Edwin T. Parlevliet

GLP-1 treatment reduces endogenous insulin resistance via activation of central GLP-1 receptors in mice fed a high-fat diet

Glucagon-like peptide-1 (GLP-1) improves insulin sensitivity in humans and rodents. It is currently unknown to what extent the (metabolic) effects of GLP-1 treatment are mediated by central GLP-1 receptors. We studied the impact of central GLP-1 receptor (GLP-1R) antagonism on the metabolic effects of peripheral GLP-1 administration in mice. High-fat-fed insulin-resistant C57Bl/6 mice were treated with continuous subcutaneous infusion of GLP-1 or saline (PBS) for 2 wk, whereas the GLP-1R antagonist exendin-9 (EX-9) and cerebrospinal fluid (CSF) were simultaneously infused in the left lateral cerebral ventricle (icv). Glucose and glycerol turnover were determined during a hyperinsulinemic euglycemic clamp. VLDL-triglyceride (VLDL-TG) production was determined in hyperinsulinemic conditions. Our data show that the rate of glucose infusion necessary to maintain euglycemia was significantly increased by GLP-1. Simultaneous icv infusion of EX-9 diminished this effect by 62%. The capacities of insulin to stimulate glucose disposal and inhibit glucose production were reinforced by GLP-1. Simultaneous icv infusion of EX-9 significantly diminished the latter effect. Central GLP-1R antagonism alone did not affect glucose metabolism. Also, GLP-1 treatment reinforced the inhibitory action of insulin on VLDL-TG production. In conclusion, peripheral administration of GLP-1 reinforces the ability of insulin to suppress endogenous glucose and VLDL-TG production (but not lipolysis) and boosts its capacity to stimulate glucose disposal in high-fat-fed C57Bl/6 mice. Activation of central GLP-1Rs contributes substantially to the inhibition of endogenous glucose production by GLP-1 treatment in this animal model.

GLP-1 receptor activation inhibits VLDL production and reverses hepatic steatosis by decreasing hepatic lipogenesis in high-fat-fed APOE*3-Leiden mice.

Objective In addition to improve glucose intolerance, recent studies suggest that glucagon-like peptide-1 (GLP-1) receptor agonism also decreases triglyceride (TG) levels. The aim of this study was to evaluate the effect of GLP-1 receptor agonism on very-low-density lipoprotein (VLDL)-TG production and liver TG metabolism. Experimental Approach The GLP-1 peptide analogues CNTO3649 and exendin-4 were continuously administered subcutaneously to high fat diet-fed APOE*3-Leiden transgenic mice. After 4 weeks, hepatic VLDL production, lipid content, and expression profiles of selected genes involved in lipid metabolism were determined. Results CNTO3649 and exendin-4 reduced fasting plasma glucose (up to −30% and −28% respectively) and insulin (−43% and −65% respectively). In addition, these agents reduced VLDL-TG production (−36% and −54% respectively) and VLDL-apoB production (−36% and −43% respectively), indicating reduced production of VLDL particles rather than reduced lipidation of apoB. Moreover, they markedly decreased hepatic content of TG (−39% and −55% respectively), cholesterol (−30% and −55% respectively), and phospholipids (−23% and −36% respectively), accompanied by down-regulation of expression of genes involved in hepatic lipogenesis (Srebp-1c, Fasn, Dgat1) and apoB synthesis (Apob). Conclusion GLP-1 receptor agonism reduces VLDL production and hepatic steatosis in addition to an improvement of glycemic control. These data suggest that GLP-receptor agonists could reduce hepatic steatosis and ameliorate dyslipidemia in patients with type 2 diabetes mellitus.

The dopamine receptor D2 agonist bromocriptine inhibits glucose-stimulated insulin secretion by direct activation of the α2-adrenergic receptors in beta cells.

