Spirulina and TGF-β/SMAD Signalling.

TGF-β Ligands, Receptors, and SMAD Phosphorylation

TGF-β superfamily ligands (TGF-β1/2/3; BMPs; Activins; GDF members) signal through type II/type I receptor serine/threonine kinase pairs. TGF-β1 binds TβRII (TGFBR2; constitutively active kinase) → TβRI/ALK5 (TGFBR1) recruitment and trans-phosphorylation by TβRII Ser165/Thr200 → ALK5 GS domain (Ser165-Ser215 stretch) phosphorylated → ALK5 catalytic activation. ALK5 phosphorylates R-SMADs 2 and 3 at the C-terminal SSXS motif (SMAD2 Ser465/467; SMAD3 Ser423/425). Phospho-SMAD2/3 dissociate from the receptor, associate with SMAD4 (Co-SMAD; MH2 domain L3 loop trimeric complex), translocate to nucleus, and recruit transcriptional co-activators (p300/CBP HAT; SRC-2; PCAF) at SMAD-binding elements (SBE; 5′-AGAC-3′ or GTCT) in promoters of α-SMA (ACTA2), collagen I (COL1A1/COL1A2), fibronectin (FN1), CTGF/CCN2, PAI-1, Snail/SLUG (EMT drivers). BMP signals through ALK2/3/6 → SMAD1/5/9 → SMAD4 → ID1/2/3 (cell fate; bone formation; anti-fibrotic in liver).

Negative Regulators: SMAD7, SMURF1/2, and SKI/SnoN

SMAD7 (inhibitory SMAD; I-SMAD) is rapidly induced by SMAD3/4 itself (negative feedback; SBE in SMAD7 promoter) and by NF-κB (two κB sites); SMAD7 competes with R-SMADs for ALK5 binding, recruits SMURF1/2 (HECT E3 ligases) to promote K48-Ub degradation of ALK5/TβRI (closes the receptor signal). SMURF2 also ubiquitinates SMAD2/3 for degradation. SKI and SnoN are nuclear proto-oncoproteins that bind the SMAD2/3-SMAD4 complex and recruit HDAC1/2/3 co-repressors, blocking p300/CBP co-activation. TGIF1/2 (homeodomain; recruit HDAC) compete with SRC-2 at SMAD2/3 transcriptional complexes. BAMBI (BMP and activin membrane-bound inhibitor) is a pseudoreceptor decoy for TβRI, induced by BMP signalling and Wnt/β-catenin, that competitively inhibits TGF-β receptor complex assembly.

NF-κB–TGF-β Crosstalk

NF-κB and TGF-β/SMAD engage in extensive bidirectional crosstalk. NF-κB drives TGFB1 transcription (two κB sites in TGFB1 promoter at −140 and −225 bp) and SMAD7 transcription (SMAD7 limits TGF-β signalling, creating an NF-κB-negative feedback on TGF-β). Conversely, TGF-β→SMAD3 binds the NF-κB p65 transactivation domain and inhibits p65 Lys310 acetylation, suppressing inflammatory genes. TAK1 (MAP3K7; activated by TGF-β→TRAF6 and by IL-1/TNF-α) phosphorylates IKKβ Ser177 (NF-κB activation) and MKK3/6 (p38 activation) and is a key non-canonical TGF-β effector promoting inflammation and fibrosis. SMAD3 also cooperates with Sp1 at collagen I and α-SMA promoters synergistically with AP-1 (TGF-β→RAS→ERK→c-Jun).

Spirulina’s Mechanistic Actions

• NF-κB ↓ → TGF-β1 production ↓: PCB→IKKβ↓→NF-κB↓→TGFB1 mRNA ↓ 25–40% in hepatic stellate cells (HSCs) and macrophages→autocrine TGF-β1 fibrogenic drive ↓.
• AMPK → ALK5 Ser165 phosphorylation ↓: AMPK activation decreases ALK5 GS domain trans-phosphorylation efficiency (via indirect mTORC1/mTORC2-dependent kinase suppression); ALK5 kinase activity ↓ 20–30%→SMAD3 Ser423/425 ↓ 20–35%→nuclear SMAD3/4 complex ↓→α-SMA ↓ 25–45%; collagen I ↓ 25–40% in CCl4-fibrosis rodent model.
• Nrf2 → SMAD7 ↑: Nrf2 activation increases SMAD7 expression (Nrf2-ARE element proposed in SMAD7 promoter; also indirect via NF-κB↓ which drives SMAD7 as negative feedback) → ALK5 ubiquitination via SMURF2 ↑→receptor degradation ↑→TGF-β signal self-limiting.
• Nrf2 → HO-1/CO → TAK1 ↓: CO inhibits TRAF6 auto-ubiquitination→TAK1 activation ↓→non-canonical NF-κB/p38 TGF-β effectors ↓ 20–30%.
• Wnt/β-catenin (partial) → BAMBI ↑: AMPK→GSK-3β Ser9→β-catenin→BAMBI ↑ 10–20% in hepatocytes→TβRI decoy competitive inhibition→TGF-β receptor complex formation ↓.
• Downstream readouts: α-SMA ↓ 25–45%; fibronectin ↓ 20–35%; hydroxyproline content ↓ 20–35% (collagen); MMP-2/9 ↓ via SMAD3/AP-1 ↓ (paradox: TGF-β/SMAD3 also induces TIMP-1 → anti-MMP; spirulina net effect is normalisation of MMP/TIMP balance).

Clinical Correlates and Dosing

Animal models: spirulina (100–400 mg/kg) in CCl4, bile-duct ligation, and high-fructose NAFLD models reduced fibrosis score (Metavir F-score equivalent) 1–2 grades, SMAD3 nuclear staining ↓ 25–40%, α-SMA ↓ 25–45%, collagen I ↓ 25–40%. In diabetic nephropathy models (streptozotocin), TGF-β1 ↓ 30–45%, fibronectin ↓ 25–35%. Human surrogate: 4–8 g/day spirulina reduces ALT/AST (hepatic fibroinflammatory markers) in NAFLD RCTs by 20–35%, consistent with TGF-β/NF-κB suppression. Interactions: anti-fibrotic drugs (pirfenidone, nintedanib for IPF) — mechanistically complementary (pirfenidone suppresses TGF-β1 and NF-κB; additive with spirulina plausible); insufficient human data; use caution.