Search

Ghrelin acts through the ghrelin receptor (GhrR) to play an important role in the control of energy balance. Genetic ablation of both ghrelin and GhrR results in resistance to diet-induced obesity and increased insulin sensitivity. Ghrelin is believed to act primarily through central mechanisms to induce food intake and promote adiposity. It is unclear whether there are direct effects on peripheral tissues and, in particular, whether GhrR is expressed by adipocytes. The aim of our work was to explore the possibility that ghrelin may have a direct action on adipocytes via GhrR. To examine ghrelin signaling, we first assessed insulin sensitivity in the murine 3T3-L1 adipocyte cell-line treated with ghrelin for 4 days. Insulin sensitivity was assessed by examining Akt phosphorylation after a 5 minute insulin treatment. We found that 10nM and 100nM ghrelin caused a repression of insulin-induced Akt phosphorylation in response to 1nM insulin (3-fold and 22-fold, respectively) or 10nM insulin (3-fold and 5-fold, respectively). Secondly, we compared GhrR mRNA expression in adipose tissue, isolated adipucytes from C57B16 mice and 3T3-L1 adipocytes, using quantitative RT-PCR with pituitary and hypothalamus as positive controls. Data are expressed as mean ± SEM relative to pituitary (= 1.0). As expected, pituitary and hypothalamus had the highest expression (1.00 ± 0.11,1.06 ± 0.13, respectively). GhrR mRNA was clearly detectable in freshly isolated adipocytes (0.030±0.004). However, GhrR mRNA was barely detectable in epididymal adipose tissue. Because adipocytes comprise only 20% of adipose tissue, it is probable that the adipocyte signal is diluted in the total tissue mRNA. 3T3-L1 cells expressed GhrR at higher levels than the isolated adipocytes (0.23 ± 0.04). This level remained constant throughout adipocyte differentiation. In summary, chronic ghrelin treatment reduces insulin signaling in adipocytes and the detection of GhrR in isolated adipocytes and 3T3-L1 cells suggests ghrelin acts directly through GhrR on adipocytes to regulate energy homeostasis. [ABSTRACT FROM AUTHOR]