Identification of a subset of trace amine-associated receptors and ligands as potential modulators of insulin secretion.

The worldwide prevalence of diabetes has reached 8.5% among adults, and this is characterised by elevated glucose concentrations and failing insulin secretion. Furthermore, most people with type 2 diabetes are either obese or overweight, with the associated dyslipidaemia contributing to the development of insulin resistance and increased cardiovascular risk. Here we incubated INS-1 pancreatic ß-cells for 72 h in RPMI-1640 media, or media supplemented with 28 mM glucose, 200 µM palmitic acid, and 200 µM oleic acid as a cellular model of diabetic glucolipotoxicity. Illumina HiSeq gene expression analysis showed the trace amine-associated receptor (TAAR) family to be among the most highly downregulated by glucolipotoxicity. Importantly, MetaCore integrated knowledge database, from Clarivate Analytics, indicated potential TAAR impact on insulin secretion through adenylyl cyclase signalling pathways. We therefore investigated the effect of TAAR ligands on cAMP signalling and insulin secretion, and found that only the branch of the TAAR family tree that is activated by isopentylamine, 2-phenylethylamine, p-tyramine, and agmatine significantly increased intracellular cAMP and resulted in increased insulin secretion from INS-1 cells and primary mouse islets under normal conditions. Crucially however, this enhancement was not evident when the receptor family was downregulated by glucolipotoxic conditions. This data indicates that a subset of TAARs are regulators of insulin secretion in pancreatic ß-cells, and that their downregulation contributes to glucolipotoxic inhibition of insulin secretion. As such they may be potential targets for treatment of type 2 diabetes.

Carnosine scavenging of glucolipotoxic free radicals enhances insulin secretion and glucose uptake.

The worldwide prevalence of diabetes has risen to 8.5% among adults, which represents a staggering rise in prevalence from 4.7% in 1980. Whilst some treatments work by increasing insulin secretion, over time their effectiveness decreases. We aim to increase insulin secretion by developing strategies that work through mechanisms independent of current treatment options. Isolated CD1 mouse islets, INS-1 pancreatic ß-cells, or C2C12 mouse myotubes were incubated in standard tissue culture media, or media supplemented with 28 mM glucose, 200 µM palmitic acid, and 200 µM oleic acid as a cellular model of diabetic glucolipotoxicity. Intracellular reactive species content was assayed using 2',7'-dichlorofluorescein diacetate dye, inducible nitric oxide synthase levels determined by Western blot, 3-nitrotyrosine and 4-hydrpxnonenal both assayed by ELISA, insulin secretion quantified using ELISA or radioimmunoassay, and glucose uptake determined through 2-deoxy glucose 6 phosphate luminescence. Our data indicate that carnosine, a histidine containing dipeptide available through the diet, is an effective scavenger of each of the aforementioned reactive species. This results in doubling of insulin secretion from isolated mouse islets or INS-1 ß-cells. Crucially, carnosine also reverses glucolipotoxic inhibition of insulin secretion and enhances glucose uptake into skeletal muscle cells. Thus, carnosine, or non-hydrolysable carnosine analogs, may represent a new class of therapeutic agent to fight type 2 diabetes.