Treatment with the dopamine receptor D2 (DRD2) agonist bromocriptine improves metabolic features in obese patients with type 2 diabetes by a still unknown mechanism. In the present study, we investigated the acute effect of bromocriptine and its underlying mechanism(s) on insulin secretion both in vivo and in vitro. For this purpose, C57Bl6/J mice were subjected to an intraperitoneal glucose tolerance test (ipGTT) and a hyperglycemic (HG) clamp 60min after a single injection of bromocriptine or placebo. The effects of bromocriptine on glucose-stimulated insulin secretion (GSIS), cell membrane potential and intracellular cAMP levels were also determined in INS-1E beta cells. We report here that bromocriptine increased glucose levels during ipGTT in vivo, an effect associated with a dose-dependent decrease in GSIS. During the HG clamp, bromocriptine reduced both first-phase and second-phase insulin response. This inhibitory effect was also observed in INS-1E beta cells, in which therapeutic concentrations of bromocriptine (0.5-50nM) decreased GSIS. Mechanistically, neither cellular energy state nor cell membrane depolarization was affected by bromocriptine whereas intracellular cAMP levels were significantly reduced, suggesting involvement of G-protein-coupled receptors. Surprisingly, the DRD2 antagonist domperidone did not counteract the effect of bromocriptine on GSIS, whereas yohimbine, an antagonist of the alpha2-adrenergic receptors, completely abolished bromocriptine-induced inhibition of GSIS. In conclusion, acute administration of bromocriptine inhibits GSIS by a DRD2-independent mechanism involving direct activation of the pancreatic alpha2-adrenergic receptors. We suggest that treatment with bromocriptine promotes beta cells rest, thereby preventing long-lasting hypersecretion of insulin and subsequent beta cell failure.

CNTO736, a Novel Glucagon-Like Peptide-1 Receptor Agonist, Ameliorates Insulin Resistance and Inhibits Very Low-Density Lipoprotein Production in High-Fat-Fed Mice

CNTO736 is a glucagon-like peptide (GLP) 1 receptor agonist that incorporates a GLP-1 peptide analog linked to the Mimetibody platform. We evaluate the potential of acute and chronic CNTO736 treatment on insulin sensitivity and very low-density lipoprotein (VLDL) metabolism. For acute studies, diet-induced insulin-resistant C57BL/6 mice received a single intraperitoneal injection of CNTO736 or vehicle. Chronic effects were studied after 4 weeks of daily intraperitoneal administration. A hyperinsulinemic-euglycemic clamp monitored insulin sensitivity. A single dose of CNTO736 reduced fasting plasma glucose levels (CNTO736, 4.4 ± 1.0; control, 6.3 ± 2.4 mM) and endogenous glucose production (EGP) (CNTO736, 39 ± 11; control, 53 ± 13 μmol/min/kg) and increased insulin-mediated glucose uptake (CNTO736, 76 ± 25; control, 54 ± 13 μmol/min/kg). Chronic administration of CNTO736 reduced fasting glucose levels (CNTO736, 4.1 ± 0.8; control 6.0 ± 1.0 mM), improved insulin-dependent glucose uptake (CNTO736, 84 ± 19; control, 61 ± 15 μmol/min/kg), and enhanced inhibition of EGP (CNTO736, 91 ± 18; control, 80 ± 10% inhibition). In addition, chronic dosing with CNTO736 reduced fasting EGP (CNTO736, 39 ± 9; control, 50 ± 8 μmol/min/kg) and VLDL production (CNTO736, 157 ± 23; control, 216 ± 36 μmol/h/kg). These results indicate that CNTO736 reinforces insulins action on glucose disposal and production in diet-induced insulin-resistant mice. In addition, CNTO736 reduces basal hepatic glucose and VLDL output in these animals. The data suggest that CNTO736 may be a useful tool in the treatment of type 2 diabetes.

