits fructose-1,6-bisphosphatase. Because gluconeogenesis is increased in diabetes, our observations warrant a search for related inhibitors of f ructose-1,6-bisphosphatase, which may have therapeutic potential in this disorder.

Concentration Dependence and Time Course of the Effects of Glucose on Adenine and Guanine Nucleotides in Mouse Pancreatic Islets

Changes in the ATP:ADP ratio in pancreatic B cells may participate in the regulation of insulin secretion by glucose. Here, we have investigated the possible role of guanine nucleotides. Mouse islets were incubated in a control medium (when K+-ATP channels are the major site of regulation) or in a high K+ medium (when glucose modulates the effectiveness of cytosolic Ca2+ on exocytosis). Glucose induced a concentration-dependent (0-20 mM) increase in GTP and a decrease in GDP in both types of medium, thus causing a progressive rise of the GTP:GDP ratio. ATP and ADP levels were 4-5-fold higher but varied in a similar way as those of guanine nucleotides. Insulin secretion was inversely correlated with ADP and GDP levels and positively correlated with the ATP:ADP and GTP:GDP ratios between 6 and 20 mM glucose in control medium and between 0 and 20 mM glucose in high K+ medium. The increases in the GTP:GDP and ATP:ADP ratios induced by a rise of glucose were faster than the decreases induced by a fall in glucose, but the changes of both ratios were again parallel. In conclusion, glucose causes large, concentration-dependent changes in guanine as well as in adenine nucleotides in islet cells. This raises the possibility that both participate in the regulation of nutrient-induced insulin secretion.

Adenylosuccinase Deficiency - An Inborn Error of Purine Nucleotide Synthesis

Clinical and biochemical data are presented on eight children with adenylosuccinase deficiency. This newly discovered inborn error of purine metabolism is characterized by an accumulation in body fluids of succinyladenosine (S-Ado) and succinylaminoimidazole carboxamide riboside (SAICA riboside), the dephosphorylated derivatives of the two substrates of adenylosuccinase. Six living children (three boys and three girls) and one deceased sibling displayed severe psychomotor retardation. Epilepsy was documented in five cases, autistic features in three, and growth retardation associated with muscular wasting in a brother and sister. In the cerebrospinal fluid, plasma and urine of these patients, the S-Ado/SAICA riboside ratio was between 1 and 2. In striking contrast, the eighth patient (a girl) was markedly less mentally retarded. Most noteworthy, the S-Ado/SAICA riboside ratio in her body fluids was around 5, suggesting that her milder psychomotor retardation was causally linked to this higher ratio. Adenylosuccinase deficiency was demonstrated in the liver of all seven living children, in the kidney of three patients in whom the enzymatic activity was measured, and in the muscle of three patients, including the two with muscular wasting. In fibroblasts of the six severely retarded patients, adenylosuccinase activity was reduced to approximately 40% of normal; in the patient with the higher S-Ado/SAICA riboside ratio, it reached only 6% of normal. The clinical heterogeneity of adenylosuccinase deficiency justifies systematic screening for the enzyme defect in unexplained neurological disease.

Metabolic Effects of Fructose in the Liver

Publisher Summary This chapter explores the metabolic effects of fructose in the liver. The main mechanism of action of fructose can be traced to its very rapid metabolization, resulting in modification of the concentration of metabolites, most notably ATP, GTP, and P i , which have important regulatory functions. Fructose metabolism bypasses the regulatory phosphofructokinase step. This provides high amounts of substrates that can be readily utilized in the absence of normal metabolic control. Fructose should thus not be considered a substitute for glucose in the human diet and in parenteral nutrition. It constitutes, however, a very useful tool for the study of metabolic regulations. It also appears that fructose favors the release of hepatic triglycerides by stimulation of their biosynthetic pathway, associated with inhibition of the catabolism of fatty acids. Fructose increases the hepatic consumption of oxygen by up to 100% in vivo , in slices, in the perfused organ, and in isolated cells. Ethanol by itself has no effect on this parameter, but if fructose is added while ethanol is being metabolized, the increase in oxygen consumption is approximately doubled as compared with the effects of adding fructose alone.

Effects of Nicardipine and Nisoldipine on Myocardial Metabolism, Coronary Blood Flow and Oxygen Supply in Angina Pectoris

The effects of the calcium antagonists nicardipine and nisoldipine on left ventricular (LV) metabolism were analyzed in 32 patients with angina pectoris. Measurements were made at a fixed heart rate under the basal state and during a cold pressor test (CPT). After administration of the drugs, coronary blood flow increased significantly and the mean aortic pressure decreased by 10% (p less than 0.01) in the basal state and by 11% (p less than 0.01) during CPT. Despite the reduction in pressure-rate product, myocardial oxygen consumption was unchanged in the basal state (18 +/- 4 vs 19 +/- 4 ml/min, difference not significant) and during CPT (21 +/- 5 vs 21 +/- 5 ml/min, difference not significant); this discrepancy between a reduced pressure-rate product and an unchanged oxygen consumption was also noted when nicardipine was given after propranolol (0.1 mg/kg; 12 patients). Both agents also increased LV lactate uptake, particularly during CPT (+13 mumol/min, p less than 0.05 vs control CPT) and reduced LV glutamine production. In 10 patients in whom 14C-lactate was infused, the chemical LV lactate extraction ratio increased more than the 14C-lactate extraction ratio after administration of the drugs, indicating a reduction in LV lactate production. The data are consistent with the hypothesis that nicardipine and nisoldipine improve perfusion and aerobic metabolism in chronically ischemic areas, resulting in an augmented oxygen consumption and in a reduced lactate production.

