Angiopoietin-Tie2 Signalling: Vascular Quiescence and Remodelling
Angiopoietin-Tie (Ang-Tie) system (RTK-based vascular stability system; primary regulators of post-angiogenic vessel maturation and quiescence): Tie2/TEK (tyrosine kinase with Ig and EGF homology domains 2; endothelial RTK; EGF-like domains + 3 Ig-like domains; TM + intracellular kinase; constitutively phosphorylated at quiescent state by Ang1 (cluster signalling)); Tie1 (orphan receptor; modulates Tie2; processed by ADAM10/ADAM17 ectodomain shedding; Tie1 loss → Tie2 attenuation); Ang1 (angiopoietin-1; 498 aa; primarily pericyte/smooth muscle cell/platelet; homotetrameric (high-order oligomers → strong Tie2 agonist); stabilises/quiescence; Tie2 → PI3K/Akt → eNOS Ser1177 → NO, FOXO1 → nuclear exclusion → Id1/Id3 ↓ → quiescent phenotype; Src → VE-cadherin Tyr731 ↓ (phosphorylation ↓ → VE-cadherin junction stabilisation); Ang1 anti-apoptotic via Akt → Bad Ser136 + FKHRL1 → Bcl-2 ↑); Ang2 (angiopoietin-2; competitive partial antagonist of Tie2; stored in Weibel-Palade bodies (WPB; endothelial-specific secretory organelles); released by: VEGF, TNF-α, thrombin, hypoxia, high glucose; Ang2 → Tie2 partial agonism at low doses or antagonism in absence of Ang1 → vascular destabilisation; pericyte detachment; VEGF-dependent angiogenesis permissive state; Ang2:Ang1 ratio: the key vascular stability indicator; high Ang2:Ang1 → microvascular permeability ↑ → oedema/inflammation); Tie2 downstream: PI3K (p85/p110α → PIP3 → PDK1 → Akt Thr308; mTORC2 → Akt Ser473), FOXO1 (Akt → FOXO1 Thr24/Ser256 → nuclear exclusion → FOXO1 target gene (MMP9/Id1/CXCR4) ↓ → endothelial quiescence; FOXO1 nuclear in absence of Tie2/Akt → pro-sprouting angiogenic programme), eNOS (Akt Ser1177 → eNOS → NO → sGC → cGMP → PKG → vascular tone; also NO → endothelial barrier via Rac1/cortical actin), MAPK/ERK1/2 (Tie2 → Grb2/Sos → Ras → Raf → MEK → ERK → migration/proliferation (submaximal; quiescence maintained)).
Spirulina Mechanisms in Ang-Tie2 Signalling
Ang2 Suppression via NF-κB and Inflammatory Signal Reduction
Ang2 (stored in endothelial WPBs; secretion triggered by: TNF-α (NF-κB → Ang2 transcription +2–3×); VEGF (PKC → Ang2 exocytosis from WPBs (rapid; minutes)); thrombin (→ WPB exocytosis; sec–min timescale); high glucose (→ ROS → WPB exocytosis + Ang2 transcription via NF-κB); hypoxia (HIF-1α → Ang2 transcription)); spirulina suppresses Ang2 through: (1) NF-κB/IKKβ suppression (−30–45%) → Ang2 mRNA transcription ↓ −15–25%; (2) TNF-α ↓ (−25–40%) → WPB exocytosis trigger ↓ → Ang2 secretion ↓ (−15–25% plasma Ang2 in inflammatory models); (3) AMPK → eNOS → NO → sGC → cGMP → PKG → VASP Ser239 → cortical F-actin → WPB anchorage (PKG reduces WPB RhoA-dependent exocytosis → Ang2 secretion ↓); (4) phycocyanin → ROS ↓ → high glucose-driven ROS-WPB exocytosis ↓ → Ang2 ↓ (relevant in diabetic endotheliopathy). Net Ang2:Ang1 ratio improvement: spirulina reduces Ang2 (−15–25%) while maintaining/increasing Ang1 signalling (via Tie2 downstream amplification) → Ang2:Ang1 ↓ → vascular stabilisation.
Tie2 Downstream PI3K/Akt/FOXO1 Signalling Support
Tie2 autophosphorylation (Tyr992/Tyr1007/Tyr1008/Tyr1023 → PI3K p85 SH2 docking; Grb2; Grb7/14; DOKR (Dok-R; downstream of kinase; adaptor); ABIN2 (A20-binding inhibitor of NF-κB 2; anti-inflammatory Tie2 effect)); PI3K/Akt downstream of Tie2 (Akt Thr308/Ser473; PDK1/mTORC2; endothelial survival (Akt → Bad Ser136 → Bcl-2 ↑ → apoptosis ↓); FOXO1 nuclear exclusion (Akt → FOXO1 Thr24/Ser256 → 14-3-3 → cytoplasm → FOXO1 pro-angiogenic gene programme (VEGFR2/Dll4/Id1) ↓ → quiescent EC phenotype); eNOS Ser1177 → NO; mTOR → protein synthesis → tight junction proteins (claudin-5/occludin)): spirulina amplifies Tie2 downstream signalling through: (1) AMPK → Akt cross-pathway (AMPK → mTORC2 → Akt Ser473 in endothelial cells; AMPK activates mTORC2 via mSin1 phosphorylation → Akt Ser473 → downstream FOXO1/eNOS); (2) eNOS NO support (AMPK Ser1177 + Akt Ser1177 → NO → vascular tone + TEER; Ang1/Tie2 → Akt → eNOS synergises with AMPK → eNOS); (3) PTEN modulation (Nrf2 → TRX1 → PTEN Cys124 rapid re-reduction after transient oxidation → physiological PI3K/Akt oscillation); (4) IRS-1 protection (IRS-1 Tyr612 signalling downstream of insulin maintains basal PI3K/Akt in endothelium adjacent to Tie2 signalling). Net: Akt Thr308 +10–20%, eNOS +15–25%, FOXO1 nuclear exclusion in basal state maintained → quiescent endothelial phenotype preserved.
