A novel formula Sang-Tong-Jian improves glycometabolism and ameliorates insulin resistance by activating PI3K/AKT pathway in type 2 diabetic KKAy mice

Sang-Tong-Jian (STJ), a novel formula composed of flavonoids and alkaloids derived from mulberry leaf, has been found to reduce blood glucose levels in rats with type 2 diabetes mellitus (T2DM) in our previous studies. However, the precise mechanisms remain unknown. Insulin resistance is the main characteristic of T2DM, which may be due to impairment of the PI3K/AKT signaling pathway. In this study, we investigated the effects of STJ on glycometabolism and insulin resistance in KKAy mice. A total of 50 KKAy male mice were randomly divided into five groups: model, metformin at 260 mg/kg, and STJ at 105, 210 and 420 mg/kg. C57BL/6J mice were used as the control group. Random blood glucose levels in KKAy mice were determined every 10 days after treatments. At the 10th and 13th week, oral glucose tolerance test (OGTT) and insulin tolerance test (ITT) were conducted after a 12 h overnight fast, respectively. After 13-week treatments, glycosylated hemoglobin (GHb) and serum insulin were measured using a colorimetric method and ELISA kits. Liver glycogen and muscle glycogen levels were analyzed using a colorimetric method. The morphology of pancreas, liver, skeletal muscle and epididymal fat were visualized by haematoxylin and eosin staining. The gene level of GLUT2 (liver) and GLUT4 (skeletal muscle, epididymal fat) were detected by real-time PCR. The proteins of GLUT2, GLUT4, IRS1, PI3K, AKT and their phosphorylation were assayed by Western blot analyses. STJ significantly decreased the random blood glucose and GHb levels, and increased liver and muscle glycogen levels. The results of OGTT and ITT and measurement of serum insulin indicated that STJ ameliorated insulin resistance in KKAy mice. STJ treatments also ameliorated the histopathological alterations in pancreas, liver, skeletal muscle and epididymal fat in KKAy mice. Furthermore, STJ upregulated the gene and protein expression of GLUT2 (liver) and GLUT4 (skeletal muscle, epididymal fat). Meanwhile, GLUT4 translocation and phosphorylation of IRS1, p85-PI3K and AKT were significantly increased by STJ treatments. Our results indicated that STJ ameliorated glycometabolism and insulin resistance in KKAy mice, which might be due to activation of PI3K/AKT pathway.