Fig. 1
Structures of two marker compounds in mulberry leaf extract and the effect of these two compounds on glucose uptake in differentiated C2C12 myotubes (c). Structures of trans-caffeic acid (a) and syringaldehyde (b). Differentiated C2C12 cells were incubated in serum-starved DMEM for 1 h and then treated with trans-caffeic acid or syringaldehyde 1 h. Thereafter, the medium was changed to Krebs-Ringer phosphate-HEPES buffer containing the prescribed concentrations of trans-caffeic acid or syringaldehyde for 30 min, and then cells were treated with 100 nM insulin for 30 min. After treatment with 2-deoxy-[3H]–glucose (0.5 μCi/mL) and 2-deoxy-D-glucose (100 μM) for 10 min, the reaction was terminated and glucose uptake activity was measured in lysed cells with a liquid scintillation counter (see Materials and methods). Rosiglitazone (10 μM) was used as a positive control for glucose uptake in differentiated C2C12 myotubes. Data are expressed as the mean ± SEM of three independent experiments. * P < 0.05 and ** P < 0.01 versus insulin-stimulated glucose uptake in the untreated control