Acylated and Unacylated Ghrelin Promote Proliferation and Inhibit Apoptosis of Pancreatic β-Cells and Human Islets: Involvement of 3′,5′-Cyclic Adenosine Monophosphate/Protein Kinase A, Extracellular Signal-Regulated Kinase 1/2, and Phosphatidyl Inositol 3-Kinase/Akt Signaling

Abstract

Among its pleiotropic actions, ghrelin modulates insulin secretion and glucose metabolism. Herein we investigated the role of ghrelin in pancreatic beta-cell proliferation and apoptosis induced by serum starvation or interferon (IFN)-gamma/TNF-alpha, whose synergism is a major cause for beta-cell destruction in type I diabetes. HIT-T15 beta-cells expressed ghrelin but not ghrelin receptor (GRLN-R), which binds acylated ghrelin (AG) only. However, both unacylated ghrelin (UAG) and AG recognized common high-affinity binding sites on these cells. Either AG or UAG stimulated cell proliferation through Galpha(s) protein and prevented serum starvation- and IFN-gamma/TNF-alpha-induced apoptosis. Antighrelin antibody enhanced apoptosis in either the presence or absence of serum but not cytokines. AG and UAG even up-regulated intracellular cAMP. Blockade of adenylyl cyclase/cAMP/protein kinase A signaling prevented the ghrelin cytoprotective effect. AG and UAG also activated phosphatidyl inositol 3-kinase (PI3K)/Akt and ERK1/2, whereas PI3K and MAPK inhibitors counteracted the ghrelin antiapoptotic effect. Furthermore, AG and UAG stimulated insulin secretion from HIT-T15 cells. In INS-1E beta-cells, which express GRLN-R, AG and UAG caused proliferation and protection against apoptosis through identical signaling pathways. Noteworthy, both peptides inhibited cytokine-induced NO increase in either HIT-T15 or INS-1E cells. Finally, they induced cell survival and protection against apoptosis in human islets of Langerhans. These expressed GRLN-R but showed also UAG and AG binding sites. Our data demonstrate that AG and UAG promote survival of both beta-cells and human islets. These effects are independent of GRLN-R, are likely mediated by AG/UAG binding sites, and involve cAMP/PKA, ERK1/2, and PI3K/Akt.

Ghrelin Gene Products in Acute and Chronic Inflammation

“…In a rat model in which ghrelin and GOAT genes are overexpressed in pancreatic b cells under the control of the rat insulin II promoter, the serum insulin levels, pancreatic insulin mRNA expression, and b-cell numbers in islets are increased (Bando et al 2013), suggesting that acylated ghrelin can directly stimulate b-cell proliferation in vivo after islet injury even without its orexigenic or GH-stimulating activities. In other studies, both acylated and unacylated ghrelin prevent apoptosis in the b-cell lines HIT-T15 and INS-1E, as well as in human islets (Granata et al 2007). Similarly, obestatin prevents bcell apoptosis, preserves b-cell mass, and stimulates insulin secretion in vitro and in vivo in animals, in both normal and diabetic conditions (Egido et al 2009;Granata et al 2008Granata et al , 2012.…”

“…Among the wide range of common biological functions, acylated and unacylated ghrelin exert a protective activity on several cell lines including cardiac and endothelial cells, pancreatic b cells, islets, and islet microendothelial cells, cortical neurons, vascular smooth muscle cells, and visceral adipocytes (Baldanzi et al 2002;Chung et al 2008;Favaro et al 2012;Granata et al 2007;Hwang et al 2009;Rodríguez et al 2012;Zhan et al 2008). Both peptides promote the differentiation of skeletal myoblasts, preadipocytes, and embryonic stem cells toward cardiomyocytes (Filigheddu et al 2007;Gao et al 2012Gao et al , 2013Giovambattista et al 2008;Miegueu et al 2011).…”

“…Both peptides promote the differentiation of skeletal myoblasts, preadipocytes, and embryonic stem cells toward cardiomyocytes (Filigheddu et al 2007;Gao et al 2012Gao et al , 2013Giovambattista et al 2008;Miegueu et al 2011). Moreover, they affect, either positively or negatively, proliferation of numerous cell types including osteoblasts, pancreatic b cells, islets and islet microendothelial cells, myoblasts, and adrenocortical tumor cells (Delhanty et al 2006(Delhanty et al , 2007Favaro et al 2012;Filigheddu et al 2007;Granata et al 2007).…”

“…Since unacylated ghrelin shares with ghrelin common high affinity binding sites in various cell lines, including cells lacking GHS-R1a (Baldanzi et al 2002;Cassoni et al 2001Cassoni et al , 2004Filigheddu et al 2007;Granata et al 2007;Jeffery et al 2002;Muccioli et al 2004), and they share biological activities in vivo, also in Ghsr null mice (Porporato et al 2013), both peptides act in all probability through a common, although yet unidentified, receptor.…”

Ghrelin-producing epsilon cells in the developing and adult human pancreas

“…Little is known about the role of ghrelin within the pancreas, but the hypothesis that ghrelin could play a paracrine/autocrine role in the regulation of beta cell survival and function [27] appears reasonable. The production of ghrelin in a prominent endocrine cell population in the fetal pancreas suggests a potential role in islet development.…”