Adenosine Is an Agonist of the Growth Hormone Secretagogue Receptor

Rat pituitary tumor cells (GC cells) exhibit spontaneous oscillations of intracellular free calcium concentration ([Ca²⁺]_i) that allow continuous release of growth hormone (GH). Of the somatostatin (SRIH) receptor subtypes (sst receptors) mediating SRIH action, sst₁ and sst₂ receptors are highly expressed by GC cell membranes. In the present study, the effects of sst₁ or sst₂ receptor activation on single-cell [Ca²⁺]_i were investigated in GC cells by confocal fluorescence microscopy. In addition, the effects of sst₁ or sst₂ receptor activation on GH secretion were also studied. Our results demonstrate that SRIH decreases [Ca²⁺]_i baseline and almost completely blocks Ca²⁺ transients through activation of sst₂ but not of sst₁ receptors. In contrast, SRIH effectively inhibits GH secretion through activation of both sst₁ and sst₂ receptors. Blocking Ca²⁺ transients is less efficient than SRIH to inhibit GH release. The cyclic octapeptide, CYN-154806, antagonizes sst₂ receptors at [Ca²⁺]_i since it abolishes the sst₂ receptor-mediated inhibition of [Ca²⁺]_i without affecting single-cell Ca²⁺ signals. On the other hand, CYN-154806 alone potently inhibits GH secretion through the involvement of pertussis toxin-sensitive G proteins. In conclusion, the present results demonstrate that SRIH inhibition of GH release in GC cells involves mechanisms either dependent or independent on SRIH modulation of [Ca²⁺]_i. The implications of CYN-154806 inhibition of GH secretion are discussed.

Section: Discussionmentioning

confidence: 99%

Inhibitory Control of Growth Hormone Secretion by Somatostatin in Rat Pituitary GC Cells: sst2 but Not sst1 Receptors Are Coupled to Inhibition of Single-Cell Intracellular Free Calcium Concentrations

Cervia

Petrucci²,

Bluet-Pajot³

et al. 2002

“…By contrast, UAG does not bind GHS-R1a [69, 70, 118]. Adenosine was initially proposed to be a partial agonist for GHS-R1a [119,120,121], but this has been questioned [122, 123]. A series of other molecules apparently unrelated to ghrelin have also been shown to bind GHS-R1a, such as the natural SRIH-like neuropeptide cortistatin, some synthetic SRIH-mimetic octapeptides (octreotide, lanreotide and vapreotide) [124,125,126] and the atypical L-type Ca 2+ channel blocker diltiazem [127].…”

Section: Type 1a Ghs Receptormentioning

confidence: 99%

Heterogeneity of Ghrelin/Growth Hormone Secretagogue Receptors

Muccioli¹,

Baragli²,

Granata³

et al. 2007

Ghrelin is a gastric polypeptide displaying strong GH-releasing activity by activation of the type 1a GH secretagogue receptor (GHS-R1a) located in the hypothalamus-pituitary axis. GHS-R1a is a G-protein-coupled receptor that, upon the binding of ghrelin or synthetic peptidyl and non-peptidyl ghrelin-mimetic agents known as GHS, preferentially couples to G_q, ultimately leading to increased intracellular calcium content. Beside the potent GH-releasing action, ghrelin and GHS influence food intake, gut motility, sleep, memory and behavior, glucose and lipid metabolism, cardiovascular performances, cell proliferation, immunological responses and reproduction. A growing body of evidence suggests that the cloned GHS-R1a alone cannot be the responsible for all these effects. The cloned GHS-R1b splice variant is apparently non-ghrelin/GHS-responsive, despite demonstration of expression in neoplastic tissues responsive to ghrelin not expressing GHS-R1a; GHS-R1a homologues sensitive to ghrelin are capable of interaction with GHS-R1b, forming heterodimeric species. Furthermore, GHS-R1a-deficient mice do not show evident abnormalities in growth and diet-induced obesity, suggesting the involvement of another receptor. Additional evidence of the existence of another receptor is that ghrelin and GHS do not always share the same biological activities and activate a variety of intracellular signalling systems besides G_q. The biological actions on the heart, adipose tissue, pancreas, cancer cells and brain shared by ghrelin and the non-acylated form of ghrelin (des-octanoyl ghrelin), which does not bind GHS-R1a, represent the best evidence for the existence of a still unknown, functionally active binding site for this family of molecules. Finally, located in the heart and blood vessels is the scavenger receptor CD36, involved in the endocytosis of the pro-atherogenic oxidized low-density lipoproteins, which is a pharmacologically and structurally distinct receptor for peptidyl GHS and not for ghrelin. This review highlights the most recently discovered features of GHS-R1a and the emerging evidence for a novel group of receptors that are not of the GHS1a type; these appear involved in the transduction of the multiple levels of information provided by GHS and ghrelin.

