Enhancement of glucose uptake in 3T3-L1 adipocytes by Toona sinensis leaf extract

The effects of substances extracted from Toona sinensis leaves, using 50% alcohol/water, on cellular [3H]-2-deoxyglucose uptake in differentiated cultured 3T3-L1 adipocytes were investigated. Following treatment of cells with 0.001, 0.01, or 0.1 mg/mL extracts for 60 minutes, [3H]-2-deoxyglucose uptake increased from a basal value of 0.23 nmol/min/mg protein to 0.30, 0.33, and 0.38 nmol/min/mg protein, respectively. In insulin-stimulated cells, cellular [3H]-2-deoxyglucose uptake was enhanced by Toona sinensis leaf extract from a basal value of 0.35 nmol/min/mg protein to 0.41, 0.46, and 0.52 nmol/min/mg protein, respectively. Cellular glucose uptake was also enhanced by Toona sinensis leaf extract after incubation of cells with 20 mM glucose for 48 hours. Cellular glucose uptake with a combination of Toona sinensis leaf extract and insulin was significantly inhibited by pretreatment of cells with the protein synthesis inhibitor cycloheximide and the protein kinase C inhibitor calphostin C in normal-, medium- and high-glucose media. However, the glucose uptake-enhancing effect of Toona sinensis leaf extract was not diminished by cycloheximide and calphostin C in the absence of insulin. These results indicate that enhancement of cellular glucose uptake by Toona sinensis leaf extract in basal and insulin-stimulated 3T3-L1 adipocytes may be mediated by distinct mechanisms.