An Antioxidative Exopolysaccharide–Protein Complex of Cordyceps Cs-HK1 Fungus and Its Epithelial Barrier-Protective Effects in Caco-2 Cell Culture

The exopolysaccharides (EPS) from the mycelial fermentation of Cordyceps sinensis Cs-HK1, especially the low-molecular weight, protein-rich exopolysaccharide fractions (EPS-LM), have previously exhibited significant antioxidant activity. This study further investigated the antioxidant and protective effects of EPS-LM on intestinal epithelial barrier integrity in Caco-2 monolayers challenged with hydrogen peroxide (H₂O₂, 550 μM). EPS-LM contained two major molecular-weight fractions, 25 kDa and 1.7 kDa, with 19.3% total carbohydrate and 28.7% protein content (w/w). Treatment of the cells with EPS-LM (50–200 μg/mL) showed concentration-dependent protective effects against ROS-induced losses of cell viability and epithelial barrier integrity. EPS-LM treatment enhanced the activities of major antioxidant enzymes (SOD, GSH-Px, and CAT) and modulated NRF2 and its downstream target NQO1, consistent with alleviated oxidative stress. It also improved several indicators of intestinal barrier function, including increased transepithelial electrical resistance and upregulation of tight junction proteins (Occludin, ZO-1, and Claudin-1). These results provide new experimental evidence and theoretical basis for the nutraceutical potential of EPS-LM to mitigate oxidative stress and preserve intestinal epithelial barrier integrity.