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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