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M Guerre-Millo

Publications and source records attributed to M Guerre-Millo.

At least 37 records · Page 2Linked to original sources
PubMed1998

A high glycemic index starch diet affects lipid storage-related enzymes in normal and to a lesser extent in diabetic rats.

The of this study was to evaluate the chronic effects of a high (waxy corn) vs. a low (mung beans) glycemic index starch diet on the lipogenic enzymes, fatty acid synthase (FAS) and lipoprotein lipase (LPL). Normal and diabetic (streptozotocin-injected on d 2 of life) male Sprague-Dawley rats consumed a diet containing 575 g/kg carbohydrates either as waxy cornstarch (WCS) or as mung bean starch (MBS). After 3 wk, neither body weights nor relative epididymal fat pad weights differed. In diabetic rats, the WCS diet induced high basal plasma insulin levels. Plasma triglycerides were not significantly affected by diet in either normal or diabetic rats. Adipose tissue and liver LPL activities were not modified by the type of starch in the diet. In normal rats, FAS activity and gene expression in epididymal adipose tissue but not in liver were greater in rats consuming the WCS diet than in those consuming MBS. To evaluate the implication of insulin in this regulation, two genes regulated by insulin [GLUT4 and phosphoenolpyruvate carboxykinase (PEPCK)] were also studied. The high glycemic index WCS diet compared with the low glycemic index MBS diet resulted in lower hepatic PEPCK mRNA in both normal and diabetic rats. Normal, but not diabetic rats fed WCS had greater GLUT4 gene expression in adipocytes than did those fed MBS. We conclude that the total replacement of 575 g/kg low glycemic index starch by a high glycemic index starch for 3 wk caused the following in normal rats: 1) high FAS activity and mRNA in adipose tissue but not in liver and 2) high GLUT4 gene expression in adipose tissue. In both normal and diabetic rats this same diet resulted in lower hepatic PEPCK mRNA. Therefore, high glycemic index starch diet is implicated in stimulating FAS activity and lipogenesis and might have undesirable long-term metabolic effects.

Adipose Tissue↗
PubMed1998

Inhibition of insulin secretion by leptin in normal rodent islets of Langerhans.

The recently discovered adipose cell-specific hormone called leptin decreases food intake and increases energy expenditure in rodents through a pathway involving hypothalamic leptin receptors, OB-R. In addition, leptin decreases insulin circulating levels independent of the reduction in food intake. Whether or not the hormone has a direct effect on pancreatic beta-cells is not clear, because previous in vitro studies have led to controversial results depending on the animal model used. The present study was designed to investigate the effects of leptin in islets of Langerhans isolated from normal rodents. Three isoforms of the leptin receptor, OB-Ra, b, and f, were detected by RT-PCR analysis of total RNA from rat islets. In static incubations, leptin (10 ng/ml) did not alter basal insulin secretion nor insulin secretion stimulated by glucose alone, potassium chloride, or ketoisocaproic acid. In contrast, insulin secretion stimulated by glucose + 3-isobutyl 1-methylxanthine (IBMX) was inhibited by 34 +/- 15% (n = 4, P < 0.05). This was further substantiated in perifusion experiments, in which leptin decreased by 31 +/- 3% (n = 5, P < 0.01) glucose + IBMX-stimulated insulin release. Similarly, in mouse islets a significant inhibitory effect of leptin (-31 +/- 4%, n = 6, P < 0.05) was observed only on glucose + IBMX-stimulated insulin secretion, with no effect of the hormone on basal nor glucose-stimulated secretion. Finally, leptin was totally inefficient in islets isolated from obese fa/fa rats, which bear a mutation in OB-R. These results suggest that, in normal rodent islets, leptin specifically inhibits IBMX-potentiated glucose-induced insulin secretion, through a direct effect involving at least one of the three isoforms of OB-R expressed in islets.

1-Methyl-3-isobutylxanthine↗
PubMed1998

Moderate intake of n-3 fatty acids for 2 months has no detrimental effect on glucose metabolism and could ameliorate the lipid profile in type 2 diabetic men. Results of a controlled study.

