Obestatin promotes survival of pancreatic β-cells and human islets and induces expression of genes involved in the regulation of β-cell mass and function

Abstract

OBJECTIVE-Obestatin is a newly discovered peptide encoded by the ghrelin gene whose biological functions are poorly understood. We investigated obestatin effect on survival of β-cells and human pancreatic islets and the underlying signaling pathways. RESEARCH DESIGN AND METHODS-β-cells and human islets were used to assess obestatin effect on cell proliferation, survival, apoptosis, intracellular signaling, and gene expression. RESULTS-Obestatin showed specific binding on HIT-T15 and INS-1E β-cells, bound to glucagon-like peptide-1 receptor (GLP-1R), and recognized ghrelin binding sites. Obestatin exerted proliferative, survival, and antiapoptotic effects under serum-deprived conditions and interferon-γ/tumor necrosis ffactor-α/ interleukin-1β treatment, particularly at pharmacological concentrations. Ghrelin receptor antagonist [D-Lys3]-growth hormone releasing peptide-6 and anti-ghrelin antibody prevented obestatin-induced survival in β-cells and human islets. β-cells and islet cells released obestatin, and addition of anti-obestatin antibody reduced their viability. Obestatin increased β-cell cAMP and activated extracellular signal-related kinase 1/2 (ERK1/2) and phosphatidylinositol 3-kinase (PI 3-kinase)/Akt; its antiapoptotic effect was blocked by inhibition of adenylyl cyclase/cAMP/ protein kinase A (PKA), Pi 3-kinase/Akt, and ERK1/2 signaling. Moreover, obestatin upregulated GLP-1R mRNA and insulin receptor substrate-2 (IRS-2) expression and phosphorylation. The GLP-1R antagonist exendin-(9-39) reduced obestatin effect on β-cell survival. In human islets, obestatin, whose immunore-activity colocalized with that of ghrelin, promoted cell survival and blocked cytokine-induced apoptosis through cAMP increase and involvement of adenylyl cyclase/cAMP/PKA signaling. Moreover, obestatin 1) induced PI 3-kinase/Akt, ERK1/2, and also cAMP response element-binding protein phosphorylation; 2) stimulated insulin secretion and gene expression; and 3) upregu-lated GLP-1R, IRS-2, pancreatic and duodenal homeobox-1, and glucokinase mRNA. CONCLUSIONS-These results indicate that obestatin promotes β-cell and human islet cell survival and stimulates the expression of main regulatory β-cell genes, identifying a new role for this peptide within the endocrine pancreas. © 2008 by the American Diabetes Association.