Ghrelin vs IGF-2

Must be acylated at Ser3 (by GOAT enzyme) for GHSR-1a binding. Receptor activation in the pituitary stimulates GH release; hypothalamic action via NPY/AgRP neurons increases appetite and reduces energy expenditure.

Ghrelin is an endogenous 28-amino-acid peptide hormone produced primarily by X/A-like cells of the gastric fundus, characterized by a unique octanoyl modification at Ser3 required for GHS-R1a receptor binding; it is the endogenous ligand for the growth hormone secretagogue receptor and functions as a dual regulator of GH secretion and energy homeostasis. Ghrelin acts centrally via hypothalamic GHS-R1a receptors to potently stimulate GH release from the pituitary and to promote appetite through NPY/AgRP pathway activation, and has peripheral effects on gastric motility and insulin secretion, establishing it as a key integrator of nutritional status, GH axis activity, and energy balance. The pharmacology of GHS-R1a activation in humans is validated through macimorelin (Macrilen), an FDA-approved oral ghrelin receptor agonist; Phase 1 and Phase 2 randomized controlled trials of macimorelin demonstrated robust and reliable GH stimulation in adults, supporting FDA approval in 2017 for the diagnosis of adult GH deficiency and confirming the human physiological relevance of ghrelin receptor activation. Ghrelin itself is not therapeutically administered; it has a very short plasma half-life and the active acylated form is rapidly degraded in circulation; FDA-approved ghrelin receptor agonists require prescription and are indicated for diagnostic rather than therapeutic use, while ghrelin peptide is used exclusively as a research tool compound in neuroendocrine pharmacology studies.

Binds IGF-1R and insulin receptor variant A; promotes anabolic signaling in muscle and fat; activates PI3K/Akt/mTOR pathway; different receptor binding profile than IGF-1

Insulin-like growth factor 2 (IGF-2) is a naturally occurring peptide growth factor structurally homologous to IGF-1 that plays a central role in embryonic development and remains expressed in adult skeletal muscle, where it functions as an autocrine regulator of myoblast differentiation and myocyte maturation downstream of MyoD activation. IGF-2 binds both the IGF-1 receptor (IGF-1R), which mediates PI3K/Akt anabolic signaling, and the mannose-6-phosphate/IGF-2 receptor (M6P/IGF-2R), which targets bound ligand for lysosomal degradation rather than intracellular signal transduction; the balance between these receptor populations influences net downstream anabolic signaling. Preclinical and cell biology research demonstrates that autocrine IGF-2 signaling through IGF-1R is required for normal myocyte maturation, and that TGF-β-mediated suppression of IGF-2 autocrine pathways impairs skeletal muscle differentiation, establishing IGF-2 as a functionally important endogenous anabolic signal in muscle regeneration. IGF-2 has no FDA-approved applications in performance enhancement or muscle anabolism; exogenous administration as a research compound is investigational and no human clinical trials have established safety or efficacy for these uses.