OBJECTIVE: To evaluate the effect of a moderate dose of fish oil on glycemic control and in vivo insulin action in type 2 diabetic men with elevated plasma triacylglycerols and to determine the effect of the same treatment on gene expression of GLUT4, lipoprotein lipase (LPL), and hormone-sensitive lipase (HSL) in the abdominal adipose tissue. RESEARCH DESIGN AND METHODS: A total of 12 type 2 diabetic men were randomly allocated to 2 months of 6 g daily of either fish oil or sunflower oil, separated by a 2-month washout interval, in a double-blind crossover design. RESULTS: For glucose metabolism, 2 months of fish oil supplementation compared with sunflower oil led to similar fasting plasma insulin, glucose, and HbA1c. Basal hepatic glucose production did not increase after fish oil. There was no difference in insulin suppression of hepatic glucose production nor in insulin stimulation of whole-body glucose disposal measured by the euglycemic-hyperinsulinemic clamp. Fish oil did not ameliorate the low mRNA level of GLUT4 in adipose tissue of these patients. For lipid profile, fish oil lowered plasma triacylglycerol more than sunflower oil (P < 0.05) and tended to increase the amount of mRNA of both LPL and HSL in adipose tissue. CONCLUSIONS: A moderate dose of fish oil did not lead to deleterious effects on glycemic control or whole-body insulin sensitivity in type 2 diabetic men, with preserved triacylglycerol-lowering capacities.

Basal Metabolism↗
PubMed1998

Does leptin regulate insulin secretion?

The hormone leptin secreted by adipocytes plays a major role in body weight homeostasis. Its main target is the hypothalamus, but it also affects several peripheral tissues directly. The direct effect of leptin on insulin secretion by pancreatic beta cells has been investigated in several studies, though with controversial results. Interpretation of these data must take into account the animal model and the leptin concentrations used. Experiments carried out on islets from ob/ob mice harbouring a mutation in the leptin gene are not representative of the leptin effect in normal animals because ob/ob islets are very sensitive to the hormone and show altered regulation of insulin secretion. In normal rodent islets, physiological concentrations of leptin seem to inhibit insulin secretion only when the islets are maximally stimulated with high concentrations of glucose associated with secretion potentiators. Several isoforms of the leptin receptor are expressed in pancreatic beta cells. Indirect experimental evidence suggests that leptin signalling in islets requires the long isoform of the receptor. The molecular mechanisms underlying the effect of leptin on insulin secretion are unknown. Our hypothesis is that physiological concentrations of leptin in normal rodents do not affect the direct pathway (coupling a rise in glucose concentration to insulin secretion) but modulate a potentiation of glucose-induced insulin secretion involving cyclic AMP or phospholipase C/protein kinase C activation.

Adipocytes↗
PubMed1997

Rab 3D in rat adipose cells and its overexpression in genetic obesity (Zucker fatty rat).

Members of the Rab 3 subfamily of low-molecular-mass GTP-binding proteins have been functionally implicated in regulated exocytosis. The aim of the present study was to examine the subcellular distribution of a member of this family, Rab 3D, in rat adipose cells, given the hypothesis that this protein might be involved in insulin-stimulated GLUT4 exocytosis. We show that Rab 3D immunoreactivity is associated predominantly with the high-density microsomal fraction, where the signal intensity is 3- and 7-fold greater than that in plasma membranes and low-density microsomes respectively. Rab 3D does not co-localize with GLUT4 on immuno-isolated intracellular vesicles and, unlike GLUT4, it is not redistributed in response to insulin. Thus, if Rab 3D plays a role in GLUT4 trafficking, it relies on mechanisms independent of relocation. We observed that Rab 3D is overexpressed in adipose cells of obese (fa/fa) Zucker rats, in a tissue- and isoform-specific manner. The pathophysiological significance of this defect remains elusive. This could form the molecular basis for altered adipose secretory function in obesity.

Adipose Tissue↗
PubMed1997

Regulation of ob gene and overexpression in obesity.

The ob gene product, called leptin, is a recently discovered hormone secreted by the adipose cells. By acting as a satiety factor and increasing energy expenditure, leptin plays a major role in body weight homeostasis in mice. Ob gene and leptin production by the adipose cells are under the control of various hormonal and metabolic factors. Ob mRNA levels are markedly reduced by fasting and restored to normal by refeeding. High-fat feeding increases ob gene and plasma leptin, and induces a state of resistance to leptin. Two hormones, insulin and corticosterone, increase leptin production in rodent and human adipose cells. In contrast, the activity of the sympathetic nervous system exerts an opposite effect, mainly through activation of the adipose beta 3-adrenergic receptors. Leptin synthesis is also decreased by thiazolidinediones, a new class of antidiabetic drugs. The obese Zucker fa/fa rats bear a mutation in the leptin receptor gene (OB-R) and are leptin resistant. In these rats, ob mRNA levels are increased early in life and are not reduced by fasting. This suggests that functional OB-Rs are required for the generation of the signal(s) that downregulates ob gene expression in the adipose cell. The extent to which this is relevant to human obesities, which are characterized by increased leptin levels, remains to be determined.

