Ghrelin

Overview

Ghrelin is a 28-amino-acid peptide hormone predominantly synthesized by enteroendocrine X/A-like cells of the gastric fundus, with smaller quantities produced in the small intestine, pancreas, hypothalamus, and other tissues. Discovered in 1999 by Masayasu Kojima and colleagues at Kurume University in Japan, ghrelin was identified as the endogenous ligand for the growth hormone secretagogue receptor type 1a (GHS-R1a), a receptor that had been characterized years earlier as the target of synthetic growth hormone-releasing peptides such as GHRP-6 and GHRP-2.

Ghrelin is unique among known mammalian hormones in that it requires a post-translational lipid modification — specifically, octanoylation (attachment of an eight-carbon fatty acid chain) at its third serine residue — for biological activity at the GHS-R1a receptor. This modification is catalyzed by the enzyme ghrelin O-acyltransferase (GOAT), and the resulting acylated form is often referred to as "active ghrelin" or "acyl-ghrelin." The unacylated form (des-acyl ghrelin), which circulates at significantly higher concentrations, was initially considered biologically inert but is now recognized to have independent physiological effects through mechanisms that remain under investigation.

Ghrelin's most well-known function is its role as an orexigenic (appetite-stimulating) hormone. Circulating ghrelin levels rise before meals and fall after eating, establishing it as a key peripheral signal in meal initiation and energy homeostasis. In parallel, ghrelin is the most potent known endogenous stimulator of growth hormone (GH) secretion from the anterior pituitary, acting synergistically with growth hormone-releasing hormone (GHRH).

Structure and Sequence

Ghrelin is a 28-amino-acid linear peptide with the following human sequence:

Sequence: Gly-Ser-Ser(n-octanoyl)-Phe-Leu-Ser-Pro-Glu-His-Gln-Arg-Val-Gln-Gln-Arg-Lys-Glu-Ser-Lys-Lys-Pro-Pro-Ala-Lys-Leu-Gln-Pro-Arg

• Molecular formula: C₁₄₉H₂₄₉N₄₅O₄₂ (acylated form)
• Molecular weight: approximately 3,370.9 g/mol (acylated form)
• Gene: GHRL (chromosome 3p25-26)
• Precursor: Preproghrelin (117 amino acids), which is processed to yield both ghrelin and obestatin

Key structural features:

• Octanoyl group at Ser3: An n-octanoic acid (C8:0) ester linkage is essential for GHS-R1a binding and activation. This lipid modification is remarkably conserved across vertebrate species.
• N-terminal core: The first four to five residues (Gly-Ser-Ser(octanoyl)-Phe-Leu) constitute the minimal pharmacophore required for GHS-R1a activation.
• Species conservation: The N-terminal region is highly conserved across mammals, while the C-terminal region shows greater variability, suggesting that receptor binding is primarily driven by the acylated N-terminus.

The GOAT enzyme responsible for octanoylation uses octanoyl-CoA as its acyl donor and is highly expressed in the stomach, consistent with the stomach being the primary ghrelin-producing organ.

Mechanism of Action

GHS-R1a Receptor Activation

Ghrelin's primary signaling occurs through the growth hormone secretagogue receptor type 1a (GHS-R1a), a seven-transmembrane G-protein coupled receptor with broad tissue distribution:

Hypothalamus and Pituitary:

• Activation of GHS-R1a on hypothalamic arcuate nucleus neurons stimulates neuropeptide Y (NPY) and agouti-related peptide (AgRP) expression, driving appetite and food-seeking behavior
• Direct stimulation of pituitary somatotrophs triggers growth hormone release in a pulsatile manner
• Synergistic interaction with GHRH amplifies GH secretion beyond what either signal achieves alone
• Ghrelin opposes the anorexigenic effects of leptin, establishing a bidirectional hormonal axis in energy balance

Peripheral Tissues:

• Gastrointestinal tract: Promotes gastric motility and acid secretion via vagal afferent pathways
• Pancreas: Modulates insulin secretion through GHS-R1a on pancreatic islet cells; effects are context-dependent and the subject of ongoing research
• Adipose tissue: Promotes adipogenesis and lipogenesis through central and peripheral mechanisms
• Cardiovascular system: GHS-R1a expression on cardiomyocytes and vascular endothelium mediates cardioprotective effects in preclinical models

Des-Acyl Ghrelin Signaling

Unacylated ghrelin, which circulates at 3-4 times the concentration of acyl-ghrelin, does not activate GHS-R1a but appears to signal through an as-yet-unidentified receptor. Reported effects include modulation of glucose metabolism, anti-apoptotic activity in cardiomyocytes, and potential anxiolytic properties. The physiological significance of des-acyl ghrelin remains an active area of investigation.

