Molecular Characterization of Growth Hormone Secretagogue Receptors

Abstract

The synthetic hexapeptide growth hormone releasing peptide 6 (GHRP-6) mediates growth hormone (GH) release from primary pituitary cells through a distinct mechanism from that controlled by growth hormone releasing hormone (GHRH) or somatostatin (1–3). Biochemical and pharmacological evidence supports the notion that GHRP-6 and the nonpeptide growth hormone secretagogs (GHSs) act through the same receptor. Numerous attempts to characterize the GHRP or GHS receptors (GHS-Rs) biochemically were frustrated by a low GHS-R abundance. The development of procedures for high-specificactivity [35S] radiolabeling of the nonpeptide GHS MK-0677 in conjunction with improved receptor preparation procedures led to the identification of a GHS-R binding site (4,5). The GHS-R bound [35S]-MK-0677 with high affinity, and the rank order of potency of diverse peptide and nonpeptide ligands for [35S]-MK-0677 displacement correlated with their in vivo GH secretory activity. Based on its binding characteristics the authors assumed that the GHS-R was a G protein-coupled receptor (GPC-R) found in low abundance in the anterior pituitary and hypothalamus. This data facilitated the development of a strategy to clone the GHS-R (Fig. 1). The assay for identification of the GHS-R relied on the knowledge that GHS-R activation leads to G protein-mediated activation of phosphoinositol-specific phospholipase C (PI-PLC) and subsequent calcium mobilization. Expression cloning rationale. Schematic representation of GHS-S coupling to Gα11 and PLC leading to intracellular Ca2+ release, which can be measured with the photoprtein aequorin KeywordsGrowth HormoneXenopus OocyteGrowth Hormone SecretagogueGrowth Hormone Secretagogue ReceptorGrowth Hormone Release HormoneThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.