Potchanapond Graidist and Juntipa Purintrapiban

pp. 247 - 253

Abstract

Plant derivatives with purported hypoglycemic properties have been used in traditional medicine around the world. Coccinia indica (ivy gourd) is used in traditional medicine to treat diabetics in many countries. C. indica is able to cause a reduction in blood glucose level and has shown hypoglycemic activity in vitro and in vivo. However, the mechanism of this effect remains unknown. In this study, we generated the pGL3-glucose transporter 1 (GLUT1) promoter to elucidate the molecular mechanism of the regulation of GLUT1 gene expression in response to a water extract of C. indica stem (CI extract). A fragment of 2.1 kb of rat GLUT1 promoter, located at -2,106 to +134, was linked to firefly luciferase. The regulating transcription was analyzed in transient expression assay after transfection and exposure of L6 myocytes with the GLUT1 promoter system and CI extract, respectively. Under normal condition (5 mM glucose), promoter activity induced
by 0.15 mg CI extract was markedly increased by 5.71 fold from the basal value. CI extract was more effective than 2 mM metformin. Surprisingly, promoter activity in hyperglycemic condition (15 mM glucose) induced by 0.50 mg CI was increased by 1.63 fold from the basal value. In addition, CI extract increased the 2-deoxyglucose (2-DG) uptake in L6 myocytes in a dose-dependent manner in both conditions, 5 mM and 15 mM glucose. GLUT1 protein was determined by Western blot analysis and the level also increased in a dose-dependent fashion. Interestingly, the activity of the -273 to +134 of GLUT1 promoter was increased by 2.12 fold from the basal value. This site is the transcription initiation site containing GC box and TATA box. These observations suggest that the hypoglycemic action of C. indica may regulate through the activation of GLUT1 promoter resulting in an increase of the GLUT1 protein expression.