Ghrelin and Des-Acyl Ghrelin Both Inhibit Lipolysis in Rat Adipocytes via Novel Non-GHS-R1a Receptor
Background
Lipolysis regulation in adipose tissue is a key determinant of fat mobilization, and disruptions to this process are implicated in metabolic disease and obesity. Ghrelin, a gastrointestinal octanoylated peptide, is well established as a potent GH-releasing hormone acting via the GHS-R1a receptor. However, the major circulating isoform — des-acyl ghrelin — lacks the octanoyl modification, does not activate GHS-R1a, and was not known to share ghrelin's peripheral metabolic actions. The gap in understanding whether des-acyl ghrelin exerts direct antilipolytic effects on adipose tissue, and which receptor mediates this action, represents a critical unanswered question in ghrelin biology and adipose tissue pharmacology.
Study Design
Using rat epididymal adipocytes and adipose tissue membrane preparations in vitro, the study tested des-acyl ghrelin, ghrelin, MK-0677, hexarelin, ghrelin-(1-5), and ghrelin-(1-10) across concentrations ranging from 1 to 1000 nM. The primary endpoint was glycerol release — a direct index of lipolysis — stimulated by the non-selective beta-adrenoceptor agonist isoproterenol. mRNA expression for GHS-R1a in isolated adipocytes was assessed to confirm receptor identity, and binding experiments on adipocyte membranes were conducted to characterize the putative novel binding site shared across all test compounds.
Results
All six compounds — des-acyl ghrelin, ghrelin, MK-0677, hexarelin, ghrelin-(1-5), and ghrelin-(1-10) — significantly reduced isoproterenol-stimulated glycerol release in rat epididymal adipocytes across the 1–1000 nM concentration range. The potency order established was: des-acyl ghrelin = ghrelin > MK-0677 = hexarelin > ghrelin-(1-5) = ghrelin-(1-10). Isolated adipocytes and adipose tissue membranes did not express GHS-R1a mRNA, ruling out the canonical ghrelin receptor as the mediator.
The single most important finding: des-acyl ghrelin — which lacks endocrine activity and cannot bind
GHS-R1a— matched full acylated ghrelin in antilipolytic potency, proving that the lipid modification is dispensable for this effect.
Binding experiments confirmed a common high-affinity binding site on adipocyte membranes recognized by all tested compounds, both acylated and des-acylated forms, as well as synthetic GHS and truncated ghrelin fragments, consistent with the functional potency ranking observed in the lipolysis assay.
Key Findings
- Des-acyl ghrelin matched acylated ghrelin in antilipolytic potency (1–1000 nM range) in rat adipocytes.
- Potency order: des-acyl ghrelin = ghrelin > MK-0677 = hexarelin > ghrelin-(1-5) = ghrelin-(1-10).
- Rat epididymal adipocytes did not express
GHS-R1amRNA, ruling out the canonical ghrelin receptor. - A common high-affinity binding site distinct from
GHS-R1awas identified on adipocyte membranes for all tested compounds. - Short C-terminal ghrelin fragments ghrelin-(1-5) and ghrelin-(1-10) retained significant antilipolytic activity.
Why It Matters
Des-acyl ghrelin is not metabolically inert — it directly suppresses fat breakdown at nanomolar concentrations equal to acylated ghrelin, fundamentally revising how practitioners should interpret circulating ghrelin isoforms. For clinicians and researchers evaluating GH secretagogues like MK-0677 or hexarelin in body-composition protocols, this work signals that antilipolytic effects at the adipocyte level may occur independently of GHS-R1a signaling and the GH axis — suggesting an additional fat-retention mechanism that stacking with other agents may not easily overcome. The existence of a distinct, high-affinity antilipolytic receptor for ghrelin-related peptides opens a new drug target for obesity and metabolic syndrome therapeutics. However, translation to clinical dosing protocols requires identification and characterization of this novel receptor before meaningful human predictions can be made.
