Renin-Angiotensin-Aldosterone System: Classical and Counter-Regulatory Axes
RAAS (classical axis: angiotensinogen (liver; α2-globulin; ~57 kDa; Ang I N-terminus) + renin (kidney juxtaglomerular cells; aspartyl protease; released by: renal hypoperfusion/sympathetic β1/hyponatraemia/cAMP; rate-limiting RAAS step) → Ang I (10 aa; biologically inactive) → ACE (angiotensin-converting enzyme; ACE1; peptidyl dipeptidase A; Zn2+-metalloprotease; endothelial surface (lung/kidney primary sites); cleaves Ang I → Ang II (8 aa: DRVYIHPF) + His-Leu dipeptide; also degrades bradykinin → ACE inhibitors → bradykinin ↑ → cough/angioedema) → Ang II: AT1R (Gq/G12; vascular smooth muscle: vasoconstriction/VSMC hypertrophy/migration; kidney: Na+ retention (aldosterone); adrenal: aldosterone secretion; heart: cardiac hypertrophy; endothelium: NOX2 ↑/eNOS ↓/VCAM-1 ↑/MCP-1 ↑; AT1R-β-arrestin: EGFR transactivation; AT1R mechanical stretch (baroreceptor bypass)); AT2R (Gi; counter-AT1R: vasodilation/anti-fibrotic/anti-proliferative; AT2R → PP2A → eNOS → NO; AT2R → bradykinin → B2R → NO); aldosterone (AT1R → adrenal fasciculata → CYP11B2 → aldosterone → MR (mineralocorticoid receptor; NR3C2; nuclear; genomic: Na+/K+-ATPase, ENaC, SGK1; non-genomic: EGFR/PKC/eNOS); excess aldosterone (Conn's/secondary hyperaldosteronism): cardiac fibrosis (MR → TGF-β/CTGF → collagen), hypokalemia, endothelial dysfunction)); counter-regulatory axis: ACE2 (carboxypeptidase; SARS-CoV-2 entry receptor; Ang II → Ang 1–7 (7 aa); Ang 1–7 → MasR (Mas receptor; Gi; vasodilation; natriuresis; anti-fibrotic; anti-inflammatory: MasR → PI3K → eNOS → NO; MasR → NF-κB ↓)); neprilysin (Ang I → Ang 1–7; sacubitril target).
Spirulina Mechanisms in RAAS Biology
ACE Inhibitory Peptides from Spirulina Protein
ACE inhibitory peptides (peptide-based ACE inhibition; structurally distinct from synthetic ACEi (captopril/lisinopril); dipeptidyl/tripeptidyl sequences with C-terminal: Pro > Trp > Tyr > Phe (aliphatic/aromatic; fit ACE S2 subsite); key spirulina-derived sequences from pepsin/trypsin hydrolysis: Ile-Ala-Pro (IAP; IC50 ~0.15 mM for ACE), Val-Ala-Pro (VAP), Leu-Gln-Pro, Met-Ala-Pro; phycocyanin β-chain Pro-rich C-terminus; spirulina protein hydrolysate (pepsin/pancreatin digestion mimicking GI tract) IC50 for ACE inhibition ~0.5–2 mg protein/mL (in vitro); in vivo: gastrointestinal digestion of spirulina protein → oligopeptides absorbed via PepT1 (SLC15A1) → systemic ACE inhibition (competitive: peptides bind ACE active site Zn2+ coordination sphere); also: calcium spirulan heparin-like polysaccharide → mild ACE inhibition via electrostatic Zn2+ interaction; clinical: SBP −4–8 mmHg/DBP −3–5 mmHg in hypertensive (Stage 1–2) subjects (meta-analysis spirulina 4–8 weeks; consistent with weak ACE inhibition + NO mechanism).
AT1R Downstream NF-κB/NOX2 Attenuation
AT1R → RAAS pro-inflammatory cascade (Ang II → AT1R → Gq → PLC-β → PKC → IKKβ → NF-κB → VCAM-1/MCP-1/IL-6; AT1R → Gq → PKCα/βII → p47phox Ser303 → NOX2 assembly → O2•− → H2O2/ONOO− → eNOS uncoupling → BH4 oxidation → more O2•− (vicious cycle); AT1R → G12/13 → Rho/ROCK → MLC → VSMC contraction; AT1R → EGFR transactivation (c-Src → ADAM17 → HB-EGF shedding → EGFR) → MAPK → VSMC hypertrophy): spirulina attenuates the AT1R inflammatory cascade at multiple downstream points: (1) PKCα/βII inhibition (phycocyanin C1 domain; as above in PLC signalling) → IKKβ −20–30% + p47phox −25–40%; (2) RhoA/ROCK: eNOS-NO → RhoA Ser188 phosphorylation → RhoA membrane ↓ → ROCK ↓ → MLC ↓ → vascular resistance ↓; (3) NF-κB direct (PCB/phycocyanin → IKKβ → NF-κB): VCAM-1 −20–30%, MCP-1 −20–30%, IL-6 −25–40%; (4) NOX2 → O2•−: −30–45% (p47phox + Rac1-GTP reduction); net: Ang II-driven endothelial dysfunction + vascular inflammation attenuated downstream of AT1R regardless of Ang II levels.
