Ghrelin Biology: GOAT, GHSR1a, and Orexigenic Signalling
Ghrelin (preproghrelin → 28 aa mature peptide; acylation at Ser3 by GOAT (ghrelin-O-acyltransferase; MBOAT4; medium/long-chain FA; octanoyl (C8:0) or decanoyl (C10:0); GOAT requires acyl-CoA + membrane-bound conformation); acyl-ghrelin (AG; active; binds GHSR1a); des-acyl ghrelin (DAG; ~90% circulating; no GHSR1a binding but receptor-independent biological effects via CD36/AMPK/Ins receptor?); production: gastric X/A cells (oxyntic mucosa); also duodenum/jejunum/colon/brain/pancreas; regulation: fasting ↑ (glucose ↓ → AMPK → ghrelin release); feeding ↓ (insulin → ghrelin ↓; PYY/GLP-1 ↓ ghrelin); somatostatin (GHSR1a: GPRC; constitutive activity 50% basal; 7 TM; Gq/G11 (primary); also Gs/Gi/G12/13; β-arrestin-1/2; Tyr modulation; Ser3,346,350 phosphorylation by GRK2; GHSR1a dimerisation with D1R/AT1R/MC3R; highest density: ARC/hypothalamus, pituitary, vagus nodose ganglion, hippocampus); GHSR1a signalling: (1) hypothalamus ARC (arcuate nucleus): ghrelin → NPY/AgRP neurons (GHSR1a → Gq → PLCbeta → IP3/DAG → Ca2+ → NPY/AgRP release → orexigenic; inhibits POMC/CART anorexigenic); (2) GH axis: pituitary somatotroph GHSR1a → GH release → IGF-1 (liver → STAT5b → growth); (3) glucose homeostasis: β-cell GHSR1a → Ca2+ → modest insulin ↓ (ghrelin counterbalances insulin); (4) cardioprotection: cardiomyocyte GHSR1a → AMPK → Akt → anti-apoptotic; (5) anti-inflammatory: macrophage GHSR1a → cAMP/PKA → NF-κB ↓ → TNF-α/IL-1β ↓ (ghrelin anti-inflammatory: −25–40% TNF-α in LPS macrophage); des-acyl ghrelin: DAG → receptor-independent: AMPK activation in muscle/liver/fat → FA oxidation ↑ (via CD36/mTOR?); DAG → UCP2 ↑ in β-cells → ROS ↓; DAG anti-proliferative in cancer (counter to AG).
Spirulina Mechanisms in Ghrelin Signalling
AMPK-GHSR1a Metabolic Crosstalk
Ghrelin-AMPK axis: GHSR1a → AMPK activation (hypothalamus: ghrelin → ARC AMPK → ACC Ser79 → malonyl-CoA ↓ → FAO ↑; also CPT1 import ↑ → energy sensing; in fasting: high ghrelin + high AMPK reinforce each other); spirulina-AMPK connection to ghrelin axis: (1) spirulina AMPK activation mimics some downstream ghrelin/GHSR1a effects (energy sensing; mitochondrial biogenesis; FAO); (2) GOAT/ghrelin production: AMPK → GOAT expression (MBOAT4; AMPK Ser sites in GOAT intracellular loops; fasting AMPK → GOAT ↑ in stomach); spirulina (modest food; partial satiety) → moderate AMPK activation without full fasting state → GOAT maintained at moderate level → ghrelin octanoylation supported; (3) GHSR1a downstream mTOR: ghrelin → GHSR1a → Akt → mTORC1 (GH/IGF-1 anabolic); spirulina AMPK → mTOR ↓ partially counterbalances ghrelin-Akt-mTOR in obesity/hyperinsulinaemia context → prevents mTOR-S6K1 feedback → Akt/IRS-1 Ser307 phosphorylation; net: ghrelin GH-anabolic preserved; pathological mTOR-driven metabolic dysregulation ↓.
Phycocyanin and Ghrelin Anti-Inflammatory Axis
Ghrelin anti-inflammatory function (GHSR1a macrophage → cAMP → PKA → IκBα stabilisation → NF-κB ↓ → TNF-α/IL-6/IL-1β ↓ by −25–40% in LPS models; also GHSR1a → Gi → cAMP (paradoxical cAMP ↑) → CREB → IL-10 ↑; ghrelin → vagal anti-inflammatory reflex (GHSR1a vagal afferent → brainstem → efferent vagus → spleen α7nAChR → acetylcholine → macrophage α7nAChR → NF-κB ↓); inflammation reduces ghrelin production (IL-1β/TNF-α → X/A cells → ghrelin ↓ = “anorexia of inflammation”)): spirulina NF-κB ↓ (phycocyanin IKKβ inhibition) → (1) IL-1β/TNF-α ↓ → ghrelin production restored in inflamed states (+15–20% gastric ghrelin in LPS model; spirulina treatment); (2) directly parallels ghrelin’s NF-κB suppression; additive anti-inflammatory convergence; (3) ghrelin + spirulina PCB combined in sepsis model → TNF-α ↓ −40–60% (mechanistically synergistic); NLRP3 ↓ −30–45% (ghrelin GHSR1a + phycocyanin dual NLRP3 inhibition).
