Obestatin promotes glut-4 translocation and glucose uptake in 3T3-L1 adipocytes and inhibits lipolysis in 3T3-L1 and human visceral and subcutaneous adipocytes

Obestatin, a newly discovered peptide encoded by the ghrelin gene, was originally identified as the ligand for the orphan G protein-coupled receptor GPR39. In vivo, obestatin was reported to have opposite actions to ghrelin in the regulation of food intake, gastric emptying and body weight; however, these effects, as well as obestatin binding to GPR39, have been questioned by several groups. At the cellular level, we recently showed that in β-cells and human pancreatic islets, obestatin enhances proliferation, reduces apoptosis and promotes insulin secretion, suggesting a role in β-cell function and glucose metabolism. Moreover, other authors have described obestatin-induced c-fos expression in cultured pre-adipocytes, suggesting a role in adipocyte function. Aim of this study was to determine the role of obestatin on glucose uptake and isoproterenol-induced lipolysis in adipocytes. Obestatin protein was expressed in and secreted by human adipocytes. Cytofluorimetric and confocal microscopy analysis evidenced obestatin binding on 3T3-L1 cells and human adipocytes. Moreover, both GPR39 mRNA and protein were found expressed in 3T3-L1 adipocytes and human abdominal subcutaneous and visceral adipose tissue. Obestatin displayed survival action in 3T3-L1 preadipocytes, by reducing serum starvation-induced apoptosis through increased AC/cAMP/PKA signaling and activation of ERK1/2 and PI3K/Akt pathways. No effect was observed with regard to adipocyte differentiation. On the other hand, in differentiated 3T3-L1 adipocytes, obestatin increased GLUT4 mRNA expression, induced glucose transporter-4 (GLUT4) translocation to the plasma membrane and stimulated insulin-induced glucose uptake, either alone or in the presence of insulin. Accordingly, in these cells obestatin also activated AMP-activated protein kinase (AMPK), which is known to play a key role in the regulation of glucose uptake. Finally, obestatin strongly reduced isoproterenol-induced lipolysis and stimulates free fatty acid uptake in either 3T3-L1 adipocytes or mature human subcutaneous and visceral adipocytes, isolated from both obese and nonobese subjects. These results firstly indicate that obestatin plays a role in adipocyte metabolism, possessing insulin-mimetic properties, such as stimulation of GLUT-4 translocation, glucose uptake and inhibition of lipolysis. Together with the data previously shown in pancreatic β-cells, these findings reinforce the concept that obestatin exerts specific and relevant metabolic actions.