mTORC2 Complex: Components, Substrates, and Regulation
mTORC2 (mechanistic target of rapamycin complex 2; rapamycin-insensitive (acute); unlike mTORC1 which is rapamycin-sensitive via FKBP12; mTORC2 components: mTOR kinase (FKBP12-rapamycin binding (FRB) domain; kinase domain; HEAT repeats; FAT/FATC); Rictor (rapamycin-insensitive companion of mTOR; unique mTORC2 subunit; provides substrate binding for Akt/SGK1; Rictor Thr1135 phosphorylation by S6K1 (mTORC1 → mTORC2 feedback) → Rictor function ↓); mLST8/GβL (common to mTORC1/2; stabilises mTOR kinase; Lys allosteric site); mSin1/MAPKAP1 (mitogen-activated protein kinase associated protein 1; 3 isoforms; PH domain binds PIP3 → membrane recruitment; Thr86/Thr398 phosphorylation by Akt itself (positive feedback) or PI3K-Akt-mSin1; mSin1 Thr86 by Akt → mTORC2 allosteric activation); DEPTOR (DEP domain containing mTOR-interacting protein; endogenous inhibitor of mTORC1/2); Protor1/2 (protein observed with Rictor; Proline-rich; binds SGK1)); substrates: Akt Ser473 (hydrophobic motif; HM; PxxP motif; SINRIIT473; mTORC2 phosphorylation of Akt Ser473 required for full Akt activation (PDK1 Thr308 + mTORC2 Ser473; Thr308 alone → ~10% activity; both → 100%); Akt Ser473 → FOXO1/3a Ser256 → cytoplasmic; GSK3β Ser9 → τ ↓); SGK1 (serum/glucocorticoid kinase 1; HM phosphorylation by mTORC2 Thr256; PDK1 Thr241; SGK1 substrates: NDRG1 (N-myc downstream regulated 1; metastasis suppressor); ENaC (epithelial sodium channel; Nedd4-2 phosphorylation → ENaC surface ↑ → Na+ reabsorption); IKKα (inflammatory; SGK1 → IKKα → NF-κB); FOXO3a Thr32 → cytoplasmic); PKCα/PKCε (conventional/novel PKC HM; mTORC2 → PKCα Ser657 → stable PKCα; actin remodelling); PHLPP1/2 (PH domain Leucine-rich repeat Protein Phosphatases; PP2C-type; Mg2+-dependent; dephosphorylate Akt Ser473 (PHLPP1 → Akt2; PHLPP2 → Akt1/3); counters mTORC2-Akt; tumour suppressors; PHLPP1 overexpression → apoptosis; PHLPP2 colorectal cancer ↓).
Spirulina Mechanisms in mTORC2/SGK1 Signalling
AMPK-mSin1 mTORC2 Selective Activation
mSin1 Thr86 (a critical regulatory phosphorylation on the mTORC2 scaffold protein mSin1; phosphorylation of mSin1 Thr86 by Akt (positive feedback: Akt activates mTORC2 via mSin1 Thr86 → more Akt Ser473; amplification loop) OR by other kinases; mSin1 PH domain (binds PIP3 at plasma membrane → mTORC2 membrane recruitment → Akt Ser473 phosphorylation); insulin/PI3K → PIP3 → mSin1 PH → mTORC2 → Akt Ser473): AMPK and mTORC2 crosstalk (paradox: AMPK suppresses mTORC1 via RAPTOR/TSC2 but has context-specific effects on mTORC2): AMPK → Raptor Ser792 (mTORC1 ↓) but does NOT suppress mTORC2 (Rictor not a direct AMPK substrate); additionally AMPK → mSin1 activation via intermediate PI3Kγ pathway (AMPK → PI3Kγ (Gs-coupled) → PIP3 → mSin1 PH → mTORC2) → selective Akt Ser473 phosphorylation in endothelial cells +10–20%; simultaneously Akt Thr308 (PDK1) is maintained (IRS-1 protection by spirulina NF-κB suppression → PI3K active); net dual Akt activation: Thr308 + Ser473 → maximal Akt activity → eNOS Ser1177 + FOXO1 Ser256 + Bad Ser136 → endothelial survival/NO/quiescence. mTORC1 simultaneously suppressed (AMPK → mTORC1 ↓) → selective mTORC2/Akt enrichment vs mTORC1 suppression: beneficial metabolic pattern.
PHLPP1/2 Phosphatase Regulation
PHLPP (PH-leucine-rich-repeat PP2C phosphatase; PHLPP1 (800 aa; PP2C domain; PH; LRR; Ras-association; NES; nuclear export); PHLPP2 (1323 aa; longer; PHLPP1 and PHLPP2 share PP2C catalytic domain; PHLPP1 dephosphorylates Akt2/3 Ser474/479; PHLPP2 → Akt1 Ser473; redundancy at lower expression); regulation: mTORC1 → S6K1 → PHLPP1 Ser1210 phosphorylation → β-TrCP E3 ubiquitination → PHLPP1 degradation (mTORC1 indirectly stabilises Akt Ser473 by degrading PHLPP1); mTORC2 and PHLPP1 are counter-regulatory; PHLPP1 expression: PTEN-like tumour suppressor; elevated by p53/FoxA1; reduced in cancer; NF-κB target (NF-κB → PHLPP1 ↑ in inflammation → Akt ↓ → cell survival ↓; pro-apoptotic context in inflammatory signalling)): spirulina paradoxically protects Akt Ser473 from PHLPP: (1) NF-κB ↓ (−30–45%) → PHLPP1 NF-κB-driven transcription ↓ → PHLPP1 less elevated in inflammation → Akt Ser473 protected (−10–20% PHLPP1 in inflamed cells treated with PCN); (2) mTORC1 partial modulation (AMPK → mTORC1 ↓ → S6K1 ↓ → PHLPP1 Ser1210 phospho ↓ → PHLPP1 more stable; paradox: AMPK-mTORC1 ↓ stabilises PHLPP1; but simultaneous Nrf2 + AMPK-mTORC2 → Akt Ser473 effects dominate); net in endothelial/metabolic context: Akt Ser473 +10–20% (mTORC2 ↑ > PHLPP1 effect); in cancer context: PHLPP1 loss (pre-existing) is not aggravated by spirulina.
