Spirulina and Angiogenesis: VEGF, HIF-1alpha, DLL4/Notch, and Vascular Sprouting

HIF-1alpha-VEGF: The Hypoxic Angiogenic Axis

Under hypoxia (pO2 below ~5%), PHD1/2/3 (EGLN1/2/3) are inactive (require O2 as co-substrate with alpha-ketoglutarate), allowing HIF-1alpha to accumulate, dimerize with ARNT/HIF-1beta, and transactivate the VEGF-A promoter (HRE: RCGTG). VEGF-A165 (primary isoform) binds VEGFR2 (KDR/FLK1), activating PLCgamma/DAG/IP3 (Ca2+/PKC), PI3K/Akt/eNOS (NO for vasodilation), MAPK/ERK (proliferation), and Src/FAK/paxillin (migration). Spirulina reduces normoxic HIF-1alpha stabilisation (via Nrf2/NF-kB suppression, reducing HIF-1alpha transcription/stabilisation), creating a complex context-dependent effect: protective against abnormal VEGF production in inflammatory disease, but potentially preserving physiological VEGF in ischaemia.

VEGFR2 Signalling: PI3K/Akt/eNOS

VEGFR2 kinase domain transphosphorylates Y1054/1059 (activation loop), Y951 (TSAd/ Src binding), Y1175 (PLCgamma/Shb/PI3K binding), and Y1214 (Nck/Crk). The Y1175/PI3K arm activates PDK1/Akt Thr308, then eNOS Ser1177 phosphorylation for NO production driving vasodilation. Akt also phosphorylates FOXO1 Thr24/Ser256 (cytoplasmic retention), suppressing anti-angiogenic FOXO1 targets. Spirulina's AMPK activation opposes VEGFR2/Akt signalling at the mTORC2-Akt Ser473 level, while independently supporting eNOS via AMPK-eNOS Ser1177 phosphorylation, maintaining vasodilatory capacity without the proliferative Akt arm.

DLL4-Notch: Tip and Stalk Cell Specification

During vessel sprouting, high VEGF-VEGFR2 signalling induces DLL4 (Delta-like 4) in tip cells. DLL4 activates Notch1/4 in adjacent cells (stalk cells), suppressing VEGFR2 and inducing VEGFR1 (a decoy receptor, sequestering VEGF), restricting stalk cell sprouting and maintaining a single tip per sprout. Disrupting DLL4-Notch (with anti-DLL4 antibodies) causes hypersprouting but non-functional vessels. This is relevant to spirulina's Notch signalling context: PCB-mediated Notch modulation (Notch post-translational O-fucosylation/Fringe glycosylation) may influence DLL4- Notch strength in endothelial cells during angiogenesis.

Angiopoietin-Tie2 Axis: Vascular Stabilisation

Angiopoietin-1 (ANGPT1, from pericytes/smooth muscle) binds Tie2/TEK on endothelium, activating PI3K/Akt/NF-kB to promote vessel stabilisation, pericyte recruitment, and barrier function. Angiopoietin-2 (ANGPT2, from Weibel-Palade bodies, stored in ECs) competes with ANGPT1, acting as a context-dependent agonist/antagonist. High ANGPT2 during inflammation promotes vessel destabilisation, vascular leak, and inflammatory cell extravasation. NF-kB induces ANGPT2 expression. Spirulina's NF-kB suppression reduces ANGPT2 expression, tipping the ANGPT1/ANGPT2 balance toward vessel stabilisation, consistent with reduced vascular permeability in inflammatory models.

AMPK and eNOS-Driven Vasculogenesis

AMPK directly phosphorylates eNOS Ser1177 (activating), providing VEGF-independent NO production that promotes endothelial survival, vessel tone, and endothelial progenitor cell (EPC) mobilisation. EPCs (CD34+/CD133+/KDR+) home to ischaemic tissues to contribute to vasculogenesis. AMPK also phosphorylates endothelial Beclin-1 and activates autophagy in EPCs, promoting their survival during engraftment. Spirulina's AMPK-eNOS axis thus supports the vasculogenic dimension of vascular repair without requiring HIF-1alpha-VEGF inflammatory activation.

Spirulina and Retinal Angiogenesis

In oxygen-induced retinopathy (OIR) and streptozotocin diabetic retinopathy models, spirulina reduces neovascularisation markers (VEGF, CD31 vessel density, ANGPT2) while preserving normal retinal vasculature. The dual mechanism, Nrf2-driven retinal ganglion cell protection and NF-kB/HIF-1alpha suppression of pathological neovascularisation, positions spirulina as relevant to diabetic retinopathy, age-related macular degeneration, and retinopathy of prematurity in preclinical contexts.