Search

International audience; ABSTRACT Background The role of hepatoportal glucose sensors is poorly understood in the context of insulin resistance. Objectives We assessed the effects of glucose infusion in the portal vein on insulin tolerance in 2 rat models of insulin resistance, and the role of capsaicin sensitive nerves in this signal. Methods Male Wistar rats, 8 weeks old, weighing 250–275 g, were used. Insulin and glucose tolerance were assessed following a 4-hour infusion of either glucose or saline through catheterization in the portal vein in 3 paradigms. In experiment 1, for diet-induced insulin resistance, rats were fed either a control diet (energy content: proteins = 22.5%, carbohydrates = 64.1%, and lipids = 13.4%) or a high-fat diet (energy content: proteins = 15.3%, carbohydrates = 40.3%, and lipids =44.4%) for 4 months. In experiment 2, for centrally induced peripheral insulin resistance, catheters were inserted in the carotid artery to deliver either an emulsion of triglycerides [intralipid (IL)] or saline towards the brain for 24 hours. In experiment 3, for testing the role of capsaicin-sensitive nerves, experiment 2 was repeated following a periportal treatment with capsaicin or vehicle. Results In experiment 1, when compared to rats fed the control diet, rats fed the high-fat diet exhibited decreased insulin and glucose tolerance (P ≤ 0.05) that was restored with a glucose infusion in the portal vein (P ≤ 0.05). In experiment 2, infusion of a triglyceride emulsion towards the brain (IL rats) decreased insulin and glucose tolerance and increased hepatic endogenous production when compared to saline-infused rats (P ≤ 0.05). Glucose infusion in the portal vein in IL rats restored insulin and glucose tolerance, as well as hepatic glucose production, to controls levels (P ≤ 0.05). In experiment 3, portal infusion of glucose did not increase insulin tolerance in IL rats that received a periportal pretreatment with capsaicin. Conclusions Stimulation of hepatoportal glucose sensors increases insulin tolerance in rat models of insulin resistance and requires the presence of capsaicin-sensitive nerves.

International audience; One main mechanism of insulin resistance (IR), a key feature of type 2 diabetes, is the accumulation of saturated fatty acids (FAs) in the muscles of obese patients with type 2 diabetes. Understanding the mechanism that underlies lipid-induced IR is an important challenge. Saturated FAs are metabolized into lipid derivatives called ceramides, and their accumulation plays a central role in the development of muscle IR. Ceramides are produced in the endoplasmic reticulum (ER) and transported to the Golgi apparatus through a transporter called CERT, where they are converted into various sphingolipid species. We show that CERT protein expression is reduced in all IR models studied because of a caspase-dependent cleavage. Inhibiting CERT activity in vitro potentiates the deleterious action of lipotoxicity on insulin signaling, whereas overexpression of CERT in vitro or in vivo decreases muscle ceramide content and improves insulin signaling. In addition, inhibition of caspase activity prevents ceramide-induced insulin signaling defects in C2C12 muscle cells. Altogether, these results demonstrate the importance of physiological ER-to-Golgi ceramide traffic to preserve muscle cell insulin signaling and identify CERT as a major actor in this process.

