Inhibition of glucose uptake by Silybin and Dehydrosilybin

Abstract

Type 2 diabetes (T2D) is a common disease in industrialized countries. Pathological changes in cellular glucose and fatty acid uptake are hallmarks of T2D. Medical plants from traditional medical systems of China and India are considered as a promising source for new anti-diabetic drugs. Here, we screened a panel of plant extracts and single chemical compounds from traditional medical plants with reported anti-diabetic effects for their ability to modulate cellular glucose and fatty acid uptake rates. Cellular glucose uptake, but not cellular fatty acid uptake, was significantly reduced by several candidate drugs. Of those drugs, we chose Silybin (SIL) and its derivative Dehydrosilybin (DHS), two main flavonoids of Silybum marianum , for further investigations. SIL is reported to improve blood sugar control in several clinical trials and also possesses strong anti-cancer effects. We could show that SIL and DHS inhibit glucose transport in 3T3-L1 fibroblasts and adipocytes. Both flavonoids reduce basal glucose uptake in a dose-dependent manner (52% and 36% of basal uptake for 40 μM SIL, 5% and 20% of basal uptake for 40 μM DHS in fibroblasts and adipocytes). Insulin-stimulated glucose uptake is inhibited by SIL and DHS at concentrations of 20 and 10 μM, respectively. Using time course experiments, immunofluorescence microscopy and subcellular fractionation/immunoblotting, we demonstrated that both substances do not interfere with insulin-triggered translocation of GLUT4 to the plasma membrane. Onward metabolism seems unaffected since there are no inhibitory effects of SIL and DHS on hexokinase activity in 3T3-L1 adipocytes. To determine whether SIL and DHS may affect intrinsic transport activity of GLUT4, we heterologously expressed this isoform in CHO-K1 cells. CHO-K1 expressing GLUT4 shows a 2-fold increase in basal glucose uptake compared to control cells. This increase in glucose transport is completely abolished by 40 μM SIL or DHS. SIL and DHS therefore inhibit glucose uptake by interfering with glucose transporters including GLUT4. Stable expression of GLUT4 in CHO-K1 significantly reduces the cytotoxic effect of both SIL and DHS as determined by MTT assay, indicating that restriction of glucose uptake is an important mechanism for the anti-cancer effect of the flavonoids. Our results show that inhibition of glucose transporters is a potential mechanism by which anti-diabetics from medical plants could work.