Vitamin D receptor and metabolite effects on corneal epithelial cell gap junction proteins.

Vitamin D is a fat-soluble prohormone that can be activated both systemically and within individual tissues. Our lab has previously demonstrated that the corneal epithelium can activate vitamin D and that the vitamin D metabolites 1,25(OH)2D3 and 24R,25(OH)2D3 can affect corneal epithelial migration, proliferation, and tight and gap junction function. These vitamin D-derived metabolites signal through the vitamin D receptor (VDR). The purpose of this study was to specifically determine the effects of 1,25(OH)2D3 and 24R,25(OH)2D3 on corneal epithelial cell gap junction proteins. Connexin (Cx) 26, 30 and 43 protein expression was detected in a human corneal epithelial cell line (HCEC), wild type and vitamin D receptor knockout (VDR-/-) mouse corneas, and cultured mouse primary epithelial cells (MPCEC). In vitro gap junction function was assessed using the scrape loading/dye transfer assay. HCEC and MPCEC were treated with 1,25(OH)2D3 or 24R,25(OH)2D3. Western blotting was used to detect gap junction proteins. Vitamin D3 effects on epithelial intracellular Ca++ (Ca++i) were determined using the dye Cal-520. Cx26 and Cx43 protein levels were significantly increased in HCEC and MPCEC treated with both 1,25(OH)2D3 and 24R,25(OH)2D3. Cx30 and Cx43 protein levels were also significantly increased in VDR-/- MPCEC. In vitro gap junction connectivity was significanlty enhanced in HCEC and MPCEC cultured with 24R,25(OH)2D3 and 1,25(OH)2D3. Ca++i was not affected by 1,25(OH)2D3 or 24R,25(OH)2D3 in HCEC or MPCEC. We conclude that both 1,25(OH)2D3 and 24R,25(OH)2D3 are positive regulators of connexin proteins and gap junction communication in the corneal epithelium. These vitamin D metabolites appear to signal through both VDR-dependent and -independent pathways. The effects of vitamin D on corneal epithelial gap junctions do not seem to be dependent on Ca++i.