Background: The role of the type 2 diabetes risk gene SLC30A8, encoding ZnT8, in the control of glucagon secretion is not clearly established. Results: Inactivation of ZnT8 in a subset of α-cells leads to increased glucagon secretion in vivo and in vitro. Conclusion: ZnT8 is involved in normal glucagon release. Significance: SLC30A8 diabetes risk alleles may influence glucagon secretion., SLC30A8 encodes a zinc transporter ZnT8 largely restricted to pancreatic islet β- and α-cells, and responsible for zinc accumulation into secretory granules. Although common SLC30A8 variants, believed to reduce ZnT8 activity, increase type 2 diabetes risk in humans, rare inactivating mutations are protective. To investigate the role of Slc30a8 in the control of glucagon secretion, Slc30a8 was inactivated selectively in α-cells by crossing mice with alleles floxed at exon 1 to animals expressing Cre recombinase under the pre-proglucagon promoter. Further crossing to Rosa26:tdRFP mice, and sorting of RFP+: glucagon+ cells from KO mice, revealed recombination in ∼30% of α-cells, of which ∼50% were ZnT8-negative (14 ± 1.8% of all α-cells). Although glucose and insulin tolerance were normal, female αZnT8KO mice required lower glucose infusion rates during hypoglycemic clamps and displayed enhanced glucagon release (p < 0.001) versus WT mice. Correspondingly, islets isolated from αZnT8KO mice secreted more glucagon at 1 mm glucose, but not 17 mm glucose, than WT controls (n = 5; p = 0.008). Although the expression of other ZnT family members was unchanged, cytoplasmic (n = 4 mice per genotype; p < 0.0001) and granular (n = 3, p < 0.01) free Zn2+ levels were significantly lower in KO α-cells versus control cells. In response to low glucose, the amplitude and frequency of intracellular Ca2+ increases were unchanged in α-cells of αZnT8KO KO mice. ZnT8 is thus important in a subset of α-cells for normal responses to hypoglycemia and acts via Ca2+-independent mechanisms.

High plasma copeptin, a marker of vasopressin (VP) secretion, has been shown to be associated with the metabolic syndrome and development of type 2 diabetes in humans. The present study was designed to determine the long-term influence of plasma VP concentration in a rodent model prone to metabolic dysfunction. Obese Zucker rats and their lean counterparts were submitted for 4 weeks to one of three protocols inducing different levels of VP. Circulating VP was either reduced by increasing the daily water intake (low-VP), or increased by a chronic i.p. infusion of VP (high-VP). The control rats had normal VP levels that depended on their own regulation of water intake and VP secretion. Compared with controls with normal VP, lean rats with high-VP had a higher fasting glycaemia after 4 weeks. In obese rats, high-VP promoted hyperinsulinaemia, glucose intolerance, assessed by glucose and insulin tolerance tests, and an impaired response to a pyruvate challenge. Conversely, treatment with a selective arginine vasopressin receptor 1A (V1aR) antagonist reduced glucose intolerance. Low-VP obese rats had unchanged glucose tolerance but exhibited a drastic decrease in liver steatosis compared with control obese rats, associated with low hepatic triacylglycerol and cholesterol content, and reduced expression of hepatic lipogenic genes. These effects were independent of changes in body adiposity, and plasma sodium and osmolality did not differ among groups. These findings show a causal relationship between the VP–hydration axis and the metabolic risk. Therapeutic perspectives include diet recommendations regarding hydration, but also potential pharmacological interventions targeting the VP V1aR.

International audience; Aims/hypothesis Regulation of energy balance involves the participation of many factors, including nutrients, among which are circulating lipids, acting as peripheral signals informing the central nervous system of the energy status of the organism. It has been shown that neuronal lipoprotein lipase (LPL) participates in the control of energy balance by hydrolysing lipid particles enriched in triacylglycerols. Here, we tested the hypothesis that LPL in the mediobasal hypothalamus (MBH), a well-known nucleus implicated in the regulation of metabolic homeostasis, could also contribute to the regulation of body weight and glucose homeostasis.Methods We injected an adeno-associated virus (AAV) expressing Cre-green fluorescent protein into the MBH of Lpl-floxed mice (and wild-type mice) to specifically decrease LPL activity in the MBH. In parallel, we injected an AAV overexpressing Lpl into the MBH of wild-type mice. We then studied energy homeostasis and hypothalamic ceramide content.Results The partial deletion of Lpl in the MBH in mice led to an increase in body weight compared with controls (37.72 +/- 0.7 g vs 28.46 +/- 0.12, p < 0.001) associated with a decrease in locomotor activity. These mice developed hyperinsulinaemia and glucose intolerance. This phenotypealso displayed reduced expression of Cers1 in the hypothalamus as well as decreased concentration of several C18 species of ceramides and a 3-fold decrease in total ceramide intensity. Conversely, overexpression of Lpl specifically in the MBH induced a decrease in body weight.Conclusions/interpretation Our study shows that LPL in the MBH is an important regulator of body weight and glucose homeostasis.

