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F Assimacopoulos-Jeannet

Publications and source records attributed to F Assimacopoulos-Jeannet.

At least 37 records · Page 2Linked to original sources
PubMed1997

Fatty acids rapidly induce the carnitine palmitoyltransferase I gene in the pancreatic beta-cell line INS-1.

Fatty acids are important metabolic substrates for the pancreatic beta-cell, and long term exposure of pancreatic islets to elevated concentrations of fatty acids results in an alteration of glucose-induced insulin secretion. Previous work suggested that exaggerated fatty acid oxidation may be implicated in this process by a mechanism requiring changes in metabolic enzyme expression. We have therefore studied the regulation of carnitine palmitoyltransferase I (CPT I) gene expression by fatty acids in the pancreatic beta-cell line INS-1 since this enzyme catalyzes the limiting step of fatty acid oxidation in various tissues. Palmitate, oleate, and linoleate (0.35 mM) elicited a 4-6-fold increase in CPT I mRNA. The effect was dose-dependent and was similar for saturated and unsaturated fatty acids. It was detectable after 1 h and reached a maximum after 3 h. The induction of CPT I mRNA by fatty acids did not require their oxidation, and 2-bromopalmitate, a nonoxidizable fatty acid, increased CPT I mRNA to the same extent as palmitate. The induction was not prevented by cycloheximide treatment of cells indicating that it was mediated by pre-existing transcription factors. Neither glucose nor pyruvate and various secretagogues had a significant effect except glutamine (7 mM) which slightly induced CPT I mRNA. The half-life of the CPT I transcript was unchanged by fatty acids, and nuclear run-on analysis showed a rapid (less than 45 min) and pronounced transcriptional activation of the CPT I gene by fatty acids. The increase in CPT I mRNA was followed by a 2-3-fold increase in CPT I enzymatic activity measured in isolated mitochondria. The increase in activity was time-dependent, detectable after 4 h, and close to maximal after 24 h. Fatty acid oxidation by INS-1 cells, measured at low glucose, was also 2-3-fold higher in cells cultured with fatty acid in comparison with control cells. Long term exposure of INS-1 cells to fatty acid was associated with elevated secretion of insulin at a low (5 mM) concentration of glucose and a decreased effect of higher glucose concentrations. It also resulted in a decreased oxidation of glucose. The results indicate that the CPT I gene is an early response gene induced by fatty acids at the transcriptional level in beta- (INS-1) cells. It is suggested that exaggerated fatty acid oxidation caused by CPT-1 induction is implicated in the process whereby fatty acids alter glucose-induced insulin secretion.

Carnitine O-Palmitoyltransferase↗
PubMed1997

Endogenous glucose production, gluconeogenesis and liver glycogen concentration in obese non-diabetic patients.

Resting, post-absorptive endogenous glucose production (EGP), fractional gluconeogenesis and liver glycogen concentration were assessed in 6 lean and 5 obese non-diabetic subjects undergoing elective abdominal surgery. During the 2 days preceding these measurements, 0.3 g/day U-13C glucose had been added to their usual diet to label their endogenous glycogen stores. On the morning of day 3, EGP was measured with 6,6-2H glucose. Their endogenous 13C glycogen enrichment was calculated from 13CO2 and respiratory gas exchanges. Fractional gluconeogenesis was assessed as 1-(13C glucose/13C glycogen)100. EGP was similar in lean subjects (113 +/- 5 mg/min) and in obese subjects (111 +/- 6). Fractional gluconeogenesis was higher in obese (59 +/- 10%) than in lean subjects (29 +/- 8%). However, overall EGP remained constant due to a decrease in glycogenolysis. Since an increased gluconeogenesis and a decreased glycogenolysis may both contribute to increase liver glycogen concentration in obesity, hepatic glycogen concentrations were assessed in hepatic needle biopsies obtained during surgery. Hepatic glycogen concentrations were increased in obese patients (515 +/- 38 mg/g protein) compared to lean subjects (308 +/- 58, p < 0.05). It is concluded that in obese patients: a) fractional gluconeogenesis is increased; b) overall EGP is unchanged due to a proportional inhibition of glycogenolysis; c) liver glycogen concentration is increased.

