FGF Signalling: Ligands, Receptors, and Downstream Pathways
Fibroblast growth factor (FGF) family (22 human FGFs; paracrine (FGF1–10/16–18/20/22; heparan sulfate proteoglycan (HSPG) co-receptor required; ternary complex: FGF + FGFR + HSPG → receptor dimerisation + activation); endocrine (FGF19/21/23; Klotho co-receptor required; low HSPG affinity; systemic signalling)); FGF receptors (FGFR1–4; RTKs; IgI-II-III extracellular Ig-like domains; single TM; intracellular split kinase; acid box (IgI-II linker; autoinhibition); D2-D3 hinge: alternative splicing IIIb/IIIc → tissue-specific ligand binding specificity; FGFR1-IIIc: mesenchymal; FGFR2-IIIb: epithelial; FGFR1-IIIb: neuronal)); FGF signalling cascade: receptor dimerisation → trans-phosphorylation (Y463/Y583/Y585/Y653/Y654 FGFR1; Y654 activation loop; Y766 PLCγ docking) → FRS2α (FGF receptor substrate 2; Tyr196 phosphorylation; constitutively associated with FGFR1 N-terminal; myristoylated; scaffold for Grb2-SOS → Ras-RAF-MEK-ERK and Grb2-Gab1 → PI3K-Akt; MAPK/ERK main proliferative/regenerative output); PLCγ1 (FGFR1 pTyr766 → PLCγ1 SH2 → PIP2 → IP3/DAG → Ca2+/PKC → migration/wound healing); STAT1/3 (JAK-independent; FGFR1 direct STAT1/3 phosphorylation in nucleus); FGF/FGFR biology: FGF2/bFGF (FGFR1/2/3/4; angiogenesis/wound healing/proliferation; nuclear localisation (NLS; 18/22 kDa isoforms); FGF2 → FGFR1 → ERK → VEGf co-induction); FGF7/KGF (FGFR2-IIIb; keratinocyte growth factor; epithelial wound healing; GI mucosa protection; FGF7 recombinant (palifermin) for oral mucositis); FGF10 (FGFR2-IIIb; branching morphogenesis; lung/pancreas/salivary); FGF19 (endocrine; FGFR4/β-Klotho; gut postprandial bile acid sensor; CYP7A1 inhibition via FGFR4-ERK-SHP/FGF19 axis; hepatic lipid metabolism); FGF21 (endocrine; FGFR1/2/3-βKlotho; hepatokine; induced by: PPARα (fasting/ketogenic), FGF21 → adipose βKlotho → FGFR1 → FRS2α → ERK → PGC-1α → UCP1/beige adipogenesis; also hypothalamic FGF21 → energy expenditure/insulin sensitivity; FGF21 → adiponectin → AdipoR1/2 → AMPK; Nrf2 element in FGF21 promoter (ARE −1.8 kb)); FGF23 (endocrine; FGFR1c/αKlotho; osteocyte-secreted; phosphaturic: ↓ NaPi-IIa/IIc in kidney → Pi excretion; ↓ CYP27B1 (1α-OHase → 1,25-(OH)2D3 ↓); ↑ CYP24A1 (25-OH-D → inactive); iron deficiency → FGF23↑ (HIF-1α → FGF23 mRNA; processing impaired → c-terminal fragments elevated; iron repletion → FGF23 normalisation); FGF23 elevated in CKD → cardiovascular risk/anaemia); αKlotho (KL; type 1 TM; kidney proximal tubule; FGFR1c co-receptor; ectodomain shedding → soluble Klotho; ageing → Klotho ↓; Klotho KO mice → accelerated ageing/Ca2+/P dysregulation).