Dietary supplement may potentially help to fight obesity and other metabolic disorders such as insulin-resistance and low-grade inflammation. The present study aimed to test whether supplementation with Agaricus blazei murill (ABM) extract could have an effect on diet-induced obesity in rats. Wistar rats were fed with control diet (CD) or high-fat diet (HF) and either with or without supplemented ABM for 20 weeks. HF diet-induced body weight gain and increased fat mass compared to CD. In addition HF-fed rats developed hyperleptinemia and insulinemia as well as insulin resistance and glucose intolerance. In HF-fed rats, visceral adipose tissue also expressed biomarkers of inflammation. ABM supplementation in HF rats had a protective effect against body weight gain and all study related disorders. This was not due to decreased food intake which remained significantly higher in HF rats whether supplemented with ABM or not compared to control. There was also no change in gut microbiota composition in HF supplemented with ABM. Interestingly, ABM supplementation induced an increase in both energy expenditure and locomotor activity which could partially explain its protective effect against diet-induced obesity. In addition a decrease in pancreatic lipase activity is also observed in jejunum of ABM-treated rats suggesting a decrease in lipid absorption. Taken together these data highlight a role for ABM to prevent body weight gain and related disorders in peripheral targets independently of effect in food intake in central nervous system.

Background The human SLC30A8 gene encodes the secretory granule-localised zinc transporter ZnT8 whose expression is chiefly restricted to the endocrine pancreas. Single nucleotide polymorphisms (SNPs) in the human SLC30A8 gene have been associated, through genome-wide studies, with altered type 2 diabetes risk. In addition to a role in the control of insulin release, recent studies involving targeted gene ablation from the pancreatic α cell (Solomou et al., J Biol Chem 290(35):21432-42) have also implicated ZnT8 in the control of glucagon release. Up to now, however, the possibility that increased levels of the transporter in these cells may impact glucagon secretion has not been explored. Methods Here, we use a recently-developed reverse tetracyline transactivator promoter-regulated ZnT8 transgene to drive the over-expression of human ZnT8 selectively in the α cell in adult mice. Glucose homeostasis and glucagon secretion were subsequently assessed both in vivo during hypoglycemic clamps and from isolated islets in vitro. Results Doxyclin-dependent human ZnT8 mRNA expression was apparent in both isolated islets and in fluorescence-activated cell sorting- (FACS) purified α cells. Examined at 12 weeks of age, intraperitoneal glucose (1 g/kg) tolerance was unchanged in transgenic mice versus wild-type littermates (n = 8-10 mice/genotype, p > 0.05) and sensitivity to intraperitoneal insulin (0.75U/kg) was similarly unaltered in transgenic animals. In contrast, under hyperinsulinemic-hypoglycemic clamp, a ~45 % (p 0.05). Over-expression of ZnT8 in glucagonoma-derived αTC1-9 cells increased granule free Zn2+ concentrations consistent with a role for Zn2+ in this compartment in the action of ZnT8 on glucagon secretion. Conclusions Increased ZnT8 expression, and a likely increase in intragranular free Zn2+ concentration, is deleterious in pancreatic α cells for stimulated glucagon release. These data provide further evidence that type 2 diabetes-associated polymorphisms in the SLC30A8/ZnT8 gene may act in part via alterations in glucagon release and suggest that ZnT8 activation may restrict glucagon release in some settings.