Adult↗
PubMed1997

Effects of beta-adrenoceptor subtype stimulation on obese gene messenger ribonucleic acid and on leptin secretion in mouse brown adipocytes differentiated in culture.

The ob gene product is known to control food intake and energy expenditure. To determine whether thermogenic agents directly control ob gene expression, the effects of beta-adrenoceptor agonists on the level of the ob gene messenger RNA (mRNA) and on leptin secretion have been studied in mouse brown adipocytes differentiated in culture. These cells highly expressed the beta 3-adrenoceptor, the uncoupling protein, and the ob gene mRNAs. The ob gene was expressed in mouse brown adipocytes earlier than in mouse white adipocytes under the same culture conditions and to a similar level. The beta 3-, beta L-, and beta 2-adrenoceptor agonists BRL 37344, dobutamine, and terbutaline inhibited ob gene expression in mouse brown adipocytes differentiated in culture with EC50 values of 0.3, 1.0, and 85 nM, respectively. Leptin secretion by the cells under basal conditions was 78 +/- 10 pg/microgram DNA-4 h and was decreased by exposure to the beta-adrenoceptor agonists. The ob gene mRNA half-life was 9.4 h and was decreased to 2.4 h by 1 nM BRL 37344, indicating that the inhibitory effect of the beta 3-agonist might be due to destabilization of ob gene mRNA. (Bu)2cAMP (10-100 microM) and forskolin (20 microM) mimicked the effect of the beta-adrenoceptor agonists. FFA (150-800 microM) had only a small inhibitory effect on ob gene mRNA expression. The results suggest the existence in brown adipose tissue of a retroregulatory pathway by which leptin production in inhibited when the sympathetic nervous system is stimulated.

Adipocytes↗
PubMed1997

Long-chain fatty acids inhibit acetyl-CoA carboxylase gene expression in the pancreatic beta-cell line INS-1.

The mechanism whereby long-term exposure of the beta-cell to fatty acids alters the beta-cell response to glucose is not known. We hypothesized that fatty acids may alter beta-cell function by changing the expression level of metabolic enzymes implicated in the regulation of insulin secretion, in particular acetyl-CoA carboxylase (ACC). This enzyme catalyzes the formation of malonyl-CoA, a key regulator of fatty acid oxidation. Using the beta-cell line INS-1 as a model, the results show that the polyunsaturated fatty acid linoleate (C18:2) inhibited both basal and glucose-stimulated ACC mRNA induction. The inhibition was detected by 4-6 h, and a maximal 60% effect occurred at 12 h after cell exposure to the fatty acid. Linoleate, as glucose, did not modify the half-life of the ACC transcript. Prolonged exposure of INS-1 cells to linoleate also inhibited ACC protein accumulation at low and high glucose. The saturated fatty acids myristate (C14:0), palmitate (C16:0), and stearate (C18:0) were also effective as well as the monounsaturated oleate (C18:1) and the short-chain fatty acids butyrate (C4:0) and caproate (C6:0); long-chain omega3 fatty acids were ineffective. The threshold concentration for long-chain fatty acids was 0.05 mmol/l, and maximal inhibition occurred at 0.3 mmol/l. 2-bromopalmitate, a nonmetabolizable analog, had no effect, suggesting that fatty acids must be metabolized to change ACC gene expression. Prolonged exposure of INS-1 cells to palmitate, oleate, and linoleate markedly altered the glucose-induced insulin response, resulting in high basal insulin release and a suppression of glucose-induced insulin secretion. This was associated with an exaggerated (twofold to threefold) rate of fatty acid oxidation at all tested glucose concentrations. The data provide a possible mechanism to at least partially explain how fatty acids cause beta-cell insensitivity to glucose, i.e., by downregulating ACC with a resulting exaggerated fatty acid oxidation.

Acetyl-CoA Carboxylase↗
PubMed1996

Evidence for an anaplerotic/malonyl-CoA pathway in pancreatic beta-cell nutrient signaling.

