Esculentin-2CHa(1–30) and its analogues: stability and mechanisms of insulinotropic action

The insulin-releasing effects, cellular mechanisms of action and anti-hyperglycaemic activity of 10 analogues of esculentin-2CHa lacking the cyclic C-terminal domain (CKISKQC) were evaluated. Analogues of the truncated peptide, esculentin-2CHa(1–30), were designed for plasma enzyme resistance and increased biological activity. Effects of those analogues on insulin release, cell membrane integrity, membrane potential, intracellular Ca2+ and cAMP levels were determined using clonal BRIN-BD11 cells. Their acute effects on glucose tolerance were investigated using NIH Swiss mice. d-Amino acid substitutions at positions 7(Arg), 15(Lys) and 23(Lys) and fatty acid (l-octanoate) attachment to Lys at position 15 of esculentin-2CHa(1–30) conveyed resistance to plasma enzyme degradation whilst preserving insulin-releasing activity. Analogues, [d-Arg7,d-Lys15,d-Lys23]-esculentin-2CHa(1–30) and Lys15-octanoate-esculentin-2CHa(1–30), exhibiting most promising profiles and with confirmed effects on both human insulin-secreting cells and primary mouse islets were selected for further analysis. Using chemical inhibition of adenylate cyclase, protein kinase C or phospholipase C pathways, involvement of PLC/PKC-mediated insulin secretion was confirmed similar to that of CCK-8. Diazoxide, verapamil and Ca2+ omission inhibited insulin secretion induced by the esculentin-2CHa(1–30) analogues suggesting an action on KATP and Ca2+ channels also. Consistent with this, the analogues depolarised the plasma membrane and increased intracellular Ca2+. Evaluation with fluorescent-labelled esculentin-2CHa(1–30) indicated membrane action, with internalisation; however, patch-clamp experiments suggested that depolarisation was not due to the direct inhibition of KATP channels. Acute administration of either analogue to NIH Swiss mice improved glucose tolerance and enhanced insulin release similar to that observed with GLP-1. These data suggest that multi-acting analogues of esculentin-2CHa(1–30) may prove useful for glycaemic control in obesity-diabetes.