Growth Hormone-Releasing Hormone

Overview

Growth hormone-releasing hormone (GHRH) is a 44-amino acid hypothalamic peptide that serves as the principal stimulator of growth hormone (GH) synthesis and secretion from somatotroph cells in the anterior pituitary. First isolated from a pancreatic tumor causing ectopic GH excess, GHRH was subsequently found to originate from neurons in the arcuate nucleus of the hypothalamus. The human form contains a conserved His-Ala-Asp-Ala active sequence at the N-terminus.

Structure and Biosynthesis

GHRH is cleaved from a 107-amino acid precursor (prepro-GHRH) following removal of a signal peptide. The mature peptide contains an N-terminal pyroglutamate residue and C-terminal amidation, both of which are essential for biological activity. The conserved residues His1, Ala2, Asp3, and Ala5 form the minimal active sequence required for receptor binding. GHRH mRNA is primarily expressed in the infundibular (arcuate) nucleus, with lower levels in the placenta and intestine.

Receptor and Signaling

GHRH binds to the GHRH receptor (GHRHR), a Gs-coupled seven-transmembrane receptor highly expressed on somatotroph cells. Receptor activation elevates intracellular cAMP, activates protein kinase A, and stimulates both GH gene transcription and exocytotic GH release. GHRH also promotes somatotroph proliferation and prevents apoptosis. Intriguingly, splice variants of GHRH (SV1, SV2, SV3) act as dominant-negative inhibitors of the full-length receptor.

Regulation of GH Secretion

GHRH acts in concert with ghrelin (the endogenous GH secretagogue) to stimulate GH release, while somatostatin provides inhibitory tone. The pulsatile pattern of GH secretion reflects the oscillatory activity of GHRH and somatostatin neurons in the hypothalamus. GH secretagogue receptor agonists synergize with GHRH to amplify GH output. Negative feedback from IGF-1 suppresses both GHRH and GH release.

Clinical Applications

Synthetic GHRH analogs such as sermorelin (GHRH(1-29)NH2) are used diagnostically to assess pituitary GH reserve. GHRH agonists are being developed for GH deficiency, sarcopenia, and age-related muscle loss. Mutations in the GHRHR gene cause isolated GH deficiency type IV. Antagonists of GHRH, including peptide derivatives like MZJ-13, show potential in treating ectopic GHRH-secreting tumors and acromegaly.