Vitamin D enhances corneal epithelial barrier function

Purpose. The purpose of this study was to determine whether 25-hydroxyvitamin D ₃ (25(OH)D ₃) and/or its active metabolite, 1α,25-dihydroxyvitamin D ₃ (1,25(OH) ₂D ₃), can enhance corneal epithelial barrier function. The authors also determined if corneas contain mRNA for the vitamin D receptor (VDR) and 1α-hydroxylase, the enzyme required to convert 25(OH)D ₃ to 1,25(OH)2D ₃, and measured vitamin D metabolite concentrations in aqueous and vitreous humor. Methods. RT-PCR was used to examine mouse, rabbit, and human corneal epithelial VDR and 1α-hydroxylase mRNA. Vitamin D metabolites were measured using a selective vitamin D derivatizing agent and mass spectroscopy. Barrier function experiments were performed by measuring inulin permeability (IP) and/or transepithelial resistance (TER) in control, 25(OH)D ₃-, and 1,25(OH)2D ₃-treated human and rabbit corneal epithelial monolayers cultured on permeable inserts. Ca ²⁺ was removed, then reintroduced to the culture medium while IP and TER readings were taken. Occludin levels were examined using Western blotting. Results. All corneal samples were positive for both VDR and 1α-hydroxylase mRNA. All vitamin D metabolites except for unhydroxylated vitamin D ₃ were detected in aqueous and vitreous humor. Epithelial cells showed increased TER, decreased IP, and increased occludin levels when cultured with 25(OH)D ₃ and 1,25(OH) ₂D ₃. Conclusions. We conclude that corneas contain mRNA for VDR and 1α-hydroxylase as well as significant vitamin D concentrations. 25(OH)D ₃ and its active metabolite 1,25(OH) ₂D ₃, both enhance corneal epithelial barrier function.