Adipocytes↗
PubMed1997

Pioglitazone induces in vivo adipocyte differentiation in the obese Zucker fa/fa rat.

Thiazolidinediones are potent antidiabetic compounds, in both animal and human models, which act by enhancing peripheral sensitivity to insulin. Thiazolidinediones are high-affinity ligands for peroxisome proliferator-activated receptor-gamma, a key factor for adipocyte differentiation, and they are efficient promoters of adipocyte differentiation in vitro. Thus, it could be questioned whether a thiazolidinedione therapy aimed at improving insulin sensitivity would promote the recruitment of new adipocytes in vivo. To address this problem, we have studied the in vivo effect of pioglitazone on glucose metabolism and gene expression in the adipose tissue of an animal model of obesity with insulin resistance, the obese Zucker (fa/fa) rat. Pioglitazone markedly improves insulin action in the obese Zucker (fa/fa) rat, but doubles its weight gain after 4 weeks of treatment. The drug induces a large increase of glucose utilization in adipose tissue, where it stimulates the expression of genes involved in lipid metabolism such as the insulin-responsive GLUT, fatty acid synthase, and phosphoenolpyruvate carboxykinase genes, but decreases the expression of the ob gene. These changes are related to both an enhanced adipocyte differentiation, as shown by the large increase in the number of small adipocytes in the retroperitoneal fat pad, and a direct effect of pioglitazone on specific gene expression (phosphoenolpyruvate carboxykinase and ob genes) in mature adipocytes.

Adipocytes↗
PubMed1996

Thiazolidinediones repress ob gene expression in rodents via activation of peroxisome proliferator-activated receptor gamma.

The ob gene product, leptin, is a signaling factor regulating body weight and energy balance. ob gene expression in rodents is increased in obesity and is regulated by feeding patterns and hormones, such as insulin and glucocorticoids. In humans with gross obesity, ob mRNA levels are higher, but other modulators of human ob expression are unknown. In view of the importance of peroxisome proliferator-activated receptor gamma (PPARgamma) in adipocyte differentiation, we analyzed whether ob gene expression is subject to regulation by factors activating PPARs. Treatment of rats with the PPARalpha activator fenofibrate did not change adipose tissue and body weight and had no significant effect on ob mRNA levels. However, administration of the thiazolidinedione BRL49653, a PPARgamma ligand, increased food intake and adipose tissue weight while reducing ob mRNA levels in rats in a dose-dependent manner. The inhibitory action of the thiazolidinedione BRL49653 on ob mRNA levels was also observed in vitro. Thiazolidinediones reduced the expression of the human ob promoter in primary adipocytes, however, in undifferentiated 3T3-L1 preadipocytes lacking endogenous PPARgamma, cotransfection of PPARgamma was required to observe the decrease. In conclusion, these data suggest that PPARgamma activators reduce ob mRNA levels through an effect of PPARgamma on the ob promoter.

Adipocytes↗
PubMed1996

Unique subcellular distribution of five annexins in resting and insulin-stimulated rat adipose cells.

Several lines of evidence suggest that annexins, a family of phospholipid-binding proteins, play a role in cellular trafficking. Five annexins (I, II, V, VI, VII) were detected in rat adipose cells. They were primarily associated with the plasma membrane in a calcium-dependent manner. None of them redistributed with insulin treatment of the cells, in contrast to the glucose transporter GLUT4, which moved from intracellular membranes to the plasma membrane. Although the actual function of annexins in adipose cells remains to be determined, our data indicate that insulin-stimulated GLUT4 trafficking does not rely on a change in subcellular location of any of the five annexins detected so far in these cells.

Adipose Tissue↗
PubMed1996

The adipocyte specific transcription factor C/EBPalpha modulates human ob gene expression.

