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

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 factor-α/interleukin-1β 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 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 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. 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.