Neuroprotectin D1 Induces Dephosphorylation of Bcl-xL in a PP2A-dependent Manner during Oxidative Stress and Promotes Retinal Pigment Epithelial Cell Survival

Abstract

Retinal pigment epithelial (RPE) cell integrity is critical for the survival of photoreceptor cells. Bcl-x L is a major anti-apoptotic Bcl-2 protein required for RPE cell survival, and phosphorylation of Bcl-x L at residue Ser-62 renders this protein pro-apoptotic. In this study, we identify serine/threonine protein phosphatase 2A (PP2A) as a key regulator of Bcl-x L phosphorylation at residue Ser-62 in ARPE-19 cells, a spontaneously arising RPE cell line in which Bcl-x L is highly expressed. We found that either PP2A inhibitor okadaic acid or depletion of catalytic subunit of PP2A (PP2A/C) by small interfering RNA enhanced Bcl-x L phosphorylation when activated with hydrogen peroxide and tumor necrosis factor -induced oxidative stress. Disruption of PP2A/C exacerbated oxidative stressinduced apoptosis. PP2A/C colocalized and interacted with S62Bcl-x L in cells stressed with H 2 O 2 /tumor necrosis factor . By contrast, the omega-3 fatty acid docosahexaenoic acid derivative, neuroprotectin D1 (NPD1), a potent activator of survival signaling, down-regulated oxidative stress-induced phosphorylation of Bcl-x L by increasing protein phosphatase activity. NPD1 also attenuated the oxidative stress-induced apoptosis by knockdown of PP2A/C and increased the association of PP2A/C with S62Bcl-x L as well as total Bcl-x L . NPD1 also enhanced the heterodimerization of Bcl-x L with its counterpart, pro-apoptotic protein Bax. Thus, NPD1 modulates the activation of this Bcl-2 family protein by dephosphorylating in a PP2Adependent manner, suggesting a coordinated, NPD1-mediated regulation of cell survival in response to oxidative stress.