Regional brain glucose utilization in adenylosuccinase-deficient patients measured by positron emission tomography.

ABSTRACT: Regional brain glucose utilization was investigated by positron emission tomography with fluorodeox-yglucose in three children with adenylosuccinase deficiency. A consistent pattern was found in the three patients, namely a marked decrease of fluorodeoxyglucose uptake in all gray structures, with the exception of the cerebellum, which was minimally affected. Anomalies predominated in the cerebral cortex, particularly in the anterior regions; they were less pronounced in thalamus and basal ganglia. The observations suggest that positron emission tomography may be a useful tool for the localization of the deleterious effects of metabolic diseases and for the investigation of their pathophysiologic mechanisms.

Activation of deoxycytidine kinase by UV-C-irradiation in chronic lymphocytic leukemia B-lymphocytes.

Deoxycytidine kinase (dCK), a key enzyme of the deoxynucleoside salvage pathway, might have a preponderant role in DNA synthesis in resting chronic lymphocytic leukemia B-lymphocytes. In these cells, two important enzymes in deoxynucleoside triphosphate production, ribonucleotide reductase and thymidine kinase (TK), both cell-cycle regulated, are indeed very weakly expressed. This study investigated the regulation of dCK activity in response to UV-C light, a condition which causes DNA lesions and DNA repair synthesis. We observed that activity of dCK in B-CLL cells was upregulated up to 3-fold, 30 min after irradiation with 30 J/m(2) UV-C, whereas TK activity was unchanged. Activation of dCK by UV-C light was caused neither by a change in concentration of a low molecular weight metabolite nor by an increase in the amount of dCK protein. Activation of dCK by UV-C was mimicked by H(2)O(2), markedly counteracted by N-acetylcysteine, a general antioxidant, and completely abolished by the growth factor receptor inhibitor suramin. Taken together, these results indicate that dCK activity is upregulated by UV-C light through a postranslational modification that may be initiated at the cell surface through oxidative mechanisms. Suramin also suppressed the increase in DNA repair synthesis elicited by UV-C irradiation, suggesting that upregulation of dCK activity could contribute to the normal completion of DNA repair synthesis elicited by UV light.

Radiochemical assay of adenylosuccinase: demonstration of parallel loss of activity toward both adenylosuccinate and succinylaminoimidazole carboxamide ribotide in liver of patients with the enzyme defect

A radiochemical assay for adenylosuccinase, an enzyme which intervenes twice in the biosynthesis of adenine nucleotides, has been developed. The two substrates of the enzyme, succinylaminoimidazole carboxamide ribotide (SAICAR) and adenylosuccinate (S-AMP), were synthesized in radioactive form by incubating [2,3-14C]fumarate and, respectively, AICAR and AMP with partially purified adenylosuccinase from yeast. Enzyme activities were determined by measuring the release of labeled fumarate after its separation from the substrate by chromatography on polyethyleneimine thin-layer plates. The ratio of the activity of adenylosuccinase measured with SAICAR compared to that with S-AMP was about 1 in crude extracts of rat liver and muscle and around 0.5 in human liver. In rat and human liver, but not in rat muscle, 20 to 40% of both activities of adenylosuccinase were lost after freezing at -80 degrees C followed by thawing. In the liver of patients with adenylosuccinase deficiency, in whom the deficiency had hitherto been measured only with S-AMP, the activity of the enzyme toward S-AMP and SAICAR was found to be lost in parallel. This is in accordance with the finding that both SAICA-riboside and succinyladenosine accumulate in adenylosuccinase-deficient patients.

ENZYME DEFECT IN PRIMARY GOUT

The rate-limiting step in the degradation of adenine nucleotides in the liver is the conversion of adenosine monophosphate (A.M.P.) to inosine monophosphate by A.M.P. deaminase, which is normally 95% inhibited. When the inhibition is released, uric acid is formed in large excess, and the biosynthesis of purines is increased. We therefore propose that congenital hyperuricaemia is caused by the presence of an abnormal A.M.P. deaminase, which is less sensitive to its physiological inhibitors. Verification of the hypothesis depends upon the availability of liver tissue from patients with congenital hyperuricaemia for kinetic analysis of A.M.P. deaminase. A call for collaboration is addressed to the medical community.

Potentiation of antitumor effects of cyclophosphamide derivatives in B-chronic lymphocytic leukemia cells by 2-chloro-2'-deoxyadenosine.

Because 2-chloro-2′-deoxyadenosine (CdA) is active in B-chronic lymphocytic leukemia (B-CLL), and may interfere with DNA repair, we investigated the potentiating effect of CdA on the cytotoxicity induced in vitro in B-CLL lymphocytes by cyclophosphamide (CP) derivatives, which induce DNA damage by DNA cross-linking. Exposure to CdA at clinically achievable concentrations for 2 h, followed by mafosfamide (MAF) or 4-hydroxycyclophosphamide (4HC) for 22 h, resulted in synergistic cytotoxicity in the majority of B-CLL samples tested. Synergy between CdA and MAF was observed in cell samples of sensitive/untreated patients, as well as in cells of resistant/pretreated patients, particularly at the highest concentrations of MAF. In the cells treated with CdA and MAF, we observed loss in ATP and hallmarks of apoptosis, as evidenced by cellular morphology and high molecular weight DNA fragmentation. The synergy could be explained neither by an influence of MAF on the phosphorylation of CdA, nor by an increase in the incorporation of CdA into DNA in the presence of MAF. The in vitro synergy between CdA and CP derivatives provides a rationale for the use of this association in B-CLL patients.