Induction of ferroptosis by HO-1 contributes to retinal degeneration in mice with defective clearance of all-trans-retinal

The accumulation of all-trans-retinal (atRAL) in photoreceptors and the retinal pigment epithelium (RPE), which is induced by chaos in visual (retinoid) cycle, is closely associated with the pathogenesis of dry age-related macular degeneration (AMD) and autosomal recessive Stargardt's disease (STGD1). Although we have reported that the induction of ferroptosis by atRAL is an important cause of photoreceptor loss, but its mechanisms still remain unclear. In this study, we identified heme oxygenase-1 (HO-1) as an inducer of photoreceptor ferroptosis elicited by atRAL. In atRAL-loaded photoreceptor cells, inhibition of Kelch-like ECH-associated protein 1 (KEAP1) at least in part by reactive oxygen species (ROS) production evoked the release of nuclear factor-erythroid 2-related factor-2 (NRF2) from KEAP1, followed by the translocation of active NRF2 into the nucleus where it promoted the transcription of the Ho-1 gene, thereby leading to HO-1 overexpression in the cytosol. A significant elevation of Fe2+ levels in photoreceptor cells resulted from activation of HO-1 by atRAL, and it facilitated ROS overproduction and then triggered ferroptotic cell death through ROS-mediated lipid peroxidation. Both treatment with HO-1 repressor Zinc protoporphyrin IX (ZnPP) and knockout of Ho-1 gene clearly rescued photoreceptor cells against ferroptosis arising from atRAL overload. Light-exposed Abca4-/-Rdh8-/- mice rapidly display severe defects in atRAL clearance, and serve as an acute model of dry AMD and STGD1. HO-1 activation was distinctly observed in neural retina of Abca4-/-Rdh8-/- mice after exposure to light, and it was visibly relieved by intraperitoneally injected ferroptosis inhibitor ferrostatin-1. More notably, intraperitoneal administration of ZnPP effectively alleviated both photoreceptor degeneration and RPE atrophy in Abca4-/-Rdh8-/- mice in response to light exposure by repressing HO-1-mediated ferroptosis. Our study suggests that HO-1 is a key factor that regulates atRAL-induced ferroptosis in photoreceptors and the RPE, and its inhibition may hold promises for the therapy of dry AMD and STGD1.