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 K_ATP 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 K_ATP 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.