Plant Derived Antioxidants and Their Protective Action on Ultraviolet-induced Corneal Injury: A Narrative Review

Cornea acts as the first line of primary defense against various harmful effects of Ultraviolet (UV) radiation. Constant UV exposure leads to various ocular surface diseases such as pterygium, photokeratitis, and may also lead to ocular surface neoplasias. Available literature suggests association between UV induced oxidative damages and corneal diseases. Prolonged exposure to UV radiation leads to cellular modifications such as activation of NF-κB leading to pro-inflammatory cytokine release and apoptosis. Upregulation of Matrix Metalloproteinases (MMPs) particularly MMP-1, MMP-2, and MMP-9, involved in tissue remodelling and degradation of extracellular matrix. Changes in cytokine expression which includes IL-1, IL-6, IL-8, and TNF-α, which are involved in inflammation and cellular injury. Genetic changes include downregulation of PAX6, a critical gene for eye development and maintenance and UVinduced activation of Deoxyribonucleic Acid (DNA) damage pathways such as JNK and MAPK, leading to apoptosis. This review mainly focuses on the use of natural antioxidants derived from plants such as Rosmarinus officinalis, Curcuma longa (Curcumin), Camellia sinensis (Green tea polyphenols), Centella asiatica, Ginkgo biloba, Allium cepa (Quercetin), Glycyrrhiza glabra (Glycyrrhizin), Zingiber officinale (Ginger), Nigella sativa, Lycopene which can be used as therapeutic targets (NF-κB signaling pathway, MAPK and JNK pathways, P53 phosphorylation, MMPs (e.g., MMP-1, MMP-2, MMP-9), oxidative stress markers] in treating UV-induced inflammation and oxidative damages in cornea. PubMed literature search was performed using keywords antioxidants, ultraviolet radiation, oxidative stress, inflammation, cornea on July 3rd 2021, which resulted in 165 papers of which 81 were chosen after applying specific filters and relevant curation by the authors. This study reviews the various biochemical pathways involved in cell injury and cell death in response to UV exposure. The study also reviews the natural antioxidants and the various medicinal plants from where these are derived. The authors conclude that inflammatory mediators have a major role in stimulating oxidative stress leading to apoptosis and cell death and ocular surface diseases such as photokeratitis, pterygium, Climatic Droplet Keratopathy (CDK), Dry Eye Disease (DED), pinguecula, corneal ulceration. Targeting these inflammatory mediators by the use of natural antioxidants which are dietary-derived plant polyphenols with anti-inflammatory potential are reported to have beneficial effects.