A metabolic model of fuel sensing has been proposed in which malonyl-CoA and long-chain acyl-CoA esters may act as coupling factors in nutrient-induced insulin release (Prentki M, Vischer S, Glennon MC, Regazzi R, Deeney J, Corkey BE: Malonyl-CoA and long chain acyl-CoA esters as metabolic coupling factors in nutrient-induced insulin secretion. J Biol Chem 267:5802-5810, 1992). To gain further insight into the control of malonyl-CoA content in islet tissue, we have studied the short- and long-term regulation of acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS) in the beta-cell. These enzymes catalyze the formation of malonyl-CoA and its usage for de novo fatty acid biogenesis. ACC mRNA, protein, and enzymatic activity are present at appreciable levels in rat pancreatic islets and clonal beta-cells (HIT cells). Glucose addition to HIT cells results in a marked increase in ACC activity that precedes the initiation of insulin release. Fasting does not modify the ACC content of islets, whereas it markedly downregulates that of lipogenic tissues. This indicates differential regulation of the ACC gene in lipogenic tissues and the islets of Langerhans. FAS is very poorly expressed in islet tissue, yet ACC is abundant. This demonstrates that the primary function of malonyl-CoA in the beta-cells is to regulate fatty acid oxidation, not to serve as a substrate for fatty acid biosynthesis. The anaplerotic enzyme pyruvate carboxylase, which allows the replenishment of citric acid cycle intermediates needed for malonyl-CoA production via citrate, is abundant in islet tissue. Glucose causes an elevation in beta (HIT)-cell citrate that precedes secretion, and only those nutrients that can elevate citrate induce effective insulin release. The results provide new evidence in support of the model and explain why malonyl-CoA rises markedly and rapidly in islets upon glucose stimulation: 1) glucose elevates citrate, the precursor of malonyl-CoA; 2) glucose enhances ACC enzymatic activity; and 3) malonyl-CoA is not diverted to lipids. The data suggest that ACC is a key enzyme in metabolic signal transduction of the beta-cell and provide evidence for the concept that an anaplerotic/malonyl-CoA pathway is implicated in insulin secretion.

Acetyl-CoA Carboxylase↗
PubMed1995

Modulation of obese gene expression in rat brown and white adipose tissues.

The ob gene mRNA expression in rat brown adipose tissue (BAT) and epididymal white adipose tissue (WAT) was measured on Northern blots hybridized with a rat ob gene probe. The level of ob gene mRNA in BAT was about 40% of that in WAT. Fasting (36 h) or semi-starvation (10 days) decreased the ob gene mRNA level in both tissues by 62-68%, and cold exposure at 6 degrees C (24 h) decreased it in BAT (-84%) but not in WAT. Acute administration of the beta 3-adrenergic agonist Ro 16-8714 decreased the ob gene mRNA level in BAT (-51%) and WAT (-28%) of lean Zucker rats and only in BAT (-74%) of obese falfa rats. This study demonstrates that, in the rat, the ob gene is not only expressed in WAT but also in BAT, and suggests that in these two tissues, the modulation of the ob gene expression might be more closely associated with known alterations in cell lipid content than with changes in sympathetic activity.

Adipose Tissue↗
PubMed1995

In vivo effects of hyperinsulinemia on lipogenic enzymes and glucose transporter expression in rat liver and adipose tissues.

Chronic hyperinsulinemia with maintenance of euglycemia was imposed on normal rats for 4 days. In white adipose tissue, hyperinsulinemia resulted in a twofold increase in GLUT4 protein and mRNA and a sixfold to 15-fold increase in fatty acid synthase (FAS) and acetyl coenzyme A (CoA) carboxylase (ACC) activity, respectively. Lipogenic enzyme mRNA was also markedly increased (20- to 30-fold). This was specific for white adipose tissue and was not observed in brown adipose tissue. In the liver, hyperinsulinemia was accompanied by a threefold increase in glucokinase (GK) activity and mRNA and by a threefold to fivefold increase in lipogenic enzyme activities and mRNA. In agreement with the changes in lipogenic activities, lipogenesis was markedly increased in white adipose tissue and liver of hyperinsulinemic rats. The data strongly suggest that in the rat, insulin is a driving force leading to increased lipid synthesis in liver and white adipose tissue.

