GHRELIN
Ghrelin is a 28-amino-acid peptide hormone primarily produced by X/A-like cells of the gastric fundus, functioning as the endogenous ligand for the growth hormone secretagogue receptor (GHS-R1a). It is the only known peripherally produced orexigenic hormone, playing critical roles in appetite stimulation, growth hormone release, energy homeostasis, and gastrointestinal motility.
Ghrelin is a 28-amino-acid peptide hormone discovered in 1999 by Kojima et al. as the endogenous ligand for the growth hormone secretagogue receptor type 1a (GHS-R1a). It is predominantly synthesized by X/A-like enteroendocrine cells of the gastric oxyntic mucosa and is unique among peptide hormones for requiring post-translational octanoylation (acylation with n-octanoic acid) at its third serine residue for receptor activation.
Overview
Ghrelin was identified in 1999 through reverse pharmacology — the receptor (GHS-R1a) was known before its endogenous ligand was discovered. The peptide derives from a 117-amino-acid precursor, preproghrelin, which is processed to yield both ghrelin and obestatin (encoded by the same gene). The name "ghrelin" is derived from the Proto-Indo-European root "ghre" meaning "to grow," reflecting its potent growth hormone (GH)-releasing activity.
Ghrelin is the only known hormone that stimulates appetite when administered peripherally. Its plasma levels rise preprandially and fall rapidly after food intake, establishing it as a key meal initiation signal. This contrasts with satiety hormones such as leptin, PYY, and cholecystokinin (CCK), which signal meal termination and satiety. The ghrelin system has thus emerged as a therapeutic target for conditions involving appetite dysregulation, including cachexia, anorexia nervosa, obesity, and Prader-Willi syndrome.
The enzyme ghrelin O-acyltransferase (GOAT) catalyzes the octanoylation of ghrelin at Ser3, which is essential for binding and activating GHS-R1a. GOAT expression is highest in the stomach and intestine, and its activity is nutritionally regulated — medium-chain fatty acids from the diet serve as acyl donors. This creates a direct link between dietary fat intake and ghrelin activation, positioning the GOAT-ghrelin system as a nutrient sensor.
Beyond appetite and GH secretion, ghrelin exerts diverse physiological effects including regulation of glucose homeostasis (promoting hyperglycemia through suppression of insulin secretion), cardiovascular protection, modulation of immune function, neuroprotection, and regulation of gastrointestinal motility. These pleiotropic actions reflect the widespread distribution of GHS-R1a across hypothalamic, brainstem, cardiovascular, immune, and gastrointestinal tissues.
Mechanism of Action
Ghrelin exerts its biological effects primarily through binding to the growth hormone secretagogue receptor type 1a (GHS-R1a), a G-protein-coupled receptor (GPCR) that signals through Gq/11-phospholipase C pathways:
Growth Hormone Secretion: Ghrelin is the most potent endogenous GH secretagogue. It acts on GHS-R1a receptors in the anterior pituitary somatotrophs to stimulate GH release through inositol trisphosphate (IP3)-mediated calcium mobilization. Ghrelin's GH-releasing activity is synergistic with growth hormone-releasing hormone (GHRH) and is partially independent of the somatostatin inhibitory tone. This synergy occurs because ghrelin and GHRH act through distinct intracellular signaling cascades — Gq/PLC for ghrelin versus Gs/cAMP for GHRH. Kojima M et al. (1999) — Nature 402, 656-660.
Orexigenic Signaling: Ghrelin crosses the blood-brain barrier and activates GHS-R1a on NPY/AgRP neurons in the arcuate nucleus of the hypothalamus. This stimulates release of NPY and AgRP — potent orexigenic neuropeptides — while simultaneously inhibiting pro-opiomelanocortin (POMC) anorexigenic neurons. The net effect is robust appetite stimulation and increased food intake. Ghrelin also activates reward-related feeding circuits through mesolimbic dopaminergic pathways. Nakazato M et al. (2001) — Nature 409, 194-198.
Vagal Afferent Signaling: Ghrelin also signals to the central nervous system via vagal afferent neurons. GHS-R1a receptors are expressed on vagal afferent cell bodies in the nodose ganglion, and vagotomy attenuates some of ghrelin's orexigenic effects. This vagal pathway provides a rapid signaling route from the gut to the brainstem nucleus tractus solitarius (NTS).
