Opening of Chloride Channels by 1α,25-Dihydroxyvitamin D3 Contributes to Photoprotection against UVR-Induced Thymine Dimers in Keratinocytes

UVR produces vitamin D in skin, which is hydroxylated locally to 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3). 1,25(OH)2D3 protects skin cells against UVR-induced DNA damage, including thymine dimers, but the mechanism is unknown. As DNA repair is inhibited by nitric oxide (NO) products but facilitated by p53, we examined whether 1,25(OH)2D3 altered the expression of nitrotyrosine, a product of NO, or p53 after UVR in human keratinocytes. 1,25(OH)2D3 and the nongenomic agonist 1α,25-dihydroxylumisterol3 reduced nitrotyrosine 16 hours after UVR, detected by a sensitive whole-cell ELISA. p53 was enhanced after UVR, and this was further augmented in the presence of 1,25(OH)2D3. DIDS (4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid), a chloride channel blocker previously shown to prevent 1,25(OH)2D3-induced chloride currents in osteoblasts, had no effect on thymine dimers on its own but prevented the 1,25(OH)2D3-induced protection against thymine dimers. Independent treatment with DIDS, at concentrations that had no effect on thymine dimers, blocked UVR-induced upregulation of p53. In contrast, reduction of nitrotyrosine remained in keratinocytes treated with 1,25(OH)2D3 and DIDS at concentrations shown to block decreases in post-UVR thymine dimers. These results suggest that 1,25(OH)2D3-induced chloride currents help protect from UVR-induced thymine dimers, but further increases in p53 or reductions of nitrotyrosine by 1,25(OH)2D3 are unlikely to contribute substantially to this protection.