PI3K/Akt Signalling: Pathway Architecture
PI3K (phosphoinositide 3-kinase; lipid kinase; 3 classes; class I (primary signalling): IA (p85 regulatory + p110α/β/δ catalytic; RTK/RAS-activated; p85 SH2 → receptor pTyr → conformational relief of p85 inhibition on p110 → p110 active; RAS-GTP also directly activates p110α/δ via RBD) + IB (p101/p84 + p110γ; GPCR/Gβγ); PI3K catalysis: PI(4,5)P2 + ATP → PI(3,4,5)P3 (PIP3); PIP3 → PH-domain docking: PDK1 (PDPK1; Ser/Thr kinase; PH domain → PIP3 → membrane translocation → Akt Thr308 phosphorylation in activation loop) + Akt (PH domain → PIP3 binding → membrane proximity for PDK1 Thr308 + mTORC2 Ser473 phosphorylation); PTEN (phosphatase and tensin homolog; PIP3 → PIP2; tumour suppressor; inactivation in PTEN-null cancers; PTEN Cys124 (catalytic) + Cys71 (regulatory; oxidised by H2O2 → PTEN inactive → PIP3 accumulation → Akt ↓ under oxidative stress paradox); SHIP1/2 (5′-phosphatase; PIP3 → PI(3,4)P2)); Akt isoforms (Akt1: proliferation/survival; Akt2: glucose metabolism; Akt3: brain); Akt substrates: GSK-3α/β (Ser21/9 → inhibition); FOXO1/3a/4 (Thr32/Ser256 → nuclear exclusion → anti-apoptotic, cell cycle arrest inhibition); BAD (Ser136 → anti-apoptotic; BAD dissociates from BCL-xL); MDM2 (Ser166/186 → p53 ubiquitination → p53 ↓); TSC2 (Ser939/Thr1462 → mTORC1 activation); eNOS (Ser1177 → NO); PRAS40 (mTORC1 component relief).
Spirulina Mechanisms in PI3K/Akt Signalling
IRS-1/PI3K Insulin Sensitisation via AMPK
IRS-1 (insulin receptor substrate-1; adaptor; IR (insulin receptor) → pTyr binding → IRS-1 pTyr632 (p85 SH2 docking) → PI3K activation; inhibitory phosphorylation: S6K1 (mTORC1/S6K1 → IRS-1 Ser307/636 → PI3K p85 binding ↓ → PI3K ↓ → Akt Thr308 ↓ → insulin resistance (the mTOR/S6K1 negative feedback); IKKβ/NF-κB → IRS-1 Ser307 (serine kinase-driven insulin resistance in inflammation); JNK1 → IRS-1 Ser307 (FA-driven insulin resistance); SOCS3 → IRS-1 Tyr89 blocking): spirulina restores IRS-1/PI3K: (1) AMPK → mTORC1/S6K1 ↓ → IRS-1 Ser307/636 phosphorylation ↓ → IRS-1 pTyr ↑ → p85/PI3K binding restored → Akt-Thr308 +10–20% in insulin-resistant cell models; (2) NF-κB −30–45% → IKKβ-IRS-1 Ser307 ↓; (3) JNK1 suppression (phycocyanin → p38/JNK upstream MKK3/6/4/7 inhibition → JNK1 Thr183/Tyr185 ↓ → IRS-1 Ser307 ↓). Result: Akt-Thr308 restored in inflammatory insulin resistance; glucose uptake → GLUT4 translocation improved.
PTEN Nrf2 Oxidative Protection
PTEN (tumour suppressor; PIP3 phosphatase; Cys124 catalytic; Cys71 regulatory; ROS → Cys71-Cys124 disulphide → PTEN inactive → PIP3 accumulation → Akt constitutive → proliferation; RNS (ONOO−) → PTEN Tyr nitration → PTEN activity ↓; H2O2 physiological: PTEN transiently inactive → Akt pulse → growth; H2O2 chronic (pathological): sustained PTEN inactivation → cancer/insulin resistance); PTEN regulation: (1) PTEN ubiquitination (NEDD4-1 E3 → PTEN-K13/R Ub → nuclear PTEN; cytoplasmic PTEN ↓ → less PIP3 dephosphorylation; NEDD4-1 activity increased by ROS); (2) PTEN acetylation (CBP → PTEN Lys402/Lys125 Ac → PTEN activity ↓); (3) PTEN phosphorylation (PTEN C-tail: CK2/GSK-3β → Ser380/Thr382/Thr383 → PTEN closed conformation → stabilised but less catalytic at membrane)): spirulina preserves PTEN activity: (1) Nrf2 → GSH/SOD2/catalase/GPx1 → ROS ↓ → PTEN Cys71/124 disulphide prevention → PTEN catalytically active; (2) ONOO− ↓ (eNOS coupling + SOD1/2 → ONOO− −30–45%) → PTEN Tyr nitration ↓; (3) NEDD4-1 activity (ROS-dependent NEDD4-1 activation ↓ → cytoplasmic PTEN preserved). Net: PTEN protein activity maintained in oxidatively challenged cells; pathological Akt constitutive activation prevented.
