ROLE OF AUTOPHAGY IN THE SURVIVAL EFFECT OF ACYLATED GHRELIN, UNACYLATED GHRELIN AND OBESTATIN IN HIT-15 AND INS1-E PANCREATIC β-CELLS

Acylated ghrelin (AG) was identified in the stomach as the endogenous ligand of the growth hormone secretagogue receptor (GHS-R)1a. AG exerts many central and peripheral actions, including negative regulation of insulin secretion and glucose metabolism. Recently, our group demonstrated that both AG and unacylated ghrelin (UAG) promote proliferation and prevent apoptosis of pancreatic β-cells, and increase glucose-induced insulin secretion from β-cells, indicating a role in β-cell survival and function. Similarly, obestatin, a recently identified product of the ghrelin gene, promotes survival and prevents apoptosis in β-cells and human islets. These findings strongly indicate a role of the ghrelin gene products in the pancreas, particularly in the maintenance of β-cell mass. According to other studies, autophagy, a cellular mechanism characterized by a massive cytoplasmic vacuolization, may represent a survival tool for β-cells, under stress conditions such as glucolipotoxicity, which largely occurs in type 2 diabetes. Indeed, in type 2 diabetic patients, alterations in the autophagic machinery, can lead to β-cell death. Aim of this study was to determine whether the survival effects displayed by the ghrelin gene peptides could be mediated by autophagic mechanisms. AG, UAG and obestatin reduced apoptosis in HIT-15 hamster beta-cells, that was induced by palmitate/glucose (glucolipotoxicity). Interestingly, rapamycin, which activates autophagy, reduced palmitate/glucose-induced apoptotic cell death in these cells. In addition, the autophagic inhibitor (mettere per esteso?) 3-MA blocked AG, UAG and obestatin survival effects in serum-free medium, which is also proapoptotic in beta-cells. In rat pancreatic INS-1E beta-cells, both AG, UAG and obestatin increased punctate (per esteso) GFP-LC3 structures, a known autophagic marker, similarly to rapamycin and to serum starvation, suggesting that the ghrelin gene peptides may activate autophagic mechanisms. However, the involvement of autophagy in AG, UAG and obestatin protective affect against glucolipotoxicity-induced apoptosis, has yet to be determined. Although most of the molecular mechanisms remain to be elucidated, there is growing interest in the role of autophagy in β-cell pathophysiology. Therefore, autophagic studies may provide new information on beta-cell death mechanisms in pathological conditions such as diabetes.