“…These findings point to an overlapping in the agonist-binding site, as receptor agonism does not require identical disposition at the ligand-binding site [16]. In addition, GHSR-1a appears to show another binding site different from the characterized GHS binding pocket, which was revealed by studies developed with adenosine [17, 18]. Ligand binding and site-directed mutagenesis studies showed that adenosine binds to a binding site different from the one characterized for GHSs and ghrelin [17, 18].…”

Section: Introductionmentioning

confidence: 99%

“…In addition, GHSR-1a appears to show another binding site different from the characterized GHS binding pocket, which was revealed by studies developed with adenosine [17, 18]. Ligand binding and site-directed mutagenesis studies showed that adenosine binds to a binding site different from the one characterized for GHSs and ghrelin [17, 18]. Functionality studies demonstrated that adenosine triggers intracellular calcium rise with a slower temporal dynamic than that observed for GHSs.…”

Section: Introductionmentioning

confidence: 99%

“…Functionality studies demonstrated that adenosine triggers intracellular calcium rise with a slower temporal dynamic than that observed for GHSs. This fact was attributed to a slower rate of formation of the adenosine/GHSR-1a complex [17, 18]. Interestingly, the agonist activity of adenosine seems to be insufficient to stimulate in vitro GH secretion from pituitary cells, either basally or during stimulation by GHSs or GHRH [17, 18].…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Agonist-Specific Coupling of Growth Hormone Secretagogue Receptor Type 1a to Different Intracellular Signaling Systems

Carreira

Camiña²,

Smith³

et al. 2004

The growth hormone secretagogue receptor subtype 1a (GHSR-1a) is involved in biological actions of ghrelin by triggering intracellular second messengers coupled to heterotrimeric G-protein complex involving Gα_q/11. Adenosine is a partial agonist of the GHSR-1a, binding to a binding pocket distinct from the one described for ghrelin. This suggests a variety of functions for the poorly understood GHSR1a receptor. In this work, a sequential analysis of the pathways involved in the regulation of GHSR-1a signaling was undertaken to characterize the intracellular calcium mobilization that is observed following adenosine binding. The results showed that adenosine induced, in a dose-dependent manner, a calcium mobilization from IP₃-sensitive intracellular stores since the IP₃ receptor blocker 2-APB was able to suppress the calcium response. However, adenosine did not show any effect in the formation of inositol phosphates. The calcium-mobilizing activity was blocked after preincubation of cells with CTX, the inhibitor of adenylate cyclase MDL-12,330A and the protein kinase A blocker H-89. Furthermore, the administration of adenosine stimulated cAMP production. Based on the experimental data, a signaling pathway is proposed involving adenylate cyclase and protein kinase A, which causes phosphorylation of the IP₃ receptor, with a cross-talk between the signaling pathways activated by ghrelin and adenosine. The data described in this report suggest that GHSR-1a is able to activate different intracellular second-messenger systems depending on the agonist that activates it. The regulation of the ghrelin-activated earliest signaling pathways by adenosine may have unexpected implications in the GHSR-1a actions.