The Role of Pituitary Ghrelin in Growth Hormone (GH) Secretion: GH-Releasing Hormone-Dependent Regulation of Pituitary Ghrelin Gene Expression and Peptide Content

Abstract

Ghrelin is a GH-releasing peptide originally purified from the rat stomach. It has been demonstrated that ghrelin expression, within the gastroenteric system, is regulated by both the metabolic and GH milieu. Our laboratory and others have previously reported that ghrelin is also produced in the pituitary. Given that the receptor for ghrelin [GH secretagogue receptor (GHS-R)] is also expressed by the pituitary, the possibility exists that locally produced ghrelin plays an autocrine/paracrine role in regulating GH release. Because we have previously reported that GHRH infusion increases pituitary levels of ghrelin mRNA, we hypothesized that GHRH could be a key regulator of pituitary ghrelin expression. In this report, we demonstrate that 4-h GHRH infusion increased pituitary ghrelin peptide content. Interestingly, under experimental conditions in which hypothalamic GHRH expression is increased, e.g. GH deficiency due to GH gene mutation, glucocorticoid deficiency, and hypothyroidism, we observed that pituitary ghrelin expression (mRNA levels and peptide content) was also increased. Consistent with this positive correlation between GHRH and ghrelin, pituitary ghrelin expression (mRNA levels and peptide content) was found to be decreased in conditions in which hypothalamic GHRH expression is decreased, e.g. GH treatment, glucocorticoid excess, hyperthyroid state, and food deprivation. Collectively, these results suggest that pituitary ghrelin expression is GHRH dependent. We also conducted functional studies to examine whether the pituitary ghrelin/GHS-R system contributes to GH release after GHRH stimulation, by challenging pituitary cell cultures with GHRH in the presence of a GHS-R-specific inhibitor ([d-Lys-3]-GHRP-6). The GHS-R inhibitor did not affect GH release in the absence of GHRH, but significantly reduced GHRH-mediated GH release. This is the first report demonstrating that endogenous pituitary ghrelin can play a physiological role in GH release, by optimizing somatotroph responsiveness to GHRH.