Antidiabetes and Anti-obesity Activity of Lagerstroemia speciosa

Abstract

The leaves of Lagerstroemia speciosa (Lythraceae), a Southeast Asian tree more commonly known as banaba, have been traditionally consumed in various forms by Philippinos for treatment of diabetes and kidney related diseases. In the 1990s, the popularity of this herbal medicine began to attract the attention of scientists worldwide. Since then, researchers have conducted numerous in vitro and in vivo studies that consistently confirmed the antidiabetic activity of banaba. Scientists have identified different components of banaba to be responsible for its activity. Using tumor cells as a cell model, corosolic acid was isolated from the methanol extract of banaba and shown to be an active compound. More recently, a different cell model and the focus on the water soluble fraction of the extract led to the discovery of other compounds. The ellagitannin Lagerstroemin was identified as an effective component of the banaba extract responsible for the activity. In a different approach, using 3T3-L1 adipocytes as a cell model and a glucose uptake assay as the functional screening method, Chen et al. showed that the banaba water extract exhibited an insulin-like glucose transport inducing activity. Coupling HPLC fractionation with a glucose uptake assay, gallotannins were identified in the banaba extract as components responsible for the activity, not corosolic acid. Penta-O-galloyl-glucopyranose (PGG) was identified as the most potent gallotannin. A comparison of published data with results obtained for PGG indicates that PGG has a significantly higher glucose transport stimulatory activity than Lagerstroemin. Chen et al. have also shown that PGG exhibits anti-adipogenic properties in addition to stimulating the glucose uptake in adipocytes. The combination of glucose uptake and anti-adipogenesis activity is not found in the current insulin mimetic drugs and may indicate a great therapeutic potential of PGG.

Keywords: PGG; diabetes; glucose transport; obesity; plant extract.

Figure 1.

Langerstroemia speciosa. Tree and tree leaf.

Figure 2.

Chemical structures of corosolic acid and Lagerstroemin. Corosolic acid (A) is a triterpenoid (21), not a tannin. Lagerstroemin (B) is an ellagitannin (22).

Figure 3.

Absence of glucose transport stimulatory activity of corosolic acid in adipocytes. Pure corosolic acid in aqueous solution was added to 3T3-L1 adipocytes grown in wells of 6-well cell culture plates to induce glucose transport by a commonly used procedure (28,29). Cells treated with either 1 μM insulin or 30 μM penta-galloyl-glucose (PGG) were used as positive controls. Cells treated with water vehicle served as negative controls. Samples were in triplicate in the experiment, and the experiment was repeated three times. No difference was found by a one-way ANOVA statistical analysis between the negative (vehicle) control and corosolic acid samples at any concentration.

Figure 4.

Glucose transport stimulatory activity of glucosol as compared with banana extract. Banaba extract (BE) was prepared in house (28). Glucosol (30) was purchased. Glucosol was compared with BE in a regular glucose uptake assay (28,29). Samples were assayed in triplicates, and the assay was repeated three times. The result of the assays was analyzed with a one-way ANOVA. *P < 0.05, **P < 0.01, ***P < 0.001. All samples were compared with the negative (vehicle) control.

Figure 5.

Chemical structures of gallotannins (tannic acid, A) and PGG (B). PGG exist in two anomeric forms.