Spirulina and Hedgehog Pathway: GLI Transcription Factors and Tumour Stroma

Canonical Hedgehog Signalling: Primary Cilia and SMO

Canonical Hedgehog (Hh) signalling proceeds through primary cilia. Absent Hh ligand: PTCH1 (Patched 1) localises to cilia and suppresses SMO (Smoothened) by preventing its ciliary entry; GLI2/GLI3 are phosphorylated by PKA (GNAS/cAMP), CK1, and GSK-3 at multiple Ser/Thr sites, targeting their C-terminal regions for partial proteasomal processing (GLI3 repressor form GLI3R; GLI2R to a lesser extent), entering nucleus as transcriptional repressors. Upon Hh binding to PTCH1: PTCH1 exits cilia, SMO enters cilia, activating SUFU (Suppressor of Fused) dissociation from GLI2/3, allowing full-length GLI2/GLI1 activators to transactivate Hh target genes: PTCH1 (feedback), GLI1 (self-amplification), HHIP, SNAI1, BCL-2, CCND1, VEGF.

Non-canonical Hh Signalling: AMPK and RAS

Non-canonical (SMO-independent) GLI activation occurs via: (1) KRAS-MEK-ERK phosphorylating and activating GLI1 (Ser408) independently of SMO; (2) TGF-beta/ SMAD3-GLI2 synergy in fibroblasts; (3) PI3K-Akt-GSK-3beta inhibition (preventing GLI phosphorylation/processing, stabilising GLI activator forms). AMPK directly phosphorylates GLI1 Ser102, promoting 14-3-3-mediated cytoplasmic sequestration and GLI1 degradation, opposing non-canonical GLI1 activation. Spirulina's AMPK activation thus suppresses non-canonical GLI1 in tumour cells with KRAS/PI3K mutations that bypass SMO.

GLI1 and Cancer Stem Cell Maintenance

GLI1 maintains cancer stem cell (CSC) populations in pancreatic, colorectal, basal cell carcinoma, and medulloblastoma by inducing stemness factors: NANOG, OCT4 (POU5F1), SOX2, and ABC transporters (ABCG2, conferring drug resistance). GLI1 also induces SNAIL/SNAI1 and ZEB1 for epithelial-mesenchymal transition (EMT). AMPK phosphorylation of GLI1 Ser102 disrupts its interaction with SUFU-releasing machinery, promoting degradation and reducing CSC markers. Spirulina's AMPK axis therefore potentially attenuates CSC maintenance through GLI1 suppression, complementing its NF-kB and HIF-1alpha-driven anti-tumour effects.

NF-kB and GLI: Bidirectional Cross-talk

NF-kB and GLI1 form a bidirectional regulatory loop in tumour stroma: NF-kB induces Sonic Hedgehog (SHH) secretion from tumour cells, which activates GLI in adjacent stroma; stromal GLI1 induces IGF2 and Wnt ligands that feedback to tumour NF-kB. This NF-kB-SHH-stromal-GLI axis amplifies tumour growth and invasion. Spirulina's PCB-driven NF-kB suppression directly disrupts the initial SHH secretion step, attenuating stromal GLI activation and tumour-stroma bidirectional amplification.

PTCH1 and Basal Cell Carcinoma

Loss-of-function PTCH1 mutations (Gorlin syndrome, sporadic BCC) constitutively activate SMO-GLI signalling. PTCH1 normally functions as a 12-pass transmembrane sterol pump, possibly transporting cholesterol away from SMO, preventing SMO oxysterol activation. Vismodegib (GDC-0449) is a clinical SMO inhibitor approved for advanced BCC. The connection to spirulina lies in its AMPK effects on cholesterol biosynthesis: altered sterol pools may influence PTCH1-SMO communication, though direct evidence in BCC is lacking.

Hh in Pancreatic Cancer Stroma and Drug Delivery

Pancreatic ductal adenocarcinoma (PDAC) has a dense desmoplastic stroma activated by Hh ligands from tumour cells. Stromal Hh activation impairs drug delivery (physical barrier, compressed vasculature). Paradoxically, complete Hh inhibition in PDAC stroma accelerated tumour progression in clinical trials, suggesting stromal Hh restrains tumour growth in some contexts. Spirulina's anti-fibrotic effects (TGF-beta/ SMAD3 and NF-kB suppression reducing stromal activation) may reduce desmoplasia through parallel non-Hh pathways, potentially improving drug delivery without the paradoxical Hh inhibitor effect.