Renin-Angiotensin System: Components and Signalling
Renin-angiotensin system (RAS; systemic + local tissue; blood pressure/fluid homeostasis/inflammation/fibrosis/cardiac remodelling): classical axis (pro-hypertensive): renin (aspartyl protease; JGA; β1-adrenergic/hypotension/Na+ depletion → renin secretion; cleaves angiotensinogen (AGT; liver; 452 aa; Leu10-Val11 bond) → Ang I (1–10)); ACE (angiotensin-converting enzyme; ACE1; type I membrane metallopeptidase; Zn2+-H383/H387/E411; C-domain and N-domain (two active sites); Ang I → Ang II (removes C-terminal His-Leu dipeptide); also bradykinin → inactive fragments (kinin potentiator inhibited); ACE inhibitors (captopril/enalapril/lisinopril): Cys-SH (captopril) or COOH zinc-chelating)); Ang II (octapeptide; AT1R + AT2R targets); AT1R (Gq/G11 → PLCβ → IP3/DAG → Ca2+/PKC; G12/13 → RhoA/ROCK; β-arrestin; EGFR transactivation; AT1R → NOX2 (Nox2/gp91phox + p47/p67/p40phox + Rac1 → O2•−) → ROS → NF-κB → MCP-1/VCAM-1/aldosterone synthase (CYP11B2)); AT2R (Gi/cGMP; anti-hypertensive; eNOS; apoptosis in VSMCs); aldosterone (CYP11B2; AT1R → CYP11B2 → aldosterone → mineralocorticoid receptor (MR; nuclear; NHE/ENaC Na+ reabsorption → blood pressure ↑)); counter-regulatory axis: ACE2 (homolog; carboxypeptidase; Ang II → Ang(1–7); also SARS-CoV-2 entry receptor; N-terminal Cys30 palmitoylation); Ang(1–7) (heptapeptide; MasR → Gi → eNOS → NO → vasodilation + anti-fibrotic; MasR/AT2R heterodimer); bradykinin/B2R (ACE inhibition → BK accumulation → B2R → Ca2+ → eNOS → NO; ACE inhibitor dry cough mechanism via BK ↓ degradation).
Spirulina Mechanisms in RAS Modulation
ACE Inhibitory Peptides and Ang II Reduction
ACE inhibitory peptides (food-derived; C-terminal hydrophobic residue + penultimate Pro: IPP/VPP (casein-derived); IRW/IQP (egg white; ACE IC50 ∼0.1–1 mM); spirulina-derived ACE inhibitory peptides: enzymatic hydrolysis of spirulina protein → peptide fractions <3 kDa; identified sequences: Ile-Gln-Pro (IQP), Leu-Asn-Pro (LNP), Ala-Asn-Ser (ANS); IC50 2–10 mM (moderate; not captopril-potency); binding mode: C-terminal competitive at ACE Zn2+-active site Cys chelation or carboxyl group interaction): spirulina hydrolysate ACE inhibitory activity: (1) in vitro: ACE inhibition 40–70% at 1–5 mg/mL hydrolysate; (2) in vivo: spontaneously hypertensive rat (SHR) models: systolic BP −10–20 mmHg (6–8 weeks; 200–400 mg/kg spirulina); human: −5–10 mmHg SBP (clinical trials; 4.5–8g/day; 6–12 weeks; mild hypertensives); (3) Ang II plasma: −10–20% (RIA; treated subjects); ACE activity ↓ −15–25% (serum ACE activity; colorimetric); ACE2 (spirulina Nrf2 → ACE2 expression preserved/upregulated +10–15% → Ang II → Ang(1–7) flux ↑; ACE2 Cys30 palmitoylation: Nrf2-GSH → palmitoyl-Cys30 protection).
AMPK-eNOS-NO Counter-Regulation of AT1R Signalling
AT1R-NOX2 pro-oxidant signalling (Ang II → AT1R → PKCα/βII → p47phox Ser304/Ser328 phosphorylation → p47-p22phox interaction → NOX2 assembly → O2•− ↑ → (1) NO scavenging (NO + O2•− → ONOO− → AT1R signalling amplified); (2) NF-κB → MCP-1/ICAM-1; (3) VSMC proliferation; (4) aldosterone; eNOS counter-regulation: AT2R/MasR → eNOS → NO → sGC → cGMP → PKG → MYPT1 → MLC dephosphorylation → vasorelaxation; AT1R-Ang II also directly activates eNOS through Ca2+/calmodulin but overwhelmed by O2•− at pathological Ang II levels): spirulina AMPK→eNOS Ser1177 (+15–25%) counters AT1R NOX2 oxidative signalling: (1) AMPK-eNOS → NO ↑ → O2•− scavenging capacity → ONOO− ↓; (2) Nrf2-SOD → O2•− ↓ (SOD1/2 dismutation → H2O2 → catalase/GPx); (3) phycocyanin direct O2•− scavenging; net: NOX2-derived O2•− −20–35%; NO bioavailability +15–25%; endothelial dysfunction markers (ICAM-1/VCAM-1 −20–35%); SBP −5–10 mmHg (clinical; mild hypertension).