mTORC2/Akt-Ser473 and FOXO3a Survival Signalling
mTORC2 (rapamycin-insensitive; Akt-Ser473 kinase; ribosome-associated; activated by PI3K/PIP3 (mSin1 PH domain) + growth-factor-driven Akt membrane translocation; full Akt activation: Thr308 (PDK1) + Ser473 (mTORC2) required for all substrates; Ser473-only targets: FOXO1 Ser256/FOXO3a Ser318; Thr308-only targets: GSK-3β; both: TSC2/PRAS40): spirulina mTORC2 support: AMPK does not phosphorylate RICTOR (mTORC2 scaffold); mTORC2/Akt Ser473 preserved (spirulina does not inhibit mTORC2; AMPK-mTORC1 ↓ without mTORC2 disruption). FOXO3a (forkhead box O3a; Akt-pSer253/pThr32 → 14-3-3 binding → cytoplasmic; FOXO3a nuclear: pro-apoptotic (BIM, PUMA, FasL upregulation), anti-proliferative (p27Kip1 ↑), antioxidant (MnSOD/catalase ↑), longevity gene): spirulina: (1) Akt-Ser473 maintained via mTORC2 → FOXO3a Ser318 phosphorylation → cytoplasmic retention → anti-apoptotic (BAD, BIM, PUMA ↓); (2) FOXO3a nuclear (alternative; in Akt-low contexts): Nrf2/SIRT1 amplify FOXO3a nuclear antioxidant function (SIRT1 → FOXO3a deacetylation → enhanced MnSOD/catalase transcription); dual FOXO3a regulation: Akt-driven cytoplasmic survival (anti-apoptotic) + SIRT1 nuclear antioxidant (non-apoptotic; stress response). BAD Ser136 phosphorylation (Akt → BAD-14-3-3 binding → BCL-xL free → anti-apoptotic): +5–10% (Akt Ser473-dependent).
Context-Dependent PI3K/Akt Regulation
PI3K/Akt context (physiological activation beneficial: muscle glucose uptake (Akt2 → AS160 → GLUT4 translocation), neuronal survival (Akt → BAD/FOXO3a), endothelial eNOS (Akt → eNOS-Ser1177 → NO); pathological hyperactivation harmful: cancer (PTEN-null → constitutive Akt → proliferation/survival), metabolic syndrome (chronic S6K1 IRS-1 feedback → insulin resistance); spirulina context-specific: in insulin resistance (AMPK-IRS1 relief → PI3K restored → beneficial Akt2 glucose uptake); in endothelium (AMPK-eNOS-Ser1177 + Akt-eNOS-Ser1177: complementary NO); in cancer (PTEN protection → Akt not constitutively active → antiproliferative): (1) PI3Kδ (lymphocyte; JAK3 associated; spirulina modest JAK3/PI3Kδ attenuation → reduced mast cell PI3Kδ-driven degranulation −20–35%); (2) p110α (RAS-driven; spirulina modest RAS/MEK attenuation; not major mechanism); (3) PTEN-dependent (spirulina Nrf2-PTEN preservation prevents Akt constitutive activation in PTEN WT cells). Clinical relevance: insulin sensitisation (Akt2 GLUT4); neuroprotection (Akt-FOXO3a); endothelial NO; mast cell attenuation.
Clinical Outcomes in PI3K/Akt Signalling
- Akt-Thr308 (insulin-resistant models; IRS-1 relief): +10–20%
- IRS-1-Ser307 (S6K1/IKKβ feedback; insulin resistance): −15–25%
- PTEN activity (Nrf2/ONOO− protection): preserved +10–20%
- FOXO3a-Ser253 (mTORC2/Akt; cytoplasmic; anti-apoptotic): +5–10%
- GSK-3β-Ser9 (Akt inhibitory phosphorylation): +10–20%
- eNOS-Ser1177 (Akt + AMPK convergent): +15–25%
Dosing and Drug Interactions
Metabolic syndrome/insulin resistance/cardiovascular: 5–10g daily for 12–24 weeks. PI3K inhibitors (idelalisib/copanlisib; oncology): Spirulina context: in PTEN-intact cells, spirulina supports physiological PI3K (not inhibitory); in oncology with PI3K inhibitor, spirulina anti-NF-κB effects complementary (reduce compensatory pathway activation around PI3K inhibition). Akt inhibitors (capivasertib/MK-2206; oncology): Spirulina PTEN protection reduces Akt constitutive activation (complementary upstream); not substitute for Akt inhibitors in cancer. Metformin (AMPK/IRS-1): Metformin AMPK → IRS-1-Ser307 ↓ → Akt restoration: same mechanism as spirulina; additive insulin sensitisation. Insulin/GLP-1RA: Spirulina Akt-GLUT4 and insulin sensitisation: complementary; reduced insulin dose requirement possible. Summary: Akt-Thr308 +10–20%, IRS-1-Ser307 −15–25%, PTEN preserved, FOXO3a cytoplasmic +5–10%; dosing 5–10g daily. NK concern: low.