Stephanie Migrenne

Intestinal Gluconeogenesis Is a Key Factor for Early Metabolic Changes after Gastric Bypass but Not after Gastric Lap-Band in Mice

Unlike the adjustable gastric banding procedure (AGB), Roux-en-Y gastric bypass surgery (RYGBP) in humans has an intriguing effect: a rapid and substantial control of type 2 diabetes mellitus (T2DM). We performed gastric lap-band (GLB) and entero-gastro anastomosis (EGA) procedures in C57Bl6 mice that were fed a high-fat diet. The EGA procedure specifically reduced food intake and increased insulin sensitivity as measured by endogenous glucose production. Intestinal gluconeogenesis increased after the EGA procedure, but not after gastric banding. All EGA effects were abolished in GLUT-2 knockout mice and in mice with portal vein denervation. We thus provide mechanistic evidence that the beneficial effects of the EGA procedure on food intake and glucose homeostasis involve intestinal gluconeogenesis and its detection via a GLUT-2 and hepatoportal sensor pathway.

Fatty Acid Signaling in the Hypothalamus and the Neural Control of Insulin Secretion

It is now clearly demonstrated that fatty acids (FAs) may modulate neural control of energy homeostasis and specifically affect both insulin secretion and action. Indeed, pancreatic β-cells receive rich neural innervation and FAs induce important changes in autonomic nervous activity. We previously reported that chronic infusion of lipids decreased sympathetic nervous system activity and led to exaggerated glucose-induced insulin secretion (GIIS), as would be expected from the known inhibitory effect of sympathetic splanchnic nerve activity on insulin secretion. Intracarotid infusion of lipids that do not change plasma FA concentrations also lead to increased GIIS. This effect of FAs on GIIS was prevented by inhibition of β-oxidation. It is noteworthy that a single intracarotid injection of oleic acid also induced a transient increase in plasma insulin without any change in plasma glucose, suggesting that FAs per se can regulate neural control of insulin secretion. Finally, using whole cell current clamp recordings in hypothalamic slices and calcium imaging in dissociated hypothalamic neurons, we identified a hypothalamic subpopulation of neurons either excited (13%) or inhibited (6%) by FAs. Thus, FAs per se or their metabolites modulate neuronal activity, as a means of directly monitoring ongoing fuel availability by central nervous system nutrient-sensing neurons involved in the regulation of insulin secretion.

Adiponectin is required to mediate rimonabant-induced improvement of insulin sensitivity but not body weight loss in diet-induced obese mice

The increase in adiponectin levels in obese patients with untreated dyslipidemia and its mRNA expression in adipose tissue of obese animals are one of the most interesting consequences of rimonabant treatment. Thus, part of rimonabants metabolic effects could be related to an enhancement of adiponectin secretion and its consequence on the modulation of insulin action, as well as energy homeostasis. The present study investigated the effects of rimonabant in adiponectin knockout mice (Ad(-/-)) exposed to diet-induced obesity conditions. Six-week-old Ad(-/-) male mice and their wild-type littermate controls (Ad(+/+)) were fed a high-fat diet for 7 mo. During the last month, animals were administered daily either with vehicle or rimonabant by mouth (10 mg/kg). High-fat feeding induced weight gain by about 130% in both wild-type and Ad(-/-) mice. Obesity was associated with hyperinsulinemia and insulin resistance. Treatment with rimonabant led to a significant and similar decrease in body weight in both Ad(+/+) and Ad(-/-) mice compared with vehicle-treated animals. In addition, rimonabant significantly improved insulin sensitivity in Ad(+/+) mice compared with Ad(+/+) vehicle-treated mice by decreasing hepatic glucose production and increasing glucose utilization index in both visceral and subcutaneous adipose tissue. In contrast, rimonabant failed to improve insulin sensitivity in Ad(-/-) mice, despite the loss in body weight. Rimonabants effect on body weight appeared independent of the adiponectin pathway, whereas adiponectin seems required to mediate rimonabant-induced improvement of insulin sensitivity in rodents.

