Abstract
Although the essential role of protein kinase B (PKB)/Akt in cell survival signaling has been clearly established, the mechanism by which Akt mediates the cellular response to hydrogen peroxide (H2O2)-induced oxidative stress remains unclear. We demonstrated that Akt attenuated neuronal apoptosis through direct association with histone 2A (H2A) and phosphorylation of H2A at threonine 17. At early time points during H2O2 exposure of PC12 cells and primary hippocampal neurons, when the cells can tolerate the level of DNA damage, Akt was activated and phosphorylated H2A, leading to inhibition of apoptotic death. At later time points, Akt delivered the NAD+-dependent protein deacetylase Sirtuin 2 (Sirt 2) to the vicinity of phosphorylated H2A in response to irreversible DNA damage, thereby inducing H2A deacetylation and subsequently leading to apoptotic death. Ectopically expressed T17A-substituted H2A minimally interacted with Akt and failed to prevent apoptosis under oxidative stress. Thus Akt-mediated H2A phosphorylation has an anti-apoptotic function in conditions of H2O2-induced oxidative stress in neurons and PC12 cells.
Highlights
Neurons are susceptible to acute oxidative stress[1]
In vitro binding assays with a series of Akt fragments expressed as GST fusions in HEK 293 cells demonstrated that the catalytic domain of Akt was required for interaction with histone 2A (H2A), raising the possibility that H2A is a kinase substrate of Akt (Fig. 1c)
We have shown that Akt phosphorylates ribosomal protein S3 (RPS3), which acts as a proapoptotic protein, and enhances its endonuclease activity in the nucleus in response to DNA damage thereby promoting neuronal survival[34]
Summary
Neurons are susceptible to acute oxidative stress[1]. Chronically elevated levels of reactive oxygen species (ROS) such as H2O2 have been implicated in neuronal cell death in many neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and amyotrophic lateral sclerosis[2,3,4,5,6,7]. H2O2-induced oxidative stress mediates phosphorylation of Akt to promote survival in neurons[19,20]. We demonstrated a new defense mechanism in neuronal cells that is regulated by Akt signaling during H2O2-induced oxidative stress. Under moderate oxidative stress Akt was activated and directly interacted with histone 2A, phosphorylating the N-terminal tail of H2A This interaction activated pro-survival signaling and postponed apoptosis. Upon continued oxidative stress the neurons failed to sustain Akt signaling and underwent apoptotic cell death. We found that Akt recruited Sirt[2], a member of a family of conserved NAD+-dependent protein deacetylases that bound to Akt-mediated phosphorylated H2A and decreased H2A acetylation, suggesting that H2A is a new deacetylation target of Sirt[2] in neuronal cells under H2O2-induced oxidative stress. Our findings indicate that Akt regulates neuronal apoptosis by fine-tuning H2A protein modification under conditions of H2O2-induced oxidative stress
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