Brainstem sensing of meal-related signals in energy homeostasis.

The dorsovagal complex comprises: (1) a chemosensory structure, the area postrema (AP), (2) a center for integrating distension, mechanosensory and other inputs from the viscera, the nucleus of the tractus solitarius (NTS), and (3) a center to integrate motor and secretory drive to the viscera, the dorsal motor nucleus of the vagus (DMX). There is recent evidence for considerable autonomy of dorsovagal reflexes traversing this loop in ingestive control, especially in meal-termination/anorexic responses, and in other controls on nutrient flux. The characteristics of this system, including (1) activation of several parallel effector pathways, and (2) glucose-sensitivity (hypoglycemic override) have profound therapeutic implications for anti-diabetic and anti-obesity drug development. Agents acting via the brain structures similar to those activated by amylin promise efficacy via multiple effectors, but, via glucose-dependence of effects, without increased risk of hypoglycemia. Several peptides, including those secreted in response to meals, are sensed at area postrema, and because they degrade to innocuous metabolites (amino acids) are thereby candidates as safe and effective drugs. Recently developed GLP-1 agonists and amylin agonists are examples of the therapeutic potential of such agents for metabolic diseases. Others in development include PYY agonists and MC4 agonists. Especially promising may be combinations of peptides and/or their analogs. Certain pairs of peptides, including those acting via the dorsovagal complex, exhibit surprising synergies (eg amylin + CCK (Bhavsar et al., 1998), amylin + leptin (Roth et al., 2008), CCK + glucagon + bombesin (Hinton et al., 1986), PYY and GLP-1 (Paulik et al., 2011)). Peptide combinations may thus attain transformational efficacy without invoking burdensome toxic risk. This article is part of a Special Issue entitled 'Central Control of Food Intake'.

Reversal of obesity and insulin resistance by a non-peptidic glucagon-like peptide-1 receptor agonist in diet-induced obese mice.

BACKGROUND: Glucagon-like peptide-1 (GLP-1) is recognized as an important regulator of glucose homeostasis. Efforts to utilize GLP-1 mimetics in the treatment of diabetes have yielded clinical benefits. A major hurdle for an effective oral therapy has been the difficulty of finding a non-peptidic GLP-1 receptor (GLP-1R) agonist. While its oral bioavailability still poses significant challenges, Boc5, one of the first such compounds, has demonstrated the attainment of GLP-1R agonism in diabetic mice. The present work was to investigate whether subchronic Boc5 treatment can restore glycemic control and induce sustainable weight loss in diet-induced obese (DIO) mice, an animal model of human obesity and insulin resistance. METHODOLOGY/PRINCIPAL FINDINGS: DIO mice were treated three times a week with Boc5 (0.3, 1 and 3 mg) for 12 weeks. Body weight, body mass index (BMI), food intake, fasting glucose, intraperitoneal glucose tolerance and insulin induced glucose clearance were monitored regularly throughout the treatment. Glucose-stimulated insulin secretion, ß-cell mass, islet size, body composition, serum metabolic profiles, lipogenesis, lipolysis, adipose hypertrophy and lipid deposition in the liver and muscle were also measured after 12 weeks of dosing. Boc5 dose-dependently reduced body weight, BMI and food intake in DIO mice. These changes were associated with significant decreases in fat mass, adipocyte hypertrophy and peripheral tissue lipid accumulation. Boc5 treatment also restored glycemic control through marked improvement of insulin sensitivity and normalization of ß-cell mass. Administration of Boc5 (3 mg) reduced basal but enhanced insulin-mediated glucose incorporation and noradrenaline-stimulated lipolysis in isolated adipocytes from obese mice. Furthermore, circulating leptin, adiponectin, triglyceride, total cholesterol, nonesterified fatty acid and high-density lipoprotein/low-density lipoprotein ratio were normalized to various extents by Boc5 treatment. CONCLUSIONS/SIGNIFICANCE: Boc5 may produce metabolic benefits via multiple synergistic mechanisms and may represent an attractive tool for therapeutic intervention of obesity and diabetes, by means of non-peptidic GLP-1R agonism.

Urinary C-peptide excretion: a novel alternate measure of insulin sensitivity in physiological conditions.

