Unacylated ghrelin is not a functional antagonist but a full agonist of the type 1a growth hormone secretagogue receptor (GHS-R)

Abstract

Recent findings demonstrate that the effects of ghrelin can be abrogated by co-administered unacylated ghrelin (UAG). Since the general consensus is that UAG does not interact with the type 1a growth hormone secretagogue receptor (GHS-R), a possible mechanism of action for this antagonistic effect is via another receptor. However, functional antagonism of the GHS-R by UAG has not been explored extensively. In this study we used human GHS-R and aequorin expressing CHO-K1 cells to measure [Ca(2+)](i) following treatment with UAG. UAG at up to 10(-5)M did not antagonize ghrelin induced [Ca(2+)](i). However, UAG was found to be a full agonist of the GHS-R with an EC(50) of between 1.6 and 2 microM using this in vitro system. Correspondingly, UAG displaced radio-labeled ghrelin from the GHS-R with an IC(50) of 13 microM. In addition, GHS-R antagonists were found to block UAG induced [Ca(2+)](i) with approximately similar potency to their effect on ghrelin activation of the GHS-R, suggesting a similar mode of action. These findings demonstrate in a defined system that UAG does not antagonize activation of the GHS-R by ghrelin. But our findings also emphasize the importance of assessing the concentration of UAG used in both in vitro and in vivo experimental systems that are aimed at examining GHS-R independent effects. Where local concentrations of UAG may reach the high nanomolar to micromolar range, assignment of GHS-R independent effects should be made with caution.

MECHANISMS IN ENDOCRINOLOGY: Regulation of glucose metabolism by the ghrelin system: multiple players and multiple actions

“…There is also evidence that dAG alone does not regulate islet cell function, but rather acts to antagonize the inhibitory action of AG on insulin secretion (21). The receptor that mediates these actions of dAG is still unknown, and although it is largely accepted that dAG does not bind and activate GHSR (1), studies on CHO-K1 cells transfected with human GHSR demonstrate that dAG can act as a full agonist of GHSR in the high nanomolar to micromolar range (38). Similarly, high nanomolar concentrations of dAG activate GHSR in HEK-293 cells transfected with the ghrelin receptor (9).…”

MECHANISMS IN ENDOCRINOLOGY: Ghrelin: the differences between acyl- and des-acyl ghrelin

“…The obvious reason for this is that DAG is unable to bind and activate the GHSR1a in the submicromolar, physiological range (1,7). Therefore, DAG has never received the level of attention in research and the literature that it deserves, although a growing number of reports strongly suggest that DAG has intrinsic activities in a variety of physiological and pathophysiological situations.…”

Ghrelin Gene Products in Acute and Chronic Inflammation

“…Indeed, the unacylated form of ghrelin, which accounts for most of the circulating peptide, does not bind to and activate GHS-R1a (Kojima et al 1999), unless used at micromolar concentrations, because at such high concentrations the unacylated form can act as a functional agonist (Bednarek et al 2000;Gauna et al 2007;Heppner et al 2014;Matsumoto et al 2001). However, starting with the first demonstration in vitro of the anti-apoptotic activity of unacylated ghrelin on cardiomyocytes and endothelial cells (Baldanzi et al 2002), and, then, in vivo on insulin secretion in humans (Broglio et al 2004), a continuously increasing number of works demonstrated that unacylated ghrelin is a biologically active peptide, sharing, often in cells lacking GHS-R1a, most of acylated ghrelin biological activities.…”