Algal polysaccharides are an essential active substance in food nutrition and even pharmaceutical activity research due to wide sources, low toxicity, and no side effects. In our previous research, we identified the chemical structure and studied the immunomodulatory mechanism of the polysaccharide from edible green algae Caulerpa racemosa var. peltata (CRVP). In this study, we further investigated the cytotoxicity and potential mechanism of CRVP against Hepa1-6 cells. Metabolomics analysis showed that CRVP might affect glutathione metabolism-related pathways. CRVP significantly up-regulated oxidized glutathione (GSSG) and down-regulated reduced glutathione (GSH) in Hepa1-6 cells, increasing the ratio of GSSG to GSH and intracellular reactive oxygen species. The potential anti-liver cancer mechanism is that CRVP may affect redox homeostasis and lead to apoptosis by inhibiting glutathione reductase or System Xc^–. Based on the known immunomodulatory mechanism of CRVP, this study can provide a material basis for adjuvant therapeutic nutritional agents with immune-enhancing anti-cancer activity.

In this study, oligosaccharide from Caulerpa racemosa var. peltate (CRVO-G) was prepared by sequential enzymatic hydrolysis with lactase, membrane filtration, dialysis, and freeze-drying, and the relative molecular mass and monosaccharide composition of CRVO-G were measured. Meanwhile, the immunoregulatory activity of CRVO-G was evaluated and its mechanism of action was studied. The results showed that CRVO-G was a mixture of two tetrasaccharides and one hexasaccharide with relative molecular masses of 617.115, 621.081, and 995.368, and was mainly composed of galactose (51.0%), mannose (21.3%), glucose (11.5%), xylose (6.5%), rhamnose (2.3%), glucuronic acid (2.0%), glucosamine hydrochloride (1.8%), fucose (0.8%), glucosamine hydrochloride (0.7%), galacturonic acid (0.7%), mannuronic acid (0.7%), and arabinose (0.6%). In addition, CRVO-G had good immunomodulatory activity, as manifested by increased release of nitric oxide and cytokines (interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α). The results of metabolomics analysis and Western blot showed that CRVO-G could promote the synthesis of thromboxane A2 (TXA2) and 15-keto prostaglandin F2α (15-keto-PGF2α) in the arachidonic acid pathway of macrophages, and increase the expression levels of cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), and the phosphorylation of nuclear factor κB (NF-κB). In summary, CRVO-G exerts immunomodulatory effects by the NF-κB/iNOS/COX-2 pathway in RAW264.7 macrophages.