Succinate-Itaconate Immunometabolism: TCA Reprogramming in Macrophages
Macrophage immunometabolism (the discovery that metabolic reprogramming is not a consequence but a driver of immune function; Warburg effect in immune cells: LPS/TLR4-activated M1 macrophages → glycolytic shift (lactate; IL-1β); two TCA breaks in M1 metabolism: (1) isocitrate dehydrogenase (IDH) → citrate accumulates (exported to cytoplasm → ACLY → acetyl-CoA → fatty acid synthesis for membrane expansion; citrate → aconitate → IRG1/ACOD1 → itaconate); (2) SDH (succinate dehydrogenase / Complex II; fumarate→fumarate inhibition OR reverse SDH (fumarate→succinate under hypoxia); succinate accumulates in M1 macrophages (from: glutamine/α-KG via GABA shunt or reverse SDH; succinate ~30-fold ↑ in LPS macrophages); succinate (1) escapes mitochondria → plasma → SUCNR1 (GPR91; succinate receptor; Gs ↓ Gi ↑ at high succinate; macrophage/DC SUCNR1 → IL-1β/TNF-α amplification; SUCNR1 on DCs → danger signal paracrine); (2) intracellular: SDH substrate → reverse electron transport (RET) at Complex I → O2•− → ROS → HIF-1α prolyl hydroxylase (PHD2) inhibition (succinate + 2-OG competition at PHD2 Fe2+ centre) → HIF-1α stabilisation → HIF-1α → IL-1β/VEGF/PDK1 (lactate); itaconate (cis-aconitate → IRG1/ACOD1 (immune-responsive gene 1; induced 30–100× by LPS); itaconate: anti-inflammatory; (1) SDH inhibitor (succinate-competitive; IC50 ~0.5–1 mM; reduces succinate accumulation); (2) Keap1 alkylation: itaconate → Cys151/Cys273/Cys288 Keap1 modification → Nrf2 derepression → HO-1/NQO1 → anti-inflammatory; (3) IkB-ζ (IKBZ) inhibition (itaconate alkylates IkB-ζ Cys → IKBZ protein ↓ → IL-6/IL-12 secondary transcription ↓); (4) A20 (TNFAIP3) → NF-κB deubiquitinase derepression; dimethyl itaconate (DMI) → 4-octyl itaconate (4-OI) cell-permeable analogues used experimentally).
Spirulina Mechanisms in Succinate/Itaconate Immunometabolism
AMPK Attenuation of Succinate Accumulation
Succinate accumulation in M1 macrophages (critical inflammatory metabolite; fuelled by: LPS → TLR4 → glycolysis → pyruvate → TCA intermediates; glutamine → α-KG → GABA shunt → succinate (bypassing succinyl-CoA); reverse SDH under transient mitophagy/hypoxia; SDH expression: SDHA/B/C/D subunits; SDHB is tumour suppressor; LPS does not significantly increase SDH expression but metabolic flux changes → succinate ↑): AMPK modulates this by: (1) AMPK → LKB1 → reduced mTORC1 → less HIF-1α translational upregulation (mTORC1 → 4E-BP1 → HIF-1α mRNA cap-dependent translation); (2) AMPK → reduced glycolytic flux (AMPK → PFKFB3 Ser461 phosphorylation → PFK-2 ↓ → fructose-2,6-bisphosphate ↓ → PFK-1 ↓ → glycolysis ↓ → pyruvate ↓ → less TCA intermediate loading via PDC); (3) AMPK → mitochondrial biogenesis → re-establishing OXPHOS capacity → normalising TCA flux (reducing succinate bottleneck); spirulina AMPK activation (phycocyanin) → succinate:fumarate ratio −15–25% in LPS macrophage models.
Nrf2 Enhancement of Itaconate Circuit
Itaconate-Nrf2 axis (the endogenous anti-inflammatory brake; IRG1/ACOD1 (aconitate decarboxylase 1; mitochondrial; aconitate → itaconate; maximally induced 30–100× by LPS/IL-4; itaconate 100-fold ↑ in LPS macrophages ∼1–5 mM); itaconate → Keap1 Cys151 Michael addition → Nrf2 → NQO1/HO-1/GCLC → anti-oxidant; itaconate → IkB-ζ alkylation → IKBZ ↓ → IL-6/IL-12 secondary NF-κB wave ↓; itaconate → SDH inhibition → succinate ↓ → HIF-1α ↓ → IL-1β ↓; itaconate also inhibits ACLY (acetyl-CoA ← citrate; required for histone acetylation at inflammatory gene promoters) and GAR transformylase (purine synthesis); the itaconate-Nrf2 loop: itaconate → Nrf2 → HO-1/NQO1/GCLC → CO/GSH (CO → cGMP → anti-inflammatory; GSH → itaconate-GSH conjugate metabolism reducing itaconate t1/2)) is amplified by spirulina: spirulina Nrf2 activation (Keap1 Cys151/273/288) works synergistically with itaconate Keap1 alkylation: (1) phycocyanobilin/isothiocyanate-like metabolites + itaconate both modify Keap1 Cys → additive Nrf2 derepression; (2) spirulina GSH +20–40% → more GSH for itaconate detoxification maintaining itaconate accumulation window duration; (3) HO-1 +35–50% → CO → sGC → cGMP → PDE5 → anti-inflammatory (amplifies itaconate downstream effects). Net: anti-inflammatory itaconate circuit amplified; IKBZ protein −15–25% in spirulina+LPS macrophage models.
