Tigerinin-1R: A potent, non-toxic insulin-releasing peptide isolated from the skin of the Asian frog, Hoplobatrachus rugulosus

Aim: Characterization of peptides in the skin of the Vietnamese common lowland frog Hoplobatrachus rugulosus with the ability to stimulate insulin release in vitro and improve glucose tolerance in vivo. Methods: Peptides in an extract of skin were purified by reversed-phase HPLC, and their abilities to stimulate the release of insulin and the cytosolic enzyme lactate dehydrogenase were determined using BRIN-BD11 clonal β cells. Insulin-releasing potencies of synthetic peptides and their effects on membrane potential and intracellular Ca²⁺ concentration were also measured using BRIN-BD11 cells. Effects on glucose tolerance and insulin release in vivo were determined in mice fed a high-fat diet to induce obesity and insulin resistance. Results: A cyclic dodecapeptide (RVCSAIPLPICH.NH₂), termed tigerinin-1R, was isolated from the skin extract that lacked short-term cytotoxic and haemolytic activity but significantly (p < 0.01) stimulated the rate of release of insulin from BRIN-BD11 cells at concentrations ≥0.1 nM. The maximum response was 405% of the basal rate at 5.6 mM ambient glucose concentration and 290% of basal rate at 16.7 mM glucose. C-terminal α-amidation was necessary for high potency and a possible mechanism of action of the peptide-involved membrane depolarization and an increase in intracellular Ca²⁺ concentration. Administration of tigerinin-1R (75 nmol/kg body weight) to high fat-fed mice significantly (p < 0.05) enhanced insulin release and improved glucose tolerance during the 60-min period following an intraperitoneal glucose load. Conclusion: Tigerinin-1R is a potent, non-toxic insulin-releasing peptide that shows potential for development into an agent for the treatment of type 2 diabetes.