PURPOSE: To investigate the role of DJ-1 in Nrf2-regulated antioxidant defense in corneal endothelial cells (CECs) at baseline and in response to ultraviolet A (UV-A)-induced oxidative stress.
METHODS: DJ-1-deficient CECs were obtained by transfection of an immortalized normal human corneal endothelial cell line (HCECi) with DJ-1 small interfering RNA (siRNA) or by isolation of CECs from ex vivo corneas of DJ-1 knockout mice. Levels of reactive oxygen species (ROS), protein carbonyls, Nrf2 subcellular localization, Nrf2 target genes, and protein interaction between Keap1/Nrf2 and Cul3/Nrf2 were compared between normal and DJ-1-deficient CECs. Oxidative stress was induced by irradiating HCECi cells with UV-A, and cell death and levels of activated caspase3 and phospho-p53 were determined.
RESULTS: DJ-1 siRNA-treated cells exhibited increased levels of ROS production and protein carbonyls as well as a 2.2-fold decrease in nuclear Nrf2 protein when compared to controls. DJ-1 downregulation led to attenuated gene expression of Nrf2 and its target genes HO-1 and NQO1. Similar levels of Nrf2 inhibitor, Keap1, and Cul3/Nrf2 and Keap1/Nrf2 were observed in DJ-1 siRNA-treated cells as compared to controls. Ultraviolet A irradiation resulted in a 3.0-fold increase in cell death and elevated levels of activated caspase3 and phospho-p53 in DJ-1 siRNA-treated cells compared to controls.
CONCLUSIONS: Downregulation of DJ-1 impairs nuclear translocation of Nrf2, causing decreased antioxidant gene expression and increased oxidative damage. The decline in DJ-1 levels leads to heightened CEC susceptibility to UV-A light by activating p53-dependent apoptosis. Targeting the DJ-1-Nrf2 axis may provide a potential therapeutic approach for enhancing antioxidant defense in corneal endothelial disorders.
The cornea is covered by a stratified epithelium that is renewed by stem cells located in the peripheral region of the cornea known as the limbus. This stroma of the limbus contains stromal keratocytes that, when expanded in culture, are termed limbal fibroblasts (LFs). It is thought that LFs exhibit similar characteristics to bone marrow mesenchymal stem cells (BM MSCs) and help maintain the epithelial stem cell phenotype in the limbal region. In this study, we aimed at reprogramming stage-specific embryonic antigen-4 (SSEA4+) LFs and BM MSCs into corneal epithelial lineage using a three-dimensional culture system and embryonic stem cell medium. After enrichment, SSEA4+ cells showed a higher level of stem cell marker expression such as Sox2, Oct4, Nanog, Rex1, ABCG2, and TRA-1-60, and colony-forming efficiency than did SSEA4- cells. SSEA4+, as compared to SSEA4- cells, had a greater propensity to form spheres that, in turn, were induced into ectodermal lineage and further differentiated into functional corneal epithelium. Results show that LFs were similar to BM MSCs in marker profiles, and together with the differences noted between SSEA4+ and SSEA4- cells, point to LFs' being tissue-specific MSCs. However, LFs showed a greater potential for differentiation into corneal epithelium, indicating the potential importance of tissue-specific adult progenitors in their reprogramming capacity into cells of interest. This study opens a new avenue for investigating the molecular mechanism involved in maintaining a limbal stem cell niche and thus a potentially important clinical application to treat corneal epithelial stem cell loss.