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

Obestatin is a newly discovered peptide encoded by the ghrelin gene whose biological functions are poorly understood. We investigated obestatin effect on survival of beta-cells and human pancreatic islets and the underlying signaling pathways. beta-Cells and human islets were used to assess obestatin effect on cell proliferation, survival, apoptosis, intracellular signaling, and gene expression. Obestatin showed specific binding on HIT-T15 and INS-1E beta-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-gamma/tumor necrosis factor-alpha/interleukin-1 beta treatment, particularly at pharmacological concentrations. Ghrelin receptor antagonist [D-Lys(3)]-growth hormone releasing peptide-6 and anti-ghrelin antibody prevented obestatin-induced survival in beta-cells and human islets. beta-Cells and islet cells released obestatin, and addition of anti-obestatin antibody reduced their viability. Obestatin increased beta-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 beta-cell survival. In human islets, obestatin, whose immunoreactivity 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) upregulated GLP-1R, IRS-2, pancreatic and duodenal homeobox-1, and glucokinase mRNA. These results indicate that obestatin promotes beta-cell and human islet cell survival and stimulates the expression of main regulatory beta-cell genes, identifying a new role for this peptide within the endocrine pancreas.