Figure 3
Main intracellular signaling pathways associated with the survival effects of the ghrelin gene peptides in β-cells and human
pancreatic islets subjected to serum deprivation or cytokine treatment. Presumably, AG, UAG, and Ob bind to a GPCR which,
through the Gsα subunit of the associated G-protein complex, stimulates AC, resulting in increased cAMP production. The involved receptors
are GHS-R1a for AG, an unknown receptor(s) for UAG, and likely GPR39 or GLP1R for Ob. In addition, the existence of common
receptor(s) for the three peptides has been suggested. In the case of Ob-induced cAMP elevation, the cAMP-dependent enzyme
PKA phosphorylates and activates the transcription factor CREB. The effect of AG and UAG on CREB phosphorylation remains to
be studied. The antiapoptotic, proliferative, and survival effects of AG, UAG, and Ob involve the canonical PI3K/AKT and MAPK
(ERK1/2) pathways. Furthermore, AG and UAG also decrease NO, which has been associated with β-cell dysfunction and death;
this effect was not examined for Ob. All these events contribute to reduced apoptosis, increased β-cell, and human islet cell
proliferation and survival. Treatment with the ghrelin gene peptides determines increased glucose-induced insulin secretion
in both rat β-cells and human pancreatic islets. Both the survival and insulinotropic actions of Ob are supported by its capacity
to increase IRS2 phosphorylation and IRS2, PDX1, GK, and GLP1R mRNAs in human islets. (AC, adenylyl cyclase; CREB, cAMP response element-binding protein; ERK1/2, extracellular signal-regulated
kinase 1/2; GHS-R1a, GH secretagogue receptor type 1a, GK, glucokinase; GLP1R, glucagon-like peptide-1 receptor; GPR39, G-protein-coupled
receptor 39; Gsα, G-coupled receptor stimulatory protein; IRS2, insulin receptor substrate-2; NO, nitric oxide; PDX1, pancreatic and duodenal
homeobox-1; PI3K, phosphatidylinositol 3-kinase; PKA, protein kinase A.