3,6-Anhydro-L-galactose suppresses mouse lymphocyte proliferation by attenuating JAK-STAT growth factor signal transduction and G₁-S cell cycle progression

Recombinant GH16B β-agarase-catalyzed liquefaction of 5-7 %(w/v) melted agarose at 50 °C completely hydrolyzed agarose into neoagarohexaose (NA6) and neoagarotetraose (NA4). Subsequent saccharification by recombinant GH50A β-agarase or recombinant GH50A β-agarase/recombinant GH117A α-neoagarobiose hydrolase at 35 °C converted NA6/NA4 into neoagarobiose (NA2) or 3,6-anhydro-L-galactose (L-AHG)/D-galactose, respectively. Purification of NA6/NA4 and NA2 was achieved by Sephadex G-15 column chromatography, while L-AHG was purified by Sephadex G-10, achieving ≥ 98 % purity. L-AHG (25-200 μg/mL), but not NA2, NA4, or NA6, inhibited the proliferation of immobilized anti-CD3/anti-CD28-activated T cells and immobilized anti-CD40 + soluble anti-IgM + interleukin (IL)-4-activated B cells. This inhibition impacted the G₁-S traverse in the cell cycle without influencing CD69 expression and p27^Kip1 down-regulation, markers of the exit from G₀ into G₁ phase in activated lymphocytes. L-AHG impeded cyclin-dependent kinases (CDKs)-driven retinoblastoma phosphorylation, necessary for the G₁-S traverse, by reducing the activating phosphorylation of CDKs (CDK4, CDK2, and CDK1) and lowering cyclin D3, cyclin A2 and cyclin B1 levels. Furthermore, L-AHG diminished the production of growth factors, including IL-2 in activated T cells and IL-6 in activated B cells. The antiproliferative effect of L-AHG on T cells was partially restored by exogenous IL-2 but was unaffected by exogenous IL-6 on B cells. L-AHG inhibited the activating phosphorylation of Janus kinase 1 (JAK1), affecting signal transducer and activator of transcription 1 (STAT1) and STAT3 signaling. These results demonstrate that L-AHG may serve as a novel immunosuppressant by impairing JAK-STAT growth factor signaling and G₁-S cell cycle progression in T and B lymphocytes.