The ob gene product, leptin, apparently exclusively expressed in adipose tissue, is a signaling factor regulating body weight homeostasis and energy balance. ob gene expression is increased in obese rodents and regulated by feeding, insulin, and glucocorticoids, which supports the concept that ob gene expression is under hormonal control, which is expected for a key factor controlling body weight homeostasis and energy balance. In humans, ob mRNA expression is increased in gross obesity; however, the effects of the above factors on human ob expression are unknown. We describe the structure of the human ob gene and initial functional analysis of its promoter. The human ob gene's three exons cover approximately 15 kb of genomic DNA. The entire coding region is contained in exons 2 and 3, which are separated by a 2-kb intron. The first small 30-bp untranslated exon is located >10.5 kb upstream of the initiator ATG codon. Three kilobases of DNA upstream of the transcription start site has been cloned and characterized. Only 217 bp of 5' sequence are required for basal adipose tissue-specific expression of the ob gene as well as enhanced expression by C/EBPalpha. Mutation of the single C/EBPalpha site in this region abolished inducibility of the promoter by C/EBPalpha in cotransfection assays. The gene structure will facilitate our analysis of ob mutations in human obesity, whereas knowledge of sequence elements and factors regulating ob gene expression should be of major importance in the prevention and treatment of obesity.

Adipocytes↗
PubMed1996

Transcription, adipocyte differentiation, and obesity.

Differentiation of adipogenic precursor cells into mature adipocytes is a complex phenomenon, characterized by an ordered expression of adipocyte-specific genes, triggered by a set of interacting transcription factors. The most important transcription factors involved in this process are the gamma form of peroxisome proliferator activated receptors (PPAR gamma) and the various members of the CCAAT enhancer binding proteins (alpha, beta, and delta). In addition to PPAR gamma and these enhancer binding proteins, several other transcription factors, including ADD-1 (SRE-BP), HMGI-C, are involved in regulating this process. Altered activity and/or expression of these transcription factors, will induce the expression of target genes in the differentiating cells, ultimately resulting in the phenotypical characteristics of the adipocytes. It is speculated that modulation of these transcription factors by either pharmacological or dietary manipulations might influence adipocyte differentiation and prove beneficial in the prevention and treatment of obesity.

Adipocytes↗
PubMed1995

Transient increase in obese gene expression after food intake or insulin administration.

Obesity is a disorder of energy balance, indicating a chronic disequilibrium between energy intake and expenditure. Recently, the mouse ob gene, and subsequently its human and rat homologues, have been cloned. The ob gene product, leptin, is expressed exclusively in adipose tissue, and appears to be a signalling factor regulating body-weight homeostasis and energy balance. Because the level of ob gene expression might indicate the size of the adipose depot, we suggest that it is regulated by factors modulating adipose tissue size. Here we show that ob gene exhibits diurnal variation, increasing during the night, after rats start eating. This variation was linked to changes in food intake, as fasting prevented the cyclic variation and decreased ob messenger RNA. Furthermore, refeeding fasted rats restored ob mRNA within 4 hours to levels of fed animals. A single insulin injection in fasted animals increased ob mRNA to levels of fed controls. Experiments to control glucose and insulin independently in animals, and studies in primary adipocytes, showed that insulin regulates ob gene expression directly in rats, regardless of its glucose-lowering effects. Whereas the ob gene product, leptin, has been shown to reduce food intake and increase energy expenditure, our data demonstrate that ob gene expression is increased after food ingestion in rats, perhaps through a direct action of insulin on the adipocyte.

Adipose Tissue↗
PubMed1995

Regulation of glucose transporters in cultured rat adipocytes: synergistic effect of insulin and dexamethasone on GLUT4 gene expression through promoter activation.

A triggering effect of insulin on GLUT4 expression in adipocytes is consistently observed in vivo, whereas GLUT1 is roughly unaffected. However, in cultured rat adipocytes, insulin increases GLUT1 but fails to increase GLUT4, suggesting that additional factors are involved in vivo. This prompted us to evaluate the potential role of glucocorticoids as coregulators with insulin of glucose transporter expression using 3T3-F442A adipose cells and primary cultured rat adipocytes. In both systems, insulin increased and dexamethasone decreased GLUT1 messenger RNA (mRNA) and protein, an effect inhibited by the glucocorticoid antagonist RU 38486. When the two hormones were added together, the effect of dexamethasone was dominant in 3T3-F442A cells, but was totally antagonized in rat adipocytes. Moreover, in rat adipocytes, the GLUT1 gene transcription rate (run-on) was identical in the absence or presence of the two hormones. With regard to GLUT4 expression, neither insulin nor dexamethasone alone had any significant effect after 2 days of treatment. In contrast, the combined hormones markedly increased GLUT4 mRNA (+550% in rat adipocytes; +130% in 3T3-F442A cells) and protein (+164% in rat adipocytes; +79% in 3T3-F442A cells) with a 24- to 48-h delay after mRNA induction. Studies of the molecular mechanism(s) showed that exposure of rat adipocytes to dexamethasone plus insulin did not affect GLUT4 mRNA stability, but increased the GLUT4 gene transcription rate 3-fold. Transient transfections of rat adipocytes with the 5'-flanking 2.2-kilobase sequence of the rat GLUT4 gene fused to luciferase demonstrated that promoter activity was unchanged by insulin, increased 50% by dexamethasone, and increased 3-fold in the presence of both. These data show that insulin elicits an increase in GLUT4 gene expression provided glucocorticoids are present. Our results indicate that the synergism between insulin and glucocorticoids on GLUT4 gene transcription is mediated through GLUT4 promoter activation.

