GOAT: a stomach enzyme that whets our appetite.

Accessible online at: www.karger.com/ofa Fax +49 761 4 52 07 14 E-mail Information@Karger.de www.karger.com mal sample collection methods and imperfect ghrelin immunoassays. Based on available information, the predominant form of serum ghrelin is desacyl ghrelin. Neither its putative functions nor a possibly existing specific receptor for this form of ghrelin are known [8]. An extensive search for the enzyme responsible for the acylation and activation of ghrelin had been ongoing in numerous laboratories since ghrelin was discovered in 1999. Very recently, two independent teams of scientists using two similar approaches were successful and have identified GOAT as the specific acyl transferase that activates ghrelin. Yang et al. [9] generated several cell cultures producing only desacyl-ghrelin by cDNA transfection. In a second step they co-transfected these cells with cDNA of 16 members of a family of related enzymes, the membrane-bound O-acyltransferases (MBOATs) which catalyze O-acylation reactions related to Wnt signaling. They measured whether the cells were able to produce acyl-ghrelin after being transfected with one of the candidates’ genes and demonstrated that MBOAT 4 was the only candidate producing acylated ghrelin at the expected serine-3 position in the cell culture. Thus, MBOAT 4 was renamed GOAT. At the same time, Gutierrez et al. [10] used gene silencing technology to identify the acyl transferase. They took advantage of the fact that TT cells derived from human medullary carcinoma cells were able to produce acyl-ghrelin when octanoic acid is added to the culture medium. The group designed silencing RNAs (siRNAs) for 12 possible candidate genes to knock down each candidate at a time in the TT cells. When treating the cell culture with siRNA of MBOAT 4, but not other candidates, a significant decrease in ghrelin acylation occurred, indicating that this enzyme was crucial for the modification of ghrelin, which they also re-named GOAT. Using human embryonic kidney (HEK-293) cells co-transfected with GOAT and ghrelin they also showed that GOAT is The discovery of ghrelin O-acetyltransferase (GOAT) is the last – but not least – chapter of a long story which started around 1980, when Cyril Bowers and colleagues [1] discovered that certain opioid peptide derivatives had weak growth hormone (GH) releasing activity. Further studies led to the identification of several synthetic peptides that potently stimulated GH release, although the receptor responsible for mediating these effects remained unknown. This receptor, named growth hormone secretagogue receptor (GHS-R), was finally discovered in 1996 by Roy Smith and colleagues [2] and identified as a typical G-protein-coupled receptor. With the use of the ‘orphan receptor strategy’, the endogenous ligand for the GHS-R1a was subsequently identified by Kojima et al. [3] in 1999 and named ‘ghrelin’. Ghrelin demonstrated GH-releasing properties similar to the synthetic GHSs. However, what has brought this hormone and the GHS-R to the forefront of metabolic research are the additional properties of ghrelin to stimulate food intake and promote adiposity [4]. Thus, the endogenous ghrelin system today is less of a target for GH-related therapies, but is an important basis for potential drugs regulating energy metabolism and body mass. Genetic models support those pharmacological data as it has been described that both ghrelin and GHS-R ‘knock-out’ mice are resistant to high fat diet [5–7]. Ghrelin is a 28-amino acid peptide hormone mainly secreted by the stomach, which has two unique characteristics: It is the only circulating peptide hormone to increase food intake, and its serine-3 residue has a hydroxyl group that requires acylation by n-octanoic acid in order to allow ghrelin to bind and activate its only known receptor [3]. This acylation has been shown to be necessary for most of the biological actions of ghrelin. Although the major active form of ghrelin possesses an octanoylation at the serine-3 position, the vast majority (80–90%) of circulating ghrelin has been found to be non-acylated. However, this result may be biased by the less than optiGOAT: A Stomach Enzyme That Whets Our Appetite