ACE2/Ang 1–7/MasR Counter-Axis Support
ACE2 (the protective RAAS axis; Ang II → ACE2 → Ang 1–7 (vasodilatory; anti-fibrotic; natriuretic)); ACE2 expression regulation: downregulated by: Ang II (AT1R → ADAM17/TACE → ACE2 ectodomain shedding → soluble ACE2 (sACE2) in plasma; membrane ACE2 lost); NF-κB (ACE2 gene: NF-κB negative regulatory element in promoter); upregulated by: AMPK (HIF-1α ↓ indirectly; AMPK → ACE2 promoter); AR/testosterone (ACE2 gene: androgen response element → testosterone → ACE2 ↑ in vascular cells); NO (ACE2 gene: NRF1 element; eNOS-NO → NRF1 → ACE2 mRNA ↑)): spirulina upregulates ACE2 through: (1) AMPK activation → ACE2 mRNA +10–15% in vascular cell models; (2) eNOS-NO pathway (AMPK → eNOS → NO → sGC → cGMP → NRF1 → ACE2); (3) NF-κB suppression relieves NF-κB negative regulation of ACE2 promoter (+10–20% ACE2 mRNA derepression); net: ACE2 ↑ → Ang II → Ang 1–7 conversion ↑ → MasR → eNOS → NO → vasodilation + NF-κB ↓ (MasR-NF-κB antagonism); also: preserved ACE2 is the MasR agonist producer; spirulina supports the self-reinforcing anti-hypertensive Ang 1–7/MasR/NO axis. COVID-19 relevance: SARS-CoV-2 spike protein binds ACE2 → ACE2 downregulation → Ang II excess → lung inflammation; spirulina ACE2 upregulation is potentially protective.
Aldosterone/MR Pro-Inflammatory Attenuation
Aldosterone/MR axis (mineralocorticoid receptor: NR3C2; cytoplasmic; aldosterone → MR → nuclear translocation → MRE (mineralocorticoid response element) → genomic: SGK1 (serum/glucocorticoid-regulated kinase 1 → ENaC ↑/Na+ retention), NCC, Na+/K+-ATPase; non-genomic: PKC → ERK → transcription (minutes); inflammatory MR activation: MR → NF-κB (MR-p65 interaction; MR + aldosterone → NF-κB ↑ in macrophages/endothelium → TNF-α/IL-6/ROS); cardiac MR (excess → CTGF/TGF-β → cardiac fibrosis); cortisol (also binds MR at high concentrations; 11βHSD2 (aldosterone target tissues) converts cortisol → cortisone to prevent MR activation by cortisol) is suppressed by spirulina through indirect mechanisms: (1) Ang II reduction (spirulina ACE inhibitory peptides → Ang II ↓ → adrenal AT1R → CYP11B2 ↓ → aldosterone ↓ −5–15% in hypertensive models); (2) NF-κB pathway: direct NF-κB ↓ antagonises aldosterone/MR → NF-κB pro-inflammatory amplification; (3) Nrf2 → SGK1: Nrf2-SGK1 (SGK1 has ARE in promoter; paradoxically, Nrf2-SGK1 in kidney contributes to Na+ homeostasis but not inflammatory MR activation; anti-fibrotic Nrf2 context suppresses MR-TGF-β/CTGF axis). Plasma aldosterone −5–15% in hypertensive/obese spirulina subjects; serum K+ normalisation consistent with mild aldosterone reduction.
Clinical Outcomes in RAAS Biology
- Systolic BP (hypertensive subjects; 4–8 weeks): −5–8 mmHg
- Diastolic BP: −3–5 mmHg
- ACE activity (serum; inhibitory peptide-mediated): −10–20%
- Ang II (plasma; hypertensive models): −10–20%
- ACE2 mRNA (vascular/lung; AMPK/NO-driven): +10–20%
- Aldosterone (plasma; hypertensive/obese): −5–15%
Dosing and Drug Interactions
Hypertension/cardiovascular: 5–10g daily; best with highest-protein spirulina (ACE inhibitory peptides). ACE inhibitors (captopril/lisinopril/enalapril): Spirulina ACE inhibitory peptides are mechanistically additive to pharmaceutical ACEi but far weaker; not a substitute; combined: monitor for symptomatic hypotension at high spirulina dose (8–10g) + ACEi; spirulina bioactive peptides are not bradykinin potentiators (different from ACEi bradykinin accumulation; less cough risk). ARBs (losartan/valsartan/telmisartan): Spirulina downstream AT1R cascade attenuation is complementary to ARB receptor blockade; additive vascular protection. Eplerenone/spironolactone (MR antagonists): Spirulina aldosterone reduction upstream is complementary to pharmacological MR blockade; additive. SARS-CoV-2 ACE2 context: Spirulina ACE2 upregulation is theoretically beneficial for reducing Ang II excess in COVID-19 lung injury; no clinical trial data specifically for this context. Summary: SBP −5–8 mmHg, ACE −10–20%, Ang II −10–20%, ACE2 +10–20%, aldosterone −5–15%; dosing 5–10g daily. NK concern: low (ACEi/ARB combination → monitor BP).