Blockade of Nitroxidative Stress by Roasted Licorice Extracts in High Glucose-exposed Endothelial Cells

Diabetes can cause a wide variety of vascular complications and endothelial dysfunction. In this study, human vascular endothelial cells were exposed to 5.5 mM and 33 mM glucose for 5 d in the absence and presence of 1 to 20 mu g/mL roasted licorice (Glycyrrhiza inflata Bat.) ethanol extracts (rLE). Caspase-3 activation and Annexin V staining revealed that high glucose induced endothelial apoptotic toxicity with a generation of reactive oxygen species (ROS) and these effects were reversed by rLE at >= 1 mu g/mL in a dose-dependent manner. Cytoprotective rLE substantially reduced high glucose-induced expression of endothelial nitric oxide synthase (eNOS), and hence attenuated the formation of peroxynitric radicals derived from NO. In addition, rLE suppressed expression of PKC beta 2 and activation of NADPH oxidase subunit of p22phox promoted by high glucose. However, rLE <= 1 mu g/mL did not modulate the high glucose-triggered activation of ASK-JNK signaling pathway. Our results suggest that PKC beta 2 expression and NADPH oxidase-dependent superoxide production and eNOS-mediated peroxynitrite generation may be essential mechanisms responsible for increased oxidative stress and endothelial apoptosis in chronic hyperglycemic conditions. Thus, rLE may be a beneficial agent most likely contributing to prevention of vascular NADPH oxidase induction and preservation of endothelial nitric oxide availability, resulting in blunting diabetes-associated endothelial dysfunction and vascular complications.