Adipocytes↗
PubMed1994

Tyrosine kinase-deficient mutant human insulin receptors (Met1153-->Ile) overexpressed in transfected rat adipose cells fail to mediate translocation of epitope-tagged GLUT4.

Insulin regulates essential pathways for growth, differentiation, and metabolism in vivo. We report a physiologically relevant system for dissecting the molecular mechanisms of insulin signal transduction related to glucose transport. This is an extension of our recently reported method for transfection of DNA into rat adipose cells in primary culture. In the present work, cDNA coding for GLUT4 with an epitope tag (HA1) in the first exofacial loop is used as a reporter gene so that GLUT4 translocation can be studied exclusively in transfected cells. Insulin stimulates a 4.3-fold recruitment of transfected epitope-tagged GLUT4 to the cell surface. Cells cotransfected with the reporter gene and the human insulin receptor gene show an increase in cell surface GLUT4 in the basal state (no insulin) to levels comparable to those seen with maximal insulin stimulation of cells transfected with the reporter gene alone. In contrast, cells overexpressing a naturally occurring tyrosine kinase-deficient mutant insulin receptor (Met1153-->Ile) show no increase in the basal cell surface GLUT4 and no shift in the insulin dose-response curve relative to cells transfected with the reporter gene alone. These results demonstrate that insulin receptor tyrosine kinase activity is essential in insulin-stimulated glucose transport in adipose cells.

Adipose Tissue↗
PubMed1994

Alteration in membrane lipid order and composition in metabolically hyperactive fatty rat adipocytes.

We have previously shown that adipose cells from young genetically obese Zucker rats are characterized by very high metabolic activity together with an increase in a wide range of membrane-mediated functions. The aim of the present study was to examine whether the physical properties of the membranes and the composition of the membrane lipids were altered in these cells. Plasma membranes and two intracellular membrane fractions were prepared by differential ultracentrifugation from inguinal adipose cells of 30-day-old obese (fa/fa) and lean (Fa/fa) littermates. The lipid order as measured by steady-state fluorescence polarization of diphenylhexatriene used as probe was markedly decreased in the plasma membranes of obese rat adipose cells. Consistent with this, the cholesterol-to-phospholipid ratio was significantly decreased, and the degree of unsaturation of the phospholipid fatty acids was significantly increased. In intracellular membranes, none of these parameters were altered by the different genotype. In fat cells from obese rats, both plasma and intracellular membranes exhibited a 2-fold decrease in the ratios of n-6/n-3 fatty acids mainly due to an enrichment in docosahexaenoic acid (22:6n-3). The data show that the fatty genotype is a determinant of membrane lipid order and composition in adipose cells. The alterations reported here for young obese Zucker rat adipocytes might be related to the metabolic hyperactivity of these cells.

Adipose Tissue↗
PubMed1993

Transfection of DNA into isolated rat adipose cells by electroporation: evaluation of promoter activity in transfected adipose cells which are highly responsive to insulin after one day in culture.

Isolated adipose cells are among the most insulin responsive cells with respect to glucose transport and metabolism. However, molecular biological techniques such as transfection of DNA have heretofore not been applied successfully in these cells in primary culture. We report a method for transfection of DNA into rat adipose cells by electroporation. Six shocks at 800 V and 25 microF in a 0.4 cm gap cuvette results in efficient transfection. We compared the ability of five promoters to drive expression of a luciferase reporter gene in transfected adipose cells. After one day in culture, promoter activity ranged from no expression to a very high level of expression. These transfected, cultured cells also displayed a 10-fold increase in 3-O-methylglucose transport with maximal insulin stimulation. The ability to transfect DNA into adipose cells which remain insulin responsive after one day in primary culture may be helpful for understanding adipose cell-specific gene regulation and elucidating the molecular mechanisms of insulin action.

3-O-Methylglucose↗