Acetyl-CoA Carboxylase↗
PubMed1994

Modifications of glucose and lipid metabolism in cold-acclimated lean and genetically obese rats.

Glucose turnover rate, 2-deoxy-D-[3H]glucose (2-DG) uptake, lipid synthesis in liver, white adipose tissue, and brown adipose tissue (BAT) were measured in lean FA/FA and genetically obese fa/fa rats either kept at 21 degrees C or acclimated to a cold environment (4 degrees C). After 10 days at 4 degrees C, lean rats increased their glucose turnover rate; 2-DG uptake as well as lipid synthesis in BAT were markedly stimulated. After cold acclimation, obese rats also increased glucose turnover; however, BAT glucose utilization was only slightly stimulated. Basal hyperinsulinemia and muscle insulin resistance of the obese group (as assessed by reduced 2-DG uptake in the soleus muscle) were present at room temperature and persisted at 4 degrees C. Total BAT lipid synthesis was increased to the same extent as in lean rats. Obese rat liver lipid synthesis, already much higher than normal at 21 degrees C, was further increased by cold exposure. We conclude that obese cold-acclimated fa/fa rats do not improve their muscle insulin resistance and barely improve BAT glucose utilization. We further suggest that an additional activation of hepatic lipid synthesis and oxidation thereof could participate in the heat production needed by the cold-acclimated obese rats.

Acclimatization↗
PubMed1993

Involvement of non-esterified fatty acid oxidation in glucocorticoid-induced peripheral insulin resistance in vivo in rats.

The mechanism by which glucocorticoids induce insulin resistance was studied in normal rats administered for 2 days with corticosterone then tested by euglycaemic hyperinsulinaemic clamps. Corticosterone administration induced a slight hyperglycaemia, hyperinsulinaemia and increased non-esterified fatty acid levels. It impaired insulin-stimulated total glucose utilization (corticosterone 15.7 +/- 0.7; controls 24.6 +/- 0.8 mg.kg-1 x min-1), as well as residual hepatic glucose production (corticosterone 4.9 +/- 1.0; controls 2.0 +/- 0.7 mg.kg-1 x min-1). During the clamps, insulin did not decrease the elevated non-esterified fatty acid levels in corticosterone-administered rats (corticosterone 1.38 +/- 0.15, controls 0.22 +/- 0.04 mmol/l). Corticosterone administration decreased the in vivo insulin-stimulated glucose utilization index by individual muscles by 62 +/- 6%, and the de novo glycogen synthesis by 78 +/- 2% (n = 8-9 muscles). GLUT4 protein and mRNA levels were either unchanged or slightly increased by corticosterone administration. Inhibition of lipid oxidation by etomoxir prevented corticosterone-induced muscle but not hepatic insulin resistance. In conclusion, glucocorticoid-induced muscle insulin resistance is due to excessive non-esterified fatty acid oxidation, possibly via increased glucose fatty-acid cycle ultimately inhibiting glucose transport, or via decreased glycogen synthesis, or by a direct effect on glucose transporter translocation or activity or both.

Animals↗
PubMed1992

Effect of a beta-adrenergic agonist on glucose transport and insulin-responsive glucose transporters (GLUT4) in brown adipose tissue of control and obese fa/fa rats.

A beta-adrenergic agonist specific for brown adipose tissue, Ro 16-8714, was administered to control and obese insulin-resistant fa/fa rats and glucose utilisation measured in brown adipose tissue using the euglycaemic hyperinsulinaemic clamp combined with the injection of 2-deoxyglucose. Treatment with the beta-agonist increased basal and insulin-stimulated glucose utilization in both groups, resulting in an increased effect of the hormone in treated animals. This effect is specific for brown adipose tissue and is not found in other insulin-sensitive tissue. The total number of insulin-responsive glucose transporters (GLUT4) measured in crude membrane preparations was similar in the two groups when expressed per total tissue. They were, however, decreased in the fa/fa group when expressed per milligram of tissue. Acute treatment with the beta-adrenergic agonist increased the total number of GLUT4 in both groups. The agonist also increased the amount of mRNA coding for GLUT4 suggesting an effect on the transcription and/or on the stability of GLUT4 mRNA.

