Rôle du glucagon like peptide-1 et de la dipeptidyl peptidase 4 dans le contrôle de la glycémie : influence des lipides nutritionnels

En réponse à une charge orale en glucose, la régulation de l'homéostasie glucidique est dépendante d'un " arc réflexe métabolique anticipateur " dont la sécrétion d'hormones intestinales et l'activation du nerf vague, sont les premières informations du statut énergétique de l'organisme alors transmises au cerveau. Le glucagon like peptide-1 (GLP-1), sécrété par l'intestin en réponse au glucose, est reconnu pour sa propriété insulinotropique mais dont le mode d'action reste à définir. En effet, on observe paradoxalement que malgré son importance biologique sur la sécrétion d'insuline ce peptide est très rapidement (moins d'une minute) dégradé par la dipeptidyl peptidase IV (DPP-IV) dans la sphère entéro-portale. Pour en étudier la signification physiologique nous avons utilisé dans un premier temps une faible dose d'un inhibiteur de la DPP-IV administrée par voie orale, la sitagliptine. Nous avons mis en évidence que seule la sécrétion entéro-portale du GLP-1 permettrait une diminution relative de la glycémie en réponse à une charge orale en glucose. Par utilisation de l'antagoniste du récepteur au GLP-1, l'exendine 9, nous avons constaté que cette diminution partielle de la glycémie par le GLP-1 intestinal permet alors le recrutement du GLP-1 cérébral insulinotropique. La mesure de l'activité électrique du nerf vague démontre que l'axe " intestin-cerveau " est bien recruté. D'autre part, le dipeptide généré par la dégradation du GLP-1 actif, l'histidine-alanine, induirait un rétrocontrôle des effets du GLP-1 sur le pancréas en diminuant la sécrétion d'insuline et en augmentant la sécrétion de glucagon. Dans un second temps, l'utilisation d'une forte dose orale de sitagliptine (40mg) pour laquelle la concentration plasmatique de GLP-1 dans le sang portal est fortement augmentée, a induit une diminution accrue de la glycémie qui n'a donc pas permis le recrutement du GLP-1 cérébral et qui n'a pas favorisé la sécrétion d'insuline. Nous avons validé que les récepteurs entéro-portaux du GLP-1, qui agissent indépendamment de la sécrétion d'hormones pancréatiques, sont bien impliqués dans la régulation de la glycémie par la sitagliptine. Ainsi, le contrôle des taux de GLP-1 et de DPP-IV semblent nécessaire au maintien de l'homéostasie glucidique. D'autre part, en physiopathologie, les lipides nutritionnels sont une cause de l'installation du diabète de type II. Néanmoins, leur rôle et leur mode d'action sur l'intestin, reste à élucider. Ainsi, sur des souris perfusées d'une solution riche en lipides par voie intragastrique ou intracarotidienne, nous avons montré l'importance de l'information provenant de l'intestin par rapport aux signaux qui arrivent au cerveau. Les lipides perfusés par voie entérale induisent alors un état d'insulinorésistance associé à un stress oxydant intestinal. Ce dernier semble être un facteur précoce dans l'installation du diabète de type II et qui perturbe l'activité nerveuse de l'axe " intestin-cerveau ".In response to a glucose load, the control the glucose homeostasis depends on a "metabolic reflex". The secretion of intestinal hormones and the activation of the vagus nerve are the first events involved. The glucagon like peptide 1 (GLP-1), is secreted by the intestinal L cells in response to the glucose. This hormone stimulates glucose-induced insulin secretion. However, GLP-1 rapidly, within less than a minute, degraded by the dipeptidyl-peptidase-IV (DPP-IV) in the entero-portal circulation and in the systemic blood. Therefore, a physiological mechanism should be issued from such a short half life. To validate this hypothesis we used a DPP4 inhibitor sitagliptine in vivo. We here showed that the entero-portal secretion of GLP-1 induces a relative diminution of the glycemia in response to an oral glucose challenge. To use the antagonist of GLP-1 receptor, exendin 9, we have shown that the gut GLP-1-induced relative diminution of the glycemia permits the recruitment of the brain GLP-1 to favor the insulin secretion. The recording of the vagus nerve confirms the activation of the "gut-brain" axis and the implication of the sitagliptine. On other hand, the dipeptide, generated by the GLP-1 (7-37) degradation, histidine-alanine, decreases the insulin secretion and increases the glucagon secretion to regulate the GLP-1(7-37) effects. In second time, the utilization of a high oral dose of sitagliptine (40mg) increases the portal concentration of GLP-1 which induces an important diminution of the glycemia to not recruit the brain GLP-1, and not increase the insulin secretion. We have valided that the entero-portal GLP-1 receptors, no insulin dependent, are implicated in the regulation of the glycemia. So, the control of the levels of GLP-1 and DPP-IV seems to be necessary for the maintain of the glucose homeostasis. On the other hand, in physiopathology, the nutritional lipids are a cause of the development of the type II diabetes. However, their role and signaling on the intestine are to define. Thus, mice are perfused with a lipid solution into the gut or through the carotid vein to compare the importance of the lipid signal from the intestine in relation to the lipid signal toward the brain. So, the intestinal lipids induce an insulin resistance state, associated to an intestinal oxidative stress. The former should be an early factor to impact on the "intestine-brain" axis to perturb the vagus nerve activity

