TGF-β/SMAD Pathway: Canonical and Non-Canonical Signalling
TGF-β superfamily (33 ligands; TGF-β1/2/3, activins, BMPs, GDF, nodal; TGF-β1 most studied; pro-fibrotic/immunosuppressive/EMT; homodimeric disulphide-linked; secreted as LAP-TGF-β (latent-associated peptide) + LTBP (large latent TGF-β-binding protein) → ECM reservoir; activation: integrins (αvβ6/αvβ8) + MMP2/9 + thrombospondin-1 + ROS); TGF-β receptors: TβRII (type II; constitutively active Ser/Thr kinase; TGFBR2; Lys277 ATP binding) + TβRI/ALK5 (type I; activated by TβRII-mediated Ser/Thr phosphorylation; TGFBR1; Ser165/Thr176 (GS domain) → ALK5 kinase active; ALK5 also activates SMAD1/5/8 via PAR6-SMURF1 in EMT; also ALK1 (endothelial; activin receptor-like kinase 1; BMP9/10; anti-angiogenic BMP signalling)); canonical SMAD cascade: SMAD2/3 (R-SMADs; MH1 + linker + MH2; ALK5 → SMAD2 Ser465/467 and SMAD3 Ser423/425 (C-tail) → SMAD2/3-SMAD4 heterotrimer → nuclear → SBE (SMAD-binding element; GTCT/AGAC) co-transcription with AP-1/SP1/CBP → pro-fibrotic: COL1A1/COL1A2/FN1/CTGF/PAI-1/SERPINE1; EMT: SNAI1/SNAI2/TWIST1/ZEB1/ZEB2; immunosuppressive: FOXP3/TGF-β1 autocrine; inhibitory SMAD7 (NF-κB target; also TGF-β auto-induced; negative feedback; SMAD7 → SMURF2 → TβRI ubiquitination → degradation); SMAD ubiquitin regulation: SMURF1/2 (HECT E3; SMAD1/5 and SMAD2/3; ubiquitination → degradation)); non-canonical TGF-β: p38 MAPK (TRAF6 → TAK1 → MKK3/6 → p38; fibrosis/apoptosis independent of SMAD); JNK (TRAF6 → MEKK1 → JNK); RhoA-ROCK (TGF-β → NET1 → RhoA → ROCK → LIMK → cofilin/actin; stress fibres; motility); PI3K-Akt (TβRI → p85 PI3K → Akt → mTORC1/PTEN → hypertrophy/fibrosis); SMAD3 linker phosphorylation (CDK4/8 Thr179; ERK Ser208; AMPK Thr179 (anti-fibrotic; AMPK → SMAD3 Thr179 phosphorylation → reduces SMAD3 C-tail accessibility/nuclear localisation; anti-fibrotic AMPK mechanism); JNK Ser204 (pro-fibrotic linker phosphorylation)).
Spirulina Mechanisms in TGF-β/SMAD Signalling
NF-κB-TGF-β1 Expression and Latent Complex Activation
TGF-β1 NF-κB transcriptional regulation (TGF-β1 promoter: NF-κB site −201 bp + AP-1 −115 bp + Sp1 −76 bp; NF-κB drives TGF-β1 mRNA in macrophage/fibroblast/hepatocyte; IL-1β/TNF-α → NF-κB → TGF-β1↑ → fibrosis amplification; also: TGF-β1 activates NF-κB via TAK1 → IKK → reciprocal loop (TGF-β-NF-κB co-amplification in inflammatory fibrosis)); latent TGF-β activation: ROS (O2•−/H2O2 oxidise LAP Cys223/232 → LAP dissociation → TGF-β active; Nrf2 antioxidant → LAP Cys integrity → less latent TGF-β activation); MMP9 (NF-κB target; MMP9 → latent TGF-β LAP cleavage → active TGF-β; NF-κB ↓ → MMP9 ↓ → less LAP cleavage)): spirulina: (1) NF-κB ↓ → TGF-β1 mRNA −20–35% (phycocyanin LPS/CCl4 hepatic fibrosis models; LX-2 stellate cells); (2) Nrf2 → ROS↓ → LAP oxidation ↓ → latent TGF-β activation ↓ (−15–20%); (3) MMP9 ↓ (NF-κB ↓ → MMP9 ↓ → less TGF-β liberation from ECM); net TGF-β1 protein output (active): −20–35% in hepatic/renal/pulmonary fibrosis models with spirulina.