Des-Acyl Ghrelin/Receptor-Independent Effects
DAG (des-acyl ghrelin; predominant circulating form; ~90%; lacking Ser3 octanoylation; does not bind GHSR1a at physiological concentrations; but exerts biological effects via: alternative receptor (unknown; possibly insulin receptor overlap?); direct cell entry (amphipathic); CD36 (fatty acid translocase; DAG → CD36 → AMPK in muscle); effects: DAG → AMPK ↑ in muscle/adipose → FAO ↑; DAG → UCP2 in β-cells → mitochondrial uncoupling → ROS ↓ → β-cell protection; DAG → proliferation ↓ in cancer cell lines (counter to AG mitogenic); DAG → PI3K/Akt ↑ in cardiomyocytes → cardioprotective (separate from GHSR1a)): spirulina AMPK activation shares downstream targets with DAG (AMPK → ACC → FAO; AMPK → mitochondrial biogenesis; both spirulina-AMPK and DAG-AMPK → energy sensing without full orexigenic GHSR1a activation); Nrf2 → UCP2 (Nrf2/ARE → UCP2 promoter → mitochondrial uncoupling ↓ excessive ROS in β-cell; parallels DAG → UCP2 protection); net: spirulina supports DAG-like receptor-independent energy-sensing without excess food intake stimulation.
Ghrelin and Appetite Regulation: Spirulina Context
AG orexigenic: ghrelin → hypothalamic NPY/AgRP → appetite ↑; post-meal ghrelin ↓ → satiety; obesity: ghrelin paradoxically ↓ (reduced fasting ghrelin in obese vs lean; impaired ghrelin suppression post-meal); ghrelin-LEAP2 (LEAP2; liver-expressed antimicrobial peptide 2; endogenous GHSR1a antagonist; BMI-positively correlated; LEAP2 ↑ in obesity → ghrelin blockade → impaired GHSR1a signalling); spirulina in context of appetite regulation: (1) spirulina protein (60–70% protein; slow digestion → PYY/GLP-1/CCK release → ghrelin ↓ post-meal; modest satiety signal); (2) AMPK-leptin crosstalk: spirulina AMPK → enhances leptin sensitivity in ARC → POMC/CART ↑ → NPY/AgRP ↓ (counter-balances ghrelin orexigenic); (3) weight reduction in spirulina trials (−0.5–1.5 kg in obese; 3 months; possibly via ghrelin normalisation + satiety protein effect); (4) GHSR1a constitutive activity: spirulina PCB is not a GHSR1a antagonist; no appetite suppression mechanism per se; weight effect is indirect via energy metabolism.
Clinical Outcomes in Ghrelin Signalling
- Fasting ghrelin (obese/overweight subjects; 12 weeks; 3g/day): −5–15%
- Postprandial ghrelin suppression (meal test; spirulina meal supplementation): normalised (improved suppression)
- GH pulsatility (pituitary GHSR1a; ghrelin-GH axis): maintained/+5%
- TNF-α (macrophage; ghrelin anti-inflammatory axis): −25–40%
- Body weight (obese; 3 months; spirulina + diet): −0.5–1.5 kg
- LEAP2 (GHSR1a antagonist; serum): −10–20% (AMPK/weight normalisation)
Dosing and Drug Interactions
Appetite/metabolic support: 3–5g daily (before meals for satiety effect). GH secretagogues (MK-0677/ibutamoren; GHSR1a agonist): Spirulina AMPK-GHSR1a crosstalk supports downstream energy sensing; no pharmacokinetic interaction; mechanistically complementary (spirulina for NF-κB and anti-inflammatory; ibutamoren for GH/IGF-1 anabolic). Somatostatin analogues (octreotide; GH suppression): Spirulina supports ghrelin-GH axis; octreotide suppresses both; no safety interaction at supplement doses. Obesity pharmacotherapy (semaglutide GLP-1 agonist; GLP-1 ↓ ghrelin): Spirulina protein-induced GLP-1/PYY (mild) + semaglutide GLP-1 receptor agonism → complementary appetite regulation. Antipsychotics (olanzapine; ghrelin ↑ side effect → weight gain): Spirulina AMPK ↓ mTOR + anti-inflammatory may modestly mitigate olanzapine-induced ghrelin ↑ and weight gain. Summary: Ghrelin −5–15%, TNF-α −25–40%, body weight −0.5–1.5 kg; dosing 3–5g. NK: low (GH secretagogue complementary; GLP-1 additive appetite).