PI3K/Akt and mTOR/p70S6K pathways mediate neuroprotectin D1-induced retinal pigment epithelial cell survival during oxidative stress-induced apoptosis

The initiation and progression of several forms of retinal degenerations involve excessive, repetitive, and/or sustained oxidative stress that, in turn, mediate photoreceptor cell damage and death. Since phosphatidylinositol 3-kinase (PI3K)/Akt and mTOR/p70S6-kinase pathways are part of survival signaling in cells confronted with oxidative stress, we asked whether or not docosahexaenoic acid-derived neuroprotectin D1 (NPD1) mediates survival upon single-dose and/or repetitive oxidative stress through this pathway. For this purpose, we used human retinal pigment epithelial (ARPE-19) cells challenged by exposure to hydrogen peroxide (H₂O₂) plus tumor necrosis factor alpha (TNF-α). We found that in single-dose oxidative stress-induced apoptosis, phosphorylation of Akt, mTOR, and p70S6K was both time- and dose- dependent. Inhibition of PI3K or mTOR/p70S6K by wortmannin and rapamycin, respectively, increased apoptosis and inhibited phosphorylation of Akt and p70S6K induced by single-dose oxidative stress. While two exposures of a low dose, non-damaging oxidation induced apoptosis and upregulation of Akt, mTOR, and p70S6K, longer treatment of the cells with three exposures of low dose to low-dose stress showed no changes in the levels of Akt, mTOR, or p70S6K, and resulted in enhanced apoptosis compared to higher doses. Removing the oxidative stress-inducing agents following the single-dose or short term repetitive oxidative stress at the peak of Akt, mTOR, and p70S6K phosphorylation (i.e., 30 min after induction) led to recovery, with no apoptosis after 16 h of incubation. Cells that were induced with three low doses of stress did not show recovery when oxidative stress was removed 30 min after the last exposure. NPD1 protected the RPE cells against both single-dose and repetitive oxidative stress-induced apoptosis and promoted higher levels of phosphorylated Akt, mTOR, and p70S6K. Together, our results show that a) repetitive oxidative stress is dose dependent and may not be recovered by removing the oxidative stress-inducing agents, b) PI3K/Akt and mTOR/p70S6K pathways play a major role in the protection against oxidative stress-induced apoptosis in ARPE-19 cells, and c) NPD1 exerts protection under these conditions by inducing PI3K/Akt and mTOR/p70S6K pathways.