(DHA), the major polyunsaturated fatty acid in the retina, promotes survival of rat retina photoreceptors during early development in vitro and upon oxidative stress by activating the ERK/MAPK signaling

This article is available online at http://www.jlr.org retinitis pigmentosa and age-related macular degeneration. Photoreceptor loss in these pathologies occurs by apoptosis and oxidative stress, and lack of trophic factors is implicated in the activation of the apoptotic mechanisms ( 1 ). Identifying the trophic factors and intracellular pathways activated to prevent this apoptosis is crucial for designing procedures to rescue photoreceptors in these diseases. Previous research from our laboratory has demonstrated that docosahexaenoic acid (DHA), the major n-3 polyunsaturated fatty acid (PUFA) in the retina, acts as a trophic factor for photoreceptor cells. DHA promotes differentiation and postpones apoptosis of photoreceptors, which otherwise occurs in the absence of trophic factors during their early development in culture ( 2–4 ); it also effectively prevents photoreceptor apoptosis due to oxidative stress induced by Paraquat (PQ) ( 5 ) and H 2 O 2 ( 6 ). This antiapoptotic effect is highly specifi c for DHA, because other saturated, monoenoic, or polyunsaturated fatty acids cannot stop photoreceptor death ( 2 ). DHA has several other protective effects in the retina, reducing neovascularization and infl ammatory processes and promoting survival in several cell types ( 7–13 ). Furthermore, n-3 fatty acid defi ciency alters recovery of the rod photoresponse in rhesus monkeys ( 14 ). An understanding of the molecular signaling pathways triggered by survival molecules like DHA is critical to design potential protective therapies and eventually provide an effective treatment for neurodegenerative diseases. Our work has shown that DHA activates the ERK/MAPK signaling pathway in photoreceptors to promote their survival and differentiation, stimulating the expression of antiapoptotic proteins such as Bcl-2 and preserving mitochondrial membrane potential ( 5, 15 ). In addition, it Abstract We have established that docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in the retina, promotes survival of rat retina photoreceptors during early development in vitro and upon oxidative stress by activating the ERK/MAPK signaling pathway. Here we have investigated whether DHA turns on this pathway through activation of retinoid X receptors (RXRs) or by inducing tyrosine kinase (Trk) receptor activation. We also evaluated whether DHA release from phospholipids was required for its protective effect. Addition of RXR antagonists (HX531, PA452) to rat retinal neuronal cultures inhibited DHA protection during early development in vitro and upon oxidative stress induced with Paraquat or H 2 O 2 . In contrast, the Trk inhibitor K252a did not affect DHA prevention of photoreceptor apoptosis. These results imply that activation of RXRs was required for DHA protection whereas Trk receptors were not involved in this protection. Pretreatment with 4-bromoenol lactone, a phospholipase A 2 inhibitor, blocked DHA prevention of oxidative stress-induced apoptosis of photoreceptors. It is noteworthy that RXR agonists (HX630, PA024) also rescued photoreceptors from H 2 O 2 -induced apoptosis. These results provide the fi rst evidence that activation of RXRs prevents photoreceptor apoptosis and suggest that DHA is fi rst released from phospholipids and then activates RXRs to promote the survival of photo receptors. —German, O. L., S. Monaco, D. L. Agnolazza, N. P. Rotstein, and L. E. Politi. Retinoid X receptor activation is essential for docosahexaenoic acid protection of retina photoreceptors. J. Lipid Res. 2013. 54: 2236–2246.