Photoreceptor outer segment phagocytosis attenuates oxidative stress-induced apoptosis with concomitant neuroprotectin D1 synthesis

Abstract

Photoreceptor cell (rods and cones) renewal is accompanied by intermittent shedding of the distal tips of the outer segment followed by their phagocytosis in the retinal pigment epithelial (RPE) cells. This renewal is essential for vision, and it is thought that it fosters survival of photoreceptors and of RPE cells. However, no specific survival messenger/mediators have as yet been identified. We show here that photoreceptor outer segment (POS) phagocytosis markedly attenuates oxidative stress-induced apoptosis in ARPE-19 cells in culture. This phenomenon does not seem to be a generalized outcome of phagocytosis because nonbiological (polystyrene microsphere) phagocytosis did not elicit protection. The free docosahexaenoic acid (DHA) pool size and neuroprotectin D1 (NPD1) content increased during POS phagocytosis but not during microspheres phagocytosis. We have also explored other lipid mediators [lipoxin A4 and 15(S)-and 12(S)-hydroxyeicosatetraenoic acids] under these conditions and found them unchanged upon POS phagocytosis. Moreover, oxidative stress challenge to RPE cells undergoing POS phagocytosis further increased DHA and NPD1 content. Under these conditions, NPD1 was found within the RPE cells as well as in the culture medium, suggesting autocrine and paracrine bioactivity. Furthermore, using deuterium-labeled DHA, we show that as the availability of free DHA increases during oxidative stress, NPD1 synthesis is augmented in ARPE-19 cells. Our data suggest a distinct signaling that promotes survival of photoreceptor and RPE cells by enhancing the synthesis of NPD1 during phagocytosis. Taken together, NPD1 may be a mediator that promotes homeostatic regulation of cell integrity during photoreceptor cell renewal. docosahexaenoic acid ͉ polystyrene microspheres ͉ retinal pigment epithelial cells ͉ survival ͉ omega-3 fatty acids

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

“…Because NPD1 counteracts oxidative stress-induced apoptosis and also up-regulates the anti-apoptotic protein Bcl-x L in ARPE-19 cells (3), it was proposed that NPD1 may act at the transcriptional, translational, and posttranslational level to regulate the anti-apoptotic activity of Bclx L . In a previous report, it had been shown that after 1 h of serum starvation and 6 h of 400 and 800 M H 2 O 2 /TNF␣ induction, total Bcl-x L protein was down-regulated, and 50 nM NPD1 up-regulated this protein (4,5). The activated (phosphorylated) form of this protein has until now not been studied.…”

“…Single plane images were taken using a DIAPHOT 200 microscope (Nikon, Melville, NY) with fluorescence optics. Images were recorded using a Hamamatsu color chilled 3CCD camera (Bridgewater, NJ) and PhotoShop software, Version 5.0 (Adobe Systems, Mountain View, CA) (5,10). (Fig.…”

Docosanoids are multifunctional regulators of neural cell integrity and fate: Significance in aging and disease

“…Experimental phagocytosis of photoreceptor outer segments and polystyrene microspheres showed that RPE cells fed only with outer segments were more resistant to oxidative stressinduced apoptosis [20]. On the other hand, RPE cells that effectively phagocytized microspheres when exposed to oxidative stress showed apoptosis.…”

“…These noxious stimuli further orchestrate pathogenic gene-expression programs in stressed brain cells, thereby linking a cascade of cell death pathways with apoptosis and neuronal demise. Neural mechanisms leading toward NPD1 generation from DHA thereby appear to redirect cellular fate toward successful preservation of RPE-photoreceptor cell integrity [10,20,26] and brain cell aging [25]. The Bcl-2 pro-and antiapoptotic gene families, sAPP alpha (and/or other neurotrophins) and NPD1, lie along a cell fate-regulatory pathway whose component members are highly interactive, and have potential to function cooperatively in brain and retina cell survival.…”

Pharmacological protection of retinal pigmented epithelial cells by sulindac involves PPAR-α

“…A property of PPAR-α which adds further clinical potential to our findings is that PPAR-α is also known to influence the activity of key functional components, such as vascular endothelial growth factor (VEGF), docosahexanoeic acid (DHA), and matrix metalloproteinases (MMP), which participate in the progression of AMD and are themselves possible targets for therapeutic intervention (13,14). DHA has been shown to be critical for maintaining the integrity of photoreceptors (34), and its protective effect on RPE cells to oxidative stress is mediated by neuroprotectin D1 (NPD1), a derivative of DHA, that possesses antiapoptotic and antiinflammatory properties (35,36). The first evidence that an IPC-like response was involved in the sulindac protective effect was that the protection was also dependent on PKCe as demonstrated by a loss of sulindac protection when cells were treated with either a nonspecific PKC blocker or a specific PKCe peptide inhibitor.…”