Okadaic acid induces Akt hyperphosphorylation and an oxidative stress-mediated cell death in serum starved SK-N-SH human neuroblastoma cells that are augmented by rapamycin.

Using a neuronal model of serum starved SK-N-SH neuroblastoma cells, we showed previously that the phosphorylation of Akt and the mTOR substrates S6K and S6 through the vascular endothelial growth factor receptor VEGFR2 was enhanced by treatments with the phosphatase PP2A inhibitor okadaic acid (OA). These findings suggested that PP2A inhibition uncouples the regulation of Akt signaling by mTOR and affects cell survival. We therefore examined the effects of mTOR inhibition on Akt phosphorylation at sites threonine 308 (T308) and serine 473 (S473) and survival in OA treated cells. OA induced a loss in cell viability, the accumulation of hyperactivated Akt as monomeric and ubiquitinated forms and an increase in the total levels of ubiquitinated proteins. These events were exacerbated by treatments with an allosteric (rapamycin) but not an active-site inhibitor (PP242) of mTOR. Notably, rapamycin augmented the OA-induced hyperphosphorylation of Akt by suppressing a negative feedback loop of Akt activation through VEGFR2 and its downstream target phosphatidylinositol 3-kinase (PI3K). Treatments with the antioxidant N-acetlycysteine but not the pan caspase inhibitor Z-VAD-FMK promoted survival. Unlike reports that rapamycin promotes survival through increased Akt activation, these findings show that rapamycin-induced hyperphosphorylation of Akt fails to rescue our neuronal model from an oxidative stress-induced and caspase-independent cell death mediated by PP2A inhibition. Moreover, the exacerbation of OA-induced events by rapamycin suggests that mTOR and PP2A work in concert to regulate cell survival, activated Akt and the levels of ubiquitinated proteins.