GDH-Dependent Glutamate Oxidation in the Brain Dictates Peripheral Energy Substrate Distribution

Glucose, the main energy substrate used in the CNS, is continuously supplied by the periphery. Glutamate, the major excitatory neurotransmitter, is foreseen as a complementary energy contributor in the brain. In particular, astrocytes actively take up glutamate and may use it through oxidative glutamate dehydrogenase (GDH) activity. Here, we investigated the significance of glutamate as energy substrate for the brain. Upon glutamate exposure, astrocytes generated ATP in a GDH-dependent way. The observed lack of glutamate oxidation in brain-specific GDH null CnsGlud1(-/-) mice resulted in a central energy-deprivation state with increased ADP/ATP ratios and phospho-AMPK in the hypothalamus. This induced changes in the autonomous nervous system balance, with increased sympathetic activity promoting hepatic glucose production and mobilization of substrates reshaping peripheral energy stores. Our data reveal the importance of glutamate as necessary energy substrate for the brain and the role of central GDH in the regulation of whole-body energy homeostasis.

Unsaturated Fatty Acids Stimulate LH Secretion via Novel PKCε and -θ in Gonadotrope Cells and Inhibit GnRH-Induced LH Release

The activity of pituitary gonadotrope cells, crucial for reproductive function, is regulated by numerous factors including signals related to nutritional status. In this work, we demonstrated, for the first time, that in vivo central exposure of rats to lipids intracarotid infusion of a heparinized triglyceride emulsion selectively increases the expression of pituitary LH subunit genes without any alteration of pituitary GnRH receptor and hypothalamic GnRH or Kiss-1 transcript levels. Furthermore, we showed that unsaturated fatty acids (UFA), oleate and linoleate, increase LH release in a dose-dependent manner as well as LHβ mRNA levels in both immortalized LβT2 gonadotrope cell line and rat primary cell cultures. In contrast, the saturated palmitate was ineffective. ACTH or TSH secretion was unaffected by UFA treatment. We demonstrated in LβT2 cells that linoleate effect is mediated neither by activation of membrane fatty acid (FA) receptors GPR40 or GPR120 although we characterized these receptors in LβT2 cells, nor through nuclear peroxisome proliferator-activated receptors. Furthermore, linoleate β-oxidation is not required for its action on LH secretion. In contrast, pharmacological inhibition of protein kinase C (PKC) or ERK pathways significantly prevented linoleate-stimulated LH release. Accordingly, linoleate was shown to activate novel PKC isoforms, PKCε and -θ, as well as ERK1/2 in LβT2 cells. Lastly, unsaturated, but not saturated, FA inhibited GnRH-induced LH secretion in LβT2 cells as well as in pituitary cell cultures. Altogether, these results suggest that the pituitary is a relevant site of FA action and that UFA may influence reproduction by directly interfering with basal and GnRH-dependent gonadotrope activity.

Physiological and pathophysiological implications of lipid sensing in the brain

Fatty acid (FA)‐sensitive neurons are present in the brain, especially the hypothalamus, and play a key role in the neural control of energy homeostasis. Through neuronal output, FA may modulate feeding behaviour as well as insulin secretion and action. Subpopulations of neurons in the ventromedial and arcuate hypothalamic nuclei are selectively either inhibited or activated by FA. Molecular effectors of these FA effects probably include chloride or potassium ion channels. While intracellular metabolism and activation of the ATP‐sensitive K+ channel appear to be necessary for some of the signalling effects of FA, at least half of the FA responses in ventromedial hypothalamic neurons are mediated by interaction with FAT/CD36, an FA transporter/receptor that does not require intracellular metabolism to activate downstream signalling. Thus, FA or their metabolites can modulate neuronal activity as a means of directly monitoring ongoing fuel availability by brain nutrient‐sensing neurons involved in the regulation of energy and glucose homeostasis. Recently, the role of lipoprotein lipase in FA sensing has also been shown in animal models not only in hypothalamus, but also in hippocampus and striatum. Finally, FA overload might impair neural control of energy homeostasis through enhanced ceramide synthesis and may contribute to obesity and/or type 2 diabetes pathogenesis in predisposed subjects.

