Unravelling the role of islet G-protein coupled receptors for the treatment of type 2 diabetes

Abstract

G-protein coupled receptors (GPCRs) play a pivotal role in cell signalling, controlling a range of processes including immunity, growth, cellular differentiation, secretions and neurological signalling. Incretin based therapies including dipeptidyl peptidase-IV (DPP-IV) inhibitors and glucagon-like peptide-1 (GLP-1) receptor agonists that target GPCRs (class B) have been identified as possible targets for the treatment of diabetes. Fatty acid agonists are having an increasing role in regulating glucose homeostasis and stimulating islet GPCRs (class A).

This thesis investigated the insulin secretory properties of various endogenous and chemically synthetic GPCR agonists for GPR55, GPR119, GPR120 and GPR39. In addition, anti-diabetic drugs were assessed for DPP-IV inhibitory properties and synergistic effects with incretin receptor agonists. Insulin release and alterations in intracellular Ca2+ and cAMP in response to glucose and GPCR ligands were assessed in clonal BRIN-BD11 cells and isolated islets. Cytotoxicity was determined by lactate dehydrogenase (LDH) release. GPCR expression was determined by double staining immunofluorescence microscopy. In-vivo effects of GPCR agonists were assessed in lean, diabetic and incretin receptor knockout mice (n=6).

Metformin reduced DPP-IV activity and enhanced insulinotropic actions of incretin receptor agonists both in-vitro and in-vivo. Dose-dependent effects on insulin secretion were observed with the GPCR agonists congruent with alterations in intracellular Ca2+ and cAMP concentrations in BRIN-BD11 cells and isolated islets. Expression of GPCRs (GPR55, GPR119, GPR120, GPR39) were confirmed on clonal BRIN-BD11 cells and pancreatic beta-cells. Oral administration of GPCR agonists improved their potency in NIH Swiss mice relative to intraperitoneal administration. Incretin receptor knockout mice implicated all of the receptors in the incretin pathway. Glucose tolerance, insulin sensitivity and lipid profiles were improved, following the 28 day treatment period with Abn-CBD (GPR55 agonist) and AS-1269574 (GPR119 agonist) in streptozotocin-induced diabetic mice. Both agonists enhanced pancreatic morphology increasing beta cell proliferation and reduced hyperglucagonaemia. Overall it was found that anti-diabetic drugs inhibited DPP-IV, GPCR agonists increased insulin release and improved glucose tolerance. This study highlights the anti-diabetic potential of fatty acid GPCR-based therapies which may have a role in diabetes treatment in the future.

Thesis is embargoed until 30th September 2016

Date of Award	Oct 2014
Original language	English
Sponsors	Vice Chancellor's Research Scholarship (VCRS)
Supervisor	Aine McKillop (Supervisor) & Yasser Abdel-Wahab (Supervisor)