The metabolic actions of the ghrelin gene-derived peptide obestatin are still unclear. We investigated obestatin effects in vitro, on adipocyte function, and in vivo, on insulin resistance and inflammation in mice fed a high-fat diet (HFD). Obestatin effects on apoptosis, differentiation, lipolysis, and glucose uptake were determined in vitro in mouse 3T3-L1 and in human subcutaneous (hSC) and omental (hOM) adipocytes. In vivo, the influence of obestatin on glucose metabolism was assessed in mice fed an HFD for 8 wk. 3T3-L1, hSC, and hOM preadipocytes and adipocytes secreted obestatin and showed specific binding for the hormone. Obestatin prevented apoptosis in 3T3-L1 preadipocytes by increasing phosphoinositide 3-kinase (PI3K)/Akt and extracellular signal-regulated kinase (ERK)1/2 signaling. In both mice and human adipocytes, obestatin inhibited isoproterenol-induced lipolysis, promoted AMP-activated protein kinase phosphorylation, induced adiponectin, and reduced leptin secretion. Obestatin also enhanced glucose uptake in either the absence or presence of insulin, promoted GLUT4 translocation, and increased Akt phosphorylation and sirtuin 1 (SIRT1) protein expression. Inhibition of SIRT1 by small interfering RNA reduced obestatin-induced glucose uptake. In HFD-fed mice, obestatin reduced insulin resistance, increased insulin secretion from pancreatic islets, and reduced adipocyte apoptosis and inflammation in metabolic tissues. These results provide evidence of a novel role for obestatin in adipocyte function and glucose metabolism and suggest potential therapeutic perspectives in insulin resistance and metabolic dysfunctions.

Traditionally, the interaction of G protein-coupled receptors has been described by models that assume that the receptor exists as a monomer coupled to G protein in a 1:1 stoichiometry. However, these classical models of receptor/G protein coupling may be oversimplified. It has now been shown that many G protein-coupled receptors can form dimers or higher order oligomers and that this phenomenon has relevance to receptor function (for a review, see Ref. 6). Dopamine receptors have also been shown to form dimers and higher order oligomers. Evidence has been provided for D 1 , D 2 , and D 3 homodimers in transfected cell lines (7-9), and D 2 receptors have been shown to exist as dimers in human and rat brain tissues (10). Moreover, Rocheville et al. (11) have recently shown that the dopamine D 2 receptors not only form homodimers but also form heterodimers with somatostatin SSTR 5 receptors. In addition, Gines et al. (12) have shown that the dopamine D 1 receptor forms hetero-oligomers with the adenosine A 1 receptor.As the issue of G protein-coupled receptor homo-and heterodimerization is becoming more and more important, it is crucial to define the mechanism(s) of receptor-receptor interactions in order to predict which receptors can interact with one another. The results obtained until now suggest that more than one mechanism exists and that one receptor can interact with another in more than one way.One of the mechanisms that have been proposed to explain receptor dimerization is the phenomenon of domain swapping

These results highlight differential subcellular localization for betaAR subtypes and indicate that betaAR may have specific roles in regulating nuclear function in cardiomyocytes.

Bioluminescence resonance energy transfer (BRET)and co-immunoprecipitation experiments revealed that heterotrimeric G proteins and their effectors were found in stable complexes that persisted during signal transduction. Adenylyl cyclase, Kir3.1 channel subunits and several Gprotein subunits (G␣ ␣ s , G␣ ␣ i , G␤ ␤ 1 and G␥ ␥ 2 ) were tagged with luciferase (RLuc) or GFP, or the complementary fragments of YFP (specifically G␤ ␤ 1 -YFP 1-158 and G␥ ␥ 2 -YFP 159-238 , which heterodimerize to produce fluorescent YFP-G␤ ␤ 1 ␥ ␥ 2 ). BRET was observed between adenylyl-cyclase-RLuc or Kir3.1-RLuc and GFP-G␥ ␥ 2 , GFP-G␤ ␤ 1 or YFP-G␤ ␤ 1 ␥ ␥ 2 . G␣ ␣ subunits were also stably associated with both effectors regardless of whether or not signal transduction was initiated by a receptor agonist. Although BRET between effectors and G␤ ␤␥ ␥ was increased by receptor stimulation, our data indicate that these changes are likely to be conformational in nature. Furthermore, receptor-sensitive G-protein-effector complexes could be detected before being transported to the plasma membrane, providing the first direct evidence for an intracellular site of assembly.

