Structural analysis of a glucoglucuronan derived from laminarin and the mechanisms of its anti-lung cancer activity

Laminarin (LA), a storage glucan, was purified from the brown alga Sargassum thunbergii. After specific oxidation using the stable nitroxyl radical, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), together with NaBr and NaClO, glucoglucuronan (LAO) was obtained. Compositional analysis of LAO showed a molar ratio of glucuronic acid (GlcA) to glucose (Glc) of 12.7: 1. Nuclear magnetic resonance (NMR) and mass spectroscopy (MS) showed LAO to have a backbone of (1 → 3)-linked β-D-GlcpA interspersed with (1 → 3, 1 → 6)-linked β-D-Glcp, that was terminated with β-D-GlcpA. LAO inhibited human lung cancer A549 cell proliferation in vitro. IC₅₀ values at 12 h and 24 h were 2.70 mg/mL and 2.85 mg/mL, respectively. Western blotting showed that TSC2 was up-regulated at an LAO concentration of 3.80 mg/mL. FAK, PI3K, P-AKT and mTOR were down-regulated, indicating LAO inhibited cancer cell proliferation through the FAK/PI3K/AKT/mTOR pathway. Surface plasmon resonance (SPR) studies revealed that LAO showed an IC₅₀ of 0.07 mg/mL inhibiting the binding of heparin to fibroblast growth factor 1 (FGF1) LAO inhibition of heparin binding to FGF2 fluctuated between 15% and 28%, suggesting that LAO inhibits A549 cell proliferation by selectively interacting with FGF1.