Circulating insulin stimulates fatty acid retention in white adipose tissue via KATP channel activation in the central nervous System only in insulin-sensitive mice

Insulin signaling in the central nervous system (CNS) is required for the inhibitory effect of insulin on glucose production. Our aim was to determine whether the CNS is also involved in the stimulatory effect of circulating insulin on the tissue-specific retention of fatty acid (FA) from plasma. In wild-type mice, hyperinsulinemic-euglycemic clamp conditions stimulated the retention of both plasma triglyceride-derived FA and plasma albumin-bound FA in the various white adipose tissues (WAT) but not in other tissues, including brown adipose tissue (BAT). Intracerebroventricular (ICV) administration of insulin induced a similar pattern of tissue-specific FA partitioning. This effect of ICV insulin administration was not associated with activation of the insulin signaling pathway in adipose tissue. ICV administration of tolbutamide, a KATP channel blocker, considerably reduced (during hyperinsulinemic-euglycemic clamp conditions) and even completely blocked (during ICV administration of insulin) WAT-specific retention of FA from plasma. This central effect of insulin was absent in CD36-deficient mice, indicating that CD36 is the predominant FA transporter in insulin-stimulated FA retention by WAT. In diet-induced insulin-resistant mice, these stimulating effects of insulin (circulating or ICV administered) on FA retention in WAT were lost. In conclusion, in insulin-sensitive mice, circulating insulin stimulates tissue-specific partitioning of plasma-derived FA in WAT in part through activation of KATP channels in the CNS. Apparently, circulating insulin stimulates fatty acid uptake in WAT but not in BAT, directly and indirectly through the CNS.

Pharmacological Modulation of Dopamine Receptor D2-Mediated Transmission Alters the Metabolic Phenotype of Diet Induced Obese and Diet Resistant C57Bl6 Mice

High fat feeding induces a variety of obese and lean phenotypes in inbred rodents. Compared to Diet Resistant (DR) rodents, Diet Induced Obese (DIO) rodents are insulin resistant and have a reduced dopamine receptor D2 (DRD2) mediated tone. We hypothesized that this differing dopaminergic tone contributes to the distinct metabolic profiles of these animals. C57Bl6 mice were classified as DIO or DR based on their weight gain during 10 weeks of high fat feeding. Subsequently DIO mice were treated with the DRD2 agonist bromocriptine and DR mice with the DRD2 antagonist haloperidol for 2 weeks. Compared to DR mice, the bodyweight of DIO mice was higher and their insulin sensitivity decreased. Haloperidol treatment reduced the voluntary activity and energy expenditure of DR mice and induced insulin resistance in these mice. Conversely, bromocriptine treatment tended to reduce bodyweight and voluntary activity, and reinforce insulin action in DIO mice. These results show that DRD2 activation partly redirects high fat diet induced metabolic anomalies in obesity-prone mice. Conversely, blocking DRD2 induces an adverse metabolic profile in mice that are inherently resistant to the deleterious effects of high fat food. This suggests that dopaminergic neurotransmission is involved in the control of metabolic phenotype.

The brain modulates insulin sensitivity in multiple tissues.

Insulin sensitivity is determined by direct effects of circulating insulin on metabolically active tissues in combination with indirect effects of circulating insulin, i.e. via the central nervous system. The dose-response effects of insulin differ between the various physiological effects of insulin. At lower insulin concentrations, circulating insulin inhibits endogenous glucose production through a combination of direct and indirect effects. At higher insulin concentrations, circulating insulin also stimulates glucose uptake and fatty acid uptake in adipose tissue, again through direct and indirect effects. High-fat diet induces insulin resistance in the central nervous system, which contributes considerably to overall insulin resistance of liver and peripheral tissues. Central insulin resistance is amendable to therapeutic intervention, reflected in the central effects of topiramate and glucagon-like peptide-1 on hepatic and peripheral insulin resistance in insulin resistant mice.

PS20 - 92. Inhibition of the central melanocortin system affects VLDL metabolism in E3L and E3L.CETP mice

The central melanocortin system is known to regulate food intake, adiposity, blood pressure, and glucose metabolism. Recently, it has been reported that the melanocortin system also regulates cholesterol metabolism. Studies in wild-type rats and mice showed increased HDL cholesterol levels upon chronic central infusion with SHU9119, a melanocortin 3/4 receptor antagonist, possibly due to modulation of hepatic pathways controlling cholesterol re-uptake.