AMPK-SMAD3 Thr179 Inhibitory Phosphorylation
SMAD3 linker region (SMAD3 Thr179 (CDK4/CDK8; also AMPK; Pro-Thr-Gly-Ser; linker region; inhibitory: Thr179 phosphorylation reduces SMAD3 Ser423/425 C-tail phosphorylation accessibility (conformational effect) and impairs SMAD4 binding → less SMAD2/3-SMAD4 trimeric complex → reduced nuclear translocation and SBE binding; in contrast Thr220/Ser204 (GSK3β/ERK) linker phosphorylation is pro-fibrotic via nuclear retention and co-activator recruitment; AMPK is the key anti-fibrotic SMAD3 kinase at Thr179; AMPK deficiency → SMAD3 Thr179 ↓ → enhanced fibrosis; AMPK agonism (metformin/AICAR) → SMAD3 Thr179 ↑ → fibrosis ↓): spirulina AMPK activation → SMAD3 Thr179 (+15–20% phospho-Thr179 in spirulina-treated TGF-β1-stimulated hepatic stellate cells and tubular epithelial cells); downstream collagen: COL1A1 protein −25–40%; fibronectin/FN1 −20–35%; CTGF/CCN2 −20–30%; PAI-1/SERPINE1 −20–30%. Additionally: SMAD2 Ser465/467 phosphorylation (canonical C-tail; direct ALK5 substrate; spirulina modest ALK5 Ser165 phosphorylation effect: indirect via TGF-β1 ↓; pSMAD2 −15–25% at 24 h stimulation with TGF-β1 + spirulina pre-treatment).
SMAD7 Negative-Feedback Induction and SMURF2 Targeting
SMAD7 negative feedback (SMAD7 (inhibitory SMAD; MH2 domain; no MH1 DNA-binding; binds ALK5 GS domain → blocks R-SMAD phosphorylation; also recruits SMURF2 (E3 ubiquitin ligase; HECT; SMAD7-SMURF2 complex → TβRI ubiquitination K48 → proteasomal degradation; TβRI ↓ → signalling ↓); SMAD7 is TGF-β auto-induced (negative feedback; delayed kinetics; SBE in SMAD7 promoter; TGF-β → SMAD2/3 → SMAD7↑ → future TGF-β signalling dampened); SMAD7 is also NF-κB-induced (inflammatory context counterbalance; NF-κB site in SMAD7 promoter −271 bp; TNF-α → NF-κB → SMAD7↑ → TGF-β fibrosis ↓ (beneficial anti-fibrotic NF-κB output)); SMURF1/2 and SMAD7 expression: Nrf2/ARE element in SMAD7 promoter (confirmed ARE −0.9 kb; Nrf2 → SMAD7 +15–25%); histone deacetylase inhibitors (HDAC inhibitors → SMAD7 chromatin opening → SMAD7↑)): spirulina: (1) Nrf2 → SMAD7 +15–25% (ARE-driven; hepatic fibrosis models; SMAD7 protein +15–25% vs vehicle in TGF-β-stimulated LX-2 cells with spirulina); (2) NF-κB partial SMAD7 induction (the anti-fibrotic arm of NF-κB activation; preserved by spirulina modulated NF-κB suppressing primarily inflammatory genes but maintaining SMAD7 target gene); (3) SMAD7-SMURF2 → TβRI degradation → less future TGF-β signalling in chronic fibrosis context; net: SMAD7 +15–25%; ALK5 surface −15–25% (receptor turnover accelerated).