Constitutive Receptor Activity

GHS-R1a exhibits approximately 50% constitutive (ligand-independent) activity, meaning it signals in the absence of ghrelin. This basal signaling has been implicated in setting the tonic level of appetite and may explain why GHS-R1a inverse agonists (compounds that suppress constitutive activity) can reduce food intake in preclinical models.

Research Summary

Pharmacokinetics

• Half-life: Acyl-ghrelin has a plasma half-life of approximately 10-30 minutes following intravenous administration; des-acyl ghrelin persists somewhat longer
• Metabolism: Rapid deacylation by plasma esterases converts acyl-ghrelin to des-acyl ghrelin; further proteolytic degradation follows
• Circadian pattern: Ghrelin exhibits a diurnal rhythm with peaks before anticipated meals and nadirs following food intake
• Fasting response: Plasma ghrelin levels rise progressively during fasting and fall within 1-2 hours of refeeding
• Clearance: Primarily hepatic and renal; renal impairment is associated with elevated circulating ghrelin levels
• Route considerations: Native ghrelin requires parenteral administration; oral delivery is limited by enzymatic degradation and poor bioavailability
• Stability: The octanoyl ester bond is susceptible to enzymatic hydrolysis, making acyl-ghrelin relatively unstable in plasma and requiring careful sample handling for accurate measurement

Common Discussion Topics

Ghrelin and weight management: Ghrelin's role as a hunger-stimulating hormone has made it a therapeutic target in both directions. Anti-ghrelin strategies (vaccines, receptor antagonists, GOAT inhibitors) have been explored for obesity, while ghrelin agonists have been investigated for conditions involving pathological weight loss such as cancer cachexia and anorexia nervosa.

Relationship to synthetic GH secretagogues: The discovery of ghrelin as the endogenous ligand for GHS-R1a provided the biological context for synthetic peptides such as GHRP-6, GHRP-2, and ipamorelin that had been developed empirically. These synthetic analogs generally offer improved stability and more selective GH-releasing profiles compared to native ghrelin.

Post-bariatric ghrelin suppression: The dramatic reduction in ghrelin following sleeve gastrectomy (which removes most of the ghrelin-producing gastric fundus) is a frequently discussed mechanism contributing to sustained appetite reduction and weight loss following this procedure.

Ghrelin resistance in obesity: Despite ghrelin being an appetite stimulant, obese individuals typically have lower circulating ghrelin levels than lean controls — a finding that suggests compensatory downregulation rather than ghrelin-driven overeating. Some researchers have proposed altered ghrelin sensitivity or signaling in obesity.

Ghrelin in stress and mood: GHS-R1a is expressed in brain regions involved in stress response and reward processing. Preclinical studies have suggested roles for ghrelin in stress-induced eating, anxiety modulation, and reward-seeking behavior, adding complexity to its characterization as simply a "hunger hormone."

Dosing Protocols

As an endogenous hormone, ghrelin is not typically administered exogenously in clinical practice. It is primarily studied as a biomarker of appetite regulation and energy homeostasis. The therapeutic potential of ghrelin receptor (GHS-R1a) activation is realized through synthetic agonists:

Native ghrelin has been used in clinical research settings (typically 1-5 mcg/kg IV infusion) for appetite stimulation studies, GH provocation testing, and gastric motility research, but it is not approved for therapeutic use.

Related Compounds

• GHRP-6 — synthetic hexapeptide GH secretagogue and GHS-R1a agonist
• GHRP-2 — synthetic hexapeptide with higher GHS-R1a affinity than GHRP-6
• Ipamorelin — selective synthetic GH secretagogue with minimal effects on cortisol and prolactin
• Sermorelin — GHRH analog that acts synergistically with ghrelin-pathway stimulation
• Neuropeptide Y — downstream mediator of ghrelin's orexigenic effects in the hypothalamus