Lipid-Induced Peroxidation in the Intestine Is Involved in Glucose Homeostasis Imbalance in Mice

BACKGROUND: Daily variations in lipid concentrations in both gut lumen and blood are detected by specific sensors located in the gastrointestinal tract and in specialized central areas. Deregulation of the lipid sensors could be partly involved in the dysfunction of glucose homeostasis. The study aimed at comparing the effect of Medialipid (ML) overload on insulin secretion and sensitivity when administered either through the intestine or the carotid artery in mice. METHODOLOGY/PRINCIPAL FINDINGS: An indwelling intragastric or intracarotid catheter was installed in mice and ML or an isocaloric solution was infused over 24 hours. Glucose and insulin tolerance and vagus nerve activity were assessed. Some mice were treated daily for one week with the anti-lipid peroxidation agent aminoguanidine prior to the infusions and tests. The intestinal but not the intracarotid infusion of ML led to glucose and insulin intolerance when compared with controls. The intestinal ML overload induced lipid accumulation and increased lipid peroxidation as assessed by increased malondialdehyde production within both jejunum and duodenum. These effects were associated with the concomitant deregulation of vagus nerve. Administration of aminoguanidine protected against the effects of lipid overload and normalized glucose homeostasis and vagus nerve activity. CONCLUSIONS/SIGNIFICANCE: Lipid overload within the intestine led to deregulation of gastrointestinal lipid sensing that in turn impaired glucose homeostasis through changes in autonomic nervous system activity

Rôle du glucagon like peptide-1 et de la dipeptidyl peptidase 4 dans le contrôle de la glycémie (influence des lipides nutritionnels)

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Apelin stimulates glucose uptake but not lipolysis in human adipose tissue ex vivo.

International audienceApelin is a peptide present in different cell types and secreted by adipocytes in humans and rodents. Apelin exerts its effects through a G-protein coupled receptor called APJ. During the last years, a role of apelin/APJ in energy metabolism has emerged. Apelin was shown to stimulate glucose uptake in skeletal muscle through an AMP-activated protein kinase (AMPK)-dependent pathway in mice. So far, no metabolic effects of apelin have been reported on human adipose tissue (AT). Thus, the effect of apelin on AMPK in AT was measured as well as AMPK-mediated effects such as inhibition of lipolysis and stimulation of glucose uptake. AMPK and Acetyl-CoA Carboxylase phosphorylation were measured by western blot to reflect AMPK activity. Lipolysis and glucose uptake were measured, ex vivo, in response to apelin on isolated adipocytes and explants from AT of the subcutaneous region of healthy subjects (BMI: 25.6 ± 0.8 kg/m2, n = 30 in total). APJ mRNA and protein were present in human AT and isolated adipocytes. Apelin stimulated AMPK phosphorylation at Thr-172 in a dose-dependent manner in human AT which was associated to increased glucose uptake since, C Compound (20 μM), an AMPK inhibitor, completely prevented apelin-induced glucose uptake. However, in isolated adipocytes or AT explants, apelin had no significant effect on basal and isoprenaline-stimulated lipolysis. Thus, these results reveal, for the first time, that apelin is able to act on human AT in order to stimulate AMPK and glucose uptake

Physiological and pharmacological mechanisms through which the DPP-4 inhibitor sitagliptin regulates glycemia in mice