2-Hydroxyphenethylamine↗
PubMed1992

Vanadate treatment markedly increases glucose utilization in muscle of insulin-resistant fa/fa rats without modifying glucose transporter expression.

The present study examined the effects of chronic treatment with vanadate on in vivo insulin-stimulated glucose uptake by various tissues of obese and insulin-resistant fa/fa rats. It further determined whether the substantial improvement induced by vanadate administration was associated with altered expression of the insulin-responsive glucose transporter (GLUT4). Since oral Na3VO4 caused decreases in food intake and body weight, vanadate-treated fa/fa rats were compared with controls, fed ad libitum, and pair-fed rats. The animals in the three groups were submitted to hyperinsulinemic clamps combined with the 2-deoxyglucose method. At similar levels of imposed hyperinsulinemia, the glucose infusion rate (milligrams per kg.min-1) required to maintain euglycemia, extremely low in controls (0.8 +/- 0.3) and pair-fed rats (1.2 +/- 0.6), was strikingly improved in vanadate-treated rats (9.5 +/- 0.3). Correspondingly, the insulin-mediated glucose utilization indices were 2- to 3-fold higher in all types of muscle in treated rats: hindlimb skeletal muscle, diaphragm, and heart. Glucose utilization remained unaffected in white adipose tissue and jejunum, whereas it was increased by mere food restriction in brown adipose tissue of pair-fed rats. The amounts of GLUT4 and GLUT4 mRNA were then measured in the insulin-sensitive tissues of the three groups of animals. Vanadate treatment induced no change in GLUT4 mRNA or GLUT4 protein levels in any of the examined tissues. It even prevented the rise in GLUT4 protein expression caused by calorie restriction in brown adipose tissue of pair-fed rats. In conclusion, chronic administration of vanadate markedly increases the insulin-mediated glucose uptake in muscle of insulin-resistant fa/fa rats without altering GLUT4 number. A functional improvement of glucose transporters due to more efficient translocation and/or increased intrinsic activity or changes in the insulin signaling pathway is, thus, likely to play a major role in the beneficial effects of vanadate.

Adipose Tissue, Brown↗
PubMed1991

Increased gene expression of lipogenic enzymes and glucose transporter in white adipose tissue of suckling and weaned obese Zucker rats.

Previous experiments have shown that insulin-induced glucose utilization is increased in white adipose tissue of young obese Zucker rats. We have investigated the possible role of over-expression of the muscle/fat glucose transporter (Glut 4) and key lipogenic enzymes in this increased insulin-responsiveness. The amount or activity and the mRNA concentrations of Glut 4, fatty acid synthase (FAS) and acetyl-CoA carboxylase (ACC) were measured before and after weaning in white adipose tissue of obese and lean Zucker rats. Comparison of the levels of Glut 4 and lipogenic-enzyme expression in 15-day-old suckling and 30-day-old weaned rats on a high-carbohydrate diet shows a marked increase in the latter group. The increase was, in lean and obese rats respectively, 6- and 7-fold for the amount of Glut 4 and 2- and 3-fold for its mRNA concentrations, 40- and 100-fold for the activity of lipogenic enzymes (FAS and ACC) and 30- and 10-fold for their mRNA concentrations. Furthermore, all these parameters, except the amount of Glut 4, were 2-5-fold higher in obese rats, both before and after weaning. Changes at weaning were largely blunted when rats were weaned on to a high-fat diet, although the differences between lean and obese rats persisted, and even became significant for the amount of Glut 4. Whatever the experimental conditions, plasma insulin levels were significantly higher in obese than in lean rats. These results indicate the existence of an enhanced expression of Glut 4, FAS and ACC in white adipose tissue of young obese fa/fa rats which could be related to the increased plasma insulin levels.

