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Jawaher A. Alsalem, Radhika Susarla, Miguel Coca-Prados, Rosemary Bland, Elizabeth A. Walker, Saaeha Rauz, Graham R. Wallace; Endogenous Production of 1,25(OH)2D3 by Ocular Barrier Cells. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2915.
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There is an increasing evidence that many cells and tissues including those at barrier sites express the vitamin D receptor (VDR) and 1-alpha-hydroxylase (CYP27B1), the enzyme required to convert inactive 25(OH)D3 into active 1,25(OH)2D3 . These can activate and locally produce 1,25(OH)2D3, which functions as a potent immunomodulatory hormone. Blood-aqueous (ciliary body and iris) and blood-retinal barriers maintain an immune privileged environement in the eye. Corneal endothelium contributes to barrier function between corneal stroma and aqueous humor. This study aims to investigate the role of 1,25(OH)2D3 in ocular barrier function.
Human adult retinal pigment epithelial (ARPE-19), non-pigmented ciliary body epithelium (ODM2), and corneal endothelial (HCEC-12) cell lines were cultured. Gene expression for elements of the vitamin D and Toll like receptor pathways were examined by conventional and real-time polymerase chain reaction (RT-PCR) both in the absence and presence of TLR ligands and vitamin D substrate (25(OH)D3). Protein expression was analysed by immunohistochemistry. Cytokine and 1,25(OH)2D3 production were measured by enzyme immunoassay.
ARPE-19, ODM2, HCEC-12 cells expressed mRNA for VDR, CYP27B1, and the catabolizing enzyme, 24-hydoxylase (CYP24A1). This was confirmed by immunohistochemistry in non-pigment ciliary body epithelium and corneal endothelium tissue sections and by immunofluorescence in ARPE-19 cells. ARPE-19 and HCEC-12 cells are able to convert 25(OH)D3 into active 1,25(OH)2D3 and HCEC-12 show higher levels than ARPE-19. ARPE-19 cells were responsive to TLR ligand stimulation but this did not have an effect on elements of vitamin D gene expression. Conversely, the addition of 25(OH)D3 did not influence cytokine production by these cells.
Our data show for the first time that retinal pigment epithelial and corneal endothelial cells have the ability to convert inactive 25(OH)D3 into active 1,25(OH)2D3 and this is not influenced by TLR ligand stimulation. Vitamin D3 produced by ocular barrier cells may possibly play a role in ocular immune privilege.
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