Characterization and hypoglycemic effects of sulfated polysaccharides derived from brown seaweed Undaria pinnatifida

The brown seaweed Undaria pinnatifida polysaccharides show various biological activities, but their hypoglycemic activity and the underlying mechanism remain unclear. Here, three fractions of sulfated polysaccharides Up-3, Up-4, and Up-5 were prepared by microwave-assisted extraction from U. pinnatifida. In vitro assays demonstrated that Up-3 and Up-4 had strong alpha-glucosidase inhibitory activity, and Up-3, Up-4, and Up-5 could improve the glucose uptake in insulin-resistant HepG2 cells without affecting their viability. In vivo studies indicated Up-3 and Up-4 markedly reduced postprandial blood glucose levels. Up-U (a mixture of Up-3, Up-4, and Up-5), reduced fasting blood glucose levels, increased glucose tolerance and alleviated insulin resistance in HFD/STZ-induced hyperglycemic mice. Histopathological observation and hepatic glycogen measurement showed that Up-U alleviated the damage of the pancreas islet cell, reduced hepatic steatosis, and promoted hepatic glycogen synthesis. These findings suggest that Up-U could alleviate postprandial and HFD/STZ-induced hyperglycemia and was a potential agent for diabetes treatment.