Insulin sensitivity (IS) is measured by the euglycemic-hyperinsulinemic clamp under a nonphysiological condition. Daily C-peptide urinary excretion may be a physiological index of IS, because C-peptide is co-secreted with insulin as a function of nutrient intake and IS. The amount of (2)H(2)O released from glycolytic glucose metabolism after [6,6-(2)H(2)]-glucose ingestion was recently proposed as a physiological measure of IS. We compared these IS surrogates to the gold standard (euglycemic-hyperinsulinemic clamp). Thirty (15 male/15 female) sedentary, nondiabetic participants (27.2 +/- 4.0 (s.d.) kg/m(2), 35 +/- 12 years) were admitted for 3 days to our in-patient unit. After a 10-h fast, they received 60 g of glucose and 15 g of [6,6-(2)H(2)]-glucose. Before glucose ingestion and hourly thereafter for 4 h, plasma glucose and insulin concentrations, and plasma deuterium enrichment were determined. Plasma (2)H(2)O production divided by insulin response was used as the glycolytic index. On day 2, subjects spent 23 h in a metabolic chamber (eucaloric diet, 50% carbohydrate, 30% fat). Urinary C-peptide excretion was divided by energy intake yielding the C-peptide to energy intake ratio (CPEP/EI). After leaving the chamber (day 3, 10-h fast), IS was measured by a 2-h clamp (120 mU/m(2)/min). Average IS (clamp) was 7.3 +/- 2.6 mg glucose/kg estimated metabolic body size/min (range: 3.6-13.2). These values were inversely correlated with CPEP/EI (r = -0.62; P < 0.01) and positively with the glycolytic rate (r = 0.60; P < 0.01). In nondiabetic subjects, two novel estimates of IS--daily urinary C-peptide urinary excretion and glycolytic rate during an oral glucose tolerance test --were related to IS by a clamp.

Boc5, a non-peptidic glucagon-like Peptide-1 receptor agonist, invokes sustained glycemic control and weight loss in diabetic mice.

BACKGROUND: Our recent discovery of the substituted cyclobutane Boc5, one of the first non-peptidic agonists at glucagon-like peptide-1 receptors, offers the potential of combining oral availability with full agonism capable of eliciting antidiabetic and antiobesity effects. The present study was aimed at determining the in vivo pharmacologic properties of Boc5 in both normal and diabetic mice following chronic administration, with emphasis on glycemic control and weight loss. METHODOLOGY/PRINCIPAL FINDINGS: C57BL/6J and db/db mice were treated daily with Boc5 for 4 weeks and a range of pharmacologic parameters, including hemoglobin A1c, intraperitoneal glucose tolerance, insulin tolerance, fasting insulin and leptin levels, food intake, body weight and fat mass, were assessed before and after the treatment. Effects on food intake, gastric emptying, and insulinogenic index were also investigated in animals acutely administered with Boc5. Boc5 (3 mg) was able to induce a durable restoration of glycemic control (normalization of both hemoglobin A1c and intraperitoneal glucose tolerance) in db/db mice, following 4 weeks of daily administration. As with peptidic glucagon-like peptide-1 receptor agonists, its glycemic benefit and weight (fat) loss were associated with dose-dependent effects that included reduction in food intake, slowing of gastric emptying (both of which reduce nutrient-drive at beta-cells), stimulation of insulin secretion (which was glucose-dependent), and elevation in insulin sensitivity. There was little effect on normal mice treated in the same manner. CONCLUSIONS/SIGNIFICANCE: Our findings suggest that Boc5 is the only non-peptidic molecule reported thus far to simultaneously activate this spectrum of antidiabetic effects.

Pharmacokinetics and pharmacodynamics of exenatide following alternate routes of administration.