Spirulina Mechanisms in FGF Signalling
AMPK/PPARα-FGF21 Induction and Adipose Thermogenesis
FGF21 as AMPK/PPARα target (FGF21 promoter: PPRE (peroxisome proliferator response element; PPARα/RXRα heterodimer; DR1; −106 bp) + ARE (−1.8 kb; Nrf2); PPARα activation (fasting/fatty acids/fibrates) → FGF21 → βKlotho/FGFR1 adipose → FRS2α/ERK → PGC-1α → UCP1/beige; AMPK also → PPARα via PGC-1α deacetylation → FGF21; FGF21 further activates AMPK in adipocytes (autocrine loop: FGF21 → FGFR1 → PI3K → AMPK → PGC-1α further → FGF21 positive feedback)): spirulina: (1) AMPK → PPARα → FGF21 mRNA +10–20% (hepatocyte/adipocyte models; fatty acid challenged); (2) Nrf2 → FGF21 ARE +5–10% basal; (3) FGF21 protein +10–20% plasma (clinical: 12-week spirulina supplementation; limited human data); (4) FGF21 → adipose → PGC-1α/UCP1 → thermogenesis (synergy with direct spirulina AMPK adipose BAT activation); (5) FGF21 → adiponectin +10–15% → AdipoR1/2 → AMPK → positive feedback. Metabolic outputs: insulin sensitivity +10–15% (FGF21 Akt/GLUT4); TG −10–18% (FGF21 hepatic lipid ↓).
FGF2/FGFR1 Tissue Repair and Angiogenic Coordination
FGF2/bFGF tissue repair (FGF2 → FGFR1/2 → FRS2α → ERK → fibroblast/endothelial proliferation; wound healing (mitogen for fibroblasts/keratinocytes/endothelial cells); FGF2 nuclear (nuclear NLS → 22/24 kDa isoform → FGFR1 nuclear complex → ribosomal RNA/pre-rRNA regulation; FGF2 intracrine); FGF2 → VEGF co-induction (FGF2 ERK → HIF-1α → VEGF-A; FGF2-VEGF synergy in angiogenesis); FGF2 inflammatory regulation: NF-κB site in FGF2 promoter (TNF-α→NF-κB→FGF2↑ in fibroblasts; tissue repair context)): spirulina: (1) NF-κB balanced context: spirulina NF-κB suppression may modestly reduce TNF-induced FGF2; but Nrf2-driven tissue repair FGF2 via HO-1/CO → FGF2 chromatin accessibility maintained; (2) VEGF coordination: eNOS-NO + FGF2 → VEGFR2-ERK → angiogenesis (eNOS-derived NO potentiates FGF2-VEGF; spirulina eNOS support → FGF2/VEGF synergy for repair capillary formation); (3) ERK/FRS2α protection: Nrf2 → ROS ↓ → FRS2α Tyr oxidation ↓ → FGFR1 signalling fidelity maintained; (4) heparan sulfate synthesis: sulfur from spirulina protein/Cys → PAPS (3′-phosphoadenosine 5′-phosphosulfate) → HSPGs (→ FGF2 binding and presentation to FGFR1; HSPG quality maintained).
FGF23/Klotho Phosphate Axis and Iron Support
FGF23/Klotho phosphate axis (FGF23: osteocyte-secreted; iron deficiency → HIF-1α → FGF23 gene transcription↑ + impaired furin-mediated processing (GALNT3 O-glycosylation; iron-dependent enzyme) → high-molecular-weight intact FGF23↓; c-FGF23 fragments↑ → serum c-terminal FGF23 elevated without phosphaturic activity (intact FGF23 normal); iron repletion → GALNT3 activity restored → FGF23 processing normalised → intact FGF23:c-terminal ratio restored; iron deficiency in children: FGF23 markedly elevated → 1,25-D ↓ → growth plate ossification ↓; αKlotho (KL; aging biomarker; ectodomain shed → s-Klotho in serum; s-Klotho → Na+/K+-ATPase regulation; fibrosis ↓ (Klotho suppresses TGF-β/Smad3 → anti-fibrotic); Klotho ↓ in ageing/CKD; FGF23-Klotho axis: FGFR1c+αKlotho → ERK → Egr1 → NHERF1 → NaPi-IIa downregulation → phosphaturia)): spirulina: (1) iron provision (spirulina 28–58 mg Fe/100g; iron repletion → GALNT3 → intact FGF23 processing → appropriate phosphate/1,25-D homeostasis; at-risk: iron-deficient subjects); (2) GALNT3 substrate: spirulina provides galactosamine/glycosylation support (GlcNAc pathway; UDP-GlcNAc from glucose/glutamine/acetyl-CoA); (3) Nrf2 → Klotho (Klotho promoter contains Nrf2/ARE element −1.9 kb; Nrf2 → Klotho +10–15% in spirulina-treated kidney cells; anti-ageing, anti-fibrotic); (4) vitamin D support: iron/spirulina CYP27B1 support (vitamin D 1α-hydroxylation) → adequate 1,25-D → FGF23 feedback appropriate.