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The ghrelin gene peptides include acylated ghrelin (AG), unacylated ghrelin (UAG), and obestatin (Ob). AG, mainly produced by the stomach, exerts its central and peripheral effects through the GH secretagogue receptor type 1a (GHS-R1a). UAG, although devoid of GHS-R1a-binding affinity, is an active peptide, sharing with AG many effects through an unknown receptor. Ob was discovered as the G-protein-coupled receptor 39 (GPR39) ligand; however, its physiological actions remain unclear. The endocrine pancreas is necessary for glucose homeostasis maintenance. AG, UAG, and Ob are expressed in both human and rodent pancreatic islets from fetal to adult life, and the pancreas is the major source of ghrelin in the perinatal period. GHS-R1a and GPR39 expression has been shown in b-cells and islets, as well as specific binding sites for AG, UAG, and Ob. Ghrelin colocalizes with glucagon in a-islet cells, but is also uniquely expressed in 3-islet cells, suggesting a role in islet function and development. Indeed, AG, UAG, and Ob regulate insulin secretion in b-cells and isolated islets, promote b-cell proliferation and survival, inhibit b-cell and human islet cell apoptosis, and modulate the expression of genes that are essential in pancreatic islet cell biology. They even induce b-cell regeneration and prevent diabetes in streptozotocin-treated neonatal rats. The receptor(s) mediating their effects are not fully characterized, and a signaling crosstalk has been suggested. The present review summarizes the newest findings on AG, UAG, and Ob expression in pancreatic islets and the role of these peptides on b-cell development, survival, and function.

OBJECTIVEAcylated ghrelin (AG) is a diabetogenic and orexigenic gastric polypeptide. These properties are not shared by the most abundant circulating form, which is unacylated (UAG). An altered UAG/AG profile together with an impairment of circulating endothelial progenitor cell (EPC) bioavailability were found in diabetes. Based on previous evidence for the beneficial cardiovascular effects of AG and UAG, we investigated their potential to revert diabetes-associated defects.RESEARCH DESIGN AND METHODSHealthy human subjects, individuals with type 2 diabetes, and ob/ob mice were AG or UAG infused. EPC mobilization in patients and mice was evaluated, and the underlying molecular mechanisms were investigated in bone marrow stromal cells. Recovered EPCs were also evaluated for the activity of senescence regulatory pathways and for NADPH oxidase activation by knocking down p47phox and Rac1. Finally, UAG modulation of human EPC vasculogenic potential was investigated in an in vivo mouse model.RESULTSNeither AG nor UAG had any effect in healthy subjects. However, systemic administration of UAG, but not AG, prevented diabetes-induced EPC damage by modulating the NADPH oxidase regulatory protein Rac1 and improved the vasculogenic potential both in individuals with type 2 diabetes and in ob/ob mice. In addition, unlike AG, UAG facilitated the recovery of bone marrow EPC mobilization. Crucial to EPC mobilization by UAG was the rescue of endothelial NO synthase (eNOS) phosphorylation by Akt, as UAG treatment was ineffective in eNOS knockout mice. Consistently, EPCs expressed specific UAG-binding sites, not recognized by AG.CONCLUSIONSThese data provide the rationale for clinical applications of UAG in pathologic settings where AG fails.

Aims/hypothesisPancreatic islet microendothelium exhibits unique features in interdependent relationship with beta cells. Gastrointestinal products of the ghrelin gene, acylated ghrelin (AG), unacylated ghrelin (UAG) and obestatin (Ob), and the incretin, glucagon-like peptide-1 (GLP-1), prevent apoptosis of pancreatic beta cells. We investigated whether the ghrelin gene products and the GLP-1 receptor agonist exendin-4 (Ex-4) display survival effects in human pancreatic islet microendothelial cells (MECs) exposed to chronic hyperglycaemia.MethodsIslet MECs were cultured in high glucose concentration and treated with AG, UAG, Ob or Ex-4. Apoptosis was assessed by DNA fragmentation, Hoechst staining of the nuclei and caspase-3 activity. Western blot analyses and pharmacological inhibition of protein kinase B (Akt) and extracellular signal-related kinase (ERK)1/2 pathways, detection of intracellular cAMP levels and blockade of adenylyl cyclase (AC)/cAMP/protein kinase A (PKA) signalling were performed. Levels of NO, IL-1β and vascular endothelial growth factor (VEGF)-A in cell culture supernatant fractions were measured.ResultsIslet MECs express the ghrelin receptor GHS-R1A as well as GLP-1R. Treatment with AG, UAG, Ob and Ex-4 promoted cell survival and significantly inhibited glucose-induced apoptosis, through activation of PI3K/Akt, ERK1/2 phosphorylation and intracellular cAMP increase. Moreover, peptides upregulated B cell lymphoma 2 (BCL-2) and downregulated BCL-2-associated X protein (BAX) and CD40 ligand (CD40L) production, and significantly reduced the secretion of NO, IL-1β and VEGF-A.Conclusions/interpretationThe ghrelin gene-derived peptides and Ex-4 exert cytoprotective effects in islet MECs. The anti-apoptotic effects involve phosphoinositide 3-kinase (PI3K)/Akt, ERK1/2 and cAMP/PKA pathways. These peptides could therefore represent a potential tool to improve islet vascularisation and, indirectly, islet cell function.Electronic supplementary materialThe online version of this article (doi:10.1007/s00125-011-2423-y) contains peer-reviewed but unedited supplementary material, which is available to authorised users.