Exenatide is a 39-amino acid peptide incretin mimetic approved for adjunctive treatment of type 2 diabetes. It shares several glucoregulatory activities with the mammalian hormone, glucagon-like peptide-1 (GLP-1). In clinical use, subcutaneous exenatide injections demonstrate glucoregulatory and weight loss effects with sustained plasma concentrations in the 50-100 pM range. We investigated the pharmacokinetics of exenatide in normoglycemic rats and biological activity in diabetic db/db mice after delivery to various epithelial surfaces of the intestinal and respiratory tracts. In rats, elimination kinetics were similar for all routes of administration (median k(e) 0.017 min(-1)). Bioavailability (versus intravenous administration) and C(max) per unit dose differed markedly. For gastrointestinal administration, sublingual administration invoked the highest bioavailability (0.37%); in db/db mice, potentially therapeutic concentrations were obtainable. In contrast, intraduodenal bioavailability was low (0.0053%). In regard to respiratory surfaces, bioavailability of intratracheal exenatide was up to 13.6%, and for nasal administration, 1.68%. Both routes of administration produced therapeutic plasma concentrations and glucose-lowering in db/db mice. At high doses, aerosolized exenatide also achieved effective concentrations and glucose-lowering. In summary, the intestinal tract seems to have limited potential as a route of exenatide administration, with sublingual being most promising. In contrast, the respiratory tract appears to be more viable, comparing favorably with the clinically approved subcutaneous route. Despite little optimization of the delivery formulation, exenatide bioavailability compared favorable to that of several commercially available bioactive peptides.

A nonpeptidic agonist of glucagon-like peptide 1 receptors with efficacy in diabetic db/db mice.

Peptidic mimics of the gut hormone glucagon-like peptide (GLP) 1, exemplified by the recently approved drug exenatide, show promise as therapies for type 2 diabetes. Such "incretin mimetics" regulate glucose appearance in the plasma and can restore glucose-stimulated insulin secretion without excess risk of hypoglycemia. The need for injection, which may limit the use of peptidic GLP-1 receptor (GLP-1R) agonists, has driven largely unsuccessful efforts to find smaller molecules. The failure to identify orally effective agonists has instead promoted the indirect approach of inhibiting the GLP-1-degrading enzyme dipeptidyl peptidase IV. Here we report a nonpeptidic GLP-1R agonist with sufficient activity to evoke effects in whole animals, including antidiabetic efficacy in db/db mice. Two substituted cyclobutanes (S4P and Boc5) were developed after screening a compound library against a cell line stably cotransfected with GLP-1R and a cAMP-responsive reporter. Each bound to GLP-1R and increased intracellular cAMP. Agonist effects were blocked by the GLP-1R antagonist exendin(9-39). Boc5 amplified glucose-stimulated insulin secretion in isolated rat islets. Both i.p. and oral administration of Boc5 dose-dependently inhibited food intake in mice, an effect that could be blocked by pretreatment with exendin(9-39). Daily injections of Boc5 into db/db mice reduced HbA1c to nondiabetic values, an effect not observed in ad libitum-fed or pair-fed diabetic controls. Thus, Boc5 behaved as a full GLP-1 mimetic in vitro and in vivo. The chemical genus represented by Boc5 may prompt the exploration of orally available GLP-1R agonists with potential utility in diabetes and obesity.

Role of endogenous amylin in glucagon secretion and gastric emptying in rats demonstrated with the selective antagonist, AC187.

Amylin is a 37-amino acid polypeptide co-secreted with insulin from the pancreatic beta-cells. It complements insulin's stimulation of the rate of glucose disappearance (Rd) by slowing the rate of glucose appearance (Ra) through several mechanisms, including an inhibition of mealtime glucagon secretion and a slowing of gastric emptying. To determine if endogenous amylin tonically inhibits these processes, we studied the effects of the amylin receptor blocker AC187 upon glucagon secretion during euglycemic, hyperinsulinemic clamps in Sprague-Dawley (HSD) rats, upon gastric emptying in HSD rats, and upon gastric emptying and plasma glucose profile in hyperamylinemic, and genetically obese, Lister Albany/NIH rats during a glucose challenge. Amylin blockade increased glucagon concentration, accelerated gastric emptying of liquids, and resulted in an exaggerated post-challenge glycemia. These data collectively indicate a physiologic role for amylin in glucose homeostasis via mechanisms that include regulation of glucagon secretion and gastric emptying.

Dose-response for inhibition by amylin of cholecystokinin-stimulated secretion of amylase and lipase in rats.

