Structural characterization of polysaccharides from Medicago sativa L. roots and lipid-lowering activity in oleic acid-induced HepG2 cells

Polysaccharides have multiple pharmacological activities without toxicity or side effects. In the present study, the structure, antioxidant and lipid-lowering activities of alfalfa (Medicago sativa L.) polysaccharides (APS) were reported. Three alfalfa root polysaccharides (RAPS), RAPS-1, RAPS-2, and RAPS-3 were isolated, purified, and characterized. The structural analyses demonstrated that the homogeneous RAPS had molecular weights of 22.60, 26.95, and 38.25 kDa, respectively. Furthermore, RAPS-1 was composed of rhamnose, arabinose, xylose, glucuronic acid, glucose, and galactose with molar ratios of 10.25%, 12.53%, 6.58%, 4.56%, 15.74%, and 36.30%, RAPS-2 comprised rhamnose, arabinose, xylose, glucose, and galactose in the molar ratios of 24.07%, 14.48%, 1.24%, 15.05%, and 39.37%, while RAPS-3 was composed of rhamnose, arabinose, xylose, glucuronic acid, mannose, glucose, and galactose in the molar ratios of 31.61%, 11.78%, 1.93%, 5.49%, 3.18%, 14.56%, and 24.63%. The SEM analysis suggested that with the increase of the molecular weight of polysaccharides, its structure is more compact and the stability is stronger. In vitro, RAPS showed strong scavenging ability against DPPH and ABTS free radicals. In addition, RAPS had potent antioxidant activity and reduced the accumulation of lipids in oleic acid-induced NAFLD in HepG2 cells, with RAPS-1 being the most effective. Furthermore, RAPS-1 decreased MDA production, improved the activities of SOD and the total antioxidant capacity of cells, demonstrating that RAPS could effectively restrict lipid accumulation in HepG2 cells induced by oleic acid by restoring the antioxidant system. Western blot analysis indicated that the RAPS-1 can activate the Nrf2/HO-1 signaling pathway to relieve oxidative stress induced by oleic acid, thus playing a role in the treatment of NAFLD.