Unsaturated Fatty Acids Disrupt Smad Signaling in Gonadotrope Cells Leading to Inhibition of FSHβ Gene Expression

Reproductive function is highly dependent on nutritional input. We recently provided evidence that the unsaturated ω6 fatty acid (FA), linoleic acid (linoleic), interferes with transcription and secretion of the gonadotropin LH, highlighting the existence of a lipid sensing in pituitary gonadotropes. Here, we show, using a combination of in vivo and in vitro models, that linoleic differentially regulates Lhb and Fshb expression. Central exposure of rats to linoleic over 7 days was associated with increase of Lhb but not Fshb transcript levels. Consistently, exposure of rat pituitary cells or LβT2 cells to linoleic increased Lhb, whereas it dramatically decreased Fshb transcript levels without affecting its stability. This effect was also induced by ω9 and ω3-polyunsaturated FA but not by saturated palmitic acid. Analysis of the underlying mechanisms in LβT2 cells using small interfering RNA revealed that early growth response protein 1 mediates linoleic stimulation of Lhb expression. Furthermore, we demonstrated that linoleic counteracts activin and bone morphogenetic protein-2 stimulation of Fshb expression. Using Western blotting and Smad-responsive reporter gene assays, linoleic was shown to decrease basal Smad2/3 phosphorylation levels as well as activin- and bone morphogenetic protein-2-dependent activation of Smad, uncovering a new FA-sensitive signaling cascade. Finally, the protein phosphatase magnesium-dependent 1A was shown to mediate linoleic inhibition of basal Smad phosphorylation and Fshb expression, identifying protein phosphatase magnesium-dependent 1A as a new target of FA in gonadotropes. Altogether, this study provides a novel mechanism by which FAs target gene expression and underlines the relevant role of pituitary gonadotropes in mediating the effects of nutritional FA on reproductive function.

Differential Effect of Fatty Acids in Nervous Control of Energy Balance

Energy homeostasis is kept through a complex interplay of nutritional, neuronal and hormonal inputs that are integrated at the level of the central nervous system (CNS). A disruption of this regulation gives rise to life-threatening conditions that include obesity and type-2 diabetes, pathologies that are strongly linked epidemiologically and experimentally. The hypothalamus is a key integrator of nutrient-induced signals of hunger and satiety, crucial for processing information regarding energy stores and food availability. Much effort has been focused on the identification of hypothalamic pathways that control food intake but, until now, little attention has been given to a potential role for the hypothalamus in direct control of glucose homeostasis and nergy balance. Recent studies have cast a new light on the role of the CNS in regulating peripheral glucose via a hypothalamic fatty acid (FA)-sensing device that detects nutrient availability and relays, through the autonomic nervous system, a negative feedback signal on food intake, insulin sensitivity and insulin secretion. Indeed, accumulating evidences suggest that FA are used in specific areas of CNS not as nutrients, but as cellular messengers which inform “FA sensitive neurons” about the energy status of the whole body (Blouet & Schwartz, 2010; Migrenne et al., 2006; Migrenne et al., 2011). Thus it has been described that up to 70% of hypothalamic arcuate nucleus (ARC) and ventromedian nucleus (VMN) neurons are either excited or inhibited by long chain fatty acids such as oleic acid (Jo et al., 2009; Le Foll et al., 2009; Migrenne et al., 2011). Within the VMN, 90% of the glucosensing neurons also have their activity altered by FA. In a large percentage of these neurons, glucose and FA have opposing effects on neuronal activity, much as they do on intracellular metabolism in many other cells (Randle et al., 1994). Neuronal FA sensing mechanisms include activation of the KATP channel by long chain fatty acid acyl CoA (Gribble et al., 1998) or inactivation by generation of ATP or reactive oxygen species during mitochondrial ┚-oxidation (Jo et al., 2009; Le Foll et al., 2009; Migrenne et al., 2011; Wang et al., 2006). Many fatty acid sensing neurons are activated by interaction of long chain fatty acids with the fatty acid transporter/receptor, FAT/CD36, presumably by activation of store-operated calcium channels by a mechanism that is independent of fatty acid metabolism (Jo et al., 2009). Importantly, most neurons utilize FA primarily for membrane production rather than as a metabolic substrate (Rapoport et al., 2001; Smith & Nagura, 2001) and only nanomolar concentrations of fatty acid are required to

