Study of the mechanisms underlying increased glucose absorption inSmilax chinaL. leaf extract-treated HepG2 cells

Abstract

Purpose: Previous studies have shown that treatment with Smilax china L. leaf extract (SCLE) produces antidiabetic effects due to α-glucosidase inhibition. In this study, we examined the mechanism underlying these antidiabetic effects by ex-amining glucose uptake in HepG2 cells cultured with SCLE. Methods: Glucose uptake and glucokinase activity were examined using an assay kit. Expression of glucose transporter (GLUT)-2, GLUT-4, and HNF-1α was measured by RT-PCR or western blot. Results: Treatment with SCLE resulted in enhanced glucose uptake in HepG2 cells, and this effect was especially pronounced when cells were cultured in an insulin-free medium. SCLE induced an increase in expression of GLUT-2 but not GLUT-4. The increase in the levels of HNF-1α, a GLUT-2 transcription factor, in total protein extract and nuclear fraction suggest that the effects of SCLE may occur at the level of GLUT-2 transcription. In addition, by measur-ing the change in glucokinase activity following SCLE treatment, we confirmed that SCLE stimulates glucose utilization by direct activation of this enzyme. Conclusion: These results demonstrate that the potential antidiabetic activity of SCLE is due at least in part to stimulation of glucose uptake and an increase in glucokinase activity, and that SCLE-stimulated glucose uptake is mediated through enhancement of GLUT-2 expression by inducing expression of its transcription factor, HNF-1α.