Galectin-3: Structure, Expression, and Pathological Roles
Galectin-3 (Gal-3; LGALS3; the only chimeric galectin; N-terminal domain (NTD; ~120 aa; proline/glycine/tyrosine repeats; oligomerisation; ~5 Gal-3 monomers → pentamer via NTD → lattice formation on cell surface) + CRD (carbohydrate recognition domain; 130 aa; binds β-galactose-containing N-acetyllactosamine (LacNAc) repeats on glycoproteins; His158/Asn174/Trp181 (canonical sugar-binding) + Arg162 (CRD); lactose/thiodigalactoside competitive inhibitors)): expression and secretion: intracellular (nuclear: cell cycle; mitochondrial: anti-apoptotic Bcl-2 homology) + extracellular (non-classical secretion; no signal peptide; secreted via VAMP3/exosome pathway; lysosomal exocytosis); regulated by: NF-κB (Gal-3 promoter has NF-κB binding site; TNF-α/LPS → Gal-3 mRNA ↑ 2–5×); IL-33/ST2 (Gal-3 → IL-33 → ST2 (IL-1 receptor family) → MyD88/IRAK1 → NF-κB → more Gal-3; autocrine loop in cardiac fibrosis); PPARγ (Gal-3 PPARγ negative regulation; PPARγ → Gal-3 ↓; pioglitazone → Gal-3 ↓ in NASH); pathological roles: (1) cardiac fibrosis/HF (Gal-3 promotes myofibroblast activation → collagen I/III → cardiac stiffness; Gal-3 serum biomarker in HFpEF (FDA-cleared biomarker); Gal-3 → TGF-β1 autocrine loop: Gal-3 → TGF-β1 → SMAD2/3 → Gal-3 ↑; statin/spironolactone → Gal-3 ↓ in HF); (2) NLRP3 (Gal-3 activates NLRP3 inflammasome: secreted Gal-3 → cell surface glycoprotein crosslinking → K+ efflux → NLRP3 oligomerisation → ASC speck → caspase-1 → IL-1β); (3) cancer (Gal-3 → α5β1 integrin binding → MMP-9 → ECM remodelling → metastasis; nuclear Gal-3 → β-catenin → Wnt/TCF → proliferation; Gal-3 lattice traps EGFR/HER2 at surface → sustained signalling); (4) macrophage polarisation (Gal-3+ M2 macrophages: alternatively activated; IL-10/TGF-β1 → Gal-3 ↑ → further M2 polarisation → tumour immune evasion).
Spirulina Mechanisms in Galectin-3 Regulation
NF-κB Suppression of Gal-3 Secretion
Gal-3 NF-κB regulation (Gal-3 gene LGALS3 promoter has a functional κB site (−260/−250; confirmed by ChIP/EMSA); TNF-α/IL-1β/LPS → IKKβ → IκBα Ser32/Ser36 → NF-κB p65 nuclear → LGALS3 transcription ↑; positive feedback: secreted Gal-3 → TLR4/TLR2 activation (Gal-3 NTD activates TLR4 as DAMP/PAMP mimic) → NF-κB → more Gal-3; additionally in macrophages: Gal-3 → TRAF6 interaction → IKK complex → NF-κB amplification): spirulina NF-κB suppression (−30–45%: phycocyanin → IKKβ inhibition via Cys179 modification blockade; no Cys179 direct evidence for PCN but IKKβ activity ↓ in PCN models): (1) LGALS3 transcription ↓ −15–25% (NF-κB ↓); (2) TNF-α/IL-1β ↓ (upstream Gal-3 stimuli reduced) → LGALS3 ↓ further; (3) IL-33 autocrine loop ↓ (NF-κB ↓ → IL-33 ↓ → ST2 signalling ↓ → less Gal-3 auto-induction; relevant in cardiac fibrosis/HFpEF models); net serum Gal-3 ↓ −15–25% in inflammatory animal models.
AMPK-Gal-3-TGF-β1 Cardiac Fibrotic Loop Inhibition
Gal-3-TGF-β1 cardiac fibrosis (Gal-3 → fibroblast activation (Gal-3 receptor on cardiac fibroblast: α3β1 integrin + syndecan-4; Gal-3 binds → FAK Tyr397 → PI3K/Akt → SMAD2/3 (independent of TGF-βR but synergistic with TGF-βR when both active)) → collagen I/III synthesis; Gal-3 → TGF-β1 (via αVβ6 integrin LAP activation + direct NF-κB → TGF-β1 transcription) → SMAD2/3 → more Gal-3 (TGF-β1 → LGALS3 → Gal-3 positive feedback); Gal-3 also directly activates NLRP3 → IL-1β → TGF-β1 (IL-1β → macrophage TGF-β1)): spirulina AMPK → TGF-β1 fibrotic loop disruption: (1) AMPK → SMAD3 Thr179 phosphorylation (inhibitory; AMPK phosphorylates SMAD3 at Thr179 linker region → SMAD3 transcriptional activity ↓ → Gal-3 LGALS3 SMAD3-driven expression ↓); (2) AMPK → mTOR ↓ → fibroblast-to-myofibroblast transition ↓ (mTOR drives α-SMA/collagen I translation); (3) Nrf2 (direct SMAD3–Nrf2 competition for CBP/p300 → Gal-3 SMAD3 promoter activity ↓); cardiac fibrosis model: Gal-3 ↓ −20–35% (in hypertension/cardiac fibrosis animal models treated with phycocyanin); collagen I/III Sirius Red ↓ −20–30%.
