Dissecting the physiology and pathophysiology of glucagon-like peptide-1

Copyright © 2018 Paternoster and Falasca. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. An aging world population exposed to a sedentary life style is currently plagued by chronic metabolic diseases, such as type-2 diabetes, that are spreading worldwide at an unprecedented rate. One of the most promising pharmacological approaches for the management of type 2 diabetes takes advantage of the peptide hormone glucagon-like peptide-1 (GLP-1) under the form of protease resistant mimetics, and DPP-IV inhibitors. Despite the improved quality of life, long-term treatments with these new classes of drugs are riddled with serious and life-threatening side-effects, with no overall cure of the disease. New evidence is shedding more light over the complex physiology of GLP-1 in health and metabolic diseases. Herein, we discuss the most recent advancements in the biology of gut receptors known to induce the secretion of GLP-1, to bridge the multiple gaps into our understanding of its physiology and pathology

GPR40 partial agonist MK-2305 lower fasting glucose in the Goto Kakizaki rat via suppression of endogenous glucose production

<div><p>GPR40 (FFA1) is a fatty acid receptor whose activation results in potent glucose lowering and insulinotropic effects <i>in vivo</i>. Several reports illustrate that GPR40 agonists exert glucose lowering in diabetic humans. To assess the mechanisms by which GPR40 partial agonists improve glucose homeostasis, we evaluated the effects of MK-2305, a potent and selective partial GPR40 agonist, in diabetic Goto Kakizaki rats. MK-2305 decreased fasting glucose after acute and chronic treatment. MK-2305-mediated changes in glucose were coupled with increases in plasma insulin during hyperglycemia and glucose challenges but not during fasting, when glucose was normalized. To determine the mechanism(s) mediating these changes in glucose metabolism, we measured the absolute contribution of precursors to glucose production in the presence or absence of MK-2305. MK-2305 treatment resulted in decreased endogenous glucose production (EGP) driven primarily through changes in gluconeogenesis from substrates entering at the TCA cycle. The decrease in EGP was not likely due to a direct effect on the liver, as isolated perfused liver studies showed no effect of MK-2305 <i>ex vivo</i> and GPR40 is not expressed in the liver. Taken together, our results suggest MK-2305 treatment increases glucose stimulated insulin secretion (GSIS), resulting in changes to hepatic substrate handling that improve glucose homeostasis in the diabetic state. Importantly, these data extend our understanding of the underlying mechanisms by which GPR40 partial agonists reduce hyperglycemia.</p></div

Effects of chronic treatment with MK-2305 in the GK rat.

<p>(A) Morning blood glucose levels in GK rats treated with vehicle, 10, or 30 mg/kg of MK-2305, or 10 mg/kg rosiglitazone for 20 days in feed. (B) Fasted blood glucose levels on days 7 and 14 of the study were significantly reduced with MK-2305 and rosiglitazone treatment compared to vehicle controls. (C) Fasted plasma insulin levels on days 7 and 14. (D) Effects on food intake and (E) body weight during the chronic study. (F) Plasma insulin levels during a OGTT in chronically treated rats on day 13. Changes in blood glucose, food intake or body weight over time with MK-2305 or rosiglitazone vs. vehicle were analyzed by two-way ANOVA with repeated measures followed by Tukeys post hoc analysis. Changes in fasted glucose or insulin of glucose AUC were analyzed by one way ANOVA comparing MK-2305 or rosiglitazone treatments with vehicle followed by Dunnetts post hoc analysis. *p<0.05, **p<0.01.</p

Effect of (A) acute treatment and (B) chronic treatment with 10 mg/kg MK-2305 or vehicle on endogenous glucose production (EGP) from various substrates in the GK or WKY rats.

<p>Data were analyzed via ANOVA followed by Tukey’s multiple comparisons test comparing MK-2305 treatment to vehicle within GK or WKY rats or comparing WKY rats treated with vehicle to GK rats treated with vehicle. *p< 0.05, **p<0.01, ***p<0.005, ****p<0.001.</p

In vitro and ex-vivo pharmacology of MK-2305.

<p>(A) Structure of the synthetic GPR40 partial agonist MK-2305. (B) Dose-response curves for MK-2305 were generated monitoring IP accumulation in CHO cells expressing rat GPR40. Data are expressed as a percentage of the control response of an in-house partial agonist, and fitted to a standard 4-parameter non-linear regression model. EC₅₀’s were determined for each test compound using a custom in-house developed software package. Each experiment was multiple times with a representative graph shown. The mean parameters of these and other individual experiments are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0176182#pone.0176182.t001" target="_blank">Table 1</a>. (C) Effect of MK-2305 on GSIS in isolated GPR40 WT and KO islets under high (15 mM) and not basal (2 mM) glucose. Data provided are means +/- SEM. Data were analyzed via ANOVA followed by Bonferroni multiple comparisons test. **p<0.01compared to DMSO treated islets under 15 mM glucose.</p

Effects of MK-2305 on glucose metabolism in perfused mouse livers.

<p>Effect of 10 μM MK-2305 or DMSO on the conversion of [2-¹³C] pyruvate to ¹³C-glucose, ¹³C-glycogen, and ¹³C-lactate in perfused db/db mouse livers. MK-2305 treatments were compared to vehicle for each endpoint via students ttest.</p

Uptake and metabolic conversion of [1-¹³C] glucose in liver (A) and muscle (B) following acute treatment with MK-2305 or vehicle.

<p>Data were analyzed via students ttest comparing MK-2305 treatment vs. vehicle for each metabolite in each tissue measured.</p

<i>In vitro</i> pharmacology of MK-2305.

<p><i>In vitro</i> pharmacology of MK-2305.</p

Discovery of Chromane Propionic Acid Analogues as Selective Agonists of GPR120 with <i>in Vivo</i> Activity in Rodents

GPR120 (FFAR4) is a fatty acid sensing G protein coupled receptor (GPCR) that has been identified as a target for possible treatment of type 2 diabetes. A selective activator of GPR120 containing a chromane scaffold has been designed, synthesized, and evaluated <i>in vivo</i>. Results of these efforts suggest that chromane propionic acid <b>18</b> is a suitable tool molecule for further animal studies. Compound <b>18</b> is selective over the closely related target GPR40 (FFAR1), has a clean off-target profile, demonstrates suitable pharmacokinetic properties, and has been evaluated in wild-type/knockout GPR120 mouse oGTT studies