Age-related cataracts: Homocysteine coupled endoplasmic reticulum stress and suppression of Nrf2-dependent antioxidant protection

To determine whether high levels of homocysteine (Hcy) induce endoplasmic reticulum (ER) stress with suppression of the nuclear factor-erythroid-2-related factor 2 (Nrf2)-dependent antioxidant protection in lens epithelial cells (LECs). ER stress was acutely induced by exposure of LECs to 100 μM Hcy without FCS and also by exposure to 5 mM Hcy with 10% FCS. After exposure to Hcy, significant changes were found in P-PERK, P-eIF2α, XBP1, Nrf2, and Keap1 within 24 h. The production of reactive oxygen species (ROS) was increased after Hcy exposure. The downstream enzymes of Nrf2 like, catalase, and glutathione reductase, were significantly decreased. These results suggested that the Hcy-induced ER stress suppressed the Nrf2-dependent antioxidant protection and simultaneously generated ROS which resulted in further oxidation and death of LECs. The loss of Nrf2 is mainly due to proteosomal degradation and m-calpain activation by the increased levels of cytoplasmic Ca⁺⁺. The caspases also play a role in the degradation of Nrf2. Our findings demonstrated that high levels of Hcy induce ER stress, chronic UPR, alter the levels of UPR specific proteins, increase the production of ROS, degrade Nrf2 and block the Nrf2-dependent antioxidant defense protection in LECs. Thus, the upregulation of ROS might exceed the Nrf2 dependent antioxidant defense protection in the LECs and result in the highly oxidized lenses and resulted in ARCs.