Dietary supplement may potentially help to fight obesity and other metabolic disorders such as insulin-resistance and low-grade inflammation. The present study aimed to test whether supplementation with Agaricus blazei murill (ABM) extract could have an effect on diet-induced obesity in rats.Wistar rats were fed with control diet (CD) or high-fat diet (HF) and either with or without supplemented ABM for 20 weeks.HF diet-induced body weight gain and increased fat mass compared to CD. In addition HF-fed rats developed hyperleptinemia and insulinemia as well as insulin resistance and glucose intolerance. In HF-fed rats, visceral adipose tissue also expressed biomarkers of inflammation. ABM supplementation in HF rats had a protective effect against body weight gain and all study related disorders. This was not due to decreased food intake which remained significantly higher in HF rats whether supplemented with ABM or not compared to control. There was also no change in gut microbiota composition in HF supplemented with ABM. Interestingly, ABM supplementation induced an increase in both energy expenditure and locomotor activity which could partially explain its protective effect against diet-induced obesity. In addition a decrease in pancreatic lipase activity is also observed in jejunum of ABM-treated rats suggesting a decrease in lipid absorption.Taken together these data highlight a role for ABM to prevent body weight gain and related disorders in peripheral targets independently of effect in food intake in central nervous system.

Introduction Des etudes recentes ont montre que l'accumulation de lipides dans l'hypothalamus serait responsable de l'installation d'une lipotoxicite centrale. Ce phenomene pourrait jouer un role dans l'apparition d'une insulinoresistance et du diabete en deregulant le controle nerveux de l'homeostasie glucidique. Il a ete montre que l'accumulation des ceramides joue un role preponderant dans le developpement d'une lipotoxicite au niveau des tissus peripheriques. Nos resultats preliminaires ont montre que la perfusion de saindoux, compose principalement d'acides gras satures, directement dans le cerveau, conduit a une insulino-resistance peripherique associee a une accumulation de ceramides dans l'hypothalamus. Materiels et methodes Afin etudier la chronologie qui existe entre l'installation d'une insulino-resistance centrale et peripherique, nous avons decide d'etudier les effets d'un acide gras sature, le palmitate sur l'insulino-resistance centrale a l'aide d'approches in vitro (cellules hypothalamiques) et in vivo (rat). Resultats Nous avons pu mettre en evidence un phenomene d'insulino-resistance centrale in vitro en reponse au palmitate qui s'accompagne d'une accumulation de ceramides. Certains ARNm codant pour les enzymes de la voie de synthese de novo des ceramides etant augmentes a la suite de ces traitements, nous avons decide d'inhiber cette voie avec la myriocine. En presence de myriocine, ce phenomene d'insulino-resistance centrale est partiellement contrecarre. Chez le rat, nous avons pu observer le meme phenomene d'insulino-resistance centrale en reponse a une perfusion dans la carotide de palmitate. Cette insulino-resistance s'accompagne a son tour d'une accumulation de ceramides dans l'hypothalamus. Conclusion Au final, notre etude revele que les acides gras satures induisent une accumulation de ceramides dans l'hypothalamus. De facon interessante, cette accumulation est responsable de l'installation d'une insulino-resistance hypothalamique. Ces resultats mettent en evidence le role cle du metabolisme des ceramides dans la deregulation du controle nerveux de l'homeostasie glucidique observe au cours de l'obesite.

Aims/hypothesis Per-Arnt-Sim kinase (PASK) is a nutrient-regulated domain-containing protein kinase previously implicated in the control of insulin gene expression and glucagon secretion. Here, we explore the roles of PASK in the control of islet hormone release, by generating mice with selective deletion of the Pask gene in pancreatic beta or alpha cells. Methods Floxed alleles of Pask were produced by homologous recombination and animals bred with mice bearing beta (Ins1Cre; PaskBKO) or alpha (PpgCre [also known as Gcg]; PaskAKO) cell-selective Cre recombinase alleles. Glucose homeostasis and hormone secretion in vivo and in vitro, gene expression and islet cell mass were measured using standard techniques. Results Ins1Cre-based recombination led to efficient beta cell-targeted deletion of Pask. Beta cell mass was reduced by 36.5% (p