Activation of Promoter Activity of the Catalytic Subunit of γ-Glutamylcysteine Ligase (GCL) in Brain Endothelial Cells by Insulin Requires Antioxidant Response Element 4 and Altered Glycemic Status: Implication for GCL expression and GSH synthesis

摘要

Our recent finding that insulin increased the expression of glutamate cysteine ligase catalytic subunit (GCLc) and coincident increases in GCL activity and cellular glutathione (GSH) in human brain microvascular endothelial cells (IHECs) suggests a role for insulin in vascular GSH maintenance. Here, using IHECs stably transfected with promoter-luciferase reporter vectors, we found that insulin increased GCLc promoter activity, which required a prerequisite increase or decrease in media glucose. An intact antioxidant response element-4 (ARE4) was essential for promoter activation that was attenuated by inhibitors of PI3kinase/Akt/mTOR signaling. Interestingly, only at low glucose conditions did promoter activation correlated with increased GCLc expression and GSH synthesis. Low tert-butyl hydroperoxide (tBH) concentrations similarly mediated promoter activation, but the maximal activation dose was decreased 10-fold by insulin. Insulin-tBH co-administration abrogated the low or high glucose requirement for promoter activation, suggesting possible ROS involvement. ROS production was elevated at low glucose without or with insulin; however GSH increases were not inhibited by tempol, suggesting that ROS did not or achieve the threshold for driving GCLc promoter activation and de novo GSH synthesis. The minor effect of pyruvate also ruled out a major role for hypoglycemia (± insulin)-induced metabolic stress on GSH induction under these conditions.