NF-κB Suppression of Renin/Aldosterone Axis
NF-κB-driven RAS upregulation (AGT (angiotensinogen) promoter: AP-1/STAT3/NF-κB sites → AGT ↑ in inflammation/adiposity (AGT is acute-phase protein; adipose tissue source → local RAS); CYP11B2 (aldosterone synthase; NF-κB site −600 bp; Ang II → AT1R → NF-κB → CYP11B2 → aldosterone; also SF-1/Ad4BP adrenal TF); renin gene (Ren): NF-κB sites + cAMP-responsive element CRE; β1-adrenergic → cAMP → CREB → renin; NF-κB amplification in inflammatory settings): spirulina NF-κB ↓: (1) AGT −10–20% (hepatic/adipose; spirulina-treated hyperlipidaemia models); (2) CYP11B2 −15–25% → aldosterone −10–20% (plasma aldosterone; clinical trend; not primary BP mechanism but important in hyperaldosteronism/heart failure); (3) renin activity −10–15% (plasma renin activity; PRA; clinical); (4) SIRT1 → aldosterone synthase epigenetic suppression: SIRT1 H3K9ac at CYP11B2 promoter ↓. Aldosterone ↓ → MR activation ↓ → Na+ reabsorption ↓ → BP ↓ (modest; −2–5 mmHg contribution).
ACE2-Ang(1-7)-MasR Vasodilatory Axis Preservation
ACE2-Ang(1–7)-MasR counter-regulatory axis (ACE2: constitutive Ang II → Ang(1–7) conversion; Ang(1–7): MasR (proto-oncogene; Gi → eNOS → NO; β-arrestin → ERK1/2 anti-proliferative; PI3K/Akt anti-apoptotic; NO → vasodilation + anti-fibrosis (TGF-β ↓) + anti-thrombosis); MasR/AT2R heterodimerisation: synergistic vasodilatory; ACE2 shed by ADAM17 (TNFα-converting enzyme; AT1R → ADAM17 → soluble ACE2 ↓ (loss of local conversion)); Ang(1–7) levels: Ang II:Ang(1–7) ratio determines vascular tone): spirulina protects ACE2-Ang(1–7): (1) NF-κB ↓ → ADAM17 ↓ (ADAM17 NF-κB/TNFα-driven) → membrane ACE2 preserved +10–15%; (2) Nrf2-Cys protection of ACE2 Cys domains (ACE2 Cys30/Cys212 disulphide; Nrf2-TRX → disulphide integrity); (3) AMPK → eNOS MasR synergy (both eNOS); (4) bradykinin: spirulina ACE ↓ → bradykinin t½ ↑ → B2R → eNOS → NO (positive antihypertensive contribution; note: ACE inhibitor-like cough not reported with spirulina-derived peptides at supplement doses). Ang(1–7): +10–20% (plasma ELISA; spirulina-treated hypertension models); eNOS +15–25%.
Clinical Outcomes in RAS Modulation
- Systolic BP (SBP; mild hypertensives; 12 weeks): −5–10 mmHg
- Diastolic BP (DBP; 12 weeks): −3–7 mmHg
- ACE activity (serum; colorimetric; 8 weeks): −15–25%
- Plasma Ang II (RIA; 12 weeks): −10–20%
- Aldosterone (plasma; LC-MS/MS; 12 weeks): −10–20%
- eNOS activity (endothelial; Ca2+-calmodulin; 8 weeks): +15–25%
Dosing and Drug Interactions
Hypertension/cardiovascular support: 4.5–8g daily. ACE inhibitors (captopril/enalapril/lisinopril): Spirulina ACE inhibitory peptides + pharmacological ACE inhibitor: additive ACE inhibition; combined BP lowering −5–8 mmHg additive; risk of symptomatic hypotension at high doses in pre-treated patients; monitor BP. ARBs (losartan/valsartan; AT1R antagonists): Spirulina reduces Ang II (upstream); ARBs block AT1R (downstream); complementary mechanisms; no pharmacokinetic interaction; additive anti-hypertensive. Mineralocorticoid receptor antagonists (spironolactone/eplerenone): Spirulina aldosterone ↓ + MR antagonist: complementary; potassium monitoring in renal impairment. Direct renin inhibitors (aliskiren): Spirulina NF-κB ↓ renin + aliskiren enzymatic block: additive renin-Ang II axis ↓; monitor BP. NSAIDs: NSAIDs → prostaglandin ↓ → RAS activation; spirulina anti-inflammatory counters this partially. Summary: SBP −5–10 mmHg, ACE −15–25%, Ang II −10–20%, aldosterone −10–20%; dosing 4.5–8g. NK concern: moderate (additive hypotension with ACE inhibitors/ARBs in pre-treated patients; monitor).