Ghrelin, an appetite-stimulatory hormone secreted by the stomach, was discovered as a ligand for the growth hormone secretagogue receptor (GHSR). Through GHSR, ghrelin stimulates growth hormone (GH) secretion, a function that evolved to protect against starvation-induced hypoglycemia. Though the biology mediated by ghrelin has been described in great detail, regulation of ghrelin action is poorly understood. Here, we report the discovery of liver-expressed antimicrobial peptide 2 (LEAP2) as an endogenous antagonist of GHSR. LEAP2 is produced in the liver and small intestine, and its secretion is suppressed by fasting. LEAP2 fully inhibits GHSR activation by ghrelin and blocks the major effects of ghrelin in vivo, including food intake, GH release, and maintenance of viable glucose levels during chronic caloric restriction. In contrast, neutralizing antibodies that block endogenous LEAP2 function enhance ghrelin action in vivo. Our findings reveal a mechanism for fine-tuning ghrelin action in response to changing environmental conditions.

Edited by Jeffrey E. PessinApelin-36 was discovered as the endogenous ligand for the previously orphan receptor APJ. Apelin-36 has been linked to two major types of biological activities: cardiovascular (stimulation of cardiac contractility and suppression of blood pressure) and metabolic (improving glucose homeostasis and lowering body weight). It has been assumed that both of these activities are modulated through APJ. Here, we demonstrate that the metabolic activity of apelin-36 can be separated from canonical APJ activation. We developed a series of apelin-36 variants in which evolutionarily conserved residues were mutated, and evaluated their ability to modulate glucose homeostasis and body weight in chronic mouse models. We found that apelin-36(L28A) retains full metabolic activity, but is 100-fold impaired in its ability to activate APJ. In contrast to its full metabolic activity, apelin-36(L28A) lost the ability to suppress blood pressure in spontaneously hypertensive rats (SHR). We took advantage of these findings to develop a longer-acting variant of apelin-36 that could modulate glucose homeostasis without impacting blood pressure (or activating APJ). Apelin-36-[L28C(30kDa-PEG)] is 10,000-fold less potent than apelin-36 at activating the APJ receptor but retains its ability to significantly lower blood glucose and improve glucose tolerance in diet-induced obese mice. Apelin-36-[L28C(30kDa-PEG)] provides a starting point for the development of diabetes therapeutics that are devoid of the blood pressure effects associated with canonical APJ activation.The apelin gene encodes a pre-pro-protein that is processed into a number of regulatory hormones. The best characterized of these peptide hormones are apelin-13, apelin-17, and apelin-36 (1). The 13 C-terminal amino acids of these peptides (comprising apelin-13) are shared, with apelin-17 extending an additional 4 amino acids from the N terminus, and apelin-36 extending a further 19 amino acids beyond the N terminus of apelin-17 (see Table 1). Apelin was discovered as an endogenous agonist of the G protein-coupled receptor APJ (2). Specifically, apelin-36 was purified from bovine stomach tissue extract based on its ability to stimulate signaling through APJ.Through a combination of pharmacological and genetic approaches, apelin has been linked to two major types of biological activities: cardiovascular (stimulation of cardiac contractility and suppression of blood pressure) and metabolic (improving glucose homeostasis and lowering body weight) (3, 4). Both of these activities have been assumed to be mediated solely through APJ, but there is substantial evidence to suggest that APJ may have apelin-independent activities (5, 6). Here, we provide evidence that this ligand-receptor promiscuity goes in both directions, and that apelin may have APJ-independent activities as well. Specifically, we demonstrate that the metabolic activity of apelin-36 can be dissociated from canonical APJ signaling. ResultsTo evaluate the chronic metabolic activity of apelin peptid...