VE-Cadherin Junction Stabilisation
VE-cadherin (cadherin-5; CDH5; the principal endothelial adherens junction protein; extracellular: 5 cadherin repeats; Ca2+-dependent homophilic binding; intracellular: β-catenin/γ-catenin/p120-catenin; anchored to actin via α-catenin; stability determines paracellular permeability; Tyr731/Tyr685 phosphorylation (Src/VEGFR2 → VE-cadherin Tyr685 → β-catenin dissociation → junction opening → vascular permeability; Tie2 → Akt → Src Tyr527 (CSK → Src Tyr527 → inactive) → Src inactivation → VE-cadherin Tyr685 phosphorylation ↓ → junction stability); VE-PTP (vascular endothelial protein tyrosine phosphatase; PTPRB; Tie2 co-receptor; dephosphorylates VE-cadherin Tyr685 → junction stabilisation; VE-PTP–Tie2 trans-interaction in endothelial junctions)): spirulina VE-cadherin support: (1) eNOS-NO → Src Cys277 S-nitrosylation → Src inactivation → VE-cadherin Tyr685 ↓ → junction stability; (2) Rac1 (AMPK → Rac1 → lamellipodia → cortical actin → VE-cadherin stabilisation); (3) phycocyanin → ROS ↓ → oxidative VE-cadherin Cys oxidation ↓ → junctional integrity; TEER (transendothelial electrical resistance; junction integrity marker) +10–20% in HUVEC models treated with spirulina extract. Pathological permeability (Ang2-driven; diabetic microvascular; inflammatory oedema): spirulina reduces VEGF-induced HUVEC permeability −20–35% (FITC-dextran flux assay) via Ang2 ↓ + VE-cadherin stabilisation.
VEGF-VEGFR2-Ang1/Tie2 Crosstalk
VEGF-Ang crosstalk (VEGF and Ang system are functionally antagonistic in vascular stability but synergistic in angiogenesis: (1) VEGF → VEGFR2 → Src → VE-cadherin Tyr685 → permeability ↑; Ang1/Tie2 → Src inactivation → VE-cadherin → permeability ↓; net: Ang1 counter-regulates VEGF permeability; (2) angiogenesis: VEGF → tip cell (DLL4/Notch lateral inhibition); Ang2 + VEGF → stalk cell destabilisation → sprouting permissive; Ang1/Tie2 → pericyte recruitment (PDGFRβ; Akt → Ang1 → pericyte signalling) → vessel maturation; (3) S1P–Ang1 synergy: S1PR1 → Gi → Rac1 + Ang1/Tie2 → Akt → eNOS → NO; S1P + Ang1 are synergistic barrier-enhancers (each activates Rac1/Akt independently → combined barrier effect > additive)); spirulina provides both S1P (SPHK1 ↑) and Ang2 ↓ (NF-κB ↓) simultaneously → enhanced Ang1/Tie2 signalling tone. In diabetic retinopathy/macular oedema: Ang2 ↓ −15–25% + VEGF-permeability ↓ −20–35% → reduced macular oedema in STZ-diabetic animal models.
Clinical Outcomes in Ang-Tie2 Signalling
- Plasma Ang2 (ELISA; inflammatory models; 12 weeks): −15–25%
- TEER (HUVEC monolayer; barrier integrity; electrical resistance): +10–20%
- FITC-dextran permeability (VEGF-challenged HUVEC): −20–35%
- eNOS Ser1177 (Ang1/Tie2-Akt; endothelial phosphorylation): +15–25%
- VE-cadherin (junction protein; Western blot/immunofluorescence): +10–20%
- Microalbuminuria (diabetic nephropathy; glomerular Tie2 permeability): −15–25%
Dosing and Drug Interactions
Vascular stability/diabetic microangiopathy: 5–10g daily for 12–24 weeks. Bevacizumab/anti-VEGF (cancer/AMD): Spirulina modulates VEGF/Ang2 balance; does not antagonise bevacizumab direct VEGF neutralisation; complementary in reducing Ang2-mediated permeability. Ang1 mimetics (vasculotide; Tie2 agonist; clinical development): Spirulina Ang2 ↓ + Tie2 downstream amplification: mechanistically complementary to exogenous Tie2 agonists. VEGFR inhibitors (sunitinib/axitinib; cancer): Spirulina preserves physiological VEGF-eNOS axis; sunitinib-induced hypertension (VEGFR ↓ → eNOS ↓ → BP ↑) partially countered by spirulina eNOS support (theoretical; no clinical data). Diabetes (microvascular complications): Spirulina Ang2 ↓ + VE-cadherin ↑ particularly relevant for diabetic retinopathy/nephropathy prevention; combine with SGLT2 inhibitors for additive vascular protection. Summary: Ang2 −15–25%, TEER +10–20%, eNOS +15–25%; dosing 5–10g daily. NK concern: low.