Acyl vs Des-acyl Ghrelin: Only acyl ghrelin (octanoylated at Ser3) activates GHS-R1a. Des-acyl ghrelin, which constitutes approximately 80-90% of total circulating ghrelin, does not bind GHS-R1a but may have independent biological activities through an as-yet-unidentified receptor. Des-acyl ghrelin has been reported to exert cardioprotective, anti-apoptotic, and metabolic effects, and may functionally oppose some actions of acyl ghrelin on glucose metabolism. Delhanty PJ et al. (2012) — Trends Endocrinol. Metab. 23, 4-8.
Glucose Homeostasis: Ghrelin suppresses glucose-stimulated insulin secretion from pancreatic beta cells through GHS-R1a signaling. This diabetogenic action is physiologically relevant — ghrelin knockout mice show improved glucose tolerance and insulin sensitivity. The ghrelin system thus functions as a counter-regulatory hormone axis that opposes insulin's effects during fasting, maintaining glucose availability for the brain.
Gastrointestinal Motility: Ghrelin stimulates gastric motility and gastric acid secretion through both central (vagal) and peripheral mechanisms. It accelerates gastric emptying and promotes the migrating motor complex (MMC) during fasting. These prokinetic effects have made ghrelin and its analogs therapeutic candidates for gastroparesis and postoperative ileus.
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Research
Ghrelin and Growth Hormone Deficiency
Ghrelin's potent GH-releasing activity led to interest in using ghrelin and its mimetics for GH deficiency states. Growth hormone secretagogue receptor agonists (GHS-R1a agonists) such as ibutamoren (MK-677) have demonstrated sustained increases in GH and IGF-1 levels with oral dosing. Macimorelin, an oral ghrelin mimetic, was approved by the FDA in 2017 as a diagnostic test for adult GH deficiency — the macimorelin stimulation test measures GH response to a standardized oral dose, replacing more cumbersome stimulation tests.
Ghrelin and Obesity
Paradoxically, despite being an orexigenic hormone, circulating ghrelin levels are typically reduced in obesity, suggesting a compensatory downregulation. However, the preprandial ghrelin surge is blunted in obese individuals, which may impair meal-related satiety signaling. Roux-en-Y gastric bypass surgery dramatically reduces ghrelin levels, and this reduction has been proposed (though debated) as one mechanism contributing to post-surgical weight loss and appetite suppression. Pharmacological blockade of the ghrelin system — through GHS-R1a antagonists, GOAT inhibitors, or anti-ghrelin vaccines — has been explored as an anti-obesity strategy, though no ghrelin-based anti-obesity therapeutic has reached clinical approval. Cummings DE et al. (2002) — N. Engl. J. Med. 346, 1623-1630.
Ghrelin in Gastroparesis
Ghrelin's prokinetic effects on gastric motility have been explored therapeutically in gastroparesis, a condition characterized by delayed gastric emptying without mechanical obstruction. Intravenous ghrelin administration accelerates gastric emptying in both diabetic and idiopathic gastroparesis. The ghrelin receptor agonist relamorelin demonstrated significant improvements in gastric emptying and vomiting frequency in Phase 2 trials for diabetic gastroparesis, though Phase 3 development has faced challenges. TZP-101 (ulimorelin) showed efficacy in postoperative ileus.
The GOAT-Ghrelin Axis as Nutrient Sensor
The discovery of GOAT (ghrelin O-acyltransferase) in 2008 by Yang et al. revealed the enzymatic mechanism for ghrelin acylation and identified a new therapeutic target. GOAT uses medium-chain fatty acids (primarily octanoic acid, C8:0) from the diet as acyl donors, creating a molecular link between dietary fat composition and ghrelin activation. GOAT inhibitors have shown promise in preclinical models — reducing acyl ghrelin while increasing des-acyl ghrelin, improving glucose tolerance, and reducing food intake without directly targeting the receptor. Yang J et al. (2008) — Cell 132, 387-396.
