GSK-3beta down-regulates the transcription factor Nrf2 after oxidant damage: relevance to exposure of neuronal cells to oxidative stress.

Oxidant injury activates the neuroprotective pathway represented by phosphatidylinositol 3 kinase (PI3K) and Akt. However, the final outcome of oxidant exposure is often associated with neuronal death. This study was aimed to identify the molecular mechanism responsible for loss of tolerance to an oxidative environment. In N2A neuroblasts, serum and H2O2 exhibited different kinetics of regulation for the Ser/Thr kinases Akt and glycogen synthase kinase 3beta (GSK-3beta) and for the transcription factor Nrf2, which governs redox homeostasis. Thus, H2O2 rapidly activated Akt, inhibited GSK-3beta, and directed the transcription factor Nrf2 to the nucleus, but after 4 h Akt was inactive, GSK-3beta was active and Nrf2 was more cytosolic than nuclear. Inhibition of the PI3K/Akt pathway by LY294002, impeded the short-term effect of H2O2 on nuclear translocation of Nrf2. GSK-3beta activation (inhibiting PI3K/Akt) or direct GSK-3beta inhibition in cerebellar granule neurons resulted in respective nuclear exclusion and nuclear accumulation of Nrf2. Moreover, in these neurons, nuclear accumulation of Nrf2 correlated with increased heme oxygenase-1 expression. Over-expression of the kinase active GSK-3beta (Delta9) mutant, induced Nrf2 cytoplasmic localization and inhibited Nrf2 transcriptional activity towards an antioxidant-response-element luciferase reporter. Moreover, GSK-3beta (Delta9) sensitized N2A neuroblasts to H2O2-induced oxidative stress and cell death. This study identifies GSK-3beta, a kinase known to participate in neurodegeneration, as a fundamental element in the down-regulation of the antioxidant cell defense elicited by Nrf2 after oxidant injury and provides a mechanism to explain the loss of oxidant tolerance that happens under persistent oxidant exposure such as those found in several neuropathologies.