In vivo studies have demonstrated that hepatic glucose production is poorly responsive to insulin in genetically obese Zucker rats. In this work, we have investigated the modulation by insulin of basal gluconeogenesis, fructose 2,6-bisphosphate levels, and pyruvate kinase and 6-phosphofructo 2-kinase activities in hepatocytes isolated from fed obese (fa/fa) or lean (Fa/-) rats. Gluconeogenesis was estimated by the conversion of a mixture of [14C]lactate-pyruvate to [14C]glucose. Basal gluconeogenesis was significantly reduced in hepatocytes isolated from obese rats compared to that measured in hepatocytes from lean animals (0.63 +/- 0.09 vs. 1.47 +/- 0.05 mumol lactate converted to glucose/g cells.20 min; n = 3-4; P < 0.001). In hepatocytes isolated from lean rats, insulin, without affecting the cellular cAMP concentration, caused a dose-dependent inhibition of the rate of gluconeogenesis, which was accompanied by a significant increase in fructose 2,6-bisphosphate levels and activation of both pyruvate kinase and 6-phosphofructo 2-kinase. In contrast, in hepatocytes isolated from obese (fa/fa) rats, neither basal gluconeogenesis nor any of the other metabolic parameters mentioned were significantly modified by insulin, even when assayed at high hormonal concentrations (10 nM). These results demonstrate a lack of responsiveness of hepatic gluconeogenesis to short term insulin action in genetically obese (fa/fa) rats.
Hereditary fructose intolerance (HFI) is an autosomal recessive metabolic disorder caused by aldolase (fructosediphosphate aldolase, EC 4.1.2.13) B deficiency.1 The B isoform of aldolase is critical for the metabolism of exogenous fructose by the liver, kidney, and intestine, since it can use fructose-1-phosphate as substrate at physiological concentrations, unlike aldolases A and C. Affected subjects suffer abdominal pain, vomiting, and hypoglycaemia after the ingestion of fructose, sucrose, or sorbitol. Continued ingestion of noxious sugars causes hepatic and renal injury, which eventually leads to liver cirrhosis and sometimes death, particularly in small infants.1 Treatment consists of strict elimination of fructose, sucrose, and sorbitol from the diet immediately after HFI is suspected. This diet exclusion therapy allows for a rapid recovery and, if liver and kidney damage is not irreversible, an uneventful course thereafter. Confirmatory diagnosis is generally made by intravenous fructose tolerance tests and assays of aldolase B activity in hepatic biopsies.Since the gene coding for human aldolase B (ALDOB) was cloned, 2 at least 22 different mutations associated with HFI have been described.3 4 Kinetic analyses of recombinant aldolase B mutants and molecular modelling studies have shown important structure-function implications for several aldolase B residues affected by HFI mutations.5-8 The first HFI mutation identified, termed A149P, 9 is a G→C transversion at the first base of codon 149, which replaces the normal alanine by a proline residue. This missense mutation accounts for more than 50% of mutant alleles in HFI patients from different populations world wide 3 ; the frequency of heterozygous carriers in the United Kingdom has been estimated to be 1.32 ± 0.49%, which allows the prediction of an incidence of HFI associated with the A149P allele of 1 in 23 000 births. 10 The three more common aldolase B mutations, A149P, A174D, and N334K, account for more than 80% of HFI alleles in populations from European countries.4 11 Although there are many reports dealing with the identification of aldolase B mutations associated with HFI in different European populations, studies on the incidence of these mutations in Spanish subjects have not been reported.In this work, we have analysed the molecular defects in the ALDOB gene in 28 HFI patients from Spain. For this purpose, we have performed PCR amplification of the aldolase B coding exons from the probands and subsequent analysis by restriction endonuclease digestion, allele specific oligonucleotide (ASO) hybridisation, and direct sequencing. Our results have allowed us to estimate the frequencies of HFI alleles in Spanish patients, as well as to discover two novel mutations in the ALDOB gene (g.4271C>G and g.1133G>A) that can cause HFI. MATERIALS AND METHODS SubjectsTwenty-eight HFI patients from 21 independent families, referred by hospitals in Spain, were studied. Probands were resident in the following regions: Madrid (11 families), Andalusia (4), Galici...
