Involvement of peroxisome proliferator activator receptors in the photoprotective activity of the di‐apo‐carotenoid norbixin on RPE cells

Purpose There are increasing evidences for the involvement of peroxisome proliferator activator receptors (PPARs) in the development of ocular diseases, in particular AMD. PPAR receptors comprise three sub‐types of nuclear receptors: PPARα, PPARβ/δ and PPARγ. The mechanism by which norbixin (NBX), a di‐apo‐carotenoid exerts in vitro and in vivo protective effects on RPE was studied. Methods The affinity of NBX for the 3 PPAR subtypes was determined by binding studies. The photo‐protective properties of NBX were evaluated by an in vitro assay using primary cultures of porcine RPE cells challenged with A2E then blue light illumination. With the same assay, NBX protective activity was then characterized by a pharmacological approach employing different PPAR subtypes’ selective agonists and antagonists. The expression level of the PPARs was assessed by western blot. Results NBX bound PPARα and PPARγ with Ki values of 16.5 and 1.15 μM, respectively. In a non‐cellular PPARβ/δ receptor functional assay, NBX displayed a Kb value of 3.2 μM. Further, PPARγ (troglitazone) and PPARβ/δ (GW0742) agonists strongly and significantly protected RPE cells. NBX protective activity was partially reversed by PPARγ (T0070907) and PPARβ/δ (GSK3787) antagonists. We showed for the first time that PPARα protein expression was rapidly and strongly reduced upon A2E treatment. Most interestingly, its expression was maintained or restored when RPE cells were treated with NBX before or after A2E exposure, respectively. Interestingly, some PPARγ and β/δ agonists displayed similar patterns, and other compounds were unable to sustain PPARα protein expression. Conclusions Taken together, this study demonstrates that NBX photoprotective activity relies potentially on both PPARβ/δ and PPARγ activation and on a sustained expression level of PPARα.