Palmitic acid mediates hypothalamic insulin resistance by altering PKC-q subcellular localization in rodents (Journal of Clinical Investigation (2009) 119, (2577-2589))

Original citation: J. Clin. Invest. 2010;120(1):394. doi:10.1172/JCI36714C1. Citation for this corrigendum: J. Clin. Invest. 2011;121(1):456. doi:10.1172/JCI45846. In the Methods section titled “Fatty acid infusion,” the dose of fatty acids delivered centrally was given incorrectly. The correct sentence appears below. The cannula was connected via a polyethylene catheter to a subcutaneous osmotic minipump (Alza Corporation) filled with either palmitic or oleic acid (equimolar concentrations, 50 μmol/l; Sigma-Aldrich) or vehicle (PBS) for continuous infusion over 3 days. The authors regret the error. Corrigendum

P240 Caractérisation phénotypique d’un nouveau modèle de souris transgéniques exprimant dans le foie une carnitine palmitoyltransférase 1 constitutivement active

Introduction La carnitine palmitoyltransferase 1 (CPT1) represente le site majeur de controle de l’oxydation des acides gras a chaine longue (AGCL) du fait de sa capacite a etre inhibee par le malonyl-CoA, premier intermediaire de la lipogenese. Dans le foie d’animaux obeses ou diabetiques, l’existence d’une lipogenese accrue conduit a une augmentation du malonyl-CoA qui entraine une diminution de l’oxydation mitochondriale des AGCL. Materiels et methodes Nous avons developpe un nouveau modele de souris transgeniques (Tg-CPT1mt) exprimant specifiquement dans le foie (promoteur de la transthyretine) une CPT1 insensible au malonyl-CoA (CPT1mt). Resultats L’expression homozygote de la CPT1mt entraine une letalite embryonnaire. Chez les souris heterozygotes a l’etat nourri, l’expression de la CPT1mt entraine une diminution de la sensibilite de la CPT1 hepatique vis-a-vis du malonyl-CoA similaire a celle observee apres un jeune chez des souris temoins (WT). A l’âge de 3 mois, les souris Tg-CPT1mt presentent a l’etat nourri des concentrations circulantes de triglycerides et d’acides gras libres significativement plus faibles que chez les souris WT. Chez les souris Tg-CPT1mt âgees de 10 mois, seule la triglyceridemie est diminuee et ceci s’accompagne d’une diminution du poids corporel. Quel que soit leur âge, les souris Tg-CPT1mt ont a l’etat nourri une diminution du poids des tissus adipeux (epididymaire, visceral et sous cutane) et du contenu hepatique en glycogene. De plus, la glycemie des souris Tg-CPT1mt est plus faible a l’etat nourri comme a l’etat a jeun. Seules les souris Tg-CPT1mt âgees de 10 mois ont une tolerance au glucose et une sensibilite a l’insuline ameliorees. Chez ces souris, des experiences de clamp euglycemique-hyperinsulinemique suggerent que la diminution de la glycemie observee ne resulte pas d’une diminution de la production hepatique de glucose, mais d’une amelioration de l’utilisation basale de glucose. Conclusion Ces resultats suggerent que les souris Tg-CPT1mt presentent un phenotype evolutif et constituent un modele pertinent pour l’etude des consequences d’une augmentation de l’oxydation des AGCL dans le foie sur l’obesite, l’insulinoresistance et la steatose hepatique.