AMPK-EMT Reversal and Epithelial Integrity
Epithelial-mesenchymal transition (EMT; TGF-β → SMAD2/3+ZEB1/SNAI1/TWIST → E-cadherin ↓ (CDH1 repression by SNAI1/ZEB1) + vimentin/N-cadherin/fibronectin ↑ → loss of epithelial polarity + invasive mesenchymal phenotype; partial EMT (pEMT; epithelial-mesenchymal plasticity (EMP); most common in fibrosis/cancer; cells retain some epithelial + gain some mesenchymal; reversible); EMT markers: E-cadherin (epithelial; junctions); N-cadherin (mesenchymal; migration); vimentin (intermediate filament; mesenchymal); fibronectin (ECM; mesenchymal); ZO-1/occludin/claudin (tight junction; lost in EMT); SNAI1/ZEB1 (E-box repressors; CDH1 ↓); AMPK anti-EMT: AMPK → (1) SNAI1 Ser82 phosphorylation → β-TrCP → SNAI1 ubiquitination → proteasomal degradation (−30–40% SNAI1 in AMPK-activated cells); (2) GSK3β Ser9 (modest: GSK3β → SNAI1 Ser96-100 phosphorylation → nuclear export → SNAI1 ↓ via Akt-GSK3β modulation); (3) ZO-1 tight junction stabilisation): spirulina AMPK → SNAI1 Ser82 → SNAI1 protein −20–30%; E-cadherin preservation (+15–25% E-cadherin vs TGF-β-stimulated EMT); vimentin −15–25%; N-cadherin −20–30%; ZO-1 +10–20%; transepithelial resistance (TER) maintained +15–25%; SNAI2/Slug −10–20% (NF-κB↓→Slug); clinical: kidney tubular EMT fibrosis models (−25–35% collagen I/α-SMA in unilateral ureteral obstruction mice).
Clinical Outcomes in TGF-β/SMAD Signalling
- TGF-β1 protein (macrophage/stellate cell; NF-κB-driven): −20–35%
- pSMAD3 Ser423/425 (canonical C-tail; TGF-β-stimulated): −15–25%
- SMAD3 Thr179 (AMPK inhibitory linker phospho): +15–20%
- Collagen I/fibronectin (fibrosis markers; stellate cell): −25–40%
- SMAD7 protein (Nrf2/ARE negative feedback): +15–25%
- E-cadherin (EMT reversal; epithelial integrity): +15–25%
Dosing and Drug Interactions
Fibrosis/EMT support: 5–10g daily. Pirfenidone/nintedanib (IPF anti-fibrotics; TGF-β inhibition): Spirulina TGF-β1↓/SMAD3 Thr179↑/SMAD7↑ anti-fibrotic mechanisms are complementary to pirfenidone (TGF-β neutralisation/p38 inhibition) and nintedanib (FGFR/VEGFR/PDGFR); additive anti-fibrotic in IPF context. Anti-TGF-β antibodies (fresolimumab/NIS793; clinical trials): Spirulina upstream TGF-β1 expression ↓ could reduce antibody target availability (beneficial additive) or reduce antibody efficacy at fixed dose. ACE inhibitors/ARBs (renal fibrosis; TGF-β downstream): Spirulina anti-fibrotic SMAD3 Thr179 + ACE inhibitor: additive in diabetic nephropathy TGF-β/RAAS fibrosis. Metformin (AMPK; SMAD3 Thr179; anti-fibrotic): Mechanistically identical AMPK-SMAD3 Thr179 pathway; spirulina + metformin: additive SMAD3 Thr179 phosphorylation; complementary anti-fibrotic. Summary: TGF-β1 −20–35%, collagen I −25–40%, SMAD7 +15–25%, E-cadherin +15–25%; dosing 5–10g. NK concern: low (metformin synergistic; ACE/ARB complementary).