Ghrelin octanoylation mediated by an orphan lipid transferase

Abstract

The peptide hormone ghrelin is the only known protein modified with an O-linked octanoyl side group, which occurs on its third serine residue. This modification is crucial for ghrelin's physiological effects including regulation of feeding, adiposity, and insulin secretion. Despite the crucial role for octanoylation in the physiology of ghrelin, the lipid transferase that mediates this novel modification has remained unknown. Here we report the identification and characterization of human GOAT, the ghrelin O-acyl transferase. GOAT is a conserved orphan membrane-bound O-acyl transferase (MBOAT) that specifically octanoylates serine-3 of the ghrelin peptide. Transcripts for both GOAT and ghrelin occur predominantly in stomach and pancreas. GOAT is conserved across vertebrates, and genetic disruption of the GOAT gene in mice leads to complete absence of acylated ghrelin in circulation. The occurrence of ghrelin and GOAT in stomach and pancreas tissues demonstrates the relevance of GOAT in the acylation of ghrelin and further implicates acylated ghrelin in pancreatic function.G hrelin is a 28-aa peptide hormone produced principally by stomach tissue with an unusual acyl modification on its critical serine-3 residue. Ghrelin is the endogenous ligand for the growth hormone secretagogue receptor 1a (GHSR1a), and its acyl modification is critical for the activation of the GHSR1a (1). In addition to stimulating growth hormone release from pituitary, ghrelin also promotes food intake, carbohydrate utilization, and adiposity (2-5). Accordingly, ghrelin levels are modulated by changes in nutritional status such as feeding and fasting or exposure to high-fat diets (5, 6). Importantly, ghrelin is the only peptide hormone of peripheral tissue origin that increases food intake (2).More recent studies have identified roles for acylated ghrelin in regulating insulin secretion and blood glucose. Acylated ghrelin occurs in pancreas tissues, and GHSR1a receptor blockade with specific antagonists or treatments with antiserum against acylated ghrelin enhance glucose-induced increases in insulin release whereas acylated ghrelin decreases insulin release (5, 7-10). These observations have further implicated acylated ghrelin in the regulation of metabolism.In stomach tissue and in circulation, acylated forms of ghrelin are modified via an ester linkage with n-octanoic acid and to a lesser extent with decanoyl and decenoyl fatty acids (1, 3). Importantly, the acyl modification in ghrelin is essential for function, with octanoyl and decanoyl fatty acids being optimal (11). Ghrelin is highly conserved in vertebrates, and the third serine residue, which is uniquely modified by the ester-linked acyl group, occurs in all mammal, avian, and fish species (3).The enzyme(s) responsible for acylation of ghrelin has remained unknown. Work by Takada et al. (12) described that porcupine, an enzyme with structural similarities to membrane-bound O-acyl transferases (MBOAT), is required for serine-209 acylation with palmitoleic acid and for transport of...

Clinical application of ghrelin administration for gastric cancer patients undergoing gastrectomy

“…Ghrelin exists as two major molecular forms: acyl ghrelin and des-acyl ghrelin. Ghrelin is octanoylated at Ser3, an unusual post-translational modification that is catalyzed by the enzyme ghrelin O-acyltransferase (GOAT) [21,22]. Des-acyl ghrelin, which lacks the Ser3 residue octanoylation, is unable to release GH or bind to the classic GHS-R1a receptor [23].…”

Inhibition of ghrelin O -acyltransferase (GOAT) by octanoylated pentapeptides

“…GOAT was first identified by transfection of candidate cDNAs into cultured endocrine cells that process proghrelin to ghrelin but fail to attach octanoyl (1). With these transfection assays, the enzyme attached octanoyl to the appropriate serine-3 in ghrelin, and the attachment reaction was shown to require the two amino acids in GOAT that are conserved in other membrane-bound O-acyltransferases (1,2). GOAT is a highly hydrophobic protein with eight postulated membrane-spanning helices.…”

“…enzyme that attaches eight-carbon octanoate to a serine residue in ghrelin, a peptide hormone (1,2). Ghrelin comprises the N-terminal 28 amino acids of proghrelin, a precursor of 94 amino acids that is cleaved proteolytically to release ghrelin.…”

Acylated and unacylated ghrelin do not directly stimulate glucose transport in isolated rodent skeletal muscle

“…Ghrelin can be posttranslationally acylated by ghrelin‐ O ‐acyl‐transferase (GOAT) and unacylated by serum esterases (Gutierrez et al. 2008; Yang et al. 2008; Satou and Sugimoto 2012).…”