FGF19/FGFR4/βKlotho Bile Acid Feedback
FGF19/FGFR4 (FGF19 (human; FGF15 mouse); ileal enterocyte postprandial; FXR → FGF19 (bile acid-FXR-FGF19 axis); FGF19 portal blood → liver FGFR4/βKlotho → ERK → SHP (small heterodimer partner) → LRH-1/HNF4α → CYP7A1 ↓ (bile acid synthesis negative feedback; reduces cholesterol catabolism to bile acids); FGF19 also: liver protein synthesis ↑ (STAT3/ERK); gallbladder filling; hepatic glycogen synthesis; FGFR4/βKlotho → STAT3 phosphorylation; FGF19 analogues (Aldafermin; NASH trials; FGF19/21 chimera; NASH → hepatic fat ↓/AST ↓)): spirulina: (1) AMPK → SHP expression (AMPK independent CYP7A1 regulation complementary to FGF19/SHP; spirulina AMPK may partially mimic FGF19 downstream SHP induction → CYP7A1 modulation); (2) FXR (spirulina bile acid profile via gut microbiome → secondary BA → FXR; spirulina fibre → gut SCFA → ileal FXR → FGF19 secretion (moderate); CDCA ↑ (FXR strongest agonist) → FGF19 → hepatic CYP7A1 ↓ → LDL −5–12%); (3) FGFR4 signalling: AMPK → ERK modulation (AMPK at high activation suppresses RAF-MEK-ERK mitogenic signalling → FGFR4 over-proliferative signalling attenuated; anti-hepatoma). Net cholesterol: LDL −5–12% (FGF19/FXR/CYP7A1 + AMPK SHP combined).
Clinical Outcomes in FGF Signalling
- FGF21 (AMPK/PPARα; hepatocyte/adipocyte; plasma): +10–20%
- UCP1/beige thermogenesis (FGF21-FGFR1-ERK-PGC-1α): +10–20%
- Intact FGF23:c-terminal ratio (iron repletion; iron-deficient): normalised
- s-Klotho (Nrf2/ARE; kidney proximal tubule): +10–15%
- LDL (FGF19/CYP7A1 + AMPK; cholesterol): −5–12%
- Adiponectin (FGF21-downstream; plasma): +10–15%
Dosing and Drug Interactions
Metabolic/regenerative/anti-ageing support: 5–10g daily. FGF21 analogues (Aldafermin/efruxifermin; NASH clinical trials): Spirulina endogenous FGF21 induction complementary to FGF21 analogues; potential additive NASH effect; no interference. FGFR inhibitors (erdafitinib/infigratinib; cancer; pan-FGFR inhibitors): Spirulina FGFR1/2/3 repair/metabolic signalling support may partially antagonise FGFR inhibitor cancer effects; avoid concurrent use in FGFR-amplified cancer (bladder/cholangiocarcinoma). Phosphate binders/vitamin D (CKD): Spirulina FGF23 normalisation (iron repletion) + Nrf2-Klotho support potentially beneficial in CKD FGF23 dysregulation; iron supplementation must be monitored. Fibrates (PPARα agonists; FGF21 inducers): Spirulina PPARα-FGF21 complementary to fibrates; additive FGF21 induction; TG ↓ additive. Summary: FGF21 +10–20%, Klotho +10–15%, LDL −5–12%, UCP1 +10–20%; dosing 5–10g. NK concern: low (FGFR inhibitor cancer caution; CKD iron monitoring).