Melatonin inhibits ferroptosis and delays age-related cataract by regulating SIRT6/p-Nrf2/GPX4 and SIRT6/NCOA4/FTH1 pathways

Cataracts are the main cause of reversible blindness worldwide. The ageing of the lens caused by ultraviolet B (UVB) radiation is mostly related to oxidative stress (OS). Little is known about whether OS induced by UVB enhances the sensitivity of lens epithelial cells to ferroptotic stress, which may be a new mechanism leading to age-related cataracts (ARCs). Ferroptosis was detected by transmission electron microscopy (TEM), iron assay, lipid peroxidation (MDA) assay, real-time PCR, western blotting, and immunofluorescence. Genetic engineering technology was used to investigate the regulatory relationship among Sirtuin 6 (SIRT6), nuclear factor erythroid 2-related factor 2 (Nrf2), nuclear receptor coactivator 4 (NCOA4), glutathione peroxidase 4 (GPX4) and ferritin heavy chain (FTH1). Knockdown and overexpression of SIRT6 locally in vivo in rats were performed to probe the regulatory mechanism of SIRT6 in ferroptosis in ARCs. Here, we observed that UVB can drastically induce ferroptosis in lens epithelial cells in vivo and in vitro. Surprisingly, inhibition of ferroptosis was the direct reason that melatonin rescued B-3, SRA01/04 and HEK-293 T cells survival; the pan-caspase inhibitor Z-Vad-FMK did not significantly reverse the death of UVB-irradiated cells compared with that in the UVB+DMSO group. SIRT6 was an upstream regulator of phosphorylated Nrf2 (p-Nrf2) and NCOA4 in B-3, SRA01/04 and HEK-293 T cells. Melatonin inhibited ferroptosis through the SIRT6/p-Nrf2/GPX4 and SIRT6/COA4/FTH1 pathways to neutralize lipid peroxidation toxicity, which protected cells against ferroptotic stress in vitro and delayed cataract formation caused by UVB exposure in rats. Our findings reveal a novel causal role of melatonin in the pathogenesis of ARCs, which raises the possibility of selectively targeting the activation of SIRT6 and ferroptotic resistance as a latent antioxidative therapeutic strategy for ARCs.