In vivo studies have demonstrated that hepatic glucose production is poorly responsive to insulin in genetically obese Zucker rats. In this work, we have investigated the modulation by insulin of basal gluconeogenesis, fructose 2,6-bisphosphate levels, and pyruvate kinase and 6-phosphofructo 2-kinase activities in hepatocytes isolated from fed obese (fa/fa) or lean (Fa/-) rats. Gluconeogenesis was estimated by the conversion of a mixture of [14C]lactate-pyruvate to [14C]glucose. Basal gluconeogenesis was significantly reduced in hepatocytes isolated from obese rats compared to that measured in hepatocytes from lean animals (0.63 +/- 0.09 vs. 1.47 +/- 0.05 mumol lactate converted to glucose/g cells.20 min; n = 3-4; P < 0.001). In hepatocytes isolated from lean rats, insulin, without affecting the cellular cAMP concentration, caused a dose-dependent inhibition of the rate of gluconeogenesis, which was accompanied by a significant increase in fructose 2,6-bisphosphate levels and activation of both pyruvate kinase and 6-phosphofructo 2-kinase. In contrast, in hepatocytes isolated from obese (fa/fa) rats, neither basal gluconeogenesis nor any of the other metabolic parameters mentioned were significantly modified by insulin, even when assayed at high hormonal concentrations (10 nM). These results demonstrate a lack of responsiveness of hepatic gluconeogenesis to short term insulin action in genetically obese (fa/fa) rats.
Genetically obese (fa/fa) Zucker rats show oral glucose intolerance, an alteration that has been attributed at least in part to an impaired suppression of hepatic glucose output after the ingestion of glucose. In this work, we studied the influence of different concentrations of glucose (2.5-30 mM) on gluconeogenesis from a mixture of [14C]lactate-pyruvate as well as on fructose 2,6-bisphosphate levels, pyruvate kinase activity, and flux through the reaction catalyzed by 6-phosphofructo-1-kinase, in hepatocytes isolated from fed obese (fa/fa) or lean (Fa/-) rats. In hepatocytes isolated from lean rats, incubation with increasing concentrations of glucose caused a dose-dependent inhibition of gluconeogenesis (5.02 +/- 0.54 and 1.82 +/- 0.33 mumol lactate converted to glucose/g cells.20 min in hepatocytes incubated in the presence of 2.5 and 30 mM glucose, respectively; n = 4 experiments; P < 0.01) together with a significant elevation of the fructose 2,6-bisphosphate content and a stimulation of the flux through 6-phosphofructo-1-kinase reaction. Glucose also provoked a dose-dependent activation of pyruvate kinase in the absence of changes in the cellular concentration of cAMP. In liver cells from obese animals, gluconeogenesis was not significantly modified by raising the glucose concentration in the incubation medium (1.26 +/- 0.11 and 0.83 +/- 0.14 mumol lactate converted to glucose/g cells.20 min in hepatocytes incubated with 2.5 and 30 mM glucose, respectively; n = 4 experiments; P = 0.11) despite significant increases in both fructose 2,6-bisphosphate levels and flux through the 6-phosphofructo-1-kinase reaction. In these cells, pyruvate kinase was only slightly activated by high glucose concentrations. These results indicate that, unlike fructose 2,6-bisphosphate levels and flux through the 6-phosphofructo-1-kinase reaction, hepatic gluconeogenesis is unresponsive to high glucose concentrations in genetically obese (fa/fa) rats.
In different types of mammalian cells, insulin has been shown to promote the release of an inositol phosphate glycan (InsP-glycan) through the hydrolysis of a glycosyl-phosphatidylinositol (glycosyl-PtdIns). This InsP-glycan, which has been demonstrated to be taken up by intact cells, may mediate some of the biological effects of insulin. We have investigated how the insulin resistance expressed in genetically obese (fa/fa) rats affects the glycosyl-PtdIns signaling system in isolated hepatocytes compared to what occurs in hepatocytes isolated from lean (Fa/-) rats. The hepatocyte content of glycosyl-PtdIns was reduced by about 30% in obese rats, with respect to that measured in lean rats (2553 +/- 138 vs. 3334 +/- 115 dpm/mg protein; P < 0.01; n = 5). This reduction was accompanied by a marked blockade of the insulin-mediated glycosyl-PtdIns hydrolysis as well as a decrease (approximately 30%) in the rate of InsP-glycan uptake by the isolated liver cells. Obese Zucker rat hepatocytes also showed a significant decrease in the effects of both insulin and InsP-glycan on the stimulation of glycogen synthesis and the activation of glycogen synthase compared to hepatocytes isolated from lean rats. Our results demonstrate that genetic obesity in Zucker (fa/fa) rats is associated with an impairment of the glycosyl-PtdIns-dependent insulin signaling system.
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