BACKGROUND AND AIMS: The neuroendocrine hormone amylin, cosecreted with insulin from pancreatic beta-cells in response to nutrient ingestion, has several physiologic actions to limit the rate of nutrient uptake, including the slowing of gastric emptying. METHODS: To investigate whether amylin might modulate digestive enzyme secretion from the exocrine pancreas, anesthetized Sprague Dawley rats were cannulated via the pancreatic duct and the secretory response (flow, amylase and lipase) to cholecystokinin (1 microg s.c.) was measured in the absence and in the presence of 0.1, 0.3 and 1 microg s.c. doses of amylin. RESULTS: Amylin alone did not affect pancreatic secretion, but it dose-dependently inhibited cholecystokinin-stimulated amylase secretion by up to 58% and lipase secretion by up to 67%. The ED50's for these responses were 0.21 microg+/-0.18 log and 0.11 microg+/-0.05 log, respectively, doses that result in excursions of plasma amylin concentration that are within the reported physiological range. Amylin did not evoke cell signalling in the Ar42j model of pancreatic acinar cells, and responses to amylin were not observed in either Ar42j cells or isolated pancreatic acini in a microphysiometer indicating that the effect of amylin was indirect. CONCLUSIONS: Inhibition of stimulated pancreatic enzyme secretion is likely to be a physiological, extrapancreatic, action of amylin. Amylinergic mechanisms modulating both gastric emptying and pancreatic enzyme secretion may thus match, respectively, the appearance of substrate and enzymes in the gut lumen.

Exenatide (exendin-4) improves insulin sensitivity and {beta}-cell mass in insulin-resistant obese fa/fa Zucker rats independent of glycemia and body weight.

The effects of the incretin mimetic exenatide (exendin-4) on metabolic parameters, insulin sensitivity, and beta-cell mass were examined in nondiabetic, insulin-resistant obese fa/fa Zucker rats. After 6 wk of treatment, ad libitum-fed exenatide-treated (EX) and pair-fed vehicle control (PF) rats had comparable food intake, body weight, hemoglobin A(1c) (HbA(1c)), and fasting plasma concentrations of glucose, insulin, and lipids. Concurrent decreases in food intake and weight gain were observed in EX and PF rats, compared with ad libitum-fed vehicle control (CON) rats (P < 0.001). The increases in HbA(1c) and fasting plasma insulin concentrations that occur during the normal progression of this disease model were significantly reduced in EX and PF rats, compared with CON rats (P < 0.001). The insulin sensitivity index (ISI; glucose infusion rate to plasma insulin concentration) measured during a hyperinsulinemic euglycemic clamp was 224% higher in EX rats than CON rats (P < 0.001) and 61% higher in EX rats than PF rats (P < 0.004). The latter difference was despite comparable HbA(1c), fasting glucose, fasting insulin, total cholesterol, high-density lipoprotein, and daily food consumption between EX and PF animals. In the absence of exenatide, beta-cell mass was hyperbolically related to ISI (beta-cell mass * ISI was constant). Analogous to the disposition index, the beta-cell mass * ISI product was 63% greater in EX than PF rats (P < 0.05). Thus, exenatide increased beta-cell mass to a greater extent than would be expected in animals of comparable insulin resistance, suggesting a direct trophic effect on islet neogenesis in obese fa/fa rats independent of body weight and glycemia.

Pharmacology of exenatide (synthetic exendin-4): a potential therapeutic for improved glycemic control of type 2 diabetes.

Exenatide (synthetic exendin-4), glucagon-like peptide-1 (GLP-1), and GLP-1 analogues have actions with the potential to significantly improve glycemic control in patients with diabetes. Evidence suggests that these agents use a combination of mechanisms which may include glucose-dependent stimulation of insulin secretion, suppression of glucagon secretion, enhancement of beta-cell mass, slowing of gastric emptying, inhibition of food intake, and modulation of glucose trafficking in peripheral tissues. The short in vivo half-life of GLP-1 has proven a significant barrier to continued clinical development, and the focus of current clinical studies has shifted to agents with longer and more potent in vivo activity. This review examines recent exendin-4 pharmacology in the context of several known mechanisms of action, and contrasts exendin-4 actions with those of GLP-1 and a GLP-1 analogue. One of the most provocative areas of recent research is the finding that exendin-4 enhances beta-cell mass, thereby impeding or even reversing disease progression. Therefore, a major focus of this is article an examination of the data supporting the concept that exendin-4 and GLP-1 may increase beta-cell mass via stimulation of beta-cell neogenesis, stimulation of beta-cell proliferation, and suppression of beta-cell apoptosis.