International audienceInhibition of dipeptidyl peptidase-4 (DPP-4) activity improves glucose homeostasis through a mode of action related to the stabilization of the active forms of DPP-4-sensitive hormones such as the incretins that enhance glucose-induced insulin secretion. However, the DPP-4 enzyme is highly expressed on the surface of intestinal epithelial cells; hence, the role of intestinal vs. systemic DPP-4 remains unclear. To analyze mechanisms through which the DPP-4 inhibitor sitagliptin regulates glycemia in mice, we administered low oral doses of the DPP-4 inhibitor sitagliptin that selectively reduced DPP-4 activity in the intestine. Glp1r(-/-) and Gipr(-/-) mice were studied and glucagon-like peptide (GLP)-1 receptor (GLP-1R) signaling was blocked by an i.v. infusion of the corresponding receptor antagonist exendin (9-39). The role of the dipeptides His-Ala and Tyr-Ala as DPP-4-generated GLP-1 and glucose-dependent insulinotropic peptide (GIP) degradation products was studied in vivo and in vitro on isolated islets. We demonstrate that very low doses of oral sitagliptin improve glucose tolerance and plasma insulin levels with selective reduction of intestinal but not systemic DPP-4 activity. The glucoregulatory action of sitagliptin was associated with increased vagus nerve activity and was diminished in wild-type mice treated with the GLP-1R antagonist exendin (9-39) and in Glp1r(-/-) and Gipr(-/-) mice. Furthermore, the dipeptides liberated from GLP-1 (His-Ala) and GIP (Tyr-Ala) deteriorated glucose tolerance, reduced insulin, and increased portal glucagon levels. The predominant mechanism through which DPP-4 inhibitors regulate glycemia involves local inhibition of intestinal DPP-4 activity, activation of incretin receptors, reduced liberation of bioactive dipeptides, and activation of the gut-to-pancreas neural axis

Activity of the vagal nerve.

<p><b>A</b>: Recordings during 5 min before (upper panel) and 5 min after (lower panel) the glucose load in isocaloric (isocal), Medialipid (ML) or, aminoguanidine treated-ML (ML+ amino) conditions. <b>B</b>: Frequency of the vagal activity recorded before and during OGTT. (n = 8). **p<0.01 vs isocaloric.</p

Effects of lipid infusion on glucose homeostasis in mice infused intragastrically for 24 hours with Medialipid (ML) or isocaloric solution (A, B, C, D, E, F) ; and in the brain (intracarotid) with ML or isocal (G, H, I).

<p><b>A</b>: Plasma TG (g/l). <b>B</b>: FFA (mM). <b>C</b>: Plasma portal GLP1 concentration (pM) at the end of the intragastric perfusion (t0) and 15 min after glucose gavage. <b>D</b>: Time course of glycemia (mM) during the OGTT. <b>E</b>: Plasma insulin (pg/ml) 20 min before and 15 min after glucose challenge. <b>F</b>: Time course of glycemia during ITT. Results in 24 h intracarotid infused mice: <b>G</b>: Time course of glycemia (mM) during OGTT. <b>H</b>: Plasma insulin (pg/ml) 20 min before and 15 min after glucose. <b>I</b>: Time course of glycemia during ITT. (n = 8) *p<0.05, **p<0.01, ***p<0.001 compared to controls.</p

Intestinal Intestinal lipid content and markers of inflammation in isocaloric or ML intragastrically infused mice.

<p><b>A</b>: TG content (mg/g of tissue) in jejunum. <b>B</b>: Electronic microscopy of jejunum (×4000). <b>C</b>,<b>D</b>,<b>E</b>, <b>F</b> : mRNA expression of TNFα (<b>C</b>), IL1β (<b>D</b>), PAI-1 (<b>E</b>) and F4/80 (F). <b>G</b>: Number of macrophages (F4/80), <b>H</b>: F4/80 immunohistochemistry. (n = 8) *p<0.05, **p<0.01, ***p<0.001 compared to controls.</p

Markers of oxidative stress in intragastric-infused mice.

<p><b>A</b>,<b>B</b>,<b>C</b>: mRNA expression of NADPH oxidase (<b>A</b>), GST (<b>B</b>), catalase (<b>C</b>). <b>D</b>: Activity of glutathione reductase in duodenum and jejunum. <b>E</b>: Lipid peroxidation by the expression of MDA (µM/µg protein). F, G, H, I: Infusion of isocaloric and Medialipid solution over 6 h. <b>F</b>: MDA production. <b>G</b>: Glutathione reductase activity in duodenum and jejunum. <b>H</b>: Plasma insulin (pg/ml) 20 min before and 15 min after glucose challenge. <b>I</b>: Time course of glycemia (mM) during OGTT. (n = 8) *p<0.05, **p<0.01, ***p<0.001 compared to controls.</p