1173 Inducible DNA repair of UV photoproducts depends on vitamin D receptor

Abstract

Ultraviolet radiation (UV) initiates vitamin D synthesis by converting 7-dehydrocholesterol to pre-vitamin D3 that is converted to 25-hydroxyvitamin D3 (25D3) and then 1,25-dihydroxyvitamin D3 (1,25D3). However, UV also generates DNA damage that is repaired by nucleotide excision repair (NER). We tested the hypothesis that vitamin D signaling elicits compensatory responses to the DNA damage incurred during vitamin D synthesis. Treatment of human keratinocytes with either UVB or with 1,25D3 or 25D3 induced the DNA damage recognition protein, XPC, and induction was suppressed by either siRNA targeting the vitamin D receptor (VDR) or ketoconazole, a broad inhibitor of oxidases that hydroxylate vitamin D3 to active metabolites. Irradiation of cells with UVC through 3 um pore filters resulted in sub-nuclear spots of DNA damage that were probed with antibodies to the major UV-induced DNA lesions, and to XPC. Relative to controls which removed >50% of 6-4 photoproduct (6-4PP) spots and reduced spot intensity by >70% over 2 hours, cells treated with siRNA targeting VDR had no significant change in either spot density or intensity over this time period (p<0.001), indicating a lack of NER. Co-staining of spots for XPC demonstrated that XPC begins accumulating at DNA damage spots within seconds, peaking by 15 minutes following UV damage, and was no longer detectable at DNA damage spots by 90 minutes, consistent with rapid 6-4PP repair. In contrast, siVDR-treated cells, while also rapidly recruiting XPC to 6-4PP, exhibited prolonged retention of XPC at 6-4PP spots at 30 minutes relative to controls. These results indicate that vitamin D and VDR are important for both induction of XPC as well as normal retention of XPC at DNA damage in keratinocytes, and suggest that vitamin D may mitigate the UV-induced DNA damage associated with its own genesis.