Signal transduction mechanism of the seabream growth hormone secretagogue receptor

Abstract

We have recently cloned the full-length cDNAs of the two growth hormone secretagogue receptor (GHSR) subtypes from a teleost species, the black seabream (Acanthopagrus schlegeli) [Mol. Cell. Endocrinol. 214 (2004) ] i ), whereas sbGHSR-1b appeared to play an inhibitory role on the signal transduction activity of sbGHSR-1a. In the present study, we have further investigated the signal transduction mechanism of sbGHSR-1a. The peptide GHS GHRP-6 and the non-peptide GHS L163,540 were able to trigger a receptor specific and phospholipase C (PLC)-dependent elevation of [Ca 2+ ] i in HEK293 cells stably expressing sbGHSR-1a. This GHS-induced calcium mobilization was also dependent on protein kinase C activated L-type calcium channel opening. It was found that sbGHSR-1a could function in an agonist-independent manner as it exhibited a high basal activity of inositol phosphate production in the absence of GHS, indicating that the fish receptor is constitutively active. In addition, the extracellular signal-regulated kinases 1 and 2 (ERK1/2) were found to be activated upon stimulation of sbGHSR-1a by GHRP-6. This observation provides direct evidence in the coupling of sbGHSR-1a to ERK1/2 activation. Neither G s nor G i proteins are coupled to the receptor, as GHS did not induce cAMP production nor inhibit forskolin-stimulated cAMP accumulation in the sbGHSR-1a bearing cells. Furthermore, the ability of the GHSR antagonist D D -Lys(3)-GHRP-6 to inhibit basal PLC and basal ERK1/2 activity suggests that this compound is an inverse agonist. In summary, the sbGHSR-1a appears to couple through the G q=11 -mediated pathway to activate PLC, resulting in increased IP 3 production and Ca 2+ mobilization from both intracellular and extracellular stores. Moreover, sbGHSR-1a may trigger multiple signal transduction cascades to exert its physiological functions.

Ghrelin stimulates phagocytosis and superoxide production in fish leukocytes

“…The effect of ghrelin was abolished by a GHS-R antagonist, ]-GHRP-6, which is known to inhibit signal transduction mechanism also in fish GHS-R (Chan et al 2004). Those facts suggest that ghrelin activates trout phagocytes through a GHS-Rmediated mechanism.…”

“…A similar isoform of ghrelin is also present in rat and man as des-Gln 14 -ghrelin, which is also a ligand for GHS-R (Hosoda et al 2000(Hosoda et al , 2003. Signal transduction mediated by G protein has been detailed also in fish GHS-R (Chan et al 2004). Carboxyl-terminus is amidated in teleost ghrelins, but not in tetrapod ghrelins (Kaiya et al 2003a).…”

“…The effect of ghrelin on energy metabolism has suggested its potential use as a therapeutic target in disorders of GH secretion, feeding and nutritional condition (Muccioli et al 2002, Yoshihara et al 2002, Broglio et al 2003. Signal transduction mediated by G protein has been detailed in fish GHS-R (Chan et al 2004). However, expression of GHS-R gene was detected in human T cells, B cells and neutrophils, suggesting the action of ghrelin on the immune system (Hattori et al 2001).…”

A novel GH secretagogue, A233, exhibits enhanced growth activity and innate immune system stimulation in teleosts fish

“…It has recently been described in two variants of GHSR1a in zebrafish (Danio rerio; Kaiya et al 2008, Olsson et al 2008, catfish, and rainbow trout (O. mykiss; Kaiya et al 2009a,b,c). Moreover, it is reported that the peptide [D-Lys 3 ]-GHRP6 inhibits the mechanism of signal transduction of synthetic GHSs (Chan et al 2004, Moulin et al 2007. Leukocytes isolated from tilapia anterior kidney were treated with the specific antagonist secretagogue receptor [D-Lys 3 ]-GHRP6 and subsequently stimulated with peptide A233.…”

Seabream ghrelin: cDNA cloning, genomic organization and promoter studies

“…However, the cognate GHSR has not been cloned and characterized from those fish species in which their ghrelins have been isolated or studied. To date, fish GHSR has only been reported in two species, the pufferfish Spheroides nephelus (Palyha et al 2000) and the black seabream Acanthopagrus schlegeli , and is functionally characterized in black seabream only , Chan et al 2004b. The putative ghrelin gene present in pufferfish, however, has not yet been isolated.…”