Ghrelin and Cardiovascular Protection
Ghrelin and des-acyl ghrelin both exhibit cardioprotective effects, though through partially distinct mechanisms. Ghrelin reduces cardiac afterload through vasodilation, improves cardiac output in heart failure models, and protects cardiomyocytes from ischemia-reperfusion injury. These effects involve both GHS-R1a-dependent and -independent pathways. Clinical studies have shown that ghrelin infusion improves cardiac function in patients with chronic heart failure without significant adverse effects. Nagaya N et al. (2004) — Circulation 110, 3674-3679.
Ghrelin in Cachexia and Wasting Syndromes
Cancer-associated cachexia affects up to 80% of advanced cancer patients and is a major cause of morbidity and mortality. Ghrelin's dual ability to stimulate appetite and promote GH/IGF-1 anabolic signaling makes it an attractive therapeutic candidate. Clinical trials with ghrelin infusions and the ghrelin receptor agonist anamorelin have demonstrated improvements in lean body mass, appetite, and food intake in cancer cachexia patients. Anamorelin (100 mg daily oral) increased lean body mass by approximately 1.5 kg over 12 weeks in the ROMANA trials, though functional improvements (handgrip strength) were not consistently observed. Temel JS et al. (2016) — Lancet Oncol. 17, 519-531.
Safety Profile
Exogenous ghrelin administration is generally well tolerated in clinical studies. The most common effects include transient increases in appetite and food intake (pharmacological extension of its physiological role), mild flushing, and a sensation of gastric warmth or fullness. Ghrelin transiently increases plasma glucose levels and suppresses insulin secretion — this diabetogenic effect is dose-dependent and typically modest but requires monitoring in diabetic patients. Growth hormone elevation is predictable and dose-dependent. Chronic administration of ghrelin receptor agonists (e.g., anamorelin, ibutamoren) has raised theoretical concerns about sustained GH/IGF-1 elevation and potential effects on glucose metabolism, though clinical trial data have not demonstrated significant safety signals with durations up to 12 weeks. Des-acyl ghrelin may partially counterbalance the diabetogenic effects of acyl ghrelin. Ghrelin receptor agonists should be used with caution in patients with diabetes mellitus, active malignancies (due to GH/IGF-1 axis activation), and in settings where appetite stimulation is contraindicated.
Pharmacokinetic Profile
GHRELIN — Pharmacokinetic Curve
Subcutaneous, intravenousQuick Start
- Route
- Subcutaneous, intravenous
Molecular Structure
- Formula
- C149H249N45O42
- Weight
- 3370.9 Da
- CAS
- 258279-04-2
- PubChem CID
- 10270228
- Exact Mass
- 365.0349 Da
- LogP
- 1.2
- TPSA
- 137 Ų
- H-Bond Donors
- 3
- H-Bond Acceptors
- 6
- Rotatable Bonds
- 4
- Complexity
- 616
Identifiers (SMILES, InChI)
InChI=1S/C14H12ClN5O3S/c15-8-3-1-7(2-4-8)10-6-24-14(20-19-10)17-11(21)5-9-12(22)18-13(23)16-9/h1-4,9H,5-6H2,(H,17,20,21)(H2,16,18,22,23)
DAGIUMVKBALSGT-UHFFFAOYSA-NResearch Indications
Metabolic
Ghrelin and ghrelin mimetics (anamorelin) improve appetite, lean body mass, and quality of life in cancer cachexia. Anamorelin showed significant effects in Phase III ROMANA trials for NSCLC cachexia.
Ghrelin is the only known orexigenic hormone. Exogenous administration robustly increases food intake, meal frequency, and caloric consumption in healthy subjects and patients with anorexia.
Ghrelin is a potent endogenous GH secretagogue acting via GHS-R1a receptor in the pituitary. Stimulates pulsatile GH release synergistically with GHRH, representing a distinct regulatory axis.
Gastrointestinal
Ghrelin accelerates gastric emptying by activating vagal afferents and enteric neurons. Clinical trials show improvement in diabetic and post-surgical gastroparesis symptoms.
Ghrelin administration promotes GI motility recovery after abdominal surgery. Phase II trials demonstrated reduced time to first bowel movement and hospital discharge.