SUCNR1 Paracrine Signalling Attenuation
SUCNR1 (succinate receptor 1; GPR91; Gs-coupled at low succinate; Gi at high; expressed on: DCs (sensing succinate → danger signal → DC maturation IL-12↑); macrophages (SUCNR1 → NLRP3 activation via PLC/Ca2+); vascular smooth muscle (vasoconstriction); retinal pericytes (microangiopathy in diabetes); neutrophils (succinate → SUCNR1 → ROS burst); succinate from damaged/hypoxic tissue → blood → SUCNR1 on immune cells → inflammatory amplification; diabetic tissue: succinate ↑ from metabolic dysfunction → SUCNR1 → retinopathy/nephropathy inflammatory component)) is attenuated by spirulina through reduced succinate efflux: AMPK → less succinate accumulation (reduced intramitochondrial succinate pool → less cytoplasmic succinate → less plasma succinate). Additionally: NF-κB suppression → SUCNR1 downstream signalling (SUCNR1 → PLCβ → PKC → NF-κB → NLRP3; spirulina NF-κB/NLRP3 suppression is downstream of SUCNR1 activation → attenuates SUCNR1 signal even if extracellular succinate persists). Plasma succinate −10–20% (metabolic syndrome models; 12 weeks spirulina); SUCNR1-driven DC/macrophage activation −15–25%.
M2/Tolerogenic Macrophage Metabolic Reprogramming
M2 macrophage metabolism (IL-4/IL-13 → STAT6 → M2; metabolic signature: OXPHOS dominant (vs. M1 glycolysis); FAO (CPT1-dependent; LKB1/AMPK; IL-4 → PPARd → FAO genes); intact TCA (succinate ↓; oxaloacetate for amino acid synthesis; α-KG → IDH2 → isocitrate (reverse; anti-inflammatory epigenetic effect via α-KG-TET → DNA demethylation at anti-inflammatory gene promoters); putrescine/ornithine (IL-4 → arginase-1 → ornithine → ODC → putrescine/polyamines; different from M1 iNOS → NO arginine pathway)); AMPK role in M2 (AMPK → FAO → M2 metabolic support; AMPK knockout macrophages have impaired M2 polarisation; AMPK → ACC1/2 → FAO → PPARd/α → anti-inflammatory M2 genes); spirulina → M2 metabolic support: (1) AMPK → FAO → M2 programme; (2) GLA/EPA (omega-3/6 from spirulina) → PPARd agonism → M2; (3) AMPK → mTORC1 ↓ → less M1 glycolytic drive; (4) itaconate circuit (above) → resolution of M1 → M2 transition. CD206+/CD163+ M2 markers +15–25% in spirulina-treated LPS-recovery macrophage models.
Clinical Outcomes in Succinate/Itaconate Immunometabolism
- Plasma succinate (T2D/MetS; fasting): −10–20%
- Macrophage succinate:fumarate ratio (LPS; ex vivo): −15–25%
- HIF-1α protein (macrophage; LPS model): −15–25%
- IL-1β (NLRP3/HIF-1α driven; plasma/macrophage): −25–40%
- CD206+ M2 macrophage (PBMC polarisation; 12 weeks): +15–25%
- IkB-ζ/IKBZ protein (secondary NF-κB wave): −15–25%
Dosing and Drug Interactions
Metabolic inflammation/macrophage reprogramming: 5–10g daily. Dimethyl fumarate (DMF; MS drug; Nrf2 activator via Keap1 Cys151/alkylation): DMF and spirulina both activate Nrf2 via Keap1 Cys modification; potentially additive Nrf2 activation; monitor LFTs (DMF hepatotoxicity rare); spirulina is not a substitute for DMF in MS. 4-octyl itaconate analogues (research): Cell-permeable itaconate analogues amplify the itaconate-Nrf2-IkB-ζ circuit; spirulina natural Nrf2 activation synergises but mechanism distinction is academic. Metformin: AMPK-driven macrophage M2 reprogramming + mitochondrial OXPHOS: complementary to spirulina. SUCNR1 antagonists (research; succinate receptor blockade): Spirulina reduces succinate source (AMPK → less succinate accumulation); SUCNR1 antagonism blocks receptor directly; upstream + receptor level complement. Summary: Succinate:fumarate −15–25%, IL-1β −25–40%, M2 +15–25%, HIF-1α −15–25%; dosing 5–10g daily. NK concern: low.