Involvement of the PI3K/AKT pathway in the hypoglycemic effects of saponins from Helicteres isora

Aim of the study: Saponins from Helicteres isora have previously been shown to exert antidiabetic effects. The present study explored the underlying mechanisms in C2C12 skeletal muscle cells. Materials and methods: C2C12 cells were incubated with saponins and sapogenin followed by Western blotting and immunofluorescence analysis. Results: Western blotting revealed that incubation with saponins (100 mu g/ml) and sapogenin (100 mu g/ml) induced the phosphorylation of the phosphatidylinositol-3-kinase (PI3K) as well as of the downstream targets protein kinase B/Akt (at Ser473) and glycogen synthase kinase GSK-3 alpha/beta (at Ser21/9) in a time-dependent manner. In contrast, no phosphorylation of the AMP-sensitive kinase AMPK (at Thr172) was observed. Within 48 h saponins/sapogenin treatment further increased the protein abundance of the insulin-sensitive glucose transporter Glut4. Confocal microscopy confirmed that saponins/sapogenin treatment stimulated Akt phosphorylation and revealed that the treatment was followed by translocation of Glut4 into the cell membrane of C2C12 muscle cells. Conclusions: Saponins and sapogenin activate the PI3K/Akt pathway thus leading to phosphorylation and inactivation of GSK-3 alpha/beta with subsequent stimulation of glycogen synthesis as well as increase of Glut4-dependent glucose transport across the cell membrane. (C) 2009 Elsevier Ireland Ltd. All rights reserved.