Research Protocols
oral
Anamorelin (100 mg daily oral) increased lean body mass by approximately 1.5 kg over 12 weeks in the ROMANA trials, though functional improvements (handgrip strength) were not consistently observed. Growth hormone secretagogue receptor agonists (GHS-R1a agonists) such as ibutamoren (MK-677) have dem
| Goal | Dose | Frequency | Duration |
|---|---|---|---|
| ROMANA trials | 100 mg | Daily | 12 weeks |
subcutaneous Injection
Administered via subcutaneous injection.
intravenous Injection
Intravenous ghrelin administration accelerates gastric emptying in both diabetic and idiopathic gastroparesis.
Interactions
Peptide Interactions
Ghrelin's GH-releasing activity is synergistic with growth hormone-releasing hormone (GHRH) and is partially independent of the somatostatin inhibitory tone.
Ghrelin receptor agonists should be used with caution in patients with diabetes mellitus, active malignancies (due to GH/IGF-1 axis activation), and in settings where appetite stimulation is contraindicated.
What to Expect
What to Expect
Rapid onset expected; half-life of ~30 minutes indicates fast-acting pharmacokinetics
Anamorelin (100 mg daily oral) increased lean body mass by approximately 1.
Due to short half-life (~30 minutes), effects are expected per-dose; consistent daily administration maintains therapeutic levels
Chronic administration of ghrelin receptor agonists (e.g., anamorelin, ibutamoren) has raised theoretical concerns about sustained GH/IGF-1 elevation.
Regular administration schedule required; effects are dose-dependent and do not persist between doses
Safety Profile
Common Side Effects
- Intense Hunger:: The most common side effect, which can lead to unwanted fat gain if diet is not strictly controlled.
- Water Retention:: Users frequently report 'moon face' or edema due to sodium and water storage.
- Lethargy:: High pulses of growth hormone can lead to daytime sleepiness or a 'heavy' feeling.
- Insulin Resistance:: Long-term use or high doses may negatively impact glycemic control.
Quality Indicators
What to look for
- Human clinical trials conducted
- Well-established safety profile
- Multiple peer-reviewed studies available
- Oral administration available
Frequently Asked Questions
References (14)
- [11]Muller TD et al — The biology and pharmacology of ghrelin Mol. Metab. (2023)
- [2]Structure and physiological actions of ghrelin
→ This research explains the unique n-octanoylation modification of ghrelin required for its biological activity and its production in the stomach and hypothalamus.
- [3]Ghrelin: Integrative Neuroendocrine Peptide in Health and Disease
→ Researchers identified ghrelin as a mediator of growth hormone release that coordinates energy needs with growth processes and body weight regulation.
- [5]
- [6]
- [7]Cummings, D. E. et al Plasma ghrelin levels after diet-induced weight loss or gastric bypass surgery N. Engl. J. Med. (2002)
- [8]Yang, J. et al Identification of the acyltransferase that octanoylates ghrelin, an appetite-stimulating peptide hormone Cell (2008)
- [9]Temel, J. S. et al Anamorelin in patients with non-small-cell lung cancer and cachexia (ROMANA 1 and ROMANA 2) Lancet Oncol. (2016)
- [10]Delhanty, P. J. et al Des-acyl ghrelin: a metabolically active peptide Trends Endocrinol. Metab. (2012)
- [12]
- [14]Pietra C et al — Ghrelin receptor agonists for gastrointestinal motility disorders Expert Opin. Investig. Drugs (2023)
- [4]Ghrelin: Structure and Function
→ A foundational study using reverse pharmacology to identify ghrelin as the natural ligand for the growth hormone secretagogue receptor in rat and human tissues.
- [13]Kuroda K et al — Ghrelin and its receptor in the central nervous system: structure, function, and therapeutic potential Pharmacol. Rev. (2023)
- [1]Molecular Mechanisms and Health Benefits of Ghrelin: A Narrative Review
→ This review details how ghrelin activates the GHSR to promote appetite, pleasure-reward eating, and various metabolic functions throughout the body.
GHK-Cu
GHK-Cu is a naturally occurring copper-binding tripeptide that modulates gene expression, attenuates inflammation, improves antioxidant responses, and promotes tissue repair through copper-mediated redox mechanisms.
GHRH (Growth Hormone Releasing Hormone)
GHRH is a 44-amino acid endogenous hypothalamic peptide that stimulates pulsatile growth hormone release from the anterior pituitary. It is the primary physiological regulator of GH secretion and the parent molecule from which synthetic analogues like sermorelin and CJC-1295 are derived.