NRF2-Dependent Glutamate-L-Cysteine Ligase Catalytic Subunit Expression Mediates Insulin Protection Against Hyperglycemia-Induced Brain Endothelial Cell Apoptosis

Increased oxidative stress and susceptibility of brain endothelium are contributing factors in the development of central nervous system complications in neurodegenerative disorders in diabetes, Alzheimer's and Parkinson's disease. The molecular mechanisms underpinning the vulnerability of brain endothelial cells to chronic oxidative challenge have not been elucidated. Here, we investigated the oxidative susceptibility of human brain endothelial cells (IHEC) to chronic hyperglycemic stress and insulin signaling and cytoprotection. Chronic hyperglycemia exacerbated IHEC apoptosis in accordance with exaggerated cytosolic and mitochondrial glutathione and protein-thiol redox imbalance, and actin/Keap-1 S-glutathionylation. Insulin attenuated hyperglycemia-induced apoptosis via restored cytosolic and mitochondrial redox. Insulin stimulated glutamate-L-cysteine ligase (GCL) activity by activation of phosphatidylinositol 3-kinase (PI3K)/Akt/mTOR signaling, increased serine phosphorylation and nuclear translocation of nuclear NF-E2-related factor 2 (Nrf2), and upregulation of Nrf2-dependent GCL-catalytic (GCLc) subunit expression. Expression of the GCLmodulatory subunit (GCLm) was unchanged. Inhibitors of insulin receptor tyrosine kinase, PI3K, Akt and mTOR abrogated insulin-induced Nrf2-mediated GCLc expression, redox balance, and IHEC survival. Collectively, these results demonstrate that human brain endothelial cells exhibit vulnerability to hyperglycemic stress which is associated with marked cytosolic and mitochondrial redox shifts. Activation of insulin signaling through PI3K/Akt/mTOR/Nrf2/ GCLc pathway affords significant cell protection by maintaining cellular redox balance.