Differential functional selectivity and downstream signaling bias of ghrelin receptor antagonists and inverse agonists

Abstract

The ghrelin receptor [growth hormone secretagogue receptor (GHSR)‐1a] represents a promising pharmacologic target for the treatment of metabolic disorders, including obesity and cachexia, via central appetite modulation. The GHSR‐1a has a complex pharmacology, highlighted by G‐protein–dependent and—independent downstream signaling pathways and high basal constitutive activity. The functional selectivity and signaling bias of many GHSR‐1a–specific ligands has not been fully characterized. In this study, we investigated the pharmacologic properties of ghrelin, MK‐0677, L692,585, and [d‐Lys3]‐growth hormone–releasing peptide‐6 (Dlys), JMV2959, and [d‐Arg(1),d‐Phe(5),d‐Trp(7, 9), Leu(11)]‐substance P (SP‐analog). We investigated their effect on basal GHSR‐1a constitutive signaling, ligand‐directed downstream GHSR‐1a signaling, functional selectivity, and signaling bias. Dlys behaved as a partial antagonist with a strong bias toward GHSR‐1a–β‐arrestin signaling, whereas JMV2959 acted as a full unbiased GHSR‐1a antagonist. Moreover, the SP‐analog behaved as an inverse agonist increasing G‐protein–dependent signaling, but only at high concentrations, whereas, at low concentrations, the SP‐analog attenuated β‐arrestin–dependent signaling. Considering the limited success in the clinical development of GHSR‐1a–targeted drugs so far, these findings provide a novel insight into the pharmacologic characteristics of GHSR‐1a ligands and their signaling bias, which has important implications in the design of novel, more selective GHSR‐1a ligands with predictable functional outcome and selectivity for preclinical and clinical drug development.—Ramirez, V. T., van Oeffelen, W. E. P. A., Torres‐Fuentes, C., Chruścicka, B., Druelle, C., Golubeva, A. V., van de Wouw, M., Dinan, T. G., Cryan, J. F., Schellekens, H. Differential functional selectivity and downstream signaling bias of ghrelin receptor antagonists and inverse agonists. FASEB J. 33, 518–531 (2019). http://www.fasebj.org

Growth hormone secretagogue receptor signalling affects high‐fat intake independently of plasma levels of ghrelin andLEAP2, in a 4‐day binge eating model

“…74 Indeed, it is likely that the interaction of these synthetic ligands with GHSR involves a variety of partially biased effects. 74,77 Moreover, we cannot rule out that handling-induced stress affected our observations to some extent because central infusions per se slightly decreased 2-hour HF intake in our experiments. Thus, the current observations should be interpreted cautiously with these considerations in mind.…”

“…Given the complexity of the biology of GHSR, a selective pharmacological manipulation of specific features of the receptor has been shown to be challenging . Indeed, it is likely that the interaction of these synthetic ligands with GHSR involves a variety of partially biased effects . Moreover, we cannot rule out that handling‐induced stress affected our observations to some extent because central infusions per se slightly decreased 2‐hour HF intake in our experiments.…”

Ghrelin regulation of glucose metabolism

“…For example, pancreatic studies using [ d ‐Lys 3 ]‐GHRP‐6 concluded that it was an inverse agonist against GHSR1a Gα i signalling, although this does not align with recent studies reporting that ghrelin‐GHSR signalling in the pancreas is coupled to Gα q , nor more recent studies demonstrating that [ d ‐Lys 3 ]‐GHRP‐6 functions as a biased antagonist against GHSR1a β‐arrestin signalling . Numerous compounds have been used to investigate GHSR1a signalling (eg, [ d ‐Lys 3 ]‐GHRP‐6, substance P, JMV2959) and these careful studies highlight the inherent complexities of GHSR1a signalling, enabling a revised understanding regarding drugs, originally assumed to be antagonists or inverse agonists, functioning instead as biased antagonists or partial agonists for specific downstream signalling cascades . Thus, the nuances of pharmacological interference with GHSR1a must be taken into account when interpreting past and designing future experiments.…”

“…Studies of GHSR1a signalling in δ‐cells did not examine constitutive activity and mostly found that ghrelin signalling acted downstream via the Gα q pathway . However, downstream GHSR1a activity may occur through Gα q , Gα 1/3 , Gα o , Gα i or β‐arrestin signalling and these differential downstream signalling cascades have been implicated for the diverse roles of ghrelin . Although the bulk of this work has been carried out in transfected cells or lipid discs, evidence also exists in vivo .…”

Estudio de la inhibición de los canales de calcio presinápticos tipo CaV2 por la actividad constitutiva del receptor de ghrelina (GHSR) y su impacto sobre la actividad sináptica en neuronas hipocampales

“…Estos comportamientos duales no son excepcionales ya que muchos ligandos sintéticos de GHSR tampoco funcionan estrictamente como agonistas, antagonistas o agonistas inversos sino que poseen características mixtas dependiendo de las vías que activan o inhiben, evidenciando la complejidad del funcionamiento de GHSR (Ramirez et al, 2018).…”