Biochemical mechanisms involved in monomethyl succinate-induced insulin secretion

Endogenous and synthetic agonists of GPR119 differ in signalling pathways and their effects on insulin secretion in MIN6c4 insulinoma cells

Background and purpose:

GPR119 is a G protein-coupled receptor that is preferentially expressed in islet cells and mediates insulin secretion. Oleoyl-lysophosphatidylcholine and oleoylethanolamide (OEA) act as endogenous ligands for this receptor, whereas PSN375963 and PSN632408 are two recently reported synthetic agonists. In this study, we explored mechanisms underlying GPR119-induced insulin secretion. In addition, we assessed the potential utility of the synthetic agonists as tools for exploring GPR119 biology.

Experimental approach:

We examined natural and synthetic GPR119 agonist activity at GPR119 in MIN6c4 and RINm5f insulinoma cells. We evaluated insulin secretion, intracellular calcium [Ca²⁺]_i, ion channel involvement and levels of cAMP.

Key results:

We report that increases in insulin secretion induced by OEA were associated with increased cAMP and a potentiation of glucose-stimulated increases in [Ca²⁺]_i. We also demonstrate that ATP-sensitive K⁺ and voltage-dependent calcium channels were required for GPR119-mediated increases in glucose-stimulated insulin secretion. In contrast to OEA, the synthetic GPR119 agonist PSN375963 and PSN632408 have divergent effects on insulin secretion, cAMP and intracellular calcium in MIN6c4 cells.

Conclusions and implications:

The endogenous ligand OEA signals through GPR119 in a manner similar to glucagon-like peptide-1 (GLP-1) and its receptor with respect to insulin secretion, [Ca²⁺]_i and cAMP. In addition, PSN375963 and PSN632408 substantially differ from OEA and from one another. These studies suggest that the commercially available synthetic agonists, although they do activate GPR119, may also activate GPR119-independent pathways and are thus unsuitable as GPR119-specific pharmacological tools.

Succinic acid monoethyl ester, a novel insulinotropic agent: effect on lipid composition and lipid peroxidation in streptozotocin-nicotin-amide induced type 2 diabetic rats

Succinic acid monoethyl ester (EMS) is recently proposed as an insulinotropic agent for the treatment of non-insulin dependent diabetes mellitus. Oxidative stress has been suggested to be a contributory factor in the development and complications of diabetes. In the present study the effect of EMS and Metformin on plasma glucose, insulin, serum and tissue lipid profile, lipoproteins and lipid peroxidation in streptozotocin-nicotinamide induced type 2 diabetic model was investigated. The carboxylic nutrient EMS was administered intraperitonially (8 micromol/g body weight) to streptozotocin diabetic rats for 30 days. The levels of thiobarbituric acid reactive substances (TBARS) and hydroperoxides in liver and kidney and serum and tissue lipids [cholesterol, triglycerides, phospholipids and free fatty acids] and very low density lipoprotein-cholesterol (VLDL-C) and low density lipoprotein-cholesterol (LDL-C), were significantly increased in diabetic rats, whereas the levels of high-density lipoprotein-cholesterol (HDL-C) and antiatherogenic index (AAI) (ratio of HDL to total cholesterol) were significantly decreased. The effect of EMS was compared with metformin, a reference drug. Treatment with EMS and metformin resulted in a significant reduction of plasma glucose with increase plasma insulin in diabetic rats. EMS also resulted in a significant decrease in serum and tissue lipids and lipid peroxidation products. These biochemical observations were supplemented by histopathological examination of liver and kidney section. Our results suggest the possible antihyperlipidemic and antiperoxidative effect of EMS apart from its antidiabetic effect.