br Acknowledgements We thank the

Acknowledgements We thank the following individuals for notable contributions to this work: Dr. Ruth Wood, Dr. Alan Watts, Dr. Casey Donovan, Andrea Suarez, Emily Nakamoto, Allison Apfel, April Banayan, and Jonathan Cheung. This study was supported by the National Institute of Health grants, DK107333 (TMH), DK111158 (EEN), DK105858 (DJR) DK105155 (MRH), and DK104897 (SEK). We report no biomedical financial interests or potential conflicts of interest.
Introduction Ghrelin, a 28–amino Rapalink-1 synthesis peptide that was originally purified from rat stomach in 1999, is the endogenous ligand of growth hormone secretagogue receptor (GHS-R) [1]. Using a reverse pharmacology approach, ghrelin was identified based on binding to stable cell lines that overexpressed GHS-R [1]. Purification of ghrelin from the stomach revealed that the release of growth hormone (GH) from the pituitary is regulated by both hypothalamic GH-releasing hormone and a peripherally derived hormone. ‘Ghrelin’ was named for its characteristic effect of stimulating growth hormone release. A distinguishing structural feature of ghrelin is N-octanoylation at its third serine residue, which is necessary for receptor binding and induction of its biological activity [1]. The attachment of octanoate is catalyzed by a membrane-bound enzyme called ghrelin O-acyltransferase (GOAT) [2]. GOAT mRNA is largely restricted to stomach and intestine, the major ghrelin-secreting tissues [2]. Subsequent studies revealed that intracerebroventricular (icv) administration of ghrelin potently stimulates appetite [3]. After central administration of ghrelin, Fos, a marker of neuronal activation, is present in regions containing neurons that produce neuropeptide Y (NPY) and agouti-related protein (AgRP); conversely, antibodies and antagonists of NPY and AgRP abolish ghrelin-induced feeding [3]. Ghrelin-related functions have also been reported in the cardiovascular, gastrointestinal, and immune systems, as well as in bone. In this review, we will discuss the characteristics of ghrelin and its potential therapeutic roles in cardiac disease.
Production and secretion of ghrelin As previously described, the stomach is a major site of ghrelin production, and an in situ analysis revealed that ghrelin and its mRNA are mainly localized in the submucosal layer [1]. There are four types of endocrine cells in the oxyntic mucosa of the stomach, ECL, D, enterochromaffin, and X/A-like cells; among them, ghrelin is present predominantly in X/A-like cells [4]. In the stomach in both rats and humans, the mature ghrelin peptide (28 amino acids) is cleaved from a precursor preproghrelin protein of 117 amino acids [1]. Ghrelin is secreted from the submucosal layer of the stomach into the blood stream. In totally gastrectomized patients, the ghrelin level is reduced by 35% relative to that in control subjects, indicating that the stomach is the major organ responsible for secretion of circulating ghrelin [5]. In rats, immunoreactive ghrelin is also present in the duodenum, jejunum, ileum, and colon [4]. Plasma ghrelin levels increase by 31% after 12 h of fasting, but decrease by 22% immediately after feeding [5]. In patients with anorexia nervosa, plasma ghrelin levels are markedly higher than those in normal controls, and are negatively correlated with body mass index [5].
Ghrelin receptor GHS-R was identified by expression cloning in 1996 [6]. It acts as a receptor for ghrelin, ghrelin-induced GH release, and appetite promotion [7]. Two transcript variants have been identified in multiple tissues in several species. GHS-R1a encodes a functionally active protein that induces GH release, whereas GHS-R1b is an alternatively spliced isoform [6]. In human, expression of GHS-R mRNA is predominantly detected in pituitary, hypothalamus, and hippocampus [8]. Expression of GHS-R in the cardiovascular system remains controversial, with some studies reporting expression in the heart and vasculature [[9], [10], [11]]. However, Callagham et al. did not detect GHS-R expression by immunohistochemistry in the aorta, mesenteric artery, cerebral artery, coronary artery, or myocardial tissue of GHS-R reporter mice, nor did they detect GHS-R1a mRNA by RT-PCR in the aorta or mesenteric vessels of rats [12]. Thus, further investigations are needed to determine the localization of GHS-R within the cardiovascular system.