Nonpeptide and peptide growth hormone secretagogues act both as ghrelin receptor agonist and as positive or negative allosteric modulators of ghrelin signaling

Two nonpeptide (L692,429 and MK-677) and two peptide [GH-releasing peptide (GHRP)-6 and ghrelin] agonists were compared in binding and in signal transduction assays: calcium mobilization, inositol phosphate turnover, cAMP-responsive element (CRE), and serum-responsive element (SRE) controlled transcription, as well as Arrestin mobilization. MK-677 acted as a simple agonist having an affinity of 6.5 nm and activated all signal transduction systems with similar high potency (0.2-1.4 nm). L-692,429 also displayed a very similar potency in all signaling assays (25-60 nm) but competed with a 1000-fold lower apparent affinity for ghrelin binding and surprisingly acted as a positive allosteric receptor modulator by increasing ghrelin's potency 4- to 10-fold. In contrast, the potency of GHRP-6 varied 600-fold (0.1-61 nm) depending on the signal transduction assay, and it acted as a negative allosteric modulator of ghrelin signaling. Unexpectedly, the maximal signaling efficacy for ghrelin was increased above what was observed with the hormone itself during coadministration with the nonendogenous agonists. It is concluded that agonists for the ghrelin receptor vary both in respect of their intrinsic agonist properties and in their ability to modulate ghrelin signaling. A receptor model is presented wherein ghrelin normally only activates one receptor subunit in a dimer and where the smaller nonendogenous agonists bind in the other subunit to act both as coagonists and as either neutral (MK-677), positive (L-692,429), or negative (GHRP-6) modulators of ghrelin function. It is suggested that an optimal drug candidate could be an agonist that also is a positive modulator of ghrelin signaling.