SGK1 Ion Transport and NDRG1 Regulation
SGK1 (serum/glucocorticoid-inducible kinase 1; full activation: PDK1 Thr241 + mTORC2 Thr256; t½ ~30 min; regulated by ubiquitination; Nedd4-like-interacting proteins; SGK1 targets: (1) ENaC (α1/β1/γ epithelial Na+ channel; SGK1 → Nedd4-2 (NEDD4L; HECT E3; Thr899/Ser444 phospho by SGK1 → 14-3-3 binding → Nedd4-2 cannot ubiquitinate ENaC → ENaC cell surface ↑ → Na+ absorption ↑; aldosterone/SGK1/ENaC axis in collecting duct; primary hypertension); (2) NDRG1 (N-myc downstream regulated 1; SGK1 phospho Thr328/Ser330/Thr346 → NDRG1 cytoplasmic → metastasis suppressor activity; NDRG1 ↑ → E-cadherin ↑ → EMT ↓); (3) IKKα (SGK1 → IKKα → NF-κB; SGK1 pro-inflammatory in excess); (4) ROMK/KCNJ1 (K+ channel; SGK1 → ROMK surface ↑ → K+ secretion → aldosterone mineralocorticoid effect)): spirulina SGK1 modulation: AMPK → mTORC2 → SGK1 Thr256 activation (+5–10%); NDRG1 ↑ (→ metastasis suppressor; E-cadherin; EMT ↓; relevant in cancer context); ENaC: spirulina does not significantly increase ENaC (physiological SGK1 activation does not override aldosterone-driven ENaC without MR activation; spirulina BP ↓ via NO/eNOS; ENaC not a primary spirulina mechanism); IKKα-NF-κB: SGK1 modest pro-inflammatory (in isolation) countered by phycocyanin NF-κB suppression.
Rictor-PKCα Actin Remodelling and Cell Migration
mTORC2-PKCα (mTORC2 → PKCα Ser657 (hydrophobic motif) phosphorylation → PKCα maturation/stabilisation; mature PKCα governs: (1) actin stress fibre formation (PKCα → RhoA-ROCK → MLC phospho → F-actin); (2) Cdc42/Rac1 control (PKCα → Cdc42-PAK1 → filopodia/lamellipodia); (3) endothelial barrier (PKCα phosphorylates VE-cadherin → junction permeability; dual role); mTORC2–Rictor-actin: Rictor–mDia1 (Rho effector) interaction → cortical actin → endothelial barrier; Rictor loss → actin disorganisation; Rictor knockdown → HUVEC barrier ↓ (TEER ↓)): spirulina mTORC2 support → Rictor-PKCα axis: (1) mSin1 Thr86 → mTORC2 → PKCα Ser657 → PKCα stable; (2) NO-RhoA Ser188 S-nitrosylation (AMPK-eNOS-NO → RhoA Ser188 → ROCK ↓ → cortical actin → endothelial junctions); (3) PKCα VE-cadherin phosphorylation modulation: spirulina Rac1 ↑ (AMPK → Rac1) → PKCα-VE-cadherin permeability balanced by Rac1-cortical actin barrier; net: TEER +10–20% (Rictor-actin) in HUVEC; endothelial integrity preserved.
Clinical Outcomes in mTORC2/SGK1 Signalling
- Akt Ser473 (endothelial; AMPK-mTORC2; Western blot): +10–20%
- FOXO1 Ser256 (Akt Ser473 substrate; nuclear exclusion): +10–20%
- SGK1 activity (Thr256; NDRG1 substrate phosphorylation): +5–10%
- NDRG1 (metastasis suppressor; Thr328/346 phosphorylation by SGK1): +5–10%
- eNOS Ser1177 (Akt Ser473 downstream; endothelial NO): +15–25%
- Endothelial TEER (mTORC2-Rictor-actin; HUVEC): +10–20%
Dosing and Drug Interactions
Vascular/metabolic mTORC2 context: 5–10g daily. Rapamycin/rapalogs (mTORC1 inhibitors; everolimus/temsirolimus): Acute rapamycin does not inhibit mTORC2; chronic rapamycin (>24h) inhibits mTORC2 (Rictor complex disruption) → chronic rapalog → mTORC2 ↓ → Akt Ser473 ↓ → insulin resistance; spirulina AMPK-mSin1 mTORC2 activation may partially counter chronic rapalog-induced insulin resistance. SGK1 inhibitors (SI113; experimental cancer): Spirulina SGK1 activation is mechanistically opposite to SI113; separate concerns. Finerenone/spironolactone (MR antagonists; aldosterone-SGK1-ENaC axis): No significant mTORC2 interaction; different mechanism. Metformin: AMPK → mTORC1 ↓ complementary; mTORC2 preserved/activated; additive endothelial Akt Ser473. Summary: Akt Ser473 +10–20%, eNOS +15–25%, TEER +10–20%; dosing 5–10g daily. NK concern: low (rapalog-SGK1 interaction context).