Photodamage Generates 7-keto- and 7-hydroxycholesterol in the Rat Retina via a Free Radical-mediated Mechanism

Albino Sprague–Dawley rats are known to undergo photoreceptor degeneration after exposure to constant light, but the molecular mechanism(s) by which the photoreceptors degenerate is not fully understood. We hypothesized that cytotoxic oxysterols are generated in situ in the retina under such conditions and may be involved in the degenerative mechanism. Thus, photodamaged and control rat retinas were analyzed for oxysterols by liquid chromatography mass spectroscopy. Elevated levels of two known cytotoxic oxysterols, 7‐ketocholesterol (7KCh) and 7αβ‐hydroxycholesterol (7HCh), were found in the photodamaged retinas, at levels six‐fold and 50‐fold greater, respectively, than those found in non photodamaged controls. Notably, two key intermediates, 5,6α,β‐epoxycholesterol (5,6‐epoxyCh) and 7αβ‐hydroperoxy‐cholesterol, were also identified, indicating that the formation of 7KCh and 7HCh is mediated by a free radical mechanism. By immunohistochemistry, 7KCh was localized to the ganglion cell layer, photoreceptor inner segments and retinal pigment epithelium (RPE), which coincides with the localization of ferritin in the retina. Exposure of a mixture of ferritin and low‐density lipoprotein to intense white light in vitro produced similar oxysterol species as seen in vivo. We propose that the increased levels of 7KCh and 7HCh, especially in photoreceptor inner segments and RPE, may arise due to ferritin‐catalyzed reactions and may be important contributors to the photoreceptor degeneration observed in photodamaged rats.