Spirulina and Vitamin D Receptor: VDR Signalling, CYP27B1, and Immune Modulation

Vitamin D Activation: CYP27B1 and CYP24A1

Vitamin D₃ (cholecalciferol) undergoes two hydroxylations: hepatic CYP2R1/CYP27A1 produces 25-hydroxyvitamin D₃ (25(OH)D₃, calcidiol), and renal (and extrarenal) CYP27B1 (1α-hydroxylase) converts it to 1,25(OH)₂D₃ (calcitriol, the active form). CYP24A1 (24-hydroxylase) catabolises calcitriol to calcitroic acid. CYP27B1 is induced by PTH, low calcium/phosphate, and by immune signals (IFN-γ, LPS in macrophages); CYP24A1 is a primary VDR target gene creating a feedback loop. NF-κB activation suppresses CYP27B1 in renal tubular cells; spirulina's NF-κB inhibition (via PCB) thus may preserve CYP27B1 expression under inflammatory conditions.

VDR-RXRα Heterodimerisation and VDRE Binding

Calcitriol binds the vitamin D receptor (VDR, NR1I1) ligand-binding domain (Kd ~0.1 nM), causing a conformational shift that releases co-repressors (NCoR/SMRT) and recruits RXRα to form the VDR–RXRα heterodimer. This complex binds vitamin D response elements (VDREs, direct repeats of RGKTCA spaced 3 bp apart, DR3) in target gene promoters, recruiting coactivator complexes: SRC-1/2/3 (NCOA family), DRIP/Mediator, and chromatin-remodelling SWI/SNF. Target genes include CAMP (cathelicidin antimicrobial peptide), DEFB4 (β-defensin 2), TRPV5/6 (calcium channels), CYP24A1, and anti-proliferative CDKN1A (p21).

VDR–NF-κB Cross-talk

Calcitriol-activated VDR suppresses NF-κB through multiple mechanisms: (1) VDR sequesters IKKβ, blocking IκBα phosphorylation; (2) VDR directly competes with p65 for DNA binding at κB sites; (3) VDR induces IκBα transcription, stabilising the cytosolic NF-κB inhibitory complex. Conversely, NF-κB activation represses VDR transcription. PCB's NF-κB suppression thus creates a permissive environment for VDR signalling—a non-genomic synergy between spirulina's anti-inflammatory actions and vitamin D pathway activity.

VDR and Nrf2 Cooperation

VDR and Nrf2 co-regulate several overlapping genes: HMOX1, NQO1, and ferritin heavy chain (FTH1). Calcitriol activates Nrf2 indirectly by downregulating thioredoxin- interacting protein (TXNIP), freeing thioredoxin to reduce Keap1 and stabilise Nrf2. PCB-driven Nrf2 activation conversely increases VDR protein stability (NRF2 target SQSTM1/p62 sequesters Keap1, and p62 also binds VDR proteasomal degradation machinery). This bidirectional stabilisation suggests spirulina and vitamin D are mechanistically complementary.

CAMP (Cathelicidin) and Innate Immunity

LL-37/CAMP is the only human cathelicidin—a broad-spectrum antimicrobial peptide induced by VDR at a VDRE in the CAMP promoter. LL-37 also acts as an immunomodulator: it activates TLR4 signalling, promotes autophagy via Beclin-1 induction, and has direct antiviral activity. Macrophage CYP27B1 allows local calcitriol synthesis during infection to induce CAMP independently of serum 25(OH)D. Spirulina's macrophage-activating polysaccharides (stimulating TLR4) and NF-κB/CYP27B1 preservation therefore synergise with vitamin D to maximise CAMP innate defence.

VDR in Metabolic Regulation

Beyond immunity, VDR signalling in adipocytes and pancreatic β-cells suppresses adipogenesis (downregulates PPARγ2), reduces lipid accumulation, and promotes insulin secretion (VDR knockout mice develop glucose intolerance). VDR also induces FGF23 expression in osteocytes, regulating phosphate homeostasis. Spirulina's PPAR-family interactions and AMPK-insulin signalling support create a metabolic context where adequate VDR activity further improves outcomes, particularly in populations with vitamin D insufficiency common in the Turkish community spirulina targets.