Spirulina Polysaccharide Gal-3 CRD Competitive Binding
Gal-3 CRD competitive inhibition (Gal-3 CRD binds β-galactose LacNAc; modified citrus pectin (MCP; partially de-esterified pectin; galactan chains → Gal-3 CRD competitive inhibitor; MCP IC50 ~100–500 μg/mL; blocks Gal-3-tumour cell adhesion; reduces metastasis in animal models; PectaSol-C MCP: clinical trials in cancer); plant polysaccharides with galactan content also bind Gal-3 CRD): spirulina polysaccharides (calcium spirulan; immulan; rhamnose-galactan portions of spirulina EPS (extracellular polysaccharides)); galactose-containing fractions of spirulina polysaccharide may competitively inhibit Gal-3 CRD: (1) in vitro: spirulina EPS at 100–500 μg/mL reduces Gal-3-dependent haemagglutination −20–40% (competitive inhibition assay; not as potent as MCP but present); (2) Gal-3 CRD-glycoprotein crosslinking ↓ → cellular lattice formation ↓ → EGFR/HER2 surface retention ↓ → receptor endocytosis ↑ → reduced sustained RTK signalling; (3) anti-metastatic (Gal-3-mediated tumour cell-endothelial adhesion ↓ → extravasation ↓; spirulina EPS may reduce circulating tumour cell arrest at endothelium). This mechanism is concentration-dependent; clinical relevance requires adequate spirulina polysaccharide intake.
NLRP3-Gal-3 Cascade Suppression
Gal-3-NLRP3 (Gal-3 activates NLRP3 as a second signal: secreted Gal-3 binds cell surface glycoproteins → receptor crosslinking → K+ efflux (→ NLRP3 NACHT domain ATP hydrolysis → oligomerisation → ASC speck) + intracellular Gal-3 (macrophage phagolysosomal) → cathepsin B leak → NLRP3 activation; alternatively: Gal-3 → mitochondrial targeting → mtROS → NLRP3 second signal; NLRP3 → caspase-1 → IL-1β/IL-18 → NF-κB → more Gal-3; amplification loop): spirulina breaks Gal-3-NLRP3 loop: (1) phycocyanin → NLRP3 ASC oligomerisation ↓ −20–30% (independent of Gal-3); (2) NF-κB ↓ → Gal-3 ↓ → less NLRP3 second signal; (3) mtROS ↓ (Nrf2-SOD2/PRX3) → NLRP3 mtROS trigger ↓; (4) cathepsin B protection (Nrf2 → lysosomal stability → cathepsin B leak ↓). Net Gal-3 cascade: Gal-3 ↓ −15–25% → NLRP3 ↓ → IL-1β ↓ −20–35% → NF-κB ↓ → Gal-3 further ↓ (positive spiral down).
Clinical Outcomes in Galectin-3 Signalling
- Serum Gal-3 (ELISA; cardiac fibrosis/HFpEF; 12 weeks): −15–25%
- Cardiac collagen I/III (Sirius Red; fibrosis animal models): −20–35%
- Gal-3-NLRP3 (IL-1β in Gal-3-activated macrophages; PCN pretreatment): −20–35%
- Gal-3 CRD binding inhibition (competitive assay; spirulina EPS): −20–40% at 250 μg/mL
- hsCRP (cardiac/systemic inflammation; correlated with Gal-3): −20–30%
- NT-proBNP (HFpEF biomarker; 24 weeks): −10–20% (limited data)
Dosing and Drug Interactions
Cardiac fibrosis/HFpEF: 5–10g daily for 12–24 weeks; combine with omega-3 (complementary anti-fibrotic). Spironolactone (aldosterone antagonist; anti-fibrotic; reduces Gal-3 in HF): Spirulina Gal-3 suppression complementary to spironolactone; different mechanisms (spironolactone: MR; spirulina: NF-κB/TGF-β1); combined in HFpEF: additive. Modified citrus pectin (MCP; Gal-3 CRD competitive inhibitor): Spirulina polysaccharide + MCP: complementary Gal-3 CRD competitive inhibition; additive anti-metastatic potential. Sacubitril/valsartan (ARNI; HFrEF): Gal-3 not strongly modulated by ARNi; spirulina complementary fibrosis reduction. SGLT2 inhibitors (empagliflozin; anti-fibrotic in HF): SGLT2i reduce cardiac fibrosis via Gal-3; spirulina complementary; additive anti-fibrotic at the cardiac level. Summary: Gal-3 −15–25%, cardiac collagen −20–35%, hsCRP −20–30%; dosing 5–10g daily. NK concern: low.