Tea seed saponin-reduced extract ameliorates palmitic acid-induced insulin resistance in HepG2 cells

Tea (Camellia sinensis) seed cake is a potential resource that contains a wealth of bioactive compounds. However, the high toxicity of tea saponins in tea seed cake restricts its applications. The present study aimed to i) develop a method of extracting bioactive compounds and reducing tea saponins during the process of tea seed cake extraction and ii) investigate the anti-insulin resistance effect of tea seed saponin-reduced extract (TSSRE) in a palmitic acid (PA)-induced insulin resistance HepG2-cell model. The concentration of tea saponins in TSSRE was similar to 10-fold lower than that in tea seed crude extract (TSCE) after the saponin-reduction process. In addition, TSSRE cytotoxicity was significantly lower than that of TSCE in HepG2 cells. TSSRE treatment improved glucose consumption as well as glucose transporter (GLUT) 2 and GLUT4 expression levels in PA-stimulated HepG2 cells. Moreover, TSSRE enhanced the phosphorylation of the insulin receptor substrate 1/protein kinase B/forkhead box protein O1/glycogen synthase kinase 3 beta and inhibited the elevated expression of phosphoenolpyruvate carboxykinase in PA-exposed HepG2 cells. The effect of TSSRE on the mediation of the insulin signaling pathway was attributed to the inhibition of PA-induced mitogen-activated protein kinase activation. The findings of the present study indicated that TSSRE ameliorates hepatic insulin resistance by ameliorating insulin signaling and inhibiting inflammation-related pathways.