Acetyl-CoA Carboxylase↗
PubMed1991

Glucose transporters: structure, function, and regulation.

Glucose is transported into the cell by facilitated diffusion via a family of structurally related proteins, whose expression is tissue-specific. One of these transporters, GLUT4, is expressed specifically in insulin-sensitive tissues. A possible change in the synthesis and/or in the amount of GLUT4 has therefore been studied in situations associated with an increase or a decrease in the effect of insulin on glucose transport. Chronic hyperinsulinemia in rats produces a hyper-response of white adipose tissue to insulin and resistance in skeletal muscle. The hyper-response of white adipose tissue is associated with an increase in GLUT4 mRNA and protein. In contrast, in skeletal muscle, a decrease in GLUT4 mRNA and a decrease (tibialis) or no change (diaphragm) in GLUT4 protein are measured, suggesting a divergent regulation by insulin of glucose transport and transporters in the 2 tissues. In rodents, brown adipose tissue is very sensitive to insulin. The response of this tissue to insulin is decreased in obese insulin-resistant fa/fa rats. Treatment with a beta-adrenergic agonist increases insulin-stimulated glucose transport, GLUT4 protein and mRNA. The data suggest that transporter synthesis can be modulated in vivo by insulin (muscle, white adipose tissue) or by catecholamines (brown adipose tissue).

Adipose Tissue↗
PubMed1991

Effects of a peroxisome proliferator on beta-oxidation and overall energy balance in obese (fa/fa) rats.

The aim of the study was to examine in the obese Zucker (fa/fa) rats the effect of a peroxisome proliferator nafenopin on liver and brown adipose tissue peroxisomal and mitochondrial beta-oxidation enzyme activities and on the overall energy dissipation. A 17-day nafenopin treatment increased liver wet weight 2.1-fold and liver total acyl-CoA oxidase and mitochondria beta-oxidative activities 32- and 4.6-fold, respectively. It increased the interscapular brown adipose tissue (IBAT) acyl-CoA oxidase activity 2.1-fold but had no effect on the mitochondria beta-oxidative activity. Because nafenopin was found to decrease food intake by 22%, obese nafenopin-treated rats were compared with a group of obese pair-fed rats. Both food restriction and nafenopin treatment decreased body weight gain, but a decrease (14%) in fat content was only observed in nafenopin-treated rats. Food restriction of obese rats decreased the mean metabolic rate by 13%, and nafenopin treatment prevented this decrease. Both food restriction and nafenopin treatment decreased the mean daily respiratory quotient (RQ). However, the RQ of nafenopin-treated rats was steadily lower than that of control, whereas that of food-restricted rats was the same as that of control animals during the feeding period and decreased when food supply was exhausted. The increase in liver and IBAT fatty acid beta-oxidative activities may be the cause of the decreased lipid accretion measured in obese rats.

Adipose Tissue↗
PubMed1990

Insulin activates 6-phosphofructo-2-kinase and pyruvate kinase in the liver. Indirect evidence for an action via a phosphatase.

The effect of insulin on hepatic glucose production has been studied in anesthetized rats in the postabsorptive state. Insulin decreases significantly hepatic glucose production within 5-10 min. It also increases the level of fructose 2,6-bisphosphate, via an increase in the Vmax of 6-phosphofructo-2-kinase and concomitantly decreased the activity of fructose-2,6-bisphosphatase, resulting in a 5-fold increase in the ratio of kinase/phosphatase. Insulin also increased the apparent Kd of pyruvate kinase for phosphoenolpyruvate. The changes in the activity of 6-phosphofructo-2-kinase and pyruvate kinase were measured after separation from possible modulators, and suggest a decrease in their phosphorylation state which cannot be attributed to a decrease in the level of cAMP or in the activity of cAMP-dependent protein kinase since these two parameters were not modified by insulin. In addition, neither the activity of phosphorylase a nor that of glycogen synthase were modified. The data strongly suggest that the increase in the glycolytic rate plays a role in the effect of insulin on hepatic glucose production and that insulin mediates its effect on the activity of these enzymes via one or more phosphatases.

Adenosine Triphosphate↗